Chemistry Reference
In-Depth Information
ω
1
N
(
z
)
M
H
PLL 1
ω
IF
ω
Stab
M
R
M
S
ω
PLL 2
IF
ω
2
φ
FIGURE 6.6
To obtain highest frequency stability in a heterodyne interferometer, both the
difference frequency ω
IF
of probing and reference signals and the probing frequency ω
1
are
phase locked to highly stable quartz oscillators at frequencies ω
IF
and ω
stab
. The difference
frequency generated in mixer
M
R
is compared in the PLL 2 circuit with a quartz oscillator at
ω
IF
resulting in a control signal which acts on the reference source at ω
2
in such a way that
the difference frequency is phase locked to the ω
IF
source. To keep the probing frequency ω
1
constant, the signal is mixed in
M
H
with a high harmonic (typically
n
=
10) of a stable source
in the 10 GHz range. The difference frequency is phase locked with the aid of PLL 1 to the
quartz oscillator as well, controlling source at ω
1
in such a way that ω
1
−
n
ω
stab
=
ω
IF
is kept
constant. (From Vowinkel, B., Private communication.)
6.2.4.2 Path Length Variations
In a similar way as frequency variations, path length variations δ
L
cause phase
fluctuations δϕ
δ
L
, a contribution often difficult to quantify in the phase data
interpretation. The mechanical movements and vibrations of beam guiding elements
in particular those inside the plasma vessels are difficult to avoid. A way out is
to probe the plasma with two separate waves of different frequency ω
1
and ω
2
,
traveling along identical paths. In this
two-color interferometer
according to (6.13)
each wave experiences a different phase shift ϕ
1,2
, the common vibration error can
be canceled. Let
=
(
ω
/
c
)
1,2
be the two measured phases of the two separate interferometers
1,2
=
ϕ
1,2
+
(
ω
1,2
/
c
)
δ
L
, the line density can be evaluated from
z
2
1
5.303
ω
1
ω
2
1
−
ω
1
ω
2
2
N
e
(
z
)
dz
=
.
(6.14)
·
10
−
6
ω
2
−
ω
1
z
1
The two sources at frequency ω
1
and ω
2
can be completely independent. The phases
need to be measured separately with two phase meters. In the so-called
2
-
or
dispersion interferometer
[118,119], the second signal is generated by frequency
doubling from the first ω
2
=
ω
2ω
1
. The plasma is probed simultaneously at identical
paths with signals at ω
1
and 2ω
1
giving rise to phase shifts ϕ
1
(
.
After the plasma passage a second frequency doubler is used to generate the second
harmonic of the probing signal at ω
1
, too. Both signals now have the same frequency
ω
1
)
and ϕ
2
(
2ω
1
)
