Phase-conjugate Beam Research Articles

Vector modulation of fully-polarized phase conjugate light field through scattering media

Vector modulation of the light field through scattering media holds significant scientific and application value in areas such as optical imaging, optical communications, nonlinear optics, and biomedicine. Digital optical phase conjugation (DOPC), grounded in the time reversal principle, has been extensively investigated for wavefront shaping in scattering media. Nonetheless, reports on vector modulation of phase conjugate beams via DOPC remain scarce. In this study, we propose a vector DOPC to recover and modulate the polarization state of the phase conjugate beams based on vector decomposition and superimposition of light field. First, pre-set two orthogonal polarization basis probe beams to pass through a multimode fiber and use digital holography to record the phase distribution of their orthogonal polarization components in the speckle field. Then, perform polarization-preserving phase conjugation and vector superposition of both components simultaneously. By altering the phase difference and amplitude ratio between the two, vector modulation of the synthesized phase conjugate beam can be achieved. Theoretical analysis aligns well with experimental results, demonstrating a positive impact on understanding the physical properties of scattering media and expanding the applications of the DOPC technique.

Automatic turbulence mitigation for coherent free-space optical links using crystal-based phase conjugation and fiber-coupled data modulation.

There are various performance advantages when using temporal phase-based data encoding and coherent detection with a local oscillator (LO) in free-space optical (FSO) links. However, atmospheric turbulence can cause power coupling from the Gaussian mode of the data beam to higher-order modes, resulting in significantly degraded mixing efficiency between the data beam and a Gaussian LO. Photorefractive crystal-based self-pumped phase conjugation has been previously demonstrated to "automatically" mitigate turbulence with limited-rate free-space-coupled data modulation (e.g., <1 Mbit/s). Here, we demonstrate automatic turbulence mitigation in a 2-Gbit/s quadrature-phase-shift-keying (QPSK) coherent FSO link using degenerate four-wave-mixing (DFWM)-based phase conjugation and fiber-coupled data modulation. Specifically, we counter-propagate a Gaussian probe from the receiver (Rx) to the transmitter (Tx) through turbulence. At the Tx, we generate a Gaussian beam carrying QPSK data by a fiber-coupled phase modulator. Subsequently, we create a phase conjugate data beam through a photorefractive crystal-based DFWM involving the Gaussian data beam, the turbulence-distorted probe, and a spatially filtered Gaussian copy of the probe beam. Finally, the phase conjugate beam is transmitted back to the Rx for turbulence mitigation. Compared to a coherent FSO link without mitigation, our approach shows up to ∼14-dB higher LO-data mixing efficiency and achieves error vector magnitude (EVM) performance of <16% under various turbulence realizations.

Application of phase-conjugate beams in beam correction and underwater optical wireless communication subject to surface wave turbulence

Water surface wave turbulence is one of the factors affecting the performances of underwater optical wireless communication (UOWC) systems. In our research, a phase-conjugate beam was used to correct the beam distortion and enhance the communication performances when a system is subject to surface wave turbulence. The phase-conjugate beam was generated by a phase-conjugate mirror (PCM), and a turbulence generator was used to generate surface wave turbulence in the experiment. We calculated the beam centroid distribution and the results showed that the phase-conjugate beam had a better propagation performance than the distorted beam at the different water depths. The root mean square (RMS) of the beam centroid for the phase-conjugate beam was 11 times less than that for the distorted beam, which meant that the phase-conjugate beam could effectively correct the beam drift. We further investigated the scintillation index and the signal-to-noise ratio (SNR); the results showed that the phase-conjugate beam was able to reduce the scintillation and an obvious improvement in SNR could be obtained. This research has the potential to be applied in UWC.Graphical

Backward SRS suppression of picosecond pulses in water upon moving the pump beam waist from the water volume through the surface

We report, for the first time to our knowledge, suppression of backward stimulated Raman scattering (BSRS) of picosecond pulses (57 ps, 532 nm) due to the development of optical breakdown in the surface (0 – 3 mm) water layer with a shift of the beam waist (lens focal length of 83 mm) to the water – air interface without changing the pump pulse energy (∼1.3 – 1.5 mJ). In this case, SRS generation in forward direction is observed even in the presence of breakdown. When the focal plane coincides with the surface, the BSRS generation is restored without optical breakdown, despite an increase in the pump radiation intensity due to a decrease in the beam diameter. It is significant that the optical breakdown threshold in the water volume was unattainable even with an increase in the pump pulse energy by more than an order of magnitude – up to 16 mJ. The mechanism of self-consistent summation of nonlinear optical processes, such as electrostriction, beam self-focusing, beam phase conjugation, and BSRS pulse compression is discussed.

