Abstract:
A medical laser apparatus for performing treatment by irradiating an affected part with a laser beam comprises: a fiber laser source with variable output power, which emits an infrared fundamental laser beam; a wavelength converting element which wavelength-converts the fundamental beam to a visible second harmonic laser beam and is placed on an optical axis of the fundamental beam; a first incident angle changing unit which changes an incident angle of the fundamental beam to the wavelength converting element; an optical axis deviation correcting unit which corrects deviation of an optical axis of the second harmonic beam caused by a change of the incident angle of the fundamental beam to the wavelength converting element; and a controller which controls the first changing unit and the correcting unit based on a change in power of the fundamental beam.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a medical laser apparatus for use in an ophthalmic field and others.  
         [0003]     2. Description of Related Art  
         [0004]     In the ophthalmic field, there is a laser apparatus adapted to wavelength-convert an infrared fundamental laser beam emitted from a laser source to a visible second harmonic laser beam by a wavelength converting element, and perform treatment with the visible beam (or a visible or ultraviolet laser beam generated by additional wavelength-conversion).  
         [0005]     Such laser apparatus has some problems if a fiber laser source is used as the laser source. Specifically, a central wavelength of the fundamental beam tends to shift due to a change in power thereof, resulting in a deterioration in wavelength-conversion efficiency of the wavelength converting element in converting the fundamental beam to the second harmonic beam.  
         [0006]     In particular, the apparatus using a Raman fiber laser source by stimulated Raman scattering effect would cause a large shift of a central wavelength of a fundamental beam due to a change in power thereof.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The present invention has been made in view of the above circumstances and has an object to provide a medical laser apparatus capable of efficiently and stably performing wavelength-conversion of a fundamental laser beam to a second harmonic laser beam even when a central wavelength of the fundamental laser beam shifts due to a change in power thereof.  
         [0008]     Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.  
         [0009]     To achieve the purpose of the invention, there is provided a medical laser apparatus for performing treatment by irradiating an affected part with a laser beam, the apparatus comprising: a fiber laser source with variable output power, which emits an infrared fundamental laser beam; a wavelength converting element which wavelength-converts the fundamental beam to a visible second harmonic laser beam and is placed on an optical axis of the fundamental beam; a first incident angle changing unit which changes an incident angle of the fundamental beam to the wavelength converting element; an optical axis deviation correcting unit which corrects deviation of an optical axis of the second harmonic beam caused by a change of the incident angle of the fundamental beam to the wavelength converting element; and a controller which controls the first changing unit and the correcting unit based on a change in power of the fundamental beam.  
         [0010]     According to another aspect, the present invention provides a medical laser apparatus for performing treatment by irradiating an affected part with a laser beam, the apparatus comprising: a fiber laser source with variable output power, which emits an infrared fundamental laser beam; a wavelength converting element which wavelength-converts the fundamental beam to a visible second harmonic laser beam and is placed on an optical path of the fundamental beam, the wavelength converting element being placed rotatably or swingablly so that an angle of an entrance face of the wavelength converting element with respect to an optical axis of the fundamental beam is changed in a range enabling incidence of the fundamental beam on the entrance face of the wavelength converting element; and an optical member which corrects deviation of an optical axis of the second harmonic beam and is placed on an optical path of the second harmonic beam, the optical member being placed rotatably or swingablly so that an angle of an entrance face of the optical member with respect to an optical axis of the second harmonic beam is changed in a range enabling incidence of the second harmonic beam on the entrance face of the optical member. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The accompanying drawings, which are incorporated in and constitute a part of this specification illustrate an embodiment of the invention and, together with the description, serve to explain the objects, advantages and principles of the invention.  
         [0012]     In the drawings,  
         [0013]      FIG. 1  is a schematic structural view of an ophthalmic laser photocoagulation apparatus in the present embodiment;  
         [0014]      FIG. 2  is an explanatory view showing an adjustment method, using two wavelength converting elements, for a shift of a central wavelength of a fundamental laser beam caused by a change in power thereof; and  
         [0015]      FIG. 3  is an explanatory view showing an adjustment method, using a single wavelength converting element and a single optical member, for a shift of a central wavelength of a fundamental laser beam caused by a change in power thereof.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     A detailed description of a preferred embodiment of the present invention will now be given referring to the accompanying drawings.  FIG. 1  is a schematic structural view of an ophthalmic laser photocoagulation apparatus in the present embodiment. The present invention may be applied to not only the ophthalmic laser photocoagulation apparatus but another laser apparatus for ophthalmic treatment or a laser apparatus for use in another medical field.  
