A 1.3 μm directional coupler polarization splitter by ionexchange

A glass waveguide polarization splitter for operation in the 1.3 μm wavelength region is reported. The device, which has a symmetric directional coupler configuration, exploits the stress-induced birefringence in K⁺-Na⁺, ion-exchanged waveguides, giving rise to an adequate difference in the coupling lengths for the two polarizations. Starting from the measured potassium concentration (refractive index) profile of the structure and utilizing a combination of the multilayer stack theory and the effective-index method, the normal mode propagation constants and mode field profiles are calculated to determine the polarization splitting length and the extinction ratio, and the results are compared with the experimental data. It is shown that in a given coupler, the splitting occurs at several wavelengths in the 1.0-1.45 μm range. A 25 mm-long coupler, fabricated by thermal diffusion of K⁺ ions in BK7 glass, exhibits an extinction ratio of 18.2 dB at 1.32 μm, in excellent agreement with the simulation results     

Wafer-fused optoelectronics for switching

Wafer fusion technique for realization of compact waveguide switches and three-dimensional (3-D) photonic integrated circuits is investigated theoretically and experimentally. Calculations based on beam propagation method show that very short vertical directional couplers with coupling lengths from 40 to 220 μm and high extinction ratios from 20 to 32 dB can be realized. These extinction ratios can be further improved using a slight asymmetry in waveguide structure. The optical loss at the fused interface is investigated. Comparison of the transmission loss in InGaAsP-based ridge-loaded waveguide structures with and without a fused layer near the core region, reveals an excess loss of 1.1 dB/cm at 1.55 μm wavelength. Fused straight vertical directional couplers have been fabricated and characterized. Waveguides separated by 0.6 μm gap layer exhibit a coupling length of 62 μm and a switching voltage of about 2.2 V. Implications for GaAs-based fused couplers for 850 nm applications will also be discussed     

ARROW-B type polarization splitter with asymmetric Y-branchfabricated by a self-alignment process

We have proposed and demonstrated a polarization splitter based on an ARROW-B type waveguide. This device has an asymmetric Y-branch configuration, connecting a waveguide with a large modal birefringence to a low-birefringent waveguide which is channeled by a stripe lateral confinement structure. The device can achieve a high extinction ratio without precise mask alignment, since these two waveguides are connected by a self-alignment process. In addition, a low insertion loss is achieved because of adiabatic Y-branching. We designed this device at the wavelength of 1.55 μm and fabricated it using RF sputtering. As a result, high extinction ratios of -19.4 dB for TE mode and -19.0 dB for TM mode were achieved     

A high-speed 4×4 Ti:LiNbO3 integrated optic switchat 1.5 μm

The operation of a fully packaged and pigtailed polarization dependent Ti:LiNbO₃ 4×4 integrated optic switch with an operating wavelength of 1.5 μm is demonstrated. The switching matrix is fast, with a measured 3-dB small-signal bandwidth greater than 1 GHz. Unwanted cross modulation between channels due to coupling between switching elements was measured and found to be less than -20 dB of the signal strength. The switch is composed of balanced bridge switching elements having a 17-V switching voltage, an 18 dB extinction ratio, and an excess optical loss of 0.6 dB     

Theoretical and experimental study of 10 Gb/s transmissionperformance using 1.55 μm LiNbO3-based transmitters withadjustable extinction ratio and chirp

This paper has experimentally and theoretically investigated transmission performance depending on chirping and extinction ratio for a 10 Gb/s transmission system with the standard single-mode fiber. The transmission performance can be dramatically degraded or improved by adjusting chirp and extinction ratio in a 1.55 μm LiNbO₃ modulator-based transmitter and erbium-doped fiber amplifier (EDFA)-pin diode receiver configuration. To estimate the transmission performance, bit error rate (BER) characteristics rather than eye-opening penalty (EOP) have been calculated by solving the nonlinear Schrodinger equation with including the model of chirping and extinction ratio for the transmitter, and noise and intersymbol interference for the receiver. This simulation can predict the measured BER characteristics well enough to see interplaying between chirping and extinction ratio     

