Differences between revisions 4 and 57 (spanning 53 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:

Project Tasks

Tasks	Who	Status	Last updated
General tasks
Make procurements list and purchase stuff	MT	Done	Oct 9
Find out the fiber mode with collimator	MT	Waist of output beam = 210um	Oct 16
Y End laser fiber setup
Waist of beam from doubling crystal	MT	Elog 10287 --> Waist = 40.2um	Oct 9
Designate space for the setup at end tables	MT	~30cm of propagation distance available	Oct 13
Design telescopes for coupling light into the fiber	MT	converging lens f=12cm	Oct 13
Assemble end telescope and couple light into fiber	MT	Status - elog 10638	Oct 24
Layout the fiber along 40m arms in insulated tubes	MT+help	Laid out	Nov 5
PSL fiber setup
Waist of beam from doubling crystal	MT	Elog 10287 --> Waist = 43.5um	Oct 23
Designate space on PSL table	MT	Lot of space available	Oct 23
Find out the fiber mode with collimator	MT	Waist of output beam = 176um	Oct 23
Design telescopes for coupling light into the fiber	MT	f_1=75mm@z=10.4cm+f_2=-1m@z=32.3cm with target @z=40cm	Oct 23
Assemble end telescope and couple light into the fiber	MT	Temporary telescope in place	Nov 5
X End laser fiber setup
Waist of beam from doubling crystal	MT
Designate space for the setup at end tables	MT
Design telescopes for coupling light into the fiber	MT
Assemble end telescope and couple light into fiber	MT
Layout the fiber along 40m arms in insulated tubes	MT+help	Laid out	Nov 21
Beat note setup
Make fiber chassis layout	MT	Done	Nov 21
Setup fibers in the fiber chassis	MT
Couple fiber to fiber	MT
Combine PSL and end laser with appropriate polarization (50/50 --> 90/10)	MT
Detecting beatnotes at the PD	MT
Frequency counter setup
Hook up frequency counter	MT
Check FC acquisition scripts	MT
Read beat frequency on StripTool	MT
Mount FC module on IOO rack	MT+help
End laser thermal actuator characterization
Digital servo
MISC tasks
Behavior of PM fiber to temperature fluctuations
Polarization controller characterization

Procurements list

Component	Quantity needed/procured	Part number	Order status
70m PM fiber	4+1/4+1	Corning PM98-U25A	Procured
6 axis fiber mount	4/4	Thorlabs K6XS	Procured
Fiber insulating tubes	1/1	-	Procured
Polarization controller	1/1	Acrobat PCM4102-333	Procured
Fiber coupled PD	2/2	Thorlabs Menlo FPD310	Procured
Frequency counter	4/2	Minicircuits UFC6000	Procured
OPTICS & OPTOMECHANICS
Fiber mounting adapters	4/4	Thorlabs SM1FCA	Procured
Fiber collimators	6/6	Thorlabs CFC-2X-C	Procured
Fiber collimator mounts	6/6	Thorlabs AD9.5F	Procured
HR1064 mirrors	used mirrors in stock	CVI Y1 mirrors	Procured
Lenses for telescopes	used lenses in stock	?	Procured
Fiber BS for splitting PSL light between the arms(PM = PM+PM in 50-50)	1/1	Afw Tech POBC-64--C-(s)PM-7-2-25 dB	Procured
PM fiber mating sleeves PM FC/APC & (FC/APC)	6,4/0	Thorlabs ADAFCPMB2&ADAFCB3	Procured
PM and SM fiber patch cables	multiple lengths	Thorlabs	Procured
Fiber BC for beat note between arm and PSL (PM+PM=SM)	2/2	Afw tech POBC-64-C-1-7-2-25 dB	Procured
Fiber BS for splitting combined light between PD and pick off (SM 10-90)	2/2	Afw Tech FOSC-1-64-10-C-(s)3m-H64-2	Procured
Fiber chassis	1/1	Front Panel Express	Procured

Notes

The transimpedance gain for the Menlo FPD 310 is hard to find. After a few emails with the company's technical team, the information we have now is that it has 50ohm RF transimpedance, and an RF amplifier with a gain of +42 dB. Assuming an InGaAs sensitivity of 0.65A/W @ 1064nm, this means that we get 4092 V/W at the nominal gain setting. If the attenuation setting on the PD is set to 20dB, the number becomes 409 V/W. This PD is actually obsolete as per the Thorlabs catalog. The manual FPD310-Manual.pdf I found from the Thorlabs website is in the Attachments section of this page. The more modern version of the manual, , is also attached.

