JPH05196825A - Input/output structure of light guide - Google Patents

Abstract

PURPOSE:To provide input/output mechanism which improves the efficiency of the light input/output terminal of a waveguide deteriorating, owing to reflection at a terrace while guaranteeing the optical quality of an etched mirror by forming a recessed part at the light incidence force and outside the etched mirror across the terrace. CONSTITUTION:The efficiency of the light input/output end depending upon a wall (etched mirror) crossing a waveguide is improved as the etch depth (h) is increased. At the light input/output end of the waveguide formed of a wall perpendicular or almost perpendicular to a substrate 9 crossing an optical waveguide in one substrate, the hollow part 10 is formed in the front surface of the shallow etched mirror having etching depth (h) is provided for high quality to substantially increase the etching depth (h). Consequently, extremely high light input/output efficiency is obtained even when the etching depth (h) is low. For example, when T is 5mu and T0 is 30mum, the hollow part of >=28mum in depth h0 is formed for the etched mirror of 4mum in etching depth to reduce the reflection loss from 34% in the absence of the hollow part to 5%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improving the efficiency of a light input / output end of a waveguide by means of a wall that traverses the waveguide (hereinafter referred to as an etched mirror).

[0002]

2. Description of the Related Art FIG. 2 shows an input / output structure of a conventional optical waveguide, where 1 is a core (refractive index n1, thickness d), 2 is a clad (refractive index n2), and 3 is a clad (refractive index n2). 4 is a cap layer, 5 is an etched mirror, 6 is an optical axis, 7 is a terrace, 8 is an element end, 9 is a substrate, h is an etching depth, and T is a terrace length.

As shown in FIG. 2, the etched mirror has a simple structure consisting of a single-step wall which is vertical or almost vertical to the substrate and a bottom surface (hereinafter referred to as a terrace) formed on the front surface of the wall. had. The light propagating through the optical waveguide feels the difference between the refractive index of the optical waveguide and the external refractive index, and is reflected and transmitted by the etched mirror surface corresponding to the wall portion cutting the waveguide. The reflectance is determined by the refractive index of the waveguide and the outside medium, the smoothness of the waveguide structure and the etched mirror surface, the frequency of light, and the like. It is known that the reflectivity of an ordinary semiconductor laser with a refractive index near 3.5 is about 0.32 for near infrared light. Since such an etched mirror can be formed at an arbitrary position in the substrate by a semiconductor microfabrication technique mainly including lithography,
It is one of the basic elements that make up an optical integrated device.

First, the problem of output efficiency from the input / output structure of the optical waveguide will be described. Generally used waveguides are designed to propagate a fundamental mode in order to enhance the optical quality of emitted light. Therefore, the normalized frequency needs to be π / 2 or less, so that the thickness of the core layer of the waveguide cannot be increased. Light emitted from such a waveguide into free space diverges in a Gaussian manner. For example, in a double heterostructure laser made of Al _0.3 Ga _0.7 As / GaAs with an active layer corresponding to the core having a thickness of 0.18 μm, the emitted light spreads with a full width at half maximum of about 47 degrees. Therefore, a part of the light emitted from the etched mirror is reflected on the terrace. Since the optical axis of such reflected light is deviated from that of the direct light, when the light from the etched mirror is condensed by a lens, the direct light and the reflected light do not form an image at one point. Therefore, the reflection on the terrace is a loss. For high power applications, such loss is a factor that degrades performance.

The reflection on the terrace is the depth from the center of the waveguide layer to the terrace (hereinafter referred to as the etch depth) when the length from the etched mirror to the edge of the terrace (hereinafter referred to as the terrace length) is constant. It decreases with the increase of. Therefore, the reflection on the terrace should be reduced by simply increasing the etch depth. However, increasing the etch depth,
That is, an increase in the amount of processing causes alteration of the processing mask, increase in damage, and amplification of minute defects in the substrate, which deteriorates the smoothness and verticality of the etched mirror. As a result, the original optical function of the etched mirror is lost. In order to form a high quality etched mirror that retains this function, it is desirable to make the etching depth as shallow as possible. Due to the above constraint conditions, there is a limit in reducing the reflection loss by increasing the etching depth.

Next, the problem of input efficiency will be described. When light is incident from the input / output structure of the optical waveguide, the numerical aperture (NA) is limited by the etching depth and the terrace length. Therefore, a lens having a large NA focuses the input light on the etched mirror surface. However, the amount of light incident on the waveguide is limited. The input efficiency at this time can be improved by increasing the NA of the etched mirror. Therefore, when the terrace length is constant, the input efficiency increases as the etching depth increases, but there is a limit to the improvement of the input efficiency due to the same factors as the above-mentioned problem of the output efficiency.

