Optical system for laser doppler homodyne detection

Abstract

Abstract 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.

Inventors

• 👤 Hans W. Mocker

Bibliographic Details