This invention relates generally to optical fibers, and more particularly to optical fiber probes that use manipulated delivery and reception regions to improve sensitivity to specific light-matter interactions.
In recent years, the use of optical fibers has become increasingly widespread in a variety of applications. Optical fiber probes have been found to be especially useful for analyzing materials by employing various types of light-scattering spectroscopy.
Optical fibers offer numerous advantages over other types of source/detection equipment. In short, the fiber provides a light conduit so that the source-generating hardware and the recording apparatus are stationed independently of the subject under investigation and the point of analysis. Thus, analyses are conducted remotely in otherwise inaccessible locations. Previously unattainable information is acquired in situ, often in real time. This capability is sought in numerous industrial, environmental, and biomedical applications. The laboratory is moved on line in the industrial realm, to the field in the environmental sector, and in vivo in the biotechnical arena. Additionally, hardware and measurements are more robust, quicker, less intrusive, more rugged, less costly, and many other advantages are realized.
While transmission spectroscopy analyzes light passing through a substance, light-scattering spectroscopy entails illumination of a measured and analyzing light that is scattered at angles relative to the incident source. The photon-matter interactions of the scattering events may be either elastic or inelastic. In an inelastic event, a photon""s energy (wavelength) changes as a result of the light-matter interaction. In an elastic event, a photon""s energy (wavelength) does not change. Absorption, the phenomena in which a fraction of photons are entirely absorbed, also plays a role in light-scattering spectroscopies. Raman, diffuse, reflectance, and fluorescence spectroscopies are of particular interest as they relate to vibrational and non-vibrational photonic responses of a material.
The Raman effect describes a subtle light-matter interaction. Minute fractions of light illuminating a substance are Raman-scattered in random directions. Raman-scattered light is color shifted from the incident beam (usually a laser). The color (frequency) shifts are highly specific as they relate to molecular bond vibrations inducing molecular polarizability changes. Raman spectroscopy is a powerful technique for chemical analysis and monitoring. The resulting low light levels require sophisticated, expensive instrumentation and technical complexity. Suitable technology and products for on-line analysis of processes and environmental contaminants are just becoming available.
Specular reflectance relates to a surface""s mirror-like aspects. Diffuse reflectance relates to light that is elastically scattered from a surface of material at diffuse angles relative to the incident team. For example, a projector screen diffusely reflects light while a glossy, new waxed car has a high specular component. Diffuse reflectance spectroscopy is important for chemical analysis as well as measuring visual perception. Among other things, it is based on particulate-scattering and absorption events.
Fluorescence relates to substances which absorb light at one wavelength then re-emit it at a longer wavelength as a result of electronic transitions. As an example, a xe2x80x9chighlighterxe2x80x9d felt-tip marker appears to xe2x80x9cglowxe2x80x9d green as it absorbs blue and ultraviolet light then emits it as green. Fluorescence provides a powerful technique for chemical monitoring.
Raman spectroscopy is a well-established laboratory technique and is generally recognized as having enormous potential for on-line monitoring and sensing. With the advent of stable lasers, cheap computing power, efficient detectors, and other new technological advancements, Raman spectroscopy is primed for widespread industrial monitoring deployment. In addition to process control monitoring, it will be utilized in specialized monitoring and sensing devices ranging from neuroimaging to environmental monitoring, to in vitro and in vivo medical testing.
Raman spectroscopy involves energizing a sample with a high-power, narrow-wavelength energy source, such as a laser. The laser photons induce low intensity light emissions as wavelengths shift from the laser""s. The Raman effect is an elastic scattering of photons The emitted Raman light is collected and analyzed with a specialized instrument.
The spectral positions (colors) of the shifts provide fingerprints of the chemicals in the sample. Thus, Raman spectroscopy provides a means for chemical identification. The intensity of the shift (the spectral peak height) correlates to chemical concentration. Thus, a properly calibrated instrument provides chemical content and concentration. In practicality, Raman spectroscopy is technically complex and requires sophisticated, expensive instrumentation.
Raman spectroscopy is well suited to aqueous-based media without sample preparation. From this standpoint, it is an ideal tool for process control medical testing and environmental applications. Thus, Raman spectroscopy has great potential for real-time monitoring and is being vigorously pursued.
The basic concept for a probe-based, on-line Raman instrument is simple. Laser light is directed down an optical fiber to a remote probe. The laser light exits the fiber and illuminates the sample medium. Another fiber picks up the Raman-emitted light and returns it to the instrument for analysis.
In practicality, the engineering challenges for a robust physical probe implementation are substantial. In addition to the optical performance expected by laboratory instruments, a probe must be hardened to withstand extreme physical and chemical conditions. Optical characteristics must also remain constant as dynamic conditions change.
Optical aspects of probe engineering require particular design finesse. The Raman effect involves very weak signals. Raman emissions may be one trillionth as intense as the exciting radiation. Subsequently, the probe must be incredibly efficient in collecting and transmitting Raman-emitted light. And, the signal must not be corrupted by extraneous influences. As an example of the sensitivity, Raman instruments typically feature cosmic ray filters. The mechanisms identify and discard measurement data samples influenced by passage of a single cosmic ray photon through the detector.
