GaAs/GaAlAs multiquantum well waveguides for all-optical switching at 1.55 mu m

The first measurements of selfphase modulation and nonlinear absorption in a GaAs/GaAlAs multiquantum well (MQW) waveguide near the half-bandgap resonance are reported. An intensity-dependent refractive index coefficient n/sub 2/ of 9*10/sup -14/ cm/sup 2//W and a two-photon absorption coefficient of 0.15 cm/Gw at 1.55 mu m wavelength for TM light were obtained. This corresponds to an intensity-independent figure of merit of 24 rad phase change in one nonlinear absorption length, high enough to allow the implementation of subpicosecond all-optical switches at 1.55 mu m wavelength.<>     

Refractive index change of GaInAs/InP disordered superlattice waveguide

The refractive index changes of disordered superlattice waveguides were measured and evaluated. A GaInAs/InP superlattice structure grown by gas-source molecular beam epitaxy (MBE) was disordered by the Si/sub 3/N/sub 4/ cap annealing method. The measurement of the wavelength response of a Bragg grating waveguide shows that the refractive indexes of the disordered region vary gradually in accordance with the development of partial disordering. Maximum refractive index changes were -1.1*10/sup -2/ for the transverse electric mode and +6.7*10/sup -3/ for the transverse magnetic mode at 1.5 mu m wavelength.<>     

Disorder-induced buried-stripe optical waveguides in GaAs/AlGaAs MQW material

Buried-stripe optical waveguides have been fabricated in GaAs/AlGaAs multiquantum-well material by masked Si/sup +/ implantation followed by annealing at 750 degrees C to produce selective-area quantum well mixing. The waveguides were found to support both TE and TM modes with propagation losses of 33 and 56 dBcm/sup -1/, respectively, at a wavelength of 1.15 mu m.<>     

Observation of field-induced refractive index variation in quantum box structure

An electric-field-induced refractive index variation in a GaInAs/InP five-layered quantum box structure was observed for the first time. The wavelength dependence of the refractive index variation was measured to confirm the quantum box effect. The size of the GaInAs/InP quantum box was estimated to be (22-30 nm)/sup 2/ with the thickness of 7.5 nm. from the spectral property of the field-induced refractive index variation. The maximum value of the refractive index variation in the quantum box was evaluated to be 7% around 1.52 mu m wavelength at an applied electric field of 8*10/sup 4/ V/Cm.<>     

Observation of self-lensing in a 4BCMU-polydiacetylene, thin-film waveguide

Strong self-lensing effects have been observed when propagating 35 ps duration, 1.06 mu m wavelength pulses over 5-10 mm distance through thin-film slab waveguides formed from 4BCMU-polydiacetylene. The results are consistent with a negative nonlinear refractive index coefficient (n/sub 2/) of magnitude 1-2*10/sup -13/ cm/sup 2/ W/sup -1/.<>     

Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers

The group refractive index dispersion in ultra-broad-band quantum cascade (QC) lasers has been determined using Fabry-Perot spectra obtained by operating the lasers in continuous wave mode below threshold. In the wavelength range of 5-8 /spl mu/m, the global change of the group refractive index is as small as +8.2 /spl times/ 10/sup -3/ /spl mu/m/sup -1/. Using the method of Hakki and Paoli (1975), the subthreshold gain of the lasers has furthermore been measured as a function of wavelength and current. At the wavelength of best performance, 7.4 /spl mu/m, a modal gain coefficient of 16 cm/spl middot/kA/sup -1/ at threshold and a waveguide loss of 18 cm/sup -1/ have been estimated. The gain evolution confirms an earlier assumption that cross-absorption restricted laser action to above 6 /spl mu/m wavelength.     

GaInAsSb/GaSb pn photodiodes for detection to 2.4 mu m

Ga/sub 0.77/In/sub 0.23/As/sub 0.20/Sb/sub 0.80//GaSb pn heterojunction photodiodes have been prepared by liquid phase epitaxy. They exhibit a long-wavelength threshold of 2.4 mu m. The room-temperature dark current at V=-0.5 V is 3 mu A (10 mA/cm/sup 2/) and the external quantum efficiency is around 40% in the wavelength range 1.75-2.25 mu m. The estimated detectivity D* at 2.2 mu m is 8.8*10/sup 9/ cm Hz/sup 1/2/ W/sup -1/.<>     

Fabrication and Evaluation of Nano-Scale Optical Circuits Using Sol-Gel Materials With Photo-Induced Refractive Index Variation Characteristics

