Whispering gallery mode microsphere resonator based on cylindrical air cavity coupling.

In this Letter, an optical fiber whispering gallery mode microsphere resonator based on cylindrical air cavity coupling is proposed and demonstrated. The cylindrical air cavity is vertical to the axis of a single-mode fiber and in touch with the fiber core, fabricated by using femtosecond laser micromachining together with hydrofluoric acid etching. A microsphere is inserted into the cylindrical air cavity and in tangential contact with the inner cavity wall, which is in touch with or inside the fiber core. The light traveling in the fiber core is coupled into the microsphere via an evanescent wave when the light path is tangential to the contacting point of the microsphere and the inner cavity wall, resulting in whispering gallery mode resonance when the phase-matching condition is satisfied. Such a device is highly integrated, robust in structure, low in cost, stable in operation, and has a good quality factor (Q) of 1.44 × 104.

All-fiber saturable absorber based on slightly tapered graded index multimode fiber for conventional and bound soliton generation.

A slightly tapered graded index multimode fiber (GIMF) is demonstrated as the saturable absorber (SA) for a conventional soliton and tightly bound soliton generation. The SA device is simply a GIMF spliced with single-mode fiber (SMF) at its two ends. The waist diameter of the slightly tapered fiber is ∼23.8µm, located at one of the splicing points of SMF and GIMF. The mode-locked output of conventional solitons and tightly bound solitons can be obtained in a ring fiber laser based on such a SA. The central wavelength of the output conventional solitons is 1563.5 nm, and the pulse width is as short as 473 fs at the fundamental frequency of 19.53 MHz. The central wavelength of the tightly bound solitons is 1563.5 nm, with a pulse width of 636 fs and a separation of 1.89 ps. The device is featured with a simple and robust structure and stable operation capability and can effectively overcome the difficulty in requirement of precise control of the GIMF length, which make it attractive in ultrafast lasers and optical fiber communication systems.

[Comparison the characteristics of serum adrenocortical hormone profile in patients with adrenal cortical carcinoma and adrenal coritcal adenoma].

Objective: To compare the characteristics of serum adrenocortical hormone profiles detected by liquid chromatography tandem mass spectrometry in patients with adrenal cortical carcinoma and adrenal adenoma. Methods: A total of 23 patients with adrenal cortical carcinoma and 119 patients with adrenal cortical adenoma/hyperplasia who visited the Department of Endocrinology and/or the Department of Urology of Peking University First Hospital from January 2018 to June 2022 were analyzed retrospectively. The imaging characteristics and serum adrenal cortical hormone profiles detected by liquid chromatography tandem mass spectrometry were analyzed retrospectively. The independent related factors of adrenal cortical carcinoma were screened by univariate analysis and multivariate logistic regression analysis. Results: The age of patients with adrenal cortical carcinoma was 46 (35, 57) years, and 15 (65.2%) were female; The age of adrenal cortical adenoma patients was 49 (40, 58) years old, and 80 (67.2%) were female. There was no significant difference in age and gender between the two groups (all P values>0.05). The maximum tumor diameter M (Q1, Q3) of patients with adrenocortical carcinoma was 7.05 (5.45, 9.78) cm, which was larger than that of patients with adrenocortical adenoma [2.1 (1.6, 3.0) cm] (P<0.001). Compared with patients with adrenal adenoma, the androstenedione (AD) of patients with adrenal cortical carcinoma [4.056 9 (1.619 5, 7.907 9) nmol/L vs 1.517 5 (0.935 1, 2.582 1) nmol/L (P<0.001)] was significantly increased; 11-ketotestosterone/11-ketoandrostenedione [0.034 3 (0.020 6, 0.079 2) vs 0.041 0 (0.028 6, 0.061 5) (P=0.089)] and 11-ketoandrostenedione/11-hydroxyandrostenedione [0.013 0 (0.006 4, 0.086 7) vs 0.063 0 (0.018 2, 0.162 5) (P=0.042)] were significantly decreased. Multivariate analysis found that AD, the largest diameter of the tumor, 11-ketotestosterone/11-ketoandrostenedione and 11-ketoandrostenedione/11-hydroxyandrostenedi-one were related factors for adrenal cortical carcinoma, with OR values (95%CI) of 1.841 (1.093-3.100), 5.130 (2.332-11.285), 0.381 (0.167-0.867) and 0.000 (0.000-0.014), respectively, all P values <0.05. Conclusions: The larger diameter of adrenal cortical tumor and the higher the level of androstenedione are independent risk factors for adrenal cortical carcinoma. The reduction conversion of 11-hydroxyandrostenedione to 11-ketoandrostenedione and 11-ketoandrostenedione to 11-ketotestosterone were independently associated with adrenal cortical carcinoma.

Long-period fiber grating based on inner microholes in optical fiber.

