Laser Physics IILaser Physics II

PH482/582-2E (Mirov)PH482/582 2E (Mirov)

C l C t LColor Center Lasers

Lectures 4-5

Spring 2009

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OUTLOOKOUTLOOK1. Types of color centers in ionic crystals and principles of operation2. Crystal hosts for active elements of color center lasers 3. Color center formation in ionic crystals

3a. Additive coloration3b. Electrolytic coloration3c. Color center formation in alkali-halide crystals under ionizing irradiation3c. Color center formation in alkali halide crystals under ionizing irradiation

LiF:F2- active element optimization

LiF:F2+ active element optimization

4. Major spectroscopic characteristics of color centers5 CW d i CW l t l ti (T 77K)5. CW and quasi-CW color center laser operation (T=77K)6. Pico and femtosecond quasi-CW color center laser operation (T=77K)7. Room temperature CCL operation

7a. CW and quasi-CWq7b. High peak power room temperature CCL operation in mode-locked regime7c. High energy and power color center lasers7d Color center energy and power amplifier

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7d. Color center energy and power amplifier7e. Narrowline tunable color center lasers

8. Practical applications

Major LiteratureMajor Literature1. "Room temperature tunable color center p

lasers", T.T.Basiev, S.B.Mirov, Laser Science & Technology book Ser., 16, 1-160, V S Letokhov C V Shank Y R Shen H WalterV.S.Letokhov, C.V.Shank, Y.R.Shen, H.Walter, Eds., Gordon and Breach Science Publ./Harwood Acad. Publ. (1994).( )

2. T.T. Basiev, P.G. Zverev, and S.B. Mirov. “Color Center Lasers”, Handbook of Laser T h l d A li ti I tit t fTechnology and Applications. Institute of Physics publishing. Eds. C.E.Webb and J.D.C.Jones, B1.8, 1-23, (2003).

2

J C Jo es, 8, 3, ( 003)

1. Types of Color CentersIn general, the major crystals where laser active CCs can be developed are

d i it d d fl id d hl id f Li N K d Rb lk lipure and impurity-doped fluorides and chlorides of Li, Na, K, and Rb alkali metals [i], [ii], [iii], [iv], as well as CaF2 and SrF2[v]. Laser oscillation has also been reported using CCs in Al2O3[vi],[vii], diamond[viii], MgF2[ix], and the compound fluorides KMgF3 [x] and YLiF4 [xi]. In the primal state these crystals are optically transparent Under irradiation with high-energy electrons neutronsare optically transparent. Under irradiation with high energy electrons, neutrons, γ-rays, X-rays, or hard UV, or calcination in alkali metal vapor (additive coloration), anionic vacancies appear in the crystal lattice and serve to localize free electrons. The absorption bands of these (F) centers (vacancy + electron) give a typical coloring to the crystals.

[i] Yu. L. Gusev, S. I. Marennikov, V. P. Chebotaev, Bull. Acad. Sci. USSR, Phys. Ser., 44, 15 (1980).[ii] I.A.Parfianovich, V.M.Khulugurov, B.D.Lobanov, and N.T.Maksimova, Luminescence and stimulated

emission of color centers in LiF, Bull. Acad. Sci. USSR, Phys. Ser., 43, 20-27 (1979). [iii] W. Gellermann J. Phys. Chem. Solids, 52, 249 (1991).[ ] y , , ( )[iv] L.F.Mollenauer, Color Center Lasers, in Tunable Lasers, L.F.Mollenauer and J.C. White (Eds.), Vol. 59,

pp. 225-277, Berlin, New York: Springer Verlag (1987).[v] V. A. Archandelskaya and P. P. Feofilov, Sov. J. Quantum Electron., 7, 657 (1980).[vi] E.F.Martynovich, V.I.Baryshnikov, V.A.Grigorov, Visible lasing of Al2O3 color center crystals at room

temperature, Opt. Commun., 53, 257-258 (1985).p , p , , ( )[vii] A.P.Voytovich, V.A.Grinkevich, V.S.Kalinov, S.A.Mikhnov, Spectroscopic and oscillation characteristics

of color center sapphire crystals in the range of 1.0 μm, Kvant.Electron., 15, 318-320 (1988); Sov. J. Quantum Electron. (1988).

[viii] S.C.Rand, L.G.DeShazer, Visible color-center laser in diamond, Opt.Lett., 10, 481-483 (1985).[ix] A.P.Shkadarevich, A.P.Yarmolkevich, New laser media on color center compound fluoride,

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[ ] , , p ,Inst.Phys.AN BSSR, Minsk, USSR, preprint 24 (1985).

[x] G.Horsch, H.J.Paus, A new color center on the basis of lead-doped KMgF3, Opt. Commun., 60, 69-73 (1986).

[xi] T.T.Basiev, F.A.Vakhidov, S.B.Mirov, Radiational transformations in a new LiYF4 laser crystal with color centers, Kratkie Soobsheniya po Fizike, 7, 3-5 (1988); Sov. Phys.Lebedev Inst.Rep., 7, 1-5 (1988).

1. Structure of the simplest color centers in LiF crystalsF F2

e-

e- e-

F2-F2

+

e-

e- e-

e

e--electron

e-

Anion F-- ions

4

Vacancy F - ions

Li+-- ions

1. Ionic configurations of other simple F type color centerscenters

FB(I) centers FA(I) centers in the ground and excited t t

B( ) ce e sin the ground and excited states

states

FA(II) centers in the ground and excited

FB(II) centers in the ground and excited states

5

statesstates

1. Models of (a) F2 CCs in alkali-halide crystals; (b) (F ) CC i lk li th fl id(b) (F2)A CCs in alkali-earth fluorides

6

1. Two possible arrangements of four F centers to form N centers.

These arrays are either a tetrahedron or a parallelogram

7

1. Principles of CCL operations1 P di ti i b b d i th id1. Pump radiation is absorbed in the wide

band of the electric-dipole electron-vibrational transition 1→2.

2. In a time on the order of 10-12-10-13 s,

E

,radiationless relaxation to the minimum of potential curve of the excited electronic state, 2→3 occurs, accompanied by a mutual

E2

2 p yrearrangement of the neighbouring ions and by phonon emission.

3. Then a radiative electron- vibrational transition (3→4) occurs with a1 3 transition (3→4) occurs with a probability of A =107-108 s-1, followed by

4. another rapid vibrational relaxation (4→1) to the potential curve minimum

E1

1 3

(4→1) to the potential curve minimum of the ground electronic state with resetting of the spatial ion configuration. Disregarding the details, one may

id thi h th f l lQ4

8

consider this scheme as the four levels laser scheme of oscillation.

1. Principles of CCL operations

The principles of CCL operation can be accounted for

i th F kusing the Frank-Condon configuration diagram whichdiagram, which describes the optical properties of an electron systemelectron system interacting with molecular or lattice vibrations: in particular, the optical transition of CCs.

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1. Two different groups of CCC.) ti t RT b) ti t T<77Ka) operating at RT; b) operating at T<77K

10

1. Major Defects ClassificationEdge Dislocations Screw Dislocations

1. Major Defects Classification

DEFECTS

LINE IMPERFECTIONSLaser active optical centers

Point Defects

f

Cation Vacancies

centers

Impurity Impurity-Vacancy Dipoles Intrinsic defects

CationCr3+; Ti3+

Nd3 E 3

AnionOH-

O

Cation-VacancyFA; FB; (F2

+)A;M F M F

Anion-VacancyO--Va

+

Anion Vacancies

HoleNd3+;Er3+

Ho3+O2

- Me+-F; Me++-F2-;

Zcenters :F-Me++Vc

-

Electron centers(Color Centers)

HoleTrappingCenters

SimpleF

AggregateF F + F F F + F F

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F F2; F2+; F2

-; F3; F3+; F3

-; F4

2. Main physicochemical, mechanical and optical

characteristics of the most promising crystal hosts with CCs

Crystal ρ ( / 3)

d H (kg/mm2)

Solubility (g/100g

Tm

(K)K

(W/mK)RT*

(W/m)n dn/dT x

105Ithr x 10-12 (W/m2)

Transparancy range (g/cm3)

(oA)

(kg/mm2) (g/100g H2O)

(K) (W/mK) (W/m) 105

(K-1) (W/m2) range

at α=1cm-1 level LiF 2.64 4.03 99-102 0.12 1121 14.2 43-143 1.387 -2.9 3600 0.11-6.6 NaF 2.79 4.62 60 4.2 1270 9.2 - 1.321 -1.8 1400 0.16-11.2 NaCl 2.17 5.64 15.2-18.2 36.0 1074 6.4 - 1.53 -3.7 200 0.17-18.0KF 2.50 5.35 - 94.9 1130 - - - - - 0.2-15.0 KCl 1.99 6.29 7.2-9.3 37.4 1049 6.0 - 1.48 -3.3 700 0.18-23.0 KBr 2.75 6.60 6-7 70.9 1007 4.8 - 1.54 -3.2 500 0.21-28.0 KI 3.13 7.07 5 144.0 959 2.1 - 1.64 -4.5 200 0.3-35.0 RbCl 2 76 6 58 94 2 717RbCl 2.76 6.58 - 94.2 717 - - - - - -MgF2 3.18 a=4.64

c=3.06 576 0.0076 1,536 21 ⊥ c

30⎥⎥ c 470 n0=1.373

ne=1.385 11.2 5.8

1000 0.1-7.0

CaF2 3.18 5.46 120-163 0.0016 1,676 9.7 - 1.429 -1.05 >1000 0.13-9.4 SrF2 4.24 5.79 144 <0.1 1,190 - - - - 0.1-9.0Al2O3 3.974 a=4.76

c=13.0 2100 0 2,313 35.0 10,000 n0=1.765

ne=1.757 1.3

0.97 1900 0.18-.5.1

Diamond 3.515 3.57 8820 0 3,770 900.0 - 2.40 0.4 12000 0.24-2.7 Y3Al5O12 4.55 12.0 1350 0 2,223 13.0 790 1.815 1.05 2000 0.21-5.3 ED-2 glass 2 539 - - - 582 1 4 140 1 56 0 3 -

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ED 2 glass 2.539 582 1.4 140 1.56 0.3

3. Methods of Color Center Formation

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3a. Formation of F centers by Additive Coloration (AD)( )

