InGaAlAs/InP structure is used, and at that, the LD emission wavelength is 1.3 μm.
Such laser diodes with thin structures are called quantum-well laser diodes (QWLD).
At present, QWLD investigations are intensively executed, and their radiofrequency modulation band becomes permanently widened. The maximal modulation frequency f m of QWLD is determined by expression: f m ¼
1=T 1
ð
Þ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
E
2
0 G 0p = 1 þ ε 0 E
2
0
À
Á
q
, where E
2
0 ¼ S p is the normalized emission power,
S p is the internal photon density in the QWLD active region, G 0p % T 1 /T OF is the
slope of the optical amplification coefficient of the active layer, ε 0 is the coefficient
characterizing the gain nonlinearity, T 1 ¼ τ e is the lifetime of electrons, T OF ¼ τ p is
the lifetime of photons in the optical resonator. It follows from this expression that
the growth of the photon density in the active layer and the gain slope leads to the
increase of the maximal modulation frequency. This can be achieved, for example, at
large exceeds of pumping above the threshold value. In modern QWLD, the
pumping (bias) current is 6–9 times more than the threshold pumping value.
The second mechanism of increase of the QWLD modulation frequency band is
the reduction of the optical amplification dependence upon temperature owing to the
temperature stabilization and by means of introduction of the thermal-compensating
selective optical filter in the QWLD structure. At negative slope of the “resonance
peak” dependence of such optical filter upon temperature, for the positive room
temperature, for example, À0.15 nm/
С leads to the growth of the optical QWLD
wavelength approximately by the same value but with the opposite sign +0.15 nm/
С, taking into account the QWLD temperature increase, we can achieve the
significant thermal compensation and the increase of the output power to 30 mW
at the pumping current of 80 mA and at the QWLD slope increase of the watt–
ampere characteristic to 0.4 W/A. At that, the modulation frequency range of QWLD
is 14 GHz at 25
C and 12 GHz at 85
С, relatively.
The author of [9] informs about the creation of QWLD on the base of InGaAsP/
InP with the output power introduced into the optical fiber at about 7 mW and the
modulation frequency band about 40 GHz. At that, in the synchronization mode of
QWLD longitudinal modes, the duration of the emission pulse is 2.8 ps with the low
level of the “jitter” equals to 73 fs.
Let us consider the QWLD operation. The structural diagram of QWLD and its
physical operation principle is explained in Fig. 7.16. This Fig. 7.16a shows the
QWLD structural diagram, which has got the largest distribution at present. This
diagram explains the operation principle of QWLD on the base of so-called “model
3N”. The equivalent electrical scheme of QWLD is shown at the same Fig. 7.16b.
The main “spurious” laser elements are shown in [10–12]: C f is the capacitance
formed by the dielectric film, C pn is the barrier capacitance of the laser p-n junction,
which is formed owing to the fact that p- and n-emitters are alloyed, and the active
medium and wave-guiding layers are non-alloyed.
Let us perform the AFC and PFC analysis of QWLD and their functions upon the
DC pumping current. We also note that the QWLD watt–ampere characteristic, the
optical spectrum of generation at various pumping currents, the power spectral
7.3 OEO DM Analysis on the Base of Abbreviated Differential Equations
395
Such laser diodes with thin structures are called quantum-well laser diodes (QWLD).
At present, QWLD investigations are intensively executed, and their radiofrequency modulation band becomes permanently widened. The maximal modulation frequency f m of QWLD is determined by expression: f m ¼
1=T 1
ð
Þ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
E
2
0 G 0p = 1 þ ε 0 E
2
0
À
Á
q
, where E
2
0 ¼ S p is the normalized emission power,
S p is the internal photon density in the QWLD active region, G 0p % T 1 /T OF is the
slope of the optical amplification coefficient of the active layer, ε 0 is the coefficient
characterizing the gain nonlinearity, T 1 ¼ τ e is the lifetime of electrons, T OF ¼ τ p is
the lifetime of photons in the optical resonator. It follows from this expression that
the growth of the photon density in the active layer and the gain slope leads to the
increase of the maximal modulation frequency. This can be achieved, for example, at
large exceeds of pumping above the threshold value. In modern QWLD, the
pumping (bias) current is 6–9 times more than the threshold pumping value.
The second mechanism of increase of the QWLD modulation frequency band is
the reduction of the optical amplification dependence upon temperature owing to the
temperature stabilization and by means of introduction of the thermal-compensating
selective optical filter in the QWLD structure. At negative slope of the “resonance
peak” dependence of such optical filter upon temperature, for the positive room
temperature, for example, À0.15 nm/
С leads to the growth of the optical QWLD
wavelength approximately by the same value but with the opposite sign +0.15 nm/
С, taking into account the QWLD temperature increase, we can achieve the
significant thermal compensation and the increase of the output power to 30 mW
at the pumping current of 80 mA and at the QWLD slope increase of the watt–
ampere characteristic to 0.4 W/A. At that, the modulation frequency range of QWLD
is 14 GHz at 25
C and 12 GHz at 85
С, relatively.
The author of [9] informs about the creation of QWLD on the base of InGaAsP/
InP with the output power introduced into the optical fiber at about 7 mW and the
modulation frequency band about 40 GHz. At that, in the synchronization mode of
QWLD longitudinal modes, the duration of the emission pulse is 2.8 ps with the low
level of the “jitter” equals to 73 fs.
Let us consider the QWLD operation. The structural diagram of QWLD and its
physical operation principle is explained in Fig. 7.16. This Fig. 7.16a shows the
QWLD structural diagram, which has got the largest distribution at present. This
diagram explains the operation principle of QWLD on the base of so-called “model
3N”. The equivalent electrical scheme of QWLD is shown at the same Fig. 7.16b.
The main “spurious” laser elements are shown in [10–12]: C f is the capacitance
formed by the dielectric film, C pn is the barrier capacitance of the laser p-n junction,
which is formed owing to the fact that p- and n-emitters are alloyed, and the active
medium and wave-guiding layers are non-alloyed.
Let us perform the AFC and PFC analysis of QWLD and their functions upon the
DC pumping current. We also note that the QWLD watt–ampere characteristic, the
optical spectrum of generation at various pumping currents, the power spectral
7.3 OEO DM Analysis on the Base of Abbreviated Differential Equations
395