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Optical Engineering

Design of 90-deg hybrid based on birefringent crystal for coherent optical communication system

Jiali Wu, Xizheng Ke, Deqiang Ding, Shangjun Yang

Published Apr 23, 2021• 0 citations
Publisher

Abstract

Based on the birefraction of crystals and phase retardation of wave plate, a 2 × 4 90-deg crystal spatial optical hybrid is designed. It is comprised of three birefringent optical plates, λ / 4 wave plate and two λ / 2 wave plates. By splitting and recombining the signal beam and local oscillator beam, four mixed beams with phase shift of 90 deg are obtained. The mathematical model of the mixer is established by Jones matrices. The results show that the hybrid has low insertion loss, and by rotating the wave plate, the phase difference and split ratio between in-phase and quadrature-phase can be changed. It proved that the crystal hybrid design method we proposed has good performance; the phase compensation and splitting ratio adjustment scheme is simple, effective, and feasible; and it can be applied to free-space coherent optical communication system well.

DOI
10.1117/1.oe.60.4.045106

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Based on the birefraction of crystals and phase retardation of wave plate, a 2 × 4 90-deg crystal spatial optical hybrid is designed. It is comprised of three birefringent optical plates, λ / 4 wave plate and two λ / 2 wave plates. By splitting and recombining the signal beam and local oscillator beam, four mixed beams with phase shift of 90 deg are obtained. The mathematical model of the mixer is established by Jones matrices. The results show that the hybrid has low insertion loss, and by rotating the wave plate, the phase difference and split ratio between in-phase and quadrature-phase can be changed. It proved that the crystal hybrid design method we proposed has good performance; the phase compensation and splitting ratio adjustment scheme is simple, effective, and feasible; and it can be applied to free-space coherent optical communication system well.
Abstract We establish a mathematical model of a crystal spatial light mixer based on the principle of crystal birefringence and splitting. The spatial light mixer consists of two crystals, three quarter-wave plates, four half-wave plates, two planar mirrors and two polarization beam splitting prisms (PBS). After the signal light and the local oscillator light pass through the spatial optical mixer, four channels of coherent light with 90° relative phase differences are output. According to the established mathematical model of spatial optical mixers, the fabrication error of components, the monochromaticity of the light source, the angle between the PBS and the light beam are simulated and analyzed. Error simulation results show that the phase noise is 4.14° when the crystal processing error is 10 nm and the refractive index error is 0.172 when the angle between the beam and the PBS is 0.0183°. The maximum phase noise that can be generated is 2.278°. The phase error of a new spatial optical mixer evolved from the mathematical model satisfies the allowable range of the Costas phase-locked loop error. It can be used in a space coherent laser communication system to improve its detection sensitivity.
In this paper, a multiplexing technique of coherent polarization states based on three-dimensional rotation group is proposed. The technique applies two Stokes parameters in one polarized light as signal and local oscillator (LO), and combines them in the single spatial mode. Three-dimensional rotation group SO(3) is used to describe the transformation of Stokes parameters. SO(3) is decomposed into three subgroups, with each subgroup represents a rotation along different axes. A multiplexer is designed with three electro-optical crystals to realize the function of three-dimensional rotation group. The multiplexer achieves an ergodicity of polarization states. Signal and LO propagates in the same spatial mode, multiplexing of polarization states is realized. Finally, the multiplexer is tested in a free space quantum communication system and its feasibility in quantum communication is confirmed.
Two-wave mixing in photorefractive crystals has been shown to combine coherently a modulated optical signal beam with a strong pump optical field.’ For a phase-modulated signal beam, the induced index of refraction phase grating inside the crystal yields a coupling of the optical fields of the pump and signal so that subsequent intensity detection results in apparent intensity modulation of the transmitted signal or pump beams. This provides an easy way to detect phase differences between two optical beams. Some of the advantages the above scheme offers over commonly used coherent optical phase detection schemes are: no loss of 3 dB in the signal beam intensity at the 50/50 beam splitter (used to combine signal and local oscillator fields); automatic alignment of the index of refraction grating with the two incident optical wavefronts; insensitivity to slow random changes in the phases of both beams. Experimental results are given, and a new definition for the resulting mixing gain for the phase-modulated beam is introduced, through which optimal conditions for input beam intensity ratios can be found. Applications to coherent homodyne detection are discussed.
A new mode of coherent optical communication for QKD was proposed in this paper， there are two quadrature Stokes parameters, one is used as observed physical quantity, the other as the carrier wave. The EOM of 45 of azimuth is changed by continuously rotating the HWP, which indirectly complete the linear modulation of 2 S and 3 S parameters. The receiver adopts the Q-Q-H wave plate rotation mode to select the measured 2 S or 3 S . The detection system is built up in combination with LabVIEW to implement the signal demodulation. The results shown that the scheme is feasible.
We propose and experimentally study a coherent optical secure transmission system based on one dual-polarization in-phase and quadrature modulator (IQM). One beam of the polarized light is used to generate broadband chaos by configuring a nonlinear opto-electronic oscillator while the other beam carries the encrypted signal. The encrypted signal is obtained through sequential encryption of the analog and digital chaos. The mutual mask of the hybrid chaotic signals can effectively enhance the security performance. Moreover, by varying the encryption depth of analog and digital vectors, the transmission performance can be flexibly adjusted. A commercial dual-polarization IQM could simultaneously generate a chaotic signal and a load message, which provides a high-integration solution. A fast independent component analysis (ICA) algorithm is adopted to compensate for the rotation of state of polarization (RSOP). 60 Gb/s encrypted quadrature phase shift keying (QPSK) signal transmission over 100 km single-mode fiber is realized, and the decrypted bit error rate (BER) performance is below the 7% forward error correction (FEC) threshold (BER = 3.8 × 10 −3 ).
Based on the cascaded four wave mixing processes, a coherent-feedback control system is constructed by utilizing a linear beam splitter as the feedback controller. Considering the loss of optical propagation in the coherent feedback loop and the absorption effect of Rb vapor cells to beams, we theoretically investigate the entanglement properties of this system under different feedback ratio, gain and phase by calculating the covariance matrix of system and applying the positivity under partial transpose (PPT) criterion to all possible bipartitions. The result shows that the genuine tripartite entanglement exists in the coherent feedback control system, but the entanglement structure of system will be destroyed by the excessive feedback. In addition, when the phase is π, we find that the tripartite entanglement can be enhanced by changing the gains and the reflectivity of the beam splitter in the range of 0.1 to 0.4. The results pave the way for manipulating multipartite entanglement by coherent feedback control and have potential application in quantum communication.