A phenomenon known as the silica-Raman effect has proven especially troublesome for those engaged in remote Raman spectroscopy. As laser light is transmitted over optical fibers, a subtle light-matter interaction inherently occurs. The laser light and the silica in the glass fiber interact generating xe2x80x9csilica-Ramanxe2x80x9d light. The extraneous silica-Raman light becomes waveguided in the fiber and hopelessly mixed with the laser light. The purity of the laser light is corrupted. Fiber fluorescence cause similar problems.
Remote Raman spectroscopy employs optical fiber between the base instrument and the remote probe or process interface. Optical fiber delivers laser light from its source to the probe. Separate fiber returns sensed light from the probe to an instrument for analysis. In both delivery and return, undesirable silica-Raman light travels in the fibers concurrently with desirable laser and sensor light. A major obstacle in fiber-optic-based Raman spectroscopy has been in separating the desirable light from the undesirable silica-Raman light.
Flat Face, Parallel Fiber Probes
Standard optical fibers deliver and receive light within narrow angular confines. Consider a xe2x80x9cprobexe2x80x9d that is formed by mounting two standard, flat face fibers (i.e., a source fiber and a collection fiber) in parallel. The functionality, operation, and limitations of this probe will be analyzed to present relevant technical requirements. The technical discussion addresses, among other things, issues of optical efficiency. Efficiency is a critical parameter concerned with the ratio between the illumination energy verses the energy of collected light.
Increased optical efficiency has significant benefits. As efficiency is increased, system performance is dramatically boosted. In sophisticated instrument systems, enormous efforts, expense and other considerations are devoted to produce small, marginal performance gains in the detector subsystem. With an optimized probe, tremendous gains are readily realized. Gains in probe efficiency vastly dominate fractional electronic and detector improvements. With increased probe performance, the overall system benefits with reduced noise, increased stability, faster response, and better repeatability. Required illumination intensity is minimized. This translates to reducing intrusive aspects and ensuring the subject under analysis is not damaged or altered. In addition, much less expensive opto-electronic components can be employed.
In the flat face, parallel fiber probe, the source fiber delivers illuminating light in the form of a diverging light beam. The collection fiber has a receptivity zone that is similar in shape to that of the illumination zone. However, the collection and illumination zones are offset from one another, each originating from its respective fiber face. As the zones expand outward from the fiber end faces, they begin to overlap. Under normal circumstances, only in this overlapping region can the source fiber deliver illumination and the collection fiber gather light from the target. The lack of overlap between these regions produces numerous troublesome effects. A second, though not entirely distinct, set of problems is associated with the angular orientation of light rays within the illumination and collection cones. These problems are described below.
In many common applications, the investigative medium is light absorbing, the probe might be deployed in a chemical mixture that is slightly black but not fully opaque. For example, various biological tissues are well known as light-absorbing matrices. And, the sample need not be dark in the traditional sense. Even visually transparent media often strongly absorb ultraviolet and/or infrared light. In a light-absorbing medium, the illumination light must penetrate some distance into the environment prior to reaching a position in which the detector fiber can actively collect returning light. Since the source light is absorbed as it traverses this distance, its intensity is diminished before it reaches an active target zone. Once the illumination light reaches an active target zone, it triggers release of potentially collectible sample light from the target. Depending on the application, the sample light may be generated by any of various photonic mechanisms. Assuming a passive target, the sample light is reduced in strength from the illuminating source light. Depending on the phenomenon of interest, the attenuation is severe. Before capture by the collecting fiber, the sample light must traverse a path through the absorbing medium further reducing the signal strength by attenuation.
Initially, this problem appears readily solved by increasing the illumination intensity. While in certain cases this technique might be effective, in many circumstances, it is not feasible. As the medium absorbs source light energy, it can be irreparable damaged. Even without damage, minimum light intensity translates to minimum intrusive attributes. And, in addition to damage, photochemical reactions are inadvertently initiated in certain circumstances. Therefore, applications that will not tolerate high intensity illumination may preclude the use of a flat-face, parallel fiber probe. In addition, the goal of minimizing illumination light intensity is desirable in almost all uses that are currently being investigated.
A second problem exists in environments that involve elastic particulate-scattering media, such as slurries, mists, aerosols, paints, and various other media. Biological tissues are well known for these types of light-scattering characteristics. Most unpurified samples scatter light to a certain degree and often intensely. Although light scattering occurs by various mechanisms, Rayleigh and Mie-scattering is common and produces strong influences. As with the previous example, the illumination energy must traverse a path of attenuation prior to reaching a target zone for which the collection fiber is receptive. And, the target-generated light must likewise traverse a path through the scattering agent prior to reaching the collection fiber. As with the example of the light-absorbing sample, minimizing delivered light intensity to prevent sample damage is a factor.
For elastic light-scattering media, additional detrimental effects are observed. Assume a distinct target is stationed within the particulate-scattering medium and is positioned within the region where the illumination and light-gathering zones overlap. Illumination is elastically scattered as it traverses a path to reach the target. Although the direct pathway may lie outside of the collection fiber""s receptivity zone, it is incorrect to surmise that this scattered light cannot be captured by the collecting fiber. The incorrect conclusion is based upon a single scattering event which primarily redirects a source ray to a new angular orientation. The population of angular orientations for an arbitrary single ray is statistically determined and is a function, among other things, of the characteristics of the scattering agent. These characteristics include, but are not limited to, particle size, shape, refractive index, and reflective qualities. Granted, for a single scattering event to generate a ray to be received by the collection fiber, the event must occur within the collection fiber""s receptivity zone. Unfortunately, light scattering, particularly Rayleigh and Mie-scattering, often is a multiple event phenomenon. Typically, a source ray undergoes multiple scattering events and is redirected many times. Thus, the ray path is complex as it interacts with various sample particles.
