JP2000121852A - Waveguide grating dispersion compensating element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a wavelength bandwidth and a dispersion quantity which are practically necessary only by an easy grating formation technology by forming a grating for dispersion compensation on two waveguides connected to one side of a 2×2 input and output 3 dB coupler waveguide and using two waveguides connected to the other of the 3 dB coupler waveguide as signal input/output waveguides. SOLUTION: Light inputted from an input waveguide 2 is equally distributed to the two 3 dB coupler waveguides 4' by a 3 dB coupler 4. Chirp gratings 5 and 6 in the same structure are formed at a distance of equal optical path length from the 3 dB coupler waveguides 4'. Then long-wavelength light is reflected where the grating pitch is large and short-wavelength light is reflected where the pitch is small; and they return to the 3 dB coupler 4 and are coupled here and outputted to an output waveguide 3 (a). Here, the grating length is short, so a sufficient dispersion quantity can not be obtained and a multistage connection with gratings 12 to 21 is made by optical fibers to increase the dispersion quantity (b).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a waveguide grating type dispersion compensating element.

[0002]

2. Description of the Related Art In recent years, dispersion compensation techniques have become important in ultra-high-speed and ultra-large-capacity optical communication systems.
The main dispersion compensator is a dispersion compensating fiber (DCF)
And a fiber grating type dispersion compensator.

[0003] A dispersion compensating fiber has an inverse dispersion to a normal fiber due to structural dispersion. This type has the advantage that the amount of dispersion can be arbitrarily adjusted by adjusting the length, and the wavelength bandwidth is wide. However, there are drawbacks such as a long length, a large loss, a large size, and a nonlinear optical effect. Therefore, as a recent tendency, the following fiber grating type is expected.

The fiber grating type is a type in which a refractive index modulation type grating is formed in a fiber utilizing the ultraviolet light sensitivity of glass, and the grating pitch is changed from the light input side in the propagation direction. It is a chirp grating.

For example, if the grating pitch on the input side is wide and the pitch gradually decreases along the propagation direction, long wavelength light is reflected on the side close to the input, and light on the short wavelength side is directed toward the back of the grating. Is reflected by In this case, it will have a negative variance. The amount of dispersion and the wavelength band are determined by the chirp rate and the grating length.

FIG. 5A shows a conventional example of a fiber grating type dispersion compensator. When actually used, it is necessary to combine the circulator 30 as shown in FIG. Light input from the input port 32 passes through the circulator 30 and passes through the chirp grating 3.
Incident on 1. The shorter wavelength light is reflected farther from the circulator 30 by the chirp grating 31, so that a large dispersion is generated. The reflected light passes through the circulator 30 and is output from the output port 33.

In order to realize a practical wavelength bandwidth and dispersion amount in this type, a length of at least about 50 cm is required. In order to form a 50 cm chirped grating, FIG. Such a forming apparatus is required. Generally, the photosensitive grating is formed by irradiating the optical fiber 34 with the UV beam 37 through the phase mask 35. When forming a chirped grating, it is simple to use a phase mask having a chirped pitch.

However, the commercially available phase mask is the longest and is about 10 cm long, and a special device is required to form a 50 cm chirped grating.

In FIG. 5B, an optical fiber 34 is irradiated with a UV beam 37 from a phase mask 35 having a uniform pitch while moving a mirror 40 to scan. The optical fiber 34 is fixed on the PZT stage 36, and moves at a speed lower than the scanning speed of the UV beam for chirping. In addition, this alone
Since only a grating having the same length as the phase mask 35 can be formed, after the formation of the grating 41, the phase mask 35 is moved onto another portion of the fiber (in this figure, the phase mask is moved by moving the fiber). Moving 35), it is necessary to form a continuation of the grating. In this case, the movement accuracy of the phase mask is required to be on the order of nm. Very high precision moving equipment is required.

