JP5273517B2 - Phase stabilization optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical apparatus stable in a disturbance environment since it includes an optical system for utilizing a two-color method principle and implementing a self compensation of a disturbance generated when two coaxial optical fluxes having different wavelengths pass (are transmitted and reflected) through the same optical system, adapted to a light source having a short coherent length since a function for adjusting the coherent length is provided, simplifying a structure, drastically miniaturized, conveniently eliminating an adjustment, and utilized so as to construct an inexpensive optical interferometer. <P>SOLUTION: The optical apparatus includes: the light source 1; an optical element or optical system for converting the wavelength of a portion of the light from the light source; a phase controlling optical element 3 or optical system for controlling a phase difference between the light from the light source and the light whose wavelength is converted; and an optical path length controlling optical element 4 or optical system for controlling an optical path length between the light from the light source and the light whose wavelength is converted. The light from the light source and the light whose wavelength is converted pass (are transmitted and reflected) through the same optical system as two coaxial optical fluxes. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention is an optical device for manipulating the optical phase, such as an optical interference measuring device, which is easy to manufacture and adjust with a simple configuration, but is not affected by disturbances such as mechanical vibration, temperature fluctuation, and air fluctuation. The present invention relates to an optical apparatus capable of measuring optical interference.

  By using the light interference phenomenon, the size, amount of minute displacement, amount of minute deformation, density, and surface shape of an object can be measured with high accuracy at about 1/10 of the wavelength of light. A typical apparatus is an optical interferometer. Hereinafter, the prior art will be described using an optical interferometer as a specific example.

  As seen in the Michelson interferometer or Macher-Zehder interferometer in Figure 4, the optical interferometer splits the light into two with a beam splitter and moves one light along with the variation of the object in the measurement optical path. Reflecting with a mirror, reflecting the other light with a fixed mirror on the reference optical path, interfering both reflected lights and receiving them with a light receiver to measure the displacement and refractive index of the object with high accuracy . As a method of separating light, there are a homodyne method in which a light wave of the same frequency is divided into two and interference is measured as a signal, and a heterodyne method in which light having a slightly different frequency is divided into two and the beat signal is measured as a signal. is there.

  There are two main problems with optical interferometer measurements. One is the stability of the interferometer and the other is operability.

  The decrease in stability due to the optical interferometer is due to the fact that the light divided into two due to disturbance during the measurement is subjected to different phase fluctuations in the respective optical paths. As the interference fringes fluctuate, the fringe contrast (visibility) decreases and the measurement accuracy decreases. In particular, when the disturbance is large and the light and darkness of the interference fringes are reversed, measurement is impossible.

Disturbances are classified into the following three categories.
(1) Air fluctuations (temperature and pressure fluctuations)
(2) Temperature fluctuation of components (3) Fluctuation due to vibration

  Fluctuations in a medium such as gas (temperature fluctuations, pressure fluctuations) in the optical path give rise to fluctuations in the refractive index and cause phase fluctuations. Temperature fluctuations and vibrations give subtle distortions to the optical element, optical element fixing holder, and optical surface plate, and the optical path length of the light divided into two changes, causing phase fluctuations. An optical interferometer that is stable in principle by constructing the interferometer in a vacuum environment, managing the temperature of the equipment machine including the interferometer at a constant level, and introducing a vibration isolation function to completely block the mechanical vibration. Can be built.

  However, such devices are large, complex and expensive and difficult to operate. Also, in an environment where severe vibration is generated by machining or the like, it is very difficult to construct a stable interferometer even if a vibration isolator is applied.

  Also, typical optical interferometers that can be measured even in a measurement environment with disturbances are those that measure interference images at high speed in a time sufficiently shorter than fluctuations, and measure disturbances separately to compensate for them. Some have a device to do. By applying feedback to the optical path length of the reference light, disturbance of interference fringes due to disturbance is prevented. However, complicated devices and analysis methods must be incorporated into the interferometer.