         [0017]     A fiber laser source  1  which emits an infrared fundamental laser beam includes a plurality of laser diodes (LD)  2  for excitation and a Yb-doped fiber  3  for a resonator. The fiber  3  is formed with for example two fiber Bragg gratings (FBG)  4   a  and  4   b  forming the resonator to emit the fundamental beam having a central wavelength λ 1  of 1160 nm by excitation light having a wavelength of 890 nm from the LD  2 . The power of the fundamental beam to be emitted from the laser source  1  is adjusted when a controller  40  serving as control means controls the power of the excitation light from the LD  2 .  
         [0018]     On an optical path of the fundamental beam emitted from the laser source  1 , there are arranged a collimator lens  11  and a first wavelength converting element  13 . The fundamental beam is made into a parallel or converged beam by the lens  11  to enter the first wavelength converting element  13  in which the beam is wavelength-converted to a visible second harmonic laser beam having a central wavelength λ 2  of 580 nm.  
         [0019]     On an optical path of the second harmonic beam emerging from the first wavelength converting element  13 , a second wavelength converting element  15  is arranged. The second harmonic beam enters the second wavelength converting element  15  and passes therethrough without conversion. Further, a remaining fundamental beam having not been wavelength-converted by the first wavelength converting element  13  enters the second wavelength converting element  15  in which the beam is wavelength-converted to the second harmonic beam.  
         [0020]     On an optical path of the second harmonic beam emerging from the second wavelength converting element  15 , there are arranged a dichroic mirror  17  serving as a beam splitter and having a property of reflecting infrared light and transmitting visible light, a half mirror  18  serving as a beam splitter and having a property of reflecting a part of visible light but transmitting most part of the visible light, a condensing lens  19 , and an optical fiber  50 . A damper  30  for the fundamental beam is placed in a reflection direction by the mirror  17 . Further, a sensor  31  for detecting the power of the second harmonic beam is placed in a reflection direction by the mirror  18 . The second harmonic beam passes through the mirror  17  and then most part of the beam passes through the mirror  18  to enter the fiber  50  via the lens  19 . A part of the second harmonic beam reflected by the mirror  18  enters the sensor  31 . A remaining fundamental beam having still not been wavelength-converted by the second wavelength converting element  15  is reflected by the mirror  17  to enter the damper  30 .  
         [0021]     The second harmonic beam having entered the fiber  50  is then irradiated to a fundus of a patient&#39;s eye E through a light delivery optical system  52  and a contact lens  65 . The light delivery optical system  52  includes a relay lens  53 , a zoom lens system  54  for changing the spot size of the beam, an objective lens  55 , and a mirror  56  which reflects the beam to the eye E. This light delivery optical system  52  is attached to a slit lamp  60  having a binocular microscope part  61  and an illumination part  62 .  
         [0022]     Used as the first wavelength converting element  13  is a quasi phase matching element (hereinafter, referred to as “QPM element”) having a periodic polarization inverting structure. As a material of this QPM, LiNb0 3 , LiTa0 3 , KTiOP0 4 , or a crystal is preferably used. The QPM element serving as the first wavelength converting element  13  is structured such that a polarization direction of a nonlinear optical material is inverted alternately at every coherence length to satisfy quasi phase matching. The periodic structure is determined so as to efficiently wavelength-convert a fundamental beam to a second harmonic beam. The QPM element as the first wavelength converting element  13  includes a beam entrance face  13   a  and a beam exit face  13   b  parallel to each other.  
         [0023]     As the second wavelength converting element  15 , an identical QPM element to the first wavelength converting element  13  is used. In this case, however, it is preferable to perform phase correction between the first and second wavelength converting elements  13  and  15 . For the second wavelength converting element  15  of a bulk type, no phase correction is required. Thus, an apparatus structure can be simplified. The QPM element serving as the second wavelength converting element  15  similarly includes a beam entrance face  15   a  and a beam exit face  15   b  parallel to each other.  
