TRAPPED ATOMS IN CAVITY QED
Chapter 6 Chapter 6 Cold Atoms and High Finesse Microcavities - Experimental
6.4 The Laser System, Cavity Locking and Het- erodyne Detection of the Intracavity Field erodyne Detection of the Intracavity Field
6.4.1 Ti:Sapphire Laser
Our optical tahlc is alrnost literally divitled in half, witlt all of the cooling anti trapping work don<: on one side. and. tlie majority of optics rieedecl to prod~lce arid detect the sirglephoton intracavity field occ~ipyirlg the other. At tire ireart of the experiment is a Coherent 8'39 ring Ti:Sappliire laser which is purnped by a Coherent, Verdi single frequency diode purnpeci solid state laser at 532 nm (int,racavit,y doubled Nd:YAG).
~ e w FOCUS 6200 locking diode laser (836 nm)
transfer cavity 1
APD to lock
wM #2
$ y ~ - 9
physics cavity-
to physicscavity locking 150 MHz (nom.) Zdiode laser
probe for Cs I
spectrometer
, + 200 MHz wM
240 MHz ("om.)
spectrometer 4 \
locked on w4.5
through the resonance
I I cavity
local oscillator cleaning cavity
I
Laser (852 nm)
-
s mw ot LO power 1 Ibalanced
' heterodyne detection @
Cti
+ 10 MHz (nominal)TUI Optics FORT diode +lo0 MHz laser (869 nm) (arbitrary)
Figme 6.18: The conlplernentary side of tile table to t,he t,rapping lasers is shown here, including t,ht: Ti:Sapph, locking diode and trapping diode lasers. This figure should bt: useti in cor~jurrction with Fig. 6.20; which explains tilo tirble layout in frequency space. The optical table (8 feet x 16 feet) is now get,ting so dense with optics that, t,wo 8 foot x 2 foot hreaciboards and rulmerous plat,fornls are rnount,ed iii a. "second layer" arourid the table.
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The history of our 899 Ti:Sapphire laser lias been well-documented 1158, 68, 159;.
The Verdi replaced a Coherent Irnnova-100 20W Ar+ ion laser about 1 year ago, and lias been an excellerli replacerncrit as a purnp laser for several reasons. The following chart compares arid contrasts the two lasers as pump sources for a Coherent 899 Ti:S;ipph based or1 roughly oxie year of exyericrice with the Verdi. Though ~liost of
Tclhlc 6.6: A comparison of the Cohererit Innova-100 h r ion laser and Colierent Verdi as pump soiu-ces for a Coherent 899 Ti :Sapphire laser.
issue dai1.y set-up daily follow up electrical poxver coolirig
maintenaxice power needed ot,her
tlie corrirrients liero itre qualitative, t.iiis ci1ar.t gives an idea of sonie of the practical advantages of the solid state punip over the gas laser. 1x1 addition, some quantitative measurements were rriadc of thc iritensity noise of the Ti:Sapph laser outpnt under different pump conditions, wliich are presented in below in Fig. 6.6. It is clear that, the ion laser purnpecl Ti:Sappli has about 30 dB rriore noise out to about 600 kHz compared to x Ti:Sapph purnped by a Verdi. Furthermore, the peals at 100, 200 and 300 kHz in the latter case also appear on tile intensity spectrum of tlie Verdi on its own. Bot,Ii pump lasers lraw the sarrie c:ffect, on the Ti:Sapph out beyond about 3 or
Coherent Verdi 1 hr. (bet1111 pointingj riever neeti to touch 120 V/10 A
Dieslab chiller new diodes in 4 yrs.
4 W
a
532 nm no stri~y fieldscompact,, sits on table quiet in operation
4 li1l-I~. It is important to understanti that this intensity noise directly wril;ten onto the Ti:Sappllire light will evcritiially coritaniinate the purity of rlie Ti:Sapph-derived LO for the ba1arict:ti iieterodyni: clet,ection set-up, and it is clearly preferable t,o be
Coherent Innova-100 Ari
5 hrs. to szabiiize power! pointing and mode tweak once every 2 hrs. after warn1 up 480Vj 20
h
tliree phase, special wiring Calt,ecli chilled water, significant plumbing new tube every 1.5 yrs., clean every 3 mons.8 W typical 62 514 nm (all lines)
st,ray magnet,ic fields ( p l a n i s confining) large framelpower supply? sit,s under table noisy power supply
pumping with tlie Verdi for this reason.
Having already conipleteci a discussiori of the diode laser systenis for the MOTS, I will now proceed ~vitli a brief overview of tlie "second h a l f of the optical table.
