JPH0150850B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for measuring baseband transmission characteristics of optical communication cables. In particular, the present invention relates to a measuring device suitable for performing tests between far separated ends of an optical cable after it has been installed.

Before installing an optical cable, if both ends of the optical cable are in close proximity and the transmitting and receiving devices of the measurement device can be directly connected using a stable measurement standard such as a resistive attenuator, the transmission characteristics of the optical cable should be determined using that measurement standard. By comparing, it is possible to measure with high precision. However, optical cables have been laid,
If the two ends are separated by several kilometers or tens of kilometers, it is not possible to set a comparison standard, so the absolute values of the signal level of the transmitting device and the signal level of the receiving device must be set or measured correctly.

Furthermore, if the two ends are far apart, it is necessary to send and receive synchronization signals, etc. in order for the receiving device to know the frequency sent from the transmitting device, and for this reason, it is necessary to send and receive a synchronization signal etc. in parallel to the optical cable under test. A cable is required to transmit electrical signals.

For this reason, conventional devices are large-scale devices with absolute precision, and are configured to utilize intervening lines or the like for transmitting and receiving synchronization signals. moreover,
Conventional devices require that operators at both ends be able to contact each other by telephone during measurements.

The present invention improves on this by making it possible to easily and accurately measure the characteristics of an optical cable after it has been laid, eliminating the need for transmitting and receiving separate signals such as synchronization signals, and basically allowing operators at both ends to contact each other by telephone. The purpose is to provide a device that does not require

In particular, the present invention is based on an application filed by the same applicant as the present application (Japanese Patent Application No. 55-107556 (Japanese Patent Publication No. 1-22894)).
This is an improvement.

The baseband frequency characteristic of an optical cable is, in principle, a characteristic proportional to a constant power of the baseband frequency (modulation frequency), and this characteristic per unit length is approximately determined by the type of cable, optical signal wavelength, etc. The baseband characteristics of such optical cables do not change significantly due to installation. Although the length may change due to installation, connection, etc., the basic characteristics remain the same.

Therefore, when testing after installation is completed, it is not necessarily necessary to measure the baseband frequency over a wide range of frequencies at many frequency points; it is sufficient to measure the characteristics of a very small number of frequencies. can. In particular, when a specific cable type and optical signal wavelength are determined, it is possible to accurately estimate the overall characteristics by measuring the transmission characteristics at several frequencies, and there is no practical problem. .

The inventors conducted tests on a large number of installed optical cables and confirmed that measuring the transmission characteristics using only a few frequencies is sufficient for cable testing after installation.

The present invention is based on such a principle, and includes an oscillator that automatically alternately switches and transmits several predetermined baseband frequencies to be measured, and a modulation input for the output of this oscillator. and an electro-optical converter whose output light is connected to the optical cable under test.The receiving side device inputs the output light of the optical cable under test, demodulates the output light, and converts the output light to the above-mentioned several basebands under test. a photoelectric converter for obtaining a frequency output signal; a frequency converter for converting this output signal into several intermediate frequencies; and a detector for detecting the level of the intermediate frequency signal obtained at the output of the frequency converter. , and an arithmetic unit that takes in the output of the detector and performs arithmetic processing.

This will be explained in detail with reference to the drawings.

FIG. 1 is a diagram showing the configuration of an apparatus according to an embodiment of the present invention. In this figure, 1 is the transmitting side device, 2 is the optical cable to be measured,
3 is a receiving side device. The transmitting device 1 includes oscillators 10, 11, and 12 that generate predetermined frequencies ₀ , ₁ , and ₂ within the baseband frequency characteristics.
and a switching device 13 that switches and sends this output,
an electro-optical converter 14 that uses this output as a modulation input;
A switching signal generator 15 that drives the switching device 13 is included. In addition, the receiving side device 3 includes an optical cable 2
an opto-electrical converter 31 that inputs the output light of and demodulates it; an amplifier 32 that amplifies the electrical signal output from the opto-electric converter; and a local section that generates a frequency of { _2-1 )/ _2-0 } _. An oscillator 33, a local oscillator 34 that generates a frequency of ( ₁ + ₂ )/2, a local oscillation switch 35 that switches the outputs of the local oscillators 33 and 34, and an output of the amplifier 32 and the output of the local oscillator 33 or 34. a mixer 36 that mixes and performs frequency conversion; a detector 37 that detects this output;
It includes an arithmetic unit 38 that processes this output, and a display 39 that displays this output.

The operation of the device configured in this way will be explained.
FIG. 2 is a signal waveform diagram at the points indicated by x marks in FIG. 1. That is, by the switching device 13 controlled by the switching signal generator 15, the oscillators 10, 11, 1
The signal No. 2 is sent out at a period as shown in FIG. 2a.
This is modulated into an optical signal by the electro-optic converter 14,
The signal is sent to the optical cable 2 to be measured.

