Systems, methods and apparatus for discrete-time coherent ranging

Abstract

1. A coherent ranging system comprising: an electromagnetic radiation source; a detector; and an optical system comprising a sample arm and a reference arm; wherein the electromagnetic radiation source is configured to generate a plurality of light pulses and send the plurality of light pulses toward the sample arm and the reference arm; and wherein the detector is configured to receive electromagnetic radiation returned from the sample arm and the reference arm and to generate a plurality of values ​​based on the optical energy of each of the plurality of light pulses of the electromagnetic radiation returned from the sample arm and the reference arm, each value of the plurality of values ​​corresponding to a respective light pulse of the plurality of light pulses.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application is based on and claims priority to U.S. Patent Application No. 63 / 481,829 (filed January 27, 2023), the disclosure of which is incorporated herein by reference.

[0002] <Statement Regarding Federally Sponsored Research> none. [Background technology]

[0003] Coherent optical ranging uses interferometry to measure sample position information and is used in many fields, including lidar, inspection, and imaging (e.g., optical coherence tomography). Traditionally, coherent ranging systems have used light sources that emit electromagnetic fields with either (i) wavelengths that do not change with time (e.g., broadband light sources) or (ii) wavelengths that change continuously with time (e.g., wavelength-swept lasers). However, current systems designed for use with these light sources have deficiencies when used with light sources that emit electromagnetic radiation fields with discontinuous tuning of wavelength, otherwise known as "discrete-time tuning." Summary of the Invention

[0004] Therefore, new systems, methods and media for discrete-time coherent ranging are desired.

[0005] In one aspect, a coherent ranging system is provided, the system comprising: an electromagnetic radiation source, a detector, and an optical system having a sample arm and a reference arm, the electromagnetic radiation source configured to generate and transmit a plurality of light pulses toward the sample arm and the reference arm, the detector configured to receive electromagnetic radiation returned from the sample arm and the reference arm, and generate a plurality of values ​​based on optical energy of each of the plurality of light pulses of the electromagnetic radiation returned from the sample arm and the reference arm, each value of the plurality of values ​​corresponding to a respective light pulse of the plurality of light pulses.

[0006] In another aspect, a method for coherent ranging is provided, the method including providing an electromagnetic radiation source, a detector, and an optical system having a sample arm and a reference arm, generating a plurality of light pulses using the electromagnetic radiation source and directing the plurality of light pulses toward the sample arm and the reference arm, receiving electromagnetic radiation returned from the sample arm and the reference arm using the detector, and generating a plurality of values ​​using a processor coupled to the detector based on optical energy of each of the plurality of light pulses of the electromagnetic radiation returned from the sample arm and the reference arm, each value of the plurality of values ​​corresponding to a respective light pulse of the plurality of light pulses.

[0007] A more complete understanding of the various objects, features and advantages of the presently disclosed subject matter can be obtained by considering the following detailed description of the presently disclosed subject matter in conjunction with the following drawings, in which like reference numerals refer to like elements. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 illustrates an example discrete-time coherent ranging system of the present disclosure. [Figure 2] 2 illustrates an example discrete-time optical signal generated by the electromagnetic radiation source of FIG. 1. [Figure 3]2A-2C illustrate various example pulse shapes of a discrete-time optical signal generated by the electromagnetic radiation source of FIG. 1. [Figure 4A-4B] 2 illustrates an example of the application of further discrete-time modulation to the optical signal after it has been generated by the electromagnetic radiation source of FIG. 1. [Figure 4C-4D] 2 illustrates an example of the application of further discrete-time modulation to the optical signal after it has been generated by the electromagnetic radiation source of FIG. 1. [Figure 5] FIG. 10 illustrates an alternative configuration of optical mixers and receivers that provides polarization-diverse detection. [Figure 6] 6A and 6B show the optical signal before optical detection in a balanced optical receiver configuration, e.g., the optical signal at the upper optical output of the balanced detector of FIG. 1 (FIG. 6A) and the optical signal at the lower optical output of the balanced detector of FIG. 1 (FIG. 6B), before detection by the balanced detector. [Figure 7] FIG. 1 illustrates crosstalk between adjacent pulses generated using a conventional receiver design in a discrete-time coherent ranging system. [Figure 8] FIG. 1 illustrates the output of a discrete-time coherent ranging system. [Figure 9] 9A and 9B show an input pulse 900a (FIG. 9A) and an output pulse 902a (FIG. 9B) in a coherent ranging system with a discrete-time receiver using Nyquist pulses. [Figure 10] FIG. 1 illustrates an embodiment in which crosstalk in the early digital sampling can be removed by deconvolution. [Figures 11A-11B] FIG. 11A is a system diagram of a system with a photodetector having integrate, hold, and dump functions, and FIG. 11B is a sample input timing diagram. [Figure 11C] 11B is a sample output timing diagram of the system of FIG. 11A. [Figure 12]FIG. 1 illustrates a discrete-time coherent ranging system with a timing clock for synchronizing / coordinated control of the operation of one or more elements of the discrete-time coherent ranging system. [Figure 13A] FIG. 1 illustrates a discrete-time coherent ranging system in which an optical receiver deinterleaves output data pulses onto at least two different output optical paths. [Figure 13B] FIG. 13B illustrates deinterleaving of the optical receivers of the discrete-time coherent ranging system of FIG. 13A. [Figure 14] FIG. 1 illustrates a procedure for reducing or eliminating unwanted signals caused by reflections from components such as lenses. [Figure 15] FIG. 1 illustrates an implementation of a discrete-time coherent ranging system with 32 separate optical channels coupled to a laparoscope. [Figure 15A] FIG. 16 is a diagram showing a continuation of FIG. [Figure 16] FIG. 16A illustrates a light source used with a discrete-time coherent ranging system implementing a phase-code mode-locked (PCML) laser, and FIG. 16B illustrates example discrete output pulses generated by the light source at wavelengths λ, λ, . . . λ. [Figure 17] FIG. 1 illustrates one implementation of a passive polarization modulator. [Figure 18] FIG. 16 illustrates phase modulation using one implementation of an active (eg, controlled by a voltage-driven electro-optic modulator) phase and polarization modulator that can be inserted into the configuration of FIG. [Figure 19] FIG. 16 illustrates polarization modulation using one implementation of an active (eg, voltage-driven electro-optical modulator) phase and polarization modulator that can be inserted into the configuration of FIG. [Figure 20] FIG. 1 shows a polarization modulator implementation of a Sagnac interferometer that generates and polarizes a pulse train (top left) after it has been sent through phase modulator PM1 to generate pulses of multiple different wavelengths λ1 and λ2, different polarities Rx and Ry, and different phases I and Q, shown in the bottom right. [Figure 21]17A-17C show phase and polarization encoded pulses at the end of the sample arm (top row) and phase and polarization encoded pulses at the end of the reference arm (bottom row), where the sample arm encoding is achieved by the modulator shown in FIGS. 18-20 and the reference arm encoding is achieved by the passive polarization modulator shown in FIG. 17. [Figure 22] FIG. 1 shows a photograph of a fingertip / fingernail imaged using the discrete-time coherent ranging system of the present disclosure (left) and a structural OCT microphotograph generated using the image (right). [Figure 23] Figure 1 shows images of a phantom generated by imaging multiple pieces of tape (top row). On the left, the phantom represents the tape pieces using structural OCT, and on the right, the phantom represents the tape pieces using polarization-sensitive OCT. [Figure 24] FIG. 22 illustrates a phase encoding scheme for the phase and polarization modulator described in FIGS. DETAILED DESCRIPTION OF THE INVENTION

[0009] In some embodiments of the present disclosure, mechanisms (which may include systems, methods, and devices) are provided for discrete-time coherent ranging.

