Frequency-locking and threshold current-lowering effects of a quantum cascade laser and an application in gas detection field

In this paper,the frequency-locking and threshold current-lowering effects of a quantum cascade laser are studied and achieved.Combined with cavity-enhanced absorption spectroscopy,the noninvasive detection of H_2 with a prepared concentration of 500 ppm in multiple dissolved gases is performed and evaluated.The high frequency selectivity of 0.0051 cm^-1 at an acquisition time of 1 s allows the sensitive detection of the（1-0） S（l） band of H_2 with a high accuracy of（96.53±0.29）%and shows that the detection limit to an absorption line of 4712.9046 cm^-1 is approximately（17.26±0.63） ppm at an atmospheric pressure and a temperature of 20 ℃.

Dual-mode stabilization for laser to radio-frequency locking by using a single-sideband modulation and a Fabry-Pérot cavity

The dual-mode stabilization scheme has been demonstrated as an efficient way to stabilize laser frequency.In this study,we propose a novel dual-mode stabilization scheme that employs a sizable Fabry-Pérot cavity instead of the microcavity used in previous studies and has enabled higher bandwidth for locking.The results demonstrate a 30-fold reduction in laser frequency drift,with frequency instability below 169 kHz for integration time exceeding 1 h and a minimum value of 33.8 kHz at 54 min.Further improvement could be achieved by optimizing the phase locking.This scheme has potential for use in precision spectroscopic measurement.

Joint frequency offset tracking and PAPR reduction algorithm in OFDM systems

This paper presents an algorithm that aims to reduce the peak-to-average power ratio（PAPR） of orthogonal frequency division multiplexing（OFDM） communication systems while maintaining frequency tracking.The algorithm achieves PAPR reduction by applying the complex conjugates of the data symbol obtained from the frequency domain to cancel the phase of the data symbol.A likelihood estimator is used to obtain the sub-carrier phase error due to the residual carrier frequency offset（RCFO） using the same complex conjugates as a pilot signal.Furthermore,a joint time and frequency domain multicarrier phase locked loop（MPLL） is developed to compensate additional frequency offset.Simulation results show that this algorithm is capable of reducing PAPR without impacting the frequency tracking performance.

A novel high precision Doppler frequency estimation method based on the third-order phase-locked loop

In deep space exploration,many engineering and scientific requirements require the accuracy of the measured Doppler frequency to be as high as possible.In our paper,we analyze the possible frequency measurement points of the third-order phase-locked loop(PLL)and find a new Doppler measurement strategy.Based on this finding,a Doppler frequency measurement algorithm with significantly higher measurement accuracy is obtained.In the actual data processing,compared with the existing engineering software,the accuracy of frequency of 1 second integration is about 5.5 times higher when using the new algorithm.The improved algorithm is simple and easy to implement.This improvement can be easily combined with other improvement methods of PLL,so that the performance of PLL can be further improved.

Laser frequency locking method for Rydberg atomic sensing

Based on the Rydberg cascade electromagnetically induced transparency,we propose a simultaneous dual-wavelength locking method for Rydberg atomic sensing at room temperature.The simplified frequency-locking configuration uses only one signal generator and one electro-optic modulator,realizing real-time feedback for both lasers.We studied the effect of the different probe and coupling laser powers on the error signal.In addition,the Allan variance and a 10 kHz amplitudemodulated signal are introduced to evaluate the performance of the laser frequency stabilization.In principle,the laser frequency stabilization method presented here can be extended to any cascade Rydberg atomic system.

