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Multi-wavelength bismuth-based erbium-doped fiber laser based on four-wave mixing effect in photonic crystal fiber
ARTICLE IN PRESS Optics & Laser Technology 42 (2010) 1250–1252
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Multi-wavelength bismuth-based erbium-doped fiber laser based on four-wave mixing effect in photonic crystal fiber R. Parvizi a, S.W. Harun a,b,n, N.S. Shahabuddin c, Z. Yusoff c, H. Ahmad a a
Photonics Research Center, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia c Faculty of Engineering, Multimedia University, 63100 Cyberjaya, Malaysia b
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 December 2009 Received in revised form 18 March 2010 Accepted 29 March 2010 Available online 15 April 2010
A stable multi-wavelength erbium-doped fiber laser based on four-wave mixing (FWM) in a photonic crystal fiber (PCF) is demonstrated in this paper. The phase matching condition for four-wave mixing in the photonic crystal fiber has been enhanced using a seed signal and a polarisation controller to control the states of polarisation in the ring laser cavity. At a maximum pump power of 1480 nm, 5 lines are observed with nearly 2.15 nm spacing between the lines, and with a signal to noise ratio of more than 20 dB. The number of channels and wavelength spacing can be controlled by varying the output coupler ratio. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Photonic crystal fiber Bismuth-based erbium-doped fiber Four-wave mixing
1. Introduction Multi-wavelength fiber lasers are useful light sources for wavelength-division-multiplexed (WDM) fiber communication systems, fiber sensors, and testing of optical instruments [1,2]. Erbium-doped fiber amplifiers (EDFAs) usually have a homogeneous gain medium at room temperature, which leads to strong mode competition and limits the number of lasing modes. In order to achieve simultaneous multi-wavelength operation in an erbium-doped fiber laser (EDFL), the homogenous line broadening characteristic at room temperature must be dealt with. Several methods have been proposed to mitigate the mode competition phenomenon, such as cooling the EDF in liquid nitrogen to reduce the homogeneous broadening [3], using special erbium-doped fiber structure [4], using configurations, which depend on nonlinear effects [5–7], etc. Among those methods, four-wave mixing (FWM) based multi-wavelength operations are attractive due to their simple structure, tunable spacing and work at room temperature. Multi-wavelength lasers have potential application in wavelength conversion systems [8]. In an earlier report, Liu et al. [9] theoretically proved that the self-stability function originated from two-pump FWM processes. In another work [10], the stabilization of waves by multiple FWM processes was proved
n Corresponding author at: Photonics Research Center, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail addresses: [email protected] (R. Parvizi), [email protected] (S.W. Harun).
0030-3992/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2010.03.018
theoretically and confirmed experimentally with the assistance of a highly nonlinear photonic crystal fiber (PCF). PCF technology has advanced significantly in recent years and has resulted in the production of high quality PCFs with low loss, ultrahigh nonlinearity, and controllable dispersion. PCFs with an effective nonlinearity of 10–100 times higher than that of standard single-mode silica fibers can be achieved due to the large refractive index contrast between silica and air. Therefore, it becomes possible to achieve a stabilized multi-wavelength EDFL using a very short length of PCF [11]. In this paper, a compact FWM-based multi-wavelength EDFL is experimentally demonstrated using a 100 m long PCF in conjunction with a 49 cm long bismuth-based erbium-doped fiber (Bi-EDF) in a ring configuration. FWM effect is used to suppress the mode competition by transferring the energy among different frequency components. The number of channels and wavelength spacing of the proposed multi-wavelength laser can be flexibly controlled by changing the ratio of the output coupler in the ring configuration.
2. Experimental set-up The schematic of the experimental set-up for the proposed multi-wavelength EDFL is shown in Fig. 1. The main components are the forward pumped bismuth-based erbium-doped fiber amplifier (Bi-EDFA), the 100 m long of PCF and a polarisation controller (PC). The Bi-EDFA consists of a 49 cm long Bi-EDF with an erbium ion concentration of 3250 ppm and a cut-off wavelength of 1440 nm and a pump absorption rate of
ARTICLE IN PRESS R. Parvizi et al. / Optics & Laser Technology 42 (2010) 1250–1252
1251
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78mW
PCF, 100m
Bi-EDF, 49cm LD, 1480nm
2
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TLS
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106mW 140mW
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Wavelength (nm) Fig. 2. Output spectra of the FWM-based multi-wavelength EDFL at different 1480 nm pump powers.
