JP2009036706A - Method and device for estimating laser damage resistance of optical material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and device for measuring laser damage resistance that can evaluate laser damage resistance nondestructively with high accuracy even when an optical material generates fluorescence by irradiation of laser beams. <P>SOLUTION: The method for estimating laser damage resistance of the optical material includes a preliminary measuring step including irradiating the optical material with laser beams of first wavelength to observe fluorescence, a measuring step of changing irradiation intensity while causing two-photon absorption in a measuring region which is a part of the optical material, to measure the change of transmittance to the irradiation intensity in the measuring region, and a first estimating step of estimating laser damage resistance in the first wavelength, referring to correlative data of laser damage resistance and the change of transmittance to the irradiation intensity when associated with fluorescence obtained beforehand, based on the change of transmittance to the irradiation intensity. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a laser damage resistance estimation method and a laser damage resistance estimation apparatus capable of nondestructively evaluating the laser damage resistance of an optical material.

  Laser systems using laser light are used in a wide range of industrial fields such as microfabrication and information communication. In this laser system, optical materials such as optical crystals are used to construct wavelength conversion, condensing / reflection, amplification elements, etc., but these optical materials are exposed to high energy density laser light. The quality of the optical material determines the performance and reliability of the system.

Optical materials used in these systems include glass materials represented by quartz mainly used for lenses, mirror substrates, window materials, etc., and simple materials such as Nd: YAG and Yb: YAG mainly used for laser oscillation. Crystal materials, mainly single crystal materials such as CLBO (CsLiB ₆ O ₁₀ ), LBO (Li ₂ B ₂ O ₄ ), BBO (BaB ₂ O ₄ ), KTP (KTiOPO ₄ ), mainly used for wavelength conversion, Examples thereof include single crystal materials such as CaF ₂ and MgF ₂ used for lenses, and translucent ceramics mainly used as window materials. These optical materials have the quality obtained by the production method and the raw materials used. Differently, it is important how to evaluate the quality and guarantee the quality.

  For example, in a single crystal material, optical characteristics change or laser damage resistance deteriorates due to crystal defects such as dislocations and the amount of impurities. In addition, in a glass material or a ceramic material, optical characteristics change or laser damage resistance deteriorates due to the uniformity of the material, the presence of bubbles, the state of grain boundaries, or the like. In general, as the crystal material becomes larger than the glass and the material becomes smaller than the small material, the production / mass production technology becomes extremely difficult.

  Therefore, it is important to evaluate optical materials such as glass, crystals, and ceramics that constitute the laser system, particularly evaluation of laser damage resistance. However, since conventional laser damage resistance evaluation cannot be measured unless the material is directly irradiated with laser light and destroyed, there is a problem that optical materials used in laser systems cannot be directly evaluated.

In view of this, the present inventors have proposed a method capable of evaluating the laser damage resistance of an optical material in a non-destructive and inexpensive manner (Patent Document 1). In this proposed method, two-photon absorption is generated in a part of the optical material by laser light irradiation, and laser damage resistance is evaluated on the basis of a decrease in transmittance caused by the two-photon absorption.
JP 2005-114720 A

  However, the method disclosed in Patent Document 1 has a problem that the laser damage resistance cannot be accurately evaluated when the optical material emits fluorescent light when irradiated with laser light.

  Therefore, the present invention provides a laser damage resistance measurement method and laser damage resistance measurement method that can accurately evaluate laser damage resistance in a non-destructive manner even when the optical material emits fluorescent light when irradiated with laser light. An object is to provide an apparatus.

In order to solve the above-mentioned problems, the present inventor has repeatedly conducted intensive studies, and as a result, the relationship between the decrease in transmittance and the laser damage resistance threshold when the optical material emits fluorescence and when fluorescence does not occur. Have been found to have different tendencies.
That is, a laser damage resistance estimation method for an optical material according to the present invention includes a preliminary measurement step including observing fluorescence emission by irradiating the optical material with laser light of a first wavelength;
A measurement step of measuring a change in transmittance with respect to the irradiation intensity in the measurement region by changing the irradiation intensity while causing two-photon absorption in a measurement region which is a part of the optical material;
Based on the change in the transmittance with respect to the irradiation intensity, the laser damage resistance at the first wavelength is estimated with reference to the correlation data between the change in the transmittance with respect to the irradiation intensity and the laser damage resistance when the fluorescence emission is obtained in advance. A first estimation step.
In the laser damage resistance estimation method according to the present invention configured as described above, the observation of the fluorescence emission and the change in transmittance with respect to the irradiation intensity can be measured nondestructively, and the laser is based on the nondestructive measurement data. Damage resistance is estimated.

