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We propose a multimode fiber laser sensor utilizing PI-SMF (polyimide-coated single mode fiber) for low-error relative humidity (RH) measurement, which is temperature compensated based on FBG. The PI-SMF in the laser cavity is used as a sensing element, and its length varies with humidity and temperature by volume-variation induced strain, which leads to frequency shift of the longitudinal mode beat frequency signal (BFS). When the 2000 MHz BFS is selected as the sensing signal, a RH sensitivity of −2.68 kHz/%RH and a temperature sensitivity of −14.05 kHz/°C are achieved. The peak shift of the FBG-based laser emission spectrum is only sensitive to temperature rather than RH with a temperature sensitivity of 9.95 pm/°C, which is used as the temperature compensation for RH measurements. By monitoring the response of the BFS and the laser wavelength, the cross-sensitivity effect of RH and temperature is overcome, and low-error RH measurement in the temperature range of 20 to 65 °C is realized with errors within ±0.67 %RH (25 to 85 %RH). The scheme does not require the design and production of complex structures and hygroscopic material coating processes, owning the advantages of simple structure, easy operation and high accuracy, and is expected to be practically applied in food safety and environmental monitoring. [ABSTRACT FROM AUTHOR]

In order to improve the signal-to-noise ratio (SNR) of vibration sensing in the phase-sensitive optical time-domain reflectometer (φ-OTDR) system, a fiber sensing signal processing method based on the FFDNet convolutional neural network is proposed in this paper. In the network, the concept of residual learning is introduced, which involves constructing a residual mapping and utilizing multi-layer convolutional neural networks to learn the noise distribution present in the original image. The denoised result can be obtained by subtracting the learned noise from the original image. We have built a φ-OTDR system based on coherent detection, using three PZTs as simulated vibration sources and a series of experiments at 200 Hz, with each experiment simulating a single vibration event or multiple vibration events by setting different intensities. The experimental results demonstrate that the FFDNet based fiber optic sensing signal processing method enhances the SNR to 37.84 dB, 37.11 dB, and 37.31 dB, respectively, while preserving vibration signal details more effectively than wavelet denoising and Gaussian filtering techniques. The trained FFDNet model has great potential for improving the performance of the φ-OTDR system and has some practical application value. [ABSTRACT FROM AUTHOR]

A frequency comb generator (FCG) based on dual-cavity Brillouin random fiber lasing oscillation in the 1.5 μm telecon spectral window is established and experimentally demonstrated. In the half-open main cavity of the dual cavity, the stimulated Brillouin scattering in highly nonlinear fiber (HNLF) and Rayleigh scattering in single-mode fiber are employed to provide sufficient Brillouin gain and the randomly distributed feedback, respectively, for random mode resonance. The sub-cavity includes an Er-doped fiber amplifier to couple back and boost lower-order Stokes and anti-Stokes light for the cascade of stimulated Brillouin scattering to generate multiple higher-order Stokes and anti-Stokes light. Meanwhile, efficient four-wave mixing is stimulated in the HNLF-based main cavity, further enhancing the number and intensity of the resonant Stokes and anti-Stokes light. By taking advantages of the unique transmission characteristics of nonlinear optical loop mirrors, the power deviation between Stokes and anti-Stokes lines is further optimized with 17 orders of stable Stokes lines and 15 orders of stable anti-Stokes lines achieved within the 10 dB power deviation, with maximum optical signal-to-noise ratio (OSNR) of ~22 dB and ~17 dB and minimum OSNR of ~10 dB and ~7.5 dB for Stokes and anti-Stokes lines, respectively. In addition, the dynamic characteristics of the proposed FCG have been experimentally investigated. Such an FCG with fixed frequency spacing will find promising applications in fields of optical communication, microwave, optical sensing, etc. [ABSTRACT FROM AUTHOR]

