Patent Description:
There are known laser processing devices for welding or processing workpieces by guiding the laser beam emitted from a laser oscillator such as a gas laser or a solid-state laser through an optical fiber.

It is common to perform alignment between the laser oscillator and the optical fiber in order to enhance their coupling efficiency, thereby ensuring the laser beam power used for processing.

A well-known alignment technique is performed as follows. A laser beam from the laser oscillator strikes one end of the optical fiber through an optical member such as a condenser lens, and the power of the laser beam emitted from the other end is measured with, for example, a power meter. The position of the condenser lens is adjusted to maximize the power (e.g., Patent Literature <NUM>).

Further prior art is known from documents <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

<CIT> discloses an optical transmission device, a solid state laser device, and a laser beam processing device having the optical transmission device or the solid state laser device for transmitting laser beam having a highly focusability used for laser beam processing for industrial processing purpose, medical laser application purpose, and the like.

There has been a growing demand for high processing precision in recent years, and therefore, for the control of the shape of the laser beam applied to workpieces.

However, the condenser lens position where the laser beam power is maximum does not necessarily coincide with the condenser lens position where the beam quality, typified by laser beam shape, is optimized. For this reason, when an alignment is completed with the condenser lens fixed at the former position, it is not always possible to obtain a desired beam quality. This may cause the alignment process to be repeated again, thus decreasing the work efficiency.

In view of these problems, an object of the present invention is to provide an alignment method performed while monitoring the power and quality of the laser beam at the same time.

To accomplish the object, according to an aspect of the present invention, the laser beam emitted from the laser oscillator through the optical fiber is divided such that the power and quality of the laser beam can be measured at the same time. The alignment between the laser oscillator and the optical fiber is performed based on these measurement results.

To be more specific, the present invention is a method for alignment between a laser oscillator and an optical fiber to be connected to the laser oscillator as defined in the appended claims.

According to this method for alignment between the laser oscillator and the optical fiber, the laser beam power can be maximized with at least a certain level of beam quality ensured while the power and quality of the beam are being monitored at the same time.

This method enables the laser beam to be narrowed and formed into to a desired shape while keeping the laser beam power at a level sufficient for processing.

The lens adjustment mechanism may include a displacement sensor configured to detect the position of the condenser lens, and an actuator configured to move the condenser lens. The first adjustment step preferably includes driving the actuator to move the position of the condenser lens detected by the displacement sensor to the first lens position. The second adjustment step preferably includes driving the actuator to move the position of the condenser lens detected by the displacement sensor to the second lens position. The fourth adjustment step preferably includes driving the actuator to move the position of the condenser lens detected by the displacement sensor to the position where the BPP is not more than the predetermined value.

This method eliminates the need for a manual operation to be performed near the laser oscillator during laser oscillation, thereby improving the safety of the alignment process.

According to the aspect of the present invention, the laser beam can be narrowed and formed into a desired shape and can also have a maximum power. The laser processing device subjected to such an alignment can perform excellent laser processing.

The embodiments of the present invention will be described in detail as follows with reference to drawings. The following embodiments do not limit the present invention, which invention is defined by the claims.

<FIG> shows the configuration of laser processing device <NUM> according to a first embodiment which is not encompassed by the wording of the claims but is considered useful for understanding the invention. Device <NUM> includes laser oscillator <NUM> for emitting a laser beam and optical fiber <NUM> for guiding the laser beam emitted from laser oscillator <NUM>. Device <NUM> further includes laser emission head <NUM> and manipulator <NUM>. Laser emission head <NUM>, which is connected to the emission end (not shown) of optical fiber <NUM>, applies laser beam <NUM> guided by optical fiber <NUM> to workpiece <NUM>. Manipulator <NUM> operates laser emission head <NUM> to move it to workpiece <NUM>.

