The compression of 200GHz DWDM channelized optically mode-locking WRC-FPLD fiber ring pulse of at 10 GHz is performed for high-capacity TDM application. To prevent temporal and spectral crosstalk, the duty-cycle of the DWDM channelized WRC-FPLD FL pulse needs to be shortened without broadening its linewidth. With dual-cavity configuration induced DWDM channelization, a shortest single-channel WRC-FPLD FL pulsewidth of 19 ps is generated, which can be linearly compensated to 10 ps and fifth-order soliton compressed to 1.4 ps. Under a maximum pulsewidth compression ratio up to 14 and a ±100m tolerance on compressing fiber length, the single-channel pulsewidth remains <2 ps (duty-cycle <2%) with spectral linewidth only broadening from 0.29 nm to 0.8 nm. In comparison, a typical SOAFL without intra-cavity TBF in fiber ring broadens its spectral linewidth from 2.4 to 3.8 nm after compressing its mode-locked pulsewidth from 21 to 2.1 ps. The duty-cycle of the DWDM channelized WRC-FPLD FL pulsed carrier is approaching 1% to satisfy at least 256 optical TDM channels.
©2009 Optical Society of America
During the past decade, an all-optical cross-gain modulation (XGM) scheme [1-3] has also been performed for mode-locking semiconductor optical amplifier (SOA) system by periodically depleting its gain via forward optical injection. Such a scheme essentially overcomes the drawback of the mode-locked SOA fiber laser (referred as SOAFL) which is typically implemented by directly gain-modulating the SOA with a limited bandwidth of electrical modulation . Taking the concept of the optical injection induced XGM in SOA, a backward dark-optical-comb injection technique was subsequently applied to mode-lock the SOAFL at repetition frequency of 1 GHz, which generate 15-ps and 1.2-ps pulsewidths before and after dispersion compensation . Except these demonstrations, the mode-locked SOAFLs have also been incorporated in various applications. In addition to providing gain, the introduction of SOAs in fiber laser cavity can have benefits related to their high nonlinearities and their fast response time. It has been shown that the dynamic SOA nonlinearity can stabilize the harmonically mode-locked fiber lasers . The fast response of SOAs has been utilized for reducing the amplitude fluctuations and pulse dropout associated with super-mode noise . Up to now, most XGM mode-locked SOAFL systems generate picoseconds (ps) pulses with broadened spectrum [8-10], whereas the multichannel XGM mode-locked SOAFL system with sub-ps single-channel output was seldom investigated. Recently, we have proposed a weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) based intra-cavity DWDM channelized fiber laser system with concurrently 200GHz/12-channel DWDM output and widely tunable wavelength from 1525 to 1565 nm . To meet the demand of high-capacity optical TDM carrier with sufficiently low duty-cycle and moderate linewidth to avoid cross-talk between adjacent channels, the WRC-FPLD FL pulsewidth has to be compressed while preserving its spectral linewidth not to be broadened. In this paper, we study the spectral properties and pulse compression dynamics of the 200GHz DWDM channelized WRC-FPLD fiber laser XGM mode-locked by dark-optical-comb pulse-train with adjustable duty cycle. To achieve sub-picosecond DWDM channelized WRC-FPLD pulse carrier with its duty-cycle approaching 1%, the parametric tuning for dispersion compensation and pulse compression of the single-channel DWDM output pulse-train at repetition frequency of 10 GHz is carefully adjusted and compare with a typical SOA fiber laser.
2. Experiment setup
The schematic diagram is illustrated in Fig. 1, in which a two-port edge-emitting WRC-FPLD with the front/back facet reflectivity of 10% driven at 280 mA is employed as the gain medium to construct a fiber ring laser with a cavity length is 14 m. Such a WRC-FPLD essentially results in an additional longitudinal mode selecting mechanism, which forces the harmonic mode-locking occurred at a least multiplication harmonic frequency of the WRC-FPLD and the fiber ring link, as shown in upper part of Fig. 2. In the electrical path (see Fig. 1), an RF synthesizer is operated at 10 GHz and 8 dBm, a 30-dB amplifier is used to drive the electric comb where the minimum driven power is 27 dBm. The optical injection is generated by seeding a single-wavelength laser through a MZM biased with the electrical comb. The dark-optical-comb pulse  can be obtained by DC-biasing the MZM at 8.2 V, as illustrated in lower part of Fig. 2. To obtain the best optical mode-locking with highest on-off extinction ratio, the optimized injection power of the dark-optical-comb at 1562.4 nm was controlled at 2 dBm by using tunable erbium-doped fiber amplifier (EDFA) driven at 46 mA. In the optical path, the isolator is used to guide the lights traveling counter-clockwise. The first polarization controller (PC) before MZM is used to optimize the input power in front of polarization dependent MZM. The second PC can ensure the best dark-optical comb injecting into WRC-FPLD which is also polarization dependent. The third PC is used to optimize the feedback lights into the polarization sensitive WRC-FPLD. The output WRC-FPLD FL pulse after channel selecting with a TBF (with 3dB bandwidth of 1.4 nm shown in Fig. 1(a)) was directly amplified by an EDFA before pulsewidth compression. To compare the WRC-FPLD FL output performance with those of a typical SOAFL, we introduce a commercial traveling-wave SOA to substitute the WRC-FPLD for similar experiments, which was DC-biased at 225 mA (well above transparent condition) with broadband ASE around 1530 nm.
