JPH10303452A - Semiconductor light detecting element, semiconductor light modulating element, and transmitter for optical transmission - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform high-speed and large capacity of optical transmission with simple constitution enough. SOLUTION: A buffer layer 12 consisting of GaN, and an n-type GaN layer 13 are made in order on a substrate 11 consisting of sapphire. In one region of the topside of the n-type GaN layer 13, a first n-type electrode 14 consisting of Ti/Au is made. In the other region of the topside of the n-type GaN layer 13, an n-type Al0.2 Ga0.8 N layer 15 and a multiple quantum well layer 16 where well layers 16a consisting of undoped GaN and barrier layers consisting of undoped Al0.2 Ga0.8 are stacked alternately, and an n-type GaN layer 17 smaller in band gap than the n-type Al0.2 Ga0.8 N layer 15 are made in order. At the topside of the n-type GaN layer 17, a second n-type electrode 18 which has an opening 18a consisting of Ti/Au and receiving an incident light is made.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor light detecting element, a semiconductor light modulating element and a transmitting apparatus for optical communication using the semiconductor light modulating element used for optical communication and the like.

[0002]

2. Description of the Related Art For optical communication, a semiconductor laser which outputs laser light in a 1.3 μm band or a 1.55 μm band with small wavelength dispersion and light loss of an optical fiber is used. Ge photodiodes and InGaAs are used as photodetectors in this wavelength band.
An s-based avalanche photodiode (APD) has been used. Ge photodiodes are simple and inexpensive to manufacture, but in recent years, InGaAs-based avalanche photodiodes have become mainstream because of their large dark current and low response speed. InGaAs-based photodetectors also include pin photodiodes, but are not being used for recent large-capacity communications of 10 Gbps or more because their response speed cannot be followed.

Hereinafter, a conventional semiconductor photodetector will be described with reference to the drawings.

FIG. 12 shows a cross-sectional structure of a conventional InGaAs avalanche photodiode. A substrate 101 made of p-type InP is formed on a substrate 101 made of p-type InP.
A buffer layer 102 for improving lattice matching of each semiconductor layer grown on 1 and a multiplying layer 103 made of p ⁻ -type InGaAs, which absorbs light and causes avalanche multiplication, are sequentially formed.

At the center of the multiplication layer 103 on the substrate 101, a p ⁺ -type InP layer 104, InAlGaAs and InP
A multi-quantum well layer 105 formed by stacking AlAs, n ⁺
Layer 106 of n-type InAlAs, n ⁺ -type In
GaAs contact layer 107 and n-electrode 10
8 are sequentially formed.

On both ends of the multiplication layer 103 on the substrate 101, a Zn diffusion region 109 in which Zn as a p-type impurity is implanted so as to reach the multiplication layer 103 is formed. A p-electrode 110 is formed on 109, and is provided between the n-electrode 108 and the p-electrode 110 on the multiplication layer 103 on the substrate 101 to ensure insulation between the two electrodes. Implanted insulating region 10
9 are formed.

An outline of the operation of the avalanche photodiode having the above-described configuration will be described. First, p electrode 1
When 10 is grounded and a voltage of about 20 V is applied to the n-electrode 108, electrons, which are minority carriers of the multiplication layer 103, begin to rapidly flow into the cap layer 106 due to the electric field, and are in a state just before avalanche breakdown occurs. In this state, light having a wavelength of 1.3 μm band or 1.55 μm band is amplified by the multiplication layer 103.
When incident on the multiple quantum well layer 1 as shown in FIG.
At 05, the incident light is absorbed, and electrons in the valence band Ev are excited in the conduction band Ec. The excited electrons are triggered to cause avalanche breakdown in the multiplication layer 103, which causes the present diode to be damaged. Electric current flows. Since this phenomenon responds to an optical signal at a very high speed, it is suitable for a photodetector for large-capacity communication.

On the other hand, as shown in FIG. 14, a conventional method for generating an optical signal has been shown in FIG.
A direct modulation method for directly modulating the drive current of the laser light and outputting a laser beam 112, and a semiconductor light modulation element 114 shown in FIG.
And an external modulation method of modulating the laser beam 112 using the above method. The direct modulation method shown in (a) has been widely used because it can be realized with the simplest configuration. However, when the laser element 111 is modulated at high speed, a chirp phenomenon occurs and the oscillation spectrum spreads. Since it has the property that the propagation speed is different,
The signal waveform deteriorates. Therefore, it is considered that the external modulation method shown in (b) is promising for large-capacity communication of 10 Gbps or more.

[0009]

However, the conventional photodetector using an InGaAs-based avalanche photodiode operates with a reverse bias voltage of 20 V to 30 V applied thereto, so that a large-capacity and expensive power supply device is required. Is required. Since avalanche breakdown does not occur with a bias voltage of a power supply device of about 5 V which is generally used, only an operation as a pin photodiode can be obtained. Therefore, the response speed is slow and 10G
It cannot follow high-speed transmission of bps or more. In addition, since light having a higher photon energy than signal light, for example, has sensitivity to visible light, when this element is incorporated in a device,
Since it is necessary to provide a device for blocking stray light, there is a problem that the device becomes complicated.

[0010] In addition, the optical modulation element in the conventional optical communication transmitting apparatus using the external modulation method is such that light is efficiently coupled to the optical modulation element also serving as an optical waveguide so that a sufficient extinction ratio of an optical signal can be obtained. In addition, since it is necessary to perform very high-precision positioning, there is a problem that a large number of steps are required for assembling the apparatus.

The present invention has been made in view of the above-mentioned conventional problems, and has as its object to enable high-speed, large-capacity optical transmission with a simple configuration.

[0012]

In order to achieve the above-mentioned object, the present invention provides a method for detecting or modulating light by using GaN or ZnSe.
And so on, using the transition between sub-bands of a multiple quantum well layer formed by laminating the respective wide-gap n-type semiconductors.

A semiconductor photodetector according to the present invention comprises a first n-type semiconductor layer formed on a substrate, and a well layer and a barrier layer alternately stacked on the first n-type semiconductor layer. A multi-quantum well layer having a plurality of well-type potentials, and a second n-type semiconductor layer formed on the multi-quantum well layer and having a band gap smaller than that of the first n-type semiconductor layer.
Carriers are injected from the second n-type semiconductor layer to the base subband level of the multiple quantum well layer by the resonance tunnel effect, and when input light is incident, the base subband level Then, the incidence of the input light is detected based on a phenomenon in which the carriers excited to the excitation subband level are injected into the first n-type semiconductor layer.

