1. Introduction

Multi-wavelength optical short pulse generation has attracted a lot of interest due to its important applications in all-optical signal processing, fiber-optical sensing and ultrahigh capacity transmission systems based on optical time-division-multiplexing (OTDM) and wavelength-division-multiplexing (WDM) systems. Moreover, a multi-wavelength pulse source providing pulsewidth tunability and wavelength reconfigurability would be highly desirable for various applications in future photonic networks.

So far, various approaches have been proposed to realize the multi-wavelength pulses generation. Several researches have reported on generation of multi-wavelength lasing in an actively mode-locked ring laser based on erbium-doped fiber (EDF) [1] or semiconductor optical amplifier (SOA) [2]. The use of mode-locked ring lasers is restricted in terms of cost, stability, and wavelength tunability, regardless the number of wavelengths. Super-continuum (SC) has often been utilized to generate multi-channel short pulse sources. However, this technique requires specialty nonlinear fibers and WDM filter for spectrum slicing [3]. Multi-channel pulse sources are also generated by using nonlinear optical loop mirror (NOLM) [4]. However, the pulsewidths of the generated pulse sources have been broaden due to the dispersion. Moreover, reference [5] have demonstrated a pulsewidth-tunable multi-wavelength synchronized pulse generation utilizing a single SOA-based delayed interferometric switch. However, it is difficult to realize pulsewidth tunability in picosecond pulsewidth range. Recently, adiabatic soliton compression techniques have attracted much attention to generate high-quality short-width pulse trains in the order of a few picoseconds. Mainly, there are two types of techniques for adiabatic soliton compression. The first technique is gradually decreasing the dispersion value along the fiber by using dispersion profiled fibers, such as dispersion decreasing fiber (DDF) [6], comb-like dispersion profiled fiber (CDPF) [7], and step-like dispersion profiled fiber (SDPF) [8]. However, this technique requires special fibers with optimized dispersion profiles. As a simpler alternative, the second technique is using a distributed Raman amplifier (DRA) to increase the peak power of the soliton pulse during the pulse propagation in an anomalous dispersion fiber [9–11]. This technique is possible to tune the pulsewidth of compressed pulse in the picosecond range by control of the Raman pump power. Recently, we have demonstrated the compression for multi-wavelength pulse trains at bit rate of 10 Gb/s by using a Raman amplification-based adiabatic soliton compressor [12] and its application to NRZ-to-RZ data format conversion with picosecond pulse [13], all-channel OTDM demultiplexing [14]. It would be flexible for bit-rate with the OTDM demultiplexing when the multi-wavelength pulse can be tuned. However, multi-wavelength pulse generation for wide channel spacing with pulsewidth tunability in picosecond range has not been demonstrated in previous schemes so far.

In this paper, we experimentally demonstrate a short pulsewidth-tunable multi-wavelength pulse generation using Raman amplification-based adiabatic soliton compressor (RA-ASC). A single distributed Raman amplifier (DRA) is used to compress simultaneously two 10 GHz pulse trains to picosecond pulsewidth range. By changing the gain of the DRA, the pulsewidth of both compressed pulse trains can be simultaneously controlled from 16.0 ps to 2.0 ps. Both compressed pulses exhibit almost same pulsewidth. Operating wavelength range of our scheme is also investigated, showing the possibility for wide channel spacing operations.

2. Principle and experimental setup

The scheme for multi-wavelength pulse generation using RA-ASC is shown in Fig. 1. The RA-ASC operates on the basis of adiabatic soliton compression in distributed Raman amplifier. The adiabatic soliton compression is based on the relation the dispersion length $(L_{\text{D}}=T_0^2/{\beta }_2)$ and the nonlinear length (L_NL = 1/(γP)) when a fundamental soliton pulse is propagated in anomalous dispersion fiber (β₂ < 0) [15]:

 

Fig. 1 Experimental setup for generation of multi-wavelength picosecond pulses with tunable pulsewidth and channnel spacing by using a Raman amplification-based adiabatic soliton compressor. ECL: external-cavity laser-diode, EAM: electroabsorption modulation, EDFA: erbium-doped fiber amplifier, TDCM: tunable dispersion-compensating module, AWG: array waveguide grating, VOA: variable optical attenuator, WDM-PC: WDM power controller, TFRL: tunable fiber Raman laser, DSF: dispersion-shifted fiber, OBPF: optical bandpass filter.

