JP3975217B2 - Grating production method - Google Patents

Description

  The present invention relates to a method of manufacturing a grating that forms a refractive index modulation by irradiating an optical waveguide such as an optical fiber or a planar lightwave circuit with ultraviolet laser light, and more particularly to a band-pass filter or a dispersion equalization device used in an optical communication system. The present invention relates to a method for manufacturing a grating of a grating device that requires highly accurate characteristics.

  For high-density wavelength division multiplexing transmission systems (DWDM transmission systems) such as 50 GHz (wavelength: 0.4 nm) and 25 GHz (wavelength: 0.2 nm) that are expected to be realized in the future, optical fibers and planar lightwave circuits (Planar) A band-pass filter in which a grating is provided in an optical waveguide such as a Lightwave Circuit (hereinafter referred to as PLC) is an indispensable device. In addition, in a future ultrahigh-speed optical transmission system with a bit rate of 10 Gbit / s or higher or 40 Gbit / s or higher, a dispersion equalization device in which a grating is provided in an optical waveguide is an indispensable device.

For example, a grating device such as a bandpass filter or a dispersion equalization device irradiates an optical waveguide such as an optical fiber in which Ge is added to a quartz core or a planar lightwave circuit (PLC) with interference fringes of ultraviolet laser light. Can be produced. Furthermore, specifically, the optical waveguide is left in high pressure hydrogen of several tens to several hundreds of atmospheres for several days to several weeks to fill the inside of the optical waveguide with hydrogen, thereby sensitizing changes in refractive index due to ultraviolet light irradiation. Then, the optical waveguide is irradiated with an interference fringe of ultraviolet laser light separated into two light beams by a phase mask or a half mirror, and a refractive index change corresponding to the interference fringe is formed. Forming the refractive index change according to the interference fringes is called refractive index modulation, and the magnitude of the change is called the refractive index modulation degree. If the grating pitch of the grating formed in the optical waveguide is Λ and the equivalent refractive index of the optical waveguide is N _eff , the light of wavelength λ _B that satisfies the following formula (1) among the light incident on the optical waveguide causes Bragg reflection. Reflected on the incident side.
λ _B = 2 · N _eff · Λ (1)
The equivalent refractive index is an equivalent refractive index received by light propagating through the optical waveguide, is determined by the interaction between the core and the cladding, and is also referred to as an effective refractive index or an effective refractive index.

When the wavelength λ _B satisfying the relationship of the expression (1) is made constant over the entire grating, only the light of a specific wavelength can be efficiently reflected, so that the band pass characteristic is very sharp. A band pass filter can be obtained. On the other hand, by changing the grating pitch Λ or the equivalent refractive index N _eff of the grating in the propagation direction to form a chirped grating in which the Bragg wavelength λ _B is changed according to the position of the grating, a dispersion compensator, a dispersion slope compensator, etc. A distributed equalization device can be obtained.

In order to obtain characteristics used for applications such as a band pass filter and a dispersion equalization device, high accuracy is required for the production of the grating. That is, in the 1.55 μm wavelength band used in the optical communication system, the grating pitch is about 500 nm, and it is necessary to make the grating regularly over a length of about several mm to about 100 mm. Furthermore, since the refractive index modulation degree and the equivalent refractive index N _eff change depending on the irradiation amount of the ultraviolet laser light, it is necessary to irradiate the ultraviolet laser light with high accuracy over the entire length of the grating. A shift in the grating pitch from such a design is called a phase error, and a shift in the refractive index modulation degree or the equivalent refractive index N _eff is called an amplitude error. These errors cause deterioration of out-of-band attenuation in the band-pass filter, and cause group delay time ripple, that is, group delay ripple, in the dispersion equalization device. This is described in “Ricardo Feded, et al.,“ Effect of Random Phase and Amplitude Errors in Optical Fiber Bragg Gradings ”,“ JOURNAL OF LIGHTWAVE TECHN. "Issuance of IEEE".

  In addition, several methods have been proposed as a method for manufacturing a grating with reduced errors. For example, in the method for manufacturing an optical waveguide diffraction grating disclosed in Japanese Patent Application Laid-Open No. 8-28666, when a grating is formed by irradiating ultraviolet light having a predetermined wavelength (near 240 nm) as shown in the perspective view of FIG. The generated fluorescence is detected, and alignment is performed so that the amount of received fluorescence is maximized. That is, when an optical fiber is irradiated with ultraviolet laser light having a wavelength of around 240 nm, fluorescence having a wavelength of 350 to 550 nm is generated in the core of the optical fiber. Part of the generated fluorescence propagates through the optical fiber, reaches the detector 8, and is received. Here, the laser beam irradiated to the core of the optical fiber 1 is maximized by adjusting the amount of light received by the detector 8 to be maximized. As an ultraviolet laser having a wavelength around 240 nm, a KrF excimer laser (wavelength 248 nm) and a second harmonic (wavelength 244 nm) of an argon laser are known.

“Tetsuro Komukai, et al.,“ Examination of the cause of group delay ripple in chirped fiber gratings ”,“ Technical Report of the Institute of Electronics, Information and Communication Engineers OF2000-49 ”, pages 31-35, published by The Institute of Electronics, Information and Communication Engineers The optical fiber so that the ultraviolet laser beam is always irradiated uniformly by monitoring the fluorescence while scanning the ultraviolet laser beam having a wavelength of 244 nm, which is the second harmonic of the argon laser, in the optical axis direction of the optical fiber. Controlling the position of is described. Furthermore, the following factors are shown on the assumption that many of the causes of the group delay ripple caused by the chirped grating used as a dispersion equalization device are in the manufacturing process.
(1) Power fluctuation or mode fluctuation (amplitude error) of ultraviolet laser light to be irradiated.
(2) Fluctuation of the composition of the core in the longitudinal direction of the optical waveguide such as an optical fiber.
(3) The incompleteness of chirp grating apodization.
(4) Position shift (phase error) between the phase mask and the optical waveguide due to mechanical vibration.
(5) Incomplete position control (amplitude error) of optical waveguide and laser beam irradiation.
(6) Insufficient cleaning of optical waveguide (amplitude error).
(7) Phase mask imperfection (amplitude error and phase error) such as stitching error.

  On the other hand, in the method of manufacturing a grating disclosed in Japanese Patent Laid-Open No. 10-90545, as shown in FIG. 18, when an optical waveguide formed on a PLC is irradiated with KrF excimer laser light having a wavelength of 248 nm, which is an ultraviolet laser. The generated heat is dissipated through the radiator. In this case, when the ultraviolet laser light is irradiated for several minutes to several tens of minutes for producing the grating, a part of the ultraviolet laser light that has passed through the cladding and the core and reached the substrate is absorbed and heated. The heat generated at this time is radiated from the PLC 1 through the radiator 5. As a result, the temperature rise of the entire PLC 1 is suppressed to ± 10 ° C. or less, and changes in the grating pitch due to the thermal expansion of the PLC 1 are suppressed.

Ricardo Feded, et al., “Effect of Random Phase and Amplitude Errors in Optical Fiber Bragg Gratings”, “JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. Tetsuro Komukai, et al., “Examination of the cause of group delay ripples in chirped fiber gratings”, “Technical Report of the Institute of Electronics, Information and Communication Engineers OF2000-49”, pages 31-35, published by The Institute of Electronics, Information and Communication Engineers JP-A-8-286066 JP 10-90545 A

However, there are phase errors that can still occur even if various error generation factors as described above are prevented. That is, when a grating is manufactured by scanning an ultraviolet laser beam having a beam width of several millimeters or less in the optical axis direction of the optical waveguide, the optical waveguide has a grating pitch shift caused by thermal expansion due to local heating, that is, distortion. Occurs. This distortion of the grating pitch causes a phase error. The cause of such distortion of the grating pitch is considered as follows. Usually, the optical waveguide, since the quartz (SiO ₂₎ as a main raw material, its thermal expansion coefficient has a small but nevertheless thermal expansion coefficient of about 10 ^-6. Therefore, for example, if a region of an optical waveguide having a length of 1 mm is irradiated with an ultraviolet laser beam having a beam width of 1 mm and the temperature of the region is increased by several degrees C., the region of 1 mm thermally expands by several nm, Extrude other areas by several nanometers. In addition, since the ultraviolet laser beam is scanned along the optical axis of the optical waveguide, when the other area of the optical waveguide is irradiated, the irradiated area is thermally expanded by several nanometers, and the other parts are several. Extrude by nm. Thus, even if the temperature rises by only a few degrees C due to irradiation with ultraviolet laser light, fluctuations of about ± several nm occur in the grating pitch. Since the grating pitch is about 500 nm, even a fluctuation of several nm, for example, has a great influence, and this becomes a phase error and causes a group delay ripple. This is not limited to the chirped grating used in the dispersion equalization device, and even a uniform grating used in the band pass filter has an adverse effect.

