JPH03213809A - Diffraction grating optical coupler - Google Patents

Abstract

PURPOSE:To improve coupling efficiency by providing a diffraction grating on an optical waveguide and successively changing the coupling coefft. in the direction where the guided light in the optical waveguide propagates. CONSTITUTION:The diffraction grating optical coupler provided on the optical element formed with strip-shaped optical waveguide 12 on a substrate 11 is provided along the optical waveguide 12 over a length d0 on one side part of the waveguide 12. The optical waveguide 12 is formed to a specified width exclusive of the diffraction grating optical coupler part thereof and is formed to the tapered shape gradually decreased in width in the diffraction grating optical coupler part. The diffraction grating 13 is constituted of plural planar scattering bodies on the tapered part of the optical waveguide 12. The coupling coefft. of the diffraction grating optical coupler is, therefore, changed by a change in the width size of the optical waveguides 12. The incident light or exit light and the guided light in the optical waveguide are optically coupled with the high efficiency in this way.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to an optical device such as an integrated optical device having an optical waveguide through which light propagates, in order to make light enter the optical waveguide or to The present invention relates to a diffraction grating optical coupler used for emitting light from an optical waveguide.

(Prior Art) In integrated optical devices such as integrated optical pickups, integrated optical scanning devices, and integrated optical Doppler velocimeters.

High performance has been achieved by using optical waveguides through which light propagates. In such an integrated optical device, in order to make the fundamental wave enter the optical waveguide.

Conventionally, the end face of the optical waveguide is optically polished, and the fundamental wave is focused using a lens having a large numerical aperture (NA) and is made to enter the optical waveguide from the optically polished end face. However, in order to make the fundamental wave enter the optical waveguide in this way, the end face of the optical waveguide must be optically polished with high precision, and the optical axis of the lens and the optical waveguide must be precisely adjusted. There is also.

In contrast to this method, recently, a diffraction grating optical coupler is used, which is small and easy to integrate.

A widely used method is to input light into an optical waveguide or to output light from an optical waveguide.

A diffraction grating optical coupler has a diffraction grating installed on an optical waveguide.One diffraction grating consists of multiple straight lines arranged at equal pitches, or multiple curved lines arranged so that the pitch gradually changes. Various configurations having a light condensing function have been considered.

FIG. 5(a) is a plan view showing an example of a conventional diffraction grating optical coupler, and FIG. 5(b) is a sectional view thereof. The diffraction grating optical coupler includes a diffraction grating 43 formed on one side of an optical waveguide 42 formed on a crystal substrate 41 such as LiNbO2.
has. The optical waveguide 42 is formed at the center of the substrate 41 in the width direction and has a constant width and thickness along the longitudinal direction.

The diffraction grating 43 is composed of a plurality of transparent flat bodies having equal lengths and perpendicular to the direction in which the optical waveguide 42 extends. When light is projected onto the diffraction grating 43, the light enters the optical waveguide 42 via the diffraction grating 43 and propagates within the optical waveguide 42.

Further, the guided light propagating within the optical waveguide 42 is emitted from the diffraction grating 43 to the outside of the waveguide 42 .

In such a conventional diffraction grating coupler, the optical waveguide 4
The coupling efficiency when the guided light 21 propagating in the first diffraction grating 43 is output from the first diffraction grating 43 is first-order. When the intensity of the guided light 21 propagating in the optical waveguide 42 is PI+ and the intensity of the output light 22 from the diffraction grating 43 is Pli, the coupling coefficient of a single diffraction grating coupler is η.

P,=-ηP,...(1) It is expressed as

Light propagating within the grating coupler gradually attenuates during its propagation. If the coordinate of the propagation direction of the guided light 21 in the optical waveguide 42 in the diffraction grating coupler is expressed as 2, then the intensity of the light emitted from the diffraction grating coupler is 9.

This is the solution to the first-order differential equation shown in equation (2).

From equation (1).

Pa~exp(-r)z)-(3) The intensity distribution of the emitted light is expressed as an exponential function, as shown in FIG. 5(b).

