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High-performance, high-volume-manufacturing Si 3 N 4 photonics requires extremely low waveguide losses augmented with heterogeneously integrated lasers for applications beyond traditional markets of high-capacity interconnects. State-of-the-art quality factors (Q) over 200 million at 1550 nm have been shown previously; however, maintaining high Qs throughout laser fabrication has not been shown. Here, Si 3 N 4 resonator intrinsic Qs over 100 million are demonstrated on a fully integrated heterogeneous laser platform. Q i is measured throughout laser processing steps, showing degradation down to 50 million from dry etching, metal evaporation, and ion implant steps, and controllable recovery to over 100 million from annealing at 250 ∘ C-350 ∘ C.

We demonstrate a high-sensitivity acetylene/methane gas sensor based on hollow-core fiber photothermal interferometry (PTI) with a pump-probe-alternating technique. This technique utilizes two distributed-feedback lasers as pump and probe beams alternatively for two gas components to facilitate photothermal phase modulation and detection through time-division multiplexing. With a 2.5-cm-long hollow-core conjoint-tube fiber, noise-equivalent concentrations of 370 ppb and 130 ppb are demonstrated for methane and acetylene, respectively. Noise characteristics of the PTI system are analyzed and experimentally tested. The proposed technique eliminates the need for an additional laser in the traditional PTI setup, enabling the construction of a sensitive yet more cost-effective multi-gas component detection system.

In this Letter, we present a high-strain resolution fiber laser-based sensor (FLS) by a novel optical phase-locked loop (OPLL) interrogation technique based on a root mean square detector (RMSD). The sensor consists of a distributed feedback (DFB) fiber laser as a master laser for strain sensing and a fiber Fabry-Perot interferometer (FFPI) as a reference. The laser carrier locks to the reference by the PDH technique, and the single sideband laser working as a slave laser locks to the DFB sensing element using the OPLL technique, respectively. A strain resolution of 8.19 pε/√Hz at 1 Hz and 35.5 pε in 10 s is achieved in the demonstrational experiments. Significantly, the noise behaves a 1∕f distribution below 0.2 Hz due to the very low pump power for the DFB sensor and an active thermostat testing environment. The proposed OPLL interrogation brings new thinking for the demodulation of FLS. This strain sensor based on FLS has a great performance in strain measurement and can be a powerful tool for geophysical research.

Soliton mode locking in high-Q microcavities provides a way to integrate frequency comb systems. Among material platforms, AlGaAs has one of the largest optical nonlinearity coefficients, and is advantageous for low-pump-threshold comb generation. However, AlGaAs also has a very large thermo-optic effect that destabilizes soliton formation, and femtosecond soliton pulse generation has only been possible at cryogenic temperatures. Here, soliton generation in AlGaAs microresonators at room temperature is reported for the first time, to the best of our knowledge. The destabilizing thermo-optic effect is shown to instead provide stability in the high-repetition-rate soliton regime (corresponding to a large, normalized second-order dispersion parameter D 2 /κ). Single soliton and soliton crystal generation with sub-milliwatt optical pump power are demonstrated. The generality of this approach is verified in a high-Q silica microtoroid where manual tuning into the soliton regime is demonstrated. Besides the advantages of large optical nonlinearity, these AlGaAs devices are natural candidates for integration with semiconductor pump lasers. Furthermore, the approach should generalize to any high-Q resonator material platform.

This paper presents a novel fiber liquid-pressure sensor that uses photopolymer glue to generate Fabry-Perot (F-P) interference, resulting in high sensitivity and a wide measurement range. The sensor comprises a single-mode fiber and photopolymer glue; the latter adheres to the fiber's end face and is decomposed by a 405-nm laser to create an air channel with a diameter of 5.9 µm and a length of 50 µm. When the air channel is placed underwater, a 17.5-µm air cavity forms between the fiber core and the air-liquid boundary due to the pressure balance, creating an F-P interferometer. Based on experimental results, the sensor has an average pressure sensitivity of 5.68 nm/kPa over 0.49-2.94 kPa. The sensitivity can be maintained at this level across different pressure measurement ranges (up to about 500 kPa) by using a 980-nm laser's radiation pressure to reset the air-liquid boundary. Besides its high sensitivity and wide measurement range, the sensor's straightforward structure, durability, affordability, compactness, and simple construction make it an appealing choice for liquid pressure measurement applications in various fields.

