JP2019203859A - Device and method for measuring brillouin frequency shift - Google Patents

Abstract

To provide a device and a method that can measure accurate Brillouin frequency shift with high spatial resolution even when the Brillouin frequency shift is larger than a frequency interval of an optical comb.SOLUTION: A device according to the present invention is a device that measures Brillouin frequency shift of probe light, and comprises: an optical comb generation unit that can generate first and second optical combs respectively having first and second frequency intervals; a branch unit that branches the optical combs into probe light and pump light; an optical frequency shift unit that shifts an optical frequency of input light; a delay unit that delays the phase of the input light; and a light receiving unit that measures light intensity to acquire a Brillouin gain spectrum. The device further includes a Brillouin frequency shift measuring unit that measures a frequency at which the peaks of gain match each other between the Brillouin gain spectra of the first and second optical combs, as Brillouin frequency shift.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an apparatus and method for measuring Brillouin frequency shift.

In stimulated Brillouin scattering that occurs when light propagates through an optical fiber, a planar optical waveguide (PLC), etc., the maximum Brillouin when the frequency of the probe light is smaller than the pump light frequency by the Brillouin frequency shift ν _BFS Gain occurs and the probe light is amplified. Here, the Brillouin frequency shift ν _BFS is the difference between the frequency of the incident light to the optical fiber, the PLC, etc. and the frequency of the probe light amplified by stimulated Brillouin scattering in the subject. Indicated.

Here, n _eff is an effective refractive index in a subject such as an optical fiber or PLC, _VA is a sound velocity in the subject, and λ is a wavelength of light. Since n _eff and V _A change depending on strain and temperature applied to the subject, the strain and temperature of the subject can be sensed by measuring the Brillouin frequency shift ν _BFS .

JP 2017-116451 A

K. Y. Song, Z. He, and K. Hotate, “Distributed strain measurement with millimeter-order spatial resolution based on Brillouin optical correlation domain analysis,” OPTICS LETTERS Vol. 31, No. 17, September 1, 2006

As one conventional technique for measuring the Brillouin frequency shift ν _BFS distribution of light propagating in a subject such as an optical fiber or an optical device with a spatial resolution of about cm or less, the Brillouin optical correlation region analysis method (hereinafter referred to as BOCDA) is used. Is known (see, for example, Non-Patent Document 1). BOCDA frequency modulates both the probe light and the pump light having a Brillouin frequency shift ν _BFS greater than the frequency of the probe light, and the modulation phase coincides only in a specific very small section in the subject, This is a method that enables measurement of the Brillouin frequency shift ν _BFS excellent in spatial resolution by utilizing the fact that the frequency correlation between the probe light and the pump light becomes large at the coincidence point.

However, in BOCDA, the frequency correlation between the probe light and the pump light is not completely zero except at the place where the coincidence point in the subject is to be measured. For this reason, in BOCDA, in addition to the correlation signal for detecting the Brillouin frequency shift ν _BFS , a signal generated at a place other than the place to be measured is generated in a large proportion. In order to exclude this, various means such as adding complex frequency modulation to the probe light and the pump light are required. As a result, in BOCDA, the apparatus configuration is complicated, and at the same time, the control of the apparatus at the time of measurement is complicated, which hinders the reduction of the apparatus price.

As another conventional technique for measuring the Brillouin frequency shift ν _BFS distribution in a subject with a spatial resolution of about cm or less, an optical pulse having a period T = 1 / Δf shorter than the acoustic phonon lifetime of the subject (light Comb is used as probe light and pump light (see, for example, Patent Document 1). The advantage of this method is that the stimulated Brillouin scattering does not occur at all other than the collision point of the two light pulses of the probe light and the pump light in the subject, so that the complex modulation means for the probe light and the pump light required for BOCDA Is no longer necessary.

