Laser illuminators and target designators for night vision devices.

Laser illuminators and target designators for night vision devices..

LASER ILLUMINATORS AND TARGET DESIGNS FOR NIGHT VISION DEVICES

At present, night vision devices (NVD) based on image intensifier tubes have become widely used to ensure vision at night and in low atmospheric transparency [1, 2]. However, when the level of natural light decreases, the range of NVDs decreases, and in conditions of complete darkness, these devices become completely inoperative. In this regard, NVDs are often equipped with infrared illuminators designed specifically for operation in adverse conditions. In addition, night sights require target designators that create a small-sized (“point”) illumination spot on the target. It is very advisable to make these illuminators and target designators based on injection semiconductor laser emitters, which compare favorably with other light sources by the small size of the luminous body and its high overall brightness, low weight, dimensions and energy consumption, significant service life and high performance characteristics. Let's consider the main types, parameters, capabilities and development prospects of laser illuminators and target designators for night vision devices.

Laser illuminators are divided into devices operating in pulse and continuous modes. Pulse laser illuminators are used as part of active-pulse night vision devices [2]. Their operating principle is based on the illumination of the observed object by the radiation of a pulse laser illuminator and synchronized with it pulse control (strobing) of the electron-optical converter installed in the receiving part of the device. This allows achieving, in comparison with traditional passive and active night vision devices, significant ranges of action, ensuring their accurate measurement, continuity of observation under the influence of intense light interference, as well as with reduced transparency of the atmosphere (haze, fog, rain, snowfall, etc.). The illuminator consists of radiation shaping optics, a pulsed laser semiconductor emitter and a pumping unit. The latter is based on transistor or thyristor circuits, which are described in sufficient detail in [3]. The issues of creating radiation shaping optics for such illuminators are described in [2]. The illuminator can be made either as a single-emitter or as a group-emitter [2]. In the first case, the illuminator has the circuit shown in Fig. 1, where (1) are elementary laser emitters or their arrays, (2) a fiberglass collector, (3) is an integrator, (4) is a radiation shaping objective. Here, in order to obtain a high average radiation power, individual single-element emitters or their arrays (1) are combined into a single luminous body using flexible bundles of a fiberglass collector (2). The cross-section of the bundle has the shape of a rectangle with the dimensions of the sides corresponding to the overall dimensions of the luminous body of the emitter. At the output of the collector (2), an integrator (3) is mounted — a section of a light guide that serves to mix the radiation from individual emitters and create a uniform distribution of the energy brightness at its output end. The shape and size of the illumination spot are determined by the shape and size of the output end of the integrator, which is projected onto the terrain using the radiation shaping lens (4). The appearance of an illuminator constructed according to this scheme is shown in Photo 1. The illuminator may contain a multi-element emitter in the form of a laser diode array, the individual emitters of which may not lie in the same plane. An integrator is also mounted at its output, onto the output end of which the radiation shaping lens is focused. The LS-410 illuminator from LDL (USA) [4] is constructed according to this scheme (Photo 2). To increase the radiation power, it is necessary to increase the number of elementary emitters. But in this case, the overall dimensions of the integrator output end inevitably increase. To maintain a constant illumination angle, it is necessary to increase the focal length of the lens. This entails a sharp increase in the longitudinal dimensions and weight of the illuminator.

 
Fig. 1. Scheme of constructing a laser illuminator

Photo 1. External appearance of the illuminator made according to the mono-emitter scheme

Photo 2. LS-410 illuminator by LDL, USA

Therefore, a more appropriate solution for significant radiation powers is a group module scheme. An illuminator made according to this scheme consists of a number of standard modules with mutually parallel optical axes. Each module has a lens and an emitter containing a laser diode array with or without an integrator. The radiation of all modules is summed up in a single illumination angle equal to the illumination angle of one module. Such a scheme of the illuminator ensures its minimum longitudinal dimensions, a simple scheme of the radiation forming lens. The scheme is also convenient due to its high maintainability, since if one module fails, it can easily be replaced with another. The modules can be spatially separated, which creates wider possibilities for their arrangement on an object, for example, on a car, helicopter or ship. The following are usually used as radiation forming objectives: two- or three-lens objectives, raster optics (photo 3 – illuminator from the Polyus Research Institute), single-lens objectives with one aspherical or kinoform surface, gradient objectives [4] or plastic Fresnel lenses [5]. Photo 4 shows a two-module illuminator from a serial active-pulse device 1PN61 [6], made on the basis of the ILPI-110 laser emitter. The dimensions of the illuminator are 287x210x120 mm. The PL-1 illuminator with dimensions of 250x170x150 (photo 5) is made on the basis of the ILPI-114 emitter. Photo 6 shows a ten-module illuminator based on pulsed laser emitters LPI-14, photo 7 shows a six-module illuminator O-245/6.

