Title: Waveguide light detecting element
Abstract: Composition wavelengths of materials of cladding layers and optical guide layers are 0.92 μm and 1.2 μm respectively. When the thickness of optical guide layers, corresponding to an extreme value in which inclination of a sensitivity curve to 1.3 μm-wavelength light and to 1.55 μm-wavelength light with respect to a change in the thickness of each of both optical guide layers changes from positive to negative, are defined as d1 and d2, respectively, the thickness, dg, of optical guide layers of a light detecting element satisfies 0.75d1≦dg≦1.25d2.
Patent Number: 7,020,375 Issued on 03/28/2006 to Nakaji, et al.
| Inventors:
|
Nakaji; Masaharu (Tokyo, JP);
Ishimura; Eitaro (Tokyo, JP)
|
| Assignee:
|
Mitsubishi Denki Kabushiki Kaisha (Tokyo, JP)
|
| Appl. No.:
|
777136 |
| Filed:
|
February 13, 2004 |
Foreign Application Priority Data
| Jul 29, 2003[JP] | 2003-281450 |
| Current U.S. Class: |
385/131; 257/43 |
| Current Intern'l Class: |
G02B 6/10 (20060101) |
References Cited [Referenced By]
U.S. Patent Documents
| 6737718 | May., 2004 | Takeuchi.
| |
| 2002/0195616 | Dec., 2002 | Bond.
| |
| 2003/0098475 | May., 2003 | Ueda.
| |
| Foreign Patent Documents |
| 10027921 | Jan., 1998 | JP.
| |
| 10-125948 | May., 1998 | JP.
| |
| 11284219 | Oct., 1999 | JP.
| |
| 11-330536 | Nov., 1999 | JP.
| |
Other References
T. Ido et al., "Highly Efficient Lens-Less Coupling of High-Speed Waveguide Photodiode
to SMF and its Application to an Extremely Thin-Surface-Mountable 10-Gbps Receiver
Module", OFC 2003, pp. 66-67, vol. 1, Mar. 24, 2003.
Ishimura et al., "40Gbps Waveguide Photodiode for 1.3 μm/1.55 μm
wavelength", Extended abstract of presentation made Mar. 27, 2003 at The Japan
Society of Applied Physics and Related Societies.
Matsuoka et al.; "Properties of λ=1.5 μm 10Gb/s Waveguide PIN-PD",
Extended Abstracts (The 50th Spring Meeting, 2003); No. 3, '27 p-H-15',
p. 1246, The Japan Society of Applied Physics and Related Societies.
Ishimura et al.; "40Gbps Waveguide Photodiode for 1.3 μm/1.55 μm
wavelength", Extended Abstracts (The 50th Spring Meeting, 2003); No.
3, '27 p-H-16', p. 1247, The Japan Society of Applied Physics and Related Societies.
|
Primary Examiner: Kang; Juliana
Attorney, Agent or Firm: Leydig, Voit & Mayer, Ltd.
Claims
The invention claimed is:
1. A waveguide light detecting element for detecting multiwavelength-band signal
light, comprising:
a semi-insulating semiconductor substrate; and
an optical waveguide layer supported by the semiconductor substrate, said optical
waveguide layer including, sequentially laminated from the semiconductor substrate
side, a first conductivity type first cladding layer connected to a first electrode,
a first conductivity type first optical guide layer, an optical absorbing layer,
a second conductivity type second optical guide layer, and a second conductivity
type second cladding layer connected to a second electrode, wherein,
when a center wavelength of a first signal light wavelength band, corresponding
to a shortest signal light wavelength band is defined as λ
1, a center
wavelength of a second signal light wavelength band is defined as λ
2
(λ
2>λ
1), and a composition wavelength of a material
for each of the first and second cladding layers is defined as λa, composition
wavelength, λg, of a material of each of the first and second optical guide
layers satisfies λa<λg<λ
1 so that the first
and second optical guide layers are transparent to the first signal light,
when the thickness of each of the first and second optical guide layers, corresponding
to an extreme value in which inclination of a sensitivity curve of λ
1
with respect to a change in the thickness of each of the first and second optical
guide layers changes from positive to negative, is defined as d
1, and the
thickness of each of the first and second optical guide layers, corresponding to
an extreme value in which inclination of a sensitivity curve of λ
2
with respect to the change in the thickness of each of the first and second optical
guide layers changes from positive to negative, is defined as d
2, the thickness,
dg, of the first and second optical guide layers satisfies 0.75d
1≦dg1.25d
2, and
each of the first and second cladding layers is InP, the composition wavelength
λg of each of the first and second optical guide layers is fixed, the composition
wavelength of the first and second cladding layers, λa, is 0.92 μm,
λ
1=1.3 μm, λ
2=1.55 μm, and the thickness,
dg, of the first and second optical guide layers satisfies 0.3 μm≦dg≦0.75
μm with d
1=0.4 μm and d
2=0.6 μm.
