multiwavelength erbium-doped fiber laser em-ploying nonlinear polarization rotation in a symmetric...

Multiwavelength Erbium-doped fiber laser em-

ploying nonlinear polarization rotation in a

symmetric nonlinear optical loop mirror

Jiajun Tian1, Yong Yao

1,*, Yunxu Sun

1, Xuelian Yu

1, and Deying Chen

1,2

1Department of Electronic and Information Engineering, Shenzhen Graduate School, Harbin Institute of

Technology, Shenzhen, Guangdong Province, 518055, China 2National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin,

Heilongjiang Province, 150001, China

*[email protected]

Abstract: A new multiwavelength Erbium-doped fiber laser is proposed and

demonstrated. The intensity-dependent loss induced by nonlinear

polarization rotation in a power-symmetric nonlinear optical loop mirror

(NOLM) suppresses the mode competition of an Erbium-doped fiber and

ensures stable multiwavelength operation at room temperature. The

polarization state and its evolution conditions for stable multiwavelength

operation in the ring laser cavity are discussed. The number and spectra

region of output wavelength can be controlled by adjusting the work states of

NOLM.

©2009 Optical Society of America

OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.2410) Fibers, erbium;

(190.3270) Kerr effect; (190.4370) Nonlinear optics, fibers; (140.3560) Lasers, ring

References and links

1. A. Bellemare, M. Karasek, M. Rochette, S. A. L. S. Lrochelle, and M. A. T. M. Tetu, “Room temperature

multifrequency erbium-doped fiber lasers anchored on the ITU Tetu, frequency grid,” J. Lightwave Technol.

18(6), 825–831 (2000).

2. C. L. Zhao, X. F. Yang, C. Lu, N. J. Hong, X. Guo, P. R. Chaudhuri, and X. Y. Dong, “Switchable

multi-wavelength erbium-doped fiber lasers by using cascaded fiber Bragg gratings written in high birefringence

fiber,” Opt. Commun. 230(4-6), 313–317 (2004).

3. X. M. Liu, X. Q. Zhou, and C. Lu, “Four-wave mixing assisted stability enhancement: theory, experiment, and

application,” Opt. Lett. 30(17), 2257–2259 (2005).

4. X. H. Feng, H. Y. Tam, and P. K. A. Wai, “Stable and uniform multiwavelength erbium-doped fiber laser using

nonlinear polarization rotation,” Opt. Express 14(18), 8205–8210 (2006).

5. Z. X. Zhang, L. Zhan, K. Xu, J. Wu, Y. X. Xia, and J. T. Lin, “Multiwavelength fiber laser with fine adjustment,

based on nonlinear polarization rotation and birefringence fiber filter,” Opt. Lett. 33(4), 324–326 (2008).

6. Z. X. Zhang, L. Zhan, and Y. X. Xia, “Tunable self-seeded multiwavelength Brillouin-Erbium fiber laser with

enhanced power efficiency,” Opt. Express 15(15), 9731–9736 (2007).

7. X. H. Feng, H. Y. Tam, H. L. Liu, and P. K. A. Wai, “Multiwavelength erbium-doped fiber laser employing a

nonlinear optical loop mirror,” Opt. Commun. 268(2), 278–281 (2006).

8. Z. X. Zhang, K. Xu, J. Wu, X. B. Hong, and J. T. Lin, “Multiwavelength figure-of-eight fiber laser with a

nonlinear optical loop mirror,” Laser Phys. Lett. 5(3), 213–216 (2008).

9. E. A. Kuzin, J. A. Andrade-Lucio, B. Ibarra Escamilla, R. Rojas-Laguna, and J. Sanchez-Mondragon, “Nonlinear

optical loop mirror using the nonlinear polarization rotation effect,” Opt. Commun. 144(1-3), 60–64 (1997).

10. N. J. Doran, and D. Wood, “Nonlinear-optical loop mirror,” Opt. Lett. 13(1), 56–58 (1988).

11. E. A. Kuzin, N. Korneev, J. W. Haus, and B. Ibarra-Escamilla, “Theory of nonlinear loop mirrors with twisted

low-birefringence fiber,” J. Opt. Soc. Am. B 18(7), 919–925 (2001).

12. O. Pottiez, E. A. Kuzin, B. Ibarra-Escamilla, and F. Méndez-Martínez, “Theoretical investigation of the NOLM

with highly twisted fibre and a λ/4 birefringence bias,” Opt. Commun. 254(1-3), 152–167 (2005).

