Patent Application
20050111515
This invention relates generally to self-induced waveguides in the atmosphere.
High power laser pulses traversing the atmosphere can self-focus due to the nonlinear index of refraction of air. At some critical power, self-focusing overcomes diffraction and the beam collapses until it is balanced by some higher order effect, usually, but not exclusively, attributed to plasma de-focusing. This balance can lead to the formation of a filament, a self-induced waveguide in air. Filaments have been seen in the infrared (IR) as well as in the ultraviolet (UV) regime. In both cases, direct measurements have faced a fundamental problem, namely that the high intensity (approx. 1 TW/cm2 in the UV and up to 100 TW/cm2 in the IR) damages optical components. So far only indirect measurements have been performed, e.g. measuring the damage spot of a UV filament on a piece of film or looking at the light reflected by a glass slide when it is hit by a filament with grazing incidence.
Numerous experiments have shown self-guiding of high peak power femtosecond pulses through the atmosphere. Many experiments were carried out in the near infrared while at least one experiment involved fs pulses at 248 nm. After reaching the focus, the light appeared to trap itself in self-induced waveguides or “filaments” of the order of 100 μm diameter. Since the first report of 1995, several experimental studies on UV filaments have been reported. The energy contained in a single filament is only of the order of a mJ. However, a theoretical study indicates that more energetic filaments (up to 1 J) could be obtained with longer pulses (up to 1 ns) than the sub-picosecond pulses that have been used so far.
Previous studies in this general area have included the following references:
Embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in these embodiments and their equivalents.
In
In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.
In an embodiment, a system includes a laser source and an aerodynamic window to generate a filament, a self-induced waveguide in air. The filament is the result of a balance between the collapse of a beam of high power laser pulses traversing an atmosphere and a higher order effect within the traversed atmosphere. An aerodynamic window, also referred to as an aerowindow, uses a fluid flow to separate different atmospheric regions without a solid window. The use of an aerodynamic window provides a opening for a laser beam to propagate between two different atmospheric regions without the use of a solid window. In an embodiment, an aerodynamic window uses a supersonic fluid flow such that a pressure gradient across the supersonic fluid flow is adapted for atmospheric pressure on one side of the fluid flow and a vacuum on the other side of the fluid flow. In an embodiment, the supersonic fluid flow is a supersonic air or nitrogen stream. In an embodiment, a region on one side of the supersonic fluid flow has a pressure less than about 50 Torr.
In an embodiment, a system includes an aerodynamic window and a detector to take measurements to characteristic a filament. As a filament passes through the aerodynamic window, its properties are not affected. However, a filament will not subside in a vacuum. In the vacuum, an originally 100 μm diameter beam will become larger by diffraction, and the peak local intensities decrease. After a sufficiently long propagation distance in the vacuum, the filament diameter is sufficiently increased that conventional attenuators (for instance, dielectric coatings) can be used to bring the intensity to a level acceptable without damage to a detector. The detector includes various optical measurement apparatus to characterize the spatial and temporal characteristics of the radiation that it receives such as beam size, energy, and spectrum. These characterizations can be mapped back to determine the characteristics of the filament from which the radiation is derived after propagation through vacuum.
Embodiments of the present invention include the production, use, and study of ultra-intense laser light pulses in the atmosphere, which are of a sufficient power as to create their own waveguides in air. This phenomenon related to the self creation of waveguides in air has been labeled successively “light bullets, “light strings, “filaments, “self-trapped-filament,” and “self-induced waveguide.” The manifestation is that a high power laser beam collapses into one or more of these channels of approximately 100 μm diameter, in which the light intensity reaches between 1012 W/cm2 (for filaments at uv wavelengths, i.e. wavelengths shorter than 300 nm) to 1014 W/cm2 (for filaments at visible and near IR wavelengths, most typically around 800 nm). It is believed that, under the proper circumstance, these filaments could propagate over distances of several km. The proper circumstances for propagation over such distances are extremely difficult to define, because the formation of filaments is one of the most difficult phenomenon to control. In a general production of filaments, any atmospheric perturbation (air current, thermal convection) would affect the position where a filament is produced, its direction, and whether or not one or more filaments are produced. Various embodiments of the present invention provide for the launching of single and multiple filaments in the atmosphere under controlled conditions due to the use of an aerodynamic window between a vacuum and air to launch the filament or filaments into the atmosphere. As can be understood by those skilled in the art, the aerodynamic window can be used in relation to two controlled environments and is not limited to a configuration between the atmosphere and a vacuum.
