Comparison of effective α parameters for multiquantum-wellelectroabsorption modulators

A new effective α parameter appropriate for evaluating the transmission performance of multiquantum-well electroabsorption (MQW-EA) modulators is proposed. It is defined by the ratio of the phase change to the loss change between the ON voltage and the voltage at which the exciton peak reaches the wavelength of the light source. Simulation reveals that along various effective α parameters it has the best correlation with the equivalent α parameter obtained from the transmission performance     

Electroabsorption properties of InGaAs/InAlAs multiple quantum wellstructures at 1.5 μm

The electroabsorption properties of InGaAs/InAlAs MQW structures are characterised in terms of Δα, Δα/F and Δα/α₀, where Δα is the electroabsorption, α₀ is the residual absorption coefficient under zero bias, and F is the applied electric field. The limitations of these structures for 1.5 μm modulators are primarily due to the relatively small Δα/F values as a result of the small well width. The results are compared with the literature     

On the figures of merit for electroabsorption waveguide modulators

ΓΔα/F and Δα/α₀ (where Δα is the absorption change, α₀ is the residual absorption, F is the applied electric field, and Γ is the optical confinement factor in the waveguide) have been separately proposed as the relevant figure of merit for electroabsorption waveguide modulators. Using a quantitative and systematic argument, the authors show that they are both necessary and important to the total performance of the modulator     

On the optimization of InGaAs-InAlAs quantum-well structures forelectroabsorption modulators

Several InGaAs-InAlAs multiple quantum-well structures grown by metalorganic vapor phase epitaxy (MOVPE), with various Ga content and quantum-well width, have been investigated for electroabsorption modulators (EAM's). The light-hole heavy-hole splitting, the chirp parameter, the insertion loss and the figure of merit ΓΔα/F of the different InGaAs-InAlAs structures have been evaluated with photocurrent, photoluminescence, absorption and X-ray measurements. It was then possible to experimentally study the influence of different parameters of the multiple quantum-well structures on the device performance. The use of tensile strained barriers are believed to be responsible for the improvement in the figure of merit. Structures with unresolved light-hole and heavy-hole transitions, with negligible chirp, with adequate insertion loss and with extremely high values for ΓΔα/F have been obtained, however, not simultaneously     

Negative-chirp electroabsorption modulator using low-wavelengthdetuning

The negative chirp of an electroabsorption modulator having an α-parameter value of 0 to -0.5, at an input light wavelength of 1.55-1.56 μm, has been developed by optimizing the bandgap energy of an InGaAsP bulk absorption layer. We have demonstrated successful transmission with 10 Gb/s NRZ modulation over a 100-km span of standard fiber without resort to dispersion compensation     

Large electroabsorption effect in GaInAs/InP multiple quantum well(MQW) optical modulator grown by OMVPE

To make short, high-speed electroabsorption modulators, it is necessary to use a material and device structure that displays a large change in absorption coefficient, Δα. The authors report a device with Δα=7800 cm^-1, which provides an on/off ratio of 44:1 at λ=1.6 μm with a drive voltage of 10 V     

On the transmission performances and the chirp parameter of amultiple-quantum-well electroabsorption modulator

The chirp parameter of a multiple-quantum-well (MQW) electroabsorption modulator was measured with accuracy for several operating wavelengths in the 1.5 μm window. It varied strongly with applied bias. Effective chirp parameter, defined as the ratio of phase change to transmission change between modulator on and off states, is about zero, or even negative. However, experimental transmission length on standard fiber at 10 Gb/s NRZ is much smaller than what is expected for such a low chirp parameter. It is demonstrated that the effective chirp parameter should not be computed from changes between on and off states, but from the average of the chirp parameter values in a 3 dB region of the most transparent states of the modulator. This simple rule allows us to predict transmission performances based on measurements of the chirp parameter, and can be used to optimize optical components without actually experimenting on a transmission system. The effective chirp parameter of the MQW electroabsorption modulator is found positive. This should be intrinsic to red-shift electroabsorption effects, such as the quantum confined Stark effect     

