JPH0815089A - Device for evaluating nonlinear optical material - Google Patents

Abstract

PURPOSE:To provide an inexpensive compact device for evaluating nonlinear optical material which can evaluate a nonlinear optical material by simultaneously using a powder method and total reflection method. CONSTITUTION:A device for evaluating nonlinear optical material is constituted of a laser light source for generating fundamental waves, semicylindrical prism 41 for totally reflecting the laser light from the light source, mechanism for closely adhering a sample to the prism 41, photoreceptor 7 for measuring the intensity of secondary higher harmonics generated from the sample, and analyzer which processes and analyzes electric signals from the photoreceptor 7. This device can be made compacter by intruding the laser light through an optical fiber 3 and simpler and easily usable by reducing the number of mobile sections by using a light receiving system having a large aperture.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-linear optical material evaluation apparatus for wavelength conversion of light, which is used for optical communication, optical computer, optical recording, optical measurement and the like.

[0002]

2. Description of the Related Art Conventionally, as a method for evaluating a nonlinear optical material, 1968, Journal of Applied Physics, Vol. 39, page 3798 (Journal of Applied Physi
The powder method described in cs, 39, 3798 (1968)) has been widely used. In addition, the measurement method using total reflection is 1
1992 Applied Physics Letter 60
Volume, 1933 (Applied Physics Letter, 60, 1933)
(1992)).

[0003]

As a conventional simple method for evaluating a non-linear optical material, Kurtz has been published in 1968.
And the powder method developed by Perry have been widely used in general. However, in the powder method, since the signal intensity depends on the sample particle size and the phase matching condition, correct evaluation was difficult.

Recently, a method using total internal reflection has been devised as a method for solving this problem. In order to perform the total reflection method, it is desirable to measure the harmonic intensity from the sample excited by the evanescent wave while changing the incident angle of the fundamental wave. Since the generated harmonics are emitted in the direction that satisfies (Equation 1) described later, it is necessary to observe the signal at an angle according to the incident angle of the fundamental wave while changing the incident angle. Needed a mechanism to rotate. Therefore, the device becomes large-scale and costly. In addition, the usability was not taken into consideration when mounting the sample, and the work efficiency was poor.

In the conventional arrangement in which the laser beam propagates in the horizontal direction, the hole for inserting the sample is oriented laterally, and therefore the sample holder must always be removed from the device when the sample is refilled. Further, the supporting portion of the holder covers a part of the cylindrical surface of the prism in order to support the upper and lower sides of the prism, which sometimes narrows the range of the incident angle and the observation angle.

An object of the present invention is to provide an evaluation apparatus for a non-linear optical material which is simpler, lower in cost and higher in work efficiency.

[0007]

The basic device configuration of the total reflection method is a laser light source that generates a fundamental wave, a prism for totally reflecting the laser light, a mechanism for bringing a sample into close contact with the prism, and a sample. It is composed of a photodetector for measuring the intensity of the second harmonic generated from the photodetector and an analyzer for processing and analyzing the electric signal from the photodetector.

For example, the second harmonic generated is n (2w) sin (m) = n (w) as described in Applied Physics Letter, Vol. 60, p. 1933, for each incident angle i. The light is emitted with directivity in the direction of the angle t represented by sin (t) (Equation 1).
Here, n (w) represents the refractive index at the fundamental wavelength of the prism, and n (2w) represents the refractive index at the second harmonic wavelength. If the conditions for total reflection are satisfied, only the harmonics emitted in the direction determined by (Equation 1) are observed, so it is not particularly necessary to perform angle decomposition for observation. Thereby, the device configuration can be simplified.

By guiding the laser beam from below using a fiber, it is possible to arrange the holder so that the hole for inserting the sample is oriented vertically, and it is possible to improve the work efficiency when refilling the sample. Further, in this case, the supporting portion which covers a part of the cylindrical surface of the prism can be removed, and the range of the incident angle and the observation angle is not narrowed.

[0010]

[Function] The fundamental laser light is guided using an optical fiber,
By arranging the tip of the fiber on the turntable, the incident angle can be changed. Thereby, the prism can be fixed. If the laser light is made incident from the lower side, the incident surface of the prism, that is, the cylindrical surface will be the lower side, and the surface on which the sample will be in contact will face upward.
Therefore, the hole for inserting the sample is oriented vertically, and the sample can be inserted as it is. It is not necessary to remove the prism even when removing the sample.

