QPI unveils lasers with circular beams
Vertically integrated fab model
After receiving funding in mid-2000, the four-year-old spin-off from the University of Maryland began the development of Fabry-Perot laser diodes at a 40,000 sq. ft production facility that includes a 15,000 sq. ft cleanroom. At this site, which began operation at the end of 2001, QPI undertakes the epitaxial growth of InGaAsP MQW lasers on InP substrates using an MBE reactor. Wafers are processed into die, assembled, packaged and tested in house.
Lasers are available as bare die, chip-on-submount and in a range of packaging options. Specially designed heat sinks are also able to facilitate rapid testing and evaluation of the gain chips in customer applications. The company also plans to offer a fiber-coupled 14-pin butterfly package in the near future.
"At QPI our designs are based on ridge-waveguide structures, which provides us with a simpler manufacturing process than buried heterostructure laser designs," says CTO and co-founder of the company, Mario Dagenais. "Our technology allows us to avoid the regrowth processes typically needed to incorporate passive and active components. We also get a higher yield, which translates into lower production costs."
Single-angled facet gain chip
The company s InGaAsP/InP gain chip features a reflective facet coating on one end, and an angled facet with a simple anti-reflection coating at the other. The optical coating is performed by electron-beam deposition. Known as a single-angled facet (SAF) device, the waveguide is tilted 6-8° away from the normal to reduce the occurrence of reflection back into the device. The SAF laser exhibits an effective reflectivity of less than 10-5, and this low residual reflectivity is the key to wide gain bandwidth (up to 150 nm). This wide operation band is being leveraged by QPI s customers to make external cavity tunable lasers, or even superluminescent diodes, which find applications in network monitoring. QPI says that currently most companies are content with either C- or L-band wavelengths.
Other key operating parameters for the gain chip include a Fabry-Perot equivalent threshold current of 50 mA, a slope efficiency of 0.4 W/A and output power in excess of 100 mW CW at room temperature.
"In our single-angled facet device, the light in the waveguide hits the small angle produced by the facet, and effectively very little is coupled back into the waveguide," says Dagenais. This leads to a very low reflectivity at the facet, resulting in higher optical power being transmitted from the chip.
"This is exactly what you want for an external cavity laser," continues Dagenais. "If you put a filter outside the cavity, such as a grating that reflects the light at a particular wavelength, the facet reflectivity remains very low over the whole spectrum of amplified spontaneous emission."
High-power laser diodes
QPI s high-power Fabry-Perot laser diode is also targeting eye-safe 1550 nm free-space optical wireless or remote sensing applications. Based on the same InGaAsP ridge-waveguide design, the laser operates in single transverse mode in the C- and L-bands. The company s standard InGaAsP device achieves 160 mW using a conventional facet coating design. Preliminary data sheets indicate an operating current of 450 mA, with threshold currents and slope efficiencies essentially the same as the SAF device used in a low loss cavity.
Circular beam properties
Both device types also benefit from the company s proprietary passive-active resonant coupler (PARC) technology. QPI monolithically integrates a passive InP waveguide with the ridge-waveguide laser, which leads to a low optical loss between passive and active components. "Our patented integration technology uses fewer processing steps and offers more robust and efficient coupling between active and passive optical elements," says Dagenais. "While this approach allows the integration of semiconductor amplifiers to make high-power devices, it also allows passive InP waveguides to control beam divergence and shape."
Both lasers feature a mode control option that produces almost circular output. For both chip designs, the transverse and parallel beam divergence angles are 26° and 25°, respectively, and a second option provides a wider divergence and a circular 30 x 30° beam. Without this option, the standard diode has beam divergence angles of 36° (transverse) and 14° (parallel).
"Using PARC we are able to shape the output of the laser so that the beam is larger and almost circular compared with that of a typical edge-emitting laser," says Dagenais. "This makes it much easier to couple to single-mode fiber, and the larger beam means that overall there is less divergence."
According to Dagenais QPI s near-term targets for the SAF device include pre-production by May and full Telcordia production by the summer. The high-power lasers are due for production during the fourth quarter of 2002. Semiconductor optical amplifiers are also on the company s development roadmap.