JPH06100519B2 - 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, as a device for evaluating power coupling between propagation modes in a transmission line, the strength of a propagation signal is evaluated at the end of the transmission line. However, in this conventional device, since the amount of power coupling is integrated with the length of the transmission line, 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. is there.

(M, Nakazawa et al, “Measurements of polarization m
ode couplings along polarization-maintaining Singl
-mode optical fibers ", J.Opt.Soc.Am.A, vol-1, pp285-2
92, 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.

[Configuration 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.
Demultiplexing means, means for variably setting the required optical path length difference between the demultiplexed lights, and both lights transmitted through each optical path set to the required optical path length difference are combined. Means for injecting into the measured transmission path having two propagation modes, means for extracting light transmitted in one of the two propagation modes in the measured transmission path, and photoelectric conversion of the extracted light And a means for detecting the amplitude of the interference signal of the light as a function of the optical path length difference.

(Operation) Coherent light emitted from the light source is divided into two, and the divided two lights are transmitted through the respective optical paths set to the required optical path length difference, and then combined to form two light beams. It is incident on the transmission line under test having a propagation mode.

The combined light is transmitted in each propagation mode through the measured transmission path. Then, of the transmitted light, only the light transmitted in one propagation mode is extracted and photoelectrically converted, and the amplitude of the interference signal that interferes in the measured transmission path is detected as a function of the optical path length.

In this way, the light transmitted through the transmission path under test is used to detect the interference signal due to both propagation mode components, and the degree of mode coupling is detected temporally separated by the group delay time difference between both propagation modes. It

Thus, the longitudinal distribution of the degree of mode coupling is quantitatively measured with high 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 is an optical path length varying device, 3 is a transmission path (waveguide), and 4 is a photodetector.

Then, the coherent light wave emitted from the light source 1 enters the transmission path 3 and propagates as mode 1 and mode 2. At this time, the mode 1 is changed by the optical path length varying device 2.
And the difference between the optical path lengths of mode 2 is adjusted to be ΔL as described later.

Now, assuming that the distance of the transmission line 3 is divided into N and the power coupling coefficient between both modes at the position of the distance Z _i is h _i , the photodetector 4
The time-averaged light intensity I _s detected by can be expressed as the following equation.

I _s = <| φ ₁₂ | ² > + <| φ ₂₂ | ² > + 2Re [<φ ₁₂ / φ ^*
₂₂ ＞]… (1) Here, ω represents the angular frequency of light, φ ₁₂ and φ ₂₂ represent the electric field mode-converted from mode 1 to mode 2 and the electric field propagated without conversion in mode 2, and Re represents the real part thereof. . β ₁ and β ₂ are propagation constants in mode 1 and mode 2, c is the speed of light in free space, ΔL
Is the optical path difference given by the optical path length varying device 2.

A (ω) is the amplitude spectrum of the light source 1, and the oscillation power spectrum S (ω) of the light source 1 (= << A (ω) · A
^* (Ω)>) is a Gaussian distribution with a half-width Δω as a general approximation I am assuming. In the equation (4), I ₀ is the light intensity of the light source 1, and ω
₀ is the oscillation center frequency.

At this time, the amplitude I of the interference term which is the third term of the equation (1)
Is calculated as follows.

From the above equations (5) and (6), the amplitude of the interference signal is R _i = 0.
Takes the maximum value at. That is, for the optical path where the power coupling occurs at the position Z _i , The interference intensity becomes maximum when the above condition is satisfied.

In this case, the position where the interference intensity has the maximum value and 1 / e of the maximum value
Difference ΔZ (coherence length of the light source 1) from
From equations (5) and (6), Given in. Therefore, the resolution for position Z is ΔZ
And h _i can be considered as a coupling coefficient per 2ΔZ. The above equation (8) is the coherence time of the light source 1. And the group velocity difference between both modes It can be expressed as the product of and.

Next, in order to normalize the value of the coupling coefficient h _i , when setting is made so that both light beams emitted from the light source 1 and propagated in two are propagated in the same mode, the equation (2) becomes Similar to the equation (3), it becomes as follows.

As a result, the interference signal at ΔL = 0 Becomes Assuming that the ratio of equation (5) and equation (10) is I _R , I _R is given by the following equation.

If the distance between the positions Z _{i where} the mode coupling occurs is sufficiently larger than the resolution ΔZ, I _{R at the} positions Z _i can be approximated by the following equation.

From the equation (12), the degree of coupling coefficient h _i is given by h _i = I _Ri ² / (1 + I _Ri ² ) ... (13).

Further, the extinction ratio η between the two propagation modes is determined by the transmission path length L.
The integrated value of the power-converted coupling coefficient Is defined as This can be approximated as N = L / 2ΔZ.

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

In FIG. 2 and FIG. 4 which will be described later, the same or equivalent devices as those in FIG. 1 are designated by the same reference numerals, and the duplicated description will be omitted.

First, the configuration will be described. In FIG. 2, reference numeral 5 is a frequency shifter, which is used to split coherent light from the light source 1 into two light waves that differ by a constant frequency difference. A demultiplexing unit is configured.

