**13. Experimental investigations**

Experimental researches were devoted for several experimental OEO of microwave range with various pumping laser diodes, which emit at wavelengths of 1310 nm or 1550 nm. The maximal output power of optical emission for used laser diodes formed about 10 … 20 mW. **Figure 11** shows the photo picture of one piece assembled on the base of the circuit in **Figure 1b**. As the photo-detector, we applied the PD on the base of InGaAs. The radiofrequency filter represented the dielectric resonator of microwave range with the loaded Q-factor of 1000. This resonator was made on ceramics and had a natural frequency 8.2 GHz. This breadboard model used the wideband (up to 12 GHz) modulation of laser emission, which was performed by the Mach-Zehnder modulator from Hitachi Co. The single-mode light guiders with lengths from 60 m to 4640 m were used for experiments. The stable generation of single-frequency oscillation at frequency close to 8.2 GHz was observed in OEO system for various OF lengths.

The delay of OEO signal was performed with the help of additional fiber-optical light guider with the 10 km-length and the additional photo-diode. The phase noise level at usage of different lasers formed the value 100 … –127 dB/Hz, for offsets 1 … 10 kHz from the microwave sub-carrier frequency under generation and it depends on the spectral line width of laser emission.

Essential reduction of the phase noise by 15 dB was observed in OEO using the differential delay line on the base of two optical fibers of different length. These experimental functions are well-agreed with theoretical at account of the stabilization effect at OF lengths more than 2000 m.

**Figure 11.**

*General view of the experimental breadboard of low-noise laser opto-electronic oscillator of microwave range. The mean oscillation frequency is 8 … 10 GHz.*

## **14. Conclusion**

At that, formation of the final phase RF noises of OEO is examined as the result of the convolution operation of the laser optical spectrum and the RF spectrum of the oscillation. Thus, when using microstructured optical fibers in OEO, it is possible to significantly reduce the dimensions of the fiber-optic delay line -FODL.

For a stable mode of OEO generation in the single-frequency mode, it is necessary to double the time constant of the electron filter (F) over the time constant of the relaxation of electrons in the active layer of the laser.

We have shown that the resonant curve of the electron-photon resonance of the laser has a significant influence on the formation of the power spectral density PSD of the phase noise in OEO. For stable operation of the OEO, the laser coherence time and the delay time in the optical fiber must be balanced. The use of microstructured fibers with low bending losses makes it possible to create compact fiber-optic delay lines for OEO.

Under assumption of the small and large oscillation amplitude at the modulator electrical input, we study OEO as a system in which two oscillation processes are developed on the optical frequency and in radiofrequency. The relatively simple expressions for phase noise PSD of the radiofrequency generation in optoelectronic generator in the mode with the single-side carrier with an account of the laser phase noise. The analysis fulfilled shows that under condition of predominance of laser noises being detected over v noises of the electronic amplifier and the OEO photo-detector of the filtering system.

For reduction of spurious influence of DC intensity component on the photodetector we offer to use the modulator operation mode with an offset of the optical channels "pi".

The suppression factor of the OEO laser phase noise at optical fiber lengths from 2 to 10 km is about �8 … -10 dB/Hz at offset of *F* ¼ 1kHz. Utilization in OEO of the highly-coherent laser with the phase noise less than *S F*ð Þ¼�100 dB/Hz (at the same offset) is the condition of OEO small phase noises less than *S F*ð Þ¼�130 dB/ Hz at the *F* ¼ 1 kHz offset. The value of the OEO power spectral density is proportional to the spectral line width of the laser optical emission.

#### **Acknowledgements**

Authors express our thanks to PhD Yu. B. Il'in for manifested interest and participation in discussions.

#### **Author details**

Alexander Bortsov NRU MPEI, Moscow, Russia

\*Address all correspondence to: laseroeo5@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Laser Opto-Electronic Oscillator and the Modulation of a Laser Emission DOI: http://dx.doi.org/10.5772/intechopen.98924*
