Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/02—Optical fibres with cladding with or without a coating
  • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
  • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
  • G02B6/02114—Refractive index modulation gratings, e.g. Bragg gratings characterised by enhanced photosensitivity characteristics of the fibre, e.g. hydrogen loading, heat treatment
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C13/00—Fibre or filament compositions
  • C03C13/04—Fibre optics, e.g. core and clad fibre compositions
  • C03C13/045—Silica-containing oxide glass compositions
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C13/00—Fibre or filament compositions
  • C03C13/04—Fibre optics, e.g. core and clad fibre compositions
  • C03C13/045—Silica-containing oxide glass compositions
  • C03C13/047—Silica-containing oxide glass compositions containing deuterium
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C25/00—Surface treatment of fibres or filaments made from glass, minerals or slags
  • C03C25/60—Surface treatment of fibres or filaments made from glass, minerals or slags by diffusing ions or metals into the surface
  • C03C25/607—Surface treatment of fibres or filaments made from glass, minerals or slags by diffusing ions or metals into the surface in the gaseous phase
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C25/00—Surface treatment of fibres or filaments made from glass, minerals or slags
  • C03C25/62—Surface treatment of fibres or filaments made from glass, minerals or slags by application of electric or wave energy; by particle radiation or ion implantation
  • C03C25/6206—Electromagnetic waves
  • C03C25/6226—Ultraviolet
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C3/00—Glass compositions
  • C03C3/04—Glass compositions containing silica
  • C03C3/06—Glass compositions containing silica with more than 90% silica by weight, e.g. quartz
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2201/00—Glass compositions
  • C03C2201/06—Doped silica-based glasses
  • C03C2201/20—Doped silica-based glasses containing non-metals other than boron or halide
  • C03C2201/21—Doped silica-based glasses containing non-metals other than boron or halide containing molecular hydrogen
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2201/00—Glass compositions
  • C03C2201/06—Doped silica-based glasses
  • C03C2201/20—Doped silica-based glasses containing non-metals other than boron or halide
  • C03C2201/24—Doped silica-based glasses containing non-metals other than boron or halide containing nitrogen, e.g. silicon oxy-nitride glasses
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2201/00—Glass compositions
  • C03C2201/06—Doped silica-based glasses
  • C03C2201/20—Doped silica-based glasses containing non-metals other than boron or halide
  • C03C2201/26—Doped silica-based glasses containing non-metals other than boron or halide containing carbon
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2201/00—Glass compositions
  • C03C2201/06—Doped silica-based glasses
  • C03C2201/30—Doped silica-based glasses containing metals
  • C03C2201/31—Doped silica-based glasses containing metals containing germanium
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
  • C03C2201/00—Glass compositions
  • C03C2201/60—Glass compositions containing organic material

Definitions

• the present invention relates to an apparatus and method for increasing the photosensitivity of glassy materials. Specifically, in one aspect, the present invention comprises a method for rapidly diffusing hydrogen or deuterium into an optical fiber from a gas mixture having a low total hydrogen content.
• Optical fibers and optical fiber devices are widely used in signal transmission and handling applications.
• Optical fiber-based devices are vital components in today's expanding high- volume optical comm mications infrastructure. Many of these devices rely on fiber Bragg gratings (FBG's) to manipulate light.
• FBG fiber Bragg gratings
• An FBG is an optical fiber with periodic, aperiodic or pseudo- periodic variations of the refractive index along its length in the light-guiding region of the waveguide.
• the ability to produce these refractive index perturbations in a fiber is a key to the manufacture of FBG's and, hence, a number of optical components, such as optical sensors, wavelength-selective filters, and dispersion compensators.
• Photosensitivity is defined as the effect whereby the refractive index of the glass is changed by actinic radiation-induced alterations of the glass structure.
• actinic radiation includes visible light, UV, PR. radiation and other forms of radiation that induce refractive index changes in the glass.
• a given glass is considered to be more photosensitive than another when a larger refractive index change is induced in it with the same delivered radiation dose.
• the level of photosensitivity of a glass detemiines how large an index change may be induced upon exposure to photonic radiation and therefore places limits on grating devices that may be fabricated practically. Photosensitivity also affects the speed in which a desired refractive index change may be induced in the glass by a given radiation intensity. By increasing the photosensitivity of a glass, one may induce larger index perturbations in it at a faster rate.
