External-cavity diode lasers (ECDLs) have been commercially available for several years. Since their introduction, they have found applications ranging from telecommunications to atomic spectroscopy. These lasers provide a combination of extremely narrow linewidth, broad tunability, and ease of use, making them some of the most versatile lasers available. Until recently, the accessible wavelengths have been limited to the 0.6-1.6-µm region where commercial laser diodes are available. Development conducted by Focused Research, a research subsidiary of New Focus, and Sarnoff Corporation, with support from NASA Goddard Space Flight Center, has extended this range, resulting in the commercialization of the first external-cavity diode laser operating beyond 2.0 µm.

Figure 1. Schematic of a Modified Littman-Metcalf Laser Cavity.

To develop this technology, researchers designed and fabricated a new strained-layer InGaAs/InP quantum-well ridge-waveguide semiconductor laser. Tensile strain was then added in the barrier region to extend the operating wavelength beyond 2.0 µm. To suppress laser oscillation between the facets of the diode, an antireflection coating was deposited onto the output facet. The quality of the antireflection coating is critical to achieving single-mode operation with narrow linewidths and wide tuning ranges, as oscillations between the facets will interfere with the effectiveness of the external cavity.

The laser is then placed in an external cavity with a grating in a Littman-Metcalf grazing incidence configuration, as illustrated in Figure 1, The grating serves as the wavelength-selective output coupler. The narrow linewidth is a result of the highly dispersive nature of the grating, which forces the laser to oscillate in a single longitudinal mode at any given setting. Adjusting the angle of the feedback mirror (or retroreflector) with respect to the grating tunes the output wavelength. There are two methods of doing this: a DC motor provides rapid (>10 nm/s) coarse tuning over the entire tuning range of greater than 70 nm. Fine wavelength adjustment is achieved with a stack of piezoelectric crystals. This piezo stack provides the user with the ability to modulate the output wavelength with frequencies as high as 2 kHz. When faster modulation is required, a direct connection to the diode itself is provided for current modulation. The total tuning range of the piezo stack is approximately 20 GHz (about 0.27 nm). The new product (Model 6332 Velocity Laser) tunes from about 1970 nm to 2040 (see Figure 2 for a typical tuning curve). Nearby custom wavelengths are also available.

Figure 2. Typical Tuning Curve of a Model 6332 Velocity 2-µm laser.

The 2.0-µm wavelength region is especially useful for spectroscopy of molecular species such as CO2, H2O, N2O, and NH3, for combustion diagnostics and environmental monitoring, and HBr for in-situ gas-phase substrate etching for the semiconductor industry. Figure 3 shows data from a survey spectrum of CO2 taken with the Model 6332 Velocity laser. The laser itself is compact and uses wall-plug power, making it useful for field applications. Until the development of these lasers, researchers had to use cryogenic color-center or lead-salt lasers or complicated nonlinear schemes. These systems require much more care and time than the new lasers, and are generally used only in research labs.

Figure 3. Survey Spectra of CO2taken with a 2-µm Velocity laser. Data courtesy of Stanford University's High-Temperature Gasdynamics Laboratory.

With their combination of simplicity, broad tuning range, and narrow linewidth, these 2-µm ECDLs are useful sources for many spectroscopic and chemical sensing applications. Further advances in materials technology will continue to fuel development in laser diodes. As longer-wavelength materials become available, New Focus anticipates their rapid integration into external-cavity tunable systems.

This work was done by Dr. Tim Day, Dr. I-Fan Wu, Dr. Bill Chapman, and Greg Feller at Focused Research, Inc., a subsidiary of New Focus Inc., Santa Clara, CA. For more information, please call New Focus at (408) 980-8088.

Photonics Tech Briefs Magazine

This article first appeared in the June, 1998 issue of Photonics Tech Briefs Magazine.

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