Abstract:
A probe including a fisheye lens is disclosed to measure the velocity distribution of a moving surface along many lines of sight. Laser light, directed to the surface and then reflected back from the surface, is Doppler shifted by the moving surface, collected into fisheye lens, and then directed to detection equipment through optic fibers. The received light is mixed with reference laser light and using photonic Doppler velocimetry, a continuous time record of the surface movement is obtained. An array of single-mode optical fibers provides an optic signal to an index-matching lens and eventually to a fisheye lens. The fiber array flat polished and coupled to the index-matching lens using index-matching gel. Numerous fibers in a fiber array project numerous rays through the fisheye lens which in turn project many measurement points at numerous different locations to establish surface coverage over a hemispherical shape with very little crosstalk.

Description:
1. STATEMENT REGARDING FEDERAL RIGHTS 
     This invention was made with government support under Contract No. DE-AC52-06NA25946 and was awarded by the U.S. Department of Energy, National Nuclear Security Administration. The government has certain rights in the invention. 
    
    
     2. FIELD OF THE INVENTION 
     This invention relates to photonic Doppler velocimetry and in particular to a method and apparatus for multipoint photonic Doppler velocimetry using optical lens elements. 
     3. RELATED ART 
     Photonic Doppler velocimetry (PDV) can be used to monitor movement of a curved surface that is moving along multiple points, such as during an experiment. The movement of the curved surface may occur due to an implosion, explosion, or any other force or factor that causes movement of the surface. Such movement often occurs during dynamic material experiments. These dynamic material experiments frequently involve complicated geometries and therefore large numbers of data points are a distinct advantage. Various solutions have been proposed to record the movement of the surface, but each of these various solutions suffer from various drawbacks. 
     One proposed prior art solution for measurement of the moving curved surface is the use of electrical shorting pins. In such a configuration, electrical shorting pins of various lengths were used to contact the moving service. An electrical pin provides a shorting signal between the surface and the pin tip when the tip of the pin comes into contact with the moving surface. Each electrical shorting pin gives a single timing point when the collapsing surface contracts the pin. Each length of a pin records one distance, so that many different lengths of pins are needed to follow the movement of an imploding surface. However, longer pins can interfere with the surface movement and interfere with the shorter pins. As a result, data may be corrupted by the interference. In addition, data is only collected by a pin when that pin contacts the moving surface. Movement prior to contact with the pin is not recorded. Moreover, a dense array of pins at many lengths is needed and this density may not be possible to achieve while also achieving desired data recordation. 
     Several different optical designs have been proposed to measure the behavior of the moving surface. One such proposed solution involved a ball shaped housing with discrete fibers pointed outward toward the surface of interest. This solution provided optic monitoring but it suffered from being size limited. As the number of optic fiber points is increased, this probe eventually comes to its limit in how small the entire group can be made, while still having reasonable optical fiber bending radii. The bend radius of the optic fibers limit size reductions in the probe and there is a limit to the number of fibers which may be packed in the probe. 
     Therefore, there is a need in the art for an accurate and cost effective light delivery and collection system for use as a collection probe in a PDV system. 
     SUMMARY 
     A new fisheye lens design is disclosed for use in probe (which may be any size or miniaturized) to measure the velocity distribution of a moving surface along many lines of sight. An optic signal is directed to the surface using a launching fiber and a fisheye lens. The optic signal may be laser light. The optic signal is scattered back along each beam projected on the surface and is Doppler shifted by the moving surface before being collected into the launching fiber. The received light is mixed with reference laser light in each optical fiber, in a technique called photonic Doppler velocimetry, providing a continuous time record. 
     An array of single-mode optical fibers sends laser light through the fisheye lens toward the surface. In one embodiment, the lens consists of an index-matching positive element, two positive doublet groups, and two negative singlet elements. The optical design minimizes beam diameters, physical size, and back reflections for excellent signal collection. The fiber array projected through the fisheye lens provides many measurement points of surface coverage over a hemisphere with very little crosstalk. The probe measures surface movement with only a small encroachment into the center of the cavity. 
     The fiber array is coupled to the index-matching element using index-matching gel. The array is bonded and sealed into a blast tube for ease of assembly and focusing. This configuration also allows the fiber array to be flat polished at a common object plane. In areas where increased measurement point density is desired, the fibers can be close packed. To further increase surface density coverage, smaller diameter cladding optical fibers may be used. 