2. The waveguide light detecting element according to claim 1, wherein, when
the thickness of the optical absorbing layer is defined as da, 0.3 μm≦da≦0.5 μm.
3. The waveguide light detecting element according to claim 1, wherein each of
the first and second optical guide layers is InGaAsP.
4. The waveguide light detecting element according to claim 1, wherein each of
the first and second optical guide layers is AlInGaAsP.
5. The waveguide light detecting element according to claim 1, wherein each of
the first and second optical guide layers is GaInNAs.
6. The waveguide light detecting element according to claim 1, including a low
refractive index layer of a material lower than the optical absorbing layer in
refractive index disposed on side faces of a waveguide.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a waveguide light detecting element, and particularly
to a waveguide light detecting element used in an optical communication system
or the like.
2. Description of the Related Art
With a leap increase in amount demanded for communications, an attempt to increase
the capacity of a communication system has been made. To this end, however, there
has been a need to speed up an optical communication apparatus and bring it into
less size/high efficiency and less cost.
In an optical communication transmission system, lights lying in two wavelength
bands have been used as signal lights. One of them is a signal light lying in a
1.3 μm band in which a center wavelength of a bandwidth of the signal light
is 1.3 μm, and the other thereof is a signal light lying in a 1.55 μm
band in which a center wavelength of a bandwidth of the signal light is 1.55 μm.
The 1.55 μm-band signal light is low in optical fiber loss and used as
a signal light for a long-distance communication system. This is called interurban
communication (trunk-line system) and is used for communications between large
cities as in the case of Tokyo-to-Osaka, for example.
On the other hand, the 1.3 μm-band signal light is large in optical fiber
loss but less in wavelength dispersion and is used as a signal light for a short-distance
communication system. This is called an in-city communication and has been used
in large city communication. Also the 1.3 μm-band signal light is used even
in communications between an access system a base station and respective homes.
In an optical communication system, as a photodiode for receiving the signal
lights
lying in the two wavelength bands, a waveguide type semiconductor photodiode adaptable
to one wavelength, which has been formed so as to be adapted to the lights lying
in the respective wavelength bands, had been used.
As a known example of a conventional waveguide type light receiving element,
there
has been disclosed a configuration wherein an n-InGaAsP optical guide layer (bandgap
wavelength: 1.3 μm), an InGaAs optical absorbing layer, a p-InGaAsP optical
guide layer (bandgap wavelength: 1.3 μm), and a p-InP layer are sequentially
laminated over an n conductivity type InP substrate (hereinafter an n conductivity
type is represented as "n-", a p conductivity type is represented as "p-", and
an intrinsic semiconductor is represented as "i-", respectively) (see, for example,
section 0001 in Japanese Patent Application Laid-Open No. H10-125948).
As another known example, there has been disclosed a semiconductor light receiving
element having a configuration wherein a material small in bandgap adapted to a
1.3 μm band and a 1.5 μm band is used for an optical absorbing layer
to receive lights lying in the 1.3 μm band and 1.5 μm band well used
in optical fiber communications, and a guide layer of n-InAlGaAs or n-InGaAsP,
an avalanche-doubling layer of n-InAlAs, an electric field relaxation layer of
p-InAlAs or p-InP, a low-concentration optical absorbing layer of p-InGaAs, a high-concentration
optical absorbing layer of p-InGaAs, a p-type guide layer, and a p-type contact
layer are sequentially laminated over an n-InP substrate to form a mesa stripe-shaped
waveguide, which is covered with a passivation film of SiOx or SiNx (see, for example,
sections 0023 to 0025 in Japanese Patent Application Laid-Open No. H11-330536).
