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Illustrates a configuration of phase conjugation (PC) with three beams in BaTiO[sub3] crystal. Diagram of the optical setup for a three-beam PC; PC characteristics for various incident beam combinations.

Examines a double 45 degree-cut mutually pumped phase conjugate mirror which has high phase conjugate reflectivities and is highly insensitive to the alignment of the two incident beams. Overview of the two-beam coupling coefficient; Angular dependent of the phase conjugate reflection; Cause of light scattering and the loss of the incident.

Examines optical phase conjugation at microwave and millimeter wave frequencies in an optomechanical medium composed of three-dimensional array of electrically small, anisotropic particles. Optical index gratings that important to the phase conjugate beam; Expression for the electrostrictive potential for each sphere.

Presents a study that examined the growth and self-pumped phase conjugation of cerium-doped crystals. Preparation of the crystal sample; Measurement of the phase conjugation reflectivities of the samples; Determination of the time required for the initiation of the phase conjugate.

Presents information on a study which investigated the optical phase conjugation by degenerate four-wave mixing in samples of pristine and iodine-doped polyphenylacetylene at ruby laser wavelength. Methodology of the study; Results and discussion of the study.

Focuses on the proposed model for phase conjugate mirror reflectivity and its application to a laser cavity with phase conjugate mirror. Information on phase conjugate mirrors (PCM); Theory of the variable aperture PCM; Equations used in the proposed model for PCM reflectivity.

Optical phase-modulated systems are promising candidates for the development of ultrahigh-bit-rate transmission links, thanks to their high spectral efficiency and increased tolerance to fiber-optic nonlinearities. However, their implementation was considerably slowed down by theoretical studies, suggesting that the transmission performance can be severely hindered by nonlinear phase noise (the so-called Gordon-Mollenauer effect). The simulations presented here show that, in realistic systems (including nonlinearity, dispersion, and attenuation), Gordon-Mollenauer noise does not represent the main source of signal distortions. We demonstrate that all nonlinear impairments can be efficiently compensated by optical phase conjugation, independent of system characteristics. The combination of optical-phase-conjugation with phase-modulation formats could enable ultrahigh-transmission-capacity and easy embedded-link upgrading.

The optical phase conjugation (OPC) phenomenon of light multiply scattered through a macroscopic scattering medium is simulated using the pseudospectral time-domain (PSTD) technique. We investigate the OPC reconstruction from scattered light at various positions in space. Specific results show that the original incident light cannot be reconstructed from singly scattered light. To reconstruct the original incident light, an “unbiased” selection of scattered light is required.

Taking optical frequency combs as WDM laser sources for coherent optical communication systems has great potential in low-cost, low-complexity, and large-capacity communications. Master–slave carrier phase recovery (MS-CPR) utilizes the phase coherence of received signals to reduce the computational complexity (CC) in frequency comb-based communication systems. In long-haul systems, chromatic dispersion (CD) weakens the phase coherence and makes MS-CPR ineffective. Besides, nonlinear effects greatly degrade the quality of received signals. A master–slave carrier phase recovery method using optical phase conjugation (OPC) is proposed for frequency comb-based long-haul coherent optical communication systems. By placing the OPC module in the middle of the link, not only the weakening of phase coherence caused by CD is solved, but also nonlinear effects are mitigated. The simulation results show that the proposed method has better performance than conventional MS-CPR and independent carrier phase recovery (ICPR). In the 1000 km long system with 16 quadrature amplitude modulation (QAM) signals, the BER of proposed method is 9.3% of conventional MS-CPR and even 40% lower than ICPR, and the BER of all channels is below the threshold for 7% overhead hard-decision forward error correction (7% HD-FEC). • A new master–slave carrier phase recovery method using optical phase conjugation is proposed. • A low-complexity carrier phase recovery method is realized. • The performance of proposed method is better than the independent carrier phase recovery. [ABSTRACT FROM AUTHOR]

Light field control through a diffuser using feedback wavefront shaping (WS) is an emerging method for sorting in orbital angular momentum (OAM) multiplexed communications. However, the feedback iterative optimization of the wavefront shaper converges slowly, reducing the real-time capability. Herein, we developed a WS scheme using a diffuser with customizable optical phase conjugation (COPC) for arbitrary OAM light control and mode sorting without iteration. The numerical and experimental results showed that the multiplexed OAM mode can be sorted into predefined spatial positions. Moreover, we found that the OAM mode with a larger topological charge | l | has stronger focusing, i.e., higher transmittance through the diffuser. Furthermore, we demonstrated that two, four, six, and eight modes can be sorted beyond 0.99, 0.93, 0.87 and 0.79 detection probabilities in the experiment. [ABSTRACT FROM AUTHOR]

