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OPTICS DESIGN FOR LASER DESIGNATOR CUM RANGE FINDER

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OPTICS DESIGN FOR LASER DESIGNATOR CUM RANGE FINDER
Conference Paper · December 2011
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Ranabir Mandal
Defence Research and Development Organisation
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OPTICS DESIGN FOR LASER DESIGNATOR CUM RANGE FINDER
Sucharita Sanyal, Ranabir Mandal, P.K. Sharma & Ikbal Singh
Instruments Research and Development Establishment, Raipur Road, Dehradun-248008
Email: [email protected] / [email protected]
Abstract:
This paper presents the optics design for laser designator cum range finder. The instrument contains three basic parts: laser transmitter,
receiver and day sight. The transmitter optics converts the raw laser beam into a beam of specific diameter with reduced divergence. The
receiver optics focuses the reflected laser beam from the target on to the detector in order to determine the range/distance of the target.
The day sight is used to precisely locate the target. As the designator laser beam is aligned with the optical axis of the sight, the target
seen at the centre of the field of view of the sight gets illuminated when the laser transmitter is switched on. In addition to it, to view the
range information simultaneously along with the sight reticle through the eyepiece of the day sight, an additional projection lens/mirror is
required in this instrument. The design constraints/criticality of the system is discussed.
Keywords: Laser guided precision munitions, Target search, Designation & Range determination, Optics design
1. INTRODUCTION
Laser designator is used as an auxiliary device for automatic homing of precision, laser guided bombs, missiles, smart
artillery munitions. It is basically a laser light source from which a series of coded pulse are fired towards the target. The
laser light reflected from the target is collected by the seeker on the guided munition and the munition is steered towards the
centre of the reflected signal from the target. Laser designators and seekers use a pulse coding/decoding system to ensure
that a specific designator and seeker combination work in harmony. By setting the same code in both the designator and the
seeker, the seeker tracks only the target designated by the particular designator. Laser designators may be mounted on
aircraft, ground vehicles, or may be ground-based. Military designators usually operate at infrared wavelength around
1.06µm. However, laser designators work best in clear atmospheric conditions. Cloud cover, rain or smoke can make
designation of targets difficult or even impossible. Sometimes the functions of designation and range finding may be
combined in the same device. Range finder calculates the precise range/distance to a target. Considerable engineering may
be needed to meet all the constraints of the battlefield. The soldier designating a target does not want the target to know that
it is being marked. Even if the soldier is observing and mapping, and not guiding weapons onto the target, he still obviously
wants to keep his designator clandestine. Some targets may have optoelectronics that can detect when laser energy is hitting
them, even if the beam, in the infrared spectrum, is invisible to the human eye. The enemy may try to jam the sensor
looking for the laser designator energy, perhaps by turning on infrared light of similar wavelength or firing flares. Modern
laser designators do not send a simple continuous wave or steady set of pulses at the target, but send a complex, changing,
and unique pulse pattern. This infrared laser energy from the designator can damage the eye. The latest practice is to use
eye-safe laser wavelengths beyond 1.54µm for laser ranging.
Laser designator cum range finder instrument usually contains three basic parts: laser transmitter, receiver and
day/night/thermal sight. Optics is an integral part of each of these subsystems. The transmitter converts the raw laser beam
into a coded pulse stream and the outgoing beam is of reduced divergence and specific diameter greater than the original.
The role of transmitter optics is to reduce the beam divergence as per the application requirement. The receiver part does
the range determination and the receiver optics focuses the reflected beam from the target on to the detector.
Day/night/thermal sight is used to precisely locate the target. Our instrument has day sight only. As the designator laser
beam is aligned with the optical axis of the sight, the target seen at the centre of the field of view of the sight gets
illuminated when the laser transmitter is switched on. Moreover, the system is so designed that the front optics is common
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for all the subsystems; viz. the transmitter, receiver and day sight. In fact, use of common lens groups in various subsystems
makes the design difficult. In addition to it, to view the range information alongside the reticle through the eyepiece of the
day sight, the seven segment LED display is projected on the reticle plane of the day sight and an additional lens group
combined with mirrors is required for this purpose. Packaging of these four subsystems in very compact manner in one
instrument is itself a demanding proposition specifically in view of the fact that instrument’s weight is very important factor
in military applications.
Our device is designed for designating tank type targets at more than 5km range. The transmitter is Nd:YAG laser based,
emitting a wavelength of 1064nm. The raw beam diameter of the laser is 5mm and divergence is 2 milliradians. The
transmitter optics which is basically a beam expander has a magnification of 11X and it reduces the beam divergence by the
same factor. A magnification of 11X is required in order to get a laser beam of desired diameter at the target distance. The
receiver optics has an EPD of 68mm in order to have sufficient collection of back reflected light from the target. The size of
the active sensor area of detector in the receiver is 0.8mm. In order to identify tank targets at 5km distance, a magnification
of 12X is required for the day sight. The day sight has a field of view of ±2º, EPD 68mm, f# 4.5 and eye clearance of
22mm.
In the following section, we present the design methodology and the layout of the optics for laser designator cum range
finder (LDRF). The design constraints/criticality of the system is discussed.
