JPH0682086B2 - Mode coupling evaluation device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention relates to a mode in which the position and degree of coupling between two propagation modes in an optical transmission line can be measured with high resolution. The present invention relates to a coupling evaluation device.

(Prior Art) Conventionally, there is a device that evaluates the strength of a propagation signal at the end of a transmission line as a device for evaluating the power coupling between the propagation modes in the transmission line. However, in this conventional device, both power couplings are integrated by the length of the transmission line, so that the dependency in the longitudinal direction of the transmission line cannot be obtained and the power coupling cannot be properly evaluated.

Further, as another conventional example, an optical pulse is incident on the transmission line in order to determine the dependence in the longitudinal direction, and the mode coupling is evaluated from the back-scattered light propagating in the opposite direction generated in the transmission line. There (M, Nakazawa et al, “Measure
ments of polarization mode couplings along p
olarization−maintaining Singl−mode optical fi
bers ", J.Opt.Soc.Am.A, vol-1, pp285-292, 1984).

However, with the one that uses this backscattered light,
First, the intensity of backscattered light is 30-40 dB with respect to the intensity of incident light.
Will fall. Therefore, it becomes necessary to increase the intensity of incident light. Further, since an optical pulse is used as the incident light, the resolution in the longitudinal direction is about the pulse width, and as a typical numerical example, when the pulse width is 1 μs, it becomes about 100 m.

In addition, the pulse height (intensity) is required to be about several kW from the light receiving sensitivity limit of the photodetector, and the extinction ratio of the optical fiber to be measured is limited to about 20 dB / km or less, and the degree of mode coupling is The dynamic range becomes narrow.

Further, when the intensity of the light pulse exceeds several kW, conversely, stimulated Raman scattering and Brillouin scattering are induced in the transmission line, and these scattered lights are detected as noise. Therefore, the correction is necessary.

(Problems to be Solved by the Invention) In the conventional example in which the strength of the propagation signal is evaluated at the end of the transmission line, the longitudinal dependence of the transmission line cannot be obtained, and the mode coupling can be appropriately performed with high resolution. There was a problem that it could not be evaluated.

Further, in the conventional example in which an optical pulse is used as the incident light and the mode coupling is evaluated from the backscattered light, the intensity of the backscattered light decreases, so the intensity of the incident light is reduced to several kW.
However, there is a problem that the distance resolution is low and the dynamic range for the degree of mode coupling is narrow.

The present invention has been made based on the above circumstances, and is capable of quantitatively measuring the distribution of the degree of mode coupling in the longitudinal direction with high resolution, and also capable of widening the dynamic range for the degree of mode coupling. The purpose is to provide an evaluation device.

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above problems, the present invention provides a light source that emits light having coherence and a light source that emits light.
Means for injecting into one of the two propagation modes of the ratio measurement transmission line having two propagation modes and light having two propagation mode components emitted from the measured transmission line. Separation means for separating into propagation modes, optical path length varying means for variably setting a required optical path length difference between both lights separated by the separating means, and optical path length varying means for setting a required optical path length difference. And a means for combining the two lights transmitted through the respective optical paths, and a means for photoelectrically converting the interference light combined by the combining means to detect the amplitude of the interference light as a function of the optical path length difference. Having it is the gist.

(Operation) Coherent light emitted from the light source is incident as one propagation mode on the measured transmission path having two propagation modes. Part of the incident light undergoes mode conversion in the measured transmission line, and light having two propagation mode components is emitted from the measured transmission line. The light emitted from the transmission line to be measured is separated into two propagation mode components by the separation means, variably set so as to have a required optical path length difference between the separated lights, and then transmitted through each optical path. Both lights are combined, and the amplitude of the interference signal between the two lights is detected as a function of the optical path length difference.

In this way, the light transmitted through the transmission path to be measured is used to detect an interference signal due to both propagation mode components, and the degree of mode coupling is detected by temporal separation by the group delay time difference between both propagation modes. .

