Single-chip optical coherence tomography device

A high-performance single-chip, integrated-optics-based OCT system is disclosed, where the length of the reference arm is digitally variable. The reference arm includes a plurality of switch stages comprising a 2×2 tunable wavelength-independent waveguide switch that can direct an input light signal onto either of two different-length output waveguides. In some embodiments, the directional couplers are thermo-optic based. Some embodiments include a solid-state scanning system for scanning a sample signal along a line of object points on the sample under test.

FIELD OF THE INVENTION

The present invention relates to medical imaging systems in general, and, more particularly, to optical coherence tomography.

BACKGROUND OF THE INVENTION

Optical coherence tomography (OCT) has become an increasingly popular diagnostic tool in areas such as the biological, biomedical, medical screening, and vision-care. It utilizes low-coherence optical interferometry to enable non-invasive imaging of micron-scale microstructure inside biological tissue. In recent years, OCT has rapidly become competitive with radiography, ultrasound and magnetic resonance imaging in the biological and biomedical imaging communities due, in part, to its relatively low cost and high-resolution, in-vivo capabilities, as well as its lack of ionizing radiation. In the vision-care arena, for example, OCT is used to non-invasively image the human eye fundus, thereby facilitating diagnosis of retinal pathologies, such as macular degeneration, glaucoma, retinitis, pigmentosa, and the like.

Early OCT systems were typically time-domain systems based on a relatively simple implementation of the classic Michelson interferometer. In such an interferometer, an input light signal is split into a reference arm and a sample arm. In the reference arm, light is directed toward a movable reference mirror, which continuously reflects light back toward the detector. The length of the reference arm depends on the position of this mirror. In the sample arm, light is directed at an object point of a sample under test and only light reflected by sub-surface features in the sample is returned toward the detector. The length of the sample arm, therefore, is based on the positions of features within the sample tissue. A beam combiner combines the two reflected light signals to form an interferometric signal that generates an interference pattern at a detector. Light that travels the same length in each of the reference arm and sample arm constructively recombines to form high-intensity signals at the detector.

A one-dimensional scan of the surface and sub-surface features of the object point is developed by scanning the reference mirror at a constant speed to change the length of the reference arm. This encodes the depths of the surface and sub-surface features in time based on the position of the reference mirror, as represented in the interference pattern that is subsequently sampled by the detector.

Unfortunately, while early time-domain OCT techniques were promising, their complexity and time-intensive nature served to limit their widespread adoption. As a result, Fourier-domain OCT (FD-OCT) was developed, which offers significant improvements over time-domain OCT.

FD-OCT is a method in which the interferometric signal is sampled by a detector as a function of wavelength rather than mirror position. FD-OCT systems are capable of faster imaging with improved sensitivity. In FD-OCT, typically, an object point in a sample is interrogated with a light source that sweeps through a range of optical frequencies (i.e., a swept source). As a result, the object point is illuminated with a monochromatic beam whose optical frequency is a function of time. This results in an interferometric signal of intensity versus wavenumber, k (k is proportional to the inverse of wavelength). A mathematical algorithm, referred to as a Fourier transform, is then used to convert the interferometric signal to a plot of intensity versus depth.

SD-OCT interrogates an object point with broad-spectrum light. Light reflected from the object point is dispersed by wavelength along a row of detectors, which simultaneously provide a different output signal for each of a plurality of wavelength components. As a result, information is collected from many depths within the object point at the same time, and a Fourier transform operation can be used to convert this information into a plot of intensity versus depth.

Unfortunately, current state-of-the-art OCT systems are rather bulky, complex, and sensitive to alignment issues, since they contain a multitude of fiber and free-space optical components. As a result, there has been a push toward miniaturized OCT systems based on integrated-optics, which have the potential for mitigating or overcoming many of these issues.