In the laser damage resistance estimation method according to the present invention, when no fluorescence emission is observed in the first estimation step based on a change in transmittance with respect to the irradiation intensity, the fluorescence emission obtained in advance is not accompanied. It is preferable that the method further includes estimating laser damage resistance at the first wavelength with reference to correlation data between change in transmittance with respect to irradiation intensity and laser damage resistance.
In this way, the laser damage resistance of both the optical material that generates fluorescence and the optical material that does not generate fluorescence can be obtained nondestructively.

  Further, in the laser damage resistance estimation method according to the present invention, the preliminary measurement step irradiates the optical material with the laser light having the first wavelength at different intensities, and performs the optical measurement based on a change in transmittance with respect to the irradiation intensity. The step of determining whether or not the nonlinear absorption amount due to the two-photon absorption generated in a part of the material can be evaluated, and the nonlinear absorption amount is increased so that the nonlinear absorption amount can be evaluated. On the other hand, it is preferable to further include an adjustment step for suppressing fluorescence.

Furthermore, after the preliminary measurement step, the method includes a step of dividing the optical material into a plurality of measurement regions using the part where two-photon absorption has occurred as a measurement region, and sequentially measuring the measurement step and the first step with respect to the plurality of measurement regions. One estimation step may be repeated.
In this way, the laser damage resistance of the entire region that needs to be measured can be obtained nondestructively, and the intensity distribution of the laser damage resistance can be imaged in three dimensions.

Further, based on the estimated laser damage resistance at the first wavelength, the laser damage resistance at the second wavelength is referred to by referring to the correlation data between the laser damage resistance at the first wavelength and the laser damage resistance at the second wavelength that is obtained in advance. The method may further include a second estimation step for estimating.
In this way, even if it is difficult to measure the fluorescence emission phenomenon and transmittance of the laser light of the second wavelength due to technical or economic reasons, the laser damage resistance to the laser light of the second wavelength is nondestructive. Can be obtained.

An apparatus for estimating laser damage resistance of an optical material according to the present invention condenses and irradiates an optical material with at least one laser light source that generates laser light having a first wavelength and the laser light emitted from the laser light source. An optical measurement device including an optical system, a transmitted light detector that measures the intensity of laser light that passes through the optical material, and a light emission detector that detects fluorescence from the irradiated laser light;
The energy density irradiated to the measurement region that is a part of the optical material is improved to cause two-photon absorption in the measurement region, and the energy density irradiated to the region other than the measurement region in the optical material is decreased. An optical measurement control device that changes the emission intensity of the laser light source in a state where the optical system is controlled to suppress the fluorescence emission of the optical sample,
Based on the intensity of the laser beam detected by the transmitted light detector and the emission intensity of the laser light source, a change in transmittance with respect to the irradiation intensity of the laser beam applied to the measurement region is calculated, and the calculated Based on the change in transmittance, a first estimate for estimating laser damage resistance at the first wavelength with reference to correlation data between the change in transmittance with respect to the irradiation intensity and laser damage resistance obtained with fluorescence emission obtained in advance. And a device.

  In the laser damage resistance estimation apparatus according to the present invention, the first estimation apparatus is configured such that when fluorescence emission is observed based on a change in transmittance with respect to the irradiation intensity, the irradiation intensity when the fluorescence emission is obtained in advance is obtained. The laser damage resistance at the first wavelength is estimated with reference to the correlation data of the change in transmittance with respect to the laser damage resistance, and when the fluorescence emission is not observed, the irradiation intensity without the fluorescence emission obtained in advance is determined. It is preferable to estimate the laser damage resistance at the first wavelength with reference to the correlation data between the change in transmittance and the laser damage resistance.

  In the laser damage resistance estimation apparatus according to the present invention, the optical measurement device preferably includes a stage for moving the optical material or the optical system in accordance with the control of the optical measurement control device. The laser damage resistance can be obtained over a wide range, and the intensity distribution of the laser damage resistance can be imaged in three dimensions.

  In the laser damage resistance estimation apparatus according to the present invention, reference is made to the correlation data between the laser damage resistance at the first wavelength and the laser damage resistance at the second wavelength obtained in advance based on the estimated laser damage resistance at the first wavelength. Then, a second estimation device that estimates the laser damage resistance at the second wavelength may be further included.

