RU2543070C1 - Analogue phase-stable fibre-optic link - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: apparatus comprises a reference signal generator, signal generators, a multiplexer, an optical transmitter, an optical fibre, an optical receiver, radio-frequency dividers, a high-pass filter, a controlled phase shifter, filters, a frequency converter, a phase detector and a scaling amplifier. The apparatus includes a fourth signal generator, a radio-frequency divider, a trimming phase shifter and a controlled attenuator; a heterodyne and one of the filters and converters are excluded; the generators are synchronised with the reference signal generator.

EFFECT: high phase stability, accuracy and reliability of transmitting a high-frequency analogue signal through the fibre-optic link.

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Description

The invention relates to optoelectronic technology, and more particularly to devices for transmitting a high-frequency analog signal with a stable phase through fiber-optic communication lines (FOCL) in phase synchronization systems of remote objects. The technical result is to increase the phase stability of the high-frequency signal transmitted via the FOCL.

Such systems are used, for example, in linear accelerators, gravity detectors, space navigation, phased array radars, phase interferometry [1-3].

A device is known "Delay compensator in a fiber optic cable", including an optical transmitter, optical receivers, splitters, optical fibers [1]. 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 a voltage-controlled oscillator (VCO) signal 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 decreases 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 of the optical transmitter due to the harmful effects of optical feedback, which will lead to disruption of the entire device.

A 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 [2]. In the known device, an optical signal that has passed through the entire length of the optical line is reflected back by the mirror for subsequent phase comparison with the input signal. This solution dramatically reduces the energy potential, the signal-to-noise ratio, increases phase noise and decreases the accuracy of phase synchronization. In addition, in the known device, it is necessary to use high-quality optical insulators and couplers, the slightest disruption of which will lead to 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 entire device.

Another disadvantage 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.

The single task to which this invention is directed is to increase stability during transmission from a high-precision source to consumers of a high-frequency analog signal at low material cost.

The essence of the invention lies in the simultaneous transmission of one optical fiber (OB) together with the reference three 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 clearly greater phase stability at the output of the organic matter compared with the actual 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. The third is the local oscillator signal for the mixer of the photodetector; its frequency is selected so that the frequency difference between the second and third signals is equal to the frequency of the first.

A single scientific and technical result that can be obtained by implementing the invention is simultaneously expressed in the following:

a) increasing phase stability and accuracy when transmitting high-frequency analog signal over fiber optic;

b) improving the reliability of the device by eliminating false signals in the phase stabilization system at the receiving end of the fiber optic link;

c) simplifying the device, reducing the level of energy consumption at the remote receiving end of the fiber optic link;

d) reducing the level of radio emissions at the receiving end of the fiber optic link and improving electromagnetic compatibility (EMC).

The specified single technical result in the implementation of the invention is achieved by the fact that, compared with the known [3], which is the closest analogue to the claimed one, with common features: a radio frequency reference high frequency signal generator, radio frequency auxiliary signal generators with frequencies f ₂ and f ₃ connected through radiofrequency combiner with an input of an optical transmitter, OB connected to a photodetector, radiofrequency divider, high-pass filter, first and second narrow-band filters, first and second th converters, local oscillator, first and second intermediate frequency filters, phase detector, scaling amplifier and controllable phase shifter, introduced a third, synchronized with the reference, auxiliary radio frequency generator with a frequency of f ₄ , a radio frequency divider, third narrow-band filter, a tuning phase shifter and a controlled attenuator, and the local oscillator, the second converter and the second intermediate frequency filter are excluded, and the output of the third auxiliary radio frequency generator with a frequency of f ₄ is connected to I ode of the RF combiner, the third narrow-band filter is connected to one of the outputs of the RF divider, the output of which is connected to the input of the tuning phase shifter connected by its output to one of the inputs of the phase detector.

Thanks to the introduction of the third auxiliary radio frequency generator with a frequency of f ₄ (local oscillator), it was possible to abandon the local oscillator at the remote receiving end of the fiber optic link, which worked on two mixers.

In the previous device [3], with the instability of the local oscillator frequency, a differential phase difference appeared in the signals at the inputs of the phase detector, which after detection acted as a false signal for compensating for the phase drift in the FOCL, which, after amplification by a scaling amplifier, was applied to a controlled phase shifter.

In the proposed device, the local oscillator in the receiving part of the device is absent, while one of the main sources of phase instability is excluded.

Thanks to the adjustment of the phase shifter, the inevitable initial phase difference at the inputs of the phase detector is compensated due to the non-identical channels.

In this case, the initial control signal of the phase error can be optimized, for example, by setting it equal to zero.

This allows you to set the operating point on the control characteristic of the main controlled phase shifter in the optimal (for example, in a symmetrical) position, thereby increasing the accuracy of maintaining the phase of the reference signal and expand the range of phase control.

