GB2448052A

GB2448052A - Semi active laser target detection method with coherent reception

Info

Publication number: GB2448052A
Application number: GB0805298A
Authority: GB
Inventors: Alfred Fendt; Rudolf Protz
Original assignee: LFK Lenkflugkoerpersysteme GmbH
Current assignee: LFK Lenkflugkoerpersysteme GmbH
Priority date: 2007-03-24
Filing date: 2008-03-20
Publication date: 2008-10-01
Anticipated expiration: 2028-03-20
Also published as: DE102007014256A1; GB2448052B; FR2914069A1; FR2914069B1; DE102007014256B4; GB0805298D0; ITMI20080312A1

Abstract

A target detection method is disclosed in which a target object Z is illuminated with coherent radiation S' from a laser designator DS and the radiation S reflected by the target is detected and coherently mixed by one or more detectors incorporated in a seeker. The differential signal obtained in a downstream detector D and lying in the frequency range between 0 and 10 GHz is narrow band filtered and processed further. The designator radiation may be modulated to allow information to be conveyed to the seeker. The disclosed heterodyne method is particularly suited for allowing a semi active laser (SAL) seeker to detect a designator using modulated continuous wave (CW) radiation in a sufficiently sensitive manner. CW designator radiation is distinctly harder to detect than the pulsed radiation of conventional laser warning systems.

Description

SAL (semi-active-laser) target-detection method with coherent reception

The invention relates to a target-detection method in which a target object is illuminated by means of a laser designator and the radiation reflected by the target is detected and evaluated by means of a seeker to determine the type, location and/or movement.

Conventional semi-active-laser (SAL) seeker heads are usually used together with laser designators with pulsed laser radiation. These radiate with vesy high peak power (in the megawatt range) with a pulse duration of typically ns and a pulse repetition rate in the 10 Hz range. The laser wavelength amounts to 1.06 pm.

On account of these features simple laser warning systems, given a target-marking, can already rapidly identify the stray light of the designator and initiate countermeasures, for example fogging or combat.

Continuous-wave (CW) laser radiation, on the other hand, is not discovered by usual laser warning systems and can only be detected with difficulty, in particular when there

is background radiation.

It is desirable to provide a laser seeker which makes it possible to detect and evaluate continuous-wave designator radiation which for its part can only be identified with very great difficulty by laser warning sensors that trigger countermeasures. Furthermore, at the same time the possibility is to be provided of transmitting information between the designator(s) and the seeker(s).

In accordance with the invention coherent radiation is generated by means of the designator for the purposes of illumination, and the radiation that is reflected by the target is coherently mixed in the seeker in one or more detectors, and in that the differential signal that is obtained in the downstream detector and lies in the frequency range between 0 and 10 GHz is filtered in a narrow band and processed further to obtain information.

By using special modulation methods in a further development of the invention it is also possible to transmit information from the designator to the seeker.

As a result of applying a heterodyne method it thus becomes possible in accordance with the invention to reaiize an SAL seeker which detects CW designator radiation in a sufficiently sensitive manner. Specific knowledge of the designator setting, which a conventional laser warning system does not have at its disposal, is expediently used for this purpose. The discoverability of the CW designator in accordance with the invention is distinctly reduced in comparison with conventional ones with pulsed radiation.

The physical bases and the technical embodiment of an SAL seeker and the associated laser designator, based on the laser heterodyne method, are described in the following.

In the drawings: Figure 1 shows the fundamental structure of the seeking device in accordance with the invention; Figure 2 shows a representation of the signal/noise ratio in the case of the application of the invention; and Figure 3 shows the structure of an embodiment with a scanning seeking head.

Figure 1 shows the fundamental structure of the seeking device. In this connection, a designator DS illuminates a target Z with coherent CW laser radiation S' which is partly reflected by the target Z and is collimated as radiation S by the receiving optics EQ and forwarded to the detector 0. The designator can be spatially independent of the seeker, or it can be connected to the latter to form a unit.

At the detector 0 the radiation of a local oscillator laser LO is coherently superimposed, expediently by means of a beam-combiner B, upon the designator radiation S that is reflected by the target Z. The detector D acts as a mixer that generates a differential signal of laser-designator frequency fD and LO frequency L10 in the RF-frequency range. This is filtered in a narrow band and processed further.

In this connection, the following relationship holds: sig = flD q (2PSjgPLO)112 / hf with isig: signal current in the detector liD: quantum efficiency of the detector q: electron charge Psig: signal power received PLO: power of local oscillator (LO) hf: photon energy.

The advantages of the heterodyne method lie in the fact that the signal-to-noise ratio S/N that can be achieved in the small-signal range in the case of coherent heterodyne reception is distinctly higher than in the case of direct

incoherent reception. The background noise and the

receiver noise are effectively suppressed by the local oscillator La. This connection is represented in Figure 2.

The S/N that can be attained is ultimately determined by the number of photons received: S/N = i2sig/12N = flD Psig / hf B with isig: signal current in the detector noise current (shot noise) riD: quantum efficiency of detector Psig: signal power received hf: photon energies B: filter bandwidth.

In order to guarantee effective superimposition of the frequencies in the case of heterodyne reception, the laser beams from the designator and the local oscillator must have good temporal and spatial coherence.

The line widths of the designator laser (DS) and the local oscillator (LO) determine their coherence lengths and thus the possible coverage or range and filter bandwidth.

Commercial erbium-fibre lasers with lambda = 1.55 Mm, which are safe for the eye, have bandwidths B below 10 kHz in the multiwatt range and below 1 kflz in the lower power range. The possible coverage or range R for SAL application is R = c/B, where c = 3 x 108 rn/s. With B = 10 kHz, this produces a range R of 30 km.

