Solid-state resonator

Abstract: In a solid-state laser resonator (4) capable of outputting a high power linearly polarized laser beam (7), a laser beam subjected to the birefringence in the laser resonator (4) is effected to minimize a component of the laser beam which is orthogonal to the direction of polarization definable by a Brewster plate (8, 9) upon traversing a quarter-wave plate (11, 12). The linear polarization output distribution of the laser beam is thus made uniform. With this, the emission efficiency and the output power of a linearly polarized laser beam are much improved.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state laser apparatus, and more particularly to a high power solid-state laser resonator which is capable of emitting a high-power linearly polarized laser beam. 2. Description of Related Art In order to output a linearly polarized laser beam from a solid-state laser resonator such as a YAG laser resonator, for example, it is a typical practice to insert an optical device represented by such as a Brewster plate which characterizes a direction of polarization thereof. According to this method, however, it is difficult to output a high-power linearly polarized laser beam due to a birefringence caused by the thermal stress occurring in the laser rod. As a method for solving such a problem, an arrangement including a quartz polarization rotator which is capable of rotating the polarizing direction by 90 degrees and which is disposed between two laser rods is disclosed in (1) APPLIED PHYSICS LETTERS (Vol.18, No.1, pp.3-4, January 1971). According to this, an influence of birefringence induced in one laser rod is effected to be canceled out by another birefringence induced in the other laser rod. To this end, it is prerequisite that a laser beam is to traverse substantially the same portions in both laser rods, and that substantially the same thermal loadings are to be applied to both the laser rods. According to the experiments conducted by the inventors, however, it was found very difficult in practice for these two prerequisite conditions to be satisfied concurrently in an actual equipment arrangement, thus it was found impossible to make the influences of the two birefringence to cancel out fully. Thereby, with an increasing thermal loading applied to the laser rod, the output of a linearly polarized laser beam decreased, and nevertheless resulting in a non-uniform intensity distribution thereof. Further, another arrangement for solving the problem is proposed in (2) USP No.4,935,932 in which a polarization plate and a quarter-wave plate are disposed at both ends of a laser rod, respectively, whereby the polarizer and the quarter-wave plate are disposed such that an incident light beam impinging on the laser rod becomes elliptically polarized or more preferably to become circularly polarized. In the arrangement of (2), it was found that the output of a linearly polarized laser beam decreased with an increasing thermal loading of the laser rod. This phenomenon is presumed to be due to a so-called thermal loading birefringence effect induced in the resonator which allows for the laser beam generated within the resonator to have a component polarized in a different direction from the polarized direction determined by a Brewster plate or the like. When a birefringence phenomenon takes place, a linearly polarized laser beam having a cross-shaped intensity distribution as shown in accompanying Fig.15 called as the cross mode is emitted from the laser resonator. An arrow P in Fig.15 designates the directions of polarization to be characterized by a Brewster inserted therein. Portions designated by symbol Br in Fig.15 are regions where a laser beam, subject to the birefringence phenomenon, having an orthogonal component relative to the direction of polarization to be characterized by a Brewster plate becomes dominant. Thereby, the overall efficiency of the linearly polarized laser beam is degraded, thus resulting in a significantly reduced output power. SUMMARY OF THE INVENTION An object of the present invention is to solve the foregoing problems associated with the prior art and to provide a solid-state laser resonator which can substantially improve its laser beam efficiency, thereby ensuring a linearly polarized higher output power to be emitted therefrom. This object is accomplished by the solid-state laser apparatus defined in claim 1. According to the invention, a laser beam subjected to the birefringence phenomenon in the laser resonator, upon traversing a quarter-wave plate, reduces its component in a direction orthogonal with the direction of polarization of the laser beam to be defined therein, thereby the linearly polarized output distribution of the laser beam becomes homogeneous, whereby the efficiency thereof is substantially improved, and in consequence a linearly polarized higher output power of laser beam is obtained. These and other objects of the invention will be seen by reference to the description, taken in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of a schematic YAG laser resonator of one embodiment of the invention. Fig. 2 is a cross-sectional view of a YAG rod illustrating the polarization state thereof which is utilized in Fig. 1. Fig. 3 illustrates a polarization state of the laser beam prior to impinging on a quarter-wave plate 11 in the YAG laser resonator of Fig. 1. Fig. 4 illustrates a polarization state of the laser beam after passing through the quarter-wave plate 11 in the YAG laser resonator of Fig. 1. Fig. 5 illustrates a polarization state of the laser beam after traversing the YAG rod 1 of Fig.1 in its forward path. Fig. 6 