Page 1

Stable and uniform dual-wavelength

erbium-doped fiber laser based on fiber Bragg

gratings and photonic crystal fiber

Xueming Liu, Xiufeng Yang, Fuyun Lu, Junhong Ng, Xiaoqun Zhou,

and Chao Lu

Institute for Infocomm Research, Unit 230, Innovation Centre, Block 2, 18 Nanyang Drive, Singapore 637723

liuxueming72@yahoo.com , xmliu@i2r.a-star.edu.sg

Abstract: Based on fiber Bragg gratings (FBGs) and high nonlinear photonic

crystal fiber (HN-PCF), a novel dual-wavelength erbium-doped fiber (EDF) laser

is proposed and demonstrated. Experimental results show that, owing to the

contributions of two degenerate four-wave mixings in the HN-PCF, the proposed

fiber laser is quite stable and two output signals are uniform at room temperature.

With adjustment of the attenuator, our fiber laser can selectively realize one

wavelength lasing.

©2005 Optical Society of America

OCIS codes: (140.3510) Lasers, fiber; (140.3500) Lasers, erbium; (060.2310) Fiber optics; (060.2320)

Fiber optics amplifiers and oscillators.

References and links

1.

P. C. Peng, H. Y. Tseng, S. Chi, “A tunable dual-wavelength erbium-doped fiber ring laser using a self-seeded

Fabry-Perot laser diode,” IEEE Photon.Technol. Lett. 15, 661-663 (2003).

2.

L. Talaverano, S. Abad, S. Jarabo, et al., “Multiwavelength fiber laser sources with Bragg-grating sensor

multiplexing capability,” J. Lightwave Technol. 19, 553-558 (2001).

3.

A. Bellemare, M. Karasek, M. Rochette, et al., “Room temperature multifrequency erbium-doped fiber lasers

anchored on the ITU frequency grid,” J. Lightwave Technol. 18, 825-831 (2000).

4.

J. Nilsson, Y. W. Lee, and S. J. Kim, “Robust dual-wavelength ring-laser based on two spectrally different

erbium-doped Fiber amplifiers,” IEEE Photon.Technol. Lett. 8, 1630–1632 (1996).

5.

Y. G. Liu, X.H. Feng, S. Z. Yuan, et al., “Simultaneous four-wavelength lasing oscillations in an erbium-doped fiber

laser with two high birefringence fiber Bragg gratings,” Opt. Express 12, 2056-2061 (2004).

http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-10-2056

6.

Y. Z. Xu, H. Y. Tam, W. C. Du, et al., “Tunable dual-wavelength-switching fiber grating laser,” IEEE Photon.Technol.

Lett. 10, 334-336 (1998).

7.

J. Yao, J. P. Yao, Z. C. Deng, “Multiwavelength actively mode-locked fiber ring laser with suppressed homogeneous

line broadening and reduced supermode

http://www.opticsexpress.org/abstract.cfm?URI=OPEX-12-19-4529

8.

S. Yamashita and K. Hotate, “Multiwavelength erbium-doped fiber laser using intracavity etalon and cooled by liquid

nitrogen,” Electron. Lett. 32, 1298–1299 (1996).

9.

A. Bellemare, M. Karasek, C. Riviere, et al. “A broadly tunable erbium-doped fiber ring laser: experimentation and

modeling,” IEEE J. Sel. Top. Quantum Electronics 7, 22-29, (2001).

10. J. Canning, N. Groothoff, et al. “All-fibre photonic crystal distributed Bragg reflector (PC-DBR) fibre laser,” Opt.

Express 11, 1995-2000 (2003) http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-17-1995

11. S. O. Konorov, A. B. Fedotov, A. M. Zheltikov, “Enhanced four-wave mixing in a hollow-core photonic-crystal

fiber,” Opt. Lett. 28, 1448-1450 (2003).