Nano-scale optical circuits with core thickness of ~ 230 nm and core width of ~ 1 mum were fabricated and evaluated, using the photo-induced refractive index variation sol-gel materials, whose refractive index gradually increases by UV light exposure and baking. Propagation loss of linear waveguides was 1.86 dB/cm for TE mode and 1.89 dB/cm for TM mode at 633 nm in wavelength, indicating that there were small polarization dependences of ~ 0.03 dB/cm. Spot sizes of guided beams along core width direction and along core thickness direction were, respectively, 0.6 and 0.3 mu m for both TE mode and TM mode. Bending loss of S-bending waveguides was reduced from 0.44 to 0.24 dB for TE mode with increasing the bending curvature radius from 5 to 60 mu m. Although the bending loss for TM mode was slightly higher than that for TE mode, the difference was less than 10%. Branching loss of Y-branching waveguides was reduced from 1.33 to 0.08 dB for TE mode, and from 1.34 to 0.12 dB for TM mode with decreasing the branching angle from 80deg to 20deg. From these results, it is concluded that the photo-induced refractive index variation sol-gel materials can realize miniaturized optical circuits with sizes of several tens of microns and guided beam confinement within a cross section area less than 1.0 mum² with small polarization dependences, indicating potential applications to intra-chip optical interconnects.     

Birefringence suppression of UV-induced refractive index with grooves in silica-based planar lightwave circuits

An effective method for suppressing the birefringence of a UV-induced refractive index change is demonstrated for phase trimming of waveguides in planar lightwave circuits. By employing grooves on both sides of the waveguide to control the UV-induced strain, a significant reduction is achieved from 1.46/spl times/10/sup -4/ to 0.11/spl times/10/sup -4/ in the UV-induced birefringence for a /spl pi/ phase shift averaged between the TE and TM modes in a 2 mm-long waveguide.     

Gain saturation coefficients of strained-layer multiple quantum-well distributed feedback lasers

The gain saturation coefficients were measured for strained and unstrained multiple quantum-well distributed feedback (MQW-DFB) lasers. The gain saturation coefficient depends on the deviation of the laser's transverse-magnetic (TM) mode gain peak wavelength from its transverse-electric (TE) mode gain peak wavelength delta lambda , which is related to the strain on the active-layer wells. The gain saturation coefficient epsilon increased with increasing compressed strain on the active-layer wells. The coefficient epsilon of the unstrained MQW DFB laser with a wavelength deviation delta lambda of -350 AA was 2.45*10/sup -17/ cm/sup 3/, and epsilon increased up to 12.6*10/sup -17/ cm/sup 3/ in the SL-MQW DFB laser with a wavelength difference delta lambda of -890 AA.<>     

Measurement of quadratic electro-optic effect in polydiacetylene optical waveguide

Planar optical waveguides of the soluble polydiacetylene 9-SMBU were formed by dip-coating on silicon substrates. The quadratic electro-optic (Kerr) response was measured by guided wave polarisation interferometry. The effective electro-optic coefficient was found to be 3.7*10/sup -20/ m/sup 2/V/sup -2/ at a wavelength of 1.3 mu m.<>     

Simple measurement of carrier induced refractive-index change in InGaAsP PIN ridge waveguide structures

The carrier induced refractive-index change Delta n in integrated InGaAsP 1.30 mu m interferometer structures is evaluated by 1.55 mu m transmission measurements. At carrier concentrations N from 8*10/sup 16//cm/sup 3/ to 3*10/sup 18//cm/sup 3/ a value of Delta n/N=-1*10/sup -20/ cm/sup 3/ is obtained. A good agreement of the experimental results with theoretical predictions on bandfilling, plasma effect and bandgap shrinkage is demonstrated.<>     

Mode Coupling and Vertical Radiation Loss for Whispering-Gallery Modes in 3-D Microcavities

The characteristics of whispering-gallery modes (WGMs) in 3-D cylindrical, square, and triangular microcavities with vertical optical confinement of semiconductors are numerically investigated by the finite-difference time-domain (FDTD) technique. For a microcylinder with a vertical refractive index 3.17/3.4/3.17 and a center layer thickness 0.2 mum, Q-factors of transverse electric (TE) WGMs around wavelength 1550 nm are smaller than 10³, as the radius R < 4 mum and reach the orders of 10⁴ and 10⁶ as R = 5 and 6 mum, respectively. However, the Q-factor of transverse magnetic (TM) WGMs at wavelength 1.659 mum reaches 7.5 times 10⁵ as R = 1 mum. The mode coupling between the WGMs and vertical radiation modes in the cladding layer results in vertical radiation loss for the WGMs. In the microcylinder, the mode wavelength of TM WGM is larger than the cutoff wavelength of the vertical radiation mode with the same mode numbers, so TM WGMs cannot couple with the vertical radiation mode and have high Q -factor. In contrast, TE WGMs can couple with the corresponding vertical radiation mode in the 3-D microcylinder as R < 5 mum. However, the mode wavelength of the TE WGM approaches (is larger than) the cutoff wavelength of the corresponding radiation modes at R = 5 mum (6 mum), so TE WGMs have high Q-factors in such microcylinders too. The results show that a critical lateral size is required for obtaining high Q-factor TE WGMs in the 3-D microcylinder. For 3-D square and triangular microcavities, we also find that the Q-factor of TM WGM is larger than that of TE WGM.     

