vcsels studied for short- haul wdm

June 1, 1998

vcsels studied for short- haul wdm

By YVONNE CARTS-POWELL

The combination of two hot topics--wavelength-division multiplexing (wdm) and vertical-cavity surface-emitting lasers (vcsels)--has produced work on using vcsels as sources for future short-haul wdm applications. Fiber applications, including three wdm papers, dominated a session on applications of vcsels at the Conference on Lasers and Electro-Optics (cleo--San Francisco, CA) in May.

vcsels are attractive because they can be made cheaply, can be designed with differing wavelengths more easily than edge-emitting lasers, and emit a circular beam that can be coupled into fiber without requiring beam-shaping optics. The lasers are usually multimode and operate at relatively low output powers, however, which makes them unsuitable for the current wdm applications involving long-distance communications over singlemode fiber. Kent Choquette of Sandia National Laboratories (Albuquerque, NM), who presided over the session at cleo, emphasizes that the vcsel wdm papers are about devices that are still in research rather than in commercial use.

Three papers at cleo discussed the possibility of using vcsels for short-haul wdm optical links over multimode fiber.

An invited paper by Syn Yem Hu and others in Larry Coldren`s group at the University of California--Santa Barbara shows results from an eight-wavelength optical link using integrated vcsels and photodetector arrays. The arrays showed a 32.9-nm lasing wavelength span. The researchers produced both a wavelength-selective photodetector array and a vscel array arranged in a pie shape (see photo). The vcsel array consists of eight bottom-emitting vcsels arranged like wedges of a pie within a 60-micron-diameter area, a size chosen to match the core of a multimode fiber. The group used a 980-nm vcsel design with InGaAs quantum wells and AlGaAs/GaAs mirrors. The cavity modes for the individual channels were adjusted during growth. Most devices had threshold currents around 1.7 mA and a maximum output power of about 7.3 mW. At the receiving end of the link, each channel was detected by another pie-shaped array, this time made of wavelength-selective resonant-cavity-enhanced photodetector arrays. These detectors demonstrated a 14-dB channel rejection ratio.

Different approaches

A slightly different approach was taken by A. Alduino and others at the University of New Mexico and Sandia National Laboratories. This group also used resonant-cavity-enhanced photodetector arrays, but not in the same geometry, and with different characteristics. The photocurrent rejection ratio was about -22 dB between channels spaced 3.5-nm apart. Also, a monolithic wavelength-chirped vcsel array acted as the source in this case.

Wupen Yuen and others in Connie Chang-Hasnain`s group at the University of California--Berkeley reported further work on GaAs/AlGaAs vcsel arrays in which the different wavelengths were achieved using patterned-substrate molecular beam epitaxy. The laser arrays were fabricated with the negative contact up. Choquette explains that the positive contact is less efficient than the negative one, and by putting the positive contact on the substrate, it can be made larger. This reduces the contact resistance of the device, which reduces the voltage required to drive the device, and thus increases the vcsel`s overall efficiency. The arrays demonstrated continuous lasing over a 24-nm wavelength span, with a threshold current of less than 5 mA and output powers of more than 3 mW.

Turning over the structure so that the negative contact is up is also reported by Hong Q. Hou and others at Sandia and Hewlett-Packard Laboratories (Dallas, TX). An 850-nm vcsel optimized for fast data links over multimode fiber demonstrated a power output of 2 mW and a low threshold voltage of about 1.5 V.

Hybrid device

Another paper, not directly wdm-influenced, describes a laser that could, nevertheless, provide a valuable tool for current wdm applications. Kevin Hsu and others at Micron Optics (Atlanta, GA) and Sandia demonstrated a novel laser that offers the benefits of a continuous tuning range without mode hopping and single-frequency output. Choquette, who is one of the paper`s authors, describes the device as "half vcsel, half fiber cavity."

The single-frequency laser tunes over 10.8 nm at 850 nm. The vcsel part of the device includes a bottom mirror (made of a distributed Bragg reflector) and a multiple quantum-well gain region. The rest of the cavity is made of an air gap and the top mirror--which is a singlemode-fiber dielectric mirror. This arrangement forms a fiber Fabry-Perot cavity, in which tuning can be performed by changing the length of the air gap (see figure). In the experiment, the gap was changed by a piezoelectric actuator. The gap could be changed by 2 to 3 microns.

Two vcsel surface areas were investigated, 3 ¥ 3 microns and 8 ¥ 8 microns. Both produced singlemode output at a threshold current of 3 mA. When the latter device was coupled to a fiber with a 98.8% reflective mirror and an 8-micron-long fiber, single-frequency output powers as high as 0.5 mW were obtained with 10-mA current. q

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

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