As an overly simple example, consider a ray exiting the source fiber parallel to the fiber axis at a zero-degree heading, and is scattered by an event perpendicularly directing it to a 90-degree heading. At this heading, it enters the collection fiber""s zone of receptivity. While in this zone, the ray undergoes a second event directing it to a new heading for intersection with the collection fiber""s end face. The ray is then captured by the collection fiber.
Light captured by the collection fiber prior to undergoing intended interaction with the target is usually highly detrimental. The negative effect transcends diminishing the intensity of source illumination delivered to the target. This light becomes indiscriminately mixed with the desired light within the collection fiber. This xe2x80x9cstray lightxe2x80x9d severely corrupts the process of various analytical measurements. Typically, the stray light becomes indistinguishable from the desired light. Stray light levels may be dependent on various environmental factors. In the aforementioned example, stray light is a function of the quantity of scattering agent present in the optical path. Assuming this quantity is an uncontrolled application variable, the effect cannot be readily eliminated by referencing of similar compensation.
The situation in which scattering medium separates an intended target from the probe tip is quite common. For example, for in vivo analysis of biological samples, various light-scattering aqueous solutions separate the probe tip from the target. For example, biological tissue is often surrounded by fluids containing scattering agents, such as tissue particulates and blood.
A distinct class of sensor measurements is concerned with analyzing particulate-scattered light to ascertain particle characteristics. In this configuration, returning light from the particles is analyzed to ascertain turbidity, particle concentration, and related parameters. These measurements are highly sought in the biotechnology field for both bioprocesses as well as in vivo and in vitro biomedical applications. Industrial applications are likewise numerous. In this instance, it is desirable to collect and analyze light that has undergone a minimal number of scattering interactions. It is understood from the previous discussion that the greater the distance from the probe end face to a zone of mutual illumination and collection, the more likely the collected light will have undergone multiple interactions. Therefore, for this application, other related criteria addressing the extent and spatial duration of the zone overlap, and various illumination and collection angles can and should be optimized.
Consider an application in clear media, which exhibits neither light absorption nor particulate scattering. As the distance from the probe end face increases, the zones of illumination and receptivity increasingly overlap and asymptotically approach full concurrence. However, it would be incorrect to assume the optimal target location is at a position removed from the probe end face, where the illumination and collection zones are basically in concurrence. An opposing factor must be considered. As distance from the probe end face increases, the relative sizes of the fibers nonlinearly decrease. At a point removed from the end face, the collection fiber possesses light-gathering abilities within a solid angle. These two opposing factors can be modeled to calculate an optimal target distance which maximizes the signal for a given set of application criteria, including beam divergence, fiber size, and fiber separation. The mechanism by which the target returns source light and the characteristics of this light are also important. Nevertheless, the solid angle effect is dominant and the collection fiber""s light-gathering ability decreases dramatically as distance from the fiber end face increases. From this perspective, it is highly advantageous to be able to position the target as close to the probe end face as possible. As with the discussions of the probe""s other limiting factors, intensity is a major factor.
Consider an application in which a flat face, parallel fiber probe is used for Raman analysis of a clear fluid. In this case, the medium through which the detection and collection beams are projected and the target are one in the same. As the collection and illumination zones extend from the probe tip, they overlap as previously described. Unfortunately, at a distance away from the probe tip at which significant overlap occurs, the illumination beam has diverged, and its intensity has diminished. For the collection fiber, a similar scenario exists. At a distance at which zone overlap occurs, the relative size of the collection fiber is reduced. The solid angle within which the fiber has the ability to collect light is severely reduced over that close to the collection fiber end face.
Along a similar line of reasoning, consider a probe investigating fluorescence characteristics of a liquid in a flat-bottom beaker. If the liquid is sufficiently transparent, a portion of the light penetrates the liquid to the beaker bottom and is reflected back to the detector fiber. This reflection manifests itself as stray light and corrupts the acquired data. If the probe had the ability to angularly control illumination and collection, then the stray light problem would be avoided by directing the reflections to miss the detector fiber.
The dependence of captured light intensity upon target distance from a flat face, parallel fiber probe tip is often utilized in the prior art to create a displacement sensor for position measurement. The dynamic range and characteristics of such sensors are limited by available source and detector pattern geometries.
Another important factor related to the probe is power density of the delivered illumination. Power density may be expressed in watts per unit area. Power density in the medium is highest at the surface of the illuminating optical fiber and decreases as the source beam diverges. Thus, fibers that do not rapidly diverge maintain power density as the source beam is projected into the medium. Unfortunately, the source beam must diverge in order to deliver illumination light into the collection fiber""s zone of receptivity. For a given quantity of light injected into the proximal end of a source fiber, power density at the fiber""s distal end face at the probe tip decreases as the fiber core diameter increases. As previously described, the lower the power density, the less intrusive the probe and the less potentially damaging the source energy.
