JP2690049B2 - Polarization dispersion measuring method and device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring polarization dispersion of an optical signal transmitted through an optical fiber, an optical component, an optical amplification repeater transmission system and the like.

[0002]

2. Description of the Related Art A conventional technique of this kind will be described with reference to the drawings. FIG. 8 is a block diagram showing a configuration example of a conventional device used for polarization dispersion measurement. In the figure, 1 is a wavelength tunable light source, 2 is a wavelength λ of an output light signal L1 from the wavelength tunable light source 1.
A wavelength controller such as a temperature controller for controlling the light source, 3 an optical branching device, 4 a wavelength meter, 5 a polarization controller for controlling the polarization state of the optical signal, 6 a measurement target, 7 Stokes
A polarization analyzer such as an analyzer or a rotary analyzer, and 8 is a recording / processing device such as a computer.

A conventional polarization dispersion measuring method will be described with reference to FIG. The optical signal L1 output of the wavelength tunable light source 1 is branched by an optical branching device 3 such as an optical fiber coupler, one optical signal is input to the measurement target 6 and the optical signal L1a for measuring the polarization dispersion thereof, and the other Of the optical signal is monitored by the wavelength meter 4 for its wavelength (or frequency. Since the frequency can be displayed as the reciprocal of the wavelength, the frequency and the wavelength are almost the same in the following explanation of the conventional method). The optical signal L1b for recording is used.

Then, the optical signal L1b to be input to the measurement object 6 is made into an arbitrary polarization state by the polarization control device 5 and is incident on the measurement object 6 such as an optical fiber or an optical component, and the Stokes analyzer or the rotary type is used. Polarizer 7 such as analyzer
Then, the polarization state for each wavelength λ (frequency f) is recorded in the recording arithmetic processing unit 8 such as a computer, and each wavelength λ (frequency f) is changed, and the same measurement is performed.

In the conventional polarization dispersion measuring method, each wavelength λ on the Poincare sphere display that can be obtained by the polarization analyzer 7 such as a Stokes analyzer or a rotary analyzer.
Polarization state with respect to (frequency f) (hereinafter referred to as SOP)
By drawing a circle as shown in Fig. 9, the phase difference Δφ
Was calculated, and the polarization dispersion τ was calculated using the following formula (1). τ = Δφ / Δω = (Δφ / 2π) (1 / Δf) = (Δφ / 2π) (λ1λn / cΔλ) (1) where Δλ = | λ1-λn | Δφ: The phase difference between λ1 and λn c: speed of light

As shown in FIG. 9, in the representation on the Poincare sphere, the wavelength λ1 is set on the plane perpendicular to the principal axis of polarization.
(Frequency f1), wavelength λ2 (frequency f2), wavelength λ3
(Frequency f3), ..., SO with wavelength λn (frequency fn)
The trajectory of P draws a circle.

[0007]

However, from a mathematical fact, a circle is defined and drawn by connecting arbitrary three points, and therefore, depending on which of the three measured results is selected, the deviation can be biased. There is a problem that the measured value of the wave dispersion is scattered.

The state of the above scattering is shown in FIG.
(B) and (c) show. 10A is a graph showing a curve when the measurement result of the optical signal L3 after passing through the measurement target 6 is ideal, and FIG. 10B is a graph showing the case where the measurement result of the optical signal L3 is scattered. FIG. 10C is a graph showing a curve, and only the locus (circle) drawn on the Poincare sphere when the measurement results are scattered is drawn on the plane.

FIG. 10 (a) shows a change in the SOP of the optical signal L3 after passing through the object 6 to be measured, 1.0 represents linearly polarized light, 0.0 represents circularly polarized light, and all of the middle thereof are shown. Represents elliptically polarized light. If the measured values are scattered, some curves and circles can be drawn as shown in FIGS. 10 (b) and 10 (c), resulting in a measurement error. In order to solve the above-mentioned problems of the prior art, the present invention provides a highly stable polarization dispersion measuring method and device for a measurement target in all polarization states.

[0010]

The object of the present invention can be attained by adopting the following novel characteristic construction methods and means of the present invention. That is, the first feature of the method of the present invention is that the optical signal of the wavelength tunable light source is made to have an arbitrary polarization state and an arbitrary polarization main axis direction by the polarization control means, is incident on the measurement target, and is emitted from the measurement target. The optical signal is analyzed by polarization analysis means, and the peak (valley) between the wavelength and the wavelength at which the phase difference of the optical signal of the cosine wave-like periodic function showing the variation of the polarization state with respect to the wavelength of the optical signal becomes a predetermined angle. ) Is a group delay time of the orthogonal polarization states of the optical signal, that is, the polarization dispersion is measured.

