WAVELENGTH COMBINATION LASER DEVICE

Abstract

Wavelength combining laser device (10) comprising the following: a semiconductor laser (1) to emit a plurality of laser beams (6) in a direction of an optical axis from a light-emitting end face (1b) of the plurality of light-emitting areas (1a) arranged in a laser beam combination direction, wherein the direction of the optical axis is perpendicular to the laser beam combination direction; a focusing element (2) that collimates the laser beams (6); a wavelength combining element (3) to combine the plurality of laser beams (6) in the laser beam combination direction into a single laser beam (6) and to output the single laser beam (6); an optical system (4) for cross-coupling reduction, which has a positive refractive power in the laser beam combination direction perpendicular to an optical axis of the individual laser beam (6) emitted by the wavelength-combining element (3); and a partially reflective mirror (5) to reflect the single laser beam (6) which passes through the optical system (4) for cross-coupling reduction, and to allow the single laser beam (6) to transmit through and exit the partially reflective mirror (5), wherein the optical system (4) is arranged to reduce cross-coupling by imaging the light-emitting face (1b) onto the partially reflective mirror (5) by causing the light-emitting face (1b) to be conjugated with the partially reflective mirror (5) in a plane spanned by the optical axis of the single laser beam (6) and the laser beam combination direction, wherein the optical system (4) for cross-coupling reduction comprises two lenses (41, 42; 43, 44) which have an effect in the laser beam combination direction perpendicular to the direction of the optical axis of the individual laser beam (6) and satisfy a relation f1>f2, where f1 is a focal length in the laser beam combination direction of the first lens (41, 43) which is arranged on a side that is closer to the wavelength-combining element (3), and where f2 is a focal length in the laser beam combination direction of a second lens (42, 44) which is arranged on a side that is closer to the partially reflective mirror (5), and a distance L between the first lens (41, 43) and the second lens (42, 44) L>f1+f2 is satisfied, wherein a common principal lens surface (H) of the first lens (41, 43) and the second lens (42, 44) is positioned on a side that is closer to the semiconductor laser (1) than to the wavelength-combining element (3).

Description

AREA

[0001] The present invention relates to a wavelength combining laser device for combining laser beams emitted from a plurality of light emitting regions of a semiconductor laser. BACKGROUND

[0002] As an example of a laser oscillator for combining light emitted from many semiconductor laser diodes (LDs) (such light is hereinafter referred to as LD rays), there exists a wavelength combining resonator that uses a diffraction grating as a wavelength combining element.

[0003] Recently, LD beams, which have a wavelength in the 900 nm or 1000 nm band and high electro-optical conversion efficiency, have been directly used for processing applications such as metal cutting or welding. However, in these applications, there is a limitation on the output of a single LD beam—that is, the light emitted from an active layer stripe, which is a light-emitting layer of a semiconductor element—so LD beams from a plurality of light-emitting layers are collected and used to generate a high-power laser beam.

[0004] Furthermore, cutting processes, in particular, require high beam quality, i.e., high light focusing performance, so that the multiple LD beams are not only collected but also combined. Processing such as metal cutting or welding requires laser power ranging from several hundred W to several kW. An LD bar equipped with multiple light-emitting layers is suitable for generating a large number of LD beams. Specifically, the light-emitting layers, each with a width of 50 µm to 200 µm, are arranged at equal intervals in the LD bar, which has a width of approximately 10 mm. Ten to 50 laser beams are emitted from the end face of the LD bar. Such an LD bar emits a power of several tens of W. LD bars with outputs of several hundred W have also become available in recent years.

[0005] The wavelength combining resonator is prone to crosstalk, or cross-coupling oscillation. This reduces beam quality and electro-optical conversion efficiency. The wavelength combining resonator superimposes multiple LD beams emitted from the LD bar to generate a single combined laser beam with high power and multiple wavelengths. Such wavelength combining utilizes a beam path of an external resonator containing the wavelength combining element. In some cases, an LD beam reflected by a partially reflecting mirror is not returned to the light-emitting layer from which the LD beam was emitted, but is guided to another light-emitting layer, such as an adjacent light-emitting layer.At this time, laser oscillation occurs between different light-emitting layers, such as adjacent light-emitting layers. This type of laser oscillation is called cross-coupling oscillation.

[0006] In order to reduce such cross-coupling oscillation, Patent Literature 1 states that it is sufficient if a partially reflecting output coupler, i.e., a partially reflecting mirror, is arranged within the Rayleigh length of a laser beam after a system for reducing cross-coupling. LITERATURE Patent literature 1 US 6 192 062 B1 Patent literature 2 US 6 665 471 B1 Patent literature 3 JP 2014- 120 560 A Literature 4 Schröder, G. Technical Optics, 7th edition, p. 138, Würzburg, 1990, ISBN 3-8023-0067-X Patent literature 5 WO 2017 / 022 142 A1 Patent literature 6 US 6 529 542 B1 Patent literature 7 DE 10 2011 003 142 A1 Patent literature 8 CN 1 04 638 513 A Patent literature 9 CN 1 04 143 763 A Patent literature 10 US 2013 / 0 208 361 A1

[0007] In particular, Patent Literature 8 discloses a laser device having partially reflecting mirrors for reflecting a single laser beam passing through the optical system and for allowing the single laser beam to transmit through and exit from the partially reflecting mirror (4).