As a further known example, there has been disclosed a mesa type having a double-core
structure of an InGaAlAs system as a configuration of a 1.5 μm-light receiving
wavelength band 10 Gb/s waveguide type PIN-PD, wherein In0.53Ga0.47As is used in
an optical absorbing layer (see, for example, "Characteristics of light receiving
wavelength-1.5 μm band 10 Gb/s waveguide PIN-PD", (The 50th Spring Lecture
Proceedings (Kanagawa University, 2003.3), 2003 (Heisei 15th Year)); The Japan
Society of Applied Physics, p.1246, 27p-H-15).
The conventional waveguide type light receiving element is configured as a photodiode
adapted to a signal light lying in a single wavelength band used in its optical
communication system. However, there is a possibility that a communication network
maintained for in-city communications will be used as for interurban communications
at this stage with enlargement of transmission capacity in the optical communication
system. In this case, the optical communication apparatus employed in the optical
communication system should be unavoidably complicated in configuration where optical
parts adapted to respective wavelengths are used as at the present time.
However, even if an optical part adapted to the signal light of one wavelength,
here a waveguide type photodiode (hereinafter called waveguide PD), the waveguide
PD adapted to the one-wavelength signal light has received two-wavelength signal
lights as it is, it was difficult to cause the waveguide PD to perform a high-speed
operation at high sensitivity.
That is, the waveguide PD has a structure wherein light is confined in a waveguide
portion having an optical absorbing layer and optical guide layers provided with
the optical absorbing layer interposed therebetween, and the light is absorbed
while the light confined in the waveguide portion is being propagated to the optical
guide layers and the optical absorbing layer, and converted into an electric signal.
This waveguide PD confines the light in the waveguide portion and allows the
waveguide portion to absorb the light by use of the difference in refractive index
between the optical absorbing layer and each of the optical guide layers and cladding
layers. Therefore, when the signal lights are different in wavelength from each
other, the optical absorbing layer, optical guide layers and cladding layers adapted
to the respective lights are different from one another in refractive index.
Thus, the waveguide PD corresponding to the single wavelength band is capable
of optimizing a device structure in conformity with a light-receiving wavelength
band. However, it happens that the waveguide PD corresponding to a multiwavelength
is excellent in sensitivity characteristic at a certain wavelength but very poor
in sensitivity characteristic at another wavelength. It can also happen that the
sensitivity characteristic is degraded in all the wavelength bands in some cases.
For instance, as it is able to increase or improve confining of light in a waveguide
by enlarging the difference in refractive index between an optical guide layer
and a cladding layer, a composition wavelength on the long-wavelength side may
preferably be selected from composition wavelengths in each of which a bandgap
signal light passes through the optical guide layer, as the composition wavelength
of the optical guide layer.
However, in order to cope with the multiwavelength, the optical guide layer
must have a composition wavelength in which a signal light lying in the shortest
wavelength band can pass through its corresponding optical guide layer. It can
happen that if the composition wavelength of the optical guide layer is simply
determined on the basis of the wavelength of the signal light lying in the shortest
wavelength band, sensitivity is significantly degraded with respect to signal lights
lying in other wavelength bands.
Thus, a problem arises in that even if the waveguide PD high in sensitivity
and capable of high-speed operation corresponding to the signal light lying in
the first wavelength band has received signal lights lying in a second wavelength
band or other wavelength bands as it is, the waveguide PD encounters difficulties
in enabling high sensitivity and high-speed operation with respect to these signal lights.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above-described problems. A
first object of the present invention is to configure a waveguide type light receiving
element capable of high-speed operation at high sensitivity with respect to a multiwavelength
signal light.
