Patent Application
20070230532
Light sources provide light for projection systems and other optical equipment. A typical projection display uses a UHP arc lamp as the light source. Unfortunately, the arc lamp has a relatively large etendue, is unpolarised, emits over a broad spectrum (but low in red content), has a relatively short lifetime and requires a ballast., Unfortunately, even the most optimized projection system has an optical throughput of less than 10 percent of the original arc lamp light output.
Recently, light emitting diodes (“LEDs”) are being looked at as a replacement for the arc lamp. Unfortunately, the etendue of LEDs is still undesirably high and the use of only a few LEDs per color quickly exceeds the etendue of the projection engine thereby impacting optical throughput.
A light source for a display includes a gain media, an output reflector, and a wavelength converter that cooperate to generate a source output beam. The gain media generates a media output beam that exits an output facet of the gain media. The media output beam has a first spectral frequency range and a relatively large number of modes. The output reflector is spaced apart from the gain media, and the output reflector forms a portion of a laser cavity. The wavelength converter is positioned within the laser cavity. The wavelength converter converts at least a portion of the media output beam from the first spectral frequency range to a converted beam having a secondary spectral frequency range.
With this design, in certain embodiments, the light source can efficiently generate the source output beam in the visible light range. For example, the light source will consume relatively low power, i.e. approximately 0.5 to 1 watt per color. This reduces the amount of heat generated by the light source. Additionally, the light source has a relatively long operational lifespan, has good power stability, and is relatively small in size. Furthermore, with the present light source, the source output beam has relatively low speckle, is highly polarized, and has a relatively low etendue.
Additionally, the light source can include a wavelength controller, e.g. a thin-film filter, positioned in the laser cavity that limits the number of modes that are lasing in the laser cavity. For example, the wavelength controller can limit the number of modes that are lasing in the laser cavity to between approximately 10 and 100 modes. Moreover, the wavelength controller can control the center wavelength that is lasing in the laser cavity. As discussed in more detail below, the wavelength controller improves the efficiency of the system and the quality of the source output beam.
In one embodiment, the wavelength controller is positioned between the gain media and the wavelength converter. Alternatively, the wavelength controller can be positioned between the wavelength converter and the output reflector.
In one embodiment, the output reflector reflects light in the infrared range and transmits frequencies of light above the infrared range. Alternatively, in other designs, the output reflector reflects light in the infrared range and reflects light above the infrared range.
In certain embodiments, the light source also includes an intermediate reflector positioned between the gain media and the wavelength converter. In this embodiment, a portion of the media output beam is directed through the intermediate reflector into the wavelength converter and converted into the converted beam. Further, a portion of the converted beam is transmitted through the output reflector as the source output beam. Additionally, a portion of the converted beam is reflected off of the output reflector as a reflected converted beam. Moreover, the reflected converted beam is directed into the wavelength converter and a portion of the reflected converted beam is transmitted through the intermediate reflector to the gain media. Additionally, a portion of the reflected converted beam is reflected off of the intermediate reflector back to the wavelength converter.
Additionally, the present invention is directed to a method for generating a source output beam. In one embodiment, the method includes the steps of: (i) generating a media output beam with a gain media, the media output beam having a first spectral range and a relatively large number of modes; (ii) reflecting light with an output reflector that is spaced apart from the gain media, the output reflector forming a portion of a laser cavity; and (iii) converting at least a portion of the media output beam from the first spectral range to a converted beam having at a secondary spectral range with a wavelength converter that is positioned within the laser cavity.
The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:
Referring initially to
In
The light source assembly 12 generates light for the projection system 10. As an overview, in certain embodiments, the light source assembly 12 generates a light beam 28 that includes blue light, green light and red light. As a result thereof, one or more components, such as a color wheel is not required for the DLP system. Alternatively, the light source assembly 12 could be designed to generate more than three or fewer than three colors.