In addition to the described criteria, the angular orientation of rays within the illumination and collection zones are of interest. Depending on the intended application, this aspect is critically important. For a flat face, parallel fiber probe, emitted illumination rays are oriented within the divergence angle of the illumination pattern and centered about the fiber""s axis. The fiber axis is, therefore, the average angular orientation of the emitted light rays. A similar scenario exists for the receiving/collection fiber.
Consider gathering light from a theoretical point source positioned a short distance from a collection fiber end face. The fiber""s cone-shaped collection pattern extends outward from its end face. If the point source is positioned outside the collection pattern, no light is collected by the fiber. At this position, light rays incident on the fiber end face are not properly angularly oriented for collection. Similarly, if the point source is positioned within the collection pattern, a portion of the point source rays are collected. For a given stand-off distance of the point source from the collection fiber end face, the fraction of collected light varies across the collection pattern. With the point source at the center axis of the pattern, the fraction collected is maximum. Moving at a right angle to the center axis of the fiber, the zone of maximum collection extends across a portion of the collection pattern. Moving further towards the outer boundary of the collection pattern, the fraction of collected light is reduced. This reduction in collected light is due to the fact that near the edges of the collection pattern most of the point source rays striking the fiber end face are improperly angularly oriented for collection. The described scenario is important in modeling and understanding the effects of collection and illumination zone overlap in fiber optic probes. In the described flat face, parallel fiber probe, the overlap occurs only in the outer fringes of the conical illumination and collection zones. The center, more critical regions of the illumination and collection patterns do not coincide with one another. Thus, efficiency is poor.
For many measurements, the angular orientation of illumination and collection light is crucial. As previously described, Rayleigh and Mie-scattered light is often angularly biased and the bias orientation is analytically important. Similarly, for measurements related to visual perception, the angular orientation is often crucial. Gloss is measured at specific angles of illumination and collection. Various material parameters such as paper brightness are likewise measured. For color measurements, illumination and receptive angles are often specified according to the material under analysis and various industry-specific standards. Often, diffuse illumination is desired. Perfectly diffuse illumination has no angular bias; the target is illuminated by light rays incident from all directions. Perfectly diffuse illumination is never fully attainable: nevertheless, it can be approached.
In addition to visually oriented measurement such as color, texture, smoothness, and gloss; diffuse reflectance measurements are widely utilized in analytical measurements. In many of these measurements, it is desirous to minimize the specular component of reflection. In so doing, collection of source light that has not undergone the desired interaction with the target is minimized. This characteristic is desired for diffuse reflectance measurements in the visible, ultraviolet, near-infrared, and infrared regions. It is also often desired for general light-scattering measurements including fluorescence and Raman spectroscopy. It is readily seen that a flat face, parallel fiber probe is limited in its capability to deliver diffuse illumination. These measurements are highly sought for a variety of industrial and biomedical applications.
In addition to attaining light-diffusion-related measurements based upon illumination of a target with highly diffuse light, another technique is of interest. In this technique, light is angularly directed at the target such that the specular light from the target surface is reflected away from the light-collection device. By this means, the collector is only receptive to light that the target scatters in a non-specular fashion and the light-collection device is not receptive to specular light. It is readily seen that a flat face, parallel fiber probe lacks the capability for angular light control to achieve this goal.
Consider a flat reflective surface placed in front of the flat face, parallel fiber probe so that it is perpendicular to the fibers. If the surface is positioned within the region of overlapping receptivity and illumination, then the collection fiber receives and transmits source light projected from the reflective surface. However, the received light is a small fraction of that available. Because the angle of incidence equals the angle of reflection, the majority of the light is directly reflected back and away from the collection fiber. The axis of the reflected light remains concurrent with the source fiber""s axis. The fact that the optical axis of the illumination from the source fiber remains fixed prevents manipulating the optical patterns to change the percentage of surface-reflected light from the collection fiber.
As previously described, light reaching the collection fiber is a function of the distance from the flat reflective surface to the probe end face. This distance dependence can be utilized for the purpose of displacement sensing. However, the lack of ability to manipulate the optical axis of the illumination and collection cones limits the controllability of the measurement dynamics. It further limits the overall ability to achieve specific application goals, such as linearity, dynamic range, sensitivity and related criteria. And as previously mentioned, the capability to manipulate the angle and axis of illumination incidence facilitates the ability to maximize or minimize, desirable or undesirable surface reflections. This capability, which a flat face, parallel fiber probe lacks, can be utilized to significant advantage.
In certain sensing applications, the parameter under investigation responds inadequately to light of desirable wavelengths. For example, suppose an arbitrary chemical has an infrared signature suitable for photonic sensor development, but the appropriate infrared light does not readily transmit with conventional optical fibers. In many situations such as these, visible light and standard optical fibers may be successfully utilized. This may be accomplished by introducing an indicator material that undergoes a visible color change upon interaction with the chemical species of interest.
To successfully employ indicator-based fiber optic sensors, fiber must illuminate the chemical indicator and collect light from it. Although this sensor methodology encompasses many techniques, one method involves coating the fiber end face with the indicator material. If a single fiber""s end face is coated and the fiber is utilized as a bidirectional light conduit, poor isolation between delivered and collected photons can result.
Due to shortcomings associated with a flat face, parallel fiber probe and its ability to control illumination and collection, complications arise in illuminating and collecting indicator light. A probe able to project illumination light onto a clearly defined indicator zone is highly preferred over a flat face, parallel fiber probe. For many situations, the ideal probe""s desirable features include the capability to project illumination light directly onto a collection fiber whose end face coated with the indicator. In this superior configuration, only light interacting with the indicator reaches the detector-thereby eliminating stray light.
