Short Cavity Research Articles

Anti-crosstalk optical fiber sensor based on polydimethyl siloxane fluid cavity and graphene oxide film

An integrated optical fiber sensor with high sensitivity and extremely low crosstalk was theoretically and experimentally demonstrated for sensing temperature and pressure. Integration of dual pressure/temperature sensitive materials on a section of hollow-core fiber (HCF) by core-offset part filling technology and dipping technology. Through the movable polydimethyl siloxane fluid (PSF)-air surface and the short cavity length of the PSF, the sensitivity of the PSF as a temperature-sensitive material reaches to 2.15 nm/°C, but it presents the immunity of wavelength to pressure. The sensitivity of graphene oxide (GO)-based air cavity as a pressure-sensitive material can reach −748.15 nm/Mpa with the GO film thickness of 713 nm. In addition, experimental results show that the cross-sensitivity between temperature and pressure are 0 °C/Mpa and 0.000318 Mpa/°C. Thus, the proposed sensor shows a strong capability of dual-parameter discriminative proficiency, anti-crosstalk and simultaneous measurement of temperature and pressure. Such a compact, robust and low-cost sensor, is significant for the applications of exploitation of marine resource and well and reservoir down-hole operations.

On‐Chip Multiwavelength Single‐Mode Lasers with CdSe Nanoribbons‐Embedded Microcavities

Integrated multiwavelength nanolasers are important in the practical development of optical circuitry for optical information processing and optical data storage. Herein, a new method for on‐chip integrated lasers is proposed, which can integrate single‐mode lasers with multiple wavelengths at room temperature. A four‐wavelength single‐mode nanoribbon laser chip is fabricated for demonstration. The mode of different laser units is controlled by the thickness of dielectric microcavity layers to select the resonant modes of excitation. Therefore, multiwavelength lasing can be coupled out based on the integrated cavity which is realized using combinatorial etching technique and can integrate tens of different‐wavelength lasers on a chip. The lasing modes of this device are all single‐mode lasing because of the short cavity length. Its lowest lasing threshold is only 5.4 kW cm−2. The gain material and the dielectric microcavity can be controlled and fabricated separately, which have strong controllability and flexibility. This new laser chip paves the way for a new class of hybrid nanolasers for chip‐integrated applications.

Linear-cavity Er-doped fiber mode-locked laser with large wavelength tunability.

A linear-type wavelength-tunable all-polarization-maintaining fiber mode-locked laser is proposed for the first time, to our knowledge, and is implemented with an Er-doped fiber and polarization-maintaining fiber components. The tuning range of the center wavelength is from 1533.7nm to 1565.6nm. The linear-type configuration makes the proposed laser simpler and more compact, allowing it to achieve the highest repetition rate of 126.5MHz among C-band wavelength-tunable mode-locked lasers due to its short cavity length. Also, its polarization-maintaining fiber components provide reliable operating robustness. The significant wavelength tunability and high repetition rate of the proposed laser can be expected to make it an attractive resource for various applications, including optical communications, broadband spectroscopic LIDAR, and high-precision ranging.

Single-longitudinal-mode short-cavity Brillouin random fiber laser via frequency auto-tracking with unpumped-EDF sagnac loop

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.

High-Density Weak in-Fiber Micro-Cavity Array for Distributed High-Temperature Sensing With Millimeter Spatial Resolution