Device <NUM> further includes controller <NUM> connected to laser oscillator <NUM> and manipulator <NUM>. Controller <NUM> includes a plurality of arithmetic processors, a controller, a storage unit and a display unit (<FIG>) so as to control the laser oscillation of laser oscillator <NUM> and the operation of manipulator <NUM>.

When alignment between laser oscillator <NUM> and optical fiber <NUM> is performed for maintenance or other purposes, manipulator <NUM> moves laser emission head <NUM> to laser beam evaluation device <NUM>, which is connected to controller <NUM>. Laser beam evaluation device <NUM> evaluates the properties of laser beam <NUM>.

Controller <NUM> receives signals from device <NUM>, stores them, processes them into desired forms, and display them on a display unit (<FIG>).

<FIG> is a schematic diagram of laser beam evaluation device <NUM>. Device <NUM> includes incident unit <NUM> of laser beam <NUM>, partial reflection mirror <NUM> and damper <NUM>. Mirror <NUM> reflects part of the incident laser beam <NUM> and transmits the rest. Damper <NUM> absorbs laser beam <NUM> reflected by partial reflection mirror <NUM>.

Laser beam evaluation device <NUM> further includes partial reflection mirror <NUM> and far field pattern (FFP) measurement unit <NUM>. Mirror <NUM> reflects part of laser beam <NUM> that has passed through partial reflection mirror <NUM> and transmits the rest. FFP measurement unit <NUM> measures the intensity distribution of laser beam <NUM> reflected by partial reflection mirror <NUM>. Laser beam evaluation device <NUM> further includes power measurement unit <NUM> for measuring the power of laser beam <NUM> that has passed through partial reflection mirror <NUM>.

In laser beam evaluation device <NUM>, the laser beam emitted from laser oscillator <NUM> passes through optical fiber <NUM>, collected by laser emission head <NUM>, and strikes incident unit <NUM>.

Laser beam <NUM> incident on device <NUM> is divided by partial reflection mirrors <NUM> and <NUM>, and the divided beams are supplied to power measurement unit <NUM>, FFP measurement unit <NUM> and damper <NUM>.

Power measurement unit <NUM> measures the power of the incident laser beam <NUM> (first laser beam), while FFP measurement unit <NUM> measures the FFP, which is the far-field image of laser beam <NUM> (second laser beam). This FFP corresponds to the laser beam shape and shows a spatial intensity distribution on the surface on which the laser beam is incident.

Partial reflection mirrors <NUM> and <NUM> have predetermined reflectances of laser beams <NUM> and <NUM>, respectively. Mirrors <NUM> and <NUM> divide laser beams <NUM> and <NUM> such that the amount of laser beams required for measurement can be transmitted to power measurement unit <NUM> and FFP measurement unit <NUM>.

For example, the reflectance and transmittance of mirror <NUM> are set so as to reflect <NUM>% of the incident laser beam <NUM> to damper <NUM> and to transmit the remaining <NUM>%. Similarly, the reflectance and transmittance of mirror <NUM> are set so as to reflect <NUM>% of the incident laser beam <NUM> to FFP measurement unit <NUM> and to transmit the remaining <NUM>%.

The placement of power measurement unit <NUM>, FFP measurement unit <NUM> and damper <NUM> in laser beam evaluation device <NUM> is not limited to the one shown in <FIG>. For example, unit <NUM> and damper <NUM> may be replaced by each other. In that case, the reflectance and transmittance of mirrors <NUM> and <NUM> can be changed appropriately.

Laser beam <NUM> is absorbed by damper <NUM> and consumed as heat.

