JP3138731B2 - Optical fiber filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber filter applicable to a narrow band filter for wavelength division multiplex communication, a distortion sensor head, and the like.

[0002]

2. Description of the Related Art An optical fiber diffraction grating can be applied to a filter or a sensor head for detecting distortion of an optical communication signal. Generally, optical fiber gratings are classified into short-period gratings having a submicron pitch and long-period gratings having a pitch of several hundred microns or more. The former is already commercialized and is used for wavelength separation of communication signals, shaping of signal waveforms, and the like, and mainly uses return light due to diffraction.

[0003] On the other hand, a long-period diffraction grating is a diffraction grating that utilizes the penetration of signal light into a cladding portion. FIG.
FIG. 3 is a cross-sectional view schematically illustrating a configuration of a long-period diffraction grating. As shown in FIG. 6, the long-period diffraction grating 2 has a high refractive index portion 21 and a low refractive index portion at a constant pitch P of several hundred microns or more on the core portion 23 of the single mode optical fiber from which the coating material has been removed. 22 are alternately formed. Core part 2
A part of the signal light S ₀ that has passed through 3 leaches into the cladding part 24 at the high refractive index part 21. The wavelength of the leaching light depends on the pitch P. If the pitch P is small, the wavelength of the leaching light is small, and if the pitch P is large, the wavelength of the leaching light is large. The leached light S ₁ is applied to the cladding 2
Since the coating material covering the outer peripheral surface of No. 4 has been removed, Fresnel reflection occurs on the outer peripheral surface, and the return light S ₂ returns toward the core portion 23. Of the return light S ₂ , return light S ₂ (shown by a solid line) that couples with the pitch P of the diffraction grating 2 is
Proceed again toward the outer peripheral surface of the clad portion 24 (S _{3 in the} figure).
), And returns toward the core portion 23 again by Fresnel reflection.

[0004] By the repetition of the above, light coupled with the pitch P of the diffraction grating 2 is confined in the clad portion 24 and travels. Therefore, most of the light out penetrates are no longer come back to the core unit 23, it is possible to use only light S ₄ passing through the diffraction grating. The light that travels inside the cladding portion 24 passes through the diffraction grating 2 and then travels into the coating material in a region covered with a coating material having a higher refractive index than the cladding portion 24.

[0005]

As described above [0008], the transmitted light S _4, although the loss peak to a specific wavelength position corresponding to the pitch P of the diffraction grating 2 occurs, the half width is, for example, below As shown in Comparative Example 1, it is as wide as several tens to several hundreds of nm. With a single long-period diffraction grating, the half width is strictly controlled, and it has been difficult to obtain a multi-channel filter that obtains transmitted light having a plurality of sharp transmission peaks.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks of the prior art, and has as its object to provide an optical fiber filter in which the spectral waveform of transmitted light is strictly controlled using a long-period fiber diffraction grating. And

[0007]

The inventor of the present invention has conducted various studies to solve the above problems, and as a result, in a single mode optical fiber, writes a long-period diffraction grating having a constant pitch at three or more locations at equal intervals. As a result, it has been found that a kind of resonance phenomenon corresponding to the interval between the diffraction gratings is obtained, and a multi-channel filter having a plurality of transmission wavelength bands in which the spectral waveform of transmitted light is strictly controlled can be formed.

That is, the present invention provides an optical fiber filter characterized in that a core portion of a single mode optical fiber is provided with a diffraction grating group in which three or more diffraction gratings are formed at regular intervals.

[0009] Then, among the diffraction grating group, the first diffraction grating counted from the incident end of _{light, κ 0 · d 0 = π} / 2
(Where κ ₀ is the coupling constant of the diffraction grating, d ₀ is the length of the diffraction grating), and the second and subsequent diffraction gratings counted from the light incident end are κ ₀ · d ₀ <π. / 2 is satisfied.

[0010] Preferably, the predetermined interval between the diffraction gratings is 5 cm or more and 50 cm or less.

Further, preferably, the pitch of the diffraction grating is 200 μm or more and 600 μm or less.

