Femtosecond laser micromachined optical waveguides in LiTaO3 crystal

By using femtosecond laser micromachining, optical wave guides in both depressed cladding and dual‐line configurations have been produced in LiTaO₃ crystal. The guiding properties and the thermal stability have been investigated for both geometries, which exhibit different performance. Depressed cladding waveguides support guidance along both extraordinary and ordinary index polarizations, while dual‐line waveguides support only extraordinary index polarization. Thermal annealing has been proved to be an effective method to reduce the propagation losses. For the cladding waveguide, the lowest propagation loss was as low as 0.38 dB/cm after the annealing treatment at 400 °C. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Design and comparison of single-mode planar and ridge type electro-optic waveguide modulators

Electro-optic waveguide modulators utilizing phase retardation of two orthogonally polarized optical modes in LiNbO₃ and LiTaO₃ waveguides have been designed taking into consideration the optical field distribution in the waveguides and the electrical properties of the electrodes. The analysis has revealed that a driving-voltage to frequency-bandwidth ratio of 1 V/GHz is attainable at the wavelength of 1.05 m using presently available embedded and ridge waveguides. Improvement in waveguide fabrication techniques may reduce the ratio by at least a half. Thus, LiNbO₃ and LiTaO₃ waveguide modulators are considered to be promising candidates for practical application to single-mode optical-fibre transmission systems of higher than 1 Gbit/s.     

Optical interconnection between different spatial modes using double phase conjugate mirror with polarization alignment system

We propose a fundamental interconnection method using a polarization alignment system for waveguides having different spatial modes. In addition, as an example for the verification of the fundamental operation, we demonstrate an interconnection between a photonic crystal fiber and a laser that have obviously different spatial modes. The polarization alignment system operates synergistically with a self-written waveguide formed with a double phaseconjugate mirror. This technique enables us to interconnect a photonic crystal fiber with a laser source without complicated and time-consuming optical alignment. In this method, although it is not necessary to perform an external control for interconnection, the waveguide most suitable for connection is formed autonomously in a Sn₂P₂S₆:Sb crystal developed for this purpose. There was a marked reduction in the polarization dependence of coupling efficiency, compared with that observed using a stand-alone double phase-conjugate mirror.     

Coupled mode analysis of light modulation in dielectric waveguides

A detailed analysis of electrooptic light modulation in optical waveguides is presented. Several important problems of a waveguide modulator, such as the difference of waveguide axes from crystalline electrooptic ones, the distribution of transverse and longitudinal field components of light modes, and the traveling-wave property of the modulating field, are discussed. The analysis is based on the coupled mode theory, regarding the modulation as the coupling among sidebands of unperturbed waveguide modes. The coupled mode equation is derived for the modulation in optical waveguides. It can be solved if the normal modes of the waveguide are given. Actually the equation is solved for the modulation in dielectric slab waveguides and the mechanism of modulation is discussed. The results of the analysis are applied to designing two types of waveguide modulators. In an example (10.6 μm modulator with a GaAs slab waveguide) a new efficient crystal orientation is found. The calculated phase retardation with this orientation is 0.13 rad/(V·cm) with a 1 μm thick slab. Another example of a 0.633 μm modulator using a LiTaO₃ crystal as a substrate is also described.     

Second-order optical non-linearity of proton exchanged lithium tantalate waveguides

A detailed correlation between the fabrication conditions, crystallographic phase state of H_xLi_1-xTaO₃ waveguides and second-order optical non-linearity has been investigated by using reflected SHG measurements from the polished waveguide end face. The non-linearity, strongly reduced after the initial proton exchange, is found to be restored and even increased after annealing. However, this apparent increase in the non-linearity is accompanied by a strong degradation of the quality of the SHG reflected beam in the region of the initial as-exchanged waveguide due to beam scattering. The high temperature proton exchange technique has been shown to produce high-quality α-phase waveguides with essentially undegraded non-linear optical properties. There is no phase transition when the α-phase waveguides are fabricated by direct exchange. This phase presents the same crystalline structure as that of LiTaO₃ and maintains the excellent non-linear properties of the bulk material. The results obtained are important for the design, fabrication and optimization of guided-wave non-linear optical devices in LiTaO₃. Received: 21 May 2001 / Published online: 23 October 2001     

Ultrafast laser waveguide writing: Lithium niobate and the role of circular polarization and picosecond pulse width

For the first time to our knowledge, ultrafast laser writing has generated room-temperature stable guided-wave optics in bulk lithium niobate for the telecommunication spectrum. Among a seven-dimensional parameter space for waveguide optimization, two frequently overlooked parameters, pulse duration and polarization, were found to be key in overcoming undesired nonlinear optical responses imposed by this material. Single-mode waveguides were best formed with circularly polarized light having a relatively long pulse duration of approximately 1.0 ps. The waveguides were highly polarization dependent and guided in both telecommunication bands near 1300 and 1550 nm, exhibiting losses as low as 0.7 dB/cm.     