1. Introduction

Multiwavelength Erbium-doped fiber lasers (MWEDFLs) aroused extensive attention in the

passed two decades, due to their potential applications in wavelength-division-multiplexing

#113197 - $15.00 USD Received 22 Jun 2009; revised 1 Aug 2009; accepted 2 Aug 2009; published 11 Aug 2009

(C) 2009 OSA 17 August 2009 / Vol. 17, No. 17 / OPTICS EXPRESS 15160

(WDM) communication systems, fiber sensors and optical instrumentations. The Erbium-

doped fiber (EDF) is a homogeneous broadening medium at room temperature, which leads to

fierce mode competition and makes it difficult to obtain stable lasing. Various techniques have

been proposed to deal with the critical issue, such as frequency shift feedback [1], polarization

hole burning effect [2], four-wave mixing [3], nonlinear polarization rotation (NPR) [4,5],

stimulated Brillouin scattering [6], intensity-dependent loss (IDL) in the nonlinear optical loop

mirror (NOLM) [7,8] and so on.

IDL is one kind of the power-dependent nonlinear output characteristics (NOCs) of

NOLM, in which a higher power beam will experience lower transmission (higher loss) than a

lower power beam. This feature can be utilized as an intensity equalizer to suppress the mode

competition of EDF. A typical approach is made by X.H. Feng [7]. In that design, the IDL

relies on self-phase modulation (SPM), which allows a differential nonlinear phase shift to

accumulate only if a power imbalance exists between the beams propagating clockwise (CW)

and counter clockwise (CCW) in the loop. In fact even in the power-symmetric NOLM, NOC

can be obtained by maintaining and accumulating the polarization asymmetry between beams

in CW and CCW [9].

In this paper, a new MWEDFL is proposed, in which a power-symmetric NOLM is used as

an intensity equalizer. The IDL induced by the NPR in the NOLM can effectively suppress the

mode competition of EDF. The number and spectra region of output wavelength can be

controlled by adjusting the work states of NOLM. The polarization and its evolution conditions

for the multiwavelength operation in the ring laser are analyzed in detailed.

2. Principle

Most NOLM designs are based on the SPM difference induced by the power imbalance

between CW and CCW beams in the loop [10]. Thus the power-asymmetric directional coupler

(DC) is extensively used in the NOLM to obtain the power imbalance. However, the studies in

recent years have shown that the NOC of the NOLM can also be obtained when a

power-symmetric DC is used. In the power-symmetric NOLM, a difference of NPR can exist

with equal power, provided that the polarization states are different. When the beams

recombine at the output of NOLM, a different NPR is responsible for an intensity-dependent

transmission characteristic, similarly to a different nonlinear phase shift [9,11]. Although the

SPM effect and the cross-phase modulation effect still exist, the difference phase shift induced

by them between counter propagation beams is zero due to the equal power in this NOLM. In

this paper, the power-symmetric NOLM model is derived based on Ref [12]. It consists of a 3

dB DC, a piece of twisted single mode fiber (SMF) connecting the two output ports of the DC,

and a quarter wave plate (QWP) inserted in the loop. The QWP can be rotated in a plane

perpendicular to the fiber. The twist on fiber generates high optical activity along the fiber, and

also causes a rapid precession of its principal axes. Both effects tend to conserve the

polarization ellipticity of each of the counter propagating beams during propagation in the loop

[12]. O. Pottiez provides a theoretical derivation of the transmission of the symmetrical NOLM

though the matrix description [12]. Based on Ref [12], we further derive the transmission as

shown in Eq. (1)~(3), and use the effective nonlinear length to substitute the absolute length of

SMF:

2 21 1

cos 2 cos 22 2 3 3

cw ccw

s eff s effA n PL A n PL

TAeff Aeff

π πβ α β α

λ λ

= − − − − −

(1)

( )21 sin 2

ccw cw

s sA A α ϕ= − − + (2)

( )1L

effL eδ δ−= − (3)



Whereb

L Lβ µ θ= + , 2 2gµ π= + and qLθ = . q , L , b

L , eff

L and δ are the twist rate,

the absolute length, the beat length, the effective nonlinear length and the attenuation of SMF,

respectively. g kγπ= is the ratio of circular to linear birefringence. ( )2 1h n qγ = − ⋅ is the

circular birefringence in the rotation frame (n is the refractive index and 0.13 0.16h ≈ − ).

bk Lπ= describes the linear birefringence. P is the absolute input power of NOLM.