     Disclosed herein is a photonic Doppler velocimetry probe comprising a housing defining an interior space between a first end and a second end. One or more optic signal conductors enter the housing through the second end. The optic signal conductors terminate within the interior space of the housing. The one or more optic signal conductors provide one or more optic signals to one or more lenses in the probe. A ferrule, located within the housing, is configured with one or more passages such that the passages receive and secure the one or more optic signal conductors to the ferrule. An index-matching lens is adjacent the ferrule and configured with a flat surface adjacent the ferrule to receive the optic signal. The index-matching lens has an index of refraction selected to match an index of refraction of the optic signal conductors. Also part of this embodiment is a fish eye lens, mounted at the first end, configured to receive the optic signal and project the optic signal outward, in a direction controlled by the configuration of the fish eye lens. 
     In one embodiment, the one or more optic signal conductors are optic fibers. The optic fibers may be single mode optic fibers. It is contemplated that the terminating end of the one or more optic fibers and a side of the ferrule are end-polished. The probe may further comprise an index-matching material between the index-matching lens and the ferrule such that the index-matching material consists of index-matching gel or index-matching epoxy. The optic signal may be at 1550 nm wavelength. The probe may also include one or more additional lens elements between the index-matching element and the fisheye lens. In one embodiment, the fisheye lens, index-matching lens, and the optic signal conductors are configured to receive a reflection of the optic signal and conduct the reflection back through the optic signal conductor. 
     The photonic Doppler velocimetry probe may be configured as a housing defining an interior space and one or more optic signal conductors having a terminating end configured to provide optic signals near or within the housing. The optic signal conductors may be fiber optic cables. Also part of this embodiment is an index-matching lens, located in the interior space, configured to receive the optic signals. The index-matching lens may have an index of refraction selected to match an index of refraction of the one or more optic signal conductors. A fisheye lens is provided and configured to receive the optic signals after the optic signals pass through the index-matching lens. The fisheye lens projects the optic signal outward, in a direction controlled by the configuration of the fish eye lens onto a curved surface that is the subject of photonic Doppler velocimetry measurement. 
     In one configuration optic signal conductors are optic fibers, which may be single mode optic fibers. The optic signal conductors may be secured by a disk having one or more passages. The optic signal conductors may pass through the passages and be end-polished flat with a side of the disk faxing the index-matching lens. In one embodiment, the probe further comprises an index-matching gel material between the index-matching lens and the one or more optic signal conductors. There may be one or more additional lens elements between the index-matching element and the fisheye lens. In one configuration, the fisheye lens, the index-matching lens and the optic signal conductors are configured to receive a reflection of the optic signal and conduct the reflection back through the optic signal conductor to measurement equipment. 
     Also disclosed is a method for use during photonic Doppler velocimetry, for presenting an optic signal to a curved surface and receiving a reflection from the curved surface during movement of the curved surface. This method includes establishing a distal end of a probe facing the curved surface such that the distal end of the probe includes a fisheye lens facing the curved surface. Then, generating an optic signal and presenting the optic signal to an optic signal conductor. This method then passes the optic signal through the optic signal conductor to one or more lenses. The one or more lens includes an index matched lens. This method directs the optic signal into a fisheye lens such that the fisheye lens changes the path of the optic signal to thereby project the optic signal onto a position on the curved surface. At this stage, movement of the curved surface is initiated and the fisheye lens receives a reflection of the optic signal. The reflection passes through the lenses to the optic signal conductor. 
     In one embodiment, the optic signal conductor is a single mode fiber optic cable. It is contemplated that the reflection will proceed along the same path through the fisheye lens, through the one or more lenses, and into the optic signal conductor as the optic signal. The step of projecting may comprise projecting the optic signal onto a position on the curved surface. 
     Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  illustrates a side view of an exemplary environment of use of the probe disclosed herein. 
         FIG. 2  illustrates a perspective view of the probe assembly  104  and the interior surface. 
         FIG. 3A  is a cut away side view of the probe assembly. 
         FIG. 3B  illustrates a perspective view of the probe assembly shown in  FIG. 3A . 
         FIG. 4  illustrates a detailed view of a ferrule as shown in element in  FIG. 3A . 