However, in stimulated Brillouin scattering, a gain peak occurs when the frequency difference between the bright line spectral components of the probe light and the pump light matches the Brillouin frequency shift ν _BFS . Therefore, in this method, the Brillouin gain spectrum due to stimulated Brillouin scattering also has a plurality of gain peaks with Δf as the frequency interval. That is, the frequency ν _p at which the gain peak occurs with respect to the Brillouin frequency shift ν _{BFS of} light propagating in the subject is expressed by the following (formula 2), where m is an integer.
ν _p = ν _BFS + mΔf (Formula 2)

As shown in the above (Formula 2), in the method described in Patent Document 1, since a plurality of peaks are generated at intervals of Δf in the Brillouin gain spectrum, which of these gain peaks is a Brillouin frequency shift ν _BFS . You will not know if there is. Therefore, coverage of this technique is the Brillouin frequency shift [nu _BFS was limited to observing the Brillouin frequency shift [nu _BFS if less frequency interval Δf of the optical comb throughout a subject.

The present invention has been made in view of the above problems, even Brillouin frequency shift [nu _BFS is a larger than the frequency interval Δf of the optical comb throughout a subject, measuring the Brillouin frequency shift [nu _BFS It is an object of the present invention to provide an apparatus and method capable of performing the above.

  An apparatus according to an aspect of the present invention is an apparatus for measuring a Brillouin frequency shift of probe light amplified by stimulated Brillouin scattering by probe light and pump light propagating in opposite directions in a subject. An optical comb generator that generates an optical comb having a period shorter than the lifetime of the acoustic phonon, a branching unit that branches the optical comb into probe light and pump light, and at least one of the probe light and pump light An optical frequency shift unit that shifts the frequency, a delay unit that delays the phase of at least one of the probe light and the pump light, and a Brillouin intensity by measuring the intensity of the probe light amplified by stimulated Brillouin scattering in the subject A light receiving unit that obtains a gain spectrum, and the optical comb generation unit includes a first frequency. A first optical comb having an interval and a second optical comb having a second frequency interval different from the first frequency interval can be generated, respectively, and the apparatus generates the first optical comb. Using the Brillouin gain spectrum acquired at the light receiving unit and the Brillouin gain spectrum acquired at the light receiving unit using the second optical comb as a Brillouin frequency shift And a Brillouin frequency shift measuring unit.

  An apparatus according to another aspect of the present invention is further characterized by further comprising a polarization scrambler that makes the input pump light non-polarized.

  An apparatus according to still another aspect of the present invention further includes an optical amplifying unit that amplifies at least one of the probe light and the pump light.

  The method according to one aspect of the present invention uses an optical comb as probe light and pump light, and the probe light that is amplified by stimulated Brillouin scattering by the probe light and the pump light propagating in opposite directions in the subject. A method of measuring a frequency shift, the method comprising: obtaining a first Brillouin gain spectrum generated in the subject using a first optical comb of a first frequency interval; and the first frequency interval; Acquiring a second Brillouin gain spectrum generated in the subject using a second optical comb of a different second frequency interval, the acquired first Brillouin gain spectrum, and the acquired first Measuring as a Brillouin frequency shift the frequency at which the gain peaks match between the two Brillouin gain spectra; Characterized in that it comprises a.

  According to one aspect of the present invention, even when the Brillouin frequency shift is larger than the frequency interval Δf of the optical comb, an accurate Brillouin frequency shift can be measured with a high spatial resolution of about cm or less.

It is a figure which illustrates the composition of the Brillouin frequency shift measuring device concerning one embodiment of the present invention. 2 (a) and 2 (b) are diagrams showing the measurement principle in the Brillouin frequency shift measuring apparatus according to the present invention. FIGS. 3A and 3B are diagrams illustrating Brillouin gain spectra observed in the first and second measurements, respectively.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<Embodiment>
FIG. 1 illustrates the configuration of a Brillouin frequency shift measurement apparatus according to an embodiment of the present invention. In FIG. 1, an optical comb generator 10, an optical branching unit 20, an optical frequency shift unit 30, a delay unit 40, a polarization scrambler 50, first and second optical amplification units 61 and 62, 1 shows a Brillouin frequency shift measuring device including a circulator 70, a light receiving unit 80, and a Brillouin frequency shift measuring unit 90.

