stable continuous wave mode of autonomous single-frequency steady-state microwave generation of the quasi-harmonic subcarrier on the frequency about 8.2 GHz.
The frequency functions of OEO with ordinary single-mode light guides of different
lengths (from several meters to 4.6 km) are investigated experimentally in the
steady-state mode. For these experiments’ fulfillment, we use the “runout” light
guide segments with the lengths 1, 3, 60, 70, and 4640 m as the light guides of the
fiber-optical system.
For the ordinary low-dispersion optical fiber with lengths up to 65 m, the singlefrequency oscillations are excited in the OEO system on the frequency closed to the
natural RFF frequency of 8.2 GHz. The measured output power of the microwave
subcarrier oscillations at the microwave output is about 0.1–1 mW.
OEO frequency functions are experimentally studied at control from the QWLD
bias current (i.e., an operating point on its watt–ampere characteristic). The watt–
ampere curve of QWLD is presented in Fig. 8.13a. The QWLD threshold bias
current is about I thr % 12 mA.
As we see, the QWLD watt–ampere curve in the whole bias current range is
closed about to the linear function, but it has a “sharp bend” at the pumping current
of about 60 mA, which relates to the generation condition fulfillment of the longitudinal modes of higher orders.
The single-mode optical fibers are selected according to their geometric length
with the aid of side component suppression in the frequency spectrum of the
generated subcarrier, which allows provision of the microwave single-frequency
mode in the whole range of subcarrier frequency control.
Explanation of OEO frequency functions versus the QWLD bias current is
described in the Chap. 4 of the present book. In Chap. 4 we can see the joined
detailed plots of experimental and theoretical amplitude-frequency and phasefrequency curves for QWLD for various bias currents. At QWLD bias current
growth, the generation frequency increases and determines from the phase balance
condition by the interception points of the QWLD phase response and the RFF phase
response.
At optical fiber length growth (up to 70 m) in experimental curves, we can
observe the frequency jumps (Fig. 8.13d), which relates to the OEO frequency
tuning to the adjacent oscillation types. If the QWLD bias current increases, the
generation frequency essentially grows for small pumping excesses over the threshold value and has the insignificant growth (stabilization from the current) for bias
current excesses above the threshold value more than I 0 /I 0thr ¼ 5–8, which corresponds to QWLD bias currents equaled to I 0 ¼ 50–80 mA.
These experimental functions of the frequency f and the amplitude U of OEO
microwave generation presented in Fig. 8.13b, d versus the bias current well agree
with calculations in Chap. 3.
The frequency of the generated microwave subcarrier can be exactly tuned to the
central frequency of the microwave RFF by means of adjustment of the optical phase
incursion in the optical phase shifter.
When using the RFF in OEO, the generation frequency determines by the
amplitude-frequency curve of the mutual two-port formed by RFF and QWLD.
484
8 Experimental Investigations and Practical Circuits of Optoelectronic. . .
The frequency functions of OEO with ordinary single-mode light guides of different
lengths (from several meters to 4.6 km) are investigated experimentally in the
steady-state mode. For these experiments’ fulfillment, we use the “runout” light
guide segments with the lengths 1, 3, 60, 70, and 4640 m as the light guides of the
fiber-optical system.
For the ordinary low-dispersion optical fiber with lengths up to 65 m, the singlefrequency oscillations are excited in the OEO system on the frequency closed to the
natural RFF frequency of 8.2 GHz. The measured output power of the microwave
subcarrier oscillations at the microwave output is about 0.1–1 mW.
OEO frequency functions are experimentally studied at control from the QWLD
bias current (i.e., an operating point on its watt–ampere characteristic). The watt–
ampere curve of QWLD is presented in Fig. 8.13a. The QWLD threshold bias
current is about I thr % 12 mA.
As we see, the QWLD watt–ampere curve in the whole bias current range is
closed about to the linear function, but it has a “sharp bend” at the pumping current
of about 60 mA, which relates to the generation condition fulfillment of the longitudinal modes of higher orders.
The single-mode optical fibers are selected according to their geometric length
with the aid of side component suppression in the frequency spectrum of the
generated subcarrier, which allows provision of the microwave single-frequency
mode in the whole range of subcarrier frequency control.
Explanation of OEO frequency functions versus the QWLD bias current is
described in the Chap. 4 of the present book. In Chap. 4 we can see the joined
detailed plots of experimental and theoretical amplitude-frequency and phasefrequency curves for QWLD for various bias currents. At QWLD bias current
growth, the generation frequency increases and determines from the phase balance
condition by the interception points of the QWLD phase response and the RFF phase
response.
At optical fiber length growth (up to 70 m) in experimental curves, we can
observe the frequency jumps (Fig. 8.13d), which relates to the OEO frequency
tuning to the adjacent oscillation types. If the QWLD bias current increases, the
generation frequency essentially grows for small pumping excesses over the threshold value and has the insignificant growth (stabilization from the current) for bias
current excesses above the threshold value more than I 0 /I 0thr ¼ 5–8, which corresponds to QWLD bias currents equaled to I 0 ¼ 50–80 mA.
These experimental functions of the frequency f and the amplitude U of OEO
microwave generation presented in Fig. 8.13b, d versus the bias current well agree
with calculations in Chap. 3.
The frequency of the generated microwave subcarrier can be exactly tuned to the
central frequency of the microwave RFF by means of adjustment of the optical phase
incursion in the optical phase shifter.
When using the RFF in OEO, the generation frequency determines by the
amplitude-frequency curve of the mutual two-port formed by RFF and QWLD.
484
8 Experimental Investigations and Practical Circuits of Optoelectronic. . .