Phase Mismatch Leads to Intensity-Modulated Blue Laser

Breck Hitz

Laser engineers usually worry about maintaining phase matching in frequency-doubled lasers, but now researchers at Technische Universität Kaiserslautern in Germany have taken advantage of the narrow tolerance for phase mismatch in a periodically poled KTP crystal to design and operate a simple, intensity-modulated blue laser. Such a laser could be useful in displays, in photofinishing and in other applications requiring low-power modulated blue light.

Figure 1. The laser may find application wherever low-power modulated blue light is required. Courtesy of Technische Universität Kaiserslautern.

The system consists of a diode-laser master oscillator power amplifier (MOPA) whose output is frequency-doubled in a single pass through a periodically poled KTP crystal (Figure 2). Because phase matching in the crystal is highly dependent on the incoming wavelength, the researchers converted a small wavelength modulation of the diode laser into a large intensity modulation of the second-harmonic output (Figure 3). By tuning the MOPA's wavelength by only 0.2 nm, they observed a 700:1 intensity modulation in the blue output beam. Moreover, they maintained that modulation depth at up to several kilohertz, and they predict that improvements in the experimental arrangement will lead to even higher frequencies.

Figure 2. The infrared beam from a diode laser master oscillator power amplifier was frequency-doubled in a single pass through the periodically poled KTP crystal to generate a blue (462 nm) output. A small modulation of the master oscillator power amplifier's wavelength produced a large modulation of the blue output intensity. Reprinted with permission from Dirk Woll, Marc A. Tremont, Harry Fuchs, Oliver Casel and Richard Wallenstein, Applied Physics Letters, 86, 151101. ©2005, American Institute of Physics.

The researchers altered the MOPA's wavelength by modulating the current passing through the distributed feedback oscillator. This changed the resonant wavelength of the distributed feedback, and, therefore, the oscillator's wavelength, but -- unavoidably and undesirably -- also affected its output power. To minimize the modulation of the MOPA's output power, they drove the oscillator hard enough so that its output saturated the gain in the power amplifier.

Figure 3. A small change in the wavelength of the fundamental radiation incident on the KTP crystal resulted in a large change in the second-harmonic power generated in the crystal. Reprinted with permission from Dirk Woll, Marc A. Tremont, Harry Fuchs, Oliver Casel and Richard Wallenstein, Applied Physics Letters, 86, 151101. ©2005, American Institute of Physics.

The phase-matching curve of Figure 3 is shown on a logarithmic scale by the solid line in Figure 4, and experimental points are represented by the circles. The divergence of the fundamental beam in the nonlinear crystal, together with inhomogeneities in the periodicity of the periodically poled crystal, prevented experimental observation of the absolute minima in the phase-matching curve.

Figure 4. The theoretical (solid lines) and experimental (circles) phase-matching curves differ because the experimental arrangement lacked the resolution to reveal the narrow minima. By modulating the master oscillator power amplifier's wavelength by 0.2 nm between the second-harmonic maximum and minimum, the scientists obtained a 700:1 modulation depth in the intensity of the second-harmonic output. Reprinted with permission from Dirk Woll, Marc A. Tremont, Harry Fuchs, Oliver Casel and Richard Wallenstein, Applied Physics Letters, 86, 151101. ©2005, American Institute of Physics.

As a result, the researchers had to modulate the wavelength between the maximum at 923.52 nm and the experimental minimum at 923.32 nm to obtain 700:1 modulation depth in the second-harmonic output. The corresponding modulation of the MOPA's output power was less than 11 percent.