According to one aspect of the invention, there is provided a waveguide
type light receiving element shared for a multiwavelength-band signal light comprising:
a semi-insulating semiconductor substrate; and an optical waveguide layer disposed
over the semiconductor substrate, the optical waveguide layer being formed by sequentially
laminating from the semiconductor substrate side, a first conductivity type first
cladding layer connected to a first electrode, a first conductivity type first
optical guide layer, an optical absorbing layer, a second conductivity type second
optical guide layer, and a second conductivity type second cladding layer connected
to a second electrode, wherein when a center wavelength of a first signal light
wavelength band corresponding to the shortest signal light wavelength band is defined
as λ1, a center wavelength of a second signal light wavelength band
is defined as λ2 (λ2>λ1), and a composition
wavelength of a material for each of the first and second cladding layers is defined
as λa, a composition wavelength λg of a material for each of the first
and second optical guide layers satisfies λa<λg<λ1
such that the first and second optical guide layers become transparent to the first
signal light, wherein when the thickness of each of the first and second optical
guide layers, corresponding to an extreme value in which an inclination of a sensitivity
curve of λ1 with respect to a change in the thickness of each of the
first and second optical guide layers changes from positive to negative, is defined
as d1, and the thickness of each of the first and second optical guide layers,
corresponding to an extreme value in which an inclination of a sensitivity curve
of λ2 with respect to the change in the thickness of each of the first
and second optical guide layers changes from positive to negative, is defined as
d2, the thickness dg of the first and second optical guide layers satisfies 0.75d1≦dg≦1.25d2.
Therefore, a high-speed operation is enabled with respect to a multiwavelength-band
signal light containing the first and second signal light wavelength bands while
high photo detecting sensitivity is being held.
Accordingly, It is thus possible to simply provide a waveguide type
light receiving element shared for a multiwavelength-band signal light, which is
high in photo detecting sensitivity and capable of high-speed operation. By extension,
an optical communication system becomes simple and hence an increase in capacity
of the communication system can be put forward at low cost.
Other objects and advantages of the invention will become apparent from the
detailed description given hereinafter. It should be understood, however, that
the detailed description and specific embodiments are given by way of illustration
only since various changes and modifications within the scope of the invention
will become apparent to those skilled in the art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a waveguide light receiving element according
to one embodiment of the present invention.
FIG. 2 is a cross-sectional view of a waveguide light detecting element as viewed
in a cross-section taken along line II—II of FIG. 1.
FIG. 3 is a cross-sectional view of the waveguide light detecting element as
viewed in a cross-section taken along III—III of FIG. 1.
FIG. 4 is a graph showing the dependence of sensitivity to signal light on the
thicknesses of optical guide layers of the waveguide light detecting element according
to the one embodiment of the present invention.
FIG. 5 is a graph showing comparisons between calculated values and actually
measured values of sensitivity characteristics of a waveguide light detecting element
according to the one embodiment of the present invention.
FIG. 6 is a graph showing frequency responses of the waveguide light detecting
element according to the one embodiment of the present invention.
In all figures, the substantially same elements are given the same reference numbers.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the invention described below will be explained using
a built-in waveguide type PIN-PD sharing a 1.3 μm-band and a 1.55 μm-band
for 40 Gbps as one example of a waveguide type light receiving element of an optical
communication system.
First Embodiment
FIG. 1 is a perspective view of a waveguide type light receiving element according
to one embodiment of the present invention.
In FIG. 1, the waveguide type PIN-PD 10 has a photodetector 12 provided
at a frontal cleavage end face, which receives a signal light 14 indicated
by arrow. The signal light 14 contains lights of two wavelength bands corresponding
to a 1.3 μm band defined as a first signal light wavelength band in which
a center wavelength λ1 is 1.3 μm, and a 1.55 μm band defined
as a second signal light wavelength band in which a center wavelength λ2
is 1.55 μm.
A waveguide mesa 16 including a waveguide into which the signal light is
introduced through the photodetector 12, is disposed on the upper surface
side of the PIN-PD 10. A p electrode 18 is disposed along the surface
of the waveguide mesa 16, and n electrodes 20 are disposed on both
side surfaces of the waveguide mesa 16 and the upper surface of the PIN-PD
10. An upper surface other than the areas where the p electrode 18
and the n electrodes 20 are disposed, is covered with an insulating film 22.
FIG. 2 is a cross-sectional view of a waveguide type light receiving element
as viewed in a cross-section taken along line II—II of FIG. 1. In other words,
FIG. 2 is a cross-sectional view as seen in the direction that intersects the signal
light and viewed in a cross-section orthogonal to the waveguide.
