RU2119719C1 - Device for phase stabilization of high- frequency analog signal which is transmitted through fiber-optical communication line - Google Patents

Abstract

FIELD: communication. SUBSTANCE: device has reference signal oscillator and oscillators of signals of f2 and f3 frequencies which are connected to coupler. Output of coupler is connected to input of optical transmitter, which output is connected to input of light guide. Output of light guide is connected to input of line photodetector. Output of line photodetector sends signal, which is filtered by filter, to input of phase shifter. Signals of oscillators are sent to converters through coupler and low-pass filters. Outputs of converters are connected to phase detector, which output is connected to input of phase shifter through scaling amplifier. EFFECT: increased transmission distance, increased signal-to-noise ratio for constant initial wavelength of fiber-optical communication line. 1 dwg

Description

 The invention relates to optoelectronic technology, and more particularly to a device for transmitting a high-frequency analog signal with a stable phase via fiber optic link in phase synchronization systems of remote objects. Such systems are used, for example, in linear accelerators, gravity detectors of space navigation, radars with a phased array, phase interferometry [1, 2, 3].

A known device "Fiber optic communication line", including a reference signal generator, an optical transmitter, splitter, optical fibers, a photodetector, phase shifter. [1]
In the known device, it is necessary to use a special yet practically unrealized three-fiber optical cable, the parameters of which must satisfy the requirement: (Q ₂ - Q ₁ ) ² (D ₃ - D ₂ ) ² - (Q ₃ - Q ₂ ) ² (D ₂ - D ₁ ) ² > 2 (Q ₂ - Q ₁ ) (Q ₃ - Q ₂ ) (D ₂ - D ₁ ) (D ₃ - D ₂ , where Q ₁ , Q ₂ , Q ₃ , D ₁ , D ₂ , D ₃ -. appropriate temperature phase sensitivity dispersion D and Q of the respective first, second and third optical fibers disadvantages in said apparatus is the necessity of branching transmitter laser optical power into three equivalent guide which leads to a decrease in the energy potential (i.e., the maximum length of the fiber optic link), signal-to-noise ratio, an increase in phase noise, and a decrease in the accuracy of phase synchronization.In addition, in this device it is necessary to use three photodetectors that are identical in phase and amplification characteristics, which is a high technical challenge at high frequencies.

 A device is known "Delay compensator in fiber optic cable", including an optical transmitter, optical receivers, splitters, optical fibers [2].

 In the known device, the signal of a highly stable reference generator (hydrogen maser) with normalized phase noise and a high signal-to-noise ratio at the output is replaced by the signal of a voltage-controlled generator (VCO) with non-normalized phase noise and signal-to-noise value. Another disadvantage of the known device is the need for reverse transmission of the signal reflected from the end of the optical cable to the optical transmitter for subsequent phase comparison with the input signal. This solution dramatically reduces the energy potential, the signal-to-noise ratio, increases phase noise and reduces the accuracy of phase synchronization. In addition, it is necessary to use high-quality optical isolators and couplers in this device, the slightest deterioration of which can lead to strong excess laser noise for the optical transmitter due to the harmful effects of optical communication, which will lead to disruption of the entire device.

 The known device "Phase correction system", including an optical transmitter, optical isolators, fiber optic modulator, optical splitters, optical detector, phase detector, amplifier, optical fiber and optical receiver [3].

 In the known device, an optical signal that has passed through the entire length of the optical line is reflected by a mirror for subsequent phase comparison with the output signal. This solution dramatically reduces the energy potential, the signal-to-noise ratio, increases phase noise and reduces the accuracy of phase synchronization. In addition, in the known device it is necessary to use high-quality optical isolators and couplers, the slightest disruption of which will result in the appearance of strong excess laser noise from the optical transmitter due to the harmful effects of optical feedback, which leads to disruption of the operation of the entire device. Another drawback is the use of a fiber optic modulator as a phase shifter, which introduces significant additional losses into the optical signal, which further reduce the energy potential of the device. In the known device, due to the peculiarities of the operation of the fiber optic modulator, the limit of compensation for phase drift in the optical fiber (optical cable) is also limited to a value of the order of ± 45 degrees. phase, therefore, the length of the phase-stabilized optical line, the frequency of the reference signal transmitted through it, and (or) the range of compensated temperature fluctuations and mechanical loads of the fiber are limited.