Thus, the longitudinal distribution of the degree of mode coupling is quantitatively measured with optical resolution, and the dynamic range for the degree of mode coupling is expanded.

(Embodiment) First, an embodiment of the present invention will be conceptually described with reference to FIG.

In FIG. 1, 1 is a light source that emits a light wave having coherence,
Reference numeral 2 denotes a measured transmission line having two propagation modes, and the two propagation modes are schematically illustrated in the figure as separated. Reference numeral 3 is an optical path length varying device as an optical path length varying means, and 4 is a photodetector for photoelectrically detecting the combined light of the lights propagating in the measured transmission path 2 in two propagation modes.

Then, the coherent light wave emitted from the light source 1 enters the measured transmission path 2 and propagates as mode 1.

Now, assuming that the distance of the measured transmission line 2 is divided into N and the coupling coefficient between both modes at the position of the distance Zi is hi, the time average light intensity I detected by the photodetector 4 is written as follows. Can be represented.

Where j is , Ψ ₁₁ and ψ ₁₂ represent the electric field propagated without conversion and the electric field converted from mode 1 to mode 2, respectively, and Re represents the real part thereof. β ₁ and β ₂ are propagation constants in mode 1 and mode 2, ω is the angular frequency of light, C is the speed of light, L is the length of the measured transmission line, and ΔL is the optical path difference given by the optical path length varying device 3. is there.

A (ω) is the amplitude spectrum of the light source 1, and the oscillation power spectra S (ω) and S (ω) = A of the light source 1
(Ω) ^* · A (ω).

Gaussian distribution with center angular frequency ω ₀ and spectral width Δω, with S (ω) as a general approximation Assuming that, the amplitude of the interference term in the above equation (1) is calculated by the following equation.

(Dβ ₁ / dω) ₀ and (dβ ₂ / dω) ₀ represent differential coefficients of the propagation constant at ω = ω ₀ .

From the above equations (5) and (6), the amplitude of the interference signal is Ri = 0
Takes the maximum value at. That is, ΔL = − {(dβ ₁ / dω) ₀ − (dβ ₂ / dω) ₀ } C for the optical path where power coupling occurs at the position Zi.
The interference intensity becomes maximum when (7) is satisfied.

In this case, the half width ΔZ (coherence length of the light source 1) of the position Zi at which the interference intensity with respect to the position Zi becomes 1 / e of the maximum value is
From equations (5) and (6), Given in. Therefore, the resolution for position Z is ΔZ
Can be treated as a degree. This ΔZ is the coherence time of the light source 1. And the group velocity difference ΔVg = {(dβ ₁ / dω) ₀ − (dβ ₂ / dω) ₀ } ⁻¹ between the two modes, which can be interpreted as a propagation distance difference between the two modes occurring at Tc time.

Next, in order to standardize the value of the coupling coefficient hi, the mode component of the light emitted from the optical fiber, which is the same as the propagation mode excited at the incident end, is spatially divided into two, and one of the light fluxes passes through the optical path length varying device 3. The case where both light fluxes are combined after that is handled. In this case, the amplitude of the interference signal obtained is the same as that of the equation (5). And the ratio between equation (5) and equation (10) is Can be calculated. From this, considering the mode coupling per ΔZ, Therefore, the coupling coefficient hi is hi = (/ ₀ ) ² / {2 + (/ ₀ ) ² } …… (13) (1/4) ・ (/ ₀ ) ² …… (14) (when hi << 1) Is given.

Further, the extinction ratio η between the propagation modes is a value obtained by integrating the coupling coefficient for the transmission line length L. The extinction ratio η is approximately Here, N = L / ΔZ can be calculated.

Next, an embodiment of the present invention will be described with reference to FIGS.

In FIG. 2, components that are the same as or equivalent to the devices and the like shown in FIG. 1 are designated by the same reference numerals as those used above, and a duplicate description will be omitted.

