Depressed Cladding Waveguides Research Articles

The investigation of Er3+ fluorescence properties on femtosecond-laser-written LiNbO3 dual-line and depressed-cladding waveguides

The dual-line and depressed-cladding waveguides are fabricated by femtosecond laser (fs-laser) in Er3+, MgO co-doped nearly stoichiometric LiNbO3 (Er3+:MgO:NSLN) crystals. Single-mode propagation at 1064 nm can be supported inside both two waveguides, in which the 980 nm excited green up-conversion emission can be enhanced. The confocal micro-photoluminescence (µ-PL) spectroscopy has been performed to investigate the changes of Er3+ fluorescence properties in fs-laser modified regions. Under 980 nm excitation, the fluorescence intensity of green up-conversion and near-infrared (NIR) emission is reduced by ~5% in modified areas. However, there is an increase for intensity of 532 nm excited Er3+ emission line (at ~1550 nm). Our work paves a way for fabricating novel waveguide amplifiers as well as lasers, and studying physical mechanism on fs-laser-processing of Er3+:MgO:NSLN crystals.

From near-UV to long-wave infrared waveguides inscribed in barium fluoride using a femtosecond laser

Depressed-cladding waveguides (DCWs) of various sizes were inscribed in barium fluoride, allowing single-mode operation in the entirety of its transmission window (λ=0.2-12µm). Using femtosecond laser pulses at 515 nm, type I laser modified tracks were overlapped to form circular waveguides, whose cross-sectional geometry and numerical aperture were tailored to accommodate 0.405, 2.85, and 10.6 µm light. The mode profile, propagation loss, refractive index profile, and numerical aperture of the optimized waveguides were analyzed and compared with theory. We particularly demonstrate the challenging inscription of a large DCW for single-mode operation at 10.6 µm with propagation loss of <0.63dB/cm.

Laser inscribed waveguide optical isolators in iron-doped lithium niobate.

There is a growing need for optical isolators that do not require a magnetic field, especially for uses such as on-chip optical devices and cold atom physics. As one approach, we propose using waveguides in photorefractive materials, such as Fe:LiNbO3, as optical isolator devices due to their unique asymmetric transmission properties that allow low loss transmission in one crystal orientation and attenuation in the flipped orientation. We utilize ultrafast laser inscription to fabricate photorefractive depressed cladding waveguides in Fe:LiNbO3 along the crystal c axis to demonstrate the operation of Fe:LiNbO3 waveguide optical isolators. We show the ability to write transmission and reflection gratings into these waveguides that provide an isolation ratio of approximately 5000:1 per cm of path length.

Femtosecond laser direct writing of depressed cladding waveguides in Nd:YAG with "ear-like" structures: fabrication and laser generation.

Low-loss depressed cladding waveguide architecture is highly attractive for improving the laser performance of waveguide lasers. We report on the design and fabrication of the "ear-like" waveguide structures formed by a set of parallel tracks in neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal via femtosecond laser writing. The obtained "ear-like" waveguides are with more symmetric mode profiles and lower losses by systematically comparing the guiding properties of two kinds of normal cladding waveguide. Efficient waveguide lasers are realized based on the designed structure in both continuous wave and pulsed regimes. Combined the high-gain from cladding waveguide and special "ear-like" structure, a passively fundamentally Q-switched laser with the narrow pulse width and the high repetition rate has been obtained by using tin diselenide (SnSe2) as saturable absorber.

Direct laser writing of depressed-cladding waveguides in extremely low expansion lithium aluminosilicate glass-ceramics

We report on direct femtosecond laser writing of depressed-cladding optical waveguides in extremely low expansion lithium aluminosilicate glass-ceramics. The waveguide cladding was formed by space-selective vitrification of nanocrystals in laser-modified regions of glass-ceramics responsible for the negative refractive index change up to 0.005. The optical properties of the laser-written waveguides were evaluated in terms of near-field and far-field intensity distribution and propagation losses at the wavelength of 1064 nm. The possibility of single-mode waveguiding with the upper limit of propagation losses 2.4 or 2.7 dB/cm for vertically or horizontally polarized light, respectively, was demonstrated. The obtained results open up new frontiers in the laser-assisted fabrication of 3D optical waveguide circuits in the media with outstanding thermal stability.

