Many applications rely on the ultra-precise timing of optical signals through fiber, such as fiber interferometers, large telescope arrays, in phase arrayed antennae, optical metrology, and precision navigation and tracking. Environmental changes, specifically those caused by temperature fluctuations, lead to variations in the propagation delay of optical signals and thereby decrease the accuracy of the system’s timing. The cause of these variations in delay is the change in the glass properties of the optical fiber with temperature. Both the refractive index of the glass and the length of the fiber are dependent on the ambient temperature. Traditional optical fiber suffers from a delay sensitivity of 39 ps/km/K. We are reducing the temperature sensitivity of the fiber delay through the application of a novel design of optical fiber, Anti-Resonant Hollow Core Fiber. The major improvement in the thermal sensitivity of this fiber comes from the fact that the light is guided in an air core, with very little overlap into the glass structure. This drastically reduces the impact that the thermally sensitive glass properties have on the propagation time of the optical signal. Additionally, hollow core fiber is inherently radiation insensitive, due to the light guidance in air, making it suitable for space applications.
Many applications rely on the ultra-precise timing of optical signals through fiber, such as fiber interferometers, large telescope arrays, in phase arrayed antennae, optical metrology, and precision navigation and tracking. Environmental changes, specifically those caused by temperature fluctuations, lead to variations in the propagation delay of optical signals and thereby decrease the accuracy of the system’s timing.
The cause of these variations in delay is the change in the glass properties of the optical fiber with temperature. Both the refractive index of the glass and the length of the fiber are dependent on the ambient temperature. Traditional optical fiber suffers from a delay sensitivity of 39 ps/km/K. We are reducing the temperature sensitivity of the fiber delay through the application of a novel design of optical fiber, Anti-Resonant Hollow Core Fiber. The major improvement in the thermal sensitivity of this fiber comes from the fact that the light is guided in an air core, with very little overlap into the glass structure. This drastically reduces the impact that the thermally sensitive glass properties have on the propagation time of the optical signal. Additionally, hollow core fiber is inherently radiation insensitive, due to the light guidance in air, making it suitable for space applications.
A simple, interferometric force sensor based on a multicore optical fiber (MCF) that operates in reflection mode is presented. The device consists of a short segment of MCF inserted at the distal end of a conventional single mode optical fiber (SMF). To demonstrate the concept we used a mechanical piece with grooves to press the MCF. In this way the external force on the MCF is converted in localized pressure on the fiber which causes attenuation losses to the interfering modes and makes the interference pattern to shrink. The changes experienced by the interference pattern can be easily monitored. The sensor here proposed is highly sensitive since it can resolve forces down to 0.01 N.
In this work, we demonstrate the use of particularly characterised multicore optical fibres (MCFs) to devise compact, compellingly simple, ultrasensitive interferometric sensors which are capable of sensing single or multiple physical parameters. Generally, our devices operate in reflection mode and consist of a few centimetres of MCF fusion spliced to standard single-mode optical fibre (SMF). The tools and instrumentation needed to fabricate our devices are a conventional fibre cleaver and a fusion splicing machine. We demonstrate a highly-sensitive bending sensor (inclinometer) with a MCF with three strongly coupled cores which is capable of distinguishing multiple bending or inclination orientations, and also a force sensor based on MCF with seven coupled cores. In both cases the devices are interrogated with a low-power LED and a miniature spectrum analyser. Bending or force on the MCF induces drastic changes of the supermodes, their excitation, and consequently, on the reflected spectrum (interference pattern).
For the first time, we demonstrate the implementation of a core pumped few mode erbium amplifier utilizing a mode selective photonic lantern for spatial modal control of the pump light. This device is able to individually amplify the first six fiber modes with low differential modal gain. In addition, we obtained differential modal gain lower than 1 dB and signal gain of approximately 16.17 dB at λs = 1550 nm through forward pumping the LP21 modes at λp = 976 nm.
