JP2012185081A - Nonlinear absorption measurement method of optical material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of eliminating influence of fluorescent emission in a thickness direction, measuring two-photon absorption, and evaluating laser damage resistance of an optical material with respect to the optical material associated with the fluorescent emission.SOLUTION: A method for measuring a two-photon absorption coefficient in a measurement region inside an optical material includes: [1] a measuring step for generating the two-photon absorption at a focal point position by irradiating pulse laser beams focused on at least two measurement regions with different depths from a surface of the material respectively while sequentially changing incident light intensity, and measuring a transmission coefficient and a total emission amount of fluorescent emission for each incident light intensity when the two-photon absorption is generated; [2] a transmission coefficient correction step for correcting the measured transmission coefficient based on measured values with respect to each incident light intensity; and [3] a two-photon absorption coefficient computing step for obtaining the two-photon absorption coefficient in the measurement region based on change in the transmission coefficient after the correction to change in the incident light intensity.

Description

  The present invention relates to a method for measuring nonlinear absorption (two-photon absorption) of an optical material accompanied by fluorescence emission, and particularly in an optical material having a thickness, by correcting the contribution of fluorescence emission in the thickness direction of the optical material, The present invention relates to a method for accurately measuring a two-photon absorption coefficient. The invention further relates to a method for evaluating the laser damage resistance of an optical material with fluorescent emission.

  Laser systems using laser light are used in a wide range of industrial fields such as information communication, ultrafine processing, and medical treatment. In a laser system, optical materials such as optical crystals are used as components of wavelength conversion, condensing, reflecting, and amplifying elements of laser light. However, since 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.

As optical materials used in these systems, glass materials represented by quartz mainly used for lenses, mirror substrates, window materials, etc., single crystal materials such as Nd: YAG, Yb: YAG mainly used for laser oscillation, 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, CaF ₂ mainly used for lenses And single crystal materials such as MgF _{2 and} translucent ceramics mainly used as window materials. Since these optical materials have different qualities obtained depending on the manufacturing method and raw materials used, it is important how quality evaluation and quality assurance are performed. In particular, the evaluation of laser damage resistance is extremely important.

  Accordingly, the present inventors have developed a method for nondestructively and noncontactly evaluating the laser damage resistance of an optical material (Patent Documents 1 and 2). This method irradiates an optical material with pulsed laser light, measures a decrease in transmittance due to a two-photon absorption coefficient at a number of positions inside the optical material, and based on the result, a laser in each region inside the optical material. This is to evaluate damage resistance. By applying these results, it has become possible to demonstrate a technology capable of non-destructive three-dimensional imaging of laser damage resistance of optical materials.

Japanese Patent No. 4528075 JP 2009-36706 A

  According to the method described in Patent Document 1, it is possible to evaluate the laser damage resistance of an optical material using nonlinear absorption, and the laser damage resistance in a region near the focal point of the irradiated laser light can be nondestructively and highly accurately determined from the nonlinear absorption amount. I was able to ask. By moving the focal position of the laser beam to irradiate vertically and horizontally within the optical material, imaging evaluation of in-plane homogeneity has become possible. On the other hand, some optical materials are accompanied by strong fluorescent light emission in the process of nonlinear absorption when irradiated with strong laser light. When an optical material with fluorescent emission is irradiated with laser light, the transmittance is not only reduced by nonlinear absorption but also reduced by fluorescent emission. Since the degree of transmittance decrease due to fluorescence emission is very large compared to the degree of transmittance decrease due to nonlinear absorption, based on the degree of transmittance decrease in the thickness direction of the optical material accompanied by fluorescence emission. The two-photon absorption amount in the region near the focal point cannot be accurately obtained, and therefore, the correlation between the nonlinear absorption amount and the laser damage resistance cannot be obtained. For this reason, the evaluation of laser damage resistance due to nonlinear absorption in an optical material with fluorescent emission is currently only possible up to an area of ± 0.5 mm from the surface of the optical material with a reference of about 3 mm, and particularly a thick lens material. Then, the originally aimed three-dimensional imaging could not be performed well (FIG. 1 (a)). Moreover, in the conventional general non-linear absorption measurement technique, a thin plate sample is used in order to minimize the influence of fluorescence emission in the thickness direction, and the evaluation and analysis inside the optical material having a thickness is performed. Not.

  In view of the above, the first object of the present invention is to provide a method capable of accurately measuring the two-photon absorption coefficient not only in the vicinity of the surface but also in the interior of an optical material having a thickness accompanied with fluorescence. . A second object of the present invention is to provide a method capable of evaluating laser damage resistance inside an optical material using a measured two-photon absorption coefficient in an optical material having a thickness accompanied with fluorescence. The third object of the present invention is to provide a method capable of three-dimensional imaging with high accuracy of laser damage resistance inside an optical material having a thickness accompanied with fluorescence (FIG. 1B).

  In order to solve the above-mentioned problems, the present inventors have analyzed in detail the energy attenuation due to two-photon absorption and the energy attenuation due to fluorescence emission in an optical material with fluorescence emission. The inventors have discovered a method for accurately measuring the two-photon absorption coefficient by performing correction to remove the contribution of fluorescence emission in the thickness direction.

That is, the two-photon absorption coefficient measuring method according to the present invention is:
A method for measuring a two-photon absorption coefficient in a measurement region inside an optical material having a thickness L, comprising:
[1] For each of at least two measurement regions having different depths from the surface of the optical material, a pulsed laser beam focused on each measurement location in each measurement region is irradiated, and the incident light intensity at each focus position is determined. Two-photon absorption is caused while changing sequentially,
(I) When two-photon absorption occurs in one measurement region close to the optical material surface of the at least two measurement regions, the total emission of fluorescence emitted in the region from the optical material surface to the one measurement region Measure the amount of each incident light intensity as a reference total emission amount,
(Ii) When two-photon absorption occurs in the other measurement region, the transmittance and the total emission amount of the fluorescence emission generated in the region from the optical material surface to the other measurement region are measured for each incident light intensity. Measuring steps;
[2] The transmittance measured in the other measurement region for each incident light intensity is measured in the reference total light emission amount measured in the one measurement region for each incident light intensity and in the other measurement region. And a transmittance correction step for correcting based on the total light emission amount and the ratio of the total light emission amount to the reference total light emission amount,
[3] A two-photon absorption coefficient calculating step for obtaining a two-photon absorption coefficient in the other measurement region based on the change in the corrected transmittance with respect to the change in incident light intensity and the thickness L of the optical material;
It is characterized by including.

