Self-Quenched Fluorophore-DNA Labels for Super-Resolution Fluorescence Microscopy.

Protein labeling through transient and repetitive hybridization of short, fluorophore-labeled DNA oligonucleotides has become widely applied in various optical super-resolution microscopy methods. The main advantages are multitarget imaging and molecular quantification. A challenge is the high background signal originating from the presence of unbound fluorophore-DNA labels in solution. Here, we report the self-quenching of fluorophore dimers conjugated to DNA oligonucleotides as a general concept to reduce the fluorescence background. Upon hybridization, the fluorescence signals of both fluorophores are restored. We expand the toolbox of fluorophores suitable for self-quenching and report their spectra and hybridization equilibria. We apply self-quenched fluorophore-DNA labels to stimulated emission depletion microscopy and single-molecule localization microscopy and report improved imaging performances.

Self-quenched Fluorophore Dimers for DNA-PAINT and STED Microscopy.

Super-resolution techniques like single-molecule localisation microscopy (SMLM) and stimulated emission depletion (STED) microscopy have been extended by the use of non-covalent, weak affinity-based transient labelling systems. DNA-based hybrid systems are a prominent example among these transient labelling systems, offering excellent opportunities for multi-target fluorescence imaging. However, these techniques suffer from higher background relative to covalently bound fluorophores, originating from unbound fluorophore-labelled single-stranded oligonucleotides. Here, we introduce short-distance self-quenching in fluorophore dimers as an efficient mechanism to reduce background fluorescence signal, while at the same time increasing the photon budget in the bound state by almost 2-fold. We characterise the optical and thermodynamic properties of fluorophore-dimer single-stranded DNA, and show super-resolution imaging applications with STED and SMLM with increased spatial resolution and reduced background.

Lab-in-a-fiber-based integrated particle separation and counting.

An all-fiber integrated device capable of separating and counting particles is presented. A sequence of silica fiber capillaries with various diameters and longitudinal cavities are used to fabricate the component for size-based elasto-inertial passive separation of particles followed by detection in an uninterrupted continuous flow. Experimentally, fluorescent particles of 1 µm and 10 µm sizes are mixed in a visco-elastic fluid and fed into the all-fiber separation component. The particles are sheathed by an elasticity enhancer (PEO - polyethylene oxide) to the side walls. Larger 10 µm particles migrate to the center of the silica capillary due to the combined inertial lift force and elastic force, while the smaller 1 µm particles are unaffected, and exit from a side capillary. A separation efficiency of 100% for the 10 µm and 97% for the 1 µm particles is achieved at a total flow rate of 50 µL min-1. To the best of our knowledge, this is the first time effective inertial-based separation has been demonstrated in circular cross-section microchannels. In the following step, the separated 10 µm particles are routed through another all-fiber component for counting and a counting throughput of ∼1400 particles per min is demonstrated. We anticipate the ability to combine high throughput separation and precise 3D control of particle position for ease of counting will aid in the development of advanced microflow cytometers capable of particle separation and quantification for various biomedical applications.

Laser restructuring and photoluminescence of glass-clad GaSb/Si-core optical fibres.

Semiconductor-core optical fibres have potential applications in photonics and optoelectronics due to large nonlinear optical coefficients and an extended transparency window. Laser processing can impose large temperature gradients, an ability that has been used to improve the uniformity of unary fibre cores, and to inscribe compositional variations in alloy systems. Interest in an integrated light-emitting element suggests a move from Group IV to III-V materials, or a core that contains both. This paper describes the fabrication of GaSb/Si core fibres, and a subsequent CO2 laser treatment that aggregates large regions of GaSb without suppressing room temperature photoluminescence. The ability to isolate a large III-V crystalline region within the Si core is an important step towards embedding semiconductor light sources within infrared light-transmitting silicon optical fibre.

High performance micro-flow cytometer based on optical fibres.

Flow cytometry is currently the gold standard for analysis of cells in the medical laboratory and biomedical research. Fuelled by the need of point-of-care diagnosis, a significant effort has been made to miniaturize and reduce cost of flow cytometers. However, despite recent advances, current microsystems remain less versatile and much slower than their large-scale counterparts. In this work, an all-silica fibre microflow cytometer is presented that measures fluorescence and scattering from particles and cells. It integrates cell transport in circular capillaries and light delivery by optical fibres. Single-stream cell focusing is performed by Elasto-inertial microfluidics to guarantee accurate and sensitive detection. The capability of this technique is extended to high flow rates (up to 800 µl/min), enabling a throughput of 2500 particles/s. The robust, portable and low-cost system described here could be the basis for a point-of-care flow cytometer with a performance comparable to commercial systems.

