The present disclosure generally relates to lasers, and more specifically, to high power infrared laser technologies and components including frequency doublers, solid state lasers, vertical cavity surface emitting lasers, and diodes used to make high power visible lasers.
Generally, red, green, blue and ultraviolet (UV) lasers have many potential uses in illumination, medical, material processing, welding, and display. Cost, reliability, efficiency, size, and power are laser parameters that may be considered when selecting a laser for use in these various markets/technology fields. Display is an example of a market that has a number of different segments that value these parameters in slightly different ways. In the consumer display market, cost, efficiency, and size may be important parameters, while in the professional display market, reliability, high power, and cost may be key parameters. Cheap, reliable, highly efficient green, red, or blue source is important for all the above applications. Such a green laser source is particularly relevant since high power direct sources, such as Vertical Cavity Surface Emitting Lasers (VCSELs) or edge emitting diodes, do not yet exist at the appropriate wavelengths for professional display applications.
Generally, a full color display uses at least red, green, and blue light sources. When employed in movie theaters, these colors have to be in certain ranges to comply with the standards set by the movie industry, and more specifically, with the Digital Cinema Initiative. The approximate accepted color ranges for movies are given by red or 616-650 nm, green or 523-545 nm, and blue or 455-468 nm. However, consumer display markets do not have such a strict wavelength requirement. As red and blue diodes have become available, interest in lasers for backlighting and use in consumer projectors has increased. Direct laser sources are cost effective, reliable, and efficient sources of light. However, currently there is no high power green direct laser source having the wavelength needed for display. Thus, green is an important laser technology, and there is a need in the art for a high power, efficient green direct source of light. In addition, red lasers currently have very severe cooling requirements and their lifetimes are relatively limited.
Disclosed herein are novel architectures for VCSEL-based laser systems, as well as related methods of doubling the generated light from a VCSEL system to generate red, green, or blue light. In advantageous embodiments employing a high power IR VCSEL array of elements, a bulk (for example, crystal) doubling material may be used to double the infrared (IR) light and generate “visible” light (red, green, blue, or UV light) in an external cavity in either continuous wave (CW) or pulsed mode. By enabling the use of a bulk doubling material and directly doubling the frequency of the VCSEL, the device can be inexpensive, simpler, have high efficiency, better reliability, and vastly improved manufacturing and alignment tolerances. Moreover, if using PPLN (periodic poled Lithium Niobate) or other periodic poled materials as the frequency doubler, the use of high power array elements allows for the use of short materials (0.2 mm to 4 mm). This is critical because the longer the material, the more sensitive it is to alignment, temperature and wavelength. With the disclosed principles, there are a number of cavity architectures that can be used to double the IR light from VCSEL lasers, and the VCSEL(s) can be single elements, or arrays with high intensity elements.
In one embodiment, architecture for a vertical cavity surface emitting laser system may include at least one vertical cavity surface emitting laser (VCSEL) element. Such an exemplary architecture may further include a bulk crystal doubling material located in a cavity adjacent to the VCSEL element and configured to receive light emitted from the VCSEL element, and to substantially double the frequency of the received light. This embodiment of architecture may also include an output coupler configured to output the doubled light from the cavity and output said visible light for use in display illumination.
In another embodiment, an architecture for a vertical cavity surface emitting laser system may include at least one vertical cavity surface emitting laser (VCSEL) element configured to emit infrared light, and a cavity defined between the at least one VCSEL element and a mirror being highly reflective of infrared light. Such an exemplary architecture may further include a bulk crystal doubling material located in the cavity and configured to receive infrared light emitted from the VCSEL element, and to substantially double the frequency of the received infrared light to output visible light.
In yet another embodiment, an architecture for a vertical cavity surface emitting laser system may include at least one vertical cavity surface emitting laser (VCSEL) element configured to emit infrared light, and a bulk crystal doubling material located in a cavity adjacent to the VCSEL element and configured to receive infrared light emitted from the VCSEL element, and to substantially double the frequency of the received infrared light to output visible light. Such an exemplary architecture may further include a coating on an end of the doubling material opposite to the at least one VCSEL element, the coating being highly reflective of infrared light. In addition, such architecture may further include an etalon or dichroic mirror orientated near Brewster's angle and used as an output coupler configured to receive the doubled light from the doubling material and output said doubled light for use.