12. G. P. Agrawal, Application of Nonlinear Fiber Optics, San Diego: Academic Press, 2001.

13. N. J. C. Libatique, L. Wang, and R. K. Jain, "Single-longitudinal-mode tunable WDM-channel-selectable fiber laser,"

Opt. Express 10, 1503-1507 (2002), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-10-25-1503

noise,” Opt. Express

12, 4529-4534 (2004).

1. Introduction

Because of potential applications of multiwavelength fiber lasers, erbium-doped fiber (EDF)

lasers emitting in multiple wavelengths simultaneously have attracted much interest recently

[1-3]. Because of the homogeneous gain broadening of EDFs, various techniques for mitigating

the mode competition have been employed to achieve stable multiwavelength (or

dual-wavelength) oscillations [4-9]. For example, a filter was inserted into the EDF laser cavity

[2, 4], and a single EDF was cooled in liquid nitrogen (77 K) to reduce the homogeneous

(C) 2005 OSA 10 January 2005 / Vol. 13, No. 1 / OPTICS EXPRESS 142

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broadening [8]. Moreover, with the assistance of four-wave mixing (FWM) of PCF, a new fiber

laser configuration has been proposed [10] and laser pulses have been enhanced [11]. In this

paper, on the basis of fiber Bragg gratings (FBGs) and FWM of HN-PCF, a stable

dual-wavelength EDF laser is proposed and demonstrated. The experimental results show that the

proposed EDF laser can stably and uniformly lase two wavelengths simultaneously.

2. Experimental setup

The schematic layout of the proposed EDF laser is shown in Fig. 1(a). This experiment is based on a

standard EDF ring laser, with HN-PCF and two FBGs. The transmission spectra of FBG1 and

FBG2 are exhibited in Fig. 1(b). A circulator and two FBGs form the ring, with EDFA for lasing

wavelengths λ1 and/or λ2 and HN-PCF for creating FWM. A variable attenuator (VA) is used to

adjust the reflection spectra of FBG1, helping to lase wavelength λ1 or λ2 or both. A 90:10 coupler

(10% output) is used for the laser output. For simultaneously lasing two wavelengths λ1 and λ2, the

ring losses at λ1 and λ2 are balanced by adjusting VA and changing polarization controller (PC).

In Fig. 1(a), the commercial EDFA (Opto-Link Corporation Limited, Model EDFA-MP),

which can offer 13 dBm output saturation power and a maximum 35 dB small signal gain,

includes the 980 nm pump source, 980/1550 WDM coupler and optical isolator. A 51-m-long

HN-PCF is with the nonlinear coefficient of ~11 /W/km and the flat dispersion (0.5-1.5

ps/nm/km) over the wavelength range of 1480-1620nm. FBG1 and FBG2 have the central

wavelength of 1563.51 nm and 1567.21 nm, the reflectivity of 12.1 dB and 13.9 dB, and the

full-width at half maximum (FWHM) of 0.12 nm and 0.13 nm, respectively (see Fig. 1(b)).

Moreover, except that Fig. 2 and Fig. 7 are measured by an optical spectrum analyzer (OSA) with

the resolution of 0.1 nm, all other results are examined by the resolution of 0.01 nm.

3. Principle and results

When the attenuation of VA is equal to zero, the amplified spontaneous emission (ASE) reflection

spectra from two FBGs are demonstrated in Fig. 2. Figure 2 shows that the normalized ASE

power reflected from FBG1 is more than that reflected from FBG2. Therefore, λ1 is lased in the

ring cavity, and the output spectrum is illustrated in Fig. 3(a). On the other hand, when the

attenuation of VA is more than zero and the reflection power from FBG1 is less than that from

FBG2, only λ2 is lased rather than λ1. In this case, Fig. 3(b) shows the output spectrum. Both

Reflection spectra (2dB/div)

EDFA

HN-PCF

PC

90:10 Coupler

Circulator

1

3

2

FBG2 FBG1

VA

1563 1564 1565 1566 1567

Wavelength (nm)

Transmission (2dB/div)

10% Output

FBG1

FBG2

(a)

(b)

Fig. 1. Experimental setup of the dual-wavelength-switching EDF laser (PC: polarization controller, VA:

variable attenuator, PCF: photonic crystal fiber, FBG: fiber Bragg gratings). (b) The transmission spectra

of two FBGs.