The preceding discussion has focused on a probe consisting of two parallel-mounted fibers (one source and one detector fiber). The progression and correlation to bundles of fibers in various configurations is readily appreciated and followed by those skilled in the art. Although bundles potentially overcome some of the pre-described limitations, significant limitations remain. And, the usage of bundles introduces additional problems and undesirable characteristics.
Attempts to Improve Probe Performance
From the preceding discussion, it is apparent that the ability to direct and manipulate illumination and receptivity zones of optical fibers is highly desired. Several prior art techniques have been employed to manipulate a probe""s illumination and receptivity characteristics and to address the input/output constraints of optical fibers. For the reasons set out below, these methods are limited in terms of their effectiveness for many desired Raman instrumentation applications.
One approach employs optical fibers with varying numerical apertures in order to gain better control over the entry/exit characteristics. For example, by employing fibers with higher numerical aperture, the light-gathering ability is increased. This approach includes several drawbacks. First, the required fiber materials have characteristics not suitable for high-end instrumentation application including environmental sensitivity, usage restrictions, and the generation of extraneous responses. Second, physical laws limit the extent to which a fiber""s acceptance characteristics can be extended. Third, a fiber""s delivery pattern/field-of-view can only be broadened or narrowed but not steered off axis or directed to view in a specific region. Fourth, a wide acceptance angle on the input end of the fiber translates to a wide divergence on the output end. While a high numerical aperture fiber increases light gathering on the collection end, it delivers its light to a detector system in a widely diverging angle. In many cases, the delivery of widely diverging light to the detector system is detrimental to achieving acceptable performance.
Another approach employs expanded-beam external elements, such as lenses and mirrors, to manipulate the illumination and receptivity characteristics. These elements are bulky, expensive, sometimes fragile, often lossy, difficult to align, and susceptible to environmental influences. Additionally, it is difficult to engineer a highly robust package. For example, larger, more robust components, have higher mass and increase system susceptibility to mechanical shock.
If an individual lens is dedicated to each collection and illumination fiber, then the ensuing device becomes large and bulky. Furthermore, the larger the assembly, the further apart and less efficient the collection and illumination devices. If, on the other hand, illumination and collection paths traverse the same optical element, a significant portion of the source energy is inadvertently reflected into the collection fiber without interacting with the sample. This stray light contaminates the measurement and is extremely detrimental. Additionally, the introduction of expanded-beam optical elements complicates the assembly, causes manufacturing difficulties, yields additional variability, and produces other undesirable results.
A special class of devices, termed confocal, involves the utilization of multiple optical elements in conjunction with optical fibers. In these devices, focusing optics create a converging illumination beam which is projected into the operating medium under investigation. The focal point, or point of ray convergence, of the illumination beam lies within the sample. The collection zone is also created with focusing optics and is likewise formed, to the extent possible, concurrent with the illumination zone.
The objective is to create matching focal points of illumination and receptivity projected into the sample medium. The underlying theory is that stimulated light originating at the focal point within the sample is collected from a large solid angle defined by the angle of beam convergence. The intent is to optically re-image the source fiber end face within the sample thereby creating a virtual fiber end face. In theory and assuming 100 percent optical efficiency and no optical distortion, re-imaging the end face as described re-creates the illumination intensity of the actual fiber end face. Although achieving the theory is physically impossible, in a perfectly transparent medium under laboratory conditions, acceptable results is a reasonable goal.
Unfortunately, in the majority of applications in which fiber optic""s remote capabilities are highly sought, the materials under investigation are complex, dark, and scattering. The situation is similar to that of the flat face, parallel fiber probe previously analyzed; the converging illumination beam is drastically attenuated and distorted before it effectively reaches the optimum point of receptivity. And for similar reasons, feeble stimulated emissions from this point cannot return to the collector optics.
As a separate disadvantage to this technique, response is collected from a potentially undesirably large sample area since measurement contributions are accumulated, to a certain degree, as the beam converges to its focal point. These devices suffer additional drawbacks including complexity, environmental sensitivity, large size, high expense, and failure in hostile environments. For example, unlike even the flat face, parallel fiber probe previously analyzed, this type of device cannot be inserted into a biomedical catheter.
As a separate consideration, suppose the focal point projecting apparatus is utilized to investigate an undulating mass, such as a heart muscle. As the muscle beats, the tissue moves in relation to the analytical zone. Thus, the measurement is difficult or unsuccessful.
In another approach, a fiber can be bent at its tip in order to point in a direction of interest. For example, one or more flat-faced optical fibers may be directed to view a common or overlapping zone of receptivity. A second group of one or more flat-faced optical fibers may be directed to illuminate the zone. By this method, receptivity and illumination overlap is achieved. Unfortunately, this method suffers from several serious drawbacks. The assembly is expensive and difficult to construct. The ensuing device also lacks repeatability due to manufacturing constraints. If the fibers are gradually bent from their converging orientations to parallel, then the assembly is very large. Even if the fibers are rapidly bent near the assembly""s distal end, the assembly is bulky and relatively large in diameter. Such an assembly is much too large to be utilized in an application such as in vivo medical. As the fibers are bent, they become inefficient and lossy at the sharp bend resulting in light escaping from the fiber at this point. In addition, bending the fibers to create a probe results in exceeding the minimum bend limitations for most optical fibers. The fiber is subsequently prone to failure and suffers increased sensitivity to environmental influences.
