Directly modulated laser drive circuit
A driver circuit 11 includes a plurality of cascode-connected NMOS transistors, a modulating signal V GN1 is applied to a gate terminal of a lowermost stage transistor T N1 located at a lowermost stage out of the NMOS transistors, and an upper stage bias potential V GN2 that is a sum of a minimum gate-source voltage V GN1min and a maximum drain-source voltage V DS1max of a transistor (T N1 ) located immediately below an upper stage transistor located at an upper stage above the lowermost stage transistor of the NMOS transistors is applied to the upper stage transistor T N2 .
1. A directly modulated laser driving circuit comprising:
a power supply circuit configured to supply a driving current to a laser diode; and
a driver circuit connected in parallel to the laser diode, configured to bypass the driving current in accordance with an input modulating signal,
wherein the driver circuit comprises a plurality of cascode-connected NMOS transistors,
the modulating signal is applied to a gate terminal of a lowermost stage transistor located at a lowermost stage out of the NMOS transistors, and
an upper stage bias potential that is a sum of a minimum gate-source voltage and a maximum drain-source voltage of a transistor located immediately below an upper stage transistor located at an upper stage above the lowermost stage transistor of the NMOS transistors is applied to a gate terminal of the upper stage transistor.
2. The directly modulated laser driving circuit according to claim 1 , wherein the power supply circuit comprises a constant current source connected between a constant voltage source and the laser diode.
3. The directly modulated laser driving circuit according to claim 1 , wherein the power supply circuit includes a high-frequency choke coil connected between a constant voltage source and the laser diode.
4. The directly modulated laser driving circuit according to claim 1 , wherein the power supply circuit includes a PMOS current control transistor connected between a constant voltage source and the laser diode, and is configured to control the driving current to be constant based on a current control bias potential.
5. The directly modulated laser driving circuit according to claim 4 , further comprising:
an upper stage decoupling circuit including an RC low-pass filter connected between the upper stage bias potential and the gate terminal of the upper stage transistor, and is configured to remove a high-frequency noise component;
a power supply decoupling circuit including an RC low-pass filter connected between the current control bias potential and the gate terminal of the PMOS current control transistor, and is configured to remove the high-frequency noise component; and
a decoupling capacitor connected between the drain terminal of the upper stage transistor and a ground potential.
6. The directly modulated laser driving circuit according to claim 4 , further comprising:
a series circuit including a resistive element and an inductor connected between a source terminal of the lowermost stage transistor and a ground potential;
a capacitor connected between the source terminal of the lowermost stage transistor and the ground potential; and
a high-pass filter including an input resistive element having one end applied with the current control bias potential and the other end connected to a gate terminal of the current control transistor, a capacitive element connected between the gate terminal of the current control transistor and a gate terminal of the lowermost stage transistor, and a series circuit including a resistive element and an inductor connected between the gate terminal of the lowermost stage transistor and the ground potential.
7. The directly modulated laser driving circuit according to claim 6 , further comprising:
an upper stage decoupling circuit including an RC low-pass filter connected between the upper stage bias potential and the gate terminal of the upper stage transistor, and is configured to remove a high-frequency noise component; and
a power supply decoupling circuit including an RC low-pass filter connected between the current control bias potential and the one end of the input resistive element, and is configured to remove the high-frequency noise component.
8. The directly modulated laser driving circuit according to claim 4 , further comprising:
a series circuit including a resistive element and an inductor connected between a source terminal of the lowermost stage transistor and a ground potential;
a capacitor connected between the source terminal of the lowermost stage transistor and the ground potential; and
a high-pass filter including an input capacitive element having one end applied with an in-phase signal of the modulating signal and the other end connected to a gate terminal of the current control transistor, a series circuit including a resistive element and an inductor connected between the one end of the capacitive element and the ground potential, and a resistive element connected between the current control bias potential and the other end of the capacitive element.