JP2019113510A - measuring device - Google Patents

Abstract

To provide a measuring device capable of measuring a complex electric field amplitude waveform of a light signal.SOLUTION: A measuring device 1 comprises a local light beam output device 11, a light circuit 12, a first conversion unit 13, a second conversion unit 14, and a signal processing circuit 15, and measures a measurement light beam OS having periodicity. The local light beam output device 11 sequentially outputs local light beams LO each of which has a different wavelength. The light circuit 12 outputs a first light signal indicating a beet corresponding to a sine component of the measurement light beam OS with respect to the local light beam LO, and a second light signal indicating a beet corresponding to a cosine component of the measurement light beam OS with respect to the local light beam LO. The first conversion unit 13 converts the output of the light circuit 12 to an analog electric signal. The second conversion unit 14 converts an output of the first conversion unit 13 to a digital signal. The output of the first conversion unit 13 indicates a band division component corresponding to a part of a spectrum waveform of the measurement light beam OS. The signal processing circuit 15 processes the signal of the output of the second conversion unit 14 to acquire band division components each of which has a different frequency band, and synthesizes the acquired band division components.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a measuring device.

  Measurement of optical spectrum waveform (waveform of frequency component of optical signal) is indispensable for analysis of optical signal to noise ratio (OSNR) etc. and is one of the important measurements in optical signal analysis. . A coherent optical spectrum analyzer (COSA) has been proposed as an apparatus for measuring an optical spectrum waveform (see, for example, Non-Patent Document 1). A coherent optical spectrum analyzer is an optical spectrum analyzer that measures an optical spectrum waveform using coherent detection. A coherent light spectrum analyzer measures the entire light spectrum waveform by sweeping the frequency of the local light, using wavelength-tunable laser light as the local light.

D. M. Baney, B .; Szafraniec, A. Motamedi, "Coherent Optical spectrum analyzer", IEEE Photonics Technology Letters, March 2002, Vol. 14, No. 3, p355-357

  However, in the known coherent light spectrum analyzer, only the intensity component of the intensity component and the phase component of the optical signal can be measured, and the phase component can not be measured. Therefore, the complex electric field amplitude waveform of the optical signal could not be measured.

  The present invention has been made in view of the above problems, and an object thereof is to provide a measuring apparatus capable of measuring a complex electric field amplitude waveform of an optical signal.

  A measuring apparatus according to the present invention includes a local light output device, an optical circuit, a first conversion unit, a second conversion unit, and a signal processing circuit. The local light output device outputs local light. The optical circuit inputs the measured light having periodicity and the local light, and outputs a first light signal and a second light signal. The first light signal indicates a beat corresponding to a sine component of the light to be measured for the local light. The second light signal indicates a beat corresponding to the cosine component of the light to be measured with respect to the local light. The first conversion unit converts the output of the optical circuit into an analog electrical signal. The second conversion unit converts the output of the first conversion unit into a digital signal. The signal processing circuit performs signal processing on the output of the second conversion unit to measure a complex electric field amplitude waveform of the light to be measured. The local light output device sequentially outputs the local light having different wavelengths. The output of the first conversion unit indicates a band division component corresponding to a part of the spectrum waveform of the light to be measured. The signal processing circuit acquires the band division components different in frequency band from one another, and combines the band division components.

  In one embodiment, the signal processing circuit Fourier-transforms the output of the second converter into a frequency domain, and divides the band-divided component after the Fourier transform into an intensity component and a phase component. Thereafter, the signal processing circuit differentiates the phase component with respect to the frequency to obtain a first-order differential phase component. Further, the signal processing circuit adjusts a constant component of the first order differential phase component so that the first order differential phase component is connected between the adjacent band division components. Then, the signal processing circuit connects the intensity component and the first derivative phase component whose constant component is adjusted between the adjacent band division components.

  In one embodiment, the local light output device changes the wavelength of the local light so that regions overlap between the adjacent band division components. The signal processing circuit detects an overlap portion where regions overlap between adjacent band division components, and removes the overlap portion from one of the adjacent band division components.

  In one embodiment, the signal processing circuit finds a cross correlation between adjacent band division components, and detects the overlapping portion based on the cross correlation.

  In one embodiment, the signal processing circuit differentiates the phase components of the band division components with respect to frequency to obtain a first order differential phase component, and cross-correlations of the first order differential phase components between the adjacent band division components. Ask for

  In one embodiment, the signal processing circuit obtains a complex conjugate of the band division component and multiplies the band division component by the complex conjugate.

  According to the present invention, the complex electric field amplitude waveform of the optical signal can be measured.