AC involves bringing the crystal in equilibrium with a bath of alkali vapor.The F-centers are formed at the crystal surfaces and fill up the body of the crystal through diffusion Assume that Nbody of the crystal through diffusion. Assume that No –equilibrium density of F centers; N′- the metal vapor densityUsually No=αN′, where α=2-3 for alkali-halides. At the beginning of coloration the F centers are concentrated at the crystal surfaces For a thin slab the behavior at later timescrystal surfaces. For a thin slab, the behavior at later times (t>τD) is

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

Do

tlxNtxN

τπ

πexpsin41),(x

Where D-diffusion constant; τD-diffusion time

⎦⎣ ⎠⎝ Dl τπ

l 2( ) ⎟

⎞⎜⎛ T

l

For KCl: T0=14,400K; D0=1.22x102cm2/sFor T=600°C; D=8x10-6cm2/s

Dl

D 2πτ = ( ) ⎟

⎠⎞

⎜⎝⎛−=

TTDTD 0

0 exp

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For T=600 C; D=8x10 cm /sFor l=2 mm; τD=8.4 min and t=30 min for practically uniform

coloration

3b. Electrolytic colorationr.

p.a. a. Thermal insulation cap

b. Quartz tube

q.

o.

c.

b. c. Crystal sample (ZnSe)thickness = 4.0mm

d. Chromium (Cr) foil thickness = 1.0mm

n.

m.

l.

d.

e.

e. Electrode stagef. Adjustment spring assemblyg. Ceramic baseh. Cathode connection

j

f.

g

i. Anode connectionj. Gas inlet portk. Threaded assembly rodl. Thermal insulation wrapping

k.

j.

i. h.

g. m. Anode plate nutn. Inert/evacuated atmosphereo. Gas exhaust portp. Cover plate and stand bracket

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q. Cover plate nutr. Steel support stand and base

3c. Color center formation in alkali-halide crystals under ionizing irradiation

• The formation of aggregate color centers in alkali-halide crystals under ionizing radiation is a complicated process involvingunder ionizing radiation is a complicated process involving emergence, separation, and recombination of primary Frenkel defects, association into aggregate F2, (F2)A, F2

+, (F2+)A, F2

-, F3, F3+,

F3- and other color centers and recharging of color centers by

electrons and band holes The defect formation may run fast orelectrons and band holes. The defect formation may run fast or slow. Decomposition of self-localized excitons into primary radiational defects and recharging of color centers are relatively fast processes (10-12 -10-7 s). Slow processes run either due to spatial diffusion of the defects and their associates or due to diffusion ofdiffusion of the defects and their associates, or due to diffusion of self-localized holes (resulting in color centers recharging).

• All the above processes determine the efficiency of formation of any type of color centers. These processes depend upon the t t f i di ti d t f th t l i ittemperature of irradiation and storage of the crystal, impurity composition of the initial material, ionizing radiation dose power and irradiation dose.

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3c. Color center formation in alkali-halide crystals under ionizing irradiationy g

• Separated electrons and holes, free and self-localized excitons are generated in alkali-halide crystals under ionizing irradiation. Self-localized excitons decompose with the emergence of pairs of F-centers and interstitial halogen atoms (H) A fast recharging of thesecenters and interstitial halogen atoms (H). A fast recharging of these pairs under flux of electrons gives rise to actually simultaneous formation of the anion vacancies Va+ and interstitial halogen ions Ia

(1)e F Ho ⇒ + (1)

(2)• The mechanisms of formation of aggregate centers through

e F Hs ⇒ +

e V Iso

a a⇒ ++ −

migration of anion vacancies are mainly going through the following route: charged F2

+ centers first appear, and then capture electrons to produce neutral F2 centers

(3)

(4)

V F Fa+ ++ ⇒ 2

F e F2 2+ + ⇒

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3c. Color center formation in alkali-halide crystals under ionizing irradiation

• Simultaneously with a fast process of electron capture F + centers may• Simultaneously with a fast process of electron capture, F2 centers may take part in a slow temperature-dependent migration process. Colliding with the F, F2

+ and F2- CCs, they form more complex CCs - F3

+, F4+ , and F4 ,

respectively: (5)F F F+ ++ ⇒ (5)

(6)

(7)

F F F2 3+ ⇒++ ⇒+ 422 FFF

422 FFF ⇒+ −+(7)

whose further aggregation leads to the appearance of colloid particles in the crystal. The processes of F2 CCs formation by scheme (3.4) are competing with the processes of their ionization due to a fast capture of free electrons:

422 FFF ⇒+

(8)or holes

(9)

F e F2 2+ ⇒ −

+⇒+ FhF (9)or due to diffusion processes involving mobile anion vacancies

(10)

⇒+ 22 FhF

V F Fa+ ++ ⇒2 3

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and self-localized holes(11)F V Fk2 2+ ⇒ +

3c. LiF:F2- active element optimization

• The LiF:F2- crystals combining unique modulation, thermal, and operational

characteristics, are now widely used as active media in tunable lasers and as nonlinear elements with saturable absorption for neodymium laser Q-switches. However, available techniques of such laser crystals preparation , q y p pfail to produce optically dense, high-contrast media required for a series of applications. Usually, the value of active absorption at the working wavelength does not exceed 0.4-0.6 cm-1 at a contrast (ratio of the active absorption coefficient to the loss coefficient at =1.06 µm) of 10-20. Under the action of γ quanta commonly used for producing LiF:F - laser crystalsthe action of γ-quanta commonly used for producing LiF:F2 laser crystals, the F2

+ CCs emerging by reaction (3) may be involved in three processes: a) fast process of electron capture to produce F2 CCs (reaction 4) and F2

-

CCs (reaction 8); b) slow diffusion processes of aggregation of the type 5, 6, 7; c) dissociation on the F center and anion vacancy F2

+→ F + Va+. , ; ) y 2 →

• The rates of the processes and final concentration of CCs depend on the impurity composition of the initial material, irradiation dose, power, and crystal temperature. Varying the ratio of these parameters, one cancrystal temperature. Varying the ratio of these parameters, one can essentially influence the processes of F2

- CC formation in LiF.

• When we optimized the procedure of LiF crystal treatment under ionizing irradiation so, that the processes 3, 4 were efficient and 5-7 – negligible,

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irradiation so, that the processes 3, 4 were efficient and 5 7 negligible, maximum active absorption of F2- CCs at the wavelength 1.064 µm exceed 1.5 cm-1 at a contrast as high as 20 – 40.

3c. Optically Dense Electron Irradiated LiF:F2-

t lcrystals

20

3c. LiF(F2→F2+) crystals optimization

21

3c. Efficient, RT Stable LiF:F2+** Crystals

F ti Of ThFormation Of The Thermostabilized F2

+** Active Color Centers

e F Hso ⇒ + ; e V Is

oa a⇒ ++ − ; V F Fa

+ ++ ⇒ 2 ; F e F2 2+ + ⇒

F F F2 3+ ++ ⇒ ; F F F2 2 4

+ ++ ⇒ ; F F F2 2 4+ + ⇒ ; V F Fa

+ ++ ⇒2 3

1. LiF crystals doped with LiOH, Li2O y pand MgF2 can be grown by any method which assures good optical quality

2. Crystals are subjected to γ -irradiation X ray or electron

e F Hso ⇒ + ; e V Is

oa a⇒ ++ −

2( )OH O V H o− − +irradiation, X -ray or electron irradiation with a dose of 2.5x103 - 1.5x104 Q/kg at a temperature lower than TV -- temperature of Va

+ mobility in LiF crystals.

2 2( )OH O e V Ha io+⇒ + + + ,

2( )OH V e O V O Ha a io− + −− + −+ + ⇒ + + ,

y y3. Crystals are heated up to TO-V <T<

TF and stored in the refrigerator for a period up to one month (TF corresponds to mobility of F2

+CC and T to O--V + dipoles

V F Fa+ ++ ⇒ 2

O V F F Oa−− + + −−+ ⇒ 2

Me V F Me V V Vc c a c2+ − + − + −+ ⇒ + ; V V F F Va c c

+ − + −+ ⇒ 2

and TO-V to O VA dipoles.4. Crystals may be reirradiated at the

temperature T< TV with a dose of 25-250 Q/kg and then subjected to the procedure described in 3.

F→Va+

22

p5. Crystals heated up to RT and are

stored for a period of some months.

O V F F Oa−− + + −−+ ⇒ 2 ; V V F F Va c c

+ − + −+ ⇒ 2 ; F O e F O2 2+ − + −−+ + ⇒

F F F2 3+ ++ ⇒ low efficiency scince NF is small

3c. Efficient, RT Stable LiF:F2+** Crystals3c c e t, Stab e 2 C ysta s

Photostability Of Active CC As Well As Other CC That MayPhotostability Of Active CC As Well As Other CC That May Be Ionized By The Powerful Pump Laser Excitation

1. The technology of LiF should provide a highest possible concentration of the active CC. The pump radiation will be p ppredominantly absorbed by the photostable F2

+ like centers.

2. Pump radiation from one hand should match the absorption band of F +** color and from another shouldabsorption band of F2 ** color, and from another, should be longer than the threshold wavelength determining the process of two step ionization of the neutral F2 centers (590 nm) ( )

3. the wavelength of the pump radiation shouldn’t match the absorption bands of the parasitic aggregate CC (500-600 nm) that may occur in the crystal and result in decreasing of the efficienc of lasing

23

of the efficiency of lasing

3c. Novel LiF:F2+** Crystals

(U S P t t N 5 796 762 )(U.S. Patent No. 5,796,762 )m

-1

8

10

F C enter 60000

70000

80000

532 nm Excitation633 nm Excitation

Abso

rptio

n C

oeffi

cien

t, cm

4

6

F 2+**

F 2+**-like

F 2 C enter

Inte

nsity

, arb

. uni

ts

20000

30000

40000

50000

F requency, H z (x1014)246810

A

0

22

Wavelength, nm600 700 800 900 1000 1100 1200 1300

0

10000

20000

uni

ts

30000

40000

New F2+**-like Center

(measured at 24 K)

F2+** Center

A

Inte

nsity

, arb

.