FIG. 1 shows the emission direction dependence of the second harmonic measured using a meta-nitroaniline powder sample and a rutile prism. The horizontal axis is the observation angle (Obserbation angle) and the vertical axis is the intensity (SH power).

(A) shows the case where the fundamental wave is not totally reflected, and in this case, the second harmonic generated by the transmitted fundamental wave is mainly observed, and this signal is obtained by the powder method. It is equivalent to a signal. (B) is the case where the fundamental wave is totally reflected, and the harmonics emitted from the prism are
It is observed only in the direction that satisfies (Equation 1).

Therefore, if the light is received through the wide aperture, it is not necessary to change the observation angle. At this time, since the transmitted fundamental wave does not exist, the signal equivalent to that of the powder method having no directivity as in (a) is not observed, but only the signal of the total reflection method is observed.

Here, the fiber may be fixed and the prism may be rotated instead. In this case, since it is relatively easy to guide the fundamental wave laser by using a mirror or a prism, it is not necessary to use a fiber on the incident side. On the light receiving side, a lens or mirror having a wide aperture may be used so that harmonics can be observed in all cases even if the prism rotates. Further, by using a mirror or the like that moves at the same time as or in conjunction with the prism, it becomes easy to realize an equivalently wide aperture.

When a prism having birefringence is used,
As can be seen from (Equation 1), the direction in which the harmonic is emitted differs depending on the polarization direction of the fundamental wave. Further, the harmonic intensity of the fundamental wave differs from that of the harmonic wave depending on the polarization direction. Therefore, conventionally, only the fundamental wave of the s-wave has been used to simplify the analysis, but if all the generated harmonics are simultaneously observed as in the present invention, the polarization of the fundamental wave is particularly increased. You don't have to choose. If the particle diameter of the powder is made smaller than the beam diameter to such an extent that the crystal orientation of the powder can be averaged, it is not necessary to select the polarization of higher harmonic waves. With these, the device configuration can be simplified. When a polarized light source is used, a depolarizing plate or the like may be used so that the averaging in the polarization direction can be more sufficiently performed and the intensity calculation can be simplified. Although all the embodiments and explanations of the present invention describe the second harmonic, the same can be applied to the third harmonic. In addition to powder, it can be applied to a film or a single crystal sample. When using a plate-shaped sample such as a film or a single crystal, it is difficult to bring the sample into good contact with the prism over the entire surface. Therefore, if the bottom surface of the prism, that is, only a small area of the sample contact surface is projected, the sample can be pressed.
You can easily get in touch.

[0016]

【Example】

(Embodiment 1) Here, the evaluation method of the second-order nonlinear optical characteristic using total reflection which is the basis of the present invention will be briefly described. When a powder sample of a nonlinear optical material is irradiated with a laser, light having a frequency twice that of the laser light is generated. Therefore, this intensity is measured and the second-order nonlinearity of the material is evaluated. This is the powder method, and its signal intensity depends on the second-order nonlinear optical constant of the material, the refractive index, the phase matching condition, the average particle diameter of the powder, and the like.
Therefore, the signal intensity is not simply proportional to the second-order nonlinear optical constant, and it is difficult to evaluate it.

What was devised to solve this problem was proposed by the inventors of the present application in Japanese Patent Application No. 03-218.
H.264, 03-314303. That is, the powder sample is brought into close contact with the prism, the laser is totally reflected using the prism, and the intensity of the second harmonic generated at that time is measured. This 2
Since the second harmonic is automatically phase-matched, its intensity can more accurately evaluate the second-order nonlinear optical constant regardless of the average particle size of the sample and the phase-matching condition.

The present invention presents a more improved device configuration that accomplishes this.

An embodiment of the present invention will be described with reference to FIG. (A) shows an overall configuration diagram, and (b) shows details of a laser light irradiation end.

Nd: Y at which a predetermined fundamental wave is generated
The output of the AG laser is input to the optical fiber 3. The laser light irradiation end 4 of the optical fiber 3 is attached to a turntable 6 driven by a stepping motor, and guides the fundamental wave to a prism 41 through a lens 5. Since the cylindrical surface of the prism 41 has a lens effect, the lens 5 is used for correction so that the laser light is parallel within the prism. Instead of the lens, the tip of the fiber 3 may be processed into a spherical shape to have a lens effect. The axis of the rotary table 6 and the center of the bottom surface of the total reflection prism 41 are aligned so that the refraction condition of light does not change depending on the incident angle or the observation angle. The laser used was linearly polarized, but this was used as it was. The intensity of the harmonic generated from the sample is measured by the light receiver 7 having a large light receiving surface. The radius of the prism 41 is 5 mm, and the light receiver 7 having an effective light receiving surface of 12 mm is arranged at a distance of 5 mm from the prism surface. As a result, the observation angle in the range of about 60 degrees can be covered. The filter 8 provided on the light receiving surface is an infrared cut filter for removing the fundamental wave. Further, the light receiver 7 uses a photoelectron multiplication sensation having low infrared sensitivity. The signal from the light receiver 7 is averaged by the boxcar integrator and transferred to the controller.
I am analyzing. In this work, it is easy to convert a control device for measurement into a microcomputer and program it to enable automatic measurement.