It is a figure showing the measuring device concerning embodiment 1 of the present invention. It is a figure which shows the structure of the optical circuit which concerns on Embodiment 1 of this invention, and a 1st conversion part. (A) is a figure which shows the time-axis waveform of to-be-measured light. (B) is a figure which shows the spectrum waveform of to-be-measured light. It is a figure which shows the principle of the band division measurement which concerns on Embodiment 1 of this invention, and a spectrum synthesis | combination. It is a figure which shows the phase noise resulting from coherent detection. (A) is a block diagram which shows the phase noise suppression process based on Embodiment 2 of this invention. (B) is a figure which shows the phase noise suppression process based on Embodiment 2 of this invention. (C) is a figure which shows the result of the phase noise suppression process based on Embodiment 2 of this invention. It is a block diagram which shows the delay compensation process which concerns on Embodiment 3 of this invention. It is a figure which shows the principle of the frequency fluctuation compensation process which concerns on Embodiment 4 of this invention. It is a figure which shows the waveform of the cross correlation strength which concerns on Embodiment 4 of this invention. It is a block diagram which shows the signal processing which concerns on Embodiment 5 of this invention. It is a figure which shows the waveform of the cross correlation strength of the primary differential phase component which concerns on Embodiment 5 of this invention. It is a figure which shows the measuring apparatus which concerns on other embodiment of this invention. (A) is a figure showing the measuring device concerning the example of the present invention. (B) is a figure which shows the to-be-measured light based on the Example of this invention, and local light. It is a figure which shows each measurement result of the spectrum waveform of the band division component which concerns on the Example of this invention, the waveform of the phase component which carried out the first differentiation of the band division component, the waveform of cross correlation intensity, and the bandwidth of an overlap area | region. (A) is a figure which shows the spectrum synthetic | combination waveform which concerns on the Example of this invention. (B) is a figure which shows the measurement result of a constellation waveform which concerns on the Example of this invention. (A) is a figure which shows the measurement result of the spectrum waveform concerning a comparative example. (B) is a figure which shows the measurement result of the constellation waveform which concerns on a comparative example.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments. In the drawings, the same or corresponding portions are denoted by the same reference characters and description thereof will not be repeated. In addition, the description may be omitted as appropriate for portions where the description overlaps.

Embodiment 1
FIG. 1 is a view showing a measuring apparatus 1 according to the present embodiment. As shown in FIG. 1, the measurement apparatus 1 includes a local light output device 11, an optical circuit 12, a first conversion unit 13, a second conversion unit 14, and a signal processing circuit 15.

  The measuring device 1 measures the light to be measured OS. Here, the measured light OS is a light signal having periodicity, and the measuring apparatus 1 measures the measured light OS a plurality of times. Specifically, the measurement apparatus 1 repeats the measurement of the light to be measured OS until the measurement of the entire frequency band of the light to be measured OS is completed. The measured light OS is, for example, an optical signal indicating a light pulse train or a pseudo-random bit sequence (PRBS).

  The local light output device 11 sequentially outputs local light LO having different wavelengths when measuring the measurement light OS. Specifically, the local light output device 11 changes the wavelength of the local light LO for each of the measurement light OSs that are repeatedly input. In other words, the local light output device 11 changes the frequency of the local light LO for each measurement of the light to be measured OS. The local light output device 11 is, for example, a wavelength tunable laser.

  The optical circuit 12 receives the light to be measured OS and the local light LO, and outputs a first light signal and a second light signal. In the present embodiment, the light to be measured OS is an optical signal of a single polarization, and the polarization of the light to be measured OS matches the polarization of the local light LO. The first light signal indicates a beat corresponding to the sine component (real phase component) of the light to be measured OS for the local light LO, and the second light signal indicates the cosine component of the light to be measured OS for the local light LO Shows a beat corresponding to the phase component).

  Specifically, the optical circuit 12 uses the local light LO to generate a first local light LO1 and a second local light LO2 having a phase difference of 90 degrees between the first local light LO1. The optical circuit 12 causes the light to be measured OS and the first local light LO1 to interfere with each other, and outputs the light to be measured OS and the first local light LO1 after the interference as a first light signal. The optical circuit 12 causes the light to be measured OS and the second local light LO2 to interfere with each other, and outputs the light to be measured OS and the second local light LO2 after interference as a second light signal. The optical circuit 12 is, for example, a phase diversity optical 90 degree hybrid circuit.

  The first conversion unit 13 converts each of the first light signal and the second light signal (the output of the light circuit 12) into an analog electrical signal. In the present embodiment, the first conversion unit 13 includes a first balanced photodiode (BPD) 13a and a second balance photodiode 13b. The balance photodiode is an example of a photoelectric conversion circuit.

  The first balance photodiode 13a converts the first light signal into a first analog electrical signal. Specifically, the measured light OS after interference and the first local light LO1 are converted into a first analog electrical signal. The first analog electrical signal indicates an in-phase component (In-Phase component: I component) of interference light between the measurement light OS and the local light LO. The second balance photodiode 13 b converts the second light signal into a second analog electrical signal. Specifically, the measured light OS after interference and the second local light LO2 are converted into a second analog electrical signal. The second analog electrical signal indicates a quadrature component (Quadrature component: Q component) of interference light between the measurement light OS and the local light LO.

  The second conversion unit 14 converts each of the first analog electric signal and the two analog electric signals (the output of the first conversion unit 13) into digital signals. In the present embodiment, the second conversion unit 14 includes a first analog-digital converter (ADC) 14a and a second analog-digital converter 14b. The first analog-to-digital converter 14a converts the first analog electrical signal into a first digital signal. The second analog-to-digital converter 14b converts the second analog electrical signal into a second digital signal. The first digital signal indicates an I component, and the second digital signal indicates a Q component.

  The signal processing circuit 15 processes the output (first digital signal and second digital signal) of the second conversion unit 14 to measure the complex electric field amplitude waveform of the light to be measured OS. Specifically, the signal processing circuit 15 Fourier-transforms the output of the second conversion unit 14 (measurement signal in the time domain) into the frequency domain by arithmetic processing, and outputs data indicating a waveform (spectral waveform) in the frequency domain. get. Further, the signal processing circuit 15 performs inverse Fourier transform on the spectrum waveform by arithmetic processing to acquire data indicating the waveform in the time domain. Then, the signal processing circuit 15 measures the complex electric field amplitude waveform of the light to be measured OS from the waveform data of the time domain. The signal processing circuit 15 is, for example, a general-purpose personal computer or a DSP (digital signal processor).

