Optical pumping improves laser output

High-power output in a circular TEM₀₀ beam has been achieved from a semiconductor laser. The design allows, through series combination, scalable powers as high as 20 W; such a compact, efficient device would be valuable for applications in optical communications and optical storage. The optically pumped vertical-external-cavity surface-emitting laser (VECSEL), conceived by Aram Mooradian and designed with coworker Mark Kuznetsov while at Micracor Inc. (Acton, MA), operated continuous-wave (CW) at a wavelength near 1004 nm with output powers of 0.69 W in a TEM₁₁ mode, 0.52 W in a TEM₀₀ mode, and 0.37 W coupled to a single-mode fiber. The developers claim all these results to be record levels.¹

Erbium-doped fiber amplifiers (EDFAs) in optical communication systems require 0.l-0.5 W of CW optical pump power in single-mode fiber; conventional semiconductor lasers, such as edge-emitting lasers, wide-stripe lasers, and vertical-cavity surface-emitting lasers (VCSELs) have various disadvantages in either beam profile or output power. Optically pumped semiconductor (OPS) VECSELs combine the approaches of diode-pumped solid-state (DPSS) lasers and semiconductor quantum-well (QW) VCSELs, drawing on the advantages of both.

OPS-VECSEL design

The OPS-VECSEL device uses an external spherical mirror as the output coupler of the cavity, producing a cavity length of about 20 mm. About 1-2 W of output from a commercial multimode diode laser was pumped onto a strain-compensated multiple-quantum-well indium gallium ar senide/gallium arsenide phosphide/gallium arsenide (InGaAs/ GaAsP/GaAs) wafer structure, which converted the pump power to the circular fundamental transverse mode with a beam diameter of about 100-110 µm. The OPS-VECSEL, with pump laser and accompanying optics, fits within a 3-cm-long package, is tunable, and can be modulated at much higher speeds than a DPSS laser. Significantly, the output power of the OPS-VECSEL can be scaled to the multiwatt range by using multiple pumps and gain elements in series.²

"This represents a milestone demonstration," said Mooradian, now at AMTEK (Winchester, MA). "For the first time you`re able to operate a semiconductor laser like a more-common run-of-the-mill laser that has a high-quality, round Gaussian beam coming out of it. With this multiple-bounce approach it`s possible to scale that up with a circular beam to power levels that were just not possible before," all in a laser with a head size that would fit into the palm of a hand. The OPS-VECSEL is comparable to a DPSS laser but with a higher efficiency—71% of the 0.52 W of TEM₀₀ output was coupled into an uncoated single-mode fiber, an amount that could be improved to as much as 80% by antireflection-coating the fiber and improving lens transmission.

Coherent Inc. (Santa Clara, CA) has acquired Mooradian`s patent on optically pumped semiconductor lasers and is working to bring the OPS-VECSEL technology to the marketplace (see Laser Focus World, June 1997, p. 133). Coherent expects to have laser sources to pump EDFAs deployable for the cable TV market in about a year, according to Jean-Michel Pelaprat, director of Coherent`s Commercial Business Unit, also, potentially, praseodymium (Pr)-doped fiber amplifiers (PDFAs), which require pump energy at 1017 nm. Products for the telecommunications market need Bellcore qualification, which would take an additional 12 to 18 months, according to Pelaprat.

REFERENCES

1. M. Kuznetsov et al., IEEE Phot. Technol. Lett. 9, 1064 (1997).

2. A. Mooradian, US Patent #5,131,002.