Tunable violet radiation in a quasi-phase-matched periodically poled stoichiometric lithium tantalate waveguide by direct femtosecond laser writing

We report on violet-light generation using the femtosecond-laser written waveguides in periodically poled MgO:LiTaO3 crystal under conditions of third-order quasi-phase matching. Ten parallel depressed cladding waveguides are successfully fabricated with different grating periods in the same sample with fan-out χ(2) grating structures. These waveguides exhibit high optical quality with minimum insertion loss as low as 0.71 dB. Temperature and wavelength tuned second harmonic generation for different waveguides are demonstrated by using a tunable CW Ti sappire laser. Tunable violet second harmonic light has been generated with a single period over the range of 396 nm to 401 nm by varying the crystal temperature from 60 °C to 200 °C. At the quasi-phase matching temperature, 0.37 mW of violet light power at 397.2 nm is generated for a fundamental power of 336.7 mW, corresponding to a normalized conversion efficiency of 0.39%/(W·cm2). Our work contributes to designing tunable and efficient on-chip violet light sources based on femtosecond-laser written waveguides.

Second harmonic generation in femtosecond laser written lithium niobate waveguides based on birefringent phase matching

We report on second harmonic generation (SHG) through birefringent phase matching in femtosecond (fs) laser fabricated lithium niobate (LiNbO3) waveguides. Based on end-face coupling system, the guiding properties (mode profiles and propagation losses) of these waveguides are studied at 1064 nm. The effects of guiding properties on SHG of 1064 nm are investigated in detail. The maximum conversion efficiency of 0.87% is acquired in a 10-mm-long depressed-cladding waveguide at room temperature. Our results pave a way for constructing novel frequency converters based on fs-laser written LiNbO3 waveguides.

Direct Laser Written Waveguide in Tellurite Glass for Supercontinuum Generation in 2 μm Spectral Range

A depressed cladding waveguide composed of 36 parallel tracks is inscribed in high purity 70TeO2-22WO3-8Bi2O3 glass by femtosecond laser operating at 1030 nm. The Sellmeier formula for the refractive index of the investigated glass is obtained for the range of 1170–2350 nm. The module of the negative refractive index change in a track is found to increase with increasing wavelength in the range of 640–2000 nm. The waveguide shows single mode behavior in the wavelength range of 1700–2100 nm with mode field diameter (MFD) of 20 μm. The propagation loss is predominantly due to scattering in the waveguide. Spectrum broadening as large as 503 nm was demonstrated in a 14 mm long waveguide pumped by femtosecond pulses at 1950 nm. In the low input pulse energy region, the spectrum broadening is well described by generalized nonlinear Shrodinger equation (GNLSE) taking into account Kerr and the Raman nonlinearities under the assumption of constant MFD along the waveguide. Under higher energy pumping, an extra broadening was observed that is attributed to the focusing effect of Kerr lens in the waveguide mode.

Waveguide-integrated three-dimensional quasi-phase-matching structures

Nonlinear photonic structures with a modulated second-order nonlinearity are used widely for quasi-phase-matched parametric processes. Creating three-dimensional (3D) nonlinear photonic structures is promising but still challenging, since standard poling methods are limited to two-dimensional structures. Light-induced quasi-phase matching (QPM) can overcome this issue by a depletion of the second-order nonlinearity with focused femtosecond laser pulses. We report, to the best of our knowledge, the first integration of a 3D QPM structure in the core of a lithium niobate waveguide applying light-induced fabrication. Depressed-cladding waveguides and embedded QPM structures are fabricated by femtosecond laser lithography. The 3D capability is exploited by splitting the QPM gratings in the waveguide core into two or four parts, respectively. These monolithic nonlinear waveguides feature parallel multi-wavelength frequency conversion. Finally, we demonstrate a concept for second-harmonic beam shaping taking advantage of a helically twisted nonlinear structure. Our results open new avenues for creating highly efficient advanced QPM devices.

飞秒激光加工凹陷包层波导及其应用

飞秒激光加工凹陷包层波导及其应用

Fabrication of Tapered Circular Depressed-Cladding Waveguides in Nd:YAG Crystal by Femtosecond-Laser Direct Inscription.