         [0024]     The first wavelength converting element  13  is mounted on a rotating base  14  whose rotation center C 1  is on a reference optical axis L of the beam. This rotation center C 1  is located at the center of the first wavelength converting element  13 . The rotating base  14  is rotated by a rotation driving part  20  including a motor, a gear, and others. Simultaneously the first wavelength converting element  13  is also rotated about the rotation center C 1 , changing the angles of the entrance face  13   a  and the exit face  13   b  with respect to the optical axis L. Similarly, the second wavelength converting element  15  is mounted on a rotating base  16  whose rotation center C 2  is on the optical axis L. This rotation center C 2  is located at the center of the second wavelength converting element  15 . The rotating base  16  is rotated by a rotation driving part  22  including a motor, a gear, and others. Simultaneously the second wavelength converting element  15  is also rotated about the rotation center C 2 , changing the angles of the entrance face  15   a  and the exit face  15   b  with respect to the optical axis L. Note that the reference optical axis L corresponds to an optical axis of the fundamental beam emitted from the laser source  1  in the case where the wavelength converting elements  13  and  15  are absent and also to an optical axis of the wavelength-converted second harmonic beam in the case where the wavelength converting elements  13  and  15  are arranged colinear (where each entrance face and each exit face are perpendicular to the optical axis).  
         [0025]     Furthermore, a temperature controller  21  for regulating (changing) the temperature of the first wavelength converting element  13  is mounted on the rotating base  14 . A temperature controller  22  for regulating (changing) the temperature of the second wavelength converting element  15  is mounted on the rotating base  16 . Each of the temperature controllers  21  and  23  is constituted of e.g. a Peltier device. The wavelength converting elements  13  and  15  are maintained at a predetermined constant temperature by the temperature controllers  21  and  23  respectively so that their periodic structures will not vary with temperature changes.  
         [0026]     The laser source  1 , the rotation driving parts  20  and  22 , the temperature controllers  21  and  23  are connected to the controller  40  which controls them individually. Further connected to the controller  40  are an input part  41  for inputting beam irradiation conditions and others such as the power of the second harmonic beam to be used as a treatment beam, a trigger switch  42  to start beam irradiation, and a memory  43 .  
         [0027]     The following explanation will be made on an adjustment method for a shift of the central wavelength of the fundamental beam caused by a change in power thereof.  
         [0028]     For adjustment for the case where the central wavelength λ 1  of the fundamental beam shifts to a long wavelength side or a short wavelength side, a reference position of the first wavelength converting element  13  is a position inclined at a predetermined angle θo (e.g. 5°) to the optical axis L as shown in  FIG. 2 . The periodic structure of the first wavelength converting element  13  is determined so that the fundamental beam is wavelength-converted to the second harmonic beam when passes through the first wavelength converting element  13  in this position. This determination is also made by incorporating a variation in optical path length by refraction, as will be mentioned later. Similarly, a reference position of the second wavelength converting element  15  is a position inclined at the angle θo in a direction opposite to the first wavelength converting element  13  with respect to the optical axis L.  
         [0029]     The case where the central wavelength λ 1  of the fundamental beam shifts to the long wavelength side is first explained below.  
         [0030]     When the rotating base  14  is rotated at a rotation angle θi with respect to the optical axis L to satisfy a relation; θi&gt;θo, an incident (entrance) angle of the fundamental beam with respect to the entrance face  13   a  of the first wavelength converting element  13  (i.e. an angle with respect to the normal to the entrance face  13   a ) is inclined to the angle θi. The fundamental beam having entered the first wavelength converting element  13  in this position travels longer through the inside of the first wavelength converting element  13  than the case where the first wavelength converting element  13  is inclined at the angle θo. Specifically, an optical path for wavelength-conversion corresponding to an equivalently long wavelength is formed in the first wavelength converting element  13 . By controlling the angle θi to provide the optical path length corresponding to the shift amount of the central wavelength λ 1  of the fundamental beam, it is possible to prevent a deterioration in wavelength-conversion efficiency. An angle θr (an angle with respect to the normal to the entrance face  13   a ) of the beam passing through the inside of the first wavelength converting element  13  inclined at the angle θi can be calculated based on a known refractive index of the first wavelength converting element  13 . Based on this angle θr, the optical path length for wavelength-conversion in the inside of the first wavelength converting element  13  can also be calculated.  