A new, to the best of our knowledge, type of long-period fiber grating based on inner microholes in an optical fiber core is proposed and demonstrated. The device is fabricated by first using a femtosecond laser to write a series of vertical short line structures in the core of a single-mode fiber, followed by discharge of the short line areas with a fusion splicer. Owing to the high temperature generated, the line structure rapidly expands and forms an inner microhole. The device is found to have good high temperature sustainability up to 1000°C, with a sensitivity of approximately 13 pm/°C and its strain and bending sensitivities are approximately -1.57 pm/µÉ› and -5.198 dB/m-1, in the ranges of 0-800 µÉ› and 0-0.77 m-1, respectively. It is expected that the device can contribute to the multi-parameter sensing and detection of high temperature in harsh environments.

Ultra-short long-period fiber grating based on a micro air-channel structure.

A new type of long-period fiber grating is proposed and demonstrated. The structure of the device consists of a few micro air-channels along a single-mode fiber, and is fabricated by using a femtosecond laser to inscribe a few groups of fiber inner waveguide arrays followed by hydrofluoric acid etching. The length of the long-period fiber grating is down to 600 µm, corresponding to only five grating periods. To the best of our knowledge, this is the shortest long-period fiber grating reported. The device has a good refractive index sensitivity of ∼587.08 nm/RIU (refractive index unit) in the refractive index range of 1.34-1.365 and a relatively small temperature sensitivity of ∼12.1 pm/°C, thus reducing the temperature cross-sensitivity. Such a device is expected to have promising photonic applications.

Multimode fiber surface plasmon resonance sensor based on a down-up taper.

A multimode fiber surface plasmon resonance sensor based on a down-up taper for refractive index measurement is proposed and demonstrated. The device is fabricated by splicing two multimode fibers in a heating and pushing process to form an up taper, followed by heating and pulling the fiber adjacent to the up-taper area to form a down taper, and then using a magnetron sputtering technique to deposit a Cr + Ag layer of 50 nm on the surface of the down-up taper. Such a structure effectively increases the taper ratio and hence improves the measurement sensitivity. The experimental results obtained show that in the refractive index range of 1.345-1.375, the refractive index sensitivity achieved is ∼3264.01 nm/RIU. The device has a compact size and its entire length is ∼2.75 mm. Moreover, the robustness of the device is better than that of the previously reported fiber surface plasmon resonance sensors due to its relatively large waist diameter of 40 µm for the down taper. The device is expected to have potential applications in biological and chemical sensing.

Coupling scheme for graphene saturable absorber in a linear cavity mode-locked fiber laser.

A saturable absorber based on a graphene layer covered single-mode fiber with inner short waveguides is proposed and demonstrated for a linear cavity Er-doped mode-locked fiber laser. A pair of short waveguides is written in the fiber by using femtosecond micromachining technology, and the propagating light is guided by one short waveguide to the cladding-air interface and interacts with the graphene layer in the form of evanescent waves before being collected back to the core by another short waveguide, and, as a result, the saturable absorption is excited. The designed saturable absorber is used in the passively mode-locked fiber laser to generate traditional soliton mode-locked pulse output with the center wavelength of 1564.9 nm and pulse width of 758 fs at the fundamental frequency of 22.58 MHz. The fabricated saturable absorber device is stable in operation, compact in structure, safe for thermal damage, and can effectively overcome the shortcomings of poor robustness of the saturable absorbers based on a tapered fiber and D-shaped fiber. This provides a new optical coupling scheme for saturable absorbers based on 2D materials such as graphene and has great potential application in the field of ultrashort pulse lasers.

Simultaneous refractive index and temperature sensing based on a fiber surface waveguide and fiber Bragg gratings.

An optical fiber sensor based on a fiber surface waveguide and Bragg grating is proposed for a simultaneous refractive index (RI) and temperature sensing. The device consists of two fiber Bragg gratings fabricated by a femtosecond laser, one of which is situated in the fiber core for temperature sensing; the other is located in the fiber surface waveguide for both temperature and RI measurements. The RI and temperature sensitivities achieved are 10.3 nm/RIU and 9.94 pm/°C, respectively. The device is featured with a compact structure, high robustness, and convenient operation.

Ultra-high sensitivity strain sensor based on biconical fiber with a bulge air-bubble.

An ultra-high sensitivity Fabry-Perot interferometer (FPI) strain sensor is proposed and experimentally demonstrated. It is composed of a biconical fiber with a bulge air-bubble at its waist. Because of the good stress concentration capability of the tapered fiber, the bulge air-bubble is easily deformed when it is stressed, resulting in an ultra-high strain sensitivity of 101.7 pm/µÉ›, which is the highest strain sensitivity among the direct wavelength interrogation-based strain sensors reported so far, to the best of our knowledge. Moreover, the proposed sensor has a low temperature sensitivity of ∼1pm/∘C; thus an extreme low temperature cross sensitivity of less than 10 nɛ/°C can be achieved.

