JP7454183B2 - Spectrometer and spectrometer measurement method - Google Patents

Description

The present disclosure relates to a spectrometer and method using an optical frequency comb.

A method has been proposed in which one of two optical frequency combs is used as a probe light and the other as a local light to perform broadband and highly accurate spectroscopic measurements (see, for example, Non-Patent Document 1).

Coddington, Ian, Nathan Newbury, and William Swann. “Dual-comb spectroscopy.” Optica 3.4 (2016): 414-426.

In the method of performing spectroscopic measurements using two optical frequency combs, the wider the optical frequency comb, the smaller the optical frequency fluctuation between the two optical frequency combs is required. Note that the conditions required for optical frequency fluctuation are expressed by Equation 1. Therefore, in Non-Patent Document 1, broadband measurement is performed by synchronizing two optical frequency combs. As described above, asynchronous spectroscopic measurements in which two optical frequency combs are not synchronized have a problem in that wideband measurements are difficult due to frequency fluctuations of the optical frequency combs.

SUMMARY OF THE INVENTION Therefore, in order to solve the above problems, it is an object of the present invention to provide a spectrometer and a spectrometer measurement method that can perform broadband measurements even when two optical frequency combs are asynchronous.

In order to achieve the above object, the spectrometer according to the present invention cuts out some bands from two asynchronous optical frequency combs, performs spectroscopic measurements for each band, and converts the measurement results to a predetermined optical frequency. We decided to connect them.

Specifically, the spectrometer according to the present invention includes:
Two optical frequency combs whose repetition frequencies are different from each other,
a bandpass filter that cuts out a band with the same center optical frequency wavelength from each of the optical combs, and uses one as a probe light and the other as a sampling light;
an impulse response measurement circuit that generates a first beat signal of the sampling light and the probe light that has passed through the optical fiber to be measured;
a continuous light source that outputs continuous light;
a reference frequency measurement circuit that generates a second beat signal of the continuous light and the probe light that has passed through the optical fiber to be measured, and a third beat signal of the continuous light and the sampling light;
changing the center optical frequency set in the bandpass filter;
causing the impulse response measurement circuit to generate the first beat signal for each of the bands having different center optical frequencies;
The reference frequency measuring circuit is provided with the second beat signal and the third beat signal in the continuous light having an optical frequency between the center optical frequencies of the two bands, for each of the two bands having different center optical frequencies. generating a beat signal;
calculating a multiplication signal obtained by multiplying the complex conjugate of the second beat signal and the third beat signal for each of the two bands having different center optical frequencies;
For each of the two bands having different center optical frequencies, multiply the first beat signal by the complex conjugate of the multiplication signal, and convert the first beat signal into a waveform having the optical frequency of the continuous light as the origin. and connecting each of the waveforms at the origin,
A control circuit that performs
Equipped with.

Furthermore, the spectroscopic measurement method according to the present invention includes:
Two optical frequency combs whose repetition frequencies are different from each other,
a bandpass filter that cuts out a band with the same center optical frequency wavelength from each of the optical combs, and uses one as a probe light and the other as a sampling light;
an impulse response measurement circuit that generates a first beat signal of the sampling light and the probe light that has passed through the optical fiber to be measured;
a continuous light source that outputs continuous light;
a reference frequency measurement circuit that generates a second beat signal of the continuous light and the probe light that has passed through the optical fiber to be measured, and a third beat signal of the continuous light and the sampling light;
A spectroscopic measurement method performed by a spectrometry device comprising:
changing the center optical frequency set in the bandpass filter;
causing the impulse response measurement circuit to generate the first beat signal for each of the bands having different center optical frequencies;
The reference frequency measuring circuit is provided with the second beat signal and the third beat signal in the continuous light having an optical frequency between the center optical frequencies of the two bands, for each of the two bands having different center optical frequencies. generating a beat signal;
calculating a multiplication signal obtained by multiplying the complex conjugate of the second beat signal and the third beat signal for each of the two bands having different center optical frequencies;
For each of the two bands having different center optical frequencies, multiply the first beat signal by the complex conjugate of the multiplication signal, and convert the first beat signal into a waveform having the optical frequency of the continuous light as the origin. and connecting each of the waveforms at the origin,
It is characterized by

The spectrometer according to the present invention divides the bands of probe light and sampling light using a bandpass filter, sequentially performs measurements by changing the center wavelength of the bandpass filter and the CW light source wavelength (reference frequency), and then performs measurements for each band. The measurement results of the divided bands are connected using separately calculated optical frequencies.

