Internal optical elements produced by irradiation-induced refractive index changes

Systems and methods are provided for forming an optical element within a transparent material using an irradiating optical beam, where the irradiating optical beam is employed to induce internal refractive index changes in the transparent substrate. Optical elements such as bulk and gradient index lenses may be formed in the transparent structure according various embodiments of the disclosure. An optical element may be formed by selecting a refractive index profile for the optical element, determining a corresponding suitable spatially dependent irradiation intensity profile for producing the selected refractive index profile, focusing an irradiating optical beam within the transparent structure, and controlling an intensity and position of the focused irradiating optical beam within the transparent structure according to the spatially dependent irradiation intensity profile.

BACKGROUND

The present disclosure relates to methods of producing optical refractive structures within transparent materials.

Conventional optical lenses, as have been made since the time of Galileo, are produced through the polishing and shaping of glass. Another form of a lens is the gradient refractive index lens, or GRIN lens. GRIN lenses employ a spatially varying refractive index to produce a focusing effect. GRIN lenses may be produced according to a number of known methods, including ion embedding, sol-gel methods, and the use of photonic crystal fibers. Unfortunately, such methods limit the degree to which the desired properties of the GRIN lens can be controlled.

SUMMARY

Systems and methods are provided for forming an optical element within a transparent material using an irradiating optical beam, where the irradiating beam is employed to induce internal refractive index changes in the transparent substrate. Optical elements such as bulk and gradient index lenses may be formed in the transparent structure according various embodiments of the disclosure. An optical element may be formed by selecting a refractive index profile for the optical element, determining a corresponding suitable spatially dependent irradiation intensity profile for producing the selected refractive index profile, focusing an irradiating beam within the transparent structure, and controlling an intensity and position of the focused irradiating beam within the transparent structure according to the spatially dependent irradiation intensity profile.

In one aspect, there is provided a method of forming an optical element having a pre-selected refractive index profile within a transparent substrate, the method comprising the steps of: (a) focusing an optical irradiating beam within the transparent substrate, wherein beam parameters of the optical irradiating beam are sufficient to generate a permanent refractive index change, and wherein the transparent substrate is substantially transparent over a wavelength region associated with the optical irradiating beam, and scanning a position of the focus of the optical irradiating beam within the transparent substrate for forming the pre-selected refractive index profile within the transparent substrate;

(b) directing an additional optical beam through the transparent substrate, such that the additional optical beam is refracted by the optical element, and measuring one of more properties of the additional optical beam after it is transmitted by the optical element; (c) comparing the one of more properties with at least one reference property to determine an error profile, the error profile associated with a difference between the refractive index profile formed by the optical irradiating beam and the pre-selected refractive index profile; and (d) controlling the beam parameters of the optical irradiating beam while scanning the optical irradiating beam to reduce the error profile.

In another aspect, there is provided a method of forming an optical element having a pre-selected refractive index profile within an acoustically active transparent substrate, the method comprising the steps of: focusing an optical irradiating beam within the acoustically active transparent substrate, wherein beam parameters of the optical irradiating beam are sufficient to generate a permanent refractive index change, and wherein the acoustically active transparent substrate is substantially transparent over a wavelength region associated with the optical irradiating beam; and wherein the optical irradiating beam is focused within the acoustically active transparent substrate such that the pre-selected refractive index profile is formed within the transparent substrate.

In another aspect, there is provided a system for forming an optical element having a pre-selected refractive index profile within a transparent substrate, said system comprising: an optical source for generating an optical irradiating beam, wherein beam parameters of the optical irradiating beam are sufficient to generate a permanent refractive index change within the transparent substrate when the transparent substrate is substantially transparent over a wavelength region associated with the optical irradiating beam; a focusing and scanning apparatus for focusing the optical irradiating beam within the transparent substrate and scanning a position of the focus of the optical irradiating beam within the transparent substrate for forming the pre-selected refractive index profile within the transparent substrate; an additional source for generating an additional optical beam and transmitting the additional optical beam through the transparent substrate, such that the additional optical beam is refracted by the optical element; and a detector configured to measure one of more properties of the additional optical beam after it is transmitted by the optical element.

In another aspect, there is provided a method of modifying the refractive index of an optical component embedded within an ultrasonic transducer, the method comprising the steps of: focusing an optical irradiating beam within the optical component, wherein beam parameters of the optical irradiating beam are sufficient to generate a permanent refractive index change, and wherein the optical component is substantially transparent over a wavelength region associated with the optical irradiating beam, and scanning a position of the focus of the optical irradiating beam within the optical component.

DETAILED DESCRIPTION

As used herein, the terms “about” and “approximately”, when used in conjunction with ranges of dimensions of particles, compositions of mixtures or other physical properties or characteristics, are meant to cover slight variations that may exist in the upper and lower limits of the ranges of dimensions so as to not exclude embodiments where on average most of the dimensions are satisfied but where statistically dimensions may exist outside this region. It is not the intention to exclude embodiments such as these from the present disclosure.

As used herein, the term “wavelength range” refers to both continuous and discontinuous wavelength ranges.

In selected embodiments as described below, an optical refractive element is produced within a transparent structure by inducing localized index of refraction changes by irradiating the structure with an irradiating optical beam. The irradiating optical beam has a wavelength, intensity and fluence that is suitable for inducing a non-transient change in the refractive index within the transparent structure when the irradiating optical beam is focused within the transparent structure. During irradiation, the position of the focus of the irradiating optical beam is positioned such that the induced refractive index change is generated with a spatial profile suitable for forming the internal optical refractive element. The optical refractive element is formed such that it acts as a lens or other refractive element when an internal optical beam is incident on the internal optical element from within the structure. In some embodiments, the internal optical element is a gradient index (GRIN) optical lens.

