RU2756383C1 - Receiving channel of the laser rangefinder - Google Patents

Abstract

FIELD: laser technology.

SUBSTANCE: invention relates to the field of laser technology and concerns the receiving channel of a laser rangefinder. The receiving channel contains a receiving lens and two photosensitive elements with amplifiers, at the outputs of which temporary signal fixation schemes are introduced. The photosensitive elements are located at the minimum possible distance from each other, and two inclined plane-parallel optical plates are inserted in front of them. The plate closest to the photosensitive elements is located perpendicular to the plane containing the axes of the photosensitive elements and is inclined to them by an angle θ. On its surface, opposite to the photosensitive elements, a dichroic coating is applied, reflecting the received radiation with a working wavelength. The second plate, installed closer to the receiving lens, is tilted at an angle of minus θ.

EFFECT: ensuring high accuracy of temporary fixation of the received signal in a wide dynamic range with minimal measurement time and without increasing the dimensions of the equipment.

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Description

The invention relates to techniques for receiving pulsed optical radiation, mainly to receivers of pulsed laser range finders and other light-locating devices.

Known receivers of pulsed optical radiation [1] for pulsed laser ranging systems, designed to convert into electrical signals reflected by distant objects of the probe pulses of laser radiation and the timing of electrical pulses to determine their delay t ₃ relative to the instant of radiation of the laser probe pulse. By this delay judged R range to the reflecting object according to the formula with R = t _3/2, where c - velocity of light. Pulsed radiation detectors [2, 3], containing a photosensitive element and a signal processing circuit, are constructed in a similar way. These devices have insufficient dynamic range, which limits the accuracy of the time fixation of the received signals and, thereby, prevents the use of such receivers in range meters and other equipment with increased accuracy requirements. Known photodetector device [4], in which this drawback is eliminated by introducing a controlled electro-optical attenuator in front of the sensitive area of the photodetector, however, this solution leads to a significant complication of the device and deterioration of the signal-to-noise ratio.

The closest in technical essence to the proposed invention is the receiving channel of the laser rangefinder, which includes a photodetector with a receiving lens, moreover, the photodetector contains a photosensitive element and an amplifier [5]. To expand the dynamic range of signals in the receiving channel [5], a controllable semitransparent shutter is introduced, the position of which depends on the level of the received signal. The disadvantage of this technical solution is the need for repeated measurements with a corresponding consumption of the device's resource, and the loss of time for removing the shutter and the second measurement.

The objective of the invention is to ensure high accuracy of the time fixation of the received signal in an extremely wide dynamic range with a minimum measurement time and without increasing the size of the equipment.

где d_p - допустимый диаметр кружка рассеяния в плоскости фоточувствительного элемента фотоприемного устройства с меньшей чувствительностью; F - фокусное расстояние приемного объектива; D₀ - световой диаметр приемного объектива.This problem is solved due to the fact that in the known receiving channel of the laser rangefinder, which includes a photodetector with a receiving lens, moreover, the photodetector contains a photosensitive element and an amplifier, a second photosensitive element with a second amplifier is introduced, at the outputs of the amplifiers, time latching circuits are introduced, representing a sequential connection of the differentiating link and the zero comparator, their outputs are connected to the input of the output signal driver through a switch controlled from the output of the threshold device introduced at the output of the differentiating link belonging to a photodetector with a higher sensitivity, while the photosensitive elements are located at the shortest possible distance b one from the other, and two inclined plane-parallel optical plates are introduced in front of them, the plate closest to the photosensitive elements is located perpendicular to the plane containing the axes of the photosensitive elements and is tilted to them at an angle θ, a dichroic coating is applied on its surface opposite to the photosensitive elements, reflecting the received radiation with a working wavelength, the second plate, installed closer to the receiving lens, is tilted by an angle of minus θ, while the thickness of each plate is d should be minimal under the condition d≥nb⋅cosθ, where n is the refractive index of the plate material, b is the distance between the photosensitive elements in the plane perpendicular to their optical axes, and the difference in the positions of the photosensitive elements Δh * along the axis of the receiving lens satisfies the relation

where d _p is the permissible diameter of the scattering circle in the plane of the photosensitive element of the photodetector with a lower sensitivity; F is the focal length of the receiving lens; D ₀ - light diameter of the receiving lens.

The thickness d _{1 of the} first plate closest to the receiving objective can be twice as large as the thickness d ₂ = d of the second plate.

The first and second plates can be mutually perpendicular.

In the drawing, FIG. 1 shows a functional diagram of the receiving channel of the laser rangefinder. FIG. 2 a) shows the optical scheme of the receiving channel. FIG. 2 b) shows the real path of the rays in the second plate, and Fig. 2 c) shows the equivalent path of the rays in the second plate on its scan. d _r = d / n is the thickness of the reduced plate [7]. FIG. 3 illustrates the waveform of signals U (t) at the output of the amplifiers (FIG. 3a) and U '(t) at the output of the first differentiating unit (FIG. 3b).