Experimental study on compensation of beam distortion caused by turbulence using a phase conjugate beam

The distortion of the laser beam caused by atmospheric turbulence has always been a major impediment in free space optical communication (FSOC) system. The atmospheric turbulence is mainly from the changes of the random refractive index in the atmosphere and leads to the random distribution of beam intensity. A phase-conjugate beam can effectively correct this type of deformation. In our research, by degenerate four-wave mixing (DFWM) method, a phase-conjugate beam is generated through a methyl orange doped polyvinyl alcohol solution to correct the mirror-reflected beam through a turbulence promoter. In comparison with the mirror-reflected beam, the phase-conjugate beam has a better near-Gaussian intensity distribution. The results show that the phase-conjugate beam has a positive effect on the correction of distortion caused by turbulence.

Optical Time Reversal from Time-Dependent Epsilon-Near-Zero Media.

Materials with a spatially uniform but temporally varying optical response have applications ranging from magnetic field-free optical isolators to fundamental studies of quantum field theories. However, these effects typically become relevant only for time variations oscillating at optical frequencies, thus presenting a significant hurdle that severely limits the realization of such conditions. Here we present a thin-film material with a permittivity that pulsates (uniformly in space) at optical frequencies and realizes a time-reversing medium of the form originally proposed by Pendry [Science 322, 71 (2008)SCIEAS0036-807510.1126/science.1162087]. We use an optically pumped, 500nm thick film of epsilon-near-zero (ENZ) material based on Al-doped zinc oxide. An incident probe beam is both negatively refracted and time reversed through a reflected phase-conjugated beam. As a result of the high nonlinearity and the refractive index that is close to zero, the ENZ film leads to time reversed beams (simultaneous negative refraction and phase conjugation) with near-unit efficiency and greater-than-unit internal conversion efficiency. The ENZ platform therefore presents the time-reversal features required, e.g., for efficient subwavelength imaging, all-optical isolators and fundamental quantum field theory studies.

A 100 Hz 3.31 J all-solid-state high beam quality Nd:YAG laser for space debris detecting

With the rapid development of space technology, human activities into space are increasing, thereby producing lots of space debris. And the space debris impact is the major cause for the mechanical damage to the space crafts and the main factor affecting the service life; it even endangers the life safety of the astronauts working outside the spacecraft and pose a threat to the astronomical observation and studies. Thus, the monitoring and early warning of space debris are gradually attracting wide attention. Obviously, laser detection as a good-directivity and strong anti-jamming active detecting means has a unique advantage in terms of a round-the-clock detection. Therefore, the developing of debris-detecting laser beam source becomes the most direct and effective means for increasing the space debris detection accuracy. The laser detecting ability is restricted by the laser beam quality, the pulse energy and the repetition frequency at the same time. The beam quality could affect the ability to detect and recognize space target. The bigger the laser pulse energy, the higher the repetition frequency and the smaller the detectable debris, the stronger the detecting ability will be. A good detection effect could be achieved at 80-100 Hz laser pulse repetition frequency. A further increase of the repetition frequency will greatly increase the difficulty and cost accordingly but the improvement of the detection performance is not obvious at all. Thus, repetition frequency around 100 Hz becomes the best choice for laser space debris detection. Based on the laser diode side-pumped rod-shaped amplifier, a high-repetition-frequency and high-beam-quality of joule level Nd:YAG nanosecond laser for space debris detection is developed in this work. The laser adopts MOPA structure, mainly including single longitudinal mode, pre-amplifier unit, SBS phase-conjugate beam control unit and energy extraction unit. In the energy extraction unit, beam splitting-amplifying-combining is adopted for reducing the thermal effect on beam quality by reducing the working current of the amplifier. Under the condition of 100 Hz high repetition frequency and 10.73 J single pulse energy injected by the single longitudinal mode seed, 3.31 J output energy is gained. The output laser beam has a 4.58 ns pulse width, far field beam spot of 2.12 times the value of the diffraction limit, and 0.87% energy stability (RMS).

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.