The transparent structure may be any structure having a transparent internal volume and exhibiting refractive index change in response to irradiation with a suitable optical beam. Examples of transparent structures include transparent substrates and optical fibers. The transparent structure may be a composite structure that is only optically transparent over a subset of its total volume or spatial extent.

The properties of the material forming the transparent structure, and the parameters of the focused irradiation beam, are selected such that a suitably large refractive index change is locally produced within the transparent structure in response to the focused irradiating optical beam. In some embodiments, the refractive index change may be on the order of 10−3to 10−2, depending on the properties of the irradiating optical beam and the irradiated material.

In some embodiments, the material may exhibit an induced refractive index change in response to the irradiating optical beam provided that the irradiating optical beam does not exceed an intensity or fluence threshold. In other embodiments, the material may exhibit an induced change in refractive index that occurs only after a threshold intensity and/or fluence has been delivered to a given focal volume within the transparent structure where the irradiating optical beam is focused. Accordingly, a two- or three-dimensional refractive index spatial profile may be written or transcribed into the transparent structure by varying the location of the focal volume within the transparent structure, thus defining the internal optical element.

The writing of the refractive index spatial profile may be achieved by scanning the irradiating optical beam relative to the transparent structure. Scanning of the beam may involve changing the position and/or orientation of the incident beam and/or the transparent structure, for example, using a scanning mechanism. The scanning mechanism may include one or more scanning mirrors and/or one or more translation devices.

It is to be understood that the irradiating optical beam may produce a change in both the real and imaginary refractive index, and that changes in the imaginary refractive index may be induced, provided that when transmitting an incident optical beam through the internal optical element, the optical beam is transmitted without substantial optical absorption.

Suitable materials for forming the transparent structure include glasses and polymers that are known to exhibit a change in refractive index in response to optical irradiation. The transparent structure, or transparent portion of the structure, may be formed at least in part from a transparent plastic such as polymethylmethylaccrylate (PMMA), PDMS, or polycarbonate. In another example implementation, the transparent structure may be formed from a glass material, such as a doped glass. For example, the glass may be doped via ion embedding or implantation, where the dose of ions is suitable for obtaining a desired level of refractive index change under irradiation by the irradiating optical beam.

In one embodiment, the induced changes in the refractive index are produced by a local change in the density of the material in response to the irradiating optical beam, provided that a threshold has been exceeded. Such a densification process is typically a multiphoton process and thus allows for the writing of features sizes diameters smaller than the focal spot. This mechanism has been demonstrated to produce optically induced changes in the refractive index on the order of 10−2to 10−3with a spatial resolution on a micron scale in some materials. In another embodiment, the transparent structure may be formed from a material that is selected to exhibit photorefractive properties.

As described above, the internal optical element is formed such that it is substantially transparent. The level of transparency of the induced internal optical element may be such that an internally propagating incident optical beam is internally directed towards, and optically refracted by, the internal optically element, without encountering substantial optical absorption by the structure.

It is to be understood that the transparent structure may only be transparent over a portion of the optical spectrum. The transparent structure may be transparent to the incident optical beam over at least a portion of one or more regions of the optical spectrum, including the ultraviolet, visible, and infrared spectral regions.

The transparent structure may be provided such that the incident optical beam is optically waveguided within the structure prior to encountering, during encountering and/or after encountering the internal optical element. For example, the transparent structure may include a passive waveguide prior to exposure to the irradiating optical beam, such a fiber optic waveguide defined by suitable core and cladding regions. Alternatively, the transparent structure may be initially free of optically confining structures.

The irradiating optical beam may have one or more properties (for example, a wavelength, intensity and/or fluence) such that the irradiating optical beam is suitable for producing a local change in the density of the structure through a linear or nonlinear optical absorption process, such that the resulting change in the density of the structure produces a corresponding change in refractive index.

In one example implementation, the irradiating optical beam includes one or more laser pulses, where the pulses are ultrafast pulses. The pulse length of the ultrafast pulses may be, for example, in the nanosecond regime, picosecond regime, or femtosecond regime, provided that the pulsewidth is sufficiently short to produce, at the beam focus within the optical structure, a focus spot that exceeds the threshold for the generation of a refractive index change. In some embodiments, the wavelength of the laser pulse may be within the extreme ultraviolet (50-250 nm), visible (400-800 nm) or infrared (800 nm-3 um) spectral range. In another example implementation, the irradiating optical beam may have a wavelength within the x-ray regime.

Suitable parameters of the optical irradiation beam for producing refractive index changes in a wide variety of transparent material include those described in U.S. Pat. No. 6,573,026, (Aitken et al.), U.S. Pat. No. 6,884,960 (Bourne et al.), and U.S. Pat. No. 6,853,785 (Dunn et al.), which are all incorporated herein by reference in their entirety. According to one example implementation, index of refraction changes required to achieve lensing conditions as described herein can be realized with 550 nm laser pulses of 150 fs duration with a writing energy of 1.3 μJ/pulse at a 250 kHz repetition rate, and at a 400 μm/s feed rate and with a beam waist of 1.5 μm into a fused silica plate at a depth of 0.5 mm. One skilled in the art will appreciate that in practicing the embodiments of the present disclosure, only reasonable and routine experimentation will be required to determine suitable parameters of the irradiating optical beam for producing a desired refractive index change in a given material.

In some example implementations, wavelengths used for the irradiating beam may include 532 nm, 790 nm, 1064 nm, 1310 nm and 1550 nm. In some embodiments, pulse duration can vary across a broad range, 30 fs to 500 fs and pulse repetition is typically seen in the range of 1 kHz to 100 MHz. In some embodiments, energy per pulse can range from approximately 0.02 μJ to as approximately 6 μJ. In some embodiments, the feed speed of the sample relative to the irradiating beam can also be varied between approximately 50 μm/s to 1000 μm/s.