The formation of such a long grating requires a very advanced forming technique.
Some proposals have been made. A plurality of (three in this case) short gratings 42 satisfying the requirement of the wavelength bandwidth but satisfying the requirement but having a dispersion amount smaller than the requirement (in this case, one third) are connected via a circulator. As a result, the amount of dispersion required for one chirped grating 42 is reduced, and the grating length can be shortened.

However, the circulator 43 is approximately 1
It has a loss of dB (approximately 2 dB from going to and from one circulator). Also, circulators are very expensive,
In terms of loss and economy, the circulator 43
There is a limit to the number that can be connected, at most about three. As a result, it is necessary to form a grating of about 16 cm, which is one third of 50 cm.

[0012]

As described above, the dispersion compensating fiber has disadvantages of being large in size, having a large loss, and having a nonlinear optical effect.

In a fiber grating type dispersion compensator, it is necessary to form a long grating in order to obtain a wavelength bandwidth and an amount of dispersion which are practically required. The forming apparatus is required to have a position control accuracy on the order of nm, and there is a problem that making such an apparatus requires advanced technology and enormous cost. At present, there are no products produced by this method that can satisfy the characteristics required for a practical system.

There is also an idea to connect short gratings in multiple stages using a circulator, but this is not realistic considering the loss and cost of the circulator.

It is an object of the present invention to position the fiber and phase mask during formation without the need for advanced techniques to move the phase mask or fiber described above on the order of nanometers and the formation of long gratings that are costly. Provided is a waveguide grating-type dispersion compensating element that can satisfy a wavelength bandwidth and a dispersion amount that are practically required only by a technique for forming a short grating that does not need to be moved, that is, only an easily formed grating technique that is usually performed. It is in.

[0016]

The gist of the present invention is 2 × 2
A dispersion compensating grating is formed on two waveguides connected to one side of the input / output 3 dB coupler waveguide, and the two waveguides connected to the other of the 3 dB coupler waveguides are connected to the signal input / output conductor. It was in the wave path.

In the above-mentioned waveguide grating type dispersion compensating circuit, it is preferable that an interval between two gratings is 30 μm or less.

Further, in the present invention, in the waveguide grating type dispersion compensating circuit, the position where the grating is formed and the equivalent optical path length up to the 3 dB coupler waveguide are equal between the two waveguides.

Further, the present invention resides in that a plurality of the above-mentioned waveguide grating type dispersion compensating circuits are integrated in one element and connected in multiple stages.

Further, the present invention resides in that in the above-mentioned waveguide grating type dispersion compensating element, the dispersion compensating part and the multi-stage connecting part are produced as separate elements and connected.

Further, in the present invention, in the waveguide grating type dispersion compensating element, the multistage connection is connected by an optical fiber.

Further, according to the present invention, in the waveguide grating type dispersion compensating element, the end of the waveguide connected to the side of the grating opposite to the 3 dB coupler waveguide extends to the element end face, and finally the element end face is It has been polished diagonally.

Further, the present invention is characterized in that in the waveguide grating type dispersion compensating element, the core width is reduced so that the end of the waveguide connected to the side of the grating opposite to the 3 dB coupler waveguide is tapered.

Further, in the present invention, in the waveguide grating type dispersion compensating element, an isolator is inserted into the fibers connected in multiple stages.

Further, in the present invention, in the waveguide grating type dispersion compensating element, the 2 × 2 3 dB coupler waveguide may be an MMI (Muti-Mode Interference) type 3 dB coupler waveguide or a directional coupler type 3 dB coupler waveguide. Any of the wave paths may be used.

Further, the present invention is that, in the waveguide grating type dispersion compensating element, the dispersion compensating grating is formed by irradiating the waveguide with a UV laser.

A chirp grating type dispersion compensator,
In order to satisfy the wide wavelength bandwidth and large dispersion required for practical systems with one grating,
The grating length becomes longer. However, by passing a short grating having a small amount of dispersion over a wide wavelength bandwidth many times, one long grating can
It is possible to realize a dispersion amount corresponding to the number of times. Therefore, a method of passing through a short grating having a small amount of dispersion in a wide wavelength bandwidth many times is sufficient. One way to do this is to use a circulator as shown in FIG. 3, but this is not practical considering the loss, price, etc. of the circulator. Thus, the present invention has realized a method of actually passing through a short grating many times.

