JP2006266797A - Apparatus for optical heterodyne interference - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus for optical heterodyne interference capable of improving precision in measuring optical frequencies and phase characteristics by improving the correspondence between phase signals acquired by phase detection and the frequency of measuring light. <P>SOLUTION: This apparatus for optical heterodyne interference is provided with a Mach-Zehnder interferometer 20 having both a light branching means 4 for branching frequency-variable measuring light, incident light, into two and emitting two beams of light having different frequencies and an optical coupler 6 for emitting interference light by combining the two beams of light entering through different optical paths; a light receiver 8 for performing heterodyne detection on interference light; an RF lock-in amplifier 11 for detecting the phase of its detection signals; and a processing means 16 for processing data on phase signals from the RF lock-in amplifier 11 and determining optical frequencies and phase characteristics of objects to be measured. The processing means 16 receives regular optical frequency interval triggers c generated at prescribed regular optical frequency intervals correspondingly to variations of the frequency of the measuring light and processes data the phase signals on the basis of the regular optical frequency interval triggers c. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical heterodyne interferometer that measures, for example, the phase characteristics of an object to be measured from the phase of interference light emitted from an interferometer, and more particularly to an acousto-optic frequency shifter (AOFS) in a Mach-Zehnder interferometer. ) To provide a frequency difference for heterodyne detection between the two branched lights.

  Conventionally, as an optical heterodyne interferometer in which an AOFS is arranged in a Mach-Zehnder interferometer and a frequency difference for heterodyne detection is given to two branched lights, the Mach-Zehnder interferometer uses an optical fiber in its optical path. There are some optical fiber interferometers configured to measure the chromatic dispersion from the phase characteristics of the object to be measured. (For example, see Non-Patent Document 1)

  A schematic configuration of this type of optical heterodyne interferometer is shown in FIG. The wavelength tunable light source 1 is, for example, a wavelength tunable laser, and generates measurement light having a predetermined frequency range variable based on a control signal a input from the control unit 13 and emits the measurement light to the optical coupler 3. For example, the frequency (wavelength) range of 196.5 THz (1525.26 nm) to 185.0 THz (1620.5 nm) is divided into 1000 points and can be varied in 1 sec. The light source 2 is a laser, for example, and oscillates a reference light having a fixed frequency different from the oscillation frequency of the wavelength variable light source 1, for example, a wavelength of 1625 nm, and emits it to the optical coupler 3. The optical coupler 3 combines the measurement light and the reference light and multiplexes them, and emits the WDM light to the AOFS 4b.

AOFS4b is driven by the ultrasonic frequency f ₁ input from the ultrasonic generator 4a, with branches the wavelength-multiplexed light incident from the optical coupler 3 into two light, heterodyne detection to the branched two optical Is given to the optical coupler 6 via the DUT 10 as light of the first path based on the 0th order diffraction, and the other is ultrasonic waves. The light is emitted to the delay device 5 as the light of the second path based on the + _1st order diffraction shifted by the frequency f1. Specifically, when the frequency relationship of incident wavelength multiplexed light is ν (the oscillation frequency of the wavelength tunable light source 1) and ν ₀ (the fixed frequency of the reference light), the frequency of the light in the first path is ν. And ν ₀ , the frequencies of light in the second path are ν + f ₁ and ν ₀ + f ₁ . When they are combined by the optical coupler 6 and then subjected to heterodyne detection, an electrical signal having the frequency f ₁ of the beat signal is output.

  The delay device 5 is constituted by, for example, an optical fiber delay line, and in order to match the optical path length as much as possible with the two arms of the Mach-Zehnder interferometer, the light of the second path is changed by the object 10 to be measured. The light is output to the optical coupler 6 with a delay approximately equal to the delay time. The optical coupler 6 multiplexes the light of the first path incident through the device under test 10 and the light of the second path incident through the delay device 5 to reference the interference light based on the measurement light and the reference light. The interference light based on the light is emitted to the optical demultiplexer 7.

  The ultrasonic generator 4a and the AOFS 4b constitute the optical branching means 4, and the optical branching means 4, the DUT 10, the delay device 5 and the optical coupler 6 constitute a Mach-Zehnder interferometer 20. Therefore, the light branching means 4 is a means for branching the incident light into two in a Mach-Zehnder interferometer for branching the incident light into two and coupling (interfering) after passing through separate optical paths. , Means for coupling (interference) through separate optical paths. When the Mach-Zehnder interferometer 20 is an optical fiber interferometer that includes an optical fiber in its optical path, or when the delay unit 5 that matches the optical path length is an optical fiber delay line, the temperature changes. It is likely to cause phase fluctuations due to fluctuations in the optical fiber due to vibration and vibration. In order to improve this, the measurement light and the reference light are wavelength-multiplexed and simultaneously incident on the Mach-Zehnder interferometer 20, and the phase fluctuation in the measurement light is canceled by the phase fluctuation in the reference light at the time of phase detection.

The optical demultiplexer 7 receives the interference light of the wavelength multiplexed light formed by wavelength multiplexing of the measurement light and the reference light incident from the optical coupler 6, demultiplexes them to the respective frequencies (wavelengths), and The interference light is emitted to the light receiver (PD) 8 and the interference light of the reference light is emitted to the light receiver (PD) 9. The light receiver 8 receives the interference light of the measurement light incident from the optical demultiplexer 7 and performs heterodyne detection, and performs the light of the frequency ν in the light of the first path and the frequency ν + f of the light of the second path. converting the frequency difference between the _first optical beat signals of (f ₁₎ to a first electrical signal. Similarly, the light receiver 9 receives the interference light of the reference light incident from the optical demultiplexer 7 and performs heterodyne detection to convert the beat signal having the frequency difference (f ₁ ) into the second electric signal. The RF lock-in amplifier 11, which is a synchronous detector, receives the second electric signal as a synchronizing signal b, and detects the phase of the first electric signal based on the synchronizing signal b. The processing unit 12 includes an AD converter 12a and a calculation unit 12b. The AD converter 12a converts the phase signal input from the RF lock-in amplifier 11 into a digital value based on the control signal a input from the control means 13, and outputs the digital value to the arithmetic means 12b. The computing means 12b computes the digital value of this phase signal and obtains chromatic dispersion from the optical frequency-phase characteristics of the device under test. The control means 13 generates a control signal a for changing the frequency of the measurement light and outputs it to the wavelength variable light source 1 and the processing means 12.

51th Japan Federation of Reciprocal Products “Evaluation of Chromatic Dispersion Using a Two-Wavelength Heterodyne Fiber Interferometer” 28p-R-12 (2004.3): Kensuke Ogawa, Titi Ray (DNRI)

  In general, in such an optical heterodyne interferometer, when the frequency of the measurement light is varied and the optical frequency-phase characteristic of the device under test is obtained from the phase of the optical heterodyne interference light, each phase data output from the synchronous detector and Correspondence with the frequency of the measurement light, that is, the accuracy of the frequency at the time of acquiring each phase data is important in accurately obtaining the optical frequency characteristic of the phase characteristic of the object to be measured.

  In the above-described conventional optical heterodyne interferometer, each phase data (phase signal) output from the RF lock-in amplifier 11 (synchronous detector) is associated with the frequency of the measurement light by using this phase signal as an AD converter 12a. Is converted into a digital value by AD conversion (synchronized) based on a control signal a that varies the frequency of the measurement light. That is, when the control signal is a signal that determines a variable start time and end time and equally divides the time between them, each phase data is acquired at equal time intervals by performing AD conversion using this signal. Alternatively, each phase data is acquired at equal time intervals by performing AD conversion at equal time intervals from a variable start time using the internal clock of the AD converter 12a. Thereby, each phase data is acquired on the assumption that the frequency change between the variable start frequency and the variable end frequency of the measurement light is linear in time. Specifically, for example, as described above, if the measurement light is divided into 1000 points and the frequency range of 196.5 to 185.0 THz is variable in 1 sec, each phase data is acquired at a variable start frequency. Assuming that the frequency changes from 196.5 THz by 11.5 GHz every 1 ms, 1001 points are performed from the variable end frequency of 185.0 THz.

  However, the method for acquiring each phase data at equal time intervals on the assumption that the relationship between the elapsed time from the variable start of the measurement light and the frequency is linear has the following problems. That is, when the variable frequency range is wide and the variable is fast (variable time is short), the nonlinearity at the time of variable frequency in the wavelength tunable light source cannot be ignored. As shown in FIG. The frequency of the measurement light associated with the frequency differs from the frequency actually used for measuring each phase data. As a result, the optical frequency-phase characteristic of the object to be measured cannot be obtained accurately. Although a method of measuring the frequency of the measurement light every time the phase data is acquired using a wavelength meter is also conceivable, the frequency measurement time per point is as long as several seconds, and there are 1000 points as described above. In such a case, the data acquisition time for one variable becomes very long. For this reason, it cannot cope with a wavelength variable light source having a short frequency variable time.

  An object of the present invention is to solve these problems and to provide an optical heterodyne interference device that improves the measurement accuracy of optical frequency-phase characteristics.