  In the measurement method of the present invention, it is preferable to set the one measurement region in the vicinity of the surface of the optical material. In this way, a more accurate two-photon absorption coefficient can be obtained.

  Further, in the [1] measurement step, the transmittance is measured for each incident light intensity in the one measurement region, and the measurement is performed based on the measured transmittance in the [2] transmittance correction step. Each of the transmitted transmittances may be corrected, and the two-photon absorption coefficient in the one measurement region may be obtained in the [3] two-photon absorption coefficient calculation step. By doing so, the two-photon absorption coefficient can be measured also in the one measurement region.

Moreover, the measurement method of the present invention includes the [1] measurement step,
A pulse focused on at least one third measurement region on a straight line connecting the at least two measurement regions and at a depth different from that of the at least two measurement regions. Similarly to the at least two measurement regions, laser light is irradiated while sequentially changing the incident light intensity to cause two-photon absorption at the focal position, and at each incident light intensity, the third measurement is performed from the surface of the optical material. Further comprising measuring the total amount of fluorescence emitted in the region up to the region,
In the [2] transmittance correction step,
Based on the depth from the surface of the optical material of each measurement region and the total amount of light emission measured in each measurement region at each incident light intensity, an approximate straight line or an approximate curve based on a linear function or higher is obtained. The distance between the approximate curve and the measured total light emission amount is set to be minimum, and the extrapolated value on the approximate line or the approximate curve when the depth of focus is zero is set as the reference total light emission amount. The value on the approximate line or the approximate curve at the depth from the optical material surface of the other measurement region was again used as the total light emission amount from the optical material surface to each measurement region, and measured in the other measurement region. The transmittance may be corrected. In this way, the two-photon absorption coefficient can be measured with higher accuracy.

in this case,
In the [1] measurement step, the transmittance is measured with respect to each incident light intensity for the one measurement region and the third measurement region,
In the [2] transmittance correction step, the values on the approximate straight line or the approximate curve in the depths of the one measurement region and the third measurement region from the optical material surface are changed from the optical material surface to the one measurement region. And, as a total light emission amount to the third measurement region, correct the transmittance measured in the one measurement region and the third measurement region,
In the [3] two-photon absorption coefficient calculation step, a two-photon absorption coefficient in the one measurement region and the third measurement region may be obtained. In this way, the two-photon absorption coefficient can be measured also in the one measurement region and the third measurement region.

  The function is preferably a function of second order or higher. In this way, the two-photon absorption coefficient can be measured with higher accuracy.

  Further, any one of the above-described methods for measuring the two-photon absorption coefficient in the at least two measurement regions and the third region on the straight line may be one or more different from the straight line and parallel to the straight line. By repeating for a straight line, the two-photon absorption coefficient can be measured in each region in the three-dimensional direction inside the optical material.

  The value of the two-photon absorption coefficient obtained by any of the above methods is stored in association with the position information of each measurement region, and the database storing the correlation between the two-photon absorption coefficient and the laser damage resistance is referred to By evaluating the laser damage resistance in each measurement region, it is possible to evaluate the laser damage resistance in each measurement region of the optical material.

Further, for a plurality of measurement regions of the first optical material that are homogeneous and have known laser damage resistance, the two-photon absorption coefficient is obtained using any one of the two-photon absorption coefficient measurement methods described above,
Storing the two-photon absorption coefficient in relation to the position information of each measurement region,
For a plurality of measurement regions of the second optical material that is the same type as the first optical material and whose laser damage resistance is unknown, a two-photon absorption coefficient is obtained using any one of the two-photon absorption coefficient measurement methods described above,
Comparing the two-photon absorption coefficient determined for each measurement region of the second optical material with the two-photon absorption coefficient determined for the corresponding measurement region of the first optical material,
By evaluating the laser damage resistance of the second optical material based on the comparison result and the database storing the correlation between the two-photon absorption coefficient and the laser damage resistance in each region of the first optical material, The laser damage resistance in the measurement area can be evaluated.

  When the method of the present invention is used, the two-photon absorption coefficient can be measured only in a narrow range of about 3 mm ± 0.5 mm from the surface in the prior art. It is possible to accurately measure the two-photon absorption coefficient. Furthermore, it becomes possible to accurately evaluate the laser damage resistance inside the material of the optical material having a thickness accompanied with the fluorescence emission, and three-dimensional imaging of the laser damage resistance inside the material can be performed.

FIG. 1 is a diagram conceptually comparing the prior art with the present invention. FIG. 2 is a model diagram showing the theory of the measurement method of the present invention. FIG. 3 is a conceptual diagram showing the relationship between the incident light intensity and the reciprocal of the transmittance. FIG. 4 is a conceptual diagram showing the relationship between the depth of focus and the total light emission amount. FIG. 5 is a block diagram showing an example of the configuration of a measuring apparatus used in the measuring method of the present invention. FIG. 6 is a diagram schematically illustrating another configuration example of the laser beam irradiation method in the measurement apparatus used in the measurement method of the present invention. FIG. 7 is a graph in which the reciprocal of the transmittance before correction measured in the example is plotted with respect to the incident light intensity. FIG. 8 is a graph showing data obtained by plotting the total light emission amount measured in the example with respect to the focal depth, and the result obtained by fitting each approximate line and approximate curve of Examples 1 to 3 of the example to the data. is there. FIG. 9 is a graph in which the reciprocal of the corrected transmittance is plotted against the incident light intensity in Example 3 of the example. FIG. 10 is a graph in which the calculation results of the two-photon absorption coefficient are plotted with respect to the depth of focus for each example of the example. FIG. 11 is a diagram comparing the result of evaluating laser damage resistance based on the transmittance before correction and the result of evaluating laser damage resistance based on the corrected transmittance in the example.

