Laser diode driver with back terminator and optical transmitter providing the same
An LD-Driver with a back termination circuit is disclosed. The back termination circuit of the invention provides a transistor as an active device, a current source to provide a bias current to the transistor, and a resistor as a passive element that couples the transmission lines carrying a differential signal thereon. Because the transistor shows a differential resistance of several tens of ohms by providing only a few milli-amperes, the output impedance of the LD-Driver may be substantially matched with the characteristic impedance of the transmission line with lesser additional power consumption.
1. A laser diode driver that provides a driving current to a semiconductor laser diode enclosed in a transmitter optical subassembly, comprising:
a pair of output terminals each outputting said driving current in a differential mode to a transmission line connecting said laser diode driver with said transmitter optical subassembly; and
a back terminator provided between said output terminals, said back terminator comprising a pair of circuit blocks and a passive device, each circuit block being connected to one of said output terminals and including a series circuit of an active device and a current source to provide a bias current to said active device, said passive device connecting said circuit blocks,
wherein said active device in each said circuit block is a bipolar transistor whose collector and base are short-circuited to operate as a forwardly biased diode.
2. The laser diode driver of claim 1 , wherein said current source in said circuit block is a variable current source.
3. The laser diode driver of claim 1 , wherein said back terminator further provides an additional passive device directly connecting said output terminals of said laser diode driver.
4. The laser diode driver of claim 3 , wherein said additional passive device has an impedance at least ten times greater than an input impedance of said transmitter optical subassembly.
5. The laser diode driver of claim 1 , wherein said back terminator has an impedance substantially equal to twice a characteristic impedance of said transmission line.
6. The laser diode driver of claim 5 , wherein said active device has an impedance substantially equal to half of a characteristic impedance of said transmission line, subtracted by an impedance of said passive device.
7. An optical transmitter, comprising:
a transmitter optical subassembly that includes a semiconductor laser diode to emit signal light by being provided with a driving current;
a pair of transmission lines each connected to said transmitter optical subassembly; and
a laser diode driver to provide said driving current in a differential mode to said transmission lines, said laser diode driver providing a pair of output terminals each coupled with one of said transmission lines and a back terminator provided between said output terminals, said back terminator comprising a pair of circuit blocks and a passive device, each circuit block being connected to one of said output terminals and including a series circuit of an active device and a current source to provide a bias current to said active device, said passive device connecting said circuit blocks,
wherein said active device in said circuit block is a bipolar transistor whose collector and base are short-circuited to operate as a forwardly biased diode.
8. The laser diode driver of claim 7 , wherein said current source in said circuit block is a variable current source.
9. The laser diode driver of claim 7 ,
wherein said back terminator further provides an additional passive device directly connecting said output terminals of said laser diode driver.
10. The laser diode driver of claim 9 , wherein said additional passive device has an impedance at least ten times greater than an input impedance of said transmitter optical subassembly.
11. The laser diode driver of claim 7 , wherein said back terminator has an impedance substantially equal to twice a characteristic impedance of said transmission line.
12. The laser diode driver of claim 11 ,
wherein said active device has an impedance substantially equal to half a characteristic impedance of said transmission line; subtracted by an impedance of said passive device.
13. A laser diode driver that provides a driving current to a semiconductor laser diode enclosed in a transmitter optical subassembly, comprising:
a pair of output terminals each outputting said driving current in a differential mode to a transmission line connecting said laser diode driver with said transmitter optical subassembly; and
a back terminator provided between said output terminals, said back terminator comprising a pair of circuit blocks and a passive device, each circuit block being connected to one of said output terminals and including a series circuit of an active device and a current source to provide a bias current to said active device, said passive device connecting said circuit blocks,
wherein said back terminator further includes an additional passive device directly connecting said output terminals of said laser diode driver.
14. The laser diode driver of claim 13 ,
wherein said additional passive device has an impedance at least ten times greater than an input impedance of said transmitter optical subassembly.
15. An optical transmitter, comprising:
a transmitter optical subassembly that includes a semiconductor laser diode to emit signal light by being provided with a driving current;
a pair of transmission lines each connected to said transmitter optical subassembly; and
a laser diode driver to provide said driving current in a differential mode to said transmission lines, said laser diode driver providing a pair of output terminals each coupled with one of said transmission lines and a back terminator provided between said output terminals, said back terminator comprising a pair of circuit blocks and a passive device, each circuit block being connected to one of said output terminals and including a series circuit of an active device and a current source to provide a bias current to said active device, said passive device connecting said circuit blocks,
wherein said back terminator further includes an additional passive device directly connecting said output terminals of said laser diode driver.
16. The laser diode driver of claim 15 ,
wherein said additional passive device has an impedance at least ten times greater than an input impedance of said transmitter optical subassembly.