Production device and production method for an optical device component having a grating structure

A production device and a production method for a grating-type optical component enabling formation of a variety types of FBGs using a single phase mask and an optical component made by the production method or production device for a grating-type optical component are provided. The method involves diffusing at least one of hydrogen or deuterium into an optical fiber and altering the refractive index of the optical fiber by irradiating the fiber with non-interfering UV lamp light.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical waveguide having photosensitivity in the core thereof, and more specifically to a production device for a grating-type optical component and a production method for a grating-type optical component, the properties of which are adjusted by changing the refractive index thereof using ultraviolet light (hereinafter “UV”) and to an optical component made using the production device for a grating-type optical component or production method for a grating-type optical component.

2. Description of the Related Art

Quartz is a material having excellent optical transmission qualities and is therefore used in a variety of applications such as optical lenses or waveguides for optical transmission or the like. The material of an optical fiber as an optical communications line is silica based, when producing functional optical components such as an optical wavelength selection filter, an optical splitter, a spectral separator or attenuator or the like in that line, there are merits, in terms of compatibility with optical fiber (the refractive index, core diameter, and fusion point when making a fusion connection), to produce such functional components of quartz.

In an optical waveguide such as a planar light wave circuit (PLC) or an optical fiber including photosensitive material such as Ge, phosphorus or boron added in the core, a grating is formed by irradiating UV light of an appropriate wavelength into the optical waveguide from the side thereof so as to alter the refractive index inside the core periodically, in a longitudinal direction; such gratings comprise optical components used as an above-mentioned wavelength selection filter. As shown inFIG. 1A, in accordance with the desired objective, the refractive index is varied at a determined periodicity following in a longitudinal direction of the optical waveguide100. Further, besides a gradual, successive alteration, this alteration may be of an irregular or discontinuous periodicity.

The grating102formed in the core101of the optical fiber100is called an optical fiber grating. Normally, the optical fiber grating is classified by the period of refractive index variation. One is long period grating whose period of refractive index variation is above 100 μm, and the other is fiber Bragg grating (hereafter “FBG”) whose period of refractive index variation is below a few micro meters. These are important optical components in the field of optical transmission.

In the description following, optical fiber refers to optical waveguides. In the same manner, FBG in the description refers to a grating formed inside an optical waveguide.

When the period of refractive index variation of an FBG formed in a core is determined as Λ, wavelength λ of light reflected at the FBG satisfying the expression
λ=2·neff·Λ0(1).

Here, neffis the effective refractive index of the FBG and neffis nearly equal to 1.46 at the silica-based core. As an example, if the above expression (1) is applied to wavelength λ=1550 nm used in public (commercial) optical transmission networks, then FBG pitch Λ0≈500 nm=0.5 μm is obtained.

A conventional method for producing an FBG will now be described. Referring toFIG. 2A, firstly optical fiber127is disposed inside pressurized container111. A kilometer or from several hundred to tens of meters of reeled optical fiber covered with protective coating, optical fiber covered with protective coating cut into several meter lengths, or optical fiber127as shown in the drawing cut into several meter lengths having a part of the covered protective coating material part129removed to expose the inner part are all suitable for use as the optical fiber127.

Next, in a condition loaded with hydrogen (H2) or deuterium (D2) and in a pressurized condition (e.g.: 10 MPa-30 MPa), high-pressure hydrogen113or deuterium is diffused through the cladding125of the optical fiber127reaching the core123. This process is known as hydrogen diffusion treatment.

The object of the above hydrogen diffusion treatment is that if hydrogen or deuterium are diffused into the core123of the optical fiber127then, as will be described subsequently, the photosensitivity of the core123can be increased when an interfering UV laser beam is radiated to the core123. In other words, it is known that when imprinting an FBG, defusing hydrogen or deuterium in a core, here core123, raises the speed of the increase of the refractive index approximately fiftyfold in comparison to a core that has not been diffused with hydrogen or deuterium. It is well known that in such a condition, raising the temperature inside the pressurized container above room temperature raises the speed of this diffusion.

When, in this hydrogen diffusion treatment, the optical fiber127has been diffused with hydrogen or deuterium, the covering material part129must be removed to radiate UV laser rays therein. This is because the covering material part129, of resin, diffused with the hydrogen, absorbs UV laser light thereby preventing the rays from reaching the core.

Next, as highly interfering UV laser light171is radiated through a phase mask173having a specific periodicity, a fringe pattern of the interference arises in the hydrogen diffused optical fiber core123; the density of energy being higher, and thereby raising the refractive index, in the bright portions of this UV pattern. Usually, interference of diffracted light of first order through the phase mask is used, the resulting interference fringe being half the period of the phase mask such that the period of the FBG is half the period of the phase mask. An FBG (hydrogen diffused)121having an uniform period can be formed in this way. The process itself is known as UV exposure processing.