High-power synchronous multi-wavelength solitons from a multimode mode-locked fiber laser system.

In this Letter, we implement a multimode fiber (MMF) laser system mode-locked by a nonlinear polarization rotation technique for controllable synchronous multi-wavelength soliton generation. The synchronization of the repetition rates for different wavelengths is realized by the special mode transmission in MMF. For dual-wavelength mode-locking at 1566.7 nm and 1617.2 nm, each of the synchronously mode-locked solitons consists of a train of quasi-periodic beat pulses with a pulse width of 84 fs and period of 162 fs. The total output power reaches 532 mW with optimally balanced two-color intensities. Furthermore, switchable dual- and tri-wavelength synchronized femtosecond pulses are also obtained. In contrast to previous reports, this synchronously mode-locked multi-wavelength is output directly from a laser oscillator, which provides a simpler candidate to achieve pulse synchronization.

Microfiber sensor probe integrated with a cascaded Fabry-Perot interferometer.

Developing micrometer-nanometer size optical fiber sensors has promising application prospects in microenvironments, such as biological cells, micro robots, and microfluids. We propose a new strategy to fabricate a microfiber sensor probe (MSP). A femtosecond laser was applied to integrate cascaded Fabry-Perot interferometers (FPIs) into a silica microfiber. And a MSP with diameter of ∼8µm, extinction ratio of 15 dB, fitness of 24.6, and Q-factor of 2310 was demonstrated in the experiment. In addition, the MSP was applied for the refractive index and thermal measurement and the sensitivity was observed to be 10 pm/°C and 18.5 nm/RIU. The two-beam approximation model was applied to analyze the spectrum, and simulations were taken to research the refractive index sensitivity influenced by the fiber size.

Integrated in-fiber coupler for a whispering-gallery mode microsphere resonator.

An integrated in-fiber coupler is proposed and demonstrated, which can excite whispering-gallery mode (WGM) in a microsphere resonator. This device is fabricated firstly by using femtosecond laser micromachining and a fusing splicing technique to create an inner air cavity with a suspended fiber core, then an open micro-channel is drilled on the top of the inner air cavity, and finally a microsphere is placed inside the air cavity, in contact with the suspended fiber core, to excite the WGM through the evanescent field. In the transmission spectrum of the device, the slopes of the two asymmetric Fano resonance lines of 41.12 dB/nm and $ - {18.46}\;{\rm dB/nm}$-18.46dB/nm, respectively, and a symmetrical Lorentz line with a quality factor of ${9.32} \times {{10}^3}$9.32×103 can be obtained simultaneously. Such an in-fiber WGM microsphere resonator has the advantages of compact structure, convenient operation, and high durability.

Parallel structured optical fiber in-line Fabry-Perot interferometers for high temperature sensing.

We propose and demonstrate parallel structured optical fiber in-line Fabry-Perot interferometers for high temperature sensing. The device consists of three Fabry-Perot cavities in parallel connection, which allows three independent fringe patterns superimposed at its output, and, as a result, a number of dominant fringe peaks/dips appear, thus enabling unambiguous measurement in a large range. The device is featured with compact size, robust structure, and excellent high temperature sustainability, which makes it promising in extreme environment monitoring.

Optical fiber surface waveguide with Fabry-Perot cavity for sensing.

A parallel structured optical fiber Fabry-Perot interferometer sensor is proposed and demonstrated for refractive index and strain sensing with low temperature cross sensitivity. The device consists of two Fabry-Perot cavities fabricated by a femtosecond laser: one is inscribed in the fiber surface waveguide and used for sensing, and the other one is located in the fiber core for referencing. Part of the light propagating in the fiber core can be directed to the fiber surface waveguide via an X coupler. Because of the evanescent field, the light traveling along the fiber surface waveguide interacts with the surrounding medium and enables external refractive index sensing. The measurement sensitivity of the device is enhanced due to the Vernier effect associated with the parallel structured two Fabry-Perot interferometers. The sensitivities of ∼843.3nm/RIU and ∼101.8pm/µÎµ have been obtained for refractive index and strain, respectively, and the corresponding temperature cross sensitivities are ∼9.6×10-6RIU/∘C and ∼7.956×10-2µÎµ/∘C, respectively. The device is featured with high robustness, compact size, and large sensitivity.

Hollow-Core-Photonic-Crystal-Fiber-Based Miniaturized Sensor for the Detection of Aggregation-Induced-Emission Molecules.

