JP2006509253A - High power low loss fiber waveguide - Google Patents

Abstract

In general, in one aspect, the present invention provides high power and low loss consisting of alternating layers of different dielectric materials (130, 140) of polymer (130) and glass (140) surrounding a core (120) extending along a waveguide axis (199). Features an article that includes a fiber waveguide (100).

Description

This application claims the priority of US Provisional Application No. 60/432059 filed on Dec. 10, 2002. The contents of the above application are incorporated herein by reference.
The present invention relates to a fiber waveguide and a method for manufacturing the waveguide.

  Waveguides play an important role in many industries. For example, optical waveguides are widely used in telecommunications networks. In such telecommunications networks, fiber waveguides such as optical fibers are used to transmit information as optical signals between different positions. Yes. Such waveguides substantially limit the optical signal to propagation along one or more preferred paths. Other uses of the optical waveguide include imaging uses such as endoscopes and light detection applications.

  The most common type of fiber waveguide is an optical fiber that utilizes a light guide by refractive index to limit the optical signal to a preferred path. Such a fiber includes a core region extending along the waveguide axis and a cladding region surrounding the core about the waveguide axis and having a refractive index smaller than the refractive index of the core region. Because of this refractive index difference, light rays propagating substantially along the waveguide axis through a relatively high refractive index core can undergo total internal reflection (TIR) at the core-cladding interface. As a result, the optical fiber directs one or more electromagnetic (EM) radiation modes to propagate through the core along the waveguide axis. The number of such guided modes increases as the core diameter increases. In particular, the index guiding mechanism eliminates the presence of any cladding modes below the lowest frequency guided mode for a given wave vector parallel to the waveguide axis. Nearly all optical fibers guided by the refractive index used commercially are silica-based with impurities in the core and / or cladding to create a refractive index difference and create a core-cladding interface. Fiber. For example, a commonly used silica optical fiber has a refractive index of about 1.45 and a refractive index difference of about 0.2% to 3% depending on the application for wavelengths in the range of 1.5 microns.

  The method of drawing a fiber from a preform is the most commonly used method for manufacturing fiber waveguides. A preform is a short (eg, 10 to 20 inches long) rod having the exact shape and composition of the desired fiber. However, the preform diameter is much larger than the fiber diameter (eg, 100 to 1000 times). In general, when drawing an optical fiber, the material composition of the preform is such that one or several of the various concentrations added to the core of the preform are made to make the refractive index of the core greater than the refractive index of the cladding. It consists of a single glass with a dopant. This ensures that the material forming the core and cladding is drawn in the same rheological and chemical manner, while at the same time providing a refractive index difference sufficient to support guided modes within the core. I will provide a. To form the fiber from the preform, the preform is heated in a furnace to a temperature at which the viscosity of the glass is low enough to draw the fiber from the preform (eg, less than 108 poise). When drawn, the preform becomes a fiber having the same cross-sectional composition and structure as the preform. The fiber diameter depends on the specific rheological properties of the fiber and the draw speed.

The preform can be manufactured using a number of techniques well known to those skilled in the art including chemical vapor deposition (MCVD), external deposition (OVD), plasma activated chemical vapor deposition (PCVD) and axial deposition (VAD). Each of these methods generally involves depositing a vaporized layer of raw material in the form of soot on a prefabricated tube or bar wall. Immediately after deposition, each soot layer fuses. This gives a preform tube, which is then crushed into a solid rod,
Cover the outside, then draw to fiber.

  Optical fiber applications are limited by wavelength and signal power. The fiber is preferably formed from a material that absorbs less energy at the guided wavelength and has minimal defects. If the absorption is large, the signal strength may be reduced to a level indistinguishable from noise during transmission over long fibers. Even with a relatively low absorption material, the fiber is heated by absorption by the core and / or cladding. Defects can scatter directed radiation out of the core, which can also heat the fiber. Beyond a certain power density, this heating causes irreparable damage to the fiber. Thus, many applications that utilize high power radiation sources use devices other than optical fibers to direct radiation from the radiation source to its destination.

  In one aspect, the invention features a photonic crystal waveguide (eg, a Bragg fiber) that includes a polymer portion and a glass portion (eg, a chalcogen glass portion). In some embodiments, the photonic crystal waveguide includes a hollow core. Radiation is confined within the hollow core by a photonic band gap defined by a plurality of alternating layers of polymer and glass (eg, a continuous polymer layer wound in a spiral and a continuous glass layer). In general, glass layers have a high refractive index and polymer layers have a low refractive index. The optical thickness of the alternating layers determines the fundamental and higher order spectral transmission windows, from visible radiation (eg radiation having a wavelength of 0.35 to 0.75 microns) to infrared radiation (eg wavelength 0.75 to about 15 microns or It is possible to scale to more radiation).

  The invention further features a method for manufacturing a fiber photonic crystal fiber waveguide. A polymer substrate is coated with a layer of glass to form a planar multilayer film. The multilayer film is then rolled to obtain a hollow multilayer tube having a spiral cross section. Subsequently, the hollow tubes are bonded together by heating, and the spiral layers are fused to obtain a hollow fiber preform. This is drawn to obtain a fiber waveguide.

  The material can be selected so that the refractive index difference between different parts of the photonic crystal waveguide is large. By increasing the refractive index difference, a fiber having a large photonic band gap and omnidirectional reflectivity can be provided. When the photonic band gap is large, the penetration depth into the portion surrounding the core of the waveguide becomes shallow, and the radiation loss and absorption loss of the radiation guided by the fiber are reduced.

  If fiber materials that are optically different are drawn together when the thermomechanical, rheological and physicochemical properties of the fiber material are compatible, a fiber with a low defect density can be obtained. Thus, in one aspect, the invention features a combination of glass and polymer that can be drawn together, as well as criteria for selecting a glass and polymer that can be drawn together.

A low loss, low defect density fiber can be used to guide high power radiation with little damage to the fiber.
In general, in a first aspect, the invention includes rolling a multilayer structure into a spiral structure to form a fiber waveguide, and forming the fiber waveguide includes drawing a fiber preform derived from the spiral structure. It features a method to do.

Embodiments of this method may include one or more of the following features and / or features of other aspects.
The multilayer structure can include at least two layers of materials having different refractive indices. These layers may include a first material layer and a pair of second material layers sandwiching the first material layer. These layers can be substantially flat layers. These different materials may include a first material that includes glass and a second material that includes a polymer. In some embodiments, the different materials include a high refractive index material and a low refractive index material, and the ratio of the refractive index of the high refractive index material to the refractive index of the low refractive index material is greater than 1.5 (eg, 1 Greater than .8).

  The method further includes at least a first layer of a first material (eg, glass such as chalcogen glass) and a second material of a second material (eg, a polymer such as PES, PEI) that is different from that of the first material. Disposed on a plurality of layers to form a multilayer structure. The first material can be disposed on both sides of the second layer. This arrangement can consist of sputtering or evaporation. Additional layers can be disposed on the first and second layers to form a multilayer article.

  This multilayer structure can be wound around a rod (eg, a hollow rod) to form a spiral structure. This method can fuse the spiral structure to form a preform. The coalescence includes heating the spiral structure. In some embodiments, coalescence includes heating the spiral structure under vacuum conditions. This method includes removing the rod from the preform (eg, by chemical etching) prior to drawing.

This spiral structure may include a core surrounded by alternating layers of a multilayer structure. The fiber waveguide may include a hollow core surrounded by multiple layers corresponding to a multilayer structure.
In general, in another aspect, the invention features an article that includes a fiber waveguide that includes alternating layers of different materials surrounding a core extending along the waveguide axis, the alternating layers defining a spiral structure.

Embodiments of the article may include one or more of the following features and / or features of other aspects.
The spiral structure may include a multilayer structure including at least two layers of different materials that surround the core multiple times. The different materials can include a high refractive index dielectric material and a low refractive index dielectric material, where the ratio of the refractive index of the high refractive index material to the refractive index of the low refractive index material is greater than 1.5 (eg, 1 Greater than .8). These different materials can include polymers (eg PES) and chalcogen glasses (eg As ₂ Se ₃ ).