The resonant fiber-optic sensor (RFOS) is well known for its high sensing resolution but usually suffers from high cost and system complexity. In this Letter, we propose an ultra-simple white-light-driven RFOS with a resonant Sagnac interferometer. By superimposing the output of multiple equivalent Sagnac interferometers, the strain signal is amplified during the resonance. A 3 × 3 coupler is employed for demodulation, by which the signal under test can be read out directly without any modulation. With 1 km delay fiber and ultra-simple configuration, a strain resolution of 28 f ε /Hz at 5 kHz is demonstrated in the experiment, which is among the highest, to the best of our knowledge, resolution optical fiber strain sensors.

We studied the effect of varying gas concentration, buffer gas, length, and type of fibers on the performance of optical fiber photothermal phase modulators based on C 2 H 2 -filled hollow-core fibers. For the same control power level, the phase modulator with Ar as the buffer gas achieves the largest phase modulation. For a fixed length of hollow-core fiber, there exists an optimal C 2 H 2 concentration that achieves the largest phase modulation. With a 23-cm-long anti-resonant hollow-core fiber filled with 12.5% C 2 H 2 balanced with Ar, phase modulation of π-rad at 100 kHz is achieved with a control power of 200 mW. The modulation bandwidth of the phase modulator is 150 kHz. The modulation bandwidth is extended to ∼1.1 MHz with a photonic bandgap hollow-core fiber of the same length filled with the same gas mixture. The measured rise and fall time of the photonic bandgap hollow-core fiber phase modulator are 0.57 µs and 0.55 µs, respectively.

This Letter proposes a highly birefringent one-air-hole panda fiber, which is fabricated by corroding a single stress zone of the traditional panda-type polarization-maintaining fiber (PMF). An additional geometric asymmetry is induced in the fiber to increase the birefringence effect and enhance the light-matter interaction, which improves the performance of the sensor and the device applications of the special fiber. A theoretical and experimental analysis of the one-air-hole panda fiber demonstrates that the birefringence of the fiber can be of the order of 10 -3 , which is one order of magnitude higher than that of the traditional panda-type fiber. The corroded region provides a microchannel to be filled with a functional material to compose optical fiber sensors; a sample of a salt solution was filled into the microchannel to measure the refractive index with a sensitivity of 3760 nm/RIU (refractive index units).

A novel, to the best of our kowledge, reflective phase-shifted fiber Bragg grating (R-PS-FBG) structure composed of a conventional fiber Bragg grating (FBG) and a micromirror with an air gap between them is proposed. The R-PS-FBG may be regarded as consisting of a Fabry-Perot interferometer and a phase-shifted fiber Bragg grating (PS-FBG), and its reflection and transmission spectra are investigated numerically and experimentally for different mirror reflectances and gap widths. The R-PS-FBG can be implemented easily and could be used in optical spectrum filtering, wavelength tuning, and multiparameter sensing with better sensitivity and resolution.

An all-nonmetal metamaterial (ANM) terahertz device with a silicon bowtie structure has been developed, which has comparable efficiency to that of its metallic counterparts, and better compatibility with modern semiconductor fabrication processes. Moreover, a highly tunable ANM with the same structure was successfully fabricated through integration with a flexible substrate, which demonstrated large tunability over a wide frequency range. Such a device can be used in terahertz systems for numerous applications, and is a promising substitute for conventional metal-based structures.

Optical switches based on phase change materials have enormous application potential in optical logic circuits and optical communication systems. Integration of all-optical switching devices with optical fibers is a promising approach for realizing practical applications in enabling the optical fiber to transmit and process signals simultaneously. We describe an all-fiber nonvolatile broadband optical switch using an all-optical method. We use a single optical pulse to modulate the phase change material deposited on the tapered fiber to achieve logical control of the transmitted light. The response time of our optical switch is 80 ns for SET and 200 ns for RESET. Our optical switches can operate in the C-band (1530-1565 nm). The optical switching contrast is 40%. Our approach paves the way for all-optical nonvolatile fiber optic communication.