As shown inFIG. 2C, the optical fiber with imprinted FBG is then placed in an oven151for a determined period of time (e.g. 12 hours) in a heated condition (120° C.) so that the hydrogen153or deuterium diffused into the optical fiber127is released to the outside. This process is called the hydrogen removal process. The optical fiber117shown inFIG. 2Cis an optical fiber with hydrogen removed through the hydrogen removal process, and the optical fiber covering part119thereof is a cladding, the hydrogen in which has been removed in the same manner.

An optical fiber having a refractive index periodically distributed at a constant pitch Λ0in the core thereof inside a cladding produced in this way, as shown inFIG. 1A, is called a uniform type FBG. In a uniform type FBG reflection occurs at multiple points in phase in relation to signal light of wavelength λisatisfying the above expression (1), among signal light propagating in the core. Appropriate applications can be found in FBG for stabilization of wavelengths of laser diodes (“LD”) or FBG for Add/Drop for adding or dropping light with specific wavelengths.

Where the pitch Λ of an FBG inside a core changes successively and gradually (e.g. Λ1-Λn), the FBG is said to be a chirped type FBG. This kind of FBG has broad bandwidth and is effective for multiple wavelengths. Appropriate applications can be found in FBG for compensating chromatic dispersion and FBG for equalizing gain after amplification by an optical amplifier.

SUMMARY OF THE INVENTION

The above-described conventional methods however, only allow for imprinting of FBG having the same type of pitch from one type of mask as the periodicity of the FBG is determined by the phase mask. Thus, in order to create FBG's having different properties, a variety of different phase masks are required leading to increased production costs.

With the foregoing in view, the present invention provides a method for production of a grating-type optical component that is superior in terms of facilitating mass production and enables formation of a variety of types of FBG using a single phase mask, a production device for producing that grating-type optical component and an optical component made using that production method or production device for a grating-type optical component.

In a first technical aspect of the present invention the method for production of a grating-type optical component includes the steps of: radiating a monochromatic light of the ultraviolet region onto a silica-based optical waveguide diffused with at least one of hydrogen or deuterium so as to alter the refractive index of the silica-based optical waveguide and radiating interfering light to the quartz optical waveguide. Further, the first aspect includes an optical component created using the production method.

In a second technical aspect of the present invention the production device for producing a grating-type optical component includes: a light source for generating a monochromatic light of the ultraviolet region, a primary irradiation system for radiating that monochromatic light to a silica-based optical waveguide diffused with at least one of hydrogen or deuterium and a secondary irradiation system for radiating interfering light to the silica-based optical waveguide. Further, the second aspect includes an optical component created using the production device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIGS. 3 and 4illustrate the production method for a grating-type optical component related to the first embodiment of the present invention.

As shown inFIG. 3Aan optical fiber27is placed inside a pressurized container111. Several kilometers or from several hundred to tens of meters of reeled optical fiber covered with protective coating, covered optical fiber cut into several meter lengths, or optical fiber27as shown in the drawing cut into several meter lengths having a part of the covering material part29removed to expose the inner part, are all suitable for use as the optical fiber27.

Step 1 Hydrogen Diffusion Process

Next, the optical fiber is left in a hydrogen (H2) or deuterium (D2) loaded, pressurized condition (e.g.: 10 MPa-30 MPa) and high-pressure hydrogen113or deuterium is diffused through the cladding25of the optical fiber27reaching the core23. Where for example the diameter of the silica-based part is 125 μm, if pressurized at 55° C., the optical fiber must remain in that condition for a period of five days.

If hydrogen or deuterium is diffused into the core23of the optical fiber27then, as will be described subsequently, the photosensitivity of the core23can be increased when an interference capable UV laser beam is radiated to the core23.

Step 2 Raising Refractive Index

As shown inFIG. 3B, non-interfering UV light (hereinafter “UV light”) is then radiated to irradiation region24aof the optical fiber27diffused with hydrogen or deuterium, using non-interfering UV lamp light131that radiates over a broad area. An excimer lamp that generates incoherent, monochromatic light is one example of a preferred light source for this non-interfering UV lamp light. Here, non-interfering refers to incoherent light that does not create interference.

In step 2 above, as the UV light is irradiated, the power of this radiation is adjusted allowing the refractive index of the optical fiber27to be adjusted. In addition to adjusting the power of light radiated from a light source, adjusting the integrated power of radiation here can be performed by adjusting the duration thereof or by using the following methods for performing adjustment of the refractive index.

For example, as shown inFIG. 5A, raising the base line level of refractive index (refractive index profiles)550, of the entire optical fiber by radiating a fixed power of non-interfering UV light501over the entire optical fiber507can be used.

Further, as shown inFIG. 5B, a changing of the refractive index in a longitudinal direction of an optical fiber507in which the degree of increase in the refractive index is adjusted in different portions of the optical fiber507by radiating varying quantities of non-interfering UV light502to those portions can also be used. This raising the refractive index551results in a transition in the refractive index as indicated by the sloping line inFIG. 5B.

