JP2001141651A - JUDGMENT METHOD FOR OPTICAL DAMAGE-RESISTANT PROPERTY OF MgO-DOPED LiNbO3 CRYSTAL AND ITS USE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method in which the existence of the optical damage- resistant property of an MgO-doped LN crystal can be judged nondestructively on the basis of an actual crystal irrespective of a prepared composition, and to provide a use for the MgO-doped LN crystal whose optical damage-resistant property is judged to be existent by the judgment method. SOLUTION: The transmittance of light in an MgO-doped LiNbO3 crystal is measured. When the width of a drop in the transmittance near a wavelength of 2.83 μm is at 5% or more and the width of the drop in the transmittance near a wavelength of 2.87 μm is less than 5% with reference to a base transmittance in the band of a wavelength of 2.5 to 3.3 μm, the crystal is judged to have an optical damage-resistant property. When the periodic polarization inversion structure is formed in the MgO-doped LN crystal whose optical damage- resistant property is judged to be existent by this method, it is possible to obtain a preferable wavelength conversion element which comprises an optical damage-resistant property.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of nonlinear optical crystals, and more particularly to a technique for determining the light damage resistance of an MgO-doped LiNbO ₃ crystal and the use of the crystal obtained thereby.

[0002]

2. Description of the Related Art A LiNbO ₃ crystal (hereinafter also referred to as “LN crystal”) and a crystal provided with a periodically poled structure (Periodically Poled LiNbO ₃ ; hereinafter also referred to as “PPLN crystal”) have a large nonlinear optical constant. And is promising as a wavelength conversion element. However, these LN crystals are easily damaged by light, and there is a problem in using them as a stable wavelength conversion element. Optical damage is that when irradiated with strong light such as laser light, the refractive index of the irradiated part changes,
This is a phenomenon that the refractive index does not return to the original one.

Therefore, various studies have been conducted on LN crystals having excellent light damage resistance, and as a result, MgO, Zn
It has been reported that LN crystals doped with O, In ₂ O ₃ , SC ₂ O _{3 and the} like have high light damage resistance. At present, among them, the MgO-doped LN crystal is considered to be the most promising in terms of crystallinity.

With respect to the light damage resistance of MgO-doped LN crystals, it has been reported that the light damage resistance changes sensitively depending on the amount of MgO doped. Previous reports have shown that, although there is some error, optical damage hardly occurs when MgO is doped at 5 mol% or more. Therefore, in order to know the degree of light damage resistance of the MgO-doped LN crystal, which is the material in front of the eyes, the amount of Mg in the crystal (= the amount of MgO)
It is necessary to know exactly.

[0005] The control of the amount of MgO doped in the LN crystal is based on the fact that in the actual growth of the MgO-doped LN crystal,
It is mainly performed by controlling the amount of MgO in the charged composition. And MgO in the obtained crystal
The display of the doping amount is not a result of analyzing the obtained crystal but a value estimated based on the MgO amount in the charged composition. This is because it is generally known how much the amount of MgO in the crystal varies from the charged composition, and it is difficult to analyze the crystal as described later.

[0006]

However, the amount of change in the amount of MgO from the charge composition described above is only a rough estimation and is not a precise value in detail. This is because even if the amount of MgO in the charged composition is constant, the amount of MgO in the obtained crystal greatly varies depending on the crystal growth temperature and the like. In the actual report examples as well, when comparing the data on the physical properties of the MgO-doped LN crystal for each report, the light damage resistance differs greatly from report to report even if the composition of MgO is the same. I have.

On the other hand, when the MgO-doped LN crystal is analyzed to determine the amount of Mg contained therein, a composition analysis method for that purpose is based on Inductively Coupled Plasma-Ato.
micEmission Spectroscopy (ICP-AES), Elect
ron Probe Micro Analysis (EPMA) can be considered. However, even with these methods, measuring the amount of Mg is basically difficult. The reason is that Mg is present in water and hands (due to sweat or the like) in large amounts, so that it tends to be mixed into the sample in the measurement of the amount of Mg, and the true value is unclear. In ICP-AES, the peak of Mg is weak, and the measurement itself is not easy. Even in EPMA, Mg
Detection is not easy with an Mg-doped amount of about O-doped LN crystal.

