JPH09166764A - Absorption type semiconductor light modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an absorption type semiconductor light modulator having an excellent optical modulation band without deteriorating of extinction ratio by making an electrode a traveling-wave type electrode. SOLUTION: A P-side electrode is made to be a traveling-wave type electrode. A polyimidelayer 8 is provided on the incident side of an optical signal, an electrode on its upper part becomes the input part I of an electric signal and an electric signal (reversely biased voltage) is inputted from a driving signal source. On the other hand, an output part II is provided on an electrode on the output side of the optical signal and a terminal resistor is connected to the output part. Namely, when light is made incident and propagates through a MQW(multiple quantum well) optical modulator, the electric signal also propagates in the same direction as light over the total length. Since the absorption end wavelength of a MQW core 4 is shifted to the side of long wavelength by the electric signal during the travel of both signals together, the light is absorbed and becomes the OFF state. By matching the velecities of the electric signal with the optical signal, the length of a semiconductor core absorbing the light at the time of the OFF state of light is lengthened and super-high speed light modulation is obtained while keeping high extinction ratio.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a semiconductor optical modulator,
In particular, it relates to an absorption type semiconductor optical modulator that operates at high speed.

[0002]

2. Description of the Related Art FIG. 3 shows an example of a conventional absorption type semiconductor optical modulator. In the figure, 1 is a p-side electrode, 2 is p ⁺ -InGaA
s cap layer, 3 is a p-InP clad layer, and 4 is a core. Here, i-InGaAlAs which is not doped is used.
A (13 nm) / InAlAs (5 nm) multiple quantum well (MQW) is shown as an example. Reference numeral 5 represents an n-InP clad layer, 6 represents an n-InP substrate, and 7 represents an n-side electrode. 8
Is a polyimide layer, and in the figure, the portion of the p-side electrode 1 above the polyimide layer 8 is called a bonding pad. A bonding wire 9 is applied with an electric signal (voltage) from a drive signal source (not shown).

In order to explain the operating principle of this conventional example,
FIG. 4 shows the optical absorption spectrum of the MQW core. For example,
When the wavelength of the signal light is 1.55 μm, the absorption peak is designed to be about 1.49 μm. Then, as shown in the figure, when the voltage is not applied to the optical modulator, the operating wavelength, that is, the wavelength of the optical signal is farther to the long wavelength side than the absorption edge wavelength as shown by the solid line A. Therefore, the incident light is emitted without being absorbed by the MQW core 4, and the light is turned on. On the other hand, when the reverse bias is applied, the absorption spectrum moves to the long wavelength side as shown by the dotted line B, so that the optical signal is absorbed by the MQW core 4 and, as a result, the light is turned off.
State.

Now, the p-side electrode 1 of the absorption type semiconductor optical modulator shown as the conventional example is an electrode for lumped constant operation. In order to explain this, an equivalent circuit including a drive signal source is simplified and shown in FIG. S _G is a drive signal source, R _G
Is the characteristic impedance of the drive signal source, R _L is the terminating resistance,
C _MQW is the capacitance of the MQW core, and C _p is the capacitance of the bonding pad that is a part of the p-side electrode described above. The electric 3 dB band Δf in this case is approximately

[0005]

[Number 1] Δf = 1 / (π · R L · C MQW) can be expressed as (1). Generally, the terminating resistance R _L is 50Ω which is the same as the characteristic impedance R _G of the drive signal source. It is assumed here that the pad capacitance C _p is small enough that the total capacitance is approximately determined by the MQW core capacitance C _MQW . By the way, the thickness d, the width w and the length L of the i-MQW core are 0.2 μm and 2 μm, respectively.
And 300 μm, the capacitance of the MQW core 4 is

[0006]

## EQU2 ## It is obtained from C _MQW = ε ₀ ε _r wL / d (2). Here, ε ₀ and ε _r are the permittivity in vacuum and the relative permittivity of the MQW core 4, respectively. From equations (1) and (2), it can be seen that the electric 3 dB band Δf in the lumped constant MQW optical modulator is 20 GHz or less. To widen this 3 dB band, C
_{Although it is sufficient} to make the _MQW small, if the length of the i-MQW core 4 is made short, the extinction ratio of the optical signal deteriorates. That is, the increase in the absorption coefficient of the i-MQW core 4 due to the voltage application is Δα,
When the confinement coefficient of the guided light in the i-MQW core 4 is Γ, the extinction ratio R of the optical signal is represented by the following equation.

[0007]

## EQU00003 ## R = exp (-. DELTA..alpha..GAMMA..multidot.L) (3) As can be seen from this equation, when the length L is shortened, the extinction ratio deteriorates. Therefore, from the viewpoint of the extinction ratio, the i-MQW core 4 The length cannot be too short.