Again, as shown inFIG. 5C, there can be a raising of the refractive index552applied to only a part of an optical fiber, achieved by radiating only specific portions of the optical fiber with UV light503by using an amplitude mask175having slits disposed in parts thereof, or using two masks175to block part of the UV light radiated.

Further, as shown inFIG. 6A, there can be a raising of the refractive index553in only a part of one side of an optical fiber507by using an amplitude mask175having slits that maintains a constant power of UV light504.

As shown inFIG. 6B, by radiating UV light505via an amplitude mask175in which the rate of permeation of light is disparate in different places, the degree of the increase in the refractive index can be adjusted in relation to a location, thereby enabling the refractive index to be altered following a longitudinal direction of the optical fiber507. This raising of the refractive index from the bottom up554can be adjusted with changes in the properties of the amplitude of the adjusting mask used, as illustrated by the sloping line in the example onFIG. 6B.

Moreover, as shown inFIG. 6C, the refractive index can be altered following a longitudinal direction of the optical fiber507by adjusting the duration of exposure of the UV light through adjusting the speed of movement of the amplitude mask175having slits, that maintains a constant power of UV light506radiated, as that mask is moved. This raising of the refractive index from the bottom up555, can be adjusted by controlling the movement178of the amplitude mask175, as illustrated by the sloping line in the example ofFIG. 6C.FIG. 3Billustrates the case where the method ofFIG. 6Cis used.

Although the required wavelengths of UV light used in order to obtain the above described changes in the refractive index is dependent to some degree on the sensitizing material added, normally wavelengths of below 280 nm are required. This is because it is difficult to obtain the desired refractive index with extremely low photosensitivity. On the other hand, at below 150 nm, there is an extremely high rate of light absorption by pure quartz having no sensitizing material, such that light cannot penetrate; accordingly the wavelength of radiated light must be above 150 nm.

For this reason light sources generating light of wavelengths in the region between 150 and 280 nm from among those excimer lamps shown inFIG. 7can be used. Among those the 222 nm lamp using KrCl and the 172 nm lamp using Xe are the most convenient, as they can be economically obtained and provide stable lamp functions because they are used for cleaning purposes on liquid crystal panels and the like.

An excimer lamp, as opposed to an excimer laser, can be used for a UV light source. Whereas the power intensity of the laser is 1000 times that of the lamp, the lamp can radiate light over a broader area, 500×80 mm. A small type excimer lamp can be as small as 20 mm×18 mmφ and is approximately one tenth the cost of a laser light source, while light radiated from an excimer lamp for a long period is not highly conducive to deterioration in the mechanical strength of silica-based material of an optical fiber, and the 100V or 200V power supply used by the lamp makes for compatibility with ordinary commercial power supply voltage. Further, as the power density of an excimer lamp is low compared to that of a laser lamp, light from the excimer lamp does not cause damage to an optical waveguide during the exposure period.

Step 3 Hydrogen Removal Process

Referring toFIG. 4A, the optical fiber27is placed inside an oven151and left therein for a set duration (12 hours), remaining in a heated condition (120° C.), enabling the hydrogen153or deuterium diffused in the optical fiber27to be released to the outside.

Numeric17inFIG. 4A, indicates the optical fiber with hydrogen removed therefrom. Numeric15inFIG. 4Bis the cladding with hydrogen removed and numeric13in that figure is the core with hydrogen removed.

As due to this step, the density of remaining hydrogen or deuterium inside the optical fiber17is lowered sufficiently, even where there is a long period of standby in which the optical fiber is kept before the subsequent step, basically no changes occur in the properties of the optical fiber during that time. Even with the hydrogen or deuterium thus removed however, photosensitivity to UV laser light during the formation of the FBG is still maintained. It is a characteristic of this invention that the photosensitivity of the core does not decrease after removal of the hydrogen or deuterium to the extent that occurs when technology of the present invention is used.

Silica-based material that has undergone the above-described steps has increased photosensitivity and the deterioration over time of that photosensitivity is small, thereby enabling creation of an optical waveguide type optical component.

Further, in addition to each of the above steps, a step 4 enables formation of an FBG by radiating interfering UV light to the optical fiber. Normally, a light source having an interfering effect such as a laser or the like that utilizes changes in amplitude through the interference of light, is used in order to change the refractive index of a core. The second harmonic of an argon ion laser or an excimer laser can be used to provide such a light source.

An example of a short period FBG formed using a phase mask173, as shown inFIG. 4B, provides an example of the first embodiment of the FBG production method of the present invention. Here, as interfering UV light171is radiated through a phase mask173having a fixed periodicity, a fringe pattern arises in the core13(having no hydrogen) of the optical fiber, the refractive index of the core being raised at points exposed to the high brightness of the UV. FBG11(having no hydrogen) having a fixed period can be formed in this way. In addition to this method of using a phase mask as a method of forming an FBG, a twin beam interference method can also be used.

Generally, as shown inFIG. 8A, the reflected center wavelength λ determined from the pitch Λ0of a uniform type FBG formed using this method, results in λ=2N1Λ0when expression (1) is applied. N1is the effective refractive index in the FBG region.