High temperature detection is a constant challenge for condition monitoring under harsh environments in optical fiber sensors research. In this study, the temperature response characteristics of guided acoustic wave Brillouin scattering (GAWBS) spectra in silica single-mode fiber (SMF) up to 800 °C are experimentally investigated, demonstrating the feasibility of the method for high-temperature monitoring. With increasing temperature, the resonance frequency of GAWBS spectra increases in a nearly linear manner, with linearly fitted temperature-dependent frequency shift coefficients of 8.19 kHz/°C for TR2,7 mode and 16.74 kHz/°C for R0,4 mode. More importantly, the linewidth of the GAWBS spectra is observed to narrow down with increasing temperature with a linearly fitted rate of −6.91 × 10−4/°C for TR2,7 modes and −8.56 × 10−4/°C for R0,4 modes. The signal-to-noise ratio of the GAWBS spectra induced by both modes increase by more than 3 dB when the temperature rises from 22 °C to 800 °C, which indicates that the proposed sensing scheme has better performance in high-temperature environments, and are particularly suitable for sensing applications in extreme environments. This study confirms the potential of high-temperature sensing using only GAWBS in silica fibers without any complex micromachining process, which has the advantages of strong mechanical strength, simple structure, easy operation, and low cost. [ABSTRACT FROM AUTHOR]

High-temperature measurements above 1000 °C are critical in harsh environments such as aerospace, metallurgy, fossil fuel, and power production. Fiber-optic high-temperature sensors are gradually replacing traditional electronic sensors due to their small size, resistance to electromagnetic interference, remote detection, multiplexing, and distributed measurement advantages. This paper reviews the sensing principle, structural design, and temperature measurement performance of fiber-optic high-temperature sensors, as well as recent significant progress in the transition of sensing solutions from glass to crystal fiber. Finally, future prospects and challenges in developing fiber-optic high-temperature sensors are also discussed. [ABSTRACT FROM AUTHOR]

A stabilized coherent narrow-linewidth Brillouin random fiber laser with low lasing noise is proposed and investigated experimentally, which employs stimulated Brillouin scattering and Rayleigh scattering in optical fibers to provide the gain and the random distributed feedback, respectively. The stabilized lasing output with suppressed noise is realized utilizing a simple structure of a double-coupler fiber ring resonator, which is composed of a section of single-mode fiber and two optical couplers with different coupling ratios. Compared with the existing free-running Brillouin random fiber lasers, much more stabilized laser output can be obtained as the double-coupler fiber ring resonator effectively acts as a narrow-bandwidth mode filter through the Vernier effect that constrains the dense random longitudinal modes competition within a small frequency range. In the experiment, a stable ultra-narrow linewidth of approximately 586.4 Hz is obtained from the laser output. Statistical analysis of mode hopping in the proposed laser indicates a great improvement in multi-mode resonances and frequency jitters with the imbedding of the ring resonator. In addition, measurement results of relative intensity noise and frequency noise of the proposed laser showcase a great suppression in mode-competition-induced lasing instability and a significant improvement in the noise level compared to the free-running Brillouin random fiber lasers. The proposed all-fiber-based stabilization technique makes a step forward for developing practically robust and reliable Brillouin random fiber lasers. Such a stabilized highly coherent narrow-linewidth laser source with low cost and low power consumption will find promising applications in high-resolution spectrometers, microwave photonics, and optical sensing. [ABSTRACT FROM AUTHOR]

The frequency response of phase-sensitive optical time-domain reflectometry ($\Phi $ -OTDR) is strictly limited by the repetition rate of the injected sampling pulses in conventional sensing scheme. To fully reconstruct an arbitrary vibration signal, the pulse repetition rate is required to be lower bounded by twice the maximum frequency of the vibration signal according to the Nyquist sampling theorem and at the same time upper bounded by the sensing fiber length to avoid generations of mixed Rayleigh backscattered signals from more than one optical pulse within the sensing fiber, making high-frequency vibration sensing over long distance impossible. In this work, we propose a novel method based on the serial multi-coset sampling (MCS) strategy which can recover sparse broadband vibration signals with sub-Nyquist sampling rates, which is thus able to break through the tradeoff between the measurable vibration frequency bandwidth and the sensing range. Pulses with non-uniform intervals are adopted to realize MCS for the Rayleigh backscattered lights along the entire sensing fiber rather than the traditional uniform pulse train. A continuous to finite (CTF) block is used to determine the active spectrum ranges of the sparse signal. By using a serial MCS sequence with an average sampling rate of 4.8 kHz, both a 4 kHz single-frequency sinusoidal signal and a 0.2 kHz bandwidth chirped signal with a center frequency of 4.6 kHz were sampled and recovered successfully in the simulation. In experiments, a single frequency vibration signal of 15 kHz and a chirped vibration signal with frequency ranging from 15 to 16 kHz were successfully identified and reconstructed over a sensing distance of 4 km using an average sampling rate of 21.6kHz, respectively, which however is not possible in conventional $\Phi $ -OTDR system. Furthermore, accurate identification of a broadband vibration signal with a maximum frequency of 6 kHz over a 10km-long sensing fiber shows that this method is also practically valid in long-distance sensing. [ABSTRACT FROM AUTHOR]