         FIG. 5  illustrates an enlarged view of the junction between the index-matching lens and the ferrule. 
         FIG. 6  illustrates an example passage layout within the ferrule. 
         FIG. 7  illustrates on exemplary lens arrangement with resulting ray traces through the lenses. 
         FIG. 8  illustrates an example arrangement of an alternative lens arrangement with resulting ray traces. 
         FIG. 9  illustrates a plot of distortion improvement realized in the embodiment of  FIG. 8  over the embodiment of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     Photonic Doppler velocimetry (PDV) with a novel light signal path is utilized to measure movement of a curved surface. PDV uses light scattered and reflected from a moving surface to continuously measure the movement of that surface. This method takes advantages of the Doppler principles. Namely, the Doppler-shifted light from the moving surface is compared to unshifted light to create fringes in a Michelson interferometer made up of fiber-optic components. Then, a fiber-optic circulator is used as the beam splitter of the interferometer. Reference (unshifted) light is provided through mixing with an external reference laser source or from back reflections in the probe itself. Surface velocities of between a few millimeters per second up to 14 km/s have been measured and it is expected that further development will allow operation at higher velocities. A PDV probe lens or bare fiber can both transmit and receive the laser light. In the embodiment disclosed herein a 1550 nm telecommunications wavelength, continuous-wave (CW) fiber laser is used with matching fiber, fiber related devices, optic detectors, and signal digitizers. 
     As discussed above, drawbacks exist in the prior art with the probe that is used to present the light signal to the moving surface and collect the light signal from the moving surface. The probe structure and associated light path disclosed herein overcomes those drawbacks.  FIG. 1  illustrates a side view of an exemplary environment of use of the probe disclosed herein. This is but one possible environment of use and as such it is contemplated that other uses for the disclosed system may be arrived at by one of ordinary skill in the art. 
     As shown in  FIG. 1 , the probe assembly  104  is provided in the interior space  108  that is bounded by a curved interior surface  112  of a structure  116 . The structure  116  is surrounded on an exterior surface  120  by a material  124 . The material  124  may comprise any material that exerts a force on the surface  120  to thereby move the material  116  inward toward the probe assembly  104 . In one embodiment, the material  124  comprises an explosive material that when detonated, implodes the interior surface  112  inward toward the probe assembly  104 . 
     In other embodiments, it is contemplated that a material  124  other than explosives may be utilized such as, but not limited to heat, air pressure, liquid pressure, radiation, or any other type of force. The material  116  may comprise any type material or barrier. The interior area  108  may comprise any type material or void including but not limited to, a vacuum, air or other gas, or even other material, such as gel or liquid, or a solid compressible material. 
     It is also contemplated that the interior surface  112  may move outward, away from the probe assembly  104  instead of inward toward the probe assembly. In other embodiments, the surface may move in a non-uniform manner with a first portion of the surface  116  moving toward the probe assembly  104  and a second part of the surface moving away from the probe assembly. 
     On the exterior of the probe assembly  104  is an exterior lens  130  which rests in a support  134 . Below the exterior lens  130  are one or more lens, discussed below in greater detail, which are contained in a housing  138 . In one embodiment, the exterior lens is a fisheye lens. A fisheye lens provides an ability to image over a very wide range of angles using optic signals which enter the lens from a single direction, such as through a planar surface. Likewise, reflections into the outer surface of a fisheye lens are directed though a single plane on the interior side of the fisheye lens. The ‘fisheye’ term refers to simulating a large angular view. In one configuration, this lens type produces a whole-sky or whole-view image as a finite circle. Some fisheye lenses ‘see’ beyond 180 degrees with darkness beyond it lens limit. In general, a fisheye lens is a wide angle lens that produces strong visual distortion intended to create a wide panoramic or hemispherical image. Fisheye lenses achieve extremely wide angles of view by forgoing producing images with straight lines of perspective (rectilinear images), opting instead for a special mapping (for example: equisolid angle), which gives images a characteristic convex non-rectilinear appearance. In various different embodiments the term fisheye lens includes a circular fisheye lens, a full frame fisheye lens, a zoom fisheye lens, a miniature fisheye lens, or any other configuration. 
