JPH1164670A - Grating built-in type optical coupler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to reduce a cost and to embody not only the multiplex sepn. of signals but a function to add the signals of specific wavelengths as well by forming gratings of a structure uniform in the pitch thereof in a longitudinal direction and specifying a grating length and induced refractive index change to specific values. SOLUTION: The fiber coupler formed with the fiber gratings in the melt extended part of the fiber coupler is formed to the structure uniform in the pitch of the gratings in the longitudinal direction. The grating length is 1.5 mm and the induced refractive index change is 0.001. If the grating length is shorter than 1.5 mm, the ratio at which the light is reflected to a port 2 and returned is low and the ratio at which the light transmits to a port 4 is high. When the grating length is longer than 1.5 mm, the ratio at which the light returns to a port 1 increases. The spectral width returning to the port 2 more widens when the induced refractive index change is larger than 0.001. The reflectivity is lower when the change is made smaller than 0.001.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical coupler formed by using a grating filter having a grating (periodic perturbation of refractive index) formed in an optical fiber core and its vicinity.

[0002] The present invention is believed to contribute in the field of optical communications. In particular, this is a technique useful for extracting only a signal of a specific wavelength or adding a signal of a specific wavelength in wavelength division multiplexing transmission.

[0003]

2. Description of the Related Art A conventional transmission type optical filter will be described below. Generally, in the field of optical communication, a transmission-type optical filter is often required, and in order to transmit a predetermined wavelength band, it is necessary to use an optical fiber grating filter in combination with an optical component such as an optical circulator. there were. FIG. 11 is a schematic diagram of an optical fiber grating filter. FIG. 12 shows general reflection characteristics of an optical fiber grating filter. The optical fiber grating filter 22 formed on the optical fiber 21 has a function of reflecting light of a predetermined wavelength and transmitting other wavelengths, has a narrow band, has excellent wavelength selectivity, and has almost no insertion loss. It is a reflection type filter having. The normal reflection wavelength band is about 1 nm, but a band of 0.2 nm to 10 nm can be realized by using a special manufacturing method. In FIG. 13, in the path of the optical fiber 31,
An example in which an optical circulator 33 and an optical fiber grating filter 32 are combined to form a transmission type optical filter will be described. The signal incident from the port is output to the port, but if an optical fiber grating filter 32 is provided in the middle of the port, a specific wavelength (here, wavelength λ
Only B) is reflected, returns to the optical circulator 33 again, and is output from the port. If a plurality of wavelength-multiplexed signals are input from the port, only the signal corresponding to the wavelength λB is output from the port, and all the signals corresponding to other wavelengths are output from the port. That is, the signal of the specific wavelength λB can be demultiplexed.

FIG. 14 shows an example in which a transmission type optical filter is formed by combining an optical fiber grating filter 42 and an optical fiber coupler 43 in the optical fiber 41 path. In this example, the signal input from the port is split into two halves by the optical fiber coupler 43 and output to the port and the port. At the port, the light is reflected by the optical fiber
The signal corresponding to B is re-input to the optical fiber coupler 43, split into two halves again, and output to the ports. Therefore, at the port, one of the input signals
/ 4 is output. Also in this case, if a plurality of wavelength-multiplexed signals are input from the port, only the signal corresponding to the wavelength λB is output from the port, all the signals corresponding to the other wavelengths are output from the port, and the specific wavelength λB Can be demultiplexed.

[0005]

The problems in the above prior art are as follows. First, in the method of FIG. 13 in which the optical fiber grating filter 32 and the optical circulator 33 are used in combination, the insertion loss is about 2 dB from port to port, and although the characteristics are excellent, the optical circulator 33 is expensive. There are problems.

In FIG. 14, which is a combination of an optical fiber grating filter 42 and an optical fiber coupler 43, the optical fiber coupler 43 is a cheaper device than the optical circulator 33, but the insertion loss is at least 6 dB ( That is, 1/4). Further, the entire transmission signal output from the port is 3 dB.
(That is, 1/2).

