Micromirrors aid wdm networks

By YVONNE CARTS-POWELL

Micromirror devices are being developed for several wavelength-division multiplexing (wdm) related applications. These precisely positioned mirrors, which are only a few microns in diameter, can be made on semiconductor substrates using a variation on conventional growth methods, and can be moved quickly with very little power. In theory, this could allow dynamic reconfiguration of wdm networks.

Connie Chang-Hasnain and others at the University of California, Berkeley, have developed a vertical-cavity surface-emitting laser (vcsel) with output wavelength that can be tuned over 30 nm by applying a voltage to an integrated tunable micromirror. Meanwhile, Joseph E. Ford and colleagues at Bell Laboratories, Lucent Technologies (Holmdel, NJ), have demonstrated an integrated switch for low-loss adding and dropping of 16 channels in wdm networks.

Tunable lasers

vcsels are attractive for wdm local area networks because they can be made in 1-D or 2-D arrays, emit normal to the substrate, and have circular mode emission that makes coupling into fibers easier than with conventional edge-emitters. The Chang-Hasnain group, which includes graduate students Melissa Y. Li, Wupen Yuen, and Gabriel S. Li, has developed a vcsel design that can be voltage-tuned. An array of these lasers could be individually tuned to provide the wavelengths needed for short-haul wdm links.

Typical vcsels have top and bottom mirrors on the laser cavity typically made of distributed Bragg reflectors (dbrs). The tunable vcsel has an air-gap in the middle of the top mirror. The top dbr layers are suspended over an airspace on a flexible cantilever. The air-gap occurs between an n-doped layer on the cantilever and a p-doped layer (see Fig. 1). By reverse-biasing the p-n junction, the cantilever can be electrostatically attracted towards the p-doped area. By increasing the voltage, the cantilever is bent closer to the substrate, which effectively shortens the length of the laser cavity and the output wavelength of the laser. Surprisingly, the tilt of the mirror results in very little power loss.

The group has been working on micromechanically tuned vcsels for the last several years. Although the first vcsel with roughly this design was announced in 1995, the work since then has wrought refinements that provide better operating characteristics. For example, the first version was a bottom-emitting laser, in which the laser beam exited the cavity through the substrate. In a bottom-emitting laser, Li explains, "The thick substrate on the bottom forms a partial mirror along the emission path. The net effect is that the intensity of the output beam in a bottom-emitting vcsel oscillates as a function of wavelength." A top-emitting vcsel sidesteps this problem.

The most recent published results from the group are devices with a continuous tuning range of 31.6 nm for a device with a tuning range centered at 935 nm. As of January, this was the widest continuous tuning range reported for a vcsel. The current threshold for continuous emission between 1.2 and 4.5 mA with output power at a 10-mA DC bias is between 0.5 and 1.6 mW.

The tunable vcsels developed so far are aimed at short-haul transmission. The wavelengths and multiple transverse mode output are acceptable for short-haul, but not long-distance transmission.

For the future, Melissa Li says, "We have absolutely no doubt about the feasibility of making the tunable laser into a singlemode device." The Berkeley group and others have grown singlemode vcsels and expect that they can incorporate singlemode designs into the micromechanically tunable laser. Making vcsels that operate at the longer wavelengths needed for long-haul transmission is expected to be straightforward as well.

Add/drop devices

Micromirrors are also being developed for use at wdm network nodes. An integrated device made by Joseph E. Ford and others in David Bishop`s group at Bell Laboratories uses 16 micromirrors and other free-space optics to add and drop channels from the signal, with a switching speed of 20 microsec. This allows fast dynamic reconfiguration of the network, while also offering lower signal losses than wdm add/drop devices made by assembling wavelength routers and discrete switches.

The free-space optics assembly is sandwiched between two fiber circulators. The heart of the device includes lenses, gratings, and a linear array of 16 micromirrors (see Fig. 2). Unlike the Berkeley laser micromirror, these mirrors are designed to tilt, as a method of redirecting the light beams. Light enters the device through the first port of the circulator, exits the fiber, is collimated and then separated into different channels (wavelengths) by a grating. Each channel bounces off a micromirror. If the channel is to be passed, the mirror is not tilted, and the beam is reflected back to the grating and back into the fiber, back to the circulator, and out onto the network again.

If the signal on a channel is to be dropped, and another signal added, then 20V are applied to the micromirror, which tilts it about 9. The light is directed to another grating, imaged into another fiber, and passes on to the second circulator. A signal can be added to the channel by sending it on the appropriate wavelength into the second fiber circulator.

In a test system, 16 channels were separated by 1.6 nm, each with 0.7-nm width (to a 3-dB rolloff). The pass and drop channel losses at each channel`s center wavelength were 7 dB and 12 dB, respectively. The contrast between states (again measured from center to center wavelengths) was 15 to 20 dB. q

Yvonne Carts-Powell writes on photonics from Belmont, MA.