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Message ID: 4940     Entry time: Tue Jul 5 17:38:46 2011
Author: kiwamu 
Type: Update 
Category: ABSL 
Subject: ABSL laser frequency-locked 

In this past weekend the ABSL laser was successfully frequency-locked to the PSL laser with a frequency offset of about 100 MHz.

In the current setup a mixer-based frequency discriminator is used for detection of the beat-note frequency.

 


Setup for frequency locking

 The diagram below shows the setup for the frequency locking.

RFsetup.png

(UGF)
  According to a brief check of the loop oscillation it implies that the UGF is around 40 kHz.
Of course I will measure the open-loop over a wide frequency range at some point.
The lock was quite robust and it was able to stay locked for more than an hour as far as I observed.
 
(Beat-note detection and its amplification)
 The fast RFPD, which is standing on the PSL table to detect the beat-note (see the picture on #4939), showed the signal level of -16 dBm.
Then the signal goes through the RF amplifier stage to have an appropriate power level at the mixer in the frequency detection stage.
Before the signal goes into the amp. stage I put a power splitter so that I can see a signal on a spectrum analyzer.
 
(Frequency detection)
 In the mixer-based frequency discriminator, the length difference between two coax. cable was chosen to be 1 m.
This length difference gives us a zero cross point (operation point) of ~100 MHz and linear range of ~ +/-100MHz, which can sufficiently cover the FSR of PRC: 22MHz.
 
(Control filter)
 After the signal goes through the freq. discriminator, a low noise amplifier, ITHACO 1201 was installed as a control filter.
The reason I chose ITHACO 1201 was that it has a larger output range than that of SR560.
ITHACO 1201 can go to +/- 15V, which means the ABSL laser should be able to track the frequency by about +/- 65 MHz without a thermal actuation.
Right now a single pole was put at 1 Hz for an easy lock.
The output from 1201 goes directly to the laser PZT, whose input is on the front panel of the laser controller.
 

 

Temperature setpoints

 During the work I found three temperature points, where we can observe the beat-note signal within a bandwidth of 1 GHz.
 Here is a summary of the temperature set points :
    PSL temperature = 31.71 deg
    ABSL temperature = 44.19 deg (not good),
                                       47.25 deg (chosen to be nominal set point),
                                       50. 23 deg
 The first one (44.19 deg) wasn't good because changing the temperature of the ABSL laser also changes the amplitude of the beat-note significantly.
In a worst case the signal completely disappeared when the ABSL laser was at a certain temperature close to 44.19 deg.
The other two temperature points seemed good. I eventually chose the second one (47.25 deg) for the nominal set point.
Note that the current readout has been 1.81 A on the laser controller.
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