JP2008241580A - Optical heterodyne interference device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable high-accuracy measurements without having to depend on polarization of light. <P>SOLUTION: An optical heterodyne interference device includes coupling emission light P of a wavelength variable light source 31 and reference light Pr with a first coupler 33 to be branched into two; being emitted to a first optical path L1, including an object to be measured 1 and a second optical path L2 without, including the object to be measured 1; giving a prescribed frequency difference between lights of both optical paths by a frequency difference giving means 35; dividing the lights through the optical paths L1, L2 by a second coupler 40 into a P polarizing component and an S polarizing component, respectively to be coupled; selecting a reference signal used in synchronous detection processing from a beat signal obtained by interference of a reference light component, obtained by a wavelength branched wave of each coupled component with a reference signal output means 55; performing synchronous detection processing on a measurement light component of P polarization and a measurement optical component of S polarization of the light passing through the first optical path L1 by two lock-in amplifiers 56, 57; and obtaining delay characteristics, or the like, of the object to be measured 1 of each polarization component, based on obtained signals (Xp, Yp) and (Xs, Ys). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technique for enabling high-precision measurement in an optical heterodyne interferometer for measuring group delay characteristics, wavelength dispersion characteristics, and the like of optical components, without depending on the polarization state of light.

  Conventionally, an optical heterodyne interferometer 10 shown in FIG. 9 has been used to measure the group delay characteristics, chromatic dispersion characteristics, and the like of optical components.

  This optical heterodyne interferometer 10 divides a measurement light P having a frequency f emitted from a wavelength tunable light source 11 into two by an optical demultiplexer 12 and a first optical path L1 including the DUT 1 as one of the lights Pa. And the other light Pb is emitted to the second optical path L2 not including the DUT 1.

  Here, on the second optical path L2 side, a frequency shifter 14 composed of an acousto-optic element or the like that receives the signal Ea of the frequency fa from the signal generator 13 and shifts the optical frequency of the incident light by the signal frequency fa is inserted. The light Pd shifted by the frequency fa by the frequency shifter 14 and the light Pc passing through the device under test 1 are combined by the optical multiplexer 15, and the combined light Pe enters the photoelectric converter 16. Thus, an electric signal Ee is obtained. Then, the signal Ee is supplied to a lock-in amplifier (synchronous detector) 17 to perform synchronous detection processing using the signal Ea as a reference signal, and the obtained orthogonal signal components X and Y are input to the arithmetic processing unit 18.

  Here, in the signal Ee output from the photoelectric converter 16, in addition to the direct current component, the light Pc that has passed through the DUT 1 and the light Pd to which the frequency difference fa has been given without passing through the DUT 1. A beat signal component caused by interference is included. This beat signal component is a sine wave having an amplitude corresponding to the intensity of the light Pc and the light Pd and a frequency equal to the frequency difference between the light Pc and Pd (in this case, fa).

  When the optical frequency f of the measurement light P emitted from the wavelength tunable light source 11 is changed, the amplitude and phase of the light Pc that has passed through the DUT 1 change, so that the amplitude and phase of the beat signal component also change.

If the group delay time tg of the device under test is defined as each frequency derivative of the phase characteristic, the group delay time tg is expressed by the following equation.
tg = dθ (ω) / dω

  Therefore, the group delay time can be obtained by measuring the phase change while varying the optical frequency f of the wavelength variable light source 11.

  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 having the above-described configuration, accurate measurement may be difficult due to the large influence of disturbance on the DUT 1 side.

  In order to solve this, it is conceivable to use a reference light different from the measurement light, such as the optical heterodyne interferometer 20 shown in FIG.

  That is, the reference light Pr having a fixed wavelength fr emitted from the reference light source 21 and the measurement light P are incident on the coupler 22 and combined, and the combined light is split into two, and one of the light Pa ′ is obtained. The first light path L1 including the DUT 1 is emitted, the other light Pb ′ is emitted to the second optical path L2 not including the DUT 1, and the light Pe emitted from the optical multiplexer 15 is separated by wavelength. A first wavelength component Pf composed of light having a frequency difference fa through the two light paths L1 and L2 by the component of the measurement light P and the two light paths L1 by the reference light P component. , L2, and the second wavelength component Pg made of light having a frequency difference fa from each other.