Embodiments are illustrated by way of example in the accompanying figures, in which like reference numbers indicate similar parts, and in which:
Generally, one embodiment of a VCSEL-based light source in accordance with the present disclosure may take the form of architecture for a VCSEL system that may use high power IR VCSEL element(s). A bulk crystal doubling material may be used to double the IR light and generate “visible” light (red, green, blue, or UV light) in an external cavity in either continuous wave or pulsed mode. The reflectivity of the output distributed Bragg reflector (DBR) of these VCSELs can be designed to increase the power in an external cavity rather than the power in the VCSEL laser. By enabling the use of shorter either bulk or periodically poled doubling material and directly doubling the power of the VCSEL, the device can be inexpensive, simpler, have higher efficiency, better reliability, and vastly improved manufacturing and alignment tolerances. There are a number of cavity architectures that can be used to double the IR light from the VCSEL(s) in accordance with the disclosed principles, and the VCSEL(s) can be single elements, or arrays with high intensity elements. Such arrays may be designed for high power per element, and designed and fabricated to increase power in the external cavity. In practice, arrays are needed to generate enough overall power out of the cavity to be commercially viable. Examples of required watts output range from 3 W to potentially hundreds of watts of visible power.
Generally, there are several approaches to green/blue/red lasers. For very low power applications in which the wavelength of the light is not as critical, direct diodes in the approximate range of 50-100 mW are available in the appropriate wavelength ranges. For example, these diodes are of interest in small, mobile, “pico” projectors for the consumer market. However, for high power applications these sources are not useful. For higher power applications, conventional approaches typically employ infrared wavelengths generated by a solid state laser and then doubled with a non-linear crystal. Typical examples may include the doubling of fiber lasers and solid state lasers using crystals and doped glasses, for example, the YAG laser, that are lamp or diode pumped. These lasers may generate high power, good quality IR light which can be doubled. High intensity and good quality are critical for efficient doubling of the IR light to make visible or UV light, for example, 1064 nm doubled to 532 nm (green light), while 1232 nm can be doubled to 616 nm (red light). While these current doubled solid state sources can generate a lot of power in the green wavelength, in the approximate range of a few Watts to thousands of Watts, solid state sources are expensive, complex, not very efficient, and are difficult to make reliable. For example, approximately 30,000 hours or more are commonly needed for professional display applications for which approximately 5 to 2000 Watts of visible light may be appropriate.
Another version of green or blue laser that has been produced uses a VCSEL array, such as the exemplary embodiment of a VCSEL array illustrated in
Looking specifically at the light on the first path which is being doubled, the frequency doubler 120 may be a periodically poled lithium niobate crystal (PPLN) as mentioned above, wherein PPLN may be employed rather than a bulk doubling crystal because it may be more efficient at doubling the frequency of the light. This may be employed due to the lower intensities of the VCSEL beams. However, long lengths of PPLN may have a number of significant concerns. First, it may be more expensive than a bulk doubling crystal. Second, in order for PPLN to work well it has very tight tolerances on alignment, wavelength of IR light, and temperature. Thus, the PPLN has to be actively temperature controlled to approximately 0.1 degree Celsius or so, depending on the length of the crystal. Such a tight temperature control system is expensive and is challenging from a reliability point of view.
The wavelength spread that the PPLN can double effectively is also very challenging. Depending on length of the PPLN this can be as little as 0.1nm, typically. This tight tolerance typically requires the use of a wavelength control device, such as a VBG in the cavity as described in the conventional architecture of
Princeton Optronics of Mercerville, N.J., has been able to manufacture VCSELs with unique properties. The individual VCSEL elements can exhibit high power, for example, greater than approximately 150 mW; are very reliable, for example, greater than 100,000 hours; and have good optical quality, which may include microlenses fabricated on the VCSEL elements for improving energy capture. The VCSELs and corresponding properties are generally discussed in U.S. Pat. No. 6,888,871, “VCSEL and VCSEL Array Having Integrated Microlenses For Use In A Semiconductor Laser Pumped Solid State System” and “High Power VCSEL Mature Into Production”, Laser Focus World, April 2011, pp. 61-65, both of which are herein incorporated by reference in their entirety for all purposes.
