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We fabricate a nonlinear optical device based on graphene-silicon microring resonator (GSMR). Using such graphene-assisted nonlinear optical device, we experimentally demonstrate up and down wavelength conversion of a 10-Gbaud quadrature phase-shift keying (QPSK) signal by exploiting degenerate four-wave mixing (FWM) progress in the fabricated GSMR. We study the conversion efficiency as a function of the pump power. In addition, the resonant wavelength of GSMR is tuned by changing the temperature from 20°C to 40°C. We evaluate the bit-error rate (BER) performance for up and down wavelength conversion. The observed optical signal-to-noise ratio (OSNR) penalties for QPSK up and down wavelength conversion are less than 1.4 dB at a BER of 1 × 10−3. The BER performance as a function of the pump power for up wavelength conversion is also assessed. The minimum OSNR penalty is less than 0.8 dB when the pump power is 13.3 dBm.
© 2016 Optical Society of America
1. Introduction
Graphene [1], a single sheet of carbon atoms in a hexagonal lattice, has attracted a high level of research interest because of its linear, massless band structure E ± (p) = V|p|, where the sign corresponds to electron (respectively, hole) band, p is the quasi-momentum, and V≈106 m/s is the Fermi velocity. The unique band structure allows graphene an ideal material platform for broadband electro-absorption modulators [2], ultrafast photodetectors [3], broadband polarizers [4], as well as saturable absorption for mode-locking [5]. Graphene has been suggested as a material that might have large χ(3) nonlinearities [6], which is due to its linear band structure allowing interband optical transitions at all photon energies. After that, optical bistability, self-induced regenerative oscillations and four-wave mixing (FWM) have been consecutively observed in graphene-silicon hybrid optoelectronic devices [7].
Recently, all-optical signal processing is considered to be a promising technology for future optical transparent networks for its strong abilities to overcoming the electronics bottlenecks, supporting the ultra-fast optical signal processing without requiring costly optical-electrical-optical equipment. High-speed all-optical signal processing functions, including wavelength conversion, signal regeneration, logic gate, format conversion, and so on, would play a crucial role in achieving flexible and low-latency management of network data traffic. Specifically, wavelength conversion is of great importance for its ability to enhance the flexibility and optimize wavelength usage of future optical network. All-optical wavelength conversions have been widely studied by many previous works based on semiconductor optical amplifiers (SOAs), highly nonlinear fibers (HNLFs), and periodically poled lithium niobate (PPLN) waveguides. Silicon-on-insulator (SOI) waveguides feature low cost, ultra-compact footprint and complementary metal-oxide-semiconductor (CMOS) compatibility [8, 9 ]. Also, the tight light confinement of silicon waveguides will greatly enhance the nonlinear effects and thus reduce the required optical power. To further enhance the nonlinear interaction in silicon waveguide devices, resonator structures such as microrings, microdisks, and photonic crystal nanocacities can be introduced. Among these integrated resonant structures, microring resonator has been attractive to researchers in the last several years mainly because of its small size and potential for telecom and datacom applications. Microring resonators have accelerated the demonstration of very low power continuous-wave (CW) nonlinear optics, and similar benefits are expected for its operation in processing high bandwidth optical signal. For applications to optical data signal processing, the main challenge is that the bandwidth of the microring must be large enough to contain all the spectral components of the optical signals. Actually, it is only very recently that the first demonstration of optical signal processing based a resonant cavity has been reported. Wavelength conversion at 2.5 Gb/s in a single microring [10] and 10 Gb/s in a silicon cascaded microring resonator [11] has been demonstrated. On the other hand, it is well known that advanced optical modulation formats have become of great importance to enable high-capacity optically routed transport networks and design of modern wavelength-division multiplexed (WDM) fiber systems [12]. However, all-optical wavelength conversion of advanced optical modulation formats have not been realized in integrated ring structures. Also, it would be valuable to combine silicon microring with graphene to further enhance the nonlinear interactions. Very recently, the FWM enhancement effect was demonstrated in a compact graphene-silicon microring resonator (GSMR) [13]. A maximum enhancement of 6.8 dB of conversion efficiency in the GSMR was observed.