Moreover, in certain embodiments, the light source assembly 12 can be designed to efficiently generate a focused light beam 28 with relatively low power, i.e. approximately 0.5 to 1 watt per color. This reduces the amount of heat generated by the light source assembly 12 and improves the performance of the precision apparatus 10. Additionally, the light source assembly 12 has a relatively long operational lifespan, has good power stability, and is relatively small in size. Furthermore, with the present light source assembly 12, the light beam 28 has relatively low speckle, is highly polarized, and has a relatively low etendue.
In one embodiment, the light source assembly 12 includes three separate light sources, namely a blue light source 30A (illustrated as a box) that generates the blue light beam, a green light source 30B (illustrated as a box) that generates the green light beam, and a red light source 30C (illustrated as a box) that generates the red light beam. Alternatively, the light source assembly 12 could be designed to have more than three or fewer than three light sources.
The beam homogenizer 16 homogenizes the light 28 that is exiting the light source assembly 12. In
The mirror 18 reflects the light 28 exiting from the beam homogenizer 16 and directs the light 28 at the imager 20.
The imager 20 creates the image 26. In one embodiment, the imager 20 is a digital light processing chip that includes anywhere from approximately 800 to more than 1 million tiny mirrors that are individually controlled to generate the image 20. Alternatively, for example, the imager 20 can be a LCD imager or a LCOS imager.
The lens 22 collects the image 26 from the imager 20 and focuses the image 26 on the screen 24. The screen 24 displays the image 26.
It should be noted that with the designs disclosed herein, with relatively small changes in the specifications of the components, the light source 230 can be alternatively designed to be (i) a blue light source 30A that generates blue light, (ii) a green light source 30B that generates green light, or (iii) a red light source 30C that generates red light. Alternatively, the light source 230 could be designed to create other colors such as yellow, cyan, or magenta, for example.
The gain media 232 generates the light for the light source 230. In one embodiment, the gain media 232 is a semiconductor laser such as a laser diode bar that contains a plurality of emitters. In alternative, non-exclusive embodiments, the laser diode bar can include an array of 5, 10, 15, 20, or 50 emitters. In one embodiment, the gain media 232 emits a multiple frequency, media output beam 247 to reduce speckle. In certain embodiments, the media output beam 247 has numerous longitudinal modes, typically in the thousands.
In certain embodiments, the laser diode bar is reliable, stable, and has a relatively long lifetime. A suitable diode bar can be made of InGaAs/AlGaAs. A suitable laser diode bar can be purchased from Bookham, located in Zurich Switzerland.
In one embodiment, the gain media 232 includes a reflector facet 232A that is coated with a high reflection (“HR”) coating 248 and an output facet 232B that is coated with an anti-reflection (“AR”) coating 250. The HR coating 248 reflects light that is directed at the reflector facet 232A back into the gain media 232. In alternative, non-exclusive embodiments, the HR coating 248 has a reflectivity of greater than approximately 90, 95, or 99 percent. With this design, the reflector facet 232A cooperates with the output reflector 246 to define a laser cavity 251.
The AR coating 250 allows light to exit the gain media 232 and allows light that has rebounded from the output reflector 246 and passed through the intermediate reflector 242 to easily enter the gain media 232. In alternative, non-exclusive embodiments, the AR coating 250 has an average reflectivity of less than approximately 1, 0.1, or 0.01 percent. In certain embodiments, the gain media 232 requires alignment with the output reflector 246 so that light can be rebounded back into the gain media 232.
In certain embodiments, one or both of the facets 232A, 232B can be angled to enhance performance of the light source 230.
In one non-exclusive embodiment, (i) to ultimately generate a blue light, the gain media 232 is designed to emit a media output beam 247 having an average wavelength of approximately 940 nm; (ii) to ultimately generate a green light, the gain media 232 is designed to emit a media output beam 247 having an average wavelength of approximately 1050 nm; and (iii) to ultimately generate a red light, the gain media 232 is designed to emit a media output beam 247 having an average wavelength of approximately 1240 nm.
