JP2008224394A - Optical heterodyne interference apparatus and method for measuring optical path-length difference therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correctly perform phase measurements with respect to an object to be measured. <P>SOLUTION: This optical heterodyne interference apparatus divides light P emitted from a wavelength variable light source 11 into a first optical path L1 containing the object to be measured 1 and a second optical path L2 not containing the object 1 with an optical demultiplexer 12, makes a predetermined frequency difference between light having passed through the first path L1 and light having passed through the second path L2 by a frequency difference making means, combines those rays of light with an optical multiplexer 15, and finds the property of the object 1 on the basis of the output signal of a photoelectric transducer 16 which receives light emitted from the optical multiplexer 15. The apparatus is provided with an optical frequency sweeping means 31 for sweeping the frequency of the outgoing light of the light source 11 at a fixed speed; a frequency detection means 32 for detecting the frequency of a signal output from the photoelectric transducer 16 while the frequency of the light P is swept at the fixed speed; and an optical path length difference calculation means 33 for finding the difference between a frequency detected by the detection means 32 and the predetermined frequency, and based on the difference, calculating the difference ΔL in length between the first path L1 and the second path L2. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technique for correctly performing phase measurement on an object to be measured in an optical heterodyne interferometer.

  Conventionally, an optical heterodyne interference device 10 having the configuration shown in FIG. 7 has been used to obtain various characteristics of optical fibers and optical components.

  This optical heterodyne interferometer 10 splits light P having a frequency fo emitted from a wavelength tunable light source 11 into two by an optical demultiplexer 12, and emits one light Pa to an optical path L1 including the device under test 1. The other light Pb is emitted to the optical path L2 side not including the DUT 1.

  Here, a frequency shifter 13 is inserted on the optical path L2 side. The frequency shifter 13 is composed of, for example, an acousto-optic element, receives an electric drive signal Ea having a predetermined frequency fa output from the signal generator 14, and converts the light Pb input at the frequency fo into light Pc having the frequency fo + fa. To do. The frequency shifter 13 may be inserted before or after the measured part 1 on the optical path L1 side.

  The light Pa ′ (hereinafter referred to as signal light) emitted from the DUT 1 on the optical path L1 side and the light Pc (hereinafter referred to as reference light) emitted from the frequency shifter 13 in the optical path L2 are optically combined. The combined light Pd is input to the photoelectric converter 16 and converted into an electric signal Ed.

  Here, if the electric field component of the signal light Pa ′ is Es, the amplitude is As, the frequency is fs, the initial phase is φs, the electric field component of the reference light Pc is Er, the amplitude is Ar, the frequency is fr, and the initial phase is φr, The electric field components of both lights are expressed as follows.

Es = As · cos (2πfst + φs)
Er = Ar · cos (2πfrt + φr)

  The light intensity I when these two lights are combined is expressed by the following equation (1).

I = <| Er + Es | ² >
= (Ar ² + As ² )
+ 2Ar · As · cos [2π (fr−fs) t + (φr−φs)]
...... (1)
Here, the symbol <> indicates a time average.

If the frequencies fr and fs are fixed, the light intensity I represented by the above formula (1) is centered on (Ar ² + As ² ), with an amplitude of 2Ar · As and a beat frequency fbo (= fr−fs). It changes sinusoidally.

  In the optical heterodyne interferometer 10 having the above configuration, the output signal Ed of the photoelectric converter 16 is proportional to the intensity I of the light. Further, since the frequencies fr = fo + fa and fs = fo, the beat frequency fbo is the frequency fa of the drive signal Ea given from the signal generator 14 to the frequency shifter 13 regardless of the frequency fo of the emitted light P of the wavelength variable light source 11. be equivalent to.

  Therefore, by obtaining changes in the amplitude and phase of the output signal Ed of the photoelectric converter 16 with respect to the wavelength of light applied to the device under test 1, various characteristics with respect to the wavelength of the device under test 1 can be grasped.

  The optical heterodyne interferometer having the above configuration is disclosed in, for example, the following Patent Document 1.

Japanese Patent Laid-Open No. 2006-266796

  However, in the optical heterodyne interferometer 10 having the above configuration, the wavelength of the light emitted from the wavelength tunable light source 11 is different in the length of the two optical paths L1 and L2 between the optical demultiplexer 12 and the optical multiplexer 15. If the phase is changed, a phase change due to the optical path length difference occurs. If this changes by 2π or more and a phase rotation occurs, the phase cannot be measured correctly.

