JP2000235126A - Waveguide type grating element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a waveguide type grating element which satisfies a loss, a wavelength band width and dispersion quantity in spite of integration to a high density and is inexpensive. SOLUTION: A pair of grating forming waveguides 31A and 31B are connected to 3dB couplers 21 and 22 of 2×2 input and output and their one side, by which light is eventually passed many times through the short gratings having the wide wavelength band width and small dispersion quantity and the dispersion quantity corresponding to the passage one long grating once may be embodied and, therefore, the waveguide type grating element which satisfies the loss, the wavelength band width and the dispersion quantity in spite of integration to the high density and is inexpensive may be embodied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a waveguide type grating element.

[0002]

2. Description of the Related Art In recent years, pulse shaping and dispersion compensation techniques for signal light have become important in ultra-high-speed, ultra-large-capacity optical communication systems. In particular, as a dispersion compensator, a dispersion compensating fiber (DCF), a fiber grating type dispersion compensator, and the like have been actively developed.

[0003] A dispersion compensating fiber has an inverse dispersion to a normal fiber due to structural dispersion. This type of fiber has the advantage that the amount of dispersion can be arbitrarily adjusted by adjusting the length and the wavelength bandwidth is wide. But,
Since the length is required to be about 1/5 of that of the dispersion compensating fiber, the overall size becomes large and the loss becomes large. Further, in general, the core diameter is smaller than that of a normal optical fiber, and the optical power density in the core region becomes extremely high. Therefore, recently, studies for compensating dispersion using a fiber grating described below have been actively conducted.

[0004] Here, a fiber grating is one in which a refractive index modulation type grating is formed in a fiber by utilizing the sensitivity of glass to ultraviolet light. (Chirped grating) changed along the propagation direction from the input side of.

FIG. 4 shows a conventional example of a fiber grating type dispersion compensator.

When a fiber grating type dispersion compensator is actually used, an optical circulator 5, an input port 6, a fiber grating 7 and an output port 8 are used in combination as shown in FIG. For example, as shown in the figure, when the grating pitch on the input side is wide and the pitch gradually decreases along the propagation direction, the light of the long wavelength is reflected on the side near the input, and the light on the short wavelength side is the fiber grating 7. It is reflected at the back of. In this case, the light will have a negative dispersion. The amount of dispersion and the wavelength band are determined by the chirp rate and length of the grating.

Light input from an input port 6 passes through an optical circulator 5 and enters a fiber grating 7. Light incident on the fiber grating 7 is reflected at a position farther from the optical circulator 5 as light having a shorter wavelength is obtained, so that a large dispersion is obtained. The reflected light is output from the output port 8 through the optical circulator 5.

In order to realize a practical wavelength bandwidth and a dispersion amount in such a configuration, the length of the fiber grating 7 needs to be at least about 50 cm, and is specially formed for writing a long fiber grating. Equipment is required.

In general, in order to write a grating on an optical fiber using the photo-sensitive effect, the optical fiber is irradiated from the lateral direction with ultraviolet light via a phase mask. When forming a chirped grating, it is common to use a method in which the pitch of the phase mask itself is chirped, whereby a chirped fiber grating can be easily formed.

However, the longest phase mask that can be actually manufactured is at most about 10 cm, and in order to form a fiber grating exceeding this length, it is necessary to write while shifting the optical fiber with respect to the phase mask. For this reason, a joint is generated in the grating, and reflection at the joint affects the dispersion, causing a large ripple in the dispersion characteristics.

In order to reduce the number of the joints of the grating as much as possible or to obtain a larger dispersion amount, a multi-stage configuration as shown in FIG. 5 has been considered.

FIG. 5 shows another conventional fiber grating type dispersion compensator.

This fiber grating type dispersion compensator has relatively short fiber gratings 71, 72,
73 are connected via optical circulators 51, 52 and 53. With these optical components, the length of one fiber grating can be shortened, and the number of seams of the grating can be reduced. This fiber grating type dispersion compensator
Although the amount of dispersion obtained with one fiber grating is small, the required amount of dispersion can be obtained by connecting in multiple stages.

[0014]

However, the optical circulator is very expensive, and the loss due to the optical circulator increases if many optical circulators are connected. Therefore, the number of steps cannot be increased unnecessarily, and the limit is about three steps as shown in FIG. As a result, the fiber grating length per fiber cannot be extremely reduced. Further, since the length of a commercially available phase mask is limited, it is not possible to eliminate the joint of the grating. Furthermore, each fiber must be formed with a similar chirped grating one by one, which greatly increases the number of manufacturing steps.

