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Message ID: 15496     Entry time: Mon Jul 20 19:21:16 2020
 Author: anchal Type: Summary Category: ALS Subject: Few proposals for Voyager ALS

I've added 4 proposed schemes for implementing ALS in voyager. Major thing to figure out is what AUX laser would be and how we would compare the different PSL and AUX lasers to create an error signal for ALS. Everywhere below, 2um would mean wavelengths near 2 um including the proposed 2128nm. Since it is not fixed, I'm using a categorical name. Same is the case for 1um which actually would mean half of whatever 2 um carries.

Higher Harmonic Generation:

• We can follow the current system of ALS with using 1.5 times PSL frequency as AUX instead of second harmonic as 1 um is strongly absorbed in Si.
• To generate 1.5 times PSL frequency, three stages would be required.
• SHG: Second Harmonic Generation mode matched to convert 2um to 1um. If we are instead making 2 um from 1um to start with, this stage will not be required.
• SFG: Sum Frequency Generation mode matched to sum 2um photon and 1um photon to give 0.65 um photon.
• DPDC: Degenerate Parametric Down Conversion mode matched to convert 0.65 um to 1.3 um (which would be 1.5 times PSL frequency).
• To compare, we can either convert pick-off from PSL to AUX frequency by doing the above 3 stages (Scheme II).
• Or we can just do SHG and SFG at PSL pick-off and do another SHG at AUX end (Scheme I) to compare the AUX and PSL both converted to 0.65 um (which would be 2 times AUX and 3 times PSL frequency).
• This method would have added noise from SHG, SFG and DPDC processes along with issues to be inefficiency of conversion.

Arbitrary AUX frequency:

• We can get away with using some standard laser near 1.5 um region directly as AUX. Most probably this would be 1550 nm.
• What's left is to devise a method of comparing 1.5 um and 2um frequencies. Following are two possible ways I could think of:

Using a frequency comb:

• Good stable frequency combs covering the wavelength region from 1.5 um to 2 um are available of the shelf.
• We would beat PSL and transmitted AUX separately with the frequency comb. The two beat note frequencies would be:
$\Delta_\text{PSL} = \nu_\text{PSL} - \nu_{CEO} - m_1 \nu_\text{Rep}$
$\Delta_\text{AUX} = \nu_\text{AUX} - \nu_{CEO} - m_2 \nu_\text{Rep}$
• Here, m1 and m2 represent the nearest modes (comb teeth) of frequency comb to PSL and AUX respectively.
• Carrier Envelope Offset frequency ($\nu_{CEO}$) can be easily generated by using an SHG crystal in front of the Frequency comb. This step is not really required since most of the modern frequency combs now comb with inbuilt zero $\nu_{CEO}$ stabilization.
• Mixing above beatnotes with $\nu_{CEO}$ would remove $\nu_{CEO}$ from them along with any noise associated with $\nu_{CEO}$.
• Further, a Direct Digital Synthesis IC is required to multiply the AUX side RF signal by m1/m2. This finally makes the two RF signals to be:
$\nu_{A} = \nu_\text{PSL} - m_1 \nu_{Rep}$
$\nu_{B} = \frac{m_1}{m_2}\nu_\text{AUX} - m_1 \nu_{Rep}$
• Which on mixing would give desired error signal for DFD as :
$\nu_\text{PSL} - \frac{m_1}{m_2}\nu_\text{AUX}$
• This method is described in Stenger et al. PRL. 88, 073601 and is useful in comparing two different optical frequencies with a frequency comb with effective cancellation of all noise due to the frequency comb itself. Only extra noise is from the DDS IC which is minimal.
• This method, however, might be an overkill and expensive. But in case (for whatever reason) we want to send in another AUX at another frequency down the 40m cavity, this method allows the same setup to be used for multiple AUX frequencies at once.

Using a Transfer Cavity:

• We can make another more easily controlled and higher finesse cavity with a PZT actuator on one of the mirrors.
• In the schematic, I have imagined it has a triangular cavity with a back end mirror driven by PZT.
• Shining PSL from one side of the transfer cavity and employing the usual PDH, we can lock the cavity to PSL.
• This lock would require to be strong and wide bandwidth. If PZT can't provide enough bandwidth, we can also put an EOM inside the cavity! (See this poster from Simon group at UChicago)
• Another laser at AUX frequency, called AUX2 would be sent from the other side of the cavity and usual PDH is employed to lock AUX2 to the transfer cavity.
• So clearly, this cavity also requires coatings and coarse length such that it is resonant with both PSL and AUX frequencies simultaneously.
• And, the FSS for AUX2 needs to be good and high bandwidth as well.
• The transmitted AUX2 from the transfer cavity now would carry stability of PSL at the frequency of AUX and can be directly beaten with transmitted AUX from the 40m cavity to generate an error signal for DFD.
• I believe this is a more doable and cheaper option. Even if we want to do a frequency comb scheme, this could be a precursor to it.

Using Mode Cleaner cavity as Transfer Cavity:

• If we coat the mode cleaner cavity mirrors appropriately, we can use it to lock AUX2 laser (mentioned above).
• This will get rid of all extra optics. The only requirement is for FSS to be good on AUX2 to transfer PSL (MC) stability to AUX frequency.
• I've added suggested schematic for this scheme at the bottom.

 Attachment 1: VoyagerALSSchemes.pdf  270 kB  Uploaded Tue Jul 21 18:19:17 2020
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