The receiving channel of the laser rangefinder (Fig. 1) contains a first photosensitive element 1 with a first amplifier 2, a second photosensitive element 3 with a second amplifier 4, differentiating links 5 and 6, zero comparators 7 and 8, a switch 9, to the inputs of which the outputs are zero -comparators 7 and 8. The switch output is connected to the input of the output signal driver 10. At the output of the first differentiating link 5, a threshold device 11 is turned on, the output of which is connected to the control input of the switch 9.

Photosensitive elements 1 and 2 are structurally placed in the focus of the receiving lens 12 (Fig. 2a). Two optical plates 13 and 14 symmetrically inclined at an angle 9 are introduced between the receiving lens and the photosensitive elements. A dichroic coating 15 is applied to the rear face of the plate 14, transparent to visible radiation and reflecting radiation with a working wavelength towards the photosensitive elements.

The device works as follows.

In the initial state, the switch 9 is open for signals from the output of the zero comparator 7. If the signals at the output of the amplifier 2 and the differentiating link 5 are within the linear range, then the output signal driver fixes the position of their maximum at the same time instant t _0, regardless of amplitude (Fig. 3b). Due to the inertia of the differentiating link, the moment t _{0 is} slightly delayed relative to the moment t _{max of the} maximum of the signal U ₁ (t). This delay does not depend on the amplitude of the signals U ₁ (t) and U ₂ (t) in their entire linear range. The response of the differentiating link to the signals of the maximum amplitude in the linear range exceeds the level U _{pores of} the threshold device 8, thereby causing the supply of a switching signal to the control input of the switch 5 in the time interval from t _org <t ₀ to t _pore <t ₀ , where t _org - the moment the threshold device is triggered from the reaction of the differentiating link 5 to the limited signal U '_1org; t _{p is the} moment of triggering the threshold device from the reaction of the differentiating link 5 to the signal U _{1max of the} maximum amplitude within the linear range. In this case, the switch 9 closes for the signal from the photosensitive element 1 and opens for the signal from the photosensitive element 3. Due to the fact that the maximum response of the differentiating link U ' _1max (Fig.3b) is ahead of the maximum of the pulse U _1org in time, the latter is blocked, and the output the switch passes a pulse U ' ₂ (t) from the photosensitive element 3, which has a significantly lower amplitude lying in the linear range, due to which the temporal position of the input signal is still fixed at time t ₀ in the practically unlimited amplitude range of the input signals.

According to the proposed invention, the input radiation is divided by the second optical plate 14 into two beams of different intensities. One beam reflected from the rear surface of the plate enters the main photosensitive element 1. A weaker beam is reflected from the front surface of the plate 14 and focuses on the less sensitive photosensitive element 3 (Fig. 2a). To correct the aberrations introduced by the plate 14, a similar plate 13 is introduced, inclined at the opposite angle minus θ. In this case, however, the plate 14, which reflects the radiation from the back surface, acts as a plate of double thickness, and the system of two plates compensates for aberrations in the straight branch (towards the intended eyepiece) but introduces distortions in the plane of the photosensitive elements - as one equivalent plate.

In this case [7], the focus shift along the axis (Fig. 2c)

where d is the thickness of the plate;

n is the refractive index of the plate material;

θ is the angle of incidence of the beam on the plate;

θ '- angle of refraction according to Snell's law

формула (1) с учетом (2) записывается в видеSince [8]

formula (1) taking into account (2) is written in the form

For small values of 0, formula (4) takes the form

Example 1

θ = 0; n = 1.5; d = 2 mm.

Example 2

θ = 45 °; n = 1.5; d = 2 mm.

The discrepancy between the Δh focal planes is compensated by placing the photosensitive elements at different heights corresponding to the Δh value (Fig. 2a). Consideration should be given to the permissible defocusing ΔF of one or both photosensitive elements within the depth of field of the lens. In this case, the required constructive difference in the heights of the photosensitive elements is

Δh * = Δh-ΔF,

where ΔF = d _p F / D ₀ ,

d _p - permissible diameter of the scattering circle,

F is the focal length of the lens,

D ₀ - light diameter of the objective.

Thus, the permissible height difference Δh * of the photosensitive elements should be within

Example 3

d _p = 0.3 mm; F = 150 mm; D ₀ = 45 mm; Δh = 0.93 mm (Example 2).

ΔF = 0.3⋅150 / 45 = 1 mm.

Δh * = 0.93-1 <0, therefore, with the parameters of example 3, the photosensitive elements can be installed at the same level.

