JP2972973B2 - Optical pulse tester - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

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

The pulse light source unit 11 of the optical pulse tester 10
A laser light source 12 that outputs laser light of a predetermined wavelength,
The laser light source 12 is constituted by a pulse generator 13 for pulse driving at a predetermined cycle, and outputs a laser pulse light having a wavelength λ ₀ and a predetermined width (for example, 1 μS) to the optical connector 15 via the directional coupler 14. The optical connector 15 is connected to the optical connector 2 at one end of the optical fiber 1 to be measured, 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 reflected) toward the directional coupler 14 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 optical fiber under measurement 1 attenuates little by little due to scattering in the fiber and proceeds. Of the scattered light, the optical connector 2
The backscattered light returning to the side passes through the directional coupler 14 and
The light is received at 8, and the received light signal is amplified and output.

[0005] backscattered light to become smaller as the laser pulse light travels in the fiber, the output signal from the light receiving portion 17, as shown in FIG. 11, the optical connector portion to the emission time t ₀ of the laser pulse light It fluctuates in a pulse-like manner due to Fresnel reflection, and thereafter, attenuates and changes at a constant attenuation rate determined mainly by fiber loss. Optical fiber under test 1
When there is a connection loss at the connection part in the middle of the above, the received light output level of the backscattered light sharply decreases at t ₁ corresponding to the position.

The received light output is converted into a digital value while being sampled at a high speed by an A / D converter 20 and sent to a processing section 21. The processing unit 21 receives the received light 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, t ₁ when the received light output level is rapidly changing. The time point is detected, and the position (distance) and the amount of change of the change point are measured.

From this characteristic, the measurer can know the transmission characteristics of the measured optical fiber 1, such as the transmission loss (total inclination), the position of the connection portion, and its loss or breakage.

[0008]

However, in the conventional optical pulse tester having the above configuration, the optical connector 1
Since there is a large difference between the level of the Fresnel reflected light due to the gaps 5 and 2 and the level of the backscattered light following the Fresnel reflection, 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 hundred meters in terms of distance) as compared with the width of the laser pulse light, and the dead zone in which the transmission characteristics cannot be measured becomes wide.

That is, for example, the Fresnel reflection of a laser pulse having a pulse width of 1 microsecond due to an air gap is -14 dB with reference to the emission level, while the level of the backscattered light subsequent thereto is almost the same as in a general optical fiber. −
50 dB, the difference being as high as 36 dB. 36dB
Is 72 dB as the voltage difference of the received light output. In practice, it is necessary to detect even lower backscattered light, and it is extremely difficult with the current technology to configure a light receiving unit having such a wide dynamic range and high speed. Therefore, when the light receiving level changes steeply from a high level like Fresnel reflection on the incident side, the output of the light receiving section is greatly delayed with respect to this change.

In order to solve the problem of the dead zone, a method of shutting off the light to the photodetector 18 only when the laser pulse light is emitted, or a method of connecting a dummy fiber between the optical connectors 15 and 2 and Although there was a method of measuring only the received light signal of the above, the former method requires an expensive and high-speed optical switch, and the latter method has a problem that connection of a dummy fiber of several hundred meters and correction of its length are troublesome. Was.

According to the present invention, there is provided a conventional measuring laser light source, and a short-wavelength laser whose wavelength is shorter than the wavelength of the measuring laser light source and whose backscattered light level near the entrance is higher than that of the measuring light source. It is an object of the present invention to provide an optical pulse tester that solves the above problem at once by using a light source.

[0012]

In order to solve the above-mentioned problems, an optical pulse tester according to the present invention comprises a pulse light source section (31) for emitting laser pulse light to an optical fiber to be measured (1).
And a light receiving unit (40) for detecting the levels of backscattered light and Fresnel reflected light from the measured optical fiber that has received the laser pulse light, and In an optical pulse tester that measures a transmission characteristic of the measured optical fiber by measuring a change with time of an output level, the pulse light source unit (31) emits a laser beam having a predetermined wavelength (λ ₁ ). A first laser light source (32) for output, 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. A second laser light source (3) that outputs a laser beam having a wavelength (λ ₂ ) at which backscattered light of
3) measuring the transmission characteristics near the entrance of the optical fiber to be measured by the output of the light receiving section with respect to the laser pulse light from the second laser light source, and using the laser from the first laser light source. A processing unit (50) for measuring transmission characteristics beyond the vicinity of the entrance of the optical fiber to be measured based on the output of the light receiving unit with respect to the pulsed light.

