23.4 Lasers
681
10
8
6
4
2
1
0
2
4
6
8
1 0
thr
2.0 10
6.5 10
2.5 10
9.0 10
2.7 10
17
17
18
18
19
N (cm )
A
-3
Fig. 23.59 Variation of turn-on delay time with the injected current for a laser at room temperature. Adapted from [2012]
10ns
(a)
1ns
(b)
Fig. 23.60 Schematic response of a LED to current pulse and b of laser to current step
Large-Signal Modulation
If a current pulse is fed to the laser, the laser radiation is emitted with a short time delay, the so-called
turn-on delay (TOD) time. This time is needed to build up the carrier density for inversion. The time
dependence of the density is (neglecting the density dependence of the lifetime)
n(t) =
I τ
e A d
1 − exp
−
t
τ
.
(23.42)
The TOD time to reach the threshold density (using (23.23)) is
τ TOD = τ ln
I
I − I thr
.
(23.43)
We note that τ TOD > 0 for I > I thr . Such a dependence is found experimentally (Fig. 23.59). The
turn-on delay time decreases with increasing pump current but typically is at least 1 ns. In order to
circumvent this limitation for more than about 1 GHz pulse repetition rate, the laser is biased slightly
below threshold.
In Fig. 23.60a, the response (light emission) of a LED to a short current pulse is shown schematically.
The monotonously decreasing transient (that is more or less exponential) corresponds to the carrier
recombination dynamics. When a laser is excited with a steep (long) current pulse, the response
681
10
8
6
4
2
1
0
2
4
6
8
1 0
thr
2.0 10
6.5 10
2.5 10
9.0 10
2.7 10
17
17
18
18
19
N (cm )
A
-3
Fig. 23.59 Variation of turn-on delay time with the injected current for a laser at room temperature. Adapted from [2012]
10ns
(a)
1ns
(b)
Fig. 23.60 Schematic response of a LED to current pulse and b of laser to current step
Large-Signal Modulation
If a current pulse is fed to the laser, the laser radiation is emitted with a short time delay, the so-called
turn-on delay (TOD) time. This time is needed to build up the carrier density for inversion. The time
dependence of the density is (neglecting the density dependence of the lifetime)
n(t) =
I τ
e A d
1 − exp
−
t
τ
.
(23.42)
The TOD time to reach the threshold density (using (23.23)) is
τ TOD = τ ln
I
I − I thr
.
(23.43)
We note that τ TOD > 0 for I > I thr . Such a dependence is found experimentally (Fig. 23.59). The
turn-on delay time decreases with increasing pump current but typically is at least 1 ns. In order to
circumvent this limitation for more than about 1 GHz pulse repetition rate, the laser is biased slightly
below threshold.
In Fig. 23.60a, the response (light emission) of a LED to a short current pulse is shown schematically.
The monotonously decreasing transient (that is more or less exponential) corresponds to the carrier
recombination dynamics. When a laser is excited with a steep (long) current pulse, the response