An FBG formed by step 1 of the method of this invention has the refractive index raised by UV lamp light, such that the reflection central wavelength is changed, becoming longer. Where the refractive index of an entire core of an optical fiber is raised,556as shown inFIG. 8B, the reflected center wavelength is λ2=2N2Λ0, and there is a shift toward longer wavelengths. Here, N2is the effective refractive index of FBG regions having raised refractive index.

FIG. 9Ashows the case where four levels of refractive index are formed using a graded form of raising of refractive index. These four levels of effective refractive index of an FBG are termed, respectively, N1, N2, N3and N4(N1<N2<N3<N4). Four values exist for the reflection central wavelength, respectively, λ1=2N1Λ0, λ2=2N2Λ0, λ3=2N3Λ0, λ4=2N4Λ0(λ1<λ2<λ3<λ4). The FBG with four refractive indexes is equivalent to a combination of four FBGs, each has one refractive index and a different period of those four ones.

Where the sloping raising of refractive index558is formed, as shown inFIG. 9B, as the refractive index gradually increases in a longitudinal direction of the optical fiber, the reflection central wavelength also gradually changes toward the longer wavelength side, such that reflection arises in some wavelength bandwidths, resulting in the appearance of transmission loss in those bandwidths.

The results of an FBG of this embodiment experimentally produced will now be described. The optical fiber of the optical waveguide was single mode optical fiber, with approximately 3.5 Wt % GeO2added, the core diameter being approximately 10 μm and the difference in a specific refractive index between the core and cladding being 0.35%. This optical fiber was placed in a 55° C., 10 MPa hydrogen atmosphere and left there for one week to allow the hydrogen to penetrate through to the center part of the optical fiber. The UV-curable resin providing a protective coating around the quartz optical fiber does not allow ultra violet light to penetrate, so that the resin was removed in parts to expose the quartz.

After the exposed quartz parts were irradiated for a fixed duration with ultra violet light from an excimer lamp having power density of 15 mW/cm2, hydrogen removal processing was performed for 12 hours at 120° C. The wavelength radiated from the excimer lamp was 172 nm. Thereafter, using the phase mask method, a FBG with a reflection central wavelength of 1550 nm was formed on the lamp irradiated portions using the second harmonic (wavelength 244 nm) from an argon ion laser. At this time a uniform mask having equal periodicity was used for the phase mask such that each period of the periodicity of the FBG was equal. The length of the FBG region was 3 mm. The results obtained by measuring the changes of the central wavelength of the FBG are shown inFIG. 10. The horizontal axis inFIG. 10shows time duration of exposure to the excimer lamp and the vertical axis, the degree of change in central wavelength of the FBG, taking the properties of the FBG without exposure to an excimer lamp as the base of measurement.

As shown inFIG. 10, as the length of time of exposure to the excimer lamp increases the refractive index rises, and notwithstanding the fact that the periodicity of the FBG is constant, changes in the central wavelength were confirmed. This relationship between the time duration of radiation exposure and changes in central wavelength sits very well above a plain curved line, and as there is a one-to-one relationship between change in refractive index and duration of radiation exposure, the desired change in refractive index can be obtained simply by controlling this time of exposure, thereby confirming that it is possible to create a grating having the desired central wavelength.

The difference between refractive index when non-interfering UV lamp light (excimer lamp light) is radiated to one side of an optical waveguide and radiated to multiple faces of an optical waveguide will be considered.

FIG. 17Ashows a core702and a cladding703disposed on a planar optical waveguide substrate700. Due to the structure of the planar optical waveguide the face of the radiation is restricted to one side of the structure. That is to say, the side and lower faces of the optical waveguide are a thick substrate, such that irradiation of the optical waveguide from the surrounding area other than from the upper surface is difficult. Accordingly there is a substantial difference δ of the light amplitude in the inside and the upper surface of the optical waveguide close to the lamp light701, thus, it can be estimated that the double refraction increases.

On the other hand, in the case illustrated inFIG. 17Bradiated light easily reaches from all around the optical waveguide and even if for example, UV lamp711irradiates only one side of the optical waveguide, the core705can still be irradiated due to the reflection and dispersal of the UV light712and713occurring within the optical fiber and coming from the material surrounding the fiber. Accordingly as the difference δ of the light amplitude in the inside and the upper surface of the optical waveguide close to the lamp light is relatively small, it can be estimated that the double refraction is small. Further, as the supplementary UV light712and713is radiated from around the optical fiber, double refraction is lowered further.

FIG. 18illustrates the relationship of double refraction and duration of radiation time when UV lamp light irradiated one side of the optical fiber and multiple faces of the optical fiber. The duration of radiation from the excimer lamp is plotted on the horizontal axis in that figure and values for double refraction are plotted on the vertical axis.

FIGS. 19A-19Edepict methods for realizing the above-described irradiation of different aspects of an optical waveguide with UV lamp light.

