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.
As a bridge connecting microwave frequency and optical frequency, femtosecond laser has important significance in optical frequency measurement. Compared with the traditional Ti-sapphire femtosecond optical frequency comb, with the advantages of compact structure, strong anti-interference ability and low cost, the fiber femtosecond optical frequency comb has a wider application prospect. An experiment of spectrum broadening in a highly nonlinear photonic crystal fiber pumped by an Er-fiber mode-locked femtosecond laser is studied in this paper. Based on optical amplification and frequency doubling, the central wavelength of the output spectrum is 780nm and the average power is 232mW. With the femtosecond pulses coupled into two different photonic crystal fibers, the coverage of visible spectrum is up to 500nm-960nm. The spectral shape and width can be optimized by changing the polarization state for satisfying the requirments of different optical frequencies 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.
The background and principle of zero-crossing point locking technology are introduced in this paper. An experimental locking system is designed to realize fast locking of zero-crossing point, and the results of locking is studied by analyzing zero-crossing point locking signal. In the distance measurement of femtosecond pulsed laser, a crystal produces the balanced cross-correlation (BCC) signal, which signifies the time offset of the target pulses with respect to the reference pulses. By continuously pulling this signal to zero-crossing point, the locking system provides a closed loop control process, which ensures the stability of the zero-crossing point and the precision of measurement. This locking system is mainly made up by five sections. As a core section of system, P-I circuit can optimize the locking state by changing parameters. A frequency counter referenced to the rubidium atomic clock is used to measure the pulse repetition rate with a stability of 10<sup>-12 </sup>in the sampling rate of 10s in 24 hours, which is helpful to analyze the measurement precision. In the experiment, the result of zero-crossing point lock can reach to 15mV, in other words, the range of amplitude variation can be reduced to less than 15mV after locking. With the repetition rate data evaluated, the jitter of the pulse repetition rate is within 25Hz in the sampling time of 15s after locking the zero-crossing point. It is proved that the locking system designed has a high practical value in the distance and vibration measurement of femtosecond pulsed laser.
Two vibration measurement methods with femtosecond pulsed laser based on the optical cross-correlation technique are presented independently in this paper. The balanced optical cross-correlation technique can reflect the time jitter between the reference pluses and measurement pluses by detecting second harmonic signals using type II phase-matched nonlinear crystal and balanced amplified photo-detectors. In the first method, with the purpose of attaining the vibration displacement, the time difference of the reference pulses relative to the measurement pluses can be measured using single femtosecond pulsed laser. In the second method, there are a couple of femtosecond pulsed lasers with high pulse repetition frequency. Vibration displacement associated with cavity length can be calculated by means of precisely measuring the pulse repetition frequency. The results show that the range of measurement attains ±150μm for a 500fs pulse. These methods will be suited for vibration displacement measurement, including laboratory use, field testing and industrial application.
The new technique known as “The femtosecond frequency comb technology” has dramatic impact on the diverse fields of precision measurement and nonlinear optical physics. In order to acquire high-precision and high-stability femtosecond comb, it’s necessary to stabilize the repetition rate f<sub>Rep</sub> and the offset frequency f<sub>0</sub>. This article presents the details of stabilizing and controlling the comb parameter f<sub>Rep</sub> and finally phase lock the repetition rate of femtosecond laser to a radio frequency reference, derived from an atomic clock. In practice, the narrower the bandwidth of lock system (close-loop system), the higher stability we can achieve, but it becomes easier to be unlocked for external disturb. We adopt a method in servo unit to avoid this problem in this paper. The control parameters P and I can be adjusted and optimized more flexibly. The lock steps depend on the special servo system make it easier to find the right parameters and the lock becomes more convenient and quickly. With this idea, the locked time of repetition rate can be as long as the mode-locking time of the laser. The stability of laser can be evaluated by allan deviation. In this research, the contrast of stability of f<sub>Rep</sub> between the locked laser and the unlocked is given. The new lock system is proved reasonable.