In this work, combining the high nonlinearity of GSMR with advanced optical modulation format, we comprehensively study the system performance of FWM based wavelength conversion of a 10-Gbaud QPSK signal. This is the first demonstration of wavelength conversion of advanced optical modulation formats in integrated resonant structure, to the best of our knowledge. We study the conversion efficiency as a function of pump power and the tunability of resonant wavelength of GSMR by changing temperature. Moreover, we also characterize the performance of QPSK wavelength conversion by measuring the bit-error rate (BER) as a function of the received optical signal-to-noise ratio (OSNR).
2. Fabrication and characterization of GSMR
Figure 1(a) illustrates the Raman spectrum of the transferred graphene on the silicon microring resonator [13]. Compared to pristine graphene, the blue shifts of the positions of the G and 2D peaks are consistent to the nature of the p-doped graphene [14]. Additionally, the intensity ratio of the 2D to the G peak is about 1.8, significantly smaller than that of the pristine graphene (4~5), which is another evidence of the p-doped graphene [14]. The heavily p-doped graphene is particularly fabricated to achieve optical transparency in the infrared with negligible linear losses, which can be explained as follows: due to the p-doping of graphene, the Fermi level (EF) is lower than half the photon energy (-νℎ/2, blue dashed line) and there are no electrons available for the interband transition [2] (Fig. 1(b)) and intraband graphene absorption is near-absent in the infrared [15]. The device consists of a silicon microring resonator coupled to a straight waveguide with a gap of 150 nm. The waveguide is bidirectional tapered up to a width of 20 μm over a length of 600 μm to connect dual TE-polarized grating couplers. A scanning electron micrograph (SEM) of the fabricated GSMR resonator with partial part of the straight waveguide is shown in Fig. 1(c). It is fabricated by standard complementary metal-oxide-semiconductor (CMOS) processes on a SOI substrate with a 3 μm-thick buried oxide layer. The width and height of the ridge waveguide in the structure are 450 and 200 nm, respectively. The total insertion loss is about 10 dB at input wavelength of 1550 nm. Here the input power is defined as the power in the straight waveguide coupled into the silicon microring resonator. We use standard polymer-based transfer method to cover a graphene sample on the top of the silicon waveguide and the detailed picture of the straight waveguide coupled with an arc region of the microring resonator is shown in the inset of Fig. 1(c). Due to the deposition of polymer on the GSMR, the gap is covered and the silicon waveguides before transfer are marked with red dashed lines.
Fig. 1 (a) Raman spectrum of the graphene sample. (b) The Fermi level (EF) is lower than half the photon energy (-νℎ/2, blue dashed line) and there are no electrons available for the interband transition. (c) SEM image of the GSGM resonator. The inset shows the detailed view of the coupling region. Red dashed lines show the outline of the silicon waveguides.
3. Concept and working principle
Figure 2 illustrate the up and down wavelength conversion processes based on degenerate FWM in GSMR (up: low frequency to high frequency; down: high frequency to low frequency). The continuous-wave (CW) pump light and the signal light carrying QPSK data are fed into the GSMR. When propagating along the GSMR, pump photons are annihilated to create signal photons and newly converted idler photons by the degenerate FWM process. At the output of the GSMR, data information carried by the input signal is converted to the idler for up and down wavelength conversions.
