JPH08148703A - Photodetector - Google Patents

Abstract

PURPOSE: To output a signal according to the reception intensity of incidence light from a photodetector by making the relation among a refractive index n of N/ a core layer, a refractive index n' of an adhesive layer, and a refractive index n" of the light reception part of the photodetector to n<=n'<=n". CONSTITUTION: Light advancing inside a core layer 31 is refracted by an end face 30a of a light pipe 30 and enters an adhesive layer 40. At this time, n'.sinθ"' is equal to n.sinθ", where a refraction angle is θ". θ"'<θ" is always true due to n<n'. Therefore, entire light reaching the end face 30a of a light pipe 30 enters the adhesive layer 40 without totally reflected on the end face 30a. The same occurs on the junction surface between the adhesive layer 40 and a photodetector 20 and total light reaching the end face 30a of the light pipe 30 reaches a light reception part 20a of the photodetector 20. In this manner, incidence light can be focused at the light reception part of the photodetector with an extremely small light loss. Also, light spot image can be extremely made small since light is focused using a waveguide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photo-detecting device most suitable for a receiving device of a spatial propagation optical communication system.

[0002]

2. Description of the Related Art In recent years, optical communication using the optoelectronic technology has been receiving attention. Among them, the optical communication system using the optical fiber has already been put to practical use because it is capable of large-capacity communication, and is expected as a next-generation wired transmission line. Related arts of this optical communication system are disclosed in JP-A-59-229509 and JP-A-61-1657.
08, etc. On the other hand, a spatial optical communication system that radiates light into the space and transmits and receives information to and from the space is also being studied intensively. This spatial optical communication system is composed of a pair of transceivers 100, 100 as shown in FIG. In each of the transceivers 100, a directional light source 101 such as a laser diode, a light receiving element 102 such as a silicon photodiode, a lens 103 that collects incident light on a light receiving surface 102a of the light receiving element 102, A half mirror 104 that reflects the light beam from the light source 101 toward the lens 103 is provided.

[0003]

By the way, in the conventional spatial optical communication system, when the distance between the light source 101 and the light receiving element 102 becomes long, it is affected by scattering of the atmosphere, rain, snow, fog, and uneven density. It is easy to receive such influence, and the light flux reaching the light receiving element 102 is disturbed by attenuation, diffusion, turbulence, dance, and the like. Therefore, it is necessary to make the diameter of the lens 103 as large as possible in order to capture as much light flux as possible.

However, if the diameter of the lens 103 is increased, the focal length of the lens 103 becomes longer, and the size of the light spot and its dance (spot dancing amount) increase accordingly. Further, when the light spot size and the spot dancing amount become large, the area M of the light receiving portion 102a of the light receiving element 102 in order to reliably capture the light flux.
Will also need to be increased.

By the way, it is a well-known fact that the smaller the area M of the light receiving portion 102a, the more suitable for high-speed response and the better S / N. However, in the conventional technique, the area M of the light receiving unit 102a cannot be reduced as described above, and therefore the response frequency becomes low and the S / N cannot be maintained above a certain level. . Therefore, the conventional spatial optical communication system cannot maintain the S / N above a certain level, resulting in a high error rate, which is unsuitable for long-distance communication.

In such a case, as shown in FIG. 7, it is a conventional means to reduce the light spot image formed by the lens 103 by using the relay lens 105 (105a to 105d) and guide it to the light receiving element 102. is there. Here, when the angle formed by the light beam incident on the relay lens 105 and the optical axis is α, and the angle formed by the emitted light beam and the optical axis is α ′, and the image magnification is M, these are , M = | sin α / sin α ′ |

If the diameter of the light spot image formed by the lens 103 is φ (including the spot dancing amount) and the diameter of the light spot image reduced by the relay lens 105 is φ ′, then M = φ ′ / φ Is.

Since | sinα '| ≦ 1, where M
≧ | sin α |, and the image magnification M does not become smaller than | sin α |. Therefore, when α is close to π / 2, there is a problem that the image magnification M also becomes large and φ ′ cannot be made so small.

It is an object of the present invention to solve the above problems and provide a photodetector capable of high-speed response and having a good S / N.

