JP2001235397A - Method for measuring group delay time - Google Patents

Abstract

PROBLEM TO BE SOLVED: To precisely measure a group delay time regardless of values of a modulation frequency. SOLUTION: In this method, a true group delay time is estimated from the group delay time actually measured by the following operations. 1) A group delay time ripple spectrum (see Fig. (a)) is obtained by reducing a linear component from the group delay time actually measured. 2) The group delay time ripple spectrum is Fourier-transformed by wavelengths (see Fig. (b)). 3) The group delay time ripple spectrum (Fourier-transformed spectrum), which is Fourier-transformed, is divided by an averaged factor spectrum (see Fig. (c)) (see Fig. (d)). 4) The Fourier-transformed spectrum, which is corrected, is inversely Fourier-transformed (see Fig. (e)). 5) The true group delay time is obtained by adding the linear component to the Fourier-transformed spectrum inversely Fourier-transformed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a group delay time measuring method for measuring a group delay time for evaluating characteristics of an optical fiber or an optical component.

[0002]

2. Description of the Related Art When pulse light having a wide wavelength width propagates through a single-mode optical fiber or optical component, waveform distortion occurs. This waveform distortion becomes a major obstacle when speeding up optical communication. Therefore, it is necessary to accurately evaluate chromatic dispersion that causes waveform distortion of optical fibers and optical components.

[0003] Chromatic dispersion is defined as the group delay time difference between light waves having different wavelengths. Therefore, if the group delay time of each light wave (incident light) incident on an optical fiber or an optical component can be measured, chromatic dispersion can be calculated, and waveform distortion can be evaluated using the chromatic dispersion.

As a method of measuring the group delay time, a phase method is widely used. Hereinafter, a group delay time measurement system using the phase method will be described. FIG. 9 is a block diagram illustrating a configuration example of a group delay time measurement system of a DCFG (dispersion compensation fiber grating). In the group delay time measurement system shown in this figure, first, the light intensity modulator 1
The output light of the variable wavelength light source 3 is amplitude-modulated using the frequency of the output signal (modulation frequency f _m ). Here, as an example, a phase-change optical modulator is used as the light intensity modulator 1 and a TLD (tunable laser diode) is used as the variable wavelength light source 3.

The amplitude-modulated output light is incident on a DCFG, which is a DUT (evaluation target) 5, via an optical coupler 4. This incident light is reflected by the DCFG, and the reflected light enters the photoelectric converter 6 via the optical coupler 4 and is converted into an electric signal by the photoelectric converter 6. And the phase comparator 7
Measures the phase difference between the output signal from the signal source and the output signal from the photoelectric converter 6. Analysis and display unit 18, an a phase difference phi and the modulation frequency f _m, by substituting the τ = φ / 2πf _m, obtaining the group delay time tau. When the group delay time is measured in this manner, chromatic dispersion can be calculated using the measured group delay time, and waveform distortion can be evaluated using the calculated chromatic dispersion.

In general, when the DUT 5 is a DCFG, the group delay time is measured using the reflected light of the DCFG. However, when the DUT 5 is an optical fiber, the transmitted light of the optical fiber is measured. Utilize to measure group delay time. Therefore, in order to configure a group delay time measurement system applicable to both the DCFG and the optical fiber, the configuration in FIG. 9 needs to be slightly changed. FIG. 10 is a block diagram illustrating a configuration example of a general-purpose group delay time measurement system. In the group delay time measurement system shown in this figure, when the DUT is a DCFG, the phase difference between the reflected light of the DCFG and the signal source output signal is used, and when the DUT is an optical fiber, the optical fiber The phase difference between the transmitted light and the signal source output signal is used.

[0007]

By the way, in the above-described conventional group delay time measuring method, as shown in FIGS. 9 and 10, it is necessary to modulate the output light of the variable wavelength light source with the amplitude modulation frequency. . However, this method has a problem that "the measurement accuracy of the group delay time differs depending on the value of the modulation frequency" (the reason will be described later).