The laser damage resistance estimation method and apparatus according to the present invention configured as described above irradiates an optical material with laser light having a first wavelength and observes fluorescence emission, and then transmits the transmittance with respect to the irradiation intensity in the measurement region. The laser damage resistance at the first wavelength is estimated by referring to the correlation data between the change in the transmittance with respect to the irradiation intensity and the laser damage resistance obtained with the fluorescence emission obtained in advance.
Therefore, according to the present invention, a laser damage resistance measuring method and laser capable of accurately evaluating laser damage resistance in a non-destructive manner even when the optical material emits fluorescent light when irradiated with laser light. An apparatus for measuring damage resistance can be provided.

Hereinafter, a laser damage resistance measuring method and a laser damage resistance measuring apparatus according to embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a functional block diagram illustrating the configuration of the laser damage resistance measuring apparatus according to the embodiment.
As shown in FIG. 1, the laser damage resistance measuring apparatus according to the embodiment includes an optical measurement unit 10, an optical measurement control unit 11, a first estimation processing unit 12 to which a first database 13 is connected, and a second database. 15 is connected to a second estimation processing unit 14 connected thereto.

In the present embodiment, each unit is configured as follows.
<Optical measurement unit 10>
As shown in FIG. 2, the optical measurement unit 10 divides the laser light 1 emitted from the laser light source 1 having the first wavelength and (b) the laser light emitted from the laser light source 1 into two parts. One of the divided lights is incident on the power monitor 5 as monitor laser light, and the other light is incident on the condenser lens unit 2 as measurement laser light, and (c) the incident measurement laser light. A condensing lens unit 2 that collects the light and enters the optical material 7; (d) a transmitted light detector 3 that detects the transmitted light intensity of the measurement laser light transmitted through the optical material 7; and (e) a laser light. A power monitor 5 that monitors the intensity of the light, and (f) a light emission detector 4 that detects the presence or absence of the fluorescence emission of the optical material 7 and a light emission amount. Output measurement data to the processing unit.
Although not shown, the optical measuring unit 10 has a moving stage that moves the optical material 7 or the condenser lens unit 2 to be measured.

<Optical measurement controller 11>
The optical measurement control unit (i) optical that adjusts the optical system so that energy can be concentrated in the measurement region so as to cause nonlinear absorption, and a decrease in transmittance due to nonlinear absorption can be observed while suppressing energy absorption due to fluorescence emission. A system control unit, and (ii) a measurement control unit that sequentially changes the incident energy intensity of the laser light and enters the sample in each measurement region, and measures the transmittance and fluorescence emission intensity with respect to each incident energy intensity; (Iii) a scanning control unit that sequentially moves the position of the measurement region so that the transmittance of all the regions that need to be measured is measured based on the size and position information of the measurement region. The optical measurement unit 10 is controlled according to the flowchart shown.

Hereinafter, based on the flowchart shown in FIG. 2, the control procedure of the optical measurement control unit 11 will be described.
(Step S1 (initial value setting))
In step S1, an initial value for the laser light source for preliminary measurement is set.
For example, in the case of a laser oscillator having a variable wavelength, the first wavelength as the measurement wavelength, the minimum intensity and maximum intensity of the emitted laser light, the step of increasing the emitted intensity, the focal length of the lens system for condensing the beam in the sample, etc. The initial value of is set.
In the case of a laser oscillator with a fixed wavelength, it is not necessary to set the initial value of the laser wavelength.

(Step S2 (preliminary measurement))
In step S2, according to the set initial value, the laser light source 1 is driven to emit laser light at a predetermined intensity step and enter the optical material, and the intensity of transmitted light and fluorescence for each incident intensity. The luminescence intensity is measured.

(Step S3)
In step S3, non-linear absorption occurs by determining whether non-linear absorption occurs or whether non-linear absorption can be measured based on a change in transmittance with respect to the incident intensity of laser light incident on the optical material. If the measurement is not possible even if it has not occurred or has occurred, the process proceeds to step S4, and if nonlinear absorption has occurred and the measurement is possible, the process proceeds to step S5.

(Step S4)
In step S4, when fluorescence is observed so that a nonlinear absorption amount is generated or increased, the optical lens system is adjusted so as to suppress energy absorption by fluorescence, and the process proceeds to step S2. For example, when the focal length of the condenser lens unit 2 is shortened, the energy density of the laser beam in the vicinity of the focal point can be increased, while the energy density can be rapidly decreased as the distance from the focal point increases. The amount of nonlinear absorption resulting from photon absorption can be increased, and fluorescence emission outside the measurement region can be relatively suppressed.
Thereafter, steps S2 and S3 are repeated until measurement of nonlinear absorption becomes possible.
As a result of the adjustment of the optical lens system in step S4, a measurement range (size) in one measurement described later is determined, and a measurement region in one measurement is determined based on the measurement range.

