Shalika, Mitsuhashi-san

This elog covers the aspects of

1. Attempting to lock the cavity. We were able to lock the cavity for 10s.  

Details:

After proper installation of the new mirrors, we observed the transmitted beam. We scanned the laser by providing a ramp temperature signal (see Fig 1). In the oscilloscope image the colors correspond to the following signals.

CH1-yellow--> Error Signal from mixer

CH2-blue-->Output from PD at Transmitted beam path

CH3-purple--> DC output from RF PD at FI

CH4-green--> Feedback signal from SR560

We then attempted maximizing the TEM00 mode by tuning the pitch and yaw of the cavity mirrors. Since we didn't have access to a camera sensitive to 1550 nm, we had to do the tuning intuitively. We tried maximizing the modes and at the end we were left with only one mode maximized, as in Fig 2. No mode was observed to be maximized at an amplitude more than this, and so we guessed that it could be the TEM00 mode. We then trying locking the cavity by providing the error signal as feedback to the laser. We were able to lock the cavity for 10s (See Fig 3)

How to lock the cavity (This is the approach we took and is also for anyone to refer who tries to attempt this in future for first time like us).

1.Make sure that you have tuned the mirrors to have reflected beam power (at the FI) to be maximum.

2.Observe the transmitted beam and Reflected beam using Photodiodes. The photodiode used to analyze the reflected beam is a RF PD. It has DC and AC output.

3.Scan the laser by providing a temperature ramp signal.

4.Initially you will see a lot of peaks corresponding to higher order modes in the transmitted beam (as in Fig 1). The zero-order beam is embedded among them, and the purpose is to tune the alignment of the mirror to maximize this and suppress other higher order modes as much as you can. Access to a camera can make this work easy, but in case if you don't have access, you may simply maximize all the beams. And at the end you will be left with only TEM00 mode maximized.

5.When you have obtained the most maximized beam (as in Fig 2), you may try to lock

6.Make the demodulation path using AC of PD, mixer and SR560.

         a.Connect the AC of PD to F terminal of mixer.

         b.Set the frequency of mixer same as EOM frequency using function generator (L terminal of mixer)

         c.Provide the demodulation phase shift to the mixer (calculated using 2*pi*L/Lambda, where L is optical length after EOM)

         d.The unfiltered mixer output from terminal I is the error signal. Filter the error signal and provide the output of filter is provided to another SR560

         e.The above output is coupled with a DC offset voltage to provide feedback to the laser.

7.Set the temperature of the laser at which you observed the TEM00 mode during the scan.

8.Tune the offset voltage to see if the cavity can lock.

9.When the cavity is locked you would see the transmitted beam PD signal maintain a constant value same as that around the TEM00 mode amplitude level (as in Fig 3).

10.The error and feedback signal will be varying as an attempt to maintain the lock.

11.To increase the lock time keep tuning the mirror alignment, laser temperature and the offset voltage.

What we will do next:

We will try tuning the alignment, offset voltage and laser temperature to improve lock time.