An integrated-optics system includes one or more optical waveguides formed on the surface of a substrate. The optical waveguides can be combined in myriad arrangements, typically referred to as planar-lightwave circuits (PLCs), which can provide complex optical functionality. Each “surface waveguide” (sometimes referred to herein as simply “waveguides”) includes a light-guiding core surrounded by cladding material that substantially confines a light signal conveyed by the surface waveguide to the core material.

Several integrated-optics-based “micro-spectrometers” have been developed based upon different configurations. For example, Yurtsever, et al., disclosed an FD-OCT system based on a swept source and an on-chip Michelson interferometer in “Integrated photonic circuit in silicon on insulator for Fourier domain optical coherence tomography,” in Proc.SPIE, Vol. 7554, pp. 1-5 (2010). This system achieved 40-μm axial resolution and 25-dB sensitivity. In addition, a partially integrated SD-OCT system that included an arrayed waveguide grating (AWG) spectrometer was disclosed by Akca and Nguyen, et al., in “Toward spectral-domain optical coherence tomography on a chip,”IEEE J. Sel. Top. Quantum Electron, Vol. 18, pp. 1223-1233 (2012) and “Spectral domain optical coherence tomography imaging with an integrated optics spectrometer,”Opt. Lett., Vol. 36, pp. 1293-1295 (2011). Jiao and Tilma, et al., disclosed SS-OCT systems based on InAs/InP quantum-dot-based waveguide photodetectors and a tunable laser source in “InAs/InP(100) quantum dot waveguide photodetectors for swept-source optical coherence tomography around 1.7 μm,”Opt. Express, Vol. 13, pp. 3675-3692 (2012) and “Integrated tunable quantum-dot laser for optical coherence tomography in the 1.7 μm wavelength region,”IEEE J. Quantum Electron, Vol. 48, pp. 87-98 (2012). It was shown that such systems could perform cross-sectional imaging with an in-depth detection range that approached the level of existing bulk optical systems.

More recently, an SS-OCT system comprising a combination of Michelson interferometer, reference arm, and directional coupler for balanced detection was disclosed by Nguyen, et al., “Integrated-optics-based swept-source optical coherence tomography,”Opt. Lett., Vol. 137, pp. 4820-2822 (2012). This system was used to demonstrate that a phantom could be imaged with 80-dB sensitivity and a 5-mm depth range in air. Unfortunately, due to the relatively short length of the reference arm of the system, scanning of the sample was required to obtain cross-sectional images.

The need for sample scanning could be overcome by employing a long on-chip reference arm, as disclosed by Yurtsever, et al., in “Photonic integrated Mach-Zehnder interferometer with an on-chip reference arm for optical coherence tomography,”Biomed. Opt. Express, Vol. 5, pp. 1050 (2014) and “Ultra-compact silicon photonic integrated interferometer for swept-source optical coherence tomography,”Opt. Lett., Vol. 39, pp. 5228-5231 (2014). Unfortunately, due to the fixed-nature of the layout of the integrated-optics components, these systems required extremely complicated dispersion compensation and relatively poor signal-to-noise ratio (SNR) as compared to fiber-based system.

Despite the advances of integrated-optics technology, and the rich palette of optical effects available for exploitation, the need for a practical, high-performance integrated-optics-based OCT system remains, as yet, unmet in the prior art.

SUMMARY OF THE INVENTION

The present invention enables a high-resolution integrated-optics-based optical coherence tomography (OCT) system that overcomes some of the costs and disadvantages of the prior art. OCT systems in accordance with the present invention include a reference arm whose total length can be digitally varied. In addition, the optical power of the reference signal carried in the reference arm can be controlled. As a result, the present invention enables OCT systems whose dispersion compensation is less complex than the prior art, as well as OCT system having improved signal-to-noise-ratio performance. Embodiments of the present invention are particularly well suited for use in optical coherence tomography systems and the like.

An illustrative embodiment includes a source, a pair of photodetectors, a processor, and a planar-lightwave circuit (PLC) that is formed on a single substrate, which collectively define an OCT system. The PLC comprises a 3-dB coupler, a plurality of surface waveguides that includes a reference waveguide, an interrogation waveguide, and a return waveguide, a beam combiner, and a delay controller. An input light signal from the source is distributed into a sample signal and a reference signal, which propagate through a sample arm and reference arm, respectively.

