KR20110034338A - Laser machining system having high transmission power stabilizer and optical attenuator - Google Patents

Abstract

PURPOSE: A laser machining system having high transmission power stabilizer and optical attenuator is provided to perform precision-machining by using an output stabilizing device. CONSTITUTION: A laser machining system having high transmission power stabilizer and optical attenuator comprises a laser unit(20), a half-wave plate(21), and a molten silica plate(22) and a PID controller. The laser unit emits a pulse laser beam. The half-wave panel converts the inflow angle of layer beam and varies a polarization value about a brewster window. The molten silica plate comprises the brewster window which is tilted in Brewster`s angle. The PID controller collects the light passing through the window by an optical detector and a peak/sample hold amplifier.

Description

Laser machining system having high transmission power stabilizer and optical attenuator

The present invention relates to a laser processing system, and more particularly, an output stabilization device for stabilizing output by an external control method based on polarization theory through a half-wave plate and Brewster's angle. A laser processing system having a stabilizer is provided.

Laser processing uses heat generated from the interaction between laser light and materials, and is a thermal processing method that uses the high energy density and high directivity of the laser.The laser oscillator technology, the machine manufacturing technology, the beam transmission system technology, and the control technology This is a technology that is sequentially connected. Laser processing has a number of advantages, in particular, because of the thermal processing and the very small condensing area, local processing of several tens of micrometers is possible, and the stress and distortion of the material is almost lower than other processing techniques.

Such laser processing can be classified into laser macro-machining that can process 100μm or more and laser micro-machining that can process 0.1 ~ 50μm.

The core technologies of laser micromachining include laser light source technology with wavelength and output suitable for the type and thickness of the workpiece, design of cutting head or welding head that can focus the laser beam to the lens, sample / beam moving system technology, and movement. CNC lathe technology that can operate within ± 1μm reproducibility, computer program for laser processing. In particular, laser micromachining technology has been put to practical use through the development of various laser light sources such as CO ₂ lasers, Nd: YAG lasers, and UV lasers, and the development of high power, ultra-short pulse, high repetition rate, and high efficiency laser technology.

Among the core technologies of the laser micromachining as mentioned above, the problem is the irregular output of the laser light, the temporal change of the oscillation wavelength, the irregular phenomenon of scattering when the laser light is incident on the medium. The instability of the laser output is difficult to make fine adjustments, which limits the application of the laser, and even if this instability is controlled, the irregular phenomenon of scattering when the light is incident on the medium limits its application. This example can be seen in the case of micromachining with a Q-switched Nd: YAG laser. In order to secure precision and uniformity of micromachining, a technique for controlling space-time by measuring characteristics of laser light in real time is required. In particular, in the case of processing using a pulse laser having an output variation rate of about 5%, measurement and control of laser power is essential for ultra-precision machining.

Methods of controlling the laser output include a method of controlling inside the laser oscillator and a method of controlling externally. First, in the case of internal control, a cavity dumper is generated by using an etalon or an acoustic optical component to make a single mode. However, this internal control method has a disadvantage in that the output loss is severe because the oscillation is suppressed and the oscillation is suppressed except for the matching frequency, and it is difficult to apply to the laser processing machine already used. In the case of external control, there is a method using an electro-optic modulator or an acoustic-optic modulator, which has a disadvantage in that the wavelength and output conditions of the applicable light source are limited.

Conventional laser processing generally employs an optical attenuator with a dielectric filter. The relevant component includes a dielectric filter for the neutral optical unit as well as beam attenuation to compensate for any beam displacement. Continuous attenuation of the combined laser beam is obtained by changing the angle of incidence of the dielectric filter in the range of 0 to 45 degrees. Systems with a dielectric filter can exhibit a maximum transmission of 90% at 248 nm wavelength.

