WO2015108507A1

WO2015108507A1 - Beam conditioner including an uncoated hollow core fiber

Info

Publication number: WO2015108507A1
Application number: PCT/US2014/011453
Authority: WO
Original assignee: Avolonte Health LLC
Priority date: 2014-01-14
Filing date: 2014-01-14
Publication date: 2015-07-23

Abstract

A beam conditioner can spatially filter light produced by a light source by removing unwanted spatial intensity variations and higher-order modes. The beam conditioner focuses light onto a first end of a single-mode fiber, and produces a cleaned-up output beam emerging from a second end of the single-mode fiber without introducing any new intensity variations. Propagation through the single-mode fiber extinguishes the spatial intensity variations and higher-order modes, so that the output beam includes only a zero-order mode that is relatively free from spatial intensity variations. The fiber can be a hollow core fiber, formed from fused silica, and devoid of metallic or dielectric optical coatings on its interior. For wavelengths between 8 and 12 microns, the optical performance of the uncoated hollow core fiber is relatively close to that of a more expensive silver-coated hollow core fiber.

Description

BEAM CONDITIONER INCLUDING AN UNCOATED HOLLOW

CORE FIBER The present invention relates generally to conditioning of an optical beam, and more specifically relates to spatially filtering an optical beam with an uncoated hollow core single-mode optical fiber.

BACKGROUND OF THE INVENTION

Spatially filtering is often used to "clean up" an optical beam. Spatial filtering can remove unwanted spatial intensity variations and higher-order modes, and can produce an output beam that includes only a zero-order mode that is relatively free from spatial intensity variations. One known spatial filtering technique focuses the beam through a pinhole in an opaque substrate. The pinhole is sized to roughly match the focused beam, so that light at locations away from the central portion of the focus are blocked. Typically, the opaque substrate is a metallic foil, which can be fragile and expensive to fabricate with sufficiently tight tolerances to be effective.

Another known spatial filtering technique uses a single-mode fiber, rather than a pinhole on a metallic foil. Fibers are relatively inexpensive and often have tighter tolerances than pinholes in metallic foil. The fiber technique focuses an optical beam onto one longitudinal end of a single-mode fiber, propagates the beam through a particular length of the single-mode fiber, and transmits a spatially filtered output beam from the second longitudinal end of the single-mode fiber. Coupling into the fiber extinguishes unwanted spatial intensity variations and high-order modes, while maintaining a desired single- order mode without significant attenuation. The emergent output beam includes only the single-order mode. In some cases, the emergent output beam is nearly or substantially a zero-order Gaussian distribution, so that an overlap integral of the emergent beam with a true zero-order Gaussian distribution exceeds 90%, 95%, 98%, 99%, 99.5%, or another suitable threshold. In some cases, using a single-mode fiber to perform spatial filtering does not introduce any additional intensity artifacts.

An optical fiber is typically formed as an elongated core surrounded by a cladding. As light propagates along the fiber, the majority of optical power is contained within the core, with relatively little power contained within the cladding. For many applications, the core and cladding are both formed from glass, where the glass in the cladding has a slightly lower refractive index than the glass in the core.

There are certain conditions in which all-glass fibers can have drawbacks. For instance, for coupling light into and out of an all-glass fiber, the interfaces between air and glass can have a significant reflection, typically on the order of 4% or 5% for each interface. Light can etalon between both ends of the fiber, as it would between two surfaces of a transparent flat window substrate, resulting in a loss of 9-12%. Anti-reflection coatings can reduce these reflections to about 0.5% per surface, or at best, about 0.25%, and angling the ends of the fiber can direct the remaining reflections away from the transmitted beam. Even with anti-reflection coatings and angled fiber ends, these reflections can still be problematic for high-power applications or where low loss of light is required for the application, e.g., where available light sources have limited power and the application requires a high signal to noise ratio. In addition, if the peak intensity in an optical beam is sufficiently high, the beam may damage the glass core of the fiber, which is undesirable. In some cases, suitable

transmissive materials may be excessively lossy, excessively expensive, may be excessively brittle or fragile, or may possess undesirable mechanical properties.

Hollow core single-mode fibers can remedy both of these potential drawbacks. Hollow core fibers use air as the core material, rather than glass or a glass-like material. As one advantage, the reflectivity can be relatively small when coupling a beam into or out of a hollow core optical fiber, because the effective (modal) refractive index of the guided mode can be very close to unity, and thus may produce a small or negligible reflection in an interface with air. As another advantage, a hollow core fiber can be resistant to damage when transmitting high-powered optical beams, because the region of peak intensity is disposed within air, and not within glass or a glass-like material that can be susceptible to damage.

