Multiwavelength erbium-doped fiber ring laser employing Fabry-Perot etalon inside cavity operating in room temperature

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Multiwavelength erbium-doped ﬁber ring laser employing Fabry–Perot etalon

inside cavity operating in room temperature

C.H. Yeh

a,*

, C.W. Chow

b

, Y.F. Wu

c

, F.Y. Shih

b

, C.H. Wang

b

, S. Chi

b,c a

Information and Communications Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan

b_{Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University Hsinchu 30010, Taiwan} c_{Department of Electro-Optical Engineering, Yuan Ze University, Chungli, Taoyuan 32003, Taiwan}

a r t i c l e

i n f o

Article history:

Received 19 December 2008 Revised 27 February 2009 Available online 28 April 2009 Keywords:

Multiwavelength Erbium-doped ﬁber Fabry–Perot

a b s t r a c t

In this investigation, we propose and demonstrate a simple and cost-effective erbium-doped ﬁber (EDF) ring laser using a Fabry–Perot etalon inside a linear cavity and employing the accurate ﬁber cavity length to satisfy the least common multiple number for generating multiwavelength in C-band at room temper-ature. Furthermore, the center wavelength of the lasing wavelength bands can be adjusted to 1541.02, 1551.32, and 1562.03 nm, respectively. The wavelength separation in each wavelength band is 0.34 nm. Moreover, the output stability of the multiwavelength laser has also been discussed and analyzed.

1. Introduction

Recently, tunable and stable multiwavelength erbium-doped fiber (EDF) laser source has considerable interest due to its impor-tant applications in wavelength-division-multiplexed (WDM) communication system, optical device testing, fiber sensing, and precise spectroscopy. For the multiwavelength EDF lasers, because of the homogeneous broadening effect of erbium at room temper-ature, gain competition among different lasing modes is the main issue for stable multiwavelength operation. The EDF could be cooled by liquid nitrogen at 77 K to reduce the homogeneous behavior[1,2]. However, this method is not practical due to its high cost and poor system durability. In order to realize the multiwave-length fiber lasers, various techniques have been proposed, such as the ring cavity with a semiconductor modulator[3], with a spectral polarization-dependent loss element[4], with cascaded fiber Bragg grating (FBG)[5–7], with multi-ring cavity design[8], and with a Sagnac loop reflector [9]. Most of these techniques employ the FBG, which is used to select lasing single wavelength or multi-wavelength. Furthermore, the room-temperature multiwavelength EDF ring laser using phase-modulation method with Sagnac loop in linear cavity has been also investigated[10]. This approach is par-ticularly interesting but needs to adjust the phase modulator and polarization status in optimal operating condition. Besides, using an acousto-optical frequency shifter to decrease homogeneous line broadening at room temperature is also proposed[11,12].

In this paper, we propose and demonstrate a simple EDF laser scheme using a Fabry–Perot etalon inside linear cavity with optimal fiber length. When the free spectral ranges (FSRs) of Fabry–Perot etalon and fiber cavity length satisfy the least com-mon multiple number, then the proposed fiber laser could generate multiwavelength in C-band window. Furthermore, based on proper control of the polarization state in the proposed laser, three lasing wavelength bands of center wavelengths located in 1541.02, 1551.30, and 1562.03 nm, respectively, are observed. The wave-length separation in each wavewave-length band is 0.34 nm. Moreover, the output stability of the multiwavelength laser has also been discussed and analyzed.

2. Experiments and results

Fig. 1 illustrates the experimental setup of multiwavelength EDF ring laser. The proposed laser scheme is consisted of an er-bium-doped fiber amplifier (EDFA), a 1 2 and 10/90 optical cou-pler (CP), a polarization control (PC), and a Fabry–Perot etalon. The PC is used to control and adjust the polarization status and main-tain the maximum output power. The EDFA used is gain-flattened. Hence,Fig. 2 shows the output amplified spontaneous emission (ASE) spectrum and gain profile of the EDFA without the ring struc-ture at different wavelengths when the 980 nm pumping power operates at 215 mW. The power difference of ASE spectrum is nearly 5 dB between 1528 and 1562 nm as also shown inFig. 2. Moreover, the total output power of ASE is measured at 3.6 dBm. Fig. 2 also presents the flattened gain spectrum and the maximum gain is 23.5 dB at 1528 nm for the 20 dBm input

*Corresponding author.

E-mail addresses:[email protected],[email protected](C.H. Yeh).