[0007]

SUMMARY OF THE INVENTION The present invention provides an input / output structure of an optical waveguide which improves the efficiency of the optical input / output end of the waveguide which is deteriorated by reflection on the terrace while guaranteeing the optical quality of the etched mirror. To provide.

[0008]

[Means for Solving the Problems] In the present technology for forming an etched mirror, in order to improve the above-mentioned problems, a depression is provided at the light input / output end through a terrace on the outside of the etched mirror.

[0009]

The efficiency of the light input / output end by the etched mirror is improved with the increase of the etching depth as described in the prior art. In the input / output end structure of the optical waveguide of the present invention, on the front surface of the etched mirror with a shallow etching depth for high quality,
Providing the depression substantially increases the etch depth. Therefore, extremely high light input / output efficiency can be realized even if the etching depth is shallow. Such an effect of the present invention is shown in FIG.
AlGaAs / GaAs with optical input / output terminals as shown in
A waveguide will be described as an example.

First, after clarifying the relationship between the reflection loss and the etching depth at the input / output end of the conventional one-stage structure,
The reflection loss of the input / output structure of the optical waveguide according to the present invention is quantitatively evaluated. The profile of the emitted light from the waveguide composed of the layered core and the clad can be obtained by the electric field distribution of the light in the waveguide and the wave equation as described below (Reference: HCCasey, MBPanish: Heterost
ructure Lasers, P.75, Academic Press, 1978). When the z-axis is taken as the optical axis and the x-axis is taken as a direction perpendicular to the substrate, the electric field amplitude of light at a certain position (x, z) outside the waveguide is E
If (x, z) (the width of the waveguide in the direction horizontal to the substrate is sufficiently wider than the width in the vertical direction, the intensity distribution of light in the horizontal direction is uniform). Using the amplitude distribution E (x, 0) of the radio wave of light,

[Equation 1] Can be expressed as When the terrace is up to z = T,
The intensity of the reflected light is

[Equation 2] Becomes Here, as shown in FIG. 1, θ is the angle between light and the z axis, d is the thickness of the active layer, h is the etch depth, T is the terrace length,

[Equation 3] Is. If the total amount of emitted light is P ₀ , the reflection loss γ is γ
= P _r / P ₀ .

Here, the active layer corresponding to the core has a thickness d of 0.18 μm of GaAs and the clad of Al _0.3 Ga _0.7.
Take a double heterostructure laser of As as an example. The normalized frequency v of this waveguide is 0.826, and the normalized transmission refractive index b is
Since it is 0.3575, the guided mode is the 0th mode,

[Equation 4] Becomes Here, γ = 7.25 and k = 5.41. The radiation profile for this mode can be calculated by equation (1). As a result, the divergence angle of the beam is approximately full width at half maximum as described above.
It is 47 degrees. FIG. 4 shows the relationship between the reflection loss of such a laser and the end face depth.

In the conventional etched mirror, the terrace length is 30
At μm, the reflection loss is reduced to 5% or less.
Etch depth of m or more is required. On the other hand, the terrace length is 5
When μm, etching depth is 4 μm or more and reflection loss is 5
% Or less.

Based on the above results, the reflection loss of the input / output structure of the optical waveguide of the present invention is evaluated. In the present invention, the reflection of the radiated light is the sum of the reflections of both the terrace on the first stage immediately before the etched mirror and the bottom of the depression (the terrace on the second stage). This total reflection loss is caused by the reflection loss of the input / output structure of the optical waveguide having the conventional structure having only the first terrace and the etching depth caused by digging the first terrace to the second terrace. Compared with the reflection loss of the input / output structure of the optical waveguide of the conventional structure which is equal to the depression depth, it is equal to the larger one. Therefore, when the length of the terrace on the first stage (the distance between the etched mirror and the depression) is sufficiently short, the reflection loss occurs on the bottom surface of the depression.

FIG. 3 shows the input / output structure of the optical waveguide of the present invention, in which 10 is a depression and h ₀ is a depression depth, and the same reference numerals as those in FIG. 2 indicate the same. As shown in FIG.
= 5 μm and T ₀ = 30 μm, the etch depth h = 4
By setting the recess depth (depth from the waveguide layer to the bottom of the recess) h ₀ to 28 μm or more for the etched mirror of μm, the reflection loss is reduced from 34% of the case without the recess to 5%. To do.