10000

20000

F2 Center(measured at 13 K)

B

24W avelength, nm

700 800 900 1000 1100 1200 1300 14000

3c. Proposed model of F2+** stabilized CC in LiF

Four defect configurations with different symmetries are possible for closest (110) neighboring positions of the F center vacancy site (labeled 1 to 4) with respect to the

charge compensating vacancy

F2+**

−−++−− ⎯→⎯+ RTa OFFVO 2)1

O--

Va+

Li+ 1

+−−+−

−−

→++

+⎯→⎯RT

oi

a

VOVeO

HOOH

2

)2

)

γ

e-

F-F- Li+

Li+

2 +−−

−−++−−

+++⎯→⎯

⎯→⎯+

⎯→⎯++

oia

RTa

aa

HVeOOH

OFFVO

VOVeO

2

2

2)(2 γ

Va+

Li+

Li+ Vc-

−

−+−−+−

⎯→⎯+

++⎯→⎯++

ioi

oiaa

HeH

HOVOeVOH )(2 γ

F-Li+

Li+

F-

−++−++

−−+−+

+⎯→⎯+

⎯→⎯++

RT

Rt

VVMgFVMg

OFeOF 22

)4

)3

25

34−+−+ ⎯→⎯+

+→+

cRT

ca

cac

VFFVV

VVMgFVMg

2

)4

3c. Key Advantages of the Optically Dense C l C t A ti M diColor Center Active Media

Th k d f h i ll d i di h• The key advantages of the optically dense active media are the high efficiency of the pump energy conversion into the output generation and an opportunity for further miniaturization of the laser devices on their basis. The aforesaid advantageous are undeniably t d f th i i l t t l H hi htrue and for the ionic color center crystals. However, high-concentrated color center crystals exhibit some new special features.

• light to light efficiency increasingg g y g• color center crystal photostability may be essentially improved• it is possible to reach high gain coefficients and laser threshold

conditions on the colored layers with a thickness of about 0.01 1mh thi l d fil f di l t i t l• such a thin colored films of dielectric crystals are

semicanductor in character. This fact provides a theoretical reason enough to develop principally new lasers based on the dielectric crystals with an electrical pumping

26

3c. LiF:F2 ultimately high concentrated media3c 2 u t ate y g co ce t ated ed a

e 150 keV

I= 20 A

d=90 microns

LiFk=1000 cm-1

•A highly-concentrated (k = 400 cm-1) LiF:F2 crystal with a 90 m thickness of coloration was developed under low energy electron irradiation. This enables the threshold of generation of the LiF:F2 crystals to be significantly decreased to such a low level of pump radiation ( 3 μJ), that the probability of the F2 color centers two step photoionization started to be negligible.

27

3c. LiF:F2 ultimately high concentrated media3c 2 u t ate y g co ce t ated ed a

X rays

YAG:Nd

1.06

LiFE=1J k=1500 cm 1t=4 nsf=10Hz

k=1500 cm-1

d=4 microns

A new method of radiational coloring of the LiF crystals with quanta of soft X-ray radiation, generated with the use of a laser (1.06 μm) plasma source. The absorption coefficient in the F2 band was about 1500 cm-1,

28

thickness of coloration - 4 μm.

4. Major spectroscopic characteristics of color centers

29

4. Luminescence bands of (a) FA(II) and FB(II) centers; (b) F2

+ centers, and (c) FA(Tl) centers in alkali-halides t 77Kat 77K

30

4. Absorption (solid lines) and luminescence (dashed lines) bands of CCs in Al2O3crystal at 4.2 and 300 K,

respectively.Numbers near the bands are wavelengths of zero

phonon linesp

31

5. CW and quasi-CW color center laser operation (T=77K)p ( )

• The pump density of several kW⋅cm-2 is needed to reach p p ythe threshold of CCL oscillation. This is easily achieved for pulsed pumping by a flash lamp emitting few tens of joules during a millisecond pulse. In this case the lasing j g p gvolume of color center crystal can be as large as 1 cm-3

or even more.• In order to get the same pump densities for cw laserIn order to get the same pump densities for cw laser

pumping with power of 0.1-10 W, one have to use strong focusing of the pump radiation and put the CCL crystal at the focal point or cavity waist of 0 1-1 mm in diameterthe focal point or cavity waist of 0.1 1 mm in diameter. The crystal length is limited by the confocal parameter of focused beam and usually does not exceed 1 cm (typically it is a few mm)

32

(typically it is a few mm).

5. CW and quasi-CW color center laser operation (T=77K)operation (T=77K)

• The most popular optical scheme used now for cw CCL ti i thCCL operation is the Kogelnik scheme with a Brewster cut active element and with V-shape or Z-shape folded cavity to compensate spherical lens or mirror astigmatism [i]astigmatism.[i]

•[i] H. W. Kogelnik, E. P. Ippen, A. Dienes, and C.

sin tann

n n

tf

θ θ⋅ ⋅− +

=4

2 2 121 1( )( )Ippen, A. Dienes, and C.

V. Shank, IEEE J. of Quantum Electron., QE-8, 373 (1972).

n n f+1 1( )( )

W f λ b f b f⎛⎜

⎞⎟

2

The beam waist diameter W0 and confocal length b can befound from the formulas

33

W f l022= λ

π t b fl f

fl

≤ = =⎝⎜

⎠⎟

2 2 2

5. Cavities of CW CCL5 Ca t es o C CC

34

5. CW and quasi-CW color center laser ti (T 77K)operation (T=77K)

• There are two sets of the most popular regimes p p gof operation in cw mode.

1. The first one is the narrow line (single f ) b dl bl CCL ill i ffrequency) broadly tunable CCL oscillation for visible and IR spectroscopy, analytical application and fundamental metrologyapplication, and fundamental metrology.

2. The second is the mode-locked pico- and femtosecond pulse operation regime for time resolved transient spectroscopy, high peak power nonlinear conversion, and fast telecommunication systems

35

telecommunication systems.

5. CW and quasi-CW color center laser operation (T=77K)ope at o ( )

• Single-mode, single-frequency operation of a KCl:Li FA(II) CCL under krypton ion laser pumping at 647 nm with linewidth less than 260 KHz and tunability in 2 5 2 9 μm spectral range

150tunability in 2.5-2.9 μm spectral range realized with the pair of birefringent plates and an intracavity solid etalon was obtained in Ref. [[i]]. By the active frequency stabilization, linewidth as low as 2 KHz was realized later in [ii] w

er, m

W

100

KCl:Li+

low as 2 KHz was realized later in [ii]. To obtain a single mode operation and to improve linewidth and wavelength stability, the ring cavity operation can be also applied [iii], [iv]. Typical output power of FA(II) CCL varies from tens to O

utpu

t Pow

50RbCl:Li+

power of FA(II) CCL varies from tens to hundreds milliwatts in 2.3 - 3.5 mm spectral region

25 30 35O

0

• [i] R. Beigang, G. Liftin, and H. Welling Opt. Commun. 22, 269 (1977).

• [ii] Ch. Breant, T. Baer, D. Nesbitt, and J. L. Hall in Laser Spectroscopy VI (Edited by H. P. Weber and W. Luthy) p.138, Springer, Berlin (1983).[iii] T F J h t d W P ffitt IEEE J Q t

Wavelength, μm

2.5 3.0 3.5

Tuning curves of KCl:Li+ and

36

• [iii] T. F. Johnston and W. Proffitt IEEE J. Quantum Electron., QE-16, 483 (1980).

• [iv] C. R. Pollock and D. A. Jennings Appl. Physics, B28, 308 (1982).

Tuning curves of KCl:Li+ and RbCl:Li+ FA(II) CCL

5. CW and quasi-CW color center laser ti (T 77K)operation (T=77K)

• With the Kr laser pump source, LiF:F2+, CCL operation p p 2 p

with a very high slope efficiency of 60%, close to the physical limit (output to pump photon energy ratio) 70%, was obtained. A wide tuning range 0.84÷1.04 μm was g g μcovered with the cw output power up to 1.1 W in pure cw regime and under synchronous picosecond pumping 6. In the last case, 25 times shortening from 100 ps pump , g p p pto 4 ps CCL output was demonstrated [i].

• [i] L F Mollenauer Color Center Lasers in Tunable• [i]. L.F.Mollenauer, Color Center Lasers, in Tunable Lasers, L.F.Mollenauer and J.C. White (Eds.), Vol. 59, pp. 225-277, Berlin, New York: Springer Verlag (1987).

37

5. CW and quasi-CW color center laser ti (T 77K)operation (T=77K)

• One of the main problems of the LiF(F2+) CCL is fast p ( 2 )

optical bleaching due to orientational diffusion of F2+

CCs. The maximum cw lasing output (2.7 W) was recorded in the KF(F2

+) CCL under 1.064 μm Nd3+:YAG ( 2 ) μpumping with the tuning range of 1.22÷1.5 μm. [i]

• In the case of (F2+)A color center, the KCL:Li crystal

showed a very good performance: Pout=400 mW undershowed a very good performance: Pout 400 mW under 1.32 μm cw Nd laser pumping and Eout=2.3 mJ under 1.41 μm pulsed pumping with the tuning range from 2 to 2 5 μm [i]2.5 μm. [i]

• [i] K. R. German, in Handbook of Solid State Lasers (Edited by P. K. Cheo, Chap. 5, Marcel Dekker, New York (1989)

38

York (1989).

5. CW and quasi-CW color center laser ti (T 77K)operation (T=77K)

• The operation of (F2+)H color centers in NaCl crystals doped by OH

ions looks very promising. For the first time it was reported in [i].

• Output power as high as 3.05 W at the slope efficiency of 33% was reported in [ii] under 9W pumping at 1.064 μm and 200 mW collinear auxiliary light of 514 nm.auxiliary light of 514 nm.

• A very wide tuning range, 1.4÷1.92 μm, important for telecommunication study and test with output up to 1.3 W was obtained in [iii] at 6 W 1 064 m laser pumping and only 2 mW ofobtained in [iii] at 6 W 1.064 μm laser pumping and only 2 mW of 532 nm collinear auxiliary light. This laser can also easily operate in the mode-locked regime with picosecond pulses as short as 5 ps.

• [i] F. Rong, Y. Yang, and F. Luty Cryst. Latt. Def. Amorph Mater. 18, 1, (1989).• [ii] R. Beigang, J. J. Wynne, Opt. Lett. 6, 295 (1981).

[iii] J F Pi t E St tt d C R P ll k O t L tt 10 384 (1985)

39

• [iii] J. F. Pinto, E. Stratton, and C. R. Pollock, Opt. Lett., 10, 384 (1985).