  The present invention solves this problem, and can easily determine the difference in length between the two optical paths between the optical demultiplexer and the optical multiplexer, and eliminates this optical path length difference to reduce the phase relative to the object to be measured. An object of the present invention is to provide an optical heterodyne interferometer capable of correctly performing measurement.

In order to achieve the above object, an optical heterodyne interferometer according to claim 1 of the present invention comprises:
A wavelength tunable light source (11);
Optical demultiplexing means (12) for receiving the light emitted from the wavelength tunable light source and emitting the light to the first optical path (L1) including the object to be measured and the second optical path (L2) not including the object to be measured;
Optical multiplexing means (15) for receiving and multiplexing the light having passed through the first optical path and the light having passed through the second optical path;
Frequency difference providing means (13, 43, which gives a predetermined frequency difference between the light incident on the optical multiplexing means via the first optical path and the light incident on the optical multiplexing means via the second optical path. 14, 44),
A photoelectric converter (16) that receives the light combined by the optical combining means,
In the optical heterodyne interferometer for obtaining the characteristics of the device under test based on the output signal of the photoelectric converter,
Optical frequency sweeping means (31) for sweeping the frequency of the emitted light of the wavelength tunable light source at a constant speed;
Frequency detection means (32) for detecting the frequency of a signal output from the photoelectric converter while the frequency of the light emitted from the wavelength tunable light source is swept at a constant speed by the optical frequency sweep means;
An optical path length difference calculating means (33) for obtaining a difference between the frequency detected by the frequency detecting means and the predetermined frequency, and calculating a difference in length between the first optical path and the second optical path based on the difference; It is characterized by providing.

An optical heterodyne interference device according to claim 2 of the present invention is the optical heterodyne interference device according to claim 1,
The frequency difference giving means is
It is characterized by comprising a signal generator (14) and a frequency shifter (13) that emits light by shifting the optical frequency of incident light by the frequency of the output signal of the signal generator.

An optical heterodyne interference device according to claim 3 of the present invention is the optical heterodyne interference device according to claim 1 or 2,
An optical path length varying means (21) is inserted in at least one of the first optical path and the second optical path, and the optical path length varying means can cancel the optical path length difference calculated by the optical path length difference calculating means. It is characterized by.

Moreover, the optical path length difference measuring method of the optical heterodyne interferometer according to claim 4 of the present invention is:
The wavelength tunable light is demultiplexed and divided into a first optical path (L1) that includes the object to be measured and a second optical path (L2) that does not include the object to be measured, and passes through the first optical path and the second optical path. In an optical heterodyne interferometer for obtaining a characteristic of the device under test based on an output signal of the combined light that enters a photoelectric converter (16) by combining a predetermined frequency difference between the lights. An optical path length difference measuring method for measuring a difference in length between the first optical path and the second optical path,
Sweeping the frequency of the wavelength tunable light at a constant speed and detecting the frequency of the signal output from the photoelectric converter during the sweep (S1 to S3);
Calculating a difference between the detected frequency and the predetermined frequency and calculating a difference between the lengths of the first optical path and the second optical path based on the difference (S4, S5). .

  As described above, in the present invention, when the frequency of the light emitted from the wavelength tunable light source is swept at a constant speed, the photoelectric converter has a length difference between the two optical paths between the optical demultiplexing unit and the optical multiplexing unit. Since the frequency of the output signal changes from a predetermined frequency to another frequency, the optical path length difference can be easily obtained from the amount of frequency change, and the optical path length difference can be estimated or canceled. Thus, the phase measurement for the object to be measured can be performed correctly.

(First embodiment)
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows the configuration of an optical heterodyne interference apparatus 20 to which the present invention is applied.

  As shown in FIG. 1, the optical heterodyne interferometer 20 uses the light P having the frequency fo emitted from the wavelength tunable light source 11 as light splitting means, as in the conventional optical heterodyne interferometer 10 described above. The light is branched into two by the branching filter 12, and one light Pa is emitted to the optical path L 1 including the device under test 1 such as an optical fiber, and the other light Pb is emitted to the optical path L 2 side not including the device under test 1. The drive signal Ea having the frequency fa from the signal generator 14 is given to the frequency shifter 13 inserted as a frequency difference providing means on the optical path L2 side, and converted to the reference light Pc having the frequency fo + fa, and emitted from the DUT 1. The signal light Pa ′ is incident on an optical multiplexer 15 as optical multiplexing means and combined, and the combined light Pd is incident on a photoelectric converter 16 to be converted into an electrical signal (interference signal) Ed. Yes.