 A device is known "Device for transmitting a reference phase-stable RF signal through a single-mode fiber-optic line", including a reference generator, a directional coupler, a controlled phase shifter, a frequency converter, three optical directional couplers, two filters, a frequency doubler, a frequency converter, an optical transmitter, a mixer , filter, phase detector, light guide (fiber optic line), two optical receivers. Frequency converter, mirror, local oscillator. This device employs a method for adjusting the phase of the reference signal modulating the optical transmitter using a phase shifter controlled by a signal obtained by comparing the phase of the signal at the inputs of the phase detector, which comes directly from the output of the reference generator, the phase of the signal transmitted through the phase shifter, and the phase of the signal transmitted through the optical transmitter, and the phase of the signal that passed through the optical line from beginning to end and back (phase-feedback system).

 In the known device, large power losses of the optical signal significantly reduce the energy potential of the FOCL, reducing the signal-to-noise ratio at its output and limiting the maximum length. These additional losses occur due to the need, when using this method of phase stabilization, in transmitting an optical signal from the input of the fiber to the output and vice versa and the presence of optical directional couplers. The specially created strong optical feedback from the mirror at the receiving end of the fiber determines the high directivity requirements for the optical couplers used in the prototype and the optical isolation for optical isolators, when their performance deteriorates, the noise on the photodetector call increases sharply and the entire phase stabilization system is disrupted .

 The problem of transmitting a high-frequency analog signal over a fiber optic cable with a stable phase for the purposes of phase synchronization has been known for a long time. The use of fiber optical fibers as a medium for the propagation of such signals contributes to the solution of the problem of their transmission over considerable distances, however, the problem of phase stability at the output when environmental conditions and, above all, temperature remains unresolved. Currently, there are two main directions for the development of phase-stable fiber optic links. The first is aimed at changing (decreasing) the phase sensitivity of the optical fibers themselves — the “fiber” direction. In the future, it can radically solve the problem of phase-stable transmission of a high-frequency signal over fiber optic links. However, such phase-stable optical cables are still very far from industrial production and it is still too early to talk about their estimated cost (presumably very high due to complex manufacturing techniques and expensive materials). The second direction is “hardware” based on the use of standard commercially available fibers and standard designs of connected optical cables (by the way, constantly improved in the direction of decreasing phase sensitivity as well) and hardware-implemented phase drift compensators at the output of such FOCLs. This approach makes it possible to reduce phase drift to an acceptable level at fairly low costs for an additional amount of equipment and, most importantly, to begin the development of industrial production almost immediately. However, the potential for the rapid development of the industrial production of such systems and their widespread adoption in various fields of science and technology is constrained by the lack of a technically successful implementation of such an approach.

 The only task this invention is directed to is to transmit from a precision source to consumers a high-frequency analog highly stable signal with minimal overcoming of various kinds of instabilities, drifts (primarily phase), noise and distortion of the spectrum with relatively low material costs.

 For this, a device is proposed whose operation is based on the fact that the phase deviation of the signal in the fiber with temperature and mechanical loads is directly proportional to its frequency (see, for example, relations (4), (5) in [3]).

 The essence of the invention lies in the simultaneous transmission along one fiber along with the reference two more signals of different frequencies, synchronized with it by phase drift. One of them, due to the choice of a sufficiently low frequency, has a obviously greater phase stability at the output of the fiber than is actually necessary for practical applications. The frequency of the other signal is chosen so that the departure of its phase in any case certainly does not exceed the limits of the working range of the phase detector.