Narrow Linewidth Terahertz Generation Engined by All-Fiber Parametric Optical Source

Narrow linewidth terahertz (THz) generation based on difference frequency generation pumped by an all-fiber parametric light source is reported. A pulse source with an ultra-narrow bandwidth is built up as the pump seed source for the succeeding optical parametric amplification (OPA). A pair of phase-conjugated beams is generated as the seed light through all-fiber OPA. The amplified phase-conjugated light beams are injected into 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) to stimulate the emission of terahertz radiation. A THz source with linewidth less than 512 MHz at 1.748 THz pumped by an all-fiber light source is achieved.

Slowing down of light pulses using backward-wave four-wave mixing with local response

The slowing down of light pulses is achieved using backward-wave four-wave mixing in a medium with local response. A Bi12TiO20 crystal with an external dc field is used in the experiment as a proof-of-concept material. The delay and shape transformation of output pulses are studied and compared for the transmitted and phase conjugate channels. It is shown that the phase conjugate pulse achieves a longer delay under typical experimental conditions with equal intensities of the pump beams. This advantage of the phase conjugate beam is especially pronounced for short pulses with half-widths smaller than the response time of the medium. The agreement of the experimental results with numerical calculations confirms the validity of the previously developed theoretical model. Analytical solutions are derived for input pulses with triangular shapes.

One-Wave Optical Phase Conjugation Mirror by Actively Coupling Arbitrary Light Fields into a Single-Mode Reflector.

Rewinding the arrow of time via phase conjugation is an intriguing phenomenon made possible by the wave property of light. Here, we demonstrate the realization of a one-wave optical phase conjugation mirror using a spatial light modulator. An adaptable single-mode filter is created, and a phase-conjugate beam is then prepared by reverse propagation through this filter. Our method is simple, alignment free, and fast while allowing high power throughput in the time-reversed wave, which has not been simultaneously demonstrated before. Using our method, we demonstrate high throughput full-field light delivery through highly scattering biological tissue and multimode fibers, even for quantum dot fluorescence.

Dynamically reconfigurable characteristics of a double phase conjugate mirror using Sn2P2S6 crystals and their application to optical inter-satellite communication

A double phase conjugate mirror (DPCM), created by two mutually incoherent beams entering photorefractive nonlinear materials, can generate a phase conjugate beam whose reflectivity may be greater than 100%. Even though the conditions of the incident beams are changed, the DPCM can be dynamically reconfigured by using a Sn2P2S6 crystal with a high response speed. These features of the DPCM are advantageous, particularly in an optical inter-satellite communication system. In particular, use of the phase conjugate beam from the DPCM offers wavefront compensation and amplification in satellite communication. In addition, the dynamically reconfigurable DPCM using a Sn2P2S6 crystal relaxes the acquisition accuracy of the signal beam in the system. In this study, the temporal and spatial operating characteristics of the DPCM using a Sn2P2S6 crystal were first clarified. Next, an inter-satellite system based on the DPCM was proposed, and it was demonstrated that our system significantly improves the tolerance of the acquisition accuracy and tracking time.

Local heating of a liquid-like medium by a phase-conjugate ultrasound beam with self-adaptive focusing

Local heating of a liquid-like medium by means of self-adaptive focusing of a high-power phase conjugate ultrasound wave has been experimentally implemented. A sample of organic silicon polymer with a reflecting inhomogeneity was placed in water and exposed to a plane ultrasound beam so as to make the reflected acoustic field partially to enter the phase conjugation device. A parametrically amplified and conjugate wave was self-adaptively focused back to the inhomogeneity. Thermocouple measurements revealed ultrasound heating, which was localized near the inhomogeneity and depended weakly on its position within the aperture of the incident ultrasound beam. Themaximum heating value was about 6.5°C in 120 s with a frequency of ultrasound of 4.75MHz and a mean power of phase conjugate wave of about 0.1W.

Phase-conjugated twin waves for communication beyond the Kerr nonlinearity limit

The transmission of a pair of phase-conjugated beams is shown to mitigate nonlinear distortion during optical fibre communication, allowing a 400 Gbit s−1 superchannel to be sent over 12,800 km of optical fibre.