       FIG. 2  illustrates a perspective view of the probe assembly  104  and the interior surface  112 . As compared to  FIG. 1 , similar elements are identified with identical reference numbers. As shown, the housing  138  of probe assembly extends near or into the interior of the structure  116 . The exterior lens is at the distal end of the probe structure which faces the interior surface  112 . The structure  116  has an interior surface  112  and an outer surface  120  which form a dome or spherical shape. As discussed below in greater detail, the exterior lens (not shown in  FIG. 2 ) projects one or more optic signals on the interior of the inner surface  112 . These points of projection are shown by reflection points  150 . These projections points  150  may be arranged at any point on the interior surface subject to the monitoring preferences. As discussed below in greater detail, the location of the reflection points is determined by the location of the fiber optic cables and the lenses within the housing  138 . This is but one configuration for the surface  112 . 
     Turning to  FIG. 3A , a cut away side view of the probe assembly  104  is shown. This is but one possible configuration for the probe assembly and it is contemplated that one of ordinary skill in the art, after reading this disclosure, may arrive at different configurations which do not depart from the claims that follow. In general, the probe assembly  104  includes an outer housing  344  which defines an interior area as shown. While the housing  344  may be of any shape, in this embodiment the housing is cylindrical on its interior surface. As shown, at a distal end  302  of the housing  344  is an exterior lens mount  300  configured to hold or otherwise support an exterior lens  304  which may comprise a fisheye lens. Below the exterior lens  304  are one or more additional lens and open spaces or voids. Although shown in a particular configuration it is contemplated that other arrangements of lens may be arrived at without departing from the scope of the claims that follow. The lens arrangement is configured to convey optic signals from the exterior lens  304  to the interior of the housing  344 . The lens disclosed herein may have one or more coatings on the lens which contacts air or the void. These coatings decrease optical reflection, dispersion, and scattering and thereby increase optic signal transmission into and out of the lens. Anti-reflection coatings for glass-to-air interfaces and the use of a minimum of lens elements are important for the operation of the external reference light. 
     In this example embodiment, opposing the exterior lens  304  is a plano-concave lens  308 . Below the plano-concave lens  308  is a space  312  and then a doublet lens  316 . Below the doublet lens  316  is an aperture spacer  320  and then another doublet lens  324 . Below the doublet lens  324  is a space  328  and below the space  328  is an index matched lens  340 . These lenses operate in combination to conduct light signals through the interior of the housing. 
     Below the index-matching lens  340  is a ferrule  332  configured as a cylindrical disk having a top surface and bottom surface contained by an outer circumferential surface. The ferrule may also be a disk, washer, plate or any other element configured to function as disclosed herein. The ferrule  332  is discussed below in greater detail in connection with  FIGS. 4 ,  5 , and  6 . The top surface is configured to fit flush with or nearly flush with a bottom surface of the index-matching lens  340 . Within the ferrule  332  are one or more cylindrical holes, referred to herein as passages, which pass from the first side to the second side to form a passage. Within the passages are optic signal conductors  350 , such as fiber optic cables, optic channels, vacuum, gas, lenses, or any other medium capable of carrying an optic signal. The optic signal conductors  350  have a first end which is adjacent the index-matching lens  340  and second opposing end which interfaces or connects to an optic signal generator and an optic signal detector (not shown). 
     As an advantage of this configuration over prior art devices, the optic fibers  350  are established and maintained in linear alignment to the planar bottom surface of the index-matching element  340 . This alignment maximizes optic signal transmission. In addition, by maintaining a generally linear configuration for the optic fibers, unwanted bending of the optic fibers is avoided. This allows use of thinner optic fibers and a high density of optic fibers as compared to prior art configurations which in turn increases measurement point density. 
     To maximize transmission of the optic signal between the index matched lens  340  and the optic signal conductors  350 , the index matched lens is indexed matched to the optic signal conductor. For example, if the optic signal conductor  304  is an optic fiber then the material of the lens  340  is selected to have the same index of refraction as the fiber optic cable. This minimizes back reflections at the interface between optical fiber ends and the index matched lens. This design may use an index-matching, fused silica lens element in contact with the fibers. The index-matching lens element performs the multiple duties of keeping back reflections low, bending the light to be telecentric out of and into the optical fibers from the rest of the fisheye lens, and flattening the image plane. The index-matching element and optical fiber array can also be adjusted as a unit for fine focusing. 