[0008] Similar systems are known from patent literatures 9 and 10, which select the focal lengths of the lenses in a ratio that reduces crosstalk. SUMMARYTechnical problem

[0009] However, the method described in Patent Literature 1 cannot prevent cross-coupling oscillation. Patent Literature 1 stipulates that Lcplr_eff, the distance from a diffraction grating to the partially reflecting mirror, is greater than or equal to Zr, which is the Rayleigh length of the laser beam after the lens. Furthermore, a specific value for Zr is 160 m. This is unrealistic, so the partially reflecting mirror is positioned to reduce the distance to a realistic distance of about 1 m using an afocal telescope. However, the position of the partially reflecting mirror that can reduce cross-coupling oscillation does not occur in reality over such a wide range, but only at a very narrow point.Thus, simply placing the partially reflecting mirror within the Rayleigh length of the laser beam after the cross-coupling reduction system is not sufficient to prevent cross-coupling oscillation. Furthermore, cross-coupling oscillation cannot be prevented with an afocal telescope.

[0010] The present invention was conceived against this background. An object of the present invention is to provide a wavelength-combining laser device that does not generate cross-coupling oscillation and has high beam quality and high electro-optical conversion efficiency. Solution to the problem

[0011] The solution to the problem according to the invention results from a wavelength-combining laser device having the features of claim 1. Advantageous further developments emerge from the subclaims. Advantageous effects of the invention

[0012] The present invention produces an effect capable of providing a wavelength combining laser device that does not generate cross-coupling oscillation and has high beam quality and high electro-optical conversion efficiency. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing the configuration of a wavelength combining laser device according to a first embodiment of the present invention. Fig. 2 is a diagram showing the behavior of a plurality of laser beams in a wavelength combining element according to the first embodiment. Fig. Figure 3 is a diagram explaining the behavior of laser beams under cross-coupling oscillation. Fig. 4 is a diagram for explaining the function of an optical system for cross-talk reduction in the wavelength combining laser device according to the first embodiment. Fig. Figure 5 is a graph showing a wavelength spectrum of a wavelength combining laser device used in Fig. 3 is shown. Fig. Fig. 6 is a graph showing a wavelength spectrum of the wavelength combining laser device used in Fig. 4 is shown. Fig. 7 is a diagram showing a state in which a λ / 2 plate is arranged in the wavelength combining laser device according to the first embodiment. Fig. 8 is a diagram showing the configuration of a wavelength combining laser device according to a second embodiment of the present invention. Fig. 9 is an enlarged view of an example of an LD bar and a beam-shaping element according to the second embodiment. Fig. 10 is an enlarged view of another example of the LD bar and the beam-shaping element according to the second embodiment. Fig. 11 is a diagram for explaining how the beam quality is reduced in the wavelength combining laser device according to the first embodiment. Fig. 12 is a diagram showing the configuration of a wavelength combining laser device according to a third embodiment of the present invention. Fig. 13 is a diagram showing the configuration of another wavelength combining laser device according to the third embodiment. Fig. 14 is a diagram showing the configuration of a wavelength combining laser device according to a fourth embodiment of the present invention. Fig. 15 is a diagram showing the configuration of another wavelength combining laser device according to the fourth embodiment. DESCRIPTION OF THE EMBODIMENTS

[0013] Hereinafter, a wavelength combining laser device according to embodiments of the present invention will be described in detail with reference to the drawings. It should be noted that the present invention is not limited by the embodiments. Embodiment 1

[0014] Fig. 1 is a diagram showing the configuration of a wavelength combining laser device 10 according to a first embodiment of the present invention. The wavelength combining laser device 10 includes: an LD array 1, which is a semiconductor laser having a plurality of light-emitting layers 1a as light-emitting regions; a focusing element 2; a wavelength combining element 3; a crosstalk reducing optical system 4; and a partially reflecting mirror 5. A specific example of each of the focusing element 2 and the crosstalk reducing optical system 4 is a lens. A specific example of the wavelength combining element 3 is a diffraction grating.

[0015] Approximately ten to fifty of the light-emitting layers 1a are arranged in the LD bar 1 and arranged at equal intervals in the width direction of the LD bar 1. The width direction of the LD bar 1 corresponds to the laser beam combining direction in which the wavelength combining element 3 combines laser beams 6. Fig. For simplicity, FIG. 1 shows only three of the light-emitting layers 1a. The laser beam 6 is emitted from a light-emitting end face 1b of each of the light-emitting layers 1a in the direction of an optical axis perpendicular to the laser beam combining direction. The laser beam combining direction is the direction relative to the optical axis of the laser beam 6 and is defined as the direction perpendicular to both the optical axis and the thickness direction of the LD bar 1. Thus, when the optical axis direction of the wavelength combining element 3 is changed, as described later, the laser beam combining direction is also changed.Here, the width direction of the LD bar 1 in which the light-emitting layers 1a are arranged is defined as an X direction; the thickness direction of the LD bar 1 is defined as a Y direction; and the direction in which the laser beam 6 is emitted is defined as a Z direction.

[0016] The LD bar 1 is fabricated by performing a semiconductor process such as lithography on an InGaAs substrate or an AlGaAs substrate. Fig. 1 omits the following: a heat sink for cooling the LD bar 1, a submount for connecting the LD bar 1 to the heat sink and an electrode and a gold wire for the conduction.

[0017] If the focusing element 2 is formed from a lens, the focusing element 2 is made of quartz or glass and has a convex effect, i.e., a positive refractive power in at least the laser beam combining direction (X direction). The focusing element 2 collimates the laser beams 6 emitted from the light-emitting layers 1a and propagating divergingly. At the same time, the focusing element 2 collects principal rays 6a of the laser beams 6 at a point A on the wavelength combining element 3.

[0018] When the wavelength combining element 3 is formed of a diffraction grating, the wavelength combining element 3 diffracts the laser beams 6, of which the principal rays 6a are collected at a point A, in the first-order diffraction direction. Note that the direction in which the wavelength combining element 3 diffracts the laser beams 6 is not limited to the first-order diffraction direction.