  Specifically, in the present embodiment, the frequency bandwidths of the first balance photodiode 13a and the second balance photodiode 13b are narrower than the frequency bandwidth of the light to be measured OS. Therefore, the frequency band of the light to be measured OS measured in one measurement is a part of the entire frequency band of the light to be measured OS. As a result, the output (first analog signal and second analog signal) of the first conversion unit 13 and the output (first digital signal and second digital signal) of the second conversion unit 14 are spectrum waveforms of the light to be measured OS The band division component corresponding to a part of (frequency axis waveform) is shown. In the following description, the frequency bandwidth may be described as “bandwidth”. Similarly, a frequency band may be described as a "band."

  The local light output unit 11 changes the wavelength (frequency) of the local light LO so that the signal processing circuit 15 can acquire band division components in different bands. Specifically, the local light output device 11 changes the wavelength of the local light LO such that the entire band (spectral waveform) of the light to be measured OS is divided into a plurality of bands.

  The signal processing circuit 15 converts the time axis waveform of each band division component into a spectral waveform by Fourier transform. Further, the signal processing circuit 15 combines the spectrum waveforms of the band division components in the digital domain to reproduce the spectrum waveform of the light to be measured OS. Specifically, the signal processing circuit 15 sequentially measures each band division component by arithmetic processing, and combines data of the measured band division component in the digital domain. Hereinafter, measuring a band division component may be described as "band division measurement", and combining a band division component may be described as "spectral synthesis". In addition, a spectral waveform obtained by synthesizing the band division components may be described as a "spectral synthesized waveform".

  The signal processing circuit 15 arithmetically processes the spectrum synthesized waveform to measure a complex electric field amplitude waveform of the light to be measured OS. Specifically, the spectrum synthesized waveform is inverse Fourier transformed to measure the time axis waveform of the light to be measured OS, and the time axis waveform of the light to be measured OS is resampled to measure the constellation waveform.

  Heretofore, the measuring apparatus 1 of the present embodiment has been described with reference to FIG. According to the present embodiment, the band of the light to be measured OS is divided to measure the complex electric field amplitude waveform. Therefore, complex electric field amplitude waveforms can be measured using narrow bandwidth photoelectric converters or slow digital to analog converters.

  Subsequently, an example of the configuration of the optical circuit 12 and the first conversion unit 13 will be described with reference to FIG. FIG. 2 is a diagram showing the configuration of the optical circuit 12 and the first conversion unit 13 according to the present embodiment.

  As shown in FIG. 2, the optical circuit 12 according to the present embodiment is a phase diversity light 90 degree hybrid circuit, and includes a first light demultiplexing unit 121, a second light demultiplexing unit 122, and a 90 degree phase shift unit 123. And a first light interference unit 124 and a second light interference unit 125.

  The first light separation unit 121 separates the light to be measured OS into two. One measured light OS is input to the first light interference unit 124. The other measured light OS is input to the second light interference unit 125.

  The second light separation unit 122 separates the local light LO into two. One local light LO is input to the first light interference unit 124 as the first local light LO1. The other local light LO is input to the 90 ° phase shift unit 123.

  The 90-degree phase shift unit 123 shifts the phase of the other local light LO by 90 degrees and outputs a second local light LO2. The second local light LO2 is input to the second light interference unit 125.

  The first light interference unit 124 causes the measurement light OS and the first local light LO1 to interfere with each other and outputs the interference light. The second light interference unit 125 causes the measurement light OS and the second local light LO2 to interfere with each other and outputs the interference light.

  Subsequently, the first balance photodiode 13a will be described. As shown in FIG. 2, the first balance photodiode 13 a includes a first photodiode 131, a second photodiode 132, and a differential amplifier 133. The first photodiode 131 converts the measured light OS after interference with the first local light LO1 into an analog signal. The second photodiode 132 converts the first local light LO1 after interference with the light to be measured OS into an analog signal. The differential amplifier 133 amplifies the difference between the output of the first photodiode 131 and the output of the second photodiode 132. The output of the differential amplifier 133 indicates the I component of interference light between the light to be measured OS and the local light LO. The configuration of the second balance photodiode 13b is the same as that of the first balance photodiode 13a, so the description thereof will be omitted.

  Subsequently, a spectral waveform of the light to be measured OS will be described with reference to FIGS. 3 (a) and 3 (b). FIG. 3A shows a time axis waveform of the light to be measured OS. FIG. 3B is a diagram showing a spectral waveform of the light to be measured OS. As shown in FIGS. 3 (a) and 3 (b), when the light to be measured OS has periodicity, the spectrum waveform of the light to be measured OS is a comb-like waveform consisting of comb components discrete on the frequency axis It becomes.

  Subsequently, band division measurement and spectrum synthesis according to the present embodiment will be described with reference to FIGS. 1 and 4. FIG. 4 is a diagram showing the principle of band division measurement and spectrum synthesis according to the present embodiment. Here, the band division measurement will be described by way of an example where the spectrum waveform of the light to be measured OS is divided into six.