High-spatial-resolution distributed sensing at high temperature is crucial in many industrial areas, such as aero-engines, nuclear power, furnaces, and fuel cells. Here, we propose and demonstrate a large-scale multiplexed high-density weak in-fiber micro-cavity (MC) array for distributed high-temperature sensing with millimeter spatial resolution. The proposed in-fiber MC, featured by an intrinsic Fabry-Perot interferometer (IFPI) with a short cavity length of 100 μm and a low peak reflectivity of ∼−55 dB, was formed by two weak reflectors created in a conventional single-mode fiber (SMF) by using femtosecond laser point-by-point inscription. Several high-density MC arrays, consisting of identical weak IFPIs over 1000, were fabricated by using different femtosecond laser pulse energy to investigate the transmission loss (TL) of MC arrays. The experimental result shows that the TL induced by a single MC could be low as 0.0009 dB. Moreover, the high-temperature performance of the MCs was studied via cyclic heating and cooling between room temperature and 1000 °C, showing a temperature sensitivity of −2.29 GHz/°C (i.e., 18.4 pm/°C). Furthermore, distributed high-temperature sensing was demonstrated by employing the fabricated in-fiber MC array with the demodulation of optical frequency domain reflectometry, and a high spatial resolution of 1 mm was achieved at a high temperature of 1000 °C. As such, the proposed high-density weak in-fiber MC arrays are suitable for distributed high-temperature sensing in harsh environment, and hence have wide prospection of application in the fields of aerospace, nuclear power, metallurgy, and electrochemical industry.

Nonparaxial corrections for short cavities and fibers

This paper describes optical propagation beyond the paraxial limit in rotational symmetric short Fabry-P\'erot cavities and long fibers with parabolic-index profiles. Frequency shifts due to paraxial and nonparaxial effects, the so-called spectral fine structure and vector corrections, are calculated with perturbation theory and expressed in a single dimensionless expansion parameter. The results obtained for short cavities and fibers have similar functional forms but are surprisingly different. These similarities and differences are pointed out and discussed, including their consequences for mode multiplexing in fibers.

Narrow-Linewidth Single-Frequency Ytterbium Laser Based on a New Composite Yb3+-Doped Fiber

Fiber single-frequency lasers are currently being actively developed, primarily due to the growing number of applications that require compact and reliable narrow-band sources. However, the most developed single-frequency fiber lasers based on phosphate fibers have the disadvantages of low mechanical strength of both the phosphate fibers themselves and their splices. In this paper we demonstrate a single-frequency laser based on a new composite Yb3+-doped active fiber. The core of this fiber is made of phosphate glass with a high concentration of ytterbium ions and its cladding is made of standard silica glass. This structure ensures a higher splicing strength of the fiber compared to the phosphate fibers and provides high resistance to atmospheric moisture. Despite the multimode structure of this fiber, we achieved stable single-frequency lasing with an average power of 10 mW and a spectral contrast of more than 60 dB in the scheme with a short (1.1 cm) cavity formed by two fiber Bragg gratings. We believe that further optimization of this fiber will make it possible to create powerful and reliable single-frequency lasers in the one-micron wavelength range.

In-situ construction of photoanode with Fe2O3/Fe3O4 heterojunction nanotube array to facilitate charge separation for efficient water splitting

Hematite (α-Fe 2 O 3 ), as a promising material in photoelectrochemical water splitting, has been given much attention to enhance its performance through a series of modifications. In this work, anodized Fe 2 O 3 nanotubes were annealed in air and then in argon atmosphere. The samples annealed in argon underwent partial escape of oxygen atoms, thus generating Fe 3 O 4 and eventually forming Fe 2 O 3 /Fe 3 O 4 heterojunction in situ. The built-in electric field constructed by the heterojunction establishes a channel for effective charge transfer. In addition, the formation of heterojunction not only accelerates the charge transfer at the interface, but also shortens the band gap and promotes light absorption. As a result, the two-step annealed sample showed the best photoelectrochemical performance with a photocurrent of 2.5 mA cm −2 photocurrent at 1.6 V νs . RHE, threefold that of pure Fe 2 O 3 nanotubes (0.75 mA cm −2 ). The highest hydrogen production rate and oxygen production rate reached 23.84 and 15.66 μmol cm −2 h −1 , respectively. The facile two-step annealing method to generate Fe 2 O 3 /Fe 3 O 4 heterojunction in-situ contributes to solving the problem of short cavity diffusion length and slow water oxidation kinetics. • Fe 2 O 3 /Fe 3 O 4 heterojunction was in situ constructed by two-step annealing. • Mechanism of phase transition from Fe 2 O 3 to Fe 3 O 4 is explained. • In-situ constructed heterojunctions improve separation and transport of carriers.

