Dual-polarization wavelength conversion of 16-QAM signals in a single silicon waveguide with lateral p-i-n diode [Invited]
1. INTRODUCTION
The stable growth of data traffic in current optical networks is pushing the need to provide higher throughput at a reduced cost per bit. All-optical signal processing (OSP) is a promising direction to achieve simultaneous processing of signals over a large optical bandwidth, thus potentially reducing the overall cost [1,2]. Among the several promising OSP functionalities that have been proposed, optical wavelength conversion is particularly interesting, both to improve network usability and to implement more complex operations [3]. The possibility to resolve wavelength contentions within a network without converting signals back to the electrical domain could significantly decrease the blocking probability as well as enable the processing of multiple wavelength-division multiplexed (WDM) channels simultaneously [3]. A favorable scaling in terms of cost and energy consumption may therefore be achievable [1]. Furthermore, several more advanced processing operations can be implemented starting from a basic wavelength converter. The phase-sensitive properties of four-wave mixing (FWM)-based wavelength conversion enables low-noise amplification [4,5], signal regeneration [68" target="_self" style="display: inline;">–
The potential advantages from all-optical wavelength converters have led to a strong research effort focused on nonlinear materials in order to evaluate which material platform may provide the most efficient wavelength converter. Currently, most of the demonstrations focusing on wavelength conversion in a state-of-the-art system scenario are based on highly nonlinear fibers (HNLFs) [10] or use cascaded second-order nonlinearity in periodically poled lithium niobate (PPLN) waveguides [5,11]. Whereas these media enable the processing of wavelength- and polarization-division multiplexed (PDM) signals currently deployed in coherent transmission systems, i.e., using quadrature amplitude modulation (QAM) signaling, the limited potential for integration has pushed the search towards materials based on silicon compounds which are more suited for photonics integration.
Promising results have been shown in crystalline [12,13] and amorphous silicon [14] as well as silicon-germanium [14], silicon nitride [15], AlGaAs-on-insulator [16], high-index doped glass [17], chalcogenide glass [18], and silicon–organic hybrids [19]. Among these, crystalline silicon is particularly promising as it benefits the most from the existing CMOS foundry fabrication of electronic circuits and can be easily integrated with high-speed electronics. A main challenge of using crystalline silicon for nonlinear application is caused by its material bandgap, which results in strong two-photon absorption (TPA) at telecom wavelengths (around 1550 nm) leading to free-carrier absorption (FCA). FCA depletes the pump power, thus strongly hindering the Kerr nonlinear interaction. To effectively reduce FCA and thus enable more efficient FWM, the use of a reverse-biased p-i-n diode fabricated across the silicon waveguide has been demonstrated [8,12,2022" target="_self" style="display: inline;">–
Even though the above-mentioned results are fundamental in paving the way toward practical wavelength converters, the polarization sensitivity of the devices limited most of the reported demonstrations to single-polarization applications. Being able to provide transparent processing of both polarizations of PDM signals is fundamental for applying wavelength conversion to existing optical systems. Along this direction, in Refs. [23,24], dual-polarization wavelength conversion has been demonstrated by using single-pump FWM with the pump polarization angled to balance the CE in TE and TM waveguide modes. While successful, this approach requires careful design of the waveguide to match dispersion and nonlinear properties of the waveguide such that the two orthogonal modes are not severely limited by the mode with the lowest CE. Alternatively, in Ref. [13], an on-chip polarization splitter and rotator was used to separate the two signal polarizations, process them independently in two silicon waveguides, and then recombine them in a polarization combiner. This approach relaxes the complexity in terms of dispersion engineering of the waveguide but requires additional components to be designed and fabricated.
In this work, we follow a different approach and demonstrate dual-polarization wavelength conversion in a single silicon waveguide optimized for single-polarization operation by employing an off-chip polarization diversity loop [25
The paper is structured as follows. In Section
2. DEVICE FABRICATION
The silicon waveguides have been fabricated in the SiGe-BiCMOS pilot line of IHP, and a sketch of the waveguide structure is shown in Fig.
Fig. 1. Sketches of (a) the silicon waveguide structure ( , , and ) including the p-i-n lateral diode ( ) and metal contacts and (b) the polarization-diversity loop setup based on the silicon waveguide highlighting CW (red arrows) and CCW (dark yellow arrows) propagation for signal (solid) and idler (dashed).
Results on devices with a similar cross section have already been reported in Refs. [8,20]. In Refs. [8,20], the slab height was 50 nm. The higher slab (100 nm) used in this work had the potential to improve the p-i-n diode efficiency [30] as well as relax the fabrication tolerances. Comparing the results reported here with Ref. [20], no relevant changes in achievable performance can be seen. The difference in slab height thus has a negligible impact.
3. POLARIZATION INSENSITIVE FOUR-WAVE MIXING
The polarization-insensitive setup demonstrated here is based on a standard-polarization diversity loop as commonly used when HNLFs are considered as the nonlinear medium of choice [25
The scheme is shown in Fig.
The conversion bandwidth of the system outlined in Fig.
Fig. 2. Experimental setup for conversion bandwidth measurements using the polarization-insensitive wavelength converter.
A weak continuous-wave signal from an external cavity laser (ECL) is polarization scrambled and coupled together with a strong continuous-wave pump generated by amplifying a second ECL in a high-power erbium-doped fiber amplifier (EDFA). Before combining signal and pump, out-of-band amplified spontaneous emission (ASE) noise from the pump EDFA is suppressed with a narrowband (0.8 nm full width at half-maximum) optical bandpass filter (OBPF). The two waves propagate through the circulator and enter the diversity loop of Fig.
