External Cavity Mid-Infrared Semiconductor Lasers
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Abstract Sb mid-IR laser can be used in external configuration to achieve wide wavelength tuning range. At
low temperature, gain media with band-edge wavelengths between 3.3 to 4 pm have been demonstrated with wavelength tuning up to ~90/o of the center wavelength. Power output from few tens of mW to 0.2W peak, 20-mW average was achieved. Type-II Sb laser promises the possibility of such performance at higher temperature, e. g. 200 K. However, significant trade-off must be considered between tuning range and power and efficiency. Optimization requires consideration of both basic wafer design and cavity geometry.
1. Introduction Infrared (2-20 pm) spectroscopy is an essential technique in analytical chemistry. The method of IR
spectral pattern recognition has been extensively applied to identify not only chemical substances but also complex biological matters [1]. JR laser is desirable for high brightness and spectral resolution, but a crucial requirement is the capability to cover a broad wave-
4.0 1.0
thick poly-propylene-rich commercial-bottle plastic in
the 2500-3000 cm' band. The arrows p to absorpthe f500-u000 thtbare tcalrof astpoint t absorption features that are typical of plastics; and measurement of many such features over several bands are necessary for spectral pattern identification. It is
3.3
00.8 o (A • 0.6 0.4
length range. The reason is that complex systems often
have broad spectral features extending over many widely separated wavelength bands. As an example, Fig. I shows the transmission spectrum of -0.5-mm-
Wavelength (micrometer)
S-emiconductor
0.2
laser transmission
through commercial bottle plastic
2500 Wavenumber (cm") 3000 Fig. I Mid-infrared transmission spectrum of plastic
obtained with tunable Sb-based external cavity laser
(not corrected for Fresnel reflection). The arrows point to absorption features that are typical of plastics.
not cost-efficient to use a large array of lasers with discrete wavelength over a wide spectral range to measure broad spectra. Using a few tunable lasers is more practical. Figure 1 was obtained with a single external-cavity wavelength-tuned Sb laser that covered - 3.4-3.7 pm. An attractiveness of the narrowgap mid-JR semiconductor laser is the capability of broad wavelength tunability. This property is a direct consequence of the low effective electron mass, leading to a high Fermi level and wide gain band [2,3]. However, there are several challenging technical issues in exploiting this capability for practical application. One significant issue is the trade-off between power, efficiency, and tuning range. There are several mechanisms affecting this trade-off, some of which have been discussed previously [2,3]; this paper examines this issue in the context of latest progress in Sb material technology.
159 Mat. Res. Soc. Symp. Proc. Vol. 607 © 2000 Materials Research Society
2. Basic Sb laser properties for wavelength tunability
3.4
3.5
3.8
3.6
Wavelength (prm)
Wavelength (pm)
Fig. 2 Tuning range of Sb-based mid-IR lasers [2,31. A
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