illustrates a polarization state of the laser beam after traversing the quarter-wave plate 12 of Fig. 1 in its forward path. Fig. 7 illustrates a polarization state of the laser beam after retraversing the quarter-wave plate 12 in its return path. Fig. 8 illustrates a polarization state of the laser beam after retraversing the YAG rod 1 in its return path. Fig. 9 illustrates a polarization state of the laser beam after retraversing the quarter-wave plate 11 in its return path. Fig. 10 shows an example of oscillation characteristics of a YAG laser resonator pertaining to the invention. Fig. 11 is a side view of a YAG laser resonator of another embodiment of the invention. Fig. 12 is a side view of a YAG laser resonator of still another embodiment of the invention. Fig. 13 is a side view of a YAG laser resonator of further another embodiment of the invention. Fig. 14 illustrates a YAG laser resonator of another embodiment of the invention as applied to a laser marker. Fig. 15 is a cross section view of a typical laser beam output in order to explain the birefringence thereof. DESCRIPTION OF THE PREFERRED EMBODIMENTS One preferred embodiment of the invention will be set forth in the following by way of example of a YAG laser as shown in Fig.1. With reference to Fig.1, numeral 1 is a YAG rod which contains Nd ion of about 1 %, which in combination with a total reflection mirror 2 and an output mirror 3 constitutes a laser resonator 4. A laser beam 6 is generated in the YAG rod 1 by light excitation by means of an excitation lamp 5, and a portion of the laser beam 6 is emitted out of a resonator 4 as a laser beam 7. In order to characterize or define a direction of polarization within the resonator 4, Brewster plates 8, 9 are disposed in the optical path of the laser beam 6, whereby in Fig. 1, the direction of polarization of the laser beam 6 is effected to become parallel with the plane of the paper. Numerals 11 and 12 designate quarter-wave plates, each of which is disposed such that the direction of an optic axis of the crystal thereof becomes substantially parallel with the direction of polarization of the laser beam 6 to be characterized by the Brewster plates 8, 9. Next, the operation of the laser oscillator according to this embodiment will be described in the following. Upon light excitation in the YAG rod 1 by means of the excitation lamp 5, laser oscillation is started. Although the polarizing direction of the laser beam 6 while passing through the Brewster plates 8, 9 is effected to become parallel with the plane of the paper, with an increasing temperature in the YAG rod 1, a birefringence phenomenon due to the thermal stress is caused to take place therein. As the result of loading of the thermal stress, a relationship between an index of refraction (n r ) in the radial direction and an index of refraction (n Φ ) in the tangential direction as shown in Fig. 2 becomes n r <n Φ , thereby the phase of a laser beam having its polarization plane in the tangential direction advances from the phase of a laser beam having its polarization plane in the radial direction. The Y axial direction in Fig.2 coincides with a direction parallel to the plane of the paper in Fig.1, and the X axial direction coincides with a direction perpendicular to the plane of the paper thereof. An influence resulting from a difference between these two directions becomes maximum on lines 13, 14 which make ±45 degrees with respect to the polarization direction of the laser beam 6 shown by P in Fig.2. Figs.3 through 9 indicate how the polarization states of a linearly polarized laser beam change while it passes through the line 13 in the direction perpendicular to the plane of the paper as it makes one round trip traversing the YAG rod 1. A polarization state of a laser beam 6 immediately prior to its incidence on the quarter-wave plate 11 is shown in Fig.3. That of the laser beam after passing through the quarter-wave plate becomes linearly polarized as shown in Fig. 4, and further, it becomes elliptically polarized as shown by Z in Fig. 5 upon passing through the YAG rod 1 in its forward path. Upon transmission through the quarter-wave plate 12, the laser beam is linearly polarized as shown in Fig. 6, then is reflected by the total reflection mirror 2 so as to pass through once again the quarter-wave plate 12 thereby elliptically polarized right-handedly as shown by Z in Fig. 7. Upon retraversing the YAG rod 1 in its return path, the laser beam is further elliptically polarized as shown in Fig.8, then while retraversing the quarter-wave plate 11, the beam becomes linearly polarized as shown in Fig. 9. As described hereinabove, the linearly polarized incident laser beam is polarized to have a right-handed elliptical polarization while traversing the YAG rod 1 in its forward path, where Δ is a phase difference in radian caused by traversing the lasing rod 1. Reflected by the total reflection mirror 2, and through retraversing the YAG rod 1 in its return path, the laser beam is elliptically polarized in the right hand with a different tilt angle, then upon retraversing the quarter-wave plate 11, it becomes a linearly polarized beam with an azimuth angle of -Δ. Therefore, an azimuth angle of a laser beam which has traveled one round trip within the resonator becomes Δ. When Δ=±nπ/2, it coincides with the direction of polarization of an incident laser beam the direction of which is shown in Fig.3 by P. Then, when Δ=±(n+1)π/2, it orthogonally intersects with the direction of polarization of the incident laser beam the direction of which is shown in Fig.3 by P, where, n=0, 2, 4, 6..... Depending on a value of the phase difference