Solid-State Laser System for Color Picture Projection

The application relates to a laser system according to the preamble of Claims 1 and 2. Such a laser system is known from International Patent Document WO 91/12556. That document describes the producing of suitable red/green/blue beams by means of at least one--in this case, two--IR continuous-wave lasers (laser diode) in the wavelength range of 850-1,320 nm, in which the colors are obtained by using frequency doubling and summation frequency formation, and an optical parametric oscillator is used for the frequency conversion.

From German Patent Document DE 43 06 797 C1, a laser system for producing the suitable red/green/blue beams for a general color picture projection by means of at least one--in this case, three, lasers is known.

From Appl. Phys. Lett. 58 (12), Mar. 25, 1991, pages 1227-1229, a laser system is known for producing the suitable red/green/blue beams for particularly high-resolving laser printers having at least one--in this case, two--IR continuous-wave lasers (laser diode) in the wavelength range of 850 to 1,320 nm, in which the colors are obtained by frequency conversion, specifically by using frequency doubling and summation frequency formation.

From Appl. Phys. Lett. 56 (9), Feb. 26, 1990, Page 811-813, a laser system is known for producing the suitable red/green/blue beams for a general color picture projection by means of a single laser.

From Optics Letters, Oct. 15, 1990, Vol. 15, No. 20, Page 1129-1131, it is known to use the signal wave as well as the idler wave of an OPO and from J. Opt. Soc. Am.B., Volume 10, No. 11, November 93, Page 2151, it is known to enlarge the tuning range of a light source by combining frequency doubling and/or summation frequency forming steps with a stimulated parametric interaction. Optical parametric oscillators (OPO's) are known, for example, from German Patent Document DE 42 19 169 A1.

The invention is based on the object of further developing a laser system of the initially mentioned type such that it has a high beaming quality with a beam diameter.times.beam divergence product close to the diffraction limit and has certain colors in the visible wavelength range (RGB) which permit a good overlapping of the color sensitivity range of the human eye by color mixing; has a median optical power of several hundred milliwatts to a few watts and is optimized by a small construction with a good efficiency. All three colors are to be produced in a single laser system with the above-mentioned requirements.

This object is achieved by the measures indicated in Claims 1 and 2. Developments and further developments are indicated in the subclaims and are described in the following by means of embodiments.

In this concept, an efficient IR continuous-wave laser, which uses rare-earth or transition elements, forms the single larger active electro-optical componentry of the system. For reaching the required long life and high efficiency, it is suggested that semiconductor laser diodes be used for exciting the IR continuous-wave laser which operates with rate-earth or transition elements.

The RGB (red, green, blue) effective beams are derived from the IR emission of the IR continuous-wave laser, which operates with rare-earth or transition elements, by means of a simple passive non-linear frequency conversion. By means of the common excitation source for all colors, the complexity of the system is minimized. By means of the intermittent IR continuous-wave laser, which operates with rare-earth or transition elements, of a high pulse peak power, a high efficiency of the frequency conversion is achieved. In the OPO, the intermediate frequencies and thus also the wavelength of the effective radiation can be continuously tuned for the color adaptation.

The FIGURE shows a schematic representation of a possible embodiment of the laser system. The green radiation in the wavelength range of 510-550 nm is obtained by a direct frequency doubling of the radiation of an IR solid-state laser, such as the well-known Nd:YAG laser or related lasers, such as the Nd:YLF or Yb:YAG. From the emission wavelength .lambda..sub.l =1,047 nm (for example, for the Nd:YLF), the suitable line is formed at .lambda..sub.g =524 nm. After passing through the frequency doubler (SHG), the beam consists of a mixture of the two wavelengths of the IR radiation and of the green radiation.

By means of a dichroitic beam splitter (S1), the two beams of different wavelengths are separated from one another. In another beam splitter (S2), the green effective beam part is then split off. The second part of the green beam, which is used as a pumping beam of the wavelength .lambda..sub.g for the OPO, is converted there at a high efficiency into a signal beam of the wavelength .lambda..sub.s and an idler beam of the wavelength .lambda..sub.i. By the selection of a suitable crystal orientation and/or crystal temperature of the non-linear crystal in the OPO, the output wave lengths .lambda..sub.s and .lambda..sub.i can be determined (taking into account the energy maintenance condition .omega..sub.g =.omega..sub.s +.omega..sub.i)

The IR beam of the laser is divided by means of a beam splitter (S3); the signal and idler beam respectively is then superimposed on the two partial beams by means of two dichroitic mirrors (S4 and S5); and a mixing then takes place at a high efficiency in non-linear crystals (SFGS and SFGI).

Frequencies .omega..sub.s and .omega..sub.i are selected such that the sum of their frequencies with the frequency of the fundamental wave of the IR laser .omega..sub.l result in the frequencies of the required colors blue, .omega..sub.b =.omega..sub.l +.omega..sub.i, or red, .omega..sub.r =.omega..sub.l +.omega..sub.s in the required wavelength ranges.

