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Based on four-wave mixing (FWM) with an optical comb source (OCS), we experimentally demonstrate 26-way or 15-way wavelength multicasting of 10-Gb/s differential phase-shift keying (DPSK) data in a highly-nonlinear fiber (HNLF) or a silicon waveguide, respectively. The OCS provides multiple spectrally equidistant pump waves leading to a multitude of FWM products after mixing with the signal. We achieve error-free operation with power penalties less than 5.7 dB for the HNLF and 4.2 dB for the silicon waveguide, respectively.
© 2017 Optical Society of America
1. Introduction
Wavelength multicasting enables efficient simultaneous delivery of a data stream from one single element to a group of destinations in an optical network [1–3]. Additionally, the parallelism afforded by wavelength multicasting makes this operation essential to a variety of ultrahigh-speed optical signal processing systems [4–8]. For example, demonstrated signal processing applications based on wavelength multicasting include a channelized radio-frequency receiver [6], an optical tapped delay line [7], and serial-to-parallel conversion [8]. A critical challenge for wavelength multicasting is to provide a multitude of high-quality channels while minimizing the needed resources. Various approaches to wavelength multicasting have been demonstrated through several nonlinear optical effects in diverse devices. In addition to the four-wave mixing (FWM) process used here, these approaches include cross-phase modulation (XPM) in a silicon nanowire [9], cross-gain modulation (XGM) in a semiconductor optical amplifier (SOA) [10], cross-absorption modulation (XAM) in an electroabsorption modulator (EAM) [11,12], and self-phase modulation (SPM) in a photonic-crystal fiber (PCF) [13].
FWM is a highly desirable mechanism for achieving wavelength multicasting as the generated FWM idlers inherently enable wavelength conversion of a signal. An early on-chip demonstration by Biberman et al. showed 16-way wavelength multicasting through degenerate FWM in a silicon waveguide by mixing an on-off keying (OOK) data-encoded partially-degenerate pump with an array of 16 continuous wave (CW) laser sources [14]. Each CW laser source mixed with the OOK encoded pump to provide an individual idler carrying the OOK data from the pump [14,15]. However, this approach requires a large number of individual laser sources and is not modulation format transparent due to the use of a data-carrying degenerate pump, which, for example, will non-linearly transfer phase modulation to the multicast signals. More recently, FWM generation of phase preserving replicas has been investigated through dual-pump FWM that uses two individual CW laser sources as the pumps and the data-carrying wave as a seed signal to obtain six multicast channels [16]. Adding more CW pump lasers allows this basic approach to generate a larger number of multicast channels [17]. However, in addition to being resource intensive, a non-uniform pump wavelength spacing is required to avoid noisy interference among the phase incoherent but spectrally coincident FWM idlers due to the phase incoherence of the array of free-running pump lasers [17,18].
Rather than using multiple independent lasers, comb sources generated using optical parametric processes efficiently provide a multitude of pump lines suitable for multicasting operations [19,20]. For example, more than 60 phase-preserved multicast channels can be achieved by using higher-order mixing processes in a dual-pump parametric mixer with compression/mixing stages [20]. In this existing approach, only two CW pump waves and a data-carrying signal wave are combined and injected into the compression/mixing stages leading to the efficient generation of numerous multicast channels through cascaded FWM. However, using this approach the multicast channels are interleaved with the replicas of pumps, and therefore this scheme requires a corresponding filter arrangement to reject the unused pumps. More importantly, this approach requires highly cascaded FWM of a wide-bandwidth data-carrying signal making it difficult to implement in, for example, chip-scale devices. While highly cascaded FWM has been demonstrated in microresonator-based devices [21], the narrow resonance bandwidth required to achieve sufficient resonant enhancement of the FWM process makes these devices unable to accommodate a high-speed signal in the interaction.