Δ resulting from the transmission through the YAG rod, a difference arises in the effects to be brought about by insertion of the quarter-wave plates 11, 12. However, according to the inventors' experiment the result of which is shown in Fig. 10 utilizing the arrangement of Fig.1 where a YAG rod of 10 mm in diameter and 152 mm in length was utilized, a laser output A obtained with an input power of 1.2 kW according to the embodiment of the invention was observed to increase approximately by 20% in comparison with a prior art laser output B. Although it was anticipated at first, in particular, for the intensity distribution of an emitted laser beam to become nonuniform as shown in Fig.15 in consideration of the result of the foregoing discussions, the cross-mode polarization obtained in the experiment prior to insertion of the quarter-wave plate has unexpectedly changed to a circular mode having substantially a uniform intensity distribution as shown in Fig.2. This has been presumed to be due to that Figs. 3 through 9 dealt only with the results of discussions on limited cases involving a linearly polarized laser beam having a particular direction of polarization and making one round trip, which in consequence suggests existence of some other laser beams having other directions of polarization in the resonator capable of serving advantageously to the object of the invention. Further, with reference to Fig.10, a laser output C designates a property obtained by the arrangement according to USP 4,935,932 in comparison with the output A of the present invention. As clearly seen, the latter output is lower by about 30% than the former. This difference in the output is judged to result from the fact that whereas according to the present embodiment the quarter-wave plate and Brewster plate are disposed such that an incident beam entering a laser rod becomes linearly polarized, a quarter-wave plate and a polarization plate according to USP 4,935,932 are disposed such that an incident laser beam entering a laser rod becomes elliptically polarized or circularly polarized, thereby causing the difference in the states of polarization of laser beams entering laser rod, thus adversely affecting the output. According to the embodiment as set forth hereinabove, the laser beam having been subjected to the birefringence in the laser resonator, upon transmission through the quarter-wave plate, is effected to minimize a component in its laser beam which intersects orthogonally with a polarization direction to be defined by the Brewster plate, and thus enables the linearly polarized output distribution of the laser beam to be distributed uniformly, with an improved efficiency such that the overall laser beam output can be increased approximately by 20% in terms of the linearly polarized output in comparison with a case without the use of the quarter-wave plate. Fig. 11 shows another preferred embodiment of the invention in which a quartz polarization rotator 15 for rotating the direction of polarization by 90 degrees is interposed between the two YAG rods 1. Such an arrangement of a laser resonator including the two YAG rods 1 and the quartz polarization rotator 15 is known as an effective method for canceling out birefringence influences generated in both the YAG rods. According to the results of the experiments conducted by the present inventors, however, it was found very difficult to ensure a precise alignment to be made between the two YAG rods 1, and in addition, when a thermal loading in the YAG rods 1 increases significantly, the influences of birefringence in the respective YAG rods could not have been canceled out. Therefore, with respect to the foregoing configuration, according to this embodiment, a direction of polarization definable by the Brewster plate 8 and a direction of an optic axis of crystal of the quarter-wave plate 11 are made essentially in parallel with each other, and a direction of polarization definable by the Brewster plate 9 and a direction of an optic axis of crystal of the quarter-wave plate 12 become essentially parallel with each other. These two polarization directions are rendered to hold a relationship such that they intersect orthogonally each other via the quartz polarization rotator 15 interposed therebetween. Since this embodiment of the invention operates to compensate the influences of birefringence in the two YAG rods 1 by means of the quarter-wave plates 9, 11, which could not have been canceled out even by insertion of the quartz polarization rotator 15, a substantial effect can be obtained by insertion of a quarter-wave plate even for a higher input power than available by the arrangement of Fig.1. In an experiment utilizing two YAG rods of 10 mm in diameter and 152 mm in length, the laser output increased by 20% for 2.4 kW of an input power in comparison with the prior art, and moreover the intensity distribution of the emission laser beam changed unexpectedly from the cross-mode to a circular-mode having approximately a uniform intensity distribution with no cost of decreased intensity which has been inevitable in the prior art. Fig.12 illustrates still another embodiment of the invention in which the quarter-wave plate 12 is disposed between the YAG rod 1 and total reflection mirror 2, and the Brewster plate 8 between the YAG rod 1 and the output mirror 3. Although the same effect cannot be obtained according to this embodiment of the invention for particularly high-power inputs as exhibited in the foregoing two embodiments of the invention, it is confirmed that a substantial effect is obtainable in the level of a 0.7 kW input power. According to this embodiment