Patent Application
20170115449
The present invention relates to an apparatus for writing a grating in an optical waveguide and a method for writing a grating in an optical waveguide.
By irradiating an optical waveguide of an optical fiber or the like having the core or a clad formed of silica glass containing photosensitive materials such as GeO2 and B2O3 with ultraviolet light which has been intensity modulated in an axial direction of the core, a grating having a distribution of refractive index corresponding to a distribution of the intensity of the ultraviolet light in the axial direction of the core can be manufactured. Such a grating can be used as, for example, a gain equalizer that equalizes a gain of an erbium-doped fiber amplifier (EDFA) including an amplifying optical fiber that contains erbium (Er) in its core.
Techniques for manufacturing a grating are descried in JP 2003-4926A (PTL 1), WO2003/093887 (PTL 2), JP 10-253842A (PTL 3), JP 2001-166159A (PTL 4), and JP 2004-170476A (PTL 5). Examples of the ultraviolet light include a second harmonic of an argon ion laser light (244 nm), a KrF excimer laser light (248 nm), a fourth harmonic of a YAG laser light (265 nm), a second harmonic of a copper vapor laser light (255 nm), and so forth.
Examples of a method for irradiating the optical waveguide with the ultraviolet light which has been intensity modulated in the axial direction of the core include a phase mask method, a method in which the optical waveguide is directly exposed to the laser light, and a dual-beam interference exposure method. With the phase mask method, positive/negative first-order diffracted beams generated by using a chirp-type grating phase mask are caused to interfere with each other. With the dual-beam interference exposure method, the laser light is divided into two beams and these divided beams are caused to interfere with each other. With the phase mask method, compared to other methods, the grating can be easily manufactured with good repeatability.
With the technique for manufacturing a grating disclosed in PTL 3 and PTL 4, after the grating has been formed in the optical waveguide with the phase mask method, the phase mask is replaced with a dimmer filter having a distribution of transmittance in the axial direction of the optical waveguide, and the optical waveguide is irradiated with non-interference light having passed through the dimmer filter. Thus, the effective refractive index is caused to vary in the axial direction of the optical waveguide so as to manufacture the grating having a desired attenuation wavelength characteristic. With this technique for manufacturing a grating, the step of adjusting the effective refractive index by irradiation with the non-interference light is required in addition to the step of forming the grating by using the phase mask. Consequently, there is a problem in that a manufacturing cost and manufacturing time are increased.
PTL 5 describes that, in the phase mask method, an amplitude of refractive index modulation of the grating can be adjusted by adjusting the distance between the phase mask and the optical waveguide. PTL 5 also describes that, as the distance between the phase mask and the optical waveguide is reduced, the amplitude of the refractive index modulation of the grating can be increased, and as the distance between the phase mask and the optical waveguide is increased, the amplitude of the refractive index modulation of the grating can be reduced. However, according to calculation, conducted by the inventor, of the behavior of the positive/negative first-order diffracted beams with respect to the distance between the phase mask and the optical waveguide, the increase in the distance between the phase mask and the optical waveguide is not necessarily able to reduce the amplitude of the refractive index modulation of the grating, and the behavior of the diffracted beams are complex. Furthermore, only by adjusting the distance between the phase mask and the optical waveguide, versatility in forming the grating is low, and it is difficult to realize a characteristic specific to the optical waveguide and variation in the longitudinal direction.
Accordingly, an object of the present invention is to provide an apparatus for manufacturing a grating and a method for manufacturing a grating with which a grating having a desired attenuate wavelength characteristic can be easily manufactured.
An apparatus for manufacturing a grating writes a grating in an optical waveguide. The apparatus includes a laser source, mirror position adjusting means, mask position adjusting means, and a synchronous controller. The laser source outputs laser light. The mirror position adjusting means is movable in an axial direction of the optical waveguide and adjusts a position of a scanning mirror, which deflects the laser light to the optical waveguide, so as to adjust a grating write position in the optical waveguide. The mask position adjusting means adjusts a position of a phase mask, which is disposed between the scanning mirror and the optical waveguide, so as to adjust a distance between the phase mask and the optical waveguide. The synchronous controller controls an adjustment of the position of the scanning mirror performed by the mirror position adjusting means and an adjustment of the position of the phase mask performed by the mask position adjusting means in a manner in which they are associated with each other.
The apparatus may further include beam diameter adjusting means that is provided between the laser source and the scanning mirror and that adjusts a beam diameter and a wavefront of the laser light. In this case, the synchronous controller also associates and controls an adjustment of the beam diameter of the laser light performed by the beam diameter adjusting means. The apparatus may further include lens position adjusting means that adjusts a distance between the optical waveguide and a cylindrical lens which receives the laser light having been deflected by the scanning mirror. In this case, the synchronous controller also associates and controls an adjustment of a position of the cylindrical lens performed by the lens position adjusting means. A focal length of the cylindrical lens may be from 100 to 200 mm.
