Studies On Measuring The Mapping Relationship Between Instantaneous Frequency And Time Of Frequency Modulated Continuous-wave Light Sources

There are more and more broadband linearly chirped light sources that are used in various precision measurement systems as the kind of laser systems have been constantly and significantly developed.In order to determine the performance of the measurement systems,the key parameters of this type of light sources,such as frequency sweep range,frequency sweep rate,frequency sweep period,instantaneous linewidth and frequency sweep linearity,should be accurately measured in advance.In this paper the methods and techniques for calibration of the broadband linearly chirped lasers are studied thoroughly.By establishing the mapping relationship between the instantaneous frequency and time of this type of light source,the time-frequency distribution curve of the light source can be obtained accurately,then the key parameters can be retrieved from the accurately known time-frequency distribution curve.Based on the theory of conventional coherent optical spectrum analyzer,it is true that the analyzer was only suitable for measuring chirped continuous light with a very slow sweep rate.However,we creatively proposed a new approximation condition,and theoretically derived an analytic expression of the interference pattern which is generated by the same coherent optical spectrum analysis system operating under the condition of fast sweep rate.Due to our work described above,it has been proved both theoretically and experimentally that the time-frequency mapping relationship can be determined by Gaussian Fitting.The interference patterns for the situations at sweep rates of ?65GHz/s and ?25THz/s are obtained,respectively,by an experiment set in which a low-pass filter bandwidth of 600 k Hz is needed.It has been confirmed that our new method is more accurate than the traditional methods by our comparing the experimental results with the theoretical predictions and our performing a standard error analysis.In this paper the effect of the sweep rate on the signal-to-noise ratio of the interference pattern has been studied experimentally by comparing the signal-to-noise ratios of the interference patterns at the sweep rate of ?65GHz/s,?12THz/s and?25THz/s,respectively.It was found that the signal-to-noise ratio would be deteriorated while the sweep rate increases.In order to solve this issue,we removed off the lowpass filter in the experiment set and combined the coherent optical spectrum analysis method with digital signal processing techniques such like "waveform cutting ",digital filtering,and Gaussian fitting on the interference patterns.By using the modified method the accuracy for measuring the time-frequency mapping relationship of an ultrafast frequency modulated continuous wave light source has been improved significantly.The frequency uncertainty can be kept less than 100 k Hz within a sweep period of 8?s,even for the measurement of swept laser at the sweep rate of 5000THz/s.Finally,we proposed a new concept and the generation method of a continuous wave frequency comb,and apply the method of determining time-frequency mapping proposed in this paper to the measurement of linearly chirped continuous wave light with sweep frequency range of 200 GHz and sweep rate of 5000THz/s.The timefrequency distribution curve is reconstructed by simulation.