• the intrinsic photosensitivity of silica-based glasses, the main component of high-quality optical fibers, is not very high. Typically index changes of only about 10 "5 are possible using standard germanium doped fiber.
• gratings written today by industry involve about 5 cm (2 inches) or less of the length of a fiber, depending on the type of grating to be written.
• Traditionally it has been taught to place an entire length of optical fiber in a vessel containing pure hydrogen or pure deuterium atmospheres at certain temperatures and pressures.
• the grating manufacturing process usually entails a first process of placing a fiber spool in a hydrogen or deuterium containing vessel, placing the vessel in an oven and loading the entire fiber with hydrogen through the fiber's polymer coating. Once the length of fiber has been hydrogen-loaded, the coating is stripped
• the fiber is typically exposed to a pure hydrogen atmosphere for several days and, in some cases, for several weeks. Exemplary exposures include 600 hours (25 days) at 21°C and 738 atm or 13 days at 21°C at 208 atm. Obviously, such long exposures extend the time required to fabricate optical devices that rely on photosensitive glass.
• 3M Company developed a process for accelerated hydrogen loading using higher temperatures and/or pressures. United States Application Serial No. 10/028,837, which is herein incorporated by reference, describes such a process. Pure hydrogen or deuterium loading atmospheres, under any conditions, present safety and cost concerns. Hydrogen is highly flammable.
• Deuterium is often used due to the resulting improvements in loss due to absorption at wavelengths of interest in telecommunications applications.
• the cost of deuterium is high, and a more efficient use of the gas is preferred. It would be desirable to be able to benefit from the hydrogen loading photosensitization effect while reducing some of the associated risks and costs.
• At least one aspect of the current invention is a method for increasing the photosensitivity of a glassy material comprising placing the glassy material in a pressure chamber; pressurizing the chamber with a mixture of gases, the mixture comprising hydrogen and at least one diluent gas; wherein the hydrogen in the mixture has a first partial pressure and the diluent gas in the mixture has a second partial pressure; and exposing the glassy material to the gas mixture at a prescribed temperature and total pressure.
• At least one aspect of the current invention is a method for manufacturing an optical device comprising placing a glassy material in a pressure chamber; pressurizing the chamber with a mixture of gases, the mixture comprising hydrogen and at least one diluent gas; exposing the glassy material to the gas mixture at a prescribed temperature and total pressure; and irradiating the glassy material with actinic radiation, h at least one embodiment of the invention, the optical device may be an optical grating and the actinic radiation may be patterned.
• At least one aspect of the current invention is an optical fiber having increased photosensitivity produced by the method comprising placing the optical fiber in a pressure chamber; pressurizing the chamber with a mixture of gases, the mixture comprising hydrogen and at least one diluent gas; hydrogenating the optical fiber at a prescribed temperature and total pressure; and exposing the optical fiber to a pattern of actinic radiation.
• At least one aspect of the current invention is an optical device prepared by a method comprising placing the glassy material in a pressure chamber; pressurizing the chamber with a mixture of gases, the mixture comprising hydrogen and at least one other diluent gas; exposing the glassy material to the gas mixture; and irradiating the glassy material with actinic radiation.
• the optical device may be an optical grating and the actinic radiation may be patterned.
• At least one aspect of the current invention relates to a method of utilizing hydrogen partial pressures near or below 0.1 MPa (one atmosphere) in a high pressure gas mixture, which can generate changes in the refractive index of a glassy material in excess of 7x10 "5 , using conventional fibers. Sensitizing fibers with such low partial pressures of H 2 or D 2 allows significant cost savings and a reduction of some safety concerns while permitting fibers to be sufficiently sensitized.
• hydrogen as used herein generally refers to hydrogen gas (H ), but also includes deuterium gas (D 2 ) and hydrogen-deuterium gas (HD).
• liquid gas as used herein is a gas or supercritical fluid that does not chemically react with hydrogen or the glassy material under the process conditions.
• partial pressure refers to the molar fraction of a component in a mixture of gasses or supercritical fluids multiplied by the total pressure of the mixture.
• enhancement factor or “amplification factor” as used herein is defined as the ratio of the pressure of a pure hydrogen-loaded sample divided by the partial pressure of hydrogen in a mixed gas sample, which would result in the same level of internal hydrogen content in the fiber (as measured by either loss measurements, or photosensitivity measurements).
• a refractive index change due to UV exposure above about 5xl0 "5 is desirable.