Accordingly, an object of the present invention is to solve the above problems, and an object thereof is an output stabilization device for stabilizing an output by an external control method based on polarization theory through a half-wave plate and Brewster's angle. To provide a laser processing system with a power stabilizer.

Another object of the present invention is to use a fused silica laminate which is not affected by laser wavelength or power, and is suitable for ultra-short wavelength execution and high damage by using pulse laser such as aximmer laser or high power UV laser for laser processing. It is to provide a laser processing system showing prevention.

It is still another object of the present invention to provide a laser processing system that lowers the system cost by employing a Brewster window, i.e., a pair of uncoated fused silica, instead of a coated large diameter high power laser optical device, i.e., a polarizer.

It is still another object of the present invention to provide a laser processing system having improved optical permeability during optical attenuation by PID control of a high transmission power stabilization device using a half-wave plate and a Brewster window for a high power UV pulse laser.

The present invention for achieving the above object is to change the polarization degree when having a Brewster window at the rear end by changing the incident angle of the laser device that emits a laser beam, attached to the rotation stage rotatably supported and passing through the laser beam At least 2 having a half-wave plate, each of which has a Brewster window tilted at the Brewster angle and which changes the polarization direction of the laser beam incident through the half-wave plate to output the laser beam passing through as a laser beam used for laser processing Fused silica plates, and light scattered through the window of the polarized light through a photodetector and a peak / sample hold amplifier, and then compensates for errors between the scattered laser signal collected and a given set value PID control for rotationally driving the half-wave plate to minimize variations It provides a laser processing system comprising a group.

It is preferable that the said laser apparatus consists of a pulse laser.

In addition, the fused silica plate is positioned so that a high transmittance is obtained and the loss of polarization degree is minimized.

Moreover, as the number of windows increases, the polarization ratio increases.

As the angle between the half-wave plate and the optical axis is rotated from 0 to 45 °, the degree of polarization decreases.

In addition, as the number of windows increases, the output of the polarized laser beam decreases, and the polarization degree and the light transmittance increase.

Moreover, the PID controller controls to realize the PID control algorithm. It may be configured as a personal computer (PC) on which a program is mounted.

The half-wave plate is attached to a rotatable stage rotatably supported, and the PID controller preferably drives the rotation stage and the half-wave plate to rotate by rotating the motor through a motor controller to minimize variations in laser beam output. .

Preferably, the apparatus further comprises a beam dump for receiving the vertically polarized laser beam.

As described above, the present invention includes a power stabilizer for stabilizing the output by an external control method based on polarization theory through a half-wave plate and Brewster's angle. This can be done.

In addition, the present invention uses a fused silica laminate which is not affected by the laser wavelength or power, and is suitable for ultra-short wavelength execution by using a pulse laser such as an aximmer laser or a high power UV laser for laser processing, and prevents high damage. Indicates.

Furthermore, in the present invention, it is possible to employ a Brewster window, i.e., a pair of uncoated fused silica, in place of expensive polarizers to lower the system cost.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Figure 1 is a schematic block diagram showing a laser processing system according to an embodiment of the present invention, Figure 2 is a graph showing the relationship between the number of windows and the polarization angle when the angle of the half-wave plate is increased, Figure 3 is a half-wave plate Graph showing the relationship between the number of windows and the attenuated light transmittance as the angle increases, FIGS. 4 (a) and 4 (b) are graphs showing the variation of the output of the pulsed laser for a short period of 30 seconds and a period of 15 minutes, respectively, 5 is a graph showing stabilized long term output variation of a pulsed laser under PID control.

Referring to FIG. 1, a laser processing system including an optical stabilization device and an optical component for a high power UV pulse laser according to an embodiment of the present invention includes a laser device 10, an output stabilization device 20, and a control device 30. It consists of).

The laser device 10 uses a KrF aximmer laser (eg, GAM Laser, EX10) having a wavelength of 248 nm, and has a maximum pulse repetition rate of 300 Hz, a pulse width of 14 ns, and an average power of 3.2 W (100 Hz, 13 kV). ) And polarization ratio 100: 1.