[0019] The laser beams 6 diffracted by the wavelength combining element 3 are transmitted through the crosstalk reduction optical system 4 and partially reflected by the partially reflecting mirror 5. The portions of the laser beams 6 reflected by the partially reflecting mirror 5 travel along the beam path in a direction opposite to the direction in which the beams have propagated up to that point, back to the crosstalk reduction optical system 4, the wavelength combining element 3, and the focusing element 2, and are returned to the corresponding light-emitting layers 1a from which the laser beams 6 were emitted.

[0020] That is, the wavelength combining laser device 10 is a resonator with the light-emitting layers 1a as an oscillation source, which is a gain medium; with the end face 1c of the LD bar 1 opposite the light-emitting end face 1b as a totally reflecting mirror; and with the partially reflecting mirror 5 as a mirror facing the end face 1c. The area from the focusing element 2 to the partially reflecting mirror 5 outside the LD bar 1 is called the beam path of an external resonator.

[0021] The combining of the plurality of laser beams 6, i.e. the wavelength combining, is described below. Fig. 2 is a diagram showing the behavior of the plurality of laser beams 6 in the wavelength combining element 3 according to the first embodiment. Fig. For simplicity, Figure 2 shows the plurality of laser beams 6 as three main beams 6i, 6j, and 6k. The main beams 6i, 6j, and 6k are collected by the focusing element 2 at a point A on the wavelength combining element 3.

[0022] The incident angles of the principal rays 6i, 6j, and 6k with respect to the wavelength combining element 3 are determined as θi, θj, and θk, respectively, based on the pitch of the light-emitting layers 1a in the LD bar 1, the focal length of the focusing element 2, and the placement angle of the wavelength combining element 3.

[0023] In order for the principal rays 6i, 6j, and 6k to be returned to the corresponding light-emitting layers 1a from which the rays were emitted, the principal rays 6i, 6j, and 6k must enter the partially reflecting mirror 5 at a right angle and be reflected therefrom at a right angle after being diffracted by the wavelength combining element 3. Under such conditions, the emission angles, i.e., the diffraction angles, of the principal rays 6i, 6j, and 6k from the wavelength combining element 3 are all set to the same angle ψ.

[0024] The wavelengths of the principal rays 6i, 6j and 6k are determined as λi, λj and λk, respectively, based on the angles of incidence θi, θj and θk and the angle of emission ψ, which is determined as described above.

[0025] That is, the wavelength at which the laser beam 6 emitted from each of the light-emitting layers 1a can oscillate is automatically selected so that the wavelength-combining laser device 10 oscillates. At this time, the wavelength increases from λi to λk as the angle of incidence increases from θi to θkn. The wavelengths of the laser beams 6 emitted from the light-emitting layers 1a thus shift gradually and discretely along the X direction of the LD strip 1.

[0026] The emission angle ψ of the laser beam 6 from the wavelength combining element 3 is fixed regardless of the incident angle. In other words, the wavelength combining element 3 combines the plurality of laser beams 6 in the X direction, which is the laser beam combining direction, and outputs a single laser beam 6. As a result, the laser beams 6 of all wavelengths are superimposed between the wavelength combining element 3 and the partially reflecting mirror 5 into a single beam. The wavelength combining laser device 10 thereby emits a single combined multi-wavelength laser beam from the partially reflecting mirror 5.

[0027] If the optical system 4 for cross-coupling reduction is not provided or is improperly placed, the laser beam 6 emitted from one light-emitting layer 1a may, in some cases, not be returned to the light-emitting layer 1a from which the beam was emitted, but instead be returned to another light-emitting layer 1a. In such a case, oscillation occurs between two different light-emitting layers 1a. Such a phenomenon is called cross-coupling oscillation.

[0028] Fig. Figure 3 is a diagram explaining the behavior of the laser beam 6 during cross-coupling oscillation. Fig. The wavelength combining laser device 20 shown in Figure 3 does not have an optical system 4 for cross-coupling reduction. Fig. 3 also simplifies the effect of diffraction in the wavelength combining element 3, and the LD strip 1 and the partially reflecting mirror 5 are arranged in a straight line with each other. Regarding the light-emitting layers of the LD strip 1, only two light-emitting layers 1p and 1q of interest are shown. Principal beams 6p and 6q, indicated by solid lines, are laser beams emitted from the corresponding light-emitting layers 1p and 1q, and represent beam paths of normal oscillation, which is not cross-coupling oscillation. Beams 6r, indicated by dashed lines, represent beam paths of cross-coupling oscillation.

[0029] The beam 6r emitted at an angle from the light-emitting layer 1p enters the partially reflecting mirror 5 at an angle of incidence ε, is reflected therefrom at a reflection angle ε, and is returned to the light-emitting layer 1q. Conversely, the beam 6r emitted from the light-emitting layer 1q is returned to the light-emitting layer 1p. The beam 6r thus travels back and forth between the light-emitting layer 1p and the light-emitting layer 1q. At this time, a portion of the main beams 6p and 6q is directly transmitted through the partially reflecting mirror 5, whereas a portion of the cross-coupling oscillation beam 6r is transmitted through the partially reflecting mirror 5, offset by the angle ε.This means that when cross-coupling oscillation occurs, the laser beam reflected by the partially reflecting mirror 5 spreads more than the laser beam during normal oscillation. The beam quality is thus reduced.

[0030] If the wavelengths of the principal beams 6p and 6q are λp and λq, respectively, then the wavelength of the cross-coupling oscillation beam 6r is approximately midway between λp and λq, or (λρ+λq) / 2. Therefore, one knows whether the cross-coupling oscillation occurs or not by measuring the wavelength spectrum of the emitted laser beam.