Photo 3. Pulse laser illuminator with raster optics, Research Institute “Polus”, Russia

Photo 4. Two-module illuminator of the 1PN61 device

Photo 5. Illuminator PL-1 TsKB “Peleng”, Belarus

Photo 6. Ten-module illuminator
based on pulsed laser emitters LPI-14

Photo 7. Six-module illuminator O-245/6

Pulsed electron-pumped semiconductor lasers (PESELs) can be used as pulsed laser illuminators [4, 8, 9]. Although they have a lower efficiency compared to semiconductor injection lasers, they have a significant pulse power — from 4×106 to 3×108 W with a pulse duration of 3 — 5 ns [8, 9]. In combination with a low operating frequency measured in tens of Hz, this allows for a high duty cycle of the active-pulse NVD and a small depth of the viewed space. This makes it possible to dramatically increase the range of the NVD, the accuracy of its measurement and the degree of protection from light interference. PESELs were first developed for such purposes and used at the State Unitary Enterprise «NPO Orion» [4]. Currently, the Platan Research Institute is successfully developing PESELs [8, 9]. Depending on the type of the semiconductor laser target bombarded by the electron beam, the laser can operate at discrete wavelengths in the range from 0.38 to 1.7 μm: 0.395, 0.471, 0.51, 0.53, 0.58, 0.61, 0.66, 0.89, 1.3, 1.7 μm. The overall dimensions of the emitter are O160x500 mm, the power supply unit is 310x310x120 mm, the power consumption is 50-100 W, the total weight is 22 kg [8, 9]. The appearance of the emitting head of the IPLEN is shown in photo 8.

Photo 8. The appearance of the emitting head of the IPLEN

The main parameters of pulsed laser illuminators are given in Table 1.

Table 1. Main parameters of pulsed laser illuminators for active-pulse NVDs

Note:
1. Рср. – average radiation power.
2. tи – radiation pulse duration.
3. F – operating frequency.
4. l – wavelength.
5. q – illumination angle.
6. m – mass.
7. РЕ – power consumption.

For conventional NVDs, laser illuminators operating in continuous mode are used. Compared to pulse emitters, continuous emitters have significantly smaller dimensions of the glow body, a significant service life and higher efficiency. Due to this, “continuous” illuminators have smaller dimensions, weight and power consumption. With their help, it is possible to obtain smaller illumination angles, reaching units of minutes. Illuminators also have a simpler pumping scheme. The main parameters of “continuous” illuminators are given in Table 2.

Table 2. Illuminators based on semiconductor lasers for NVDs

Note:

U – supply voltage.
P – radiation power.
All other designations – see note to Table 1.

Photo 9 shows the RT-5A illuminator [7], in which the illumination angle is changed by refocusing the lens with a variable focal length. The original solution was proposed by the Rostov Optical and Mechanical Plant (ROMZ), which created a whole series of illuminators built into the NVG handle. An example is the NZT-1 NVG (photo 10). However, in general, it is more expedient to create illuminators in the form of independent modules in a separate housing and with an autonomous primary power source. Such modules are connected to the NVG using standardized mounting units. If necessary, they can be easily separated and replaced with illuminators with other parameters.

Photo 9. RT-5A illuminator

Photo 10. Night vision device NTZ-1
with an illuminator built into the handle, ROMZ, Russia

Small-sized laser designators based on continuous laser emitters are also used in night vision technology. Their main parameters are given in Table 3.

Target designators are used in sighting systems consisting of night vision goggles for observation at night and a laser target designator mounted on a weapon that creates a light “point” spot of illumination on the target. Both the target and the spot on it are observed through night vision goggles. The target designator is mounted parallel to the weapon barrel and is sighted in together with it. The position of the target designator is adjusted using a verification system. Due to this, bullets accurately hit the target at which the illumination spot is aimed. Such a sighting system eliminates the need for aiming — it is enough to simply give the weapon a position in which the illumination spot coincides with the target. Aimed shooting can be carried out from any position of the weapon, including when moving. Such sighting systems are used when shooting from pistols, machine guns, grenade launchers, hunting and sporting weapons of any type. Initially, target designators used semiconductor lasers operating at a wavelength of 0.82 — 85 µm so that illumination was carried out covertly, and radiation at such wavelengths would be observed only in night vision goggles. In practice, such radiation, firstly, is still visible from the target as a red dot, and secondly, this circumstance is even useful, since it produces a strong psychological effect on the enemy. In addition, operation at the specified wavelengths excludes the use of the complex in daytime conditions, unless night vision goggles with diaphragm lenses are used. Therefore, in recent years, target designators based on laser emitters operating at wavelengths of 0.635 — 0.67 µm have become widespread. For a sighting system with such an emitter, it is possible to observe the illumination spot on the target during the day with the naked eye, and at night — in night vision goggles at an even greater distance than in the case of using target designators with a wavelength of 0.82 — 0.85 μm, since the sensitivity of the photocathode of the electron-optical converter of night vision goggles in the spectral region of 0.635 — 0.67 μm is 1.5 — 2 times higher than in the region of 0.82 — 0.85 μm.

