Wide-range pH sensor based on a cascaded MZI up-down tapered hetero-core structure.

In this paper, we describe the first demonstration of an optical fiber sensor based on two cascaded architectures of the Mach-Zehnder interferometer (MZI) with an up-down-tapered (UDT) hetero-core fiber structure for simultaneous pH measurement at two different spatial locations. The two fiber structures, namely structures I and II, were fabricated by sandwiching a 45 mm and 60 mm long piece of no-core fiber (NCF) between two single-mode fibers (SMFs), respectively. By inserting a down-taper between two adjacent up-tapers in the NCF section using the over-fusion splicing technique, the UDT hetero-core fiber structure was achieved. The down-taper works as an optical attenuator, while the two up-tapers each function as a fiber splitter/combiner. By cascading the two sensor structures I and II, two distinct interference dips were obtained, resulting in dual sensing points (so-called sensing points I and II). According to experimental results, it was shown that the proposed sensing points I and II are pH-sensitive, and exhibit sensitivities of 1.0428 nm/pH and -1.7857n m/p H for simultaneous measurement for each point as the pH ranged from 8 to 14 and from 1 to 7 pH, respectively. The obtained results show that the proposed dual point pH sensor has the potential to be used for the simultaneous detection of pH parameters in any environment and at various places.

Skin-like and highly elastic optical fiber strain sensor based on a knot-bend shape for human motion monitoring.

A simply designed, highly sensitive, stretchable, compact wearable, and skin-like optical fiber sensing instrument is designed and demonstrated for joint motion monitoring. The fiber sensing scheme comprises only a section of single-mode fiber (SMF) deformed in the knot-like configuration, which performs as a Mach-Zehnder interferometer (MZI) based on a modal coupling mechanism between the core and cladding modes of the deformed SMF section. This proposed optical fiber sensor based on a knot-like configuration is mounted onto wearable woven fabric and then garments on the limbs of a healthy human's body. As the flexion angle of the human limb is varied, the interference fringe coding based on the spectral shift difference of the periodical transmission spectra is perceived. The proposed wearable optical fiber sensor exhibits excellent sensitivities from around -0.431 to -0.614n m/∘ realized for elbow and knee joint flexion between a range of motion around 0°-90°. Additionally, this sensor also displays high repeatability and stability and a fast response time of 1.4 ms, combined with a small standard deviation of about 2.585%. The proposed sensor device possesses manufacturing simplicity, high processing accuracy, lightness, and elasticity, as well as certain improvements over other goniometers and optical fiber sensors. These attributes of the proposed sensor prove its applicability for human joint angle monitoring.

Evaluation of the laser wavelength role on tooth bleaching in terms of color change, roughness, and microhardness with pulpal rapid temperature monitoring: an in vitro study.

This in-vitro study evaluates the efficacy of the teeth bleaching approach using different laser wavelengths (405 nm blue diode, 940 nm infrared diode, and Er,Cr:YSGG 2780 nm lasers) in comparison to the conventional method using light-emitting diode (LED) sources (420-480) nm. Eighty caries-free sound human premolars were randomly divided into four groups (N=20). Each group received a different bleaching procedure. Then each group was further subdivided into two subgroups (N=10) stained with different solutions. The pulp chamber temperature rise was recorded using an optical fiber sensor with a novel design and fabrication. The color was analyzed using a digital spectrophotometer. Five samples of each subgroup were tested for surface roughness, while the others were tested for Vickers microhardness. The bleaching process with a short wavelength 405 nm blue diode laser showed the best results for the shade, with a minimum pulpal temperature increase indicating no possible necrosis and hence maintaining tooth vitality. Additionally, a remarkable reduction in bleaching time was achieved compared to the conventional approach. This process also yielded the highest color change (Δ E) and increased microhardness, with no noticeable change to the tooth roughness. The 405 nm blue diode laser applied for bleaching showed the best bleaching activity against tested stains and negligible pulpal temperature rise with a noticeable reduction in the bleaching time. The proposed novel method to measure temperature change could be used to develop a promising smart sensor for quick, effective, repeatable, and in-situ monitoring of human body temperature.

Mach-Zehnder interferometer comb filter for multi-wavelength mode-locked generation from erbium-doped fiber laser.

