ti:sapphire laser

Ultrafast Chromium-Forsterite Laserand its Application to Frequency Metrology

Ahmer Naweed

Group: M. Faheem, K. Knabe, R. Thapa, A. Pung,

B. R. Washburn, and K. L. Corwin

Thanks: M. Wells, R. Reynolds, and JRM Staff (KSU)

S. Diddams and N. Newbury (NIST)J. Nicholson (OFS)

Funding: NSF

AFOSR

Ti:sapphire Laser Verdi

5 - 10 W 530 nm

= 800 nm

Cr:forsterite LaserFiber Laser

10 W1075 nm

= 1270 nm


10 W1075 nm

= 1270 nm

Frequency standards for the telecom wavelengthsFrequency standards for the telecom wavelengths


10 W1075 nm

= 1270 nm


Cr doped forsterite


10 W1075 nm

= 1270 nm


Cr doped forsterite

Poor thermal conductivity


10 W1075 nm

= 1270 nm


Cr doped forsterite

Poor thermal conductivity

Sensitive to environmental perturbations

OutlineOutline

• Fundamentals of ultrafast lasers– Mode locking– Dispersion management

• Frequency combs and their realization

• Chromium-forsterite lasers: – Benefits and Challenges

• Optimizing Chromium-forsterite laser – Operation at KSU

• Supercontinuum generation

• Laser performance• Future work

Ultrafast Lasers: BasicsUltrafast Lasers: Basics

t

f

S. Diddams et al., Science 306, 1318 (2004)

Tr

Constant depends upon the pulse shape

For a Gaussian pulse,

Time Bandwidth ProductTime Bandwidth Product

constantpulset

0.441pulset f

t

Propagation of Ultrafast Laser PulsesPropagation of Ultrafast Laser Pulses

20 0exp( )exp( )inE E i t t

20 0 0

1( ) ( ) .....

2k k k k

0

g

dk v

dk

0 0

2

2

1

g

d dk

dk d v

xx

exp( )i k x

2

0 2 2

22

2 2

α exp exp1 4

2exp

1 4

out

p g

g

x xE i t t

v k x v

k x xi t

k x v



xx

Propagation of an ultrafast laser through a transparent material can lead to:

• Pulse broadening• Pulse delay• Chirp


• Material dispersion is positive.• A prism (or a grating) pair can have both positive or negative dispersion• By using a pair of prisms (or gratings) one can control net cavity dispersion.

Frequency CombsFrequency Combs

tr.t = 1/fr

t

E(t)Time domain

Frequency domain

0fn = nfr + fo

I(f)

f

fo fr

Supercontinuum generation in microstructure fiber preserves frequency comb.

T. Udem, J. Reichert, R. Holzwarth, and T.W. Hänsch, OL 24, 881, (1999).D. J. Jones, et al. Science 288, 635 (2000).

Carrier-envelopephase slip from pulseto pulse because:

vg vp

It is critical to have an octave spanning spectrum.

1( ) ( ) ( )g p

c dnv v

n dn d n d

( )p

cv

n

www.nobel.se

Existing portable wavelength references for the telecom industry

laser

or LEDPressure-broadened

Line centers:±130 MHz or ±13 MHzUsed to calibrate Optical Spectrum

Analyzers (OSA’s)Line widths ~5 GHz (OSA resolution)pressure → broadening & shift

C2H2

W.C. Swann and S.L. Gilbert, JOSA B 17, 1263 (2000)

Saturation spectroscopy in hollow optical fiber

zPump Probe

Saturation spectroscopy in hollow optical fiber

-1000 -500 0 500 1000

0.0

0.2

0.4

0.6

0.8

1.0 112 mW (+ 0.4) 83 mW (+ 0.3) 40 mW (+ 0.2) 20 mW (+ 0.1) 10 mW

Fra

cti

on

al A

bso

rpti

on

Frequency (MHz)

-400 -200 0 200 400 600 800-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

112 mW (- 0.2) 83 mW (- 0.1) 40 mW (- 0.1) 20 mW (- 0.05) 10 mW

Fra

ctio

nal

Ab

sorp

tio

n

Frequency (MHz)

Significant signal strength at 10 and 20 mW pump powers!

10 m core

R. Thapa, K. Knabe, M. Faheem, K. L. Corwin

I(f)

f

fo

0

fr

Self-Referenced Optical Frequency Comb

• fo is generated from a heterodyne beat between the second harmonic of the nth mode and the 2nth mode.

