Multi-wavelength pulse sources by slicing continuum spectrum have attracted interest for use in wavelength-division multiplexed (WDM) systems [1]–[6]. Supercontinuum (SC) generation is based on the interplay among several nonlinear effects in SC medium fiber, and offers a broadened and flattened spectrum that can be sliced over a wide wavelength range. Especially, in order to equalize channel power levels of multi-wavelength pulse, the flattened spectrum with power uniformity is required. However, the profile of SC spectrum depends mainly on dispersion design of SC medium fiber [2]–[4]. On the other hand, several techniques based on adiabatic compression in Raman amplifier have been developed to compress soliton pulses [7]–[10]. The adiabatic compression means that a soliton pulse evolves adiabatically into a shorter pulse in anomalous dispersion region with a gradually distributed Raman gain medium. These techniques, which can offer high-compression ratios, are expected to broaden SC spectrum effectively and increase channel numbers. Although Lewis et al. reported SC generation based on nonlinear parametric and stimulated scattering processes in the vicinity of zero dispersion region in Raman gain medium, [11], SC generation based on adiabatic compression and its application for multi-wavelength pulse generation have not been reported.

One approach that generates a highly stable soliton-like pulse is the direct modulation of light from a continuous-wave (CW) laser diode using an electroabsorption modulator (EAM). However, such pulse sources are not suitable for SC seed pulse, because of their long pulse duration. Although a few SC generations based on EAMs have been demonstrated so far, special fibers such as dispersion decreasing fiber [5] or pulse compression stage to generate picosecond seed pulses [6] were also required.

In this paper, we demonstrate a simple SC spectrum generation utilizing adiabatic compression based on an electroabsorption modulated seed pulse source. SC medium consists of only conventional dispersion-shifted fiber (DSF) as a counter-pumped Raman gain medium. Input seed pulses with a peak-power much higher than that of fundamental soliton pulses generate SC spectrum, which results from adiabatic compression, modulational instability, broad Raman gain bandwidth, and nonlinear parametric effects. Since these effects depend on profile of the Raman gain spectrum, optimum adjustment of the wavelength and power of Raman pump source improves the power uniformity of SC spectrum for multi-wavelength pulse generation. We show that this technique enables to generate 16-nm-wide uniform SC spectrum with the power fluctuation less than 3 dB. The combination of slicing spectrum technique and seed pulse source produced by EAM leads to highly stable operation of the multi-wavelength pulse source with a simple configuration.

 

Fig. 1. Experimental configuration of the proposed multi-wavelength pulse source generation.

Download Full Size | PDF

Figure 1 shows the experimental configuration of the proposed multi-wavelength pulse source. A seed pulse source was realized with CW source as an external cavity laser-diode (ECL) by using an EAM. The spectral linewidth of the ECL was broadened to 50 MHz to suppress stimulated Brillouin scattering. A pulse train of 18.0 ps pulse-width with a center wavelength of 1538 nm at 10 GHz repetition rate was obtained. As a pre-amplifier of the seed pulse, an erbium-doped fiber amplifier (EDFA) was employed. In order to eliminate amplified spontaneous emission (ASE), a bandpass filter (BPF) with 3 nm-bandwidth was employed. The average launched power of the seed pulse source to Raman gain medium was 18.5 dBm, which is much higher than the calculated fundamental soliton of this configuration 6.2 dBm. The Raman gain was distributed over a span of 17.0 km of DSF with a dispersion of 3.0 ps/nm/km at seed pulse wavelength. The zero dispersion wavelength and dispersion slope of this fiber are 1486 nm and 0.059 ps/nm²/km, respectively. A wavelength tunable fiber Raman laser (TFRL) manufactured by IPF Technology, Ltd. was employed for the CW Raman pump source and has a spectral linewidth of 1.4 nm. The measured 3 dB gain bandwidth of this amplifier was 25 nm. The seed pulse experienced the exponential gain profile along the Raman gain medium. The SC pulse was filtered by using an arrayed-waveguide grating (AWG) to obtain multi-wavelength outputs. The bandwidth of each channel was approximately 0.24 nm, and the channel spacing was 0.8 nm (100 GHz). This configuration is similar to that of adiabatic compression in Raman amplifier [8]–[10]. However, our target is not providing ideal adiabatic soliton compression with a low pedestal ratio but generating a proper SC spectrum for multi-wavelength pulse source. In this scheme, a higher launched power of the seed pulse is also required in order to give rise to nonlinear parametric interactions for SC generation.