(Step S5)
In step S5, the entire measurement range in the sample can be measured based on the measurement range obtained as a result of the optical adjustment in step S4 (the range where two-photon absorption occurs). Dividing into a plurality of measurement areas, a measurement area at the start position, a measurement area at the end position, and a scan step (stage movement step) are set.

(Step S6)
In step S6, the initial value and end value of the incident energy intensity of the laser light incident on the sample and the energy increasing step (number of measurement points) in each measurement region are set.

(Step S7)
In step S7, laser light is incident on the optical material 7 while monitoring the energy incident on the optical material 7 by the power monitor 5, and the transmittance and fluorescence emission intensity are measured.

(Step S8)
In step S8, the measured transmittance and fluorescence emission intensity are output in association with the position information of the measurement region.

(Step S9)
In step S9, it is determined whether or not the incident energy intensity of the laser beam incident on the sample has reached the end value in the measurement region being measured. If the incident energy intensity has not reached the end value, step S9 is performed. The process proceeds to S10, and if the incident energy intensity has reached the end value, the process proceeds to Step S11.

(Step S10)
In step S10, the incident energy intensity is increased by increasing the output of the laser oscillator by a certain value.
Thereafter, steps S7 to S9 are repeated.

(Step S11)
In step S11, it is determined whether or not the measurement of all the measurement areas is completed. If the measurement of all the measurement areas is not completed, the process proceeds to step S12, and the measurement of all the measurement areas is completed. If so, the measurement is terminated.

(Step S12)
In step S12, the stage is moved by a predetermined amount to move to an adjacent measurement region, and steps S6 to S11 are repeated.

<First estimation processing unit 12>
Under the control of the optical measurement control unit 11, the first estimation processing unit 12 is sequentially input with the energy value, the transmittance, and the fluorescence emission intensity that are output from the optical measurement unit 10 and incident on the optical material 7. Based on the input data, the first estimation processing unit 12 calculates the slope of the reciprocal of the transmittance with respect to the incident energy for each measurement region, and previously calculates the slope based on the calculated slope and the fluorescence emission intensity. The laser damage resistance at the first wavelength is estimated with reference to the correlation data between the change in transmittance and the laser damage resistance stored in one database 13.

Here, the first database 13 includes (i) correlation data of a change in transmittance with respect to incident energy intensity and laser damage resistance when accompanied by fluorescence emission, and (ii) incident energy intensity when not accompanied by fluorescence emission. Correlation data of transmittance change and laser damage resistance against
In addition, the correlation between the change in transmittance with respect to the incident energy intensity and the laser damage resistance in the case of accompanied with the fluorescence emission is stored for each of the different fluorescence emission intensities, and thus, the fluorescence emission intensity with respect to the incident energy intensity. By having a plurality of correlation data related to, it is possible to estimate the laser damage threshold with higher accuracy.

Hereinafter, the process in the 1st estimation process part 12 is demonstrated according to the flow shown in FIG.
The flowchart shown in FIG. 4 shows a processing flow for one measurement region, and the processing shown in FIG. 4 is repeated for each measurement region.

(Step S101)
In step S101, the energy value, transmittance, and fluorescence emission intensity incident on the optical material 7 output from the optical measurement unit 10 are acquired, and the slope of the reciprocal of the transmittance with respect to the incident energy is obtained for each measurement region. calculate.

(Step S102)
In step S102, it is determined whether or not fluorescence emission is involved. If fluorescence emission is involved, the process proceeds to step S103. If fluorescence emission is not involved, the process proceeds to step S104.

(Step S103)
In step S103, based on the slope of the reciprocal of the transmittance with respect to the incident energy and the fluorescence emission intensity, with reference to the correlation data of the change in the transmittance with respect to the incident energy intensity and the laser damage resistance when accompanied by the fluorescence emission, the laser Estimate the damage threshold. Here, the laser damage threshold is estimated by referring to the correlation data of the transmittance change and the laser damage resistance with respect to the incident energy intensity created corresponding to the fluorescence emission intensity closest to the measured fluorescence emission intensity. Estimate the laser damage threshold with high accuracy.

(Step S104)
In step S104, based on the slope of the reciprocal of the transmittance with respect to the incident energy, the laser damage threshold is estimated while referring to the correlation data between the change in the transmittance with respect to the incident energy intensity and the laser damage resistance when there is no fluorescence emission. To do.

(Step S105)
Then, the laser damage threshold value at the first wavelength estimated in step S105 is output to the second estimation processing unit in association with the position information of the measurement region output from the measurement control unit.