[0031] Fig. Fig. 4 is a diagram for explaining the function of the optical system 4 for cross-talk reduction in the wavelength combining laser device 10 according to the first embodiment. Fig. 4 is, as with Fig. 3, the effect of diffraction in the wavelength combining element 3 is shown in simplified form. The LD bar 1 and the partially reflecting mirror 5 are also aligned in a straight line with each other.

[0032] If the cross-coupling reducing optical system 4 is formed as a lens, the cross-coupling reducing optical system 4 is made of quartz or glass. The cross-coupling reducing optical system 4 has a laser beam combining direction 400 (see Fig. 1) A convex effect, i.e., a positive refractive power, which is perpendicular to both the optical axis direction of the single laser beam 6 combined in and emitted from the wavelength combining element 3 and the thickness direction of the LD bar 1. The laser beam combining direction 400 is a direction in the crosstalk reduction optical system 4 that corresponds to the width direction of the LD bar 1 that emits the plurality of laser beams 6 combined by the wavelength combining element 3.

[0033] A lens system of the beam path of the external resonator, in which the optical system 4 for cross-coupling reduction and the focusing element 2 are combined, enables the light-emitting end face 1b of the LD bar 1 to be conjugated with the partially reflecting mirror 5 in a plane spanned by the direction of the optical axis of the single laser beam 6 and the laser beam combining direction 400, i.e., in an XZ plane. In other words, the lens system images the light-emitting end face 1b in the XZ plane onto the partially reflecting mirror 5.

[0034] As in Fig. 3 are the principal rays 6p and 6q, which are drawn by the solid lines in Fig. 4, the laser beams emitted from the corresponding light-emitting layers 1p and 1q, which represent the beam paths of normal oscillation. Beams 6v, which are indicated by the dashed lines in Fig. 4 are imaginary rays which are defined with reference to the rays 6r in Fig. 3. They enter the partially reflecting mirror 5 at the entrance angle ε and are reflected therefrom at the reflection angle ε. Fig. 4 shows that beam 6v is returned to a central point B between light-emitting layer 1p and light-emitting layer 1q. The position of point B to which the beam is returned does not depend on the size of the angle ε. Thus, beam 6v, which is the laser beam of the cross-coupling oscillation, cannot exist.

[0035] The function of the crosstalk reduction optical system 4 is described by another method. As described above, the lens system of the external resonator beam path, which combines the crosstalk reduction optical system 4 and the focusing element 2, images the light-emitting end face 1b of the LD strip 1 in the XZ plane onto the partially reflecting mirror 5. When crosstalk oscillation occurs, the light-emitting layer 1p and the light-emitting layer 1q oscillate at the same wavelength (λρ+λq) / 2, so the images of the light-emitting layer 1p and the light-emitting layer 1q on the partially reflecting mirror 5 must be separated by the distance between the light-emitting layer 1p and the light-emitting layer 1q. This means that the images of the light-emitting layer 1p and the light-emitting layer 1q on the partially reflecting mirror 5 do not overlap.At this time, the laser beam 6 reflected by the partially reflecting mirror 5 is not returned to the light-emitting layer 1a that is different from the light-emitting layer from which the beam was emitted, but can only be returned to the light-emitting layer 1a from which the beam was emitted. Therefore, cross-coupling oscillation does not occur, and the laser beam 6 emerging from the partially reflecting mirror 5 is only the laser beam 6 of normal oscillation.

[0036] Fig. Fig. 5 is a graph showing the wavelength spectrum of the wavelength combining laser device 20 used in Fig. 3 is shown. Fig. Fig. 6 is a graph showing the wavelength spectrum of the wavelength combining laser device 10 used in Fig. 4 is shown. In Fig. 5 and Fig. 6 The horizontal axis represents the wavelength and the vertical axis represents the intensity. Fig. 6 shows the effect of the wavelength combining laser device 10 according to the first embodiment.

[0037] The Fig. 3 wavelength combining laser device 20 and the one shown in Fig. 4 both use the LD bar 1 with 19 light-emitting layers 1a. It can be seen that in Fig. 5 the wavelength spectrum is disturbed because the cross-coupling oscillation is not successfully reduced. In Fig. 6 However, 19 separate peaks can be clearly identified because the cross-coupling oscillation is successfully reduced.

[0038] That is, the wavelength combining laser device 10 according to the first embodiment is equipped with the cross-coupling reduction optical system 4. This causes the light-emitting end face 1b of the LD bar 1 to be conjugated with the partially reflecting mirror 5. Thus, the light-emitting end face 1b is imaged on the partially reflecting mirror 5 in the plane defined by the optical axis direction and the laser beam combining direction 400. This reduces cross-coupling oscillation, and high beam quality can be achieved. In addition, the electro-optical conversion efficiency originally exhibited by the LD bar 1 can be achieved by reducing the cross-coupling oscillation, although the electro-optical conversion efficiency of the LD bar 1 decreases when the cross-coupling oscillation occurs.In addition, the laser beam is wasted light due to the cross-coupling oscillation, which reduces the beam quality, so the utilization efficiency of the laser beam, which contributes to laser processing, can be increased by reducing the cross-coupling oscillation.

[0039] In the wavelength combining laser device 10 according to the first embodiment, the light-emitting end face 1b of the LD strip 1 is imaged onto the semi-reflective mirror 5 simply by arranging the crosstalk reduction optical system 4. Thus, the wavelength combining laser device 10 has a simple and easy configuration, eliminating the need for adding an aperture or slit to block the crosstalk oscillation beam path, a locking arm, which is an additional beam path for fixing the oscillation wavelength of the laser beam 6, or the like. Thus, the wavelength combining device 10 is inexpensive and can stably reduce crosstalk oscillation even with manufacturing defects.

