Phase-conjugate Mirror Research Articles

Rotational Doppler shift of a phase-conjugated photon

The rotational Doppler shift of a photon with orbital angular momentum $\pm \ell \hbar$ is shown to be an even multiple of the angular frequency $\Omega$ of the reference frame rotation when photon is reflected from the phase-conjugating mirror. We consider the one-arm phase-conjugating interferometer which contains $N$ Dove prisms or other angular momentum altering elements rotating in opposite directions. When such interferometer is placed in the rotating vehicle the $\delta \omega=4 (N+1/2) \ell \cdot \Omega$ rotational Doppler shift appears and rotation of the helical interference pattern with angular frequency $\delta \omega /{2 \ell}$ occurs. The accumulation of angular Doppler shift via successive passage through the $N$ image-inverting prisms is due to the phase conjugation, for conventional parabolic retroreflector the accumulation is absent. The features of such a vortex phase conjugating interferometry at the single photon level are discussed.

Performance of large aperture tapered fiber phase conjugate mirror with high pulse energy and 1-kHz repetition rate

A large aperture fused silica tapered fiber phase conjugate mirror is presented with a maximum 70% stimulated Brillouin scattering (SBS) reflectivity, which is obtained with 1 kHz repetition rate, 15 ns pulse width and 38 mJ input pulse energy. To the best of our knowledge, this is the highest SBS reflectivity ever reported by using optical fiber as a phase conjugate mirror for such high pulse repetition rate (1 kHz) and several tens of millijoule (mJ) input pulse energy. The influences of fiber end surface quality and pump pulse widths on SBS reflectivity are investigated experimentally. The results show that finer fiber end surface quality and longer input pulse widths are preferred for obtaining higher SBS reflectivity with higher input pulse energy. Double passing amplification experiments are also performed. 52 mJ pulse energy is achieved at 1 kHz repetition rate, with a reflected SBS pulse width of 1.5 ns and a M(2) factor of 2.3. The corresponding peak power reaches 34.6 MW. Obvious beam quality improvement is observed.

Time-reversed ultrasonically encoded optical focusing in biological tissue

We report an experimental investigation of time-reversed ultrasonically encoded optical focusing in biological tissue. This technology combines the concepts of optical phase conjugation and ultrasound modulation of diffused coherent light. The ultrasonically encoded (or tagged) diffused light from a tissue sample is collected in reflection mode and interferes with a reference light in a photorefractive crystal (used as a phase conjugation mirror) to form a hologram. Then a time-reversed copy of the tagged light is generated and traces back the original trajectories to the ultrasonic focus inside the tissue sample. With our current setup, we can achieve a maximum penetration depth of 5 mm in a chicken breast sample and image optical contrasts within a tissue sample with a spatial resolution approximately equaling 1/√2 of the ultrasound focal diameter.

High-resolution four-photon intracavity laser spectroscopy with the external phase conjugation mirror

The phase conjugation regime is experimentally implemented in a cell with a liquid (water, ethanol, carbon tetrachloride) at the four-wave mixing of counter-propagating beams of diode lasers. Intracavity enhancement of the conjugate wave and the wave of the coherent anti-Stokes Raman scattering (CARS) arising in the interaction of the conjugate wave and one of the pumping waves in CARS scheme is detected. The latter process occurs in a narrow spectral range when the geometry of the wave vectors of the interacting waves in CARS scheme ensures arrival of the CARS signal at the laser aperture. It is found out that the quality of the four-photon phase conjugate mirror in the given interaction scheme is very sensitive to any perturbations (mechanical, acoustic, thermal, etc.) of the induced grating in the sample, which manifests itself in the distortion of the signal spectrum.

Modulation response characteristics of a semiconductor laser with phase-conjugate feedback

Modulation response characteristics of a semiconductor laser with a phase-conjugate feedback mirror is studied. Modulation response equations about the amplitude and frequency chirping of the laser are deduced in detail. It is demonstrated that the amplitude and frequency chirping responses are the function of the modulation angular frequency, optical feedback level, and delayed time. Numerical results illustrate that the amplitude response and modulation bandwidth of the laser are larger than that of the solitary laser when the optical amplitude reflectivity is <0.0022. When the optical amplitude reflectivity is increased further, the peaks of the amplitude response of the laser show periodic variability while the modulation angular frequency at the maximal value of the amplitude response is added; however, the frequency chirping can be enlarged or reduced. The large drive current or the large detuning can enhance the modulation response and frequency chirping.