Further, in the above analysis method, it is necessary to destroy the sample for analysis, and it is difficult to evaluate the MgO-doped LN crystal as it is on a wafer. Because of the above points, the exact relationship between the amount of MgO in the MgO-doped LN crystal and the light damage resistance has not been clarified yet.

An object of the present invention is to solve the above-mentioned problems,
An object of the present invention is to provide a method capable of non-destructively determining the presence or absence of light damage resistance of an O-doped LN crystal based on the actual crystal regardless of the charged composition. Still another object of the present invention is to provide a use of an MgO-doped LN crystal determined to have light damage resistance by the determination method according to the present invention.

[0010]

Means for Solving the Problems The present inventors have found that, among the light transmittances of MgO-doped LN crystals, specific wavelengths (2. 1) near 2.8 µm attributed to the presence of OH ^- ions are considered. 83 μm and 2.87 μm) and found that there is a strong relationship between the transmittance and the amount of MgO,
Further, the present invention has been completed by combining the transmittance with the light damage resistance. The present invention has the following features.

(1) The light transmittance of the MgO-doped LiNbO ₃ crystal was measured, and the base transmittance in the wavelength band of 2.5 μm to 3.3 μm was compared with the wavelength of 2.83 μm.
It is determined that the crystal has light damage resistance when the degree of decrease in transmittance in the vicinity is 5% or more and the degree of decrease in transmittance near the wavelength of 2.87 μm is less than 5%. Do
A method for determining light damage resistance of MgO-doped LiNbO ₃ crystal.

(2) MgO-doped LiNb of the following (A)
A wavelength conversion element in which an O ₃ crystal is used and a periodically poled structure is formed in the crystal. (A) the transmittance of light of the crystal is 5% or more in a decrease in transmittance near a wavelength of 2.83 μm with respect to a base transmittance in a wavelength band of 2.5 μm to 3.3 μm; And a wavelength of 2.8
Mg having a transmittance decrease of less than 5% near 7 μm.
O-doped LiNbO ₃ crystal.

The LN crystal and MgO-doped L in the present invention
The composition of each N crystal is a congruent composition.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In describing a judgment method according to the present invention, first, important properties serving as a basis for the judgment will be described. It is known that when the transmittance of light for each wavelength of the MgO-doped LN crystal is roughly observed, there is a peak portion near 2.8 μm where the transmittance of light locally decreases.
This is attributed to light absorption by OH ^- ions in the crystal. The determination method according to the present invention includes the above-mentioned 2.8
This is based on the following properties (a) and (b) newly discovered by examining the properties of crystals near μm for finer wavelengths. (A) The wavelength (peak wavelength) at the peak where the light transmittance is reduced is 2.87 μm at a certain MgO content.
The property of changing stepwise from around m to around 2.83 μm. (B) In accordance with the stepwise change from about 2.87 μm to about 2.83 μm described in the above property (a),
The property that the light damage resistance of the crystal also changes from low light damage resistance to high light damage resistance.

The term "around 2.87 μm" is more strictly equivalent to 2.87 μm.
The range is from 865 μm to 2.874 μm. Also, 2.
More specifically, the vicinity of 83 μm means 2.825 μm to 2.825 μm.
834 μm. Hereinafter, two peak wavelengths having these widths are simply referred to as “2.87 μm” and “2.83 μm”.
m ”.