It should be noted that the media output beam 247 has a first spectral range. For example, (i) to ultimately generate a blue light, the first spectral wavelength range of the media output beam 247 can be between approximately 800 and 1000 nm; (ii) to ultimately generate a green light, the first spectral wavelength range of the media output beam 247 can be between approximately 1000 and 1100 nm; and (iii) to ultimately generate a red light, the first spectral wavelength range of the media output beam 247 can be between approximately 1200 and 1400 nm.
In certain embodiments, the exact temperature of the gain media 232 is not critical for wavelength control. In these embodiments, active temperature control of the gain media 232 is not necessary. Alternatively, the temperature of the gain media 232 can be actively controlled with a media controller 232C that can include a heat pipe, water or air, for example.
Further, in certain embodiments, the gain media 232 can be modulated in time at or near a resonant frequency of the cavity to enhance conversion by the wavelength converter 244.
The first optical assembly 234 improves the shape of the media output beam 247 exiting the output facet 232B of the gain media 232. For example, the first optical assembly 234 can eliminate both astigmatism/beam waist asymmetry to maximize the conversion efficiencies and cavity stability. In one embodiment, the first optical assembly 234 includes an anamorphic lens or an array of lenses in the case of a multi-stripe gain media 232. For example, the first optical assembly 234 can be spaced apart from the gain media 232 approximately 0.1 mm.
The second optical assembly 236 collimates and focuses the media output beam 247 on the wavelength controller 238. For example, the second optical assembly 236 can include one or more optical lens. It should be noted that the second optical assembly 236 is positioned a SOE focal length 252 from the output facet 232B of the gain media 232. Further, the beam waist of the media output beam 247 can be changed by changing the SOE focal length 252. In one non-exclusive embodiment, the SOE length can be approximately 1.5 mm.
In another embodiment, the first optical assembly 234 and the second optical assembly 236 can be combined into a single optical element by using an aspherical lens or series of lenses.
The wavelength controller 238 reduces and limits the number of longitudinal modes that are lasing in the laser cavity 251. Further, the wavelength controller 238 can reduce the spectral width of the light that is directed to the wavelength converter 244 and can control the center wavelength of the light. In this embodiment, the wavelength controller 238 is used to precisely control the number of longitudinal modes, the wavelength, and the spectral width of the light that is lasing in the laser cavity 251. Stated in another fashion, the media output beam 247 that passes through the wavelength controller 238 has a center wavelength and spectral width defined by the wavelength controller 238.
In alternative, non-exclusive embodiments, the wavelength controller 238 reduces the number of longitudinal modes that are lasing in the laser cavity 251 to less than approximately 100, 90, 80, 70, 60, 50, 40, 30, 20, or 10 longitudinal modes.
Stated in another fashion, in alternative, non-exclusive embodiments, the wavelength controller 238 reduces the number of longitudinal modes that are lasing in the laser cavity 251 to between approximately 10-100 longitudinal modes, 30-70 longitudinal modes, or 40-60 longitudinal modes.
Additionally, in alternative, non-exclusive embodiments, the wavelength controller 238 reduces the spectral width to less than approximately 5, 1, 0.5, 0.4, 0.3, 0.2, or 0.1 nanometers. In one embodiment, the wavelength controller 238 reduces the number of modes to approximately 60. This number assumes an effective spectral width for the wavelength controller 238 of approximately 0.3 nanometers and a cavity length that gives a mode spacing of approximately 0.05 nanometers.
Referring back to
Further, as discussed above, the wavelength controller 238 can be used to control the center wavelength of the light that is directed at wavelength converter 244. With this design, the wavelength controller 238 can be designed to control the average wavelength to be at or near optimum/designed wavelength for conversion by the wavelength converter 244. Stated in another fashion, the wavelength controller 238 forces the laser cavity 251 to lase at or near the optimum wavelengths for conversion with the wavelength converter 244. As a result thereof, the wavelength converter 244 can efficiently convert the light. Thus, the multimode output when used with the wavelength controller 238 enhances conversion efficiency, reduces speckle and eliminates mode hop sensitivity.