We have experimentally analyzed and compared the performance of optical frequency domain reflectometry (OFDR) assisted by image processing methods including wavelet transform and Gaussian filter arithmetic in terms of strain measurement range. The incomplete space similarity in the distributed strain measurement is used to recover the accurate strain gradient information by the image processing methods. The distributed spectrum shifts of the cross-correlation calculation along the sensing fiber between reference signal and measurement signal is regarded as a two-dimension (2D) image which is disposed by the proposed methods rather than extracting the spectrum shifts immediately. Compared with the traditional processing method, the proposed image processing method is able to remove and smoothen the fake peaks or outliers caused by the spatial mismatch of stretched fiber in OFDR system. In the experiments, strain gradient information up to 7000 μϵ can be identified by the proposed methods with a 4 mm spatial resolution. Furthermore, the wavelet transform is discussed in detail in order to determine the value of the suitable decomposition level in OFDR data processing scheme. The proposed methods provide a new idea to achieve the wide strain measurement range in OFDR system, which is a significant step toward a high-performance distributed sensing system, especially for occasions when a wide range of strain measurement is required. [ABSTRACT FROM AUTHOR]

Measurement accuracy of the optical frequency domain reflectometry (OFDR) will be deteriorated severely when the spatial resolution is improved due to the influence of the random noise, especially in long range distributed fiber optical sensing system. In order to eliminate the random noise, we have experimentally compared and analyzed the performance of OFDR system with and without the image processing methods in term of spatial resolution. The data arrays of the cross-correlation results are obtained after processing the collected distributed Rayleigh scattering signals along the sensing fiber, which can be used to construct a two-dimension (2D) image with the spectrum shift information induced by the strain. Thus, the image denoising methods can be applied to remove the random noise and improve the measurement accuracy. Based on this principle, two image denoising methods including the total variation method and the 2D Gaussian filter arithmetic are introduced to realize distributed sensing with high spatial resolution in long distance OFDR system. Experimental results show that the random noise can be suppressed and the outliers can be removed by the proposed methods. The strain gradients with high spatial resolution are identified effectively when the image denoising methods are used. Furthermore, spatial resolution with 1.3mm is achieved by the proposed methods with a wavelength sweeping rate of 20nm/s in 52m sensing fiber. The proposed methods provide a potential solution for OFDR system in long distance sensing with high spatial resolution, which is a significant step toward a high-performance OFDR system for practical applications. [ABSTRACT FROM AUTHOR]

This paper reports an intracavity terahertz parametric source based on the stimulated polariton scattering in RbTiOPO4 crystal with a high repetition rate and a high average power. The side pumping for the gain medium by laser diodes is adopted to get a higher fundamental power and a larger fundamental laser beam size than those in the diode-end-pumping in order to improve the THz output power. A non-collinear convex-plane fundamental cavity is adopted so that the thermal effect can be offset in some degree and the fundamental beam size is further increased. The obtained maximum average THz output power is 367 μW at 3.88 THz when the diode pump power is 105 W and the pulse repetition frequency (PRF) is 7 kHz. The terahertz power of 367 μW is the highest ever obtained in SPS sources. [ABSTRACT FROM AUTHOR]