 

Fig. 2. Measured SC output spectra for several values of (a) Raman pump wavelength and (b) pump power.

Download Full Size | PDF

To examine the characteristics of the SC spectrum generation, the Raman pump wavelength and power dependence of the spectral profile were measured at the output of Raman gain medium before AWG. The SC spectra for four values of the Raman pump wavelength at 1.5 W pump power are shown in Fig. 2(a). The measured pulse-width and peak-to-pedestal ratio of the autocorrelation traces were 0.975 ps and 14.2 dB, respectively. In case of 1438 nm pump wavelength (dashed curves), the peak of the Raman gain spectrum was 1538 nm, which corresponds to the peak of the output SC spectrum. By adjusting the pump wavelength, the SC spectrum could be broadened and flattened. The best profile of the SC spectrum could be obtained at 1446 nm pump wavelength (solid curves) that has a gain peak 8 nm longer wavelength than that of the seed pulse. This means that the Raman gain contributes to not only generation of adiabatic compression, but also to gain improvement for the profile of the SC spectrum. Especially in the case of 1446 nm pump, the gain improvement contributes greatly to the broadening of the spectrum. It was thus found that the profile of the SC spectrum could be improved by adjusting the pump wavelength.

Figure 2 (b) depicts the measured SC spectra for six values of the Raman pump power in the range 0 - 1.5W. The pump wavelength was set to 1446 nm based on the wavelength adjustment as mentioned before. As the pump power is increased, the SC spectrum can be broadened and flattened. In addition, the asymmetry in the SC spectrum becomes also remarkable. The asymmetric spectrum-broadening is mainly attributed to Raman effects such as self-Raman, broad gain bandwidth, and gain saturation. In the range from 1541 nm to 1557 nm, a very flat SC spectrum bandwidth with the power fluctuation less than 3 dB was obtained. The spectral bandwidth in this experiment was 16 nm.

 

Fig. 3. Measured spectra of sliced pulses from several output channels.

Download Full Size | PDF

Figure 3 shows the sliced pulse spectra from several output channels. Numbers above peaks of the each spectrum indicate the output channel number. Many output spectra with variation of the peak power as small as 3 dB could be obtained. The side lobes of the each channel are the adjacent-channel crosstalks, while the superposed spectral profile less than 50 dBm output power is the background crosstalks determined by the power profile of the SC spectrum.

 

Fig. 4. Measured autocorrelation trace at channel 25. The circles correspond to the Gaussian fitting. Inset, the oscilloscope traces.

Download Full Size | PDF

The pulse-width and time-bandwidth product were measured by using an intensity autocorrelator. The autocorrelation trace of channel 25 as an example is shown in Fig. 4. Its pulse-width and time-bandwidth product were 17.9 ps and 0.531, respectively. These values depend mainly on the bandwidth of the AWG. By using another AWG with a broader bandwidth, a shorter pulse train would be obtained. The circles correspond to the Gaussian fitting. The autocorrelation trace could be fitted by that of Gaussian pulse. Inset shows an oscilloscope trace of the SC pulse, which was measured with a 20 GHz-bandwidth digital sampling oscilloscope.

 

Fig. 5. Measured output power and pulse-width of SC pulses from each channel.

Download Full Size | PDF

Figure 5 summarizes the wavelength channel dependence of the measured output power and pulse-width of SC pulse duration. Over 20 channels, the variations of the output powers were as small as 3 dB. The pulse-widths were distributed from 17.8 ps to 19.4 ps. At channel 15, which overlaps seed pulse wavelength, the output power was much larger than other channels. In order to improve the uniformity of the output power and suppress the channel crosstalks as mentioned before, the use of a notch filter at the front of AWG to reduce the power of the SC spectrum at seed pulse wavelength will be effective.

In conclusion, we demonstrated a supercontinuum generation utilizing adiabatic compression in Raman amplifier. By adjusting the wavelength and power of the Raman pump source, we improved the power uniformity of the SC spectrum, and generated multi-wavelength pulses with the power fluctuation less than 3 dB over 20 channels. Such a highly stable and cost-effective multi-wavelength light source with a simple configuration is useful and practical for multi-channel transmission systems.

The authors would like to thank Dr. M. Onishi with Sumitomo Electric Industries, Ltd. for providing us the dispersion-shifted fiber.