Manufacturing of Nd:Gd3Ga5O12 ridge waveguide lasers by pulsed laser deposition and ultrafast laser micromachining

Laser radiation is used both for the deposition of the laser active thin films and for the micro structuring to define wave guiding structures for the fabrication of waveguide lasers. Thin films of crystalline and amorphous neodymium doped Gd₃Ga₅O₁₂ are grown on single crystal yttrium aluminium garnet by pulsed laser deposition using excimer laser radiation.Manufacturing of the laser active waveguides by micro structuring is done using femtosecond laser ablation of the deposited films. The structural and optical properties of the films and the morphology of the structured waveguides are determined in view of the design and the fabrication of compact and efficient diode pumped waveguide lasers. The resulting waveguides are polished, provided with resonator mirrors, pumped using diode lasers and the waveguide lasers are characterized. The spectroscopic properties of the amorphous waveguide are investigated and an infrared waveguide laser is demonstrated. To our knowledge, there have been no reports by other groups of the successful operation of a structured waveguide laser fabricated by this technique or of a waveguide laser made from amorphous neodymium doped Gd₃Ga₅O₁₂.     

Frequency Doubling of a Laser Diode Using a Domain-Inverted LiTaO3Waveguide with a Monolithic Bragg Reflector

We report wavelength stabilization of a laser diode using a highly efficient distributed Bragg reflector (DBR) grating formed on a LiTaO₃ quasi-phase-matched second harmonic generation (QPM-SHG) device. Fabricating the second-order DBR on the LiTaO₃ waveguide, the reflectivity of 90% and FWHM wavelength bandwidth of 0.2 nm were obtained. By stabilizing the oscillation wavelength of the laser diode, 3.1 mW of blue light was generated in the QPM-SHG device with the monolithic grating.     

Metastable phases in HxLi1-xTaO3 waveguide layers and pure LiTaO3

The formation of high-temperature phases in low-doped H:LiTaO₃ waveguide layers in Z-cut LiTaO₃ has been observed both by refractive-index and IR-spectra measurements. This permits us to correlate the index jumps to the changes of the OH bonds in the crystal lattice. Reversible phase transitions were detected in the temperature interval T=50–200 °C over a wide range of hydrogen content including as-grown LiTaO₃. The high-temperature phases are metastable close to room temperature. This was demonstrated by tracing the time evolution of the refractive-index change. It was shown that the high-temperature phases are responsible for the long-term refractive-index instabilities in both H:LiTaO₃ waveguides and virgin LiTaO₃. Received: 7 May 2001 / Revised version: 10 August 2001 / Published online: 30 October 2001     

Photorefraction and anisotropic light scattering in LiNbO3-Fe crystals

By holographic recording in LiNbO₃-Fe and LiTaO₃-Fe crystals a new light scattering effect has been observed with an optical indicatrix along the optical axis. The kinetics of the light scattering depends on the intensity, wavelength as well as the polarization of the incident light The holographic volume grating is created by the interference of an incident light and light scattered by crystal inhomogeneities.     

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     

WKB analysis of guided and semileaky modes in graded-index anisotropic optical waveguides

WKB analysis of graded-index anisotropic optical waveguides is briefly presented. The angular dependencies of the propagation characteristics of both guided and semileaky modes are calculated numerically for the out-diffused and in-diffused LiNbO₃ waveguides with parabolic index profile. The results for the out-diffused waveguide are compared with those obtained from the rigorous theory and the applicability of the WKG method is discussed for semileaky mode loss calculations. The angular dependence of the propagation constants of a TiO₂-diffused LiNbO₃ waveguide is demonstrated experimentally, too.     

Mechanism and process of fabricating fluorinated polyimide optical waveguide by CO2 laser direct-writing

Fluorinated polyimide waveguides were fabricated by CO₂ laser direct-writing. The poly(amic acid) micro-region irradiated by CO₂ laser beam was studied with FT-IR micro-spectroscopy. The FT-IR spectra implied that the laser induced thermal-imidized polyimide was of optical anisotropy, and the imidization degree of exposed micro-region increased with the rising of output laser power. The increased aspect ratio of waveguide and smoothness of surface can be achieved by increasing the pre-cured temperature (below 120°C) and writing speed, and optimizing laser power and the distance between the lens and the annular aperture. The guided light was clearly confined to the core of the fabricated waveguide, which means this technique can be used for fluorinated polyimide waveguide fabrication.     

Fast, reconfigurable light-induced waveguides

Fast and reconfigurable one-dimensional waveguides are produced by interband photorefraction. The index barriers are induced by ultraviolet light. The guiding of a red laser beam with a full width at half-maximum of 15 mum is demonstrated. Buildup and decay times of the waveguide in pure KNbO(3) are of the order of 100 mus and 10 ms, respectively. The intensity of the guided light has no influence on the guiding properties over the range from 4 mW/cm(2) to 200 W/cm(2) . By reconfiguration of the waveguide, deflection angles of as much as 1.75 deg inside the crystal are achieved.     