<Second estimation processing unit 14>
The second estimation processing unit 14 is configured to output the first laser damage threshold at the first wavelength and the second laser damage threshold at the second wavelength based on the first laser damage threshold in the measurement region output from the first estimation processing unit 12. The second laser damage threshold value at the second wavelength is estimated with reference to the correlation data with, and the second laser damage threshold value is output in association with the position information of the measurement region.
The correlation between the first laser damage threshold at the first wavelength and the second laser damage threshold at the second wavelength is stored in the second database 15.

<First database>
In the first database 13, as described above, (i) the correlation between the change in transmittance with respect to the incident energy intensity and the laser damage resistance when accompanied by fluorescence emission, and (ii) the case where fluorescence emission is not accompanied. The change in transmittance with respect to the incident energy intensity and the correlation between laser damage resistance are stored.
Here, the change in transmittance with respect to the incident energy intensity is specifically represented by the slope of a straight line when the reciprocal of the transmittance (1 / transmittance) is plotted against the incident energy. The correlation between the change in transmittance and the laser damage resistance with respect to is a correlation between the inclination and the laser damage threshold.

Hereinafter, the correlation between the change in the transmittance with respect to the incident energy intensity and the laser damage resistance will be described in the case of the CaF ₂ crystal. The CaF ₂ crystal is irradiated with a laser beam having a wavelength of 213 nm to evaluate the change in transmittance and the resistance to laser damage. First, an example of a CaF ₂ crystal accompanied by fluorescence emission will be described.
The following correlations are findings that have been found for the first time by the present inventors, and the present invention has been completed only by new findings that have been found by the present inventors.
FIG. 5 shows the fluorescence emission intensity when the incident energy is changed for each of nine CaF ₂ crystals (Nos. 1 to 9) having different laser damage thresholds. As apparent from FIG. 5, the fluorescence emission intensity increases as the incident energy increases for any crystal.

In addition, FIG. 1-No. 9 CaF ₂ respectively, for crystals is a graph showing the transmittance at the time of changing the incident energy at the inverse of the transmittance. As shown in FIG. 6, the reciprocal of the transmittance increases linearly as the incident energy increases, and the two are in a proportional relationship. Therefore, it can be seen that the increase in incident energy and the decrease in transmittance are not in a simple proportional relationship but are in a nonlinear relationship, and the decrease in transmittance is caused by the nonlinear absorption amount.

  As shown in the table below, the laser damage threshold increases as the slope of the reciprocal transmittance with respect to the incident energy increases, that is, as the nonlinear absorption amount increases, and the slope of the reciprocal transmittance with respect to the incident energy increases. There is also a correlation between the laser damage threshold.

FIG. 7 is a graph showing the transmissivity of the CaF ₂ crystal with substantially no fluorescence emission when the incident energy is changed as the reciprocal of the transmissivity. Here, the transmittance with respect to incident energy was measured for three samples A, B, and C having different laser damage thresholds. Of the three samples, sample A has the lowest laser damage threshold and sample C has the highest laser damage threshold.

  As shown in FIG. 7, the reciprocal of the transmittance increases linearly as the incident energy increases in any sample, and both are in a proportional relationship. Therefore, it can be seen that the increase in incident energy and the decrease in transmittance are not in a simple proportional relationship but are in a nonlinear relationship, and the decrease in transmittance is caused by the nonlinear absorption amount.

  The laser damage threshold decreases as the slope of the reciprocal transmittance with respect to the incident energy increases, that is, as the nonlinear absorption amount increases, and there is a correlation between the slope of the reciprocal transmittance with respect to the incident energy and the laser damage threshold. is there. Therefore, it is possible to estimate the laser damage threshold based on this correlation.

However, in the CaF ₂ crystal without fluorescence emission, the laser damage threshold decreases as the slope of the reciprocal transmittance with respect to the incident energy increases, and in the CaF ₂ crystal with fluorescence emission, the incident energy with the reciprocal transmittance. The tendency of the two is completely different in that the laser damage threshold increases as the inclination with respect to increases.

Furthermore, the correlation between presence / absence of fluorescence emission, non-linear absorption, and laser damage threshold for CaF ₂ crystals of various qualities is summarized in the following table. As described above, in the CaF ₂ crystal with fluorescence, the laser damage threshold increases as the slope of the reciprocal of the transmittance with respect to the incident energy increases, that is, as the nonlinear absorption amount increases, and the reciprocal of the transmittance. There is also a correlation between the slope for incident energy and the laser damage threshold. On the other hand, in the CaF ₂ crystal without fluorescence emission, the laser damage threshold decreases as the slope of the reciprocal transmittance with respect to the incident energy increases, and in the CaF ₂ crystal with fluorescence emission, the incident energy with the reciprocal transmittance. The greater the slope relative to, the greater the laser damage threshold. In order to show these correlations with high accuracy, it is necessary to further consider the fluorescence emission intensity.