Fig. 1. Schematic of the set-up for FWM-based multi-wavelength EDFL.
10
3. Results and discussion Since conventional EDFLs are homogeneously broadened, different wavelengths trying to oscillate in the same laser compete, so one emerges as the dominant wavelength. In the proposed EDFL, Bi-EDF is used as a gain medium and PCF is used to suppress the mode competition in the EDFL so that the simultaneous multi-wavelength operation can be achieved with assistance from the seed signal. Fig. 2 shows the output spectrum of the proposed FWM-based EDFL at different 1480 nm pump power. The inset of Fig. 2 shows the output spectrum of the laser without the seed TLS signal in which case the laser oscillates at only one wavelength at around 1561 nm, and the lasing wave is very unstable. In the experiment, a 90/10 output coupler is used where 90% of the light is allowed to oscillate in the ring cavity. As the 1561 nm TLS signal is launched into the ring in clockwise direction, the signal interacts with the oscillating free running laser in the opposite direction to enhance the phase matching condition and generate FWM stokes signals. With the use of highly nonlinear PCF in the ring cavity, a wave with stronger energy can transfer part of the energy into another wave with weaker energy via FWM processes [6]. This can effectively improve the gain competition in the Bi-EDF and significantly
0 Output power (dBm)
130 dB/m at 1480 nm. It is pumped by a 1480 nm laser diode with the maximum power of 140 mW. The PCF uses a micro-structured cladding region with air holes to guide light in a pure silica core; therefore it has a very high nonlinearity characteristic. The nonlinearity coefficient of the PCF is approximately 11 W/km, the zero dispersion wavelength of the PCF is around 1550 nm. The dispersion at 1561 nm is approximately 0.5 ps/(km nm). Initially, the amplified spontaneous emission (ASE) from the Bi-EDFA oscillates in the ring cavity to generate laser output. The output from a tunable laser source (TLS) is injected via optical circulator to the ring cavity to control the wavelength of laser operation and realize stable simultaneous multi-wavelength oscillation. An optical isolator is used to block the TLS signal from oscillating in the cavity and to ensure unidirectional operation of the laser. A PC is used to control the polarisation state in the cavity and thus maximize the FWM effect. The output of the EDFL is tapped out from the coupler with a larger portion of the light allowed to oscillate in the ring cavity. The output spectrum is measured using an optical spectrum analyzer (OSA) with a resolution of 0.015 nm.
80% 90%
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-60 1550
1555
1560
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Wavelength (nm)
Fig. 3. Multi-wavelength FWM spectra based on different ratios of couplers.
increase the stability and uniformity of the output multiwavelength lines due to the self-stability function of FWM effect [6,12]. By adjusting the PC to obtain a proper polarisation state, the FWM effect can be maximized and the mode competition in the EDF can be further reduced. As a result, a stable multiwavelength laser comb is obtained as shown in Fig. 2. The number of lines and its flatness are expected to be improved if the FWM effect can be maximized. The FWM in the PCF is propotional to the product of gPL, where g is the nonlinear coefficient of the PCF, P is the optical power passing through the PCF and L is the effective length of the PCF. The effectiveness of the FWM can be enhanced by using a longer length of PCF L, and/ or using a fiber with larger g, and/or using higher gain/larger saturation power EDFA to get a high cavity power P. Fig. 2 demonstrates that the output power of the multi-wavelength output increases as the 1480 nm pump power increases, which verifies the above expectation. At maximum pump power of 1480 nm, 5 lines are observed with nearly 2.15 nm spacing between the lines and signal to noise ratio of more than 20 dB. The number of lines decreases when the pump power decreases. It is also expected that when a coupler with a different coupling ratio is used, the bandwidth and the spacing between laser output lines will change. Fig. 3 shows the laser output spectrum at different output couplers. As shown in the figure, the FWM lines of spacings of 1.15, 2.15, and 0.57 nm are dominant with the use of 80/20, 90/10, and 95/5 couplers, respectively. The optimum coupling ratio with the highest signal to noise ratio is obtained with the 90/10 coupler. This is attributed to the oscillating laser power in the laser cavity, which has been optimised so that the product of gPL is largest in the cavity. The use of 90/10 coupler
ARTICLE IN PRESS 1252
R. Parvizi et al. / Optics & Laser Technology 42 (2010) 1250–1252
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–10.0
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Wavelength (nm) Fig. 4. Spectral evolutions of the proposed FWM-based multi-wavelength EDFL against time. (The laser spectra scanned over time.)
