Optical waveguide fabrication and integration with a micro-mirror inside photosensitive glass by femtosecond laser direct writing

Photosensitive glass is a potentially important material for micro-fluidic devices that can be integrated with micro-optical components for biochemical analysis. Here, we demonstrate the fabrication of optical waveguides inside glass by femtosecond laser direct writing. The influence of the laser parameters on the waveguide properties is investigated, and it is revealed that the waveguide mode can be well controlled. The single mode is achieved at a low writing energy, while the multimode is achieved with increasing energy. In spite of a longitudinally elongated elliptical shape of the cross-sectional profile, the far-field pattern of the single-mode waveguide shows an almost symmetric profile. The measured propagation loss and the coupling loss are evaluated to be ∼0.6 dB/cm and ∼1.6 dB at a wavelength of 632.8 nm, respectively, under the conditions of 1.0–2.0 μJ pulse energy and 200–500 μm/s scan speed. The increased optical loss is associated with a higher waveguide mode at higher writing energy. Furthermore, the integration of waveguides and a micromirror made of a hollow microplate inside the glass is demonstrated to bend the laser beam at an angle of 90° in a small chip. The bending loss is estimated to be smaller than 0.3 dB. PACS 42.62.-b; 42.82.Cr; 82.50.Pt; 42.79.Gn; 42.81.Qb     

Tunable mid-infrared,high-energy femtosecond laser source for glyco-protein structure analysis

We have developed a 6–12 μm mid-infrared (MIR) femtosecond laser source for glyco-protein structure analysis. The MIR femtosecond laser pulses are generated by a differential frequency generation (DFG) configuration with a combination of Ti:sapphire based regeneratively amplified femtosecond laser pulses (780 nm, 160 fs, 1 mJ) and a β-BaB₂O₄ (BBO) based optical parametric amplifier (OPA). The MIR pulse energy exceeds 4.5 μJ, where a glyco-protein molecule has resonant absorption lines due to the vibrational–rotational transitions. The pulse width is estimated to be less than 1 ps according to the cross correlation measurement between the two OPA output pulses. Using the MIR femtosecond laser pulses, we demonstrated photo-dissociation of the sialyl Lewis X (sLeX) proton added ion, which is the first time to the best of our knowledge. PACS 42.65.Re; 42.62.-b; 42.60.-b; 42.65.-k; 87.50     

Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining

We report on the fabrication of hollow optical waveguides in fused silica using femtosecond laser micromachining. We show that in such hollow waveguides, high-intensity femtosecond laser beams can be guided with low optical loss. Our technique, which was established earlier for fabrication of optofluidic structures in glass, can ensure a high smoothness at the inner surfaces of the hollow waveguides and provide the unique capability of fabrication of hollow waveguides with complex geometries and configurations. A transmission of ∼90% at 633 nm wavelength is obtained for a 62-mm-long hollow waveguide with an inner diameter of ∼250 μm. In addition, nonlinear propagation of femtosecond laser pulses in the hollow waveguide is demonstrated, showing that the spectral bandwidth of the femtosecond pulses can be broadened from ∼27.2 to ∼55.7 nm.     

Investigations on ultrafast welding of glass–glass and glass–silicon

Using ultrafast laser radiation glass substrates are welded with glass and silicon plates. The pump beam is focused by a microscope objective with large NA=0.4 (beam diameter 4 μm) into the glass. After partial absorption of the optical energy, the glass is heated and melted. Procedures for high-quality welding of glass–glass and glass–silicon substrates with high-repetition ultra-fast laser radiation have been derived at the repetition rate 700 kHz. The dependencies of the dimension and geometry of the welding seam on scan velocity, repetition rate and pulse energy have been investigated defining a process window. Adding a noninterferometric technique for quantitative phase detection with the welding setup, the interaction zone of the welding seam for the welding partners glass–glass is detected. A change in refractive index is induced by heating and compression of the glass and has been detected by phase detection up to 2 μs after irradiation with 100 fs time resolution.     

