JPS6263833A - Method and instrument for measuring optical transmission loss - Google Patents

Abstract

PURPOSE:To measure an optical transmission loss in each separate mode by observing independently the intensity of an interference caused in each mode due to a group delay difference between plural modes. CONSTITUTION:A luminous flux which has been emitted from a light source 1 passes through a polarizer 2, and separated into two luminous fluxes by an acousto-optical modulator 3. The first luminous flux is made incident on an optical fiber to be measured 8. The second luminous flux is made incident on a reference use optical fiber 9. An emitted light of the fiber 9 passes through a semi-transparent mirror 10, reflected by a movable mirror 11, multiplexed with the first luminous flux by the semi-transparent mirror 10, and made incident on a photodetector 12. The photodetector 12 detects the interference intensity of the luminous flux of an LP01 mode and an LP11 mode, which has been emitted from the fiber 8, and a reference light of an LP01 mode, which has been emitted from the fiber 9, as a beat signal of ad riving frequency (f) of the modulator 3. In this way, the transmission losses of the LP01 mode and the LP11 mode are measured independently, respectively.

Description

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

〔overview〕

The present invention provides a method and apparatus for measuring optical transmission loss of an optical fiber, in which a plurality of modes propagated through an optical fiber to be measured and a single mode propagated through a reference optical fiber are caused to interfere with each other. The transmission loss of each mode is measured independently using the group delay difference.

[Conventional technology]

Conventionally, to measure transmission loss due to bending or length of an optical fiber, the intensity change of propagating light including multiple modes, especially the lowest order LP01 mode and the next lower order mode LP11 mode, has been measured. That's what I was looking for.

[Problems to be Solved by the Invention] However, the losses caused by bending and length are essentially different between the LP01 mode and the LP mode.

However, it has been impossible to measure the LPOI mode and the LP mode independently. In addition, when measuring a short optical fiber under test, the dynamic range of the measurement becomes small because there is little difference in loss depending on the mode.
The drawback was that quantitative evaluation was difficult. Regarding this, Masaharu Ohashi et al.
z modelloss 01easement
in the two-+mode-propagate
ionregior+ of optical fib
ers) J, Optics Letters (Opt, L
ett, Volume 9 (1984), pages 303-305.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method and apparatus for independently measuring the transmission loss of a plurality of modes propagating in an optical fiber, particularly LP mode and LP mode.

[Means for solving problems]

The optical transmission loss measurement method of the present invention splits light emitted from a light source into two beams, enters one of the two beams into an optical fiber to be measured, and propagates in multiple modes, and then splits the light emitted from a light source into two beams. enter the reference optical fiber and propagate in one mode, causing the light beam propagated through the measured optical fiber and the light beam propagated through the reference optical fiber to interfere;
The present invention is characterized in that the optical transmission loss is measured for each mode by independently observing the intensity of interference caused by each mode based on the group delay difference between the plurality of modes.

In practice, the multiple modes propagating in the optical fiber under test are LP01 mode and LP, mode, and one mode propagating in the reference optical fiber is L P o
This is I mode.

The optical transmission loss measuring device of the present invention includes a light source that emits coherent light, a light branching means for branching the light flux emitted by the light source into two light fluxes, and a light branching means for branching one of the branched light fluxes. means for inputting the light into the optical fiber to be measured; a reference optical fiber for propagating the other of the light beams branched by the light branching means in a single mode; and a light beam emitted from the optical fiber to be measured and the reference optical fiber. optical means for guiding the emitted light beams to the same observation surface; means for changing the optical path length of at least one of the two light beams; and measuring the amplitude of an interference signal generated on the observation surface by the two light beams. It is characterized by comprising means.

[Effect]

The present invention utilizes interference between a plurality of modes propagated in an optical fiber, particularly LP01 mode and LP mode, and an LP01 mode propagated in a reference optical fiber, to group a plurality of modes propagated in an optical fiber under test. Separate in time by delay difference. The interference intensity is proportional to the square root of the light intensity of each mode.

Therefore, by measuring this interference intensity, we can determine the loss of each mode, which depends on the bending and length of the optical fiber.
In particular, the loss in the LP11 mode can be measured. Furthermore, since the interference intensity is proportional to the square root of the optical intensity, the sensitivity of the optical intensity of higher-order modes is improved by a factor of two.