Triple-wavelength laser from a femtosecond laser directly-written fiber cavity for microwave generation.

We report an all-fiber triple-wavelength single-frequency distributed Bragg reflector (DBR) laser for 1-10 GHz microwave (MW) generation. The DBR cavity contains a non-polarization-maintaining Er fiber and a pair of fiber Bragg gratings (FBGs) made using a femtosecond laser line-by-line (LbL) direct-writing method. Such a configuration combining a short cavity and radial asymmetry leads to frequency locking and phase controlling of multi-wavelength fiber lasers. A 1.59-µm triple-wavelength laser with high coherence, spectrum purity, and polarization purity has been demonstrated; 3-6 GHz triple-frequency MW was generated.

Fundamental and high-order cross-correlation combined demodulation method for absolute cavity length measurement of fiber-optic Fabry–Perot sensors

For the accurate cavity-length demodulation of fiber-optic Fabry–Perot (FP) sensors, a combined correlation method based on the fundamental cross-correlation and a higher-order one is proposed, simulated, and experimentally verified. By extending the reflection spectrum eightfold through continuous frequency-doubling three times, cross-correlation using both the original and eightfold spectra, and determination of the main peak of the fundamental cross-correlation coefficient function with the assistance of the eighth-order cross-correlation coefficient, the cavity-length demodulation resolution for fiber-optic FP sensors can be significantly improved even when the spectral bandwidth of the source is limited or the cavity length is relatively short. A cavity length resolution better than 1.8 nm is achieved for an FP sensor with a cavity length of ∼162 μm. The proposed demodulation method can effectively reduce the bandwidth requirement of the light source for the cavity-length extraction of fiber-optic FP sensors, particularly those with relatively short cavity lengths.

Cavity length dependence on strain sensitivity for Fabry–Perot sensors

This study presents the dependence of strain sensitivity on cavity length in conventional Fabry–Perot (F-P) sensors. A high number of F-P sensors were required and to ensure their reproducibility, a manufacturing process was developed to obtain similar sensors but with different types of lengths. A hollow-core silica tube was used to fabricate several F-P cavities by fusion splicing it between two sections of SMF28 fiber. The fabricated F-P has a varying length ranging from 15 to 2500 μm. The cavities were measured under a microscope and the reflected spectrum was acquired for each one. Strain measurements were performed for a maximum strain of 1000 με. The strain sensitivity showed a highly linear correlation with ∆λFSR. Small length variations for short cavities heavily affect the FSR value. The smallest and longest cavities present sensitivities of 8.71 and 2.68 pm/με, respectively. Thermal characterization for low- and high-temperature regimes was also performed and is constant for tested sensors.

Inverse-cavity structure for low-threshold miniature lasers

Creating micro and nano lasers, high threshold gain is an inherent problem that have critically restricted their great technological potentials. Here, we propose an inverse-cavity laser structure where its threshold gain in the shortest-cavity regime is order-of-magnitude lower than the conventional cavity configurations. In the proposed structure, a resonant feedback mechanism efficiently transfers external optical gain to the cavity mode at a higher rate for a shorter cavity, hence resulting in the threshold gain reducing with decreasing cavity length in stark contrast to the conventional cavity structures. We provide a fundamental theory and rigorous numerical analyses confirming the feasibility of the proposed structure. Remarkably, the threshold gain reduces down by a factor ~ 10−3 for a vertical-cavity surface-emitting laser structure and ~ 0.17 for a lattice-plasmonic nanocavity structure. Therefore, the proposed approach may produce extremely efficient miniature lasers desirable for variety of applications potentially beyond the present limitations.

