Broadband C- plus L-band double-ring fiber baser based on a two-stage hybrid amplifier

(1)

Broadband C- plus L-band double-ring fiber laser

based on a two-stage hybrid amplifier

Chien-Hung Yeh

National Chiao Tung University Department of Photonics and Institute

of Electro-Optical Engineering Hsinchu 300, Taiwan

and

Industrial Technology Research Institute Computer & Communications Research

Laboratories

Transmission System Department Chutung, Hsinchu 300, Taiwan E-mail: [email protected]

Bing-Chung Cheng

and

Fu-Jen Catholic University Department of Physics Taipei 242, Taiwan

Chih-Yang Chen

Sien Chi

and

Yuan Ze University

Institute of Electrical Engineering Chung-Li 320, Taiwan

Abstract. A stable and wavelength-tunable C- plus L-band fiber double-ring laser, which uses a two-stage hybrid amplifier with a semiconductor optical amplifier and an erbium-doped fiber amplifier, has been proposed and experimentally demonstrated. Based on the double-ring configura-tion, the proposed fiber ring laser exhibits more stable output wave-lengths and powers than the single-ring laser. A wide tunable range of 1540 to 1620 nm, a side-mode suppression ratio 共SMSR兲 of ⬎31.2 dB/0.05 nm over a wide tuning range from 1550 to 1612 nm, and an output power of ⬎2 dBm over the operation range of 1546 to 1608 nm have been achieved. © 2005 Society of Photo-Optical Instru-mentation Engineers.关DOI: 10.1117/1.2083307兴

Subject terms: erbium-doped fiber_{共EDF兲; fiber ring laser; L band; double ring}. Paper 040848R received Nov. 11, 2004; revised manuscript received Mar. 16, 2005; accepted for publication Mar. 21, 2005; published online Oct. 14, 2005.

1 Introduction

Broadband tunable fiber lasers are the major optical devices in optical communication systems. Stable output frequency and power of optical fiber ring lasers are essential for wave-length division multiplexing共WDM兲 networks and sensor systems. In general, a fiber Fabry-Perot共FFP兲 filter inside the ring cavity of the fiber ring lasers can be used to pro-vide wavelength selection. However, that is not enough to provide stable output wavelength and power of a fiber ring laser. Recently, several techniques have been studied, such as integrating two cascaded FFP filters of different wide free spectral ranges 共FSRs兲 into a cavity, to provide full tunability and single-longitudinal-mode共SLM兲 selection,1,2

using a compound ring resonator composed of a dual-coupler fiber ring共DCFR兲 or passive multiple-ring cavity to guarantee SLM laser oscillation.3,4 Due to the bandwidth limitation of erbium-doped fiber 共EDFs兲, EDF ring lasers can only be operated at S 共1480 to 1530 nm兲, C 共1530 to 1560 nm兲, or L band 共1560 to 1610 nm兲; see Refs. 3, 5, and 6, respectively.

In this paper, we propose and experimentally investigate a widely stable and wavelength-tunable C- plus L-band fi-ber double-ring laser using a two-stage hybrid amplifier module, which is composed of a semiconductor optical am-plifier共SOA兲 and an erbium-doped fiber amplifier 共EDFA兲. The behavior of the output power, wavelength stability, tun-ing range, and side-mode suppression ratio 共SMSR兲 has also been experimentally studied.

Optical Engineering 44共10兲, 104201 共October 2005兲

Optical Engineering 104201-1 October 2005/Vol. 44共10兲

(2)

2 Experiments

Figure 1 shows the experimental setup for the widely stable and wavelength-tunable C- plus L-band fiber double-ring laser. This proposed apparatus consists of two 2⫻2 and 50:50 optical couplers共C₁and C₂兲, a polarization controller 共PC兲, two FFP filters, and a two-stage hybrid amplifier module. In the hybrid amplifier module, the first 共SOA兲 stage has 250-mA bias current, and the second 共EDFA兲 stage is composed of a high-concentration 18-m-long EDF 共High Wave 742兲, a 1550 to 980-nm WDM coupler 共W兲, two optical isolators, and a 980-nm pump laser, as shown in Fig. 1. According to the proposed structure, we only em-ploy an SOA and a shorter EDF length to achieve a wide operation range of 1540 to 1620 nm. Actually, the optical output of the SOA can be used to pump the second stage 共EDFA module兲 for extending the bandwidth from C to L band. The two FFP filters are all-fiber devices having a wide tuning range, a low insertion loss of ⬍0.5 dB, and polarization-dependent loss 共PDL兲 of ⬃0.1 dB. Two FFP filters having a free spectral range共FSR兲 of 80 nm and a finesse of 200 can provide wavelength selection in the ring laser cavity by applying an external voltage of 0 to 12 V to the piezoelectric transducer 共PZT兲 of the two FFP filters. Due to the remaining PDL of the passive components 共op-tical filters, isolators, etc.兲 and the polarization-dependent gain 共PDG兲 in the EDF, proper adjustment of the PC is necessary. In addition, an optical spectrum analyzer共OSA兲 and a power meter 共PM兲 are used to measure the output spectra and powers for this proposed fiber ring laser at point “a” in Fig. 1.

