JPH06167424A - Light pulse tester - Google Patents

Abstract

PURPOSE:To shorten the dead zone in the vicinity of an incident port in a light pulse tester emitting a laser pulse to an optical fiber to be measured and testing the transmission characteristics thereof. CONSTITUTION:The laser pulses with wavelengths lambda1, lambda2 outputted from first and second laser beam sources 32, 33 are emitted to an optical fiber 1 to be measured through optical connectors 37, 2. The beam with a wavelength lambda1 among the beams returned from the connection part of the connectors is detected by a first photodetector 42 and the beam with the wavelength lambda2 is detected by a second photodetector 43. The level of the Fresnel reflection at the connection part of the connectors is equal independent of a wavelength and the level of back scattering beam with the wavelength lambda2 continuous thereto is higher than the level of back scattering beam with the wavelength lambda1. A processing part 50 measures the characteristics in the vicinity of the incident port of the optical fiber l to be measured on the basis of the beam detection output on the side of the second photodetector 43 on which back scattering beam reduced in level difference is incident in succession to the Fresnel reflection and measures remoter characteristics on the basis of the beam detection output on the side of the first photodetector 42.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention emits a laser pulse light to an optical fiber to be measured and measures the change in the received output level of backscattered light or Fresnel reflected light from the optical fiber over time, The present invention relates to an optical pulse tester that measures transmission characteristics such as the length of an optical fiber under test, transmission loss, and connection loss at a connection point.

[0002]

2. Description of the Related Art Conventionally, an optical pulse tester 10 having a configuration shown in FIG. 10 has been used to measure the transmission characteristics of an optical fiber.

The pulse light source section 11 of the optical pulse tester 10
Is a laser light source 12 that outputs laser light of a predetermined wavelength,
A pulse generator 13 that pulse-drives the laser light source 12 at a predetermined cycle, and outputs a laser pulse light having a predetermined width (for example, 1 μS) at a wavelength λ ₀ to the optical connector 15 via the directional coupler 14. An optical connector 2 on one end side of the optical fiber 1 to be measured is connected to the optical connector 15, and the laser pulse light is incident on the optical fiber 1 to be measured via the optical connectors 15 and 2.

A part of the laser pulse emitted from the optical connector 15 is reflected (Fresnel reflection) to the directional coupler 14 side by a slight air gap between the optical connectors, and is received by the light receiver 18 of the light receiver 17. The received light signal is amplified by the amplifier 19. On the other hand, the laser pulse light incident on the measured optical fiber 1 is gradually attenuated by the scattering in the fiber and advances. Of this scattered light, the optical connector 2
The backscattered light returning to the side passes through the directional coupler 14 and the light receiver 1
Light is received at 8, and the received light signal is amplified and output.

Since the backscattered light becomes smaller as the laser pulse light travels in the fiber, the output signal from the light receiving portion 17 is, as shown in FIG. 11, at the optical connector portion at the time t ₀ of emission of the laser pulse light. The Fresnel reflection causes a pulse-like increase / decrease, and thereafter, the attenuation changes with a constant attenuation rate mainly determined by the loss of the fiber. Optical fiber under test 1
If there is a connection loss in the connection part in the middle of, the received light output level of the backscattered light sharply decreases at t ₁ corresponding to that position.

The received light output is converted into a digital value while being sampled at high speed by the A / D converter 20, and is sent to the processing unit 21. The processing unit 21 receives the light reception output data from the time when the laser pulse light is emitted, displays the attenuation characteristic as shown in FIG. 11 on the display device 22, and, for example, the light reception output level is rapidly changing t ₁ The time point is detected, and the position (distance) of the change point and the change amount are measured.

From this characteristic, the measurer can know the transmission loss (overall inclination) of the optical fiber 1 to be measured, the position of the connecting portion, and the transmission characteristic such as loss or breakage.

[0008]

However, in the conventional optical pulse tester having the above structure, the optical connector 1 is used.
Since the level difference between the Fresnel reflected light level due to the gaps 5 and 2 and the level of the backscattered light following the Fresnel reflection is large, the light receiving unit 17 cannot fully respond to this change, and A in FIG.
The response time B of the received light output becomes very long (several hundreds of meters in terms of distance) as compared with the width of the laser pulse light, and the dead zone where the transmission characteristics cannot be measured becomes wide.

That is, for example, the Fresnel reflection due to the air gap of a laser pulse having a pulse width of 1 microsecond is -14 dB with reference to the emission level, whereas the level of the backscattered light that follows is approximately in the case of a general optical fiber. −
It is 50 dB, and the difference is 36 dB. 36 dB
Power difference is 72 dB due to the voltage difference of the received light output. Actually, it is necessary to detect even lower backscattered light, and it is extremely difficult to construct a light receiving section having such a wide dynamic range and high speed with the current technology. Therefore, when the light-receiving level sharply changes from a high level like Fresnel reflection on the incident side, the output of the light-receiving unit is greatly delayed with respect to this change.