         [0031]     When the first wavelength converting element  13  is inclined at the angle θi, the optical axis of the second harmonic beam (and the remaining fundamental beam) emerging from the exit face  13   b  will deviate from the optical axis L. To correct this deviation, accordingly, the rotating base  16  is rotated at the angle θi with respect to the optical axis L in the opposite direction to the rotation angle θi of the rotating base  14 , thereby inclining an incident (entrance) angle (an angle with respect to the normal to the entrance face  15   a ) of the second harmonic beam (and the remaining fundamental beam) emerging from the exit face  13   b  of the first wavelength converting element  13  to an angle θi in the same opposite direction as above with respect to the entrance face  15   a  of the second wavelength converting element  15 . Thus, the second harmonic beam (and the remaining fundamental beam) entering the entrance face  15   a  of the second wavelength converting element  15  is deflected at the angle θr. Since the first and second wavelength converting elements  13  and  15  are equal in length, the optical axis of the second harmonic beam (and the remaining fundamental beam) emerging from the exit face  15   b  of the second wavelength converting element  15  is returned to the optical axis L.  
         [0032]     When the optical axis of the second harmonic beam emerging from the second wavelength converting element  15  is returned to the optical axis L, the second harmonic beam is made by the lens  19  to enter the fiber  50  while the beam is in the same condition as before the optical path length is changed. This makes it possible to solve disadvantages resulting from entrance deviation. In other words, if the beam as deviated enters the fiber  50 , a beam transmission efficiency of the fiber  50  will become down. Further, this may cause inhomogeneous beam intensity distribution, resulting in difficulty in forming an uniform coagulation spot.  
         [0033]     Next, the case where the central wavelength λ 1  of the fundamental beam shifts to the short wavelength side is explained below.  
         [0034]     In this case, contrary to the above mentioned case, the rotating base  14  is rotated at the rotation angle θi with respect to the optical axis L to satisfy a relation; θi&lt;θo. Accordingly, the optical path length of the fundamental beam which travels through the inside of the first wavelength converting element  13  is shorter than the case where the first converting element  13  is inclined at the angle θo. The rotating base  16  is thus rotated at the angle θi in the opposite direction to the rotation angle θi of the rotating base  14 . This makes it possible to correct the deviation of the optical axis caused by inclination of the first wavelength converting element  13 .  
         [0035]     Inclining each of the wavelength converting elements  13  and  15  as mentioned above makes it possible to adjust the problem that the central wavelength λ 1  of the fundamental beam shifts. As another adjustment method for this problem, a method of controlling the temperature of the first wavelength converting element  13  is also conceivable. However, in the temperature control, it is necessary to largely change the temperature as the power of the fundamental beam is largely changed. It therefore problematically takes long before the changed temperature becomes stable at a constant temperature. On the other hand, the method that the wavelength converting elements  13  and  15  are inclined as described above can rapidly adjust such problem by electrical driving of a motor or the like.  
         [0036]     In the above embodiment, the wavelength converting elements  13  and  15  are maintained at the predetermined temperatures by the temperature controllers  21  and  23  respectively. Further, addition of a temperature control function is more convenient. For instance, in order to adjust a fine wavelength-shift over a long term resulting from drifts, the temperature controllers  21  and  23  may be adapted to precisely control the temperature to be maintained constant so that the power of the second harmonic beam to be detected by the sensor  31  is maximum.  
         [0037]     Operations of the apparatus during treatment will be explained below. For treatment, the beam irradiation conditions are set with the input part  41 . In the present embodiment, the power of the second harmonic beam serving as the treatment beam can be set in a range of 50 mmW to 1.4 W. Assuming that the wavelength-conversion efficiency of each of the wavelength converting elements  13  and  15  is 20%, the power of the fundamental beam from the laser source  1  is adjusted in a range of 250 mmW to 7 W. Stored in the memory  43  is a relationship between a change in power of the fundamental beam from the laser source  1  (a change in power of the excitation light, i.e. a change in driving current of the LD  2 ), which have been obtained in advance by experiment or the like, and the rotation angle θi of each of the rotating bases  14  and  16  to incline the associated wavelength converting elements  13  and  15 . When the power of the treatment beam (the second harmonic beam) is set, the controller  40  calculates the corresponding driving current of the LD  2  and reads out the angle θi corresponding to the power of the fundamental beam to be emitted from the laser source  1  by driving of the LD  2 , and controls the driving of the rotation driving parts  20  and  22  to rotate the rotating bases  14  and  16  respectively, thereby inclining the associated wavelength converting elements  13  and  15 .  
