JPH0658291B2 - Optical transmission loss measuring method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is used for measuring an optical transmission loss by an optical fiber. In particular, the present invention relates to a method and apparatus for independently measuring the transmission loss due to the bending and the length of an optical fiber for each propagation mode of the optical fiber.

〔Overview〕

The present invention provides a method and an apparatus for measuring an optical transmission loss of an optical fiber, in which a plurality of modes propagated through an optical fiber under measurement and a single mode propagated through a reference optical fiber are interfered to thereby cause The transmission loss of each mode is measured independently by using the group delay difference of.

[Conventional technology]

In order to measure the transmission loss due to bending or length of an optical fiber, conventionally, a change in the intensity of propagating light including a plurality of modes, in particular, the lowest LP ₀₁ mode and the LP ₁₁ mode that is the next lower mode is used. Was measured and sought.

[Problems to be solved by the invention]

However, the LP ₀₁ mode and the LP ₁₁ mode have essentially different losses with respect to bending and length. But,
It was not possible to measure LP ₀₁ mode and LP ₁₁ mode independently. Further, in the case of measuring a short length of optical fiber to be measured, there is a drawback that the dynamic range of the measurement becomes small because the difference in loss due to modes is small, which makes quantitative evaluation difficult. Regarding this, by Masaharu Ohashi et al., "LP ₁₁ mode loss measurement (LP ₁₁ mode in the two-mode propagation region of an optical fiber
loss measurement in the two-mode-propagation regi
on of optical fibers, "Opt. Letters, Vol. 9, 1984, pp. 303-305.

It is an object of the present invention to provide a method and apparatus for independently measuring the transmission loss of a plurality of modes propagating in an optical fiber, in particular, LP ₀₁ mode and LP ₁₁ mode.

[Means for solving problems]

The optical transmission loss measuring method of the present invention splits light emitted from a light source into two light fluxes, one of the two light fluxes is incident on an optical fiber to be measured, and propagated in a plurality of modes. The other of the two is incident on the reference optical fiber and propagated in one mode, and the luminous flux propagating through the measured optical fiber and the luminous flux propagating through the reference optical fiber are interfered with each other,
Utilizing the fact that the interference caused by each mode occurs at different positions due to the group delay difference between the plurality of modes, a group of the target mode of the plurality of modes and the light flux propagating through the reference optical fiber The optical path length difference of at least one of the two light fluxes is changed so that the delay difference approaches zero, and the intensity of interference caused by the target mode is independently observed, and the optical transmission loss is measured for each mode. And

Practically, a plurality of modes propagating in the optical fiber to be measured are LP ₀₁ mode and LP ₁₁ mode, and one mode propagating in the reference optical fiber is LP ₀₁ mode.

The optical transmission loss measuring device of the present invention includes a light source that emits coherent light, a light splitting unit that splits the light flux emitted by this light source into two light fluxes, and one of the light fluxes split by this light splitting unit. Means for entering the optical fiber to be measured, reference optical fiber for propagating the other of the light beams branched by the optical branching device in a single mode, light beam emitted from the optical fiber to be measured, and the reference optical fiber Optical means for guiding the light beam emitted from the same to the same observation surface, means for changing the optical path length of at least one of the two light rays, and the amplitude of the interference signal generated on the observation surface by the two light rays. And means.

[Action]

The present invention utilizes the interference between a plurality of modes propagated in an optical fiber, particularly LP ₀₁ mode and LP ₁₁ mode, and an LP ₀₁ mode propagated in a reference optical fiber, and a plurality of modes propagated in a measured optical fiber. Are temporally separated by the group delay difference. The interference intensity is proportional to the square root of the light intensity of each mode. Therefore, the loss of each mode depending on the bending and length of the optical fiber, especially the loss of the LP ₁₁ mode can be measured by measuring the interference intensity. Further, since the interference intensity is proportional to the square root of the light intensity, the sensitivity of the light intensity in the higher order mode is squared.

〔Example〕

FIG. 1 is a block diagram of an optical transmission loss measuring apparatus according to an embodiment of the present invention.

The light beam emitted from the light source 1 passes through the polarizer 2 and is separated into two light beams by the acousto-optic modulator 3. The first light flux enters the optical fiber 8 to be measured. The light emitted from the optical fiber 8 to be measured enters the light receiver 12. This emitted light is the receiver
A reflector 7 and a semi-transparent mirror 10 are used to lead to 12. The second light flux enters the reference optical fiber 9. In order to guide the second light flux to the reference optical fiber 9, the reflecting mirror 5,
6 is used. Light emitted from the reference optical fiber 9 passes through the semitransparent mirror 10, is reflected by the movable mirror 11, is combined with the first light flux by the semitransparent mirror 10, and is incident on the light receiver 12.

A driving oscillator 4 is connected to the acousto-optic modulator 3. The light receiver 12 is connected to the waveform storage device 15 via the amplifier 13 and the filter 14. The waveform storage device 15 is connected to the movable mirror 11 and the recorder 16.