  In order to solve the above-described problem, in the optical heterodyne interferometer according to claim 1 of the present invention, the light having a variable frequency, which is incident light, is branched into two, and the light of the first path having different frequencies from each other and The light branching means (4) that emits light as the second path light and the light of the first path incident through the object to be measured and the light of the second path incident without passing through the object to be measured. A Mach-Zehnder interferometer (20) having optical coupling means (6) for combining and emitting interference light, and frequency of the light of the first path and the light of the second path after receiving the interference light and performing heterodyne detection. A photoreceiver (8) for converting the beat signal of the difference into an electrical signal, a synchronous detector (11) for detecting the phase of the electrical signal based on a synchronization signal having substantially the same frequency as the beat signal, and the measurement Corresponding to the optical frequency of light, In the optical heterodyne interference device comprising processing means (16) for receiving the phase signal output from the synchronous detector and processing the data of the phase signal to obtain the optical frequency-phase characteristic of the device under test. The means receives an equal optical frequency interval trigger generated at a predetermined equal optical frequency interval corresponding to the variable frequency of the measurement light, and performs data processing of the phase signal based on the equal optical frequency interval trigger. Yes.

  In the optical heterodyne interferometer according to claim 2 of the present invention, in the optical heterodyne interferometer according to claim 1 described above, the Mach-Zehnder interferometer is further provided between the optical branching unit and the optical coupling unit. The time until the light of the second path emitted from the light branching means reaches the optical coupling means, and the time until the light of the first path reaches the optical coupling means via the object to be measured. A delay device (5) is provided for delaying the light of the first path or the light of the second path so that the time is substantially equal.

  In the optical heterodyne interferometer according to claim 3 of the present invention, the wavelength-multiplexed light obtained by wavelength-multiplexing the frequency-variable measurement light and the reference light having a fixed frequency different from the optical frequency of the measurement light, An optical branching means (4) for branching the multiplexed light and outputting the light of the first path and the light of the second path, wherein the light of the first path and the light of the second path are measured light components, respectively. And the reference light component, the frequency of the measurement light component included in the light of the first path is different from the frequency of the measurement light component included in the light of the second path, and included in the light of the first path The light branching means (4) in which the frequency of the reference light component and the frequency of the reference light component included in the light of the second path are different from each other, and the light of the first path passing through the device under test and the device under test Light that combines the light of the second path that does not pass through A Mach-Zehnder interferometer (20) having a coupling means (6), an optical demultiplexer (7) for frequency demultiplexing the measurement light component and the reference light component, and demultiplexed by the optical demultiplexer A first light receiver (8) for converting a first interference light composed of a measurement light component of the light of the first path and a measurement light component of the light of the second path into a first electric signal; A second interfering light that is demultiplexed by the optical demultiplexer and composed of the reference light component of the light of the first path and the reference light component of the light of the second path is converted into a second electric signal. Receiver (9), a synchronous detector (11) for detecting the phase of the first electric signal using the second electric signal as a synchronous signal, and the synchronous detection corresponding to the optical frequency of the measurement light And processing means (16) for processing the phase signal output from the measuring device to obtain the optical frequency-phase characteristic of the object to be measured. In the optical heterodyne interferometer, the processing means receives an equal optical frequency interval trigger generated at a predetermined equal optical frequency interval corresponding to a change in the frequency of the measurement light, and based on the equal optical frequency interval trigger. Data processing of the phase signal is performed.

  Moreover, in the optical heterodyne interference device according to claim 4 of the present invention, in the optical heterodyne interference device according to claim 3 described above, the Mach-Zehnder interferometer is further provided between the optical branching unit and the optical coupling unit. The time until the light of the second path emitted from the light branching means reaches the optical coupling means, and the time until the light of the first path reaches the optical coupling means via the object to be measured. A delay device (5) is provided for delaying the light of the first path or the light of the second path so that the time is substantially equal.

  The optical heterodyne interferometer according to claim 5 of the present invention is the optical heterodyne interferometer according to claim 3 or 4, wherein the Mach-Zehnder interferometer includes an optical fiber in its optical path. It is an interferometer.

  In the optical heterodyne interference device according to claim 6 of the present invention, in the optical heterodyne interference device according to claim 1 or 2, the wavelength tunable light source that sequentially oscillates the frequency-variable measurement light within a predetermined frequency range ( 1), a control means (13) for outputting a control signal for varying the frequency of the measurement light from the wavelength tunable light source, and the measurement light incident from the wavelength tunable light source is branched into two, Receiving the first optical coupler (14) that emits the incident light to the optical branching means as the incident light and the other measurement light emitted from the first optical coupler to cope with the variable frequency of the measurement light And an equal optical frequency interval trigger generating means (15) for generating an equal optical frequency interval trigger at a predetermined equal optical frequency interval and outputting it to the processing means.

  Further, in the optical heterodyne interference device according to claim 7 of the present invention, in the optical heterodyne interference device according to any one of claims 3 to 5 described above, a wavelength for sequentially oscillating the measurement light whose frequency is variable within a predetermined frequency range. A variable light source (1), a control means (13) for outputting a control signal for changing the frequency of the measurement light from the wavelength variable light source, and the reference light having a fixed frequency different from the frequency of the measurement light is oscillated. A light source (2) to be emitted, a first optical coupler (14) that divides and emits the measurement light incident from the wavelength tunable light source, and one of the ones that is emitted from the first optical coupler. A second optical coupler (3) for wavelength-multiplexing the measurement light and the reference light emitted from the light source by combining them and emitting the obtained wavelength-multiplexed light as the incident light to the optical branching means; , The first optical power An equal optical frequency interval trigger generating means (15) which receives the other measurement light emitted from the laser and generates a predetermined equal optical frequency interval trigger corresponding to the change in the frequency of the measurement light and outputs it to the processing means. ).

  Also, in the optical heterodyne interference device according to claim 8 of the present invention, in the optical heterodyne interference device according to claim 1 or 2, the processing means has a predetermined reference optical frequency corresponding to the variable frequency of the measurement light. In response to the reference optical frequency trigger generated in step (1), data processing of the phase signal is performed based on the reference optical frequency trigger.

  Moreover, in the optical heterodyne interference device according to claim 9 of the present invention, in the optical heterodyne interference device according to any one of claims 3 to 5, the processing means has a predetermined value corresponding to the variable frequency of the measurement light. In response to a reference optical frequency trigger generated at the reference optical frequency, data processing of the phase signal is performed based on the reference optical frequency trigger.

  Also, in the optical heterodyne interference device according to claim 10 of the present invention, in the optical heterodyne interference device according to claim 1 or 2, the processing means has a predetermined reference optical frequency corresponding to the variable frequency of the measurement light. The absolute optical frequency of the equal optical frequency interval trigger is corrected from the relative time relationship between the reference optical frequency trigger and the equal optical frequency interval trigger.

  Further, in the optical heterodyne interference device according to claim 11 of the present invention, in the optical heterodyne interference device according to any one of claims 3 to 5, the processing means has a predetermined value corresponding to the variable frequency of the measurement light. In response to the reference optical frequency trigger generated at the reference optical frequency, the absolute optical frequency of the equal optical frequency interval trigger is corrected from the relative time relationship between the reference optical frequency trigger and the equal optical frequency interval trigger.

  The optical heterodyne interference device according to claim 12 of the present invention is the same as the above-described optical heterodyne interference device according to claim 8 or 10, but further includes a wavelength tunable light source that sequentially oscillates frequency-variable measurement light within a predetermined frequency range. 1), a control means (13) for outputting a control signal for varying the frequency of the measurement light from the wavelength tunable light source, and the measurement light incident from the wavelength tunable light source is branched into two, A first optical coupler (14) that emits light to the optical branching means, and a third optical coupler that receives the other measurement light incident from the first optical coupler and branches and emits it into two (17) and receiving one of the measurement lights emitted from the third optical coupler, and generating an equal optical frequency interval trigger of a predetermined equal optical frequency interval corresponding to the variable frequency of the measurement light Isometric light output to the processing means A number interval trigger generating means (15) and the other measurement light emitted from the third optical coupler are received, and a reference frequency trigger of a predetermined reference optical frequency is generated corresponding to the variable frequency of the measurement light. Reference optical frequency trigger generation means (18) for generating and outputting to the processing means.

  The optical heterodyne interference device according to claim 13 of the present invention is the same as the optical heterodyne interference device according to claim 9 or 11, further comprising a wavelength variable light source that sequentially oscillates frequency-variable measurement light within a predetermined frequency range. 1), a control means (13) for outputting a control signal for varying the frequency of the measurement light from the wavelength tunable light source, and a light source for oscillating the reference light having a fixed frequency different from the frequency of the measurement light ( 2), a first optical coupler (14) that splits and emits the measurement light incident from the wavelength tunable light source, and one of the measurement lights emitted from the first optical coupler; A second optical coupler (3) for wavelength-multiplexing the reference light emitted from the light source by multiplexing and emitting the obtained wavelength-multiplexed light as the incident light to the optical branching unit; First light A third optical coupler (17) that receives the other measurement light emitted from the optical fiber and diverges into two, and receives the one measurement light emitted from the third optical coupler. And an equal optical frequency interval trigger generating means (15) for generating a predetermined equal optical frequency interval trigger corresponding to the change in the frequency of the measurement light and outputting it to the processing means, and emitted from the third optical coupler. Reference light frequency trigger generating means (18) that receives the other measurement light, generates a reference frequency trigger of a predetermined reference light frequency corresponding to the change in the frequency of the measurement light, and outputs the reference frequency trigger to the processing means. ing.

