Abstract:
In traveling-wave ring-resonator an optically nonlinear crystal for converting visible radiation to ultraviolet (UV) radiation has an input face and two output faces. The visible light propagates through the crystal from the input face to one of the output faces. That output face is coated with a dichroic optical coating that transmits unconverted visible light and reflects the ultraviolet light. The reflected ultraviolet light exits the optically nonlinear crystal via the other output face and is coupled out of the resonator at an angle to the resonator axis.

Description:
PRIORITY 
   This application claims priority of United Kingdom Patent Application No. 0608805.8, filed May 4, 2006, which is incorporated herein by reference. 
   TECHNICAL FIELD OF THE INVENTION 
   The present invention relates in general to frequency-converting laser radiation to provide output radiation at a wavelength shorter than the wavelength of the laser radiation being frequency converted. The invention relates in particular to generating ultraviolet radiation by frequency converting longer-wavelength radiation and coupling the ultraviolet radiation out of a resonator in which the longer-wavelength radiation is being frequency-converted. 
   DISCUSSION OF BACKGROUND ART 
   Intra-resonator frequency multiplication of solid-state or optically pumped semiconductor (OPS) radiation laser radiation in optically nonlinear crystals is commonly used to generate ultraviolet UV laser radiation. The frequency multiplication may be carried out within an active resonator in which the fundamental laser radiation is being generated or may be carried out separately in a passive traveling-wave ring resonator that provides enhancement of the frequency conversion by re-circulating the radiation being converted through the optically linear crystal such that radiation not converted to UV radiation generated after one pass can generate further UV radiation on a subsequent pass. 
   UV radiation must be separated from radiation being converted to provide UV radiation output of the resonator. This is usually effected by including a mirror including a dichroic coating in the laser resonator. Such a mirror may be one of the mirrors defining the resonator, i.e., either a terminating mirror or a mirror folding the resonator axis, in which case the dichroic coating would be arranged to transmit the UV radiation and reflect the radiation being converted. The mirror may also be a separate mirror with the dichroic coating arranged to transmit the radiation, usually visible radiation, being converted, and to reflect the UV radiation out of the resonator, transverse to the resonator axis. 
   Problems are often encountered with dichroic-coated elements as such elements cannot be made entirely loss free, and are typically more lossy the shorter the wavelength of the radiation. Further, optically nonlinear crystal materials used for converting radiation to UV wavelengths are subject to degradation by the UV radiation being generated. Such optically nonlinear crystal materials include, but are not limited to, β-barium borate (BBO) and cesium lithium borate (CLBO). These problems must be taken into account when deciding how to best accomplish the UV output separation. As UV degradation can not be entirely avoided, most commercial lasers in which such optically nonlinear crystals are used for UV generation usually include an arrangement for periodically moving the crystal as degradation appears on parts of the crystal through which the UV radiation passes. 
     FIG. 1  schematically illustrates one prior-art arrangement  10  that is used to generate UV radiation having a wavelength of 266 nanometers (nm) by frequency-doubling radiation having a wavelength of 532 nm. In arrangement  10  an optically nonlinear crystal  12 , has an entrance face  12  and an exit face  14 . The crystal, here, is assumed to be a BBO crystal. The crystal is cut such that 532 nm radiation incident on face  12  at about the Brewster angle θ B  (for that wavelength) travels along a longitudinal axis  16  of the crystal and exits face  14  also at the Brewster angle, i.e., faces  14  and  16  are parallel to each other. The 532 nm radiation is plane-polarized with the polarization plane being parallel to the plane of incidence of face  14  (p-polarized), as indicated by arrow P 1 . 266 nm radiation generated in the crystal follows the same path as the 532 nm radiation, and exits face  14  of the crystal at the Brewster angle for the 266 nm wavelength. There is a difference of approximately 1° between the exit angles of the 532 nm and 266 nm radiation. The 266 nm radiation is plane-polarized with the polarization plane being perpendicular to the plane of incidence of face  14 . 