A sub-Nyquist random single-pulse sampling method based compressive sensing technique is proposed firstly to reconstruct signals under test in phase-sensitive optical time domain reflectometry (ϕ-OTDR). The key information of Rayleigh backscattering curves can be recognized after sparse transformation, which can be successfully reconstructed with far smaller number of measurements by orthogonal matching pursuit algorithm. Unlike the uniform sampling in conventional ϕ-OTDR system, pulse intervals are randomly modulated to generate a random single-pulse sequence in the proposed method in order to achieve random single-pulse sampling along every sensing point of the fiber. Experimental results verify that pulse sequences with average pulse repetition rate less than 2-kHz can be efficiently utilized to reconstruct an 8-kHz signal under test and obtain its frequency information, as well as to realize the detection of high frequency signals at different locations along the sensing fiber. It is demonstrated that the sampling rate is reduced greatly without any hardware modification compared to that required in the conventional method. This method can break the limits of the traditional Shannon-Nyquist sampling theorem to reconstruct signal, which is of great significance for the sensor performance enhanced in traditional ϕ-OTDR system. [ABSTRACT FROM AUTHOR]

We proposed a new method for gas detection in photoacoustic spectroscopy based on acousto-optic Q-switched fiber laser by merging a transmission PAS cell (resonant frequency f 0 = 5.3 kHz) inside the fiber laser cavity. The Q-switching was achieved by an acousto-optic modulator, achieving a peak pulse power of ~679 mW in the case of the acousto-optic modulation signal with an optimized duty ratio of 10%. We used a custom-made fiber Bragg grating with a central wavelength of 1530.37 nm (the absorption peak of C2H2) to select the laser wavelength. The system achieved a linear response (R2 = 0.9941) in a concentration range from 400 to 7000 ppmv, and the minimum detection limit compared to that of a conventional intensity modulation system was enhanced by 94.2 times. [ABSTRACT FROM AUTHOR]

The characteristics of a tunable Stokes laser based on the cascaded stimulated polariton scattering and stimulated Raman scattering in KTiOPO4 crystal were studied experimentally and theoretically. When the pumping wavelength was 1064 nm, the Stokes laser output wavelength was able to be tuned discontinuously from 1112.08 nm to 1113.64 nm, from 1114.94 nm to 1115.77 nm, and from 1117.37 nm to 1119.92 nm, and the maximum output power appeared at 1118.86 nm. With a pulse repetition frequency of 7 kHz and a pump power of 6.0 W, the maximum output power of the Stokes laser reached 734 mW, and the corresponding diode to laser conversion efficiency was 12.2%. The rate equations describing the temporal evolutions of the fundamental and Stokes waves by noncollinear stimulated polariton scattering and the Stokes wave by collinear stimulated Raman scattering were derived. They were used to simulate the tunable Stokes laser. The calculated results were in agreement with the experimental results on the whole. [ABSTRACT FROM AUTHOR]

This paper reports the characteristics of an injection-seeded terahertz parametric oscillator (TPO) with a ring-cavity configuration based on the MgO:LiNbO3 crystal. The ring cavity is constituted of three mirrors and the THz wave output surface where the pump and Stokes beams are totally reflected. The THz pulse energies and the Stokes pulse energies as functions of the pump pulse energy for different seed powers of 47.5 mW, 150.7 mW, and 312.8 mW were investigated. The experimental results showed that the injection-seeded ring cavity TPO exhibited the benefits of lower pump thresholds and higher output energies for the Stokes and THz waves. The smaller the pump pulse energy, the more obvious the effect of the seed injection. The reasons for the laser performance improvement were analyzed. [ABSTRACT FROM AUTHOR]