Optical Fiber Technology 15 (2009) 344–347

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Optical Fiber Technology

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signal power. The gain value is between 21.2 and 23.5 dB with 2.3 dB gain variation at the wavelengths of 1526 to 1564 nm. And the saturated output power of the EDFA is 15 dBm. Further-more, the FSR1 (FSR1 = c/2d, where c is light speed in vacuum and d is cavity length between two reflected plates) of Fabry–Perot eta-lon is 0.34 nm (42.5 GHz). Thus,Fig. 3shows the output spectrum when the gain-flattened ASE light source has passed through the Fabry–Perot etalon filter. It is also worth to note that the output power is very low ( 60 dBm) in this case.

In this experiment, the total fiber cavity length (L) of the pro-posed laser scheme is nearly 24 m long. Hence, the FSR2 of the la-ser, FSR2 = c/nL, where c is the speed of light in vacuum, n is the average refractive index of the single-mode fiber of 1.468 and L is the total cavity length, is nearly 8.5 MHz. Therefore, when the value of effective FSRs of etalon and fiber ring cavity become the least common multiple number for both FSR1 and FSR2, the multi-mode laser could be oscillated that satisfies the resonant conditions of the etalon cavity length and the fiber ring cavity simultaneously.

Compared with the past studies, our proposed EDF laser does not need to cool the EDF at 77 K[1,2]or use the ﬁber frequency shifter[11,12]inside ﬁber ring cavity to reduce the homogeneous broadening effect and maintain the multiwavelength lasing. In the

proposed laser, while the FSR1 and FSR2 satisfy the least common multiple number and the proper polarization control is adjusted, the multiwavelength can be retrieved easily. Therefore, Fig. 4

shows the output spectra of the proposed multiwavelength EDF ring laser under different and proper polarization status in the effective operating range of 1520–1570 nm. When the PC is ad-justed in a proper position, there are three multiwavelength lasing bands observed as shown inFig. 4. However, when a cavity length is changed to 20 m, then the proposed ﬁber laser scheme could not generate multiwavelength lasing and also has unstable output wavelength due to the homogeneous broadening and gain compe-tition, as shown inFig. 5.

When the operating conditions for the proposed laser are achieved, three lasing wavelength bands are obtained at center wavelength of 1541.02, 1551.32, and 1562.03 nm, respectively, having 0.34 nm mode spacing, as shown inFig. 6(a)–(c). As illus-trated inFig. 6(a)–(c), with the lasing wavelength increases, the

Fig. 1. Experimental setup of the proposed multiwavelength EDF ring laser architecture.

Fig. 2. Output ASE spectrum and gain proﬁle of the commercially gain-ﬂattened EDFA used under different wavelength.

Fig. 3. Received output spectrum when the ASE source of EDFA injects into the Fabry–Perot etalon ﬁlter. The insert is the FPEF output spectrum between 1542 and 1546 nm.

Fig. 4. Output spectra of the proposed multiwavelength EDF ring laser with 24 m ﬁber cavity length in the effective operating range of 1530–1570 nm under different polarization status.

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lasing multiwavelength distribution would change from ﬂat to sharp gradually.Fig. 6(a)–(c) shows that the maximum peak pow-ers and optical signal to noise ratios (OSNRs) are 1.9, 1.3, and 0.3 dBm and 48.2, 49.0. and 48.4 dB/0.1 nm at the wavelength of 1541.02, 1551.32, and 1562.03 nm, respectively. We can see that the output power is greatly enhanced when compared with the non-lasing case (Fig. 3). Besides,Fig. 6(a)–(c) also presents the las-ing multiwavelength of 29-line, 22-line, and 18-line while the OSNR is larger than 30 dB between 1536.58 and 1545.50 nm (Dk= 8.92 nm), 1548.42 and 1555.18 nm (Dk= 6.76 nm), and 1559.09 and 1564.65 nm (Dk= 5.56 nm), respectively. And the perk powers are larger than 26.6, 26.9, and 26.0 dBm, respec-tively, in the same wavelength bandwidth as mentioned.

In order to realize and investigate the performance of output stability, the short-term stability measurement of the proposed la-ser is performed, as shown inFig. 7. The maximum output peak

power and output wavelength are 0.3 dBm and 1562.03 nm ini-tially and the observing time is over 60 min for the stability obser-vation. We observed that the output wavelength variations and the output power fluctuations of the lasing central lightwave are smal-ler than 0.7 nm and 1.1 dB, respectively, as also shown inFig. 7. During 4 h measurement and observation, the stable output of the proposed fiber laser is still maintained. Besides,Fig. 8shows the output spectra under 60 min observation time. And the output profiles are nearly the same. The power and wavelength fluctua-tion could be due to the optical feedback produced by the back-reflection of fiber connectors of the Fabry–Perot etalon and the pump instability of the EDFA[13]. As a result, the proposed fiber laser not only can generate multiwavelength output, but also has good output stability in a long-term observing time. In addition, the proposed laser has the advantages of simply architecture and cost-effective for stable multiwavelength operation.