[0010] Coherent ranging based on discrete-time wavelength-stepped light sources offers numerous advantages. For example, when implemented in a circular ranging configuration, such light sources can achieve higher speeds and longer depth ranges for a given electronic acquisition bandwidth. In such systems, the discrete wavelength steps mean that the optical signal incident on the receiver is in discrete-time format. However, current discrete-time coherent ranging systems are based on receivers configured and optimized for continuous-time signals, and the lack of such properly designed receivers can contribute to performance and cost penalties. Therefore, novel receiver and / or light source configurations are needed to fully realize the technical and performance advantages of discrete-time coherent ranging. Here, "coherent ranging" refers to interferometry, including optical coherence tomography (OCT). Also included are optical frequency domain ranging, optical frequency domain reflectometry (OFDR), any other frequency modulated laser based optical detection and ranging method, and LIDAR, if based on the interference of coherent light.

[0011] In conventional swept-source-based coherent ranging systems, the output optical signal of the system is in the form of an analog / continuous-time signal in which the depth position in the sample is encoded by the modulation frequency content of the output signal. As a result, the optical receiver used in this type of system, which is responsible for optical detection and digital conversion of the output signal, is designed to function as an RF spectrum analyzer. The optical receiver itself follows several well-known principles. Given that signal detection is based on the frequency content of the signal, the sampling clock is set so that the measured RF frequency content in the signal is below the Nyquist frequency (equal to half the sampling clock frequency). This is most commonly achieved by placing an electronic anti-aliasing or low-pass filter in the detection / measurement system to attenuate signals above the Nyquist frequency. Modern OCT techniques using pulsed light sources, such as circular ranging (Lippok et al., Optics Letters 47, 1903-1906 (2022), the entire contents of which are incorporated herein by reference for all purposes), also follow the same principles, specifically treating the signal as encoded in the RF spectrum over a specific bandwidth and consequently using a receiver designed to capture that bandwidth accurately without electronic aliasing.

[0012] This disclosure discloses a coherent ranging system based on a pulsed light source and a system using a discrete-time optical receiver. Unlike the conventional systems described above, the output optical signal in this system is a discrete-time train of pulses derived from a train of independent, time-separated pulses returned from the sample to the optical receiver. The optical receiver is thus dedicated to generating a train of digital measurements, each digital measurement being based on the optical energy of each pulse returned from the sample, with the measured energy being associated with only one of the returned sample pulses (i.e., the digital measurement of a particular pulse returned from the sample is not substantially contaminated by the energy of adjacent pulses returned from the sample). The optical receiver is not configured to perform RF spectral analysis when generating the digital measurement sequence, and in fact is configured to avoid such an objective. This is because the optical receiver in the disclosed system is configured such that the RF spectral content of the signal sent to the analog-to-digital converter is above the Nyquist frequency associated with the digital stream output rate (i.e., half the digital stream output rate). As a result, embodiments of the disclosed systems are configured to induce electronic aliasing in a specific manner so that each time-separated pulse can be measured accurately and independently of its neighbors.

[0013] In various embodiments, the present disclosure provides an optical receiver for a discrete-time coherent ranging system. The present disclosure also provides apparatus and methods for phase and polarization modulation of an optical signal, which can be used with a discrete-time optical receiver, including the optical receiver of the present disclosure. The present disclosure also provides apparatus, methods, and systems for reducing unwanted back-reflected signals, which can be used with a discrete-time optical receiver, including the system of the present disclosure.

[0014] In some embodiments of the present disclosure, a discrete-time optical signal is a signal whose optical properties, including wavelength, phase, or polarization, vary discontinuously over time, but the optical properties are not limited to the above examples. Generally, a discrete-time optical source generates output in the form of a train of multiple distinct pulses that are distinguishable from one another in time. Furthermore, the discrete-time pulses can be emitted at regular intervals, and the pulse train can include a pulse at each predetermined time interval ( FIG. 2A ), and / or some time intervals in the pulse train can contain no pulses ( FIG. 2B ). Furthermore, the discrete-time pulses can span the entire predetermined time interval (e.g., FIG. 2A , “100% duty cycle”), or alternatively, can span only a portion of each time interval (e.g., FIG. 8 , left panel), resulting in a duty cycle of less than 100%. In some embodiments, the duty cycle of the pulses can be approximately 10%, 20%, 25%, 30%, 40%, 50%, 75%, 90%, or any other suitable percentage.

[0015] An example of a discrete-time optical signal is a wavelength-stepped frequency comb source, whose output is discretely stepped through the spectral lines of a frequency comb. This contrasts with a wavelength-swept source, which produces an output wavelength that varies continuously over time; as the term "swept" suggests, wavelength-swept sources differ from the "stepped" output of comb and related sources. For example, circular ranging (CR) optical coherence tomography can use a source that discretely steps wavelengths through the spectral lines of an optical frequency comb and skips wavelengths inside (between) those lines.

[0016] In certain embodiments, the disclosed optical receiver design embodiments can be applicable to a wide range of discrete-time encodings, i.e., patterns of wavelength, phase, polarization, or other optical parameters are modulated in a discrete-time fashion, and thus the specific encodings are provided to illustrate the optical receiver and are examples of specific coherent ranging methods, not limitations of the present invention.

[0017] In certain embodiments, the disclosed optical receivers and associated procedures can be used to convert discrete-time coherent ranging optical signals into signals such as digital signals (as described in more detail below, the term "digital signal" as used herein can refer to a value based on the optical energy of the measured output signal). Thus, the described optical receivers can be used in a variety of coherent ranging system architectures, which may include, for example, an electromagnetic radiation source, an interferometer, an optical mixer configuration, an optical receiver, and / or a modulator. The various system configurations disclosed herein are examples of how the described optical receivers can be used and are not intended to limit the scope of the invention.

[0018] It should be noted that in this disclosure and the embodiments illustrated therein, the terms "discrete-time signal" and "continuous-time signal" refer to a coding format applied to an optical or electrical signal, consistent with the use of the terms in the communications field. For optical signals, the coding may be of amplitude, phase, wavelength, polarization, or at least one other optical parameter; for electronic signals, the coding may be of voltage, current, or at least one other suitable electrical parameter.