of the invention, since the quarter-wave plate 11 and the Brewster plate 9 were eliminated, equipment can be made compacter accordingly. Further, it was confirmed with this embodiment of the invention that no significant change occurred in the effect obtainable even by placing the quarter-wave plate 12 between the YAG rod 1 and the Brewster plate 8. Fig. 13 shows a further embodiment of the invention as applied to a slab type YAG laser, where since the direction of polarization is definable by the YAG slab itself (in the plane of the paper in Fig. 13), the quarter-wave plate 12 is inserted in such a manner that the direction of an optic axis of crystal of a quarter-wave plate becomes approximately parallel with the direction of the polarization. In this arrangement, no Brewster plate is needed. Typically, in a slab type lasing medium, its linearly polarized laser output characteristic varies depending on its adiabatic performance at both ends in cross direction thereof. However, by adopting the arrangement of this embodiment, a linearly polarized laser at a higher input power can be prevented from decreasing its output power resultant of a temperature gradient at both ends in cross direction. Further, also this configuration can be adapted to have only one of the quarter-wave plates 11 and 12. Further, with respect to the foregoing four embodiments of the invention, they have been described referring to such particular cases where quarter-wave plates having only one optic axial direction are utilized. However, there is also known the use of such an equivalent quarter-wave plate which has been prepared by laminating two quartz plates in such a manner that their respective optic axial directions intersect orthogonally, and as having a thickness equivalent to a difference between the two quartz plates laminated. When utilizing this type of a quarter-wave plate, it may be inserted in the optical path in such a manner that either one of the optic axial directions thereof becomes approximately parallel with the direction of polarization to be defined by the Brewster plate. With reference to Fig. 14, there is disclosed a laser oscillator 4 according to the invention which is applied to a trimming apparatus for use in resistance trimming of resistor films mounted on semiconductor devices. In this figure, a laser beam 7 from a laser oscillator 4 is exposed onto a liquid crystal mask 102 via a beam expander 101. On the liquid crystal mask 102, pattern information to be marked on a work piece is displayed, which is transmitted from a laser marker controller 103 through a liquid crystal drive unit 104. Then, a laser beam 106 which reflects the pattern information is separated by a polarizing beam splitter 105 in order to transcribe the pattern onto a work piece 111 via a mirror 109 and through a lens 110. An unnecessary beam 107 is absorbed in an absorber plate 108. A movable work stage 112 is controlled by the laser marker controller 103. Further, a laser beam machine utilizing the laser oscillator 4 according to the invention exhibits a significant advantage when applied to the surface treatment, but not limited thereto, and can be applied to cutting and the like as well.

Claims: A solid-state laser apparatus comprising a laser resonator (4) which includes a solid-state laser medium (1; 16) for generating a laser beam (6) with a particular direction of polarization, a total reflection mirror (2) disposed on one side of the laser medium (1; 16), an output mirror (3) disposed on the other side of the laser medium (1; 16) and a quarter-wave plate (11, 12) located in the optical path of the laser beam (6), characterised in that an optical axis of the crystal of said quarter-wave plate (11, 12) is substantially parallel with the particular direction of polarization. The apparatus of claim 1, wherein the laser medium (1) is a laser rod, the apparatus further including a Brewster plate (8, 9) for defining the particular direction of polarization of the laser beam (6). The apparatus of claim 2, wherein the laser rod (1) is disposed between said Brewster plate (8, 9) and said quarter-wave plate (11, 12). The apparatus of claim 2, wherein the quarter-wave plate (11, 12) and the Brewster plate (8, 9) are disposed on opposite ends of said laser rod (1). The apparatus of claim 2, wherein the quarter-wave plate (12) is disposed between said laser rod (1) and said total reflection mirror (2), and the Brewster plate (8)- is disposed between the laser rod (1) and the output mirror (3). The apparatus of any one of claims 1 to 5, further including another laser rod and, a quartz polarization rotator (15) which is disposed between the two laser rods (1). The apparatus of claim 1, wherein said laser medium comprises a laser slab (16) which defines the particular direction of polarization of the laser beam. The apparatus of claim 7, wherein quarter-wave plates (11, 12) are disposed at opposite ends of the laser slab (16). The apparatus of claim 7, wherein the quarter-wave plate (12) is disposed between said laser slab (16) and said total reflection mirror (2). Use of the apparatus of any one of claims 1 to 9 in a marking apparatus, wherein the laser output beam (7) is passed through a mask (102) having pattern information so as to transfer said pattern information onto a work piece (111) to be processed. Use of the apparatus of any one of claims 1 to 9 in a trimming apparatus, wherein the laser output beam (7) is adapted for use in trimming work.

Inventors: KUWABARA KOUJI, TAKEHISA KIWAMU, YANO MAKOTO, KUWABARA, KOUJI, TAKEHISA, KIWAMU, YANO, MAKOTO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: H01S3
IPC codes: H01S3