The most severe challenge to the controlled production of a single filament is the transition phase between the macroscopic large diameter beam and the self-guided channel. It is during that phase of propagation that the main beam loses most of its energy. It is also during that phase that any atmospheric perturbation may distort the wavefront, distortion that will be amplified by the nonlinear interaction, resulting in a modification of the position or pointing of the filament, or in the formation of multiple filaments. In an embodiment, this problem is addressed by focusing a well corrected plane wave in a vacuum down to about 100 μm onto a vacuum/air interface of an aerodynamic window.
Aerodynamic window 320 provides a unique position to circumvent the highly unstable and uncontrollable formation phase, and in addition enables a system to launch high power filaments in a controlled environment, such as in dry air, at sea level pressure or at high altitude pressure, oxygen or nitrogen. In an embodiment, aerodynamic window 320 can be configured to generate filaments that solve the problem of the large aperture optics needed to launch a high power beam from an airplane, such as required for the airborne iodine laser program. Because the filament does not diffract, instead of an optical port of 30 cm to 1 m in diameter, only a 100 μm diameter aperture of aerodynamic window 320 is required for each filament. In an embodiment, aerodynamic window 320 may be an integral part of an aircraft, from which high power laser beams may be sent. The aircraft may be a supersonic airplane.
A single filament carries a well defined amount of energy. The lethality of a filamented beam will increase with the number of filaments. A beam shaping system, combined with the aerodynamic window, will make it possible to create a predetermined wavefront as an initial condition for the filament leading to the production of a predetermined pattern of filaments.
In an embodiment, use of an aerodynamic window between the atmosphere and vacuum may be used to study the properties of the light inside the filament. One of the main technical difficulties associated with the study of filaments is that air is one of the medium with the lowest non-linearity. Any optical component put in the path of the filament will generally be destroyed. In the rare circumstances that the component is not destroyed, it will generally have a larger influence on the filamented field than air.
Air being one of the media with the highest damage threshold, it is not surprising that laser radiation sufficiently intense to cause high nonlinear response in air will cause severe damage and/or strong self-phase modulation in any solid optical material. It is therefore very difficult to study the properties of the filament, since there is no material that can be used to sample a portion of the field inside a filament.
The aerowindow 420 of
w=w0{square root}{square root over (1+(z/z0)2)}=19×w0=1.9 mm
where w0=100 μm is the beam waist and z0=πw02/λ=12.6 cm is the Rayleigh range. The intensity is reduced by a factor of 360 and can be further attenuated by reflecting the diffracted beam out of the chamber using a glass slide. The remaining intensity I is then (0.05/360=1.4×10−4×I0) 140 MW/cm2 in the UV and 14 GW/cm2 in the IR, sufficiently low for optical components. In an embodiment, an aerowindow is configured for a pressure on the vacuum side of <50 Torr with a 3 mm entrance hole.
Ultrashort light pulses (100 fs) of a few millijoule energy have sufficiently high peak power to self-focus in air. Even more interestingly, such self-focused pulses have been observed to create their own waveguide or filament in air, and propagate over tens of meters. The intense white light continuum generated with this process has been observed in backscattering over 13 km. In a reported experiment, a light filament was directed towards the sky, and the recorded spectra indicated that the white light created by the filament can be successfully used for absorption measurements and monitoring of the atmospheric components.
Studies have been limited to femtosecond pulses around 800 nm. At that wavelength, the filamentation process is complex, and cannot be scaled to long pulsewidth. Recent measurements, supported by theoretical simulations, indicate indeed that the stabilizing process in UV filaments is mainly a balance between self-focusing in air and the self-defocusing of the electron plasma created by 3-photon ionization of air. Since this is an intensity dependent process these filaments could be scaled to longer pulse duration and higher pulse energies, making them a means to transport pulses of the order of Joules, in a 100 micron diameter channel, over kilometers distances, with intensities of tens of GW/cm2.