  The theory of the two-photon absorption coefficient measurement method of the present invention will be described below with reference to the drawings.

[theory]
As shown in FIG. 2 (a), a system for measuring a two-photon absorption coefficient at each of points A, B, and C inside an optical material by irradiating an optical material having a thickness L accompanied by fluorescence with laser light. Think. The distances of points A, B and C from the optical material surface on the incident side (hereinafter also simply referred to as the optical material surface) are a, b and c, respectively.
The surface of the optical material is irradiated with a laser beam having an intensity I _in focused on the point A, and two-photon absorption is generated in the measurement region near the focus. In this case, the transmitted light intensity IA _out, the total emission amount of fluorescence occurring in the region of an optical material surface to measurement area (hereinafter, simply referred to as a total light emission amount) and PL _A is measured. Here, the total light emission amount means a value obtained by integrating the light emission intensity of the fluorescent light generated when the optical material is irradiated with laser light from the optical material surface to the focal position. At this time, a distance from the surface of the optical material to the focal position (hereinafter, this distance is also referred to as a focal depth) is a. In such a system, how the incident light attenuates due to nonlinear absorption and fluorescence emission inside the optical material can be described based on the model diagram shown in FIG.

First, the incident light having the intensity I _in is strongly absorbed near the surface of the material, so that fluorescent emission is generated, thereby causing a sharp decrease in light intensity (loss of light energy) (1-1). Thereafter, the light intensity gradually decreases, and at point A, the light intensity becomes IA _in . The two-photon absorption near the point A (non-linear absorption) further reduces the light intensity more rapidly (1-2), resulting in the light intensity IA _out . After the two-photon absorption occurs, the light intensity becomes very weak, so the amount of fluorescence emission depending on the intensity is considered to be negligibly small. Following this assumption, the light can then be neglected in absorption due to fluorescence emission, proceeding at approximately the same intensity (1-3), and exiting the back surface of the optical material as transmitted light of intensity IA _out .

Ａ点と焦点深さの異なるＢ点およびＣ点についても同様に、透過率の逆数Ｔ_Ｂ~^−１およびＴ_Ｃ~^−１が下記式で表される。

一方、測定対象の光学材料が蛍光発光を伴わない場合、２光子吸収係数は、透過率の逆数を入射光強度に対してプロットしたグラフの傾きから下記式

を用いて計算することができる。 At this time, the reciprocal number T _A ⁻¹ of the transmittance T _A ˜ (symbol “˜ (tilde)” which should be formally described at the top of “T”, and so on) is actually measurable. It is represented by the following formula.

Similarly, for points B and C having different focal depths from point A, the reciprocal transmittances T _B ⁻¹ and T _C ⁻¹ are expressed by the following equations.

On the other hand, when the optical material to be measured does not emit fluorescence, the two-photon absorption coefficient is expressed by the following equation from the slope of the graph in which the reciprocal of the transmittance is plotted against the incident light intensity:

Can be used to calculate.

Ｂ点およびＣ点においても同様に、透過率の逆数Ｔ_Ｂ ^−１およびＴ_Ｃ ^−１を下記式で表すことができる。

蛍光発光の寄与が排除された上記の透過率の逆数Ｔ_Ａ ^−１、Ｔ_Ｂ ^−１およびＴ_Ｃ ^−１を求めることができれば、式（４）を用いて２光子吸収係数を算出することができる。 In order to obtain the two-photon absorption coefficient using Equation (4), it is necessary to exclude the contribution of light intensity attenuation caused by fluorescence emission from the measured transmittance.
When the contribution of fluorescence emission is eliminated, it is considered that the light intensity attenuation due to fluorescence emission does not occur and only the light intensity attenuation due to two-photon absorption occurs. Thus, the reciprocal T _A ⁻¹ of the transmittance at the point A when the light intensity is attenuated due to the two-photon absorption alone can be expressed as follows.

Similarly, at points B and C, the reciprocal transmittances T _B ^-1 and T _C ^-1 can be expressed by the following equations.

If the reciprocal numbers T _A ⁻¹ , T _B ⁻¹ and T _C ⁻¹ of the above-described transmittance from which the contribution of fluorescence is excluded can be obtained, the two-photon absorption coefficient can be calculated using Equation (4). it can.

Below, inverse _{T A} ~ ^-1 of the measured transmittance, the _{T B} ~ ^-1 and ^{_{T C ~ -1, T A -1}} , illustrating a method of correcting to ^{T B -1} and _T ^{C -1} .
First, for point _B , FIG. 3 shows a conceptual diagram in which T _B ⁻¹ and T _B ⁻¹ are plotted against the incident light intensity. When there is a contribution of fluorescence, since I _in > IB _{in as} can be seen from FIG. 2, the reciprocal number T _B ⁻¹ of the actually measured transmittance is larger than the reciprocal number T _B ^{−1 of the} transmittance to be obtained. Value. Assuming that there is no reflection of light on the surface, the intercept of the graph should be 1 originally, but when there is a contribution of fluorescence, intercept T ₀ ⁻¹ > 1. Therefore, it can be seen that it is necessary to remove the contribution of fluorescence emission from the reciprocal number T _B ⁻¹ of the measured transmittance and correct it to the reciprocal number T _B ⁻¹ of the target transmittance.

