Half-spectrum OFDM to quadruple the spectral efficiency of underwater wireless optical communication with digital power division multiplexing.

Improving the spectrum efficiency (SE) is an effective method to further enhance the data rate of bandwidth-limited underwater wireless optical communication (UWOC) systems. Non-orthogonal frequency-division multiplexing (NOFDM) with a compression factor of 0.5 can save half of the bandwidth without introducing any inter-carrier-interference (ICI) only if the total number of subcarriers is large enough, and we termed it as half-spectrum OFDM (HS-OFDM). To the best of our knowledge, this is the first reported work on a closed-form HS-OFDM signal in the discrete domain from the perspective of a correlation matrix. Due to the special mathematical property, no extra complex decoding algorithm is required at the HS-OFDM receiver, making it as simple as the conventional OFDM receiver. Compared with traditional OFDM, HS-OFDM can realize the same data rate, but with a larger signal-to-noise ratio (SNR) margin. To fully use the SNR resource of the communication system, we further propose a digital power division multiplexed HS-OFDM (DPDM-HS-OFDM) scheme to quadruple the SE of conventional OFDM for the bandwidth-starved UWOCs. The experimental results show that HS-OFDM can improve the receiver sensitivity by around 4 dB as opposed to conventional 4QAM-OFDM with the same data rate and SE. With the help of the DPDM-HS-OFDM scheme, the data rate of multi-user UWOC can reach up to 4.5 Gbps under the hard-decision forward error correction (HD-FEC) limit of a bit error rate (BER) of 3.8×10-3. Although there is some performance degradation in comparison with single-user HS-OFDM, the BER performance of multi-user DPDM-HS-OFDM is still superior to that of conventional single-user 4QAM-OFDM. Both single-user HS-OFDM and multi-user DPDM-HS-OFDM successfully achieve 2 Gbps/75 m data transmission, indicating that the DPDM-HS-OFDM scheme is of great importance in bandwidth-limited UWOC systems and has guiding significance to underwater wireless optical multiple access.

Ppb-level methane detection sensitivity based on a homemade Raman fiber amplifier and differential photoacoustic technology.

A high-power near-infrared wavelength-modulated differential photoacoustic spectroscopy sensor for parts-per-billion (ppb) level methane detection is reported by using a homemade Raman fiber optical amplifier. A commercial 1653.7 nm continuous wave distributed feedback laser is employed as a seed source to excite a high light power of ∼550m W, which greatly improves sensor performance. Wavelength modulation spectroscopy and differential techniques are applied to further improve the signal-to-noise ratio of the photoacoustic signal. A 1σ minimum detection limit of ∼10p p b for methane detection is achieved with an integration time of 10 s.

Optical Fiber Fabry-Pérot Microfluidic Sensor Based on Capillary Fiber and Side Illumination Method.

In this paper, an optical fiber Fabry-Pérot (FP) microfluidic sensor based on the capillary fiber (CF) and side illumination method is designed. The hybrid FP cavity (HFP) is naturally formed by the inner air hole and silica wall of CF which is side illuminated by another single mode fiber (SMF). The CF acts as a naturally microfluidic channel, which can be served as a potential microfluidic solution concentration sensor. Moreover, the FP cavity formed by silica wall is insensitive to ambient solution refractive index but sensitive to the temperature. Thus, the HFP sensor can simultaneously measure microfluidic refractive index (RI) and temperature by cross-sensitivity matrix method. Three sensors with different inner air hole diameters were selected to fabricate and characterize the sensing performance. The interference spectra corresponding to each cavity length can be separated from each amplitude peak in the FFT spectra with a proper bandpass filter. Experimental results indicate that the proposed sensor with excellent sensing performance of temperature compensation is low-cost and easy to build, which is suitable for in situ monitoring and high-precision sensing of drug concentration and the optical constants of micro-specimens in the biomedical and biochemical fields.

Novel Broadband Cavity-Enhanced Absorption Spectrometer for Simultaneous Measurements of NO2 and Particulate Matter.

A novel instrument based on broadband cavity-enhanced absorption spectroscopy has been developed using a supercontinuum broadband light source, which showcases its ability in simultaneous measurements of the concentration of NO2 and the extinction of particulate matter. Side-by-side intercomparison was carried out with the reference NOx analyzer for NO2 and OPC-N2 particle counter for particulate matter, which shows a good linear correlation with r2 > 0.90. The measurement limits (1σ) of the developed instrument were experimentally determined to be 230 pptv in 40 s for NO2 and 1.24 Mm-1 for the extinction of particulate matter in 15 s. This work provides a promising method in simultaneously monitoring atmospheric gaseous compounds and particulate matter, which would further advance our understanding on gas-particle heterogeneous interactions in the context of climate change and air quality.

A New Photoacoustic Soot Spectrophone for Filter-Free Measurements of Black Carbon at 880 nm.