[0013]

In this manner, in the vicinity of the entrance of the optical fiber to be measured, the level of the backscattered light with respect to the laser pulse of the second laser light source becomes higher than the level of the backscattered light by the first laser light source, and the wavelength , And the level difference between the Fresnel reflected light and the backscattered light by the second laser light source is reduced, and the response time of the light receiving unit is shortened.

[0014]

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 one embodiment.

The pulse light source unit 3 of the optical pulse tester 30
Reference numeral 1 denotes a first laser light source 32 which is composed of a laser diode and outputs a laser light having a wavelength λ ₁ and a second laser light source 33 which outputs a laser light having a wavelength λ ₂ shorter than λ _1. The two laser light sources 32 and 33 are pulse-driven by a common pulse generator 34, and the laser pulses output from both laser light sources 32 and 33 are multiplexed by a multiplexer 35 and transmitted to a directional coupler 36.

FIG. 2 shows a change in the amount of loss with respect to the wavelength of the optical fiber 1 to be measured. In order to obtain a long transmission distance, the wavelength of the light used for this fiber is usually the lowest loss wavelength region C. The first laser light source 32 uses the wavelength λ ₁ (for example, 1) in the low-loss area C.
(300 nm).

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

The pulse light from the multiplexer 35 is transmitted to the optical connector 37 via the directional coupler 36, and the pulse light emitted from the optical connector 37 to the optical connector 2 enters the optical fiber 1 to be measured. Is done.

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

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

The processing unit 50 synchronizes with the pulse driving of the two laser light sources 32 and 33 by the pulse generator 34,
The light receiving outputs of the second light receivers 42 and 43 are taken in at predetermined time intervals, the characteristics from the vicinity of the entrance to the far end of the measured optical fiber 1 are measured, and the results are displayed on the display device 51.

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

When the switch 34 is connected to the second light receiver 43, 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 connection portion is sent to the duplexer 41 via the directional coupler 36, and the Fresnel reflected lights of the wavelengths λ ₁ and λ ₂ are converted into the first and second light receivers 42 and 4 respectively.
3 is incident.

The level of the Fresnel reflected light is constant at 4% (-14 dB) of the emission level without depending on the wavelength if the output of each laser pulse is equal.
The light receivers 42 and 43 receive pulse light having a peak of approximately −14 dB based on the emission level.

Since the light having the wavelength λ ₁ has low loss with respect to the optical fiber 1 to be measured, as described above, the level of the backscattered light following the Fresnel reflected light is reduced to approximately −50 dB. Due to this sharp decrease in the level, the received light output of the first photodetector 42 has a wavelength λ ₁ after an extremely long response time Ta (dead zone) has elapsed since t ₀ , as shown in FIG. At a constant slope corresponding to the fiber loss at

On the other hand, the wavelength λ ₂ is shorter than the wavelength λ ₁ ,
For high loss for the measured optical fiber 1, a second level of the backscattered light that follows the Fresnel reflection light on the light receiver 43 is greater than the level of the backscattered light having a wavelength of lambda _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) level is increased, and the level difference for Fresnel reflection is reduced to 25 dB. For this reason, as shown in FIG. 4B, the received light output of the second light receiver 43 is t _0.
After a very short response time Tb (dead zone) has elapsed from the time, decays with a constant slope corresponding to the loss of the fiber at that wavelength lambda _2.

The processing unit 50 receives the trigger pulse from the pulse generator 34, between time t ₀ of slightly longer set time than Ta T _1, the output data of the second photodetector 43 side N
(T) is taken in (S1-4).

After the set time T ₁ has elapsed, the switch 46
It is switched to the first light receiver 42 side, T ₂ hours minus T ₁ from the measurement period T ₀ which is determined, a first light receiver 42
The output data F (t) on the side is fetched (S5-7).

Next, the data N (t) taken in the period T ₁
Respect, performing a correction process corresponding to the wavelength to be displayed on the period T ₁ and period T ₂ consecutive displays a series of characteristics device 51 (S8,9).