  Therefore, if the two light beams are coaxial and pass through the same optical system (common path) (transmission and reflection), the two beams are subject to the same fluctuations, so that disturbance is canceled out and stable optical interference that is self-compensated. You can build a total.

  As shown in FIG. 5, there is a two-color method that uses light having a different frequency from a Sagnac interferometer as an optical interferometer in which disturbance is corrected automatically. The Sagnac interferometer is also called a ring interferometer, and two light beams rotate clockwise and counterclockwise on the same ring optical path, respectively, so that there is no need for special correction and stabilization mechanisms. Interference can be obtained. However, since the optical paths are common, the two optical path lengths are completely coincident with each other, so that a phase sweep is not possible.

  In the two-color method, instead of dividing the light into two spatially, a part of the light can be converted into light having different frequencies, and different phase operations can be performed on the light at each frequency. In the two-color method using light with different frequencies, a coaxial two-beam system is realized by wavelength-converting part of the light source without spatially separating part of the light source. The phase fluctuations caused by the two beams are automatically compensated through almost the same optical system (see “Patent Document 1” and “Patent Document 2”). As a result, it is possible to provide an inexpensive optical interferometer that is simple to operate and has a simple structure that does not require a function for correcting the disturbance and a device that removes the disturbance.

On the other hand, a problem with the operation of the optical interferometer is that it is difficult to make fine adjustments when the two light beams divided by the two-beam interferometer are overlapped again. The light interference conditions are as follows: (1) the center of the light beam matches, (2) the optical axis (traveling direction) of the light beam matches, (3) the light beam The polarizations match, and (4) they are overlapped within the coherence distance (within the coherence time). In order to satisfy all of these requirements, fine adjustment of a large number of complicated optical elements is required at the time of production, and highly skilled techniques and a great deal of adjustment time are required. Also, the fine adjustment often changes due to vibration or aging.
Japanese Patent Publication No.6-229922 Patent Publication 9-79811

  The two-color method is an excellent method in which disturbance is self-corrected because two coaxial light fluxes of different wavelengths pass through the same optical system (transmission and reflection), and conditions for further light interference (1) (2) (3) is automatically met at the same time. This indicates that fine adjustment to spatially superimpose the two light beams is unnecessary. (4) is not an important issue for laser light sources with long coherence distances, but for light sources with short coherence distances such as short pulse lasers and lamps, the coherence distance of the beam divided into two is accurate to a micrometer. Fine adjustment is required.

  The introduction of a light source having a short coherent distance is important from the viewpoint of miniaturization of the apparatus and cost reduction, and also leads to improvement of resolution. However, if the conventional optical system that finely adjusts the over-interference distance between the two beams is introduced, it is difficult to pass through the same optical system, and the self-correction function for disturbance, which is the greatest feature of the two-color method, and the need for fine adjustment are not required. There was a problem of being lost.

  The subject of the present invention is the use of the principle of the two-color method (1) a self-compensation optical system for disturbance in which two coaxial light beams of different wavelengths pass through (transmit and reflect) the same optical system, so even in an environment with disturbance It is stable and (2) has a coherence distance adjustment function, so it can be applied to light sources with a short coherence distance, and (3) it has a simple structure and can be significantly reduced in size, and no adjustment is required (or It is an object of the present invention to provide an optical device that can be used for constructing a simpler and more inexpensive optical interferometer.

In order to solve the above problems, the apparatus of the present invention is a light source, an optical element or optical system that converts a part of light from the light source, and a phase that can control a phase difference between the light from the light source and the wavelength-converted light. A control optical element or optical system, and an optical path length control optical element or optical system for controlling the optical path length of light and wavelength-converted light from the light source are provided. The light from the light source and the wavelength-converted light are coaxial 2 The light beam passes through the same optical system (transmitted and reflected).

