1. Introduction

Silicon photonics provides the opportunity for optoelectronic components that can be integrated at low cost for optical communications and interconnects [1-3]. The high index contrast of silicon waveguides offers much smaller optical modal areas than in single mode fibers, so that higher optical power densities can be achieved for observing nonlinear effects in compact devices. Recently, silicon photonic devices, such as lasers [4-7], modulators [8-13] and wavelength converters [14-21], have been demonstrated. One principle element in optical networks is the amplification of an optical signal to overcome the various losses. Silicon has a ∼10⁴ times higher Raman scattering efficiency than silica fiber [22], which has inspired investigations of Raman amplification in silicon waveguides [23-27]. Recently, stimulated Raman scattering (SRS) in silicon waveguides has been demonstrated to achieve net continuous-wave optical gain using a reverse biased p-i-n waveguide, which reduces nonlinear losses due to two-photon absorption (TPA) induced free carrier absorption (FCA) in the waveguide [28,29]. The inherent Raman gain profile of silicon has a width of <100 GHz [24,30], and the net gain depends on the signal wavelength detuning from the center of the gain profile, which is 15.6 THz red-shifted from the pump wavelength. This wavelength-dependent gain provides a method for selectively amplifying individual optical channels but presents a potential bandwidth limitation for high speed optical signals.

Here we demonstrate amplification of a high speed optical signal using stimulated Raman scattering in a reverse biased silicon p-i-n waveguide. The optical signal at 1544.9 nm is modulated at 40 Gb/s, and the pump beam is at 1430 nm. Using 230 mW of pump power coupled into the waveguide, we achieve 2.3 dB on/off gain and observe minimal change in the signal-to-noise ratio of the eye diagram. We perform measurements using both co- and counter-propagating pump and probe beams. For the co-propagating configuration, we observe additional noise introduced most likely due to contributions from four-wave mixing, cross-phase modulation and noise transfer from the pump [31,32]. In addition, we measure the on/off gain as a function of detuning from the Stokes wavelength. This demonstration of Raman amplification of data rates as high as 40 Gb/s shows the capability of silicon-based amplifiers for next generation networking, such as OC-768.

2. Device description

The device is a silicon-on-insulator p-i-n rib waveguide, fabricated on the (100) surface of a silicon-on-insulator (SOI) substrate using standard photolithographic patterning and reactive ion etching techniques. The rib waveguide width is 1.5 μm, the rib height is 1.55 μm, and the etch depth is 0.76 μm. The waveguide has an S-bend shape with a total length of 4.6 cm. The straight sections of the waveguides are oriented along the [011] direction, and both bends have a bend radius of 400 μm. The p- and n-doped regions, situated along either side of the waveguide, are separated by 6 μm and contacted with an aluminum film. Reverse biasing the p-i-n diode decreases the free carrier density in the waveguide and therefore reduces the nonlinear optical loss due to TPA induced FCA [28]. The linear optical transmission loss of the waveguides at low optical powers is 0.25 ± 0.05 dB/cm, measured using the Fabry-Perot resonance technique [1].

3. Raman amplification in silicon waveguides

We measure the amplification of an optical signal at 1544.9 nm from an external cavity laser that is modulated by a lithium niobate modulator driven by a 40 Gb/s pseudo-random bit sequence (PRBS 10³¹-1) signal. Figure 1 shows a schematic of the experimental set-up for the counter-propagating pump and probe beam configuration. The 1544.9 nm signal is coupled into the waveguide with a lensed fiber. The coupling loss from the lensed fiber into the waveguide is approximately 4 dB. The pump beam at 1430 nm is amplified up to 570 mW and then coupled into the opposite end of the silicon rib waveguide with another lensed fiber, allowing us to achieve coupled pump powers of up to 230 mW. Polarization controllers are used to set the polarizations of the pump and signal beams to maximize the polarization-dependent Raman gain [27]. Add/drop wavelength multiplexer filters are used on both sides of the waveguide to separate the residual pump and extract transmitted signal beams. In addition, the transmitted signal is sent through a 1550 nm filter to further reduce the background noise before being sent to a power monitor and a digital communications analyzer (DCA). For the measurements, the device is maintained at 25°C with a thermo-electric cooler, and a reverse bias of -25 V is applied to the p-i-n diode to minimize the nonlinear loss caused by TPA-induced FCA.

