Adaptive Compensation for Angular Deflection of Beams Carrying Orbital Angular Momentum on Mode Sorting

Beams carrying orbital angular momentum (OAM) has attracted attention as a potential candidate to expand the transmission capacity in the optical communication. Mode sorting is an efficient method to demultiplex the OAM multiplexed lightwaves. We have reported the mode sorting performance for OAM beams suffering lateral displacement and angular deflection. It was found that the angular deflection was severe, namely, only 100-μrad deflection deteriorated the performance in terms of crosstalk. Thus, the strict angular alignment would be required in the free-space optical communication using OAM multiplexing. In this paper, we propose an adaptive compensation method based on detection of the angular deflection of beams carrying OAM on mode sorting in order to relax such a strict requirement. Numerical simulation results reveal that the compensation in every 100 μrad effectively suppresses the crosstalk caused by the angular deflection

Analysis of optical OAM mode conversion using elastic vortex wave in graded-index optical fiber

We theoretically analyze acousto-optic (AO) mode conversion between optical orbital angular momentum (OAM) modes using an elastic vortex wave (EVW) carrying OAM in a squared graded index (GI) fiber. The AO mode conversion from the fundamental mode to the higher order optical OAM mode is a useful technology to generate optical OAM modes in the GI fiber. This paper clarify the contribution of each component of the dielectric constant perturbation caused by the EVW to the AO mode conversion

All-Optical Modulation Format Conversion Between OOK, QPSK and 8QAM

Modulation format conversion is useful when the signal is transmitted through networks employing different modulation formats. It is also expected to increase the spectral efficiency and reduce power consumption in network nodes by applying all-optical signal processing to the modulation format conversion. In this paper, we propose an all-optical modulation format conversion system between on-off keying (OOK), quadrature phase shift keying (QPSK) and 8-ary quadrature amplitude modulation (8QAM) using optical nonlinear effects in semiconductor optical amplifier (SOA) and optical threshold device. We also evaluate the conversion performances of the system by numerical simulation in terms of optical signal-to-noise ratio, optical signal power, and accumulated chromatic dispersion

Light masking of the circatidal activity rhythm in the mangrove cricket Apteronemobius asahinai

Apteronemobius asahinai shows two rhythms in its locomotor activity. One is the circatidal rhythm, which produces active and inactive phases, and the other is the circadian rhythm, which suppresses the intensity of the activity of the circatidal active phase during the subjective day. In the present study, to reveal whether light suppresses locomotor activity by masking the circatidal rhythm without interfering with the circadian rhythm, we observed the activity under 23-h light–dark cycles before and after RNA interference (RNAi) of the circadian clock gene period (per RNAi), which disrupts the circadian rhythm but not the circatidal rhythm. Hyperactivity just after lights-on and lights-off was observed both before and after per RNAi. However, the activity levels were suppressed in the photophase as compared to the scotophase before per RNAi, but not suppressed in the photophase after per RNAi. Thus, the activity of A. asahinai is not directly suppressed by light

All-Optical Waveguide-Type Switch Using Saturable Absorption in Graphene

An all-optical switch using saturable absorption that accompanies refractive-index change such as in graphene films is proposed. The switching conditions are theoretically derived for arbitrary values of absorption and refractive-index change. It is found that switching can be performed by weaker control light when the refractive-index change is accompanied in saturable absorption. Switching conditions for various combinations of absorption and refractive-index change are confirmed by FD-BPM simulation. As an experimental demonstration of saturable absorption in graphene, we discuss measured nonlinear absorption in a graphene-loaded waveguide. The saturable absorption and refractive-index change through vertically placed multilayered graphene sheets are also discussed based on reported experimental results

Highly accurate spatial mode generation using spatial cross modulation method for mode division multiplexing

We propose a spatial mode generation technology using spatial cross modulation (SCM) for mode division multiplexing (MDM). The most well-known method for generating arbitrary complex amplitude fields is to display an off-axis computer-generated hologram (CGH) on a spatial light modulator (SLM). However, in this method, a desired complex amplitude field is obtained with first order diffraction light. This critically lowers the light utilization efficiency. On the other hand, in the SCM, the desired complex field is provided with zeroth order diffraction light. For this reason, our technology can generate spatial modes with large light utilization efficiency in addition to high accuracy. In this study, first, a numerical simulation was performed to verify that the SCM is applicable for spatial mode generation. Next, we made a comparison from two view points of the coupling efficiency and the light utilization between our technology and the technology using an off-axis amplitude hologram as a representative complex amplitude generation method. The simulation results showed that our technology can achieve considerably high light utilization efficiency while maintaining the enough coupling efficiency comparable to the technology using an off-axis amplitude hologram. Finally, we performed an experiment on spatial modes generation using the SCM. Experimental results showed that our technology has the great potential to realize the spatial mode generation with high accuracy