  The innermost layers of the alternating layers can have a thickness that is less than the thickness of subsequent layers of the same material. The alternating layer thickness can be selected to direct EM radiation along the waveguide axis at a wavelength in the range of about 8-12 microns (eg, a wavelength of about 10.6 microns). In some embodiments, the alternating layer thickness is selected to direct EM radiation with a wavelength in the range of about 2-5 microns along the waveguide axis.

  The core can be hollow. For straight fibers, this fiber waveguide can exhibit a transmission loss of less than about 1 dB / m at selected wavelengths (eg, wavelengths in the range of about 0.75 to about 10.6 microns). . In some embodiments, the selected wavelength is about 10.6 microns.

  When bent 90 degrees with any bend radius in the range of about 4-10 cm, the fiber waveguide is about 1. at the selected wavelength (eg, in the range of about 0.75 to about 10.6 microns). It is possible to show a transmission loss smaller than 5 dB.

This fiber waveguide is capable of directing EM radiation along the waveguide axis at selected wavelengths (eg, wavelengths in the range of about 0.75 to about 10.6 microns) with a power density of about 300 W / cm ² or higher. It is possible to have In some embodiments, the selected wavelength is about 1
0.6 microns. Even when the fiber waveguide is smoothly bent at 90 degrees with a bending length of at least 0.3 m, the fiber waveguide conducts EM radiation at selected wavelengths with a power density of about 300 W / cm ² or more. It is possible to have the ability to guide along the waveguide axis.

  The fiber waveguide may have the ability to direct EM radiation along the waveguide axis at selected wavelengths (eg, wavelengths ranging from about 0.75 to about 10.6 microns) with a power of about 25 W or greater. Is possible. In some embodiments, the selected wavelength is about 10.6 microns.

  In general, in another aspect, the invention features an article that includes a high power, low loss fiber waveguide that includes alternating layers of different dielectric materials including a polymer and glass surrounding a core that extends along the waveguide axis.

Embodiments of the article may include one or more of the following features and / or features of other aspects.
The alternating layers can define a spiral structure. The spiral structure may include a multilayer structure consisting of at least two layers of different materials that surround the core multiple times. The different materials can include a high index dielectric material and a low index dielectric material, wherein the ratio of the refractive index of the high index material to the index of refraction of the low index material is greater than 1.5. The different materials can include a high index dielectric material and a low index dielectric material, and the ratio of the refractive index of the high index material to the index of refraction of the low index material is greater than 1.8. The glass can include chalcogen glass (eg, As ₂ Se ₃ ). The polymer can comprise PES or PEI. The innermost layers of the alternating layers can have a thickness that is less than the thickness of subsequent layers of the same material. The thickness of the alternating layers can be selected to direct EM radiation along the waveguide axis at wavelengths in the range of about 8-12 microns (eg, a wavelength of about 10.6 microns). In some embodiments, the alternating layer thickness is selected to direct EM radiation with a wavelength in the range of about 2-5 microns along the waveguide axis.

The core may be hollow.
For straight fiber waveguides, the fiber waveguide can exhibit transmission losses of less than about 1 dB / m at selected wavelengths (eg, wavelengths in the range of about 0.75 to about 10.6 microns). It is. This selected wavelength can be about 10.6 microns.

  When bent 90 degrees with any bend radius in the range of about 4-10 cm, the fiber waveguide is about 1. at the selected wavelength (eg, in the range of about 0.75 to about 10.6 microns). It is possible to show a transmission loss smaller than 5 dB. This selected wavelength can be about 10.6 microns.

This fiber waveguide is capable of directing EM radiation along the waveguide axis at selected wavelengths (eg, wavelengths in the range of about 0.75 to about 10.6 microns) with a power density of about 300 W / cm ² or higher. It is possible to have This selected wavelength can be about 10.6 microns.

Even when the fiber waveguide is smoothly bent at 90 degrees with a bending length of at least 0.3 m, the fiber waveguide conducts EM radiation at selected wavelengths with a power density of about 300 W / cm ² or more. It is possible to have the ability to guide along the waveguide axis.

The fiber waveguide may have the ability to direct EM radiation along the waveguide axis at selected wavelengths (eg, wavelengths ranging from about 0.75 to about 10.6 microns) with a power of about 25 W or greater. Is possible. This selected wavelength can be about 10.6 microns.

Embodiments of the invention can have one or more of the following advantages.
Photonic crystal fiber waveguides can have low transmission loss with both straight and bent fibers. Photonic crystal fiber waveguides can be used to direct high power EM radiation. A photonic crystal fiber waveguide can be used to direct EM radiation having a high power density. A photonic crystal fiber waveguide can be used to direct EM radiation at IR wavelengths (eg, wavelengths from 0.75 to about 12 microns or more).

  The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

Like reference numerals in the drawings denote like elements.
US patent application Ser. No. 10/121452. US Pat. No. 6,130,780

  Referring to FIG. 1A, a photonic crystal fiber waveguide 100 includes a core 120 extending along the waveguide axis, and a dielectric confinement region 110 (eg, an alternating layer of a high refractive index layer and a low refractive index layer) surrounding the core. . The confinement region 110 is surrounded by a support layer 150 that provides mechanical support to the confinement region.

The confinement region 110 includes continuous layers 130 and 140 of dielectric material (eg, polymer, glass) having different refractive indices, whereas in other embodiments, it includes a plurality of concentric discontinuous layers that form the confinement region. . Continuous layers 130 and 140 form spirals about axis 199 along which the photonic crystal fiber waveguide guides electromagnetic radiation. One layer, for example, the layer 140 is a high refractive index layer having a refractive index n _H and a thickness d _H , and the layer, for example, the layer 130 is a low refractive index layer having a refractive index n _L and a thickness d _L. However _{n H> n L (n H} -n L , for example 0.01 or more, 0.05 or more, 0.1 or more, 0.2 or more, it is possible to 0.5 or higher). Since layers 130 and 140 draw a spiral around axis 199, radial cross section 160 extending from axis 199 intersects each layer more than once, and includes a radius that includes alternating layers of high and low refractive index layers. Provide a direction profile.

Referring to FIG. 1B, the layers depicting the spirals optically provide periodic refractive index variations along the radial cross section 160 having a period corresponding to the optical thickness of layers 130 and 140. To do. That is, the confinement region 110 has a double layer optical period n _H d _H + n _L d _L. “R” refers to the radial position measured from axis 199.

The thickness (d _H and d _L ) and optical thickness (n _H d _H and n _L d _L ) of layers 130 and 140 can be varied. In some embodiments, the optical thickness of layer 130 and the optical thickness of layer 140 are the same. The layer thickness is usually selected based on the desired optical performance of the fiber (eg, according to the waveguiding wavelength). The relationship between layer thickness and optical performance will be discussed later. The layer thickness is generally from submicron size to tens of microns. For example, the thickness of layers 130 and 140 can be about 0.1 μm to 20 μm (eg, about 0.5 to 5 μm).

In the embodiment shown in FIG. 1A, the confinement region 110 has a thickness of five double layers. In practice, however, the confinement region 110 has more double layers (for example, more than about 8 as double layers, more than 10, more than 15, more than 20, more than 25, for example 40, etc. Minutes or more).

  The layer 140 includes a material having a high refractive index such as chalcogen glass. Layer 130 is made of a material having a lower refractive index than the high refractive index material of layer 140 and is generally mechanically flexible. For example, layer 130 is often made of a polymer. The material forming layer 130 and layer 140 is preferably a material that can be drawn together. The criteria for selecting materials that can be drawn together are discussed below.

  In this embodiment, the core 120 is hollow. Optionally, the hollow core can be filled with a fluid such as a gas (eg, air, nitrogen and / or noble gas), a liquid (eg, an isotropic liquid or a liquid crystal). Alternatively, the core 120 can be composed of any material or combination of materials that is rheologically compatible with the material forming the confinement region 110. In one embodiment, the core 120 is “HIGH INDEX-CONTRAST FIBER WAVEGUIDES AND APPLICATIONS”, filed on Apr. 12, 2002, which is incorporated herein by reference, and is now published as publication number US-2003-0044158-A1. One or several dopant materials, such as those described in US patent application Ser. No. 10 / 121,452.