The sample arm includes an interrogation waveguide and a return waveguide, where the interrogation waveguide provides the sample signal to a sample under test and the return waveguide couples sample-signal light reflected by the sample based on its surface and sub-surface structure. The return waveguide conveys the reflected signal to a beam combiner.

The reference arm comprises two reference waveguide portions that are optically coupled with the delay controller, which is located between them. The delay controller has a length that can be digitally controlled by the processor to effect a desired total length of the reference arm and, therefore, a desired delay for the reference signal. The length of the delay controller is based on a plurality of switch stages, each of which can be configured to have one of two different lengths. Furthermore, the delay controller includes waveguide switch at its output, which can direct the reference signal to the beam combiner and/or controller photodetector for providing a dispersion signal to the processor. In addition, the delay controller can control the amount of optical power in the reference signal as it is received at the beam combiner.

The beam combiner combines the reference signal and the reflected signal to generate an interference signal whose intensity is based on the structure of the sample. The interference signal is detected at the photodetector which provides an output signal to the processor. The processor processes the output signal to develop an estimate of the structure of the sample.

In some embodiments, the interrogation waveguide includes a scanning system at its output facet, which enables the sample signal to be scanned along a row of object points on the sample to generate a “B-scan” of the sample.

An embodiment of the present invention is an integrated-optics-based optical coherence tomography (OCT) system having a sample arm and a reference arm, the OCT system comprising a photonic lightwave circuit (PLC) that is monolithically integrated on a substrate, wherein the PLC includes: (i) a coupler for distributing the input light signal into a sample signal on the sample arm and a reference signal on the reference arm; (ii) a beam combiner for combining the reference signal and a reflected signal from a sample to generate an interference signal, the reflected signal being based on the sample signal and the sample; and (iii) a reference arm for conveying the reference signal from the coupler to the beam combiner, wherein the reference arm includes a delay controller that is operative for digitally controlling the length of the reference arm within a range from a minimum length to a maximum length.

Another embodiment of the present invention is a method for performing optical coherence tomography (OCT), the method comprising: providing a planar-lightwave circuit (PLC) that includes a coupler, a reference waveguide, a delay controller, and a beam combiner, wherein the PLC is monolithically integrated on a substrate; distributing an input light signal received at the coupler into a sample signal on a sample arm and a reference signal on a reference arm, wherein the reference arm includes the reference waveguide and the delay controller; providing the sample signal to a sample; receiving a reflected signal at a beam combiner, wherein the reflected signal is based on the sample signal and the sample; conveying the reference signal from the coupler to the beam combiner via the reference waveguide and the delay controller; combining the reference signal and the reflected signal at the beam combiner to generate an interference signal; and digitally controlling a length of the reference arm within a range from an initial length to a maximum length.

DETAILED DESCRIPTION

FIG. 1depicts a schematic diagram of an optical coherence tomography system in accordance with an illustrative embodiment of the present invention. System100includes source102, processor104, photodetectors106and108, and PLC110.

Source102is a conventional coherent-light source that is operative for providing light signal112to PLC110. In the depicted example, source102is an edge-emitting laser that emits light signal112such that it has a center wavelength of approximately 1300 nm. In some embodiments, source102comprises a different coherent-light source and/or emits light at centered at a different wavelength.

Processor104is a conventional instrument controller and processing system operative for providing control signal CV to PLC110based on output signal142and dispersion signal144received from photodetector106and photodetector108, respectively. Typically, processor104is also operative for processing the output signal received from PLC110to develop an estimate of the structure at object point136of sample134.

Photodetector106is a conventional photodetector that is operative for providing an electrical output signal (i.e., output signal142) based on the intensity of interference signal140received from beam combiner126, as discussed below.