A picosecond pulse multi-wavelength erbium-doped fiber ring laser based on an optically switchable/tunable Mach-Zehnder interferometer (MZI) filter is proposed and experimentally demonstrated. The MZI configuration is constructed with two 3 dB couplers and two short pieces of endlessly single-mode photonic crystal fiber with different lengths in each arm. The combination of the MZI and a polarization controller (PC) acts as a selective comb filter and a mode-locking device. By adjusting the pump power or the first PC (PC1) state, the laser can emit up to 10 channels with a 20 nm tuning range (1530 to 1550 nm). Furthermore, by changing the PC1 condition, stable picosecond mode-locking generation with a repetition rate of 13.5 MHz is realized. To achieve wavelength spacing tunable multiple-channel laser emissions, the MZI is configured by incorporating a second PC (PC2) in one arm of the conventional MZI. Four channels of picosecond mode-locking pulses with a repetition rate of 11.6 MHz in the range of 1530-1550 nm are realized by carefully controlling the pump power and/or PC2 state. With the manipulation of intracavity birefringence through PCs with comb filtering via MZI, the proposed structure may be a potential tool in various photonics applications.

Highly efficient optical fiber sensor for instantaneous measurement of elevated temperature in dental hard tissues irradiated with an Nd:YaG laser.

In this in vitro experiment, the effect of 1.064 µm pulsed laser on both enamel- and dentin-dental tissues has been investigated. A total of fifty-five dental hard tissue samples were exposed to Nd:YAG laser that possesses a pulse width of 9 ns and 850 mJ of total energy. An optical fiber sensor was put behind the samples to measure the temperature instantaneously. A novel, to the best of our knowledge, fiber sensor has been proposed and used to measure the heat generated in dental hard tissues instantaneously after the application of laser irradiation on the tissue surface. This optical sensor exhibits a fast response time of about 1 ms and high sensitivity with about 1.975 nm/°C. The findings of this study in decreasing the probability of pulpal necrosis structure while handling the tooth, whether for ablation, welding, or tooth resurfacing purposes, may establish standards for dentists and laser manufacturers (healthcare professionals) that should be followed.

Stable evanescent wave mode-locked laser based on a photonic-crystal-fiber-induced Mach-Zehnder filter as a gain-tilt equalizer.

In this paper, a switchable multichannel ytterbium-doped fiber laser based on a Mach-Zehnder interferometer (MZI) has been experimentally demonstrated. The MZI is fabricated by splicing a short piece of nonlinear photonic-crystal fiber (NLPCF) between two segments of a single-mode fiber. MZI-based NLPCF gathering with the intracavity birefringence polarization controller offers functions as both the selective comb filter and the mode locker. The laser can emit a single, dual, triple, or quadruple channel that can be varied in the spectral span around1030-1050 nm by only adjusting the pump power or polarization state. A stable evanescent wave, picosecond mode-locking generation with variable repetition rate (24.5 to 18.23 MHz and 34.64 MHz) has been realized via only inserting the polarization controller and adjusting its state inside the cavity. To the best of the authors' knowledge, this the first demonstration of a passive filter based on a NLPCF-MZ interferometer to generate switchable mode-locked pulses delivered from a single laser oscillator.

Tellurium-nanorod-based saturable absorber for an ultrafast passive mode-locked erbium-doped fiber laser.

In this paper, we propose and demonstrate ultrafast Te nanorods as a saturable absorber (SA) for producing mode locking from an erbium-doped fiber laser for the first time, to the best of our knowledge. The Te nanorods were fabricated by a simple green chemical method with energy conservation and without a purification process. The morphology and structure measurements confirm uniform Te nanorods with a constant aspect ratio. The synthesized SA has a saturation intensity and modulation depth of $25.44\, {\rm MW/cm}^{2} $25.44MW/cm2 and 4%, respectively. By integrating the proposed SA into an erbium-doped all fiber-based ring cavity, the mode-locked fiber laser was readily generated. The conventional soliton pulses of ${3.56}\;{\rm ps}$3.56ps pulse width were obtained at 1566.7 nm central wavelength and a pulse repetition rate of 1.87 MHz. The results show that the moderate saturable-absorption characteristics of Te nanorods have superior performance in the ultrafast optics field, which is eligible in many applications, such as optical communications.

Stable L-band multiwavelength erbium-doped fiber laser based on four-wave mixing using nickel nanofluid.

A simple continuous-wave multiwavelength erbium-doped fiber laser based on four-wave mixing has been successfully demonstrated utilizing nickel nanofluid (Ni-NF) for the first time, to the best of our knowledge. By fine adjustment of the laser diode pump power up to 196 mW and without any intracavity filtering, stable dual-, triple-, and quadruple-lasing lines in the L-band have been observed at 1595.6 nm, 1596.8 nm, 1598 nm, and 1599.2 nm, respectively, with a signal-to-noise ratio ∼43 dB. The induced L-band wavelengths showed high stability with wavelength shifts <0.07 nm and power fluctuation of <3 dB by monitoring the output spectra for a duration of 30 min at room temperature. Taking into account the superiority of Ni-NF in terms of compactness, low cost, and easy fabrication, this design can be practically used in a variety of nonlinear photonic applications.