KR20100011147A - Compact solid-state laser with nonlinear frequency conversion using periodically poled materials - Google Patents

Abstract

PURPOSE: A compact solid state laser is provided to improve the reliability and effectiveness by providing low prices solid state laser. CONSTITUTION: A pump diode laser(1) emits a pump beam(2) of wavelength between 800nm and 900nm for efficient absorption through a gain material(8). An beam optical instrument(3) changes the pump beam to the beam in order to form a circular cross-section with a desirable diameter on the surface(7) of the gain medium. A transparent optical material(6) such as a sapphire, an undoped YVO4, and an undoped YAG has a high thermal conductance to maximize efficiency. The optical material is combined with the gain material. The optical material is operated as a heat sink. A nonlinear crystal(10) is combined with the gain material through the chemically activated direct coupling.

Description

Compact solid-state lasers with nonlinear frequency conversion using periodic polarization materials {COMPACT SOLID­STATE LASER WITH NONLINEAR FREQUENCY CONVERSION USING PERIODICALLY POLED MATERIALS}

FIELD OF THE INVENTION The present invention relates to a system and method for manufacturing a laser source, in particular a compact, efficient visible or near-ultraviolet laser source with an output power level of at least several hundred milliwatts and more, which is a level not attainable with conventional techniques. It relates to a manufacturing system and method thereof.

There has long been a need for small, efficient, low cost laser sources in the visible and ultraviolet spectral regions for a variety of applications. Such applications include laser-based projection displays, optical storage devices, bio-analytical devices, semiconductor inspection and spectroscopy. Semiconductor lasers, which provide a low cost, compact and efficient platform, rely on material systems such as InGa (Al) P, which most efficiently laser in the near infrared spectral region. Efficient operation below 650 nm (red) can be achieved without significant technical problems, and some semiconductor lasers can extend below 635 nm while reducing efficiency and reliability. On the shorter wavelength side of the visible region, GaN systems have been recently developed and lasers in the ultraviolet spectral region (˜400 nm to 445 nm) have been commercialized. However, achieving> 470 nm wavelength efficiently and reliably remains a very difficult challenge. Therefore, most visible spectra (ie, from 470 nm blue to 635 nm red) do not currently have efficient semiconductor laser solutions.

Of these colors (wavelengths), the absence of green is probably the most important because this color is the most sensitive to the human eye. Indeed, there is no direct solution currently available for green semiconductor lasers. Indirect solutions, commercially available since 1990, are based on nonlinear frequency doubling (known as second-harmonic generation, or SHG) of neodymium (Nd) based solid state lasers, such as Nd: YAG, or Nd: YVO ₄ It was set as. This solid state gain material is pumped by an infrared semiconductor laser (eg ˜808 nm) and yields laser radiation at ˜1064 nm wavelength. This 1064 nm radiation can then be frequency doubled to a green 532 nm wavelength using a nonlinear crystal such as Potassis Titanyl Phosophate (KTP), or Lithium Borate (LBO). Similar techniques can be used to obtain a blue color of, for example, 473 nm by frequency doubling a 946 nm solid-state laser. A review of this approach can be found in WP Risk, TR Gosnell, and AV Nurmikko's book, "Compact Blue-Green Lasers", Cambridge University Press (2003). In addition, low cost platforms can be achieved using so-called microchip technology, in which gain chips and nonlinear crystals are combined to form a monolithic laser cavity. The microchip concept was first proposed by Mooradian (US Pat. No. 5,365,539).

However, currently available microchip lasers lack the efficiency and flexibility required in many applications, especially at high power levels, for example from hundreds of milliwatts to several watts. This is mainly due to the inefficiency of the frequency conversion of conventional nonlinear materials such as KTP. To obtain hundreds of milliwatts of green light from KTP-based microchip lasers, a significant power margin must be provided for the basic infrared laser, which imposes temperature, size, and cost constraints on the overall laser system design. In addition, traditional bulk nonlinear materials such as KTP are limited to their range of frequency conversions. For example, KTP is used for frequency doubling to green, but cannot actually be used for frequency doubling to blue light, and therefore must find other nonlinear materials that have their own limitations in efficiency, reliability, and cost.

Laurell (US Pat. No. 6,259,711) suggested that most of these limitations can be overcome by the use of periodic polarized nonlinear crystals. Such crystals can be engineered to provide high nonlinearity for the desired conversion wavelength. Therefore, such laser designs implemented with microchip architectures can address numerous limitations associated with conventional bulk nonlinear materials.