Broadband measurement can be achieved by dividing the band of the optical frequency comb, performing spectroscopic measurements, and connecting the measurement results in the divided bands. By dividing the band, it is possible to relax the conditions required for the frequency fluctuation of the optical frequency comb required for a wide band. In other words, wideband measurement can be achieved without stabilizing frequency fluctuations. This spectrometer can ease the frequency fluctuation conditions of the optical frequency comb, and enables broadband measurements with two asynchronous optical frequency combs.
Furthermore, since this spectrometer can utilize an asynchronous optical frequency comb, it can perform measurements even in situations where the probe light and sampling light are installed far from each other (for example, on an optical transmission line).

Therefore, the present invention can provide a spectrometer and a spectrometer measurement method that can perform broadband measurements even when two optical frequency combs are asynchronous.

Note that the above inventions can be combined as much as possible.

The present invention can provide a spectrometer and a spectrometer measurement method that can perform broadband measurements even when two optical frequency combs are asynchronous.

FIG. 1 is a diagram illustrating a spectrometer according to the present invention. FIG. 3 is a diagram illustrating light bands used by the spectrometer according to the present invention. FIG. 3 is a diagram illustrating a spectroscopic measurement method according to the present invention. FIG. 2 is a diagram illustrating a spectroscopic measurement method according to the present invention. FIG. 1 is a diagram illustrating a spectrometer according to the present invention.

Embodiments of the invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components with the same reference numerals in this specification and the drawings indicate the same components.

FIG. 1 is a diagram illustrating a spectrometer 301 of this embodiment. The spectrometer 301 cuts out a predetermined band from the two optical frequency combs, and performs linear optical sampling measurement of the optical fiber 50 to be measured using the comb in the band. Then, the spectrometer 301 changes the band to be cut out and performs the linear light sampling measurement again. The spectrometer 301 repeats this an arbitrary number of times and connects the obtained measurement results to obtain broadband measurement results. The optical fiber 50 to be measured is, for example, a multi-core optical fiber, and includes a fan-in fan-out method in which optical combs are distributed to each core and optical combs from each core are multiplexed.

The specific configuration of the spectrometer 301 is as follows. The spectrometer 301 is
Two optical frequency combs (11-1, 11-2) whose repetition frequencies are different from each other,
bandpass filters (13-1, 13-2) that cut out a band with the same center optical frequency from each of the optical combs, and use one as probe light and the other as sampling light;
an impulse response measurement circuit 18 that generates a first beat signal of the sampling light and the probe light that has passed through the optical fiber 50 to be measured;
Continuous light sources (20-1, 20-2) that output continuous light;
a reference frequency measurement circuit 28 that generates a second beat signal of the continuous light and the probe light that has passed through the optical fiber 50 to be measured, and a third beat signal of the continuous light and the sampling light;
a control circuit 30 that controls these operations and calculates measurement results;
Equipped with.

It is assumed that the spectrum of the optical combs output by the optical frequency combs (11-1, 11-2) is spread over an equivalent wavelength range F. Since the impulse response of the optical fiber 50 to be measured is measured by linear optical sampling, the ideal condition is that both the probe light and the sampling light are Fourier transform limited pulses (the phase relationship of each longitudinal mode is flat). For this reason, the optical frequency combs (11-1, 11-2) are preferably passive mode-locked fiber lasers.

f _rep in FIG. 1 is the repetition frequency of the optical comb, which is approximately several MHz to several GHz in the case of a passive mode-locked fiber laser. Since equivalent time sampling is performed as linear optical sampling, the optical frequency combs (11-1, 11-2) output optical combs whose repetition frequencies are offset from each other by Δf. That is, the repetition frequency f _rep of the optical frequency comb 11-1 that outputs the probe light is set to f rep +Δf, whereas the repetition frequency of the optical frequency comb 11-2 that outputs the sampling light is f _rep +Δf. Note that the ratio between f _rep and Δf is approximately Δf/f _rep =1e ⁻⁵ .
Also, since the optical frequency combs (11-1, 11-2) are not locked to the reference frequency, the center frequency (absolute frequency) fluctuates, but the repetition frequency f _rep and the repetition frequency f _rep +Δf are stable. It is assumed that

With linear optical sampling, the optical frequency characteristics of the optical fiber to be measured can only be measured in a region where the spectra of two optical frequency combs overlap. Therefore, the center frequencies of BPFs (band pass filters) (13-1, 13-2) that cut out the probe light and the sampling light are the same.