allows 90% of the light to oscillate in the cavity and thus provide the optimum gain to compensate for the loss in the cavity. The stability of the multi-wavelength operation is of vital importance for practical applications. Fig. 4 shows the spectral evolutions of the proposed EDFL with PCF in terms of time. In the experiment, the output spectral is repeatedly scanned for every 10 min. The multi-wavelength EDFL lases stably with power fluctuations of less than 0.5 dB over 2.5 h. There are two main factors that contribute to the operation instability in the conventional EDFL. They are mode competition in the laser cavity and the drift of the spectral profile as a result of the thermal fluctuation. In the proposed set-up, a 100 m length of PCF is used to reduce the mode competition inside the ring cavity via the nonlinear effect. Therefore, multi-wavelength operation may be self-stabilized by taking the combination of PCF and Bi-EDFA as a good candidate for nonlinear and linear gain media. In addition, temperature variations and mechanical vibrations were small as the experiment was conducted in a laboratory; thus the drift of the spectral profile was minimal. These results show that the proposed configuration is suitable for wavelength conversion application in optical communication networks.
4. Conclusion A multi-wavelength EDFL is experimentally demonstrated at room temperature based on FWM effect in the PCF. At maximum pump power of 1480 nm, 5 lines are stably produced with nearly 2.15 nm spacing between the lines and a signal to noise ratio of more than 20 dB. The phase matching condition for four-wave mixing in the PCF has been enhanced using a seed signal and a suitable state of polarisation in the ring laser cavity. The number
of channels and wavelength spacing can be controlled by varying the output coupler ratio.
References [1] Chen DR, Fu H, Ou H, Qin S. Wavelength-spacing continuously tunable multiwavelength SOA-fiber ring laser based on Mach–Zehnder interferometer. Opt Laser Technol 2008;40(2):278–81. [2] Pan S, Zhao X, Yu W, Lou C. Dispersion-tuned multiwavelength actively mode-locked fiber laser using a hybrid gain medium. Opt Laser Technol 2008;40(6):854–7. [3] Yamashita S, Hotate K. Multiwavelength erbium-doped fiber laser using intracavity etalon and cooled by liquid nitrogen. Electron Lett 1996;32:1298–9. [4] Graydon O, Loh WH, Laming RI, Dong L. Triple-frequency operation of an Erdoped twincore fiber loop laser. IEEE Photon Technol Lett 1996;8:63–5. [5] Harun SW, Emami SD, Abd Rahman F, Muhd-Yassin SZ, Abd-Rahman MK, Ahmad H. Multi-wavelength brillouin/erbium-ytterbium fiber laser. Laser Phys Lett 2007;4(8):601–3. [6] Liu X, Yang X, Lu F, Ng J, Zhou X, Lu C. Stable and uniform dual-wavelength erbium-doped fiber laser based on fiber Bragg gratings and photonic crystal fiber. Opt Express 2005;13:142–7. [7] Shahi S, Harun SW, Shahabuddin NS, Shirazi MR, Ahmad H. Multi-wavelength generation using a bismuth-based EDF and brillouin effect in a linear cavity configuration. Opt Laser Technol 2009;41(2):198–201. [8] Yu J, Jeppesen P. Wavelength conversion by use of four-wave mixing in a novel optical loop configuration. Opt Lett 2000;25:393–5. [9] Liu X, Zhou X, Lu C. Multiple four-wave mixing self-stability in optical fibers. Phys Rev A 2005;72:013811. [10] Liu XM. Theory and experiments for multiple four-wave-mixing processes with multifrequency pumps in optical fibers. Phys Rev A 2008;77:043818. [11] Zhang AL, Liu H, Demokan MS, Tam HY. Stable and broad bandwidth multiwavelength fiber ring laser incorporating a highly nonlinear photonic crystal fiber. IEEE Photon Technol Lett 2005;17:2535–7. [12] Liu X, Zhou X, Tang X, Ng J, Hao J, Chai ’TY, et al. Switchable and tunable multiwavelength erbium-doped fiber laser with fiber Bragg gratings and photonic crystal fiber. IEEE Photon Technol Lett 2005;17(8):1626–8.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Multi-wavelength bismuth-based erbium-doped fiber laser based on four-wave mixing effect in photonic crystal fiber R. Parvizi a, S.W. Harun a,b,n, N.S. Shahabuddin c, Z. Yusoff c, H. Ahmad a a
Photonics Research Center, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia c Faculty of Engineering, Multimedia University, 63100 Cyberjaya, Malaysia b
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 December 2009 Received in revised form 18 March 2010 Accepted 29 March 2010 Available online 15 April 2010