  The wavelength components Pf and Pg are received by the photoelectric converters 24 and 25, respectively, and the first beat signal Ef generated by the difference between the frequency f and the frequency f + fa, and the second beat signal Eg generated by the difference between the frequency fr and the frequency fr + fa. Then, the lock-in amplifier 17 synchronously detects the first beat signal Ef using the second beat signal Eg as a reference signal.

  By using the reference light Pr in this way, the influence of the disturbance on the measurement light component passing through the DUT 1 is equally applied to the reference light component, and the fluctuation of the amplitude and phase of the synchronous detection output due to the disturbance is prevented. Is possible.

  However, even in the optical heterodyne interferometer that is not affected by disturbance using the reference light as described above, the interference signal level is significantly lowered depending on the polarization state of the light emitted from the DUT 1. Measurement may be difficult.

  An object of the present invention is to provide an optical heterodyne interferometer that can improve this point and perform highly accurate measurement without depending on the polarization state of light.

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 (31) that emits wavelength tunable measurement light (P);
A reference light source (32) for emitting reference light (Pr);
A first coupler (33) for combining the measurement light and the reference light, and dividing the combined light into a first optical path including the object to be measured and a second optical path not including the object to be measured; ,
A frequency difference providing means (35) for giving a predetermined frequency difference between the light passing through the first optical path and the light passing through the second optical path;
The light having the predetermined frequency difference is combined by the frequency difference providing unit, and the first combined light including the P-polarized component of the light passing through the first optical path and the S-polarized light of the light passing through the first optical path. A second coupler (40) for emitting a second combined light including a component;
From the first combined light emitted from the second coupler, the first measurement light component having the predetermined frequency difference in the measurement light component and the first measurement light component having the predetermined frequency difference in the reference light component. First wavelength demultiplexing means (45) for extracting one reference light component;
From the second combined light emitted from the second coupler, the second measurement light component having the predetermined frequency difference in the measurement light component and the second measurement light component having the predetermined frequency difference in the reference light component. Second wavelength demultiplexing means (46) for extracting two reference light components;
A first photoelectric converter (51) that receives the first measurement light component and outputs a first beat signal having a frequency equal to the predetermined frequency difference caused by interference of light included in the first measurement light component;
A second photoelectric converter (52) that receives the first reference light component and outputs a second beat signal having a frequency equal to the predetermined frequency difference caused by interference of light included in the first reference light component;
A third photoelectric converter (53) that receives the second measurement light component and outputs a third beat signal having a frequency equal to the predetermined frequency difference caused by interference of light included in the second measurement light component;
A fourth photoelectric converter (54) that receives the second reference light component and outputs a fourth beat signal having a frequency equal to the predetermined frequency difference caused by interference of light included in the second reference light component;
A reference signal output means (55) for outputting, as a reference signal, a signal selected from the signals of the predetermined frequency including the second beat signal and the fourth beat signal;
A first lock-in amplifier (56) for synchronously detecting the first beat signal with the reference signal;
A second lock-in amplifier (57) for synchronously detecting the third beat signal with the reference signal;
An arithmetic processing unit (60) for obtaining characteristics of the device under test based on output signals of the first lock-in amplifier and the second lock-in amplifier.

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
A signal generator (36, 36a, 36b);
It has a frequency shifter (37, 37a, 37b) for receiving the output signal of the signal generator and shifting the frequency of the incident light by the frequency of the signal.

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,
The frequency difference giving means is
A first signal generator (36a) for outputting a signal of a first frequency (fa);
A first frequency shifter (37a) that is inserted into one of the first optical path and the second optical path, receives the signal of the first frequency, and shifts the frequency of the incident light by the frequency of the signal;
A second signal generator (36b) for outputting a signal having a second frequency (fa ′) different from the first frequency;
A second frequency shifter (37b) that is inserted into one of the first optical path and the second optical path, receives the signal of the second frequency, and shifts the frequency of the incident light by the frequency of the signal;
A difference in frequency between the first frequency and the second optical path is provided between the light in the first optical path and the light in the second optical path.

An optical heterodyne interference device according to claim 4 of the present invention is the optical heterodyne interference device according to any one of claims 1 to 3,
The reference signal output means outputs the higher one of the second beat signal and the fourth beat signal as the reference signal.