15601563

Wavelength (nm)

1566 1569 1572

Fig. 2. Normalized ASE reflection spectra of two FBGs at the case that the attenuation of VA is

equal to zero.

#5962 - $15.00 US Received 3 December 2004; revised 22 December 2004; accepted 22 December 2004

(C) 2005 OSA 10 January 2005 / Vol. 13, No. 1 / OPTICS EXPRESS 143

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Figures 3(a) and (b) are measured at the pump current I=55 mA. The wave without lasing in Fig.

3(b) is from the reflection of FBG, while the wave at the wavelength of 1567.21 nm in Fig. 3(a) is

somewhat lased besides the FBG-reflected ASE. The experimental results also show that the

output spectrum performance at other pump currents is similar to Fig. 3. Therefore, by adjusting

VA in the EDF ring laser, the lased wavelength λ1 or λ2 is selective.

By adjusting VA and PC, both λ1 and λ2 can be lased simultaneously when the reflection

powers from FBG2 and FBG1 are approximate equal. Unfortunately, the experiments show that

the lased wavelengths λ1 and λ2 are unstable and nonuniform if without PCF. To improve the

stability and increase the uniformity of dual-wavelength, the HN-PCF is introduced. The

operation principle is as follows.

The continuous waves (CW) with wavelengths λ1 and λ2 (corresponding to frequencies ω1

and ω2) are reflected from FBG1 and FBG2, respectively, and are launched into a 51-m-long

HN-PCF. Due to higher nonlinear coefficient and lower dispersions of PCF, two new waves at

frequencies ω0=2⋅ω1-ω2 and ω3=2⋅ω2-ω1 are created at the output by two degenerate-FWM

processes of HN-PCF (ω0, ω1, ω2 and ω3 are shown in Fig. 4). Successively, four waves are

inputted to EDFA and are amplified. Because of the mode competition caused by the

homogeneous gain broadening of EDF [5], the power of one signal (e.g., ω1) is greater than that

of other signal (e.g., ω2). After the reflection of FBGs with signals ω1 and ω2 and launching into

HN-PCF again, two degenerate-FWMs (i.e., ω0+ω2=2⋅ω1 and ω3+ω1=2⋅ω2) in the HN-PCF lead

to the energy transfer from the higher-power signal ω1 to the lower-power signal ω2. Therefore,

FWM can effectively alleviate the gain competition in the EDF and significantly increase the

stability and uniformity of two signals ω1 and ω2.

Figure 4 demonstrates the experimental result of the proposed dual-wavelength EDF laser at

I=240 mA, where two inserted figures are zoomed in two created waves ω0 and ω3. From Fig. 4,

we can see that two signals ω1 and ω2 produced by our EDF laser are great uniform and OSNR is

15601563

Wavelength (nm)

156615691572

-60

-50

-40

-30

-20

Power (dBm)

I=55 mA

1560 1563

Wavelength (nm)

1566 15691572

-60

-50

-40

-30

-20

Power (dBm)

I=55 mA

(a) (b)

Fig. 3. Output spectra for different VA value at pump current I=55 mA at the case that (a) the

attenuation of VA is equal to zero, and (b) the reflection power from FBG1 is less than that from

FBG2.

15601563

Wavelength (nm)

1566 1569 1572

-70

-60

-50

-40

-30

-20

-10

0

ω3

ω0

Power (dBm)

I=240 mA

ω2

ω1

Fig. 4. Output spectra at pump current I=240 mA. Two inserted figures denote the magnification of

waves ω0 and ω3 created by two degenerate-FWMs. ω0=2⋅ω1-ω2 and ω3=2⋅ω2-ω1.

