January 27, 2009

Semiconductor Today

Alfalight unveils surface-emitting DFB laser for lower-cost, high-power systems

In the session ‘Novel In-Plane Semiconductor Lasers VIII’ at the Photonics West 2009 conference in San Jose, CA this Thursday, Manoj Kanskar, VP R&D of high-power diode laser manufacturer Alfalight Inc of Madison, WI, USA, will present an invited talk ‘High Brightness Surface-emitting Distributed Feedback (SE-DFB) Laser’ giving the first details of the firm’s new SE-DFB technology.


A significantly different approach to high-power semiconductor lasers at the wafer level provides what is claimed to be a lower-cost system solution with enhanced brightness and ruggedness compared to existing edge-emitting laser technology.


SE-DFB lasers are made of the same material used for Alfalight’s edge-emitting lasers. But, whereas standard edge-emitting lasers emit through a small facet on the edge of the chip, SE-DFB lasers emit through a large optical window on the surface. A curved grating patterned on the base layer of the chip has four main functions:


to form the laser cavity, providing optical feedback precisely at the design wavelength, locking the emission spectrum on a narrow peak for any combination of operating temperatures and currents;

to couple the laser beam out of the laser chip, through an anti-reflection(AR)-coated optical window while collimating it in one direction without the aid of external optics;

to shape the optical wavefront to enhance brightness and suppress filamentation; and

to protect the device from external optical feedback.


The optical density at the emission window is four orders of magnitude lower than at the facet of edge-emitting lasers. This makes SE-DFB lasers immune to catastrophic optical mirror damage (COMD), which is the main reliability issue plaguing existing laser diode technology.


Several SE-DFB laser chips can be combined on a common heat sink and coupled into an optical fiber using simple optical elements. No micro-lenses or beam transformation optics are needed.


The lasers offer the same 0.07nm/°C wavelength stability as Alfalight’s proven Wavelength Stabilization Technology (WST) - a factor of five more stable over temperature than standard semiconductor lasers, the firm claims. Consequently, there is no need to control the temperature of the chip precisely. SE-DFB diodes can also be driven by low-current power supplies, further reducing system costs.


Alfalight says that the SE-DFB laser's key features - complete immunity to facet damage, on-chip integrated wavelength stabilization, and intrinsically high brightness that eliminates the need for expensive, precision optical elements - can benefit fiber laser, solid-state laser and direct-diode laser applications (e.g. materials processing and infrared illumination).


“SE-DFB lasers have unmatched power scaling and wavelength stabilization capability besides a singular brightness advantage over edge-emitting laser diodes,” says Kanskar. The technology will reach beyond the capabilities of bar-based optical sources, he claims. “This technology has the potential to displace ubiquitous laser diode bars in many important applications.”


Kanskar’s talk also discusses experimental results and outlines configurations for combining chips into high-power laser arrays. Prototype SE-DFB chips, and demonstrations of a SE-DFB-based array and a fiber-coupled module, are also on display at Photonics West.