[0010]

In order to solve the above-mentioned problems, the first photodetector of the present invention is such that the diameter of one end is larger than the diameter of the other end and the cross-sectional area is smooth from one end to the other end. A waveguide comprising a decreasing rod-shaped core layer and a clad layer that covers the side surface of the core layer and has a refractive index lower than the refractive index n of the core layer, and an end face of the waveguide on the side where the diameter of the core layer is narrow. The light-receiving unit is provided with the light-receiving unit facing each other, and an adhesive layer formed between the waveguide and the light-receiving element to bond the waveguide and the light-receiving element. The relationship between the refractive index n of the core layer, the refractive index n ′ of the adhesive layer, and the refractive index n ″ of the light receiving portion of the light receiving element is
It is characterized in that n ≦ n ′ ≦ n ″.

Here, a reflection film may be formed on the outer peripheral surface of the clad layer, and the cross-sectional shape of the waveguide may be elliptical or polygonal.

In the second photodetector of the present invention, the diameter of one end is larger than the diameter of the other end, and the cross-sectional area smoothly decreases from one end to the other end. A waveguide including a reflection film that covers the side surface on the smaller diameter side and a cladding layer that covers the side surface of the core layer that is not covered by this reflection film and has a refractive index lower than the refractive index n of the core layer, and the diameter of the core layer. A light-receiving element in which the end face of the waveguide on the thin side and the light-receiving portion are arranged so as to face each other; and an adhesive layer formed between the waveguide and the light-receiving element to bond the waveguide and the light-receiving element. Is equipped with.

Here, the relationship between the refractive index n of the core layer, the refractive index n'of the adhesive layer, and the refractive index n "of the light receiving portion of the light receiving element is as follows.
It may be n ≦ n ′ ≦ n ″, and the cross-sectional shape of the waveguide may be elliptical or polygonal.

[0014]

According to the first photodetector of the present invention, when light enters from the end surface of the waveguide where the diameter of the core layer is large, the light is totally reflected at the boundary surface between the core layer and the clad layer. Meanwhile, the core layer advances toward the end face of the waveguide where the diameter of the core layer is smaller. Due to this progress, the light is focused, and the focused light is emitted from the end surface of the waveguide where the core layer has a small diameter. The emitted light passes through the adhesive layer and enters the light receiving portion of the light receiving element, and the light receiving element outputs a signal corresponding to the received light intensity of the incident light.

The refractive index of the core layer of the waveguide is n, the refractive index of the adhesive layer is n ', and the incident angle of light from the core layer to the adhesive layer is θ, and the adhesive layer to the adhesive layer is Letting θ ′ be the refraction angle refracted by the incidence of light, there is a relationship of nsin θ = n ′ sin θ ′. Here, since n <n ', θ'<
θ holds. Therefore, the light traveling toward the adhesive layer in the core layer enters the adhesive layer without being totally reflected at the boundary surface between the core layer and the adhesive layer.

Similarly, the refractive index of the light receiving portion of the light receiving element is n ″.
And θ is the refraction angle refracted by the incidence of light from the adhesive layer to the light receiving portion of the light receiving element, there is a relationship of n'sin θ '= n "sin θ". Here, since n'<n" , Always θ ″
<Θ'is established. Therefore, the light traveling in the adhesive layer toward the light receiving portion of the light receiving element enters the light receiving portion of the light receiving element without being totally reflected at the boundary surface between the adhesive layer and the light receiving portion of the light receiving element.

Further, according to the second photodetector of the present invention, similarly to the first photodetector, the light incident from the end face of the waveguide on the side where the diameter of the core layer is thicker is in the core layer. It is converged as it travels, and is emitted from the end face on the side where the diameter of the core layer is narrow. In particular, since the side surface of the core layer on the smaller diameter side is covered with the reflection film, the light traveling inside the core layer on the smaller diameter side is totally reflected by the reflection film and is converged with almost no light loss. It

[0018]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a cross-sectional view showing the configuration of the photodetector 1 according to the first embodiment of the present invention. From the figure,
The photodetector 1 according to the present embodiment has a light receiving portion 2 on the upper surface of the base 10.
The light receiving element 20 is arranged with 0a facing upward. Above the light receiving portion 20a of the light receiving element 20, a frustum-shaped light pipe 30 with the end surface 30a having the smaller area facing is disposed at a slight distance from the light receiving portion 20a. Light receiving portion 20a of light receiving element 20 and end face 30a of light pipe 30
An adhesive agent is filled in the space between and to form an adhesive agent layer 40. A cylindrical support tool 50 is attached to the side wall of the light pipe 30, and the support tool 50 is fixed on the base body 10. By the support of the support tool 50, the light pipe 30 is fixed on the light receiving element 20 so that the central axis A and the upper surface of the light receiving element 20 are substantially perpendicular to each other.