[0040] A λ / 2 plate may be arranged between the LD bar 1 and the wavelength combining element 3, more precisely between the focusing element 2 and the wavelength combining element 3. Fig. 7 is a diagram showing a state in which a λ / 2 plate 8 is disposed in the wavelength combining laser device 10 according to the first embodiment. When the λ / 2 plate 8, disposed between the focusing element 2 and the wavelength combining element 3, causes the polarization direction of the laser beam 6 to rotate 90 degrees around the optical axis, the diffraction efficiency of the wavelength combining element 3 is more easily increased, so that the wavelength combining laser device 10 with high efficiency can be achieved.

[0041] The wavelength combining device 10 may further comprise a plurality of LD bars 1 as semiconductor lasers. The LD bars 1 are arranged such that the main beams 6a of the laser beams 6 emitted from all the LD bars 1 converge at a point A on the wavelength combining element 3. The laser beams 6 of the plurality of LD bars 1 are superimposed into a single laser beam. The wavelength combining laser device 10 can thereby achieve high power. Embodiment 2

[0042] Fig. Fig. 8 is a diagram showing the configuration of a wavelength combining laser device 11 according to a second embodiment of the present invention. Fig. 8 are those components that are connected to the components in Fig. 1 and Fig. 4 are identical, are assigned the same reference numerals as those in Fig. 1 and Fig. 4. The wavelength combining laser device 11 shown in the second embodiment includes a beam-shaping element 7. That is, the LD bar 1 of the wavelength combining laser device 11 includes the beam-shaping element 7 facing the light-emitting end face 1b. The beam-shaping element 7 collimates the plurality of laser beams 6 emitted from the LD bar 1 as divergent light from the light-emitting layers 1a arranged at equal intervals in the X direction, which is the width direction of the LD bar 1, and reduces the divergence angle before the plurality of laser beams 6 overlap with each other. The plurality of laser beams 6 collimated by the beam-shaping element 7 enter the wavelength combining element 3. Fig. For simplicity, Fig. 8 shows only three of the light-emitting layers 1a. Details of the beam-shaping element 7 are described below with reference to Fig. 9 and Fig. 10 described.

[0043] Fig. 9 is an enlarged view of an example of the LD bar 1 and the beam-shaping element 7 according to the second embodiment. Fig. 10 is an enlarged view of another example of the LD strip 1 and the beam-shaping element 7 according to the second embodiment. Fig. 9 and Fig. 10 illustrate two different types of beam-shaping elements 7.

[0044] The width of the light-emitting layer 1a of the LD bar 1 is typically 50 µm to 200 µm. The divergence angle of the laser beam 6 emitted from the light-emitting layer 1a in the width direction (X direction) of the LD bar 1 is 7 degrees to 11 degrees. The thickness of the light-emitting layer 1a is approximately 1 µm. The divergence angle of the laser beam 6 emitted in the thickness direction (Y direction) of the light-emitting layer 1a is 45 degrees to 60 degrees. The direction in which the divergence angle of the laser beam 6 is small is called the slow axis direction. The direction in which the divergence angle of the laser beam 6 is large is called the fast axis direction. Thus, the width direction (X direction) of the LD bar 1 corresponds to the slow axis direction, and the thickness direction (Y direction) of the light-emitting layer 1a corresponds to the fast axis direction.That is, the plurality of laser beams 6 are emitted from the plurality of light-emitting layers 1a arranged in the width direction (slow axis direction) of the LD strip 1, and the individual laser beams 6 have two directions corresponding to different divergence angles, namely, the slow axis direction and the fast axis direction.

[0045] In order to collimate such laser beams 6, for example, the Fig. 9 is used. The beam-shaping element 7 has two lenses, namely a fast axis collimator (FAC) 71 for performing collimation in the fast axis direction and a slow axis collimator (SAC) 72 for performing collimation in the slow axis direction. The FAC 71 is a cylindrical lens with a generatrix in the X direction. An exit surface 72b of the SAC 72 is a lens array, i.e., a microlens array in which a plurality of cylindrical lens surfaces with generatrixes in the Y direction are arranged in the X direction, which is the laser beam combining direction. An incident surface 72a of the SAC 72 is a flat surface.

[0046] On the other hand, the Fig. 10 includes the FAC 71 and a beam path conversion element 73 instead of the SAC 72. The beam path conversion element 73 functions to switch the slow axis direction and the fast axis direction of the laser beam 6. In other words, once the laser beam 6 passes through the beam path conversion element 73, the slow axis direction of the beam is switched to the Y direction from the X direction, and the fast axis direction of the beam is switched to the X direction from the Y direction. In the beam path conversion element 73, both an incident surface 73a and an exit surface 73b are a lens array, that is, a microlens array in which a plurality of cylindrical lens surfaces are arranged in the X direction, which is the laser beam combining direction. Each of the lens surfaces forming the microlens array has a generatrix in the direction of 45 degrees in the XY plane.The individual lens surfaces constituting the microlens arrays are arranged to face each other between the incident surface 73a and the exit surface 73b. The positions of the facing lens surfaces coincide with each other in the X direction, which is the slow axis direction. The beam path conversion element 73 is also arranged so that the positions of the individual lens surfaces coincide with the positions of the laser beams 6 irradiated from the light-emitting layers 1a.

[0047] The lens surface of each of the FAC 71, the SAC 72, and the beam path conversion element 73 is one of the following: a circular arc, an ellipse, a hyperbola, or a non-circular arc obtained by combining a polynomial with them, with the cross-sectional shape perpendicular to the generatrix being convex. Note that when the beam path conversion element 73 is used, a lens for collimating the laser beam 6 in the slow axis direction may additionally be arranged in the beam path of the external resonator.

