Studies On Light Source With Controlled Frequency Based On External Radio Frequency Modulation

As the development of microwave photonics,the modulation of optical carrier by radio frequency signal outside the laser cavity provides a new idea for the accurate control of optical signal frequency.External modulation separates the seed light source from the frequency modulation part,does not change the structure of the seed light laser cavity,guarantees the stability of the parameters such as the linewidth,wavelength and power of the seed light output,and is convenient for replacing the seed light source to upgrade the system.The mature and stable radio frequency signal is used to modulate the optical carrier to ensure the accuracy and stability of the frequency modulation.In this paper,the scheme of precise control of the optical carrier frequency outside the cavity based on dual parallel Mach-Zehnder modulator(DPMZM)is studied.The seed light is modulated by DPMZM outside the cavity,and generates various sidebands.By controlling the RF signal and DC bias voltage,the precise control of the power and frequency interval of the DPMZM output is realized.Based on the research of the precise control of the frequency by DPMZM,this paper designs and studies three kinds of light sources whose frequencies can be accurately controlled.The main contents are as following:1.Frequency sweeping light source based on recirculating frequency shifter outside the cavity.DPMZM is used to shift the seed light out of the cavity,and the high speed and high linearity swept frequency output of the seed light is realized.Furthermore,by precisely controlling the swept range of seed light through the radio frequency signal,the multi-wavelength parallel swept spectrum can be equivalent spliced,which overcomes the technical bottleneck between the sweep rate and the sweep range of single-wavelength swept light source.The experimental results show that the maximum swept rate is 1.23 MHz and the swept linearity in wavenumber domain is0.99999.The potential of expanding the swept range to 60 nm without losing the swept rate is explored.2.Dual-frequency light source with adjustable frequency difference based on external radio frequency controlThe same seed light is modulated at different frequencies by two DPMZMs outside the cavity via a Mach-Zehnder structure to achieve dual-frequency output,which overcomes the limitation of single modulator bandwidth on dual-frequency optical output frequency difference.Experimentally,a wide range of continuous and precise tuning of frequency difference from 40 k Hz to 30 GHz is realized,and the application potential of the frequency difference adjustable dual-frequency light source in the laser interferometry measurement system is preliminarily explored.3.Multi-wavelength light source with controllable frequency spacing based on external radio frequency modulation and cascaded four-wave mixingBased on the principle of DPMZM modulation to generate multiple sidebands,the number of the sidebands is expanded by four-wave mixing and cascaded four-wave mixing,and the multi-wavelength output is achieved.The frequency spacing can be precisely controlled by the RF driving signal loaded on the DPMZM.Experimentally,for phased array antenna system,the frequency spacing can vary linearly and sinusoidally with time,and the tuning range is from 4GHz to 16 GHz.