In FIG. 4, a graph 41 shows a spectral waveform X (f) of the light to be measured OS. The graph 42 shows spectral waveforms X _LO (f ₁ ) to X _LO (f ₆ ) of _six local light LO ( _{1st LO to 6 th LO} ). Specifically, a spectral waveform X _LO (f _k ) of each local light LO sequentially output from the local light output device 11 at different timings is shown. Note that X _LO (f _k ) indicates the spectral waveform of the local light LO output from the local light output device 11 at the k-th measurement. The graph 43 shows a spectrum synthetic waveform of the light to be measured OS.

As shown in FIG. 4, the six local light LOs (1st LO to 6 th LO) have different frequencies from one another. Specifically, the local light output device 11 changes the frequency of the local light LO so that the entire band of the measured light OS is divided into a plurality of bands. As a result, the signal processing circuit 15 sequentially acquires band division components X _k (f) having different bands. Note that X _k (f) indicates a band division component measured at the k-th measurement.

The signal processing circuit 15 sequentially measures each of the band division components X _k (f) by arithmetic processing, and combines the measured band division components X _k (f) in the digital domain. In the example shown in FIG. 4, the signal processing circuit 15 combines _six band division components X ₁ (f) to X ₆ (f). Specifically, the signal processing circuit 15 spectrally synthesizes the six band division components based on the following equation (1).
X ₁ (f−f ₁ ) + X ₂ (f−f ₂ ) +... + X ₆ (f−f ₆ ) (1)

The band division component X _k (f) needs to have a bandwidth B that can measure a plurality of comb components. Preferably, the bandwidth B of the band division component X _k (f) satisfies the condition shown in the following equation (2). In equation (2), f ₀ is the repetition frequency of the light to be measured OS.
B> 2f ₀ (2)

Second Embodiment
A second embodiment of the present invention will now be described with reference to FIGS. 1, 5 and 6 (a) to 6 (c). However, items different from the first embodiment will be described, and description of the same items as the first embodiment will be omitted. The second embodiment differs from the first embodiment in that the signal processing circuit 15 executes phase noise suppression processing. The phase noise suppression process is a process for suppressing the phase noise of the band division component and is executed before the spectrum synthesis.

FIG. 5 is a diagram showing phase noise due to coherent detection. In FIG. 5, the graph 51 shows the spectrum waveform X _LO (f) of the local light LO, and the graph 52 shows the spectrum waveform X (f) of the light to be measured OS. Specifically, a spectrum waveform X (f) of the light to be measured OS received by the optical circuit 12 described with reference to FIG. 1 is shown. A graph 53 shows a spectrum waveform of the light to be measured OS received by the signal processing circuit 15 described with reference to FIG.

In the coherent detection, phase fluctuation between the measured light OS and the local light LO is added to the local light LO. This phase fluctuation is a random walk noise that fluctuates with time. When random walk phase fluctuation is added to the local light LO, the spectral waveform X _LO (f) of the local light LO becomes a Lorentzian function waveform having a half width δf as shown in the graph 51. Coherent detection is modeled in the time domain by multiplication of the measured light OS and the local light LO. The multiplication in the time domain corresponds to the convolution integral in the frequency domain, as shown in FIG. Therefore, the spectral waveform of the measured light OS received by the signal processing circuit 15, as shown in the graph 53, and a spectrum waveform X _LO of the local light LO and the spectrum and the X of the measured light OS (f) (f) A waveform X (f) * X _LO (f) resulting from convolutional integration is obtained, and each comb component is a waveform expanded by a half width δf. In addition, "*" shows a convolution integral.

Subsequently, phase noise suppression processing will be described with reference to FIGS. 1, 6A and 6B. FIG. 6A is a block diagram showing phase noise suppression processing according to the present embodiment, and FIG. 6B is a view showing phase noise suppression processing according to the present embodiment. The signal processing circuit 15 according to the present embodiment suppresses the phase noise by executing the processes shown in FIGS. 6A and 6B for each measurement of the band division component X _k (f). Below, the signal which the signal processing circuit 15 receives may be described as a "received signal." The reception signal has a waveform obtained by multiplying the light to be measured OS and the local light LO in the time domain. Further, in the frequency domain, a waveform obtained by convoluting the measured light OS and the local light LO is shown.

As shown in FIG. 6 (a), the signal processing circuit 15 first separates the received signal X _k (t) · X _LO (t _k ) into two, and specifically copies it. Fourier transform the received signal. X _k (t) represents a time axis waveform of the band division component at the time of the k-th measurement. X _LO (t _k ) represents a time axis waveform of the local light LO at the time of the k-th measurement.

The signal processing circuit 15 extracts one of the comb components included in the Fourier-transformed received signal X _k (f) * X _LO (f _k ) by filtering (see the left diagram of FIG. 6B). The filtering bandwidth is set to the comb interval f ₀ . In other words, the repetition frequency of the light to be measured OS is set. Next, the signal processing circuit 15 shifts the frequency of the extracted comb component so that the center frequency thereof indicates “0” (see the central view of FIG. 6B).

The signal processing circuit 15 performs inverse Fourier transform on the frequency-shifted comb component. That is, the extracted comb component is subjected to baseband conversion by frequency shift and inverse Fourier transform. As a result, as shown in the right diagram of FIG. 6B, the phase noise component is extracted. Next, as shown in FIG. 6A, the signal processing circuit 15 obtains the complex conjugate of the phase noise component by arithmetic processing, and multiplies the received signal X _k (t) · X _LO (t _k ) by the complex conjugate. Do. As a result, phase noise of the received signal is suppressed. The signal processing circuit 15 Fourier transforms the received signal after suppressing the phase noise to measure the band division component X _k (f).