In one embodiment, the wavelength controller 238 is an optical filter that transmits or blocks a portion of the spectrum of the media output beam 247 that is directed at the wavelength controller 238. The portion of the media output beam 247 that passes through the wavelength controller 238 is referred to herein as the transmitted beam 256. The transmitted beam 256 is subsequently directed to the wavelength converter 244.
In one embodiment, the optical filter 238 is a band pass type filter that transmits a band of wavelengths (“the passband”) and blocks wavelengths outside of the passband. The passband has a center wavelength that is at the center of the passband. For example, in alternative non-exclusive embodiments, the filter is a narrow band pass filter with a passband having a bandwidth of less than approximately 0.1, 0.2, 0.3, 0.4, 0.5, 1, or 5 nanometers. As a result thereof, the transmitted beam 256 has a relatively narrow spectral width compared to the spectral width of the media output beam 247.
Further, the transition from transmitting to rejection can be sharp. In alternative, non-exclusive embodiments, the optical filter has a spectral slope capable of transitioning between 10% and 90% transmission in less than 0.2 nm, 0.5 nm, 1 nm, or 5 nm.
In certain embodiments, the optical filter 238 is designed so that the center wavelength of the passband is near the optimum/designed wavelength for conversion by the wavelength converter 244. As a result thereof, the wavelength converter 244 can efficiently convert the transmitted beam 256. The optical filter 238 can be rotated to align the center wavelength to the wavelength converter 244.
With this design, the optical filter 238 controls the transmitted beam 256 to match the wavelength of acceptance of the wavelength converter 244. Further, in certain embodiments, the transmitted beam 256 is passively controlled by the optical filter 238. As a result thereof, the wavelength and temperature control issues of the gain media 232 are reduced or eliminated.
Additionally, the shape of the optical filter 238 can be adjusted to improve the performance of the light source 230. Simple filters such as single cavity thin film filters have a roughly Gaussian peak which gives a certain performance. In certain embodiments, multiple cavity filters can be used to make the shape flat or arbitrarily shaped in such a way as to enhance the performance of the light source 230. In addition, the bandwidth of the filter can be optimized to improve such laser parameter performance such as speckle reduction. In one embodiment, the optical filter 238 is a bat ear filter having a relatively flat center with peaks at each edge. With the present invention, the shape and characteristics of filters can be changed to improve the performance of the light source 230, such as operation on a selected combination of wavelength modes to improve speckle.
In certain embodiments, the optical filter 238 can be rotated to align to the wavelength converter 244. Additionally, the filter 238 can be designed to be temperature sensitive in the same direction as the wavelength converter 244 so that the center wavelength changes in the same fashion as the wavelength of acceptance of the wavelength converter 244 as temperature changes.
Additionally, the light source 230 can include a wavelength adjuster 262 (any mechanism) that adjusts the wavelength controller 238. For example, the wavelength adjuster 262 can adjust the angle of the filter 238 with temperature (or adjusts the filter via heat or stress or some other way) to achieve the desired center wavelength to maintain optimum conversion efficiency of the wavelength converter 244. The wavelength adjuster 262 can tilt the filter 238, rotate the filter 238, adjust the temperature (heat and/or cool) of the filter 238, add or release stress to the filter 238, and/or add or remove an electric field to the filter 238 to accurately control and adjust the center wavelength of the filter 238.
A suitable optical filter 238 can be purchased from Bookham, located in Santa Rosa, Calif.
The third optical assembly 240 focuses the light beam 228 on the wavelength converter 244. For example, the third optical assembly 240 can include one or more optical lens. It should be noted that the third optical assembly 240 is positioned a TOE focal length 254 from a second side 244B of the wavelength converter 244. Further, the beam waist of the light beam 258 at the second side 244B and exiting the light source 230 can changed by changing the TOE focal length 254. For example, as the TOE focal length 254 is increased, the beam waist is decreased. In one non-exclusive embodiment, the TOE focal length 254 is approximately 13 mm.
