Polarization-Diversified, Multichannel Orbital Angular Momentum (OAM) Coherent Communication Link Demonstration using 2D-3D Hybrid Integrated Devices for Free-Space OAM Multiplexing and Demultiplexing

We present dual-polarization QPSK link transmission performance below the FEC limit with simultaneous transmission of three OAM states carrying 14×10-GBd WDM channels using silica 2D-3D hybrid integrated devices for OAM state multiplexing/demultiplexing capacity of 1.68 Tb/s. OCIS codes: (060.1660) Coherent communications; (050.4865) Optical vortices; (060.4230) Multiplexing Currently, space-division multiplexing (SDM) has generated great interest as a means to increase the spectral efficiency and communication capacity of free-space and telecommunications networks [1]. In particular, SDM based on orbital angular momentum (OAM) is attractive due to the natural orthogonality of the OAM states and the potential compatibility of the cylindrically symmetric beams with various types of multimode fiber (e.g., ring core, etc.). Light beams carrying OAM show an azimuthal phase variation of φ(r,φ) = exp(ilφ) where φ is the azimuthal angle, integer l is the charge number or OAM state [2], and φ(r,φ) is the phase of the wavefront. Multiple demonstrations of OAM links based on bulk-optic OAM beam generators and sorters have been previously reported [3]. However, practical implementations would greatly benefit from integrated, polarization-diversified OAM state multiplexers and demultiplexers. Our group [4] and others [5] have recently demonstrated OAM mux/demux devices based on 2D (planar) photonic integrated circuits in silicon that utilize grating structures that are polarization dependent. More recently, we reported the single-polarization performance of a OAM mux/demux device based on hybrid integration of a silica planar lightwave circuit (PLC) and a direct laser inscribed 3-D waveguide structure [6]. Here, we significantly extend that work and demonstrate a polarization diversified free-space OAM link with simultaneously three OAM states and 14 wavelength channels using two OAM hybrid devices as an OAM mux/demux pair. Each wavelength channel has 10-GBd, dual-polarization quadrature phase shift keying (DPQPSK) data for an aggregate capacity of 1.68 Tb/s and spectral efficiency of 9.6 bits/s/Hz. Fig. 1. (a) Concept of the OAM photonic integrated circuit. (b) Conceptual view of the 2D-3D hybrid integrated device for OAM mux/demux. (c) Normalized transmission of the OAM mux/demux pair for each polarization. (d) Experimental arrangement of the multichannel OAM polarization-diversified free-space communication link testbed. FPR: free-propagation region; ECL: external cavity laser; OFC: optical frequency comb; WSS: wavelength-selective switch; EDFA: erbium-doped fiber amplifier; ATT: attenuator; PC: polarization controller; PBC: polarization beam combiner; BPF: bandpass filter. EAWG: electrical arbitrary waveform generator. Fig. 1(a) shows a conceptual diagram of a photonic integrated circuit (PIC) for OAM state multiplexing/demultiplexing. The free-propagation region (FPR) applies a linear phase tilt on the input light. The phase-matched waveguides sample the output of the FPR and transform the linear phase tilt to azimuthal phase variations at the output apertures. Illuminating multiple inputs simultaneously generates OAM states that spatially overlap at the output apertures, providing true OAM state multiplexing. Alternatively, when operated in reverse, the circular-patterned apertures sample and demultiplex spatially overlapped OAM states [6]. Fig. 1(b) shows a drawing of the 2D-3D hybrid integrated device where the 2D silica PLC is mated with a 3D waveguide circuit fabricated by direct laser inscribing in borosilicate glass (further device details are in [6]). Fig. 1(c) shows the normalized transmission from OAM mux input (A) to OAM demux output (B) for both polarizations. The total loss for OAM state 0 is 19.4 dB (device TE) and 19.3 dB (device TM). Fig. 1(d) shows the experimental arrangement where a 10-GBd QPSK signal (2-1 PRBS) is applied to all 14 WDM channels (12.5-GHz spacing). The modulated signal is polarization multiplexed with a decorrelation of 500-ns between the two polarization states. After multiplexing up to three OAM states through the first hybrid device, the spatially overlapped beams are transmitted over a 81.8 cm free-space link, demultiplexed by the second hybrid device and sent to the polarization diversified coherent receiver with offline DSP. The DSP uses an adaptive equalizer, which includes a 17-tap finite impulse response (FIR) filter and power-of-four methods for phase and frequency recovery and direct-decision based bit-error counting. Fig. 2. (a) BER performance for the multichannel OAM link with 14×10-GBd, DP-QPSK WDM channels. (b) Averaged BER for three OAM states (0, -3 and -6) for two received orthogonal polarizations (x-pol and y-pol not necessarily the same as device TE and TM). The averaged bit-error-rate (BER) of two polarization states for a multichannel free space transmission link was measured. We use OAM states −6, −3 and 0 for the transmission link experiment. The measured crosstalk values at all three OAM states outputs are between −10.5 dB and −11.6 dB for TE, between −10.2 dB and −14.1 dB for TM. Fig. 2(a) shows the measured BER for the following cases: i) back to back transmission without WDM or OAM state crosstalk (XT); ii) back to back transmission with all the wavelength channels; iii) single OAM state and single wavelength channel transmission without crosstalk from other OAM states; iv) all three OAM states transmission with a single wavelength channel; v) single OAM state with all the wavelength channels but without crosstalk from other OAM state; vi) all three OAM states transmission with all the wavelength channels. There is ~3-dB OSNR penalty for OAM state 0 and −3 transmission with OAM states crosstalk. However, the OSNR penalty for OAM state −6 is much higher due to a joint effect of crosstalk and polarization dependent loss (PDL). In the future, more precise phase error correction can further reduce the crosstalk. PDL is mainly from the submicron misalignment in the device setup. Fig. 2(b) shows that the measured averaged BER values with three OAM states and all wavelength channels are below HD-FEC limit of 3.8×10 (BCH(1020,988) super FEC code, 7% overhead) [7]. In summary, we have demonstrated a multichannel OAM free-space coherent communication link using two separate photonic hybrid devices with polarization diversity. We transmitted 14×10 GBd DP-QPSK signals simultaneously on three OAM states that shows the averaged BER of both polarization states for all three OAM states at all wavelength channels are below HD-FEC limit. Future work includes device packaging to improve device stability and device design changes to reduce crosstalk for additional OAM state multiplexing. [1] P. J. Winzer, IEEE Photon. J., 4, 647-651 (2012). [2] L. Allen, et al., Phys. Rev. A, 45, 8185-8189 (1992). [3] J. Wang, et al., Nature Photon., 6, 488-496 (2012). [4] T. Su, et al., Opt. Express, 20, 9396-9402 (2012). [5] N. K. Fontaine, et al., in OFC/NFOEC, (2012). [6] B. Guan, et al., Opt. Express, 22, 145-156 (2014). [7] ITU-T, in G.975.1 : Forward error correction for high bit-rate DWDM submarine systems, Appendix I.9, (2004). This work was supported in part by DARPA DSO under the contracts HR0011-11-1-0005 and W911NF-12-1-0311.