Research On Photonic Microwave Technology Based On Optical Heterodyne

As an interdisciplinary of microwave and laser,microwave photonics aims at realizing high-frequency microwave signal generation,transmission,and processing in the light field,breaking through the limitation of traditional electronics.Microwave photonics has been successfully applied in the radio-over-fiber system,microwave photonics radar,satellite communication,and other fields.A high-quality microwave signal source is the foundation of such applications.Therefore,how to effectively generate a stable and low-noise microwave,becomes a research focus in the current.In this paper,combining with single-frequency fiber laser for its narrow linewidth and low noise,the photonics microwave research based on optical heterodyne technology is implemented.The specific research contents and results are as follows:(1)A 1-μm short-cavity dual-wavelength single-frequency laser output based on distributedBragg reflection structure is established by using the fiber Bragg grating with twostep exposure and a home-made phosphate gain fiber,as the foundation of the high-quality microwave.Further,an optoelectronic feedback is demonstrated to suppress the intensity noise of the laser source.With the AM-FM transfer,the microwave has a noise-suppression of about 6 dB at 1 k Hz and 3 dB at 10 k Hz,reaching-38.98 dBc/Hz and-66.07 dBc/Hz respectively.(2)Using a 1.5-μm k Hz-linewidth single-frequency fiber laser as Brillouin pump source and a 1-km single-mode fiber as Brillouin gain loop,a switchable microwave signal output of 10.84 and 21.68 GHz is realized.Resulting from the long loop,the Brillouin laser is at multilongitudinal-mode status,which induces the spectrum broadening and poor performance of the microwave.Thus,with analysis and calculation,a dual-wavelength single-longitudinal-mode Brillouin laser output is realized with an 8-m high nonlinear fiber.The linewidth of the generated microwave signal is 1.6 k Hz and its phase noise reaches-62.4 dBc/Hz at 10 k Hz.(3)Aiming at the problem that the relaxation oscillation of the laser source would induce noise sidebands in the microwave signal,the self-injection locking is introduced to suppress the relaxation oscillation.The suppression of noise sidebands in the microwave signal generated by dual-wavelength Brillouin fiber laser and short-cavity fiber laser are realized respectively,which have an improvement of 15 dB and 30 dB in signal-to-noise ratio respectively.The influence of the self-injection loop on the linewidth and phase noise of the photonic microwave signal is also investigated.When using a 100 m locking loop,the phase noise on short-cavity dual-wavelength fiber laser reaches-87.13 dBc /Hz at 10 k Hz with an improvement of 24 dB.