• a typical grating-quality fiber in a pure high pressure hydrogen atmosphere, can be photosensitized by exposure at 60°C for 3 days to give an index change of 1 x 10 "3 .
• accelerated photosensitization of the same fiber may be done at high temperatures, e.g., 260°C, for 10 minutes in a pure high pressure hydrogen atmosphere, resulting in an index change of 4x10 "4 .
• the present invention utilizes hydrogen partial pressures less than 1 MPa, more typically near or below 0.1 MPa (one atmosphere) to generate changes in the refractive index of a glassy material in excess of 7x10 "5 using conventional fibers in a high pressure gas mixture that includes hydrogen.
• Sensitizing fibers with such low partial pressures of H 2 , D , or HD allows significant cost savings and a reduction of some safety concerns while permitting fibers to be sufficiently sensitized. This photosensitization process applies to other material systems, such as planar waveguides.
• FIG. 1 is a simplified schematic diagram of an embodiment of a hydrogen loading apparatus 10 in accordance with the present invention.
• the hydrogen loading apparatus 10 includes a pressure vessel 12, a hydrogen source 14 and a diluent gas source, h one exemplary embodiment of the present invention, the vessel 12 is a high-pressure gas chamber, capable of withstanding gas pressures as large as about 20 MPa.
• the apparatus 10 further includes a heater unit 16 and accompanying insulation 18 placed around the pressure vessel 12.
• a source of a purge gas 22, such as N 2 may also be provided.
• a glassy material such as planar waveguides, optical fiber, and the like is placed in a pressure chamber.
• the chamber is then typically pressurized to at least about 40 MPa, more preferably at least 100 MPa, with a mixture of gases, the mixture comprising hydrogen and at least one diluent gas.
• the pressure, temperature, and composition of this mixture are selected such that the fugacity of the hydrogen in the mixture is greater than the fugacity of pure hydrogen under the same partial pressure and temperature conditions.
• the fugacity of the hydrogen in the mixture is at least twice that of pure hydrogen. In another embodiment, it is at least five times greater.
• This mixture is used to hydrogenate the glassy material.
• the initial partial pressure of hydrogen in the mixture is typically less than 1 MPa and the total pressure and the volume concentration of hydrogen in the gas mixture is typically less than or equal to 4%.
• the gas mixture can be formed either by pressurizing the chamber from a source containing already mixed gasses, with a predetermined concentration ratio, or by forming the mixture in situ, hi the case where the chamber is first filled with pure hydrogen and then a diluent gas is added, the initial pressure of pure hydrogen will be less than the initial partial pressure of hydrogen after it has been mixed with the diluent gas.
• This excluded volume effect is an increase in the partial pressure of a component in a mixture of gasses (not predicted by the ideal gas law), which is observed when a diluent gas is added and the pressure of the mixture is increased.
• the increases in the fugacity of hydrogen in dilute high pressure mixtures which are taught in this invention are distinct from the excluded volume effect.
• the diluent gas is selected from the group consisting of noble gases (argon (Ar), neon (Ne), and the like), partially or completely halogenated (especially fluorinated) hydrocarbons, carbon monoxide, carbon dioxide (CO 2 ), nitrogen (N 2 ), nitrous oxide (N 2 O), small hydrocarbons (methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ) and the like), sulfur hexafluoride and other substantially non-reactive gasses which significantly increase the fugacity of hydrogen when used as the major component in a mixture with hydrogen.
• noble gases argon (Ar), neon (Ne), and the like
• partially or completely halogenated (especially fluorinated) hydrocarbons carbon monoxide, carbon dioxide (CO 2 ), nitrogen (N 2 ), nitrous oxide (N 2 O), small hydrocarbons (methane (CH 4 ), ethane (C 2 H 6 ), propane (C 3 H 8 ) and the like
• the method of photosensitizing a glassy material may comprise a further step of heating the gas mixture to at least 50°C, more preferably to at least 80°C.
• the gas mixture may be heated to at least 250°C.
• the resulting photosensitive glassy material typically has the first overtone hydrogen absorption peak value greater than lxl 0 "3 dB/m.
• the hydrogen loading is in direct relation to the fugacity of the hydrogen during the loading process, hi systems with diluent gasses, the fugacity of the hydrogen will depend on the total pressure, the temperature of the system, the identity of the gasses, and the partial pressure of the hydrogen.
• the resulting photosensitive glassy materials, or portions thereof, may then be exposed to actinic radiation to alter their refractive index.
• actinic radiation, or portion of fiber exposed to actinic radiation may be patterned.