[0019] However, while the disclosed procedure may be similar to work in other fields, such as communications, there are a number of differences that make the present work unique. For example, the present procedure requires a method for reducing crosstalk to levels significantly lower than those required in communications systems. Furthermore, while the transmission channel of a communications system (which may be several meters or kilometers long) introduces unknown distortions into the pulses, in the disclosed system the transmission channel is relatively short (e.g., one meter or less) and its perimeter can be controlled / shielded, so that the pulse arriving at the receiver can be assumed to have the same shape as the pulse emitted by the light source.

[0020] Furthermore, in communications systems, the "clock" (i.e., frequency and phase of the received pulses) must be derived from the pulse train using clock recovery circuitry, whereas in the disclosed system, an electrical connection (see, e.g., element 1200 in FIG. 12) can be used to directly / physically synchronize the receiver clock to the source clock. Furthermore, the disclosed procedure allows the wavelength characteristics of the pulses to be used to help reduce intersymbol interference (ISI), as shown, for example, in FIG. 13, whereas in communications, a given channel is assumed to have a consistent wavelength and therefore pulses cannot be distinguished based on wavelength.

[0021] Finally, while communication systems require maximizing spectral efficiency to make the best use of available channels, the disclosed device does not have such a constraint. Thus, the disclosed system can implement pulses that are shorter (sometimes much shorter) than the pulse spacing (see, e.g., FIG. 3B). In communication systems, using pulses shorter than the spacing ΔT results in the spectral linewidth of the channel being larger than the nominal required width, thereby limiting the density of spectral channels that can be packed into a communication line. This is not a problem in the disclosed system, and in fact, short pulses can be advantageous insofar as they can be used to filter out unwanted reflections (FIG. 14).

[0022] 1 illustrates one embodiment of an example discrete-time coherent ranging system. Electromagnetic radiation source 100 generates a discrete-time optical signal 170, which is sent to optical beam splitter 105. Beam splitter 105 splits optical signal 170 into a sample arm optical signal 171a and a reference arm optical signal 171b. In the sample arm, another beam splitter 109a is used to send the sample arm light to optical path 172a, which sends it to sample 140. Reflection or light scattering from sample 140 is collected by optical path 172a, which can be an optical fiber, an optical waveguide, or a free-space optical path. A portion of this reflected light is sent along optical path 173a to mixer 115, or a mixing optical coupler. The reference beam is similarly manipulated and sent to path 173b rather than to sample 140, but using reflector or mirror 145. Mixer 115 is an optical coupler with a nominal 50% coupling that combines optical fields 173a and 173b to generate interference signals at respective outputs 174a and 174b of the optical coupler. These interference signals 174a and 174b are the optical outputs of the coherent ranging system and are sent to photodetectors within balanced detector 125. Balanced detector 125 has two inputs that receive signals 174a and 174b, converts the optical energy at each photodetector (labeled "+" and "-"), and subtracts the electrical signals to remove optical intensity noise.

[0023] Balanced detector 125 generates an electrical output signal 175 that is sent to an analog-to-digital converter (ADC) 130. ADC 130 generates a digital signal 176 that is sent to processing system 135. Processing system 135 determines sample position information using any known method for analyzing the digital signal and provides output 177. This may include, for example, a Fourier transform, and the algorithms of processing system 135 are generally configured according to the optical encoding of discrete-time optical signal 170.

[0024] The electromagnetic radiation source 100 can be, for example, a frequency comb stretched pulse mode-locked (SPML) laser, a frequency comb phase code mode-locked (PCML) laser, or an integrated photonic frequency comb laser, and can be any optical source technology that provides such a discrete-time optical signal.

[0025] The optical paths shown in the system of FIG. 1 may be implemented using free-space optics, or alternatively may be based on waveguides such as optical fibers or integrated photonic waveguides.

[0026] 2A and 2B illustrate an example discrete-time optical signal, such as signal 170, generated by electromagnetic radiation source 100 of FIG. 1. In FIG. 2A, the discrete-time optical signal is characterized by a sequence of non-overlapping optical pulses. Three consecutive pulses, 201a, 201b, and 201c, are labeled. The time interval between pulses is ΔT, which is consistent for all pulses. FIG. 2B illustrates a discrete-time optical signal characterized by null pulses, i.e., time slot 221b contains no pulses, while adjacent time slots 221a and 221c contain pulses. In FIG. 2B, the discrete-time optical source can provide an agile signal in which, for some periods 226, each time slot separated by ΔT contains a pulse, while for other periods 225, some time slots may contain no pulses. In this manner, the discrete-time optical source can provide pulses spaced apart by NΔT, where N is a positive integer that may vary during operation of the optical source.

[0027] 3A, 3B, and 3C illustrate various embodiments of various pulse shapes for the discrete-time optical signal 170 generated by the electromagnetic radiation source 100. In each of FIGS. 3A and 3B, the pulse amplitude drops to zero at the boundaries between pulses. In FIG. 3A, the peak pulse power is twice the average power of the pulse in the time slot of duration ΔT that contains the pulse. In FIG. 3B, the peak pulse power is four times the average power of the pulse in the time slot of duration ΔT that contains the pulse. The amplitude of the pulse shown in FIG. 3C does not drop to zero at the edges of the pulse. In general, the amplitude of the discrete-time optical pulses can be in either a return-to-zero (RZ) format (FIGS. 3A and 3B) or a non-return-to-zero (NRZ) format (FIG. 3C). In the RZ format, the ratio of peak power to average power in the time slot can be anything greater than 1.0, including, for example, values ​​between 2 and 10 or any value greater than 1.0, depending on the design of the electromagnetic radiation source. This pulse amplitude shaping can be used in combination with specific photoreceiver configurations described below to optimize signal acquisition performance. The electromagnetic radiation source can be configured to inherently provide a specific pulse shape, or an amplitude modulator can be added within the electromagnetic radiation source to provide additional shaping functionality to the pulse. The amplitude modulator can be implemented using any of a number of technologies known to those skilled in the art, such as lithium niobate or other electro-optic intensity modulators or current-modulated semiconductor optical amplifiers.