The concept of the present invention is to use a 3 dB coupler + waveguide grating composed of a silica glass waveguide (PLC) instead of the above-described circulator + fiber grating. The loss of the 3 dB coupler with the optimized structure is less than 0.1 dB, which is almost negligible. Specifically, a grating is formed at the same position of two waveguides extending from one side of a two-input two-output 3 dB coupler waveguide formed by a PLC (if the equivalent optical path length from the 3 dB coupler waveguide to the grating is the same). Do). Although this grating has a wide wavelength bandwidth, the amount of dispersion is small, and its length is 5 cm.
It is about. The waveguide connected to the grating end on the opposite side of the 3 dB coupler waveguide is tapered to emit light to the cladding and to prevent reflected light returning from the cladding. The two waveguides on the side opposite to the side where the grating is formed and the 3 dB coupler waveguide are used for inputting and outputting light. This is one unit of the dispersion compensator. By connecting these in multiple stages, the required amount of dispersion can be obtained. The 3 dB coupler waveguide is an MMI (Muti-Mode Inter
directional) type or directional coupler type.

[0029]

FIG. 1 is a view showing one embodiment of the present invention. FIG. 1 (a) is a plan view showing a basic unit, and FIG. 1 (b) is a waveguide grating type of the present invention. FIG. 3 is a plan view of a dispersion compensation element.

The operation will be described with reference to FIG. Light input from the input waveguide 2 is equally distributed to the two waveguides by the 3 dB coupler 4. At this time, the power is equally distributed, but the phase differs by π / 2 radians. Chirped gratings 5 and 6 having the same structure are formed at an equal optical path length away from the 3 dB coupler waveguide 4 '.

Here, light outside the wavelength band covered by the grating propagates as it is and is radiated to the cladding 1 at the tapered waveguide end face 7. Light in the wavelength band covered by the grating reflects long wavelength light where the grating pitch is wide and short wavelength light where the grating pitch is narrow, and returns to the 3 dB coupler. The returned light is coupled by the 3 dB coupler and output to the output waveguide 3. Here, since the grating length is short, a sufficient amount of dispersion cannot be obtained. Therefore, as shown in FIG. 1B, the dispersion amount is increased by connecting the optical fiber in multiple stages as shown in FIG. If all of the gratings 12 to 21 in FIG. 1B have the same structure, the amount of dispersion can be increased by a multiple thereof. For example, in this example, since the same structure is connected in ten stages, it is possible to obtain a dispersion amount ten times as large as the dispersion obtained in one unit.

One embodiment of the present invention shown in FIG. 1B includes a quartz glass waveguide and chirp gratings 12 to 21 formed by irradiating the waveguide with UV light. Fig. 1 (a)
The two chirped gratings 5 and 6, and the 3 dB coupler 4 and the input / output waveguides 2 and 3 connected to the chirped gratings 5 and 6 are one basic unit. In the present embodiment, M is used as a 3 dB coupler.
An MI coupler was used. Although the MMI type has a slightly larger loss than the directional coupler type, the allowable range of process accuracy for accurately forming a 3 dB coupler can be widened. The interval between the two chirped gratings is 20 μm. FIG. 1 (b) shows that this basic unit is
The input / output waveguides 9 and 10 are connected one at a time, and have one input / output waveguide 9 and 10 as a whole. Chirp grating 1
2 to 21 have the same structure and a length of 50 mm. The chirp grating is formed at the same position of two waveguides on one side of the multi-waveguide of the MMI type 3 dB coupler 11 having two inputs and two outputs. The waveguide connected to the end of the grating on the opposite side of the MMI type 3 dB coupler is tapered (FIG. 1).
7 (a). Waveguide core height is 6μ
m, the core width other than the MMI type 3 dB coupler 11 is 6 μm. The relative refractive index difference between the core and the clad is Δ = 0.8%.

