Variable focusing due to refractive-index gradients in a diode-array traveling-wave amplifier

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Abstract
Focusing of beams injected into a diode‐array traveling‐wave amplifier has been observed. The gradient in the complex index of refraction responsible for this lensing is well described by a quadratic profile centered over the array. Measurements both resonant and nonresonant with the gain spectrum indicate that the real part of the refractive index associated with a thermal gradient is the dominant source of the index gradient. The strength of the quadratic refractive‐index coefficient varies from 0 to 0.19 mm−2 for amplifier currents between 0 and 550 mA. Adjustment of the focusing of the injected signal by translating a single cylindrical lens in an anamorphic telescope allows for near‐diffraction‐limited performance at any current. A far‐field lateral beam divergence of 0.37° was observed for a 110‐μm aperture (the calculated diffraction limit is 0.36°).