• Once fr and fo are referenced to a known oscillator, all the frequency modes of the fs comb are fixed.

D. J. Jones, et al. Science 288, 635 (2000)

fnf2n

fn = n fr + fo f2n = 2nfr + fo

fo

2nfr + 2fox2

lasing medium Ti:sapphire Cr:forsterite

pump laser 10 W Green (>$ 60,000) 10 W fiber laser (<$ 15,000)

optical fiber microstructured highly-nonlinear Dispersion-shifted

frequency range 500 – 1100 nm 1100 – 2200 nm

Crystal temp room temp -5 oC

Ti:sapphire vs. Cr:forsteriteTi:sapphire vs. Cr:forsterite

S. Diddams et al., Science 293 (2001) I. Thomann et al., OL 28, 1368 (2003)

Zhang et al, 90 nm FWHM; 20 fs; 60 mW

IEEE J Q. Electronics 1997

V. Yanovsky et al, 90 nm FWHM; 80 nm FWHM; 25 fs, 400 mW

OL 1993

Haus et al., 90 nm FWHM; 250 nm FWHM; 14 fs, 80 mW, OL

Chromium-forsterite Lasers: A Brief HistoryChromium-forsterite Lasers: A Brief History

Net cavity dispersion = Cr:f dispersion + prism (SF6 ) dispersion

+ angular dispersion

net cavity dispersion* = - 260 fs2

Cr:f dispersion = 277 fs2 Prism dispersion = - 588 fs2

angular dispersion = -1155.13 fs2

optimal prism separation = 32.5 cm

third order dispersion = 240.77 fs2

Optimizing Cr:fr Laser: DispersionOptimizing Cr:fr Laser: Dispersion

Pump laser

Cr:forsterite Laser

*I. Thomann et al., OL 28, 1368 (2003)

Ray transfer matrix (ABCD) analysis is performed to yield optimal cavity parameters that is essential for stable laser operation.

Optimizing Cr:fr Laser: StabilityOptimizing Cr:fr Laser: Stability

1 /

0 1

d n

h

refractive index n

d

1 0

1/ 1f

Lens of focal length f

Ray transfer matrix (ABCD) analysis is performed to yield optimal cavity parameters that is essential for stable laser operation.


1 1 2 2

1 1 2 2

........ n n

n n

A BA B A B A B

C DC D C D C D

Ray matrix (ABCD) analysis performed to yield optimal cavity parameters that is essential for stable laser operation.


Self consistentsolution:

2

2

0 12

B

n A Dq

Pump laser

Cr:forsterite Laser

Optimizing Cr:fr Laser: AstigmatismOptimizing Cr:fr Laser: Astigmatism

Because of a lack of axial symmetry, the beam waist along the sagittal and tangential planes may not necessarily be equal and spatially overlap (astigmatism). Therefore, the effects of astigmatism must be taken into account in cavity stability analysis.

2 1/ 21 /( sin )

0 1ct n

2 2 2 3/ 21 (1 sin ) /( sin )

0 1c ct n n


4.5 5.5 6 6.5dcm

-0.2

-0.1

0.1

0.2

beam waist mmBeam diameter (mm)

d2 (cm)

Mode Locking Cr:fr LaserMode Locking Cr:fr Laser

Unlike Ti-sapphire laser, no well established method for mode-locking the Cr:fr laser is known.

Observation of strong and periodic fluctuation in output laser power. This is an indication that the laser is close to ML regime.

I. Thomann et al., OL 28, 1368 (2003)

76.43 nm FWHM Bandwidth 59 nm FWHM Bandwidth

1100 1200 1300 1400 1500 1600 1700

-80

-70

-60

-50

-40

-30

-20

-10

0

Inte

nsi

ty (

dB

m/n

m)

Wavelength (nm)

103.452 nm FWHM Bandwidth

Rep. Rate Measurements: 115 MHzRep. Rate Measurements: 115 MHz

Hyperbolic Secant Pulse: Hyperbolic Secant Pulse: 38 fs.38 fs.

Transform limited pulse for Transform limited pulse for 105 nm bandwidth: 16.5 fs.105 nm bandwidth: 16.5 fs.