The output stabilization device 20 includes a half-wave plate 21, two fused silica plates 22, a photodetector (PD) 23 using a photodiode and a beam dump 24. Controller 30 includes a peak / sample hold amplifier 31, a data acquisition board (DAQ) 32, a controller (PC) 33, and a motor controller 34; have.

First, before describing the detailed configuration of the laser processing system according to the present invention, the design theory required for designing the output stabilization device 20 and the control device 30 used in the system will be described.

1. Polarization theory

Polarization refers to a phenomenon in which an electric or magnetic field constituting a wave vibrates in a specific direction when electromagnetic waves progress. Polarization is a wave of light that vibrates only in a certain direction, and the vibration direction of the electric field of light composed of an electric field and a magnetic field is called a polarization direction. Polarization is linear polarization in which the electric field oscillates in only one direction and non-polarization state without any direction, such as sunlight or lamp, and phase difference of linearly polarized light having the same orthogonal direction Circular polarization occurs when π / 2, and elliptical polarization occurs when phase difference is π / 4.

When light in a polarization state is incident at a specific angle of incidence, it is not reflected at the interface. This particular angle of incidence is referred to as the Brewster angle (θ _B ). At this angle, the electric field of the polarized light is not reflected because it is parallel to the incident plane, and the light in this polarized state is horizontally polarized (p-polarization) and is vertically polarized if it is polarized perpendicular to the plane where the light is incident. It is called a s-polarization state.

When the optical component is applied to the Brewster angle set at a specific angle, the output fluctuates while transmitting only light polarized in a specific direction. That is, when unpolarized light is incident on a plane at Brewster's angle, the reflected light is always s-polarized.

A tilted window at Brewster's angle reflects some vertically polarized light, not a horizontally polarized light. That is, the gain for the vertical polarization is attenuated, but the gain for the horizontal polarization is not attenuated. This allows the laser output to be horizontally polarized so that no loss of horizontal polarization occurs due to the window.

2. Brewster window

Using Brewster's window, the polarization ratio theoretically increases when compared to the situation with polarizers. A laser with a polarization ratio of 100: 1 exhibits a polarization ratio of only 100: 1 when incident on a combination of a half wave plate and a polarizer. However, it can be seen that if the same laser is incident on the combination of half-wave plate and Brewster window, the polarization ratio is greater than 100: 1.

This is due to the reflection of the s-polarized light in the window, the s-polarized (vertically polarized) light is reflected in the first window with reflectance R _s and the transmitted light has the same reflection in the second window. When this process is repeated, the amount of s-polarized light transmitted (T _s ) is given by T _s = (1-R _s ) ⁿ , where n is the number of Brewster windows. Thus, the polarization ratio, defined as the ratio of p-polarized light to s-polarized light, can be increased with these procedures.

3. Degree of polarization (偏光 度)

로 정의되고, 여기서 I_p 및 I_n은 각각 편광된 빛과 편광되지 않은 빛의 구성성분 플럭스 밀도이다. 편광되지 않은 빛은 I_p=0 및 V=0 이며, 반대로 만약 I_n=0 및 V=1인 경우, 빛은 완전 편광된 것이다. 따라서,

이다.Sometimes it is desirable to use the concept of polarization degree (V). The polarization degree (V)

Where I _p and I _n are the constituent flux densities of polarized and unpolarized light, respectively. Unpolarized light is I _p = 0 and V = 0 and conversely, if I _n = 0 and V = 1, the light is fully polarized. therefore,

to be.

이다. 편광도(V)는 빛이 임의 종류의 편광자에 도달하기 전에 이미 부분 또는 완전 편광될 수 있는 빔의 성질을 가진다.In this case, if we rotate the analyzer on the beam, we know the direction in which the transmitted radiation reaches the maximum of I _max , and we know the direction perpendicular to this orientation and the minimum, I _min . As a result, I _p = I _max -I _min . therefore,

to be. The degree of polarization V has the property of a beam that can already be partially or fully polarized before light reaches any kind of polarizer.

