JPH055056B2 - - Google Patents

Description

Detailed Description of the Invention [Field of the Invention] The present invention relates to a polarization-maintaining optical fiber that stably maintains linearly polarized light or elliptically polarized light over long distances. The present invention relates to a method and apparatus for easily and highly accurately measuring the amount of mode coupling between modes of a principal axis (referring to the amount converted from one mode to another mode).

[Description of prior art]

As a conventional device of this type, there is one that utilizes backward Rayleigh scattering light. The feature of this conventional device is that it uses a Q-switch N _d ³⁺ :YAG laser as the light source.
One mode of the polarization-maintaining optical fiber is excited with high-output light from this YAG laser, and back Rayleigh scattered light from the other mode excited by mode coupling is received at each point along the length of the optical fiber.
The amount of mode coupling is determined from the value of the power of this backward Rayleigh scattered light in the longitudinal direction of the optical fiber.

However, the power of the backward Rayleigh scattered light in a single mode optical fiber is about -50 dB smaller than the optical power that causes the backward Rayleigh scattered light, and the polarized light with low crosstalk with a mode coupling amount of less than -10 dB In a holding optical fiber, it has been impossible to measure the longitudinal distribution of its mode coupling because the level of optical power to be detected is too low.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method and apparatus for easily and highly accurately measuring the amount of coupling between modes in the longitudinal direction of the fiber in a polarization-maintaining optical fiber with a small amount of mode coupling.

[Features of the invention]

A first aspect of the present invention is a method for measuring the characteristics of a polarization-maintaining optical fiber, in which one of the two orthogonal main axes of the polarization-maintaining optical fiber is measured from one end of the polarization-maintaining optical fiber to be measured. The linearly polarized light of the two principal axes of the polarization-maintaining optical fiber is detected by providing the linearly polarized light of the two principal axes of the output light obtained at the other end of the polarization-maintaining optical fiber as the input light, and the mode coupling of the two principal axes of the polarization-maintaining optical fiber is performed. In the method of measuring the characteristics, the emitted light is spatially separated into linearly polarized lights of the two principal axes, one of the linearly polarized lights is rotated at a nearly right angle, and then combined with the other linearly polarized light,
While changing the relative length of the path from when the linearly polarized lights of the two main axes are separated until they are combined at a constant speed, the peak value of the intensity change due to interference of the combined light is calculated based on the above relative distance. It is characterized by observing against.

A second invention of the present invention is a polarization-maintaining optical fiber characteristic measuring device, which includes a light source and an output light of the light source that is connected to one of the two orthogonal main axes of the polarization-maintaining optical fiber to be measured. means for polarizing the light in the polarization direction of and inputting the polarized light into one end of the polarization-maintaining optical fiber; and means for comparing the intensities of the linearly polarized light of the two principal axes of the output light appearing at the other end of the polarization-maintaining optical fiber to be measured. In the polarization-maintaining optical fiber characteristic measuring apparatus, the comparing means includes a polarizing prism that separates the emitted light into linearly polarized light of the two main axes, and one of the two linearly polarized lights of the polarizing prism. means for rotating the polarization angle to approximately a right angle; a half mirror for combining the one linearly polarized light and the other linearly polarized light that have passed through the means; and the half mirror for combining the two linearly polarized lights separated by the polarizing prism. means for changing the relative distance of the paths of the two linearly polarized lights at a constant speed until they are combined; The present invention is characterized by comprising a means for observing and recording distance.

It is desirable to use a light source with low coherence.

[Explanation based on examples]

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention.
1 is a superluminescent diode (super
This is a low-coherence light source using a luminescent diode. 2 is a polarizer, and 3 is an objective lens. 4
is the polarization-maintaining optical fiber to be measured, 5 is the objective lens,
6 is a polarizing prism using a rotation prism, 7
is a 1/2 wavelength plate, 8 is a total reflection mirror, 9 is a half mirror,
10 is a total reflection mirror, 11 is a photodetector using a germanium photodiode, 12 is an amplifier, 13 is an AC voltmeter, and 14 is a recorder.

Output light from a light source 1 is made to enter one end of a polarization-maintaining optical fiber 4 to be measured from an objective lens 3 via a polarizer 2 . The emitted light appearing at the other end of the polarization-maintaining optical fiber 4 to be measured enters a polarizing prism 6 via an objective lens 5. Here, the emitted light is split for each polarization mode, one of the emitted lights is reflected by a total reflection mirror 8 via a 1/2 wavelength plate 7, and the other emitted light is reflected directly by a half mirror. 9. The above-mentioned one outgoing light that has passed through this half mirror 9 is reflected by a total reflection mirror 10. This structure is equivalent to a Twymandalene interferometer. This total reflection mirror 10 is configured so that the distance in the direction of incident light (arrow direction) can be changed. The one output light that has passed through the half mirror 9 and the other output light that has been reflected by the total reflection mirror 10 and reflected by the half mirror 9 are configured to be input to a photodetector 11. .

