JP2967637B2 - Laser interferometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for separating laser light into reference light and measurement light, causing the measurement light to participate in an object to be measured, and then measuring the phase information of interference light obtained by combining with the reference light. The present invention relates to a laser interferometer for obtaining a physical quantity to be measured of an object.

[0002]

2. Description of the Related Art As a laser interferometer, for example,
JP-A-2-115701, JP-A-2-24750
No. 2, JP-A-3-45761 and the like are known.

[0003] In any of the devices disclosed in these publications, light emitted from a laser light source is separated into reference light and measurement light, and the measurement light is caused to participate in a measurement object. The interference light is generated by multiplexing with the reference light, the interference light is detected by a photodetector, and the difference in the optical path length between the reference light and the measurement light is obtained from the phase information to measure the displacement of the measurement object. The target physical quantity is obtained.

[0004]

By the way, in each of the above-mentioned conventional devices, basically, the intensity of the interference light that changes depending on the phase change of the measurement light is measured by a photodetector, and the intensity of the measurement light is measured. This is for obtaining a phase difference with respect to the reference light. here,
A change in the intensity of the interference light based on a change in the phase of the measurement light, that is, a change in the output of the photodetector is not necessarily sufficiently large. Therefore, for example, in order to clearly determine whether or not the phase difference between the measurement light and the reference light is equal to or greater than a specific value, various electrical processes are performed on the output of the photodetector to perform photodetection. It is necessary to take measures such as enlarging a change in the output of the detector and enabling a determination as to whether or not the output value of the photodetector is equal to or greater than an intensity value corresponding to a specific phase difference.

However, no matter how much the output of the photodetector is increased by taking such measures, it is impossible to obtain a resolution higher than the resolution of the photodetector, and the device becomes complicated. .

The present invention has been made in view of the above background, and has made it possible to convert a phase change of measurement light into a binary light intensity signal and detect it with a relatively simple configuration. It is an object of the present invention to provide a laser interferometer.

[0007]

In order to solve the above-mentioned problems, the present invention provides (1) separating light emitted from a laser light source into reference light and measurement light, and applying the measurement light to a measurement object. After the laser light is involved, the light is multiplexed with the reference light by an optical multiplexer to generate interference light, the interference light is detected by a photodetector, and a laser interference for obtaining a physical quantity to be measured of the measurement object from its phase information. In the measuring device, between the optical multiplexer and the photodetector, the relationship between the amount of outgoing light and the amount of outgoing light is discontinuous when the amount of outgoing light changes in accordance with the amount of incoming light and the amount of incoming light is a predetermined critical value. The configuration is characterized in that an optical bistable element having a property that greatly changes is provided.

Further, as an aspect of the first aspect, (2) the laser interferometer of the first aspect is characterized in that a laser light source capable of modulating the intensity of emitted light is used as the laser light source.

[0009]

According to the above construction, the interference light is detected by the photodetector after passing through the optical bistable element.

Here, the optical bistable element has such a property that the outgoing light quantity changes in accordance with the incident light quantity, and that when the incident light quantity has a predetermined critical value, the relationship between the incident light quantity and the outgoing light quantity greatly changes discontinuously. Have Therefore, when the optical bistable element, the light source, and other conditions are set such that the critical value becomes equal to the output intensity value of the photodetector corresponding to a specific phase difference between the measurement light and the reference light, the optical bistable is set. The intensity of the light passing through the stable element and reaching the photodetector depends on whether or not it is equal to or more than the specific phase difference.
w ". Therefore, a binary output of “high” and “low” can be obtained depending on whether or not the output value of the photodetector is also equal to or more than the specific phase difference. Thus, it is possible to determine whether or not the output of the photodetector is equal to or more than a specific phase difference without performing special processing. In addition, since “high” and “low” are optically distinguished before reaching the photodetector, accurate distinction can be made without being affected by the resolution of the photodetector.

