Patent Description:
Embodiments of the application relate to waveguides having designs aimed at compensating for bi-refringence effects.

Low coherence interferometry (LCI) is often used in the medical imaging field to provide depth-resolved information of both internal and external tissue. Example LCI techniques include optical coherence reflectometry (OCR) and optical coherence tomography (OCT), which can each provide depth resolved information with high axial resolution by means of a broadband light source and an interferometric detection system.

When using these LCI imaging techniques with medical devices, the devices often require optical links to deliver light between a light source and the target sample, and between the sample and a detector. One problem with many of these optical links is that they exhibit the optical property of bi-refringence. In these bi-refringent optical links, the propagating medium/waveguide presents a different refractive index for the different polarization states of light propagating along it, causing a differential group delay (DGD) between light components on each of the polarization states. Whenever an interface between two bi-refringent optical links occurs within a system, very precise alignment between the optical axis of both links is needed in order to prevent cross-talk between polarization components that have accumulated a DGD.

Alignment precision when connecting optical links is often limited by mechanical or process-related tolerances. Typical angular tolerances range from ±<NUM>° for standard precision to ±<NUM>° for high precision connections. When a misalignment occurs at an interconnection of two optical links, the extinction ratio measuring the portion of light which goes into the undesired axis on the receiving link can be calculated as ER=<NUM>*log<NUM> (tan<NUM>θ), where θ is the angular misalignment on the interconnection. ER values corresponding to the above-mentioned ±<NUM>° and ±<NUM>° are <NUM> dB and <NUM> dB, respectively. While these values might be good enough for many applications, for high dynamic range measuring techniques like OCR and OCT, where the imaging dynamic range can be as high as <NUM>-<NUM> dB, those values of ER are not enough for ensuring that there are no measurement artifacts due to the cross-coupled polarization components.

<CIT> discloses an optical fiber capable of performing polarization dispersion compensation. The optical fiber includes two segments that are coupled in series. The second segment is rotated about the optical axis of the optical fiber with respect to the first segment, so that birefringence occurring in the first and second segments is reduced to zero after light propagates through the entire length of the optical fiber.

<CIT> discloses a catheter system that allows safely crossing coronary occlusions. The system includes a catheter comprising a proximal section, a distal section provided with a flexible substrate, and a sheath coupled between the proximal section and the distal section. A plurality of waveguides are patterned or otherwise formed on the flexible substrate to provide multiple scanning beams of light for performing OCT. The system further includes a processing device and an optical fiber for transmitting light generated from an optical source.

<CIT> discloses a system to be used in performing optical coherence tomography. The system may include a polarization splitter comprising a splitting element and a recombination element of an interferometer. The splitting element splits an incoming beam of radiation between an upper arm and a lower arm. Each of the upper arm and lower arm includes waveguide segments having varying properties (e.g., a different geometry) to provide a specific light modulation in each arm. Waveguide lengths are defined to obtain desired cross-coupling relations.

<CIT> discloses a catheter scope comprising a waveguide having a lens assembly provided at one end. The waveguide may have a rib-like structure including a photoresist layer having a higher plateau and a lower plateau. The photoresist layer is arranged on a substrate, e.g., made of silicon oxide (SiO2).

<CIT> discloses another example of a waveguide coupler.

An optical integrated circuit according to the invention is disclosed in claim <NUM>. Further disclosed are different optical waveguide devices for reducing detrimental effects on an optical signal due to accumulated DGD are described.

In an embodiment, a catheter system includes a catheter, a processing device, and an optical fiber disposed in the processing device, or between the processing device and the catheter, or within a sheath of the catheter. The catheter includes a proximal section, a distal section, and the sheath connecting the proximal section to the distal section. The distal section includes a substrate having one or more waveguides patterned upon the substrate. A first waveguide of the one or more waveguides is designed to guide a beam of radiation. The processing device includes an optical source designed to generate a source beam of radiation and a communications interface designed to transmit electrical and optical signals to the proximal section of the catheter. The optical fiber includes a first segment and a second segment. The second segment is rotated about an optical axis relative to the first segment.

