OPTICAL DEVICE, METHOD OF FORMING AN OPTICAL DEVICE, AND METHOD FOR DETERMINING A PARAMETER OF A FLUID

According to embodiments of the present invention, an optical device is provided. The optical device includes an optical fiber comprising a core for propagation of light and a cladding surrounding the core, and at least one microchannel defined in the optical fiber extending at least partially through the cladding, wherein the at least one microchannel has a concave-shaped surface arranged to interact with an optical field of the light. According to further embodiments of the present invention, a method of forming an optical device and a method for determining a parameter of a fluid are also provided.

DETAILED DESCRIPTION

Embodiments described in the context of one of the methods or devices are analogously valid for the other method or device. Similarly, embodiments described in the context of a method are analogously valid for a device, and vice versa.

In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element includes a reference to one or more of the features or elements.

In the context of various embodiments, the phrase “at least substantially” may include “exactly” and a reasonable variance.

In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.

As used herein, the phrase of the form of “at least one of A or B” may include A or B or both A and B. Correspondingly, the phrase of the form of “at least one of A or B or C”, or including further listed items, may include any and all combinations of one or more of the associated listed items.

Various embodiments may relate to a microchannel optical fiber device configuration that may feature serially cascaded microchannels. Various embodiments of the device scheme may be applicable to passive and active device applications including but not limited to lasers, sensors and detectors.

Various embodiments may provide a low loss microchannel optical fiber device with large light-channel interaction surface and volume for active and passive device applications.

Various embodiments may provide a device scheme that may achieve low loss microchannel device configuration with a large light-channel interaction surface and a large light-channel interaction volume. The device concept of various embodiments may incorporate a series of cascaded microchannels with optimized dimensions, shapes and separations between them for maximum light-channel interaction and minimum insertion loss. The series of cascaded microchannels may be coupled to a fiber, e.g. coupled to the fiber core. The series of cascaded microchannels may be embedded with the fiber. As a non-limiting example, each microchannel may feature a biconcave shape in order to induce a lensing effect, enabling more light to be guided within the fiber core with less scattered light loss into the cladding of the fiber. With a low optical insertion loss property, in conjunction with a large light-channel interaction surface and volume, the microchannel fiber device concept of various embodiments may enable practical active and passive device schemes, not achievable before using conventional devices.

In various embodiments, multiplexing operation may be realized in such a microchannel fiber device configuration as the optical power may not be compromised by the cascaded array of channels or microchannels. It may be possible to incorporate one or more non-intrinsic materials, for example an optical gain medium or gain material such as a dye (e.g. an organic dye) into the microchannel array or one or more microchannels, such that fiber resonators with a high optical gain and a low insertion loss may be achieved, thereby enabling fiber laser operation. Therefore, fiber grating lasers not limited by the intrinsic fiber material may be realized with ease.

The microchannel device design of various embodiments may be incorporated into conventional active and passive fiber device schemes. Fiber devices such as fiber gratings may be inscribed in proximity to the microchannels to realize microchannel fiber grating devices. Also, as an example, the device concept of various embodiments may be incorporated into conventional distributed Bragg reflector (DBR) fiber grating laser design, thereby enabling ultra-high resolution microfluidic fiber laser sensors.

Further, materials such as magneto-optic materials and/or electro-optic materials may be incorporated into the microchannel fiber device of various embodiments, for example into the microchannel array or one or more microchannels, to manipulate or change the optical properties, such as polarization, of the light propagating through the microchannel fiber device.

Also, optical material such as a saturable absorber and/or a semiconductor material may be infused or provided into one or more microchannels to enable optical properties such as optical switching, spectral filtering or wavelength tuning Such integration of non-intrinsic materials into the microchannel fiber device may lead to at least one of light generation, light modulation/manipulation, or light detection, within the fiber device itself.

The device concept of various embodiments may provide localized transverse access into the fiber and may be applicable to all types of fibers, including but not limited to doped fibers and photonic crystal fibers.

Various embodiments may provide an optical device (e.g. an optical fiber device) having one or more microchannels formed therein. The optical device may include an optical fiber having a fiber core and a cladding. The microchannel(s) may extend close to the fiber core or at least partially into the core or extend through the core, across the entire diameter of the core. Further, the microchannel(s) may extend across the entire fiber, for example from one outer surface of the cladding to an opposed surface of the cladding.

In various embodiments, each microchannel may have at least one curved surface or shape (e.g. concave-shaped). Each microchannel may for example have a concave shape or a biconcave shape. In further embodiments, each microchannel may for example have a square or rectangular shape.

In various embodiments having a plurality of microchannels, the microchannels may be spaced apart. The microchannels may be arranged in cascade or in series. Each of the microchannels may have a concave shape, a biconcave shape or a rectangular shape. In further embodiments, the microchannels may have different shapes, selected from a concave shape, a biconcave shape or a rectangular shape.

FIG. 1Ashows a schematic diagram of an optical device100, according to various embodiments. The optical device100includes an optical fiber102including a core104for propagation of light (as represented by arrow112) and a cladding106surrounding the core104, and at least one microchannel108defined in the optical fiber102extending at least partially through the cladding106, wherein the at least one microchannel108has a concave-shaped surface110arranged to interact with an optical field (as represented by curve114) of the light112.

In other words, the optical device100may have an optical fiber102. The optical fiber102may have a fiber core104where light112may at least substantially propagate in or through the core104, and a cladding106encircling the core104. The core104may have a refractive index (RI) that is higher than the refractive index (RI) of the cladding106, so as to at least substantially confine the optical signal or light112in the core104. The core104may be arranged centrally of the optical fiber102. The optical device100may further include at least one microchannel108formed in the optical fiber102, for example embedded within the optical fiber102. The at least one microchannel108may extend at least partially into the cladding106. The at least one microchannel108may extend from an outer diameter of the optical fiber102on one side of the optical fiber102, through the cladding106towards the core104. The at least one microchannel108may have a concave-shaped surface110, where the concave-shaped surface110is arranged or positioned to interact or overlap with an optical field (or mode field or optical mode)114of the light112. As illustrated inFIG. 1A, a tail of the optical field114may extend into the cladding106and may interact with the concave-shaped surface110of the at least one microchannel108.