   The 532 nm radiation and 266 nm radiation are incident at the Brewster Angle for 532 nm radiation on a beamsplitter  18  having front and rear surfaces  20  and  22  respectively. There is a dichroic coating  24  on front surface  20 . Rear surface  22  is uncoated. The dichroic coating reflects more than 95% of the incident 266 nm radiation and transmits more than 95% of the 532 nm radiation. By way of example, with commercially available coatings, the transmission of 532 nm radiation may be as high as 99.7% and the reflection of the 266 nm radiation may be as high as 96%. There is essentially no reflection of 532 nm radiation from surface  22  of the beamsplitter because of the Brewster-angle incidence of the radiation at the surface. Those skilled in the art will recognize, without further illustration or detailed description, that the path of the 532 nm-radiation depicted in  FIG. 1  would be collinear with the longitudinal axis of a resonator in which the crystal was located. 
   Exit surface  16  of crystal  12  creates about 20% loss of the 266 nm radiation. This is because the 266 nm radiation is polarized in a plane perpendicular to the plane of incidence of surface  16  (s-polarized) as indicated by arrowhead P 2 . It is possible, in theory at least, to reduce this loss by adding a suitable antireflection coating to surface  16 . It has been found, however, that in a passive ring-resonator, such a coating rapidly fails. It is believed that this failure is due to local heating in the coating by the 532 nm radiation. 
   Preferably coating  24  is deposited by a Q-Plate™ process. This process is an ion-assisted deposition process capable of producing coatings with very low surface roughness, for example, on the order of about 1.4 Ångstrom units (Å.U). Such coatings are available from Coherent, Inc., of Santa Clara, Calif., the assignee of the present invention. Dichroic coating  24 , deposited by the Q-Plate™ process, has exhibited a long lifetime in a passive ring resonator. Eventual UV degradation is, however, inevitable. This long lifetime, was also observed in a similar arrangement wherein 488 nm radiation is frequency-doubled to provide 244 nm UV radiation. The coating is highly efficient at extracting the 266 nm UV power with very loss of the 532 nm radiation. There is a need, however, for a UV out-coupling arrangement for a frequency-doubling resonator that does not exhibit the UV loss of the arrangement of  FIG. 1  and does not require an optically nonlinear crystal having antireflection coatings in the path of radiation circulating in the resonator. Preferably, the arrangement should allow for translation of the crystal when parts of the crystal surfaces are degraded by UV radiation. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to arrangements for coupling UV radiation out of a unidirectional optical resonator in which an optically nonlinear crystal is arranged to accept radiation having a first wavelength and propagating in one direction only and to convert a portion of the first-wavelength radiation to radiation having a second wavelength that is shorter than the first wavelength. One aspect apparatus in accordance with the present invention comprises an optical resonator having a resonator axis and arranged to cause a beam of optical radiation having a first-wavelength to circulate therein in one direction only along the resonator axis. An optically nonlinear crystal is located in the optical resonator on the resonator axis. The optically nonlinear crystal is arranged to convert the circulating first-wavelength wavelength radiation the second wavelength radiation, and to reflectively couple the second-wavelength radiation out of the resonator at an angle to the resonator axis. 
   In another aspect of the invention, the optically nonlinear crystal includes an input-face, and first and second output-faces. An optical coating is deposited on the first output-face. The optical coating is a dichroic coating that is transmissive for the first wavelength radiation and reflective for the second wavelength radiation. The input face and the first output face of the optically nonlinear crystal are arranged such that the first-wavelength radiation is transmitted through the optically nonlinear crystal to the first output face of the crystal. The first and second output faces of the optically nonlinear crystal are arranged such that an unconverted portion of the first wavelength radiation is transmitted out of the optically nonlinear crystal through the first output-face thereof, and such that second wavelength radiation reflected from the optical coating is transmitted out of the optically nonlinear crystal via the second output face thereof. 
   In a preferred embodiment of the invention described hereinbelow, the optically nonlinear crystal is arranged for frequency-doubling the first-wavelength radiation in a passive ring-resonator. The input face and the first output face of the optically nonlinear crystal are arranged such that the first-wavelength radiation is incident on the input face and the first output face at about the Brewster angle. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
       FIG. 1  schematically illustrates one prior-art arrangement for coupling UV radiation out of a frequency-doubling, traveling-wave, ring-resonator, the arrangement including an optically nonlinear crystal having an entrance face and an exit face for generating 266 nm (UV) radiation from 532 nm radiation by frequency-doubling, and a beamsplitter including a dichroic coating configured to transmit residual 532 nm along the resonator axis and reflect the 266 nm radiation away from the resonator axis as output radiation. 