  The core and confinement regions 120 and 110 can include a plurality of dielectric materials having different refractive indices. In such cases, the “average refractive index” of a given region can be considered. This represents the sum of the weighted refractive indices of the components in the region, with each refractive index being weighted by the area occupied by that component in the region. However, the boundary between layers 130 and 140 is defined by a change in refractive index. This change can be caused by the interface of two different dielectric materials, or by different dopant concentrations within the same dielectric material (eg, different dopant concentrations within silica).

  The dielectric confinement region 110 directs EM radiation included in the first wavelength range to propagate through the dielectric core 120 along the waveguide axis 199. The confinement mechanism is based on the photonic crystal structure of the region 110 that forms the band gap including the first wavelength range. Since this confinement mechanism is not a refractive index guide type, the refractive index of the core need not be higher than the refractive index of the confinement region adjacent to the core. Conversely, the average refractive index of the core 120 may be lower than the average refractive index of the confinement region 110. For example, the core 120 can be air, other gases, such as nitrogen, or substantially vacuum. In such cases, the EM radiation that is directed through the core reflects the absorption of the silica core and nonlinear interaction constants, which is less than that of silica or other such solid materials. It has much less loss and much less nonlinear interaction than EM radiation guided through it. In additional embodiments, for example, the core 120 includes a porous dielectric material that provides structural support for the surrounding confinement region while defining an air-based core. Thus, the core 120 need not have a uniform refractive index profile.

The layers 130 and 140 of the confinement region 110 form a fiber known as a Bragg fiber. The periodic optical structure of these layers wound in a spiral is similar to the alternating layers of planar dielectric stack reflectors (also known as Bragg mirrors). Both the confinement region 110 layer and the flat stack of dielectric stack reflectors are examples of photonic crystal structures. The photonic crystal structure is described by John D. John D. Joanopoulos et al. “Photonic Crystals” (Princeton University Press), Princeton, NJ, 199
5 years).

  As used herein, a photonic crystal is a dielectric structure having a refractive index modulation that creates a photonic band gap in the photonic crystal. As used herein, a photonic bandgap is a wavelength (or frequency if reciprocal) range where there is no extended (ie, propagation, delocalization) state accessible in a dielectric structure. In general, this structure is a periodic dielectric structure, but can also include, for example, more complex “quasicrystals”. By combining photonic crystals with “defect” regions that deviate from the bandgap structure, this bandgap can be used to confine, direct, and / or localize light. In addition, wavelengths above and below this gap have accessible extended states that can confine light even in the low index region (as opposed to index guided TIR structures such as those previously described). ) The term “accessible” state means a state in which binding is not prohibited by some symmetry or conservation law of the system. For example, in a two-dimensional system, the polarization is preserved, so that only the same polarization state need be excluded from the band gap. For waveguides with a uniform cross-section (such as a common fiber), the wave vector β is conserved, so that to support a photonic crystal guided mode, only the state with a given β is removed from the band gap. It can be eliminated. Furthermore, in a waveguide with cylindrical symmetry, the “angular momentum” index m is preserved, so only modes with the same m need be excluded from the band gap. In short, in a highly symmetric system, the requirements for the photonic band gap are considerably relaxed compared to a complete band gap where all states are excluded regardless of symmetry.

  Therefore, since EM radiation cannot propagate through the dielectric stack reflector, the reflectivity of the dielectric stack reflector is high in the photonic band gap. Similarly, the layer of confinement region 110 provides confinement because it is highly reflective to incident light in the band gap. Strictly speaking, the photonic crystal is completely reflective in the band gap only when the refractive index modulation in the photonic crystal has an infinite extent. Otherwise, incident radiation can “tunnel” the photonic crystal through a transient mode that couples the propagation modes on both sides of the photonic crystal. In practice, however, the rate of such tunneling decreases exponentially with the photonic crystal thickness (eg, the number of alternating layers). This further decreases with the magnitude of the refractive index difference in the confinement region.

Furthermore, the photonic band gap can only widen over a relatively small region of the propagation vector. For example, a dielectric stack is highly reflective to normal incident light, but is only partially reflective to obliquely incident light. A “perfect photonic bandgap” is a bandgap that spans over all possible wave vectors and all polarizations. In general, a complete photonic bandgap is only associated with photonic crystals with refractive index modulation along three dimensions. However, in the context of EM radiation incident on a photonic crystal from an adjacent dielectric material, the `` omnidirectional '' where the adjacent dielectric material is a photonic bandgap for all possible wave vectors and polarizations that support propagating EM modes. It is possible to define a “photonic band gap”. In the same sense, an omnidirectional photonic bandgap can be defined as a photonic bandgap for all EM modes above the light line. This light line defines the lowest frequency propagation mode supported by the material adjacent to the photonic crystal. For example, in air, the light line is approximately given by ω = cβ. In the above equation, ω is the angular frequency of radiation, β is a wave vector, and c is the speed of light. A description of omnidirectional planar reflectors is disclosed in US Pat. No. 6,130,780, which is incorporated herein by reference. In addition, using dielectric alternating layers to provide omnidirectional reflection (planar limit) to a cylindrical waveguide geometry is incorporated herein by reference.
U.S. Pat. No. 6,463,200 to Yoel Fink et al. Entitled "MULTILAYER DEVICE FOR ENHANCED OPTICAL WAVEGUIDING".

  When the alternating layers 130 and 140 of the confinement region 110 create an omnidirectional band gap with respect to the core 120, the guided mode is strongly confined because, in principle, EM radiation incident on the confinement region from the core is completely reflected. However, such complete reflection only occurs when the number of layers is infinite. When the number of layers is finite (eg, about 10 as a double layer), the omnidirectional photonic bandgap is, for example, of EM radiation having frequencies at all incident angles from 0 ° to 80 ° and omnidirectional bandgap. For all polarizations, the planar geometry corresponds to at least 95% reflection. Furthermore, even when the photonic crystal fiber waveguide 100 has a confinement region with a band gap that is not omni-directional, this waveguide is still 0.1 GHz / dB for the frequency range of the strong waveguide mode, eg, the band gap. Supports modes with radiation loss of less than km. In general, whether a bandgap is omnidirectional depends on the size of the bandgap created by the alternating layers (generally varying with the refractive index difference between the two layers) and the component with the smallest refractive index of the photonic crystal.

  In a configuration similar to the Bragg configuration, the refractive index and thickness of the high refractive index layer can be changed and / or the refractive index and thickness of the low refractive index layer can be changed. The confinement region can further include a periodic structure including three or more layers per period (eg, three or more layers per period). Furthermore, the refractive index modulation can be changed continuously or discontinuously in the confinement region as a function of the fiber radius. In general, the confinement region can be based on any refractive index modulation that creates a photonic band gap.

Since the multilayer structure 110 has a periodic refractive index variation with respect to the radial axis, the multilayer structure 110 forms a Bragg reflector in this embodiment. Appropriate refractive index variation is about a quarter wavelength condition. For normal incidence, the maximum bandgap for a “¼ wavelength” stack where each layer has equal optical thickness λ / 4 or the same but d _H / d _L = n _L / n _H It is well known that d and n refer to the thickness and refractive index of the high and low refractive index layers. These correspond to layers 240 and 230, respectively. Normal incidence corresponds to β = 0. In a cylindrical waveguide, the desired mode is generally near the light line ω = cβ (at the large core radius limit, the lowest order mode is essentially a plane wave propagating along the z-axis or waveguide axis. is there). In this case, the 1/4 wavelength condition is as shown in the following equation.

  Strictly speaking, this equation is not necessarily optimal because this quarter wavelength condition is modified by a cylindrical geometry. In a cylindrical geometry, the optical thickness of each layer needs to change smoothly with its radial coordinates. Nevertheless, this equation has been found to be an excellent guide for optimizing many desirable characteristics, especially for core radii that are larger than the central bandgap wavelength.

Some embodiments of photonic crystal fiber waveguides have been filed on January 25, 2002 and are now incorporated by reference in this application, which is published as publication number US-2002-0164137-A1. It is described in US patent application Ser. No. 10/057258, entitled “LOW-LOSS PHOTOTONIC CRYSTAL FIBER HAVING LARGE CORE RADIUS” by Steven G. Johnson et al.