  Further, in the optical heterodyne interference device according to claim 14 of the present invention, in the optical heterodyne interference device according to any of claims 6, 7, 12 and 13, the equal optical frequency interval trigger generating means includes the first optical frequency interval trigger generating means. Interfering light that generates triggering interference light whose output light intensity periodically changes at predetermined equal optical frequency intervals in response to variable frequency of the measuring light incident from the optical coupler or the third optical coupler A generating means (15a), a third light receiver (15b) for receiving the trigger interference light incident from the interference light generating means and converting it into the periodically changing third electric signal; Each time the local maximum voltage / minimum voltage of the periodically changing third electric signal input from the third light receiver is detected, and each time the local maximum voltage, the minimum voltage, or both are detected, the first is detected. Pal It was generated, and a first pulse generating means and (15c) for outputting a first pulse as said equal optical frequency interval trigger.

  Also, in the optical heterodyne interference device according to claim 15 of the present invention, in the optical heterodyne interference device according to any one of claims 6, 7, 12 and 13, the equal optical frequency interval trigger generating means is the first light. A plurality of output light intensities relatively periodically with a predetermined phase difference at predetermined equal optical frequency intervals corresponding to a change in frequency of the measurement light incident from a coupler or the third optical coupler. Interference light generating means for generating the trigger interference light, and a plurality of trigger interference lights incident from the interference light generating means, respectively, and converting the plurality of trigger interference lights into the periodically changing electrical signals A third pulse that is phase-divided is generated by using a light receiver and a plurality of electrical signals that are input from the plurality of light receivers and periodically change with the predetermined phase difference, and the third pulse The isolight And a third pulse generating means for outputting a wave number interval trigger.

  In the optical heterodyne interference device according to claim 16 of the present invention, in the optical heterodyne interference device according to claim 12 or 13, the reference optical frequency trigger generation means is configured to receive the measurement incident from the third optical coupler. Frequency reference light generating means (18a) for generating frequency reference light whose output light intensity is maximized or minimized at a predetermined reference light frequency corresponding to the variation of the light frequency, and incident from the frequency reference light generating means A fourth light receiver (18b) that receives the frequency reference light and converts it into a fourth electric signal, and detects a maximum or minimum voltage of the fourth electric signal input from the fourth light receiver; And a second pulse generation means (18c) for outputting a second pulse as the reference optical frequency trigger each time the maximum or minimum voltage is detected.

  In order to solve the above-mentioned problem, in the optical heterodyne interference device according to claim 17 of the present invention, receiving the frequency-variable measurement light as the incident light and branching it into two, and the light of the first path having different frequencies and The light branching means (4) that emits light as the second path light and the light of the first path incident through the object to be measured and the light of the second path incident without passing through the object to be measured. A Mach-Zehnder interferometer (20) having optical coupling means (6) for combining and emitting interference light, and frequency of the light of the first path and the light of the second path after receiving the interference light and performing heterodyne detection. A photoreceiver (8) for converting the beat signal of the difference into an electrical signal, a synchronous detector (11) for detecting the phase of the electrical signal based on a synchronization signal having substantially the same frequency as the beat signal, and the measurement Corresponding to the optical frequency of light An optical heterodyne interferometer comprising processing means (19) for receiving a phase signal output from the synchronous detector and performing data processing on the phase signal to obtain an optical frequency-phase characteristic of the device under test. The processing means has an optical frequency table (19c) that holds the relationship between the variable elapsed time from the variable start to the end and the frequency of the measurement light incident in a variable manner.

  Further, in the optical heterodyne interference device according to claim 18 of the present invention, in the optical heterodyne interference device according to claim 17 described above, the optical frequency table indicates a relationship between the variable elapsed time and the frequency of the measurement light at an equal optical frequency. Hold at intervals.

  The optical heterodyne interferometer according to claim 19 of the present invention is the optical heterodyne interferometer according to claim 17 or 18, wherein the Mach-Zehnder interferometer is further provided between the optical branching means and the optical coupling means. Furthermore, the time until the light of the second path emitted from the light branching means reaches the optical coupling means, and the time until the light of the first path reaches the coupling means via the object to be measured. And a delay device (5) for delaying the light of the first path or the light of the second path so that the time is substantially equal.

  In the optical heterodyne interferometer according to claim 20 of the present invention, the wavelength-multiplexed light obtained by wavelength-multiplexing the frequency-variable measurement light and the reference light having a fixed frequency different from the optical frequency of the measurement light, An optical branching means (4) for branching the multiplexed light and outputting the light of the first path and the light of the second path, wherein the light of the first path and the light of the second path are measured light components, respectively. And the reference light component, the frequency of the measurement light component included in the light of the first path is different from the frequency of the measurement light component included in the light of the second path, and included in the light of the first path The light branching means (4) in which the frequency of the reference light component and the frequency of the reference light component included in the light of the second path are different from each other, and the light of the first path passing through the device under test and the device under test Combines the light of the second path that does not pass through A Mach-Zehnder interferometer (20) having a coupling means (6), an optical demultiplexer (7) for demultiplexing the measurement light component and the reference light component, and demultiplexed by the optical demultiplexer A first light receiver (8) for converting a first interference light composed of a measurement light component of the light of the first path and a measurement light component of the light of the second path into a first electric signal; A second interfering light that is demultiplexed by the optical demultiplexer and composed of the reference light component of the light of the first path and the reference light component of the light of the second path is converted into a second electric signal. Receiver (9), a synchronous detector (11) for detecting the phase of the first electric signal using the second electric signal as a synchronous signal, and the synchronous detection corresponding to the optical frequency of the measurement light And processing means (19) for processing the phase signal output from the measuring device to obtain the optical frequency-phase characteristic of the object to be measured. In the optical heterodyne interferometer, the processing means has an optical frequency table (19c) that holds the relationship between the variable elapsed time from the variable start to the end and the frequency of the measurement light that is variably entered. .

  Further, in the optical heterodyne interference device according to claim 21 of the present invention, in the optical heterodyne interference device according to claim 20, the optical frequency table indicates the relationship between the variable elapsed time and the optical frequency at equal optical frequency intervals. keeping.

  The optical heterodyne interferometer according to claim 22 of the present invention is the optical heterodyne interferometer according to claim 20 or 21, wherein the Mach-Zehnder interferometer is further provided between the optical branching means and the optical coupling means. Furthermore, the time until the light of the second path emitted from the light branching means reaches the optical coupling means, and the light of the first path reaches the optical coupling means via the object to be measured. A delay device (5) for delaying the light of the first path or the light of the second path so that the time until the time becomes approximately equal.

  The optical heterodyne interferometer according to claim 23 of the present invention is the optical heterodyne interferometer according to any of claims 20 to 22, wherein the Mach-Zehnder interferometer includes an optical fiber in its optical path. An optical fiber interferometer.

  Further, in the optical heterodyne interference device according to claim 24 of the present invention, in the optical heterodyne interference device according to any one of claims 17 to 19 described above, the measurement light variable in frequency within a predetermined frequency range is sequentially oscillated, A wavelength tunable light source (1) that is emitted as the input light to the light branching means, and a control means (13) that outputs a control signal for changing the frequency of the measurement light from the wavelength tunable light source.

  In the optical heterodyne interferometer according to claim 25 of the present invention, in the optical heterodyne interferometer according to any one of claims 20 to 23 described above, a wavelength for sequentially oscillating the measurement light whose frequency is variable within a predetermined frequency range. A variable light source (1), a control means (13) for outputting a control signal for changing the frequency of the measurement light from the wavelength variable light source, and the reference light having a fixed frequency different from the frequency of the measurement light is oscillated. The light source (2) to be multiplexed, the measurement light incident from the wavelength tunable light source and the reference light incident from the light source are multiplexed to multiplex the wavelength-multiplexed light obtained by the optical branching And a second optical coupler (3) that emits the incident light to the means.

  In the optical heterodyne interferometer according to the first aspect of the present invention, an equal optical frequency interval trigger having a predetermined equal optical frequency interval is generated in response to a change in the frequency of the measurement light, and a phase signal is generated based on the equal optical frequency interval trigger. Therefore, the detected phase signal (each phase data) can be accurately associated with the frequency of the measurement light, and the measurement accuracy of the optical frequency-phase characteristic can be improved.

  In the optical heterodyne interferometers according to claims 2 and 4 of the present invention, the two optical path lengths of the Mach-Zehnder interferometer are matched by the delay unit, so that the phase measurement accuracy can be improved.