Hollow core single mode fibers typically have an internal coating of silver or another suitable metal, optionally with a dielectric coating. The metallic coating has a high reflectivity, and reflects a high fraction of incident light back into the hollow core. As a specific example, Opto-Knowledge of Torrance, California, produces a hollow glass waveguide formed as a glass capillary tube, a reflective layer formed from Ag disposed within the interior of the glass capillary tube, a dielectric layer formed from Agl disposed within the interior of the reflective layer, and a roughly 300-micron-diameter hollow core disposed within the interior of the dielectric layer. The example hollow glass waveguide sold by Opto-Knowledge provides good transmission from wavelength between 2.5 and 18 microns, and single-mode performance with a Gaussian beam output for wavelengths between 5 and 18 microns.

SUMMARY OF THE DISCLOSURE

A beam conditioner can spatially filter light produced by a light source by removing unwanted spatial intensity variations and higher-order modes. The beam conditioner focuses light onto a first end of a single-mode fiber, and produces a cleaned-up output beam emerging from a second end of the single- mode fiber. Propagation through the single-mode fiber extinguishes the spatial intensity variations and higher-order modes, so that the output beam includes only a zero-order mode that is relatively free from spatial intensity variations. The fiber can be a hollow core fiber, formed from fused silica or an optical glass, and devoid of metallic or dielectric optical coatings, such as silver, on its interior. For wavelengths between 8 and 12 microns, or between 8.5 microns and 11 microns, the optical performance of the uncoated hollow core fiber is relatively close to that of a more expensive silver-coated hollow core fiber, such as the hollow glass waveguide sold by Opto-Knowledge. In the wavelength range of 8 to 12 microns, using an uncoated fiber in an optical system, such as a beam conditioner, can achieve the performance of a silver-coated fiber, but with a significant cost savings that arises from not depositing a silver coating on the interior of the fiber. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of an example of a beam conditioner. FIG. 1 A is a schematic drawing of another example of a beam conditioner.

FIG. 2 is a perspective drawing of an example of a longitudinal end of a hollow core fiber.

FIG. 3 is an end-on cross-sectional drawing of an example of an elongate body having a circular cross-section.

FIG. 4 is an end-on cross-sectional drawing of an example of an elongate body having an elongated cross-section.

FIG. 5 is an end-on cross-sectional drawing of an example of an elongate body having a square cross-section.

FIG. 6 is an end-on cross-sectional drawing of an example of an elongate body having a circular cross-section and a reduced core diameter.

FIG. 7 is an end-on cross-sectional drawing of an example of an elongate body having an irregular cross-section.

FIG. 8 is a flow chart of an example of a method for conditioning a beam.

DETAILED DESCRIPTION

FIG. 1 is a schematic drawing of an example of a beam conditioner 10. A light source 11, such as a laser, a light emitting diode, a blackbody source, or an FTIR source, emits a source beam 12. The light source 11 can optionally include one or more optical elements that can affect the collimation of the source beam 12, such as a collimating lens, a near-field lens, or a mirror. The source beam 12 can be collimated or diverging. The source beam 12 can have a wavelength between 8 and 12 microns, between 8.5 and 11 microns, or within other suitable ranges of wavelengths. A focusing optic 13, such as a lens or a mirror, focuses the source beam 12 to form an input beam 14. The input beam 14 converges to a focus 15. The light from the input beam 14 couples into a hollow core fiber 20 at a first longitudinal end 16, propagates along a longitudinal length of the hollow core fiber 17 to a second longitudinal end 18, couples out of the second longitudinal end 18 and emerges as the output beam 19.

The hollow core fiber 17 has a characteristic mode size, which corresponds to the size of a beam inside the hollow core fiber 17 as it transmits along the longitudinal length of the hollow core fiber 17. The focal length of the focusing optic 13 can be selected such that the size of the focus 15 matches or roughly matches the characteristic mode size. Matching the size of the focus 15 to the characteristic mode size of the hollow core fiber 17 improves the coupling efficiency at the first longitudinal end 16 of the hollow core fiber 17.

The source beam 12 can include undesirable spatial intensity variations, which can be caused by dust on an optical element, by wavefront aberrations, and/or by higher-order modes produced by the light source 11. For these reasons, the source beam 12 is said to be spatially noisy. The beam conditioner 10 spatially filters the source beam 12, removes the spatial noise, and produces an output beam 19 that has a zero-order beam profile that varies smoothly and is generally devoid of spatial intensity variations. In some examples, the output beam 19 has a zero-order rotationally symmetric Gaussian distribution. For these reasons, the output beam 19 is said to be spatially clean or spatially filtered. Such a spatially filtered beam can be useful as an input beam for an optical instrument, a measurement device, an imaging device, a device that uses laser delivery, a spectroscopy device, a device that performs chemical identification and detection, or other suitable systems.

