0198506961.pdf

8.5 Calibration in laser spectroscopy 171

Molecular iodine

Carbon dioxide laser

Methanol laser

Helium−neon laser

Colour centre laser

X

X

X X

X

Microwave source

Microwave source Cs frequency

standard

Counter

X

X

X

X

Methanol laser

Visible

Infra-red

Far infra-red

Microwave

Nonlinear

mixing diode

Nonlinear

crystal

Fig. 8.14A frequency chain. The fre-

quency of a line in the visible spectrum

of molecular iodine is measured by set-

ting up a chain of oscillators. The fre-

quency of each oscillator is compared

to a multiple or sum of the frequencies

of the other oscillators. At the bot-

tom of the chain is a microwave source

whose frequency is measured with ref-

erence to a caesium frequency stan-

dard (the primary time standard as de-

scribed in Section 6.4.2). A diode mixer

produces a high number of harmonics

of the microwaves, so that the first few

stages achieve multiplication by a fac-

tor of 7. Nonlinear crystals are used for

mixing mid-infra-red and visible radi-

ation. The scheme shown here is one

of the least complicated but it still in-

volves a large number of devices. When

the whole chain is operating it deter-

mines the multiplication factor that re-

lates the optical frequency to the cae-

sium frequency standard at 9 GHz. Af-

ter Jenningset al. (1979).

measurements are a major undertaking. Recently, a new method has
been invented that supersedes cumbersome frequency chains and makes
the measurement of optical frequencies more straightforward.

8.5.3 Optical frequency combs

Recently, a new method of measuring optical frequencies has been in-
vented that has revolutionised optical metrology. The new method re-
lies on the ability to generatefrequency combsusing laser techniques,
i.e. laser radiation that contains a set of regularly-spaced frequencies, as