In various embodiments, the at least one microchannel108may be spaced a distance from the core104. The at least one microchannel108may be arranged in proximity to the core104so that the concave-shaped surface110may interact with the optical field114of the light112propagating in the core104. In this way, the at least one microchannel108may be offset or away from the core104. Such an arrangement may, for example, induce polarization-dependent effects and/or to effectuate cladding devices.

In the context of various embodiments, the light112propagating in the core104may have an optical field114that may extend beyond the physical dimension or diameter of the guiding core104. For example, the optical field114may be at least substantially maximum within the core104, with a tail, containing optical power, that may extend out of the core104, for example in the form of an evanescent field. Therefore, by arranging the at least one microchannel108in proximity to the core104, the at least one microchannel108and its concave-shaped surface110may interact or overlap with the optical field114of the propagating light112, by means of the evanescent field.

In the context of various embodiments, a “concave-shaped surface” may mean a surface that is at least substantially curved inwardly, in a direction towards the inside of the at least one microchannel108.

In various embodiments, the concave-shaped surface110, in interacting with an optical field (or mode field or optical mode)114of the light112propagating in the core104, may act as a lens to induce a lensing or focusing effect, e.g. to focus the optical field114. The concave-shaped surface110may enable reduced-scattering and/or a focal point to be provided within the at least one microchannel108.

In various embodiments, the at least one microchannel108may extend at least partially into the core104, wherein the concave-shaped surface110overlaps with the core104. This may mean that the at least one microchannel108may extend from an outer diameter of the optical fiber102on one side of the optical fiber102, through the cladding106and at least partially into the core104. In this way, the concave-shaped surface110of the at least one microchannel108may interact with the optical field114in or within the core104.

In various embodiments, the concave-shaped surface110of the at least one microchannel108may intersect with the core104, such that the concave-shaped surface110may intersect the light112propagating in the core104.

In various embodiments, the concave-shaped surface110of the at least one microchannel108may focus the light112propagating in the optical fiber102, e.g. the light112propagating in the core104.

In various embodiments, the concave-shaped surface110may reduce scattering of the light112within the at least one microchannel108and/or may focus the light112to a focal point within the at least one microchannel108.

In various embodiments, the at least one microchannel108may pass through or extend across the core104. This may mean that the at least one microchannel108may extend across the dimension or diameter of the core104.

In various embodiments, the at least one microchannel108may extend through the cladding106on one side of the optical fiber102, through the core104and at least partially into the cladding106on the opposite side of the optical fiber102. This may mean that the at least one microchannel108may be accessed from one side of the optical fiber102.

In various embodiments, the at least one microchannel108may extend through the cladding106on one side of the optical fiber102, through the core104and through the cladding106on the opposite side of the optical fiber102. This may mean that the at least one microchannel108may be accessed from opposite sides of the optical fiber102.

In various embodiments, the surface of the at least one microchannel108opposite to the concave-shaped surface110may be at least substantially flat or planar. Therefore, the at least one microchannel108may have a plano-concave shape or geometry.

In various embodiments, the at least one microchannel108may have another concave-shaped surface opposite to the concave-shaped surface110. Therefore, the at least one microchannel108may have a biconcave shape or geometry.

In various embodiments, the other concave-shaped surface may be arranged to interact or overlap with the optical field (or mode field)114of the light112.

In embodiments where the at least one microchannel108extends at least partially into the core104and the concave-shaped surface110overlaps with the core104, the other concave-shaped surface may also overlap with the core104.

In various embodiments, the at least one microchannel108may be defined or formed orthogonally (or perpendicularly) to the core104. This may mean that the at least one microchannel108may be defined transversely across the optical fiber102, e.g. along a transverse axis perpendicular to the longitudinal axis of the optical fiber102.

In various embodiments, the optical device100may further include an optical filter arranged adjacent or in proximity to the at least one microchannel108. The optical filter may be provided overlapping or within the core104.

In various embodiments, the optical filter may be in the form of a fiber grating, for example formed or defined in the core104.

In various embodiments, the optical device100may include two optical filters (e.g. two fiber gratings) arranged on opposite sides of the at least one microchannel108.

In the context of various embodiments, an optical gain medium may be arranged in the at least one microchannel108. The optical gain medium may be a dye, for example an organic dye. In various embodiments, the dye may include but not limited to Rhodamine or Fluorescein.

By incorporating an optical gain medium, optical gain may be achieved in the optical device100. Therefore, the optical device100may function as an optical resonator, e.g. a fiber resonator. The optical device100may enable laser operation.

In the context of various embodiments, at least one of a saturable absorber or a semiconductor material may be arranged in the at least one microchannel108.

In the context of various embodiments, the term “saturable absorber” may mean an optical material where the absorption of light decreases with increasing light intensity.

In various embodiments, the saturable absorber may include but not limited to carbon (e.g. carbon nanotubes), indium gallium arsenide, or gallium arsenide.

In various embodiments, the semiconductor material may include but not limited to silicon, germanium, gallium arsenide, or indium phosphate.

In various embodiments, the incorporation of a saturable absorber and/or a semiconductor material may enable one or more optical properties such as optical switching, spectral filtering or wavelength tuning Such integration of non-intrinsic material(s) into the optical device100may lead to light generation and/or light modulation/manipulation and/or light detection within the optical device100.

