JP2002048927A - Method for manufacturing chirped fiber grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a chirped fiber grating which reduces ripples and has excellent dispersion characteristics by reducing fluctuations generated in a method for manufacturing a chirped fiber grating and reducing ununiformity of the grating. SOLUTION: In the method for manufacturing the chirped fiber grating wherein a phase mask 6 is arranged closely to an optical fiber 3 the partial coating layer of which is removed and the coating layer removed part of the optical fiber 3 is irradiated with ultraviolet rays through the phase mask 6 to form the grating, the phase mask 6 is scanned with ultraviolet beams 7 and the optical fiber 3 is repeatedly irradiated with the ultraviolet rays to form the grating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a chirped fiber grating used in fields such as optical communication and optical sensors.

[0002]

2. Description of the Related Art In optical communication, chromatic dispersion accumulated in a transmission optical fiber affects the width of an optical pulse to be transmitted, which is a factor in limiting the transmission capacity of optical communication. Particularly, in a transmission line using an optical amplifier, since transmission is performed without being converted into an electric signal (regeneration relay) by a repeater from transmission to reception of an optical signal, chromatic dispersion over the entire length accumulates. Affects transmission characteristics. Also, when the transmission speed of the optical signal increases, the width and interval of the transmitted optical pulse become narrower, so even if the spread of the optical pulse due to chromatic dispersion is slight, the adjacent optical pulses interfere with each other, The transmission characteristics in the transmission path are degraded. Therefore, a technique of compensating for chromatic dispersion accumulated in the transmission line of optical communication is required, and the following two methods exist as such methods.

One method uses a dispersion compensating optical fiber. In this method, an optical fiber having a chromatic dispersion having a sign opposite to that of the chromatic dispersion accumulated in the transmission line is inserted in front of the receiver. This is a method of compensating for the chromatic dispersion caused by the above.
In this method, it is possible to completely compensate for the accumulated chromatic dispersion by appropriately selecting the type of optical fiber and its length, but in proportion to the magnitude of the dispersion compensation amount,
There are problems that the insertion loss due to the dispersion compensating optical fiber increases and the required length of the dispersion compensating optical fiber ranges from several km to several + km, so that the volume when mounted on the device becomes very large. In particular, by combining light of multiple wavelengths,
In the case of wavelength division multiplexing communication (WDM communication) in which transmission is performed using a single optical fiber, the amount of chromatic dispersion generated at each wavelength is different. The amount of dispersion compensating optical fiber required in proportion to the number was required, and the size of the apparatus was further required.

[0004] Another method of compensating the dispersion of the transmission line is to use a chirped fiber grating. A fiber grating is an optical component in which a refractive index of a core of an optical fiber is periodically changed by irradiating ultraviolet rays to form a grating, and reflects light of a specific wavelength. A grating whose period continuously changes in the longitudinal direction of the grating is particularly called a chirped fiber grating. By using such a chirped fiber grating, chromatic dispersion compensation in a transmission optical fiber can be performed.

Next, the principle of the chirped fiber grating will be described with reference to FIGS. FIG. 3 schematically shows an example of a chirped fiber grating in which refractive index increasing portions 4 and 4 are periodically formed in a part of a core 1 in a longitudinal direction, and light enters from a left end. The following shows the case. In such a chirped fiber grating 5, since the grating period changes continuously in the longitudinal direction, the reflection position differs depending on the wavelength of the incident light. In the case of the chirped fiber grating 5 shown in FIG. 3, light having a wavelength of λ1 is reflected at a portion near the incident end of the chirped fiber grating 5,
Light λ2 having a wavelength longer than λ1 is reflected at a portion farthest from the incident end. For this reason, there is a difference in the time (delay time) until the lights of the wavelengths λ1 and λ2 enter, are reflected by the grating portion where the refractive index increasing portions 4 and 4 are formed, and return to the incident end again. Actually, the delay time is the wavelength λ1
To λ2 continuously.