Generation and application of high power femtosecond pulses in the vibrational fingerprint region

We present a novel high power femtosecond infrared laser source, based on a three-stage chirped-pulse amplification scheme. Owing to the high power output of the Ti:sapphire amplifiers, it becomes routinely possible to produce femtosecond infrared laser pulses in the wavelength region of 2.6–20 μm with minimum pulse energies of 15 μJ, to our knowledge roughly an improvement of an order of magnitude. With such pulses we have performed femtosecond second-order nonlinear optical surface spectroscopy in the fingerprint region. We have probed the skeletal modes of the first few monolayers of a polymer/air interface in a femtosecond vibrational sum frequency generation experiment. This development opens up new possibilities to investigate surface structures and dynamics of, e.g., organo-metallic compounds, proteins, and peptides. PACS 42.65.-k; 68.35.-Ja; 82.35.Ps     

Waveguides and quasi-waveguides based on pyrromethene 597-doped poly(methyl methacrylate)

The characteristics of amplified spontaneous emission (ASE) from asymmetric planar waveguides and quasi-waveguides consisting of thin films of poly(methyl methacrylate) incorporating lasing dye pyrromethene 597 deposited onto quartz and glass substrates, respectively, are investigated. The variable stripe length and moving constant stripe methods, together with appropriate theoretical expressions which take into account gain saturation and a simple model based on a four-level laser, allow for obtaining the net gain coefficients as a function of pump intensity, losses, pump thresholds for the onset of ASE, effective stimulated emission cross sections, pump saturation intensities, and saturation lengths. Net gain coefficients of up to 84±3 cm⁻¹ at a pump intensity of 404 kW/cm² (28 μJ/pulse) for quasi-waveguides and up to 59±6 cm⁻¹ at a pump intensity of 360 kW/cm² (25 μJ/pulse) for waveguides were obtained, with pump thresholds of 15.7 kW/cm² (1.1 μJ/pulse) and 6.3 kW/cm² (0.43 μJ/pulse), respectively. When waveguides 8 μm thick were irradiated with pulses of 200 kW/cm² at 10 Hz repetition rate, the ASE remained at 79% of its initial value after 1000 pump pulses in the same position of the sample. In quasi-waveguides 10 μm thick, the emission remained at 82% of the initial value under the same conditions.     

Interference microscopy of femtosecond laser written waveguides in phosphate glass

By focusing fs-laser radiation in the volume of a transparent material the refractive index can be changed locally, leading to 3-dimensional waveguiding structures. Waveguides are written in phosphate glass (IOG from Schott) at a depth of 100 μm below the surface. The pulse energy and the scan velocity are varied. For the first time the optical path difference caused by the waveguides and therefore the refractive index distribution of the waveguides and their cross sections are determined using interference microscopy. The optical path difference measured in the written structures and their cross sections is analyzed by a phase-shift algorithm. Thus, the refractive index distribution both along a line perpendicular to the waveguide and in the plane of a cross section is determined. The results are visualized as 2-dimensional graphics. Several regions of opposite sign of the refractive index change are observed in the cross sections of waveguides generated by femtosecond laser pulses. The number and the size of these regions are increasing with increasing pulse energy and decreasing scan velocity.     

High-efficiency optical compression of Ti:sapphire laser pulses at 800 nm using a silver-coated hollow fiber

A 50 cm silver coated hollow fiber with inner diameter of 250 μm and filled with argon has been used to compress optical pulses from a Ti:sapphire laser at 800 nm. Input pulses with energy of 250 μJ and duration of 110 fs were used and compressed pulses with energy of 220 μJ and duration of 20 fs were generated by using a prism compressor. Numerical and experimental results are compared. There is good agreement between the measured beam diameters of the hollow-fiber output pulse and the calculated values obtained from propagation of the HE₁₁ mode into free space. For comparison, a similar uncoated fused-silica hollow fiber was also used to obtain 20 fs compressed pulses with an energy of 190 μJ. Received: 7 September 2002 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-780/492-1811, E-mail: mohebbi@ee.ualberta.ca     

Fabrication of polaritonic structures in LiNbO<Subscript>3</Subscript> and LiTaO<Subscript>3</Subscript> using femtosecond laser machining