  The radius of the core 120 can be changed depending on the end use of the fiber 120. The radius of the core depends on the wavelength or wavelength range of the energy guided by the fiber and whether the fiber is a single mode fiber or a multimode fiber. For example, if the fiber is a single mode fiber that guides visible wavelengths (eg, wavelengths from about 400 nm to 800 nm), the core radius is on the order of sub-microns to several microns (eg, about 0.5 μm to 5 μm). However, if the fiber is a multimode fiber that guides IR wavelengths (about 2 μm to 15 μm, eg 10.6 μm), the core radius will be on the order of tens to thousands of microns (eg about 10 μm to 2,000 μm, eg 500 μm). To 1,000 μm). The core radius is greater than about 5λ (eg, greater than about 10λ, greater than 20λ, greater than 30λ, greater than 50λ, greater than 100λ). Where λ is the wavelength of the energy to be introduced.

  As previously discussed, the support layer 150 provides mechanical support to the confinement region 110. The thickness of the support layer 150 can be changed as desired. In some embodiments, the support layer 150 is significantly thicker than the confinement region 110. For example, the thickness of the support layer 150 can be about 10 times or more (for example, more than 20 times, more than 30 times, more than 50 times) the thickness of the confinement region 110.

  The composition of the support layer 150 is typically selected to provide the desired mechanical support and protection for the confinement region 110. In many embodiments, the support layer 150 is formed from a material that can be drawn with the confinement region 110. Criteria for selecting materials suitable for drawing together will be discussed later. In some embodiments, the support layer is formed from the same material that is used to form the confinement region 110. For example, if layer 130 is formed from a polymer, support layer 150 can be formed from the same polymer.

  Next, the composition of the layers 130 and 140 in the confinement region 110 will be described. Materials having a refractive index suitable for forming a high refractive index portion (eg, layer 140) include chalcogen glass (eg, glass containing a chalcogen element such as sulfur, selenium and / or tellurium), heavy metals Oxide glasses, amorphous alloys and combinations thereof are included.

  In addition to chalcogen elements, chalcogen glasses can contain one or more of the following elements: boron, aluminum, silicon, phosphorus, sulfur, gallium, germanium, arsenic, indium, tin, antimony, thallium, lead, bismuth , Cadmium, lanthanum and halides (fluorine, chlorine, bromide, iodine).

  The chalcogen glass is a two-component or three-component glass, such as As-S, As-Se, Ge-S, Ge-Se, As-Te, Sb-Se, As-S-Se, S-Se-Te, As-. Se-Te, As-S-Te, Ge-S-Te, Ge-Se-Te, Ge-S-Se, As-Ge-Se, As-Ge-Te, As-Se-Pb, As-S- Tl, As-Se-Tl, As-Te-Tl, As-Se-Ga, Ga-La-S, Ge-Sb-Se, or As-Ga-Ge-S, Pb-Ga- based on these elements A composite multicomponent glass such as Ge—S can be used. The ratio of each element in chalcogen glass can be changed. A chalcogen glass having an appropriate refractive index is formed of, for example, arsenic 5 to 30 mol%, germanium 20 to 40 mol%, and selenium 30 to 60 mol%.

Examples of heavy metal oxide glasses having a high refractive index include Bi ₂ O ₃ containing glass, PbO containing glass, Tl ₂ O ₃ containing glass, Ta ₂ O ₃ containing glass, TiO ₂ containing glass and TeO ₂ containing glass. It is.

Amorphous alloys having an appropriate refractive index include Al-Te and R-Te (Se) (R = alkali).
Materials having a low refractive index suitable for forming the low refractive index portion (eg, layer 130) include oxide glasses, halide glasses, polymers, and combinations thereof. Carbonate esters (eg polycarbonate (PC)), sulfones (eg poly (ether sulfone) (PES)), ether imides (eg poly (ether imide) (PEI)), and acrylate esters (eg poly (methyl methacrylate) ( PMMA)) polymers, as well as polymers containing fluoropolymers, are also good candidates.

Suitable oxide glasses include, for example, glasses containing one or several of the following compounds: M ₂ O 0-40 mol% (M is Li, Na, K, Rb or Cs); M′O 0-40 mol% (M ′ is Mg, Ca, Sr, Ba, Zn or Pb); M ″ ₂ O ₃ 0-40 mol% (M ″ is B, Al, Ga, In, Sn or Bi); P ₂ O ₅ 0 to 60 mole%; and _SiO 2 0 to 40 mol%.

  The portion of the photonic crystal fiber waveguide can optionally include other materials. For example, any portion may include one or several materials that change their refractive index. The portion can include a material that increases its refractive index. Such materials include, for example, germanium oxide that increases the refractive index of the portion containing borosilicate glass. Alternatively, the portion can include a material that reduces its refractive index. For example, boron oxide reduces the refractive index of the portion containing borosilicate glass.

  The portion of the high index difference fiber waveguide can be homogeneous or inhomogeneous. For example, one or more parts of one type of material nanoparticles embedded in a host material to form inhomogeneous parts (eg, sufficiently small particles that minimize light scattering at the guided wavelength). May be included. An example of this is a high refractive index polymer composite formed by embedding high refractive index chalcogen glass nanoparticles in a polymer host. Other examples include CdSe and / or PbSe nanoparticles in an inorganic glass matrix.

The portion of the fiber waveguide may include materials that change its own mechanical, rheological and / or thermodynamic behavior. For example, one or more portions can include a plasticizer. The portion can include materials that suppress crystallization or other undesirable phase behavior within the fiber. For example, crystallization of the polymer can be suppressed by including a crosslinking agent (eg, a photosensitive crosslinking agent). As another example, if a glass-ceramic material is desired, a nucleating agent such as TiO ₂ , ZrO ₂ can be included in the material.

The portion can further include a compound designed to affect the interface between adjacent portions of the fiber (eg, between the low and high refractive index layers). Such compounds include adhesion promoters and compatibilizers. For example, organosilane compounds can be used to promote adhesion between silica-based glass portions and polymer portions. For example, phosphorus or P ₂ O ₅ can be compatible with both chalcogenide and oxide glasses, and promote adhesion between parts formed from these glasses.

The fiber waveguide can include additional materials that are specific to a particular fiber waveguide application. For example, in a fiber amplifier, any dopant or combination of dopants that has the ability to interact with an optical signal in the fiber to enhance absorption or emission of one or more light wavelengths by the fiber, such as erbium ions, ytterbium ions, Any portion can be formed from at least one rare earth ion, such as neodymium ions, holmium ions, dysprosium ions and / or thulium ions.

The portion of the high index difference waveguide may include one or several nonlinear materials. A nonlinear material is a material that enhances the nonlinear response of the waveguide. Specifically, nonlinear materials have a greater nonlinear response than silica. For example, the nonlinear material is the Kerr nonlinear index of silica (Kerr).
Nonlinear index) greater than (i.e. 3.5 × ¹⁰ -20 ^m 2 / W, such as more than 5 × ¹⁰ -20 ^m 2 / W ^{greater, 10 × 10 -20 m 2 /} W , greater than 20 × ^{10 -20} m ² / W, over 100 × 10 ⁻²⁰ m ² / W, over 200 × 10 ⁻²⁰ m ² / W ⁾ .

  When manufacturing a rugged fiber waveguide using a drawing process, not all material combinations with the desired optical properties are suitable. In general, a material that is compatible rheologically, thermomechanically and physicochemically must be selected. Some criteria for selecting compatible materials are discussed next.

  The first criterion is to select materials that are rheologically compatible. In other words, materials with similar viscosities must be selected over a wide temperature range corresponding to the temperatures encountered at various stages of fiber drawing and operation. Viscosity is the resistance of a fluid to flow under applied shear stress. In the present specification, the viscosity is expressed in units of poise. Before elaborating on rheological compatibility, it is useful to define a set of characteristic temperatures for a given material. This characteristic temperature is the temperature at which a given material has a certain viscosity.