FIG. 4 is a graph showing the relationship between the wavelength and the delay time in the chirped fiber grating 5, and FIG. 4A shows the relationship between the position L in the grating section and the reflection wavelength λ. b) shows the relationship between the reflection wavelength λ and the delay time τ. The characteristic shown in FIG. 4B is called a group delay time characteristic. The slope of the graph in FIG. 4B indicates the chromatic dispersion. The chromatic dispersion has a constant value D as shown in FIG. 4C, and the chromatic dispersion in the chirped fiber grating 5 can be compensated. Become. The value D is represented by D = (τ2-τ1) / (λ2-λ1). Such chirped fiber gratings for dispersion compensation are especially
persion Compensation Fiber Grating (DCFG)
Call. A desired value of the wavelength dispersion characteristics of the DCFG can be obtained by appropriately designing the amount of change (chirp rate) of the grating period in the DCFG. Further, by changing the light incident direction, the slope of the delay time characteristic in the DCFG is reversed, so that both positive and negative dispersion compensation can be realized.

[0007] Other features of DCFG include the following. 1. It operates only in the reflection band of the grating, and its band is usually about 1 to 10 nm. 2. Since the grating length is 10 to 100 cm, the device becomes very small. 3. Low insertion loss. Also, the insertion loss is not proportional to the amount of dispersion obtained. 4. The nonlinear characteristic with respect to the incident power of light as seen in the dispersion compensating optical fiber is very small.

[0008] The phase mask method is used for the fabrication of the DCFG, similarly to the fabrication of a normal fiber grating. This phase mask is a quartz substrate on which periodic grooves corresponding to the period of the grating written on the optical fiber are formed. A method of manufacturing a fiber grating by the above-described phase mask method will be described with reference to FIG. The optical fiber 3 is disposed so as to be in contact with or close to the phase mask 6 by removing the coating only at the portion where the grating is to be written. The ultraviolet beam 7 is applied through the phase mask 6 to the portion of the optical fiber 3 where the clad 2 is exposed, where the grating is formed. The ultraviolet beam 7 having passed through the phase mask 6 is diffracted and separated into two diffracted lights 8, 8. Where the two diffracted lights 8, 8 overlap each other, the periodicity of the ultraviolet light is caused by interference fringes 9. A pattern occurs. By placing the optical fiber 3 here, the refractive index of the core 1 of the optical fiber 3 periodically rises in accordance with the intensity distribution of the ultraviolet beam 7, and a fiber grating can be manufactured.

In the case of DCFG, the longer the grating length, the larger the amount of chromatic dispersion or wavelength bandwidth that can be obtained. Therefore, it is desirable to manufacture a grating as long as possible, but this is limited by the size of the phase mask. The largest commercially available phase mask is about 15 cm.

[0010]

As a problem of such a DCFG, there is a point that the group delay time characteristic fluctuates (group delay time ripple, sometimes simply referred to as ripple). The characteristic of the group delay time ripple (hereinafter sometimes referred to as ripple characteristic) will be described. Figures 6-9
Shows an example of DCFG characteristics. FIG. 6 shows the reflection characteristics of DCFG. In this case, 1546.3 to 1547.
It is possible to reflect light having a wavelength of 3 nm and compensate for dispersion in that band. FIG. 7 shows a group delay time characteristic of reflected light, which is a characteristic representing the performance of dispersion compensation of DCFG. The slope of the graph in FIG. 7 indicates the amount of dispersion that can be compensated for by DCFG, and in this case, it is 850 ps / nm.
Such a group delay time characteristic is ideally a straight line as shown in the graph of FIG. However, the characteristics of the DCFG that can be actually produced include fine fluctuation components and do not form a straight line. FIG. 9 is a graph in which only fine fluctuation components in this DCFG are plotted, and shows a ripple characteristic. Such a ripple characteristic shown in FIG. 9 is obtained by performing a linear approximation to the group delay time characteristic measured by DCFG, and taking a difference between the approximated line and the measured group delay time characteristic. .

In order to actually use DCFG as a dispersion compensating element, it is desirable that the above-mentioned ripple (represented by the amplitude of the graph) be as small as possible. However, in the conventional DCFG fabrication technology, the obtained DC
The FG has a very large ripple of several tens to several hundreds of ps, making it difficult to use it as a dispersion compensating element in an actual optical communication system. The causes of the ripple in the DCFG include a change in the refractive index of the fiber grating and non-uniformity of the grating period.

