Phase conjugate mirror and diffraction grating yield stable, collimated, coherent, high-power diode laser.

Oregon Medical Laser Center Newsletter, Dec. 1997. Steven Jacques

The Risoe National Laboratory of Denmark has developed a single-mode laser diode array with phase conjugate mirror feedback for single-mode operation and a diffraction grating for wavelength stabilization. The result is a high-power diode laser array which is collimated, coherent, and frequency stabilized. Paul Michael Petersen (paul.michael.petersen@risoe.dk) and Martin Loebel (martin.lobel@risoe.dk) have developed the new diode array.

The significance of this new laser for biomedical optics is found in the following new capabilities:

• The ability to efficiently focus the laser into a small core-diameter optical fiber.
• The ability to fine tune a stable laser wavelength to match a particular interference filter so that fluorescence emission and inelastic scattering can be distinguished from the source.
• The long coherence length to enable such applications as speckle imaging and holography.

Single-mode semiconductor lasers provide spectrally and spatially coherent light. High optical power density of quantum-well laser diodes, however, can thermally destroy the sensitive nanostructures of these devices - especially the facets. Consequently the quantum-well laser diodes are limited to an optical output power of maximum 200 mw. In many applications, however, much higher output power is needed. One approach to increase the output power is to form an array of laser elements. Such laser diode arrays are now commercially available with output power exceeding 20 W. Unfortunately, these devices suffer from a multimode non-diffraction limited radiation pattern and have a very short coherence length of a few hundred micrometers, which limits the usefulness of the output beam.

Petersen and Loebel have invented a new high-power single-mode laser system that has an output beam with unique coherence properties. The system, which is shown in Figure 1, is based on a high-power GaAlAs laser diode array coupled to a photorefractive phase conjugator. The phase conjugate feedback is made frequency selective by placing a grating in the external cavity between the conjugator and the laser diode array. The frequency selective feedback system forces the multimode laser diode array to oscillate in a single spatial and a single longitudinal mode with a conversion efficiency exceeding 90%. In comparison with the freely running laser diode array, the linewidth has been reduced to less that 0.02 nm, the coherence length has been increased by a factor of 70 and the output has become diffraction limited (1.4 x the diffraction limit). The latter implies that the output beam can be focused to a spot at the size of a wavelength. More than 80% of the total energy provided by the laser diode array can be extracted from the single-mode laser system.

Figure: A GaAlAs laser diode array with phase conjugate feedback from a BaTiO₃ crystal.

The system has the following properties:

• Single spatial mode (close to diffraction limited).
• Single longitudinal mode.
• Long coherence length of several centimeters (maybe meters in the future).
• Tunable laser system with center wavelength at 800 nm and a minimum tuning range of 5 nm.
• No temperature control is needed, in contrast to conventional laser diode arrays.

The response time of the photorefractive crystal is on the order of a few seconds and will therefore be capable of compensating for slow temperature changes of the cavity and the laser array itself.

Prepared Dec. 15, 1997 by Steven Jacques