In addition, light at 1310nm is much less damaging to the eye than light at 850nm, so it's much easier to design links that are eye safe -- much more of a problem when arrays of VCSELs are used, as the combined power of the array must be considered.
But before they become a viable commercial proposition, 1310nm VCSELs must equal the performance of DFB lasers. That means they need to operate uncooled over a wide temperature range -- many telecom applications require –40 to +85 degrees C. (Adding a cooler to a VCSEL would wipe out any advantages a VCSEL has in terms of cost and power consumption.)
Recently, several vendors have made technology announcements about 1310nm VCSELs. Unlike short-wavelength devices, there are several distinct options for making 1310nm VCSELs. So, this report will compare vendors according to the materials technology they have adopted. To understand some of the technical issues involved, it is necessary to read the earlier section on Challenges.
Gore Photonics Inc., a subsidiary of W.L. Gore and Associates Inc., was first with a 1310nm VCSEL announcement in 1998. Its device comprises a 1310nm laser with an integrated optical pump (a second laser at 850nm). The active region of the 1310nm device is based on indium phosphate, while the mirrors and the 850nm laser are based on gallium arsenide. The regions are joined by wafer bonding, an industry-proven process of fusing two semiconductors with dissimilar lattice constants, using heat and pressure.
E2O Communications Inc. is also working on an optically pumped 1310nm VCSEL, though it hasn't published any results yet.
Single epitaxy means that the device can be fabricated in one process. This is advantageous in term of manufacturing -- it's simpler and cheaper than growing several layers and then bonding them. There is no need for an optical pump, which saves on space and materials and helps bring costs down, too. All of the single epitaxy devices are compatible with GaAs mirrors, and benefit from its good thermal conductivity and established manufacturing techniques. However, in general, the active regions do not work as efficiently as those based on InP. There are several approaches:
1. GaInNAs active region
The main drawback is that nitrides have low gain, which means that the laser has to be pushed quite hard to give out a decent amount of power.
Cielo Communications Inc. and Sandia National Laboratories are collaborating on 1310nm VCSEL development (see Cielo Shows Laser with Sandia Labs). Ultimately -- summer 2001 -- Cielo will manufacture the devices. Best results so far are 0.4 milliwatts of singlemode output power and 0.8 mW of multimode power, according to Bob Mayer, Cielo's VP of product development. He also claims that Cielo's devices are stable at 80 degrees C.
E2O Communications Inc. is also working on this type of VCSEL. It claims 1 mW of output power and 1 milliamp threshold current. "It's a little bit shorter than 1310 nm -- actually it's at 1220 nm," says Wenbin Jiang, E2O's VP of advanced technology. But it's better than PicoLight, which has only achieved 1170 nm, he adds.
PicoLight Inc. is working on a single epitaxy device. Stan Swirhun, PicoLight's CEO declined to confirm that the device is based on nitrides, but Light Reading's sources say this seems quite likely. Like Cielo, PicoLight is aiming to introduce 1310nm VCSELs in summer 2001.
2. InGaAs active region
This is the material from which the active regions of 850nm lasers are made. Normally, there's too much stress in the material to grow it on top of a GaAs substrate, without many defects (atomic-level cracks) in the material. Light Reading believes that one startup, Nova Crystals Inc., has found a way around this. Nova has developed a "compliant universal substrate," which is a stretchy layer that can accommodate the mismatch between the crystal lattices. This would make it possible to grow InGaAs at 1310 nm (see Nova Crystals Demos High-Power VCSEL and Nova Crystal's Little Secret). Nova declined to comment.
3. Quantum dots
Quantum dots are particles of semiconductor (in this case, InAs) typically just 100 atoms wide, which are embedded in another material. The wavelength of light they throw out depends on the size of the particle, not just the material, so there's more scope in the design of the laser structure. What's more, the particles are so small they can stretch to fit in with the crystal lattice of GaAs. Researchers at the Air Force Institute of Technology and Imperial College in London are exploring this approach. These are very early stage devices.
We're working on it…
Several vendors have said that long-wavelength VCSELs are on the agenda, but are vague about technology and time to market. They include: