Intensity evaluation using a femtosecond pulse laser for absolute distance measurement

Impact of Laser Intensity Noise on Dual-Comb Absolute Ranging Precision

Noise in mode-locked lasers has been a central issue for dual-comb metrological applications. In this work, we investigate the laser intensity noise on dual-comb absolute ranging precision. Two different dual-comb schemes based on linear optical sampling (LOS) and nonlinear asynchronous optical sampling (ASOPS) have been constructed. In the LOS scheme, the ranging precision deteriorates with the increase in laser relative intensity noise (RIN). This effect can be corrected by implementing a balanced photo-detection (BPD). In the ASOPS scheme, the experiment shows that the conversion from laser RIN to dual-comb ranging precision is negligible, making a balanced detection unnecessary for ranging precision improvement. The different manners of RIN's impact on absolute ranging precision are attributed to the distinct cross-correlation signal patterns and the underlying time-of-flight (TOF) extraction algorithms.

Measurement of the distance between two parallel precision apertures using time-of-flight method for cryogenic radiometry

Time-of-flight method was adopted to measure the distance between two parallel precision apertures utilized in a vacuum chamber for cryogenic radiometry. The diameters of the apertures are 9 mm and 8 mm, respectively. A 1550-nm femtosecond pulse laser, a 70-GHz photodetector, and a 30-GHz oscilloscope were used to measure the round-trip flight time difference between the flat front surfaces of the two precision apertures. The distance between the apertures was analyzed to be 0.36423 m with a relative standard uncertainty of 0.004%. The non-contact distance measurement method is useful for applications such as low background infrared radiance measurement system based on an absolute cryogenic radiometer.

A High-Resolution Ultrasonic Ranging System Using Laser Sensing and a Cross-Correlation Method

Ultrasound has been proven to be a valid tool for ranging, especially in water. In this paper, we design a high-resolution ultrasonic ranging system that uses a thin laser beam as an ultrasonic sensor. The laser sensing provides a noncontact method for ultrasound detection based on acousto-optic diffraction. Unlike conventional methods, the ultrasound transmitted from the transducer is recorded as the reference signal when it first passes through the laser. It can be used to improve the accuracy and resolution of the time-of-flight (TOF) by a cross-correlation method. Transducers with a central frequency of 1 MHz and diameters of 20 mm and 28 mm are used in the experiment. Five targets and a test piece are used to evaluate the ranging performance. The sound velocity is measured by the sound velocity profiler (SVP). The repeatability error of TOF is less than 4 ns, and the theoretical resolution of TOF is 0.4 ns. The results show a measurement resolution within one-tenth of the wavelength of ultrasound and an accuracy better than 0.3 mm for targets at a distance up to 0.8 m. The proposed system has potential applications in underwater ranging and thickness detection.

Glass thickness and index measurement using optical sampling by cavity tuning

In this paper, we describe a method based on optical sampling by cavity tuning, which is capable of high-accuracy glass thickness and index measurement. By tuning the repetition frequency of the frequency comb, a series of cross-correlation patterns can be obtained that correspond to the front and rear surfaces of the specimen and the co-operation mirror. Both the geometrical thickness and the optical thickness of the specimen can be measured via the cross-correlation patterns, and consequently, the glass refractive index can be determined at the same time. The comparison with the reference value shows an agreement within 1.3 μm for the thickness measurement, and within 5×10^-4 for the refractive index measurement.

Absolute distance measurement by multi-heterodyne interferometry using a frequency comb and a cavity-stabilized tunable laser

In this paper, we develop a multi-heterodyne system capable of absolute distance measurement using a frequency comb and a tunable diode laser locked to a Fabry-Perot cavity. In a series of subsequent measurements, numerous beat components can be obtained by downconverting the optical frequency into the RF region with multi-heterodyne interferometry. The distances can be measured via the mode phases with a series of synthetic wavelengths. The comparison with the reference interferometer shows an agreement within 1.5 μm for the averages of five measurements and 2.5 μm for the single measurement, which is at the 10^-8 relative precision level.

Absolute distance measurement by chirped pulse interferometry using a femtosecond pulse laser

We propose here a method for absolute distance measurement by chirped pulse interferometry using frequency comb. The principle is introduced, and the distance can be measured via the shift of the widest fringe. The experimental results show an agreement within 26 μm in a range up to 65 m, corresponding to a relative precision of 4 × 10^-7, compared with a reference distance meter.