FIG. 19Aillustrates the case where UV lamp light801irradiates optical fiber811from one side.FIGS. 19B to 19Eillustrate radiation of UV lamp light to multiple faces of the optical waveguide. InFIG. 19B, the case of having two lamps802and803juxtaposed, irradiating both sides of optical fiber811is illustrated.FIG. 19Cshows the case where two excimer lamps804and805are arranged on opposite sides of optical fiber811, so as to radiate to both sides thereofFIG. 19Dshows a reflection plate820disposed opposing excimer lamp806, such that as light radiated from the single excimer lamp is reflected and dispersed from this plate, the optical fiber811is irradiated with light from multiple directions. Finally,FIG. 19Eillustrates that the shape cross-sectionally of a curved reflection plate821may be elliptical. Further, the reflection plate may be an ellipsoid having the optical fiber811and the excimer lamp807disposed respectively in two different focal points therein.

The maximum value of double refraction of an FBG formed according to the above method is, in the case of irradiation from one side, 0.7×10−4. Further, it was confirmed that double refraction can be below 0.7×10−4where radiation from the excimer lamp is directed to multiple surfaces of the optical waveguide. Accordingly, polarization dependent loss (PDL) and polarization mode dispersion (PMD) arising due to double refraction can be reduced.

Second Embodiment

FIGS. 11 and 12illustrate a method for producing a grating-type optical component related to the second embodiment of the present invention. As step 1 ofFIG. 11Ais the same as step 1 of the first embodiment depicted inFIG. 3Aa description of step 1 ofFIG. 11Ais omitted here.

As shown inFIG. 11B, in step 2 of this embodiment interfering UV laser light171is radiated, via phase mask173, to optical fiber27to form FBG21with hydrogen diffused on the optical waveguide core part23.

According to this second embodiment, the optical fiber is single mode having a specific refractive index difference of 0.35%, with approximately 3.5 Wt % GeO2added, the core diameter being approximately 10 μm.

Descriptions of step 3 of the second embodiment shown inFIG. 12Aand of step 4 of that embodiment shown inFIG. 12Bare omitted here, those steps being the same respectively as step 2 of the first embodiment shown inFIG. 3Band step 3 of the first embodiment shown inFIG. 4A.

According to this second embodiment, the order of irradiation using the excimer lamp and the laser light to form an FBG, performed under the same conditions as described with respect to the first embodiment, was changed. Moreover, removal of the hydrogen was performed as the last step. That is to say, the steps hydrogen diffusion, irradiation with laser light, irradiation from an excimer lamp and hydrogen removal were performed in that order and changes in the central wavelength of the laser light were confirmed. The results are shown inFIG. 13. It was evident that changes in the central wavelength of reflected light from the FBG were the same as those apparent in the case of the first embodiment even where irradiation with the excimer lamp was performed at this stage among the order of the steps, thus confirming that the results for central wavelength of reflected light from the FBG changes were the same regardless of the order in which the excimer lamp irradiation and FBG formation steps were performed.

The maximum value of double refraction of an FBG created according to the above method is, in the case of radiation with an excimer lamp from one side, 0.7×10−4. Further, in the same manner as applied with respect to the first embodiment it was confirmed that double refraction can be below 0.1×10−4where radiation from the excimer lamp is directed to multiple surfaces of the optical waveguide. Because these values are the same as those obtained using the FBG formed in accordance with the method of the first embodiment, it was confirmed that the same results were obtained with respect to the properties of double refraction regardless of whether the excimer lamp irradiation step or the FBG formation step is performed first. Accordingly, PDL and PMD arising due to double refraction can be reduced.

Third Embodiment

A production method for a grating-type optical component related to a third embodiment of the present invention will now be described. According to this third embodiment, an experimental single mode optical fiber having a specific refractive index difference of 0.85%, with approximately 8.5 Wt % GeO2added and a core diameter of approximately 4 μm was produced.

In step 1, this optical fiber was placed in a 55° C., 10 MPa hydrogen atmosphere and left there for one week to allow the hydrogen to penetrate through to the core of the optical fiber. The UV-curable resin, providing a protective coating around the quartz optical fiber, does not allow ultra violet light to penetrate, so this was removed in parts to expose the quartz.

At step 2, non-interfering UV lamp light131was uniformly radiated over the entire optical fiber as shown inFIG. 12A, raising the refractive index as shown inFIG. 5A. However, at this step a amplitude mask was not used. Further, the amplitude of the UV lamp light providing the light source was 110 mW/cm2.

At step 3, after the exposed quartz parts were irradiated for a fixed duration with ultra violet light from an excimer lamp having power density of 10 mW/cm2, hydrogen removal processing was performed for 12 hours at 120° C. The wavelength radiated from the excimer lamp was 172 nm. Thereafter, using the phase mask method, a reflection central wavelength 1550 nm FBG was formed on the lamp irradiated portions using the second harmonic (wavelength 244 nm) from an argon ion laser. At this time a uniform mask having equal periodicity was used for the phase mask such that each period of the periodicity of the FBG was equal. The length of the FBG region was 3 mm.