Optimizing experimental parameters on OPC in PMMA polymer doped azo dye

In this paper, the optical phase conjugation (OPC) using degenerate four wave mixing (DFWM) technique in a nonlinear DR1 chromophore doped in polymer PMMA thin films (by 8% W/W) is experimentally investigated. A CW Nd:YAG laser (532nm) with 50mW incident power has been used for (DFWM) technique to produce optical phase conjugation beam. The optimized reflectivity (R) relates to signal, forward and backward pump intensity titled with parameters M=IsignalIFP and q=IFPIBP. Furthermore, we have discussed temporal behavior and the maximum of phase conjugation reflectivity. The photo-induced reorientation of the DR1 by trans–cis–trans photoisomerization is the dominant mechanism for the OPC wave generation.

Compensating for Target Motion Between Seed Pulse and Phase Conjugate Beam Arrival Times at the Target

In nonmechanical beam steering with phase conjugate (PC) mirrors, the scattered and reflected beams from a foam target are utilized. A scattering and reflection model has been made that coincides with the experimental results with an error between ±15%. In the case where four probe beams irradiated a foam target, the three-dimensional intensity distribution was simulated. The peak-to-minimum variation of the reflected plus the scattered energy per unit solid angle was [approximately]20% of the average energy.The PC beam direction needs to compensate for target motion (v [approximately] 300 m/s) to accurately irradiate an injected target with laser beams in the method of beam steering with PC mirrors. The compensation with four-wave mixing utilized as a PC mirror is shown. A PC beam can be adjusted by properly setting the angle between two pump beams.

Coherent beam combination laser system using SBS-PCM for high repetition rate solid-state lasers

In this paper, the coherent beam combination technique using SBS-PCM for a high repetition rate laser will be demonstrated. It is a novel concept capable of achieving a high energy and high repetition rate laser. A phase conjugation mirror and beam combination laser using a self-phase-controlling technique for coherent beam combination is introduced in this paper. And, a practical demonstration of the coherent beam combination laser using SBS-PCM is presented. With a wave-front dividing scheme, the relative phases are measured to be less than λ/26 even when the amplifier is operating. The coherent beam combination laser using SBS-PCMs has the additional advantage of providing accurate high speed/high rate injection of the fusion target in the LIFE reactor system. The coherent beam combination of 25 100J/10Hz laser modules for the 2.5kJ/10Hz laser beam line for the fusion energy driver is suggested, which is available with the current laser technologies.

Target Separation from Sabot with Permanent Magnetic Array and Beam Steering Method with Four-Wave Mixing

We developed the permanent magnet array to separate a target from a sabot and analyzed the deceleration of the sabot velocity per unit length. The injection accuracy is about ± 12mm at one meter from the injector muzzle when the sabot velocity is around 30m/s. The method of compensation of a phase conjugate beam with four-wave mixing is shown. As one pump beam for four-wave mixing has a bit of longitudinal angle from the plane that another pump beam and a seed beam propagate, the phase conjugate beam tilts at the same angle from the plane. The displacement angle of the phase conjugate beam is compensated by adequately adjusting the angle between two pump beams. c

Slow light with photorefractive four-wave mixing

A slowing down of light pulses using backward-wave four-wave mixing is achieved in photorefractive crystals with different coupling strength. The delay and width of the output pulse are studied as a function of the input pulse width and pump intensity ratio for the amplified transmitted beam and for the phase-conjugated beam. The delay characteristics are compared with those of the two-beam coupling. It is demonstrated that the four-wave mixing process ensures a larger slowing down of short pulses (pulses with width shorter than the photorefractive response time) as compared to the photorefractive two-beam coupling scheme and guarantees the elimination of forerunners. The delay of long pulses in the four-wave mixing configuration is almost the same as the one that can be reached with two-beam coupling. The effect of absorption on the nonlinear shape transformation of the output pulses is discussed qualitatively.

Behaviours of excitonic optical nonlinearity associated with a large detuning rate photonic echo in biased asymmetric coupled quantum well structure

The third-order optical nonlinear susceptibility χ(3) responsible for phase-conjugate beam in non-degenerate four-wave mixing for a biased asymmetric coupled quantum well (ACQW) structure are given by the doubled-side Feynman diagram technique. It is shown that the photonic echo (PE) in excess of one terahertz pump-probe detuning rate can be achieved under low temperature condition due to electronic wavepaket oscillation when two laser beams interacts with the delocalized excitons in the ACQW structure. Also, the resonant peak position in the frequency domain of the large detuning rate PE depends strongly on an external inverse electric field strength but its peak intensity is insensitive to this one, resulting in the frequency conversion peak maintains a constant in a small bias range.