JPH0651144A - Optical fiber coupler - Google Patents

Abstract

PURPOSE:To enable a pair of many pieces of data to be transmitted by receiving the light emitted by a diffraction grating coupler with photodetectors. CONSTITUTION:This optical coupler has the diffraction grating coupler 3 within a waveguide structure and is disposed with a photodetector (photodiode) 6 array in parallel with the waveguide direction on its rear surface. The light emitted from the diffraction grating coupler 3 is received by the photodetectors 6. All the photodiodes 6 on the waveguide sense light when the exit light of a semiconductor laser 7 is coupled to the waveguide. The intensity of the received light is, however, higher with the photodiodes 6 nearer the incident end in such a case. For example, the intensity modulation of the exit light of the laser diode is detected by the arbitrary photodiode 6 by taking out the output of this photodiode 6. The path as a photocoupler changes eventually by changing the photodiode 6 out of which the output is taken.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical connection, that is, an optical coupler.

[0002]

2. Description of the Related Art Conventionally, an optical coupler has been used to electrically separate a circuit in an electric circuit. This is a module in which a semiconductor light emitting element and a light receiving element are combined, and the element emits light when a current flows through the first circuit connected to the light emitting element, and the light receiving element receives the light and the second circuit is connected thereto. An electrical signal is generated at. Although the first circuit and the second circuit are connected by light, they are electrically independent and therefore, for example, electrical noise generated in the circuit 1 is not transmitted to the circuit 2.

[0003]

However, the conventional optical coupler is for data transmission using one-to-one light, and is composed of a semiconductor light emitting element and a light receiving element which opposes the semiconductor light emitting element. Meanwhile, the light propagates through a material transparent in space or the emission wavelength of the light emitting element and reaches the light receiving element. Therefore, it is necessary to shorten the distance between the light emitting element and the light receiving element. If this distance is made longer, the emitted light spreads even if a semiconductor laser is used as the light source, resulting in a large loss.

[0004]

An optical coupler according to the present invention has, in an optical waveguide, a diffraction grating coupler of a second order or more having a radiation loss coefficient which is not 0 with respect to the guided light in the waveguide. A light receiving element array is arranged on the surface of the substrate in parallel with the waveguide direction, and the light emitted by the diffraction grating coupler is received by the light receiving element.

The optical coupler of the present invention is preferably characterized by having a clad layer or a clad layer and a buffer layer having a thickness not smaller than the evanescent light does not leak to the surface of the waveguide on the waveguide.

In the optical coupler of the present invention, more preferably, the diffraction gratings are arranged at a constant interval in the waveguide direction and at a constant length, and the light emitted from each diffraction grating coupler corresponds to one to one. The light receiving element of the light receiving element array receives the light.

In the optical coupler of the present invention, more preferably, the lengths of the diffraction gratings arranged at regular intervals become longer from the incident end face of the waveguide toward the inside of the substrate, and the total radiation loss of the diffraction grating coupler is increased. When the coefficient is represented by α and the length of each diffraction grating coupler is represented by L, αL of each diffraction grating coupler is not constant but is designed so that the emitted light intensities become equal.

In the present invention, in the optical waveguide, the diffraction grating coupler is provided in the waveguide structure, and the light receiving element (photodiode) array is arranged on the front surface or the back surface of the substrate in parallel with the waveguide direction. Provided is an optical coupler having a structure in which emitted light is received by the light receiving element. When the emitted light of the semiconductor laser is coupled to the waveguide, all photodiodes on the waveguide sense the light. However, in this case, the closer the photodiode is to the incident end, the higher the intensity of light received. For example, by extracting the output of an arbitrary photodiode, the intensity modulation of the light emitted from the laser diode can be received by that photodiode. Also, by changing the photodiode that takes out the output, the path as an optical coupler will change. Of course, this optical connection is not limited to one-to-one, and one-to-many connection is possible.

The present invention has described the case where the diffraction grating is formed over the entire area of the waveguide. However, the diffraction grating between each bit of the photodiode array causes waveguide loss. Therefore, the diffraction grating couplers inside the waveguide are arranged at a constant length in the waveguide direction at a constant length, and the light emitted from each diffraction grating coupler is received by the corresponding bits of the photodiode array in a one-to-one correspondence. It should be done. It is also possible to increase the distance between the light source and the photodiode by providing a region without a diffraction grating coupler near the incident end.