In the context of various embodiments, at least one of a magneto-optic material or an electro-optic material may be arranged in the at least one microchannel108.

In the context of various embodiments, the term “magneto-optic material” may mean a material whose one or more optical properties may change in response to a magnetic field.

In various embodiments, the magneto-optic material may include but not limited to terbium doped borosilicate, terbium gallium garnet, or yttrium iron garnet.

In the context of various embodiments, the term “electro-optic material” may mean a material whose one or more optical properties may change in response to an electric field.

In various embodiments, the electro-optic material may include but not limited to lithium niobate, beta-barium borate, or potassium titanyl phosphate.

In various embodiments, the magneto-optic material and/or the electro-optic material may be employed to manipulate or tune one or more optical properties, such as polarization, of the light propagating through the optical fiber102.

In the context of various embodiments, the concave-shaped surface110may be aspherical. In various embodiments, the other concave-shaped surface may be aspherical. An aspherical surface may provide for chromatic dispersion compensation.

In the context of various embodiments, at least one of the concave-shaped surface110or the other concave-shaped surface may have a radius of curvature, R, of between about 10 μm and about 30 μm, for example between about 10 μm and about 20 μm, between about 20 μm and about 30 μm, or between about 15 μm and about 25 μm, e.g. a radius of curvature of about 15 μm, about 20 μm, or about 30 μm.

In the context of various embodiments, a width, W, of the at least one microchannel108may be between about 10 μm and about 100 μm, for example between about 10 μm and about 50 μm, between about 10 μm and about 30 μm, or between about 20 μm and about 30 μm, e.g. a width of about 20 μm, about 24 μm, about 26 μm, about 30 μm, or about 50 μm. In various embodiments, the width of the at least one microchannel108may be larger than a diameter of the core104.

In the context of various embodiments, a length, L, of the at least one microchannel108may be between about 20 μm and about 100 μm, for example between about 20 μm and about 50 μm, between about 20 μm and about 40 μm, or between about 20 μm and about 30 μm, e.g. a channel length, L, of about 20 μm, about 30 μm, about 40 μm, or about 50 μm.

In various embodiments, the optical device100may include a plurality of spaced apart microchannels108defined in the optical fiber102extending at least partially through the cladding106, wherein each microchannel108of the plurality of spaced apart microchannels108has a concave-shaped surface110arranged to interact with the optical field114of the light112.

In various embodiments, the plurality of spaced apart microchannels108may extend at least partially into the core104, wherein the concave-shaped surface110of each microchannel108may overlap with the core104.

In various embodiments, the plurality of spaced apart microchannels108may be arranged in series or in cascade along the optical fiber102, meaning that the plurality of spaced apart microchannels108may be arranged one after another along the optical fiber102.

It should be appreciated that any one of or each microchannel108of the plurality of spaced apart microchannels108may be as described above in the context of the at least one microchannel108. Further, incorporation of material(s) such as the optical gain medium, the saturable absorber, etc. may be provided in any one of or each microchannel108of the plurality of spaced apart microchannels108.

In various embodiments, the plurality of spaced apart microchannels108may be oriented at least substantially parallel to each other.

In the context of various embodiments, a sum, Lsum, of respective lengths, L, of the plurality of spaced apart microchannels108may be between about 40 μm and about 900 μm, for example between about 40 μm and about 500 μm, between about 100 μm and about 500 μm, between about 100 μm and about 300 μm or between about 150 μm and about 250 μm, e.g. a sum of about 100 μm, about 210 μm, about 500 μm, or about 900 μm.

In the context of various embodiments, a number of the plurality of spaced apart microchannels108may be between 2 microchannels and 30 microchannels, for example between 2 microchannels and 20 microchannels, between 2 microchannels and 10 microchannels, between about 5 microchannels and 20 microchannels or 5 microchannels and 10 microchannels, e.g. 5 microchannels, 7 microchannels, 10 microchannels, 20 microchannels or 30 microchannels.

In the context of various embodiments, adjacent microchannels108of the plurality of spaced apart microchannels108may be spaced apart by a separation, s, of between about 10 μm and about 100 μm, for example between about 40 μm and about 80 μm, between about 50 μm and about 70 μm, or between about 55 μm and about 65 μm, e.g. a separation of about 50 μm, about 54 μm, about 58 μm, about 64 μm, or about 70 μm.

In the context of various embodiments, the optical fiber102may be a single mode fiber.

In the context of various embodiments, the optical fiber102may be or may include a doped fiber or a photonic crystal fiber (PCF).

In various embodiments, a fluid (e.g. a liquid) may be provided into the at least one microchannel108, or any one of or each of the plurality of spaced apart microchannels108, so that the fluid may interact with the optical field114of the propagating light112. Such an interaction may cause a change in an optical property of the light112, e.g. a transmission characteristic or power of the light112.

In the context of various embodiments, the optical device100may be a microchannel device, for example a microchannel optical fiber device.

In the context of various embodiments, the optical device100may be integrated on a substrate or a chip. In various embodiments, at least one of a reservoir, control means such as a valve, or delivery means such as a pump, interconnection(s), or microchannel(s) may be provided or integrated on the substrate or chip, for delivery and/or control of material (e.g. fluid or liquid) to the at least one microchannel108or the plurality of spaced apart microchannels108.

FIG. 1Bshows a flow chart120illustrating a method of forming an optical device, according to various embodiments.

At122, an optical fiber including a core for propagation of light and a cladding surrounding the core is provided.

At124, at least one microchannel is formed or defined in the optical fiber extending at least partially through the cladding, the at least one microchannel having a concave-shaped surface arranged to interact with an optical field of the light.

FIG. 1Cshows a flow chart140illustrating a method for determining a parameter of a fluid, according to various embodiments.