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In the experimental setup, to generate two seed pulse trains of 10 GHz repetition rate for the pulse compression, two continuous waves (CWs) at wavelengths: λ₁ =1546.1 nm (channel 1) and λ₂ =1549.3 nm ∼ 1557.3 nm (channel 2) from external-cavity lasers (ECLs) were simultaneously modulated by an electroabsorption modulator (EAM), driven by a 10 GHz synthesizer. An erbium-doped fiber amplifier (EDFA) was used after EAM to compensate for the EAM insertion loss. To compensate the frequency chirp induced by the EAM for multi-wavelength pulse trains, conventional technique using a dispersion-compensating fiber (DCF) [11] is no longer suitable due to its large dispersion slope. Here, we utilized a tunable dispersion-compensating module (TDCM) for simultaneous chirp compensation for two pulse trains. It is important to ensure fundamental soliton powers of the two seed pulse trains for adiabatic soliton compression. Here, a WDM power controller (WDM-PC) was constructed with following an EDFA for power adjustment before the RA-ASC. This WDM-PC consisted of an arrayed waveguide grating (AWG), variable optical attenuators (VOAs) and a coupler. Wavelength setting in the AWG was done in accordance with the wavelength of two input channels. Before injected into the RA-ASC, the pulsewidths of the two pulse trains at a bit-rate of 10 Gb/s were around 18 ps full width at half maximum (FWHM). The RA-ASC operates on the basis of adiabatic soliton compression in distributed Raman amplifier. In the RA-ASC, a 17 km dispersion-shifted fiber (DSF) was employed for adiabatic soliton compression technique. The second- and third-order dispersions of the DSF are 3.8 ps/nm/km and 0.059 ps/nm²/km, respectively. The Raman pump signal generated by a tunable fiber Raman laser (TFRL) was injected into the counter-propagating direction by using a WDM coupler. The wavelength of TFRL can be tuned in the range between 1425 nm and 1495 nm. To achieve high quality compression performance, Raman pump wavelength was optimized for the two seed pulse trains. The pulsewidths of the two seed pulse trains are compressed as its peak power increases with the increase of the Raman pump power since the soliton condition is maintained in the DSF during the amplification. At the output of RA-ASC, the compressed two pulses are filtered by two 3-nm optical bandpass filters (OBPFs) for pulse measurements. The output spectrum and waveform were measured by a spectrum analyzer and an autocorrelator, respectively.

3. Experimental results and discussion

To investigate the performance of the RA-ASC, we evaluate the compressed pulses in terms of pulsewidth and peak-to-pedestal ratio, which are the important parameters to characterize the pulse’s quality. The former parameter gives strong influence to the transmission performance in the photonic networks. The latter parameter is defined by the ratio of the peak to the second peak of the autocorrelation trace of the pulse. In a high-speed optical system, a less pedestal is beneficial for avoiding intersymbol interference (ISI) and improving the signal quality. Moreover, at the same average optical power, a pulse with a larger pedestal would have smaller peak power, which is crucial for the high-power pulse applications. In our scheme, the output pulsewidth and peak-to-pedestal ratio of the compressed pulses were measured independently by using two 3.0-nm OBPFs after the RA-ASC. Figure 2 shows (a) the optical spectra after the RA-ASC, and the autocorrelation traces at (b) channel 1 and (c) channel 2 of the output pulses as a function of Raman pump power (P_r) with channel spacing (Δλ = λ₂ – λ₁) of 3.2 nm. Both the optical spectra and autocorrelation traces of the compressed pulses were well fitted to sech² functions. It can be seen that over the adiabatic soliton compression with the increase in the Raman pump power, output spectra of the two pulses were broadened at the same time, and their pulsewidths were compressed. The input pulsewidths (18.02 ps, 18.08 ps) at (channel 1, channel 2) were considerably compressed to (5.58 ps, 5.84 ps), (3.03 ps, 2.92 ps) and (2.63 ps, 2.65 ps) as the Raman pump power was set at 0.75 W, 0.90 W and 0.95 W, respectively.

