Record low tuning voltage for long wavelength VCSEL


Last month, BeamExpress SA said it had demonstrated 1.5µm waveband optically pumped tunable vertical-cavity surface-emitting lasers (VCSELs) with 38nm tunability at tuning voltages below 4V. Devices with 5µm apertures show 1mW continuous wave single-mode output with 40dB side-mode suppression ratio. Demonstration of widely tunable VCSELs covering the whole C or L bands at tuning voltages below 4V opens up new perspectives for cost-effective, tunable lasers in metro area networks, it says.

"Tunable lasers are key elements for reconfigurable metro networks, but will be commercially viable components only if their cost is lowered to match that of current fixed-wavelength telecom lasers," says chief scientist Eli Kapon.

BeamExpress was founded in 2001 by a team from the Laboratory of Physics of Nanostructures, Institute of Quantum Electronics and Photonics of the Swiss Federal Institute of Technology (EPFL), led by Kapon, a professor at EPFL's Department of Physics, with Dr Alexei Sirbu, VP of core technology development.

The company is based at the Scientific Park on the EPFL campus in Lausanne. It currently focuses on developing wavelength-controlled emitters and filters based on a 1550nm-wavelength VCSEL fabricated by wafer fusion.

The EPFL's Industrial Relations Office helped to transfer its unique localised wafer fusion technology, which was developed by the founders over several years at EPFL. It allows simple, efficient and high-yield monolithic integration of a long-wavelength VCSELs with a micro-electro-mechanical system (MEMS) tuning structure which can be electrostatically activated at very low tuning voltage using standard low-cost electronics.

The gain medium for laser emission contains InGaAlAs and/or InGaAsP quantum wells and barriers based on indium phosphide with composition, strain and thickness adjusted for 1.5µm emission. Then 36 pairs of AlGaAs/GaAs distributed Bragg reflectors (DBRs) are grown on gallium arsenide with absolute reflectivities of >99.8% and thickness uniformities of >±1% over 40mm diameter.

The VCSEL is assembled by wafer fusion of the InP-based active region to first the top GaAs-based DBR then, after selectively etching the active region, the bottom DBR, overcoming the mismatch of the crystal structures.

In Figure 1 the left side represents standard fabrication of fused vertical-cavity devices, with electrical and optical confinement obtained in post-fusion processing by etching deep mesas and lateral oxidation. The right side represents localised wafer fusion, i.e. structuring one wafer before fusion, giving in-situ optical and electrical confinement.

Wafer fusion allows the optimal combination of DBRs and active cavities: AlGaAs-based DBRs outperform other DBRs; InP-based InGaAsP and InGaAlAs multi-quantum wells have the best performance for 1550nm lasers.

Also, by structuring the wafers, localised wafer fusion gives built-in lateral electric and optical confinement similar to single-wavelength VCSELs. An advantage is that, in post-fusion processing, the wafer's surface remains planar, with no need for deep etching through the top AlGaAs/GaAs DBR and wet lateral oxidation.

BeamExpress recently demonstrated 1550nm VCSELs with 3mW output power at room temperature and 0.6mW at 80°C. But the technique also enables devices that are difficult to obtain reliably solely with post-fusion processing, e.g. mesa trimming (Fig 2) to precisely determine the cavity length in multi-wavelength VCSEL arrays. The alternative — multi-step epitaxial growth — has lower yield.

BeamExpress is currently developing 1550nm VCSEL-based multi-channel transmitters with high data rates for metro CWDM. Features include:

  • Data rates up to 2.5Gbit/s per channel.
  • CWDM wavelengths of 1470–1610nm, spaced by 20nm.
  • Minimum output power per channel of 1mW.
  • An integrated wavelength multiplexer .
  • Long-reach transmission (>50km).

BeamExpress is also developing widely tunable metro DWDM sources. Features include:

  • High output power, suitable for long-distance, metro-compatible applications.
  • Emission in the C or L bands.
  • 2.5Gbit/s direct modulation capabilities.
  • Tuning voltage below 10V.
  • A tuning range of 20–40nm.
  • 100GHz channel spacing.

Also monolithic structures can be formed that include air-gaps, which allow simple fabrication of MEMS-based devices such as tunable Fabry-Perot filters, tunable VCSELs, resonant-cavity semiconductor amplifiers etc.

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