A second feature of the method of the present invention is that the predetermined angle of the phase difference of the optical signal in a cosine wave-like periodic function showing the variation of the polarization state with respect to the wavelength of the optical signal in the first feature of the method is: This is a polarization dispersion measurement method in which polarization dispersion is obtained from the number of peaks (valleys) between wavelengths that are π or π / 2.

A third feature of the method of the present invention is that the optical signal of the wavelength tunable light source is made to have an arbitrary polarization state and an arbitrary polarization main axis direction by the polarization control means, is made incident on the measuring object, and is emitted from the measuring object. Analyzing the optical signal with the polarization analysis means, obtained by the polarization analysis means, the cosine wave periodic function of the polarization state with respect to the wavelength of the optical signal is converted from the wavelength region of the optical signal to the frequency region, further, It is converted into the time domain by inverse Fourier transforming the periodic function of the polarization state with respect to the frequency domain of the optical signal, and the polarization dispersion is obtained from the half width of the optical signal obtained by the conversion into the time domain. This is a polarization dispersion measurement method.

A fourth feature of the method of the present invention is that the wavelength tunable light source according to the first, second or third feature of the method comprises a modulation means for emitting modulated light source light, and the polarization analysis means is synchronized. This is a polarization dispersion measuring method which is a polarization analyzing means capable of detection.

A fifth feature of the method of the present invention is that the polarization analyzing means in the first, second, third or fourth feature of the method is a polarization analyzer using a Stokes analyzer or a rotary analyzer. It is a certain polarization dispersion measurement method.

The first feature of the device of the present invention is that the wavelength tunable light source capable of changing the wavelength of the output optical signal and the optical branching for branching the optical signal output from the wavelength tunable light source into two. Means and one of the optical signals from the optical branching means, the polarization state of the optical signal can be converted into any polarization state, and the polarization main axis of the optical signal can be controlled in any direction. Polarization control means input to the measurement target as
Monitoring means for monitoring the other of the optical signals from the optical branching means, polarization analysis means for analyzing the optical signal output from the polarization control means and passing through the measurement object, and outputting an electrical signal, analyzed In the optical signal, a recording calculation process for obtaining the polarization dispersion from the number of peaks (valleys) between wavelengths at which the phase difference of the optical signal has a predetermined angle in a cosine wave-like periodic function showing the variation of the polarization state with respect to the wavelength. A polarization dispersion measuring apparatus comprising:

The second feature of the device of the present invention is that the wavelength tunable light source capable of changing the wavelength of the output optical signal and the optical branching for branching the optical signal output from the wavelength tunable light source into two. Means and one of the optical signals from the optical branching means, the polarization state of the optical signal can be converted into any polarization state, and the polarization main axis of the optical signal can be controlled in any direction. Polarization control means input to the measurement target as
Monitoring means for monitoring the other of the optical signals from the optical branching means, polarization analysis means for analyzing the optical signal output from the polarization control means and passing through the measurement object and outputting an electrical signal, and the polarization analysis. A record obtained by converting the periodic function on the cosine wave of the polarization state with respect to the wavelength of the optical signal obtained by the means from the wavelength domain of the optical signal of the optical signal to the frequency domain, and from the frequency domain to the time domain by the inverse Fourier transform. A polarization dispersion measuring apparatus comprising an arithmetic processing means.

A third feature of the device of the present invention is that the wavelength tunable light source according to the first or second feature of the device comprises a modulation means for emitting modulated light source light, and the polarization analysis means is capable of synchronous detection. It is a polarization dispersion measuring device which is a polarization analyzing means.

A fourth feature of the device of the present invention is that the polarization analyzing means in the first, second or third feature of the device is a polarization analyzer using a Stokes analyzer or a rotary analyzer. It is a dispersion measuring device.

[0019]

Since the present invention employs the means and methods described above, the method of the present invention is one in which the polarization dispersion is obtained from the phase difference Δφ between two wavelengths on the Poincare sphere in the conventional method. In regard to the measurement of the polarization state, is it calculated from the number of peaks (valleys) where the phase difference Δφ of the periodic function is π (or π / 2) and the interval between the peaks (valleys)? , Or S
Since the polarization dispersion τ is obtained by performing an inverse Fourier transform on the function of the frequency that becomes a periodic function of OP, it is possible to provide a stable polarization dispersion measurement method and device.

