JP3021338B2 - Extinction ratio measurement method and extinction ratio measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an extinction ratio measuring method and an extinction ratio measuring device for measuring the extinction ratio of a linear polarizer.

[0002]

2. Description of the Related Art Heretofore, linear polarizers that transmit or reflect only linearly polarized light polarized in a certain direction of incident light have been conventionally known. Thomson prisms, Gran-Taylor prisms, dichroic polarizers and the like are known. Hereinafter, outlines of these various linear polarizers will be described.

(Gran-Thomson Prism) FIG.
It is a schematic diagram of a Gran-Thomson prism. Gran-
The Thomson prism is considered to be close to ideal because transmitted light goes straight and the field of view is maximum, and is the most widely used. The Glan-Thompson prism has two pieces of calcite divided at an angle as shown in FIG. The optical axis is perpendicular to the plane of the paper. The ordinary ray O is totally reflected on the balsam surface and exits the optical path, and the ray passing therethrough is the extraordinary ray E.

(Gran-Taylor Prism) FIG.
It is a schematic diagram of a Gran-Taylor prism. In this Gran-Taylor prism, the bonding surface of the Gran-Thomson prism is made into an air layer so that it can be used in the ultraviolet region up to about 215 nm. As shown in FIG. 5, the optical axes of the two pieces of calcite are perpendicular to the paper surface, the ordinary ray O is totally reflected by the air layer, and the extraordinary ray E is transmitted.

(Dichroic Polarizing Plate) A dichroic polarizing plate absorbs almost one of linearly polarized light (either an ordinary ray or an extraordinary ray) having polarization planes orthogonal to each other, and hardly absorbs the other. It is made of a dichroic material, a type of refractive material. State-of-the-art dichroic polarizers have a molecular structure in which molecules are aligned (thus producing birefringence) by stretching a plastic sheet, and are made of a material in which dye molecules selectively adhere. . The selective absorption of either the ordinary ray or the extraordinary ray is determined by the chemical bond direction.

[0006] The polarized light extracted by the linear polarizer is
Ideally, it is desirable that the polarization state be a single polarization state, but in practice, it always includes a non-polarization part. As shown in FIG. 6, two linear polarizers are often used as one set as a linear polarizer and the other as a linear analyzer. The linear polarizer extracts linearly polarized light having a vibration plane coincident with the transmission axis, and the linear analyzer further extracts linearly polarized light only in the transmission axis direction and enters the detector. The intensity varies depending on the angle θ between the transmission axes of the linear polarizer and the linear analyzer. When θ is 0, the intensity becomes maximum and this state is historically referred to as (parallel Nicol state). In this state, the intensity transmittance for the light incident on the linear polarizer is referred to as a parallel transmittance, and when natural light is incident, the value is 0.5 for an ideal linear polarizer. On the other hand, when θ is 90 °, the linearly polarized light emitted from the linear polarizer does not have a component in the transmission axis direction of the linear analyzer, and the intensity is ideally zero. This is called quenching, and this state of quenching has historically been called the orthogonal Nicol state.
In an actual device, the transmission intensity is finite even in the extinction state, and the intensity transmittance of light extracted from a linear analyzer with respect to light incident on a linear polarizer is referred to as orthogonal transmittance. When the ratio between the orthogonal transmittance and the parallel transmittance is calculated, the transmittance of the linear analyzer in the extinction state is obtained.
term ratio).

In the arrangement of FIG. 6, a linear polarizer is fixed and the optical axis is
Linear analyzer transmittance when the linear analyzer is rotated to the center
Fig. 7 shows the change of
Swell. In order from the lower graph in FIG.
And the extinction ratio of the linear polarizer is 10 ^-6, 10^-Five, 10^-FourPlace
It is. The smaller the extinction ratio, the more the extinction responds to the change in θ.
And the extinction position is determined accurately.
Call Among the commercially available linear polarizers,
N-Thomson Prism, Gran-Taylor Prism
Then 10^-Five-10^-6And 10 for a polarizing plate utilizing dichroism.
^-3-10^-FourAn extinction ratio of the order is obtained.