According to the semiconductor photodetector of the present invention, the first
And a multiple quantum well layer sandwiched between a second n-type semiconductor layer having a band gap smaller than that of the first n-type semiconductor layer, and a bias between the first and second n-type semiconductor layers. When a voltage is applied, electrons as carriers are injected from the second n-type semiconductor layer to the base subband level of the multiple quantum well layer by the resonance tunnel effect, and
Electrons excited from the base subband level to the excited subband level upon receiving the input light are injected into the first n-type semiconductor layer, whereby the input light is detected as a current. As described above, since the inter-subband transition of electrons in the quantum well is used for light detection instead of the interband transition in which electrons transition from the valence band to the conduction band, the bias voltage is 1 V to 2 V.
Electrons can be injected into the base subband level of the multiple quantum well layer at a low voltage of about V. Further, since the inter-subband transition is used, the resonance tunnel current is extremely fast, and the input light can be selectively absorbed.

In the semiconductor photodetector of the present invention, the first
It is preferable that all of the semiconductor layer, the second semiconductor layer, and the multiple quantum well layer contain GaN or ZnSe.

In the semiconductor photodetector of the present invention, it is preferable that the excitation sub-band level is a third quantum level among the sub-band levels.

In the semiconductor photodetector of the present invention, the wavelength of the input light is preferably in a 1.3 μm band or a 1.55 μm band.

A first semiconductor light modulation device according to the present invention is:
An n-type semiconductor layer formed on a substrate and containing GaN, and a well layer and a barrier layer each containing GaN are alternately stacked on the n-type semiconductor layer to form a multiple quantum well having a plurality of well-type potentials. A well layer and a Schottky electrode formed on the multiple quantum well layer, and when no bias voltage is applied between the n-type semiconductor layer and the Schottky electrode, the multiple quantum well layer is depleted and n When a bias voltage is applied between the n-type semiconductor layer and the Schottky electrode, carriers are injected from the n-type semiconductor layer to the base subband level of the multiple quantum well layer, and the n-type semiconductor layer and the Schottky electrode , The input light is modulated by changing the bias voltage applied during the period to change the transition probability of carriers excited from the base sub-band level to the excited sub-band level.

According to the first semiconductor light modulation device, GaN
A multiple quantum well layer containing GaN sandwiched between an n-type semiconductor layer containing GaN and a Schottky electrode. When no bias is applied, the multiple quantum well layer is depleted. Electrons, which are carriers, are injected into the base sub-band level of, and the input light is modulated by changing the bias voltage to change the transition probability of electrons excited from the base sub-band level to the excited sub-band level. You. As described above, since the inter-subband transition of electrons in the quantum well is used for the light modulation instead of the inter-band transition in which electrons transition from the valence band to the conduction band, the bias voltage is about 1 V to 2 V. Electrons can be injected into the ground subband level of the depleted multiple quantum well layer at low voltage. In addition, since transition between sub-bands of a semiconductor including GaN, which is a wide gap semiconductor, is used, by selecting an appropriate material, film thickness or impurity concentration for the multiple quantum well layer, a wavelength at which the optical fiber is most efficient can be obtained. A certain 1.3 μm band or 1.5 μm band can be selectively absorbed. In addition, since a wide gap semiconductor is used, the effective mass of electrons increases, so that the absorption coefficient between subbands increases by about one digit.

[0020] The second semiconductor light modulation device according to the present invention comprises:
A first n-type semiconductor layer formed on the substrate and containing GaN; and a well layer and a barrier layer each containing GaN are alternately stacked on the first n-type semiconductor layer. A multi-quantum well layer having a n-type potential, and a second n-type semiconductor layer formed on the multi-quantum well layer and having a lower energy in the conduction band than the barrier layer. When no bias voltage is applied between the second n-type semiconductor layer and the second n-type semiconductor layer, the multiple quantum well layer is depleted, and a bias voltage is applied between the first n-type semiconductor layer and the second n-type semiconductor layer. In this state, carriers are injected from the first n-type semiconductor layer to the base subband level of the multiple quantum well layer and applied between the first n-type semiconductor layer and the second n-type semiconductor layer. Excitation from ground subband level by changing bias voltage Modulating an input light by varying the transition probabilities of carriers excited in the subband level.

According to the second semiconductor light modulation device, GaN
A multi-quantum well layer containing GaN sandwiched between a first n-type semiconductor layer and a second n-type semiconductor layer, the depletion of the multi-quantum well layer when there is no bias, and the depletion of the multi-quantum well layer when no bias is applied. Electrons are injected from the n-type semiconductor layer 1 into the base subband level of the multiple quantum well layer, and the transition voltage of the electrons is changed from the base subband level to the excited subband level by changing the bias voltage. This modulates the input light. As described above, since the inter-subband transition of electrons in the quantum well is used for the light modulation instead of the inter-band transition in which electrons transition from the valence band to the conduction band, the bias voltage is about 1 V to 2 V. Electrons can be injected into the ground subband level of the depleted multiple quantum well layer at low voltage. In addition, since transition between sub-bands of a semiconductor including GaN, which is a wide gap semiconductor, is used, by selecting an appropriate material, film thickness or impurity concentration for the multiple quantum well layer, a wavelength at which the optical fiber is most efficient can be obtained. A certain 1.3 μm band or 1.5 μm band can be selectively absorbed. In addition, since a wide gap semiconductor is used, the effective mass of electrons increases, so that the absorption coefficient between subbands increases by about one digit.

In the first or second semiconductor light modulation device, the well layer or the barrier layer preferably has an n-type conductivity and an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ or more.

In the first or second semiconductor light modulation device, the excitation subband level is preferably the third quantum level among the subband levels.

In the first or second semiconductor light modulation element, the wavelength of the input light is 1.3 μm band or 1.55 μm.
It is preferably a band.

