Photoacoustic spectroscopy has the advantage of multi-gas sensing with only one sound detector because of its non-selectivity. A photoacoustic cell (PAC) is an important part of the system as it amplifies the intensity of sound pressure. In this paper, we report on the development of a multi-gas sensing system based on a specially designed cruciform photoacoustic cell (CF-PAC), whose structure contains two beam paths and is suitable for multiple laser beams incident from different directions independently. It is especially suitable where beams are not easy to combine due to large wavelength differences. The structure of the cell is carefully designed using the finiteelement method (FEM) and the resonant frequency of it is about 5050 Hz and the external dimension is only 67x60x40 mm with the inner volume of about 24 cm 3 . We also present a contrast experiment between the CFPAC and the conventional "H-type PAC " with similar parameters and the result shows that the former sacrifices only a small amount of performance, but adds beam path effectively. The sensor is designed to detect methane (CH 4 ), nitrogen dioxide (NO 2 ), and carbon dioxide (CO 2 ), and the minimum detection limit (MDL) with a 5 s integration time are 113 ppb, 2.1 ppm, and 1.6 ppm, respectively. The 1 sigma normalized noise equivalent absorption coefficients (NNEA) of the three gases are 2.54x10-9 cm- 1 W Hz-1/2 , 5.70x10-8 cm-1 W Hz-1/2 , and 2.31x10-6 cm- 1 W Hz-1/2 , respectively. Modified formulas based on nonlinear regression will be given to make the detection results more accurate in the multi-component gas environment.

Integration of multiwavelength distributed feedback quantum cascade lasers (QCLs) in C-mount packages is demonstrated in this paper, which is applicable to portable gas sensors. The thermal management is performed by modeling and experimentally verifying. The QCLs with the wavelength of 4492, 5459, 6224, and 9640 nm are integrated. The wavelength tuning range and optical power of each QCL are no smaller than 6.5 nm and 65 mW, respectively. The dimensions of the integrated sources are 40 mmx40 mmx80 mm $40\unicode{x0200A}\text{mm}\times 40\unicode{x0200A}\text{mm}\times 80\unicode{x0200A}\text{mm}$.

This letter proposes a simple method for measuring frequency responses of photodetectors (PDs) based on the multiple harmonic responses of a direct-modulated laser (DML). This method provides numerous harmonics as carriers for subsequent Mach–Zehnder modulator (MZM) modulation based on the nonlinear effect of the direct modulation laser. The modulated signal is injected into the photodetector under test, and the frequency response of the photodetector detector is obtained by sampling the MZM modulation component around multiple harmonics on the electronics spectrum analyzer (ESA). By measuring the relative amplitude of the modulation components around different harmonics, measurement error caused by harmonic amplitude differences can be eliminated. This method greatly expands the measurement range of the frequency response from DC to 2nfm. Moreover, since the modulator modulation frequency range occupies only $\frac {1}{2{\mathrm {n}}}$ of the entire measurement range, the uneven frequency response caused by the modulator is reduced, and extra calibration is avoided. According to this method, we use a 35GHz Mach-Zehnder modulator to measure the frequency response of photodetectors with the bandwidth of 35GHz and 50GHz, respectively. We also compared our method with those conventional methods, and the results show that the method which has a wide frequency response can accurately measure the frequency response of the PD under the condition of the method.

In this paper, a compact ambient gas sensor with an optimized photoacoustic cell is reported. The relationship between the geometric dimensions (usually radius and length) of the photoacoustic cell (PAC) and the acoustic signal was studied through theoretical and finite element analysis. Then an optimized H-type PAC with a volume of 80 mm × 30 mm × 30 mm was machined out. The gas capacity is only 18.85 millilitres. The performance of the photoacoustic cell has been verified experimentally by the detection of nitrogen dioxide (NO2) standard gas. With an electret microphone and an economically self-designed 450 nm laser module, the detection of NO2 concentration was executed. The experimental results show good linearity with a fitting R-square of 0.9991. With an SNR (signal-to-noise ratio) of 41.247, the minimum detection limit (MDL) of the system can reach 4.85 ppb (1σ). With an analysis of allan variance, the MDL can achieve 0.11 ppb with a 228 s integration time. By replacing the light source, the system shows great potential for sensitive and compact detectors for other ambient gasses as well.