[0048] The wavelength combining laser device 11 can be constructed using the Fig. 9 or Fig. 10 can be made compact to collimate the laser beams 6 and reduce the divergence angles. This means that the wavelength-combining laser device 11 can be manufactured cost-effectively.

[0049] The laser beams 6 collimated by the beam-shaping element 7 are emitted from the exit surface 72b of the SAC 72 or from the exit surface 73b of the beam path conversion element 73 and then redistributed by diffraction. Thus, with the beam-shaping element 7 present, the wavelength-combining laser device 11 can treat the exit surface 72b or 73b as a new light-emitting face of the LD strip 1 and form an image thereof on the semi-reflective mirror 5. This can reduce cross-coupling oscillation. That is, the exit surface 72b or 73b is imaged on the partially reflecting mirror 5 by arranging the crosstalk reducing optical system 4 so as to cause the exit surface 72b or 73b to be conjugated with the partially reflecting mirror 5 in the plane spanned by the direction of the optical axis and the laser beam combining direction.The dashed lines in . Fig. 8 are the divergent light, indicating that the exit surface 72b or 73b is imaged on the partially reflecting mirror 5. The solid lines are the principal rays 6a of the laser beam 6. In reality, the beam-shaping element 7 is sufficiently small relative to the wavelength-combining laser device 10 that the crosstalk reduction optical system 4 can be arranged to image the light-emitting end face 1b of the LD bar 1 onto the partially reflecting mirror 5 while ignoring the beam-shaping element 7.

[0050] Furthermore, the main rays 6a of the laser beam 6 emitted from the light-emitting layers 1a can be collected at a point A on the wavelength combining element 3 by changing the distance of the SAC 72 or by tilting the generatrix of the FAC 71 in the XY plane. That is, the focusing element 2 included in the wavelength combining laser device 10 according to the first embodiment is not a necessary component and, as shown in Fig. 8. If the focusing element 2, as shown in Fig. 8 is not used, the crosstalk reducing optical system 4 alone images the exit surface 72b of the SAC 72 or the exit surface 73b of the beam path conversion element 73 on the partially reflecting mirror 5 in the XZ plane.

[0051] As described above, according to the wavelength combining laser device 11 of the second embodiment, the crosstalk reduction optical system 4 causes the exit surface 72b of the SAC 72 or the exit surface 73b of the beam path conversion element 73 to be conjugated with the partially reflecting mirror 5. Thus, the exit surface 72b or 73b is imaged on the partially reflecting mirror 5 in the plane defined by the optical axis direction and the laser beam combining direction. This reduces crosstalk oscillation. At the same time, the wavelength combining laser device 11 can be manufactured compactly and inexpensively. Embodiment 3

[0052] Fig. 11 is a diagram for explaining how beam quality is reduced in the wavelength combining laser device 10 according to the first embodiment. Although crosstalk oscillation can be reduced by the crosstalk reducing optical system 4 included in the wavelength combining laser device 10 according to the first embodiment and the wavelength combining laser device 11 according to the second embodiment, the beam quality may be reduced under the following conditions.

[0053] Fig. 11 is a diagram for explaining the reduction in beam quality due to the crosstalk reducing optical system 4 in the wavelength combining laser device 10 according to the first embodiment. Fig. For simplicity, Fig. 11 shows only three of the light-emitting layers 1a. Considering only the laser beam 6 emitted from the central light-emitting layer 1a, the solid line is the main beam 6a, and the dashed lines are the divergent light, indicating that the light-emitting end face 1b is imaged on the partially reflecting mirror 5. The crosstalk reduction optical system 4 in the wavelength combining laser device 10 is a single lens. As described above, the crosstalk reduction optical system 4 has a convexity, that is, a positive refractive power, in the laser beam combining direction 400, which is perpendicular to both the optical axis direction of the single laser beam 6 combined in the wavelength combining element 3 and the thickness direction of the LD strip 1.

[0054] In Fig. 1, it is assumed that “f” is the focal length of the cross-coupling reducing optical system 4 in the laser beam combining direction, “L1” is the distance between the wavelength combining element 3 and the cross-coupling reducing optical system 4, and “L2” is the distance between the cross-coupling reducing optical system 4 and the partially reflecting mirror 5. In the wavelength combining laser device 10 according to Fig. 11, the optical system 4 for reducing cross-coupling and the partially reflecting mirror 5 are arranged to satisfy L1=L2=f.

[0055] If the optical system 4 for cross-coupling reduction and the partially reflecting mirror 5 as in Fig. 11, a plurality of beams 6s (with the wavelength of λs), 6t (with the wavelength of λt), and 6u (with the wavelength of λu) diffracted by the wavelength combining element 3 and having different wavelengths can be returned to the original light-emitting layer 1a. Thus, one light-emitting layer 1a oscillates in a wide continuous wavelength range, so that the beam diameter of the laser beam 6 is widely distributed on the partially reflecting mirror 5. For this reason, the beam quality of the laser beam emitted from the partially reflecting mirror 5 is reduced.

[0056] The reduction of beam quality can be prevented if the relation L1< <L2 erfüllt ist. Mit zunehmender Differenz zwischen L1 und L2 kann die Wellenlängenbreite pro lichtemittierender Schicht 1a verringert werden. Dadurch wird der Strahldurchmesser des Laserstrahls 6 auf dem teilreflektierenden Spiegel 5 verringert und die Strahlqualität des Laserstrahls 6, der von dem teilreflektierenden Spiegel 5 emittiert wird, wird verbessert. In Fig. 11, the difference between L1 and L2 increases as the crosstalk reduction optical system 4 is brought closer to the wavelength combining element 3, so that L1 is close to zero and L2 (=f) is increased. However, since L1 is not zero or becomes smaller, the wavelength combining laser device 10 as a whole is increased in size.

