VCSELs continue to push back technological barriers

Papers on 1310 and 1550 nm VCSELs also generated great interest. Long-wavelength devices are eagerly anticipated as cost-effective replacement sources for edge emitters. These devices are slated for 10-40 km optical links in high-growth areas at the edge of the network, including metro access and enterprise communications.

Manufacturing In his invited talk, Honeywell s Jim Tatum suggested doing away with proprietary packaging in favor of standardized packages. This should enable VCSEL makers to move on to the next step of automated assembly and manufacturing. The standardization of packaging will enable the vendors of automated manufacturing tools to design products specifically for VCSEL producers if the market is large enough. Currently, each manufacturer has its own package, and the number of variations may be slowing market development.

Tatum also considered replacements for the traditional TO-46 can, which still features leads that can reflect power during 2.5 GHz operation. Controlled-impedance ceramic packages could be the answer owing to their essentially flat impedance response up to 12 GHz, which would be adequate for 10 Gbit/s applications. Tatum said he would also like to see an industry consensus on a 10 Gbit/s integrated VCSEL/TIA/pin photodetector package.

Flip-chip arrays B Schneider of ULM Photonics stressed the importance of flip-chip bonding to large-scale VCSEL array manufacturing (see figure 1). Flip-chip processing is seen as vital to the manufacture of large arrays: it simplifies assembly and test, and eliminates complicated wire-bonding and bond-pad schemes. ULM s 850 nm VCSELs employ a coplanar waveguide structure that allows all the contacts to be placed on the top, while bottom emission requires removal of the GaAs substrate.

The company s devices exhibit a series resistance of 70 Ω at 4 mA, and provide an output power of 2 mW. The threshold current is typically 1 mA, and even after substrate removal thermal resistance is only 2.6 K/mW.

Large-scale arrays undergoing accelerated aging at 160 °C and 7 mA drive current typically fail at around 1350 hours, which extrapolates to more than 10 million hours at room temperature. Schneider concluded by saying that singlemode operation at 10 Gbit/s is still desirable, and that processing and reliability issues still remain the main challenges for VCSELs to overcome.

Long-wavelength VCSELs Long-wavelength VCSELs have developed more slowly than their shorter-wavelength 850 nm cousins as a result of fundamental materials issues. Mirror structures with high reflectivity can be grown on GaAs, but 1300 nm active regions are difficult to grow in this material system. However, good InP-based long-wavelength active regions can be grown, but at the expense of good mirrors.

One way around this is to use wafer bonding to fuse the different materials that give the desired emission wavelength and high-reflectivity mirrors. Alternatively, quaternary alloys such as InGaAsN can be used in the active region to overcome the problems of lattice matching GaAs-based mirrors to InP.

J Klem of Sandia National Laboratories discussed advances in 1300 nm VCSELs based on InGaAsN with nitrogen contents up to 1.5%. Sandia s devices are grown by MBE on 3 inch GaAs substrates. Several new oxide-confined monolithic VCSEL designs were also described. These incorporated compositionally graded mirror interfaces and doping profiles, which reduce the operating voltage to 4.3 V. Adding nitrogen allows the indium content to be increased in the InGaAs QWs, although too much indium causes strain relaxation and limits the number of QWs that can be introduced. Sandia has thus opted for 34% indium in InGaAsN wells that are 6 nm thick.

One scheme discussed employed a monolithically grown GaAs tunnel junction and two n-type DBRs that are designed to reduce free-carrier absorption losses. This led to output powers of 2 mW at room temperature for large-aperture devices. A second design with a smaller aperture operated at a much lower voltage of 2.7 V. This device has a threshold current of 1.1 mA and demonstrated 0.68 mW of CW output power. The slope efficiency was reported to be 0.24 W/A.

T Miyamoto of the Tokyo Institute of Technology also discussed the fabrication of GaInAsN/GaAs VCSELs for telecom applications. Miyamoto described a process that involved growing the Al0.7Ga0.3As/GaAs DBRs (35 pairs on the bottom and 24 on top) by MOCVD, and depositing the three QWs separating the mirrors using CBE. Apertures of 9 µm2 were formed using oxidation at 450 °C and devices exhibited 1 mW in CW mode at a wavelength of 1185 nm. The threshold current was 2.6 kA/cm2, and the maximum pulsed power output was 4 mW for a slope efficiency of 0.22 W/A.