Here we demonstrate a wavelength multicasting approach that is both laser source efficient and amenable to chip-scale integration. Furthermore, the format transparency provided by this approach is crucial since modern optical communication systems are increasingly relying on advanced modulation formats (e.g. DPSK, QPSK) to increase the information capacity of the lightwave carrier. In this architecture, a FWM optical comb source (OCS) is mixed with a data-carrying signal in a separate FWM interaction from the comb generation process. In this second interaction, the comb lines serve as both degenerate and non-degenerate pumps leading to a multitude of spectrally equidistant multicast idlers. We demonstrate error-free (bit error rate, BER ≤ 10−9) 26-way and 15-way wavelength multicasting of 10-Gb/s differential phase-shift keying (DPSK) data through FWM in a highly-nonlinear fiber (HNLF) and a silicon waveguide, respectively. The majority of the 26/15 multicast channels possess a power penalty lower than 3 dB. The use of an OCS as the pump wave prevents noisy interference from the spectrally coincident FWM products. Furthermore, only two CW laser sources are used in the OCS representing a resource efficient and scalable approach to wavelength multicasting. Moreover, with the recent development of chip-based microresonator frequency comb sources [21] the approach validated here provides a path towards a compact entirely on-chip wavelength multicasting device for applications in optical communications networks and ultrahigh-speed optical signal processing.
2. FWM-based wavelength multicasting using multiple pump waves
For FWM-based wavelength multicasting, adding more pump waves yields a greater number of generated idlers. Here we use uniformly spaced lines from an OCS which serve as both degenerate and non-degenerate pump waves in mixing with an input signal to generate a multitude of idlers. These idlers represent the wavelength multicast replicas of the signal and the approximate number of replicas is given by the relation n = 4m – 3, where m is the number of pump waves (comb lines) used in the FWM processes and n is the number of generated multicast replicas. For many of these generated idlers multiple FWM pathways exist for their generation as depicted in Fig. 1. To illustrate this effect, we denote the frequencies of the three uniformly-spaced pump waves and the input signal wave shown in Fig. 1 as ω1, ω2, ω3, and ωs, respectively. Each newly generated wave via a FWM process can be expressed as ωijk = ωi + ωj – ωk, where i, j, and k indicate three waves participating in the FWM process. The corresponding spectral phase of the FWM generated wave is represented by φijk = φi + φj – φk, where φi, φj, and φk are the spectral phases of the three waves involved in the process. In this example case (m = 3), ten multicast data-carrying idlers (including the output signal at ωs and nine replicas) are generated.
Fig. 1 An illustration of spectrally coincident FWM products. Given three uniformly-spaced pumps (in green) and an input signal (in blue), nine multicast replicas (in red) carrying the original input signal are generated. Among these replicas, each of a, b, and c consists of multiple FWM products at a coincident frequency. For example, two FWM products, ω13s (dash) and ω22s (solid), both contribute to the replica c. If the pump waves are provided by independent free-running lasers, then the multiple FWM pathways will lead to noisy interference and thus poor multicast signal quality.
Importantly, each of the newly generated replicas a, b, and c results from spectrally coincident contributions from distinct FWM pathways. For example, the replica c is the result of the two pathways depicted in Fig. 1 with coincident frequencies at ω13s and ω22s. The spectral phases of these two components are φ13s = φ1 + φ3 – φs and φ22s = 2φ2 – φs, respectively. For ideal constructive interference these two spectral phases, φ13s and φ22s, should be equal indicating that a linear phase relationship amongst the pump waves φ1 + φ3 = 2φ2 is desired. Most critically, if the relative phase of the pump waves is not fixed, as is the case for independent CW pump lasers, the relative phase noise will lead to noisy interference from the multiple FWM pathways resulting in loss of the data quality in the multicasting process. Therefore, it is crucial that the pump waves possess a fixed (preferably linear) phase relationship as is the case for the OCS used here.
3. Experimental setup
Figure 2 depicts the multicasting architecture used here. A fiber-based FWM OCS provides pump waves with a fixed relative phase relationship. In this OCS two independent CW lasers (New Focus TLB-6728, linewidth < 200 kHz) with a wavelength spacing of 0.4 nm are combined by a 3-dB coupler, amplified by an erbium-doped fiber amplifier (EDFA), and sent into two cascaded FWM stages separated with a designed length of single-mode fiber (SMF) used for temporally compressing the generated pulses after the first FWM stage [20,22–24]. The lengths of the two segments of HNLF, which serve as the FWM stages, are 100 m and 160 m, respectively, and the SMF length is 400 m. To enhance the mixer bandwidth, a longitudinally varying tension plan [22,25] with 10-step stair-ramp distribution is applied to both HNLFs (OFS, with nonlinear coefficient of 11.3 W−1km−1, and a dispersion parameter of 1.93 ps/nm/km at 1550 nm). The total power at the output of the OCS is 480 mW. This process leads to the generation of a large number of equidistant spectral lines through cascaded FWM. We note that the use of independent CW lasers in our OCS results in an arbitrary and time-varying linear spectral phase across the OCS spectrum, which will not impact the FWM multicasting efficiency. However, it does result in increased phase noise on lines generated at the edges of the OCS spectrum, which can be transferred to the multicast replicas. Phase-locking the lasers can be implemented to reduce this effect [26]. The central six adjacent lines from the OCS are spectrally isolated by a tunable optical band-pass filter (OBPF) for use as pump waves for the multicasting stage. These pump waves are amplified by an EDFA, and then filtered by a 5.3-nm OBPF to remove the amplified spontaneous emission (ASE) noise introduced by the EDFA. The pump power injected into the HNLF and the silicon waveguide is 607 mW and 36.5 mW, respectively. The optical signal-to-noise ratio (OSNR) of pump is found to be 41.2 dB from its spectrum measured with a 0.05 nm resolution bandwidth. This OSNR is limited by the ASE noise of the EDFAs in the OCS and by the parametric fluorescence accompanying the FWM interaction in the OCS.
Fig. 2 Experimental setup of the proposed wavelength multicasting scheme using a fiber-based OCS. OCS: optical comb source, OBPF: optical band-pass filter, EDFA: erbium-doped fiber amplifier, ISO: optical isolator, CW LD: continuous-wave laser diode, PC: polarization controller, EOM: electro-optic modulator, HNLF: highly-nonlinear fiber, Silicon WG: silicon waveguide, VOA: variable optical attenuator, OSA: optical spectrum analyzer, DLI: delay interferometer, and BPD: balanced photodetector. For BER measurement the received power is monitored immediately at the 10% port of a 90/10 splitter and adjusted to report the power in the 90% arm.
The signal wave is produced by another CW laser at 1558.98 nm, which is modulated in a lithium niobate electro-optic modulator (EOM) with a 10-Gb/s DPSK signal encoded by a 231 – 1 pseudo-random binary sequence (PRBS). The data-carrying signal wave is amplified by an EDFA, filtered by a narrow-band OBPF, and then combined with the pump waves using a WDM coupler with a 9-nm passband. In the signal (lower) branch, the power at the output of the CW laser and the EOM is 1.5 mW and 0.7 mW, respectively, and the power after the signal EDFA is 150 mW for the HNLF and 260 mW for the silicon waveguide, respectively; the signal OSNR is measured to be 45.9 dB.
After being combined, the total power including pumps and signal into the HNLF and the silicon waveguide is 660 mW and 44 mW, respectively. Subsequently, these pump waves and the signal wave mix through FWM in either a 30-m long HNLF (OFS, with nonlinear coefficient of 11.5 W−1km−1, a dispersion parameter of 0.1 ps/nm/km at 1550 nm, and a zero dispersion wavelength of 1543 nm) or a 15-mm long silicon waveguide (described below) leading to the generation of a multitude of wavelength multicast replicas of the signal. The total power at the output of the multicasting stages is 550 mW for the HNLF and 2.4 mW for the silicon waveguide, respectively.
To validate the BER performance for each multicasting channel, an OBPF is used to reject the pump waves. All of the multicast channels are then amplified together and each individual channel is isolated by a narrow-band tunable OBPF. Finally, the selected individual channel is amplified by a 30-dB gain pre-amplifier, filtered with another narrow-band tunable OBPF, decoded by an asymmetric Mach-Zehnder delay interferometer (DLI) with 100-ps delay between the paths, detected by a balanced photodetector (BPD) and then sent into a bit-error-rate tester (BERT). The received power for BER measurement is monitored immediately at the 10% port of a 90/10 splitter and adjusted to report the power in the 90% arm.
