Finally lock my homemade diode laser onto the super stable cavity!
It is not that easy to do this because the cavity is a ULE cavity with finesse as high as 300,000. The line width of this little monster is as low as 5kHz, which is far below the typical ECDL bandwidth. (So the PDH dispersive signal we get from the oscilloscope is actually representing the line width of.our unlocked laser!) We also need to fine tune the loop filter and stabilize our modulators to get a stable lock.
Our homemade high speed photo detector. Powered by a LEMO connector with very soft silica cable. The shielding is done by CNC the copper box, which works really good that no interference from the modulator can be seen at the PD output in the spectrum analyzer down to -90dBm.
In order to further determine the actual line width, we need to setup another set of laser and measure the beat notes between the two lasers.
Our preliminary scheme is to collect the beat notes with a high speed photodetector and down covert it to kilohertz region using a mixer. The final result will be readout by a dynamic analyzer (SR785) and a frequency counter. The signal generator, dynamic analyzer and the frequency counter will all be synchronized to a rubidium frequency standard.
P.S. Our GIANT SR785 just arrived!