The present invention has been made under such a background, and has as its object to provide a group delay time measuring method capable of accurately measuring the group delay time regardless of the modulation frequency. I do.

[0009]

According to the first aspect of the present invention,
The method is characterized in that the group delay time spectrum measured by the phase method is Fourier-transformed by wavelength, the Fourier-transformed group delay time spectrum is corrected using an averaging factor spectrum, and the correction result is inversely Fourier-transformed. According to a second aspect of the present invention, in the method for measuring a group delay time according to the first aspect,
The correction is performed by dividing the Fourier-transformed group delay time spectrum by an averaging factor spectrum. According to a third aspect of the present invention, in the group delay time measuring method according to any one of the first and second aspects,
The group delay time spectrum is a group delay time ripple spectrum obtained by subtracting an approximate polynomial obtained by a least squares method from a group delay time measured by a phase method.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

§1. First, a description will be given of a problem of the related art (that is, a reason why a difference in measurement accuracy of the group delay time occurs depending on a value of a modulation frequency). Assuming that the wavelength characteristic of the group delay time of the optical fiber or the optical component is f (λ), f (λ) can be expressed by the following equation (1).

(Equation 1) In equation (1), λ is the wavelength of the incident light, _pn is the frequency of the group delay time on the wavelength axis (hereinafter referred to as “group delay frequency”), and _An is the amplitude corresponding to each group delay frequency component. is there.

Here, for simplicity of discussion, attention is paid to one group delay frequency p _k (where n = k). That is, a component represented by the following equation (2) is considered. f _k (λ) = A _k sin (2πp _k λ) (2)

On the other hand, as described above, since the incident light is amplitude-modulated in the phase method, the spectrum width Δλ of the incident light is obtained by the following equation (3). Δλ = c / (f ₀ −f _m ) −c / (f ₀ + f _m ) ≒ 2f _m c / f ₀ ² (f ₀ ≫f _m ) (3) In equation (3), c is light speed of, f ₀ is the frequency of the incident light, is f _m is the modulation frequency.

Due to the spread of the spectral width Δλ, the group delay time actually measured is such that the wavelength is from λ−Δλ / 2 to λ
It is assumed to be the average value of the group delay time up to + Δλ / 2. Therefore, the actually measured group delay time f _k
(Λ) _meas is obtained by the following equation (4).

(Equation 2)

From equation (4), it can be seen that "the actually measured group delay time f _k (λ) _meas is larger than the true group delay time f _k (λ) by the averaging effect due to the spread of the spectral width Δλ. Only a predetermined coefficient, that is, the averaging factor sin (πpΔ
λ) / πpΔλ ”.

FIG. 1 shows different modulation frequencies (f _m = 100
9 is a graph showing an example of an averaging factor spectrum corresponding to (MHz, 500 MHz, 1 GHz). Equation (3) Equation (4) As is clear is substituted, the larger the modulation frequency f _m is or larger group delay frequency p is the average factor is reduced (i.e., its value is far from 1 , The effect as a coefficient increases). Therefore, the actually measured group delay time f _k (λ) _meas is the modulation number wave number f
_{As m} increases, it becomes smaller than the true group delay time f _k (λ). The above is the reason why the difference in the measurement accuracy of the group delay time by the value of the modulation frequency f _m may occur.

For reference, a specific example of the measurement result according to the prior art will be described below. Figure 2 is a graph showing an example of a different modulation frequency (f _m = 50 MHz, 1 GHz) ripple characteristic corresponding to the group delay time measured in (group delay time ripple spectrum). As shown in FIG. 3, the group delay time ripple spectrum shown in this figure is an approximate polynomial obtained from the actually measured group delay time (the upper graph) by the least square method from the group delay time. (In the middle graph, here, using a first-order approximation formula). As can be seen from Figure 2, when the modulation frequency is high (f _m = 1GHz),
Due to the averaging operation, the high-frequency fluctuation component of the group delay time ripple spectrum is distorted.

