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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 12, DECEMBER 2005
Multiwavelength Erbium-Doped Fiber Laser With 0.8-nm Spacing Using Sampled Bragg Grating and Photonic Crystal Fiber Xiufeng Yang, Xinyong Dong, Shumin Zhang, Fuyun Lu, Xiaoqun Zhou, and Chao Lu
Abstract—We propose and demonstrate a novel room-temperature multiwavelength erbium-doped fiber laser (EDFL) with a 0.8-nm wavelength spacing based on four-wave mixing (FWM) in a length of high-nonlinear photonic crystal fiber and sampled-fiber Bragg grating. The FWM processes suppress the wavelength competition of the EDFL and increase the number of lasing wavelength. By adjusting the FWM efficiency, the number of the concurrent lasing wavelengths can be changed, and the peak power differences among the main oscillation wavelengths are less than 2.0 dB. Index Terms—Erbium-doped fiber lasers (EDFLs), four-wave mixing (FWM), multiwavelength fiber lasers.
I. INTRODUCTION
M
ULTIWAVELENGTH erbium-doped fiber lasers (EDFLs) have attracted considerable interest for their potential applications in optical test and measurement, optical wavelength-division-multiplexing communication and sensing systems. The requirements for the multiple wavelength sources include small equal-wavelength spacing, a large number of peaks within broad-band, and high output uniformity over the channels. Meeting all the requirements is a very challenging task. In order to achieve concurrent multiple wavelength operation in EDFLs, the behavior of the homogenous gain broadening (HGB) of the erbium-doped fiber (EDF) at room temperature must be overcome. Various techniques have been proposed to achieve simultaneous lasing at several wavelengths [1]–[9]. The most straightforward method is to cool the EDF in liquid nitrogen (77 K) to reduce the HGB and subsequently promote the multiwavelength operation [1]. Many different approaches of the multiple wavelength EDFLs at room temperature such as polarization or spatial hole burning [2]–[5], independent gain media [6], [7], frequency shift [8], and phase modulator [9] have also been reported. Except for the above-mentioned methods, nonlinear effects in optical fiber have been used to Manuscript received April 21, 2005; revised July 26, 2005. The work of X. Dong was supported by the Singapore Millennium Foundation (SMF) under a postdoctoral scholarship award. X. Yang, X. Zhou, and C. Lu are with the Lightwave Department, Institute for Infocomm Research, Singapore 119613, Singapore (e-mail:
[email protected];
[email protected];
[email protected]). X. Dong is with the Network Technology Research Centre, Nanyang Technological University, Singapore 637553, Singapore (e-mail: exydong@ ntu.edu.sg). S. Zhang and F. Lu are with the Lightwave Department, Institute for Infocomm Research, Singapore 119613, Singapore, and also with the Nakai University, Tianjin 300071, China (e-mail:
[email protected]; lufuyun1 @hotmail.com). Digital Object Identifier 10.1109/LPT.2005.858076
Fig. 1.
Schematic diagram of the multiwavelength EDFL.
generate multiple wavelength lasers. For example, a hybrid Brillouin/erbium fiber laser combining Brillouin gain and EDF gain has been demonstrated, in which the multiple wavelength oscillations at room temperature were achieved although the wavelength spacing determined by the Brillouin shift was too small ( 0.08 nm) [10]. Photonics crystal fibers (PCFs) have in recent years attracted much scientific and technological interest for their obtainable high nonlinear coefficient [11]. Furthermore, the dispersion profiles may be designed to facilitate different nonlinear effects for special applications. In our recent work, a large wavelength spacing of 4.2-nm dual-wavelength EDFL was achieved, where the nonlinear effect of four-wave mixing (FWM) generated in the cavity fiber stabilized the laser powers very well by a dynamic energy transfer process [12]. In this letter, a stable and power uniform multiwavelength laser with 0.8-nm wavelength spacing is achieved through FWM process in a piece of high-nonlinear photonic crystal fiber (HNL-PCF). The polarization controller (PC) and sampledfiber Bragg grating (SFBG) with 0.8-nm wavelength spacing initiated the multiple wavelength operation. The number of lasing wavelength increases through the FWM process and the laser is stabilized by an FWM-induced dynamic gain flattening mechanism. II. EXPERIMENTAL SETUP The schematic diagram of the multiwavelength EDFLs is shown in Fig. 1. In this setup, a commercial erbium-doped fiber amplifier (EDFA) was used, which may provide 13-dBm output saturation power and a maximum 35-dB small signal gain at the conventional wavelength band. A 101-m-long HNL-PCF ) was (manufactured by Crystal Fiber A/S, inserted between the EDFA and the output coupler, where the intracavity laser power is the highest so that high-power nonlinear products can be expected. The HNL-PCF has a zero-dispersion
1041-1135/$20.00 © 2005 IEEE
YANG et al.: MULTIWAVELENGTH EDFL WITH 0.8-nm SPACING USING SAMPLED BRAGG GRATING AND PCF
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Fig. 2. Reflection spectrum of the SFBG.