       FIG. 2  schematically illustrates one preferred arrangement in accordance with the present invention for coupling UV radiation from a frequency-doubling ring-resonator, similar to the arrangement of  FIG. 1 , but wherein the beamsplitter is omitted and the dichroic coating is deposited on the exit face of the optically nonlinear crystal, with the 532 nm radiation being transmitted through the exit face along the resonator axis, and the reflected 266 nm radiation propagating laterally in the crystal and exiting the crystal via a lateral face thereof. 
       FIG. 3A  is a front elevation view schematically illustrating another preferred arrangement in accordance with the present invention for coupling UV radiation from a frequency-doubling ring-resonator, similar to the arrangement of  FIG. 2 , but wherein the lateral face of the optically nonlinear crystal is inclined in planes both perpendicular and parallel to the resonator axis. 
       FIG. 3B  is a right hand side elevation view schematically illustrating the arrangement of  FIG. 3A . 
       FIG. 4  is a three-dimensional view schematically illustrating further detail of faces of the optically nonlinear crystal in the arrangement of  FIGS. 3A and 3B . 
       FIG. 5A  is a front elevation view schematically illustrating yet another preferred arrangement in accordance with the present invention for coupling UV radiation from a frequency-doubling ring-resonator, similar to the arrangement of  FIGS. 3A and 3B , but wherein the lateral face of the optically nonlinear crystal is inclined only in a plane parallel to the resonator axis. 
       FIG. 5B  is a right hand side elevation view schematically illustrating the arrangement of  FIG. 5A . 
       FIG. 6  is a three-dimensional view schematically illustrating further detail of faces of the optically nonlinear crystal in the arrangement of  FIGS. 5A and 5B . 
       FIG. 7  schematically illustrates a preferred embodiment of a passive ring-resonator in accordance with the present invention, the resonator including the UV output coupling arrangement of  FIG. 2   
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Continuing with reference to the drawings, wherein like components are designated by like reference numerals,  FIG. 2  schematically illustrates a preferred arrangement  30  in accordance with the present invention for coupling UV radiation from a traveling wave frequency-doubling ring-resonator in which radiation being frequency doubled circulates in one direction only. Arrangement  30  is similar to above discussed arrangement  10  with an exception that dichroic beamsplitter  10  is omitted and the dichroic coating  24  is deposited instead on exit face of an optically nonlinear crystal  12 A. Crystal  12 A is similar to crystal  12  of  FIG. 1  with an exception that a lateral face  26  of the crystal, not intercepted by the resonator axis, is polished. Coating  24  preferably has specifications discussed above with reference to the dichroic beamsplitter of  FIG. 1 . 
   266 nm radiation generated in optically nonlinear crystal  12 A is reflected from exit face  16  back though the crystal and exits the crystal through lateral face  26  thereof. Face  26  can optionally be furnished with an antireflection coating  28  optimized for the appropriate polarization state and incidence angle of the 266 nm radiation. This coating, being on a face that is not intercepted by the resonator axis, will not be degraded by the 532 nm radiation circulating in the resonator. Such an antireflection coating may in fact provide a measure of protection for surface  28  from atmospheric moisture and the like. 
   It should be noted here that while Brewster θ B  is indicted in  FIG. 2 , and in other drawings referred to hereinbelow, as being on the air-side of faces of the optically nonlinear crystal, there is a corresponding Brewster angle within the crystal material which is 90-θ B . Reference to the Brewster angle in the following description and in the appended claims is applied interchangeably to both the air-side and material-side Brewster angles. Those skilled in the art will recognize from the context in which of the Brewster angles is referred to. 
     FIGS. 3A and 3B  schematically illustrate another arrangement  40  in accordance with the present invention for coupling UV radiation from a frequency-doubling resonator. Arrangement  40  is similar to above discussed arrangement  30  of  FIG. 2  with an exception that, in a crystal  12 B of arrangement, 266 nm-exit-face  26  of crystal  12 A is replaced by an exit face  26 A that is inclined in a plane of incidence perpendicular to the resonator axis such that the 266 nm radiation is incident thereon, in that plane, at about the Brewster angle for that wavelength (see  FIG. 3B ). Face  26 A is also inclined in a plane parallel to the resonator axis such that the 266 nm radiation is incident normally in that plane (see  FIG. 3A ). The arrangement of the crystal faces can be seen to advantage in the three-dimensional representation of  FIG. 4 . In this arrangement it is intended of course that face  26 A be uncoated. 
