Patent Application
20140177036
Optical systems with optical parametric oscillators (OPOs) may be used to generate multiple wavelengths of laser light from one pump laser beam. In the OPO, parametric amplification in a nonlinear crystal converts the pump laser wavelength into two more wavelengths of light, so an optical system with one pump laser and one OPO may produce at total of three wavelengths of visible light which are useful for applications such as full-color digital image projection.
In general, in one aspect, an optical system including an optical parametric oscillator (OPO), a second-harmonic generator (SHG), a first lens which passes light between the OPO and the SHG, a second lens which passes light between the OPO and the SHG, and a third lens which passes light between the OPO and the SHG.
Implementations may include one or more of the following features. A fourth lens may pass light between the OPO and the SHG. The first lens may pass a collimated beam segment to the second lens and the third lens may pass a collimated beam segment to the fourth lens. The first lens, the second lens, the third lens, and the fourth lens may be arranged to form beam segments with a non-rectilinear shape. There may be a recirculating optical subsystem which focuses the beam waist in the OPO to the same location for 10 circuits around the recirculating optical subsystem with a variation of less than 10% of the width of the beam waist. There may also be a first short wave pass (SWP) filter, a second SWP filter, a minor, and a third SWP filter arranged to form a recirculating optical subsystem. The first SWP filter and the second SWP filter may be mounted in an optical module with the OPO and the minor and the third optical SWP filter may be mounted in an optical module with the SHG. The optical system may generate two infrared beams. Each infrared beam may be frequency doubled to make a visible beam. One visible beam may be in the range of 430 nm to 480 nm and the other in the range of 600 nm to 680 nm. One visible beam may be at 452 nm and the other at 621 nm. The OPO may be pumped by a beam in the range of 510 nm to 550 nm. The pump beam may be 523.5 nm or 540 nm.
In general, in one aspect, an optical system including a first SWP filter, an OPO, a second SWP filter, a first lens, a second lens, a minor, an SHG, a third SWP filter, a third lens, and a fourth lens. The OPO passes light from the first SWP filter to the second SWP filter, the second SWP filter passes light from the OPO to the first lens, the first lens passes light from the second SWP filter to the second lens, the second lens passes light from the first lens to the minor, the minor passes light from the second lens to the SHG, the SHG passes light from the mirror to the third SWP, the third SWP passes light from the SHG to the third lens, the third lens passes light from the third SWP to the fourth lens, the fourth lens passes light from the third lens to the first SWP filter, and the first SWP filter passes light from the fourth lens to the OPO.
In general, in one aspect, an method of generating light including the steps of focusing a pump beam into an OPO, forming an idler beam in the OPO, focusing the idler beam through a first lens and a second lens into a first SHG, and recirculating the idler beam through a third lens and a fourth lens back into the OPO.
Implementations may include one or more of the following features. The beam segment between the first lens and the second lens may be collimated and the beam segment between the third lens and the fourth lens may be collimated. Additional steps may include forming a signal beam in the OPO, separating the idler beam from the signal beam and the pump beam, forming a first second-harmonic beam from the idler beam in the first SHG, separating and outputting the first second-harmonic beam, separating and outputting the remaining pump beam, focusing the signal beam into a second SHG, forming a second second-harmonic beam in the second SHG, separating and outputting the second second-harmonic beam, and dumping the remaining signal beam.
A detailed description of an optical system generating three colors of visible light using an optical parametric oscillator may be found in U.S. Pat. No. 5,740,190, the complete disclosure of which is incorporated herein by reference. Starting with a visible pump beam, an OPO makes an infrared signal beam and an infrared idler beam. By choosing a specific temperature and other characteristics of the OPO crystal, the signal beam and idler beam may be tuned as desired. For example, if the pump beam has a wavelength of 523.5 nm, the two additional beams created may be at 898 nm and 1252 nm which are in the infrared region. By using two second-harmonic generators, the 898 nm light can be frequency doubled to 449 nm light which is blue, and the 1252 nm light can be frequency doubled to 626 nm which is red. This produces red, green, and blue colors of visible light that may be used by a digital image projection system. By choosing a different temperature of the OPO, the infrared wavelengths may be 904 nm and 1242 nm which can be frequency doubled to visible wavelengths of 452 nm and 621 nm which fit the color gamut specified for digital cinema. Other wavelengths within the blue or red range may give acceptable color performance for digital cinema projection or other types of digital image projection. Blue wavelengths may be in the range of 430 nm to 480 nm and red wavelengths may be in the range of 600 nm to 680 nm. The pump beam may be in the middle of the green region of visible light which extends from 510 nm to 550 nm. The pump laser may be a neodymium-doped yttrium lithium fluoride (Nd:YLF) laser which emits light at 1047 nm that can be frequency doubled to 523.5 nm, a neodymium-doped yttrium aluminum perovskite (Nd:YAP) laser which emits light at 1079.5 nm that can be doubled to 540 nm, or other lasers with other wavelengths.