The present invention has been made in view of the above circumstances, and an optical fiber grating is formed in a fused portion of an optical fiber coupler, and a signal having a specific wavelength can be transmitted without using an optical component such as the optical circulator. It is an object of the present invention to provide an optical coupler with a built-in grating that can be demultiplexed and the resulting device is inexpensive, and can realize not only demultiplexing of a signal but also a function of adding a signal of a specific wavelength. I do.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above object, an optical coupler with a built-in grating according to the present invention is a fiber coupler in which a fiber grating is formed in a melt-stretched portion of the fiber coupler. It has a uniform structure, a grating length of 1.5 mm, and a change in induced refractive index of 0.001.

Further, the optical coupler with a built-in grating according to the present invention is characterized in that, in a fiber coupler in which a fiber grating is formed in a melt-drawn portion of the fiber coupler, the pitch of the grating is not uniform in the length direction. is there.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration explanatory view of an embodiment of the present invention. FIG. 2 is a side view of an optical fiber coupler having a tapered shape, and FIG. 3 is a cross-sectional view of a melt-drawn optical fiber coupler. Here, 11 is an optical fiber, 12 is a filter on which an optical fiber grating is formed, Co is the width of two optical fibers that are not melted, Cmin is the width of the thinnest portion of the melt-stretched optical fiber coupler, and Lc is the melted length. The length of the tapered portion of the drawn optical fiber coupler (here, 0.9C
o The length of the following part).

The method of producing the optical coupler with a built-in grating is as follows. First, the two optical fibers 11 are melt-drawn by a heat-melt drawing method to form an optical fiber coupler. Next, a grating is formed on the thinnest portion of the melt-stretched optical fiber coupler. The grating is formed by irradiating ultraviolet light having a wavelength of around 244 nm from a side surface using a phase mask and diffracting the phase mask to form intensity fringes of the irradiated light. Since the induced refractive index of the optical fiber core changes according to the intensity of the ultraviolet light, a grating is formed. In the present embodiment, the target signal wavelength is in the 1.55 μm band, so that the pitch of the phase mask is about 1 μm.

First, in producing an optical fiber coupler, the optical fiber 11 used is an optical fiber in which Ge (germanium) is added to a core and Ge and F (fluorine) are added to a clad. The refractive indexes of the core and the cladding of the optical fiber 11 are 1.4624 and 1.4580, respectively. The produced optical fiber coupler is a coupler having a wavelength dependency, and for example, has a characteristic that when a signal in the 1.55 μm wavelength band is input from a port in FIG. 1, the signal is output to the port. The length Lc of the tapered portion of the optical fiber coupler is about 30 mm.

First, a grating length of 1 mm, 1.5 mm
mm and 2 mm, a built-in grating type optical coupler was prepared. At this time, the induced refractive index change is 0.00
It was set to 1. Output characteristics from each port to in each case are shown in FIG. 4, FIG. 5, and FIG. The center wavelength is the set value (= 1.5
50 μm) and 1.545 μm.
When the grating length is 1 mm, the rate of reflection at the port and return is small, and the rate of transmission to the port is high. When the grating length is 2 mm, the ratio of returning to the port increases. Therefore, a grating length of about 1.5 mm is the optimum length.

When a signal is input from a port of the optical coupler with a built-in grating, the signal is reflected by the grating and output from the port. That is, it is possible to interrupt a signal of a specific wavelength into a signal group.

Next, an optical coupler with a built-in grating was prepared by changing the induced refractive index change by setting the grating length to 1.5 mm. The induced refractive index change is 0.001,
For the cases of 0.002 and 0.005, FIGS.
As shown in FIG. As can be seen from FIGS. 8 and 9, as the induced refractive index change increases, the spectral width returning to the port increases. Normal 1.55μm band WDM
In the system, the signal channel spacing is 100 GHz (0.
8 nm), the spectrum spread of FIG. 7 is appropriate. When the change in the induced refractive index is 0.001 or less, the spectrum width is further narrowed, but the reflectivity decreases, so that it is necessary to increase the grating length, which is not practical.