FIG. 3 is a cross-sectional view of the waveguide type light receiving element
as viewed in a cross-section taken along III—III of FIG. 1. In other words,
FIG. 3 is a cross-sectional view as seen in a cross-section taken along the traveling
direction of the signal light and as viewed in a cross-section taken in the direction
in which the waveguide extends. Incidentally, the same reference numerals in the
figures correspond to the same ones or equivalent ones.
Referring to FIG. 2, an n-contact layer 26 composed of n-InGaAs
is disposed on a semi-insulating Fe-doped InP substrate 24 used as a semi-insulating
semiconductor substrate. The waveguide mesa 16 into which the signal light
14 is introduced through the photodetector 12, is disposed over the
n-contact layer 26.
The waveguide mesa 16 includes a waveguide 16
a used as an
optical waveguide layer formed by sequentially laminating from the n-contact layer
26 side, an n-cladding layer 28 composed of n-InP, which is used
as a first cladding layer disposed on the surface of the n-contact layer 26,
an n-optical guide layer 30 composed of n-InGaAsP, which is used as a first
optical guide layer disposed on the central surface of the n-cladding layer 28,
an optical absorbing layer 32 composed of i-InGaAs, which is disposed on
the surface of the n-optical guide layer 30, a p-optical guide layer 34
composed of p-InGaAsP, which is used as a second optical guide layer disposed on
the surface of the optical absorbing layer 32, a p-cladding layer 36
composed of p-InP, which is used as a second cladding layer disposed on the surface
of the p-optical guide layer 34, and a p-contact layer 40 composed
of p-InGaAs, which is disposed on the surface of the p-cladding layer 36,
and a block layer 38 composed of Fe-doped InP, which is disposed on both
sides of the waveguide 16
a excluding the p-contact layer 40
and used as a low refractive index layer that forms the side faces of the waveguide
mesa 16.
The block layer 38 disposed on both sides of the waveguide 16
a
is formed of a material lower than the optical absorbing layer 32 in
refractive index so that the difference in refractive index between the waveguide
16
a and the block layer can be made large. Thus, the confining efficiency
of light is enhanced and the light-receiving sensitivity of the light-receiving
element can be increased.
Further, the p electrode 18 is disposed on the surface of the p-contact
layer 40, and the n electrodes 20 brought into contact with the surface
of the n-contact layer 26 are respectively disposed so as to cover both
side faces of the block layer 38. The insulating film 22 is disposed
on the surface of the waveguide mesa 16, which does not cover the p electrode
18 and the n electrodes 20. The p electrode 18 and the n electrodes
20 are electrically separated from one another with the insulating film
22 interposed therebetween.
Referring to FIG. 3, the block layer 38 composed of the Fe-doped
InP, having a cleavage end face 38
a is disposed on the frontal light-receiving
side of the waveguide 16
a. The block layer 38 composed of
the Fe-doped InP is disposed even in the rear of the waveguide 16
a.
That is, the waveguide 16
a is embedded in the block layer 38
composed of the Fe-doped InP and is cleaved in the block layer 38 and formed
as a chip. The signal light is introduced into the waveguide 16
a through
the photodetector 12 provided at the cleaved end face of the block layer 38.
In the present embodiment, the length of the waveguide 16
a, which
extends in its longitudinal direction, i.e., the traveling direction of light,
is 16 μm. The thickness of the n-cladding layer 28 is set to 1.5 μm,
the thickness of the p-cladding layer 36 is set to 0.8 μm, and the
thicknesses of the n-optical guide layer 30 and p-optical guide layer 34
are respectively set to 0.4 μm
The thickness of the optical absorbing layer 32 can be brought into a
broad band by shortening the traveling time of a carrier. Since, however, the absorption
of the light is reduced when the thickness thereof is thinned, the thickness da
of the optical absorbing layer 32 is represented as 0.3 μm≦da≦0.5
μm as a suitable range. In the present embodiment, however, da=0.5 μm.
A composition wavelength λa of InP used as the material for the n-cladding
layer 28 and the p-cladding layer 36 was set to 0.92.