 

Fig. 1. Schematic of the experimental set-up for 40 Gb/s Raman amplification using counter-propagating pump and probe beams.

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We use the DCA to analyze the amplification and signal integrity of the transmitted 40 Gb/s pseudo-random data with both co- and counter-propagating pump and probe beam configurations. In Fig. 2, we show eye diagrams of the 40 Gb/s PRBS data taken with the DCA for the waveguide with co-propagating pump and probe beams. For this data, with coupled pump power of 228 mW, we observe 2.3 dB on/off gain, but the signal-to-noise ratio decreases from 6.6 for the transmitted signal without pump power to 4.9 for the amplified signal with pump power, corresponding to a noise figure of 1.3 dB.

 

Fig. 2. Eye diagrams from the digital communications analyzer (DCA) for (a) transmitted (pump off) and (b) amplified (pump power = 228 mW) signals at 40 Gb/s data rate using co-propagating pump and probe beams.

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Measurements taken using counter-propagating pump and probe beams do not exhibit this amplitude noise at high pump powers. In Fig. 3(a), we show the eye diagram for the transmitted signal without any pump beam. The eye amplitude is 2.1 mW, and the signal-to-noise ratio is 6.6. Figure 3(b) presents the eye diagram for the amplified signal with 222 mW of coupled pump power in the waveguide, illustrating 2.2 dB on/off gain with a signal-to-noise ratio of 5.5 which is an improvement over the co-propagating pump-probe configuration. The noise figure is reduced to 0.8 dB in this case. In addition, there is no appreciable increase in the jitter observed in the amplified signals. Since the Raman gain is independent of the relative propagation directions of the pump and signal beams, the counter-propagating pump and signal configuration should be used to reduce potential noise contributions from four-wave mixing and cross-phase modulation, which are more pronounced for co-propagating pump and signal beams [31,32] and which we observe at high pump powers (>190 mW).

 

Fig. 3. Eye diagrams from the DCA for (a) transmitted (pump off) and (b) amplified (pump power = 222 mW) signals at 40 Gb/s data rate using counter-propagating pump and probe beams.

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Figure 4 shows the on/off gain for the 40 Gb/s eye amplitude as a function of the coupled pump power for both co- and counter-propagating pump and probe beam configurations. For the coupled pump power up to 230 mW, we observe that the on/off gain increases to 2.3 dB monotonically. For higher pump powers, we anticipate higher gain; however, the gain will eventually saturate due to increasing TPA-induced FCA [28,29] in this simple waveguide. To mitigate the saturation effect, the free carrier lifetime must be further reduced or a special amplifier design is required, e.g., a cascaded ring cavity [33] or cladding-pumped amplifier [34].

 

Fig. 4. On/off gain as a function of coupled pump power in the waveguide for co-propagating (open symbols) and counter-propagating (filled symbols) pump and probe beams.

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The Stokes frequency shift is 15.6 THz for silicon, and the peak of the stimulated Raman scattering for the 1430 nm pump beam is around 1544.9 nm, which is the wavelength at which we observe maximum gain for the signal. We compare the relative amplitudes of the eye diagrams for different signal wavelengths to obtain the on/off gain as a function of wavelength detuning for 123 mW coupled pump power in Fig. 5. For the signal detuned 2 nm from the Stokes wavelength, where there should be no Raman gain, the coupled pump power of 123 mW introduces 0.5 dB on/off loss which is due to TPA-induced FCA. The on/off gain as a function of wavelength has a Lorentzian profile of approximately 80 GHz linewidth (FWHM), which agrees with previous measurements [24,30]. In addition, the positive on/off gain is only observed within ~80 GHz from the first-order Stokes wavelength. Therefore, this method of amplification can be used for selectively amplifying signals in individual DWDM optical channels.

 

Fig. 5. Measured on/off gain as a function of signal wavelength with 123 mW coupled pump power.

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4. Conclusion

To our knowledge, we have demonstrated, for the first time, Raman amplification at 40 Gb/s data rate in silicon waveguides. We measure Raman amplification of an optical signal modulated at a data rate of 40 Gb/s and observe on/off gain of up to 2.3 dB with minimal signal distortion. The linewidth of the amplified spontaneous emission of approximately 80 GHz does not significantly deteriorate the observed eye diagrams at 40 Gb/s data rate but provides applicability for the selective amplification of individual optical channels.