Carbon nanotube mode-locked optically-pumped semiconductor disk laser.

An optically pumped semiconductor disk laser was mode-locked for the first time by employing a single-walled carbon nanotube saturable absorber. Stable passive fundamental mode-locking was obtained at a repetition rate of 613 MHz with a pulse length of 1.23 ps. The mode-locked semiconductor disk laser in a compact geometry delivered a maximum average output power of 136 mW at 1074 nm.

High-energy picosecond OPO based on PPKTP.

Output energy of 1 mJ is obtained for the 380 ps long idler pulses at 2800 nm from a short cavity singly resonant 500 Hz optical parametric oscillator employing PPKTP and a near-diffraction-limited, single frequency, sub-nanosecond pump source at 1064 nm.

Actively Q-switched all-fiber laser with an electrically controlled microstructured fiber.

Actively Q-switching of an all-fiber laser system is demonstrated. The active element is a polarization switch with nanosecond risetime based on a microstructured fiber with electrically driven internal electrodes. Optical feedback between two 100% reflectors is inhibited until a nanosecond current pulse Q-switches the laser. After a short optical pulse develops several roundtrips later, the fiber switch is turned off, removing the short optical pulse from the cavity through a polarization splitter. Pulses of 50 W peak power and approximately 12 ns duration are obtained with 400 mW pump power at 100 Hz.

All-fiber cavity dumping.

Cavity dumping of an all-fiber laser system is demonstrated. The active element is a pulse-picker with nanosecond rise time consisting of a microstructured fiber with electrically driven internal electrodes. The device is used for intracavity polarization rotation and dumping through a polarization splitter. The optical flux is removed from the cavity within one roundtrip and most of the amplified spontaneous emission, spiking and relaxation oscillation that follow during the gain recovery phase of the laser are blocked from the output signal.

Highly efficient temporally stable narrow linewidth cryogenically cooled Yb-fiber laser.

Cryogenic cooling is an effective way of increasing the efficiency in many solid-state lasers. In fiber lasers however, while the efficiency is increased, a reduced reabsorption in combination with reduced homogeneous broadening tends to broaden the linewidth, yielding a low spectral power density of the laser emission. In this work we lock a cryogenically-cooled Yb-doped fiber laser with a volume Bragg grating to overcome this problem and achieve a temporally stable narrow linewidth highly efficient laser. We extract 11.4-W of output power in spectral window of less than 0.4-nm with 14.5-W of launched pump light.

Improved photodarkening resistivity in ytterbium-doped fiber lasers by cerium codoping.

We show that the photodarkening resistivity of ytterbium-doped fiber lasers can be greatly improved by cerium codoping. It is suggested that the coexistence of the redox couple Ce(3+)/Ce(4+) in the glass provides means for trapping both hole- and electron-related color centers that are responsible for the induced optical losses in Yb-doped fiber lasers.

High-power, continuous-wave, second-harmonic generation at 532 nm in periodically poled KTiOPO(4).

We report efficient generation of high-power, cw, single-frequency radiation in the green in a simple, compact configuration based on single-pass, second-harmonic generation of a cw ytterbium fiber laser at 1064 nm in periodically poled KTiOPO(4). Using a crystal containing a 17 mm single grating with period of 9.01 microm, we generate 6.2 W of cw radiation at 532 nm for a fundamental power of 29.75 W at a single-pass conversion efficiency of 20.8%. Over the entire range of pump powers, the generated green output is single frequency with a linewidth of 8.5 MHz and has a TEM(00) spatial profile with M(2)<1.34. The demonstrated green power can be further improved by proper thermal management of crystal heating effects at higher pump powers and also by optimized design of the grating period to include thermal issues.

High-power and wavelength-tunable operation of an Er,Yb fiber laser using a volume Bragg grating.

Efficient high-power operation of double-clad Er,Yb-doped fiber lasers with fixed-wavelength and wavelength-tunable resonator configurations using volume Bragg gratings for wavelength selection are reported. The fixed-wavelength laser yielded a maximum output power of 103 W at 1552.6 nm with a linewidth of ~0.4 nm (FWHM) for a launched pump power of 290 W at 976 nm. The wavelength-tunable laser could be tuned from 1528 to 1550 nm with a linewidth of 0.2 nm (FWHM) and with output power in the range 30-38 W for a launched pump power of 120 W. The prospects for further improvement in performance are considered.