First, the configuration of the mode-coupling evaluation device will be described. In FIG. 2, 5 is a polarizer, 6 is a birefringent optical fiber under test (transmission path under test) having two propagation modes for measuring mode coupling. ) And the incident light on the optical fiber 6 to be measured is made into linearly polarized light that coincides with its birefringence axis by the polarizer 5. Thus, the polarizer 5 constitutes a means for causing the light emitted from the light source 1 to enter one of the two propagation modes of the optical fiber 6 to be measured.

Reference numeral 7 is a prism having the same action as a Rochon prism, which spatially separates two orthogonal propagation mode components, and the two propagation mode components emitted from the optical fiber 6 under measurement by this prism. Means for separating the light flux having the above into two propagation modes is configured. Reference numeral 8 denotes a frequency shifter for providing a constant frequency difference between the two light beams split by the prism 7 and realizing optical heterodyne detection of an interference signal. A typical example of the frequency shifter 8 is an acousto-optic modulator, which shifts the frequency of light due to the interaction between ultrasonic waves and light.

9a, 9b and 9c are reflecting mirrors, 10 is a semi-transparent mirror as a multiplexing means, 11
Is an analyzer, and the analyzer 11 has a function of combining two orthogonally polarized light beams. 12 is an intermediate frequency filter for extracting the shift frequency component given by the frequency shift device 8,
Reference numeral 13 is an amplifier, and 14 is a waveform storage device. The photodetector 4, the intermediate frequency filter 12, the amplifier 13 and the waveform storage device 14 constitute means for detecting the amplitude of the interference light as a relationship of the optical path length difference.

Further, as the optical path length varying device 3, for example, a device composed of a movable reflecting mirror 15 and two fixed reflecting mirrors 16 as shown in FIG. 3 is used.

Next, the operation will be described.

The coherent light emitted from the light source 1 is transmitted to the polarizer 5
Transmitted through the optical fiber 6 and incident on the optical fiber 6 to be measured with polarized light that coincides with one of the birefringence axes, and then H ▲ E ^x ₁₁ ▼ or H ▲ E ^y ₁₁
▼ Propagated as a mode. Hereinafter, in this embodiment, H
E ^y ₁₁ ▼ Described below is the case where the mode is excited.

The excited H ▲ E ^y ₁₁ ▼ and the H ▲ E ^x ₁₁ ▼ mode generated in the measured optical fiber 6 are spatially separated by the prism 7, and one of the separated light beams is a frequency shifter. 8
Gives a frequency shift of a constant angular frequency Δωb,
The light is reflected by the reflecting mirror 9a and enters the semi-transparent mirror 10. The other light flux enters the semi-transparent mirror 10 via the optical path length varying device 3. Then, the two light beams that are divided and orthogonal to each other are combined by the semi-transparent mirror 10, extracted by the analyzer 11 as the same polarized light, and photoelectrically detected by the photodetector 4.

At this time, H ▲ E generated in the measured optical fiber 6
^As described with reference to FIG. 1, the interference intensity corresponding to the position Z on the optical fiber 6 to be measured can be obtained depending on the degree of power coupling between the ^x ₁₁ mode and the H E ^y ₁₁ mode. However, in this embodiment, since a constant angular frequency difference Δωb (Δωb << Δω) is given between the two interfering light beams, the equation (11) can be written as follows.

That is, the angular frequency Δωb component of the interference signal is detected by the intermediate frequency filter 12, and its amplitude value is associated with the mode coupling at the position Z.

Next, FIGS. 4A to 4D show specific examples of the measurement results based on the measurement system of FIG.

The optical fiber 6 to be measured has a total length of 100 m, and the spectrum width of the light source 1 used is about 500 GHz.

(A) to (C) of FIG. 4 are the amplitudes of the interference signals represented by the equation (17), and the abscissa represents the optical path length in the space given by the optical path length varying device shown in FIG. The difference ΔL is 8.19 mm / div. The vertical axis represents the amplitude of the interference signal on a linear scale.

