JP2006058678A - Optical fiber grating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber grating with which a high cut-off rate and miniaturization of an element can be accomplished at the same time. <P>SOLUTION: In the core 1 of an optical fiber, there is formed a first chirped fiber grating FG1 in a manner that a cut-off wavelength varies from a short wave to a long wave along a light travelling direction. Also, connected to the long wave side of the first chirped fiber grating FG1, a second chirped fiber grating FG2 is formed in a manner that the cut-off wavelength varies from a long wave to a short wave along the light travelling direction. In addition, a distance d between the ends at the long wave side of the first and the second chirped fiber grating FG1, FG2 is made not longer than 50 mm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical fiber grating used in the fields of optical information communication, optical measurement, and the like.

  The optical fiber grating transmits or reflects the propagating light according to the wavelength by giving a periodic refractive index change in the optical axis direction to the core region of the optical fiber. Various applications such as compensators have been made.

  Conventionally, when a high cutoff rate is to be obtained with a single-stage fiber grating, a core region having a large refractive index change is produced.

For example, Patent Document 1 in which two stages of long-period fiber gratings are connected is reported as an optical fiber grating connected in multiple stages, and Non-Patent Document 1 is reported as an example in which chirped fiber gratings are connected in series. However, none of them is intended to obtain a high blocking rate by multistage connection.
JP 2001-91759 A Y. Painchaud et al. “ECOC'01, Th.M.1.2, Vol.4, pp490-491 (2001)”.

  However, in the conventional optical fiber grating, in order to reduce the size of the element, when the distance between the chirped fiber gratings connected in multiple stages is reduced, the cutoff rate of the optical fiber grating is caused by the interference of the adjacent chirped fiber gratings. However, the sum of the cutoff rates of the chirped fiber grating cannot be reached, and a high cutoff rate cannot be obtained.

  The purpose of the optical fiber grating described in Non-Patent Document 1 is not to obtain a high cutoff rate, and the chirped fiber gratings connected in multiple stages are not close to each other, and the element can be miniaturized. could not.

  In addition, the optical fiber grating described in the above-mentioned Patent Document 1 is a product in which a long-period fiber grating having a function different from that of a chirped fiber grating is produced close to the optical fiber grating, and is intended to obtain specific transmission characteristics. It was not intended to obtain a high blocking rate.

  On the other hand, in order to fabricate an optical fiber grating having a high cutoff rate with a single-stage fiber grating instead of a multi-stage connection, a large refractive index change is caused in the core region. In the vicinity of this saturation point, the speed of refractive index change is slow. In the conventional technology, since a high cutoff rate has been achieved in the vicinity of such a saturation point, it cannot be manufactured, or it has been forced to manufacture in a state where the manufacturing time is very long and the yield is extremely poor. .

  The present invention has been made in view of the above, and an object of the present invention is to provide an optical fiber grating that can simultaneously achieve a high blocking rate and a reduction in the size of the element.

  The optical fiber grating of the present invention is an optical fiber grating in which two gratings are formed on one optical fiber so that the cutoff wavelength changes from a short wave to a long wave along the light traveling direction on the optical fiber. The first chirped fiber grating formed, and connected to the long wave side of the first chirped fiber grating on the optical fiber, and the cutoff wavelength changes from the long wave to the short wave along the light traveling direction. And a second chirped fiber grating formed as described above.

  In the optical fiber grating of the present invention, the distance between the long-wave end of the first chirped fiber grating and the long-wave end of the second chirped fiber grating is 50 mm or less. And

  The optical fiber grating of the present invention is formed on an optical fiber such that the first chirped fiber grating is changed from a short wave to a long wave along the light traveling direction, and the second chirped fiber grating is formed. Since it is connected to the long wave side of the first chirped fiber grating and is formed such that the cutoff wavelength changes from the long wave to the short wave along the traveling direction of the light, a high cutoff rate can be achieved.

  Further, since the distance between the long wave side end of the first chirped fiber grating and the long wave side end of the second chirped fiber grating can be reduced to 50 mm or less, miniaturization of the element is achieved. Can do.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(How to make)
A phase mask method is used to manufacture the optical fiber grating of the present embodiment. This phase mask is a quartz substrate on which periodic grooves corresponding to the period of the grating written in the optical fiber are formed. A method of manufacturing an optical fiber grating by this phase mask method will be described with reference to FIGS.

  First, as shown in FIG. 1A, the optical fiber 3 is disposed so as to be in contact with or close to the phase mask 4 after removing the coating only on the portion where the grating is written (step S10). At this time, it arrange | positions so that a cutoff wavelength may change from a short wave to a long wave along the advancing direction of light.