A first optical communication transmitting apparatus according to the present invention comprises:
A semiconductor laser element that outputs laser light, and a semiconductor light modulation element that is provided on the output side of the laser light and that modulates and outputs the input laser light, and the semiconductor light modulation element is formed on a substrate; An n-type semiconductor layer containing GaN, and well layers and barrier layers each containing GaN are alternately stacked on the n-type semiconductor layer, and a multiple quantum well layer having a plurality of well-type potentials, A multi-quantum well layer having a Schottky electrode formed on the well layer and no bias voltage being applied between the n-type semiconductor layer and the Schottky electrode; When a bias voltage is applied between the n-type semiconductor layer and the Schottky electrode, a carrier is injected from the n-type semiconductor layer to the base subband level of the multiple quantum well layer. By changing the bias voltage,
The input laser light is modulated by changing the transition probability of carriers excited from the base subband level to the excitation subband level.

According to the first optical communication transmitter, the semiconductor optical modulation element that receives and modulates the output light of the semiconductor laser element uses the transition between sub-bands of electrons in the quantum well, so that the bias voltage is reduced. Electrons can be injected into the depleted multiple quantum well layer at a low voltage of about 1 V to 2 V.
In addition, since the transition between sub-bands of a semiconductor including GaN, which is a wide gap semiconductor, is used, the wavelength of 1.3 μm or 1.
If a laser beam in the 5 μm band is used, the laser beam can be selectively absorbed. In addition, since a wide gap semiconductor is used, the effective mass of electrons increases, so that the absorption coefficient between subbands increases by about one digit.

A second optical communication transmitting apparatus according to the present invention comprises:
A semiconductor laser element that outputs laser light, and a semiconductor light modulation element that is provided on the output side of the laser light and that modulates and outputs the input laser light, and the semiconductor light modulation element is formed on a substrate; A first n-type semiconductor layer containing GaN;
Well layers and barrier layers each containing GaN are alternately stacked on the first n-type semiconductor layer, and formed on the multiple quantum well layer having a plurality of well potentials and on the multiple quantum well layer. A second n-type semiconductor layer whose energy at the lower end of the conduction band is smaller than that of the barrier layer, wherein no bias voltage is applied between the first n-type semiconductor layer and the second n-type semiconductor layer. In this case, the multiple quantum well layer is depleted, and in a state where a bias voltage is applied between the first n-type semiconductor layer and the second n-type semiconductor layer, the multiple quantum well layers are separated from the first n-type semiconductor layer. Are injected into the base sub-band level of the first sub-band and the bias voltage applied between the first n-type semiconductor layer and the second n-type sub-layer is changed to change the excitation sub-band from the base sub-band level. Varies the transition probability of carriers excited to the level Modulating the laser beam inputted by Rukoto.

According to the second optical communication transmitter, the semiconductor optical modulation element that receives and modulates the output light of the semiconductor laser element uses the transition between sub-bands of electrons in the quantum well. Electrons can be injected into the depleted multiple quantum well layer at a low voltage of about 1 V to 2 V.
In addition, since the transition between sub-bands of a semiconductor including GaN, which is a wide gap semiconductor, is used, the wavelength of 1.3 μm or 1.
If a laser beam in the 5 μm band is used, the laser beam can be selectively absorbed. In addition, since a wide gap semiconductor is used, the effective mass of electrons increases, so that the absorption coefficient between subbands increases by about one digit.

In the first or second optical communication transmitting device, a collimating lens provided between the semiconductor laser element and the semiconductor light modulation element for converting the laser light into a plane wave is provided on the output side of the semiconductor light modulation element. It is preferable that the apparatus further includes a focus lens provided to collect output light output from the semiconductor light modulation element.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) A first embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view showing a semiconductor photodetector according to a first embodiment of the present invention. As shown in FIG.
On a substrate 11 made of sapphire, made of GaN,
A buffer layer 12 for lattice matching between a sapphire crystal and a semiconductor layer grown on a substrate 11 and an n-type GaN layer 13 doped with Si are sequentially formed.

On one region on the upper surface of the n-type GaN layer 13, a first n-type electrode 14 made of, for example, Ti / Au
Are formed. In the other region on the upper surface of the n-type GaN layer 13, an n-type Al _0.2 Ga _0.8 N layer 15 as a first n-type semiconductor layer doped with Si, an undoped G
aN well layer 16a and undoped Al _0.2 Ga
A multi-quantum well layer 1 having a plurality of well-type potentials is formed by alternately stacking barrier layers 16b of _0.8 N.
6, and n-type GaN as a second n-type semiconductor layer having a band gap smaller than that of the n-type Al _0.2 Ga _0.8 N layer 15
Layers 17 are sequentially formed. On the upper surface of the n-type GaN layer 17, a second n-type electrode 18 made of Ti / Au and having an opening 18a at the center for introducing an optical signal 19 into the multiple quantum well layer 16 is formed.

Hereinafter, the operation of the photodetector configured as described above (hereinafter referred to as an intersubband transition type photodetector) will be described with reference to the drawings.

FIG. 2 shows the energy band of the intersubband transition type photodetector according to the present embodiment.
(B) shows a non-biased state when a predetermined voltage is not applied to the n-type electrode 14 and the second n-type electrode 18 of FIG.
This shows a bias time when a predetermined voltage is applied to the mold electrode 14 and the second n-type electrode 18. As shown in FIG. 2A, a plurality of quantum levels exist in the conduction band Ec of the multiple quantum well layer 16, and these quantum levels are arranged in ascending order of energy from the base subband level. Are the first quantum level E1, the second quantum level E2, and the third quantum level E3.
Here, assuming that the thicknesses of the well layer 16a and the barrier layer 16b are 3 nm and 5 nm, respectively, the energy difference between the quantum levels is about 300 eV between the second quantum level E2 and the first quantum level E1. And the distance between the third quantum level E3 and the first quantum level E1 is about 800 meV. This transition energy of 800 meV has a wavelength of 1.55 μm to be detected.
Coincides with the photon energy of the incident light.

At the time of no bias, n-type Al _0.2 Ga
_The multiple quantum well layer 16 has an electron between the _0.8 N layer 15 and the n-type GaN layer 17 having a band gap smaller than that of the n-type Al _0.2 Ga _0.8 N layer 15, that is, the conduction band E c has an energy smaller by ΔE c. No electrons flow because they act as a barrier to The conduction band Ec of the multiple quantum well layer 16
Since there are almost no electrons at each quantum level,
Even if an optical signal in the 1.55 μm band enters from the opening 18a of the second n-type electrode 18, the incident light is not absorbed.