As a conventional method for accurately measuring the extinction ratio of a linear polarizer, a linear analyzer to be measured is disposed at the position of the linear polarizer in FIG. 6, and the transmitted light power is directly detected by a detector. It is to detect.

[0009]

The extinction ratio of a linear polarizer having good performance is 10 ^-6 or less. In such a case, even if the measured incident light amount is 10 mW, the transmitted light intensity in the extinction state is on the order of several nW. At present, there are various optical power meters that can measure the light amount on the order of nanowatts.However, in order to measure the accurate dimming degree of the linear polarizer body, the light transmitted from the measurement system and the background light in the room must be measured. It must be completely removed, and devices and environments need to be devised.

Further, if the incident light amount is set to, for example, 1 W in order to increase the accuracy of the light amount measurement in the extinction state, the transmitted light amount in the extinction state is less than 1 μmW. It is hard to be targeted. As a result, measurement accuracy is improved. However, 1W to 1μW
There is no power meter with a wide dynamic range that can measure the light amount over the following,
Two or more power meters must be used. At this time, calibration between power meters to be used is required.

In view of such circumstances, the present invention provides an extinction ratio measuring method capable of measuring the extinction ratio of a linear polarizer with high accuracy, and an extinction ratio measuring apparatus suitable for implementing the method. With the goal.

[0012]

The extinction ratio measuring method of the present invention, which achieves the above object, divides light emitted from a light source into two so that the light follows two optical paths, and divides the light into two. After shifting the frequency of the light that follows at least one of the optical paths of the optical path, prepare an optical heterodyne detection device that superimposes the light that has followed these two optical paths and guides the light to a photodetector,
The object to be measured is arranged on one of the two optical paths, and the object to be measured is rotated around the one of the optical paths, while the beat signal detected by the photodetector is rotated. An extinction ratio, which is a ratio between the maximum value and the minimum value, is obtained by detecting a maximum value and a minimum value during rotation of the measured object.

An extinction ratio measuring apparatus according to the present invention, which is suitable for carrying out the extinction ratio measuring method according to the present invention, divides light emitted from a light source into two so that the light follows two optical paths, and divides the light into two. An optical heterodyne detection system that shifts the frequency of light following at least one of the two optical paths and superimposes the light following these two optical paths and guides the light to a photodetector; and A rotation unit for rotating a device to be measured disposed on one of the optical paths around the one of the optical paths, and a beat signal detected by the photodetector, while the device to be measured is rotating. And an arithmetic unit that detects a maximum value and a minimum value of the above and calculates an extinction ratio that is a ratio between the maximum value and the minimum value.

In the extinction ratio measuring apparatus of the present invention, at least one position on the optical path between the light emitted from the light source and the light following the two optical paths is superimposed, It is preferable to include a polarization adjustment unit that adjusts the direction or state of polarization. In this case, it is preferable that the polarization adjustment unit is disposed downstream of the frequency shifter that shifts the frequency of light that follows one of the two optical paths in the light traveling direction. Alternatively, the polarization adjustment unit may be arranged on the optical path of light before being bisected after being emitted from the light source.

It is preferable that the polarization adjusting unit is constituted by a pair of a half-wave plate and a linear polarizer, which are disposed relatively upstream and downstream in the light traveling direction, respectively. In the extinction ratio measuring device of the present invention, the rotating unit may be a device that automatically rotates the object to be measured by a motor or the like, but is not limited thereto, and may be a jig or the like that is rotatably fixed. The measurement object may be rotated manually.