We introduce dispersion compensating fiber (with dispersion parameter of D = -80 ps/km/nm) of different lengths to reduce the linear chirp accompanied with the 200GHz DWDM channelized WRC-FPLD FL (or SOAFL) pulse. For nonlinear soliton with pulse compressing ratio proportional to the soliton order, we employ the formula Pn=3.11β2n2/γτ2  to estimate the peak power of the DWDM channelized WRC-FPLD FL pulse, and use the equation Zopt=Z0(0.32n+1.1)/n2=0.322(0.32n+1.1)πτ2/2β2n2  to determine the fiber length required to obtain the nth-order soliton , where Pn is the peak power of the WRC-FPLD FL pulse and n is the soliton order. For a specific fiber (Corning SMF-28, with D = 20 ps/km/nm) with core diameter of 9.3 μm used as soliton compressor, β2 is the group velocity dispersion (GVD) parameter, γ =1.3 W-1 km-1 is the nonlinearity coefficient. To perform soliton effect compression, the dispersion compensated pulse-train was amplified using a high-power EDFA (IPG EAU2M), and then coupled into the SMF-28 fiber with changing lengths.
3. Results and discussion
Figure 2 shows the continuous-wave (CW) ASE spectrum of SOA and WRC-FPLD before inserting into the fiber ring. In comparison with the mode-free SOA spectrum with linewidth up to 30 nm, the WRC-FPLD with longitudinal mode spacing of about 0.25 nm exhibits a linewidth of 22 nm. After optically injection-mode-locking at 10 GHz, both the mode-locked spectra of SOA and WRC-FPLD FL shown in Fig. 3 become narrower due to the fiber ring cavity effect. The mode-locked SOAFL spectrum centered at 1540 nm exhibits 3dB linewidth of 2.9 nm, whereas the WRC-FPLD FL spectrum reveal a channelized effect due to the WRC-FPLD and fiber ring dual-cavity, which induces a wider longitudinal mode spacing than each individual cavity by least-common multiplication of the two original mode-spacing frequencies. Such a hybrid-cavity mode-frequency multiplication scheme can achieve mode-spacing detuning by simply changing the fiber ring length. In principle, the longitudinal mode spacing of a WRC-FPLD is defined as ΔλFPLD = λFPLD 2/(2nLFPLD), where LASE is the WRC-FPLD cavity length (~1.7 mm), λFPLD denotes the peak wavelength of WRC-FPLD (~1540 nm), n denotes the reflective index of WRC-FPLD active region (~3.5). The same formula is also applied to calculate the mode spacing of fiber ring with cavity length of 20.8 m as ΔλFiber = 7.7×10-5 nm (or Δν = 9.63 MHz). With fine adjustment on fiber ring length, the least-common multiple of ΔλFPLD and ΔλFiber results in an enlarged mode spacing of 1.6 nm perfectly coincident with the DWDM channel spacing of 200 GHz from ITU-T specification. A single-channel TDM carrier can simply be obtained by using a 200GHz DWDM demultiplexer or a TBF after the WRC-FPLD FL output port.
We employ an optical PRBS NRZ data stream at 1555.7 nm which simulates as the 200GHz AWG filtered single-channel incoming data before injecting into the WRC-FPLD based fiber ring. The harmonic mode locking of WRC-FPLD FL is relatively easy to be established via the backward injection of an optically sinusoidal wave or a large-duty-cycle transistor-transistor logic (TTL) data stream like dark-optical-comb waveform instead of using an optical pulse injection . With the TBF selection, the peak channel of the mode-locked WRC-FPLD FL is at 1543 nm with its original pulsewidth of 19 ps obtained from auto-correlator trace, as shown in Fig. 4. The channelized WRC-FPLD FL pulse can be linearly dispersion compensated after propagating through a 70 m-long DCF, providing a pulsewidth shortened from 19 to 10 ps without changing its spectral linewidth. To meet the demand of the future high-capacity optical TDM application without cross-talk both in temporal and spectral domains, the duty-cycle of the DWDM channelized WRC-FPLD FL pulse needs to be reduced with preserving linewidth.