In another approach, the illumination and collection zones may be manipulated by shaping the fibers"" end faces to create a refractive surface. For example, a center fiber may be encircled by a ring of fibers with tapered end faces. This tapering creates a refractive surface on the ring fibers to manipulate their field-of-view inward and toward the center fiber""s axis. A key aspect of this refractive-end-face approach is that light manipulation occurs at the fiber end face boundary, and rays are bent as they enter or exit the fiber and cross the boundary of the fiber core end face. Several problems are associated with this approach and limit its effectiveness.
In manipulation of accepted and/or emitted light by the method of forming shaped end faces into optical fibers, the refraction is due to the refractive index differential between the fiber core and the medium surrounding the fiber end face. The extent of refraction is a function of the difference between the two refractive indices and the angular orientation of the light relative to the surface of the interface. Optical fiber cores are typically glass or similar materials with relatively high refractive indices. In order to achieve significant refraction at the fiber end face, it is usually desired to have a gaseous medium, such as air surrounding the fiber end face. This type of medium has a low refractive index thereby facilitating sufficient ray refraction. Most fluids have relatively high refractive indices with values approaching those of common optical fiber core materials. Therefore, media such as fluids, fluid-filled matrix, biological tissue, and melts typically provide insufficient characteristics to achieve the requisite refractive index differential. In addition, shaped end faces typically protrude beyond the protective housing in which the fiber is mounted. This delicate protrusion is susceptible to physical or mechanical damage.
In order to address the dependence on refractive index, the fiber end face must be surrounded by a medium with a known refractive index. The medium is preferably air or a similar gaseous material. This may be accomplished by situating the probe tip behind a window in a sealed chamber. However, use of a window causes numerous problems.
The assembly encompassing fiber, fiber mount, window, window housing, and sealing mechanism is expensive and difficult to construct. The necessity of the sealed chamber also forces substantial increases in the size of the assembly. Thermal expansion and sealing problems also plague the windowed mechanism. The window""s optical material possesses low thermal expansion characteristics while the housing to which the window is bound is typically of metal or other high thermal expansion material. Bonding and sealing the window to the metal housing presents numerous difficult engineering challenges.
As light enters or exits the sealed chamber it traverses the window. In many instances, the window material has undesirable spectral characteristics. For example, diamond windows produce strong spectral peaks of Raman scattered light as laser light is transmitted. As a second example, sapphire windows often contain impurities that fluoresce.
The window forces the fiber end face to be removed from the application environment by, at least, the thickness of the window. Although the window may be only a few millimeters thick, it remains large relative to the size of the fibers. On the optical fiber scale, positioning the fiber tip even this distance from the physical target often correlates to excessive light intensity losses.
For the proper optical performance, the fiber end face should be positioned as close as possible to, and preferably touching, the window. Accomplishing this feat requires a complex means to adjust the distance and lock the assembly in place. As previously described, the shaped end face is mechanically feeble and prone to physical damage. Thus, the assembly is prone to damage not only during positioning but also as a result of thermal expansion, vibration, and general operations.
As the light is incident upon the inner and outer boundaries of the window, it is refracted and reflected. The refracted aspect is either a boon or a hindrance depending on application specifics. The reflection aspects are often highly disadvantageous. For source fibers, window reflections not only weaken the emitted light but also are directed back within the sealed chamber. Depending on configuration and application specifics, these reflections are captured by the source fiber and thereby redirected towards the source. For many applications, this back propagated light is significantly detrimental. The window reflections also tend to interfere with elements adjacent to the optical fiber. For example, a detector fiber positioned in proximity to the source fiber captures a portion of the source light that is back reflected by the window. Light captured in this manner potentially mixes with and contaminates the desirable light. Similar circumstances surround applications in which the shaped end face fiber""s principal role is to capture source light generated outside the confines of the fiber. The housing to which the window is fixed, together with the window, forms a sealed chamber. Undesirable light tends to bounce around in this chamber amplifying and exasperating the described stray light problem.
A standard optical fiber, properly mounted in a typical fiber optic connector, withstands tremendous hydrostatic pressure prior to failure. The fiber""s small surface area translates even high pressures into very small forces. Thus, extreme pressures are required to generate sufficient force to cause the fiber to piston back into its connector and fail. Conversely, a window is typically much larger in diameter than the fiber positioned behind it. Hence, for a given environmental pressure, the window is subjected to much higher forces than would be an exposed fiber. Additionally, the window is thin and only supported around its outer rim. Therefore, it is susceptible to breakage. Strong, thin windows can be produced from materials such as diamond, sapphire, and similar materials. Unfortunately, these materials not only suffer from the pre-described drawbacks but also have high refractive indices. A high-refractive-index window intensifies the pre-described reflection/refraction problem.
Another drawback to relying on refractive end faces results from the nature of the refractive effect, which limits the extent to which light can be manipulated. This is readily investigated and studied by applying Snell""s Law through ray tracing. Due to the nature of refraction, light cannot be aggressively steered off axis to achieve optimal response.