Next, as shown in FIG. 2B, the first n
When a voltage of about -1 V is applied to the second n-type electrode 18 with the n-type electrode 14 grounded, the energy at the conduction band Ec end of the n-type GaN layer 17 becomes the first energy of the first well layer 16 a of the multiple quantum well layer 16.
It almost coincides with the quantum level E1. At this time, electrons are injected from the n-type GaN layer 17 into the first quantum level E1 of the well layer 16a by the resonance tunnel phenomenon. When electrons are injected into the first quantum level E1 of the well layer 16a, the wavelength is 1.55.
When an optical signal 19 in the .mu.m band is incident, the optical signal 19 is absorbed by the well layer 16a of the multiple quantum well layer 16, and the energy level of the electron changes from that of the first quantum level E1 to that of the photon. A transition is made to a third quantum level E3 corresponding to energy. The electron that has transitioned to the third quantum level E3 is an n-type A
It flows into the l _0.2 Ga _0.8 N layer 15 by the resonance tunnel phenomenon. The flow rate of electrons flowing into the n-type Al _0.2 Ga _0.8 N layer 15 becomes a detection signal indicating the intensity of the optical signal 19.

In general, the light absorption spectrum between subbands has a sharp shape with a peak at a wavelength of 1.55 μm as shown in FIG. For this reason, noise due to stray light other than the signal light does not occur, so that it is not necessary to provide a filter or the like for suppressing stray light in the present element.

Further, since the effective mass of electrons is larger than that of a narrow gap semiconductor because the wide gap semiconductor is used, the electron density of the first quantum level E1 is 1 × 10 ¹⁷ cm ^-3 . Sometimes, it is as large as about 40000 cm ^−1, and sufficient light absorption can be obtained even if the number of well layers 16 a of the multiple quantum well layer 16 is smaller than that of a normal APD.

Also, since electrons are injected into the multiple quantum well layer 16 by using the resonance tunnel effect, it is possible to operate at a very low bias voltage and at a very high speed. It can support high-speed, large-capacity communication of 10 Gbps or more.

In this embodiment, the wavelength is 1.55 μm.
Although light absorption is generated for light in the m band, it is possible to detect light in the 1.3 μm band by reducing the thickness of the well layer 16a or changing the material of the well layer 16a or the barrier layer 16b. Become.

Although the light absorption is performed using the transition between the first quantum level E1 and the third quantum level E3, the thickness of the well layer 16a may be reduced, By changing the material of the barrier layer 16b, the first quantum level E
It is also possible to cause light absorption due to the transition between the first quantum level E2 and the second quantum level E2.

However, since the transition between the first quantum level E1 and the third quantum level E3 has a higher electron transition probability,
It is easy to obtain a larger absorption coefficient.

Hereinafter, a modification of the first embodiment of the present invention will be described with reference to the drawings.

FIG. 4 is a perspective view showing an intersubband transition type photodetector according to a modification of the first embodiment. FIG.
As shown in FIG. 3, on a substrate 21 made of n-type GaN,
a buffer layer 22 made of nSe for lattice matching with a semiconductor layer grown on the substrate 21; n doped with Cl;
Type ZnSSe layer 23, first n-type Z doped with Cl
The nMgSSe layer 24, the well layer made of undoped ZnSe and the barrier layer made of undoped ZnMgSSe are alternately stacked, and the multiple quantum well layer 25 having a plurality of well-type potentials and the first n-type ZnMgSSe layer Second n-type ZnMg with small band gap
The SSe layers 26 are sequentially formed.

A first n-type electrode 27 made of Ti / Au is formed on the lower surface of the substrate 21, and an optical signal 19 made of Ti / Au is formed on the upper surface of the substrate 21.
A second n-type electrode 28 having an opening 28a for incidence on the fifth electrode 5 is formed.

Also in this modification, since the photodetector element contains ZnSe of the II-IV group which is a wide gap semiconductor and uses an intersubband transition, the first photodetector using the semiconductor layer containing GaN is used. It has the same effect as the embodiment.

(Second Embodiment) Hereinafter, a second embodiment according to the present invention will be described with reference to the drawings.

FIG. 5 is a perspective view showing a semiconductor light modulation device according to a second embodiment of the present invention. As shown in FIG.
On a substrate 31 made of sapphire, made of GaN,
A buffer layer 32 for lattice matching between the sapphire crystal and the semiconductor layer grown on the substrate 31;
Type GaN layers 33 are sequentially formed.

In one region on the upper surface of the n-type GaN layer 33, an n-type electrode 34 made of, for example, Ti / Au is formed. In the other region on the upper surface of the n-type GaN layer 33, an n-type Al _0.2 Ga _0.8 N layer 35 doped with Si, and an n-type doped impurity with an impurity concentration of 1 × 10 ¹⁷ cm ^−3.
Layer 36a made of p-type GaN and undoped Al _0.2 G
a _0.8 N barrier layers 36 b are alternately stacked, a multiple quantum well layer 36 which is a light absorption layer having a plurality of well-type potentials, and an undoped Al _0.2 Ga _0.8
N layers 37 are sequentially formed. Undoped Al _0.2 G
On the upper surface of the a _0.8 N layer 37, a Schottky electrode 38 made of Au is formed.

Hereinafter, the operation of the light modulation device configured as described above (hereinafter, referred to as an intersubband transition type light modulation device) will be described with reference to the drawings.

FIGS. 6A and 6B show the energy bands of the intersubband transition type light modulation device according to the present embodiment. FIG. 6A shows the state when no predetermined voltage is applied to the n-type electrode 34 and the Schottky electrode 38 when no bias is applied. (B) shows the n-type electrode 34
5 shows a state in which a predetermined voltage is applied to the Schottky electrode 38 and a bias is applied. As shown in FIG. 6A, a plurality of quantum levels exist in the conduction band Ec of the multiple quantum well layer 36, and these quantum levels are determined in order from the lower energy to the base subband level. The first quantum level E1,
It is assumed that the second quantum level is E2 and the third quantum level is E3. here,
The thickness of each of the well layer 36a and the barrier layer 36b is 3 n
Assuming m and 5 nm, the energy difference between each quantum level is about 3 between the second quantum level E2 and the first quantum level E1.
00 eV, the third quantum level E3 and the first quantum level E1
Is about 800 meV. This transition energy 8
00meV matches the photon energy of the incident light having a wavelength of 1.55 μm to be modulated.

