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A laser Doppler homodyne detection system includes transmitting-receiver optics including a roof reflector and a twin off-axis Cassegrain reflector system. BACKGROUND OF THE INVENTION With the development of laser sources with good frequency stability, optical systems for Doppler homodyne detecti

A laser Doppler homodyne detection system includes transmitting-receiver optics including a roof reflector and a twin off-axis Cassegrain reflector system. BACKGROUND OF THE INVENTION With the development of laser sources with good frequency stability, optical systems for Doppler homodyne detection have become possible. Laser Doppler systems are especially useful for flow measurement, true air speed measurement, vortex detection, moving target indication, and remote atmospheric measurements. In a typical laser Doppler homodyne system, the laser beam is expanded in diameter and is directed at a target. The radiation back reflected and scattered by the target is received by receiver optics and directed to a detector. A portion of the laser beam is split off prior to transmission and forms the local oscillator beam. This local oscillator beam is also directed to the detector to produce Doppler homodyne signal. One form of laser Doppler system is shown in R. Munoz et al., Applied Optics, 13, 2890 (Dec, 1974). This type of system is termed a "co-linear off-axis system". In this system, the laser beam is split by a beam splitter into the output and local oscillator beams. The output beam is directed to an off-axis Cassegrain telescope which provides beam expansion. The returning beam is reduced by the same Cassegrain telescope and is directed to a detector. The local oscillator beam is combined with the returning beam by means of a beam splitter. An important disadvantage of the co-linear transmitter-receiver system is the inherent 6 db signal loss. This signal loss is due to the 50% beam splitter which is shared by the output and returning beams. On-axis systems (systems in which the beam expander optics are located on-axis rather than off-axis) have additional disadvantages. These on-axis systems, which typically use a Cassegrain telescope, can reflect a large amount of laser energy under variable phase and frequency back into the laser and thus broaden the frequency spectrum of the laser. The performance of the system, therefore, is degraded. One system which overcomes the 6 db signal loss problem uses a plane mirror having a hole through it. Both sides of the plane mirror are reflecting surfaces. The laser beam is expanded by beam expanding optics and the expanded beam is directed to the plane mirror, which is oriented at 45° to the direction of propagation of the expanded beam. Most of the beam is reflected by the first surface of the mirror and is directed to a cube corner reflector. The cube corner reflector reflects the beam to the target. A small portion of the beam, however, passes through the hole to become the local oscillator beam. The returning beam from the target is reflected off the second surface of the mirror and is directed along a common path with the local oscillator beam. The returning beam and the local oscillator beam pass through beam reducing lenses which focus the returning beam and the local oscillator beam on to the detector. The disadvantages of this system are that it requires on-axis beam expanding optics. The on-axis arrangement results in back reflection which can degrade laser performance. In addition, the system uses several lenses to expand the output beam and to reduce the returning beam. When an infrared laser such as a CO 2 laser is used, these lenses can be very expensive. SUMMARY OF THE INVENTION The optical system of the present invention overcomes many of the problems associated with prior art laser Doppler homodyne detection systems. The optical system of the present invention includes light source means, roof reflector means, transmitter means, receiver means, and detector means. The light source means produces a light beam. A first reflecting surface of the roof reflector means separates the light beam into first and second portions. The transmitter means receives the first portion from the first reflecting surface, expands the beam diameter of the first portion, and transmits the expanded first portion. The receiver means receives incident radiation and directs the incident radiation to a second reflecting surface of the roof reflector means. The detector means receives the incident radiation from the second reflecting surface and receives the second portion of the light beam. Detector means produces a signal which is indicative of the second portion of the light beam and the incident radiation.

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1. An optical system comprising: light source means for producing a light beam; roof reflector means having first and second reflecting surfaces, the first reflecting surface for separating the light beam into an output beam and a local oscillator beam, and the second reflecting surface for refl