We report a new design method for miniature dual-resonance photoacoustic (PA) structure, mainly consisting of a miniature T-type PA cell and a piezoelectric ceramics slice. Both the T-type PA cell and the piezoelectric ceramics slice have the resonance frequencies, and the resonance frequencies of the T-type PA cell and the piezoelectric ceramics slice exhibit the same variation pattern with size. It provides support for the design of miniature photoacoustic sensors. As an example, we developed a miniature dual-resonance photoacoustic sensor, the resonance frequency of the miniature T-type PA cell is matched with the natural frequency of the piezoelectric ceramics slice to achieve double resonance of the acoustic signal. The volume of the designed photoacoustic structure is only about 3.75 cubic centimeters. The performance of this system was evaluated through Methane detection. Experimental results show good linearity, the linear fitting R square is 0.9988. The minimum detectable limit of CH4 is calculated to be 542 ppb with an integration time of 217 s according to the Allan-Werle deviation analysis. The performance demonstrated a great potential for the design of miniature photoacoustic sensors.

A compact system for methane monitoring based on Photoacoustic Spectroscopy has been designed. In this paper, we theoretically derived the photoacoustic pressure and the first-order longitudinal resonant frequency of different photoacoustic cells. Based on theoretical analysis, we improved the acoustic signal amplification capability of the photoacoustic (PA) cell by optimizing the radius and length of the resonator and buffer through the simulation. A compact optimized H-type PA cell was designed, whose total volume was 60*30*30 mm3. The performance of this system was evaluated through Methane detection. Experimental results show good linearity, and the linear fitting R square is 0.9993. The minimum detectable limit of CH4 is calculated to be 4 ppm with an integration time of 10 s.

A frequency division multiplexing (FDM) WMS system for multi-gas detection is reported in this paper. The distributed feedback lasers with different wavelengths adopt different modulation frequencies to achieve multi-components gas detection by the FDM technology. Combining optical path extension, normalized harmonic detection and wavelength modulation spectroscopy technology, the highly self-made system realized high sensitivity, high precision and short response time. The performance of this system was evaluated through the mixed gas of methane and hydrogen chloride. Experimental results show great linear response between gas concentration and normalized second harmonic (2f/1f) signal, the minimum detection limit (MDL) of the system is calculated to be 30.16 ppb for CH4 and 31.6 ppb for HCl with the integration time of 15 s. The MDL could be further enhanced to 11 ppb for CH4 and 13.1 ppb for HCl with an extended integration time. The maximum relative error is 2.16% and 3.19% for CH4 and HCl respectively.

In this paper, a single humidity sensor for water vapor and heavy oxygen water vapor detection is presented. The sensor is based on tunable diode laser absorption spectroscopy (TDLAS) and thus has high sensitivity, good selectivity, and a short response time. A 1372 nm distributed feedback (DFB) diode laser is utilized as the light source, the wavelength tuning range of which covers the absorption lines of water vapor and heavy oxygen water vapor. A Herriott gas cell with 12 m optical length is designed for signal-to-noise ratio (SNR) enhancement. The sensor can distinguish between water vapor and heavy oxygen water vapor effectively. The accuracy of water detection is within ±0.5% RH. The accuracy of heavy oxygen water vapor detection is within ±1.0% RH.