Therefore, in the present invention, the CaF ₂ crystal with fluorescence, the correlation data of the slope the laser damage threshold of the reciprocal of the transmittance, the CaF ₂ crystal without fluorescence, the reciprocal of the transmittance slope and the laser damage threshold Two different correlation data of the correlation data are used to estimate the laser damage threshold so that the laser damage threshold can be estimated for both CaF ₂ crystals with fluorescence and CaF ₂ crystals without fluorescence. Yes.

  As described above, according to the present invention, there are two different correlation data of an optical material with fluorescence emission and an optical material without fluorescence emission. In the case of an optical material with fluorescence emission, the amount of fluorescence emission is absorbed nonlinearly. By measuring the transmittance with the amount measured to the extent possible, it is possible to determine the laser damage threshold based on the transmittance measured non-destructively, with or without fluorescence. Became.

<Second database>
The second database 15 is connected to the second estimation processing unit 14 and stores the correlation between the first laser damage threshold at the first wavelength and the second laser damage threshold at the second wavelength. The correlation data includes the first laser damage threshold at the first wavelength obtained by the destructive measurement and the second laser damage threshold at the second wavelength obtained by the destructive measurement for the optical material to be measured (for example, CaF ₂ crystal). It is a database.

  FIG. 8 is a graph showing the correlation between the laser destruction threshold value due to laser light having a wavelength of 213 nm and the laser destruction threshold value due to laser light having a wavelength of 193 nm. Based on the data indicating such correlation, the laser destruction threshold value for the 193 nm wavelength laser beam can be estimated based on the first laser damage threshold value for the 213 nm wavelength laser beam obtained in the non-destructive manner as described above. As a result, it becomes possible to determine the laser destruction threshold value with a laser beam having a wavelength of 193 nm in a non-destructive manner.

  When the correlation shown in FIG. 8 is used, the laser damage threshold for the 213 nm ultraviolet laser light and the 193 nm are calculated based on the transmittance measured nondestructively with the 213 nm ultraviolet laser light that can generate nonlinear absorption, is easy to handle, and is inexpensive. A laser damage threshold for the laser light can be obtained. In this way, not only the laser damage threshold for 213 nm ultraviolet laser light but also the laser damage threshold for 193 nm laser light can be determined nondestructively, and at present, the lithography exposure wavelength has reached 193 nm, the technical significance is large.

In the above description, it is possible to obtain the laser damage threshold for the 213 nm ultraviolet laser light and the laser damage threshold for the 193 nm laser light based on the transmittance measured nondestructively with the 213 nm ultraviolet laser light by using the CaF ₂ crystal example. showed that.
However, the present invention is not limited to the method and apparatus for determining the laser damage threshold for specific wavelengths (213 nm ultraviolet laser light and 193 nm laser light) in the CaF ₂ crystal.

  For example, as shown in Patent Document 1 (Japanese Patent Laid-Open No. 2005-114720), generally, an optical material has a wavelength in a specific range related to a transmission limit wavelength unique to the optical material. When laser light is irradiated at an energy density within a predetermined range, a change in transmittance occurs due to nonlinear absorption due to two-photon absorption, and the change in transmittance correlates with laser damage resistance. In addition, it has been confirmed that many optical materials are accompanied by fluorescence emission, and it is clear that the correlation between the change in transmittance and the laser damage resistance differs depending on the presence or absence of fluorescence emission and the fluorescence emission intensity. .

  Therefore, based on the technical idea of the present invention, a database of correlations between changes in transmittance inherent in optical materials and laser damage resistance enables nondestructive measurement of laser damage resistance of various optical materials. Become.

  As is clear from the above description, the wavelength range in which the first wavelength laser light in this specification can cause nonlinear absorption due to two-photon absorption by irradiating the optical material with an energy density within a specific range. The wavelength range differs depending on the optical material.