Gain-switching in CsPbBr3 microwire lasers

All-inorganic perovskite microwire lasers, which have intrinsic high material gain and short cavity, especially favor the generation of ultrashort optical pulses via gain switching for various potential applications. Particularly, the ultrashort gain-switched pulses may extend perovskite microwires to previously inaccessible areas, such as ultrafast switches, and chipscale microcombs pumping souces in photonic integrated circuits. Here, we show 13.6-ps ultrashort single-mode green pulses from the gain-switched CsPbBr3 microwire lasers under femtosecond optical pumping. The gain-switching dynamics is experimentally investigated by a streak camera system. The excitation fluence dependences of pulse width, delay time and rise time of the output pulses show good agreements with the rate equation simulations with taking gain nonlinearities and carrier recombination ABC model into account. Our results reveal that perovskite microwire lasers have potential for ultrashort pulse generation, while the low transient saturated gain, which may result from the high transient carrier temperature under femtosecond pumping is a significant limitation for further pulse shortening.

Modeling of surface grating-loaded VCSEL with slowing light

We propose single-mode vertical cavity surface-emitting lasers (VCSEL) based on the surface grating with various cavity lengths ranging from 10 μm to 1 cm. We present the modeling of surface grating-loaded VCSEL based on the coupled-mode theory with slowing light. Thanks to the low effective index caused by the slow-wave propagation, a shallow surface grating of a few tens nm provides large optical feedback. A narrow stop band in the surface grating-loaded VCSEL waveguide appears because of the large waveguide dispersion of slow-waves. The numerical results on coupling coefficient, mode selectivity and threshold gain are given for single-mode, long-cavity and short-cavity grating-loaded VCSELs with different grating parameters. A wide range of grating coupling strength enables a short cavity of 10 μm and a long cavity of 1 cm with a single-mode operation. The proposed VCSEL structure may open up new VCSEL platforms for low-power consumption and high-power applications.

A 658-W VCSEL-pumped rod laser module with 52.6% optical efficiency

A high-efficiency and high-power vertical-cavity surface-emitting laser (VCSEL) side-pumped rod Nd:YAG laser with temperature adaptability are demonstrated. The VCSEL side-pumped laser module is designed and optimized. Five VCSEL arrays are symmetrically located around the laser rod and a large size diffused reflection chamber is designed to ensure a uniform pump distribution. Furthermore, the absorbed pump power distribution of the rod is simulated to verify the uniformity of the pump absorption. Finally, a proof-of-principle experiment is performed in short linear cavity laser with single laser module. A continuous-wave output power of 658 W at 1064 nm is obtained, the corresponding optical-to-optical efficiency is 52.6%, and the power variations are ±0.7% over 400 s and ±3.1% over the temperature range from 16 °C to 26 °C. To the best of our knowledge, this is the highest output power and the highest optical-to-optical efficiency ever reported for VCSEL pumped solid-state lasers. By inserting a telescopic module into the cavity and optimizing the TEM00 mode volume, the average beam quality is measured to be M 2 = 1.34 under an output power of 102 W. The experimental results reveal that such a high power rod laser module with temperature stability is appropriate for field applications.

Gigahertz femtosecond laser-by a novel asymmetric one-dimensional photonic crystal saturable absorber device with defect layer

Abstract High repetition frequency (HRF) ultrashort pulse fiber laser has been widely used in laser cold processing. The technical solutions such as short cavity length fiber laser have been proposed to achieve HRF ultrashort pulse output recently. However, the application of material-based saturable absorbers in this field has been astricted due to the low modulation depth, low damage resistance threshold, and high saturation fluence. Here, we designed a one-dimensional asymmetric photonic crystal with defect layer (1D-APCDL) as a novel saturable absorber, where the defect layer is Bi1.6Sb0.4Te3 with high modulation depth. The harmonic pulse with 3.82 GHz repetition frequency is achieved at the wavelength of 1562 nm, which is the highest repetition frequency of the topological insulator-based ring fiber laser so far to the best of our knowledge. The research provides a new saturable absorber solution, and provides a new idea for the application of material-based nonlinear optical chip in high-repetition frequency ultrashort pulse fiber lasers.