Next, another embodiment will be described. This corresponds to claim 2. The optical fiber coupler was formed to be 50 mm or more sufficiently longer than usual.
A chirp grating was formed on this optical fiber coupler. Here, the chirp grating is a grating whose reflection wavelength changes in the length direction of the grating. Here, the phase mask used for device fabrication has a chirp structure in which the wavelength changes by 0.2 nm per 50 mm length. FIG. 10 shows a result of measuring a relative delay time with respect to a wavelength when a signal is input from a port of the created grating coupler and output to the port.
Shown in From this result, it was found that this device was effective for compensating chromatic dispersion.

In the above two embodiments, it is more effective to mount the entire device in a temperature-controllable housing in order to suppress the fluctuation of the wavelength or realize the function of changing the wavelength. .

[0018]

As described above, according to the present invention, it is possible to realize a device capable of extracting a signal component of a specific wavelength or adding an additional signal component in a wavelength division multiplexing system. Moreover, the device can be significantly lower in cost as compared with the conventional technology. Further, it can be used as an inexpensive device that can compensate for chromatic dispersion.

[Brief description of the drawings]

FIG. 1 is a configuration explanatory diagram showing an embodiment of the present invention.

FIG. 2 is a side view showing a tapered optical fiber coupler according to an embodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating a melt-drawn optical fiber coupler according to an embodiment of the present invention.

FIG. 4 is an output wavelength characteristic (grating length: 1 mm, induced refractive index change: 0.00) according to an embodiment of the present invention.
It is a characteristic view which shows 1).

FIG. 5 is an output wavelength characteristic (grating length: 1.5 mm, induced refractive index change: 0.0) according to an embodiment of the present invention.
01) is a characteristic diagram.

FIG. 6 shows an output wavelength characteristic (grating length: 2 mm, induced refractive index change: 0.00) according to an embodiment of the present invention.
It is a characteristic view which shows 1).

FIG. 7 is an output wavelength characteristic (grating length: 1.5 mm, induced refractive index change: 0.0) according to an embodiment of the present invention.
01) is a characteristic diagram.

FIG. 8 shows output wavelength characteristics (grating length: 1.5 mm, induced refractive index change: 0.0) according to an embodiment of the present invention.
02) FIG.

FIG. 9 shows an output wavelength characteristic (grating length: 1.5 mm, induced refractive index change: 0.0) according to an embodiment of the present invention.
FIG.

FIG. 10 is a characteristic diagram showing wavelength dependence of a relative delay time according to another embodiment of the present invention.

FIG. 11 is a configuration explanatory view showing a conventional optical fiber grating filter.

FIG. 12 is a characteristic diagram showing reflection characteristics of a conventional optical fiber grating filter.

FIG. 13 is a configuration explanatory view showing an example in which a transmission optical filter is configured in combination with a conventional optical circulator.

FIG. 14 is a configuration explanatory view showing an example in which a transmission optical filter is configured in combination with a conventional optical fiber coupler.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Optical fiber 12 Filter which formed optical fiber grating 21 Optical fiber 22 Optical fiber grating filter 31 Optical fiber 32 Optical fiber grating filter 33 Optical circulator 41 Optical fiber 42 Optical fiber grating filter 43 Optical fiber coupler

Claims (2)

[Claims] 1. A fiber coupler in which a fiber grating is formed in a melt-drawn portion of a fiber coupler, the grating pitch is uniform in the length direction, the grating length is 1.5 mm, and the induced refractive index change is 0.001. An optical coupler with a built-in grating. 2. An optical coupler with a built-in grating, wherein a pitch of the grating is non-uniform in a length direction in a fiber coupler in which a fiber grating is formed in a melt-drawn portion of the fiber coupler.