[0028] In some embodiments, it is useful to apply further discrete-time modulation to the optical signal after it is generated by light source 100. This further discrete-time modulation is achieved by passing the incident discrete-time optical signal 400 (FIGS. 4A and 4B) through an optical modulator 402, which generates an additionally modulated output discrete-time signal 401. The modulator 402 can be driven by an electrical signal 403 that generates a discrete-time encoding synchronized with the arrival of the discrete-time optical pulses at the modulator. FIG. 4C (panel 420a) shows the power and phase of the output discrete-time optical signal when the modulator 402 is a phase modulator configured to modulate the phase by 90° between adjacent pulses. The phase modulator can be an electro-optic phase modulator, such as a lithium niobate phase modulator. FIG. 4D (panel 420b) shows the power and polarization of the output discrete-time optical signal when the modulator 402 is a polarization modulator configured to modulate polarization between horizontal (H) and vertical (V) states. The polarization modulator may be an electro-optic polarization modulator, such as a lithium niobate phase modulator. Modulator 402 may be placed before the interferometer (e.g., before beam splitter 105, 171a or 173a in the sample arm, or 171b or 173b in the reference arm). Multiple modulators may be incorporated into the system to apply complex discrete-time modulation to radiation 170 or a radiation field derived from radiation 170. Other optical modulator embodiments are shown in Figures 17-20 and are described below.

[0029] FIG. 5 illustrates an alternative configuration of optical mixers and receivers that provides polarization-diverse detection. Here, optical field 1011a corresponds to optical field 174a, and 1011b corresponds to optical field 174b. Optical device 1012 is a nominally 50% coupling optical coupler. Output interference signals 1013a and 1013b are coupled into polarizing beam splitters 1014a and 1014b to generate horizontally polarized optical signals 1015a and 1015d and vertically polarized optical signals 1015b and 1015c. These signals are connected to balanced optical receivers 1016a and 1016b, where balanced receiver 1016a generates electrical signal 1017a representing the sample response for horizontal polarization, and balanced receiver 1016b generates electrical signal 1017c representing the sample response for vertical polarization. These signals are coupled into digitizers 1018a and 1018b to generate digital signals 1019a and 1019b. Thus, shown in Figure 5 are additional optical mixer configurations that can be included in the discrete-time optical receiver designs described below. Additional coherent ranging mixer designs, such as phase diversity (in-phase and quadrature detection), can also be used.

[0030] 6A and 6B illustrate optical signals before detection by balanced detector 125 in a balanced optical receiver configuration, such as the optical signal at optical output 174a (FIG. 6A) and optical signal at optical output 174b (FIG. 6B) of the balanced detector of FIG. 1. This disclosure provides devices, methods, and systems for generating a digital signal 176 including a sequence of digital values ​​1010, 1011, and 1012, where digital value 1010 is proportional to the optical energy of pulse 1100a integrated from Ta to Tb (e.g., integrated over the duration of the pulse) minus the optical energy of pulse 1100b integrated from Ta to Tb. Digital value 1011 is similarly proportional to the optical energy of pulse 1101a integrated from Tb to Tc minus the optical energy of pulse 1101b integrated from Tb to Tc. Importantly, procedures are provided to maximize correlation of digital value 1011 to the difference between the optical energy of pulse 1101a and the optical energy of pulse 1101b as described above, while minimizing the influence of digital value 1011 from the optical energy of preceding and succeeding pulses 1100a, 1100b, 1102a, and 1102b, as described in more detail below. Thus, the present disclosure identifies a photoreceiver design specifically configured for discrete-time signals, such that information about each pulse (as a digital value) can be measured without crosstalk from adjacent pulses. The term "digital value" or "digital value" herein refers to a signal measured and digitized by a detector in a discrete-time system based on the optical energy of the detected pulse, as opposed to detectors used in known coherent ranging systems that generate spectral measurements based on the RF spectrum of the pulse.

[0031] 7A and 7B illustrate the crosstalk between adjacent pulses generated using a conventional optical receiver design in a discrete-time coherent ranging system. A set of RZ optical pulses 700a (FIG. 7A) is incident on a photodetector, such as 125. Each of the three pulses shown (which are excerpts from a longer pulse train, the illustration of which is omitted for clarity and convenience) is designated 705a, 705b, and 705c. These pulses are non-overlapping and are spaced apart by a distance ΔT. Traditionally, the information contained in this pulse train is captured entirely within the RF frequency range from DC to 1 / (2ΔT) according to the Nyquist theorem. Therefore, conventional optical receiver designs digitize the signal at twice this frequency, i.e., the sampling rate 1 / ΔT, and use an analog low-pass filter with a corner frequency near the Nyquist frequency 1 / (2ΔT). However, filtering the pulses with such a low-pass filter results in a significant elongation of the time domain 702a (FIG. 7B), resulting in substantial pulse overlap and high crosstalk. For example, measurement Db is heavily influenced by the wave tail of the response to pulse 705a. Thus, discrete-time coherent ranging requires measuring each pulse independently, which necessitates the use of multiple different receiver designs. Various embodiments of such receiver designs are presented below.

[0032] 8A and 8B show the output of a discrete-time coherent ranging system configured with an electromagnetic radiation source having an analog bandwidth substantially greater than the Nyquist value 1 / (2ΔT) and providing RZ pulses 800a (FIG. 8A), where the pulse width of the RZ pulses 800a is substantially shorter than the pulse interval, or equivalently, the peak power is at least twice the average power of the pulses. The resulting electrical output 802a (FIG. 8B) of a detector (such as the balanced detector 125 of FIG. 1) has pulses with a shorter temporal spread, e.g., the digital sampling Db (the sampling time of the output pulse corresponding to the input pulse 805b) is not affected by the response of pulses 805a and 805c. Thus, a receiver design with an analog bandwidth greater than 1 / (2ΔT) and RZ optical pulses shorter than the pulse interval can be used to eliminate or mitigate crosstalk in a discrete-time coherent ranging system. In one embodiment, a system with a ΔT=10 ns pulse spacing and a 4 ns pulse width and an analog bandwidth of 100 MHz (twice 1 / (2ΔT)) can be used to reduce crosstalk. In another embodiment, to reduce crosstalk (also known as "inter-symbol interference (ISI)") without compromising the signal-to-noise ratio (SNR), a receiver low-pass filter with a corner frequency in the range of 1.2 / (2ΔT) to 1.5 / (2ΔT) can be used. For pulses much shorter than the pulse repetition time, the receiver bandwidth can be relaxed (increased) to find the optimal low-pass filter that cancels pulse overlap (ISI) without compromising the SNR. The exact discrete-time receiver bandwidth is system-specific (pulse shape, pulse repetition rate, and type of low-pass filter) and must be determined for each individual system. It should be noted that conventional approaches to coherent ranging use a light source with a nearly constant power, and having such a light source with long periods of no optical power is generally considered suboptimal due to light loss at such times or the need for higher peak power to maintain a particular average power.