We present here a method to create spectrally addressable phase masks by encoding phase profiles into volume Bragg gratings, allowing these holographic elements to be used as phase masks at any wavelength capable of satisfying the Bragg condition of the hologram. Moreover, this approach enables the capability to encode and multiplex several phase masks into a single holographic element without cross-talk while maintaining a high diffraction efficiency. As examples, we demonstrate fiber mode conversion with near-theoretical conversion efficiency as well as simultaneous mode conversion and beam combining at wavelengths far from the original hologram recording wavelength.
Output performances of fiber-based optical systems, in particular fiber lasers and amplifiers, can be controlled using tailored fiber designs, gain profiles, and pump light overlap with the gain medium. Here, the performances of 2-μm light, propagating in three large-mode area fibers, a step-index fiber, a photonic crystal fiber (PCF), and a leakage channel fiber (LCF), designed to deliver a single-mode (SM) beam at this wavelength, were compared. Using the S2 imaging technique, the transverse mode content has been decomposed, and propagation losses, SM purity, and mode-field area (MFA) were measured for various input mode overlap and coiling diameters. It was experimentally demonstrated that coiling the PCF and LCF to 40 and 20 cm in diameter, respectively, resulted in efficient higher-order mode suppression, pure SM beam delivery, moderate (∼1 dB) coil-induced losses in the fundamental mode, and nondistorted, large MFA (∼1600 μm2) beam delivery.
To scale to power levels of up to tens of kW with a few fiber lasers, the best candidates are large core fibers guiding a few large-area higher order modes with the outputs of these fibers combined into a single beam. However, in many applications it is desirable to convert these higher order modes into a Gaussian profile. Here, we propose a method to accomplish this task via single volume phase element. This element accepts multiple higher order mode beams and simultaneously converts and combines them to a single Gaussian profile in the far field.
Optical fibers that support single mode operation while achieving large mode areas are key elements for scaling up
optical powers and pulse energies of fiber lasers. Here we report on a study of the modal properties of a new-generation
of polarization maintaining large-mode-area photonic crystal fibers based on the spectrally and spatially resolved (S2)
imaging technique. A fiber designed for Tm fiber laser system single mode operation in the 2μm spectral range is
demonstrated for coiling diameters smaller than 40cm. At shorter wavelengths in the 1.3μm range, efficient higher order
mode suppression requires tide coiling to about 20cm diameters.
The noise power spectrum of solid-state lasers - including fiber lasers - exhibits a characteristic peak at the relaxation
oscillation frequency. The tails associated with this peak extend to neighboring spectral ranges and may increase the
noise level above acceptable limits in applications using weak signals. One of the key factors to reduce the relative
intensity noise (RIN) amplitude is a low loss laser resonator. We describe a method to ultimately reduce the intensity
noise in single frequency phosphate fiber lasers by minimizing intra-cavity losses caused by fusion splices between
fibers made of different materials. Conventional fiber Bragg gratings written in silica fibers have been replaced with
gratings written in phosphate glass fibers. The quality of the intra-cavity fusion splice has been improved due to material
similarity. All-phosphate fiber laser devices have been built and tested utilizing the new gratings. The results show
relative intensity noise amplitudes that are very similar to those of conventionally fabricated devices. Challenges in the
grating writing process are currently preventing the new devices from surpassing their commercial counterparts in terms
of performance. However, this type of all phosphate glass fiber lasers may ultimately lead to a new generation of
commercial single frequency fiber lasers with improved intensity noise performance.
We performed extensive spectroscopy of tellurite glasses doped with high concentration of Tm ions for laser emission at
around 2 micron wavelength. The aim of the work is to develop a glass suitable for single-frequency fiber laser. In fact
such a kind of laser require the use of short cavity length and therefore high gain per unit length medium. Tellurite
glasses allows high-doping concentration and are therefore an excellent candidate. In these paper we review our recent
results. In particular we address the optical and thermo-mechanical properties of several tellurite glasses
(75mol%Te02.20mol%ZnO. 5mol%Na2O) with Tm3+ doping up to 111,564 ppm.