An optical integrated circuit includes a substrate and a waveguide patterned upon the substrate. The waveguide guides a beam of radiation and includes a first waveguide segment designed to impart a first differential group delay on the beam of radiation, a second waveguide segment designed to impart a second differential group delay on the beam of radiation, and a coupling region between the first and second waveguide segments. A sum of the first differential group delay and the second differential group delay is substantially zero.

A medical instrument includes an optical source, an optical fiber, and a waveguide patterned upon a substrate. The optical fiber receives radiation from the optical source and includes a first segment and a second segment. The second segment is rotated about an optical axis relative to the first segment. The waveguide receives radiation from the optical source and guides a beam of radiation. The waveguide includes a first waveguide segment designed to impart a first differential group delay on the beam of radiation and a second waveguide segment designed to impart a second differential group delay on the beam of radiation. A sum of the first differential group delay and the second differential group delay is substantially zero.

The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.

Embodiments of the present invention will be described with reference to the accompanying drawings.

Although specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations and arrangements can be used without departing from the scope of the present invention. It will be apparent to a person skilled in the pertinent art that this invention can also be employed in a variety of other applications.

It is noted that references in the specification to "one embodiment," "an embodiment," "an example embodiment," etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Further, when a particular feature, structure or characteristic is described in connection with an embodiment, it would be within the knowledge of one skilled in the art to effect such feature, structure or characteristic in connection with other embodiments whether or not explicitly described.

Described herein are embodiments of a medical device, such as a catheter, that uses optical signals to visualize the placement and/or movement of the device within a patient's body. Low coherence interferometry (LCI) may be used with the optical signals to provide depth-resolved information about the sample tissue being imaged. Although portions of the application may focus on catheters and the transmittance of optical signals through parts of the catheters, it should be understood that the embodiments discussed herein may apply to any medical device that utilizes optical signals. Furthermore, embodiments herein relating to reducing or eliminating the effect that the differential group delay (DGD) has on the information extracted from the optical signals may be used in any system or device that transmits and/or receives light for the purpose of measurement or analytical study.

Herein, the terms "electromagnetic radiation," "light," and "beam of radiation" are all used to describe the same electromagnetic signals propagating through the various described elements and systems.

<FIG> illustrates a catheter <NUM> according to an embodiment. Catheter <NUM> includes a proximal part <NUM>, a distal part <NUM>, and a sheath <NUM> coupled between proximal part <NUM> and distal part <NUM>. In an embodiment, sheath <NUM> includes one or more radiopaque markers for navigation purposes. In one embodiment, catheter <NUM> includes a communication interface <NUM> between catheter <NUM> and a processing device <NUM>. Communication interface <NUM> may include one or more wires and/or optical patch cords between processing device <NUM> and catheter <NUM>. In other examples, communication interface <NUM> is an interface component that allows wireless communication, such as Bluetooth, WiFi, cellular, etc. Communication interface <NUM> may communicate with one or more transceiver elements located within either proximal part <NUM> or distal part <NUM> of catheter <NUM>.

In an embodiment, sheath <NUM> and distal part <NUM> are disposable. As such, proximal part <NUM> may be reused by attaching a new sheath <NUM> and distal part <NUM> each time a new procedure is to be performed. In another embodiment, proximal part <NUM> is also disposable.

Proximal part <NUM> may house various electrical and optical components used in the operation of catheter <NUM>. For example, a power supply may be included within proximal part <NUM> to apply RF energy to an electrode located at distal part <NUM> for tissue ablation. The power supply may be designed to generate an alternating current at frequencies at least between <NUM> and <NUM>. As such, one or more conductive wires (or any electrical transmission medium) may lead from the power supply to distal part <NUM> within sheath <NUM>. Furthermore, proximal part <NUM> may include an optical source for generating a beam of radiation.

In another embodiment, various electrical and optical components such as the power supply, optical source, and interferometer elements are located in processing device <NUM>. Optical signals may be transferred between the optical source and interferometer elements using optical fibers within processing device <NUM>. The electrical and optical signals from these components may be sent to proximal part <NUM> via communication interface <NUM>. By housing these components in processing device <NUM>, the whole of catheter <NUM> may be disposable.