と表すことができる。この総発光量の比を発光増大比γ_ＡＢとする。
同様に、Ａ点に対するＣ点の発光増大比γ_ＡＣは、下記式

で表すことができる。
（８）式を変形すると、

となり、両辺をＩＢ_ｏｕｔで割ると、求めるべきＴ_Ｂ ^−１は

と表すことができる。Ｃ点についても同様に、Ｔ_Ｃ ^−１は、（９）式を変形して

と表すことができる。 Next, when the optical material is irradiated with the laser light focused on the points A, B, and C, a graph in which the total light emission amount from the optical material surface to the focal position is plotted with respect to the focal depth. As shown in FIG. The total light emission amount is proportional to the light absorption amount, and the increase in absorption due to the fluorescence emission is considered to be a loss of incident light energy and appear in the form of a decrease in light intensity. Therefore, the amount of decrease in light intensity associated with light emission at point A is I _in -IA _in . Similarly at point B, I _in −IB _in . Since the ratio of the decrease in light intensity is considered to be equal to the ratio of the total light emission,

It can be expressed as. The ratio of the total light emission amount is defined as a light emission increase ratio γ _AB .
Similarly, the emission increase ratio γ _AC of point C to point A is given by the following equation:

Can be expressed as
When formula (8) is transformed,

When both sides are divided by IB _out , T _B ^{−1 to be obtained} is

It can be expressed as. Similarly for point _C , TC ^-1 is obtained by modifying equation (9).

It can be expressed as.

Ａ点が光学材料表面近傍に存在する場合、ＩＡ_ｉｎをＩＯ_ｉｎで近似して差し支えないと考えられる。即ち、ＩＡ_ｉｎ／Ｉ_ｉｎをＩＯ_ｉｎ／Ｉ_ｉｎで置き換えることが可能である。式（１３）を用いて式（１１）および式（１２）を書き直すと、

となる。更に、Ｔ_Ａ ^−１については以下のように表すことができる。

ここで、Ｔ_Ａ~^−１、Ｔ_Ｂ~^−１およびＴ_Ｃ~^−１は透過率測定から直接得られる値であり、γ_ＡＢ、γ_ＡＣおよびＴ_０は、測定可能な値を用いて求めることができる値である。 Subsequently, in order to express T _B ⁻¹ and T _C ⁻¹ by measurable quantities, the following modifications are performed. First, in FIG. 3, the intercept T ₀ ^-1 is considered to mean a decrease in light intensity due to fluorescence emission in the vicinity of the surface of the optical material (that is, (1-1) in FIG. 2). Therefore, the value IO _in obtained by subtracting the attenuation amount of the light intensity due to the fluorescence emission near the surface of the optical material from the incident light intensity can be defined by the following equation.

When the point A exists in the vicinity of the optical material surface, it is considered that IA _in may be approximated by IO _in . That is, IA _in / I _in can be replaced with IO _in / I _in . When Expression (11) and Expression (12) are rewritten using Expression (13),

It becomes. Furthermore, T _A ^-1 can be expressed as follows.

Here, T _A to ⁻¹ , T _B to ⁻¹ and T _C to ⁻¹ are values obtained directly from the transmittance measurement, and γ _AB , γ _AC and T ₀ are obtained using measurable values. Is a value that can be.

  By applying the above theory, it is possible to perform correction to remove the contribution of fluorescence emission from the measured transmittance, and to obtain the two-photon absorption coefficient using the reciprocal of the corrected transmittance and Equation (4). it can.

[Optical materials]
Examples of optical materials that can be measured by the method of the present invention include glass materials typified by quartz used mainly for lenses, mirror substrates, window materials, etc., Nd: YAG, Yb: YAG used mainly for laser oscillation, and the like. Single crystal materials such as CLBO (CsLiB ₆ O ₁₀ ), LBO (Li ₂ B ₂ O ₄ ), KTP (KTiOPO ₄ ) and the like mainly used for wavelength conversion, CaF ₂ mainly used for lenses, Examples thereof include single crystal materials such as MgF _{2 and} translucent ceramics mainly used as window materials.
The thickness of the optical material that can be measured by the method of the present invention varies depending on the type of material. In the case of an optical material having a relatively small fluorescence emission, an optical material having a thickness of up to about 30 mm can be measured by the method of the present invention. Even an optical material having a remarkable fluorescence emission can measure an optical material having a thickness of up to about 20 mm by the method of the present invention. If the thickness of the optical material is larger than the above value, the attenuation of the transmittance due to the fluorescence emission becomes extremely large, and it becomes difficult to determine the attenuation of the transmittance due to the slight nonlinear absorption, which is not preferable.

[measuring device]
An example of a measuring apparatus that can be used in the measuring method of the present invention will be described below. This example does not limit the invention, and the measurement can be performed using another device.

An example of the configuration of the measuring apparatus according to the present invention is shown in FIG. In this example, the measuring device is: (a) a laser light source 1 that emits laser light;
(B) The laser light emitted from the laser light source 1 is divided into two, one of the divided light is incident on the power monitor 5 as a monitoring laser light, and the other light is used as a measuring laser light and a condensing lens A beam splitter 6 incident on the unit 2;
(C) A condensing lens unit 2 that condenses incident measurement laser light and enters the optical material 7;
(D) a transmitted light detector 3 for detecting the transmitted light intensity of the measurement laser light transmitted through the optical material 7;
(E) a power monitor 5 for monitoring the intensity of the laser beam; and (f) a light emission detector 4 for detecting the fluorescence emission of the optical material 7.
Consists of.

  The optical measurement device is controlled by an optical measurement control unit, and measurement data is output. Although not shown, the optical measuring device has a moving stage that moves the optical material 7 or the condenser lens unit 2 to be measured. By moving the optical material 7 or the condenser lens unit 2 on the moving stage, the focal position of the laser light inside the optical material can be changed.