In order to solve the problem of this dead zone, a method of cutting off the light to the photodetector 18 only when the laser pulse light is emitted, or a dummy fiber is connected between the optical connectors 15 and 2 and the dummy fiber However, the former method requires an expensive and high-speed optical switch, and the latter method has a problem in that it is troublesome to connect a dummy fiber for several 100 m and correct its length. It was

The present invention relates to a conventional laser light source for measurement, and a short-wavelength laser light whose wavelength is shorter than that of the laser light source for measurement and the level of backscattered light near the entrance is higher than that of the light source for measurement. It is an object of the present invention to provide an optical pulse tester that solves the above problems all at once by using a light source.

[0012]

In order to solve the above-mentioned problems, an optical pulse tester of the present invention comprises a pulse light source section (31) for emitting laser pulse light to an optical fiber (1) to be measured.
And a light receiving section (40) for detecting the level of backscattered light and Fresnel reflected light from the optical fiber under measurement that has received the laser pulse light, and the light receiving section of the light receiving section after the laser pulse light is emitted. In an optical pulse tester that measures the transmission characteristics of the optical fiber under test by measuring the change in output level over time, the pulse light source unit (31) emits laser light of a predetermined wavelength (λ ₁ ). A first laser light source (32) for outputting and a level shorter than the wavelength of the first laser light source and higher than the backscattered light by the laser pulse of the first laser light source near the entrance of the optical fiber to be measured. Second laser light source (3) that outputs laser light of a wavelength (λ ₂ ) that can obtain backscattered light of
3) and the transmission characteristic near the entrance of the optical fiber to be measured is measured by the output of the light receiving unit with respect to the laser pulse light from the second laser light source, and the laser from the first laser light source is measured. A processing unit (50) for measuring the transmission characteristics of the measured optical fiber beyond the vicinity of the entrance is provided by the output of the light receiving unit with respect to the pulsed light.

[0013]

As a result, in the vicinity of the entrance of the optical fiber to be measured, the level of backscattered light with respect to the laser pulse of the second laser light source becomes higher than the level of backscattered light from the first laser light source, and The level difference between the Fresnel reflected light not depending on and the backscattered light by the second laser light source is reduced, and the response time of the light receiving unit is shortened.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of an optical pulse tester 30 according to an embodiment.

The pulse light source unit 3 of the optical pulse tester 30
Reference numeral 1 denotes a laser diode, which has a first laser light source 32 which outputs a laser beam of wavelength λ ₁ and a second laser light source 33 which outputs a laser beam of wavelength λ ₂ shorter than λ _1. , Both laser light sources 32 and 33 are pulse-driven by a common pulse generator 34, and laser pulses output from both laser light sources 32 and 33 are combined by a combiner 35 and sent to a directional coupler 36.

FIG. 2 shows changes in the loss amount with respect to the wavelength of the optical fiber 1 to be measured. The wavelength of the light used in this fiber is usually the lowest loss wavelength region C in order to obtain a long transmission distance. wavelength of the inner is used, the first laser light source 32, the wavelength lambda ₁ (e.g. 1 of the low loss region C
Laser light of 300 nm) is output.

The second laser light source 33 outputs a laser beam having an extremely short wavelength λ ₂ (for example, 680 nm) which is not usually used for the optical fiber 1 to be measured. Laser light with a wavelength of 680 nm can be output by an inexpensive laser diode.

The pulsed light from the multiplexer 35 is transmitted to the optical connector 37 via the directional coupler 36, and the pulsed light emitted from the optical connector 37 to the optical connector 2 is incident on the optical fiber 1 to be measured. To be done.

The Fresnel reflected light due to the gap between the optical connectors and the backscattered light from the optical fiber 1 to be measured are sent to the demultiplexer 41 of the light receiving section 40 via the directional coupler 36.

The demultiplexer 41 separates the light of wavelength λ _{1 and} the light of wavelength λ ₂ of the incident light into the first light receiver 42 and the second light receiver 42.
To the light receiver 43 of each. First and second light receiver 4
The light receiving signals of 2, 43 are amplified by the amplifiers 44, 45, and input to the A / D converter 47 via the switch 46. The switching of the switch 46 is performed by a processing unit 50, which will be described later, and the received light signal selected by the switch 46 is converted into a digital value while being high-speed sampled by the A / D converter 47 and is sent to the processing unit 50.