Wannier-Stark localization in a 1.55 μm InGaAs/InAlAssuperlattice waveguide modulator structure

The authors present an optical waveguide modulator structure based on Wannier-Stark localization in a InGaAs-InAlAs superlattice. Optical waveguide transmission below the superlattice bandgap displays expected F^-1 oscillatory behavior leading to various modulation schemes. An 11 dB extinction ratio was obtained by applying a 0.7 V drive voltage to a 100 μm long waveguide device operating at 1.55 μm under a transverse-electric (TE)-polarization mode. On-state attenuation was 5 dB. Lower open-state attenuation (3 dB) can be obtained simultaneously with a higher extinction ratio (13 dB) but in that case a larger drive voltage (1.6 V) is needed     

2×2 optical waveguide switch with bow-tie electrode based oncarrier-injection total internal reflection in SiGe alloy

Based on the total internal reflection and the plasma dispersion effect of SiGe alloy, a 2×2 intersectional rib optical waveguide switch with bow-tie electrode has been proposed and fabricated for the wavelength of 1.3-μm operation. The thickness of the SiGe layer is 2.6 μm and the width is 9 μm. The branch angle of the switch is 2° and the bow-tie angle is 1.5°. The on-state crosstalk is -19.6 dB, the off-state extinction ratio is 38.5 dB and the off-state insertion loss is less than 1.70 dB. The switching time is about 180 ns     

25 GHz polarization-insensitive electroabsorption modulators withtraveling-wave electrodes

Electroabsorption modulators with traveling-wave electrodes have been designed and fabricated using MOCVD grown InGaAsP-InGaAsP quantum wells. A modulation bandwidth of 25 GHz is achieved for a 2 μm-wide 30 μm-long device. Driving voltage of 1.20 V is achieved for an extinction ratio of 20 dB for operation at 1.55 μm     

Polymeric digital optical modulator based on asymmetric branch

A digital optical modulator based on an asymmetric Y-branch waveguide is proposed and fabricated by using an electro-optic polymer. The operating point is initially shifted to the off-state utilising the asymmetry in the branch to provide an initial zero-state with no electrical bias. It has been confirmed that the high extinction ratio can be obtained with a low drive voltage. An extinction ratio of 25 dB is demonstrated for a drive voltage of 20V using a polymer PMMA-DR1 with r_ss of 5 pm/V at 1.3 μm     

High-speed continuously tunable liquid crystal filter for WDMnetworks

We describe a high-speed wavelength tunable liquid crystal filter which can be utilized as the tuning element at the receiving end of wavelength division multiaccess (WDMA) optical networks. The filter uses chiral smectic A electroclinic liquid crystals as the active cavity material in a Fabry-Perot etalon in order to obtain a microsecond switching speed. Using the commercially available BDH764E liquid crystal material, we demonstrate a tunable optical filter both in a bulk Fabry-Perot and a fiber Fabry-Perot (FFP) configuration. A bandwidth of about 0.7 mm and effective finesse of 70 were obtained in the FFP configuration. A FFP tuning range of 13 nm with a switching time of less than 10 μs were measured at the operating wavelength of 1.55 μm. A theoretical analysis of the expected filter performance in the FFP configuration is given. Diffraction in the Fabry-Perot cavity is identified as the dominant loss factor, resulting in reduced throughput and finesse broadening. It is calculated theoretically that an effective finesse of 130 and a throughput loss of 2.2 dB are achievable for a mirror finesse of 200 and a liquid crystal cavity thickness of 5 μm. A short waveguide piece is assumed to be included in the cavity. Other expected loss sources for the filter in the FFP configuration have been calculated, showing negligible effect on the filter performance     

Reflection-type optical waveguide switch with bow-tie electrode

Based on the total internal reflection and the plasma dispersion effect of SiGe alloy, an intersectional rib optical waveguide switch with bow-tie electrode has been proposed and fabricated for operating wavelengths of 1.3 and 1.55 μm. The thickness of the SiGe layer is 2.6 μm and the width is 9 μm. The branch angle of the switch is 2° and the bow-tie angle is 1.5°. The crosstalks are -19.6 dB for 1.3 μm and -21.8 dB for 1.55 μm. At both wavelengths, the extinction ratio is larger than 38.5 dB and the insertion loss is less than 1.70 dB. The switching time is about 180 ns     