Aromatic polyureas: a new class of nonlinear optical polymer with large second-harmonic generation

The second-order nonlinear optical properties of aromatic polyureas are reported. From Maker fringe measurements, the nonlinear d/sub 33/ coefficient of poled polyurea films with and without pendant chromophores was found to be 20*10/sup -9/ esu and 12*10/sup -9/ esu, respectively, at 1.064 mu m. Aromatic polyurea having no pendant chromophores shows a cutoff wavelength of transmission at 307 nm which is the first example of organic NLO materials to be optically transparent at such low wavelengths.<>     

Single-layer coating for angled facet amplifier

A three-dimensional model for the calculation of the reflectivity of an angled facet amplifier is presented. It is predicted that a reflectivity as low as 5*10/sup -5/ can be obtained simultaneously for both the TE and the TM modes. For the same coating an angled facet amplifier has a reflectivity which is approximately 100 times lower than that of a normal facet amplifier for the investigated structure.<>     

High-performance 1.06-/spl mu/m selectively oxidized vertical-cavity surface-emitting lasers with InGaAs-GaAsP strain-compensated quantum wells

We present the first room-temperature continuous-wave operation of high-performance 1.06-/spl mu/m selectively oxidized vertical-cavity surface-emitting lasers (VCSEL's). The lasers contain strain-compensated InGaAs-GaAsP quantum wells (QW's) in the active region grown by metalorganic vapor phase epitaxy. The threshold current is 190 /spl mu/A for a 2.5/spl times/2.5 /spl mu/m/sup 2/ device, and the threshold voltage is as low as 1.255 V for a 6/spl times/6 /spl mu/m/sup 2/ device. Lasing at a wavelength as long as 1.1 /spl mu/m was also achieved. We discuss the wavelength limit for lasers using the strain-compensated QW's on GaAs substrates.     

GaAs/GaAlAs asymmetric Mach-Zehnder demultiplexer with reduced polarization dependence

A two-channel GaAs/AlGaAs asymmetric Mach-Zehnder wavelength demultiplexer with reduced polarization dependence was demonstrated. The device was fabricated on a single heterostructure comprising a 1.45- mu m-thick layer of GaAs on a 6.0- mu m-thick Ga/sub 0.85/Al/sub 0.15/As buffer layer. The epitaxial layers were grown by MOCVD (metalorganic chemical vapor deposition) on an n/sup +/ GaAs substrate. The single-mode rib waveguides, 3 mu m wide and 0.29 mu m high, were fabricated using standard photolithographic techniques followed by chemical etch and removal of the resist mask. Extinction ratios of 24.1 dB for transverse electric (TE) and 22.5 dB for transverse magnetic (TM) polarized light were measured on a device with an anti-reflection coating on its input and output facets. The active length of the device is approximately 6.5 mm and total loss of 1.1 dB was obtained in a 16-mm-long chip.<>     

Photoconductivity measurements in optical fibre preforms

The AC photoconductivity and the photodielectric shift in UV-illuminated germano-silicate preforms and optical fibres has been measured using a noncontact microwave resonator technique at 2.9 GHz. The highest photoconductivity measured was 1.5*10/sup -6/ ( Omega cm)/sup -1/ and the largest measured photodielectric shift corresponded to an increase of the refractive index of 1.6*10/sup -2/. A permanently induced increase of the refractive index, 6.1*10/sup -3/, was also observed. The dark DC conductivity was measured to be 10/sup -15/ ( Omega cm)/sup -1/.<>     

Analysis of mode quality factors and mode reflectivities for nanowire cavity by FDTD technique

The mode frequency and the quality factor of nanowire cavities are calculated from the intensity spectrum obtained by the finite-difference time-domain (FDTD) technique and the Pade/spl acute/ approximation. In a free-standing nanowire cavity with dielectric constant /spl epsiv/=6.0 and a length of 5 /spl mu/m, quality factors of 130, 159, and 151 are obtained for the HE/sub 11/ modes with a wavelength around 375 nm, at cavity radius of 60, 75, and 90 nm, respectively. The corresponding quality factors reduce to 78, 94, and 86 for a nanowire cavity standing on a sapphire substrate with a refractive index of 1.8. The mode quality factors are also calculated for the TE/sub 01/ and TM/sub 01/ modes, and the mode reflectivities are calculated from the mode quality factors.     

On the Mode and Square Correspondence Between Circular Multimode Tapered Waveguides

In an axially straight multimode circular waveguide taper excited with a pure TE/sup /spl circle// /sub 11/ dominant mode, the first and only converted mode at and near cutoff is the TM/sup/spl circle// /sub 11/ mode. It is shown that in an axially straight multimode square waveguide taper excited with a pure TE/sup/spl square// /sub 10/ dominant mode, the TM/sup/spl square// /sub 12/ mode corresponding to the TM/sup/spl circle// /sub 11/ mode in circular case is not the only first converted mode at and near cutoff. The overall behavior or coupling mechanism of waveguides is similar whether the waveguide is rectangular, square, circular, or elliptical: i.e., the overall coupling coefficient at cutoff of a converted mode or modes approaches an ininfinity of the order 0/sup -1/4/.