JPH03246510A - Formation of diffraction grating optical coupler - Google Patents

Abstract

PURPOSE:To enhance coupling efficiency and to miniaturize the above optical coupler by working a thin film or optical waveguide layer constituting a diffraction grating to a specific shape and changing the coefft. of coupling according to places. CONSTITUTION:The thin film 3 is laminated by gradually changing the thickness thereof in the propagating direction of the guided light in the optical waveguide layer and in succession, the thin film 3 is removed until the optical waveguide layer 2 is exposed, by which the groove width in the propagation direction of the respective grooves dug in the thin film 3 is so formed as to have the same length without depending upon the places. The height in the thin film patterns for each of the respective pitches in the thin-film patterns is successive ly changed in the propagation direction. The coefft. of coupling of the diffraction grating optical coupler is, therefore, changed along the propagation direction. The small-sized diffraction grating optical coupler which can make the light having an ordinary intensity distribution incident on the inside of the optical waveguide with good efficiency is obtd. in this way.

Description

Detailed Description of the Invention (a) Industrial Application Field The present invention provides a method for creating a diffraction grating optical coupler that uses a diffraction grating to input light into an optical waveguide or output light in an optical waveguide to the outside. It is related to.

(b) Conventional technology Conventionally, diffraction grating optical couplers are small, have almost no thickness, and are easy to integrate, so they have been used to integrate integrated optical pickups, integrated optical scanning elements, integrated optical Doppler velocimeters, etc. It has been widely used in optical devices. Diffraction gratings used as optical couplers include those that are composed of straight lines with a constant pitch, and those that are curved and have a gradually changing pitch to collect light like a lens. Various patterns and pitches, such as t-shaped ones, are used, and their usefulness is increasing. These diffraction grating optical couplers can be fabricated using the normal photolithography method if the pitch is long, or by the holographic exposure method if the pitch is short. The so-called lift-off method, in which an optical coupler pattern is created, an appropriate material is deposited on it, and unnecessary films are removed by a lift-off method, etc., or a diffraction grating optical coupling base material is first deposited on the optical waveguide layer. After that, a photoresist pattern is created and the material is etched. Alternatively, the optical waveguide layer itself may be etched.

(c) Problems to be Solved by the Invention However, the coupling efficiency of such a diffraction grating optical coupler is limited to approximately 80% when the intensity distribution of incident light is symmetrical. The reason for this is explained as follows. Since the input of light into the optical waveguide layer by the diffraction grating optical coupler is considered to be the exact opposite effect to the output of light from the diffraction grating optical coupler, the output efficiency will be explained.

FIGS. 8(a) and 8(b) are diagrams illustrating the structure of a conventional diffraction grating optical coupler.

In FIG. 8(b), the Fr grating optical coupler 53 includes:
A diffraction grating 5 is provided on the optical waveguide layer 52 on the LiNb0a substrate 51.
8 is formed. This diffraction grating is formed by the optical waveguide layer 52.
An optical waveguide pattern in which the groove width M of the groove 60 dug in the direction of propagation of the guided light 54 (T direction) is the same regardless of the location, and the grating pitch P is also the same. It is formed by leaving behind.

In addition, the optical waveguide layer pattern portion 58+L for each pitch
The height H is also the width N of the pattern part in the rough direction.
are also set to the same length.

The light that has propagated through the optical waveguide layer (guided light) is radiated into space through the grating optical coupler, so the intensity of this emitted light is very small compared to the intensity of the propagating light. It is considered that it is proportional to the product of P and the coupling coefficient η of the diffraction grating optical coupler. Assuming that the coupling efficiency η of the diffraction grating optical coupler is constant in all parts, the propagating light gradually attenuates while passing through the diffraction grating optical coupler, so the intensity of the light at the diffraction grating part gradually attenuates. I will do it. This can be expressed as follows: dP/dz=77F (1). Here, P represents the intensity of light at an arbitrary position in the diffraction grating portion, and 2 represents the propagation direction of the light.