〔Example〕

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

The light beam emitted from the light source 1 passes through a polarizer 2 and is separated into two light beams by an acousto-optic modulator 3.

The first light beam enters the optical fiber 8 to be measured. The light emitted from the optical fiber 8 to be measured is incident on the light receiver 12 . In order to guide this emitted light to the light receiver 12, a reflecting mirror 7 and a semi-transparent mirror 10 are used. The second beam is a reference optical fiber 9
incident on . A reflector 5.6 is used to guide the second beam into the reference optical fiber 9. Reference optical fiber 9
The emitted light passes through the semi-transparent mirror 10, is reflected by the movable mirror 11, is combined with the first light beam by the semi-transparent mirror 10, and enters the light receiver 12.

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

A light source 1 emits a coherent light beam. Acousto-optic modulator 3
spatially separates the light beam from the light source 1 into two light beams whose frequency is shifted by the driving frequency from the driving oscillator 4.

The optical fiber 8 to be measured has an LP at the emission wavelength of the light source 1.
The offset wavelength is set to be equal to or greater than the wavelength of the light source I so that the z mode is propagated. The reference optical fiber 9 has a cutoff frequency equal to or lower than the wavelength of the light source 1 so as to propagate the LP01 mode as the reference light.

The light receiver 12 receives the LP emitted from the optical fiber 8 to be measured.
The interference intensity between the light beams in the 01 mode and the LP01 mode and the LP01 mode reference light emitted from the reference optical fiber 9 is detected as a beat signal of the driving frequency f of the acousto-optic modulator 3. The beat signal detected by the light receiver 142 is stored in the waveform storage device 15 after being amplified and detected.

Here, by sweeping one optical path length with the movable mirror 11, the amplitude of the beat signal is measured as a function of the optical path length.

The amplitude of the beat signal is maximum when the optical path length difference between the two beams is zero, and in LP01 mode and LpH mode,
Since the group delay difference is different, the amplitude of the beat signal becomes maximum at different positions.

L propagated through the optical fiber 8 to be measured in the optical receiver 12
P 01l mode intensity is ■. , LP, , where the intensity of the mode is 11 and the intensity of the reference light is IR, the output current i (t) of the photoreceiver 12 is 1(t)oci. +1. +(,l +2E call lr(τ.1)10°3φ.

+2ζπI7(τ.su l cos(2πft+φox)
+2 J] MaI r (τ11I) l Co5('2
It is expressed as ffft+φ18)-...-(1). Here, τIl+, τOR, and τ, R are the group delay difference between the LPG mode and the L P + l mode propagated in the optical fiber under test, LP, the group delay difference between the l mode and the reference light, and LP, , are the group delay differences between the mode and the reference light, and 1γ(τ.1) 1.1γ(τ.i and lr(τ+*)l are the group delay differences τ.8, τ, respectively.
is the degree of coherence for oR and τ, and φoI
, φOR and φ are their respective phase differences. (1
)2 frequency f component tr(t) is i,(t)oc
'l (VAT'';; l r (τ11J1) 1
+2 flower--IT(τ18)l ・-・・-・(
2). When the movable mirror 11 is swept, the group delay difference τON
, τ, are zero, the amplitudes of the beat signals of the LP01 mode and the reference light, and the amplitudes of the beat signals of the LP+ mode and the reference light, respectively, become maximum. From the moving distance d of the movable mirror 11 between these maximum values, the group delay difference τ (τ-2d/c, where C is the speed of light in free space) between the two modes can be determined. Regarding this, N. Shibata et al. [Measurement of polarization mode dispersion by optical heterodyne detection]
1arizatton mode dispersio
nby optical heterodyne
tection) J, Electronics Letters (11!1ectron, Lett,) Volume 20 (1
984), pages 1055-1057.