The optical source may include one or more laser diodes or light emitting diodes (LEDs). The beam of radiation generated by the optical source may have a wavelength within the infrared range. In one example, the beam of radiation has a central wavelength of <NUM>. The optical source may be designed to output a beam of radiation at only a single wavelength, or it may be a swept source and be designed to output a range of different wavelengths. The range of wavelengths may include any wavelengths found in the near-infrared or mid-infrared spectral range. The generated beam of radiation may be guided towards distal part <NUM> via an optical transmission medium connected between proximal part <NUM> and distal part <NUM> within sheath <NUM>. Some examples of optical transmission media include single mode and multimode optical fibers and integrated optical waveguides. In one embodiment, the electrical transmission medium and the optical transmission medium are provided by the same hybrid medium allowing for both electrical and optical signal propagation.

In an embodiment, proximal part <NUM> or processing device <NUM> includes one or more components of an interferometer in order to perform LCI using the light generated from the optical source. Further details of the LCI system are discussed with reference to <FIG>. Due to the nature of interferometric data analysis, in an embodiment the optical transmission medium used for guiding the light to and from distal part <NUM> does not affect the state and degree of light polarization. In another embodiment, the optical transmission medium affects the polarization in a constant and reversible way.

Proximal part <NUM> may include further interface elements with which a user of catheter <NUM> can control the operation of catheter <NUM>. For example, proximal part <NUM> may include a deflection control mechanism that controls a deflection angle of distal part <NUM>. The deflection control mechanism may require a mechanical movement of an element on proximal part <NUM>, or the deflection control mechanism may use electrical connections to control the movement of distal part <NUM>. Proximal part <NUM> may include various buttons or switches that allow a user to control when RF energy is applied at distal part <NUM>, or when the beams of radiation are transmitted from distal part <NUM>, allowing for the acquisition of optical data. In some examples, these buttons or switches are located at a separate user interface coupled to processing device <NUM>.

Distal part <NUM> may include one or more external electrodes for ablation, according to some embodiments. Distal part <NUM> may also include a plurality of optical view ports to transmit/collect light at various angles from distal part <NUM>. Distal part <NUM> may include a substrate with patterned waveguides for guiding light to/from each of the plurality of optical view ports. The substrate may be a flexible (including a partially flexible) substrate made from a material such as polyimide, polyethylene glycol, Parylene, or polydimethelsiloxane (PDMS).

The optical view ports may be distributed over the outside of distal part <NUM>, resulting in a plurality of distinct viewing directions, according to an embodiment. In an embodiment, each of the plurality of viewing directions is substantially non-coplanar.

<FIG> illustrate cross-section views of sheath <NUM>, according to embodiments. Sheath <NUM> may include all of the elements interconnecting proximal part <NUM> with distal part <NUM>. Sheath 106a illustrates an embodiment that houses an irrigation channel <NUM>, RF conductive medium <NUM>, deflection mechanism <NUM>, electrical connections <NUM>, and optical transmission media <NUM>. RF conduction medium <NUM> and irrigation channel <NUM> may not be necessary if catheter <NUM> is not being used for ablation. <FIG> illustrates a protective cover <NUM> wrapped around both electrical connections <NUM> and optical transmission media <NUM>. Electrical connections <NUM> may be used to provide signals to optical modulating components located in distal part <NUM>. One or more optical transmission media <NUM> guide light generated from the optical source (exposure light) towards distal part <NUM>, while another subset of optical transmission media <NUM> guides light returning from distal part <NUM> (scattered or reflected light) back to proximal part <NUM>. In another example, the same one or more optical transmission media <NUM> guides light in both directions. According to an embodiment, optical transmission media <NUM> include polarization maintaining (PM) fibers.