In an experiment with UV beams, a 20 to 50 mJ UV beam is collimated with a diameter of about 1 cm. A manifestation of the filament is to put an obstacle in the beam and the size of the hole made by the filament is measured as shown in
Measurements of multiphoton ionization combined with conductivity measurements have indicated an electron concentration of 2×1015 e−/cm3. This is in agreement with theoretical simulations showing that the 3-photon ionization of oxygen in the filament produced an electron plasma which stabilizes the filament by defocusing, balancing the self-focusing action of air. This mechanism is purely intensity dependent.
Comparative measurements of the ionization with IR filaments have been made and showed the electron ionization in these filaments to be twenty times smaller, even though the energy in the IR filament is 50 times larger. The mechanism of stabilization of IR filaments is much more complex, and specifically involves the femtosecond duration of these pulses. Another difference between IR and UV filaments is that in the latter case there is no loss of energy from the filament into “conical emission”.
A series of experiments with pulse durations varying from 500 fs to 2.5 ps have been conducted. In all these cases, the peak intensity in the filament is the same (1.4 TW/cm2). The theory, developed by the group under Professor Moloney in Tucson, Ariz., corroborates the experimental findings, namely that longer pulses produce filaments with the same peak intensity, but higher energy is stored, and they should propagate over a longer distance.
It has been determined experimentally that the only loss mechanism is 3-photon ionization. The light trapped in the filament looses energy at a rate of 40 μ/m. This is quite significant for a filament of only 150 μJ created by a 500 fs pulse.
The pulse duration limit is reached when the multiphoton ionization process is overpowered by avalanche ionization. This takes place if the energy gained by one electron by inverse Bremstrahlung equals the ionization energy. The maximum pulse duration, for a given ionization energy, scales as 1/(I×λ2), where I is the intensity in the filament and λ is the wavelength. Clearly, the shorter the wavelength, the longer the pulse duration limit at a given filament intensity. As compared to the IR filament, the pulse duration limit imposed by this condition is about 2,000 times longer for UV pulses. The limit for the UV pulse is estimated on the order of a nanosecond, which implies that a pulse energy close to a Joule could be trapped in a filament, propagating for 1 J/(40 10−6 J/m)=25 km! A filament of that energy can obviously produce strong laser damage at far distances. One of the most exciting properties of these filaments is that they are much smaller than the characteristic size of atmospheric turbulence, hence they do not seem to be affected by such turbulence.
The analysis of filaments has been limited to the crude measurements as cited above, because any material put in the path of the filament suffers some damage or transformation. Spectroscopic measurements have been plagued by the fact that reflections off solid or liquid targets produce plasma with much brighter emission than the one associated with the filament itself. In an embodiment, this problem is addressed through the use of aerodynamic windows between air and vacuum. Once in vacuum the 100 μm diameter filament will diffract with an angle of at least 10−4 radian. After a propagation distance of 2 m in vacuum, the intensity is reduced 250 times, hence below the damage threshold of good optics. An embodiment of an arrangement of a system 1000 for studying and measuring parameters of a filament with an aerodynamic window is shown in
In the embodiment of
A system using an aerodynamic window makes it possible to make measurements of the spectrum, duration, shape, and phase modulation of the pulse inside the filament. From the diffracted pattern, the spatial field distribution inside the filament can be inferred, allowing for much more quantitative and accurate comparisons with theoretical calculations than have been possible previously.
An aerodynamic window in a system provides the means and method for controllably launching filaments into the atmosphere. Systems using filaments launched utilizing an aerodynamic window have a wide variety of applications that use high intensity energy in a small spot size from a source of electromagnetic radiation such as provided by a laser. In addition, an aerodynamic window in a system provides the means and method for characterizing filaments propagating through the atmosphere using conventional diagnostic tools for studying properties of electromagnetic energy propagating through a medium.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive. Combinations of the above embodiments, and other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention includes any other applications in which the above structures and fabrication methods are used.
This application claims priority under 35 U.S.C. 119(e) from U.S. Provisional Application Ser. No. 60/524,242 filed Nov. 21, 2003, which application is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60524242 | Nov 2003 | US |