In this paper, a KTiOPO4 (KTP) crystal is used as the nonlinear medium in an intracavity-pumped terahertz parametric oscillator (TPO) based on stimulated polariton scattering (SPS). Almost all the reported intracavity SPS sources adopt the diode-end pumped configuration, and the THz output power is limited by the serious thermal effect and small fundamental beam size. For improving the THz output power, we propose diode-side pumping for the laser medium to get a higher fundamental power and a larger fundamental beam size. A convex–plane fundamental laser resonator is used to further offset the thermal effect and increase the fundamental beam size. The THz frequency of the intracavity-pumped KTP terahertz parametric oscillator can be discontinuously tuned from 3.19 THz to 5.94 THz with three gaps. The fundamental beam diameter in the KTP crystal is about 1.3 mm. The maximum average THz power is 166 μW at 5.74 THz under a pulse repetition frequency (PRF) of 6 kHz and a diode pump power of 98 W. By means of the diode-side pumped configuration, the maximum THz output power is more than two-fold higher compared to the diode-end pumped configuration reported using the KTP crystal. [ABSTRACT FROM AUTHOR]

An efficient 1185 nm Nd:KLu(WO 4 ) 2 self-Raman laser was demonstrated for the first time. The characteristics of the self-Raman laser at different pulse repetition rates were studied. With an incident pump power of 15.4 W and a pulse repetition frequency of 20 kHz, the maximum output power of 1.5 W was obtained. The optical conversion efficiency from the diode to the Stokes radiation was 9.8%. At a pulse repetition frequency of 10 kHz, the pulse width was 9.5 ns. The corresponding pulse energy and peak power were 112 μJ and 12.3 kW. [ABSTRACT FROM AUTHOR]

Abstract: A diode-side-pumped actively Q-switched Nd:YAG/BaWO4 intracavity anti-Stokes Raman laser is demonstrated for the first time. The oscillation directions of the fundamental and the Stokes beams are separated at a phase-matching angle by introducing an additional coupled Raman cavity in the fundamental resonator. The highest 968nm anti-Stokes output power of 0.94mW is obtained with a 170W 808nm pumping power and a 7.5kHz pulse repetition frequency. Meanwhile, the Stokes power obtained is 4.2W. [Copyright &y& Elsevier]

• Scanning cycle is optimized in detail for photoacoustic energy accumulation. • Optimum modulation index is verified to be ∼5 by experiments instead of 2.2. • Frequency and efficiency of QTF are evaluated in a long-term (∼8 months) experiment. • Photoacoustic spectrophones are discussed and compared in their optimum geometric dimensions. Efforts were made in this study to evaluate a series of pivotal techniques in standard quartz tuning fork (QTF) based quartz enhanced photoacoustic spectroscopy (QEPAS) systems for engineering applications. In current QEPAS systems, a scanning ramp combined with high frequency sinusoidal signal is used to drive the light source for photoacoustic excitation, the scanning cycle was optimized for the first time in experiments guaranteeing that photoacoustic energy accumulation finished to achieve the largest photoacoustic signal, and the optimum modulation index was verified by experiments to be ∼5 instead of the simulated result 2.2. As an important step for engineering applications, a long-term (∼8 months) experiment was also carried out to evaluate the stability of QTF's property. In addition, several photoacoustic spectrophone configurations were compared in the optimum geometry parameters, dual-tubes on beam was verified in experiments to be the most efficient one compared with half on-beam and off-beam structures in standard QTF based systems, which is consist with the summary of published papers. [ABSTRACT FROM AUTHOR]

Abstract An efficient signal processing method based on compressive sensing (CS) is presented for vibration sensing in phase sensitive optical time domain reflectometry system (φ -OTDR), which allows for compressing and denoising the raw data simultaneously. The main information of Rayleigh backscattering curves can be identified by sparse transformation. Then the data is compressed by observation matrix while the unwanted information is discarded. Orthogonal matching pursuit (OMP) algorithm is used to recovery the signal under the sparsity determined by a threshold rule. Experiment results show that the SNR is increased to 34.39 dB with the compression ratio of 18.9 for the vibration event of 100 Hz along 3 km sensing fiber which indicates the prominent performance of the novel method for vibration detection. [ABSTRACT FROM AUTHOR]