Fabrication of patterned materials in ferroelectric LiNbO₃ and LiTaO₃ crystals using femtosecond laser micromachining is presented and discussed. Damage feature sizes in the 10–100 μm range were achieved using 800-nm, 50-fs (FWHM) ultra-fast laser pulses with energies ranging from 10 μJ up to 350 μJ. Fabrication of polaritonic devices such as waveguides, resonators, focusing reflectors, diffractive and dispersive elements, photonic band gap materials, and other microstructures is demonstrated. PACS 77.84.Dy; 42.62.Cf; 71.36.+c     

Optical waveguide writing inside Foturan glass with femtosecond laser pulses

We investigated the femtosecond laser writing of optical waveguides inside Foturan glass at various pulse energies and focusing depths. An optimal waveguide fabricated solely by femtosecond laser irradiation showed a refractive index modulation of ∼1.7×10^-3 and a minimum transmission loss of ∼0.80 dB/cm. This type of waveguide had lower transmission loss than those fabricated by a hybrid process of femtosecond laser exposure and following thermal treatment. An optical splitter was also fabricated at high pulse energy. PACS 42.65.Re; 42.82.Et; 42.70.Gi     

Femtosecond laser writing of optical waveguides with controllable core size in high refractive index glass

Optical waveguides have been produced by femtosecond-laser writing in a high linear and nonlinear refractive index glass (SF57). Light guiding occurs nearby a central damaged zone due to the collateral densification caused by shockwaves generated in the focal volume. High pulse energies are required to induce a modified region capable of supporting a guided mode. An alternative processing method, based on using multiple structures, has been successfully used for the production of waveguides with controllable core size at low energies. PACS 42.82.Et; 42.65.Re; 42.70.Ce; 81.20.–n     

Pearl-chain waveguides written at megahertz repetition rate

Waveguides in the form of connected pearls, written with sub-30 fs pulses centered at 800 nm at a repetition rate of 10 MHz, show high changes of the refractive index and, as a consequence, simple in-coupling. The value of the refractive index modification of these waveguides is as high as 10^-2, optical losses are 6 dB/cm (at 1.55 μm) and the mode field diameter is 8 μm at 670 nm. PACS 42.62.-b; 87.80.Mj; 42.82.Et     

Femtosecond laser direct writing of multiwavelength Bragg grating waveguides in glass

Novel Bragg grating waveguide structures have been fabricated in bulk borosilicate glass through a type II photosensitivity mechanism driven by single femtosecond laser pulses. Low-loss single-mode waveguides and narrow-linewidth Bragg gratings were generated simultaneously by forming an array of refractive index voxels in a single laser scan. Laser pulse duration is shown to significantly affect the grating strength and waveguide loss. Bragg wavelengths, defined by the periodicity of laser-modified volumes, were fully controlled by the sample scan speed to provide resonances anywhere in the 1200-1620 nm telecommunication bands. Four linear Bragg filters with distinct resonant wavelengths are presented that define the first demonstration of laser writing of multiple-wavelength and cascaded Bragg grating waveguides in a single process step.     

All organic DFB laser enhanced by intermediate high refractive index polymer layer

Performances of a distributed feedback (DFB) organic dye laser were enhanced by introducing an intermediate high refractive index layer of poly(N-vinylcarbazole) (PVCz) laminated between a glass substrate and a laser dye doped active polymer layer. The active layer is consisted of rhodamine 6G and cellulose acetate (CA). Introduction of an intermediate layer allows a single mode lasing. Slope efficiencies of 2.2 and 4.7% and thresholds of 0.3 and 0.14 mJ/cm²/pulse were measured for the waveguides with 1.7 and 3.4 μm active layers, respectively. Furthermore, permanent relief grating on an intermediate layer gave rise to the reduction of the threshold. With increasing in amplitude of the relief grating from 20 to 45 nm, lasing threshold was reduced from 0.18 to 0.04 mJ/cm²/pulse for the waveguide with 1.7 μm active layer. The slope efficiency increased from 3.5 to 4.2%.     