The processing unit 50 synchronizes with pulse driving of both laser light sources 32 and 33 by the pulse generator 34, and
The light reception outputs of the second light receivers 42 and 43 are fetched every predetermined time, the characteristics of the measured optical fiber 1 from the vicinity of the entrance to the far end are measured, and the result is displayed on the display device 51.

FIG. 3 shows an example of the processing procedure of the processing section 50, and the operation of the optical pulse tester 30 will be described below with reference to this flowchart.

With the switch 34 connected to the second light receiver 43 side, a drive pulse of, for example, 1 μS is output from the pulse generator 34 of the pulse light source unit 31 to the first and second laser light sources 32 and 33. Then, two laser pulses having different wavelengths are emitted from the optical connector 37 to the optical connector 2 side. The Fresnel reflected light at the connector connecting portion is sent to the demultiplexer 41 via the directional coupler 36, and the Fresnel reflected light having the wavelengths λ ₁ and λ ₂ is respectively received by the first and second photodetectors 42 and 4.
3 is incident.

If the output of each laser pulse is equal, the level of the Fresnel reflected light is constant at 4% (-14 dB) of the emission level regardless of the wavelength.
The pulsed light having a peak of approximately -14 dB with respect to the emission level is incident on the photodetectors 42 and 43.

Since the light of the wavelength λ _{1 has} a low loss with respect to the optical fiber 1 to be measured, the level of the backscattered light following the Fresnel reflected light is lowered to approximately -50 dB as described above. Due to this steep drop in level, the received light output on the side of the first photodetector 42 has this wavelength λ ₁ after a very long response time Ta (dead zone) has elapsed from t ₀ , as shown in FIG. It attenuates with a constant slope corresponding to the fiber loss at.

On the other hand, the wavelength λ ₂ is shorter than the wavelength λ ₁ ,
Since the optical fiber 1 to be measured has a high loss, the level of the backscattered light following the Fresnel reflected light to the second photodetector 43 becomes larger than the level of the backscattered light of the wavelength λ ₁ . That is, since the level of the backscattered light is almost proportional to the reciprocal of the fourth power of the wavelength, λ ₁ = 1300 nm and λ ₂ = 6
In the case of 80nm, the backscattered light lambda ₂ is (lambda ₁ / lambda ₂₎ to light of lambda ₁ ⁴ times (approximately 13 times, 1 dB in terms
1 dB) and the level difference for Fresnel reflection is reduced to 25 dB. Therefore, the light reception output on the second light receiver 43 side is t _{0 as} shown in FIG.
After a very short response time Tb (dead zone) has elapsed from time, the light is attenuated with a constant slope corresponding to the loss of the fiber at the wavelength λ ₂ .

The processing section 50 receives the trigger pulse from the pulse generator 34 and outputs the output data N on the second photodetector 43 side for a set time T ₁ slightly longer than Ta from t _0.
Take in (t) (S1-4).

After the set time T ₁ has passed, the switch 46
Is switched to the first optical receiver 42 side, and the first optical receiver 42 is operated for T ₂ time obtained by subtracting T ₁ from the determined measurement period T _0.
The output data F (t) on the side is fetched (S5-7).

Next, the data N (t) fetched in the T ₁ period
On the other hand, a correction process according to the wavelength is performed to cause the display device 51 to display a series of characteristics continuous for the T ₁ period and the T ₂ period (S8, 9).

In this correction process, the difference H (of the linear equation K = Qt-39 of the backscattered light of FIG. 4B from the linear equation J = Pt-50 of the backscattered light of FIG. 4A is used. t) = K-J
From the time t _{0 to the} time T ₁
(T) was the correction data for each time within period T _1, taken in period T ₁ data N (t) from H (t)
Is performed by subtraction correction. As a result of this correction processing, as shown in FIG. 5, the characteristics in the T ₁ period and the characteristics in the T ₂ period are continuously displayed with the same slope, and it is constant as if only pulsed light of wavelength λ ₁ was used. The characteristic that decays and changes with the slope is displayed. The dead zone of this characteristic is
The time Tb is the same as before the correction and only a short time is required, and the measurable range in the vicinity of the entrance is expanded by Ta-Tb.

It should be noted that, within the period T ₁ , for example, R in FIG.
As described above, when a level step change occurs due to the connection point loss, the backscattered light of wavelength λ ₁ is also R ′ in FIG.
As shown in FIG. 5, the corrected characteristic is continuously displayed as indicated by R ″ in FIG.

[0033]

Other Embodiments In the above embodiment, the characteristics from the vicinity of the entrance of the optical fiber to be measured to the far end were measured by the output of one laser pulse, but as shown in the flow chart of FIG. Each time the pulse is output once, the acquired data is added, the average value is calculated after M times, and the characteristic of the optical fiber to be measured is measured by this averaged data. Measurements can be made on the suppressed data.