As the optical path length varying device 2, for example, a device including a movable reflecting mirror 20 and two fixed reflecting mirrors 21 as shown in FIG. 3 is used.

6 is a mirror, 7 is a retarder, 8 and 9 are polarizers, respectively
11 is a semi-transparent mirror. The semi-transparent mirror 11 constitutes means for recombining the two demultiplexed and polarized light waves.

Reference numeral 12 is an optical fiber under test (transmission line under test) having two propagation modes, 13 is an analyzer, and the analyzer 13 constitutes means for extracting an optical component of only one of the two propagation modes. To be done. Reference numeral 14 is an intermediate frequency filter (band pass filter), 15 is an amplifier, and 16 is a waveform recording device, and means for detecting (detecting) the amplitude of the interference signal as a function of the optical path length difference by the intermediate frequency filter 14 and the waveform recording device 16. Is configured.

Next, the operation will be described.

Coherent light emitted from the light source 1 passes through the frequency shifter 5 and is split into two light waves that differ by a constant frequency difference Δν.

One of the split light waves passes through the phase shifter 7 and the polarizer 8 and is incident on the semi-transparent mirror 11, and the other light wave passes through the optical path length varying device 2, the reflecting mirror 6 and the polarizer 9, and then enters the semi-transparent mirror 11. Is incident on. Then, the two divided and polarized light waves are recombined by the semi-transparent mirror 11 and are incident on the measured optical fiber 12 having a double diffraction property.

At this time, the polarization directions of the respective divided light waves are set so as to be orthogonal to each other and coincide with the double diffraction axis of the optical fiber 12 to be measured.

Thus, the two light waves propagate in the optical fiber 12 to be measured as ▲ HE ^X ₁₁ ▼ and ▲ HE ^Y ₁₁ ▼ modes, respectively, and after exiting the optical fiber 12 to be measured, the analyzer 13 causes one of the birefringences of one of them. Only the component in the analysis axis direction is extracted and photoelectrically detected by the photodetector 4.

At this time, ▲ HE ^X occurs in the optical fiber 12 to be measured.
_As described with reference to FIG. 1, it is possible to obtain the interference intensity corresponding to the position Z on the optical fiber 12 to be measured, depending on the degree of power coupling between the ₁₁ mode and the HE ^Y ₁₁ mode.

However, in this embodiment, a constant angular frequency difference Δω _b (Δω _b << Δω) is given between the two interfering light waves, and the interference signal is observed as the amplitude of the beat signal oscillating at Δω _b .

The intermediate frequency filter 14 is used to extract the [Delta] [omega _b component, the amplitude value of [Delta] [omega _b components are recorded in the waveform recording device 16.

Next, FIG. 4 shows another embodiment of the present invention.

In this embodiment, it is possible to evaluate the relative change with respect to the distance dependency in the degree of mode coupling.

In this embodiment, the arrangement of the phase shifter and the polarizer in the one embodiment (FIG. 2) is omitted, and the mode filter 17 is provided instead of the analyzer.

Then, the coherent light wave emitted from the light source 1 is
After passing through the frequency shifter 5, it is split into two light waves that differ by a constant frequency difference Δν.

One of the split light waves is directly incident on the semi-transparent mirror 11, and the other light wave is incident on the semi-transparent mirror 11 via the optical path length varying device 2 and the reflecting mirror 6. And the two divided light waves are
The light is multiplexed again by the semi-transparent mirror 11 and is incident on the optical fiber 12 to be measured.

The two light waves are transmitted through the measured optical fiber 12 in the LP ₀₁
After propagating as a mode and LP ₁₁ mode and exiting the optical fiber 12 to be measured, only one mode light passes through the mode filter 17 and is photoelectrically detected by the photodetector 4.

The normalized value of the interference signal detected by the photodetector 4 is
Similar to the equation (12), it is represented by the following equation.

Here, C is the deterioration coefficient of the interference efficiency based on the difference in the spatial electric field distributions of the LP ₀₁ mode and the LP ₁₁ mode, and its value is a constant value for the same optical fiber, and therefore for different positions Z _i . The relative change of the mode coupling coefficient can be observed.

Next, FIGS. 5A to 5D show specific examples of the measurement results based on the measurement system of FIG.

The optical fiber 12 to be measured has a total length of 100 m, and the spectrum width of the light source 1 used is about 500 GHz.

5 (A) to (C) are interference signals (optical beat signals).
And the horizontal axis is the optical path length difference ΔL. The horizontal axes of FIGS. 5 (A), (B), and (C) are 8.2 and 4.1 mm / div from the measured values, respectively. The vertical axis represents the amplitude of the optical beat signal on a linear scale.

First, (A) of FIG. 5 shows the beat amplitude when the same polarization (▲ HE ^Y ₁₁ ▼) is set in the optical fiber 12 of the two light waves in FIG. 2 and the total power is transmitted by the analyzer 13. It is ΔL-dependent and corresponds to the temporal coherence degree curve of the light source 1 (reference: N. Shibata et al. “Measurements of Pol”).
arisation Mode Dispersion by Optical Heterodyne De
tection ", Electron Lett, 20, pp1055 to 1057, 1984)." ▲ HE ^X ₁₁ ▼ + ▲ HE ^Y ₁₁ ▼ "shown in FIG. 5 (A) represents a two-beam propagation mode that causes an interference signal. .

