IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 21, NOVEMBER 1, 2009
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Multiwavelength Fiber Optical Parametric Oscillator Zhengqian Luo, Wen-De Zhong, Senior Member, IEEE, Zhiping Cai, Chenchun Ye, Huiying Xu, Xinyong Dong, and Li Xia
Abstract—We demonstrate, for the first time to the best of our knowledge, a multiwavelength fiber ring laser using a continuouswave dual-pump fiber optical parametric amplifier (FOPA) as gain medium. In the ring cavity, two high-power pumps are injected into a spool of 1-km highly nonlinear dispersion-shifted fiber to generate the parametric gain, and a superimposed chirped fiber Bragg grating is used as the comb-like filter. Benefiting from the high and flat gain of the dual-pump FOPA, seven lasing lines around 1.55 m with wavelength spacing of 1 nm have been obtained. Each lasing line has a linewidth of as narrow as 20 pm and an extinction ratio of 30 dB. The proposed mechanism using a dual-pump FOPA as gain medium is a promising alternative for generating multiwavelength laser. Index Terms—Dual-pump, multiwavelength, optical fiber, parametric amplifier, parametric oscillator.
I. INTRODUCTION
Fig. 1. Schematic diagram of the proposed MFOPO.
F
IBER optical parametric oscillators (FOPOs) relying on the gain of highly efficient four-wave mixing (FWM) in optical fibers have been intensively investigated [1]–[7] in recent years because of their remarkable features, such as the wide wavelength tunability, ultrashort pulse generation, and arbitrary wavelength operation, etc. Using a pulsed Ti : sapphire laser to pump a short-piece microstructure fiber, Sharping et al. successfully implemented a pulsed FOPO with output pulsewidth of 570 fs and the wavelength tunability 200 nm [1]. A continuous-wave (CW) FOPO with as high as 40 mW of idler output was also achieved by Marhic et al. [5]. Using the laser intracavity pump technique, we recently studied another simple and cost-efficient CW FOPO [6]. However, all of them [1]–[7] achieved only a single-wavelength oscillation with one-pump fiber optical parametric amplifier (FOPA). It is demonstrated that dual-pump FOPA can offer a high, wideband and flat gain. Using a highly nonlinear dispersion-shifted fiber (HNL-DSF) with the low variation of zero-dispersion wavelength (ZDW), a CW dual-pump FOPA with 20 dB gain over 155 nm was recently demonstrated [8] by Boggio et al. In principle, there are reasons to believe that it is feasible to realize a multiwavelength lasing using a dual-pump FOPA as the gain medium. Furthermore, owing to the unique Manuscript received May 20, 2009; revised July 02, 2009. First published August 25, 2009; current version published October 09, 2009. The work of Z. Luo was supported by the Chinese Scholarship Council (CSC). Z. Luo is with Xiamen University, Xiamen 361005, China and also with Nanyang Technological University, Singapore 639798, Singapore (e-mail:
[email protected]). W.-D. Zhong and L. Xia are with Network Technology Research Centre, Nanyang Technological University, Singapore 639798, Singapore. Z. Cai, C. Ye, and H. Xu are with the Department of Electronic Engineering, Xiamen University, Xiamen 361005, China (e-mail:
[email protected]). X. Dong is with China Jiliang University, Hangzhou 310018, China. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2009.2030778
features of dual-pump FOPAs, such multiwavelength oscillators may essentially possess some intriguing advantages over the rare-earth-doped fiber amplifier-based multiwavelength lasers: 1) an arbitrary wave-band operation by choosing proper pumps and fiber dispersion [1]; 2) a larger lasing-channel count due to the ultrawide parametric gain bandwidth [8]; and 3) a high output power with high conversion efficiency [4] and idler output [5]. However, to the best of our knowledge, an FOPA-based multiwavelength laser has not yet been demonstrated. In this letter, we report what we believe to be the first demonstration of a multiwavelength all-fiber optical parametric oscillator. The oscillation cavity consists of a 1-km HNL-DSF and a superimposed chirped-fiber Bragg grating (SCFBG) as the comb-like filter. By pumping the HNL-DSF with two highpower laser sources whose wavelengths are symmetrically allocated on the two sides of the ZDW of the fiber, a high and flat parametric gain can be obtained, and the multiwavelength parametric oscillations are thus generated. II. EXPERIMENTAL SETUP The experimental setup of the proposed multiwavelength fiber optical parametric oscillator (MFOPO) is shown in Fig. 1. Two tunable lasers (TL1 and TL2) used as the pump seeds and 1568.70 nm are combined by a at 1551.88 nm 3-dB optical coupler (OC), and then phase-modulated with a 3-GHz pseudorandom bit sequence (PRBS) signal by a phase modulator (PM) to suppress the stimulated Brillouin scattering (SBS). To maximize the suppression of SBS, two polarization controllers (PC1 and PC2) are used to optimize the pumps’ state of polarization (SOP), respectively. The power of both the pumps is further boosted by a high-power erbium-doped fiber amplifier (EDFA), filtered by a pair of 0.5-nm-bandwidth highly reflective ( 99%) fiber Bragg gratings (FBG1 and
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 21, NOVEMBER 1, 2009
Fig. 2. Reflective spectrum of CFBG (dashed line) and the transmission spectrum of SCFBG (solid line).