The intensity of the light at the point z-〇 where the guided light enters the diffraction grating optical coupler 53 is P. Then, the solution to equation (1) is P-Poexp (77Z) (2),
As shown in Fig. 8, the intensity distribution 56 of the guided light 54 on the diffraction grating is exponential. If light with an exponential intensity distribution such as It is difficult to create light with an intensity distribution, and the light emitted from most light sources has a symmetrical intensity distribution.

Because of this, a diffraction grating optical coupler with a constant coupling coefficient cannot achieve 100% coupling efficiency for normal laser light.

Therefore, in order to increase the coupling efficiency of the diffraction grating optical coupler, it is better to make the intensity distribution of the guided light in the diffraction grating part a Gaussian distribution type, which is often seen in ordinary laser light. . For this purpose, the coupling coefficient may be designed to vary linearly, as previously shown by the inventors. As a method of changing the coupling coefficient, the previous invention (
There is a method of changing the equipolarized refractive index of the optical waveguide layer as shown in Japanese Patent Application No. 1-38686, but since the change in the coupling coefficient is small, the length of the diffraction grating optical coupler must be increased. There was a problem.

(d) Means and Effects for Solving the Problems This invention forms a plurality of grooves by removing an optical waveguide layer or a thin film formed on the optical waveguide layer in stripes at a period comparable to that of light. A diffraction grating is created by forming an optical waveguide layer pattern or an N film pattern by leaving it on or leaving it behind, so that light can enter the optical waveguide layer or the light within the optical waveguide layer can be emitted to the outside. When creating a diffraction grating optical coupler, the following (A), (B) or (
Either of C), that is, (A) laminating thin films while gradually changing the film thickness in the propagation direction of guided light in the optical waveguide layer, and then removing the thin film until reaching the optical waveguide layer; The groove width in the propagation direction of the groove dug in the thin film is formed to be the same length regardless of the location, and the height of the thin film pattern portion for each pitch in the thin film pattern is sequentially changed in the propagation direction. or,
(B) After laminating the thin films, the groove width in the propagation direction of the grooves dug in the thin film is formed to be the same length regardless of the location, and the depth of each groove in the thin film pattern is adjusted as described above in the propagation direction. (C) After laminating thin films, the thin film is removed until it reaches the optical waveguide layer, thereby changing the propagation direction of the guided light in the optical waveguide layer. The groove width in the propagation direction of the grooves dug in the optical waveguide layer is sequentially changed by sequentially changing the groove width in the propagation direction and removing the optical waveguide layer using the obtained thin film pattern as a diffraction grating pattern. The pattern remains and the pattern width in the propagation direction of the optical waveguide layer pattern portion for each pitch is sequentially changed, thereby changing the coupling coefficient of the diffraction grating optical coupler along the propagation direction. The present invention provides a method for producing a diffraction grating optical coupler characterized by the following.

From another perspective, in a diffraction grating optical coupler that allows light to enter an optical waveguide layer formed on a substrate or emit light within the optical waveguide layer to the outside, After forming the thin film, the thin film is sequentially irradiated with an electron beam along the propagation direction of the guided light in the optical waveguide layer,
The area of the electron beam irradiated part on the thin film is changed for each grating pitch, thereby changing the refractive index of the electron beam irradiated part, and the rate of change in the refractive index also changes according to the area of the electron beam irradiated part. The present invention provides a method for producing a diffraction grating optical coupler, characterized in that the coupling coefficient of the diffraction grating optical coupler is changed along the propagation direction.

That is, the present invention improves the coupling of a diffraction grating optical coupler by processing the thin film or optical waveguide layer constituting the diffraction grating into a specific shape or by changing the proportion of the portion where the refractive index is changed. By changing the coefficient depending on the location, the intensity distribution of the guided light in the diffraction grating portion can be made equal to the intensity portion of normal light, and by increasing the coupling efficiency, it is possible to reduce the size.

In this invention, an optical waveguide layer, an optical waveguide layer pattern,
Alternatively, the thickness (layer thickness) of the /I pattern, NH film pattern, resist film or diffraction grating pattern (remaining portion of the resist film) means the length thereof in the height direction.

