Coherent detection of the rotational Doppler effect measurement based on triple Fourier transform.

In recent years, the rotational Doppler effect (RDE) has been widely used in rotational motion measurement. However, the performance of existing detection systems based on the RDE are generally limited by the drastic reduction of signal-to-noise ratio (SNR) due to the influence of atmospheric turbulence, partial obscuration of the vortex beam (VB) during propagation, and misalignment between the optical axis of VB and the rotational axis of the object, which poses a challenge for practical applications. In this paper, we proposed a coherent detection method of the RDE measurement based on triple Fourier transform. First, the weak RDE signal in backscattered light is amplified by using the balanced homodyne detection method, and the amplified signal still retains the same characteristic of severe broadening in the frequency domain as the original signal. Furthermore, we proposed the triple Fourier transform to extract the broadened RDE frequency shift signal after the coherent amplification. The proposed method significantly improves the SNR of RDE measurement and facilitates the accurate extraction of rotational speed, which helps to further improve the RDE detection range and promote its practical application.

Simulating GPS radio signal to synchronize network--a new technique for redundant timing.

Currently, many distributed systems such as 3G mobile communications and power systems are time synchronized with a Global Positioning System (GPS) signal. If there is a GPS failure, it is difficult to realize redundant timing, and thus time-synchronized devices may fail. In this work, we develop time transfer by simulating GPS signals, which promises no extra modification to original GPS-synchronized devices. This is achieved by applying a simplified GPS simulator for synchronization purposes only. Navigation data are calculated based on a pre-assigned time at a fixed position. Pseudo-range data which describes the distance change between the space vehicle (SV) and users are calculated. Because real-time simulation requires heavy-duty computations, we use self-developed software optimized on a PC to generate data, and save the data onto memory disks while the simulator is operating. The radio signal generation is similar to the SV at an initial position, and the frequency synthesis of the simulator is locked to a pre-assigned time. A filtering group technique is used to simulate the signal transmission delay corresponding to the SV displacement. Each SV generates a digital baseband signal, where a unique identifying code is added to the signal and up-converted to generate the output radio signal at the centered frequency of 1575.42 MHz (L1 band). A prototype with a field-programmable gate array (FPGA) has been built and experiments have been conducted to prove that we can realize time transfer. The prototype has been applied to the CDMA network for a three-month long experiment. Its precision has been verified and can meet the requirements of most telecommunication systems.

Three-mode optoacoustic parametric amplifier: a tool for macroscopic quantum experiments.

We introduce the three-mode optoacoustic parametric amplifier (OAPA), a close analog of the optical parametric amplifier, for macroscopic quantum mechanics experiments. The radiation pressure reaction of light on the reflective surface of an acoustic resonator provides a nonlinearity similar to the Kerr effect in the optical parametric amplifier. The OAPA can be tuned to operate in a positive gain regime where acoustic signals are amplified or in a negative gain regime where acoustic modes are cooled. Compared with conventional optoacoustic devices, (i) the OAPA incorporates two transverse cavity modes such that the carrier and sideband fields simultaneously resonate, and (ii) it is less susceptible to the laser phase and amplitude noise. These two features significantly ease the experimental requirements for cooling of acoustic modes to their quantum-ground state and creating entangled pairs of phonons and photons.

Direct measurement of absorption-induced wavefront distortion in high optical power systems.

Wavefront distortion due to absorption in the substrates and coatings of mirrors in advanced gravitational wave interferometers has the potential to compromise the operation and sensitivity of these interferometers [Opt. Lett.29, 2635-2637 (2004)]. We report the first direct spatially-resolved measurement, to our knowledge, of such wavefront distortion in a high optical power cavity. The measurement was made using an ultrahigh sensitivity Hartmann wavefront sensor on a dedicated test facility. The sensitivity of the sensor was lambda/730, where lambda=800 nm.

[Structural variation of o-amino-benzoic acid induced by free electron laser].

Free electron laser (FEL) have been developed as tunable lasers over a wide range of wavelenths. Equipment in the region of 6-16 microns (1,660-630 cm-1) is available in the Beijing free electron laser erected by Institute of High Energy Physics, CAS. In this study we report first measurements of infrared-induced molecular change under irradiation by FEL of 9.414 microns and 6.228 microns, respectively. Taking o-amino-benzoic acid as an example, FTIR characterization indicates that the peak positions of both N-H and C=O stretching vibrations shift before and after the irradiation of FEL. It demonstrates that FEL induces the re-arrangement of H-bonds between COOH and NH2 by stimulating the vibration of carboxyl groups. It suggests that the presence of stable/metastable phases in the title compound.