In any of the above cases, the light intensity reaching each photodiode, that is, the number of photons, becomes weaker as the distance from the incident end increases. As means for solving this, the following two methods or a combination thereof can be considered. One is to increase the size of the photodiode as the distance from the incident end increases. Another method is to make αL of each diffraction grating coupler unequal. Here, α is the radiation loss coefficient of the diffraction grating coupler, and L is the length of the diffraction grating coupler. For this purpose, the simplest method is to lengthen L as the distance from the incident end increases, but other than this, a method of deepening the depth of the diffraction grating as the distance from the incident end increases α gradually, A method of gradually changing the waveguide structure with respect to the waveguide direction can be considered.

[0011]

The diffraction grating provided in the optical waveguide functions as an output coupler and distributes an optical signal to each photodiode.

[0012]

The present invention will be described in detail below with reference to the drawings. FIG. 1 is a perspective view for explaining a manufacturing process of the optical coupler of the present invention. FIG. 2 is a graph showing the calculation result of the light intensity distribution in the optical waveguide shown in the example of the present invention. FIG. 3 is a schematic sectional view of the optical coupler of the present invention. Figure 4
It is a figure for explaining an electric circuit used for taking out a signal from a photodiode. FIG. 5 is a perspective view for explaining a part of the manufacturing process of the diffraction grating in the manufacturing process of the optical coupler of the present invention.

As shown in FIG. 1 (a) or FIG. 5 (a), a GeO 2 film 2 having a thickness of 0.5 μm was formed on a quartz glass substrate 1 by CVD to form a waveguide layer (waveguide). Figure 5
As shown in (b), a Cr film 20 used as an etching mask in a later step was formed on the GeO2 film 2 by a sputtering method. As shown in FIG. 5C, a diffraction grating pattern 21 of photoresist having a period of 0.5 μm was formed thereon by an interference exposure method. C using this pattern 21 as a mask
The r film 20 was wet-etched, and the Cr film was processed into a lattice shape as shown in FIG. As shown in FIG. 5 (e), a depth of 0.
A 15 μm diffraction grating 3 was formed on the GeO 2 film 2. As shown in FIG. 1B, after removing the lattice-like Cr film,
As shown in FIG. 1C, a part of the waveguide layer was formed into a rib structure 4 for confining light in the lateral direction, and a depth of 0.2 μm was etched while leaving a width of 5 μm. As shown in FIG. 1D, a SiO2 clad layer 5 having a thickness of 1 μm was formed again by the CVD method. In the optical waveguide manufactured in this way, the diffraction grating in the waveguide functions as an outgoing coupler (diffraction grating coupler). That is, the light propagating through the waveguide is diffracted by the diffraction grating inside the waveguide and exits outside the waveguide. FIG. 2 shows the calculation result of the light intensity distribution of the guided light in the present optical waveguide. In this calculation, quartz glass substrate 1, G
The refractive indexes of the eO2 film 2 and the CVDSiO2 cladding layer 5 are 1.460, 1.603, and 1.450, respectively.
The wavelength of the guided light in vacuum was 780 nm. This waveguide is a single mode and has an equivalent refractive index of the fundamental wave of 1.530.
become. When the diffraction grating is provided in the waveguide, the guided light is diffracted by the diffraction grating and emitted in the direction of the angle θ that satisfies sin θ = N−mλ / Λ. Here, N is the equivalent refractive index of the waveguide, λ is the wavelength of the incident light in vacuum, and Λ is the period of the diffraction grating. Further, m is called a diffraction order, and is an integer satisfying -1 <N-mλ / Λ <1. In the case of the present embodiment, the above formula is established only when m = 1, and the emitted light has an angle of −1.23 ° with respect to the vertical line of the substrate. The radiation loss coefficient at this time, that is, the diffraction efficiency is 25.7 cm ^-1 on the surface side and 2 on the substrate side.
It became 5.4 cm ^-1 . Therefore, in total 51.
The radiation loss is 1 cm ^-1 . As shown in FIG. 1 (e),
Finally, an array of amorphous silicon pin photodiodes 6 was formed on the waveguide. As shown in FIG.
Each bit of the photodiode array has a length of 50 μm, and an interval between the bits is 20 μm. The light intensity I received by the n-th photodiode is I / I ₀ = exp [− (n−1) α (L + d)] × [1−exp (−α _s L when normalized by the incident light intensity I _0. )] It can be expressed as. Where α is the total radiation loss coefficient, αs
Is a radiation loss coefficient toward the surface side, L is the length of the photodiode, and d is the distance between the photodiodes. Waveguide losses other than the radiation loss coefficient are ignored. Using this, the intensity of light incident on the photodiode is 12% of the intensity of incident light when n = 1, 8% when n = 2, 6% when n = 3, and n = 4.
At 4%, n = 5 at 3%, and n = 10 at 0.47%
become. To check the optical connection, as shown in Figure 3,
Light emitted from a 780 nm semiconductor laser 7 having an output of 10 mW was coupled to the waveguide. Data was taken out from each photodiode equivalently by the circuit shown in FIG. When sampling data from the photodiode, the field effect transistor (FET) 8 is turned on and the FET 9 is turned on.
The current between the signal extraction electrodes 10 was monitored as ff. On the other hand, when data is not monitored, FET8 is turned on.
ff, FET9 was turned on and both ends of the photodiode were short-circuited to prevent carrier accumulation. The drive of the FET 8 is controlled by the bias value of the control electrode 12,
Driven with the bias inverted by the inverter 11. The output ratio from each photodiode is approximately 12% for n = 1 and 8 for n = 2, as calculated above.
%, 6% when n = 3, 4% when n = 4, and 3% when n = 5.