An optical heterodyne interference device according to claim 5 of the present invention is the optical heterodyne interference device according to any one of claims 1 to 4,
The second coupler is
A beam splitter (41) for receiving the light having passed through the first optical path and the light having passed through the second optical path and emitting the light separately;
A first polarizer (42) that selectively passes a P-polarized component of the light emitted from the beam splitter;
And a second polarizer (43) that selectively allows the S-polarized light component of the light emitted from the beam splitter to pass therethrough.

An optical heterodyne interference device according to claim 6 of the present invention is the optical heterodyne interference device according to any one of claims 1 to 4,
The second coupler is
A polarization beam splitter (41 ′) that receives the light that has passed through the first optical path and the light that has passed through the second optical path, and divides the light into a P-polarized component and an S-polarized component, respectively;
Of the light emitted from the polarization beam splitter, the transmission axis for selectively passing the same polarization component upon receiving the P polarization component of the light passing through the first optical path and the S polarization component passing through the second optical path is P A first polarizer (42 ') that makes an angle of 45 with respect to the polarized light;
Of the light emitted from the polarization beam splitter, the transmission axis for selectively passing the same polarization component upon receiving the S polarization component of the light passing through the first optical path and the P polarization component passing through the second optical path is P And a second polarizer (43 ') having an angle of 45 with respect to the polarized light.

An optical heterodyne interference device according to claim 7 of the present invention is the optical heterodyne interference device according to any one of claims 1 to 6,
The second optical path is configured using a polarization maintaining fiber.

An optical heterodyne interference device according to claim 8 of the present invention is the optical heterodyne interference device according to claim 7,
The main axis of the polarization-maintaining fiber is configured to form 45 ° with respect to the P-polarized component of the light passing through the first optical path.

An optical heterodyne interference device according to claim 9 of the present invention is the optical heterodyne interference device according to any one of claims 1 to 8,
The arithmetic processing unit includes:
Based on the output signal of the first lock-in amplifier, a group delay is obtained for the P-polarized light of the light passing through the first optical path, and S of the light passing through the first optical path is determined based on the output signal of the second lock-in amplifier. It is characterized in that a group delay with respect to polarization is obtained.

An optical heterodyne interference device according to claim 10 of the present invention is the optical heterodyne interference device according to claim 9,
The arithmetic processing unit includes:
A group delay for the P-polarized light and a group delay for the S-polarized light are averaged to obtain an average group delay.

  As described above, the optical heterodyne interferometer of the present invention uses the reference light and multiplexes the light that has passed through the two optical paths into the P-polarized component and the S-polarized component, respectively, and obtains it by wavelength demultiplexing of each combined component. Among the beat signals obtained by interference of the obtained reference light components, a signal that is preferable as a reference signal for synchronous detection processing is selected, and the measurement light component measured object obtained by wavelength demultiplexing of the combined component is included. Synchronous detection processing is performed on the P-polarized light component of the light passing through the first optical path and the S-polarized light component of the light passing through the first optical path including the device under test, and the device under test for each polarization component based on the signal obtained thereby. I want to find the characteristics.

  Therefore, it is possible to obtain the measurement object characteristics for each polarization component with high accuracy while preventing measurement errors due to disturbances, and to obtain measurement results that are not affected by polarization by averaging processing etc. Become.

  Further, a path that does not include the object to be measured, which is a reference optical path of the interferometer, is configured using a polarization maintaining fiber, and the measurement light component and the reference light component are guided along the main axis of the polarization maintaining fiber, and the second By measuring the principal axis of the coupler 45 ° with respect to the P-polarization axis and the S-polarization axis, the measurement light component and the reference light combined with the P-polarization component and the S-polarization component of the light passing through the path including the object to be measured, respectively. Since the components have almost the same intensity and the phases are synchronized, the characteristics of the object to be measured for each polarization component can be measured stably and accurately.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.
FIG. 1 shows a configuration of an optical heterodyne interference device 30 to which the present invention is applied.

  This optical heterodyne interferometer 30 is similar to the optical heterodyne interferometer 20 described above. The measurement light P having the optical frequency f emitted from the wavelength variable light source 31 and the reference of the fixed optical frequency fr emitted from the reference light source 32 are used. The light Pr is incident on the polarization-maintaining first coupler 33 to combine both lights, the combined light is split into two, and one of the lights Pa ′ is the first including the DUT 1. The light Pb ′ is emitted to the optical path L1, and the other light Pb ′ is emitted to the second optical path L2 that does not include the DUT 1. Here, the measurement light P and the reference light Pr are linearly polarized light.