-  ⇤ ← Revision 4 as of 2014-07-14 18:27:54 → 
  Size: 4038
  Editor: AndrewhallATligoDOTorg
  Comment:
+   ← Revision 57 as of 2018-05-05 04:10:31 → ⇥
  Size: 5820
  Editor: GautamvenugopalanATligoDOTorg
  Comment:
-Deletions are marked like this.
+Additions are marked like this.
 Line 1:
-= FREQUENCY OFFSET LOCKING =
+= Frequency Offset Locking (FOL) for Dual Wavelength Laser Stabilisation =
 Line 3:
-'''Overview'''
+== Overview ==
 Line 5:
-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.
 Line 7:
-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.
 Line 9:
-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.
 Line 11:
-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:
-Line 13:
+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.
+----
-Line 17:
+Line 18:
-Through this scheme, we hope to effectively achieve a constant frequency offset between the AUX and PSL.
+== Project Tasks ==
||'''Tasks'''||Who||Status||Last updated||
||||||''General tasks''||
||Make procurements list and purchase stuff||MT||Done||Oct 9||
||Find out the fiber mode with collimator||MT||Waist of output beam = 210um ||Oct 16||
||||||''Y End laser fiber setup''||
||Waist of beam from doubling crystal||MT||Elog 10287 --> Waist = 40.2um|| Oct 9||
||Designate space for the setup at end tables||MT||~30cm of propagation distance available||Oct 13||
||Design telescopes for coupling light into the fiber||MT||converging lens f=12cm||Oct 13||
||Assemble end telescope and couple light into fiber||MT||Status - elog 10638||Oct 24||
||Layout the fiber along 40m arms in insulated tubes||MT+help ||Laid out||Nov 5||
||||||''PSL fiber setup''||
||Waist of beam from doubling crystal||MT||Elog 10287 --> Waist = 43.5um|| Oct 23||
||Designate space on PSL table||MT ||Lot of space available||Oct 23||
||Find out the fiber mode with collimator||MT||Waist of output beam = 176um ||Oct 23||
||Design telescopes for coupling light into the fiber||MT||f_1=75mm@z=10.4cm+f_2=-1m@z=32.3cm with target @z=40cm||Oct 23||
||Assemble end telescope and couple light into the fiber||MT||Temporary telescope in place ||Nov 5||
||||||||''X End laser fiber setup''||
||Waist of beam from doubling crystal||MT|| || ||
||Designate space for the setup at end tables||MT|| || ||
||Design telescopes for coupling light into the fiber||MT|| || ||
||Assemble end telescope and couple light into fiber||MT|| || ||
||Layout the fiber along 40m arms in insulated tubes||MT+help ||Laid out||Nov 21||
||||||''Beat note setup''||
||Make fiber chassis layout|| MT || Done || Nov 21||
||Setup fibers in the fiber chassis||MT || || ||
||Couple fiber to fiber||MT|| || ||
||Combine PSL and end laser with appropriate polarization (50/50 --> 90/10)||MT|| || ||
||Detecting beatnotes at the PD||MT || || ||
||||||||''Frequency counter setup''||
||Hook up frequency counter||MT || || ||
|| Check FC acquisition scripts||MT || || ||
||Read beat frequency on StripTool|| MT|| || ||
||Mount FC module on IOO rack|| MT+help|| || ||
||||||''End laser thermal actuator characterization''||
||||||''Digital servo''||
||||||''MISC tasks''||
||Behavior of PM fiber to temperature fluctuations|| || || ||
||Polarization controller characterization|| || || ||
-Line 19:
+Line 58:
-<<TableOfContents(4)>>
<<BR>>
<<BR>>
<<BR>>
--------
+== Procurements list ==
||'''Component'''||'''Quantity needed/procured'''||'''Part number'''||'''Order status'''||
||70m PM fiber||4+1/4+1||Corning PM98-U25A||Procured||
||6 axis fiber mount||4/4||Thorlabs K6XS||Procured||
||Fiber insulating tubes||1/1||-||Procured||
||Polarization controller||1/1||Acrobat PCM4102-333|| Procured||
||Fiber coupled PD||2/2||--(Thorlabs)-- Menlo FPD310||Procured||
||Frequency counter||4/2||Minicircuits UFC6000||Procured||
||'''OPTICS & OPTOMECHANICS'''||
||Fiber mounting adapters||4/4||Thorlabs SM1FCA||Procured||
||Fiber collimators ||6/6||Thorlabs CFC-2X-C||Procured||
||Fiber collimator mounts||6/6||Thorlabs AD9.5F||Procured||
||HR1064 mirrors||used mirrors in stock||CVI Y1 mirrors||Procured||
||Lenses for telescopes||used lenses in stock||?||Procured||
||Fiber BS for splitting PSL light between the arms(PM = PM+PM in 50-50)||1/1||Afw Tech POBC-64--C-(s)PM-7-2-25 dB|| Procured||
||PM fiber mating sleeves PM FC/APC & (FC/APC)||6,4/0||Thorlabs ADAFCPMB2&ADAFCB3||Procured||
||PM and SM fiber patch cables||multiple lengths||Thorlabs||Procured||
||Fiber BC for beat note between arm and PSL (PM+PM=SM)||2/2||Afw tech POBC-64-C-1-7-2-25 dB||Procured||
||Fiber BS for splitting combined light between PD and pick off (SM 10-90)||2/2||Afw Tech FOSC-1-64-10-C-(s)3m-H64-2||Procured||
||Fiber chassis||1/1||Front Panel Express||Procured||
== Other links ==
-Line 25:
+Line 80:
-== Optics ==
+[[\Frequency Counter Characterization]]
-Line 27:
+Line 82:
-=== Purpose ===