In FIG. 2, the casing 1 is shown in a simplified manner, but the whole is covered with a shielding plate so that external light does not actually enter, and the infrared light is incident on the laser light entrance. A transmission (visible light blocking) filter is provided. However, only the upper part of the sample holder is projected from this case, and it is not necessary to open the cover of the case to replace the sample and the prism.

Since the fibers are arranged below the prism, the sample can be refilled without removing the sample cell simply by pressing the sample against the prism and removing the portion having the mechanism. The sample cell 2 will be described in detail in Example 4.

Although a Q-switched Nd: YAG laser is used here to generate a predetermined fundamental wave, a more compact device can be constructed by using a semiconductor laser or a semiconductor laser pumped Nd: YAG laser. Moreover, if a wavelength tunable light source such as a titanium sapphire laser, a dye laser, or an optical parametric oscillator is used, the wavelength dispersion characteristic of the nonlinear optical constant can be measured.

(Embodiment 2) Another embodiment for guiding the signal light to the light receiver will be described with reference to FIG. In this embodiment,
The cylindrical waveguide 61 was used in contact with the prism 41. The waveguide 61 is a cylinder whose inner surface is a mirror, and the incident light is reflected on the inner surface and guided to the photomultiplier tube built in the light receiver 7. If the incident angle can be changed by about 60 degrees, an ordinary sample can be sufficiently measured. For that purpose, the inner diameter of the cylinder may be set to 5 mm or more. Here, an inner diameter of 6 mm was used. In addition to the cylindrical type, a rectangular parallelepiped or parallel plate type may be used. By using these waveguides, the distance between the prism and the light receiver can be increased, the degree of freedom in design can be increased, and an inexpensive photomultiplier with a small light receiving surface can be used.

Further, when the exit side surface of the prism 41 is a prism 42 processed into a prism as shown in FIG. 4, it has a condensing property with respect to the outgoing light, so that it is easy to cover a wider observation angle. At the same time, it is advantageous in terms of contact surface with the waveguide. The corners are arcuate with a radius of 0.5 mm to prevent chipping of the prism and scattering of light.

FIG. 5 shows an example of the prism 43 which is easier to process. The light-collecting characteristic of the emitted light is inferior to that of the prism 42 in FIG. 4, but in this case, it is preferable to use a light-receiving system having a larger aperture that uses the entire surface of the prism 43 on the emitting side.

Further, the incident surface may be flat. However, in this case, it is necessary to obtain the incident angle to the sample by Snell's law, and since the position of the beam hitting the sample surface changes depending on the incident angle, it is necessary to take measures such as enlarging the sample surface. A cylindrical shape is preferable.

(Embodiment 3) Another embodiment of the present invention will be described with reference to FIG. In this embodiment, the fundamental wave is guided using a mirror 71 and a lens 72 instead of the fiber. At this time, in order to adjust the incident angle, the sample cell 2 moves together with the rotary table 73. Other configurations are similar to those of the first embodiment.

(Embodiment 4) The structure of the sample cell 2 will be described with reference to FIG. A total reflection prism 41 having a high refractive index is placed at a predetermined position of a prism holder 23 made of stainless steel via a prism carrier 22 and fixed with a prism fixing screw 24. A pressing jig 25 is placed on the upper part of the prism holder 23, and both are integrally fixed with a fixing screw 29. A sample 27 was packed in a hole 26 provided in the pressing jig 25 and penetrating to the upper surface of the prism 41, and the sample was pressed against the prism with a screw 28. Screw 2
No. 8 measures the torque value using a torque driver. As a result, the pressing pressure between the sample and the prism can be adjusted, and the reproducibility of the results is improved. In addition, as a mechanism for pressing the sample against the prism, a spring, air pressure,
Hydraulic pressure or the like may be used.