<FIG> and <FIG> are schematic diagrams of the joint between laser oscillator <NUM> and optical fiber <NUM>. Laser processing device <NUM> further includes fiber joint <NUM> disposed between laser oscillator <NUM> and optical fiber <NUM>. Fiber joint <NUM> includes aperture <NUM>, condenser lens <NUM> and lens adjustment mechanisms <NUM> to <NUM>. Aperture <NUM> is located on the plane perpendicular to the optical axis of the laser beam. Condenser lens <NUM> is movable along each of the X, Y and Z axes. The optical axis is along the Z axis. When the plane perpendicular to the optical axis is referred to as x-y plane, two axes perpendicular to each other in the x-y plane are referred to as the X axis and the Y axis. Lens adjustment mechanisms <NUM> to <NUM> adjust the position of condenser lens <NUM> along these axes. In the present embodiment, lens adjustment mechanisms <NUM> to <NUM> are operated manually. Fiber joint <NUM> further includes connector <NUM> for fixing the end of optical fiber <NUM> such that incident end surface <NUM> of optical fiber <NUM> can be perpendicular to the optical axis of the laser beam.

<FIG> is a functional block diagram of the laser processing device according to the present embodiment. Laser oscillator <NUM> includes laser oscillation unit <NUM> and fiber joint <NUM> shown in <FIG> and <FIG>.

Manipulator <NUM> includes servomotor <NUM> for rotating each joint of manipulator <NUM> and encoder <NUM> connected to servomotor <NUM>.

For simplification, <FIG> includes only one servomotor <NUM> and only one encoder <NUM>.

Controller <NUM> includes laser controller <NUM>, motor controller <NUM>, storage unit <NUM>, power supply <NUM>, display unit <NUM>, input unit <NUM> and calculation unit <NUM>.

Laser controller <NUM> transmits a control signal to power supply <NUM> according to the control program received from input unit <NUM> so as to control the output of laser oscillation unit <NUM>.

Motor controller <NUM> controls the speed and amount of rotation of servomotor <NUM> of manipulator <NUM> according to the control program received from input unit <NUM> and to the feedback signal obtained from encoder <NUM>.

Storage unit <NUM> stores the following information: the information about the laser beam power received from power measurement unit <NUM> of laser beam evaluation device <NUM>, and the information about the laser beam intensity distribution received from FFP measurement unit <NUM> of laser beam evaluation device <NUM>.

Power supply <NUM> supplies electric power to laser oscillation unit <NUM> according to the control signal received from laser controller <NUM> and the control signal received from the output monitor (not shown) of laser oscillator <NUM>.

Display unit <NUM> displays the following: the output of laser oscillator <NUM>, the power and FFP of the laser beam measured by laser beam evaluation device <NUM>, and the beam parameter product (BPP) of the laser beam calculated by calculation unit <NUM>.

Input unit <NUM> receives the control program and numerical values that determine the output of laser oscillator <NUM> and the speed and amount of travel of manipulator <NUM>.

Calculation unit <NUM> performs arithmetic processing using the information received from input unit <NUM> and stored in storage unit <NUM>. The arithmetic processing is used for the control performed by laser controller <NUM> and motor controller <NUM>. For example, calculation unit <NUM> fetches the information about the laser beam intensity distribution from storage unit <NUM> and calculates the BPP of the laser beam. Furthermore, calculation unit <NUM> can process various kinds of information into charts and graphs in appropriate forms and can make them displayed on display unit <NUM>.

<FIG> only illustrates the functional blocks used for the alignment, and the other functions are not illustrated. For example, a safe stop function block and the storage of the control program are not shown.

Whether signals and commands are supplied to laser controller <NUM>, motor controller <NUM> and calculation unit <NUM> either directly from input unit <NUM> or after being stored in storage unit <NUM> is determined appropriately by the specification of laser processing device <NUM> or controller <NUM>, and is not limited to the procedure described in the present embodiment. Similarly, the signal flow in controller <NUM> is not limited to that described in the present embodiment, either.

<FIG> and <FIG> show flowcharts of processes of alignment between laser oscillator <NUM> and optical fiber <NUM> according to the present embodiment.