To implement a system that will efficiently and effectively sample and beat light from both PSL and AUX, to act as an input to the digital PID loop, which will actuate temperature servos in the AUX lasers.

=== Methods ===

==== Razor Blade Measurement ====

Razor Blade, or knife edge beam profiling, is a technique which allows manual measurement of a gaussian beam's properties (read: beam waist).
A razor blade, or other reliably straight edge, is translated across a beam which is then incident upon a photodiode or power meter. As the blade is translated, corresponding measurements of blade location and power supplied to photodiode are taken. The measurements should then be repeated in the direction orthogonal to both optical axis, and original direction or translation, to give a two dimensional measurement of spot size. This measurement is repeated many times along the optical axis, preferably somewhere near the beam waist. If this is impossible, a lens may be used to create a new waist (w_f), from which the original beam waist may be calculated using ABCD matrices.

The blade position vs beam power data is then fit to V(x) = (.5*Vmax)*(1-erf((sqrt(2)*(x-x0))/wz))+c . x0 and c are offsets in x and V, correspondingly, with the independent variable being x. The other fit parameters being Vmax (maximum power available to photodiode), and wz, the spot size at the current value of z.

The spot size data is then fit to w(z) = w_o*sqrt(1 + ((z-b)lambda) / (pi*w_o^2))^2), where z is the independent variable (blade position along optical axis), b corresponds to an offset from zero in the x, lambda the laser wavelength, and w_o the beam waist. b and w_o are fit parameters that should be returned, lambda should be known (1064nm in our case).

This technique returns beam waist and location, relative to some point along z chosen to be zero.

==== Beam Profiler ====

Measurement using a beam profiler essentially replaces the razor blade setup, and takes measurements of spot size far more efficiently.
The profilers is translated along the optical axis, where values of wz and z should be taken, again preferably near the beam waist.
These data are then fit in the same way as in the razor blade measurement (see above) to yield a beam waist and location.

=== Measurements ===

==== Razor Blade ====

The razor blade measurement proved difficult, and ended up requiring six trials.
Details of the final round of measurement are included here:

''Setup''

{{attachment:RazorBladeSchematic.png|alt txt|width=500 height = 200}}

''Data''

[[/RazorBladeData | Razor Blade Data]]

==== Beam Profiler ====

== Electronics ==
+== Notes ==
 * The transimpedance gain for the Menlo FPD 310 is hard to find. After a few emails with the company's technical team, the information we have now is that it has 50ohm RF transimpedance, and an RF amplifier with a gain of '''+42 dB'''. Assuming an InGaAs sensitivity of 0.65A/W @ 1064nm, this means that we get '''4092 V/W''' at the nominal gain setting. If the ''attenuation'' setting on the PD is set to 20dB, the number becomes '''409 V/W'''. This PD is actually obsolete as per the Thorlabs catalog. The manual [[attachment:FPD310-Manual.pdf]] I found from the Thorlabs website is in the Attachments section of this page. The more modern version of the manual, , is also attached.