Recently, a novel approach to satisfy the nonlinearity compensation criteria by mid-span optical-phase-conjugation (OPC) was proposed and excellent transmission performance was achieved, there the OPC propagation symmetry was obtained through optimized dispersion management with inverse-dispersion fiber (IDF). In this work, the author numerically investigates the possibilities of using dispersion-compensating fiber (DCF) instead of IDF for construction of the link. Nonlinearity compensation effectiveness of the two links are compared with respected to the intrachannel four-wave mixing (IFWM) induced intensity fluctuation of the '1' bits and the generation of the ghost pulse. It is found that, by launching different powers (or energy of a limited bits) into the spans before and after the OPC, nonlinearity mismatch between the spans of the DCF-managed link can be counterbalanced with significantly improved transmission performance. For a given input energy, there exists an optimum ratio of the energies into the spans before and after the OPC, at which optimum nonlinearity compensation can be achieved, and the compensation effectiveness is very close to that of the IDF-managed link. Further studies show that the optimum energy ratio increases with the input energy but is independent of the span number of the link. • Recently, a novel approach (J. Lightwave Technol. 39 (9), 2820–2829, 2021) to satisfy the nonlinearity compensation criteria by mid-span optical-phase-conjugation (OPC) was proposed and excellent transmission performance was achieved, where the OPC propagation symmetry was obtained through optimized dispersion management with inverse-dispersion fiber (IDF). This paper numerically demonstrates for the first time the possibility of using dispersion-compensating fiber (DCF) instead of IDF for construction of the link, which allows for a relatively simple modification and for upgrading of already installed fiber-optic links. • A set of four coupled nonlinear equations describing the evolution of four pulses (bits) is solved exactly for the first time by the split-step Fourier method, which includes group velocity dispersion (GVD), self-phase modulation (SPM), fiber loss, EDFA gain, EDFA noise, intrachannel cross-phase modulation (IXPM), and intrachannel four-wave mixing (IFWM). The advantage of the model is that detailed dynamic pulse shape and spectrum evolutions can be observed. • It is find that, by launching different powers (or energy of a limited bits) into the spans before and after the OPC, nonlinearity mismatch between the spans of the DCF-managed link can be counterbalanced with significantly improved transmission performance. For a given input energy, there exists an optimum ratio of the energies into the spans, at which optimum nonlinearity compensation can be achieved and the compensation effectiveness is very close to that of the IDF-managed link. [ABSTRACT FROM AUTHOR]

Focusing light though scattering media beyond the ballistic regime is a challenging task in biomedical optical imaging. This challenge can be overcome by wavefront shaping technique, in which a time-reversed (TR) wavefront of scattered light is generated to suppress the scattering. In previous TR optical focusing experiments, a phase-only spatial light modulator (SLM) has been typically used to control the wavefront of incident light. Unfortunately, although the phase information is reconstructed by the phase-only SLM, the amplitude information is lost, resulting in decreased peak-to-background ratio (PBR) of optical focusing in the TR wavefront reconstruction. A new method of TR optical focusing through scattering media is proposed here, which numerically reconstructs the full phase and amplitude of a simulated scattered light field by using a single phase-only SLM. Simulation results and the proposed optical setup show that the time-reversal of a fully developed speckle field can be digitally implemented with both phase and amplitude recovery, affording a way to improve the performance of light focusing through scattering media.

The dissemination of the digital coherent technology has enabled the recent growth of information networks, which has obviated signal processing in the optical domain such as optical dispersion compensation. However, the further scaling of the digital coherent technology will eventually suffer from the slowdown of the Moore's law and/or the energy crunch as a result of the longstanding relentless traffic increase. The use of all-optical signal processing, free from the electronic limitations, will then inevitably be reevaluated but in a slightly different way from how it was previously expected. In this paper, we highlight the unique features of all-optical signal processing that outperforms digital signal processing, and review the two latest results: one is upgrade-free, multi-channel wavelength conversion; and the other, all-optical back propagation for WDM channel nonlinearity compensation. Both of these functions are exploiting a versatile physical phenomenon of parametric processes in highly nonlinear fiber that is highly efficient in terms of cost and energy and is as cascadable as EDFAs. This paper discusses their practical aspects with emphasis on cascadability. In particular, the promising prospects for “cascaded phase-conjugating amplifier chains” to compensate nonlinear signal distortion will be highlighted after discussing the parameter tolerances for all-channel nonlinearity compensation. [ABSTRACT FROM PUBLISHER]