FIG. 6C shows the result of the phase noise suppression process. Specifically, the left diagram of FIG. 6C shows the spectrum waveform of the band division component before phase noise suppression processing, and the right diagram of FIG. 6C shows the spectrum waveform of the band division component after phase noise suppression processing Indicates As shown in the left diagram of FIG. 6C, the comb component before the phase noise suppression processing becomes a waveform having a half width δf due to the phase noise. On the other hand, as shown to the right figure of FIG.6 (c), a swelling can be removed from a comb component by performing a phase noise suppression process. The spectral line width of the laser light used for the local light LO and the measurement light OS needs to be sufficiently smaller than the comb interval f ₀ .

The second embodiment has been described above. In the present embodiment, the signal processing circuit 15 obtains the complex conjugate of the band division component _{X k (t) · X LO} (t k), band splitting component X _k (t) · X _LO and (t _k) complex Phase noise is removed by multiplying with the conjugate. Therefore, it becomes possible to measure the complex electric field amplitude waveform of the light to be measured OS more accurately. In addition, since phase noise caused by coherent detection is random walk noise, different phase noise may be added to each of the band division components. On the other hand, according to this embodiment, it is possible to suppress phase noise for each of the band division components.

Third Embodiment
A third embodiment of the present invention will now be described with reference to FIGS. 1 and 7. However, matters different from the first and second embodiments will be described, and the description of the same matters as the first and second embodiments will be omitted. The third embodiment differs from the first and second embodiments in that the signal processing circuit 15 executes delay compensation processing. The delay compensation process is performed before spectral synthesis. The delay compensation process is a process for suppressing the degradation of the spectrum synthesized waveform due to the difference in the delay with respect to the measured light OS between the band division components.

Each band division component is measured asynchronously with the measured light OS. Thus, each band division component has a different delay τ _k . In other words, the value of delay τ _k differs for each band division component. However, as shown in the following equations (3) and (4), the time delay τ _k causes only linear phase rotation in the frequency domain and does not affect the intensity.
X _k (f) = X (f) exp (j2πfτ _k ) (3)
f _{LO k} −B / 2 ≦ f ≦ f _{LO k} + B / 2 (4)

In equation (3), X _k (f) represents the spectral waveform of the band division component measured at the k-th time. X (f) represents a spectral waveform of the light to be measured OS. τ _k indicates the delay of the band division component measured at the kth time. Further, in Equation (4), f _LOk indicates the frequency of the local light LO output at the time of the k-th measurement. B indicates the bandwidth of the band division component.

If Expression (3) is divided into an amplitude component A _k (f) and a phase component _{k k} (f), Expression (5) below is obtained. The amplitude component A _k (f) and the phase component _{k k} (f) are real numbers.
X _k (f) = A _k (f) exp [jΦ _k (f)] (5)

When the phase component _{k k} (f) is subjected to polynomial expansion, the following equation (6) is obtained.

In equation (6), the time delay τ _k appears only in the first order term and does not affect the other terms. When Φ _k (f) is differentiated with respect to the frequency, the following equation (7) is obtained.

  Formula (7) is a function which shows the waveform of the phase component which carried out the primary differential. Hereinafter, the first-order differentiated phase component may be described as a “first-order differential phase component”. Moreover, the waveform of the phase component which carried out primary differential may be described as "a primary differential phase waveform."

As shown in equation (7), the function of the first derivative phase component only changes the constant component (2π [τ _k + Φ ₁ ]) by the time delay τ _k , and even if the time delay τ _k changes, the first order The differential phase waveform does not change. Therefore, the influence of the time delay τ _k can be removed by adjusting the constant component of the first order differential phase component so that the first order differential phase component smoothly connects between the adjacent band division components.

Subsequently, delay compensation processing will be described with reference to FIGS. 1 and 7. FIG. 7 is a block diagram showing a delay compensation process according to the present embodiment. The signal processing circuit 15 according to the present embodiment executes the process shown in FIG. 7 for each measurement of the band division component X _k (f) to remove the influence of the time delay τ _k .

As shown in FIG. 7, the signal processing circuit 15 receives the band division component X _k (t−τ _k ). The band division component X _k (t−τ _k ) at the k-th measurement has a delay component τ _k . The delay component τ _k shows different values among the band division components.

The signal processing circuit 15 Fourier-transforms the band division component X _k (t−τ _k ) and calculates the spectrum waveform X _k (f) of the band division component based on the above equations (5) and (6). Ask by. Then, the signal processing circuit 15 divides the band division component after Fourier transform, that is, the spectral waveform X _k (f) of the band division component into an amplitude component A _k (f) and a phase component _{k k} (f).

Next, the signal processing circuit 15 obtains the absolute value of the amplitude component A _k (f) by calculation, and obtains the phase angle of the phase component _{k k} (f) by calculation. Then, the signal processing circuit 15 differentiates the phase angle of the phase component _{k k} (f) with respect to the frequency based on the above equation (7) to acquire the first derivative phase component.

  When acquiring the first derivative phase component, the signal processing circuit 15 adjusts the constant component of the first derivative phase component so that the first derivative phase component is smoothly connected between the adjacent band division components.

The signal processing circuit 15 performs spectrum synthesis by connecting the absolute value of the amplitude component A _k (f) and the first derivative phase component whose constant component has been adjusted, between adjacent band division components.