Deflection mechanism <NUM> may include electrical or mechanical elements designed to provide a signal to distal part <NUM> in order to change a deflection angle of distal part <NUM>. The deflection system enables guidance of distal part <NUM> by actuating a mechanical control placed in proximal part <NUM>, according to an embodiment. This system may be based on a series of aligned and uniformly spaced cutouts in sheath <NUM> aimed at providing unidirectional deflection of distal part <NUM>, in combination with a wire which connects the deflection mechanism control in proximal part <NUM> with the catheter tip at distal part <NUM>. In this way, a certain movement of the proximal part may be projected to the distal part. Other embodiments involving the combination of several control wires attached to the catheter tip may enable the deflection of the catheter tip along different directions.

<FIG> illustrates a cross-section of sheath 106b. Sheath 106b depicts an embodiment having most of the same elements as sheath 106a from <FIG>, except that there are no electrical connections <NUM>. Sheath 106b may be used in situations where modulation (e.g., multiplexing) of the generated beam of radiation is performed in proximal part <NUM> or in processing device <NUM>.

Further details of an ablation catheter that may utilize the embodiments described herein can be found in co-pending <CIT>.

Various embodiments of the present application include a LCI system integrated within a medical device such as catheter <NUM> for optical interrogation of a sample. <FIG> illustrates an example LCI system <NUM> for imaging a sample <NUM>, according to an embodiment. For example, sample <NUM> may be a tissue surface within a patient's body. A delay unit <NUM> may include various light modulating elements. These modulating elements may perform phase and/or frequency modulation to counteract undesired optical effects in the light, and to select one or more depths of sample <NUM> to be imaged. The use of the term "light" may refer to any range of the electromagnetic spectrum. In an embodiment, the term "light" refers to infrared radiation at a wavelength of about <NUM>.

LCI system <NUM> further includes an optical source <NUM>, a splitting element <NUM>, a sample arm <NUM>, a reference arm <NUM>, and a detector <NUM>. In the embodiment shown, delay unit <NUM> is located within reference arm <NUM>. However, it should be understood that delay unit <NUM> may instead be located in sample arm <NUM>. Alternatively, various elements of delay unit <NUM> may be present in both sample arm <NUM> and reference arm <NUM>. For example, elements of delay unit <NUM> that introduce a variable delay to the light may be located in sample arm <NUM>, while elements that modulate different polarization modes of the light may be located in reference arm <NUM>. In another example, elements of delay unit <NUM> that modulate different polarization modes of the light may be located in sample arm <NUM>, while elements that introduce a variable delay to the light may be located in reference arm <NUM>. In one example, sample arm <NUM> and reference arm <NUM> are optical waveguides, such as patterned waveguides or optical fibers. In an embodiment, all of the components of LCI system <NUM> are integrated onto a planar lightwave circuit (PLC). In another embodiment, at least the components within delay unit <NUM> are integrated on the same substrate of a PLC. Other implementations may be considered as well, such as, for example, fiber optic systems, free-space optical systems, photonic crystal systems, etc..

In an embodiment, delay unit <NUM> and splitting element <NUM> are integrated on a PLC while optical source <NUM> and detector <NUM> are provided separately from the PLC. In this embodiment, light may be coupled from optical source <NUM> to splitting element <NUM> via one or more PM fibers, and light may be coupled from splitting element <NUM> to detector <NUM> via one or more PM fibers or by direct free-space coupling.

It should be understood that LCI system <NUM> may include any number of other optical elements not shown for the sake of clarity. For example, LCI system <NUM> may include mirrors, lenses, gratings, splitters, micromechanical elements, etc., along the paths of sample arm <NUM> or reference arm <NUM>. Splitting element <NUM> is used to direct light received from optical source <NUM> to both sample arm <NUM> and reference arm <NUM>. Splitting element <NUM> may be, for example, a bi-directional coupler, an optical splitter, an adjustable splitting-ratio coupler, or any other modulating optical device that converts a single beam of light into two or more beams of light.

Light that travels down sample arm <NUM> ultimately impinges upon sample <NUM>. Sample <NUM> may be any suitable sample to be imaged, such as tissue. The light scatters and reflects back from various depths within sample <NUM> and the scattered/reflected radiation is collected back into sample arm <NUM>. In another embodiment, the scattered/reflected radiation is collected back into a different waveguide than the transmitting waveguide. The scan depth may be chosen via the delay imposed on the light within delay unit <NUM>.