  In general, ultraviolet laser light having a wavelength in the vicinity of 240 nm is used as shown in the method for producing an optical waveguide diffraction grating described in JP-A-8-286066. As such an ultraviolet laser, a KrF excimer laser (wavelength 248 nm) and an argon laser second harmonic (wavelength 244 nm) are known. However, since the KrF excimer laser has poor beam coherence and temporal and spatial stability of energy, it is not suitable for manufacturing a highly accurate grating. Further, the second harmonic of the argon laser has high coherence, but since it is continuous oscillation, the efficiency of changing the refractive index is poor, and a large energy density is required to obtain a sufficient degree of refractive index modulation. For this reason, since an argon laser condenses a laser beam and irradiates the optical waveguide, a slight positional deviation causes a large variation in the irradiation amount of the ultraviolet laser light, resulting in an amplitude error. Further, although the optical waveguide is irradiated with a large energy density by condensing, local thermal expansion of the optical waveguide is not considered. For this reason, as described above, a slight fluctuation occurs in the grating pitch, and a phase error also occurs.

  Next, considering the above-described prior art, the grating manufacturing method described in Japanese Patent Application Laid-Open No. 10-90545 keeps the temperature of the entire PLC on which the optical waveguide is formed constant. Therefore, it is good to uniformly irradiate the entire optical waveguide with ultraviolet laser light, but local thermal expansion due to local heating when irradiating and scanning the optical waveguide with ultraviolet laser light with a beam width of several mm or less. Is not considered. That is, even if a heat radiator is provided on the entire PLC or forcibly cooled, the portion of the optical waveguide irradiated with the ultraviolet laser light thermally expands locally. In other words, for example, when a radiator is not provided, the temperature rise to the periphery of the portion irradiated with the ultraviolet laser light is constant even if a radiator is provided or a cooling mechanism is provided. It is not sufficient to eliminate the temperature difference that occurs at the periphery of the ultraviolet laser beam irradiated portion. Therefore, a slight fluctuation occurs in the grating pitch as described above, and a phase error occurs.

  Accordingly, a first object of the present invention is to provide a method for manufacturing a grating that suppresses local thermal expansion and reduces phase error when a grating is manufactured by scanning an ultraviolet laser beam onto an optical waveguide. . A second object of the present invention is a method for producing a grating with reduced amplitude error using an ultraviolet laser beam having high efficiency for causing a change in refractive index in an optical waveguide and excellent in temporal stability and spatial stability. Is to provide.

The method for producing a grating according to the present invention includes an optical waveguide including an optical waveguide composed of a core made of a material whose refractive index changes when irradiated with ultraviolet light, and a clad surrounding the core. , And forming a grating by forming a refractive index modulation in the core,
In the core, the irradiation range is controlled so that a predetermined irradiation amount distribution is obtained in the optical axis direction of the grating, and the scanning of the ultraviolet laser light is performed a plurality of times.

The method for producing a grating according to the present invention includes an optical waveguide including an optical waveguide composed of a core made of a material whose refractive index changes when irradiated with ultraviolet light, and a clad surrounding the core. , And forming a grating by forming a refractive index modulation in the core,
The ultraviolet laser light is scanned along the optical axis of the optical waveguide at a scanning speed equal to or higher than the scanning speed defined by the energy density E per unit time and the beam diameter B of the ultraviolet laser light. .

Further, the method for producing a grating according to the present invention is a method for producing the grating, wherein the ultraviolet laser beam is defined by an energy density E (W / cm ² ) per unit time and a beam diameter B (mm). Scanning speed B ² / (85 · E ^-1.2 ) (mm / sec)
The scanning is performed at the above scanning speed.

  Further, the method for producing a grating according to the present invention is the method for producing a grating, wherein in the core, the irradiation range is controlled so as to have a predetermined irradiation amount distribution with respect to the optical axis direction of the grating, and Scanning with ultraviolet laser light is performed a plurality of times.

  Furthermore, the method for producing a grating according to the present invention is a method for producing the grating, wherein the optical waveguide is disposed on a thermally conductive substrate.

  Further, the method for producing a grating according to the present invention is a method for producing the grating, wherein the energy density E per unit time of the ultraviolet laser light, the beam diameter B, and the thermal conductivity k of the thermally conductive substrate The ultraviolet laser beam is scanned along the optical axis of the optical waveguide at a scanning speed that is equal to or higher than the scanning speed defined in (1).

Furthermore, the method for producing a grating according to the present invention is the method for producing a grating, wherein the ultraviolet laser beam is converted into an energy density E (W / cm ² ), a beam diameter B (mm), and the heat. Scanning speed defined by the thermal conductivity k (J / (m · K)) of the conductive substrate B ² / [(115 · k) ^0.5 · E ^−1.2 ] (mm / sec)
The scanning is performed at the above scanning speed.

  Still further, the method for producing a grating according to the present invention is the method for producing a grating, wherein the ultraviolet laser beam is an ultraviolet pulsed laser beam.

  The method for producing a grating according to the present invention is the method for producing a grating, wherein the light source of the ultraviolet pulse laser beam is a semiconductor light source.

  Furthermore, the method for producing a grating according to the present invention is the method for producing a grating, wherein the ultraviolet pulse laser beam has an energy density at a changing point where a gradient of a refractive index increase coefficient with respect to an energy density per pulse changes. It has the above energy density.

Furthermore, the method for producing a grating according to the present invention is a method for producing the grating, wherein the optical waveguide is disposed on a mirror substrate that reflects ultraviolet laser light, and
Monitoring the reflected light reflected from the mirror substrate when the ultraviolet laser beam is irradiated onto the optical waveguide, and adjusting the irradiation position of the ultraviolet laser beam and the relative position of the optical waveguide It is characterized by.

  Further, the method for producing a grating according to the present invention is the method for producing a grating, wherein the ultraviolet laser beam is scanned while locally cooling the portion irradiated with the ultraviolet laser beam in the optical waveguide. It is characterized by.

  Furthermore, the method for manufacturing a grating according to the present invention is a method for manufacturing the grating, wherein the optical waveguide is locally cooled by an air cooling method.

  As described above in detail, according to the method for manufacturing a grating according to the present invention, the grating is controlled by performing a plurality of scans by controlling the irradiation range of the ultraviolet laser beam scanned along the longitudinal direction of the optical waveguide. A predetermined dose distribution can be obtained with respect to the optical axis direction. Thereby, apodization in which a distribution of a predetermined refractive index modulation degree is formed can be performed. In addition, since a predetermined dose distribution is obtained by scanning a plurality of times, the dose per shot can be reduced, and local thermal expansion can be suppressed and distortion of the grating pitch can be suppressed.

  According to the grating manufacturing method of the present invention, the ultraviolet laser beam is scanned at a scanning speed equal to or higher than the scanning speed defined by the energy density E per unit time and the beam diameter B of the ultraviolet laser beam. The thermal expansion can be suppressed, the phase error can be reduced, and the distortion of the grating pitch can be suppressed.

In addition, according to the method for producing a grating of the present invention, B ² / defined by the energy density E (unit: W / cm ² ) per unit time of the ultraviolet laser beam and the beam diameter B (unit: mm). Since the ultraviolet laser beam is scanned at a scanning speed of (85 · E ^−1.2 ) (mm / sec) or more, local thermal expansion of the optical waveguide can be suppressed, phase error can be reduced, and distortion of the grating pitch can be suppressed. .

  Furthermore, according to the method for producing a grating of the present invention, the irradiation range of the ultraviolet laser light scanned along the longitudinal direction of the optical waveguide is controlled, and scanning is performed a plurality of times, so that the optical axis direction of the grating is Thus, a predetermined dose distribution can be obtained. Thereby, apodization in which a distribution of a predetermined refractive index modulation degree is formed can be performed. In addition, since a predetermined dose distribution is obtained by scanning a plurality of times, the dose per shot can be reduced, and local thermal expansion can be suppressed and distortion of the grating pitch can be suppressed.

  Furthermore, according to the method for producing a grating of the present invention, since the optical waveguide is disposed on the thermally conductive substrate, local heat generated by the irradiation of the ultraviolet laser light is diffused through the thermally conductive substrate, and the optical waveguide is Local thermal expansion due to local heating of the waveguide can be suppressed, phase errors can be reduced, and distortion of the grating pitch can be suppressed.

  Further, according to the method for producing a grating of the present invention, the scanning speed is equal to or higher than the scanning speed defined by the energy density E per unit time of the ultraviolet laser beam, the beam diameter B, and the thermal conductivity k of the thermally conductive substrate. Since the ultraviolet laser beam is scanned, the local thermal expansion of the optical waveguide can be suppressed, the phase error can be reduced, and the distortion of the grating pitch can be suppressed.

Furthermore, according to the method for producing a grating of the present invention, the energy density E (unit: W / cm ² ), the beam diameter B (unit: mm) per unit time of the ultraviolet laser beam, and the thermal conductivity of the thermally conductive substrate. Since ultraviolet laser light is scanned at a scanning speed of B ² / [(115 · k) ^0.5 · E ^−1.2 ] (mm / sec) or more, which is defined by k (W / (m · K)) The local thermal expansion of the waveguide can be suppressed, the phase error can be reduced, and the distortion of the grating pitch can be suppressed.

