Infrared laser spotlights.

Infrared laser projectors.

ARKHUTIK Stepan Trofimovich,
VOLKOV Viktor Genrikhovich, Candidate of Technical Sciences, Associate Professor
KOZLOV Konstantin Valerievich
SALIKOV Vyacheslav Lvovich
UKRAINSKY Sergey Alekseevich

INFRARED LASER PROJECTORS

The night vision device (NVD) must operate not only in passive mode, but also in active mode with floodlight illumination to ensure the required range in particularly dark nights.

The proposed replacement of infrared (IR) spotlights on incandescent and gas-discharge lamps with a unified small-sized spotlight (type PL-1) based on an effective pulsed laser semiconductor emitter (ILPI) allows not only to increase the visibility range in active mode, but also to improve noise immunity and the overall efficiency of the complex under operating conditions.

The advantage of using the PL-1 floodlight is the possibility of implementing the active-pulse mode of operation of the NVD [1].

The PL-1 floodlight is a new generation of IR illumination sources based on the ILPI. It contains a single unit that functionally combines the ILPI, a power supply unit (ILPI pumping unit) and a forming optical system, as well as a heating system for the protective glass.

The PL-1 floodlight forms a rectangular radiation spot with a uniform distribution of the radiation energy brightness, which is convenient for the operator, while lamp floodlights form a bell-shaped distribution (Fig. 1).

The PL-1 floodlight is characterized by lower energy consumption and an increased service life compared to existing analogues, and has a smaller weight and dimensions [2]. In particular, the PL-1 floodlight has a weight of 7.0 kg, dimensions of 246x174x177 mm, and an energy consumption of 50 W, while the L4 lamp floodlight has similar parameters: 20.5 kg, W300x280 mm, and 400 W.

This eliminates such defects of lamp floodlights as lamp explosion, failure to start, instability of brightness, destruction of the reflector and light filter during lamp explosion, etc. The main parameters of laser floodlights based on the ILPI are given in Table 1.

Fig. 1. Distribution of intensity and shape of the spot
of the illumination of the IR illuminator radiation

As a result of full-scale tests of the NVD based on the II+ and III generation image intensifier tubes, having a lens with a focal length of 150 mm and a relative aperture of 1:1.7, it was established that the target recognition range at night, with an atmospheric transparency coefficient of at least 0.8 per 1 km and a level of natural night illumination of the terrain of (3 – 5) x 10-3 lx, corresponded to: 1100 m – 1200 m in passive mode and 1200 m – 1300 m in active mode with the PL-1 floodlight. This is no worse than in the case of using the L4 IR lamp floodlight.

It should also be noted that the PL-1 laser spotlight has good spectral matching with the II+ and III generation image intensifier photocathode. At the same time, due to the decrease in image contrast due to the effect of backscattering of illumination radiation, it is necessary to use an active-pulse mode of operation in the NVG.

In addition to increasing the recognition range to 2 km, the presence of an active-pulse mode allows for an increased degree of protection against local light interference, ensuring observation in low-transparency conditions and accurate measurement of the distance to the object of observation.

The PL-1 floodlight operates in a pulse mode, which meets the requirement for implementing the active-pulse mode of operation of the NVD. Minor modifications to the floodlight are required to ensure synchronization with the NVD strobing unit.

The floodlight is based on the ILPI-114 type ILPI with the following parameters: average radiation power of 0.2 W (0.15 W at the output of the illuminator optics) in a cone with an apex angle of 400, operating wavelength of 850 nm, operating maximum pulse repetition rate of illumination of 5.2 kHz, radiation pulse duration of 130 ns, operating pump current in a pulse of 50 A at a resistance of 3.2 Ohm.

The optical diagram of the floodlight is shown in Fig. 2. ILPI 1 creates a radiation flux. Filter 2 is designed to correct the radiation spectrum of ILPI 1 in order to reduce the unmasking of the operating floodlight.

The range of the IR illuminator's radiation visibility with the naked eye does not exceed 200 m.

Lens 3 is used to form the illumination spot of the required dimensions. It has a focal length of 114.5 mm with a relative aperture of 1:1.4.

The rectangular shape of the illumination spot is determined by the shape of the ILPI 1 glow body.

Fig. 2. Optical diagram of the PL-1 IR illuminator:
1 – pulsed semiconductor laser emitter;
2 – filter;
3 – lens;
4 – protective glass

The issue of the optics for forming the radiation of the laser spotlight [2] should be especially considered. In the simplest case, the above-mentioned lens, designed for a minimum of spherical aberration (Fig. 2), can be used as such optics.

Such optics are simple, but due to significant aberrations, the radiation is strongly scattered outside the specified illumination angle, which leads to energy losses reaching 25 – 30%.

To reduce aberrations and increase the efficiency of optics, lenses with one aspherical surface can be recommended (Fig. 3) [2]. But to further reduce the weight of the lens, one of the optical surfaces can be made kinoform (Fig. 4). These lenses have a focal length of 115 mm (Fig. 4a) and 245 mm (Fig. 4b) and a relative aperture of 1:1.25 (Fig. 4a) and 1:1.4 (Fig. 4b). The maximum diameter of the scattering circle is 0.25 mm for the lens according to Fig. 3a, 0.17 mm for Fig. 3b, 0.1 mm for Fig. 4a, and 0.2 mm for Fig. 4b.

