News Article

Commercializing Long-wavelength VCSELs (Cover Story - VCSELs)

In this two-part feature, Richard Dixon describes how a number of obstacles have been overcome to make long-wavelength VCSELs a commercial reality. Starting overleaf, Tim Whitaker discusses the use of 850 nm VCSELs in 10 Gbit/s transponders.
VCSELs operating at the telecom wavelengths of 1310 and 1550 nm have the potential to become low-cost alternatives to edge-emitting lasers. However, despite strong demand for VCSELs at long wavelengths, the ability to grow an optimal combination of a good quality active region and high-reflectivity mirrors is technologically challenging (see ). A number of different approaches have been tried. Gore Photonics has used wa-fer fusion to combine a 1300 nm active region with GaAs-based DBR mirrors and an integrated optical pump (see Compound Semiconductor July 2000, p54). As described below, several companies - Cielo, Infineon and E2O - have used InGaAsN active regions lattice-matched to GaAs-based DBRs, while Nova Crystals and Bandwidth9 have developed monolithic InP-based VCSELs. Also, CoreTek is having success with its MEMS-based tunable VCSEL structure. Nova reaches 10 Gbit/s at 1310 nm Last year Nova Crystals introduced a 2.5 Gbit/s 1310 nm surface emitter providing CW operation up to 1 mW in a single spatial mode. At this year s Optical Fiber Communication conference (OFC) in March, the company demonstrated an updated 10 Gbit/s version of this electrically injected device with an output of 7 mW, achieved by integrating an optical amplifier into the device. The VCSEL is a monolithic InP-based device grown by a single MOVPE sequence. A number of other components have also been monolithically integrated on-chip, including a power monitor diode and an electroabsorption modulator. All functions were integrated into a chip size of just 600 300 m. The company aims to extend the range of its VCSELs into the metro domain and later to longer reaches for telecom applications. According to CEO Felix Ejeckam, this will mean addressing limitations in spatial mode operation and reducing polarization in the device. "The polarization state is particularly significant for these applications, as increased polarization leads to dispersion which affects long distance operation," says Ejeckam. Nova says both single spatial mode and single polarization state operation have been achieved over the entire current and temperature range (from 40 to 85C). The company is presently ramping up production of its 2.5 Gbit/s VCSELs, and plans to have its 10 Gbit/s devices ready for sampling as early as July/August, with volume production around the end of the year. Bandwidth9 demos 1550 nm VCSEL As well as manufacturing a MEMS-based 2.5 Gbit/s tunable VCSEL (see p49), Bandwidth9 has also developed a single wavelength monolithic InP-based 1550 nm VCSEL. Single mode fiber operation at 2.5 Gbit/s has been achieved over 150 km up to 75C. The output power is 0.5 mW and the single mode side suppression ratio is > 35 dB. Active regions prove popular Unlike Nova Crystals, Cielo Communications has developed a monolithic VCSEL lattice-matched to GaAs, which is grown by MBE. The active region consists of three 6 nm thick InGaAsN/ GaAs QWs and 30 undoped pairs of AlAs/GaAs for the bottom DBR stack. A 24-pair C-doped AlGaAs/GaAs stack forms the top DBR. In May, the company reported at CLEO error-free 10 Gbit/s operation over 10 km of single mode fiber with an emission peak centered on 1289 nm. Lasing was sustained up to 125C with a spectral line width of 0.05 nm. The maximum output power was 1 mW at room temperature, with 2.5 mW available in multi-mode operation. "We investigated both antimonide- and nitride-based GaAs active regions, and feel that the nitrides have provided better photoluminescence intensity and make better lasers," says marketing manager Jess Bisberg. "The wafer bonding approach complicates manufacture, and makes it more difficult to design array structures." Cielo plans to incorporate its new 10 Gbit/s VCSELs into a series of integrated optical subsystems to be announced later this year. Infineon backs quaternary QWs Infineon s new 1300 nm VCSEL employs two 6.5 nm In0.35Ga0.65As0.982N0.018 QWs in the active region, which are lattice matched at this composition to GaAs/ AlGaAs top and bottom DBR mirrors. Infineon uses RF-coupled plasma source MBE to grow the nitride layers. Operating at OC-192 data rates, the maximum wavelength is 1308 nm (at 8 mA), with a CW output power of 1 mW at 25C, although lasing can be sustained up to 80C. The threshold current is 2.02.2 mA and typical side mode suppression is 40 dB. Henning Riechert of Infineon s corporate research group says the company decided that the wafer fusion was not ideally suited to volume production. "We chose the InGaAsN route for the active region because of the fast progress we have already made with this materials system." (See Compound Semiconductor July 2000, p71.) E2O evaluates different approaches E2O Communications also recently announced a long-wavelength VCSEL which operates at 1320 nm with a single-mode CW output power of more than 2 mW at room temperature. According to Wenbin Jiang, vice-president of advanced technology, E2O is working on a number of approaches, although its current device is based on standard long-wavelength materials. "We are currently studying InGaAsN for the active region, but there are still concerns about reliability because nitrogen is difficult to incorporate and tends to degrade," says Jiang. CoreTek leads the way in high power CoreTek, which is now part of Nortel Networks, is poised to introduce a new 40 Gbit/s tunable laser capable of high-power operation in the 15141564 nm wavelength range. Known as MEM-LASE, the device employs a MEMS-based curved top mirror and a deposited dielectric bottom mirror around the InGaAsP QW active region (see Compound Semiconductor July/August 1999, p32). The device is tuned by applying a voltage to the top MEMS mirror, which changes its curvature and hence alters the cavity length. The device is optically pumped by a 1300 nm laser diode which allows a very high peak power of 30 mW, and up to 20 mW across the entire 50 nm tuning band. According to CoreTek s president Parvis Tayebati, this is the first such device to offer the requisite output power for telecom applications at 10 and 40 Gbit/s data rates in a tunable format. "The key to the power lies in the optical pumping technique employed," says Tayebati. "The high power leads to the high quality spatial line width of around 24 MHz, which is maintained in single mode operation by our curved MEMS mirror. In system trials this had produced a modulated signal that only needs amplification every 80 km and regeneration every 600 km." At present, the device is externally modulated, although there are plans to integrate a GaAs-based MachZehnder modulator into a smaller package in future. The tuned signal can be locked using an integrated front-end locking system. Tayebati says that beta testing is currently underway, and product introduction is expected by the last quarter of the year.
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