RU2434247C1 - Method of generating interference signal in doppler lidars - Google Patents

Abstract

FIELD: physics. ^ SUBSTANCE: advantage of the disclosed method becomes apparent when using pulsed lasers with pulse duration shorter than one nanosecond. The method involves generation of a continuous reference signal by using backward scattered light which is picked up by a directional coupler, with propagation of an input light pulse in an annular fibre-optic delay line with a repeating generation cycle and regeneration of the sequence of such input light pulses. ^ EFFECT: high accuracy and noise-immunity of measuring Doppler signals, broader functionalities; method does not have any limitations when using lasers with short atmospheric probing pulses; simple and cheap optical and electronic parts of the device. ^ 2 dwg

Description

The invention relates to the field of construction of the optical part - Doppler lidars, designed to measure wind speed and detect turbulent processes in the atmosphere, and in particular to the question of the formation of the reference signal necessary to obtain an interference signal of Doppler frequency. The process of measuring wind speed in lidars consists of sensing the atmosphere with a powerful short pulse of light, receiving scattered and reflected light, generating a reference signal, summing the received signal with the reference signal, and then detecting the resulting interference signal.

There are a large number of methods for generating an interference signal in which attempts are made to find effective solutions to a number of the most complex problems. These include obtaining an interference signal with a short probe pulse and obtaining estimates of the frequency of the Doppler signal at the required points of spatial coordinates.

As an example, patents US 2009073417 A1; W02007009759 A1; US 2008/0024756 A1, G01S 17/00. The first patent describes a laser pumped circuit through which scattered light is received at the same time. The second one proposes an optical scheme in which the light intensity distribution of the speckle structure of the image is measured at different instants of time of the received light, and the last patent offers optical schemes for obtaining quadrature components of Doppler signals. At the same time, the question of how to form a reference signal is not addressed so that a continuous interference signal can be obtained during the entire propagation time of the probe light pulse.

The signal received by the lidar is formed mainly due to the reflection of the probe beam from aerosol particles scattering light in the atmosphere and is a light pulse modulated in frequency and amplitude with a light scattering medium. As a rule, this is a noise-like signal having a Doppler frequency shift, since it is obtained from a plurality of light-scattering particles distributed randomly. In addition, the process of estimating the Doppler frequency in such a signal is substantially complicated due to the short duration of both the received signal and the required spatial resolution time. Since the duration of the probing pulse of compact and reliable laser sources is about 1 nanosecond or less, the noise frequency band of the signal processing unit after the photodetector will be determined by the duration of the probing pulse. In addition, measurements over such short periods of time are technically difficult to implement and require expensive equipment.

A known method of generating an interference signal (RF patent No. 2338223, issued November 10, 2008, G01S 17/88, and an article by the same authors: GG Matvienko, SN Polyakov, and VK Oshlakov; Basic Principles of Pulse-Wind Doppler Lidaras with Multitime Coherencse ISSN 1054-0660X, Laser Physics, 2008, Vol. 18, No. 11, pp. 1246-1250), in which the scattered light from the probe pulse interferes with the reference signal obtained by repeatedly delaying part of the probe pulse in separate segments of the fiber optic lines and the subsequent summation of the delayed pulses by substituting them one after another. Such an operation is called by the authors spatio-temporal multiplication. However, in real devices it is impossible to ensure the accuracy and stability of delays in different segments of the fiber delay lines. Therefore, it is impossible to match the phases of the end of the previous and the beginning of the subsequent delayed pulses to ensure the phase continuity of the resulting multiplied reference signal, which leads to the appearance of phase jumps. As a result, the phase difference of the scattered light signals from the probing pulse and the reference signal is completely transferred to the output signal of the photodetector and the measured Doppler signal will have phase jumps within 0-360 degrees, distributed randomly. Under these conditions, the Doppler frequency can only be measured if at least two samples of the phase incursion of the measured signal can be generated during the duration of the initial probe pulse. With a pulse duration of 1 ns, such measurements become impossible, since the actual phase advance of the Doppler signal is less than 1 degree already for Doppler frequencies of the order of 3 MHz.

Closest to the claimed solution is a prototype method for generating an interference signal in Doppler lidars, published by Adrian A. Dorrington, Rainer Kunnemeyer, and Paul M. Danehy "Refrence-beam storage for long-range low-coherence pulsed Doppler lidar ", Appl. Opt. vol. 40, No. 18, 2001; Juli-Lai Shen and Rainer Kunnemaer, "Amplified refrence pulse storage for low coherence pulsed Doppler lidar", Appl. Opt. vol. 45, No. 32, 2006. The essence of this known method for producing an interference signal is that a pulse obtained using an optical coupler from an atmosphere-sensing pulse is used for the reference signal. This branched pulse is repeated many times in an annular fiber optic delay line with a period equal to the delay time in it. The delayed pulses form a time sequence in the form of a series, which are extracted using a coupler and then added to the received optical signal reflected from the atmosphere. The resulting interference signal is detected.