  Furthermore, according to the method for producing a grating of the present invention, since an ultraviolet pulse laser beam is used as the ultraviolet laser beam, a high energy density can be obtained and a refractive index change can be efficiently generated.

  Further, according to the method for producing a grating of the present invention, since an ultraviolet pulse laser beam from a semiconductor light source is used as the ultraviolet laser beam, the ultraviolet laser beam to the optical waveguide caused by temporal and spatial intensity fluctuations of the ultraviolet laser beam is used. There is almost no fluctuation in the dose distribution. For this reason, an amplitude error can be reduced and the refractive index change by ultraviolet light irradiation can be performed efficiently.

  Furthermore, according to the method for producing a grating of the present invention, the ultraviolet pulse laser beam has an energy density equal to or higher than the changing point at which the gradient of the refractive index increase coefficient with respect to the energy density per pulse changes. Changes can be made more efficiently.

  Furthermore, according to the method for producing a grating of the present invention, the optical waveguide is disposed on the mirror substrate, and the reflected light from the mirror substrate is monitored, and the relative position between the irradiation position of the ultraviolet laser beam and the optical waveguide is monitored. Therefore, it is possible to irradiate the optical waveguide in a range where the energy density of the ultraviolet laser light is high and the energy fluctuation is small. Thereby, the amplitude error can be reduced and the refractive index change can be efficiently generated.

  Further, according to the method for producing a grating of the present invention, since the ultraviolet laser beam is irradiated while locally cooling the portion of the optical waveguide irradiated with the ultraviolet laser beam, the local heat generated by the local heating of the optical waveguide is obtained. Expansion can be suppressed, phase errors can be reduced, and grating pitch distortion can be suppressed.

  Furthermore, according to the method for producing a grating of the present invention, ultraviolet laser light is irradiated while the optical waveguide is cooled by an air cooling method, so that local thermal expansion due to local heating of the optical waveguide is suppressed and phase error is reduced. Thus, distortion of the grating pitch can be suppressed.

  A method for manufacturing a grating according to an embodiment of the present invention will be described below with reference to the accompanying drawings. In addition, what attached | subjected the same code | symbol in an accompanying drawing is the same or it corresponds. In the first aspect of the grating manufacturing method according to the embodiment of the present invention, the scanning speed of the ultraviolet laser beam is controlled to suppress distortion of the grating pitch caused by thermal expansion due to local heating of the optical waveguide. That is, the ultraviolet laser light is scanned along the optical axis of the optical waveguide at a scanning speed equal to or higher than the scanning speed defined by the energy density E per unit time of the ultraviolet laser light and the beam diameter B. When the ultraviolet laser light is scanned at a scanning speed equal to or higher than the predetermined speed in this manner, local thermal expansion occurring at the beam end of the ultraviolet laser light can be suppressed, and distortion of the grating pitch to be formed can be suppressed.

  In the second aspect of the grating manufacturing method according to the embodiment of the present invention, the irradiation range of the ultraviolet laser light is controlled, and scanning is performed a plurality of times to manufacture the grating. Thus, a predetermined irradiation amount distribution can be obtained by controlling the irradiation range of the ultraviolet laser light. In addition, since scanning is performed a plurality of times, the irradiation amount per time can be reduced. As a result, local thermal expansion can be suppressed, so that grating pitch distortion can be suppressed.

  In a third aspect of the method for manufacturing a grating according to the embodiment of the present invention, an optical waveguide is disposed on a mirror substrate, and the reflected light of the ultraviolet laser beam by the mirror substrate is monitored to detect the ultraviolet laser beam and the optical waveguide. Align with. As a result, the energy density of the ultraviolet laser light is the highest, and the optical waveguide can be irradiated in a range where the energy fluctuation is small. This can also reduce the amplitude error.

  In the fourth aspect of the method for manufacturing a grating according to the embodiment of the present invention, the portion of the optical waveguide that has been irradiated with the ultraviolet laser light is locally cooled. As a result, local thermal expansion is suppressed, and deviation of the grating pitch, that is, distortion is suppressed.

Embodiment 1 FIG.
In the grating manufacturing method according to the first embodiment of the present invention, the scanning speed of the ultraviolet laser light is controlled to suppress distortion of the grating pitch caused by thermal expansion due to local heating of the optical waveguide. That is, the ultraviolet laser light is scanned along the optical axis of the optical waveguide at a scanning speed equal to or higher than the scanning speed defined by the energy density E per unit time of the ultraviolet laser light and the beam diameter B. When the ultraviolet laser light is scanned at a scanning speed equal to or higher than the predetermined speed as described above, local thermal expansion occurring in the irradiated portion of the ultraviolet laser light can be suppressed, and distortion of the grating pitch to be formed can be suppressed.

  A specific method for producing this grating will be outlined. In this grating manufacturing method, as shown in the perspective view of FIG. 1, an optical fiber 1 is used as an optical waveguide. The optical fiber 1 is accommodated in a groove formed in the substrate 2, and a phase mask 3 for forming a grating is provided on the groove in which the optical fiber 1 is accommodated, and ultraviolet light is substantially perpendicular to the optical axis of the optical fiber 1. The laser beam 7 is scanned. The mechanism for producing this grating is as follows. When the optical fiber 1 is irradiated with the ultraviolet laser light 7 through the phase mask 3, the ultraviolet laser light 7 is mainly ± 1st order by the phase mask 3 as shown in the sectional view in the optical axis direction of FIG. Diffracted by the diffracted lights 23 and 24, the + 1st order diffracted light 23 and the −1st order diffracted light 24 interfere with each other to form interference fringes. When this interference fringe is irradiated onto the optical fiber 1, the refractive index changes according to the energy intensity of the ultraviolet light of the interference fringe, and the grating 20 is produced on the core 21 of the optical fiber 1. The pitch of the grating 20 formed at this time is half of the pitch of the phase mask, and the grating pitch is about 500 nm for 1.55 μm band optical communication.

  The scanning of the ultraviolet laser light 7 is performed by moving the moving stage 5 holding the optical fiber 1, the substrate 2, and the phase mask 3 by the holder 4 relative to the ultraviolet laser light 7. By moving the moving stage 5, the optical fiber 1, the substrate 2, and the phase mask 3 held on the moving stage 5 can be moved left and right. The moving stage 5 may be controlled by a controller such as a personal computer (not shown). When the moving stage 5 is moved in the optical axis direction of the optical fiber 1, the optical fiber 1, the substrate 2 and the phase mask 3 are moved, and the position irradiated with the ultraviolet laser light 7 is moved along the optical axis of the optical fiber 1. Can be made. By irradiating the core 21 of the optical fiber 1 with the ultraviolet laser light 7, a grating is produced along the optical axis of the optical fiber 1. At this time, the moving stage 5 is moved at a scanning speed that can suppress a local temperature rise of the optical fiber 1 due to the irradiation of the ultraviolet laser beam 7. Thus, by scanning the ultraviolet laser light 7 at a scanning speed equal to or higher than a predetermined scanning speed, local thermal expansion of the optical fiber 1 can be suppressed, and a highly accurate grating can be manufactured. A more detailed condition of this scanning speed will be described later.

  Next, a description will be given of each component of an optical system that scans the peripheral portion of the optical fiber 1 serving as an optical waveguide and ultraviolet laser light. First, the peripheral part of the optical fiber 1 will be described. The optical fiber 1 includes a core 21 made of a material that causes a change in refractive index by ultraviolet light, and a clad surrounding the core 21. In addition, a groove slightly larger than the diameter of the optical fiber 1 is formed in the substrate 2, and the optical fiber 1 is accommodated in this groove. The substrate 2 is preferably a substrate made of a material having high thermal conductivity such as a semiconductor such as Si or a metal such as Au or Ag. A phase mask 3 for producing a grating in the optical fiber 1 is provided in the vicinity of the optical fiber 1 and the substrate 2. In FIG. 1, the respective structures are shown separated from the optical fiber 1 and the substrate 2 for easy understanding, but in actuality, a cross-sectional view taken along line AA ′ in FIG. ), The phase mask 3 is provided close to the optical fiber 1 and the substrate 2. As shown in FIG. Further, the optical fiber 1, the substrate 2, and the phase mask 3 are fixed to each other so that their positional relationship does not change. Further, the optical fiber 1, the substrate 2, and the phase mask 3 are installed on the moving stage 5 by the holder 4. By this moving stage 5, the optical fiber 1, the substrate 2, and the phase mask 3 can be moved left and right. In this grating manufacturing method, an optical fiber is used as an optical waveguide, but the present invention is not limited to this, and a planar lightwave circuit (PLC) can also be used.