A stable multi-wavelength erbium-doped fiber laser based on four-wave mixing (FWM) in a photonic crystal fiber (PCF) is demonstrated in this paper. The phase matching condition for four-wave mixing in the photonic crystal fiber has been enhanced using a seed signal and a polarisation controller to control the states of polarisation in the ring laser cavity. At a maximum pump power of 1480 nm, 5 lines are observed with nearly 2.15 nm spacing between the lines, and with a signal to noise ratio of more than 20 dB. The number of channels and wavelength spacing can be controlled by varying the output coupler ratio. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Photonic crystal fiber Bismuth-based erbium-doped fiber Four-wave mixing
1. Introduction Multi-wavelength fiber lasers are useful light sources for wavelength-division-multiplexed (WDM) fiber communication systems, fiber sensors, and testing of optical instruments [1,2]. Erbium-doped fiber amplifiers (EDFAs) usually have a homogeneous gain medium at room temperature, which leads to strong mode competition and limits the number of lasing modes. In order to achieve simultaneous multi-wavelength operation in an erbium-doped fiber laser (EDFL), the homogenous line broadening characteristic at room temperature must be dealt with. Several methods have been proposed to mitigate the mode competition phenomenon, such as cooling the EDF in liquid nitrogen to reduce the homogeneous broadening [3], using special erbium-doped fiber structure [4], using configurations, which depend on nonlinear effects [5–7], etc. Among those methods, four-wave mixing (FWM) based multi-wavelength operations are attractive due to their simple structure, tunable spacing and work at room temperature. Multi-wavelength lasers have potential application in wavelength conversion systems [8]. In an earlier report, Liu et al. [9] theoretically proved that the self-stability function originated from two-pump FWM processes. In another work [10], the stabilization of waves by multiple FWM processes was proved
n Corresponding author at: Photonics Research Center, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail addresses: [email protected] (R. Parvizi), [email protected] (S.W. Harun).
0030-3992/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2010.03.018
theoretically and confirmed experimentally with the assistance of a highly nonlinear photonic crystal fiber (PCF). PCF technology has advanced significantly in recent years and has resulted in the production of high quality PCFs with low loss, ultrahigh nonlinearity, and controllable dispersion. PCFs with an effective nonlinearity of 10–100 times higher than that of standard single-mode silica fibers can be achieved due to the large refractive index contrast between silica and air. Therefore, it becomes possible to achieve a stabilized multi-wavelength EDFL using a very short length of PCF [11]. In this paper, a compact FWM-based multi-wavelength EDFL is experimentally demonstrated using a 100 m long PCF in conjunction with a 49 cm long bismuth-based erbium-doped fiber (Bi-EDF) in a ring configuration. FWM effect is used to suppress the mode competition by transferring the energy among different frequency components. The number of channels and wavelength spacing of the proposed multi-wavelength laser can be flexibly controlled by changing the ratio of the output coupler in the ring configuration.
2. Experimental set-up The schematic of the experimental set-up for the proposed multi-wavelength EDFL is shown in Fig. 1. The main components are the forward pumped bismuth-based erbium-doped fiber amplifier (Bi-EDFA), the 100 m long of PCF and a polarisation controller (PC). The Bi-EDFA consists of a 49 cm long Bi-EDF with an erbium ion concentration of 3250 ppm and a cut-off wavelength of 1440 nm and a pump absorption rate of
ARTICLE IN PRESS R. Parvizi et al. / Optics & Laser Technology 42 (2010) 1250–1252
1251
10
WSC
78mW
PCF, 100m
Bi-EDF, 49cm LD, 1480nm
2
Circulator 1
Isolator
TLS
Output power (dBm)
0
106mW 140mW
-10 -20 -30 -40 -50
3 -60 1550
OSA PC
1555
10% Coupler
1560
1565
1570
Wavelength (nm) Fig. 2. Output spectra of the FWM-based multi-wavelength EDFL at different 1480 nm pump powers.
Fig. 1. Schematic of the set-up for FWM-based multi-wavelength EDFL.