[0020]

【Example】

(Example of Device) An example of the device of the present invention will be described with reference to the drawings. FIG.
2 is a block diagram showing a configuration example of a polarization dispersion measuring apparatus of this apparatus example. In the figure, 9 is a modulator that modulates the outgoing light signal S1 of the variable wavelength light source 1. The same members as those in the conventional example are denoted by the same reference numerals.

(Example of Method) The method of the present invention will be described in detail with reference to the drawings. 2 (a), (b), and (c) are graphs conceptually explaining the process of carrying out the method of the present invention.
Here again, the problems of the conventional method will be described. In the conventional method, a circumscribing circle was drawn on a Poincare sphere with at least three wavelengths (frequency) and polarization dispersion was obtained from it, but the reproducibility of measurement was poor because the number of samples was small.

In the method of the present invention, the following method was adopted in consideration of the problems of the conventional method. In the method of the present invention, as shown in FIG. 2A, the phase difference of the cosine wave-like periodic function of the polarization state with respect to at least the wavelength is π [peak (valley)].
To the peak (valley)] or π / 2 [peak (valley) to valley (peak)], the number N (phase difference π) of peaks or troughs from wavelength λ1 to λn is measured, and the wavelength difference is calculated from these wavelength differences. This is a measuring method characterized by obtaining a group delay time (polarization dispersion τ) of orthogonal polarization states. The polarization dispersion τ is calculated by the following equation (2). τ = (N / 2) (1 / Δf) = (N / 2) (λ1λn / cΔλ) (2) where N: the number of peaks (valleys) from λ1 to λn (the phase difference is π
Equivalent to)

However, even in this method, the reproducibility of measurement is deteriorated depending on which peak or valley is selected. For example, FIG.
To give an example in (a), the distance between λ1 and λ2 is Δλ1, but the distance between λ5 and λ4 is Δλ4, clearly Δλ
4 is wider in terms of intervals. Therefore, on the Poincare sphere, when a measurement result of Δλ4 is adopted, a circle with a large radius is naturally drawn, and when a measurement result of Δλ1 is adopted, a circle with a smaller radius than that when Δλ4 is adopted. Will be drawn.

Therefore, as shown in FIG. 2B, first,
The cosine wave-shaped periodic function of the polarization state with respect to the wavelength obtained by the measurement of the optical signal S3 of the polarization analyzer 7 is converted into the cosine wave-shaped periodic function of the frequency domain.

Then, the periodic function of the polarization state with respect to the frequency shown in FIG. 2 (b) is transformed into the time domain by inverse Fourier transform as shown in FIG. 2 (c), and from its half-value width d, The group delay time (half the polarization dispersion: the phase difference is π) of orthogonal polarization states is calculated. This is another major feature of the method of the present invention.

As the polarization analyzer 7 in FIG. 1, there is a Stokes analyzer or a polarization analyzer using a simple rotary analyzer, and respective configuration examples are shown in FIGS. 3 and 4. FIG. 3 is a diagram showing a configuration example of a Stokes analyzer as the polarization analyzer 7 used in the present invention, and FIG. 4 is a diagram showing a configuration when a rotary analyzer is used as the polarization analyzer 7 used in the method of the present invention. is there.

In the figure, 7a is a Stokes analyzer, 7
b is a polarization analyzer using a rotary analyzer, 10a, 10b
Is a beam splitter (BS), 11 is a polarization beam splitter (PBS), 12 is a π / 2 phase element such as a λ / 4 wave plate, 13 is an analyzer, 13 'is a rotary analyzer, and 14 is light detection. And 15 is an optical receiver such as an optical power meter.

FIG. 5 shows an analysis of the polarization state of the optical signal on the Poincare sphere. Stokes parameter S0
(SO), S1 (-S1), S2 (-S2) and S3
(-S3) is a variable indicating the polarization state, and the light intensity, the horizontal linearly polarized light component, the 45-degree linearly polarized light component, and the right-handed circularly polarized light component are shown in [(), respectively. Linearly polarized light component, -45 degree linearly polarized light component and left-handed circularly polarized light component] are shown.

Generally, the light I input to the ellipsometer 7 is
(Δ, γ, α) can be represented by the light intensity of any combination of the phase plate and the polarizer. Light intensity I obtained by the combination of the azimuth γ phase plate (phase difference δ) and the azimuth α analyzer
(Δ, γ, α) is expressed by the following equation (3). I (δ, γ, α) = S0 / 2 + [(S1cos 2γ + S2sin 2γ) cos 2 (γ−α)] / 2 + {[(S1sin 2γ−S2cos 2γ) cos δ−S3sin δ] sin 2 (γ− α)} / 2 (3)

The principle of measurement of the Stokes parameters will be described with reference to FIG. 3 showing an example of the structure of the Stokes analyzer. The incident optical signal S3 from the measurement target 6 is
The light is reflected by the splitter 10a, and the λ / 4 wave plate 12 and the analyzer 13 having an azimuth of 45 degrees act on each other, and the light intensity I4 is obtained by the photodetector 14.