2n is

the Kerr coefficient. λ is the signal wavelength. eff

A is the effective mode area of SMF. The

input polarization state inφ of beam is defined by Stokes parameter s

A and polarization

directionϕ . 2 2 2 2

( ) ( )sA C C C C+ − + −= − + is the Stokes parameter, where C

+ and C−

are the complex amplitudes of the right-handed and left-handed circular polarization,

normalized to the power P, in such a way that 2 2

1C C+ −+ = . cw

sA and ccw

sA are the Stokes

parameters for CW and CCW, respectively. It is assumed that DC introduces no birefringence,

thus cw inφ φ= . α is the angle of QWP.

Fig. 1. The various NOCs of NOLM for the different input polarizations ( cw

sA ,ϕ ) and angles of

QWPα

The transmission of NOLM is a function of the input power. It also strongly depends on

both the input polarization state and the QWP angle. Thus the different combinations of inφ

and α lead to different power-dependent NOCs, as shown in Fig. 1. In this paper, typical

values in calculation are 20L km= , 15b

L m= , 1.45n = , 0.14h = , 20 2

23.2 10n m W−= × ,

1550nmλ = , 0.0461/ kmδ = , 6q = turn/m and2

50eff

A mµ= . By adjusting inφ and α , the

IDL can be obtained. As a result, the balance between the inhomogeneous loss induced by NPR

in the symmetric NOLM and the mode competition effect of the EDF can lead to stable

multiwavelength oscillations. it must be noticed that for the circular input ( 1cw

sA = ± ) as shown

in Fig. 1, the polarization direction ϕ can be any value.

3. Polarization state and its evolution conditions

Generally, the output polarization of the NOLM depends on not only the input polarization, but

also the input power. The output polarization is independent of input power when the special

settings are made on NOLM [12]. When NOLM is used in a laser, such as ring cavity laser, the

input polarization and output polarization of NOLM mutually determine each other. Thus the

power fluctuation in the ring cavity will change the output polarization and finally affect the

transmission of NOLM. In order to obtain IDL to suppress the mode competition in EDF,

special polarization and its evolution conditions in the cavity must be satisfied.



Firstly, it is necessary to make proper settings on the NOLM to ensure that the output

polarization is independent of the input power. The conclusions of power-independent output

polarization in Ref [12]. are used to discuss the polarization conditions in the ring laser cavity.

For the circular (right or left) input polarization ( 1cw

sA = ± ), when 2 2mα β π= + ( m is

integer and is same for the remainder of this paper), the output polarization is independent of

power. In this case, low-power transmission is zero and maximum transmission is unity.

Apparently, this NOC is not the IDL. For the linear input polarization ( 0cw

sA = ), when

2 2mα β π= + , 2 4mϕ β π= − + or ( )2 2 1 4mϕ β π= − + + , the output polarization is

independent of input power. Equation (1)~(3) show that under the first condition

( 2 2mα β π= + , 2 4mϕ β π= − + ), transmission is always zero for any input power, while

transmission increases until the input power increases to the critical power under the second

condition ( 2 2mα β π= + , ( )2 2 1 4mϕ β π= − + + ). The DOCs under these two

conditions are also not suitable to suppress mode competition of EDF. For the elliptic input

polarization ( 1 1cw

sA− < < and 0cw

sA ≠ ), the power-independent output polarization

condition is cw ccw

s sA A= . By utilizing Eq. (2), a fixed value of ϕ can be solved to make

cw ccw

s sA A= [12]. In this case, the output polarization is independent of power, and is given by

out cw

s sA A= − , 2outϕ ϕ π= − + . Thus there are a series of elliptic input polarizations of NOLM

that make the output polarization independent of power.

Fig. 2. The transmission of NOLM for different angle of QWP α under the condition of

power-independent output polarization. 0.3cw

sA = ,ϕ is determined by cw ccw

s sA A= and Eq.

(2)



Fig. 3. The transmission of NOLM for different elliptic input polarizations ( cw

sA ,ϕ ) under the

condition of power-independent output polarization. 0.65α π= , ϕ is determined by

cw ccw

s sA A= and Eq. (2)

Secondly, to ensure the same NOC of the NOLM at every oscillation in ring laser, the

polarization evolution of beam in the cavity must satisfy the Eq. (4)~(5):

1

in in

n nPC M NOLMφ φ+ = × × × (4)

1

in in

n nφ φ+ = (5)

where the roles of components in the cavity are treated as operators. PC and

NOLM represent the polarization controller (PC) and the NOLM, respectively. M represents

the effects of other components besides NOLM and PC. n represents the nth oscillation.

Equation (4)~(5) can be obtained by adjusting PC in the cavity.