§2. Group delay time measuring method according to the present invention From equation (4), it can be seen that “the actually measured group delay time f
_k (λ) _meas is smaller than the true group delay time f _k (λ) by the averaging factor sin (πpΔλ) / πpΔλ ”. Therefore, the present invention “corrects the actually measured group delay time f _k (λ) _meas by the averaging factor sin (πpΔλ) / πpΔλ to obtain the true group delay time f _k (λ)”. It is characterized by the following. Specifically, the actually measured group delay time f _k
(Λ) _meas is averaged by sin (πpΔλ) / πpΔ
Divide by λ.

Hereinafter, an embodiment for realizing the above features will be described. FIG. 4 is a block diagram illustrating a configuration example of the group delay time measurement system according to the present embodiment. This figure corresponds to FIG. 9 (DCFG group delay time measuring system), but the present invention uses the same method to make FIG. 10 (general group delay time measuring system). It is also applicable. The group delay time measurement system shown in FIG. 4 differs from the group delay time measurement system shown in FIG. 9 only in the analysis / display unit 8. That is, until the phase difference is measured (ie,
Since the configuration and operation up to the phase comparator 7 are the same,
Here, the description is omitted.

Here, the analyzing / displaying unit 8 is, specifically,
It is a computer device controlled by a control program recorded on a predetermined recording medium (magnetic disk, semiconductor memory, or the like), or a hardware device that performs the same operation as the program.

The analysis / display unit 8 obtains (approximate value of) the true group delay time from the actually measured group delay time by performing the following operations (1) to (4). The phase difference φ is input from the phase comparator 7. Use τ = φ / 2πf _m, determine the group delay time from the phase difference. The group delay time ripple spectrum is obtained by subtracting the linear component from the obtained group delay time (see FIG. 3). Fourier transform is performed on the obtained group delay time ripple spectrum by wavelength. The Fourier-transformed group delay time ripple spectrum (hereinafter referred to as “Fourier-transformed spectrum”) is corrected (specifically, divided) using the averaging factor spectrum (see FIG. 1). Inverse Fourier transform is performed on the corrected Fourier transform spectrum. The true group delay time (approximate value) is obtained by adding a linear component to the inverse Fourier transformed Fourier transform spectrum (that is, the corrected group delay time ripple spectrum).

FIG. 5 is an explanatory diagram showing an example of the above-described operation of the analysis / display unit 8 for a modulation frequency f _m = 500 MHz. In this figure, (a) shows the result of the above operation (group delay time ripple spectrum), (b) shows the result of the above operation (Fourier transform spectrum),
(C) shows the averaging factor spectrum used in the above operation, (d) shows the result of the above operation (corrected Fourier transform spectrum), and (e) shows the result of the above operation (corrected group delay). (Time ripple spectrum).
As can be seen by comparing FIG. 5 (a) and FIG. 5 (e),
It can be seen that the above operations 1 to 5 increase the fine frequency components and correct the smoothed fine fluctuation components.

Hereinafter, specific examples of the measurement results obtained by the present method will be described. FIG. 6 shows different modulation frequencies (f _m = 10
5 is a graph showing an example of a Fourier transform spectrum before and after correction for 0 MHz, 500 MHz, and 1 GHz). In this figure, (a) is before correction, and (b)
Is after the correction. As can be seen from FIG. 6A, the waveform of the Fourier transform spectrum before the correction substantially matches the waveform of the averaging factor spectrum (see FIG. 1). on the other hand,
As can be seen from FIG. 6 (b), the waveform of the Fourier transform spectrum of the corrected, the modulation frequency (f _m = 100MH
(z, 500 MHz, 1 GHz). From this, it can be confirmed that each Fourier transform spectrum is correctly corrected by the present method.

