RELATED APPLICATIONS

This application claims the benefit of the priority date of 62/275,423 filed on January 6, 2016, and 62/275,869 filed on January 7, 2016, the contents of which are herein incorporated by reference. FIELD OF INVENTION

This invention relates to imaging, and in particular, to holographic imaging. BACKGROUND

Holographic imaging involves illuminating a sample with a sample beam and mixing the sample beam with a reference beam. Any changes in index of refracting along the path of the sample beam will change its phase velocity. Thus, interference between the sample beam and the reference beam provides information on structures along the sample beam's path. This means that these interference patterns can provide information about structures underneath an exposed surface of a sample. However, to obtain a good image, one must minimize noise. SUMMARY

The invention is based on the recognition that the

interference pattern, which is spatially oscillating, is

superimposed on a non-oscillating, "DC" component, and that removing this DC component prior to detection significantly reduces shot noise and improves signal to noise ratio (SNR) . In one aspect, the invention features an apparatus for holographic imaging. Such an apparatus includes first and second image planes, and a low-pass filtering stage therebetween. The low-pass filtering stage has a mask that is disposed to remove a central portion of a Fourier transform (FT) pattern on the first Fourier plane and to cause a filtered interference pattern to be incident on the second image plane. Some embodiments also include a high-pass filtering stage disposed between the second Fourier plane and a third image plane. The high-pass filtering stage includes a mask having a central aperture . In one aspect, the invention features a holographic- imaging apparatus having first and second image planes with a low-pass filtering stage between them. The low-pass filtering stage includes a first mask disposed to remove a central portion of an interference pattern on the first image plane and to cause a filtered interference pattern to be incident on the second image plane. Some embodiments also include a high-pass filtering stage disposed between the second image plane and a third image plane, the high-pass filtering stage comprising a mask having a central aperture. Among the embodiments are those in which the low-pass filtering stage further includes first and second relay lenses, with the first mask being disposed between the relay lenses. Among these are embodiments in which the first relay lens is one focal length from the first image plane and forms a Fourier transform of the image plane at a first Fourier transform plane that is located one focal length away from the first relay lens, and in which the first mask is located at or proximal to the Fourier transform plane. In these embodiments, the second relay lens is located one focal length away from the Fourier transform plane, and forms an image on the second image plane. Yet other embodiments include a high-pass filtering stage that follows the second image plane. Among these are those in which the high-pass filtering stage includes a second mask. In some of these embodiments, the second mask includes a central aperture whose diameter is larger than that of the first mark. Also among these embodiments are those in which the second mask is configured to filter out diffraction-related artifacts introduced by the first mask, as well as those in which the first and second masks are configured to pass an annulus having inner and outer radii that are a function of the diameter of the first mask and the diameter of an aperture in the second mask. Additional embodiments also include an interferometer disposed to illuminate a sample with a sample beam, to mix the sample beam with an image beam to form a mixed beam, and to direct the mixed beam towards a first image plane. In another aspect, the invention features a method

comprising obtaining a holographic image by low-pass filtering a mixture of an image beam and a reference beam. Low-pass

filtering includes providing a first mask between first and second image planes, directing the mixture of beams towards this first mask, and, using this same first mask, removing a central portion of an interference pattern on the first image plane and causing a filtered interference pattern to be incident on the second image plane. Among the foregoing practices are those that also include high-pass filtering a beam between the second image plane and a third image plane. In these practices, high-pass filtering the beam includes passing the beam through a mask having a central aperture. Yet other practices include those in which low-pass filtering includes placing the first mask between first and second relay lenses. Among these are practices that also include placing the first relay lens one focal length from the first image plane, causing the first relay lens to form a Fourier transform of the image plane at a first Fourier transform plane that is located one focal length away from the first relay lens, locating the first mask is located at or proximal to the Fourier transform plane, locating the second relay lens one focal length away from the Fourier transform plane, and causing the second relay lens to form an image on the second image plane. Additional practices of the invention are those that also include high-pass filtering a beam following the second image plane. Among these are practices in which high-pass filtering a beam following the second image plane includes passing the beam through a second mask, those in which high-pass filtering a beam following the second image plane includes passing the beam through a central aperture that has a diameter larger than a diameter of the first mask, and those that include passing an annulus of light, with this annulus having inner radius and outer radii that are a function of the diameter of the first mask and the diameter of an aperture in a second mask. In another aspect, the invention features an apparatus for use in surgery. Such an apparatus includes an electro-cauterizer having a portable handle and an electro-cauterizing attachment extending from the handle. The attachment includes a pair of support conductors, a filament that extends between distal ends of the support conductors, and a pipe. The filament has a higher impedance than the support conductors. The pipe has an opening disposed adjacent to the filament for removing gas and smoke generated during cauterization. The handle has a rechargeable power supply connected to the support conductors . These and other features of the invention will be apparent from the following detailed description and the accompanying figures, in which: DESCRIPTION OF THE FIGURES FIG. 1 shows an electro-cauterization apparatus; FIG. 2 shows an attachment for the electro-cauterization apparatus of FIG. 1 ; FIGS. 3-5 shows alternative embodiments of an attachment for the electro-cauterization apparatus of FIG. 1 ; FIG. 6 shows structures in the handle of the electro- cauterization apparatus shown in FIG. 1; and FIG. 7 shows a recharging apparatus for the electro- cauterization apparatus shown in FIG. 1. FIG. 8 shows an embodiment of an apparatus that removes a DC offset from an interference pattern; and FIG. 9 shown an alternative embodiment that removes

