The advantages offered by vertical-cavity surface-emitting lasers (VCSELs) for testing and integration make them appealing for telecom and datacom applications. Still, to be truly useful, devices should operate at telecom wavelengths or across broader spectral bands. Groups in Europe are developing 1.55-µm devices, while researchers in Asia have broadened the spectral bandwidth of short-wavelength VCSELs to 192 nm. advances in Europe
Although short-wavelength VCSELs have been commercialized for many years, long-wavelength devices present fabrication challenges in terms of materials, thermal performance, and mirror layer reflectivity. Several research groups are working to overcome these challenges, such as Eli Kapon and colleagues at the Swiss Federal Institute of Technology (EPFL; Lausanne, Switzerland). "We have developed a method called localized wafer fusion that allows us to produce devices operating at 1.55 µm by monolithically integrating wafers of different materials," explains Kapon.
Figure 1. Localized wafer fusion system at the EPFL can unite dissimilar wafers to yield VCSELs with optimized components. (Beam Express)
The basic problem is that materials such as indium phosphide (InP) that make good VCSEL active regions at long wavelengths are typically weak reflectors with poor thermal conductivity as distributed Bragg reflectors (DBRs). Ideally, a 1.55-µm VCSEL would integrate the thermal and reflective performance of DBRs made from aluminum gallium arsenide/gallium arsenide (AlGaAs/GaAs) with an InP active region. These different wafer structures require distinct materials and processes, however, so they cannot be grown together. Localized wafer fusion allows devices to be produced from wafers of different materials for an end product with the benefits of each.
"Fusing wafers of dissimilar materials requires key know-how in wafer surface preparation and micro-structuring, and fusion recipes that are carried out in a custom-built industrial fusion machine," says Jean-Claude Charlier, president of Beam Express (Lausanne, Switzerland), an EPFL spinout commercializing the technology. The first step in the process is to fuse the top DBR to the InP active region. Next, the InP module undergoes selective etching to delineate the length of the laser cavity, which determines the emission wavelength and the cross-sectional area that define the waveguide and current-confinement properties. A second fusion process attaches the bottom DBR to the active region.
Recently, Kapon's group demonstrated 1.55-µm VCSELs with 3-mW output power at room temperature and 0.6-mW at 80°C, higher powers than currently possible without wafer fusion. Beam Express is now using a similar type of structure to develop electrically pumped VCSELs, multi-wavelength arrays, and tunable devices. By building multiple active regions of slightly different lengths, the group has fabricated VCSELs producing four discrete wavelengths in the 1.55-µm region and are working on an eight wavelength array, both for coarse wavelength division multiplexing (CWDM) applications.
By suspending the top DBR on four micro-electro-mechanical systems (MEMs) beams, the EPFL group produced an optically-pumped, tunable VCSEL at 1.55 µm. The device tuned across 38 nm when the voltage was changed from 1.5 to 3.4 V. Adjusting the pump level yielded 1 mW of singlemode output power across the tuning range, with side mode suppression ratios of 40 dB. XMM-Newton eyes the Universe
Figure 2. Image from the European Space Agency's XMM-Newton x-ray observatory shows neutron star 1E1207.4-5209 (bright yellow object at center). The image was taken with the observatory's European Photon Imaging Camera. (ESA/CESR)
Researchers at Vertilas (Munich, Germany) have overcome the technical challenges of long wavelength VCSELs using a different approach. They have developed a hybrid metallic/dielectric mirror to provide the high thermal conductivity and reflectivity. All layers of their VCSEL are grown in InP using two epitaxial steps, first growing the top mirror and active region, then proceeding with a regrowth step for the second mirror. Singlemode output powers around 1 mW, low threshold currents, and wavelength tuning over 3 nm by adjusting current allow them to address applications in telecom and spectroscopy. "Our focus is on increasing power, pushing wavelengths beyond 2 µm, and developing broadly tunable, long-wavelength VCSELS by integrating MEMs structures," says Robert Shau, R&D Manager of Vertilas. The broadly tunable VCSEL involves a two-chip approach in which the active region and one mirror are grown together and the tunable MEMs structure is grown separately. The two chips are fixed together in a final device using non-fusion techniques. advances in Asia
Meanwhile, a research team at the Tokyo Institute of Technology (TITECH; Tokyo, Japan) is concentrating on constructing gallium indium arsenide/gallium arsenide (GaInAs/GaAs) quantum-well VCSELs with a wider wavelength window to increase the possible number of wavelengths handled by a single chip.
Using non-planar metal-organic chemical vapor deposition, the group grew an active region consisting of a series of alternating grooves and mesas, each wide at one end and tapering to zero at the other. "We found that the resonant wavelength of vertical cavities formed on mesas became longer than for those on a planar area, and that the lasing wavelength is dependent on the pattern width and pattern height," says Fumio Koyama of the TITECH group. "By the same token, the wavelength of vertical cavities formed in grooves became shorter, and we found that the photoluminescence peak wavelength of quantum wells also changes at the same time the cavity frequency changes." The shift in the peak wavelength of the gain was smaller than the cavity resonance shift, however. "That was because the energy-level shift of quantum wells is not as sensitive to well thickness changes," explains Koyama, noting that the main limiting factor in expanding the wavelength span in VCSEL arrays is the offset between the gain peak and the resonant wavelength.
The secret, according to Koyama, is pressure control during growth of VCSEL wafers on the substrate. "We found that growth rate enhancement on patterned substrates is very much dependent on the growth pressure, which means that varying the growth pressure allowed us to match the wavelength shift of gain peak and cavity mode while growing the wafer," he says.
Fabrication involves using standard photolithography and wet etching to create a substrate with 15-µm mesas. The key is changing pressure before and after growth of the quantum wells. The group grew the bottom n-type DBR at 695°C and 24 torr, but stopped before growth of the quantum wells. After stabilizing pressure and temperature, they deposited a GaAs barrier and quantum wells at 540°C and 200 torr. Before growing the p-type DBR, they changed temperature and pressure back to the original numbers.
At a bias current of 2 mA, the chip showed lasing wavelengths ranging from 969 to 1161 nm. "We clearly demonstrated a multiple-wavelength VCSEL array with highly strained GaInAs/GaAs quantum wells on a patterned GaAs substrate in a new wavelength band of 0.9 to 1.2 µm," says Koyama. "We [also] demonstrated a record wavelength span of 192 nm."
"This looks like an interesting technology," says Jim Tatum, strategic marketing manager of VCSEL Optical Products at Honeywell (Richardson, TX), that several research groups are pursuing this goal, though exact details may differ. "I would view this technology as most interesting for CWDM applications running 10 Gigabit Ethernet."
Koyama says optimization of the pattern shape of the non-planar substrate will extend the wavelength window even further, and expects that VCSELs such as those his group produced may open low-cost ultra-wideband WDM data links and networking in the future.