However, as can be seen from FIG. 6B, for the modulation frequency f _m = 1 GHz, even if the Fourier transform spectrum after the correction is used, the spectrum protrudes around p = 62 [nm ⁻¹ ]. ing. This is because, as is clear from FIG. 1, the averaging factor of f _m = 1 GHz is p = 62.
This is because the value is zero near [nm ^-1 ], and the error of the correction (that is, division by the averaging factor) becomes large.

FIG. 7 is a graph showing an example of a ripple characteristic (group delay time ripple spectrum) of DCFG at a modulation frequency f _m = 50 MHz. FIG. 8A shows the ripple characteristic of FIG. 7 before correction. Wavelength A in FIG.
It is a graph which shows an example of the modulation frequency dependence of the ripple amplitude in B, C ± 50nm. As can be seen from FIG. 8 (a), in front correction, by the action of the averaging factor, the ripple amplitude as the modulation frequency f _m is large would be estimated to be smaller.

On the other hand, FIG. 8B shows the corrected ripple characteristics of FIG. 7 at wavelengths A, B, C ± 50 nm in FIG.
6 is a graph showing an example of the modulation frequency dependence of ripple amplitude in FIG. As can be seen from FIG. 8 (b), the post-correction according to the present method, ripple amplitude modulated frequency f _m is changed is substantially constant, the utility of this method has been demonstrated.

However, as can be seen from FIG. 8B, for the modulation frequency f _m = 1 GHz, the ripple amplitude is large also in the corrected ripple characteristic. This is considered to be due to the above-mentioned error near p = 62 [nm ^-1 ]. Therefore, in this method, care must be taken in correction in a region where the averaging factor is small (小 さ い 0).

The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and a design change or the like may be made without departing from the gist of the present invention. Even if there is, it is included in the present invention.

[0029]

As described above, according to the present invention, the group delay time can be accurately measured regardless of the value of the modulation frequency.

[Brief description of the drawings]

FIG. 1 is a graph showing an example of an averaging factor spectrum.

FIG. 2 is a graph showing an example of a group delay time ripple spectrum.

FIG. 3 is an explanatory diagram illustrating an example of a method of obtaining a group delay time ripple spectrum.

FIG. 4 is a block diagram illustrating a configuration example of a group delay time measurement system according to an embodiment of the present invention.

FIG. 5 is an explanatory diagram showing an example of the operation of the analysis / display unit 8;

FIG. 6 is a graph showing an example of a Fourier transform spectrum before and after correction according to the present invention.

FIG. 7 is a graph showing an example of a ripple characteristic of DCFG.

8 is a graph showing an example of the modulation frequency dependence of the ripple amplitude for the ripple characteristics of FIG. 7 before and after correction.

FIG. 9 is a block diagram illustrating a configuration example of a DCFG group delay time measurement system according to the related art.

FIG. 10 is a block diagram illustrating a configuration example of a general-purpose group delay time measurement system according to the related art.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light intensity modulator, 2 ... Signal source, 3 ... Variable wavelength light source, 4 ... Optical coupler, 5 ... DUT, 6 ... Photoelectric converter, 7 ... Phase comparator, 8 ... Analysis・ Display

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Reiko Kojima 1440, Mukurosaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Plant, Inc. 72) Inventor Kenji Nishiide 1440 Mutsuzaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Works

Claims (3)

[Claims] 1. A method of performing a Fourier transform on a group delay time spectrum measured by a phase method using a wavelength, correcting the Fourier transformed group delay time spectrum using an averaging factor spectrum, and performing an inverse Fourier transform on the correction result. A group delay time measuring method. 2. The group delay time measuring method according to claim 1, wherein the correction is performed by dividing the Fourier-transformed group delay time spectrum by an averaging factor spectrum. . 3. The group delay time measuring method according to claim 1, wherein said group delay time spectrum is an approximate polynomial obtained by a least squares method from a group delay time measured by a phase method. A group delay time ripple spectrum obtained by subtracting the following.