FIG. 1 A is a schematic drawing of another example of a beam conditioner 10A. In this configuration, a mirror 104 is placed near the second longitudinal end 18 of the fiber 17 to reflect light emergent from the second longitudinal end 18 of the fiber 17. The mirror 104 may be curved, so that the second longitudinal end 18 of the fiber 17 is at its center of curvature.

Alternatively, the mirror 104 can be flat, with an additional lens between the second longitudinal end 18 of the fiber 17 and the mirror 104. Spatially filtered light returns through the fiber 17, emerges from the first longitudinal end 16 of the fiber, passes through the focusing optic 13, and is reflected by a beamsplitter 102 to form output beam 19 A. In the configuration of FIG. 1 A, the output beam 19A is collimated; passing the output beam 19 of FIG. 1 through an additional lens can also produce a collimated beam.

FIG. 2 is a perspective drawing of an example of a hollow core fiber 20. The hollow core fiber 20 includes an elongated body 21 having a longitudinal axis 23. In some examples, the elongated body 21 has a uniform cross-section at each longitudinal location along its length. In other examples, the elongated body 21 has cross-section that varies continuously, and/or varies in discrete sections.

The elongated body 21 extends from a first longitudinal end 25 to a second longitudinal end 27. In some examples, the first and second longitudinal ends 25, 27 are both perpendicular to the longitudinal axis 23. In other examples, one or both of the first and second longitudinal ends 25, 27 are angled with respect to the longitudinal axis 23.

The elongated body 21 has a lumen 29 extending therethrough. In some examples, the lumen 29 has a diameter between 0.2 mm and 0.5 mm, inclusive. Such a range of diameters can allow for single-mode propagation of light in the wavelength range of 8 to 12 microns.

The elongated body 21 is devoid of dielectric and metallic optical coatings on its interior (e.g., the wall of the lumen 29). The wall of the lumen 29 includes a bare interface between air and the material of the elongated body 21.

It is particularly advantageous to form the elongated body 21 from fused silica. Pure fused silica has a little-known Reststrahlen Band for wavelengths in the range of 8 to 12 microns. In this Reststrahlen Band, a fundamental solid state effect prevents light in the wavelength range of 8 to 12 microns from entering fused silica at an interface between fused silica and air. As a result, fused silica is a high reflector, in air, in the wavelength range of 8 to 12 microns. Because of the high reflectivity, light that enters lumen 29 through first longitudinal end 25 of a fused silica elongated body 21 transmits efficiently through the lumen 29 to the second longitudinal end 27. Due to the high reflectivity, in air, of fused silica in the range of 8 to 12 microns, the fused silica material itself can duplicate the function of silver in the known Opto-Knowledge hollow glass waveguides, described above. It is found that a purely fused silica elongated body, devoid of internal metallic and dielectric coatings, transmits light in the range of 8 to 12 microns about as efficiently as the Opto-Knowledge hollow glass waveguide that uses an internal coating of silver. The purely fused silica elongated body is significantly less expensive than the Opto-Knowledge hollow glass waveguide, because the difficult manufacturing step of coating an interior of the elongated body is absent.

In other examples, the elongated body 21 can be formed from an optical glass that includes a large fraction of silica, such as BK7 or other borosilicate glasses. The reflectivity at an interface between optical glass and air, in the range of 8 to 12 microns, is not as high as at a comparable interface between fused silica and air. As a result, an elongated body 21 formed from uncoated optical glass will still exhibit spatial filtering characteristics, but may exhibit some intensity loss between its longitudinal ends.

The cross-sectional shapes of the elongated body can be varied as needed, in order to produce a desired intensity profile for the output beam. For example, FIG. 3 is an end-on cross-sectional drawing of an elongate body 30 having a circular cross-section, as in FIG. 2. After transmission though the elongate body 30, the output beam is rotationally symmetric.

FIG. 4 is an end-on cross-sectional drawing of an elongate body 40 having an elongated cross-section. After transmission though the elongate body 40, the output beam is elongated.

FIG. 5 is an end-on cross-sectional drawing of an elongate body 50 having a square cross-section. After transmission though the elongate body 50, the output beam exhibits a pair of orthogonal spikes.

FIG. 6 is an end-on cross-sectional drawing of an elongate body 60 having a circular cross-section and a reduced lumen diameter. After transmission though the elongate body 60, the output beam has a greater divergence angle than the comparable output beam for FIG. 3.