FBG2) with the respective center wavelengths of and to eliminate the excess amplified spontaneous emission (ASE). An isolator (ISL1) is inserted into the output port of EDFA to isolate any possible reflection. After being filtered, the combined pump power with a maximum value of 1.3 W is injected into a 1-km HNL-DSF by a circulator (CIR1) to generate the parametric gain. The nominal ZDW, loss coefficient, dispersion slope, and nonlinear parameter of the HNL-DSF are 1559.5 nm, km , respectively. 0.75 dB/km, 0.02 ps/nm /km and10 W The comb-like wavelength selection for multiwavelength generation is completed using an SCFBG in combination with a highly reflective chirped fiber Bragg grating (CFBG). Using a broadband ASE light source, we measured that the reflective spectrum of the CFBG and the transmission spectrum of the SCFBG as given in Fig. 2. The CFBG has a bandwidth of 7.5 nm (1554.8–1562.3 nm) and a reflectivity 99% to select out the possible oscillating range from the parametric gain profile. Then, in the selected spectral range, the SCFBG realizes the comb-like wavelength selection of nine channels with a spacing of 1 nm. Each channel has a 3-dB bandwidth of 0.05 nm and 21-dB extinction ratio. The average loss dB, and the at transmission peaks of the SCFBG is insertion loss of the CFBG is 0.8 dB. A 10/90 OC is used to couple out 10% of oscillating light. The output is measured by an optical spectrum analyzer (OSA) and power meter. PC3 is used to align the oscillating light’s SOP with the pumps so as to maximize the gain. III. EXPERIMENTAL RESUTLS AND DISCUSSION The cavity round-trip loss was measured to be 11.6 dB at one of the transmission peaks of the SCFBG (1558.49 nm), mainly originating from the losses of the SCFBG, two circulators, HNL-DSF, and CFBG. To realize multiwavelength oscillation, such large cavity loss may be a real challenge. To solve this issue, we purposely designed a smaller pump separation of 17 nm between the two pumps ( and ) and a sufficient length (1 km) of HNL-DSF so as to obtain a flat and sufficient gain. The central wavelength of the two pumps at 1560.29 nm is slightly deviated from the ZDW of the HNL-DSF into the anomalous dispersion region. To evaluate the feasibility of the MFOPO, we first investigated the gain characteristics of the dual-pump FOPA. To measure the parametric gain, the ring cavity was disconnected at points and as shown in Fig. 1. A signal light from the
Fig. 3. Gain spectrum of the dual-pump FOPA with P
= 1 W.
