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Amplifier Boosts Efficiency of Microlaser

Photonics Spectra
Mar 2004
Breck Hitz

Miniature, longitudinally pumped, Q-switched solid-state lasers have relatively low efficiency, in part because much of the diode pump light passes through the laser chip without being absorbed. Recently, scientists at Massachusetts Institute of Technology's Lincoln Laboratory in Lexington have shown that adding an amplifier to scavenge the otherwise-wasted pump power can increase laser efficiency by more than a factor of two without appreciable increase in the laser's cost or complexity.

Amplifier Boosts Efficiency of Microlaser

Figure 1. Laser efficiency can be increased by a factor of two by adding an amplifier to use pump power that would have been wasted.

The concept is shown schematically (Figure 1) and mechanically (Figure 2). A coupling lens focuses both the oscillator output and the residual pump power into the amplifier. Careful positioning of the lens -- an aspheric doublet is used -- ensures good overlap between the laser and pump beams in the amplifier. Although the oscillator chip is Nd:YAG, the amplifier is Nd:YVO4 to take advantage of the latter material's high gain and absorption cross sections.

Amplifier Boosts Efficiency of Microlaser

Figure 2. A commercial aspheric doublet focuses pump power into the laser chip, and an identical doublet focuses both laser power and pump power into the amplifier.

Experimentally, the researchers started with a passively Q-switched microlaser oscillator consisting of an undoped YAG end cap, a 1.5-mm-thick Nd:YAG chip, a 1-mm-thick Cr4+:YAG saturable absorber and another undoped YAG end cap, all of which were diffusion-bonded in series. This is a standard laser design, and similar lasers are in use in ladar systems. In this work, the laser's output mirror had 55 percent reflectivity at the 1.06-µm laser wavelength and was antireflection-coated at the 808-nm pump wavelength.

Amplifier Boosts Efficiency of Microlaser
Figure 3. The entire oscillator/amplifier assembly fits into a package barely three inches long.

The scientists then added the amplifier, a 4.5-mm-thick piece of Nd:YVO4 that was antireflection-coated at both the pump and laser wavelengths. The coupling doublet, however, was antireflection-coated only at the pump wavelength, resulting in a loss of approximately 9 percent at the laser wavelength.

Amplifier Boosts Efficiency of Microlaser
Figure 4. The amplifier increases output energy by more than 200 percent across all repetition frequencies.

The pulsed diode lasers that pumped the system delivered 25 W of peak power, slightly more than half of which was absorbed in the Nd:YAG chip. The rest was focused into the Nd:YVO4 amplifier.

The oscillator alone produced 370-ps pulses containing about 30 µJ. When the amplifier was added, the pulse energy was more than doubled (Figure 4). By detuning the pump wavelength from the peak of the Nd:YAG absorption, the investigators increased the percentage of the pump light passing through the Nd:YAG chip and reaching the Nd:YVO4 amplifier. The Nd:YVO4 still absorbed this off-peak light effectively because it has a broad absorption spectrum. In this case, the scientists saw the amplifier gain increase from slightly greater than two to nearly four.

To demonstrate the versatility of the energy-scavenging amplifier, the team applied it to a number of microlasers, each designed for and currently operating in the field in different applications. Pulse energies from these lasers ranged from 8 to 60 µJ. With the amplifier added, the lasers' pulse energies increased by 25 percent to 280 percent.

The scientists point out that none of these experiments employed an optimized design for an oscillator-amplifier system. When their findings are taken into account, they believe that the performance will improve.


GLOSSARY
solid-state laser
A laser using a transparent substance (crystalline or glass) as the active medium, doped to provide the energy states necessary for lasing. The pumping mechanism is the radiation from a powerful light source, such as a flash lamp. The ruby and Nd:YAG lasers are solid-state lasers.
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