Maximum Index Change Research Articles

Direct UV written integrated planar waveguides using a 213 nm laser.

We present the first demonstration of integrated waveguides in planar silica devices fabricated using direct UV writing with 213 nm laser light. Waveguides were produced with different writing fluences and the NA and MFD of each were measured. Single mode waveguides were achieved at fluence values one-tenth that typically required when operating with a 244 nm laser, allowing for more rapid fabrication. A maximum in-plane index change of 2.4 ×10-3 for a writing fluence of 5 kJ cm-2 was estimated from NA measurements. Finally cutback measurements were performed and a propagation loss of 0.42 ± 0.07 dB cm-1 was directly measured, though losses as low as 0.2 ± 0.03 dB/cm are indicated through calculations.

Femtosecond laser direct inscription of mid-IR transmitting waveguides in BGG glasses

A detailed investigation of the photo-inscription of waveguides in barium gallo-germanate (BGG, BaO, GeO2, Ga2O3) glass is presented. Upon irradiation of BGG glass samples of different contents of germanium dioxide with a femtosecond laser pulse train, positive refractive index changes are produced over a wide range of exposure conditions. Waveguides with a controllable diameter ranging from 4 to 35 µm and a maximum index change up to 10−2 were inscribed. A glass sample with custom molecular composition was purified to remove hydroxyl ions and reduce the strong absorption band near 3 µm. A careful tailoring of the writing conditions allowed for the inscription of low-loss waveguides supporting only two transverse modes at the wavelength of 2.78 µm. An upper bound for the propagation losses of 0.5 ± 0.1 dB/cm was determined, showing the great potential of the BGG glass family for the fabrication of core waveguides operating in the 2-4 µm spectral range. Our results actually open a pathway towards the integration of mid-IR photonic devices based on the BGG glass family.

Study of the photosensitivity of GeS binary glasses to 800nm femtosecond pulses

We present the first study of the photosensitivity of GeS binary glasses in response to irradiation to femtosecond pulses at 800 nm. A maximum positive refractive index change of 3.5x10(-3) is demonstrated with the possibility to control the waveguide diameter from ~8 to ~50 µm by adjusting the input pulse energy. It is also demonstrated that under different exposure conditions, a maximum negative index change of -7.5x10(-3) can be reached. The present results clearly illustrate the potential of this family of glasses for the fabrication of mid-infrared waveguides.

Fabrication of dynamic planar waveguide array in SBN:Cr crystals by structure-light irradiation

A planar waveguide array, whose structure is similar to a volume phase grating,can be fabricated in a congruent SBN:Cr (Sr0.61Ba0.39Nb2O6:Cr) crystal by irradiation of two-beam interference pattern,and,simultaneously, by applying an appropriate direct electrical field along the c-axis of the crystal. The transversal index profile of the waveguide array is measured in real-time using a Mach-Zehnder interferometer,and the maximum index change in the waveguide region is close to 10-4. The initial guiding tests show that it is feasible to write a dynamic planar waveguide array in the SBN:Cr crystal by using irradiation of period structure-light,due to the rapid response property of SBN:Cr crystals. And the dynamic control of the waveg uide can be realized by switching the writing lights on or off,or adjusting the angle of the two interferent beams. Moreover,the waveguide parameter such as the index profile in the waveguide zone can be effectively controlled by varying the external electric field.

Photosensitive GeO2-SiO2 films for ultraviolet laser writing of channel waveguides and bragg gratings with Cr-loaded waveguide structure.

Irradiation with intense ultraviolet laser pulses induced a large refractive-index change in 30GeO2-70SiO2 waveguide-grade thin films prepared by the plasma-enhanced chemical vapor deposition method, which contained a large amount of photoactive Ge2+ defects. The maximum index change in the as-deposited films by KrF and XeF excimer laser irradiation was estimated to be 1.2 x 10(-3) and 0.28 x 10(-3), respectively. These results clearly indicate that the photorefractivity of GeO2-SiO2 glasses is due to a Ge2+ defect in origin. The channel waveguide and the planar Bragg gratings were directly written in the photoactive Ge(2+)-enriched GeOs-SiO2 thin films by pulsed ultraviolet laser irradiation with a Cr-metal-loaded-type waveguide structure.

Optical gas sensing by evaluating ATR leaky mode spectra

The angular reflectivity spectrum of a glass-silver-polymer-air multilayer structure is investigated for gas-sensor applications. Minima of reflectance are caused by surface plasmon polariton (SPP) excitation at the silver-polymer interface and by attenuated total reflection (ATR) leaky modes coupled into the polymer film. Changes in refractive index of the polymer (here Teflon ®AF) due to absorption of vapour molecules in the surrounding atmosphere lead to line shifts of the resonances in the reflectivity spectrum. For several vapours (benzene, toluene, xylene, etc.) maximum index changes of about Δ n = 0.02 are observed with an accuracy better than 0.0001. A dynamic and more accurate measurement is obtained by recording the reflected intensity as a function of time at a fixed observation angle, the angle of the SPP resonance. The recorded intensity is then a measure for the refractive-index changes of the Teflon ®AF coating. In a modified set-up for application purposes, a divergent reflected beam is recorded by a CCD camera and the line shifts of two leaky modes are monitored simultaneously with a time resolution of a few microseconds. On-line evaluation of these shifts allows the time-resolved observation of the refractive-index changes. A first miniaturization step is achieved by laser-beam patterning of PMMA substrates.