In order to indicate how much the transmittance of light decreases at the two peak wavelengths, a transmittance (base transmittance) which is a reference value of the degree of the decrease is required. In the present invention, in the transmittance in the wavelength band of 2.5 μm to 3.3 μm, a fluctuation portion around 2.8 μm is excluded.
The average value of the remaining flat transmittance portions is used as the base transmittance. However, this is the above-mentioned wavelength band of 2.5 μm.
m to 3.3 μm is not necessarily required to be measured over the entire range, but the wavelength is 2.5 μm to 3.3 μm.
If the reference transmittance in the m band can be obtained, the value of the wavelength to be measured is not limited and may be outside this range. The calculation method may be based on continuous data or discrete data, and the number of measurement points is not limited. However, in the measurement for obtaining the base transmittance, it is necessary to keep the relationship between the crystal axis and the light incident direction constant. Regardless of the MgO content or the like, the base transmittance shows almost the same value for every crystal and every production lot. And two peak wavelengths 2.87 μm, 2.83
The degree of decrease in the transmittance at μm from the base transmittance (that is, the difference between the transmittance at each peak wavelength and the base transmittance).
And the degree of decrease in transmittance at each peak wavelength.

FIG. 2 is a graph showing the light transmittance of a certain MgO-doped LN crystal sample in order to explain the base transmittance and the transmittance at the peak wavelength. As is apparent from the graph of FIG.
An absorption peak at which the transmittance is greatly reduced is seen. The base transmittance in this example is 76% when the average value of the remaining flat portions excluding the variation of the transmittance at the absorption peak is calculated. The transmittance at 2.83 μm is 70.
%. Therefore, the width of decrease from the base transmittance is 6% of the difference.

The property (a) will be described in more detail. As shown in the graph of FIG. 1, when the amount of MgO contained in the MgO-doped LN crystal is increased from 0 mol%, the peak wavelength at which the light transmittance decreases changes as follows. MgO plotted on the graph of FIG.
The amount is not the charged composition but a value obtained by analyzing the MgO-doped LN crystal. When the amount of MgO is 0 to 4 mol%, the peak wavelength of the transmittance decrease is almost stable at 2.87 μm. At this time, the width of decrease in transmittance is 5% or more from the base transmittance. Also,
At this time, the decrease in the transmittance at the other peak wavelength of 2.83 μm is less than 5% from the base transmittance. The MgO content of 4 to 5 mol% is a critical region, during which the peak wavelength of the transmittance decrease is 2.87 μm.
Greatly changes to 2.83 μm. When the amount of MgO is 5 mol% or more, the peak wavelength of transmittance decrease is almost stable at 2.83 μm. At this time, the width of decrease in transmittance is 5% or more from the base transmittance. Also,
At this time, the decrease in transmittance at the other peak wavelength of 2.87 μm is less than 5% from the base transmittance. As described above, the graph of FIG. 1 clearly shows two steps.

The property (b) shows a strong correlation between the property (a) and the light damage resistance.
That is, while the MgO content in the crystal is small, the degree of decrease in transmittance appears as a peak at 2.87 μm, but the light damage resistance at that time is low, and the MgO content in the crystal is 5 mol%.
When the number is larger than the above, the degree of decrease in transmittance is 2.83 μm.
m, the light damage resistance at that time is high.

Therefore, if the properties (a) and (b) described above are utilized, the degree of light damage resistance (good or bad) can be inspected nondestructively based on the actual crystal. That is, for the MgO-doped LN crystal as a material in front of the eyes, the degree of decrease in light transmittance was measured, and the difference (decrease width) between the transmittance at 2.83 μm and the base transmittance was 5% or more, and If the difference (decrease width) between the transmittance at 2.87 μm and the base transmittance is less than 5%, it is determined to have light damage resistance. In other cases, it is determined that there is no light damage resistance. This is shown in (1) above,
The crystal which is the determination method of the present invention and is determined to have light damage resistance by this method is the crystal of the above (A).

As a method of examining how much the light transmittance at the two peak wavelengths is lower than the base transmittance, there is an infrared spectroscopy method. In particular, Fourier transform infrared spectroscopy (FT-IR) is a preferable method because of its good sensitivity.