As examples of the influence of such non-uniformity, FIGS. 10 and 11 show the results of simulations of the ripple characteristics for the case where there is no fluctuation in the grating period and the case where there is. As shown in FIG.
When there is no fluctuation in the grating period of the DCFG, there is no change in the ripple characteristics as shown in FIG. On the other hand, as shown in FIG. 11A, when the grating period fluctuates with a width of about 0.05 nm,
It can be seen that a ripple of about 50 ps occurs in the width as shown in FIG. As described above, in DCFG, the ripple characteristics are greatly affected by the non-uniformity of the grating.

[0013] Fluctuation of the grating (non-uniformity)
The following can be considered as factors that cause the occurrence. (1) Fluctuation caused by a phase mask used for manufacturing a grating. (2) Fluctuation caused by the outer diameter of the optical fiber used for the grating or the refractive index of the core. (3) Fluctuation caused by an exposure apparatus and an exposure method used for forming a grating.

It is considered that a composite result of these fluctuations results in the occurrence of a riffle. Until now, (1),
Regarding the cause of the fluctuation shown in (2), several improvement methods are described. (Ref. 2000 IEICE General Conference Proceedings C-3-62, IEEE Photonics Tec
hnology Letters, vol. 11pp572-574, 1999., Technical D
igest of Fiber Cnference 99 (CFC99), FA, 1999.s) However, these fluctuations shown in (1) and (2) do not fluctuate with time, and are determined by the phase mask and optical fiber used. In practice, these fluctuations cannot be reduced when fabricating fiber gratings. On the other hand, the fluctuation shown in (3) is a fluctuation that fluctuates with time, such as external power such as the power intensity of the ultraviolet laser for irradiation, the quality of the laser beam, and vibration applied to the exposure apparatus. The method of reducing has not been established yet.

The present invention has been made in view of the above circumstances, and reduces ripples by reducing fluctuations generated in the above-described method of manufacturing a chirped fiber grating to reduce non-uniformity of the grating.
It is an object of the present invention to obtain a chirped fiber grating having excellent dispersion characteristics.

[0016]

In order to solve the above-mentioned problems, the present inventors have focused on the fluctuation (3) occurring in the method of manufacturing a chirped fiber grating and reduced the fluctuation to reduce the chirp. A method for reducing ripples in a fiber grating has been devised. That is, according to the present invention, a phase mask is arranged close to an optical fiber from which a coating layer has been partially removed, and a grating is formed by irradiating the optical fiber with an ultraviolet beam through the phase mask on the coating layer removed portion of the optical fiber. In the method for producing a chirped fiber grating, a UV beam is scanned on the phase mask, and the optical fiber is repeatedly irradiated with ultraviolet light to form a grating. And to obtain a chirped fiber grating excellent in quality.

In the above-described method of manufacturing a grating, it is desirable that the number of scans of the ultraviolet beam is 5 or more. Further, in the above-described method of fabricating the fiber grating, the scanning speed of the ultraviolet beam is set to 0.
It is desirable that it be 2 mm / sec or more.

[0018]

The general method of forming a grating is as shown in FIG. 5 described above. At this time, the irradiation of ultraviolet light is performed by an ultraviolet beam 7 by an ultraviolet laser.
The ultraviolet beam 7 moves along the longitudinal direction of the optical fiber 3. This is because the size of the ultraviolet beam 7 is generally smaller than the size of the phase mask 6, so that a grating having the same size as the phase mask 6 cannot be manufactured unless the ultraviolet beam 7 is moved.

As a result of intensive studies, the present inventors have found that, in the above-described method of manufacturing a fiber grating, the number of times of irradiating the optical fiber with ultraviolet rays, that is, the number of times of scanning the ultraviolet beam, and the ripple characteristic of the chirped fiber grating. Has a correlation, that is, the ripple decreases as the number of UV beam scans increases, and succeeds in reducing the ripple in the chirped fiber grating by increasing the number of UV beam scans, completing the present invention. did.

The reason why the ripple in the chirped fiber grating can be reduced as described above is considered as follows. First, when a grating is produced on an optical fiber by one scan of an ultraviolet beam, fluctuations that occur during scanning remain in the grating as they are. However, by repeatedly irradiating the optical fiber with the ultraviolet light by scanning the ultraviolet beam, the time-varying fluctuation component is averaged, and finally the fluctuation remaining in the grating is reduced. Ribbles in the chirped fiber grating can be reduced.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The method for manufacturing a chirped fiber grating of the present invention can apply the conventional phase mask method shown in FIG. 5 described above. The feature of the method is that an ultraviolet beam irradiated from above the phase mask is repeatedly scanned, This is to repeatedly irradiate the optical fiber with ultraviolet rays. Hereinafter, a method of manufacturing a chirped fiber grating of the present invention will be described with reference to FIG.

