Laser apparatus

A laser apparatus capable of emitting laser beams of a plurality of different wavelengths includes: a solid-state laser medium which emits light of a plurality of different peak wavelengths; a resonance optical system which resonates the emitted light of the plurality of different peak wavelengths and converts respective light to oscillate the laser beams of the plurality of different wavelengths; and a ¼ wave plate for a wide band, which is placed in the resonance optical system and has a property of providing a uniform phase difference to the light of the plurality of different peak wavelengths to be converted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser apparatus capable of emitting laser beams of a plurality of different wavelengths.

2. Description of Related Art

As a laser apparatus capable of emitting laser beams of a plurality of different wavelengths, there have been known an apparatus using a solid-state laser and a wavelength converting element (a wavelength selecting element), in addition to an apparatus using an argon dye laser, a krypton laser, or the like. In the laser apparatus using the solid-state laser, the effect of thermal birefringence (or thermally induced birefringence) may be caused by heat generation of a laser crystal (a laser medium) itself, leading to a loss in a resonator, with the result that laser output power is reduced.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a laser apparatus capable of efficiently and stably emitting laser beams different in wavelength.

To achieve the purpose of the invention, there is provided a laser apparatus capable of emitting laser beams of a plurality of different wavelengths, the apparatus including: a solid-state laser medium which emits light of a plurality of different peak wavelengths; a resonance optical system which resonates the emitted light of the plurality of different peak wavelengths and converts respective light to oscillate the laser beams of the plurality of different wavelengths; and a ¼ wave plate for a wide band, which is placed in the resonance optical system and has a property of providing a uniform phase difference to the light of the plurality of different peak wavelengths to be converted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A detailed description of a preferred embodiment of a laser apparatus embodying the present invention will now be given referring to the accompanying drawings. The following explanations are made on a laser apparatus which selectively emits laser beams of three wavelengths.FIG. 1is a perspective view of an ophthalmic photocoagulation laser apparatus using a slit lamp.FIG. 2is a schematic diagram of an optical system and a control system in the apparatus.

A main body1of the laser apparatus houses a laser oscillator (a resonator)10, a part of a light delivery optical system for delivering a laser beam to an affected part of a patient's eye to irradiate the affected part, a control section20, and others. A control box2is provided thereon with a wavelength selection switch2ato be used for selecting a wavelength of a laser beam and other various switches for setting laser irradiation conditions. A footswitch3is used for generating a trigger signal to start laser irradiation.

A slit lamp4contains an observation optical system for allowing an operator to observe the patient's eye and a part of the light delivery optical system. An optical fiber5is used to deliver the laser beam from the main body1to the slit lamp4. A stand6mounts thereon the slit lamp4and is moved in a vertical direction.

The laser oscillator10is internally provided with an Nd:YAG crystal11(hereinafter, referred to as “rod”) which is a solid-state laser medium serving as a laser oscillating source, a laser diode12(hereinafter, referred to as “LD”) serving as an exciting light source, nonlinear crystals13a,13b, and13c(hereinafter, referred to as “NLC”) serving as wavelength converters (wavelength converting elements), a ¼ wave plate16for a wide band, total reflection mirrors (high reflectors)14ato14f(hereinafter, referred to as “HR”), and an output mirror15.

The Nd:YAG crystal emits light having a plurality of oscillation lines (peak wavelengths) in the infrared region by an exciting light from the exciting light source. Therefore, the apparatus in the present embodiment is constructed such that each second harmonic light of three oscillation lines; about 1064 nm, about 1123 nm, about 1319 nm (hereinafter, “about” is omitted), which are the wavelengths with high power among the plural oscillation lines emitted from the above crystal, is generated with the use of the nonlinear crystal, thereby emitting (oscillating) laser beams of three different wavelengths (three different colors) of about 532 nm (green), about 561 nm (yellow), and about 659 nm (red) (hereinafter, “about” is omitted), respectively. It is to be noted that the nonlinear crystals may be selected from among KTP crystal, LBO crystal, BBO crystal, or the like. In the present embodiment, the KTP crystal is used for 532 nm and the LBO crystal is used for 561 nm and 659 nm, respectively.