     An index-matching gel may be utilized between the index matched lens  340  and the optic signal conductors  350  to further improve index-matching. Other substances may also be utilized at the junction such as index-matching epoxy. 
     It is contemplated that the lens and ferrule are sized to fit snuggly within the interior of the housing to maintain optical and special alignment between elements. The interior of the housing may also be lined or coated with a material to minimize reflection or light scattering. 
       FIG. 3B  illustrates a perspective view of the probe assembly shown in  FIG. 3A . As compared to  FIG. 3A , common elements are identified with identical reference numbers. As shown, probe assembly  302  includes the housing  344  having a distal end  312 . At the distal end  312  is the exterior lens  304  configured as a fisheye lens or any other lens shape capable of carrying two or more optic signals out of the lens and receiving two or more reflections. Supporting the exterior lens  304  is the lens mount  300 . 
       FIG. 4  illustrates a detailed view of a ferrule as shown in element  332  in  FIG. 3A . This is but on possible configuration provided for purposes of discussion. As shown, the ferrule  332  includes a top surface  404  on one or more cylindrical passages  408  between the top surface  404  and the opposing bottom surface (not shown). Any number and arrangement of passages  408  may be established in the ferrule  332 . It is also contemplated that the passages  408  may be of various different sizes and shapes as shown to accommodate various different arrangements of optic signal conductors  350  which reside within the passages  408 . 
     In one embodiment, the optic fibers are spatially positioned by being mounted into passages or holes in the optical fiber ferrule  332 . The optic fibers are bonded into the ferrule and then the ferrule and optic fibers are polished together. To compensate for imperfections in contact across the fiber ferrule, index-matching gel is added for better coupling at the fiber-to-lens surface. Commercial optical fiber ferrules for MT connectors are made of glass-filled polyphenylene sulfide (PPS) based thermoplastic. Other ferrule material may include PPS, Macor, Vespel, Torlon and Photoveel II. Photoveel II performs well for the micro-hole drilling used to spatially position our single-mode optical fibers. It enables clean, burr-free holes at 125-micron diameter. It also polishes well with the optical fibers. Flat polishing is performed to ensure good coupling with the index-matching element. Photoveel II is a fine-grain, machinable nitride ceramic that is used in the probe card industry. The arrangement of the passages  408  within the ferrule  332  is discussed below in greater detail in connection with  FIG. 6 . 
       FIG. 5  illustrates an enlarged view of the junction between the index-matching lens and the ferrule. As shown, the index-matching lens  340  is adjacent the ferrule  332 , both of which are contained within the housing  344 . The ferrule  332  has one or more passages  408  through which an optic signal conductor  350 , such as a fiber optic cable, passes. The index-matching lens  340  has a top surface that is opposite a bottom surface which defines the junction or interface  516  between the index-matching lens  340  and the ferrule  332 . At this junction, the index-matching gel or index-matching epoxy may be utilized to reduce refraction, back reflection, and any other index of refraction mismatches. The ends of the optic signal conductor  350  may be flat polished. 
       FIG. 6  illustrates an example passage layout within the ferrule. This lay out also controls the position that the optic signal impacts and exits the fisheye lens. Due to the optic behavior of a fisheye lens, where the optic signal enters the fisheye lens controls where the optics signal exists from the fisheye lens, and consequently where the optic signal will strike the interior surface under measurement. This is but one possible layout arrangement for passages this layout may be modified based on the optic system, the surface to be monitored as part of the PDV process and the desired location of the reflection points on the surface which is being monitored. Referring back to  FIG. 2 , the reflection points  150  ( FIG. 2 ) are controlled by the location of the passages  408  in the ferrule  332  (and the optic system, which control projection of the optic signals on to the inner surface  112  of  FIG. 2 ). 
     In this example embodiment, the passages are defined by two generally straight lines  604  of individual passages  408 . In the center of the ferrule is an offset linear opening  608  in which numerous adjacent optic signal conductors may be placed as shown within passages  408 . Extending outward from the offset linear opening  608  are groups of individual passages  612  and two large openings  616  into which multiple optic signal conductors are placed. The optic signal conductors may be grouped or packed into the larger openings  616 . 