The introduction of a controlled attenuator between the tuning phase shifter and the phase detector ensures optimal conditions for the phase detector by equalizing not only the initial phases, but also the amplitudes of the signals at its inputs. Such alignment is necessary, since there is always no identity of the channels with respect to the transmission coefficient, including due to the inclusion of a frequency converter in one of the channels of the device, which, as a rule, has a low transmission coefficient.

For example, in phase meters, equalizing the differential amplitudes of signals at the inputs from several times to their approximate equality gives a five-fold increase in the accuracy of measuring the relative phase of the signal [4, 5]. Due to this, a significantly greater accuracy of phase detection is achieved. Thus, a signal is supplied to the input of the controlled phase shifter, which provides more accurate phase compensation of the phase change in the fiber optic link.

Thus, the overall phase stability of the analog FOCL is increased.

The achieved, in this regard, reduction in size, mass, cost, energy consumption and facilitating the solution of the EMC problem at the receiving end is especially important in multichannel systems for wiring a phase-stable signal along FOCLs, for example, for phased antenna arrays (PAR) and active phased antenna arrays ( AFAR) with the number of emitters up to a thousand or more.

The present invention is presented in figure 1, it contains: a reference signal generator 1, signal generators 2, 3 and 4, a radio frequency combiner 5, an optical transmitter 6, OV 7, an optical receiver 8, a radio frequency divider (1: 2) 9, radio frequency divider (1: 3) 10, high-pass filter 11, controlled phase shifter 12, narrow-band filters 13, 14, converter 16, intermediate frequency filters 15 and 17, tuning phase shifter 18, controlled attenuator 19, phase detector 20, scaling amplifier 21.

The proposed analog phase-stable fiber optic link operates as follows: the reference high-frequency (HF) signal of generator 1 with a frequency f ₁ together with the signals of generators 2, 3 and 4, with frequencies f ₂ , f ₃ and f _4, respectively, is fed to signal combiner 5, and him to the input of the linear optical transmitter 6 and modulates it.

The optical signal modulated by them is fed to the input of a single-mode OB 7 and received by a linear photodetector 8. From the output of the optical receiver, through the radio frequency divider 9, filtered by a high-pass filter 11, the reference RF signal is fed to the input of the phase shifter 12.

The signals of the generators 2, 3 and 4 through the radio frequency dividers 9, 10 and narrow-band filters 13, 14 are fed to the converter 16, and a signal with a frequency f ₂ through a filter 13 is fed to the input of the converter 16, and a signal with a frequency f ₄ through a filter 14 is fed to LO input of inverter 16, and the signal with frequency f _3, which coincides with the intermediate, through filter 15, phase shifter 18 and trimmer controllable attenuator 19 is supplied to one input of a phase detector 20, the other input of which is supplied through the filter 17 with the intermediate frequency signal O and inverter 16, the output of the phase detector signal dependent on the difference between the input signal phase to its input through a scaling amplifier 21 is supplied to the control input of the phase shifter 12 which compensates the phase of the reference care RF or microwave signal.

The device operates as follows: in static mode, i.e. at normal (calibration) temperature of the environment surrounding a single-mode OM, and in the absence of mechanical loads on it, the phase difference of the signals with a frequency of f ₂ and f _{3 is} practically zero. Therefore, the voltage at the output of the phase detector will also be almost zero, and the controlled phase shifter will have only the 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 _o .

The frequencies of the second, third and fourth generators f ₂ , f ₃ and f ₄ are selected from the relations:

max {Δφ _Tf2 , Δφ _σf2 } <90 deg. phase; (one)

max {Δφ _Tf3 , Δφ _σf3 } << Δφ ₀ deg phase; (2)

max {Δφ _Tf4 , Δφ _σf4 } << Δφ ₀ deg phase; (3)

where Δφ _Tf2 , Δφ _Tf3 , Δφ _σf2 , Δφ _σf3 , Δφ _Tf4 , Δφ _σf4 are the phase incursion changes in the fiber-optic communication line with temperature and mechanical loads at frequencies f ₂ , f ₃ and f _4, respectively, and f ₄ = f ₂ -f ₃ , Δφ ₀ is the required accuracy of phase synchronization.

The gain of the scaling amplifier is:

$$K_m=m{f}_{\mathrm{one}}/f_2,\text{(four)}$$

where m is the scaling factor.