This embodiment is a general quartz glass waveguide.
Manufacturing process and photo-sensitive grating
It was produced by a forming method. Specifically, first, on the quartz substrate 8,
GeO_Two-SiO_TwoSputtered film and lower cladding layer
And a core film TiO on it _Two-GeO_Two-SiO_TwoTo
The film is formed by sputtering. Then, photolithography, R
Patterning of optical circuits by IE,
The waveguide section is completed through processes such as
You. At this time, the lower cladding layer, core, and upper cladding layer
The amount of Ge contained is almost the same, and the relative refractive index difference is
A) is determined by the amount of Ti contained in the

FIG. 2A shows a chirp grating 12.
21A to 21C are diagrams illustrating a method of forming the first through fourth embodiments. Waveguide element fixing base 2
4, a phase mask 26 is placed on the element 25 placed on the excimer laser light 2 having a wavelength λ = 248 nm.
3 is irradiated with a mirror 27 while scanning. The phase mask 26 was used for forming a chirp grating. At this time, the phase mask 26 and the waveguide element 25 remain fixed, and need not be moved. During irradiation, optical fiber a
Light is input and output through the optical interface to monitor optical characteristics. The chirp gratings 12 to 21 were simultaneously formed by one irradiation process.

FIG. 2B is a diagram showing the wavelength group delay characteristic of one embodiment of the present invention. Dotted line 28 is the wavelength group delay characteristic of one basic unit, and solid line 29 is the characteristic of the ten-stage connection shown in FIG. The dispersion amount when 10 stages were connected was about 200 ps / nm, which is 10 times as large as that of one basic unit. The loss when the MMI type 3 dB coupler reciprocated once was about 0.1 dB, and the loss of the entire 10-stage connected element including the propagation loss was 3 dB, which was sufficient for practical use. The wavelength bandwidth is at least 20 nm.

If the intervals between the gratings connected to the same 3 dB coupler are too wide, it becomes difficult to form the gratings at the same location of the two waveguides. If the interval is 30 μm or less, it can be easily formed.

As described above, the 3 dB coupler can be applied to both the MMI type and the directional coupler type.

The number of multistage connections may be ten or more stages or less depending on the required characteristics.

In the embodiment, the chirp gratings 12 to 21 are the same, but it is also possible to connect different chirp gratings having different dispersion characteristics and apply the dispersion slope compensation.

The refractive index may be adjusted by adding fluorine to the cladding.

The chirping is not based on the grating pitch, but may be performed by changing the refractive index or the core width.

The multi-stage connection is not limited to the connection using the waveguide, but the single element may be connected via the optical fiber 22 as shown in FIG.

As shown in FIG. 4, when the optical fiber 22 is connected in multiple stages, a method of inserting an isolator 47 to prevent characteristic deterioration due to resonance is conceivable.

The waveguide grating type dispersion compensating element of the present invention is used as a dispersion compensator and a dispersion slope compensator in an optical communication system.

[0045]

(1) A dispersion compensator for compensating a large amount of dispersion over a wide wavelength bandwidth, which has been considered to be achievable only by a long grating that requires advanced forming technology, has been realized by a general PLC forming technology and It has become possible to obtain easily and with low loss using only the photo-sensitive grating forming technique.

(2) In the past, formation of long gratings required position control on the order of nm, and since this was not sufficiently realized, dispersion ripples were generated, which could be put to practical use. In the present invention, no special technique is required for the formation, and ideal characteristics can be obtained.

(3) Since the product of the present invention is easy to produce,
It has become possible to provide a high performance dispersion compensator at a very low price.

[Brief description of the drawings]

FIG. 1 (a) is a plan view showing a basic unit, and FIG. 1 (b) is a plan view of a waveguide grating type dispersion compensating element showing one embodiment of the present invention.

FIG. 2A is a diagram illustrating a manufacturing method, and FIG. 2B is a diagram illustrating a wavelength dispersion characteristic.