Note that, whereas a silicon-waveguide-based polarization-diversity loop does not suffer from enhanced stimulated Brillouin scattering as do HNLF-based loops [28], care is required to minimize reflections at the fiber–chip interface. Using a polarization-diversity loop results in the reflection from the CW (CCW) waves at the input of the waveguide interfering with the CCW (CW) waves at the output of the waveguide. The impact of reflections rises mainly from the use of flat cleaved fibers, which result in approximately
The conversion bandwidth has been measured by varying the signal wavelength for a fixed pump wavelength of 1545.5 nm. To test the polarization dependence of the scheme, three measurement conditions have been considered: both signal and pump aligned to the CW propagation direction and the polarization scrambler switched off (CW case), both signal and pump aligned to the CCW propagation direction and the polarization scrambler switched off (CCW case), and signal polarization scrambled and pump polarization aligned such that the output idler power variations (measured at the OSA) are reduced below 0.5 dB (scrambled case). The different losses in CW and CCW paths and their fluctuations over time, mainly due to coupling drifts, are the main limiting factors to the achievable idler power variations in the scrambled case. Nevertheless, the power fluctuations could be kept below 0.5 dB for several minutes, thus enabling reliable measurements. Future packaging of the device with fiber pigtails is expected to further improve the stability.
The conversion bandwidths in the three cases are shown in Fig.
Fig. 3. (a) Conversion bandwidth as a function of the signal wavelength at constant power per waveguide input (22 dBm at grating coupler) and (b) diode current as a function of the reverse bias applied to the diode for constant combined power at both grating couplers.
Additionally, the similar values of CE for the scrambled and single-polarization cases hint that the p-i-n diode is effective in providing free-carrier removal even in the case of bi-directional propagation of the pump through the waveguide. This is confirmed by the measurements of Fig.
Comparing the curves for CCW and polarization-scrambled signal propagation (the pump is once again 45° polarization aligned), similar current levels are measured. The curves for CW propagation show slightly lower current levels that are consistent with the higher coupling loss for the CW side discussed for the CE of Fig.
Finally, the results in Fig.
4. SYSTEM SETUP
The system setup used to characterize the dual-polarization wavelength converter based on the diversity-loop scheme of Fig.
The wavelength converter has been optimized by measuring the performance of the central channel (channel 4) as a function of the input signal power. The pump power at the grating couplers has been set to approximately 22 dBm/direction. The BER and the received idler optical signal-to-noise ratio (OSNR) per channel (reference bandwidth of 12.5 GHz) are shown in Fig.
Fig. 5. (a) BER and received OSNR of channel 4 as a function of the signal power in input to the circulator and (b) input and output spectra for the WDM PDM wavelength conversion (resolution bandwidth of 0.1 nm).
The input and output spectra measured at port 1 and port 3 of the circulator are shown in Fig.
5. WAVELENGTH CONVERSION RESULTS
The quality of the generated idlers has been evaluated by measuring the BER as a function of the received OSNR, which is varied by adding ASE at the receiver input. The idler quality has then been benchmarked against the signal performance measured directly at the transmitter output. The BER curves as a function of the receiver OSNR, measured over the standard 12.5 GHz reference bandwidth, are shown in Fig.
Fig. 6. (a) BER as a function of the received OSNR/channel for signal and idler channels and (b) required receiver OSNR/channel and OSNR penalty for a (HD-FEC).
All the curves are within 1 dB of OSNR variations even though a 12 dB degradation in maximum received OSNR is caused by the wavelength converter. The back-to-back signal received OSNR of 29 dB/channel is decreased to approximately 23 dB/channel (31.3 dB total) at the output of the wavelength converter. Higher CE would enable decreasing such an OSNR loss. In this demonstration, the modest CE was limited by the available pump power and not by the power handling of the waveguide. Using the same waveguide design with the lateral p-i-n diode, CE values up to
The required receiver OSNR to reach the hard decision forward error correction (HD-FEC) threshold (
Finally, in Fig.
Fig. 7. Polarization dependence shown as average effective SNR difference between and polarizations for the signal and idler channels.
The low OSNR penalty and polarization dependence of the scheme show the potential of this wavelength converter for optical communication systems. Further improvements are expected if higher pump power levels than what was available for this work are available. Higher pump power enables further increasing the CE [20], thus decreasing the loss of OSNR during conversion. Finally, more optimized dispersion engineering of the waveguide (for the TE mode only) is expected to enhance the conversion bandwidth as partially demonstrated in Ref. [12].
6. CONCLUSIONS
Wavelength conversion of dual-polarization WDM 16-QAM signals is reported in a single silicon waveguide by using a polarization-diversity loop configuration. The high CE enabled by the lateral p-i-n diode and the low polarization dependence achievable with the scheme are key to provide high-quality idlers showing an OSNR penalty below 0.7 dB for all seven channels, and a polarization dependence below 0.5 dB.
Article Outline
Francesco Da Ros, Andrzej Gajda, Erik Liebig, Edson P. da Silva, Anna Pęczek, Peter D. Girouard, Andreas Mai, Klaus Petermann, Lars Zimmermann, Michael Galili, Leif K. Oxenløwe. Dual-polarization wavelength conversion of 16-QAM signals in a single silicon waveguide with lateral p-i-n diode [Invited][J]. Photonics Research, 2018, 6(5): 05000B23.