WO1991003888A1

WO1991003888A1 - Remote site optical transponder communication and tracking system using a laser aerial

Info

Publication number: WO1991003888A1
Application number: PCT/AU1990/000378
Authority: WO
Inventors: John Leonard Hughes
Original assignee: Australian Electro Optics Pty. Ltd.
Priority date: 1989-08-31
Filing date: 1990-08-29
Publication date: 1991-03-21

Abstract

This invention relates to a remote site optical transponder system capable of detecting weak laser beam signals (1), amplifying them, impressing on them stored information and re-directing the said laser beam signals directly back to their source, said system consisting of a wide angle, 360° ''fly-eye'' optical detector aperture (2) formed by an array of micro lenses, each micro lens within said arrays being connected to the core of a single mode optical fiber (3), said optical fibers being bundled so that their free ends form an array (4), itself coupled to a focusing lens system via a second array of micro lens (5), said laser beam signals being focussed by said lens (6) onto a second lens (7) so that the said laser signal beam enters a diode pumped, fiber coupled (10) laser gain medium (9) which amplifies said laser signal after which it is reflected off a mirror (16) back to its source along its path of incidence being also amplified on its return path through the said laser gain medium. The invention has applications in the defence and optical communications fields where information has to be transmitted to a remote site.

Description

Remote Site Optical Transponder Communication and Tracking System Using a Laser Aerial

Field of the Invention

This invention relates to a remote site optical transponder system capable of detecting weak laser beam signals, amplifying them, impressing on them stored information and re directing the said laser beam signals directly back to their source, said system consisting of a wide angle, 360° "fly-eye" optical detector aperture formed by an array of micro lenses, each micro lens within said array being connected to the core of a single mode optical fiber, said optical fibers being bundled so that their free ends form an o array, itself coupled to a focusing lens system via a second array of micro lens, said laser beam signals being focussed by said lens onto a second lens so that the said laser signal beam enters a diode pumped, fiber coupled laser gain medium which amplifies said laser signal after which it is reflected off a mirror back to its source i s along its path of incidence being also amplified on its return path through the said laser gain medium. The invention has applications in the defence and optical communications fields where information has to be transmitted to a remote site.

Summary of the Prior Art

2o Prior art optical transponders pioneered by the inventor (see

US Patent No. 4,058,774, issued November 15, 1977) did not teach the art of a wide angle laser signal collection/transmission which was in the form of a flexible laser aerial, and randomly positioned along line of sight relative to the source of laser beam. The present invention overcomes defects of the prior art by coupling an array of micro lenses to the laser gain medium via a bundle of single mode optical fibers. Furthermore, the present invention also allows for impressing information directly onto the laser signal being amplified via direct optical switching of the diode pump light.

Background of the Invention

Laser beam communication systems have been extensively developed over the past thirty years using both directed ι o atmospheric transmission and via optical fibers. A problem exists in the case of direct atmospheric transmissions the narrow beamwidth of laser beams have been fully utilised to pin point a direct line of sight path between two points. However, this prior art optical communications technology relied on the laser beam

15 transmitter and optical detector being fixed relative to each other any relative movement between the two leading to a loss of signal once the beam began to over lap the detector and a complete lack of signal as soon as the beam failed to hit the detector. The need for this invention arose in battlefield communications where the foot

2o soldier or a military vehicle such as a troop carrier or a tank moved over rough terrain at speed so that direct line of sight contact between a fixed transmitter and fixed detector became impossible. The present invention provides for a wide angle detector system which can be orientated at any angle relative to a fixed laser beam

25 transmitter and yet remain in direct line of sight contact with the said transmitter. The invention allows for either a foot soldier or a tank on which it is mounted to traverse very rough terrain whilst remaining in full communications contact with a laser beam transmitter which is remotely sighted.

Summary of the Invention It is an object of the invention to provide an array of optical detectors which can provide direct line of sight contact with a laser beam communication link from a remote site despite the fact that individual detectors within the said detector array do not continuously intersect the said transmitted beam. Another object of the invention is to provide an array of optical detectors thought a 360° field of view in azimuth and up to 180° in elevation.

It is an object of the invention to provide an array of optical detectors of relatively large light collection area in the form of micro lenses each of which concentrates the light signal collected so that it can be directed along the core of a single mode optical fiber.

Another object of the invention is to bundle the free ends of the said optical fibers together so as to form a single optically polished aperture whose outputs can be further concentrated via a lens system so that any transmitted signal can be directed through a laser gain medium prior to being reflected off a mirror to retrace its return journey to the source from which it was transmitted. It is an object of the invention to provide means of sampling the collected laser signal and decoding its information content. Another object of the invention is to code information into the amplified laser beam by modulating the diode pumps used to excite the laser gain medium.