Crystalline materials are excellent substrates for the integration of compact photonic devices benefiting from the unique optical properties of these materials. The technique of direct inscription with femtosecond lasers, as an advantage over other techniques, has opened the door to the fabrication of true three-dimensional (3D) photonic devices in almost any transparent substrate. Depressed-cladding waveguides have been demonstrated to be an excellent and versatile platform for the integration of 3D photonic circuits in crystals. Here, we present the technique that we have developed to inscribe tapered depressed-cladding waveguides with a circular section for the control of the modal behavior. As a proof of concept, we have applied the technique to fabricate structures in Nd:YAG crystal that efficiently change the modal behavior from highly multimodal to monomodal, in the visible and near infrared, with reduction factors in the waveguide radius of up to 4:1. Our results are interesting for different devices such as waveguide lasers, frequency converters or connectors between external devices with different core sizes.

Femtosecond Laser Written Depressed-Cladding Waveguide 2 × 2, 1 × 2 and 3 × 3 Directional Couplers in Tm3+:YAG Crystal.

Ion-doped crystal-based compact devices capable of beam splitting and coupling are enthralling for a broad range of classical and quantum integrated photonics applications. In this work, we report on the fabrication of depressed-cladding waveguide 2D 2 × 2, 1 × 2 and 3D 3 × 3 directional couplers in Tm:YAG crystal by femtosecond laser writing. The performances of the couplers are characterized at 810 nm, showing single-mode guidance, polarization independence, finely matched splitting ratios. These results open up new opportunities in the beneficial fabrication of 3D circuits and devices in crystals.

Ultrashort pulse propagation through depressed-cladding channel waveguides in YAG crystal: Spatio-temporal characterization

Inscription of optical waveguides by direct femtosecond laser irradiation has become a very versatile tool for the development of integrated photonic devices, such as waveguide lasers, frequency converters or photonic lanterns, among many others. The potential application of such devices for the control and manipulation of ultrashort pulses requires the precise knowledge of the temporal distortions that may be induced in the pulse propagation. Currently, research in this topic is scarce, and to our knowledge there is no previous experimental study on the spatio-temporal characterization at the output of waveguides inscribed inside crystals. Here, we have firstly fabricated depressed-cladding waveguides with different modal behavior in YAG crystal by direct femtosecond laser irradiation. Then, we implemented an experimental method based on the fiber coupler assisted spectral interferometry technique, that allows obtaining: (1) the temporal dispersion of a pulse at the output of an inscribed waveguide and, (2) full spatio-spectral and spatio-temporal characterization of the output of single-mode and multi-mode waveguides. Our results suggest that the main contribution to the pulse dispersion is due to the material dispersion. Moreover, we found that multimodal waveguides may induce an appreciable inhomogeneity in the temporal features of the pulses that needs to be taken into account in the design of complex devices.

Femtosecond laser direct writing of few-mode depressed-cladding waveguide lasers.

We report on mirrorless laser operation of Nd:YVO4 single- and double-cladding waveguides fabricated by femtosecond laser direct writing. Fundamental- (LP01) and high-order-mode (LP03, LP05) guiding and lasing have been observed in waveguides with different geometries and sizes. Double-cladding waveguides exhibit good guiding and lasing performance via inheriting advantages respectively from individual single cladding. As a result, continuous-wave lasing with a threshold as low as 59 mW is obtained, depending on the optical feedback provided only by Fresnel reflections at the waveguide end faces. By using few-layer graphene as saturable absorber, passively Q-switched operation in fabricated waveguides is also achieved.

Multi-watt, broadband second-harmonic-generation in MgO:PPSLT waveguides fabricated with femtosecond laser micromachining.

We demonstrate optical waveguides fabricated in periodically poled MgO-doped stoichiometric lithium tantalate crystals using an fs-laser direct-write process. Two different waveguide architectures were developed: depressed cladding and stress-induced waveguides. Our strain-optic simulations confirmed the guiding mechanism for either case. We demonstrate designs optimized for low propagation loss (0.52 dB/cm) for both fundamental (1050 nm) and second-harmonic wavelengths (525 nm). Low-power CW second-harmonic-generation studies show normalized efficiencies comparable to that of annealed reverse-proton-exchange waveguides in lithium niobate. High-power studies demonstrate second-harmonic power levels up to 8.5 W in a single-pass configuration, using a 1-nm bandwidth CW IR fiber laser as a pump.

