JPS61269037A - System for measuring dispersing characteristics of optical fiber - Google Patents

Abstract

PURPOSE:To make it possible to instantaneously measure the relative delay time difference of multiple wavelengths, by providing a light incident means, a light condensing means, a mode separation means and a display means. CONSTITUTION:A light pulse during oscillation is incident to an optical fiber 3-3 with good efficiency by using a lens 3-2A and the emitted light of the optical fiber 3-3 is condensed in the output side thereof by a lens 3-2B to be guided to a monochrometer 3-4. When the slit provided to the emitting end of said fiber 3-3 is perfectly opened, all of modes are spatially separated and emitted. Next, because the modes are further timewise separated, said modes are guided to a streak camera 3-5 and a SIT camera 3-7. The signal of each mode plotted on a primary picture is read by an analyser 3-9 to be imparted to a television monitor 3-10. The delay time difference between the modes having different wavelength is visibly displayed on a monitor 3-10 on the basis of the output signal of the analyser 3-9 while the reference signal formed by a delay circuit 3-8 is triggered.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to an apparatus for measuring dispersion characteristics of an optical fiber.

(Prior art and its problems) The wavelength of light used in optical fiber communication and the relay distance during relay transmission are determined by the transmission loss and dispersion characteristics of the optical fiber. In particular, the dispersion characteristics of optical fibers cause waveform distortion and limit the transmission speed during digital transmission. Therefore, □1
iti *ゝ7 y 4 i#c'#J°゛T<>'l
t, 7y4i<0)@j i The relay distance may be limited by the dispersion characteristics, so measuring the dispersion characteristics of an optical fiber is very important, as is the measurement of transmission loss.

Dispersion in optical fiber is a phenomenon in which the speed of light propagating within an optical fiber differs depending on the wavelength.
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Several methods have been proposed. This type of conventional
□.

A typical example of a 11° measuring device is shown in FIGS. 1 and 2.
;□In Figure 1, light source 1 such as a semiconductor laser
-1 to optical fiber 1 with a pulse width of several hundred picoseconds.
-2, the Raman effect causes the first Stokes light, the second Stokes light...
The nth Stokes light is generated. One Stokes light, for example, the first Stokes light, from among the Stokes lights with different wavelengths is extracted by the wavelength selection circuit 1-5, and the extracted first Stokes light is split by the spectrometer 1-7, and one After propagating through the optical fiber to be measured 1-3, the optical receiver 14
is converted into an electrical pulse by the oscilloscope 1-6
is input. The other first Stokes light branched by the spectrometer 1-7 is converted into an electric pulse by the photoreceiver l-4', and a certain delay is given by the delay circuit 1-8 so that it becomes a reference signal. -6 is input. The oscilloscope 1-6 can determine the relative delay time difference from the pulse positions of the reference electric pulse of the reference signal and the electric pulse to be measured that has been converted into an electric signal after propagating through the optical fiber to be measured.

Next, the wavelength selection circuit 1-5 selects a second Stokes beam having a different wavelength and performs the same operation to determine the relative delay time difference with the reference signal. By repeating this operation up to the n-th Stokes light, the relationship between the relative delay time for each wavelength of the light propagating through the optical fibers 1-3 is measured, and the dispersion characteristics of the optical fiber 130 are determined (L, G, Cohen
et al, Appl, oct.

Vol　16. p 3136.1977).

Note that a high-output laser such as a YAG laser is used as the light source 1-1, and a stimulated Raman phenomenon is generated within the optical fiber 1-2.