The light pipe 30 is composed of a core layer 31 processed into a truncated cone shape and a cladding layer 32 covering the side surface of the core layer 31 and having a substantially uniform layer thickness. Core layer 3
1 and the clad layer 32 are formed of a single crystal, polycrystal, or solid solution of a dielectric or semiconductor that becomes transparent in the signal light wavelength range to be used. Then, the refractive index n of the core layer 31
Is higher than the refractive index n ″ of the cladding layer 32, and therefore the light traveling in the core layer 31 is totally reflected at the boundary surface between the core layer 31 and the cladding layer 32.

Here, the diameter of the core layer 31 at the end surface 30b having the larger area of the light pipe 30 is D, and the end surface 30a.
Let D ′ be the diameter of the core layer 31 at
Image reduction ratio of 0 (concentration of light by the light pipe 30)
M becomes M = D '/ D ... Further, when the refractive index of the adhesive layer 40 is n ′ and the refractive index of the light receiving portion 20a of the light receiving element 20 is n ″, the core layer 3
Since the refractive index of 1 is n and the refractive index of the cladding layer 32 is n ″ ″, the relationship of n ″ ″ <n <n ′ <n ″ ...

The thickness t of the cladding layer 32 must be at least twice the wavelength used in order to prevent evanescent waves from leaking out during total reflection (Kazumi Nagao "Optical Fiber"). (See page 36).

The core layer 3 on the end face 30b of the light pipe 30
The light which is incident on the light source 1 at an angle θ with respect to the central axis A is refracted at the end face 30b and has an angle θ ′ with the central axis A. Here, between θ and θ ′, there is a relationship of sin θ = n · sin θ ′ ...

Light incident on the core layer 31 at an angle θ'with respect to the central axis A travels in the core layer 31 while repeating total reflection at the boundary surface between the core layer 31 and the cladding layer 32, and the end face 30a.
At an angle θ ″ with the central axis A. Here, when the diameter D of the core layer 31 is sufficiently smaller than the length L of the light pipe 30, it is between the angle θ ′ and the angle θ ″. , D ′ · sin θ ″ = D · sin θ ′ ... Therefore, M = sin θ ′ / sin θ ″ ...

Further, the light traveling in the core layer 31 is refracted at the end face 30a of the light pipe 30 and enters the adhesive layer 40. If the refraction angle at this time is θ ″ ′, θ ″ ″
And θ ″ have a relationship of n ′ · sin θ ″ ″ = n · sin θ ″ ... Here, since n <n ′, θ ″ ″ <θ ″ is always established. All the light that has reached the end surface 30a of the pipe 30 enters the adhesive layer 40 without being totally reflected by the end surface 30a.

The same applies to the adhesive layer 40 and the light receiving element 2.
It also occurs at the joint surface of 0, and eventually the end surface 3 of the light pipe 30
All the light reaching 0a is received by the light receiving section 20 of the light receiving element 20.
a will be reached.

The incident angle θ of the incident light on the end face 30b of the light pipe 30 is equal to or more than the angle corresponding to the case where the refraction angle θ ″ is 90 °. Arrives at the light receiving portion 20a). That is, light incident at an incident angle θ which is equal to or larger than the angle corresponding to the case where the refraction angle θ ″ is 90 ° is in the process of traveling in the core layer 31. The reflection at the boundary surface with the clad layer 32 makes an angle of 90 ° or more with respect to the central axis A, and after that, it starts returning in the direction of the end face 30b and cannot reach the end face 30a. . Therefore, the condition that the refraction angle θ ″ is less than 90 ° is a condition for effectively producing the above-mentioned action. Here, when the formula is substituted into the formula, M = sin θ / (n · sin θ ″) Is obtained, and θ ″ <90 °, so that M> | sin θ / n |.