Annealing point _{T a} is the temperature at which the viscosity of the material is ^{10 13} poise. T _a may be measured using a Model SP-2A System of Orton Ceramic Foundation (Orton Ceramic Foundation), Inc. (Ohio Westerville (Westerville)). T _a is generally the temperature at which the viscosity of the glass piece is low enough to allow the residual stress to be released.

The softening point T _s is the temperature at which the viscosity of the material is 10 ^7.65 poise. T _s is a softening point measuring instrument such as Orton Ceramic Foundation (Orton Ceramic Foundation).
Measurements can be made using Model SP-3A from Foundation (Westerville, Ohio). The softening point is related to the temperature at which the flow properties of the material change from plastic to viscous.

Working point _{T w} is the temperature at which the viscosity of the material is 10 ⁴ poise. T _w can be measured using a glass viscometer, such as a Model SP-4A from Orton Ceramic Foundation (Westerville, Ohio). The processing point relates to the temperature at which the glass can be easily drawn into a fiber. For example, in some embodiments where the material is inorganic glass, the processing point temperature of the material can be greater than 250 ° C., such as about 300 ° C., 400 ° C., 500 ° C. or higher.

Melting point _{T m} is the temperature at which the viscosity of the material is 10 ² poise. T _m can also be measured using a glass viscometer, such as a Model SP-4A from Orton Ceramic Foundation (Westerville, Ohio). The melting point is related to the temperature at which the glass becomes liquid and the fiber drawing process becomes very difficult to control with respect to maintaining the fiber geometry.

In order to be rheologically compatible, there are two types of temperatures over a wide temperature range, for example, the temperature at which the fiber is drawn, to the temperature at which the fiber can no longer release stress at a discernable rate (eg, T _a ). The material must have a similar viscosity. Therefore, the processing temperatures of the two compatible materials must be similar so that the two materials flow at similar rates when drawn. For example, when the viscosity η ₁ (T) of the first material is measured at the processing temperature T _w2 of the second material, η ₁ (T _w2 ) is at least 10 ³ poise, such as 10 ⁴ poise or 10 ⁵ poise. And no more than 10 ⁶ poise. Furthermore, as the drawn fiber cools, the behavior of both materials must change from viscous to elastic at similar temperatures. In other words, the softening temperatures of the two materials must be similar. For example, at the softening temperature Ts ₂ of the second material, the viscosity η ₁ (T _s2 ) of the _first material must be at least 10 ⁶ poise, such as 10 ⁷ poise or 10 ⁸ poise, and not more than 10 ⁹ poise. I must. In a preferred embodiment, it must be possible to anneal both materials together, so that the viscosity η ₁ (T _a2 ) of the first material at the annealing temperature T _a2 of the second material Must be at least 10 ⁸ poise (eg, at least 10 ⁹ poise, at least 10 ¹⁰ poise, at least 10 ¹¹ poise, at least 10 ¹² poise, at least 10 ¹³ poise, at least 10 ¹⁴ poise).

In order to achieve rheological compatibility, it is further preferred that the change in viscosity of both materials with respect to temperature (i.e., the slope of the viscosity) match as much as possible.
The second selection criterion is that the coefficient of thermal expansion (TEC) of each material must be similar at temperatures between the annealing temperature and room temperature. In other words, as the fiber cools and its rheology changes from liquid to solid, the volume of both materials must change by a similar amount. If the TEC match between the two materials is not sufficient, a large volume change difference between the two fiber sections can result in a large amount of residual stress, thereby cracking one or more sections. And / or peeling may occur. Even if the stress is sufficiently lower than the fracture stress of the material, the residual stress may cause delayed fracture.

TEC is a measure of the very slight change in sample length with temperature change. This parameter can be calculated from the slope of the temperature-length curve (or equivalent temperature-volume curve) for a given material. The temperature-length curve of the material can be measured, for example, using an dilatometer such as the Model 1200D dilatometer from Orton Ceramic Foundation (Westerville, Ohio). Is possible. The TEC can be measured over a selected temperature range or as an instantaneous change at a given temperature. The unit of this amount is ° C- ¹ .

For many materials, there are two linear regions with different slopes in the temperature-length curve. There is a transition region where the curve changes from the first linear region to the second linear region. This region is associated with a glass transition where the behavior of the glass sample transitions from the behavior normally associated with solid materials to the behavior typically associated with viscous fluids. This is a continuous transition and is characterized by a gradual change in the slope of the temperature-volume curve as opposed to a discontinuous change in the slope. The glass transition temperature _Tg is defined as the temperature at which the extrapolated glass solid line and the viscous fluid line intersect. The glass transition temperature is the temperature associated with the change in material rheology from a brittle solid to a flowable solid. Physically, the glass transition temperature is related to the thermal energy required to excite various translational and rotational modes of molecules in the material. The glass transition temperature is often the approximate annealing point at which the viscosity is 10 ¹³ poise, but in practice the measured Tg is a relative value and depends on the measurement method.

It is also possible to measure the dilatometer softening point T _ds using a dilatometer. The dilatometer works by applying a small compressive load to the sample and heating the sample. When the temperature of the sample becomes high enough, the material begins to soften and the compressive load causes the sample to bend as observed in volume or length reduction. This relative value is called the dilatometer softening point, which usually occurs when the material has a viscosity of 10 ¹⁰ to 10 ^12.5 poise. The exact T _ds value of the material usually depends on the instrument and measurement parameters. When similar equipment and measurement parameters are used, this temperature provides a useful measure of the rheological compatibility of various materials in this viscosity range.

As previously mentioned, matching the TECs is an important consideration for obtaining a fiber that does not have excessive residual stresses that occur in the fiber during the drawing process. In general, when the TECs of the two materials are not well matched, the residual stress occurs as an elastic stress. The elastic stress component results from the difference in volume shrinkage between the various materials of the fiber as it cools from the glass transition temperature to room temperature (eg, 25 ° C.). Volume change is determined by TEC and temperature change. In embodiments where the fiber materials are fused or joined at any interface during the drawing process, differences in the TECs of the materials cause stress at the interface. One material is pulled (positive stress) and the other material is compressed (negative stress), so the total stress is zero. Mild compressive stress is usually not a major problem for glass fibers, but tensile stress is undesirable and can lead to failure over time. Therefore, it is desirable to minimize the TEC difference of the component materials in order to minimize the elastic stresses that occur in the fiber during drawing. For example, in a composite fiber formed from two different materials, the absolute difference in TEC of each glass between _Tg measured at a heating rate of 3 ° C./min using a dilatometer and room temperature is 5 × 10 ^{− 6} ° C. ⁻¹ or less (eg 4 × 10 ⁻⁶ ° C. ⁻¹ or less, 3 × 10 ⁻⁶ ° C. ⁻¹ or less, 2 × 10 ⁻⁶ ° C. ⁻¹ or less, 1 × 10 ⁻⁶ ° C. ⁻¹ or less, 5 × 10 ⁻⁷ ° C.- ¹ or less, 4 × 10 ⁻⁷ ° C ⁻¹ or less, 3 × 10 ⁻⁷ ° C ⁻¹ or less, 2 × 10 ⁻⁷ ° C ⁻¹ or less).

While it is possible to minimize the elastic stress component by selecting a material with a similar TEC, residual stress can also be generated from the viscoelastic stress component. Viscoelastic stress components occur when there is a sufficient difference in strain point temperature or glass transition temperature of the component materials. When cooled to a temperature below the T _g, the material is subjected to considerable size of the volume contraction. As the viscosity changes during this transition during cooling, the time required to relieve stress increases from zero (instantaneous) to several minutes. For example, consider a composite preform made of glass and polymer having different glass transition ranges (and different T _g ). At the beginning of drawing, glass and polymer behave as viscous fluids, and the stress due to drawing strain is immediately relieved. Upon exiting the hottest part of the draw furnace, the fiber quickly loses heat and exponentially increases the viscosity and stress relaxation time of the fiber material. When allowed to cool to T _g, the glass and the polymer can not be released virtually more stress. This is because the stress relaxation time is much longer than the drawing speed. Thus, if the component materials have different T _g values, the first material cooled to T _g can no longer reduce the stress and the second material is still higher than its T _g , It is possible to release the stress generated between the materials. When the second material is cooled to T _g, stress generated between the material will not be relaxed longer effective. Further at this time, volume shrinkage of the second glass (temperature below its T _g, behaving as a brittle solid) much larger than the volume shrinkage of the first material. This situation results in sufficient stress between the glass and the polymer, resulting in mechanical failure of one or both parts. This leads to a third selection criterion for selecting the fiber material that it is desirable to minimize the _Tg difference between the component materials in order to minimize the viscoelastic stress that occurs in the fiber during drawing. It is burned. The glass transition temperature T _g1 of the first material is preferably within 100 ° C. from the glass transition temperature T _g2 of the second material (for example, | T _g1 −T _g2 | is less than 90 ° C., less than 80 ° C., 70 Less than 60 ° C, less than 60 ° C, less than 50 ° C, less than 40 ° C, less than 30 ° C, less than 20 ° C, less than 10 ° C).