The initial phase shift Δφ _1,2 and the signal level difference ΔA _1,2 at the inputs of the phase detector (20) should be equal to zero:

$$\Delta {\varphi }_{\mathrm{one},2}\text{and}\Delta {{\rm A}}_{\text{1,2}}=0\text{(5)}$$

When conditions (1) - (5) are fulfilled, the increased accuracy required to compensate the phase shifter 12 for changes in the phase incursion at the frequency of the reference oscillator f ₁ due to changes in temperature ΔT and mechanical stress Δσ in the optical fiber, and increased phase stability of the fiber optic link as a whole.

If we choose the frequencies of the generators f ₂ and f ₄ close enough, i.e. low intermediate frequency f ₃ , then the phase difference will be minimal and, therefore, the transmission line will maintain high accuracy in a wide range of temperatures and mechanical loads.

The presence of generators with respect to low-frequency signals having the same direction of phase drift with the reference generator is not associated with significant additional hardware costs, because three of the four signal frequencies used in the device are standard at the outputs of all generators - frequency standards that are usually the reference signal generators [4].

Thus, the proposed transmission system can transmit a phase-stable RF or microwave signal with high accuracy over significant distances without using optical feedback and a local oscillator in the receiving part, which significantly increases its quality and reliability, as well as reduces its radio emission and cost.

Information sources

1. Primas L.E. Cable delay compensator for microwave signal distribution over optical fibers // Microwave Journ. - 1990. - V.33, No. 12 - P.81-92.

2. Smith P., Phase stabilization of reference signals in analog fiber - optic links // Electron. Letters. - 1997. - V.33, No. 13. - P.1164-1165.

3. Device for stabilizing the phase of a high-frequency analog signal transmitted via FOCL: Russian Patent. 2119719 / D.F. Zaitsev - No. 97119141; Claim 11/27/1997; Publ. 09/27/98 // Descriptions of Russian inventions. - 1998 - Part 2.

4. Measurements in electronics. Handbook Ed. V.A. Kuznetsova - M .: Energoatomizdat, 1987 .-- 286 p.

5. The meter of the phase difference and the ratio of the levels of FC2-33. Technical description and instruction manual. 1.405.007 TO.

Claims (1)

An analog phase-stable fiber optic link containing a signal generator with a frequency of f ₁ , signal generators with a frequency of f ₂ , f ₃ , a radio frequency combiner, an optical transmitter, an optical fiber, an optical receiver, a 1: 2 radio frequency divider, a high-pass filter, a controlled phase shifter, filters , a frequency converter, a phase detector and a scaling amplifier, characterized in that a generator with a frequency of f ₄ , a radio frequency divider 1: 3, a tuning phase shifter and a controlled attenuator are introduced, and the generators with h The frequencies f ₂ , f ₃ and f _{4 are} synchronized with the reference signal generator, the outputs of all four generators are combined by an RF combiner and connected to an optical transmitter whose output is connected to an optical fiber whose output is connected to the input of the optical receiver, the output of which is connected to the input of the divider 1: 2, the first output of which is connected to the input of the high-pass filter, the output of which is connected to the input of the controlled phase shifter, and the second output of the first divider 1: 2 is connected to the input of the divider 1: 3, outputs which is connected to the inputs of three narrow-band filters, and the outputs of the first and second narrow-band filters are connected to the input and the heterodyne input of the converter, respectively, and the output of the third narrow-band filter is connected to the input of the tuning phase shifter, the output of the converter is connected to the input of the intermediate frequency filter, the output of which is connected to the first input phase detector, and the output of the tuning phase shifter is connected to the input of the controlled attenuator, the output of which is connected to the second input of the phase d detector, whose output is connected to the input of the scaling amplifier, whose output is connected to the control input of the phase shifter, the output of which is an output device, wherein the frequency of the second, third and fourth generators f _2, f ₃ and f ₄ are selected from the relations:
max {Δφ _Tf2 , Δφ _σf2 } <90 deg. phase; (one)
max {Δφ _Tf3 , Δφ _σf3 } << Δφ ₀ deg phase; (2)
max {Δφ _Tf4 , Δφ _σf4 } << Δφ ₀ deg phase; (3)
where Δφ _Tf2 , Δφ _Tf3 , Δφ _σf2 , Δφ _σf3 , Δφ _Tf4 , Δφ _σf4 are the phase incursion changes in the fiber-optic communication line with temperature and mechanical loads at frequencies f ₂ , f ₃ and f _4, respectively, and f ₄ = f ₂ -f ₃ , Δφ ₀ - the required accuracy of phase synchronization, and the gain of the scaling amplifier is equal to:
$$K_m=m{f}_{\mathrm{one}}/f_2,\text{(four)}$$

where m is the scaling factor,
and the initial phase shift and the difference in signal levels at the inputs of the phase detector should be equal to zero:
$$\Delta {\varphi }_{\mathrm{one},2}=0;\text{and}\Delta {\text{A}}_{\text{1,2}}=0.\text{(5)}$$