3 Results and Discussion

The double-ring configuration can serve as a mode filter so that only the particular resonant mode that coincides with the central frequencies of the two filters can oscillate. The double-ring configuration can be viewed as the combina-tion of the main and subsidiary ring cavities, which have free spectral ranges 共FSRs兲 of FSRm and FSRs,

respec-tively. Owing to the vernier effect of the double-ring

cavity,4 the value of the effective FSR becomes the least common multiple of FSRm and FSRs, and mode

suppres-sion can be achieved. Besides, the two lasing spectra from FFP filters 1 and 2 are nearly overlapped to provide further restriction on possible laser modes. As a result, only the mode fsis selected for oscillation and mode stability can be

guaranteed. Thus, the two beams from the two ring cavities can interfere mutually to produce a stabilized single-frequency output when the PC is properly controlled.

Figure 2 shows the amplified spontaneous emission 共ASE兲 spectra of the first 共SOA兲 stage, the second 共EDFA兲 stage, and the hybrid amplifier when the EDFA and SOA were operated at 100-mW pump power and 250-mA bias current, respectively. In Fig. 2, the maximum peak power levels of ASE for the EDFA and SOA are −20.1 and −33.2 dBm at near 1559 and 1561 nm, respectively. In ad-dition, when a hybrid amplifier consisting of an SOA and an EDFA is used, the medium gain will be enhanced at the longer wavelength. Therefore, an 80-nm ASE bandwidth of 1540 to 1620 nm can be achieved, and the −18.7-dBm peak power level occurs at 1566 nm, as shown in Fig. 2.

Figure 3 shows the optical spectra of the proposed fiber double-ring laser over the operating region of

Fig. 1 Experimental setup of the proposed fiber double-ring laser.

Fig. 2 Optical spectra of the ASE for the hybrid amplifier, the first

共SOA兲 stage, and the second 共EDFA兲 stage.

Fig. 3 The output wavelength spectra of the proposed double-ring

laser over the wavelength range of 1540 to 1620 nm. Yeh et al.: Broadband C- plus L-band double-ring fiber laser…

(3)

1540 to 1620 nm when voltages of 0 to 12 V are applied to the PZTs of the two FFP filters. Figure 4 shows the output power and SMSR versus the tuning wavelength for this double-ring laser over the bandwidth from 1540 to 1620 nm. As seen in Fig. 4, the maximum output power of 5.1 dBm is obtained at around 1570 nm, and the output power drops to 3.8 and 1.1 dBm at 1596 and 1614 nm, respectively. The output power level can be kept above 2.0 dBm over the tuning range of 1546 to 1608 nm. Owing to the ASE compression and gain competition, the maximum SMSR value can be up to 48.3 dB at 0.05 nm near 1572 nm. The SMSR can be kept larger than 31.2 dB at 0.05 nm over a tuning range of 62 nm from 1550 to 1612 nm.

The threshold current of the SOA is 50 mA in the pro-posed configuration. The measured slop efficiencies are 2.13% and 1.25% for the double-ring and the traditional single-ring cavity.7 When the pumping current of an SOA is above about 200 mA, then the output power will be saturated.

To investigate the behavior of the output power and the wavelength stability, the short-term stability of the pro-posed configuration共in Fig. 1兲 was measured and compared with the traditional architecture,7 as shown in Fig. 5. The lasing wavelength is 1570.1 nm initially, and the observa-tion time is more than 900 s. In Fig. 5, the output power fluctuations for the proposed 共double-ring兲 and traditional 共single-ring兲 configuration are 0.02 and 0.42 dB, respec-tively. Figure 5 also shows that the wavelength variations of two configurations are 0 and 0.1 nm 共readout resolution= 0.01 nm兲, respectively. During 4-h observation, the stable output of the proposed double-ring laser is still maintained. Therefore, compared with the traditional fiber single-ring laser, this proposed laser has better stability.