Light within sample arm <NUM> and reference arm <NUM> is recombined before being received at detector <NUM>. In the embodiment shown, the light is recombined by splitting element <NUM>. In another embodiment, the light is recombined at a different optical coupling element than splitting element <NUM>. Detector <NUM> may include any number of photodiodes, charge-coupling devices, and/or CMOS structures to transduce the received light into an electrical signal. The electrical signal contains depth-resolved optical data related to sample <NUM> and may be received by a processing device for further analysis and signal processing procedures. As used herein, the term "depth-resolved" defines data in which one or more portions of the data related to specific depths of an imaged sample can be identified.

In an embodiment, optical source <NUM>, detector <NUM> and delay unit <NUM> are located within proximal part <NUM> of catheter <NUM>. In another embodiment, optical source <NUM>, detector <NUM> and delay unit <NUM> are located within processing device <NUM>. Splitting element <NUM> and at least part of one or both of sample arm <NUM> and reference arm <NUM> may be located in processing device <NUM> or in either proximal part <NUM> or distal part <NUM> of catheter <NUM>. In another embodiment, all of the elements of LCI system <NUM> are located in distal part <NUM> of catheter <NUM>. Optical source <NUM> may include one or more light emitting diodes (LEDs) or laser diodes. For example, LEDs may be used when performing time domain and/or spectral domain analysis, while tunable lasers may be used to sweep the wavelength of the light across a range of wavelengths. In another embodiment, any of the components of LCI system <NUM> are located external to catheter <NUM>, for example, within processing device <NUM>.

LCI system <NUM> is illustrated as an interferometer design similar to a Michelson interferometer, according to an embodiment. However, other interferometer designs are possible as well, including Mach-Zehnder or Mireau interferometer designs.

As discussed above, medical devices such as catheters may use optical fibers to transmit and receive light. These optical fibers may include a PM fiber designed to maintain a given polarization state of the incident light as the light propagates through the PM fiber. With reference to <FIG>, a PM fiber may be used within processing device <NUM>, between processing device <NUM> and catheter <NUM>, or within sheath <NUM>. Depending on how accurately light is coupled into the PM fiber, a beam of radiation propagating down the length of the PM fiber may accumulate a differential group delay (DGD), which causes cross-talk between the polarized components of the light as the light is coupled to another PM fiber or waveguide. This cross-talk will degrade the optical signal (e.g., cause double images) ultimately being detected, especially for systems utilizing sensitive measurement techniques like LCI.

According to an embodiment, a PM fiber design is provided to substantially negate any DGD introduced into the beam of radiation propagating through a PM fiber. <FIG> illustrates an example PM fiber <NUM> having a first segment <NUM> identified by a first length (L<NUM>) and a second segment <NUM> identified by a second length (L<NUM>), according to an embodiment. Light may be coupled into PM fiber <NUM> on one end and exit PM fiber <NUM> at the other end. The light exiting PM fiber <NUM> may be received by either a sample to be imaged, or by other optical components, such as additional waveguides.

Because PM fiber <NUM> maintains a given polarization state of the propagating beam of radiation, the fiber includes regions to introduce a non-uniform stress within the cladding surrounding the fiber core. Cross-section <NUM> illustrates a cross-section of first segment <NUM> having a first fiber core <NUM> and first stress-inducing regions <NUM>. First fiber core <NUM> may have a diameter between about <NUM> microns and <NUM> microns. First stress-inducing regions <NUM> may be rods of a different material than the remainder of the cladding around first fiber core <NUM>. Other shapes are designs of stress-inducing regions <NUM> may be used as would be understood to a person skilled in the relevant art. According to an embodiment, second segment <NUM> is coupled to first segment <NUM> via a coupling region <NUM>, and is rotated along an optical axis passing through the center of PM fiber <NUM> with respect to first segment <NUM>. Cross-section <NUM> illustrates a cross-section of second segment <NUM> having a second fiber core <NUM> and second stress-inducing regions <NUM>. Second fiber core <NUM> and second stress-inducing regions <NUM> may be the same as first fiber core <NUM> and first stress-inducing regions <NUM>. In this example, second segment <NUM> is rotated <NUM>° around the optical axis with respect to first segment <NUM>. In other embodiments, second segment <NUM> may be rotated at any angle with respect to first segment <NUM>. According to an embodiment, first segment <NUM> and second segment <NUM> have substantially the same length. The term "substantially" as it is used herein to pertain to length means that the lengths are close enough to one another so as to be insignificant with regards to any effect on the propagating beam of radiation.