The third embodiment has been described above. In the present embodiment, the signal processing circuit 15 suppresses the deterioration of the spectrum synthesized waveform due to the difference in the delay τ _k with respect to the light to be measured OS between the band division components. Therefore, it becomes possible to measure the complex electric field amplitude waveform of the light to be measured OS more accurately.

Fourth Embodiment
A fourth embodiment of the present invention will now be described with reference to FIGS. 1, 8 and 9. However, matters different from the first to third embodiments will be described, and the description of the same matters as the first to third embodiments will be omitted. The fourth embodiment differs from the first to third embodiments in that the signal processing circuit 15 executes frequency fluctuation compensation processing. The frequency fluctuation compensation process is performed before spectrum synthesis. The frequency fluctuation compensation process is a process for suppressing the deterioration of the spectrum synthesized waveform due to the fluctuation of the frequency of the local light LO.

In this embodiment, the frequency of the local light LO is changed for each measurement in order to perform the band division measurement. However, since the frequency accuracy of general laser light is about 10 MHz, the frequency f _{LO of the} laser light has a fluctuation component Δf _LO . Therefore, when laser light is used for the local light LO, the frequency of each comb component of the band division component also fluctuates, which causes deterioration of the spectrum synthesis waveform.

Subsequently, frequency fluctuation compensation processing will be described with reference to FIGS. 1 and 8. FIG. 8 is a diagram showing the principle of the frequency fluctuation compensation process according to the present embodiment. Specifically, FIG. 8 shows a process of combining the band division component X ₁ (f) measured at the first measurement and the band division component X ₂ (f) measured at the second measurement.

The local light output device 11 according to the present embodiment changes the frequency (wavelength) of the local light LO so that the regions partially overlap between adjacent band division components as shown in FIG. In addition, as shown in FIG. 8, the signal processing circuit 15 according to the present embodiment detects an overlap portion (overlap component) in which regions overlap between adjacent band division components, and detects an overlapping portion of the adjacent band division components. Remove the overlap from one side. The signal processing circuit 15 combines adjacent band division components after removing the overlap portion. In the example shown in FIG. 8, after removing the overlap portion from the band division component X ₁ (f) measured at the first measurement, the band division component X ₁ (f) measured at the first measurement; and the band was measured during the second measurement divided component X ₂ and (f) shows a process for combining.

  The frequency fluctuation compensation process has been described above with reference to FIGS. 1 and 8. According to the present embodiment, it is possible to suppress the deterioration of the spectrum synthesized waveform due to the frequency of the local light LO having a fluctuation component.

However, due to the fluctuation of the frequency of the local light LO, the bandwidth of the overlap region may fluctuate for each measurement. For example, even in the case of providing an overlap region having a bandwidth (B / 2) that is half the bandwidth B of the band division component, every measurement due to the fluctuation component Δf _LO of the frequency of the local light LO In addition, the bandwidth of the overlap region may change within the range of (B / 2) ± Δf _LO .

  On the other hand, the signal processing circuit 15 according to the present embodiment detects an overlap area (overlap portion) based on the cross correlation between adjacent band division components. As a result, due to the fluctuation of the frequency of the adjacent local light LO, even if the bandwidth of the overlap region changes for each measurement, the bandwidth of the overlap region can be detected accurately.

Subsequently, detection processing of the overlap area will be described with reference to FIGS. 1 and 9. FIG. 9 is a diagram showing the waveform of the cross correlation strength. The signal processing circuit 15 determines the cross correlation between the phase components of adjacent band division components based on the cross correlation function. As shown in FIG. 9, the intensity of this cross correlation has a sharp peak due to the overlap component. The phase φ _{p of the} peak value corresponds to the data length of the overlap region. Therefore, even if the overlap area fluctuates due to the frequency fluctuation of the local light LO, the data length (bandwidth) of the overlap area can be accurately detected based on the phase φ _{p of the} peak value. .

  In the present embodiment, the process of removing the overlap portion from the band division component measured earlier among the adjacent band division components has been described, but the band measured later among the adjacent band division components is described. The overlap portion may be removed from the split components.

Fifth Embodiment
A fifth embodiment of the present invention will now be described with reference to FIGS. 1, 10 and 11. However, matters different from the first to fourth embodiments will be described, and the description of the same matters as the first to fourth embodiments will be omitted. The fifth embodiment differs from the first to fourth embodiments in that the signal processing circuit 15 executes the three signal processing (phase noise suppression processing, delay compensation processing, and frequency fluctuation compensation processing) described in the second to fourth embodiments. .

  FIG. 10 is a block diagram showing signal processing according to the present embodiment. The signal processing circuit 15 according to the present embodiment executes each process shown in FIG. In order to execute frequency fluctuation compensation processing, as described in the fourth embodiment, the local light output device 11 according to the present embodiment is a local light LO such that regions overlap between adjacent band division components. Change the frequency (wavelength) of

As shown in FIG. 10, the signal processing circuit 15 generates data X ₁ (t), X ₂ (t), X ₃ (t),..., X _N (t) of the time domain of each band division component. On the other hand, the phase noise suppression process described in the second embodiment is performed. Thereafter, the data of each band division component after the phase noise suppression processing is subjected to Fourier transform to be converted into data of the frequency domain, and peak values are extracted at comb frequency intervals from the data of the frequency domain of each band division component.

  Next, the signal processing circuit 15 calculates the first derivative of the phase component in the frequency domain for the delay compensation process described in the third embodiment.