The impulse response measurement circuit 18 includes an optical 90-degree hybrid 15 and an analog-to-digital converter 17. The reference frequency measurement circuit 28 includes an optical 90-degree hybrid (25-1 to 25-4) and an analog-to-digital converter 17.

The continuous light sources (20-1, 20-2) output continuous light at an optical frequency instructed by the control circuit 30. FIG. 2 is a diagram illustrating the relationship between the center frequency and band set for the BPF (13-1, 13-2), and the optical frequency of continuous light.
First, the continuous light source 20-1 outputs continuous light with a center optical frequency set to the BPF (13-1, 13-2) at the first time and a center light frequency set at the BPF (13-1, 13-2) at the second time. An optical frequency CW#1 between the set center optical frequency is set.
Then, the continuous light source 20-2 outputs continuous light with a center optical frequency set to BPF (13-1, 13-2) in the second time and a center light frequency set in BPF (13-1, 13-2) in the third time. An optical frequency CW#2 between the set center optical frequency is set.
Note that when the BPF (13-1, 13-2) cuts out only two bands (two set center optical frequencies (first and second)), there is only one continuous light source (continuous light Only one frequency is required.

Furthermore, when the center optical frequency is set to BPF (13-1, 13-2) four or more times, the continuous light source 20-1 outputs continuous light to BPF (13-1, 13-2) for the third time. ) and the center optical frequency set in BPF (13-1, 13-2) for the fourth time. The continuous light source 20-2 outputs continuous light with a center light frequency set to BPF (13-1, 13-2) at the fourth time and a center light frequency set to BPF (13-1, 13-2) at the fifth time. The optical frequency CW#4 is set between the center optical frequency and the center optical frequency. In this way, each time the center optical frequency is set in the BPF (13-1, 13-2), the continuous light source (20-1, 20-2) switches the optical frequency of the continuous light output.

It is desirable that the bands cut out by the BPFs (13-1, 13-2) partially overlap adjacent bands as shown in FIG. 2, and that the optical frequency of the continuous light is in a range where the bands overlap.

The optical branching unit 14-1 inputs the probe light that has passed through the optical fiber to be measured 50 to the 90-degree optical hybrid 15, the 90-degree optical hybrid 25-1, and the 90-degree optical hybrid 25-2.
The optical branching unit 14-2 inputs the sampling light to the 90-degree optical hybrid 15, the 90-degree optical hybrid 25-3, and the 90-degree optical hybrid 25-4.
The light branching unit 14-3 inputs the continuous light from the continuous light source 20-1 to the 90-degree optical hybrid 25-1 and the 90-degree optical hybrid 25-3.
The light branching unit 14-4 inputs the continuous light from the continuous light source 20-2 to the 90-degree optical hybrid 25-2 and the 90-degree optical hybrid 25-4.

である。 The impulse response measurement circuit 18 uses the sampling light obtained by cutting out the optical comb from the optical frequency comb 11-2 by the BPF 13-2, and the optical comb from the optical frequency comb 11-1 by cutting out the optical comb by the BPF 13-1 into the optical fiber to be measured. A first beat signal is generated with the probe light that has passed through 50. The impulse response measurement circuit 18 generates first beat signals for each band cut out by the BPF and for the number of propagation modes of the optical fiber 50 to be measured. Note that the first beat signal is expressed as follows.

It is.

である。 The reference frequency measurement circuit 28 uses the probe light which has cut out the optical comb from the optical frequency comb 11-1 by the BPF 13-1 and passed through the optical fiber 50 to be measured, and the continuous light from the continuous light sources (20-1, 20-2). Generate a second beat signal with light. Furthermore, the reference frequency measurement circuit 28 extracts the sampling light extracted from the optical comb 11-2 by the BPF 13-2 and the continuous light from the continuous light sources (20-1, 20-2). Generate a beat signal. The impulse response measurement circuit 18 generates a second beat signal and a third beat signal for each band cut out by the BPF. Note that the second beat signal and the third beat signal are expressed as follows.

It is.

The control circuit 30 adjusts the band cut out by the BPF (13-1, 13-2) and the optical frequency of the continuous light output by the continuous light source (20-1, 20-2), and measures the optical fiber 50 under test. conduct. In other words, the control circuit 30 sets the first center optical frequency in the BPF (13-1, 13-2), and sets the probe light of an arbitrary band from the optical frequency comb (11-1, 11-2). and cut out the sampling light. At the same time, the control circuit 30 causes the impulse response measurement circuit 18 to measure the impulse response using the probe light and the sampling light, and causes the reference frequency measurement circuit 28 to measure the reference frequency. Subsequently, the control circuit 30 sets the next center optical frequency in the BPF (13-1, 13-2), similarly causes the impulse response measurement circuit 18 to measure the impulse response, and causes the reference frequency measurement circuit 28 to set the reference frequency. have the students measure the The control circuit 30 sequentially measures this by changing the center optical frequency set in the BPF (13-1, 13-2) and the continuous light source (20-1, 20-2). Finally, the control circuit 30 connects the impulse response measurement results of all bands at the measured reference frequency.