diffraction-related artifacts introduced by the mask from the apparatus of FIG. 8. DETAILED DESCRIPTION FIG. 1 shows an electro-cauterization apparatus 10 having a handle 12 coupled to an attachment 14 for cutting and

cauterizing tissue. The handle 12 is designed to be easy to grip with one hand and easy to advance, retract, and rotate. A sanitary cover 16 covers the handle 12. In some embodiments, the sanitary cover 16 is disposable and in others it is reusable. In either case, the sanitary cover 16 functions as a fluid barrier and a dust barrier. Preferable, it includes an opening so that the attachment 14 can engage various

electronics inside the handle 12. FIG. 2 shows an embodiment of the attachment 14 in which first and second support conductors 17, 18 having parallel proximal sections extend longitudinally in a distal direction from the handle 12. At a distal end thereof, each support conductor 17, 18 bends away from the longitudinal axis to form a flared section. A cutting filament 20 extends from between distal tips of the support conductors 17, 18. The distal tips hold the cutting filament 20 under tension. The support conductors 17, 18 are low impedance conductors, such that they dissipate minimal power and deliver most energy efficiently to the filament 20. The support conductors 17, 18 will have minimal impedance when a direct current is applied. The support conductors 17, 18 will have minimal impedance when a pulsed current between 0.1 Hz and 10 Hz is applied. Suitable materials for the support conductors 17, 18 include copper, silver, brass, and aluminum. An insulating support 22 maintains physical separation of the two support conductors 17, 18 along their length. In the illustrated embodiment, the insulating support 22 is coaxial with a longitudinal axis of the attachment 14. The filament 20 is a high impedance conductor that accounts for most of the voltage drop and energy dissipation in a circuit that includes the support conductors 17, 18 and the filament 20. It is made of a material that has high temperature resistance and that also functions as a good thermal conductor, and is capable of radiating with high energy density. Suitable

materials include Nichrome, which is an alloy of nickel and chromium, KANTHAL, which is an alloy of iron, chromium, and aluminum, tungsten, Nitinol, which is an alloy of nickel and titanium, Inconel, Constantan, which is an alloy of copper and nickel, MANGANIN, which is an alloy of copper, manganese, and nickel, WASPALOY, which is an alloy of nickel, chromium, cobalt, molybdenum, titanium, and aluminum, and Monel, which is an alloy of nickel and copper, stainless steel, and SUPERTHERM, which is an alloy of nickel, chromium, iron, cobalt, and tungsten. The energy density at the surface of the filament 20 can be greater than lxlO ³ Watts/m ² [W/m ² ] ; greater than lOxlO ³ W/m ² ;

greater than lOOxlO ³ W/m ² ; greater than lxlO ⁶ W/m ² ; or greater than 10x10 ^s W/m ² . An energy density between lxl0 ^e and 10x10 ^s W/m ² on the filament effectively cuts and coagulates mammalian tissues at room temperature. Cutting is achieved when the radiating filament 20 contacts tissues causing them to vaporize. Filaments 20 of reduced radius are advantageous as they increase energy density and provide sharp cuts. In some

embodiments, the filament 20 has a cross section diameter between 5xl0 ^~6 meters and 5xl0 ^~3 meters. The energy density of the support conductors 17, 18 can be less than 100 W/m ² ; less than 10 W/m ² ; or less than 1 W/m ² . When the filament 20 is energized but not in contact with a sample to help dissipate energy or conduct heat away, it may heat to beyond its melting temperature. This may be the case when the filament 20 is initially being advanced into a tissue sample to be cut. To avoid this difficulty, some embodiments pulse current flowing through the filament 20. During the intervals when no current flows, the filament has an opportunity to cool down. In this pulsed mode the filament still provides cutting and