  In this arrangement direction, the light with the shortest wavelength in the cutoff band is reflected at a portion near the incident end of the chirped fiber grating, and the light with the longest wavelength is reflected at a portion farthest from the incident end. Hereinafter, the end portion of the chirped fiber grating that reflects the light having the shortest wavelength is referred to as the short wave side, and is denoted as S. The end portion that reflects the light having the longest wavelength is referred to as the long wave side. Call it and express it as L.

  Next, the ultraviolet beam 5 is irradiated to the formation part of the first chirped fiber grating FG1 of the optical fiber 3 where the cladding 2 is exposed through the phase mask 4 (step S20). As a result, the ultraviolet beam 5 that has passed through the phase mask 4 is diffracted and separated into two diffracted lights 6a and 6b. Where the two diffracted lights 6a and 6b overlap, the period of the ultraviolet light is caused by the interference fringes 7. Pattern occurs. By placing the optical fiber 3 here, the refractive index of the core 1 of the optical fiber 3 periodically rises corresponding to the intensity distribution of the ultraviolet beam 5, and the first chirped fiber grating FG1 is formed.

  Thereafter, as shown in FIG. 1B, the optical fiber 3 is inverted to write the second chirped fiber grating FG2, so that the cutoff wavelength of the second chirped fiber grating FG2 changes from a long wave to a short wave. The optical fiber 3 is disposed (step S30).

  Then, in the same manner as in step S20, the optical fiber grating of the present embodiment can be manufactured by irradiating the formation portion of the second chirped fiber grating FG2 with the ultraviolet ray 5 (step S40).

  By the way, when two chirped fiber gratings are manufactured in series on the same optical fiber, the transmission spectrum does not necessarily reach the sum of the transmission spectra of the respective chirped fiber gratings. This is due to light interference by the two chirped fiber gratings.

(Transmission spectrum)
Hereinafter, the relationship between the arrangement of the chirped fiber grating and the transmission spectrum will be described.

  The experiment for examining the relationship between the arrangement of the chirped fiber grating and the transmission spectrum is performed using the apparatus shown in FIG. The apparatus shown in FIG. 3 includes an SLD light source 8 having a wavelength of 1600 to 1680 nm, an optical fiber 9, and an optical spectrum analyzer 10, and produces a chirped fiber grating in the optical fiber 9.

  First, the light from the SLD light source 8 is incident on the optical fiber 9 and its transmission spectrum is detected by the optical spectrum analyzer 10. Then, after the chirped fiber grating is manufactured, the light of the SLD light source 8 is passed through the optical fiber 9 and its transmission spectrum is detected by the optical spectrum analyzer 10.

  Then, the transmission characteristics of the chirped fiber grating are examined by taking the difference between the transmission spectra before and after the production of the chirped fiber grating.

  In the chirped fiber grating, the cutoff wavelength continuously changes in the longitudinal direction of the fiber. When two chirped fiber gratings are arranged in series, the chirped fiber grating on the incident side is called the first chirped fiber grating FG1, and the chirped fiber grating on the emitting side is called the second chirped fiber grating FG2. I will do it. When two chirped fiber gratings are produced in series, they are always produced from FG1. Further, the distance between the emission side end of FG1 and the incidence side end of FG2 is d, and when d is a negative value, it indicates that FG1 and FG2 overlap.

(How to place)
Here, FIG. 4 shows how the two chirped fiber gratings are arranged. 4A, 4B, and 4C show a state in which the first and second chirped fiber gratings FG1 and FG2 are formed on the optical fiber 3. FIG.

  FIG. 4A shows that the cutoff wavelength of FG1 changes from a short wave to a long wave and the cutoff wavelength of FG2 changes from a long wave to a short wave along the light traveling direction, and this arrangement is represented as SL-LS. .

  In FIG. 4B, the cutoff wavelength of FG1 changes from a short wave to a long wave and the cutoff wavelength of FG2 changes from a short wave to a long wave along the traveling direction of light, and is denoted as SL-SL.

  In FIG. 4C, the cutoff wavelength of FG1 changes from a long wave to a short wave along the traveling direction of light, and the cutoff wavelength of FG2 changes from a short wave to a long wave, which is expressed as LS-SL.

  Since it is known that the characteristic of LS-LS is equal to that of SL-SL, the arrangement of arranging two chirped fiber gratings in series may be considered in three ways shown in FIG.

(Experimental example 1)
The relationship between the arrangement of the two chirped fiber gratings and the transmission spectrum will be described with reference to FIG.