A miniature sensor for detection of aggregation-induced-emission (AIE) molecules is proposed in this work. The sensing head is fabricated by use of hollow-core photonic crystal fiber with a core diameter of about 4.8 µm. The cladding holes are sealed with a fusion splicing technique, and the central hole remains open to allow the filtration of solution with AIE molecules. When the solution is excited by an ultraviolet lamp, the fluorescence is received by a fiber-optic spectrometer. The fluorescence intensity is associated with the concentration of AIE molecules and the infiltrated-core length. In the whole process of the experiments, the output-peak wavelength is stable, which indicates that the existing forms of AIE particles are stable, and the fluorescence reabsorption can be neglected. The experimental results obtained are in accordance with traditional microplate-spectrophotometer methods. The most exciting result is that the amount of sample measured can be as low as 0.36 nL, which allows the detection of AIE molecules at only 0.02 pmol. In addition, the miniature sensor was successfully applied to the detection of an AIE-based bioprobe for evaluating the activity of the dipeptidyl-peptidase 4 (DPP-4) inhibitor sitagliptin with an IC50 of 59.80 ± 3.06 nM. The advantages of small device size and nanoliter-scale sample volumes suggest that the proposed sensor is promising for many biosensing applications.

Construction of cascaded Fabry-Perot interferometers by four in-fiber mirrors for high-temperature sensing.

An optical fiber high-temperature sensor is proposed and demonstrated by use of cascaded Fabry-Perot interferometers based on four in-fiber mirrors fabricated by femtosecond laser inscription. The output fringe pattern of the device exhibits dominant/highly distinguishable dip wavelength, which helps in unambiguous measurement beyond the free spectral range. The device has excellent thermal stability in high temperature up to 1100°C, and the temperature sensitivity obtained is 9.91 pm/°C within the temperature range of 100°C to 400°C, and 15.88 pm/°C within the temperature range of 400°C to 1100°C, respectively. The device features compact size, simple fabrication, and convenient operation, which make it attractive for monitoring of extreme environment.

In-line reflective Mach-Zehnder interferometer based on a tilted in-fiber beam splitter.

A fiber in-line reflective Mach-Zehnder interferometer is proposed and experimentally demonstrated, which is based on a tilted in-fiber beam splitter inscribed by a femtosecond laser. The beam splitter splits the incident light beam into two parts; one is directed to the fiber cladding-air interface, where it experiences total internal reflection and is directed to the fiber core. The other part of the light beam keeps traveling in the fiber core. The two parts of the light beams are recombined in the fiber core to generate interference before being reflected at the fiber cut end face. The device proposed is compact in size, robust in mechanical strength, easy in fabrication, and can be used for environmental refractive index sensing in a convenient manner.

Cascaded multiple Fabry-Perot interferometers fabricated in no-core fiber with a waveguide for high-temperature sensing.

An optical fiber high-temperature sensor is proposed and demonstrated by use of cascaded multiple Fabry-Perot interferometers in no-core fiber with a waveguide fabricated by femtosecond laser pulse inscription. The device can sustain the high temperature up to 1100°C, and the temperature sensitivity obtained is 8.9 pm/°C within the temperature range 100°C-400°C, and 16.36 pm/°C within the temperature range 400°C-1100°C, respectively. Such a no-core fiber-based device can be fabricated in a simple way and operated reliably, which makes it attractive for extreme environment monitoring.

Generation of pulse-width controllable dissipative solitons and bound solitons by using an all fiber saturable absorber.

A passively mode-locked fiber laser with controllable pulse width is demonstrated by use of an all-fiber saturable absorber based on a hybrid no-core fiber (NCF)-graded index multimode fiber (GIMF) structure incorporated into an Er-doped fiber ring cavity. Such a hybrid NCF-GIMF structure has a tunable intracavity filtering effect. As a result, the mode-locking operation is achieved with controllable pulse width and spectral bandwidth in the normal dispersion regime by only stretching the fiber device. Soliton pulses with the pulse width of 7.7-23 ps are generated, and bound solitons with variable widths are also experimentally demonstrated. The results obtained reveal the versatility and flexibility of the NCF-GIMF structured device in controlling the pulse dynamics for different experimental requirements.

Highly sensitive PDMS-filled Fabry-Perot interferometer temperature sensor based on the Vernier effect.

A highly sensitive temperature sensor is demonstrated experimentally, which is fabricated based on a Fabry-Perot interferometer (FPI) filled with polydimethylsiloxane (PDMS). The sensor's sensitivity is -0.653 nm/°C by utilizing the thermal expansion effect of PDMS, which has been greatly improved compared to that of the traditional FPI temperature sensor. Moreover, in order to further improve the sensitivity, a scheme where two parallel FPI structures are used to form the Vernier effect is proposed, which are a sensing FPI and reference FPI, respectively. Such a temperature sensor based on the FPI filled with PDMS and the Vernier effect exhibits a high temperature sensitivity of 17.758 nm/°C. Meanwhile, the proposed sensors show the advantages of high sensitivity, simplicity, and low cost.