In
With the present design, in
The intermediate reflector 242 can be tilted to allow the visible beam to come out of the laser at an angle. This is particularly useful in other embodiments where the output reflector 246 is designed to reflect both IR and visible light because then all of the light comes out after bouncing off of the intermediate reflector 242. This is shown more clearly in
Referring back to
The wavelength converter 244 changes the wavelength of the transmitted beam 256 and any reflected beam that was reflected off of the output reflector 246. In each of the embodiments illustrated herein, the wavelength converter 244 is located in the laser cavity 251. As a result of the intra-cavity conversion, the enhanced energy in the laser cavity 251 improves the performance of the light source 230. Further, a shorter wavelength converter 244 can be used because of the improved efficiency of intra-cavity conversion. In
In one embodiment, the wavelength converter 244 is a highly nonlinear/robust crystal such as MgO doped periodically poled lithium niobate (PPLN). The PPLN has good long term stability and is very robust with a very high optical intensity damage threshold.
The wavelength converter 244 can function as a second harmonic generation crystal that efficiently doubles the frequency of the transmitted beam 256. In this embodiment, the transmitted beam 256 is converted into a converted beam 258 by the wavelength converter 244 and the converted beam 258 has a wavelength that is one half of the wavelength of the transmitted beam 256; and the converted beam 258 has a frequency that is twice the frequency of the transmitted beam 256.
In this embodiment, the media output beam 247 has a first spectral frequency range while the converted beam 258 is in a secondary spectral frequency range that is approximately twice the first spectral frequency range. Thus, the wavelength converter 244 is an intra-cavity frequency doubler.
In
In certain embodiments, the angular sensitivity of the PPLN is such that the FWHM is 2 degrees. Further, the FWHM of the wavelength range for efficient conversion for a 10 millimeter long PPLN can be approximately 0.4 nanometer. Thus, a PPLN of approximately 10 millimeters can be used. The relatively short length of PPLN is enabled by the intra-cavity approach disclosed herein. This reduces the cost and size.
In one embodiment, the temperature of the wavelength converter 244 is actively controlled with a converter temperature controller 261. The converter temperature controller 261 can include heater or cooler. Because the wavelength controller 238 forces the laser operation over multiple modes (approximately 60 in one example), the temperature stability of the wavelength converter 244 can be improved to approximately one degree Celsius. Accordingly, only crude temperature control of the wavelength converter 244 is necessary. This simplifies the design of the light source 230.
It should be noted that the wavelength of acceptance of the wavelength converter 244 can be controlled by controlling the temperature of the wavelength converter 244 with the converter temperature controller 261 to tune the light source 230.
In one embodiment, the temperature of the wavelength converter 244 is tied to the location of the center wavelength as controlled by the wavelength controller 238. For example, as discussed above the wavelength adjuster 262 can be used to adjust the wavelength controller 238 to maintain optimum conversion efficiency.
The wavelength converter 244 has an optimum conversion wavelength that varies, roughly linearly, with temperature. Any simple mechanism, such as a piece of material attached to one edge of the filter 238 which causes it to tilt slightly about its center as the material expands and contracts with temperature, could cause the filter center wavelength to adjust and match the change in the wavelength converter 244 for optimum conversion wavelength. Another novel approach is to have the optical filter 238 on a substrate such that a change in temperature causes a change in stress and/or cavity thickness which causes the optical filter 238 to tune its wavelength with temperature to match that of the wavelength converter 244. In principle, these give totally passive ways to keep conversion efficiency high without any need for active control of the temperature of the wavelength converter 244.
In certain embodiments, the operating wavelength of the wavelength converter 244 can also be adjusted by tilt, rotation, adjustment of the temperature, the addition or release of stress, and/or the application of an electric (or magnetic) field to the wavelength converter 244 to optimize performance.