The light source 1 emits a coherent light beam. Acousto-optic modulator 3
Spatially separates the luminous flux from the light source 1 into two luminous fluxes whose frequency is shifted by the driving frequency from the driving oscillator 4.

The measured optical fiber 8 is an LP at the emission wavelength of the light source 1.
_The offset wavelength is the light source 1 so that the ₁₁ modes are propagated.
Is set to be equal to or longer than the wavelength of. The reference optical fiber 9 has a cutoff frequency equal to or less than the wavelength of the light source 1 so as to propagate the LP ₀₁ mode as the reference light.

The light receiver 12 is the LP emitted from the optical fiber 8 to be measured.
The interference intensity between the ₀₁ mode and LP ₁₁ mode light beams and the LP ₀₁ mode reference light emitted from the reference optical fiber 9 is detected as a beat signal of the drive frequency f of the acousto-optic modulator 3. The beat signal detected by the light receiver 12 is
It is stored in the waveform storage device 15 after amplification and detection.

Here, the amplitude of the beat signal is measured as a function of the optical path length by sweeping one optical path length with the movable mirror 11.
The amplitude of the beat signal becomes maximum when the optical path length difference between the two light fluxes is zero, and since the group delay difference is different between the LP ₀₁ mode and the LP ₁₁ mode, the amplitude of the beat signal becomes maximum at different positions. . That is, LP ₀₁ mode and L
Since the interference caused by the P ₁₁ mode and the P ₁₁ mode causes different interferences at different positions, for example, the group delay difference between the LP ₀₁ mode propagated through the optical fiber 8 to be measured and the reference light from the reference optical fiber 9 is If the movable mirror 11 is changed so as to approach zero, the intensity of interference caused by the LP ₀₁ mode can be independently observed. Also, if the movable mirror 11 is changed so that the group delay difference between the LP ₁₁ mode propagating through the optical fiber 8 to be measured and the reference light from the reference optical fiber 9 approaches zero, the LP ₁₁ mode is also caused by the LP ₁₁ mode. The intensity of interference can be observed independently. That is, the optical transmission loss can be measured for each mode.

LP propagated through the optical fiber 8 to be measured in the light receiver 12
The intensity of the ₀₁ mode is I ₀ , the intensity of the LP ₁₁ mode is I ₁ ,
If the intensity of the reference light is I _R , the output current i of the photodetector 12
(t) is Is expressed as Where τ ₀₁ , τ _0R and τ _1R are
The group delay difference between the LP ₀₁ mode and the LP ₁₁ mode propagated through the optical fiber to be measured, the group delay difference between the LP ₀₁ mode and the reference light, and the group delay difference between the LP ₁₁ mode and the reference light, respectively, | γ ( τ ₀₁ ) |, | γ (τ _0R ) | and | γ (τ _1R ) | are group delay differences τ ₀₁ and τ _0R , respectively.
And τ _1R for coherence, φ ₀₁ ,
φ _0R and φ _1R are respective phase differences. (1)
The frequency f component _if (t) of the equation is Becomes When the movable mirror 11 is swept, the group delay differences τ _0R , τ
_{At the} position where _1R becomes zero, the amplitude of the beat signal with the reference light becomes maximum in the LP ₀₁ mode, and the amplitude of the beat signal of the LP ₁₁ mode and the reference light becomes maximum. From the moving distance d of the movable mirror 11 between these maximum values, the group delay difference τ between two modes (τ = 2d / c, where c is the speed of light in free space) can be obtained. Regarding this, N Shibata et al., “Measurement of Polarization mode dispersion by optical heterodyne detection”
by optical heterodyne detection) ", Electronic Letters (Electron. Lett.) Volume 20 (1984)
This is explained in detail on pages 1055-1057.

On the other hand, the second term on the right side of the equation (2) represents the amplitudes of the beat signals of the LP ₁₁ mode and the reference light. Since this value changes depending on the bending (bending diameter 2R) of the optical fiber 8 to be measured, the bending dependence of the transmission loss of the LP ₁₁ mode can be obtained.

2 to 4 are diagrams showing the amplitude of the beat signal with respect to the optical path length difference. Bending diameter 2R is 200 and 7 respectively
Each case of 0 and 50 mm is shown. The wavelength λ of light source 1 is 1.29
μm, and the length L of the measured optical fiber 8 is 2.8 m.

When the group delay difference τ _0R is zero, the degree of coherence | γ (τ _0R ) |
Is 1, the amplitude of the beat signal in the LP ₀₁ mode becomes maximum. Further, when the group delay difference τ _1R is zero and the coherence degree | γ (τ _1R ) | is 1, the amplitude of the beat signal in the LP ₁₁ mode becomes maximum. Each of these is indicated by an arrow. With this maximum value, for diameter 2R The bending dependence of the value of is obtained. Where the intensity of the reference light I _R
Is constant, and the bending diameter 2 of the strength I ₁ of the LP ₁₁ mode is 2
The dependence on R is obtained with a high sensitivity of squared.

