Intensity Noise Reduction Methods and Apparatus for Interferometric Sensing and Imaging Systems

Abstract

In part, aspects of the invention relate to methods, apparatus, and systems for intensity and/or pattern line noise reduction in a data collection system such as an optical coherence tomography system that uses an electromagnetic radiation source and interferometric principles. In one embodiment, the noise is intensity noise or line pattern noise and the source is a laser such as a swept laser. One or more attenuators responsive to one or more control signals can be used in conjunction with an analog or digital feedback network in one embodiment.

What is claimed is: 
     
         1 .- 29 . (canceled) 
     
     
         30 . A method of reducing or suppressing noise in an interferometric imaging system, the method comprising:
 combining light scattered from a sample with light from a reference reflector, wherein the combined light comprises a plurality of frequencies;   directing the combined light to one or more photodetectors,   generating a plurality of photocurrents, using the one or more photodetectors, in response to the combined light; and   applying a first control signal to an attenuator in communication with the one or more photodetectors;   attenuating at least one photocurrent of the plurality of photocurrents in response to the first control signal, wherein attenuation of at least one photocurrent reduces or suppresses noise.   
     
     
         31 . The method of  claim 30 , wherein the one or more photorecievers is a first photoreciever and a second photoreciever. 
     
     
         32 . The method of  claim 31  further comprising actively balancing the first photoreciever and the second photoreciever. 
     
     
         33 . The method of  claim 30  further comprising the step of generating the first control signal by filtering an amplified signal, integrating the filtered amplified signal, and amplifying the integrated filtered amplified signal. 
     
     
         34 . The method of  claim 30  wherein attenuation of at least one of the plurality of photocurrents is changed in response to the first control signal being received by an attenuator selected from the group of a photoreceiver, a transistor, a diode, a resistor, a variable optical attenuator, a variable gain amplifier, a pair of resistors in electrical communication with a pair of transistors, and combinations thereof. 
     
     
         35 . The method of  claim 30  further comprising actively balancing a first photo receiver and a second photoreceiver by repeating the step of transmitting the first control signal until the laser noise component reaches a minimum. 
     
     
         36 . The method of  claim 30  wherein the imaging system is a frequency-domain optical coherence tomography system. 
     
     
         37 . The method of  claim 30  wherein the light is generated using a laser, wherein the first control signal is generated using a laser intensity noise component. 
     
     
         38 . The method of  claim 37  wherein the laser intensity noise component is within a frequency range of from about 1 MHz to about 250 MHz. 
     
     
         39 . The method of  claim 30  wherein the plurality of frequencies received at the one or more photodetectors corresponds to differences between light traveling from the sample and the reference reflector. 
     
     
         40 . The method of  claim 30  further comprising generating a second control signal to control attenuation of at least one of the plurality of photocurrents. 
     
     
         41 . An imaging system comprising
 a first photoreceiver for receiving light from an interferometer;   a first attenuator in communication with the first photoreciever, wherein the first attenuator is configured to change a first electrical property of the first attenuator in response to a first control signal;   a second photo receiver for receiving light from the interferometer;   a second attenuator in communication with the second photoreciever;   an amplifier having a first input in electrical communication with the first photoreciever and a first output, the amplifier configured to transmit, from the first output, an amplified signal comprising a noise component; and   a feedback network in electrical communication with the amplifier, the feedback network configured to receive the amplified signal and generate the first control signal and the second control signal.   
     
     
         42 . The system of  claim 40  wherein the feedback network comprises (a) a filter that transmits frequencies below an interference signal band, (b) an integrator in electric communication with the filter, and (c) a pair of rectifying amplifiers in electrical communication with integrator. 
     
     
         43 . The system of  claim 41  wherein the feedback network comprises (a) a bandpass filter that transmits frequencies either below or above an interference signal band, (b) an RMS detector in electrical communication with the bandpass filter, (c) an analog-to-digital converter in electrical communication with the RMS detector, (d) a microprocessor in electrical communication with the analog-to-digital converter, and (e) a digital-to-analog converter in electrical communication with the microprocessor. 
     
     
         44 . The system of  claim 41  wherein the feedback network is configured to transmit one of the first or second control signals using the digital-to-analog converter to maintain a null in an RMS noise signal received at the RMS detector. 
     
     
         45 . The system of  claim 41  wherein the feedback network is configured to operate synchronously with a sweep period of the laser. 
     
     
         46 . The system of  claim 45  wherein the feedback network computes the first control signal during a non-imaging cycle of the interferometer and transmits the first control signal during an imaging cycle to reduce or suppress the noise component. 
     
     
         47 . The system of  claim 41  wherein light emitted by the laser is amplitude modulated at a fixed frequency to create a first signal in a reference arm of the interferometer and wherein a second signal at an output of the system is demodulated to generate a phase-sensitive error signal that is processed by the feedback network. 
     
     
         48 . The system of  claim 41  wherein the first attenuator is a first transistor and the second attenuator is a second transistor, wherein the first and second transistors are configured to differentially shunt photocurrents at the first input of the amplifier in response to the first and second control signals. 
     
     
         49 . The system of  claim 41  wherein the first attenuator is a first variable-gain amplifier having an input and an output and wherein the second attenuator is a second variable-gain amplifier having an input and an output.