A single technical result that can be obtained by carrying out the invention is simultaneously expressed in the following:
a) in a significantly greater energy potential, that is, in a greater transmission range of a high-frequency signal with a stable phase along the fiber optic link; b) in a higher signal-to-noise ratio (low intrinsic phase noise) at the output while maintaining the original fiber length; c) that the device is a direct translator of the signal of the reference generator without replacing it with other generators with unnormalized noise and spectral characteristics; d) the device does not use optical feedback in the fiber optic link for its functioning; e) only commercially available components and tested blocks and units, as well as circuitry solutions are used in the device for its operation; f) the device is simple and adaptable and has a relatively low cost in production and reliability in operation; h) has small dimensions and weight along with low power consumption.

где Δφ_Tf2 , Δφ_Tf3 , Δφ_σf2 , Δφ_σf3 - изменения набега фазы в ВОЛС при колебаниях температуры и механической нагрузки соответственно, Δφ_o - требуемая точность фазовой синхронизации.The specified single technical result in the implementation of the invention for the device is achieved in that in comparison with the known device [1], which is the closest analogue to the claimed, with common features: a reference signal generator connected to the input of the transmitter, a fiber connected to the photodetector, and controlled phase shifter, introduced generators with frequencies f ₂ and f ₃ , combiner, high-pass filter, splitter, first and second narrow-band filters, first and second converters, local oscillator, first and second fil There are three intermediate frequencies, a phase detector and a scaling amplifier, and generators with frequencies f ₂ and f _{3 are} synchronized with the reference signal generator, the outputs of all three generators are connected through the combiner to the input of the transmitter, the output of which is connected through the optical fiber to the input of the photodetector, the output of which is through the upper filter frequency is connected to the input of the controlled phase shifter, and through the splitter to the inputs of narrow-band filters, the outputs of which are connected to the corresponding inputs of the first and second converters, to each CB of which is connected a corresponding output of the local oscillator, the outputs of inverters connected input intermediate frequency filters, whose outputs are connected to inputs of the phase detector, the output of which is connected through a scaling amplifier to the control input of the phase shifter, the frequencies f ₂ and f ₃ generators are selected from the relations:

where Δφ _Tf2 , Δφ _Tf3 , Δφ _σf2 , Δφ _σf3 are the phase incursion changes in the fiber-optic link with temperature and mechanical load fluctuations, respectively, Δφ _o is the required accuracy of phase synchronization.

Thanks to the introduction of generators with frequencies f ₂ and f _3, it was possible to abandon a special, practically unrealized three-fiber optical cable with unique characteristics of each fiber, because in the proposed device, the phase difference (phase error) is compared between the signals of the generators with frequencies f ₂ and f ₃ passing simultaneously through the same fiber as the signal of the reference generator, and the error signal generated in this way, appropriately scaled, is fed to compensate for the phase drift ( drift) of the reference signal to the phase shifter located at the output of the photodetector. The application of the method of comparing the phases between two relatively low-frequency signals (compared to the frequency of the reference oscillator) of the generators, and not between the phases of the signal of the reference oscillator and one of them, allows not to go beyond the working area of the phase detector by selecting the appropriate frequencies most unambiguity of the error signal with virtually no limitation of the frequency of the reference signal.

 The absence of a three-way optical splitter at the output of the optical transmitter and a large number of optical connectors in the optical path reduces the energy loss of the optical signal — both of these factors significantly increase the energy potential, increase the signal-to-noise ratio at the output of the device, increase the stability and reliability of the entire device, and reduce its cost.

 The proposed device directly transmits the original signal of the reference generator without replacing it with a signal of another generator and without its non-linear conversion - this factor has a beneficial effect on the signal-to-noise ratio and the receipt of its predicted spectrally pure signal characteristics close to the calculated ones.

 The presence of a generator with respect to low-frequency signals synchronized by phase drift with a reference signal generator is not associated with additional costs, because they are the standard outputs of all generators - frequency standards that are set for reference signal generators [5].