At the time of no bias, the multiple quantum well layer 3
6, an undoped Al _0.2 Ga _0.8 N layer 37 is provided on the opposite side of the n-type Al _0.2 Ga _0.8 N layer 35.
As each semiconductor layer of the multiple quantum well layer 36 is depleted, the conduction band Ec rises from the n-type Al _0.2 Ga _0.8 N layer 35 toward the undoped Al _0.2 Ga _0.8 N layer 37. The multiple quantum well layer 36 is formed by the inclination of the conduction band Ec.
And a Schottky electrode 38, a potential barrier is formed. In the well layer 36a of the multiple quantum well layer 36, electrons hardly exist at each quantum level.
Even if the light in the 5 μm band enters the multiple quantum well layer 36, the incident light is transmitted without being absorbed.
As a result, the incident light is not modulated when there is no bias.

Next, as shown in FIG. 6B, when the Schottky electrode 38 is grounded and a voltage of about -1 V is applied to the n-type electrode 34, the depletion layer near the multiple quantum well layer 36 becomes smaller. Therefore, electrons can be present at the first quantum level E1. The electron density of the first quantum level E1 substantially coincides with the doping concentration, and is 10 ¹⁷ cm ^-3 here.

In this state, when light having a wavelength of 1.55 μm is incident, the incident light is absorbed by the well layer 36a of the multiple quantum well layer 36, and the energy level of the electrons is changed to the first energy level. The energy difference transits from the quantum level E1 to the third quantum level E3 corresponding to the photon energy. Since the electron that has transitioned to the third quantum level E3 has no gradient in the electric field, it returns to the first quantum level of the base subband level after a lapse of a predetermined time. Thus, by controlling the bias voltage,
The incident light can be modulated and output.

In general, the light absorption spectrum between subbands has a sharp shape with a peak at a wavelength of 1.55 μm as shown in FIG. Further, the light absorption coefficient of the first quantum level E
When the electron density of 1 is 1 × 10 ¹⁷ cm ^-3 , about 40,000
cm ^{−1, which} is an order of magnitude larger than that of a conventional light modulation element, so that a sufficient extinction ratio can be secured without forming a waveguide as in the case of a conventional semiconductor light modulation element. It is not necessary to precisely adjust the incident position of the light.

In this embodiment, the wavelength is 1.55 μm.
Although light absorption is generated for light in the m band, it is possible to detect light in the 1.3 μm band by reducing the thickness of the well layer 36a or changing the material of the well layer 36a or the barrier layer 36b. Become.

Although the light absorption is performed using the transition between the first quantum level E1 and the third quantum level E3, the thickness of the well layer 36a may be reduced, or the well layer 36a or By changing the material of the barrier layer 36b, the first quantum level E
It is also possible to cause light absorption due to the transition between the first quantum level E2 and the second quantum level E2.

However, since the transition between the first quantum level E1 and the third quantum level E3 has a higher electron transition probability,
It is easy to obtain a larger absorption coefficient.

In this embodiment, n-type GaN is used for the well layer 36a of the multiple quantum well layer 36, but the transition energy corresponding to a desired wavelength can be obtained as the transition energy due to optical absorption. If there is undoped Ga
N may be used, and undoped Al
_{Although 0.2} Ga _0.8 N was used, similarly, n-type Al _0.2 Ga
_It may be _0.8 N.

Hereinafter, a first modification of the second embodiment of the present invention will be described with reference to the drawings.

FIG. 7 is a perspective view showing an intersubband transition type light modulation device according to a first modification of the second embodiment. As shown in FIG. 7, as in the first embodiment, an n-type Al _0.2 Ga _0.8 N layer 35 as a first n-type semiconductor layer and a well layer made of n-type GaN are formed on a substrate 31 made of sapphire. A multiple quantum well layer 3 in which barrier layers 36b made of undoped Al _0.2 Ga _0.8 N are alternately stacked.
6 and an undoped Al _0.2 Ga _0.8 N layer 37 are sequentially formed, and the undoped Al _0.2 Ga _0.8 N layer 37 is further formed.
, An n-type GaN layer 39 serving as a second n-type semiconductor layer in which the energy at the bottom of the conduction band is smaller than that of the barrier layer 36b.
Are formed. The description of the same members as those shown in FIG. 5 will be omitted by retaining the same reference numerals.

FIG. 8 shows the energy band of the intersubband transition type light modulation element according to this modification. FIG. 8A shows the state when no predetermined voltage is applied to the n-type electrode 34 and the Schottky electrode 38 when no bias is applied. 3B shows a state in which a predetermined voltage is applied to the n-type electrode 34 and the Schottky electrode 38 at the time of bias. At the time of no bias shown in FIG. 8A, the n-type Al _0.2 Ga in the multiple quantum well layer 36 is formed.
Undoped Al _0.2 Ga _0.8 N on the opposite side of the _0.8 N layer 35
A conduction band Ec instead of the layer 37 and the Schottky electrode 38;
-Type GaN whose energy is smaller than that of the barrier layer 36b
Since the layer 39 is provided, each semiconductor layer of the multiple quantum well layer 36 is depleted.

As a result, the same effect as in the second embodiment can be obtained.

Hereinafter, a second modification of the second embodiment of the present invention will be described with reference to the drawings.

FIG. 9 is a perspective view showing an intersubband transition type light modulation device according to a second modification of the second embodiment.
As shown in FIG. 9, on a substrate 41 made of n-type GaN, a buffer layer 42 made of ZnSe for achieving lattice matching with a semiconductor layer grown on the substrate 41, an n-type ZnSSe layer doped with Cl 43, n-type Z doped with Cl
nMgSSe layer 44, n-type ZnSe doped with Cl
A multi-quantum well layer 45 having a plurality of well-type potentials, and an undoped ZnMgSe layer.
MgSSe layers 46 are sequentially formed.

On the lower surface of the substrate 41, n made of Ti / Au
A mold electrode 47 is formed, and a Schottky electrode 48 made of Au is formed on the upper surface of the substrate 41.

Also in this modification, since the light modulation element contains ZnSe of the II-IV group, which is a wide gap semiconductor, and uses an intersubband transition, the second element using the semiconductor layer containing GaN is used. It has the same effect as the embodiment.

(Third Embodiment) Hereinafter, a third embodiment according to the present invention will be described with reference to the drawings.

