Shalika, Mitsuhashi-san,

 

In continuation of the experiment in elog, this elog covers the following aspects:

1. Aligning the optics for transmitted beam path of the cavity

2. Aligning the end mirror of the cavity. 

(Details below)

 

1. To begin with, we set the lens (located before the input mirror of the cavity). Since we achieved diffraction efficiency of the AOM at 38%, we first set the path for the transmitted beam and placed the photodiode outside the chamber. The transmitted beam was extended to reach the photodiode using a mirror and a low focal length lens (see Image 1). The output of the photodiode was connected to the oscilloscope to keep a constant check on our setup. The position of  the mirror used was optimized to observe a constant voltage on the oscilloscope. 

2. Furthermore, we placed the end mirror and aligned it properly so that the beam is incident at its center. We made sure that the reflected and the incident beam overlapped and we could simultaneously see the reflected beam from the Faraday Isolator(FI) (see Image 2)

 

Next step;

We will align the input mirror of the cavity and tune its position to continue observing the reflection of the beam at the FI. We will place the RF photodiode at the FI. 

 

Also, we are listing the steps to refer to while trying to set up a cavity (for anyone who tries this for the first time in the future, like us). 

1. Make sure that the incident beam entering the chamber is not clipping in any way. The chamber's window is usually small and it could happen if you are not careful. 

2. Place the lens for your cavity (located before the input mirror of the cavity). This position shall be left unchanged from here on. 

3. If you are using an AOM and the diffraction efficiency is low, you won't see any transmitted beam from the mirrors. In such a case you need to set the transmitted beam path before you place the highly reflective end or input mirrors. 

4. Make sure that the transmitted beam is reaching the photodiode outside the chamber. You can use a mirror and also place a lens(of less focal length) to ensure that the transmitted beam is incident on the photodiode even after you place the cavity mirrors in the subsequent steps. 

5. Optimise the position of the mirror placed before the photodiode (from 3) by using a connection to the oscilloscope. The voltage observed would be stable for an optimum position of the mirror. You can tune its pitch and yaw to observe this. 

6. Place the end mirror of the cavity at the desired position. Using a sensor card observe the reflected beam from this mirror. You can also see a shadow of the incident beam on the sensor card.

   a. Try to align the mirror to overlap the reflected and incident beam as much as possible before fixing the mirror.

   b. For minor adjustments at the end you might use the screws on the mirror frame to adjust the pitch and yaw.

But it's better to overlap as much as possible when you place the lens in the first part. Also, this step is an iterative one. You might have to do the "a" step again after "b" if the incident beam is not centered or is vulnerable to a possible clipping from the next steps. 

7. For the optimum position when the incident and reflected beam overlaps you can see the reflected beam from the Faraday Isolator. 

8. Place your input mirror as in step 6 and ensure you can still see the reflected beam from the Faraday Isolator. 

 

We will mention the steps on how to lock the laser to the cavity in our future elogs.