Based on the above theory, the polarization degree of the output stabilization device can be modeled using a Jones matrix. This can basically be explained using the above formula. For example, for a laser with a polarization ratio of 100: 1, the number of Brewster windows is changed in accordance with the relationship between the number of Brewster windows and the degree of polarization. The intensity of the s-polarized component of the laser decreases as each laser passes through each Brewster window, while the intensity of the p-polarized component remains constant. In addition to the increase in polarization ratio, the degree of polarization increases as the Brewster window increases.

Table 1 below shows the relationship between the number of Brewster windows and the intensity and polarization degree of the s-polarized component (s-pol).

Number of windows strength of s-pol Polarization degree @ 0 ° 0 0.01 × I _{0, s} 0.9900 2 0.0075 × I _{0, s} 0.9925 6 0.0042 × I _{0, s} 0.9958 10 0.0023 × I _{0, s} 0.9976

Where I _{0, s} indicates the original intensity of s-pol.

For a device with two Brewster windows, the polarization degree is almost 0.9925 when the angle between the half-wave plate and the optical axis is 0 °.

When the angle of the half wave plate is changed between 0 and 45 °, the intensities of the p-polarized component (p-pol) and the s-polarized component (s-pol) also change in a constant ratio. The transmission for p-pol is 1 and the transmission for s-pol is 0.8651 at Brewster's angle. For two Brewster windows, the transmission for p-pol is 1 and the transmission for s-pol is (0.8651) ² = 0.7484. In the case of two Brewster windows, the raw p-pol intensity (i.e. power) is 0.99 × I _{0, p} and the raw s-pol intensity indicates that the laser has a polarization ratio of 100: 1 (p-pol: s-pol). Due to the fact that it is 0.01 × I _{0, s} . When the angle of the half-wave plate is 0 °, the change in p-pol intensity is 0.99 × I _{0, p} and the change in s-pol intensity is 0.01 × 0.7484 × I _{0, s} .

At a 45 ° angle the polarization state changes from vertical to horizontal or vice versa. The transmittance of the original p-pol is 0.01 × 0.7484 × I _{0, p} and the transmittance of s-pol is 0.99 × I _{0, s} . Under this assumption, the intensities of p-pol and s-pol can be calculated at each angle.

When a laser beam exhibiting a polarization ratio of 100: 1 (p-pol: s-pol) passes through an output stabilization device, the p-pol intensity corresponds to Equation 1 from 0.99 × I _{0, p} × T _p ^m Decrease by value. Conversely, when T _s is the transmission of s-pol and m is the number of windows, the s-pol intensity, described by 0.01ⅹI _{0, s} × T _s ^m at 0 °, is 0.0075 × I _{0, s} according to Equation 2 below. To increase.

I _p = I _{O, p} × (T _p ^m ) × (0.99 × cos ² 2θ + 0.01 × sin ² 2θ)

I _s = I _{O, s} × (T _s ^m ) × (0.99 × sin ² 2θ + 0.01 × cos ² 2θ)

Where I _{O, p} is the original p-pol intensity, I _{O, s} is the original s-pol intensity, T _p is the p-pol transmittance, T _s is the s-pol transmittance, θ is the angle of the half wave plate, m Is the number of windows. Here, T _p ^m and T _s ^m were introduced to explain the transmission of the p-polarized component and the s-polarized component after the m window. After the p-pol and s-pol intensities are calculated, the polarization degree can also be calculated using Equation 3 as follows.

