JPH02287411A - Optical coupling device - Google Patents

Abstract

PURPOSE:To obtain high coupling efficiency by having a conical type reflecting prism via a dielectric layer on a waveguide and specifying a coupling length to <=0.4mm, the thickness of the waveguide to >=0.5mum and the difference in the refractive index between the waveguide and the dielectric layer to <=0.3. CONSTITUTION:This coupling device is constituted of the waveguide 2 provided on a transparent substrate 1 and the conical type reflecting prism 4 disposed via the dielectric layer 3 on the waveguide 2. The coupling length thereof is specified to <=0.4mm, the thickness of the waveguide to >=0.5mum and the difference in the refractive index between the waveguide 2 and the dielectric layer 3 to <=0.3. The light emitting distribution of a laser is, therefore, inverted by the conical type reflecting prism 4 and the input coupling is executed with the light distribution approximate to the radiation characteristic distribution of the waveguide 2. The high input efficiency is obtd. and further, the tolerance to a fluctuation in the wavelength of the incident light and a change in incident angle is widened and the stable coupling efficiency is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical coupling device for guiding light into a waveguide of an optical integrated circuit or the like and converting it into guided light that propagates radially.

2. Description of the Related Art Conventionally, optical couplers for radially propagating guided light include those in which a concentric grating is formed on a waveguide, and those in which a conical waveguide is formed.

Problems to be Solved by the Invention In a conventional optical coupler using a grating, an efficiency of only 50% at maximum can be obtained due to the diffraction characteristics of the grating. Furthermore, since the incident light has a Gaussian distribution, the efficiency becomes even lower. Similarly, an optical coupler using a conical waveguide 111 as shown in FIG. 11 has problems such as low efficiency due to Gaussian distribution and waveguide loss due to bending of the waveguide 112 at its outer periphery. Furthermore, no countermeasures have been taken to prevent a decrease in coupling efficiency due to wavelength fluctuations of incident light or changes in incidence angle. Here, 113 is incident light, 114 is a thin dielectric layer, 115 is a dielectric layer, and 116 is guided light.

The present invention aims to solve the problems of the prior art.

Means for Solving the Problems The present invention consists of a waveguide provided on a transparent substrate and a conical reflecting prism disposed on the waveguide via a dielectric layer, and the coupling length is reduced to 0. .4mm or less, waveguide thickness 0.4mm or less. 5 μm or more, and the refractive index difference between the waveguide and the dielectric layer is 0. This is an optical coupling device characterized in that the number of optical fibers is 3 or less. The optical coupling device is also characterized in that the refractive index nP of the conical reflecting prism is determined by the following equation.

nP: (2N”-no”) blind”
(1) However, N is the equivalent refractive index, fl
a is the refractive index of the dielectric layer. Furthermore, the radius rg at which the intensity of the incident light having a Gaussian distribution is 1/e2
The ratio of the aperture radius rA to 0. This is an optical coupling device characterized in that the ratio is within the range of 3 to 0.6.

Effect of the Invention In the present invention, the emission distribution of the semiconductor laser is inverted by the conical reflecting prism, and input coupling is performed with a light distribution close to the radiation characteristic distribution of the waveguide, so that high input efficiency can be obtained.

Furthermore, the coupling length was set to 0.4 mm or less, and the waveguide thickness was set to 0.85 mm.
μm or more, and the refractive index difference between the waveguide and the gap is 0.
By setting it to 5 or less, the allowable range for wavelength fluctuations and incidence angle changes of incident light is expanded, and stable coupling efficiency can be obtained. By determining the refractive index nP of the conical reflecting prism according to equation (1), error sensitivity characteristics that are symmetrical with respect to positive and negative errors are obtained, so that the coupling efficiency is stabilized. Further, by setting the ratio of the radius rE and the aperture radius r^ at which the intensity of the incident light having a Gaussian distribution is 1/e2 to be in the range of 0.3 to 0.6, high coupling efficiency can be achieved.

Embodiments Below, embodiments of the present invention will be explained based on FIGS. 1 to 10. FIG. 1 shows the configuration of an optical coupling device according to an embodiment of the present invention, and FIG. 2 is an external view thereof. As shown in FIG. 1, the optical coupling device of the present invention is composed of a waveguide 2 provided on a transparent substrate 1 and a conical reflecting prism 4 disposed with a dielectric layer 3 interposed therebetween. Transparent substrate! Incident light 6 that enters from the side through the aperture stop 5 is reflected by the conical surface of the conical reflecting prism 4 and enters the waveguide 2 through the dielectric layer 3 as guided light 7 . Guided light 7 is guided through waveguide 2
The optical energy propagates radially within the same substrate and is supplied to an optical integrated circuit formed on the same substrate.

