Multiwavelength Switchable Erbium-Doped Fiber Laser Employing Fiber
Bragg Gratings
Yan Ju Chiang
*Abstract
A multiwavelength switchable erbium doped fiber laser (EDFL) employing fiber Bragg gratings is proposed in this paper. The spectral profile for the active fiber in the EDFL is specifically tailored to enhance the switching capability between two closely-spaced wavelengths. The experimental results demonstrate that the proposed EDFL has advantages of moderate power with high stability (4
0.04 dBm), wide wavelength switchable range (covering the C-band), high side-mode suppression ratio (>37 dB), and narrow separation between two switchable wavelengths(0.15 nm).Keywords: Erbium-doped fiber laser, fiber Bragg
grating, tunable laser.
I Introduction
Widely tunable, dense wavelength-switchable and narrow linewidth erbium doped fiber lasers (EDFLs) have attracted great interests for two decades because of their widespreading applications in testing performances of DWDM systems, optical fiber sensors and sophisticated fiber-optic instruments. They offer a number of advantages over semiconductor lasers, such as low insertion loss with the optical components in WDM networks, high conversion efficiency, and availability of low-cost. In case of multiwavelength EDFL with one section of Er-doped fiber (EDF) as the
common gain medium, the stable operation in switching between two closely spaced wavelengths is difficultly achievable because strong gain competition effect then occurs on the active fiber.
Traditional methodology for investigating this topic is inserting an optical chopper into the laser cavity and the chopper functions as the loss modulator corresponding to each lasing wavelength [1]. The experiment demonstrates that for such an EDFL the minimal separation between two switchable lasing wavelengths is 0.35 nm. Other investigations include exploiting pump-controlled bistability effect in the L-band EDFL via inserting a homogeneously saturable absorber into overlapping linear cavities [2], using high birefringence fiber Bragg gratings (FBGs) [3], and applying thermal heating on polarization dependent few-mode FBGs [4].
In this paper, we propose a new structure of multiwavelength switchable EDFL with a section of active fiber as the common gain medium. In addition, we propose a novel idea that a tilt gain profile in the C-band realized by specifically designing the length of active fiber enables the stable operation in switching between two closely spaced lasing wavelengths.
II Principle of Operation
Fig. 1(a) shows the configuration of the proposed multiwavelength switchable EDFL. It consists of overlapping Fabry-Perot (FP) cavities with one section
of Er-doped fiber as the gain medium. Each FP cavity Ci (i=1, 2,…6) is structured as a common Er-doped fiber amplifier (EDFA) between FBGi and a highly reflective broadband loop mirror M1. A 1: 9 fiber coupler FC is outputting 9/10 lasing power circulating along M1. The EDFA is pumped via a laser source at 980 nm with a fixed power of 84 mW. As shown in Fig.1 (a) and (b), FBGi (i=1, 2,…6) is spliced into EDFA in order of magnitude with respect to round-trip gain derived by its corresponding traveling beam passing through EDFA.
Derived gain differences between traveling beams at
i (i=1, 2,…6) in each round-trip propagation around Ci will in steady-state lead to single wavelength lasing at 6 in case that we do not intentionally bendthe fiber line between any specific pair of FBGs. On the other hand, bending the fiber line at A, B, C, D or E will correspond to the operation of lasing wavelength switched from6 to1,2, 3,4or5, respectively.
Conversely releasing these bent fiber lines will result in the lasing wavelength switched in the reverse order.
III Experimental Results
In the experimental configuration for our proposed EDFL, Six C-band FBGs with Bragg wavelengths at 1532.69, 1539.51, 1548.06, 1550.52, 1550.98, 1559.89 nm, are chosen, corresponding to FBG1 to FBG6 in Fig. 1(a). The tilt gain profile mentioned in Fig. 1(b) may be derived by adjusting erbium-doped fiber in laser cavity in EDFL. As shown in Fig. 2, the tilt ASE profile (i.e., small signal gain spectra with a tilt slope) covering the C-band is derived while erbium-doped fiber length reaches 6-m. The piecewise slopes in the C-band are estimated to be 2 dB/nm ranging from 1530 to 1535 nm, 0.5 dB/nm ranging from 1535 to 1540 nm , and 0.88 dB/nm from 1540 to 1560 nm. While we keep loose fiber line at A to E in Fig. 1(a), EDFL is lasing at
1559.89 nm, with respect to 3.93 dBm output lasing power,
0.04 dBm output power instability and 45 dB side-mode suppression ratio. These results illustrate that our EDFL has stable and moderate power, which may be compatible with the performance of commercially available laser sources in the modern WDM systems [5].To measure minimal separation between two switchable lasing wavelengths, we operate the EDFL to switch continuously between Bragg wavelengths of FBG4 and FBG5. In the meantime FBG4 was thermally heated and FBG5 is kept at a fixed temperature (i.e. Bragg wavelength of FBG5 is fixed to be 1550.98 nm). The overlapping spectra in Fig. 3(a) (measured by using Anritsu optical spectrum analyzer MS9710 C with 0.08 nm detectable resolution for two close-separated wavelengths) shows that when Bragg wavelength at FBG 4 is thermally red-shift to 1550.83 nm, lasing still occurs and lasing wavelengths is switchable between 1550.83 nm and 1550.98 nm. Such a close separation (0.15 nm) for two switchable lasing wavelengths satisfies the requirements for channel separation for DWDM systems (i.e., 0.8 nm to 0.4 nm) [5] and thus it implies that proposed laser is a candidate to become a source operated in wavelength-switching mode in DWDM optical communication system. Unstable and simultaneous dual-wavelength lasing is observed at the output of EDFL while Bragg wavelength at FBG 4 is thermally red-shift to longer than 1550.83 nm. It is due to the splicing loss on the fiber line between FBG4 and FBG5 compensates the tilt gain over a substantially tiny range for lasing wavelengths between 1550.98 nm and <1550.83 nm.