The group delay difference between the two modes when the length L of the optical fiber 8 to be measured is 2.8 m is obtained by actually measuring the optical path difference 2d between peaks, that is, twice the moving distance of the movable mirror 11. To be In this example, the value of 2d was 5.84 mm and the group delay difference was 19.5 picoseconds.

FIG. 5 shows the measurement result and the calculated value of the loss of the LP ₁₁ mode with respect to the bending diameter. The calculated values are indicated by a straight line, a dash-dotted line and a broken line, and the measured values are indicated by a circle, a triangle and a cross, and the wavelength λ of the light source 1 is 1.2 μm, 1.29 μm and 1.31 μ, respectively.
The value when m is shown. From this figure, it can be seen that measurement is possible in a dynamic range of about 300 dB / m.

In the above embodiments, the transmission loss in the LP ₀₁ mode and the LP ₁₁ mode, which is a problem when using a normal optical fiber, has been described as an example. However, in principle, the present invention can independently measure the transmission loss of the modes having different group delay times.

〔The invention's effect〕

As described above, the optical transmission loss measuring method and apparatus of the present invention can temporally separate the modes of the light beam propagating through the optical fiber under measurement with sub-picosecond resolution. Therefore, the bending loss of the LP ₁₁ mode can be measured with high sensitivity independently of the LP ₀₁ mode even with a short measured optical fiber of, for example, several meters.

Since the LP ₁₁ mode loss can be measured independently of the LP ₀₁ mode, the optical fiber length dependency of the LP ₁₁ mode loss can be evaluated by changing the length of the optical fiber to be measured. Therefore, the effective cutoff wavelength, which is a characteristic parameter of the optical fiber, can be evaluated.

Furthermore, at the same time as measuring the LP ₁₁ mode loss, the group delay difference between modes can be obtained with sub-picosecond accuracy from the difference in optical path length that separates propagation modes.

Therefore, the present invention has a great effect when used for performance evaluation of an optical fiber.

[Brief description of drawings]

FIG. 1 is a block diagram of an optical transmission loss measuring device according to an embodiment of the present invention. FIG. 2 is a diagram showing the amplitude of the beat signal with respect to the optical path length difference. FIG. 3 is a diagram showing the amplitude of the beat signal with respect to the optical path length difference. FIG. 4 is a diagram showing the amplitude of the beat signal with respect to the optical path length difference. FIG. 5 is a diagram showing measurement results and calculated values of LP ₁₁ mode loss with respect to bending diameter. 1 ... Light source, 2 ... Polarizer, 3 ... Acousto-optic modulator, 4
...... Driving oscillator, 5,6,7 ...... Reflecting mirror, 8 …… Measured optical fiber, 9 …… Reference optical fiber, 10 …… Semi-transparent mirror, 11 …… Movable mirror, 12 …… Receiver, 13 ... Amplifier, 14 ...
… Filter, 15… Waveform storage device, 16… Recorder.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shoji Ohashi 162 Shirahane, Shirahoji, Tokai-mura, Naka-gun, Ibaraki Prefecture Nippon Telegraph and Telephone Corporation, Ibaraki Telecommunications Research Institute (72) Yoshimi Aomi Naka, Ibaraki Prefecture 162, Shirahone, Shirahata, Tokai-mura, Gunma, Ibaraki Telecommunications Research Institute, Nippon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. A light emitted from a light source is split into two light beams, one of the two light beams is made incident on an optical fiber to be measured and propagated in a plurality of modes, and the other of the two light beams is used for reference. It is incident on the optical fiber and propagated in one mode, and the light beam propagating through the measured optical fiber and the light beam propagating through the reference optical fiber are interfered with each other by the group delay difference between the plurality of modes. By utilizing the fact that interference caused by occurs at different positions, the group delay difference between the target mode of the plurality of modes and the light beam that has propagated through the reference optical fiber is close to zero. An optical transmission loss measuring method, characterized in that at least one optical path length difference is changed, the intensity of interference caused by the target mode is independently observed, and the optical transmission loss is measured for each mode. 2. A plurality of modes propagating in an optical fiber to be measured include an LP ₀₁ mode and an LP ₁₁ mode, and one mode propagating in a reference optical fiber is LP ₀₁ mode.
Mode The optical transmission loss measuring method according to claim (1). 3. A light source that emits coherent light, a light branching unit that splits the light beam emitted by this light source into two light beams, and one of the light beams split by this light branching unit is applied to an optical fiber to be measured. Incident means, reference optical fiber that propagates the other of the light beams branched by this light branching device in a single mode, the light beam emitted from the measured optical fiber and the light beam emitted from the reference optical fiber And an optical means for guiding the two to the same observation surface, a means for changing the optical path length of at least one of the two light fluxes, and a means for measuring the amplitude of the interference signal generated on the observation surface by the two light fluxes. Optical transmission loss measuring device.