Control of phase locking in a set of lasers with self-pumped phase-conjugate gain-grating mirrors using a passive Q-switch

We studied the possibility of controlling phase-locked lasing of a set of lasers with self-pumped phase-conjugate gain-grating mirrors by using a passive Q-switch and selecting the gain of one of the lasers. The ranges of stable operation are calculated by mathematical simulation and well agree with the experimental results. Single-mode radiation in the form of trains of phase-locked laser pulses are experimentally obtained for the first time in a three-channel diffraction-coupled gain-grating Nd : YAG laser system with a LiF :F −2 passive Q-switch. The space-time pattern of the trains of 1.064-μm nanosecond pulses is the same in the channels with and without Q-switch and their interference pattern contrast reaches 0.81.

Loop laser cavities with self-pumped phase-conjugate mirrors in low-gain active media for phase-locked multichannel laser systems

It is proved that lasers with different loop cavities with self-pumped phase-conjugate mirrors in low-gain active media can operate under injection of external laser radiation and can be used for the development of diode-pumped phase-locked multichannel neodymium laser systems operating both on the fundamental laser transition with the wavelength λ = 1.06 μm and on the transition with λ = 1.34 μm. The phase-conjugate oscillation thresholds in the case of injection of an external signal are determined for a multiloop cavity configuration and an increased number of active elements in the cavity. It is shown that phase-conjugate oscillation can occur even if the single-pass gain of the active element is as low as only ∼2. Under high-power side diode pumping of a multiloop Nd:YAG laser, single-mode output radiation was achieved at λ = 1.064 μm with a pulse energy up to 0.75 J, a pulse repetition rate up to 25 Hz, an average power up to 18.3 W, and an efficiency up to 20%. In a multiloop Nd:YAG laser with three active elements in the cavity, single-mode radiation at λ = 1.34 μm was obtained with a pulse energy up to 0.96 J, a pulse repetition rate up to 10 Hz, and an average power up to 8.5 W.

The physical optics of phase-conjugate mirrors: Scalar theory

A scalar theory of physical optics is developed for the scattered waves by finite or infinite phase-conjugate mirrors. The method is applied to the scattering problem of waves by a semi-infinite phase-conjugate mirror. The results are examined numerically and compared with the scattered waves by a conducting half-plane. The effect of the phase-conjugation to the location of the transition region is outlined.

Reflection-mode time-reversed ultrasonically encoded optical focusing into turbid media

Time-reversed ultrasonically encoded (TRUE) optical focusing was recently proposed to deliver light dynamically to a tight region inside a scattering medium. In this letter, we report the first development of a reflection-mode TRUE optical focusing system. A high numerical aperture light guide is used to transmit the diffusely reflected light from a turbid medium to a phase-conjugate mirror (PCM), which is sensitive only to the ultrasound-tagged light. From the PCM, a phase conjugated wavefront of the tagged light is generated and conveyed by the same light guide back to the turbid medium, subsequently converging to the ultrasonic focal zone. We present experimental results from this system, which has the ability to focus light in a highly scattering medium with a round-trip optical penetration thickness (extinction coefficient multiplied by round-trip depth) as large as 160.

Time-reversed ultrasonically encoded optical focusing into tissue-mimicking media with thickness up to 70 mean free paths.

In turbid media such as biological tissue, multiple scattering hinders direct light focusing at depths beyond one transport mean free path. As a solution to this problem, time-reversed ultrasonically encoded (TRUE) optical focusing is proposed based on ultrasonic encoding of diffused laser light and optical time reversal. In TRUE focusing, a laser beam of long coherence length illuminates a turbid medium, where the incident light undergoes multiple scattering and part of it gets ultrasonically encoded within the ultrasonic focal zone. A conjugated wavefront of the ultrasonically encoded light is then generated by a phase conjugate mirror outside the medium, which traces back the trajectories of the ultrasonically encoded diffused light and converges light to the ultrasonic focal zone. Here, we report the latest experimental improvement in TRUE optical focusing that increases its penetration in tissue-mimicking media from a thickness of 3.75 to 7.00 mm. We also demonstrate that the TRUE focus depends on the focal diameter of the ultrasonic transducer.

用于千赫兹高能量MOPA激光系统的大口径锥度光纤相位共轭镜特性研究

用于千赫兹高能量MOPA激光系统的大口径锥度光纤相位共轭镜特性研究

Optical interconnection between different spatial modes using double phase conjugate mirror with polarization alignment system

We propose a fundamental interconnection method using a polarization alignment system for waveguides having different spatial modes. In addition, as an example for the verification of the fundamental operation, we demonstrate an interconnection between a photonic crystal fiber and a laser that have obviously different spatial modes. The polarization alignment system operates synergistically with a self-written waveguide formed with a double phaseconjugate mirror. This technique enables us to interconnect a photonic crystal fiber with a laser source without complicated and time-consuming optical alignment. In this method, although it is not necessary to perform an external control for interconnection, the waveguide most suitable for connection is formed autonomously in a Sn2P2S6:Sb crystal developed for this purpose. There was a marked reduction in the polarization dependence of coupling efficiency, compared with that observed using a stand-alone double phase-conjugate mirror.

