Study On The Intensity Balance Ratio Tuning Of The Microchip Dual-frequency Lasers

Over the past decades,the photonics-based coherent radio-frequency(RF)signal sources are widely developed and show great potentials in the applications of wireless communication,material spectroscopy,frequency measurement and hybrid lidar-radar fields.Among various methods of producing photonic RF signal with high spectral purity,the frequency beating at photodiode based on dual-frequency lasers(DFLs)with RF frequency separations currently provides an effective way.With the obtained DFL signals for heterodyne frequency beating,the relative intensities of the two frequency components are very important factors for the beat-noting efficiency.A continuous-wave microchip DFL with well balanced intensity was presented.The intensity balance ratio(IBR)tuning of the microchip DFL was easily realized based on the thermally induced laser wavelengths and emission cross section(ECS)spectra of gain media shift.Based on rate-equation theory,the Lorenz type coupled rate equations that described the dual-frequency operation of homogeneously-broadened DFLs were established.Since the relative intensities of the Nd3+ based DFLs are strongly due to the wavelength dependent ECS values of the two frequency components,the IBR can be tuned by shifting the wavelengths in the unflat ECS spectral profiles.Above all,the influence of temperature on the output relative intensities is due to the combination of both the temperature dependent laser wavelengths and ECS spectra of gain media.The monolithic microchip DFL output spectra and the ECS spectra of an uncoated Nd:YVO4 crystal with same dimension and Nd3+ doping concentration at different temperature were studied.The principle of the ECS spectra of gain media and laser wavelengths variation versus temperature were experimentally revealed.The experimental results clearly showed that when increased the heat sink temperature the monolithic microchip DFL output spectra shifted toward higher wavelengths,the overall laser intensity had a decreasing trend,the phenomenon of mode hopping happened over our range of temperature.And the ECS spectral peak wavelength red-shifted,the ECS spectral peak value decreased,the ECS spectral full width at half maximum(FWHM)broadened for increasing temperatures.The red-shifting rate of the centre wavelength of the DFL spectral envelope and the ECS spectral peak wavelength were measured as 3.88 pm/°C and 3.84 pm/°C,respectively.For analyzing the principle of the relative intensities variation of the DFL signals versus temperature,the relationship among the DFL wavelengths,ECS spectra and their interaction to the output relative intensities were comprehensively discussed.After comparing the fitted expressions the centre wavelength of the DFL spectral envelope and the ECS spectral peak wavelength,the temperature difference between the monolithic microchip DFL and the uncoated Nd:YVO4 crystal was numerically calculated as 54°C with the same wavelength.Considering the DFL signals only emitted within the ECS spectral range of the laser crystal,the centre wavelength of the DFL spectral envelope and the ECS spectral peak wavelength should be identical.Here,the temperature difference can be explained as the different pump and laser powers,the heat deposited in the fluorescence spectra acquisition experiments were much smaller than that in the laser spectra acquisition experiments.As a result,the monolithic microchip DFL temperature was higher than the uncoated Nd:YVO4 crystal which was equal to the heat sink temperature.The relative position of the DFL average wavelength and the compensated Nd:YVO4 crystal ECS spectral peak wavelength determined the IBR of the DFL signal.In order to obtain such a balanced intensity distribution of the two frequency components,the DFL wavelengths were precisely tuned and spectrally matched with the ECS spectrum of the gain medium by employing a temperature controller.Finally,when the heat sink temperature was controlled at-5.6°C,a 264 mW DFL signal was achieved with frequency separation at 67.52 GHz and IBR at 0.991.A lower heat sink temperature of the microchip DFL controlled,a higher power of the balanced intensity DFL signal was achieved.