  In the optical heterodyne interferometer according to the third aspect of the present invention, an equal optical frequency interval trigger having a predetermined equal optical frequency interval is generated in response to a change in the frequency of the measurement light, and a phase signal is generated based on the equal optical frequency interval trigger. Therefore, the detected phase signal (each phase data) can be accurately associated with the frequency of the measurement light, and the measurement accuracy of the optical frequency-phase characteristic can be improved. In addition, the measurement light and the reference light are wavelength-multiplexed and simultaneously incident on the Mach-Zehnder interferometer so that the phase fluctuation in the measurement light caused by the instability of the interferometer is canceled by the phase fluctuation in the reference light. Even if the Mach-Zehnder interferometer is an optical fiber interferometer configured to include an optical fiber in its optical path, it is possible to improve the phase fluctuation due to fluctuations caused by temperature change and vibration of the optical fiber. .

  In the optical heterodyne interferometer according to claim 5 of the present invention, since the Mach-Zehnder interferometer is an optical fiber interferometer that includes an optical fiber in its optical path, an interferometer in which the optical path is a space is used. Compared to this, the apparatus can be made smaller and more economical.

  In the optical heterodyne interference apparatus according to claims 8 and 9 of the present invention, a reference optical frequency trigger generated at a predetermined reference optical frequency corresponding to a change in the frequency of the measurement light is received, and based on the reference optical frequency trigger. Since phase signal data processing is performed, the detected phase signal (each phase data) and the frequency of the measurement light can be accurately associated with the absolute optical frequency, and the measurement accuracy of the optical frequency-phase characteristic is improved. Can be planned.

  In the optical heterodyne interference apparatus according to claims 10 and 11 of the present invention, a reference optical frequency trigger generated at a predetermined reference optical frequency corresponding to a change in the frequency of the measurement light is received, Since the absolute optical frequency of the equal optical frequency interval trigger is corrected from the relative time relationship with the frequency interval trigger, the correspondence between the detected phase signal (each phase data) and the frequency of the measurement light is the absolute optical frequency. On the other hand, the accuracy can be improved and the measurement accuracy of the optical frequency-phase characteristic can be improved.

  In the optical heterodyne interference apparatus according to claims 17 and 18 of the present invention, the optical frequency that maintains the relationship between the variable elapsed time from the start to the end of the variable specified by the control signal and the frequency of the measurement light that is variably entered. Since the phase signal data processing is performed based on the optical frequency table, the correlation between the detected phase signal (each phase data) and the frequency of the measurement light can be made with high accuracy. The measurement accuracy of the phase characteristics can be improved.

  In the optical heterodyne interferometers according to claims 19 and 22 of the present invention, the two optical path lengths of the Mach-Zehnder interferometer are matched by the delay unit, so that the phase measurement accuracy can be increased.

  In the optical heterodyne interferometers according to claims 20 and 21 of the present invention, the optical frequency that maintains the relationship between the variable elapsed time from the start to the end of the variable specified by the control signal and the frequency of the measurement light that is variably entered. Since the phase signal data processing is performed based on the optical frequency table, the correlation between the detected phase signal (each phase data) and the frequency of the measurement light can be made with high accuracy. The measurement accuracy of the phase characteristics can be improved. In addition, the measurement light and the reference light are wavelength-multiplexed and simultaneously incident on the Mach-Zehnder interferometer so that the phase fluctuation in the measurement light caused by the instability of the interferometer is canceled by the phase fluctuation in the reference light. Even if the Mach-Zehnder interferometer is an optical fiber interferometer configured to include an optical fiber in its optical path, it is possible to improve the phase fluctuation due to fluctuations caused by temperature change and vibration of the optical fiber. .

  In the optical heterodyne interferometer according to claim 23 of the present invention, since the Mach-Zehnder interferometer is an optical fiber interferometer including an optical fiber in its optical path, an interferometer in which the optical path is a space is used. Compared to this, the apparatus can be made smaller and more economical.

  Embodiments of the present invention will be described below.

[First Embodiment]
The configuration of the optical heterodyne interference apparatus according to the first embodiment of the present invention is shown in FIG. The same elements as those of the conventional optical heterodyne interference device are denoted by the same reference numerals, and detailed description thereof is omitted. The wavelength tunable light source 1 is, for example, a wavelength tunable laser, and generates measurement light (frequency ν) having a predetermined optical frequency range variable based on a control signal a input from the control unit 13 and emits it to the optical coupler 14. To do. For example, the frequency (wavelength) range of 196.5 THz (1525.26 nm) to 185.0 THz (1620.5 nm) is divided into 1000 points and can be varied in 1 sec. The optical coupler 14 divides the measurement light from the wavelength tunable light source 1 into two and outputs one to the optical coupler 3 and the other to the optical coupler 17. The optical coupler 17 branches the measurement light from the optical coupler 14 into two, one to the equal optical frequency interval trigger generating means 15 (described later) and the other to the reference optical frequency trigger generating means 18 (described later). Each is emitted. The light source 2 is, for example, a laser, and oscillates a fixed frequency different from the oscillation frequency of the wavelength tunable light source 1, for example, a reference light (frequency ν ₀ ) having a wavelength of 1625 nm, and emits it to the optical coupler 3. The optical coupler 3 combines the measurement light and the reference light and multiplexes them, and emits the WDM light to the AOFS 4b.

AOFS4b is driven by the ultrasonic frequency f ₁ input from the ultrasonic generator 4a, with branches the wavelength-multiplexed light incident from the optical coupler 3 into two light, heterodyne detection to the branched two optical For one of them, a wavelength-multiplexed light of 0th-order diffracted light is emitted to the optical coupler 6 as light of the first path through the object to be measured, and the other is Emits wavelength-multiplexed light of + _1st order diffracted light shifted by an ultrasonic frequency f by ₁ to the optical coupler 6 through the delay device 5 as light of the second path. The light of the first path and the light of the second path are each composed of a component (measurement light component) composed of measurement light and a component (reference light component) composed of reference light. In the first embodiment, the frequencies of the measurement light component and the reference light component in the light of the first path are ν and ν ₀ , respectively, and the measurement light component and the reference light component of the light of the second path are The frequencies are ν + f ₁ and ν ₀ + f ₁ , respectively. Then, they are multiplexed by the optical coupler 6, demultiplexed into the respective frequency components by the optical demultiplexer 7, and then subjected to heterodyne detection, respectively, so that an electrical signal of frequency f ₁ is output.

  The delay unit 5 is configured by, for example, an optical fiber delay line, and corresponds to the time when the light of the first path is delayed by the DUT 10 in order to match the optical path length of the Mach-Zehnder interferometer. The light is delayed by the time to be output to the optical coupler 6. Note that the delay unit 5 may be capable of varying the delay time, or may be provided in the first path. However, the delay unit 5 is effective in increasing the phase measurement accuracy by matching the optical path length of the interferometer, but is not necessarily essential. The optical coupler 6 multiplexes the first path light (wavelength multiplexed light) incident through the device under test 10 and the second path light (wavelength multiplexed light) incident through the delay device 5. Then, the interference light of the wavelength multiplexed light is emitted to the optical demultiplexer 7.

  The ultrasonic generator 4a and the AOFS 4b constitute the optical branching means 4, and the optical branching means 4, the DUT 10, the delay device 5 and the optical coupler 6 constitute a Mach-Zehnder interferometer 20. Therefore, the light branching means 4 is a means for branching the incident light into two in a Mach-Zehnder interferometer for branching the incident light into two and coupling (interfering) after passing through separate optical paths. , Means for coupling (interference) through separate optical paths. When the Mach-Zehnder interferometer 20 is an optical fiber interferometer that includes an optical fiber in its optical path, or when the delay unit 5 that matches the optical path length is an optical fiber delay line, the temperature changes. It is likely to cause phase fluctuations due to fluctuations in the optical fiber due to vibration and vibration. In order to improve this, the measurement light and the reference light are wavelength-multiplexed and simultaneously incident on the Mach-Zehnder interferometer 20, and phase fluctuations in the measurement light caused by instability of the interferometer during phase detection are detected. It is offset by fluctuations. The device under test 10 is an optical component such as an optical fiber or a dispersion compensator.

The optical demultiplexer 7 receives the first interference light of the measurement light components incident from the optical coupler 6 and the second interference light of the reference light components, and separates them into respective optical frequency (wavelength) components. The interference light between the measurement light components is emitted to the light receiver (PD) 8, and the interference light between the reference light components is emitted to the light receiver (PD) 9. The light receiver 8 receives the interference light between the measurement light components incident from the optical demultiplexer 7 and performs heterodyne detection, and outputs a beat signal having a frequency difference f ₁ between the light having the frequency ν + f _{1 and} the light having the frequency ν. 1 electrical signal. Similarly, the light receiver 9 receives the interference light between the reference light components incident from the optical demultiplexer 7 and performs heterodyne detection to convert the beat signal having the frequency difference f ₁ into a second electric signal. The RF lock-in amplifier 11 that is a synchronous detector receives the second electric signal as the synchronizing signal b, detects the phase of the first electric signal based on the synchronizing signal b, and outputs the phase signal (variable). Each phase data detected sequentially according to the frequency change of the measured light) is output to the processing means 16.