However, embodiments of this invention, as you know, are experiencing significant limitations that have prevented the commercialization of such platforms, and to this day, visible wavelength microchip lasers continue to rely on bulk nonlinear materials such as KTP, and KNbO ₃ , The latter material is used to produce blue light (see, eg, international patent application WO2005 / 036,703). The cause of this limitation is due to the selection of the cyclically polarized nonlinear crystals proposed in Laurell's invention, namely KTiOPO ₄ (KTP), LiNbO ₃ (LN), and LiTaO ₃ (LT). Such materials have high nonlinearity and can be easily polarized into periodic structures for frequency doubling. However, the practical use of such materials is very limited. Similar to bulk KTPs, cyclically polarized KTPs can perform well at low power levels (several milliwatts of visible area, or possibly even tens of milliwatts), but at the higher power levels passive and inductive absorption ("gray tracking") Will suffer. In addition, KTP crystal production is not easily scaled up to mass production at low cost as required by some applications such as consumer electronic displays. LiNbO ₃ , and LiTaO ₃ are scalable to mass production and may facilitate periodic polarization, but do not allow the use of such crystals to yield several milliwatts of visible light without severe degradation ("). Optical refraction damage)). Light refractive damage can be reduced at heated temperatures (> 150 ° C.). However, this requires the use of an oven to keep the nonlinear crystals at a high temperature. Such ovens are inconsistent with low cost, efficient laser fabrication, in particular microchip geometry. Therefore, the laser design described by Laurell cannot be implemented with a high power, low cost, compact, and efficient architecture. Similarly, Brown (US Published Patent Application 2005 / 0,063,441) proposed a design for a small laser package that appeared to be suitable for low cost applications. However, Brown's teachings still revolve around conventional nonlinear materials such as KTP, and LBO. Although the use of PPLN and PPKTP is mentioned as possible, it has not been taught how to overcome the limitations of this decision, in particular the reliability limitations described above.

It is known that joint LiNbO ₃ , and LiTaO ₃ suffer photorefractive damage due to visible light, and several methods have been proposed to overcome this problem. The high temperature operation described above partially solves this problem, but is not suitable for most applications. Another proposed solution is to dope the joint material during crystal growth to suppress optical refractive damage mechanisms (T. Volk, N. Rubinina, M. Wohlecke, "Optical-damage-resistant impurities in lithium niobate", Journal of the Optical Society of America B, vol. 11, p. 1801 (1994). Growing bulk crystals at high levels of stoichiometry has been proposed as another method to suppress photorefractive damage (Y. Furukawa, K. Kitamura, S. Takekawa, K.Niwa, H. Hatano, “Stoichiometric Mg: LiNbO”). ₃ as an effective material for nonlinear optics ", Optics Letters, vol. 23, p. 1892 (1998)).

However, those skilled in the art have not taught the means of achieving a high power, stable room temperature operable frequency doubled laser suitable for producing green and blue light with low cost, high volume design. If the periodic polarized LiNbO ₃ or LiTaO ₃ crystals were stoichiometrically within 0.05%, they found that no dopant was needed to stabilize at high powers up to 500 mW. For stoichiometrically within 0.6% of doping, doping of ZnO or MgO of approximately 0.1 to 0.6 mole percent achieves substantially the same beneficial results as obtained with periodically polarized LiNbO ₃ or LiTaO ₃ crystals. The present invention teaches small, efficient, low cost frequency conversion lasers based on periodic polarized materials that contain dopant MgO or ZnO and / or have a certain degree of stoichiometry that ensures high reliability for such materials. ZnO or MgO doped stoichiometric LiNbO ₃ and LiTaO ₃ are materials that are very different from their joint counterparts, and their altered ferroelectric properties are several micrometers long domains necessary for frequency conversion of these materials into the short spectral, visible spectral range This makes it very difficult to polarize. The technical problems of producing periodic polarized ZnO or MgO doped stoichiometric LiNbO ₃ and LiTaO ₃ have recently been overcome and such new materials are expected to be manufacturable. Crystals that whistle for periods suitable for laser conversion to blue, green, and longer wavelengths have been produced, and techniques for this production process are co-pending, co-assigned published US patent application Ser. No. 2005 / 0,133,477 It is described in

In short, well-known technical approaches do not provide reliable, cost effective, small frequency converted lasers.

The present invention solves this problem and is based on periodically polarized LiNbO ₃ or LiTaO ₃ containing a dopant such as MgO or ZnO, and / or having a specified degree of stoichiometry that ensures high reliability for such materials. A low cost, efficient and reliable solid state laser architecture is disclosed. The invention is also compact, efficient and frequency-converted into wavelength ranges that are not available through direct semiconductor lasers, ie, in the blue, green, yellow, orange, and near-ultraviolet wavelength ranges, ie, wavelengths from approximately 275 nm to 635 nm. And a reliable low cost solid state laser. The present invention teaches a method of making a compact and efficient visible or near ultraviolet laser source having an output power level of at least several hundred milliwatts and more, which is a level not attainable with conventional techniques.

