Polarization-maintaining Single-mode Fiber Research Articles

A waveguide-typed plasmonic mode converter

Waveguide-typed plasmonic mode converters (WPMCs) at a wavelength of 1.55 μm are presented. The WPMC is composed of an insulator-metal-insulator waveguide (IMI-W), a 1st reversely tapered insulator-metal-insulator-metal-insulator waveguide (RT-IMIMI-W), an insulator-metal-insulator-metal-insulator waveguide (IMIMI-W), a 2nd RT-IMIMI-W with lateral silver mirrors (LSMs), and a metal-insulator-metal waveguide (MIM-W) in series. The mode sizes for the IMI-W, IMIMI-W, and MIM-W via the IMIMI-W with LSMs were not only calculated using a finite element method but were also experimentally measured. The input mode size of 10.3 μm × 10.3 μm from a polarization-maintaining single-mode fiber was squeezed to the mode size of ~2.9 μm × 2.9 μm in measurement by converting an s0 mode to an Sa0 mode via an Ss0 mode. The WPMC may be potentially useful for bridging micro- to nano-plasmonic integrated circuits.

18 W single-stage single-frequency acoustically tailored Raman fiber amplifier

A single-mode polarization-maintaining fiber doped to increase the Raman gain while suppressing stimulated Brillouin scattering (SBS) was utilized in a single-stage counter-pumped Raman fiber amplifier. The SBS suppression was achieved through the acoustic tailoring of the core. A pump probe experiment was conducted to characterize the Brillouin gain and indicated the existence of multiple Brillouin peaks. When the amplifier was seeded with approximately 15 mW of 1178 nm light, 11.5 W of cw output power was obtained with a linewidth ≤2 MHz. The application of a thermal gradient to further mitigate the SBS process increased the output power to 18 W, thus providing a net amplifier gain >30 dB.

Theoretical Analysis of a Non-Symmetric Polarization-Maintaining Single-Mode Fiber for Sensor Applications

An asymmetric polarization-maintaining single-mode fiber with one side-hole being incorporated into the fiber cladding has been investigated analytically in this work for potential pressure measurements. The material birefringence of the fiber is calculated using a thermo-elastic displacement potential method through the superposition of sectional displacement potentials. The results obtained are generic and are thus applicable to any one-hole fiber structures, should the hole diameter or position vary in the fiber cladding, or the fiber hole be empty or filled in with any material. This enables the analysis to be applied more widely in a range of optical fiber sensor applications.

Blue laser via IR resonant doubling with 71% fiber to fiber efficiency

We report on high efficiency resonant doubling to 486 nm using periodically poled KTP. The IR laser source is an FBG stabilized semiconductor laser with a maximum polarization maintaining (PM) fiber coupled output of 840 mW. An output power of 680 mW at 486 nm was obtained from our optimized cavity, giving net efficiency of 81%. Also, we report an 87.5% net efficiency in coupling of this blue light from the servo locked cavity into a single-mode PM fiber. This gives a total of 71% fiber to fiber efficiency. Furthermore, a wall plug efficiency of 21.4% was obtained.

Power monitoring in dielectric-loaded surface plasmon-polariton waveguides

We report on propagating mode power monitoring in dielectric-loaded surface plasmon-polariton waveguides (DLSPPWs) by measuring the resistance of gold stripes supporting the DLSPPW mode propagation. Inevitable absorption of the DLSPPW mode in metal causes an increase in the stripe temperature and, thereby, in its resistance whose variations are monitored with an external Wheatstone bridge being accurately balanced in the absence of radiation in a waveguide. The investigated waveguide configuration consists of a 1-µm-thick and 10-µm-wide polymer ridges tapered laterally to a 1-µm-wide ridge placed on a 50-nm-thin and 4-µm-wide gold stripe, all supported by a magnesium fluoride substrate. Using single-mode polarization-maintaining fiber for in- and out-coupling of radiation, DLSPPW mode power monitoring at telecom wavelengths is realized with the responsivities of up to ~1.8 µV/µW (showing weak wavelength dependence) being evaluated for a bias voltage of 1 V.

Endoscopic cross-polarization spectrometer as an instrument for puncture diagnostics

A modification of the polarization reflection spectroscopy method is proposed with the use of a polarization-maintaining single-mode fiber. The choice of the fiber made it possible to create an optimal self-consistent system for the illumination of a biological tissue and the reception of scattered radiation and to create a probe compatible with any endoscopic equipment. The first clinical results of diagnosing tumor diseases with the created instrument showed that it is possible to determine the tumor-norm boundary, which is very important for organ-preserving operations. All of the results obtained by this method agree well with the results obtained by the method of polarization-sensitive optical coherence tomography.