The disadvantage of this method is the discontinuity of the reference signal, which makes it impossible to continuously measure the phase of the Doppler signal.

In the method proposed by the authors for generating an interference signal in Doppler lidars, the problem of obtaining a continuous reference optical signal is solved. This is achieved due to the fact that the annular fiber optic delay line is divided into two parts using an additionally inserted optical isolator that transmits light in only one direction, light propagating in the opposite direction from the first part of the annular fiber optic delay line using a directional fiber optic coupler, and use it as a reference signal.

This allows the formation of a continuous reference signal in the ring fiber delay line, designed for multiple scrolling of the initial part of the probe pulse, in contrast to the known method, where short signals transmitted through the ring delay line in the forward direction, the duration of which corresponds to the probe length, are used as the reference signal momentum. In the present invention, a continuous reference signal is generated, propagating in the opposite direction from the first part of the annular fiber optic delay line. Moreover, the duration of such a reference signal is equal to twice the delay value of the first part of the annular fiber optic line to the point where the optical isolator is turned on. As a result of adding the reference and received light scattered from aerosol particles in the atmosphere, a continuous interference signal is obtained. The resulting interference signal allows you to create a lidar, in which it becomes possible not only to continuously measure the phase of the Doppler frequency signal, but also it becomes possible to select any value of spatial resolution, which is determined only by the time of estimation of the Doppler frequency. On the other hand, by increasing the duration of the obtained optical reference signal, it is possible to reduce the bandwidth of the electronic circuits of the electric signal processing unit and thereby improve the signal to noise ratio. This extends the functionality of the equipment and reduces the measurement errors of the Doppler frequency. This is the novelty of the proposed method. In: Adrian A. Dorrington, Rainer Kunnemeyer, and Paul M. Danehy, "Refrence-beam storage for long-range low-coherence pulsed Doppler lidar", Appl. Opt. vol. 40, No. 18, 2001; Juli-Lai Shen and Rainer Kunnemaer, "Amplified refrence pulse storage for low coherence pulsed Doppler lidar," Appl. Opt. vol. 45, No. 32, 2006, a description is given of a device implementing the known method for generating an interference signal in Doppler lidars. The disadvantage of this device is the discreteness of measuring the phase of the signal, since measurements can be carried out only at the moments of the existence of the pulse of the reference signal and the shorter these pulses are, the more difficult it is to carry out the measurements. This follows from the disadvantages noted in the known method.

The following is a description of the structural diagram of the device based on the proposed method. In this device for generating an interference signal in Doppler lidars made in accordance with the proposed method, this drawback noted above is eliminated. This is achieved due to the fact that a fiber optic insulator is additionally included in the ring fiber-optic delay line, transmitting light in only one direction and dividing the ring line into two parts, a directional fiber coupler connected to the input of the primary line with the remaining end of the secondary line of the second fiber coupler, and the output from the input of the annular fiber optic delay line, and the output of the secondary line by the working arm is connected to the remaining input of the optical adder. The output of the directional fiber optic coupler becomes the output of the reference signal of the ring fiber optic delay line.

The proposed method for generating an interference signal in lidars has significant advantages compared with known analogues in terms of accuracy and noise immunity. It allows even with a pulse duration of 1 ns to take measurements of the phase incursion of the frequency of the Doppler signal, since from any short pulse it is possible to generate a reference signal of any desired duration. This method allows the use of high-power lasers with any short duration of the probe pulse. This circumstance simplifies and reduces the cost of both the optical and electronic parts of the device, and makes it possible to significantly increase the signal-to-noise ratio in electronic signal processing units.

The invention is illustrated by drawings.

Figure 1 presents a structural diagram of a device for generating an interference signal. Figure 2 shows the temporal sequence of the envelopes of the main signals.

The structural diagram (Fig. 1) contains a laser 1 that generates a light pulse, a first fiber optic coupler 2, which branches out part of the energy of the laser pulse I ₀ (t), a transmit-receive unit 3, which generates and emits a sensing pulse I _o (t) and carries out reception of reflected scattered light I _in (t) from the atmosphere. The annular fiber optic delay line 4 comprises a series-connected optical amplification unit 5, a second fiber optic coupler 6, a directional fiber optic coupler 7, which forms a reference signal I _r (t) at the output of its secondary shoulder, and an optical isolator 8 separating the annular optical fiber line the length into two parts L1 and L2. The optical adder 9, which generates an interference signal from the sum of the scattered light and the reference signal, is connected by its output to the input of the detection unit 10, which converts the interference signal into electrical U (t). In this case, the branching part of the laser pulse energy is input into the annular fiber optic delay line through the secondary line of the second fiber optic coupler.

