Publications

We report on the development of single-frequency VCSELs (vertical-cavity surface-emitting lasers) for sensing the position of a moving MEMS (micro-electro-mechanical system) object with resolution much less than 1nm. Position measurement is the basis of many different types of MEMS sensors, including accelerometers, gyroscopes, and pressure sensors. Typically, by switching from a traditional capacitive electronic readout to an interferometric optical readout, the resolution can be improved by an order of magnitude with a corresponding improvement in MEMS sensor performance. Because the VCSEL wavelength determines the scale of the position measurement, laser wavelength (frequency) stability is desirable. This paper discusses the impact of VCSEL amplitude and frequency noise on the position measurement.

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We report on the development of 850-nm high-speed VCSELs optimized for low-power data transmission at cryogenic temperatures near 100 K. These VCSELs operate on the n=1 quantum well transition at cryogenic temperatures (near 100 K) and on the n=2 transition at room temperature (near 300 K) such that cryogenic cooling is not required for initial testing of the optical interconnects at room temperature. Relative to previous work at 950 nm, the shorter 850-nm wavelength of these VCSELs makes them compatible with high-speed receivers that employ GaAs photodiodes. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

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Vertical-cavity surface-emitting lasers (VCSELs) are well suited for emerging photonic microsystems due to their low power consumption, ease of integration with other optical components, and single frequency operation. However, the typical VCSEL linewidth of 100 MHz is approximately ten times wider than the natural linewidth of atoms used in atomic beam clocks and trapped atom research, which degrades or completely destroys performance in those systems. This report documents our efforts to reduce VCSEL linewidths below 10 MHz to meet the needs of advanced sub-Doppler atomic microsystems, such as cold-atom traps. We have investigated two complementary approaches to reduce VCSEL linewidth: (A) increasing the laser-cavity quality factor, and (B) decreasing the linewidth enhancement factor (alpha) of the optical gain medium. We have developed two new VCSEL devices that achieved increased cavity quality factors: (1) all-semiconductor extended-cavity VCSELs, and (2) micro-external-cavity surface-emitting lasers (MECSELs). These new VCSEL devices have demonstrated linewidths below 10 MHz, and linewidths below 1 MHz seem feasible with further optimization.

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We have been investigating the use of coaxial multimode VCSEL/PD (vertical cavity surface emitting laser/photodiode) pairs for positional sensing with emitter to target mirror distances on the order of 1mm. We have observed large variations in signal levels due to the strong optical feedback in these close-coupled systems, employing either heterogeneously integrated commercial components or our own monolithically integrated devices. The feedback effect is larger than anticipated due to the annular geometry of the photodetector. Even though there is very little change in the measured VCSEL total output power, the optical feedback induces variations in the transverse mode distributions in these multimode VCSELs. The higher order modes have a larger divergence angle resulting in changes in the reflected light power incident upon the active detector area for a large range of emitter/mirror separations. We will review the experimental details and provide strategies for avoiding these variations in detected power. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

We report the demonstration of a fully micro-fabricated vertical-external-cavity surface-emitting laser (VECSEL) operating at wavelengths near 850 nm. The external-cavity length is on the order of 25 microns, and the external mirror is a dielectric distributed Bragg reflector with a radius of curvature of 130 microns that is micro-fabricated on top of the active semiconductor portion of the device. The additional cavity length, relative to a VCSEL, enables higher output power and narrower laser linewidth, and micro-fabrication of the external mirror preserves the manufacturing cost advantages of parallel lithographic alignment. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

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A future generation of high-performance low-power atomic systems is expected to require VCSEL linewidths below 10 MHz for compatibility with the natural atomic linewidth (5 MHz for cesium) that is realized with atomic beams, trapped atoms, and trapped ions. This paper describes initial efforts at Sandia to reduce VCSEL linewidth by increasing the effective cavity length of an 850-nm monolithic VCSEL. In particular, two aspects of VCSEL design will be discussed: the Q of the VCSEL cavity, and the linewidth enhancement factor of the active region material. We report a factor of two linewidth reduction, from 50 MHz for our standard oxide-aperture VCSEL to 23 MHz for an extended-cavity VCSEL. ©2009 SPIE.