JP2929481B2 - Optical function element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical coupling device for converting a spot diameter of a light wave propagating through an optical waveguide of an optical functional device into another optical functional device with low loss.

[0002]

2. Description of the Related Art When optically coupling optical functional devices having different structures from each other, for example, when optically coupling between a semiconductor laser diode (LD) and a single mode fiber, the end face of the laser diode element and the fiber are connected. When the butt joints are directly joined to each other, the light waveguide spot sizes of the optical waveguides are different from each other, so that there is a problem of the coupling loss of the directly joined portions. Usually, the light spot size (mode radius: W) of the laser diode is about 1 μm, and the spot size of the fiber is about 5 μm.
m, this coupling loss is about 10 dB. Therefore, a method of reducing the coupling loss by converting the spot size by a lens is generally adopted.

FIG. 1 shows an example of a conventional optical coupling configuration in which a single lens is used to optically couple an optical function element having a plurality of laser diodes and an array fiber. In FIG. 1, reference numeral 804 denotes a semiconductor substrate;
Denotes an active region (optical waveguide portion) of a laser diode, 812
Is a lens, 806 is an optical fiber, and 807 is a v-groove array for fixing the fiber at regular intervals.
In such a configuration, as the integration scale of the laser diode increases, the coupling loss increases due to the influence of lens aberration and the like, so that the number of laser diodes that can be integrated in one semiconductor is limited.

Further, by using an optical coupling device for converting the spot size of light with a tapered optical waveguide as shown in FIGS. 2 and 3 in place of a lens, the loss between the laser diode and the fiber can be reduced. There is a method of optical coupling. FIG. 2 is a top view of such a conventional optical coupling device, and FIG. 3 is a cross-sectional view of the same device.

FIG. 4 is a diagram for explaining the operation principle of the optical coupling device. That is, as can be seen from FIG. 4, the normalized refractive index difference Δn [= of the core layer 908 of the optical waveguide.
(N ₁ −n ₂ ) / n ₁ , n ₂ : cladding layers 901, 90
9 and n ₁ : refractive index of the core layer 908] are fixed to a certain size, and when the thickness t and the width w of the core layer 908 are gradually increased from 0, the guided light (basic The spot size W of the (mode light) gradually decreases from infinity, takes a minimum value, and then increases again. Here, if the thickness t and the width w are too large, the waveguide becomes a multimode waveguide, and the loss due to higher-order mode conversion increases. Therefore, the dimensions of this region are not usually used. Utilizing this relationship, in designing the size (t, w) of the core layer 908 of the optical coupling device, the spot size (about 1 μm) of the laser diode light on the light incident end side (coupling side with the laser diode) ) sized to give the same degree of spot size W _i (thickness t _i, width w _i is the number 100nm~ number μ
m), the dimensions (thickness t ₀ and width w ₀ are 10 nm to 10000 nm) that give a size W ₀ similar to the spot size (about 5 μm) of the fiber on the light emitting end side are set. In addition, the length L of the region where the size of the core layer 908 is tapered is about 10 to reduce radiation loss.
The length is set from 0 μm to several mm or more. However, when the dimension t ₀ or w ₀ on the light emitting end side is reduced and W ₀ is increased, the guided light intensity distribution of the optical fiber has an almost Gaussian distribution shape as shown in FIG. ,
The light intensity distribution of the waveguide has an exponential shape because the normalized frequency v, which is a well-known parameter determined by the waveguide width, the refractive index difference, and the wavelength of the propagating light, is smaller than 1. For this reason, there was a disadvantage that the coupling loss due to the shape mismatch was increased.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical coupling device capable of low-loss optical coupling between two different optical functional devices, particularly an optical functional device in which a plurality of devices are integrated. An optical coupling method for an optical functional device;
An optical functional element obtained by being integrated by this optical coupling;
Is to provide.

[0007]

The optical functional device according to the present invention comprises:
A single-mode optical fiber or a single-mode optical waveguide component ; an optical functional element in which an optical input / output semiconductor waveguide and a semiconductor optical functional element unit optically coupled to a single-mode optical waveguide component are formed on the same substrate; An optical waveguide layer constituting the optical input / output semiconductor waveguide,
In at least a part of the substrate, at least the size or the refractive index is changed along the light propagation direction, so that the magnitude of the normalized frequency at the input / output end of the optical input / output semiconductor waveguide is 0. It is set in the range of 6 to 1.0.