  However, in this optical heterodyne interferometer 20, the optical path length varying means 21 capable of changing the optical path length is provided in the optical path L2 in which the frequency shifter 13 side is inserted.

  The interference signal Ed output from the photoelectric converter 16 is input to the lock-in amplifier 17 and subjected to synchronous detection (vector detection) using the signal Ea, and the output X and Y are used to calculate the characteristics of the DUT 1. It is given to the arithmetic processing unit 40 that performs processing.

  The interference signal Ed is converted into a digital value by the A / D converter 18 and input to the optical path length difference measuring unit 30.

  The optical path length difference measuring unit 30 obtains a difference ΔL in the lengths of the optical paths L1 and L2 between the optical demultiplexer 12 and the optical multiplexer 15 before performing measurement on the DUT 1, and calculates the difference ΔL. The optical frequency sweeping means 31 sweeps the frequency fo of the emitted light P of the wavelength tunable light source 11 at a constant speed, and photoelectrically operates while the frequency fo of the emitted light P is swept at a constant speed. The frequency detection means 32 for detecting the frequency (beat frequency) fb of the interference signal Ed output from the converter 16; the detected beat frequency fb; and the frequency difference given by the frequency difference giving means during non-sweep (this In this case, the difference Δf with respect to the frequency fa) of the output signal of the signal generator 14 is obtained, and the length difference between the two optical paths L1 and L2 between the optical demultiplexer 12 and the optical multiplexer 15 based on this difference Δf. Optical path length difference calculating means 3 for calculating ΔL 3.

Next, the relationship between the optical path length difference ΔL and the phase change will be described.
When the optical path length difference ΔL, the difference Δθ between the phase of the interference signal Ed obtained when the light P having the wavelength λ1 is incident from the wavelength variable light source 11 and the phase of the interference signal Ed when the light P having the wavelength λ2 is incident. Is represented by the following equation (2).

Δθ = [(ΔL / λ1) − (ΔL / λ2)] · 2π · n
= ΔL [(λ2−λ1) / λ1 · λ2] · 2π · n (2)
Where n is the refractive index

On the other hand, the difference Δf between the frequencies f1 and f2 of light of wavelengths λ1 and λ2 is expressed by the following equation (3):
Δf = f1−f2 = c (λ2−λ1) / λ1 · λ2 (3)
c is represented by the speed of light.

Substituting equation (3) into equation (2), the phase difference Δθ is
Δθ = ΔL · Δf · 2π · n / c (4)
It becomes.

Therefore, the optical path length difference ΔL is
ΔL = c · Δθ / (Δf · 2π · n) (5)
Thus, if the phase change amount Δθ with respect to the frequency difference Δf is known, the optical path length difference ΔL can be obtained.

  However, when the optical path length difference ΔL is large, even a slight frequency change Δf causes a phase change of 2π or more, necessitating minute and precise wavelength control, and it is difficult to actually determine the amount of phase change. .

  Therefore, in the present embodiment, as described above, the frequency fo of the emitted light P from the wavelength tunable light source 11 is swept at a constant speed, so that even a large optical path length difference ΔL can be obtained accurately.

  That is, when the frequency fo of the output light P of the wavelength tunable light source 11 is swept in a direction that increases, for example, at a constant speed in a state where there is no optical path length difference ΔL in FIG. 1, as shown in FIG. The frequency difference (beat frequency fb) between the light Pa ′ incident on the optical multiplexer 15 from the optical path L1 and the light Pc incident on the optical multiplexer 15 from the optical path L2 is the frequency of the signal Ea input to the frequency shifter 13. equal to fa.

  When the optical path L1 is longer than the optical path L2, as shown in FIG. 2B, the time corresponding to the optical path length difference ΔL is the frequency change of the light Pa ′ incident on the optical multiplexer 15 from the optical path L1. Delay by Δt (shift to the right).

  Therefore, the frequency difference (beat frequency fb) between the outgoing lights Pa ′ and Pc of both optical paths L1 and L2 is fa + Δf.