First, lens adjustment mechanisms <NUM> to <NUM> move condenser lens <NUM> to the designed center position with respect to the X, Y and Z axes, so that laser emission head <NUM> and laser beam evaluation device <NUM> can be set in position. Controller <NUM> makes laser oscillator <NUM> oscillate at an output power close to actual use conditions (Step S1).

Next, the laser beam power, which has been evaluated by power measurement unit <NUM> of laser beam evaluation device <NUM>, is monitored (Step S2).

Lens adjustment mechanisms <NUM> to <NUM> of fiber joint <NUM> move condenser lens <NUM> based on the monitoring results so as to perform alignment (Step S3).

Whether the laser beam power is maximized or not is monitored and determined by moving condenser lens <NUM> (Step S4). When a lens position P1 where the power is maximum is found, the lens position P1 is either recorded on recording paper or stored in storage unit <NUM> (<FIG>) of controller <NUM> (Step S5).

The details of Step S3 above will be described with reference to <FIG>.

As shown in <FIG> and <FIG>, when lens adjustment mechanisms <NUM> to <NUM> are turned clockwise when viewed along their axes, condenser lens <NUM> moves in the positive direction of these axes. When mechanisms <NUM> to <NUM> are turned counterclockwise, condenser lens <NUM> moves in the negative direction of these axes.

Referring to the laser beam power displayed on display unit <NUM> of controller <NUM>, for example, lens adjustment mechanism <NUM> moves condenser lens <NUM> on the designed center position along the negative direction of the X axis.

As shown in <FIG>, if the power measured by power measurement unit <NUM> increases as condenser lens <NUM> is moved in the negative direction of the X axis, then condenser lens <NUM> is moved in the negative direction of the X axis until the power begins to decrease (route "a").

When condenser lens <NUM> is moved in the negative direction of the X axis until the power begins to decrease, condenser lens <NUM> is moved again in the positive direction of the X axis until reaching the position where the power is maximum (route "b").

Meanwhile, as shown in <FIG>, if the power measured by power measurement unit <NUM> decreases as condenser lens <NUM> is moved in the negative direction of the X axis (the end of route "c"), then condenser lens <NUM> is moved in the positive direction of the X axis.

The power is confirmed to increase as condenser lens <NUM> is moved in the positive direction of the X axis. Condenser lens <NUM> is moved in the positive direction of the X axis until the power begins to decrease (route "d"). When the power begins to decrease, condenser lens <NUM> is again moved in the negative direction of the X axis until reaching the position where the power is maximum (route "e").

Thus, condenser lens <NUM> is moved along the X axis to the position where the power is maximum. This operation may be repeated two or more times.

Lens adjustment mechanisms <NUM> and <NUM> move condenser lens <NUM> to the position where the power is maximum along each of the Y and Z axes in the same manner as along the X axis.

When the positioning of condenser lens <NUM> is completed, the position P1 of condenser lens <NUM> where the power is maximum along each of the X, Y and Z axes is either recorded on recording paper or stored in storage unit <NUM> of controller <NUM>.

Referring back to the flowcharts of <FIG> and <FIG>, subsequent to Step S5, FFP measurement unit <NUM> measures the FFP of the laser beam (Step S6). Next, the BPP of the laser beam is calculated based on the FFP (Step S7). Lens adjustment mechanisms <NUM> to <NUM> of fiber joint <NUM> move condenser lens <NUM> based on the obtained BPP so as to perform alignment (Step S8).

Whether the BPP of the laser beam is minimized or not is monitored and determined by moving condenser lens <NUM> (Step S9). When a lens position P2 where the BPP is minimum is found, the lens position P2 is either recorded on recording paper or stored in storage unit <NUM> of controller <NUM> (Step S10).

The BPP is an index of the laser beam quality, which is generally expressed by Formula (<NUM>). <MAT> where.

In Step S7, the BPP can be derived from the FFP as follows.

