Monitoring of a laser source with front and rear output photodetectors to determine frontal laser power and power changes over laser lifetime
View Patent ↗A power monitoring and correction to a desired power level of a laser or group of lasers utilizes two photodetectors which are employed to accurately determine the amount of output power from the front end or “customer” end of a laser or a plurality of such lasers. During power detection, which may be accomplished intermittently or continuously, the laser is modulated with a tone of low frequency modulation. One photodetector at the rear of the laser is employed to detect the DC value of the frequency tone, i.e., a value or number representative of the AC peak-to-peak swing, amplitude or modulation depth of the tone. Also, the rear photodetector may be employed to determine the optical modulation index (OMI). In either case, these values may be employed in a closed loop feedback system to adjust or otherwise calibrate the value of the low tone frequency relative to the total desired bias current applied to the laser. A front photodetector is employed to receive a portion of the total output of the laser, or of each laser, and the average output power of the laser, or of each laser, is determined from already knowing the optical modulation index (OMI) via the rear photodetector. Thus, by measuring and/or calibrating the laser OMI with the use of a rear photodetector, the average output power from the front end output can be unambiguously determined from detection of the AC peak-to-peak swing or amplitude of the low frequency tone received via the front photodetector.
1. A monitoring method, comprising the steps of:
superimposing a frequency tone on a bias current to generate a modulation current;
modulating a laser in accordance with the modulation current to generate a first optical signal output from a first facet of the laser and a second optical signal output from a second facet of the laser;
detecting a first output power associated with the first optical signal with a first photodiode;
providing a first detection signal based on the first output power;
determining a first optical attribute based on the first detected signal;
transmitting the second optical signal through a second photodiode;
detecting a second output power associated with the second optical signal with the second photodiode;
providing a second detection signal based on the second output power;
determining a second optical attribute based on the second detection signal; and
determining an average output power associated with the second optical signal based on the first and second attributes,
wherein said first optical attribute is an optical modulation index (OMI) of the frequency tone and the second optical attribute is a modulation depth.
2. The method of claim 1 wherein the modulation depth of the frequency tone is a predetermined percentage of the bias current.
3. The method of claim 2 wherein the predetermined percentage is in a range of about 0.1% to about 10%.
4. The method of claim 1 comprising the further step of calibrating the modulation depth to be a predetermined percentage of the bias current.
5. A monitoring method in accordance with claim 1 , further including a step of modulating the second optical signal with an external modulator.
6. A monitoring method in accordance with claim 5 , wherein the external modulator is a Mach-Zehnder modulator.
7. A monitoring method in accordance with claim 1 , wherein the first and second photodiodes are integrated on a common substrate.
8. A monitoring method in accordance with claim 1 , wherein the first and second photodiodes are discrete components.