That is, the dispersion compensating fiber has problems that it is long, has a large size, a large loss, and is apt to cause a nonlinear optical effect.

By the way, in the fiber grating type dispersion compensator, it is necessary to form a long grating in order to obtain a practically necessary wavelength bandwidth and dispersion amount. In order to form the joints of the gratings as smoothly and continuously as possible, a grating forming apparatus that requires high-precision position control is required.

However, nonetheless, the ripples in the dispersion characteristics caused by the joints of the gratings cannot be completely eliminated, and at present, the characteristics required in a practical system cannot be satisfied.

There is also a configuration in which short gratings are connected in multiple stages by using a plurality of optical circulators. However, considering the loss and cost of the optical circulators, it is not practical to use multiple stages.

An object of the present invention is to solve the above-mentioned problems and to provide an inexpensive waveguide grating element which satisfies the loss, the wavelength bandwidth and the amount of dispersion even when integrated at a high density. .

[0020]

In order to achieve the above object, a waveguide type grating element according to the present invention comprises a pair of waveguides connected to one side of a 2 × 2 input / output coupler. A grating whose refractive index changes periodically is formed, and one of a pair of waveguides connected to the other side of the coupler is connected to an input / output end or one side of another 2 × 2 input / output coupler. A grating whose refractive index periodically changes in the longitudinal direction is formed in a pair of waveguides that are connected and connected to the other side of another coupler.

In addition to the above configuration, in the waveguide grating element of the present invention, it is preferable that the position where the grating is formed and the equivalent optical path length up to the coupler be equal between the pair of waveguides.

In addition to the above configuration, the waveguide type grating element of the present invention may be configured such that 2 × 2 input / output couplers connected to a waveguide on which a grating is formed are connected in multiple stages in a cascade.

In addition to the above configuration, the waveguide type grating element of the present invention is preferably arranged so that the odd-numbered coupler and the even-numbered coupler from the input side face each other.

In addition to the above configuration, in the waveguide type grating element of the present invention, the odd-numbered coupler and the even-numbered coupler from the input side are connected in the same direction by connecting each coupler with a 180-degree bent waveguide. Preferably, they are arranged such that

In addition to the above configuration, the waveguide type grating element of the present invention comprises a pair of waveguides connected to odd-numbered couplers from the input side and a pair of waveguides connected to even-numbered couplers. It is preferable that the gratings formed in the wave path are formed together with the even-numbered and odd-numbered gratings.

In addition to the above configuration, in the waveguide type grating element of the present invention, it is preferable that all the gratings formed on a pair of waveguides connected to the coupler are formed collectively.

In addition to the above configuration, the waveguide type grating element according to the present invention comprises a pair of waveguides connected to odd-numbered couplers from the input side and a pair of waveguides connected to even-numbered couplers. It is preferable that the grating pitches of the gratings are different from each other.

In addition to the above configuration, in the waveguide type grating element of the present invention, it is preferable that the ends of the pair of waveguides on which the grating to be connected to the coupler is formed be slanted or tapered to reduce the width of the waveguide.

In addition to the above configuration, in the waveguide type grating element of the present invention, the coupler is an MMI (Multi-Mod).
e Interference) type 3 dB coupler or a directional coupler type 3 dB coupler.

In addition to the above configuration, in the waveguide type grating element of the present invention, it is preferable that the grating is formed by irradiating the waveguide with ultraviolet laser light.

Here, in order to realize a dispersion compensator using a chirped grating, it is necessary to use a single grating to satisfy a wide wavelength bandwidth and a large amount of dispersion required in a practical system. The length will be long. However, by passing light many times through a short grating having a small amount of dispersion in a wide wavelength bandwidth, a dispersion equivalent to passing through one long grating once can be realized.

Such a configuration can be configured by using the multi-stage circulator and the fiber grating shown in FIG. 5, but in the present invention, a 2 × 2 input / output 3 dB coupler and a pair of one side connected to one side thereof are used. A similar function is realized by forming a grating in the waveguide. Such a configuration can be easily realized by a planar optical circuit using silica glass, and the loss of the 3 dB coupler whose structure has been optimized can be reduced to 0.1 dB or less. It is advantageous. Specifically, a grating is formed at the same position of a pair of waveguides extending from one side of a 2 × 2 input / output 3 dB coupler formed of a quartz-based planar optical circuit (equivalent optical path from the 3 dB coupler waveguide to the grating). As long as the length is the same).

Although this grating has a wide wavelength band, the amount of dispersion is small, and its length is about several cm. Since a chirped phase mask having a length of 10 cm or less can be easily obtained, a seamless waveguide grating can be formed with one mask.