The present invention uses an optical heterodyne detection technique in an optical system for measuring the amount of light transmitted from a linear polarizer. The optical heterodyne detection technology is a technology for detecting a beat (beat signal) generated by superimposing light waves having slightly different frequencies and having the same phase with the same polarization plane, and observing the intensity and frequency. . The intensity of the beat signal is proportional to the intensity of the combined light. The optical heterodyne detection method is characterized by having an extremely wide dynamic range. It is possible to detect a signal level difference of usually 12 digits, and thus the problem of replacing the power meter, which has been described above, is essentially solved. Further, since the optical heterodyne detection method detects a signal by focusing on the beat frequency, it is essentially not affected by stray light or background light. As described above, by using the optical heterodyne detection method for the extinction ratio measurement, a highly accurate measurement that has not been achieved in the past can be achieved.

[0017]

Embodiments of the present invention will be described below. FIG. 1 is a schematic configuration diagram of one embodiment of the present invention using a Mach-Zehnder optical heterodyne detection system. As the light source 1, a linearly polarized laser oscillating at a single frequency is preferably used. Light 1a emitted from light source 1
Is converted into a parallel light beam by the collimator lens system 2 and split into two light beams by the beam splitter 3 to follow the optical path 1 and to travel along the optical path 2.

The light traveling along the optical path 1 is reflected by the mirror 4 and then undergoes a frequency shift by the frequency shifter 5, and thereafter,
The polarization state or the polarization direction is adjusted by the polarization adjustment unit 6 composed of a pair of the wavelength wave 6a and the linear polarizer 6b,
A beam combiner 9 passes through a linear polarizer 7 as an object to be measured.
To the photodetector 10 via. Here, the polarization adjustment unit 6 adjusts the polarization state and the polarization direction of the light emitted from the light source 1 and following the optical path 1 because the linear polarization state or the linear polarization direction of the light may be disturbed by the frequency shifter 5. It is for. The linear polarizer 7 is rotated by the rotation device 8 about the optical axis of the optical path 2 as a central axis.

On the other hand, the light split into two by the beam splitter 3 and following the optical path 2 is transmitted by the frequency shifter 11 to the frequency shifter 5.
Are shifted by a frequency shift amount different from the frequency shift amount of
Polarization adjusting unit 1 composed of 3a and linear polarizer 13b
3, the polarization state or the polarization direction is adjusted. The polarization adjusting unit 13 is for adjusting a linear polarization state or a linear polarization direction which may be disturbed by the frequency shifter 11 by the polarization adjusting unit 13 composed of 13b disturbed by the frequency shifter. The light following the optical path 2 exits the polarization adjusting unit 13 and is then superimposed on the light following the optical path 1 by the beam combiner 9 so that the light detector 1
Incident at 0. Since the two lights following the optical paths 1 and 2 have been frequency-shifted by different frequency shift amounts by the frequency shifters 5 and 11, the photodetector 10 detects a beat signal. This beat signal is input to the processor 14, and the processor 14 performs signal processing described later.

In the example shown in FIG. 1, the optical path 1 and the optical path 2
Are provided with frequency shifters. In optical heterodyne detection, lights having frequencies different from each other may be multiplexed. Therefore, a frequency shifter may be provided only on one of the optical paths. By the way, in the embodiment of FIG. 1, the following adjustment is performed after removing the linear polarizer 7 which is the object to be measured.