In order to test the lasing performance of our EDFL operating in wide wavelength switchable range, we bend the fiber line at A to E in Fig. 1(a) to observe the stability of the output power as well as the side-mode
suppression ratio for each lasing wavelength. Fig. 3(b) from L1 to L6 represent the measured lasing profile from Bragg wavelength of FBG1 to FBG6, respectively. 3.38 dBm for minimum lasing power at 1539.51 nm and 4.15 dBm for maximum at 1550.98 nm with power instability of
0.04 dBm are derived. The worst case of side-mode suppression ratios is derived among the lasing profiles in Fig. 3(b) to be ~37 dB at 1539.51 nm.IV Conclusion
The proposed multiwavelength switchable erbium doped fiber laser employing fiber Bragg gratings has been experimentally demonstrated to have a dense (0.15 nm separation between two closely lasing wavelengths) wavelength-switchable capability and a wide wavelength-switchable range (ranging from 1532.69 nm to 1559.89 nm). Moderate lasing power with high stability (~4
0.04 dBm) for each lasing wavelength has also been observed as well as the side-modesuppression ratio of over 37 dB.
References
[1] Y. Z. Xu, H. Y. Tam, W. C. Du, and M. S. Demokan, “Tunable dual-wavelength switching fiber grating laser,” IEEE Photon. Technol. Lett.,
10, pp. 334-336 (1998).
[2] Q. Mao, and J. W. Y. Lit, “Multiwavelength erbium-doped fiber lasers with active overlapping linear cavities,” J. Lightw. Technol., 21, pp. 160-169 (2003).
[3] C. Zhao, X. Yang, C. Lu, J. H. Ng, X. Guo, J. H. Ng, X. Guo, R. C. Partha, and X. Dong, “Switchable multiwavelength erbium-doped fiber lasers by using cascaded fiber Bragg gratings written in high birefringence fiber,” Opt. Commun., 230, pp. 313-317 (2004).
[4] D. S. Moon, U. C. Paek, Y. Chung, X. Dong, and P. Shum, “Multi-wavelength linear-cavity tunable
fiber laser using a chirped fiber Bragg grating and a few-mode fiber Bragg grating,” Opt. Exp.,
13, pp. 5614-5620 (2005).
[5]Biswanath Mukherjee, “WDM optical communication networks: progress and Challenges,“ IEEE J. on Selected Areas in Commun., 18, (2000).
Fig. 1(a) Multiwavelength switchable EDFL. FC: fiber coupler. EDFA: Er-doped fiber amplifier.
Fig. 2 ASE spectrum exhibited from EDFA
Fig. 3 (a) The lasing exhibited from the EDFL is switched between 1550.83 nm and 1550.98 nm. (b) The EDFL operates in multiwavelength switching mode. The spectra from L1 to L6 denote lasing wavelength is switched from Bragg wavelength of FBG 1 to FBG 6.
多波長可調式摻鉺光纖雷射
蔣彥儒
*摘要
我們提出一個多波長且波長可切換的 C-band 摻鉺光纖雷射。其雷射共振腔是以布拉格式光纖光柵以及 Circulator Loop Mirror 所構成。藉由串接多個不同 C-band 波長的布拉格式光纖光柵在單一增益介質中，且調整 共振腔中損耗可達成多波長切換功能。實驗證明此種雷射具有中等功率、高穩定度(4 ± 0.04 dBm)，全 C-band 的波長可調範圍，以及可作為 DWDM 高密度光纖通訊系統之光源的切換波長精確度(0.15 nm)
關鍵字：摻鉺光纖，布拉格式光纖光柵，光纖雷射