Referring now toFIG. 1a, the fabrication of an internal optical element within a transparent structure using an irradiating optical beam is illustrated. As shown in the Figure, irradiating optical beam54is directed through an external focusing element, such as objective lens55, and is focused to an internal focal volume53within transparent structure51. As described above, irradiating optical beam produces a change in the refractive index of transparent structure51within the local focal volume53. In some embodiments, such as those involving processes that produce local densification, the refractive index change within the focal volume may be positive. In other embodiments, such as processes that produce voids or optical filaments, the refractive index change within the focal volume may be negative.

Transparent structure51may have one or more curved external surfaces. For example, structure51may be an optical fiber. In such a case, the densification pattern shown inFIG. 1ais written along the longitudinal axis of the structure51. InFIG. 1b, the transparent structure51may be a cylindrical structure, such as an optical fiber. Accordingly, in the case shown inFIG. 1b, the irradiating optical beam54is directed through an end-face of the cylindrical structure.

FIG. 1(c)provides a flow chart illustrating an example method for forming an optical element within a transparent structure. In step210, an optical source is provided for generating an irradiating optical beam, where the irradiating optical beam has a wavelength suitable for transmission within the transparent substrate. In step220, a suitable refractive index spatial profile for forming the optical element within the transparent substrate is determined. In step230, a suitable spatially dependent irradiation intensity profile is determined for producing the suitable refractive index spatial profile. This step may be achieved, for example, based on a known relationship between the irradiation beam intensity and the induced change in the refractive index profile, or, for example, based on pre-determined calibration data. In step240, the irradiating optical beam is focused with a profile sufficient to generate a suitable refractive index change within the transparent substrate. In step250, the a position and beam profile of the irradiating optical beam is controlled according to the suitable spatially dependent irradiation beam profile, optionally based on a feedback signal, as further described below.

FIG. 2illustrates an example method for producing an internal optical element within a transparent structure. In the embodiment shown, irradiating optical beam54is focused by lens55and is directed into substrate51, where the focusing is provided such that the focal volume within the substrate is of a size suitable for producing the internal optical element without requiring further relative translation and/or scanning of the irradiating optical beam. Such an embodiment may be employed to generate an internal optical element having a spatially dependent refractive index profile. The focusing lens55, or another optical focusing or beam shaping element, may be provided to generate a suitable refractive index profile. In one example embodiment, the spatially varying refractive index profile may be generated to provide a gradient index profile over at least a portion of the internal optical element, which may be employed to generate an internal GRIN lens, as further described below. In particular, the intensity roll-off characteristic of the focused irradiating optical beam, and the corresponding roll-off characteristic of the induced refractive index change (for example, due to densification), may be selected to provide a suitable GRIN refractive index profile. It has been reported that densification roll-off characteristics from femtosecond lasers approximate a Gaussian curve. The present method is desired for its simplicity in manufacturing, but requires very specific beam characteristics.

Referring now toFIG. 3, an illustration is provided of an example system100for producing an internal optical element within a transparent structure using an irradiating optical beam. System100includes irradiating optical beam source102for producing irradiating optical beam104, irradiating optical beam focusing lens106, and control and processing unit125, which are employed for irradiating transparent structure108and producing internal optical element110.

Irradiation beam source102may be an optical source such as a laser, although it is not necessary for irradiation beam source102to emit coherent light. Irradiation beam104is directed onto transparent structure108, where it induces the formation of internal optical element110having a different refractive index than that of transparent structure108.

Internal optical element110is shown, by way of example only, as a focusing bulk lens formed from irradiating a volume of the transparent structure. In the presently illustrated example embodiment, internal optical element110is formed by focused irradiating optical beam104without requiring further positioning of irradiating optical beam104. As shown in the Figure, internal optical element110is formed such that an incident optical beam114is focused upon transmission through internal optical element110.

Control and processing unit125, which is described in further detail below, is employed for the control of irradiation beam source102. As shown in the Figure, control and processing unit may provide a feedback signal to irradiation beam source102based on a signal detected by detector112, where the signal is associated with a property of the transmitted optical beam113, as further described below. Detector112may be an array detector for detecting a beam profile. Alternatively, detector112may be a single element detector for detecting a local intensity, optionally further including a spatial filter. System100may also include optical beam source116for producing optical beam114. Control and processing subsystem125may be integrated with one or more of the other subsystems of system100.

Some aspects of the present disclosure can be embodied, at least in part, in software. That is, the techniques can be carried out in a computer system or other data processing system in response to its processor, such as a microprocessor, executing sequences of instructions contained in a memory, such as ROM, volatile RAM, non-volatile memory, cache, magnetic and optical disks, or a remote storage device. Further, the instructions can be downloaded into a computing device over a data network in a form of compiled and linked version. Alternatively, the logic to perform the processes as discussed above could be implemented in additional computer and/or machine readable media, such as discrete hardware components as large-scale integrated circuits (LSI's), application-specific integrated circuits (ASIC's), or firmware such as electrically erasable programmable read-only memory (EEPROM's).

FIG. 3(andFIG. 5below) provides an example implementation of control and processing unit125, which includes one or more processors130(for example, a CPU/microprocessor), bus170, memory135, which may include random access memory (RAM) and/or read only memory (ROM), one or more internal storage devices140(e.g. a hard disk drive, compact disk drive or internal flash memory), a power supply145, one more communications interfaces150, external storage155, a display160and various input/output devices and/or interfaces165(e.g., a receiver, a transmitter, a speaker, a display, an imaging sensor, such as those used in a digital still camera or digital video camera, a clock, an output port, a user input device, such as a keyboard, a keypad, a mouse, a position tracked stylus, a position tracked probe, a foot switch, and/or a microphone for capturing speech commands).

