Abstract:
The present invention provides for a low noise, intra-cavity frequency-doubling; diode pumped micro chip laser with wide temperature range comprising pumping diode, gain medium, nonlinear crystal and temperature compensation element. The gain medium pumped by diode generates fundamental wavelength and the nonlinear crystal acts as second harmonic generator. The condition for the laser to work in “low noise operation” is that the nonlinear crystal formed by birefringent materials keeps a quarter wave plate to fundamental wavelength. Since the phase retardation of the plate is strongly relied on temperature, the temperature range of the laser with low noise is narrow. To expend the working temperature range, an additional birefringent material is utilized to compensate variation in phase retardation of the nonlinear material with temperature. It makes the laser in low noise operation in wider temperature range. The additional birefringent plate compresses the number of longitudinal modes also that further reduces “green noise”.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to an intra-cavity laser source. More particularly, this invention relates to a low noise intra-cavity frequency doubling microchip laser operable over expanded temperature range.  
       BACKGROUND OF THE INVENTION  
       [0002]     Practical applications of the intra-cavity frequency-doubling laser source are still limited by a technical difficulty that the temperature for operating the laser sources must be controlled within a very limited range. The tight temperature control is necessary in order to circumvent a technical problem generally known as the “green noise”. With recent advancements made in many technical fields, there is an urgent demand to resolve such limitation. Since such laser sources have advantages of compact size, high-energy efficiency, stable frequency, high quality in light beam, low thermal effect, and long lifetime, the lasers source can be readily applied in biomedical and display. With these advantages, the diode pumped, intra-cavity frequency-doubling lasers have found ever-increasing fields of potential applications once the laser sources with low noise level are available. Specifically, the problems of the green noises are generated from the coupling of the longitudinal modes through cross saturation of the gain and sum-frequency mixing. However, even that such lasers have the above-mentioned advantages and that the laser sources are developed, the usefulness of the diode pumped, intra-cavity frequency-doubling solid state lasers are still limited due to the hindrance of the green noises. In order to resolve the problem of “green noise” as a long time headache, many technical approaches as further discussed below have been attempted.  
         [0003]     A seemingly simplistic way to overcome the problem is to create a laser system that operates in a single frequency. Such system by its own nature would more likely provide an operation condition that could minimize or even totally eliminate the problems of green noise. The drawback of single frequency operation is obviously the low energy efficiency, high cost and much tighter operation conditions. Single frequency laser operation thus is a self-defeating and non-practical proposition due to these intrinsic drawbacks.  
         [0004]     Another solution to eliminate the noises is to deal with the root cause of noise generation based on detail investigations of the characteristics of the optical interactions in the processes of optical resonance and frequency doubling taking place in the intra-cavity. In general, a diode pumped, multimode intra-cavity frequency-doubling laser with low noise as available now typically includes a birefringent gain medium, specially orientated birefringent nonlinear crystal. If the optical thickness and orientation of both gain medium and nonlinear crystal meet certain conditions, the green noise in second harmonic output laser radiation is compressed. However, since the geometric length and diffraction index of nonlinear crystal is strongly temperature-dependent, the conditions for low noise operations are easily broken with variation of environmental temperature. Normally, such laser requires expensive, high precision temperature controller to keep operation temperature of the laser in around 0.1° C.  
         [0005]     Therefore, a need still exists in the art of manufacturing and designing the laser sources to provide improved new and improved configuration and method to remove such stringent temperature control limitations.  
       SUMMARY OF THE PRESENT INVENTION  
       [0006]     It is therefore an object of the present invention to provide an improved design and configuration to extend temperature range for low noise operation to several decade times wider by introducing a piece of birefringent crystal such that the aforementioned difficulties and limitations in the prior art can be overcome.  
         [0007]     Specifically, the intra-cavity frequency-doubling laser includes a specially designed and configured birefringent crystal to compensate the variation in optical length of the nonlinear crystal with temperature and reduces the numbers of longitudinal modes in cavity.  