Measurement of dissolved oxygen (DO) in liquid samples is of vital importance in both industrial and biomedical fields. In this paper, a DO sensor based on the fluorescence quenching method has been built. The measurement principle is based on fluorescence lifetime detection, which is indicated by the phase difference between an excitation light signal and a fluorescence signal. The nonlinear effect of the fluorescent material has been taken into consideration to obtain a more accurate fitting model. The performance of the system varying with the modulation frequency of excitation light signals is also reported. Modulation frequency mainly affects the sensitivity and phase resolution ratio of the system. The system at the optimized modulation frequency has a good degree of fitting with R 2 value of 0.9981 and a small relative error of 0.79%. The study shows that this kind of sensor with optimal modulation frequency has good performance, which can be used in many important fields.

A scheme of dual-loop optoelectronic oscillator (OEO) based on a directly modulated distributed feedback (DFB) laser is proposed and experimentally demonstrated. In the proposed scheme, thanks to the DFB laser with enough output optical power utilized as an optical source and a modulator at the same time, high-performance microwave signals are generated without external modulator and erbium-doped fiber amplifier (EDFA). A theoretical model based on the control theory is detailed to assess the single sideband (SSB) phase noise performance of the dual-loop OEO. The laser frequency noise under small-signal direct modulation is proposed and introduced to make the phase noise model more complete for the first time. According to the theoretical analysis, the close-in (<1 MHz) SSB phase noise floor of the dual-loop OEO is dominated by the flicker noise when the offset frequency is below 10 kHz, and determined by the laser's relative intensity noise (RIN) when the offset frequency is beyond 10 kHz. Microwave signals with the frequency tuned from 8 to 12 GHz and 3-dB linewidth less than 100 Hz are realized. The side mode suppression ratio (SMSR) of the generated microwave signals are measured to be 55 dB based on the dual-loop configuration. Meanwhile, within the whole frequency tuning range, the SSB phase noises of about -125 dBc/Hz at 10 kHz offset frequency are obtained. The experimental results of the SSB phase noise agree very well with the simulation results based on the theoretical model.

In this letter, a wavelength-tunable Optical Time-Domain Reflectometer (OTDR) for Dense Wavelength Division Multiplexing Passive Optical Network (DWDM-PON) is proposed and experimentally demonstrated. An Integrated Tunable Laser Assembly (ITLA) serves as the light source permitting measurements of 80 DWDM channels in the C-band. Dither modulation is added to ITLA to depress coherent noise resulted from the narrow linewidth of the light source. Semiconductor Optical Amplifier (SOA) modulates the continuous light into pulsed light and meanwhile amplifies peak optical power of the pulsed light to 13 dBm. In addition, we optimize the wavelet denoising algorithm for further signal-noise ratio (SNR) enhancement. Dynamic range and spatial resolution of the proposed OTDR reach up to 16.2 dB and 2 m respectively. A DWDM-PON fiber link including an Arrayed Waveguide Grating (AWG) is measured to verify the system performances, and the OTDR profiles of different channels coincide with each other. The designed wavelength-tunable OTDR is shown to be appropriate for DWDM-PON.

A scanning near-field fluorescence microscopy for in situ test is demonstrated in this letter. The scanning near-field fluorescence microscopy can be applied to commercial environmental scanning electron microscopy (ESEM) without changing the ESEM performance. The designed scanning near-field fluorescence microscopy combines the advantage of the ESEM and scanning near-field optical microscopy (SNOM). The CdSe/ZnS quantum dot samples are prepared to verify system performance. The system offers fluorescence image and topography image simultaneously with a high depth and high resolution, thus permitting further ingredient and functional research. The topography image spatial resolution and the fluorescence image spatial resolution are below 130 nm and 115 nm, respectively, with a tip aperture of similar to 100 nm. The image range reaches 300 x 300 mu m by virtue of the micromanipulators and image stitching technology.

We demonstrate a method based on an ICL with tunable wavelength covering ethanol absorption peak, water absorption peak and a reference point around 3.345um to make a stand-off detection of ethanol vapor in the space. The detection model is established using the ratios of reference signal and detection signal at three target wavelengths, which help to eliminate the influence of laser power and the cross interference from water vapor in the space. The intrinsic error caused by detectors and optical elements have been corrected, and availability of this approach has been proved both in theory and in experiment.

