Plasmonic photonic crystal fiber polarization filter with asymmetry around Au-coated and liquid-filled air holes

A novel polarization filter based on plasmonic photonic crystal fiber with Au-coated and liquid-filled air holes is presented in this paper. The coupling theory is introduced to explain the complete coupling and incomplete coupling. We can adjust the resonance point to the communication band by optimizing the parameters of the fiber structure. Numerical simulation results demonstrate that the resonance strength can reach 489.78 dB cm\(^{-1}\) at the communication wavelength of 1.55 um in x-polarized direction. By filling liquid analyte the confinement loss can reach 572.55 dB cm\(^{-1}\) at the wavelength of 1.55 um. When the fiber length of L equals to 800 um, the peak value of the crosstalk can reach 394.75 dB at the wavelength of 1.55 um, and the bandwidth of the crosstalk better than 20 dB is up to 180 nm when the length of the fiber L is 200 um. These features make it a promising candidate for designing new types of polarization filter devices.     

Modeling,design, growth and characterization of red wavelength range microcavity emitters for plastic optic fiber applications

Resonant cavity light emitting diodes (RC-LEDs) and vertical cavity surface emitting lasers (VCSELs) for the red wavelength range are presented in the paper. A wide variety of simulation tools were employed in the device design and optimization and the good agreement between simulations and measurements enabled effective device development. Our red wavelength range RC-LEDs were mainly intended for short-haul communication systems based on polymethyl methacrylate plastic optical fiber and were optimized accordingly. They are achieving, under different structure and working regime variants, high output power (15 mW), high external efficiency (9.5%), record small-signal modulation bandwidth (f _-3dB up to 350 MHz), error-free back-to-back transmission rates beyond 622 Mbits/s, adjustable far-field pattern and good POF coupling efficiency with reasonably wide tolerances and without using auxiliary optics. In view of the possible use of graded-index POFs, free-space transmission and other high bandwidth or high spectral purity applications, VCSELs in the red wavelength range were also realized. They have achieved sub-milliamp room-temperature (RT) continuous-wave (CW) lasing for an 8 m diameter emission window and exhibited an external quantum efficiency of 6.65% for 10 m devices in RT CW operation. The VCSEL structures were far from optimal – not even incorporating all the RC-LED structure refinements – and significant improvement in performance characteristics are predicted for the optimized layer structure.     

Spectral characteristics for a fiber grating external cavity laser

A numerical investigation has been carried out on the influence of the antireflection (AR) coating of laser, the coupling efficiency of laser and fiber (), and the reflectivity of fiber grating (R _g) on the side-mode suppression ratio (SMSR) of the fiber grating external cavity laser (FGECL). The FGECL was fabricated by the assembly of the multimode Fabry–Perot (FP) laser chip and fiber grating. The results showed that the FGECL with a lower AR coating, higher and R _g exhibited a better SMSR. A comparison of the SMSR dependence on the and R _g showed that SMSR increased more rapidly with increasing than SMSR for R _g. These spectral characteristic studies of the SMSR dependence on the device parameters of the AR coating, , and R _g may provide useful device designs for the practical fabrication of a FGECL for use in dense wavelength division multiplexing (DWDM) applications.     

Waveguide sub‐wavelength structures: a review of principles and applications

Periodic structures with a sub‐wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high‐precision lithography techniques that sub‐wavelength structures have found widespread application in the field of photonics. This review first provides an introduction to the physics of sub‐wavelength structures. An overview of the applications of sub‐wavelength structures is then given including: anti‐reflective coatings, polarization rotators, high‐efficiency fiber–chip couplers, spectrometers, high‐reflectivity mirrors, athermal waveguides, multimode interference couplers, and dispersion engineered, ultra‐broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub‐wavelength structures and their impact in photonics.

     

New fluorescent labels for time-resolved detection of biomolecules

New dyes with characteristic fluorescence lifetimes have been developed for bioanalytical applications. Based upon the concept of multiplex dyes, we have designed rhodamine dyes with nearly identical absorption and emission spectral characteristics but different fluorescence lifetimes. Extending this principle to applications with laser diodes, new rhodamines with functional groups for covalent coupling of analytes have been developed. The new labels exhibit absortion and fluorescence beyond 600 nm and have a high quantum efficiency, even in aqueous buffer systems.     