The optical fiber 3 forming the grating is not particularly limited, and those used for a normal chirped fiber grating can be used.
In other words, a core 1 made of quartz glass doped with germanium oxide, a clad 2 made of quartz glass provided on the core 1, and a single mode optical fiber 3 provided with a coating layer on the clad 2 are used. Can be. In the method of manufacturing the chirped fiber grating, first, the cladding is removed by removing the coating layer of the portion of the optical fiber 3 where the grating is formed. Next, the phase mask 6 is disposed in contact with or close to the grating forming portion. The phase mask 6 has a periodic groove corresponding to the period of the grating.

Then, an ultraviolet beam 7 is applied to the grating portion of the optical fiber 3 through the phase mask 6. At this time, the ultraviolet beam 7 is scanned (moved) to repeatedly irradiate the grating forming portion with ultraviolet light. At this time, the number of repeated irradiations of the ultraviolet beam, that is, the number of scans is set to 5 or more, preferably 5 to 15 times. If the number of scans is less than 5, the effect of reducing fluctuations during the formation of the grating cannot be sufficiently obtained. Further, even if the number of times exceeds 15, the effect is saturated, and the production time is required.

The moving speed of the ultraviolet beam 7 can take an arbitrary value in consideration of the number of scans of the ultraviolet beam 7 and the like, but it is desirable that the moving speed be as fast as possible from the viewpoint of the manufacturing speed. For example, in the case of manufacturing a 100 mm long DCFG, if the number of scans of the ultraviolet beam 7 is 5 times and the exposure time of the ultraviolet beam is within 1 hour, the ultraviolet beam 7
It is desirable to set the scanning speed to 0.2 mm / sec or more, preferably in the range of 0.2 to 1 mm / sec.

The irradiation device of the ultraviolet beam 7 includes:
Excimer lasers such as F ₂ , ArF, XeF, KrF,
An excimer laser, a third-order high frequency of a Q-switched YAG laser or the like can be used. The ultraviolet beam 7 preferably has a wavelength of 190 to 400 nm and a beam width of 1 to 10 mm.

According to the method of manufacturing a chirped fiber grating, fluctuations due to an exposure apparatus and an exposure method used for forming the grating can be reduced, and the non-uniformity of the grating, that is, the group delay time ripple can be reduced. Can be reduced. Therefore, the reflection / transmission characteristics and dispersion characteristics of the chirped fiber grating can be improved, and can be effectively used as a chromatic dispersion compensation element in optical communication.

This method of manufacturing a chirped fiber grating can be applied to any grating length. For example, a grating for multiplexing / demultiplexing used in wavelength division multiplexing communication has a length of about several centimeters, but a similar technique can be expected to reduce group delay time ripple.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments. Example 1, Comparative Example 1 A chirped fiber grating was manufactured under the following conditions. Grating length: 10 cm Grating central wavelength: 1551.3 nm Dispersion: 850 ps / nm Reflectance: 90% Irradiation ultraviolet wavelength: 244 nm Irradiation ultraviolet beam width: 1 mm The first time was taken as Example 1. In order to make the reflectance the same, the moving speed of the ultraviolet beam was set to 0.1 mm / sec for one scan and 0.6 mm / sec for six scans.

The rubble characteristics of the obtained chirped fiber grating are shown in FIG. 1 for Example 1 and in FIG. 12 for Comparative Example 1. In each figure, (a) shows the reflection characteristics and (b) shows the lip characteristics. From FIG. 12 (b),
In Comparative Example 1, the maximum ripple was 200
psp-p. Further, the reflection characteristics of FIG. On the other hand, in the case of the first embodiment, the ripple characteristics shown in FIG.
It can be seen that it has been reduced to Also, it can be seen that the fluctuation in the reflection characteristic of FIG. Thus, it can be confirmed that in the chirped fiber grating of Example 1, not only the fluctuation of the reflection characteristic but also the fluctuation of the reflection characteristic can be reduced. This is because the reflection characteristics and the group delay time characteristics are closely related to the optical characteristics of the grating.