First, the position coordinate where the light intensity has a peak value and the position coordinate where the light intensity is <NUM>/e<NUM> of the peak value are obtained from the FFP measured by FFP measurement unit <NUM>. The distance between the two positions corresponds to the laser beam radius D.

FFP measurement unit <NUM> measures the FFP in the direction of travel of the laser beam at regular spacings so as to derive the beam radius D. The obtained beam radius D is plotted with respect to the direction of travel of the laser beam. In the present embodiment, the laser beam travels along the Z axis.

The plot can be fitted to a hyperbola so as to find the beam waist radius r of the laser beam.

When the laser beam is collected on FFP measurement unit <NUM>, an fθ lens (not shown) can be used. In this case, the divergence angle distribution of the laser beam can be converted into a positional distribution on the light-receiving surface, thus facilitating the finding of the divergence angle θ of the laser beam.

The beam waist radius r and the divergence angle θ obtained as above can be substituted into the above-mentioned formula (<NUM>) to obtain the BPP.

The procedure in Step S10 is the same as that in Step S3 (<FIG>) except for finding the position, of the condenser lens <NUM>, where the BPP is minimum.

This operation may also be repeated two or more times.

Referring back to the flowcharts of <FIG> and <FIG>, laser beam <NUM> again strikes laser beam evaluation device <NUM>. This time, attention is paid to both the power and quality of the laser beam.

First, lens adjustment mechanisms <NUM> to <NUM> move condenser lens <NUM> at predetermined spacings along a three-dimensional coordinate vector that points from the position P2 (Xb, Yb, Zb) of condenser lens <NUM> where the BPP is minimum toward the position P1 (Xp, Yp, Zp) of condenser lens <NUM> where the power is maximum (Step S11).

The predetermined spacings, which are different between the X, Y and Z axes, are in the range of several hundred nanometers to several micrometers. These values, however, can be changed appropriately according to the processing conditions, the desired laser beam shape and other conditions.

Laser beam evaluation device <NUM> measures the power of laser beam <NUM> (Step S12) and the FFP of laser beam <NUM> so as to find the BPP of laser beam <NUM> (Step S13).

It is determined whether condenser lens <NUM> has reached the position P1 (Step S14). If not, condenser lens <NUM> is moved by a predetermined spacing, and Steps S12 and S13 are performed again. If condenser lens <NUM> has reached the position P1, the process goes to Step S15.

It is monitored and determined whether the BPP of laser beam <NUM> is not less than a predetermined value (Step S15). If the BPP is not less than the predetermined value, the position immediately before the position where the BPP has the predetermined value is determined, and the determined position is fixed as the final position of condenser lens <NUM> (Step S16). As a result, the position where the BPP is less than the predetermined value and which is closest to the position P1 is fixed as the final position.

If the BPP remains less than the predetermined value even after the condenser lens is moved from the position P2 to the position P1, the position P1 is fixed as the final position of condenser lens <NUM> (Step S17).

In Steps S16 and S17, the final position of condenser lens <NUM> may be stored in storage unit <NUM> of controller <NUM>. In this case, the value can be used in the next alignment.

In the present embodiment, laser oscillator <NUM> is a solid-state YAG laser. Whether the process goes to Step S16 or Step S17 is determined by whether the BPP is not less than <NUM> (mm·mrad). This value, however, a mere example. For example, if laser oscillator <NUM> is a multi-wavelength laser formed of semiconductor lasers with different wavelengths, then the above determination is made by whether the BPP is not less than a predetermined value in the range of <NUM> to <NUM> (mm·mrad). The BPP value used as the criteria can be changed appropriately according, for example, to the oscillation wavelength of the laser, or the desired processing condition.

