23.4 Lasers
679
Fig. 23.56 Wavelength as a function of temperature (with mode hopping) for a GaAs-based DFB laser
(a)
(b)
1.52
1.54
1.56
1.58
W
0.3
0.2
0.1
0
R1 R2
Fig. 23.57 a Schematic representation of SGDBR (sampled-grating DBR) laser with four sections. Adapted from [2008].
b Reflectance of two sampled gratings DBR mirrors
refraction and subsequently the longitudinal modes. The index of refraction increases with increasing
temperature at typically ∼ 3 × 10
−4 K
−1 . Generally, a red-shift is the consequence.
Another possibility to vary the index of refraction (and thus the optical path length) is a variation
of the carrier density. The coefficient dn r /dn is about −10
−20 cm
3 . In a two-section laser, separate
regions (with separately controlled currents) for gain and tuning are present. The regions are separated
with deep-etched trenches to avoid crosstalk. The tuning range is limited to about 10 nm. For a modehopping free tuning, the control of the phase in the cavity is important and requires an additional
section for the phase control. Such a three-section laser has separate regions (and current control) for
the reflection, phase and amplification (or gain) region.
Using sampled gratings, the tuning range can be strongly increased to about 100 nm. A sampled
grating is a nonperiodic lattice that has several (∼ 10) reflection peaks. The laser structure has four
sections (Fig. 23.57) with two mirrors that have slightly different sampled gratings. Via the carrier
densities in the two mirror sections, different maxima can be brought to overlap (Vernier effect) and
the position of the selected maximum can be tuned over a wide spectral range (Fig. 23.58).
23.4.13 Dynamics and Modulation
For transmission of information in the time domain, the laser intensity must be modulated. This can be
accomplished by direct modulation, i.e. modulation of the injection current, or external modulators,
for example using the voltage-induced shift of the absorption spectrum due to QCSE (cf. Sect. 15.1.2).
679
Fig. 23.56 Wavelength as a function of temperature (with mode hopping) for a GaAs-based DFB laser
(a)
(b)
1.52
1.54
1.56
1.58
W
0.3
0.2
0.1
0
R1 R2
Fig. 23.57 a Schematic representation of SGDBR (sampled-grating DBR) laser with four sections. Adapted from [2008].
b Reflectance of two sampled gratings DBR mirrors
refraction and subsequently the longitudinal modes. The index of refraction increases with increasing
temperature at typically ∼ 3 × 10
−4 K
−1 . Generally, a red-shift is the consequence.
Another possibility to vary the index of refraction (and thus the optical path length) is a variation
of the carrier density. The coefficient dn r /dn is about −10
−20 cm
3 . In a two-section laser, separate
regions (with separately controlled currents) for gain and tuning are present. The regions are separated
with deep-etched trenches to avoid crosstalk. The tuning range is limited to about 10 nm. For a modehopping free tuning, the control of the phase in the cavity is important and requires an additional
section for the phase control. Such a three-section laser has separate regions (and current control) for
the reflection, phase and amplification (or gain) region.
Using sampled gratings, the tuning range can be strongly increased to about 100 nm. A sampled
grating is a nonperiodic lattice that has several (∼ 10) reflection peaks. The laser structure has four
sections (Fig. 23.57) with two mirrors that have slightly different sampled gratings. Via the carrier
densities in the two mirror sections, different maxima can be brought to overlap (Vernier effect) and
the position of the selected maximum can be tuned over a wide spectral range (Fig. 23.58).
23.4.13 Dynamics and Modulation
For transmission of information in the time domain, the laser intensity must be modulated. This can be
accomplished by direct modulation, i.e. modulation of the injection current, or external modulators,
for example using the voltage-induced shift of the absorption spectrum due to QCSE (cf. Sect. 15.1.2).