Specifically, in order to increase the change in the coupling efficiency of the diffraction grating optical coupler, as an example of processing a thin film into a specific shape, (i) the diffraction grating optical coupler is configured as shown in FIG. TtO, I! The thickness of the i33 is gradually changed in the diffraction grating portion [see FIG. 6(c)], and the height of the pattern portion 33b for each grating pitch P is sequentially changed in the propagation direction of the guided light.

(ii) Gradually change the ratio of the thickness of the lines forming the diffraction grating to the grating pitch. In other words, Fig. 5 (
As shown in d), the removed portion of the optical waveguide layer (ill
) 22b or the remaining part (pattern part) 302
change the proportion of

(iii) As shown in Fig. 1(c), the coupling efficiency of the diffraction grating optical coupler can be changed by sequentially changing the depth of the grooves too and tot dug in the ITO film in the above propagation direction. This is what I did.

The coupling efficiency of the diffraction grating optical coupler is preferably determined when the thickness of the thin film constituting the diffraction grating optical coupler is sufficiently thin (preferably 0 to 2000), as shown in the curve G (T
In the case of M mode), the layer thickness increases in proportion to the square of the layer thickness.For example, in the example shown in Fig. 3, when the thickness increases by 01 wavelength, the
Become twice as large. Therefore, a sufficient change in coupling efficiency can be obtained by only slightly changing the layer thickness, and the size of the diffraction grating optical coupler can be reduced.

On the other hand, the change in coupling efficiency when the ratio of the thickness of the lines constituting the diffraction grating to the grating pitch is changed is almost symmetrical when the aspect ratio is 0.5, as shown in Figure 4. It becomes a parabolic curve K (in case of TM mode). In this case, the change in coupling efficiency is smaller than when the layer thickness is changed, but it is larger than when the refractive index is changed, and the controllability is better than when the layer thickness is changed.

Furthermore, as a method for increasing the coupling efficiency, as shown in FIG. 7, the area of the part of the chalcogenide glass thin film 43 irradiated with the electron beam 44 is changed without changing the pitch P, thereby changing the refractive index. parts 43a to 43
The ratio of g may be changed.

(e) Examples Examples of the present invention will now be described in detail with reference to the drawings.

Example 1 (First Example) Figure 1 shows a method for creating a diffraction grating optical coupler on a lithium niobate base [1] on which an optical waveguide layer 2 was created in the form of a stripe by a normal ion exchange method. It is something that

In FIG. 1, first, on a substrate 1 on which an optical waveguide layer 2 with a thickness of 4000 nm is formed, an IrO film 3 having a thickness of, for example, 1500 nm is deposited by ordinary magnetron sputtering as a thin film constituting a diffraction grating optical coupler. After f, an electron beam resist 4 is applied, and a diffraction grating optical coupler pattern is drawn by electron beam exposure.

At this time, by gradually changing the exposure amount of the electron beam 5 for each pitch, the remaining thickness of the resist film after development is made to change little by little (see Figure 1 (a)).
.

Thereafter, when the electron beam resist 4 and ITO@3 are successively etched by the etching method using the Ar ion beam 6 (see FIG. 1(b)), the thickness of the remaining portion (diffraction grating pattern) 4a of the resist film 4 is reduced. The ITO film 3 is etched in proportion to the etching rate, and the obtained TO film pattern 3a can be processed as a diffraction grating (see FIG. 1(c)).

That is, among the plurality of grooves formed by the removed portion of the ITO film 3,
A portion where the film portion between the groove bottom surface and the top surface of the ITO film 3 in the diffraction grating pattern 4& is thicker, for example, the portion indicated by +42 which is thicker than the portion indicated by the symbol 141 (indicated by a phantom line) in FIG. 1(b). I corresponding to (indicated by phantom line)
In the etched portion of the TO film, the ITO film is hardly etched to form, for example, the shallowest groove 100 having the minimum depth, and in the etched portion of the ITO film corresponding to the thinner portion, the ITO II is etched deeply, for example, to the maximum depth. The deepest groove 101 with the highest depth is formed so as to reach the optical waveguide layer 2 [see FIG. 1(c)]. For this reason, a diffraction grating optical coupler is created in which the groove depth gradually changes from shallow groove 100 to long JIOI as TO film pattern 3a that constitutes the diffraction grating (Figure 1). (See (C)).

