Having to use stacked edge-emitting diodes or multijunction edge-emitting diode lasers as sources for laser range finders and illuminators has numerous disadvantages, particularly related to poor beam quality and limited output power. A single-laser diode that produces high beam quality at high power would be ideal, as it would simplify collimating optics (i.e., running the rays parallel) and extend the useful range of laser rangefinders.
In the past, numerous diodes have been stacked one on top of the other to achieve the power needed for range-finding applications. This method compromises beam quality because the sources are separated by hundreds of microns. There has been a move toward a multijunction approach, which uses, for example, three diodes that are grown one on top of the other on a single substrate. The emitters are still separated by up to 10μm, thus limiting their beam quality. In the lateral direction, power scaling is achieved by using large aperture sizes of ~350μm. We use a single-laser diode with a curved second-order grating in a distributed feedback configuration. This architecture allows two independent knobs. One is used to scale power by increasing the emission area. The other maintains high beam quality by tailoring a cylindrical wavefront in the lateral direction, which causes the laser beam to emanate from a much smaller virtual source.
Figure 1. Schematic illustration of beam from a surface-emitting distributed feedback (SE-DFB) laser transmitting through the substrate. Cross section of an SE-DFB laser showing the propagation direction of a fundamental mode (0), surface-normal output (-1), and feedback (-2). N, P: Negative, positive.
The basic surface-emitting distributed feedback (SE-DFB) laser (Figure 1) has a second-order grating etched into the top cladding that is overcoated with gold. A patterned contact delineates a pumped stripe that generates laser light. The zeroth-order mode traveling along the waveguide has two diffraction orders (i.e., two different diffraction angles): one for surface emission and a second for feedback.1 Reflections from the ends and sides are suppressed by an absorbing overlayer so that feedback for the laser comes only from the grating. Consequently, the output spectral width is very narrow and typically less than 0.3nm near 975nm. The grating also ensures that the center wavelength does not drift at a rate greater than 0.065nm/°C. The surface-diffracted beam transmits through the substrate by means of an antireflection-coated output window in the backside metallization (see Figure 1). This output beam is always surface normal and is collimated in the longitudinal direction (see Figure 2), and the beam quality is nearly diffraction limited.
Figure 2. A collimated longitudinal beam from an SE-DFB laser requires no optics.
The beam exits from an aperture that is many millimeters long and hundreds of microns wide, making optical power density several orders of magnitude smaller in this laser as compared to edge emitters. As a result, we can extract very high peak powers from a single laser diode without catastrophic optical damage. We have demonstrated 312W of peak power at 1μs pulse duration from a single SE-DFB laser (see Figure 3). In pulsed operation, edge emitters are limited to several tens of watts per diode laser before the onset of facet damage. In the lateral direction, the divergence is approximately 8 degrees at full angle. Simple and inexpensive bulk cylindrical optics enable the laser beam to be fully collimated.
Figure 3. A single SE-DFB laser produces 312W of peak power.
The unique feature of SE-DFB lasers is that gratings are curved (see Figure 4). Mode control is realized through the phase of the grating. The shape of the grating is designed to make the laser cavity analogous to a classic unstable resonator so that it provides a lateral mode with good beam quality while in a wide-stripe configuration. The curved grating introduces a cylindrical wavefront in the lateral direction of the output beam. Thus, this beam appears to emanate from a narrow virtual line source even though a much wider stripe is pumped. The ratio of the pumped stripe width to the virtual line source width roughly characterizes the brightness improvement achieved by the use of the curved grating. As a result, this source is much brighter than wide-aperture edge emitters. The grating curvature also suppresses filamentation in the wavefront propagation, which is common to broad stripe lasers. By design, the laser is linearly polarized.
Figure 4. An underside view of a curved-grating SE-DFB laser showing the central pumped stripe, grating, and absorber regions (right).
Combination of an order of magnitude higher peak power, 10-fold narrower spectral width, sixfold smaller sensitivity to temperature drift of the center wavelength, higher brightness beam quality in both directions, and linearly polarized light make the SE-DFB laser a better source for direct-diode-laser range finders. All these attributes provide a much higher signal-to-noise ratio, which translates to accessing longer distances with compact range finders. At shorter pulse durations in the range of tens of nanoseconds, these lasers are expected to produce even higher peak powers. In the future, we intend to extend this architecture to eye-safer wavelengths.
Manoj Kanskar received his PhD in condensed matter physics. He is currently vice president of research and development at Alfalight, where he is spearheading development of high-power, high-efficiency, and high-brightness laser diodes, fiber lasers, and diode-pumped solid-state lasers. He has over 20 years of experience in photonics, optoelectronics, and high-power laser systems.
1. M. Kanskar, J. Cai, D. Kedlaya, T. Klos, M. Martin, D. Olson, Y. Xiao, S. H. Macomber, 975nm surface-emitting distributed feedback (SE-DFB) laser and array, 22nd Ann. Solid State Diode Laser Technol. Rev., pp. 84-88, 2009.