Apparatus and method for modulating a laser beam and sensing the optical power thereof

1. An apparatus for determining an optical power of a laser beam during modulation, comprising:

an electro-absorption driver;

an electro-absorption modulator configured to modulate the laser beam according to a signal from the electro-absorption driver;

a back current detector comprising a first current detector connected between a first voltage source and the electro-absorption driver to detect a first current flowing to the electro-absorption driver, and a second current detector connected between the electro-absorption driver and a second voltage source to detect a second current flowing from the electro-absorption driver, the first and the second voltage sources powering the electro-absorption driver, the back current detector configured to measure, based on the detected first and second currents, a back current generated by the electro-absorption modulator in response to modulating the laser beam;

a memory storing a transfer function; and

a processor in communication with the memory and the back current detector, the processor configured to determine the optical power of the laser beam based on the back current and using the transfer function.

2. The apparatus of claim 1 , wherein the transfer function comprises a back current value correlated with an optical power value.

3. The apparatus of claim 2 , wherein the back current value is a voltage value.

4. The apparatus of claim 1 , wherein the processor is configured compare the back current to the transfer function to determine the optical power of the laser beam.

5. The apparatus of claim 1 , wherein the transfer function is calibrated for the electro-absorption modulator.

6. The apparatus of claim 1 , wherein the first voltage source is a high-voltage source, and the second voltage source is ground.

7. The apparatus of claim 1 , wherein the first and second current detectors each comprise:

a resistor connected in series between the electro-absorption driver and the voltage source;

an analog difference amplifier electrically connected to the resistor for measuring a voltage difference across the resistor; and

an analog-to-digital converter for digitizing the voltage difference.

8. The apparatus of claim 7 , comprising a precision current source between the voltage source and the electro-absorption driver.

9. A method for determining an optical power of a laser beam during modulation, comprising:

projecting the laser beam onto an electro-absorption modulator;

generating a signal with a driver;

modulating the laser beam with the electro-absorption modulator according to the signal;

receiving a back current at the driver, the back current in response to the laser beam being projected onto the electro-absorption modulator; and

measuring the back current responsive to a first current received by the electro-absorption driver from a high-voltage source, and a second current provided by the driver to a low-voltage source; and

determining the optical power of the laser beam based on the back current and using a transfer function in a memory.

10. The method of claim 9 , wherein measuring the first and second currents comprises measuring a voltage drop across a resistor, respectively.

11. The method of claim 9 , wherein determining the optical power comprises comparing the back current to a value in the transfer function to identify the optical power.

12. The method of claim 9 , wherein said measuring the back current comprises taking the difference between the first current and the second current.