Identical optical plates, mutually inclined in opposite directions, eliminate astigmatism and coma introduced by each of the plates [9]. It should be borne in mind that when the received radiation is reflected from the back surface of the plate, its thickness is equivalently doubled. If the amount of aberrations in the forward channel is not critical, then to eliminate aberrations in the receiving channel, the thickness of the plate 13 can be doubled, due to which the aberrations of the second plate 14 are corrected.

The magnitude of the astigmatism of the inclined plate Δh _A [8]

Example 4

Under the conditions of example 2 (θ = 45 ° = π / 4)

Δh _A ~ - d (n ² -1) θ ² / n ³ - - 2 (2.25 - 1) 9.86 / (16 - 3.38) = -0.46 mm.

Third-order meridional coma Δh _C [8]

Example 5

Under the conditions of examples 3, 4

Δh _C ~ 3d (n ² -1) (D ₀ / F) ² θ / 2n ³ ~ 6⋅1.25 (45/150) ² 0.785 / (2⋅3.38) = 0.08 mm.

As the above examples show, depending on the parameters of the optical system, one can choose such a combination of them, in which the negative effect of the optical plates may be insignificant, primarily for a highly sensitive photodetector.

The described technical solution provides an almost unlimited expansion of the linear dynamic range in the entire working dynamic range of the first and second photosensitive elements. At the same time, the maximum achievable accuracy of the temporal fixation of the signal with single measurements is ensured, that is, without deterioration in performance. The equipment has minimal dimensions and is housed in the same case as the previous model.

In accordance with the proposed invention, a prototype receiver was developed. The studies carried out confirmed the fulfillment of the specified technical requirements - both in single and in frequency operation.

Thus, the proposed technical solution provides high accuracy of time fixation of the received signal in an extremely wide dynamic range with a minimum measurement time and without increasing the size of the equipment.

Sources of information

1.V.A. Volokhatyuk et al. Questions of optical location. - M .: Soviet radio, 1971. - p. 213.

2. V.G. Vilner et al. Analysis of the input circuit of a photodetector with an avalanche photodiode and anti-noise correction. "Optical and mechanical industry". No. 9, 1981 - p. 593.

3. V.A. Afanasyev et al. Sensitivity threshold of a pulsed optical radiation receiver with a high input impedance. Electronic equipment. Series 11. "Laser technology and optoelectronics". 1988, V.Z. - with. 78 - 83.

4. Radiation receiver with active optical protection system. US patent No. 6,548,807.

5. V.G. Vilner et al. Laser rangefinder. RF patent No. 2655003 for the application for invention No. 2017123345 dated July 03, 2017 - prototype.

6. G.I. Tsukanov. Applied optics. Part 1. ITMO University - St. Petersburg: 2008. - p. 16-18.

7.V.A. Panov et al. Handbook of the designer of optical-mechanical devices. - L .; Mechanical engineering, 1980. - p. 127-128.

8. G.B. Dwight. Integral tables and other mathematical formulas. - M .; Science, 1973 .-- p. 83.

9.T.S. Voropai et al. Correction of astigmatism in spectral instruments using an inclined plane-parallel plate. "Bulletin of BSU". Ser. 1. 2007, No. 3. - with. 12-17.

Claims (3)

1. Receiving channel of the laser rangefinder, including a photodetector with a receiving lens, and the photodetector contains a photosensitive element and an amplifier, characterized in that a second photosensitive element with a second amplifier is introduced, a second photosensitive element with a second amplifier is introduced at the outputs of the amplifiers, time-latching circuits of the signal, which are the sequential inclusion of the differentiating link and the zero comparator, their outputs are connected to the input of the output signal driver through a switch controlled from the output of the threshold device introduced at the output of the differentiating link belonging to a photodetector with a higher sensitivity, while photosensitive the elements are located at the minimum possible distance b from one another, and in front of them two inclined plane-parallel optical plates are introduced, the plate closest to the photosensitive elements is located perpendicular ular to the plane containing the axes of the photosensitive elements and is inclined to them at an angle θ, a dichroic coating is applied to its surface opposite to the photosensitive elements, reflecting the received radiation with a working wavelength, the second plate, installed closer to the receiving lens, is inclined by an angle minus θ, in this case, the thickness of each plate d should be minimal under the condition d≥nb⋅cosθ, where n is the refractive index of the plate material, b is the distance between the photosensitive elements in the plane perpendicular to the optical axis of the receiving lens, and the difference in the positions of the photosensitive elements Δh * along the axis of the receiving lens satisfies the ratio

where d _p is the permissible diameter of the scattering circle in the plane of the photosensitive element of the photodetector with a lower sensitivity; F is the focal length of the receiving lens; D ₀ - light diameter of the receiving lens. 2. A device according to claim 1, characterized in that the thickness d _{1 of the} first plate closest to the receiving objective is twice the thickness d ₂ = d of the second plate. 3. The device according to claim. 1, characterized in that the first and second plates are mutually perpendicular.

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