Miyamoto s colleague T Kondo described 1160 nm VCSELs based on an active region with three strained In0.36Ga0.64As quantum wells. Using an aperture size of 9 µm2, these devices achieved a threshold current of 3 mA and current density of 3 kA/cm2. The output power was 2 mW and slope efficiency was 0.3 W/A at 25 °C. The dependence of current on temperature was nearly constant up to 70 °C, which led Konda to suggest that the devices could be used as uncooled sources in LAN applications.

Cielo ships a range of long-wavelength VCSELs that can be driven at up to 10 Gbit/s in eight- and twelve-channel arrays, while its singlemode devices give up to 1 mW output power. Lance Thomson gave an update on Cielo s latest 1300 nm device, which now offers 1 mW at temperatures up to 90 °C with a wavelength drift of 0.08 nm/°C. VCSELs operating at a current density of 35 kA/cm2 and at 90 °C have functioned reliably for 1550 hours, he added.

Emcore s long-wavelength VCSELs Hong Hou gave an overview of Emcore s 1300 nm singlemode VCSEL, which is intended for telecom applications. The device features an In0.34Ga0.66As0.99N0.01 QW active region surrounded by high-index GaAs/AlAs DBR mirrors (39 pairs on the bottom and 25 pairs on top).

A device with a wavelength of 1275 nm has been designed with an 8 µm aperture, and provides a threshold current of 3 kA/cm2. The singlemode output is 0.4 mW, and more than 1 mW in multimode operation. The company s target is 0.7 mW for 2 km optical links.

Hou explained that higher-power singlemode devices still remain a major challenge, and that currently only around 60% of the optical power is coupled into a singlemode fiber with optics. In addition, the series resistance in the long-wavelength emitter needs to be reduced from its current level of 100 Ω to 30 Ω demonstrated by the company s 850 nm products. Emcore is also working to remove the wavelength s dependence on temperature.

Developments in 10 Gbit/s VCSELs Swiss VCSEL specialist Avalon Photonics is manufacturing singlemode VCSELs for absorption spectroscopy applications, and developing 1 x 4 and 1 x 12 arrays using multimode fiber at rates of up to 3.125 Gbit/s (see Compound Semiconductor May 2001, p70).

John Humphries described Avalon s new 10 Gbit/s devices, which consist of small (4 µm) apertures designed to lower parasitic resistance and reduce power consumption at these high modulation rates. Humphries noted that reducing the dimensions of the oxide region could lower the series resistance from 100 Ω to around 40 Ω.

Avalon is targeting 10 Gigabit Ethernet applications, in addition to cable television and remote antenna addressing for mobile-phone systems. The last two applications use analog modulation schemes that require low noise, high linearity and fast modulation, and may benefit from Avalon s VCSELs, which feature a low relative intensity noise (RIN) of -130 dB/Hz at 10 GHz.

Emcore s Hou gave a paper on the company s VCSEL arrays, which together with single devices constitute a capacity of 1.5 million VCSELs per month. The company s roadmap includes 4 x 8 and 4 x 12 arrays, in addition to 32 x 32 arrays at 2.5 and 3.3 Gbit/s. VCSELs capable of 10 Gbit/s performance are also under development. Their mesa structure reduces series resistance, which is currently 60 Ω. The power output is 1 mW. Initial results of accelerated reliability studies (at 70 °C and 6 mA) for 10 Gbit/s 1 x 12 arrays led to lifetimes of 4 x 105 hours.

S Chiou reviewed United Epitaxy Company s (UEC s) recent VCSEL activities. Development began in October 2000, and GaAs-based VCSELs that operate in the 780-850 nm range were first announced last June (see Compound Semiconductor June 2001, p15). These devices are available as epiwafers on 3 inch GaAs substrates, or as chips. Red 650 nm devices are also being developed.

UEC now has a third facility at an industry park in Tainan, Taiwan and has a total of 30 MOCVD reactors, including five dedicated to nitride-based materials. By June this year, the company plans to offer 10 Gbit/s 850 nm single VCSELs and 4 x 2.5 Gbit/s arrays.

UEC s arrays contain VCSELs with 10 µm apertures placed at 250 µm intervals (figure 2). The measured series resistance is 35 Ω, and the devices deliver 2 mW of multimode output power at 5 mA for a forward voltage of 1.8 V. To date, these devices have operated at 5 mW for 1000 hours at 70 °C. In contrast with the multimode devices, UEC s singlemode 850 nm VCSELs have a relatively high series resistance of 75 Ω and an output of 1.5 mW at 5 mA. The threshold current lies at 1.5 mA and the singlemode suppression ratio is 40 dB.