Optical transmitting and receiving system

A method and apparatus for transmitting and receiving optical signals in a bi-directional transmission system (1) using a common transmission medium (3). Laser radiation at the first wave length and intensity is directed from a laser source (4) to a non-linear optical device (6). The non-linear optical device (6) is responsive to radiation at the first wavelength and intensity is directed from a laser source (4) to non-linear optical device (6). The non-linear optical device (6) is responsive to radiation at the first wavelength and intensity to emit radiation of a second wavelength which is transmitted through the medium (3). The non-linear optical device (6) transmits incident radiation of the second wavelength have less than a selected intensity. A selective mirror (5) is interposed between the source (4) and non-linear optical device 6 to pass radiation of the first wavelength from the source (4) and directed towards a target (7) radiation of a second wavelength passing from the medium through the non-linear optical device (6) in the opposite direction to radiation from the source (4).

FIELD OF THE INVENTION
 This invention relates to an optical transmitting and receiving system for
 use in bi-directional transmission in a common transmission medium. In the
 present context, a common transmission medium refers to the common volume
 of any suitable medium, such as air, space or an optical fibre which is
 used to transmit optical signals in opposite directions at the same time.
 BACKGROUND ART
 In many optical applications, it is desirable to utilise the same optical
 path for signals travelling in opposite directions. The advantages that
 flow from this configuration include a reduction in the amount of optical
 alignment required, and where the path is through a dedicated medium such
 as an optical fibre, a reduction in the amount of medium required.
 Prior attempts to achieve bi-directional transmission of optical signals,
 have utilised laser sources operating through partially reflective mirrors
 or, in the case of transmission through optical fibres, directional
 couplers. In both cases, the losses incurred by the use of the partially
 reflective mirrors and the directional couplers respectively have lowered
 the efficiency of operation.
 DISCLOSURE OF THE INVENTION
 It is an object of this invention to provide an improved optical
 transmitting and receiving system and an improved method for
 bi-directional transmission of optical signals in a common transmission
 medium.
 Accordingly, the present invention provides an optical transmitting and
 receiving system for a bi-directional transmission system using a common
 transmission medium, said transmitting and receiving system including a
 source of a laser radiation of a first wavelength and intensity,
 non-linear means responsive to radiation at said first wavelength and
 intensity to emit radiation of a second wavelength, said non-linear means
 transmitting incident radiation of said second wavelength having less than
 a selected intensity, selective reflecting means interposed between said
 source and said non-linear means to pass radiation of said first
 wavelength from said source and reflect toward a target radiation of said
 second wavelength passing through the non-linear means and travelling in
 the opposite direction to radiation from said source.
 Preferably, the non-linear means is an optical parametric oscillator (OPO).
 The active material of such an oscillator can be chosen to have a
 threshold intensity for incoming radiation below which conversion to the
 other wavelength will essentially not occur.
 The optical transmitting and receiving system of this invention has
 particular application to fibre optic communications systems. The
 radiation generated by the non-linear means at the second frequency is
 launched into an optical fibre. At the other end of a typical fibre, the
 signal will be much weaker and the intensity will be less than the chosen
 operating threshold of the non-linear device. The signal will thus pass
 through the non-linear device and be reflected to the detector.
 Preferably, the selective reflecting means includes a dichroic mirror which
 reflects radiation of the second wavelength to a target in the form of a
 suitable detector.
 The optical transmitting and receiving system also preferably further
 includes means to launch the radiation of said second wavelength emitted
 by the non-linear means for transmission through a medium to a like
 optical transmitting and receiving system, and means to direct an optical
 signal from said like optical transmitting and receiving system through
 said non-linear means to said reflecting means.
 The medium used for transmission can be any suitable propagating medium
 such as air, space or an optical fibre. Where required, adjustable
 attenuation can be provided for the radiation of said second wavelength
 emitted from said non-linear means to allow the intensity level of the
 optical signal reaching the like optical transmitting and receiving system
 to be reduced below said selected intensity.
 Preferably, the laser source is a Nd:YAG laser operating at 1.06 .mu.m. The
 non-linear means preferably converts the laser emission to 1.54 .mu.m. In
 this form, the invention allows free space laser communications at the eye
 safe frequency of 1.54 .mu.m, operating bi-directionally in the same
 space. This has the significant advantage of avoiding the need for
 alignment of multiple optical apertures.

BEST MODE FOR CARRYING OUT THE INVENTION
 Referring to the drawing, the laser communication system 1 comprises two
 optical transmitting and receiving systems 2, which respectively launch
 signals for transmission through a medium 3. Each of the systems 2
 comprises an Nd:YAG laser 4 appropriately pumped by a flash lamp or laser
 diode in the conventional manner to produce a laser emission at a
 wavelength of 1.06 .mu.m. A dichroic mirror 5 is interposed between the
 laser 4 and a non-linear optical device 6 in the form of an OPO. The
 dichroic mirror 5 passes radiation at wavelengths around 1.06 .mu.m, but
 is highly reflective at wavelengths around 1.5 .mu.m. The laser radiation
 emitted by laser 4 passes through dichroic mirror 5 to OPO 6. The
 intensity of the incident laser radiation is above the threshold intensity
 for the OPO, which converts the incident radiation to radiation at a
 wavelength of 1.5 .mu.m for launching into the transmission medium 3. Any
 suitable form of optics can be used for launching the optical signal for
 transmission according to the type of medium.
 The signals transmitted through the medium by each of the transmitting and
 receiving systems 2 travel along the same optical path and are thus
 received at the other of the transmitting and receiving systems. If the
 launch intensity of the signal is appropriately chosen, the transmission
 of the signal through the medium reduces the intensity of the radiation to
 below the operational threshold of the OPO. The incoming optical signal
 thus passes through the OPO substantially unchanged and is reflected by
 dichroic mirror 5 to a detector 7 in each case. It will be apparent that a
 practical laser transmission system would include suitable modulating
 equipment associated with each laser 4 and demodulating equipment
 associated with each detector 7.