  As shown in FIG. 1, a subject 1 such as an optical fiber or PLC is connected between the first optical amplifying unit 61 and the circulator 70. Probe light is coupled to one end of the subject 1, and pump light is coupled to the other end opposite to the one end of the subject 1. Probe light is amplified by stimulated Brillouin scattering by probe light and pump light propagating in opposite directions in the subject.

  The optical comb generator 10 can generate an optical comb having a plurality of bright line spectral components arranged at a constant frequency interval Δf (= 1 / T). The optical comb generated by the optical comb generator 10 has a period T = 1 / Δf shorter than the acoustic phonon lifetime that causes stimulated Brillouin scattering in the subject 1. The optical comb generator 10 is configured to change the frequency interval Δf of the generated optical comb.

As shown in FIG. 1, the optical comb generator 10 includes a light source 11 that generates continuous light such as a laser diode, an intensity modulator 12 such as a LiNbO ₃ modulator, and a frequency that generates a pulse signal having a variable period. A variable pulse generator 13. The optical comb generator 10 can generate an optical comb by intensity-modulating continuous light from the light source 11 by an intensity modulator 12 driven by a pulse signal generated by the frequency variable pulse generator 13. By changing the period of the pulse signal generated by the frequency variable pulse generator 13, the period T of the optical comb generated by the optical comb generator 10 can be changed. When the length of the subject 1 is L and the group velocity of the optical comb in the subject 1 is V _g , the period T of the optical comb is set to satisfy the following (Equation 3).
V _g T / 2> L (Formula 3)

  The optical branching unit 20 can branch the optical comb output from the optical comb generation unit 10 into two. One branch of the optical branching unit 20 is coupled to one end of the subject 1, and the other branch of the optical branching unit 20 is coupled to the other end provided on the side opposite to the one end of the subject 1.

  The optical frequency shift unit 30 can shift the optical frequency of the input light. By adjusting the shift amount of the optical frequency of the light input in the optical frequency shift unit 30 and changing the relative frequencies of the probe light and the pump light, the Brillouin gain spectrum can be acquired in the light receiving unit 80. .

  As shown in FIG. 1, the optical frequency shift unit 30 includes, for example, a frequency variable sine wave generator 31 that generates a sine wave signal having a variable frequency, and an approximately Brillouin frequency shift corresponding to incident probe light. And a single sideband modulator (SSB modulator) 32 that provides a downshift on the order of 11 [GHz]. The optical frequency shift unit 30 can adjust the amount of downshift in the SSB modulator 32 by adjusting the frequency of the sine wave signal generated by the frequency variable sine wave generator 31. In the example shown in FIG. 1, the optical frequency shift unit 30 is arranged in the probe light path, but changes the relative frequency of the probe light and the pump light. It may be arranged in both paths.

  The delay unit 40 can give a predetermined delay to the phase of the input light. Since the relative phase of the probe light and the pump light can be adjusted by adjusting the delay amount of the light in the delay unit 40, the optical pulse of the probe light and the pump light in the subject 1 collides to induce Brillouin. It is possible to adjust the position where scattering occurs. By manipulating the delay amount of the incident light in the delay unit 40 and changing the collision position of the probe light and the pump light over the entire subject 1, it is possible to measure the distribution of the Brillouin frequency shift in the entire subject 100. From that point of view, it is obvious that the delay unit 40 may be arranged in either or both of the probe light and the pump light.

  The polarization scrambler 50 makes the input light non-polarized by making the polarization state of the input light random. That is, since the light output from the polarization scrambler 50 includes all polarization states equally, it always includes polarization that matches the polarization state of the probe light. Therefore, it is possible to obtain a sufficient beat signal intensity for the probe light amplified by the stimulated Brillouin scattering by the probe light and the pump light in the subject 1.

  The first and second optical amplifying units 61 and 62 can control the intensity of the Brillouin gain generated in the subject 1 by amplifying the input light, respectively. From that point of view, it is obvious that the first and second light amplifying units 61 and 62 may be arranged in either one of the probe light and the pump light.

  The circulator 70 outputs the input pump light to the subject 1, and outputs the probe light amplified by stimulated Brillouin scattering at the collision position of the probe light and the light pulse of the pump light to the light receiver 80. .