Design of segmented cladding fiber for femtosecond laser pulse delivery at 1550 and 1064 nm wavelengths

Segmented cladding fiber (SCF) is capable of single mode operation over an extended range of wavelengths while maintaining large mode area. In this paper we report the design of an SCF with mode area as large as 1,825  $\upmu \hbox {m}^{2}$ , suitable for delivery of high peak power femtosecond laser pulses at 1550 and 1064 nm wavelengths. An SCF with such a large-mode area is a few-moded fiber and its design requires careful choice of design parameters to have robustness against mode-coupling effects and bend loss. In this paper we address these issues and report a design of an SCF showing near distortion-free propagation of 100-fs, 53-kW peak power pulses at 1550-nm wavelength with 1,825- $\upmu \hbox {m}^{2}$ mode area through fundamental mode. The same fiber can also deliver 250-fs, 15-kW peak power pulses at 1064-nm wavelength with 1,793- $\upmu \hbox {m}^{2}$ mode area. The fiber has been analyzed by using the radial effective-index method in conjunction with transfer matrix method and the pulse propagation has been studied by solving the nonlinear Schroedinger equation by split-step Fourier method. Such a fiber would find applications in multiphoton microscopy and in biomedical engineering.     

Manipulating light–matter interaction in a gold nanorod assembly by plasmonic coupling

The interaction of light with a single gold nanorod (GNR) depends strongly on the polarization and wavelength of the light. For isolated GNRs, the maximum of the polarization (wavelength)‐dependent linear and nonlinear absorption appear at the same excitation polarization (wavelength). Here, it is demonstrated that these relationships can be manipulated in a GNR assembly composed of randomly distributed and oriented GNRs by controlling the plasmonic coupling strength between GNRs. It is revealed that the strongly localized modes resulting from the plasmonic coupling of GNRs play a crucial role in determining these relationships. For a GNR tetramer, it is shown by numerical simulation that the maximum two‐photon absorption achieved at a particular polarization can be switched to the minimum absorption and vice versa by controlling the coupling strength. More importantly, it is demonstrated both numerically and experimentally that the two‐photon‐absorption peak of a GNR assembly can be made to be different from its single‐photon‐absorption peak by increasing the coupling strength. Both properties are distinct from previous experimental observations. Our findings provide a useful guideline for engineering the interaction of light with complex plasmonic systems.

     

Mid‐infrared supercontinuum covering 2.0–16 μm in a low‐loss telluride single‐mode fiber

A mid‐infrared (MIR) supercontinuum (SC) has been demonstrated in a low‐loss telluride glass fiber. The double‐cladding fiber, fabricated using a novel extrusion method, exhibits excellent transmission at 8–14 μm: < 10 dB/m in the range of 8–13.5 μm and 6 dB/m at 11 μm. Launched intense ultrashort pulsed with a central wavelength of 7 μm, the step‐index fiber generates a MIR SC spanning from ∼2.0 μm to 16 μm, for a 40‐dB spectral flatness. This is a fresh experimental demonstration to reveal that telluride glass fiber can emit across the all MIR molecular fingerprint region, which is of key importance for applications such as diagnostics, gas sensing, and greenhouse CO₂ detection.

     

Four-element fiber Bragg grating acceleration sensor array

A four-element fiber Bragg grating acceleration sensor array is presented in this paper. The system combines an unbalanced Mach–Zehnder interferometer and three coarse wavelength division multiplexers (WDMs). The totally passive scheme has the advantages of high operation frequency and loose requirements for the working environment. An acceleration resolution of better than for the four channels is achieved.     

Performance of optical fibers for transmission of high-peak-power XeCl excimer laser pulses

The simple and efficient fiber delivery of 5-ns pulses from a XeCl excimer laser operating at a wavelength of 308 nm is demonstrated. The coupling scheme uses all of the output energy of the XeCl excimer laser and benefits from a simple and easy-to-adjust fiber coupling. Experiments on the 308-nm fiber delivery for more than 2.5 million laser pulses of 8-ns pulse width (FWHM) and up to 8-mJ stabilized pulse energy are performed. The long-time pulsed UV laser transmission is found to be different for individual samples of optical fibers that perform very similarly in low-intensity UV light applications. For applications with strict demands on the long-time stability, a critical evaluation of the fiber performance with the 308-nm laser under operating conditions is necessary. Measurements between 1 and 200 Hz show a negligible dependence of the fiber delivery performance on the repetition rate of the transmitted laser pulses. PACS 42.55.Lt; 42.81.Cn; 07.69.Vg     