According to the second aspect, the intensity of light emitted from the laser light source can be modulated. Therefore, by applying this modulation, the output light of the optical bistable element is changed to “high”, “lo” at a frequency corresponding to the modulation frequency.
w ”, and outputs“ high ”,“ lo ”
There is no phase difference between the reference light and the measurement light by utilizing the fact that the switching point of "w" on the phase in the modulated wave changes depending on the phase difference between the reference light and the measurement light. The phase difference between the reference light and the measurement light can be obtained by obtaining the amount of phase shift on the modulated wave between the “high” and “low” switching points in the case and the switching point in a certain case.

[0012]

EXAMPLES First Embodiment FIG. 1 is a diagram showing the structure of a laser interference measuring apparatus according to a first embodiment of the present invention, FIG 2 is characteristic diagram of an optical bistable device,
FIG. 3 is a diagram showing a change in the amount of light emitted from the LD (modulated light), and FIG.
Is a vector diagram of light incident on the optical bistable element, and FIG. 5 is a diagram showing a relationship between LD modulated light and a photodetector output. Hereinafter, the laser interferometer according to the first embodiment will be described in detail with reference to these drawings.

In the drawing, reference numeral 1 denotes a laser diode (LD) as a laser light source, reference numeral 2 denotes a corner cube which also serves as a light separating means and optical multiplexing means, reference numeral 3 denotes a corner cube for reflecting measurement light, and reference numeral 4 denotes an optical bistable. Element 5 is a photodetector.

[0014] LD1 is driven by the control device 1a, by applying modulation to the current supplied from the control unit 1a, thereby making it possible to apply the intensity modulation of the predetermined period on the exit light L _0. The traveling direction of the emitted light L ₀ from the LD1, the corner cube 2 and 3 are arranged.

The corner cube 2 is formed of transparent quartz glass or the like in the shape of a right-angled triangular prism, and one of two surfaces orthogonal to each other is a half mirror 2a and the other is a total reflection surface 2b. Then, the surface of the half mirror 2a is LD
No exit light L ₀ and 45 ° from the orientation and LD1 to 1 side in which are arranged to intersect.

The corner cube 3 has substantially the same configuration as the corner cube 2 except that two surfaces orthogonal to each other are total reflection surfaces 3a and 3b. Are symmetrically arranged so as to face each other.

The optical bistable element 4 has such a property that the amount of emitted light changes in accordance with the amount of incident light and the relationship between the amount of emitted light and the amount of incident light changes discontinuously greatly when the amount of incident light is a predetermined critical value. in, is arranged in the path of progress as the reference light L ₁ emitted light L ₀ is changed 90 ° to the traveling direction is reflected by the half mirror 2 from LD1.

The photodetector 5 detects the light emitted from the optical bistable element 4, and is constituted by a photoelectric conversion element or the like.
Although not shown, the output from the photodetector 5 is subjected to electrical processing by a suitable processing device, and is converted into a recordable or displayable signal.

The laser light emitted from the LD ₁ is reflected by a half mirror 2a formed on one of two orthogonal surfaces of the corner cube 2 so as to change its course by 90 °, and a half mirror 2a. It is separated into the measurement light L ₂ having passed through the. Measuring light L ₂ having passed through the half mirror 2a are each reflecting surface and the corner cube 2 is reflected by the reflecting surface 3a and 3b of the arranged corner cube 3 so as to be opposed, folded enters the corner cube 2 again , is reflected by the reflecting surface 2b of the corner cube 2 is transmitted through the half mirror 2a, the reference light L ₁ and the multiplexing described above. Here, optical distance tracing the measurement light L ₂ is, is larger than the optical distance to follow the reference light L ₁ by the light path fraction reciprocates between the corner cube 2 and 3. For this reason,
Reference light L ₁ that has been combined by the half mirror 2a and has caused a phase difference corresponding to the optical path difference between the measurement light L _2, interference occurs by this are multiplexed, interference light L ₃ It has become. The interference light L ₃ of the phase depends on the optical path difference between the reference light L ₁ and the measurement light L _2. In this case, since the optical path of the reference beam L ₁ is constant, the optical path difference between the reference light L ₁ and the measurement light L ₂ depends on the optical distance measuring light L _2. That is, for example, when the distance between the corner cube 3 and the corner cube 2 changes, the optical path difference changes in accordance with the change in the distance, and as a result, the interference light L ₃
Changes. Therefore, it is possible to know the distance change of the corner cube 3 by knowing the phase change of the interference light L _3, if attach the corner cube 3 in the measured object allows the distance measurement of the object to be measured. Also,
The corner cube 3 leave fixed, if an intervening measured medium between the corner cube 2 and 3, since the optical distance measuring light L ₂ corresponding to the refractive index of the medium changes,
Similarly, it is possible to know the refractive index of a measured medium by knowing the phase change of the interference light L _3.