When the optical material is irradiated with laser light to generate two-photon absorption, the wavelength of the irradiated laser light is preferably longer than the transmission limit wavelength of the optical material and not more than 5 times the transmission limit wavelength. Furthermore, in the case of pulsed laser light having a pulse width of 10-18 s or more and 10-7 s or less, the energy density in the optical material is increased to generate two-photon absorption, and the transmittance of incident laser light is measured. It is possible to obtain a decrease in transmittance due to two-photon absorption.
For example, for a CLBO crystal that is a nonlinear optical crystal, two-photon absorption can be more easily generated in the CLBO crystal by condensing and irradiating pulsed laser light (266 nm or less) in the ultraviolet region.
In quartz glass, a decrease in transmittance due to two-photon absorption could not be observed remarkably when irradiated with laser light having a wavelength of 266 nm, and a correlation between change in transmittance and laser damage resistance could be confirmed at a wavelength of 213 nm. . Compared with crystalline materials, defects in glass materials, impurity density, etc. may be reduced in glass materials. In this case, it is necessary to increase the photon energy of the laser light to be emitted, and irradiation with laser light with a short wavelength is indispensable. is there.
In addition, for CaF ₂ crystal, for example, a correlation between the change in transmittance and laser damage resistance has been confirmed by irradiation with pulsed laser light having a wavelength of 213 nm, but two-photon absorption is generated by a femtosecond laser having a wavelength of 860 nm. It is also possible to measure the laser damage threshold.

In the above embodiment, by adjusting (shortening) the focal length of the condenser lens unit 2, the fluorescence emission outside the measurement region is relatively suppressed, and the transmittance due to the nonlinear absorption amount in the measurement region is measured. Made possible.
However, in the present invention, as shown in FIG. 9, two laser beams are overlapped in the measurement region to increase the amount of nonlinear absorption due to the two-photon absorption in the measurement region, and two lasers are out of the measurement region. The laser pulse width and timing may be adjusted so that the light does not overlap, and the amount of fluorescence emitted outside the measurement region may be suppressed.

That is, in the configuration shown in FIG. 9, two laser light sources and two transmitted light detectors 3a and 3b are provided, and two laser beams La and Lb emitted from the laser light sources respectively correspond to the respective condensing lens units (FIG. It enters the optical material 7 from a different direction via a not-shown). In this configuration, the two laser light sources have optical pulse generation timings so that the two laser pulses overlap in the measurement region in consideration of the optical path length until the light emitted from the light sources reaches the measurement region of the optical material. Transmitted light detectors 3a and 3b, which are controlled by the measurement control unit and are provided corresponding to the laser beams La and Lb, detect the transmittance of the laser beams La and Lb in accordance with the pulse generation timing.
In the configuration shown in FIG. 9, the measurement region is determined based on the pulse width of the irradiated pulse laser and the diameter of the beam condensed by the condenser lens unit.

  In this configuration, it is preferable to set the pulse period and the duty ratio to be the same, irradiate the laser pulse a plurality of times in the same measurement region, and measure the transmittance, respectively, thereby enabling more accurate transmittance measurement. It becomes possible.

  As described above, the laser damage resistance estimation apparatus including the optical measurement unit can measure transmittance without being affected by fluorescence emission in regions other than the measurement region, and can measure transmittance with higher accuracy. The laser damage resistance can be estimated with high accuracy.

  In the above description, two laser light sources are provided and two laser beams La and Lb emitted from each laser light source are used. However, the laser beams emitted from one laser light source are divided and used. You may do it.

  In the above embodiments, specific preferred examples have been described. However, the laser damage resistance estimation method and apparatus according to the present invention can be variously modified within the scope of the present invention.

  According to the laser damage resistance estimation method and apparatus according to the present invention described above, it is possible to evaluate the laser damage resistance in a nondestructive manner, so that it is possible to guarantee the quality of the manufactured optical material, and the high-quality optical material It is possible to provide a highly reliable laser system that is configured using the above.

It is a functional block diagram which shows the structure of the laser damage tolerance estimation apparatus of embodiment which concerns on this invention. It is a block diagram which shows the structure of the optical measurement part 10 of FIG. It is a flowchart which shows the flow of a process in the optical measurement control part 11 of FIG. It is a flowchart which shows the flow of the process in the 1st estimation process part 12 of FIG. For CaF ₂ crystals with the fluorescence emission, which is a graph showing the fluorescence emission intensity with respect to incident energy. For CaF ₂ crystals with the fluorescence emission, which is a graph showing the transmittance of incident energy. For CaF ₂ crystal without fluorescence is a graph showing the transmittance of incident energy. For CaF ₂ crystal is a graph showing the correlation of the laser damage threshold to the laser damage threshold and 193nm laser light to 213nm laser beam. It is a schematic diagram which shows typically the irradiation method of the laser beam in the laser damage tolerance estimation apparatus which concerns on the modification of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Laser light source, 2 Condensing lens unit, 3, 3a, 3b Transmitted light detector, 4 Light emission detector, 5 Power monitor, 6 Beam splitter, 7 Optical material, 10 Optical measurement part, 11 Optical measurement control part, 12th 1 estimation process part, 13 1st database, 14 2nd estimation process part, 15 1st database.