We self-injection-lock a diode laser to a 1.41 m long, ultra-high Q integrated resonator. The hybrid integrated laser reaches a frequency noise floor of 0.006 H z 2 / H z at 4 MHz offset, corresponding to a Lorentzian linewidth below 40 mHz-a record among semiconductor lasers. It also exhibits exceptional stability at low-offset frequencies, with frequency noise of 200 H z 2 / H z at 100 Hz offset. Such performance, realized in a system comprised entirely of integrated photonic chips, marks a milestone in the development of integrated photonics; and, for the first time, to the best of our knowledge, exceeds the frequency noise performance of commercially available, high-performance fiber lasers.

While Moore's law predicted shrinking transistors would enable exponential scaling of electronic circuits, the footprint of photonic components is limited by the wavelength of light. Thus, future high-complexity photonic integrated circuits (PICs) such as petabit-per-second transceivers, thousand-channel switches, and photonic quantum computers will require more area than a single reticle provides. In our novel approach, we overlay and widen waveguides in adjacent reticles to stitch a smooth transition between misaligned exposures. In SiN waveguides, we measure ultralow loss of 0.0004 dB per stitch, and produce a stitched delay line 23 m in length. We extend the design to silicon channel waveguides, and predict 50-fold lower loss or 50-fold smaller footprint versus a multimode-waveguide-based method. Our approach enables large-scale PICs to scale seamlessly beyond the single-reticle limit.

We demonstrate a high sensitivity all-fiber spectroscopic methane sensor based on photothermal interferometry. With a 2.4-m-long anti-resonant hollow-core fiber, a 1654 nm distributed feedback laser, and a Raman fiber amplifier, a noise-equivalent concentration of ${\sim}{4.3}\;{\rm ppb}$ methane is achieved at the room temperature and pressure of ${\sim}{1}\;{\rm bar}$. The effects of temperature on the photothermal phase modulation as well as the stability of the interferometer are studied. By introducing a temperature-dependent compensation factor and stabilizing the interferometer at quadrature, signal instability of ${\sim}{2.1}\%$ is demonstrated for temperature variation from 296 to 373 K.

We demonstrate a novel method to optically tune the pulse advancement and delay based on stimulated Raman gain in hydrogen. With a frequency-chirped pump, the generated signal pulse is selectively amplified at the leading or trailing edge of the pump pulse, depending on whether the frequency difference between the pump and the signal beam is blue or red detuned from the Raman transition, which results in advancement or delay of the signal peak. Different from the method of slow/fast light, where advancement and delay are accompanied with power loss and gain, respectively, for a single resonance, both the advancement and delay are realized in the gain region for the method here. With a piece of 48-mm-long optical nanofiber in hydrogen, the time-shift for a signal peak ranging from 3.7 to -3.7 ns is achieved in a Raman-generated pulse with width of ∼12 n s .

We demonstrate sensitive gas detection with mode-phase-difference photothermal spectroscopy assisted by a long period grating (LPG) inscribed on a dual-mode negative-curvature hollow-core fiber (NC-HCF). The LPG is inscribed using a pulsed C O 2 laser, which enables pump propagation in the fundamental L P 01 mode to achieve maximum photothermal phase modulation while exciting both the L P 01 and L P 11 modes at the probe wavelength to form a dual-mode interferometer for detection of the phase difference. With a 1533 nm pump and a 1620 nm probe, a noise equivalent concentration of ∼2.2 ppb acetylene is achieved with an 85-cm-long NC-HCF gas cell and 1 s lock-in time constant.

Ultra-low-loss waveguide fabrication typically requires high-temperature annealing beyond 1000°C to reduce the hydrogen content in deposited dielectric films. However, realizing the full potential of an ultra-low loss will require the integration of active materials that cannot tolerate high temperature. Uniting ultra-low-loss waveguides with on-chip sources, modulators, and detectors will require a low-temperature, low-loss dielectric to serve as a passivation and spacer layers for complex fabrication processes. We report a 250°C deuterated silicon dioxide film for top cladding in ultra-low-loss waveguides. Using multiple techniques, we measure propagation loss below 12 dB/m for the entire 1200-1650 nm range and top-cladding material absorption below 1 dB/m in the S, C, and L bands.

We demonstrate the first, to the best of our knowledge, photothermal carbon monoxide (CO) sensor using a hollow-core negative curvature fiber. The hollow-core fiber features a typical structure of one ring cladding containing eight nontouching capillaries to form a negative curvature core-surround. The photothermal effect in a 40-μm hollow core is induced by CO absorption at 2327 nm and detected by a Mach-Zehnder interferometer operating at 1533 nm. By using wavelength modulation spectroscopy, we achieve a normalized noise equivalent absorption coefficient of 4.4×10 -8   cm -1  WHz -1/2 . As CO has a very slow vibrational-translational relaxation process, we enhance the photothermal signal by enhancing the relaxation with the water vapor additive.