(C) 2005 OSA 10 January 2005 / Vol. 13, No. 1 / OPTICS EXPRESS 144

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more than 60 dB. Moreover, Figs. 3 and 4 illustrate that the FWHM bandwidth in each

wavelength is approximately 0.09-0.11 nm, and then they are the multi-longitudinal mode lasers

(the detailed explanations are shown in Appendix). To give a clearer understanding of the

uniformity of signals ω1 and ω2, we present a movie that illustrates the evolution of

dual-wavelength EDF laser in terms of the EDFA pump current I (see Fig. 5).

The movie exhibits that (1) when the pump current I<45 mA, the oscillator does not produce

any signal except noises because of the loss of the cavity; (2) if 45 mA <I<50 mA, two waves are

created by the reflection of two FBGs; (3) when I>50 mA, only one signal is lased in the

oscillator firstly, and successively two signals are generated simultaneously; (4) during the

process of 50 mA <I<70 mA, two lased signals ω1 and ω2 are unstable and nonuniform; (5) when

I>70 mA, ω1 and ω2 are great stable and uniform. Above results can be found from Fig. 6 in

detail.

Figure 6 shows the power difference ∆P of two signals ω1 and ω2 in terms of the pump

current I. The inset is zoomed in the red dashed-dot frame. From Fig. 6 and movie, one can see

that (1) although the power difference ∆P is small when I<50 mA, no any signal is lased; (2) only

one signal is lased when I=51 mA and 52 mA, as leads to ∆P>27 dB; (3) if I>70 mA, two lased

signals are great uniform and their power difference ∆P is less than 1.8 dB (see the inset of Fig.

6); (4) ∆P is less than 0.6 dB when I exceeds 165 mA. Therefore, with the increase of the EDF

gain (correspond to increase I), the lased spectra of the dual-wavelength tend to be more uniform.

Our experiments also show that lengthening the length of nonlinear fiber can effectively equalize

the lased spectra of two waves. The experimental results exhibit that, when I>70 mA, our

dual-wavelength EDF laser is much stable. To show this advantage, Fig. 7 offers an experimental

example at I=170 mA.

The variation of power in terms of time at I=170 mA is demonstrated in Fig. 7, where the unit

15601563

Wavelength (nm)

15661569 1572

-70

-60

-50

-40

-30

-20

-10

0

Power (dBm)

Beginning

Fig. 5. (658 KB) Movie showing the procedure of dual-wavelength EDF laser in terms of pump current I.

50 100

Pump current I (mA)

150 200

0

5

10

15

20

25

100 150 200

0.0

0.4

0.8

1.2

1.6

Power difference ∆P (dB)

Fig. 6. Power difference ∆P of two signals ω1 and ω2 in terms of the

pump current I. The inset is zoomed in the red dashed-dot frame.

(C) 2005 OSA 10 January 2005 / Vol. 13, No. 1 / OPTICS EXPRESS 145

#5962 - $15.00 USReceived 3 December 2004; revised 22 December 2004; accepted 22 December 2004

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of abscissa is second. One can see that, from Fig. 7, the signal powers fluctuate slightly and their

relative fluctuation is less than 0.34%. Therefore, the proposed EDF laser with the assistance of

two degenerate-FWMs are not only much uniform for two signals but also great stable. The

experiments also show that the output power of each wavelength in the single-wavelength (e.g.,

Fig. 3) and dual-wavelength lasing is of the excellent stability.

From Figs. 4-7, we can see that the proposed fiber laser has the capacity of lasing the

dual-wavelength with the excellent uniformity and stability. The physical reasoning can be

explained as follow. When two degenerate-FWM processes are produced, we name as

ω1+ω1=ω0+ω2 and ω2+ω2=ω3+ω1. Then, two photons of frequency ω1 are annihilated in order to

create one photon of frequency ω0 and another photon of frequency ω2. In the same way, two

photons of frequency ω2 are annihilated in order to produce one photon of frequency ω3 and

another photon of frequency ω1. Here, we designate P1 and P2 as the powers at ω1 and ω2,

respectively. To balance the number of photons annihilated and created in FWM processes,

variation of power at frequency ω1, ∆P1, and at ω2, ∆P2, can be determined as

ω

α

ω

⎝

1

121

2

2

PPP

⎛

⎜

⎞

⎟

⎠ (1)