First SWP filter 104, OPO 106, second SWP filter 110, first lens 114, first minor 118, first SHG 122, third SWP filter 126, and second lens 130 form recirculating optical subsystem 100. Recirculating optical subsystem 100 recirculates the idler beam so it passes multiple times through OPO 106 and first SHG 122. The optical input to recirculating optical subsystem 100 is pump beam 102 and the optical outputs are colocated signal and remaining pump beam segment 134 and first second-harmonic beam segment 140.
After leaving recirculating optical subsystem 100, colocated signal and remaining pump beam segment 134 passes to fourth SWP filter 136 which reflects first signal beam segment 150 and passes remaining pump beam segment 138. First signal beam segment 150 reflects from second mirror 152 to form second signal beam segment 154. Second signal beam segment 154 is focused by third lens 156 to form third signal beam segment 158. Third signal beam segment 158 passes into second SHG 160 which consists of first SHG crystal 162 and second SHG crystal 164. First SHG crystal 162 and second SHG crystal 164 form a walk-off SHG system which converts part of third signal beam 158 to form colocated second harmonic and remaining signal beam 166. Colocated second harmonic and signal beam 166 passes to long wave pass (LWP) filter 168 which reflects second second-harmonic beam segment 172 and passes remaining signal beam 170. Remaining signal beam 170 passes into beam dump 178 and is absorbed in beam dump 178. Second second-harmonic beam segment 172 reflects from third mirror 174 to form third second-harmonic beam segment 176.
Fourth SWP filter 136, second mirror 152, third lens 156, first SHG crystal 162, second SHG crystal 164, LWP filter 168, third mirror 174, and beam dump 178 form beam separation and conversion system 180. The optical input to beam separation and conversion system 180 is colocated signal and remaining pump beam segment 134 and the optical outputs are remaining pump beam segment 138 and third second-harmonic beam segment 176.
A detailed description of OPOs may be found in U.S. Pat. No. 5,740,190. The wavelengths of the pump, signal, and idler beam are related by the following mathematical expression: 1/λp=1/λs+1/λi, where λp is the wavelength of the pump beam, 1/λs is the wavelength of the signal beam, and 1/80i is the wavelength of the idler beam. The wavelengths also depend on various parameters of the crystal such as its size, orientation, and temperature. Some of the requirements for high efficiency conversion include phase matching, good beam quality, and sufficiently high beam density. Q-switched lasers may be used achieve sufficient beam density by using short pulses and low duty cycles. The OPO may be an x-cut lithium triborate (LBO) crystal with noncritical phase matching and temperature controlled at 134.7 degrees Celsius to obtain signal and idler beams at 898 nm and 1252 nm. Alternately, if the temperature is controlled to 135.9 degrees Celsius the signal and idler beam may be at 904 nm and 1242 nm.
A detailed description of SHGs may be found in U.S. Pat. No. 4,019,159, the complete disclosure of which is incorporated herein by reference. SHGs use non-linear optical processes to convert the wavelength of the original light beam into a harmonic wavelength such as half the original wavelength. This is equivalent to doubling the frequency of the light beam. The SHGs shown in
Phase matching in OPOs and SHGs may be divided into two types. Type I is defined as the condition where two input beams have the same polarization and type II is defined at the condition where two input beams have orthogonal polarization. In the case where there is one input beam, it can be considered two input beams with the same polarization. In
SWP and LWP filters may be formed by conventional methods such as the deposition of multilayer interference filters with alternating layers of high index and low index of refraction materials that are designed to transmit certain wavelengths while reflecting other wavelengths. The SWP and LWP filters shown in
Optical lenses are used to focus the beams into the nonlinear crystals of the OPOs and SHGs. A narrow focal point (beam waist) is helpful to reach the high power density required for nonlinear optical processes. Additionally, in order for the recirculating optical subsystem to work efficiently, the beam must go around the recirculation path repeating the position of each beam waist within a variation of approximately 10% of the width of the beam waist after 10 circuits. For example, if the beam waist is 80 micrometers wide, the idler beam must go around the recirculating optical subsystem 10 times while drifting less than 8 micrometers.
The recirculating optical subsystem shown in
A second advantage of the four-lens system is that alignment is much easier because more configurations are possible solutions for optical alignment of the recirculating optical subsystem. Two-lens systems such as the one shown in
A third advantage of the four-lens system is that the issue of keeping the beam waist centered in the crystal can be separated from the issue of keeping the beam waist coming back into the same position each time it travels around the recirculating system. These two issues are closely interrelated during the alignment of the two-lens system.
Other implementations are also within the scope of the following claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12553890 | Sep 2009 | US |
| Child | 14194984 | US |