To operate this, first, out of the light emitted from the light source 1, linearly polarized light in a specific direction is extracted using a polarizer 2, and the polarization of the polarization-maintaining optical fiber 4 to be measured is maintained using the objective lens 3. One of the two principal axis polarization modes (referred to as the HE ^X ₁₁ mode) is excited. The light propagating in the polarization-maintaining optical fiber to be measured is made into parallel light using the objective lens 5,
The polarizing prism 6 _spatially separates the light propagating both the HE ^X ₁₁ mode of the polarization-maintaining optical fiber to be measured and the ^HE .

In Figure 1, the light propagating in the ^HEX ₁₁ mode travels straight as linearly polarized light parallel to the plane of the paper and passes through the half mirror 9.
After passing through, it is reflected by the total reflection mirror 10 and travels again toward the half mirror 9, where it is reflected and travels toward the photodetector 11. On the other hand, the light propagated in the HE ^Y ₁₁ mode is refracted by the polarizing prism 6 as linearly polarized light perpendicular to the plane of the paper, becomes linearly polarized light parallel to the plane of the paper by the half-wave plate 7, and then passes through the half mirror 9. Combine with the previous HE ^Y ₁₁ mode light.

The total reflection mirror 10 is moved at a constant speed (10 μm/s). This allows both modes (HE ^X ₁₁ mode and
HE ^Y ₁₁ mode), the interference intensity of the combined light of the respective propagated lights changes alternately. The AC component of the combined light whose intensity changes is converted into an AC current using a photodetector, amplified by an amplifier 12, and the peak value (root mean square) of this current is measured using an AC voltmeter 13. This is recorded on the recorder 14.

The above AC current value corresponding to each position of the total reflection mirror 10 is determined at each point of the fiber as explained below.
Since it is proportional to the mode coupling coefficient between the HE ^X ₁₁ and HE ^Y ₁₁ modes, it is possible to determine the mode coupling distribution within the fiber.

HE ^X ₁₁ mode at the output end of the optical fiber (fiber length L) when only the HE ^X ₁₁ mode is excited at the input end of the polarization maintaining optical fiber to be measured
_The electric fields C _{X and} C _Y of HE ^Y _{11 mode} are as _follows : C
_{dβ _ _} ^_ _{_ _ _ _ _ _} ×f[t
_{−dβ _ _ _} Here, E ₀ , ω ₀ are the amplitudes (constants) of the incident electric field, the center frequency, β _X , β _Y are the propagation constants of the HE ^X ₁₁ mode and _HE ^Y ₁₁ mode, and _dβ Propagation constant ω=ω
The differential coefficient at _{0 ,} Z _K is the distance from _the input end of _the fiber to the point where coupling occurs between ^{the HE} Therefore, as shown in _the example above, ^HE
When the light propagating in the HE ^Y ₁₁ mode is spatially separated, the total reflection mirror is moved, and both mode lights are combined with a time difference of T from each other, the interference intensity is as follows: .

I=<|C _X (L, t+T) + C _Y (L, t) | ² >=<|C
_X (L, t+T) | ² > + < | C _Y (L, t) | ² > +2Re {C _X (L, t + T) C _Y (L, t) ^* }< | C _X (
L,t+T) | ² > {1+2Re[ν(T)]}...(2) Here, the mode coupling between HE ^X ₁₁ and HE ^Y ₁₁ is sufficiently weak, so <|C _X (L,t+T)| ² ＞＞＜｜C _Y (L,t)｜ ² ＞ is assumed to hold. Here, ν(T) is given by the following equation.

ν(T)＝jexp(jω ₀ T)・ 〓 〓 ^K Γ _K exp〔jΔβ(L−Z _K )〕・F〜[T+D(L−Z _K )
]...(3) Here, ^Δβ =β _Y −β _X D=dβ _Y ／ _dω −dβ
f(t) ^* >.

Therefore, when using a light source with low coherence, that is, a short coherence time, such as a superluminescent diode, |ν(T)| becomes T _K = −D (L−Z _K ) Take the peak value |Γ _K | only for . This is F~
This is because (r) is zero except for τ=0. Therefore, by moving the total reflection mirror 10 at a constant speed, T changes at a constant speed. For this reason,
From equation (3), τ(T) changes at a constant period over time.
The AC voltmeter 13 measures the amplitude (root mean square) of τ(T) which changes periodically. Therefore, since the AC voltmeter 13 takes a peak value proportional to |Γ _K | at the location corresponding to T _K = -D(L-Z _K ), these peak values determine the mode coupling coefficient |Γ _K | It becomes possible to determine the distribution in the fiber longitudinal direction.

From the above equation (2), the output from the AC voltmeter at T _K =-D(L-Z _K ) is proportional to P _X |Γ _K |.

Here, P _X is the power of light propagating in the ^HEX ₁₁ mode, and is approximately equal to the power of the incident light. However, in reality, the loss is 1 dB due to the loss within the optical fiber.
degree decreases.