At142, a fluid is provided into at least one microchannel defined in an optical fiber including a core for propagation of light and a cladding surrounding the core, the at least one microchannel extending at least partially through the cladding and having a concave-shaped surface.

At144, a light is provided into the core, wherein an optical field of the light interacts with the fluid. The optical field of the light may also interact with the concave-shaped surface of the at least one microchannel.

At146, a transmission characteristic of the light after interaction between the optical field and the fluid is determined.

At148, a parameter of the fluid is determined based on the determined transmission characteristic.

In various embodiments, the transmission characteristic may be the transmission power of the light.

In various embodiments, the parameter of the fluid may be the refractive index (RI).

While the method described above is illustrated and described as a series of steps or events, it will be appreciated that any ordering of such steps or events are not to be interpreted in a limiting sense. For example, some steps may occur in different orders and/or concurrently with other steps or events apart from those illustrated and/or described herein. In addition, not all illustrated steps may be required to implement one or more aspects or embodiments described herein. Also, one or more of the steps depicted herein may be carried out in one or more separate acts and/or phases.

Various embodiments may provide an optical fiber device with serially cascaded transverse microchannels passing through the fiber core. Each microchannel wall may feature a curved geometry to induce focusing lens effect, so as to reduce light scattering away from the fiber core. As a result, the transmitted power through the device may not be compromised by the number of cascaded microchannels. The curved geometries may include but not limited to biconcave and plano-concave shapes.

In various embodiments, a minimum optical insertion loss with maximum light-channel interaction may be achieved through optimized microchannel physical design. These optimized parameters may include dimensions, shapes as well as separations. For an identical channel volume that interacts with the propagating core light, the cascaded microchannels design concept of various embodiments may reduce the optical transmission loss by an order of magnitude as compared to a single-microchannel design.

The microchannel fiber device design or structure may be highly suited for biomedical applications as the transmission power loss with respect to the channel refractive index (RI) in the range of 1.333-1.4 may be very low.

FIG. 2Ashows a schematic diagram of an optical device200, according to various embodiments, illustrating a microchannel fiber device. The optical device200includes an optical fiber202having a core204surrounded by a cladding206. The size of the core204is exaggerated compared to the size of the cladding206for clarity purposes. As illustrated inFIG. 2A, the fiber202may have a longitudinal axis along the z-direction. A light provided to the optical device200, for example to the optical fiber202, may propagate through the core204(the propagating light represented as the dashed line arrow208), at least substantially within the core. This may mean that the optical field or the power of the propagating light208may be at least substantially contained within the core204or overlap with the core204. The light208may propagate along the longitudinal axis of the fiber202.

The optical device200may include a microchannel array210defined in the fiber202. The microchannel array210may include one or more microchannels formed or defined in the fiber202, for example embedded in the fiber202. As a non-limiting example as illustrated inFIG. 2A, the microchannel array210may have 7 microchannels, as represented by212for one such microchannel. However, it should be appreciated that any number of microchannels212may be provided, for example one, two, three, four or any higher number of microchannels212.

Referring toFIG. 2A, the plurality of microchannels212may be arranged along the longitudinal axis of the fiber202. The plurality of microchannels212may be arranged at least substantially parallel relative to each other. The plurality of microchannels212may be spaced apart, such that adjacent microchannels212may be separated by a separation distance, s. The plurality of microchannels212may be arranged in series or in cascade, one after another. Therefore, the optical device200may have a cascaded microchannel fiber device design.

Each microchannel212may pass through the fiber core204. For example, the optical device200may include serially cascaded microchannels212passing through the fiber core204. In various embodiments, each microchannel212may have opposed surfaces, for example a first surface220and a second surface222, arranged facing the core204. In this way, a respective portion of each of the first surface220and the second surface222may form a respective interface with the core204. This may also mean that a respective portion of each of the first surface220and the second surface222may overlap with or intersect the core204. Therefore, the light208propagating through the core204may also pass through the microchannel array210, through each microchannel212. In various embodiments, each microchannel212may be defined across the entire core204.

Each microchannel212may be defined through the cladding206, for example across the entire cladding206. Therefore, in various embodiments, each microchannel212may be defined through the diameter of the fiber202, extending between two opposed sides214,216, of the peripheral surface218of the fiber202.

Each microchannel212may be arranged transversely across the fiber202, along a transverse axis that is perpendicular to the longitudinal axis of the fiber202. This may mean that each microchannel212may be positioned orthogonal to the core204or the light propagation axis along the core204. In this way, the first surface220and the second surface222of each microchannel212may be arranged at least substantially perpendicular to the core204or the light propagation axis. However, it should be appreciated that any one or more or all of the plurality of spaced apart microchannels212may be arranged slightly angled to the transverse axis, for example about 1° to 10° offset from the transverse axis.

Each microchannel212may be defined by its height, H, defined along the y-direction, its width, W, defined along the x-direction, and its length, L, defined along the z-direction. Each microchannel212may have a biconcave shape. This may mean that each microchannel212may have two opposed surfaces that may be curved inwardly into the microchannel212, in the form of a concave shape. Referring toFIG. 2, each microchannel212, of a biconcave shape, may have a first surface220that may be curved and a second surface222that may be curved. The channel length, L, of each microchannel212may be defined as the distance between the first surface220and the second surface222. Each of the first surface220that and the second surface222may be curved across the entire respective surface. The first surface220and the second surface222may be curved inwardly into the microchannel212, such that the first surface220and the second surface222may be curved towards each other, thereby defining concave shapes. Therefore, the light208propagating in the core204may encounter the first surface220that is curved away from the light208, and then the second surface222that is curved towards the light208. In various embodiments, by having a biconcave shape, each microchannel, aided by its geometry, may focus the propagating light208within the core204. In this way, minimal or no light may be lost through the cladding206. Therefore, in various embodiments, the optical device200may include an array of biconcave microchannels212positioned orthogonal to its light propagation axis along the core204. The microchannels212may extend through the fiber cladding206, passing through the fiber core204. Each microchannel212may reduce scattering of the light208within the microchannel212and/or may focus the light208to a focal point within the microchannel212.