Since there are two mechanisms (i.e., elastic and viscoelastic) that generate permanent stresses in the drawn fiber due to differences between the component materials, these mechanisms can be used to cancel each other. For example, if the mismatch of the material T _g of that produces opposite signs of stress, the material constituting the fibers, the stress caused by the thermal expansion mismatch can be canceled naturally. Conversely, if the thermal expansion of the material reduces the overall permanent stress greater difference in T _g of the inter-material it is acceptable. One method for evaluating the combined effect of thermal expansion and glass transition temperature difference is a method of comparing the temperature-length curves of the respective component materials. After finding the T _{g of} each material using the slope-tangent method described above, one curve is moved along the vertical axis so that the curves match at the lower T _g temperature value. The difference in the y-intercept at room temperature gives the expected strain ε when the glass is not bonded. The expected tensile stress σ of a material that exhibits a greater amount of shrinkage in the temperature range from T _g to room temperature can be simply calculated from the following equation:

σ = E · ε
Where E is the elastic modulus of the material. In general, residual stress values below 100 MPa (eg below 50 MPa, below 30 MPa) are small enough to show that the two materials are compatible.

The fourth selection criterion is that the thermal stability of the candidate materials is consistent. A measure of thermal stability is given by the temperature interval (T _x -T _g ). Where T _x is the temperature at which crystallization begins when the material is cooled slowly enough to allow each molecule to find its lowest energy state. Thus, the crystalline phase is an energetically more favorable state for the material than the glass phase. However, for fiber waveguide applications, the glass phase of the material generally has performance and / or manufacturing advantages over the crystalline phase. The closer the crystallization temperature is to the glass transition temperature, the easier the material will crystallize during drawing, which is detrimental to the fiber (eg, introduces optical inhomogeneities into the fiber that can increase transmission loss). Typically, a thermal stability interval (T _x -T _g ) of at least 80 ° C. (eg, at least 100 ° C.) is sufficient to draw the material from the preform into a fiber. In a preferred embodiment, the thermal stability interval is at least 120 ° C, such as 150 ° C, 200 ° C or higher. T _x can be measured using a thermal analysis instrument such as a differential thermal analyzer (DTA) or a differential scanning calorimeter (DSC).

Another matter to consider when selecting materials that can be drawn together is the melting temperature T _m of the material. At the melting temperature, the viscosity of the material is too low to maintain the correct geometry well during the fiber drawing process. Thus, in a preferred embodiment, the melting temperature of one material is higher than the processing temperature of a second material that is rheologically compatible. In other words, when the preform is heated, the temperature of the preform reaches a temperature at which the preform can be successfully drawn before any material of the preform melts.

One example of a pair of materials that can be drawn together and provide a photonic crystal fiber waveguide with a high index difference between the layers of the confinement region is As ₂ Se ₃ and polymer PES. As ₂ Se _{3 has} a glass transition temperature (T _g ) of about 180 ° C. and a coefficient of thermal expansion (TEC) of about 24 × 10 ⁻⁶ / ° C. At 10.6 μm, As ₂ Se _{3 has} a refractive index of 2.7775 and an absorption coefficient α of 5.8 dB / m. This refractive index is calculated according to Proc. The absorption coefficient, “Physics and Applications of Non-Cryst” was determined by the measurement of Hartouni et al. Described in SPIE, 505, 11 (1984).
alline Semiconductors in Optical Electronics, "A. A. Andriesh and M.C. By measurement of Voitt and Linke described in the edition of M. Bertolotti, NATO ASI Series, 3, High Technology, Vol. 36, p. 155 (1996). Both of these references are incorporated herein by reference. The TEC of PES is about 55 × 10 ⁻⁶ / ° C., and the refractive index is about 1.65.

  In some embodiments, a photonic crystal fiber waveguide, such as waveguide 100, is manufactured by rolling a planar multilayer article into a spiral structure and drawing the fiber from a preform resulting from the spiral structure.

  Refer to FIG. 2A. To prepare the preform, glass is deposited 220 on the surface 211 of the polymer film 210. The glass can be deposited by several methods including thermal evaporation, chemical vapor deposition or sputtering. Refer to FIG. 2B. This attachment process yields a multilayer article 240 comprising a glass layer 230 on the polymer film 210.

Refer to FIG. 2C. Following this attachment step, the multilayer film 240 is wrapped around a mandrel 255 (eg, a hollow glass or polymer tube such as borosilicate glass) to form a spiral tube. The spiral tube is then wrapped with several (eg, about 3 to 10) polymer films to form a preform wrap. In some embodiments, the polymer film is made from the same polymer or glass used to form the multilayer article. Under vacuum conditions, the preform wrap is heated to a temperature above the glass transition temperature of the polymer and glass forming the multilayer film 240 and the film wrapped around the spiral tube. The preform wrap is heated for a time sufficient for the layers of the vortex tube to fuse together and the vortex tube to fuse with the polymer film wrapped around the vortex tube. The heating temperature and heating time depend on the composition of the preform wrap. For example, when this multilayer is composed of As ₂ Se ₃ and PES and the wrapping film is composed of PES, it is generally sufficient to heat at 200 to 300 ° C. (eg about 250 ° C.) for 15 to 20 minutes (eg about 18 minutes). It is. This heating fuses the various layers together and fuses the spiral tube and wrapping film. The structure after coalescence is shown in FIG. 2D. The spiral tubes are joined together to form a multilayer region 260 corresponding to the rolled multilayer film 240. The wrapped polymer film is fused into a single support cladding 270. This bonded structure holds the hollow core 250 of the mandrel 255.

Instead of winding the polymer film around the spiral tube to form the support cladding 270, the spiral tube can be inserted into a hollow tube having an inner diameter that matches the outer diameter of the spiral tube.
The mandrel 255 is removed from the bonded structure to obtain a hollow preform. This is then drawn into a fiber. The preform has the same composition and the same relative dimensions as the final fiber (eg, core radius to confinement region layer thickness). The absolute dimensions of the fiber depend on the draw ratio used. Long fibers (eg thousands of meters) can be drawn. The drawn fiber can then be cut to the desired length.

  The coalescence is preferably performed at a temperature lower than the glass transition of the mandrel so that the mandrel provides a solid support for the spiral tube. This ensures that the multilayer film does not collapse under vacuum conditions. The composition of the mandrel can be selected to deviate from the innermost layer of the multilayer tube after coalescence. Alternatively, if the mandrel adheres to the innermost layer of the multilayer tube at the time of coalescence, the mandrel can also be removed chemically, for example by etching. For example, in embodiments where the mandrel is a glass capillary, the mandrel can be etched to obtain a preform using, for example, hydrofluoric acid.

  In embodiments where a solid core is desired, it is possible to fuse the multi-layer tube around a solid mandrel that is drawn together with other portions of the fiber. Alternatively, in other embodiments, it is possible to roll a multilayer film without using a mandrel to obtain a self-supporting spiral tube.

  In some embodiments, both sides of the polymer film 210 can be coated with glass. This method is advantageous because the thickness of each glass layer may be half the thickness of the glass layer deposited on only one side. Thin glass layers are generally less susceptible to mechanical stress damage that can occur during rolling.