Using high power IR VCSEL element(s), a bulk crystal doubling material or short periodically poled crystal or other doubling materials can be used to double the IR light and generate “visible” light, such as red, green, blue, or UV light, in an external cavity in either continuous wave or pulsed mode. By using a bulk crystal (such as KTP) all the elements of a large (possibly square) two-dimensional VCSEL array can be frequency doubled simultaneously in the same large doubling crystal, which results in a very high second harmonic power. Since PPLN is typically manufactured in 500 um thick wafers due to the restrictions of the poling process, this limits the dimensions of the conventional VCSEL array that can be doubled with a single PPLN crystal, resulting in lower total second harmonic power. With the disclosed principles, the reflectivity of the output distributed Bragg reflector of these VCSELs can be designed to increase the power in an external cavity, rather than the power in the VCSEL laser. By enabling the use of a short length or bulk doubling material and directly doubling the frequency of the light output from VCSEL, a device constructed in accordance with the disclosed principles can be inexpensive, simpler, high efficiency, better reliability, and have vastly improved manufacturing and alignment tolerances. There are a number of cavity architectures that can be used to double the IR light from such high power VCSEL(s). The VCSEL(s) can be single elements, or arrays with high intensity elements. The arrays may be designed for high power per element and may be designed and fabricated to increase power in the external cavity, which is defined by the disclosed principles as being between the VCSEL element(s) and a frequency-dependent highly reflective/anti-reflective (HR/AR) or highly reflective/highly reflective (HR/HR) (depending on the application, as described in further detail below) structure immediately following the bulk doubling material. The arrays can be fabricated and appropriately cooled so that the spread in wavelength is within the acceptable use of the doubling material.
The 1D or 2D array of high power VCSEL element(s) 210 generate IR light and can have integrated micro-lenses (not illustrated) fabricated on top to improve the beam intensity in the cavity 220. The doubling material 230 may generate the visible light by a non-linear conversion process such as, but not limited to, frequency doubling or second harmonic generation, and can include type I and/or type II phase matching. Examples of typical bulk doubling crystals for the doubling material 230 may include, but are not limited to, BBO (barium borate), KDP (potassium dihydrogen phosphate), KTP (potassium titanyl phosphate—as illustrated), lithium niobate, LBO (lithium triborate), KnbO3 (potassium niobate), and so forth. Additionally, other types of doubling materials 230 besides crystals exist and can be used including, but not limited to, non-linear polymers, organic materials, and so forth.
An IR mirror 270 may be included on the other side of the doubling material 230. This IR mirror 270 may have a high reflective (HR) coating to the IR wavelengths and/or an anti-reflective (AR) coating for the visible wavelengths. With these coatings, non-doubled light (e.g., 1064nm) will be reflected back from the IR mirror 270 into the doubling material 230 and to the VCSEL array 210, while frequency doubled light (e.g., 532nm) passes through the IR mirror 270 to be output from the device 200. In this embodiment, the IR mirror 270 forms one end of the cavity as the reflected light can also be doubled and reflected out of the cavity.
The VCSEL array system 200 may employ 1064 nm and 532 nm as examples of IR and visible wavelengths, respectively, but many other wavelengths can be generated. These lasers may be directly doubled, thus a wide range of IR wavelengths can be used to generate visible light from the red to the UV wavelength, or in the approximate range of 700 nm to 350nm. In the embodiment shown in
In other embodiments, instead of an IR mirror 270 with a HR coating for IR light and an AR coating for visible light, these coatings (280) can be fabricated directly onto the surface of the doubling material 230. The coating 280 may be fabricated on the side away from the VCSEL array 210. This may eliminate one element (e.g., the IR mirror 270) and may increase the likelihood of alignment with the doubling material 230. In addition to the AR and HR coatings, a protective coating (not illustrated) such as SiO2 may be located on the outside and/or inside of these HR and AR coatings 280. The protective coatings and/or layers can be applied to either the separate mirror element IR mirror 270, or in the case of the coatings 280 being incorporated, into the doubling material 230.
The doubling material 230 may also have AR coatings on both sides, or may be Brewster cut (in which the cut surface has an angle which may be at or near the Brewster angle) to substantially minimize reflections. A Brewster cut doubler may also improve the polarization purity of the intra-cavity power and thereby improve doubling efficiency.