4. Experimental setup
Figure 3 shows the experimental setup for degenerate FWM based up and down wavelength conversion using a GSMR. The CW output from an external cavity laser 1 (ECL1) serves as the signal light for the degenerate FWM and is modulated with QPSK signal at 10 Gbaud by a single-polarization optical I/Q modulator. An arbitrary waveform generator (AWG) is used to produce the electrical signal to drive the optical I/Q modulator. The modulated 10-Gbaud QPSK signal is then amplified by an erbium-doped optical fiber amplifier 1 (EDFA1) followed by a tunable filter 1(TF1) to suppress the amplified spontaneous emission (ASE) noise. Another CW light from ECL2 serving as the pump light is amplified by EDFA2 followed by TF2. Afterwards, the 10-Gbaud QPSK signal is combined with the pump light through a 3-dB coupler and launched into the GSMR. The polarization states of the QPSK signal and CW pump are adjusted to achieve optimized conversion efficiency of degenerate FWM in GSMR. The amplified QPSK signal and CW pump take part in the degenerate FWM process when passing through the GSMR and a newly converted idler is generated. After the FWM up/down wavelength conversion, the converted idler is selected using two tunable filters (TF3, TF4) for coherent detection. First, the converted idler is selected using TF3. Since the power level of the converted idler is relatively low, the selected converted idler is amplified by EDFA3. Second, in order to suppress the ASE noise originated from EDFA3, another TF4 is employed. Hence, the TF3 is used to select the converted idler, and TF4 is used to suppress the ASE noise. The CW output from ECL3 serves as a reference light (λidler_Ref.) for coherent detection. A variable optical attenuator (VOA) and a low noise EDFA (EDFA4) are employed to adjust the received OSNR for BER measurements. The optical spectra at different taps in the experimental setup are monitored by use of an optical spectrum analyzer (OSA) (YOKOGAWA-AQ6370C).
Fig. 3 Experimental setup for degenerate FWM based up and down wavelength conversion in GSMR. ECL: external cavity laser; AWG: arbitrary waveform generator; EDFA: erbium-doped optical fiber amplifier; TF: tunable filter; OC: optical coupler; PC: polarization controller; OSA: optical spectrum analyzer; VOA: variable optical attenuator.
5. Experimental results
In the experiment, the radius of the silicon microring resonator is 10 μm, and the corresponding free spectral range is around 10 nm. The grating coupler exhibits a 50-nm coupling range with 3-dB coupling loss and the central wavelength of the grating is 1550 nm. Based on these characterizations of the fabricated GSMR, two neighboring resonant wavelengths of 1548 and 1558 nm are chosen as the pump and signal light for up wavelength conversion, and the converted idler wavelength is around 1538 nm. Similarly, for down wavelength conversion, the signal and pump light wavelengths are chosen as 1538 nm and 1548 nm, and the converted idler wavelength is around 1558 nm. Figures 4(a) and 4(b) show typical output degenerate FWM spectra obtained after the GSMR for up and down wavelength conversions of QPSK signal.
We define the conversion efficiency as the power ratio of converted idler to signal. Experimentally measured and fitted conversion efficiency as a function of input pump power for up and down wavelength conversion are plotted in Figs. 5(a) and 5(b) , respectively. The nonlinear Kerr coefficient increment caused by graphene is responsible for the enhanced FWM in the GSMR [13]. One can clearly see that the conversion efficiency increases with the pump power. The saturation of the conversion efficiency at relatively high pump power level results from the two-photon absorption and free carrier absorption in silicon. The device can be tuned by thermo-optic effect. The resonant wavelength of GSMR as a function of temperature is depicted in Fig. 6 . The resonant wavelength can be linearly tuned from 1556.80 to 1559.11 nm when the temperature changes from 20°C to 40°C. With future improvement, the tuning range of the device might be also remarkably increased by using micro-heater structures for temperature tuning [16, 17 ].
Fig. 5 Experimentally measured and fitted conversion efficiency versus input pump power for (a) up and (b) down wavelength conversion.
To further characterize the performance of QPSK wavelength conversion, we measure the BER curves as a function of the received OSNR for back-to-back signals and up/down converted idler. Figure 7 plots measured BER performance for QPSK wavelength conversion with the converted idlers generated at 1538.64 nm (up conversion) and 1558.15 nm (down conversion), respectively. The output power of EDFA1 and EDFA2 are set to be around 25.1 and 25.3 dBm, respectively. The measured conversion efficiencies for converted idlers at 1538.64 nm (up conversion) and 1558.15 nm (down conversion) are −38.34 and −40.2 dB, respectively. The observed OSNR penalties for QPSK up and down wavelength conversion are less than 1.4 dB at a BER of 1 × 10−3 (7% forward error correction (FEC) threshold). The insets of Fig. 7 depict corresponding constellations of the back-to-back signals and converted idlers. We also evaluate the BER performance for up wavelength converted idler when the pump power increases from 9.3 dBm to 15.3 dBm. As shown in Fig. 8 , the minimum penalty is less than 0.8 dB when the pump power is 13.3 dBm. The OSNR penalty is around 2 dB with pump power 9.3 dBm. The obtained results shown in Figs. 4-8 imply favorable performance achieved for up and down wavelength conversion of QPSK signal using the fabricated GSMR.
