Optical Phase Conjugation Research Articles

Mitigation of Nonlinear Effects on WDM QAM Signals Enabled by Optical Phase Conjugation With Efficient Bandwidth Utilization

We present nonlinear impairment mitigation of wavelength division multiplexed signals, through optical phase conjugation (OPC). We conduct our experiments on a 400-km-long installed fiber link equipped with erbium-doped fiber amplifiers, with the OPC placed close to the middle of the link. Our OPC configuration realizes efficient reuse of the signal bandwidth, avoiding the loss of half of the spectral band typical of most phase conjugating schemes. We demonstrate the operation of the system using both 16- and 64-quadrature amplitude modulation (QAM) signals and report Q-factor improvements up to 0.5 and 2.5 dB for 16- and 64-QAM, respectively.

Dispersion compensated optical amplifier in Ti-diffused PPLN ridge waveguide.

Nonlinear optical phase conjugation is known to be an effective technique for dispersion compensation in optical fibers. It requires an optical filter to separate out only the phase conjugated signal at the output to achieve signal pulse re-narrowing by its subsequent propagation through an identical medium. In this paper, a compact design, which integrates the optical phase conjugator and the filter on a single substrate of periodically poled lithium niobate and facilitates the use of a single pump source for both phase conjugation and amplification, is proposed and analyzed using computer simulations.

Effective suppression of nonlinear distortion in optical phase conjugation of optical OFDM signals using backward Raman pumping

ABSTRACTThe use of backward Raman amplification to mitigate nonlinear distortion in optical phase conjugation (OPC) of orthogonal frequency-division multiplexing (OFDM) signals in a fibre communication system is investigated in this paper. A ∼29 dB reduction in the nonlinear distortion is achieved, while comparable conjugated output signal power is maintained through the Raman gain. By numerically solving the nonlinear Schrodinger equation (NLSE) along the fibre, a detailed theoretical model is developed to elaborate the mechanism in Raman-assisted OPC. Different optical network scenarios have been analysed using this model. The mechanism of Raman-assisted OPC is further explained by comparing the power profiles of both the conjugated signal and the nonlinear distortion with those in the conventional scheme. In addition to investigating the nonlinear distortion caused by spurious four-wave mixing (FWM) products, the waveform distortion resulting from pump depletion in the OPC system is also analysed. The ...

SPM Supression in Long Haul CO-OFDM System using SPWR-OPC Modules

Objective: The main aim of this research paper is to enhance the reach of Long-haul Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) communication system. Methods/Statistical Analysis: The purpose of SPWR and OPC in CO-OFDM system is to palliate dispersion and (Non-linearity) SPM respectively. In this proposed system Symmetrical Pulse Width Reduction (SPWR) and Optical Phase Conjugate modules (OPC) are used. The CO-OFDM system is investigated to compensate SPM by using up to 6 OPCs. Findings: While analyzing results it reveals that with the increase in the number of OPCs up to 6 enhances the transmission distance of CO-OFDM system and improves the Bit Error Rate (BER) performance but if increased beyond 6 OPCs deteriorates its performance. When using six OPCs in comparison to a single OPC, the system reach extends six times. Application/Improvement: The proposed system is simulation based and in future can be implemented commercially in Digital Terrestrial Television (DTT) and real-time audio and video communication.Keywords: Coherent Optical Orthogonal Frequency Division Multiplexing, Optical Phase Conjugate, Self Phase Modulation, Symmetrical Pulse Width Reduction

Optical back propagation for compensating nonlinear impairments in fiber optic links with ROADMs.

An optical back propagation (OBP) technique is investigated to compensate for nonlinear impairments in fiber optic communication systems with reconfigurable optical add-drop multiplexers (ROADMs). An OBP module consisting of an optical phase conjugator (OPC), amplifiers and dispersion-decreasing fibers (DDFs) fully compensates for the nonlinear impairments of a transmission fiber. The OBP module can be placed after each transmission fiber (inline OBP case) or at each network node (node OBP case). For a wavelength division multiplexing (WDM) system with 2400 km transmission distance and 32-quadrature amplitude modulation (QAM) format, inline OBP and node OBP bring Q-factor improvements of 4.9 dB and 5.6 dB as compared with linear compensation, respectively. In contrast, receiver-side digital back propagation (DBP) only provides 1.3 dB Q-factor gain, due to its incapability of mitigating inter-channel nonlinear effects in fiber optic networks.