From the equation, it can be seen that the image reduction magnification M of the light pipe 30 can be reduced by 1 / n as compared with the case where the conventional relay lens 105 shown in FIG. 7 is used. On the contrary, when the light pipe 30 is used to form a light spot having the same size as the light spot formed using the conventional relay lens 105, the relay lens 1
As compared with the case of 05, the light amount of n ² times can be sent to the light receiving element 20.

In particular, when a semiconductor light receiving element made of silicon, germanium, or the like is applied, the refractive index n ″ thereof is as high as 3 to 4, so that the effect is extremely high. In this case, as the material of the light pipe 30, Is preferably sapphire, ZnS, ZnSe having a high refractive index, etc. Thus, if the light spot image becomes small, the area of the light receiving portion 20a of the light receiving element 20 may be small, the response frequency increases, and S / N is also improved.

Next, FIG. 2 shows an example in which the photodetector 1 of this embodiment is applied to a spatial optical communication system. As shown in the figure, this spatial optical communication system includes a pair of transceivers 60,
It consists of 60. And which transceiver 6
0, a directional light source 61 such as a laser diode
And a half mirror 62 that reflects the light emitted from the light source 61 and transmits the incident light.
Further, the light reflected by the half mirror 62 is collimated and emitted to the outside, and a lens 63 for condensing the incident light from the outside, and light for receiving the incident light arranged at the condensing position of the incident light by the lens 63. The detection device 1 is provided.

This spatial optical communication system operates as follows. First, the light emitted from the light source 61 of one transceiver 60 is reflected by the half mirror 62, becomes a parallel light beam by the lens 63, and is emitted toward the other transceiver 60. The emitted light propagates through the space and is then condensed by the lens 63 of the other transmitter / receiver 60 to form the half mirror 62.
The end surface 3 of the light pipe 30 of the photodetector 1
A light spot having a diameter φ is formed at 0b (the diameter φ includes turbulence and spot dancing amount). Light pipe 30
The diameter of the end surface 30b of the core layer 31 is adjusted in advance so as to be equal to or slightly larger than the diameter φ. By this adjustment, all the incident light surely enters the core layer 31 of the light pipe 30. In this way, the light that has entered the core layer 31 is focused with almost no light loss and reaches the light receiving element 20. Therefore, the light receiving element 20 of the other transmitter / receiver 60 can receive the optical signal propagated through the space from the one transmitter / receiver 60 and transmitted with almost no optical loss.

(Embodiment 2) Next, a photodetector 2 according to a second embodiment of the present invention will be described with reference to the sectional view of FIG. The second embodiment is different from the first embodiment shown in FIG. 1 only in that the side surface of the core layer 31 of the light pipe 30 is covered with the cladding layer 32 and the reflection film 33. The configuration is the same as that of the first embodiment. In addition,
The same or equivalent components as those in the first embodiment are designated by the same reference numerals and the description thereof will be omitted.

As shown in FIG. 3, the photodetector 2 of this embodiment has a light receiving portion 20a of the light receiving element 20 arranged on the upper surface of the substrate 10.
The light pipe 30 is placed on the upper surface of the via the adhesive layer 40. The light pipe 30 includes a core layer 31 processed into a truncated cone shape, a clad layer 32 having a substantially uniform layer thickness covering the side surface of the core layer 31 having a larger diameter, and the core layer 3
The reflective film 33 has a substantially uniform film thickness and covers the side surface of the thin film 1 having a small diameter. The light incident on the core layer 31 of the end surface 30b is totally reflected at the boundary surface between the core layer 31 and the cladding layer 32 and travels toward the end surface 30a.
Similarly, total reflection is also performed at the boundary surface between the reflection film 33 and the reflection film 33. Therefore, the light traveling in the core layer 31 surely reaches the end surface 30a.

The reflection angle when the light traveling in the core layer 31 is reflected by the boundary surface gradually increases as it approaches the end surface 30a, but the reflection film 3 is formed near the end surface 30a.
Since 3 is formed, total reflection can be reliably performed. Therefore, most of the light incident on the core layer 31 of the end face 30b is received by the light receiving section 20a of the light receiving element 20.