[0033] 9A and 9B illustrate input pulse 900a (FIG. 9A) and output pulse 902a (FIG. 9B) in a coherent ranging system with a discrete-time optical receiver using Nyquist pulses. (Note that a "Nyquist pulse" is a shaped pulse that differs from the Nyquist frequency reference above.) FIG. 8 illustrates an embodiment for suppressing crosstalk by limiting the temporal response of the photodetector system to less than the pulse interval. FIGS. 9A and 9B illustrate an alternative approach, in which the photodetector allows the pulses to extend beyond the interval between pulses, but the photodetector filter response and the shape of the input optical pulse are configured so that the resulting response at preceding and succeeding times is zero. As a result, each output value determined by the detector is based on the optical energy of each pulse, with minimal contribution from adjacent pulses. A train of optical pulses, shown as 900a (FIG. 9A), is incident on a photodetector, such as the balanced detector 125 of FIG. 1. Three pulses in the long pulse train are shown as 905a, 905b, and 905c, with a pulse separation of ΔT. The photodetector is configured (FIG. 9B) with an analog bandwidth and filter response such that pulse 905a produces the electronic response shown at 902a in FIG. 9B. Note that although the temporal response exceeds ΔT, sampling at times 903, 904, 905, and 906 results in a maximum response at sampling time 904 and zero response at 905. The responses at 903 and 906 are similarly zero. That is, the pulses emitted by the light source are set short enough so that the output pulse striking the detector is short relative to the detector's response time, and the detector's response to a given pulse accurately measures that pulse but does not affect the readings of its neighboring pulses. This allows for the use of a lower analog bandwidth for better noise suppression without introducing crosstalk, such as that observed in the example of FIG. 8.

[0034] 10A and 10B illustrate an embodiment in which crosstalk in the initial digital sampling can be removed after digital conversion using a deconvolution algorithm implemented on a processing system, such as processing system 135 of FIG. 1. The calculation is based in part on a characterization of the detector's response pattern to an incident pulse, allowing deconvolution to be used to effectively cancel the effects of the system response to adjacent pulses. Photodetector 125 (panel 1400a, FIG. 10A) is hit by a train of optical pulses 1403a, 1404a, 1405a, and 1406a. Due to the analog bandwidth / system response of the detection system, each pulse in this train of optical pulses 1403a, 1404a, 1405a, and 1406a causes significant pulse broadening in the electrical output, with only pulse 1405a shown in 1402a (FIG. 10B). 1402a also shows specific sampling times 1404, 1405, 1406, and 1407, spaced apart by ΔT, with 1404 preceding 1405. The amplitude of the electrical signal at these times is A for sampling 1404. +1 , A0 for 1405, A for 1404 -1 , 1406A for 1407 -2 Therefore, the digital value measured by sampling at time 1405 is D 1405 From the above, D 1405 =A0P 1405 +A -1 P 1404 +A -2 P 1403 This becomes:

[0035] where P i is the energy of pulse i. More generally, a digital measurement D i is expressed as follows: D i =A0P i +A -1 P i-1 +A i-2P i-2

[0036] For convenience, 0 <j<-2のとき、A j is assumed to be zero, but A j It is easy to see that the above equation can be extended with additional terms to cover scenarios where additional values ​​of D are non-zero. i is Pulse P i Although it is intended to measure only signals related to A -1 and A -2 The value of is non-zero, i.e., the signals generated by adjacent pulses overlap, resulting in D i will ultimately contain significant crosstalk from adjacent pulses.

[0037] In this embodiment, the digital sample D i A processing system operating on the corrected set of digital samples C i This C can be calculated i HA P i and P for adjacent pulses, i.e., j ≠ i j This calculation minimizes the dependency or correlation on the value of C i =(1 / A0)D i -(A -1 / A0)C i-1 -(A -2 / A0)C i-2 This can be done by calculating

[0038] where C j-1 (=P i-1 ) and C j-2 (=P i-2 ) is assumed to be known from a previous calculation. First, to perform the above calculation at a specific index j, we use C j-1 and C j-2 The value of can be assumed to be zero. The error resulting from this assumption is -1 / A0) and (A -2 / A0) is less than zero, so it disappears immediately. i can be calculated in real time by a programmable logic computer such as a field programmable gate array, ASIC, CPU, GPU, or other suitable computing device capable of simple digital storage, arithmetic operations, and multiplication.

[0039] In one embodiment shown in FIGS. 11A-11C, an optical signal 1100 is incident on a photodetector 1101, which generates an electrical signal 1102 that is coupled to an ADC 1103, which generates digital data 1104 (FIG. 11A). The photodetector 1101 is configured to integrate, hold, and provide a dump / reset function, where the electrical output 1102 is the integral of the optical energy over a predetermined period defined by inputting a control signal to the photodetector. The photodetector 1102 is further configured to provide a dump function based on a control signal 1107 so that the integrated electrical signal can be reset to a baseline value, such as ground. In this manner, when the control signal places the photodetector in the integrate state, the electrical signal integrates the photocurrent generated by the photoelectric conversion, and when the control signal places the photodetector in the reset state, the electrical output is returned to and held at the baseline signal (e.g., ground). Alternatively, the control signal can place the photodetector in a third "hold" state, which holds the electrical signal at its current value during analog-to-digital sampling. During this "hold" state, the electrical output does not respond to the optical input.

[0040] Panels 1100a (FIG. 11B) and 1102a (FIG. 11C) illustrate an example of the operation of this embodiment. 1100a (FIG. 11B) shows three input optical pulses, with pulse time slots starting at 1120a for pulse 1105a, followed by pulse time slot 1120b for pulse 1105b, and pulse time slot 1120c for pulse 1105c. The optical pulses are shown as RZ pulses, but can optionally be in NRZ format. 1102a (FIG. 11C) illustrates the logic states of the control signals and resulting electrical outputs for the integrate and dump configuration. In this embodiment, control signal 1107 is configured to place the photodetector in integrate mode at times 1120a, 1120b, and 1120c, and to place the photodetector in reset / dump mode at times 1140a, 1140b, and 1140c. The resulting electrical signal 1133 rises to a value proportional to the pulse energy over the duration of the optical pulse. The ADC is configured via the control signal to sample the electrical signal 1133 at times 1130a, 1130b, and 1130c, before the reset signals 1140a, 1140b, and 1140c. In this way, digital conversion values ​​D associated with pulses 1105a, 1105b, and 1105c, respectively, are obtained. a ,D b ,D c is proportional to the energy of the corresponding pulse, but is generated to minimize response to the energy of adjacent pulses. The integrate function, however, effectively filters out noise signals at high frequencies, i.e., RF frequencies above (½ΔT). In another embodiment, the control signal provides a hold function that precedes the dump / reset function, allowing time for the signal to be held to a steady state, facilitating alignment of the ADC sampling time to the appropriate position in time.