  The frequency difference giving means 35 is for giving a predetermined frequency difference fb (in this case, fb = fa) to the light passing through the first optical path L1 and the light passing through the second optical path L2. In this embodiment, the frequency difference giving means 35 A signal generator 36 that outputs a signal Ea, an acousto-optic crystal element that is inserted into the second optical path L2, receives the signal Ea, and shifts the optical frequency of incident light higher (or lower) by the frequency fa and emits it. The frequency shifter 37 is used.

  The light Pc emitted from the first optical path L1 via the DUT 1 and the delay means 38 and the light Pd emitted from the second optical path L2 via the frequency shifter 37 are incident on the second coupler 40. The

  The delay means 38 is arranged to adjust the optical path length difference between the optical paths L1 and L2. When the optical path length difference is large, the phase change increases with respect to the change in optical frequency.

  As shown in FIG. 2, for example, the second coupler 40 includes a non-polarization type beam splitter 41, a 0 ° polarizer 42, and a 90 ° polarizer 43, and the light Pc incident from the first optical path L1 is as follows. The beam is split by the beam splitter 41, the P-polarized component Pcp of one light is selectively emitted through the 0 ° polarizer 42, and the S-polarized component Pcs of the other light is selected through the 90 ° polarizer 43. The light is emitted.

  Further, the linearly polarized light Pd incident from the second optical path L2 is incident on the beam splitter 41 in a state where its polarization axis is inclined by 45 ° with respect to the P polarization axis and the S polarization axis. The P-polarized component Pdp of the second light is selectively emitted through the 0 ° polarizer 42, and the S-polarized component Pds of the other light is selectively emitted through the 90 ° polarizer 43.

  At this time, the measurement light component and the reference light component included in the P-polarized component Pdp and the S-polarized component Pds are almost equal in intensity and synchronized in phase.

  In the second coupler 40 described above, the non-polarization type beam splitter 41, the 0 ° polarizer 42, and the 90 ° polarizer 43 are used to pass the P polarization component Pcp of the light passing through the first optical path L1 and the second optical path L2. The light including the P-polarized component Pdp of the light is the first combined light Pp, and the light including the S-polarized component Pcs of the light passing through the first optical path L1 and the S-polarized component Pds of the light passing through the second optical path L2 is second. As shown in FIG. 3, the second coupler 40 has a polarization beam splitter 41 'and two transmission axes whose transmission axes are inclined by 45 ° with respect to the P polarization axis and the S polarization axis. It can also be configured using polarizers 42 'and 43'.

  In this case, the light including the P-polarized component Pcp of the light passing through the first optical path L1 and the S-polarized component Pds of the light passing through the second optical path L2 is emitted as the first combined light Pp, and the light passing through the first optical path L1 Light including the S-polarized light component Pcs and the P-polarized light component Pdp of the light passing through the second optical path L2 is emitted as the second combined light Ps.

  Even in this case, the measurement light component and the reference light component of the light passing through the second optical path L2 combined with each of the P-polarization component Pcp and the S-polarization component Pcs of the light passing through the first optical path L1 are intensities, respectively. Are almost equal and the phase is synchronized.

  The polarization of light Pd is stable 45 with respect to the transmission axis of the 0 ° polarizer 42 and the 90 ° polarizer 43 of the second coupler 40 or the P polarization axis and the S polarization axis of the polarization beam splitter 41 ′. In order to make the light incident at an angle of °, the second optical path L2 is composed of the polarization maintaining fiber 34 as shown in FIG. 4, and its principal axes x and y are relative to the P polarization axis and S polarization axis of the light beam splitter 41 '. It is inclined 45 degrees.

  The first combined light Pp emitted from the second coupler 40 is incident on the first wavelength demultiplexer 45, and the second combined light Ps is incident on the second wavelength demultiplexer 46.

  The first wavelength demultiplexer 45 converts the input first combined light Pp into a first measurement light component Pp1 composed of light having a predetermined frequency difference fb as a component of the measurement light P and a component of the reference light Pr. And the first reference light component Pp2 made of light having a predetermined frequency difference fb.