There are a series of elliptic input polarizations of the NOLM that can satisfy the

polarization and its evolution conditions. Various NOCs can be obtained by adjusting the input

polarization and the angle of QWP, as shown in Fig. 2 and Fig. 3. When the IDL is acting, the

NOLM can be used to suppress the mode competition in the MWEDFL.

4. Experimental results and discussion

Using the mechanism described above, a MWEDFL is obtained with the ring cavity as shown

in Fig. 4. A power-symmetric NOLM is inserted in the cavity, which consists of a 3 dB DC and

a 20 km SMF (Coring: SMF-28e) twisted by 3 turn/m. The SMF is wrapped on a cylinder with

suitable diameter to form a QWP which is inserted in the NOLM. A single direction pumping

(Amonics: ALD1480-400-B-FA) at 1480nm is used. A 15 m EDF (Nufern: EDFL-980-HP) is

used as the active medium. A PC is used to adjust the input polarization of NOLM. The optical

isolator is used to prohibit the backward amplified spontaneous emission. The Fabry-Pérot

(F-P) filter (MOI: FFP-TF2) is used to provide periodic loss in the spectrum domain to generate

multiwavelength lasing. A 10 dB DC is used for output. The laser output is taken via the 10%

output port of coupler and measured by an optical spectrum analyzer (ANDO: 6317B) with

0.05nm resolution. The pump power is at 410mW for all experimental results. All outputs have

a wavelength spacing of 1nm, which is determined by the F-P filter. So the wavelength spacing

can be changed by different F-P filter.



Fig. 4. Schematic diagram of proposed laser.

Fig. 5. Output spectra of the laser at two different adjustments of NOLM (a) 13-wavelength

operation (b) 8-wavelength operation within 3-dB bandwidth.

The laser system can be easily set to the proper work state by monitoring its output as PC

and QWP are adjusted. Figure 5(a) shows the output spectra of the laser containing 22 laser

lines. The power difference among the 13 oscillation wavelengths is <3 dB within a spectra

range of 1590-1603 nm. The total output power is −1.28 dBm. Eight wavelength operation with

the total output power of −1.95 dBm is also obtained by adjusting the PC and the QWP, as

shown in Fig. 5(b). The output power fluctuation of the lasing line at 1600 nm in Fig. 5(a) is

<0.6 dB over a 0.5-h period, as shown in Fig. 6.

It can be observed from Fig. 2 and Fig. 3 that the transmission of the NOLM is generally far

less than unity when its input power is zero. This means that except for the power-dependent

inhomogeneous loss (IL) IDL includes the power-independent homogeneous loss (HL). The

values of HL and IL can be respectively or synchronously controlled by adjusting the input

polarization and the angle of QWP. As the mechanism described above, the IL can suppress the

mode competition of EDF. However, the HL is no use to suppress the mode competition, it

actually affects the total intrinsical loss in the laser cavity. So under the condition of the same

pump power and the same length of EDF, the number of multiwavelength operation is

determined by the IL, while the spectra range is determined by the HL. In our experiments, the

number of output wavelength varies from 2 to 13. And the positions of multiwavelength

operation in spectra vary from 1570 nm to 1605 nm. Figures 7(a) and 7(b) show two typical

output spectra of the laser with 5 wavelengths in different spectra range when NOLM is at the

different work states. The total output powers are −2.95dBm and −3.01dBm in Figs. 7(a) and

7(b), respectively. Thus the total output power of the laser varies with the number and spectra

range of output wavelength.



Fig. 6. Output power fluctuation at 1600nm versus time.

Fig. 7. . 5-wavelength output spectra within 3-dB bandwidth are near (a) 1570 nm, and (b) 1600

nm

5. Conclusion

In conclusion, a new MWEDFL has been proposed and demonstrated. A power-symmetric

NOLM, which consists of 3 dB DC, a span of twist SMF and a QWP, is used as an intensity

equalizer. The IDL induced by NPR in the NOLM is effective to suppress the mode

competition of EDF. The polarization and its evolution conditions for the NOLM in the

MWEDFL are discussed in detail. 13 wavelengths lasing operation in a 3 dB bandwidth has

been achieved. The power fluctuation of the output wavelength is <0.6 dB within a 0.5-h

period. The number and spectra range of the multiwavelength operation can be controlled by

adjusting the degree of the IDL.

Acknowledgments

This work is financially supported by Natural Science Foundation of Guangdong Province

(8151805707000004) and Development Program for Outstanding Young Teachers in Harbin

Institute of Technology (HITQNJS.2008.60). The authors appreciate the help of Key

Laboratory of Network Oriented Intelligent Computation, HIT.



multiwavelength erbium-doped fiber laser em-ploying nonlinear polarization rotation in a symmetric...

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