Hongxing Yang

A Capacitive Sensor for the Measurement of Departure From the Vertical Movement

A capacitive sensor is proposed to measure the departure from the vertical movement. It has a special structure consisting of a long silica filament static plate and a cylindrical-surface moving plate. Now it is adopted to measure the horizontal displacement of the movable coil moving along the vertical direction in a joule balance. For the consideration of avoiding the error caused by the transfer process of direction reference, the sensor is designed to have a direct vertical direction reference. The measurement errors caused by the imperfection of the capacitive sensor plates are discussed. In addition, the characteristics of the sensor, such as sensitivity, resolution, and short-time stability, have been analyzed through experiments. The experimental results show that the resolution of the sensor is better than 0.05 μm, and the short-time stability is about 0.1 μm peak to peak, which is better than that of the alcohol mirror. The combined uncertainty of the capacitive sensor obtained through analysis and experiments is 1.68 μm.

Doubly Q-switched Ho:LuAG laser with acoustic-optic modulator and Cr 2+ :ZnS saturable absorber

A doubly Q-switched (DQS) Ho:LuAG laser resonantly pumped by a 1.91-μm laser was first presented with an acoustic-optic modulator (AOM) and a Cr2+:ZnS saturable absorber. A comparison among the active Q-switched (AQS), passively Q-switched (PQS), and DQS laser performances was carried out. The maximum continuous wave (CW) output power of 6 W with the central wavelength of 2100.65 nm was obtained at an incident pump power of 35.2 W. Compared with CW laser, the AQS, PQS, and DQS lasers shared the same central wavelength of 2098.34 nm under the same incident pump power. The central wavelength of the AQS and DQS lasers remained constant with the change of AOM repetition frequency (RF). When the incident pump power was 35.2 W and the AOM RF was 15 kHz, the DQS Ho:LuAG laser at a maximum RF of 2.13 kHz achieved the maximum average output power of 4.95 W. At the AOM RF of 10 kHz, the DQS Ho:LuAG laser achieved the shortest pulse width of 40.4 ns with the highest peak power of 61.5 kW. At an incident pump power of 35.2 W, the PQS Ho:LuAG laser obtained the shortest pulse width of 46.1 ns, corresponding to the RF of 2.25 kHz. Experiment results showed that the pulse width could be compressed effectively with a significant increase of peak power for a 2-μm DQS laser.

Frequency stabilization of an internal mirror He-Ne laser with a high frequency reproducibility.

A method has been developed for the stabilization of an internal mirror He-Ne laser to achieve a high frequency reproducibility that is mainly influenced by the temperature of the stabilized laser. However, it is difficult to achieve a reproducible temperature in a short time under different ambient temperatures. In this paper, the He-Ne laser is stabilized based on the relationship between the laser mode number and the laser cavity temperature where a reproducible temperature can be rapidly achieved under different ambient temperatures, resulting in a high frequency reproducibility. Experiments have demonstrated that the He-Ne laser used can be stabilized in approximately 10 min, typically 6 min; the frequency stability is less than 2×10(-10); the frequency reproducibility is less than 1×10(-9).

Identification and elimination of half-synthetic wavelength error for multi-wavelength long absolute distance measurement

A multi-wavelength absolute distance measurement method based on synchronous occurrence and phase measurement of coarse and refined synthetic wavelength is proposed to remove the effect of vibration on the measurement distance. The phase value of refined synthetic wavelength is monitored to see if half-synthetic wavelength error occurs by judging whether it is near to 2π or 0. If yes, a new suitable refined synthetic wavelength is generated to re-measure the distance with phase near π. A measuring system has been built with a He–Ne laser source and three acousto-optic frequency shifters for implementation of this method. A comparative measurement has been performed using a counting laser interferometer at a distance of 20 m. Actual measurements indicate that half-synthetic wavelength error has been identified and eliminated with uncertainty of smaller than 60 µm under laboratory conditions.

Ultrastable offset-locked frequency-stabilized heterodyne laser source with water cooling

An ultrastable frequency-stabilized He-Ne laser with a water-cooling structure has been developed for a high-speed and high-accuracy heterodyne interferometer. To achieve high frequency stability and reproducibility, a two-mode He-Ne laser was offset locked to an iodine-stabilized laser. An improved synchronous multi-cycle offset frequency-measurement method with a gate time of an integer multiple of the modulation period was employed to remove the frequency-modulation effect on the offset-frequency counter. A water-cooling structure based on the double-helix structure was established to provide a stable and low-temperature working environment. This structure can remarkably reduce the frequency instability arising from the environmental temperature variation and the thermal pollution released from the laser to the environment. The experimental results indicate that the frequency stability according to the Allen variance is better than 2.3×10-11 (τ=10  s) and the frequency reproducibility is better than 4.5×10-10.

Long-Distance Measurement Applying Two High-Stability and Synchronous Wavelengths

A high-precision laser range finder based on highly stable synchronous measuring wavelengths with a large range is proposed in this paper. In this method, the coarse and fine measuring wavelengths are generated by the beat signals of lasers in different frequencies. The lasers are produced by frequency shifting one laser mode with three acousto-optic modulators working at different frequencies. To realize an appropriate coarse and fine measuring wavelengths for the required measurement range and resolution, the driving frequencies of the acousto-optic modulators are controlled by direct digital synthesizer (DDS) in high precision. At the same time, the frequency drift of the laser source and other common errors are reduced by applying the optical heterodyne effect. Consequently, the stability of measuring wavelengths is significantly improved. Furthermore, an improved digital method of phase measurement with same basic clock DDS is proposed to achieve synchronous phase measurement with a little error. Results from the experiments show that the measuring frequency stability is better than 1.75 x 10-9 (τ = 100 s), and a measurement with relative uncertainty of 0.63 x 10-6 at a distance of 100 m is achieved under laboratory conditions.

Alignment technology for coil movement of Joule Balance

To estimate the misalignment error caused by the horizontal movement of fixed coil in the Joule Balance project, a high precision measuring method based on self-alignment capacitive displacement sensor is proposed to measure the displacement of the Joule Balance in the horizontal direction. Results show that the measurement resolution can reach 1 μm.

Frequency stabilized laser for length metrology system in Joule Balance

This paper shows a frequency offset-locked laser designed for the length metrology system in the next-generation Joule Balance. This laser integrates most of the components in a limited volume and avoids serious thermal pollution. The frequency stability of this laser is about 4.2×10-10 at the sampling rate of 0.1s, which totally meets the demands of length measurement with the relative uncertainty better than 1×10-8 in Joule Balance.

Methods for measuring and controlling the movable coil position of the joule balance

A system based on laser heterodyne interferometer is proposed in this paper to measure and control the movable coil position of a joule balance. A damping system is used to suppress the movement of movable coil in the horizontal direction while a piezoelectric ceramic control unit with PID controller is used to inhibit the vibration and long term drift of movable coil in the vertical direction. The effectiveness of the proposed method in measuring and controlling the movable coil position of a joule balance is proved through experiments. Experiment results indicate that the displacement of movable coil in the vertical direction can be reduced from 400 nm to 50 nm while the drift of movable coil is successfully inhibited.

High stable remote photoelectric receiver for interferometry

A high sensitive and high stable remote photoelectric receiver has been developed to reduce noise and phase delay drift caused by thermal pollution and environmental interference. The phase delay drift model is analyzed and built based on a traditional photoelectric receiver. According to the model, a new mechanical isolation structure and a temperature control system are designed to keep the photoelectric receiver in a low constant temperature. Comparison experiments with traditional bias voltage compensation method and temperature control method are carried out between photoelectric receivers. The results verify that the output voltage fluctuation of photoelectric receiver used is reduced by 65% while the phase drift between measurement and reference photoelectric receivers decreases from 1.05° to 0.02°.