Figure 2 shows the following signals: I ₀ (t) - at the output of a pulsed laser, I _sum (t) - at the output of a second fiber optic coupler, I _r (t) - at the output of a directional fiber optic coupler, I _s (t) - damped a signal from atmospheric aerosol particles at the output of the optical transmission receiving unit.

The device operates as follows. The laser light pulse is divided into two parts in the first fiber optic coupler. The main part of the energy is supplied to the transmit-receive unit, where a probe pulse is formed, the other part is sent to the ring fiber optic delay line through the second fiber optic coupler. Next, the branched light pulse passes the directional fiber optic coupler along the primary line and enters the fiber optic line, which is divided into two parts using fiber optic insulator 8 (on L1 - before the insulator and L2 - after the insulator), which transmits light in only one direction. In this case, two processes occur in the fiber optic line: the process of multiple regeneration of part of the laser pulse I ₀ (t) and its reproduction in the form of a temporary pulse sequence of signals I _sum (t), propagating in the forward direction, with a period equal to the delay time of the fiber optic ring ring, and the process of releasing light propagating in the opposite direction. As you know, when a pulse propagates in a fiber line, light is partially absorbed, and also reflected and scattered in the opposite direction. In the considered circuit, the indicated processes will be observed both in the first part of the L1 line and in the second L2, but the fiber optic insulator will not let back light from the second part of the fiber optic line. Therefore, only light from the first part will return, which will be separated from the direct wave in the directional fiber optic coupler and fed to the output of the working arm of the secondary line in the form of a reference signal I _r (t). From this it follows that the requirements for the first and second parts of the annular fiber optic delay line will be different. The first part must be selected with high losses, so that the intensity of the return light flux is sufficient to obtain the required level of the reference signal, and the second part is lossless. As you know, fiber optic transmission lines are performed with minimal losses for specific wavelengths. Therefore, for this case, it is necessary to select a fiber optic line with the required loss factor from the list of known types of transmission lines produced by the industry, or to create such a line. In order to avoid the re-arrangement of pulses of backscattered light from each subsequent pulse I _sum (t), the fiber-optic delay line using optical isolator 8 is divided into two parts L1 and L2, so that the condition τ1≤τ2 + ΔT is satisfied, where τ1 is the delay by the length L1, τ2 is the delay on the length L2, ΔT is the duration of the input pulse. To compensate for the loss of light pulse, an optical amplification unit 5 is installed at the end of the ring line. The gain factor should not exceed the loss factor from the condition of stability of the operation of the ring fiber optic delay line.

The technical result from the use of the proposed method is to increase the accuracy of measurements, expand the functionality of the equipment, the reliability and stability of its operation due to the formation of a continuous reference signal from a laser light source with any pulse duration. The proposed method uses a continuous signal of backscattered light from the first part of the annular delay line. In this case, a continuous interference signal is obtained, which makes it possible to create a lidar in which it becomes possible to continuously measure the phase of the Doppler frequency signal.

The feasibility of the proposed method is confirmed by the widely used quality meters of fiber-optic lines — reflectometers, in which the light scattered in the opposite direction, generated by the input light pulse, propagating in the forward direction, is measured.

The difference of the proposed method from existing methods is that instead of a short pulse reference signal, the duration of which is equal to the duration of the probe pulse, a continuous signal is used, generated by the short pulse itself when it propagates in the fiber optic line in the form of reflected and scattered light.

A lidar that implements the proposed method can be manufactured using currently existing components and parts, such as directional couplers, amplifiers, isolators, etc., commercially available from well-known companies (for example, Telecommunications Inc. or Thorlabs), in a compact form .

Claims (1)

The method of generating an interference signal in the Doppler circuits of the lidar, namely, that a part of the main probe pulse is branched and a time series of pulses is formed from it using a ring fiber optic delay line equipped with an optical amplification unit, delayed pulses are extracted and used as a reference signal, which then summed with scattered light from the probe pulse and detect, characterized in that the annular fiber optic delay line is divided into two parts using olnitelno inputted optical isolator that transmits light only in one direction emit light propagating in the opposite direction from the first portion of the annular optical fiber delay line optical fiber via the directional coupler and use it as a reference signal.

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