  Photonic crystal fiber waveguides prepared using the techniques discussed above have a low defect density. For example, the waveguide has less than about 1 defect per 10 meters of fiber (eg, less than about 1 per 20 meters of fiber, less than about 1 per 50 meters of fiber, less than about 1 per 100 meters of fiber). Defects include both material defects (eg, impurities) and structural defects (eg, delamination, layer cracks), both of which cause scattered radiation to scatter from the core, causing signal loss and localizing the fiber. Heating can occur. Therefore, it is desirable to reduce fiber defects in applications that are sensitive to signal loss (eg, high power applications where the fiber can be damaged by radiation absorbed by the fiber).

  A photonic crystal fiber waveguide formed from a glass film covering both sides of the substrate provides a slightly different refractive index profile than a fiber waveguide where the glass film covers only one side. For example, referring to FIGS. 3A and 3B, a confinement region 310 of fiber formed from a multilayer film coated on both sides has a continuous spiral polymer layer 330 / glass layer 340. FIG. The innermost region 340A and the outermost region 340B of the glass layer 340 correspond to a single glass layer covering the polymer, while the other regions have twice the layer thickness. Its refractive index profile taken at radial cross section 360 is shown in FIG. 3B.

  In some embodiments, two or more multilayer films are prepared and superimposed before rolling. In this way, it is possible to increase the number of layers in the confinement region without increasing the size of the film.

As discussed above, photonic crystal fiber waveguides can be used to direct IR radiation. The IR radiation has a wavelength of about 0.7 to 20 microns (eg, about 2 to 5 microns, or about 8 to 12 microns). In some embodiments, a photonic crystal fiber waveguide may be used to direct radiation from an IR laser such as a CO ₂ laser that emits radiation having a wavelength of about 6.5 microns or 10.6 microns. Is possible. Other lasers capable of emitting IR energy include Nd: YAG laser (for example, wavelength 1.064 microns), Er: YAG laser (for example, wavelength 2.94 microns), Er, Cr: YSGGG (erbium, chromium). Doped yttrium scandium gallium garnet) laser (eg 2.796 microns wavelength), Ho: YAG laser (eg 2.1 microns wavelength), free electron laser (eg 6 to 7 microns wavelength), quantum cascade laser (For example, wavelength 3 to 5 microns).

In some embodiments, a photonic crystal fiber waveguide can be used to direct radiation having a very high power density. For example using a waveguide, a power density of greater than about 100W / cm ² (e.g. about 300 W / cm ^2, greater than 500 W / cm ^2, greater than 1 kW / cm ^2, such as more than about 10 kW / cm ² or more power density) of It is possible to guide radiation having. In particular, hollow core waveguides are well suited for such applications because of the low absorption of radiation directed through the core. The absorption loss can be further reduced by selecting a material having a small absorption at the waveguide wavelength as the material for the confinement region of the waveguide. As discussed above, for example, chalcogen glass has low absorption at IR wavelengths and is well suited for high power IR waveguides. Radiation loss that not only degrades waveguide performance but can also damage the waveguide can be further reduced by selecting a material with a high index difference as the material of the confinement region.

  It is possible to create a high power density in a fiber waveguide by coupling radiation from a high power laser into the fiber. For example, radiation from high power IR lasers, such as some of the lasers listed above, can be directed using photonic crystal fiber waveguides. The output power of the laser can be greater than about 1 watt (eg, about 5 watts, 10 watts, 25 watts or more). In some applications, the output energy of the laser can be greater than about 100 watts, such as several hundred watts (eg, greater than about 200 watts, greater than 300 watts, greater than 500 watts, greater than 1 kilowatt).

In some embodiments, the photonic crystal fiber waveguide has a relatively low transmission loss. For example, the transmission loss can be less than about 2 dB / m (eg, less than about 1 dB / m, less than 0.5 dB / m, eg, 0.2 dB / m or less). This fiber waveguide can have low transmission loss at certain wavelengths, such as IR wavelengths, eg about 3-5 microns (eg about 3.5 microns) or about 10-12 microns (eg about 10.6 microns) It is. It is possible to significantly reduce transmission loss (for example, 1 to 3 digits or more) as compared with TIR optical fibers made of similar materials. For example, a photonic crystal fiber waveguide having a hollow core and a chalcogen glass / polymer confinement region has significantly lower transmission loss than a TIR fiber having a chalcogen glass core and a polymer cladding. For example, the loss at As ₂ Se ₃ is reported to be about 7-10 dB / m at 10.6 microns and the loss at PES is about 100,000 dB / m at 10.6 microns. In contrast, the loss of a photonic crystal fiber waveguide with an As ₂ Se ₃ / PES confinement region can be less than about 1 dB / m. This relatively low loss is thought to be possible due to the shallow penetration depth of the guided electromagnetic wave into the confinement region of the fiber. Thus, even if the confinement region material has a relatively high absorption at the guided wavelength, the interaction between the directed radiation and the material is minimal.

  Photonic crystal fiber waveguides also suffer from relatively small losses due to fiber bending. For example, if the fiber is bent 90 degrees with a radius of curvature less than about 10 cm (eg, less than about 5 cm, eg, 4 cm or less), the loss is less than about 2 dB (eg, 1.5 dB, 1 dB, 0.5 dB or less). The relatively small loss associated with bending is advantageous in many fiber applications where it is desirable that the relative strength of the transmitted signal does not change significantly when the fiber bends during use.

  A low transmission loss (eg, loss due to intrinsic loss and / or bending) is generally advantageous even in high power applications. In high power applications, in addition to reducing the power delivered from the radiation source to its destination due to power lost along the fiber, fiber damage can also occur.

Various fibers were prepared by depositing 5-10 micron thick As ₂ Se ₃ using thermal evaporation on a 25-50 micron thick PES film, followed by wrapping the coated film around a hollow glass mandrel. Manufactured. The tube was covered with an outer layer of thick PES, which was heated and coalesced under vacuum conditions. After coalescence, the mandrel was etched away by introducing hydrofluoric acid into the hollow core of the mandrel. A multilayer preform was obtained by this etching. Each of these was put into an optical fiber drawing tower and drawn into a fiber of several tens to several hundreds of meters.

  The nominal position of the photonic band gap of each fiber was determined by monitoring the outer diameter (OD) of the fiber during the drawing process. The photonic band gap position was determined from the draw ratio provided by the OD measurement. The typical standard deviation of the fiber OD was about 1 percent of the fiber OD.

Reference is made to FIGS. 4A and 4B. According to scanning electron microscope (SEM) analysis of a single section of the fiber, the drawn fiber generally maintains a proportional layer thickness ratio, and PES and As ₂ Se ₃ film during thermal cycling and stretching associated with the manufacturing process. Adhered well. In the multilayer structure shown in FIGS. 4A and 4B, the thickness of the PES layer (gray) was about 900 nm, and the thickness of the As ₂ Se ₃ layer (light color) was about 270 nm (first and last As ₂ Se _3). Excluding the layers, the thickness of these layers was 135 nm).

  Broadband fiber transmission spectra were measured using a Fourier Transform Infrared (FTIR) spectrometer (Nicolet Magna 860) using a parabolic mirror that couples light to the fiber and an external detector. The results of these measurements for fibers having two different layer structures are shown in FIG. For each spectrum, fundamental and higher order photonic bandgap light was derived.

  Several fibers were prepared having a hollow core with a diameter of 700-750 microns, an OD of 1300-1400 microns, and a basic photonic band gap spanning 10-11 micron wavelengths. FIG. 6A shows the FTIR transmission spectrum of one of these fibers, measured using a straight fiber about 30 cm long.

In order to quantify the transmission loss of these fibers, fiber cutback measurements were performed. These measurements include comparing the intensity of radiation transmitted through an approximately 4 meter straight fiber to the intensity transmitted through the same fiber cut to a shorter length (see FIG. 6B). This test was performed on multiple fibers and the results were found to be about the same for different test fibers. Measurements were performed using 25 watt CO ₂ laser (GEM-25, Coherent-DEOS ) and a high power detector (818T-10 detector, Newport (Newport), Inc.). In order to reduce variations in input coupling and propagation conditions during fiber cutting, the fiber was held straight and fixed at both ends of the fiber and at multiple locations between the ends. The laser beam was coupled to the fiber through a focusing lens and a 500 micron diameter pinhole aperture. Furthermore, the input end face of the fiber was coated with a metal film to reduce accidental laser damage due to poor alignment.