On the other hand, the second term on the right side of (2) 2 represents the amplitude of the beat signal of the LP++ mode and the reference light. Since this value changes depending on the bending of the optical fiber 8 to be measured (bending diameter 2R), the bending dependence of the transmission loss of the LP mode can be determined. FIG. 3 is a diagram showing the amplitude of a beat signal versus the difference. In each case, the bending diameter 2R is 200.70.50 m5+. The wavelength λ of the light source 10 is 1.29 μm, and the length of the optical fiber 8 to be measured is 2.8 m.

Group delay difference τ. The coherence degree is 1r (τ.lI) when the image is zero.
When is 1, the amplitude of the beat signal in LP01 mode is maximum. Further, when the group delay difference τl is zero and the degree of coherence 1γ (τIK)1 is 1, the amplitude of the beat signal in the LP11 mode becomes maximum. These are each indicated by an arrow. This maximum value provides the bending dependence of the value of the second lower shingle on the diameter 2R. Here, the intensity of the reference light Ill
is constant and LP, .

The dependence of the mode intensity I on the bending diameter 2R can be obtained with a square-fold high sensitivity.

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

FIG. 5 shows the measured and calculated loss of the i=P + + mode with respect to the bending diameter. Calculated values are shown by straight lines, dot-dashed lines, and broken lines, and measured values are shown by ○, Δ, and
The values are shown when m and 1.3br+m. It can be seen from this figure that measurements can be made in a dynamic range of about 300 dB/m.

In the above embodiments, the transmission loss between the LP moat 4' and the LPz moat, which is a problem when using a normal optical fiber, was explained as an example. However, the present invention can, in principle, independently measure the transmission loss of modes with different group delay times.

〔Effect of the invention〕

As described above, the optical transmission loss measuring method and apparatus of the present invention can temporally separate the modes of the light flux propagating through the optical fiber to be measured with sub-picosecond resolution. Therefore, even with a short optical fiber to be measured of, for example, several meters, the bending loss in the LpH mode can be measured with high sensitivity independently from the L13° mode.

Since the LP mode loss can be measured independently of the LP mode, it is also possible to evaluate the optical fiber length dependence of the LP mode loss 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 mode loss, the group delay difference between modes can be determined with sub-picosecond accuracy from the optical path length difference between the propagation modes.

Therefore, the present invention is highly effective when used for evaluating the performance of optical fibers.

[Brief explanation of drawings]

FIG. 1 is a configuration 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 for I-P++ mode one member loss with respect to bending diameter. DESCRIPTION OF SYMBOLS 1... Light source, 2... Polarizer, 3... Acousto-optic modulator, 4... Drive oscillator, 5.6.7... Reflector,
8... Optical fiber to be measured, 9... Optical fiber for reference, 10... Semi-transparent mirror, 11... Movable mirror, 12... Light receiver, 13... Amplifier, 14... Filter , 15・
...Waveform storage device, 16...Recorder. Patent applicant: Nippon Telegraph and Telephone Corporation, -1, Agent
Patent attorney Naotaka Ide, ゛1 Example device Figure 1 Light W & Negative! LmlTIJ

Claims (3)

[Claims] (1) The light emitted from the light source is split into two beams, one of these two beams enters the optical fiber to be measured and propagates in multiple modes, and the other of these two beams is sent to the reference optical fiber. The light beam propagating through the optical fiber under test and the light beam propagating through the reference optical fiber are caused to interfere with each other, and the difference in group delay between the plurality of modes is caused by each mode. An optical transmission loss measuring method characterized by measuring optical transmission loss for each mode by independently observing the intensity of interference caused by interference. (2) The multiple modes propagating in the optical fiber under test are L
One mode propagating in the reference optical fiber is LP_0_1, which includes P_0_1 mode and LP_1_1 mode.
The optical transmission loss measuring method according to claim (1), which is a mode. (3) a light source that emits coherent light; a light branching means that branches the light beam emitted by the light source into two light beams; and one of the light beams branched by the light branching means is input into the optical fiber to be measured. means, a reference optical fiber that propagates the other of the light beams branched by the light branching means in a single mode, and a light beam emitted from the optical fiber to be measured and a light beam emitted from the reference optical fiber. Optical transmission comprising optical means for guiding to the same observation surface, means for changing the optical path length of at least one of the two light beams, and means for measuring the amplitude of an interference signal generated on the observation surface by the two light beams. Loss measurement device.