Circular polarization shift-keying modulation based on orbital angular momentum division multiplexing in free space optical communication

Highlights

• Vortex beam is used for multiplexing circular polarization shift-keying signals.

• Reference beams convert the modulation signal to intensity changes for demodulation.

• Differential operation can improve the communication performance of the system.

• Phase modulation is compatible with this modulation system for complex modulation.

Abstract

Circular polarization shift-keying (CPolSK) modulation is crucial for optical communication in free space optical communication. In this paper, we propose a new optical communication system with multiple orbital angular momentum (OAM) modes for division multiplexing CPolSK modulation signals. A well-designed reference beam is employed to interfere with the CPolSK-OAM-division multiplexing (CPolSK-OAM-DM) signals for coherent detection. Then, the complex modulation signal is converted into intensity change directly detected by photo-detector. The communication performance can be further improved by subtracting the two interference beams detected. In numerical simulation, the demodulated bit error rate (BER) is 2.25 $\times$ 10⁻⁴ with the signal noise ratio (SNR) of 12-dB, and it can be reduced to 1.25 $\times$ 10⁻⁴ after difference operation. Through mapping the signals to both the initial phases and circular polarization states of the multiplexing channels, we construct the phase-CPolSK-OAM-DM system, the demodulated BER is 1.07 $\times$ 10⁻⁴ with the SNR of 15-dB. The results demonstrate that the CPolSK-OAM-DM modulation can effectively expand the communication capacity of the CPolSK system. The CPolSK-OAM-DM system has great potential in the field of high-speed broadband optical communications.

Introduction

Circular polarization shift-keying (CPolSK) modulation system maps binary digital signals to optical rotation states (left-handed and right-handed) of circularly polarized beam [1], [2]. Comparing the bit error rate (BER) performance of CPolSK modulation with other more straightforward binary modulation methods, one can find that the BER of CPolSK system is much less than that of on–off keying (OOK) system with the same signal to noise ratio (SNR) [3]. And the theoretical BER expression of CPolSK is consistent with that of binary phase shift keying (BPSK) [4], [5]. But due to the weak influence of the atmospheric channel on the polarization state, CPolSK is more suitable for free space optical (FSO) communication than BPSK [6], [7]. Moreover, the two advantages of “no need for polarization axis alignment” and “more uniform scattered light intensity distribution” make CPolSK modulation can obtain more ideal communication performance than linear polarization shift-keying (LPolSK) modulation [3]. Thus, the CPolSK modulation technology has great potential for optical information transmission in free space. However, the CPolSK modulation seems hampered for that circularly polarized beam contains only two states of rotation, the communication capacity needs to be significantly increased. Optical vortex (OV) beam carrying orbital angular momentum (OAM) provides additional degrees of freedom in free space optical (FSO) communication according to the researches [8], [9], [10], [11], [12]. The field expression of OV beam possesses an azimuthal-dependent phase structure of $exp\left(il\theta \right)$, where l represents the topological charge of the OAM mode, $\theta$ represents the azimuth angle [8], [12], [13], [14]. Theoretically, OV beams with different OAM modes have inherent orthogonality and the topological charge l can take any integer values, these paramount features make OAM a new dimension for modulation and multiplexing in optical communication [15], [16], [17], [18], [19], [20]. Therefore, OAM division multiplexing (OV beams with different topological charge l are taken as independent multiplexing channels) can significantly increase the communication capacity of the CPolSK system [21].

Recently, OAM division multiplexing has been adopted in FSO communication to increase the system capacity with the addition of OAM modes, the communication rate of 2.5-Gbit/s has been accomplished in the 2-OAM ($l=0$, 1) multiplexing system [17]. On this basis, the researchers constructed a FSO communication system that multiplexing two orthogonal polarization states and 4-OAM ($l=-8,-10,12,14$) modes and the communication rate achieves 1.37-Tbit/s [18], [22]. However, only linearly polarized beam has been involved in these researches, the combination of CPolSK and OAM has not been studied and the application of CPolSK modulation is still limited to binary digital modulation.

In order to resolve the separation of different OAM modes that used as multiplexing channels, two effective methods for demultiplexing are demonstrated as follow: interferometer method and diffraction method. Placing a Dove prism in each arm of the Mach–Zehnder interferometer to produce desired phase shift between the two interfering arms, the OV beams with different OAM modes can be split at the single-photon level by adjusting the angle between Dove prisms [17], [23], [24], [25], [26], [27]. And phase mask such as forked diffraction grating phase hologram also can be used to split the OV beam, it divides the OV beam into multiple beams transmitting at different angles and adds phase structure $exp(-il\theta )$ to convert the OV beam into Gaussian beam without ring-like structure [28], [29], [30], [31], [32]. After splitting the OV beams with different OAM modes, the binary signals mapped to the polarization states of the multiple circularly polarized vortex beams will be separately demodulated. However, the interferometer system requires extremely pinpoint accuracy and a large number of secondary subsystems when processing multiple OAM modes simultaneously. In addition, the pinhole filter is used to remove excess annular intensity distribution of OV beams mixed with the Gaussian beams, that filters out majority of the energy in the communication system, and symmetrical diffracted beams generally appear after passing through the phase mask, which will interfere with each other. Thus, these disadvantages limit the subsequent polarization demodulation of the CPolSK signals. How to extract the signals mapped on the CPolSK-OAM division multiplexing (CPolSK-OAM-DM) system simply and accurately is a great challenge to increase the communication capacity of high-speed CPolSK system.

In this paper, the CPolSK-OAM-DM system is proposed to map and encode signals to OAM modes and circular polarization states, and a reference beam is designed for demodulating the CPolSK-OAM-DM signals in the receiver. The quarter-wave plate (QW) and polarization beam splitters (PBS) are employed to identify and split two circular polarization states of the circularly polarized vortex beam, and then convert the polarization information into amplitude information, the split OV beam can be determined as an OAM shift-keying signal. A reference beam which is composed of several designed linearly polarized vortex beams is proposed to interfere with the split OV beam for coherent detection, and then the whole information mapped on the OAM modes and circular polarization states is converted into intensity changes in the receiver that can be directly detected. In numerical simulation, two random binary signals are respectively mapped to the circular polarization states (left-handed, right-handed) of the two OV beams with different topological charges ($+1$, $+3$) to generate CPolSK-OAM-DM signals. In the receiver, the reference beam is well-designed with the identical OAM modes of $+1$, $+3$ and amplitude weight coefficients of −1.5, 0. Upon 100 m transformation in free space, the BER of demodulated signal is 2.25 $\times$ 10⁻⁴ with the SNR of 12-dB, the BER can be reduced to 1.25 $\times$ 10⁻⁴ with the difference operation on the two split signals. Furthermore, the phase-CPolSK-OAM-DM system which maps the signals to both the initial phases and circular polarization states is also constructed, and the demodulated BER of that is 1.07 $\times$ 10⁻⁴ with the SNR of 15-dB.