Rings on a Chip Portend Tiny Laser Gyroscopes
Researchers in New Mexico have designed and fabricated a pair of semiconductor ring lasers on a single chip and have observed a beat note when the outputs of the two lasers are combined. Led by Marek Osinski, a professor of electrical engineering, physics and computer science at the University of New Mexico in Albuquerque, the researchers are developing monolithically integrated ring laser devices. They believe that their new device paves the way for tiny rotation sensors, or gyroscopes, based on the Sagnac effect.
Figure 1. This drawing of the two semiconductor ring lasers shows the widths and heights of the ridge waveguides exaggerated for clarity. The four integrated photodetectors measure the directional circulating power in the rings. The detector labeled "PD1/R2," for example, measures the clockwise-circulating power in ring R2. The Y-coupler on the right combines clockwise power from R2 with counterclockwise power from R1, and directs it out of the device for external analysis.
The Sagnac effect predicts a frequency shift between the two counterpropagating waves in a ring laser when the laser is rotated about an axis perpendicular to the plane of the ring. If the ring is rotated clockwise, for example, the clockwise-propagating wave is shifted to a lower frequency, and the counterclockwise-propagating wave is shifted to a higher one. The frequency of the beat note between the two waves is a measurement of the angular velocity of the rotation.
Laser gyroscopes based on large, frequency-controlled ring lasers are a fairly mature technology, but a difficulty with any laser gyroscope is that crosstalk -- in the form of scattered light inside the resonator -- between the two waves tends to lock their frequencies together, defeating the Sagnac effect. Semiconductor ring lasers have been fabricated, but success in developing them into gyroscopes has been elusive as a result of the lock-in effect of intracavity scattering.
Figure 2. The spectrum of one of the rings shifted by approximately 0.1 nm when a 90-mA current passed through the resistance heater on the ring. Inset: The shift was linear with current, with a slope of ~0.0011 nm/mA.
The team, which included scientists from the university and from Sandia National Laboratories, also in Albuquerque, avoided the frequency-locking problem by fabricating two separate rings so that there would be minimal opportunity for crosstalk.
The ring laser features two ridge-waveguide rings adjacent to straight ridge waveguides that evanescently couple a small amount of the circulating power from each of the rings (Figure 1). Also monolithically integrated into the device are four photodetectors, which measure the power coupled into each of the four straight waveguides.
Figure 3. The beat frequency when the outputs of the two rings are combined was observed on a radio-frequency spectrum analyzer. The beat note was shifted by ~20 MHz/mA when a current was passed though the resistance heater on one of the rings.
The researchers fabricated the device using wafers grown by metallorganic chemical vapor deposition with InGaAs/GaAs/AlGaAs, double-quantum-well heterostructures whose emission wavelength was 1.02 µm. They deposited Ti/Pt/Au resistance heaters on the sides of the rings to provide a mechanism to fine-tune the lasers' wavelengths.
The spiral structures on the outside of R2 and on the inside of R1 attenuated the counterclockwise- and clockwise-propagating beams in the respective rings. Although the rings showed some directional instabilities as the current increased above laser threshold, both lasers oscillated stably in the direction favored by the spiral attenuating structures at the operational level: Power circulated counterclockwise around R1 and clockwise around R2.
Figure 4. In this fully mounted optoelectronic integrated circuit package, the beat note is collected from the pair of semiconductor ring lasers at the left (spiral waveguides are not visible in this photograph). The two smaller ring lasers on the right are for testing purposes only. Two gold-coated silicon mirrors, mounted at opposite ends in front of the branched output waveguides, reflect the output light upward for direct optical measurements.
The output spectrum of either laser can be tuned by adjusting the current through its resistance heater (Figure 2). When the outputs of the two rings are combined, the beat note shifts in proportion to the heating current applied to one of the rings (Figure 3). In the case illustrated, the tuning rate was ~20 MHz/mA, much less than that for one of the rings alone because the heater, although closer to one of the rings, heated them both. The researchers were able to tune the beat note smoothly through 0 MHz without observing any frequency-locking of the two lasers.
The team has yet to demonstrate the Sagnac effect in the device. Nevertheless, it has demonstrated that counterpropagating waves in semiconductor ring lasers can produce a beat note without locking together.
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