Alignment-free fabrication of a hybrid electro-optic polymer/ion-exchange glass coplanar modulator

A hybrid electro-optic (EO) polymer phase modulator with a 6 μm coplanar electrode gap was realized on ion exchange glass substrates. The critical alignment steps which may be required for hybrid optoelectronic devices were eliminated with a simple alignment-free fabrication technique. The low loss adiabatic transition from glass to EO polymer waveguide was enabled by gray scale patterning of novel EO polymer, AJLY. Total insertion loss of 5 dB and electrode gap of 8 μm was obtained for an optimized device design. EO polymer poling at 135 °C and 75 V/μm was demonstrated for the first time on a phosphate glass substrate and was enabled by the sol-gel buffer layer.

Nonmechanical bifocal zoom telescope

We report on a novel zoom lens with no moving parts in the form of a switchable Galilean telescope. This zoom telescope consists of two flat liquid-crystal diffractive lenses with apertures of 10mm that can each take on the focal lengths of -50 and +100cm, with a spacing of 50cm and, hence, a zoom ratio of 4x. The lenses are driven using a low-voltage ac source with 1.6V and exhibit millisecond switching times. The spectral characteristic of this diffractive zoom system is evaluated for light sources of various bandwidths. Potential applications for this technology include a zoom lens with no moving parts for camera phones and medical imaging devices.

Kilowatt-level stimulated-Brillouin-scattering-threshold monolithic transform-limited 100 ns pulsed fiber laser at 1530 nm

We demonstrate a high-stimulated-Brillouin-scattering-threshold monolithic pulsed fiber laser in a master oscillator power amplifier configuration that can operate over the C band. In the power amplifier stage, we used a newly developed single-mode, polarization maintaining, and highly Er/Yb codoped phosphate fiber with a core diameter of 25 microm. A single-frequency actively Q-switched fiber laser was used to generate pulses in the hundreds of nanoseconds at 1530 nm. We have achieved peak power of 1.2 kW for 105 ns pulses at a repetition rate of 8 kHz, corresponding to a pulse energy of 0.126 mJ, with transform-limited linewidth and diffraction-limited beam quality.

Enhanced terahertz source based on external cavity difference-frequency generation using monolithic single-frequency pulsed fiber lasers

We demonstrate a resonant external cavity approach to enhance narrowband terahertz radiation through difference-frequency generation for the first time (to our knowledge). Two nanosecond laser pulses resonant in an optical cavity interact with a nonlinear crystal to produce a factor of 7 enhancement of terahertz power compared to a single-pass orientation. This external enhancement approach shows promise to significantly increase both terahertz power and conversion efficiency through optical pump pulse enhancement and effective recycling.

Grating dynamics in a photorefractive polymer with Alq(3) electron traps

The electron transporting molecule tris(8-hydroxyquinoline) aluminum (Alq(3)) was added in low concentrations to a photorefractive polymer composite to provide trapping sites for electrons. This sample exhibited larger two-beam coupling gain, higher diffraction efficiency at lower voltages, and an increased dielectric breakdown strength compared to a control sample. The dynamics also revealed the presence of a competing grating, and a bipolar charge transport model is shown to fit the data. Overall, Alq(3) improves the response time, efficiency, and breakdown voltage without a significant increase in absorption or loss of phase stability. This has applications for reflection displays and pulsed writing, where charge trapping and generation are major factors limiting the usefulness of photorefractive polymers.

Coherent beam transformations using multimode waveguides

Physical insights and characteristics of beam transformations based on multimode interference (MMI) in multimode waveguides are illuminated and analyzed. Our calculations show that, utilizing a short piece of cylindrical multimode waveguide, an input Gaussian beam can be readily transformed to frequently desired beams including top-hat, donut-shaped, taper-shaped, and Bessel-like beams in the Fresnel or the Fraunhofer diffraction range, or even in both ranges. This is a consequence of diffractive propagation of the field exiting the waveguide. The performance of the beam shaper based on MMI can be controlled via tailoring the dimensions of the multimode waveguide or changing the signal wavelength. This beam shaping technique is investigated experimentally using monolithic fiber devices consisting of a short piece of multimode fiber (approximately 10 mm long) and a single-mode signal delivery fiber.

Adjustable adaptive compact fluidic phoropter with no mechanical translation of lenses

We demonstrate a compact optical phoroptor consisting of adjustable astigmatic and defocus lenses. The lenses are fluidically controlled and allow for an arbitrary refractive error to be corrected without mechanically moving lenses. Shack-Hartmann measurements were used to characterize the optical properties of the individual lenses. The lenses were then assembled into the phoropter and controlled with three separate fluid controls. The phoroptor was verified by correcting the vision of a model eye with an induced refraction error.