A fiber reflector based round-trip structure is proposed in TDLAS gas sensing system to increase absorption path-length, and a residual amplitude modulation removal (RAMR) technique is introduced, improving the second harmonic profile significantly. Fiber coupled Herriott cell (3 m path-length) is used in experiments, the transmitted light is reflected back by a fiber reflector, realizing double absorption path-length. The double-pass light is split in 1:1 to two photodetectors for second harmonics lock-in, one is for gas concentration demodulation and another for residual amplitude modulation (RAM) measurement by tuning the reference signal in different phases, then a waveform recovery technique based on subtraction is applied to the dual path lock-in for RAMR. The theory is simulated, and experimental results verify that the new structure increases absorption signal efficiently, almost 2 times, the RAM effects are eliminated successfully without increasing the system noise. R-square of 0.9987 is achieved in linearity test of the improved gas sensing system. [ABSTRACT FROM AUTHOR]

Measuring the phase shift of luminescence is always used to achieve oxygen concentration detection. A robust phase demodulator is proposed in this paper to measure luminescent phase, overcoming common faults of bad SNR in optical fiber oxygen sensor. Its phase detection linearity of 0.99999 R-square is calibrated by a commercial lock-in amplifier (Model 7230). Modulation frequency of 16 kHz is used in the oxygen measurements and a phase resolution of 0.02° is achieved by the demodulator. Phase noises of 0.03, 0.06, 0.08 and 0.1° were measured in different oxygen levels of 0.99%, 3%, 5% and 8% in experiments. A resolution of better than ±0.005% in 0.99% O 2 is realized. In addition, the demodulator which is simple but robust can be used in many luminescence phase based applications. [ABSTRACT FROM AUTHOR]

• A single-longitudinal-mode (SLM) Brillouin random fiber laser (BRFL) with the shortest cavity structure ever reported in BRFL configuration is proposed and experimentally demonstrated. • The proposed BRFL lases with a narrow linewidth of ∼802.42 Hz and a high SLM operation probability of ∼90.35 % by employing a HNLF-assisted short-cavity structure and an unpumped-EDF sagnac loop. • Highly stabilized lasing output is obtained with significantly alleviated mode hopping and multi-mode resonances. • Relative intensity noise and frequency noise are excellently suppressed compared to long-cavity BRFLs which are either incorporated with ultra-narrow filters or not. • Correlation of intensity fluctuations among different traces of laser output is statistically investigated. • Output wavelength of the proposed BRFL can be tuned within the range of 1530 ∼ 1590 nm. A single-longitudinal-mode high-coherence Brillouin random fiber laser (BRFL) with low-noise and narrow linewidth is proposed and experimentally demonstrated, which owns the shortest cavity structure ever reported in BRFL configuration by the combined utilization of highly nonlinear fiber (HNLF) and unpumped Er-doped-fiber sagnac loop (UESL). In this BRFL, stimulated Brillouin scattering and Rayleigh scattering are exploited to provide the resonant gain and random distributed feedback, respectively. The UESL, with a narrow bandpass width and a capability of auto-tracking the center frequency of the transmitted light, is realized by forming a dynamic fiber grating (DFG) filter with periodically modulated refractive index through the saturable absorption effect and hole-burning effect along the Er-doped fiber. The UESL can effectively act as a center-frequency-auto-tracking bandpass filter with narrow bandwidth to filter out the undesired dense random modes and simultaneously restrict the resonant modes within a very limited frequency range. It thus suppresses the modes hopping and multi-mode resonances within the Brillouin gain bandwidth so that the frequency jitter and instability of laser output are greatly alleviated. The short cavity was realized by utilizing a section of HNLF with high nonlinearity coefficient (≥10 W−1·km−1 @1550 nm), which can provide enough Brillouin gain for the BRFL when short length is employed. Compared with the currently prevalent BRFLs based on long-cavity configurations, the short-cavity structure enables a significant reduction in the number of random cavity modes so as to guarantee a high probability for the single-longitudinal-mode random lasing output. In the experiment, a stable single-longitudinal-mode lasing output with an ultra-narrow linewidth of ∼802.42 Hz is obtained from the proposed BRFL. Statistical characteristics of the mode resonances, laser noise performance and statistical analysis of the lasing output are also experimentally investigated for the proposed BRFL. In addition, broadband wavelength-tunable output is realized in the proposed BRFL. [ABSTRACT FROM AUTHOR]