 

Fig. 2 (a) Optical spectra after RA-ASC, and autocorrelation traces at (a) channel 1 and (b) channel 2 of the output pulses measured in various case of Raman pump power (P_r) with 3.2 nm channel spacing.

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Figure 3(a), (b) show the output pulsewidth and peak-to-pedestal ratio of the compressed pulses at both channels for different Raman pump powers, respectively. It is found that the compressed pulses at both channels have similar characteristics. In Fig. 3(a), the output pulsewidth can be tuned from 16 ps to 2 ps by adjusting the Raman pump power from 0.4 W to 0.95 W. Small difference in pulsewidth was observed between two channels. The peak-to-pedestal ratio of the output pulse decreases slightly as the Raman pump power becomes larger in both channels as seen in Fig. 3(b). The pedestals associated with the compressed pulses were observed due to the slight deviation from adiabatic conditions in the experiment. However, throughout the tuning range, the peak-to-pedestal ratio was above 11.5 dB.

 

Fig. 3 Raman pump power dependency of (a) the pulsewidth, (b) the peak-to-pedestal ratio of the compressed pulses with 3.2 nm channel spacing.

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To investigate the operating channel spacing of the RA-ASC, we fixed λ₁ at 1546.1 nm and tuned λ₂ from 1549.3 nm to 1557.3 nm corresponding for channel spacing from 3.2 nm to 11.2 nm. It should be noted that the pulse waveforms were distorted due to the deviations from the adiabatic condition when the Raman pump power was over an optimum Raman pump power (P_r,opt). Figure 4(a) shows the optical spectra of the output compressed pulses when the Raman pump power was set to P_r,opt at each case of channel spacing. Figure 4(b), (c) show corresponding autocorrelation traces of the output compressed pulses at channel 1, channel 2, respectively. In Fig. 4(b), (c), the solid line shows the measured waveform whereas the dashed line shows the sech² fitting waveform. In all cases of Raman pump power and channel spacing setting, the output compressed pulses could be fitted with sech² waveform both in spectrum and waveform. The time-bandwidth product of the compressed pulses at all Raman pump power and channel spacing were found to be less than 0.41. These results show good-quality output pulses were obtained for both channels at the same time by our scheme.

 

Fig. 4 (a) Optical spectra after RA-ASC and autocorrelation traces at (b) channel 1, (c) channel 2 of the output pulses measured in various case channel spacing. Raman pump power was set to P_r,opt in each case.

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As only the case of channel spacing of 3.2 nm shown in Fig. 3, Raman pump power dependency of the pulsewidth and peak-to-pedestal ratio of the compressed pulse were investigated in various cases of channel spacing. Figure 5(a) shows the output pulsewidth of channel 1 as function of Raman pump power in various cases of channel spacing from 3.2 nm to 11.2 nm. Our measurements show small pulsewidth differences between channel 1 and 2 in all cases. These results show that both channels have similar characteristics. As seen in Fig. 5(a), pulsewidth of channel 1 was compressed to picosecond range between 16 ps and 2 ps by increasing the Raman pump power. Moreover, larger channel spacing required larger Raman pump power to obtain output pulsewidth. For example, in order to obtain pulsewidth of around 2.5 ps, while Raman pump power of 0.95 W was needed with 3.2 nm channel spacing, it was 1.15 W in the case of 11.2 channel spacing. Figure 5(b) shows the optimum Raman pump power P_r,opt and peak-to-pedestal ratio of the output pulse at both channels as the function of channel spacing. P_r,opt increased and the peak-to-pedestal ratio decreased slightly as the channel spacing increase from 3.2 nm to 11.2 nm. The peak-to-pedestal ratio was above 7.7 dB for both channels in all setting of Raman pump power and channel spacing.

 

Fig. 5 (a) Raman pump power dependency of the pulsewidth of the compressed pulse (at channel 1) and (b) channel spacing dependency of the optimum Raman pump power (P_r,opt) and peak-to-pedestal ratio of the output pulse.

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4. Conclusion

We have successfully demonstrated a short pulsewidth-tunable multi-wavelength pulse generation by means of adiabatic soliton compression in a single distributed Raman amplifier. Both pulse trains were compressed with flexibly pulsewidth-tunable range from 16 ps to 2 ps by controlling the Raman pump power. High quality compressed pulses were achieved in all operating Raman pump powers in wide channel spacing operations.