JP3568366B2 - Optical pulse generator - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical pulse generator that performs high-speed modulation in a high frequency band.
[0002]
[Prior art]
FIG. 5A is a conceptual diagram showing a conventional optical pulse generator. As shown in the figure, reference numeral 1 denotes a semiconductor gain unit, and 2 denotes an optical modulator. The semiconductor gain unit 1 receives a constant current from an external DC current source 3 to generate an optical output. The optical modulator 2 is connected to a strip line 7 for transmitting a high-frequency voltage from an external high-frequency power supply 4 via a bias tee 6 via a gold wire 8 having an inductance, and applies a high-frequency voltage to the optical modulator 2. This modulates the light intensity. Also, a bias voltage can be applied from the external voltage source 5 via the bias tee 6. When the optical modulator 2 is modulated at a frequency determined by the reciprocal of the optical round-trip time in the optical resonator including the semiconductor gain unit 1 and the optical modulator 2, a short pulse light output is obtained by so-called active mode locking. Reference numeral 9 denotes a resistor (normally 50 ohms) for impedance matching with the strip line 7, and is connected in parallel with the optical modulator 2. FIG. 5B shows an equivalent circuit thereof, which uses a model in which the optical modulator 2 includes a resistor and a capacitor. FIG. 6 shows the calculation results of the frequency characteristics of the response and the return loss of the optical modulator 2.
[0003]
[Problems to be solved by the invention]
However, in such a conventional optical pulse generator, as the frequency increases, the electrical reflection increases, and high-frequency power is not effectively supplied to the optical modulator 2, and an optical pulse can be obtained effectively. There was a problem that it was difficult.
[0004]
The present invention has been made to solve the above-described problems, and has as its object to provide an optical pulse generator having high conversion efficiency in a high frequency band and low electrical reflection.
[0005]
[Means for Solving the Problems]
In an optical pulse generator including an optical modulator that modulates the intensity or phase of light by applying a high-frequency voltage, a line that propagates the high-frequency voltage and the optical modulator are connected by a first conductor having an inductance, The optical modulator and the capacitor are connected by a second conductor having inductance, the other electrode of the capacitor is directly grounded, and the length of the second conductor is set to 5 times the length of the optical modulator. Double or less.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1A is a conceptual diagram showing an embodiment of an optical pulse generator according to the present invention. As shown in the figure, a constant current is injected from an external DC current source 3 into a semiconductor gain section 1 to generate an optical output. The optical modulator 2 having a length of about 100 microns has a strip line 7 for transmitting a high-frequency voltage from an external high-frequency power supply 4 via a bias tee 6 and a gold wire 8 having a length of about 250 microns having inductance. And a metal wire 10 (second conductor) having a length of about 300 microns having an inductance with one electrode of a chip capacitor 11 having a size of about 250 microns arranged near the optical modulator 2. Connected, and the other electrode of the capacitor 11 is directly grounded. The light intensity is modulated by applying a bias voltage from the external DC voltage source 5 to the optical modulator 2 via the bias tee 6 and superimposing a high frequency voltage from the external high frequency power supply 4. When the optical modulator 2 is modulated at a frequency determined by the reciprocal of the optical round-trip time in the optical resonator including the semiconductor gain unit 1 and the optical modulator 2, a short pulse light output is obtained by so-called active mode locking. The capacitor 11 connected to the optical modulator 2 makes it possible to apply a bias voltage from the external DC voltage source 5 via the bias tee 6. The capacitor 11 is provided to prevent a DC component from passing.
[0009]