In this way, the light propagating within the optical waveguide 42 is emitted from the two-diffraction grating combiner with an exponential light intensity distribution. However, since the two-diffraction grating is a dead phase element, when making light enter the optical waveguide 42, if the incident light has such an exponential light intensity distribution, the optical waveguide 4
The intensity of light propagating within 2 becomes constant. As a result, the coupling efficiency of the single diffraction grating combiner is significantly improved.

(Problem to be solved by the invention) However, in reality, the light incident on a single diffraction grating combiner,
Alternatively, it is difficult to give the above-mentioned exponential intensity distribution to the light propagating in the optical waveguide; 1. Usually, the light propagates in the optical waveguide has a symmetrical light intensity distribution like semiconductor laser light. For this reason.

The coupling efficiency of a diffraction grating optical coupler is limited to about 80%.

The present invention solves the above-mentioned conventional problems.

The purpose is to provide a diffraction grating optical coupler with excellent coupling efficiency.

(Means for Solving the Problems) A diffraction grating optical coupler of the present invention includes a diffraction grating provided on an optical waveguide in order to optically couple the inside and outside of the optical waveguide through which light is propagated. , the coupling coefficient changes sequentially in the propagation direction of the guided light in the optical waveguide, thereby achieving the above object.

(Example) The present invention will be described below with reference to Examples.

The diffraction grating optical coupler of the present invention is shown in FIGS. 1(a) and (b).
), for example, a striped optical waveguide 12 is provided on an optical element formed on a LiNbO3 substrate 11. The optical waveguide 12 is formed to have a constant thickness by a common ion exchange method.

The grating optical coupler is mounted on one side of the optical waveguide 12, e.g.
It is provided along the optical waveguide 12 over a length d8.

The optical waveguide 12 has a constant width except for the diffraction grating light coupling area, where the width gradually narrows (becomes tapered).

A diffraction grating 13 is formed on the tapered portion of the optical waveguide 12 by a plurality of flat scatterers having equal lengths and perpendicular to the direction in which the optical waveguide 12 extends.

The optical waveguide 12 is constructed by applying an electron beam resist such as polymethacrylate on the substrate Ill, then drawing a mask pattern having the shape described above by electron beam exposure, and further developing the resist. It is formed by the proton exchange method. The diffraction grating 13 is made by depositing a transparent film such as a 543N4 film on the substrate 11 on which the optical waveguide 12 is formed, applying an electron beam resist to this transparent film, and then drawing a diffraction grating pattern using an electron beam exposure method. later.

It is formed by etching using an etching solution such as Buffer HF. The length of the diffraction grating optical coupling 2g is set at 50 mm to avoid aberrations of the Ti beam exposure device.
Approximately 0 μm is preferable.

In the diffraction grating optical coupler, the coupling coefficient of the diffraction grating optical coupler changes as the width dimension of the optical waveguide 12 changes. FIG. 2 is a graph showing the relationship between the equivalent refractive index and the coupling coefficient of the diffraction grating optical coupler.

If the width of the optical waveguide is thick (the equivalent refractive index increases almost proportionally), the width of the grating optical coupling type portion of the optical waveguide 12 in this embodiment is equal to that of the optical waveguide of the one-diffraction grating optical coupler. The coupling coefficient is set so that it is maximum at the narrow end of the width dimension and changes linearly.As is clear from the graph in Figure 2, the guided light does not propagate ( As the equivalent refractive index increases from the cut-off point (the point where the equivalent refractive index becomes equal to the refractive index of the substrate).

The coupling coefficient is monotonically decreasing. In this example.

The width of the optical waveguide 12 is tapered so that the width of the optical waveguide with the maximum coupling coefficient is the width of the narrow end.

In the diffraction grating optical coupler of this embodiment with such a configuration, when light is projected onto one diffraction grating 13, the light is transmitted to the diffraction grating 1.
3 and is introduced into the optical waveguide 12, and propagates within the optical waveguide 12. Further, the light propagating within the optical waveguide 12 is diffracted by one diffraction grating 13 and output from the diffraction grating 13.