In this embodiment, the reference light L 3 is detected by detecting the interference light L ₃ by the photodetector 5 through the optical bistable element 4.
Depending on whether the phase difference between ₁ and the measuring light L ₂ is given above phase difference, the output of the photodetector 5 is "high", "lo
w), so that it is possible to detect whether the phase difference is greater than or equal to the above-mentioned phase difference by converting it into a binary signal at the stage of optical output. It is. Less than,
This will be described in detail.

The optical bistable element 4 is an element having the characteristics shown in FIG. 2 and includes BSO (B ₁₂ SiO ₂₀ ), BGO
It is made of (B ₁₂ GeO ₂₀ ) or a crystal of GaAs, GaP, Si, or the like, or an MQW (multiple quantum well structure) semiconductor. This device utilizes the fact that the factor affecting the multiple interference phenomenon due to multiple reflection between the input and output surfaces varies discontinuously depending on the amount of incident light. When light having a coherent length enough to cause interference is included in the incident light, the relationship between the incident light amount Ii and the outgoing light amount Io has a characteristic shown by a curve shown in FIG. That is, as shown by the arrow pointing upward in the drawing, when the incident light amount Ii gradually increases from near zero, the incident light amount Ii becomes the first critical value I.
_The output light quantity Io gradually increases in proportion to the incident light quantity Ii until it reaches _M1 , but when the incident light quantity Ii exceeds the first critical value I _M1 , the output light quantity increases discontinuously and largely. That is, there is a property that the value of the emitted light amount Io is greatly different from the critical value I _M1 . By utilizing this property, the case where the incident light amount is less than the critical value I _M1 and the case where the incident light amount is equal to or more than the critical value I _M1 are clearly distinguished as a binary signal of ON / OFF of the emission light amount Io. It is possible to get. When the incident light amount Ii is gradually reduced from a sufficiently large value, as shown by a downward arrow in FIG. 2, the output light amount Io becomes inadequate when reaching the second critical value I _M2. It continuously decreases greatly. In this embodiment, the case where the incident light amount Ii is increased is used.

Now, assuming that the electric field of the measuring light L ₂ (α light) is Eα, Eα = E ₀ e ^{−γ eiψ} (T ^1/2 ) ² + E ₀ e− ^2γ e ^{2 iψ} (T ^1/2 ) ² = R ^1/2 +... = E ₀ Te ^{−γ + iψ} {1−R− ^{1 / 2} e ^{−γ + i} } ^{ψ −1} (1)

If the electric field of the reference light L ₁ (β light) is Eβ, then Eβ = E ₀ R ^1/2 (2)

[0024] However, the above (1) and (2), E0; field ψ of the incident light L _0; phase shift amount when the measurement light L ₂ is one round the corner cube 2 and 3 e ^{- gamma;} the reflectance of the half mirror 2a; transmittance of the half mirror 2a R; the measuring light L ₂ transmitted light quantity ratio T of time of one round and a corner cube 2 and 3.