Example 2 The relationship between the number of scans of the ultraviolet beam of the manufactured chirped fiber grating and the magnitude of the ripple was examined. The manufacturing conditions were Example 1.
A chirped fiber grating was manufactured in the same manner as described above, and the number of scans of the ultraviolet beam was increased up to 20 times, and a change in the magnitude of the ripple was observed. The scanning speed of the ultraviolet beam was controlled so that the reflectance was constant as in the first embodiment. The results are shown in FIG.

As the number of scans of the ultraviolet beam increases, the size of the rubble decreases. However, when the number of scans decreases to a certain extent, the size of the ripple does not decrease even if the number of scans is increased. This is considered to be because the fluctuation that does not fluctuate over time remains as a factor of the ripple. However, it can be seen that the ripple can be reduced to a certain point. From FIG. 2, it can be seen that when the number of scans of the ultraviolet beam is 5 or more, the ripple level is settled to a substantially steady level.

In the second embodiment, if the number of times of scanning of the ultraviolet beam is five and the exposure time is within one hour, the scanning speed of the ultraviolet beam should be 0.2 mm / sec or more. You can see that.

[0033]

As described above, according to the method for manufacturing a chirped fiber grating of the present invention, it is possible to reduce the fluctuation at the time of forming the grating and to greatly reduce the ripple in the chirped fiber grating. it can. Therefore, the chirped fiber grating obtained by the present invention has excellent dispersion compensation characteristics and can be suitably used as DCFG.

[Brief description of the drawings]

FIG. 1 is a graph showing characteristics of a chirped fiber grating according to a first embodiment of the present invention, wherein FIG.
Shows the reflection characteristics, and (b) shows the ripple characteristics.

FIG. 2 is a graph showing results in Example 2 of the present invention.

FIG. 3 is a schematic configuration diagram for explaining a structure of a chirped fiber grating.

FIG. 4 is a graph for explaining a group delay ripple characteristic in a chirped fiber grating;
(A) shows the relationship between the grating position and the reflection wavelength, (b) shows the relationship between the wavelength and the delay time, and (c) shows the relationship between the wavelength and the chromatic dispersion.

FIG. 5 is a schematic configuration diagram for explaining a method of manufacturing a chirped fiber grating.

FIG. 6 is a graph showing reflection characteristics of DCFG.

FIG. 7 is a graph showing the performance of DCFG dispersion compensation.

FIG. 8 is a graph showing ideal delay time characteristics.

FIG. 9 is a graph in which only fine fluctuation components in DCFG are plotted, and shows group delay time ripple characteristics.

FIG. 10 is a graph simulating and calculating a ripple characteristic of a chirped fiber grating when there is no fluctuation in the grating period.

FIG. 11 is a graph obtained by simulating and calculating the ripple characteristics of a chirped fiber grating when the grating period fluctuates.

FIGS. 12A and 12B are graphs showing characteristics of a chirped fiber grating of Comparative Example 1 in an example of the present invention, in which FIG. 12A shows reflection characteristics and FIG. 12B shows ripple characteristics.

[Explanation of symbols]

 Reference Signs List 1 core 2 clad 3 optical fiber 5 chirped fiber grating 6 phase mask 7 ultraviolet beam

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kenji Nishiide 1440, Mukurosaki, Sakura-shi, Chiba Pref. F-term (Reference) 2H049 AA02 AA06 AA33 AA51 AA62 2H050 AA07 AC84 AD16

Claims (3)

[Claims] 1. A chirped fiber grating in which a phase mask is arranged close to an optical fiber from which a coating layer has been partially removed, and a grating is formed by irradiating ultraviolet rays through the phase mask to a portion of the optical fiber from which the coating layer has been removed. The method of manufacturing a chirped fiber grating according to claim 1, wherein an ultraviolet beam is scanned on the phase mask, and the optical fiber is repeatedly irradiated with ultraviolet light to form a grating. 2. The method for producing a chirped fiber grating according to claim 1, wherein in the method for producing a chirped fiber grating, the number of times of scanning the ultraviolet beam is five or more. 3. The method of manufacturing a chirped fiber grating according to claim 1, wherein the scanning speed of the ultraviolet beam is 0.2 mm / sec or more.
Or a method for producing a chirped fiber grating according to item 2.