Tunable-focus flat liquid-crystal diffractive lens

We demonstrate an innovative variable-focus flat liquid-crystal diffractive lens (LCDL) with 95% diffraction efficiency and millisecond switching times using a +/-2.4 V ac input. This lens is based on the electrical modulation of a 3 mum layer of nematic liquid-crystal sandwiched between a Fresnel zone electrode structure and a reference substrate. Each zone is divided into 12 subzones to digitize the phase profiles and define the phase wrapping points. The focusing power can rapidly be switched by electrically changing the number of subzones and re-establishing the wrapping points. Potential applications include zooms with no moving parts and autofocus lenses for compact imaging devices.

Nonlinear optical transmission of lead phthalocyanine-doped nematic liquid crystal composites for multiscale nonlinear switching from nanosecond to continuous wave

We have formulated composites of lead (II) tetrakis (4-cumylphenoxy) phthalocyanine (PbTCPc) doped into nematic liquid crystal (LC), 4(')-pentyl-4-biphenylcarbonitrile (5-CB), that has received a 90 degree twisted alignment and investigated the nonlinear transmission properties using both pulsed (Nd:YLF 524 nm, 5 ns) and cw (532 nm) lasers. In the nanosecond regime, this compound is a reverse saturable absorber performing similarly to low-concentration solutions of PbTCPc. Under cw conditions, we observe optically self-activated polarization switching with low threshold input energy. Our results suggest the potential for an all-optical switch working from the nanosecond time scale to cw.

High Deltan strip-loaded electro-optic polymer waveguide modulator with low insertion loss

We demonstrate a novel electro-optic polymer modulator design which shows a record low optical insertion loss of 5.7 dB at 1550 nm. The modulator consists of a high numerical aperture passive waveguide which is converted to a strip-loaded electro-optic polymer waveguide through refractive index tapers. The device is fabricated using all wet-etch techniques which results in low excess loss from roughness created during fabrication and, employs new low loss passive sol-gel materials. The fabricated device also shows a low half-wave voltage of 2.8 V.

Detailed investigation of self-imaging in large-core multimode optical fibers for application in fiber lasers and amplifiers

Properties of the self-imaging effect based on multimode interference (MMI) in large-core passive optical fibers are investigated and analyzed in detail, with the purpose of using multimode active fibers for high power single-transverse-mode emission. Although perfect self-imaging of the input field from a standard single-mode fiber (SMF-28) in a multimode fiber becomes practically impossible as its core diameter is larger than 50 microm, a quasi-reproduction of the input field occurs when the phase difference between the excited modes and the peak mode inside the multimode fiber is very small. Our simulation and experimental results indicate that, if the length of the multimode fiber segment can be controlled accurately, reproduction of the input field with a self-imaging quality factor larger than 0.9 can be obtained. In this case, a low-loss hybrid fiber cavity composed of a SMF-28 segment and a very-large-core active multimode fiber segment can be built. It is also found that for the hybrid fiber cavity, increasing the mode-field diameter of the single-mode fiber improves both the self-imaging quality and the tolerance on the required length accuracy of the multimode fiber segment. Moreover, in this paper key parameters for the design of MMI-based fiber devices are defined and their corresponding values are provided for multimode fibers with core diameters of 50 microm and 105 microm.

Time-resolved studies of photoinduced birefringence in azobenzene dye-doped polymer films

We measured transient photoinduced birefringence (delta n) in various azobenzene dye films by pumping with a nanosecond pulse at 532 nm and probing at 633 nm. The switch-on times for the photoinduced birefringence range from nanoseconds to milliseconds and are systematically related with the lowest optical transition energies for those films. Moreover, our results suggest that the transient photoinduced birefringence measurement is a convenient way to determine the relative energies of pi-pi(*) and n-pi(*) states in azo-based materials.

Local electric field enhancement and polarization effects in a surface-enhanced Raman scattering fiber sensor with chessboard nanostructure

A surface-enhanced Raman scattering fiber sensor with chessboard nanostructure on a cleaved fiber facet is studied by finite-difference time-domain method. Surface plasmons at the metal coated nanostructured fiber facet can be effectively excited and strong local electric field enhancement is obtained. Studies on the influence of light polarization demonstrate a large polarization dependence of the field enhancement factor while the polarization effects on the plasmon resonance wavelength are relatively small.