Here, since it can be approximated to e− ^γ = 1, when this is put into the equation (1), Eα = {E ₀ Te ⁱ } / {1-R- ^{1 / 2} e ⁱ }} (3) Becomes

Therefore, from the above equations (2) and (3),
Light intensity of the interference light L ₃ by interference with the reference light L ₁ (beta light) and the measurement light L ₂ (alpha light) ^{is, | Eα + Eβ | 2 =} E 0 2 R + {(T 2 E 0 2) + 2E 0 2 TR ^1/2 (cos ψ-R ^1/2 )｝ / (1 + R-2R ^1/2 cos ψ) (4) The (4) As is apparent from the equation, the intensity of the interference light L ₃ changes depending on the phase difference ψ between the reference light L ₁ and the measurement light L _2. Therefore, when the phase difference を takes a specific value, the intensity of the interference light L ₃ incident on the optical bistable element 4 becomes I
When the characteristics of the optical bistable element 4 and the intensity of the emitted light from the LD ₁ and other conditions are set so as to be M1, the intensity of the emitted light from the optical bistable element 4 is set at the specific phase difference. One of the binary values of “high” and “low” is taken. Therefore, by detecting by the photodetector 5 this "high" whether the phase difference is a specific value or more and the reference light L ₁ and the measurement light L ₂ "lo
w "can be obtained as a binary output value. In addition,
Here, the transmittance and the reflectance are different between the case where the light enters the medium from the air and the case where the light enters the air from the medium, but these differences are ignored here.

By the way, according to the above-mentioned method, it is determined whether or not the phase difference is equal to or more than a specific value by the output of the photodetector 5 being "hig".
h or low, but when the photodetector indicates, for example, “high” or “low”, the phase difference value itself cannot be obtained.
Therefore, as shown in FIG. 3, the intensity modulation of the frequency f is applied to the LD1. This makes it possible to determine the phase difference.
Hereinafter, this method will be described. Note that the intensity modulation is LD1
In addition to changing the injection current to the LD1, it is also possible to provide a chopper in front of the LD1.

FIG. 4 is a vector diagram of the incident light on the optical bistable element 4. The reference light L ₁ (β light) and the measurement light L ₂ (α
When the phase difference with light is θ, when no modulation is performed, the intensity relationship shown by the dotted line is obtained, and at this time, the light emitted from the optical bistable element 4 is “high”. When the modulation is applied, the light incident on the optical bistable element 4 changes every moment,
Now, when the intensity is indicated by the solid line in FIG.
Output becomes “low”. The phase of the modulation at the time of “low” depends on the value of θ. Therefore, at any point in the modulation phase, the output of the optical bistable element 4 becomes “h”.
By determining whether to change from “high” to “low” or vice versa, the reference light L ₁ (β light) and the measurement light L
₂ (α light).

FIG. 5 is a diagram showing the relationship between the modulated light amount of the LD 1 and the output of the photodetector 5. As shown in FIG. 5, the output of the photodetector 5 becomes "hig" at a point of time .DELTA.T from the phase when the amount of modulated light is minimized.
h ”to“ low ”, θ can be obtained by the following equation (5).

.Theta. = 2.pi..DELTA.T.f (5) When the moving distance is obtained by fixing the corner cube 3 to the moving distance measuring object, assuming that the moving distance is .DELTA.L, the following equation (5) is obtained. The following equation (6) is obtained, and the moving distance can be obtained from the equation (6).

ΔL = {θ / (2π)} · (λ / 2) = (λ / 2) · f · ΔT (6) For example, the laser beam emitted from the LD 1 has a wavelength of 63
When using a laser beam of 0 nm, assuming that the measurement was performed using a measuring device having an accuracy of 10 ns, ΔT = 1 μs,
If f = 100 KHz, the accuracy of about 30 nm and 30
A movement distance of 0 nm can be measured. The accuracy can be improved by lowering the modulation frequency f, but there is a certain limit in consideration of the moving speed of the object. However, in the case of refractive index measurement and the like, there is no such restriction, so that extremely high accuracy can be ensured.

According to the above-described embodiment, since the phase information can be made into a binary value of “high” or “low” at the stage of optical output, special electrical signal processing is performed on the output of the photodetector. The phase difference can be obtained by a relatively simple process without performing. In addition, since "high" and "low" are optically distinguished before reaching the photodetector, accurate distinction can be made regardless of the resolution of the photodetector.