Design of InGaAsP multiple quantum-well Fabry-Perot modulators forsoliton control

The use of synchronous optical modulators is effective in reducing the pulse timing jitter in long-distance soliton transmission. The inherently polarization-insensitive characteristics of the Fabry-Perot multiple quantum-well (MQW) electroabsorption modulator make it a potentially suitable device for this application. We investigate the intensity and phase modulation characteristics of symmetric and asymmetric Fabry-Perot modulators, and show that, by positioning the resonant wavelength <30 nm away from the exciton absorption peak to obtain negative chirp operation, both configurations can be used to successfully reduce timing jitter in a 20 Gb/s soliton system     

Measurement of field-induced refractive index variation in GaAs/AlGaAs superlattice using monolithic Fabry-Perot etalon

The electrorefractive effect in a GaAs/AlGaAs superlattice was measured by using a monolithic Fabry-Perot structure, where the superlattice active region is embedded inside a cavity, formed by two quarter-wave stacks, all grown by molecular beam epitaxy. At the wavelength approximately 7675 AA, there is a large absorption change ( approximately 5400/cm) and an accompanying index change ( approximately 0.015) when the applied field change is approximately 100 kV/cm. In addition, the chirp parameter at this wavelength is less than 1. This result shows that superlattice electroabsorption optical modulation can offer relatively very low chirp.<>     

Heterodyne technique for measuring the amplitude and phase transferfunctions of an optical modulator

We propose a technique based on heterodyne detection for accurately and simultaneously measuring the amplitude and phase transfer functions of an optical modulator. The technique is used to characterize an InGaAsP multiple quantum-well electroabsorption modulator. From the measurements we derive the small-signal α-parameter and the time-dependent chirp for different operation conditions     

Strain dependence of the linewidth enhancement factor inlong-wavelength tensile- and compressive-strained quantum-well lasers

Values of the linewidth enhancement factor α in InGaAs/InGaAsP tensile- and compressive-strained quantum-well (QW) Fabry-Perot lasers are measured and compared to calculated values. The strain dependence of the measured values agrees with that of the calculated values: Values of α are smaller for tensile-strained QW lasers than for compressive-strained QW lasers, and α decreases with the increase of tensile or compressive strain. According to the model used in the calculation, short-wavelength-composition barriers reduce α in compressive-strained QW lasers, and α for such lasers is expected to be as low as that for the tensile-strained QW lasers     

The effect of modulator nonlinearity on measurements of chirp inelectroabsorption modulators

Traditional techniques for measuring the chirp in external modulators assume that the optical intensity output of the modulator is a faithful representation of the applied voltage. For electroabsorption modulators, which can have highly nonlinear transmission- vs voltage characteristics, this is a poor approximation, especially when they are operated at high modulation indices. We demonstrate a new technique that makes use of the actual measured T(V) for the device under test, and show that this new technique permits measurement of chirp in modulators generating either NRZ signals or soliton pulses. We apply this technique to both bulk and MQW electroabsorption modulators, and show that traditional measurements significantly underestimate the α values of these devices     

Laser-power stabilization using a quantum-well modulator

A novel method for laser-power stabilization using a multiple quantum-well reflection electroabsorption modulator with a Fabry-Perot cavity is demonstrated. Stable operation of the modulator produces a linear relationship between control current and optical absorption which permits the realization of noninterferometric optical subtraction and the implementation of a negative feedback architecture. Using this stabilization technique and a modulator with a 70% reflectivity change, intensity noise as large as ±75% of the incident level can be eliminated from a noisy laser beam     

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.<>     

Simultaneous Measurements of Dispersion Parameter and Fiber Length Using the Self-Seeding Laser Oscillation of a Fabry–PÉrot Laser Diode

A simultaneous measurement technique of the dispersion parameter and the length of an optical fiber has been demonstrated by using the self-seeding laser oscillation of a Fabry-Perot laser diode. We measured the dispersion parameter and the length of an optical fiber from the modulation frequency changes required to induce single-mode laser oscillations through an optical closed-loop path. The dispersion and fiber length measurements were within 1.5% and 0.2%, respectively, of the values measured by commercial instruments.     