FIG. 3 is a diagram illustrating the operation of the control circuit 30. The control circuit 30 is
changing the center optical frequency set in BPF (13-1, 13-2) (step S01);
causing the impulse response measurement circuit 18 to generate the first beat signal for each of the bands having different center optical frequencies (step S02);
The reference frequency measuring circuit 28 is provided with the second beat signal and the third beat signal in the continuous light having an optical frequency between the center optical frequencies of the two bands for each of the two bands having different center optical frequencies. generating a beat signal (step S03);
calculating a multiplication signal obtained by multiplying the complex conjugate of the second beat signal and the third beat signal for each of the two bands having different center optical frequencies (step S04);
For each of the two bands having different center optical frequencies, multiply the first beat signal by the complex conjugate of the multiplication signal, and convert the first beat signal into a waveform having the optical frequency of the continuous light as the origin. (step S05), and connecting each of the waveforms at the origin (step S06).
I do.

ここで、“ＸＹ^＊”と記載されている“^＊”は信号Ｙの複素共役を示し、“ＸＹ”は信号Ｘと信号Ｙの乗算を意味する。 The measuring method shown in FIG. 3 will be explained in detail using FIG. 4.
(1) Steps S01 to S04
The center optical frequency i (in this example, i = 1 to 3) is set in order for the BPF (13-1, 13-2), and the optical frequency CW#1 is set for the continuous light source (20-1, 20-2), respectively. and set CW#2. Then, the first beat signal, second beat signal, and third beat signal are measured in each band (BPF #1 to BPF #3). Then, a multiplication signal of the following equation is calculated by multiplying the complex conjugate of the second beat signal and the third beat signal.

Here, " ^* " in "XY ^* " indicates the complex conjugate of the signal Y, and "XY" means the multiplication of the signal X and the signal Y.

When the signals measured and calculated as described above (first beat signal, multiplication signal) are Fourier-transformed, the result is as shown in FIG. 4(A).

(2) Step S05
Calculate the signal of the following equation.

When the three signals calculated as described above are Fourier transformed, the result is as shown in FIG. 4(B). The signals of the first and second equations in Equation 5 are obtained by converting the impulse response (first beat signal) at BPF #1 and BPF #2 into a relative frequency spectrum with respect to CW1. The signal expressed by the third equation of Equation 5 is obtained by converting the impulse response (first beat signal) at BPF #3 into a relative frequency spectrum with respect to CW2.

数６のＭは、ｉ＝２（ＢＰＦ＃２、ＣＷ＃２）のときの乗算信号（数７、図４（１）の信号Ｒ）をＣＷ１に対する相対周波数のスペクトルに変換したものである。

Now, perform the following calculation.

M in Equation 6 is obtained by converting the multiplication signal (Equation 7, signal R in FIG. 4 (1)) when i=2 (BPF#2, CW#2) into a spectrum of relative frequencies with respect to CW1.

(3) Step S06
The upper signal in Figure 4 (2) (first equation of Equation 5) and the middle signal in Figure 4 (2) (Second equation of Equation 5) are combined at 0Hz (relative frequency to CW #1). Connect as points.
The middle signal in FIG. 4 (2) (the second equation of Equation 5) and the signal in the lower part of FIG. Connect as a turning point.
FIG. 4(3) is a diagram illustrating the result of concatenating three signals.

の信号を計算する。数８の信号は、第１ビート信号をＣＷ＃３に対する相対周波数のスペクトルに変換したものである。
そして、この信号と図４（２）の下段の信号（数５の第３式）とを、数９の周波数（ＣＷ＃２に対する相対周波数）を転結点として連結する。

以下、同様にｉを増加していく。 (4) Supplement In this embodiment, the case where i=1 to 3 has been described, but when i=4 or more as shown in FIG. 2, the following processing is performed.
Optical frequency CW #3 is set in the continuous light source 20-1, and the first beat signal, second beat signal, and third beat signal are measured.
As mentioned above,

Compute the signal for . The signal of Equation 8 is obtained by converting the first beat signal into a relative frequency spectrum with respect to CW #3.
Then, this signal and the signal in the lower part of FIG. 4 (2) (the third equation of Equation 5) are connected using the frequency of Equation 9 (relative frequency to CW #2) as a turning point.