  The equal optical frequency interval trigger generating means 15 receives the variable measurement light from the optical coupler 17 and, as shown in FIG. An optical frequency interval trigger c is generated and output to the processing means 16. The equal optical frequency interval trigger c is used to obtain each phase data at equal optical frequency intervals when obtaining the optical frequency-phase characteristics by processing each phase data with the processing means 16 or each phase data. It is used when processing data and solves the conventional problem of data acquisition at regular time intervals. The equal optical frequency interval trigger generating means 15 includes an interference light generating means 15a, a light receiver (PD) 15b, and a pulse generating means 15c.

  The interference light generating means 15a is composed of, for example, an asymmetric Mach-Zehnder interferometer, and generates interference light whose light intensity periodically changes at a predetermined equal optical frequency interval Δν corresponding to the change in the frequency of incident measurement light. To do. The light receiver 15b receives the interference light and converts it into an electrical signal that changes periodically. The pulse generating means 15c detects the voltage that is the maximum or minimum of each of the periodically changing electric signals, and each time the respective maximum or minimum voltage or both are detected, a pulse (iso-optical frequency interval trigger) is detected. c) is generated and output. Specifically, the electric signal of the interference light is differentiated, and the pulse generator is driven using the differentiated signal as a trigger input. Alternatively, if the peak voltage of each electric signal of the interference light is substantially constant, the pulse generator is driven using the electric signal as a direct trigger input. It is also possible to generate the equal optical frequency interval trigger c at intervals smaller than the period of the interference light by phase-dividing the electrical signal from the light receiver 15b. The waveform of each part is shown to Fig.7 (a), (b), (c), (d). 7A shows the interference light, FIG. 7B shows the electrical signal of the interference light, FIG. 7C shows the differential waveform of the electrical signal, and FIG. 7D shows the pulse (iso-optical frequency interval trigger c). is there.

Here, an asymmetric Mach-Zehnder interferometer constituting the interference light generating means 15a will be described. FIG. 8A shows a schematic configuration, and FIG. 8B shows the light intensity of the emitted interference light. If the optical path difference between the two optical paths for splitting the incident light into two to cause interference is ΔL, the light intensity I of the interference light emitted from the interferometer is expressed by equation (1), and the incident light (variable) Is periodically changed at equal optical frequency intervals with respect to the change in the frequency of the measured measurement light), and the equal optical frequency interval Δν is given by equation (2). If ΔL = 2 cm and n = 1.5, the light intensity of the emitted interference light changes with a period of 10 GHz.
I = A + B · cos (2πν · n · ΔL / c) (1)
Δν = c / (n · ΔL) (2)
Here, ν is the optical frequency, n is the refractive index, c is the speed of light in vacuum, π is the circular ratio, and A and B are constants determined by the intensity of the incident light and the degree of coherence. Note that the optical frequency at which the above-mentioned equal optical frequency interval trigger c is output is determined by the relationship of the equation (1).

  The asymmetric Mach-Zehnder interferometer includes (a) an interferometer spatially assembled using optical components such as a beam splitter and a mirror, (b) an optical fiber interferometer using an optical fiber and an optical fiber coupler, and (c) a glass substrate. And (d) an interferometer using a crystal axis of a birefringent crystal or a polarization axis of a polarization-maintaining optical fiber, or the like. The asymmetric Mach-Zehnder interferometer controls the temperature of the entire interferometer or feedback control so that the fixed-frequency light is incident simultaneously with the variable incident light and the phase of the interference signal due to the fixed-frequency light is constant. By applying, it is possible to further stabilize the equal optical frequency interval Δν of the interference light.

  Next, the reference optical frequency trigger generation means 18 receives the changed measurement light from the optical coupler 17 and generates a reference optical frequency trigger d of a predetermined reference optical frequency corresponding to the change of the frequency of the measurement light. Output to the processing means 16. The reference optical frequency trigger d is configured such that when each phase data described above is processed by the processing means 16 to obtain an optical frequency-phase characteristic, the correspondence between each phase data and the frequency of the measurement light is relative to the absolute optical frequency. In order to improve the accuracy, each phase data is used in combination with the above-mentioned equal optical frequency interval trigger c when calculating the phase data, or the absolute optical frequency of the equal optical frequency interval trigger c is corrected (or calibrated). Used. The reference optical frequency trigger generating means 18 includes a frequency reference light generating means 18a, a light receiver (PD) 18b, and a pulse generating means 18c.

  The frequency reference light generating means 18a is composed of an absorption cell serving as a wavelength reference by enclosing atoms or molecules that cause saturated absorption with respect to light having a specific frequency, or a wavelength filter using a Fabry-Perot etalon. Corresponding to the change in the frequency of the measurement light, frequency reference light having a maximum or minimum light intensity at a predetermined reference light frequency is generated. The light receiver 18b receives the frequency reference light and converts it into an electrical signal. The pulse generating means 18c detects a voltage at which the electric signal is maximized or minimized, and generates and outputs a pulse (reference optical frequency trigger d) each time the detected maximum voltage or minimum voltage is detected. Specifically, the electric signal of the frequency reference light is differentiated, and the pulse generator is driven using the differentiated signal as a trigger input. Alternatively, the pulse generator is driven using the electrical signal of the frequency reference light as a direct trigger input. The waveform of each part is shown to Fig.9 (a), (b), (c), (d). 9A is the frequency reference light, FIG. 9B is the electrical signal of the frequency reference light, FIG. 9C is the differential waveform of the electrical signal, and FIG. 9D is the pulse (reference optical frequency trigger d). It is.

  The processing unit 16 includes an AD converter 16a and a calculation unit 16b, and receives a phase signal (each phase data sequentially detected according to a change in the frequency of the variable measurement light) input from the RF lock-in amplifier 11. Data associated with the control signal a input from the control unit 13, the equal optical frequency interval trigger c input from the equal optical frequency interval trigger generation unit 15, and the reference optical frequency trigger d input from the reference optical frequency trigger generation unit 18. Processing (conversion to digital values, various arithmetic processes, etc.) is performed, and physical quantities such as propagation delay time, chromatic dispersion, and temperature are obtained from the optical frequency-phase characteristics of the object to be measured. In this case, as shown in FIG. 10, the control signal a (specifies the variable start and end times, or the time of each point when variable is divided into a plurality of points), each phase data, AD The conversion timing, the equal optical frequency interval trigger c, and the reference optical frequency trigger d are processed with a common clock, for example, a measurement clock that determines variable start and end times, and their relative time relationships are managed. Then, the control means 13 generates a control signal a for changing the frequency of the measurement light and outputs it to the wavelength variable light source 1 and the processing means 16.

  Here, the correspondence between the acquisition of each phase data (in other words, each AD-converted phase data) and the variable frequency of the measurement light will be described. First, when the equal optical frequency interval trigger c output from the equal optical frequency interval trigger generating means 15 is input to the AD converter 16a (solid line in FIG. 1), every time the equal optical frequency interval trigger c is input. Since phase data is acquired at equal optical frequency intervals, the phase data can be accurately associated. Then, phase data between triggers (inter-frequency) are associated by calculation based on the relative time relationship between the equal optical frequency interval trigger c and the measurement clock. Next, when the iso-optical frequency interval trigger c is input to the arithmetic means 16b (broken line in FIG. 1), the timing of the measurement clock (or a clock synchronized with that) input as a trigger to the AD converter 16a Since the phase data is acquired, the phase data is correlated by calculation based on the relative time relationship between the measurement clock and the equal optical frequency interval trigger c.

  In addition, when using the reference optical frequency trigger d to associate each acquired phase data with the absolute optical frequency, or when correcting (or calibrating) the equal optical frequency interval trigger c to the accuracy of the absolute optical frequency, the reference optical frequency Correspondence or correction is performed by calculation based on the relative time relationship between the trigger d and the measurement clock and / or the equal optical frequency interval trigger c. If the equal optical frequency interval trigger generating means 15 is stable, the correction for the equal optical frequency interval trigger c does not need to be performed at every measurement.

  In the optical heterodyne interferometer of the first embodiment configured as described above, the equal optical frequency interval trigger generating means 15 generates the equal optical frequency interval trigger c at a predetermined equal optical frequency interval corresponding to the change in the frequency of the measurement light. The phase signal input from the RF lock-in amplifier 11 is converted into a digital value by this equal optical frequency interval trigger c, and the optical frequency-phase characteristic of the device under test is obtained from the digital value, or the measurement clock Since the optical frequency-phase characteristic of the object to be measured is obtained by performing arithmetic processing using the equal optical frequency interval trigger c after being converted into a digital value by means of the optical signal, the detected phase signal (each phase data) and the frequency of the measuring light And the optical frequency-phase characteristics are required with high accuracy. As a result, physical quantities such as propagation delay time, chromatic dispersion, and temperature based on optical frequency-phase characteristics are also accurately obtained.

Here, the case where the chromatic dispersion is obtained by calculating the difference of the group delay time from the phase characteristic will be described as an example of the improvement in accuracy. The phase signal input from the RF lock-in amplifier 11 to the AD converter 16a is converted into a digital value of the phase data φ (n) by the nth equal optical frequency interval trigger c (generated every equal optical frequency interval Δν). It is assumed that the data is converted and input to the calculation means 16b. When the variable start frequency of the measurement light is ν ₀ , the group delay time τ (n) at the frequency ν ₀ + nΔν is [τ (n) = (φ (n + 1) −φ (n)) / (2πΔν)]. Calculated. The chromatic dispersion is obtained by differentiating the group delay time τ (n) with respect to the wavelength. Therefore, since Δν (equal optical frequency interval) in this equation is accurate, chromatic dispersion is also obtained with high accuracy.