By rotating two segments of a PM fiber of substantially equal length by <NUM>°, any accumulated DGD on each of the polarization states will cancel out after the light has propagated through the entire length (i.e., L<NUM> + L<NUM>) of PM fiber <NUM>, according to an embodiment. In other words, the light propagating through the "slow" polarization axis of first segment <NUM> accumulates a delay DGD<NUM> with respect to the light in the "fast" axis of the same first segment <NUM>. The "slow" axis light from first segment <NUM> is then, thanks to the <NUM>° rotation in the transition, coupled into the "fast" polarization axis of second segment <NUM>, where it accumulates -DGD<NUM> with respect to the light propagating through the "slow" axis of second segment <NUM> which, thanks to the same <NUM>° rotation in the transition, had previously propagated through the "fast" axis of first segment <NUM>. As a result, the total DGD between the two polarization components entering PM fiber <NUM>, as accumulated in PM fiber <NUM> as a whole, is DGD<NUM> - DGD<NUM> = <NUM>, or is substantially zero. The term "substantially zero" as it is used herein to pertain to DGD means that the DGD is small enough so as to be insignificant with regards to any effect on the propagating beam of radiation.

Coupling region <NUM> may represent a fusion splice between first segment <NUM> and second segment <NUM>. Other coupling techniques may be used to couple first segment <NUM> and second segment <NUM>, such as using an index-matching material between the two fiber segments or free space coupling with one or more lenses.

Although PM fiber <NUM> illustrates an example having only two segments of substantially equal length, in other embodiments, any number of fiber segments may be created with each segment characterized as having a given rotation about the optical axis. For example, multiple matching pairs of fiber segments may be spliced together. Furthermore, the fiber segments do not have to be the same length to cancel out the accumulated DGD. In some other embodiments, first segment <NUM> has a different propagation medium than second segment <NUM>, such that the accumulated DGD per unit length of first segment <NUM> is different than the accumulated DGD per unit length of second segment <NUM>. In these instances, the fiber segments may have different lengths to ultimately cancel the accumulated DGD.

Embodiments of the present disclosure are not limited to optical fibers. Waveguides patterned on a substrate may also accumulate DGD in each of the polarization modes as light propagates down the length of the patterned waveguide, potentially causing the same problems discussed above.

According to an embodiment, a patterned waveguide is designed to provide substantially zero accumulated DGD on the polarization modes of a propagating beam of radiation. <FIG> illustrate three views of an optical integrated circuit <NUM> that includes a waveguide <NUM> patterned on a substrate <NUM>, according to an embodiment. Waveguide <NUM> may include silicon as a core material. Substrate <NUM> may be a silicon substrate, or may be another semiconducting material such as gallium arsenide or indium phosphide. Substrate <NUM> may also be a glass substrate. In some embodiments, substrate <NUM> is flexible polymer, such as PDMS, polyimide, Parylene, or polyethylene glycol.

Optical integrated circuit <NUM> may include any number of waveguides and/or other optical elements. For example, optical integrated circuit <NUM> may include an optical multiplexer (not illustrated) for switching light between various outputs or an optical amplifier. Other modulating elements such as phase modulators, frequency modulators, and delay elements may be included as well. Further details regarding other optical components that may be included with waveguide <NUM> on optical integrated circuit <NUM> may be found in co-owned <CIT> and <CIT>.