5. CW and quasi-CW color center laser ti (T 77K)operation (T=77K)

• A stable cw laser operation i h i h ili li h P =5 Wwithout using the auxiliary light

was also obtained with the help of FA(Tl) color centers in KCl:Tl+, KBr:Tl+, and KF:Tl+

0.8

1.0 T=22%

W 0.15

0.20Pin=5 WPabs.=1.7 W

Pin=6.8 WPabs.=2.8 W

T=22%

KCl:Tl , KBr:Tl , and KF:Tlcrystals. The first of these crystals provides about 1.4 W output at the maximum of the t i ith t bilit 0 4

0.6

tput

pow

er,

0.10

tuning curve with tunability from 1.4 to 1.6 μm at the absorbed pump power ~ 3.5 W of 1.064 μm Nd:YAG pump

0.2

0.4

ou0.05T=5%T=8%

μ p plaser). [i]

• [i] W. Gellermann J. Phys. Chem. Solids, 52,249 (1991).

1.4 1.5 1.6 1.70.0 0.00

40

Tuning curves of FA(Tl) color center laser in KCl:Tl+ (solid lines) and KBr:Tl+ (dashed lines) pumped by Nd:YAG laser radiation (1.064 mm).

6. Pico and femtosecond quasi-CW color center laser operation (T=77K)ce te ase ope at o ( )

• One of the most interesting and important regimes of CCL operation is the mode locked regime when many longitudinal modes of ais the mode-locked regime , when many longitudinal modes of a broadband laser radiate with «coordinated phase». According to the Fourier transformation, this multifrequency cw laser operation leads to a short pulse high peak power quasi-CW lasing with the pulse-to-pulse temporal interval equaling to the laser cavity round trip timepulse temporal interval equaling to the laser cavity round trip time and pulse duration being inversely proportional to the number of longitudinal mode operating in mode-locking regime.

• All types of the mode-locking techniques developed for dye lasers li bl f d l i d l k d CCL C l tare applicable for developing mode-locked CCL. Color center

mode-locked operation is quite similar to the dye laser mode-locking regime with the only one distinction: the excited state lifetime of CC is about τCC=15-1500 ns and is usually larger than the laser cavity

d t i ti t 6 10 hil f th d it i llround trip time, tcavity=6÷10ns, while for the dyes it is smaller (τdye=3÷5ns).

41

6. Pico and femtosecond quasi-CW color center laser operation (T=77K)ce te ase ope at o ( )

• One of the most popular and effective techniques for realization of pico- and femtosecond lasing is synchronous pumping, when the pump laser mode-locking regime is transferred to CCL. In this case, the cavity round trip times g g , y pof both pump and CCLs should be synchronized and the synchronized CCL gain modulation results in pulsed CCL lasing and pulse shortening to duration much shorter than the pump pulse duration.

• Expression for the pulse width [i] shows that it should be proportional to the d CCL i i i h d i l i lpump and CCL cavities mismatch parameters Δ and inversely proportional

to the logarithmic gain loss product .• This means that the shortest pulse duration of CCL can be realized at the

highest gain, pump energy, and power. When using 90 ps pulsed krypton laser synch pumping the LiF:F + CCL pulses had duration of 2 3 ps inlaser synch-pumping, the LiF:F2

+ CCL pulses had duration of 2-3 ps in linear cavities and 0.7 ps in ring cavities.[ii]

• Similar results were obtained in linear cavities of KF(F2+), NaCl(F2

+), and KCL:FA(Tl+) lasers under Nd:YAG synch-pumping and for KCl:Li(FA) and RbCl:Li(F ) CCL under modelocked argon laser pumping)RbCl:Li(FA) CCL under modelocked argon laser pumping).

•[i] Z. A. Yasa, Optics Lett., 8, 277 (1983).

• [ii] N. Langford, K. Smith, and W. Sibbett, Opt. Commun., 64, 274 (1987).

42

6. Pico and femtosecond quasi-CW color center laser operation (T=77K)ce te ase ope at o ( )

• In the passive mode-locking regime with a dye saturable absorber, pulse duration of 180 fs was achieved in a LiF:F + CCL ring cavity atpulse duration of 180 fs was achieved in a LiF:F2

+ CCL ring cavity at the lasing bandwidth of about 60 cm-1.[i] Using a multiple quantum well saturable absorber, a NaCl(F2

+) CCL demonstrates 260 fs pulse operation with 50 cm-1 bandwidth.[ii]B d l i dditi l d l ki t h i f l• By developing new additive pulse mode-locking technique for pulse shortening, 127 fs pulse duration instead of 23 ps from KCl:Tl CCL was realized.[iii] In a similar regime 50fs pulses have been generated by so-called «soliton laser» on KCL:Tl at 1.5 μm. [iv]

•[i] N. Langford, K. Smith, and W. Sibbett, Opt. Lett., 12, 903 (1987).

• [ii] M. N. Islam, E. R. Sundermann, I. Bar-Joseph, N. Sauer, T. Y. Chang Appl Phys Lett 54 1203 (1989)Chang, Appl. Phys. Lett. 54, 1203 (1989).

• [iii] J. Mark, L. Y. Liu, K. L. Hall, H. A. Haus, and E. P. Ippen, Opt. Lett. 14, 48 (1989).

• [iv] L. F. Mollenauer, and R. H. Stolen, Opt. Lett., 9, 13 (1984).

43

7. Room temperature CCL operation7a. CW and quasi-CW

• Room temperature tunable CCLs color center lasers have special importance in comparison with dye and cryogenic CCLs due to theirimportance in comparison with dye and cryogenic CCLs due to their compactness, simplicity and easy to use both for laboratory and field conditions. Lithium fluoride with F2

- CCs is one of the most reliable active media for room temperature tunable operation. D i l t t d d l l d di t d t th• During last two decades only several papers were dedicated to the study of CW and quasi-cw laser operation on LiF:F2

- crystals at T=300 K 3,[i],[ii] Most of them used neodymium solid state lasers as a pumping source.

• [i] T. T. Basiev, Yu. L. Gusev, S. V. Kruzhalov, S. B. Mirov, V. Ya. Petrunkin, Sov. J. Quantum Electron., 12 (1988).

• [i] W Gellermann A Muller D Wandt S Wilk F Luty J Appl[i] W. Gellermann, A. Muller, D. Wandt, S. Wilk, F. Luty, J. Appl. Phys. 61, 1297 (1987).

44

7. Room temperature CCL operation7a. CW and quasi-CW

• For the first time, pure cw LiF:F2- lasing

has been attained using the crystals with kabs=1.3 cm-1 at a pump wavelength of 1 064 mm and low

L iF :F-wavelength of 1.064 mm and low

losses (KL = 0.01cm-1) in the lasing region of 1.1-1.2 μm [i]. A ring cavity with 1 m base and astigmatism compensation incorporated two plane

L iF :F2

p p pmirrors (2 and 3) with 99.7% and 99% reflectance in the lasing region and two 100% reflectance spherical mirrors (1) with the curvature radius of 10cm. For a pump power of 3 W the lasinga pump power of 3 W, the lasing output power was 60 mW. These results were further improved: the maximum output CW power of LiF:F2

-

laser exceeded 100 mW at a pump p ppower of 5 W. The LiF:F2- active element preserved all its properties after a 30 h performance.

•[i] T T Basiev Yu L Gusev S V Kruzhalov S Typical cavity for CW or Quasi CW

45

[i] T. T. Basiev, Yu. L. Gusev, S. V. Kruzhalov, S. B. Mirov, V. Ya. Petrunkin, Sov. J. Quantum Electron., 12, (1988).

Typical cavity for CW or Quasi-CW ring color center laser

7. Room temperature CCL operation7 CW d i CW7a. CW and quasi-CW

• The first results on frequency tuning and picosecond generation at room temperature of CW LiF:F − laser were obtained underroom temperature of CW LiF:F2

− laser were obtained under synchronous pumping with a continuous train of picosecond pulses of CW-pumped Nd3+:YAG laser. The use of a ring cavity and Lyo filter as a dispersive element provided lasing over 1.1-1.2 μm spectral region LiF:F − oscillation under Nd3+:YAG laserspectral region. LiF:F2 oscillation under Nd3+:YAG laser synchronous pumping provided picosecond pulses with duration of 10 ps and 7 ns axial interval for continuous train of pulses [i]

[i] T T B i Y L G S V K h l S B Mi V Y• [i] T. T. Basiev, Yu. L. Gusev, S. V. Kruzhalov, S. B. Mirov, V. Ya. Petrunkin, Abst. Rep. XIII Inter. Conf. Coherent Nonlinear Opt., Minsk, USSR, Part 2, 256 (1988).

46

7. Room temperature CCL operation7 CW d i CW7a. CW and quasi-CW

50y,

% 30

402

Effi

cien

cy

20

30

1

0

10

Wavelength, μm

1.05 1.10 1.15 1.20 1.25 1.30

Tuning curves of quasi cw oscillation of LiF:F - CCL

47

Tuning curves of quasi-cw oscillation of LiF:F2- CCL

pumped by Nd3+:YAG (1) and Nd3+:YLF (2) lasers.

7b. High peak power room temperature CCL ti i i d f t d ioperation in pico and femtosecond regime.

• Since middle 70th there were a lot of studies on picosecond operation of F2

+; F2+ O-; F2

- and F3-

color centers in LiF and NaF crystals at room temperature Synchronous pumping by the traintemperature. Synchronous pumping by the train of picosecond pulses of ruby, Nd:YAG or Nd:glass lasers was used as the most effective gscheme of CCL pulse shortening. Fast CC gain switching (modulation) leads to the effective CCL pulse shortening depending on degree ofCCL pulse shortening depending on degree of pump power exceeding threshold, number of pulses and length of the train.

48

7b. High peak power room temperature CCL ti i i d f t d ioperation in pico and femtosecond regime.

E i t l t f1

67

Experimental setup of picosecond LiF:F2

- CCL. (1, 2, 3) – end, spherical and cavity mirrors, (4) active 67

5

y , ( )element (LiF:F2

- crystal), (5) prism compensator of the group velocity dispersion, (6) aperture (7) Lyo filter (8)

λ μ= 1 .0 5 5 m

59

aperture, (7) Lyo filter, (8) streak camera, (9) -PC.