On the optical path of an optical axis L1on which the rod11is placed, an HR14ais disposed at one end thereof, and the output mirror15is arranged at a predetermined inclination angle at the other end. The HR14ain the present embodiment has the property of totally reflecting the light of wavelengths of 1064 nm, 1123 nm, and 1319 nm. Instead of the HR14a, another reflector capable of widely reflecting the light of wavelengths in the infrared region including 1064 nm, 1123 nm, and 1319 nm may be used. The output mirror15has the property of totally reflecting the light of wavelengths of 1064 nm, 1123 nm, and 1319 nm, while transmitting the light of wavelengths of 532 nm, 561 nm, and 659 nm.

On the optical path of an optical axis L2in a reflecting direction of the output mirror15, an NLC13aand an HR14bare placed. The NLC13agenerates the light of 659 nm which is the second harmonic light from the light of 1319 nm. The HR14bhas the property of totally reflecting the light of 1319 nm and the light of 659 nm.

The above optical arrangement constitutes a first resonance optical system including a resonator configuration constructed of a pair of the HR14aon the optical axis L1and the HR14bon the optical axis L2, arranged as the rod11is located between them. Thus, the light of 659 nm generated by the NLC13acan be emitted through the output mirror15without being blocked by the rod11. It is preferable that the angle (the reflection angle) formed by the optical axis L1and the optical axis L2is as small as possible in light of the influence of aberration.

HR14cand HR14e, which are plane mirrors, are arranged to be movable onto/off from the optical path of the optical axis L2between the output mirror15and the NLC13a. The HR14chas the property of totally reflecting the light of 1064 nm and the light of 532 nm. The HR14ehas the property of totally reflecting the light of 1123 nm and the light of 561 nm.

On the optical path of the optical axis L3in a reflecting direction of the HR14c, an NLC13band an HR14dare placed. The NLC13bgenerates the light of 532 nm which is the second harmonic light from the light of 1064 nm. The HR14dhas, as with the HR14c, the property of totally reflecting the light of 1064 nm and the light of 532 nm.

On the optical path of the optical axis L4in a reflecting direction of the HR14e, an NLC13c, a wavelength selecting element60, and an HR14fare placed. The NLC13cgenerates the light of 561 nm which is the second harmonic light from the light of 1123 nm. As with the HR14e, the HR14fhas the property of totally reflecting the light of 1123 nm and the light of 561 nm.

In the above optical arrangement, when the HR14cis moved onto the optical path of the optical axis L2, the HR14a, the rod11, and the output mirror15of the first resonance optical system are also used to construct a second resonance optical system including a resonator constructed of a pair of the HR14aand the HR14darranged as the rod11is located between them. When the HR14eis moved onto the optical path of the optical axis L2, on the other hand, the HR14a, the rod11, and the output mirror15of the first resonance optical system are also used to construct a third resonance optical system including a resonator constructed of a pair of the HR14aand the HR14farranged as the rod11is placed between them.

The movement (insertion/removal) of the HR14cand HR14ewith respect to the optical path of the optical axis L2is conducted by an insertion/removal device30.FIG. 3a schematic structural view of the insertion/removal device30.

A driving unit31in the present embodiment is a step motor. Instead of the step motor, furthermore, any unit of which a rotation angle is controllable can be used. The driving unit31is connected with a shaft32through a joint33. Thus, the rotation of the driving unit31causes the shaft32to rotate. Two mirror holders34and35are mounted to the shaft32respectively at different axial angles to the rotation axis of the shaft32. The HR14cand HR14eare attached to the mirror holders34and35respectively so that each reflection plane of the HRs14cand14eis perpendicular to the rotation axis of the shaft32. With this structure, the HRs14cand14eare moved onto/off from the optical path of the optical axis L2by the rotation of the driving unit31, without changing the angle of each reflection plane to the optical axis L2.