High-power linearly-polarized operation of a cladding-pumped Yb fibre laser using a volume Bragg grating for wavelength selection.

In this work a volume Bragg grating is used as a wavelength selective element in a high-power cladding-pumped Yb-doped silica fiber laser. The laser produced 138 W of linearly-polarized single-spatial-mode output at 1066 nm with a relatively narrow linewidth of 0.2 nm for approximately 202 W of launched pump power at 976 nm. The beam propagation factor (M(2)) for the output beam was determined to be 1.07. Thermal limitations of volume Bragg gratings are discussed in the context of power scaling for fiber lasers.

Efficient doubling of femtosecond pulses in aperiodically and periodically poled KTP crystals.

Efficient doubling of femtosecond pulses in periodically and aperiodically poled KTP crystals is demonstrated by employing the quasiphasematched frequency conversion technique. Conversion efficiencies as high as 60% were achieved using either aperiodically or periodically poled KTP nonlinear crystals in an extracavity, single-pass configuration using a diode-pumped femtosecond Yb:KYW laser as the pump source. The temporal characteristics of the frequency-doubled pulses as a function of focusing conditions in a "thick" nonlinear crystal regime have been investigated experimentally and pulses as short as 177 fs have been generated at around 520 nm under strong focusing conditions using a KTP crystal with aperiodic poling.

Frequency-doubling in femtosecond laser inscribed periodically-poled potassium titanyl phosphate waveguides.

Frequency doubling has been achieved in femtosecond-laser-inscribed single-mode waveguides written in two periodically-poled potassium titanyl phosphate crystals. A conversion efficiency of 0.22 %W(1) was obtained for first-order quasi-phase matching at 980 nm and an efficiency of 0.02 %W(-1) for third-order quasi-phase matching at 800 nm.

Active and passive hybrid mode-locking of a Nd:YVO(4) laser with a single partially poled KTP crystal.

We report on a Nd:YVO(4) laser mode-locked with a hybrid active and passive modulator consisting of a single partially poled KTP crystal. The periodically poled part provides negative cascaded Kerr-lensing, which together with intracavity soft and hard apertures gives passive modulation. Active phase modulation comes from the electro-optic effect by applying a voltage over the unpoled part of the crystal. The active modulation provides pulse lengths of about 95 ps, which initiate pulse shortening and self-sustained passive mode-locking by the cascaded Kerr effect. The repetition rate of the laser was 94 MHz and the output power was 350 mW, with a bandwidth of 0.235 nm and pulse lengths down to 6.9 ps.

Broadly tunable picosecond narrowband pulses in a periodically-poled KTiOPO(4) parametric amplifier.

A picosecond double-pass periodically poled KTiOPO(4) (PPKTP) noncollinear parametric amplifier that delivers tunable and narrowband outputs between 1.1 mum and 1.65 mum is reported. The seed source is an ultra-broadband emitting PPKTP collinear parametric generator, which is spectrally narrowed, from a bandwidth of 80 THz to about 0.3 THz, by a Fourier-filtering arrangement. A parametric gain of 70 dB is measured with preserved spectral bandwidth, resulting in a signal energy of 6.5 muJ at a pump energy of 60 muJ from the Ti:sapphire regenerative amplifier. The total energy-budget for the setup is less than 100 muJ with an optical-to-optical efficiency of 12 %. This particular system is scalable to even lower pump energies by tighter focusing arrangements.

Broadband nondegenerate optical parametric amplification in the mid infrared with periodically poled KTiOPO4.

We theoretically and experimentally demonstrate that the bandwidth in a nondegenerate optical parametric amplifier can be substantially increased by noncollinear interaction in a quasi-phase-matched single-periodicity structure. Broadband amplification of signals between 1540 and 1720 nm was realized in periodically poled KTiOPO4. The achieved signal bandwidth of 6.9 THz at 1680 nm is large enough to accommodate sub-100 fs optical pulses.

Generation of 2.8 ps pulses by mode-locking a Nd:GdVO4 laser with defocusing cascaded Kerr lensing in periodically poled KTP.

A Nd:GdVO4 laser mode locked by self-defocusing cascaded Kerr lens in PPKTP is presented. A strong pulse shortening mechanism is produced by the interplay of group velocity mismatch and the cavity design. The cavity had a repetition rate of 200 MHz and the mode-locked output power was 350 mW. Pulses as short as 2.8 ps were obtained with a bandwidth of 0.6 nm.