Babushkin A V, Basiev T T, Vorob'ev N S Mirov S B

λ μ= 1 .1 0 - 1 .2 2 m

24

3 8

Vorob ev N S, Mirov S B, Prokhorov A M, Serdyuchenko Yu N, Shchelev M Ya., Sov. J.

49

Quantum Electron. 16, 1492 (1986).

7b. High peak power room temperature CCL ti i i d f t d ioperation in pico and femtosecond regime.

• A precise matching of resonator's lengths gave rise to LiF:F2

− femtosecond oscillation with a pulse duration of less then 500 fs atwith a pulse duration of less then 500 fs, at a full train duration of 300-400 ns and a train energy of 250 mJ [i].

• The durations of the subpicosecond pulses were measured directly on the screen of th t k (2) ith l ti fthe streak camera (2) with a resolution of 0.6-0.7 ps as well as with an autocorrelator which provided an average pulse duration in the LiF:F2

- train of less than 0.5 ps. A conservative estimation of th k f 0 5 t t lthe peak power for 0.5 ps output pulses gives a minimum power of 10 MW, which greatly (by 3-4 orders) exceeds the standard peak power generated by ordinary quasi-CW dye and CC lasers.

•[i] Babushkin A V, Basiev T T, Vorob'ev N S, Mirov S B, Prokhorov A M, Serdyuchenko Yu N, Shchelev M Ya., Sov. J. Quantum Electron. 16, 1492 (1986).

Delay AK, ps

-2 -1 0 1 2

50Auto correlation curve of a pulse duration of synchronously pumped LiF:F2

-

7c. High energy and power color center lasersc g e e gy a d po e co o ce te ase s

• Success in the growth of big size and good quality alkali-halide optical crystals and in the development of homogeneous high contrast colorationcrystals and in the development of homogeneous, high contrast coloration technique gives rise to a unique opportunity to design a large scale high power lasers and amplifiers based on CCCs.

• Developing a large scale homogeneously colored LiF:F2− CC active crystals

with a size up to 40×80×200 mm3 with 4 large polished faces allowedwith a size up to 40×80×200 mm with 4 large polished faces allowed utilization of a huge (700 J in 120 ns pulse duration) pump energy from a multistage Nd-glass laser system. In a two pass pumping scheme with the 5 J/cm2 pump energy density a transversely directed lasing of F2

- CCs was realized in the nonselective 300 mm long Fabri-Perot cavity with 50%

t t li U d th diti bl t h th doutput coupling. Under these conditions we were able to reach the record output energy of 100 J and peak power of 1 GW at 1.12 - 1.16 μm and 100 ns pulse duration.[i]

[i] T T B i S V D l h k B V E h S B K t S B Mi• [i] T. T. Basiev, S.V. Dolzhenko, B. V. Ershov, S. B. Kravtsov, S. B. Mirov, V. A. Spiridonov, and V. B. Fedorov, Bull. Acad. Sci. USSR, 52, 164 (1988).

51

7c. High energy and power color center llasers

• For smaller (40×40×20 mm3) LiF:F2− crystal in transversal LiF and ( ) 2 y

Nd:glass coupled cavity arrangement the Nd laser pump to CCL output conversion efficiency was increased up to 80 % with the CCL output energy of 8 J at 1.15 μm [i].

•

52

[i] T. T. Basiev, S.V. Dolzhenko, B. V. Ershov, S. B. Kravtsov, S. B. Mirov, V. A. Spiridonov, and V. B. Fedorov, Bull. Acad. Sci. USSR, 52, 164 (1988).

7c. Dependence of LiF:F2- Laser Output Energy on Mirror (R4) Reflectivity for Different LiF Crystal Optical Densities,y y p ,

Measured in a Transverse-Oriented Coupled Cavity Schemeer

gy fo

rD

ensi

ties,

6

7

Out

pyt E

neF

Opt

cal D

J

3

4

5 E out for LiF OpticalDensity 1.15E out for LiF OpticalDensity 0.7

iF L

aser

Off

eren

t LiF

0

1

2E out for LiF OpticalDensity 0.45

LiF Laser Output MirrorReflectivity (R4), %

Li

Dif 0

0 20 40 60 80 100

53

7c. Dependence of LiF:F2- Laser Output Energy on Mirror (R4) Reflectivity for Different LiF Crystal Optical Densities,y y p ,Measured in a Longitudinal Coupled Cavity Scheme

54

7d. Color center energy and power amplifier

• In many cases it is necessary and convenient to amplify a weak laser radiation with special spectral spatial or temporal properties to a highradiation with special spectral, spatial or temporal properties to a high energy or peak power values. The four-level energy diagram describing electronic-vibrational transitions of CCs together with a considerable Stokes shift and quasi-homogeneous nature of their wide absorption and fluorescence bands, , as well as a high quantum efficiency and a large , , g q y gamplification cross section are very favorable factors for tackling this amplification task. CCC can be effectively used in master oscillator power amplifier (MOPA) schemes or for pico- and femtosecond pulse amplification in near IR spectral region.Hi h i i ti ( 7 10 17 2) d lif ti (t 100 ) f• High emission cross section (σem = 7·10-17 cm2) and lifetime (t = 100 ns) of F2

- CCs allows to predict a high amplification gain and efficiency of LiF:F2-

amplifier. Comparing to dye solutions the concentration of F2- CCs in LiF

crystal is rather low. Due to this the longitudinally pumped amplifier scheme is preferable for good spatial overlapping of the pump and probe radiation inis preferable for good spatial overlapping of the pump and probe radiation in the bulk CCC with 40÷80 mm length

55

7d. Color center energy and power amplifierEnergy dependencies of amplification gain and output energy of LiF:F2

-

single-pass CCL amplifier with 40 mJ pump pulse.

4.016

ergy

, mJ

3.0

3.5

4.0n,

G

12

14

16

utpu

t ene

2.0

2.5

atio

n G

ain

8

10

mpl

ified

ou

0 5

1.0

1.5

Am

plifi

ca

2

4

6

Am

0.0

0.5

Probe energy mJ0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

2

56

Probe energy, mJ

7d. Energy dependencies of amplification gain and output energy of LiF:F2

- two-pass amplifierand output energy of LiF:F2 two pass amplifier with 40 mJ pump pulse.

J1260

ergy

, mJ

10

ain,

G 50

utpu

t ene

6

8

atio

n G

a

30

40

plifi

ed o

u

2

4

Am

plifi

ca

10

20

Probe energy mJ0.0 0.5 1.0 1.5 2.0 2.5

Am

p

0

A

0

57

Probe energy, mJ

7d. LiF:F2+ Color center energy and

lifipower amplifier• The feasibility of amplification of broadband radiation of diode lasers and

LEDs with the aid of LiF:F + CCCs was demonstrated in [i] Single passLEDs with the aid of LiF:F2 CCCs was demonstrated in [i]. Single-pass amplifier configuration was used. The active centres were excited by the second harmonic (532 nm) of a multimode pulsed Nd3+:YAG laser emitting pulses of 10 ns duration. A GaAs laser diode generated 85 ns radiation pulses with linear polarization at the wavelength 900 nm (close to the p p g (maximum of the fluorescence spectrum of F2

+ CCs) with the spectral width was about 55 cm-1.

• In this experiment a single-pass gain G=5 in the short crystal (l = 0.8 cm) and gain G=20 in the long crystal (l = 2 cm) was reached with the pump

l 1 J N t ti f th i l ti t th f thpulse energy 1 mJ. No saturation of the gain relative to the energy of the amplified radiation from the laser diode was observed. The optical gain was constant when the pump energy was in the range 0.1-1 mJ and the energy of the amplified IR radiation was 0.4-33 nJ.

••[i] Basiev T T, Mirov S B, Ter-Mikirtychev V V Proc. SPIE Int. Soc. Opt. Eng.1839 227 (1992).

58

7e. Narrowline tunable color center lasers

• The main advantage of CCL is the possibility to obtain a g p ycoherent radiation tunable over a wide near IR spectral range. Rough selection of the oscillating wavelength can be made by choosing appropriate color center active y g pp pmedium. In the nondispersive cavity the oscillation occurs at the wavelengths near the maximum of fluorescence curve. The width of the oscillating spectra g pcan be up to several hundreds of cm-1. Tunable radiation of CCLs in dispersive cavities is obtained when prisms, diffraction gratings, Fabri Perrot etalons, Lyo filters and g g , , yother wavelength selective elements are utilized.

59

7e. A Schematic diagram of the commercial tunable color center lasers MALSAN-201 tu ab e co o ce te ase s S 0

(bottom) and MALSAN-203 (top)

LiF:F2-

LiF(F ->F22+

"MALSAN-203"

1.06

0.82 -1.1

1 08 1 26

0.53

1.08 -1.26

0.53

1.06LiF:F2

-LiF(F ->F

2 2+ NaF

YAG:Nd

KDPCDA

0.82 - 1.1

1 1 - 1 34

60

1.1 1.34

1.08 - 1.25

"MALSAN-201"

7e. Dependencies of the efficiency of MALSAN laser on the wavelength of tunable radiation: 1. the active medium is LiF:F2

- - fourth harmonic; 2. LiF(F2 →F2

+) - second harmonic; 3. LiF:F2- - third harmonic; 4. ( 2 2 ) 2

LiF:F2+*(Mg); 5. LiF(F2 →F2

+); 6. LiF:F2- ; 7. NaF:F2

+*; 8,9. LiF:F2- -

first and second Stokes components, respectively

18

20 12

NC

Y, %

12

14

16

83456

EFF

ICIE

N

4

6

8

10 789

0

2

4

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

61

WAVELENGTH, microns

7e. The dependence of the conversion efficiency of the LiF:F2

- laser crystal in the non selective cavity versus 2 y ythe reflectivity of the output mirror.

55

60

IEN

CY

, %

50

55

EFF

ICI

45

4010 20 30 40

62OUTPUT MIRROR REFLECTIVITY, %

7e. Dependence of the LiF:F2- laser efficiency on the wavelength of

tunable radiation: 1,2-for a grazing incidence schemes, 1. - two ti 2 ti d i d fl t f itgratings, 2. - one grating and a mirror as an end reflector for its

first order diffraction; 3,4 in a scheme of the MALSAN laser, 3.-for the usual pump source and active element; 4. - for the optimized

oneone.