The present invention solves the problem of not providing a reliable, cost-effective, compact frequency-converted laser of known technical approaches, includes dopants such as MgO or ZnO, and / or for such materials It provides a low cost, efficient and reliable solid state laser architecture based on periodically polarized LiNbO ₃ or LiTaO ₃ with a certain degree of stoichiometry to ensure high reliability. According to the present invention, there is also a small, frequency-converted wavelength band which is not usable via a direct semiconductor laser, i.e., in the blue, green, yellow, orange, and near-ultraviolet wavelength range, i.e., in the wavelength range of approximately 275 nm to 635 nm. It is possible to provide an efficient, reliable and low cost solid state laser. According to the present invention, it is also possible to provide a method for producing a compact and efficient visible or near-ultraviolet laser source having an output power level of at least several hundred milliwatts and more, which is a level not attainable with conventional techniques.

1 illustrates a preferred embodiment of the present invention. The pump diode laser 1 emits a beam 2 at a wavelength between 800 and 900 nm, for example ˜808 nm or 885 nm, for efficient absorption by the gain material (element) 8. The beam 2 is often astigmatism and the beam shaping optics 3 beam the pump beam 2 such that the beam 4 forms a circular cross section of the desired diameter on the surface 7 of the gain medium 8. It is advantageous to be used to convert to 4). This type of pumping arrangement is known in the art and can effectively overlap the pump area within the gain element with an internal cavity circularizing beam, which must be a single spatial mode (or TEM ₀₀ ) for efficient nonlinear frequency doubling. Suitable diameters for the pump spots on the gain element 8 are in the range of 100 to 300 microns. The beamforming optics can be a microlens, a gradient index lens, or a combination of such optical elements. The beam shaping optics 3 can be eliminated when efficiency can be sacrificed for simplicity and miniaturization. Another part of the assembly 3 may be a volume Bragg grating used to narrow the spectral emission of the diode laser 1. The narrowing of the spectral output of the pump laser may be beneficial for the efficiency of the laser system. Methods of achieving such spectral narrowing are described, for example, in paper by L. Glebov., "Optimizing and Stabilizing Diode Laser Spectral Parameters." Photonics Spectra, described in January 2005.

However, creating high laser source efficiency is a key advantage of the present invention. To maximize the efficiency, a transparent mineral material 6 having high thermal conductivity such as sapphire, undoped YVO ₄ or undoped YAG is used. Thus, element 6 is coupled to gain element 8 and acts as a heat sink. Surfaces 5 and 7 are coated for high conductivity at pump laser wavelengths, for example 808 nm. The coating of surface 7 also provides high reflectivity at the fundamental laser wavelength, such as 1064 nm, and acts as the first mirror of the solid state laser cavity. This coating may be selected for lasing at the desired wavelength supported by the solid state material 8, for example 1342 nm. In this example, care must be taken to reduce the reflectivity of the first mirror 70 or the second cavity mirror 12 at the main laser transition wavelength (1064 nm in the case of Nd: YVO ₄ pump laser). Some examples of light transmissive heat sink materials suitable for use as element 6 include sapphire, undoped YVO ₄ or undoped YAG. Among these elements, sapphire is most efficient for heat sinking due to its high thermal conductivity and good thermal expansion match for Nd: YVO ₄ . In the low power version of this laser design (less than the absorbed pump power of 1 W), traditional heat sinking methods such as mounting gain elements on copper or other high thermally conductive metal mounts are acceptable and within the scope of the present invention. It is.

The gain medium 8 is preferably an Nd doped element with a higher gain in one axis, such as NdYVO ₄ or NdGdVO ₄ such that the element 8 provides both gain and polarization control for the laser cavity. In the present invention, the level of Nd doping to maximize the laser efficiency will usually be in the range of 0.5% to 3% atm (atomic percent). Element 8 also provides transverse mode control in a flat-flat laser cavity via thermal lensing effect and gain guiding.

Nonlinear crystal 10 is a periodic polarized nonlinear crystal belonging to a family of doped or stoichiometric nonlinear materials that ensure reliable crystal behavior both at the secondary harmonic wavelength (usually visible) and at the primary wavelength (ie near infrared). . Specifically, these materials include PPMgOLN (periodic polarization MgO doped LiNbO ₃ ), PPMgOLT (periodic polarization MgO doped LiTaO ₃ ), PPZnOLN (periodic polarization ZnO doped LiNdO ₃ ), PPZnOLT (periodic polarization ZnO doped LiTaO ₃ ) PPSLN (cyclic polarization stoichiometric lithium niobate) or PPSLT (cyclic polarization stoichiometric lithium tantalate). The level of doping and stoichiometry is chosen to suppress light degradation effects such as photorefractive damage and visible light induced infrared absorption (also known as GRIIRA and BLIIRA for green and blue light, respectively). Recent discussion on this topic is described in Y. Furukawa, K. Kitamura, A. Alexandrovski, RK Route, MM Fejer, G. Foulon, "Green-induced infrared absorption in MgO doped LiNbO ₃ " Applied Physics Letters, vol. 78, p. Seen in 1970 (2001) paper. A method for making such periodic polarized crystals is described by S. Essaian, one of the co-inventors of the present invention, in US Patent Application 2005 / 0,133,477, assigned to the same assignee as the present application.