Here, since the delay time Δt is Δt = n · ΔL / c, the optical path length difference ΔL is
ΔL = (c / n) · Δt (6)
It can be expressed as.

  When the frequency sweep speed of the light P is constant at Δf / Δt = α, the equation (6) becomes the following equation (7).

  ΔL = c · Δf · / (α · n) (7)

  Therefore, the optical path length difference ΔL can be obtained regardless of the phase by detecting the change amount Δf of the beat frequency fb from the frequency fa.

The optical path length difference measuring unit 30 obtains the optical path length difference ΔL based on the above principle.
That is, as shown in the processing procedure of FIG. 3, the frequency fo of the light P emitted from the wavelength tunable light source 11 is swept at a constant speed α by the optical frequency sweeping unit 31 (S1), and output from the photoelectric converter 16 during that time. The waveform data of the interference signal Ed is stored in a memory (not shown) (S2).

  Next, the frequency detecting means 32 performs, for example, FFT (Fast Fourier Transform Processing) or frequency counting processing on the stored data to detect the beat frequency fb ′ (S3).

  Then, the optical path length difference calculating means 33 obtains a difference Δf between the detected beat frequency fb ′ and the beat frequency fbo during non-sweep (in this case, equal to the frequency fa) (S4), and this difference Δf is calculated from the above equation. By substituting into (7) (other parameters are known), the optical path length difference ΔL is calculated (S5).

  Then, the optical path length correction process corresponding to the calculated optical path length difference ΔL is automatically performed by the optical path length varying means 21 (S6) or manually, so that the optical demultiplexer 12 and the optical multiplexer 15 are The difference in length between the two optical paths is eliminated, and even if the wavelength of the wavelength tunable light source 11 is varied in order to measure the characteristics of the DUT 1 in this state, the phase change due to the optical path length difference hardly occurs. The characteristics of the DUT 1 can be accurately obtained by a predetermined calculation process for the outputs X and Y of the lock-in amplifier 17 for each wavelength.

  In this embodiment, the optical path length varying means 21 is provided on the optical path L2 side where the frequency shifter 13 is inserted, but may be provided on the optical path L1 side where the device under test 1 is inserted.

  Further, like the optical heterodyne interferometer 20 ′ shown in FIG. 4, the optical path length varying means 21 is omitted, and the optical path length difference ΔL calculated in the same manner as described above by the optical path length difference calculating section 30 is given to the arithmetic processing section 40. When measuring the characteristics of the DUT 1, the calculation process may be performed in consideration of the calculated optical path length difference ΔL.

  Further, in the above embodiment, the frequency shifter 13 and the signal generator 14 are inserted on the optical path L2 side not including the DUT 1, and a predetermined frequency difference is generated between the lights combined by the optical multiplexer 15. However, the frequency shifter 13 may be inserted on the optical path L1 side where the DUT 1 is included.

(Second Embodiment)
In the above embodiment, only one set of the frequency shifter 13 and the signal generator 14 is used to shift the frequency of light in one optical path. However, as shown in FIG. The frequency shifter 43 inserted on the optical path L1 side is supplied with the signal Ea ′ of the frequency fa ′ from the signal generator 44, and the frequency difference of | fa−fa ′ | between the lights combined by the optical multiplexer 15 May be given.

  In this case, the beat frequency can be greatly reduced, and higher phase resolution can be obtained. In this case, however, the signals Ea and Ea ′ are mixed by the mixer 45, and a signal Ea ″ having a difference frequency component is extracted from the mixed component by the filter 46 and is supplied to the lock-in amplifier 17.

(Third embodiment)
Further, as shown in FIG. 6, the fixed wavelength reference light Pu emitted from the light source 51 is combined with the output light P from the wavelength variable light source 11 by the optical multiplexer 52, and the combined light P ′ is optically demultiplexed. The output light Pd from the optical multiplexer 15 is incident on the wavelength demultiplexer 53, and the light Pe composed of components obtained by frequency shifting the output light P of the wavelength variable light source 11 and the reference light Pu are frequency shifted. Separated from the light Pf consisting of only the components, the light is incident on the photoelectric converters 16 and 54, respectively, and interference signals Ee and Ef of the beat frequency fb output from the photoelectric converters 16 and 54 are obtained. The interference signal Ef corresponding to the reference light Pu may be used as a reference signal to synchronously detect the interference signal Ee, and the signals X and Y representing the characteristics of the device under test 1 may be obtained.