Although only one of each component is illustrated inFIG. 3, any number of each component can be included control and processing unit100. For example, a computer typically contains a number of different data storage media. Furthermore, although bus170is depicted as a single connection between all of the components, it will be appreciated that the bus170may represent one or more circuits, devices or communication channels which link two or more of the components. For example, in personal computers, bus170often includes or is a motherboard.

In one embodiment, control and processing unit125may be, or include, a general purpose computer or any other hardware equivalents. Control and processing unit125may also be implemented as one or more physical devices that are coupled to processor130through one of more communications channels or interfaces. For example, control and processing unit125can be implemented using application specific integrated circuits (ASIC). Alternatively, control and processing unit125can be implemented as a combination of hardware and software, where the software is loaded into the processor from the memory or over a network connection.

Control and processing unit125may be programmed with a set of instructions which when executed in the processor causes the system to perform one or more methods described in the disclosure. For example, control and processing unit125may be programmed to scan the irradiating optical beam104within the sample over a controlled scan profile with controlled beam parameters for forming an internal optical element. Control and processing unit125may also be programmed to control the parameters of the irradiating optical beam104in response to feedback signals received from a detector112, where the feedback signals are associated with the profile of an optical beam transmitted114through the transparent structure108during or after the formation of the internal optical element110. Control and processing unit125may include many more or less components than those shown.

Referring now toFIG. 4, an illustration is provided showing an alternate embodiment in which the irradiating optical beam54is scanned within the transparent substrate to generate the internal optical element. As shown in the Figure, the beamwidth of the irradiating optical beam is smaller than the size of the internal optical element. Irradiating optical beam54is scanned relative to the optically transparent substrate51. During scanning, parameters of the irradiating optical beam may be varied in order to produce different local changes in the refractive index. Some examples of the parameters that may be adjusted include: pulse length, pulse power, pulse repetition frequency, optical wavelength, beam width, beam shape, feed rate, and number of passes. These parameters may be varied in response to a feedback signal, as further described below.

The scanning pattern59shown in the Figure illustrates an example profile in which irradiating optical beam54is scanned in a scanning path defined by concentric circular paths. As noted above, during each pass, one or more of many parameters may be varied to achieve a different index of refraction for that path.

Referring now toFIG. 5, an illustration is provided of another example system200for producing an internal optical element within a transparent structure using an irradiating optical beam. System200includes irradiating optical beam source102for producing irradiating optical beam104, scanning and focusing assembly118for focusing irradiating optical beam104and scanning irradiating optical beam104, and control and processing unit125, which are employed for scanning and irradiating transparent structure108and producing internal optical element110. System200may further include a translation assembly122for translating the position of the transparent structure during scanning, for example, to vary the relative depth of the focused irradiating optical beam120.

In one embodiment, scanning and focusing assembly118may include a two-dimensional galvanometer and a focusing lens (such as a telecentric or f-theta lens) for scanning the focus of the irradiating optical beam120in the two-dimensional lateral plane, and translation assembly122may include a motorized translation stage for controlling the depth of the beam focus within the transparent structure. In another example implementation, translation assembly122may further include, or be replaced by, a rotation assembly, for example, for rotating a cylindrical transparent structure during scanning.

Referring now toFIG. 6, an illustration is provided of an example method and apparatus for providing a feedback signal during formation of the internal optical element53. Unlike other feedback-based methods, the present embodiment employs an additional optical beam56that propagates internally within substrate51and is incident upon internal optical element53, without inducing refractive index changes within optical element53. As described further below, in some embodiments, optical element53is formed to focus or otherwise influence the propagation of an optical beam propagating internally within substrate51and having pre-selected beam parameters, and additional optical beam56may be provided having substantially the same beam parameters. Such an embodiment provides feedback that that is directly related to the performance of optical element53.

A camera or other imaging device57(such as a CCD camera, or a commercial beam profiler) is provided to generate a signal associated with the transverse profile of additional optical beam56after additional optical beam56has been transmitted through the generated optical element53. Imaging device57may be placed at a location corresponding to a desired focal plane of a fully generated internal optical element53.

The image or beam profile detected by imaging device57may be employed to actively control the formation of internal optical element53. For example, the spatial profile of the transmitted additional beam may be processed (e.g. by control and processing unit125) in order to detect any potential errors. Control and processing unit125may then be used to compare to a feedback signals to previously measured or simulated beam profiles. Alternatively, mathematical models may be derived to determine ideal irradiating parameters based on feedback signals.

In one example implementation, the measured beam profile may be compared with a reference beam profile (e.g. a previously measured or simulated beam profile) in order to infer a deviation between the spatial profile of the refractive index variations formed within substrate51and the desired refractive index profile. A spatially dependent error profile, based on, for example, the difference or ratio between the measured beam profile and the reference beam profile, may be calculated. This error profile, or another such comparative measure, may be employed to determine an additional amount of fluence and/or intensity that is delivered to substrate51.

For example, the error profile may be employed to determine, on a local (e.g. per pixel of imaging device57) basis, an additional amount of exposure of the optical irradiating beam (e.g., in terms of fluence and/or intensity) that is needed in order to further process substrate51such that the measured spatial profile (and hence the refractive index profile) is modified to a correct or suitable level, for example, such that the error profile is reduced. This may be employed to correct for imperfections in a pre-formed optical element.

In one example implementation, only a fraction of the amount of additional exposure is delivered in order to avoid overexposure, and the process is repeated one or more times until a suitable level of convergence is achieved. This embodiment may also be practiced when performing the initial exposure, such that overexposure is avoided in all processing steps.