If 3 dB couplers having a pair of waveguide gratings connected to one side are connected in multiple stages in a cascade, and odd-numbered and even-numbered ones are arranged in parallel from the input side and put together. Gratings having the same pitch can be formed individually or together. At this time, the odd-numbered waveguide grating and the even-numbered waveguide grating may have the same chirp or different chirps.

The ends of the pair of waveguides on which the grating connected to the 3 dB coupler is written are made oblique or tapered to reduce the width of the waveguide, thereby preventing unnecessary reflected light from the waveguide grating terminal. be able to.

Note that the 2 × 2 input / output 3 dB coupler is MM
A planar optical circuit can be easily formed by using either the I-type or the directional coupler type.

[0037]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing one embodiment of the waveguide grating element of the present invention.

A grating whose refractive index periodically changes in the longitudinal direction is formed on one side (right side in the figure) of a 2 × 2 input / output 3 dB coupler (hereinafter referred to as “coupler”) 21 shown in FIG. A pair of grating forming waveguides 3
1A and 31B are connected, and the input waveguide 1 and the waveguide 10 are connected to the other side (the left side in the figure) of the coupler 21. On the other side of the waveguide 10, another 2 × 2 input / output coupler 22 is connected, and on the other side of the coupler 22, a grating is formed in which a grating whose refractive index periodically changes in the longitudinal direction is formed. Waveguides 32A, 32
B is connected. Similarly, a 2 × 2 input / output coupler 23 is connected to the coupler 22 via the waveguide 11, and the other side of the coupler 23 is provided with grating forming waveguides 33 </ b> A and 33 </ b> B.
Is connected. The coupler 23 is connected to a 2 × 2 input / output coupler 24 via the waveguide 12, and the other side of the coupler 24 is connected to grating forming waveguides 34 A and 34 B. The output waveguide 4 is connected to the side of the coupler 24 to which the waveguide 12 is connected. The input waveguide 1 may be used as an output waveguide, and the output waveguide 4 may be used as an input waveguide.

The light input from the input waveguide 1 of the waveguide type grating element shown in FIG.
As a result, the pair of grating forming waveguides 31A, 31
B is equally distributed. When the light is distributed, the power is equally distributed, but the phase is different by π / 2 radians. A chirped grating having the same structure is formed at a position distant from the coupler 21 by an equal optical path length. Light in the wavelength band determined by the chirp rate of the grating, long wavelength light is reflected where the grating pitch is wide, short wavelength light is reflected where the grating pitch is narrow,
It returns to coupler 21. Light outside the wavelength band is propagated to the end of the waveguide without being reflected by the grating, but when this light is reflected at the end, the return light deteriorates the dispersion characteristics. Therefore, in order to suppress the return light of unnecessary light other than the wavelength band reflected by the grating, the ends of the pair of waveguides on which the grating is formed are slanted or tapered to reduce the width of the waveguide and radiate to the cladding region. It is supposed to be.

The light reflected by the grating and returned is coupled again by the coupler 21, propagates through the waveguide 10 different from the input waveguide 1, and is input to the second coupler 22. Thereafter, similarly, the optical signal propagates through the waveguide grating via the coupler at each stage, and is finally output from the output waveguide 4.

Although FIG. 1 shows four stages for the sake of simplicity, it goes without saying that the number of multistage connections is determined according to the required characteristics.

Since the waveguide grating of each stage has a short length, a sufficient dispersion cannot be obtained with only one stage. However, by connecting in multiple stages, the amount of dispersion can be increased. If all the gratings shown in FIG. 1 have the same structure with the same pitch, the amount of dispersion can be increased by a multiple thereof.

In the embodiment shown in FIG. 1, it is constituted by a planar optical circuit made of quartz glass. In this embodiment, an MMI coupler is used as the coupler. MMI
Although the coupler has a slightly larger loss than the directional coupler type, the allowable range of the process deviation for accurately forming the coupler can be widened. The interval between a pair of waveguide gratings connected to the coupler was 20 μm.

Next, the manufacturing process of the waveguide grating device shown in FIG. 1 will be described.

This waveguide type grating element was manufactured by a general quartz glass waveguide manufacturing process and a grating forming method using a photo-sensitive effect.

A GeO ₂ —SiO ₂ film is formed by sputtering to form a lower clad layer, on which a core film TiO ₂ —GeO _{2 is} formed.
_2- SiO _{2 is formed} by sputtering. Thereafter, the waveguide is completed through processes such as photolithography, patterning of an optical circuit by reactive ion etching, and formation of over cladding by flame deposition. The amounts of Ge contained in the lower cladding layer, the core and the upper cladding layer of this waveguide are substantially the same, and the relative refractive index difference between the core and the cladding is determined by the amount of Ti contained in the core. The core height of the waveguide is 6 μm, and the core width other than the MMI type 3 dB coupler is 6 μm.
μm. The relative refractive index difference Δ between the core and the clad is 0.8%.