This embodiment shows an optical head device that converts guided light 7 into radiation light 9 using a ring-shaped grating 8 formed on the same substrate. Furthermore, this optical head device was developed in Japanese Patent Application Laid-Open No. 8
As described in Japanese Patent No. 3-198588, it has excellent light condensing properties. Now, the incident light 6 once passes through the waveguide 2 and the dielectric layer 3, and then is reflected by the conical surface of the conical reflecting prism 4. The reflection angle .theta.1 and the apex angle .theta.- of the conical reflecting prism 4 have the following relationship as shown in FIG.

θ1 = π/2− θ@/2 After reflection, the incident light 6 travels at an angle of θ2 = 201, and if the thickness of the dielectric layer 3 is appropriate, it passes through the dielectric layer 3 and satisfies the phase matching condition in the waveguide 2. The light in the mode is excited, and this becomes the guided light 7. Note that the phase matching condition at this time is expressed by the following equation, assuming that the refractive index of the conical reflecting prism 4 is n.

n, si no2=N However, N is the equivalent refractive index of the waveguide 2.

Originally, the angle θ2 satisfies the total reflection condition determined by the refractive indexes n6 and n of the dielectric layer 3, and the transmission of optical energy from the conical reflecting prism 4 to the dielectric layer 3 and further to the waveguide 2. There isn't. However, when the thickness of the dielectric layer 3 becomes thinner, the evanescent wave that slightly leaks out of the prism during total reflection causes light energy to be transmitted to the waveguide 2 and becomes guided light 7 (hereinafter referred to as this). , called combined light). Similarly,
Transmission of light from the waveguide 2 to the conical reflecting prism 4 also occurs, which results in a loss from the perspective of the guided light 7 (hereinafter referred to as loss light).

On the other hand, the outside of the conical reflecting prism 4, that is, the dielectric layer 3
In regions where the waveguide has a free surface, the guided light 7 is completely confined within the waveguide layer. That is, the above phase matching condition cannot be satisfied in the atmosphere (n=1, the same applies in vacuum). Therefore, in order to efficiently excite the guided light 7, it is necessary to maintain an appropriate balance between coupled light and lost light in the region where the dielectric layer 3 is in contact with the conical reflecting prism 4. For this purpose, it is of course necessary to optimize the thickness of the dielectric layer 3, but it is also necessary to optimize the intensity distribution of the incident light.

For this purpose, a configuration using a conical reflecting prism 4 is suitable. That is, as shown in FIG. 4, by using the conical reflecting prism 4, the incident light distribution 10A of the incident light 6, which had a Gaussian distribution at the time of incidence, is inverted and becomes a reflected light distribution 10B. This reflected light distribution 10B has characteristics very similar to the radiation characteristics 10C of the waveguide, and is the optimum characteristic for maximizing the incident coupling efficiency.

Note that the reflected light distribution area shown in FIG. 4 is defined as the coupling length.

In addition, the relationship between the light distribution of the incident light 6 and the aperture stop is also included,
The incident coupling efficiency can be optimized by appropriately selecting the ratio of the radius rA of the aperture stop 5 shown in FIG. 5 to the radius rE at which the light intensity of the input light Gaussian distribution 10A becomes 1/e2.

FIG. 6 shows an example of the results of calculating the incident coupling efficiency for rE/ra, and from this it can be seen that rE/r11 is 0. It can be seen that high efficiency exceeding 80% can be obtained in the range of 3 to 0.6.

On the other hand, as a measure to stabilize the incident coupling efficiency against the occurrence of error factors such as wavelength fluctuations and incidence angle changes of the incident light, the coupling length is reduced to 0.
．． 4 mm or less, the waveguide thickness is 0.5 μm or more, and the refractive index difference between the waveguide and the dielectric layer is 0.3 or less.

The incident coupling efficiency is uniquely determined by the parameter D determined by the structure of the waveguide and the parameter F defined by the magnitude of error Δe, and has characteristics as shown in FIG.

F=DΔe (2) Although this is an analysis result for a conventional one-dimensional optical coupler, it is qualitatively applicable to the optical coupler of the present invention. If no error occurs (Δe=o), F=0, and the theoretical maximum efficiency is 81.4.
% is obtained. It can be seen that an error occurs and the efficiency monotonically decreases as F increases.

Now, the parameter D is defined as the following formula for each wavelength error and incident angle error, D(Δλ), D(80)
A waveguide configuration with a small value of is a stabilizing condition for errors.

Here, λ is the wavelength of light, W is the thickness of the waveguide, C+1 constant (1, 25), and other parameters are as follows.