       FIG. 7  illustrates one exemplary lens arrangement with resulting ray traces through the lenses. This is but one possible arrangement of lenses and it is contemplated that one of ordinary skill in the art or those familiar with optics may arrive at different lens arrangements. In this configuration, working from the right hand side of the figure, the index-matching lens  340  receives optic signals from one or more optic signal conductors or generators, such as optic fibers, lasers, optic channels, lenses, or any other optic signal source. 
     A curved image plane is very inconvenient for mounting an array of optical fibers. Just like the eyepiece lens and reflective lens designs, the panoramic type lens also has the problem of wrapping around near 90 degrees from the lens center axis as angular coverage is increased. This design uses more elements to image onto a flat plane. 
     However, off-axis field points for those designs come into the image plane at a significant angle, as shown in  FIG. 4 . To get maximum light signal coupled both out of and back into the optical fibers, the optical fibers would have to be mounted at different angles across the image plane. That would make fabrication and assembly more time consuming. Therefore, preference for this design was to make the image light telecentric so that off-axis field points (or optical fibers) are normal to the image surface, such as the index matched lens  430   
     In this embodiment, the index-matching lens  340  has a flat surface  704  which receives the optic signal from the light source(s). By making the surface  704  flat, maximum signal transmission into the lens  340  may occur. This configuration also provides the benefit of reducing the complexity of the interface between optic fibers (not shown) and the flat surface  704  of the lens  340 . As a result, angled end polishing may be eliminated. Index-matching gel (not shown) reduces any index mismatch between optic fibers and the lens  340 . 
     Opposing the index-matching lens is a first doublet lens group  324 . A focusing gap  708  is provided between lens  324  and lens  340 . This focusing gap  708  may increase or decrease in length to obtain optimal focus of the optic signal through the lenses onto the inner surface shown in  FIGS. 1 and 2 . 
     Opposing the first doublet lens group  324  is a second doublet lens group  316 , and then a single lens element  308 , followed by the external lens element  304 . In this embodiment, the external lens element  304  is a fisheye lens. Use of a fisheye lens provides the benefit of a single external lens element and an ability to project optic signals onto all locations of the internal surface of the object subject to movement measurement. For example, light signal input at point  720 A passes through the shown lens elements to generate light output location  720 B. The optic signal projected by the lens  304  from point  720 B is projected onto the inner surface. Concurrently, with the single external lens element and using the same lens arrangement, an optic signal reflection from the inner surface is received by the fisheye lens  304  at point  720 B and optically directed back into the same point  720 A in the index-matching lens  340 . This reflection is conveyed into the same optic signal conductor (fiber optic cable) which presented the optic signal to the lens for processing. 
     Likewise, an optic signal presented by a fiber optic cable to position  724 A, which is at or near the edge of the index-matching lens  340 , is directed through the lenses  340 ,  324 ,  316 ,  308 ,  304  to point  724 B. From point  724 B, the optic signal is projected onto the inner surface as shown in  FIGS. 1 and 2 . As can be appreciated, although the optic signals are input into the flat surface  704  at points  720 A and  724 B, which are only a small distance away and in the same flat plane, the projection of these optic signals occurs at vastly different angles from the lens  304 . By adjusting the location that the input signal is provided on the surface  704  (see  FIG. 6 ), the optic signal may thus be projected to any location on the inner surface of the material that is having its motion tracked due to the behavior of the fisheye lens. 
       FIG. 8  illustrates an example arrangement of an alternative lens arrangement with resulting ray traces. The functionality of the embodiment of  FIG. 8  is generally similar to the functionality of  FIG. 7 . As shown, an index-matching lens  804  receives the optic signals. Opposing the index-matching element are one or more lens  808 ,  812 ,  816  followed by an external lens  820 , such as a fisheye lens. 
     As compared to the embodiment of  FIG. 7 , the angle  824  which the optic signals are or can be projected from the external lens  820  is greater than the embodiment of  FIG. 7 . This provides the benefit of greater coverage and capability to project the optic signals beyond 180 degree range and likewise, receive reflections from a greater area of the inner surface. 
       FIG. 9  illustrates a plot of distortion improvement realized in the embodiment of  FIG. 8  over the embodiment of  FIG. 7 . This figure shows plots of the optic fiber position versus angular position in a hemisphere. In this plot, the vertical axis  904  represents fiber R value in millimeters while the horizontal axis  908  represents angle, in degrees, of distortion. The distortion is the difference between the linear fiber position and the model fiber position. The plot shows how further optimization has decreased the distortion in the improved design. 