[0008]

According to the optical coupling device having the above-mentioned structure, the normalized frequency of the optical waveguide at the end face to be connected to another optical functional element can be set to an appropriate value, thereby achieving low-loss optical coupling. Can be realized.

[0009]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of the present invention;

(Embodiment 1) FIGS. 6A and 6B show an embodiment of an optical coupling device according to the present invention. An optical coupling device according to the present invention is provided between an array laser diode element and a fiber. It is a block diagram in the case of inserting a device and performing optical coupling with low loss.

FIG. 6A is a top view, FIG. 6B is a sectional view, 101 is a semiconductor substrate of the optical coupling device according to the present invention, 102 is a spot size conversion waveguide, 103 is an antireflection film, 104 Is a semiconductor substrate, 105 is a laser diode active layer (optical waveguide section), 106 is a single mode optical fiber, and 107 is a v-groove array.

In this configuration, the size of the light spot of the laser diode 104 is gradually converted by the optical waveguide 102 of the optical coupling device, and is converted to an appropriate size at the light emitting end.

(Embodiment 2) FIG. 7 shows another embodiment of the present invention, and is a perspective view of a structure in which the optical coupling device and the semiconductor laser shown in FIG. 6 are monolithically integrated.
8 to 11 are graphs for explaining the principle of the present invention.

In FIG. 7, reference numeral 201 denotes a semiconductor substrate made of InP, which serves as a clad of an optical waveguide. 20
2, 205 are InGaAs, InGaAsP, InA
A portion 202 is a spot size conversion portion, and a portion 205 is a light emitting portion (active layer, semiconductor functional element portion) having the same structure as a normal semiconductor laser. Reference numeral 209 denotes a cladding layer made of InGaAsP, InP or the like, and 211 denotes outgoing light. The width w and the thickness t of the core layer 202 in the tapered portion are set to w _i and t _i that give the same size as the spot shape of the semiconductor functional element at the connection with the laser. Then, in the light emitting portion, an optical functional device (for example, an optical fiber) connected thereto
Are set to w ₀ and t ₀ at which the coupling loss with respect to is small. The magnitudes of the refractive indexes of the core layer and the cladding layer are n ₁ and n ₂ , respectively.

The magnitudes of n ₁ and n ₂ can be arbitrarily set by selecting a semiconductor material. For example, I
When nP is used, n = 3.166 for light in the wavelength λ = 1.55 μm band. Further, the refractive index of InGaAsP can be set to any value from about 3.2 to about 3.5 depending on its composition. The refractive index can be arbitrarily set by using a multiple quantum well layer as the core layer and selecting the materials and thicknesses of the well layer and the barrier layer. further,
For example, by using a selective growth mask, an epitaxial selective growth technique, a photolithography technique, or the like, the refractive index n ₁ of the core layer 202 and the waveguide dimensions (w,
The size of t) can be set and manufactured in a tapered shape.

In such a configuration, when the size (w ₀ , t ₀ ) of the output portion of the optical waveguide and the refractive index n ₁ of the core layer are used as parameters, the coupling loss when directly coupling to the optical fiber is used. 8 shows the calculation results. Here, it is assumed that the core has a circular shape, the wavelength λ is 1.55 μm, and the refractive index n ₂ of the cladding layer is 3.166. As can be seen from the figure, in order to obtain low loss characteristics with respect to the refractive index n ₁ of the core layer, there is an optimum core diameter d _w , and good characteristics of 1 dB or less are realized. We see what is possible. According to calculations, the light spot shape of the waveguide from which these low coupling loss characteristics can be obtained has an exponential function shape as shown in FIG. 5, but the spot diameter is equal to the spot diameter of the optical fiber. Are of different sizes.