Stability of Mode Locked LaserStability of Mode Locked Laser

0 2 4 6 830

45

60

75

90

105

120

Mod

eloc

ked

Spe

ctra

l Ban

dwid

th (

nm)

hours

Spectral width: 90-105 nm Pulse Duration: 38 fsRep. Rate: 115 MHzOutput Power: 220 mWCenter Wavelength: 1275 nm

Laser ParametersLaser Parameters

Supercontinuum GenerationSupercontinuum Generation

Nonlinear Effects cause creation of new optical frequencies

Honeycomb Microstructure Optical Fiber

J. Ranka, R. Windeler, A. Stentz, Opt. Lett. 25, 25 (2000).

courtesy of Jinendra Ranka

Aeff =13.9 mm2

Dispersion slope = 0.024 ps/(nm2 km)

Nonlinear coefficient = 8.5 ( W km)-1

J. W. Nicholson et. al, Opt. Lett 28, 643, 2003

• Broadest continuum is generated by the fiber when the ultrafast laser pulse is in the anomalous dispersion region.

• The pulse intensity begins to self Raman shift to longer wavelengths.

• Due to break up of these higher order solitons, four-wave mixing generates frequencies at wavelengths shorter than zero dispersion wavelength.

Highly Nonlinear FiberHighly Nonlinear Fiber

1000 1100 1200 1300 1400 1500 1600 1700 1800

-80

-70

-60

-50

-40

-30

-20

-10

0

Inte

nsi

ty (

dB

m/n

m)

Wavelength (nm)

1000 1200 1400 1600 1800

-80

-70

-60

-50

-40

-30

-20

-10

0

Inte

nsi

ty (

dB

m/n

m)

Wavelength (nm)

Laser output

88.892 nm FWHM Bandwidth

Supercontinuum

Supercontinuum Generation from Cr:fr LaserSupercontinuum Generation from Cr:fr Laser

Current Research StatusCurrent Research Status

1800 1900 2000 2100 2200 2300 24000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Po

we

r (a

rb. u

nits

)

Wavelength (nm)

Fiber in

Fiber Laser10 W

1075 nm

Cr:forsterite Laser


Fiber in

Fiber Laser10 W

1075 nm

Cr:forsterite Laser

Fiber out

HNLF

SC BS

DMnonlinearcrystal

stabilized opticalfrequency comb

Synthesizer

frep LoopFilter

PhaseDetector

Synthesizer

f0 LoopFilter


Saturation SpectroscopySaturation Spectroscopy

0

1 ( / )s

s

dP

Pdz P P

0 0( )( )zP P A B z

zPump Probe

B =

A =

2P0P0PsP02Ps

arctanhP0PsPs Ps P0 Ps

P0 PsP0 Pz

2PzPsPzPsPz2

arctanhPsPzPs Ps Ps Pz

P0 PzPs Pz


0.25 0.5 0.75 1 1.25 1.5 1.75Distance m5

10

15

20

25

30

35

40mW Probe

sat no sat

no saturation

saturation

Pump Power (mW)

Distance (m)


ConclusionsConclusions

Future WorkFuture Work

Robust and efficient Cr:fr femto second laser.

FWHM bandwidth of up to 105 nm and output energy of about 220 mW.

Realized supercontinuum generation by coupling Cr:fr pulses to a HNLF.

Octave spanning spectrum.

Laser Stabilization.

Installation of piezo mounted mirror in laser cavity.

1( ) ( ) ( )g p

c dnv v

n dn d n d

( )p

cv

n

0 2CE

rf f

ULTRAFAT LASER BASICSULTRAFAT LASER BASICS2


2

0 2 2

22

2 2

α exp exp1 4

2exp

1 4

out

x xE i t t

v k x v

k x xi t

k x v

20 0 0

1( ) ( ) .....

2k k k k

0

g

dk v

dk

0 0

2

2

1

g

d dk

dk d v

Chromium-forsterite Lasers: A Brief HistoryChromium-forsterite Lasers: A Brief History

4.5 5.5 6 6.5dcm

-0.2

-0.1

0.1

0.2

beam waist mm


3 2

2 22

d nk

c d

Frequency Combs for frequency metrology

• Transfer stability and accuracy between optical and microwave regimes.

• Ti:sapph comb commercially available.• Fiber lasers at 1.5 m increasingly interesting.

– near IR (telecom)– cheaper– more portable– will require portable references

• near-IR comb being developed at Kansas State for characterization of new standards.

Microwave OpticalFrequency Comb5 x 104 (500 THz)(9.2 GHz)

ti:sapphire laser

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