FIG. 7shows another example system, in which irradiation beam54is directed through an end face of transparent structure51for inducing refractive index changes along the longitudinal direction of transparent structure51(for example, along a longitudinal axis of a cylindrical structure, such as an optical fiber). Incident optical beam56is directed through at least a portion of transparent structure56(which may include an optical waveguide such as a fiber optic core) and is transmitted by optical element53and is detected by detector array57in order to assess the resulting beam profile after transmission and refraction. The resulting signal is provided to control and processing unit125for generating a feedback signal to control parameters of irradiation beam54, as noted above.

Dichroic mirror58, or another suitable beam sampling optical element (such as an optical window for generating a Fresnel reflection), is provided between objective lens55and optically transparent structure51. Beam sampling element58substantially transmits irradiating optical beam54, while reflecting at least a portion of optical beam56towards detector or imaging device57for assessing the beam produced by the lens writing procedure. The dichroic mirror58is selected to be predominantly transparent to irradiating optical beam54and is located sufficiently remote from the focal point of the irradiating optical beam after passing through lens55. The imaging information received from optical imaging device57can be used as feedback for tuning the internal optical element53as it is being written.

In another embodiment, an alternative optical arrangement may be provided in which a dichroic mirror may be employed to reflect, rather than transmit, irradiating optical beam54, while transmitting, rather than reflecting, optical beam56. It will be understood that the illustrated feedback embodiments are merely provided as examples, and other suitable optical arrangements may be employed without departing from the scope of the present embodiments.

In other embodiments, the preceding feedback-based embodiments may be employed as a method of tuning, re-working, trimming, or otherwise modifying the refractive index of optical element53as a post-processing step. For example, such embodiments may be performed for modifying optical element53in order to correct for fabrication errors. In another example implementation, substrate51may include a pre-formed optical element that is formed according to another process, such as a conventional lens making process (including, but not limited to, polishing, molding, and embossing), and the preceding feedback-based embodiments may be employed to post-process the pre-formed optical element. For example, optical element53may be a gradient index lens that is formed via an ion exchange process, and the post-processing steps may be performed for correcting and/or fine tuning the optical properties or performance of the gradient index lens.

FIGS. 8 to 15illustrate a non-limited set of optical elements that may be formed within a transparent structure according to various example embodiments provided herein.FIG. 8aillustrates a bulk optical element52fabricated inside a substrate51according to an embodiment of the present disclosure, where the refractive index of internal optical element52is modified relative to the baseline refractive index of substrate51. Element52consists of two convex surfaces such that when an internal incident optical beam is incident on the element, from within the substrate, it is focused to a depth as determined by the curvatures of each side of the feature and the difference between the index of refraction of the irradiated and non-irradiated portions of the substrate. Line A illustrates a cross-section through the substrate (including internal optical element52) that is portrayed inFIG. 8b.

FIG. 9aillustrates an optical element90fabricated inside a substrate51according to another example embodiment. This element consists of a planar surface formed in the substrate on one side of element along with a curved surface on the other side of the element, thus defining an internal plano-convex lens. Lines2a,2b,2cillustrate cross-sections portrayed inFIGS. 9b, 9cand 9drespectively.

FIG. 10illustrates an optical element91fabricated inside a substrate51according to another example embodiment. This element is composed of two concave surfaces formed on each side of the element, thus defining an internal double concave lens. This element is suitable for negatively focusing, or diverging, an incident optical beam propagating within substrate51, where the focusing or diverging is dependent on the curvature of each of the sides of the structure and the change in index of refraction of between the irradiated and non-irradiated substrate.

FIG. 11illustrates an optical element92fabricated inside a substrate51according to another example embodiment. This element is composed of a planar surface on one side and a concave surface on the other, thus defining a plano-concave lens. Similar to the element described inFIG. 10, this element acts to negatively focus, or diverge, an incident optical beam.

FIG. 12aillustrates an optical element93fabricated inside a substrate51according to another example embodiment. This element is composed of a planar surface on one side and another surface, which is conical in shape. This element defines an internal axicon lens, such that, for example, when an optical beam is incident on the lens from within substrate51, the lens acts to create a Bessel beam from the incident light. Lines5a,5b, and5cillustrate cross sections that are illustrated inFIGS. 12b, 12cand 12drespectively.

FIGS. 13a-13eillustrate alternative example implementations of the embodiments shown inFIGS. 8-12, where in each example illustration, the internal optical element is shown orientated at an angle θ relative to an axis or surface plane of the substrate.

Although the preceding embodiments illustrate a single internal optical element, it is to be understood that two or more internal optical elements may be formed within the transparent substrate or structure.FIG. 14aillustrates a multi-element example implementation in which two optical elements94and95are fabricated within substrate51. Similarly,FIG. 14billustrates another multi-element example implementation in which optical elements96and97are fabricated within substrate51. As shown inFIG. 14b, a telescopic configuration is provided where the curvature is differed on each geometric lens.

In one embodiment, the optical element may be internal into a substrate, and the substrate may be subjected to stretching and/or compression to induce optical changes to the internal element. Stretching and/or compression of the substrate results in a change of shape and/or density of the optical element, thereby changing the propagation of light through the optical element.

FIG. 14cillustrates an example embodiment in which stretching (expansion) and/or compression are electrically applied via piezoelectric forces. As shown in the Figure, substrate80which can be compressed or rarified under electrical or mechanical stimulation is an optically transparent substrate with an internal optical element98which, for example, may be of elements52,53,90-97and110. A voltage may be applied between the positive electrode89and negative electrode99to generate an internal electric field and thereby induce compression and/or stretching in the material via piezoelectric forces.