The design of electroabsorption modulators with negative chirp and very low insertion loss

Electroabsorption modulators (EAMs) with negative chirp and very low insertion loss are numerically designed with asymmetric intra-step-barrier coupled double strained quantum wells (AICD-SQWs) based on InGaAlAs material. For this purpose, the electroabsorption coefficient is calculated over a range of wells layer strain from compressive (CS) to tensile (TS). The chirp parameter and insertion loss for TE input light polarization are evaluated from the calculated electroabsorption spectra, and their Kramers-Krönig transformed refractive index changes. The results of the numerical simulation show that the best range of left and right wells strain for EAMs based on AICD-SQWs with negative chirp and very low insertion loss are from 0.032% to 0.05% (TS) and -0.52% to -0.50% (CS), respectively.     

A simple method for extraction of multiple quantum well absorptioncoefficient from reflectance and transmittance measurements

A simple model is presented which takes into account interference effects in the common three-layer p-i-n diode epitaxial structure used for measurement of electroabsorption effects in multiple quantum well material. Antireflection coatings or surface roughening are not required to extinguish Fabry-Perot oscillations in the reflectance and transmittance spectra which improves the accuracy of the extracted absorption coefficient. Sources of error in the extracted absorption coefficient are examined. The absorption spectra of bulk GaAs and a 10-nm GaAs/10-nm AlGaAs multiple quantum well obtained using this method are compared with other measured results     

InGaAsP electroabsorption modulator for high-bit-rate EDFA system

The total performance of an InGaAsP electroabsorption modulator was characterized. Low-driving voltage of 1.2-2.7 V, low fiber-to-fiber insertion loss of 8.4-7.8 dB, and small α-value of 0.1-0.4 were obtained in 40-nm-wide wavelength range from 1.53 to 1.57 μm. Static and dynamic modulation characteristics showed small dependence on the polarization and the power level of input light. Successful applications of a packaged EA modulator to ultralong-distance Er-doped fiber amplifier systems were confirmed by 16000-4550-km transmission experiments at 2.5-10 Gb/s     

Eye-opening improved electroabsorption modulator/DFB laser diodewith optimized thickness of the separate-confinement heterostructurelayers

The separate-confinement heterostructure (SCH) of an electroabsorption modulator integrated with a distributed feedback laser diode (EAM-LD) was optimized to obtain a clear optical waveform (eye opening) without penalty of chirp characteristics. The electric field applied to the multiple-quantum-well (MQW) structure was controlled by employing the proper thickness of undoped SCH layers to attain gentle absorption characteristics of the EAM at the optical mark level. As a result, the eye opening of the modulated emission at 10 Gb/s was improved and the intersymbol interference was reduced. No severe drawback on chirp characteristics was found in an α-parameter measurement. EAM-LD's with the optimized SCH structure exhibited good transmission characteristics of power penalties under 1.5 dB at 10 Gb/s, and the reliability of 10 Gb/s transmission characteristics was also confirmed. We have also investigated how the optical waveform after transmission was affected by the dependence of the α-parameter on bias voltage. Measurement and numerical calculation of the optical waveform after transmission showed that the optical waveform was severely affected by the α-parameters at deep reverse voltages     

Nearly chirp-free electroabsorption modulation usingInGaAs-InGaAlAs-InAlAs coupled quantum wells

We present an electroabsorption modulator based on slightly asymmetric InGaAs-InGaAlAs-InAlAs coupled quantum wells operated in the normally-off mode. The device exhibits a large change of the absorption coefficient in the vicinity of the zero-bias exciton peak wavelength with a very small change of the refractive index. The maximum excursion of the chirp parameter over the entire span of the bias voltage is less than ±0.1 for a specific wavelength or ±0.4 over a 12~14 -nm range. This structure is promising for low-chirp, high bit-rate electroabsorption modulators