  Further, each component of the optical system that scans the ultraviolet laser light will be described. The ultraviolet laser beam 7 is output from the ultraviolet laser device 6 as shown in the perspective view of FIG. The ultraviolet laser device 6 converts the wavelength of pulsed laser light excited by a semiconductor light source such as a light emitting diode (LED) or a laser diode (LD) and outputs ultraviolet laser light 7 having a beam diameter of about 1 mm. As such an ultraviolet laser beam 7, the third harmonic (wavelength 355 nm), the fourth harmonic (wavelength 266 nm), and the fifth harmonic (wavelength 213 nm) of an Nd-YAG laser excited by a semiconductor light source are used. Can do.

Next, detailed conditions regarding the scanning speed of the ultraviolet laser light will be described. First, the time required for the local temperature rise that occurs in the optical fiber 1 due to irradiation with ultraviolet laser light will be described. The irradiation time when the temperature rise of the optical fiber 1 is 1 ° C. or less is, for example, a transient characteristic of the temperature rise of the optical fiber with respect to the irradiation time of the ultraviolet laser light having an energy density E per unit time of 100 W / cm ² shown in FIG. Thus, it takes about 0.35 seconds for a quartz substrate, about 0.5 seconds for a Si substrate, and about 0.75 seconds for an Au substrate. The local temperature rise of the optical fiber 1 should be as low as possible. However, if the temperature is approximately 1 ° C. or less, the change in grating pitch due to local thermal expansion is 1 nm or less (0.2% or less). Degradation of the grating characteristics is negligible. Therefore, in the case of ultraviolet laser light having an energy density E per unit time of 100 W / cm ² , if the irradiation time is 0.35 seconds or less on the quartz substrate, the local temperature rise of the optical fiber is 1 ° C. or less. be able to. In addition, when the optical fiber 1 is installed on a substrate having a thermal conductivity lower than that of quartz, the heat generated by the laser light irradiation is conducted to the surroundings through the optical fiber, so that it is about the same as when installed on the quartz substrate. Rising temperature.

Furthermore, as shown in FIG. 4, the irradiation time at which the temperature rise of the optical fiber with respect to the irradiation energy density per unit time of the ultraviolet laser light becomes 1 ° C. is rapidly shortened as the irradiation energy density per unit time increases. The irradiation energy density E (W / cm ² ) per unit time and the irradiation time t ( ^second ) at which the temperature rise becomes 1 ° C. are approximately the following formulas (2) and ( 3) and the approximate expression (4).
t = 85 · E ^-1.2 (quartz substrate) (2)
t = 130 · E ^-1.2 (Si substrate) (3)
t = 190 ・ E ^-1.2 (Au substrate) (4)
Therefore, the local temperature rise of the optical fiber can be suppressed to 1 ° C. or less if the irradiation time is equal to or shorter than the irradiation time shown in the above formulas (2), (3), and (4) for each of the various substrates. The change of the grating pitch due to the thermal expansion can be made 1 nm or less. As described above, even a substrate having a lower thermal conductivity than quartz can be handled in the same manner as a quartz substrate. Therefore, the irradiation time of the ultraviolet laser beam is t ≦ 85 · E ^−1.2 regardless of the material of the substrate (including the case where the optical fiber is installed in the hollow).
If the moving stage is moved at a speed that satisfies the following conditions, the change in grating pitch due to local thermal expansion can be made 1 nm or less.

In addition, although the above shows an ultraviolet laser beam having a beam diameter of 1 mm, in general, when a grating is manufactured using an ultraviolet laser beam having a beam diameter B (mm), the end of the beam diameter is scanned during scanning. Irradiation continues to the part. Since the irradiation time of the beam end in this case is a time obtained by dividing the beam diameter B by the scanning speed v,
B / v ≦ 85 ・ E ^-1.2
It becomes. From this equation, when converted to an equation representing the scanning speed v,
v ≧ B / (85 · E -1.2) (mm / sec) (5)
It becomes. When ultraviolet laser light is irradiated at a scanning speed that satisfies this formula (5), the irradiation time from one end of the beam diameter to the end is in a range where the rising temperature can be suppressed to 1 ° C. or less. Can be suppressed. Therefore, it is preferable to scan the ultraviolet laser beam at a scanning speed that satisfies the condition of the formula (5).

Further, as shown in FIG. 16, when a beam having a beam diameter B (mm) is irradiated for a certain period of time, thermal expansion accumulates at the beam end compared to the beam center. Therefore, even if the irradiation time is the same, the deviation from the original position due to thermal expansion at the beam end increases as the beam diameter increases. For example, as shown in FIG. 16A, when the first position is irradiated with ultraviolet laser light, the optical waveguide at the irradiated portion is thermally expanded along the optical axis. Further, as shown in FIGS. 16B and 16C, when the ultraviolet laser beam moves to the next position, the first irradiated portion is cooled and contracted, and the next irradiated portion is thermally expanded. To do. In this case, when scanning an ultraviolet laser beam having a beam diameter of B (mm), the thermal expansion at the beam end is accumulated to be about B (nm) even during the irradiation time t that normally results in thermal expansion of 1 nm. It is considered to be. Therefore, in order to suppress the thermal expansion at the beam end to 1 nm or less,
B / v ≦ (85 · E ^−1.2 ) / B
It is necessary to satisfy the conditions. When the scanning speed v is calculated from this equation,
v ≧ B ² / (85 · E ^−1.2 ) (6)
It becomes. When the ultraviolet laser beam is scanned at a scanning speed that satisfies this formula (6), the beam is irradiated at a predetermined position for a certain period of time, and even in the case of a step scan in which the beam is moved to a jump position, the beam end is scanned. Can be suppressed to 1 nm or less. Therefore, it is more preferable to scan the ultraviolet laser beam at a scanning speed v (mm / second) that satisfies Expression (6).
When a Si substrate is used, it is more preferable to scan at a scanning speed that satisfies the following formula (7).
v ≧ B ² / (130 · E ^−1.2 ) (7)
Further, when an Au substrate is used, it is more preferable to scan at a scanning speed that satisfies the following formula (8).
v ≧ B ² / (190 · E ^-1.2 ) (8)

Further, from the equations (7) and (8), when a substrate having a high thermal conductivity such as a semiconductor or metal is used, the thermal conductivity of the material is expressed as k (W / (m · K)). Then, it is preferable to scan at a scanning speed that satisfies the following formula (9).
v ≧ B ² / {(115 · k) ^1/2 · E ^−1.2 } (9)
The beam diameter is the diameter or width of the laser beam at which the energy density is half of the maximum value in the spatial energy distribution of the laser beam.

  By the way, in order to suppress the temperature rise due to the irradiation of the ultraviolet laser light during the preparation of the grating in this way, in addition to the method of shortening the irradiation time by moving the relative position of the ultraviolet laser light and the optical waveguide at high speed, the unit of the ultraviolet laser light There is a method of reducing the irradiation energy density per hour. However, if the irradiation energy density per unit time is reduced, an extra irradiation time is required to obtain a sufficient change in the refractive index (refractive index modulation), so that the refraction can be performed without increasing the irradiation time. Conditions that efficiently cause rate changes are required.

  Under such conditions, pulsed ultraviolet laser light is used in comparison with continuous wave ultraviolet laser light that uses the second harmonic (wavelength: 244 nm) of an Ar laser that has been used in the production of high-precision gratings. Is more effective. Among these, ultraviolet laser light excited by a semiconductor light source is most suitable because it has good temporal and spatial stability of energy. As such a pulse oscillation ultraviolet laser beam, the third harmonic (wavelength: 355 nm), the fourth harmonic (wavelength: 266 nm), and the fifth harmonic (wavelength: wavelength) of an Nd-YAG laser excited by a semiconductor light source. 213 nm) is preferred. Among them, the fourth harmonic (wavelength: 266 nm) is close in wavelength to the second harmonic (wavelength: 244 nm) of Ar laser and KrF excimer laser (wavelength: 248 nm), and optical components such as phase masks can be used as they are. Therefore, it is advantageous in terms of cost reduction of the optical component.

  In addition, the pulsed ultraviolet laser beam used to manufacture the grating has an energy density equal to or higher than the energy density at the changing point where the gradient of the refractive index modulation degree changes with respect to the energy density in the relationship between the refractive index modulation degree and the energy density. It is preferable to use an ultraviolet laser beam. When the pulsed ultraviolet laser beam has an energy density equal to or higher than a predetermined energy density, the gradient of the refractive index modulation degree with respect to the energy density becomes larger. Therefore, for each of the ultraviolet laser light having an energy density lower than the change point and the ultraviolet laser light having an energy density higher than the change point, the ultraviolet laser light having the same energy density per unit time by adjusting the oscillation frequency. Even in the case of the above, the refractive index modulation degree obtained when the energy density is equal to or higher than the changing point is increased.