In order to combine the signals in one channel for their subsequent simultaneous transmission through one fiber, which guarantees only a quantitative change in the phase difference between them when the entire path passes under the influence of the environment (temperature changes, mechanical stresses, etc.), i.e. unambiguous compensation of the phase of the reference signal, a signal combiner is used in the device. For accurate and continuous comparison of the phase difference of the generator signals with frequencies f ₂ and f ₃₂ at the output of the fiber-optic line, it is necessary to apply to the input of a phase detector that generates a phase error signal, which controls the phase shifter that compensates for the phase deviation of the reference signal at the receiving end, signals of one frequency - for this purpose, the device uses the first and second frequency converters. For precision amplification of the phase error signal, which is generated by the phase detector and must be of such a magnitude that, by controlling the phase shifter, provide an exact match between small changes in the phase difference of the relatively low-frequency signals of the second and third generators (i.e., the small output voltage of the phase detector) and large compensating phase changes of the controlled phase shifter, a scaling amplifier is used in the device.

 Information confirming the possibility of implementing the claimed device.

 The proposed device contains a reference signal generator 1, signal generators 2 and 3, signal combiner 4, optical transmitter 5, optical fiber 6, optical receiver 7, high-pass filter 8, splitter (1: 2) 9, narrow-band filters 10 and 11, phase shifter 12 , converter 13, local oscillator 4, converter 15, filters of the intermediate part 16 and 17, phase detector 18, scaling amplifier 19.

The device operates as follows: the reference RF signal of the generator 1 with a frequency f ₁ together with the signals of the generators 2 and 3, with the frequencies f ₂ and f _3, respectively, is fed to the signal combiner 4, and from it to the input of the linear optical transmitter 5 and modulates it . The optical signal modulated by them is fed to the input of the optical fiber 6 and received by the linear photodetector 7. From the output of the optical receiver, the reference RF signal filtered by the high-pass filter 8 is fed to the input of the phase shifter 12. The signals of the generators 2 and 3 through the splitter 9 and narrow-band filters 10 and 11 are fed to converters 13 and 15, respectively, to the heterodyne inputs of which a signal is received from a single local oscillator 14. From the outputs of the mixers, the signals at an intermediate frequency through the filters 16 and 17 are supplied for phase comparison to a phase detector 18, the output of which is a signal depending on the phase difference of the output signals at its inputs, through a scaling amplifier 19 is fed to the control input of the phase shifter 12, which compensates for the phase shift of the RF signal.

где φ - фаза сигнала опорного генератора, T - температура среды, f - частота сигнала опорного генератора, L - длина линии, c - скорость света, n - групповой показатель преломления, σ - аксиальные напряжения в оптическом волокне, E - модуль Юнга оптического волокна.In static mode, i.e. at a typical temperature of the medium surrounding the optical fiber and in the absence of mechanical loads on it, changes in the phase difference of the signals with frequencies f ₂ and f _{3 of the} generators are practically zero, therefore, the voltage at the output of the phase detector is also practically zero and the controlled phase shifter has only an initial phase shift of the reference signal. Thus, the reference signal of the generator at the FOCL output will have an initial phase shift φ _n.
A change in temperature or mechanical load on an optical fiber (optical fiber) causes a corresponding change in the phase incursion in it [3]:

where φ is the phase of the signal of the reference generator, T is the temperature of the medium, f is the frequency of the signal of the reference generator, L is the line length, c is the speed of light, n is the group refractive index, σ are the axial stresses in the optical fiber, E is the Young's modulus of the optical fiber .

The change in the phase incursion of the signal is directly proportional to the frequency of the signal transmitted along the optical line and its length. The frequency f _{2 is} calculated by formulas (1) and (2) so that the change in phase _incidence in a fiber of a given length at maximum temperature fluctuations Δφ _Tf2 and mechanical load Δφ _σf2 during its operation does not exceed 90 degrees of the phase (working range of the phase detector), those.

