Novel Enhancements Demonstrated for Intracavity Nonlinear Optics
Intracavity nonlinear optics has long been the pathway to additional wavelengths from established lasers. The most common example is the internally doubled neodymium laser in laboratories and hospitals around the world that is being marketed by the tens of thousands as green laser pointers. Recently, a pair of researchers at Lehigh University in Bethlehem, Pa., demonstrated techniques to enhance intracavity nonlinear optics and applied them to build an efficient blue Nd:YAG laser.
The researchers employed third-harmonic generation, rather than the more common second-harmonic generation, to generate a blue output from the 1319-nm Nd:YAG line. Fundamental crystal symmetry issues make third-harmonic generation in a single step very inefficient. Instead, the third harmonic must be generated in two steps: first generating the second harmonic, which then is mixed with the fundamental to create the third harmonic.
In the past, this has required two nonlinear crystals, but the Lehigh scientists designed a single, partly periodically poled KTP crystal to accomplish both processes. Half the crystal was periodically poled for quasiphase-matching the second-harmonic process, and the other half was a single domain for phase-matching the mixing process. Because different phase-matching temperatures were required in the two halves, the crystal was housed in a dual-temperature oven.
Figure 1. The intracavity KTP crystal incorporated a periodically poled region to phase-match second-harmonic generation and a single-domain region to phase-match mixing of the second-harmonic and fundamental waves to generate the third harmonic. Images ©OSA.
The researchers configured the blue laser in a three-mirror, folded configuration with the nonlinear crystal at the waist between two spherical mirrors (Figure 1). The 10-mm-long, 5-mm-diameter, 1 percent atomic-doped Nd:YAG rod was axially pumped with up to 10 W at 808 nm from a fiber-coupled laser diode. An intracavity polarizer and half-wave plate controlled the polarization of the intracavity circulating power and, as a result, the output coupling as well. The output coupling depended on the polarization because the output coupling was simply the conversion to the second and third harmonics, and this conversion was directly dependent on the polarization of the fundamental wavelength. A dielectric coating on the front face of the nonlinear crystal -- the face that was closer to the concave mirror in Figure 1 -- reflected the 440-nm blue light toward the output coupler, which transmitted more than 90 percent of it.
Because both second-harmonic generation and mixing are two-photon, nonlinear processes, the power generated by these processes scales as the square of both the input powers and the interaction length. The researchers used two KTP crystals, one twice as long as the other. All other things being equal, the longer might have been expected to generate four times as much second-harmonic and up to 16 times as much third-harmonic light. In practice, however, the two crystals produced similar results at a repetition rate of 4 kHz (Figure 2).
Figure 2. At a repetition rate of 4 kHz, the blue output was essentially the same from two crystals, one of which was twice as long as the other. Output from the shorter crystal was enhanced by resonating the 660-nm second harmonic in an intracavity Fabry-Perot etalon.
The researchers attribute this to the 660-nm coatings on both faces of the shorter crystal, which created a Fabry-Perot etalon for the second harmonic. This not only enhanced second-harmonic generation by making it resonant, but also generated the third harmonic in both the forward and backward directions, doubling the effective interaction length for the mixing. (Although the second harmonic is generated in both directions with or without the coating, only the second harmonic traveling right to left in Figure 1 passes through the mixing section of the nonlinear crystal.)
The scientists believe that the enhancements resulting from the 660-nm Fabry-Perot etalon increased the third-harmonic output by at least an order of magnitude.
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