In these waveguides, their normalized frequency (normalized waveguide width) v [= kn ₁ d _w (2Δ)
^{1/2 / 2, k = 2π /} λ, Δ = (n 1 2 -n 2 2) / 2
and n ₁ ^2], the relationship of coupling loss between the fiber and the parameter v and the normalized equivalent refractive index b ^{_{[= (n 2 -n 2 2)}} / (n
^{_{1 2 -n 2 2), n}} : 9, shown in Figure 10 the calculation results of the relationship between the effective refractive index of the waveguide].

FIG. 9 shows that the magnitude of the normalized frequency v is set to 0.
When it is set to about 7 to 0.8, characteristics of low coupling loss can be obtained,
It can be seen that the smaller the value of n _1, the larger the optimum normalized frequency v becomes, and the more the manufacturing tolerance of the normalized frequency v for obtaining low loss characteristics tends to become loose. In practice, the normalized frequency v is 0.6 to 1.
If it is 2 , good bonding characteristics can be obtained.

At this time, it is understood from FIG. 10 that the magnitude of the normalized equivalent refractive index b should be about 0.0001 to 0.1 .

From these results, in order for the optical coupling device to couple with the optical fiber with low loss, the normalized frequency v of the waveguide of the device is set to an optimum value instead of adjusting the light spot diameter of each other. Next, it is understood that the material and structure should be designed.

Normally, a single mode semiconductor waveguide or an optical fiber used for an optical device has a structure having a normalized frequency v of about 2 in order to strengthen light confinement.

FIGS. 8 and 9 show InP as a cladding layer.
(N ₂ = 3.166) has been described,
It has been confirmed by calculation that the larger the value of n _{2, the} smaller the optimum normalized frequency v that gives low coupling loss characteristics. Therefore, the optimum normalized frequency v may be set according to the material to be used.

In the above calculation, the case where the shape of the waveguide is circular is shown, but usually, the waveguide that can be manufactured has a shape such as a square, a trapezoid, or a triangle as shown in FIG. In such a case, when n ₁ and n ₂ are equal to each other, it can be confirmed by calculation using the finite element method, the difference method, or the like that if the core has the same cross-sectional area, the same characteristics as in the case of the circular shape can be obtained. For example, n ₁ = 3.3, n ₂ = 3.166, w ₀ = 0.4 μ
FIG. 11 shows the light intensity distribution of the waveguide calculated for a square having m and t ₀ = 0.3 μm. In this case, the characteristics are the same as those of the circular waveguide (d _w ≒ 0.4 μm), and low-loss optical coupling with the optical fiber can be achieved. After that, the waveguide shape
The waveguides other than circular have the same core cross-sectional area.
The standardized frequency of a circular waveguide
And the rated frequency.

In the embodiment shown in FIG.
And with use of InP cladding layer 209, InGaAsP core layer _{202 (n 1 = 3. 3} ) using a laser unit is normally laser diode structure (w _i = 2μm, t
_i = 0.3 μm), and at the exit of the tapered waveguide, w
₀ = 0. 4 [mu] m, the experimental results when the t ₀ = _0. 3μm, shown in FIGS. 12 and 13. (A) of FIG.
(B) and (c) are near-field images of the light 211 emitted from the waveguide 202, and it can be seen that the spot diameter is about 10 μm.

FIG. 13 shows allowable axis deviation characteristics in a direction perpendicular to the optical axis of the optical fiber when the emitted light 211 is coupled to the optical fiber. From the figure, the experimental results are in good agreement with the calculations, and almost the same results as the allowable characteristics between fibers are obtained. At this time, the total insertion loss (including the propagation loss of the tapered waveguide and the coupling loss with the fiber) of the optical coupling device could be a low loss characteristic of 3 dB or less.

In the above description, the case where the waveguide layer for spot size conversion is formed on the InP substrate has been described. However, it is obvious that other semiconductor materials, for example, GaAs-based materials can be similarly manufactured.

Although the semiconductor waveguide material has been mainly described as an optical waveguide material, it is needless to say that the present invention can be applied even when an organic material such as a polymer or a glass material such as quartz is used as the optical waveguide material.

In the above description, the case where the material of the cladding layer and the material of the cladding layer of the optical waveguide are the same is described. However, these materials are combined with each other to form an asymmetric waveguide structure. Also, the same principle as in the above embodiment can be used.