Here, (I _p + I _s ) _max represents the accumulated intensity when the angle of the half wave plate is 0 °, which means that the optical axis and the fast axis of the half wave plate coincide. (I _p + I _s ) _θ is the intensity sum when the _angle of the half-wave plate is θ, and K _m is the maximum degree of polarization when the m window. K _m was introduced to explain the relationship between the number of Brewster windows and the increase in polarization degree when the angle of half wave plate is 0 °.

4. Optical Attenuation

A half-wave plate is a phase retardation plate having a 180 ° relative phase difference between an extraordinary wave and an ordinary wave. The half-wave plate changes the polarization direction of light according to the angle when the angle between the linearly polarized light and the optical axis is defined. Using an optical component with half-wave plate and Brewster angle, you can adjust the output of incident linearly polarized light.

The final output is determined by the fixed Brewster's angle, which changes the angle of light incident on the linear polarizer needed to adjust the output. At this time, the rotatable half-wave plate is placed in front of the optical component to which the Brewster angle is applied to change the angle of incident light. A device configured in this way and capable of adjusting the output is called an optical attenuator. By knowing the incident and adjusted outputs of the optical attenuator, the angle of rotation of the half-wave plate can be known.

That is, when the incident beam has an angle θ with respect to the fast axis of the half wave plate, after passing through the half wave plate, the polarization direction of the light is rotated by 2θ. If the Brewster window is aligned so that the incident beam is completely transmitted, the addition of the half wave plate causes the output to be dependent on cos ² 2θ and is derived from Jones vectors and Jones matrices. In this case, θ refers to the angle between the optical axis of the half wave plate and the incident polarization.

Malus' law states that the intensity (I) of light passing through an incident beam having an angle of θ and passing through a half-wave plate and linear polarizer is given by I = I _o cos ² 2θ. Explain. Where I ₀ is the initial intensity and θ is the angle between the incident polarization and the axis of the polarizer, i.e. the substitution for the Brewster window.

Therefore, when the output of incident light, that is, the intensity I _O and the output of the light that is desired to be adjusted, that is, the intensity I, are determined, the rotation angle θ of the half-wave plate can be obtained.

Further, on the basis of the basic theory and calculations described above, the attenuation of the intensity (i.e. output) at each angle can be derived from Equations 1 and 2 described above. After the p-pol and s-pol intensities are calculated, the degree of polarization may be derived from Equation 4 below.

5. PID control theory

As described above, the output variation of the pulsed laser is inevitable. This can be attributed to many variables including air instability and unstable mains power.

For various reasons, it is difficult to predict the behavior of each new laser pulse, which makes it impossible to control all pulses using simple error compensation algorithms, similar to on / off controllers. An excimer laser is a type of pulsed laser with a very narrow pulse width that lasts only a few nanoseconds.

On the other hand, laser pulses or various kinds of pulse lasers of an aximmer laser show a certain type of flow for a relatively long time. As mentioned above, a simple error compensation algorithm does not work effectively to stabilize the pulsed laser. Therefore, a closed-loop feedback controller is employed, and the pulse laser output can be stabilized by employing a PID controller. PID controllers are a common loop feedback system widely used in industrial control systems.

PID control is a control method that compensates for the error between the current output value and the set value in order to control the output value of the target to be controlled to the desired set value. In this case, PID means proportionality, integration, and derivative of an error.

In proportional control, the output of the controller is controlled in proportion to the error between the current measured value and the set value, and is controlled in a direction to reduce the deviation between the measured value and the set value. At this time, if the proportional constant for the proportional control is large, there is a disadvantage that the access to the set value is quick, but disturbance occurs, and there is a disadvantage that the deviation is continuously present when only the proportional control is used. Integral control is a control method that solves this problem because the deviation cannot be zero when only proportional control is used. By integrating the deviation that occurs over the entire time, the deviation is finally zero.

When used in combination with proportional control, proportional-integral control is used to quickly move to a desired target value through proportional control and eliminate the deviation of proportional control itself through integral control. Derivative control is a way to report the deviation of disturbance in order to compensate for the poor response performance in proportional control and integral control, and to react quickly with a large amount of manipulation when the difference with the previous deviation is large.