Photodetector108is a conventional photodetector that is operative for providing dispersion signal144to processor104, where the dispersion signal is based on optical signal146, which includes at least a portion of the reference signal as it leaves delay controller124. This enables any dispersion mismatch between the two arms of the OCT system to be compensated by processor104, as discussed below.

PLC110is an arrangement of surface waveguides monolithically integrated on substrate114. PLC110includes coupler116, interrogation waveguide118, return waveguide120, reference waveguide122, delay controller124, beam combiner126, output waveguide128and dispersion control waveguide130. In some embodiments, at least one of source102, photodetectors106and108is integrated with PLC110either by forming it on substrate114in monolithically integrated fashion or bonding it with the substrate using hybrid integration techniques.

FIG. 2depicts a schematic drawing of a cross-sectional view of a surface waveguide structure that is representative of each of the surface waveguides of PLC110. Waveguide200includes lower cladding202, core204, and upper cladding206, all of which are disposed on substrate114.

Substrate114is a conventional silicon substrate. In some embodiments, substrate114comprises a suitable material other than silicon, such as a different semiconductor, glass, lithium niobate, a silicon compound (e.g., silicon germanium, silicon carbide, etc.), and the like.

Lower cladding202is a layer of silicon dioxide having a thickness of approximately 8 microns, which is formed on substrate114via thermal oxidation or another appropriate method.

Core204is a layer of light-guiding material208, which has been sculpted to define a single-mode channel-waveguide structure having width, w1, of approximately 2.2 microns and a height, h1, of approximately 1 micron.

In the depicted example, material208is silicon oxynitride (SiON) that is characterized by a refractive index of approximately 1.535 at a wavelength of 1300 nm and a thermo-optic coefficient of approximately 2.35 K−1. In the depicted example, material208is deposited on lower cladding202using plasma-enhanced chemical-vapor deposition (PECVD); however, any suitable deposition process can be used to form core204.

Upper cladding206is a layer of silicon dioxide having a thickness of approximately 500 nm.

The material in each of lower cladding202and upper cladding206is characterized by a refractive index of approximately 1.4485. The effective refractive index of entire material stack of waveguide200, therefore, is approximately 1.472 (for TE polarized light).

In the depicted example, core204comprises silicon oxynitride (SiON) and each of lower cladding202and upper cladding206comprises silicon dioxide; however, one skilled in the art will recognize, after reading this Specification, that myriad materials (e.g., silicon nitride, silicon, lithium niobate, compound semiconductors, glasses, silicon oxides, etc.) can be used in the core and cladding layers of waveguide200without departing from the scope of the present invention. It should be noted that, as discussed below, the illustrative embodiment includes a delay controller that employs waveguide switches that operate based on the thermo-optic effect and, as a result, it is preferable that this embodiment employs surface waveguides that are characterized by a relatively high thermo-optic coefficient. Some embodiments of the present invention, however, employ waveguide switches based on a different switching principle, such as the electro-optic effect, induced stress, etc. In such embodiments, it is preferable that their included waveguides are designed to facilitate the use of the respective switching principle employed.

It should be noted that, although the illustrative embodiment comprises surface waveguides that are single-mode channel waveguides, surface waveguides having different propagation characteristics, geometries, structures, and/or materials can be used in embodiments of the present invention without departing from the scope of the present invention.

Returning now toFIG. 1, coupler116is a conventional, adiabatic 3-dB directional coupler that evenly distributes light signal112into sample signal112A and reference signal112B and provides them to sample arm132A and reference arm132B, respectively. In some embodiments, the split ratio of coupler116is other than 50:50. In some embodiments, coupler116is a different optical-power-splitting element, such as a y-coupler, etc. In some embodiments, coupler116includes an element, such as a spotsize converter, lens, etc., at its input port to mitigate coupling loss. In some embodiments, coupler116receives input signal112via an edge coupler, a vertical-grating coupler, and the like. In some embodiments, coupler116receives input signal112from an external element, such as an optical fiber, a bulk lens, a lensed optical fiber, a high-numerical-aperture fiber, the output port of a photonic-integrated circuit (PIC) or PLC, etc.