The polling period of the nonlinear crystal 10 is chosen to maximize the efficiency of the second harmonic generation of the base beam. For example, the polling period of PPMgOLN for frequency doubling of 1064 nm to 532 nm is approximately 7 microns. The effective nonlinear coefficient for this material is approximately 16 pm / V and can be as high as 20 pm / V when perfect lattice structure and material stoichiometric uniformity are achieved. High nonlinearity and high reliability of nonlinear crystals are key advantages of the laser system of the present invention. Because the efficiency of nonlinear conversion is proportional to the square of the nonlinear coefficient, using a material such as PPMgOLN instead of traditional materials such as KTP (~ 3.5 pm / V for conversion to green wavelength) or LBO (~ 1 pm / V) It is possible to make a smaller size than the bulk material, and to configure a lower power and higher power output system. For example, an additional advantage of using cyclic polarization materials over KTP is that only a single polarization of the primary beam is needed for the second harmonic generation process. In KTP (the most widely used crystal for SHG in the green wavelength range), the two orthogonal polarizations at the fundamental wavelength must be excited in the crystal (this constitutes a so-called type-II phase matched SHG), which is an internal cavity It creates a possibility for the polarization extinction of the laser beam, and thus a possibility for the loss of both power and efficiency.

Using optimal doping and stoichiometry for high reliability, it is possible to produce reliable laser products without the need to use expensive and space-consuming ovens for heating nonlinear crystals to suppress deterioration of the nonlinear crystals. As a result, the mass production of PPMgOLN and other crystals used in the embodiments of the present invention enables the mass production of small visible lasers for the mass consumer market. It is important to point out that colors not available directly from semiconductor diode lasers can be achieved.

It is within the scope of the present invention to use nonlinear crystals with non-periodic (chirped) or non-parallel (fanout) polling patterns. Another advantage provided by the high efficiency materials of the present invention, such as PPMgOLN, is that they provide design headroom. This means that effective nonlinearity can be compromised for other parameters such as angular acceptance bandwidth or temperature for secondary harmonic generation without significant penalty in the generated secondary harmonic power. This is because the internal cavity secondary harmonic generation can be limited by the maximum amount of power the laser can emit at the primary wavelength. This is described by Smith (R. Smith, "Theory of intracavity optical second-harmonic generation", IEEE Journal of Quantum Electronics, vol. 6, p. 215, (1970). After the laser limit is reached, No further increase in secondary harmonic power can be achieved by increasing the crystal nonlinearity, the length, or the beam focusing .. Conventional bulk nonlinear crystals never reach this situation in normal wave laser operation, but The high nonlinear cyclic polarization crystal of the invention reaches it, as a result of which there are some efficiency limitations by reducing the nonlinear crystal length, by modifying the polling pattern, and especially because of its inherent thermal curing even when the entire assembly is controlled as a whole. Low cost, monolithic microchip laser cavity assemblies provide improved laser cost and performance Thus, in a preferred embodiment, the nonlinear crystal 10 is bonded to the laser gain element 8 by, for example, chemically activated direct coupling, the input surface 9 of the nonlinear crystal is first order. It has a coating to ensure high conductivity at wavelength and high reflectance at the second harmonic wavelength This arrangement also prevents the generated visible light from entering the gain element, which can be detrimental to laser operation. It should be noted that the epoxy-free bonds, which are preferred for, have made significant advances in recent years and thus the monolithic assemblies disclosed herein can be readily manufactured.A review of the direct bond technology is incorporated herein by reference C. Myatt, N Traggis and K. Dessau paper "Optical contacting grows more robust", Laser Focus World, January 2005, p. 95.

The output surface 12 of the nonlinear crystal serves as the second mirror of the cavity. Therefore, it is desirable to coat for high reflectivity at the primary laser wavelength and for high conductivity at the secondary harmonic wavelength. The longitudinal and lateral dimensions of the described arrangement are optimized for high efficiency as is known in the art of laser design. It has been found that the nonlinear crystal length need not exceed 5 mm to obtain hundreds of milliwatts of power at 532 nm (green) wavelength. Ray 11 represents an internal cavity laser beam at the primary wavelength. These rays describe the cavity mode propagating away from the gain element 8. The rear propagation cavity mode overlaps this front propagation beam and thus is not shown. Similarly, secondary harmonic beams are produced both in the front and in the rear. The rear generated secondary harmonic beam is reflected by the light surface 9 and recombined with the secondary harmonic beam generated forward so that a single beam 13 exits the laser cavity.

Because both the front and rear generated secondary harmonic beams are coherent with each other (ie because they have a finite phase relationship, they may optically interfere with each other, thus somewhat reducing the efficiency of nonlinear conversion. Several methods are used in the present invention: One method is to control the crystal temperature (the optimum point between maximizing interference and maximizing nonlinear conversion efficiency to be as close as possible to constructive interference). It can be easily achieved with the aid of a low cost resistive heater element in the range from 0 ° C. to about 80 ° C. and positioned under a nonlinear crystal When the laser cavity is long enough to operate in a plurality of longitudinal modes, another method is ( Relying in part on some longitudinal mode which distinguishes itself in destructive interference It said, on the other hand, another mode is to strengthen the whole second harmonic output from the constructive interference. In mode laser having a plurality of longitudinal direction, which is because it will be a mode ring most efficiently out coupling advantageous in the constructive interference is automatically achieved.