coagulating capability during the times when current flows through it. Pulsing frequencies can range between 0.1 Hz to 10 Hz . As one cuts or cauterizes, considerable gas and smoke can be generated. This tends to fill up a confined space and obscure vision. As a result, the attachment 14 also includes a first tube 24 having a distal opening that is close to the filament 20. An in-line filter 26 along the first tube 24 between the filament 20 and the handle 12 captures particulate matter as gas is drawn through the first tube 24. The first tube 24 and the support conductors 17, 18 each engage the handle 12 at corresponding first and second handle interfaces 28, 30. FIG. 3 shows an alternative embodiment in which the first support conductor 17 remains straight and the second support conductor 18 has a right-angle bend at a distal region thereof. As a result, instead of being oriented perpendicular to the longitudinal axis, as was the case for the embodiment shown in FIG. 2, the filament 20 makes a 45 degree angle relative to the longitudinal axis. Additionally, the first tube 24 is no longer between the support conductors 17, 18 but is instead off to the side. However, the distal opening of the first tube 24 remains very close to the filament 20. FIG. 4 shows an embodiment similar to that shown in FIG. 3 but with an insulating material filling a space between the support conductors 17 , 18. FIG. 5 is an embodiment similar to that in FIG. 4 but with the first tube 24 moved between the support conductors 17, 18. FIG. 6 shows the structures inside the handle 12. To carry out the tube's evacuation function, the handle 12 includes a tube connector 32 at a first end of the handle 12 to couple the first tube 24 to a second tube 34. The second tube 34 extends from the first end of the handle 12 to a pump 36

oriented to move air away from the filament 20. The pump 36 can be a fan or a turbine. A third tube 38 then extends from the pump 36 to a vent 40 at a second end of the handle 12 opposite the first end. These components operate to direct gas and smoke away from the visual field. The handle 12 also accommodates a power supply 41 that includes an energy storage element 42. Particular examples of the energy storage element 42 include a battery and a capacitor. A suitable type of battery is a lithium ion battery. Suitable capacitors include double-layer capacitors. In some cases, the energy storage element 42 is a combination of batteries and capacitors. Preferably, the energy storage element 42 is one that maintains a low voltage and is capable of delivering a very high current for extended times. Embodiments of the energy storage element 42 include those that deliver current on the order of one amp, two amps, three amps, five amps, or ten amps. Embodiments of the energy storage element 42 include those that deliver such current for one minute, two minutes, five minutes, and ten minutes . Also among the embodiments are energy storage elements 42 that maintain a voltage of three volts, four volts, or five volts. The power supply 41 also includes recharge electronics 44 and discharge electronics 46. The recharge electronics 44

connects to an independent charger through the connector 60, manages the recharge rate, and protects the energy storage element 42 from outside influences. The discharge electronics 46 prevents energy storage over-discharge, and limits current. In some cases, the discharge electronics 46 also interrupts current flow to the support conductors 17, 18. Between the discharge electronics 46 and the attachment support conductors 17, 18 are a pulsed current flow enabler 48 and a DC current flow enabler 50. The DC current flow enabler 50 is capable of being actuated from outside the handle 12. It is open circuited when not actually turned on. The pulsed current flow enabler 48 is either in parallel or in series with the DC current flow enabler 50. It too is capable of being actuated from outside the handle 12. Examples of a pulsed current flow enabler 48 include a self -oscillating circuit, a flyback converter, a forward converter, a bimetallic switch, a joule thief, and a mechanical oscillating switch.

The power supply 41 ultimately connects to the support conductors 17, 18 via a low- impedance connector 52 having two or more electrical contacts. FIG. 7 shows a charger 54 having a recharging current generator 56 that generates for recharging the energy storage element 42 through a recharging connector 58. In some

embodiments, the recharging connector 58 is a USB connector. Variants of the a recharging current generator 56 include a solar panel or a dynamo, both with appropriate power

conditioning electronics, a connection to an AC wall outlet with an AC to DC converter, and a connection to a battery with a suitable DC-DC converter. FIG. 8 shows an apparatus 110 in which an interferometer 112 illuminates a sample 114 with a sample beam S. The

interferometer 112 mixes the sample beam S with a reference beam R and directs the resulting mixed beam towards a first image plane 116. FIG. 8 also shows a first coordinate system

associated with the sample 114 and a second coordinate system associated with the first image plane 116. At the image plane 116, the reference beam R and the sample beam S can be mixed such that one dimension that is