Mode selection in InGaAs/InGaAsP quantum well photonic crystal lasers based on coupled double-heterostructure cavities.

Photonic crystal lasers with a high-Q factor and small mode volume are ideal light sources for on-chip nano-photonic integration. Due to the submicron size of their active region, it is usually difficult to achieve high output power and single-mode lasing at the same time. In this work, we demonstrate well-selected single-mode lasing in a line-defect photonic crystal cavity by coupling it to the high-Q modes of a short double-heterostructure photonic crystal cavity. One of the FP-like modes of the line-defect cavity can be selected to lase by thermo-optically tuning the high-Q mode of the short cavity into resonance. Six FP-like modes are successively tuned into lasing with side mode suppression ratios all exceeding 15 dB. Furthermore, we show a continuous wavelength tunability of about 10 nm from all the selected modes. The coupled cavity system provides a remarkable platform to explore the rich laser physics through the spatial modulation of vacuum electromagnetic field at submicron scale.

Sensing performance of U-shaped fiber in chaotic correlation fiber loop ring down system

We experimentally demonstrated a high sensitivity chaotic correlation fiber loop ring down sensing based on U-shaped fiber optical sensor. The U-shaped fiber optical sensors with different bending diameters were spliced into fiber loops with different cavity lengths to study the influence of cavity length and bending diameter on detection sensitivity. The relationship between cavity length and sensitivity was investigated theoretically and experimentally. The limit of sensitivity was analyzed theoretically and the highest sensitivity of 2253.390 μs−1RIU−1 in the refractive index range of 1.3347–1.3721 was realized experimentally. A handheld probe measuring device was obtained by combining short cavity fiber loop ring down and U-shaped fiber optical sensor, and the probe device can achieve the detection of hemoglobin level with the high sensitivity.

Polarization Properties of InGaN Vertical-Cavity Surface-Emitting Laser With Pipe Distributed Bragg Reflector

Indium gallium nitride (InGaN) vertical-cavity surface-emitting lasers (VCSELs) with top dielectric and bottom pipe-gallium nitride (GaN) distributed Bragg reflectors (DBRs) were demonstrated. Pipe-GaN DBR structure consisted of a lateral wet-etched pipe-GaN layer and a GaN layer in the 20 pair stack structure as an embedded reflector. The optical birefringence behavior of the anisotropic pipe-GaN structure induced the polarization properties in the reflectance and electroluminescence (EL) spectra. Polarization EL emission peaks with narrow line-widths were measured at 430.8 nm (perpendicular to pipe-GaN) and 432.5 nm (along with pipe-GaN), respectively. The EL emission peaks with high polarization ratios were observed due to the short resonance cavity effect with the birefringence pipe-GaN DBR structure.

Peculiaritie of Polarization Waves Behavior under Excitation of an Extended Resonant Medium by Overlapping Extremely Short Light Pulses

With the coherent interaction of a sequence of extremely short light pulses with a resonant medium (when the duration of the pulses and the delays between them are shorter than the relaxation times of the population difference T1 and polarization T2 of the medium), it is possible to create spatial electromagnetically induced gratings (EMIG) of polarization and population difference in the medium, as well as their ultrafast control. The dynamics of such gratings is of interest in connection with the possibility of ultrafast control over the properties of a medium at times of the order of the pulse duration and their influence on the stability of generation of extremely short pulses in a laser with a short linear cavity. In this paper, based on the numerical solution of the system of Maxwell-Bloch equations, an unusual and interesting possibility of the appearance of narrow and alternating sections in the medium is shown and studied in detail, in which harmonic polarization waves of the medium are formed, propagating in opposite directions relative to neighboring sections. This situation occurs when two extremely short attosecond pulses collide in a two-level resonant medium. Keywords: attosecond pulses, electromagnetically induced gratings, polarization waves.