When m is the output of the controller, the final form of the PID algorithm is expressed by Equation 5 below.

Where K _p is the proportional gain, K _i is the integral gain, K _d is the derivative gain, X _sp is the target value, and x is the measured value.

According to the PID control theory, the error between the target value and the measured data over the extended period can be calculated, and the correction provided by the controller for the subsequent period is determined.

To construct an algorithm for pulse laser output stabilization using a PID controller, the angle (θ) is controlled using Malus' law that affects the output. Typical aforementioned PID control equation (5) and the end Russ law, that is, rotation of the half wave plate from I = I _o cos ² 2θ angle (θ) is expressed by the following equation (6).

Where θ _n is the rotation angle at the nth step and y is the controlled output. In case of proportional control only K _S is used to calculate the rotation angle θ.

6. Description of system operation

The detailed structure of the output stabilization apparatus 20 and the control apparatus 30 which comprise the laser processing system of this invention is demonstrated below based on the design theory mentioned above.

In the laser processing system of the present invention, the control device 30 controls to configure a PID control algorithm. The program includes a control unit (PC) 33 mounted thereon, and the control unit 33 can be implemented using a well-known general purpose personal computer (PC) and forms a PID controller as the control program is loaded.

When the data is captured from the peak / sample hold amplifier 31 through the photodetector (PD) 23 of the output stabilization device 20, the controller 33 controls the control unit through the data acquisition board (DAQ) 32. 33).

The control unit 33 performs the control as a PID controller and drives the motor M 25 to the calculated value so as to drive the rotation stage to which the half-wave plate 21 is attached through the motor controller 34. .

The laser beam emitted from the laser device 10 passes through a half-wave plate (HWP) 21 and Brewster windows, which are rotationally driven by the motor 25, and then scattered beyond the window. . The scattered beam is collected by the photodetector (PD) 23 and accumulated for 16 ns in the peak / sample hold amplifier 31. The resulting data serves as input to the controller 33, i.e., the PID controller, via the data acquisition board (DAQ) 32.

The PID controller essentially compensates for the error between the scattered laser signal collected from the peak / sample hold amplifier 31 and a given set point. This result rotates the half wave plate 21 by the angle at which the rotation stage rotated by the motor 25 is calculated.

The output stabilization device 20 is composed of a half-wave plate 21 attached to the rotating stage and a laminated fused silica plate 22 of 0.5 mm thickness. The half-wave plate 21 is attached to the rotating stage for control of the output and the rotating stage is rotatably supported by a motor M 25 controlled by the motor controller 34.

In the optical system having the above structure, first, the laser beam emitted from the laser device 10 passes through the half-wave plate 21 to generate a phase difference. This laser beam is installed in place of the polarizer and is incident on the two fused silica plates 22 to which the Brewster window is applied at a Brewster angle with respect to the optical axis and separated into p-polarized light and s-polarized light. The light used for laser processing among the separated p- and s-polarized light is horizontally polarized, i.e., p-polarized, and therefore, stabilization of the output is made according to the PID control method for the horizontally polarized component.

That is, the two fused silica plates 22 are positioned so that a high transmittance is obtained and at the same time minimizes the loss of degree of polarization. The pulsed laser output is stabilized by controlling the p-polarized component by PID control.

The rotational precision of the rotating stage, ie the motor, is 0.005 degrees per pulse and the maximum rotational speed is 30 degrees per second. Data is obtained by receiving the light scattered by the photodetector 23 having a response time of 10 ns.

On the other hand, in order to control the output of the pulsed laser, the output of each pulse should be measured. The pulse width of the pulsed laser is generally short in nanoseconds, and during the time, it is difficult to accurately measure the peak intensity. it's difficult. Therefore, in order to accurately measure the peak power of each pulsed laser, a peak and sample hold amplifier 31 should be used.

