Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-08-22
2002-08-20
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S237000, C359S238000, C257S728000
Reexamination Certificate
active
06437899
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an opto-electric conversion semiconductor device, and more particularly to, an opto-electric conversion semiconductor device having a superior opto-electric conversion characteristic over a wide frequency range for optical communication.
2. Background Art
An improvement in the performance of an opto-electric conversion semiconductor device for interconnecting a terminal electric line and an optical-fiber network plays an important role in attaining prevalence of a public communication network using optical fibers.
High-speed modulation for responding to an increase in the volume of transmitted information is indispensable for improving the performance of a semiconductor laser device or a photodiode that serves as an opto-electric conversion device. Further, an important requirement is that the semiconductor device or the photodiode has a good opto-electric conversion characteristic over a wide frequency range.
In order to effect high-speed modulation of a semiconductor laser device, an external modulation method is employed. According to this method, in order to enable long-distance transmission of a signal while reducing variations in the wavelength of the signal, which would occur during modulation, a signal is modulated by being passed through an optical modulator which is capable of controlling the amount of light passing therethrough through on-off operation in accordance with an optical signal, by means of causing a semiconductor laser to oscillate at a given intensity.
Control of the amount of light passing through the optical modulator through on-off operation, which is employed for the external modulation method, is achieved by means of the Franz-Keldysh effect, which arises when a reverse electric field is applied to an absorbing layer of an optical modulator, or by means of the quantum confinement Stark effect.
In the optical modulator, the absorption of a laser beam is changed according to a voltage to be applied to the optical modulator. Hence, a modulated signal voltage is applied to a high-frequency electric circuit connected to the optical modulator, and the laser beam to be output from an exit-end face of the optical modulator is subjected to intensity modulation in accordance with the signal voltage.
In a case where such an optical modulator is connected to a high-frequency electric circuit for enabling propagation of a modulated signal voltage, impedance matching must be ensured over the frequency range from d.c. to a modulation frequency, because the modulated signal voltage usually assumes a rectangular waveform. To this end, the amount of reflection attenuation, which would arise at a modulation frequency, must be increased, and the optical modulator must have a cut-off frequency sufficiently exceeding the modulation frequency.
The external modulation method encounters difficulty in establishing optical coupling between an optical modulator and a semiconductor laser and requires a large number of components, which adds to cost. In order to eliminate such a difficulty, there has been developed an optical modulator integrated semiconductor laser device which is formed by monolithic integration of a semiconductor laser and an optical modulator.
As in the case where the optical modulator is connected to a high-frequency electric circuit, the optical modulator integrated semiconductor laser device requires impedance matching over a wide frequency range from d.c. to a modulation frequency.
The foregoing description applies to a device for converting an electric signal into an optical signal. However, a photodiode for converting an optical signal into an electric signal also requires impedance matching over a wide frequency range from d.c. to a modulation frequency.
FIG. 25
is a plan view showing a commonly-known optical modulator described in, for example, Japanese Patent Laid-Open No. 38373/1999.
In
FIG. 25
, reference numeral
200
designates an optical modulator;
202
designates an optical modulation element;
204
designates a high-frequency electric circuit;
206
designates a high-frequency substrate;
208
designates a transmission line;
210
designates a matching circuit;
212
designates an open stub; and
214
designates a metal wire.
Arrow
216
designates incident light which corresponds to continuous light and originates from a laser; arrow
218
designates signal light modulated by the optical modulation element; and arrow
220
designates an electric signal which is applied to the optical modulation element
202
in the form of variations in a voltage by way of the high-frequency electric circuit
204
.
The optical modulator
200
comprises the high-frequency electric circuit
204
having the matching circuit
210
formed from the open stub
212
; the optical modulation element
202
provided at the tip end of the high-frequency electric circuit
204
; and the metal wire
214
for connecting the optical modulation element
202
with the high-frequency electric circuit
204
.
The commonly-known optical modulator
200
having the foregoing configuration operates as follows:
Upon efficient receipt of the incident light
216
by way of a coupling optical system (not shown), the optical modulation element
202
modulates the intensity of the incident light
216
in accordance with a variation in the voltage of the electric signal
220
by way of the Matching circuit
210
of the high-frequency electric circuit
210
and the metal wire
214
, thereby emitting the signal light
218
.
In this case, before entering the metal wire
214
, the electric signal
220
is subjected to impedance matching performed by the matching circuit
210
formed from the open stub
212
.
The open stub
212
corresponds to a capacitive matching circuit, and impedance matching is effected by means of only the matching circuit
210
formed from the open stub
212
, and hence impedance matching can be achieved in the vicinity of only a certain frequency. Accordingly, the open stub
212
can improve the modulation characteristic of the optical modulator
200
within a narrow range but encounters difficulty in improving the modulation characteristic of the optical modulator
200
over a wide range.
Impedance matching is commonly achieved by use of only a terminating resistor. Use of only a terminating resistor may lead to matching for d.c. However, because of a parasitic capacitance of the optical modulation element
202
or the inductance of the metal wire used for electrical connection, an impedance mismatch arises in, particularly, a high frequency range, thereby rendering difficult attainment of a match over a wide frequency range.
The present invention has been conceived to solve the above-described drawbacks and is aimed at providing an opto-electric conversion semiconductor device which comprises a resistive matching circuit connected to a capacitive matching circuit by way of an opto-electric conversion semiconductor element and achieves impedance matching over a wide frequency range.
For reference, Japanese Patent Laid-Open No. 75003/1998 describes a semiconductor laser module for directly receiving a microwave frequency signal as a modulated signal, in which an impedance matching circuit unit utilizing capacitance is interposed between a laser diode chip and a signal input circuit.
Further, Japanese Patent Laid-Open No. 221509/1995 describes an invention which uses, as a terminating resistor formed from a chip resistor, a capacitive matching circuit for canceling the inductance component of a chip resistor.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, an opto-electric conversion semiconductor device comprises a semiconductor element for opto-electric conversion of a signal which has an signal input electrode. A high-frequency electric signal circuit is provided which has one end positioned in proximity with the semiconductor element. The high-frequency electric signal circuit has a connection point in the one end at a location closest to the signal in
Epps Georgia
McDermott & Will & Emery
Mitsubishi Denki & Kabushiki Kaisha
Thompson Tim
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