Sample arm132A includes interrogation waveguide118, return waveguide120, and the portion of sample134that is interrogated by sample signal112A (i.e., object point136). Interrogation waveguide118conveys sample signal112A from coupler116to sample134. Return waveguide120receives light of sample signal112A reflected by the sample and conveys the reflected light to beam combiner126. It should be noted that the sample arm also includes the round-trip free-space path between PLC110and sample134; however, since the travel distance in free-space is typically very short, its contribution to sample arm132A is considered insignificant to the operation of system100and is excluded from the consideration of sample arm132A, including in the appended claims.

As discussed above, prior-art OCT systems include reference arms that are of fixed length. Unfortunately, due to the static nature of such reference arms, as well as the fact that the light in the reference arm is normally inaccessible, dispersion compensation becomes challenging in these systems. In addition, a static reference arm precludes adjustment of the optical power of the reference signal to improve signal-to-noise ratio (SNR). Embodiments of the present invention, however, include a reference arm that comprises a delay controller that enables control over both the length of the reference arm and the optical power of the reference signal that propagates through it. In addition, the delay controller is dimensioned and arranged to optically couple the reference signal in the reference arm to photodetector108, which provides processor a direct indication of the dispersion in the system. As a result, embodiments of the present invention are afforded significant advantages over prior-art OCT systems, such as higher SNR and dispersion compensation that is more easily achieved.

Each of reference waveguide portions122-1and122-2is a fixed-length waveguide for conveying reference signal112B. Waveguide portion122-1optically couples coupler116and delay controller124, while waveguide portion122-2optically couples delay controller124and beam combiner126.

Delay controller124is a network of waveguide switches and waveguide portions that enables the length of reference arm132B to be digitally varied between its initial length and a maximum length that is based on the number of switch stages included in the delay controller, as discussed below. Delay controller124is operative for receiving reference signal112B from coupler116and providing reference signal112B′ (where reference signal112B′ is the potentially delayed version of reference signal112B) and light signal146based on the configuration of the plurality of waveguide switches it contains.

FIG. 3depicts a schematic drawing of a delay controller in accordance with the illustrative embodiment. Delay controller124includes switch stages302-1through302-4and waveguide switch306-5. Although the illustrative embodiment comprises a delay controller having four switch stages, delay controllers in accordance with the present invention can include any practical number of switch stages without departing from the scope of the present invention.

Each of switch stages302-i, where i=1 through 4, includes waveguide portions304A-i and304B-i, which are optically coupled with waveguide switch306-i. Each waveguide switch306-icontrols the path of light signal112B through its respective waveguide portions304A-i and304B-i based on the magnitude of control voltage CV-i provided by processor104.

FIG. 4Adepicts a schematic drawing of waveguide portion304A-i in accordance with the illustrative embodiment. Waveguide portion304A-i is a substantially straight waveguide section having length L1. In the depicted example, waveguide portions304A-1through304A-4(referred to, collectively, as waveguide portions304A) are substantially identical and L1is equal to 1 cm; however, in some embodiments, at least one of waveguide portions304A has a different shape and/or length.

FIG. 4Bdepicts a schematic drawing of waveguide portion304B-i in accordance with the illustrative embodiment. Waveguide portion304B-i is a curved waveguide that includes waveguide sections402, waveguide sections404, and waveguide sections406.

Each of waveguide sections404is a quarter-circle-shaped waveguide having radius R.

Each of waveguide sections406is a straight waveguide having length Ls.

In the depicted example, the magnitude of R is selected such that waveguide sections402and404have a combined length that equals the length of waveguide portion304A-i (i.e., L1). The overall length of waveguide portion304B-i, therefore, is equal to 2Ls+L1. In the depicted example, waveguide portions304B-1through304B-4(referred to, collectively, as waveguide portions304B) are substantially identical and Ls is 1 cm; however, in some embodiments, at least one of waveguide portions304B has a different shape and/or length. One skilled in the art will recognize, after reading this Specification, that the length of Ls determines the resolution with which the length of reference arm132B can be tuned.

