A 1.55 mm vertical-cavity surface-emitting laser (VCSEL) that can supply high bit rates for the metro market was reported in a postdeadline paper at ECOC 2002 (Copenhagen, Denmark), by Markus Ortsiefer of VertiLas (Munich, Germany) and others.1 Transmission experiments using single-mode buried-tunnel-junction (BTJ) VCSELs demonstrated error-free data transmission at modulation frequencies up to 10 Gbit/s.
VCSELs are attractive for communications because they offer low power consumption, entire wafers of devices can be tested at once, packaging and fiber coupling are straightforward, and arrays can be easily fabricated. VCSELs at 850 nm have been used for metro applications, but 1.55-mm VCSELs would take advantage of the eye-safe wavelength, lower fiber absorption and lower voltage drivers. "By tailoring the device dimensions, polarization- and transverse-mode control should also be easier to control," said Ortsiefer.
Problems holding back development of 1.55-µm VCSELs have included the low refractive-index contrast and low thermal conductivity of InP-based distributed Bragg reflectors. Several methods have been proposed to remedy the heat problems in the devices. Because the heating problems predominately occur in the p-doped materials, the researchers from the Technical University of Munich developed a device with a buried tunnel junction that shortens the p-doped region to a very thin layer (see figure).
The front mirror consisted of 34.5 pairs of epitaxial InGaAlAs layers and the back mirror of 2.5 pairs of dielectric CaF2 and amorphous Si. The InP substrate was removed and epitaxial layer became the front mirror. An electroplated layer at the bottom provided mechanical stability and good thermal conductivity.
The BTJ-VCSEL had an elliptic aperture of 3 × 4 µm. When not modulated, the maximum output power of the single-mode device was 0.73 mW at 4.5 mA, with an initial slope efficiency of 0.24 W/A. The threshold voltage was measured as low as 0.9 V and the differential series resistance as low as 72 ohm. The laser operated in a single mode with at least 30-dB side-mode suppression over the entire current and temperature range.
The modulation bandwidth in earlier versions of the BTJ-VCSEL was limited by large resistance and capacitance produced from the contacts. This version reduces external parasitic capacitance by using small contact pads. In transmission experiments, the device demonstrated error-free data transmission with bit-error rates (BER) better than 10-12 at data rates up to 10 Gbit/s. For transmission at 5 Gbit/s over 20.5 km, the measured power penalty was 1 dB.
For more information contact Markus Ortsiefer at email@example.com.
- M. Ortsiefer et al., Postdeadline paper 4.9, ECOC 2002, Copenhagen, Denmark (Sept. 8-12, 2002).