To form a grating utilizing the photosensitive effect, an element having an optical circuit formed thereon is fixed to a waveguide element fixing base, a phase mask is arranged on the element, and a wavelength λ 248 nm is set from above the phase mask. Of excimer laser light. A chirp grating forming mask was used as the phase mask.

In the present embodiment, the MMI type is used as the coupler, but the MMI type or the directional coupler type can be applied as described above.

In this embodiment, the chirp gratings are the same for both the odd-numbered and even-numbered chirp gratings. However, since the odd-numbered and even-numbered chirped gratings have different dispersion characteristics, they are connected. It is also possible to control the dispersion slope. In this case, after the odd-numbered (or even-numbered) ones are collectively irradiated, the even-numbered (or odd-numbered) ones are collectively irradiated by exchanging the phase mask.

In the embodiment shown in FIG. 1, the odd-numbered couplers and the even-numbered couplers from the input side are arranged so as to face each other, but are not limited thereto.

FIG. 2 is a view showing another embodiment of the waveguide grating element of the present invention.

The difference from the embodiment shown in FIG. 1 is that the odd-numbered coupler and the even-numbered coupler from the input side are connected in the same direction by connecting with a 180-degree curved waveguide. It is the point where it was arranged.

Couplers 21 to 24 are arranged in parallel, and a pair of grating forming waveguides 31A, 31B, 32A, 32 is provided on one side (right side in the drawing) of each coupler 21 to 24.
B, 33A, 33B, 34A, and 34B are connected respectively. The first coupler 21 and the second coupler 22 from the input side are connected by a 180-degree bent waveguide 40, and the second coupler 22 and the third coupler 2 from the input side are connected.
3 is connected by a 180-degree bent waveguide 41. Third coupler 23 and fourth coupler 2 from the input side
4 are connected by a 180-degree bent waveguide 42. The 180th bend waveguide 43 connects between the fourth coupler 24 from the input side and the output waveguide 4.

In such a waveguide type grating element, since the waveguide gratings can be arranged in parallel on the same side of the optical circuit, the length of the waveguide grating per stage is smaller than that of the waveguide type grating element shown in FIG. 2
Can be doubled.

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

The difference from the embodiment shown in FIG. 2 is that the output waveguides are perpendicularly intersected with the bent waveguides connecting the couplers and output, so that the odd-numbered waveguide grating group and the even-numbered waveguide gratings are output. This is the point that the group of waveguide gratings is arranged close to each other.

Couplers 21 to 24 are arranged in parallel, and a pair of grating forming waveguides 31A, 31B, 32A, 32 is provided on one side (right side in the drawing) of each of the couplers 21 to 24.
B, 33A, 33B, 34A, and 34B are connected respectively. The first coupler 21 and the second coupler 22 from the input side are connected by a 180-degree bent waveguide 40, and the second coupler 22 and the third coupler 2 from the input side are connected.
3 is connected by a 180-degree bent waveguide 41. Third coupler 23 and fourth coupler 2 from the input side
4 are connected by a 180-degree bent waveguide 42. The output waveguide 4 is connected to the fourth coupler 24 from the input side so as to intersect perpendicularly with the bent waveguides 40 to 42 connecting the couplers 21 to 24.

In the waveguide-type grating element having such a configuration, the odd-numbered waveguide grating and the even-numbered waveguide grating can be formed together.

Here, the total width of a pair of odd-numbered or even-numbered parallel waveguides for writing gratings is about 1 mm even when 10 stages are connected. This is sufficiently smaller than the beam diameter of the excimer laser to be irradiated or the width of the phase mask. Therefore, all of the waveguide gratings can be formed in a batch irradiation step by combining the odd-numbered and even-numbered waveguide gratings or by arranging them as shown in FIG. .

Further, when the length of the waveguide grating per stage is set to 5 cm, and the dispersion is connected in ten stages, the dispersion amount is about 200 ps / nm, which is ten times that in the case of one stage. M
The loss of one round trip of the MI coupler is about 0.1 dB
The total loss of the elements connected in 10 stages, including the propagation loss, was 3 dB, which was sufficient for practical use. The wavelength bandwidth is determined by the chirp rate and is about 20.
nm.

