RU136660U1 - OPTICAL REFLECTOMETER - Google Patents

Abstract

An optical reflectometer, comprising in series a receiving optical system, a photodetector, an amplifier and a recording unit, as well as an emitter with a transmitting optical system and a gating unit, characterized in that an adaptation unit, a correction unit and a scaling unit are connected to it, the output of which is connected to the third the input of the registration unit, the output of which is connected to the second input of the scaling unit, the input of the adaptation unit is connected to the output of the photodetector, the first input of the correction unit is connected to the output the gating unit, the second input with the amplifier output, the third input of which is connected to the output of the adaptation unit, the output of the correction unit is connected to the fourth input of the registration unit, and a tunable filter is also introduced, the input of which is connected to the output of the photodetector, and the output is from the second input of the amplifier and with the input of the adaptation block.

Description

The utility model relates to fiber-optic communication technology, and can be used to determine the distribution of the beat length of an optical fiber in a section of a transmission line, which makes it possible to evaluate such characteristics of a linear path as the correlation length, polarization mode dispersion, etc.

The quality control of fiber-optic transmission lines is carried out by measuring light loss in them. For these purposes, equipment was developed that allows one to find not only the total losses in the line (optical testers), but also the distribution of losses and reflection coefficients along the line (optical reflectometers).

In trunk lines, they strive to make the regeneration sections of the line as long as possible, which reduces the number of repeaters and reduces the cost of servicing the line. At the same time, the requirements for the reliability of the line and the value of losses in it increase significantly. In this case, it is not enough to measure the total losses in the line, but it is still necessary to measure the losses in the construction lengths of the optical cables, in fiber splices, and in the optical connectors. Currently, this can only be done using a fiber-optic reflectometer.

A known fiber-optic reflectometer containing a series-connected receiving optical system, a photodetector, an amplifier and a recording unit, as well as an emitter with a transmitting optical system and a gating unit. (Borodulin V.I. et al. Fiber Optic OTDR. - Radio Engineering and Electronics, 1981, No. 4, p. 866-869.)

The disadvantage of this device is the low reliability of distance measurement. This is due, firstly, to the possibility of saturating the FPU photodetector with a powerful optical scattering signal from the near zone of an imperfectly transparent propagation medium. In this case, a weak pulse reflected from the medium-object boundary (from the place where the fiber under study is cut off) cannot be detected due to the “blindness" of the FPU, and accordingly, the pulse delay time and scattering time to the object cannot be reliably measured. Secondly, when the amplifier is gated, an impulse with a steep front will be "cut out" from the exponentially decaying scattering signal, along which the recording unit will make a false measurement.

The purpose of the utility model is to increase the accuracy of measuring the distance to the object in a wide dynamic range in the presence of an interfering signal.

This is achieved by the fact that a device comprising a receiving optical system in series, a photodetector, an amplifier and a recording unit, as well as an emitter with a transmitting optical system and a gating unit, a tunable filter, an adaptation unit and a correction unit are introduced, while the input of the adaptation unit is connected to the photodetector output, and the output to the control input of the amplifier, the first input of the correction unit is connected to the output of the amplifier, and the output to the control input of the registration unit, tunable by a filter with is dined at the input with the output of the photodetector, and at the output with the input of the adaptation unit, the optical output of the emitter is connected to the input of the transmitting optical system, and the sync output is connected to the input of the gating unit, the output of which is connected to the gate input of the amplifier and to the second input of the correction unit, the output of the unit scaling is connected to the second input of the registration unit, the third input of which is connected to the sync output of the emitter.

In FIG. 1 shows a structural diagram of a device; in FIG. 2 is a diagram explaining the operation of a tunable filter, adaptation unit, and amplifier; in FIG. 3 frequency response of the tunable filter (1 and 2) for the corresponding spectra of the scattering signal (3 and 4); in FIG. 4 shows a schematic diagram of a tunable filter.

The device comprises a receiving optical system 1, a photodetector 2, an amplifier 3, a recording unit 4, a transmitting optical system 5, an emitter 6, a strobing unit 7, a tunable filter 8, an adaptation unit 9, a correction unit 10, a scaling unit 11.

The device operates as follows.

The emitter 6 forms an optical pulse and a sync pulse coinciding in time with its front. The optical pulse through the transmitting optical system 5 enters the propagation medium (measured signal). The clock pulse enters the gating block 7, where a strobe pulse with the necessary time characteristics (delay, duration) is formed. The optical signal reflected from the object and the propagation medium is supplied to the photodetector 2 by means of the receiving optical system 1, where it is converted into an electrical signal. The registration unit 4, made in the form of a time-amplitude-code converter, generates an electric signal (voltage), the amplitude of which is proportional to the time interval between the clock pulse and the received signal, and then the generated analog signal is converted into binary code. This allows you to accurately change the time intervals in the range of a few thousand nanoseconds without the use of ultra-wideband digital devices.

The linearity of the conversion, and, accordingly, the high reliability of measuring the distance to the object, the registration unit 4 provides for a limited time interval, to expand the limits of the high-frequency measurement in the scaling unit 11, a coarse distance measurement is made, the discrete intervals of which are measured using the registration unit 4 with a given accuracy.