In vivo Ultrahigh-Resolution Ophthalmic Optical Coherence Tomography Using Gaussian-Shaped Supercontinuum

We have demonstrated the in vivo ultrahigh-resolution optical coherence tomography (OCT) imaging of the human eye using a Gaussian-shaped supercontinuum source. Using an ultrashort-pulse Ti:sapphire laser and a polarization-maintaining single-mode fiber, a linearly polarized, high-power, low-noise, Gaussian-shaped supercontinuum is generated in the wavelength region from 700 to 950 nm. For ophthalmic imaging, a wideband Gaussian-shaped supercontinuum with a bandwidth of 140 nm is generated at a center wavelength of 830 nm. The observed axial resolutions are 2.9 µm in air and 2.1 µm in tissue. The generated supercontinuum is combined with a modified ophthalmic OCT system, and sidelobe-free ultrahigh-resolution OCT images of the human retina and cornea are obtained.

Waveguiding properties of a photonic crystal fiber with a solid core surrounded by four large air holes

The polarization-dependent guiding properties of a hexagonal-lattice photonic crystal fiber with a solid-core surrounded by four large air holes are investigated. The appearance of a polarization dependent cutoff frequency, together with several parameters as the birefringence, the modal effective area, the group velocity dispersion and the polarization dependent loss are analyzed. A collection of fibers with different structural parameters were fabricated and characterized. An effective anti-guide structure from at least 450 nm to 1750 nm, a polarizing fiber with a polarization dependent loss of 16 dB/m at 1550 nm, and an endlessly singlemode polarization-maintaining fiber with group birefringence of 2.1x10(-3) at 1550 nm are reported. Experimental results are compared with accurate numerical modeling of the fibers.

Delivery of sub-10-fs pulses for nonlinear optical microscopy by polarization-maintaining single mode optical fiber

Broadband, sub-10-fs pulses, can be propagated through polarization-maintaining single mode fiber (PMF) for use in nonlinear optical microscopy (NLOM). We demonstrate delivery of near transform-limited, 1 nJ pulses from a Ti:Al(2)O(3) (75 MHz repetition rate) oscillator via PMF to the NLOM focal plane while maintaining 120 nm of bandwidth. Negative group delay dispersion (GDD) introduced to pre-compensate normal dispersion of the optical fiber and microscope optics ensured linear pulse propagation through the PMF. The minimized time-bandwidth product of the laser pulses at the NLOM focus allowed the nonlinear excitation of multiple fluorophores simultaneously without central wavelength tuning. Polarization sensitive NLOM imaging using second harmonic generation in collagen was demonstrated using PMF delivered pulses. Two-photon excited fluorescence spectra and second harmonic images taken with and without the fiber indicates that the fiber based system is capable of generating optical signals that are within a factor of two to three of our traditional NLOM.

Low-loss polarization-maintaining fusion splicing of single-mode fibers and hollow-core photonic crystal fibers, relevant for monolithic fiber laser pulse compression

We report on highly reproducible low-loss fusion splicing of polarization-maintaining single-mode fibers (PM-SMFs) and hollow-core photonic crystal fibers (HC-PCFs). The PM-SMF-to-HC-PCF splices are characterized by the loss of 0.62 +/- 0.24 dB, and polarization extinction ratio of 19 +/- 0.68 dB. The reciprocal HC-PCF-to-PM-SMF splice loss is found to be 2.19 +/- 0.33 dB, which is caused by the mode evolution in HC-PCF. The return loss in both cases was measured to be -14 dB. We show that a splice defect is caused by the HC-PCF cleave defect, and the lossy splice can be predicted at an early stage of the splicing process. We also demonstrate that the higher splice loss compromises the PM properties of the splice. Our splicing technique was successfully applied to the realization of a low-loss, environmentally stable monolithic PM fiber laser pulse compressor, enabling direct end-of-the-fiber femtosecond pulse delivery.