• hydrogen-loaded fibers may be used to create optical devices including optical gratings such as Bragg gratings and Bragg grating-based devices.
• the concentration of hydrogen that diffused into the fibers was measured and quantified using two different methods.
• the absorbance peak ( ⁇ ) at 1.24 microns due to hydrogen in the fiber core was measured using the cutback method described hereafter.
• Hydrogen absorption in germano-silicate fibers has a characteristic absorbance peak due to a first absorption overtone at 1.24 microns.
• the concentration of hydrogen diffused into the fiber under different loading conditions may be calculated by measuring the absorbance peak ( ⁇ ).
• the measurement requires launching a broadband light source having a wavelength ( ⁇ ) between about 1.2 microns and 1.3 microns (83437-A, Agilent Technologies, Palo Alto, CA) through the fiber and measuring the changes over distance using an optical spectrum analyzer (AQ 6315-A, Ando Electric Co., Ltd. Tokyo, Japan).
• the method known as the cutback method involves coupling fiber to the source and measuring the power out of the far end. The fiber is then cut near the detector, reconnected, and the power measured again.
• the absorbance peak (in dB/m) may be detennined by calculating [(P B - Ps)/L] .
• the average values for calculating the enhancement are compared.
• the refractive index change due to UV-writing a Bragg grating in the photosensitized fiber was calculated from a measurement of the grating strength (in transmission) as a function of the write time.
• the UV- induced refractive index change in the fiber is directly related to its photosensitivity and to the concentration of hydrogen diffused into the fiber.
• a frequency doubled C Ar+ laser (Sabre® FreDTM laser, Coherent, Santa Clara, CA) operating at 244 nm was the UV source used for writing grating in the fibers hydrogen loaded under different conditions.
• An FBG (fiber Bragg grating) fabrication system based on a Talbot interferometer was used to write gratings in the test fibers at a fixed UV power of 100 mW with a spot size of 1 mm x 0.1 mm.
• the grating growth was monitored in transmission as a function of time during the grating inscription using a computer controlled optical spectrum analyzer (Q 68384, Advantest Corporation, Tokyo, Japan).
• the refractive index modulation An induced in fibers as a function of time during the grating inscription may be calculated from the Bragg wavelength ⁇ # , the grating length L g and the transmission minimum of the grating T m in as
• An(t) [ ⁇ B / ⁇ L g jtanh "1 [jl -T mh ⁇ ⁇ . _
• a glassy material such as a spool 30 of silica glass optical fiber having a Ge and/or B-doped core and/or cladding was provided.
• Suitable fibers may be readily obtained from companies such as Corning, Inc. of Corning, NY. Methods for the manufacture, doping and coating of optical fibers are well known to those skilled in the art.
• a glassy material is defined as a material having no long-range structural order and being sufficiently solid and rigid enough not to exhibit flow on an observable time scale.
• the control samples were prepared using a contemporary hydrogen loading process with a pure, high pressure hydrogen atmosphere.
• the coated optical fibers to be hydrogenated were wound on a spool 30 and the spool 30 was placed into a pressure vessel 12.
• the vessel was then purged with nitrogen (Air Products and Chemicals Inc., Allentown, PA) tliree times and heated up to 80°C.
• the 80°C vessel was filled with hydrogen (Air Products and Chemicals Inc., Allentown, PA) up to the desired pressure (as indicated in Tables 1 and 2), and the fiber was then exposed to the hydrogen-containing atmosphere for 24 hours.
• the pressure vessel was vented and the spools 30 were removed quickly from the vessel 12 and cooled rapidly by placing into a freezer at -40°C where the fiber was stored until the degree of photosensitization or hydrogen incorporation could be evaluated or until used to create an optical device.
• the absorbance per unit length at 1.24 microns (“loss”, or ⁇ ) was determined using the first test method, described above.
• the sensitivity to grating writing (“dn”, or “ ⁇ n”) was determined using the second test method, described above.
• some fibers from the same lot of each fiber type were loaded with one part-per-thousand (ppt) concentration of high-purity grade compressed hydrogen gas pre-mixed with either argon or carbon dioxide gases (Air Products and Chemicals Inc., Allentown, PA).
• ppt part-per-thousand
• the purged 80°C vessel was filled with the mixture at various pressures up to about 100 MPa.