FIG. 14shows the results obtained after the above steps were performed. The horizontal axis inFIG. 14shows time duration of exposure to the excimer lamp and the vertical axis, the degree of change in central wavelength of the FBG, taking the properties of a sample not exposed to excimer lamp irradiation as the base of measurement. It is apparent that as the length of time of exposure to the excimer lamp increases, the refractive index rises, and notwithstanding the fact that the periodicity of the FBG is constant, changes in the central wavelength were confirmed. This relationship between the time duration of radiation exposure and changes in central wavelength sits very well above a plain curved line, and as there is a one-to-one relationship between change in refractive index and duration of radiation exposure, the desired change in refractive index can be obtained simply by controlling this time of exposure, thereby confirming that it is possible to form a grating having the desired central wavelength.

The trend of increase in the degree of change in central wavelength (a curve shaped line) is largely the same in comparison to the results obtained with respect to the first embodiment. However, the absolute values for exposure time to the light and central wavelength change are different. This is because the amount of added photosensitive material (here, Ge) as well as the structure of the optical fiber were different and also because the respective amplitudes of the excimer lamp radiation were different.

Generally, the speed of change in refractive index increases as the amplitude of light radiated from an excimer lamp increases and the maximum degree and speed of change in refractive index increases in line with the amount of photosensitive material added. As shown with respect to the first and second embodiments however, even where the amount of photosensitive material added and the optical fiber structures differ, a relationship of the power of excimer lamp radiation and the degree of central wavelength change, in other words, refractive index change, exhibits a relationship conforming to the same curve shaped line. That is to say, even where the amplitude of excimer lamp radiation or the amount of added photosensitive material are changed, it is possible for the desired properties to be easily obtained from the relationship between radiation time and refractive index change in accordance with those conditions. Further, those desired properties can be readily obtained in a short time by setting the appropriate structure for the optical fiber, such as the amount of added photosensitive material, and amplitude of excimer lamp radiation.

The maximum value for double refraction of an FBG created according to the above method is 0.8×10−4when only one side of the optical fiber is exposed to excimer lamp radiation. Further, double refraction can be brought below 0.1×10−4by irradiating multiple surfaces of the optical fiber using the excimer lamp as described. Accordingly PDL and PMD arising due to double refraction can be reduced.

After the above described step 3 of this third embodiment, step 4 for hydrogen removal is performed; however, this step 4 is the same as the step 4 of the second embodiment of this invention shown inFIG. 12B. Therefore, a description of this step 4 is omitted here.

Fourth Embodiment

A production method for a grating-type optical component according to a fourth embodiment of the present invention will now be described. Step 1 of this fourth embodiment is the same has step 1 of the first embodiment shown inFIG. 3Atherefore a description of this step one is omitted here.

As shown inFIG. 6C, at step 2 of this fourth embodiment the duration of blocking non-interfering UV lamp light131was successively altered by moving the amplitude mask175at a predetermined speed, thereby changing the duration of exposure to light of the optical fiber core. As shown inFIG. 6C, a sloping line was obtained for refractive index under these conditions. Further, a single periodicity phase mask was used producing a substantially chirped FBG. The length of the change in refractive index represented by this refractive index sloping line, that is a length of the entire length of the FBG, was 100 mm.

The experimental optical fiber used for this fourth embodiment was a single mode optical fiber having a specific refractive index difference of 0.35%, with approximately 3.5 Wt % GeO2added and a core diameter of approximately 10 μm. A light source having UV lamp light of a power of 15 mW/cm2was used.

For this embodiment the required exposure time to UV light in a lengthwise direction of the optical fiber was obtained as a linear function of a position in a lengthwise direction of the optical fiber at an accuracy confining wavelength deviation to a range of 2 nm at the location of the maximum and 0 nm at the location of the minimum, based on the results of a second embodiment. Further, based on these results the desired movement of the amplitude mask175was obtained and for step 2, the amplitude mask175was moved based on those results.

At step 3, hydrogen was removed from the optical fiber. This step of the process employed here was the same as that employed in step 3 of the first embodiment as shown inFIG. 4Atherefore a description of this step is omitted here.

Thereafter, at step 4, in the region for refractive index change, a 100 mm FBG was formed by exposure to light using the phase mask method using a uniform mask. The laser used for this light exposure was an argon ion laser radiating light of the second harmonic (wavelength 244 nm).FIG. 15Bshows the characteristics of transmission loss of the FBG obtained by this process.FIG. 15Ashows the transmission spectral after exposure to the same light without irradiation with an excimer lamp being performed. As is apparent fromFIG. 15B, it was confirmed that radiation with the excimer lamp enables formation of an FBG having a broad band, and even where a uniform mask is used a substantially chirped FBG can be formed. This kind of chirped FBG can be applied for a chromatic dispersion compensator or the like.