FIG. 10 schematically shows a transmitting apparatus for optical communication according to a third embodiment of the present invention. As shown in FIG. 10, an oscillation wavelength of 1.
A 55 μm DFB (distributed feedback) laser element 52,
An inter-subband transition type light modulation element 53 according to the second embodiment of the present invention is provided at an interval of about 100 μm from the emission surface of the laser element 52 in the optical axis direction.

The element length of the light modulation element 53 in the optical axis direction is 10
0 μm, the substrate of the laser element 52 and the light modulation element 5
If the thickness of the light modulation element 53
Adjustment of the bonding position in the vertical direction with respect to the main surface of the base 51 is unnecessary.

The position adjustment of the light modulation element 53 in the direction parallel to the main surface of the base 51 is sufficient if it is about several tens μm.

The feature of this embodiment is that the light modulating element 53
Contains GaN or ZnSe, which are wide gap semiconductors, and modulates and outputs input light by optical absorption using intersubband transitions. Therefore, the effective mass of electrons in a wide gap semiconductor is larger than that in a narrow gap semiconductor. , The light absorption coefficient becomes very large. Thus, if the laser light 54 slightly overlaps the multiple quantum well layer of the light modulation element 53, a sufficient extinction ratio can be obtained for the laser light 54, so that the laser light 54 is guided between the laser element 52 and the light modulation element 53. Since there is no need to form a wave path, when the laser element 52 and the light modulation element 53 are mounted on an apparatus, the configuration of the apparatus can be simplified.

Further, since optical absorption is performed using the transition between sub-bands, the time for electrons to return to the base sub-band level is extremely short, so that high-speed modulation of 10 Gbps or more can be supported.

Hereinafter, a modification of the third embodiment of the present invention will be described with reference to the drawings.

FIG. 11 schematically shows an optical communication transmitting apparatus according to a modification of the third embodiment. As a feature of this modified example, as shown in FIG. 11, a laser element 52 and a space in the optical axis direction from an emission surface of the laser element 52 are provided on a substrate 51 made of Si, An inter-subband transition type optical modulator 53 according to the embodiment and an optical fiber 55 for transmitting a laser beam 54 output from the optical modulator 53 to the outside of the device are provided. Further, a collimating lens 56 for converting the laser light 54 output from the laser element 52 into a plane wave is provided between the laser element 52 and the light modulating element 53.
A focus lens 57 for condensing the laser light 54 output from the light modulation element 53 is provided between the focus lens 57 and the light modulation element 53.

As described above, according to the present modification, the coupling efficiency of the laser light 54 modulated by the light modulation element 53 to the optical fiber 55 can be improved. In this case, a high-precision submicron adjustment is required only for the alignment of the optical fiber 55, but the optical modulator 53 does not require a precise adjustment as described above, so that the manufacturing process of the present apparatus is greatly simplified. Can be

[0077]

According to the semiconductor photodetector according to the present invention, the transition between sub-bands in the quantum well is used.
Electrons as carriers can be injected into the base subband level of the multiple quantum well layer at a low bias voltage of about 1 V to 2 V. Thus, the power supply device for generating the bias voltage can be simplified. In addition, since the inter-subband transition is used, the resonance tunnel current is extremely high speed, so that high-speed large-capacity communication can be supported, and a visible light filter or the like needs to be provided because input light is selectively absorbed. And the device can be simplified.

In the semiconductor photodetector according to the present invention,
When all of the first semiconductor layer, the second semiconductor layer, and the multiple quantum well layer are semiconductors containing GaN or ZnSe,
Since each of GaN and ZnSe is a wide-gap semiconductor, transition between subbands of electrons can be reliably realized.

In the semiconductor photodetector according to the present invention, if the excited subband level is the third quantum level of the subband levels, the transition probability of electrons from the base subband is large. The light absorption coefficient can be reliably increased.

Further, in the semiconductor photodetector according to the present invention, when the wavelength of the input light is in the 1.3 μm band or the 1.55 μm band, the transmission efficiency is the highest with respect to the optical fiber for transmitting the signal light.

According to the first or second semiconductor optical modulation device of the present invention, since the intersubband transition of electrons in the quantum well in the semiconductor including GaN, which is a wide gap semiconductor, is used, the bias voltage is 1V. Electrons can be injected into the base subband level of the depleted multiple quantum well layer at a low voltage of about 2 V, so that a power supply device for generating a bias voltage can be simplified. In addition, since the transition between subbands is used, by selecting an appropriate material, film thickness, or impurity concentration for the multiple quantum well layer, the 1.3 μm band or the 1.5 μm band which is the most efficient wavelength for the optical fiber is selected. Can be selectively absorbed, and since it is extremely fast, it can sufficiently cope with high-speed and large-capacity communication. In addition, since a wide gap semiconductor is used, the effective mass of electrons increases, so that the absorption coefficient between subbands increases by about one digit. This eliminates the need for precise alignment during the manufacture of a communication device or the like, so that the number of manufacturing steps can be significantly reduced.

In the first or second semiconductor light modulating element, if the well layer or the barrier layer has an n-type conductivity and an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ or more, no voltage is applied when a bias is applied. Since electrons are efficiently injected into the depleted multiple quantum well layer at the time of bias, the modulation efficiency of input light is improved.

In the first or second semiconductor optical modulator, when the excitation subband level is the third quantum level among the subband levels, the transition probability of electrons from the base subband is large. Therefore, the light absorption coefficient can be reliably increased.

In the first or second semiconductor optical modulator, the wavelength of the input light is 1.3 μm band or 1.55 μm.
When the band is used, the transmission efficiency is highest for an optical fiber that transmits signal light.

According to the first or second optical communication device according to the present invention, since the first or second semiconductor optical modulation device according to the present invention is used, the power supply device for generating a bias voltage can be simplified. In addition, the alignment of the light modulation element can be simplified, and high-speed transmission can be sufficiently performed.

In the first or second optical communication device, the device is provided between the semiconductor laser device and the semiconductor optical modulation device,
When a collimator lens for converting laser light into a plane wave and a focus lens provided on the output side of the semiconductor light modulation element and condensing output light output from the semiconductor light modulation element are provided, the semiconductor light modulation element Output light to be transmitted to the optical fiber.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a semiconductor photodetector according to a first embodiment of the present invention.