  Similarly, the second wavelength demultiplexer 46 converts the incident second combined light Ps into a second measurement light component Ps1 made of light having a predetermined frequency difference fb as a component of the measurement light P, and a reference light. A second reference light component Ps2 made of light having a predetermined frequency difference fb from the Pr component is emitted separately.

  The first photoelectric converter 51 that has received the first measurement light component Pp1 includes the first beat signal Ep1 having the frequency fb that is generated by the interference of the measurement light components that are included in the first measurement light component Pp1 and have a predetermined frequency difference fb. The second photoelectric converter 52 that receives the first reference light component Pp2 outputs the first reference light component Pp2 having the frequency fb that is generated by the interference of the reference light components that are included in the first reference light component Pp2 and have a predetermined frequency difference fb. A 2-beat signal Ep2 is output.

  Similarly, the third photoelectric converter 53 that has received the second measurement light component Ps1 has the third frequency fb generated by the interference of the measurement light components included in the second measurement light component Ps1 and having a predetermined frequency difference fb. The fourth photoelectric converter 54, which outputs the beat signal Es1 and receives the second reference light component Ps2, generates a frequency caused by interference of reference light components included in the second reference light component Ps2 and having a predetermined frequency difference fb. The fourth beat signal Es2 of fb is output.

  The second beat signal Ep2 and the fourth beat signal Es2 are input to the reference signal output means 55. The reference signal output means 55 is for outputting a signal selected from the input beat signals Ep2 and Es2 as a reference signal Er of a predetermined frequency fb. For example, the higher one of the input beat signals is selected. Then, it is input to the first lock-in amplifier 56 and the second lock-in amplifier 57 as the reference signal Er.

  The first lock-in amplifier 56 synchronously detects the first beat signal Ep1 with the reference signal Er, and the second lock-in amplifier 57 synchronously detects the third beat signal Es1 with the reference signal Er.

  For example, the arithmetic processing unit 60 varies the frequency f of the measurement light P emitted from the wavelength tunable light source 31 by Δf steps, and outputs the output signals Xp and Yp of the first lock-in amplifier 56 and the second lock-in amplifier 57 for each frequency. Based on the output signals Xs and Ys, the characteristics of the DUT 1 for P-polarized light and S-polarized light are obtained.

For example, the phase values φp and φs for each frequency f are calculated as follows:
φp = tan ⁻¹ (Yp / Xp)
φs = tan ⁻¹ (Ys / Xs)
And group delays τp and τs for each polarized light are obtained by the following calculation.

τp (n) = dφp / dω
= [Φp (n) −φp (n−1)] / (2πΔf)
τs (n) = dφs / dω
= [Φs (n) −φs (n−1)] / (2πΔf)

The amplitudes Ap and As for each polarization are
Ap = (Xp ² + Yp ² ) ^1/2
As = (Xs ² + Ys ² ) ^1/2
Can be obtained by the following calculation.

  The group delay for each polarization obtained in this way is displayed as a graph on the frequency axis as shown in FIG. 5, for example, on a display screen (not shown) by the arithmetic processing unit 60, for example.

  In addition, the arithmetic processing unit 60 calculates an average value of the group delays τp (n) and τs (n) of the two polarizations at each frequency, and the average group delay value τa (n) = [τp (n) + τs (n )] / 2 may be obtained and displayed in a graph on the frequency axis as shown in FIG. 6, for example.

  As described above, the optical heterodyne interferometer 30 according to the embodiment uses the reference light Pr and multiplexes the light that has passed through the two optical paths L1 and L2 into the P-polarized component and the S-polarized component, respectively. Measurement signal component obtained by selecting the preferred signal as the reference signal for synchronous detection processing from the beat signals obtained by interference of the reference light component obtained by wavelength demultiplexing of The synchronous detection processing is performed on the P-polarized component and the S-polarized component, and the characteristic of the object to be measured for each polarized component is obtained based on the signal obtained thereby.

  Therefore, it is possible to obtain the measurement object characteristics for each polarization component with high accuracy while preventing measurement errors due to disturbances, and to obtain measurement results that are not affected by polarization by averaging processing etc. Become.

  Here, the case where the DUT 1 passes the component of the reference light Pr has been described. However, when the DUT 1 does not pass the reference light Pr, the reference signal candidate for the synchronous detection processing is used as a candidate. Since the second beat signal Ep2 and the fourth beat signal Es2 cannot be obtained, in this case, the signal Ea is directly output from the signal generator 36 to the reference signal output means 55 as shown by the dotted line in FIG. Thus, the signal Ea having a frequency equal to the predetermined frequency difference fb may be selected as the reference signal Er.

