Aeronautics and astronautics – Missile stabilization or trajectory control – Automatic guidance
Reexamination Certificate
1977-08-22
2001-03-13
Moskowitz, Nelson (Department: 3662)
Aeronautics and astronautics
Missile stabilization or trajectory control
Automatic guidance
C244S003130, C250S341100, C356S152200, C089S041060, C089S041220
Reexamination Certificate
active
06199794
ABSTRACT:
A multi-color, multi-pulse laser operating at a number of wavelengths is provided with a like number of cavities to produce a like number of output pulses from a single pumping pulse. The spacing between the pulses is easily controlled by controlling the timing of Q-switches, each in a different cavity, so that information is imparted by the pulse spacing. In a two color embodiment, a beam splitting device is positioned along the longitudinal axis of the laser medium to produce two beams which are directed into different cavity resonators, each tuned to a different wavelength. Reflecting surfaces positioned at the end of each cavity redirect each beam back to the beam splitting device where the beams are spatially recombined and subsequently two pulses, one of one color and the other of the second color, are sequentially coupled out of the laser system. The colors may be sufficiently close in wavelength such that detection does not discriminate against the two colors, and a double pulse output is obtained at the detector. In this manner, the laser system produces two output pulses from a single laser medium and a single pumping pulse. The pulse width of each of the pulses may also be controlled by pulse stretching means in one or more of the optical paths.
FIELD OF THE INVENTION
This invention relates to laser systems, and more particularly to a laser system for providing a multiple light-pulse output in which interpulse spacing and/or pulse width are independently controlled for laser target designation.
BACKGROUND OF THE INVENTION
In military operations involving the designating and destroying of a target, laser systems are used to illuminate a target. This light is reradiated in all directions by the target and is used by a missile for homing in on the target. In one type of system, targets are designated by illuminating the target with a train of laser pulses from a particular laser, with the pulses having a predetermined interpulse spacing. A typical method of countermeasuring this laser target illumination is by detecting the pulses at the target, determining their pulse repetition frequency (PRF) or pulse interval modulation (PIM) and producing like laser pulses which are aimed at a reflective object removed from the target.
In an effort to prevent the countermeasuring of the illuminating laser, the subject invention involves a new type laser which is modulated to produce pulse pairs or doublets which cannot be easily detected and reproduced by the target. In order to be effective the interpulse spacing between the pulses of the doublet must be small and the spacing must be very accurate since it is extremely difficult to detect the fact that the laser radiation is pulsed when pulse spacings are very small. Moreover, even if the double laser pulses are detected, it is extremely difficult to generate like laser pulses because it is difficult to duplicate exactly the small interpulse spacings. Therefore the combatants who have apparatus for producing double pulse laser beams with extremely short accurately controlled interpulse spacings will have a virtually uncountermeasurable system.
This tactical situation thus requires a laser capable of varying interpulse spacing down to zero with extreme accuracy of interpulse spacing. Moreover, this tactical situation requires that pulse spacing be easily varied so that the illuminating laser can quickly change its output waveform to avoid detection.
Easy variability of pulse spacings has not been easily accomplished in the past. Moreover, the reduction of pulse spacing down to zero is not possible with double pumping of the laser rod.
In the prior art, techniques for obtaining multiple output pulses can be divided into two classes; (1) systems in which the separation between pulses is directly proportional to the time required to restore the population inversion in the lasing medium and (2) those system which are independent of the restoration time of the lasing medium. Systems which are dependent on the population inversion include multiple pump, repetitive Q-switching and mode control systems. An example of the multiple pump system is illustrated in U.S. Pat. No. 3,783,403. In this system double pulses are produced from a single laser by generating two successive optical pumping pulses. However, pulse spacing cannot be decreased to zero in this system.
An example of a laser system which is independent of the restoration time is described in an article by M. J. Landry, entitled “Variably Spaced Giant Pulses for Multiple Laser Cavities In a Single Lasing Medium”,
Applied Physics Letter,
Vol. 18, pages 494-496. In this method, a prism is used to effectively split the laser rod longitudinally into two rods. This requires a rod of relatively large cross section. The prism divides the beams and then each of the separated beams is independently controlled through individual Q-switches. However, in the Landry system only one wavelength or color is utilized and the stress birefringent effect to be described hereinafter is not used.
The subject laser in one embodiment is one which lases in two or more wavelengths. The colors result from radiative transitions which can occur in two different ways: (1) from a single high energy level to multiple different lower energy levels or (2) from multiple different upper energy levels to a single low energy level. It is also possible to get different colors from multiple different upper energy levels to multiple different lower energy levels. The situation will be discussed later in connection with Er:YLF lasers. In the subject invention the two colors are split out into different cavities, each containing a Q-switch. The Q-switches are activated sequentially with a predetermined delay such that light of one color is coupled back through the laser rod to produce a first pulse, while the activation of the second Q-switch couples light of the other color back through the laser rod to produce a second pulse. Pulse spacing is controlled by the delay in activation of the second Q-switch. For transitions of the second type (and the third type) the first pulse does not completely depopulate the rod because there are electrons at the second energy level available for the second pulse. This sytem is useable with ruby rods and some Nd:YLF rods.
When, however, there are two transitions both starting from the same excited level, stress birefringence induced by the pumping of the rod may play a major part in permitting the production of two pulses. When two transitions start from the same excited level, it is possible that the production of the first pulse will completely depopulate the laser rod making two pulse production impossible. In some instances this can be prevented by very rapid Q-switching. However, stress birefringence provides simpler, more effective way of preventing complete depopulation during the production of the first pulse.
Stress birefringence occurs because some of the energy pumped into the laser rod is converted to heat which changes the characteristics of the rod such that two beams are produced each having a different polarization. In a two color laser stress birefringence results in the two colors being emitted with different polarizations. When the beams are separated into two differently polarized beams by a polarizing beam splitter then one polarized beam only partially depopulates the rod for the first pulse, thereby leaving enough of a population inversion for the formation of the second pulse. The Q-switching of the second cavity then can couple the remaining energy out of the rod to produce the second pulse.
It is a finding of the present invention that stress birefringence when the laser is pumped produces beams having orthogonal polarizations. The beams are separated according to polarization and each are Q-switched at different times to produce multiple pulses. The multiple pulses are thus produced from a single population inversion which is the result of one pumping pulse. Each polarity beam selectively depopulates the laser host material so that only a portion of the rod
Naiman Charles S.
Pompian Stuart D.
Gomes David W.
Moskowitz Nelson
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