  The phase stabilization optical device according to the present invention has (1) a self-compensation optical system for disturbance in which two coaxial light beams having different wavelengths pass through the same optical system (transmission and reflection), and thus is stable even in an environment with disturbance. . Since a disturbance removing device such as a vibration isolator and a disturbance correcting mechanism such as a feedback device can be omitted, the device is small and simple, and costs can be reduced. A stable optical interferometer can be constructed even in the case where it is difficult to adapt the vibration isolator in an environment where severe vibration is generated particularly in machining. (2) Since it has a coherence distance adjustment function, it can be applied to light sources with a short coherence distance, and high resolution can be achieved. In addition, since a light source with a short coherence distance is often small, the entire apparatus can be reduced in size and cost. (3) Since the coaxiality of the two lights with different wavelengths is automatically maintained, there is no operation to superimpose the two divided light beams, which is a problem with a normal two-beam optical interferometer. Is absolutely unnecessary. Since the coaxiality of two lights with different wavelengths is automatically maintained, there is no aging error. Highly skilled technology and a great deal of adjustment time can be omitted. Further, as shown in FIG. 1, since it is composed of only five optical elements, simplification and cost reduction can be achieved in terms of production, structure, adjustment and the like.

[Example 1]
FIG. 1 is an overall schematic view of a phase stabilizing optical device according to the present invention. The light source 1 was a femtosecond pulse laser (wavelength 800 nm, pulse time width 100 femtoseconds: 1 femtosecond = 10 ^-15 seconds) with a short coherence distance. The coherence distance converted from the pulse time width is 30 micrometers. Part of the light from the light source is converted into the second harmonic (wavelength 400 nm) by the first nonlinear optical crystal 2 (example is BBO crystal, thickness: 1 mm). Strictly speaking, when the wavelength is converted by the first nonlinear optical crystal, the converted light undergoes a lateral shift of the optical axis, but the shift amount is very small and can be ignored and can be regarded as coaxial.

  The light from the light source and its second harmonic propagate in the air as two coaxial light beams, and first pass through the phase control optical element 3 (in the drawing, the two light beams pass through different optical paths for the sake of expression). It is drawn as if, but it actually passes through the same optical path.) As the optical element for phase control, a parallel plate type optical medium (in the example, a synthetic quartz plate, thickness: 10 mm) capable of changing the incident angle was used. At this time, because of the wavelength dependence of the refractive index, the light path length of the light having a wavelength of 800 nm and the light having a wavelength of 400 nm are different.

ただし、d₁は位相制御用光学素子の厚さ、n₁は波長800nmにおける屈折率、n₂は波長400nmにおける屈折率である。θは入射角である。厳密にはいわゆる色収差が生じ、図2に示されるように波長800nmの光と波長400nmの光は、ビームの軸の平行性は保たれるが同軸ではない。しかし、θを小さくしてほぼ垂直入射にすれば、波長800nmの光と波長400nmの光は同軸とみなしうる。光路長は入射角θを０度付近で変化させることにより波長800nmと波長400nmの光を空間的に分離することなくほぼ同軸性を保ったまま相対位相差を変化させることができる。 FIG. 2 is a detailed view of the optical element 3 for phase control. Assuming that the optical path lengths of the light having the wavelength of 800 nm and the light having the wavelength of 400 nm defined in FIG. 2 are L ₁ and L ₂ , the optical path length difference ΔL ₁ = L ₂ −L ₁ is expressed by the following equation.

Here, d ₁ is the thickness of the optical element for phase control, n ₁ is the refractive index at a wavelength of 800 nm, and n ₂ is the refractive index at a wavelength of 400 nm. θ is an incident angle. Strictly speaking, so-called chromatic aberration occurs, and as shown in FIG. 2, the light of the wavelength of 800 nm and the light of the wavelength of 400 nm maintain the parallel of the beam axis but are not coaxial. However, if θ is made small and almost perpendicular incidence occurs, light with a wavelength of 800 nm and light with a wavelength of 400 nm can be regarded as coaxial. By changing the incident angle θ around 0 degree, the optical path length can change the relative phase difference while maintaining almost coaxiality without spatially separating the light of wavelength 800 nm and wavelength 400 nm.