As the reflection film 33, various metal films, dielectric multi-layered films, or combination films of metal films and dielectric multi-layered films are used depending on the wavelength of incident light used. It is formed on the side surface of the core layer 31 by a technique such as plating.

(Embodiment 3) Next, a photodetector 3 according to a third embodiment of the present invention will be described with reference to the partial sectional view of FIG. The third embodiment differs from the first embodiment shown in FIG. 1 in that the reflective film 34 is formed on the outer peripheral surface of the cladding layer 32.
Is the same as that of the first embodiment. The same or equivalent components as those of the first embodiment are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 4, when the light flux traveling in the core layer 31 reaches the boundary surface with the cladding layer 32, the angle i formed by the normal line k of the tangent plane of the boundary surface and the light flux has an equational relationship. Then, the total reflection condition is not satisfied.

I <arcsin (n ″ ′ / n) ... In this case, a part of the light flux leaks into the inside of the cladding layer 32, and further the light pipe 30 is emitted from the outer peripheral surface of the cladding layer 32.
Since it goes out of the space, the light collection efficiency decreases.

In order to improve this decrease in light collection efficiency,
In addition to increasing the refractive index n of the core layer 31, the cladding layer 3
The value of (n ″ ″ / n) may be reduced by decreasing the refractive index n ″ ″ of 2. If the value of (n ″ ′ / n) is small,
This is because the total reflection condition is satisfied with the incident angle i in a wide range. However, since the refractive index n is restricted by the physical properties of the material forming the core layer 31, there is a limit in increasing the refractive index n. For the same reason, there is a limit to reducing the refractive index n ″ of the cladding layer 32.
For this reason, the value of (n ″ ″ / n) cannot be made so small as it is, and the core layer 31 and the cladding layer 32 cannot be made.
It is difficult to completely eliminate the light flux that leaks into the cladding layer 32 beyond the boundary surface of.

Therefore, in this embodiment, by covering the outer peripheral surface of the clad layer 32 with a reflective film 34 having a high reflectance, the light flux leaking into the clad layer 32 is also reflected by this reflective film 34 and again the core layer 31. Returned inside. Therefore, the light flux reaches the light receiving element 20 with extremely high efficiency without leaking from the light pipe 30 to the outside.

As the reflection film 34 having a high reflectance, various metal films, a dielectric multilayer film, or a combination film of a metal film and a dielectric multilayer film is used depending on the wavelength of the incident light used. It is formed on the outer peripheral surface of the cladding layer 32 by a method such as vapor deposition or plating.

Since some light flux leaks into the cladding layer 32 and travels while being repeatedly reflected in the cladding layer 32, the thickness t of the cladding layer 32 should be as thin as twice or more the used wavelength.

The first to third embodiments of the present invention have been described above, but the present invention is not limited to these embodiments and various modifications can be made. For example, the cross-sectional shape of the core layer 31 of the light pipe 30 does not need to be a perfect circle, and may be an elliptical shape or a polygonal shape. In particular,
When the photodetector of the present invention is applied to a spatial optical communication system, the end face 30b of the light pipe 30 on the light incident side is
It is practical that the vertical direction is an ellipse or a rectangle with a long vertical direction, and the end face 30a on the light emitting side is a circle, a square, or the like according to the shape of the light receiver 20. This is because when the light beam propagating in space passes near the surface of the earth, the temperature gradient of the atmosphere is large in the vertical direction, and therefore the light beam tends to be bent in the vertical direction. Therefore, the movement of the light spot in the focal plane of the light receiving lens is larger in the vertical direction than in the horizontal direction,
This is because if the end surface 30b on the light incident side is long in the vertical direction, the light flux can be surely incident.

The outer shape of the light pipe 30 may be such that the cross-sectional area smoothly decreases from the end surface 30b having a larger area to the end surface 30a having a smaller area, and in particular, a conical shape is used. It does not have to be trapezoidal. This helps to make the entire device compact while maintaining the substantial total length of the light pipe 30 (the longer the total length with respect to the diameter, the less the uneven illuminance at the light emission port). That is, as shown in FIG. 5, if the light pipe 30 is spirally wound, the total length can be shortened.

Further, although it is shown in FIG. 2 that the photodetector 1 can be applied to the spatial optical communication system, the photodetectors 2 and 3 can also be applied to the spatial optical communication system like the photodetector 1.