As can be seen from the structural diagram of FIG. 3 and the equivalent circuit of FIG. 5, the electrode of the conventional example is a lumped constant electrode, and the electric signal and the light do not run in the same direction.

[0009]

As described above, in the conventional absorption type semiconductor optical modulator having the lumped constant type electrode, there is a severe trade-off between the electric 3 dB band Δf limited by the CR constant and the extinction ratio R. I have a relationship. Therefore, there is a problem that it is difficult to realize ultrahigh-speed optical modulation of 50 GHz or higher while maintaining a high extinction ratio.

Therefore, an object of the present invention is to solve these problems and provide an absorption type semiconductor optical modulator excellent in the optical modulation band.

[0011]

In order to achieve the above object, an absorption type semiconductor optical modulator according to the present invention comprises a semiconductor core having an absorption edge wavelength shorter than the wavelength of an optical signal, and the semiconductor core when a voltage is applied. In an absorption type semiconductor optical modulator including an electrode for voltage application for absorbing the optical signal in the semiconductor core by moving the absorption edge wavelength to a long wavelength side, the electrode is a traveling waveform electrode. It is characterized by

Here, the characteristic impedance of the drive signal source for applying a voltage to the electrodes and the characteristic impedance of the optical modulator may be equal.

A non-doped semiconductor layer may be provided between the semiconductor core and the upper clad layer above it, or a non-doped semiconductor layer may be provided between the semiconductor core and the lower clad layer below it. . Further, it is preferable that a non-doped semiconductor layer is provided between the semiconductor core and the upper clad layer above the semiconductor core and between the lower clad layer below the semiconductor core.

Further, it is preferable that the equivalent refractive index of the signal light and the equivalent refractive index of the electric signal are substantially equal.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a voltage is applied to move the absorption edge wavelength of the semiconductor core to the long wavelength side, thereby causing the voltage application electrode for absorbing an optical signal to the semiconductor core to have a traveling waveform. It is used as an electrode. More specifically, an electric signal (voltage) input section is provided on the optical signal incident side, while an electric signal output section is provided on the optical signal outgoing side. With such a configuration, the electric signal and the optical signal run in parallel. Such a configuration of the present invention eliminates the problem that the electric 3 dB band Δf is limited by the CR constant.
Therefore, especially if the speed matching of the electrical signal and the optical signal is taken,
It is possible to increase the length of the semiconductor core that absorbs light when the light is turned off, and it is possible to realize ultrahigh-speed optical modulation while maintaining a high extinction ratio.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an embodiment of the present invention. In the figure, as in the conventional example, 1 is a p-side electrode, 2 is p ⁺ -InG
aAs cap layer, 3 is a p-InP clad layer, and 4 is a core layer. In the present embodiment as well, i-InGaAlAs is used.
Take (13 nm) / InAlAs (5 nm) multiple quantum wells (MQW) as an example. 5 is an n-InP clad layer, 6
Is an n-InP substrate, and 7 is an n-side electrode. Reference numeral 8 is a polyimide layer, and 10 is an i-InP layer. Such a structure
It can be manufactured by an ordinary semiconductor device manufacturing technique.

Here, the most different point of this embodiment from the conventional example shown in FIG. 3 is that the p-side electrode 1 is a traveling waveform electrode. The polyimide layer 8 is provided on the incident side of the optical signal, the upper electrode thereof serves as an input portion I for the electric signal, and the electric signal (reverse bias voltage) is input from a drive signal source (not shown). On the other hand, the output part II is provided on the electrode on the output side of the optical signal.
Is provided, and a terminating resistor is connected to this output section. That is, while the light is incident and propagates through the MQW optical modulator, the electric signal also propagates in the same direction as the light over the entire length of the optical modulator. While the both run in parallel, the absorption edge wavelength of the MQW core is shifted to the long wavelength side by the electric signal, so that the light is absorbed and turned off.

In the present embodiment, the p-side electrode 1 is a traveling waveform electrode, but since the characteristic impedance of the electric signal source S _G is 50Ω, the characteristic impedance of the absorption type MQW is also equal to or close to 50Ω. Is desirable. Therefore, in this embodiment, the i-InP layer 10 is used as the MQW core 4.
And the p-InP clad layer 3 are provided between the p-side electrode 1 and
The characteristic impedance of the MQW modulator including the above is set to 50Ω. The value of the terminal resistance connected to the output part II of the electrode 1 is 50Ω.

FIG. 2 shows an equivalent circuit of this embodiment. S _G is the drive signal source, R _G is the characteristic impedance of the drive signal source,
R _L is the terminating resistor, Z _M is the characteristic impedance of the optical modulator. As can be seen from the comparison with FIG. 5, this equivalent circuit is very different from the equivalent circuit of the conventional lumped constant optical modulator.