This embodiment allows either a user or system to dynamically change the focusing properties of the lens98and therefore dynamically change the focusing of beam. A feedback signal, generated in response to changes in the properties of a beam transmitted through optical element98, may be employed to actively vary the optical properties of element98.FIG. 14dillustrates how the substrate80and optical element98would change during compression or stretching. It should also be noted that the positive and negative electrodes89and99may additionally or alternatively located on the front and back surfaces that are perpendicular to the incoming beam60(for example, via the application of transparent conductive electrodes).

FIG. 15aillustrates an optical element53fabricated inside a substrate51according to another example embodiment. As illustrated in the drawing, a gradient index (GRIN) lens is formed by spatially varying the density of the transparent in a graded fashion. The induced GRIN lens53acts to focus an incident beam. Here, optically transparent substrate51is optically irradiated to form an optical element53with a variable refractive index pattern.

This pattern can be produced in a number of different ways, as discussed above. For example when forming a lens within a cylindrical substrate, the GRIN pattern may be formed by interrogating the optical substrate end face and scanning using a trepanning or spiral pattern described inFIG. 4, while adjusting the irradiating optical beam properties according to the desired gradient (i.e. one may increase the intensity of the irradiating optical beam gradually the center of the desired lens is approached). Alternatively, a constant beam profile may be used, and areas of increased desired density may be irradiated with greater frequency. In these example cases, it is assumed that the depth of field of the irradiating optical beam is sufficiently deep such that index changes are constant with depth throughout the desired thickness of the lens. Yet another alternative approach is to use an optical irradiating beam that has a very short depth of field and irradiating the substrate radially. In this case, either the laser source, or more preferably, the substrate may be rotated during irradiation. The focal depth and beam parameters are modified to create an index gradient with respect to the depth within the substrate, creating a gradient index shift from the outermost regions to the innermost regions. An example profile of the change in refractive index62due to the influence of the irradiating optical beam54is shown inFIG. 15b.

As shown inFIG. 15b, the index of refraction of the formed GRIN lens is highest at the center. As distance from the center of the lens increases radially outward, the index of refraction has a gradient change from high to low. A typical gradient profile from the center of the lens can be calculated based on the following formula:

N⁡(r)=No⁡[1-k2⁢r2],
where Nois the base index of refraction at the center of the lens, k is the gradient constant (conventional gradient constants are in the 0-5.5 mm−1range) and r is the distance from the center of the lens measured in millimeters. For example, an index of refraction at the center for a GRIN lens may be in the range of 1.4715 and can decrease down to ˜1.4570. Because of the gradient in the index of refraction, the light propagates in a sinusoidal fashion where one full sinusoidal path (2 π) is known as 1 pitch, and the physical pitch length (p) is determined by:

Because of the sinusoidal path, if the GRIN lens were to be varied to different lengths, this would cause the propagating light beams to exit the gradient medium at different angles causing changes in how the light will be focused.FIG. 16ashows an optical waveguide87coupled to a GRIN lens,53, that is exactly 1 pitch in length. Beam path61shows the sinusoidal beam within the lens while external beam path63demonstrates how the beam propagates once it exits the gradient medium.FIG. 16bshows GRIN lens length of ¼pitch, causing63to be collimated.FIGS. 16cand 16dexhibits how the numerical aperture of the lens is affected by varying the lens length. A well-defined numerical aperture of the lens can be achieved by designing an exact fractional pitch length.

In one embodiment, the transparent structure may be or include an optically transparent acoustically active material. Non-limiting examples of optically transparent acoustically active material may be PVDF, lithium niobate, and single crystal piezoelectrics such as PMN-PT, PZN-PT or PNN-PT. Such materials may be modified to enhance their photosensitivity and/or to increase the change in refractive index that is generated under irradiation by the irradiating optical beam. For example, a lithium niobate crystal may be ion doped to increase a photorefractive effect within the crystal.

In one embodiment, an internal optical element is formed within an optically transparent and acoustically active material, such as that of an ultrasound transducer, such that the acoustically active material may be employed both for the generation and/or detection of ultrasound radiation, and also for the transmission and collection of an optical beam. Example materials having suitable optically transparency within the visible spectrum and/or the near infrared spectrum include PVDF, lithium niobate, single crystal piezoelectric materials such as PMN-PT, PZN-PT or PNN-PT

An example of such an embodiment is provided inFIG. 17a-c, where an optical irradiation beam is employed to produce an internal optical element83within an optically transparent and acoustically active material80via optically induced refractive index changes.

FIG. 17ashows a side view of an acoustically active material disc80prior to optical irradiation. An internal optical element83, such as a lens (for example, a GRIN lens) is then directly and optically formed in disc80according to the aforementioned embodiments.FIG. 17billustrates the acoustically active disc80after irradiation, including internal optical element83.

Without intending to be limited by theory, it is believed that the refractive index change produced within the acoustically active substrate may arise from one or more of several possible mechanisms. Some mechanism may be temporary, while others may be long-term or permanent. In some embodiments, the irradiating optical beam is configured to produce permanent refractive index changes.

The specific beam parameters for processing a given transparent acoustically active material will vary depending on the material. In one example embodiment, suitable beam parameters may be determined, for example, by focusing an optical irradiating beam within the transparent acoustically active material and varying the beam parameters (for example, varying the beam parameters among the ranges provided and/or referenced above) and inferring the induced refractive index change and its dependence on the beam parameters. The refractive index may be measured, for example, via elipsometry, and/or the refractive index may be indirectly assessed by measuring the transmission and/or deflection of a beam transmitted through optical element. The beam parameters may be varied in order to obtain a suitable modified refractive index.