The relationship between the refractive index modulation degree and the energy density will be described below. First, FIG. 5 shows one pulse of ultraviolet pulsed laser light that irradiates an optical waveguide when a grating is manufactured using a fourth harmonic (wavelength: 266 nm) of a pulsed Nd-YAG laser excited by a semiconductor light source. This shows the relationship between the energy density per unit and the refractive index modulation degree of the manufactured grating. A dispersion-shifted single mode optical fiber having a zero dispersion wavelength in the 1.55 μm band was used for the optical waveguide. Here, the oscillation frequency of the laser is constant at 200 Hz, and the irradiation time of the ultraviolet pulse laser light is constant. Further, in FIG. 5, the energy density per unit time with respect to the energy density per pulse, that is, the laser output is indicated by a broken line. This laser output (unit: W) is a product of energy per pulse (unit: J) and oscillation frequency (unit: Hz), and is proportional to the energy density per pulse as shown in FIG. On the other hand, the gradient of the degree of refractive index modulation increases with the energy density per pulse of the ultraviolet pulsed laser light applied to the optical waveguide being 70 mJ / cm ² (change point) as a boundary. That is, the energy density of one pulse in the 70 mJ / cm ² or more, the refractive index modulation degree at a ratio of more than the increase rate of the laser output is changed.

Furthermore, when a normal single mode optical fiber having a zero dispersion wavelength in the 1.3 μm band is used as another optical fiber, the energy density per pulse of the ultraviolet pulsed laser beam is 100 mJ / cm ² (change point). Thus, the inclination is greatly different (not shown). In addition, in the planar lightwave circuit (PLC) in which Ge and Sn are added to the core 21 to improve the photosensitivity by ultraviolet light, the inclination becomes large with a boundary of 50 mJ / cm ² (change point) (not shown). That is, the phenomenon in which there is an energy density where the refractive index change rapidly increases with respect to the energy density per pulse of the ultraviolet pulsed laser light irradiated to the optical waveguide is different in the energy density value at the change point, but the type of optical waveguide It is the same regardless of (single mode optical fiber, dispersion shifted optical fiber, PLC, etc.). The reason for this is unknown, but it is considered that a large amount of light energy is required in an extremely short time in order to efficiently change the refractive index by irradiation with ultraviolet laser light. Therefore, continuous-wave ultraviolet laser light such as the second harmonic of the Ar laser (wavelength: 244 nm) cannot supply sufficient light energy in an extremely short time, and thus the efficiency of changing the refractive index by ultraviolet laser light irradiation. Is bad.

Also, when irradiating ultraviolet pulse laser light, the core 21 of the optical waveguide is irradiated with ultraviolet pulse laser light having an energy density equal to or higher than the energy density (change point) at which the refractive index change rapidly increases with respect to the energy density per pulse. Is preferably irradiated. For example, when a dispersion shifted single mode optical fiber having a zero dispersion wavelength in the 1.55 μm band is used, the energy density per pulse irradiated to the core 21 of the optical waveguide is 100 mJ / cm ² as shown in FIG. In an ultraviolet pulse laser beam with an oscillation frequency of 200 Hz and an energy density per pulse of 50 mJ / cm ² and an ultraviolet pulse laser beam with an oscillation frequency of 400 Hz, the energy per unit time irradiated to the core of the optical waveguide density are both 20W / cm ^2. In this case, when calculated based on FIG. 5, the 100 mJ / cm ² , 200 Hz ultraviolet pulse laser beam has a refractive index modulation degree approximately twice that of the other in the same irradiation time. On the other hand, with an ultraviolet pulse laser beam of 50 mJ / cm ² and 400 Hz, approximately twice the irradiation time is required to obtain the same degree of modulation. In order to obtain the same refractive index modulation with the same irradiation time as the ultraviolet pulse laser beam having an energy density of 50 mJ / cm ² and the energy density of 100 mJ / cm ² and 200 Hz, the oscillation frequency is set to It is necessary to set it to 800 Hz. However, if this is done, the energy density per unit time is 40 W / cm ² , which not only requires twice the power, but also increases the local temperature rise of the optical waveguide.

In order to prevent a local temperature rise in the optical waveguide, it is necessary to move the ultraviolet laser light at a relatively high speed with respect to the optical waveguide as described above, and 100 mJ / cm ² , In the case of 200 Hz, 20 W / cm ² ultraviolet pulse laser light, scanning may be performed at a scanning speed of about 0.43 mm / second or more, but in the case of 50 mJ / cm ² , 800 Hz, 40 W / cm ² ultraviolet pulse laser light. Needs to be scanned at a scanning speed of about 0.98 mm / second or more. An ultraviolet laser beam having an energy density of 70 mJ / cm ² or more per pulse irradiated to the core of the optical waveguide is advantageous in that the allowable scanning speed is widened. For this reason, an Nd-YAG laser excited by a semiconductor light source is preferable for producing this grating. The Nd-YAG laser excited by this semiconductor light source has excellent temporal stability and spatial stability of the laser beam, and is highly accurate because it is more efficient in changing the refractive index than the second harmonic of the Ar laser. Most suitable for producing gratings.

  The same can be said for the KrF excimer laser, which is an ultraviolet pulsed laser beam. However, since the KrF excimer laser has poor time stability and spatial stability of the laser beam, the amplitude error of the fabricated grating is large, and the grating pitch It does not reach a level where the fluctuation of about 1 nm is discussed. There is also a discharge lamp-excited Nd-YAG laser, but it is not suitable for producing a highly accurate grating for the same reason as the KrF excimer laser.

  Further, in this grating manufacturing method, the light source of the ultraviolet laser light is fixed at a predetermined position, the optical fiber 1 is moved by the moving stage 5, and the relative position between the optical fiber 1 and the ultraviolet laser light 7 is changed. The case of scanning with ultraviolet laser light has been described. However, the scanning method of the ultraviolet laser light is not limited to the above method. Conversely, the ultraviolet laser light may be scanned by fixing the optical fiber at a predetermined position and moving the ultraviolet laser light. That is, it is only necessary that the irradiation positions of the optical fiber and the ultraviolet laser light change relatively.

  Furthermore, it is preferable that the optical waveguide 1 which is an optical fiber for producing a grating is housed in a groove formed in the substrate 2 having high thermal conductivity. The substrate 2 having a high thermal conductivity preferably has a thermal conductivity higher than the thermal conductivity of the material constituting the optical waveguide 1. Specifically, it is more preferable to have a thermal conductivity of 1 W / (m · K) or more. Since it is housed in the groove of the substrate 2 having a high thermal conductivity in this way, the heat generated in the portion irradiated with the ultraviolet pulsed laser light 7 is diffused to the substrate 2 to prevent a local temperature rise in that portion. be able to. By suppressing local heating of the optical fiber 1 in this way, a grating in which local thermal expansion is prevented and the pitch of the grating is regularly formed can be produced.

In this grating manufacturing method, in order to suppress a local temperature rise of the optical fiber 1, detailed conditions regarding the substrate that houses the optical fiber 1 will be described below. When the optical fiber is irradiated with ultraviolet laser light, the temperature rises. That is, when the optical fiber 1 is irradiated with the ultraviolet laser beam 7 having a beam diameter of 1 mm without moving the moving stage 5 as shown in FIG. 3, the temperature rise ΔT of the optical fiber 1 with respect to the irradiation time of the laser beam as shown in FIG. The temperature rises almost linearly up to about 5 seconds after irradiation, and then the temperature rise is saturated. The ultraviolet laser beam 7 used for the measurement is a pulse oscillation ultraviolet laser beam and has a Gaussian distribution type energy distribution, and the energy per pulse is 1 mJ. Therefore, the energy density per pulse of the ultraviolet laser light 7 in the irradiated portion of the optical fiber 1 is about 200 mJ / cm ² . In this case the ultraviolet laser beam 7 is irradiated to the optical fiber 1 at an oscillation frequency of 500 Hz, the energy density E per unit is applied to the optical fiber time is 100W / cm ^2. When the optical fiber is installed on the quartz substrate, the temperature rise ΔT after 1 second is about 10 ° C. from FIG. 3, about 7 ° C. for the Si substrate and about 5 ° C. for the Au substrate. It was. As described above, when the optical fiber is installed on a metal or semiconductor substrate having a high thermal conductivity, the local temperature rise of the optical fiber can be suppressed to a low level.

  In this method for manufacturing a grating, a method for manufacturing a grating by forming interference fringes of ultraviolet laser light using a phase mask has been described. However, the present invention is not limited to this. Interference fringes by other methods such as two-beam interference method, in which ultraviolet laser light is separated into two light beams by a half mirror and combined on an optical waveguide to form interference fringes, and prism interference method using prisms to form interference fringes Alternatively, a method of forming a grating by forming a film may be used.

Embodiment 2. FIG.
In the grating manufacturing method according to the second embodiment of the present invention, the irradiation range of the ultraviolet laser light is controlled on the core for manufacturing the grating to scan the ultraviolet laser light. Thus, by controlling the irradiation range of the ultraviolet laser light, it can be adjusted to a distribution of a predetermined refractive index modulation degree. In this grating manufacturing method, the refractive index modulation is formed by scanning the ultraviolet laser light a plurality of times. By performing scanning a plurality of times in this way, it is possible to avoid a local temperature rise and a local thermal expansion that accompany it when an attempt is made to form a refractive index modulation by only one irradiation. This makes it possible to form a highly accurate grating when obtaining a refractive index modulation degree having a predetermined distribution.