Fig. 7 Measured BER versus received OSNR for up and down wavelength conversion of QPSK signal. Insets show constellations of QPSK.
Fig. 8 Measured BER versus received OSNR for up wavelength conversion of QPSK signal when pump power increases from 9.3 to 13.3 dBm.
6. Discussions
It can be seen from Fig. 5 that the conversion efficiency of the GSMR is still relatively low. The relatively low conversion efficiency of the device might be explained as follows.
- 1) In the FWM process, if the pump is located around the zero-dispersion wavelength (ZDW) of the nonlinear medium, the phase matching condition can be satisfied and a maximum conversion efficiency can be achieved. However, the waveguide dispersion in this work is optimized.
- 2) The waveguide loss is estimated to be ~6 dB/cm, which is still relatively larger than the devices fabricated in CMOS foundaries.
- 3) The nonlinear loss induced by two-photon absorption and free carrier absorption in silicon might also degrade the FWM process at high power region.
- 4) In the experimental, fundamental quasi-TE mode of the silicon-graphene hybrid waveguide is employed. The interaction of light with graphene is not maximumly enhanced.
With future improvement, the conversion efficiency might be increased by adopting the following methods.
- 1) Optimizing the waveguide dispersion to achieve broadband high efficiency FWM.
- 2) Optimizing the fabrication condition of the waveguide to reduce the linear loss mainly caused by side wall scattering.
- 3) Active removing of the generated free carriers by incorporating reverse biased p-i-n junctions in the waveguide [18].
- 4) Tuning the input signal to TM-polarization to provide more evanescent light interaction with graphene and enhance the conversion efficiency [19].
- 5) Combining graphene with slow-light silicon photonic crystal waveguides to improve evanescent light interaction with graphene and enhance the conversion efficiency [19].
Additionally, FWM response is also dependent on the different graphene layer. Previous work has demonstrated that the nonlinear response is sensitive to the number of graphene layers [20]. It is expected that for a few graphene layers the nonlinearity increases in proportion to the number of layers. Thus it is possible to further enhance the FWM response generated from different graphene layer by appropriately increasing the number of graphene layers employed in the experiment.
7. Conclusions
In summary, we fabricate a GSMR and observe the FWM nonlinear effect in the GSMR. The degenerate FWM based up and down wavelength conversions of advanced modulation format signal (e.g. QPSK) are further demonstrated in the experiment. The resonant wavelength of the fabricated GSMR can be flexibly tuned simply by changing temperature. Moreover, we evaluate the BER performance for up and down wavelength converted idlers. The observed OSNR penalties for QPSK up and down wavelength conversion are less than 1.4 dB at a BER of 1 × 10−3. The BER performance as a function of the pump power for up wavelength conversion of QPSK signal is also studied. The minimum OSNR penalty is less than 0.8 dB when the pump power is 13.3 dBm. The obtained results show favorable performance of QPSK up and down wavelength conversion using degenerate FWM in the fabricated GSMR. It is expected that the GSMR might find more interesting optical signal processing applications.
Acknowledgment.
This work was supported by the National Natural Science Foundation of China (NSFC) under grants 61222502, 11574001, 11274131 and 61077051, the Program for New Century Excellent Talents in University (NCET-11-0182), the Wuhan Science and Technology Plan Project under grant 2014070404010201, and the seed project of Wuhan National Laboratory for Optoelectronics (WNLO). The authors thank the engineer in the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for the support in the fabrication of graphene-assisted nonlinear optical device (GSMR) and the facility support of the Center for Nanoscale Characterization and Devices of WNLO.
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