TL3 was injected into the HNL-DSF from point . The ISL2 is used to prevent the leaking ASE light into the TL3. The amplified signal was output at point and attenuated by a 15-dB optical attenuator (ATT), then monitored by an OSA. With the W and the input signal power total pump power 21 dBm, we measured the gain spectrum of the FOPA by tuning the signal wavelength. The results are shown in Fig. 3. The circles represent the measured gain, and the solid line shows theoretical estimation by numerically solving the coupled wave equations [9]. In the numerical model, we took into account the fluctuation of ZDW, the loss of HNL-DSF, pump depletion and self- and cross-phase modulation. The experimental results are in good agreement with the theoretical result, except for signal wavelengths located near the pumps. This disagreement around the pumps is related to other FWM processes as discussed in dB [10]. One can find from Fig. 3 that the flat gain with can be obtained from 1555.5 to 1566 nm, which can almost cover the entire operation passbands of the SCFBG and CFBG used in our experiment. and into the ring cavity We next connected the points to examine the multiwavelength oscillation. By increasing the total pump power, we observed and recorded the three different operation states of the MFOPO as shown in Fig. 4. mW, all the signal channels selected by the With 50 dBm in Fig. 4(a), SCFBG had very low power level of which manifests the threshold of MFOPO was not reached. Noted that the marked area in the spectrum of Fig. 4(a) is the leaking pump power at . When the total pump power mW, as can be clearly seen was increased to in Fig. 4(b), one channel at 1557.43 nm abruptly started the strong oscillation, which indicates the FOPO reached to its threshold. The marked area in the spectrum of Fig. 4(b) is due to a weak transmission of the SCFBG in this area, as shown in Fig. 2. As further increasing the total pump power, other signal channels also reached their thresholds and started to oscillate mW, seven-wavelength simulsuccessively. With taneous oscillations with a spacing of 1 nm were observed with a power nonuniformity less than 3-dB by carefully adjusting the PC3, as shown in Fig. 4(c). The channel spacing could be potentially tuned by the cantilever beam-based chirp-tuning technique as described in [11]. However, one may be concerned that the FOPA as a homogeneous gain broadening medium may induce the mode competition affecting stable multiwavelength lasing. In our experiment, because the oscillated wavelengths of the MFOPO are very close to the ZDW of HNL-DSF,
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LUO et al.: MULTIWAVELENGTH FIBER OPTICAL PARAMETRIC OSCILLATOR
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Fig. 5. Measured linewidth of the signal channel at 1558.49 nm. FWHM: fullwidth at half-maximum.
IV. CONCLUSION Using a dual-pump FOPA as gain medium and an SCFBG as comb-like filter, we proposed and demonstrated a novel CW allfiber multiwavelength FOPO. Seven-wavelength simultaneous oscillations with spacing of 1 nm were observed. The MFOPO has a 20-pm linewidth, a 30-dB extinction ratio and an internal conversion efficiency of 20%. The results obtained with this preliminary design are encouraging. We believe that with further development MFOPOs could provide an important new type of multiwavelength light sources for optical communication and other applications.
Fig. 4. Output spectra of the MFOPO at different pump powers. (a) P mW; (b) P mW; (c) P mW.
525
= 562
= 892
=
the interchannel FWM among these lasing channels would be generated, which can counteract and eliminate the mode competition, and hence the stable MFOPO with equalized peak power can be obtained. In the case of Fig. 4(c), we measured the signal linewidth of the MFOPO at the channel of 1558.49 nm using an OSA with a spectral resolution of 10 pm. As shown in Fig. 5, the oscillated signal has the respective linewidth and extinction ratio of 20 pm and 30 dB. The 10-pm resolution of the used OSA limits the linewidth measurement, and the true linewidth may be narrower. Such narrow linewidth is mainly benefited from the narrow transmission-peak bandwidth 0.05 nm of the SCFBG and the signal recirculation in the cavity. The output power of the MFOPO was also measured by a power meter. The maximum output power of 4.5 dBm was obW. Accordingly, considering the 10-dB tained with incoupling loss of 10/90 OC and the total insertion losses 8 dB of SCFBG, CIRC2 and CFBG, the delivering signal power at the end of HNL-DSF has 23 dBm, and hence the internal conversion efficiency of the MFOPO is 20%. However, with the further increase of , the output power remained almost unchanged. A possible reason is that the oscillating signals have suffered from the strong SBS in the HNL-DSF because of the narrow signal linewidth.
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