In the past decade, the research on optical frequency comb and its applications has achieved rapid development, bringing revolutionary progress to the metrology field. Frequency stabilization and absolute frequency measurement of the CW laser through optical frequency combs is very important for establishing new length standards. A tunable near-infrared CW laser is phase locked to a commercial optical frequency comb referring to a Rubidium atomic clock for frequency stabilization. A home-made comb is phase locked to a hydrogen maser for measuring the absolute frequency of the frequency stabilized CW laser obtained. The procedure of frequency stabilization and absolute frequency measurement based on optical frequency combs is demonstrated and the uncertainty and stability of the frequency stabilized CW laser are estimated.
In order to solve the problem that the length of gain fiber is difficult to be determined, and implement the femtosecond pulse features of high average power and narrow pulse width in optical measurement, the effect of erbium-doped fiber length on the pulse features is studied. In the simulation analysis, combined the rate equation of two-level system with the nonlinear Schrodinger equation describing the ultrafast pulse propagation, the variation trend of average power and pulse width with erbium-doped fiber length is numerically studied. According to the two-level model, a variable gain coefficient curve with the fiber length is obtained and applied to ultrafast pulse propagation model by polynomial curvefitting method to improve the simulation accuracy. In the experiment, a 980nm pump source is used to build a singlestage forward amplification system, and the fiber truncation method is employed to verify the numerical simulation. The results show that the simulation is consistent with the experimental data. When the average power of femtosecond pulse signal light is 50mW and the pump light is 1000mW, the optimum fiber length is 55~100cm. Compared with traditional simulation method of using fixed gain coefficient, the introduction of variable gain coefficient has an impact on simulation parameters p and d, which present the gain effect and dispersion effect in transmission model.
An optical frequency comb based on a 250 MHz home-made Er-doped fiber femtosecond laser is presented in this paper. The Er-doped fiber laser has a ring cavity and operates mode-locked in femtosecond regime with the technique of nonlinear polarization rotation. The pulse duration is 118 fs and the spectral width is 30 nm. A part of the femtosecond laser is amplified in Er-doped fiber amplifier before propagating through a piece of highly nonlinear fiber for expanding the spectrum. The carrier-envelope offset frequency of the comb which has a signal-to-noise ratio more than 35 dB is extracted by means of f-2f beating. It demonstrates that both carrier-envelope offset frequency and repetition frequency keep phase locked to a Rubidium atomic clock simultaneously for 2 hours. The frequency stabilized fiber combs will be increasingly applied in optical metrology, attosecond pulse generation, and absolute distance measurement.
We demonstrate a time-of-flight absolute distance measurement method based second harmonic generation using dual-comb with different repetition rates. A distance of about 8m is measured, compared with a laser absolute tracer, the maximum deviation is 19μm at 100ms acquisition time.
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