This paper reviews recent results on the power scaling of solid-state lasers using phase conjugation. We present the basic ideas involved in employing phase conjugation to compensate thermally induced aberrations in the laser medium, and we discuss the results of recent experiments in which birefringence in an active laser amplifier was successfully compensated.
We demonstrate phase-conjugate reflectivities on the order of 1000 at frequency detunings on the order of 0.5 GHz in intracavity nondegenerate four-wave mixing in a semiconductor laser. Our measurements show that, unlike a third-order nonlinearity, the conjugate reflectivity does not scale like the pump power squared, indicating that saturation and cavity effects are important in the generation of the conjugate.
We investigate the phase-conjugated signal reflectivity and conjugation fidelity in the degenerate four-wave mixing process in a monomode birefringent optical fiber, in the nonlinear (depleted pumps) regime. Exact solutions are displayed for linearly polarized beams in a high-birefringence fiber, whereas for a low-birefringence fiber similar results hold for a scheme employing circular polarizations. The reflectivity is found to exhibit multistable isolated branches of solutions, and the deterioration of ideal phase conjugation condition are discussed. Whenever the birefringence gives phase shifts comparable to the nonlinear ones, spatial instabilities may originate chaotic power exchange between the waves. The scale of length and the powers required for the observation of the instability are estimated.
We propose a new technique for optical phase conjugation of broad band laser beams using stimulated Brillouin scattering. The application of the technique for correcting phase aberrations in pump beams with bandwidths up to 30 GHz is demonstrated. Extension to greater bandwidths is discussed.
Analytical theory of the enhanced stimulated Brillouin scattering arranged in four wave mixing configuration has been developed in the strong interaction approximation as well as in the transient regime.
Four wave mixing (FWM) performance of sodium vapor was investigated in the strong pump regime (IÃ‚Â»I at) necessary to achieve good phase conjugate reflectivity, R. Reflectivities >230% were observed using narrowband CW pump powers less than 1 W. Degenerate FWM spectral response was measured with R as a parameter, and shown to depend on self-focusing effects at higher R. The field of view of the sodium FWM was determined under narrowband high R conditions and found to behave as expected, except for nearly collinear geometries. Faith-ful imaging through a severe optical aberration was demonstrated at moderate R, but experimental observations and analysis indicate potential fidelity problems at large R. Reflect-ivity and field of view were also measured for wideband (2 GHz) laser pumping.
Phase conjugate reflectivity scaling by four wave stimulated Brillouin scattering (FWSBS) was investigated experimentally and compared to analytic expressions for the reflectivity in the weak probe and saturated pump regimes. Experiments were performed in acetone near 1.06 Lin to study the dependence of reflectivity on probe beam intensity, forward pump beam intensity and the ratio of forward to backward pump beam intensity. We observed reflectivities as high as 1000x and efficiencies as high as 16%,. We found good agreement with simple theoretical predictions by Shih1 for reflectivity and efficiency.
We studied the evolution of patterns of scattered light in a barium titanate crystal, during the process of self-pumped phase conjugation in a system with no external beam-reflecting elements. We also studied the effect on the back-propagating phase-conjugate output of erasing small successive portions of the photorefractive volume grating. We interpret the results of these studies as indicating the formation of reflection gratings by feedback, from back-propagating beams produced by initial transmission gratings. The transmission gratings are initiated by power in reflective optical cavities between cube corners. The light in these cavities is the result of forward holographically amplified scattering (fanning). Transmission gratings, resulting from light back-scattered off the microrough edge of the cube, also contribute to the back-propagating beam.
We describe a new type of phase-conjugate mirror based on Brillouin enhanced four-wave mixing that has many attractive features distinguishing it from previous types of phase-conjugate mirrors. Chief among these features are its high reflectivity, ease of alignment, and its insensitivity to aberrations on the pump waves. Both theoretical and experimental results are presented.
Crosstalk between two beams incident on a phase-conjugating four wave mixer (FWM) was investigated as a function of beam input angles and beam intensities for the case where both input beams and the FWM pumps were the same frequency. Intensity crosstalk in the conjugated return beams was observed. Spatial image crosstalk was also observed. Analysis shows that spatial and intensity crosstalk arise from the same mechanism, a competition for pump intensity that occurs when the sum of the probe and noise intensities is more than a few percent of the pumps. A model of this mechanism has been developed that permits quantitative prediction of intensity crosstalk.