However, another advantage of the microchip assembly of the present invention is that it can utilize periodic polarized crystals that are thick enough to be easily handled and bonded to other crystals. Until recently, a common accepted comment was that this PPMgOLN-like material could be polarized only on thin-film wafers (less than 0.5 mm thick) to convert to blue-green at best, and not at all in production, not in the lab environment. It could not be. Now, using the method described by S. Essaian in published US patent application 2005 / 0,133,477, it is possible to produce crystals with a thickness of 1 mm in high yield. This is a significant advantage in building microchip lasers. Thus, using this recent achievement in crystal technology, it is possible to obtain new laser platforms that outperform existing platforms in their performance: power, efficiency, reliability, and cost.

With respect to the embodiment of the invention illustrated in FIG. 2 and the subsequent figures, many elements and their functions are basically similar to those in the embodiment illustrated in FIG. Thus, although similarity can be understood from the description of FIG. 1, the above differences will be emphasized in the following description of the embodiments.

The embodiment of FIG. 2 is particularly useful when the gain medium (element 15 of FIG. 2) does not have the preferred direction for polarization to enable higher gain. A known example of such a gain medium is Nd: YAG. One advantage of using Nd: YAG is that it can provide a laser wavelength, such as 946 nm, which is not available with Nd: YVO ₄ or Nd: GdVO ₄ . This is desirable for obtaining different colors by nonlinear frequency conversion, such as blue, for example, at 473 nm wavelength. The gain material may also be a glass based material such as Yb: glass or Nd: glass, Yb: YAG glass, crystals and other materials based on glass.

Although a number of elements and techniques described in the embodiment of FIG. 1 can be applied to FIG. 2, the design of FIG. 2 provides polarization control through different means than the gain medium. Polarization control is an essential part of laser design because the second-harmonic generation process is sensitive to polarization. In order to maintain the low cost, compact design concept of the present invention, one preferred embodiment of the present invention utilizes an additional birefringent element 16. Element 16 is a birefringent crystal suitable for internal cavity laser design and is cut at an angle to provide a large walkoff between the two polarizations supported by the crystal. An example of a material suitable for use with element 16 is undoped yttrium vanadate (YV0 ₄ ). The walkoff in crystal 16 can be used to distinguish between two polarizations, for example by using aperture 18, which results in a higher loss for unwanted polarizations. Although the example in FIG. 2 shows individual elements 15 (gain determination), (16: polarization control determination for walkoff generation), (18: aperture), and (19: nonlinear crystal) It can be combined into a noisy assembly. In this case, a significant walkoff can be designed such that the aperture can be aligned manually, ie before the laser is turned on.

Another way to distinguish between two polarizations is to use a spherical mirror or lens on the right side of a nonlinear crystal (not shown in the figure), where one of the polarizations is defined by a lens or mirror on one side and the other On the plane it deviates from alignment with respect to the optical axis defined by the gain aperture. The concept is basically similar to the embodiment with apertures in that it provides a higher loss for unwanted polarization. The other elements and coatings shown in FIG. 2 are similar to those of FIG. 1.

The design of FIG. 3 is similar to the design of FIG. 2, in particular useful when the gain medium (element 15 of FIG. 2) does not have the desired orientation for polarization with higher gain. In order to control the laser polarization for efficient nonlinear frequency doubling, the design is based on the hollow cavity Brewster surface 52, which can be left uncoated. One way to obtain the Brewster surface in the cavity without adding additional elements is to cut the gain determination 51 at the Brewster angle. The Brewster surface has high conductivity for p-polarized light and low conductivity for s-polarized light. This fact can be used to tilt the gain determination at an appropriate angle to form a laser cavity. The crystal shown in FIG. 3 looks thinner than the other figures. This is to indicate that when the Brewster surface appears, the thinner cross section (wafer) of the periodic polarization crystal is generally in the horizontal plane of the figure. Designs similar to those illustrated in FIG. 3 have been used in the past (see, eg, international patent application WO2005 / 036,703), but do not have the advantages of the high reliability, periodic polarization crystals taught in the present invention.

It is to be understood that FIG. 3 only illustrates one possible scenario of component placement with an internal cavity Brewster surface. As in FIG. 1, the design can be built monolithically, for example, by cutting the surface 54 of the nonlinear crystal 10 at a constant angle and combining the gain element with the nonlinear crystal. In this case, the Brewster angular cut may be designed for an interface between the optical elements 51, 10, which is not for the interface between any of the elements and air.