The correction process is performed by calculating the difference H () between the linear expression K = Qt-39 of the backscattered light shown in FIG. 4B and the linear expression J = Pt-50 of the backscattered light shown in FIG. t) = KJ
From t ₀ to T ₁ , and this H
(T) was the correction data for each time within period T _1, taken in period T ₁ data N (t) from H (t)
Is subtracted and corrected. As shown in the results Figure 5 this correction process, and the characteristics of the characteristics and period T ₂ of the period T ₁ is continuously displayed on the same slope, fixed as if it were using only pulsed light having a wavelength lambda ₁ The characteristic that the attenuation changes with the slope is displayed. The dead zone of this characteristic is
Only a short time Tb is required as before, and the measurable range near the entrance is expanded by Ta-Tb.

[0032] Incidentally, FIG. 4, for example in the period T ₁ in (b) R
As shown in FIG. 4, when the level changes due to the connection point loss, the backscattered light having the wavelength λ ₁ is also changed to R ′ in FIG.
As shown in FIG. 5, the corrected characteristics are continuously displayed as shown by R ″ in FIG.

[0033]

[Other Embodiments] In the above embodiment, the characteristics from the vicinity of the entrance to the far end of the optical fiber to be measured are measured by the output of one laser pulse, but as shown in the flowchart of FIG. Each time a pulse is output, the acquired data is added, and the average value is obtained after M times, and the characteristics of the optical fiber to be measured are measured based on the averaged data. Measurements can be made with the suppressed data.

In the above-described embodiment, laser pulses having different wavelengths are simultaneously output. However, as in the optical pulse tester 60 shown in FIG.
2, the first and second laser light sources 32 and 33 may be alternately pulse-driven. In this case, the light receiving section 64 is set to 1
It is also possible to configure with one light receiver 65, and the processing unit 6
In the 6, the first when the trigger takes in the data of only period T _1, the second when the trigger, or performing the same processing takes in the data of only the period T _2, or the odd-numbered and even-numbered The data of each period may be alternately taken and averaged.

Further, like the pulse light source unit 71 of the optical pulse tester 70 shown in FIG.
2, 33 independent pulse generators 72,
The processor 73 may be provided so that the measurement near the entrance and the measurement beyond the vicinity of the entrance are asynchronously performed by the processing unit 73 without switching.

In the above-described embodiment, the characteristic of the 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 obtained by the two laser light sources may be displayed so as to be connected at the intersection as shown in FIG.

[0037]

As described above, the optical pulse tester of the present invention comprises a first laser light source for outputting a laser beam of a predetermined wavelength, which has been conventionally used for measurement, and an entrance of an optical fiber to be measured. A second laser light source that outputs short-wavelength laser light in which the level of the backscattered light in the vicinity is higher than the level of the backscattered light from the first laser light source; The near-field characteristics are measured by pulse light from the second laser light source, and the properties beyond the vicinity of the entrance are measured by pulse light from the first laser light source.

For this reason, 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 is reduced, and the response time of the light receiving portion to this change is shortened. Therefore, extremely small dead zones can be measured without using an expensive optical switch or a dummy fiber that complicates the operation.

[Brief description of the drawings]

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

FIG. 2 is a graph showing loss characteristics 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 a configuration of another embodiment of the present invention.

FIG. 8 is a block diagram showing a 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]

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

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) G01M 11/00-11/02 JICST file (JOIS)

Claims (1)

(57) [Claims] An optical fiber to be measured is a laser pulse light.
A light source section (31) for emitting the laser pulse light, and a light receiving section (40) for detecting a level of backscattered light and Fresnel reflected light from the optical fiber to be measured that has received the laser pulse light. In an optical pulse tester for measuring a transmission characteristic of the optical fiber to be measured by measuring a change with time of an output level of the light receiving unit from the time of light emission, the pulse light source unit (31) A first laser light source (32) that outputs a laser beam having a wavelength (λ ₁ ); and a first laser light source that is 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 A transmission characteristic near the entrance of the optical fiber to be measured is measured by an output of the light receiving unit with respect to the pulse light, and an incident light of the optical fiber to be measured is measured by an output of the light receiving unit with respect to 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 mouth.