Research On Surface Profile Measurement By Coherence Scanning Interferometry Based On A Femtosecond Laser

As one of the most concerned technologies in this century,fs laser provides breakthrough new methods and technologies for the improvement of spatiotemporal resolution in the field of precision measurement.The prosperity of fs laser-based precision metrology is built upon advances in two significant areas:(1)The repetition rate of the fs laser can be locked to the atomic clock frequency reference.As a result,the spatial distance from pulse to pulse is precisely traced.The use of the pulse to pulse distance as a ruler for geometric measurement can achieve ultra-high spatial resolution.(2)the pulse width of the fs laser is on the order of 10-15 s,which is between the time of the cosmic unity force disintegration and weak symmetry.During the pulse width,a large number of photons gathered and resulted in ultra-high instantaneous power.The use of a single femtosecond pulse for measurement target expose and capture can reach the ultra-high temporal resolution.In this paper,to realize ultra-high spatial and temporal resolutions in 3D surface metrology,two methods based on time-space and coherence characteristics of an fs laser were proposed for static and dynamic surface measurements,respectively.1)A method for quantitative measurement of coherence of fs laser sources is proposed.The measurement results show that the fiber fs laser source has high spatial coherence and low temporal coherence.The interference system based on femtosecond laser can eliminate the influence of parasitic noise fringes on the measurement,and verify the feasibility of fs laser to improve the field of view and fringe resolution in optical interference surface metrology.2)The method of large field of view rough surface profiling based on low coherence scanning is proposed.According to the coherence characteristics of fs laser,the theory of fs laser coherent scanning interference is analyzed.The zero-order fringe recognition algorithm of interference fringes is studied.For the rough surface,the technique of intensity polarization modulation is proposed to ensure the interference contrast.Taking the silicon carbide substrate of the giant Magellan telescope in the rough polishing as an example,the three-dimensional profile measurement of a rough surface with high precision and large field-of-view is realized.3)A parallel measurement method for multi-target surfaces profiling based on an fs laser was proposed.The measurement was carried out by the use of the repetition rate sweeping of fs laser,which resulted in scanning interference between different pulses in separate interferometers traced to the atomic clock.The method realized the measurement accuracy conversion from the spatial to the frequency and solved the difficulty of vertical resolution traceability of the existing coherent scanning interferometers.Nano-precision surface profiling of a silicon wafer and a gauge block were carried out by the built system,which verified the high measurement accuracy and online parallel detection capability of the method.4)The method of single pulse digital holographic interferometry using a fiber fs laser was proposed.Based on the ultra-short pulse width of fs laser,the technology of singlepulse imaging was combined with digital holographic interferometry to achieve ultrahigh time resolution in dynamic surface profiling.The target exposure and imaging were completed by synchronously matching the pulse flight time and camera exposure time.The ASE interference imaging noise from low-repetition rate fs laser amplification was investigated and removed.Moreover,the comparison and characteristics of different methods and techniques were summarized.For instance,the reflective off-axis digital holographic interference system for the dynamic surface profiling of an acoustic film vibration was demonstrated.5)Combined with the pump-probe technology,a method for instantaneous measurement of photoacoustic waves based on single-pulse digital holographic interference was proposed.The principles of photoacoustic wave excitation and propagation were investigated.The requirements of thermal diffusion and pressure diffusion limitation for photoacoustic imaging are analyzed.A transmissive single-pulse digital holographic pump-probe system was designed and constructed to measure the instantaneous photoacoustic excited by an fs laser in the Acetone solution.Compared with traditional photoacoustic wave measurement methods,this method was free from time-consuming area scanning or averaging-out of electronic noise.For the first time,three-dimensional characterization of a photoacoustic wave with ultra-high time resolution and high spatial resolution was realized.