When the HR14cis set in a position shown by a solid line inFIG. 3, that is, when the HR14cis placed on the optical path of the optical axis L2, the second resonance optical system is formed. On the other hand, when the HR14eis set in a position shown by a dashed line inFIG. 3, that is, the HR14eis placed on the optical path of the optical axis L2, the third resonance optical system is formed.

For reducing the thermal birefringence effect, the wave plate16is placed on a resonance optical path between the rod11and the HR14aboth arranged on the optical axis L1. This wave plate16is used to polarize the light of wavelengths in a wide region including a plurality of oscillation lines (peak wavelengths: 1064 nm, 1123 nm, 1319 nm) that are emitted from the rod11.

Now, the need to reduce the thermal birefringence effect is described below. It is known that if any element for restricting polarization is not provided on the resonance optical path, bring a fundamental wave into a random polarized light, a polarization ratio (P/S) varies due to the thermal birefringence effect when the laser beam is emitted from the rod11. The variation in the polarization ratio has a direct influence on a wavelength converting efficiency of the NLCs13a,13b, and13c, which would become a major factor that causes unstable output of the second harmonic light. To remove such factor causing the unstable output of the second harmonic light, the wave plate16is placed on the resonance optical path.

The wave plate16is constructed of a combination of a quartz plate and a magnesium fluoride plate and has the property of providing a flat phase difference to light of wavelengths in a wide region. The wave plate16used in the present embodiment has the property of providing a uniform phase difference (including an allowable range) to light of three wavelengths (1064 nm, 1123 nm, 1319 nm) to be converted.FIG. 5is a graph showing the characteristics of the wave plate16, in which a lateral axis indicates wavelengths in microns and a vertical axis indicates retardation (phase difference) in waves. As seen inFIG. 5, the wave plate16provides a uniform retardation of 0.25 to the light of three wavelengths (1064 nm, 1123 nm, 1319 nm). Optical contact is used for joining between the quartz plate and the magnesium fluoride plate in order to improve durability. The wave plate16, on both sides (through which the optical axis L1passes), is applied with an AR (anti reflective) coating to enhance transmittance to the light of three wavelengths (1064 nm, 1123 nm, 1319 nm).

The wave plate16has only to be placed on the resonance optical path between the rod11and the HR14a. In light of durability, it is preferably placed in a position near the rod11where a beam diameter is maximum, that is, a power density is minimum.

The selection of the resonance optical systems is not limited to the selecting method in the present embodiment and alternatively may be performed by a method disclosed in U.S. Pat. No. 6,636,537 (corresponding to Japanese patent unexamined publication No. 2002-151774) and other well known methods.

The laser apparatus having the above described structure will be explained in relation to operation to selectively emit laser beams of three colors (532 nm (green), 561 nm (yellow), 659 nm (red)).

[A method of Emitting a Laser Beam of 659 nm]

An operator operates the switch2ato select “red” (659 nm) as the color (wavelength) of a laser beam to be used in a surgical operation. When the red is selected, the HRs14cand14eare held out of the optical path of the optical axis L2.

Upon receipt of a trigger signal from the footswitch3, the control section20applies electric current to the LD12to thereby excite the rod11. It is to be noted that both end faces of an Nd:YAG crystal used as the rod11are applied with an AR (anti reflective) coating for enhancing transmittance with respect to the light of 1064 nm, 1123 nm, and 1319 nm.

When the rod11is excited, the light of 1319 nm is resonated between the HRs14aand14band converted to the second harmonic light thereof, that is, the light of 659 nm, by the NLC13adisposed on the optical path of the optical axis L2. The thus produced laser beam of 659 nm is allowed to pass through the output mirror15and enter the fiber5. Then, the laser beam delivered into the slit lamp4through the fiber5is irradiated from an irradiation port of the slit lamp4toward the patient's eye.