10
3. Results and discussion Since conventional EDFLs are homogeneously broadened, different wavelengths trying to oscillate in the same laser compete, so one emerges as the dominant wavelength. In the proposed EDFL, Bi-EDF is used as a gain medium and PCF is used to suppress the mode competition in the EDFL so that the simultaneous multi-wavelength operation can be achieved with assistance from the seed signal. Fig. 2 shows the output spectrum of the proposed FWM-based EDFL at different 1480 nm pump power. The inset of Fig. 2 shows the output spectrum of the laser without the seed TLS signal in which case the laser oscillates at only one wavelength at around 1561 nm, and the lasing wave is very unstable. In the experiment, a 90/10 output coupler is used where 90% of the light is allowed to oscillate in the ring cavity. As the 1561 nm TLS signal is launched into the ring in clockwise direction, the signal interacts with the oscillating free running laser in the opposite direction to enhance the phase matching condition and generate FWM stokes signals. With the use of highly nonlinear PCF in the ring cavity, a wave with stronger energy can transfer part of the energy into another wave with weaker energy via FWM processes [6]. This can effectively improve the gain competition in the Bi-EDF and significantly
0 Output power (dBm)
130 dB/m at 1480 nm. It is pumped by a 1480 nm laser diode with the maximum power of 140 mW. The PCF uses a micro-structured cladding region with air holes to guide light in a pure silica core; therefore it has a very high nonlinearity characteristic. The nonlinearity coefficient of the PCF is approximately 11 W/km, the zero dispersion wavelength of the PCF is around 1550 nm. The dispersion at 1561 nm is approximately 0.5 ps/(km nm). Initially, the amplified spontaneous emission (ASE) from the Bi-EDFA oscillates in the ring cavity to generate laser output. The output from a tunable laser source (TLS) is injected via optical circulator to the ring cavity to control the wavelength of laser operation and realize stable simultaneous multi-wavelength oscillation. An optical isolator is used to block the TLS signal from oscillating in the cavity and to ensure unidirectional operation of the laser. A PC is used to control the polarisation state in the cavity and thus maximize the FWM effect. The output of the EDFL is tapped out from the coupler with a larger portion of the light allowed to oscillate in the ring cavity. The output spectrum is measured using an optical spectrum analyzer (OSA) with a resolution of 0.015 nm.
80% 90%
-10
95%
-20 -30 -40 -50
-60 1550
1555
1560
1565
1570
Wavelength (nm)
Fig. 3. Multi-wavelength FWM spectra based on different ratios of couplers.
increase the stability and uniformity of the output multiwavelength lines due to the self-stability function of FWM effect [6,12]. By adjusting the PC to obtain a proper polarisation state, the FWM effect can be maximized and the mode competition in the EDF can be further reduced. As a result, a stable multiwavelength laser comb is obtained as shown in Fig. 2. The number of lines and its flatness are expected to be improved if the FWM effect can be maximized. The FWM in the PCF is propotional to the product of gPL, where g is the nonlinear coefficient of the PCF, P is the optical power passing through the PCF and L is the effective length of the PCF. The effectiveness of the FWM can be enhanced by using a longer length of PCF L, and/ or using a fiber with larger g, and/or using higher gain/larger saturation power EDFA to get a high cavity power P. Fig. 2 demonstrates that the output power of the multi-wavelength output increases as the 1480 nm pump power increases, which verifies the above expectation. At maximum pump power of 1480 nm, 5 lines are observed with nearly 2.15 nm spacing between the lines and signal to noise ratio of more than 20 dB. The number of lines decreases when the pump power decreases. It is also expected that when a coupler with a different coupling ratio is used, the bandwidth and the spacing between laser output lines will change. Fig. 3 shows the laser output spectrum at different output couplers. As shown in the figure, the FWM lines of spacings of 1.15, 2.15, and 0.57 nm are dominant with the use of 80/20, 90/10, and 95/5 couplers, respectively. The optimum coupling ratio with the highest signal to noise ratio is obtained with the 90/10 coupler. This is attributed to the oscillating laser power in the laser cavity, which has been optimised so that the product of gPL is largest in the cavity. The use of 90/10 coupler
ARTICLE IN PRESS 1252
R. Parvizi et al. / Optics & Laser Technology 42 (2010) 1250–1252
0 1
–10.0 2
3
4
5
–30.0
6 7
8
9 10 11 12 13 14 15
–50.0
Output power(dBm)
–10.0
–70.0 1555.00nm
1562.50nm
1570.00nm
Wavelength (nm) Fig. 4. Spectral evolutions of the proposed FWM-based multi-wavelength EDFL against time. (The laser spectra scanned over time.)