The MgO-doped LN crystal determined to have “light damage resistance” by the determination method of the present invention, that is, the crystal of the above (A), is used to form a periodically poled structure to provide light resistance. Various wavelength conversion elements excellent in damage property can be obtained.

The periodic domain inversion structure is used to perform quasi-phase matching in various wavelength conversions (optical parametric oscillation, second harmonic generation, sum frequency generation, difference frequency generation, etc.) using a nonlinear optical crystal. A special polarization state formed in a nonlinear optical crystal, in which the polarization of the crystal is inverted periodically like a stripe pattern, as the name suggests. For example, in optical parametric oscillation by quasi-phase matching using a periodically poled structure, by changing the poled period,
It is possible to change the wavelength of light that oscillates easily.

The MgO-doped LN crystal determined to have light damage resistance according to the present invention and having a periodically poled structure formed thereon is a single independent wavelength conversion element. Although wavelength conversion such as generation of two harmonics is possible, a wavelength conversion element capable of oscillating such as optical parametric oscillation may be added by adding an optical resonator to the crystal.

The optical resonator may be one using a mirror member separate from the MgO-doped LN crystal determined to have light damage resistance according to the present invention, one using the end face of the crystal as it is, or one using the end face of the crystal. It may be formed by applying a coating or the like. For the aspect of the mirror itself, the arrangement relationship of the pair of mirrors, and the like, a known technique may be referred to.

[0026]

Example 1 In this example, samples of MgO-doped LN crystals having different amounts of MgO were prepared, and the transmittance was checked to confirm the property (a) above. Further, the light damage resistance of these samples was checked. An experiment was conducted to check and confirm the properties of (b) above.

[Preparation of MgO-doped LN crystal sample]
The composition of O is 0 mol%, 1 mol%, 2 mol%,
.., 8 mol% and 9 mol% of MgO-doped LN crystals were used as samples, and a part of each sample was destructively measured by the ICP method to obtain the amount of MgO actually contained in each sample.

[Measurement of Peak Wavelength of Decrease in Light Transmittance] The above sample was subjected to Fourier transform infrared spectroscopy (FT-I
R), the transmittance of each sample was measured, and the wavelength was 2.8 μm
The state of transmittance in the vicinity was examined. As a result, in the sample used in the present example, as shown in the graph of FIG. 1, the amount of MgO actually contained in the crystal was 0 mol% to 4.21 mol%.
, The degree of decrease in transmittance from the base transmittance at 2.87 μm is 7%, and at that time, 2.83 μm
The degree of decrease in transmittance from the base transmittance was 3%. In addition, the MgO content is 4.89 mol% to 7.14 mol%.
, The degree of transmittance decrease from the base transmittance at 2.83 μm is 8%, and at that time, 2.87 μm
2.5% decrease in transmittance from base transmittance
Met.

From the results of these measurements, it was found that the peak wavelength of the transmittance decrease clearly separated into 2.87 μm and 2.83 μm according to the amount of MgO as shown in the graph of FIG.

[Evaluation Experiment for Light Damage Resistance] The above two types of samples (the peak wavelength of transmittance decrease was 2.87 μm)
m and 2.83 μm) were irradiated with Ar laser light (488 nm) to evaluate the light damage resistance of each sample.

As shown in FIG. 3, the system of the apparatus used for the evaluation collects Ar laser light using a lens, irradiates and transmits each sample, and transmits the transmitted Ar laser light to the optical damage. In this configuration, the output is monitored by a power meter through a pinhole having a hole diameter substantially equal to the beam diameter of the Ar laser light transmitted in the absence of the laser beam. The Ar laser used was linearly polarized light, and its polarization direction and M
The sample was placed on the irradiation optical path so that the gO-doped LN crystal coincided with the Z-axis direction.