In the above embodiment, only the light emitted from the diffraction grating coupler in the surface direction is received. However, since light is also emitted from the diffraction grating coupler to the substrate side, in order to effectively use this light, a multilayer film reflecting mirror is inserted between the backside of the substrate or the waveguide and the substrate, and the light emitted to the substrate side is used. Can also be reflected in the surface direction so that the photodiode can efficiently receive the light. Alternatively, by installing a photodiode array on the substrate side, it is possible to double the number of light receiving elements as compared with the case of the above embodiment.

[0015]

According to the present invention, an optical coupler capable of one-to-many data transfer can be realized. Further, the distance between the light emitting element and the photodiode can be set arbitrarily.

[Brief description of drawings]

FIG. 1 is a perspective view for explaining a manufacturing process of an optical coupler of the present invention.

FIG. 2 is a graph showing a calculation result of a light intensity distribution in the optical waveguide shown in the example of the present invention.

FIG. 3 is a schematic sectional view of an optical coupler of the present invention.

FIG. 4 is a diagram for explaining an electric circuit used for extracting a signal from a photodiode.

FIG. 5 is a perspective view for explaining a part of the manufacturing process of the diffraction grating in the manufacturing process of the optical coupler of the present invention.

[Explanation of symbols]

 1 Quartz glass substrate 2 GeO2 film 3 Diffraction grating 4 Rib structure 5 SiO2 cladding layer 6 Photodiode 7 Semiconductor laser 8, 9 FET 10 Signal extraction electrode 11 Inverter 12 Control electrode 20 Cr film 21 Photoresist diffraction grating pattern

Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location H01S 3/18

Claims (4)

[Claims] 1. An optical waveguide having a diffraction grating coupler of a second order or higher having a radiation loss coefficient not equal to 0 with respect to the guided light in the waveguide, and a light receiving element parallel to the waveguide direction on the substrate surface. An optical coupler in which an array is arranged and light emitted from a diffraction grating coupler is received by the light receiving element. 2. The optical coupler according to claim 1, further comprising a clad layer or a clad layer and a buffer layer having a thickness not exceeding the waveguide surface on which the evanescent light leaks. 3. The diffraction gratings are arranged at a constant interval in the waveguide direction at a constant length, and the light emitted from each diffraction grating coupler is received by a light receiving element of a corresponding one of the light receiving element arrays. The optical coupler according to claim 1, wherein 4. The length of the diffraction grating arranged at a constant interval in claim 3 becomes longer from the incident end face of the waveguide toward the inside of the substrate, and the total radiation loss coefficient of the diffraction grating coupler is α, 3. The optical coupler according to claim 1, wherein when the length of each diffraction grating coupler is represented by L, .alpha.L of each diffraction grating coupler is not constant and the emitted light intensities are equal.