A Scanning Near-field Optical Microscope (SNOM) devoted to a commercial Environmental Scanning Electron Microscope (ESEM) is demonstrated in this paper. The system can be applied to biological samples and optoelectronic devices at the nanoscale to obtain their surface topography information and fluorescence spectroscopic information in situ. Optical microscopes can characterize function and material components by means of fluorescence spectroscopic technology. However, the spatial resolution of conventional optical microscopes is limited by the diffraction limit. Owing to fine surface topography information of the sample, a SNOM combined with environmental scanning electron microscopy can obtain fluorescence images and fine surface topography images simultaneously. An Atomic Force Microscope (AFM), which utilizes the same optical fiber probe with the SNOM, is added to the system. The fluorescence signal acquired by the SNOM and the surface topography signal acquired by the AFM have the same coordinate. The surface topography images of the AFM are matched to the surface topography images of the ESEM. Therefore, the fluorescence images are located to the ESEM images. A quartz tuning fork offers feedback signals to control the distance between the optical fiber probe and the samples. The samples are scanned by a high-precision scanner with 0.3 nm resolution in X and Y directions. Coase approach to samples and selection of scanning area are achieved by a micromanipulator. Quantum dots samples and polystyrene (PS) spheres samples are prepared, and their surface topography images and fluorescence images are obtained. The spatial resolution of the SNOM applied to the commercial ESEM is less than 100 nm.

With the progress of the laser manufacturing technology, trace gas sensors based on tunable interband cascade lasers (ICLs) and quantum cascade lasers (QCLs) have been widely used to detect organic compounds with high sensitivity. Compared with overtone and combination bands in the near infrared region, for many species, the intensities of fundamental rotational-vibrational absorption bands in the mid-infrared region are much stronger. In this paper, we demonstrate an ethanol sensor using a room-temperature continuous-wave (CW) tunable ICL laser as a light source to detect ethanol vapor concentration with high sensitivity. Combined with the first harmonic (1f) normalized second harmonic (2f) wavelength modulation spectroscopy (WMS) technology, the characteristics of the harmonics of the system are analyzed, and the amplitude of the first harmonic decrease with an increased concentration of ethanol has been demonstrated both theoretically and experimentally. As a result, a detection limitation of 28 ppb is achieved.

A quartz-enhanced photoacoustic spectroscopy-based central wavelength stabilized distributed feedback diode laser is demonstrated in this letter. A quartz tuning fork is used as a sharp transducer to generate an error signal for wavelength tuning and stabilization without photodetector, which contributes to low cost, compact size, and freedom from ambient acoustic noises. The central wavelength of the diode laser obtains quickly fine-tuning at various modulation depths because it varies in time with the driving current. The wavelength stability of the diode laser is thus enhanced effectively. The wavelength stabilized diode laser is suited for absorption spectroscopy-based gas sensors.

A compact and highly linear quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for the measurement of water vapor concentration in the air is demonstrated. A cost-effective quartz tuning fork (QTF) is used as the sharp transducer to convert light energy into an electrical signal based on the piezoelectric effect, thereby removing the need for a photodetector. The short optical path featured by the proposed sensing system leads to a decreased size. Furthermore, a pair of microresonators is applied in the absorbance detection module (ADM) for QTF signal enhancement. Compared with the system without microresonators, the detected QTF signal is increased to approximately 7-fold. Using this optimized QEPAS sensor with the proper modulation frequency and depth, we measure the water vapor concentration in the air at atmospheric pressure and room temperature. The experimental result shows that the sensor has a high sensitivity of 1.058 parts-per-million.

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text P. Gong, L. Xie, R. Wang, and H. Yang, "A QEPAS sensor with a ~1.37 μm DFB laser," in Asia Communications and Photonics Conference 2013, OSA Technical Digest (online) (Optica Publishing Group, 2013), paper AF4H.4. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article