With the above configuration, if no optical damage occurs in the sample, the power meter continues to show the intensity of the initial Ar laser light, but if optical damage occurs, the refraction induced inside the sample will occur. The optical path of the laser light is bent by the rate distribution, and the beam diameter gradually expands in the polarization direction as shown in FIG. Therefore, the expanded portion cannot pass through the pinhole, and the intensity of the Ar laser beam passing through the pinhole decreases. That is, when the intensity of the Ar laser beam is monitored by a power meter, if optical damage occurs, the intensity of the laser beam is reduced and observed. In this example, the relationship between the intensity of Ar laser light monitored by a power meter and the passage of time was recorded using a pen recorder.

In the above measurement, the peak wavelength at which the transmittance decreases is 2.
For the sample of 87 μm, the irradiating Ar laser beam was applied with an intensity of 15.7 mW and a power density of 200 W / cm.
^{And 2} . As shown in the graph of FIG. 4, immediately after the irradiation with the laser light, a decrease in the intensity reflecting the spread of the beam due to optical damage was observed, and the measured intensity of the Ar laser light was about 30 seconds from the start of the irradiation. It can be seen that the value is less than half of the value. Visual observation also clearly showed that the beam was immediately and clearly expanding.

For the sample in which the peak wavelength of the transmittance decrease was 2.83 μm in the previous measurement, the intensity of the Ar laser beam was set to 100 mW in order to examine the good light damage resistance in more detail. (Power density 1273 W / cm ² ),
400 mW (power density 5093 W / cm ² ), 900
The same sample was irradiated with three types of mW (power density of 11459 W / cm ² ) and the intensity was sequentially increased.

As shown in the graph of FIG. 5, when the intensity of the irradiation light is 100 mW, the intensity of the monitor does not decrease even after irradiation for about 30 minutes, and the intensity of the irradiation light is 400 mW.
In the case of W, no decrease in the intensity of the monitor was observed even with irradiation for about 15 minutes, and no light damage occurred. When the intensity of the irradiation light was 900 mW, no decrease in the intensity of the monitor was observed even after irradiation for about 20 minutes, and no light damage occurred.

From the above experiment, the property of (a) and the property of (b) were confirmed. Thus, in order to determine the light damage resistance of the MgO-doped LN crystal, the peak wavelength of 2.83 μm is used instead of examining the amount of Mg in the crystal.
m, 2.87 μm, it was confirmed that the degree of decrease in transmittance could be examined.

Example 2 In this example, the same type of MgO-doped LN crystal as the ten types of samples used in Example 1 (ie, a crystal whose light damage resistance was determined) was used, and each sample was subjected to a periodic test. An optically parametric oscillation (OPO) experiment by quasi-phase matching was performed by forming a domain-inverted structure.

FIG. 6 shows an optical system of this OPO experiment. A Nd: YLF laser device was used as the pump light source device 1, and a laser beam having a wavelength of 1.047 μm was used as the pump light. The sample MgO-doped LN crystal 2 has
A periodic domain-inverted structure was formed as shown by a striped pattern. The shape of the crystal 2 is a rectangular parallelepiped having a dimension in the optical path direction of 30 mm and a cross section perpendicular to the optical path direction of 0.5.
mm × 0.5 mm. The inversion period of the circumferentially domain-inverted structure was 29.4 μm. An optical resonator 3 is added to the crystal 2 to make an element capable of optically parametrically oscillating 3.4 μm idler light by a quasi-phase matching method, and the intensity of the idler light is measured by a power meter through a filter that removes light other than the target wavelength. Configuration.