[Optical measurement controller]
The optical measurement controller
(1) an optical system controller that adjusts the optical system so that the incident laser beam is focused in each measurement region;
(2) In each measurement region, a measurement control unit that sequentially changes the incident energy intensity of laser light and enters the sample to measure the transmittance and the total light emission amount for each incident energy intensity, and (3) optical material A scanning control unit that sequentially moves the position of the measurement region so that the transmittance in the entire region is measured.

[Measurement condition]
Measurement conditions of the measurement method of the present invention will be described below.

  The measurable range of the optical material that can measure the two-photon absorption coefficient using the method of the present invention is that the transmittance is linear as the incident energy increases when the transmittance is measured while increasing the incident energy of the pulse laser beam. It decreases due to absorption, and then the decrease in transmittance changes to a non-linear decrease, and when the incident energy increases, the optical material is destroyed. This is a range determined to be caused by two-photon absorption at the focal position.

  On the other hand, the region near the surface of the optical material is structurally weak compared to the inside of the optical material. Therefore, when focused on the region and irradiated with pulsed laser light, excessive energy is applied to the surface of the optical material, and the surface of the optical material Is destroyed and the two-photon absorption coefficient cannot be measured. The weak laser damage resistance in the region near the optical material surface is due to the high defect density on the optical material surface. For example, when the optical material is a single crystal, the defect density on the surface is high due to the discontinuous crystal structure on the surface of the optical material. In addition, the presence of impurities such as adhesion of the polishing agent to the surface of the optical material by polishing and the finished state thereof can also cause structural weakness of the material surface. When the laser damage resistance inherent to the optical material near the optical material surface is high, the focal position at which the two-photon absorption coefficient can be measured without destroying the optical material approaches the optical material surface, and thus the measurable range is widened. On the other hand, when the laser damage resistance inherent to the optical material is low, the focal position at which the two-photon absorption coefficient can be measured without destroying the optical material is far from the surface of the optical material, and thus the measurable range is narrowed. For example, in the case of quartz, the measurable range of the optical material is preferably in the range of 1 mm or more from the optical material surface, and more preferably in the range of 2 mm or more.

The type of pulsed laser beam that can measure the two-photon absorption coefficient using the method of the present invention varies depending on the type of optical material to be measured. When measured, it is necessary to select appropriately so that the transmittance decreases nonlinearly and the decrease in the nonlinear transmittance is caused by the two-photon absorption at the focal position of the pulse laser beam. When the wavelength of the pulsed laser beam is shorter than the transmission limit wavelength of the optical material, the transmission of the pulsed laser beam becomes extremely small, making it difficult to evaluate the laser damage resistance due to a decrease in transmittance. When the wavelength of the pulse laser beam is too long, the occurrence of two-photon absorption is reduced, and an effective laser damage evaluation method for an optical material becomes difficult. In addition, it is difficult to achieve a pulse laser beam with a pulse width shorter than 10 ⁻¹⁶ s with the current technology. If the pulse width of the pulse laser beam is longer than 10 ⁻⁶ s, laser damage due to irradiation with the pulse laser beam may occur. It may occur.
The incident laser light intensity is changed from the minimum light intensity to reach the maximum light intensity while increasing the intensity at a predetermined rising step. If the incident laser light intensity is too high or the beam is too narrow, the material will be destroyed due to an increase in energy density. If the light intensity is too low, sufficient energy cannot be obtained to cause two-photon absorption in the region near the focal point.

In the measurement method of the present invention, in addition to the configuration shown in FIG. 5, as shown in FIG. 6, two laser beams are overlapped in the measurement region, and the nonlinear absorption amount due to the two-photon absorption in the measurement region is reduced. The amount of fluorescence emission outside the measurement region may be suppressed by adjusting the laser pulse width and timing so that the two laser beams do not overlap outside the measurement region.
That is, the configuration shown in FIG. 6 includes two laser light sources and two transmitted light detectors 3a and 3b, and the two laser beams La and Lb emitted from the laser light sources respectively correspond to the corresponding 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 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 in the optical material. Transmitted light detectors 3a and 3b, which are controlled by the optical measurement control unit and are provided corresponding to the respective laser beams La and Lb, detect the transmittance with respect to the respective laser beams La and Lb in accordance with the pulse generation timing. .
In the configuration shown in FIG. 6, the measurement region is determined based on the pulse width of the pulse laser to be irradiated and the beam diameter collected by the condenser lens unit.
In this configuration, it is preferable to set the pulse period and duty ratio to be the same, irradiate the laser pulse multiple times in the same measurement area, and measure the transmittance respectively, thereby enabling more accurate transmittance measurement become.
As described above, when using a measuring device in which the optical measuring unit is configured, it is possible to significantly reduce the influence of fluorescence emission in places other than the measurement region, and to measure the transmittance with higher accuracy, The two-photon absorption coefficient can be obtained with high accuracy.

[Measuring method]
The first aspect of the two-photon absorption coefficient measuring method of the present invention will be described below.

[1] Measurement step A plurality of measurement regions are set in the optical material having a thickness L.
Each measurement region is irradiated with a pulsed laser beam focused on each measurement location in each measurement region, and two-photon absorption is generated while sequentially changing the incident light intensity at each focus position. When two-photon absorption occurs in FIG. 2, the transmittance and the total emission amount of the fluorescence emission generated in the region from the optical material surface to each measurement region are measured. The measured transmittance and the total light emission amount are output in association with the position information of the measurement region.
Thereafter, the moving stage is moved by a predetermined amount, and the focal position of the pulse laser beam is adjusted within the next measurement area. The above measurement and measurement value output are repeated.