[0041] In one embodiment of the present disclosure, the timing characteristics of the electromagnetic radiation source and ADC sampling are controlled via a common electronic clock device, such that the frequency and phase of the ADC clock are controlled to be specifically aligned with the arrival time of the optical pulse at the photoreceiver. In another embodiment, the common electronic clock is extended to also control at least one of the frequency and phase of the signal or control signal 1107 input to the optical modulators such as 402 and 502. It will be understood that in this and other embodiments, the synchronization is configured to take into account optical and electronic delays between the electromagnetic radiation source, modulator, photodetector, and ADC. Such an embodiment is shown in Figure 12, in which a common electronic device 1200 outputs at least a signal 1200a for controlling the output timing of the pulses of the electromagnetic radiation source 100 and a signal 1200c for controlling the timing of the analog-to-digital conversion of the ADC 130. Optionally, electronic device 1200 outputs at least signal 1200b for controlling the timing of optical modulator 402, which is shown on signal path 173b, but more generally can be located anywhere that provides optical modulation for the light output by electromagnetic radiation source 100. Note that while existing coherent ranging systems can sometimes synchronize a digital conversion clock with the characteristics of the light source, there is no demonstration of a coherent ranging system that utilizes frequency and phase clock control between the digital conversion clock and the optical pulse train generated by the electromagnetic radiation source. In previous publications of coherent ranging using discrete-time electromagnetic radiation sources, the digital conversion clock is synchronized to a frequency relative to the electromagnetic pulse generation rate, but the phase of the digital conversion clock is not controlled (Siddiqui, M., Nam, AS, Tozburun, S. et al. High-speed optical coherence tomography by circular interferometric ranging. Nature Photon 12, 111-116 (2018), the entire disclosure of which is incorporated herein by reference).In one embodiment of the present invention, the frequency and phase of the digital conversion clock are synchronized with the optical pulse generation rate of the electromagnetic radiation pulse train, and the frequency of electromagnetic pulse generation in the light source is equal to the digital conversion rate in the digitizer.

[0042] In one embodiment, the optical receiver (FIG. 13A) is configured to deinterleave the optical pulse train into at least two separate output optical paths (FIG. 13B), each to its own photodetector and ADC. In this embodiment, the inter-pulse spacing at the photodetector and ADC is increased, thereby reducing ISI. The input optical pulse train 174a enters a deinterleaver device 1390a with two outputs, which send light to either photodetector 1325a or photodetector 1325b. Because this is a balanced configuration, the matching deinterleaver device 1390b ​​operates on pulse train 174b, sending light to photodetectors 1325c and 1325d. The top plot labeled 174a in FIG. 13B shows the pulse train on 174a (and 174b). The deinterleaved optical pulse trains sent to photodetectors 1325a and 1325b are shown below. Note that in this configuration, the deinterleaved output doubles the time interval between pulses. While this example deinterleaves the pulse train into two outputs, a deinterleaver with three, four, or any other integer number of output pulses can be used to further increase the interval between pulses at the outputs. In FIG. 13, the same pulses selected for photodetector 1325a are also selected for photodetector 1325c. Two balanced electrical signals 1375a and 1375b are generated and sent to two separate ADC channels 1330a and 1330b within ADC unit 1330. Digital signal 1376 includes sampling of pulses from 1330a and 1330b, and these signals are sent to processing system 1335, where the digital signals can be interleaved as needed for further processing.

[0043] Deinterleaver devices 1390a and 1390b ​​shown in FIG. 13A can be conventional optical frequency-based deinterleavers that provide optical frequency v + 2N(FSR) at one output and optical frequency v + (2N-1)(FSR) at the other port, where N is an integer greater than 0. Optical frequency-based deinterleavers are common in communications and can be based on free-space optical filters or integrated photonic filters, including Mach-Zehnder interferometers with optical path imbalance. In this configuration, the electromagnetic radiation source 174a is configured to generate a pulse train having an optical frequency such that adjacent pulses are separated at the deinterleaver output. For example, the electromagnetic radiation source can generate pulse trains with the following optical frequencies: [νo,νo+100GHz,νo+200GHz,νo+300GHz]

[0044] Here the deinterleaver is configured with an FSR=100 GHz so that one output contains v o and v o +200 GHz and the other output contains v o +100 GHz and v o +300 GHz.

[0045] 13A can also be implemented using an active 1×N optical switch that uses a voltage signal, such as that generated by electronic controller 1200, to send pulses to one of N outputs, with the time interval between pulses at each output being augmented by the interval between pulses at input optical ports 174 a and 174 b. The 1×2 optical switch can be based on electro-optic modulation, such as lithium niobate or thin-film lithium niobate integrated photonic platforms, or other 1×2 optical switches with switching speeds faster than ΔT.

[0046] In one embodiment, a discrete-time coherent ranging system uses RZ pulses with an optical pulse width δt that is set to reject signals from specific structures along the sample arm optical path. In FIG. 14A, a sample arm light beam from optical fiber 1400 enters focusing lens 1401 and is sent to sample 1402. Light backscattered from sample 1402 couples back into fiber 1400 and is sent to a coherent ranging receiver, as shown in FIG. 1, where 1400 is similar to 172a. However, in some embodiments, lens 1401 can also reflect light that is an unwanted reflected signal back into fiber 1400. To address this, electromagnetic radiation source 100 (see FIG. 1) can be configured to provide pulses with a pulse width δt that is shorter than the time between pulses ΔT (FIG. 14B), resulting in a walk-off between reflections from lens 1401 and reflections from sample 1402. When such a reflected pulse (e.g., reflected from a lens or other component) returns to the receiver, the system is configured to superimpose a reference field pulse only on the sample reflection, which has the effect of preventing the reference field pulse from overlapping with the lens reflection. This is possible because the reflected pulse arrives at the receiver at a different time than the reference field pulse, and the pulse width δt is sufficiently short relative to the time between pulses ΔT that the reflected signal can be essentially "masked" by superimposing the signal scattered from the reference field and the signal scattered from the sample. In this way, this configuration can retain interference from the sample while rejecting interference from the lens.

[0047] In this example, the temporal separation between the lens reflection and the sample reflection is given by 2×ΔZ / c, where c is the speed of light and ΔZ is the distance between the lens and the sample, as shown in FIG. 14A. For example, if ΔZ is 2 cm, δt should be less than 2×(0.02 m) / c=133 ps. If ΔZ is 10 cm, δt should be less than 667 ps. The electromagnetic radiation source can optionally include a high-speed amplitude modulator, such as a 1=50 GHz lithium niobate intensity modulator, to generate output pulses of the required pulse width.

[0048] FIG. 15 illustrates a specific embodiment of a discrete-time coherent ranging system, such as that shown in FIG. 1. The system of FIG. 15 is an implementation of a discrete-time coherent ranging system that includes 32 separate optical channels and is coupled to a laparoscope, with the goal of providing high-quality images at high speeds / frame rates using a relatively small and thin probe size. In some embodiments, the number of optical channels can vary from as few as one or two to as many as 100 or more. The system of FIG. 15 can include a phase-code mode-locked (PCML) laser implementation of a programmable pulsed (discrete-time) electromagnetic radiation source (FIG. 16A), which can be configured to operate at wavelengths λ, λ, ...λ. 72 16B). The pulses of the light source of FIG. 16 can have a duty cycle of less than 100%, which helps reduce or eliminate overlap of the output signals from adjacent pulses. In one particular embodiment, the center wavelength is 1588 nm, the spectral bandwidth is 48 nm, the free spectral range (FSR) is 80 GHz, the A-line rate is 164.4 kHz, the output power can be 5 mW, and the pulses have a duty cycle of 50% (FIG. 16B).