A method for manufacturing a grating, the method with which a grating is written in an optical waveguide, includes deflecting laser light having been output from a laser source to the optical waveguide by using a scanning mirror movable in an axial direction of the optical waveguide, irradiating the optical waveguide through a phase mask disposed between the scanning mirror and the optical waveguide with the laser light having been deflected by the scanning mirror, associating an adjustment of a position of the scanning mirror and an adjustment of a position of the phase mask with each other controls the adjustment of the position of the scanning mirror and the adjustment of the position of the phase mask, and writing the grating in the optical waveguide.
A radius of curvature of a wavefront of the laser light with which the phase mask is irradiated may be 20 mm or larger. The scanning mirror may be moved in the axial direction of the optical waveguide while a beam width of the laser light with which the phase mask is irradiated is varied from 500 to 3000 μm. A cylindrical lens which receives the laser light having been deflected by the scanning mirror may be used. A beam width of the laser light incident upon the cylindrical lens may be from 500 to 3000 μm.
According to the present invention, a grating having a desired attenuation wavelength characteristic can be easily manufactured.
An apparatus for manufacturing a grating and a method for manufacturing a grating according to the present invention are described in detail below with reference to the accompanying drawings. In description of the drawings, the same elements are denoted by the identical reference numerals, thereby omitting duplicate description. It should be noted that the present invention is not limited to these examples. The present invention is indicated by the scope of Claims and is intended to embrace all the modifications within the scope of Claims and within meaning and range of equivalency.
The laser source 11 outputs laser light of a wavelength at which the refractive index of a core of the optical fiber 2 can be varied (for example, 244 nm). The beam diameter adjusting means 12 adjusts the beam diameter and the wavefront of the laser light having been output from the laser source 11 and outputs the adjusted laser light. The scanning mirror 21 is movable in the axial direction of the optical fiber 2 and deflects the laser light having been output from the beam diameter adjusting means 12 toward the optical fiber 2. The mirror position adjusting means 22 adjusts the position of the scanning mirror 21 so as to adjust a grating write position in the optical fiber 2. The cylindrical lens 31 receives the laser light having been deflected by the scanning mirror 21 and causes the laser light to converge in the axial direction of the optical fiber 2. The lens position adjusting means 32 adjusts the distance between the cylindrical lens 31 and the optical fiber 2.
The phase mask 41 is disposed between the cylindrical lens 31 and the optical fiber 2. The phase mask 41 has a grating having projections and recesses with a period of about 1 μm on a surface facing the optical fiber 2. The phase mask 41 receives the laser light having been output from the cylindrical lens 31 so as to generate positive/negative first-order diffracted beams and causes these positive/negative first-order diffracted beams to interfere with one another in the core of the optical fiber 2, thereby forming a distribution of optical intensity so as to form a grating in the core of the optical fiber 2. The mask position adjusting means 42 adjusts the position of the phase mask 41 so as to adjust the distance between the phase mask 41 and the optical fiber 2. The optical fiber 2 is fixed onto the stage 51 by the fixing jig 52.
The synchronous controller 60 controls the adjustment of the position of the scanning mirror 21 performed by the mirror position adjusting means 22 and the adjustment of the position of the phase mask 41 performed by the mask position adjusting means 42 in a manner in which they are associated with each other. Preferably, the synchronous controller 60 also associates the adjustment of the beam diameter of the laser light performed by the beam diameter adjusting means 12 so as to control the adjustment of the beam diameter of the laser beam. Furthermore, preferably, the synchronous controller 60 also associates the adjustment of the position of the cylindrical lens 31 performed by the lens position adjusting means 32 so as to control the adjustment of the position of the cylindrical lens 31.
Preferably, the focal length of the cylindrical lens 31 is from 100 to 200 mm, the radius of curvature of the wavefront of the laser light with which the phase mask 41 is irradiated is 20 mm or larger, the scanning mirror 21 is moved in the axial direction of the optical fiber 2 while the beam width of the laser light with which the phase mask 41 is irradiated is varied from 500 to 3000 μm, and the beam width of the laser light incident upon the cylindrical lens 31 is from 500 to 3000 μm. Furthermore, the mirror position adjusting means 22, the lens position adjusting means 32, and the mask position adjusting means 42 preferably include, for example, a linear motor, a stepping motor, and a piezoelectric element, respectively.
For convenience of description, the xyz orthogonal coordinate system is indicated in
The calculation results described below are calculated based on the assumption that the laser light incident upon the phase mask 41 has a Gaussian distribution and the beam diameter of the laser light is 200 μmφ. Furthermore, the calculation results described below may represent only one side of the center (the center of the Gaussian distribution) of an incident beam. An actual distribution is symmetric about the center of the incident beam.