The embodiment shown in FIG. 4 is for illustrating and dealing with the optimization of the second harmonic power extraction. As discussed in the description of FIG. 1, secondary harmonic light is generated in two opposing propagation directions. In many cases, the rear generation beam can be recombined with the front generation beam via a high reflection mirror coating for the rear generation beam, and possible cancellation of interference between the two beams by using an operation in a multi-length direction mode. Can be prevented by thermal adjustment. In some cases, however, it is more efficient to use the design shown in FIG.

Element 23 is a wave plate (for example made of quartz) that rotates the polarization of both the primary and secondary harmonic beams. In the design, the waveplate is selected such that the polarization of the primary beam is rotated by 90 degrees after a single pass, and the polarization of the secondary harmonic beam is rotated by 45 degrees after a single pass. This type of wave plate is called a dual wave plate and is commercially available. Surface 23 is antireflective coated for both primary and secondary harmonic beams. Surface 22 is antireflective coated for the primary beam and highly reflective coated for the secondary harmonic beam. Since the primary light traverses the waveplate twice in one cavity round trip, its polarization does not change, so the waveplate does not disturb the operation of the primary laser. However, the secondary harmonic light that also traverses the waveplate twice changes for nonlinear crystals that orthogonally polarize their polarization without interfering with the forward generated secondary harmonic beams and returns back through the nonlinear crystals 10 ( Surface 24 is now antireflective coated for both primary and secondary harmonic beams). This design is particularly useful for applications where polarization of the output secondary harmonic beam is not critical. One such application is the use of the laser of the present invention in projection displays, which is based on digital light processing technology.

The embodiment of FIG. 5 illustrates another method for extracting back-generated secondary harmonic beams in the use of undesirable waveplates. The extraction is carried out through a tuning mirror 28 which is now coated, which has high reflectivity for secondary harmonic light and high conductivity for primary light. One example where the turning mirror design of FIG. 5 is preferred over the waveplate design of FIG. 4 is when the laser polarization is not fixed by the gain element 6, for example when Nd: YAG is used. In this case, provided efficient secondary harmonic conversion, one can also design polarization discrimination in the coating of element 28 so that the laser operates only at the desired polarization. The reorientation, post-generated secondary harmonic beam 30 is redirected again by another mirror, which travels in the same direction as the forward-generated secondary harmonic beam. Unlike the design of FIG. 4, the design yields a linearly polarized secondary harmonic beam. This is desirable for applications such as using lasers for projection displays, based on liquid crystal spatial light modulators such as LCDs or LCOS.

The embodiment of FIG. 6 combines the front and rear generating secondary harmonic beams by reflecting the front generating beams from the surface 37 of the nonlinear crystal 36. The coated glass plate 35 is preferably directed at an angle of 45 degrees to the gain crystal surface and coated for high reflectivity at the primary laser wavelength and high conductivity at the secondary harmonic wavelength. The single, linearly polarized secondary harmonic beam 39 is outcoupled from the surface 35. As mentioned above, having a slanted surface in the cavity makes it easier to distinguish polarization by designing a polarization selective coating. This is advantageous for gain determinations that do not define the laser polarization direction, such as Nd: YAG gain determinations. Similar to other embodiments of the present invention, the design is modular and is combined with the concepts illustrated in other embodiments, such as the "wavelength plate design" of FIG. 4 set to rotate the polarization of the secondary harmonic beam. Can be. One embodiment uses a waveplate to fill the cavity below the surface 36, which in this case is suitably dual anti-reflective coated.

The configuration shown in FIG. 7 is similar to the design of FIG. 1 and can be combined with the designs of FIGS. 2-6. The different element in this embodiment is the curved mirror 13, which has high reflectivity at the primary laser wavelength and high conductivity at the secondary harmonic wavelength. The design is somewhat more expensive than the other designs illustrated, but can be used in high power applications when the stability of the thermal lens in the cavity crossing mode is less efficient than it would be at a lower power level. Note that the curved mirror can be used in another aspect of the cavity as well as in a non-monolithic arrangement.