), тем меньше граничная частота перестраиваемого фильтра ((

).The tunable filter 8 passes a pulse from the object to the input of the amplifier 3, supplying an electrical signal due to the scattering of radiation from the near zone of an imperfect transparent propagation medium. Moreover, the cutoff frequency of the tunable filter changes according to information about the amplitude and the steepness of the decay of the scattering signal: the slower the level of the scattering signal decreases (according to the law

), the lower the cutoff frequency of the tunable filter ((

)

Accordingly, there is less loss of pulse energy from the object due to the suppression of the low-frequency part of its spectrum. In the adaptation unit 9, a control voltage is generated from the unfiltered electric signal of scattering, which compensates for the slow unfiltered signal of scattering, due to which the operating point of amplifier 3 is not wrinkled and transmission from the object with the maximum coefficient is not ensured. A strobe pulse is fed to the gate input of amplifier 3 from the gate unit 7 and, if it coincides with the pulse to the object, the latter after amplification is fed to the signal input of the registration unit 4 and to the first input of the correction unit 10. So that the transient voltage during gating amplifier 3 and the voltage of its noise were not accumulated by the correction unit 10, a strobe pulse is also supplied to its second input. Thus, according to the amplitude of the pulse from the object, the correction unit 10 generates a regulatory voltage supplied to the regulatory input of the registration unit 4.

As a result of this moment of arrival of the pulse from the object, and hence the distance to the object, it is measured with high accuracy regardless of the amplitude of the pulse.

, обусловленную фронтом полезного импульса в 5-10 раз. Значение коэффициента к определяется точностью воспроизведения формы фронта импульса блоком коррекции 10.From a consideration of the stress diagrams illustrating the operation of the proposed device (see Fig. 2), it can be seen that the elimination of the influence of the scattering signal due to the tunable filter 8 and the adaptation unit 9 prevents false corrections 10, makes it possible to reduce the error of distance measurement

due to the front of the useful pulse by 5-10 times. The value of the coefficient k is determined by the accuracy of reproduction of the shape of the pulse front by the correction unit 10.

A specific implementation of the proposed device is as follows.

A tunable filter 8 (see FIG. 4) attenuates the extended exponentially scattered signal with a slight attenuation of the short pulse reflected from the object. In order to minimize the indicated pulse power loss from the object, the boundary part of the filter is rearranged in accordance with the spectral width of the scattering signal (see Fig. 3). With a significant amplitude of the scattering pulse, the boundary part of the tunable filter also increases, which makes it possible to improve the ratio of the object signal / scattering signal.

The registration unit consists of threshold formers, a time-amplitude converter, and an ADC. With the arrival of the "Start" clock from the emitter 6, the comparator generates a pulse of negative polarity. At the same time, the trigger is tipped over, the generator’s work permits, the counters at the parallel load input also reset. For each pulse of the generator, the capacitance C is discharged, then its discharge from the stable current generator begins (not shown in Fig.). As a result, an eighteen-digit binary code is formed on the digital output of the meter, the lower ten digits of which are the result of an accurate count, the higher 8 are the result of a rough count.

при грубом отсчете расстояние измеряется с точностью ΔR₁=100 м в пределах R₁=R_o - 25600 м, где V - скорость распространения излучения; R_o, - «мертвая зона».Thus, during the switching period of the scaling unit

with a rough reading, the distance is measured with an accuracy of ΔR ₁ = 100 m within the range of R ₁ = R _o - 25600 m, where V is the speed of radiation propagation; R _o - "dead zone".

в пределах R=R₁+100 м, что позволяет получить высокую точность измерения в широком диапазоне измерения расстояния до объекта.With the beginning of each switching period of the scaling unit, a registration unit is launched through the generator element, and the interval time meter measures the distance

within R = R ₁ +100 m, which allows to obtain high measurement accuracy in a wide range of measuring the distance to the object.

The emitter 6 is assembled on a transistor VT1 generates power pulses for the avalanche diode (S-dida) VD1. The avalanche diode VD1 generates current pulses with a duration of 10-100 not for pumping a semiconductor laser VD2. At the time of the breakdown of the avalanche diode, a synchronizing pulse is obtained with the help of a differentiating circuit, which starts the gate driver 7. The strobe pulse is fed to the second (control) input of the correction unit 10, allowing the useful signal to pass to the output of the peak detector, the output voltage of which is supplied to the control input of the recording unit 4. As a result of this, the threshold level of the comparator changes in proportion to the amplitude of the useful signal.

The photodetector 3 is assembled on the basis of the avalanche photodiode LFD-6 and a preliminary video amplifier on transistors.

The adaptation unit 9 is based on two emitter repeaters of type VT8. The adaptation voltage supplied to the control input of the amplifier 3 (emitter of the transistor) finally eliminates the scattering signal (see Fig. 2, e).

Serial OLP-M type lenses (for laser range finders) or a fiber optic splitter with optical connectors on the output arms can be used as the receiving 1 and transmitting 5 optical systems.

Correction block 7 consists of an emitter follower VT10, a peak detector VD2, C3 and a source follower VT1. The output voltage of the correction unit 10, proportional to the amplitude of the useful signal, is fed to the regulatory input of the registration unit 4.

Claims (1)

An optical reflectometer, comprising in series a receiving optical system, a photodetector, an amplifier and a recording unit, as well as an emitter with a transmitting optical system and a gating unit, characterized in that an adaptation unit, a correction unit and a scaling unit are connected to it, the output of which is connected to the third the input of the registration unit, the output of which is connected to the second input of the scaling unit, the input of the adaptation unit is connected to the output of the photodetector, the first input of the correction unit is connected to the output the gating unit, the second input with the amplifier output, the third input of which is connected to the output of the adaptation unit, the output of the correction unit is connected to the fourth input of the registration unit, and a tunable filter is also introduced, the input of which is connected to the output of the photodetector, and the output is from the second input of the amplifier and with the input of the adaptation block.

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