         [0038]     When a command signal for beam irradiation is inputted with a switch on the input part  41 , the controller  40  drives the LD  2  to emit the fundamental beam from the laser source  1  and open a shutter not shown placed between the mirror  18  and the lens  19  (or move the shutter out of the optical path). Thus, the second harmonic beam is allowed to enter the fiber  50 . The power of the second harmonic beam is detected by the sensor  31 . The controller  40  fine adjusts the driving of the LD  2  to make the power stable at the set power. Further, it is preferable to further control the driving of the rotation driving parts  20  and  22  in accordance with the fine adjustment of the LD  2  to adjust the inclination of the wavelength converting elements  13  and  15  respectively.  
         [0039]     The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof.  
         [0040]     For instance, in the above explanation, the rotation center C 1  of the rotating base  14  is centrally located in the first wavelength converting element  13  and the rotation center C 2  of the rotating base  16  is centrally located in the second wavelength converting element  15 . However, if the rotation angle θi is too large, the entrance face  13   a  of the first wavelength converting element  13  and the exit face  15   b  of the second wavelength converting element  15  will deviate from the optical axis L. To avoid this, the rotation center C 1  may be located on the entrance face  13   a  and the rotation center C 2  may be located on the exit face  15   b.    
         [0041]     As the means for correcting deviation of the optical axis of the second harmonic beam, an optical member having a refractive index and length equal to those of the first wavelength converting element  13  and having parallel faces serving as a beam entrance face and a beam exit face (i.e. an optical member enabling at least the second harmonic beam to pass therethrough) may be used instead of the second wavelength converting element  15  identical to the first wavelength converting element  13 . In this case, the control of inclination of the optical member may be performed in a similar way to the control over the second wavelength converting element  15 .  
         [0042]     In the case where an optical member different in refractive index from the first wavelength converting element  13  is used instead of the second wavelength converting element  15 , the difference in refractive index may be adjusted based on at least one of the length (a distance between the parallel entrance and exit faces) and the inclination of the optical member.  FIG. 3  shows an example that, as the means for correcting the optical axis deviation, an optical member (an optical element)  70  different in refractive index from the first wavelength converting element  13  is used instead of the second wavelength converting element  15 . In this example, the rotation center C 1  is located on the entrance face  13   a  of the first wavelength converting element  13  and the rotation center C 2  is located on a beam exit face  70   b  of the optical element  70 . In this figure, the rotating bases and their rotation driving parts are not shown.  
         [0043]     The optical member  70  is inclined at an angle θi 2  to return the deviated optical axis of the second harmonic beam emerging from the exit face  13   b  of the first wavelength converting element  13  inclined at an angle θi 1  to the optical axis L. This angle θi 2  is determined based on a relationship between a refractive index of the optical member  70  to the second harmonic beam and the length of the optical member  70  (a distance between a beam entrance face  70   a  and the exit face  70   b ). The angle θi 2  with respect to the angle θi 1  has been calculated in advance and stored in the memory  43 .  
         [0044]     In the above explanation, the optical axis of the lens  19  whereby the second harmonic beam is allowed to enter the fiber  50  is located to be coaxial with the optical axis L (the optical axis of the lens  11 ). However, both optical axes do not always have to be coaxial. In the case where the optical axis of the lens  19  has been deviated originally from the optical axis L, the inclination of the correcting member for optical axis deviation such as the second wavelength converting element  15  and the optical member  70  has only to be adjusted to conform with the optical axis of the lens  19 .  
         [0045]     In the above explanation, the first and second wavelength converting elements  13  and  15  and the optical member  70  are configured to have a beam entrance face and a beam exit face parallel to each other. This is because when the entrance face and the exit face are parallel to each other, an outgoing beam from the exit face shifts in parallel from an incoming beam on the entrance face, so that the inclination angle for correcting the optical axis deviation can be calculated easily; however, another configuration may also be adopted. Even when the entrance face and the exit face are not parallel to each other, if a relation between them is well known, the optical axis direction of the outgoing beam from the exit face can be calculated based on that relation and the inclination angle of the optical axis deviation correcting member such as the second wavelength converting element  15  and the optical member  70  can be determined to correct the optical axis deviation.  
         [0046]     In the above explanation, the first and second wavelength converting elements  13  and  15  and the optical member  70  are inclined by rotation, but they may be swung in a range that at least the beam may enter it.  
         [0047]     While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.