A composition wavelength λg of InGaAsP used as the material for the n-optical
guide layer 30 and the p-optical guide layer 34 is larger than 0.92
corresponding to the refractive index of the material for the n-cladding layer
28 and the p-cladding layer 36. InGaAsP whose composition wavelength
λg is 1.2 μm, was used in such a manner that it became transparent
with respect to light of the 1.3 μm band, i.e., λg assumed λa<λg<λ1,
more desirably λa<λg<0.965λ1.
As an n type impurity for each layer, the group IV element, e.g., Si, S or the
like is added. The group II element, e.g., Be, Zn or the like is added as a p type
impurity. The optical absorbing layer 32 of an intrinsic semiconductor layer
is not added with an impurity in particular.
Thus, in the waveguide 16
a, the p-optical guide layer 34,
the n-optical guide layer 30 and the optical absorbing layer 32 interposed
therebetween form a p/i/n junction.
An outline of a method for manufacturing the PIN-PD 10 according to the
present embodiment will next be explained.
First, an n-InGaAs layer used as an n-contact layer 26, an n-InP layer
used as an n-cladding layer 28, an n-InGaAsP layer used as an n-optical
guide layer 30, i-InGaAs used as an optical absorbing layer 32, p-InGaAsP
used as a p-optical guide layer 34, a p-InP layer used as a p-cladding layer
36, and a p-InGaAs layer used as a p-contact layer 40 are sequentially
laminated over a semi-insulating Fe-doped InP substrate 24 by a chemical
vapor deposition method, e.g., a MOCVD method.
Next, an SiO
2 film is formed on the surface of the p-InGaAs layer
used as the p-contact layer 40 corresponding to the top layer in the laminated
layer of these. A part of the insulating film corresponding to the upper surface
of a waveguide 16
a to be formed is left behind and insulating film
patterns with openings around the part of the insulating film are formed. With
the insulating film pattern as masks, the waveguide 16
a is formed.
At this time, the insulating film patterns are processed stepwise to thereby form
portions to stop etching at places where the n-InP layer used as the n-cladding
layer 28 is perfectly exposed, i.e., portions corresponding to the frontal
surface and both side faces of the waveguide 16
a, and a portion to
be etched until the InP substrate 24 is exposed, i.e., a rear portion of
the waveguide 16
a.
Next, a damage layer formed upon dry etching is removed by wet etching, and
the growth of embedding the waveguide 16
a with Fe-doped InP is performed
to form a block layer 38.
Next, an insulating film is formed and a waveguide mesa 16 is formed
by wet etching. Then n electrodes 20 and an insulating film 22 are
formed and a p electrode 18 is further formed.
Thereafter, the back surface of the InP substrate 24 is etched
to a suitable thickness to form a back metal for bonding, whereby a wafer process
is completed.
A description will next be made of a method of determining the thicknesses of
the
n-optical guide layer 30 and the p-optical guide layer 34 though
the thicknesses of the n-optical guide layer 30 and the p-optical guide
layer 34 have respectively been set to 0.4 μm in the PIN-PD 10
shared for the 1.3 μm band and 1.55 μm bands referred to above.
First of all, device's design values are set except for the thicknesses of
the n-optical guide layer 30 and the p-optical guide layer 34. And
then, in the case that the thicknesses of the n-optical guide layer 30 and
the p-optical guide layer 34 are changed from 0.1 μm to 0.8 μm,
the dependence of sensitivity on the thicknesses of optical guide layers relative
to the respective signal lights whose wavelengths are 1.3 μm and 1.55 μm
is determined through simulation performed by a BPM (beam propagation method) method.
FIG. 4 is a graph showing the dependence of sensitivity of signal lights on
the thicknesses of optical guide layers of the waveguide type light receiving element
according to the one embodiment of the present invention. In FIG. 4, a curve a
shows the dependence of sensitivity on the thickness of an optical guide layer
related to light whose wavelength is 1.3 μm, and a curve b shows the dependence
of sensitivity on the thickness of an optical guide layer related to light whose
wavelength is 1.55 μm, respectively.
In FIG. 4, the thicknesses d1 and d2 of the optical guide layers
corresponding to extreme values in which the inclinations of the curves a and b
change from positive to negative, are in the vicinity of 0.4 μm in the case
of the curve a and in the vicinity of 0.6 μm in the case of the curve b,
respectively. It is understood that the sensitivity degrades even when the optical
guide layers become thin or thick from their thicknesses d1 and d2
corresponding to the extreme values.
