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14 Feb 2006
Alight Technologies is tackling the need for powerful long-wavelength, singlemode VCSELs by marrying its photonic-bandgap technology with Infineon's dilute-nitride platform. Dan Birkedal and Dirk Jessen detail the hybrid design and reveal why it will benefit datacom and telecom networks.
Multimode VCSELs operating at 850 nm are the dominant source for today's short-range datacom applications. However, despite advantages such as on-chip testing and straightforward fibre coupling, these surface-emitting devices are still to impact the longer-range and higher-speed datacom and telecom applications.
Infineon Technologies, Picolight and Optical Communication Products have all developed 1.3 μm VCSELs, but their singlemode output powers are limited, and this has hindered market penetration in more demanding applications. Instead, current networks are served with edge-emitting sources - either Fabry-Pérot lasers that are limited in range by modal dispersion at higher modulation frequencies or distributed-feedback lasers that usually require an additional optical isolator.
However, recent efforts at our company, Alight Technologies, have revealed that a VCSEL's singlemode output power can be increased to fulfil the requirements of communications applications through the addition of a photonic-bandgap (PBG) structure. Our team, which is based in Copenhagen, Denmark, has made the breakthrough by combining this photonic technology with Infineon's 1.3 μm dilute-nitride VCSEL design, which we acquired late last year.
The singlemode output of a conventional VCSEL is primarily limited by the oxide aperture that confines the electrical current and the optical modes. This aperture has to be quite small (<7 μm) to ensure fundamental-mode operation, but this restriction limits the output power. It also degrades the laser's lifetime and reliability, while the large electrical resistance causes local heating and hinders high-speed operation.
However, our colleague Svend Bischoff, who is a senior staff engineer at Alight, has reported that it is possible to produce high-speed, high-power singlemode VCSELs by combining a PBG structure for lateral optical confinement with a large oxide aperture providing current confinement. The PBG is formed by etching an array of holes in the VCSEL's top mirror. The modified structure produces a wavelength shift in the cavity resonance that leads to an effective refractive-index change, and this produces the lateral optical mode confinement that ensures singlemode operation.
Etching effects
The shift in cavity resonance wavelength is very small for shallow etch depths of a few hundred nanometres into the VCSEL's top-mirror surface, so the VCSELs fabricated up until now (at the University of Ulm in Germany and Korea's Advanced Institute of Science and Technology) have featured holes with a depth of 10-20 mirror periods. The holes deliver the required shift in cavity resonance wavelength but also increase the optical losses and reduce the photon lifetime (the average time that photons spend in the cavity). This means that these lasers have low output powers and a small modal volume because the lateral mode confinement is determined by both the photon lifetime and the shift in cavity resonance wavelength.
At Alight we have circumvented this problem by shallow etching into a layer close to the active region, before depositing a dielectric top mirror. This approach produces very large cavity resonance wavelength shifts (>10 nm) for etch depths of only a few tens of nanometres. We have already produced 850 nm VCSELs and are now extending the method to 1.3 μm GaInNAs VCSELs (see 'VCSELs'). These lasers are produced by shallow etching a tri-diagonal array of rods into the semiconductor surface, just below the dielectric top mirror. The PBG lasing defect is formed by omitting several rods in the centre of this lattice (see 'SEM image').
As well as increasing the cavity wavelength's sensitivity to the etch depth, our design has additional advantages resulting from the close proximity of the PBG layer to the active region. In particular, the number of DBR mirror pairs is constant over the entire structure, which means that reflectivity is high and unmodulated. Consequently, the VCSEL's lateral guiding mechanisms are determined solely by local variations in wavelength or effective index and not by loss/gain guiding. Loss guiding would increase the internal optical losses, which is highly undesirable due to the low saturated gain of the active material. Gain guiding is negligible since the index guide of the PBG is much stronger and completely governs the cavity's optical mode. In addition, our shallow etch avoids exposure of the aluminum-rich layers to the ambient environment during processing, which simplifies manufacturing.
Our VCSELs use the dielectric top-mirror structure that featured in Infineon's highly reliable, qualified lasers produced by Steinle and colleagues. The dielectric mirrors deliver lower optical losses than DBRs do due to the absence of free-carrier absorption, which improves VCSEL performance.
The VCSEL development started with the fabrication of 850 nm lasers. This work was never completed, because of a customer-driven switch to longer wavelengths, but singlemode VCSELs were produced, delivering 3-5 mW. Power levels in these devices were limited by ohmic heating due to a non-optimized contact process. However, research showed that it would be possible to construct 10 mW singlemode VCSELs.
Switching to longer-wavelength VCSELs required a redesign of the PBG structure, with emphasis on low scattering losses. Initial results show that the devices exhibit singlemode behaviour up to 3 mW at 20 °C, that they can deliver 1.4 mW single-mode power at 90 °C, and that they produce sidemode suppression ratios exceeding 30 dB.
A foundry approach
We believe that it is essential to minimize the VCSEL's time to market, so Alight is working with a foundry, and in close co-operation with a customer, to decrease the time taken from producing a prototype to manufacturing a qualified laser. The company understands that it is essential to establish a credible and reliable supply chain. Although prototyping is performed in a class 10 cleanroom facility in Copenhagen, parallel work at foundry partners validates our volume production processes at an early stage.
Based on the promising results that have been obtained so far, we are planning to release 2.5 Gbit/s 1.3 μm VCSELs later this year, targeting datacom and telecom access applications. However, we believe that the transition to higher-speed datacom applications in local storage-area networks and optical interconnects, as well as an increased focus on fibre in telecom access networks, will drive the company's future product portfolio.
The PBG technology is generic and can be applied to VCSELs operating at various wavelengths serving many different applications. For example, high single-mode power is also attractive for sensing applications, printing, passive optical fibre networks and consumer electronics. Our company's strategy is to pursue these opportunities outside the telecom and datacom markets through partnerships that will enhance the penetration of its proprietary technology.
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