23.4 Lasers
683
Fig. 23.62 Eye pattern of
a single-mode VCSEL in
response to a 10 Gb/s
random bit pattern. The
patterns are measured a
with an offset current well
above threshold and b with
an offset current above but
close to threshold.
Adapted from [2015]
100 ps
Fig. 23.63 Frequency
response of a DFB-laser for
various output powers as
labeled. Adapted
from [2013]
12
9
6
3
0
-3
-6
-9
-12
3
5
1
2
1
6
9
5mW
10 15
20
α Factor
Another important quantity is the α factor, also called the linewidth enhancement factor [2016, 2017].
Due to the coupling of amplitude and phase fluctuations in the laser, the linewidth f is larger than
expected.
f =
v g R spont ln R
8π P out L
(1 + α
2
) .
(23.45)
The linewidth enhancement is described via (1 + α
2 ) with
α =
dn r /dn
dκ/dn
= −
4π
λ
dn r /dn
g
,
(23.46)
where κ denotes the imaginary part of the index of refraction (9.4). Typical values for α are between
1 and 10. The linewidth is inversely proportional to the output power (Fig. 23.64).
683
Fig. 23.62 Eye pattern of
a single-mode VCSEL in
response to a 10 Gb/s
random bit pattern. The
patterns are measured a
with an offset current well
above threshold and b with
an offset current above but
close to threshold.
Adapted from [2015]
100 ps
Fig. 23.63 Frequency
response of a DFB-laser for
various output powers as
labeled. Adapted
from [2013]
12
9
6
3
0
-3
-6
-9
-12
3
5
1
2
1
6
9
5mW
10 15
20
α Factor
Another important quantity is the α factor, also called the linewidth enhancement factor [2016, 2017].
Due to the coupling of amplitude and phase fluctuations in the laser, the linewidth f is larger than
expected.
f =
v g R spont ln R
8π P out L
(1 + α
2
) .
(23.45)
The linewidth enhancement is described via (1 + α
2 ) with
α =
dn r /dn
dκ/dn
= −
4π
λ
dn r /dn
g
,
(23.46)
where κ denotes the imaginary part of the index of refraction (9.4). Typical values for α are between
1 and 10. The linewidth is inversely proportional to the output power (Fig. 23.64).