When using lenses with a kinoform surface, minimum weight is achieved with minimum aberrations. Specifically for the lens according to Fig. 4a the mass in glass is 0.446 kg, and according to Fig. 4b – 1.2 kg. In addition, the lenses according to Fig. 4 are made of non-scarce glass grade K8.

The loss of radiation energy due to aberrations does not exceed 10%. For work at different distances, it is advisable to change the illumination angle accordingly, which can be functionally linked to the corresponding change in the time delay in the AI ​​NVG.

For this purpose, a varifocal lens with a smoothly variable focal length can be used instead of a lens (Fig. 5) [2].

Its focal length varies within the range from 60 mm to 240 mm with a relative aperture of 1:1.4.

The illumination angle varies from 6×30 to 1.5×0.750. The lens weight in glass is 3.2 kg, dimensions are W180x354.5 mm.

Fig. 3. Aspherical lenses for forming the radiation of an IR illuminator:
a — lens with one parabolic surface;
b — lens with one hyperbolic surface

Fig. 4. Lens for forming the radiation of an IR spotlight with one kinoform surface: with a focal length of 115 mm (a) and 245 mm (b); K-kinoform surface

Fig. 5. Scheme of a varifocal lens

The functional diagram of the PL-1 floodlight is shown in Fig. 6. The low-frequency interference filter is designed to suppress interference in the power supply circuit when the ILPI power supply unit is operating. The filter reduces the interference level to values ​​that do not affect the normal operation of the radio station and the intercom of the floodlight installation site.

The ILPI power supply unit is designed to generate ILPI pump current pulses. The power supply unit has a pump current pulse amplitude control system depending on the ambient temperature. The amplitude is adjusted individually for each specific ILPI in accordance with its temperature characteristic.

The floodlight design is shown in Fig. 7. Its main components are the housing 1, rear 2 and protective 3 covers.

The floodlight housing contains ILPI 4, light filter 5 in a frame, lens 6 in a frame, ILPI power supply 7, moisture absorber 10 and thermostat 12. The front part of the case is covered with protective glass 8 in a frame.

To protect the glass 8To prevent fogging and frosting, a thermostat is used in combination with a heating element, a conductive coating applied to the inner surface of the glass 8.

A throttle 9 is installed in the cover 2, which is a low-pass filter. The protective cover 3 is fastened to the body with screw 11 in the closed and open state.

All floodlight systems are operational at an on-board network voltage of 27+2-5 V, and also remain operational after exposure to pulse overvoltage with an amplitude of up to 70 V for up to 3 ms, a decrease in the on-board network voltage to 10 V for up to 1 min, and after exposure to a reverse polarity voltage of 30 V for up to 1 min.

Fig. 6. Functional diagram of the PL-1 IR floodlight

Fig. 7. Design of the PL-1 IR illuminator:
1 – body;
2 – cover;
3 – protective cover;
4 – laser emitter;
5 – filter in frame;
6 – lens in frame;
7 – power supply;
8 – protective glass;
9 – throttle;
10 – desiccant;
11 – screw;
12 – thermostat

The further development of the PL-1 searchlight circuit should be considered the OU-6 and OU-6-01 laser searchlights (table 1). Their important advantage is the ability to track the optical axis of the NVD horizontally and vertically. This is achieved using an electromechanical drive. In this case, it is possible to automatically hold the object of observation moving along the front within the illumination spot by forming auxiliary illumination spots located along the perimeter of the main spot. For this purpose, four auxiliary ILPIs should be located along the perimeter of the main ILPI. Their operating frequency should be negligibly small compared to the operating frequency of the main ILPI, so that the operator does not see these illumination spots through the NVD.

Let the frequency of the auxiliary ILPI located to the left and right of the main ILPI be 3 Hz and 5 Hz, respectively, and those located above and below the main ILPI be 7 Hz and 9 Hz, respectively. In this case, a photodetector is installed at the output of the NVD to record signals with these frequencies. The photodetector is connected to the comparator, which is connected via the control register to the drive for moving the floodlight axis horizontally and vertically.

Let's assume that the observed object has shifted to the right of the illumination spot formed by the main ILPI.

Then the object will fall into the illumination spot formed by the auxiliary ILPI with an operating frequency of 5 Hz. The photodetector will receive this signal from the image intensifier screen and convert it into an electrical signal, which will be sent to the comparator.

The latter will generate a difference signal in relation to the frequency of the main ILPI and will control the operation of the drive via the register in the direction of eliminating the signal with a frequency of 5 Hz. This will happen when the object is again within the main illumination spot.

The drive is controlled in a similar manner when an object enters other auxiliary illumination spots. Thus, the drive tracks the observed object, ensuring its constant position within the main illumination spot.

Table 1.