Unrepeatered transmission delivers data over a few hundred kilometers without in-line active elements and it can effectively reduce the line complexity and the overall system cost. In this paper, we propose all optical phase-conjugated copy (OPC) to improve optical signal noise ratio (OSNR) margin and nonlinear tolerance for unrepeatered transmission of polarization division multiplexing (PDM) optical superchannel with EDFA only amplification. Orthogonal pumps FWM scheme is utilized to generate the optical phase-conjugated copy. The original superchannel and the phase-conjugated copy are simultaneously transmitted and received. The phases of the copy symbols are conjugated and summed with the original superchannel symbols to suppress both linear noise and nonlinear phase noise at the receiver. The proposed OPC scheme is simple and effective in phase-conjugated copy generation and digital signal processing (DSP). What is more, it is transparent to signal bit rate and modulation format, which applies to optical superchannel transmission. We experimentally verify the proposed scheme on a 400 Gb/s optical polarization division multiplexing orthogonal frequency division multiplexing (PDM-OFDM) superchannel. A Q-factor improvement of 2.1 dB is achieved after 180 km SSMF unrepeatered transmission. The optimum launch power in OPC scheme increases from −3 dBm to −2 dBm. To verify the maximum reach, we extend fiber length and realize 240 km SSMF unrepeatered transmission. [ABSTRACT FROM AUTHOR]

Monolithic photonic integrated circuits (PIC) transmitters using the prefixed optical phase switching concept for BPSK modulation format have been shown promising at 1.55 μm band. These devices could also be crucial for short reach connections and access networks. With this aim, we are studying basic quantum well designs for a laser and an electro-absorption modulator switch to be integrated by selective area growth into PICs at 1.3 μm. Photocurrent measurements and band offset modeling have been performed to determine the MQW stack well-fitted for this application. Broad area laser measurements have also been checked on these structures to verify the material lasing properties. A 6 nm thick well with low barrier seems to be the best trade-off between absorption and shift for 1.3 μm EAM and it also gives good lasing properties. [ABSTRACT FROM AUTHOR]

Optical phase conjugation (OPC)-based wavefront shaping techniques focus light through or within scattering media, which is critically important for deep-tissue optical imaging, manipulation, and therapy. However, to date, the sample thickness in OPC experiments has been limited to only a few millimeters. Here, by using a laser with a long coherence length and an optimized digital OPC system that can safely deliver more light power, we focused 532-nm light through tissue-mimicking phantoms up to 9.6 cm thick, as well as through ex vivo chicken breast tissue up to 2.5 cm thick. Our results demonstrate that OPC can be achieved even when photons have experienced on average 1000 scattering events. The demonstrated penetration of nearly 10 cm (∼100 transport mean free paths) has never been achieved before by any optical focusing technique, and it shows the promise of OPC for deep-tissue noninvasive optical imaging, manipulation, and therapy. [ABSTRACT FROM AUTHOR]

It is shown that the direction-of-arrival (DoA) information carried by an incident electromagnetic (EM) wave can be encoded into the evanescent near field of an electrically small resonance antenna array with a spatial rate higher than that of the incident field oscillation rate in free space. Phase conjugation of the received signal leads to the retrodirection of the near field in the antenna array environment, which in turn generates a retrodirected far-field beam toward the original DoA. This EM phenomenon enables electrically small retrodirective antenna arrays with superdirective, angular super-resolution, auto-pointing properties for an arbitrary DoA. A theoretical explanation of the phenomenon based on first principal observations is given and full-wave simulations demonstrate a realizability route for the proposed retrodirective terminal that is comprised of resonance dipole antenna elements. Specifically, it is shown that a three-element disk-loaded retrodirective dipole array with $0.15\lambda $ spacings can achieve a 3.4-dBi maximal gain, 3-dBi front-to-back ratio, and 13% return loss fractional bandwidth (at the 10-dB level). Then, it is demonstrated that the radiation gain of a three-element array can be improved to approximately 6 dBi at the expense of the return loss fractional bandwidth reduction (2%). [ABSTRACT FROM AUTHOR]

In this paper, we investigate the effectiveness of using phase-conjugated twin waves (PCTWs) technique to mitigate fiber nonlinear impairments in spatially multiplexed all-optical orthogonal frequency division multiplexing (AO-OFDM) systems. In this technique, AO-OFDM signal and its phase-conjugated copy are directly transmitted through two identical fiber links. At the receiver, the two signals are coherently superimposed to cancel the phase noise and to enhance signal-to-noise ratio (SNR). To show the effectiveness of proposed technique, a spatially multiplexed AO-OFDM system is demonstrated by numerical simulation. AO-OFDM signal and its phase conjugated copy are optically generated by using optical coupler-based inverse fast Fourier transform (OIFFT)/fast Fourier transform (OFFT). The generated signal includes 29 subcarriers where each subcarrier is modulated by 4-quadrature amplitude modulation (4QAM) format at a symbol rate of 25 Gsymbol/s. The results reveal that transmission performance is considerably improved where the transmission distance of the proposed system is increased by ∼45% as compared to that of original system without PCTWs technique. [ABSTRACT FROM AUTHOR]

