External Resonator Sharpens Diode Array Bandwidth
Monolithic arrays of laser diodes find wide application as pumps for solid-state lasers, but their natural bandwidth of several nanometers usually is wider than the absorption of the active ion in the solid-state laser. This mismatch often leads to inefficient pumping. Many researchers around the world have investigated techniques to reduce an array’s bandwidth, with varying degrees of success. One approach has been to add an external resonator, whose narrowband feedback forces the lasers to oscillate in a narrower bandwidth.
Figure 1. The bandwidth-narrowing external resonator comprised a cylindrical lens, a cylindrical microlens array that rotated the beams 90° and a pair of volume Bragg gratings (a). A side view of the arrangement shows the Y-shaped path of the intracavity laser power (b). Images ©OSA.
Recently, scientists at Hamamatsu Photonics KK’s Central Research Laboratory in Hamamatsu, Japan, demonstrated an external resonator with two volume Bragg gratings whose reflectivities were slightly offset from each other. The offset was a bit less than the gratings’ bandwidth, so the reflectivities overlapped slightly. Because the intracavity laser light had to bounce off both gratings, the laser oscillated only within the narrow range of frequencies that was reflected from both.
The researchers arranged the gratings so they were slightly above and below the plane of the diode array (Figure 1). They had previously used a similar arrangement to reduce the divergence of a diode array (see “Mirror Enhances Diode Laser Bar’s Beam,” Photonics Spectra, April 2004, page 23), but this time they used cylindrical lenses and a beam-rotating optical system from Lissots-chenko Mikrooptik GmbH of Dortmund, Germany, to control the divergence. The beam-rotating system was an array of cylindrical microlenses configured such that, after refractions, the directions of the fast and slow axes were exchanged.
The laser beams emitted from the array (as indicated on the left side of Figure 1a) were highly divergent -- approximately 40° -- in the vertical (Y) direction, but the divergence in the horizontal (X) direction was only ~8°. A cylindrical lens next to the array reduced the vertical divergence to ~0.35° but did nothing to the horizontal. This 8° horizontal divergence would have caused the beams to overlap when they hit the gratings so that light from one external resonator would feed back into its neighbors. Such coupling of the resonators would have resulted in undesirable instabilities.
Figure 2. The laser’s bandwidth was nominally 3.3 nm when neither grating was in place (a). With either one of the gratings in place, the bandwidth decreased to ~0.33 nm, centered on the peak reflectivity of the grating (b and c). With both gratings in place, the bandwidth dropped to ~0.24 nm, centered midway between their peak reflectivities (d).
To avoid overlapping the beams at the gratings, the scientists inserted the commercial beam-rotating system between the cylindrical lens and the gratings. Thus, the beams entering the system from the array had divergences of 0.35° and 8° in the vertical and horizontal directions, respectively, and the emerging beams were switched -- i.e., the vertical divergence was 8°, and the horizontal divergence was only 0.35°. This small horizontal divergence allowed the beams to travel to the gratings and back without overlapping.
The intracavity power followed a somewhat Y-shaped path (Figure 1b). It reflected off one grating, off the rear facet of the array and then off the other grating, before returning to reflect again off the rear facet. The output coupling was accomplished by the slight physical spacing between the gratings.
Figure 3. At maximum power, the output of the bandwidth-narrowed laser (squares) was 87 percent of the output of the laser without bandwidth-narrowing components (triangles). “Output efficiency” (circles) is defined as the ratio of the two.
The volume Bragg gratings from PD-LD Inc. of Pennington, N.J., had center reflectivities at 808.26 and 808.01 nm, respectively, and spectral widths slightly narrower than 0.5 nm. Operating without these gratings, the laser bandwidth was approximately 3.3 nm (Figure 2). With both gratings in place, the bandwidth was reduced to 0.24 nm, a fourteenfold reduction. The bandwidth-narrowed laser operated with 87 percent of the output power of the non-narrowed laser (Figure 3).
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