用于千赫兹高能量MOPA激光系统的大口径锥度光纤相位共轭镜特性研究

用于千赫兹高能量MOPA激光系统的大口径锥度光纤相位共轭镜特性研究

Double Phase Conjugate Mirror with Sb-Doped Sn2P2S6 Crystal for Different Incident Angles, Crystal Thickness, Total Incident Intensity and Polarization

We developed photorefractive Sb-doped Sn2P2S6 samples of different crystal thickness to investigate the properties of the media for double phase conjugation. The Sn2P2S6:Sb crystals have high coupling coefficient of 20 cm−1, and show fast response time of 1.3 ms at total incident intensity of 0.3 W/cm2. The maximum phase conjugate reflectivity of 32% was observed with a 4 mm thickness sample. To investigate the characteristics of Sn2P2S6:Sb for double phase conjugation, on the basis of the coupled wave equations, we showed the dependence of phase conjugate reflectivity on incident angle, crystal thickness, total incident intensity and polarization. From these results, the optimum incident condition was clarified for obtaining the highest phase conjugate reflectivity.

用于千赫兹高能量MOPA激光系统的大口径锥度光纤相位共轭镜特性研究

用于千赫兹高能量MOPA激光系统的大口径锥度光纤相位共轭镜特性研究

Transparency in Bragg scattering and phase conjugation

Reflectionless transmission of light waves with unitary transmittance is shown to occur in a certain class of gain-grating structures and phase-conjugation mirrors in the unstable (above-threshold) regime. Such structures are synthesized by means of the Darboux method developed in the context of supersymmetric relativistic quantum mechanics. Transparency is associated to superluminal pulse transmission.

Large aperture tapered fiber phase conjugate mirror in MOPA laser systems with high repetition rate and high pulse energy

A large aperture tapered fused silica fiber phase conjugate mirror with a maximum 50.7% stimulated Brillouin scattering (SBS) reflectivity is presented, which is operated with 400 Hz pulse repetition rate and 36.5 mJ input pulse energy. To the best of our knowledge, it is the first time that over 50% SBS reflectivity is achieved by using solid-state phase conjugate mirror under such high pulse repetition rate and high pulse energy. With much higher pulse repetition rate of 500 and 1000 Hz, the maximum SBS reflectivity is 41.2% and 33.3%, respectively. A single-longitudinal-mode Nd:YAG laser is experimentally studied with master oscillator power amplifier (MOPA) scheme using such a tapered fiber as a phase conjugate mirror. A 101 mJ pulse energy is achieved at 400 Hz repetition rate, with a pulse width of 6 ns and a M 2 factor of less than 2. The corresponding peak power reaches 16.8 MW.

400-Hz pulsed single-longitudinal-mode Nd:YAG laser with more than 100-mJ pulse energy and good beam quality

An all solid state master-oscillator-power-amplifier laser system was experimentally investigated using a pulsed-diode-pumped single-longitudinal-mode Nd:YAG laser as a master oscillator. The system was operated with 400 Hz pulse repetition rate. The amplifiers contain a pre-amplifier and a power amplifier. A specially designed large aperture tapered fiber as a solid state phase conjugation mirror was used in the power amplifier to improve the beam quality. The maximum pulse energy of 101 mJ is obtained after double-pass amplification. The pulse duration is less than 6 ns. The corresponding peak power reached 16.8 MW for each single pulse.

Theory of Electromagnetic Time-Reversal Mirrors

The theory of monochromatic time-reversal mirrors (TRM) or equivalently phase conjugate mirrors is developed for electromagnetic waves. We start from the fundamental time-symmetry of the Maxwell's equations. From this symmetry, a differential expression similar to the Lorentz reciprocity theorem is deduced. The radiating conditions on TRM are expressed in terms of 6-dimension Green's functions. To predict the time reversal focusing on antenna arrays, a formalism that involves impedance matrix is developed. We show that antenna coupling can dramatically modify the focal spot. Especially, we observe, that in some circumstances, sub-wavelength focusing on a bi-dimensional array may arise.

Four-Beam Coherent Combination by Stimulated Brillouin Scattering with Wavefront Division

Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea(Received 1 February 2010, in final form 10 July 2010)Using stimulated Brillouin scattering phase conjugate mirrors (SBS-PCMs), we conducted a co-herent beam combination of wavefront-divided beams. “Self-phase-controlled” SBS-PCMs wereused to lock and control the phases of the SBS wave. Using an interferometer, we monitored therelative phase differences of the four spatially divided beams in real time. With long-term phasestabilization, the phase differences between the reference beam and the other three beams were wellstabilized at standard deviations of λ/36.3, λ/35.8, and λ/43.4 for a duration of 2500 shots (250 s).