The paper studies the interaction of Lamb waves with the free edge of a plate. The reflection coefficients of a Lamb mode at a plate free edge are calculated using a semi-analytical method, as a function of frequency and angle of incidence. The conversion between forward and backward Lamb modes is thoroughly investigated. It is shown that at the zero-group velocity (ZGV) frequency, the forward S1 Lamb mode fully converts into the backward S2b Lamb mode at normal incidence. Besides, this conversion is very efficient over most of the angular spectrum and remains dominant at frequencies just above the ZGV-point. This effect is observed experimentally on a Duralumin plate. First, the S1 Lamb mode is selectively generated using a transducer array, second the S2b mode is excited using a single circular transducer. The normal displacement field is probed with an interferometer. The free edge is shown to retro-focus the incident wave at different depths depending on the wave number mismatch between the forward and backward propagating modes. In the vicinity of the ZGV-point, wave numbers coincide and the wave is retro-reflected on the source. In this frequency range, the free edge acts as a perfect phase conjugating mirror. [ABSTRACT FROM AUTHOR]

The pseudospectral time-domain (PSTD) technique is advantageous for modeling macroscopic light scattering problems but falls short to model a light source of realistic dimensions. We report a novel simulation technique that enables modeling a finite-width, continuous-wave (CW) light source within PSTD simulations that was infeasible before. By exploiting the mathematical characteristics of Maxwell's equations, the instability problem of the PSTD algorithm can be overcome. We demonstrate the reported simulation enables modeling the CW optical phase conjugation (OPC) phenomenon of light propagation through a scattering medium. Specific simulation findings show that monochromatic phase-conjugated light can be focused through scattering medium. On a wider scope, the reported method enables implementing a finite-width, CW, plane-wave light source in a PSTD simulation for general optics and electromagnetic problems. [ABSTRACT FROM AUTHOR]

Classic holography involves a reference beam and an object beam (which bounces off an object) which interact and form a holographic interference pattern on a holographic plate (grating). This method has been used for numerous applications and theoretical models. Holographic information storage is often utilized in the field of opto-electronics. The goal of this research is to understand how to transmit/receive, code/decode, modulate and focus optical signals. [ABSTRACT FROM AUTHOR]

We report optical phase conjugation in C-band by counter-propagating dual pumped non-degenerate four-wave mixing in a semiconductor optical amplifier (SOA). The co-propagating signal and pump waves create a grating inside SOA which diffracts counter-propagating pump and generates the conjugate wave. Since the signal and conjugate waves appear at opposite ends, the conjugate is easily filtered out from the rest of spectrum with minimal spectral shift of the conjugate with respect to the incoming signal. With pump powers of −3.2 dBm each and signal input power of −7 dBm, conjugate power was of −27.2 dBm, giving a conversion efficiency of 1% at 18 GHz pump-signal detuning. By modulating the signal by a periodic pattern ‘1000’ at 10 Gbps using a non-zero chirp intensity modulator and resolving the temporal profile of the electric field envelope of the conjugate wave, we demonstrate spectral inversion. [ABSTRACT FROM AUTHOR]

A joule-level Nd:YAG nanosecond laser of high repetition frequency and high beam quality is developed out. The laser is designed as a MOPA system mainly including single longitudinal mode seed, pre-amplifier unit an d power amplifier unit. In order to obtain the high-quality laser beam output, phase conjugation is adopted to compensate the laser beam distortion. Under the condition of 200 Hz high repetition frequency and 8.19 μJ single pulse energy injected by the single longitudinal mode seed, 1.53 J output energy is gained. The output laser beam is of 9 mm diameter, 7.41 ns pulse width, the far field beam spot 1.32 times the value of the diffraction limit, 1.2% energy stability (RMS) and less than 13 μrad far field beam spot angle shift. [ABSTRACT FROM AUTHOR]

