Combined Franz-Keldysh and quantum-confined Stark effect waveguidemodulator for analog signal transmission

A novel approach to the design of electroabsorption waveguide modulators for use in broadband, analog radio frequency (RF) links is presented. The approach utilizes a linear combination of two electroabsorption effects (the Franz-Keldysh effect and the quantum-confined Stark effect) to improve the spurious-free dynamic range of the waveguide modulator. Measured transfer curves verify simultaneous improvement in linearity and modulation efficiency for an exemplary combination structure. Residual absorption is modeled to determine the impact on modulation efficiency     

40 Gbit/s lumped-element modulator driver in GaAs pHEMTs

A high-speed and high-gain modulator driver circuit using 0.15 /spl mu/m gate length GaAs pHEMT technology is presented. The IC was developed for driving electroabsorption modulators in 40 Gbit/s optical fibre systems. To meet application requirements a lumped-element approach was used with differential configuration. Measured results show the circuit operates at 40 Gbit/s with a swing of 3 V/sub p-p/ for single-ended and 6 V/sub p-p/ for differential output, and 8/10 ps rise/fall times.     

Optical intensity modulators for digital and analog applications

This tutorial describes the basic principles and performance analysis of optical intensity modulators using electrooptic and electroabsorption effects, for use in analog and digital communication systems. These include lithium niobate modulators, semiconductor electroabsorption modulators, semiconductor Mach-Zehnder modulators, and polymer modulators.     

Simulation of Saturation Effects in Electroabsorption Modulators

Saturation phenomena have a deep impact on the performance of electroabsorption (EA) modulators. It is important to develop algorithms which as well describe the phenomenon of electroabsorption as they include nonlinear carrier dependent effects, band filling and reduction of the band gap. Thus, a dynamical transport model has to be coupled to the density matrix equation for excitons.The density matrix equation for interband processes determines quantum optical effects, while a drift-diffusion model (DDM) adequately describes transport phenomena in the device. Within this approach quantum transport is considered by a correction of the classical potential, the Bohm potential.To demonstrate the presented algorithm, waveguide EA-modulators including strained lattice and full Fermi statistics are investigated.     

Electroabsorption in narrow coupled double quantum wells: Coulombiccoupling effects

The effects of Coulombic coupling between different subband pairs on the electroabsorption spectra of narrow coupled double quantum wells (QWs) have been studied. It is shown via detailed comparison between electroabsorption spectra calculated using a full excitonic Green's function method, a decoupled excitonic Green's function method, a variational method, and experimental data that inclusion of the Coulombic coupling between different subband pairs is required for correct prediction of electroabsorption in the narrow well system. It is also shown that, due to Coulombic coupling, it is necessary to include unbound QW states, above the QW edge, in the simulation of electroabsorption and electrorefraction. These results are of particular significance for the accurate calculation of electrorefraction, by Kramers-Kronig transformation of QW electroabsorption spectra, in coupled QW structures containing narrow QWs     

Fabry-Perot electroabsorption modulators for high-speed free-space optical communication

Retroreflective modulators are key components in free-space optical communication systems between mobile platforms and users. Wide-aperture surface-normal electroabsorption modulators based on GaAs-AlGaAs quantum wells embedded in an asymmetric Fabry-Perot cavity are designed and fabricated with a high yield process on 10-cm wafers. It is shown that the modulator yield and its speed are improved significantly by a pixellated approach in which monolithic modulators are divided into 4-64 pixels. The fabricated 1.5/spl times/1.5 cm/sup 2/ devices exhibit contrast ratios of 8 dB at a driving voltage of 8 V and a modulation frequency higher than 10 MHz, which provides low noise and fast data transmission for long distance free-space optical communication.     