1 (b) is an equivalent circuit, the characteristic impedance of the strip line 7 50 ohms, the inductance _{L 1} of the gold wire 8 180PH, series resistance R of the optical modulator 2 is 10 ohms, the capacitor C 0. 27pF, the inductance _{L 2} of the gold wire 10 is 220PH, capacitance of the capacitor 11 is set to 100 pF. FIG. 2 shows a calculation result of the frequency characteristics of the response and the return loss of the optical modulator 2. As shown in the figure, it can be seen that the reflection sharply drops around 26 GHz and the modulation response also increases from 16 GHz to around 26 GHz. Frequency reflection is minimized substantially determined by the inductance L ₂ of the capacitor C and the gold wire wire 10 of the optical modulator 2, the frequency is increased the capacitance C and the inductance L ₂ is decreased. Band of the optical modulator 2 is substantially proportional to its length, also the inductance L ₂ is proportional to the length of the gold wire 10, in order to reduce the reflection in the band of the optical modulator 2 It is necessary to reduce the length of the gold wire 10.
[0010]
The frequency at which the reflection is minimized is approximately given by f _res = 1 / (2π√ (L ₂ C)). On the other hand, the modulator band is approximately given by f _m = 1 / (πR _Z C). Here, _RZ is the characteristic impedance of the transmission line, which is usually 50 ohms. The condition of L ₂ = R _Z ² C / 4 is derived from the condition that the frequency where the reflection is minimized and the modulator band are almost equal. The capacity C of the optical modulator ₂ is 0.2 to 0.4 pF per 100 μm of the length of the optical modulator 2, and L2 is calculated to be 125 to 250 pH from the above equation. On the other hand, the inductance _{L 2} of the gold wire 10 is 50~100pH per 100 [mu] m. From these results, by setting the length of the gold wire 10 to be approximately five times or less the length of the optical modulator 2, a circuit configuration with low reflection at high frequency and high modulation efficiency can be realized.
[0011]
The above circuit configuration is similarly effective for an optical modulator alone or an optical modulator integrated with a laser.
[0012]
FIG. 3A is a conceptual diagram showing a reference example of the optical pulse generator according to the present invention. As shown in the figure, in addition to the same configuration as in FIG. 1A, a chip-shaped resistor 9 having a resistance of about 50 ohms is connected in series between the optical modulator 2 and the capacitor 11 to provide a reflection. The configuration is such that a small band is expanded. The equivalent circuit is shown in FIG. FIG. 4 shows the calculation results of the frequency characteristics of the response and the return loss of the optical modulator 2. As shown in the figure, as in the case of the embodiment, a circuit configuration with little reflection in a high frequency band and high modulation efficiency can be realized. The same effect can be obtained by inserting the resistor 9 between the capacitor 11 and the ground.
[0013]
The above circuit configuration is similarly effective for an optical modulator alone or an optical modulator integrated with a laser.
[0014]
【The invention's effect】
As described above, in the optical pulse generator according to the present invention, by connecting a conductor and a capacitor having a length of 5 times or less of the optical modulator to the optical modulator, the modulation efficiency is high in the high frequency band, An optical pulse generator with a small reflection loss can be realized .
[Brief description of the drawings]
FIG. 1A is a conceptual diagram showing an embodiment of an optical pulse generator according to the present invention, and FIG. 1B is an equivalent circuit thereof.
FIG. 2 is a calculation result of a frequency characteristic of a response and a return loss of the optical modulator of FIG. 1;
3A is a conceptual diagram illustrating a reference example of an optical pulse generator according to the present invention, and FIG. 3B is an equivalent circuit thereof.
FIG. 4 is a calculation result of a frequency characteristic of a response and a return loss of the optical modulator of FIG. 3;
5A is a conceptual diagram showing a conventional optical pulse generator, and FIG. 5B is an equivalent circuit thereof.
FIG. 6 is a calculation result of a frequency characteristic of a response and a return loss of the optical modulator of FIG. 5;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Semiconductor gain part 2 Optical modulator 3 DC current source 4 High frequency power supply 5 DC voltage source 6 Bias tee 7 Strip line 8 Gold wire 9 Resistance 10 Gold wire 11 Capacitor

Claims (1)

In an optical pulse generator including an optical modulator that modulates the intensity or phase of light by applying a high-frequency voltage, a line that propagates the high-frequency voltage and the optical modulator are connected by a first conductor having an inductance, The optical modulator and the capacitor are connected by a second conductor having inductance, the other electrode of the capacitor is directly grounded, and the length of the second conductor is 5 times the length of the optical modulator. An optical pulse generator characterized in that the number is not more than twice.

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