• the 80°C vessel was filled with hydrogen up to about 1 MPa and then vented to about 0.1 MPa (atmospheric pressure). The vessel was then pressurized to about 100 MPa total pressure with the diluent gas, and the pressure was recorded.
• Peng-Robinson equations of state were used to calculate the partial pressure of the hydrogen.
• the fiber was then exposed to the low partial pressure hydrogen-containing atmosphere for 24 hours at the desired photosensitization temperature. After 24 hours, the pressure vessel was vented and the fibers were removed quickly from the vessel 12 and cooled rapidly by placing into a freezer at -40°C where the fiber was stored until the degree of photosensitization or hydrogen incorporation could be evaluated or until the fiber is used to create an optical device.
• Some SMF-28TM optical fibers and PureModeTM HI 1060 optical fibers were exposed to pressures of about 100 MPa of mixtures formed from 0.1 MPa hydrogen with addition of one of the following gasses to a total pressure of 100 MPa: N 2 O (nitrous oxide), CH 4 (methane) or C 2 H 6 (ethane).
• the resulting fibers were measured both for their absorbance per unit length at 1.24 microns ("loss") and their sensitivity to grating writing ("dn"). These were compared to data from fibers sensitized with pure hydrogen. As before, these results were used to calculate the pressure of pure hydrogen required to achieve the observed level of both the loss and the dn.
• the loss amplification factor for nitrous oxide is about 11. For methane, it is about 45, and for ethane it is about 54. The values obtained from the ⁇ n comparisons were typically even larger.
• Tables 1 and 2 show the photosensitization results comparing the effects of the contemporary hydrogen loading process to the presently disclosed low pressure hydrogen loading process of SMF-28TM optical fiber and PureModeTM HI 1060 optical fiber, respectively.
• the first column in the tables designates the means by which the gas mixture was generated.
• the "pure” designation indicates that the chamber was pressurized with pure H 2 .
• the "premix” designation indicates that the gas mixture was prepared by the gas supplier and used as supplied to pressurize the chamber.
• the “in situ” designation indicates the process of adding about 1 MPa of hydrogen to the vessel, venting the chamber to about 0.1 MPa, and finally pressurizing the chamber to the desired level with the chosen diluent gas.
• the second column in tables 1 and 2 indicate the diluent gas used in the given experiment.
• the third column is the mole percent of H 2 charged to the vessel.
• the fifth column is the partial pressure of H 2 at the prescribed pressure (column 4) and the loading temperature of 80°C.
• the sixth column is the loss as measured using the cutback method.
• the seventh column is a calculated value of the hydrogen pressure that would be required in a pure hydrogen atmosphere to achieve the same loss numbers as measured for the low partial pressure system.
• the eighth column is the loss amplification factor, which is a ratio of the calculated hydrogen pressure number from column 6 and the actual partial pressure of hydrogen in the system (column 5).
• the ninth column is the measured change in refractive index of the glass after writing a Bragg grating with U-V radiation for five minutes.
• the tenth column is a calculated value of the hydrogen pressure that would be required in a pure hydrogen atmosphere to achieve the same refractive index change as measured for the low partial pressure system.
• the eleventh column is the dn amplification factor, which is a ratio of the calculated hydrogen pressure number from column 9 and the actual partial pressure of hydrogen in the system (column 5).
• the photosensitivity of a hydrogen loaded material may be influenced significantly by using mixtures of gasses rather than pure hydrogen during the loading process.
• hydrogen concentrations in the glassy material were thirty to fifty times higher than expected based on the partial pressure of hydrogen present in the system.
• the concentration of hydrogen in the core of the fiber was discovered to strongly influence the selection of the diluent gas, with some diluent gasses showing 30 times the effect of other diluent gasses at similar conditions of time, temperature, and pressure. Because the choice of diluent gas influenced the sensitization, this amplification effect cannot be attributed to increases in partial pressure and the excluded volume effect alone.
• these enhancements are due to the increased fugacity of the hydrogen in the mixtures relative to the fugacity of pure hydrogen at the same partial pressure and temperature.
• Such a level of amplification allows the hydrogen or deuterium to be used at very low partial pressures, even at or below 0.1 MPa (one atmosphere) under conditions where the sensitization of pure hydrogen is negligible.
• Such low partial pressures allow significant cost savings as well as a reduction of some safety concerns while pe ⁇ nitting fibers to be sufficiently sensitized to permit the manufacture of an optical device in the photosensitized material.
• pe ⁇ nitting fibers may be used in the manufacture of a variety of optical components.

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