<FIG> illustrates a cross-section view of waveguide <NUM> having a first waveguide segment <NUM> and a second waveguide segment <NUM>, according to an embodiment. <FIG> illustrates a top-down view of the same waveguide <NUM>. <FIG> illustrates a side view of the same waveguide <NUM>. First waveguide segment <NUM> may be characterized as having a width W<NUM>, a height H<NUM> and a length L<NUM> while second waveguide segment <NUM> may be characterized as having a width W<NUM>, a height H<NUM>, and a length L<NUM>. W<NUM> is different from W<NUM>, and H<NUM> is different from H<NUM>. W<NUM> is designed to be greater than width W<NUM> and height H<NUM> is designed to be greater than height H<NUM>. For example, width W<NUM> may be between about <NUM> and <NUM> and width W<NUM> may be between about <NUM> and <NUM>. Height H<NUM> may be between about <NUM> and <NUM> and height H<NUM> may be between about <NUM> and <NUM>. According to an embodiment, length L<NUM> is substantially equal to length L<NUM>.

First waveguide segment <NUM> and second waveguide segment <NUM> may be coupled together via a coupling region <NUM> where the geometry of first waveguide segment <NUM> shifts to match the different geometry of second waveguide segment <NUM>. Coupling region <NUM> may have any length required to smoothly transition waveguide <NUM> from the height H<NUM> of first waveguide segment <NUM> to the height H<NUM> of second waveguide segment <NUM> and from the width W<NUM> of first waveguide segment <NUM> to the width W<NUM> of second waveguide segment <NUM>. In one example, coupling region <NUM> may have a height that tapers from the first height H<NUM> to the second height H<NUM>, and a width that tapers from the second width W<NUM> to the first width W<NUM>. In another example, coupling region <NUM> includes a step change between H<NUM> and H<NUM> rather than a gradual change in height. Coupling region <NUM> may be formed from the same material as first waveguide segment <NUM> and second waveguide segment <NUM>. According to an embodiment, coupling region <NUM> is formed at the same time as first waveguide segment <NUM> and second waveguide segment <NUM>.

Waveguide <NUM> may be formed via a variety of fabrication techniques known to those skilled in the relevant art. For example, a reaction ion etch (RIE) procedure may be used to etch a layer of silicon masked by a patterned photoresist layer to form waveguide <NUM>. The patterned photoresist may also have a non-uniform height, such that the non-uniform height topography is transferred to the silicon layer as the etch proceeds through the thickness of the photoresist. In another example, two masking steps may be used: a first masking step to form the waveguide having a changing width in the coupling region from W<NUM> to W<NUM>; and a second masking step to gradually or abruptly change the height in the coupling region between H<NUM> and H<NUM>.

By changing the cross-section geometry of the two coupled waveguide segments of substantially equal length, any accumulated DGD on each of the polarization states of a beam of light propagating through waveguide <NUM> will cancel out after the light has propagated through the entire length (i.e., L<NUM> + L<NUM>) of waveguide <NUM>.

In other words, the light propagating through the "slow" polarization axis of first waveguide segment <NUM> accumulates a delay DGD<NUM> with respect to the light in the "fast" axis of the same first waveguide segment <NUM>. The "slow" axis light from first waveguide segment <NUM> is then, thanks to the change in both width and height of the waveguide, coupled into the "fast" polarization axis of second waveguide segment <NUM>, where it accumulates -DGD<NUM> with respect to the light propagating through the "slow" axis of second waveguide segment <NUM> which, thanks to the same change in geometry, had previously propagated through the "fast" axis of first waveguide segment <NUM>. As a result, the total DGD between the two polarization components entering waveguide <NUM>, as accumulated in waveguide <NUM> as a whole, is DGD<NUM> - DGD<NUM> = <NUM>, or is substantially zero.

Other modifications made to first waveguide segment <NUM> and second waveguide segment <NUM> are also possible to result in substantially zero accumulated DGD. Further, first waveguide segment <NUM> has a different core refractive index than second waveguide segment <NUM>, such that the accumulated DGD per unit length of first waveguide segment <NUM> is different than the accumulated DGD per unit length of second waveguide segment <NUM>. In these instances, the waveguide segments may have different lengths to ultimately cancel the accumulated DGD. The refractive index of each waveguide segment may be altered via introducing a dopant material into a given waveguide segment, or by applying heat or stress to a given waveguide segment.