1. An optical system comprising: light source means for producing a light beam; roof reflector means having first and second reflecting surfaces, the first reflecting surface for separating the light beam into an output beam and a local oscillator beam, and the second reflecting surface for reflecting incident radiation; transmitter means positioned to receive the output beam from the first reflecting surface, the transmitter means for expanding the beam diameter of the output beam and transmitting the output beam; receiver means for receiving incident radiation and directing the incident radiation to the second reflecting surface; and detector means positioned to receive the local oscillator beam and to receive the incident radiation from the second reflecting surface, the detector means for producing a signal indicative of the local oscillator beam and the incident radiation. 2. The optical system of claim 1 wherein the second reflecting surface directs the incident radiation to the detector means along an essentially common path with the local oscillator beam. 3. The optical system of claim 2 wherein the roof reflector means includes an opening through which the local oscillator beam passes. 4. The optical system of claim 3 and further comprising attenuator means positioned in the opening for attenuating the local oscillator beam. 5. The optical system of claim 4 wherein the attenuator means is a Brewster attenuator. 6. The optical system of claim 1 wherein the roof reflector means is positioned to allow the local oscillator beam to pass the roof reflector means without being reflected by the first and second reflective surfaces. 7. The optical system of claim 1 wherein the transmitter means collimates the output beam after expanding the beam diameter of the output beam. 8. The optical system of claim 7 wherein the transmitter means comprises: third reflector means for diverging the output beam; and fourth reflector means for receiving the output beam from the third reflector means and for collimating the output beam. 9. The optical system of claim 8 wherein the third reflector means receives the output beam from the first reflector means. 10. The optical system of claim 9 wherein the third and fourth reflector means form a Cassegrain reflector system. 11. The optical system of claim 10 wherein the third reflector means has an essentially spherical reflecting surface. 12. The optical system of claim 8 wherein the fourth reflector means has an essentially parabolic reflecting surface. 13. The optical system of claim 1 wherein the receiver means converges the incident radiation and collimates the converged incident radiation. 14. The optical system of claim 13 wherein the receiver means comprises: fifth reflector means for converging the incident radiation; and sixth reflector means for receiving the incident radiation from the fifth reflector means and for collimating the incident radiation. 15. The optical system of claim 14 wherein the sixth reflector means directs the incident radiation to second reflecting surface. 16. The optical system of claim 15 wherein the fifth and sixth reflector means comprise a Cassegrain reflector system. 17. The optical system of claim 16 wherein the fifth reflector means has an essentially parabolic reflective surface and the sixth reflector means has an essentially spherical reflective surface. 18. The optical system of claim 14 wherein the transmitter means comprises: third reflector means for diverging the output beam; and fourth reflector means for receiving the output beam from the third reflector means and for collimating the output beam. 19. The optical system of claim 18 wherein the third reflector means receives the output beam from the first reflecting surface. 20. The optical system of claim 19 wherein the sixth reflector means directs the incident radiation to second reflecting surface. 21. The optical system of claim 20 wherein the fourth and fifth reflector means comprise portions of a first common reflector. 22. The optical system of claim 21 wherein the third and sixth reflector means comprise portions of a second common reflector. 23. The optical system of claim 22 wherein the first common reflector has an essentially parabolic reflective surface and the second common reflector has an essentially spherical reflective surface. 24. An optical system comprising: light source means for producing a light beam; roof reflector means having first and second nonparallel reflecting surfaces, the first reflecting surface for separating the light beam into first and second portions, and the second reflecting surface for reflecting incident radiation; beam expander means for expanding the beam diameter of the first portion; beam reducer means for reducing the beam diameter of incident radiation; and detector means for receiving the incident radiation and the second portion of the light beam and for producing a signal in response to the incident radiation and the second portion. 25. The optical system of claim 24 wherein the beam expander means comprises: third reflector means for diverging the first portion; and fourth reflector means for receiving the first portion from the third reflector means and for collimating the first portion. 26. The optical system of claim 25 wherein the third reflector means receives the first portion from the first reflecting surface. 27. The optical system of claim 26 wherein the beam reducer means comprises: fifth reflector means for converging the incident radiation; and sixth reflector means for receiving the incident radiation from the fifth reflector means and for collimating the incident radiation. 28. The optical system of claim 27 wherein the sixth reflector means directs the incident radiation to the second reflecting surface. 29. The optical system of claim 28 wherein the fourth and fifth reflector means comprise portions of a first common reflector. 30. The optical system of claim 29 wherein the third and sixth reflector means comprise portions of a second common reflector. 31. The optical system of claim 30 wherein the first common reflector has an essentially parabolic reflective surface and the second common reflector has an essentially spherical reflective surface. 32. The optical system of claim 31 wherein the roof reflector means is positioned to allow the second portion of the light beam to pass the roof reflector means without being reflected by the first and second reflective surfaces. 33. The optical system of claim 32 wherein the roof reflector means defines an opening through which the second portion passes. 34. The optical system of claim 30 wherein the first common reflector defines an aperture. 35. The optical system of claim 34 wherein the first portion is directed from the first reflective surface through the aperture to the third reflector means and wherein the incident radiation is directed from the sixth reflector means through the aperture to the second reflective surface. 36. The optical system of claim 34 wherein the roof reflector means is positioned in the aperture. 37. The optical system of claim 24 wherein the first and second reflecting surfaces are essentially perpendicular to one another. 38. In an optical system having light source means for producing a light beam and detector means for receiving a local oscillator beam and incident radiation and for producing a signal in response to the local oscillator beam and the incident radiation, a transmitter-receiver system comprising: first reflector means for separating the light beam into an output beam and the local oscillator beam; second reflector means for reflecting incident radiation to the detector means; third reflector means positioned to receive the output beam, the third reflector means for diverging the output beam; fourth reflector means positioned to receive the output beam from the third reflector means, the fourth reflector means for collimating the output beam; fifth reflector means for converging the incident radiation; and sixth reflector means positioned to receive the incident radiation from the fifth reflector means, the sixth reflector means for collimating the incident radiation and directing the incident radiation to the second reflector means.