[0049] The system of Figure 15 can also include an amplifier and a polarization modulator, such as the passive polarization modulator of Figure 17. Figure 17 shows an implementation of a passive polarization modulator using a passive delay line for use with a discrete-time coherent ranging system (i.e., modulating the polarization state of each pulse), where the upper left insert shows a pulse train at two wavelengths λ1 and λ2 that is input to the polarization modulator after being emitted from the source amplifier, and the upper right insert shows a pulse train at the two wavelengths λ1 and λ2 that is output from the polarization modulator to a sample-sized interferometer, where each input pulse is split into a pair of output pulses of different polarities.

[0050] The system of Figure 15 can also include active phase and / or polarization modulators, such as those shown in Figures 18 and 19. Figure 18 illustrates phase modulation using one implementation of an active (e.g., controlled by a voltage-driven electro-optic modulator) phase and polarization modulator that can be inserted into the configuration of Figure 15 as a "Phase / Polarization Modulation Circuit" (see section "5. Signal Modulation" of Figure 15), where input pulses of wavelengths λ1 and λ2 (left) are first modulated by the phase modulator to produce in-phase pulses (I) and quadrature pulses (Q) under the control of a voltage signal V (bottom).

[0051] FIG. 19 shows phase modulation using one implementation of an active (e.g., voltage-driven electro-optical) phase and polarization modulator that can be inserted into the configuration of FIG. 15 as a “phase / polarization modulation circuit.” The input pulses at wavelengths λ1 and λ2 (top row) are in-phase (I) and quadrature (Q) pulses from the phase modulator of FIG. 18, and the output pulses are polarity R. x and R y (right), where Q is I shifted by 90° and R x = x polarization, R y = y polarization, and the right panel of Figure 19 shows two sets of eight pulses, all of which are at the same wavelength.

[0052] FIG. 20 shows a pulse train (top left) generated and polarized to produce multiple pulses of different wavelengths λ1 and λ2, different polarities R, shown at bottom right, after being sent through a phase modulator PM1. x and R y , and a polarization modulator of a Sagnac interferometer-based implementation that generates pulses of different phases I and Q. To allow the CW and CCW pulses from a given input pulse to strike the phase modulator at different times and produce different phase shifts, the Sagnac interferometer-based implementation of FIG. 20 includes a phase modulator PM2 asymmetrically positioned within the Sagnac loop, thereby causing polarization modulation of the output pulse. Such a design is advantageous because the Sagnac interferometer eliminates DC drift in the phase modulator.

[0053] Figure 21 shows the phase- and polarization-encoded pulses at the end of the sample arm and the reference arm of an exemplary system, where the sample arm encoding (top left) is achieved by the modulators shown in Figures 18-20, and the reference arm encoding (bottom left) is achieved by the passive polarization modulator shown in Figure 17. The text on the right side of Figure 21 explains how the sample arm pulses S1 and S2 shown in the top left are modulated by the reference arm pulse R shown in the bottom left. x ,R y (See also Figure 24.)

[0054] FIG. 22 shows a photograph of a fingertip / fingernail imaged using the discrete-time coherent ranging system of the present disclosure (left) and a structural OCT microphotograph generated using the imaged image (right).

[0055] Figure 23 shows images of a phantom created with multiple tape pieces (top row), with the left side showing the phantom with tape pieces using structural OCT imaging and the right side showing the phantom with tape pieces using polarization-sensitive OCT imaging.

[0056] Figure 24 shows the phase encoding scheme for the phase and polarization modulators described in Figures 18-21. The columns are time slots of width equal to the optical pulse separation, and show the phase provided by each of the two modulators at each discrete time slot, and t p ,2t p ,3t p The resulting phase I / Q and polarization state (R x , R y ) and includes the voltages applied to PM1 and PM2.

[0057] Thus, while the present invention has been described above with reference to particular embodiments and examples, the present invention is not necessarily limited thereto, and numerous other embodiments, examples, uses, modifications, and variations from the above embodiments, examples, and uses are intended to be encompassed within the scope of the appended claims.

Claims

1. a source of electromagnetic radiation; A detector; an optical system having a sample arm and a reference arm; A coherent ranging system comprising: the electromagnetic radiation source is configured to generate and direct a plurality of light pulses toward the sample arm and the reference arm; the detector is configured to receive electromagnetic radiation returned from the sample arm and the reference arm and generate a plurality of values ​​based on the optical energy of each of a plurality of optical pulses of the electromagnetic radiation returned from the sample arm and the reference arm; Each of the plurality of values ​​corresponds to a respective one of the plurality of light pulses. A coherent ranging system comprising:

2. each value of the plurality of values ​​is independent of all other values ​​of the plurality of values; The coherent ranging system of claim 1 .

3. the light energy of each of the plurality of values ​​is independent of the light energy of each of the other values ​​of the plurality of values; The coherent ranging system of claim 1 .

4. the plurality of values ​​includes a plurality of digital values; each of the digital values ​​is based on an amount of optical energy contained in each of the plurality of optical pulses; A coherent ranging system according to any one of claims 1 to 3.

5. further comprising a processor configured to receive the plurality of values ​​and generate structural information for the sample based on the plurality of values. A coherent ranging system according to any one of claims 1 to 4.

6. each value of the plurality of values ​​representing at least one of phase quadrature, polarization state, or reflectivity of the sample for a particular wavelength of the electromagnetic radiation; A coherent ranging system according to any one of claims 1 to 5.

7. a duty ratio of each of the plurality of light pulses is less than 100%; A coherent ranging system according to any one of claims 1 to 6.

8. each of the plurality of light pulses is sent at a corresponding one of a plurality of time points; at least one time point among the plurality of time points does not include a light pulse; A coherent ranging system according to any one of claims 1 to 7.

9. the detector comprises a balanced detector having a pair of inputs; the balanced detector is further configured to receive electromagnetic radiation returned from the sample arm and the reference arm and generate the plurality of values ​​based on detecting a difference between the signals input to the pair of inputs. A coherent ranging system according to any one of claims 1 to 8.

10. further comprising an optical modulator configured to modulate at least one of the phase or polarization of the plurality of light pulses.

10. The coherent ranging system of claim 9.

11. the optical modulator comprises at least one of a passive polarization modulator, a voltage-driven electro-optic modulator, or a Sagnac interferometer polarization modulator; The coherent ranging system of claim 10.

12. the signals input to the pair of inputs of the balanced detector include signals based on at least one of a plurality of pairs of phase-modulated optical pulses or a plurality of pairs of polarization-modulated optical pulses; The coherent ranging system of claim 10.

13. the detector is configured to generate the plurality of values ​​based on an integral of light energy during the duration of each light pulse of the plurality of light pulses. A coherent ranging system according to any one of claims 1 to 12.