3540

20253035

IEN

CY

, %

4 051015

EFF

ICI

1.051.085 1.12 1.155 1.19 1.225 1.26

WAVELENGTH, microns

12

3

63

Alexandrite laser pumped LiF:F2- laser

64

SummarySummary

F th fi t ti t k l d LiF FFor the first time to our knowledge LiF:F2-

lasing in 1-1.3 μm spectral region wasli d d 740 795 l d it lrealized under 740-795 nm alexandrite laser

pumping. Experimental data on LiF:F2-

l i t d d li iti flasing are reported and peculiarities ofmechanism of F2

- excitation via F3- centers

t di dare studied.

65

MOTIVATIONMOTIVATION

1 B d i f LiF F C l C t t i1. Broadening of LiF:F2- Color Center tuning

range.2 Development solid state system continuously2. Development solid state system continuously

tunable in 0.2-1.3 μm spectral range.

66

Shortcoming of the LiF:F2- lasing under

1 064 it ti1.064 μm excitationE

N1

E2

{ } 21

212211221

12122

))()(())()(((

))()((

NII

NIIt

N

pppggg

ggpp

−++++−

+=

τυσυσυσυσ

υσυσ∂

∂

N1

2*

)))(2

))()((2(

12

122121

passactg

gggcavg

lN

lNItt

I

ββυσ

υσυσ∂

∂

−−−

+= −

2

E11*

B12*

B2*1

1/τ

N1

1 2 1

67Q2

Effective F2- Color Center Cross Section2

8 ( )( ) ( )

pabsat NNλσ

=2λ1=965 nm λ 1064

m2

6

1

2

( ) ( )pempab

NNλσλσ +2

λ excitation N2sat/N0

λ1 965 nm

λ2=1029 nm

λ4=1064 nm

λ5=1079 nm

σeff ,*

10-1

7 cm

4

4

3

e c onm

2 0

950-980 ~0.97

1 047 0 43

λ3=1047 nm

2

4

5

1.047 0.43

1.064 0.26

1.0 1.1 1.2 1.30

Wavelength, nm

68

( ) ( ) ( ) ( )( ) ( )pempab

ospabpempabosemeffem λσλσ

λσλσλσλσσ

+

−=

Comparison of main spectroscopic properties of LiF:F2

-, LiF:F3- , LiF:F2

+ and Ti-Sproperties of LiF:F2 , LiF:F3 , LiF:F2 and Ti S laser media

Mediaλab

nmΔλab

nmλem

nmλem

nmσem

cm2

τns

η % Damage J/cm2

K α/mK

LiF:F2- 960 157 1130 180 6*10-17 55 60 10 14.2

LiF:F3- 800 150 920 170 2*10-17 10 10 10 14.2

LiF:F2+ 600 140 906 185 1.5*10-16 20 20 10 14.2

Ti-Sapphire

480 100 760 175 2.7*10-19 3150 80 10 35S pp

69

Structure of Color Centers in LiF crystalsy

F - center F - center

e e

F2- center F3

- center

e

ee

70e-Vacancy-F- -Electron-Li+

Idea of the lasing mechanism of the LiF:F2- crystal under

l d it l ialexandrite laser pumping

Absorption andEmission spectra

of F2-

C l C t

F3-+hν793 → F3

-*

F3-*→F3

-+hν920

F +h F *Color Center

700 800 900 1000 1100 1200 1300

F2-+hν920 → F2

-*

F2-*→F2

-+hν100-1300

Wavelength, nm

Absorption andEmission spectra of F3

-

Color Center

71Wavelength, nm

700 800 900 1000 1100 1200 1300

Absorption spectrum of the LiF active medium

6

cm-1

4

5

6

D=108 radso

rptio

n,

2

3 F2-

F3-

600 700 800 900 1000 1100 1200 1300

Ab

0

1

Wavelength, nm600 700 800 900 1000 1100 1200 1300

72

LiF:F2- Dispersive CavityLiF:F2 Dispersive Cavity

Prism

Mirror

GratingPump

LiF:F2- β

Mirror

73

Experimental Resultspe e ta esu ts

10 LiF:FLiF:F -- tuning Curve under 793 nmtuning Curve under 793 nm

Lase

r Eff

icie

ncy

4

6

8LiF:FLiF:F22 tuning Curve under 793 nm tuning Curve under 793 nm Alexandrite laser pumpingAlexandrite laser pumping

Wavelength, nm1.00 1.05 1.10 1.15 1.20 1.25 1.30

L

0

2

• Normalized Tuning Curves of LiF:F2-

Color Center Laser under different pumping wavelength0.8

1.0

1-λ=1064nm YAG:Nd3+ laser2-λ=1047nm YLF:Nd3+ laser3-λ=793nm Alexandrite laser

0 0

0.2

0.4

0.6 123

74Wavelength, nm

1.0 1.1 1.2 1.30.0

Photostability of LiF:F2- Laserotostab ty o 2 ase

Spectral Dependence of Photoionization P b bili (W) f F d F C l CProbability (W) of F2

- and F3- Color Centers

in LiF at RT

n.

80

100 Alexandrite Laser two photon/ two step excitation

F2-

W, a

rb. u

n

40

60 F3-

320 340 360 380 400 4200

20

620-780nm310-390nm

310-380nm

75

wavelength,nmF2

-F3-

310 380nm620-760nm

The kinetics of F3- color center destruction under 740

it tinm excitation

0.7

cm-1

)

0.4

0.5

0.637.7 MW/cm2

25.7 MW/cm2

ΔK (c

0.1

0.2

0.3 15.6 MW/cm2

Rate of F3- concentration change

F -+2 hν → F -*+ hν → F + e

time (seconds)0 50 100 150 200

0.0

s-1)

2

1F3 +2 hν740 → F3 + hν740 → F3+ e

ΔK

/dt (

cm-1

s

0.4

0.60.70.8

1

KΔ

76 Intensity @ 740 nm (MW/cm2)

20 30 40 50100.22~ I

tK σ

ΔΔ

LiF:F2- Oscillation Efficiency Decay under different

pumping wavelength

ecay

0 8

1.0λpump=793 nm

atio

n D

e

0 4

0.6

0.8

λpump=760 nmO

scill

a

0 0

0.2

0.4

λpump=750 nm

Time, min0 10 20 30 40 50 60

0.0

77

ConclusionConclusion

• For the first time to our knowledge a direct alexandriteFor the first time to our knowledge a direct alexandrite laser pumping of LiF:F2

- laser was realized• Tunable oscillation of LiF:F2

- laser in 1-1.3 μm spectral 2range with maximum efficiency 10% was achieved

• two-step mechanism of photoionization of F3- Color

C t d λ 740 it ti d t t dCenter under λ=740 nm excitation was demonstrated experimentally.

• Stable Oscillation of LiF:F2- laser at RT under λ=793 nmStable Oscillation of LiF:F2 laser at RT under λ 793 nm

excitation was predicted and shown experimentally

78

All-Solid-State System Based On Alexandrite - LiF:F2+**

Laser For Deep UV (196 nm) And Mid-IR (4000 nm) SpectralRangesRanges B. Boczar, R. Frost, B. Pryor, B. Boczar, R. Frost, B. Pryor,

I. Rousseva I. Rousseva S.B.Mirov, V.V.FedorovS.B.Mirov, V.V.Fedorov

Dept. of Physics, University of Alabama at Dept. of Physics, University of Alabama at Birmingham, Birmingham AL 35294Birmingham, Birmingham AL 35294l (20 ) 934l (20 ) 934 8088 (20 ) 9348088 (20 ) 934 80428042 Light Age, IncLight Age, Inc

Two Riverview Drive, Somerset, NJ 08873Two Riverview Drive, Somerset, NJ 08873Tel: (732) 563Tel: (732) 563--0600;Fax: (732) 5630600;Fax: (732) 563--1571; 1571;

EE mail:mail: lightage@aol comlightage@aol com

Tel: (205) 934Tel: (205) 934--8088; Fax: (205) 9348088; Fax: (205) 934--8042; 8042; EE--mail: mail: mirov@uab.edumirov@uab.edu

A Yu DergachevA Yu Dergachev EE--mail: mail: lightage@aol.comlightage@aol.com

Highlights of Current FindingsHighlights of Current FindingsOUTLINEOUTLINE

A.Yu. DergachevA.Yu. DergachevQQ--Peak, Inc., 135 South Road, Bedford, MA Peak, Inc., 135 South Road, Bedford, MA

0173001730 EE--mail:mail: dergachev@qpeak.comdergachev@qpeak.com

Highlights of Current FindingsHighlights of Current FindingsOUTLINEOUTLINE1. Motivation 2.Introduction3. Room Temperature Stable LiF:F2

+** Lasers· · Optical properties of LiF:F2

+** crystals, ultrabroadband tuning capability (>400

We present a novel all solid-state lasersystem that exploits the best features

f th hi h i “d lik ” LiF F +**g p y (nm)?

· · nonselective cavity, dispersive cavity, operational characteristics

· 4. Alexandrite-CCL System Design,

of the high gain, “dye-like” LiF:F2+

medium and the high energy storagealexandrite laser. It is ideal for efficientdeep UV (196 204 nm η=23%) and

79

y g ,Frequency up-conversion (CLBO); Down-conversion (LiNbO3 Ag3AsS3)

5. Conclusions

deep UV (196-204 nm, η=23%) andmid-IR (2.2-5.5 μm, η=10%) lasing.

2002 OSA Annual Meetingand Exhibit/LS-XVIII

Motivation• Pulsed Laser sources with deep UV (below 200 nm) and mid IR

(3-5 μm) output radiation have attracted significant attention foruse in integrated circuit processing, medicine, molecularspectroscopy and remote sensingspectroscopy, and remote sensing.

• CCL with frequency up- and down conversion are very attractivecandidates: easy to scale up, high efficiency (tens of %), wideamplification band (hundreds of nm) high wavelength stabilityamplification band (hundreds of nm), high wavelength stability,extremely narrow oscillation spectral width, achievable virtuallywithout power loss, low threshold, high gain, short oscillationbuilt-up time, and absence of temporal delay between pump and

t t loutput pulses.• Our goal was to demonstrate that Alexandrite - LiF:F2

+** CCLcombination exploits the best features of both the high gain,“dye like” CCL and the high energy storage alexandrite laserdye-like CCL and the high energy storage alexandrite laserand is a promising drive source for a number of efficientnonlinear processes, including harmonic, sum-frequency anddifference-frequency generation that offers simple and

80

economical way to achieve widely tunable narrow-linewidthperformance in the UV-middle IR spectral range.