New applications of a sinusoidally driven InGaAsP electroabsorptionmodulator to in-line optical gates with ASE noise reduction effect

The authors propose new applications of a sinusoidally driven InGaAsP electroabsorption modulator to an inline optical gate for a 2R (reshape and retiming) repeater in optical amplifier systems, an n :1 optical demultiplexer in time division multiplexing systems, and an optical switch. The small polarization dependence of the modulator is essential for inline use. By utilizing the monotonic increase of the extinction ratio with increasing applied voltage, the electroabsorption modulator driven by a large-signal sinusoidal voltage can produce a time domain square-shaped gate function with variable gate width. Furthermore, amplified spontaneous emission noise of optical amplifier systems can be reduced in both time and wavelength domains at the off-state of the modulator, due to noninterferometric wide wavelength operation of the modulator. Experimental results for a 2R repeater, an n:1 (n=4, 8) optical demultiplexer, and optical gates for switching are also demonstrated at over 10-Gb/s repetition rate     

Electric-field dependence of linear optical properties in quantum well structures: Waveguide electroabsorption and sum rules

We summarize the electric-field dependence of absorption and luminescence in quantum wells for fields perpendicular to the layers, present extended discussion of electroabsorption spectra and devices in waveguide samples, and derive sum rules for electroabsorption. Optical bistability, self-linearized modulation, and optical level shifting are demonstrated in self-electrooptic effect device configurations, with good modulation contrast and polarization-dependent properties. The electroabsorption spectra enable quantitative comparison of theory and experiment for absorption strengths in quantum wells with field. The sum rules enable excitonic effects to be included in the comparison, and good agreement is seen. One sum rule is also more generally applicable to electroabsorption in semiconductors.     

Enhancement in electroabsorption waveguide modulator slopeefficiency at high optical power

An increase in transfer curve slope efficiency for a Franz-Keldysh effect InGaAsP-InP electroabsorption waveguide modulator is observed as the incident optical power is increased from 5.8 to 17 dBm. However, high-frequency RF measurements agree with the gain predicted from the low-power transfer curve at all optical powers. We attribute the increase in dc slope efficiency to a temperature-induced bandgap shrinkage of the electroabsorption material     

Fabrication and Packaging of 40-Gb/s AlGaInAs Multiple-Quantum-Well Electroabsorption Modulated Lasers Based on Identical Epitaxial Layer Scheme

High-speed AlGaInAs multiple-quantum-well (MQW) electroabsorption modulated lasers (EMLs) based on an identical epitaxial layer (IEL) integration scheme are developed for 40-Gb/s optical fiber communication systems. A self-aligned planarization technique has been adopted to reduce the capacitance of the electroabsorption modulator (EAM). The IEL structure EML chips exhibit a small signal modulation bandwidth around 40 GHz. The influence of residual reflection at the modulator facet on the small signal modulation response is investigated. Submount containing a grounded coplanar waveguide (GCPW) transmission line is used for packaging the EML chips into transmitter modules. The optimization of the GCPW structure to suppress resonances in frequency response due to parallel-plate modes is presented. Clear eye opening under 40-Gb/s nonreturn-to-zero (NRZ) modulation has been demonstrated for the packaged EML module.     

Harmonic signals from electroabsorption modulators for bias control

Harmonic signals reflected from electroabsorption modulators are measured and analyzed as a function of modulator bias. The second-harmonic signal exhibits a dip close to the bias where the maximum of RF link gain occurs, over a significant optical power range. Using an equivalent circuit analysis we show this is caused by the inherent electroabsorption effect. The second-harmonic signal can be exploited for dynamic self-bias control of electroabsorption modulators in analog fiber-optic links     

Variation of frequency chirp with wavelength in an InGaAsP/InP multiple-quantum-well (MQW) waveguide electroabsorption modulator

Through measurements of the modulation index and the optical power spectra, the chirp parameter for an InGaAsP quantum-well electroabsorption waveguide modulator has been determined. The result shows that the frequency chirp parameter achieves a minimum value of 0.6 in the wavelength range 1509-1545 nm. This is significantly less than what is obtained from direct intensity modulation of injection lasers, making this a useful device in high-bit-rate, long-haul systems.<>     

An electroabsorption modulator module for digital and analogapplications

This paper presents an electroabsorption modulator (EAM) module for digital and analog (D/A) applications. Optically broad-band operation of the EAM module is studied for such digital application as wavelength division multiplexing (WDM) systems. Utilizing anisotropic electroabsorption of a multiple quantum-well (MQW) EAM, 40- and 100-nm bandwidth operations in 2.5-Gb/s digital signal transmission over 200-km standard fiber are confirmed by the experiments and the simulations, respectively. For analog applications, low distortion and high link gain characteristics of the EAM module are investigated at the wavelength of 1535 nm. High spurious-free dynamic range of 123 dB·Hz^4/5 and high link gains of -10.3 dB with matching circuit and -20.6 dB without matching circuit are obtained using the EAM module     

High-speed tandem of MQW modulators for coded pulse generation with 14-dB fiber-to-fiber gain

We report on a tandem of electroabsorption modulators integrated with an amplifier by the identical active layer (IAL) approach. A 14 dB fiber-to-fiber is obtained by the use of constant As-P ratio MQW active layer and operation at 10 Gb/s is demonstrated. A record modulator saturation power of 35 mW is also reported.     

Integrated tandem traveling-wave electroabsorption modulators for >100 Gbit/s OTDM applications

Integrated tandem traveling-wave electroabsorption modulators are demonstrated as high-speed optical short pulse generators and demultiplexers for >100 Gbit/s optical time-division-multiplexed systems. The tandem significantly increases the extinction ratio and further compresses the optical pulses in comparison to a single modulator. An extinction ratio of /spl sim/50 dB is achieved while optical pulses of 4-6 ps width at 30-40 GHz are generated.     

4.5 Gbit/s modelocked extended-cavity laser with a monolithically integrated electroabsorption modulator

An electroabsorption modulator has been monolithically integrated with an extended-cavity laser, which incorporates a Bragg reflector. Actively modelocked at the fundamental cavity resonance frequency of 4.5 GHz, the laser provides a train of 6.3 ps pulses with a centre wavelength of 1544 nm onto which data is encoded by the modulator. This 4.5 Gbit/s single-chip transmitter is suitable for systems employing short optical pulses.<>     

Field effects on the refractive index and absorption coefficient in AlGaAs quantum well structures and their feasibility for electrooptic device applications

Electroreflectance and electroabsorption measurements have been carried out to clarify field effects on the refractive index and absorption coefficient in AlGaAs quantum well structures. The observed electroreflectance spectra show very clear exciton-induced features at room temperature. A maximum variation of the refractive index in each quantum well at a photon energy near the lowest excitonic transition gap is obtained to be 4 percent induced by a 10⁵V/cm field modulation. Electroreflectance and electroabsorption spectra are shown to demonstrate a relation between dispersion curves of the field-induced variations in the refractive index and absorption coefficient in the quantum well structure. Based on the obtained results, operation of an electroabsorption modulator with the capability of small frequency chirping and an efficient optical switch are discussed from the practical point of view.     

Measurement of chirp parameter in GaInAs/GaInAsP quantum wellelectroabsorption modulators by using intracavity modulation

A useful technique for evaluating the chirp parameter α of electroabsorption modulators is presented. The α parameter of a GaInAs/GaInAsP quantum-well (QW) electroabsorption modulator integrated with an active segment in a Fabry-Perot cavity has been evaluated by measuring the Fabry-Perot fringes. It has been shown that the relationship between absorption changes and refractive index changes is nonlinear and the α parameter is dependent on operating wavelengths     

Analysis of segmented traveling-wave optical modulators

A simple and comprehensive modeling approach is developed for analyzing the frequency response of segmented traveling-wave optical modulators. The approach is based on the microwave transmission (ABCD) matrix theory. The case study for a GaAs traveling-wave Mach-Zehnder modulator (MZM) verifies this analysis approach with excellent agreement to the reported experimental results; the analyses for the quantum-well-based MZMs and electroabsorption modulators indicate that the segmented traveling-wave design can provide much better bandwidth than the lumped-element or the continuous-traveling-wave counterparts, with a few decibels penalty in the electrical-to-optical (E/O) conversion gain if low-loss optical waveguides are available. Meandered transmission line design, which provides more design freedom, is also analyzed using this modeling approach.     

Mode-Locking in a Semiconductor Fiber Laser Using Cross-Absorption Modulation in an Electroabsorption Modulator and Application to All-Optical Clock Recovery

We demonstrate the use of cross-absorption modulation in an electroabsorption modulator to actively mode-lock a semiconductor fiber laser at 10 GHz. The laser cavity also comprises a linearly chirped fiber Bragg grating to obtain wavelength tunable operation based on the dispersion tuning approach. The laser exhibits uniform output pulse characteristics throughout the tuning range. We also investigate the impact of using different types of optical pump signals on the output pulses. Finally, we demonstrate the application of the mode-locked laser for all-optical clock recovery.