An InGaAs/GaAs MQW optical switch based on field-induced waveguides

A novel optical waveguide switch containing InGaAs/GaAs multiple-quantum wells (MQW) is proposed. In this structure, a large field-induced refractive index increase (0.1%) due to the quantum-confined Stark effect (QCSE) is utilized to generate electrically controllable waveguides. Switching operation of a first fabricated device has been investigated at wavelengths of about 1 μm. A crosstalk ratio of -18.8 dB and an extinction ratio of 20.9 dB was achieved at a reverse voltage of -7 V. Within an operational wavelength region of 9 nm, crosstalk was found to be less than -13 dB for both switching conditions. Further, the proposed switch structure seems to be well suited for monolithic integration with laser diodes and exhibits the potential for high-speed operation     

A scheme of picosecond pulse shaping using gain saturationcharacteristics of semiconductor laser amplifiers

To obtain high power, well shaped picosecond pulses from gain-switched semiconductor lasers, the use of dynamic gain saturation characteristics of semiconductor laser amplifiers was investigated theoretically and experimentally. A configuration of a reflected-wave amplifier (RWA) with single-side external coupling is introduced for pulse shaping, which is found to be suitable for enhancing dynamic gain saturation. By a combination of a distributed feedback laser oscillator at 1.3 μm in wavelength and a reflected-wave amplifier of 400 μm cavity length with asymmetric facet reflectivities of 0.01% and 30%, single-mode optical pulses with almost no tailing, full width at half maximum of 15 ps, and peak power exceeding 50 mW were obtained without pulse broadening, despite the considerable tail structure of the incident pulse     

External-cavity semiconductor laser tunable from 1.3 to 1.54 μmfor optical communication

Using semiconductor optical amplifiers with properly designed nonidentical quantum wells made of InGaAsP-InP materials in the external-cavity configuration, the semiconductor laser is broadly tunable. The tuning range covers from 1.3 μm to 1.54 μm. Without additional filtering techniques, the laser beam emitted from the linear external cavity has the sidemode suppression ratio better than 30 dB. Also, the power ratio of the lasing mode to the total output power is 90%-99%, indicating the dominance of the lasing mode in the amplification process due to the broad gain spectrum     

Photonic crystal line defect waveguide directional coupler

A three-port directional coupler based on photonic crystal line defect waveguides is demonstrated. The directional coupler has only a 21 μm long coupling region. Wavelength-selecting operation around 1.5 μm wavelength was confirmed     

Linear and quadratic electrooptic effects in symmetric andasymmetric quantum-well structures

In the first part of the paper, the changes in the complex index of refraction of quantum-well structures proportional to the applied dc electric field (F) and its square are obtained from stationary perturbation theory. Expressions are obtained for the linear and quadratic changes in index and absorption coefficients of asymmetric and symmetric quantum well structures. The triple resonance term does not contribute to the quadratic Kerr effect for a symmetrical well in an expansion about F=0. In the second part of the paper, these results are applied to several types of modulators. First we consider two kinds of silicon/silicon-germanium high-barrier asymmetric quantum well structures at 1.55 μm. For the step well, the dominant triple resonant term of the quadratic Kerr effect adds to the linear electrooptic effect (LEG) to yield an intensity extinction ratio of 15 dB in switching from F=+8 V/μm to -8 V/μm and a very small insertion loss of 0.04 dB, for a modulator length of 200 pm and for narrow linewidths (FWHM=7.6 meV). For the case of asymmetric barriers confining silicon wells, a comparable insertion loss is obtained but with an extinction ratio of 44 dB for FWHM=20 meV, a clearly superior performance. For symmetrical silicon-based wells, the double-resonance modulation, though small at λ=1.55 μm, is appreciable at λ=10.6 μm. For the Kerr effect in symmetrical gallium arsenide wells at λ=10.6 μm reported by Sa'ar et al., we show that it is the double-resonance term which yields the six order-of-magnitude enhancement with respect to bulk and that an even larger enhancement of the Kerr effect can be obtained in an asymmetric GaAs structure together with the LEO     