The peak / sample hold amplifier 31 is an electronic device that grabs the largest signal among unit pulse signals from outside. The pulse of a laser with a pulse width of nanoseconds actually has a Gaussian form. At this time, when the signal is input to the peak / sample hold amplifier 31, the largest signal can be caught for a unit time to have a desired delay time. Since the signal maximum value at this time has the same value as the laser output maximum value, data acquisition is easy. The peak sample amplifier 31 may set a unit time of 1 to 16 ms.

7. Experiment Method

Two Brewster windows were used to attempt pulse laser power stabilization based on the PID control algorithm. As a result, proper optical attenuation occurred.

Based on this method, only proportional control is used among the PID controls, and the proportional gain (Kp) is set to 5 to attempt output stabilization using the PID control.

The output variation of the laser itself was performed for 15 minutes, and the change of output of the pulsed laser was stabilized through the PID control. In addition, optical attenuation was attempted based on the above-mentioned polarization principle.

8. Experimental Results

Hereinafter, the operation of the output stabilization device 20 and the control device 30 constituting the laser processing system according to the present invention will be described in more detail with reference to the above experimental results.

8.1 Polarization and Optical Attenuation Modeling

Equations 1 through 4 above can be used to model polarization and attenuation of intensity for different numbers of Brewster windows.

2 shows the relationship between the angle of the half-wave plate and the degree of polarization. The results of FIG. 2 show that the degree of polarization decreases as the half-wave plate is rotated, in particular from 0 ° to 45 °, and changes from linear polarization in the vertical direction to linear polarization in the horizontal direction and vice versa. 2 also shows that the degree of polarization decreases more rapidly as the number of windows at an angle increases.

3 shows that the light transmittance of the laser is attenuated as the half-wave plate is rotated from 0 ° to 45 °. 3 shows that the light transmittance of the laser is attenuated more rapidly as the number of windows increases at a predetermined angle. Although the output stabilization device attenuates the light transmittance of the laser, it also changes the degree of polarization.

Using two Brewster windows and a 45 ° half-wave plate from FIGS. 2 and 3 the polarization degree changes to 14.11% and the light transmittance is attenuated by 24.72%. In the six Brewster windows, the degree of polarization changes to 40.15% and the light transmittance is attenuated by 57.25%. For 10 Brewster windows, the degree of polarization changes to 60.79 and the light transmittance is attenuated by 75.57%.

Thus, practitioners in the field of laser beam processing use Brewster windows and can use this result as a reference when designing optical components for high power UV lasers.

8.2 Laser Power Stabilization

As a result of calculating the optical transmission of the pulse laser power stabilization device, the laser beam was set to have an output of 0.33 mW before being input to the power stabilization device. After passing through the power stabilization device, the laser showed an output of 0.32mW when the half-wave plate was 0 °.

The light transmittance was calculated to be 96.97%, which is an advantage compared to conventional power stabilizers or optical attenuators.

8.3 Power stabilization

4 (a) and 4 (b) show the pulse variation of the excimer laser for 30 seconds and 15 minutes, respectively. 4 (a) shows a short time change of the pulsed laser: there is no evidence of any trend that the pulse follows. On the other hand, the long time flow of FIG. 4B is regarded as a long time action, and PID control is required to control the pulse laser output.

The fluctuations in the power of the laser itself were compared with the fluctuations after output stabilization at pulse repetition rates of 120, 150 and 180 Hz. 5 shows the fluctuation of the output stabilized at 180 Hz, and the output fluctuation rate was improved from 2.73% to 0.97% when compared to FIG. 4.

At 120Hz, the free running laser's output variation is 2.78%, but its stabilized level is 0.92%. This suggests that the output stabilizer can improve the variation by 66.91%. At 150Hz, the free running laser's output variation is 2.92%, but the stabilized level is 0.94%, an improvement of 67.81%. On the average, the RMS stability of the free running laser is 2.81% and the RMS stability of the stabilized laser is 0.94%, an improvement of about 66.55%.

