Abstract:
A length of oxynitride optical fiber is exposed to actinic radiation that is modulated by an interference technique to form a pattern of refractive index variations that functions as a reflective grating.

Description:
This application claims priority to U.S. Provisional Application No. 60-053,863, filed Jul. 25, 1997, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the application of the photorefractive effect to the fabrication of optical devices based on oxynitride glass, and more particularly, to photoinduced Bragg gratings in oxynitride optical fibers. 
     Reflection gratings are often implemented as waveguides which have a path region having a modulated refractive index. The waveguiding structure is often in the form of a fiber. The modulation preferably takes the form of alternate regions of higher and lower refractive index. These periodic variations in refractive index act as a Bragg grating, and they selectively reflect light having a wavelength of twice the spacing. Such gratings can be used to filter, to define laser cavities and as components in multiplexers and demultiplexers. 
     Photoinduced Bragg gratings have been made in a variety of ways. One approach, which is disclosed in U.S. Pat. No. 4,725,110, is to direct two interfering beams of ultraviolet radiation through the cladding of an optical fiber to form an interference pattern along the germania-doped glass core. Other techniques involve subjecting regions of a fiber core to ultraviolet radiation through an amplitude mask or a phase mask. U.S. Pat. No. 5,287,427 teaches that the refractive index effect is enhanced by exposing that part of the glass that is to be subjected to actinic radiation to hydrogen or deuterium. 
     Germania-doped silica has shown the greatest refractive index change (Δn) after being subjected to actinic radiation. For various reasons attempts have been made to make gratings from photosensitive materials other than germania, a relatively scarce, expensive constituent. An object of the invention is to provide reflective gratings that are formed from commonly occurring, inexpensive materials. Another object of the invention is to provide a germania-free glass from which reflection gratings can be made. 
     Reflective gratings have been made from other UV sensitive oxides that are less effective than germania. It is disclosed in WO 94/00784 that photoinduced ratings can be made from B 2 O 3  in combination with SiO 2  or GeO 2 . The publication, Kitagawa et al., OFC Vol.4 of 1994 OSA Technical Digest Series, paper PD-17 teaches that gratings can be made by pulsing optical fibers having P 2 O 5 —SiO 2  cores with 193 nm radiation. U.S. Pat. No. 5,478,371 teaches a technique for forming gratings in P 2 O 5  doped optical fiber with 248 nm radiation. Such photosensitive oxides can be used alone or in combination with other photosensitive oxides such as germania. A further object of the invention is to provide a photosensitive material that can be used in combination with other photosensitive materials to form reflection gratings. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention relates to an optical device comprising a nitrogen-doped silica glass region having a pattern of photo-altered refractive index variations. The pattern of refractive index variations preferably takes the form of alternate regions of higher and lower refractive index, the period of which is such that the pattern constitutes a reflection grating. The nitrogen-doped silica glass region can be the core region of an optical waveguide, the core region being at least partially surrounded by a cladding, the optical waveguide comprising a portion wherein the core region has a refractive index that varies in a longitudinal direction, the index varying such that the portion of the waveguide reflects radiation of a predetermined wavelength propagating longitudinally in the waveguide. The optical waveguide can be an optical fiber or a planar device. 
     The present invention also relates to a method of making an optical component. The method comprises (a) providing a body at least a portion of which comprises silicon oxynitride glass, and (b) exposing at least a part of the portion to actinic radiation such that the refractive index of the exposed part is changed. The change in refractive index of the irradiated region is enhanced by impregnating the irradiated region with an atmosphere comprising hydrogen or deuterium. To make a reflective grating, the irradiated region is exposed to a modulated intensity of actinic radiation whereby the refractive index thereof is modulated to reproduce the intensity pattern of the radiation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates an optical fiber having a grating pattern in its core. 
     FIG. 2 schematically illustrates a planar optical waveguide having a grating pattern in its core. 
     FIG. 3 is a schematic diagram of a reflectivity measuring optical circuit. 
     FIGS. 4 and 5 are graphs of reflectivity plotted as a function of wavelength for the reflective gratings in an oxynitride optical fibers of Examples 1 and 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to an optical device comprising a region of silicon oxynitride glass exhibiting a photorefractive effect resulting from the alteration of the refractive index of the glass region resulting from exposing it to at least one beam of actinic radiation for sufficient time to increase the refractive index of that portion of the glass region upon which the beam impinges. The oxynitride glass device is formed by conventional techniques. Silicon oxynitride planar waveguides are usually synthesized by plasma and nonplasma CVD processes, e.g. see F. Bruno et al. “Plasma-enhanced chemical vapor deposition of low-loss SiON optical waveguides at 1.55 m wavelength”, Applied Optics, vol. 30, pp. 4560-4564, 1991. Nitrogen doped optical fibers have been drawn from preforms synthesized by reduced-pressure plasmachemical deposition (SPCVD), see, eg. E. M. Dianov et al. “Low-Hydrogen Silicon Oxynitride Optical fibers Prepared by SPCVD”, Journal of Lightwave Technology, 1995 LT13, (7), pp. 1471-1474. Such a SPCVD process may result in trace amounts of chlorine (less than 1 atomic %). 
