Research On Frequency Offset Locking Of Semiconductor Laser With Reference To Femtosecond Optical Frenquency Comb

With the continuous development of sophisticated aerospace technology and leading-edge basic scientific research,people's demand for absolute distance measurement is increasing.Taking the inter satellite ranging requirement in the measurement of gravity field in the double star as an example,in order to achieve the measurement accuracy of microns in the range of hundreds of kilometers,the relative accuracy of the laser interferometry system is required 10-10 order.Considering that the space environment has no influence on air refractive index,Laser's vacuum wavelength accuracy(Laser's frequency)is the key to restricting the accuracy of measurement,the accuracy of laser frequency will reach 10-12 orders of magnitude.The He-Ne laser based on iodine frequency standard is a typical standard device for the traditional laser wavelength traceability,and its frequency accuracy is limited to 10-11 magnitude,which can not meet the requirements mentioned above.The frequency accuracy of the single comb can reach 10-16 order after the frequency control of the femtosecond optical frequency comb referenced to frequency standard.However,due to the small power of single comb,it is difficult to directly measure laser distance.The output power of butterfly type of distributed feedback(DFB)semiconductor laser can reach more than tens of milliwatts.At the same time,the wavelength is easy to adjust through current and temperature,and its volume is small and suitable for laser ranging.Combining the characteristics of the above two lasers,it is of great significance for the realization of high precision inter satellite laser ranging to refer to the semiconductor laser bias locking technology of femtosecond optical comb.Combining the characteristics of the above two lasers,it is of great significance for the realization of high precision inter satellite laser ranging to refer to the semiconductor laser bias locking technology of femtosecond optical comb.Under the above research background and technical requirements,the following research works are carried out in this paper.(1)For the demand of offset lock control of the semiconductor lasers that refer to femtosecond optical comb,the signal-to-noise ratio of the beat signals should be higher than 30 d B,which the direct interference is difficult to satisfy.This paper first establishes the mathematical model of the signal-to-noise ratio of the beat frequency signals.Based on this,the quantitative analysis of the parameters which affect the signal-to-noise ratio is achieved.The optimized design of the interference optical path for grating spatial spectral separation is achieved,and the optimized interference is achieved.The optical path can achieve beat signal output with a signal-to-noise ratio of 43 d B,which is consistent with the theoretical calculation of the optical signal SNR limit.(2)Aiming at the problem of fast frequency jitter and large-scale drift in the free running state of DFB semiconductor lasers,this paper develops a digital frequency prestabilization control key unit based on current tuning to perform digital frequency discrimination,digital filtering and signal shaping on beat frequency signals.And highprecision control of its frequency is achieved through precise adjustment of the drive current of the DFB semiconductor laser.Thus the locking and tracking accuracy of the DFB semiconductor laser to the reference femtosecond optical comb can achieve ± 455 k Hz(1 hour).(3)In view of the large linewidth of the DFB semiconductor laser and the high current tuning sensitivity,the digital pre-frequency stabilization accuracy is difficult to meet the requirements.The key unit of feedforward control based on acousto-optic frequency shifter(AOFS)is developed.The mechanism of feed forward control using the acousto-optic frequency shifter is analyzed in detail.The experimental setup is designed and implemented,and the inhibiting effect of phase noise of DFB semiconductor laser is verified.Combining this feedforward control unit with the digital pre-stabilization control unit,the offset lock control of the DFB semiconductor lasers that refer to femtosecond optical comb is realized,the error of locking while tracking is less than ±6.8 Hz,and when the frequency stability of the femtosecond optical comb is 10-16,the relative accuracy of the corresponding semiconductor laser frequency can reach 3.5×10-14,which can satisfy most of the inter-satellite laser ranging requirements.(4)For the limit demand of inter-satellite laser ranging represented by gravitational wave detection,the optical phase-locking technology of the DFB semiconductor lasers that refer to femtosecond optical comb is further explored.The principle of the phaselocked loop is studied in depth,a high-performance phase-locked loop circuit is developed and equivalent experiments were performed.Based on this,using AOFS as an optical voltage controlled oscillator,an optical phase-locked system is built and its system model is built.The three key parameters of the laser linewidth,loop gain and loop delay are analyzed for the key role of optical phase-locking.