The linear polarizer 6b and the linear polarizer 13b are placed in the optical path 1 and the optical path 2, respectively, so that the polarization components passing therethrough are parallel to each other. The half-wave plate 6a is rotated so that the intensity of light transmitted through the linear polarizer 6b is adjusted to be maximum. Similarly, the half-wave plate 13
By rotating a, the intensity of light transmitted through the linear polarizer 13b is adjusted to be maximum. As described above, these adjustments are performed by changing the polarization planes of the two beams following the optical path 1 and the optical path 2 after passing through the frequency shifters 5 and 11 to the linear polarizers 6b and 1b.
3b. After such adjustments,
The beam passing through the optical path 1 and the beam passing through the optical path 2 are superimposed via the beam combiner 9. Two beams having slightly different optical frequencies interfere with each other by the two frequency shifters 5 and 11, and this is converted into an electric signal by the photodetector 10 to obtain a beat signal. It is important to sufficiently adjust the angle of the mirror so that the beat signal is maximized. Further, while observing the beat signal, the half-wave plate 13a and the linear polarizer 13 whose positions are determined as described above.
It is also necessary to further adjust the polarization adjustment unit 13 composed of b and the optical path 2 so that the beat signal is maximized. These adjustments are important because the dynamic range of optical heterodyne detection is supported by perfect alignment of the optical axes of the two beams and perfect alignment of the polarization planes. Now, after the above adjustment is made, the signal level is changed by arranging and rotating the linear polarizer 7 which is the object to be measured on the optical path 1. A beat signal for this rotation is sent to the processor 14, and the maximum and minimum values of the beat signal level are recorded. The extinction ratio of the linear polarizer 7 can be obtained by calculating the ratio between the two.

The other beam (indicated by a dotted line in FIG. 1) combined by the beam combiner 9 is detected by a detector 15, and the detection signal and the signal of the detector 10 are electrically added. By matching, double-balanced detection for improving sensitivity may be performed. In this embodiment, the rotating device 8 is, for example, a motor or the like, and automatically rotates the linear polarizer 7. However, the linear polarizer 7 is fixed to a rotatable jig, and the jig is manually operated. It may be configured to rotate with.

FIG. 2 is a schematic configuration diagram of another embodiment of the present invention. The differences from the embodiment shown in FIG. 1 will be described. This configuration includes a half-wave plate 21a and a linear polarizer 2
Only one set of each of the polarization adjustment unit 21 and the frequency shifter 11 combined with 1b is used to reduce the cost. The laser beam 1a emitted from the light source 1 is collimated, and then guided to a polarization adjusting unit 21 system including a half-wave plate 21a and a linear polarizer 21b. Child 21
Adjustment is made so that the intensity of light transmitted through b is maximized. The polarization adjustment unit 21 in this embodiment does not mean to correct the polarization disturbed by the frequency shifter 11, but aims to remove the non-polarized light component mixed into the linearly polarized light emitted from the light source 1. The linearly polarized light that has passed through the polarization adjustment unit 21 is introduced into a Mach-Zehnder optical heterodyne detection system, and is split into an optical path 1 and an optical path 2 by a beam splitter 3. The beam transmitted through the linear polarizer 7 as the object to be measured following the optical path 1 and the beam whose optical frequency is slightly changed by the frequency shifter 11 following the optical path 2 are superimposed by the beam combiner 9, and will be described with reference to FIG. A beat signal is detected according to the same principle as that described above. This signal level is proportional to the transmittance of the object to be measured, and the extinction ratio can be obtained in the same manner as described with reference to FIG. If cost permits, a polarization adjustment unit including a half-wave plate and a linear polarizer for efficiently performing optical heterodyne detection also on the downstream side of the frequency shifter 11 in the optical path 2 in the light traveling direction. It is desirable to arrange.

FIG. 3 is a schematic configuration diagram of another embodiment of the present invention. Compared with the embodiment shown in FIG. 2, the polarization adjustment unit 21 is omitted. It is preferable that pure linearly polarized light beams are superimposed on each other by the beam combiner 9 and incident on the photodetector 10, but the optical heterodyne method originally has a strong property against disturbance light and is not required with extremely high precision. In this case, a linearly polarized laser beam may be used as the light source 1 and the linearly polarized laser beam emitted from the linearly polarized laser beam may be used without removing the non-polarized component.

[0025]

The feature of the optical heterodyne detection method is that it has an extremely wide dynamic range. Normally, dynamic range 120
dB (measurement range of 12 digits), converted minimum detection sensitivity 3
A high performance of × 10 ^-17 W is obtained. by this,
Even a linear polarizer having a very good extinction ratio can measure an extinction ratio of 10 ⁻¹⁰ or more in principle with an incident light amount on the order of microwatts.