FIG. 2Bshows a schematic cross-sectional view of a section of the optical device200taken along a plane defined by the line A-A′. The size of the light208is exaggerated for ease of understanding to illustrate the focusing effect induced by the geometry of the biconcave microchannels212. As the light208propagates in the core204, the light208encounters the first surface220of the biconcave microchannel212. From the perspective of the light208, the light208sees the first surface220as a convex surface or shape, complementary to the concave shape or geometry of the microchannel212. Therefore, the light208propagating into the microchannel212may, as a result of the lensing effect, be significantly less scattered and/or even focused within the microchannel212. As the light208passes through the microchannel212, the light208encounters the second surface222of the biconcave microchannel212. From the perspective of the light208, the light208sees the second surface220as a convex surface or shape, complementary to the concave shape of the microchannel212, and the light208become coupled and propagate within the core204. Such a lensing or focusing effect may be enabled by each biconcave microchannel212of the optical device200.

WhileFIGS. 2A and 2Bshow that the entire first surface220is curved (concave-shaped) and the entire second surface222is curved (concave-shaped) for each microchannel212, it should be appreciated that in further embodiments, the portion of the first surface220overlapping with the core204may be curved (concave-shaped) while the remaining portion of the first surface220may be at least substantially flat or planar, and the portion of the second surface222overlapping with the core204may be curved (concave-shaped) while the remaining portion of the second surface222may be at least substantially flat or planar, for one or more of the microchannels212.

While it is shown inFIGS. 2A and 2Bthat the optical device200includes a plurality of biconcave microchannels212as an illustrative example embodiment, it should be appreciated that optical devices with different configurations may be provided, for example having a single biconcave microchannel, or a single plano-concave microchannel, or a plurality of plano-concave microchannels, or a plurality of rectangular microchannels, or a combination of biconcave microchannel(s) and/or plano-concave microchannel(s) and/or rectangular microchannel(s).

Fabrication of the optical device of various embodiments will now be described by way of the following non-limiting example. The optical device (e.g. microchannel fiber device) may be realized through a hydrofluoric acid (HF) etching-assisted femtosecond (fs) laser processing fabrication methodology.

The process may include two main steps: (1) inscription of the desired structure (e.g. the channel structure) into the fiber by using a tightly focused femtosecond laser beam, and (2) etching of the fiber in a solution of 8% hydrofluoric acid (HF) for selective removal of the laser-modified regions.

In the laser inscription process, the femtosecond laser pulses, having a center wavelength of about 830 nm, may be focused into a fiber (e.g. a silica fiber) by using an objective lens with a numerical aperture (NA) of about 0.55 and a working distance of about 4 mm. The laser pulse width may be about 150 fs, and the repetition rate may be at about 100 kHz. The focused spot size diameter may be approximately 1 μm, with an average pulse energy at about 100 nJ. The fiber may be mounted on a three-axes air-bearing translation stage, so that the desired microchannel structure may be inscribed into the fiber by moving the fiber with respect to the stationary laser beam of the femtosecond laser. The translation velocity of the fiber may be maintained at about 80 μm/s. The laser inscription process may involve a continuous helical rectangular path along the transverse axis of the fiber to create the microchannel structure.FIG. 2Cshows microscopy images illustrating a top view (left image) and a side view (right image) of a fiber202with a femtosecond-inscribed 3-channel structure, where the laser modified regions, as represented by250, may allow for eventual formation of microchannels212.

Subsequently, the femtosecond-inscribed fiber202may be subjected to a HF etching process for a duration of about 30 minutes, which thereafter may reveal the removal of the laser modified regions250.FIG. 2Dshows microscopy images illustrating a top view (left image) and a side view (right image) of the fiber202after the HF etching process, which show the removal of the laser modified regions250of the 3-channel structure through HF etching. As shown inFIG. 2D, there are HF removed regions252, as well as residual fiber material254at portions where the microchannels212are to be formed.

Ultrasonic bath treatment of the fiber may be carried out in water, which may lead to the final microchannel device as shown inFIG. 2E, which shows microscopy images illustrating a top view (left image) and a side view (right image) of a completed 3-channel cascaded transverse microchannel fiber device (TMFD) structure. As shown inFIG. 2D, etched microchannels212are formed in the fiber202.

It should be appreciated that the fabrication methodology as described is independent of the microchannel geometry as well as the number of microchannels. This means that the process may be used to form one or more microchannels of any geometry or shape.

Microchannel optimization will now be described by way of the following non-limiting examples, with reference toFIGS. 3A to 3C. For the purpose of simulations, the following global parameters may be used: launch wavelength of about 1550 nm; core and cladding diameters of about 8.39 μm and about 125 μm, respectively; core and cladding refractive indices (RIs) of about 1.4503 and about 1.4436, respectively; microchannel RIs set to about 1.3333, meaning that a fluid having a RI of about 1.3333 is introduced or contained in the microchannel. With these parameters, the input light power confined within the fiber mode field diameter may be about 94.17%, and the insertion loss may be calculated by: Loss=(0.9417−T)/0.9417, where T is the transmitted power. The extended fiber length may be fixed at about 2 mm from the end-face of the last microchannel. The optimal parameter value for each microchannel may be chosen such that the transmitted power is maximum.