Novel LiF:F2+** Crystals

(U S P t t N 5 796 762 )(U.S. Patent No. 5,796,762 )m

-1

8

10

F C enter 60000

70000

80000

532 nm Excitation633 nm Excitation

Abso

rptio

n C

oeffi

cien

t, cm

4

6

F 2+**

F 2+**-like

F 2 C enter

Inte

nsity

, arb

. uni

ts

20000

30000

40000

50000

F requency, H z (x1014)246810

A

0

22

Wavelength, nm600 700 800 900 1000 1100 1200 1300

0

10000

20000

uni

ts

30000

40000

New F2+**-like Center

(measured at 24 K)

F2+** Center

A

Inte

nsity

, arb

.

10000

20000

F2 Center(measured at 13 K)

B

81W avelength, nm

700 800 900 1000 1100 1200 1300 14000

Major physicochemical and mechanical properties of LiF t lLiF crystal

Parameters Value Parameters Value

Tm, melting point (oC) 870 E, Young’s modulus (kg/mm2); 8820

Solubility, (g/100g H2O) 0.14 μ, Poisson’s coefficient 0.28

H, Knupp hardness (kg/mm2) 99 S, compression (tension) or

b di t th (k / 2)

1.2-4.0

bending strength (kg/mm2)

d, lattice constant (A); 4.03 R, thermal shock parameter (W/m) 43-143

ρ, density (g/cm3); 2.64 n, refractive index 1.38

K, coefficient of thermal

conductivity (W/m oC)

14.2 n2, nonlinear index (10–22 m2/V2) 2.7

ffi i t f li 32 0 d /dT t t d i ti 1 2α, coefficient of linear

expansion (10-6/ oC)

32.0 dn/dT, temperature derivative

refractive index ( 10-5/ oC)

-1.2

Transparency region (μm) 0.1-7.0

82

Input/output dependence for LiF:F2+** laser at 300 K

with nonselective resonator under pumping with: (1) λ =740 nm (η =31%; η =28%)(1) λpump=740 nm, (ηreal=31%; ηdiff.=28%)

(2) λpump=683 nm, (ηreal=53%; ηdiff.=58%; ηquant.=81%) (3) is linear fit.

100se

, mJ

80

100123

gy p

er p

uls

60

put e

nerg

20

40

0 50 100 150 200 250

Out

0

83

Input enegy per pulse, mJ

Alexandrite laser pumped tunable LiF F +** lLiF:F2

+** laser Mirror M3

HR 0 8Mirror M3 HR 0 8 m

J 20

25

Mirror M1

LiF:F2+**

HR at 0.8 μm-HR at 0.8 μm-

Ene

rgy

per p

ulse

, m

5

10

15

1 2Mirror M1 HR at 0.8 μm-1.2μm

Grating 1200 gr/mm

P

Wavelength, nm800 900 1000 1100 1200

0

1 2

βPump

Telescope

Mirror M3

ity

30

40

Δν=1cm-1

HR at 0.8 μm-

Reflective Conner

Inte

nsi

10

20 δν=0.26 cm-1

84pixel

300 350 400 450 5000

Fundamental, SHG, THG, and SFG conversion efficiency with respect to alexandrite laser pump

energy as a function of wavelength for LiF:F2+** laser

% 14

drite

Bea

m,%

10

12

to A

lexa

nd

4

6

8 1ω

2ω

3

ncy

rela

tive

0

2

4 3ω

4ωpump

Wavelength, nm0 200 400 600 800 1000 1200 1400Effic

ien

85

Typical temporal shapes and delays between the pump and the LiF:F2

+** p p 2

oscillating pulsesn. 1 4

1.6

1.8

2.0

t1+t2 t3

nal,

arb.

un

0.8

1.0

1.2

1.4

Sign

0.2

0.4

0.6

time, ns

550 600 650 700 750 800 850

0.0

86

,

Optical arrangement for sum frequency

generation of deep-UV radiationgeneration of deep UV radiation800-1200 nm

T2

P2 P3

M6

Alexandrite Laser

P1 M1 M3

G

+**

780-720 nm

260-240 nm

T1M4

M5

LiF:F2+**

NC12ωNC12ω NC1

PR1 PR2

DP1 M2

2ω

NC3Σ

2ω NC13ω

800-1200 nm

PR1

DP2

87196-205 nm

800 00

196-205 nm

Theoretically calculated angular tuning curves in CLBO crystal for different wavelengths of the pump radiation 240 nm, 243

nm, and 246 nm versus the SFG spectral outputnm, and 246 nm versus the SFG spectral outputSFG Phase matching for CLBO. Alexandrite laser radiation (3w) is mixed (type 1, ooe

interaction) with the tunable radiation of alexandrite laser pumped LiF:F2+** laser

100

80

90

3w=0.246

50

60

70

Ang

le, d

egre

es

3w=0.2433w=0.240

30

40

50

Phas

e m

atch

ing

10

20

88

00.192 0.193 0.194 0.195 0.196 0.197 0.198 0.199 0.2 0.20

SFG Wavelength, microns

Deep UV Frequency Upconversion in CLBOUpconversion in CLBO

22

233.66nm(701nm/3)+1110 nm=193 nm (900)

16

18

20

22245 nm+1035 nm (θ=900) 247 nm+1030 nm (θ=850)

ency

, PΣ/

P 3ω

10

12

14

16

248 nm+1035 nm (θ=820)

Effic

ie

4

6

8

10

247 nm+1110 nm (θ=730)

243 nm+1049 nm (θ=900)

195 196 197 198 199 200 201 202 203 204 2050

2

4

241 nm+1061 nm (θ=900)247 nm+1173 nm (θ=690)

89

Wavelength, nm

FREQUENCY DOWNCONVERSION(40-60 ns pump pulse duration)

Polarization Rotator (4)Glan Prism (5)

M2

Polarization Rotator (4)Glan Prism (5)

M2

DigitalBeam exp. 3:1 (12)

Input mirror (M7)

LiF:F2+** (8) Joulemeter

HR M6 Slit

E=80 mJ at λ3=740nm τ=60 ns; f=10 Hz

Digital Oscilloscope

Beam comp. 5:1 (12)

Beamsplitter (M1)80/20 or 90/10Grating (9)

Joulemeter J4-10

Ge Filter (16)

AlexandriteLaser

Beam compressor 5:1 (12)

80/20 or 90/10Grating (9)

LiNbO3 M11

M15 M15 M10λ3

λ1=2.2-5 μm

e (λ3)

ooe interaction

p ( )

M11Beamcombiner M3 M27 λ2

e (λ3)

k

k

o (λ )

k

o (λ )

90

Ge filter-Spectrograph-InSb o (λ2) o (λ1)

Experimental and theoretical DFG quantum efficiency versus DFG wavelength for LiNbO3 (Li/Nb=1) nonlinear crystal. Power density

of alexandrite laser was about 3 MW/cm2. Power density of LiF:F2+

laser was not constant. It varied from 6MW/cm2 for 3μm DFG output to 0.3 MW/cm2 for spectral regions around 2.2 and 5 μm.

40

30

35

IR (m icron) vs Q . E ff(Th) IR (m icron) vs Q .E ff(E xp)

cien

cy (%

)

20

25

IR (m icron) vs Q .E ff(E xp)

Effic

10

15

2 3 4 5 60

5

91

D FG W ave length , λ 1,(μm )

The external phase matching angle for Ag3AsS3. Inset: DFG wavelengths as a function of LiF:F2

+** wavelengths at a pump wavelength, λ3 of 736.7 nm

A g 3 A s S 3 D F G 7 3 6 . 6 9 n m p u m p

2 0λ 1 ( D F G ) v s λ 2 ( F 2

+ ) a t λ 3 ( p u m p ) = 7 3 6 . 6 9 n m

1 0

es)

0

FG W

avel

engt

h, λ

1 (μm

)

4

6

8

T h e o r e t i c a lE x p e r im e n t a l

chin

g an

gle

(deg

re

- 2 0λ 2 ( μ m ) ( F 2

+ )0 . 7 0 . 8 0 . 9 1 . 0 1 . 1 1 . 2 1 . 3 1 . 4

D

0

2

Exte

rnal

pha

se m

atc

- 4 0

T h e o r e t ic a l A n g leE x p e r im e n t a l A n g leE

- 6 0

E x p e r im e n t a l A n g le

92λ 2 ( μ m ) ( F 2+ )

0 . 7 0 . 8 0 . 9 1 . 0 1 . 1 1 . 2 1 . 3 1 . 4- 8 0

FREQUENCY DOWNCONVERSION500-600 ns pump pulse duration obtainable with

Light Age, Inc. pulsed stretched 101PALLight Age, Inc. pulsed stretched 101PAL

Results• Pulsewidth: 500 ns3-4 μm

800-1200 nm

T2

P2

M3 LiNbO3

M5

• Pump wavelength: 737 nm• LiF wavelength: 919 nm• DFG wavelength: 3.7μm

PR1

T1

P1

G

M3 LiNbO3

Alexandrite• Pump energy: 440 mJ

Mixed pulses:• LiF energy: 27 mJ

780-720 nm M1

M2

M4 LiF:F2

+**

780-720 nm

Alexandrite Laser

gy• Residual pump: 76 mJ

Nonlinear material:• Two 24 mm LiNbO3 with V 0.04

0.05

0.06Temporal overlapping of LiF:F +**color 3

walk-off compensationDFG 3.7 μm energy: 1.04 mJSi

gnal

, V

0.01

0.02

0.03

LiF:F2 color center laser oscillation (red) and oscillation of

93Time, μs1 2

0.00the Alexandrite laser (black).

