Diode Laser Emits at 2 μm, Tunable Across 177 nm
GaSb-based laser produces milliwatts over entire tuning range.
Tunable lasers that emit in the 2- to 3-μm spectral region have many applications in molecular spectroscopy, military countermeasures, and medical diagnostics and therapy. With an eye toward medical diagnostics — noninvasive blood glucose monitoring in particular — researchers at Fraunhofer Institute for Applied Solid State Physics and at the University of Freiburg, both in Freiburg, Germany, have demonstrated a GaSb diode laser capable of generating up to 16.5 mW at 2.23 μm, and they have tuned the laser across a 177-nm range centered on that wavelength.
They built an external cavity with a grating to provide the wavelength selectivity for their laser (Figure 1). But unlike most diode lasers arranged in a Littrow configuration, this one did not use the first-order diffracted beam for output coupling. Instead, they took the output from the 10 percent reflector at the other end of the laser, avoiding beam steering with wavelength tuning (because the grating — and, hence, the first-order beam — is rotated to tune the wavelength).
Figure 1. An external grating provided the wavelength selectivity, enabling the laser to be tuned across 177 nm. SMA = subminiature version A. Images reprinted with permission of IEEE Photonics Technology Letters.
To minimize the fast-axis divergence of the beam emerging from the edge-emitting laser, the researchers designed an internal waveguide. Without compromising the laser performance, the waveguide structure reduced the fast-axis divergence from the ~65° common in similar lasers to 44°. This increased the coupling efficiency not only to the output fiber, but also to the external cavity, resulting in better feedback from the grating and a wider wavelength tuning range.
Figure 2. The laser threshold varied asymmetrically with wavelength (top). For reference, the threshold of the laser without the grating is also shown in this plot. In the latter case, the laser resonator was formed by the 0.8 percent and 10 percent reflecting facets of the diode itself. The asymmetric wavelength dependence of the threshold was due to the asymmetric gain (bottom). This plot also shows the total loss (normal intracavity loss αi plus grating loss αm). When the grating was tuned to an off-peak wavelength (broken line), a greater current was required to reach threshold.
Laser threshold varied with the wavelength, with a higher current required to reach threshold at wavelengths farther from the peak (Figure 2, top). The asymmetrical shape of the threshold curve resulted from the asymmetry in the unsaturated gain (Figure 2, bottom). The bottom plot also shows the wavelength-dependent loss introduced by the grating in the external cavity.
As indicated in the figure, threshold was reached when the unsaturated gain had increased until it was equal to the total intracavity loss. When the grating was set to an off-peak wavelength, a greater current was required to produce the unsaturated gain.
Figure 3. At a maximum pump current of 500 mA, the laser could be tuned over a 177-nm spectral range.
As one might expect, the laser output also varied with wavelength, although other asymmetries in the gain-saturation mechanism tended to mask the shape of the unsaturated gain (Figure 3). When they increased the current to a maximum value of 500 mA, the researchers could tune the laser across 177 nm, from 2.213 to 2.389 μm. At 2.23 μm, they obtained the highest power, 16.5 mW, and the output dropped to 9 mW at 2.39 μm.
IEEE Photonics Technology Letters, Sept. 15, 2006, pp. 1913-1915.
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