At this time, the groove width M is formed to have the same length as the numbers 100 and 101, or the valley grooves (the numbers are omitted) arranged between them, and on the other hand, the heights of the rectangular pattern parts are different from each other. For example, the height of the pattern 3C is higher than that of the pattern 3b (numerals of other pattern parts are omitted). Further, the pattern width W in the guided light propagation direction is formed to have the same length. As shown in Figure 3, the coupling efficiency of a diffraction grating optical coupler is proportional to the square of the film thickness when the film thickness is about 0.1 or less of the wavelength of light, so the film thickness is proportional to the square root of the position. Just make sure it's proportionate.

Alternatively, a plurality of grooves may be formed into blade shapes having different sizes, thereby forming an ITO film pattern having a sawtooth cross-sectional shape.

That is, as shown in FIG. 2, in the region R where a diffraction grating is to be formed by changing the exposure amount within one period, blade-shaped grooves ( For example, an ITO film pattern portion 3 having a sawtooth cross-sectional shape is formed by forming
The pattern 3a having a plurality of d can also be processed as a diffraction grating. Thereby, the coupling efficiency can be made higher than in the case of the pattern portion 3c having a rectangular cross-sectional shape, and the size can be reduced.

It goes without saying that the cross-sectional shape of the ITO film pattern is not limited to the above two shapes, but can be processed into any shape as long as it increases the coupling efficiency.

Example 2 (Second Example) In the above example, an ITO film, which is a conductive film, was used to prevent charge-up due to electron beam exposure, but this invention uses an ordinary metal film. A second example will be described.

In FIG. 5, first, an optical waveguide layer 22 is formed by the proton exchange method as in the above embodiment.
For example, an AI film 23 is deposited on the y-substrate 21 to a thickness of approximately 200 mm using a resistance heating method, and an electron beam resist 24 is coated on the Al film 23. Electron beam 2 from above
5 (see FIG. 5(a)) to form a diffraction grating pattern 2.
4a was drawn (see FIG. 5(b)).

At this time, the coupling efficiency was changed by gradually increasing the proportion of the exposed portion within one cycle. After developing the resist pattern 24a, the A IH 23 is used as a CC1, ion beam 26 using CC1 as a reactive gas.
The AI film pattern 23a is formed by etching using the reactive ion etching method (see FIG. 5(b)).
(See Figure 5(c)).

At this time, the resist pattern 24a is removed using an organic solvent such as acetone. Next, using the diffraction grating pattern 23a of the At film 23 as a mask, an optical waveguide is etched using a reactive ion beam etching method using a C5Fs ion beam 27 using 03 Fm or the like as a reactive gas (see FIG. 5(c)). The layer 22 is etched to form an optical waveguide layer pattern 22a (see FIG. 5(c)), and a diffraction grating optical coupler is formed using the pattern 22a as a diffraction grating (see FIG. 5(d)).

At this time, each optical waveguide layer pattern portion, for example, 50
1 and 502 are rectangular and have a pattern width of d.
,, d 2 (d t < d ,), and the width of the valley groove 22b is naturally different (numerals of other pattern parts are omitted). The depth of the groove and the height of the pattern width (pattern layer thickness) are all the same length for each pattern portion.

Example 3 (Third Example) In the case of this example, as shown in FIG.
LiNb0. Transparent T t O on the substrate 31
The film 33 is deposited by a conventional directional electron beam deposition method.

At the time of vapor deposition, the shutter 35 disposed immediately in front of the substrate 31 is gradually opened in the S direction.
The thickness of the iOx film was controlled (see FIG. 6(a)).

That is, since the thickness of the T i O* film 33 is proportional to the time the shutter 35 is open, this proportionality coefficient was determined in advance through experiments, and the speed at which the shutter 35 was opened was determined based on it.