Laser beam transformation utilizing the effect of multimode interference in multimode (MM) optical fiber is
thoroughly investigated. When a Gaussian beam is launched to an MM fiber, multiple eigenmodes of the MM fiber are
excited. Due to interference of the excited modes, optical fields that vary with the MM fiber length and the signal
wavelength are generated at the output facet of the MM fiber. Diffractive propagation of these confined fields can yield
various desired intensity profiles in free space. Our calculations show that, an input fundamental Gaussian beam can be
transformed to frequently desired beams including top-hat, donut-shaped, taper-shaped, and low-divergence Bessel-like
within either the Fresnel or the Fraunhofer diffraction range, or even in both ranges. Experiments on a monothic fiber
beam transformers consisting of a short piece of MM fiber (~ 10 mm long) and a single-mode signal delivery fiber were
carried out. The experimental results indicate the functionality and high versatility of this simple fiber device. The
performance of this fiber device can be easily and widely manipulated through parameters including the ratio between
the core diameters of the SM and MM fiber segments and the length of the MM fiber segment. In addition, the intensity
profile of the output beam can be controlled by tuning the signal wavelength even after the fiber device is fabricated.
Most importantly, this technique is highly compatible with the technology of high power fiber lasers and amplifiers and
fiber delivery systems.
Recent advances in the field of phosphate glass fiber lasers are reviewed. Fabrication of microstructured fiber and
writing of fiber Bragg gratings in passive and active phosphate glass fiber are demonstrated. Based on these novel
components we fabricate cm-long, Watt-level fiber lasers that allow for tunable, single longitudinal mode operation.
An all-fiber approach is utilized to phase lock and select the in-phase supermode of compact multicore fiber lasers.
Based on the principles of Talbot imaging and waveguide multimode interference, the fundamental supermode is
selectively excited within a completely monolithic fiber device. The all-fiber device is constructed by simply fusion
splicing passive non-core optical fibers of controlled lengths at both ends of a piece of multicore fiber. Experimental
results upon in-house-made 19- and 37-core fibers are demonstrated, which generate output beams with high-brightness
far-field intensity distributions. The whole fabricated multicore fiber laser device can in principle be a single fiber chain
that is only ~10 cm in length, aligning-free in operation, and robust against environmental disturbance.
We report the photorefractive properties of tetraphenyldiaminobiphenyl (TPD) based polymer composites
that have been developed for single pulse laser operation at 532 nm. With an optimized composite, we
demonstrate more than 50% diffraction efficiency using 4 mJ/cm2 single shot writing and 633 nm
continuous wave (cw) beam reading. The present devices showed a 300 μs fast response time. This
reveals the potential for these polymer devices in applications which require fast writing and erasure. Since
the writing pulse-width is in nanosecond time scale, the recording is totally insensitive to vibrations. These
devices can also be used as a stepping stone to realize all-color holography since they are sensitive to both
green (532nm) and red (633nm) wavelengths. The holograms can be written with either of these two
wavelengths and can be read by the same wavelength or the other wavelength with high diffraction
efficiency. This demonstrates that these devices have the advantage of performing two-color holography, a
step closer to a dynamic full-color holographic recording medium.
Optical and electron confinement are utilized to tailor the optical characteristics of active materials and photonic devices. A technique to incorporate semiconductor quantum dots into planar glass waveguides with low propagation loss is demonstrated. The waveguides are fabricated by potassium-sodium and silver-sodium ion exchange processes in glasses that contain PbS quantum dots with radii of a few nanometers. The unique optical properties of the quantum dots are preserved throughout the waveguide fabrication process. We also demonstrate novel compact fiber lasers based on active, highly doped fibers with photonic crystal cladding. The flexibility provided by microstructuring the fiber enables improved fiber laser performance and several Watts of laser output are generated from few centimeters of active fiber.