In this Letter, we demonstrate a low loss gallium arsenide and aluminum gallium arsenide on an insulator platform by heterogenous integration. The resonators on this platform exhibit record high quality factors up to 1.5×10 6 , corresponding to a propagation loss ∼0.4  dB/cm. For the first time, to the best of our knowledge, the loss of integrated III-V semiconductor on insulator waveguides becomes comparable with that of the silicon-on-insulator waveguides. This Letter should have a significant impact on photonic integrated circuits (PICs) and become an essential building block for the evolving nonlinear PICs and integrated quantum photonic systems in the future.

We propose the use of graphene as the electrode heater material for a polymer waveguide thermo-optic (TO) device. Because a graphene electrode can be buried in a polymer waveguide without introducing a significant loss to the transverse magnetic polarized light, we can do away with the buffer layer that is required in a conventional TO device to isolate the metal electrode heater from the waveguide and, hence, reduce the driving electric power of the device. To demonstrate the principle, we fabricate and compare two polymer waveguide TO mode switches based on the configuration of a balanced Mach-Zehnder interferometer, which are identical except that one uses a buried graphene electrode and the other uses an aluminum electrode deposited on the waveguide surface. Our experimental device that uses a graphene electrode has a switching power almost four times lower and also responds faster. The use of buried graphene electrodes is an effective approach to reducing the power consumption of TO devices.

We propose and demonstrate the use of graphene electrodes for lithium-niobate electro-optic (EO) devices to exempt the need of incorporating a buffer layer between the waveguide and the electrodes. Using graphene electrodes, our experimental mode converter, based on an EO-generated long-period grating in a LiNbO 3 waveguide, shows a reduction in the half-π voltage by almost three times, compared with the conventional electrode design using metal. With the buffer layer exempted, the device fabrication process is also significantly simplified. The use of graphene electrodes is an effective approach to enhancing the efficiency of EO devices and, at the same time, reducing their fabrication cost.

We present an all-fiber intracavity photothermal (IC-PT) spectroscopic gas sensor with a hollow-core photonic bandgap fiber (HC-PBF) gas cell. The gas cell is placed inside a fiber-ring laser cavity to achieve higher laser light intensity in the hollow core and hence higher PT modulation signal. An experiment with a 0.62-m-long HC-PBF gas cell demonstrated a noise equivalent concentration of 176 ppb acetylene. Theoretical modeling shows that the IC-PT sensor has the potential of achieving sub-ppb (parts-per-billion) acetylene detection sensitivity.

Gamma radiation-induced birefringence (RIB) variations in a solid core photonic crystal polarization-maintaining fiber (Pc-PMF) and a Panda PMF (Pa-PMF) are investigated. Experiments show that the birefringence of the fibers changes approximately linearly with a radiation dose over the range from 0 to 400 krad(Si), with negative slope coefficients of 8.91×10 -10 /krad(Si) and 4.38×10 -9 /krad(Si) for the Pc-PMF and Pa-PMF, respectively. Models of RIB variations for the two fibers are established, and the theoretical results agree well with the experimental ones.

We demonstrate the two-dimensional tunable orbital angular momentum (OAM) generation in a ring-core (vortex) fiber. The LP 11 mode generated by an all fiber fused coupler is coupled into a vortex fiber. Because the vector modes of the LP 11 mode group in the vortex fiber are no longer degenerate, the mode status will change between linearly polarized modes (LPMs) and complex OAM modes periodically during propagation. The generated OAM can be tuned smoothly by filtering the mixed mode with different polarization directions or changing the wavelength at a certain polarization directions. The two-dimensional tuning of OAM from l=-1 to l=+1 is experimentally demonstrated in an all fiber OAM generator.

We propose a mode switch that operates on modulating the optical phases of a three-dimensional balanced four-arm waveguide Mach-Zehnder interferometer. We design and fabricate the device with polymer material to achieve thermo-optic switching between any two of the E 11 , E 21 , E 12 , and E 22 modes of the waveguide. Our experimental device shows an extinction ratio higher than 14 dB and a switching time shorter than 3.7 ms, measured with the E 11 mode switched to any of the other modes at 1550 nm. This mode switch can operate over a wide range of wavelengths with weak polarization dependence and could be used in reconfigurable fiber-based mode-division-multiplexing systems where mode routing is required.