• ·A low-noise Brillouin random fiber laser with a narrow linewidth (less than 2kHz) is achieved by incorporating an auto-tracking dynamic fiber grating. • ·The auto-tracking dynamic fiber grating is realized based on a saturable absorption ring structure that provides an ultra-narrow reflection bandwidth of ∼ 0.633 MHz. • ·Highly stabilized lasing output is obtained with significantly alleviated mode hopping and multi-mode resonances. • ·Relative intensity noise and frequency noise are excellently suppressed. A low-noise Brillouin random fiber laser (BRFL) with stabilized lasing output and narrow linewidth is proposed and demonstrated experimentally. Stimulated Brillouin scattering and Rayleigh scattering are exploited to work as the gain and random distributed feedback mechanisms, respectively. A simple unpumped Erbium-doped fiber (EDF) loop structure which consists of a section of EDF and an optical coupler is introduced into the BRFL cavity, forming a saturable absorption ring (SAR). When light with proper intensity is incident into the SAR, a dynamic fiber grating (DFG) can be built up to work as a narrow-bandwidth filter with the ability of auto-tracking the center frequency. The SAR can effectively filter out dense random longitudinal modes and suppress multi-mode resonances and frequency jitters by restricting the mode-competition-induced mode hopping within a limited frequency range of ∼ 1 MHz and auto-tracking the center frequency of light in the cavity, simultaneously. Thus, the output of BRFL with SAR is much more stabilized than existing free-running BRFLs. In the experiment, a stable lasing output with an ultra-narrow linewidth (less than2kHz) is obtained from the proposed BRFL. The measured relative intensity noise and frequency noise demonstrate that the noise level of the proposed laser is significantly lower than that of the free-running BRFLs. This proposed technique not only provides a solution to the dilemma of previously reported BRFLs with severe lasing instabilities, but also makes a step towards the development of practical and reliable BRFLs. This high-quality laser source with advantages of high stability, low cost, low threshold power, high coherence, narrow linewidth, and low noise can find useful applications in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing fields. [ABSTRACT FROM AUTHOR]

A method for maintaining a fixed phase relationship between the driving signal of acousto-optic modulator (AOM) and the original mode-locked seed laser is proposed and realized, which stabilizes the amplitude of diffracted signal output from the AOM for subsequent amplification. A field-programmable gate array (FPGA), combined with external summing amplifiers, is used to directly synthesize high-timing-precision driving signals that are synchronized with the seed laser pulses, and the accuracy of signal timing control reaches 160 ps. Using this driver, the standard deviation of the diffracted signal output from the AOM is significantly decreased from 0.52% to 0.18%. This pulse-picking solution also includes a control system that can flexibly control the frequency, gating width, etc., of the driving signal, which makes it more convenient for subsequent laser amplification and makes it suitable for a variety of mode-locked lasers.

This paper presents a modeling method for extracavity-pumped terahertz parametric oscillators (TPO) based on stimulated polariton scattering, in which the pumping beam is from a different laser, and the Stokes beam oscillates in its cavity. After suitable approximations and assumptions, the average THz wave amplitude in the nonlinear crystal is expressed as a function of the fundamental and Stokes wave amplitudes. Then the rate equation for the Stokes wave is obtained based on the Stokes wave increment within a cavity roundtrip timescale. After solving the Stokes wave rate equation, the Stokes wave temporal evolution is considered as a known parameter, and the properties of the residual fundamental and terahertz waves are obtained by numerically solving the coupled wave equations. This modeling method is applied to an extracavity-pumped TPO based on MgO:LiNbO 3 crystal. The simulation results are basically consistent with the experimental results. The main reasons causing the deviations of the simulation results from the experimental results are analyzed. To the best of our knowledge, this is the first time to perform the modeling for extracavity-pumped Q-switched TPOs.