Next, the optical signal transmitted through the beam splitter 10a is reflected by the beam splitter 10b, and the analyzer 13 having an azimuth of 45 degrees acts on the optical signal to cause the photodetector 13 to be detected.
Thus, the light intensity I3 is obtained.

Next, the optical signal transmitted through the beam splitter 10b is split into two orthogonal components by a polarizing beam splitter (PBS) 11, and a vertical polarization component light intensity I2 and a horizontal polarization component polarization intensity I1 are obtained. can get.

From the thus measured I1, I2, I3 and I4, the Stokes parameters (S0, S1, S
2, S3) is obtained by the following equation (4). S0 = I1 + I2 S1 = I1-I2 S2 = 2I3- (I1 + I2) S3 = 2I4- (I1 + I2) (4)

The Stokes parameter (S
0, S1, S2, S3), the SOP and the ellipticity (extinction ratio) η are expressed by the following equations (5) and (6), respectively. SOP = | (1−η ² ) / (1 + η ² ) | (5) η = tan [0.5tan ⁻¹ {S3 / (S1 ² + S2 ² ) ^1/2 }] ... (6)

To obtain the SOP using the polarization analyzer 7b using the rotary analyzer shown in FIG. 4, the rotary analyzer 1 is used.
By rotating 3 ', the major axis (maximum value) Imax and the minor axis (minimum value) Imin of the elliptical plane of polarization are obtained, and are obtained by the following equation (7). SOP = (Imax−Imin) / (Imax + Imin) = (1−η ² ) / (1 + η ² ) ... (7)

(Measurement example) FIGS. 6 (a) and 6 (b)
An example of performing polarization dispersion measurement of a polarization-maintaining optical fiber (PMF) using the present invention is shown. FIG. 6A shows a change in the SOP on the Poincare sphere when the wavelength difference is changed by about 3 nm (converted to a frequency of about 375 GHz).
FIG. 6 (b) is a graph in which it is converted into a change in SOP with respect to wavelength. Then, the polarization dispersion τ is about 9.873.
It can be understood that it is ps.

(Comparative Example) The table of FIG. 7 shows a comparison between the conventional method and the method of the present invention. Σ in the table indicates the standard deviation. As a result, in the case of the polarization-maintaining optical fiber (PMF), the agreement is relatively good, but the ordinary optical fiber (SMF: ordinary 1.31 μm band optical fiber, DSF:
It can be understood that the present invention shows more stable measurement results in the case of dispersion-shifted optical fiber).

[0038]

As described above, according to the present invention, highly accurate polarization dispersion measurement can be realized by converting the polarization dispersion value measured by the Stokes analyzer into SOP and performing inverse Fourier transform. The optical amplifier, the optical device, and the long-distance optical amplification repeater transmission system having a small polarization dispersion are achieved, and the excellent usefulness is achieved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of a measuring apparatus used for implementing a polarization dispersion measuring method of the present invention.

FIG. 2 is a graph illustrating an implementation process of the polarization dispersion measuring method of the present invention.

3 is a block diagram showing a configuration of a Stokes analyzer which is an example of the polarization analyzer in FIG.

FIG. 4 is a block diagram showing the configuration of a polarization analyzer using a rotary analyzer.

FIG. 5 is a diagram showing the concept of polarization analysis using a Poincare sphere.

6A and 6B show results of actual measurement using the method of the present invention. FIG. 6A is a diagram showing Poincare sphere display, and FIG. 6B is a graph showing changes in SOP in the wavelength region.

FIG. 7 is a table showing a comparison of measurement results by the conventional method and the method of the present invention.

FIG. 8 is a diagram showing a configuration of a conventional device used for measuring polarization dispersion.

FIG. 9 is a diagram showing how a polarization dispersion is obtained by displaying a Poincare sphere according to a conventional method.

FIG. 10 is a diagram for explaining the problems of the conventional method, (a) is an ideal graph when the measurement error is relatively small, (b)
Shows a graph when there are many errors, and (c) shows only the trajectory drawn on the Poincare sphere when there are many errors on a plane.

