Unfortunately, the single-frequency model restricts possibilities in investigation
of phase offsets and QWLD time constant offsets at the multi-frequency optical
generation. Therefore, we examine the calculation features of the laser diode’s
transfer function for the multi-frequency mathematical model.
We can note that the group phase incursion in RF FODL: “QWLD—single-mode
optical fiber with the length 1.5 m—the photodetector” is 7.5 ns (taking into account
the 1.5-m segment of the single-mode FOS). Experimentally measured AFC and
PFC obtained in Fig. 7.17 have well qualitative and quantitative agreement with
theoretical ones. The discrepancy of calculated and experimental resonance peaks
and amplitudes up to 2–4 dB can be attributed to limitations of the velocity equation
model, which, in general case, do not take into consideration the temperature
variations of the operating substance and the energy exchange between the optical
modes participating in the generation, as well as the situation when we do not take
into account the own small temperature variations of the active region and the
pumping current.
On the basis of the calculation and experimental studying of AFC and PFC of
mesa-strip QWLD, we can make the conclusion that appearance (together with the
fundamental mode) of other optical modes at large pumping currents (the emission
multi-frequency properties) does not qualitatively distort of AFC and PFC in the
frequency band from 5 to 12 GHz.
Small non-monotonies in AFC and PFC on the level 1–2 dB and 2–4
, are,
relatively, manifested in the existence region of sharp resonance peaks from 1 to
5 GHz. Observing in the experiment, non-monotonies in AFC on the level 1–3 dB in
the frequency band of 5–12 GHz are related to matching elements of the microwave
paths.
On the base of calculations and experimental investigations of the operation and
characteristic features of AFC and PFC of QWLD, we can make the following
conclusions:
1. The influence of the pumping current upon the slope of PFC is different in the
whole frequency band. At small exceeds of the pumping current (1.2–2.0), at
pumping current variation of 1 mA, the maximal PFC slope is 30–70
/mA at
fixed radio frequency in the range 1–5 GHz. At large pumping currents, the
maximal PFC slope is 1–5
/mA at the fixed frequency in the range 5–10 GHz.
Experimental measurements of PFC qualitatively and quantitatively coincide
with the calculated values obtained on the base of velocity equations.
2. Appearance (together with the fundamental mode) of other optical modes at large
pumping currents (the emission multi-frequency property) does not qualitatively
distort of AFC and PFC in the frequency band from 5 to 12 GHz. Small
non-monotonies in AFC and PFC are manifested in the existence region of
sharp resonance peaks from 1 to 5 GHz.
In Fig. 7.18, AFC (а) and PFC (b) of QWLD transfer function are shown for
different pumping currents in the generation mode at various ratios of the pumping
current to the threshold current. Designation SE is concerned to AFC and PFC of the
transfer function owing to spontaneous emission (the luminescence) and it is shown
7.3 OEO DM Analysis on the Base of Abbreviated Differential Equations
401
of phase offsets and QWLD time constant offsets at the multi-frequency optical
generation. Therefore, we examine the calculation features of the laser diode’s
transfer function for the multi-frequency mathematical model.
We can note that the group phase incursion in RF FODL: “QWLD—single-mode
optical fiber with the length 1.5 m—the photodetector” is 7.5 ns (taking into account
the 1.5-m segment of the single-mode FOS). Experimentally measured AFC and
PFC obtained in Fig. 7.17 have well qualitative and quantitative agreement with
theoretical ones. The discrepancy of calculated and experimental resonance peaks
and amplitudes up to 2–4 dB can be attributed to limitations of the velocity equation
model, which, in general case, do not take into consideration the temperature
variations of the operating substance and the energy exchange between the optical
modes participating in the generation, as well as the situation when we do not take
into account the own small temperature variations of the active region and the
pumping current.
On the basis of the calculation and experimental studying of AFC and PFC of
mesa-strip QWLD, we can make the conclusion that appearance (together with the
fundamental mode) of other optical modes at large pumping currents (the emission
multi-frequency properties) does not qualitatively distort of AFC and PFC in the
frequency band from 5 to 12 GHz.
Small non-monotonies in AFC and PFC on the level 1–2 dB and 2–4
, are,
relatively, manifested in the existence region of sharp resonance peaks from 1 to
5 GHz. Observing in the experiment, non-monotonies in AFC on the level 1–3 dB in
the frequency band of 5–12 GHz are related to matching elements of the microwave
paths.
On the base of calculations and experimental investigations of the operation and
characteristic features of AFC and PFC of QWLD, we can make the following
conclusions:
1. The influence of the pumping current upon the slope of PFC is different in the
whole frequency band. At small exceeds of the pumping current (1.2–2.0), at
pumping current variation of 1 mA, the maximal PFC slope is 30–70
/mA at
fixed radio frequency in the range 1–5 GHz. At large pumping currents, the
maximal PFC slope is 1–5
/mA at the fixed frequency in the range 5–10 GHz.
Experimental measurements of PFC qualitatively and quantitatively coincide
with the calculated values obtained on the base of velocity equations.
2. Appearance (together with the fundamental mode) of other optical modes at large
pumping currents (the emission multi-frequency property) does not qualitatively
distort of AFC and PFC in the frequency band from 5 to 12 GHz. Small
non-monotonies in AFC and PFC are manifested in the existence region of
sharp resonance peaks from 1 to 5 GHz.
In Fig. 7.18, AFC (а) and PFC (b) of QWLD transfer function are shown for
different pumping currents in the generation mode at various ratios of the pumping
current to the threshold current. Designation SE is concerned to AFC and PFC of the
transfer function owing to spontaneous emission (the luminescence) and it is shown
7.3 OEO DM Analysis on the Base of Abbreviated Differential Equations
401