[A Method of Emitting a Laser Beam of 532 nm]

An operator operates the switch2ato select “green” (532 nm) as the color (wavelength) of a laser beam to be used in a surgical operation. The control section20drives the insertion/removal device30to move the HR14conto the optical path of the optical axis L2(in a position indicated by a solid line inFIG. 3). Upon receipt of a trigger signal from the footswitch3, the control section20applies electric current to the LD12to thereby excite the rod11.

When the rod11is excited, the light of 1064 nm is resonated between the HRs14aand14dand converted to the second harmonic light thereof, that is, the light of 532 nm, by the NLC13bdisposed on the optical path of the optical axis L3. The thus produced laser beam of 532 nm is allowed to pass through the output mirror15and enter the fiber5. Then, the laser beam is irradiated from the irradiation port of the slit lamp4toward the patient's eye.

[A Method of Emitting a Laser Beam of 561 nm]

An operator operates the switch2ato select “yellow” (561 nm) as the color (wavelength) of a laser beam to be used in a surgical operation. The control section20drives the insertion/removal device30to move the HR14eonto the optical path of the optical axis L2(in a position indicated by a dashed line inFIG. 3). The control section20then applies electric current to the LD12in response to a trigger signal from the footswitch3, thereby exciting the rod11.

When the rod11is excited, the light of 1123 nm is resonated between the HRs14aand14fthrough the wavelength selecting element60and converted to the second harmonic light thereof, that is, the light of 561 nm, by the NLC13cdisposed on the optical path of the optical axis L4. The thus produced laser beam of 561 nm is allowed to pass through the output mirror15and enter the fiber5. Then, the laser beam is irradiated from the irradiation port of the slit lamp4toward the patient's eye.

The wave plate16is always placed between the rod11and the HR14awhen the laser beams of the above three colors (532 nm (green), 561 nm (yellow), 659 nm (red)) are selectively emitted. Thus, the light of three wavelengths (1064 nm, 1123 nm, 1319 nm) emitted from the rod11passes through the wave plate16and is reflected by the HR14a, and passes through the wave plate16again. As a result, the light whose polarization direction is rotated 90° is returned to the rod11. This returning of the light having a 90°-rotated polarization direction to the rod11can compensate the thermal birefringence effect. Accordingly, the variation in the polarization ratio (P/S) caused by the thermal birefringence effect are reduced, so that such reduced variation has little influence on the wavelength converting efficiency of the NLCs13a,13b, and13cto generate second harmonic light.

FIG. 4is a table showing the output power of a laser beam of each wavelength (532 nm, 561 nm, 659 nm) and the variation (stability) in the output power in a case that the wave plate16is placed on the resonance optical path and in another case without the wave plate16. The output power of the laser beam of each wavelength increased by as much as ten percent or more. Variations in output power of the laser beam were measured in a limited frequency band of 0 KHz to 1 KHz. From this measurement, it was found that the output power variation was within ±8%. The reason why the frequency band for measurement was limited to the above range is that a laser beam in the frequency band of more than 1 KHz would have output power variations with a smaller amplitude and a lower output energy, so that such laser beam is not worth consideration as a laser beam for surgical operation. As can be seen from this result, the wave plate16placed on the resonance optical path allows stable emission of the laser beam of each of the three colors (532 nm, 561 nm, 659 nm).

The above apparatus does not need to be provided with a plurality of wave plates for different wavelengths respectively and to change the wave plates according to the wavelength of light to be emitted. Accordingly, the apparatus of a simple structure can reduce the thermal birefringence effect without needing a complex control and structure.

The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For instance, the apparatus in the above embodiment is arranged to emit a laser beam of a wavelength selected from among three different wavelengths, but it is not limited thereto. The wavelength of the laser beam to be emitted can be selected from among multiple wavelengths, for example, two wavelengths, four wavelengths, and more.

Further, although the above embodiment exemplifies the ophthalmic photocoagulation laser apparatus, the present invention is not limited thereto and may be applied to any laser apparatus capable of emitting laser beams of a plurality of different wavelengths.