It is an object of the invention to provide a series of micro lens arrays along a flexible mount so as to provide a flexible laser aerial.

Brief Description of the Drawings

A better understanding of the invention may be obtained from the following considerations taken in conjunction with the ι o drawings which are not meant to limit the scope of the invention in any way.

Figure 1 shows a schematic layout of the invention with a micro lens array arranged to collect signals over 360° in azimuth and 180° in elevation. The collected signal is then channelled into

15 a single aperture and focussed into a laser gain medium which is diode pumped said diode bump array being capable of being modulated into communication frequency rates to as to impress a message onto the collected laser beam as it is returned to its source.

2o Figure 2 shows a schematic layout of the invention with a series of micro lens arrays mounted into a flexible stem to form a laser aerial.

Detailed Description of the Invention

In Figure 1, numeral 1 indicates both the incident and return 25 paths of a laser beam signal processed by the invention. Numeral 2 indicates a curved array of micro lenses a lens arrangement sometimes referred to as a "fly-eye" array. Each micro lens in the array 2 is connected to a single mode optical fiber, the bundle of such fibers being indicated by numeral 3. Numeral 4 indicates the array of fiber ends whilst numeral 5 indicates a second micro lens array which collimates the laser beam signal passing through any portion of array 4 onto the lens indicated by numeral 6 which in turn directs the laser beam signal into the second lens indicated by numeral 7 which collimates said laser signal beam and directs it ι o along the path indicated by numeral 8 in a laser gain medium indicated by numeral 9 which is fiber coupled, as indicated by numeral 10 to the laser diode array indicated by numeral 11 which, in turn, is connected to a supply indicated by numeral 12. Power supply 12 is connected to a system control indicated by numeral 13

15 which is connected to a computer indicated by numeral 14 which can be updated via the link indicated by numeral 15.

Numeral 16 indicates a phase conjugate mirror as known in the art which directs the laser beam signal beam to its source along its path of incidence, said signal being amplified twice in the laser

2o gain medium in the process. The invention can also operate with conventional mirrors.

In Figure 2, numeral 17 indicates a laser aerial with a series of micro lens apertures indicated by numeral 18 connected to the optical fiber bundle indicated by numeral 19.

25 The invention has application in the detection, amplification and the modulation of an incident laser beam signal at a remote site and directing it back to its source. Furthermore, this can be - 6

achieved for most orientation of the invention relative to one or more sources of the incident laser beam either sequentially or simultaneously. The invention can be activated using a trigger pulse sequence in the incident laser beam. The invention has application in the laser beam identification and tracking of vehicles onto which it is mounted, independently of the orientation of the said vehicle to the said laser beam source or laser beam sources as the case may be.

Furthermore, the incident laser beam can be used to both ι o deliver and receive information from the vehicle or object upon which the invention is mounted because it is a relatively simple process to use an optical diode to detect the message on the incoming pulse.

The invention has application in air, sea or space rescue

15 because a weak laser beam signal incident from any direction on the invention will be amplified and coded to identify the object on which it is mounted at distances normally beyond visible observation.

The invention can also be used to monitor the properties of

2o objects on which it is mounted which are themselves mounted in inaccessible or hostile environments.

It should also be noted that the invention can have a series of optical detectors along a laser aerial (Figure 2) allowing for a vision format, high data rate optical communication system under

25 battle field conditions.

The line of sight between the laser beam source and the- invention can be direct, via a land, sea or air borne vehicle or via earth satellites.

Claims

I Claim,

1. A remotely sited laser beam detection system which collects, decodes, codes and amplifies a laser beam signal before returning it directly to its source, said system consisting of: (a) An array of micro lens distributed so as to collect an incoming signal up to 360° in azimuth and up to 180° in elevation, said micro lens within the said array then focusing their position of the detected optical signal into the cores of single mode optical fibers. ι o (b) A bundle of single mode optical fibers which transmit the laser beam signal collected by the micro lens array via a common aperture formed by packing together all of the optically polished free ends of said fibers.

(c) A telescope to couple the output of said fiber bundle into a 15 laser gain medium.

(d) A diode pumped laser gain medium whose pump light can be modulated so as to impress a signal into the light being amplified within said gain medium.

(e) A mirror to re direct the detected optical signal back to 20 its source along its path of incidence.

(f) A means of sampling a scattered portion of the incident signal so as to decode the incoming signal as the need arises.

2. A system as claimed in Claim 1 with micro lens arrays mounted in sections along a flexible mount to act as a flexible

25 laser aerial.