The AC voltmeter 13 used in this device was able to observe the peak value even with an incident power of 0.1 μW to the optical fiber. From this, 1
For light incidence of mW, it is possible to observe values up to |Γ _K |=10 ⁻⁴ mW/1 (mW)=10 ⁻⁴ .

Since the amount of mode coupling determined from the mode coupling coefficient Γ _K at Z=Z _K is |Γ _K | ² , it has become possible to determine the mode coupling value up to -80 dB in the longitudinal direction of the fiber using this device.

Note that although a superluminescent diode is used as the light source 1 in this device, a high-intensity white light source can also be applied to this measurement.

FIG. 2 is a diagram showing the measurement results of the fluctuation of propagating light for a 1 km long stress-applied polarization-maintaining optical fiber. This optical fiber, abbreviated as PANDA fiber, was measured at Δ=0.6%, beat length 2 mm, crosstalk -30.5 dB, loss 1.3 dB/Km, and wavelength 1.3 μm. The wavelength of the superluminescent diode is 1.3 μm, the fiber input power is 1 mW, and the coherent length is 50 μm. The fiber length of 0 m in FIG. 2 indicates the first part of the optical fiber when it is drawn, and the 1.0 km point is the final part of the drawing. From FIG. 2, it can be seen that in the first part of the line drawing, there is a 1 dB variation in orthogonal polarization components due to periodic waveguide fluctuations of the PANDA fiber.

Additionally, the waveguide fluctuation is larger at the last part of the fiber drawing, by 6 dB compared to the first part.
It can be seen that large orthogonal polarization components are generated.

Regarding PANDA fiber, Y.Sasaki, T.Hosaka K.Takada, T.Noda,
“8Km long polarization maintaining
fiberwith highly stable palarization state，”
Electron.Lett, vol.19, No.19, PP.792−794
(1983) has a detailed description.

Regarding the superluminescent diode used as a light source in the above column, see IPKaminov, et.al, “Lateral confinement
In Ga AsP superluminescent diode at 1.3μ
m,”IEEE J.Quantum Electron., vol.QE−19,
There is a detailed description in PP.78-82.Jan.1983.

[Explanation of effects]

As explained above, according to the present invention, the output light of the polarization-maintaining optical fiber to be measured is separated into two principal axes of polarized light, and comparative measurements are performed using the interference of these polarized lights. The amount of binding can be measured easily and with high precision. According to the method of the present invention, the amount of mode coupling can be detected up to about -80 dB.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an embodiment of the apparatus of the present invention. Figure 2 shows the measurement results of fluctuations in a 1 km long stress-applied polarization-maintaining optical fiber. DESCRIPTION OF SYMBOLS 1...Light source, 2...Polarizer, 3...Objective lens, 4...Polarization-maintaining optical fiber for measurement, 5...
Objective lens, 6...Polarizing prism, 7...1/2 wavelength plate, 8...Total reflection mirror, 9...Half mirror, 1
0...Total reflection mirror, 11...Photodetector, 12...Amplifier, 13...AC voltmeter, 14...Recorder.

Claims (1)

[Claims] 1. Linearly polarized light of one of the two orthogonal principal axes of the polarization-maintaining optical fiber is applied as incident light from one end of the polarization-maintaining optical fiber to be measured; In the method of measuring the mode coupling characteristics of the two principal axes of this polarization-maintaining optical fiber by detecting the linearly polarized light of the two principal axes of the emitted light obtained at the other end, spatially separate each linearly polarized light, rotate one linearly polarized light almost at right angles, and combine it with the other linearly polarized light.The two main axis linearly polarized lights are separated and then combined. A polarization-maintaining optical fiber characterized in that the peak value of the intensity change due to interference of the combined light is observed with respect to the above-mentioned relative distance while changing the relative length of the path to the polarization-maintaining optical fiber at a constant speed. Characteristic measurement method. 2. a light source; means for polarizing the output light of the light source in the polarization direction of one of the two orthogonal principal axes of the polarization-maintaining optical fiber to be measured, and causing the polarization to enter one end of the polarization-maintaining optical fiber; In a polarization-maintaining optical fiber characteristic measuring device comprising means for comparing the intensities of the linearly polarized light of the two principal axes of the output light appearing at the other end of the polarization-maintaining optical fiber to be measured, the comparing means comprises: a polarizing prism that separates the emitted light into the linearly polarized light of the two main axes; a means for rotating the polarization angle of one of the two linearly polarized lights of the polarizing prism to approximately a right angle; and one of the linearly polarized lights that has passed through the means. and the other linearly polarized light, and a constant relative distance between the paths of the two linearly polarized lights until the two linearly polarized lights separated by the polarizing prism are combined on the half mirror. Polarization-maintaining light characterized by comprising: means for changing the velocity; and means for observing and recording the peak value of the intensity change due to interference of the light combined by the half mirror with respect to the relative distance. Fiber characteristic measuring device. 3. The polarization-maintaining optical fiber characteristic measuring device according to claim 2, wherein the light source is a light source with low coherence.