Fig. 4. Output spectra of dual- (a), triple- (b) and five-wavelength (c) laser operation with 0.8-nm wavelength spacing.
Fig. 3.
Typical output spectra of the multiple wavelength EDFLs.
wavelength at 1570 nm, and a flat and low dispersion ( 0.8 to 0.5 ps/nm/km) over 1520–1610 nm. The nonlinear coefficient and the attenuation at 1550 nm are 11 /W/km and 9 dB/km, respectively. One SFBG, which has 15 reflection peaks within the 3-dB bandwidth, was used in conjunction with an optical circulator that also served as an optical isolator. The reflection spectrum of the SFBG is plotted in Fig. 2. The free spectral range and central wavelength of the SFBG are 0.8 and 1555.16 nm, respectively. An inline PC is used to obtain the balance of the gain and loss among the peak wavelengths of the SFBG and initiate the multiple lasing operations and also adjust the FWM efficiency through the polarization state adjustment and subsequently the multiple wavelengths laser operation can be achieved. An optical spectrum analyzer with the resolution of 0.01 nm was used to do all the measurement. III. PRINCIPAL AND RESULT DISCUSSIONS When there was no HNL-PCF in the cavity, normal lasing action of the EDFLs occurred with one or dual-wavelengths at various peaks of the SFBG depending on the PC control although the lasing dual-wavelength is very unstable because the EDF is a homogenous gain-broadening medium at room temperature. Fig. 3 shows the typical laser spectra with HNL-PCF inside the laser cavity. There are four lasing wavelengths whose power are more than 12 dBm. The wavelength spacing of the laser is 0.8 nm and the power difference between the main oscillation wavelength and others is up to 8 dB. With fine adjustment of the PC setting, three multiwavelength lasers with fair uniformity are obtained where the power difference over the main oscillation wavelengths is less than 2.0 dB, as shown in Fig. 4(a), (b), and (c). The 3-dB bandwidth is measured to be 0.05 nm with an
Fig. 5. Output spectra of triple-wavelength laser operation with various wavelength spacing.
identical wavelength spacing of 0.8 nm. To examine the stability of the laser output power, we scanned the laser output spectrum at the five-wavelength operation state eight times at a time interval of 3 min. The results are shown in Fig. 5. The maximum power fluctuation is less than 1.2 dB, which is relatively stable with narrow wavelength spacing of 0.8 nm. Furthermore, the wavelength spacing, for the cases of dual- and triple-wavelength operations, can be adjusted by adjusting the PC in the cavity from 0.8 nm to its integral multiples, such as 1.6, 2.4, 3.2, and 4.0 nm. Fig. 6 shows the measured spectra for the triple-wavelength operation with different spacing. In all the cases shown in Fig. 6, the peak power difference among the main oscillation wavelengths is less than 2.0 dB also. The HNL-PCF in the laser cavity introduces the partially degenerated FWM and then the energy transfer among the involved waves takes place. In such process, two photons at the pump frequency are annihilated with a simultaneous creation and with relationship of two photons at the frequencies of ; energy of the wave at is, therefore, transand [13]. Here, we assume that the ferred to waves at partially degenerated FWM process happens among any three equally spaced oscillation peaks of the multiwavelength EDFL.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 12, DECEMBER 2005
length is increased. The number of the nearly uniform peaks is limited by the reflection profile of the SFBG and the FWM , which is dependent on the polarization state of efficiency the involved waves [13]. For a high , the transferable power is large, as indicated by (2), so more wavelengths can be involved in the degenreted FWM processes to produce stable mul, the involved peak tiple wavelengths lasing. For a small number will be limited by the low transferable power. That is why different numbers of main oscillation peaks in the laser are observed when the PC setting is adjusted. The switchable wavelength spacing of the multiwavelength EDFL is caused by the polarization-dependent loss and the birefringence-induced wavelength-dependent loss in the laser cavity. IV. CONCLUSION
Fig. 6. Repeated scan output spectra of the five-wavelength laser operation.