Organic-inorganic hybrid sol-gel materials have attracted increasing attention in recent years as low-cost, rugged materials for integrated optical devices such as optical couplers, splitters, and electro-optic modulators. These materials can be easily processed by spin-coating, wet-etching photolithography, and low-temperature baking. Precise control of waveguide core-cladding refractive indices produces well-confined low-loss propagation and good matching of the absolute refractive index to that of fused silica results in low optical coupling loss to optical fiber. The increased thermal and mechanical stability of these materials, relative to optical polymers, results in numerous packaging options and improved reliability. However organic-inorganic hybrid sol-gel materials have not yet been often used as host of active dopants such as erbium (III) ions for 1550nm optical amplification. This limitation owes primarily to matrix and chelate dominated nonradiative relaxation processes, as high phonon energy OH and OH-like oscillators can bridge off the energy from the excited erbium (III) ions at very high rates. Different strategies have been proposed to protect erbium (III) ions from matrix and chelate quenching, including host and ligand fluorination, and inorganic microstructure shielding. Here we report on our work of encapsulating erbium (III) ions in transparent, refractive index matched, and highly re-dispersible lanthanum phosphate nanoparticles and the work of examining the optical properties of these nanoparticles as active dopants in organic-inorganic hybrid sol-gels adopting 2-methacryloxypropyl trimethoxysilane (MAPTMS) as a precursor. 980nm laser pumped photoluminescence at 1535nm was obtained from solid bulk samples of 300mg La.99Er.01PO4 nanoparticles doped in 1mL hybrid sol-gel. Thick bulk samples of this composition exhibited exceptional clarity and little trace of nanoparticle scattering effects. The lifetime of the nanoparticle doped hybrid sol-gel composite was measured to be 220μs, indicating an intermediate relaxation rate between that of an erbium organic complex and annealed erbium doped glass. La.99Er.01PO4 nanoparticle doped hybrid sol-gel films were also prepared and the refractive index was measured to be 1.4966 at 1550nm, which is very close to that of optical fiber and provides a suitable index difference from an undoped and metal oxide tuned sol-gel at 1.4870 to comprise an efficient single-mode waveguide system.
A three element, 15.3 cm, fiber Bragg grating array (FBGA) operating at 1550 nm wavelength is fabricated using a single mode photosensitive fiber. The FBGA is initially simulated using in-house developed software based on the Transfer Matrix Method, then fabricated using a double frequency Argon laser and a phase mask technique, and interrogated using Optical Frequency Domain Reflectometry. A single fiber Bragg grating (FBG) is accurately strain calibrated using a Fabry-Perot interferometer and piezoelectric actuation. The piezoelectric is linearly ramped, and the shifts in the Bragg wavelength along with the fringe count from the Fabry-Perot interferometer are recorded. The fringe count is then used to determine the strain on the FBG and compared to changes in the Bragg wavelength in-order to calculate the strain gage factor. This result is used to calibrate the FBGA for strain measurements. The FBGA is then bonded to a cantilever beam with three electric strain gages attached next to each FBG in the array. The axial strain results obtained from the electric strain gages and FBGA are compared for various displacements of the cantilever beam. The Fabry-Perot interferometer and piezoelectric calibration method is a non-destructive process that eliminates the need to bond the FBG to an external support during the calibration process, and can also be used to calibrate electric strain gages.
Wavelength of 2000 nm single mode microsphere laser from highly
thulium doped tellurite glass microsphere was demonstrated by means of
fiber taper coupling. Laser wavelength was red shift from the emission
peak of thulium ions at 1800 nm.
A self-calibrating fluorescence spectroscopy technique was applied to study cross-relaxation 3H4, 3H6 → 3F4, 3F4, and energy migration 3H4, 3H6 → 3F4, 3F4, of the Tm3+ Ions doped in the tellurite glass. These glasses are investigated for their use in realization of 2 micron fiber lasers. Micro and macro-parameters of the energy transfer and migration were calculated by the means of the model of phonon-assistant multi-polar interaction and hoping mode. Steady rate equation analysis was used to fit the experimental fluorescence ratio of samples with different concentrations. We found that high-order (dipole-quadrupole) interaction was the dominant mechanism in the energy transfer of Thulium ions.