Further, in the above description, the case where the optical fiber is connected has been described. In addition to the above, the connection portion with any optical waveguide component such as another semiconductor optical waveguide component or a glass waveguide component may be used. However, if the normalized frequency of the optical coupling device waveguide according to the present invention is set so as to match the light intensity distribution of those waveguides, it is apparent that the characteristics of low coupling loss can be realized.

Since the optical coupling device of the present invention is made of a semiconductor material, the optical coupling device of the present invention is connected to the light input / output end of an optical functional element such as a semiconductor laser, a laser diode amplifier, or an optical switch. It is also possible to realize an optical device monolithically integrated on the same substrate. In this case, when forming the optical functional device waveguide on the semiconductor substrate, the optical coupling waveguide of the present invention is formed at the same time, or after forming the optical functional device portion, the waveguides are directly butted against each other. As described above, a tapered waveguide for optical coupling may be formed.

[0031]

As described above, according to the present invention, the size or the refractive index of the core portion of the optical waveguide is formed in a tapered shape, and the standardization of the optical waveguide at the end face portion connected to another optical functional element is performed. By setting the frequency to an appropriate value, low-loss optical coupling is enabled.

[Brief description of the drawings]

FIG. 1 is a plan view showing a conventional optical coupling method.

FIG. 2 is a plan view of a conventional optical coupling device.

FIG. 3 is a side sectional view of a conventional optical coupling device.

FIG. 4 is a graph showing a relationship between a core size and a spot size in a conventional optical coupling device.

FIG. 5 is a graph showing the operation principle of a conventional optical coupling device.

FIGS. 6A and 6B show an embodiment of the present invention, wherein FIG. 6A is a top view of the optical coupling device of the present invention, and FIG.

FIG. 7 shows another embodiment of the present invention, and is a perspective view of the optical coupling device of the present invention.

FIG. 8 is a graph for explaining the operation principle of the optical coupling device of the present invention, and is a graph showing the relationship between the structure of the output end of the tapered waveguide and the coupling loss of the optical fiber.

FIG. 9 is a graph for explaining the operation principle of the optical coupling device of the present invention, and is a graph showing the relationship between the normalized frequency of the waveguide and the coupling loss of the optical fiber.

FIG. 10 is a graph for explaining the operation principle of the optical coupling device of the present invention, and is a graph showing a relationship between a normalized frequency of a waveguide and a normalized equivalent refractive index of an optical fiber.

FIG. 11 is a graph showing a light intensity distribution of a rectangular waveguide.

FIG. 12 is a graph showing light intensity characteristics of the optical coupling device of the present invention, in which (a) is a graph showing light intensity contours;
(B) is a graph showing a light intensity distribution along a line 1 in (a), and (c) is a graph showing a light intensity distribution along a line 2 in (a).

FIG. 13 is a graph showing a change in coupling loss with respect to the amount of axial deviation in the optical coupling device of the present invention.

[Explanation of symbols]

 Reference Signs List 101 semiconductor substrate 102 spot size conversion waveguide 103 antireflection film 104 semiconductor substrate 105 laser diode active layer 106 optical fiber 107 v groove array 201 semiconductor substrate 202 spot size conversion waveguide 209 clad layer 211 outgoing light 804 semiconductor substrate 806 optical fiber 807 v-groove array 812 lens 901 semiconductor substrate 908 core layer 909 cladding layer 910 incident light 911 outgoing light

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Mitsuru Naganuma 1-6, Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-61-232406 (JP, A) JP-A-Hei 1-288802 (JP, A) JP-A-2-195309 (JP, A)

Claims (1)

(57) [Claims] 1. A single mode optical fiber or single mode
An optical functional element in which an optical input / output semiconductor waveguide optically coupled to an optical waveguide component and a semiconductor optical functional element section are formed on the same substrate, and an optical waveguide layer forming the optical input / output semiconductor waveguide. In at least a part of the substrate, at least its size or refractive index is changed along the light propagation direction, whereby the magnitude of the normalized frequency of the input / output end of the optical input / output semiconductor waveguide is reduced. An optical functional device, wherein the optical functional device is set in a range of 0.6 to 1.0.