To investigate the potential for chip-scale integration, a crystalline silicon (c-Si) waveguide is used as a FWM medium. The c-Si waveguide is fabricated using standard CMOS processes in a silicon-on-insulator (SOI) platform. The SOI wafer consists of 500 nm c-Si on top of 3 µm buried silicon dioxide (BOX). Thermal oxidation followed by hydrofluoric acid wet-etching of the oxide thins the top c-Si to 270 nm. Electron beam lithography and inductively coupled plasma (ICP) reactive ion etching are used to define the waveguide pattern. A thick (1 µm) silicon dioxide layer is deposited by plasma-enhance chemical vapor deposition (PECVD) for cladding and protection. The fabricated waveguide (270 nm by 600 nm) has low and anomalous group velocity dispersion, allowing for the wide-bandwidth FWM process [27,28]. The propagation loss of the c-Si waveguide is characterized by cut-back method to be ~3 dB/cm. Both ends of the waveguide have inversed taper structures to improve the coupling efficiency [29]. Fiber-to-waveguide coupling is achieved through a lensed fiber and lens-collimator assembly.
4. Experimental results
Figures 3(a)-3(c) show the optical spectra measured at the output of the OCS, HNLF, and silicon waveguide, respectively. As shown in Fig. 3(a), the uniformly-spaced spectral lines generated by the OCS are symmetric about the two CW seed laser sources with wavelength spacing of 0.4 nm. This wavelength spacing is chosen to correspond with the 50-GHz ITU grid. In both cases of the HNLF and the silicon waveguide, the wavelength of the signal wave carrying the testing 10-Gb/s DPSK data is λs = 1558.98 nm; the setup uses CW laser diodes set at λ1 = 1549.6 and λ2 = 1550 nm to seed the OCS. Using the six pump waves from the OCS, the HNLF and silicon waveguide output 26 and 15 multicast channels (including the channel at the input signal) transmitting DPSK data, respectively, with sufficient power for error-free operation. In both cases, all of the uniformly-spaced multicast channels are distributed over both sides of the OCS pump. As shown in Figs. 3(b) and 3(c), compared to the HNLF multicasting, the silicon waveguide solution possesses fewer error-free channels due to the coupling loss. The input and output coupling loss for the silicon waveguide is 13 dB and 8.1 dB, respectively. This can be improved by better design of the coupling taper and optimization of fabrication. In the case of the HNLF, the number of error-free multicast channels is limited by the gain bandwidth of the EDFAs after the FWM stage, which limits the quality of channels received below C1 and above C26. Thus, we anticipate that a greater number of multicast channels can be realized by improving the amplification as well as the FWM conversion efficiency in these regions.
Fig. 3 Measured optical spectra immediately after (a) the OCS. The six adjacent comb lines within the dashed region around 1550 nm are then isolated and used as pump sources to mix with a data-carrying signal for the multicasting experiment in (b) the 30-m long HNLF, and (c) the 15-mm long silicon waveguide. In both spectra of the HNLF and the silicon waveguide, multicast replicas are generated on both sides of the central comb pumps. Note: the wavelength spacing for all spectra is 0.4 nm. The resolution bandwidth of the optical spectra is 0.05 nm.
To illustrate the system performance, the BER of 10-Gb/s back-to-back (B2B) and multicast signals are shown in Figs. 4(a) and 4(b). Error-free (BER ≤ 10−9) operation is acquired for all of the multicast channels in both the HNLF and silicon waveguide. For some multicast channels (C18, C19 for the HNLF, and C10-C12 for the silicon waveguide), the BER measurement is unavailable due to the lack of proper filters in this region where a strong signal is present; however, error-free operation is expected to be achieved since these channels possess adequate OSNR due to higher conversion efficiency. As shown in Figs. 4(c) and 4(d), HNLF achieves error-free operation with a maximum power penalty of 5.7 dB relative to the B2B measurement, while the silicon waveguide scheme acquires a maximum 4.2-dB power penalty in the worst performing channel. For both cases, the majority of channels possess a power penalty lower than 3 dB indicating that the FWM process in the HNLF and silicon waveguide are of equivalent quality from a communications perspective.