Next, the dispersion measuring device shown in FIG. 2 will be explained. Emitted light from the semiconductor laser (LD) 2-1 having several different wavelengths is sinusoidally modulated at a frequency f by a synthesizer 2-6. One of the modulated semiconductor lasers 2-1 is selected by the optical switch 2-2, and its output is input into the optical fiber 2-3. Optical fiber 2-
The emitted light of No. 3 is converted into an electrical signal 2-7 by a light receiver 2-4, and the phase difference with the reference electrical signal 2-8 is determined by a phase shifter 2-5. Similarly, the phase changes of the modulated signal for several wavelengths are determined, and from this result, the dispersion characteristics of the optical fiber 2-3 are determined (K
, Tatekura eL at, Sympos
ius on optical fiber? s+e
asure+wents, Boulder.

U, S, A, 1984”).

As explained above, the conventional optical fiber dispersion measuring device measures the optical delay or optical delay difference within the optical fiber separately for each wavelength.

With this measurement method, the following problems arise: ■ Measurement takes time, and fluctuations in the length of the optical fiber due to temperature changes during measurement may cause errors in the measurement results. ■ The light source side and the light receiving side are located in different locations. There are disadvantages such as difficulty in making measurements at the far end, that is, far-end measurements; and (2) the need for light sources with different wavelengths, which makes the light source complicated.

(Object of the Invention) The present invention has been made in view of the above-mentioned shortcomings of the prior art, and is based on the dispersion characteristics of an optical fiber that enables instantaneous measurement of relative delay time differences of multiple wavelengths with high precision, and enables far-end measurement. The purpose is to provide a measurement method.

(Structure and operation of the invention) The method for measuring the dispersion characteristics of an optical fiber according to the present invention includes a light input means for generating a longitudinally multimode light pulse with a narrow pulse width as a measurement light and making it incident on an optical fiber to be measured. , a focusing means for focusing the light emitted from the optical fiber, a mode separating means for separating the output of the focusing means into each mode, and a delay time difference between each mode separated by the mode separating means. and display means for simultaneously visually displaying the dispersion characteristics of the optical fiber on the same display surface.

The present invention will be described in detail below using the drawings.

(Example) FIG. 3 shows an example of the dispersion measuring device according to the present invention. In this embodiment, a synthesizer 3 generates a sine wave with a frequency f by converting the semiconductor laser 3-1 which is oscillating in longitudinal multimode mode into a sine wave having a frequency f.
-6 and a comb generator or pulse generator 3-11 for generating a pulse waveform.
An optical pulse having a pulse width of several tens of picoseconds and a repetition frequency of fHz and having longitudinal multiple modes is generated. This oscillating light pulse is efficiently incident on the optical fiber 3-3, which is the medium to be measured, using the lens 3-2A. On the output side of the optical fiber 3-3, the light emitted from the optical fiber 3-3 is collected by a lens 3-2B and guided to a monochromator 3-4 for separating the light into wavelengths. This monochromator 3-4 has a built-in grating and spatially separates the incident light into each mode for each wavelength.
By leaving the slit at the output end of No. 4 completely open, all modes are spatially separated and emitted. Note that a prism may be used as a means for spatially separating each mode. Next, each spatially separated mode is connected to a streak camera 3-5 and an SI for further temporal separation.
Guided by T camera 3-7.

Both cameras 3-5 and 3-7 use an optical fiber 3-3, which is a medium to be measured, to display electric signals on the primary screen with different delay times for each wavelength on the vertical axis and each wavelength on the horizontal axis. This is a device for displaying images. Also, both cameras 3
-5 and 3-7, it is possible to temporally separate each mode with an accuracy of several picoseconds. - The signals of each mode plotted on the next screen are read out by the analyzer 3-9 and given to the television monitor 3-IO having image analysis capability.

Note that the output light from the synthesizer 3-6 is branched, one of which is sent to a pulse generator or comb generator 3-11 for generating an optical pulse with a pulse width of several tens of picoseconds, and the other is
A constant delay time is given by the delay circuit 3-8, and the signal becomes a reference signal. This reference signal is the streak camera 3
-5 trigger pulse. That is, the delay time difference between the modes having different wavelengths is visually displayed on the television monitor 3-10 using the output signal of the analyzer 3-9 as a trigger with the reference signal created by the delay circuit 3-8 described above. The dispersion characteristics of the optical fiber 3-3 for each mode can be determined from the delay time difference.

