A prototype compact, rugged optomechanical module contains a high-power, wideband laser-diode transmitter. The laser diode is of a commercial single-quantum-well AlGaAs type. Each laser diode of this type is manufactured for a specific nominal wavelength in the range from 810 to 860 nm; the one in the prototype module lases at a nominal wavelength of 824 nm. The laser diode can be operated to emit continuous-wave power of 150 mW or with amplitude modulation at average and peak powers of 150 and 300 mW, respectively. The power consumption of the entire module in dc operation is 400 mW. The laser diode is mounted on a copper plate, which conducts heat from the laser to a cold plate on which the module is mounted. The cold plate is maintained at a temperature of 15 °C.

A circuit board mounted on the copper plate next to the laser diode incorporates both dc and ac modulation electronics. Also included on the circuit board are a thermistor and a resistive heater for sensing and regulating the temperature of the laser diode. The modulation electronics include a reactive network matching circuit that enables the use of modulation frequencies up to a 3-dB-falloff frequency of 2.5 GHz. The laser diode, circuit board, and copper plate are all epoxied to a block of low-thermal-expansion glass, providing a stable platform from which to collimate and point the laser beam.

The laser diode emits a widely diverging, diffraction-limited, single-spatial-mode beam. A glass etalon in front of the laser provides wavelength-selective feedback and thus enables single-wavelength operation even in the presence of a large modulation signal. A molded glass aspherical lens with a focal length of 2 mm and an aperture diameter of f/1 roughly collimates the diverging laser beam to a divergence of about 0.5 by 1.5 milliradians. The roughly collimated laser beam also passes through a matched pair of lenses comprising a long-focal-length positive and a long-focal-length negative lens; the distance between these two lenses is adjusted to achieve fine adjustment of the collimation of the beam. The divergence achievable with fine adjustment of the collimation is low enough to make the laser beam useful at distances of the order of kilometers.

A pair of wedge prisms is used for fine adjustment of the pointing of the beam. A cubic beam splitter picks off a small fraction of the beam and directs it to another aspherical lens, which focuses this sample of the beam into a single-mode optical fiber for use in monitoring the modulation waveform or as a local-oscillator source for an optical receiver. To reduce the effects of optical feedback from the passive optical components into the laser, a quarter-wave plate is placed immediately after the first aspherical collimating lens, and adjusted such that the polarization of any reflected light passing through is rotated to be orthogonal with the original polarization of the laser. Because the laser has almost no gain for this orthogonal polarization, the reflected light exerts no measurable effect on the laser.

This work was done by Donald M. Cornwell, Jr., and Pamela S. Millar of Goddard Space Flight Center, Daniel X. Hopf of SSAI, and Anthony W. Yu of HSTX. GSC-13824

Photonics Tech Briefs Magazine

This article first appeared in the January, 2000 issue of Photonics Tech Briefs Magazine.

Read more articles from the archives here.