In this way, as shown in No. 611(b), Ti
O! A film 33 is formed, and then a photoresist film 37 is applied on this inclined surface 33b, and a diffraction grating pattern 3 is formed by a normal two-beam interference exposure method using two light beams 361 and 36b.
7a (see Fig. 6(c)), the TiO1 film 33 is etched with a mixed solution of HF, NH, and F to form a TiO*
A diffraction grating of the Mi pattern 33a is formed (FIG. 6(d)
)reference).

At this time, the L i N b O2 substrate 31 contains HF and NH,
Since the mixed solution of F is hardly etched, the film thickness of the created diffraction grating optical coupler, that is, TiO OtR33 (D film thickness d
(The height of the portion 333 indicated by the imaginary line in FIG. 6(c)) is approximately equal to (Dad), and gradually changes depending on the location. For this reason, the coupling coefficient will also change, but in order to obtain the desired value, it is necessary to sufficiently control the thickness of the TiO2 film 33. The groove width ~f in the guided light propagation direction (T direction) of the valley grooves (indicated by reference numeral 33b, other grooves are omitted) of the diffraction grating and the width W of the pattern portion are the same length. The height of the pattern portion for each grating pitch P (for example, indicated by a symbol in the pattern portion 601) changes sequentially in the propagation direction.

Furthermore, since this method does not use an electron beam, there is no need to use a conductive film. On the other hand, since the pattern 33a is fixed by an optical system, it is suitable for mass production of the same pattern.

Example 4 (Fourth Example) In the case of this example as well, as shown in FIG. Lithium oxide substrate 41
For example, a so-called chalcogenide glass thin film 43 composed of atoms such as arsenic, selenium, and sulfur is placed on top of the
It is made to a thickness of 1500 mm using the F sputtering method. When the chalcogenide glass thin film 43 is irradiated with an electron beam 44, its refractive index changes. Therefore, as shown in FIG. 7(a), if the area of the part irradiated with the electron beam 44 is changed without changing the pitch p, the refractive index of the part 4 changes accordingly.
The ratio (aspect ratio) of 3+L to 43g (shaded area in FIG. 7(b)) will also change.

At this time, the coupling efficiency of the diffraction grating optical coupler is determined by the ratio (aspect ratio) of the portions 43 & ~ 43g where the refractive index has changed as shown in Fig. 4, and the coupling efficiency is parabolic, with the ratio being maximum at approximately 0°5. Therefore, a desired distribution of coupling efficiency can be obtained by gradually increasing or decreasing the area exposed by the electron beam 44.

In each of the above embodiments, a LiNbO5 substrate was used, but other substrates and optical waveguide layers may be used, or other materials may be used for the thin film that is the diffraction grating layer other than the optical waveguide layer. It goes without saying that the diffraction grating material may be used. In short, various substrates, optical waveguide layers, and diffraction grating layers can be used in combination without departing from the spirit of the present invention.

(f) Effects of the Invention According to the present invention, the optical waveguide layer or the N film formed on the optical waveguide layer is removed in a stripe shape at a period comparable to that of light, or left or refracted. Creating a diffraction grating by changing the ratio and injecting light into the optical waveguide layer,
Or, when creating a diffraction grating optical coupler that outputs the light in the optical waveguide to the outside, the thickness of the thin film constituting the diffraction grating may be changed, or in order to configure the diffraction grating optical coupler. By changing the depth of the grooves dug, the thickness of the layer constituting the diffraction grating optical coupler is gradually changed, or the proportion of the removed portion or the remaining portion of the thin film constituting the diffraction grating is changed. The coupling coefficient of the diffraction grating optical coupler can be changed depending on the location by changing the ratio of the part with the changed refractive index to configure the diffraction grating optical coupler. It is possible to create a small-sized diffraction grating optical coupler that can efficiently input light with an intensity distribution into an optical waveguide. Application to devices can bring great industrial effects. ,