substantially perpendicular to the beam propagation direction in the space defined by the first coordinate system, e.g. the y or x direction, is imaged directly to a corresponding dimension substantially perpendicular to the beam propagation direction in the space defined by the second coordinate system. In such a case, either the relative phase of the reference beam R with respect to sample beam S varies predictably along a known direction on the first image plane 116, for example along the x' direction, or the relative phase of two copies of the combined reference and sample beams R, S varies predictably along a known direction on the first image plane 116, for example along the x' direction. Interference between the reference beam R and the sample beam S manifests itself as an interference pattern formed on the first image plane 116. The interference pattern includes

oscillating interference fringes disposed symmetrically around a brightly- illuminated region 118 superimposed on a bright, non- oscillating background. The brightly illuminated region 118 corresponds to zero relative phase between the mixed beams. The "DC" offset

background corresponds to where all of the energy from the sum of the returns from the reference beam R and the sample beam S would have struck the first image plane 116 in the absence of any interference. This background energy is thus of little interest since it contains essentially no information about interference. If this energy is not suppressed before detection and conversion to electrons, it will generate quantum noise, also known as "shot noise," that may prevent detection of small interference signals. Additionally, failure to suppress this background energy before detection may result in requiring very high dynamic range from the detector and processing electronics to be able to detect such small interference signals are against a large amount of background noise. Such dynamic range

requirements are undesirable because they are impractical to implement and sometimes impossible to achieve. To suppress the effect of energy incident from this offset before detection, the apparatus includes a low-pass filtering stage 120 between the first image plane 116 and a second image plane 122. The low-pass filtering stage 120 features a first mask 124 between first and second relay lenses 126, 128, or Fourier lenses. The first relay lens 126 is located one focal length away from the first image plane 116. The first relay lens 126 forms a Fourier transform of the image plane 116 at a first Fourier transform plane 123. The Fourier transform plane 123 is located one focal length away from the first relay lens 126. The first mask 124 is located at or proximal to the Fourier

transform plane 123. The second relay lens 128 is located one focal length away from the Fourier transform plane 123. The second relay lens 128 forms an image on the second plane 122. Without the first mask 124, the image that forms on the second image plane 122 would be a magnified, de-magnified or relayed image of the interference pattern that is present on the first image plane 116. However, as a result of the first mask 124, energy from the "DC" offset background has been blocked. The first mask 124 has thus carried out the function of a low- pass filter that removed the DC component. The image on the second plane 122 is then provided to a detector 130 for further processing in the conventional manner. The first mask 124 can be implemented such that only frequencies corresponding to the interference pattern are passed and imaged by the second relay lens 128 at the second plane 122. The first mask 124 can also be implemented such that an optimal amount of "DC" offset background is passed and imaged by the second relay lens 128 at the second image plane 122. An optimal amount of offset may be an offset that is minimal or does not result in frequencies other than those already in the

interference pattern of interest being observed by detector 130. A difficulty that arises in the embodiment shown in FIG. 8 is that sharp edges of the first mask 124 create a diffraction pattern. This diffraction pattern introduces artifacts into the image formed on the second image plane 122. To suppress these diffraction patterns, an alternative apparatus 132 shown in FIG. 9 adds a high-pass filtering stage 134 following the second image plane 122 . The high-pass filtering stage 134 features a second mask 136 that lies between third and fourth relay, or Fourier, lenses 138 , 140 . The third Fourier lens 138 is located one focal length away from the second image plane 122 . The third Fourier lens 138 forms a Fourier transform of the image plane 122 at the second Fourier transform plane 135 . The second Fourier transform plane 135 is located one focal length away from the third Fourier lens 138 . The second mask 136 is located between the second Fourier transform plane 135 and the fourth Fourier transform lens 140 . The fourth Fourier transform lens 140 is located one focal length away from the second Fourier transform plane 135 . The fourth Fourier transform lens 140 forms an image on the third image plane 144 . The second mask 136 complements the first mask 124 . In particular, the second mask 136 features a central aperture 142 that is larger than the diameter of the mask 124. As such, the combination of the first and second mask 124 , 136 passes an annulus whose inner and outer radii are a function of the diameter of the first mask 124 and the aperture 142 in the second mask 136 . This has the effect of filtering out most of the diffraction-related artifacts introduced by the first mask 124 . Light passing through the aperture 142 is then directed by the fourth relay lens 140 toward a third image plane 144 , and ultimately, to the detector 130 . Having described the invention, and a preferred embodiment thereof, what is claimed as new, and secured by letters patent is :