We report performance optimization of hollow-core photonic bandgap fiber (HC-PBF) photothermal (PT) gas sensors. The PT phase modulation efficiency of a C 2 H 2 filled HC-PBF (HC-1550-02) is found independent of the pump modulation frequency for up to ∼330  kHz, but starts to drop quickly to 10% of the maximum value at a couple of megahertz. With a 1.1 m long HC-PBF gas cell with angle-cleaved single-mode fiber/HC-PBF joints to reduce reflection and a modified 3×3 Sagnac interferometer with balanced detection for phase demodulation, a noise equivalent concentration of ∼67  ppbC 2 H 2 is achieved with a 1 s time constant, and it goes down to ∼18  ppb with 145 s integration time. The system has good long-term stability and exhibits signal fluctuations of <1% over a ∼5  h period.

We demonstrate mid-infrared photothermal interferometry (PTI) in a hollow-core fiber (HCF) for trace gas detection. Compared with free-space PTI, the optical intensity could be increased by orders of magnitude by confining the pump and probe lights in the hollow core. We coupled the pump light at 4.46 μm from a quantum cascade laser and the 1555.14 nm probe light into the HCF with a bore size of 200 μm. The HCF was filled with nitrous oxide (N 2 O) which has strong absorption at the pump wavelength. The probe light detects the N 2 O absorption-induced phase change in the HCF via a fiber-optic Mach-Zehnder interferometer. With a 25 cm fiber length and 6 mW of pump power, we have achieved a minimum detection limit of 0.8 ppm for N 2 O, corresponding to a normalized noise equivalent absorption coefficient of 4.9×10 -7   cm -1  WHz -1/2 .

We propose an approach to directly measure the bending effect on individual modes in few-mode fibers using a wavelength-scanning spatially and spectrally resolved imaging technique. By collecting a fiber output beam profile with a scanning wavelength at different bending diameters and analyzing the peaks of its Fourier transformation, we have distinguished higher-order modes (HOMs) of the fiber and investigated their group delay, beam profile, bend loss, and mode splitting individually. We have experimentally verified with a multilayer core fiber at 1-8 cm bending diameters that its HOMs experience more loss compared to lower-order modes, but delay at approximately the same speed as the bending diameter decreases. Mode splitting of the LP 11 mode at a small bending diameter due to bending-induced birefringence is also observed and discussed. This method provides an efficient tool to study the bending effect on individual HOMs in fibers and could be extended to studying fiber stretching and twisting.

We report measurements and numerical simulations of ultrafast laser-excited carrier flow across a scanning tunneling microscope (STM) junction. The current from a nanoscopic tungsten tip across a ∼1  nm vacuum gap to a silver surface is driven by a two-color excitation scheme that uses an optical delay-modulation technique to extract the two-color signal from background contributions. The role of optical field enhancements in driving the current is investigated using density functional theory and full three-dimensional finite-difference time-domain computations. We find that simulated field-enhanced two-photon photoemission (2PPE) currents are in excellent agreement with the observed exponential decay of the two-color photoexcited current with increasing tip-surface separation, as well as its optical-delay dependence. The results suggest an approach to 2PPE with simultaneous subpicosecond temporal and nanometer spatial resolution.

We present a method in this Letter to generate optical vortices with tunable orbital angular momentum (OAM) in optical fibers. The tunable OAM optical vortex is produced by combining different vector modes HE2,meven (HE2,modd) and TE 0,m (TM 0,m ) when l=1 or combining HEl+1,meven (HEl+1,modd) and EHl-1,modd (EHl-1,meven) when l>1 with a π/2 phase shift. The vortex can be regarded as a result of overlapping two orthogonal optical vortex beams of equal helicity but opposite chirality with a π/2 phase shift. We have experimentally demonstrated the smooth variation of OAM from l=-1 to l=+1 by adjusting a polarizer at the output end of the fiber.