Additionally, in one embodiment, the optical beam parameters may be selected according to a constraint that one or more acoustic properties of the acoustically active material remain substantial constant, or change within a pre-determined interval, or below a pre-determined threshold. For example, the optical beam parameters may be selected such that a crystal phase within the irradiated region of the transparent acoustically active material remains constant. According to another example, the beam parameters may be selected such that a resonance frequency changes by less than a preselected threshold. In yet another example, the beam parameters may be selected such that an amplitude of ultrasound waves generated in response to a given excitation waveform exceeds a pre-selected threshold. In still another embodiment, the beam parameters may be selected such that the ultrasound transducer is capable of receiving reflected ultrasound waves with a minimum signal to noise ratio. In another example, the beam parameters may be selected such that the directivity of the ultrasound transducer satisfies a pre-determined criterion. Such embodiments allow the use of a substantial portion of the acoustic aperture for ultrasound generation and/or detection while allowing passage of an optical imaging signal through the formed optical element.

In another example implementation, the beam parameters may be selected such that the crystalline phase of the transparent acoustically active material changes from piezoelectric to non-piezoelectric, such that the irradiated zone no longer plays a role in the generation of ultrasound waves.

As noted above, and as shown inFIGS. 6 and 7, the optical element53may be formed while monitoring performance of an additional optical beam. In embodiments in which the optical element is formed within a transparent acoustically active material, various additional methods of online monitoring are also possible.

In one example embodiment, one or more acoustic signals and/or properties of the acoustically active material are measured and monitored during formation of optical element53. Such an embodiment is shown inFIG. 17c, in which transparent acoustically active material300is irradiated by optical beam54to form internal optical element53. Acoustically active material300is electrically excited via wires320, resulting in the emission of acoustic waves310. The emitted acoustic waves310are received and detected by acoustic detector315. In other embodiments, an acoustic reflector may reflect the emitted acoustic waves back to acoustically active material300for direct detection (e.g. temporally multiplexed acoustic excitation and detection).

FIG. 17dshows an embodiment in which both acoustic and optical monitoring are performed (for example, serially or in parallel) while forming optical element53. Beam redirection element330(e.g. a mirror, beam pick-off, or dichroic beamsplitter) directs additional optical beam56onto imaging device57, as described above, while acoustic waves310transmitted by ream redirection element330are detected by acoustic detector315.

The detected signal associated with the acoustic waves may be compared with a reference signal (e.g. a previously measured or simulated signal) in order to infer a deviation between the acoustic properties of the processed material and the initial acoustic properties. If such a deviation is detected, for example, a deviation beyond a given threshold, an action may be taken, such as the termination of the process, or a change in one or more parameters of optical irradiation beam54. As noted above, the acoustic properties that may be inferred from the detected acoustic signal include a crystal phase, resonance frequency, signal to noise ratio, and directivity.

As shown inFIG. 17c, acoustically active material80with internal optical element83can be assembled with one or more additional optical elements or components, such that the internal optical element is in optical communication with at least one of the one or more additional optical elements. For example, the acoustically active material may be interfaced with an optical waveguide, such as an optical fiber, such that a focal plane of internal optical element83is positioned to align with the entrance or exit aperture of the waveguide. Other example optical elements for integration with the internal optical element include light-bending or reflective elements such as prisms, mirrors, gratings or an additional conventional lens. Alternatively a combination of optical elements produced in the manner described in this disclosure may be used in sequence either contained in an entire substrate or in separate individual substrates.

The acoustically active material80may be attached to an optical device or component including an optical waveguide, such as an optical fiber, which may be a single mode fiber. The optical waveguide may also be formed via the irradiating optical beam, and the combination of the internal optical element and optical waveguide may be formed in a composite and/or monolithic structure.

FIG. 17eillustrates an example embodiment in which acoustically active material80with internal optical element83is interfaced with an acoustic backing material86and an optical waveguide87(for example, a fiber optic). In the Figure, optical waveguide87is shown as interfacing with optical element83through a beam deflecting element88.

The acoustically active substrate may include a piezoelectric element with electrodes covering the two opposing faces of the piezoelectric element. The direction of acoustic energy propagation is normal to the plate of the electrodes. The electrodes used to electrically interrogate or excite the acoustically active material may be formed from optically transparent electrode materials such as aluminum zinc oxide. Alternatively, in embodiments where the region an optical refractive element is to be formed within the acoustic substrate is not constrained to be acoustically active, the electrodes do not need to cover the region, and hence do not need to be optically transparent. In these cases, the acoustic substrate may be processed to remove the electrodes in the region using various processing techniques including laser machining/ablation, chemical etching, masking, etc. Typical electrode materials not constrained to optical transparency used may include gold, chrome-gold, silver, aluminum, among others. These may be deposited using manual application with epoxies or conductive inks, sputtering, e-beam deposition, evaporators, among others.

At least one acoustic matching layer81may be added to improve acoustic beam properties such as bandwidth. These layers typically have acoustic impedances ranging preferably in the range between 2-12 for piezoelectrics such as ceramics and single crystal elements, and are in this case preferably formed from optically transparent materials such as quartz glass, polymers such as parylene, nylon, transparent epoxies such as Epo-tek301or Araldite etc. In one embodiment, optical element83formed within the acoustically active material80is a GRIN lens (as shown inFIGS. 17bandd). GRIN lenses that are commonly used in imaging systems typically involve changes in refractive index between of 10−3to 10−1between the center of the lens and the outer-most region. Typical thicknesses of lenses are between of 30 and 300 um, with lens diameters between 50 and 500 um. When coupled with a coreless spacer the working distances and depth of field are often between 200 um-10 mm, with spot sizes being as small as 28 um. The thickness for generating an imaging transducer are typically between 30 and 500 um in PMN-PT, which is similar to the depth of a usable GRIN lens, as noted above. Accordingly, as described above, both the GRIN lens and the acoustic transducer may be integrated within a single optically transparent and acoustically active material.