Figure 5 (B) is in the second optical fiber to be measured 12 two light waves in Figure, ▲ HE ^X ₁₁ ▼ and ▲ HE ^Y ₁₁ ▼ propagate as mode, ▲ HE ^Y ₁₁ ▼ mode only the analyzer 13 Shows the ΔL dependence of the beat amplitude when the light is transmitted by. ▲ HE in the figure
^X ₁₁ ^{→ Y} ▼ represents the propagation mode component converted from the ▲ HE ^X ₁₁ ▼ mode to the ▲ HE ^Y ₁₁ ▼ mode in the optical fiber. The vertical axis is magnified 18 times compared to (A) of FIG. The two peaks at the left and right ends are due to the mismatch between the birefringence axis of the measured optical fiber 12 and the polarization direction of the incident light when the measured optical fiber 12 is excited. Corresponds to both ends. In the example in the figure, the peak-to-peak distance is 39.0 mm, and the group delay difference between the two modes is τ (= L / δVg).
Equivalent to 130ps.

The peak-to-peak amplitude level in FIG. 5 (B) is −35 dB or less with respect to the peak level in FIG. 5 (A). Therefore, the coupling degree in the longitudinal direction of the optical fiber 12 to be measured is , It turns out to be almost constant.

The distance resolution in this measurement is given by the above equation (8), and from the measured value τ _c (0.67ps) and the group velocity difference δVg,
The distance resolution ΔZ is about 0.5 m.

5C and 5D show the external pressure applied to the measured optical fiber 12 at positions 36 m and 37 m from the incident end of the measured optical fiber 12 under the same conditions as in FIG. 5B. In addition, the beat amplitude when mode coupling is generated is shown. Two peaks showing the external pressure applying portion can be confirmed in the central portion of FIG. 5 (C). This (2) is shown in FIG.
It is a horizontal axis enlarged view of one peak part. From this figure, two points separated by 1 m where mode coupling has occurred are detected, and the coupling coefficient h _i can be planned using equation (13) from the height of each peak. In the embodiment of FIG. 5 (D), the height of the left peak (corresponding to I _Ri in the equation (13)) is about 4 × 10 ^-2.
As a result, a coupling coefficient of 1.6 × 10 ⁻³ was obtained.

As described above, in this example, the degree of mode coupling in the longitudinal direction of the optical fiber can be measured with a distance resolution of about 0.5 m, and the coupling coefficient can be quantitatively evaluated.

[Effects of the Invention] As described above, according to the configuration of the present invention, only the light transmitted in one propagation mode is extracted from the lights transmitted in the two propagation modes through the measured transmission path. The amplitude of the interference signal that is photoelectrically converted and interferes in the measured transmission path is detected as a function of the optical path length difference set by the means for setting the optical path length difference. In this way, the interference signal due to both propagation mode components is detected by using the light transmitted through the measured transmission line, and the degree of mode coupling is detected by the group delay time difference between both propagation modes separated in time, The longitudinal distribution of the degree of mode coupling is quantitatively measured with high resolution, and there is an advantage that the dynamic range for the degree of mode 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 block diagram showing an example of a length varying device, FIG. 4 is a block diagram showing another embodiment of the present invention, and FIG. 5 is a diagram showing an example of measured values of beat amplitude with respect to the optical path length difference obtained in the same embodiment. is there. 1: Light source, 2: Optical path length variable device, 4: Photodetector, 5: Frequency shift device (demultiplexing means), 8, 9: Polarizer, 11: Semi-transparent mirror (combining means), 12: Target Measurement optical fiber (measured transmission line), 13: Analyzer (means for extracting one propagation mode component), 14: Intermediate frequency filter, 16: Detects the amplitude of the interference signal together with the intermediate frequency filter as a function of optical path length difference Waveform recording device constituting means, 17: mode filter (means for extracting one propagation mode component).

Front page continuation (72) Inventor Yoshiyuki Aomi 162 Shirahane, Shikataji, Tokai-mura, Naka-gun, Ibaraki Nippon Telegraph and Telephone Corporation, Ibaraki Telecommunications Research Institute (56) Reference JP-A-60-147627 (JP) , A) JP-A-61-47533 (JP, A) JP-A-53-81146 (JP, A) JP-B-55-27293 (JP, B2)

Claims (1)

[Claims] 1. A light source that emits light having coherence, a demultiplexing unit that splits the light emitted from the light source into two, and a required optical path length difference between the demultiplexed lights is variably set. Means for combining the two lights transmitted through the respective optical paths set to have a required optical path length difference, and causing the two lights to enter a measured transmission path having two propagation modes, and two means in the measured transmission path. And a means for extracting the light transmitted in one of the propagation modes and a means for photoelectrically converting the extracted light and detecting the amplitude of the interference signal of the light as a function of the optical path length difference. Characteristic mode coupling evaluation device.