Birefringent Tuners | previous | next | feedback |
Definition: device for wavelength tuning of lasers, based on birefringence and polarization rotation
A birefringent tuner is an optical filter device used in a laser resonator for tuning of the output wavelength and/or for narrowing the optical bandwidth (linewidth). There are different tuner designs (see below), all exploiting the birefringence of some optically anisotropic crystalline plates. A laser containing such a filter will typically operate at a wavelength near a point with minimum transmission loss, if the filter curve is sufficiently sharp. The wavelength of minimum loss can be shifted by modifying the tilt angle of the birefringent filter via a micrometer screw.
Birefringent tuners are often used in vibronic lasers, such as titanium–sapphire lasers, in dye lasers, and sometimes in optical parametric oscillators. They make it possible to tune the output wavelength over hundreds of nanometers while introducing only small losses into the laser resonator.
Types of Birefringent Tuners
The simplest kind of birefringent tuner has a single birefringent plate (e.g. of crystalline quartz), through which the intracavity laser beam propagates, inserted with an angle near Brewster's angle in order to minimize reflection losses for p polarization. The birefringence leads to wavelength-dependent polarization changes and thus transmission losses, arising from the Fresnel reflection of s-polarized light at the surfaces or at an additional polarizer. The induced losses vary approximately sinusoidally with the wavelength, and the spectral position of the loss minima, where laser operation can take place, can be tuned by rotating the plate around an axis which is perpendicular to the surface.
A sharp filter response can in principle be obtained just by using a thick birefringent plate. However, this causes the transmission minima to be relatively close in wavelength, so that the laser may not operate at the desired loss minimum. That problem can be solved by using multiple birefringent plates with different thickness [1] (multi-crystal birefringent filters). A typical air-spaced filter stack contains 2–4 plates.
The optical losses for unwanted wavelengths can be increased by inserting polarizers between the different plates. With additional constraints on the plate thickness values, this leads to the design of a Lyot filter. However, the losses via Fresnel reflections, and possibly also the polarization dependence of optical gain in the laser crystal, are usually sufficient, so that additional polarizers are not required.
Bibliography
| [1] | A. L. Bloom, “Modes of a laser resonator containing tilted birefringent plates”, J. Opt. Soc. Am. 64 (4), 447 (1974) |
| [2] | D. R. Preuss and J. L. Gole, “Three-stage birefringent filter tuning smoothly over the visible region: theoretical treatment and experimental design”, Appl. Opt. 19 (5), 702 (1980) |
| [3] | S. Zhu, “Birefringent filter with tilted optic axis for tuning dye lasers: theory and design”, Appl. Opt. 29 (3), 410 (1990) |
See also: birefringence, Lyot filters, optical filters, wavelength tuning, lasers
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