Unique End-Pumping Design Shrinks Laser

Breck Hitz

The technique of placing a frequency-doubling crystal inside a vertical external cavity surface-emitting laser (VECSEL) has been investigated in many laboratories, and several internally doubled VECSELs are available commercially. They’re compact and efficient devices, but now scientists at Samsung Advanced Institute of Technology in Gyeonggi-Do, South Korea, have devised a novel optical-pumping scheme with the potential for stuffing a 1-W green laser into a package smaller than 1 cubic inch.

Figure 1. The scientists positioned the quantum wells several hundred microns in front of the pump laser, at a point where the beam emerging from it was symmetric and well-matched to the intracavity laser mode. Images reprinted with permission of Optics Letters (LBO = lithium triborate, HR = high reflectance, AR = antireflection, BRF = birefringent filter, ROC = radius of curvature).

The key to the new approach is the elimination of optics between the single-emitter diode laser pump and the semiconductor chip that is the VECSEL’s gain medium. Typically, the asymmetrically diverging beam from the edge-emitting pump laser requires nontrivial optics to match it to the intracavity laser beam in the gain medium. The Korean team eliminated the optics by placing the pump diode almost against the semiconductor chip; the separation was only a few hundred microns.

The beam that is emerging from the pump diode is elliptical, much smaller in the vertical direction than in the horizontal. But as it propagates away from the diode, the rapid divergence in the vertical direction soon switches the ellipticity, so the vertical dimension is larger. At a point several hundred microns from the diode edge, however, the beam is symmetric. The scientists positioned the pump diode several hundred microns from the semiconductor chip so that the pump beam in the chip was symmetric and well-matched to the intracavity laser mode.

Figure 2. The small kinks in the laser’s power transfer function probably were the result of longitudinal mode hopping.

Placing the pump diode that close to the chip means that the diode must be positioned behind the chip; it would block the resonator if it were in front of the chip. So the scientists devised an end-pumping geometry for a folded-cavity resonator (Figure 1). The distributed Bragg reflector was highly reflective at the ~1071-nm laser wavelength but transmissive at the 808-nm pump wavelength. The gain medium comprised 10 pairs of 6.7-nm-thick InGaAs quantum wells, positioned to align with the peaks of the standing wave in the laser resonator. The diamond heat spreader was bonded directly to the gain medium, providing a significant enhancement to thermal management.

Figure 3. The 1 W of green power was centered at ~535.5 nm, with a bandwidth of ~0.27 nm.

The folded resonator facilitated second-harmonic generation by combining the green beams generated in each direction in the lithium tribo-rate crystal and coupling them out through the dichroic folding mirror. The two arms of the resonator were 23 and 21 mm in length, with the nonlinear crystal in the shorter arm, so it is possible that the entire device could fit into a cubic-inch package. The birefringent filter controlled the fundamental laser wavelength and bandwidth and, because it was oriented at Brewster’s angle, forced the laser into a single polarization, as required for type I phase-matching in the lithium triborate.

The laser produced in excess of 1 W of green output from 7 W of optical pump power, with no thermal rollover at the high end (Figure 2). The green output had a bandwidth (full width half maximum) of ~0.27 nm at ~535.5 nm (Figure 3)

Optics Letters, Sept. 1, 2007, pp. 1325-1327.