As described above, according to the present invention, (1) a dispersion compensator for compensating a large amount of dispersion in a wide wavelength band, which cannot be achieved except for a long fiber grating which conventionally requires a high-precision forming technique, This can be realized by a planar optical circuit, and as a result, a small dispersion compensator can be realized.

(2) Since short waveguide gratings connected in multiple stages can be collectively formed, the number of manufacturing steps of the grating can be reduced, and an inexpensive dispersion compensator can be realized.

(3) In a conventional dispersion compensator using a long fiber grating, discontinuous joints of the grating cause large dispersion ripples, which hinders practical use. However, in the present invention, a multi-stage short waveguide is used. By using the wave grating, it is possible to remove the dispersion ripple due to the joint of the gratings.

[0065]

In summary, according to the present invention, the following excellent effects are exhibited.

Even when integrated at a high density, it is possible to satisfy the loss, the wavelength bandwidth, and the amount of dispersion, and to provide an inexpensive waveguide grating element.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of a waveguide grating element of the present invention.

FIG. 2 is a diagram showing another embodiment of the waveguide grating element of the present invention.

FIG. 3 is a diagram showing another embodiment of the waveguide grating element of the present invention.

FIG. 4 is a conventional example of a fiber grating type dispersion compensator.

FIG. 5 is another conventional example of a fiber grating type dispersion compensator.

[Explanation of symbols]

Reference Signs List 1 input waveguide 4 output waveguide 21, 22, 23, 24 3dB coupler (coupler) 31A, 31B, 32A, 32B, 33A, 34A, 3
4B grating forming waveguide

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hideaki Arai 5-1-1, Hidaka-cho, Hitachi City, Ibaraki Prefecture Hitachi Cable, Ltd. Opto-System Research Laboratory (72) Inventor Yasuyuki Nagao 2 Nishishinjuku, Shinjuku-ku, Tokyo No. 3-2, Kaididi Research Institute, Inc. (72) Noboru Egawa, Inventor No. 2-3-2, Nishishinjuku, Shinjuku-ku, Tokyo Inside, Kaididi Institute, Ltd. (72) Inventor, Masashi Usami 2-Nishishinjuku, Shinjuku-ku, Tokyo No. 3 in the Cadidi Research Laboratories (72) Inventor Masatoshi Suzuki 2-3-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo F-term in the Cadidi Research Laboratories (reference) 2H047 KA13 LA02 LA12 PA30 RA00 TA31 TA41 5K002 AA07 BA04 BA07 BA21 CA01 FA01

Claims (11)

[Claims] 1. A pair of waveguides connected to one side of a 2 × 2 input / output coupler is formed with a grating whose refractive index periodically changes in a longitudinal direction, and the other side of the coupler is provided with a grating. Is connected to one side of an input / output end or another 2 × 2 input / output coupler, and is connected to a pair of waveguides connected to the other side of the other coupler. A waveguide type grating element, wherein a grating whose refractive index periodically changes in the longitudinal direction is formed. 2. The waveguide-type grating element according to claim 1, wherein an equivalent optical path length between the formation position of the grating and the coupler is equal between the pair of waveguides. 3. The waveguide-type grating element according to claim 1, wherein 2 × 2 input / output couplers connected to the waveguide on which the grating is formed are connected in multiple stages in a cascade. 4. The waveguide-type grating element according to claim 3, wherein the odd-numbered coupler and the even-numbered coupler from the input side are arranged to face each other. 5. The coupler according to claim 3, wherein the odd-numbered couplers and the even-numbered couplers from the input side are arranged in the same direction by connecting the respective couplers with a 180-degree bent waveguide. Waveguide grating element. 6. A grating formed in a pair of waveguides connected to an odd-numbered coupler from the input side and a grating formed in a pair of waveguides connected to an even-numbered coupler are even-numbered, The waveguide-type grating element according to claim 4, wherein the odd-numbered waveguide elements are collectively formed. 7. The waveguide-type grating element according to claim 5, wherein all the gratings formed on the pair of waveguides connected to the coupler are formed collectively. 8. A grating formed on a pair of waveguides connected to odd-numbered couplers from the input side and a grating formed on a pair of waveguides connected to even-numbered couplers have grating pitches of respectively. The waveguide type grating element according to claim 4, which is different from the waveguide type grating element. 9. The waveguide-type grating element according to claim 1, wherein the ends of the pair of waveguides on which the grating connected to the coupler is formed have a slender or tapered width. 10. The waveguide type grating element according to claim 1, wherein the coupler is one of an MMI type 3 dB coupler and a directional coupler type 3 dB coupler. 11. The waveguide-type grating element according to claim 1, wherein the grating is formed by irradiating an ultraviolet laser beam to the waveguide.