[0045]

The photodetector of the present invention has a waveguide having a rod-shaped core layer having a diameter at one end thicker than the diameter at the other end, and a cladding layer covering the side surface of the core layer, and the diameter of the core layer. Is provided with a light-receiving element in which the end face of the waveguide on the narrow side and the light-receiving portion are arranged so as to face each other, and an adhesive layer for bonding the waveguide and the light-receiving element. Then, the light incident from the end surface of the waveguide having the larger diameter core layer is condensed while being totally reflected at the boundary surface between the core layer and the clad layer, and emitted from the end surface having the smaller diameter. The light passes through the layer and enters the light receiving portion of the light receiving element. Due to this incident light, the light receiving element outputs a signal corresponding to the received light intensity of the incident light.

Refractive index n of core layer, refractive index n'of adhesive layer
And the refractive index n ″ of the light receiving portion of the light receiving element, n ≦
Since there is a relationship of n ′ ≦ n ″, the light reaching the boundary surface between the core layer and the adhesive layer and the boundary surface between the adhesive layer and the light receiving portion of the light receiving element passes through these surfaces without being totally reflected. To do.
For this reason, the incident light is focused on the light receiving portion of the light receiving element with very little light loss.

Further, since the photodetector of the present invention collects the incident light using the waveguide, the light spot image formed at the collecting position becomes very small. Therefore, it becomes possible to use a light receiving element having a small area of the light receiving portion, the response frequency is increased, and the S / N is also improved.

[Brief description of drawings]

FIG. 1 is a sectional view showing a configuration of a photodetector according to a first embodiment.

FIG. 2 is a diagram showing an example in which the photodetector of the first embodiment is applied to a spatial optical communication system.

FIG. 3 is a sectional view showing a configuration of a photodetector according to a second embodiment.

FIG. 4 is a sectional view showing a configuration of a photodetector according to a third embodiment.

FIG. 5 is a view showing an example in which a light pipe is spirally wound as an application example of the first to third embodiments.

FIG. 6 is a diagram showing a conventional spatial optical communication system.

FIG. 7 is a diagram showing a conventional transceiver using a relay lens.

[Explanation of symbols]

20 ... Light receiving element, 20a ... Light receiving part, 30 ... Light pipe (waveguide), 30a, 30b ... End face, 31 ... Core layer, 3
2 ... Clad layer, 33, 34 ... Reflective film, 40 ... Adhesive layer.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H04B 10/00

Claims (5)

[Claims] 1. A rod-shaped core layer having a diameter at one end larger than the diameter at the other end and a cross-sectional area smoothly decreasing from one end to the other end, and a core layer covering the side surface of the core layer and having a refractive index n of the core layer. A waveguide including a clad layer having a low refractive index, a light receiving element in which an end face of the waveguide on the side of the core layer having a small diameter and a light receiving portion are arranged to face each other, the waveguide and the light receiving An adhesive layer formed between the waveguide and the light receiving element, the refractive index n of the core layer, the refractive index n'of the adhesive layer, and the light reception of the light receiving element. The relation with the refractive index n ″ of the part is n ≦ n ′
A photodetector characterized in that ≦ n ″. 2. The photodetector according to claim 1, wherein a reflective film is formed on the outer peripheral surface of the cladding layer. 3. A rod-shaped core layer whose one end has a diameter larger than that of the other end and whose cross-sectional area decreases smoothly from one end to the other end; and a reflective film which covers the side surface of the core layer on the side where the diameter is small. A waveguide including a clad layer that covers the side surface of the core layer not covered with the reflective film and has a refractive index lower than the refractive index n of the core layer, and the waveguide having a smaller diameter of the core layer. A light-receiving element having an end face and a light-receiving portion facing each other, and an adhesive layer formed between the waveguide and the light-receiving element to bond the waveguide and the light-receiving element to each other. A photodetector characterized by. 4. The relationship between the refractive index n of the core layer, the refractive index n ′ of the adhesive layer, and the refractive index n ″ of the light receiving portion of the light receiving element is n ≦ n ′ ≦ n ″. Claim 3 characterized by
The photodetector described. 5. The photodetector according to claim 1, wherein the waveguide has a cross-sectional shape that is an ellipse or a polygon.