VCSEL for Telecom Features Curved Mirror for Single-Mode Operation
Vertical-cavity surface-emitting lasers (VCSELs) based on microelectromechanical systems (MEMS) are attractive candidates for telecom lasers because they can be tested while still in wafer form, their inherently short resonators ensure single-longitudinal-mode oscillation, and the wavelength tuning is straightforward, requiring only a single, variable voltage. But a drawback to these devices is their tendency to oscillate, at least over part of the tuning range, in more than a single transverse mode unless complex techniques are employed to suppress side modes.
Now a collaboration of scientists from the University of Kassel and the University of Technology Darmstadt, both in Germany, and from the Royal Institute of Technology in Kista, Sweden, has fabricated an optically pumped MEMS-based VCSEL with a curved mirror that enhances single-transverse-mode operation.
As with other MEMS-based VCSELs, the device is fundamentally two distinct pieces: a laser body comprising a bottom distributed Bragg reflector (DBR) and the gain region, and a separate, movable top DBR. The two DBRs define the length of the resonator, and the laser's wavelength is tuned by moving the top one to change the resonator's length.
Figure 1. A white-light interferometer image reveals the structure of the laser's suspended top distributed Bragg reflector. Horizontal and vertical distances are not on the same scale. In the profile (inset), the vertical axis spans 50 µm, and the horizontal axis spans 700 µm. Images courtesy of IEEE.
Building on a concept introduced in 1998 by researchers at CoreTek Inc. in Burlington, Mass., and at Ohio State University in Columbus, the European scientists have fabricated a curved top DBR, which, together with a carefully defined active-gain region, forced the laser to oscillate only in a single transverse mode.
In a white-light interferometer image of the new laser (Figure 1), horizontal distances are magnified by a factor of 14 compared with the vertical distances; the separation between the curved membrane and the body of the laser is actually 1/14 of what it appears to be in the image. The separation is so small that it does not appear in a conceptual drawing of the laser (Figure 2).
Figure 2. The optical distance between the bottom distributed Bragg reflector and the curved top one defines the length of the vertical-cavity surface-emitting laser's resonator. Wavelength tuning is accomplished by moving the top DBR to change the resonator length.
The membranelike top DBR in Figure 1 is 300 µm in diameter, suspended by four beams 600 µm long and 70 µm wide. Its radius of curvature is 4.5 mm, and the air gap between it and the body of the laser (L') is 16 µm. Precisely tailoring the intrinsic stress of the dielectric layers across the DBR during fabrication produced the desired curvature (Figure 3).
Translating the DBR to tune the laser is accomplished by passing a current through a metallic coating on the suspension beams, heating them and causing them to expand.
Figure 3. The vertical design of the top distributed Bragg reflector yields the desired curvature. Stress values of the dielectric layers vary from –150 to 400 MPa.
A lensed fiber focuses 30 to 50 mW of 980-nm pump light into a 20-µm-diameter, Gaussian-profile spot in the VCSEL's active region. The pumping region acts geometrically as an intracavity aperture, which, together with the two DBRs, creates a pla.noconvex resonator capable of oscillation in only the fundamental transverse mode.
To assess the performance of the laser, the researchers coupled the VCSEL's output power through the bottom DBR and analyzed it with an optical spectrum analyzer. In their tests, the output of 300 to 400 µW could be tuned from approximately 1550 nm to about 1576 nm, and the side mode suppression ratio across this range was at least 57 dB. Wavelength tuning was linear with current heat dissipated in the beams' metallic coatings, with a coefficient of 7 nm/mW.
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