W-rr ” W+ 1 /γs+1/γOkp =k
(nP"-N"-nG2)1-"β = kN γs = k(N2-ns"-nG2)1-"7a
"k(N"-no2)1/2, where k is the wave number 2π
/λ, ns is the refractive index of the transparent substrate 1.

FIGS. 8 and 9 are graphical representations of the values of D(Δλ) and D(Δθ). From this, the bond length L = 0.4mm
It can be seen that both D(Δλ) and D(Δθ) take extremely large values, and it can be said that it is desirable that the bond length is 0.4 mm or less.

Further, regarding the film thickness W of the waveguide, it is desirable that it be thicker for wavelength error and thinner for incident angle error.

However, since a thick waveguide has a large effect on wavelength error, it is desirable that the optimum design condition for the film thickness be 0.5 μm or more. In addition, regarding the refractive index of the waveguide, D(Δλ), D(Δθ
) It can be seen that a low refractive index is desirable in both cases. this is,
It has been found through calculations that it is better to have a smaller difference from the refractive index nl11 of the dielectric layer 3, rather than the absolute value of the refractive index of the waveguide. In the calculation of this example, no=1.5,
Therefore, it can be seen that it is useful to set the refractive index difference between the waveguide and the dielectric layer to 0.3 or less.

In addition, in an optical coupler using a normal prism, the first
As shown by the broken line in Figure 0, the sensitivity differs depending on the sign of the error that occurs. This is because the phase matching condition of the waveguide is disturbed by the proximity of the prism. This disturbance is canceled by determining the refractive index nP of the conical reflecting prism according to the following equation.

nP = (2N”-no2-nG2)1-2(5)
However, N is a uniform refractive index, and no is a refractive index of the dielectric layer. Under these conditions, it is possible to realize the error sensitivity characteristics as shown by the solid line in FIG. 10, and the incident coupling efficiency can be stabilized.

As described in detail, the present invention enables highly efficient input coupling by optimizing the input light intensity distribution by the conical reflecting prism, and further reduces the incidence coupling due to wavelength error and incident angle error. A stable optical coupler that is resistant to errors and prevents a decrease in efficiency can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a schematic structural diagram showing the configuration of an optical coupling device according to an embodiment of the present invention, FIG. 2 is a perspective view thereof, FIG. 3 is a sectional view of the optical coupling device according to the same embodiment, and FIG. A principle diagram showing how the light distribution is converted by the conical reflecting prism in the same example. Figure 5 is a graph showing the relationship between the incident light distribution and the aperture stop in the same example. Figure 6 is the same example. FIG. 7 is a characteristic diagram of the incident coupling efficiency with respect to the incident light distribution and the radius ratio of the aperture stop in the same example. FIG. 8 is a characteristic diagram of the incident coupling efficiency with respect to the error parameter F in the same example. FIG. (Δλ) characteristic diagram, FIG. 9 is a characteristic diagram of parameter D (Δθ) with respect to the incident angle error in the same embodiment, FIG. 10 is a principle diagram of stabilization against error factors in the same embodiment, and FIG. 11 is FIG. 2 is a schematic structural diagram showing the configuration of a conventional optical coupling device. DESCRIPTION OF SYMBOLS 1... Transparent substrate, 2... Waveguide, 3... Dielectric layer, 4... Conical reflective prism, 5... Aperture stop, 6
... Incident light, 7 ... Guided light, 8 ... Ring-shaped grating, 9 ... Synchrotron radiation, IOA ... Incident light distribution, 10B ... Reflected light distribution, 10C ... Waveguide radiation properties. Name of agent: Patent attorney Shigetaka Awano and one other person Figure 1 4 circle image i- counterattack 7) Sumuro construction attack 9 Figure 2 compiled diagram figure ρ5 /d- stone figure figure to5 θriO 4ri6ρ 8 , θ 10.0! , 5 1.7 t, 8 2Δ Z, / IF7 Liquid conduction flow and flow folding diagram Tsuka dredging Raku expansion vinegar 1st diagram Public light # envy

Claims (3)

[Claims] (1) A waveguide provided on a transparent substrate and a conical reflecting prism disposed on the waveguide via a dielectric layer, the coupling length is 0.4 mm or less, and the thickness of the waveguide is 0
．． 5 μm or more, and a difference in refractive index between the waveguide and the dielectric layer is 0.3 or less. (2) The refractive index n_P of the conical reflecting prism is expressed as n_
A claim characterized in that P=(2N^2-n_G^2)^1^-^2 (where N is an equivalent refractive index and n_G is a refractive index of the dielectric layer). 1. The optical coupling device according to 1. (3) A claim characterized in that the ratio of the radius r_E and the aperture radius r^A at which the intensity of the incident light having a Gaussian distribution becomes 1/2 is in the range of 0.3 to 0.6. 1. The optical coupling device according to 1.