FIG. 2 is an energy band diagram of the semiconductor photodetector according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a light absorption spectrum between quantum levels in the semiconductor photodetector according to the first embodiment of the present invention or the semiconductor optical modulator according to the second embodiment of the present invention.

FIG. 4 is a perspective view showing an inter-subband transition type photodetector according to a modification of the first embodiment of the present invention.

FIG. 5 is a perspective view showing a semiconductor light modulation device according to a second embodiment of the present invention.

FIG. 6 is an energy band diagram of a semiconductor light modulation device according to a second embodiment of the present invention.

FIG. 7 is a perspective view showing a semiconductor light modulation device according to a first modification of the second embodiment of the present invention.

FIG. 8 is an energy band diagram of a semiconductor light modulation device according to a first modification of the second embodiment of the present invention.

FIG. 9 is a perspective view showing a semiconductor light modulation device according to a second modification of the second embodiment of the present invention.

FIG. 10 is a schematic configuration diagram illustrating a transmission device for optical communication according to a third embodiment of the present invention.

FIG. 11 is a schematic configuration diagram illustrating a transmission device for optical communication according to a modification of the third embodiment of the present invention.

FIG. 12 is a configuration sectional view of a conventional InGaAs-based avalanche photodiode.

FIG. 13 is an energy band diagram of a conventional avalanche photodiode.

FIG. 14 is a schematic configuration diagram illustrating an optical signal generation method in optical communication.

[Explanation of symbols]

Reference Signs List 11 substrate 12 buffer layer 13 n-type GaN layer 14 first n-type electrode 15 n-type Al _0.2 Ga _0.8 N layer (first n-type semiconductor layer) 16 multiple quantum well layer 16a well layer 16b barrier layer 17 n-type GaN Layer (second n-type semiconductor layer) 18 second n-type electrode 18a opening 19 optical signal 21 substrate 22 buffer layer 23 n-type ZnSSe layer 24 first n-type ZnMgSSe layer (first n-type semiconductor layer) Reference Signs List 25 multiple quantum well layer 26 second n-type ZnMgSSe layer (second n-type semiconductor layer) 27 first n-type electrode 27 28 second n-type electrode 28a opening 31 substrate 32 buffer layer 33 n-type GaN layer 34 n-type electrode 35 n-type Al _0.2 Ga _0.8 N layer (first n-type semiconductor layer) 36 multiple quantum well layer 36 a well layer 36 b barrier layer 37 undoped Al _0.2 Ga _0.8 N layer 38 Key electrode 39 n-type GaN layer (second n-type semiconductor layer) 41 substrate 42 buffer layer 43 n-type ZnSSe layer 44 n-type ZnMgSSe layer 45 multiple quantum well layer 46 undoped ZnMgSSe layer 47 n-type electrode 48 Schottky electrode 51 substrate 52 laser element 53 light modulation element 54 laser light 55 optical fiber 56 collimating lens 57 focus lens

Claims (19)

