Broadband Watt-Level Mid-Infrared Fiber-Coupled Variable Optical Attenuator

Direct intensity modulation of laser systems in the mid-infrared is marred by instabilities in power output, emission wavelength, and mode profile. In particular, emission characteristics of current implementations of quantum cascade lasers are especially susceptible to modulations in the driving current. We present a broadband mid-infrared variable optical attenuator with large dynamic range and sub-millisecond response time. The modulator has demonstrated 52 dB dynamic range (limited by the detector noise) and
${>}{\rm 1}~{\rm kHz}$
frequency response, and has been tested at various wavelength covering 3.5–4.8
$\mu{\rm m}$
. The system is composed of a piezoelectric controlled tilt-mirror that images the laser beam into a mid-infrared transmitting optical fiber. The use of broadband anti-reflection optics in the system maintains a high throughput of
${>}{90\%}$
. Power handling of the system has been confirmed up to 4 W (limited by the laser output power).