The requirements with respect to the wavelength stability of the designator and LO can be distinctly reduced by the simultaneous use of two closely adjacent laser wavelengths f1 and f2 with defined spacing f in the designator.

In the SAL seeker-head detector in the case of super-heterodyne reception two intermediate frequencies f and fz2 that are shifted by the LO frequency fLo are produced with = f1 -f and fz2 = f2 -fLo. These are superimposed in an HF mixer and the differential frequency f -fz2 is produced. This amounts to precisely f and is independent of the absolute values of f1 and f2, the LO frequency and the Doppler shift of f1 and f2 caused by movement.

The wavelength stabilization of f and fLo need merely be effected within the detector bandwidth (for example in the range up to 10 GHz) . The useful signal or signal content can be filtered in a very narrow band with the line width of the laser radiation.

It is to be noted that in the case of this method the amplitude of the designator radiation is modulated with f in a narrow band.

With a scanner, in the seeker, the detector field of vision (IFOV) scans the field of vision of the seeker (FOV) in seek mode. The detector field of vision amounts to, for example, 1 mrad, the seeker field of vision 100.

This then results in 3 x 1O4 pixels per scan.

With a scanning period for the seeker field of vision of 101 s, a frequency of 10 "images" per second results.

During the scanning period for 1 pixel, a sufficiently large number of signal photons are received for the discovery of the target.

After a target has been discovered in seek mode, the seeker locks onto this and tracks it -possibly in the first instance with microscan. After locking onto the target with an assumed stabilization precision of the seeker of < 1 mrad, a signal from the target is then permanently received.

One simple possibility of realizing a scanning seeker head is, in accordance with Figure 3, a rosette scanner with two rotating wedge plates. The transmission of the signal and the coupling-in of the LO laser radiation can then be effected in an uncritical manner in terms of adjustment by means of light-guide components, for example by means of a photonic crystal fibre with large NA and large core diameter and an X fibre coupler.

The realization of the light guide as an erbium-doped fibre-laser amplifier (EDFA) is proposed as an advantageous further development of the invention. In this connection, the input-coupling fibre is realized as an optical amplifier. Erbium-doped fibre lasers (EDFAs) are used, for example, as amplification stages in glass-fibre transmission links.

As a result of the use of an EDFA as an optical pre-amplifier, the power demand on the local oscillator is distinctly reduced, that is, in accordance with the amplification factor of the EDFA.

The same applies to the use of avalanche diodes as the detector.

Claims

Claims: 1. A target-detection method in which a target object is

illuminated by means of a laser designator and the radiation reflected by the target is detected and evaluated by means of a seeker, characterjsed in that coherent radiation (5') is generated by means of the designator (DS) for the purposes of illumination, and the radiation (S) that is reflected by the target is coherently mixed in the seeker in one or more detectors, and in that the differential signal Lhat is obtained in tho downstream detector (D) and lies in the frequency range between 0 and 10 GHz is filtered in a narrow band and processed further to obtain information.
2. A target-seeking method according to claim 1, in which the seeker includes a local oscillator (LO) for mixing the radiation coherently.
3. A target-seeking method according to claim 1 or 2, in which the coherent radiation that is generated by the designator is so-called continuous-wave (CW) or modulated CW radiation.
4. A target-seeking method according to any preceding claim, in which the designator radiation is modulated for the purpose of conveying information to the seeker.
5. A target-seeking method according to any preceding claim, in which in the designator (DS) two closely adjacent laser wavelengths f1 and f2 are simultaneously produced with defined spacing f, and as a result of a coherent mixing process two intermediate frequencies f1 and f2 shifted by the LO frequency fLo are produced in the seeker-detector D, with f = f1-f0 and fz2 = f2-fLo, and are superimposed in an HF mixer, and produce the differential frequency fzl-fz2 that amounts to precisely f and is independent of the absolute values of f1 and f2, the LO frequency and any Doppler shift of f1 and f2 caused by movement.
6. A target-seeking method according to any preceding claim, in which the seeker includes an optical scanning device with which the field of vision of a detector

unit scans the field of vision of the seeker.
7. A target-seeking method according to claim 6, in which a rosette scanner with two rotating wedge plates is used as the optical scanning device.
8. A target-seeking method according to any preceding claim, in which the transmission of the signal in the seeker from the optics as far as the detector is effected in a manner insensitive to mounting tolerances by means of light-guide components, for example by means of a photonic crystal fibre with large NA and large core diameter and an X fibre coupler.
9. A target-seeking method according to any preceding claim, in which fibre amplifiers are used for optical signal amplification.
10. A target-seeking method according to any preceding claim, in which avalanche diodes or PIN diodes are used as detectors.
11. A target-seeking method according to any preceding claim, in which the wavelengths of the designator (DS) and/or of the local oscillator (LO) are stabilized absolutely by suitable measures, for example the use of reference etalons (wavelength lock).
12. A target-seeking method according to any preceding claim, in which the evaluation of the target determines its type, location and/or movement.
13. A device for carrying out a method according to any preceding claim, having a designator (DS) to apply coherent radiation (S') to a target, a receiver for the radiation CS) reflected by the target, a local oscillator to produce spatially and temporally coherent radiation (LO), a mixing device to bring the two lots of radiation (S, LO) together and obtain a low-frequency differential signal, and an electronic signal-processing device.
14. A method or apparatus substantially as described herein with reference to the attached drawings.