Based on the foregoing discussion, it is highly desirable to redirect light by means other than refraction at the fiber""s end face. Specifically, it is desirable to manipulate light within the confines of the fiber assembly""s light path. Light manipulation can be accomplished by creating light-shaping structures within the confines of the optical fiber assembly. Thus, light that enters the fiber and would normally be rejected can be redirected for propagation via total internal reflection. Similarly, light propagating via total internal reflection can be directed to otherwise unfeasible paths. By creating the light-shaping artifices within the confines of the fiber assembly""s internal structure, effects similar to those found in fibers with shaped end faces are produced without the disadvantages or constraints associated with shaped end faces.
One method of achieving light bending within the confines of the optical fiber is to include a light-manipulating surface between two adjoining waveguide sections. This can be accomplished by inserting a light-altering component between two sections of fiber. A highly advantageous method is to construct the light-shaping artifice into or onto the fiber end face that adjoins another fiber segment or section. For example, light-shaping contours are readily constructed into a fiber end face that is butted to a second fiber. The second, adjoining fiber end face can be flat faced or also encompass light-altering surfaces or characteristics. As an alternative to light-shaping by refraction, the light shaping can occur via diffraction, reflection, scattering, interference, or other methodology. If light-shaping refractive surfaces are employed which are not symmetrical about the fiber""s central axis, the light tends to be steered or bent off axis. Thereby, illumination and/or collection zones are directed off axis.
A second method of achieving light manipulation and bending within the confines of the optical fiber is based upon reflection. In this method, the fiber core""s exterior surface is modified to create a reflective surface other than the standard core-cladding interface. For example, an optical fiber whose end face is formed into a sharply inclined planar surface will exhibit these characteristics. Suppose the angular inclination of the end face is sufficiently inclined to generate a totally internally reflective internal surface. As light propagates within the fiber core towards this distal tip, it encounters the special surface. The propagating light is re-directed by total internal reflection to exit the fiber through its side or outer cylindrical surface. Variations on this theme include creation of surface contours which do not typically yield total internal reflection but to which internally reflective coatings are applied. Additionally, various complex contours can be generated which mix various optical effects.
Another method for fiber optic light manipulation entails forming a group of flat-faced optical fibers consisting of illuminating source and collection fibers. A typical orientation is a single source fiber surrounded by a ring of collection fibers. This grouping of fibers is butted up to a single, large-core optical fiber. The single fiber""s large core has a diameter equal to or greater than the collective grouping of smaller fibers. The large-core fiber is utilized in a bi-directional capacity. Its distal end both delivers illuminating energy and captures target light. This method suffers from several drawbacks. A certain degree of source light is reflected from the fiber""s end face before the light exits the fiber. This light is prone to back propagation within the large core fiber and returns to the collection fibers as stray light. Secondly, as the source light traverses the large-core fiber segment, detrimental signals are often generated. For example, Raman-scattered light is produced and radiates in all directions. Unfortunately, the large-core fiber accumulates the Raman-scattered light and efficiently waveguides it to the collection fibers where it is mixed with the desired target light. Fluorescence generated within the large-core fiber is likewise delivered to the collection fibers and corrupts the measurement process.
Therefore, there is a need in the art for an improved fiber optic probe assembly that allows effective and efficient manipulation of the light delivery and reception regions. The light manipulation should take place within the fiber assembly""s light path and should allow significant off-axis steering of the fibers"" viewing areas. The probe assembly should be compact and easy to manufacture, and should not rely on expanded optics and other complicated features found in the prior art.
The present invention satisfies the above-described need by providing an improved method and apparatus for fiber optic light management. The invention provides a number of novel fiber optic light manipulation and management techniques, which are individually important for diverse fiber optic applications. For example, the present invention provides an improved fiber optic probe assembly for low light spectrographic analysis. The invention improves response to subtle light-matter interactions of high analytical importance and reduces sensitivity to otherwise dominant effects, thereby overcoming the technical difficulties associated with light-based characterization in complex media. This is accomplished by adjusting the illumination and collection fields of view in order to optimize the probe""s sensitivity. Light manipulation is applied internal to the fiber so that the probe""s delivery pattern and field of view do not require external manipulation and are not adversely affected by investigated media. This allows the light delivery pattern or field of view or both to be aggressively and reliably steered off-axis to achieve significant increased performance levels. Aggressive beam steering is accomplished by employing internally reflective surfaces in the fiber. A reflective metal coating or a low refractive index coating or encapsulant can be used to ensure total internal reflection. The fibers also incorporate filters, cross-talk inhibitors and other features that provide a high performance probe in a robust package. Design variations provide side viewing, viewing through a common aperture, viewing along a common axis, and other features.
Generally described, the present invention provides a probe having selective sensitivity to specific photon-matter interactions. This selective sensitivity is achieved by delivering light at one angle and collecting light at the appropriate angle to maximize the response. The delivery and collection paths are re-directed off-axis to intersect with one another at specific angles while the delivery and collection fibers remain in close proximity to one another.
In another aspect, the present invention provides a means for segregating inelastic and elastic photon-matter responses of a material by angularly manipulating the delivery and collection patterns in relation to one another. The elastic response is directionally biased such that its collection, in relation to the inelastic response, is minimized.
In another aspect, the present invention provides a fiber with a tip having a portion that is internally reflective and a portion that is internally non-reflective. The reflective portion of the tip delivers light at angular orientations beyond the fiber""s normal propagation limits. Incoming light, incident on the reflective surface, is angularly steered, so that light is received at angular orientations beyond the fiber""s normal propagation limits.