Next, the signal processing circuit 15 obtains the cross-correlation of the first derivative phase components among the data of the adjacent band division components for the frequency fluctuation compensation processing described in the fourth embodiment. Then, the signal processing circuit 15 detects the data length of the overlap area based on the phase φ _p of the peak value of the cross correlation strength, and removes the data of the overlap area from one of the data of the adjacent band division components. .

  Next, the signal processing circuit 15 adjusts the constant component of the first order differential phase component so that adjacent band division components are connected smoothly. Specifically, among the adjacent band division components, constant components included in the first derivative phase components of the band division components measured later are adjusted. Specifically, the constant component is adjusted by shifting the frequency and phase of the band division component. The adjustment of the constant component uses the average of the measured values in the overlap region. Thereafter, the signal processing circuit 15 combines the first derivative phase component and the intensity component of the adjacent band division components.

  In the present embodiment, unlike the fourth embodiment, the cross-correlation of the first derivative phase components is obtained. FIG. 11 is a diagram showing the waveform of the cross-correlation strength of the first derivative phase component. In FIG. 11, a graph 61 shows the first derivative phase waveform of the band division component obtained in the k-th measurement. A graph 62 shows the first derivative phase waveform of the band division component obtained in the (k + 1) th measurement. The graph 63 shows the waveform of the cross correlation strength of the first derivative phase component.

As shown in the graph 63, also in the cross-correlation intensity waveform of the first derivative phase components between the adjacent band division components, a sharp peak due to the overlap component occurs. The signal processing circuit 15 cross-correlates the primary differential phase component of the band division component obtained in the k-th measurement and the primary differential phase component of the band division component obtained in the k + 1-th measurement according to the following equation (8) Ask for In other words, as shown in FIG. 11, the signal processing circuit 15 obtains cross correlation of the first derivative phase components by convolution integration.

  The fifth embodiment has been described above. According to this embodiment, since the phase noise suppression process, the delay compensation process, and the frequency fluctuation compensation process are performed, it is possible to further suppress the degradation of the spectrum synthesized waveform.

  The embodiments of the present invention have been described above with reference to the drawings. However, the present invention is not limited to the above embodiment, and can be implemented in various modes without departing from the scope of the invention.

  For example, in the embodiment according to the present invention, the measured light OS is a single polarization optical signal, but the measurement light OS is not limited to a single polarization optical signal. FIG. 12 is a view showing a measuring apparatus 1 according to another embodiment. Hereinafter, the measuring device 1 shown in FIG. 12 will be described.

  The measurement apparatus 1 shown in FIG. 12 includes a local light output device 11, two polarizing beam splitters (PBSs) 16, two optical circuits 12, two first conversion units 13, and two first A 2 conversion unit 14 and a signal processing circuit 15 are provided. One of the two polarization beam splitters 16 splits the light to be measured OS into vertical polarization and horizontal polarization, and the other splits the local light LO into vertical polarization and horizontal polarization. One of the two optical circuits 12 inputs the vertical polarization of the light to be measured OS and the vertical polarization of the local light LO, and the other is the horizontal polarization of the light to be measured OS and the horizontal polarization of the local light LO And enter. According to the measuring apparatus 1 shown in FIG. 12, the vertical polarization and the horizontal polarization of the light to be measured OS are coherently detected using the same local light LO, and the complex electric field amplitude of each polarization component is measured. be able to.

  Hereinafter, the present invention will be more specifically described using examples and comparative examples. In addition, this invention is not limited at all to the range of an Example.

  In the present embodiment, the measured light OS is measured 11 times while changing the wavelength of the local light LO sequentially to perform band-split measurement. FIG. 13A is a view showing a measuring apparatus 100 according to the present embodiment. Moreover, FIG.13 (b) is a figure which shows to-be-measured light OS and local light LO (1st LO-11th LO) which concern on a present Example. The measuring apparatus 100 according to the present embodiment is configured the same as the measuring apparatus 1 described with reference to FIG. Specifically, as shown in FIG. 13A, the measuring apparatus 100 includes a first variable wavelength laser 101, an IQ modulator 102, an arbitrary waveform generator (AWG) 103, and a second. A wavelength tunable laser 104, a phase diversity light 90 degree hybrid circuit 105, two balance photodiodes 106, and an oscilloscope 107 are provided.

In the present embodiment, using the first variable wavelength laser 101, the IQ modulator 102, and the arbitrary waveform generator 103, a single polarization QPSK representing a pseudorandom bit string having a period 2 ⁹ -1 as the light to be measured OS. (Quadrature Phase Shift Keying) signal was generated. The baud rate (Baudrate) of the QPSK signal was 12.5 Gbaud. Further, the spectrum waveform of the QPSK signal is a Nyquist pulse waveform (see FIG. 13B). The IQ modulator 102 is a modulator that modulates the in-phase component (In-phase) of the waveform and the quadrature phase component (Quadrature) of the waveform.

  In the present embodiment, the second wavelength tunable laser 104 is used to generate the local light LO. In addition, the frequency of the local light LO (laser light) was changed by 1.25 Hz for each measurement of the measurement light OS. The spectral line width of the local light LO was 10 kHz.

  In the present embodiment, the bandwidth of the balance photodiode 106 is 40 GHz (> 12.5 GHz) or more. Therefore, the two balance photodiodes 106 can output the entire band (bandwidth 12.5 GHz) of the measured light OS in one measurement. Therefore, the measured light OS was divided into 11 by filtering the bandwidth to 2.5 GHz in the digital domain using the oscilloscope 107, and the band division measurement was performed. The analog bandwidth of the oscilloscope 107 was 8 GHz.