  This grating manufacturing method is different from the grating manufacturing method according to the first embodiment in that the range of irradiation with ultraviolet laser light is controlled and scanning is performed a plurality of times. By controlling the range in which the ultraviolet laser light is irradiated in this way, a predetermined ultraviolet light dose distribution, that is, a distribution of a predetermined refractive index modulation degree is obtained. Apodization that forms such a predetermined refractive index modulation degree distribution is usually used in a grating device used in an optical communication system in order to increase the out-of-band attenuation of a band-pass filter, or in a group delay of a dispersion equalization device. Needed to reduce ripple. As this apodization, various function types suitable for use such as a Gaussian distribution type, a cosine function type, and a sinc function type are adopted. In this grating manufacturing method, a case where apodization is performed by scanning ultraviolet laser light along the optical axis of an optical waveguide will be described.

  For example, in a method of manufacturing a grating that performs refractive index modulation of a sinc type function (sink function: sinc (x) = sin (x) / x) as apodization, two-step refractive index modulation is applied as shown in FIG. It is carried out. That is, the grating is first manufactured (a), and then the equivalent refractive index is uniformed (b) to form the grating (c). This grating can be manufactured using the same optical system as shown in the first embodiment. First, the phase mask 3 is brought close to the optical waveguide 1 and irradiated with ultraviolet laser light. At this time, the ultraviolet laser light is irradiated so as to have a predetermined irradiation amount distribution with respect to the optical axis direction of the optical waveguide. In FIG. 6A, the broken line is the “equivalent refractive index of the grating”. Since the refractive index change corresponds to the irradiation amount, irradiation with ultraviolet laser light so as to obtain a predetermined irradiation amount distribution that causes such a refractive index change has a refractive index change having a predetermined period. A refractive index modulation (grating) is formed (FIG. 6A). The distance between the peaks of the refractive index is set to about 500 nm in an optical communication system using a wavelength of 1.55 μm band. On the other hand, the broken line indicates the equivalent refractive index of the grating received by the light propagating through the optical waveguide. Since this equivalent refractive index is required to be constant with respect to the entire grating, thereafter, the equivalent refractive index is uniformized (FIG. 6B). In this homogenization process, the phase mask 3 is removed from the optical system of FIG. 1, and ultraviolet laser light is irradiated in the longitudinal direction of the grating produced by the above method so as to have a dose distribution as shown in FIG. 6B. To do. As a result, the equivalent refractive index of the portion where the refractive index modulation is small as shown in FIG. 6C increases, and the entire grating has a constant equivalent refractive index as shown by the broken line. It should be noted that the same refractive index distribution shown in FIG. 6C is finally obtained regardless of which is equivalent to the process for equalizing the equivalent refractive index and the preparation of the grating, so that the order opposite to the above order is obtained. You may go on.

  Next, a method for manufacturing this grating will be described. In order to give a dose distribution of a sinc type function as apodization to the part where the grating is formed, as shown in FIG. 7, the scanning range of the ultraviolet laser light is controlled and scanned. The optical system shown in FIG. 1 can be used as an optical system for irradiating ultraviolet laser light. Here, the scanning trajectory of the ultraviolet laser light is conceptually shown as a polygonal line that reciprocates a plurality of times from the lower left to the lower right in FIG. In addition, it has been conceptually shown that the irradiation dose is accumulated as the scanning trajectories are stacked, and the irradiation dose distribution of the ultraviolet laser light for apodization is obtained. FIG. 7B shows the scanning trajectory for two reciprocations of the optical waveguide and the ultraviolet laser beam.

  Further, a method for controlling the scanning range of the ultraviolet laser light in manufacturing the grating will be described. As shown in FIG. 7B, the irradiation amount distribution accumulated by controlling the irradiation position of the ultraviolet laser light can be adjusted in the range in which the ultraviolet laser light is irradiated onto the optical fiber. That is, the ultraviolet laser beam is scanned from the left end at a substantially constant scanning speed from the outside of the optical waveguide grating production range. On the first forward path, irradiation is performed in the scanning range up to the position a on the right, and on the return path, the left end is irradiated up to the position b. In the next round trip, the right end narrows the scanning range by c, and the left end reaches d. Thereafter, similarly, by gradually narrowing the scanning range of the ultraviolet laser light, the apodization was performed such that the irradiation amount of the ultraviolet laser light was small at both ends of the grating and the irradiation amount of the ultraviolet laser light was large at the central portion of the grating. A grating is produced. By controlling the scanning range of the ultraviolet laser light in this way, the cumulative irradiation dose distribution can be made a predetermined irradiation dose distribution. Thereafter, the ultraviolet laser beam is moved out of the range for producing the grating of the optical waveguide to complete the production of the grating. Although not shown in FIG. 7, light is shielded by a light shielding mask or the like so that ultraviolet laser light is not irradiated onto the optical waveguide outside the range in which the grating is manufactured.

  By controlling the irradiation range of the ultraviolet laser light as described above, it is possible to adjust the cumulative ultraviolet light irradiation amount distribution and perform apodization. In addition, since a predetermined irradiation dose distribution is obtained not by a single irradiation but by a cumulative irradiation dose distribution by a plurality of times of ultraviolet laser light irradiation, the irradiation dose per time can be reduced. For this reason, since a scanning speed can be made high, the local heating which arises by locally irradiating an optical fiber can be suppressed, and local thermal expansion can be suppressed.

  The scanning speed of the ultraviolet laser light is preferably a scanning speed that satisfies the formula (5) shown in the first embodiment. When the ultraviolet laser beam is scanned at a scanning speed that satisfies the formula (5), local thermal expansion can be suppressed to 1 nm or less. Further, a scanning speed satisfying the formula (6) is more preferable. When the ultraviolet laser beam is scanned at a scanning speed that satisfies Expression (6), the shift of the grating pitch due to local thermal expansion due to local heating of the optical waveguide can be reduced to 1 nm or less. Further, if the substrate on which the optical waveguide is installed, the type of ultraviolet laser, and the irradiation energy density per pulse of the ultraviolet pulse laser light are also those shown in the first embodiment, the local heat due to the local heating of the ultraviolet laser light is further increased. Expansion can be suppressed.

  Further, the irradiation energy intensity of the ultraviolet laser light is preferably constant during scanning from the viewpoint of temporal stability of the laser output. Note that the irradiation energy of ultraviolet laser light when moving from both ends of the grating, that is, from outside the range for producing the grating to within the range for producing the grating, and when moving from the range for producing the grating to outside the range for producing the grating, etc. Adjustments such as reducing the strength may be performed. In order to improve the temporal stability of the laser output, it is preferable to use a thermally stable laser device in which the laser is oscillated at a constant output before being irradiated within the range in which the grating is manufactured. In this grating manufacturing method, particularly the central portion of the optical fiber is reciprocally scanned many times to irradiate the ultraviolet laser light, so that the temporal variation of the irradiation energy of the ultraviolet laser light is offset by integration, so that the time The influence of global fluctuation can be reduced.

  Since the optical system used in this grating manufacturing method is the optical system shown in FIG. 1, the optical waveguide is moved by a moving stage, the ultraviolet laser beam is fixed at a fixed position, and the ultraviolet laser beam is scanned and irradiated. As described above, the optical waveguide may be fixed at a certain position, the ultraviolet laser beam may be moved, and the ultraviolet laser beam may be scanned and irradiated. That is, it is only necessary that the irradiation position of the optical waveguide and the ultraviolet laser light move relatively.

Embodiment 3 FIG.
In the production of the grating according to the third embodiment of the present invention, the irradiation range of the ultraviolet laser light is controlled by the moving light shielding plate, and a plurality of scans are performed to form the refractive index modulation. Thus, by controlling the scanning range with the moving light shielding plate, it is possible to eliminate the fluctuation portion of the scanning speed when the scanning direction of the ultraviolet laser light is reversed.

  This grating manufacturing method is different from the grating manufacturing method according to the second embodiment in that the irradiation range of the ultraviolet laser light is controlled by the movable light shielding plate 31 as shown in FIG. Another difference is that the optical waveguide 1 is fixed and the ultraviolet laser beam 7 is moved for scanning. Specifically, in this grating manufacturing method, as shown in FIG. 8, the ultraviolet laser beam 7 is output from the beam scan laser irradiation device 30. The beam scan laser irradiation device 30 is a device including an optical system that scans the ultraviolet laser light 7 output from the ultraviolet laser device 6 in the optical axis direction of the optical waveguide. Between the beam scanning laser irradiation device 30 and the phase mask 3, a movable light shielding plate 31 that is movable along the optical axis direction of the optical waveguide 1 is installed. As shown in the sectional view in the optical axis direction of FIG. 9, the moving light shielding plate 31 shields the ultraviolet laser light 7 outside the predetermined range, and controls the range irradiated with the ultraviolet laser light 7. The optical waveguide 1, the substrate 2, and the phase mask 3 are fixed by the holder 4 so that the positional relationship does not change. Here, unlike the case of the first embodiment, the holder 4 is fixed at a fixed position.

