Differences between revisions 7 and 20 (spanning 13 versions)

Frequency Offset Locking (FOL) for Dual Wavelength Laser Stabilisation

Overview

In order for the Arm Length Stabilization (ALS) system to be effective in its purpose, the beat frequency between the AUX lasers and the PSL must be within the efficient working range of ALS (< 50 MHz). Thus, our purpose in Frequency Offset Locking (FOL) is to design a feedback-control loop that will keep this beat frequency well within the working range of ALS, so that manual tuning of auxiliary (AUX) laser frequencies may be avoided.

Frequency offset locking works by sampling light from each laser source, PSL and AUX laser, and combining them to produce a beat note which corresponds to the difference between the frequencies of either laser.

The combined PSL and AUX light is sensed at an RF photodiode and an RF frequency counter is used to measure the beat frequency. A digital PID control loop compares the detected beat frequency with the desired beat frequency to produce an error signal. The error signal is then converted back to an analog signal, to actuate on the temperature of the crystal in the AUX laser to keep its frequency within the desired range.

Schematic

A schematic of the setup for FOL is shown below:

Tasks

Step 1

Laying out the fiber

Find out the fiber mode
Designate space for the setup at end tables and at PSL
Design telescopes for coupling light into the fiber
Layout the fiber through 40m in insulated pipes

Shopping list

Component	Quantity needed/procured	Part number	Order status	As of
70m PM fiber	4/3	?	Procured	Oct 7
6 axis fiber mount	6/?	?	?	Oct 7
Fiber couplers	6/?	?	?	Oct 7
HR1064 mirrors	?/plenty	Procured	Oct7
Lenses for telescope	?/lens kit available	?	Oct7

-  ⇤ ← Revision 7 as of 2014-07-15 16:20:28 → 
  Size: 1375
  Editor: ManasadevithirugnanasambandamATligoDOTorg
  Comment:
+   ← Revision 20 as of 2014-10-08 03:44:04 → ⇥
  Size: 1976
  Editor: ManasadevithirugnanasambandamATligoDOTorg
  Comment:
-Deletions are marked like this.
+Additions are marked like this.
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-= FREQUENCY OFFSET LOCKING =
+= Frequency Offset Locking (FOL) for Dual Wavelength Laser Stabilisation =
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-'''Overview'''
+== Overview ==
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-In order for Absolute Length Measurement to be effective, the differential frequency (beat frequency) between prestabilized laser (PSL) and Auxiliary Laser (AUX) must remain constant.
+In order for the Arm Length Stabilization (ALS) system to be effective in its purpose, the beat frequency between the AUX lasers and the PSL must be within the efficient working range of ALS (< 50 MHz). Thus, our purpose in Frequency Offset Locking (FOL) is to design a feedback-control loop that will keep this beat frequency well within the working range of ALS, so that manual tuning of auxiliary (AUX) laser frequencies may be avoided.
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-However, over time, the AUX frequency tends to wander due to things like temperature change. Thus, we employ our frequency offset locking (FOL) system.
+Frequency offset locking works by sampling light from each laser source, PSL and AUX laser, and combining them to produce a beat note which corresponds to the difference between the frequencies of either laser.
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-Frequency offset locking works by sampling light from each laser source, PSL and AUX, and combining them, in-line, to produce a beat note. The value of which corresponds to the difference between the frequencies of either laser.
+The combined PSL and AUX light is sensed at an RF photodiode and an RF frequency counter is used to measure the beat frequency. A digital PID control loop compares the detected beat frequency with the desired beat frequency to produce an error signal. The error signal is then converted back to an analog signal, to actuate on the temperature of the crystal in the AUX laser to keep its frequency within the desired range.
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-This light is then fed into a photodiode, the signal from which is digitized, and sent into a digital PID control loop. The PID Controller uses this signal as the current state of the system, which is further analyzed to produce an error signal.
+== Schematic ==
A schematic of the setup for FOL is shown below:
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+Line 14:
-The error is then converted back to an analog signal, which actuates upon the frequency of the AUX laser, keeping it within the desired range.
+{{attachment:FOLschematic.png||height=600}}
-Line 15:
+Line 16:
-The actuator is a temperature control, which controls the dimensions of the crystal resonator within the NPRO via thermal expansion.
+== Tasks ==
-Line 17:
+Line 18:
-Through this scheme, we hope to effectively achieve a constant frequency offset between the AUX and PSL.
+=== Step 1 ===
'''Laying out the fiber'''
 *Find out the fiber mode
 *Designate space for the setup at end tables and at PSL
 *Design telescopes for coupling light into the fiber
 *Layout the fiber through 40m in insulated pipes
-Line 19:
+Line 25:
+'''Shopping list'''
||''Component''||''Quantity needed/procured''||''Part number''||''Order status''||''As of''||
||70m PM fiber||4/3||?||Procured||Oct 7||
||6 axis fiber mount||6/?||?||?||Oct 7||
||Fiber couplers||6/?||?||?||Oct 7||
||HR1064 mirrors||?/plenty||Procured||Oct7||
||Lenses for telescope||?/lens kit available||?||Oct7||
-Line 20:
+Line 33:
-{{attachment:projectPlan.png}}
+=== Step 2 ===
'''
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+Line 36:
-[[\Fiber characterization]]
+== Other links ==
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+Line 38:
-[[\Fiber coupling]]

[[\Beat Note Setup]]
+[[\Frequency Counter Characterization]]