The embodiment of the present invention in FIGS. 1-7 shows a low cost and compact laser design for continuous wave (cw) operation. As will be apparent from FIG. 8, a compact and low cost design for pulsed (passive Q-switched or manually locked mode) operation can be obtained while enjoying all the advantages of the non-linear crystals described in the present invention. . 8 illustrates the design of FIG. 5 modulated for operation with a saturable absorber 71. Element 71 is a suitable solid-state or semiconductor saturable absorber. Solid-state example of a saturable absorber is Cr ^{+ 4:} there is a _{^{(MgAl 2 O 4 V 3+:}} : YAG, Co 2+) YAG ( chromium doped yttrium aluminum garnet), and determine this and other saturable absorber. Examples of semiconductor based saturable absorbers are epitaxially grown single quantum wells or a plurality of quantum wells (eg based on InGaAs material structure). Quantum well absorbers are also grown with an epitaxial mirror stack, also known as a distributed Bragg reflector or DBR. Similarly, the solid-state saturable absorber crystals can be coated with a mirror coating to define a secondary cavity mirror. Methods of passive Q-switching and locked mode are known in the art of laser design and are described, for example, in R. Paschotta and U. Keller, "Ever higher power from mode-locked lasers." Optics and Photonics News, p. 50, May 2003; DH Lee et al., “Intracavity-doubled self-Q-switched Nd, Cr: YAG 946/473 nm microchip laser,” Chinese Physics Letters, vol. 19, p. 504 (2002); JJ Zayhowski, "Passively Q-switched microchip lasers and applications," Rev. Laser Eng., Vol 26, p. 841 (1998). In addition, the saturable absorber and gain elements can be combined into a single element 26, for example, by doping the YAG crystal with Nd and Cr together. The pulsed embodiment is advantageous for applications that do not require cw operation. An additional advantage of the pulsed laser configuration is that there is a much higher peak power in the pulse compared to the average power at the primary wavelength. This further increases the efficiency of secondary harmonic generation and mitigates temperature endurance for periodic polarized nonlinear crystals.

In addition, designs with active Q-switching can be realized without causing a significant increase in cavity cost and complexity. It is known that both periodic polarized lithium niobate (PPLN) and lithium tantalum acid (PPLT) can be used as electro-optic Q-switching elements. A recent discussion of this problem is described in papers by YH Chen, YC Huang, YY Ling, and YF Chen, "Intracavity PPLN crystals for ultra-low-voltage laser Q-switching and high-efficiency wavelength conversion," Applied Physics B: Lasers and Optics, vol. 80, p. 889 (2005). Again, a preferred and advantageous embodiment of the present invention is the use of periodic polarized nonlinear materials with optimized doping or stoichiometry on which reliable and efficient commercial laser products depend. The example provided in FIG. 8 provides a compact, efficient and reliable active Q-switching laser, wherein the element 71 is currently used as an electro-optical Pockels cell element, ie an electro-optic Q-switch. Is another periodic polarized nonlinear crystal. The teachings of the following references are incorporated into the text by reference.

1.W. P. Risk, T. R. Gosnell and A.V. Nurmikko, "Compact Blue-Green Lasers", Cambridge University Press (2003).

2. A. Mooradian, "Microchip laser", US Patent 5,365,539.

3. F. Laurell, "Laser", US Patent 6,259,711.

4. T. Georges, "Laser diode-pumped monolithic solid-state laser device and method of application of said device", International Patent Application WO2005 / 036,703.

5. T. Volk, N. Rubinina, M. Wohlecke, "Optical-damage-resistant impurities in lithium niobate", Journal of the Optical Society of America B, vol. 11, p. 1681 (1994).

6. Y. Furukawa, K. Kitamura, S. Takekawa, K. Niwa, H. Hatano, "Stoichiometric Mg: LiNbO ₃ as an effective material for nonlinear optics", Optics Letters, vol. 23, p. 1892 (1998).

7. D. C. Brown, "High-density methods for producing diode-pumped microlasers", US patent application 2005 / 0,063,441.

8. Spectralus Corporation Web Site: http://www.spectralus.com.

9. S. Essaian, "Method for the fabrication of periodically poled lithium niobate and lithium tantalate nonlinear optical components", US patent application 2005 / 0,133,477.

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According to the present invention, a low cost, based on periodically polarized LiNbO ₃ or LiTaO ₃ , comprising dopants such as MgO or ZnO, and / or having a specified degree of stoichiometry that ensures high reliability for such materials It can provide an efficient and reliable solid-state laser architecture. Furthermore, according to the invention, it is compact and frequency-converted into wavelength ranges that are not usable through direct semiconductor lasers, ie, in the blue, green, yellow, orange, and near-ultraviolet wavelength ranges, ie wavelengths of approximately 275 nm to 635 nm. It is possible to provide an efficient, reliable and low cost solid state laser. According to the present invention, it is also possible to provide a method for producing a compact and efficient visible or near-ultraviolet laser source having an output power level of at least several hundred milliwatts and more, which is a level not attainable with conventional techniques.

1 illustrates one microchip embodiment of the present invention.

2 shows one embodiment of the invention with a gain medium with undesirable polarization and a crystal with birefringent walk-off.

3 illustrates one embodiment of the invention with a gain medium with undesirable polarization, and an internal cavity Brewster face.

4 shows one embodiment of the invention with a gain medium with preferred polarization, and a polarizing plate for polarization rotation of the backward traveling secondary harmonic beam.

Figure 5 shows one embodiment of the present invention for the recovery of a secondary harmonic beam running backwards through a turning mirror.

6 shows one embodiment of the invention with a folded cavity.

7 shows one embodiment of the invention with a curved mirror on either side of the cavity.

8 shows one embodiment of the invention with a saturable absorber.