In this study, a long-distance phase-sensitive optical time domain reflectometry (Φ-OTDR) with a flexible frequency response based on time division multiplexing is proposed and experimentally demonstrated. Distributed flexible frequency vibration sensing over long distance can be realized by reconfiguring the system layout in a time-division-multiplexed manner by re-routing the Rayleigh backscattered signals for segmented processing with extra erbium-doped fiber amplifiers added only instead of any other complex signal amplification or pulse modulation mechanisms. Through time-division-multiplexed reconfiguration, the tradeoff between sensing distance and vibration frequency response in Φ-OTDR system is largely relieved. Compared with the traditional system layout, the proposed system allows a flexible frequency response in each sensing fiber segment without any crosstalk among them. In experiments, distributed vibration sensing with a frequency response up to 4.5 kHz is achieved over a sensing distance of 60km by the proposed system, which is not possible in a conventional Φ-OTDR system. Furthermore, the frequency response flexibility of the proposed system is further verified by successfully identifying a vibration event with a frequency of up to 20 kHz at the end of a 52-km-long fiber.

A linearly polarized low-noise single-frequency fiber laser was demonstrated by using a homemade 1.2-cm-long Yb:YAG crystal derived silica fiber. A maximum output power of greater than 60 mW was obtained with a signal-to-noise ratio of ∼80 d B and a polarization extinction ratio of 27.8 dB. Additionally, the relative intensity noise was measured to be -145 d B / H z above 6.5 MHz. A frequency fluctuation of less than 20 MHz was also obtained. The output power was scaled up to 14.5 W with a one-stage all-fiber amplifier scheme with a slope efficiency of 56.4%.

The maximum detectable vibration frequency of an optical frequency domain reflectometry (OFDR) system is limited by the tunable rate of the laser source. Unlike uniform sampling with the time-resolved method, the sampling frequency is randomly modulated so that the vibration signal applied on the interrogation fiber is sampled by a multi-frequency sub-Nyquist sampling method and reconstructed by the compressive sensing technique. First, we give a full treatment to prove that the proposed method has the same performance as the conventional method. Second, in a further proof-of-concept experiment, the measurable frequency of a sparse signal is achieved up to 200 Hz with a sweeping rate of 40 nm/s. This method can recover the vibration signal with sampling rates less than that required by the Nyquist sampling theory, which is a significant step toward a high-performance OFDR system, especially for evaluating the intrinsic frequency of the object's structural condition.

The maximum detectable vibration frequency response range is inversely related with the sensing fiber length in direct-detection intensity-measuring coherent optical time domain reflectometry (DI-COTDR). Unlike the conventional uniform sampling, the pulse repetition rate is modulated in a time-division manner so that a multi-frequency sub-Nyquist sampling is realized along every point of the sensing fiber. A 24-kHz vibration signal can be detected and recovered by a compressive sensing technique using sampling pulses with repetition rate lower than 5-kHz, which is ten-fold lower compared to that required in the conventional uniform sampling method. Also, a multi-frequency vibration signal can be identified and recovered by this technique. The proposed method can break through the theoretical maximum detection frequency of traditional systems without any hardware modification. Therefore, such a method is of great significance for broadening the frequency response range of the distributed sparse-wideband vibration sensing.

In this paper, the rate equations describing the operation of intracavity-pumped Q-switched terahertz parametric oscillators based on stimulated polariton scattering are given for the first time. The rate equations are obtained under the plane-wave approximation, the oscillating fundamental and Stokes waves are supposed to be round uniform beam spots. Considering the fact that the terahertz wave nearly traverses the pump and Stokes beams and using the coupled wave equations, the terahertz wave intensity is expressed as the function of the fundamental and Stokes intensities. Thus, the rate equations describing the evolution processes of the fundamental and Stokes waves are obtained in the first step. The THz wave properties are then obtained. Several curves based on the rate equations are generated to illustrate the effects of the nonlinear coefficient, the THz wave absorption coefficient, and pulse repetition rate on the THz laser characteristics. Taking the intracavity-pumped Mg:LiNbO 3 TPO as an example, the THz frequency tuning characteristic and the dependences of the fundamental, Stokes, and THz wave powers on the incident diode pump power are calculated. The theoretical results are in agreement with the experimental results on the whole.