Morphology and lightguide property investigation of a high quality DAST single crystal

A high quality 4^′-dimethylamino-N-methyl-4-stilbazolium tosylate single crystal with dimension of 7×7×1 mm³ was grown in this study. The polarized optic microscopy photograph showed that the surface of this crystal was ordered and flat. Three-dimensional luminance image analysis demonstrated that the intensity of a guided laser light passing through the single crystal along its molecular structure alignment is stronger than that guided along a standard 8.5/125 μm silica optic fiber. The diameter of the guided light and the space between the two guided lights through the single crystal were estimated at around 25 and 65 μm, respectively. The FTIR spectrum implied that the single crystal is more suitable for use as a communication window than the disordered polycrystalline samples. The result reveals that without further processing the single crystal can be used as a waveguide and is a promising candidate for electro-optic modulators and other active optical devices.     

Fs-Laser structuring of ridge waveguides

Thin films made by PLD from Er:ZBLAN and Nd:Gd₃Ga₅O₁₂ are micro machined to form optical wave guiding structures using Ti:sapphire and Yb:glass fiber laser radiation. For the manufacturing of the ridge waveguides grooves are structured by ablation using femtosecond laser radiation. The fluence, the scanning velocity, the repetition rate, and the orientation of the polarization with respect to the scanning direction are varied. The resulting structures are characterized using optical microscopy and scanning electron microscopy. Damping and absorption coefficients of the waveguides are determined by observing the light scattered from the waveguides due to droplets in the thin films and the surface roughness of the structured edges. To discriminate between damping due to droplets and the structured edges, damping measurements in the non-structured films and the structured waveguides are performed. Ridge waveguides with non-resonant damping losses smaller than 3 dB/cm are achieved. Due to the high repetition rate of the Yb:glass fiber laser, the manufacturing time for one waveguide has been decreased by a factor of more than 100 compared to earlier results achieved with the Ti:sapphire laser.     

会聚激光扫描铌酸锂晶体写入光波导时的最佳曝光间距

对利用激光精细加工技术在铌酸锂(LiNbO₃)晶体中写入光波导时的最佳曝光间距进行了详细地理论分析和实验研究.通过数值求解简化后的光折变动力学方程组，对会聚激光束沿不同方向扫描晶体时的最佳曝光间距进行了详细的数值模拟.结果表明：最佳曝光间距的选取与写入光束的扫描方向无关；当聚焦激光束平行于光轴扫描晶体时，得不到波导结构，但可以采用“三明治”辐照方式得到对称的折射率分布.采用会聚的绿激光束扫描掺铁铌酸锂(LiNbO₃：Fe)晶体进行了实验研究，晶体中的光致折射率变 关键词： 激光精细加工 最佳曝光间距 光波导 3晶体')" href="#">LiNbO₃晶体     

Proton-exchange influence on the polar phonons of LiTaO3 optical waveguides

Refractive index change and Raman scattering of Z-cut proton-exchanged LiTaO₃ optical waveguides with different composition have been studied. Probing of the waveguide depth was carried out by using micro-Raman spectroscopy in order to distinguish layers with different phase states. Varying the proton-exchange and post-exchange annealing regimes, two phases –α and β– in addition to the pure LiTaO₃, were clearly observed. The behaviour of the A ₁ vibrations of the TaO₆ groups was found to be very sensitive to the phase changes. Using data from waveguide Raman spectra, normal and oblique E(TO, LO) phonons were sorted and their properties were followed in the α and β phases. The OH-stretchings are considered to correspond to dipoles turned out of the oxygen planes. Received: 9 July 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Optical waveguide engraving in a LiNbO3 crystal fiber

This paper reports the engraving of an optical waveguide inside a LiNbO₃ crystal fiber via the photorefractive effect and an optical vortex. Afterwards this waveguide was successfully tested and its properties evidenced.     

Pulsed-laser deposition of a Nd:KGd(WO4)2 waveguide in Ar and O2 environments

Thin-film Nd-doped potassium gadolinium tungstate (Nd:KGW or KGd(WO₄)₂) waveguides are deposited on (1102)sapphire or (100)YAG substrates by KrF laser ablation of potassium-rich ceramic targets in Ar and O₂. The dependence of the stoichiometry, crystallinity and waveguide properties of the films on the environmental gas pressure and substrate temperature is studied. Highly textured crystalline (110) KGW films are grown. An optical waveguide loss as low as 3 dB/cm is obtained for the films grown in Ar. The as-grown films are optically active. Upon annealing at 900 °C in air, the crystallinity and the properties of the emission spectra are dramatically improved. PACS 81.15.Fg; 42.70.Hj; 78.20.Ek