Characterisation of planar waveguides formed by proton implantation in lithium iodate

Abstract Permanent optical waveguides were produced by proton implantation in lithium iodate crystals. After implantation the samples present a good optical quality. No annealing procedure is required. The index profiles have been measured by dark modes spectroscopy and the ordinary refractive index has been obtained with an inverse WKB method. Moreover, the optical barrier is relevant with a maximum index change of about 0.04. The effects of ion fluence and beam energy have been investigated on z-cut samples.

Nonelectrooptic nonlocal photorefractive effect in a polymer composite

The grating spacing dependence of photorefractive diffraction efficiency in an organic polymer composite is presented and discussed. The analysis of the grating spacing dependence of the refractive index modulation with the standard model of photorefractive grating dynamics is shown to lead to inconsistencies in determination of the maximum attainable index change. It is shown that the effects resulting from orienting of the composite’s optically second-order nonlinear chromophores in the periodic local field affect the formation of the refractive index grating.

Radial gradient-index glass prepared by the sol-gel process

Radial gradient-index glass has been prepared by the sol-gel process using tetrabutoxytitanium as the source of index- modifying cation. Two systems have been studied. Wet gels were obtained by first partially hydrolyzing tetraethoxysilane (TEOS), then adding tetrabutoxytitanium alone or together with Y(NO 3) 3 into the mixture and using another part of water to hydrolyze the salts thoroughly and make it gel. The wet gels were then put into a H 2SO 4 bath to partially leach out the index-modifying cations and form the concentration gradient. Through drying and sintering the gels were turned into transparent and colorless glass rods of about 4.5∼7.5 mm in diameter. The resulting refractive index profile of the glass has been measured and the maximum index change from the center to the perimeter was about 0.03.

Potassium- and silver-double-ion-exchanged slab glass waveguides: characterization and modeling

Double-ion-exchanged glass waveguides are made first by a potassium-ion exchange followed by a silver-ion exchange. The inverse WKB method is used to determine the index profile. It is shown that a combination of a truncated quadratic function and a complementary error function represents the refractive index best. A direct WKB method is employed to find the maximum index change and the diffusion coefficient of the second exchange process. The concentration of silver, potassium, and sodium ions are measured by an x-ray photoelectron spectroscopy technique. The results agree well with the index profile when a quadratic function is used.

Effect of zinc impurity-induced disordering on the refractive index of GaAs/AlGaAs multiquantum wells

We report on the variation of the refractive index of a GaAs/AlGaAs multiquantum well of well width 6.5 nm and barrier width 19 nm, introduced by zinc impurity-induced disordering. We employ a structure consisting of several uncoupled, multiquantum well ridge waveguides with tapered disordering across the transverse direction. The refractive index changes have been deduced as a function of the interdiffusion length of Ga/Al by the use of an interference technique. We measured a maximum index change of 0.083 and 0.062 for significant disordering (Ld=6.6 nm) at 35 and 100 meV below the band edge of the undisordered multiquantum well, respectively.

Waveguides in lithium iodate by means of proton implantation

Planar waveguides are built up (around 300 K) in lithium iodate by means of proton implantation. The waveguide properties are investigated by a prism coupling technique (m-lines spectroscopy). Both TE and TM modes are used to investigate the ordinary and extraordinary indices. The reconstructed index profiles obtained with inverse WKB method, are compared to the ion concentration profile as determined by simulation (TRIM). All the measured effective index values are smaller than these of the bulk indices. Moreover, the optical barrier is relevant to a maximum index change of about 0.03.

Control of the radial gradient-index profile by leaching of a gel

A new method for controlling the shape and increasing the maximum index change in the radial gradient-index profile of sol-gel bodies is demonstrated. This is accomplished by modifying the relative rates of the intricate reactions responsible for forming the gradient. In practice the changes reported here are effected solely by varying the leaching conditions. Through the proper choice of a solution to arrest the leaching action, the index change can be maximized for a given shape to double that of the previous technique. A detailed composition analysis of the data reveals that dopant diffusion and precipitation are the dominant factors influencing the observed results.