FIG. 3 is a plan view of a waveguide grating type dispersion compensating element showing another embodiment of the present invention.

FIG. 4 is a plan view of a waveguide grating type dispersion compensator according to another embodiment of the present invention.

5A and 5B are views showing a conventional example, FIG. 5A is a plan view, and FIG. 5B is a view showing a manufacturing method.

FIG. 6 is a diagram showing another conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Quartz substrate (cladding) 2 Input waveguide 3 Output waveguide 4 3dB coupler 5, 6 Chirp grating for dispersion compensation 7 Antireflection end 8 Quartz substrate (cladding) 9 Input (output) power waveguide 10 Output (input) power Waveguide 11 3 dB coupler 12 to 21 Chirp grating for dispersion compensation 22 Optical fiber 23 UV light 24 Waveguide element fixing base 25 Waveguide grating dispersion compensation element of the present invention 26 Phase mask 27 Mirror

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akifumi Hongo 5-1-1, Hidaka-cho, Hitachi City, Ibaraki Prefecture Within the Opto-System Laboratory, Hitachi Cable, Ltd. (72) Inventor Naoto Uezuka Hidaka, Hitachi City, Ibaraki Prefecture 5-1-1, Hitachi-cho, Opto-Systems Research Laboratory, Hitachi Cable, Ltd. (72) Inventor Yasuyuki Nagao 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation (72) Inventor Masami Usami Tokyo 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation (72) Inventor Noboru Egawa 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation F-term (reference) 2H038 AA01 AA24 BA01 2H047 KA04 KA13 LA00 LA01 MA05 PA04 QA04 TA13 5K002 BA04 BA07 BA21 CA01 FA02

Claims (11)

[Claims] 1. A dispersion compensating grating is formed on two waveguides connected to one side of a 2 × 2 input / output 3 dB coupler waveguide, and is connected to the other of the 3 dB coupler waveguides.
A waveguide grating type dispersion compensation circuit, wherein the waveguides are used as signal input / output waveguides. 2. The waveguide grating type dispersion compensation circuit according to claim 1, wherein an interval between the two gratings is 30 μm or less. 3. The waveguide grating type dispersion compensating circuit according to claim 2, wherein:
A waveguide grating type dispersion compensation circuit, characterized in that the equivalent optical path length up to the dB coupler waveguide is equal between the two waveguides. 4. A waveguide grating type dispersion compensating element, wherein a plurality of the waveguide grating type dispersion compensating circuits according to claim 3 are integrated in one element and connected in multiple stages. 5. A waveguide grating type dispersion compensating element according to claim 4, wherein the dispersion compensating part and the multi-stage connection part are formed as separate elements and connected. 6. The waveguide grating type dispersion compensating element according to claim 4, wherein the multistage connection is connected by an optical fiber. 7. The waveguide grating type dispersion compensating element according to claim 4, wherein the end of the waveguide connected to the side of the grating opposite to the 3 dB coupler waveguide extends to the element end face, and finally the element end face. A waveguide grating-type dispersion compensating element, characterized in that is polished obliquely. 8. The waveguide grating type dispersion compensating element according to claim 4, wherein an end of the waveguide connected to the side of the grating opposite to the 3 dB coupler waveguide is tapered to have a narrow core width. Waveguide grating type dispersion compensating element. 9. The waveguide grating type dispersion compensating element according to claim 5, wherein an isolator is inserted in the fiber connected in multiple stages. 10. The waveguide grating type dispersion compensating element according to claim 1, wherein the 2 × 2 3 dB coupler waveguide is a MMI (Muti-Mode Interference) type 3 dB coupler waveguide or a directional coupler type 3 dB coupler. A waveguide grating type dispersion compensating circuit characterized by being one of waveguides. 11. The waveguide grating type dispersion compensating element according to claim 1, wherein the dispersion compensating grating has a U shape.
A waveguide grating type dispersion compensating circuit characterized by being formed by irradiating a waveguide with a V laser.