A highly sensitive, compact, and low-cost trace gas sensor based on photothermal effect in a hollow-core fiber Fabry-Perot interferometer (FPI) is described. The Fabry-Perot sensor is fabricated by splicing a piece of hollow-core photonic bandgap fiber (HC-PBF) to single-mode fiber pigtails at both ends. The absorption of a pump beam in the hollow core results in phase modulation of probe beam, which is detected by the FPI. Experiments with a 2 cm long HC-PBF with femtosecond laser drilled side-holes demonstrated a response time of less than 19 s and noise equivalent concentration (NEC) of 440 parts-per-billion (ppb) using a 1 s lock-in time constant, and the NEC goes down to 117 ppb (2.7×10 -7 in absorbance) by using 77 s averaging time.

We propose a mode-conversion platform based on cascaded long-period waveguide gratings. We demonstrate two specific devices by cascading a surface-corrugated grating and a sidewall-corrugated grating. One device allows conversion among the LP 01 , LP 11b , and LP 11a modes in a cyclic manner. The maximum conversion efficiency of a typical experimental device fabricated with polymer materials is about 94% at 1540 nm. The other device allows conversion between the LP 01 and LP 21a modes. The maximum conversion efficiency of a typical experimental device is estimated to be higher than 90% at 1550 nm. The performance of both devices is polarization-insensitive. The proposed platform could be developed into various mode converters for mode-division-multiplexing systems and other mode-sensitive applications.

Lasers based on monolayer (ML) transition-metal dichalcogenide semiconductor crystals have the potential for low threshold operation and a small device footprint; however, nanophotonic engineering is required to maximize the interaction between the optical fields and the three-atom-thick gain medium. Here, we develop a theoretical model to design a direct bandgap optically pumped nanophotonic integrated laser. Our device utilizes a gap-surface-plasmon optical mode to achieve subwavelength optical confinement and consists of a high-index GaP nanowire atop an ML MoS 2 film on an Ag substrate. The optical field and material medium are analyzed using a three dimensional finite-difference time-domain method and a first-principles calculation based on the density functional theory, respectively. The nanolaser is designed to have a threshold of ∼0.6  μW under quasi-continuous wave operation on an excitonic transition at room temperature.

To efficiently promote the phase retrieval in quantitative phase imaging, a new approach of quantitative phase extraction is proposed based on two intensities with dual wavelength after filtering the corresponding dc terms for each wavelength, in which a special phase shift is used. In this approach, only the combination of the phase-shifting technique and subtraction procedures is needed, and no additional algorithms are required. The thickness of the phase object can be achieved from the phase image, which is related to the synthetic beat wavelength. The feasibility of this method is verified by the simulated experiments of the optically transparent objects.

We demonstrate a mode demultiplexer with two cascaded few-mode polymer waveguide directional couplers fabricated on the same substrate along the horizontal and vertical directions, respectively. The three waveguides that form the two couplers have the same core size. The horizontal and vertical couplers are designed to provide complete power transfer for the LP(11a) and LP(11b) modes, with the LP(01) mode staying in the central core that incorporates a biconical taper to suppress any remaining LP(11) modes. A typical fabricated demultiplexer, which is 18.5 mm long, shows a coupling ratio higher than ∼96% in the wavelength range of 1530-1570 nm for both couplers. The device shows negligible crosstalk to the LP(01)-mode channel, while the crosstalks to the LP(11)-mode channels are lower than -15.6 and -13.4  dB for the TE and TM polarizations, respectively. The device can be considered polarization insensitive. The propagation losses for the three modes are about 2.0  dB/cm. This device could find applications in mode-division-multiplexing systems.

We propose a mode switch based on an electro-optic long-period grating formed in a lithium-niobate (LN) two-mode waveguide. Our fabricated device consists of an 8 mm long grating in a proton-exchanged z-cut LN waveguide. At a driving voltage of 35 V, it can switch between the fundamental mode and the higher-order mode with a mode extinction ratio of -18  dB and a 3-dB bandwidth of ∼25  nm at the wavelength 1544 nm. The performance of the device is insensitive to temperature variations. The experimental results agree well with the simulation. This mode switch could find applications in reconfigurable mode-division-multiplexing systems.

Near-infrared to ultraviolet multiphoton upconversion photoluminescence in ultrasmall Tm3+/Yb3+-codoped CaF2 nanocrystals (∼6.7  nm in size) was observed and further significantly enhanced by growing an active shell of NaYF4:Yb3+. Owing to the active shell, the lanthanide emitters inside the core are effectively prevented from the surface quenchers, and the excitation energy is absorbed more efficiently via the additional luminescence sensitizer Yb 3+ embedded in the shell. The details of underlying physics were investigated and discussed. The results present a good ultrasmall luminescent material system for achieving efficient multiphoton upconversion, which shows great potential in versatile industrial and biological applications.