Частота f₃ рассчитывается по формулам (1), (2) так, чтобы изменение набега фазы в оптической линии заданной длины при максимальных колебаниях температуры Δφ_Tf3 и механической нагрузки Δφ_σf3, во время ее эксплуатации было заведомо меньше требуемой точности фазовой синхронизации Δφ_o, т.е.

The frequency f _{3 is} calculated according to formulas (1), (2) so that the change in phase _incidence in the optical line of a given length at maximum temperature fluctuations Δφ _Tf3 and mechanical load Δφ _σf3 , during its operation, is obviously less than the required phase synchronization accuracy Δφ _o , those.

Коэффициент усиления масштабирующего усилителя 19 рассчитывается по формуле
K_м = mf₁/f₂
где m - коэффициент масштабирования.

The gain of the scaling amplifier 19 is calculated by the formula
K _m = mf ₁ / f ₂
where m is the scaling factor.

When conditions (3), (4), and (5) are satisfied, the required compensation by the phase shifter 12 of the phase incursion changes at the frequency of the reference oscillator f ₁ is provided due to changes in temperature ΔT and mechanical stress Δσ in the optical fiber.

 Therefore, when the temperature and / or mechanical load changes, due to the different frequencies of the signals of the second and third generators, a phase difference between them arises at the output of the optical line. This difference is detected by a phase detector and a phase error signal appears at its output, which is amplified by an appropriately scaling amplifier and fed to a controllable phase shifter, thereby compensating for even greater phase deviation of the reference oscillator signal that passed through the optical line.

Bibliographic data
1. Auth. testimonial. USSR N 1690294 A1, cl. H 04 B 10/00, 1989

 2. L. P. Primas, "Cable delay compensator for microwave signal distribution over optical fibers", Microwave Journal, V. 33, N 12, 1990, pp. 81 - 92.

 3. M. Shadaram, J. Medrano, SA Pappert, MH Berry, DM Gookin "Technique for stabilizing the phase of the reference signals in analog fiber - optic lincks", Applied Optics, V. 34, N. 36, 1995, pp. . 8283-8288.

 4. E. Peschardt, J. P. H. Sladen "Transmission of a stabilized RF phase reference over a monomode fiber - optic link", Electronics Letters, V. 22, N. 16, 1986, pp. 868 - 869.

 5. Measurements in electronics. Handbook Ed. V.A. Kuznetsova, M., Energoatomizdat, 1987, p. 286.

Claims (1)

A device for stabilizing the phase of a high-frequency analog signal transmitted via a fiber-optic communication line containing a reference signal generator, a transmitter, a light guide, a photodetector and a controlled phase shifter, characterized in that generators with frequencies f ₂ and f ₃ , a combiner, a high-pass filter are introduced , splitter, first and second narrow-band filters, first and second converters, local oscillator, first and second intermediate frequency filters, phase detector and scaling amplifier, and generators with h The frequencies f ₂ and f _{3 are} synchronized with the reference signal generator, and the outputs of all three generators are combined by a combiner and connected to the input of the transmitter, the output of which through the optical fiber is connected to the input of the photodetector, the output of the photodetector through the high-pass filter is connected to the input of the controlled phase shifter, and through the splitter - to the inputs of narrow-band filters, the outputs of which are connected to the corresponding inputs of the first and second converters, each of which is connected to the corresponding output of the local oscillator, to the outputs of the conversion the inputs of the filters of the intermediate frequency are connected, the outputs of which are connected to the inputs of the phase detector, the output of which through the scaling amplifier is connected to the control input of the phase shifter, and the frequencies f ₂ and f _{3 of the} generators are selected from the relations

Where

changes in the phase incursion in the fiber under fluctuations in temperature and mechanical load, respectively;
Δφ ₀ - the required accuracy of the phase synchronization.