TDLAS–WMS based near-infrared methane sensor system using hollow-core photonic crystal fiber as gas-chamber

A near-infrared methane (CH₄) sensor system was implemented using a hollow-core photonic crystal fiber (HC-PCF) as gas-chamber. Coupling joints including ceramic ferrules and ceramic mating sleeve were used to realize butt coupling between hollow-core fiber and single-mode fiber. A near-infrared distribute feedback laser was used for CH₄ detection based on wavelength modulation spectroscopy technique. CH₄ measurements were conducted to derive the sensor-system performances. Using a 5.3 mW laser power and a 0.8 m-long HC-PCF, a minimum detection limit of ~8.7 ppm at 0.1 s averaging time was obtained and it can be further improved to 1.4 ppm at an averaging time of 10 s. A good linear calibration curve between the amplitude ratio (2f/1f) and the CH₄ concentration was obtained within the concentration range of 0–1000 ppm. This sensor system shows potential applications in distributed field measurements on CH₄ in industrial process control, environmental monitoring, etc.     

Optical time-domain reflectometry for distributed sensing of the structural strain and deformation

A unique structure of microbend optical fiber sensor (MOFS) for measuring tensile and compressive strain is described in this paper. The average measuring sensitivity for tensile strain is 35μ using 3 MOFS arrays. The repeatability and stability of MFOS are better than 18μ. The loss sensitivity of single-mode (SM) fiber and multi-mode (MM) fiber used in MOFS, as well as the relationship between the pulse width of diode laser and loss sensitivity are also studied in this paper. From these studies, some conclusions have been obtained. They are (1) the loss sensitivity and repeatability of SM fiber are better when compared to MM fiber in MOFS, and (2) the variation of pulse width of laser would only influent the signal-to-noise ratio and dynamic range, but has no contribution to loss sensitivity. Experimental results also show that loss of SM fiber highly depends on the wavelength of laser, but MM fiber has no such property. The loss of SM fiber between the wavelength of 1550 and 1310 nm is about the ratio of 6.5. Therefore, the experiments reported in this paper used a wavelength of 1310 nm to measure tensile strain and 1550 nm to measure compressive strain based on the above property of SM fiber, without changing the configuration of MOFS.     

A true fiber optic refractometer

The best instrument to measure the refractive index of liquids is the Abbe refractometer which can only provide accuracies of the order of 10⁻⁵ at visible wavelengths and 10⁻⁴ in the near infrared. Here we present a technique by which the exact wavelength positions in the near infrared frequency comb of a tilted grating inscribed in the core of an optical fiber can be used to measure the absolute value of the refractive index of a liquid in which the fiber is inserted, with an accuracy of ±5×10⁻⁵. This is in contrast to typical fiber optic‐based “refractometry” where only refractive index variations can be measured accurately, hence the appellation of “true” fiber optic refractometer here. In addition to the increased accuracy, the fiber refractometer proposed here offers the additional advantages associated with in situ measurements. The performance of this refractometer is demonstrated by measurements in water from room temperature down to near freezing at wavelengths in the 1550 nm window.

     

Theoretical analysis of acoustic-optic frequency shifter by dual mode fiber

The frequency shifting between two modes in an optical fiber resulting from acoustic wave propagation has been studied theoretically. Our theoretical results confirm the earlier experimental work. The polarization effect introduced by acoustic waves has been examined as well. The analysis shows that the polarization characteristic and symmetry property of the modes are not preserved in the coupling process of acoustic-optic frequency shifting for arbitrary ratio of the fiber cladding radius to the acoustic wavelengthd/ _a, in contrast to stable mechanical microbending, and that for the givend/ _a = 0.397, the polarized light signal at frequency ₂, shifted from that at frequency ₁, is quasi-linearly polarized for any acoustic-optic interaction length. However, the polarization effect can be neglected when the fiber cladding radius is much smaller than the acoustic wavelength.     

Coupling efficiency of laser beam to multimode fiber

The coupling efficiency of laser beam to multimode fiber is given by geometrical optics, and the relation between the maximum coupling efficiency and the beam propagation factor M² is analyzed. An equivalent factor for the multimode fiber is introduced to characterize the fiber coupling capability. The coupling efficiency of laser beam to multimode fiber is calculated in respect of the ratio by the overlapping integral theory. The optimal coupling efficiency can be roughly estimated by the ratio of M² to but with a large error range. The deviation comes from the lacks of information on the detail of phase and intensity profile in the beam factor M².     