As can be seen from these drawings, as the distance from the phase mask 41 is increased, the peak position of the interference light intensity is separated from the origin. At a position separated by the distance of any value, variation in refractive index due to bias light and the variation in refractive index due to the interference pattern are superposed on one another. The bias light may cause degradation of the visibility of the interference pattern. Furthermore, as the distance from the phase mask 41 is increased, the ratio of the bias light increases. In addition, it has been found that, as the distance is further increased, the interference light is outgoing at an angle of about 14 degrees while the peak of the interference light intensity grows.
In this calculation, the grating period of the phase mask 41 is set so that the period of the interference pattern is about 0.5 μm. As can be seen from
As can be seen from these graphs, as the width of the incident beam is increased, rise of the ratio between the area of the bias light and the area of the interference pattern with respect to the distance Gap from the phase mask 41 is delayed and the degree of the inclination of the rise is reduced. For convenience of a calculation area, the calculation herein is limited to a range up to an incident beam width of 200 μm here. However, it is inferred that the degree of the inclination of the rise of the ratio between the area of the bias light and the area of the interference pattern is further reduced by further increasing the incident beam width. That is, as the incident beam width is increased, the ratio between the area of the bias light and the area of the interference pattern becomes less sensitive to the variation in Gap. This is advantageous for writing the grating.
As can be seen from these graphs, the ratio between the area of the bias light and the area of the interference pattern oscillates with a period of about 1 μm, and a variation width Δ is small, that is, from 7 to 8%, around 0 μm in Gap and when the Gap is large. The variation width Δ when the diameter of the incident beam is 200 μm has a similar shape (the ratio is 0 to 3% on a small side of the oscillation and A is 12 to 14%) around 150 μm in Gap to around 250 μM in Gap. Thus, a change with respect to the variation in Gap is small. That is, disturbance in the oscillation due to stage scanning in writing the grating can be absorbed in this range. This is advantageous for writing the grating. Although the degree of the variation width Δ is similar around 300 μm in Gap, tendency of the intensity of the bias light is significantly observed in this region. Thus, this is not advantageous for writing the grating.
As can be clearly understood from the above-described calculation results, the variation width of the ratio between the area of the bias light and the area of the interference pattern and the Gap length where the variation width is suppressed are uniquely determined along the Gap axis depending on the diameter of the incident beam.
Also in
When the light is not absorbed by the optical fiber 2, the distributions of the laser light intensity in the fiber sections A′ to C′ are the same as those when the optical fiber 2 is not disposed. That is, in the fiber section A′, the light power density is larger on the far side than on the phase mask side. In the fiber section B′, the light power density on the far side and on the phase mask side are substantially the same. In the fiber section C′, the light power density is smaller on the far side than on the phase mask side.
When the light is absorbed by the optical fiber 2, the distributions of the laser light intensity in the fiber sections A″ to C″ are determined in accordance with the light absorption by the optical fiber 2 in addition to the distributions of the laser light intensity without the optical fiber 2. That is, in the fiber section A″, although the laser light attenuates due to the light absorption by the optical fiber 2 as the laser light advances to the far side, the optical power density is equalized due to a convergence effect produced by the cylindrical lens 31. In the fiber section B″, since the laser light attenuates due to the light absorption by the optical fiber 2 as the laser light advances to the far side, and the laser light can be regarded as parallel light around this position, the optical power density is smaller on the far side than on the phase mask side. In the fiber section C″, since the laser light attenuates due to the light absorption by the optical fiber 2 as the laser light advances to the far side, and the laser light is divergent around this position, the optical power density is smaller on the far side than on the phase mask side, and the difference in the optical power density between the far side and the phase mask side increases.
According to the present embodiment, the adjustment of the position of the scanning mirror 21 and the adjustment of the position of the phase mask 41 are associated with each other so as to control the adjustment of the position of the scanning mirror 21 and the adjustment of the position of the phase mask 41. Thus, the ratio between the interference light and the bias light can be appropriately set at positions in the axial direction of the optical fiber 2. Accordingly, a grating having a desired attenuation wavelength characteristic can be easily manufactured. Furthermore, according to the present embodiment, in addition to the adjustment of the position of the scanning mirror 21 and the adjustment of the position of the phase mask 41, the adjustment of the beam diameter of the laser light is also associated so as to control the adjustment of the beam diameter of the laser light. Furthermore, the adjustment of the position of the cylindrical lens 31 is also associated so as to control the adjustment of the position of the cylindrical lens 31. This can increase versatility of writing of the grating corresponding to the size of a photosensitive region and the magnitude of the photosensitivity specific to an optical fiber.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-080174 | Apr 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2015/060835 | 4/7/2015 | WO | 00 |