Multi-parameter harsh environment sensing using asymmetric Bragg gratings inscribed by IR femtosecond irradiation

In this paper, we review our recent work on multi-parameter harsh environment sensing using asymmetric Bragg gratings inscribed by IR femtosecond irradiation, which includes the investigations of fiber Bragg gratings inscribed by graded-index silica multimode fibers and polarization maintaining (PM) single mode fibers. The unique features of this type of grating are: (1) formed by multi-photon ionization, this kind of grating can withstand temperature as high as 1000 °C, which makes it suitable for harsh environment sensing; (2) bending (or pressure, strain) and ambient temperature fluctuations affect the shapes of multiple transmission dips in different ways, which enables multi-parameter sensing capability. The principle of multi-parameter sensing of the multimode fiber gratings is based on the fact that multimode fiber gratings have multiple resonant dips in spectrum and the change of spectral shape is different for different types of perturbations (e.g., temperature and pressure). The main challenge of sensing using multimode fiber is that the shape of spectrum also depends on the excitation condition, which makes the analysis of the sensing data complicated. On the other hand, the PM single mode fiber supports only two linearly polarized modes, which results in two resonant dips in the transmission spectrum. Since measured parameters such as the temperature and bending affect the transmission spectrum of two dips in different ways, one can implement multi-parameter sensing in harsh environment using PM fiber. Note that it is much easier to analyze the data of two dips (in the case of PM fibers) instead of multiple dips (in the case of multimode fibers). Finally, we also investigate the ultra thin multimode and PM single mode harsh environment fiber gratings for multi-parameter sensing applications. After etching the most part of the cladding, both multimode and PM single mode fiber grating spectra become sensitive to ambient refractive index changes, and their sensitivity to bending and strain is also enhanced. This enables the application for multi-parameter chemical/bio and pressure sensing.

Generation of Optical Bistability in a Fiber Fabry–Perot Resonator Using Mode-locked Picosecond Pulses

We demonstrate the generation of optical bistability in a Fabry–Perot resonator made of a fused-silica single-mode polarization-maintaining fiber. To suppress the undesirable generation of stimulated Brillouin scattering, a pulse train from a mode-locked and Q-switched picosecond laser was used as an incident pulse and the fiber resonator was synchronized with it. We observed first-order bistability, higher order bistability, and power limiting depending on the initial cavity detuning and mirror reflectivity. The obtained results could be well explained by our previous transient analysis based on an iterative method.

Demonstration of an all-digital 7-bit 33-channel photonic delay line for phased-array radars

A 7-bit multichannel photonic delay line for phased-array antenna control is demonstrated. Multichannel (33-pixel) ferroelectric liquid-crystal (FLC) devices are used as polarization rotation elements, and polarization beam-splitter cubes are used as polarization elements that route the optical signals to different paths. The controller is remotely fed by a unique fiber-optic-array design that uses gradient-index lens collimators for the input single-mode polarization-maintaining fibers. The optical signal is collected by a similar fiber array that uses multimode fibers for improved coupling efficiency. Photonic delay-line (PDL) design issues such as multiport assemblies, multipixel FLC designs, and delay-line architectures are discussed. Furthermore, various PDL parameters are examined. High electrical isolation numbers are obtained for both the within-channel leakage noise (e.g., less than -70 dB) and the interchannel cross talk (e.g., less than -90 dB). Optical and electrical insertion loss is examined for the PDL as well as for the overall system. A high-compression dynamic range of 149 dB x Hz and a spurious free dynamic range of 105 dB x Hz(2/3) are presented.

Power-scalable system of phase-locked single-mode diode lasers

The direct use of diode lasers for high-power applications in material processing is limited to applications with relatively low beam quality and power density requirements. To achieve high beam quality one must use single-mode diode lasers, however with the drawback of relatively low optical output powers from these components. To realize a high-power system while conserving the high beam quality of the individual emitters requires coherent coupling of the emitters. Such a power-scalable system consisting of 19 slave lasers that are injection locked by one master laser has been built and investigated, with low-power diode lasers used for system demonstration. The optical power of the 19 injection-locked lasers is coupled into polarization-maintaining single-mode fibers and geometrically superimposed by a lens array and a focusing lens. The phase of each emitter is controlled by a simple electronic phase-control loop. The coherence of each slave laser is stabilized by computer control of the laser current and guarantees a stable degree of coherence of the whole system of 0.7. An enhancement factor of 13.2 in peak power density compared with that which was achievable with the incoherent superposition of the diode lasers was observed.