The target designator can be mounted either on the barrel or under the weapon barrel. The appearance of a typical laser target designator (1) and night vision goggles (2) is shown in Photo 11, the PL-1 target designator is shown in Photo 12, the REM 007 is shown in Photo 13, the CL-05 is shown in Photo 14, the TSL-10 is shown in Photo 15, and the Korsak-3 is shown in Photo 16. One of the possible design options for the target designator is shown in Fig. 2. Here, the dotted line shows the attachment of the target designator body to the weapon. The target designator is aligned with the weapon using an eccentric frame. Photo 17 shows the ASR-2 laser target designator, mounted on the operator’s finger and used to ensure night landings of helicopters [10]. The illumination angle of this target designator can be adjusted within the range from 0.5 mrad to 100.

Photo 11. Typical laser designator with night vision goggles

Photo 12. PL-1 designator, VOMZ, Russia

Photo 13. REM 007 from Wild Heerbrugg, Switzerland

Photo 14. Laser target designator TsL-05, BELOMO, Belarus

Photo 15. Target designator TSL-10, BELOMO, Belarus

Photo 16. Target designator “Korsak-3”, BELOMO, Belarus

Fig . 2. Design diagram of the target designator

Photo 17. Miniature target designator
ACP-2 from Night Vision Equipment Inc ., USA

The small weight, dimensions and divergence of the laser target designator combined with the simplicity of modulating its radiation by the pump current make it possible to transform the target designator into a portable rangefinder built into the NVD. In particular, ROMZ has developed an illuminator-rangefinder that can provide observation in a wide illumination angle and measure the distance to an object in a narrow angle by creating a “point” illumination spot on it [11]. Such a device is compatible with any NVD that has a standard 1/4 inch plug socket. The illuminator-rangefinder is powered by built-in batteries of the 10D-0.26S type with a total voltage of 12 V. Weight — 0.9 kg (excluding batteries), overall dimensions 220x180x50 mm. The range of measured distances is 5–150 m with a measurement error of 1–2 m. Foreign NVDs have portable built-in laser rangefinders based on semiconductor lasers. For example, the observation NVD MC31 from Litton (USA) has a rangefinder that measures distances up to 1000 m with an accuracy of ±5 m [4]. The weight of the entire NVD together with the rangefinder does not exceed 1.3 kg, and its dimensions are 170x140x70 mm. The portable NVD from Night Vision Equipment (USA) also has a built-in rangefinder that measures distances up to 5000 m [4].

Further development of illuminators and target designators is associated with the creation of new night vision devices operating in the spectrum range of 1–1.8 μm [13]. Compared with the traditional spectrum range of 0.4–0.9 μm, the spectrum range of 1–1.8 μm is characterized by higher atmospheric transmittance due to lower radiation scattering, a higher level of natural night illumination and natural contrasts, and greater stability of the latter. For this spectrum range, it is possible to use illuminators and target designators based on semiconductor lasers operating at wavelengths of 1.25–1.3 and 1.55 μm [4]. Their weight and dimensions do not exceed similar parameters of traditional illuminators operating at wavelengths of 0.82–0.85 μm. Their radiation power can be 0.1–1 W [4]. It is also possible to use an illuminator based on a solid-state laser based on erbium with a wavelength of 1.54 μm and based on holmium with a wavelength of 1.7 μm. In particular, the EAD-500D and EAD-1000D models [12], generating a radiation power of 0.5 and 1 W, respectively, at wavelengths of 1.58 and 1.57 μm with pumping from a semiconductor laser with a wavelength of 0.95 — 0.97 μm and with fiber optic radiation output, weigh 5 kg and have dimensions of 250x100x325 mm. Laser radiation at wavelengths of 1.3 — 1.7 μm is invisible to the eye and safe for vision [14].

To achieve high recognition probabilities in a wide range of changing external conditions, it is necessary to use multi-channel NVDs [15]. In this regard, it is advisable to create multi-color illuminators operating in a wide range of the spectrum at discrete wavelengths (Fig. 3). Here, due to the use of a mirror (2) with a dichroic coating, the radiation of semiconductor laser emitters (1) and (3) generating in different regions of the spectrum of 0.85 and 1.55 μm is summed up. Mirror (2) transmits the radiation of emitter (3), but reflects the radiation of emitter (1). Objective (4), focused on both emitters, sums their radiation and collimates it. Total losses in mirror (2) do not exceed 10-15%.

Fig. 3. Scheme of constructing a laser illuminator
using two laser emitters

It is also possible to manufacture a multi-element array consisting of elementary semiconductor lasers with different wavelengths. Such compact emitters can be controlled by a programmable logic circuit, which provides, depending on the need, simultaneous or separate operation of the elementary emitters. The prospects for the development of illuminators are also associated with the creation of pumping in an integrated design with a laser emitter according to the model type [16]. In this case, the operating mode of the pumping circuit changes over time in accordance with a given program. The illuminator is controlled directly or remotely. The latter is necessary for the creation of robotic complexes. To optimize the optics of radiation formation, it can be grown directly on laser emitters in an integrated design with them [17]. Based on the achievements of microelectronics, in the coming years it is expected that fully integrated illuminators will be created, which can also perform the role of target designators of rangefinders. Laser target designators-rangefinders can be functionally combined with microprocessor control devices that change the parameters of this laser device depending on external conditions and provide their built-in control.

Literature

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