[0057] Fig. 12 is a diagram showing the configuration of a wavelength combining laser device 12 according to a third embodiment of the present invention. Fig. 13 is a diagram showing the configuration of another wavelength combining laser device 13 according to the third embodiment. The wavelength combining laser devices 12 and 13 according to the third embodiment of the present invention can achieve good beam quality by avoiding the above-mentioned conditions without lengthening the beam path of the external resonator.

[0058] In the wavelength combining laser device 12 according to Fig. 12, the optical system 4 for cross-coupling reduction comprises two lenses, ie a convex lens 41 and a concave lens 42. The convex lens 41 and the concave lens 42 have effects in a laser beam combining direction 401 (see Fig. 12) that is perpendicular to both the optical axis direction of the single laser beam 6 combined in the wavelength combining element 3 and the thickness direction of the LD bar 1. The convex lens 41 as the first lens is arranged on the side close to the wavelength combining element 3. The concave lens 42 as the second lens is arranged on the side close to the partially reflecting mirror 5. If the focal length of the convex lens 41 in the laser beam combining direction 401 is defined as a first focal length f1 and the focal length of the concave lens 42 in the laser beam combining direction 401 is defined as a second focal length f2, then the distance L3 between the convex lens 41 and the concave lens 42 satisfies L3>f1+f2. In other words, the optical system for cross-coupling reduction is not an afocal telescope that satisfies L3=f1+f2.With such a lens arrangement, when the convex lens 41 and the concave lens 42 are combined, a main lens surface H can be positioned on the side closer to the LD bar 1 than to the wavelength combining element 3. The wavelength combining laser device 12 according to . Fig. 12 may thus have a configuration equivalent to the arrangement in which the optical system 4 for cross-coupling reduction is provided with a lens in Fig. 11 is arranged on the main lens surface H. The wavelength combining laser device 12 according to Fig. 12 can therefore realize the configuration equivalent to L1<0 and the difference between L1 and L2 in Fig. 11 enlarge.

[0059] In the wavelength combining laser device 13 according to Fig. 13, the optical system 4 for cross-coupling reduction comprises two convex lenses 43 and 44. The convex lenses 43 and 44 have effects in a laser beam combining direction 402 (see Fig. 13) that are perpendicular to both the optical axis direction of the single laser beam 6 combined in the wavelength combining element 3 and the thickness direction of the LD bar 1. The convex lens 43 as the first lens is arranged on the side close to the wavelength combining element 3. The convex lens 44 as the second lens is arranged on the side close to the partially reflecting mirror 5. When the focal length of the convex lens 43 in the laser beam combining direction 402 is defined as a first focal length f3 and the focal length of the convex lens 44 in the laser beam combining direction 402 is defined as a second focal length f4, the distance L4 between the convex lenses 43 and 44 satisfies L4>f3+f4. In other words, the optical system 4 for cross-coupling reduction is not an afocal telescope that satisfies L4=f3+f4. The relation f3>f4 is also satisfied.The convex lens 44 has a convex effect, i.e., a positive refractive power, in the laser beam combining direction 402. A distance S between the convex lens 44 and the partially reflecting mirror 5 satisfies S>L4-f3. With such a lens arrangement, according to the invention, when the convex lenses 43 and 44 are combined, the main lens surface H is positioned on the side closer to the LD bar 1 than to the wavelength combining element 3. The wavelength combining laser device 13 of the . Fig. 13 can thus have a configuration equivalent to the arrangement in which the optical system 4 for cross-coupling reduction is provided with a lens in Fig. 11 is arranged on the main lens surface H. The wavelength combining laser device 13 according to Fig. 13 can therefore realize the configuration equivalent to L1<0 and the difference between L1 and L2 in Fig. 11 enlarge.

[0060] As described above, according to the wavelength combining laser devices 12 and 13 of the third embodiment, the cross-coupling reducing optical system 4 is configured using two lenses. This reduces cross-coupling oscillation while achieving good beam quality without lengthening the beam path of the external resonator, thus without increasing the size of the wavelength combining laser devices 12 and 13.

[0061] Although Fig. 12 and Fig. 13 do not depict the beam-shaping element 7 shown in the second embodiment, the wavelength-combining laser devices 12 and 13 according to the third embodiment can achieve the above-described effect without the beam-shaping element 7. When the beam-shaping element 7 is employed, the wavelength-combining laser devices 12 and 13 according to the third embodiment have good beam quality without the focusing element 2. Embodiment 4

[0062] Fig. 14 is a diagram showing the configuration of a wavelength combining laser device 14 according to a fourth embodiment of the present invention. Fig. 15 is a diagram showing the configuration of another wavelength combining laser device 15 according to the fourth embodiment.

[0063] On Fig. Referring to FIG. 14, a partially reflecting surface 5a of the partially reflecting mirror 5 faces the inside of the beam path of the external resonator. A totally transmitting surface 5b of the partially reflecting mirror 5 faces the outside of the beam path of the external resonator. When the wavelength combining laser device 14 has a plurality of LD bars 1, the laser beams 6 of higher power are collected on the partially reflecting surface 5a of the partially reflecting mirror 5 than when it has only one LD bar 1. Note that the totally transmitting surface 5b has an antireflection layer formed by vacuum evaporation or another method.