         [0008]     In a preferred embodiment, this invention discloses a compact, high efficiency visible laser system that includes a slice of laser gain crystal (e.g., Nd:YVO 4  or Nd:YAG) and a slice of nonlinear materials (e.g., KTP). The outside surfaces of the crystals of the combination are properly coated to form oscillation cavity. The two crystals form the laser core that is pumped longitudinally pumped by a laser diode. The Nd:YVO 4  is preferred as it is birefringent which benefits stability of intensity of the laser. 1  The KTP is prepared and orientated to facilitate type-II or type I phase matching for the doubling of the fundamental frequency of radiation. For the purpose of reducing the noises, research results have shown that when the principle axes of the gain and nonlinear crystals are at 45° angles to each other, and the optical thickness, the product of geometric length and refractive index, of the nonlinear crystal is a quarter wave plate (QWP) to fundamental wavelength, the polarizations of adjacent modes are orthogonal. Since the lights with orthogonal polarization do not couple each other and the coupling between modes are the source of “green noise, the micro chip laser generate stable output. This invention further provides a method to compensate the temperature effects of the length and refractive index since these parameters are strong temperature dependent. While temperature varies, the nonlinear crystal is not quarter wave plate anymore, additionally, the output wavelength changes with temperature also that contribute some shifting from exact QWP also. So the mode coupling occurs and green noise increases. For stable output, the present invention provides a method for this type of laser to expand temperature range of low noise operation. An additional birefringent material (e.g., YVO 4 ) is introduced into the cavity. Its principle axis is parallel to nonlinear plate (KTP). Since extraordinary index is larger than ordinary index (n e &gt;n o ) in KTP and YVO 4 , the introduced YVO 4  and KTP form a higher order of QWP. However, the variations of index difference (Δn=n e −n o ) with temperature have opposite signs in the two crystals, a proper length can be designed to keep the crystals as a QWP in much wider temperature range. So the microchip laser outputs are maintain as a stable radiation in wider range. In this invention, both YVO 4  and Nd: YVO 4  work as wavelength selector in cavity also, the function is similar to a Lyot filter. If a birefringent filter (e.g., Brewster plate) is added into cavity between gain material and combined QWP, the laser will work in single frequency.  
         [0009]     Briefly, in a preferred embodiment, the present invention includes a diode pumped, intra cavity frequency doubled microchip laser that includes a gain medium, a nonlinear frequency-doubling medium and at least one birefringent medium for temperature compensation. In a preferred embodiment, the gain medium further includes a gain medium composed of Nd:YVO 4  In another preferred embodiment, the non-linear frequency-doubling medium further includes a frequency-doubling medium composed of KPT. In another preferred embodiment, the temperature compensation birefringent medium further includes a temperature compensation birefringent medium composed of YVO 4.  In another preferred embodiment, the temperature compensation birefringent medium and the non-linear frequency-doubling medium optically constituting a combined quarter wave plate relative to a fundamental frequency of an intra-cavity of the microchip laser. In another preferred embodiment, the temperature compensation birefringent medium and the non-linear frequency-doubling medium optically constituting a combined quarter wave plate relative to a fundamental frequency of an intra-cavity of the microchip laser. And, a thickness of the temperature compensation birefringent medium and a thickness of the non-linear frequency-doubling medium are designed for maintaining operation characteristics of the combined quarter wave plate in a predefined temperature range. In another preferred embodiment, the gain medium further includes a gain medium composed of Nd:YAG. In another preferred embodiment, the gain medium further includes a gain non-linear frequency-doubling medium further includes a frequency-doubling medium composed of KN b O 3 . In another preferred embodiment, the non-linear frequency-doubling medium further includes a frequency-doubling medium composed of LBO. In another preferred embodiment, the non-linear frequency-doubling medium further includes a frequency-doubling medium composed of KTP. In another preferred embodiment, the temperature compensation birefringent medium further includes temperature compensation birefringent of YVO4. In another preferred embodiment, the temperature compensation birefringent medium further includes a temperature compensation birefringent crystal having a parallel optical axis relative to an optical axis of the non-linear frequency-doubling medium.  
         [0010]     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various drawing figures. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  presentation of intra cavity frequency doubled laser with stable output in broader temperature range  
         [0012]      FIG. 2 : Orientations of crystals in the laser of  FIG. 1   
         [0013]      FIG. 3 : Single longitudinal mode laser in broader temperature range 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]      FIG. 1  is functional block diagram for depicting the configuration of an intra-cavity frequency doubling laser source  100  of this invention. The frequency doubling laser source  100  includes a pumping laser diode  110  for projecting a laser in a frequency with a corresponding wavelength 808 nm or other shorter wavelength than the fundamental wavelength. In a nm or other shorter wavelength than the fundamental wavelength. In a prefer design, the pumping laser is 808 nm diode, the fundamental wavelength is 1064 nm and doubled frequency laser is 532 nm. The laser is projected onto a laser gain medium  120  composed of gain materials such as Nd:YVO 4 . Opposite to the gain medium  120  is a non-linear material  130  for generating a second harmonic radiation. The non-linear second harmonic generating medium  130  is composed of materials such as KTP. For the purpose of expanding the temperature range of the laser source applications, a temperature compensation crystal  125  is disposed between the laser gain medium  120  and the non-linear material  130 . The temperature compensation crystal  125  may be an YVO 4  crystal. The frequency-doubling laser  100  as shown may be implemented as a microchip to generate a laser output of different wavelengths such as red, green and blue laser. For the purpose of generating output laser of different wavelengths, the gain materials can be Nd:YAG, the nonlinear materials may be KN b O 3  and LBO or other kinds of non-linear materials. The temperature compensation plate may be made of other birefringent materials also. It is understood in the art that there are many possible combination of materials when implemented according to the configuration as disclosed in the present invention are able to generate the frequency doubling lasers as will be further discussed below. The specific examples and the use of designated materials as shown in the embodiments should not be considered as to limit the scopes of this invention.  