Claims (9)

A preliminary measurement step comprising irradiating the optical material with laser light of a first wavelength and observing fluorescence emission;
A measurement step of measuring a change in transmittance with respect to the irradiation intensity in the measurement region by changing the irradiation intensity while causing two-photon absorption in a measurement region which is a part of the optical material;
Based on the change in the transmittance with respect to the irradiation intensity, the laser damage resistance at the first wavelength is estimated with reference to the correlation data between the change in the transmittance with respect to the irradiation intensity and the laser damage resistance when the fluorescence emission is obtained in advance. A first estimation step, and a laser damage resistance estimation method for an optical material. In the first estimation step,
When fluorescence emission is observed based on the change in transmittance with respect to the irradiation intensity, the first change is made with reference to correlation data between the change in transmittance with respect to the irradiation intensity and laser damage resistance obtained in advance when accompanied by fluorescence emission. When the laser damage resistance at the wavelength is estimated and no fluorescence emission is observed, the first wavelength is referred to by referring to the correlation data between the change in transmittance with respect to the irradiation intensity and the laser damage resistance obtained without the fluorescence emission obtained in advance. The laser damage resistance estimation method for an optical material according to claim 1, wherein the laser damage resistance is estimated.   The preliminary measurement step is caused by two-photon absorption generated in a part of the optical material by irradiating the optical material with the first wavelength laser beam with different intensity and based on a change in transmittance with respect to the irradiation intensity. The method further comprises the steps of determining whether or not the nonlinear absorption amount to be evaluated can be evaluated, and an adjustment step for suppressing the fluorescence while increasing the nonlinear absorption amount so that the nonlinear absorption amount can be evaluated. Or the laser damage tolerance estimation method of the optical material of 2.   After the preliminary measurement step, the method includes a step of dividing the optical material into a plurality of measurement regions using the part where two-photon absorption has occurred as a measurement region, and sequentially measuring the measurement step and the first estimation for the plurality of measurement regions. 4. The method for estimating laser damage resistance of an optical material according to claim 3, wherein the steps are repeated.   Based on the estimated laser damage resistance at the first wavelength, the laser damage resistance at the second wavelength is estimated by referring to the correlation data between the laser damage resistance at the first wavelength and the laser damage resistance at the second wavelength that is obtained in advance. The laser damage tolerance estimation method for an optical material according to claim 1, further comprising a second estimating step. At least one laser light source that generates laser light having a first wavelength, an optical system that focuses the laser light emitted from the laser light source and irradiates the optical material, and the intensity of the laser light that passes through the optical material An optical measurement device including a transmitted light detector for measuring the emission, and a light emission detector for detecting fluorescence by the irradiated laser light,
The energy density irradiated to the measurement region that is a part of the optical material is improved to cause two-photon absorption in the measurement region, and the energy density irradiated to the region other than the measurement region in the optical material is decreased. An optical measurement control device that changes the emission intensity of the laser light source in a state where the optical system is controlled to suppress the fluorescence emission of the optical sample,
Based on the intensity of the laser beam detected by the transmitted light detector and the emission intensity of the laser light source, a change in transmittance with respect to the irradiation intensity of the laser beam applied to the measurement region is calculated, and the calculated Based on the change in transmittance, a first estimate for estimating laser damage resistance at the first wavelength with reference to correlation data between the change in transmittance with respect to the irradiation intensity and laser damage resistance obtained with fluorescence emission obtained in advance. And an apparatus for estimating laser damage resistance of an optical material. The first estimation device includes:
When fluorescence emission is observed based on the change in transmittance with respect to the irradiation intensity, the first change is made with reference to correlation data between the change in transmittance with respect to the irradiation intensity and laser damage resistance obtained in advance when accompanied by fluorescence emission. When the laser damage resistance at the wavelength is estimated and no fluorescence emission is observed, the first wavelength is referred to by referring to the correlation data between the change in transmittance with respect to the irradiation intensity and the laser damage resistance obtained without the fluorescence emission obtained in advance. 7. The apparatus for estimating laser damage resistance of an optical material according to claim 6, wherein the laser damage resistance is estimated.   8. The laser damage resistance estimation apparatus for an optical material according to claim 6, wherein the optical measurement apparatus includes a stage for moving the optical material or the optical system according to the control of the optical measurement control apparatus.   Based on the estimated laser damage resistance at the first wavelength, the laser damage resistance at the second wavelength is estimated by referring to the correlation data between the laser damage resistance at the first wavelength and the laser damage resistance at the second wavelength that is obtained in advance. The apparatus for estimating laser damage resistance of an optical material according to any one of claims 6 to 8, further comprising a second estimation device.

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