  FIG. 14 is a view showing the measurement results of the spectrum waveform of the band division component, the first differential phase waveform of the band division component, the waveform of the cross correlation intensity, and the bandwidth of the overlap region according to the present embodiment. In this embodiment, the digital output of the oscilloscope 107 described with reference to FIG. 13 is input to a general-purpose personal computer, and the spectrum waveform of the band division component, the first differential phase waveform of the band division component, and the waveform of the cross correlation strength are It was measured. In the present embodiment, the signal processing described with reference to FIG. 10 is performed using a general-purpose personal computer.

  As shown in FIG. 14, it can be confirmed that a sharp peak occurs in the waveform of the cross correlation intensity. Note that although the bandwidth of the overlap region is calculated to be 1.25 GHz, as described in the fourth embodiment, the bandwidth of the overlap region fluctuates.

  FIG. 15A is a diagram showing a spectrum synthesis waveform according to the present example. As shown in FIG. 15A, the spectral waveform of the light to be measured OS could be regenerated by performing the spectral synthesis.

  Further, in this embodiment, inverse Fourier transform is performed on the spectrum synthesis waveform (waveform obtained by synthesizing the band division components on the frequency axis) by arithmetic processing using a general-purpose personal computer to convert it into a time domain waveform. The waveform of the region was resampled to obtain a constellation waveform. FIG. 15 (b) is a view showing the measurement results of the constellation waveform according to the present example.

As shown in FIG. 15 (b), four types of codes of the QPSK signal could be recognized sufficiently. This indicates that the spectrum waveform of the light to be measured OS was correctly reproduced. In the constellation waveform shown in FIG. 15B, the fluctuation σ _amp in the amplitude direction was “0.235”. Further, the fluctuation σθ in the phase direction was “0.094 rad”.

  FIG. 16A is a diagram showing the measurement results of the spectral waveform according to the comparative example. In the comparative example, unlike the example, the entire band (bandwidth 12.5 GHz) of the light to be measured OS was measured without filtering the bandwidth by the oscilloscope 107. In other words, the entire band of the measured light OS was measured in one measurement. As shown in FIGS. 15 (a) and 16 (a), it was confirmed that the waveform obtained by combining the band division components was not deteriorated as compared with the waveform obtained in one measurement.

FIG. 16 (b) is a view showing the measurement results of the constellation waveform according to the comparative example. In the constellation waveform shown in FIG. 16 (b), the fluctuation σ _amp in the amplitude direction was “0.232”. Further, the fluctuation σθ in the phase direction was “0.105 rad”. Therefore, as shown in FIGS. 15 (b) and 16 (b), the constellation waveform obtained by combining the band division components is not deteriorated as compared with the constellation waveform obtained in one measurement. Was confirmed.

  The present invention is useful for measuring the waveform of frequency components of an optical signal.

DESCRIPTION OF SYMBOLS 1 measurement device 11 local light output device 12 light circuit 13 first conversion unit 13a first balance photodiode 13b second balance photodiode 14 second conversion unit 14a first analog-to-digital converter 14b second analog-to-digital converter 15 signal processing Circuit 100 measuring apparatus 101 first wavelength variable laser 102 IQ modulator 103 arbitrary waveform generator 104 second wavelength variable laser 106 balance photodiode 107 oscilloscope LO local light LO1 first local light LO2 second local light OS measured light

Claims (6)

A local light output device that outputs local light;
A first light signal indicating a beat corresponding to a sine component of the light to be measured with respect to the local light, and a cosine of the light to be measured with respect to the local light An optical circuit that outputs a second optical signal indicating a beat corresponding to the component;
A first conversion unit that converts an output of the optical circuit into an analog electrical signal;
A second converter that converts the output of the first converter into a digital signal;
A signal processing circuit that processes the output of the second conversion unit to measure the complex electric field amplitude waveform of the light to be measured;
The local light output device sequentially outputs the local light having different wavelengths,
The output of the first conversion unit indicates a band division component corresponding to a part of the spectrum waveform of the light to be measured,
The measurement apparatus, wherein the signal processing circuit acquires the band division components different in frequency band from one another and synthesizes the band division components. The signal processing circuit
Fourier transforming the output of the second transformation unit into a frequency domain,
Dividing the band division component after the Fourier transform into an intensity component and a phase component;
Differentiating the phase component with respect to frequency to obtain a first derivative phase component;
Adjusting a constant component of the first differential phase component such that the first differential phase component is connected between the adjacent band division components;
The measuring apparatus according to claim 1, wherein the intensity component and the first derivative phase component in which the constant component is adjusted are connected between the adjacent band division components. The local light output device changes the wavelength of the local light so that regions overlap between the adjacent band division components,
The signal processing circuit
Detecting overlapping portions in which regions overlap between adjacent band division components;
The measurement apparatus according to claim 1 or 2, wherein the overlapping portion is removed from one of the adjacent band division components.   The measurement apparatus according to claim 3, wherein the signal processing circuit obtains a cross correlation between adjacent band division components, and detects the overlapping portion based on the cross correlation.   The signal processing circuit differentiates the phase component of the band division component with respect to frequency to obtain a first order differential phase component, and obtains cross-correlation of the first order differential phase component between adjacent band division components. The measuring device as described in 4. The signal processing circuit
Obtain the complex conjugate of the band split component,
The measuring apparatus according to any one of claims 1 to 5, wherein the band division component and the complex conjugate are multiplied.

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