14. each light pulse of the plurality of light pulses has a pulse duration; the detector is further configured to have an analog bandwidth that exceeds a Nyquist limit based on the pulse duration; Each light pulse of the plurality of light pulses comprises: a pulse width less than said pulse duration; or a peak power that is at least twice the average power of said light pulses having at least one of A coherent ranging system according to any one of claims 1 to 13.

15. each light pulse of the plurality of light pulses is shaped to match a system response of the detector such that each value of the plurality of values ​​is based substantially solely on the light energy of each corresponding light pulse; A coherent ranging system according to any one of claims 1 to 13.

16. a value of the plurality of values ​​corresponding to one pulse of the plurality of pulses is processed using a deconvolution algorithm to remove signals based on at least one other pulse of the plurality of pulses; A coherent ranging system according to any one of claims 1 to 13.

17. the detector is further configured to reset to a baseline after detecting each of the light pulses. A coherent ranging system according to any one of claims 1 to 13.

18. further comprising a controller coupled to the electromagnetic radiation source and the detector; the controller is configured to control operation of the electromagnetic radiation source and the detector to coordinately control timing of generating the plurality of light pulses by generating the plurality of values ​​based on the plurality of light pulses. A coherent ranging system according to any one of claims 1 to 17.

19. the detector includes a first detector; the coherent ranging system further comprising a second detector and a deinterleaver coupled to the first detector and the second detector; the deinterleaver is configured to send a first pulse of the plurality of pulses to the first detector and to send a second pulse of the plurality of pulses to the second detector. A coherent ranging system according to any one of claims 1 to 18.

20. each pulse of the plurality of pulses having a pulse width and a pulse duration; the pulse width is shorter than the pulse duration such that reflection of one of the pulses in the optical system is separated in time from reflection from the sample; A coherent ranging system according to any one of claims 1 to 19.

21. the optical system includes at least one of an optical fiber or a free space optical system; A coherent ranging system according to any one of claims 1 to 20.

22. the optical system includes an optical coherence tomography (OCT) system; A coherent ranging system according to any one of claims 1 to 21.

23. 1. A method for coherent ranging, comprising: Providing an electromagnetic radiation source, a detector, and an optical system with a sample arm and a reference arm; generating a plurality of light pulses using the electromagnetic radiation source and directing the plurality of light pulses toward the sample arm and the reference arm; receiving electromagnetic radiation returned from the sample arm and the reference arm with the detector; generating, with a processor coupled to the detector, a plurality of values ​​based on the optical energy of each of a plurality of optical pulses of the electromagnetic radiation returned from the sample arm and the reference arm; Including, Each of the plurality of values ​​corresponds to a respective one of the plurality of light pulses. A method characterized by:

24. each value of the plurality of values ​​is independent of all other values ​​of the plurality of values; 24. The method of claim 23.

25. the light energy of each of the plurality of values ​​is independent of the light energy of each of the other values ​​of the plurality of values; 24. The method of claim 23.

26. the plurality of values ​​includes a plurality of digital values; generating the plurality of values generating said plurality of digital values; further comprising each of the digital values ​​is based on an amount of optical energy contained in each of the plurality of optical pulses; 26. The method of any one of claims 23 to 25.

27. receiving the plurality of values ​​by the processor; generating structural information for the sample based on the plurality of values; 27. The method of any one of claims 23 to 26, further comprising:

28. each value of the plurality of values ​​representing at least one of phase quadrature, polarization state, or reflectivity of the sample for a particular wavelength of the electromagnetic radiation; 28. The method of any one of claims 23 to 27.

29. a duty ratio of each of the plurality of light pulses is less than 100%; 29. The method of any one of claims 23 to 28.

30. sending the plurality of light pulses sending each of said plurality of light pulses at a corresponding one of a plurality of time points; further comprising at least one time point among the plurality of time points does not include a light pulse; 30. The method of any one of claims 23 to 29.

31. the detector comprises a balanced detector having a pair of inputs; The method comprises: receiving electromagnetic radiation returned from the sample arm and the reference arm by the pair of inputs of the balanced detector; further comprising generating the plurality of values generating the plurality of values ​​based on detecting a difference between signals input to the pair of inputs; 31. The method of any one of claims 23 to 30, further comprising:

32. further comprising modulating at least one of the phase or polarization of the plurality of light pulses using an optical modulator.

32. The method of claim 31.

33. the optical modulator comprises at least one of a passive polarization modulator, a voltage-driven electro-optic modulator, or a Sagnac interferometer polarization modulator; 33. The method of claim 32.

34. the signals input to the pair of inputs of the balanced detector include signals based on at least one of a plurality of pairs of phase-modulated optical pulses or a plurality of pairs of polarization-modulated optical pulses; 33. The method of claim 32.

35. generating the plurality of values generating the plurality of values ​​based on an integral of light energy during the duration of each light pulse of the plurality of light pulses; 35. The method of any one of claims 23 to 34, further comprising:

36. each light pulse of the plurality of light pulses has a pulse duration; Providing the detector comprises: providing the detector with an analog bandwidth that exceeds the Nyquist limit based on the pulse duration; further comprising Each light pulse of the plurality of light pulses comprises: a pulse width less than said pulse duration; or a peak power that is at least twice the average power of said light pulses having at least one of 36. The method of any one of claims 23 to 35.

37. shaping each light pulse of the plurality of light pulses to match a system response of the detector such that each value of the plurality of values ​​is based substantially solely on the light energy of each corresponding light pulse.

36. The method of any one of claims 23 to 35.

38. further comprising processing a value of the plurality of values ​​corresponding to one pulse of the plurality of pulses using a deconvolution algorithm to remove signals based on at least one other pulse of the plurality of pulses.

36. The method of any one of claims 23 to 35.

39. further comprising resetting the detector to a baseline after detecting each of the light pulses.

36. The method of any one of claims 23 to 35.

40. using a controller coupled to the electromagnetic radiation source and the detector; generating the plurality of values ​​based on the plurality of light pulses, thereby controlling operation of the electromagnetic radiation source and the detector to coordinately control timing of generating the plurality of light pulses.

40. The method of any one of claims 23 to 39, further comprising:

41. the detector includes a first detector; The method further comprises: using a deinterleaver to direct a first pulse of the plurality of pulses to the first detector and direct a second pulse of the plurality of pulses to a second detector; 41. The method of any one of claims 23 to 40, further comprising:

42. each pulse of the plurality of pulses having a pulse width and a pulse duration; the pulse width is shorter than the pulse duration such that reflection of one of the pulses in the optical system is separated in time from reflection from the sample; 42. The method of any one of claims 23 to 41.

43. the optical system includes at least one of an optical fiber or a free space optical system; 43. The method of any one of claims 23 to 42.

44. the optical system includes an optical coherence tomography (OCT) system; 44. The method of any one of claims 23 to 43.

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