[Second Embodiment]
The configuration of the optical heterodyne interference apparatus according to the second embodiment of the present invention is shown in FIG. In the second embodiment, even if the Mach-Zehnder interferometer 20 is not an optical fiber interferometer, and even if it is an optical fiber interferometer, phase fluctuations due to fluctuations in the optical fiber can be ignored, and the phase in the measurement light using the reference light can be ignored. This is a configuration when it is not necessary to cancel out the fluctuations. It differs from FIG. 1 of the first embodiment only in the following (1) and (2), and the others are the same. Therefore, detailed description is omitted. (1) Elements related to oscillation, multiplexing, demultiplexing, and heterodyne detection of the reference light, that is, the light source 2, the optical coupler 3, the optical demultiplexer 7, and the light receiver 9 are not provided. (2) ultrasound frequency f ₁ output from the ultrasonic generator 4a is input as the synchronization signal b to the RF lock-in amplifier 11 as a synchronous detector.

  In the first and second embodiments described above, the interference light generating means 15a is configured by an asymmetric Mach-Zehnder interferometer. However, a Fabry-Perot interferometer, a Michelson interferometer, an orthogonal two-beam interferometer, or the like is used. May be. These interferometers will be described below. The Michelson interferometer has a different configuration from the asymmetric Mach-Zehnder interferometer, but the description of the optical intensity I of the interference light and the formula of the equal optical frequency interval Δν are omitted here.

First, the Fabry-Perot interferometer will be described. FIG. 11A shows the schematic configuration, and FIG. 11B shows the light intensity of the emitted interference light. Assuming that the interval between the partial reflectors is Δd, if the intensity reflectances of the two partial reflectors are equal and there is no loss in the reflectors, the light intensity I of the interference light emitted from the interferometer is (3 ) And periodically changes at equal optical frequency intervals with respect to changes in the frequency of incident light (variable measurement light), and the equal optical frequency interval Δν is given by equation (4). If ΔL = 2 cm and n = 1.5, the light intensity of the emitted interference light changes at a period of 5 GHz.
I = A / (1 + 4R · sin ² (2πν · n · Δd / c) / (1-R) ² )
(3)
Δν = c / (2 · n · Δd) (4)
Here, ν is the optical frequency, n is the refractive index, c is the speed of light in vacuum, π is the circumference, R is the intensity reflectance of the partial reflector, and A is the light intensity of the incident light. Note that the optical frequency at which the above-mentioned equal optical frequency interval trigger c is output is determined by the relationship of the equation (3). When generating the equal optical frequency interval trigger c using the interference light, the pulse generator is driven using the electrical signal of the interference light as a direct trigger input. When the finesse of the Fabry-Perot interferometer is small, the electric signal of the interference light may be differentiated and the pulse generator may be driven using the differentiated signal as a trigger input.

  The Fabry-Perot interferometer includes (a) an interferometer that is spatially assembled using optical components such as individual partial reflectors, and (b) a reflective film on both ends as necessary using a bulk type optical crystal. (C) an interferometer in which a reflection film is formed at both ends using an optical fiber, if necessary. (D) an optical waveguide is formed on a crystal substrate such as a glass substrate, and light is transmitted as necessary. An interferometer or the like in which a reflection film is formed at both ends of the waveguide can be used. The Fabry-Perot interferometer controls the temperature of the entire interferometer, or feedback control so that the fixed-frequency light is incident simultaneously with the variable incident light and the phase of the interference signal due to the fixed-frequency light is constant. By applying, it is possible to further stabilize the equal optical frequency interval Δν of the interference light.

Next, the orthogonal two-beam interferometer will be described. FIG. 12A shows a schematic configuration, and FIG. 12B shows the light intensity of emitted interference light. The incident light is divided into two linearly polarized beams whose vibration planes are orthogonal to each other, combined with an optical path difference to cause interference, the light intensity periodically changes with the change in the frequency of the incident light, and the change phase of the light intensity If an orthogonal two-beam interferometer that emits a plurality of light beams having different wavelengths is used to obtain two interference light beams whose phase change in light intensity is 90 degrees different from the change in the frequency of incident light, the interferometer emits light. The light intensities I ₁ and I ₂ of the interference light are expressed by equations (5) and (6), and periodically change at equal optical frequency intervals with respect to the change in the frequency of the incident light (variable measurement light). The equal optical frequency interval Δν is given by equation (7). If ΔL = 2 cm and n = 1.5, the light intensity of the emitted interference light changes with a period of 10 GHz.
I ₁ = A + B · cos (2πν · n · ΔL / c)
(5)
I ₂ = A ′ + B ′ · cos (2πν · n · ΔL / c + π / 2)
= A ′ + B ′ · sin (2πν · n · ΔL / c) (6)
Δν = c / (n · ΔL) (7)
Where ν is the optical frequency, n is the refractive index, c is the speed of light in vacuum, π is the circular index, ΔL is the optical path difference of the interferometer in the polarized light passing through the low refractive index axis of the λ / 4 plate, A, B, A ′ and B ′ are constants determined by the intensity of the incident light and the degree of coherence. I ₁ is the interference light intensity due to the polarization passing through the low refractive index axis of the λ / 4 plate, and I ₂ is the interference light intensity due to the polarization passing through the high refractive index axis of the λ / 4 plate.

Note that the optical frequency at which the above-mentioned equal optical frequency interval trigger c is output is determined by the relationship of the equations (5) and (6). When the equal optical frequency interval trigger c is generated using these two interference lights, these interference lights are converted into electrical signals to obtain two electrical signals having a phase change of 90 degrees. Then, by using these two electrical signals to obtain a signal that has been phase-divided by several tens to several hundreds by an interpolator, iso-light having a repetition interval shorter than the period of intensity change of the emitted light obtained from the interferometer A frequency interval trigger c can be generated. Further, when the orthogonal two-beam interferometer is used, a fine equal optical frequency interval trigger c can be obtained with a smaller optical path difference for use of the interpolator, and there is an advantage that the optical system can be made small. In addition, since two electrical signals having a phase change of 90 degrees are obtained, the direction of increase or decrease of the optical frequency can also be known. Also, as shown in FIGS. 12C and 12D, an orthogonal two-beam interferometer is configured to output interference lights I ₃ and I ₄ that are 180 degrees out of phase with respect to the interference lights I ₁ and I _2. Thus, even when the intensity of the incident light changes as the optical frequency changes, the equal frequency interval trigger c can be obtained accurately.

[Third Embodiment]
The configuration of the optical heterodyne interferometer according to the third embodiment of the present invention is shown in FIG. The third embodiment is the same as the conventional optical heterodyne interferometer shown in FIG. 14 except for the following points. That is, the processing means 19 (corresponding to the processing means 12 in FIG. 14) maintains the relationship between the variable elapsed time from the start to the end of the variable designated by the control signal a and the frequency of the measurement light that is variably entered. When the optical frequency table 19c is provided and the phase signal (each phase data) input from the RF lock-in amplifier 11 is processed to obtain the optical frequency-phase characteristic, the variable elapsed time and measurement of the optical frequency table 19c are measured. The calculation processing is performed using the relationship with the light frequency, the correspondence between each detected phase data and the frequency of the measurement light is improved, and the measurement accuracy of the optical frequency-phase characteristic is improved. . Therefore, the description of the same part as FIG. 14 is omitted, and the optical frequency table 19c will be mainly described.

  In the optical frequency table 19c, the relationship between the variable elapsed time of the control signal a and the frequency of the measurement light sequentially emitted from the wavelength tunable light source 1 as shown in FIG. Remember. At this time, the relationship between the variable elapsed time and the frequency of the measurement light may be stored at equal time intervals as shown in FIG. 13 (c), or the same light as shown in FIG. 13 (d). It may be stored at frequency intervals. In this case, measurement and confirmation of the frequency of the measurement light can be performed using a wavelength meter, the above-described absorption cell, a Fabry-Perot etalon, or the like. At the time of measurement, each phase data detected corresponding to the variable elapsed time as shown in FIG. 13B is associated with the optical frequency table 19c (FIG. 13A) with the variable elapsed time. Then, the frequency of the measurement light in each phase data is obtained. When each phase data is detected more finely than the interval of the variable elapsed time in the optical frequency table 19c (FIG. 13A), the interval is obtained by an interpolation method.

[Fourth Embodiment]
FIG. 4 shows the configuration of the optical heterodyne interferometer according to the fourth embodiment of the present invention. In the fourth embodiment, even if the Mach-Zehnder interferometer 20 is not an optical fiber interferometer, and even if it is an optical fiber interferometer, phase fluctuations due to fluctuations in the optical fiber can be ignored, and the phase in the measurement light using the reference light can be ignored. This is a configuration when it is not necessary to cancel out the fluctuations. It differs from FIG. 3 of 3rd Embodiment only in following (1) and (2), and others are the same. Therefore, detailed description is omitted. (1) Elements related to oscillation, multiplexing, demultiplexing, and heterodyne detection of the reference light, that is, the light source 2, the optical coupler 3, the optical demultiplexer 7, and the light receiver 9 are not provided. (2) ultrasound frequency f ₁ output from the ultrasonic generator 4a is input as the synchronization signal b to the RF lock-in amplifier 11 as a synchronous detector.