In this embodiment, the guided light 2 propagating in the optical waveguide 12
Consider the case where 1 is emitted from a 1 grating coupler. The intensity of the guided light 21 propagating in the optical waveguide 12 is P5.
, when the intensity of the emitted light 22 is PlI, the coupling coefficient of the diffraction grating optical coupler is η, and the propagation direction coordinate of the guided light is 2, the intensity PII of the emitted light is calculated by the first-order differential equation expressed by equation (4). This is the solution.

d PH7d z = -r) z Po- (4) Since the coupling coefficient η of the diffraction grating optical coupler is expressed as a linear function of Z.

P2~exp(-22/2)-(5), and the intensity of the emitted light Pl! is a Gaussian distribution (Fig. 1 (
b)).

In this way, when the coupling coefficient of the nine-diffraction grating optical coupler changes at a constant rate in the propagation direction of the guided light.

For guided light of constant intensity, the intensity of the emitted light has a Gaussian distribution, and the distribution of the coupling efficiency of the single diffraction grating optical coupler has a Gaussian distribution. Therefore, the diffraction grating optical coupler has a coupling efficiency distribution similar to that of light having a Gaussian intensity distribution such as a laser beam.9 If the diffraction grating optical coupler is sufficiently long in the propagation direction of the guided light.

Even though the overall coupling coefficient is low, light attenuation is reduced. As a result, in the single-diffraction grating optical coupler, incident light to or output from the optical waveguide and guided light within the optical waveguide are optically coupled with high efficiency.

FIGS. 3(a) and 3(b) show nine embodiments of the diffraction grating optical coupler according to the present invention. In this embodiment, the width of the optical waveguide I2 is kept constant and the thickness of the portion of the optical waveguide 12 where one diffraction grating 13 is formed is varied.

By changing the equivalent refractive index, the coupling coefficient of the single diffraction grating optical coupler is changed. Based on the fact that as the thickness of the optical waveguide 12 increases, the 1-equivalent refractive index also increases sequentially. Thickness is set. The other configurations are the same as the embodiment shown in FIG.

The thickness of the optical waveguide 12 can be changed by changing the ion exchange time, considering that it is proportional to the square root of the ion exchange time. As a method of changing the thickness of the optical waveguide 12 by changing the ion exchange time, as shown in FIG. It can be pulled up from the exchange melt 131.

In the above example, an optical waveguide was formed on the LiNbO3 substrate by a proton exchange method, and a diffraction grating was formed by a Si3N4 film laminated on the optical waveguide.
It is not limited to this configuration.

For example, the diffraction grating may be formed by applying a suitable etching method to a transparent film such as an ITO film or an electron beam resist.

In addition, as a substrate, such as LiTaO3 (lithium tantalate), KTiOPO4 (KTP), etc.
A material that can easily form an optical waveguide by proton exchange at a low temperature of about 200"C may be used.

(Effects of the Invention) As described above, in the diffraction grating optical coupler of the present invention, the intensity distribution of the incident light to the optical waveguide or the output light substantially matches the coupling efficiency, and the coupling efficiency is significantly improved.

Figure 1(a) is a plan view showing an example of the diffraction grating optical coupler of the present invention, Figure 1(b) is its cross-sectional view, and Figure 2 shows the width of the waveguide and diffraction. A graph showing the relationship between the grating optical coupler and the coupling coefficient, FIG. 3(a) is a plan view showing another example of the diffraction grating coupler of the present invention, FIG. 3(b) is a sectional view thereof, and FIG. FIG. 4 is a schematic diagram showing an example of the manufacturing method.

FIG. 5(a) is a plan view of a conventional diffraction grating coupler, and FIG. 5(b) is a sectional view thereof.

11... Board.

12... Optical waveguide.

13... Diffraction grating.

21... Guide Wave light.

22... Outgoing light.

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Claims (1)

[Claims] 1. A diffraction grating is provided on the optical waveguide to optically couple the inside and outside of the optical waveguide through which light is propagated, and the coupling coefficient changes sequentially in the propagation direction of the guided light within the optical waveguide. A diffraction grating optical coupler characterized by: