The Transfer Cavity and Locking Diode Laser

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

6.4.2 The Transfer Cavity and Locking Diode Laser

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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.

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slnt as sitown in Table 6.5. This laser is stabilized relative to v,,,,,,, by way of a t,eciiniilne using the a~~xiliary "trarxfer cavity" meritioried above [161]. This is done by separating tlie 9 rn\V diode lascr output irito two separate patiis. Iri the first path, the diode laser carrier is locked ill reflection usirig a Pouiid-Drevw-l'lail tecltnique to another longitudinal mode of ilie transfer cavity by irrjectiag the light frorn the opposite (:rid of tlrc cavity to tlie Ti:Sa;pptt input port. The second path is sent through a second T\VU (referred to here as TWh1*2), whose purpose is t o "bridge"

the residual frequency gap of typically 150 MHz between the piiysics cavit,y resonance at 836.43 lirn and the transfer cavity resonance closest to this wavelength. This light is mode-nlat,chetl irito the physics cavity arid detected in t,ransmission usirig art APD/transirnpedanco amplifier motinle (EGScG C30998).

Tlte physics cavit,y error sigrtal is derived once again using the Ponnd-Drever-Iiall tmhnique, ~vltere t,he locking sidebands are superimposed as an FM signal on top of t,he rf driving signal sent to the cliode laser T W I " 2 . F41 spectroscopy of the physics cavit,y will yield two error signals out-of-phase with one another, corresponding to each of the TWM#2 sidebands. The carrier produces no error signal. Furtllerrnore, t i e use of a l7W?U[#2 sideband for locking the piiysics cavitv has t,he advant.age of permitting easy frequency and anipiitude turiabilit,~ by way of an rf syntitesizer (HE' ESG 4000A). The use of approximately 10 nW for locking creates a small AC Stark shift of 50 kJlz in v,,,,,,,, anti t,he non-intrusive nature of this lock is further verified by tile absence of a noticeable effect of the atoms on tlie cavity locking error signal

(see Sec. 7.2.8). Tlie perforruance of this lock has been discilssed abo~ic in Sec. 6.3.5.

Fig. 6.20 prt>ssc?ntetl below give? a liictoral represe~itat~iori of wltere all of the laser frequencies reside with respect to one anotl-ier. The middle "physics cavity lirie"

in the diagralri contairis tltc physics cavity longitudinal xilode structure, wliere ttic probe, lock arsd FORT lasers reside at rriodes n

+

2 (836 nm) and n - 2 (869 nm) respectively, centered about rriode n at tlie noniinal probe cvavelerigth of 852 rinr. Tlie FORT laser, which1 will he discusseci extensively in Sec. 7.2.3, is shoxvn here and in Fig. 6.18 for reference. Tire optical ei~rriers of the probe anti lockiilg beams are tied t,ogether in frecll~ency space by the motic: stnsct,lre of the transfer cavity to ~vhich eaclr

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is i~idepeiiclenrly locked. This striicture is shown scl~ematically on the top Peference cavity line" anti reinforces the first key point of this laser stabilization method that the distance in frequency space between tliese two carriers is aiwmys

find,

^connected

by the transfer cavity lorigitiitlinal motle strircture. The bottom "atom line" shoxvs the fixed (by natnre!) atorriic resonance frequency; and tlie frequency of the LO bearn, arhic:li also nex7er c!la,nges fro111 Yro = I / ~ , , , , , , ,

+

40 MHz in orr set-up.

The second point about. Fig. 6.20 is to emphasize its role in helping to uriderstand the trlnability of the system through the use of TWkljfl and TWL4#2 to cont,rol the three frequencies ( u

,,,,,

,<:, u

;,,,,,,,,

Y,,,,~,, ). It is clear from the expanded view of the dashed area that T W L J I ~ ~ can be useti t,o cor~trol the detuning 6,,, = v ~ ~ ~ , > ~ - uatoli, of t,lie probe Iight from tile atomic frequency. The inLensity and detuning of this p\V-level field arc. set by rf syrjthesizt:r control of TW1VI"l; becarise the probe field is the lower sidebarid (LSB#1 in Fig. 6.20) of Ti:Sapph carrier after this modulator.

It is perhaps a hit more subtle to see how TWMSf2 is uyed t,o control the detuning fi,, = Y

.;,,

^{,it? -} u

,,,,,,,,

of the cavity wit11 respect to the aton~ic frequency. The LSB#2 of tlie locking diode laser carrier: whose frequency is deterrninecl by T W k P 2 , is locked to t,he Iongitudinai mode n

+

² of the physics cavity. At the present, neither of the upper sidebancis (USBjf1 and USB#2) of these modnlators are ised. When T W W 2 is tuned (e.g., increased), the lriodes {.

.

. n - 2, n - 1, n , n

+

^{1, n i- 2 . .} .)of the rniddle

"physics cavity line" slide to the left u.it,l~ respect to the bottom "at.om line." Note that the FORT diode laser carrier also "slides" to the left with respect t,o the fixed at,omic freqnency by virtue of the fact that it is locket1 to the physics cdvity (see Sec. 7.2.3). The locking tiiode laser carrier, Ti:Sapph carrier and its LSU"l/USB+l do not mow ~vitli respect to the atorn line becaiise of their fixed relationship t,o the upper "reference cavity line7" w1iile the USB1:2 of the locking diode laser iriovcs to the left as TlVhIk2 is increased (for exarrrple) liere. Hence, mode 7 , , having a fxed relationship to mode n + 2 by ~ i r t n e of tlie fact that they are both modes of the sanie '$lrysics'' cavity; is triried with respect to the atoniic frerluency. Using a cornbination of these two controls, any arbitrary 6,,;, can he accessed, and, for fixed h,,,,, any line profile (see) e.g. Eqs. (7.7) and (7.27)) scarlncd using

distance fixed bv transfer cavity

longitudinal modes of the transfer cavity 1

FORiiascr

~ a n > a i l 8 6 9 "mi

iJatom + 40 MHz = OLO

=fixed LO frequency physics

cavity line

i atom 1

line

-4

wavelength increases ,--,

Figixe 6.20: This figme shoill(l be cox~sitleretl orie of tlic most nseful in tliis tilesis, because it demonstrates exactly liow r~ll of tile frequencies used in the experirnerlt axe generated. rriade tlmablc, arid. rriosi irnportanlly, referericcd to one another. Ttie t,ext, goes tl-irough it ixl cletaill ant1 Fig. 6.18 is a conrplenlentary diagrlun wtiicl-1 shows the physical layout of all of tile lasers on tile opt,ical t,ahle.

distance fixed bf phynics cavity

i

r$

!

w

n-2 n - I i n ' n i l d+2 n+3

longiiudinal rn des ok the physics cavity

, ,

matom = fixed Cs atomic resonance frequency increases (852,359 nml

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Do~vnstream frorn the cavity; dichroic beamsplitters are used t.o separate the beams arid send tlier~i to their respective tlctectors. First? tile 836 ririi liglit is separated arid sent, to the APL) det,ector .sitli am optic ~vllich is apl~roxirnately (85, 15) 7a trarisniis- siori/reflcction at 836 rlln arid better tlia11 99% rcfiectioli at 852 arid 869 nm. The 869 rirrl light is tlicri separated by a second optic wtiicki is approxiriiai-ely (10; 90)

76

traixniission/reflectiori at 869 rim arid (85: 15) % transmissiori]reflecttiori af 852 rim.

The remaining 832 ilrn light, is smit to rhe heterodyne cietectioil set-up.