[0064] In the wavelength combining laser device 14 according to the fourth embodiment, the partially reflecting surface 5a is an uncoated surface. That is, a layer that potentially absorbs the laser beams 6 is removed, so damage is less likely to occur even when the laser beams 6 are collected at high power. As a result, in the wavelength combining laser device 14, the laser beams 6 are reflected at a reflectance of approximately 4% at the interface between the glass material of the partially reflecting mirror 5 and the air and returned to the light-emitting layers 1a, causing the wavelength combining laser device 14 to oscillate.

[0065] At the Fig. In the wavelength combining laser device 15 according to the fourth embodiment shown in FIG. 15, the partially reflecting surface 5a of the partially reflecting mirror 5, which faces the inside of the beam path of the external resonator, is a concave surface. Thus, the partially reflecting surface 5a has a negative refractive power. The concave surface of the partially reflecting surface 5a distributes the laser beams 6 onto the totally transmitting surface 5b, thereby reducing the energy density and reducing the likelihood of damage to the totally transmitting surface 5b.

[0066] That is, the partially reflecting surface 5a of the partially reflecting mirror 5 is formed as an uncoated surface in the wavelength combining laser device 14 according to the fourth embodiment. The partially reflecting surface 5a is formed as the concave surface in the wavelength combining laser device 15 according to the fourth embodiment. This reduces crosstalk oscillation. At the same time, damage to the partially reflecting mirror 5 is reduced, thus achieving a long service life.

[0067] It should be noted that the partially reflecting surface 5a and the totally transmitting surface 5b can be arranged in reverse positions. Thus, the totally transmitting surface can face the inside of the beam path of the external resonator, and the partially reflecting surface can face the outside of the beam path of the external resonator. Furthermore, although the focusing element 2 is arranged in the Fig. 14 and Fig. 15 is not shown, the above-described effect can be achieved with or without the focusing element 2. When the focusing element 2 is used, the wavelength-combining laser devices 14 and 15 according to the fourth embodiment can reduce damage to the partially reflecting mirror 5 without the beam-shaping element 7.

[0068] The configurations shown in the embodiments described above are merely examples of one aspect of the present invention. They may therefore be combined with other known technology or partially omitted and / or modified without departing from the scope of the present invention. LIST OF REFERENCE SYMBOLS 1 LD bar 1a, 1p, 1q light-emitting layer 1b light-emitting front side 1c front side (totally reflecting mirror) 2 focusing element 3 wavelength combining element 4 optical system for cross-coupling reduction 5 partially reflecting mirror 5a partially reflecting surface 5b totally permeable area 6 laser beam 6a, 6i, 6j, 6k, 6p, 6q main beam 6r, 6s, 6t, 6u, 6v beam 7 beam-shaping element 8 λ / 2 plate 10, 11, 12, 13, 14, 15, 20 wavelength combining laser device 41, 43, 44 convex lens 42 concave lens 71 FAC 72 SAC 72a, 73a Incident surface 72b, 73b Exit surface 73 Beam path conversion element θi, θj, θk angle of incidence ψ radiation angle (diffraction angle) A single point B point H principal lens surface

Claims

[1] A wavelength combining laser device (10) comprising: a semiconductor laser (1) for emitting a plurality of laser beams (6) in an optical axis direction from a light-emitting end face (1b) of the plurality of light-emitting regions (1a) arranged in a laser beam combining direction, the optical axis direction being perpendicular to the laser beam combining direction; a focusing element (2) which collimates the laser beams (6); a wavelength combining element (3) for combining the plurality of laser beams (6) in the laser beam combining direction into a single laser beam (6) and outputting the single laser beam (6); an optical system (4) for cross-coupling reduction having a positive refractive power in the laser beam combining direction perpendicular to an optical axis of the single laser beam (6) output from the wavelength combining element (3); and a partially reflecting mirror (5) for reflecting the single laser beam (6) passing through the crosstalk reducing optical system (4) and for allowing the single laser beam (6) to transmit through and exit from the partially reflecting mirror (5), wherein the optical system (4) for cross-coupling reduction is arranged to image the light-emitting end face (1b) onto the partially reflecting mirror (5) by causing the light-emitting end face (1b) to be conjugated with the partially reflecting mirror (5) in a plane spanned by the optical axis of the individual laser beam (6) and the laser beam combining direction, wherein the optical system (4) for cross-talk reduction comprises two lenses (41, 42; 43, 44) which have an effect in the laser beam combining direction perpendicular to the direction of the optical axis of the single laser beam (6) and satisfy a relation f1>f2, where f1 is a focal length in the laser beam combining direction of the first lens (41, 43) arranged on a side closer to the wavelength combining element (3), and where f2 is a focal length in the laser beam combining direction of a second lens (42, 44) arranged on a side closer to the partially reflecting mirror (5), and a distance L between the first lens (41, 43) and the second lens (42, 44) satisfies L>f1+f2, wherein a common main lens surface (H) of the first lens (41, 43) and the second lens (42, 44) is positioned on a side closer to the semiconductor laser (1) than to the wavelength combining element (3). [2] Wavelength combining laser device (13) according to claim 1, wherein the second lens (44) has a positive refractive power in the laser beam combining direction, and a distance S between the second lens (44) and the partially reflecting mirror (5) satisfies S>L-f1. [3] A wavelength combining laser device (14) according to any one of claims 1 or 2, wherein a partially reflecting surface (5a) on the partially reflecting mirror (5) is uncoated. [4] A wavelength combining laser device (15) according to claim 3, wherein the partially reflecting surface (5a) of the partially reflecting mirror (5) has a negative refractive power. [5] A wavelength combining laser device (10) according to any one of claims 1 to 4, wherein a plurality of semiconductor lasers (1) are provided. [6] A wavelength combining laser device (10) according to any one of claims 1 to 5, further comprising a λ / 2 plate (8) arranged between the semiconductor laser (1) and the wavelength combining element (3).

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