         [0015]     Referring to  FIG. 2  for the orientations of the crystals as depicted in  FIG. 1 . The principle axes of the temperature compensation crystal  125 , e.g., YVO 4 , and the non-linear frequency-doubling medium  130 , e.g., the KTP crystal, are parallel and are 45° degree against the optical axes of the gain medium  120 , e.g., the Nd:YVO 4 . The left end of the gain medium  120 , e.g., the Nd:YVO 4  slice, is coated to transmit the diode laser radiation projected from the diode laser  110 . The coating placed on the left surface of the gain medium slice  120  also reflects a laser at the fundamental wavelength of the gain medium, e.g., a fundamental wavelength 1.06 μm for the gain medium Nd:YVO 4 , and also reflects a laser of a second harmonic wavelength, e.g., a laser of wavelength 0.532 μm. The right side of the gain medium  120  is coated to transmit both fundamental and second harmonic wavelengths, but reflect diode laser radiation. Both side surface of the temperature compensation crystal  125 , e.g., the YVO 4  crystal, and left side of the frequency doubling non-linear medium  130 , e.g., the KTP slice, are coated with thin film to transmit both fundamental and second harmonic wavelengths. A method of applying optical glue can also be used and an antireflection (AR) coating is not required on the optically glued surfaces since the crystals have nearly a same refraction index and the reflection loss is small.  
         [0016]     The right side of frequency-doubling medium  130 , e.g., the KTP crystal, is coated with a thin film layer to transmit a laser output at a second harmonic wavelength and reflect the laser projection at the fundamental wavelength. The gain crystal  120 , the nonlinear frequency doubling crystal  130  and the birefringent temperature compensation crystal  125  thus constitute a resonant cavity to double the fundamental frequency. The relative positions of all three slices can be arbitrarily exchanged each other, provided their surface coating should be changed correspondingly to form resonant cavity for fundamental wavelength and output second harmonic radiations. Additionally, the two end surfaces of end crystals can be curved to form special types of resonant cavity. Many possible reconfigurations and variations of the basic concept as disclosed are within the scope of the present invention.  
         [0017]     Further investigations have indicated noises of output lasers from a laser cavity are closely related to the mode coupling. For the intra-cavity frequency-doubling laser source as described above, there is a strong correspondence between the relative polarizations of modes in the microchip laser cavity and the presence or absence of “green noise”. The wavelength and polarizations of modes in the cavity can be described by Jones matrix. In order to generate a stable output from a microchip laser, there is a requirement that the eigenvectors of the Jones matrix must have orthogonal polarizations. In order to achieve the conditions to have orthogonal polarization of eigenvector, it is required that the temperature compensation crystal  125 , e.g., the YVO 4  plate and the non-linear frequency doubling crystal  130 , e.g., the KTP plate, are combined to optically become a quarter wave plate to fundamental wavelength. Under that condition, two eignestates of the Jones matrix are orthogonal each other wherein each eigenstate corresponds to one longitudinal mode of the laser cavity. With longitudinal modes orthogonal to each other, the laser cavity as shown provides an advantageous effect to compress the “green noise” that arises from the coupling of longitudinal modes through cross saturation of the gain and sum-frequency mixing since the modes with orthogonal polarization do not couple to each other according to principles of optics.  
         [0018]     In order to optically generate a combined quarter wave plate in a wider temperature range, the optical thickness of the KTP crystal and the YVO 4  crystal must first satisfy the following equation:  
             nl   =           l   k     ·     (       n   ke     -     n   ko       )       +       l   y     ·     (       n   ye     -     n   yo       )         =       (     m   +     1   4       )     ⁢   λ               (   1   )             
 
         [0019]     Where l k  and l y  are geometric thickness of KTP and YVO 4  respectively, n ke  and n ye  are extraordinary index, n ko  and n yo  are ordinary index of KTP and YVO 4  respectively. All above factors are function of temperature. The m is an integer. There is a freedom to select m based on consideration of wavelength selection. Solving above equation can provide the ratio of geometric thickness of KTP to YVO 4 . If m is selected, the thickness of KTP and YVO 4  can be determined. It is hard to keep combined crystal be QWP in all temperature range; however, test results show that the temperature range of low noise operation in the microchip laser is expanded from 0.1° C. to 4° C. compared to single piece of KTP. In this prefer embodiment, d(nk ke −n ko )/dT&gt;0 and d(n ye −n yo )/dT&lt;0, that guarantees above equation has solutions.  