8.4 Optical Attenuation

As mentioned above, the output stabilization device can be used as an optical attenuator by rotating the angle of the half wave plate. Maximum and minimum outputs were measured by controlling the angle between the optical axis and the fast axis of the half wave plate at different pulse repetition rates.

At 150 Hz, the laser beam showed an output of 0.33 mW before being input into the output stabilizer. After passing through the stabilizing device, the laser showed an output of 0.32 mW when the angle? Of the half-wave plate was 0 ° and an output of 0.21 mW when 45 °. The optical attenuation was calculated from the measured output. This resulted in attenuated light transmission of 65.63% at 150 Hz when using an output stabilization device.

According to the present invention, a high transmittance power stabilization device and an optical attenuator are implemented using Brewster windows in a high power UV pulse laser environment.

In the absence of popular high power UV laser optics, such as polarized optics, Brewster windows can be a good substitute. It is proposed to model polarization and optical attenuation by varying the number of Brewster windows.

The present invention is applied to a laser processing system that can be finely processed by stabilizing the output of the laser processing system by PID control.

1 is a schematic block diagram showing a laser processing system according to an embodiment of the present invention;

2 is a graph showing the relationship between the number of windows and the polarization angle as the angle of the half-wave plate increases;

3 is a graph showing the relationship between the number of windows and the attenuated light transmittance as the angle of the half-wave plate increases;

4 (a) and 4 (b) are graphs showing the variation of the output of the pulsed laser for a short period of 30 seconds and a period of 15 minutes, respectively,

5 is a graph showing stabilized long term output variation of a pulsed laser under PID control.

<Description of the symbols for the main parts of the drawings>

10: laser device 20: power stabilization device

21: half-wave plate 22: fused silica

23: photodetector 24: beam dump

30: Control Unit 31: Peak / Sample Hold Amplifier

32: DAQ 33: Control Unit (PC)

34: motor controller 25: motor

Claims (10)

A laser device for emitting a pulsed laser beam, Half-wave plate for changing the angle of incidence of the laser beam passing through to change the degree of polarization of the Brewster window of the rear stage, At least two fused silicas each having a Brewster window tilted at Brewster's angle and changing the polarization direction of the laser beam incident through the half-wave plate to output the laser beam passing through the laser beam used for laser processing. Plate, and The light scattered through the window of the polarized light is collected through a photodetector and a peak / sample hold amplifier, and then the half wave is compensated for minimizing the variation of the output by compensating an error between the collected scattered laser signal and a given set value. A laser processing system comprising a PID controller for rotationally driving a floor plate. The method of claim 1, And said laser device comprises a pulsed laser. The method of claim 1, And the fused silica plate is positioned to minimize loss of polarization while at the same time obtaining high transmittance. The method of claim 1, And a polarization ratio increases as the number of windows increases. The method of claim 1, And a degree of polarization decreases as the angle between the half-wave plate and the optical axis is rotated from 0 to 45 °. The method of claim 1, And said at least two fused silica plates are comprised of first and second fused silica plates, said second fused silica plates compensating for beam paths moved by said first fused silica plates. The method of claim 1, The PID controller controls to realize the PID control algorithm Laser processing system comprising a signal processing device equipped with a program. The method of claim 1, And the pulsed laser output is stabilized by controlling the horizontal polarization component by PID control. The method of claim 1, And the fused silica plate vertically and horizontally polarizes a laser beam incident through the half-wave plate, and outputs a laser beam passing through the horizontally polarized light as a laser beam used for laser processing. The method of claim 1, The half-wave plate is attached to a rotating stage rotatably supported, And the PID controller rotates the rotating stage and the half-wave plate by rotating the motor through the motor controller to minimize the variation of the laser beam output.

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