Thereafter, i is increased in the same way.

(Effect of the invention)
The spectrometer of the present invention does not use the entire band of the optical frequency comb as it is, but divides the band of the optical frequency comb and performs impulse response measurements, and each measurement result is obtained using a continuous light source as a reference point at the connection point of the divided bands. Concatenate without contradiction.
The spectrometer of the present invention can relax the conditions required for frequency fluctuations of the optical frequency comb by dividing the band of the optical frequency comb and reducing the band during measurement. Further, by connecting measurement results in divided bands, wideband measurement can also be realized. That is, the spectrometer of the present invention can realize broadband measurement without stabilizing frequency fluctuations.
The spectrometer of the present invention allows broadband measurement even when two optical frequency combs are asynchronous by relaxing the frequency fluctuation conditions of the optical frequency comb. Therefore, in situations where the optical frequency comb for probe light and the optical frequency comb for sampling light are installed remotely (for example, in a situation where the optical fiber under test is an optical transmission line as shown in Figure 5) However, the spectrometer of the present invention can measure it.

11-1, 11-2: Optical frequency comb 13-1, 13-2: Band pass filter (BPF)
14-1, 14-2, 14-3, 14-4: Optical branch 15: Optical 90 degree hybrid 17: Analog-digital converter 18: Impulse response measurement circuit 20-1, 20-2: Continuous light source 25-1 , 25-2, 25-3, 25-4: Optical 90 degree hybrid 28: Reference frequency measurement circuit 50: Optical fiber to be measured 301: Spectrometer

Claims (2)

Two optical frequency combs whose repetition frequencies are different from each other,
a bandpass filter that cuts out a band with the same center optical frequency from each of the optical combs, and uses one as a probe light and the other as a sampling light;
an impulse response measurement circuit that generates a first beat signal of the sampling light and the probe light that has passed through the optical fiber to be measured;
a continuous light source that outputs continuous light with a single optical frequency ;
a reference frequency measurement circuit that generates a second beat signal of the continuous light and the probe light that has passed through the optical fiber to be measured, and a third beat signal of the continuous light and the sampling light;
changing the center optical frequency set in the bandpass filter;
causing the impulse response measurement circuit to generate the first beat signal for each of the bands having different center optical frequencies;
The reference frequency measurement circuit includes, for each of the two bands having different center optical frequencies, the second beat signal in the continuous light having the single optical frequency between the center optical frequencies of the two bands. and generating the third beat signal;
calculating a multiplication signal obtained by multiplying the complex conjugate of the second beat signal and the third beat signal for each of the two bands having different center optical frequencies;
For each of the two bands having different center optical frequencies, the first beat signal and the complex conjugate of the multiplication signal are multiplied, and the first beat signal is set at the single optical frequency of the continuous light. converting into waveforms, and connecting each of the waveforms at the origin;
A control circuit that performs
A spectrometer equipped with Two optical frequency combs whose repetition frequencies are different from each other,
a bandpass filter that cuts out a band with the same center optical frequency from each of the optical combs, and uses one as a probe light and the other as a sampling light;
an impulse response measurement circuit that generates a first beat signal of the sampling light and the probe light that has passed through the optical fiber to be measured;
a continuous light source that outputs continuous light with a single optical frequency ;
a reference frequency measurement circuit that generates a second beat signal of the continuous light and the probe light that has passed through the optical fiber to be measured, and a third beat signal of the continuous light and the sampling light;
A spectroscopic measurement method performed by a spectrometry device comprising:
changing the center optical frequency set in the bandpass filter;
causing the impulse response measurement circuit to generate the first beat signal for each of the bands having different center optical frequencies;
The reference frequency measurement circuit includes, for each of the two bands having different center optical frequencies, the second beat signal in the continuous light having the single optical frequency between the center optical frequencies of the two bands. and generating the third beat signal;
calculating a multiplication signal obtained by multiplying the complex conjugate of the second beat signal and the third beat signal for each of the two bands having different center optical frequencies;
For each of the two bands having different center optical frequencies, the first beat signal and the complex conjugate of the multiplication signal are multiplied, and the first beat signal is set at the single optical frequency of the continuous light. converting into waveforms, and connecting each of the waveforms at the origin;
A spectroscopic measurement method characterized by:

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