Circular Depressed Cladding Waveguides in Mechanically Robust, Biocompatible nc-YSZ Transparent Ceramics by fs Laser Pulses

We report on the fabrication of depressed cladding waveguides inscribed by direct femtosecond laser pulses irradiation in transparent nc-yttria stabilized zirconia (YSZ) polycrystalline ceramics. The waveguides consisted of two different circular structures with diameters of 30 and 100 $\mu \text{m}$ , and different lengths. Optical characterization at 633 $\text{}$ and 1568 nm was performed. The obtained modal profiles show dependence on the structure and size of the waveguides. We estimate a laser-induced refractive index contrast of nearly 10 $^{-2}$ , larger than can be explained by induced residual stresses alone. We conjecture that laser interactions lead to localized physical damage, i.e., microvoids cracks and possibly a decreased degree of crystallinity in the laser processed zones, which is supported by Raman measurements. Our results suggest a promising potential use of nc-YSZ in photonic bio-applications.

Reconfigurable directional coupler in lithium niobate crystal fabricated by three-dimensional femtosecond laser focal field engineering

For crystals, depressed cladding waveguides have advantages such as preservation of the spectroscopic as well as non-linear properties and the capability to guide both horizontal and vertical polarization modes, but fabrication is always quite time consuming. In addition, it is usually difficult to couple modes propagating in different depressed cladding waveguides through evanescent field overlap, so it is often required to dynamically reconfigure photonic waveguide devices using external fields for classical or quantum applications. Here, we experimentally demonstrate the single-scan femtosecond laser transverse writing of depressed cladding waveguides to form a 2×2 directional coupler inside lithium niobate crystal, which is integrated with two deeply embedded microelectrodes on both sides of the interaction region to reconfigure the coupling. By focal field engineering of the femtosecond laser, we specially generate a three-dimensional longitudinally oriented ring-shaped focal intensity profile composed of 16 discrete spots to simultaneously write the entire cladding region. The fabricated waveguides exhibit good single guided modes in two orthogonal polarizations at 1550 nm. By applying voltage to the deeply embedded microelectrodes fabricated with the femtosecond laser ablation followed by selective electroless plating, we successfully facilitate the light coupling from the input arm to the cross arm and thus actively tune the splitting ratio. These results open new important perspectives in the efficient fabrication of reconfigurable complex three-dimensional devices in crystals based on depressed cladding waveguides.

Laser-written depressed-cladding waveguides deep inside bulk silicon

Laser-written waveguides created inside transparent materials are important components for integrated optics. Here, we demonstrate that subsurface modifications induced by nanosecond pulses can be used to fabricate tubular-shaped “in-chip” or buried waveguides inside silicon. We first demonstrate single-line modifications, which are characterized to yield a refractive index depression of ≈2×10−4 compared to that of the unmodified crystal. Combining these in a circular geometry, we realized 2.9-mm-long, 30-μm core-diameter waveguides inside the wafer. The waveguides operate in a single-mode regime at a wavelength of 1300 nm. We use near- and far-field imaging to confirm waveguiding and for optical index characterization. The waveguide loss is estimated from scattering experiments as 2.2 dB/cm.

Second harmonic generation of femtosecond laser written depressed cladding waveguides in periodically poled MgO:LiTaO3 crystal

We report on the fabrication of depressed cladding waveguides in periodically poled MgO doped LiTaO3 by using low-repetition-rate femtosecond laser writing, and their use for guided-wave second harmonic generation (SHG). The cladding waveguides exhibit different guiding performance along the extraordinary and ordinary polarizations. The temperature-dependent quasi-phase-matching (QPM) is realized to obtain SHG in the depressed cladding waveguides. The results show that the QPM temperature was dependent on the poling period and on the features of the cladding waveguides. The highest nonlinear conversion efficiency (0.74%W-1cm-2) was found in the waveguide fabricated with large scanning velocity (0.75 mm/s) and small radius (15 μm).

Femtosecond laser inscription of depressed cladding single-mode mid-infrared waveguides in sapphire.

Mid-infrared optical waveguides were inscribed in sapphire with femtosecond pulses at 515nm. We show that such pulses induce a smooth negative refractive index change allowing for the inscription of a depressed cladding waveguide by closely overlapping the corresponding type I modification traces. The resulting structure consists of a highly symmetrical, uniform, and homogeneous waveguide. The size and numerical aperture of the waveguides were tailored to achieve efficient transmission in the mid-infrared. Single mode operation at a wavelength of 2850nm and propagation loss of <0.37 dB/cm are reported for a 33mm long depressed cladding waveguide. Thermal annealing was performed, and the refractive index contrast was still preserved to 50% (i.e., Δn=∼2.5×10-3) up to 1400°C.