When exchanging the sample, the pressing jig 25 can be easily removed by removing the screw 29, and the sample can be removed by removing the screw 28. Also,
When the prism 41 becomes dirty, it needs to be taken out and washed, but the prism can be removed by removing the mounting screw 24. Prism carrier 2
Reference numeral 2 is for easily attaching and detaching the prism, and also serves as a buffer so that the prism is not pressed against the prism holder 23 and damaged.
Although it is made of polystyrene here, it may be made of synthetic resin such as Teflon, diflon, or polyethylene, or a soft metal such as aluminum.

The total internal reflection method may be measured as in the above embodiment. When the measurement is performed by the conventional powder method, the incident angle may be set to an angle smaller than the total reflection critical angle. As a result, both the total reflection method and the powder method can be measured using the same device and sample cell.

(Embodiment 5) Another structure of the sample cell 2 will be described with reference to FIGS. Here, the screw portion 30 that presses the sample against the prism 41 is adapted to monitor the torque value. The detailed view is shown in FIG. 9, and the relationship between the holder and the prism is shown in FIG.

As shown in FIG. 10, the prism holder 5
0, the prism 41 is mounted using the prism carrier 42. A notch 51 for penetrating the prism is provided in the central portion of the prism holder 50, and the surface in contact with the prism carrier 42 is dented only by the thickness thereof, and further a dent 52 is provided. Reference numeral 53 is a screw hole, as shown in FIGS. 8 and 9, the pressing jig 40 is placed on the upper portion of the prism holder 50, and both are integrally fixed by the fixing screw 54. The sample was packed in a hole 60 provided in the pressing jig 40 and penetrating to the upper surface of the prism 41, and the sample was pressed against the prism with a screw 37. The screw 37 is attached to the handle shaft 32 via a coil spring 35. Pointer 3
3 is fixed to the handle shaft 32. The scale plate 34 is integrated with a screw 37 via an outer cylinder 36. By turning the handle 31, the screw 3
Although 7 rotates, the rotation angle of the screw 37 deviates from the rotation angle of the handle 31, so that the torque value at that time can be measured. A scale 38 of the calibrated torque value is displayed on the scale plate 34.

FIG. 11 is a bird cage diagram after the holder is assembled.
Shown in

(Embodiment 6) A structure in which the prism for total reflection can be applied to a plate sample will be described with reference to FIG. The bottom of the prism has a large curvature so that the center of the prism pops out. The height is different by 0.1 mm between the center and the end. FIG. 13 shows an example in which similar processing is performed not only in the radial direction of the prism but also in the height direction.

In the case of a plate-shaped sample having insufficient flatness, it is difficult to achieve good contact with the entire bottom surface of the prism, but this processing makes it easy to bring the central portion of the prism bottom surface into close contact with the sample. .

This may be applied to the prism having the shape shown in the second embodiment. These prisms can be commonly used for powder samples and plate samples.

[0038]

According to the present invention, it is possible to provide a compact, low-cost and easy-to-use device. The result of measurement at an incident angle smaller than the total reflection critical angle is equivalent to that of the conventional powder method. Therefore, if the sample is set in the cell once, it is possible to perform both the powder method and the total reflection method without changing the measuring device. By comparing the results of the two methods, it is possible to obtain information regarding whether or not the phase matching of the sample is possible.

Since the observation angle is determined only by the refractive index of the total reflection prism and the incident angle without depending on the sample, it is not necessary to change the measurement system for each sample, and the automation of the measurement is easy. This automation improves work efficiency and improves reproducibility and reliability.

[Brief description of drawings]

FIG. 1 is a diagram showing the angle dependence of second harmonic generation intensity in a reflective arrangement.

FIG. 2 is a configuration diagram showing an embodiment of a nonlinear optical material evaluation apparatus of the present invention.

FIG. 3 is a configuration diagram showing an optical system using a waveguide in an embodiment of the nonlinear optical material evaluation apparatus of the present invention.

FIG. 4 is an example diagram of a prism in an embodiment of the nonlinear optical material evaluation apparatus of the present invention.

FIG. 5 is an example diagram of a prism in an embodiment of the nonlinear optical material evaluation apparatus of the present invention.

FIG. 6 is a block diagram showing an apparatus in the case where a fiber is not used in the embodiment of the nonlinear optical material evaluation apparatus of the present invention.

FIG. 7 is a configuration diagram of an example of a sample holder for a nonlinear optical material evaluation apparatus of the present invention.