As described above, according to the present embodiment, laser beam <NUM> emitted from laser oscillator <NUM> through optical fiber <NUM> is divided into laser beams <NUM>, <NUM> and <NUM>. Laser beam <NUM> is led to power measurement unit <NUM>, whereas laser beam <NUM> is led to FFP measurement unit <NUM>. As a result, the power of laser beam <NUM> and the spatial intensity distribution corresponding to the laser beam shape can be monitored at the same time. With this configuration, alignment between laser oscillator <NUM> and optical fiber <NUM> can be achieved such that the power can be maximized with at least a certain level of beam quality while the power and quality of the laser beam are monitored at the same time.

In general, an increase in the output of the laser beam often causes changes in the intensity distribution. In the present embodiment, laser beam <NUM>, which is not used for the measurement, is absorbed by damper <NUM>, so that the laser beam can be measured under high output conditions used in actual processing. For example, the laser beam can be evaluated at kilowatt levels, and alignment can be performed based on the evaluation results.

<FIG> is a functional block diagram of a laser processing device according to a second embodiment which is not encompassed by the wording of the claims but is considered useful for understanding the invention. The present embodiment greatly differs from the first embodiment in that lens adjustment mechanisms <NUM> to <NUM> include actuator <NUM> and displacement sensor <NUM>.

Actuator <NUM>, which is disposed on each of the X, Y and Z axes, moves condenser lens <NUM> based on the control signal from actuator controller <NUM> of controller <NUM>.

Displacement sensor <NUM> detects the position of condenser lens <NUM> on the three-dimensional coordinate with a designed center position as the origin. The detected location information is transmitted as an electrical signal to storage unit <NUM> of controller <NUM>, and is fed back to actuator controller <NUM>.

Displacement sensor <NUM> may be disposed on each of the X, Y and Z axes. For simplification, <FIG> includes only one actuator <NUM> and only one displacement sensor <NUM>. In fiber joint <NUM> shown in <FIG>, aperture <NUM>, condenser lens <NUM> and connector <NUM> are omitted.

Unlike controller <NUM> shown in <FIG>, controller <NUM> shown in <FIG> has actuator controller <NUM>.

Actuator controller <NUM> generates a control signal for adjusting the position of condenser lens <NUM> and transmits the control signal to actuators <NUM>. The control signal is generated based on the information about the power and intensity distribution of the laser beam stored in storage unit <NUM>, and the location information of condenser lens <NUM> transmitted from displacement sensor <NUM>. Actuator controller <NUM> has the function of calculation unit <NUM> shown in <FIG>. Actuator controller <NUM> can fetch the information about the intensity distribution of the laser beam from storage unit <NUM> and calculates the BPP of the laser beam.

Controller <NUM> in the present embodiment may alternatively include calculation unit <NUM> as an independent component, like in the first embodiment. <FIG> only illustrates the functional blocks used for the alignment, similar to <FIG>.

When alignment is performed using the above configuration, the process of alignment shown in <FIG> and <FIG> can be performed automatically by receiving the control program from input unit <NUM> and executing the program. For example, in Step S3, S8 and S11 of the flowcharts of <FIG> and <FIG>, condenser lens <NUM> is moved by actuator <NUM> disposed in each of lens adjustment mechanisms <NUM> to <NUM>. In Steps S4, S9, S14 and S15, various kinds of determinations are performed either by actuator controller <NUM> or calculation unit <NUM>. In Steps S5, S10, S16 and S17, the positions of condenser lens <NUM> in these cases are stored in storage unit <NUM>.

In the present embodiment, the travel of condenser lens <NUM> during the alignment can be automatically performed using controller <NUM> of laser processing device <NUM>. This eliminates the need for the alignment operator to work near the emission of the laser beam.

In particular, when alignment is performed with several kilowatts of output power close to the actual use conditions, the safety of the alignment operator should be ensured. The present embodiment can avoid problems that can occur during alignment.

Whether signals and commands are supplied to controllers <NUM> to <NUM> either directly from input unit <NUM> or after being stored in storage unit <NUM> is determined appropriately by the specification of laser processing device <NUM> or controller <NUM>, and is not limited to the procedure described in the present embodiment. Similarly, the signal flow in controller <NUM> is not limited to that described in the present embodiment, either.