[Brief Description of the Drawings] Figure 1 is a diagram showing a method of manufacturing an embodiment of the present invention;
The figures show a modification of the above embodiment, Figure 3 is a characteristic diagram showing the relationship between the thickness of the thin film forming the diffraction grating layer and the coupling efficiency (coefficient) of the diffraction grating optical coupler, and Figure 4 The figure shows the line width of the thin film pattern that makes up the diffraction grating.
A diagram showing the relationship between the ratio to the period length and the coupling coefficient of the diffraction grating, FIG. 5 is a diagram showing a method for creating another embodiment of the present invention, and FIG. 6 is a diagram showing still another embodiment of the present invention. FIG. 7 is a diagram showing the method of creating another embodiment of the present invention, and FIGS. 8(a) and (b) each show the intensity characteristics of the guided light. FIG. 2 is a diagram showing a characteristic diagram and a conventional diffraction grating optical coupler. 1.21.31.41...L i N b O
s substrate, 2.22, 32.42... optical waveguide layer,
3...ITO film, 3a...ITO film pattern, 3b, 3c, 3d, 40--ITO film pattern portion, 4.24...Electron beam resist, 5.
25, 37.44...electronic, 6...
・Ar ion beam, 22a... Optical waveguide layer pattern, 22b, 33
b, 102, 103, 501... Groove, 23...
...AI film, 26...CCl, ion beam, 27...
...C3Fll ion beam, 33...Ti0
z film, 33a...TiO film pattern, 35...
...Shutters 36a, 36b...Light flux, 43...Chalcogenide glass thin film, 43a~
43g... Chalcogenide glass thin film portion exposed to electron beam, lOO... Shortest groove formed in ITO film,
101...The longest groove formed in the ITO film,
01... Optical waveguide layer pattern part with the longest width, 02... Optical waveguide layer pattern part with the shortest width, 01 502... Optical waveguide layer pattern part, 01... ...'fi O2 film pattern part.

Claims (1)

[Claims] 1. The optical waveguide layer or the thin film formed on the optical waveguide layer is removed in stripes at a period comparable to that of light to form a plurality of grooves, or the optical waveguide is formed by leaving a plurality of grooves. layer pattern,
Or, when creating a diffraction grating optical coupler in which a diffraction grating is created by forming a thin film pattern, and light is input into the optical waveguide layer, or light within the optical waveguide layer is emitted to the outside, Any of the following (A), (B) or (C), i.e. (
A) Each groove dug in the thin film is stacked by gradually changing the film thickness in the propagation direction of the guided light in the optical waveguide layer, and then removing the thin film until it reaches the optical waveguide layer. The groove width in the propagation direction is formed to be the same length regardless of the location, and the height of the thin film pattern portion for each pitch in the thin film pattern is sequentially changed in the propagation direction, or (B) the thin film is After laminating, the width of each groove in the propagation direction of each groove dug in the thin film is formed to be the same length regardless of the location, and the depth of each groove in the thin film pattern is sequentially changed in the propagation direction. , or (C) After laminating thin films, the thin films are removed until reaching the optical waveguide layer, thereby sequentially changing the groove width of each groove dug in the thin film in the propagation direction of the guided light in the optical waveguide layer. By using the obtained thin film pattern as a diffraction grating pattern and removing the optical waveguide layer, the optical waveguide layer pattern is left so as to sequentially change the groove width in the propagation direction of each groove dug in the optical waveguide layer. A diffraction grating light characterized in that the pattern width in the propagation direction of the optical waveguide layer pattern portion for each pitch is sequentially changed, thereby changing the coupling coefficient of the diffraction grating optical coupler along the propagation direction. How to create a combiner. 2. In a diffraction grating optical coupler that allows light to enter an optical waveguide layer formed on a substrate or emit light within the optical waveguide layer to the outside, after forming a thin film on the optical waveguide layer. , the thin film is sequentially irradiated with an electron beam along the propagation direction of the guided light in the optical waveguide layer, and the area of the electron beam irradiated part on the thin film is changed for each grating pitch, thereby making the electron beam irradiated part In addition to changing the refractive index of the electron beam, the rate of change in the refractive index is also changed in accordance with the area of the electron beam irradiated portion, and the coupling coefficient of the diffraction grating optical coupler is changed along the propagation direction. How to create a diffraction grating optical coupler.