JP2017116451A - Brillouin frequency shift distribution measurement system and brillouin frequency shift distribution measurement method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide means of actualizing distribution measurement of a Brillouin frequency shift which is superior in spatial resolution with very simple device constitution, according to the present invention.SOLUTION: A Brillouin frequency shift distribution measurement system according to the present invention comprises: an optical comb generation part 1 which generates a coherent optical comb having a period T=1/Δf shorter than the lifetime of an acoustic phonon as a subject; a branch output part 11-1 which branches the optical comb into pump light and probe light respectively; a delay control part 3 which delays output time of at least one of the probe light and pump light input through the branch output part 11-1; an optical frequency part 2 which is arranged behind the delay control part 3, and repeatedly varies the optical frequency of at least one of the probe light and pump light; and a power measurement part 4 which measures power of the probe light each time the optical frequency is varied.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a Brillouin frequency shift distribution measuring system and a Brillouin frequency shift distribution measuring method.

  As a technique for measuring a Brillouin frequency shift distribution in an optical fiber or optical device with a spatial resolution of about mm or less, a Brillouin optical correlation region analysis method (hereinafter referred to as related technology) is known (for example, Non-Patent Document 1). ,reference.).

  In the related technology, both the probe light and the pump light having a frequency larger than the Brillouin frequency are frequency-modulated, and the modulation phase coincides only in a specific very small section such as an optical fiber to be measured. Is a method that enables measurement of Brillouin frequency shift with excellent spatial resolution by utilizing the fact that the frequency correlation between pump light and probe light is large.

K. Y. Song, Z .; He, and K. Hotate, Distributed strain measurement with millimeter-order spatial resolution based on Brillouin optical correlation analysis, OPTICS LETTER. 31, no. 17 September 1, 2006

  However, in the related art, the correlation between the frequencies of the pump light and the probe light is not completely zero except at the place where measurement is desired in the optical fiber. For this reason, in the related art, in addition to the correlation signal for detecting the target Brillouin frequency shift, a large percentage of signals are generated in places other than the place where measurement is desired, and various means for excluding this are required. Become. As a result, the configuration of the measuring instrument is extremely complicated and many components are required.

  In order to solve the above-described problems, an object of the present invention is to provide means for realizing Brillouin frequency shift distribution measurement excellent in spatial resolution with a very simple apparatus configuration.

  In order to achieve the above object, the present invention measures the Brillouin frequency shift distribution in an optical fiber or optical device with a spatial resolution of about mm or less. Measures the distribution of dopants and strains in relatively small-sized devices and structures such as optical integrated circuits and precision instruments, with high resolution.

Specifically, the Brillouin frequency shift distribution measuring system according to the present invention is:
An optical comb generator that generates a coherent optical comb having a period T = 1 / Δf (Δf: frequency interval of the optical comb) shorter than the lifetime of the acoustic phonon of the subject;
A branch output unit for branching the optical comb into pump light and probe light, and
A delay control unit that delays an output time of at least one of the probe light or the pump light input via the branch output unit;
An optical frequency shift unit that is arranged at a subsequent stage of the delay control unit and repeatedly changes the optical frequency of at least one of the probe light and the pump light;
A power measuring unit that measures the power of the probe light each time the optical frequency is changed.

Specifically, the Brillouin frequency shift distribution measuring method according to the present invention is:
An optical comb generation procedure for generating a coherent optical comb having a period T = 1 / Δf (Δf: optical comb frequency interval) shorter than the acoustic phonon lifetime of the subject;
A branch output procedure in which the branch output unit branches the optical comb into pump light and probe light, and
A delay control procedure for delaying an output time of at least one of the probe light and the pump light input via the branch output unit by a delay control unit;
The optical frequency shift is arranged at the subsequent stage of the delay control unit, and the optical frequency shift procedure for repeatedly changing the optical frequency of at least one of the probe light or the pump light,
A power measurement procedure for measuring the power of the probe light every time the optical frequency is changed;
I do.