FREQUENCY DOWNCONVERSION(500-600 ns pump pulse duration)( p p p )

G o

utpu

t, m

J

1

G ra

diat

ion,

mJ

1

η=0.25% η=4%

gy o

f 3.7

um

DF

y of

3.7

um

DFG

η

Alexandrite (737 nm) pump energy , mJ100 150 200 250 300 350 400 450 500

Ene

rg

0

Energy of LiF pump radiation, mJ0 5 10 15 20 25 30E

nerg

y

0

J 30 J

m L

iF o

utpu

t, m

J

15

20

25

30

m D

FG o

utpu

t, m

J

1η=6.2% η=1.5%

Ene

rgy

of 9

19 n

m

5

10

15E

nerg

y of

3.7

um

0

94Alexandrite (737nm) pump energy, mJ

100 150 200 250 300 350 400 450 500

E 0

Energy of residual 737 nm, mJ20 30 40 50 60 70 80

E 0

CONCLUSIONSCONCLUSIONS1. Photo and thermostable regime of LiF:F2

+** lasing was realized due to a novel technology of LiF:F2

+ stabilized crystalrealized due to a novel technology of LiF:F2 stabilized crystal formation.

2. We demonstrated solid-state laser system for deep UV andmid IR spectral range.p g

3. The tunable radiation in 196- 204 nm with 23 % efficiency andup to 0.5 mJ pulse energy has been realized.

4 The difference frequency generation between radiation of4. The difference frequency generation between radiation ofalexandrite and LiF:F2

+** lasers in 2.2 –5.5 μm was achievedwith 10% efficiency in LiNbO3 crystal at 3 μm for 50 ns pulses.

5 Output energies of 1 04 mJ at wavelength of 3 7 μm for a5. Output energies of 1.04 mJ at wavelength of 3.7 μm for along, 600 ns, pulse duration was obtained in LiNbO3 crystal.

95

Tunable Room Temperature Stable Color Center Di t ib t d F db k LDistributed Feedback Laser

Jason G. Parker, Mentor: Sergey B. MirovCollaborators: Dmitri Martyshkin, Vladimir V. Fedorov

Physics Department, University of Alabama at Birmingham1300 University Blvd., Birmingham, AL 35294-1170, mirov@uab.edu

y ,

Laser and Photonics Research Center

96

IntroductionTunable room temperature distributed feedback color center lasers were studied

Motivation

Tunable room temperature distributed feedback color center lasers were studied.

• Current tunable lasers: utilize external cavities with dispersive elements that provide tunability across the amplification band. However, utilization of an external cavity makes the overall system not compact and bulky. It is very advantageous to combine inside laser crystals all laser elements (gain medium and positive feedback – mirrors and/or dispersive elements).

•The distributed feedback (DFB) phenomenon makes it possible to do this while still achieving tunable oscillation.

•Newly discovered LiF:F2+** crystals combine the thermal- and photo-stability of

impurity doped laser crystals with the high absorption and emission cross-sections of laser dyes.

•Until now, no tunable room temperature stable DFB lasers had been created using color center crystals (CCCs).

97

Distributed feedback (DFB) lasers•The DFB laser uses a periodic modulation of its medium to attain selective oscillationThe DFB laser uses a periodic modulation of its medium to attain selective oscillation.

This means that this modulation acts as a diffraction grating and thus follows the grating equation.

( ) mλθθ =Λ sinsin( ) lmi mλθθ =−Λ sinsin

Which in our case (θi=90˚, θm=-90˚) reduces to:

Λ=period, θi=angle of incident beam from surface normal, θm=angle of reflected beam from normal, m=order of diffraction, λl=lasing wavelength

Periodic modulations (excited LiF:F2

+)

( i , m )

lmλ=Λ2

LiF F +

98

LiF:F2+

Grating construction by interference•The interference of two equal laser beams creates a fringe normal to the point of•The interference of two equal laser beams creates a fringe normal to the point of intersection and sinusoidal in nature. Our setup uses this fringe to penetrate the crystal periodically and create our (dynamic) grating.

Initial laser beam

Beam split into nearly equal parts

Beam rejoined to obtain interference

LiF crystal

99

Picture of setupLaser BeamLaser Beam Beam splitter

Cylindrical lensMirror

Mirror

DFB output Mirror

Samplebeam Spectroscopic

fiber

100

Spectroscopic analysis•Spectroscopy was used to know if DFB was realized Typical lasing with LiF:F2

+Spectroscopy was used to know if DFB was realized. Typical lasing with LiF:F2produces a spectrum approximately 15 nm in width. However, because feedback is selective in the DFB laser, DFB lasing should occur at a very specific wavelength, determined by the grating equation. The graph below shows one instance of DFB

1.2+

lasing transposed over one instance of typical LiF:F2+ broadband lasing. The linewidth

of DFB lasing was measured and found to be less than 0.2 cm-1 (0.017 nm).

units

0.8

1.0Broad LiF:F2+ lasingDFB lasing

nsity

, arb

.

0 4

0.6

Inte

0 0

0.2

0.4

101Wavelength, nm900 920 940 960

0.0

Proof of lasing due to DFB•If a particular peak is due to DFB it should completely disappear when either shoulderIf a particular peak is due to DFB, it should completely disappear when either shoulder is blocked.

6000

ity, a

rb. u

nits

400

600

ty, a

rb. u

nits

3000

4000

5000

6000

ity, a

rb. u

nits

400

600

W l h

900 920 940 960

Inte

nsi

0

200

W l th

900 920 940 960

Inte

nsi

0

1000

2000

W l th

900 920 940 960

Inte

nsi

0

200

DFB lasing at 945 nm Right shoulder blocked Left shoulder blockedWavelength, nmWavelength, nm Wavelength, nm

This technique was employed in all instances in which DFB occurred.

102

Tuning results•The LiF DFB laser was fully tunable from 962 to 882 nm This is a large range of•The LiF DFB laser was fully tunable from 962 to 882 nm. This is a large range of tuning for any laser, and we suspect this range can grow considerably. The following figure shows instances taken throughout a tuning run from 962 to 882 nm. The small hump centered around 940 nm is due to normal LiF:F2

+ broadband lasing (with

1 0

1.2oscillation from the crystals faces).

arb.

uni

ts

0.8

1.0

882 nm

nten

sity

, a

0.4

0.6 900 nm910 nm923 nm932 nm955 nm962 nm

In

0.0

0.2962 nm

103Wavelength, nm

880 900 920 940 960

Power relationship•The following graph shows the relationship between pumping power and output•The following graph shows the relationship between pumping power and output power for our laser. A linear dependence of output energy on pumping energy was determined. The maximum efficiency reached was 3%. Also, the optical threshold of pumping was found to be 1.2 mJ.

80Pump Energy, mJ vs OutPut Energy, um Pump-fit vs Output-fit

nerg

y, u

J

40

60

Out

put E

n

20

40

0

20

104Pump Energy, mJ0 2 4 6 8 10

Conclusion•For the first time tunable room temperature stable color center DFB lasing has beenFor the first time, tunable room temperature stable color center DFB lasing has been realized.

•A dynamic grating was constructed inside the CCC using interference.

•The laser showed a large tuning range and we believe this range can grow considerably.

•A linear relationship between pump power and output power was determined, with a i i ffi i f 3% Al h i l i h h ld f dmaximum pumping efficiency of 3%. Also, the optical pumping threshold was found

to be 1.2 mJ.

•The results were prepared for presentation in the Photonics West LASE 2004 f d bli ti i th h i j l A li d Ph i L ttconference and publication in the physics journal Applied Physics Letters.

105

Persistent photon-gated spectral hole burning in LiF:F2- color

center crystal (V Fedorov S Mirov Appl Phys Lett 79 2318 2320center crystal (V.Fedorov, S.Mirov Appl. Phys. Lett. 79, 2318-2320(2001).

Alexandrite laser, Li ht A I

LiF:F2+** Color

C t l Light Age, Inc. Center laser

The Q-switched alexandrite laser λ=720-800 nm, f= 25, Hz,

τ=80ns, E=80 mJ

ARC-750 spectrometer (Δν=0.2 cm-1)

Close Cycle helium refrigerator, Janis Research Co Model CCS-450 (14K-425K)

LiF:F2- crystals was γ-

irradiated by a 60Co source

106

450 (14K 425K).with a dose D=107rad

Persistent photon-gated spectral hole burning in LiF:F2-

l t t lcolor center crystal

ZPL absorptionF2

- + hνν 1.04μm F2-* + h νν0.52μ m F2 + e

tical

Den

sity E⊥ E2ω

h

hνν 0.52μm

1038 1039 1040 1041 1042

Op E// E2ω

Δ 1 7 1

hνν 1.04μm

Wavelength,nm

ΔK

Δν = 1.7cm-1

P1.04 μm=5 MW/cm2 , P0.52 μm=0.1 MW/cm2 .

Wavelength, nm1039.0 1039.5 1040.0 1040.5 1041.0

μtime -20 min, T=14K.Δνlaser=0.2-0.3 cm-1

107

2P3/2

3d105s

3d104p

2S1/2

LASER ATOMIC FLUORESCENCE SPECTROSCOPY

2P1/2

2D

3d 4p

λex λreg(1)

λreg(2)

• RESEARCH GOAL: Develop novel laser atomic fluorescence instrument for heavy metal detection at sub-ppt level

3d104s 2S1/2

2D3/2

2D5/2 3d94s2

λex = 324.754 nm λreg1 = 510.554 nm λ 2 = 570 024 nm

for biomedical and industrial applications.

SpectrographCCD

PC

HV Pulse GeneratorPG 200 λreg2 570.024 nm

b.un 50000

60000

70000RO water Distilled water Cu - 10 μg/l

r) er)

Spect og apARC-750

CCPG-200

FiberGuide

Inte

nsity

, arb

0

10000

20000

30000

40000

CuCu

Cu

3 ω (2

nd o

rder

)

3ω (2

nd o

rder

3ω (2

nd o

rde

Lens

Lens

Sample505 510 515

Wavelength, nm505 510 515505 510 515

0

LAF detection of Cu atoms in water sample (Cu-10μg/L), distilled water, and deionized water (prepared using reverse

Tunable UVlaser

p(in graphite furnace)

Aperture

108

, (p p g

osmosis at 18 megohm resistance)

Application of LiF laser in Opto-i di l i iacoustic medical imaging

104

1000

a (cm

-1)

HbO2

Hb 0

10

10

100

Coe

ffici

ent,

µ

757 nm 1060 nm

800 nm 20

30

Axi

al in

-Dep

th, m

m

1

Abs

orpt

ion

C

920 nm 0 10 20 30 40

40

Lateral, mm

0.1200 400 600 800 1000 1200

Wavelength, nm

0.500 0.550 0.600 0.650Contrast Bar

109