Optimization of the dimension of a microchannel will now be described. An individual microchannel length, L, of about 30 μm may be considered. The microchannel shape may be of the curved-lens type that may induce a focusing effect, and therefore the biconcave shape may be employed. In this regard, the radius of curvature, R, may be a property that may influence the focusing effect, and hence, the amount of transmitted power. It may be set to be in the range of W/2≦R≦∞, where ∞ refers to a flat surface, with the width, W, and the length, L, of the microchannel set to about 26 μm and about 30 μm, respectively.FIG. 3Ashows a plot300of transmission characteristics of a biconcave-shaped microchannel as a function of the radius of curvature, R, of the microchannel, according to various embodiments. As may be observed, the transmission increases with the radius of curvature, R, up to a maximum value of approximately 0.91 at R of approximately 20 μm, and then decreases monotonically thereafter. Thus, the optimal radius of curvature, Ropt, may be chosen to be about 20 μm.

The width, W, of the microchannel may be studied by varying the width, W, for a fixed radius of curvature, R, of about 20 μm, as well as a fixed channel length, L, of about 30 μm.FIG. 3Bshows a plot320of transmission characteristics of a biconcave-shaped microchannel as a function of the width of the microchannel, according to various embodiments. As may be observed, when the width, W, of the microchannel is less than the fiber core size of about 8.39 μm, the microchannel may induce a large scattering loss and thus, a low transmitted power. The transmission increases with the width, W, and stabilizes to a maximum value of approximately 0.91 for a channel width, W, of approximately 24 μm, which therefore may be taken to be the optimal width, Wopt, for the biconcave microchannel.

Optimization of channel separation will now be described. The channel separation, s, is the distance between two adjacent microchannels, and may be studied by monitoring the transmitted power after passing through two serially cascaded channels.FIG. 3Cshows a plot340of transmission characteristics of biconcave-shaped microchannels as a function of the channel separation between adjacent microchannels, according to various embodiments. As may be observed, the transmission characteristic is similar to that for the radius of curvature, R. The transmission increases with the separation, s, up to a maximum value of approximately 0.9 at separation of approximately 64 μm, and then decreases thereafter. Thus, the optimal separation, sopt, is approximately 64 μm.

With the optimized physical parameters for the biconcave microchannels, a performance comparison may be carried out, for example a comparative study on the power transmission characteristics between: (i) a biconcave and a rectangular microchannel of identical channel length, and between (ii) a single long-length microchannel and an optimized microchannel array.

Performances relating to an optical device having a single optimized channel will now be described by way of the following non-limiting examples.FIGS. 4A and 4Bcompare the light intensity distribution and the transmitted power between a single biconcave microchannel optical device and a single rectangular microchannel optical device of the same physical channel length, L, of about 30 μm.

FIG. 4Ashows a plot400of simulated light intensity distribution and a plot420of simulated transmitted power characteristics for a biconcave microchannel device structure402, according to various embodiments. As shown in plot400, the optical device402includes an optical fiber having a core404and a cladding406. The optical device402further includes a single biconcave microchannel408extending through the core404, perpendicular to the core404. The plot440shows the simulated light intensity distribution in the vicinity of or around the biconcave microchannel408.

The plot420shows result422representing the power distribution within the core404(e.g. core power variation along the length of the fiber), result424representing the power distribution within the core404and the cladding406(e.g. core and cladding power variation along the length of the fiber) and result426representing the power distribution within the microchannel408. The plot420also shows result428illustrating the power distribution within the mode field diameter of the propagating optical mode.

FIG. 4Bshows a plot450of simulated light intensity distribution and a plot470of transmitted power characteristics for a rectangular microchannel device structure452, according to various embodiments. As shown in plot450, the optical device452includes an optical fiber having a core454and a cladding456. The optical device452further includes a single rectangular microchannel458extending through the core454, perpendicular to the core454. The plot490shows the simulated light intensity distribution in the vicinity of or around the rectangular microchannel458.

The plot470shows result472representing the power distribution within the core454(e.g. core power variation along the length of the fiber), result474representing the power distribution within the core454and the cladding456(e.g. core and cladding power variation along the length of the fiber) and result476representing the power distribution within the microchannel458. The plot470also shows result478illustrating the power distribution within the mode field diameter of the propagating optical mode.

Based on the simulation results, the insertion losses introduced by the biconcave microchannel408and the rectangular microchannel458are about 3.74% and about 6.49%, respectively. That is, for a single optimized microchannel, the focusing effect of the biconcave microchannel408may reduce the loss by approximately 2.75%. The in-fiber biconcave-shape of the microchannel408may act like two focusing lenses since the microchannel408may have a lower refractive index (RI) than that of the fiber core404. The focusing may effectively reduce the amount of scattered light loss into the cladding406, thereby achieving a lower overall transmission loss.

While the benefit of the focusing effect may not seem to be significant for a single microchannel configuration, there may be a large transmission loss improvement when multiple channels are cascaded together as will be described later below.

Performances relating to an optical device having a single channel with a long channel length will now be described by way of the following non-limiting examples.

The respective lengths of the biconcave microchannel408, as shown inFIG. 4A, and the rectangular microchannel458, as shown inFIG. 4B, provide a relatively short light-channel interaction length. Consequently, measurand(s) within the respective microchannels408,458may not produce significant perturbation to the interacting light in the respective fiber cores404,454, which may limit their practical use, for example, as effective microfluidic sensors. As a non-limiting example, in order to increase the light-channel interaction length, the channel length may be increased to, for example, 210 μm, which may be equivalent to 7 times of the desired length.

FIGS. 5A and 5Bshow the light intensity distribution and the transmitted power of a biconcave microchannel optical device and a rectangular microchannel optical device, respectively, where each optical device includes a microchannel having a channel length, L, of about 210 μm.