前記表面に最も近い測定領域を除く各測定領域において、各入射光強度に対して、測定した透過率の逆数Ｔ~^−１、前記切片Ｔ_０ ^−１の逆数Ｔ_０および発光増大比γを下記式

に代入して、測定した透過率を補正する。
更に、前記表面に最も近い測定領域において、各入射光強度に対して、測定した透過率の逆数Ｔ~^−１、前記切片Ｔ_０ ^−１の逆数Ｔ_０を（１６）式に代入することにより、前記表面に最も近い測定領域において測定した透過率を補正することができる。 [2] Transmittance correction step For each measurement region, the transmittance T ~ measured at each incident light intensity and the total light emission amount PL are called.
For each measurement region, the reciprocal T ~ ^-1 of the measured transmittance is plotted against the incident light intensity, and the obtained straight line intercept T ₀ ^-1 is obtained.
For each incident light intensity, the total light emission amount measured in the measurement region closest to the surface among the measurement regions is substituted into the following equation (17) as the reference total light emission amount PL ₀ at each incident light intensity, and is the most on the surface. By substituting each total light emission amount PL measured in each measurement region excluding the near measurement region into PL of the following equation (17), for each incident light intensity, light emission in each measurement region excluding the measurement region closest to the surface The intensity ratio γ is obtained.

In each measurement area, except the nearest measurement region to the surface, for each incident light intensity, inverse T ~ ^-1 of the measured ^{transmittance,} the reciprocal T ₀ and emission increase ratio γ of the sections T ₀ ^-1 below formula

And the measured transmittance is corrected.
Furthermore, the closest measurement region to the surface, for each incident light intensity, the inverse of the measured transmittance T ~ ^-1, by substituting the inverse T ₀ of the sections T ₀ ^-1 to (16) The transmittance measured in the measurement region closest to the surface can be corrected.

[3] Two-photon absorption coefficient calculation step In each measurement region, the reciprocal T ⁻¹ of the transmittance corrected in this way is plotted against the incident light intensity, and the above equation (4) is calculated from the slope of the obtained straight line. Is used to calculate the two-photon absorption coefficient in each measurement region.

  According to a second aspect of the two-photon absorption coefficient measuring method of the present invention, three or more measuring regions that are in a straight line and have different depths from the surface of the optical material are set and the [1] measuring step is performed. Instead of the [2] transmittance correction step in the first aspect, the [2 ′] transmittance correction step described below is performed, and then the [3] two-photon absorption coefficient calculation step is performed.

[2 ′] Transmittance Correction Step For each measurement region, the transmittance T˜ and the total light emission amount PL measured at each incident light intensity are called up.
For each measurement region, the reciprocal T ~ ^-1 of the measured transmittance is plotted against the incident light intensity, and the obtained straight line intercept T ₀ ^-1 is obtained.

  For each incident light intensity, plot the total light emission amount in each measurement area against the focal depth of each measurement area, and measure the approximate straight line or approximate curve by a linear function or higher. It is applied so that the distance from the total light emission amount is minimized.

で表される２次関数である。２光子吸収による入射光の吸収量は入射光強度の２乗に比例するので、２次の項を含む関数を用いると近似曲線を精度良く当てはめることができる。 入射光強度が更に大きくなると、３光子吸収等の多光子吸収が起こる確率が高くなる。従って、そのような場合、３次以上の項を含む関数を用いると、近似曲線をより高い精度で当てはめることができる。 The approximate straight line or the approximate curve is preferably a quadratic or higher function, and more preferably the following formula:

Is a quadratic function. Since the amount of incident light absorbed by two-photon absorption is proportional to the square of the incident light intensity, an approximate curve can be accurately applied using a function including a quadratic term. When the incident light intensity is further increased, the probability that multiphoton absorption such as three-photon absorption occurs will increase. Therefore, in such a case, the approximate curve can be applied with higher accuracy by using a function including a third-order term or more.

For each incident light intensity, an extrapolated value on the approximate line or approximate curve when the depth of focus is zero is the reference total light emission amount PL _0, and the approximation at the depth from the surface of each measurement region The value on the straight line or the approximate curve is again referred to as the total light emission amount PL in each measurement region. For each incident light intensity, PL ₀ and PL thus obtained in each measurement region are substituted into the equation (17), and the emission intensity ratio γ in each measurement region is obtained for each incident light intensity.
In each measurement area, for each incident light intensity, inverse T ~ ^-1 of the measured ^{transmittance,} the sections T ₀ ^-1 inverse T ₀ and the emission increase ratio γ (18) are substituted into equation measurement Correct the transmittance.

  Further, three or more measurement regions which are parallel to the straight line and which are on one or more straight lines different from the straight line and have different depths from the surface of the optical material are set, and the above-described second aspect is described above. The [1] measurement step, the [2 ′] transmittance correction step, and the [3] two-photon absorption coefficient calculation step are similarly performed to measure the two-photon absorption coefficient in each region in the three-dimensional direction inside the optical material. be able to.

  The value of the two-photon absorption coefficient obtained by any of the above methods is stored in association with the position information of each measurement region, and the database storing the correlation between the two-photon absorption coefficient and the laser damage resistance is referred to By evaluating the laser damage resistance in each measurement region, it is possible to evaluate the laser damage resistance in each measurement region of the optical material.

By using the method of the present invention, it is also possible to construct a database showing the correlation between the two-photon absorption coefficient and the laser damage resistance for optical materials with fluorescent emission whose laser damage resistance has been extremely difficult to evaluate. It is. By utilizing the database, it is possible to evaluate the laser damage resistance inside the non-homogeneous optical material.
For example, for a plurality of measurement regions of the first optical material that are homogeneous and have known laser damage resistance, a two-photon absorption coefficient is obtained using any of the methods described above, and the two-photon absorption coefficient is calculated for each measurement region. Are stored in relation to the position information. For a plurality of measurement regions of a second optical material that is the same type as the first optical material and whose laser damage resistance is unknown, a two-photon absorption coefficient is obtained using any of the methods described above, and the second optical material The two-photon absorption coefficient obtained for each measurement region is compared with the two-photon absorption coefficient obtained for the corresponding measurement region of the first optical material. By evaluating the laser damage resistance of the second optical material based on the comparison result and the database storing the correlation between the two-photon absorption coefficient and the laser damage resistance in each region of the first optical material, The laser damage resistance in the measurement area can be evaluated. By utilizing such a method, three-dimensional imaging of laser damage resistance inside the optical material can be performed.