First, in FIG. 4A, the same propagation mode as the excited mode of the light emitted from the optical fiber 6 to be measured is measured by the prism 7
Is the ΔL dependence of the amplitude of the interference signal detected by the photodetector 4 when it is spatially divided into two. This can be realized by rotating the main axis of the prism 7 from a predetermined position in FIG. 2, and at this time, the influence of the other propagation mode can be ignored.

FIG. 4B shows the ΔL dependence of the amplitude of the interference signal obtained when the light emitted from the optical fiber 6 to be measured is divided into two orthogonal propagation modes by the prism 7. The two peaks seen at the left and right ends are caused by an error in the angle setting of the polarizer 5 and the prism 7, and correspond to both end positions of the optical fiber 6 to be measured. In the example of the figure, the optical path length difference in space between both peaks is 39 mm, and when this is divided by the luminous flux, the group delay difference Tp130ps between the two propagation modes is obtained.
This corresponds to the length L of the measured optical fiber 6 = 100 m.
The distance resolution in this measurement is given by the equation (8), and the measured value Tc (0.67ps) and the group velocity difference ΔVg (L / T
From p), ΔZ is about 0.5 m.

(C) and (D) of FIG. 4 show lateral pressure on the measured optical fiber 6 at positions 36 m and 37 m from the incident end of the measured optical fiber 6 under the same conditions as in (A) of FIG. It shows the beat amplitude with respect to the optical path length difference when the mode coupling is generated by giving it. Two peaks indicating the lateral pressure applying portion can be confirmed at the center of (C) of FIG. FIG. 4D is an enlarged horizontal axis view of these two peak portions.
The vertical axis of (D) in FIG. 4 is enlarged 18 times compared to (A) of FIG. 4, and the ratio of the peak heights in (D) and (A) of FIG. 4 is about 0.1. Met. The ratio of peak height is (1
Since it corresponds to the amount given by equation (1), the mode coupling coefficient h generated by the application of lateral pressure is approximately 2.5 × 10 ^-3 from equation (14)
was gotten.

[Effects of the Invention] As described above, according to the configuration of the present invention, one propagation mode light excited in the measured transmission line and the other propagation mode light generated by mode coupling in the measured transmission line. The amplitude of the interference signal due to is detected as a function of the optical path length difference set between the two propagation mode lights by the optical path length varying means, and thus the interference due to the two propagation mode components is performed by using the light transmitted through the measured transmission path. Since the amplitude of the signal is detected and the degree of mode coupling is detected with time separation by the group delay time difference between both propagation modes, the longitudinal distribution of the mode coupling degree is quantitatively measured with high resolution. The advantage is that the dynamic range for the degree of coupling is expanded.

[Brief description of drawings]

FIG. 1 is a block diagram for conceptually explaining a mode coupling evaluation apparatus according to the present invention, FIG. 2 is a block diagram showing an embodiment of the present invention, and FIG. 3 is an optical path applied to the same embodiment. FIG. 4 is a configuration diagram showing an example of a length varying device, and FIG. 4 is a diagram showing an example of measured values of beat amplitudes with respect to optical path length differences obtained in the same embodiment. 1: Light source, 3: Optical path length variable device, 4: Photodetector, 5: Polarizer, 6: Optical fiber to be measured (transmission path to be measured), 7: Prism (separation means), 8: Frequency shifter, 10: Semi-transparent mirror (combining means), 11: Analyzer, 12: Intermediate frequency filter, 14: Waveform storage device.

Claims (1)

[Claims] 1. A light source that emits light having coherence and a light emitted from the light source into one of the two propagation modes in a transmission line under test having two propagation modes. An incident path, a separating means for separating the light having two propagation mode components emitted from the measured transmission path into the two propagation modes, and a required optical path between the two lights separated by the separating means. Optical path length varying means for variably setting the length difference, combining means for interfering both lights transmitted through the respective optical paths set to the required optical path length difference by the optical path length varying means, and the combining means for combining Means for photoelectrically converting the generated interference light and detecting the amplitude of the interference light as a function of the optical path length difference.