In the first and second embodiments, power measurement unit <NUM> is preferably a photoelectric sensor such as a photodiode. Photoelectric sensors have a rapid time response to the incident laser beam, so that the alignment takes less time. However, it is alternatively possible to use a thermal sensor such as an ordinary power meter although its time response is slower.

FFP measurement unit <NUM> is preferably a camera with a two-dimensional photosensor array such as a CCD or a CMOS image sensor. A two-dimensional photosensor array can obtain an accurate intensity distribution when it has a rapid time response to the incident laser beam and the array and pixel sizes are appropriate.

When both power measurement unit <NUM> and FFP measurement unit <NUM> are photoelectrical conversion devices, the power of the incident laser beam should be sufficiently narrowed in order to prevent damage or deterioration of the devices. Furthermore, the reflectance and transmittance of partial reflection mirrors <NUM> and <NUM> should be set carefully.

For example, the power of the laser beam incident on FFP measurement unit <NUM> is preferably reduced to the range of several microwatts to several hundreds of microwatts, when unit <NUM> is a CCD camera. The power of the laser beam incident on power measurement unit <NUM> is preferably reduced to the range of several milliwatts to several hundreds of milliwatts when unit <NUM> is a photodiode.

In the first and second embodiments, laser beam evaluation device <NUM> measures the laser beam emitted from laser emission head <NUM> so as to achieve the alignment between laser oscillator <NUM> and optical fiber <NUM>. Alternatively, however, device <NUM> may directly measure the laser beam emitted from the emission end of optical fiber <NUM>.

Claim 1:
A method for alignment between a laser oscillator (<NUM>) and an optical fiber (<NUM>) to be connected to the laser oscillator (<NUM>), the method comprising:
placing a condenser lens (<NUM>) and a lens adjustment mechanism (<NUM>-<NUM>) between a light emission part of the laser oscillator (<NUM>) and an incident end of the optical fiber (<NUM>), the condenser lens (<NUM>) being configured to collect a laser beam (<NUM>) emitted from the laser oscillator (<NUM>), and the lens adjustment mechanism (<NUM>-<NUM>) being configured to adjust a position of the condenser lens (<NUM>),
the method further comprising:
a measurement step of
dividing the laser beam (<NUM>) emitted from an emission end of the optical fiber (<NUM>) into a plurality of laser beams including at least a first laser beam (<NUM>) and a second laser beam (<NUM>), and
measuring a power of the first laser beam (<NUM>) and a far field pattern FFP of the second laser beam (<NUM>);
a first adjustment step of
adjusting the position of the condenser lens (<NUM>) such that the measured power of the first laser beam (<NUM>) is maximum, and
storing the position of the condenser lens (<NUM>) as a first lens position (P1);
a second adjustment step of
calculating a beam parameter product BPP of the second laser beam (<NUM>) from the FFP of the second laser beam measured in the measurement step,
adjusting the position of the condenser lens (<NUM>) such that the BPP is minimum, and
storing the position of the condenser lens (<NUM>) as a second lens position (P2);
a third adjustment step of
measuring an FFP of the second laser beam (<NUM>) and calculating therefrom a BPP, each time the condenser lens (<NUM>) is successively moved by a predetermined spacing from the second lens position (P2) in the direction of the first lens position (P1); and
a fourth adjustment step of
when the position of the condenser lens (<NUM>) has reached the first lens position (P1), and the BPP calculated from the FFP of the second laser beam (<NUM>) measured in the third adjustment step is not less than a predetermined value,
fixing the condenser lens at the position immediately before the position where the BPP has the predetermined value, as the final position of the condenser lens (<NUM>) (S16) and
when the BPP is less than the predetermined value, fixing, as the final position of the condenser lens (<NUM>), the first lens position (P1) (S17).