  In the above embodiment, the frequency shifter 37 of the frequency difference providing unit 35 is inserted on the second optical path L2 side. However, the frequency shifter 37 is inserted on the first optical path L1 side (which may be the front stage or the rear stage of the DUT 1). May be.

(Second Embodiment)
Further, as shown in FIG. 7, a frequency shifter 37a that receives the signal Ea of the frequency fa output from the signal generator 36a is inserted into the second optical path L2, and the signal Ea ′ of the frequency fa ′ output from the signal generator 36b. Is inserted into the first optical path L1 (which may be the front stage or the rear stage of the DUT 1), and a predetermined frequency difference fb = fa−fa ′ is given to the light incident on the second coupler 40. May be.

  Further, as shown in FIG. 8, even if the two frequency shifters 37a and 37b are inserted into one of the two optical paths L1 and L2, the frequency difference fb = fa−fa ′ can be given. In this case, however, the shift directions of the two frequency shifters 37a and 37b are reversed.

  When configured in this way, the frequency of each beat signal Ep1, Ep2, Es1, Es2 can be remarkably lowered, and phase detection can be performed with a low-frequency lock-in amplifier.

  Also in this case, considering the situation in which the DUT 1 that does not pass the reference light Pr is taken into account, as shown by the dotted lines, the signals Ea and Ea ′ are mixed by the mixer 61 and the beat signal Eb having the difference frequency fb is obtained. It may be configured so that it can be generated and selected as the reference signal Er.

The figure which shows the structure of the 1st Embodiment of this invention. The figure which shows the structural example of the principal part of embodiment. The figure which shows the other structural example of the principal part of embodiment. The figure which shows the relationship of the axis | shaft at the time of using a polarization maintaining fiber in the 2nd optical path side The figure which shows the example of a display of the measurement result about each polarization The figure which shows the example of a display of the measurement result obtained by the average of polarization The figure which shows a part of structure of embodiment which can make beat frequency low. The figure which shows a part of structure of embodiment which can make beat frequency low. Configuration diagram of conventional equipment Diagram showing configuration using reference light

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Object to be measured, 30 ... Optical heterodyne interference device, 31 ... Wavelength variable light source, 32 ... Reference light source, 33 ... First coupler, 35 ... Frequency difference imparting means, 36, 36a, 36b ... Signal generator, 37, 37a, 37b ... frequency shifter, 38 ... delay, 40 ... second coupler, 41 ... beam splitter, 41 '... polarizing beam splitter, 42 ... 0 degree polarizer, 42 ′ …… 45 ° polarizer, 43 …… 90 ° polarizer, 43 ′ …… 45 ° polarizer, 45 …… first wavelength demultiplexer, 46 …… second wavelength demultiplexer, 51 …… first Photoelectric converter, 52... Second photoelectric converter, 53... Third photoelectric converter, 54... Fourth photoelectric converter, 55... Reference signal output means, 56. ... second lock-in amplifier, 60 ... arithmetic processing unit, 61 ... mixer

Claims (10)