  In the present specification, the “homogeneous” optical material means an optical material in which the nonlinear absorption amount is constant over the entire region inside the material, and thus the two-photon absorption coefficient is constant. In contrast, an “inhomogeneous” optical material means an optical material with a non-constant two-photon absorption coefficient due to different laser damage resistance in each region within the material.

The two-photon absorption coefficient was determined using the method of the present invention for a homogeneous CaF ₂ crystal having a thickness of 20 mm that emits very strong fluorescence. A CaF ₂ crystal is irradiated with pulsed laser light having a wavelength of 213 nm or less to cause two-photon absorption at each focal point of points A to D, and the incident light energy intensity is between 0.054 and 2.921 GW / cm ² . While increasing, the transmitted light T _A ˜T _D ˜ and the total emission amounts PL _A ˜PL _{D of the} fluorescence emitted in the region from the crystal surface to the focal position were measured. The distance (focus depth) from the crystal surface to the focal position was as follows.
Depth of focus: Point A (5 mm), Point B (9 mm), Point C (12 mm), Point D (18 mm)

  A graph in which the reciprocal of the measured transmittance is plotted against the incident light intensity is shown in FIG. As can be seen from FIG. 7, the reciprocal value of the transmittance apparently increases as the depth of focus increases. The deviation from the result at a focal depth of 5 mm increased as the focal depth increased.

  The two-photon absorption coefficient was calculated by correcting the transmittance measured using the methods of Examples 1 to 3 below.

[Example 1]
For each measurement area of A~D point, determine the sections were approximated by a straight line plot of FIG. 7, and the average value of the intercept and T ₀ ^-1. As a result of the calculation, T ₀ ⁻¹ was 1.15.

で表される線形関数を当てはめた。焦点深さがゼロであるときの近似直線上の外挿値を参照総発光量ＰＬ_０とし、さらにＡ〜Ｄ点における近似直線上の値をあらためてＡ〜Ｄ点における総発光量ＰＬ_Ａ、ＰＬ_Ｂ、ＰＬ_ＣおよびＰＬ_Ｄとした。ＰＬ_０と、ＰＬ_Ａ、ＰＬ_Ｂ、ＰＬ_ＣまたはＰＬ_Ｄとを（１７）式に代入して、Ａ〜Ｄの各点について発光強度比γを求めた。Ａ〜Ｄの各点について、測定した透過率、発光強度比およびＴ_０ ^−１を（１８）式に代入して、測定した透過率の値を補正し、補正後の透過率の逆数Ｔ_Ａ ^−１、Ｔ_Ｂ ^−１、Ｔ_Ｃ ^−１およびＴ_Ｄ ^−１を得た。他の入射光強度についても同様の補正を行った。 For each incident light intensity, the transmittance was corrected by the procedure described below.
For certain incident light _intensity, plotted PL _A, PL B, a PL _C and PL _D with respect to the focal depth, the following equation (20)

A linear function represented by The extrapolated value on the approximate line when the depth of focus is zero is set as the reference total light emission amount PL _0, and the values on the approximate line at the points A to D are re-established and the total light emission amounts PL _A and PL at the points A to D are obtained. _B, was PL _C and PL _D. And PL _0, by substituting PL _A, PL B, and a PL _C or PL _D in equation (17) to determine the emission intensity ratio γ for each point of the to D. For each of points A to D, the measured transmittance, emission intensity ratio, and T ₀ ⁻¹ are substituted into the equation (18) to correct the measured transmittance value, and the inverse number T _A of the corrected transmittance T _A ⁻¹ , T _B ⁻¹ , T _C ⁻¹ and T _D ⁻¹ were obtained. Similar corrections were made for other incident light intensities.

  For each measurement region, the corrected transmittance value was plotted against the incident light intensity, and the two-photon absorption coefficient was calculated from the slope of the obtained straight line using equation (4).

で表される２次関数を当てはめて、焦点深さがゼロであるときの近似曲線上の外挿値を参照総発光量ＰＬ_０とし、Ａ〜Ｄ点における近似曲線上の値をあらためてＡ〜Ｄ点における総発光量ＰＬ_Ａ、ＰＬ_Ｂ、ＰＬ_ＣおよびＰＬ_Ｄとして補正を行う以外は例１と同様の方法で、２光子吸収係数を算出した。 [Example 2]
The following formula (21) instead of formula (20)

And the extrapolated value on the approximate curve when the depth of focus is zero is taken as the reference total light emission amount PL _0, and the values on the approximate curve at points A to D are anew. the total emission amount _PL a at point _D, by PL B, the same way as example 1 except that corrects a PL _C and PL _D, was calculated two-photon absorption coefficient.

[Example 3]
A two-photon absorption coefficient was calculated in the same manner as in Example 2 except that the equation (19) was used instead of the equation (21).

  The result of fitting the functions of Examples 1 to 3 is shown in FIG. Further, FIG. 9 is a graph in which the reciprocal of the corrected transmittance for Example 3 is plotted against the incident light intensity. As can be seen from FIG. 9, by performing the correction using the equation (19), all the data are collected on substantially the same straight line regardless of the focal depth.

  The two-photon absorption coefficients calculated in Examples 1 to 3 are shown in Table 1 and FIG. From Table 1 and FIG. 9, it was found that when the function fitting of Example 3 was performed, the variation in the value depending on the focal depth was the smallest, and therefore the two-photon absorption coefficient could be obtained with the highest accuracy.