In another embodiment, shown inFIGS. 18a-c, the optical waveguide may be interfaced with the acoustically active material such that a longitudinal axis of the optical waveguide is parallel to an axis of acoustic beam generated by the acoustic transducer.

It is to be understood that the internal optical element may be formed prior to, or after, assembly of one or more optical components. In one embodiment, the formation of the internal optical element can be performed and tuned in situ during or following assembly using techniques such as those shown inFIGS. 6 and 7, and 17band17c.

FIG. 19shows an embodiment in which the opto-acoustic assembly is arranged prior to the formation of an internal optical structure within an optically transparent acoustically active structure.FIG. 19ashows the acoustically active substrate80. This substrate is then assembled/attached with a combination of acoustic elements, such one or more matching layers80, and a backing layer87(and electrodes). In this embodiment, an optical waveguide87, which may be an optical fiber, is assembled into the backing layer87to be in optical communication with optically transparent acoustic substrate80, and subsequently, optically transparent matching layer81.

Optical refractive element83is then formed using techniques previously described herein into optically transparent acoustically active substrate83. This may be preferentially done using feedback methods, such as those described inFIG. 6,FIG. 7, 17band/or17c. Such an embodiment may be employed within a combined optical-ultrasound imaging system, which may include an optical lens internal with an ultrasound imaging system, such as that described in U.S. patent application Ser. No. 12/010,208, filed by Courtney et al., and titled “Imaging Probe with Combined Ultrasound and Optical Means of Imaging”, which is incorporated herein by reference in it its entirety.

In some cases, it may be beneficial or advantageous to form and/or tune refractive elements in situ following an assembly procedure. The assembly process of an optical system—particularly miniaturized optical systems can introduce significant problems, such as alignment errors, into the optical path. For example, in a combined intravascular ultrasound/optical coherence tomography system as described in Courtney et al. (referenced above), an optical element may be placed within an acoustically active component. In such an embodiment, the imaging probes are typically constrained to diameters of 0.7-5 mm. This optical system may include elements including, but not limited to an optic waveguide such as a fiber optic, optical spacer(s), mirror(s), lens(es), etc. Several embodiments include the formation of a hole within the acoustically active substrate in which at least a portion of the optical system can be embedded (for example, into a recess) to allow the delivery of optical energy (such as imaging energy) and to emit the optical energy from a surface co-aligned with the acoustic emitting face, for the purpose of registering images generated using acoustic energy and those generated using optical energy, as well as for generating images using techniques including photoacoustics or sonoluminescence.

FIG. 20illustrates an example implementation of such an embodiment. Acoustic substrate202, which need not be optically transparent, is provided. Suitable acoustic substrates include materials such as PZT, PZT-5H, lithium niobate, composite devices, PVDF, single crystal piezoelectrics, BiNaTiO, and numerous others known in the art. The acoustic substrate is processed to form a physical void, via, recess, or hole204, which has a diameter suitable for embedding one or more optical elements, as shown inFIG. 20b.

Hole204can be formed by mechanical drilling, milling, laser machining, etching, or other known processes. Alternatively, the piezoelectric substrate can be formed around the optical element or a mandrel using film deposition techniques, casting, molding, or other processes known in the art.

FIG. 20cshows the product of a combined optical-acoustic structure where optical waveguide85, which may be an optical fiber, is embedded into the backing86of the transducer, while at least a portion of optical component206is embedded within the hole204of the transducer (the figure shows an example case in which a portion of optical component206is embedded in acoustic substrate202, while another portion protrudes from the surface of acoustic substrate202. There may optionally be components such as mirrors or optical spacers (not shown) embedded either within the backing86or hole204that are used to direct the optical energy from the waveguide85into optical element206.

In one embodiment, optical component206may be initially substantially free of refractive features (e.g. a cylindrical optical blank), and which may be processed in-situ in order to form an internal optical component according to the processing methods described above. One or more surfaces of optical component206, other than a top surface through which optical irradiating beam enters component206, may be coated with a material suitable for prohibiting the propagation of the irradiating optical beam into another region of the device, such as within backing layer86. Suitable coatings include metallic coatings and dielectric coatings. In the case of a metallic coating, an aperture of via should be present to allow passage of light from waveguide85. In the case of a dielectric coating, the dielectric coating may be reflective over a small wavelength that is suitable for reflecting the optical irradiation beam.

In another embodiment, optical component206may be a refractive optical component. The process of assembling such a system may require numerous process steps including curing of adhesives or epoxies using thermal cycles (including heating and/or cooling cycles), pressure, energy deposition (i.e. UV energy, electrical energy). These processes may uncontrollably induce undesired alterations to the optical properties and/or alignment of optical structures in the optical path. In order to compensate for these alterations, it may be desirable to create or tune the refractive properties of the refractive optical element206into tuned refractive element208shown inFIG. 20d.

The processes for inducing changes in the refractive index in optical substrates as described herein allow for the ability to perform the creation or tuning of refractive optical elements in situ. These elements can be formed using known parameters under open loop control, or they can alternatively be formed using feedback of refractive properties as described inFIGS. 6,FIG. 7, 17band/or17c.

In another embodiment, one or more hollow features may be produced in the substrate. Such features generate local refractive index changes on the order of unity. For example, the irradiation beam may be selected to cause depth patterning such that the material is removed rather than densified, such as by producing internal voids within the transparent structure.

The embodiments described herein may be employed for a wide variety of applications. For example, the preceding embodiments involving an internal optical feature within a transparent structure may be includes in devices such as optical communications device, a camera, optical imaging catheters such as an endoscope, and a fiber optic probe.