Self-stabilized nonlinear lateral modes of broad area lasers

The lateral modes of broad area lasers are investigated theoretically. The nonlinear interaction between optical field and effective refractive index leads to a saturable nonlinearity in the governing field equation, so that self-modulated solutions are found to be stable with increased current injection above saturation intensity. We derive approximate analytical solutions for traveling wave fields within the broad area laser. The field amplitude consists of a small ripple superimposed on a large dc value. Matching fields at the boundary determines the modulation depth and imparts an overall phase curvature to the traveling wave mode. There are multiple lateral modes for a given set of operating conditions, and modes with successively more lobes in the ripple have greater overall phase curvature. In contrast to the linear problem, several lateral modes can achieve the same modal gain, for a given injected current density, by saturating the gain to different extent. Thus, these modes would exhibit slightly different optical powers.     

Carrier-induced lensing in broad-area and tapered semiconductoramplifiers

Wavefront lensing due to nonuniform carrier saturation in rectangular (broad-area) and tapered GaAlAs amplifiers is described. Lensing dependence on current and input power, measured interferometrically and by a lens displacement method, agree with calculations based on a beam propagation model. Lensing induced astigmatism, calculated for a representative optical focusing system, exhibited minimal variation of less than 0.3 μm/A under saturated high power conditions in both amplifier types     

High power, diffraction-limited emission from monolithically integrated active grating master oscillator power amplifier (lasers)

A novel monolithically integrated master oscillator power amplifier with a distributed amplifier/grating output coupler is demonstrated which emits 370 mW in a single-frequency diffraction-limited beam without any form of active phase control. The distributed amplifier/grating output coupler results in a guided mode power that saturates along the direction of propagation, providing a uniform carrier density to preserve the flatness of the radiated phase front.<>     

Spontaneous filamentation in broad-area diode laser amplifiers

The stability of a plane wave propagating in single- and double-pass broad-area diode laser amplifiers is analyzed. It is found that laterally periodic perturbations exhibit a longitudinal variation that is a mixture of exponential growth and periodic oscillation. The latter oscillation has a spatial period related to the Talbot distance. Due to the interaction between these two effects, double-pass amplifiers become unstable against lateral perturbations, and the field spontaneously collapses into a periodic array of filaments. Experimental results are presented that show good agreement with the theoretical analysis     

Coherent operation of monolithically integrated master oscillator amplifiers

Monolithically integrated master oscillator power amplifiers have been fabricated. The oscillator is a second order distributed Bragg reflector (DBR) laser. The epitaxial layer structure consists of a double quantum well active region bonded by Al/sub 0.3/Ga/sub 0.7/As confining layers and Al/sub 0.4/Ga/sub 0.6/As cladding layers. The resultant output is a single longitudinal mode in a nearly diffraction limited output to a power level of 485 mW.<>     

Interferometric measurement of lateral phase profile and thermallensing in broad-area diode amplifiers

A Mach-Zehnder imaging interferometric is used to directly measure the lateral phase profile in the near field of broad-area semiconductor optical amplifiers with λ/25 phase resolution, or 0.1°C in temperature. The quadratic thermally induced phase distortion and its equivalent thermal lens focal length are characterized. Anomalous localized phase variations are correlated to thermal bond nonuniformities in devices mounted under nonoptimal conditions. It is demonstrated for an unbiased 600-μm-wide by 1000-μm-long amplifier device that a phase uniformity across the stripe of better than λ/25 can be obtained     

1 W, coherent, monolithically integrated master oscillator power amplifier

Monolithically integrated master oscillator power amplifiers (M-MOPA) have been fabricated consisting of a single mode distributed Bragg reflector oscillator coupled to a chain of nine single mode amplifiers and detuned second order grating output couplers. The M-MOPA emits in excess of 1 W in a coherent, single longitudinal mode, beam.<>     

Sensitivity of grating-coupled surface emitters to length-inducedphase variations

The longitudinal mode structure of a two-gain section, three-grating surface emitting laser is studied as a function of length-induced phase variations. It is found that several mode hops occur in the course of changing the length of one of the gain sections by one grating period. Although the number of mode hops can be reduced by optimizing the design, stable, single-mode laser operation appears to require control of the optical length of each gain and grating section to better than one wavelength     

Analysis of monolithic integrated master oscillator poweramplifiers

An analysis of a novel, monolithic integrated master oscillator power amplifier (M-MOPA) is presented. The M-MOPA consists of a DBR master oscillator which injects power into a linear chain of amplifiers and detuned second-order grating output couplers. The analysis self-consistently includes amplified spontaneous emission buildup and residual reflections throughout the amplifier stages. It predicts that output powers in excess of 1 W can be expected from a single-lateral-mode waveguide multistage amplifier less than 1 cm in length, injected with less than 15 mW of input power. In addition to the signal gain of >25 dB, the signal-to-noise ratio at 1-W output exceeds 15 dB. Because of the small reflections associated with the grating output couplers, and gain saturation by the injected signal, the amplifier self-oscillation threshold is suppressed to current densities above 15 kA/cm²     

2.0 W CW, diffraction-limited tapered amplifier with diode injection

The authors have demonstrated an all-semiconductor discrete-element 1.5 mm length tapered-amplifier MOPA (master oscillator power amplifier) emitting up to 2.0 W CW in a diffraction-limited pattern. The tapered single-pass amplifier MOPA exhibits a very high differential efficiency of 72% and a total energy conversion efficiency of 39% at 2.0 W CW output power. The high efficiency of the MOPA is attributed to the tapered amplifier design, which maximises extraction efficiency with a low-power injected beam of 25 mW from a single-mode laser diode. Such an MOPA will have applications where compact, high-efficiency, diffraction-limited sources of radiation are required, such as frequency doubling and free-space communication.<>