     FIG. 1 shows an optical fiber  12  having a silicon oxynitride core  14  and a silica cladding  16 . The core  14  contains a Bragg reflection grating  18  written therein by application of actinic radiation having a linear sequence of intensity peaks. A Bragg reflection grating can similarly be written into the core of a planar waveguide as shown in FIG. 2 wherein planar waveguide  22  includes a core  24  in the surface of substrate  26 . Core  24  includes a pattern  28  of refractive index variations that function as a Bragg grating. 
     EXAMPLE 1 
     This example illustrates that a grating can be formed in an oxynitride optical fiber without impregnating it with hydrogen. 
     The optical fiber employed in this example had a core diameter of about 2 μm and an outside diameter of about 124 μm. The composition of the core was silica doped with 3.15 atomic percent nitrogen, and the cladding consisted of silica, whereby the value of Δn was about 0.042. 
     A grating was formed in the oxynitride optical fiber using a KrF excimer laser operating at a wavelength of 248 nm and a Lasiris uniform phase mask (Λ=1069 nm). The exposure was 10 minutes at 10 Hz with a fluence of about 120 mJ/cm 2 /pulse. The grating length was 19 nm, and the peak reflectivity was about 0.1%. Therefore, the total index change (assuming perfect fringe contrast) was about 1.5×10 −6 . 
     The circuit of FIG. 3 was employed to analyze the reflectivity of the resultant grating. The oxynitride fiber  12  having grating  18  was fused to an output pigtail of 3 dB coupler  46 . An Er-doped fiber amplifier  48  and an optical signal analyzer  50  were respectively connected to the two input pigtails of coupler  46 . The end of oxynitride fiber  12  and the remaining coupler output pigtail  52  were provided with antireflection terminations  56  and  58 , respectively. Coupler  46  coupled a portion of the amplified spontaneous emission from fiber amplifier  48  to fiber  12 . A portion of the signal that reflected from grating  18  was coupled to optical signal analyzer  50 . As shown in FIG. 4, the reflected signal is centered about 1536.6 nm. 
     EXAMPLE 2 
     A grating was formed in an oxynitride optical fiber  12  by the following method. The optical fiber was made by the SPCVD process described in the E. M. Dianov et al publication. The core diameter and outside diameter of the fiber were about 8 μm and 125 μm, respectively. The composition of the core  12  was silica doped with 0.9 atomic percent nitrogen, and the cladding  16  consisted of silica. 
     The fiber was subjected to hydrogen loading to increase the refractive index change in accordance with the teachings of U.S. Pat. No. 5,287,427, which is incorporated herein by reference. The hydrogen loading was done at room temperature at 100 atmospheres pressure. 
     The fiber was then exposed to an interference pattern in a side exposure geometry in accordance with the teachings of U.S. Pat. Nos. 4,725,110 and 4,807,950, which are incorporated herein by reference. The beam was derived from an excimer-pumped frequency doubled dye laser. The grating as written using a 10 minute exposure at 240 nm at a pulse rate of 10 Hz. The energy density is estimated to be 0.1 to 0.2 Joules per cm 2 . 
     The reflectivity of the grating produced in accordance with Example 2 was analyzed in the circuit of FIG.  3 . As shown in FIG. 5, the reflectivity obtained from FIG. 5 is about 0.2%, which corresponds to a refractive index change of Δn=4.5×10 −6  in grating  18  as compared with the unmodified refractive index of core  14 . 
     The reflectivity of gratings formed by the above-described methods can be changed by modifying various parameters. The hydrogen concentration in the fiber during UV exposure could be increased to an extent that reflectivity is increased by about 3-4 times that achieved in Example 2. Furthermore, a higher exposure could be employed to increase reflectivity; both peak fluence and total dose could be increased. Moreover, a shorter wavelength exposure, e.g. 215 nm exposure, might improve reflectivity; this is the case for the Si—O—P bond in the SiO 2 —P 2 O 5  system. Approximately 0.1 to 10 atomic percent nitrogen is a preferred range of the nitrogen doped silica glass, with about 0.5 to 4 atomic percent nitrogen more preferred, and about 8 to 3.25 atomic percent nitrogen most preferred.