  The fiber fundamental bandgap transmission loss at about 10.6 microns was measured to be about 0.95 dB / m, as shown in FIG. 6B, with an estimated measurement uncertainty of about 0.15 dB / m. According to a bending analysis of a fiber with a band gap centered at about 10.6 microns (discussed next), the bending loss when bending 90 degrees with a bending radius of 4-10 cm is less than about 1.5 dB / m. It was.

Bend loss measurements were performed using a broadband FTIR source and a CO ₂ laser operating at 10.6 microns. In each measurement, the fiber was bent 90 degrees around a metal cylinder of various radii. The amount of fiber after bending was about 15 cm in each case. This part was kept straight in each case. FIG. 7 shows the relative strength of a straight fiber about 50 cm long and the same fiber bent at various bend radii, measured using an FTIR spectrometer.

The FTIR bend measurement shown in FIG. 7 gives a total bend loss value of less than 1 dB for the bend at the largest radius. To reinforce these results, a similar test was performed using CO ₂ laser measurements using a fiber about 2.5 m long. FIG. 8 shows the average bend loss in dB for a 90 degree bend for an unbend fiber with a CO ₂ laser.

This CO ₂ laser bending loss result represents an average of several trials, and the observed loss variation was of the order of 0.2 dB. The results obtained using the CO ₂ laser had the same qualitative characteristics as those obtained using the FTIR instrument. Note that the different sources used had different coherencies, numerical apertures and polarization states. They were therefore expected to couple to modes with various loss characteristics.

The maximum laser power density coupled from the CO ₂ laser to the fiber was about 300 W / m ² , which was sufficient to burn the hole through paper and PES film (the main component of the fiber). When the radiation was properly coupled to the hollow fiber core, no fiber damage was observed. CO ₂ laser (GEM-25, Coherent-DEOS ) was aligned using HeNe laser. A HeNe laser was used to track the path of the CO ₂ laser. This allows the device to be aligned with a relatively low power laser. A ZnSe beam splitter was placed between the laser and the fiber to split the reference beam. The beam that passed through the beam splitter was concentrated using a lens assembly through the pinhole aperture before being coupled to the fiber. A Newport dual channel power meter with a GPIB / Labview computer interface was used to collect data simultaneously from the beam splitter reference beam and fiber output.

  For cutback measurements, the fiber was cut using a quick razor cutting action to produce a reasonably reproducible cut. In order to take into account residual variations in the cut, a short cut (eg, 1-2 mm) was performed around each data point where the cutback measurement data shown in FIG. 6B was recorded. The power level did not change much with the cut, but the data from the apparently inferior cut were discarded. The data shown in FIG. 6B was obtained by averaging the measurement data for each cut of one cut length. In addition, the power ratio between the transmitted beam and the reference beam measured after each cut was time averaged over several minutes.

By varying the CO ₂ laser input coupling conditions, a fiber variable mode output pattern was also observed for a fiber about 4 meters long and about 700 microns in core diameter. The fiber was nominally kept straight. The modal output pattern was observed by imaging the output beam using a Spiricon Pyrocam III. Imaging of various mode patterns suggested that the fibers operate in a relatively small number of modes (eg, these fibers have about 10 or fewer guided modes).

Additional Embodiments Several embodiments of the invention have been described. Nevertheless, it should be understood that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the claims.

1 is a cross-sectional view of one embodiment of a photonic crystal fiber waveguide. The figure which shows the refractive index profile of a part of photonic crystal fiber waveguide shown to FIG. 1A. Schematic showing one step of a method for manufacturing a photonic crystal fiber waveguide. Schematic showing one step of a method for manufacturing a photonic crystal fiber waveguide. Schematic showing one step of a method for manufacturing a photonic crystal fiber waveguide. Schematic showing one step of a method for manufacturing a photonic crystal fiber waveguide. 1 is a cross-sectional view of a confinement region of one embodiment of a photonic crystal fiber waveguide. The figure which shows the refractive index profile of the confinement area | region shown to FIG. 3A. An SEM micrograph of an example of a photonic crystal fiber waveguide. An SEM micrograph of an example of a photonic crystal fiber waveguide. The figure which shows the transmission spectrum of two different examples of a photonic crystal fiber waveguide. The highest transmission peak corresponds to the primary photonic band gap in each case, and the arrows indicate higher order band gaps. The figure which shows the transmission spectrum of one example of a photonic crystal fiber waveguide. The figure which shows the logarithm of the transmission power with respect to fiber length of one example of the photonic crystal fiber waveguide cut | disconnected by different length. The slope of the figure is dB / m. The figure which shows the transmission spectrum of the fiber bent by various curvature radii. The loss value is obtained by comparing the total power transmitted through the bent fiber with the same fiber held straight, and the bending loss as EM radiation from the CO ₂ laser passes through the photonic crystal fiber waveguide. The figure shown as a function of curvature.

Claims (26)

An article comprising a high power, low loss fiber waveguide consisting of alternating layers of different dielectric materials such as polymer and glass surrounding a core extending along the waveguide axis. The article of claim 1, wherein the alternating layers form a spiral structure. The article according to claim 2, wherein the spiral structure is a multilayer structure composed of at least two layers of the different materials surrounding the core a plurality of times. The said different material consists of a high refractive index dielectric material and a low refractive index dielectric material, The ratio of the refractive index of this high refractive index material and the refractive index of this low refractive index material is larger than 1.5. Goods. The said different material consists of a high refractive index dielectric material and a low refractive index dielectric material, The ratio of the refractive index of this high refractive index material and the refractive index of this low refractive index material is larger than 1.8. Goods. The article of claim 1, wherein the glass comprises chalcogen glass. The article of claim 6, wherein the chalcogen glass comprises As ₂ Se ₃ . The article of claim 6, wherein the polymer comprises PES or PEI. The article of claim 1, wherein the innermost layers of the alternating layers have a thickness that is less than the thickness of subsequent layers of the same material. The article of claim 1, wherein the alternating layer thickness is selected to direct EM radiation at a wavelength of about 10.6 microns along the waveguide axis. The article of claim 1, wherein the thickness of the alternating layers is selected to direct EM radiation at a wavelength in the range of about 8-12 microns along the waveguide axis. The article of claim 1, wherein the alternating layer thickness is selected to direct EM radiation with a wavelength in the range of about 2-5 microns along the waveguide axis. The article of claim 1, wherein the core is hollow. The article of claim 1, wherein, for the straight fiber waveguide, the fiber waveguide exhibits a transmission loss of less than about 1 dB / m at a selected wavelength. The article of claim 14, wherein the selected wavelength is a wavelength ranging from about 0.75 to about 10.6 microns. The article of claim 15, wherein the selected wavelength is about 10.6 microns. The article of claim 1, wherein the fiber waveguide exhibits a transmission loss of less than about 1.5 dB at a selected wavelength when bent 90 degrees at any bend radius in the range of about 4-10 cm. The article of claim 17, wherein the selected wavelength is a wavelength ranging from about 0.75 to about 10.6 microns. The article of claim 18, wherein the selected wavelength is about 10.6 microns. The article of claim 1, wherein the fiber waveguide is capable of directing EM radiation at a selected wavelength with a power density of about 300 W / cm ² or greater along the waveguide axis. 21. The article of claim 20, wherein the selected wavelength is a wavelength ranging from about 0.75 to about 10.6 microns. The article of claim 21, wherein the selected wavelength is about 10.6 microns. Even when the fiber waveguide is smoothly bent at about 90 degrees with a bending length of at least 0.3 m, the fiber waveguide has a power density of about 300 W / cm ² or more at the selected wavelength of the selected wavelength. 21. The article of claim 20, having the ability to direct EM radiation along the waveguide axis. The article of claim 1, wherein the fiber waveguide is capable of directing the EM radiation at a selected wavelength with a power of about 25 W or greater along the waveguide axis. 25. The article of claim 24, wherein the selected wavelength is a wavelength ranging from about 0.75 to about 10.6 microns. 25. The article of claim 24, wherein the selected wavelength is about 10.6 microns.

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