The output reflector 246 reflects at least a portion of the light that travels through the wavelength converter 244 and cooperates with the HR coating 248 to form the laser cavity 251. One beam waist in the laser cavity is formed by the TOE 240 and is positioned at or near the surface of the output reflector 246. Thus, the laser cavity 251 is formed by means of a retro-reflection off of the output reflector 246. This position of the beam waist at the output reflector reduces the sensitivity of the light source 230 to any positioning errors and movement (such as due to vibration) of the laser cavity components. The output reflector 246 can also be referred to as an output coupler.
In
In an alternative, non-exclusive embodiment, the output reflector 246 reflects wavelengths of the converted beam 258 that are in the visible range as well as wavelengths of the converted beam 258 that are in the infrared range. An example of this is discussed below.
In alternative embodiments, the light source 230 is designed so that (i) the light source output 260 is a blue light, (ii) the light source output 260 is a green light, or (iii) the light source output 260 is a red light.
In
In another embodiment, the output reflector 246 can be constructed of separate segments to allow the performance of each group of emitters in the bar to be optimized independently.
One improvement that could be placed before the output reflector 246 (or at any other beam waist in the cavity) is an aperture (not shown) which could act to keep the laser operating in a single transverse mode while allowing multiple axial modes to lase.
Alternatively, for example, the output reflector 246 can be positioned against the second side 244B of the wavelength converter 244, and the output reflector 246 could even be coated directly onto the wavelength converter 244 reducing the number of components that need to be mounted as well as reducing weight and size.
Additionally, in
Any long or short term variations in the gain media 232 can lead to mode hops or variations in the optical power of the particular longitudinal modes on which the laser cavity 251 is currently lasing. These can result in intensity fluctuations of the beam transmitted by the output coupler. Within the wavelength of acceptance of the wavelength converter 244, many longitudinal modes are excited such that the effects of a mode hop are eliminated. This feature allows the cavity length to change freely and reduces the temperature, vibration, shock stability criteria of the cavity, as well as providing highly beneficial increases in frequency doubling conversion efficiencies.
A brief summary of the light path is provided herein. In
Next, the light from the wavelength converter 244 is directed at the output reflector 246. In
Moreover, in
Further, in
In the embodiment illustrated in
In the embodiment illustrated in
In
In the embodiment illustrated in
Further, in
In the embodiment illustrated in
However, in this embodiment, the wavelength controller 738 and the intermediate reflection 742 are combined to reduce size, weight, cost, complexity, etc., are tilted to reflect the visible light out of the cavity, and are spaced apart from the wavelength converter 744. Further, the output reflection 746 reflects light in both the visible and infrared range. Further, the output reflection 746 is secured to the wavelength converter 744 or coated directly onto converter 744 to reduce size, weight, cost, complexity, etc.
In
In the embodiment illustrated in
However, in this embodiment, the intermediate reflector 842 is angled at approximately one degree, for example. Moreover, the wavelength controller 838 is angled at approximately one degree for example, and the back surface of the wavelength controller 838 is wedged to reduce interference effects. Further, in this embodiment, the wavelength converter 844 is rotated, approximately two degrees or less, for example.
In
Further, because of the off-set light, two source output beams 860 (that may partly overlap) exit the output reflector 846. Because the intermediate reflector 842 is positioned at or near the effective focal length of the third optical assembly 840, the output beam from the light that is retro-reflected by the intermediate reflector 842 will exit the third optical assembly 840 and be parallel or nearly parallel to (and spaced apart from) the light that first travels through the wavelength converter 844. This improves the optical quality and ease of use of the visible output of the cavity.
While the particular apparatus 10 as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.
This Application claims the benefit on U.S. Provisional Application Ser. No. 60/789,321 filed on Apr. 4, 2006. The contents of U.S. Provisional Application Ser. No. 60/789,321 are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60789321 | Apr 2006 | US |