Mitigation of nonlinear effects through frequency shift free mid-span spectral inversion using counter-propagating dual pumped FWM in fiber

We propose and numerically verify a scheme of frequency-shift free mid-span spectral inversion (MSSI) for nonlinearity mitigation in an optical transmission system. Spectral inversion is accomplished by optical phase conjugation, realized by counter-propagating dual pumped four-wave mixing in a highly nonlinear fiber. We examine the performance of MSSI due to critical parameters such as nonlinear fiber length, pump and signal power. We demonstrate the near complete nonlinearity mitigation of 40 Gbps DQPSK modulated data transmitted over 1000 km standard single mode fiber using our method of MSSI. We perform simulation of bit-error rate as a function of optical signal to noise ratio to corroborate the effect of frequency-shift free MSSI.

Minimizing inter-channel cross-phase modulation with optical phase conjugation in asymmetric fibre links.

Using analytic and numerical modelling of fibre transmission systems that employ optical phase conjugation (OPC), we show inter-channel cross-phase modulation depends on the integrated square error between nonlinear profiles before and after OPC and that arranging amplifiers and tuning power levels is crucial to minimizing noise. We derive modulation transparent formulas for phase noise and optimal power settings. Examples are shown for 16 and 64 quadrature amplitude modulation.

Focusing light through biological tissue and tissue-mimicking phantoms up to 9.6cm in thickness with digital optical phase conjugation.

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.

The Effects of the Number of Fiber Spans and Residual Dispersion per Span on the System Performance in Dispersion-Managed Optical Links Combined with Optical Phase Conjugation

The Effects of the Number of Fiber Spans and Residual Dispersion per Span on the System Performance in Dispersion-Managed Optical Links Combined with Optical Phase Conjugation

Simultaneous nonlinearity mitigation in 92 × 180-Gbit/s PDM-16QAM transmission over 3840 km using PPLN-based guard-band-less optical phase conjugation.

We experimentally demonstrated the simultaneous nonlinearity mitigation of PDM-16QAM WDM signals using complementary-spectrally-inverted optical phase conjugation (CSI-OPC). We achieved reserved-band-less, guard-band-less, and polarization independent OPC based on periodically poled LiNbO3 waveguides. By employing the CSI-OPC, 2.325-THz-band (93 × 25 GHz) complementary spectral inversion was achieved while retaining the original WDM bandwidth. A Q2-factor improvement of over 0.4 dB and a 5120 km transmission with a Q2-factor above the FEC limit were confirmed using a 10-channel WDM transmission at the signal band center and signal band edge. We then demonstrated the mitigation of the nonlinear impairments in a 3840 km long-haul WDM signal transmission for all 92-channel 180-Gbit/s PDM-16QAM quasi-Nyquist-WDM signals.

Some examples of photorefractive oscillators

The photorefractive effect presents a periodical change of the refractive index of an optical environment under the influence of a coherent light. An interesting phenomenon which occurs at this effect is optical phase conjugation (PC). Photorefractive oscillators, that is photorefractive mirrors present important decides in photorefractive optics and their function is based on photorefractive effect. In these oscillators, a phase-conjugated light beam occurs. The basic characteristics of photorefractive oscillators, such as reflectivity, the existance of the oscillation threshold and the threshold of the coupling strength are explained by the so-called grating-action method. This is analysed on a ring oscillator, semilinear mirror and linear mirror.

Link optimization for DWDM transmission with an optical phase conjugation.

We characterize in-span signal power asymmetry in random distributed feedback ultralong Raman laser-amplified WDM transmission and numerically optimize fiber span length and operating band to achieve the lowest inter-span signal power asymmetry between transmitted and optically conjugated channels in systems relying upon mid-link optical conjugation to combat fiber nonlinear impairments.

Inverse focusing inside turbid media by creating an opposite virtual objective.

Limited by the penetration depth, imaging of thick bio-tissues can be achieved only by epi-detection geometry. Applications based on forward-emitted signals or bidirectional illumination are restricted by lack of an opposite objective. A method for creating an opposite virtual objective inside thick media through phase conjugation was first proposed. Under forward illumination, the backward scattering light from the media was collected to generate a phase conjugate wave, which was sent back to the media and formed an inverse focusing light. Samples combined with a diffuser or a mouse skin were used as specimens. Inverse focusing was successfully demonstrated by applying holography-based optical phase conjugation with a BaTiO3. This result indicates the capability to create an opposite virtual objective inside live tissues. The proposed method is compatible with current coherent imaging and super-resolution imaging technologies. It creates a possible way for forward-emitted signals collection and bidirectional illumination in thick specimens.

Dispersion-Managed Optical Links Combined with Asymmetrical Optical Phase Conjugation for Compensating for Distorted WDM Signals

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.