[0012]

An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a sectional view showing a schematic configuration of an optical fiber filter used as a filter for an optical communication signal. FIG. 1A is an enlarged view of the entire configuration, and FIG. 1B is an enlarged view of one diffraction grating 2. FIG.

In order to strictly control the wavelength of the transmission peak of transmitted light and its half-value width, it is necessary to form diffraction gratings at at least three places. As shown in FIG. 1, in an optical fiber filter 1 of the present embodiment, long-period diffraction gratings 2 are formed at five locations at equal intervals L in a core portion 23 of an optical fiber that is single mode in a communication wavelength band. The long-period diffraction grating 2 has a constant pitch P and a high refractive index portion 21.
And the low refractive index portions 22 are formed alternately. The coating material is removed from the outer periphery of the clad portion 24, and the diffraction grating 2
The light oozing out of the clad portion 24 can be Fresnel-reflected on the outer peripheral surface of the clad portion 24.

The optical fiber filter 1 according to the present embodiment is used as a filter for an optical communication signal.
An optical fiber that is single mode in the communication wavelength band is used. The present invention is not limited to this. For example, when used as a strain sensor or the like, an optical fiber that is single-mode in a wavelength band outside the communication wavelength band can be used. Also in this case, it is necessary to satisfy the condition that the light leaking from the diffraction grating 2 into the clad portion 24 is Fresnel-reflected on the outer peripheral surface of the clad portion 24 and returns to the core portion 23 again.

In principle, the interval L between the diffraction gratings 2 only needs to be larger than the diffraction grating length d ₀ . However, as the interval L increases, the interval between the peaks of the transmitted light decreases, and as the interval L increases, the interval between the peaks of the transmitted light decreases. Further, if the diffraction grating 2 is deformed during use, the spectrum of the transmitted light changes, so that it is necessary to reduce the diffraction grating length d ₀ and the interval L to some extent so that the diffraction grating 2 can be easily held straight. Therefore, it is necessary to appropriately determine the interval L in consideration of the intended use and practical aspects such as the wavelength band to be used and the number of required peaks of transmitted light. From this point, in the case of the optical fiber filter 1 of the present embodiment used as a filter for an optical communication signal, the length d ₀ of each diffraction grating 2 is set to ₅ to 30 mm, and the interval L between the diffraction gratings 2 is set to 5 cm to 50 cm. It is preferable to set the following. In addition, each diffraction grating 2
Is preferably 200 μm or more and 600 μm or less. Even if the pitch P is out of this range, it can be used for a strain sensor or the like, but it is difficult to use it for communication because the diffraction peak is out of the communication wavelength band of 1.3 to 1.55 μm. It is.

[0016]

EXAMPLES Examples and comparative examples are shown below to further clarify the features of the present invention.

Example 1 8 mol% of GeO ₂ was added to the core part.
Is doped, and a cladding part having a diameter of 125 μm is provided on the outer periphery thereof. After removing a coating material of an optical fiber designed to be a single mode in a 1.55 μm band,
A diffraction grating having a pitch of 500 μm and a length of 20 mm is set to 10 cm.
Written in 5 places at intervals of. The fiber before writing was left in hydrogen at 150 atm for 7 days. As shown in FIG. 2, a KrF excimer laser is used as a writing light source for the diffraction grating, and the first diffraction grating is κ ₀ d ₀ = π through a metal mask having the same pattern as the target diffraction grating.
/ 2 (κ ₀ is the coupling constant of the diffraction grating, d ₀ is the length of the diffraction grating)
At an irradiation power density of 40 mJ / cm ² to satisfy
Irradiation of 2000 shots was performed. The second and subsequent diffraction gratings were irradiated at the same power density so that κ ₀ d ₀ = 0.315.

FIG. 3 shows a spectrum of light transmitted through an optical fiber having a group of diffraction gratings formed at equal intervals formed as described above. As shown in FIG. 3, the transmission wavelength is 1.55 μm.
, And a plurality of split spectra at equal intervals were obtained, and it was confirmed that the spectrum functioned as a multi-channel filter.