We describe the material characteristics and photorefractive properties of novel tetraphenyldiaminobiphenyl (TPD) based polymer composites that were developed for operation wavelengths up to 1 micron. With an optimized composite, we demonstrated more than 50% external diffraction efficiency coupled with a fast response time of about 35 ms at 980 nm. In addition to this high performing composite, we have developed a composite with high two beam coupling gain (300 cm-1). To accomplish these attractive photorefractive properties in the near-infrared, we explored the chemical flexibility of the guest-host approach. We employed a new dye with enhanced near-infrared absorption to extend the sensitivity into this long wavelength range. Styrene-based chromophores were utilized to enable high refractive index modulation. We explored ellipsometry as well as photo-conductivity measurements to optimize the composition of the composites. In addition to the composites that contain a single chromophore species, we also analyzed samples prepared with a mixture of chromophores. Our studies reveal the potential of this new polymer-composite family to extend the operation wavelength of the photorefractive materials to even longer wavelengths. Attractive photorefractive properties coupled with long wavelength sensitivity make these materials potential candidates for imaging and communication applications.
Data are presented which show eight excitonic density (Rabi) oscillations in an In0.1Ga0.9As/GaAs multiple quantum well at 5 K. Our time resolved, two-color pump-probe experimental technique for observing these oscillations is described. The quantum well sample geometry and linear spectrum are shown along with data characterizing the pump and probe pulses. Experimental data is shown to be in excellent agreement with our theoretical calculation of exciton density versus time, verifying the important affect of the Coulomb interaction between carriers in renormalizing the Rabi frequency. The theoretical calculations include the two-fold degenerate light-hole, heavy-hole, and valence bands in the Hartree-Fock form of the semiconductor Block equations, with all variables selected to conform to the experiment.
We report on the excitation and propagation of an exciton density front inside a semiconductor that is characterized by high absorption and large optical nonlinearity. Femtosecond optical pulses are used for both the excitation of the density front and the probe of the front propagation. We analyze in detail the spectra of reflected probe pulses that carry information about the propagating density front due to partial internal reflection of light at the boundary between regions of high and low excitation density. Time resolved data show that the Doppler shift of the internal reflection is limited to the duration of the pump pulse indicating the highly transient character of the exciton front propagation.
We study the emission properties of various laser cavities under pulsed optical excitation of the active semiconducting conjugated polymer material. Physical origin, magnitude, and dynamics of optical gain in these novel active laser materials are discussed leading to a selection of suitable cavity configurations for laser applications. We demonstrate laser action for various planar and ring resonator configurations that can be achieved in the regimes of transient inversion and quasi stationary excitation of the polymer material pumping with femtosecond and nanosecond pulses, respectively.
We observed laser emission in whispering gallery modes using a microring composed of a light-emitting semiconducting polymer poly[2,5-bis-(2'-ethylhexyloxy)-p- phenylenevinylene] (BEH-PPV) coated on an etched fiber under transient and quasi steady-state pumping conditions. The threshold for laser oscillation was 1 mJ/cm2 (0.1 MW/cm2) and 30 (mu) J/cm2 (300 MW/cm2) for nanosecond and femtosecond excitation, respectively. The laser output showed superlinear dependence on the excitation energy above the threshold. The demonstration of lasing under quasi steady-state pumping shows the possibility to develop electrically pumped polymer lasers. Preliminary results on the line narrowing in tripheny dilamine (TPD) films under nanosecond optical pumping are also presented. 23
We report the first experimental observation of a self- reflected wave inside a very dense saturable absorber. An intense femtosecond pulse saturates the absorption and causes a density front moving into the semiconductor sample. Due to the motion of the boundary between saturated and unsaturated areas of the sample the light reflected at this boundary is red-shifted by the Doppler effect. The spectrally shifted reflection makes it possible to distinguish between surface reflection and self-reflection and is used to proof the concept of the dynamic nonlinear skin effect experimentally. Quite well agreement with model calculations is found.
This paper studies excitons and bi-excitons in ternary (Zn,Cd)Se/ZnSe quantum wells, widely used as active region in blue-green laser diodes. Localization on alloy disorder characteristically influences the electronic structure of these excitations and their dynamical behavior. The low-temperature lasing is controlled by bi-excitons. Gain as large as 2 (DOT) 104 cm-1 and optical threshold densities as low as 2 kW cm-2 are observed. Due to their localization-enhanced binding energy, bi-exciton signatures are present up to 150 K.