Comparison of the compensation effects of fiber nonlinear impairments with mid-span optical phase conjugation between PDM CO-OFDM system and PDM QPSK system**Project supported by the National Natural Science Foundation of China (Grant Nos. 61271192, 61427813, and 61331010) and the National Basic Research Program of China (Grant No. 2013AA013401).

The compensation effects of fiber nonlinearity in 112 Gb/s polarization division multiplexing (PDM) coherent optical systems by mid-span optical phase conjugation (OPC) based on four wave mixing (FWM) effect are studied. Comparisons of the compensation results between PDM coherent optical-orthogonal frequency division multiplexing (CO-OFDM) system and the single carrier (SC) PDM quadrature phase shift keying (QPSK) system are provided as well. The results demonstrate that nonlinear compensation effect with mid-span OPC in PDM CO-OFDM system is much more obvious than that in SC PDM QPSK system.

Transmission of independent signals through a multimode fiber using digital optical phase conjugation.

Multimode fibers are attractive for a variety of applications such as communication engineering and biophotonics. However, a major hurdle for the optical transmission through multimode fibers is the inherent mode mixing. Although an image transmission was successfully accomplished using wavefront shaping, the image information was not transmitted individually for each of the independent pixels. We demonstrate a transmission of independent signals using individually shaped wavefronts employing a single segmented spatial light modulator for optical phase conjugation regarding each light signal. Our findings pave the way towards transferring independent signals through strongly scattering media.

Possibilities of joint application of adaptive optics technique and nonlinear optical phase conjugation to compensate for turbulent distortions

The efficiency of integrating the nonlinear optical technique based on forming a reverse wavefront and the conventional adaptive optics into a unified complex (for example, for adaptive focusing of quasi-cw laser radiation) is demonstrated. Nonlinear optical phase conjugation may provide more exact information about the phase fluctuations in the corrected wavefront in comparison with the adaptive optics methods. At the same time, the conventional methods of adaptive optics provide an efficient control of a laser beam projected onto a target for a rather long time.

Optical phase conjugation assisted scattering lens: variable focusing and 3D patterning.

Variable light focusing is the ability to flexibly select the focal distance of a lens. This feature presents technical challenges, but is significant for optical interrogation of three-dimensional objects. Numerous lens designs have been proposed to provide flexible light focusing, including zoom, fluid, and liquid-crystal lenses. Although these lenses are useful for macroscale applications, they have limited utility in micron-scale applications due to restricted modulation range and exacting requirements for fabrication and control. Here, we present a holographic focusing method that enables variable light focusing without any physical modification to the lens element. In this method, a scattering layer couples low-angle (transverse wave vector) components into a full angular spectrum, and a digital optical phase conjugation (DOPC) system characterizes and plays back the wavefront that focuses through the scattering layer. We demonstrate micron-scale light focusing and patterning over a wide range of focal distances of 22–51 mm. The interferometric nature of the focusing scheme also enables an aberration-free scattering lens. The proposed method provides a unique variable focusing capability for imaging thick specimens or selective photoactivation of neuronal networks.

Calibration of digital optical phase conjugation setups based on orthonormal rectangular polynomials

Digital optical phase conjugation (DOPC) has proven to be a promising technique in deep tissue fluorescence imaging. Nonetheless, DOPC optical setups require precise alignment of all optical components to accurately read the wavefront of scattered light in a turbid medium and playback the conjugated beam toward the sample. Minor misalignments and possible imperfections in the arrangement or the structure of the optical components significantly reduce the performance of the method. In this paper, a calibration procedure based on orthogonal rectangular polynomials is introduced to compensate major imperfections including the optical aberration in the wavefront of the reference beam and the substrate curvature of the spatial light modulator without adding extra optical components to the original setup. The proposed algorithm also provides a systematic calibration procedure for mechanical fine tuning of DOPC systems. It is shown experimentally that the proposed calibration process improves the peak-to-background ratio when focusing light after passing through a highly scattering medium.

Focusing light through scattering media by full-polarization digital optical phase conjugation.

Digital optical phase conjugation (DOPC) is an emerging technique for focusing light through or within scattering media such as biological tissue. Since DOPC systems are based on time reversal, they benefit from collecting as much information about the scattered light as possible. However, existing DOPC techniques record and subsequently phase-conjugate the scattered light in only a single-polarization state, limited by the operating principle of spatial light modulators. Here, we develop the first, to the best of our knowledge, full-polarization DOPC system that records and phase-conjugates scattered light along two orthogonal polarizations. When focusing light through thick scattering media, such as 2 mm and 4 mm-thick chicken breast tissue, our full-polarization DOPC system on average doubles the focal peak-to-background ratio achieved by single-polarization DOPC systems and improves the phase-conjugation fidelity.