<FIG> illustrates an example of a medical device <NUM> that uses optical signals to interrogate a sample <NUM>, according to an embodiment. Medical device <NUM> may include a processing module <NUM> that includes an optical source <NUM> and an optical detector <NUM>. Processing module <NUM> may also include additional circuitry to measure the signal generated at optical detector <NUM> and use it to produce data that can be delivered to a user of medical device <NUM>. The data may include visual information regarding sample <NUM> or material characteristics of sample <NUM> at a given depth. Processing module <NUM> may also include interferometer components used to perform LCI with the light received by optical detector <NUM>. Optical source <NUM> and optical detector <NUM> may be similar to optical source <NUM> and detector <NUM> discussed above with reference to <FIG>.

According to an embodiment, optical source <NUM> generates an incident beam of radiation <NUM> that is coupled into one or more PM fibers <NUM>. One or more PM fibers <NUM> may have a design similar to that described for PM fiber <NUM>, such that any accumulated DGD is substantially cancelled out by the time the light reaches the end of one or more PM fibers <NUM>.

The light propagating through one or more PM fibers <NUM> exists as beam of radiation <NUM> where it is coupled into a waveguide <NUM> patterned on substrate <NUM>, according to an embodiment. Waveguide <NUM> may have a design similar to that described for waveguide <NUM>, such that any accumulated DGD due to coupling misalignment of beam of radiation <NUM> into waveguide <NUM> is substantially cancelled out by the time the light reaches the end of waveguide <NUM>. Waveguide <NUM> may be one component of an optical integrated circuit formed on substrate <NUM>. The optical integrated circuit may include any number of other waveguides and/or other optical elements. In one example, substrate <NUM> is a flexible (including a partially flexible) substrate that includes a polymer material. Example polymer materials include polyimide, polyethylene glycol, Parylene, or PDMS.

The light propagates through waveguide <NUM> and may propagate through one or more other waveguides before exiting from substrate <NUM> towards sample <NUM> as beam of radiation <NUM>. Scattered or reflected radiation <NUM> is received from sample <NUM> back towards substrate <NUM> where it is coupled into one or more waveguides on substrate <NUM>. The one or more waveguides that receive scattered or reflected radiation <NUM> may include the same waveguide <NUM>.

The light received from the sample exits from substrate <NUM> as beam of light <NUM> that is coupled back into one or more PM fibers <NUM>. Beam of light <NUM> may be coupled into the same one or more PM fibers used to propagate incident beam of radiation <NUM>, or it may be coupled into different PM fibers. After traversing the length of one or more PM fibers <NUM>, the light exits as beam of radiation <NUM> where it is received ultimately by an optical detector <NUM>. In some embodiments, medical device <NUM> includes an optical interferometer such that the light returning from sample <NUM> is combined with light that traversed a separate reference path as discussed above with reference to <FIG>. The light returning from sample <NUM> may be combined with the reference light either on substrate <NUM> or in processing module <NUM>.

Embodiments of the present invention have been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof.

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the termino logy or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance.

Claim 1:
An optical integrated circuit (<NUM>), comprising:
a substrate (<NUM>); and
one or more waveguides (<NUM>) patterned on the substrate (<NUM>), wherein a first waveguide (<NUM>) of the one or more waveguides (<NUM>) is configured to guide a beam of radiation and comprises a first waveguide segment (<NUM>), configured to impart a first differential group delay on the beam of radiation, a second waveguide segment (<NUM>), configured to impart a second differential group delay on the beam of radiation, and a coupling region (<NUM>) between the first and second waveguide segments (<NUM>, <NUM>), wherein the first waveguide segment (<NUM>) has a different core refractive index than the second waveguide segment (<NUM>), and wherein a sum of the first differential group delay and the second differential group delay is substantially zero,
wherein a cross-sectional area of the first waveguide segment (<NUM>) is different from a cross-sectional area of the second waveguide segment (<NUM>), and
wherein the first waveguide segment (<NUM>) has a first height (Hi) and a first width (W<NUM>), and the second waveguide segment (<NUM>) has a second width (W<NUM>) greater than the first width (W1), characterized in that the second waveguide segment (<NUM>) has a second height (H2) less than the first height (H1).