  The above inventions can be combined as much as possible.

  According to the present invention, it is possible to provide means for realizing Brillouin frequency shift distribution measurement with excellent spatial resolution with a very simple apparatus configuration.

It is a figure which shows an example of the block diagram of the Brillouin frequency shift distribution system which concerns on this embodiment. It is a figure which shows an example of the block diagram of the optical frequency shift part which concerns on this embodiment. It is a figure which shows an example of the spectrum of probe light and pump light when the frequency shift which concerns on this embodiment is 0. It is a figure which shows an example of the spectrum of probe light and pump light in case the frequency shift which concerns on this embodiment is (DELTA) f / 4.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited to embodiment shown below. These embodiments are merely examples, and the present invention can be implemented in various modifications and improvements based on the knowledge of those skilled in the art. In the present specification and drawings, the same reference numerals denote the same components.

(Embodiment 1)
FIG. 1 shows an embodiment of the present invention. 1 is an optical comb generator, 2 is an optical frequency shift unit, 3 is a delay control unit, 4 is a power measurement unit, 5 is an optical circulator, and 6 is an optical fiber to be measured. The optical comb generator 1 literally generates an optical comb. An optical comb is an optical signal having a spectrum arranged at a constant frequency interval Δf, and is a coherent optical pulse train having a period T = 1 / Δf in the time domain. The pulse waveform of this frequency comb is thin in many cases. In this case, if the spectrum width is ΔF and the pulse width is Δt, the following relational expression is established.

  The optical frequency comb can be generated by, for example, a mode-locked laser. The reciprocal of the period or frequency interval of the optical comb used in the present invention needs to be shorter than the lifetime of the acoustic phonon that causes Brillouin scattering in the optical fiber 6 to be measured. Since the lifetime of an acoustic phonon in an optical fiber placed in a room temperature environment is about 20 ns, the frequency spacing of the optical comb needs to be 50 MHz or larger.

The length of the optical fiber 6 to be measured is selected under the restriction that the time required for the optical pulse to pass through the optical fiber 6 to be measured must be shorter than the period of the optical comb. That is, since the speed of the pulse propagating through the optical fiber is about 2 × 10 ⁸ m / s, the optical pulse requires 5 ns to travel 1 m. Assuming that the frequency interval of the optical comb is 50 MHz, the period is 20 ns, so the length of the optical fiber 6 to be measured must be within 4 m.

  Thus, although the length of the optical fiber measured by the present invention is limited, as described later, the gist of the present invention is to measure the range of about several meters with extremely high spatial resolution of about mm or less. In particular, for example, it is possible to measure the distribution of dopants and strains in relatively small-sized devices and structures, such as optical integrated circuits and precision instruments, with high resolution.

  In addition, it is assumed that the period of the optical comb used in the present embodiment is variable within a certain range. The necessary variable range is at most 1% of the period of the optical comb depending on the amount of Brillouin frequency shift to be measured, and can be realized by adjusting the resonator length of the mode-locked laser or the like.

  The optical comb from the optical comb generator 1 is branched into two at the branch output unit 11-1, and is input as pump light and probe light from opposite ends of the measured optical fiber 6, respectively. The optical frequency shift unit 2 is disposed in the probe light path. As will be apparent from the following description, the relative frequency of the pump light and the probe light is changed. You may be placed in

  The delay control unit 3 adjusts the relative delay between the pump light and the probe light, and adjusts the position where the two pulses collide in the optical fiber 6 to be measured to generate stimulated Brillouin scattering. By operating this, the collision position is scanned over the entire optical fiber, thereby realizing distribution measurement.

  From the point of view, it is natural that it may be placed in either the pump or probe light path. The power measuring unit 4 is for measuring the power of the probe light amplified by stimulated Brillouin scattering at the collision position.