Second Embodiment FIG. 6 is a view showing a configuration of a laser interferometer according to a second embodiment of the present invention, and FIG. 7 is a schematic sectional view of a measurement chamber.
Hereinafter, the second embodiment will be described with reference to these drawings. This embodiment is an example in which the present invention is applied to a liquid refractometer.

In FIG. 6, reference numeral 10 denotes a base. A plate-shaped Peltier element 11 is fixed on the surface of the base 10, and a measurement chamber 13 is provided on the Peltier element 11 via a substrate 12. Fixed.

The base 10 is made of a block of a good heat conductor such as copper, and has appropriate radiating fins (not shown).

The Peltier device 11 includes a temperature controller 11a
Is a temperature control element driven by the

The substrate 12 is made of a material having a small coefficient of thermal expansion and thermal conductivity, such as invar, and transmits the heat of the Peltier element 12 to the measuring chamber 13 to bring the measuring chamber 13 to a predetermined temperature. So that they can be maintained.

The measuring chamber 13 is made of low expansion glass.
The liquid supply pipe 14 has a liquid storage
And the liquid discharge pipe 15, the optical fiber cable 16 for incidence and the optical fiber cable 1 for emission of laser light for measurement
7 and the like are connected. The input optical fiber cable 16 and the output optical fiber cable 17 are connected to the measurement chamber 13 via a fiber connection device 16a.

FIG. 7 is a schematic sectional view of the measuring chamber 13.
As shown in this figure, a liquid container 130 is formed inside. Two sets of orthogonal inner surfaces 13a, 13d and 13b, 13c are formed in the liquid container 130, respectively, and these two sets of surfaces are arranged so as to face each other. The surfaces 13b, 13c, and 13d are all total reflection surfaces except that the surface 13a is a half mirror. Accordingly, when the incident laser light L ₀ is incident on the half mirror 13a through the half mirror 13a at an angle of 45 °,
The incident light L ₀ is split by the half mirror 13a into the reference light L ₁ and the measurement light L _2, and the measurement light L ₂ is reflected by the surfaces 13b, 13c, and 13d and passes through the half mirror 13a. Combined with L ₁ , interference light L ₃
After that, the light passes through the optical bistable element 24 and is detected by a photodetector (not shown). The laser device (not shown) emits a laser beam L ₀ is those capable of intensity modulation.

According to the above configuration, similarly to the first embodiment described above, by detecting by a photodetector (not shown) the interference light L _3, to obtain the refractive index of the liquid in the liquid containing portion 130 Can be.

FIG. 8 is a schematic sectional view of a measuring chamber 33 which is a modification of the measuring chamber 13. In FIG. 8, surfaces 33a and 33b and surfaces 33c and 33d are formed so as to be parallel, and surfaces 33a and 33d are half mirrors, and surfaces 33b and 33c are total reflection surfaces. . The laser light L ₀ is converted into the reference light L ₁ by the half mirror 33a.
It is separated into 2 Application of the measurement light L ₂ and. Measuring light L ₂ reaches the half mirror 33d through the liquid storage portion 330, whereas the reference beam L ₁ reaches the half mirror 33d through the glass constituting the measuring chamber 33 is reflected by the half mirror 33a, after these became interference light L ₃ are multiplexed, it is detected by a photodetector (not shown) through the optical bistable device 34. Also in this case, the refractive index of the liquid can be measured as in the second embodiment.

Third Embodiment FIG. 9 is a diagram showing a configuration of a laser interference measuring apparatus according to a third embodiment of the present invention. In this embodiment, the phase difference between the measurement light and the reference light is obtained more accurately by using two optical bistable elements having different characteristics and detecting the interference light through these two optical bistable elements. .

