High-capacity free-space optical communications using wavelength- and mode-division-multiplexing in the mid-infrared region

Abstract

Due to its absorption properties in atmosphere, the mid-infrared (mid-IR) region has gained interest for its potential to provide high data capacity in free-space optical (FSO) communications. Here, we experimentally demonstrate wavelength-division-multiplexing (WDM) and mode-division-multiplexing (MDM) in a ~0.5 m mid-IR FSO link. We multiplex three ~3.4 μm wavelengths (3.396 μm, 3.397 μm, and 3.398 μm) on a single polarization, with each wavelength carrying two orbital-angular-momentum (OAM) beams. As each beam carries 50-Gbit/s quadrature-phase-shift-keying data, a total capacity of 300 Gbit/s is achieved. The WDM channels are generated and detected in the near-IR (C-band). They are converted to mid-IR and converted back to C-band through the difference frequency generation nonlinear processes. We estimate that the system penalties at a bit error rate near the forward error correction threshold include the following: (i) the wavelength conversions induce ~2 dB optical signal-to-noise ratio (OSNR) penalty, (ii) WDM induces ~1 dB OSNR penalty, and (iii) MDM induces ~0.5 dB OSNR penalty. These results show the potential of using multiplexing to achieve a ~30X increase in data capacity for a mid-IR FSO link.

Fig. 1. Concept for the mid-infrared (IR) wavelength-division-multiplexing (WDM) and orbital angular momentum (OAM)-based mode-division-multiplexing (MDM) free-space optical (FSO) communication system.

Mid-IR WDM signals are generated by wavelength converting C-band signals using the difference-frequency generation (DFG) process, and detected at the C-band after being converted via another DFG process. Mid-IR OAM beams are generated by passing mid-IR Gaussian beams through spiral phase plates (SPPs), and converted back to Gaussian beams using SPPs of inverse orders.

Fig. 2. Experimental setup of the free-space mid-infrared WDM and MDM communication system.

At the transmitter, C-band WDM signals are combined with a 1064 nm pump and coupled into a PPLN waveguide. The generated mid-IR beam is split into two paths and transmitted through different SPPs to generate two OAM beams. At the receiver, an SPP with an inverse OAM order is used to convert one of the OAM beams back to the fundamental Gaussian beam. The converted beam is combined with the 1064 nm pump and coupled into another PPLN waveguide. Finally, the generated C-band signal is detected and processed by a coherent receiver. PC: polarization controller, Col.: collimator, EDFA: erbium-doped fiber amplifier, YDFA: ytterbium-doped fiber amplifier, PPLN: periodically poled lithium niobate, M: mirror, SPP: spiral phase plate, HPF: high-pass filter, BPF: tunable band-pass filter, LO: local oscillator. VOA: variable optical attenuator, OSA: optical spectrum analyzer, DSO: digital storage oscilloscope, C: fiber-based optical coupler, BS: free-space beam splitter.

Fig. 3. The generation of mid-IR WDM signals through the difference-frequency generation process in the periodically poled lithium niobate (PPLN) waveguide.

a Spectrum of the generated mid-IR WDM signals with a resolution of ~1 nm. Arrows indicate the three mid-IR WDM channels. b Generated mid-IR beam power as a function of the 1064 nm pump power with different signal power values. The C-band signal wavelength is set at 1550 nm. c Generated mid-IR beam power as a function of PPLN temperature with different C-band signal wavelengths. d Generated mid-IR beam power as a function of C-band signal wavelength with a PPLN temperature of 49.5 °C.

Fig. 4. Demonstration of a three-channel mid-IR WDM FSO communication system with the Gaussian beam.

a Spectrum of the WDM signals that are converted back to the C-band. b Normalized optical crosstalk matrix of WDM. c Measured bit error rate (BER) as a function of the received optical signal-to-noise ratio (OSNR) for a C-band generation/detection (gen./det.) and mid-IR Gaussian beam transmission. In the C-band generation/detection case, C-band signals are detected by the coherent receiver without wavelength conversion and free-space propagation. d Measured BER as a function of the received OSNR for the three mid-IR WDM channels.

Fig. 5. Demonstration of a 300 Gbit/s mid-IR FSO communication system using WDM and a combination of WDM and MDM.

a Measured beam profile of the mid-IR OAM beams. Intensity profile and interferogram with a Gaussian beam of the OAM + 1 and OAM + 3 beam, respectively. Intensity profile of the data-carrying multiplexed OAM + 1 and +3 beams. Intensity profile of the multiplexed OAM beam after passing through the second SPP with OAM order −3. b Normalized crosstalk matrix of MDM. c Measured BER of the OAM + 3 channel as a function of the received OSNR for the mid-IR OAM beam transmission when sending both OAM modes and sending a single OAM mode. d Measured BER and OSNR of all the channels, including two OAM modes with three wavelengths on each mode.