  The light receiving unit 80 receives the probe light output from the circulator 70 and amplified by stimulated Brillouin scattering at the collision position of the light pulse of the probe light and the pump light in the subject 1 and measures the light intensity. As described above, the Brillouin gain spectrum is obtained in the light receiving unit 80 by adjusting the shift amount of the optical frequency of the light input in the optical frequency shift unit 30 and changing the relative frequencies of the probe light and the pump light. be able to.

  The Brillouin frequency shift measuring unit 90 obtains a gain between the Brillouin gain spectrum obtained using the first optical comb having the frequency interval Δf1 and the Brillouin gain spectrum obtained using the second optical comb having the frequency interval Δf2. The frequency at which the peaks coincide is measured as the Brillouin frequency shift.

  Hereinafter, the optical signal processing in the Brillouin frequency shift measurement apparatus according to an embodiment of the present invention will be described using the configuration shown in FIG. 1 as an example.

  The optical comb having the frequency interval Δf generated by the optical comb generator 10 is branched into two at the optical branching unit 20, one of which is input to the optical frequency shift unit 30 as probe light and the other is input to the delay unit 40 as pump light. The

  The probe light input to the optical frequency shift unit 30 is downshifted by about 11 [GHz] corresponding to the Brillouin frequency shift in the optical frequency shift unit 30 and input to the first optical amplification unit 61. 1 is amplified by one optical amplification unit 61 and input to one end of the subject 1.

  The pump light input to the delay unit 40 is given a predetermined delay in the delay unit 40 and input to the polarization scrambler 50. The pump light input to the polarization scrambler 50 is depolarized in the polarization scrambler 50, input to the second optical amplification unit 62, amplified in the second optical amplification unit 62, and circulator 70. To the other end of the subject 1.

  The probe light input to one end of the subject 1 collides with the pump light input from the other end of the subject 1 in the subject 1 and is amplified by stimulated Brillouin scattering. The probe light amplified by the stimulated Brillouin scattering is input to the light receiving unit 80 via the circulator 70, and the light intensity is measured in the light receiving unit 80.

FIG. 2 shows the measurement principle in the Brillouin frequency shift measuring apparatus according to the present invention. In the first measurement shown in FIG. 2A, the Brillouin gain spectrum is measured using the optical comb having the frequency interval Δf ₁ (= 1 / T ₁ ) generated by the optical comb generator 10 as the probe light and the pump light. I do. The Brillouin gain spectrum also has a plurality of peaks with a frequency interval Δf ₁ (= 1 / T ₁ ). At this time, the frequency at which the gain peak occurs in the Brillouin gain spectrum is given by the following (formula 4), where m is an arbitrary integer.
ν _p = ν _BFS + mΔf ₁ (Formula 4)

In the present invention, as shown in FIG. 2B, an optical comb having a frequency interval Δf ₂ (= 1 / T ₂ ) different from the frequency interval Δf ₁ (= 1 / T ₁ ) used in the first measurement. Is used as the probe light and the pump light. Similar to the first measurement, the frequency of the gain peak of the Brillouin gain spectrum obtained at this time is given by the following (formula 5).
ν _p = ν _BFS + mΔf ₂ (Formula 5)

As is clear from the above (Formula 4) and (Formula 5), the gain peaks of both coincide only when m = 0, and it is clear that the frequency at this time gives the Brillouin frequency shift ν _BFS . Therefore, each Brillouin gain spectrum is obtained by performing the first and second measurements using optical combs having frequency intervals Δf ₁ and Δf ₂ , respectively, and a plurality of gains are obtained by finding a gain peak having a frequency coincident with each other. From the peak frequency, the Brillouin frequency shift ν _BFS can be found. That is, the frequency at which the peaks match between the two Brillouin gain spectra among the plurality of peaks in each of the two Brillouin gain spectra measured by the light receiving unit 80 is measured as the Brillouin frequency shift ν _BFS .