More particularly described, the internally reflective portion may be the result of an internally reflective coating, or may be essentially total internal reflection. Total internal reflection may be induced by placing a low refractive index material into contact with the fiber. The low refractive index material may include a low index coat or encapsulant, or the ambient medium. The internally reflective surface may include a variety of shapes, which can be used to control the field of view with great precision.
In another aspect, the present invention provides a probe that incorporates a reflective surface for steering the light path. The probe includes at least one delivery fiber and at least one collection fiber. The delivery fiber or collection fiber include an internally reflective surface for causing the light delivery path and light collection path to converge.
In another aspect, the present invention provides a probe assembly that includes filters applied directly to the interior end faces of distal fiber segments.
In another aspect, the present invention provides a method for mass producing fibers with high performance filters.
In another aspect, the present invention provides a fiber optic probe that collects light directly in front of, or at, the delivery aperture. The viewing angle is directed in response to the extent of the elastic response, the strength of the inelastic response, the desired depth of investigation, and the absorption of the medium.
In another aspect, the present invention provides a probe comprising a plurality of fibers essentially parallel to each other and in close proximity to one another. The coupling efficiency between the probe and the investigative medium is enhanced by fusing the bundle of collection and delivery fibers together. The fusing process entails heating the fibers and compressing the fibers so that no gap exists.
In another aspect, the present invention provides a means of optically isolating two or more fibers that are in close proximity to each other, such that the signals from each fiber do not mix. A light impenetrable barrier is stationed between the fibers in the area prone to cross talk.
In another aspect, the present invention provides an optical fiber enhanced at its tip to collect or deliver light beyond the fiber""s normal limits of propagation. The fiber adjoins another short fiber segment. The short fiber shuttles light between the beam-steered fiber end face and the distal end face of the assembly. The short fiber segment has the ability to carry angularly oriented light beyond that of the unmodified, primary fiber.
In another aspect, the present invention provides a fiber optic probe assembly that includes a central fiber and a plurality of fibers surrounding the central fiber. The central fiber has a flat end face at its distal end. The plurality of fibers surrounding the central fiber and have a shaped end face at their distal ends. The plurality of fibers are parallel to the central fiber at their distal ends. The shaped end faces provide an internally reflective surface for steering the fields of view associated with the plurality of fibers toward the central fiber.
The present invention also provides a fiber optic probe assembly that includes a first fiber and a second fiber. The first fiber includes an end face having a first shape. The second fiber includes an end face having a second shape. The first and second fibers are parallel to each other at their end faces. The second shape provides an internally reflective surface for directing the field of view associated with the second fiber toward the first fiber.
In another aspect, the present invention provides a fiber optic assembly having a common axis for delivering and collecting light. The assembly includes a light delivering fiber and a light collecting fiber. The light delivering fiber has a filter at its end face. The light collecting fiber has a reflector at its end face and is mounted parallel to the light delivery fiber. The first filter is operative to reflect delivered light through its side wall and to allow collected light to pass through to the light collecting fiber. The collecting fiber reflector directs light along the axis of the light collecting filter.
Alternatively, the present invention provides a fiber optic assembly having a common axis for delivering and collecting light. The assembly includes a light delivery fiber and a light collecting fiber. The light delivering fiber has a filter at its end face. The light collecting fiber has a reflector at its end face and is mounted parallel to the light delivery fiber. The filter is operative to pass delivered light and to reflect collected light to the reflector on the collection filter. The reflector directs collected light along the axis of the light collecting filter.
In another aspect, the present invention provides a fiber optic probe assembly using a common aperture for delivering and collecting light. This is achieved by transmitting desirable light through a fiber""s sidewalls. The assembly includes a central fiber having a flat end face at its distal end and a plurality of fibers surrounding the central fiber. The plurality of fibers have a shaped end face at their distal ends. The plurality of fibers are parallel to the central fiber at their distal ends. The shaped end faces provide an internally reflective surface for steering the fields of view associated with the plurality of fibers through the side wall of the plurality of fibers and through the end face of the central fiber.
In another aspect, the present invention provides a fiber optic probe assembly for side delivery and collection of light. The assembly includes a first fiber and a second fiber. The first fiber has a shaped first end face. The second fiber has a shaped second end face and is parallel to the first fiber. The shaped first end face and the shaped second end face direct light toward a common region.
In yet another aspect, the present invention provides a method for fabricating a fiber optic probe assembly. The method includes forming a bundle of fibers including a center fiber surrounded by a ring of fibers. The bundle of fibers is bound together. A cross-talk inhibitor mechanism is incorporated into the probe. The bundle of fibers is shaped to form a pencil tip or cone. The cone is then flattened so that the center fiber has a flat end face.
In another aspect, the present invention provides a fiber optic probe that incorporates an integral reference material. The probe includes fibers for delivering light to an investigative sight and fibers for collecting light from an investigative sight. In addition to exciting a response from the medium under investigation, the delivered light excites a response from the reference material. The light from the reference material maybe collected and used to calibrate the system, compensate for drift, establish accuracy, and verify functionality.
In another aspect, the present invention provides a means for manufacturing low cost, high performance probes for inclusion in a comprehensive analytical system. The probes are disposable following their utilization.
The various aspects of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the appended drawings and claims.
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