  In the case of this embodiment, the signal input as the reference signal to the lock-in amplifier 17 is the interference signal Ef obtained by the light passing through the optical path equivalent to the interference signal Ee. Even if the interference signal Ee fluctuates, the interference signal Ef is also affected by the same effect, and there is an advantage that the influence on the signals X and Y obtained can be suppressed.

  In this embodiment, the frequency shifters 13 and 43 constituting the frequency difference providing means are inserted in the two optical paths L1 and L2. However, as in the first embodiment, the frequency shifter is inserted only in one optical path. May be.

Configuration diagram of an embodiment of the present invention The figure for demonstrating the operation principle of embodiment The flowchart which shows the procedure of the optical path length difference measuring method of this invention Configuration diagram of another embodiment Configuration diagram of another embodiment Configuration diagram of another embodiment Main part configuration diagram of optical heterodyne interferometer

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Object to be measured, 11 ... Wavelength variable light source, 12 ... Optical demultiplexer, 13, 43 ... Frequency shifter, 14, 44 ... Signal generator, 15 ... Optical multiplexer, 16 ... Photoelectric Converter: 17 ... Lock-in amplifier, 18 ... A / D converter, 20, 20 '... Optical heterodyne interference device, 21 ... Optical path length variable means, 30 ... Optical path length difference measuring unit, 31 ... Optical frequency sweeping means, 32 ... Frequency detecting means, 33 ... Optical path length difference calculating means, 40 ... Arithmetic processing section, 45 ... Mixer, 46 ... Filter, 51 ... Light source, 52 ... Optical multiplexer, 53 …… Wavelength demultiplexer, 54 …… Photoelectric converter

Claims (4)

A wavelength tunable light source (11);
Optical demultiplexing means (12) for receiving the light emitted from the wavelength tunable light source and emitting the light to the first optical path (L1) including the object to be measured and the second optical path (L2) not including the object to be measured;
Optical multiplexing means (15) for receiving and multiplexing the light having passed through the first optical path and the light having passed through the second optical path;
Frequency difference providing means (13, 43, which gives a predetermined frequency difference between the light incident on the optical multiplexing means via the first optical path and the light incident on the optical multiplexing means via the second optical path. 14, 44),
A photoelectric converter (16) that receives the light combined by the optical combining means,
In the optical heterodyne interferometer for obtaining the characteristics of the device under test based on the output signal of the photoelectric converter,
Optical frequency sweeping means (31) for sweeping the frequency of the emitted light of the wavelength tunable light source at a constant speed;
Frequency detection means (32) for detecting the frequency of a signal output from the photoelectric converter while the frequency of the light emitted from the wavelength tunable light source is swept at a constant speed by the optical frequency sweep means;
An optical path length difference calculating means (33) for obtaining a difference between the frequency detected by the frequency detecting means and the predetermined frequency, and calculating a difference in length between the first optical path and the second optical path based on the difference; An optical heterodyne interferometer characterized by comprising: The frequency difference giving means is
2. A signal generator (14), and a frequency shifter (13) that emits light after shifting the optical frequency of incident light by the frequency of the output signal of the signal generator. Optical heterodyne interferometer.   An optical path length varying means (21) is inserted in at least one of the first optical path and the second optical path, and the optical path length varying means can cancel the optical path length difference calculated by the optical path length difference calculating means. The optical heterodyne interferometer according to claim 1 or 2. The wavelength tunable light is demultiplexed and divided into a first optical path (L1) that includes the object to be measured and a second optical path (L2) that does not include the object to be measured, and passes through the first optical path and the second optical path. In an optical heterodyne interferometer for obtaining a characteristic of the device under test based on an output signal of the combined light that enters a photoelectric converter (16) by combining a predetermined frequency difference between the lights. An optical path length difference measuring method for measuring a difference in length between the first optical path and the second optical path,
Sweeping the frequency of the wavelength tunable light at a constant speed and detecting the frequency of the signal output from the photoelectric converter during the sweep (S1 to S3);
Obtaining a difference between the detected frequency and the predetermined frequency, and calculating a difference in length between the first optical path and the second optical path based on the difference (S4, S5). Optical path length difference measuring method of optical heterodyne interferometer.

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