[Explanation of symbols]

 1 ... Wavelength variable light source 2 ... Wavelength control device 3 ... Optical branching device 4 ... Wavelength meter 5 ... Polarization control device 6 ... Measurement target 7 ... Polarization analyzer 7a ... Stokes analyzer 7b ... Polarization analysis using rotary analyzer Device 8 ... Recording operation processing device 9 ... Modulator 10a, 10b ... Beam splitter (BS) 11 ... Polarization beam splitter (PBS) 12 ... .pi. / 2 phase element 13 ... Analyzer 13 '... Rotating analyzer 14 ... Photodetector 15 ... Optical receiver

Claims (9)

(57) [Claims] 1. An optical signal of a variable wavelength light source is made into an arbitrary polarization state and an arbitrary polarization main axis direction by a polarization control means, is incident on a measurement target, and an optical signal emitted from the measurement target is analyzed by a polarization analysis means. Analyzing, from the number of peaks (valleys) between the wavelength at which the phase difference of the optical signal becomes a predetermined angle in the cosine wave-like periodic function indicating the fluctuation of the polarization state with respect to the wavelength of the optical signal, A polarization dispersion measuring method, characterized in that a group delay time of polarization states of signals orthogonal to each other, that is, polarization dispersion is obtained. 2. A cosine wave-shaped periodic function showing a variation of a polarization state with respect to a wavelength of an optical signal, the predetermined angle of the phase difference of the optical signal is π or π / 2, and a peak (valley) between the wavelengths.
The polarization dispersion measuring method according to claim 1, wherein the polarization dispersion is obtained from the number. 3. An optical signal of a variable wavelength light source is made into an arbitrary polarization state and an arbitrary polarization main axis direction by a polarization control means, is incident on an object to be measured, and the optical signal emitted from the object is measured by a polarization analyzing means. The cosine wave-shaped periodic function of the polarization state with respect to the wavelength of the optical signal obtained by the polarization analysis means is converted from the wavelength region of the optical signal into the frequency region, and the polarization state of the optical signal with respect to the frequency region is converted. The polarization dispersion measurement method is characterized in that the periodic function of is transformed into the time domain by inverse Fourier transform, and the polarization dispersion is obtained from the half width of the optical signal obtained by the transformation into the time domain. 4. The variable wavelength light source comprises a modulation means for emitting modulated light source light, and the polarization analysis means is a polarization analysis means capable of synchronous detection. Polarization dispersion measurement method. 5. The polarization dispersion measuring method according to claim 1, wherein the polarization analyzing means is a polarization analyzer using a Stokes analyzer or a rotary analyzer. 6. A wavelength tunable light source capable of changing the wavelength of an output optical signal, an optical branching unit for branching an optical signal output from the wavelength tunable light source into two, and an optical branching unit. Polarization that is input to one of the measurement targets by inputting one of the optical signals of the optical signal and converting the polarization state of the optical signal into an arbitrary polarization state and controlling the polarization main axis of the optical signal in any direction. Control means, monitoring means for monitoring the other of the optical signals from the optical branching means, and polarization analysis means for analyzing the optical signal output from the polarization control means and passing through the measurement target to output an electrical signal. , In the analyzed optical signal, the polarization dispersion is calculated from the number of peaks (valleys) between the wavelength at which the phase difference of the optical signal becomes a predetermined angle in the cosine wave-like periodic function showing the variation of the polarization state with respect to the wavelength. The required recording calculation processing means Polarization dispersion measuring apparatus, characterized by. 7. A variable wavelength light source capable of changing the wavelength of an optical signal to be output, an optical branching unit for branching an optical signal output from the variable wavelength light source into two, and an optical branching unit. Polarization that is input to one of the measurement targets by inputting one of the optical signals of the optical signal and converting the polarization state of the optical signal into an arbitrary polarization state and controlling the polarization main axis of the optical signal in any direction. Control means, monitoring means for monitoring the other of the optical signals from the optical branching means, and polarization analysis means for analyzing the optical signal output from the polarization control means and passing through the measurement target to output an electrical signal. , The periodic function on the cosine wave of the polarization state for the wavelength of the optical signal obtained by the polarization analysis means is converted from the wavelength region of the optical signal of the optical signal to the frequency domain, and further from the frequency domain to the time domain by the inverse Fourier transform. Convert to Polarization dispersion measuring apparatus characterized by comprising a recording processing means. 8. The polarized light according to claim 6, wherein the variable wavelength light source comprises a modulation means for emitting modulated light source light, and the polarization analysis means is a polarization analysis means capable of synchronous detection. Dispersion measuring device. 9. The polarization dispersion measuring apparatus according to claim 6, wherein the polarization analyzing means is a polarization analyzer using a Stokes analyzer or a rotary analyzer.