Thinking big in projection technology

01 Feb 2010

Laser projectors loom large at Photonics West

Laser projectors, both big and small, were the focus of several technical sessions at the Photonics West conference in San Francisco, CA, last week. Such projectors need three laser colours – red, blue and green – for brilliant, high-resolution displays.

Herein lies the rub: red and blue diode lasers are readily available, but there are no diode lasers that emit directly in the green. Numerous green-laser concepts have been investigated in the past (see Green lasers gear up for display markets), and several more were advanced at Photonics West.

By far the largest laser projector on the market today is the unit described by Jesse Anderegg of Evans and Sutherland (Salt Lake City, UT). Intended primarily for planetariums, it boasts a green laser based on an ytterbium-doped fibre master-oscillator-power-amplifier (MOPA).

The MOPA's 1 µm output is frequency-doubled in an external resonant cavity. The bow-tie cavity resonates the fundamental wavelength through an LBO crystal, generating as much as 32 W of green power with 78% conversion efficiency, in what Anderegg described as a "hero experiment".

In practice, the projector produces about 20 W. Careful "impedance matching" of the external cavity is important, Anderegg explained, to minimize back-reflection from the cavity. The emerging green beam has a small divergence, with a measured M² value of 1.05.

Interestingly, the Evans and Sutherland projector also generates the red and blue beams with nonlinear optics rather than with diode lasers. A combination of sum-frequency generation and second-harmonic generation – always using LBO in a bow-tie external cavity – generates about 20 W each in the red and blue spectral regions, from a 1 µm fibre-MOPA input.

All three wavelengths are single frequency, with a bandwidth less than 400 kHz FWHM. The power in all three wavelengths is limited by the available fundamental-wavelength power, rather than by any limitations in the nonlinear processes.

There are about 45 Evans and Sutherland laser projectors installed worldwide, Anderegg said.

Asked how his projector differed from others on the market (or approaching the market), he emphasized that the Evans and Sutherland unit is intended for very-large-area displays, mainly planetariums, and is quite expensive. It would probably be overkill for a typical movie screen.