In the above embodiment, laser pulses having different wavelengths were simultaneously output. However, as in the optical pulse tester 60 shown in FIG. 7, the switch 6 is provided on the pulse light source 61 side.
2 may be provided, and the first and second laser light sources 32 and 33 may be pulse-driven alternately. In this case, set the light receiving unit 64 to 1
It is also possible to configure one light receiver 65, and the processing unit 6
6, the data of only the T ₁ period is fetched at the first trigger, and the data of only the T ₂ period is fetched at the _second trigger to perform the same process as described above, or at the odd number and the even number of times. The data of each period may be alternately fetched and averaged.

Further, like the pulse light source unit 71 of the optical pulse tester 70 shown in FIG. 8, the first and second laser light sources 3 are used.
Independent pulse generators 72 and 33,
It is also possible to provide 73 and to perform the measurement in the vicinity of the entrance and the measurement beyond the entrance in the processing unit 73 asynchronously without switching the switch.

In the above embodiment, the characteristic of the received output of the backscattered light from the second laser light source is corrected so as to match the characteristic of the backscattered light from the first laser light source. However, the measurement results of the two laser light sources may be displayed so as to be connected at the intersecting position, as shown in FIG.

[0037]

As described above, the optical pulse tester of the present invention has the first laser light source for outputting the laser light of the predetermined wavelength, which has been conventionally used for measurement, and the entrance of the optical fiber to be measured. A second laser light source for outputting a laser light of a short wavelength, in which the level of the back scattered light in the vicinity is higher than the level of the back scattered light by the first laser light source, and the entrance of the optical fiber to be measured. The characteristic in the vicinity is measured by pulsed light from the second laser light source, and the characteristic in the vicinity of the vicinity of the entrance is measured by pulsed light from the first laser light source.

Therefore, in the measurement in the vicinity of the entrance, the level difference between the Fresnel reflected light and the backscattered light at the connection portion becomes small, and the response time of the light receiving portion to this change is shortened. Therefore, it is possible to perform measurement with a very small dead zone without using an expensive optical switch or a dummy fiber that complicates the work.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

FIG. 2 is a loss characteristic diagram of an optical fiber with respect to wavelength.

FIG. 3 is a flowchart illustrating a processing procedure of a processing unit according to an embodiment.

FIG. 4 is a light reception output diagram for light of two wavelengths.

FIG. 5 is a characteristic display diagram after correction.

FIG. 6 is a flowchart including an averaging process.

FIG. 7 is a block diagram showing the configuration of another embodiment of the present invention.

FIG. 8 is a block diagram showing the configuration of another embodiment of the present invention.

FIG. 9 is a characteristic display diagram when no correction is performed.

FIG. 10 is a block diagram showing a configuration of a conventional device.

FIG. 11 is a characteristic display diagram of a conventional device.

[Explanation of symbols]

 1 optical fiber to be measured 2 optical connector 30 optical pulse tester 31 pulse light source section 32 first laser light source 33 second laser light source 34 pulse generator 35 multiplexer 36 directional coupler 37 optical connector 40 light receiving section 41 minutes Wave device 42 First light receiver 43 Second light receiver 44, 45 Amplifier 50 Processing unit

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] February 9, 1993

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

[Fig. 2]

[Figure 3]

[Figure 4]

[Figure 5]

[Figure 7]

[Figure 8]

[Figure 9]

[Figure 10]

FIG. 11

[Figure 6]

Claims (1)

[Claims] 1. An optical fiber (1) for measuring laser pulse light
A pulse light source unit (31) for emitting to the laser pulse light, and a light receiving unit (40) for detecting the levels of backscattered light and Fresnel reflected light from the optical fiber under measurement that has received the laser pulse light, and the laser pulse In an optical pulse tester for measuring the transmission characteristic of the optical fiber under test by measuring the change in the output level of the light receiving unit from the time of light emission, the pulse light source unit (31) is A first laser light source (32) for outputting a laser light of a wavelength (λ ₁ ), and a wavelength of the first laser light source (32) shorter than the wavelength of the first laser light source and near the entrance of the optical fiber to be measured. a second laser light source (33) a high level of the backscattered light from the backscattering light by the laser pulse outputs laser light having a wavelength (lambda ₂₎ obtained, Les from the second laser light source The transmission characteristic of the measured optical fiber near the entrance is measured by the output of the light receiving unit for the pulsed light, and the incident optical fiber is made incident by the output of the light receiving unit for the laser pulse light from the first laser light source. An optical pulse tester comprising a processing unit (50) for measuring transmission characteristics beyond the vicinity of the mouth.