allows 90% of the light to oscillate in the cavity and thus provide the optimum gain to compensate for the loss in the cavity. The stability of the multi-wavelength operation is of vital importance for practical applications. Fig. 4 shows the spectral evolutions of the proposed EDFL with PCF in terms of time. In the experiment, the output spectral is repeatedly scanned for every 10 min. The multi-wavelength EDFL lases stably with power fluctuations of less than 0.5 dB over 2.5 h. There are two main factors that contribute to the operation instability in the conventional EDFL. They are mode competition in the laser cavity and the drift of the spectral profile as a result of the thermal fluctuation. In the proposed set-up, a 100 m length of PCF is used to reduce the mode competition inside the ring cavity via the nonlinear effect. Therefore, multi-wavelength operation may be self-stabilized by taking the combination of PCF and Bi-EDFA as a good candidate for nonlinear and linear gain media. In addition, temperature variations and mechanical vibrations were small as the experiment was conducted in a laboratory; thus the drift of the spectral profile was minimal. These results show that the proposed configuration is suitable for wavelength conversion application in optical communication networks.
4. Conclusion A multi-wavelength EDFL is experimentally demonstrated at room temperature based on FWM effect in the PCF. At maximum pump power of 1480 nm, 5 lines are stably produced with nearly 2.15 nm spacing between the lines and a signal to noise ratio of more than 20 dB. The phase matching condition for four-wave mixing in the PCF has been enhanced using a seed signal and a suitable state of polarisation in the ring laser cavity. The number
of channels and wavelength spacing can be controlled by varying the output coupler ratio.
References [1] Chen DR, Fu H, Ou H, Qin S. Wavelength-spacing continuously tunable multiwavelength SOA-fiber ring laser based on Mach–Zehnder interferometer. Opt Laser Technol 2008;40(2):278–81. [2] Pan S, Zhao X, Yu W, Lou C. Dispersion-tuned multiwavelength actively mode-locked fiber laser using a hybrid gain medium. Opt Laser Technol 2008;40(6):854–7. [3] Yamashita S, Hotate K. Multiwavelength erbium-doped fiber laser using intracavity etalon and cooled by liquid nitrogen. Electron Lett 1996;32:1298–9. [4] Graydon O, Loh WH, Laming RI, Dong L. Triple-frequency operation of an Erdoped twincore fiber loop laser. IEEE Photon Technol Lett 1996;8:63–5. [5] Harun SW, Emami SD, Abd Rahman F, Muhd-Yassin SZ, Abd-Rahman MK, Ahmad H. Multi-wavelength brillouin/erbium-ytterbium fiber laser. Laser Phys Lett 2007;4(8):601–3. [6] Liu X, Yang X, Lu F, Ng J, Zhou X, Lu C. Stable and uniform dual-wavelength erbium-doped fiber laser based on fiber Bragg gratings and photonic crystal fiber. Opt Express 2005;13:142–7. [7] Shahi S, Harun SW, Shahabuddin NS, Shirazi MR, Ahmad H. Multi-wavelength generation using a bismuth-based EDF and brillouin effect in a linear cavity configuration. Opt Laser Technol 2009;41(2):198–201. [8] Yu J, Jeppesen P. Wavelength conversion by use of four-wave mixing in a novel optical loop configuration. Opt Lett 2000;25:393–5. [9] Liu X, Zhou X, Lu C. Multiple four-wave mixing self-stability in optical fibers. Phys Rev A 2005;72:013811. [10] Liu XM. Theory and experiments for multiple four-wave-mixing processes with multifrequency pumps in optical fibers. Phys Rev A 2008;77:043818. [11] Zhang AL, Liu H, Demokan MS, Tam HY. Stable and broad bandwidth multiwavelength fiber ring laser incorporating a highly nonlinear photonic crystal fiber. IEEE Photon Technol Lett 2005;17:2535–7. [12] Liu X, Zhou X, Tang X, Ng J, Hao J, Chai ’TY, et al. Switchable and tunable multiwavelength erbium-doped fiber laser with fiber Bragg gratings and photonic crystal fiber. IEEE Photon Technol Lett 2005;17(8):1626–8.