Optical waveguides – Optical fiber bundle – Imaging
Reexamination Certificate
2000-02-14
2002-07-02
Ngo, Hung N. (Department: 2874)
Optical waveguides
Optical fiber bundle
Imaging
C385S147000, C359S900000
Reexamination Certificate
active
06415086
ABSTRACT:
This invention relates to the use of optics to produce a directed beam of radiation by grazing-incidence reflection. More particularly, an optical element is disclosed which is produced by assembling a collection of separate tapered-monocapillary optical elements to form a polycapillary optic. The individual monocapillary channels are created in a batch process which allows for the optimization of the shape, smoothness, and material choice of the radiation-reflecting interior surfaces. The resultant optics can be used to produce either a collimated beam or a focused spot of photons, neutrons, or charged particles. In the case of photons, the use of x-rays is the most important application.
BACKGROUND OF THE INVENTION
In recent years, there have been dramatic developments in the field of x-ray optics. Many different types of optical elements have been introduced or improved for the manipulation of short wavelength photons. In the specific area of focusing optics, several different types of lenses and focusing mirrors have been produced. These optics. include: zone plates, Bragg-Fresnel zone plates, multilayer focusing mirrors, grazing incidence mirrors, compound refractive lenses, and capillary optics. In addition to photons, some of these optical elements can be used for the focusing of neutrons and charged particle beams.
The introduction of x-ray analysis has been one of the most significant developments in twentieth-century science and technology. The use of x-ray diffraction, spectroscopy, imaging, and other techniques has led to a profound increase in our knowledge in virtually all scientific fields. The capabilities of x-ray analysis have expanded consistently with the availability of ever more powerful sources of radiation. The standard x-ray tube has seen a relatively gradual increase in performance over many decades. Notable improvements in x-ray tube technology include the introduction of rotating anode sources and microfocus tubes. The advent of synchrotron radiation sources over the past few decades has led to a true revolution in x-ray science. Although the use of synchrotron radiation has become an extremely important research tool, the need to travel to large and extremely expensive central facilities to perform experiments during a limited time interval is a distinct disadvantage. Thus, the vast majority of work is still performed using x-ray tubes.
Many experiments are now performed using rotating anode sources which have significantly higher power capabilities than stationary anode tubes. These sources are quite expensive and can consume over ten kilowatts of input power. Recently, with the introduction of improved x-ray focusing optics, the ability to use small, low power microfocus x-ray sources to achieve x-ray beam intensities comparable to that achieved with rotating anode tubes has been demonstrated. It has been shown that a microfocus source running at a few tens of watts input-power, in conjunction with focusing optics, can produce beams with a brightness comparable to a multi-kilowatt rotating anode source. Such combined small sources and collection optics will greatly expand the capabilities of x-ray analysis equipment in small laboratories. The optimization of x-ray optics for these applications is of crucial importance for realizing the potential of these laboratory instruments.
In addition to sources of x-rays, there has been a significant increase in the capabilities of neutron sources, many of which exist as user facilities in a manner similar to synchrotron radiation sources. Both reactor and spallation sources have been built with ever increasing neutron fluxes. For certain applications such as prompt-gamma activation analysis and neutron depth profiling, it would be very desirable to produce a focused beam of thermal or cold neutrons. The use of small beams is also advantageous for neutron diffraction applications, although the increased divergence of the focused beam can be detrimental in some cases. There have been some advances in neutron focusing optics over the past few years. Improvements to these optics will have a large impact on the capabilities of these neutron facilities.
PRIOR ART
Although there exist x-ray optics which utilize diffraction and refraction for their operation, we are concerned with reflective optics in this invention. It is well known that x-rays incident on a surface at sufficiently small angles of incidence will be reflected by total external reflection. The largest angle of incidence for reflection (critical angle) is determined by the refractive index of the material:
n=
1−&dgr;−
i&bgr;
Using Snell's Law, we can derive this angle as:
&thgr;
c
=(2&dgr;)
½
(assuming &bgr;=0)
The theoretical value for &dgr; is:
&dgr;=½(
e
2
/mc
2
)(
N
0&rgr;/
A
)
Z&lgr;
2
=2.70×10
10
(
Z/A
)&rgr;&lgr;
2
The angles are quite small since the refractive index for x-rays is very close to unity for all materials. For example, the critical angle for borosilicate glass at&lgr;=1Å is less than 3 milliradians. For achieving the highest critical angles, high density materials such as gold or platinum are desirable.
In the case of neutrons, grazing incidence reflection can also be used for producing optical devices. The critical angle for reflection of neutrons is:
&thgr;
c
=&lgr;(
Nb/&pgr;)
½
Where b is the coherent scattering length and N is the number of nuclei per cm
3
. The best natural material for achieving the highest critical angle for neutron reflection is nickel. The isotope Ni-58 is especially good, having a critical angle of approximately 2.1 milliradians/Å.
In addition to single-layer reflecting materials, multilayer coatings can be produced which rely on Bragg reflection to achieve high reflectivity. These layers are composed of a high-Z material which exhibits large coherent scattering for the radiation being reflected, with an alternating low-Z material that functions as a spacer. In the case of x-rays, the most common high-Z materials are tungsten or molybdenum, while the low-Z spacer is usually silicon, carbon, or beryllium. In the case of neutron reflection, these multilayer coatings are often referred to as “supermirrors.” Neutron Supermirrors differ from standard x-ray multilayers in that the d-spacing of the layers is not constant, but increases for the layers towards the surface of the mirror. Supermirror structures are generally composed of layers of nickel or a nickel alloy, with spacer layers of titanium.
Reflective x-ray optics can be classified into several different categories. One class of optics uses grazing reflection from extended mirror surfaces. The most common mirror arrangements have an ellipsoidal or toroidal surface figure for two dimensional focusing of radiation. Another common geometry uses two spherical mirrors oriented sequentially in the vertical and horizontal planes, in an arrangement known as a Kirkpatrick-Baez configuration. An absolute requirement for all reflective x-ray optics is the need to have exceedingly smooth reflecting surfaces due to the small wavelength of the radiation. In general, the surface roughness should be better than 1 nanometer rms.
A different approach for reflective x-ray optics uses the ability of fine glass capillaries to act as reflective guide tubes for x-rays, in a similar manner to fiber optics. Several different configurations of these capillary optics exist. One type of optic, sometimes known as a “Khomakhov Lens”, uses a number of discreet curved glass-capillary tubes which are precisely mounted in a frame which independently holds each capillary's curved position along the device. In some optics, each carefully positioned capillary fiber is actually a bundle of many much smaller capillary tubes. X-rays are guided through each capillary by multiple reflections along the outer arc of the capillary tube's interior surface. This mode of reflection is sometimes referred to as a “Whispering Gallery”. With such optics, the divergent radiation from an x
Hynes William Michael
Ngo Hung N.
Townsend and Townsend / and Crew LLP
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