Method and apparatus for controlling a fiber optic phased...

Optical: systems and elements – Optical amplifier – Beam combination or separation

Reexamination Certificate

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C372S029011

Reexamination Certificate

active

06813069

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to laser systems and more particularly to control systems and methods for controlling the phase of optical signals output by laser systems by utilizing frequency shifting techniques.
BACKGROUND OF THE INVENTION
Lasers are presently employed for a wide variety of applications. For example, lasers are employed to process materials, such as by cutting, welding, heat treating, drilling, trimming and coating materials, stripping paint, removing coatings, cleaning surfaces, and providing laser markings. Lasers are also used in many medical applications for precision surgery. Additionally, lasers are used in military applications, including laser weapon and laser ranging systems. Laser communication systems have also been developed in which laser signals are transmitted in a predetermined format to transmit data.
Along with the ever increasing number of applications in which lasers are used, the demands on the laser systems are also ever increasing. For example, a number of applications, including military, materials processing, medical, and communications applications, demand continuous wave lasers which emit increasingly higher power levels. In addition, a number of applications demand that the laser system produce an output beam which is of high quality, such as by being diffraction limited and/or exhibiting predominantly or entirely fundamental or TEM
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mode characteristics. Accordingly, the output beam can be more definitely focused to achieve higher brightness. At the same time, many applications require that the laser system produce an output beam which is adaptable or dynamically controllable.
One example of the need for high power, high quality laser beams is illustrated by the laser devices used to focus on remote targets. In these applications, it is advantageous for the laser beam to achieve a maximum brightness at the location of the target. For example, in military applications, it is advantageous to generate a laser beam that is focused on the remote target with maximum intensity. Similarly, in medical applications, it is essential that the laser beam be focused on the target tissue such that surrounding tissue is not affected.
Several different types of laser devices that generate high power laser beams have been developed by The Boeing Company, assignee of the present application. Examples of these laser device are discussed in detail in U.S. Pat. No. 5,694,408 to Bott et al. and U.S. Pat. No. 5,832,006 to Rice et al., the contents of which are incorporated herein by reference.
The basic approach of these laser devices is to amplify a coherent signal emitted from a master oscillator using a phased array of fiber optic amplifiers. A sample of the output optical signal is extracted for comparison to a reference laser beam that has also typically been output by the master oscillator. The sample of the output optical signal and the reference signal are combined by interference, and the interference signal is sampled by an array of detectors. The difference in phase between the sample of the output optical signal and the reference signal is recorded by the detector, and is used as feedback for altering the phase modulation of the output optical signal via an array of phase modulators that are in optical communication with respective fiber optic amplifiers.
In one example, it may be desired that the plurality of output optical signals be capable of being combined into a diffraction limited signal, thereby requiring that the output optical signals emitted by the fiber optic amplifiers have a constant phase front. Unfortunately, optical path disturbances including those attributable to variations in the optical path length may differently affect the elements of the phased array, thereby requiring independent modulation of the optical signals propagating through the respective fiber optic amplifiers in a manner that is capable of being varied over time as the optical path disturbances vary. These optical path disturbances may be due, for example, to platform shock and vibration, turn-on transients and pumping power fluctuations. Moreover, these optical path disturbances may be many wavelengths and, in some instances, thousands of wavelengths in magnitude and may occur very quickly so that a wide control bandwidth is required.
While a flat phase front is often desirable, some applications will require other types of phase fronts. For example, in one application, a reference beam is initially transmitted to a target of interest. By monitoring the reflection of the beam, atmospheric turbulence in the path of the output laser beam may be detected. To counteract this turbulence, the laser device may desirably alter the phase of the signals emitted by the various fiber optic amplifiers such that the output laser beam has a wavefront that compensates for the atmospheric turbulence.
To provide the desired phase front, the laser devices described by U.S. Pat. Nos. 5,694,408 and 5,832,006 have a feedback loop and an array of phase modulators that control the phase modulation of the output laser beam. Specifically, as discussed, a portion of the output laser beam is combined through interference with a reference signal to determine the phase difference for the signals emitted by each fiber optic amplifier. By use of the feedback signal representative of the phase of the output laser beam and knowledge of the desired wavefront, the output laser beam can be generally controlled via the array of phase modulators to produce the desired wavefront and/or to appropriately steer or tilt the wavefront.
Although these laser systems, for the most part, provide reliable and accurate control of the output laser beam, U.S. Pat. No. 6,233,085 to Bartley C. Johnson, the contents of which are also incorporated by reference herein, describes the feedback loop and the associated array of phase modulators in more detail. In this regard, the control methodology described by U.S. Pat. No. 6,233,085 patent can provide for a wide range of phase modulation by avoiding saturation and uncontrolled modulation changes in the output signal.
Although generally effective, the control methodologies described heretofore in conjunction with fiber optic phased arrays have typically been implemented utilizing digital signal processing techniques. In some applications, such as those applications potentially involving military combat, the reliability of digital signal processing techniques is still questioned. As such, it would be desirable to develop a control system and method for providing wide band phase control for the output optical signals of a fiber optic phased array utilizing conventional analog electronics.
SUMMARY OF THE INVENTION
A fiber optic phased array, as well as associated methods and apparatus for controllably adjusting the frequency of the optical signals emitted by a fiber optic phased array, are provided that permit wide band phase control and may be implemented, if desired, utilizing conventional analog electronics. In this regard, the method and apparatus of the present invention can independently control the phase of the optical signals propagating through each fiber optic amplifier of a fiber optic phased array, even in instances in which optical phase disturbances that are many wavelengths, and perhaps thousands of wavelengths, in magnitude occur, such as due to platform shock and vibration, turn-on transients, pumping power fluctuations and the like. As such, the control method and apparatus of the present invention permit a fiber optic phased array to generate a flat phase front that, in turn, can provide a diffraction limited output laser beam. Alternatively, the control method and apparatus may be designed such that the output signals emitted by an array of fiber optic amplifiers has any other desired phase front, such as to compensate for atmospheric perturbations or the like. Since the control method and apparatus of the present invention are capable of being implemented by conventional analog electronics, the control method a

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