  Using the two-photon absorption coefficient calculated based on the transmittance before correction and the two-photon absorption coefficient calculated based on the corrected transmittance in Example 3, respectively, the correlation between the two-photon absorption coefficient and the laser damage resistance is calculated. With reference to the stored database, imaging of laser damage resistance in each measurement region was performed. The results are shown in FIG. When imaging of laser damage resistance was performed based on the transmittance before correction, the value of laser damage resistance increased at a focal depth of 5 mm as the depth of focus increased even though it was actually a homogeneous material. Deviated from the value and evaluated laser damage resistance lower than actual. On the other hand, when imaging of laser damage resistance was performed based on the corrected transmittance, laser damage resistance was shown to be almost the same as laser damage resistance at a focal depth of 5 mm even when the depth of focus increased. It was possible to evaluate accurately.

  According to the analysis method of the present invention, it is possible to accurately evaluate the laser damage resistance of non-destructive optical materials with fluorescence emission, and therefore it is possible to directly guarantee the quality of the optical materials.

DESCRIPTION OF SYMBOLS 1 Laser light source 2 Condensing lens unit 3 Transmitted light detector 4 Luminescence detector 5 Power monitor 6 Beam splitter 7 Optical material

Claims (9)

A method for measuring a two-photon absorption coefficient in a measurement region inside an optical material having a thickness L, comprising:
[1] For each of at least two measurement regions having different depths from the surface of the optical material, a pulsed laser beam focused on each measurement location in each measurement region is irradiated, and the incident light intensity at each focus position is determined. Two-photon absorption is caused while changing sequentially,
(I) When two-photon absorption occurs in one measurement region close to the optical material surface of the at least two measurement regions, the total emission of fluorescence emitted in the region from the optical material surface to the one measurement region Measure the amount of each incident light intensity as a reference total emission amount,
(Ii) When two-photon absorption occurs in the other measurement region, the transmittance and the total emission amount of the fluorescence emission generated in the region from the optical material surface to the other measurement region are measured for each incident light intensity. Measuring steps;
[2] The transmittance measured in the other measurement region for each incident light intensity is measured in the reference total light emission amount measured in the one measurement region for each incident light intensity and in the other measurement region. And a transmittance correction step for correcting based on the total light emission amount and the ratio of the total light emission amount to the reference total light emission amount,
[3] A two-photon absorption coefficient calculating step for obtaining a two-photon absorption coefficient in the other measurement region based on the change in the corrected transmittance with respect to the change in incident light intensity and the thickness L of the optical material;
A two-photon absorption coefficient measuring method.   The two-photon absorption coefficient measuring method according to claim 1, wherein the one measurement region is set in the vicinity of the surface of the optical material. [1] In the measurement step, the method further includes measuring the transmittance for each incident light intensity in the one measurement region,
In the [2] transmittance correction step, the measured transmittance is corrected based on the measured transmittance,
The two-photon absorption coefficient measuring method according to claim 1 or 2, wherein, in the [3] two-photon absorption coefficient calculation step, a two-photon absorption coefficient in the one measurement region is obtained. In the [1] measurement step,
A pulse focused on at least one third measurement region on a straight line connecting the at least two measurement regions and at a depth different from that of the at least two measurement regions. Similarly to the at least two measurement regions, laser light is irradiated while sequentially changing the incident light intensity to cause two-photon absorption at the focal position, and at each incident light intensity, the third measurement is performed from the surface of the optical material. Further comprising measuring the total amount of fluorescence emitted in the region up to the region,
In the [2] transmittance correction step,
Based on the depth from the surface of the optical material of each measurement region and the total amount of light emission measured in each measurement region at each incident light intensity, an approximate straight line or an approximate curve based on a linear function or higher is obtained. The distance between the approximate curve and the measured total light emission amount is set to be minimum, and the extrapolated value on the approximate line or the approximate curve when the depth of focus is zero is set as the reference total light emission amount. Transmission value measured in the other measurement region using the value on the approximate line or the approximate curve at the depth from the surface of the other measurement region as the total light emission amount from the surface of the optical material to each measurement region. The two-photon absorption coefficient measuring method according to claim 1, wherein the rate is corrected. In the [1] measurement step, the method further includes measuring the transmittance with respect to each incident light intensity for the one measurement region and the third measurement region,
In the [2] transmittance correction step, the values on the approximate straight line or the approximate curve in the depths of the one measurement region and the third measurement region from the optical material surface are changed from the optical material surface to the one measurement region. And, as a total light emission amount to the third measurement region, correct the transmittance measured in the one measurement region and the third measurement region,
5. The two-photon absorption coefficient measurement method according to claim 4, wherein, in the [3] two-photon absorption coefficient calculation step, a two-photon absorption coefficient in the one measurement region and the third measurement region is obtained.   The two-photon absorption coefficient measuring method according to claim 4 or 5, wherein the function is a function of second order or higher.   A two-photon absorption coefficient measurement method comprising repeating the two-photon absorption coefficient measurement method according to any one of claims 4 to 6 for one or more straight lines that are parallel to the straight line and different from the straight line. .   The value of the two-photon absorption coefficient obtained by the method according to any one of claims 4 to 7 is stored in association with the position information of each measurement region, and the two-photon absorption coefficient and the laser damage resistance A laser damage resistance evaluation method including evaluating laser damage resistance in each measurement region with reference to a database storing correlations. The two-photon absorption coefficient is obtained using the two-photon absorption coefficient measuring method according to any one of claims 4 to 7 for a plurality of measurement regions of the first optical material that are homogeneous and have known laser damage resistance. When,
Storing the two-photon absorption coefficient in relation to the position information of each measurement region,
The two-photon absorption coefficient measurement according to any one of claims 4 to 7, wherein the second optical material has the same composition and the same shape as the first optical material, and the measurement region is located at the same position as the plurality of measurement regions. Using a method to determine a two-photon absorption coefficient;
Comparing the two-photon absorption coefficient determined for each measurement region of the second optical material with the two-photon absorption coefficient determined for the corresponding measurement region of the first optical material;
And evaluating the laser damage resistance of the second optical material based on the comparison result and a database storing the correlation between the two-photon absorption coefficient and the laser damage resistance in each region of the first optical material. Laser damage resistance evaluation method.

Images