The refractive index of the FBG formed in this way was 0.3×10−4when one side of the optical fiber was exposed to the radiation using the excimer lamp. Further, it was confirmed that double refraction can be below 0.1×10−4where radiation from the excimer lamp is directed to multiple surfaces of the optical fiber. Accordingly, PDL and PMD arising due to double refraction can be reduced.

The FBG produced for this experiment using a uniform type phase mask having single periodicity has the reflection properties illustrated inFIG. 16B. In portions of the FBG of substantial refractive index change through exposure to UV light irradiation was largest, reflection central wavelength λnwas longest, as the refractive index becomes successively, gradually lower, the reflection central wavelength also becomes shorter in proportion thereto, such that in the FBG portions at the lowest point of the sloping line indicating refractive index the lowest value for reflection central wavelength is λ1. As shown inFIG. 16A, this is the same result as a chirped type FBG having successively altered FBG pitch using methods of the prior art.

As can be seen from the description of the above embodiments, it is irrelevant whether the step for changing refractive index using UV light irradiated from a non-interfering light source or the step for creating the FBG using exposure to interfering light is performed first.

Control Systems

Control systems for the present invention are used for controlling operations as the refractive index of an optical fiber is altered.

These control systems can be classified as refractive index adjustment methods as shown inFIGS. 5A and 6B. That is to say, a method that does not use the amplitude mask or a method in which the amplitude mask is not moved by using a fixed type amplitude mask having a plurality of slits of different widths, or an adjustment method as shown inFIG. 6Cthat involves moving the amplitude mask.

First Control System

FIG. 23Ais a main flow chart showing operations when an amplitude mask is not used or when the amplitude mask is not moved.

Referring toFIG. 23A, after the control processes commence (S251) firstly the excimer lamp comes on (S252) and control process A for adjusting refractive index is performed (S253). When that step is completed the excimer lamp goes off (S254) and the series of processes is complete (S255). As the details of the process A differs in accordance with the structure of the control system, these are described following.

FIG. 20provides an example of a control system in which an amplitude mask is not used or in which an amplitude mask is not moved for the adjusting of refractive index.

This control system comprises an optical fiber17or27, a lamp driver part31for radiating non-interfering UV lamp light131from an excimer lamp30, a control part32for controlling the on/off conditions of the excimer lamp30and a timer33that operates as a determining means for determining the degree of refractive index change.

When this control system is utilized, the required duration of irradiation time in order to obtain the desired refractive index is first estimated, and once the time elapsed from commencement of irradiation of the light is detected, by the timer33, as having exceeded the set duration of irradiation time, radiation of the light is stopped by the control part32.

FIG. 23Bis a flow chart depicting process A of the above control system. As shown inFIG. 23B, as lighting of the excimer lamp30is confirmed, operation of a clock commences from the timer33(S2531a) and once the duration of time elapsed exceeds the set duration of radiation time (S2532a: No), the process is completed (S2533a) and the excimer lamp30stops lighting (S254).

Further, the structure of this kind of control system may be a structure having an optical power meter as shown inFIG. 21. When this kind of control system is used the required power of irradiation (the integrated power of irradiated light) in order to obtain the desired refractive index is first estimated and once the estimated power of light from commencement of radiation of the light is detected, by the optical power meter40, as having reached the power of light set, radiation of the light is stopped by the control part32.

FIG. 23Cis a flow chart depicting process A of the above control system. As shown in thatFIG. 23C, as lighting of the excimer lamp30is confirmed, measurement of light by an optical power meter40commences (S2531b) and once the power of light irradiated exceeds the set estimated power (S2532b: No), the process is completed (S2533b) and the excimer lamp30stops lighting (S254).

A control system may be of a structure having a measuring part for central wavelength measurement of reflected light from an optical fiber grating or the like. This kind of control system is shown inFIG. 22.

An optical fiber grating38for a central wavelength detector is formed on an optical fiber39. This optical fiber39can be an optical fiber which has the same photosensitivity as the optical fiber17,27or an optical fiber the degree of change of the refractive index is known. The central wavelength measurement part34measures the change of central wavelength of reflected light from the optical fiber grating due to the radiated UV lamp light131.

With this structure of control system, radiation of the UV light can be stopped at the point in time at which the desired refractive index change is achieved because the change of refractive index in the optical fiber17or the optical fiber27can be estimated in real-time from the change of central wavelength arising as the light is radiated to the optical fiber grating38.

Further, a structure can also be configured in which the degree of change of central wavelength indicating the desired change in refractive index is set in advance and radiation of the light is stopped when this degree of change in central wavelength is reached. A process flow chart for this kind of system is shown inFIG. 23D.

As shown inFIG. 23D, as lighting of the excimer lamp30is confirmed, central wavelength measurement commences from a central wavelength measurement part34(S2531c), and once the degree of measured central wavelength change, exceeds the set degree of wavelength change (S2532c: No), the process is completed (S2533c) and the excimer lamp30stops lighting (S254).