  As an example, it is very easy to generate an optical comb having a Gaussian waveform with a spectrum width of 50 GHz and a period of about 10 ns (frequency interval 100 MHz) by a mode-locked laser. Since the optical comb has a bright line spectrum component having an interval of Δf, in this example, about 500 bright line spectra are included. If the power per bright line is 1 mW, the average signal power is 0.5 W. From (1), the pulse width is about 9 ps.

  Since the duty ratio of the pulse train is 1000, the peak power of the pulse is 500W. The non-linear interaction length of this peak power pulse in a standard single-mode optical fiber is about 2 m, which is approximately the same as the assumed optical fiber 6 to be measured. Therefore, the spectrum does not change dramatically due to nonlinear effects other than Brillouin scattering in the optical fiber 6 to be measured.

  In the above example, when the optical pulse width is 9 ps, the spatial spread of the pulse in the optical fiber is about 2 mm. When the pump light and the probe light collide with each other in the optical fiber, the range in which the stimulated Brillouin scattering efficiently occurs is about 1 mm, which is half of this, and if the optical comb of the above example is used, about 1 mm is obtained. Brillouin frequency shift can be measured as spatial resolution.

  What should be emphasized at this time is that, unlike the related technique using frequency modulation, since the optical pulse having a large extinction ratio is used in this embodiment, the stimulated Brillouin scattering is not caused except at the collision point of the pulse, that is, the measurement point of the Brillouin frequency shift. It does not happen at all.

  Therefore, the power of the probe light measured by the power measuring unit 4 is to reflect stimulated Brillouin scattering at a specific point (in this example, about 1 mm in width) in the optical fiber as it is. This means that there is no need for so-called unnecessary signal removing means required in the related art in the present embodiment, which is greatly related to the fact that the configuration of FIG. 1 is extremely simple and has few components.

  The optical frequency shift unit 2 is realized, for example, with a configuration as shown in FIG. 7 is a photoelectric conversion unit, 8 is a low-pass filter, 9 is a phase modulation unit, and 10 is a variable attenuation unit. A pulse from the optical comb is converted into an electric signal by the photoelectric conversion unit 7 and is shaped into a sine wave having a frequency Δf by the low-pass filter 8 to drive the phase modulation unit 9.

As shown in FIG. 2, by adjusting the delay of either the optical pulse or the sine wave, the input is made so that the portion with the largest slope of the sine wave matches the timing of the optical pulse. In this way, the optical pulse undergoes phase modulation proportional to time, so that its frequency spectrum shifts uniformly. The frequency change amount is given by the following relational expression (2).

Here, V is the amplitude of the driving voltage, and _Vπ is a voltage necessary for the phase modulation unit 9 to give a phase change of π. Thus, by adjusting the amplitude of the drive voltage V by the variable attenuating unit 10, the frequency shift amount of the optical pulse can be arbitrarily changed.

  In order to explain the operation of the present embodiment, FIGS. 3 and 4 respectively show the probe light and the pump light when the frequency shift given to the probe light is 0 and Δf / 4 in this embodiment. The spectrum is shown.

At this time, if the Brillouin frequency of the optical fiber 6 to be measured is Δf _B , the frequency Δf of the optical comb is adjusted so that the remainder when Δf _B is divided by Δf is about Δf / 4. That is, the following relational expression (3) is established.

は余りである。このためには、光コム発生部１はパルスの周波数を変更する機能を持つことが必要であるが、この時必要な可変量はおよそ次式（４）で表される。

N represents the quotient when Δf _B is divided by Δf,

Is the remainder. For this purpose, the optical comb generator 1 is required to have a function of changing the frequency of the pulse. At this time, the necessary variable amount is approximately expressed by the following equation (4).

  Since the typical value of the Brillouin frequency shift amount in the optical fiber is about 10 GHz, assuming that the frequency interval of the optical comb is 50 MHz, the required variable amount is 250 kHz. It is considered possible by adjusting the length of the instrument.

