Multiple channel acousto-optic modulators

#The invention relates to the field of optics and more particularly to acousto-optic modulators and systems for their use.

#Acousto-optic modulators (AOMs) are often used to split and/or modulate a laser beam. The basic principle of AOMs is that a transparent medium subject to a stress undergoes a change in the index of refraction. When the stress is high frequency sound waves, such as might be produced by a piezoelectric transducer, the change in refraction is periodic. The periodic refraction pattern can act as a diffraction grating producing what is called Bragg diffraction. The undiffracted beam is called the zeroth order beam and the primary diffracted beam is called the first order beam.

#Amplitude modulation of laser beams is another requirement for many applications. The amount of light diffracted in an AOM is a function of the acoustic power being applied and can, therefore, be modulated by varying the applied power. Generation of a second modulated beam from one original incident beam using prior art techniques requires a second AOM in series with the first. The second modulated beam created this way may vary significantly from the first modulated beam in intensity or size and may contain an admixture of the modulation characteristics of the first modulated beam.

#FIG. 1 is a block diagram of a system using a two channel AOM according to the invention.

#FIG. 2 illustrates the control functions for a two channel AOM according to the invention.

#FIG. 3 illustrates a prior art crystal for use in an AOM.

#Since conventional AOMs are typically designed to operate with essentially single wavelength light, the material for the crystals are selected based on the wavelength. Materials are known which are suitable for use over the wavelength span from 0.2 to over 20 microns. Selection of the material for the MCAOMs of the invention is made according to these known principles. This implies that MCAOMs may be constructed for any wavelength for which crystals can be found for AOMs. The number of channels for which an MCAOM can be designed is limited only by the requirements of the geometry of the acoustic fields in the crystal as will be shown. The invention will first be described in a dual channel embodiment.

#FIG. 1 illustrates a block diagram of a DCAOM according to the invention. Laser 11 produces a beam 16 which is directed to crystal 12. The laser may be of any type, pulsed or continuous. The wavelength of the light must be one at which the crystal is transparent. Crystal 12 has transducers 13a, 13b mounted on opposing faces for producing acoustic waves from electrical signals. The transducers are typically piezoelectric devices which are attached to the crystal by adhesive or mechanical means. Each transducer is controlled by it own modulation signal designated as A and B. Both amplitude and frequency modulation of electrical signals driving the transducers are possible using standard techniques. A single zeroth order beam 15 emerges from the crystal. When neither transducer is activated the zeroth order beam 15 is the same as the incident beam. Activating transducer 13a causes a portion of the incident beam to be diffracted into beam 14a which is a first order beam which will be called the A1-beam. Likewise activating transducer 13b causes a portion of the incident beam to be diffracted into the B1-beam 14b which is also a first order beam. The zeroth order beam or residual beam will always exist at some level of amplitude even when the typical maximum diffraction of 80-90% is achieved.

#Although it is possible and desirable for some applications to operate a MCAOM with more than one transducer being driven at a time, the energy in the resulting beams will necessarily be reduced from the maximum possible from the incident beam, since diffracted beams are fractional portions of the incident beam. If only one transducer is allowed to be activated at a time, the diffracted beam can be driven to its maximum level. When operated in this mode an MCAOM can produce a set of pulsed first order beams which are maximally similar and have 80-90% of the intensity of the incident beam and are thus superior for some applications to a set of beams produced by prior art techniques. Note also that the beam size for each first order beam can be made essentially identical, since the path for each beam is extremely similar. It should also be noted that although prior art AOMs have two output beams, i.e., a zeroth order beam and 1st order beam, which have similar paths and beam sizes, they are not well matched in amplitude due to fact that 100% diffraction is not possible.