FIG. 7 is an end-on cross-sectional drawing of an elongate body 70 having an irregular cross-section. After transmission though the elongate body 70, the output beam exhibits corresponding irregularities. Other cross-sectional shapes can also be used.

FIG. 8 is a flow chart of an example of a method 80 for conditioning a beam. The method can be executed using a suitable beam conditioner, such as the beam conditioner 10 of FIG. 1. Step 81 generates a source beam having a wavelength between 8 microns and 12 microns. The source beam can be generated by a light source, such as light source 11 from the beam conditioner 10 of FIG. 1. Step 83 focuses the source beam to form an input beam. The source beam can be focused by a focusing optic, such as focusing optic 13 from the beam conditioner 10 of FIG. 1. Step 85 receives the input beam at a first longitudinal end of a single-mode hollow core fiber. The hollow core fiber comprises an elongated cylindrical body formed from fused silica and devoid of optical coatings on its interior, such as the elongated body 21 of FIG. 2. Step 87 transmits the input beam from the first longitudinal end of the hollow core fiber to a second longitudinal end of the hollow core fiber to form an output beam. Step 89 transmits the output beam from the second longitudinal end of the hollow core fiber.

In this document, the terms "a" or "an" are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of "at least one" or "one or more." In this document, the term "or" is used to refer to a nonexclusive or, such that "A or B" includes "A but not B," "B but not A," and "A and B," unless otherwise indicated. In this document, the terms "including" and "in which" are used as the plain-English equivalents of the respective terms "comprising" and "wherein." Also, in the following claims, the terms "including" and "comprising" are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms "first," "second," and "third," etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.

The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various

combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

What is claimed is:

1. A beam conditioner, comprising:

a light source producing a source beam having a wavelength between 8 microns and 12 microns;

a focusing optic that focuses the source beam to produce an input beam; a single-mode hollow core fiber comprising an elongated body extending from a first longitudinal end to a second longitudinal end and having a lumen extending therethrough, the elongated body being devoid of optical coatings on its interior, the first longitudinal end receiving the input beam, the elongated body transmitting the input beam from the first longitudinal end to the second longitudinal end to form an output beam, the output beam emerging from the second longitudinal end.

2. The beam conditioner of claim 1, wherein the elongated body is formed from fused silica.

3. The beam conditioner of claim 1, wherein the elongated body is formed from borosilicate optical glass.

4. The beam conditioner of any one of claims 1-3, wherein the elongated body has a uniform cross-section at each longitudinal location along its length.

5. The beam conditioner of any one of claims 1-4, wherein the lumen has a diameter between 0.2 mm and 0.5 mm, inclusive.

6. The beam conditioner of any one of claims 1-5, wherein the output beam has a zero-order rotationally symmetric Gaussian distribution.

7. The beam conditioner of any one of claims 1-6, wherein the wavelength is between 8.5 microns and 11 microns.

8. The beam conditioner of any one of claims 1-7, wherein the focusing optic is a lens.

9. The beam conditioner of any one of claims 1-7, wherein the focusing optic is a mirror.

10. A method for conditioning a beam, comprising:

generating a source beam having a wavelength between 8 microns and 12 microns;

focusing the source beam to form an input beam;

receiving the input beam at a first longitudinal end of a single-mode hollow core fiber, the hollow core fiber comprising an elongated cylindrical body formed from fused silica and devoid of optical coatings on its interior; transmitting the input beam from the first longitudinal end of the hollow core fiber to a second longitudinal end of the hollow core fiber to form an output beam; and

transmitting the output beam from the second longitudinal end of the hollow core fiber.

11. The method of claim 10, wherein transmitting the input beam from the first longitudinal end of the hollow core fiber to the second longitudinal end of the hollow core fiber to form the output beam comprises spatially filtering the input beam to form the output beam.

12. The method of any one of claims 10-11, wherein the output beam has a zero-order rotationally symmetric Gaussian distribution.

13. The method of any one of claims 10-12, wherein the wavelength is between 8.5 microns and 11 microns.

14. A beam conditioner, comprising:

a light source producing a source beam having a wavelength between 8 microns and 12 microns; a lens that focuses the source beam to produce an input beam;

a single-mode hollow core fiber comprising an elongated cylindrical body extending from a first longitudinal end to a second longitudinal end and having a lumen extending therethrough, the elongated body being formed from fused silica, the elongated body being devoid of optical coatings on its interior, the elongated body having a uniform cross-section at each longitudinal location along its length, the first longitudinal end receiving the input beam, the elongated body transmitting the input beam from the first longitudinal end to the second longitudinal end to form an output beam, the output beam emerging from the second longitudinal end.