For the sake of simplicity, the 3 dB modulation band Δf of this traveling-wave absorption semiconductor optical modulator has an electric propagation loss of the traveling-wave electrode of 0 and a characteristic impedance Z _M of 50.
Assuming Ω,

[0022]

Δf = 1.4c ₀ / (π (| n _m −n ₀ |) L) (4) Here, c ₀ is the speed of light, _nm is the equivalent refractive index of the electric signal of the MQW optical modulator, n ₀ is the equivalent refractive index of the signal light, and L is the length of the MQW core. Therefore, L is 30
When 0 μm, _nm are 3.4, and n ₀ is 3.2, Δf is about 2200 GHz, which means that ultrawideband optical modulation can be realized.

Further, if the speed matching of the electric signal and the optical signal is taken, that is, if n _m = n ₀ , even if the length L of the MQW core 4 is increased to increase the extinction ratio R of the light. ,
The band limiting factor is only the electrical propagation loss of the electrodes. Since the electric propagation loss can be reduced by increasing the thickness of the electrodes, the ultra-wide band operation of the optical modulator can be realized.

In the present embodiment, an optical modulator having a so-called high-mesa structure in which the entire laminated structure of the lower cladding layer 5 and above has a ridge shape and the side surface of the MQW core is exposed to the atmosphere is shown. Of the structure, only a part of the i-InP layer 10, the upper clad layer 3, the InGaAs cap layer 2 and the p-side electrode 1 may have a ridge shape, that is, a so-called strip loading type.

In this embodiment, the i-InP layer 10 is provided between the MQW core 4 and the upper clad layer 3, but i
The -InP layer may be provided between the MQW core 4 and the lower clad layer 5. By doing so, it is possible to avoid an increase in the signal voltage for generating a depletion layer in the MQW core, even when the purity of the undoped MQW core and the InP layer is not sufficiently high. Of course, non-doped InP layers may be provided above and below the MQW core.

In the present invention, since the electrode may be a traveling waveform electrode, it goes without saying that the p-side and n-side electrodes may have any configuration, and a semi-insulating substrate may be used as the substrate. Further, the core 4 is i-InGaAsP / I
Other MQW compositions such as nP may be used, or i-InGaAs
It is also possible to use a quaternary bulk composition such as P.

[0027]

As described above, according to the present invention,
By making the electrode a traveling waveform electrode, it is possible to realize an absorption type semiconductor optical modulator excellent in the light modulation band without deteriorating the extinction ratio.

[Brief description of the drawings]

FIG. 1 is a diagram showing the structure of an embodiment of an absorption type semiconductor optical modulator according to the present invention.

FIG. 2 is an equivalent circuit diagram for operating the embodiment of FIG.

FIG. 3 is a diagram showing a structure of a conventional absorption type semiconductor optical modulator.

FIG. 4 is a diagram illustrating an operation principle of a conventional example.

FIG. 5 is an equivalent circuit diagram for operating a conventional example.

[Explanation of symbols]

1 p-side electrode 2 p ⁺ -InGaAs cap layer 3 p-InP clad layer 4 i-InGaAlAs / InAlAs multiple quantum well (MQW) core 5 n-InP clad layer 6 n-InP substrate 7 n-side electrode 8 polyimide layer 9 bonding Wire 10 i-InP layer

Claims (6)

[Claims] 1. A semiconductor core having an absorption edge wavelength shorter than the wavelength of an optical signal, and for moving the absorption edge wavelength of the semiconductor core to a longer wavelength side when a voltage is applied, thereby absorbing the optical signal in the semiconductor core. 2. An absorption type semiconductor optical modulator having an electrode for voltage application according to claim 1, wherein said electrode is a traveling waveform electrode. 2. The absorption type semiconductor optical modulator according to claim 1, wherein the characteristic impedance of a drive signal source for applying a voltage to the electrode is equal to the characteristic impedance of the optical modulator. 3. The absorption-type semiconductor optical modulator according to claim 2, wherein a non-doped semiconductor layer is provided between the semiconductor core and an upper clad layer above the semiconductor core. 4. The absorption type semiconductor optical modulator according to claim 2, wherein a non-doped semiconductor layer is provided between the semiconductor core and a lower clad layer below the semiconductor core. 5. The absorption-type semiconductor light according to claim 2, wherein a non-doped semiconductor layer is provided between the semiconductor core and an upper clad layer above the semiconductor core and between a lower clad layer below the semiconductor core. Modulator. 6. The equivalent refractive index of the signal light and the equivalent refractive index of the electric signal are substantially equal to each other.
5. An absorption type semiconductor optical modulator according to any one of 1.