Beam combining of laser arrays with high efficiency and good beam quality for power and radiance (brightness) scaling is a long-standing problem in laser technology. Recently, significant progress has been made using wavelength (spectral) techniques and coherent (phased array) techniques, which has led to the demonstration of beam combining of a large semiconductor diode laser array (100 array elements) with near-diffraction-limited output (M/sup 2//spl sim/1.3) at significant power (35 W). This paper provides an overview of progress in beam combining and highlights some of the tradeoffs among beam-combining techniques.

Laser beam combining for high-power, high-radiance sources

We report degenerate four-wave mixing (DFWM) of visible radiation in borosilicate glasses doped with crystallites of the mixed semiconductor CdSxSe1−x. These semiconductor-doped glasses—available commercially in the form of colored glass filters—exhibit third-order nonlinearities of ~10−9–10−8 esu for DFWM with short (~10-nsec) laser pulses at various visible wavelengths. Our studies on the temporal decay of the transient gratings indicate that the nonlinearity is not thermal in origin but may be attributed to the generation of a short-lived electron–hole plasma. In contrast with DFWM experiments in other semiconductors invoking gratings of optically generated carriers (or other mobile particles), we report unique diffusion-independent decay of the gratings in these glasses; this is deduced from the dependence of the intensity and polarization of the DFWM signal on the polarization combinations of the input beams. Finally, we report detailed data on the aberration-correction properties of these isotropic glasses.

Degenerate four-wave mixing in semiconductor-doped glasses

Nonlinear optical mixing can be used to perform a variety of new optical functions, as well as real-time holography. The theory and some of the first experiments are described.

Phase conjugate optics and real-time holography

Phase conjugate optics and real time holography (A)

Active optical systems are those in which real-time control over optical wavefronts is employed to optimize system perference in the presence of random distrurbances. Applications include 1) outgoingwave systems for maximizing the power density of a laser beam on a target and 2) received-wave systems for maximizing the angular resolving power of a telescope viewing a distant object through a turbulent atmosphere. In this tutorial-review paper, the basic concepts of active optics systems and their historical evolution are discussed, from early figurecontrol systems with a servo bandwidth of less than 1 Hz to the recently developed high-bandwidth systems for atmospheric compensation with bandwidths of several hundred hertz. A critical comparison of the various approaches to wavefront sensing is then made covering both coherent (laser) and incoherent (white-light)systems. Current techniques for wavefront correction including Bragg cells, segmented mirrors, thin-plate deformable mirrors, monolithic mirrms, and membrane mirrors are described. The performance analysis and optimization of closed loop systems is covered using two basic models. The paper concludes with a review of the design and performance of five current experimental active optical systems, with some comments on future applications.

Active optics: A new technology for the control of light

We have experimentally demonstrated a laser energy-scaling technique, based on a phase-conjugate oscillator/amplifier scheme that permits the coupling of several parallel amplifiers to form a single coherent output beam. This, is achieved despite dramatic differences in the individual amplifier energies and optical path lengths. When many amplifiers are coupled together, energy can be scaled significantly beyond the limit imposed by the maximum available volume of a single active medium. Although the present demonstration utilized Nd:YAG, this technique is quite general and can be applied to a variety of laser media.

Coherent coupling of laser gain media using phase conjugation.

We have obtained quantitative measurements on the correction of severely aberrated laser beams using stimulated Brillouin scattering (SBS) at 0.69 μm. We have shown that under certain conditions SBS can be used to restore an aberrated optical beam to its original unaberrated condition. When an optical beam double passes an aberrating region after reflecting from an “ordinary” mirror (i.e., a plane mirror) the aberration is twice that obtained from a single pass. However, when the aberrated beam enters a medium that allows SBS to occur, it emerges from its second pass through the aberrating medium in the same condition as that in which it originally entered. Quantitative experiments are described in which a single-mode ruby laser beam is intentionally aberrated by passing it through an etched plate. When the beam is allowed to double-pass the plate using an ordinary reflector (i.e., plane mirror), the beam divergence is more than 10 times the diffraction-limited divergence. However, when we replace the ordinary reflector with a cell in which SBS can take place, the SBS reflected beam is restored to diffraction-limited divergence when it is allowed to pass back through the aberrating medium. Applications of this time-reversal or phase-reversal technique for correcting aberrations in optical trains and atmospheric turbulence are discussed.

Correction of phase aberrations via stimulated Brillouin scattering

We report experimental observation of polarization rotation in resonant degenerate four‐wave mixing (DFWM) arising from the third‐order coupling of the electric fields. This effect allows investigation of molecular motion and its effect on the efficiency of the DFWM. By controlling the pulse width and therefore γ=T0/τp (where T0≡λ/vth) we show quantitative agreement with a third‐order density matrix perturbation calculation. In particular, using a CO2 laser at 10.6 μm and SF6 as the nonlinear medium, we show that for γ≫1 (stationary limit) we see no molecular motion and for γ≈1, we see a significant ’’washout’’ of one third–order polarization.

Polarization‐rotation and thermal‐motion studies via resonant degenerate four‐wave mixing

A resonantly enhanced phase‐conjugate 10.6‐μm signal has been generated in the saturable absorber SF6. Direct evidence of aberration correction is shown through the use of far‐field photographs taken with a pyroelectric vidicon camera. Reflection coefficients of 7% have been achieved in a few Torr of SF6 for a 2‐cm interaction length.

Phase conjugation at 10.6 μm by resonantly enhanced degenerate four‐wave mixing

Laser apparatus which employs the principles of phase conjugation to provide for short-pulse operation. A phase-conjugate reflector (21), lasing medium (22), and output coupling device (23) are arranged to form a laser resonator. The lasing medium may be pumped by a conventional pump source (25) to produce population inversion in the lasing medium. The phase-conjugate reflector is pulsed by means of a pulsed pump source (24) which applies a periodic train of short laser pulses having high peak power and low average power thereto. The short-duration pulses create high reflective efficiencies in the phase-conjugate reflector, while minimizing the overall energy demands on the system. Additionally, the pump source which pumps the lasing medium may also provide pulsed energy thereto which is synchronous with the pulsed energy provided by the pulsed pump source to the phase-conjugate reflector. Accordingly, the system provides for a series of short pulsewidth laser pulses at its ouput.

Concetto R. Giuliano

Papers

Coherent coupling of laser gain media using phase conjugation.

Correction of phase aberrations via stimulated Brillouin scattering

Polarization‐rotation and thermal‐motion studies via resonant degenerate four‐wave mixing

Phase conjugation at 10.6 μm by resonantly enhanced degenerate four‐wave mixing

Synchronously-pumped phase-conjugate laser