Mirror Enhances Diode Laser Bar's Beam

Breck Hitz

The diode laser bar, which consists of multiple diode lasers on a single substrate, is the workhorse of semiconductor lasers. Commercial units with outputs of 100 W and higher are on the market, and laboratory devices have demonstrated outputs of many hundreds of watts.

Figure 1. Feedback from the external mirror, which had alternating high- and low-reflectivity stripes with a period equal to the spacing of the diodes on the bar, reduced the lasers' slow-axis divergence by as much as a factor of five.

An advantage of these lasers is their good efficiency: They typically convert half of the input electrical power into optical output. But a drawback is their poor beam quality: an asymmetric, elliptical beam whose divergence is typically several degrees in the "slow" axis and tens of degrees in the "fast" one. The use of a tiny cylindrical lens can compensate the fast-axis divergence, but the slow axis presents more of a challenge.

Now a research group at Hamamatsu Photonics KK's Central Research Laboratory in Hamakita, Japan, has demonstrated that a unique external-mirror configuration can improve the slow-axis divergence.

The quality of a laser diode's beam can be improved by adding a third, external mirror to form an external resonator. The longer resonator provides more directional feedback and reduces beam divergence. This approach cannot be used with a diode laser bar, however, because the high divergence and close spacing of the individual lasers would cause light from any one laser to reflect into its neighbors.

Figure 2. The bar's slow-axis divergence with the external mirror (squares) was significantly less than without the mirror (triangles) across all pumping currents.

In the new work, the researchers used the unique laser mirror shown in Figure 1. Their GaAlAs bar comprised 19 lasers, each separated from its neighbor by 500 µm. A cylindrical lens in front of the bar reduced divergence in the laser's fast axis from 40° to 0.2°. The mirror had alternating stripes of high- and antireflection coatings with a spacing period of 500 µm, so the output of each laser was fed back to itself but not to its neighbors. And because the mirror was offset by a small angle from perfect alignment with the bar, the antireflection-coated stripes served as output windows.

The slow-axis beam divergence from a normal laser bar without the striped mirror was approximately 6.2°. When the researchers positioned the mirror in front of the bar, the divergence was reduced to 1.3°. They observed similar results across the entire range of pump currents (Figure 2).

Figure 3. The bar's output power with the external mirror (squares) was less than the output without it (triangles). "Output efficiency" (circles) is defined as the ratio of the two.

Although the external mirror reduced the laser threshold from 14 to 11 A, the more selective feedback supplied by the mirror resulted in lower outputs at higher pump powers. Figure 3 shows the output power with and without the external mirror. At the highest pump current, the striped mirror reduced the output power by approximately 30 percent, but the reduction was lower at lower pump currents. (The bar was designed for continuous-wave operation, but for these measurements, the researchers operated it at a 400-Hz repetition rate with 400-µs pulses to facilitate beam-divergence measurements. The powers and currents represented are peak values.)