A surface-emitting laser with an external cavity mirror produced more than 500 mW of 980-nm TEM00 output with 90% coupling efficiency into a single-mode fiber. The erbium-doped fiber amplifiers (EDFAs) require laser pumps at 980 nm, and in general higher powers from the pump laser are desirable. Usually the pumps are edge-emitting diode lasers, which have their output power limited by catastrophic optical damage and which require external gratings for wavelength stabilization. Aram Mooradian at Novalux (Sunnyvale, CA) reported on an alternative approach that might provide higher powers from cost-efficient vertical-cavity surface-emitting lasers (VCSELs).
Other groups have reported high powers from VCSELs with apertures of about 320 µm in diameter but these lasers emit high-order modes which are undesirable for pumping EDFAs. The Novalux lasers are also capable of multimode operation, with higher powers than single-mode operation.
Mooradian's Gallium Indium Arsenide (GaInAs) quantum-well laser uses a three-mirror, coupled cavity design (see figure).Typical VCSELs have the cavity defined by two distributed Bragg reflectors deposited epitaxially on the substrate. This laser includes those two DBR mirrors but also adds a third, curved external mirror beyond the substrate. Mooradian says, "A key to achieving high performance was reducing or eliminating carrier crowding effects in the gain region for efficient power extraction in a fundamental or low-order spatial mode while at the same time keeping both the substrate optical absorption and the electrical resistance low." The n mirror reduced loss from band-edge absorption in the substrate. Electrical resistance was reduced by using graded interface structures for both the internal mirrors.
The p mirror was bonded to a heat sink to aid heat removal. Because of the large aperture, the optical intensity was far below the catastrophic limit. Instead, the power limit depends on the junction temperature. The short Fabry-Perot cavity stabilized the wavelength, which shifted with temperature at about 0.07 nm/°C.
One advantage to the design is that, unlike edge-emitters, a whole wafer full of the surface-emitters can be tested before die separation, which makes them more cost-efficient to manufacture.
The output beam is circular, with low divergence of about 3 mrad (full-angle half-maximum). Mooradian demonstrated fiber coupling efficiencies of about 90% and coupled powers of more than 420 mW cw from lasers with 150-µm-diameter apertures and uncoated single-mode fibers.
For more information contact Aram Mooradian at email@example.com.
- A. Mooradian, OFC 2001 postdeadline paper 17.
Yvonne Carts-Powell is a science and technology writer based in Belmont, MA.