The 120 rrilV Ti:SappEi outpnt is first double-passed through an external acousto-
0 200 400 600 800 '1
Founer Frequency [kHz] L c ~ c t r n n t s D O ~ S S
0 2 4 6 8 \ . , ,,,,x,,,,..,,, Fourier Frequency [MHz1
0 : 50 $00 150
t ! Fourier Frequency LMHrl
"*/I*
Figurc 6.19: Arr interesting sories of urc;~~l~rcrnerrt~s discussetl in tlie text s21o\v that the excliangt: of tlic Innova-100 Ar+ iorr for a Vercli solid state pump laser had a r~lajor irrippact on tho iriier~ity noise spectrunr of the Ti:Sapphire. Clearly, tlie best irrrprove rnerit occurrt:tl in a bandwidth up to about 1 MHz as in (a), while the improvcrrreilts are loss cirarriatic ht:yoiid this freclucrrcy in (h) arid (c).
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optical rrlodulator (A011) fc~r a iasge band~vidth (1-100 HA) loop to its frequency servol and tlieri split into four diff(;rent paths. The first pat,li is sellt to a 100 k1-Iz wide spatial-lllocle-cIcaning cavity and prociuces the approsi~~~at,ely 4 lrlW local of oscillator beam necessary for the balanccci heterodyne detection setup. This light is ai a frequcnc:.y of 40 ALIIz abow the Cs D2 (6Sir2> F = 4 -i 6pYl2, F = 5) resoliiu~t frequency at .A,$...5
=
X;,,,,,,;
= 852.359 iim.The secontl path is tlic locking patii for the Ti:Sapphire, which gets conpled i~?to the 20 kHz wide "t,ransfer ca,vity." ,211 error signal for the I'ouncl-Drever-Hall j52]
t,ype lock is derived usirrg the refiected light froni the cavity and is fed back to the esterrtal AOM, the tweeter mirror, the etalons and the Breavster platre galvo inside tlie laser cavity. The purpose of the transfer cavity will be explained more fully in Sec. 6.4.2.
Tlie third path goes to a Cs spect,rometer .Nbich uses a modulation transfer sat- urated absorption spectroscopy [l52] t,echmique plus an inlra-spectrornet,er AOM a i -80 MIIz to offset-lock the "trarisfer cavity." Incidentally. this -80 AIHz A 0 h I is the source of the -1-40 LIHL offset of t,he Ti:Sapphire from the Cs resonance.
The firral pat11 is the light that ever~tually gets coupled into the pliysics cavit,y It first gets upshifted by +200 \I& wit11 tlie first order beam frorn ail AOM t60 produce a carrier freqilancy of ~ 4 - 5
+
240 = q,,,,,,, t 240 IVIHZ. It is tliell sent through a I~oriiematle broadi~ancl trmlling waw: c:lectro-optic rnodlilator (TWkf); driver1 at 240 kffHz ant1 denoted for our pluposes liere as TW11#1. The lower sideband of TWM#lj wliicll is denotcd as t,he "probe light" ilt a frequency v,,,,,,,,:, is now rionlinnily on resonance wit21 the Cs D2 line and can be exactly tuned sirriply by changing the rf frequency of the syiitliesizer (ITP 8656B) wllicli tirives tlie modillator. Finthermore, the intensity of this light can be easily corrtrolled by cllar~girig the ~nodulatiorr index via the aniplit~icte of the sarrle synthesizer. The details of the co~lplirlg of the light into the cavity !rave been explained in Scc. 6.3.4 and t,hc detection teclinicpes will be tiiscussed shortly in Sec. 6.4.3.There 11ave been several ongoing issues with the Ti:Sapph'ire laser which should he rnerrtioned. To begin with, the crystal itself is not in tile greatest corictition, and a
112
replacemerit is expected 'o result, in an imrrletliate 50% irnprovemer~t in output power jl6O]. Oile problerri xipas that tiis cooliug water for the crystal had been far too cold in the past, allowing rrroisturc from the air in tile lab to condense on the crystal and leitvt: dirty cieposits upon evaporixtion. Tile coolirlg linc is presently phmihed into tlie sarne Neslab chiller used to cool t,he htiseplate for the Verdi laserl and is kept at a constarkt t,em1ier:it,m.c of 18 "C (above thc: lab tcrriperatnre of 16 "C). It is also t,he case that the Ti:Sapphire needs to he cleaned approximately once every two leeks urider contirl~~ous me. The alcohols (acetone; methanol) and &rasive cleaning rriotioris tend to take their t-011 on tire lifetiine of the optics, and plaris are under way t,o put, the whole laser in a box under slightly positive pressure of dry air to keep tile dust arid moisture out. This has beer1 sho~vrl in ollier labs t,o recluce tile required clearling to as little as once? every 3 or 4 months. The laser itself is in excellerit shape, however;
as was proven by a Coherent service technician [I601 who was able to get t,he laser to scan the specified 25 GHz by sett,irlg all of t,he feedforward gains correctly. This is something that needs to be done every time the laser is cleaned.