In the optical heterodyne detection method, since a beat signal generated by wavefront matching is observed, there is no influence of stray light or background light. Therefore, the measurement can be performed without considering their removal. As described above, the erasing light measuring method and the extinction ratio measuring device provided by the present invention can grasp the performance of a linear polarizer having a large extinction ratio in an environment where measurement was impossible conventionally.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention using a Mach-Zehnder optical heterodyne detection system.

FIG. 2 is a schematic configuration diagram of another embodiment of the present invention. The differences from the embodiment shown in FIG. 1 will be described.

FIG. 3 is a schematic configuration diagram of another embodiment of the present invention.

FIG. 4 is a schematic view of a Gran-Thomson prism.

FIG. 5 is a schematic view of a Gran-Taylor prism.

FIG. 6 is an explanatory diagram of a general method of using a linear polarizer.

FIG. 7 is a diagram showing a graph of transmittance with respect to a rotation angle of a linear analyzer.

[Description of Signs] 1 optical path 1a light 2 collimator lens system 3 beam splitter 4,12 mirror 5,11 frequency shifter 6 polarization adjustment unit 6a 1/2 wavelength plate 6b linear polarizer 7 linear polarizer 8 rotating device 9 beam combiner DESCRIPTION OF SYMBOLS 10 Photodetector 13 Polarization adjustment unit 13a 1/2 wavelength plate 13b Linear polarizer 14 Processor 21 Polarization adjustment unit 21a 1/2 wavelength plate 21b Linear polarizer

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-325010 (JP, A) JP-A-62-285080 (JP, A) JP-A-6-221993 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G01M 11/00-11/02 G01N 21/21 G01J 4/00 G01B 9/02 JICST file (JOIS)

Claims (6)

(57) [Claims] 1. A light emitted from a light source is bisected so that the light follows two optical paths, and the frequency of light that follows at least one optical path of the two divided optical paths is shifted. An optical heterodyne detection device is provided that superimposes light following two optical paths and guides the light to a photodetector. An object to be measured is arranged on one of the two optical paths. By detecting the maximum value and the minimum value of the beat signal detected by the photodetector while rotating the object under measurement while rotating about the one optical path as an axis, the maximum value and the minimum value are detected. An extinction ratio, which is a ratio of the extinction ratio to the extinction ratio. 2. The light emitted from the light source is bisected such that the light follows two optical paths, and the frequency of the light that follows at least one of the two optical paths is shifted. An optical heterodyne detection system that superimposes light following two optical paths and guides the light to a photodetector; and an object to be measured arranged on one of the two optical paths, with the one optical path as an axis. A rotation unit for rotating, a maximum value and a minimum value of the beat signal detected by the photodetector during rotation of the measured object, and a ratio between the maximum value and the minimum value. An extinction ratio measuring device comprising: an arithmetic unit for calculating an extinction ratio. 3. A polarization adjusting unit for adjusting the direction or state of linearly polarized light at at least one position on an optical path between the light emitted from the light source and the light following the two optical paths until the light is superimposed. The extinction ratio measuring device according to claim 2, further comprising: 4. The apparatus according to claim 1, wherein the polarization adjustment unit is disposed downstream of a frequency shifter that shifts a frequency of light that follows one of the two optical paths in a traveling direction of the light. The extinction ratio measuring device according to claim 3. 5. The extinction ratio measuring device according to claim 3, wherein the polarization adjusting unit is disposed on an optical path of light before being divided into two after being emitted from the light source. 6. The polarization adjustment unit according to claim 3, wherein the polarization adjustment unit comprises a pair of a half-wave plate and a linear polarizer, which are disposed relatively upstream and downstream of the light traveling direction, respectively. The extinction ratio measuring device according to the above.