Long-range Schmidt-Cassegrain laser velocimeter for large wind-tunnel applications

A two-component dual-beam laser velocimeter was designed and fabricated for use in the NASA Ames 40 X 80 ft wind tunnel. The instrument design was predicated on the use of inexpensive, high-quality, large-aperture Schmidt-Cassegrain telescope optics to perform both the laser beam focusing and scattered light collection functions. The instrument was configured to measure the streamwise and cross-stream components of velocity and enclosed in a streamlined optical assembly mounted on a two-dimensional traverse on the floor of the test section. The laser velocimeter probe volume was translated in the vertical direction by means of zoom optics. Coaxial backscattered light was collected through the same optical system and analyzed for Doppler shift. Single-mode polarization-maintaining fiber optics transmitted light from the remotely located laser into the optical assembly. A multimode fiber conveyed the scattered light to remotely located photodetectors. Standard frequency domain processing permitted analysis of Doppler signals with very poor signal-to-noise ratio. The system was evaluated in an optics laboratory in preparation for wind-tunnel test program applications. Sample data from this evaluation were presented as representative of the capability of the instrument. Statement of the Problem I N recent decades the laser Doppler velocimeter1 (LV) has seen increasing application to the study of aerodynamic phenomena. The LV provides a noninvasive measurement capability that is critical for the study of many phenomena. The dual-beam or differentialDoppler configuration of the instrument2 has been shown to have many practical advantages and is used extensively in small-scale wind-tunnel testing. Although several new techniques offer increased potential for global measurement capability,3'4 the LV remains a very important instrument for any aeronautical research facility. Although significant progress has been made,5 LV has not been widely used in large-scale wind-tunnel testing. To maintain the noninvasive quality of the measurement, it is necessary to place the instrument outside the wind-tunnel test section, or at least far enough away to limit its effect on the flowfield of interest. In largescale wind tunnels, this means large separation between the probe volume (laser beam intersection volume) where Doppler shifted light is scattered and the receiving optics where that light is collected and focused on a photodetector. This large separation or longrange operation requirement demands high laser power density in the probe volume to achieve high scattered intensity of the Doppler shifted light and a large collection solid angle to concentrate it on the photodetector. For a closed jet wind tunnel, it is further necessary to use windows of sufficient quality that the laser beams launched into the test section as well as the scattered light collected from the probe volume are not significantly degraded. In particular, the laser beams must not be refracted by striae in the windows so as to significantly reduce beam overlap in the probe volume. The window aperture must be sufficient to access the entire flow region of interest. The National Full-Scale Aerodynamics Complex at NASA Ames Research Center is composed of two large wind tunnels and a static (hover) test facility. The 40 x 80 ft wind tunnel is the smaller of the two wind tunnels, with a speed capability up to 300 kn (154 m/s).

Polarization-maintaining fiber Fabry–Perot tunable filters

We report what are to our knowledge the first high-performance polarization-maintaining single-mode fiber Fabry-Perot tunable filters. These filters simultaneously exhibited a loss of <2.5 dB, a finesse between 150 and 310, and an extinction ratio of >30 dB. A unique fabrication technique is developed that utilizes the tunability and the polarization resonance splitting inside the filter to align the birefringent axes at a pigtail-cavity joint. A general criterion is established for aligning polarization-maintaining fibers with elliptical inputs.

Secure communications by optical homodyne

It is shown that optical homodyne techniques offer a family of schemes for secure communications. Following some background in secure optical communications, a particular scheme is described and compared to cryptography. The scrambler built is also described, and security is discussed. Experimental results for point-to-point links in single-mode polarization-maintaining fibers are presented. Various enhancements and network compatibility are analyzed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of macro-strain transfer and complete strain state measurement with embedded fiber-optic sensors

Summarizes the normal strain coupling from an isotropic material into an embedded optical fiber which serves as a sensor. The sensor is based on single-mode polarization-maintaining fibers and is sensitive to all three normal strains. It is shown analytically that six such sensors can monitor the complete strain state of the host material provided that the sensor direction and polarization axes are properly orientated.

Properties of fiber optics for application in astronomical interferometry

We tested such properties of single-mode and multimode fiber optics that are relevant for use in a long-baseline astronomical interferometer, and we give quantitative values for modulation, transmission, and stability of the fiber optics as measured in a Mach-Zehnder interferometer configuration. Only polarization-maintaining single-mode fibers gave satisfying results. Good preservation of modulation and ease of beam combining are strong features of these fibers, whereas the light losses that are due to imperfect coupling may present a serious problem.

Effect of the refractive index dip on the cutoff frequencies in stress-applied polarization-maintaining fibers

In this paper we have presented a new model of stress-applied polarization-maintaining single-mode optical fibers to study the cutoff characteristics of such fibers with the refractive index dip. The feasibility of this model has been confirmed compared with the results obtained by the FEM. It has been shown that the central dip has a considerable effect on the cutoff values.