∆=−

2

212

1

2

PPP

ω

ω

α

⎛

⎜

⎝

⎞

⎟

⎠

∆=−

(2)

where the parameter α denotes the efficiency of FWM processes. From Eqs. (1) and (2), variation

of the quotient P2/P1 can be achieved as

()

2

22 1 2

P

P

1221

2

111 2 1

1

1

P

P

P P

∆

P P

∆

P

ωω

ω

αω

⎡

⎢

⎢

⎣

⎤

⎥

⎥

⎦

⎛

⎜

⎝

⎞

⎟

⎠

⎛

⎜

⎝

⎞

⎟

⎠

∆=−=−

. (3)

From Eq. (3), it can be seen that 1 if ω1P2>ω2P1 (or P2>P1, approximately), ∆(P2/P1)<0 and then

P2→P1; 2 if ω2P1>ω1P2 (or P1>P2, approximately), ∆(P2/P1)>0 and so P1→P2; 3 and if

ω1P2=ω2P1 (or P2=P1, approximately), ∆(P2/P1)=0. Therefore, both powers can be equalized and

stabilized by FWM processes.

4. Conclusions

On the basis of two FBGs and a flat dispersion HN-PCF, a new dual-wavelength EDF laser is

proposed and demonstrated in this paper. With the assistance of two degenerate-FWMs in the

HN-PCF, the lased dual-wavelength has great stability, e.g., the relative fluctuation of signal

power is less than 0.34% in our experiments. At the same time, the two lased signals ω1 and ω2

are quite uniform; e.g., their power difference ∆P is less than 1.8 dB for the pump current I>70

mA and even it is less than 0.6 dB for I>165 mA at room temperature. By adjustment of the

attenuator, the proposed fiber laser can easily realize one selection wavelength lasing.

0 100 200 300 400 500 600 700 800 900

Time (second)

Fig. 7. Fluctuation of power with time at I=170 mA.

-5.280

-5.276

-5.272

-5.268

-5.264

Power (dBm)

I=170 mA

(C) 2005 OSA10 January 2005 / Vol. 13, No. 1 / OPTICS EXPRESS 146

#5962 - $15.00 US Received 3 December 2004; revised 22 December 2004; accepted 22 December 2004

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Appendix

The longitudinal-mode spacing ∆vL in the laser can be achieved by use of the phase-matching

condition [12], i.e.,

[ (2) ( )]

LR

vL

β ωπ β ω+∆−

where LR is the fiber length and β is the propagation constant. Usually, ∆vL can be approximately

given by ∆vL=1/TR [12], where TR is the round-trip time within the resonator. In our EDF laser,

the total length is ~70 m, which includes the HN-PCF length of 51 m, the EDFA length of ~10 m

and some jumpers and connecting fibers. As a result, the longitudinal-mode spacing ∆vL ≅3 MHz.

On the other hand, the effective linewidth of the EDF laser is less than 100 MHz [9, 13].

However, the FWHM of the spectral envelope of the lasers in Figs. 3-5 is ~12 GHz (i.e., ~0.1

nm). Therefore, such a spectral envelope should include any modes and our EDF laser is a

multi-longitudinal-mode laser.

Actually, to realize a single-longitudinal-mode fiber laser, some special methods and

techniques have to be implemented, e.g., inserting a linewidth-narrowing saturable absorption

filter [13]. In our proposed EDF laser, such techniques are excluded. Therefore, the excellent

stability and uniformity of our dual-wavelength laser are realized by the assistance of the FWM

effect of HN-PCF.

2

π=

, (4)

(C) 2005 OSA 10 January 2005 / Vol. 13, No. 1 / OPTICS EXPRESS 147

#5962 - $15.00 USReceived 3 December 2004; revised 22 December 2004; accepted 22 December 2004