  Here, the beam scan laser irradiation apparatus 30 can be realized, for example, by a configuration as shown in FIG. A method of scanning the ultraviolet laser beam 7 in the optical axis direction of the optical waveguide 1 using the moving mirror 32 is shown in FIG. By disposing the moving mirror 32 inclined by 45 ° with respect to the optical axis of the ultraviolet laser beam output from the ultraviolet laser, the ultraviolet laser beam is reflected by changing the optical axis by 90 ° and is perpendicular to the phase mask 3 and the optical waveguide 1. Incident. Here, the moving mirror 32 is an ultraviolet light mirror that reflects ultraviolet light having a wavelength to be used. By moving the moving mirror 32 in parallel with the optical axis direction of the optical waveguide 1, the ultraviolet laser light 7 can be scanned and irradiated in the optical axis direction of the optical waveguide 1.

  On the other hand, FIG. 11 shows a method using the same four wedge-shaped prisms 33, and each of the four prisms 33 is a pair, and constitutes a deflector 1 and a deflector 2. ing. A pair of prisms of each deflector rotate in opposite directions, at opposite angular speeds and with the same absolute value (when one rotation angle is θ, the other rotation angle is −θ). A pair of prisms having the same rotation direction of the deflector 1 and the deflector 2 rotate synchronously with the same rotation angle. By rotating the four wedge-shaped prisms 33 synchronously in this way, the optical axis direction of the ultraviolet laser light 7 output from the ultraviolet laser device 6 is changed every time it passes through the wedge-shaped prism 33, and the optical It is possible to scan along the optical axis direction of the waveguide. In this way, by precisely controlling the four wedge-shaped prisms 33 in the positional relationship as shown in FIG. 11, the phase mask 3 and the optical waveguide 1 are irradiated perpendicularly to the optical axis direction of the optical waveguide. It is possible to scan in parallel.

  Next, in this grating manufacturing method, a method of controlling the scanning range of the ultraviolet laser light by the moving light shielding plate 31 will be described with reference to FIG. In FIG. 12A, the curve indicated by the broken line is the irradiation amount distribution of the ultraviolet laser light for apodization. In addition, the broken line from the lower left to the lower right in the figure and then reciprocating multiple times to the left and right, and reaching the upper right arrow is the scanning trajectory of the ultraviolet laser light that irradiates the optical waveguide, and the dose distribution accumulated thereby This is a conceptual illustration of the relationship. FIG. 12B shows a scanning trajectory for two reciprocations of the ultraviolet laser light into the optical waveguide.

  In this case, the ultraviolet laser beam 7 is passed a plurality of times between the left end L and the right end R along the optical axis direction of the optical waveguide at a constant scanning speed over a wider range including the range for producing the grating of the optical waveguide. Scan. In this case, the accumulated dose distribution can be adjusted by limiting the range in which the ultraviolet laser beam 7 is applied to the optical waveguide by the movable light shielding plate 31. That is, in the first forward path, the ultraviolet laser beam is scanned from the left end L, and the light guide plate 31 irradiates the optical waveguide in the scanning range from A to a. At this time, in the range of a to R, the light is blocked by the moving light blocking plate 31 and is not irradiated to the optical waveguide 1. In the return path, the moving light shielding plate 31 is moved at the left end to irradiate the position b. Also in this case, in the range of b to L, the light is blocked by the moving light blocking plate 31 and is not irradiated to the optical waveguide 1. In the next round trip, the scanning range is narrowed by the moving light shielding plate 31 with the right end up to c, and the left end is scanned within the scanning range up to d. Thereafter, by similarly moving the moving light shielding plate 31 to gradually narrow the scanning range, the irradiation amount of the ultraviolet laser light is small at both ends of the grating, and the irradiation amount of the ultraviolet laser light is large at the center of the grating. An apodized grating is produced. By controlling the scanning range of the ultraviolet laser light in this way, the cumulative irradiation dose distribution can be made a predetermined irradiation dose distribution. Thereafter, the ultraviolet laser beam is moved out of the range for producing the grating of the optical waveguide to complete the production of the grating. Although not shown in FIG. 12, light is shielded by a light shielding mask or the like so that the ultraviolet laser beam is not irradiated onto the optical waveguide outside the range in which the grating is manufactured.

  By controlling the irradiation range of the ultraviolet laser light as described above, it is possible to adjust the cumulative ultraviolet light irradiation amount distribution and perform apodization. In addition, since a predetermined irradiation dose distribution is obtained not by a single irradiation but by a cumulative irradiation dose distribution by a plurality of times of ultraviolet laser light irradiation, the irradiation dose per time can be reduced. For this reason, since a scanning speed can be made high, the local heating which arises by locally irradiating an optical fiber can be suppressed, and local thermal expansion can be suppressed.

  Thus, the method of controlling the irradiation range of the ultraviolet laser light by controlling the position of the movable light shielding plate 31 has the following effects. That is, if the moving stage or the beam scanning mechanism is moved at a high speed to attempt reciprocal scanning, it is necessary to decelerate at the turning point, so that the scanning speed may fluctuate. Moreover, the irradiation time of the ultraviolet laser light increases due to this deceleration, which may cause apodization to deviate from the design. On the other hand, when the irradiation range is controlled by the moving light shielding plate 31 as described above, the portion decelerated for the direction change can be cut, so that apodization with a predetermined irradiation amount distribution can be applied.

  In the present embodiment, the method of fixing the optical waveguide and scanning the ultraviolet laser beam 7 has been described. However, the optical waveguide may be moved in the same manner as the grating manufacturing method according to the first embodiment. In this case, the irradiation range may be controlled by moving the movable light shielding plate 31 in accordance with the movement of the optical waveguide. Although the ultraviolet laser beam is shielded by the moving light shielding plate 31, the light shielding means is not limited to this, and laser oscillation may be stopped in a region where it is desired to shield light.

Embodiment 4 FIG.
In the grating manufacturing method according to the fourth embodiment of the present invention, the optical fiber is housed in a groove formed in the mirror substrate, and the reflected light of the irradiated ultraviolet laser beam is monitored to maximize the amount of light received. Perform alignment. This makes it possible to irradiate the optical fiber with ultraviolet laser light having an energy density sufficient to form a grating.

  This grating manufacturing method is different from the grating manufacturing method according to the first embodiment in that the substrate for storing the optical fiber is a mirror substrate as shown in FIGS. Another difference is that a CCD camera 8 for monitoring the reflected light 9 is provided. In this grating manufacturing method, as shown in the perspective view of FIG. 13, the 0th-order diffracted light of the ultraviolet laser light incident on the optical waveguide is reflected by the mirror substrate 2, and is reflected light 9 emitted to the ultraviolet laser device 6 side. Is monitored by the CCD camera 8. In addition, although the case where 0th-order diffracted light is monitored will be described, diffracted light of any order of ± 1st order, ± 2nd order, ± 3rd order,... In this case, as shown in FIG. 14A, which is a sectional view in the radial direction of the optical fiber, the ultraviolet laser beam 7 is incident with a tilt of about 1 ° from the direction perpendicular to the substrate 2 in the radial direction of the optical waveguide 1. The The reflected light 9 is monitored by the CCD camera 8. At this time, as shown in FIG. 14B, the ultraviolet laser beam deviating from the width of the optical fiber is specularly reflected by the mirror substrate. On the other hand, the portion irradiated with the optical fiber is irregularly reflected. For this reason, the reflected light 9 has an intensity distribution obtained by cutting the light irradiated on the inner side of the width of the optical waveguide 1. By monitoring the reflected light 9 with the CCD camera 8, the center of the ultraviolet laser light can be accurately irradiated onto the optical waveguide 1. For example, in a Gaussian distribution type ultraviolet laser beam having a beam diameter of 1 mm, the fluctuation of the energy distribution in the region of ± 100 μm from the beam center is within 3%. On the other hand, since the diameter of a normal single mode optical fiber is 125 μm, the center of the ultraviolet laser beam can be irradiated onto the optical waveguide 1 with sufficient accuracy by this method. As a result, the irradiation distribution of the ultraviolet laser light applied to the optical fiber can be made as designed, and a grating with reduced amplitude error can be manufactured.

  Here, it is preferable that the mirror substrate 2 has a reflectivity such that a portion of the reflected light 9 from the optical fiber is clearly discriminated. In order to accurately match the irradiation position of the optical waveguide with the ultraviolet laser beam by this method, it is preferable that the beam diameter of the ultraviolet laser beam is large. In this case, the ultraviolet laser beam using the second harmonic of the Ar laser, which has been often used in the past, has a low energy intensity and must be focused and irradiated, which is not very suitable. On the other hand, the fourth harmonic of the Nd-YAG laser excited by the semiconductor light source is preferable because it has a large energy intensity and does not need to be condensed.