We describe the use of a single crystal of BaTiO3 as a phase-conjugator/modulator. This class of retromodulator is alignment-free, simple, and passive. By applying a time-varying electric field across a self-pumped phase conjugate mirror, simultaneous wave-front reversal and temporal modulation encoding occur within the same volume. With modest signal fields (-4120 V/cm) we observe 100% depth-of-phase modulation, with a measured bandwidth in excess of 1 MHz (scalable to >10 GHz), and with excellent conjugation fidelity. Furthermore, we observe a conjugate reflectivity of -73% using this simple geometry. Applications of such devices to remote free-space and fiber sensors and to phase-conjugator resonator configurations are discussed.
Electrostrictive Kerr effects are analyzed and formal relationships are derived which relate the nonlinear refractive index n2 to photoelastic coefficients. It is shown that information on the electrostrictive contributions to n2 may be readily obtained from acousto-optic figures-of-merit.
We have obtained coherent operation of multiple pulsed dye oscillators by using a single phase conjugate mirror (PCM) as a common end mirror for up to three physically distinct cavities. The PCM operates via the photorefractive effect in a crystal of BaTiO3, through a four-wave mixing process which requires no external pumping beams. The system has been operated in both a narrow band (0.05 nm) and a broadband (0.25 nm) configuration.
Ce-doped tungsten bronze Ba2-xSrxK1-yNayNb5O16 (BSKNN) ferroelectric crystals have A been grown by the Czochralski technique, Rand arefound to be of optical quality with excellent photorefractive properties. Although the BSKNN crystals studied have a tetragonal bronze structure at room temperature, their growth habits are different: BSKNN-1 grows in a square shape with four well-defined facets, while BSKNN-2 crystals have an octahedron shape with eight well-defined facets. Ferroelectric and optical measurements show these crystals to have strong longitudinal effects similar to perovskite BaTiO3 and excellent self-pumped phase-conjugate behavior.
In this paper we examine particle density fluctuations as an intrinsic source of noise in an artificial Kerr medium. A general method is developed for determining spontaneous grating fluctuations (identified here as a form of thermal noise) directly from the equilibrium grating response to an applied electromagnetic field. Calculations are given for the predicted root-mean-square (RMS) signal fluctuations in four-wave mixing and compared with experiment. Grating fluctuations are shown to account for the previous discrepancy between predicted and measured RMS fluctuations in the conjugate wave signal generated from an aqueous polystyrene microsphere suspension.
A Michelson Interferometer using two photorefractive BaTiO3 crystals was studied in which one BaTiO3 was used to generate a self-pumped phase-conjugate wave. This conjugate wave forms theone arm of the interferometer and was also used as one of the pumping beams to pump the other BaTiO3 crystal which generated the other phase-conjugate wave through four-wave mixing. The output from the two arms are therefore both phase-conjugate waves. The interference pattern shows excellent spatial intensity distribution and is insensitive to the reciprocal noises and optical path length. Absolute phase of the phase conjugator was also measured which is a function of the crystal orientation and can be used to bias the interferometer by simply rotating the crystal to obtain an operating point of highest sensitivity and linear response.
A portion of photorefractively fanned light, in the shape of a ring, is absent (i.e., appears as a dark ring) when two laser beams of extraordinary polarization are transmitted through a BaTiO3 crystal. These dark rings are due to two sets of index gratings destructively interfering to eliminate part of the fanning hologram. Bragg phase-matching conditions determine the shape and size of dark rings.
The laser wavefront compensation system (LWCS) is a closed-loop adaptive optics sytem whose purpose is to clean up optical aberrations in low power visible laser beams. The LWCS employs no moving parts and operates with either continuous-wave (CW) or pulsed inputs. In the pulsed mode, it will operate asynchronously over a wide range of pulse rates. The electronic data processing is entirely digital, including the wavefront reconstruction function. The deformable mirror employs a square array of 64 actuators, with a wavefront correction capability of ±600 nanometers. This paper describes the operating principles and hardware of the LWCS, together with data on the performance of the system.
This paper describes a Hartmann type wavefront sensor designed for astronomical image compensation. The sensor uses real-time digital electronics to first compute the wavefront phase gradients over 37 subapertures and then perform a matrix multiply phase reconstruction.
A small low speed wavefront sensor has been built for Harvard Smithsonian Observatory for astronomical observing. The sensor measures wavefronts using a small lenslet array to focus light as spots on a detector array. The position of these spots are used to measure the wavefront distortion. The detector array output is digitized and processed by a high speed image processor. The processor outputs the directional derivatives of the wavefront in the X and Y directions.