となる。実施例である厚さ10mmの合成石英板ではn₁＝1.452, n₂＝1.471であるので実施例の場合、ΔL₁＝0.19mmとなる。 The synthetic quartz plate as an example is a so-called positive dispersion medium in which n ₂ > n ₁ , the optical delay of light having a wavelength of 400 nm is larger than the wavelength of 800 nm, and the optical path length difference ΔL ₁ is positive.
When θ = 0, Equation (1) can be approximated by the following equation.

It becomes. In the synthetic quartz plate having a thickness of 10 mm, which is an example, n ₁ = 1.452 and n ₂ = 1.471, so in the example, ΔL ₁ = 0.19 mm.

This is not a problem for a light source whose coherence distance is sufficiently longer than ΔL _1, but in a light source with a short coherence distance (30 micrometers in the embodiment), no interference occurs. Therefore, the light then enters the optical path length control optical element 4 for correcting this optical delay.

  FIG. 3 is a detailed view of the optical element for controlling the optical path length. Parallel plate type optical medium (example is a synthetic quartz plate, thickness: 0.5 mm) The surface reflects almost 100% of light with a wavelength of 400 nm (99% in the example) and almost 100% of light with a wavelength of 800 nm (in the example 98%) Dielectric multilayer film that transmits light, and the back surface of the dielectric multilayer film that reflects almost 100% (99% in the embodiment) of light having a wavelength of 800 nm and transmits almost 100% (98% in the embodiment) of 400 nm light Is deposited.

The light is perpendicularly incident on the optical path length controlling optical element 4 and reflected. At this time, light having a long wavelength (light having a wavelength of 800 nm in the embodiment) propagates a distance that is twice as long as the thickness of the optical element compared to light having a short wavelength (light having a wavelength of 400 nm in the embodiment). So-called negative dispersion characteristics. The optical path length difference ΔL ₂ between the light with a wavelength of 800 nm and the light with a wavelength of 400 nm in the optical element 4 for controlling the optical path length is

Here, n ₃ is the refractive index of the optical element for controlling the optical path length at a wavelength of 800 nm, and d ₂ is the thickness. In the case of the synthetic quartz plate having a thickness of 0.5 mm as an example, ΔL ₂ = −1.47 mm. By reflecting with the optical element for controlling the optical path length, it is possible to compensate for the optical path length generated in the optical element for phase control while maintaining the coaxiality without spatially separating the light of wavelength 800 nm and wavelength 400 nm. .

As a result, it is possible to adapt to a light source having a short coherent distance. ΔL ₁ and ΔL ₂ are not matched in the embodiment. The reason for this is that when the phase stabilizing optical device is combined with another device, two light fluxes having different wavelengths often pass through a positive dispersion optical medium, and ΔL ₂ is designed to be large in order to guarantee that much. is there. Fine adjustment of the optical path length can be performed by changing the thickness and incident angle of the optical element for phase control.

  Strictly speaking, the light having a wavelength of 800 nm propagates through a different space from the light having a wavelength of 400 nm as much as reciprocating in the optical element for controlling the optical path length, and does not pass through the common path. However, the change in the thickness of the optical element for controlling the optical path length due to mechanical vibration cannot occur in principle, and fluctuations due to air do not occur. Further, since the temperature fluctuation can be easily controlled because of the small solid medium, no disturbance is generated in the optical element for controlling the optical path length. As a result, it can be considered that the two light beams are coaxial and pass through the same optical system (common path) (transmission and reflection).

  Then, the light is extracted by the mirror 5, and a part of the light having a wavelength of 800 nm is converted again to the second harmonic by the second nonlinear optical crystal 6 (example is BBO crystal, thickness: 1 mm) Interference with 400nm wavelength light generated by the nonlinear optical crystal. The interference pattern is detected by the photodetector 7.

  Note that the measurement object is disposed between the mirror 5 and the second nonlinear optical crystal 6 so as to transmit light. However, since it is assumed that the optical interferometer itself or a part of the optical interferometer is incorporated, there is a case where there is no measurement object, and therefore the position of the measurement object is not shown in FIG.