A new photoacoustic soot spectrometer (PASS) operating at 880 nm was developed, for the first time, for filter-free measurements of black carbon (BC). The performance of the developed PASS was characterized and evaluated using a reference aethalometer AE51 on incense smoke in the air. An excellent correlation on the measurement of incense smoke was found between the two instruments in comparison with a regression coefficient of 0.99. A 1 σ detection limit of 0.8 µg m-3 was achieved for BC measurement at a time resolution of 1 s. It can be further reduced to 0.1 µg m-3, using a longer integration time of 1 min.

Compact photoacoustic spectrophone for simultaneously monitoring the concentrations of dichloromethane and trichloromethane with a single acoustic resonator.

Chlorinated hydrocarbons are frequently used as reagents and organic solvents in different industrial processes. Real-time detection of chlorinated hydrocarbons, as toxic air pollutants and carcinogenic species, is an important requirement for various environmental and industrial applications. In this study, a compact photoacoustic (PA) spectrophone based on a single acoustic resonator for simultaneous detection of trichloromethane (CHCl3) and dichloromethane (CH2Cl2) is first reported by employing a low-cost distributed feedback (DFB) laser emitting at 1684 nm. In consideration of the significant overlapping of absorption spectral from trichloromethane and dichloromethane, the multi-linear regression method was used to calculate the concentrations of CHCl3 and CH2Cl2 with special characterization of the absorption profile. The current modulation amplitude and detection phase in the developed PA spectrophone was optimized for high sensitivity of individual components. The measurement interference of CHCl3 and CH2Cl2 on each other was investigated for accurate detection, respectively. For field measurements, all optical elements were integrated into a 40 cm × 40 cm × 20 cm chassis. This paper provides an experimental verification which strongly recommends this sensor as a compact photoacoustic field sensor system for chlorinated hydrocarbon detection in different applications.

Transportable mid-infrared laser heterodyne radiometer operating in the shot-noise dominated regime.

A transportable laser heterodyne radiometer (LHR) based on an external cavity quantum cascade laser, operating in the mid-infrared (mid-IR) around 8 µm, was developed for ground-based remote sensing of multiple greenhouse gases. A newly available novel flexible mid-IR polycrystalline fiber was first exploited to couple solar radiation, real-time captured using a portable sun-tracker, to the LHR receiver. Compared to free space coupling of sunlight, the technique usually used nowadays in the mid-IR, such fiber coupling configuration makes the LHR system readily more stable, simpler, and robust. Operation of the LHR with quasi-shot-noise limited performance was analyzed and experimentally achieved by optimizing local oscillator power. To the best of our knowledge, no such performance approaching the fundamental limit has been reported for a transportable LHR operating at a long mid-IR wavelength around 8 µm. CH4 and N2O were simultaneously measured in the atmospheric column using the developed mid-IR LHR. The experimental LHR spectrum of CH4 and N2O was compared and is in good agreement with a referenced Fourier-transform infrared spectrum from the Total Carbon Column Observing Network observation site and with a simulation spectrum from atmospheric transmission modeling.

Incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS)-based strategy for direct measurement of aerosol extinction in a lidar blind zone.

In this Letter, the development of a custom-designed incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS) and its application to in situ measurement of aerosol extinction near the ground surface are described in an effort to address the issue of missing data in the light detection and ranging (lidar) blind zone in the first hundreds of meters of the observation range. Combined measurements of aerosol extinction at the same location using lidar remote sensing at 355 nm and in situ IBBCEAS operating in the UV spectral region around 370 nm showed results with a good correlation (${{\rm R}^2} = {0.90}$R2=0.90) between the two measurement techniques. This Letter highlights a new strategy for near-end lidar calibration, using a ground-based compact and robust IBBCEAS located at the lidar measurement site to determine the vertical profile of the aerosol extinction coefficient with a higher accuracy.

Circular Regression in a Dual-Phase Lock-In Amplifier for Coherent Detection of Weak Signal.

Lock-in amplification (LIA) is an effective approach for recovery of weak signal buried in noise. Determination of the input signal amplitude in a classical dual-phase LIA is based on incoherent detection which leads to a biased estimation at low signal-to-noise ratio. This article presents, for the first time to our knowledge, a new architecture of LIA involving phase estimation with a linear-circular regression for coherent detection. The proposed phase delay estimate, between the input signal and a reference, is defined as the maximum-likelihood of a set of observations distributed according to a von Mises distribution. In our implementation this maximum is obtained with a Newton Raphson algorithm. We show that the proposed LIA architecture provides an unbiased estimate of the input signal amplitude. Theoretical simulations with synthetic data demonstrate that the classical LIA estimates are biased for SNR of the input signal lower than -20 dB, while the proposed LIA is able to accurately recover the weak signal amplitude. The novel approach is applied to an optical sensor for accurate measurement of NO 2 concentrations at the sub-ppbv level in the atmosphere. Side-by-side intercomparison measurements with a commercial LIA (SR830, Stanford Research Inc., Sunnyvale, CA, USA ) demonstrate that the proposed LIA has an identical performance in terms of measurement accuracy and precision but with simplified hardware architecture.