In the illustrative embodiment, the initial length of reference arm132B is approximately 20 cm. Since delay controller124includes four switch stages, each enabling an additional 2 cm to be added, the maximum length of the reference arm is 28 cm. The length of the reference arm, therefore, can be digitally controlled within the range of approximately 20 cm to approximately 28 cm, in increments of 2 cm, by providing each of waveguide switches306-iwith the appropriate control voltage CV-i. It should be noted that these values are merely exemplary, and any practical lengths can be used for any dimension of system100, including the initial length of reference arm132B, as well as the length of any of waveguide portions304A and304B.

The inclusion of straight waveguide sections406in waveguide portions304B affords design flexibility by adding optical length while curved waveguide sections402and404help keep the total chip footprint required for each waveguide portion304B reasonably small enabling a compact design for system100.

Using the exemplary dimensions provided above, the entirety of PLC110can be contained within a 2 cm×2 cm region, which is readily formed on a single chip. It should be noted, however, that by using a high-index-contrast waveguide material system for waveguide200(e.g. based on a core of silicon or silicon nitride, etc.), the chip area required for PLC110can be reduced significantly.

FIG. 5Adepicts a schematic drawing of a waveguide switch in accordance with the illustrative embodiment. Waveguide switch306-iis a 2×2 cross-bar switch that controls the optical coupling between input ports In1and In2and output ports Out1and Out2based on the magnitude of the control voltage CV-i. Waveguide switch306-iis a Mach-Zehnder-type interferometric coupler that includes a pair of directional couplers502-1and502-2, which reside on either side of delay section504. Waveguide switch306-iis representative of each of waveguide switches306-1through306-5(referred to, collectively, as switches306).

Each of directional couplers502-1and502-2(referred to, collectively, as couplers502) is a conventional directional coupler having length L2. Directional couplers502-1and502-2are “complimentary directional couplers.” For the purposes of this Specification, including the appended claims, the term “complimentary directional couplers” is defined as directional couplers that operate in concert such that any deviations introduced in one directional coupler are substantially compensated for in the other directional coupler. In some embodiments, directional couplers502-1and502-2are not complimentary directional couplers.

Delay section504includes delay waveguide506, trunk waveguide508, and heater510, which is disposed on trunk waveguide508and electrically connected to control signal CV-i.

Waveguide switch306-iis a thermo-optic waveguide switch; therefore, as light propagates through one of delay waveguide506and trunk waveguide508, the coupling between them is controlled by the temperature of heater510. In some embodiments, delay controller124includes waveguide switches based on other effects such as electro-optic, piezoelectric, stress, liquid-crystal-based, and the like.

Waveguide switch306-ihas a cross-coupling ratio that can be tuned anywhere within the range from 0% (no cross-coupling) to 100% (full cross-coupling). The coupling ratio of the waveguide switch is given by:
S=cos2(θ)sin2(φ1+φ2)+sin2(θ)sin2(φ1−φ2)
where φ1and φ2are the half-phase differences between the fundamental and first-order system modes existing in the parallel-waveguide coupler sections of each of couplers502-1and502-2, and 2θ=β(λ)ΔL is the relative phase delay introduced in the delay section by the path-length difference, ΔL, between delay waveguide506and trunk waveguide508and the propagation constant of the waveguide mode β(λ).

To provide waveguide switch306-i(i.e., the full coupler) with a maximally flat wavelength response, the parameters can be chosen as φ1=π/2 (full coupler), φ2=π/2 (full coupler), and 2θ=2π/3 for ease of design and fabrication. Based on these design parameters, in the depicted example, the length, L2, of each of directional couplers502-1and502-2is 155 microns, and the path-length difference, ΔL, is 0.29 microns. The total length of the waveguide switch306-iis 1.6 mm.