A wavelength tunable light source (31) that emits wavelength tunable measurement light (P);
A reference light source (32) for emitting reference light (Pr);
A first coupler (33) for combining the measurement light and the reference light, and dividing the combined light into a first optical path including the object to be measured and a second optical path not including the object to be measured; ,
A frequency difference providing means (35) for giving a predetermined frequency difference between the light passing through the first optical path and the light passing through the second optical path;
The light having the predetermined frequency difference is combined by the frequency difference providing unit, and the first combined light including the P-polarized component of the light passing through the first optical path and the S-polarized light of the light passing through the first optical path. A second coupler (40) for emitting a second combined light including a component;
From the first combined light emitted from the second coupler, the first measurement light component having the predetermined frequency difference in the measurement light component and the first measurement light component having the predetermined frequency difference in the reference light component. First wavelength demultiplexing means (45) for extracting one reference light component;
From the second combined light emitted from the second coupler, the second measurement light component having the predetermined frequency difference in the measurement light component and the second measurement light component having the predetermined frequency difference in the reference light component. Second wavelength demultiplexing means (46) for extracting two reference light components;
A first photoelectric converter (51) that receives the first measurement light component and outputs a first beat signal having a frequency equal to the predetermined frequency difference caused by interference of light included in the first measurement light component;
A second photoelectric converter (52) that receives the first reference light component and outputs a second beat signal having a frequency equal to the predetermined frequency difference caused by interference of light included in the first reference light component;
A third photoelectric converter (53) that receives the second measurement light component and outputs a third beat signal having a frequency equal to the predetermined frequency difference caused by interference of light included in the second measurement light component;
A fourth photoelectric converter (54) that receives the second reference light component and outputs a fourth beat signal having a frequency equal to the predetermined frequency difference caused by interference of light included in the second reference light component;
A reference signal output means (55) for outputting, as a reference signal, a signal selected from the signals of the predetermined frequency including the second beat signal and the fourth beat signal;
A first lock-in amplifier (56) for synchronously detecting the first beat signal with the reference signal;
A second lock-in amplifier (57) for synchronously detecting the third beat signal with the reference signal;
An optical heterodyne interference device comprising: an arithmetic processing unit (60) for obtaining characteristics of the device under test based on output signals of the first lock-in amplifier and the second lock-in amplifier. The frequency difference giving means is
A signal generator (36, 36a, 36b);
The optical heterodyne interference according to claim 1, further comprising frequency shifters (37, 37a, 37b) for receiving the output signal of the signal generator and shifting the frequency of incident light by the frequency of the signal. apparatus. The frequency difference giving means is
A first signal generator (36a) for outputting a signal of a first frequency (fa);
A first frequency shifter (37a) that is inserted into one of the first optical path and the second optical path, receives the signal of the first frequency, and shifts the frequency of the incident light by the frequency of the signal;
A second signal generator (36b) for outputting a signal having a second frequency (fa ′) different from the first frequency;
A second frequency shifter (37b) that is inserted into one of the first optical path and the second optical path, receives the signal of the second frequency, and shifts the frequency of the incident light by the frequency of the signal;
3. The optical heterodyne according to claim 1, wherein a frequency difference of a difference between the first frequency and the second frequency is provided between the light on the first optical path and the light on the second optical path. Interfering device.   The optical heterodyne interference according to any one of claims 1 to 3, wherein the reference signal output means outputs the higher one of the second beat signal and the fourth beat signal as the reference signal. apparatus. The second coupler is
A beam splitter (41) for receiving the light having passed through the first optical path and the light having passed through the second optical path and emitting the light separately;
A first polarizer (42) that selectively passes a P-polarized component of the light emitted from the beam splitter;
The optical heterodyne interference according to any one of claims 1 to 4, further comprising a second polarizer (43) that selectively passes an S-polarized component of the light emitted from the beam splitter. apparatus. The second coupler is
A polarization beam splitter (41 ′) that receives the light that has passed through the first optical path and the light that has passed through the second optical path, and divides the light into a P-polarized component and an S-polarized component, respectively;
Of the light emitted from the polarization beam splitter, the transmission axis for selectively passing the same polarization component upon receiving the P polarization component of the light passing through the first optical path and the S polarization component passing through the second optical path is P A first polarizer (42 ') that makes an angle of 45 with respect to the polarized light;
Of the light emitted from the polarization beam splitter, the transmission axis for selectively passing the same polarization component upon receiving the S polarization component of the light passing through the first optical path and the P polarization component passing through the second optical path is P The optical heterodyne interferometer according to any one of claims 1 to 4, further comprising a second polarizer (43 ') having an angle of 45 with respect to the polarized light.   The optical heterodyne interference device according to claim 1, wherein the second optical path is configured using a polarization maintaining fiber.   8. The optical heterodyne interference device according to claim 7, wherein the main axis of the polarization maintaining fiber is configured to form 45 [deg.] With respect to the P-polarized component of the light passing through the first optical path. The arithmetic processing unit includes:
Based on the output signal of the first lock-in amplifier, a group delay is obtained for the P-polarized light of the light passing through the first optical path, and S of the light passing through the first optical path is determined based on the output signal of the second lock-in amplifier. 9. The optical heterodyne interference device according to claim 1, wherein a group delay with respect to polarized light is obtained. The arithmetic processing unit includes:
10. The optical heterodyne interferometer according to claim 9, wherein the group delay for the P-polarized light and the group delay for the S-polarized light are averaged to obtain an average group delay.

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