As described above, according to the present invention, since the Brillouin frequency shift ν _BFS can be clearly found from a plurality of peaks in the Brillouin gain spectrum, the Brillouin frequency shift ν _BFS is _transmitted through the entire subject 1 to the frequency of the optical comb. Even when the distance is larger than Δf, the Brillouin frequency shift ν _BFS can be measured.

  Hereinafter, the method for measuring the Brillouin frequency shift according to an embodiment of the present invention will be described using an example in which an optical fiber is used as the subject 1.

Since the acoustic phonon lifetime in the optical fiber as the subject 1 is approximately 20 ns, the frequency interval Δf of the optical comb generated by the optical comb generator 10 needs to be set to be larger than about 50 MHz. Since the group velocity V _g of the pulses propagating through the optical fiber is about 2 × 10 8 ^{m /} s, the length L of the optical fiber as subject 1 from the above equation (3) is limited to below about 2m.

FIG. 3 illustrates the Brillouin gain spectrum observed in the first and second measurements, respectively. When selecting the two pulse frequencies Δf ₁ = 1 / T ₁ and Δf ₂ = 1 / T ₂ , it is necessary to pay attention to the Brillouin gain spectrum spread Δν in the light propagating through the optical fiber as the subject 1 There is. That is, the waveform of the Brillouin gain spectrum has a Δν spread as shown in FIG. 3, but Δν is determined by the reciprocal of the lifetime of the acoustic phonon in the optical fiber as the subject 1, and is typically about 30 MHz. is there. In order to clearly distinguish which peaks match and which peaks do not match between the first and second measurements, Δf ₁ and Δf ₂ need to be approximately different by about Δν. In consideration of the above conditions, Δf ₁ = 50 MHz and Δf ₂ = 80 MHz satisfy all these conditions as an example when an optical fiber is used as the subject 1.

The gist of the present invention is to measure the Brillouin frequency shift ν _BFS distribution with a very high spatial resolution of about cm or less in the range of the length of the subject 1 of about 2 m as described above. Therefore, according to the present invention, there is an advantage that the distribution of dopants and strains in devices and structures having relatively small dimensions such as optical integrated circuits and precision instruments can be measured with high resolution. The present invention is applicable to, for example, health monitoring of structures.

Claims (4)

An apparatus for measuring a Brillouin frequency shift of probe light amplified by stimulated Brillouin scattering by probe light and pump light propagating in opposite directions in a subject,
An optical comb generator that generates an optical comb having a period shorter than the lifetime of the acoustic phonon of the subject;
A branching portion for branching the optical comb into probe light and pump light, and
An optical frequency shift unit that shifts an optical frequency of at least one of the probe light and the pump light;
A delay unit that delays a phase of at least one of the probe light and the pump light;
A light receiving unit that obtains a Brillouin gain spectrum by measuring the intensity of probe light amplified by stimulated Brillouin scattering in the subject, and
The optical comb generator is configured to generate a first optical comb having a first frequency interval and a second optical comb having a second frequency interval different from the first frequency interval,
The device is
The gain peak matches between the Brillouin gain spectrum acquired in the light receiving unit using the first optical comb and the Brillouin gain spectrum acquired in the light receiving unit using the second optical comb. An apparatus further comprising a Brillouin frequency shift measurement unit for measuring a frequency as a Brillouin frequency shift.   The apparatus according to claim 1, further comprising a polarization scrambler that makes the input pump light unpolarized.   The apparatus according to claim 1, further comprising an optical amplification unit that amplifies at least one of the probe light and the pump light. A method for measuring a Brillouin frequency shift of probe light amplified by stimulated Brillouin scattering by the probe light and the pump light propagating in opposite directions in a subject using an optical comb as probe light and pump light,
Obtaining a first Brillouin gain spectrum generated in the subject using a first optical comb of a first frequency interval;
Obtaining a second Brillouin gain spectrum generated in the subject using a second optical comb of a second frequency interval different from the first frequency interval;
Measuring as a Brillouin frequency shift a frequency at which a gain peak coincides between the acquired first Brillouin gain spectrum and the acquired second Brillouin gain spectrum;
A method comprising the steps of:

Images