JPH0132455B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for measuring transmission characteristics of an optical fiber.

The frequency response characteristic of an optical fiber, called the baseband frequency characteristic, has conventionally been measured by the following destructive method. That is, a signal light whose amplitude is modulated in a sinusoidal manner is input into the fiber under test, first the modulation degree of the signal light on the output side (output side modulation degree) is measured, and then the modulation degree of the signal light on the output side (usually Cut the fiber at a distance of about 1 m from the end and measure the modulation degree of the signal light from the cut end (incidence side modulation degree),
By comparing the two modulation degrees, the baseband frequency characteristics of the optical fiber were measured. However, in this measurement method, it is necessary to cut the input side of the fiber under test into a mirror surface for each measurement, and when attempting to automate the measurement of baseband frequency characteristics, the fiber under test must be replaced after each measurement. was a major obstacle. Such a problem based on the destructive measurement method is that the modulation degree on the incident side is not based on the signal light that passes through a part of the fiber under test, but is based on the signal light derived directly from the excitation system (light source system) using an optical splitter. It is thought that this problem can be resolved by measuring it.

On the other hand, optical fibers have inherent losses such as losses due to scattering and absorption, so-called transmission losses, and it is impossible to measure this by the absolute light intensity of the light emitted from the far end of the fiber under test. It was believed that non-destructive measurement was impossible because the amount of light incident on the optical fiber depends on the structural parameters (core diameter and refractive index difference) of the optical fiber. However, it has recently been discovered that the backscattered light method is effective for non-destructive measurement of transmission loss. This backscattered light method is the ratio of propagating light to reflected light at each point in the longitudinal direction of the optical fiber (Po = propagating light/reflected light).
This takes advantage of the fact that if is constant, the power of reflected light from each point in the longitudinal direction is proportional to e ^-2 〓 ^l . However, α is the transmission loss, and l is the distance from the point of incidence to the point of reflection. In actual measurements, pulsed light (light that forms a pulse due to pulse modulation or a light emitting mechanism) is used as the incident light, and the reflected light from each point is sent to a detector installed at the input end of the optical fiber. The transmission loss is measured by observing the intensity of the reflected light in the time domain, using the fact that the delay time until reaching the reflection point differs depending on the reflection point. In this case, in modern optical fibers, due to the progress of loss reduction, the loss factor is mostly due to Lehre scattering, with the exception of some absorption by OH groups, and the refractive index distribution in the longitudinal direction is Since it is uniform, the condition that the ratio of propagated light/reflected light is constant, which is the prerequisite for the backscattered light method, is satisfied.

The present invention was devised from the above considerations, and
It eliminates the need to cut the fiber under test and prepare the cut surface on the input side, which has traditionally been a hindrance in measurement work.Moreover, once the fiber under test is set, transmission loss and baseband frequency characteristics can be measured from the beginning. We provide a transmission characteristic measuring device that can perform a series of operations up to the end. Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention, in which 1 is an optical fiber to be measured, 2 and 3 are adjustment stages to which the input end and output end of the optical fiber 1 are respectively coupled, and 4 5 is a continuously driven light source (a light source that continuously generates an optical signal by continuously applying an electric current) that generates a signal light whose amplitude is modulated in the form of a sine wave. A light source that generates signal light (pulse-driven light source),
6 is a light source switcher provided between the light sources 4 and 5 and the axis adjustment stage 2; 7 is provided between the light source switch 6 and the axis adjustment stage 2; An optical splitter, such as a half mirror, 1 that branches to the photodetector 9 and guides the reflected light from the optical fiber 1 that has passed through the alignment stage 2 to the photodetector 8.
0 is an optical switch provided between the optical splitter 7 and the adjustment stage 2; 11 is an optical switch provided between the adjustment stage 3, the optical splitter 7, and the photodetector 9; An optical splitter such as a half mirror that guides the signal light from the light source 4 or 5 branched by the light source 4 or 5 to the photodetector 9, 12 is an optical switch provided between the optical splitters 7 and 11, and 13 is an axis adjustment stage. A controller for adjusting the axis that issues a command to move the adjusting stages 2 and 3 based on the output of the photodetector 9 so that the photodetector 9 detects the maximum amount of light emitted from the optical fiber 1 after passing through the optical fiber 1. be. In addition, 6a is an optical splitter such as a half mirror, 6b is an optical switch, 14 is a lens, 15 is a light receiving element, 16 is a light emitting element, 17 is a sine wave modulator, and 18 is a pulse modulator.

A self-aligning mechanism is composed of the alignment stages 2 and 3, the photodetector 9, and the controller 13, but a high-precision pulse drive motor is usually used as the operating part of the alignment stages 2 and 3, and this pulse The motor is driven and controlled by a controller 13 consisting of a microcomputer or the like. This controller 13 is incorporated into a feedback circuit which inputs the light emitted from the optical fiber 1 and outputs an instruction signal to the pulse motor.
Then, a series of operations are performed to bring the coupling between the photodetectors 9 to an optimal state. This self-aligning mechanism makes it easy and accurate to set the optical fiber 1 into the measuring instrument prior to measurement, resulting in improved measurement workability and measurement accuracy. Furthermore, assuming that the optical fiber 1 has multiple cores, a fiber holder for holding a multi-core optical fiber is attached to each of the alignment stages 2 and 3, and the next fiber is automatically moved after the measurement of each fiber core is completed. By using this mechanism, each core of a multi-core fiber can be measured continuously, contributing to improved work efficiency. Furthermore, if the optical system is adjusted with sufficient precision, the photodetectors 8 and 9 may be fixedly installed, but if they are placed on an axis adjustment stage such as 2 or 3 and the position is controlled by a controller, the optical axis alignment will be difficult. can be performed easily and accurately, further improving measurement accuracy.