Thus, it can be expected that the thickness dg of optical guide layers indicative
of sensitivity satisfactory for both signal lights of a 1.3 μm band and a
1.55 μm band, for example, falls within a range of 0.4 μm≦dg≦0.6
μm, and the thicknesses of optical guide layers capable of becoming equal
in sensitivity and both high in sensitivity with respect to the signal lights in
the two wavelength bands exist. It can be expected that they exist in places where
their thicknesses are slightly thick from 0.4 μm in FIG. 4.
Generally, in other words, it can be expected that if the thickness dg
of first and second optical guide layers falls within a range of d1≦dg≦d2
when a center wavelength of a first signal light wavelength band corresponding
to the shortest signal light wavelength band is defined as λ1, a center
wavelength of a second signal light wavelength band is λ2 (λ2>λ1),
the thickness of each of the first and second optical guide layers, corresponding
to an extreme value in which the inclination of a sensitivity curve of λ1
with respect to a change in the thickness of each of the first and second optical
guide layers changes from positive to negative, is defined as d1, and the
thickness of each of the first and second optical guide layers, corresponding to
an extreme value in which the inclination of a sensitivity curve of λ2
with respect to the change in the thickness of each of the first and second optical
guide layers changes from positive to negative, is defined as d2, the thicknesses
of the optical guide layers capable of becoming equal in sensitivity and both high
in sensitivity with respect to a multiwavelength signal light contained between
the first signal light and the second signal light exist.
Since, however, the sensitivity curves respectively gently change with respect
to the changes in the thicknesses of the optical guide layers in the vicinity of
the extreme values, the thickness of the optical guide layers can be set to within,
substantially, a range of 0.3 μm≦dg≦0.75 μm, generally,
in other words, a range of 0.75d1≦dg≦1.25d2 without
any problem substantially.
FIG. 5 is a graph showing comparisons between calculated values and actually
measured values of sensitivity characteristics of the waveguide type light receiving
element according to the one embodiment of the present invention.
In FIG. 5, s1 indicates a sensitivity calculated value with respect to
the signal light lying in the 1.3 μm band, and s2 indicates a sensitivity
calculated value with respect to the signal light lying in the 1.55 μm band.
Further, m1 indicates a sensitivity actually-measured value with respect
to the signal light in the 1.3 μm band, and m2 indicates a sensitivity
actually-measured value with respect to the signal light lying in the 1.55 μm band.
As understood from the sensitivity actually-measured values m1 and m2,
an element having a sensitivity characteristic of about 0.8 A/W is obtained, and
the sensitivity calculated values and the sensitivity actually-measured values
approximately coincide with one another.
FIG. 6 is a graph showing frequency responses of the waveguide type light receiving
element according to the one embodiment of the present invention.
In FIG. 6, a curve a indicates the response with respect to the signal light
lying
in the 1.3 μm band, and a curve b indicates the response with respect to
the signal light lying in the 1.55 μm band. As is understood from FIG. 6,
the bandwidth becomes broad because the thickness da of the optical absorbing layer
32 is formed thin.
While the above description has been made of the embedded waveguide type PIN-PD
using the InGaAsP material, an AlInGaAsP material and a GaInNAs material may be
used in addition to an InGaAsP material.
Since these materials are mixed crystals containing a plurality of elements,
their lattice constants and bandgaps can be changed. Therefore, they are made identical
to the substrate material in lattice constant, and the bandgaps can be changed
in a very wide range. Therefore, the degree of freedom of design becomes high and
a higher sensitive light receiving element can be designed.
The InGaAsP material is a material system which has been investigated and developed
from a long time ago and is currently the most common material as the material
for the light receiving element for communication. A stable characteristic can
easily be obtained.
At the mention of the AlInGaAsP materials, the cladding layer, optical guide
layers
and optical absorbing layer are respectively constituted by using materials such
as InAlAs, InGaAlAs and InGaAs to thereby make it possible to obtain a predetermined
refractive index difference, whereby a similar effect can be obtained.
