Linear Frequency Sweep Technology With Ontinous Phase

Linear frequency sweep signal with continuous phase is the key technology of opticalfrequency domain reflectometry (OFDR), frequency modulated continuous-wave (FMCW)and ultra-wideband (UWB) wireless communication. With the development of these areas,phase continuous linear frequency sweep signal with high linearity, wide sweep range andadjustable center frequency has promising future applications.The direct method to generate linear frequency sweep signal is direct digital synthesizer(DDS). It has a high linearity, but faces the narrow sweep range, generally less than1GHz.Voltage controlled oscillator (VCO), another way to realize linear frequency sweep signal, canachieve the wide sweep range, but its severe nonlinearity requires complex nonlinearitycorrection system. Combining the high linearity of DDS and wide sweep range of VCO,phase locked loop (PLL) has successfully achieved the frequency sweep with high linearityand wide range. Based on PLL, we designed and constructed the linear frequency sweepsystem, as the followings.1. Integer-N PLL: Realize the linear frequency sweep signal through frequency linearlychirping the reference signal (generated by DDS): in-band phase noise80dBc/Hz and linearsweep range10~19.2GHz.2. Frac-N PLL: Realize the linear frequency sweep signal through linearly changing thefractional divider: in-band phase noise66dBc/Hz and linear sweep range10~18.5GHz.Restricted by the performance of electrical devices, linear frequency sweep signal basedon PLL has a low center frequency and narrow sweep range. To extend these two parameters, we proposed the method of extracting and beating the frequency linearly swept comb lines toobtain signals with high center frequency and wide sweep range. Phase continuous linearfrequency sweep signals based on PLL, whose sweep range is24.3~26.3GHz, are amplified tomodulating the laser beams, generating frequency linearly swept optical frequency comb(OFC). Then we can extract the comb lines through approaches proposed in this paper, asfollows.1. Cascaded interleavers: Utilizing the cascaded interleavers, common-path extracting the±1stand±2ndcomb lines with optical harmonics distortion suppression ratio (OHDSR)exceeds40dB. Through beating the extracted comb lines, we can obtain the linear frequencysweep signal with sweep range48.6~52.6GHz and97.2~105.2GHz.2. Suppressing the optical path induced noise: Separate-path extraction of comb linesthrough two optical filters, will bring uncorrelated optical path noise, we build a feedbackcontrol scheme to suppress the path noise. The extracted±5thcomb lines with OHDSRexceeds40dB are beat to generate frequency multiplied linear frequency sweep signal withsweep range243~263GHz.Choosing the appropriate center frequency, linear frequency sweep signal can locate inthe atmospheric communication window which frequency band ultra-wideband wirelesscommunications are focusing on, for example, linear frequency sweep signal with centerfrequency of100GHz is in W-band. High center frequency and wide sweep range will furtherpromote the applications of linear frequency sweep signal in W-band communication,detection system, and instrumentation.