FIG. 5Bdepicts simulation results of cross-coupled optical power and relative refractive index between delay waveguide506and trunk waveguide508for a waveguide switch306-i. Plot512shows the optical power at output port Out2for a light signal injected into input port In1as a function of the refractive index difference between delay waveguide506and trunk waveguide508. As seen in plot512, for Δn=2.4×10−3, 96% of the light signal injected into trunk waveguide508will cross-couple to the delay waveguide506, which occurs at a heater temperature of approximately 102 K.

FIG. 5Cdepicts simulation results for the splitting ratio of waveguide switch306-i. Plot514shows that cross-coupling is substantially non-existent in the switch when 0 voltage is applied to heater510and the switch is at room temperature. Plot516shows that full cross-coupling existents in the switch when sufficient voltage is applied to heater510and the switch is at an elevated temperature.

FIG. 5Ddepicts simulation results for the splitting ratio of waveguide switch306-i. Plots518and520demonstrate that the splitting ratio between delay waveguide506and trunk waveguide508is substantially wavelength independent over a 100-nm wide bandwidth in both the cross and bar states of the switch.

After reference signal1128passes through switch stages302-1through302-4, it propagates through switch306-5which directs some or all of the optical power of reference signal1128to either photodetector108as light signal146or to beam combiner126as reference signal1128′. Waveguide switch306-5is substantially identical to each of waveguide switches306-1through306-4.

Beam combiner126is a surface-waveguide element operative for recombining sample signal112A and reflected signal138to generate interference signal140and provide it to photodetector106.

FIG. 6depicts a schematic drawing of a beam coupler in accordance with the illustrative embodiment. Beam combiner126comprises slab602and output waveguide128, each of which is analogous to waveguide200described above and with respect toFIG. 2.

Slab602is a region of a waveguide structure analogous to that of waveguide200, but which is dimensioned and arranged to recombine the light signals conveyed by reference waveguide122-2and return waveguide120and provide the recombined light signal to output waveguide128as interference signal140. Slab602has width, w2, and length, L3, which are selected to enable two-mode interference (TMI) within the slab region. The structure of beam combiner126is preferred over a more-conventional Y-junction because it is more fabrication tolerant and, therefore, its performance is more reproducible. In the depicted example, w2is 6 microns, L3is 33 microns, and reference waveguide122-2and return waveguide120are separated by separation distance, d1, which is 1.4 microns.

FIGS. 7A-Cshow BPM simulation results for beam combiner126under different input conditions. The radiation loss of the beam combiner is dependent on the phase difference between reference signal112B′ and reflected signal138, as received at slab602, as well as their relative power.

Plot700shows the coupling of the light signals when reference signal112B′ and reflected signal138are in phase and have equal power. As seen in the plot, all input power is coupled into the first-order even mode and, therefore, transmitted through slab region602into output waveguide128. It is estimated that, in this case, beam combiner126will provide interference signal140having approximately 96% of the optical power received from reference waveguide122-2and return waveguide120. In other words, the minimum optical loss through the beam combiner is approximately 4%.

Plot702shows the coupling of the light signals when reference signal112B′ and reflected signal138are in phase and have a power ratio of 0.5. This power disparity gives rise to a power reduction of only 4%, relative to the equal power case depicted in plot700(i.e., interference signal140has approximately 92% of the optical power received from reference waveguide122-2and return waveguide120).

Plot704shows the coupling of the light signals when light signals112B′ and138have a phase difference of π/6. In this case, the output power of interference signal140is reduced to approximately 89% of the combined input power of reference signal112B′ and reflected signal138(i.e., a reduction of 7% from the in-phase, equal power case depicted in plot700).

In the depicted example, beam combiner126is based on two-mode interference (TMI); however, other beam combiners can be used in PLC110without departing from the scope of the present invention.

FIG. 8depicts operations of a method suitable for performing optical coherence tomography in accordance with the illustrative embodiment. Method800begins with operation801, wherein source102provides light signal112.