  Although the present invention has been described based on the embodiments, the present invention is not limited to the above embodiments. The present invention can be implemented in any way as long as the configuration described in the claims is not changed. For example, in the embodiment, the pulse laser and the wavelength conversion to the second harmonic are used, but it is also possible to use a wavelength conversion mechanism that can maintain coaxiality with another light source. In the embodiment, the optical element for controlling the optical path length is configured based on a parallel plate type solid medium. However, it is also possible to introduce liquid crystal into the parallel gap and electrically control the refractive index. In the embodiment, the optical element for phase control is realized by changing the optical path length by changing the incident angle to the parallel plate type solid medium. As shown in the equation (1), the refractive index of the medium is used. It is also possible to sweep the phase by controlling. For example, it is possible to use a method of controlling the refractive index by changing the temperature of a solid medium, liquid medium, or gas medium, a method of controlling the refractive index by changing the pressure of the gas, or a method of electrically controlling the refractive index of the liquid crystal. It is.

  The present invention can be operated as an optical interferometer itself, and it is also effective to be incorporated as a part of an optical system of a conventional optical interferometer or a part of a larger optical device.

  An optical interferometer that measures the length, displacement, refractive index, and surface shape of an object with high accuracy from the phase change of light under a more adverse condition than the environment required for conventional optical interference, part of its optical system, light It can be used for a method of generating an optical pulse whose optical phase is precisely controlled in the communication field.

  The present invention is an optical interferometer or a part of its optical system used in the field of microfabrication in the electronics industry, machine industry, etc., lithography manufacturing of integrated circuits, a glass substrate, a polymer film, an optical element, a liquid crystal element, etc. An optical interferometer or part of its optical system that measures the size, thickness, refractive index, and surface shape of an object with high accuracy, or an optical interferometer that measures the displacement or deformation of an object with high precision It can be used in an optical system. In particular, when cutting with a machine tool or the like, in-process measurement can be performed in an environment where a vibration isolator cannot be used and a conventional interferometer cannot be used. In the field of information communication, it can also be used to generate pulse pairs and pulse trains in which the phase between optical pulses is precisely controlled.

Schematic of the entire phase stabilizing optical device according to the present invention Detailed view of optical element for phase control according to the present invention Detailed view of optical element for optical path length control according to the present invention Conventional interferometer 1 Conventional interferometer 2

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 First nonlinear optical crystal 3 Optical element for phase control 4 Optical element for optical path length control 5 Mirror 6 Second nonlinear optical crystal 7 Photodetector

Claims (8)

A phase stabilizing optical device,
And the light source,
A first wavelength converting optical element for wavelength conversion of part of the light from the light source,
A phase-controlling optical element for controlling the phase difference of the light light and a wavelength conversion from the light source,
Is phase-controlled by the phase controlling optical element, the light and the wavelength converted light, the optical element for the optical path length control for controlling the optical path length from the light source,
The light from the optical path length controlled light source by the optical path length controlling optical element, the wavelength of the wavelength converted light, and a second optical wavelength conversion element for wavelength conversion,
A phase stabilizing optical device characterized in that the light from the light source and the wavelength-converted light are transmitted or reflected by two coaxial light beams through the same optical system.   2. The phase stabilizing optical device according to claim 1, wherein the light source is a pulse laser. 2. The phase stabilizing optical device according to claim 1, wherein the wavelength converting optical element is a wavelength converting element for a second harmonic. 2. The phase stabilizing optical device according to claim 1, wherein the wavelength converting optical element is a nonlinear optical element.   2. The phase stabilizing optical device according to claim 1, wherein the optical path length controlling optical element is a parallel plate type solid medium.   2. The phase stabilizing optical device according to claim 1, wherein the optical element for controlling an optical path length introduces liquid crystal into a parallel gap and electrically controls a refractive index thereof.   2. The phase stabilizing optical device according to claim 1, wherein the optical element for phase control changes an optical path length by changing an incident angle to a parallel plate type solid medium.   2. The phase stabilizing optical device according to claim 1, wherein the phase controlling optical element introduces liquid crystal into a parallel gap and electrically controls a refractive index thereof.

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