The combination of dispersion management (DM) and midway optical phase conjugation (OPC) is one of the promising techniques for compensating for optical signal distortion due to group velocity dispersion and nonlinear fiber effects. However, in this combination technique, midway OPC restricts the flexible optical link configuration. Therefore, the possibility of implementing the flexible optical link configuration with non-midway OPC applied to complete inline DM links is investigated in this study. It is confirmed that although the compensation using non-midway OPC for the distorted WDM channels is less effective than that using midway OPC, when non-midway OPC is placed at positions closer to the transmitters, the deployment of precompensation (i.e., the sequence of DCF + SMF)-OPC-postcompensation (i.e., the sequence of SMF + DCF) is more advantageous for the compensation. On the other hand, inverse deployment with respect to OPC (i.e., postcompensation-OPC-precompensation) is more advantageous when non-midway OPC is placed at positions closer to the receivers. [ABSTRACT FROM AUTHOR]

We present an in-depth look at the challenges involved in using analog retrodirective arrays for satellite communications (SATCOM). The main technical issues surrounding the development of a retrodirective (self-steering) SATCOM system are given and techniques for mitigating these issues are provided. Detailed results are given for a prototype high-performance circularly polarized retrodirective array architecture suitable for mounting on an unstabilized mobile platform. This paper concludes with practical retrodirective L-band array results with the array used to acquire actual broadband satellite data signals from a commercial L-band satellite system. Received satellite signals as low as -\130 dBm at the antenna elements are tracked. Accurate self-tracking occurs over the azimuth range of up to \pm \40^\circ. [ABSTRACT FROM PUBLISHER]

We report on a potentially deleterious issue regarding the four-wave mixing (FWM)-based processing of dispersion-altered signals. We estimate the baudrate-dependent pump linewidth tolerances by calculating the extra optical signal to noise ratio (OSNR) penalty with respect to the propagation distance. We find that the issue is not important for 10 Gbaud differential quadrature phase shift keying (DQPSK) signals, though for 28 Gbaud (and 56 Gbaud) DQPSK signals we find that the pump linewidth requirements to implement FWM based optical signal processing needs to be in the sub-MHz range in order to avoid excessive OSNR penalties for the case of dispersion-altered signals. These results are pertinent for systems employing FWM, which could be all—optical wavelength converters for packet switching or mid-span spectral inversion techniques. [ABSTRACT FROM AUTHOR]

We demonstrate and characterize the performance of two heterodyned optical phase locked loop (PLL) laser systems for use in characterizing photodetector RF frequency response and nonlinearities. Descriptions of PLL circuit parameters for Nd:YAG non-planar ring oscillator lasers at 1064 nm and 1319 nm, and Er ion fiber lasers from 1530 nm to 1565 nm are presented. Both laser systems have piezoelectric transducer wavelength control over the PLL voltage controlled oscillator circuit. Offset frequency phase locking from 1.5 kHz to 51+ GHz is demonstrated. Frequency stability at 10 MHz is measured to be ± 50 μHz, limited by the stability of the Rb stabilized crystal oscillator. Phase noise of the phase-locked 1319 nm laser system is discussed where we find that the phase noise is dominated by the input source noise at frequency offsets below 100 Hz and by the laser's RIN noise at frequency offsets >100 Hz. Comparing nonlinearity data from an InGaAs p-i-n photodiode using both 1319 nm and 1550 nm PLL nonlinearity measurement systems, we find two new separate photodetector nonlinearity mechanisms. Measurements of the harmonic components of a 11 MHz sinusoidal heterodyned optical beat note signal are found to be at or below 1 nW/mW for the second harmonic (at 22 MHz) and at or below 0.25 nW/mW for the 3rd harmonic (at 33 MHz), confirming the nearly pure sinusoidal nature of the optically generated microwave beat note. [ABSTRACT FROM AUTHOR]

The response of the nanolasers subject to the phase conjugate feedback (PCF) has been analyzed. Calculations have been performed using rate equations, which include the Purcell cavity-enhanced spontaneous emission factor $F$ and the spontaneous emission coupling factor $\beta $ . In the analysis, the influence of $F$ and $\beta $ is evaluated for varying the feedback rate. It is observed that, in general, the onset of chaos occurs at a higher feedback rate for PCF than for conventional mirror feedback (CMF), where PCF has lower intensity noise in comparison with the CMF. [ABSTRACT FROM AUTHOR]

We performed near-diffraction limited two-photon fluorescence (TPF) imaging through a lensless, multicore-fiber (MCF) endoscope utilizing digital phase conjugation. The phase conjugation technique is compatible with commercially available MCFs with high core density. We demonstrate focusing of ultrashort pulses through an MCF and show that the method allows for resolution that is not limited by the MCF core spacing. We constructed TPF images of fluorescent beads and cells by digital scanning of the phase-conjugated focus on the target object and collection of the emitted fluorescence through the MCF. [ABSTRACT FROM AUTHOR]