2. THE OPTICAL DESIGN & LAYOUT
Fig-1: The basic layout of the optics for the LDRF
The raw beam from the Nd:YAG laser in the transmitter passes through the beam splitter (BS) and lens group-2, then gets
reflected at the dichroic prism beam splitter (DPBS) and its path is deviated by 90º. It then passes through the lens group-1
and emerges as a beam of required diameter with reduced divergence. So, lens group-2 and 1 together constitute the
transmitter optics (see Fig-1). Lens group-1 is common to the three basic subsystems of the device; i.e. the transmitter,
receiver and day sight.
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For the day sight this lens group-1 serves as the objective. The visible spectrum and the laser light reflected from the target
both enter the system through lens group-1, but follow different paths after the dichroic prism beam splitter (DPBS). The
DPBS transmits the visible spectrum and reflects the laser wavelength. A prism beam splitter is preferable to a semireflecting parallel plate beam splitter for mainly two reasons. First, if used in a convergent or divergent beam, a parallel
plate BS would introduce astigmatism, and second, the reflection from the second surface, although faint, would produce a
ghost image displaced from the primary image. The beam splitter cube avoids these difficulties. It is composed of two right
angle prisms cemented together. The hypotenuse of one prism is coated with a semi-reflecting coating before cementing.
The only concern in using such prism beam splitter is the weight and the light absorption of the cube. After passing through
the dichroic beam splitter the visible light passes through a Schmidt prism and gets focused at the reticle plane. The prism
has been arranged so that the axial ray enters and leaves the prism normal to the prism faces and all reflections within the
prism are total internal reflections. Schmidt prism deviates the beam axis through an angle of 45º and the beam itself gets
inverted. It may be mentioned in this connection that in an ordinary telescope, the objective lens forms an inverted image of
the object, which is then viewed through the eyepiece (same is the case of day sight). The image seen by the eye is upside
down and reversed from left to right. To eliminate the inconvenience of viewing an inverted image, an erecting system is
usually provided in between the objective and eyepiece to re-invert the image to its proper orientation. This may be a lens
system or a prism system. In this type of use, the prism is carefully arranged so that it will not separate the different colors.
This is exactly the function of the Schmidt prism here. Moreover, Schmidt prism reduces the tube length of the day sight as
the beam traverses an optical path longer than the prism base-length due to multiple reflections within the prism. Lens
group-1(objective), the Schmidt prism (image erector as well as beam deviator) and the eyepiece together form the day
sight optics. The objective has EPD 68mm, field of view 4º, f# 4.5 and it is an achromatic air-spaced positive crown
negative flint doublet. The eyepiece has EPD 5.6mm, FOV 48º, focal length 25.46mm, eye clearance of 22mm and consists
of 5 elements (modified Erfle type). The eye clearance is quite long compared to the focal length of the eyepiece along with
wide FOV and this makes its design difficult. Combining the objective and eyepiece, the day sight has a magnification of
12X, FOV of 4º and f# 4.5. In order to keep cost & weight of optics low, use of high index glasses has been avoided to the
maximum extent possible.
The laser light reflected from the target after passing through lens group-1, gets reflected by the dichroic beam splitter,
passes through lens group-2, again gets reflected at the 2nd beam splitter (BS), passes through the interference filter and lens
group-3 and focuses on the detector plane. The purpose of using the inference filter is to eliminate any unwanted radiation
other than the specified laser wavelength (1064nm). Interference filters are very efficient in narrow spectral band selection.
The only problem with this kind of filter is that it is very sensitive to angle of incidence of the beam. Interference filters
work best under normal incidence of light. For best performance of the interference filter, the optics is so designed that lens
group-1 & 2 together make the input beam parallel and lens group-3 focuses this parallel beam on to the detector. Thus lens
group-1, 2 & 3 together constitute the receiver optics. The receiver optics has EPD of 68mm which is required for sufficient
collection of back reflected laser radiation. Actually the central part of the lens aperture is used by the transmitter beam
(55mm in diameter) and the annular part is used by the receiver. A BK7 glass window is placed in front of the entire system
as a protective cover as well as for sealing the system from environmental hazards. The wedge plates (see fig-2) help in
aligning the transmitter laser beam with the optical axis of the day sight during assembly.
In addition to it, to view through the eyepiece of the day sight the range information alongside the reticle, the seven
segment LED display is projected on the reticle plane of the day sight and an additional lens group consisting of two
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components along with two mirrors is required for this purpose (see fig-1 & 2). When eye is placed at the exit pupil of the
eyepiece range information will be seen at one corner (see fig-3).
3. CONCLUSION
This paper presents the optics design for laser designator cum range finder. The instrument contains three basic parts:
transmitter, receiver and day sight. In addition to it, optics is also designed to view through the eyepiece of the day sight the
range information alongside the sight reticle. The design philosophy and detailed layout is given. The design
constraints/criticality of the system is also discussed.
ACKNOWLEDGEMENT: Authors are grateful to Director IRDE for his inspiration and encouragement.
Fig-2: Detailed layout of the optics for LDRF
Reticle
Range information
Fig-3: View of the reticle and range information as seen by the eyepiece
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