  In this grating manufacturing method, the case where an optical fiber as an optical waveguide is installed in the groove of the mirror substrate 2 has been described. However, an optical fiber may be installed on the mirror substrate 2 where no groove is formed. . In addition, even in a PLC with an optical waveguide formed on a Si substrate, the ultraviolet laser light is slightly diffusely reflected at the portion where the core is formed. Similarly, the irradiation position of the optical waveguide and the ultraviolet laser light must be precisely matched. Can do. Furthermore, even with a PLC in which an optical waveguide is formed on a quartz substrate, the mirror waveguide substrate is installed on the back surface so that the irradiation position of the optical waveguide and the ultraviolet laser light can be accurately aligned in the same manner as the PLC in which the optical waveguide is formed on the Si substrate. Can do. Instead of the CCD camera 8, a monitor means such as a beam profiler for measuring the energy distribution of the laser beam may be used.

  Further, a groove may be formed in parallel with the optical waveguide at a predetermined distance from the optical waveguide of a planar lightwave circuit (PLC) in which the optical waveguide is formed on a Si or quartz substrate. The reflected light 9 when the groove is irradiated with the ultraviolet laser light is monitored by the CCD camera 8 and alignment is performed, and then the optical waveguide is shifted by a predetermined distance to accurately irradiate the optical waveguide with the ultraviolet laser light. Can do.

Embodiment 5 FIG.
In the grating manufacturing method according to Embodiment 5 of the present invention, the ultraviolet laser light is scanned while locally cooling the portion irradiated with the ultraviolet laser light. By locally cooling in this way, local temperature rise of the optical fiber can be suppressed and local thermal expansion can be suppressed.

  This grating manufacturing method is different from the grating manufacturing method according to the first embodiment in that the ultraviolet laser light is scanned while locally cooling the portion irradiated with the ultraviolet laser light. As this local cooling method, for example, as shown in the sectional view in the radial direction in FIG. 15A and the sectional view in the optical axis direction in FIG. 11 may be blown and cooled by an air cooling method. In this case, a hole for blowing the gas flow 11 to the air or the like is formed on the back surface of the substrate 2 along the optical fiber 1, and the blower 10 serving as a cooling unit is collinear with the ultraviolet laser beam 7. Is provided. Further, a gas flow 11 such as air is blown out from the blower 10 and is blown onto the optical waveguide 1. In addition, you may move the air blower 10 according to the scan of the ultraviolet laser beam 7. FIG. With such a configuration, the gas flow 11 is blown only on the portion of the optical waveguide 1 to which the ultraviolet laser light 7 is locally irradiated, so that local thermal expansion due to local heating of the optical waveguide 1 can be suppressed. . The gas flow 11 is adjusted to such a flow rate that the local portion of the optical waveguide 1 is not excessively cooled from its peripheral portion, and the ultraviolet laser light 7 of the optical waveguide 1 is locally irradiated. The temperature of the exposed part and the part not irradiated are kept substantially the same. As a result, phase errors caused by grating pitch distortion due to local thermal expansion of the optical waveguide 1 can be reduced.

  In this grating manufacturing method, the optical fiber is installed on the substrate having a hole on the back surface. However, the optical waveguide may be installed in a hollow state without using the substrate, and may be cooled by air cooling from the back surface. The cooling means is not limited to the air cooling method. For example, a method may be used in which an optical waveguide is installed on a substrate on which a plurality of Peltier elements are arranged along the optical axis direction of the optical waveguide, and a portion irradiated with ultraviolet laser light is selectively cooled. . Among various cooling methods, the method of suppressing local heating by air cooling by blowing is very simple in configuration, and adjustment only needs to change the flow rate of the gas flow. it can.

It is a perspective view which shows the positional relationship of the optical waveguide and optical system in the manufacturing method of the grating which concerns on Embodiment 1 of this invention. (A) is the sectional view along the A-A 'line in FIG. 1, and (b) is the sectional view along the B-B' line in FIG. It is a graph which shows the relationship between the laser irradiation time and the raise temperature of an optical fiber at the time of irradiating an ultraviolet laser beam to the optical fiber arrange | positioned on the board | substrate with which thermal conductivity differs. It is a graph which shows the relationship between irradiation energy density and the irradiation time when the temperature of an optical fiber raises 1 degreeC at the time of irradiating an ultraviolet laser beam to the optical fiber arrange | positioned on the board | substrate with which thermal conductivity differs. It is a graph which shows the relationship between the energy density per pulse and a refractive index modulation degree change when an optical fiber is irradiated with ultraviolet laser light for a fixed time. In the method for manufacturing a grating according to the second embodiment of the present invention, in the procedure for manufacturing a grating in which an optical waveguide is apodized, (a) is a conceptual diagram showing the relationship between the grating and the equivalent refractive index, (b ) Is a conceptual diagram of the distribution of refractive index change given by the equivalent refractive index uniformization process performed to make the equivalent refractive index of the grating constant, and (c) is a state where the equivalent refractive index of the grating is made uniform. FIG. It is a conceptual diagram which shows the relationship between the scanning locus | trajectory of an ultraviolet laser beam and the cumulative irradiation amount distribution in the manufacturing method of the grating which concerns on Embodiment 2 of this invention. It is a perspective view which shows the outline of the manufacturing method of the grating which concerns on Embodiment 3 of this invention. It is sectional drawing along the optical axis of the optical waveguide of FIG. It is a conceptual diagram which shows operation | movement of an example of the beam scan laser irradiation apparatus used in the manufacturing method of the grating which concerns on Embodiment 3 of this invention. It is a conceptual diagram which shows operation | movement of an example of the beam scan laser irradiation apparatus used in the manufacturing method of the grating which concerns on Embodiment 3 of this invention. It is a conceptual diagram which shows the relationship between the scanning locus | trajectory of an ultraviolet laser beam and the cumulative irradiation amount distribution in the manufacturing method of the grating which concerns on Embodiment 3 of this invention. It is a perspective view which shows the relationship between the optical waveguide in the manufacturing method of the grating which concerns on Embodiment 4 of this invention, and an optical system. (A) is sectional drawing of the radial direction of the optical waveguide of FIG. 13, (b) is a conceptual diagram of reflected light. It is the conceptual diagram which showed a mode that the irradiation part expanded by the scan of the ultraviolet laser beam of the beam diameter B. FIG. (A) is sectional drawing of the radial direction of the optical waveguide which shows the local cooling method in the grating preparation method concerning Embodiment 5 of this invention, (b) is sectional drawing of the optical axis direction of an optical waveguide. . It is a figure which shows an example of the preparation methods of the conventional grating. It is a figure which shows an example of the preparation methods of the conventional grating.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical waveguide, 2 Substrate, 3 Phase mask, 4 Holder, 5 Moving stage, 6 Ultraviolet laser device, 7 Ultraviolet laser beam, 8 CCD camera, 9 Reflected light, 10 Blower, 11 Gas flow, 19 Reflection from optical waveguide Light, 20 grating, 21 core, 22 clad, 23 + 1st order diffracted light, 24 -1st order diffracted light, 30 beam scan laser irradiation device, 31 moving light shielding plate, 32 moving mirror, 33 wedge prism, 34 deflector 1, 35 deflector 2

Claims (7)

A laser beam is scanned along the optical axis of the optical waveguide onto an optical waveguide composed of a core made of a material whose refractive index changes when irradiated with ultraviolet light, and a clad surrounding the core, and the core is scanned A method of forming a grating by forming a refractive index modulation,
Scanning speed defined by energy density E (W / cm ² ) per unit time and beam diameter B (mm)
B ² / (85 · E ^-1.2 ) (mm / sec)
A method for producing a grating , wherein the ultraviolet laser beam is scanned along the optical axis of the optical waveguide at the above scanning speed . And controls the irradiation range to a predetermined distribution of irradiation amount with respect to the optical axis direction of the grating in the core, the grating according to claim 1, characterized in that a plurality of times scanning of the ultraviolet laser beam Manufacturing method. Grating manufacturing method of claim 1 or 2, characterized in that the optical waveguide is placed on the thermally conductive substrate. The ultraviolet laser beam per unit time of the energy density E (W / cm ^2), is defined out with beam diameter B (mm) and thermal conductivity k of the heat conducting board (J / (m · K) ) Scanning speed B ² / [(115 · k) ^0.5 · E ^-1.2 ] (mm / sec)
The method for manufacturing a grating according to claim 3 , wherein the ultraviolet laser beam is scanned along the optical axis of the optical waveguide at the above scanning speed. The ultraviolet laser light is 1 to 4 claim 1, characterized in that the ultraviolet pulse laser beam inclination of refractive index increase factor for the pulse per energy density having an energy density more than the energy density at the change point that changes The method for producing a grating according to any one of the above. The optical waveguide is disposed on a mirror substrate that reflects ultraviolet laser light, and
The reflected light reflected from the mirror substrate when the ultraviolet laser light is irradiated onto the optical waveguide is monitored, and the irradiation position of the ultraviolet laser light and the relative position of the optical waveguide are adjusted. The method for producing a grating according to any one of claims 1 to 5 . The method for producing a grating according to any one of claims 1 to 6 , wherein in the optical waveguide, the ultraviolet laser light is scanned while locally cooling a portion irradiated with the ultraviolet laser light. .

Images