Fig. 4 BER validation, record of power penalty at BER = 10−9, and conversion efficiency for the wavelength multicasting of 10-Gb/s DPSK data using (a), (c), (e) 30-m long HNLF, and (b), (d), (f) 15-mm long silicon waveguide. In (e) and (f), the input signal power is 52.5 mW for HNLF and 7.5 mW for silicon waveguide, respectively. Inset: eye diagram of the 10-Gb/s back-to-back DPSK data.
The center and edge channels with higher power penalties possess a reduced OSNR due to lower conversion efficiency. From Figs. 4(e) and 4(f), compared to the input signal power, the conversion efficiency of the error-free multicast copies ranges from −37.6 dB (C1) to −17.9 dB (C20) for the HNLF case, and from −43.6 dB (C2) to −32 dB (C10) for the silicon waveguide case, respectively. Here we define conversion efficiency as the ratio of the multicast channel power after the FWM interaction to the signal power at the start of the FWM interaction. Thus the propagation loss (4.5 dB) of the silicon waveguide is accounted for in the stated conversion efficiency and partially contributes to the lower observed conversion efficiency of the silicon waveguide system. Additionally, silicon waveguides exhibit nonlinear losses resulting from two-photon absorption and free-carrier absorption, which impact the achievable conversion efficiency. We note that our scheme achieves conversion efficiency comparable to that presented in [16,17] and that higher conversion efficiency can be expected in silicon waveguides incorporating free-carrier removal [30].
The relative conversion efficiency for each multicast channel is determined by both the power and phase of the pump comb lines that contribute to their generation. Therefore, the equalization of the multicast channels can be improved by adjusting either the relative power or phase of the comb lines. We note that it is preferable to control the flatness via the power profile of the comb lines since the ideal comb spectral phase is linear and yields maximum conversion efficiency. Since the channels at the edge of the multicast spectrum are generated from fewer FWM pathways, a pump comb power profile with a flat center and more powerful lines towards the edges is ideal. However, we expect the primary equalization impairment in our experimental demonstration comes from the uncompensated (quadratic, cubic, etc.) spectral phase in the pump comb lines and this can be improved by better dispersion management of the pump comb source.
Figure 5 shows the eye diagrams of some selected multicast channels carrying the DPSK data for both the HNLF and the silicon waveguide. For both nonlinear media, clear and open eye diagrams are achieved for all of the channels indicating that, as expected, this approach faithfully transfers the phase characteristics of the signal and allows for modulation format-transparent operation.
Fig. 5 Eye diagrams of 10-Gb/s DPSK data for select isolated multicast channels using (a) 30-m long HNLF and (b) 15-mm long silicon waveguide. All eye diagrams are taken at a BER of 10−9.
5. Conclusion
We experimentally demonstrate error-free 26-way or 15-way wavelength multicasting of 10-Gb/s DPSK data based on FWM with an OCS in a HNLF or a silicon waveguide, respectively. The OCS supplies multiple pump waves with uniform wavelength spacing to achieve FWM multicasting without noisy interference between the spectrally coincident FWM products. Our approach allows the phase information of the data-carrying signal to be preserved and replicated on all of the multicast channels. Furthermore, it linearly replicates both the amplitude and phase of the input signal onto the multicast channels and therefore can be extended to higher order modulation like QPSK or 16 QAM, provided sufficient OSNR is achieved. In the BER validation of DPSK multicasting presented here, error-free performance is achieved for all of the 26/15 channels with a typical power penalty of 3-dB in both the HNLF and silicon waveguide investigation. Promisingly, the approach validated here provides a path towards full integration of the multicasting system on-chip through the incorporation of a chip-based comb source and hybridly integrated lasers and amplifiers [21,31]. Such a system is highly desirable for future WDM communications applications and ultrahigh-speed optical signal processing.
Funding
National Science Foundation (NSF) (ECCS-1443936); Office of Naval Research (ONR) (N000141210730); Defense Advanced Research Projects Agency (DARPA) (N66001-12-1-4248).
Acknowledgments
The silicon devices were fabricated in part at the Center for Nanoscale Science and Technology’s NanoFab at the National Institute of Standards and Technology. Portions of this work were presented at the Conference on Lasers and Electro-Optics in 2014 (paper STu2I.5).
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