   It is possible in any of the above-described arrangements  30 , and  40  that the polarization plane P 2  of the 266 nm radiation reflected from face  16  undergoes some unpredictable rotation due to the birefringence of the crystal material and the length of the path traveled in the crystal by the 266 nm radiation. In this case, these arrangements may not be suitable, either because it would not be possible to select an appropriate inclination of the 266 nm exit face, or to design a suitable antireflection coating for the surface, each of which requires a precise knowledge of the polarization orientation.  FIG. 5A ,  FIG. 5B  and  FIG. 6  schematically illustrate another arrangement  50  in accordance with the present invention for coupling UV radiation from a frequency-doubling resonator. This arrangement is designed to accommodate such unpredictability of the polarization plane of the 266 nm radiation at the exit face. 
   Arrangement  50  is similar to arrangement  40  of  FIGS. 3A ,  3 B, and  4  with an exception that arrangement  50  includes a crystal  12 C in which has a 266 nm radiation exit face  26 B that is inclined only in a plane perpendicular to the resonator axis, and such that the 266 nm radiation is incident thereon at normal incidence in any two mutually perpendicular planes. This provides that the incident radiation will not be resolved into p-polarized and s-polarized components whatever the polarization orientation of the radiation. Accordingly, an antireflection coating thereon can be designed with very low reflection for unpolarized 266 nm radiation, and will be equally effective whatever the polarization orientation of the 266 nm radiation incident thereon. 
     FIG. 7  schematically illustrates a preferred embodiment of a passive ring-resonator in accordance with the present invention for enhancing conversion of 532 nm radiation to 266 nm formed by a plane input mirror  624 , two concave mirrors  64 , and a plane mirror  66  the axial position of which can be adjusted by a piezoelectric driver  68  or the like. Frequency doubling is achieved by an optically nonlinear crystal  12 A in the configured and arranged as described above within reference to  FIG. 2  The crystal has an axial length of about 10.0 mm and is located axially mid-way between concave mirrors  64 . All of the mirrors have high reflectivity for 532 nm radiation as is known in the art. 
   532 nm radiation to be frequency doubled is injected into the resonator via mirror  62 . The resonator length is adjusted by moving mirror  66  such that 532 nm radiation circulating along the longitudinal axis of the resonator is in-phase on subsequent round trips, i.e., such that the resonator is in a resonant condition. When the resonator is adjusted to this, essentially all 532 nm radiation incident on mirror  62  from outside of the resonator enters the resonator. One well known technique for providing this phase adjustment is the Pound-Drever technique which, briefly described, involves monitoring and minimizing back reflection from reflection the mirror via a closed loop electronic arrangement with driver  68 . A detailed description of this technique is not necessary for understanding principles of the present invention. Accordingly, no such description is presented herein. In one preferred configuration of resonator  60 , mirrors  64  have radius of curvature of about 50.0 mm and are spaced apart to form a unit magnification relay that focuses the circulating 532 nm beam to a narrow waist inside crystal  12 A. A particular advantage of the crystal arrangement is that the crystal and the beamsplitter are fixedly aligned. This simplifies shifting the crystal from time to time for exposing fresh portions of surfaces of the crystal to the 532 nm and 266 nm radiation. 
   Those skilled in the art will recognize that the while the above-discussed inventive arrangements are described in terms of converting 532 nm to 266 nm radiation by frequency doubling in a Brewster-cut optically nonlinear crystal (a BBO crystal in the above examples), principles of the invention are applicable to other resonant enhanced frequency-multiplication schemes wherein radiation is frequency converted in an optically nonlinear crystal for example sum-frequency mixing fundamental and second-harmonic radiation to provide third-harmonic radiation. It is also not necessary that entrance and exit faces of the optically nonlinear crystal be Brewster-cut. Any such arrangement, however, must be configured such that residual portions of radiation being converted are transmitted out of the crystal via one exit face, and that resultant frequency-converted radiation is reflected, from that exit face, out of the optically nonlinear crystal via another exit face that is not intercepted by the resonator axis. 
   It should also be noted that the resonator arrangement of  FIG. 7  is merely exemplary and should not be construed as limiting the present invention. In particular, it should be noted that while the resonator of  FIG. 7  is a passive resonator, with radiation to be converted injected into the resonator from without, principles of the invention are also applicable to an active unidirectional (traveling-wave) ring-resonator in which the first wavelength radiation is generated by energizing a gain-medium located in the resonator. 
   In summary, the present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.