FIGS. 4A and 4B are characteristic diagrams showing an example of measurement of dispersion characteristics using the above-mentioned measuring device of the present invention.

1mIg (a) 1″゛3+゛″” (7)′<’v″゛″
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A streak image captured by the television monitor 3-IO when the emitted light from the oscillating semiconductor laser is directly input to the monochromator 3-4 without passing through the optical fiber 3-3, which is the medium to be measured. It is. When the optical fiber 3-3 is not passed through, longitudinal multimode oscillation with different wavelengths occurs1.
It can be seen that the optical pulses of 00 ps or less are lined up on the horizontal axis, and there is no delay time even if the wavelengths are different. Note that in the figure, brightness corresponds to light intensity.

Next, FIG. 4(b) shows the above-mentioned optical pulse propagated through the 4.5 (km) long optical fiber 3-3 on the TV monitor 3-1.
This is a streak image measured at 0. It can be seen from the figure that the streak images of each mode are obliquely inclined. This means that the propagation time of the optical fiber 3-3 differs depending on each wavelength (each mode), and the property that the propagation time differs for each wavelength, that is, the relative delay time difference of each wavelength is the characteristic of the optical fiber to be found. 3-3 dispersion characteristics. For example, from the figure, the dispersion value at a wavelength of 1.29 μm is as follows.

However, T1 is 1.28616μ ring (1,29-△-/2
) Delay time (ps) T is 1.29384 μa + (1,29 + Δ/2) Delay time (ps) ΔW is wavelength of T2 − wavelength of T1 (μm) l is length of optical fiber (km) It is.

Although the units of the horizontal and vertical axes in FIG. 4(b) are extremely unclear, in the actual TV monitor 3-10, the horizontal axis is 1 (nm) and the vertical axis is 1.63 (nm). ps) can be decoded.

In addition, normally, the dispersion value is not calculated by the measurer using the above formula, but is calculated by a microcomputer, etc. built into the television monitor 3-10 (not shown), and the dispersion value to be determined is The structure is such that the information is displayed digitally or printed.

(Embodiment 2) In Embodiment 1, a dispersion characteristic measuring device for near-end measurement was described in which the transmitting side and the receiving side are located at the same location, for example, in an optical fiber manufacturing factory. but,
Since the length of the optical fiber to be measured is typically several tens of kilometers, it is customary to perform measurements at points where the transmitting and receiving sides are separated from each other. When making this far end measurement,
The problem is how to extract the reference signal, or how to create a reference signal on the receiving side that is synchronized with the transmitting side.
Conventionally, there has been no disclosure of any measurement method capable of far-end measurement of dispersion characteristics.

FIG. 5 is another embodiment of the present invention, and is a block diagram in which the dispersion characteristics are measured by far-end measurement.

(1) Transmitter First, the configuration of the transmitter will be explained.

As a transmitter, one light source 3-1 that oscillates in longitudinal multimode is used.
It is sufficient if there is a modulation means so that the light emitted from the light source becomes a light pulse having a pulse width of at least several tens of picoseconds. In the figure, a combination of a synthesizer 3-6 and a pulse generator 3-11 is used as the modulation means, but a combination of a synthesizer 3-6 and a comb generator may also be used. In addition, in order to efficiently input the output light from the light source 3-1 into the optical fiber 3-3, a convex lens 3-2A is provided.
It is even more effective to use .

(2) Lens 3- for condensing the light emitted from the receiver optical fiber 3-3
TV monitor 3- that visually displays the delay time difference from 2B
Up to 10 are all (same as in FIG. 3).