[Claims] A first n-type semiconductor layer formed on a substrate; and a well layer and a barrier layer are alternately stacked on the first n-type semiconductor layer. And a second quantum well layer formed on the multiple quantum well layer and having a smaller band gap than the first n-type semiconductor layer.
A carrier is injected from the second n-type semiconductor layer to the base subband level of the multiple quantum well layer by the resonance tunnel effect, and the input light is incident upon the carrier. Detecting the incidence of the input light based on a phenomenon that carriers excited from a base subband level to an excitation subband level are injected into the first n-type semiconductor layer; element. 2. The semiconductor photodetector according to claim 1, wherein each of the first semiconductor layer, the second semiconductor layer, and the multiple quantum well layer contains GaN or ZnSe. 3. The semiconductor photodetector according to claim 1, wherein the excitation sub-band level is a third quantum level among the sub-band levels. 4. The semiconductor photodetector according to claim 1, wherein the wavelength of the input light is in a 1.3 μm band or a 1.55 μm band. 5. An n-type semiconductor layer formed on a substrate and containing GaN, and a plurality of well layers and barrier layers each containing GaN are alternately stacked on the n-type semiconductor layer. A multi-quantum well layer having a n-type potential, and a Schottky electrode formed on the multi-quantum well layer, wherein no bias voltage is applied between the n-type semiconductor layer and the Schottky electrode. The multiple quantum well layer is depleted, and when a bias voltage is applied between the n-type semiconductor layer and the Schottky electrode, the n-type semiconductor layer shifts from the n-type semiconductor layer to a base subband level of the multiple quantum well layer. Carriers are injected, and the bias voltage applied between the n-type semiconductor layer and the Schottky electrode is changed to be excited from the base subband level to an excited subband level. The optical modulator, characterized by modulating the input light by varying the transition probabilities of the carrier. 6. A first n-type semiconductor layer formed on a substrate and containing GaN, and a well layer and a barrier layer each containing GaN are alternately stacked on the first n-type semiconductor layer. A multi-quantum well layer having a plurality of well-type potentials; and a second n-type semiconductor layer formed on the multi-quantum well layer and having a lower energy of a conduction band lower than the barrier layer. When no bias voltage is applied between the first n-type semiconductor layer and the second n-type semiconductor layer, the multiple quantum well layer is depleted, and the first n-type semiconductor layer and the second n-type semiconductor layer are depleted. When a bias voltage is applied between the first n-type semiconductor layer and the first n-type semiconductor layer,
Carriers are injected from the n-type semiconductor layer to the base subband level of the multiple quantum well layer, and a bias voltage applied between the first n-type semiconductor layer and the second n-type semiconductor layer is changed. A semiconductor light modulation device that modulates input light by changing a transition probability of carriers excited from the base subband level to the excitation subband level. 7. The semiconductor optical modulator according to claim 5, wherein the well layer or the barrier layer has an n-type conductivity and an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ or more. element. 8. The semiconductor light modulation device according to claim 5, wherein the excitation sub-band level is a third quantum level among the sub-band levels. 9. The wavelength of the input light is in a 1.3 μm band or a 1.55 μm band.
3. The semiconductor light modulation device according to item 1. 10. An n layer formed on a substrate and containing ZnSe.
A multi-quantum well layer having a plurality of well-type potentials, wherein a well layer and a barrier layer each containing ZnSe are alternately stacked on the n-type semiconductor layer; A multi-quantum well layer is depleted in a state where no bias voltage is applied between the n-type semiconductor layer and the Schottky electrode. In a state where a bias voltage is applied between the Schottky electrode and the Schottky electrode, carriers are injected from the n-type semiconductor layer to the base subband level of the multiple quantum well layer, and the n-type semiconductor layer and the Schottky electrode By changing the bias voltage applied between and to change the transition probability of carriers excited from the base subband level to the excitation subband level The optical modulator, characterized by modulating the power light. 11. A first n-type semiconductor layer formed on a substrate and containing ZnSe, and a well layer and a barrier layer each containing ZnSe are alternately stacked on the first n-type semiconductor layer. A multi-quantum well layer having a plurality of well-type potentials; and a second n-type semiconductor layer formed on the multi-quantum well layer and having a lower energy in a conduction band than the barrier layer. When no bias voltage is applied between the first n-type semiconductor layer and the second n-type semiconductor layer, the multiple quantum well layer is depleted, and the first n-type semiconductor layer and the second n-type semiconductor layer are depleted. When a bias voltage is applied between the first n-type semiconductor layer and the first n-type semiconductor layer,
Carriers are injected from the n-type semiconductor layer to the base subband level of the multiple quantum well layer, and a bias voltage applied between the first n-type semiconductor layer and the second n-type semiconductor layer is changed. A semiconductor light modulation device that modulates input light by changing a transition probability of carriers excited from the base subband level to the excitation subband level. 12. The semiconductor optical modulator according to claim 10, wherein the well layer or the barrier layer has an n-type conductivity and an impurity concentration of 1 × 10 ¹⁷ cm ⁻³ or more. element. 13. The semiconductor optical modulation device according to claim 10, wherein the excitation sub-band level is a third quantum level among the sub-band levels. 14. The semiconductor light modulation device according to claim 10, wherein a wavelength of the input light is in a 1.3 μm band or a 1.55 μm band. 15. A semiconductor laser device that outputs a laser beam, and a semiconductor light modulator that is provided on an output side of the laser beam and that modulates and outputs an input laser beam, wherein the semiconductor light modulator includes: An n-type semiconductor layer formed on the substrate and containing GaN; and a well layer and a barrier layer each containing GaN are alternately stacked on the n-type semiconductor layer, and have a plurality of well-type potentials. A multi-quantum well layer; and a Schottky electrode formed on the multi-quantum well layer. In a state where no bias voltage is applied between the n-type semiconductor layer and the Schottky electrode, the multi-quantum well layer The well layer is depleted, and in a state where a bias voltage is applied between the n-type semiconductor layer and the Schottky electrode, the n-type semiconductor layer has a base sub-band of the multiple quantum well layer. The carrier is injected into the n-type semiconductor layer and the bias voltage applied between the n-type semiconductor layer and the Schottky electrode is changed to change the transition probability of the carrier excited from the ground sub-band level to the excited sub-band level. A transmitting device for optical communication, wherein the input laser light is modulated by varying the wavelength. 16. A semiconductor laser device that outputs laser light, and a semiconductor light modulation device that is provided on the output side of the laser light and that modulates and outputs the input laser light, wherein the semiconductor light modulation device A first n-type semiconductor layer formed on the substrate and containing GaN; and a well layer and a barrier layer each containing GaN are alternately stacked on the first n-type semiconductor layer. A multi-quantum well layer having a well-type potential, and a second n-type semiconductor layer formed on the multi-quantum well layer and having a lower conduction band energy than the barrier layer. When no bias voltage is applied between the n-type semiconductor layer and the second n-type semiconductor layer, the multiple quantum well layer is depleted, and the first n-type semiconductor layer and the second n-type semiconductor layer are depleted. Bias voltage between semiconductor layer The applied state, the first n
Carriers are injected from the n-type semiconductor layer to the base subband level of the multiple quantum well layer, and a bias voltage applied between the first n-type semiconductor layer and the second n-type semiconductor layer is changed. An optical communication transmitting apparatus for modulating an input laser beam by changing a transition probability of a carrier excited from the base subband level to the excitation subband level. 17. A semiconductor laser device that outputs laser light, and a semiconductor light modulation device that is provided on an output side of the laser light and that modulates and outputs an input laser light, wherein the semiconductor light modulation device An n-type semiconductor layer formed on the substrate and including ZnSe; and a well layer and a barrier layer each including ZnSe are alternately stacked on the n-type semiconductor layer, and have a plurality of well-type potentials. A multi-quantum well layer; and a Schottky electrode formed on the multi-quantum well layer. In a state where no bias voltage is applied between the n-type semiconductor layer and the Schottky electrode, the multi-quantum well layer The well layer is depleted, and in a state where a bias voltage is applied between the n-type semiconductor layer and the Schottky electrode, the base sub-band of the multiple quantum well layer is separated from the n-type semiconductor layer. Carriers are injected into the level, and the bias voltage applied between the n-type semiconductor layer and the Schottky electrode is changed to change the level of the carrier excited from the base subband level to the excitation subband level. An optical communication transmitter, wherein an input laser beam is modulated by changing a transition probability. 18. A semiconductor laser device that outputs laser light, and a semiconductor light modulation device that is provided on an output side of the laser light and that modulates and outputs the input laser light, wherein the semiconductor light modulation device A first n-type semiconductor layer formed on the substrate and containing ZnSe; and a well layer and a barrier layer each containing ZnSe are alternately stacked on the first n-type semiconductor layer. A multi-quantum well layer having a well-type potential, and a second n-type semiconductor layer formed on the multi-quantum well layer and having a lower conduction band energy than the barrier layer. When no bias voltage is applied between the n-type semiconductor layer and the second n-type semiconductor layer, the multiple quantum well layer is depleted, and the first n-type semiconductor layer and the second n-type semiconductor layer are depleted. Bias between semiconductor layer In a state where pressure is applied, the first n
Carriers are injected from the n-type semiconductor layer to the base subband level of the multiple quantum well layer, and a bias voltage applied between the first n-type semiconductor layer and the second n-type semiconductor layer is changed. An optical communication transmitting apparatus for modulating an input laser beam by changing a transition probability of a carrier excited from the base subband level to the excitation subband level. 19. A collimating lens provided between the semiconductor laser element and the semiconductor light modulation element, for converting the laser light into a plane wave, and provided on an output side of the semiconductor light modulation element, 16. A focus lens for condensing output light output from the element.
19. The optical communication transmitting device according to any one of 18.