4 Conclusion

In conclusion, we have proposed and experimentally dem-onstrated a stable and wavelength-tunable C- plus L-band fiber double-ring laser, which uses a hybrid amplifier with an SOA and an EDFA. Because of the double-ring configu-ration, the proposed fiber ring laser exhibits stability of output wavelength and power over a broader band than does the single-ring laser. We have achieved a wide tuning

range of 1540 to 1620 nm, a maximum SMSR of 48.3 dB at 0.05 nm near 1572 nm, an SMSR of 31.2 dB at 0.05 nm over a tuning range of 62 nm 共1550 to 1612 nm兲, and an output power of ⬎2 dBm over the operation range of 1546 to 1608 nm.

Acknowledgments

This work was supported in part by the National Science Council 共NSC兲 of Taiwan under grants NSC 94-2215-E– 155-001, NSC 94-2215-E–155-003, NSC 93-2215-E-115-004, and NSC 93-2215-E-115-005.

References

1. K. J. Vahala, P. Namkyoo, J. Dawson, and S. Sanders, “Tunable, single-frequency, erbium fiber ring lasers,” in IEEE LEOS ’93 Conf. Proc., pp. 708–709共1993兲.

2. K. K. Chow, C. Shu, M. W. K. Mak, and H. K. Tsang, “Widely tunable wavelength converter using a double-ring fiber laser with a semiconductor optical amplifier,” IEEE Photonics Technol. Lett. 14, 1445–1447共Oct. 2002兲.

3. R. M. Sova, K. Chang-Seok, J. U. Kang, and J. B. Khurgin, “Tunable dual-␭ fiber ring laser based on 2nd order Sagnac-Lyot fiber filter,” in IEEE CLEO 2002 Tech. Dig., Vol. 1, pp. 444–445共2002兲. 4. C. C. Lee, Y. K. Chen, and S. K. Liaw, “Single-longitudinal-mode

fiber laser with a passive multiple-ring cavity and its application for video transmission,” Opt. Lett. 23共5兲, 358–360 共1998兲.

5. S. Yamashita and M. Nishihara, “Widely tunable erbium-doped fiber ring laser covering both C-band and L-band,” IEEE J. Sel. Top. Quantum Electron. 7共1兲, 41–43 共2001兲.

6. C. H. Yeh, C. C. Lee, and S. Chi, “A tunable S-band erbium-doped fiber ring laser,” IEEE Photonics Technol. Lett. 15共8兲, 1053–1054 共2003兲.

7. C. H. Yeh, C. C. Lee, C. Y. Chen, and S. Chi, “A tunable C-plus-L band fiber ring laser based on hybrid amplifier,” Jpn. J. Appl. Phys., Part 1 43共2兲, 650–651 共2004兲.

Chien-Hung Yeh received his BS and MS degrees from the

Depart-ment of Physics, Fu Jen Catholic University, Taiwan, in 1998 and 2000, respectively. He received his PhD degree from the Institute Electro-Optical Engineering, National Chiao Tung University, Tai-wan, in 2004. His current research interests include optical fiber laser technologies, optical switching, EDFAs, the applications of op-tical amplifiers for WDM transmission, opop-tical monitoring technolo-gies for fiber communications, and Ethernet passive optical net-works. He is now working in the Transmission System Department, Computer & Communications Research Laboratories, Industrial Technology Research Institute, in Taiwan.

Fig. 4 The output power and SMSR versus the tuning wavelength

for the proposed ring laser over the tuning range from 1540 to 1620 nm.

Fig. 5 The output power fluctuation and the wavelength variation of

the proposed 共double-ring兲 and traditional 共single-ring兲 configura-tions when the wavelength is set at 1570.1 nm.

Yeh et al.: Broadband C- plus L-band double-ring fiber laser…

(4)

Bing-Chung Cheng received his MS degree from the Physics

De-partment, Fu Jen Catholic University, Taiwan, in 1994. He is cur-rently a PhD student at the Department of Photonics and the Insti-tute of Electro-Optical Engineering, National Chiao Tung University, Taiwan. His research interests are optical fiber communications and optical solitons.

Chih-Yang Chen received his MS degree from the Department of

Photonics and the Institute of Electro-Optical Engineering, National Chiao Tung University, Taiwan, in 2004. His research interests are optical fiber communications.

Sien Chi received his BSEE degree from the National Taiwan

Uni-versity, and his MSEE degree from the National Chiao Tung Univer-sity, Taiwan, in 1959 and 1961, respectively. He received his PhD in electrophysics from the Polytechnic Institute of Brooklyn, New York, in 1971, and he then joined the faculty of National Chiao Tung Uni-versity, where he was a professor of electro-optical engineering from 1972 to 2004 and the vice-president of the university from 1999 to 2002. He is a fellow of the Optical Society of America and the Pho-tonics Society of Chinese-Americans.

Yeh et al.: Broadband C- plus L-band double-ring fiber laser…