Claims (29)

A compact and efficient microchip laser providing frequency doubled visible or near ultraviolet output, comprising a pump beam at a selected wavelength and a pump laser providing a laser microchip in forming the laser cavity. a) two mirrors formed by the coated surface; b) a solid-state gain element pumped by a semiconductor diode laser and disposed between the two mirrors; And c) further comprising a bulk, periodic polarized nonlinear frequency doubling crystal, also disposed between the two mirrors and having PPMgOLN, PPMgOLT, PPZnOLN, PPZnOLT, stoichiometric PPSLN, or stoichiometric PPSLT. Compact, efficient microchip lasers. The method of claim 1, And the gain element is a crystal having a larger gain along any one of its crystal axes. The method of claim 1, The gain element is Nd: YVO ₄ , Nd: GdVO ₄ , or Nd: YGdVO ₄ , Nd: YAG, Nd: YLF, Yb: glass, Yb: YAG, or Nd: glass laser. The method of claim 1, The pump beam is transmitted to the gain element via a microlens or refractive index distributed lens. The method of claim 1, And the pump beam is delivered directly to the gain element without using beamforming optics. The method of claim 1, Wherein the pump laser is spectrally narrowed by a ballroom Bragg grating. The method of claim 1, The non-linear crystal is PPMgOLN, PPMgOLT, PPMgOSLN, PPMgOSLT, PPZnOLN, PPZnOSLN, or PPZnOLT, PPZnOSLT, wherein the MgO or ZnO dopant is present in an amount of 0.1 to 7 mole percent, and the LN and LT are combined or within 0.6% Small and efficient microchip laser, characterized in that the stoichiometry of. The method of claim 1, Wherein said periodic polarized nonlinear crystal is stoichiometric LT (PPSLT) or LN (PPSLN). The method of claim 1, Wherein said laser cavity component is in the form of a monolithic assembly achieved by chemically activated direct bonding of the individual elements without the use of adhesives. The method of claim 1, In order to create short cavities for the generation of single frequency 532 nm or 473 nm radiation, the periodic polarized nonlinear crystals are small and efficient microchips characterized in that the laser microchip has a length of less than 1 mm with a length of less than 1.3 mm. laser. The method of claim 1, And the laser cavity component is spatially separated and mounted on a common platform. The method of claim 1, And the output face of the nonlinear crystal forms a mirror at the end of the laser cavity and is coated such that it is highly reflective at its primary wavelength and highly conductive at its secondary harmonic wavelength. The method of claim 1, The input face of the nonlinear crystal and / or the input face of the gain element is coated for high reflectivity at the second harmonic wavelength to collect a back-generated secondary harmonic beam. The method of claim 1, The gain element is mounted on an optically transparent material having high thermal conductivity. The method of claim 14, Wherein said optically transparent material is sapphire, undoped YV0 ₄ , or YAG. The method of claim 1, Compact and efficient microchip laser, characterized in that the polarization control at the desired polarization axis is achieved by using a gain medium comprising Nd: YVO ₄ , Nd: GdVO ₄ , or Nd: YGdVO ₄ . The method of claim 1, Iii) a birefringent element having a larger spatial walkoff between one of the two supported polarizations; And Ii) A compact and efficient microchip laser further comprising an aperture, curved mirror or lens that imposes greater losses on unwanted polarization. The method of claim 1, Small sized and efficient microchip laser, wherein polarization control is achieved using an internal cavity Brewster surface as part of either or both gain elements or periodic polarized nonlinear crystals. The method of claim 1, Dual wavelengths that do not change the primary beam polarization at the cavity round trip, rotate the polarization of the rear-generated secondary harmonic beam by 90 degrees, reflect the secondary harmonic beam and combine it with the forward-generated secondary harmonic beam Compact and efficient microchip laser, characterized in that the plate is provided. The method of claim 1, Compact and efficient microchip laser, characterized in that an inclined, coated plate of the inner cavity is provided for extracting a back-generated secondary harmonic beam. The method of claim 20, A small and efficient microchip laser, wherein an inclined and coated plate of an internal cavity is provided to lock the polarization of the pump laser at the primary wavelength. The method of claim 1, And the cavity arrangement is folded for polarization control through the turning mirror and extraction of the second harmonic beam. The method of claim 22, And said secondary harmonic light is extracted from an inclined surface of said periodic polarized nonlinear crystal. The method of claim 1, At least one of the mirrors of the cavity is a curved surface. The method of claim 1, Wherein said pump laser is a continuous wave or pulsed laser. The method of claim 1, Wherein said laser cavity is operated in a pulsed situation, is obtained through a manual Q-switching or manual locked mode, and further comprises a saturable absorber element in said cavity. The method of claim 26, The saturable absorber element is Cr ^{4 +} : YAG, V ^{3 +} : YAG or Co ^{2 +} MgAl ₂ O ₄ Small, efficient microchip laser. The method of claim 26, And the saturable absorber element is an epitaxially grown semiconductor structure. The method of claim 1, Wherein said laser cavity is operated in an active Q-switched situation via an electro-optical Pockels cell.

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