  In the first to fourth embodiments, the optical branching unit 4 is configured by one ultrasonic generator 4a and one AOFS 4b. However, the optical coupler and one or more ultrasonic generators and AOFS are used. It is also good. Further, if the AOFS disposed in each path of the Mach-Zehnder interferometer is driven using two ultrasonic generators having slightly different frequencies, the beat frequency at the time of heterodyne detection can be reduced. .

The figure which shows the structure of 1st Embodiment of this invention. The figure which shows the structure of 2nd Embodiment of this invention. The figure which shows the structure of 3rd Embodiment of this invention. The figure which shows the structure of 4th Embodiment of this invention. Diagram for explaining nonlinearity of variable wavelength light source when frequency is variable The figure for demonstrating the equal optical frequency space | interval of this invention The figure for demonstrating operation | movement of the equal optical frequency space | interval trigger generation means of this invention Diagram for explaining an asymmetric Mach-Zehnder interferometer The figure for demonstrating operation | movement of the reference | standard optical frequency trigger generation means of this invention The figure for demonstrating operation | movement of the processing means of this invention Diagram for explaining Fabry-Perot interferometer Diagram for explaining orthogonal two-beam interferometer The figure for demonstrating the optical frequency table of this invention The figure which shows schematic structure of a prior art example

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Variable wavelength light source, 2 ... Light source, 3, 6, 14, 17 ... Optical coupler, 4 ... Optical branching means, 4a ... Ultrasonic generator, 4b ... Acoustooptic Frequency shifter (AOFS), 5 ... delay device, 7 ... optical demultiplexer, 8, 9, 15b, 18b ... light receiver (PD), 10 ... device under test, 11 ... RF lock-in amplifier, 12, 16, 19 ... processing means, 12a, 16a, 19a ... AD converter (A / D), 12b, 16b, 19b ... computing means, 19c ... optical frequency Table, 13... Control means, 15... Optical frequency interval trigger generation means, 15 a... Interference light generation means, 15 c and 18 c. 18a ... frequency reference light generating means, 20 ... Mach-Zehnder interferometer

Claims (25)