     As shown, plot  920  represents current while plot  924  represents linear R for the lens arrangement of  FIG. 7 . The plots  920 ,  924  represent performance of the lens arrangement of  FIG. 7 . Plot  930  represents improved current. Plot  934  represents linear R for the improved lens arrangement of  FIG. 8 . The plots  930 ,  934  represent performance of the lens arrangement of  FIG. 8 . As can be appreciated, significant improvements are realized by the improved lens arrangement. 
     Example Embodiment 
     A series of live dynamic PDV tests within hemispherical shells were fielded using a discrete collimator multipoint ball probe, a multiple lens array probe, and a fisheye probe design (described in the next section). All three gave high-quality data during testing. The fisheye lens&#39; performance stood out over prior art probes in several ways. 
     First, the fisheye element does not encroach into the center of the imploding hemisphere. This is important because experimentalists ideally want the measurement to record data until the shock wave impacts the probe. Therefore, the smaller the probe the better it will record late-time information. Second, the physical size of the waist near the center of the cavity is smaller. This helps to fit the probe through a small opening for blast mitigation. Third, angular coverage can be more complete. Fourth, the fisheye probe is easier to assemble than prior art multipoint probes. 
     During these tests to minimize costs, the PDV recording system was implemented using components that were obtained from the telecommunications industry. Components use light in the 1550 nm band, including CW fiber lasers, amplifiers, detectors, optical isolators, splitters, combiners, and optical fiber attenuators were selected for use. Therefore, the fisheye lens for PDV was designed for operation at the 1550 nm wavelength. Glass, such as lenses, for high transmission in this wavelength region were chosen. Some effort to reduce chromatic aberration and extend the band to visible red light was also made, but was not emphasized. 
     Optimization 
     During optimization the lens was modeled by tracing from a spherical surface through the lens to the fiber plane with all field points weighted equally. This method optimized imaging at 1550 nm from the large to the small conjugate side. Analysis was later performed with the lens model flipped to trace from the fibers to the spherical surface. An example fisheye lens was used as a starting point. Due to inherent fisheye lens barrel distortion, magnification is not constant across the field. Therefore, the numerical aperture of the outer field points was larger than that of the center field points. To address this, vignetting factors were employed so that the outer field points would not make a light cone excessively large for a standard single-mode optical fiber. As optimization progressed, and during testing, adjustments were made to the vignetting factors of these outer field points. 
     During optimization merit function weighting was high for the 1550 nm wavelength. Weight was also entered to make each field point normal to the image surface. A small weight was set to minimize the incident angle of light on the first surface. Glasses were initially allowed to vary, but were later fit to the model. A short lens effective focal length helped for small beam diameter channels and reduced physical size. A trade-off in the overall magnification or size of the optical fiber plane versus point-to-point resolution on the surface is made. 
     During assembly, the fiber ferrule and index-matching element with index-matching gel are bonded together. Index-matching epoxy did not yield as good of results as index-matching gel. After the other lenses were glued into the housing, the index-matched lens/fiber-array unit is moved to the proper distance for fine focusing of the fiber beams using an infrared camera. 
     Fisheye Probe Improvements 
     Inherently, fisheye lens images are distorted. The distortion manifests itself as pincushion distortion when going from the optic fiber plane to the curved surface and as barrel distortion when going from the curved surface to the optical fiber plane. The points at higher angles in  FIGS. 7 and 8  are spread out more than those at lower angles. This change in magnification versus field angle causes elliptical optical fiber spots on the concave inner surface of  FIG. 2  at high angles. It also manifests itself as a loss of efficiency for PDV signals at the higher angles. High incidence angles on lens elements at high angles may also have less effective anti-reflection coatings. 
     It is contemplated that further work was done to improve the fisheye lens probe&#39;s design performance. For example, global optimizations with merit functions that emphasize telecentric light at the optical fiber plane, good imaging, and minimizing magnification differences across the image field were performed to increase efficiency at the higher angles. Again, a small weight to minimize the outer surface incident angle was set. 
     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. In addition, the various features, elements, and embodiments described herein may be claimed or combined in any combination or arrangement.