The single-mode fiber spools with different lengths are introduced for soliton compression after high-power EDFA amplification. The peak power of 10.8 W and dispersion compensated pulsewidth of 12 ps are used to calculate soliton order of n = 5 and fundamental soliton length Z0 = 3.1 km in our case. The shortest soliton pulse was occurred when using 400-m long SMF, which is in good agreement with the predicted length of 334 m. With the SMF length increasing from 0 to 700 nm, the spectral linewidth of a single-channel mode-locked WRC-FPLD FL carrier only broadened from 0.29 to 0.8 nm after pulse compression, which will not cause any serious cross-talk between adjacent channels in comparison with the AWG channel bandwidth of 1.6 nm (see Fig. 5). It is mandatory not to greatly broaden the single-channel mode-locked spectrum during soliton compression, since the wider spectral linewidth inevitably causes more serious cross-talk between adjacent channels. In our case, the tolerance in SMF length is about ±100 m for obtaining the channelized optical TDM carrier from WRC-FPLD FL with pulsewidth <2 ps and duty-cycle <2%.
The shortest single-channel mode-locked WRC-FPLD FL pulsewidth of 19 ps repeated at 10 GHz is linearly compensated to 10 ps via 70m-long DCF and fifth-order soliton compressed to 1.4 ps via 400m-long SMF (see Fig. 6). When the SMF length increases to 500 m or longer, the nonlinear self-phase modulation (SPM) effect becomes dominant to broaden and to deform the channelized WRC-FPLD FL spectrum. The main soliton pulse of WRC-FPLD FL still maintains at <1.4 ps even passing through a 900m-long SMF. Although the enhanced SPM could further shorten the pulse, the compressed pulse shape further degrades with enlarging pedestal due to incomplete group velocity dispersion of side modes with different polarization experienced in SMF. The peak power of side-mode components is insufficiently high to match the SPM-GVD compensation set for principle pulse, thus leaving a less compressed side-pedestal due to the incomplete compensation. Such a pedestal is disadvantageous for the DWDM channelized pulse carrier since which inevitably causes cross-talk in time domain.
In comparison, the typical SOAFL shows a mode-locked pulsewidth and linewidth of 21 ps and 2.4 nm, respectively, as shown in Figs. 7 and 8. With linear dispersion compensation, the SOAFL pulsewidth shortens from 21 to 13.5 ps at optimized DCF length. The mode-locked SOAFL remains unchanged spectrum during compensation process, however, which is broadened from 2.4 to 3.8 nm after soliton compression in SMF with length exceeding 200 m. Shortest soliton pulsewidth of 2.1 ps appears during 200m-long SMF compression, the tolerance in SMF length for obtaining <3 ps pulse is relatively large (200-700 m). In comparison, the channelized WRC-FPLD FL exhibits shorter pulsewidth, smaller pedestal component and better pulse-energy confinement that SOAFL, as can be seen in Fig. 9. In particular, the spectral linewidth of WRC-FPLD FL is far narrower than SOAFL (see Fig. 10), which benefits from the advantage of high-capacity TDM channelization. Even with a maximum pulsewidth compression ratio up to 14, the WRC-FPLD FL pulse only broadens its single-channel spectral linewidth from 0.29 nm to 0.8 nm.
The pulse compression of 200GHz DWDM channelized TDM carrier at 10 GHz from optically mode-locking WRC-FPLD Fiber Ring is performed and compared with a typically SOA based fiber laser. To meet the demand of the future high-capacity optical TDM application without cross-talk both in temporal and spectral domains, the duty-cycle of the DWDM channelized WRC-FPLD FL pulse need to be compressed with preserving linewidth. With intra-cavity dual-cavity configuration induced DWDM channelization, a shortest single-channel mode-locked WRC-FPLD FL pulsewidth of 19 ps repeated at 10 GHz is generated, which can further be linearly compensated to 10 ps by passing through the 70m-long dispersion compensation fiber and fifth-order soliton compressed to 1.4 ps in 400m-long single-mode fiber. Even with a maximum pulsewidth compression ratio up to 14, the WRC-FPLD FL pulse only broadens its single-channel spectral linewidth from 0.29 nm to 0.8 nm. The tolerance in SMF length is relatively large (400-700 m) for obtaining the channelized optical TDM carrier from WRC-FPLD FL with pulsewidth <2 ps and duty-cycle <2%. In comparison, a typical SOAFL without intra-cavity TBF in fiber ring broadens its spectral linewidth from 2.4 to 3.8 nm after compressing its mode-locked pulsewidth from 21 to 2.1 ps. The duty-cycle of the DWDM channelized WRC-FPLD FL pulsed carrier is approaching 1% to satisfy at least 256 optical TDM channels.
The authors thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under grants NSC-97-2221-E-002-055.
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