There are two ways to increase the floodlight power:

• creation of a floodlight based on the principle of a group module” [2];
• combining in one floodlight the radiation of two ILPIs that differ in spectrum [2].

“A group module is two or more ILPIs with an individual lens, forming standard modules. Their optical axes are mutually parallel, so that the radiation of the modules is summed up in a single illumination angle equal to the illumination angle of one module. An example of such a two-module spotlight is shown in Photo 1. The spotlight was developed by TsKB “Tochpribor” for the active-pulse NVD 1PN61 [3].

Photo 1. Two-module IR laser spotlight
for NVD 1PN61 [3]

This floodlight can combine two to four modules. In this case, the dimensions, weight and power consumption of the floodlight will increase proportionally to the number of modules, but will not exceed the similar parameters of the prototype floodlight L4. It is possible to use lenses with a shorter focal length in a four-module floodlight. This will allow, while maintaining the same energy intensity of light, to increase the illumination angle of the floodlight, which will improve search capabilities with normal illumination without using the active-pulse mode of the NVD. At the same time, the dimensions and weight of the floodlight will increase insignificantly.

To increase the radiation power practically without changing the dimensions of the floodlight, the second way can be used, the scheme of which is shown in Fig. 8. Here, due to the use of a mirror 3 with a dichroic coating, the radiation of both ILPI 2 and 4 generating radiation at different wavelengths is summed up. The lens 1 focused on both ILPI covers their radiation and forms a spot of illumination. Losses in mirror 3 do not exceed 10 – 15%. Spectral characteristic of the dichroic coating of mirror 3is given in Fig. 9. Due to such a scheme, only the power consumption of the floodlight will increase. Since the ILPI with a wavelength of 820 nm can lead to an increase in the unmasking data of the floodlight, it is possible to use the ILPI with a wavelength of 880 nm. In this case, the sensitivity of the photocathode of the image intensifier tube at this wavelength will be 1.5 times less than for a wavelength of 850 nm. Therefore, the efficiency in terms of radiation power of such a floodlight will be lower, unless it is used in a generation III+ image intensifier tube, which has a spectral sensitivity extending to 1.2 μm and the difference in sensitivity at wavelengths of 850 and 880 nm will be insignificant [4]. In principle, it is possible to combine both proposed methods, which will sharply increase the spotlight's radiation power and its illumination angle.

Fig. 8. Laser illuminator with a single lens for two ILPI:
1 — radiation shaping lens;
2 — ILPI with a wavelength of 850±10 nm;
3 – dichroic mirror reflecting in the wavelength range of 840 – 920 nm, but transmitting at a wavelength of (820-20) nm;
4 – ILPI with a wavelength of (820-20) nm

Fig. 9. Spectral characteristics of the dichroic coating of the mirror:
1 – transmission of the coating in the spectral range of 800 – 820 nm;
2 – reflection in the spectral range of 840 – 900 nm

The next step in increasing the efficiency of the floodlight is to shift its spectral characteristics to a longer-wavelength region of the IR spectrum, provided that the NVD is simultaneously equipped with a generation III+ and IV image intensifier tube or, instead of an image intensifier tube, with highly sensitive TV cameras on CCD matrices operating in the spectral region of 0.4–1.1 μm or 1.0–1.7 μm [4]. In this case, the ILPI-114 can be replaced with an ILPI with a wavelength of 0.98 μm, 1.06 μm or 1.55 μm. An example of such ILPIs are the laser diode array models from Semiconductor Laser Int’I Corp. (USA):

— model SLI-QCW-LD-B1-980-60M-R: average radiation power of 1.2 W, wavelength of 0.98 μm, radiation pulse duration of 200 ns, operating frequency of 100 Hz, radiation divergence angle of 10×400, model SLI-QCW-SM-B10-980-600M-R with the same parameters, but with an average radiation power of 12 W;

— model SLI-QCW-LD-B1-1060-60M-R with an average radiation power of 1.2 W and a wavelength of 1.06 μm, model SLI-QCW-SM-B10-1060-600M-R with an average radiation power of 12 W and a wavelength of 1.06 μm [5].

The company (USA) developed a model of a single-element ILPI, generating an average radiation power of 0.01 W at a wavelength of 1.55 μm with an operating frequency of 10 kHz, a radiation pulse duration of 100 ns, and an angular divergence of 20×400 [5]. A similar emitter with an average radiation power of 0.005 W has been created in Russia [6].

Literature.

1. Volkov V.G. Night vision devices for armored vehicles./Special equipment, 2004, No. 5, p. 2 13.
2. Geykhman I.L., Volkov V.G. Fundamentals of improving visibility in difficult conditions. Moscow, Nedra, 1999, 286 p.
3. Devices for armored vehicles. Prospectus of GUP PO «NPZ», RF, Novosibirsk, 2004.
4. Volkov V.G. New generation night vision devices./Special equipment, 2001, No. 5, pp. 2 — 8.
5. Laser Focus World. The Buyers' Guide 2002, USA, 2001.
6. Laser semiconductor emitters. Prospectus of FSUE NPP «Inject». Russian Federation, Saratov, 2004.