Variable refractive index systems by sol-gel process

A radial gradient-index (r-GRIN) glass rod of about 13.5 mm in diameter having a parabolic index profile with the maximum index change of 0.055 has been prepared by the sol-gel process. A gel of about 35 mm in diameter and 50 mm in length was formed from TMOS, TEOS, boron ethoxide, and aqueous lead acetate. After aging for 5 days at room temperature, the gel was subjected to stepwise replacement of the liquid in the micropores by a mixed solution of iso-propanol and acetone. The gel was then immersed in an ethanolic solution of potassium acetate for 16 h to form the concentration gradient of lead and potassium ions. The gel, once soaked in a bath of acetone to temporarily fix the concentration profile of the ions, was again immersed in an ethanolic solution of potassium acetate for 2 h to modify the profile of the ions near the edge of the cylindrical gel. The refractive index profile of the glass rods obtained by drying and densifying the gel was parabolic in the range from the central axis to 80% of the radius, with the highest value of 1.65 at the center.

High-sensitivity holographic storage in Ce-doped strontium-barium niobate at 632.8nm

We report the high-sensitivity holographic storage in Ce-doped SBN with a low power CW He-Ne laser. A maximum index change Δnmax=1.07×10-4, a photorefractive sensitivity S=7.9×10-4cm2/J, and a maximum diffraction efficiency η=78% were obtained. The optical readout characteristics of holograms recorded by the sample were also studied.

Optical behavior of potassium ion-exchanged glass waveguides

Optical waveguides are fabricated by potassium ion-exchange in Fisher Premium and Corning 0211 glass substrates. Variation of the maximum index change and diffusion coefficient with the potassium ion concentration is determined. The propagation loss in the single-mode waveguides is measured. A loss of as low as 0.1 dB/cm is achieved for the waveguides manufactured in Corning 0211 glass. Nearly symmetrical mode profiles are obtained using the one-step diffusion process. The effect of stress on the waveguides is investigated. The implementation of a wavelength selective optical tap and combiner in glass is suggested.

Measurements of room-temperature band-gap-resonant optical nonlinearities of GaAs/AlGaAs multiple quantum wells and bulk GaAs

We present a systematic study of the dependence of the optical nonlinearities on quantum well thickness for GaAs/AlGaAs multiple quantum wells (MQW’s) at room temperature and compare them with bulk GaAs. The maximum change in the refractive index is greatest for the MQW’s with the smallest well size and decreases with increasing well size, reaching a minimum for bulk GaAs. The maximum index change per photoexcited carrier increases by a factor of 3 as the well size decreases from bulk to 76 Å MQW.

Minimum-mode-size low-loss Ti: LiNbO<inf>3</inf>channel waveguides for efficient modulator operation at 1.3 µm

Efficient operation of Ti-diffused LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> modulators and switches requires small, tightly guiding channel waveguide modes which effectively overlap the applied modulating field. This in turn requires Ti:LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> channel waveguides with minimum mode size. The small-mode-size waveguides are obtained in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</tex> -cut crystals by optimizing the fabrication parameters to achieve maximum index change in the wave-guiding region and the optimum diffusion depth and width. For 6 h diffusion at 1025°C, and Ti strip thickness of 800 Å and strip width of 4 μm, the TM mode has a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/e</tex> intensity full width of 3.9 μm and full depth of 2.8 μm. This is by far the smallest mode size at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda = 1.32 \mu</tex> m presented to date for Ti:LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> waveguides. Since these waveguides have small mode size, large <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\Delta n</tex> , and low propagation loss (0.2 dB/cm) at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda = 1.32 \mu</tex> m, it should be possible to fabricate modulators and switches with small voltage-length products and optical circuits that incorporate small bending radii curves, thus decreasing the total device length.

Theoretical study off silicon-clad planar diffused optical waveguides

A silicon-clad planar diffused optical waveguide structure is proposed and propagation characteristics for a wavelength of 0.6328 μm are investigated. The refractive index profile of the substrate is assumed to be a Gaussian profile and a maximum index change of 0.01 is also assumed. Computer modelling studies indicate that silicon-clad diffused optical waveguides exhibit a damped periodic oscillation in the attenuation and mode index curves. The attenuations of these waveguide structures are relatively small compared with those of the four-layer planar homogeneous waveguide structures reported by Batchman and McWright. The attenuation may be reduced with a low-index buffer layer. Furthermore, the attenuation and mode index are significantly altered by the conductivity changes of silicon.

Silver film-diffused glass waveguides: Diffusion process and optical properties

Diffusion of a silver film into a glass substrate in the presence of an electric field is used to fabricate a series of planar waveguides in an effort to better understand the diffusion process and to relate the process parameters to the device parameters. The results indicate that it is possible to fabricate highly multimode waveguides which exhibit no fabrication-induced surface defects and no coloration due to silver reduction provided the silver film is not depleted during the diffusion process and the diffusion temperature is low (275°C or less). In addition, scanning electron microscope (SEM) analysis and optical characterization indicate that the maximum index change in the waveguide is strongly dependent on the applied field. We demonstrate that \Delta n varies from approximately 0 to 0.075 for fields ranging from 0 to 15 V/mm, making this technique specially attractive for the fabrication of waveguide structures requiring a large, variable change in refractive index.