         [0020]     On the left side of equation 1, first item is the optical length of KTP, and the second item is the optical length of YVO 4 . Two items form an m order of quarter wave plate to fundamental wavelength. When temperature increases, since d(n ke −n ko )/dT&gt;0 and l k /dT is always larger than zero, so the first item in left side of equation increases. The combined quarter wave plate is not QWP anymore to fundamental wavelength. However, with temperature increase, the second item in the equation will decrease since d(n ye −n yo )/dT&lt;0, even the l y /dT considering that the geometrical thickness variation under temperature is much less than variation in index. That means that when temperature varies, the first item in equation increase, but the second item decreases, the total optical length will keep unchanged in or changes less in certain temperature range. The combined QWP could be kept in wider temperature range. There are many candidates of crystal scan be selected as temperature compensator.  
         [0021]     In prefer embodiment, the optical axis of nonlinear crystal and compensation crystal are parallel, that form a high order QWP. In this case the variation in laser wavelength needs to be considered. Longitudinal mode condition in laser cavity is:  
             NL   =           n     nd   ⁢     :     ⁢   y       ·     l     Nd   ⁢     :     ⁢   y         +       n   y     ·     l   y       +       n   k     ·     l   k         =     K   ·     λ   2                 (   2   )             
 
         [0022]     Since all above crystals are birefringent, and adjacent mode has orthogonal polarization, equation (2) split into two equations for orthogonal modes that:  
               NL   o     =           n     nd   ⁢     :     ⁢   yo       ·     l     nd   ⁢     :     ⁢   y         +       n   yo     ·     l   y       +       n   ko     ·     l   k         =     K   ·     λ   2                 (   3   )                 NL   e     =           n     nd   ⁢     :     ⁢   ye       ·     l     nd   ⁢     :     ⁢   y         +       n   ye     ·     l   y       +       n   ke     ·     l   k         =     K   ·     λ   2                 (   4   )             
 
         [0023]     Where n nd:yo  and n nd:ye  are ordinary and extraordinary index of gain medium of Nd:YVO 4  respectively, and l nd:y  is the geometric thickness of the crystal. For temperature stability, the variation of NL o  and NL e  should be zero.  
                   ⅆ     (     NL   0     )         ⅆ   T       =   0     ⁢     
     ⁢   And           (   5   )                   ⅆ     (     NL   e     )         ⅆ   T       =   0           (   6   )             
 
         [0024]     There will be no such ideal materials that could meet “low noise” conditions in all temperature range. It can be reasonably suppose that there is no mode hopping in working temperature range so the dK/dT=0. The variations in indexes against temperature are function of temperature also that could be basic data of materials or can be precisely measured and can be taken as constant at certain temperature in equation 5 and 6. Therefore three equations 2, 5, and 6 can determine suitable thickness of each crystal. A microchip laser formed by combination of such crystals will operate with low noise in wider temperature range.  
         [0025]     Referring to  FIG. 3  for an alternate preferred embodiment, wherein the design as that shown in  FIGS. 1 and 2  is implemented in single mode microchip laser. A polarizer  140  is inserted in the cavity to manage the difference between the single mode laser from the multimode laser. The polarizer  140  as shown can be Glan-Taylor prism or any other polarization prisms or Brewster plate. It is obvious that any other combinations of two even more crystals, which can be either same crystal with orthogonal orientations or different crystals with properly, arranged orientations might be used to achieve the same functional results. These alternates embodiments and variations of different implementation configurations are within the scope of this invention.  
         [0026]     This invention therefore discloses a diode pumped, intra cavity frequency-doubled microchip single-frequency laser that includes a gain medium, a nonlinear frequency-doubling medium and a birefringent medium for temperature compensation and birefringent filtering. In a preferred embodiment, the birefringent medium for temperature compensation and birefringent filtering further includes a polarizer. In another preferred embodiment, the polarizer further includes a polarizer including birefringent prisms. In another preferred embodiment, the polarizer further includes a Glan-Taylor polarizer including birefringent prisms. In another preferred embodiment, the birefringent medium for temperature compensation and birefringent filtering further includes a Brewster plate. In another preferred embodiment, the temperature compensation birefringent medium and the non-linear frequency-doubling medium optically constituting a combined quarter wave plate relative to a fundamental frequency of an intra-cavity of the microchip laser. In another preferred embodiment, the temperature compensation birefringent medium and the non-linear frequency-doubling medium optically constituting a combined quarter wave plate relative to a fundamental frequency of an intra-cavity of the microchip laser. And, a thickness of the temperature compensation birefringent medium and a thickness of the non-linear frequency doubling medium are designed for maintaining an operation characteristics of the combined quarter wave plate in a predefined temperature range  
         [0027]     Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.