A 2-step GPS carrier tracking loop for urban vehicle applications

Global positioning system (GPS) for vehicle applications in the urban area is challenged by low signal intensity. The carrier loop based on fast Fourier transform (FFT) can obtain a high signal to noise ratio (SNR) gain by increasing the observation time. However, this leads to a major problem that the acceleration cannot be ignored. The performance of the FFT-based loop will decline with the acceleration increasing. This paper discusses the effect of the dynamic on FFT first. Then a high performance carrier tracking loop for weak GPS L5 signals is proposed. It combines discrete chirp-Fourier transform (DCFT) and the phase fitting method to estimate Doppler frequency and Doppler rate simultaneously. First, a sequence of integration results is used to perform DCFT to estimate coarse Doppler frequency and Doppler rate. Second, the phase of the sequence is calculated and used to perform linear fitting. By the phase fitting method, the fine Doppler frequency and Doppler rate can be estimated. The computation cost is small because the integration results are used and the phase fitting method needs only coarse estimates of Doppler frequency and Doppler rate. Compared with FFT and DCFT, the precision of the phase fitting method is not limited by the resolution. Thus the proposed loop can get high precision and low carrier to noise ratio (C/N-0) tracking threshold. Simulation results show this loop has a great improvement than conventional loops for urban weak-signal applications.

Atmospheric N2O gas detection based on an inter-band cascade laser around 3.939 μm

N2O is a significant atmospheric greenhouse gas that contributes to global warming and climate change.In this work,the high sensitivity detection of atmospheric N2O is achieved using wavelength modulation spectroscopy(WMS)with an inter-band cascade laser operating around 3.939μm.A Lab VIEW-based software signal generator and software lock-in amplifiers are designed to simplify the system.In order to eliminate the interference from water vapor,the detection was performed at a pressure of 0.1 atm(1 atm=1.01325×10^5 Pa)and a drying tube was added to the system.To improve the system performance for long term detection,a novel frequency locking method and 2 f/1 f calibration-free method were employed to lock the laser frequency and calibrate the power fluctuations,respectively.The Allan deviation analysis of the results indicates a detection limit of^20 ppb(1 ppb=1.81205μg/m3)for a 1 s integration time,and the optimal detection limit is^5 ppb for a 40-s integration time.

Widely tunable laser frequency offset locking to the atomic resonance line with frequency modulation spectroscopy

A simple and robust technique is reported to offset lock a single semiconductor laser to the atom resonance line with a frequency difference easily adjustable from a few tens of megahertz up to tens of gigahertz. The proposed scheme makes use of the frequency modulation spectroscopy by modulating sidebands of a fiber electro-optic modulator output. The short-term performances of a frequency offset locked semiconductor laser are experimentally demonstrated with the Allan variance of around 3.9 × 10-11 at a 2 s integration time. This method may have many applications, such as in Raman optics for an atom interferometer.

Modulation-free laser frequency ofset locking using bufer gas-induced resonance

We experimentally demonstrate a simple modulation-free scheme for ofset locking the frequency of a laser using bufer gas-induced resonance. Our scheme excludes the limitation of low difraction efciency and laser input intensity when an acousto-optic modulator is applied to shift the laser frequency from the resonance. We show the stabilization of a strong 795- nm laser detuned up to 550 MHz from the 87Rb 5S1/2 F=2→5P1/2F'=2 transition. The locking range can be modifed by controlling the bufer gas pressure. A laser line width of 2 MHz is achieved over 10 min.

Label-free detection of single nanoparticles and biological molecules using microtoroid optical resonators

Single-molecule detection is one of the fundamental challenges of modern biology.Such experiments often use labels that can be expensive,difficult to produce,and for small analytes,might perturb the molecular events being studied.Analyte size plays an important role in determining detectability.Here we use laser-frequency locking in the context of sensing to improve the signal-to-noise ratio of microtoroid optical resonators to the extent that single nanoparticles 2.5 nm in radius,and 15.5 kDa molecules are detected in aqueous solution,thereby bringing these detectors to the size limits needed for detecting the key macromolecules of the cell.Our results,covering several orders of magnitude of particle radius(100 nm to 2 nm),agree with the‘reactive’model prediction for the frequency shift of the resonator upon particle binding.This confirms that the main contribution of the frequency shift for the resonator upon particle binding is an increase in the effective path length due to part of the evanescent field coupling into the adsorbed particle.We anticipate that our results will enable many applications,including more sensitive medical diagnostics and fundamental studies of single receptor–ligand and protein–protein interactions in real time.