Single pass Raman amplification experiments were conducted in hydrogen using two xenon fluoride laser beams originating from the same laser device. These beams were crossed at shallow angles (milliradians) using a Mach-Zehnder optical arrangement. A seed beam at the hydrogen vibrational Stokes shifted wavelength of 414 nm was aligned to bisect the angle made by the two pump beams.
We calculate the Raman gain lineshape including both the optical Stark effect and pressure broadening for various rotational transitions of N2[S(1)-S(14)] and for different laser/Stokes polarizations. At sufficiently high intensity the peak gain is lowered, and the line is both shifted and distorted.
This paper reports progress in development of a general purpose nonlinear, optical model. The long term objective of this modeling effort is to achieve three-dimensional, time-dependent analysis with engineering detail with respect to all pertinent components and processes. The near term objective of this program is to model stimulated Raman and Brillouin scattering. We require a numerical code which can be run in reasonable time on mini- or supermicrocomputers. In this paper, techniques are described for efficient computation of Raman and four-wave mixing effects and the results of representative problems are presented.
Experimental and analytical work on the effects of temporal correlation and linear dispersion in broadband and multiwavelength Raman conversion will be presented. An interferometric technique for demonstrating the influence of the correlation properties of the Stokes seed and pump beam(s) on the spatial coherence of the amplified Stokes will be discussed. Analytical work on the effect of linear dispersion on conversion efficiency and beam cleanup in two-line XeF Raman beam combination is also presented.
We examine conditions under which pump beam replication can occur in Raman beam cleanup with crossed pump beams. The dependence of pump beam replication on the small signal Raman gain and on the Stokes seed energy is explored.
The physical mechanism for Raman gain suppression due to Stokes/anti-Stokes coupling under phase matched conditions is clarified by a discussion of energy transfer in Raman resonant four-wave mixing. The fallacy of a common misconception that this mixing does not transfer energy between the light and the Raman medium is illustrated.
This paper describes a phase-shifting, self-referencing, radial-shear (10:1) interferometer, designed for testing a pulsed XeC1 laser (308 nm) that has been Raman-shifted into the blue (459 nm). It is the first report of an implementation of a general class of phase-shifting interferometers without moving parts: the phase shift and separation of beams are obtained with a stationary diffraction grating. The presentation includes results from early ex-periments, a discussion of the good and bad aspects of this type of interferometer, and a review of the systems where its use might be appropriate.
A laser beam quality diagnostic has been developed for making up to 500 measurements per second in the 1-to 5-μm spectral range. The primary sensors are a PtSi detector array and a separate PtSi total power detector. The computer-driven diagnostic includes a beam stabilization servo and focus scanning device for finding the best laser focus.
Optical spatial tracking of a laser beam can be accomplished using quadrant detector error signals obtained from either direct detection or heterodyning. In the direct detection system the received beam is focused directly on the quadrant, and error signals are generated as the beam moves over the detector. In heterodyned systems the received field is mixed with a local laser field on the quadrant detector for error generation. Expressions to evaluate angle error variance due to receiver noise are formulated. The tracking error variance depends on signal-to-noise ratio, quantum efficiency, and the slope of the loop S curve. The loop S curve is derived differently for the two cases, and its specific form is important to the overall performance analysis. Heterodyne detection receivers are less susceptible to background radiation and device noise than direct detection receivers. However, this advantage can be compensated for direct detection system by increasing the received signal power, reducing front end bandwidth, or reducing the number of spatial modes. Design considerations of the laser beam propagating in turbulent atr hphere and multi-mode results are also addressed. Degradation due to heterodyning misalignment is also considered. The results of the study allow direct comparison of both system as possible beacon tracking subsystems. Curves will be shown to relate the key parameter that will determine which method is the most advantageous.
In this paper, a new tracking approach, a laser scanning tracking method (LSTM) is proposed. The LSTM has been designed to track a cylindrical retroreflective target mounted on the object, which makes plane motion. The retroreflector pasted by scotchlite reflective sheeting (mad. in 3M ,0.) i s located by scanning a laser beam in holizontal. When the retroreflector is struck, its position that is azimuth is read by microcomputer and the aiming device is servocontrolled by microcomputer according to this azimuth immediately. This is a step-by-step tracking method. The time of servo-reponse is less than one millisecona in actual tests. The angular accuracy is less than 0.5 milliradian. The track angular velocity is greater than one radian/second.