The difference from FIG. 3 is that a means for newly generating a reference signal is provided. That is, the receiver is provided with a synthesizer 3-6' and a variable delay device 3-12 to synchronize with the synthesizer 3-6 of the transmitter. By the way, the delay amount of the variable delay device 3-12 is given by: However, C is the speed of light in vacuum, l is the length of the optical fiber 3-3, and n is the refractive index of the optical fiber 3-3. Or, as a delay measure, use the TV monitor 3-1.
Adjustment may be made so that the streak image is projected at the center of the television screen while looking at 0.

Note that the frequency f of the synthesizer 3-6' must be determined in advance by the transmitting and receiving sides.

As described above, a reference signal synchronized with the transmitter may be generated and applied as a trigger to the streak camera 3-5 in the same manner as shown in FIG.

(Embodiment 3) FIG. 6 is another embodiment according to the present invention, and is a configuration diagram of a receiver used for far-end measurement. This embodiment is characterized in that the modulation frequency f on the transmitting side is extracted and used as the reference signal, and a spectroscope 3-13 for splitting light is inserted after the condenser lens 3-2B, and one The signal goes to a measurement system similar to that shown in the figure, and the other signal is input to a light receiving element 3-14 for generating a reference signal. The light receiving element 3-14 may be a semiconductor light receiving element such as an APD, and is configured so that it is converted into an electrical signal only when it receives a light pulse with a pulse width of several tens of picoseconds. With this configuration, it is possible to easily create a reference signal on the receiving side that is synchronized with the transmitting side. In addition,
Although not shown, if the optical fiber 3-3 serving as the medium to be measured is long, an amplifier may be inserted after the semiconductor light-receiving element 3-14.

(Effects of the Invention) As described above, by using the measurement method of the present invention, the dispersion characteristics of the optical fiber for each wavelength can be visually displayed at the same time and measured instantly, and measurement errors due to the surrounding environmental temperature can be avoided. It can be eliminated. Furthermore, since a time resolution of lps or less can be expected, highly accurate dispersion characteristics can be obtained.

Furthermore, far-end measurements, which were difficult with conventional techniques, are also possible, which is extremely effective.

[Brief explanation of the drawings] Figures 1 and 2 are configuration diagrams of a conventional dispersion measuring device,
The figure is a block diagram showing one embodiment of the present invention, Figures 4(a) and (bl) are characteristic side views measured using the dispersion measuring device according to the present invention, and Figures 5 and 6 are far-reaching diagrams according to the present invention. It is a block diagram of an example for performing edge measurement. if... Light source, 1-2... Optical fiber, 1-3
...Optical fiber (for measurement), 1-4... Light receiver,
1-4'... Light receiver, 1-5... Wavelength selection circuit, 1-6... Oscilloscope, 1-7... Spectrometer, 1-8... Delay circuit, 2-1. ... Semiconductor laser, 2-2... Optical switch, 2-3... Optical fiber, 2-4... Light receiver, 2-5... Phase differencer,
2-6... Synthesizer, 2-7... Received electrical signal, 2-8... Reference electrical signal, 3-1... Light source,
3-2A... Lens, 3-2B... Lens, 3-3... Optical fiber, 3-4... Monochromator, 3-5... Streak camera, 3-
6...Synthesizer, 3-7...SIT camera,) 3-8°°° delay circuit/3-9°
°° Anara 9 the 3-10...TV monitor, 3-1
DESCRIPTION OF SYMBOLS 1... Pulse generator, 3-12... Variable delay circuit, 3-13... Spectrometer, 3-14... Light receiving element. 1st [X] Tree I illusion

Claims (1)

[Claims] a light input means for generating a vertical multi-mode optical pulse with a narrow pulse width as measurement light and inputting it into an optical fiber to be measured; a condensing means for condensing the light emitted from the optical fiber; A mode separation means for separating the output of the light condensing means into each mode; and a display for determining the delay time difference of each mode separated by the mode separation means and visually displaying the result as a dispersion characteristic of the optical fiber simultaneously on the same display surface. A method for measuring dispersion characteristics of an optical fiber, comprising means.