FIG. 8 is a sectional view of a holder having a mechanism capable of measuring torque in an example of a sample holder for a nonlinear optical material evaluation apparatus of the present invention.

9 is a diagram showing a configuration example of a screw portion capable of measuring torque in the embodiment of FIG.

10 is a diagram showing a configuration example of a prism holder in the embodiment shown in FIG.

FIG. 11 is a bird's-eye view of the material holder in the embodiment shown in FIG.

FIG. 12 is a view showing an example of a prism in the embodiment of the nonlinear optical material evaluation apparatus of the invention.

FIG. 13 is a diagram showing an example of a prism in an embodiment of the nonlinear optical material evaluation apparatus of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1: Housing, 2: Sample holder, 3: Optical fiber, 4: Fiber, lens fixing jig, 5: Lens, 6: Arm fixed to the rotating shaft of a stepping motor drive, 7: Photomultiplier tube, 22 : Prism carrier, 23: prism holder,
24: Screw for fixing prism, 25: Sample cell, 26: Screw hole for inserting sample, 27: Powder sample, 28: Screw for pressing sample, 29: Fixing screw, 30: Special screw, 31: Handle, 32: Handle Axis, 33: Scale needle, 34: Scale plate, 35: Winding spring, 36: Outer cylinder, 37: Screw, 38: Scale, 40: Sample cell, 41: Total reflection prism, 42: Prism carrier, 50: Prism holder , 51: prism carrier mounting hole, 52: dent, 53: fixing screw hole, 54: fixing screw, 55: sample holder fixing screw hole, 61: cylindrical waveguide, 64: fixing screw, 71: mirror , 72: lens.

Claims (10)

[Claims] 1. A laser light source for generating a predetermined fundamental wave, a prism for totally reflecting the laser light, a mechanism for bringing a sample into close contact with the prism, and measuring the intensity of a second harmonic generated by the sample. In a non-linear optical material evaluation apparatus composed of a light receiving device and an analyzer for processing and analyzing an electric signal from the light receiving device, the fundamental wave is made incident on a prism by using an optical fiber, and the output side end portion of the optical fiber is applied. A non-linear optical material evaluation system characterized by observing secondary harmonics emitted from a prism with a fixed light receiving system having a large aperture, and a mechanism for moving the lens to change the incident angle of the fundamental wave. 2. The non-linear optical material evaluation apparatus according to claim 1, wherein a semi-cylindrical prism is used as the prism for performing total reflection. 3. The nonlinear optical material evaluation apparatus according to claim 1, wherein a prism having a fundamental wave incident side having a cylindrical shape and a harmonic wave emitting side having a prismatic shape is used as a prism for performing total reflection. 4. As a prism for performing total reflection, a prism whose surface in contact with a sample is highly polished in the center is used so that the plate-shaped sample can be satisfactorily contacted in a narrow region in the center. The nonlinear optical material evaluation device according to claim 1, which is characterized in that. 5. The nonlinear optical material evaluation apparatus according to claim 1, wherein the fundamental wave entrance surface and the harmonic wave exit surface of the prism are arranged on the lower side and the sample surface is on the upper side. . 6. The nonlinear optical material evaluation apparatus according to claim 1, further comprising a waveguide for guiding a harmonic wave emitted from the prism to a light receiver. 7. A laser light source for generating a predetermined fundamental wave, a prism for totally reflecting the laser light, a mechanism for bringing a sample into close contact with the prism, and measuring the intensity of the second harmonic generated from the sample. A non-linear optical material evaluation apparatus comprising an optical receiver for receiving light and an analyzer for processing and analyzing an electric signal from the optical receiver, in which the fundamental wave is incident on a prism using an optical fiber, and the output side end of the optical fiber is applied. In the non-linear optical material evaluation apparatus for observing the second harmonic wave emitted from the prism with a fixed light receiving system having a large opening, a sample is placed on the prism. Non-linear optical material characterized in that the contacting mechanism has a structure in which the sample is pressed against the prism using a screw capable of measuring the tightening torque value. Sample holder for the evaluation device. 8. A nonlinear optical material evaluation apparatus according to any one of claims 2 to 5.
The sample holder according to claim 7, which has any one of the features 9. The sample holder according to claim 7, wherein a member for holding the prism and a member having a through hole for pressing the sample on one surface of the prism are combined with each other. 8. A sample holder for a nonlinear optical material evaluation device according to item 8. 10. The sample holder for a nonlinear optical material evaluation apparatus according to claim 9, wherein the prism is attached to a member for holding the prism via a prism holder.