The operation of the present invention has been described above by taking the semiconductor optical waveguide composed of the three layers of the core layer and the two cladding layers as an example, but a waveguide having a distributed refractive index also has the same operation. Further, the present invention has the same effect with respect to a dielectric waveguide such as a Ti diffusion waveguide on a LiNbO ₃ substrate and a Si ₃ N ₄ or SiO ₂ waveguide on a Si substrate.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. Embodiment 1 FIG. 5 shows a first embodiment of the present invention in which the emitted light of a semiconductor laser is condensed by a lens and used, 11 is a lens, and 12 is a lens.
Is direct light, and the same reference numerals as those in FIG. 2 denote the same components. If the semiconductor laser and the input / output end are the same as those described above, the proportion of reflected light deviating from the focus of direct light can be set to 5%.

Embodiment 2 FIG. 6 shows a second embodiment of the present invention in which two semiconductor lasers provided on a semiconductor laser substrate are optically coupled by a lens provided on the same substrate. Is a semiconductor laser, 14 is a lens, 15 is a light output end, and 16 is a light input end. The coupling loss at this time is equal to the reflection loss at one input / output end due to the structural symmetry. The combination loss of the laser and the input / output end is 5%.

Embodiment 3 FIG. 7 shows a third embodiment of the present invention for coupling a semiconductor laser and an optical fiber, in which 17 is an optical fiber, and the same symbols as those in FIG. Show. The fiber core is much larger than the laser waveguide, so the input loss to the fiber is negligible. Therefore, the coupling loss between the laser and the fiber is mainly determined by the output loss of the laser. Loss of 5 due to the same laser and input / output end structure as in the first and second embodiments.
% Can be reduced.

Embodiment 4 FIG. 8 is a fourth embodiment of the present invention in which light emitted from a Ti diffusion waveguide on a LiNbO ₃ substrate, which is one of dielectric waveguides, is input to an optical fiber through a lens. And 18 is T
The i-diffusion waveguide 19 is a LiNbO ₃ substrate, and the same symbols as those in FIG. 7 indicate the same components. Although the terrace shape of the second step is flat in each of the above-described embodiments, it goes without saying that a tapered shape or a multi-step shape is also effective.

[0020]

According to the present invention, the light input / output efficiency of the waveguide using the etched mirror can be made extremely high. Therefore, when the present invention is applied to a high power laser, the laser light can be efficiently extracted to the outside. Further, when the emitted light is used after being narrowed down by a lens, the intensity loss of the main spot due to reflection can be reduced. Even when the present invention is applied to an optical integrated device, the devices on the substrate can be coupled with low loss.

[Brief description of drawings]

FIG. 1 is a diagram showing a state of light emitted from an optical waveguide.

FIG. 2 is a diagram showing inputs and outputs of a conventional optical waveguide structure.

FIG. 3 is a diagram showing an input / output structure of an optical waveguide of the present invention.

FIG. 4 is a diagram for explaining the operation of the present invention from the relationship between the reflection loss and each structural parameter of the input / output structure of the optical waveguide.

FIG. 5 is a diagram showing a first embodiment of the input / output structure of the optical waveguide according to the present invention in which the emitted light of the semiconductor laser is condensed and used by a lens.

FIG. 6 is a diagram showing a second embodiment of the input / output structure of the optical waveguide according to the present invention, in which semiconductors on the same substrate are combined.

FIG. 7 is a diagram showing a third embodiment of the input / output structure of the optical waveguide according to the present invention for coupling a semiconductor laser and an optical fiber.

FIG. 8 is a diagram showing a fourth embodiment of the input / output structure of the optical waveguide according to the present invention for inputting light from the Ti diffusion waveguide on the LiNbO ₃ substrate to the optical fiber through the lens.

[Explanation of symbols]

1 core 2 clad 3 clad 4 cap layer 5 etched mirror 6 optical axis 7 terrace 8 element end 9 substrate 10 recess 11 lens 12 direct light 13 semiconductor laser 14 lens 15 optical output end 16 optical input end 17 optical fiber 18 Ti diffused conduction Waveguide 19 LiNbO ₃ substrate

Claims (1)

[Claims] 1. At the light input / output end of a waveguide realized by a wall that is perpendicular or nearly perpendicular to a substrate that cuts an optical waveguide in one substrate, a depression is formed outside a wall that traverses the waveguide surface via a terrace. An input / output structure of an optical waveguide characterized by being provided.