Continuous-wave all-solid-state 244 nm deep-ultraviolet laser source by fourth-harmonic generation of an optically pumped semiconductor laser using CsLiB6O10 in an external resonator

We report an all-solid-state laser system that generates over 200 mW cw at 244 nm. An optically pumped semiconductor laser is internally frequency doubled to 488 nm. The 488 nm output is coupled to an external resonator, where it is converted to 244 nm using a CsLiB(6)O(10) (CLBO) crystal. The output power is limited by the available power at 488 nm, and no noticeable degradation in output power was observed over a period of several hours.

Single-transverse-mode output from a fiber laser based on multimode interference

An alternative original approach to achieve single-transverse-mode laser emissions from multimode (MM) active fibers is demonstrated. The fiber cavity is constructed by simply splicing a conventional passive single-mode fiber (SMF-28) onto a few centimeters-long active MM fiber section whose length is precisely controlled. Owing to the self-imaging property of multimode interference (MMI) in the MM fiber, diffraction-limited laser output is obtained from the end of the SMF-28, and the MMI fiber laser is nearly as efficient as the corresponding MM fiber laser. Moreover, because of the spectral filtering effect during in-phase MMI, the bandwidth of the MMI fiber laser is below 0.5 nm.

Distributed feedback fiber laser pumped by multimode laser diodes

A distributed feedback fiber laser made of highly Er-Yb codoped phosphate glass fiber has been demonstrated experimentally. Efficient pump absorption allows for multimode pumping into the cladding of the active fiber. Output powers up to 160 mW have been achieved. The 35 mm long fiber laser device emits with >50 dB side mode suppression ratio.

An updatable holographic three-dimensional display

Holographic three-dimensional (3D) displays provide realistic images without the need for special eyewear, making them valuable tools for applications that require situational awareness, such as medical, industrial and military imaging. Currently commercially available holographic 3D displays use photopolymers that lack image-updating capability, resulting in restricted use and high cost. Photorefractive polymers are dynamic holographic recording materials that allow updating of images and have a wide range of applications, including optical correlation, imaging through scattering media and optical communication. To be suitable for 3D displays, photorefractive polymers need to have nearly 100% diffraction efficiency, fast writing time, hours of image persistence, rapid erasure, and large area-a combination of properties that has not been shown before. Here, we report an updatable holographic 3D display based on photorefractive polymers with such properties, capable of recording and displaying new images every few minutes. This is the largest photorefractive 3D display to date (4 x 4 inches in size); it can be recorded within a few minutes, viewed for several hours without the need for refreshing, and can be completely erased and updated with new images when desired.

Single-mode fiber laser based on core-cladding mode conversion

A single-mode fiber laser based on an intracavity core-cladding mode conversion is demonstrated. The fiber laser consists of an Er-doped active fiber and two fiber Bragg gratings. One Bragg grating is a core-cladding mode converter, and the other Bragg grating is a narrowband high reflector that selects the lasing wavelength. Coupling a single core mode and a single cladding mode by the grating mode converter, the laser operates as a hybrid single-mode laser. This approach for designing a laser cavity provides a much larger mode area than conventional large-mode-area step-index fibers.

Improved efficiency of a pulsed optical parametric oscillator by delayed double-pass pump

By delaying the second pass of the pump of a singly resonant optical parametric oscillator, the conversion efficiency can be improved. We show the experimental results of an intracavity-doubled singly resonant parametric oscillator pumped by a Q-switched laser. Using 0.49 mJ of pump energy at 532 nm from a diode-pumped Nd:YAG laser, 0.085 mJ of 488 nm was obtained with the optimum delay for the second pass of the pump, giving a conversion efficiency of 17%. The improvement over the case of a single-pass pump was 85%, and the improvement over the double-pass pump with a small delay was 40%.

Holographic injection locking of a broad area laser diode via a photorefractive thin-film device

We demonstrate locking of a high power broad area laser diode to a single frequency using holographic feedback from a photorefractive polymer thin-film device for the first time. A four-wave mixing setup is used to generate feedback for the broad area diode at the wavelength of the single frequency source (Ti:Sapphire laser) while the spatial distribution adapts to the preferred profile of the broad area diode. The result is an injection-locked broad area diode emitting with a linewidth comparable to the Ti:Sapphire laser.

Variation of Bragg condition in low-glass-transition photorefractive polymers when recorded in reflection geometry

Two low-glass transition photorefractive polymer composites were investigated in a symmetric reflection geometry. The holograms recorded in 105 mum thick devices have reached diffraction efficiencies as high as 60%. Unlike the gratings recorded in transmission geometry, holograms recorded in reflection geometry showed high angular selectivity and the Bragg condition was observed to be sensitive to the magnitude of the external bias field. We attribute this effect to poling-induced birefringence and give a theoretical analysis to describe the observed results.