FIG. 5Ashows a plot500of simulated light intensity distribution and a plot520of simulated transmitted power characteristics for a biconcave microchannel device structure502, according to various embodiments. As shown in plot500, the optical device502includes an optical fiber having a core504and a cladding506. The optical device502further includes a single biconcave microchannel508extending through the core504, perpendicular to the core504, where the biconcave microchannel508has a channel length of about 210 μm. The plot540shows the simulated light intensity distribution in the vicinity of or around the biconcave microchannel508.

The plot520shows result522representing the power distribution within the core504(e.g. core power variation along the length of the fiber), result524representing the power distribution within the core504and the cladding506(e.g. core and cladding power variation along the length of the fiber) and result526representing the power distribution within the microchannel508. The plot520also shows result528illustrating the power distribution within the mode field diameter of the propagating optical mode.

FIG. 5Bshows a plot550of simulated light intensity distribution and a plot570of transmitted power characteristics for a rectangular microchannel device structure552, according to various embodiments. As shown in plot550, the optical device552includes an optical fiber having a core554and a cladding556. The optical device552further includes a single rectangular microchannel558extending through the core554, perpendicular to the core554, where the rectangular microchannel558has a channel length of about 210 μm. The plot590shows the simulated light intensity distribution in the vicinity of or around the rectangular microchannel558.

The plot570shows result572representing the power distribution within the core554(e.g. core power variation along the length of the fiber), result574representing the power distribution within the core554and the cladding556(e.g. core and cladding power variation along the length of the fiber) and result576representing the power distribution within the microchannel558. The plot570also shows result578illustrating the power distribution within the mode field diameter of the propagating optical mode.

The respective insertion losses in both cases of the biconcave microchannel optical device502and the rectangular microchannel optical device552increases up to about 69.38% and about 72.58% respectively based on the simulation results.

Performances relating to an optical device having cascaded channels with a long effective light-channel interaction length will now be described by way of the following non-limiting examples.

In order to address or overcome the issue of large optical insertion loss while maintaining a long light-channel interaction length, a device configuration which contains multiple microchannels cascaded serially along the fiber may be provided. The microchannel separations in the array structure may be of an optimized distance. For example, for a sum or total effective light-channel interaction length, Lsum, of about 210 μm, 7 microchannels may be provided, each microchannel having a length of 30 μm, with optimized separation distances of about 54 μm provided for biconcave microchannels and optimized separation distances of about 58 μm provided for rectangular microchannels.

FIGS. 6A and 6Bshow the light intensity distribution and the transmitted power of a biconcave microchannel array device structure and a rectangular microchannel array device structure, respectively, where each microchannel array device structure has a total effective light-channel interaction length, Lsum, of about 210 μm.

FIG. 6Ashows a plot600of simulated light intensity distribution and a plot620of simulated transmitted power characteristics for an optimized biconcave microchannel device structure602with a cascaded array608of microchannels, as represented by609for one biconcave microchannel, according to various embodiments. As shown in plot600, the optical device602includes an optical fiber having a core604and a cladding606. The optical device602further includes an array608of spaced apart microchannels609arranged in series extending through the core604, perpendicular to the core604. As a non-limiting example, the microchannel array608includes 7 biconcave microchannels609, each microchannel609having a channel length of about 30 μm, thereby providing a total effective channel length of about 210 μm for interaction with light. The plot640shows the simulated light intensity distribution in the vicinity of or around the biconcave microchannel array608.

The plot620shows result622representing the power distribution within the core604(e.g. core power variation along the length of the fiber), result624representing the power distribution within the core604and the cladding606(e.g. core and cladding power variation along the length of the fiber) and result626representing the power distribution within the microchannels609. The plot620also shows result628illustrating the power distribution within the mode field diameter of the propagating optical mode.

FIG. 6Bshows a plot650of simulated light intensity distribution and a plot670of simulated transmitted power characteristics for an optimized rectangular microchannel device structure652with a cascaded array658of microchannels, as represented by659for one rectangular microchannel, according to various embodiments. As shown in plot650, the optical device652includes an optical fiber having a core654and a cladding656. The optical device652further includes an array658of spaced apart microchannels659arranged in series extending through the core654, perpendicular to the core654. As a non-limiting example, the microchannel array658includes 7 rectangular microchannels659, each microchannel659having a channel length of about 30 μm, thereby providing a total effective channel length of about 210 μm for interaction with light. The plot690shows the simulated light intensity distribution in the vicinity of or around the rectangular microchannel array658.

The plot670shows result672representing the power distribution within the core654(e.g. core power variation along the length of the fiber), result674representing the power distribution within the core654and the cladding656(e.g. core and cladding power variation along the length of the fiber) and result676representing the power distribution within the microchannels659. The plot670also shows result678illustrating the power distribution within the mode field diameter of the propagating optical mode.

Based on the device configurations of the optical devices602,652, the overall insertion losses for the biconcave channel array optical device602and the rectangular channel array optical device652are about 4.92% and about 8.71%, respectively. It is evident that there is a marked improvement in the power transmission over that of a single long-length microchannel device scheme for the optical devices502,552. Further, compared to the device configuration using a single, long microchannel length, the loss reduction reaches an order of magnitude based on the biconcave microchannel array device602.

With a low optical insertion loss property, the microchannel fiber device concept of various embodiments may enable practical active and passive device schemes, not achievable before. For example, multiplexing operation may be realized in such a microchannel fiber device configuration since the optical power may not be compromised by the cascaded array of channels. Such a multiplexing operation may include (a) having multiple simultaneous operations e.g. fluid detections, from individual channels within one microchannel fiber device, and/or (b) having simultaneous operations from two or more microchannel fiber devices cascaded in series. Multiplexing operation may be achieved for microchannel fiber devices cascaded in series through, for example incorporating wavelength-selective fiber gratings in proximity to each microchannel fiber device. By doing so, the respective optical response due to each microchannel fiber device may be differentiated based on the spectral responses.