Receives the variable frequency measurement light, which is incident light, and splits it into two, and also passes through the light branching means (4) that emits the light of the first path and the light of the second path having different frequencies, and the device under test. A Mach-Zehnder interferometer having optical coupling means (6) for combining the incident light of the first path and the incident light of the second path without passing through the device under test to emit interference light. (20) and
A light receiver (8) that receives the interference light and performs heterodyne detection to convert a beat signal of a frequency difference between the light of the first path and the light of the second path into an electrical signal;
A synchronous detector (11) for detecting the phase of the electrical signal based on a synchronous signal having a frequency substantially the same as the frequency of the beat signal;
Processing means (16) for receiving a phase signal output from the synchronous detector corresponding to the optical frequency of the measurement light and performing data processing on the phase signal to obtain an optical frequency-phase characteristic of the object to be measured; In an optical heterodyne interferometer comprising:
The processing means receives an equal optical frequency interval trigger generated at a predetermined equal optical frequency interval corresponding to the variable frequency of the measurement light, and performs data processing of the phase signal based on the equal optical frequency interval trigger. An optical heterodyne interferometer characterized in that:   The Mach-Zehnder interferometer further includes a time until the light of the second path emitted from the optical branching unit reaches the optical coupling unit between the optical branching unit and the optical coupling unit. A delay device (5) for delaying the light of the first path or the light of the second path so that the time until the light of the first path reaches the optical coupling means via the object to be measured is substantially equal. The optical heterodyne interference device according to claim 1, further comprising: The wavelength-multiplexed light obtained by wavelength-multiplexing the frequency-variable measurement light and the reference light having a fixed frequency different from the optical frequency of the measurement light is received, and the wavelength-division multiplexed light is branched. A light branching means (4) for outputting the light of the path, wherein the light of the first path and the light of the second path are each composed of a measurement light component and a reference light component; The frequency of the measurement light component included is different from the frequency of the measurement light component included in the light of the second path, and is included in the frequency of the reference light component included in the light of the first path and the light of the second path The light branching means (4) having the same frequency as the frequency of the reference light component and the light that combines the light of the first path that passes through the device under test and the light of the second path that does not pass through the device under test Mach-Zehnder interferometer (20) having coupling means (6) ,
An optical demultiplexer (7) for demultiplexing the measurement light component and the reference light component;
A first interference light that is demultiplexed by the optical demultiplexer and composed of the measurement light component of the light of the first path and the measurement light component of the light of the second path is converted into a first electric signal. 1 receiver (8),
A second interference light that is demultiplexed by the optical demultiplexer and composed of the reference light component of the light of the first path and the reference light component of the light of the second path is converted into a second electrical signal. Two receivers (9);
A synchronous detector (11) for performing phase detection of the first electrical signal using the second electrical signal as a synchronization signal;
Optical heterodyne interference comprising processing means (16) for processing the phase signal output from the synchronous detector corresponding to the optical frequency of the measurement light to obtain the optical frequency-phase characteristic of the object to be measured In the device
The processing means receives an equal optical frequency interval trigger generated at a predetermined equal optical frequency interval corresponding to the variable frequency of the measurement light, and performs data processing of the phase signal based on the equal optical frequency interval trigger. An optical heterodyne interferometer characterized in that:   The Mach-Zehnder interferometer further includes a time until the light of the second path emitted from the optical branching unit reaches the optical coupling unit between the optical branching unit and the optical coupling unit. A delay device (5) for delaying the light of the first path or the light of the second path so that the time until the light of the first path reaches the optical coupling means via the object to be measured is substantially equal. The optical heterodyne interference device according to claim 3.   5. The optical heterodyne interferometer according to claim 3, wherein the Mach-Zehnder interferometer is an optical fiber interferometer configured to include an optical fiber in an optical path thereof. A wavelength tunable light source (1) that sequentially oscillates measurement light having a variable frequency within a predetermined frequency range;
Control means (13) for outputting a control signal for varying the frequency of the measurement light from the wavelength tunable light source;
A first optical coupler (14) for branching the measurement light incident from the wavelength tunable light source into two and emitting one of the measurement light to the optical branching means as the incident light;
Upon receipt of the other measurement light emitted from the first optical coupler, an equal optical frequency interval trigger having a predetermined equal optical frequency interval is generated in response to a change in the frequency of the measurement light and output to the processing means. The optical heterodyne interference device according to claim 1, further comprising: an equal optical frequency interval trigger generating means (15) that performs the same. A wavelength tunable light source (1) that sequentially oscillates measurement light having a variable frequency within a predetermined frequency range;
Control means (13) for outputting a control signal for varying the frequency of the measurement light from the wavelength tunable light source;
A light source (2) for oscillating the reference light having a fixed frequency different from the frequency of the measurement light;
A first optical coupler (14) for branching and emitting the measurement light incident from the wavelength tunable light source;
One of the measurement light emitted from the first optical coupler and the reference light emitted from the light source are wavelength-multiplexed by multiplexing, and the obtained wavelength-multiplexed light is input to the optical branching unit. A second optical coupler (3) that emits as incident light;
An equal optical frequency interval which receives the other measurement light emitted from the first optical coupler, generates a predetermined equal optical frequency interval trigger corresponding to a change in the frequency of the measurement light, and outputs it to the processing means 6. The optical heterodyne interference device according to claim 3, further comprising trigger generation means (15).   The processing means receives a reference optical frequency trigger generated at a predetermined reference optical frequency corresponding to a change in the frequency of the measurement light, and performs data processing of the phase signal based on the reference optical frequency trigger. The optical heterodyne interferometer according to claim 1 or 2.   The processing means receives a reference optical frequency trigger generated at a predetermined reference optical frequency corresponding to a change in the frequency of the measurement light, and performs data processing of the phase signal based on the reference optical frequency trigger. 6. The optical heterodyne interference device according to claim 3, wherein   The processing means receives a reference optical frequency trigger generated at a predetermined reference optical frequency corresponding to a change in the frequency of the measurement light, and a relative time relationship between the reference optical frequency trigger and the equal optical frequency interval trigger The optical heterodyne interference device according to claim 1, wherein the absolute optical frequency of the equal optical frequency interval trigger is corrected.   The processing means receives a reference optical frequency trigger generated at a predetermined reference optical frequency corresponding to a change in the frequency of the measurement light, and a relative time relationship between the reference optical frequency trigger and the equal optical frequency interval trigger The optical heterodyne interference device according to claim 3, wherein the absolute optical frequency of the equal optical frequency interval trigger is corrected. A wavelength tunable light source (1) that sequentially oscillates measurement light having a variable frequency within a predetermined frequency range;
Control means (13) for outputting a control signal for varying the frequency of the measurement light from the wavelength tunable light source;
A first optical coupler (14) for branching the measurement light incident from the wavelength tunable light source into two and emitting one to the optical branching means;
A third optical coupler (17) that receives the other measurement light incident from the first optical coupler, diverges it into two, and emits it;
Upon receipt of one of the measurement lights emitted from the third optical coupler, an equal optical frequency interval trigger having a predetermined equal optical frequency interval is generated corresponding to the change in the frequency of the measurement light and output to the processing means. An equal optical frequency interval trigger generating means (15),
A reference light that receives the other measurement light emitted from the third optical coupler, generates a reference frequency trigger of a predetermined reference light frequency corresponding to a change in the frequency of the measurement light, and outputs the reference frequency trigger to the processing means 11. The optical heterodyne interference device according to claim 8, further comprising frequency trigger generation means (18). A wavelength tunable light source (1) that sequentially oscillates measurement light having a variable frequency within a predetermined frequency range;
Control means (13) for outputting a control signal for varying the frequency of the measurement light from the wavelength tunable light source;
A light source (2) for oscillating the reference light having a fixed frequency different from the frequency of the measurement light;
A first optical coupler (14) for branching and emitting the measurement light incident from the wavelength tunable light source;
One of the measurement light emitted from the first optical coupler and the reference light emitted from the light source are wavelength-multiplexed by multiplexing, and the obtained wavelength-multiplexed light is transmitted to the optical branching unit before A second optical coupler (3) that emits light as incident light;
A third optical coupler (17) that receives the other measurement light emitted from the first optical coupler, diverges into two, and emits;
An equal optical frequency interval that receives one of the measurement lights emitted from the third optical coupler, generates a predetermined equal optical frequency interval trigger corresponding to a change in the frequency of the measurement light, and outputs it to the processing means Trigger generating means (15);
A reference light that receives the other measurement light emitted from the third optical coupler, generates a reference frequency trigger of a predetermined reference light frequency corresponding to a change in the frequency of the measurement light, and outputs the reference frequency trigger to the processing means The optical heterodyne interference device according to claim 9 or 11, further comprising frequency trigger generation means (18). The iso-optical frequency interval trigger generating means is
Trigger interference light whose output light intensity periodically changes at a predetermined equal optical frequency interval corresponding to the variable frequency of the measurement light incident from the first optical coupler or the third optical coupler. Generating interference light generating means (15a);
A third light receiver (15b) that receives the trigger interference light incident from the interference light generating means and converts the trigger interference light into the periodically changing third electrical signal;
Each time the local maximum voltage / minimum voltage of the periodically changing third electric signal input from the third light receiver is detected, and each time the local maximum voltage, the minimum voltage, or both are detected, the second electric signal is detected. 14. The first pulse generating means (15c) for generating one pulse and outputting the first pulse as the equal optical frequency interval trigger, wherein the first pulse generating means (15c) is provided. The optical heterodyne interference apparatus according to any one of the above. The iso-optical frequency interval trigger generating means is
Corresponding to the variable frequency of the measurement light incident from the first optical coupler or the third optical coupler, the output light intensity is relatively periodic with a predetermined phase difference at predetermined equal optical frequency intervals. Interference light generating means for generating a plurality of trigger interference lights that change into
A plurality of light receivers that respectively receive the plurality of trigger interference lights incident from the interference light generating means and convert them into the plurality of periodically changing electrical signals;
A plurality of electrical signals periodically inputted with the predetermined phase difference input from the plurality of light receivers are used to generate a phase-divided third pulse, and the third pulse is converted into the iso-light. 14. The optical heterodyne interference device according to claim 6, further comprising third pulse generation means for outputting as a frequency interval trigger. The reference optical frequency trigger generating means includes
Frequency reference light generating means (18a) for generating frequency reference light whose output light intensity is maximized or minimized at a predetermined reference light frequency corresponding to the change of the frequency of the measurement light incident from the third optical coupler. )When,
A fourth light receiver (18b) for receiving the frequency reference light incident from the frequency reference light generating means and converting it into a fourth electrical signal;
A maximum or minimum voltage of the fourth electric signal input from the fourth light receiver is detected, and a second pulse is output as the reference optical frequency trigger each time the maximum or minimum voltage is detected. 14. The optical heterodyne interference device according to claim 12, further comprising two pulse generating means (18c). Receives the variable frequency measurement light, which is incident light, and splits it into two, and also passes through the light branching means (4) that emits the light of the first path and the light of the second path having different frequencies, and the device under test. A Mach-Zehnder interferometer having optical coupling means (6) for combining the incident light of the first path and the incident light of the second path without passing through the device under test to emit interference light. (20) and
A light receiver (8) that receives the interference light and performs heterodyne detection to convert a beat signal of a frequency difference between the light of the first path and the light of the second path into an electrical signal;
A synchronous detector (11) for detecting the phase of the electrical signal based on a synchronous signal having a frequency substantially the same as the frequency of the beat signal;
Processing means (19) for receiving a phase signal output from the synchronous detector corresponding to the optical frequency of the measurement light and performing data processing on the phase signal to obtain an optical frequency-phase characteristic of the object to be measured; In an optical heterodyne interferometer comprising:
The optical heterodyne interferometer characterized in that the processing means has an optical frequency table (19c) that holds a relationship between a variable elapsed time from a variable start to an end and a frequency of the measurement light incident in a variable manner.   18. The optical heterodyne interference device according to claim 17, wherein the optical frequency table holds a relationship between the variable elapsed time and the frequency of the measurement light at equal optical frequency intervals.   The Mach-Zehnder interferometer further includes a time until the light of the second path emitted from the optical branching unit reaches the optical coupling unit between the optical branching unit and the optical coupling unit. A delay device (5) for delaying the light of the first path or the light of the second path so that the time until the light of the first path reaches the coupling means via the device under test is substantially equal. The optical heterodyne interferometer according to claim 17 or 18, further comprising: The wavelength-multiplexed light obtained by wavelength-multiplexing the frequency-variable measurement light and the reference light having a fixed frequency different from the optical frequency of the measurement light is received, and the wavelength-division multiplexed light is branched. A light branching means (4) for outputting the light of the path, wherein the light of the first path and the light of the second path are each composed of a measurement light component and a reference light component; The frequency of the measurement light component included is different from the frequency of the measurement light component included in the light of the second path, and is included in the frequency of the reference light component included in the light of the first path and the light of the second path The light branching means (4) having the same frequency as the frequency of the reference light component and the light that combines the light of the first path that passes through the device under test and the light of the second path that does not pass through the device under test Mach-Zehnder interferometer (20) having coupling means (6) ,
An optical demultiplexer (7) for demultiplexing the measurement light component and the reference light component;
A first interference light that is demultiplexed by the optical demultiplexer and composed of the measurement light component of the light of the first path and the measurement light component of the light of the second path is converted into a first electric signal. 1 receiver (8),
A second interference light that is demultiplexed by the optical demultiplexer and composed of the reference light component of the light of the first path and the reference light component of the light of the second path is converted into a second electrical signal. Two receivers (9);
A synchronous detector (11) for performing phase detection of the first electrical signal using the second electrical signal as a synchronization signal;
Optical heterodyne interference comprising processing means (19) for processing the phase signal output from the synchronous detector corresponding to the optical frequency of the measurement light to obtain the optical frequency-phase characteristic of the object to be measured In the device
The optical heterodyne interferometer characterized in that the processing means has an optical frequency table (19c) that holds a relationship between a variable elapsed time from a variable start to an end and a frequency of the measurement light incident in a variable manner.   21. The optical heterodyne interferometer according to claim 20, wherein the optical frequency table holds a relationship between the variable elapsed time and the optical frequency at equal optical frequency intervals.   The Mach-Zehnder interferometer further includes a time until the light of the second path emitted from the optical branching unit reaches the optical coupling unit between the optical branching unit and the optical coupling unit. A delay device (5) for delaying the light of the first path or the light of the second path so that the time until the light of the first path reaches the optical coupling means via the object to be measured is substantially equal. The optical heterodyne interference device according to claim 20 or 21, further comprising:   The optical heterodyne interferometer according to any one of claims 20 to 22, wherein the Mach-Zehnder interferometer is an optical fiber interferometer configured to include an optical fiber in its optical path. A wavelength tunable light source (1) that sequentially oscillates measurement light having a variable frequency within a predetermined frequency range and emits the input light to the optical branching unit;
20. The optical heterodyne interference device according to claim 17, further comprising control means (13) for outputting a control signal for changing the frequency of the measurement light from the wavelength tunable light source. . A wavelength tunable light source (1) that sequentially oscillates measurement light having a variable frequency within a predetermined frequency range;
Control means (13) for outputting a control signal for varying the frequency of the measurement light from the wavelength tunable light source;
A light source (2) for oscillating the reference light having a fixed frequency different from the frequency of the measurement light;
Wavelength multiplexing is performed by combining the measurement light incident from the wavelength tunable light source and the reference light incident from the light source, and the obtained wavelength multiplexed light is emitted as the incident light to the optical branching unit. The optical heterodyne interference device according to any one of claims 20 to 23, further comprising a second optical coupler (3).

Images