By using an Nd:YLP laser with the IR fundamental wavelength .lambda..sub.l =1,047 nm, by means of this construction, for example the following suitable RGB wavelengths may be produced:

.lambda..sub.r =620 nm

.lambda..sub.g =524 nm

.lambda..sub.b =450 nm

In this case, the signal and idler wavelengths of the OPO would be

.lambda..sub.s =789 nm

.lambda..sub.i =1,520 nm

As further embodiments of the invention, it is possible by means of a laser wavelength in a shorter-wave range (860-920 nm) to first produce the blue wavelength in the wavelength range of 430-460 nm by frequency doubling and to use the remaining laser wave for the pumping of the OPO and then subsequently produce the other colors green and red by a frequency mixing of the IR laser radiation with the signal or idler radiation, as before.

Another possible variant would be the use of the IS laser radiation for the pumping of the OPO's and the frequency mixing of the frequency-doubled or tripled IR radiation with the signal or the idler radiation for producing the additional two colors.

The requirement of optical power or the overall efficiency for such a system in practice can be estimated as follows: By means of 40 W of medium power at .lambda..sub.l =1,047 .mu.m, 12 W of medium power at .lambda..sub.g =524 nm can be generated. An efficiency of 30% for the frequency doubling is achieved in light-pumped, mode-coupled high-power lasers. Because of the improved radiation quality, efficiencies of up to 50% in diode-pumped systems appear conceivable and were demonstrated by means of lower powers (G. P. A. Malcolm, M. Ebrahimzade, A. I. Ferguson "Efficient Frequency Conversion of Mode-Locked Diode-Pumped and Tunable All-Solid-State Laser Sources", IEEE J. Quant. Electr. 28 (1992) 1172). Photon conversion efficiencies of 30% respectively in the signal and idler beam are expected in an OPO so that approximately 2.6 W or 1.4 W are available at the wavelengths .lambda..sub.s =789 nm and .lambda..sub.i =1,520 nm. The summation frequency production with the remaining, very intense radiation of the wavelength .lambda..sub.l (20-30 W) permits a quantum efficiency of approximately 30%, which results in output powers of 1-2 W for the red and the blue effective radiation.

The optical efficiency during the conversion of the pumping diode radiation into the primary IR laser radiation today amounts to typically 30-40%; the electric-optical efficiency of the pumping diode is 30-50%. The total electric-optical efficiency of several percent should therefore be achievable.

As non-linear crystals for the frequency doubling in the OPO and for the summation or difference frequency production, in principle, all efficient non-linear materials can be used, such as potassium titanyl phosphate (KTP) or beta barium borate (BBO), lithium triborate (LBO), potassium titanyl arsenate, potassium and lithium niobate or lithium iodate. In principle, polymers with high non-linear coefficients can also be used.

Basically the frequency conversion processes described here can be implemented by means of a laser radiation with continuous waves as well as by means of a pulsed laser radiation. Since the efficiency of these processes in a first approximation is proportional to the output density of the radiation, a considerable advantage exists here in the use of pulsed systems of a high pulse peak output, as offered by mode-coupled lasers.

According to today's state of the art, the achieving of a comparable overall efficiency in continuous-wave systems is considerably more difficult, more complex and more cost-intensive.

When pulsed systems are used for the picture projection with a fast deflection of the laser beam over the picture surface and the simultaneous modulation of the radiation intensity of the three collinear RGB-beams, it is necessary for the pulse repetition frequency of the laser to be at least equal to or higher than the picture point frequency (pixel frequency). According to the used projection standard, this is in the range of from 10-100 MHz.

Mainly the mode-coupled continuous-wave lasers are suitable for this high pulse frequency. An active mode coupling can be used, for example, by means of acousto-optical modulators, or a passive mode coupling can be used with suitable fadable materials within the resonator, for achieving the high pulse frequency. With an optical length of the resonator of L.sub.o, the following is obtained as the pulse repetition frequency f of the mode-coupled laser:

wherein c is the speed of light in the vacuum. For achieving 100 MHz, the optical length must be L.sub.o =150 cm, and for achieving 1 GHz, it must be L.sub.o =15 cm. The used pulse repetition frequency will depend on the application case. However, it will be advantageous to select the pulse repetition frequency to be clearly higher than the picture point frequency, so that possible frequency, phase or intensity fluctuations will influence the quality of the picture reproduction as little as possible.

Mainly the diode-pumped lasers of the rare earths, such as neodymium and ytterbium or transition metals, such as titanium or chromium, are suitable as mode-coupled IR lasers for these applications. The future will probably provide other candidates for efficient continuous-wave lasers in the particularly suitable spectral range of from 850 to 1,320 nm. For this application, linearly constructed crystal lasers, ring lasers as well as fiber glass lasers doped with these laserable ions would be suitable.

Arrangements in which the pulsed beam is focussed directly without additional optical accessory devices in the frequency doubler crystal are suitable for the frequency doubling. In the case of mode-coupled lasers, an efficiency of the frequency doubling up to 50% can already be reached.

External resonators may also be used for improving the efficiency by increasing the dwell time of the IR wave in the frequency doubler. Linear resonators as well as ring resonators are suitable for this purpose. These two resonator structures may also be used in the case of the optically parametric oscillator. The summation frequency is preferably generated in a non-linear crystal without additional optical accessories.

For increasing the efficiency, arrangements should also be considered in which individual frequency conversion steps take place within a resonator (intracavity).