How the stimulated Brillouin scattering between the probe light and the pump light occurs when the frequency of the optical comb is set in this way will be described with reference to FIGS. As shown in FIG. 3, when the frequency difference between the probe light and the pump light is set equal by the optical frequency shift unit 2, the probe light frequency exists at a position obtained by subtracting the Brillouin frequency shift Δf _B from the pump light frequency. do not do.

In addition, there is no pump light frequency at a position obtained by subtracting the Brillouin frequency shift Δf _B from the probe light frequency. What is important at this time is that the gain bandwidth of stimulated Brillouin scattering is smaller than the spectral interval Δf of the optical comb because the period of the optical comb is set shorter than the lifetime of the acoustic phonon in the optical fiber. Therefore, in the spectral arrangement of FIG. 3, no stimulated Brillouin scattering occurs between the probe light and the pump light.

Next, as shown in FIG. 4, when the frequency difference between the probe light and the pump light is set to Δf / 4 by the optical frequency shift unit 2, the Brillouin frequency shift Δf _B is subtracted from the pump light frequency. There is a probe light frequency. Further, there is no pump light frequency at a position obtained by subtracting the Brillouin frequency shift Δf _B from the probe light frequency.

  Since this relationship holds for all pairs of emission line spectral components, stimulated Brillouin scattering using the probe light as Stokes light is induced between the pump light and the probe light is amplified throughout the spectrum.

  Similarly, when the frequency difference between the probe light and the pump light is set to Δf / 2, stimulated Brillouin scattering does not occur. When set to 3Δf / 4, stimulated Brillouin scattering using the probe light as anti-Stokes light is induced between the pump light and the entire spectrum is attenuated.

  In this way, if the power of the probe light is observed while changing the optical frequency of the probe light, the power repeatedly fluctuates every period Δf of the change amount of the optical frequency. If the amount of frequency shift giving this maximum or minimum is known, it is possible to measure at which position the Brillouin frequency shift is, with Δf as the measurement range.

  As described above, according to the present invention, the Brillouin frequency shift amount in the optical fiber can be measured with extremely high position resolution, with Δf being the measurement range.

  The present invention can be applied to the information communication industry.

1: optical comb generation unit 2: optical frequency shift unit 3: delay control unit 4: power measurement unit 5: optical circulator 6: optical fiber 7 to be measured: photoelectric conversion unit 8: low-pass filter 9: phase modulation unit 10: Variable attenuator 11-1, 11-2: branch output unit

Claims (2)

An optical comb generator that generates a coherent optical comb having a period T = 1 / Δf shorter than the acoustic phonon lifetime of the subject;
A branch output unit for branching the optical comb into pump light and probe light, and
A delay control unit that delays an output time of at least one of the probe light or the pump light input via the branch output unit;
An optical frequency shift unit that is arranged at a subsequent stage of the delay control unit and repeatedly changes the optical frequency of at least one of the probe light and the pump light;
A power measuring unit for measuring the power of the probe light every time the optical frequency is changed;
A Brillouin frequency shift distribution measuring system comprising:
Where Δf: optical comb frequency interval An optical comb generation procedure for generating a coherent optical comb having a period T = 1 / Δf shorter than the acoustic phonon lifetime of the subject;
A branch output procedure in which the branch output unit branches the optical comb into pump light and probe light, and
A delay control procedure for delaying an output time of at least one of the probe light and the pump light input via the branch output unit by a delay control unit;
The optical frequency shift unit is disposed at a subsequent stage of the delay control unit, and an optical frequency shift procedure for repeatedly changing the optical frequency of at least one of the probe light or the pump light,
A power measurement procedure for measuring the power of the probe light every time the optical frequency is changed;
A Brillouin frequency shift distribution measuring method characterized by:
Where Δf: optical comb frequency interval

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