Even a GaInNAs material is changed in composition ratio so that a predetermined
refractive index difference is obtained, thus making it possible to obtain a similar
effect. A PD using the GaInNAs material can cope with a wider range of bandgap
wavelength as compared with a PD using the InGaAsP material or the AlInGaAsP material.
Although the description of the above embodiment has been made of the PIN-PD
by way of example, the present invention may be applied to a light receiving element
having the action of amplifying a signal therewithin, e.g., an element having the
function of receiving light and amplifying a converted electric signal, like an
APD (avalanche photodiode) having a doubling or multiplication layer therewithin,
a light receiving element in which SOA (semiconductor optical amplifiers) each
having the function of amplifying a light signal are disposed at the frontal surface
of a photodetector, etc. to obtain a similar effect.
In particular, the AlInGaAsP material is used for the APD and brings about the
effect of reducing noise as compared with the InGaAs material when a signal is
amplified. It is thus possible to fabricate an APD higher in photo detecting sensitivity.
It is needless to say that a device brought into module form by mounting the
above-described
element also has a similar effect.
In the multiwavelength-shared embedded waveguide type PIN-PD according to the
present embodiment as described above, when the thickness of each of the n- and
p-optical guide layers, corresponding to the extreme value in which the inclination
of the sensitivity curve of the light of the wavelength λ1 with respect
to the change in thickness of each of the n- and p-optical guide layers changes
from positive to negative, is defined as d1 and the thickness of each of
the n- and p-optical guide layers, corresponding to the extreme value in which
the inclination of the sensitivity curve of the light having the wavelength λ2
with respect to the change in the thickness of each of the n- and p-optical guide
layers changes from positive to negative, is defined as d2 where the center
wavelength of the 1.3 μm-band signal light is defined as λ1,
and the center wavelength of the 1.55 μm-band signal light is defined as
λ2, the thickness dg of the n- and p-optical guide layers of the waveguide
type PIN-PD satisfies 0.75d1≦dg≦1.25d2. Consequently,
a high-speed operation is enabled with respect to the multiwavelength-band signal
light containing the 1.3 μm-band signal light and 1.55 μm-band signal
light while high photo detecting sensitivity is being held. It is thus possible
to simply configure a waveguide type light receiving element shared for a multiwavelength-band
signal light, which is high in photo detecting sensitivity and capable of high-speed
operation. By extension, an optical communication system becomes simple, and an
increase in capacity of the communication system can be put forward at low cost.
Further, when the thickness of the optical absorbing layer is defined as
da, it is set so as to satisfy 0.3 μm≦da≦0.5 μm. Owing
to this constitution, the traveling time of a carrier can be suppressed and an
increase in bandwidth is enabled. By extension, a broad-band light receiving element
can be simply configured. It is by extension possible to bring a communication
system into a broad band and easily put forward an increase in capacity of the system.
Furthermore, each of the n- and p-cladding layers is formed of InP,
and the composition wavelength λg of the material for each of the n- and
p-optical guide layers is fixed with the composition wavelengths of the material
for each of the n- and p-cladding layers as λa=0.92 μm and λ1=1.3
μm. With λ2=1.55 μm, the thickness dg of the n- and p-optical
guide layers is set so as to satisfy 0.3 μm≦dg≦0.75 with d1=0.4
μm and d2=0.6 μm. Owing to such a constitution, a high-speed
operation is enabled with respect to the multiwavelength-band signal light containing
the 1.3 μm-band signal light and 1.55 μm-band signal light while high
photo detecting sensitivity is being held.
Moreover, the block layer composed of Fe-doped InP corresponding to the
material lower in refractive index than the optical absorbing layer composed of
i-InGaAs is disposed on the side surfaces of the waveguide. Owing to this configuration,
the confining efficiency of light can be enhanced. It is therefore possible to
improve the confining efficiency of light and increase photo detecting sensitivity
of a light receiving element. By extension, a waveguide type PIN-PD high in photo
detecting sensitivity can be provided in a simple configuration.
While the presently preferred embodiments of the present invention have been
shown and described. It is to be understood these disclosures are for the purpose
of illustration and that various changes and modifications may be made without
departing from the scope of the invention as set forth in the appended claims.
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