In various embodiments, by incorporating a gain material such as a dye into the microchannel array, fiber resonators with high optical gain and low insertion loss may be achieved, enabling fiber laser operation where intra-cavity loss for lasing action may be <10%. Henceforth, fiber grating lasers which may not be limited by the intrinsic fiber material may be realized with ease.

Furthermore, the microchannel device design of various embodiments may be incorporated into conventional active and passive fiber device schemes. For example, the optical device or the device concept of various embodiments may be incorporated into conventional DBR fiber grating laser designs for enabling ultra-high resolution microfluidic fiber laser sensors.

Optimization of the cascaded microchannel fiber device design of various embodiments will now be described by way of the following non-limiting examples. The number of microchannels a device structure may accommodate without compromising a pre-determined overall insertion loss value may be determined.

FIG. 7shows a plot700for the result702for the overall insertion loss for a rectangular microchannel device, and the result704for the overall insertion loss for a biconcave microchannel device structure, as a function of the number of cascaded microchannels, providing a comparison of the insertion losses between the biconcave and the rectangular microchannel device structure geometry.

By considering an insertion loss of about 10% as the acceptable limit, the obtained result704shows that the biconcave microchannel array structure may enable or accommodate about 30 channels while keeping the loss at <10%. This may translate to an equivalent light-channel interaction length, Lsum, of about 900 μm, for individual microchannel lengths of about 30 μm. On the other hand, the result702shows that the rectangular microchannel array device structure may only accommodate up to 10 channels, leading to a maximum light-channel interaction length, Lsum, of about 300 μm for individual microchannel lengths of about 30 μm, before exceeding the 10% loss threshold.

The result704highlights that the cascaded biconcave microchannel array fiber device structure not only outperforms, for example in terms of the power throughput, by a factor of 14 over a device configuration based on a single long-length microchannel, but may also be able to increase the light-channel interaction length by a factor of 3 as compared to the rectangular microchannel array configuration counterpart.

The transmitted power characteristics of the microchannel fiber device structures with respect to the channel refractive indices (RI) may be determined, so as to illustrate variation with the refractive index. The channel refractive index refers to the refractive index of the fluid introduced into the channel.

FIG. 8Ashows a plot800of transmitted power for a rectangular microchannel device having a single rectangular microchannel and a plot810of transmitted power for a biconcave microchannel device structure having a single biconcave microchannel, as a function of refractive index change in the range of approximately 1-1.5. As may be observed fromFIG. 8A, the overall transmitted power variation may be <1% and <10% for the respective optical devices having the rectangular microchannel and the biconcave microchannel, respectively.

FIG. 8Bshows a plot820of transmitted power for a rectangular microchannel device having a single rectangular microchannel and a plot830of transmitted power for a biconcave microchannel device structure having a single biconcave microchannel, as a function of refractive index change in the range of approximately 1.333-1.4 corresponding to the RI of most bio-fluids. As may be observed fromFIG. 8B, the transmission power variation is <1% in both the optical device having the rectangular microchannel and the optical device having the biconcave microchannel. The results indicate that the microchannel array fiber device structure of various embodiments may be suitable for biosensing applications.

As described above, a fiber device scheme that achieves low loss microchannel device configuration with large light-channel interaction surface and volume may be provided. The device concept of various embodiments may incorporate a series of cascaded microchannels with optimized dimensions, shapes and separations between them for maximum light-channel interaction and minimum insertion loss. In various embodiments, each microchannel may feature a biconcave shape in order to induce a focusing lens effect, enabling more light to be guided within the fiber core with less scattered light loss into the cladding.

Through numerical simulations, it is shown that the optimized biconcave microchannel array fiber device configuration may reduce the overall optical insertion loss by an order of magnitude as compared to a device configuration using a single long-length microchannel. In addition, the device scheme of various embodiments may accommodate a large number of microchannels to achieve a long effective light-channel interaction length, for example an effective light-channel interaction length of about 900 μm, while keeping the overall insertion loss to be <10%. For bio-applications where the refractive index (RI) range of interest lies within the range of about 1.333 to about 1.4, the device configuration may achieve <1% transmission power variation with the RI.

It should be appreciated that the geometry and arrangement of the microchannel(s) are not limited to that as described herein. For example, the microchannel may have an aspherical curved geometry for chromatic dispersion compensation so that the device may operate over a larger optical wavelength range. For a similar purpose, orientation of the microchannel(s) and separation of the microchannels may vary along the fiber. Further, individual microchannel cross sectional dimension may vary as it approaches the fiber core for purpose of ease of infiltration of fluids into the microchannel(s).

In various embodiments, the position of the microchannel(s) may be offset or even away from the fiber core to induce, for example, polarization-dependent effects or to effectuate cladding devices. This may mean that one or more microchannels may not pass through the fiber core physically. However, the microchannel(s) may be provided and remain in close proximity to the fiber core such that the optical field (or optical mode), propagating within the fiber core may remain overlap, though to a much lesser extent, with the microchannel(s). This is because the optical field propagating within the fiber core may extend slightly beyond the guiding core physical diameter. Therefore, a portion of the optical power associated with the light propagating in the fiber core may extend out of the fiber core, for example in the form of evanescent field. For example, for a single-mode optical fiber, the optical field mode diameter may be about 10 μm while the physical core diameter may be about 8 μm. Therefore, the microchannel(s) may be formed or arranged about 1 μm away from the fiber core, not intersecting the fiber core, while still able to achieve a small overlap with the propagating optical field.

In various embodiments, multiple microchannels may be stacked transversely across the fiber. The microchannel device of various embodiments may be integrated onto a chip for ease of handling as well as for control of material (e.g. fluid) flow within the microchannel(s).