The induced power variation for an arbitrary oscillation peak at , therefore, can be described as (1) where denotes the efficiency of the FWM process taking and (corresponding frequencies place among ; and is the laser wavelength and are and and are powers at frequency spacing, respectively), the frequencies and , respectively. For an approximate analysis, we assume the situation of only, i.e., neglecting the energy transfer happened within the . It is reasonable, because the osFWM processes with cillation wavelength number in the practical multiwavelength reduces significantly with EDFL is limited, and the value of , for the the increase of . In addition, we assume value of frequency difference is much smaller than that of frequencies. Based on these assumptions, (1) can be simplified as (2) Equation (2) shows that energy transfer happens when the involved three frequencies have different powers. If , we get , i.e., energy is . On the partly transferred from the wave at to those at , we get , i.e., contrary, if energy is partly transferred from the waves at to that at . Note that indirect energy transfer from a laser wave to an arbitrary nonadjacent wave takes place through a multiple transfer process. A dynamical gain flattening mechanism is, therefore, formed that functions in any round-trip of the laser waves in the cavity, and suppressions to the wavelength competition from the HGB of the EDF. The explanation can be verified by the observation of several nearly uniform multiple wavelength laser peaks, as shown in Figs. 4, 5, and 6. It is believed that these laser waves are deeply involved in a series of degenerated FWM processes so that the powers are nearly uniform, the number of lasing wave-
A novel room-temperature multiwavelength EDFL using the FWM in a length of HNL-PCF to suppress the wavelength competition has been proposed and demonstrated. The multiple wavelength operation was initialed by an SFBG and PC adjustment and stabilized by the FWM-induced dynamic gain flattening mechanism in a length of HNL-PCF. Five-, triple-, and dual-wavelengths operation with 0.8-nm wavelength spacing have been obtained by simply adjusting the intracavity PC. The experiments showed that the FWM process was effective in suppressing the wavelength competition in the laser cavity. REFERENCES [1] D. Wei, T. Li, Y. Zhao, and S. Jian, “Multiwavelength erbium-doped fiber ring lasers with overlap-written fiber Bragg gratings,” Opt. Lett., vol. 25, pp. 1150–1152, 2000. [2] O. Graydon, W. H. Loh, R. I. Laming, and L. Dong, “Triple-frequency operation of an Er-doped twincore fiber loop laser,” IEEE Photon. Technol. Lett., vol. 8, no. 1, pp. 63–65, Jan. 1996. [3] G. Das and J. W. Y. Lit, “L-band multiwavelength fiber laser using an elliptical fiber,” IEEE Photon. Technol. Lett., vol. 14, no. 5, pp. 606–608, May 2002. [4] C.-L. Zhao, X. Yang, C. Lu, J. H. Ng, X. Guo, P. R. Chaudhuri, and X. Dong, “Switchable multi-wavelength erbium-doped fiber lasers by using cascaded fiber Bragg gratings written in high birefringence fiber,” Opt. Commun., vol. 230, pp. 313–317, 2004. [5] A. J. Poustie, N. Finlayson, and P. Harper, “Multiwavelength fiber laser using a spatial mode beating filter,” Opt. Lett., vol. 19, pp. 716–718, 1994. [6] R. Slavik, I. Castonguay, S. LaRochelle, and S. Doucet, “Short multiwavelength fiber laser made of a large-band distributed Fabry–Pérot structure,” IEEE Photon. Technol. Lett., vol. 16, no. 4, pp. 1017–1019, Apr. 2004. [7] Q. Mao and J. W. Y. Lit, “Switchable multiwavelength erbium-doped fiber laser with cascaded fiber grating cavities,” IEEE Photon. Technol. Lett., vol. 14, no. 5, pp. 612–614, May 2002. [8] A. Bellemare, M. Karasek, M. Rochette, S. LaRochelle, and M. Tetu, “Room temperature multifrequency erbium-doped fiber lasers anchored on the ITU frequency grid,” J. Lightw. Technol., vol. 18, no. 6, pp. 825–831, Jun. 2000. [9] K. Zhou, D. Zhou, F. Dong, and N. Q. Ngo, “Room-temperature multiwavelength erbium-doped fiber ring laser employing sinusoidal phasemodulation feedback,” Opt. Lett., vol. 28, pp. 893–895, 2003. [10] D. Y. Stepanov and G. J. Cowle, “Properties of Brillouin/Erbium fiber lasers,” IEEE J. Sel. Topics Quantum Electron., vol. 3, no. 4, pp. 1049–1057, Aug. 1997. [11] A. V. Husakou and J. Herrmann, “Supercontinuum generation, fourwave mixing, and fission of higher-order solitons in photonic-crystal fibers,” J. Opt. Soc. Amer. B., vol. 19, pp. 2171–2182, 2002. [12] X. Liu, X. Yang, F. Lu, J. H. Ng, X. Zhou, and C. Lu, “Stable and uniform dual-wavelength erbium-doped fiber laser based on fiber Bragg gratings and photonic crystal fiber,” Opt. Express, vol. 13, pp. 142–147, 2005. [13] G. P. Agrawal, Nonlinear Fiber Optics, 2 ed. San Diego, CA: Academic, 1995, ch. 10, p. 407.