(Embodiment 2) The same optical fiber as in Embodiment 1
After removing the covering material, the pitch is 320 μm and the length is 20 μm.
mm diffraction gratings were written at five locations at 5 cm intervals.
Note that the first diffraction grating has a κ₀d₀= Π / 2 (Example 1 and
The same conditions), the second and subsequent diffraction gratings have κ ₀d₀= 0.1
Energy density of 40 mJ / cm^Twoof
A KrF laser beam was irradiated.

When the transmission spectrum of the optical fiber on which the diffraction grating group formed in this way was written was measured, a spectrum of the transmitted light having substantially the same form as that of FIG. 3 was obtained around a wavelength of 1.31 μm. It was confirmed to function as a channel type filter.

Example 3 After removing the same optical fiber covering material as in Example 1, a pitch of 500 μm and a length of 10 μm were used.
mm diffraction gratings were written at 10 locations at 10 cm intervals. Note that the first diffraction grating is κ ₀ d ₀ = π / 2 (the same conditions as in Example 1), and the second and subsequent diffraction gratings are κ ₀ d ₀ =
Energy density of 40 mJ / c so as to be 0.467
Irradiated with m ² KrF laser light.

When the transmission spectrum of the fiber on which the diffraction grating group formed in this way was written was measured, a spectrum of the transmitted light split into a plurality of pieces was obtained as in FIG. 3 and functioned as a multi-channel filter. It was confirmed that.

Comparative Example 1 After removing the same optical fiber covering material as in Example 1, the pitch was 500 μm and the length was 20 μm.
mm diffraction grating was written in one place.

When the transmission spectrum of the optical fiber on which the diffraction grating thus formed was written was measured, only a plurality of transmission peaks having a very large half-value width were observed as shown in FIG. The half width of the peak could not be controlled.

(Comparative Example 2) After removing the same optical fiber covering material as in Example 1, under the same irradiation conditions as in Example 1, a diffraction grating having a pitch of 400 μm and a length of 20 mm was formed to 10 cm.
Written in two places at intervals of. Note that these two diffraction gratings were both written so as to satisfy κ ₀ d ₀ = π / 4.

When the transmission spectrum of the optical fiber on which the diffraction grating group formed in this way was written was measured, as shown in FIG. 5, the peak of the transmitted light split into a plurality of beams around a wavelength of 1.55 μm was obtained. Although observed, each peak was not independent and was sinusoidal.

[0027]

As described above, according to the present invention, there is provided a multi-channel filter in which the position of the transmission peak of transmitted light and the half width are strictly controlled.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a schematic configuration of an optical fiber filter according to the present invention.

FIG. 2 is a sectional view schematically showing a method of manufacturing a long-period diffraction grating.

FIG. 3 shows a spectrum of light transmitted through the optical fiber filter according to the first embodiment of the present invention.

FIG. 4 shows a spectrum of transmitted light of Comparative Example 1.

FIG. 5 shows a spectrum of transmitted light of Comparative Example 2.

FIG. 6 is a sectional view schematically showing a configuration of a long-period diffraction grating.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber filter 2 Long period diffraction grating 21 High refractive index part 22 Low refractive index part 23 Core part 24 Cladding part

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 6/00 G02B 6/10 G02B 5/18

Claims (3)

(57) [Claims] 1. A single-mode optical fiber core includes a group of diffraction gratings in which three or more long-period diffraction gratings having a constant pitch are formed at regular intervals in a core portion.
The first diffraction grating counted from the light incident end is κ ₀ · d ₀ = π / 2 (where κ ₀ is the coupling constant of the diffraction grating)
Number, d ₀ is to satisfy the condition of the diffraction grating length), the diffraction grating of the second and subsequent counted from the incident end of the light, and satisfies the _{κ 0 · d 0 <π /} 2 condition Optical fiber filter. Wherein said predetermined interval is an optical fiber filter according to claim 1, wherein the at 5cm or 50cm or less. Pitch wherein the diffraction grating is claim 1 or, characterized in that at 200μm or more 600μm or less
3. The optical fiber filter according to 2.