JPH0816728B2 - Optical coupling device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical coupling device for guiding light into a waveguide such as an optical integrated circuit and converting the light into guided light that propagates radially.

2. Description of the Related Art Conventionally, as an optical coupler for radially propagating guided light, there is a concentric grating formed on a waveguide or a conical waveguide.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention In an optical coupler using a conventional grating, an efficiency of up to 50% is obtained due to the diffraction characteristics of the grating. Further, since the incident light has a Gaussian distribution, the efficiency becomes lower. Similarly, a conical waveguide as shown in FIG.
Even in the optical coupler based on 111, there are problems such as low efficiency due to Gaussian distribution and at the same time, waveguide loss occurs due to the bending of the waveguide 112 at the outer periphery thereof, and the coupling efficiency due to wavelength fluctuation of incident light and incident angle change No measures were taken against the decline. 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 such problems of the conventional technology.

Means for Solving the Problems The present invention is constituted by a waveguide provided on a transparent substrate and a conical reflection prism arranged on the waveguide with a dielectric layer interposed therebetween, and has a coupling length of 0.4. The optical coupling device is characterized in that the thickness is 0.5 mm or less, the thickness of the waveguide is 0.5 μm or more, and the difference in refractive index between the waveguide and the dielectric layer is 0.3 or less. Further, the optical coupling device is characterized in that the refractive index n _P of the conical reflection prism is determined by the following equation.

n _P = (2N ² −n _G ² ) ^1/2 (1) where N is the equivalent refractive index and n _G is the refractive index of the dielectric layer. Furthermore, the optical coupling device is characterized in that the ratio of the radius r _E and the aperture radius r _A at which the intensity of incident light having a Gaussian distribution is 1 / e ² is in the range of 0.3 to 0.6.

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

Furthermore, by setting the coupling length to 0.4 mm or less, the waveguide thickness to 0.5 μm or more, and the refractive index difference between the waveguide and the gap to 0.5 or less, the allowable width for wavelength fluctuation of incident light and incident angle change is expanded, Stable coupling efficiency is obtained. By determining the refractive index n _P of the conical reflection prism by the equation (1), symmetrical error sensitivity characteristics with respect to the positive and negative of the error can be obtained, so that the coupling efficiency is stabilized. In addition, the radius r _E and the aperture radius at which the intensity of incident light with a Gaussian distribution is 1 / e ²
High coupling efficiency is realized by setting the ratio of r _A in the range of 0.3 to 0.6.

Embodiment An embodiment of the present invention will be described below with reference to FIGS. 1 to 10. FIG. 1 shows the configuration of an optical coupling device in 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 comprises a waveguide 2 provided on a transparent substrate 1 and a conical reflection prism 4 arranged via a dielectric layer 3. Incident light 6 that enters through the aperture stop 5 from the transparent substrate 1 side is reflected by the conical surface of the conical reflection prism 4 and enters the waveguide 2 as guided light 7 through the dielectric layer 3. Guided light 7 is waveguide 2
Light energy is supplied to the optical integrated circuits formed on the same substrate by propagating radially inside. In this embodiment, an optical head device is shown which converts the guided light 7 into the radiated light 9 by the ring-shaped grating 8 formed on the same substrate. still,
This optical head device has excellent light condensing characteristics as described in JP-A-63-196588.
Now, the incident light 6 is once transmitted through the waveguide 2 and the dielectric layer 3, and then reflected by the conical surface of the conical reflection prism 4. The reflection angle θ ₁ and the apex angle θ ₀ of the conical reflection prism 4 have the following relationship as shown in FIG.

_{θ 1 = π / 2θ 0/} 2 after reflection, the incident light 6 proceeds in theta _{2 =} 2 [Theta] ₁ becomes the angle, phase matching waveguide 2 the thickness of the dielectric layer 3 via which, if appropriate Light of a mode satisfying the conditions is excited, and this becomes guided light 7. The phase matching condition at this time is expressed by the following equation, where n _{P is} the refractive index of the conical reflection prism 4.

n _P sin θ ₂ = N where N is the equivalent refractive index of the waveguide 2.

Originally, the angle θ ₂ satisfies the total reflection condition that the refractive index of the dielectric layer 3 is determined by n _G and n _P , and the optical energy from the conical reflection prism 4 to the dielectric layer 3 and further to the waveguide 2 is There is no communication. However, when the thickness of the dielectric layer 3 becomes thin, the light energy is transmitted to the waveguide 2 due to the evanescent wave slightly leaking to the outside of the prism at the time of total reflection, and becomes the guided light 7 (hereinafter , Called the combined light). Similarly,
Light is also transmitted from the waveguide 2 to the conical reflection prism 4, and this is a loss when viewed from the guided light 7 (hereinafter, this is referred to as lost light).

On the other hand, in the outside of the conical reflection prism 4, that is, in the region where the dielectric layer 3 has a free surface, the guided light 7 is completely confined in the waveguide layer. That is, the above-mentioned phase matching condition cannot be satisfied in the atmosphere (n = 1, also in vacuum). Therefore, in order to efficiently excite the guided light 7, it is necessary to properly maintain the balance between the coupled light and the lost light in the region where the dielectric layer 3 is in contact with the conical reflection prism 4. For this, 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 light that is simultaneously incident.

The configuration using the conical reflection prism 4 is suitable for this purpose. That is, as shown in FIG. 4, by using the conical reflection prism 4, the incident light distribution 10A of the incident light 6 having the Gaussian distribution at the time of incidence is inverted and becomes the reflected light distribution 10B. This reflected light distribution 10B has a characteristic very similar to the radiation characteristic 10C of the waveguide, and is the optimal characteristic for maximizing the incident coupling efficiency. The reflected light distribution area shown in FIG. 4 is defined as the coupling length L.