Second Control System

FIG. 24shows an example of a control system for adjusting/tailoring refractive index profiles when a amplitude mask is moved (178) or when the amplitude mask is not moved.

This control system has, added to the system ofFIG. 20, an amplitude mask driver part36for driving the amplitude mask, a control part37for controlling the amplitude mask driver part36and a personal computer (PC)35, while the functions of the other components of the structure are the same as those of the control system described with respect toFIG. 20. The PC35receives signals from the timer33and operates the control parts32and37based on those signals.

FIG. 28Ais a main flow chart showing the operations of the control system when the amplitude mask is moved. Referring toFIG. 28A, after the control processes commence (S301) firstly the amplitude mask transitions to the starting point (the initial position) as necessary (S302) and once this is confirmed the excimer lamp comes on (S303) and control process B is implemented (S304) for adjusting refractive index. When that step is completed the excimer lamp goes off (S305) and the series of processes is complete (S306). The details of the process B differs in accordance with the structure of the control system.

FIG. 28Bis a flow chart depicting process B of the above control system. As shown inFIG. 28B, as lighting of the excimer lamp30is confirmed, processes commence (S3041a), operation of a clock commencing from the timer33(S3042a) while movement178of the amplitude mask175commences (S3043a). Where necessary, speed v(t) is changed in accordance with time elapsed, and once the duration of time elapsed exceeds the set duration of irradiation time (S3044a: No), the process is completed (S3045a) and the excimer lamp30stops lighting (S305).

FIG. 25shows a control system in which an optical power meter40is installed instead of the timer33shown inFIG. 24. A PC35receives signals from the optical power meter40and operates the control parts32and37based on these signals; this being the only point of difference between the control system shown inFIG. 25and the control system shown inFIG. 24and in all other respects the functions of the components comprising the structure of this control system are the same as those of the control system shown inFIG. 24.

FIG. 28Cis a flow chart depicting process B of the control system shown inFIG. 25. As shown inFIG. 28C, as lighting of the excimer lamp30is confirmed, processes commence (S3041b), measurement of light from the optical power meter40commences (S3042b) while movement of the amplitude mask175begins (S3043b). Where necessary, speed v(t) is changed in accordance with an integrated irradiated power of light, and once the integrated power of light irradiated exceeds the set integrated irradiated power of light (S3044a: No), the process is completed (S3045b) and the excimer lamp30stops lighting (S305).

FIG. 26shows a control system in which instead of the optical power meter40shown inFIG. 25, an optical fiber39, optical fiber grating38and central wavelength measuring part34are installed. A PC35receives signals from the central wavelength measuring part34and operates the control parts32and37based on these signals; these being the only points of difference between the control system shown inFIG. 26and the control system shown inFIG. 25and in all other respects the functions of the components comprising the structure of this control system are the same as those of the control system shown inFIG. 25.

FIG. 28Dis a flow chart depicting process B of the control system shown inFIG. 26. As shown inFIG. 28D, as lighting of the excimer lamp30is confirmed, processes commence (S3041c), central wavelength measurement commencing from the central wavelength measurement part34(S3042c), while movement of the amplitude mask175commences (S3043c). Where necessary, speed v(t) is changed in accordance with central wavelength and once the measured degree of change of central wavelength exceeds the set degree of change of central wavelength (S3044c: No), the process is completed (S3045c) and the excimer lamp30stops lighting (S305).

FIG. 27shows a control system in which an amplitude mask position measuring part42for measuring the position of an amplitude mask175is installed instead of the central wavelength measuring part34shown inFIG. 26. A PC35receives signals from the amplitude mask position measuring part42and operates the control parts32and37based on these signals; these being the only points of difference between the control system shown inFIG. 27and the control system shown inFIG. 26and in all other respects the functions of the components comprising the structure of this control system are the same as those of the control system shown inFIG. 26.

FIG. 28Eis a flow chart depicting the process B of the control system shown inFIG. 27. As shown inFIG. 28E, as lighting of the excimer lamp30is confirmed, processes commence (S3041d), measurement of the position of the amplitude mask commencing from the amplitude mask position measuring part42(S3042d), while movement of the amplitude mask175commences (S3043d). Where necessary, speed v(t) is changed in accordance with the position of the mask and once the mask175exceeds the prescribed position for termination of the process (S3044d: No), the process is completed (S3045d) and the excimer lamp30stops lighting (S305).

Effects of the Invention

According to the present invention an non-interfering light can be readily altered by changing the refractive index of a silica-based waveguide, enabling a substantial number of various FBG to be economically created using a single phase mask. Further, as non-interfering light of an excimer lamp is used to provide a monochromatic light source, the required light source can be easily obtained.

According to the present invention the desired FBG can be formed by controlling and altering the method of irradiation of the monochromatic light.

This application claims benefit of priority under 35 USC § 119 to Japanese Patent Application No. 2003-206061, filed on Aug. 5, 2003, the entire contents of which are incorporated by reference herein. Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the teachings. The scope of the invention is defined with reference to the following claims.