Femtosecond third-harmonic generation:. self-phase matching through a transient. Kerr grating and the way to ultrafast computing

The third harmonic of 810-nm 100-fs pulses at 130 μJ is generated very efficiently when ultrashort pulses from two noncollinear beams interfere in an optical medium to create an instantaneous transient grating via the optical Kerr effect. The grating couples two pathways for third-harmonic generation, each taking two photons from one beam and one photon from the other beam, respectively. The coupling enables self-phase matching in the complete process, resulting in a conversion efficiency of ≈3%. Scattering an independent beam at the transient grating confirms a lifetime limited by the pulse duration, with a reaction on the order of one optical cycle. Using the second harmonic of a Ti-sapphire laser at 405 nm, it is shown that the generation of the transient Kerr grating is a general feature, requiring less than 20 μJ/pulse. By introducing a third femtosecond beam we are able to emulate various digital logic units with femtosecond response. Received: 16 October 2001 / Published online: 6 June 2002     

Sub-nanosecond single-frequency 10-kHz diode-pumped MOPA laser

A single-axial-mode, passively Q-switched (PQS) diode-pumped Nd:YAG laser, generating a diffraction-limited beam train of ≈40–60 μJ, ∼500-ps pulses with adjustable repetition rate in the range 1–10 kHz, was efficiently amplified by a single side-pumped Nd:YVO₄ bounce amplifier. After double-pass amplification, ≈1-MW pulse peak power with 577-ps duration and 545-μJ energy was achieved, still maintaining diffraction-limited beam performance. The average output power was 5.45 W at 10 kHz, corresponding to 13% extraction efficiency. The high brightness of this laser system seems ideal for nonlinear optics and some particular laser processing applications.     

Yb-doped strictly all-fiber laser actively Q-switched by intermodal acousto-optic modulation

We show an actively Q-switched ytterbium-doped strictly all-fiber laser. Cavity loss modulation is achieved in a tapered optical fiber by core-to-cladding mode-coupling induced by travelling flexural acoustic waves. When the acoustical signal is switched-off, the optical power losses within the cavity are reduced, and then a laser pulse is emitted. Trains of Q-switched pulses were successfully obtained at repetition rates in the range 1–10 kHz, with pump powers between 59 and 88 mW, at the optical wavelength of 1064.1 nm. Best results were for laser pulses of 118 mW peak power, 1.8 μs of time width, with a pump power of 79 mW, at 7 kHz repetition rate.     

Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy

Using tightly focused femtosecond laser pulses of just 5 nJ, we produce optical breakdown and structural change in bulk transparent materials and demonstrate micromachining of transparent materials by use of unamplified lasers. We present measurements of the threshold for structural change in Corning 0211 glass as well as a study of the morphology of the structures produced by single and multiple laser pulses. At a high repetition rate, multiple pulses produce a structural change dominated by cumulative heating of the material by successive laser pulses. Using this cumulative heating effect, we write single-mode optical waveguides inside bulk glass, using only a laser oscillator.     

The generation of intense, transform-limited laser pulses with tunable duration from 6 to 30 fs in a differentially pumped hollow fibre

We have investigated experimentally the energy transmission and spectral broadening of 30-fs, 700-μJ laser pulses in a neon-filled, 250-μm inner diameter hollow fibre. We implement a differentially pumped fibre, where a vacuum is maintained at the fibre entrance, and compare this to a statically filled fibre. We obtain significantly higher transmission and increased spectral broadening in the differentially pumped case due to a reduction of ionisation defocusing at the fibre entrance. This arrangement provides a method for the generation of near-transform- limited pulses with smoothly varying pulse duration whilst maintaining constant pulse energy, by simple adjustment of the gas pressure. Compression of ∼450-μJ pulses from the differentially pumped fibre to a duration of 6.5 fs has been achieved for pulses with spectra spanning 650–900 nm, by use of negatively dispersive chirped mirrors. PACS 42.65.Re; 42.65.Jx; 42.65.-k     

Chalcogenide glasses as a medium for controlling parameters of ultrashort IR pulses: II

The propagation of ultrashort optical pulses in planar chalcogenide glass waveguides with uniform or periodic claddings is considered. In comparison with silica waveguides a structure based on chalcogenide glass provides higher nonlinearity. This material has a high normal dispersion in the wavelength range of 1.5–3.5 μm. Periodic waveguide structures can be used to compensate for this dispersion. High-intensity laser pulses propagating in these structures undergo spectral broadening. The influence of the phase self-modulation of a pulse on its spectrum and width is analyzed using numerical simulations.