UV generation in a pure-silica holey fiber

We report supercontinuum generation extending to 300 nm in the UV from a pure-silica holey fiber. The broad spectrum was obtained by launching ultra-short pulses (150 fs, 10 nJ at 820 nm) from an amplified Ti:sapphire laser. The extension of holey-fiber-based supercontinuum generation into the UV should prove to be of immediate application in spectroscopy. By slightly detuning the launch conditions we excited a higher order spatial mode, which produced a narrower supercontinuum, but with enhanced conversion efficiency at a series of blue/UV peaks around 360 nm. We present numerical simulations, which suggest that differences in the dispersion profiles between the modes are an important factor in explaining this enhancement. In a related experiment, using the same laser source and fiber, we demonstrate a visible supercontinuum from several subsidiary cores, with distinct colours in each core. The subsidiary cores were excited by an appropriate input coupling. Fabrication of a fiber with a range of core sizes (dispersion profiles) for tailored supercontinuum generation can therefore be envisaged for practical applications. PACS 42.72.Bj; 42.79.Nv; 42.81.Dp     

Free induction decay with radiation damping I. General treatment

The problem of the free induction decay of a two-level spin system in the presence of a strong coupling between a resonant circuit and the system is discussed under different experimental conditions. It is shown that under certain conditions the shapes of these signals are profoundly modifled by this coupling. In some cases one obtains, after a pulse excitation of the system, several emission peaks of energy from the spin system having a population difference below inversion, creating some sort of photon-bottleneck or of coherent echo.     

Elimination of environmental noise in interferometric wavelength shift demodulation for dynamic fiber Bragg grating sensor array

We present a novel reference compensation method for eliminating environmental noise in interferometric wavelength shift demodulation for dynamic fiber Bragg grating (FBG) sensors. By employing a shielded wavelength-division-multiplexed reference FBG in the system, the environmental noise is measured from the reference channel, and then subtracted from the demodulation result of each sensor channel. An approximate 40 dB reduction of the environmental noise has been experimentally achieved over a frequency range from 20 Hz to 2 kHz. This method is also suitable for the elimination of broadband environmental noise. The corresponding FBG sensor array system proposed in this paper has shown a wavelength resolution of .     

Miniature optical fiber current sensor based on a graphene membrane

The unique electronic and mechanical properties, of graphene make it an ideal material for nanoelectromechanical system (NEMS) applications. Here, a miniature optical fiber current sensor based on a quasistatic graphene NEMS with a graphene membrane covering the hole on a pre‐etched fiber tip and two gold electrodes on opposite sides of the tip has been demonstrated. The sensor overcomes the shortcomings of conventional optical fiber current sensors based on thermal effects, such as relatively low sensitivity, long response time, and huge device size; it has simultaneously a high sensitivity of 2.2 × 10⁵ nm/A², a short response time of ∼0.25 s and a compact device size of ∼15 μm, and has found practical application. Using a smaller graphene membrane with better quality can reduce the response time to submillisecond levels with a more precise measurement system. The sensor presented in this paper may pave the way for the practical usage of optical fiber current sensors based on thermal effects.

     

Precise laser ablation processing of black widow spider silk

Contact-free precise laser processing of black widow spider (Latrodectus hesperus) dragline silk by laser ablation at =157 nm is achieved. At this wavelength, the small optical penetration depth, below 100 nm, allows efficient and gentle material removal above the ablation threshold of _th=29 mJ/cm². The ablation rate in nm/pulse is measured against laser fluence and simultaneously calibrated with ultrapure polymethylmethacrylate (PMMA). Ripple formation in fiber ablation can be overcome by a suitable combination of ablation removal and laser polishing steps using exposure sequences at different irradiation angles , such as vertical at 0° followed by oblique irradiation at 70°. In this way ripples are destroyed until the fiber is automatically smoothed over the entire laser-exposed length. This allows precise laser ablation processing of spider silk fibers and other highly absorbing materials without affecting the bulk mechanical, and other material, properties. PACS 61.41.+e; 61.46.+w; 61.80.Ba; 61.82.Pv; 62.25.+g