In FIG. 9, a laser beam L ₀ emitted from a laser device 41 is separated by a beam splitter 42 into two beams, a measurement beam (α beam) and a reference beam (β beam). After the measurement light α is reflected by two orthogonal reflection surfaces 43a and 43b of the reflector 43 fixed to the object to be measured,
Reaches the half mirror 44a, which is formed on one surface of the corner cube 44, is separated into 2 Application of the transmitted light alpha ₁ and the reflected light alpha _2. On the other hand, the reference light beta, is separated into 2 Application of the reflected light beta ₁ and transmitted light alpha ₂ by the half mirror 44a. The transmitted light α ₁ and the reflected light β ₁ are combined to form a first optical bistable element 4.
After passing through No. 5, it is detected by the first photodetector 45a. On the other hand, the reflected light α ₂ and the transmitted light β ₂ are multiplexed, then reflected by the total reflection surface 44b, and
Are reflected by the reflection surfaces 43c and 43d, and pass through the second optical bistable element 46, and are detected by the first photodetector 46a.

FIG. 10 shows α ₁ and β ₁ , and α ₂ and β 1
FIG. ₄ is a composite vector diagram with FIG. The interference light S ₁ and S ₂ resulting from the combination can be freely set by changing the ratio of transmission and reflection of the half mirror 44a.
That is, when the reflectance is increased, the result is as shown in FIG. 11, and when the transmittance is increased, the result is as shown in FIG. Taking advantage of this phenomenon, the ratio between the transmission and reflection of the half mirror 44a is appropriately set according to the characteristics of the first optical bistable element 45 and the second optical bistable element 46 shown in FIG. , A “high” and “low” signal obtained depending on the phase difference between the reference light and the measurement light are output by the first optical detector 45 a through the first optical bistable element 45, More precise measurement is possible because the output is obtained by two separate outputs, the output obtained by the second photodetector 46a through the optical bistable element 46 of FIG.

[0045]

As described in detail above, the laser interferometer according to the present invention converts the interference light obtained by multiplexing the reference light and the measurement light into an emission light corresponding to the incident light. And, when the incident light amount is a predetermined critical value, the relationship between the incident light amount and the outgoing light amount is discontinuously greatly changed. With this configuration, the phase change of the measurement light can be converted into a binary light intensity signal and detected.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a laser interference measurement apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating characteristics of an optical bistable element.

FIG. 3 is a diagram showing modulated light emitted from a laser diode (LD) 1;

FIG. 4 is a vector composition diagram of reference light and measurement light.

FIG. 5 is a diagram illustrating a relationship between modulated light of an LD1 and an output of a photodetector.

FIG. 6 is a perspective view showing a configuration of a second embodiment.

FIG. 7 is a schematic sectional view of a measurement chamber 13.

FIG. 8 is a schematic sectional view showing a modified example of the measurement chamber.

FIG. 9 is a diagram showing a configuration of a third embodiment.

FIG. 10 is a vector composition diagram of reference light and measurement light.

FIG. 11 is a vector composition diagram when the transmittance is increased.

FIG. 12 is a vector composition diagram when the reflectance is increased.

FIG. 13 is an explanatory diagram of characteristics of the optical bistable elements 45 and 46.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Laser diode (LD), 2, 3, 44 ... Corner cube, 4, 24, 34, 45, 46 ... Optical bistable element, 31 ... Polarization beam splitter (PBS), 4 ... Sample to be measured, 5, 45a , 46a photodetector, 10 base,
11: Peltier element, 12: Substrate, 13, 33: Measurement chamber, 14: Liquid supply pipe, 15: Liquid discharge pipe, 16, 17
... Optical fiber cable, 42 ... Beam splitter, 43
... reflectors.

Claims (2)

(57) [Claims] 1. A light beam emitted from a laser light source is separated into reference light and measurement light, and the measurement light is caused to participate in a measurement target body. In a laser interferometer that detects the interference light with a photodetector and obtains the physical quantity to be measured of the measurement target from the phase information, the incident light amount is between the optical multiplexer and the photodetector. A laser having an optical bistable element having a property that the relationship between the amount of emitted light and the amount of emitted light varies discontinuously and greatly when the amount of emitted light changes in response to the incident light and has a predetermined critical value. Interferometer. 2. The laser interferometer according to claim 1, wherein the laser light source is capable of modulating the intensity of the emitted light.