  Here, the length of both FG1 and FG2 is 6 mm, and the distance d between FG1 and FG2 is about 2 mm. Fabrication is performed from the FG1 side. First, about 30 dB of FG1 is fabricated, and then FG2 is exposed until the cutoff rate is saturated. In the legend, the first stage is the transmission spectrum after completion of FG1, and the second stage is the transmission spectrum after completion of FG2.

  FIG. 5 (a) shows the transmission spectrum in the case of SL-LS. The blocking rate is about 30 dB for the first stage and about 50 dB for the second stage, and the transmission spectra of FG1 and FG2 are added together. Has reached the sum.

  FIG. 5B shows the case of SL-SL. The blocking rate is about 30 dB in the first stage and about 40 dB in the second stage, and does not reach the sum obtained by adding the transmission spectra of FG1 and FG2.

  FIG. 5C shows the case of LS-SL. The blocking rate is about 30 dB for both the first stage and the second stage, and there is almost no increase in the blocking rate when the FG2 is manufactured.

  From the above, it can be seen that an increase in the blocking rate of FG2 is seen in the same manner as in FG1, and it is only in the arrangement of SL-LS that the transmission spectrum reaches the sum of the transmission spectra of FG1 and FG2.

  In addition, an increase in the blocking rate of FG2 is seen, and it can be seen that the degree to which the transmission spectrum reaches the sum of the transmission spectra of FG1 and FG2 has a relationship of SL-LS> SL-SL> LS-SL. .

  Next, the relationship between the interval between FG1 and FG2 and the transmission spectrum will be described with reference to FIGS.

(Experimental example 2)
FIG. 6 shows the relationship between the distance d between FG1 and FG2 and the transmission spectrum in LS-SL.

  FIG. 6A shows a case where the distance d is 2 mm, and the blocking rate is about 30 dB in both the first stage and the second stage.

  FIG. 6B shows a case where the distance d is 50 mm, and the blocking rate is about 30 dB in both the first stage and the second stage as in FIG. 6A.

  FIG. 6 (c) shows the case where the distance d is 30 cm, the blocking rate is about 30 dB in the first stage, and about 60 dB in the second stage, and reaches the sum of the transmission spectra of FG1 and FG2 approximately. Yes.

  From the above, it can be seen that when the distance d between FG1 and FG2 is 0 mm to 50 mm, the second-stage blocking rate hardly increases. On the other hand, when the distance d is increased to 30 cm, the blocking rate increases in the second stage as in the first stage.

  That is, when the LS-SL is arranged, if the distance d between FG1 and FG2 is within 50 mm, the influence of interference between FG1 and FG2 remains, and the transmission spectrum does not reach the sum of the individual transmission spectra. It can be seen that when the distance d is 30 cm, the influence of interference between FG1 and FG2 is almost eliminated.

(Experimental example 3)
FIG. 7 shows the relationship between the distance d between FG1 and FG2 and the transmission spectrum in SL-SL.

  FIG. 7A shows a case where the distance d is 2 mm, and the blocking rate is about 30 dB in the first stage and about 40 dB in the second stage, and is not the sum of the transmission spectra of FG1 and FG2.

  FIG. 7B shows the case where the distance d is 50 mm, and the blocking rate is about 30 dB in the first stage and about 40 dB in the second stage as in FIG. 7A, and the transmission spectra of FG1 and FG2 are added. The total sum has not been reached.

  FIG. 7 (c) shows a case where the distance d is 30 cm, the blocking rate is about 30 dB in the first stage, and about 60 dB in the second stage, and reaches almost the sum of the transmission spectra of FG1 and FG2. Yes.

  The result of this experimental example shows almost the same tendency as in the case of LS-SL in Experimental example 2, but since the coherence of FG1 and FG2 is weaker in SL-SL than in LS-SL, the second-stage blocking rate The amount of increase is larger than that of LS-SL.

  From the above, it can be seen that when the distance d between FG1 and FG2 is 0 mm to 50 mm, the increase in the second-stage blocking rate reaches a peak at about 40 dB. On the other hand, when the distance d is increased to 30 cm, the blocking rate increases in the second stage as in the first stage.

  That is, in the SL-SL arrangement, when the distance d between FG1 and FG2 is within 50 mm, the influence of interference between FG1 and FG2 remains, and the transmission spectrum does not reach the sum of the transmission spectra of FG1 and FG2. When the distance d is 30 cm, the influence of interference between FG1 and FG2 is almost eliminated.

(Experimental example 4)
8 and 9 show the relationship between the distance d between FG1 and FG2 and the transmission spectrum in SL-LS.