At operation802, light signal112is distributed into sample signal112A and reference signal112B at coupler116.

At operation803, object point136of sample134is interrogated with sample signal112A. Sample134receives sample signal112A from output facet FC-1of interrogation waveguide118as a free-space light signal. Typically, a lens or other bulk-optics system is used to capture the free-space light emanating from interrogation waveguide118and focus it to a spot on sample134(not shown inFIG. 1).

When interrogated with sample signal112A, object point136reflects reflected signal138, which is optically coupled into input facet FC-2of return waveguide120. The spectral content of reflected signal138is based on the set of wavelength components in the sample signal and the surface and sub-surface features at object point136. Preferably, FC-1and FC-2are closely spaced to mitigate optical loss between PLC110and sample134.

At operation805, processor104configures delay controller124to establish the desired length of reference arm132B.

At operation806, reference waveguide122-2provides reference signal1126′ to beam combiner126.

At operation807, light signals138and112B′ are combined at beam combiner126to generate interference signal140.

At operation808, photodetector106provides output signal142based on the intensity of interference signal140.

At operation809, processor104estimates a depth-scan image of the structure at object point136based on output signal142.

At operation810, processor104configures waveguide switch306-5to provide at least a portion of reference signal112B′ to photodetector108as light signal146.

At operation811, photodetector108provides dispersion signal144based on light signal146and provides it to processor104.

At operation812, processor104determines the dispersion in system100(typically via a third-order polynomial approach) and provides dispersion compensation for system100by controlling the coupling ratios of waveguide switches306-1through306-4based on dispersion signal144.

At operation813, processor104controls the optical power of reference signal112B′ by controlling the coupling ratio of waveguide switch306-5based on output signal142. By controlling the power of reference signal112B′, system100enables output signal142to have improved SNR.

System100, as described above, is operative for generating an axial-depth scan at a single object point on sample134(typically referred to as an “A-scan”). It is often desirable, however, to scan a row of object points to generate a cross-sectional image of a linear region of a sample (a “B-scan”) or, in some cases, to scan a two-dimensional array of object points (a “C-scan”). To enable B-scans and/or C-scans, at least one of system100and sample136can be located on a scanning stage that is capable of one- or two-dimensional scanning. Unfortunately, changing the physical arrangement between an OCT system and a sample can lead to positional inaccuracy and noise in the output signal from which the imaging is constructed.

In some embodiments of the present invention, a solid-state scanning system is incorporated into sample arm132A to eliminate the need to physically scan system100or sample134to generate a B-scan.

FIG. 9depicts a schematic drawing of a solid-state beam scanner in accordance with an alternative embodiment of the present invention. Scanner900includes interrogation waveguide118, scanner waveguides904A-1through904A-M and904B-1through904B-M, waveguide switches902A-1through902A-M and902B-1though902B-M, and output port906.

Each of waveguide switches902A-1through902A-M and902B-1though902B-M is analogous to waveguide switch306-idescribed above.

Output port906includes output facet FC-1of interrogation waveguide118, as well as facets908A-1through908A-M and908B-1through908B-M. The output facets included in output port906are preferably separated by uniform spacing d2. In the depicted example, M is equal to 100 and d2is equal to 10 microns; therefore, the 101 output facets of output port enable light signal112A to be scanned over a total scan length, L3, of 1 mm. It should be noted that any of M, d2, and L3can have any practical value without departing from the scope of the present invention.

To scan sample signal112A along output port906, the appropriate set of waveguide switches902are actuated such that they enable full cross-coupling. For example, to emit the sample signal from the output facet of scanning waveguide904A-M, all of waveguide switches902A-1through902A-M are actuated. To move the sample signal to the output facet of scanning waveguide904A-(M-1), all of waveguide switches902A-1through902A-M-1are actuated but902A-M is not.

Since actuation of each waveguide switch can occur in microseconds, scanning sample signal112A over the total scan length, L3, of output port906can be done in a significantly shorter time than for mechanical scanners, such as galvanometer scanners, etc.