3. Conclusion

We have proposed and experimentally investigated a simple and cost-effective EDF ring laser conﬁguration using a Fabry–Perot

Fig. 5. Output spectrum of the proposed multiwavelength EDF ring laser scheme with 20 m ﬁber cavity length.

Fig. 6. Three lasing central wavelengths are obtained at (a) 1541.02, (b) 1551.30, and (c) 1562.03 nm, respectively, having nearly 10 nm multiwavelength bandwidth with 0.34 nm mode spacing.

Fig. 7. Observing short-term stability of the proposed laser when the maximum output peak power and output wavelength are 0.3 dBm and 1562.03 nm initially and the observing time is over 60 min.

Fig. 8. Observing output spectra of the proposed multiwavelength ﬁber laser at 60 min observation time when the central wavelength locates at 1562.03 nm. 346 C.H. Yeh et al. / Optical Fiber Technology 15 (2009) 344–347

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etalon with fixed cavity length [FSR is 0.34 nm (42.5 GHz)] inside a linear cavity and employing a accurate fiber cavity length to sat-isfy the least common multiple number, for generating multiwave-length in C-band at room temperature. Furthermore, based on properly control of polarization status in the fiber laser, the lasing wavelength bands located at 1541.02, 1551.32, and 1562.03 nm, respectively, are observed, having the total wavelength bandwidth of 8.92, 6.76, and 5.56 nm in each band and 0.34 nm mode spacing. The OSNR is above 30 dB/0.1 nm in each band. Moreover, the out-put stability of the multiwavelength laser has also been discussed and analyzed.

Acknowledgments

This work was supported in part by the National Science Coun-cil of ROC (Taiwan) under Contract NSC-96-2221-E-155-038-MY2-1, NSC-96-2221-E-155-039-MY3-NSC-96-2221-E-155-038-MY2-1, NSC 96-2218-E-009-025-MY2, and NSC 97-2221-E-009-038-MY3.

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C.H. Yeh received his Ph.D. degree from the Institute of Electro-Optical Engineering, National Chiao Tung Uni-versity, Taiwan in 2004. In 2004, he joined the Infor-mation and Communications Laboratories (ICL), Industrial Technology Research Institute (ITRI) in Tai-wan, as a Researcher. In 2008, he was promoted as a Senior Researcher in ICL/ITRI. His research interests are optical fiber communication, fiber laser and amplifier, mm-wave generator, and wireless/wire access network technology.

C.W. Chow received the B.Eng. (First-Class Hons) and Ph.D. degrees both from the Department of Electronic Engineering, the Chinese University of Hong Kong in 2001 and 2004, respectively. After graduation, he was appointed as a Postdoctoral Fellow at the CUHK, work-ing on hybrid integration of photonic components and silicon waveguides. Between 2005–2007, he was a Postdoctoral Research Scientist, working mainly on two European Union Projects: PIEMAN (Photonic Integrated Extended Metro and Access Network) and TRIUMPH (Transparent Ring Interconnection Using Multi-wave-length Photonic switches) in the Tyndall National Institute and Department of Physics, University College Cork in Ireland. In 2007, he joined the Department of Photonics, National Chiao Tung University in Taiwan, as an Assistant Professor.

Y.F. Wu is currently pursuing his M.S. degree in the Institute of Electro-Optical Engineering, Yuan Ze Uni-versity, Taiwan. His research interests are optical ﬁber communications and ﬁber access network technologies.

F.Y. Shih is currently pursuing his Ph.D. degree in the Institute of Electro-Optical Engineering, National Chiao Tung University, Taiwan. His research interests are optical fiber communications, fiber lasers, fiber ampli-fiers, WDM transmissions, and fiber access network technologies.

C.H. Wang is currently pursuing his Ph.D. degree in the Institute of Electro-Optical Engineering, National Chiao Tung University, Taiwan. His research interests are optical fiber communications, fiber lasers, fiber ampli-fiers, WDM transmissions, and fiber access network technologies.

S. Chi received his Ph.D. in electrophysics from the Polytechnic Institute of Brooklyn, New York, in 1971, and joined the faculty of National Chiao Tung University, where he is currently a professor of Electro-Optical Engineering. From 1993 to 1996 he received the Distin-guished Research Award sponsored by the National Science Council, Taiwan. Since 1996 he has been the chair professor of the Foundation for Advancement of Outstanding Scholarship. His research interests are optical fiber communications, optical solitons, and opti-cal fiber amplifiers. He is a fellow of the Optiopti-cal Society of America and the Photonics Society of Chinese-Americans. C.H. Yeh et al. / Optical Fiber Technology 15 (2009) 344–347 347