  8A is when the distance d is 2 mm, FIG. 8B is when the distance d is 50 mm, FIG. 8C is when the distance d is 30 cm, and FIG. 9A is when the distance d is 0 mm. FIG. 9B shows the case where the distance d is −2 mm, that is, the two fiber gratings partially overlap each other. In any case, the blocking rate reaches almost the sum of the transmission spectra of FG1 and FG2.

  From the above, it can be seen that in this arrangement, the second-stage blocking rate rises in the same manner as the first step regardless of the distance d between FG1 and FG2, and the blocking rate is substantially the sum of the blocking rates of FG1 and FG2. .

  That is, when the SL-LS is arranged, there is almost no influence of interference regardless of the distance d between the FG1 and FG2, and the transmission spectrum of the optical fiber grating of this arrangement is almost the same as the transmission spectrum of the FG1 and FG2. The sum of

  FIG. 10 shows the transmission spectrum when the distance d between FG1 and FG2 is 2 mm and 50 mm in the above-described three arrangement methods, and shows whether or not the objective of downsizing and high blocking rate can be achieved. Is. From the above description, when the distance d is 50 mm or less, the transmission spectrum reaches almost the sum of FG1 and FG2, and a high blocking rate of about 50 dB can be achieved only by the arrangement of the SL-LS. is there.

  When the distance d is set to 30 cm, the transmission spectrum reaches almost the sum of FG1 and FG2 in any arrangement, but in this case, the object of downsizing cannot be achieved.

  FIG. 11 is a schematic diagram showing the configuration of the final form of the optical fiber grating of the present embodiment. The optical fiber quartz part 13, the first chirped fiber grating FG1 and the second chirped fiber grating FG2 arranged at a distance d = 1 mm in the manner of the arrangement of SL-LS in the optical fiber quartz part 13, and the light A ferrule 14 that houses the fiber grating portion of the fiber quartz portion 13, a flange 15 that covers the ferrule 14 with the incident side of FG 1 exposed, and an optical fiber quartz portion 13 that extends in the emission direction of FG 2 are accommodated in the flange 15. It is composed of connected linear optical fibers 3. Since the total length of the fiber grating portion is 10 mm and the total length of the casing is 15 mm, downsizing is realized.

It is a schematic block diagram for demonstrating the manufacturing method of the optical fiber grating of this invention. It is a flowchart which shows the preparation procedures of the optical fiber grating of this invention. It is a schematic block diagram which shows the experimental apparatus for investigating the relationship between the arrangement | positioning method of two chirped fiber gratings, and a transmission spectrum in an optical fiber grating. It is the schematic which shows the method of arrangement | positioning of two chirped fiber gratings in an optical fiber grating. It is a graph which shows the relationship between the arrangement method of two chirped fiber gratings, and a transmission spectrum. It is a graph which shows the relationship between the space | interval of FG1 and FG2, and the transmission spectrum when the arrangement | positioning method of two chirped fiber gratings is LS-SL. It is a graph which shows the relationship between the space | interval of FG1 and FG2, and the transmission spectrum when the arrangement | positioning method of two chirped fiber gratings is SL-SL. It is a graph which shows the relationship between the space | interval of FG1 and FG2, and the transmission spectrum when the arrangement | positioning method of two chirped fiber gratings is SL-LS and the space | interval of FG1 and FG2 is 2 mm, 50 mm, and 30 cm. It is a graph which shows the relationship between the space | interval of FG1 and FG2, and the transmission spectrum when the arrangement | positioning method of two chirped fiber gratings is SL-LS and the space | interval of FG1 and FG2 is 0 mm and -2 mm. It is a figure which shows determination of whether the objective of size reduction and a high interruption | blocking rate can be achieved in three ways of arrangement | positioning. It is the schematic which shows the structure of the last form of embodiment of the optical fiber grating of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Core 2 Clad 3 Optical fiber 4 Phase mask 5 Ultraviolet beam 6a, 6b Diffracted light 7 Interference fringe 8 SLD light source 9 Optical fiber 10 Optical spectrum analyzer 13 Optical fiber quartz part 14 Ferrule 15 Flange FG1 1st chirped fiber grating FG2 1st 2. Chirped fiber grating

Claims (2)

In an optical fiber grating formed by forming two gratings on one optical fiber,
A first chirped fiber grating formed on the optical fiber so that a cutoff wavelength changes from a short wave to a long wave along a traveling direction of light;
A second chirped fiber that is connected to the long wave side of the first chirped fiber grating on the optical fiber so that the cutoff wavelength changes from a long wave to a short wave along the traveling direction of light. An optical fiber grating characterized by comprising: 2. The optical fiber according to claim 1, wherein a distance between an end on the long wave side of the first chirped fiber grating and an end on the long wave side of the second chirped fiber grating is 50 mm or less. Grating.

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