Wavelength Tuning of MEMS VCSEL Proposed
Engineers at the University of California, Berkeley, have designed and analyzed -- but not built -- a microelectromechanical system (MEMS) vertical-cavity surface-emitting laser (VCSEL) whose wavelength could be tuned using a unique piezoelectric actuator. Such tiny lasers could be useful for wavelength management and switching in dense wavelength division multiplexed communications systems, and for chip-based atomic clocks and medical diagnostics.
A MEMS VCSEL consists of two distributed Bragg reflectors that act as the laser's mirrors, and a semiconductor gain medium composed of multiple quantum wells (Figure 1). One of the reflectors is mounted on a cantilever arm so that the resonator length -- and hence the laser wavelength -- can be adjusted by flexing the arm. Like all MEMS devices, the VCSELs are fabricated with conventional photolithographic techniques and, accordingly, have the potential to be mass-produced at a relatively low per-unit cost.
Figure 1. The wavelength of a microelectromechanical system vertical-cavity surface-emitting laser is tuned when the cantilever arm supporting the top distributed Bragg reflector is flexed, changing the resonator length.
MEMS VCSELs have been around for the better part of a decade, and nearly all have been wavelength-tuned by applying an electrostatic force to the cantilever arm. Some work has been done with thermally tuned lasers, but that approach has a slow response time. The piezoelectric tuning technique proposed by the Berkeley engineers offers the advantage of a larger tuning range with less applied voltage.
The cantilever arm of the suggested device includes three undoped pairs of Al0.1Ga0.9As−Al0.8Ga0.2As that would provide the piezoelectric activation. Beneath these undoped layers, 18.5 pairs of P-doped Al0.1Ga0.9As−Al0.8Ga0.2As provide the ground plane for the piezoelectric voltage. If a voltage were applied across the undoped layers, they would expand outward, creating a moment that will force the arm to bend downward (Figure 2).
Figure 2. The cantilever arm of the proposed laser includes several layers of piezoelectric material that cause the arm to flex when a voltage is applied.
The engineers calculated the displacement of the upper distributed Bragg reflector as a function of the voltage applied to the piezoelectric element and compared it with that of an electrostatically activated device (Figure 3). Because the displacement of the piezoelectrically actuated device could be reversed by reversing the voltage, it would provide twice the displacement of an electrostatic device from the same nominal power supply. Also, the displacement of the piezoelectric actuator would vary linearly with voltage, simplifying the wavelength-tuning control.
Figure 3. Calculations indicate that the displacement of the top distributed Bragg reflector in a piezoelectrically tuned device with a 250-µm cantilever arm would be bidirectional and linear with voltage.
The tuning range that can be achieved in this type of laser depends on the resonator's free spectral range and on the bandwidth of the gain medium. For the 1.2-µm air gap in the theoretical laser, the engineers calculated that a single longitudinal mode may be tuned across 37 nm with an air-gap change of ~450 nm produced by a bidirectional voltage of ~5 V.
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