Nd-doped double-clad fiber amplifier at 1.06 μm

We propose and demonstrate a double-clad neodymium (Nd)-doped fiber amplifier (IVDFA) at 1.06 μm for a compact configuration of a high-power optical amplifier. The proposed 125 μm first cladding diameter in the double-clad fiber, provides the single-mode propagation of the signal lightwave into its doped core without misguiding the signal lightwave into the outer core (first cladding), by simply splicing with a standard single-mode fiber. Furthermore a fiber grating in a single-mode core of the double-clad fiber allows the double-pass configuration for the signal lightwave at 1.06 μm and also allows the pump lightwave coupling at 0.81 μm into the first cladding without employing a bulk dichroic mirror. We demonstrate the signal output power of 110 mW for a 550-mW pump input from a multimode fiber coupled pump source. Theoretical results predict an efficient high-power operation of the amplifier by improving the signal scattering loss in the double-clad fiber     

Polarization-selective mode coupling in two-mode Hi-Bi fibers

We propose and demonstrate a mode coupler which converts either of the LP₀₁ polarization states in a two-mode high-birefringence (Hi-Bi) fiber to the LP₁₁ mode with the same polarization. We use coupled-mode theory to develop design rules based on the polarization splitting of the beat length between the two lower-order modes. The device can be operated either as a narrow-band device in a region with large intermodal group delay difference or as a broadband device in a region with zero group delay difference. We use this novel device as a key component in a two-mode Hi-Bi fiber polarizer. In this configuration either of the polarization eigenstates can be selected and transmitted with an adjustable extinction ratio which can be as large as 30 dB. The coupling loss in the transmitted state of polarization can be less than 0.2 dB. In the broadband polarizer we demonstrate -20 dB extinction over 42 nm with a potential for considerable improvement. We also propose and experimentally investigate an increased differential group delay obtained by propagating one polarization state in the LP₁₁ mode instead of in the LP₀₁ mode. The largest differential group delay measured in this configuration is 14.5 ps/m which is seven times larger than the differential group delay between the polarization modes. We discuss several possible uses in fiber sensors and measure the transmission of a proposed two-coupler configuration     

Steady-state and transient characteristics of microcavitysurface-emitting lasers with compressively strained quantum-well activeregions

In conventional semiconductor lasers, the dimensions of the optical cavity greatly exceed the photon wavelength, and the photon density of states forms a continuum since it is essentially that of a bulk system. On the other hand, in an ideal laser, one would like to have a single optical mode coincident with the maximum in the gain spectrum of the active medium. We show that substantial density-of-states quantization and enhancement of the fraction of photons spontaneously emitted into the lasing mode can be obtained by reducing the lateral width of the surface-emitting laser. For emission at λ=0.954 μm, the threshold current density can be drastically reduced by increasing the coupling factor to a few percent. For a cavity structure width of 0.3 μm, the threshold current density is 50 A/cm², compared with 250 A/cm² for the 0.6-μm cavity. At lower still lateral widths, the cavity loses its vertical character, and confinement of the lateral optical mode rapidly deteriorates. The large-signal response of microcavity lasers is slightly improved primarily due to elimination of mode competition in intrinsically single-mode microcavities, with relaxation times close to 1 ns. The enhancement of the spontaneous emission coupling factor results in an increase of the relaxation oscillation frequency and improvement in the standard small-signal response of microcavity lasers. For J=10J_th, the -3 dB modulation frequency exceeds 40 GHz. Since low threshold current densities may be achieved in microcavity lasers, the gains in small-signal performance are primarily extrinsic, i.e., higher modulation bandwidths ace accessible for the same injection     

High-efficiency electroabsorption in quaternary AlGaInAsquantum-well optical modulators

Quaternary AlGaInAs quantum-well optical modulators operating at 1.55 μm are introduced and demonstrated for the first time. An electron-to-heavy-hole exciton absorption peak shift of over 600 Å is observed for a bias voltage of 6 V. An extinction ratio of 19 dB and high-speed operation over 4 GHz is obtained for this optical modulator