Since the present invention measures the baseband frequency characteristics and transmission loss of the optical fiber 1 using the same measurement system without destroying the optical fiber 1 to be measured, the continuous drive light source 4 and the pulse drive light source 5 can be Both are provided, but these must be used interchangeably. Therefore, in this embodiment, a light source switching device 6 consisting of an optical splitter 6a and two optical switches 6b and 6d is used. The optical switches 6b and 6d may be of any type, such as manual or mechanical, but optical switches using electrically controlled solenoid coils or optical switches using optical crystals such as BSO (bismuth silicon oxide) may be used. Its use allows for external electrical control and is also convenient. Of course, if a method is adopted in which the light source 4 or 5 itself stops emitting light, the optical switches 6b and 6d are of course unnecessary.
Note that the other optical switches 10 and 12 can be electrically controlled from the outside by using an optical switch using a solenoid coil or an optical switch using an optical crystal such as BSO. Furthermore, the pulse-driven light source 5 may be one that generates light in the form of pulses, one that intermittents the light emission itself (direct modulation), or one that extracts continuous light emission intermittently with a light switch (external modulation). But it doesn't matter. Similarly, the continuous drive light source 4 may be either directly modulated or externally modulated.

Measurement using the optical fiber transmission characteristic measuring device of the embodiment shown in FIG. 1 is carried out as follows. Prior to measurement, the controller 13 adjusts the axis adjustment stages 2 and 3.
is operated to optimize the coupling between the optical fiber 1 to be measured, the light source 4, and the photodetector 9. During this axis adjustment, the modulation frequency in the light source 4 may be arbitrary.

(1) When measuring baseband frequency characteristics: Select only the signal light modulated into a sine wave from the continuously driven light source 4 using the light source switch 6, turn off the optical switch 10, and turn off the optical switch 1.
Turn on 2. After that, while sweeping the modulation frequency of the continuously driven light source 4, the optical fiber 1 is
The sine wave modulated signal light before entering is detected by the photodetector 9 via the optical splitter 7, the optical switch 12, and the optical splitter 11, and the degree of modulation of the sine wave modulated signal light is measured for each modulation frequency. do. Next, the optical switch 10 is turned on and the optical switch 12 is turned off, the sine wave modulated signal light after propagating through the optical fiber 1 is detected via the optical splitter 11, and the degree of modulation for each modulation frequency is measured. .
As a result, the baseband frequency characteristic is determined by comparing the modulation degrees before and after the light enters the optical fiber 1 and after the light exits the optical fiber 1 for each corresponding modulation frequency.

(2) When measuring transmission loss: Select only the pulse-modulated signal light from the pulse-driven light source 5 using the light source switch 6, and switch the optical switch 10.
Turn on. The photodetector 8 detects backscattered light components of the pulsed modulated signal light incident on the optical fiber 1 reflected at each point in the longitudinal direction of the optical fiber 1. The intensity of the backscattered light component (reflected light) thus detected is observed in the time domain, and the transmission loss can be obtained by comparing the intensity of the reflected light from each point of the fiber.

Note that the loss calculation technique from the output signals of the photodetectors 8 and 9 in the above (1) and (2) is based on a known method.

FIG. 2 is a graph showing an example of the obtained transmission characteristics, in which the shaded area represents the amount of attenuation due to the baseband frequency characteristics of the total loss, and the blank area represents the transmission loss.

As specifically explained above with the examples,
According to the present invention, it is no longer necessary to cut the optical fiber to be measured on the incident side, process the cut surface, and replace the fiber after each measurement for cutting, which was a problem in conventional measurement operations. Once the optical fiber is set, transmission loss and baseband frequency characteristics can be measured as needed in a series of operations from start to finish.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of the present invention, and FIG.
The figure is a graph showing an example of measurement results. In the drawing, 1 is an optical fiber to be measured, 2 and 3 are adjustment stages, 4 is a light source (continuous drive light source) that generates a signal light modulated in a sine wave shape, and 5 is a signal light that generates a signal light modulated in a pulse shape. 6 is a light source switcher, 6a, 7, 11 are optical splitters, 8, 9 are photodetectors, 6b, 10, 12 are optical switches, and 13 is an axis adjustment controller.

Claims (1)

[Claims] 1. Two adjustment stages to which the input end and output end of the optical fiber to be measured are coupled, respectively, and a continuous stage that generates a signal light modulated in the form of a sine wave, which is switched and coupled to the adjustment stage on the input end side. a driving light source and a pulse-driven light source that generates a pulse-modulated signal light; a photodetector coupled to an axis adjustment stage on the output end side via an optical splitter; It is provided between the adjustment stages, and branches the signal light from both light sources to the adjustment stage on the input end side and the optical splitter, and also splits the reflected light from the adjustment stage on the input end side to the photodetector. includes an optical splitter different from the optical splitter that leads to another photodetector, an optical switch provided between these optical splitters, an axis adjustment stage on the incident end side, and both light sources. Inputs the output of an optical switch different from the optical switch provided between the optical splitter provided between the input end side adjustment stages and a photodetector on the output end side adjustment stage side. An apparatus for measuring transmission characteristics of an optical fiber, comprising: an adjustment controller that controls the positions of the two adjustment stages so that the output of the signal is maximized.