The relationship between the light distribution of the incident light 6 and the aperture stop is also an important factor in optimizing the incident coupling efficiency. That is, the fifth
The radius r _A of the aperture stop 5 shown in the figure and the Gaussian distribution of the input light
The incident coupling efficiency can be optimized by appropriately selecting the ratio of the radius r _E at which the light intensity of 10 A becomes 1 / e ² . Figure 6 shows r _E / r
In one example of results of calculating the incident coupling efficiency for _A, than this, a high efficiency r _E / r _A exceeds 80% in a range from 0.3 to 0.6 it can be seen that Tokuraru.

On the other hand, as a measure to stabilize the incident coupling efficiency against the occurrence of error factors such as wavelength fluctuation of incident light and incident angle change, the coupling length is
It is useful that the thickness of the waveguide is 0.4 mm or less, the thickness of the waveguide is 0.5 μm or more, and the refractive index difference between the waveguide and the dielectric layer is 0.3 or less.

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

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

The parameter D is defined by the following equations for the wavelength error and the incident angle error, and D (Δλ), D (Δ
A waveguide configuration having a small value of θ) is a stabilizing condition against an error.

Here, λ is the wavelength of light, W is the thickness of the waveguide, C is a constant (1.25), and other parameters are as follows.

W _eff = W + 1 / γ _S + 1 / γ _G k _F = k (n _F ² −N ² ) ^1/2 β = kN γ _S = k (N ² −n _S ² ) ^1/2 γ _G = k (N ² −n _G ² ) ^1/2 where k is the wave number 2π / λ and n _S is the refractive index of the transparent substrate 1.

The values of D (Δλ) and D (Δθ) are shown in the graphs of FIGS. 8 and 9. From this, it can be seen that when the bond length L = 0.4 mm, both D (Δλ) and D (Δθ) have extremely large values, and it can be said that the bond length L is preferably 0.4 mm or less. Further, regarding the film thickness W of the waveguide, it is desirable that the wavelength error is thicker and the incident angle error is thinner. However, since the thick waveguide has a great effect on the wavelength error, the optimum design condition of the film thickness is preferably 0.5 μm or more. Also, regarding the refractive index of the waveguide, D (Δ
It can be seen that a low refractive index is desirable for both λ) and D (Δθ). It has been found by calculation that this is not the absolute value of the refractive index of the waveguide, but the smaller the difference from the refractive index n _G of the dielectric layer 3. In the calculation of the present embodiment, n _G = 1.5, and 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 the case of an optical coupler using an ordinary prism,
The sensitivity varies depending on the sign of the error that occurs as shown by the broken line in Fig. 10. This is because the phase matching condition of the waveguide is disturbed by the proximity of the prism. This turbulence is canceled by determining the refractive index n _P of the conical reflection prism by the following equation.

n _P = (2N ² −n _G ² ) ^1/2 (5) where N is the equivalent refractive index and n _G is the refractive index of the dielectric layer. Under this condition, the error sensitivity characteristic as shown by the solid line in FIG. 10 can be realized, and the incident coupling efficiency can be stabilized.

EFFECTS OF THE INVENTION As described above, according to the present invention, the input light intensity distribution can be optimized by the conical reflection prism, so that highly efficient input coupling is possible, and further, the incident coupling due to the occurrence of the wavelength error and the incident angle error. It is possible to prevent a decrease in efficiency and obtain a stable optical coupler resistant to errors.

[Brief description of drawings]

FIG. 1 is a schematic structural view showing the structure of an optical coupling device in an embodiment of the present invention, FIG. 2 is a perspective view thereof, FIG. 3 is a sectional view of the optical coupling device in the same embodiment, and FIG. FIG. 5 is a principle view showing how the light distribution is converted by the conical reflection prism in the same embodiment, FIG. 5 is a graph showing the relationship between the incident light distribution and the aperture stop in the same embodiment, and FIG. 6 is the same embodiment. 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, FIG. 7 is a characteristic diagram of the incident coupling efficiency with respect to the error parameter F in the same embodiment, and FIG. 8 is a parameter D with respect to the wavelength error in the same embodiment. FIG. 9 is a characteristic diagram of (Δλ), FIG. 9 is a characteristic diagram of parameter D (Δθ) with respect to an incident angle error in the same embodiment, FIG. 10 is a principle diagram of stabilization against an error factor in the same embodiment, and FIG. 11 is Shows the configuration of a conventional optical coupling device It is a diagrammatic structural view. 1 ... Transparent substrate, 2 ... Waveguide, 3 ... Dielectric layer, 4 ...
... Conical reflection prism, 5 ... Aperture stop, 6 ... Incoming light, 7 ... Guided light, 8 ... Ring-shaped grating, 9
...... Synchrotron radiation, 10A …… Incident light distribution, 10B …… Reflected light distribution,
10C: Radiation characteristics of waveguide.

Claims (3)

[Claims] 1. A waveguide provided on a transparent substrate, and a conical reflection prism arranged on the waveguide with a dielectric layer interposed between them. The coupling length is 0.4 mm or less, and the thickness of the waveguide is small. But
An optical coupling device having a thickness of 0.5 μm or more and a difference in refractive index between the waveguide and the dielectric layer of 0.3 or less. 2. The refractive index n _P of the conical reflection prism is determined by the following equation: n _P = (2N ² −n _G ² ) ^1/2 (where N is the equivalent refractive index and n _G is the dielectric constant). It is a refractive index of a body layer.) The optical coupling device according to claim 1. 3. The ratio of the radius r _E and the aperture radius r _A at which the intensity of incident light having a Gaussian distribution is e ^1/2 , and the ratio is in the range of 0.3 to 0.6. Optical coupling device.