Specifications

18 Chapter 4. Operation
4.3 External modulation
Thelaserdiodeinjectioncurrentcanbemodulateddirectly,orvia
theSMA RF inputonthelaserheadboard(seesectionB.3). The
combined modulation bandwidth extends from DCto about2.5GHz,
providedthestandardconnectionfromheadboardtodiodeisre-
placedwithasuitablecoaxialcable. Evenhigherfrequenciescan
beused withadditionof anappropriatemicrowavebias-tee suchas
theMinicircuitsZFBT-6G+,betweenthelaserheadboardandthe
diode.
Directmodulation iscommonly usedforfrequency stabilisation,e.g.
the frequency modulation sideband method [8,9], Pound-Drever-
Hall[10],andalsoforoffsetlockingschemes[11,12]. Microwave
modulationisoften usedfor two-frequencypumpingofalkaliatoms,
for exampleto accessbothalaser coolingtransition andarepump
to prevent trapping in dark states [4,13,14].
Themodulationefficiencycanbeenhancedbymatchingtheexternal
cavitylengthtothemodulationfrequency. Thatis,setthecavity
lengthL = c/2ΩwhereΩisthemodulationfrequency. Thecavity
lengthcanbeadjustedslightlybyslidingthecollimationtubeinthe
monolithic block.Forexample, to access the
87
Rb hyperfine ground
states, separated by 6.8GHz, the cavity length could be 2.2cm and
the modulation at 6.8GHz, or 4.4cm with modulation at 3.4GHz so
that the two sidebands are used and the carrier is off-resonant.