Compact ferrule-top nanomechanical resonators with multilayer graphene (MLG) diaphragms as vibrating elements are demonstrated. The resonators comprise a suspended MLG film supported by a ceramic ferrule with a bore diameter of ∼125 μm. The mechanical resonance of the graphene film is excited and detected by an all-fiber optical interrogation system. Based on a beam-shape graphene mechanical resonator, a force sensitivity of ∼3.8 fN/Hz1/2 was theoretically predicted. The integration of nanomechanical graphene film with optical fiber simplifies the excitation and interrogation of the resonator and would allow the development of practical fiber-optic sensors for force, mass, and pressure measurements.

We report the first fabrication, to our knowledge, of polarization-rocking filters in highly birefringent elliptical microfibers. A rocking filter was made by periodically heating/twisting a microfiber with an ellipticity of ∼0.7 and a diameter of ∼2.8  μm along its major axis. Strong input polarization suppression of ∼20  dB was achieved at a resonant wavelength of ∼1556.4  nm with a device length of ∼3  mm.

Highly birefringent (Hi-Bi) microfiber-based fiber loop mirrors (FLMs) were studied for tunable comb filters and refractive index (RI) sensors. The use of two cascaded Hi-Bi microfibers instead of a single microfiber allows more flexibility in controlling the transmission/reflection characteristics of the FLM. The length of Hi-Bi microfibers is of the order of centimeters, one or even more than two orders of magnitude shorter than the conventional Hi-Bi fiber-based FLM devices. The transmission/reflection spectra are sensitive to the RI surrounding the microfibers, and RI sensitivity of 20,745 nm/RIU was experimentally demonstrated.

In-line fiber-optic microcells are fabricated by postprocessing NKT LMA10 photonic crystal fibers. The cells are suspended core (SC) elements created by locally inflating some of the air holes while the core is being tapered. Based on a SC microcell with six air holes, a cantilever beam accelerometer is demonstrated. The microcells could also be used as gain and absorption cells for amplifier and spectroscopy applications.

We demonstrate a fiber-tip photoacoustic spectrometric sensor for trace gas detection. The sensor head is a miniature fiber-tip hollow-cavity with a deflectable polymer diaphragm. Periodic light absorption of gas molecules within the cavity generates an acoustic pressure wave, which causes deflection of the diaphragm. The hollow cavity also is a Fabry-Perot interferometer with which the diaphragm deflection is detected with high sensitivity. Experimental test around the P(9) absorption line of C(2)H(2) achieved a minimum detectable gas concentration of 4.3 ppm with an excitation laser power of 8 mW. The miniature sensor head and fiber optic detection system make this type of spectrometers ideally suited for remote and space-limited applications as well as for multipoint detection in a multiplexed fiber optic sensor network.

A miniature fiber-tip pressure sensor was built by using an extremely thin graphene film as the diaphragm. The graphene also acts as a light reflector, which, in conjunction with the reflection at the fiber end-air interface, forms a low finesse Fabry-Perot interferometer. The graphene based sensor demonstrated pressure sensitivity over 39.4 nm/kPa with a diaphragm diameter of 25 μm. The use of graphene as diaphragm material would allow highly sensitive and compact fiber-tip sensors.

We demonstrate gas detection based on evanescent-wave photoacoustic (PA) spectroscopy with tapered optical fibers. Evanescent-field instead of open-path absorption is exploited for PA generation, and a quartz tuning fork is used for PA detection. A tapered optical fiber with a diameter down to the wavelength scale demonstrates detection sensitivity similar to an open-path system but with the advantages of easier optical alignment, smaller insertion loss, and multiplexing capability.

A compact fiber polarizer is demonstrated by the filling of selected air holes of a hollow-core photonic crystal fiber (PCF) with a liquid. The liquid-filling results in an asymmetric waveguide structure, leading to a large polarization dependent loss. A 6 mm long ethanol-filled PCF exhibits a polarization extinction ratio of ∼18 dB over a wavelength range from 1480 nm to 1600 nm., (© 2011 Optical Society of America)