In the optical parametric oscillation using each sample, the intensity of the pump light was kept constant at 260 mW, the crystal temperature was changed from 20 ° C. to 100 ° C., and the intensity of the idler light was measured. The results are shown in the graph of FIG. As shown in the graph, the sample having a transmittance decrease of 5% or more at 2.87 μm and less than 5% at 2.83 μm (that is, a crystal determined to have no light damage resistance in Example 1) 60 ° C
In the following, it can be seen that the intensity of the idler light has decreased. This is considered to be a result of a change in the refractive index induced by light due to optical damage. Further, when the temperature of the crystal is 60 ° C. or higher, it is considered that the optical damage was suppressed by the high temperature. On the other hand, a sample having a transmittance reduction range of 5% or more at 2.83 μm and less than 5% at 2.87 μm (that is, a crystal determined to have light damage resistance in Example 1) has no relation to the crystal temperature. And the intensity of the idler light was constant. Therefore, it is considered that no optical damage occurred. From the above, it was found that the MgO-doped LN crystal determined to have light damage resistance by the determination method of the present invention becomes a preferable wavelength conversion element by forming a periodically poled structure.

[0040]

As described above, according to the judgment method of the present invention, the light damage resistance of the MgO-doped LN crystal can be measured without destructive analysis and without referring to an incorrect composition. , Based on the actual crystal to be used,
Non-destructive and easy to determine. Further, according to the determination method of the present invention, a preferable wavelength conversion element having light damage resistance can be easily provided.

[Brief description of the drawings]

FIG. 1 shows the amount of MgO contained in an MgO-doped LN crystal,
4 is a graph showing a relationship between a transmittance decrease due to light absorption and a peak wavelength.

FIG. 2 is a graph showing an example of a result of measuring a light transmittance of a sample of an MgO-doped LN crystal in a wavelength band of 2.5 μm to 3.3 μm.

FIG. 3 shows an Ar laser light irradiation system for evaluating the light damage resistance of a sample having a transmittance decrease peak at 2.87 μm and a sample having a transmittance decrease at 2.83 μm in Example 1 of the present invention. FIG. 3 is a diagram illustrating a configuration.

FIG. 4 is a graph showing a change over time of a monitor value when an Ar laser beam is irradiated to a sample having a peak of a transmittance decrease at 2.87 μm in Example 1 of the present invention.

FIG. 5 is a graph showing a change over time of a monitor value when an Ar laser beam is irradiated on a sample having a peak of a transmittance decrease at 2.83 μm in Example 1 of the present invention.

FIG. 6 is a diagram showing a system configuration for an optical parametric oscillation (OPO) experiment by quasi-phase matching performed in a second embodiment of the present invention.

FIG. 7 is a graph showing the relationship between the crystal temperature and the intensity of idler light when optical parametric oscillation is performed using each sample in Example 2 of the present invention.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Koichi Taniguchi 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd. Itami Works (72) Inventor Kazuyuki Tadomo 4-3-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Wire F-term (reference) in Itami Works 2G050 AA02 BA09 EA03 EB07 2G059 AA02 AA10 BB20 CC20 EE01 EE05 EE11 GG01 GG04 GG09 HH01 HH06 JJ02 JJ11 JJ19 KK10 MM05 MM10 PP04 2K002 AB12 CA03 GA10

Claims (2)

[Claims] 1. The light transmittance of a MgO-doped LiNbO ₃ crystal is measured, and the transmittance of light near a wavelength of 2.83 μm is reduced with respect to the base transmittance in a wavelength band of 2.5 μm to 3.3 μm. When the width is 5% or more and the width of the decrease in transmittance near a wavelength of 2.87 μm is less than 5%, it is determined that the crystal has light damage resistance.
A method for determining the light damage resistance of a gO-doped LiNbO ₃ crystal. 2. The following (A) MgO-doped LiNbO ₃
A wavelength conversion element, wherein a crystal is used and a periodically poled structure is formed in the crystal. (A) the transmittance of light of the crystal is 5% or more in a decrease in transmittance near a wavelength of 2.83 μm with respect to a base transmittance in a wavelength band of 2.5 μm to 3.3 μm; And a wavelength of 2.87μ
An MgO-doped LiNbO ₃ crystal in which the decrease in transmittance near m is less than 5%.