Other concerns of a VCSEL array for this type of application may include wavelength diversity and power in the external cavity. The output DBR may be designed to increase the power in the external cavity rather than in the VCSEL laser. A lower reflectivity than what may be typically employed in a standalone VCSEL may result. In one example, to more efficiently double the IR light, it may be phase matched in the doubling crystal. The design, packaging, soldering, processing, and selecting of the VCSEL array may be designed to improve the wavelength and power uniformity across the array. For example, minimizing the heat differential across the array may improve the wavelength and power uniformity across the array. Minimizing the array area and choosing from the middle of the wafer can improve the uniformity. If sufficiently narrow spectrum in wavelength is difficult to achieve, for example, less than 3 nm, then an etalon can be used make the spectrum narrower. An etalon narrows the frequency but not as much when compared to a VBG. Additionally, etalons are much cheaper and have much larger tolerances on temperature and wavelength. The etalon may also be AR coated or coated to reflect the visible light, or angled near Brewster's angle to improve polarization. For display applications, wider spectrums may be desirable for mitigating speckle. If a wider spectrum than what a single device such as a visible laser can efficiently generate, then two or more devices can be used together that have VCSEL arrays that may be designed to operate at slightly different wavelengths.
The length of the doubling material 430 may be a tradeoff in that the longer the material, the better the doubling efficiency, but the tighter the tolerances on angle, temperature and wavelength. Typical lengths for the bulk doubler 430 may be in the approximate range of 1 mm and 30 mm, and in the illustrated embodiment is an exemplary 5 mm long when an exemplary cavity is approximately 19 mm. The output coupler 450 may again be a highly reflective window by being HR and AR coated 460 for the non-doubled and doubled wavelengths, respectively. Again, these coatings 460 can be incorporated onto at least the doubling material's 430 outside surface, away from the VCSEL 410, to eliminate the output coupler 450 in an alternative embodiment.
The IR VCSEL array 510 may be reflected approximately 45 degrees by a HR mirror 540 in the IR wavelength or folding output coupler or coated etalon. The beam waist may be relayed into the doubling material 530 by a lens 560. After the doubling material 530, another lens 570 may quasi re-collimate the light and substantially both IR and visible light may be reflected by a mirror 550. The light then may return and pass through the doubling material 530 again, which may generate more visible light that may exit the cavity 520 by the mirror 550, which may be HR in the IR wavelength and AR coated for the doubled (visible) wavelength, as illustrated.
Additionally, with respect to
For pulsed operation, a saturable absorber, an acoustic optical modulator, electro-optic modulator, and so forth can be placed in the cavity 820 to cause pulsing or to sync pulsing to another element in the system. In a system in which any of these elements may be employed, the elements may be included in a quasi-collimated space in the cavity 820. A possible location for a pulse inducing element may be illustrated as component 870 to the right of the KTP doubling material 830 in
Combinations of the various architectures and cavity elements disclosed and described herein may also be used, as will be understood by one skilled in the art.
It should be noted that embodiments of the present disclosure may be used in a variety of optical systems and projection systems. Exemplary embodiments may include or work with a variety of projectors, projection systems, optical components, computer systems, processors, self-contained projector systems, visual and/or audiovisual systems, and electrical and/or optical devices. Aspects of the present disclosure may be used with practically any apparatus related to optical and electrical devices, optical systems, display systems, presentation systems or any apparatus that may contain any type of optical system. Accordingly, embodiments of the present disclosure may be employed in optical systems, devices used in visual and/or optical presentations, visual peripherals and so on and in a number of computing environments including the Internet, intranets, local area networks, wide area networks and so on.
Before proceeding to the disclosed embodiments in detail, it should be understood that the illustrated embodiments discussed herein are not limited in application or creation to the details of the particular arrangements shown, because the embodiments are capable of other arrangements. Moreover, aspects of the embodiments may be set forth in different combinations and arrangements to define embodiments unique in their own right. Also, the terminology used herein is for the purpose of description and not of limitation.
As may be used herein, the terms “substantially” and “approximately” provide an industry-accepted tolerance for its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from less than one percent to ten percent and corresponds to, but is not limited to, component values, angles, et cetera. Such relativity between items ranges between less than one percent to ten percent.
While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of this disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages.
Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any embodiment(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the embodiment(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple embodiments may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the embodiment(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.
This application relates to and claims priority to U.S. Provisional Patent Application Ser. No. 61/598,175, filed Feb. 13, 2012 entitled “Laser architectures”, the entirety of which is herein incorporated by reference.
Number | Date | Country | |
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61598175 | Feb 2012 | US |