In the receiving device 3, the frequencies ₀ , ₁ , and ₂ appearing at the output of the amplifier 32 are transferred to the mixer 36 and the local oscillator 3.
3 and 34 to convert to one intermediate frequency.
Frequency ₀ is usually around several MHz, which can be said to be a low frequency that corresponds to the DC level for optical fibers. When the frequency of the local oscillator 33 is set to {( ₂ − ₁ )/2 − ₀ } and the local oscillation switch 35 is set so that the frequency goes into the mixer 36, the mixer 36 outputs ( ₂ − ₁ )/ An intermediate frequency of 2 is output. At that time, Figure 1b
The waveform observed at is as shown in FIG. 2b, and the amplitude A ₀ shown in the figure is stored in the calculator 38.

Next, by switching the local oscillator switch 35 so that the frequency ( ₁ + ₂ )/2 of the local oscillator 34 enters the mixer 36, the same intermediate frequency as when ₀ is applied to the frequencies ₁ and ₂ . can be obtained. That is, for frequency _{1 , 1} + ₂ / 2 - ₁ = ₂ - ₁ / 2, and for frequency ₂ , ₂ - ₁ + ₂ / 2 = ₂ - ₁ / 2, both of which are equal. _An intermediate frequency of _2-1 /2 is obtained. This intermediate frequency signal is sent to the detector 37
The signal shown in Fig. 2 b' is obtained. That is, the reception amplitude at frequency ₁ is _A1 , and the reception amplitude at frequency ₂ is _A2 . The signals shown in FIG. 2b and b' are taken into the arithmetic unit 38 and calculated.

The baseband loss L ₁ at frequency ₁ is expressed as L ₁ = 20logA ₀ /A ₁ , and the baseband loss at frequency ₂
L ₂ is expressed as L ₂ =20logA ₀ /A ₂ .

FIG. 3 shows an example of the baseband loss frequency characteristics of an optical cable. Now, as an example, the optical cable to be measured is this cable, and the frequency ₀ is 6M.
Hz, frequency ₁ is 210MHz, and frequency ₂ is 400MHz. If we know that the basic loss characteristics of this optical cable can be expressed with respect to frequency by the formula L = af + bf ² , we can measure two specific frequencies ₁ and ₂ and calculate the loss L ₁ ,
Knowing L ₂ allows us to know the constants a and b in the above equation. This allows the baseband frequency characteristics to be known with considerable accuracy. From this characteristic, it is also possible to calculate the transmission bandwidth with 6dB attenuation. These calculations can be performed by the calculator 38. The configuration of the computing unit 38 can be variously configured depending on its purpose. Preferably, it is configured by a microprocessor, stores some basic characteristics of the optical cable, and calculates and estimates the overall characteristics from the measurement results.

In the above description, the number of predetermined baseband frequencies to be measured is three, but if the number is four or more, the accuracy will be further improved. In this case, it is possible to measure not only optical cables with simple characteristics such as L=af+bf ² but also optical cables with more complex characteristics using the same principle.

As explained above, according to the present invention, the measurement frequency can be reduced to only a few frequencies, and the apparatus can be extremely simplified. Since there is no need for the transmitting side device to automatically switch the frequency and transmit, and for the receiving side to synchronize with this, there is no need to transmit a synchronization signal. Also,
It is sufficient for the transmitting side device to simply connect its output to the optical cable to be measured, and measurements can be carried out without the need for a telephone discussion between the two operators in principle. The measurement results obtained by the apparatus of the present invention have sufficient accuracy for testing actual optical cables.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of an apparatus according to an embodiment of the present invention. FIG. 2 is a waveform diagram for explaining the operation. FIG. 3 is a diagram showing an example of baseband loss frequency characteristics of an optical cable. 1... Sending side device, 2... Optical cable to be measured,
3... Receiving side device, 10, 11, 12... Oscillator, 13... Switching device, 14... Electro-optical converter, 1
5...Switching signal generator, 31...Photoelectric converter,
32...Amplifier, 33, 34...Local oscillator, 3
5... Local oscillation switch, 36... Mixer, 37...
...Detector, 38...Calculator, 39...Display device.

Claims (1)

[Claims] 1. An oscillator that automatically switches and transmits three predetermined baseband frequencies to be measured, and an electro-optic converter that uses the output of this oscillator as a modulation input and connects the output light to the optical cable to be measured. The transmitter includes an opto-electrical converter which inputs the output light of the optical cable to be measured and demodulates the output light to obtain output signals of the three baseband frequencies to be measured; a frequency converter for converting to an intermediate frequency; a detector for detecting the level of the intermediate frequency signal obtained at the output of the frequency converter; An optical cable transmission characteristic measuring device comprising, on the receiving side, a calculation unit that compares and calculates the detector outputs for two other frequencies using the detector output for the above as a reference.