Compact Solid-State Source Generates 600 mW at 488 nm
Frequency-doubled diode MOPA could supplant gas lasers.
In the decades since the first 488nm argon-ion laser was marketed by Spectra-Physics, a multibillion-dollar instrumentation market has grown around that wavelength. Flow cytometry, DNA sequencing, confocal microscopy and other analytical medical technologies are dependent on dyes whose fluorescence is excited by 488-nm light. The wavelength also is widely used in the graphics industry, in semiconductor inspection and in other applications.
Figure 1. A new solid-state source challenges the argon-ion laser’s monopoly at 488 nm.
But the venerable ion laser, with its fragile construction and terrible power efficiency, has been challenged in recent years by a number of solid-state competitors (see “Solid-State Lasers Are Gunning for Argon-Ion’s Place, Photonics Spectra, September 2003, page 54). Except for the Coherent Sapphire laser, however, these have been limited to tens of milliwatts — and even the Sapphire has a maximum rated output of 200 mW.
Now scientists at Ferdinand Braun Institut für Höchstfrequenztechnik in Berlin have demonstrated up to 600 mW of 488-nm light from a compact, solid-state source (Figure 1). The package consists of a semiconductor laser master oscillator power amplifier (MOPA) whose 4-W infrared output is frequency-doubled in a single pass through a periodically poled MgO:LiNbO3 (PPMgLN) nonlinear crystal (Figure 2).
Figure 2. The semiconductor master oscillator power amplifier generated 4 W, which was frequency-doubled to 600 mW at 488 nm in a single pass through a nonlinear crystal.
The master oscillator is a 1.5-mmlong InGaAs distributed feedback laser with a 3-µm ridge waveguide. It generates 40 mW at 976 nm in a single longitudinal and single transverse mode to seed the power amplifier, which is a 4-mm-long InGaAs, single-quantum-well tapered structure whose facets are antireflection-coated at the laser wavelength. The first 1.35 mm of the amplifier is a straight, index-guided section, and the remaining 2.65 mm is gain-guided and tapered at 6°.
Figure 3. As expected, the second-harmonic power was quadratically dependent on the fundamental. The deviation from quadratic dependence at the top of the curve was probably caused by degradation of the beam quality from the master oscillator power amplifier.
The output of the power amplifier is focused to a 100 × 60-µm spot in the 30-mm-long PPMgLN crystal from HC Photonics Corp. of Hsinchu, Taiwan. The crystal faces are antireflection-coated for both the fundamental and harmonic wavelengths, and the crystal is held at 70 °C in an oven.
Up to 600 mW of 488-nm light can be generated from the system, with an overall wall-plug efficiency of 4 percent (Figure 3). The single-pass conversion efficiency in the PPMgLN crystal is 15 percent. The scientists believe that the second harmonic’s deviation from the quadratic dependence at the upper end of Figure 3 is caused by degradation of the MOPA’s beam quality at its maximum output.
Optics Letters, March 15, 2006, pp. 802-804.
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