Optimum operating points for electroabsorption modulators in 10Gb/s transmission systems using nondispersion shifted fiber

The chirp, optical extinction ratio and insertion loss of an electroabsorption modulator (EAM) depend on the properties of the bulk or multiple-quantum-well absorption layer of the device and on the bias voltage and modulating voltage waveform. For 10 Gb/s transmission over nondispersion shifted fiber, joint optimization of the bias and modulation voltages is considered for five different EAM's. To comprehensively explore this issue, the measured dependence of the absorption and α-parameter on applied voltage is used to accurately model an EAM in a system simulator. The effects of group velocity dispersion and self-phase modulation arising from the Kerr nonlinearity are included to permit assessment of the dependence of the optimum bias and modulation voltages on the average transmitted optical power for a given fiber length. The improvement in receiver sensitivity, relative to that obtained with maximum optical extinction ratio, depends quite significantly on the transmitted optical power and the specific properties of the modulator. This makes it difficult to determine optimum operating conditions which apply generally     

Peripheral-coupled-waveguide MQW electroabsorption modulator for near transparency and high spurious free dynamic range RF fiber-optic link

Peripheral coupled waveguide (PCW) design has been deployed in InGaAsP multiple quantum-well (MQW) electroabsorption modulator (EAM) at 1.55-/spl mu/m wavelength. PCW enhances the optical saturation power and reduces the optical insertion loss and the equivalent V/sub /spl pi// simultaneously. A radio-frequency link using a 1.3-mm-long lumped-element PCW EAM has achieved experimentally a link gain of -3 dB, at 500 MHz and at input optical power of 80 mW. The corresponding two-tone multioctave spurious-free dynamic range (SFDR) at the same bias is measured at 118 dB/spl middot/Hz/sup 2/3/. The single-octave SFDR at the third-order null bias is 132 dB/spl middot/Hz/sup 4/5/.     

Analog characterization of low-voltage MQW traveling-wave electroabsorption modulators

In this paper, high-speed traveling-wave electroabsorption modulators (TW-EAMs) with strain-compensated InGaAsP multiple quantum wells as the absorption region for analog optical links have been developed. A record-high slope efficiency of 4/V, which is equivalent to a Mach-Zehnder modulator with a V/sub /spl pi// of 0.37 V and a high extinction ratio of > 30 dB/V have been measured. A detailed study of the nonlinearity and the spurious-free dynamic range (SFDR) is presented. By optimizing the bias voltage and the input optical power, the SFDR can be improved by 10-30 dB. After minimizing the third-order distortion, an SFDR as high as 128 dB-Hz/sup 4/5/ is achieved at 10 GHz. A simple link measurement was made using this EAM and an erbium-doped fiber amplifier and a 50-/spl Omega/ terminated photodetector. At 10 GHz, a link gain of 1 dB is achieved at a detected photocurrent of 7.6 mA with higher gains at lower frequencies. The dependence of link gains on bias voltage, input optical, and radio frequency powers are investigated in detail.     

Increased linear dynamic range by low biasing the Mach-Zehndermodulator

Operating a Mach-Zehnder modulator at an optical bias below the conventional 50% (quadrature) bias is investigated. Theoretical distortion curves as a function of bias have been calculated and experimentally verified. These curves show that (single octave) linear dynamic range increases as the bias is lowered, so long as optical power at the detector can be maintained just below the saturation level or laser noise limit. This method thus provides a way for very large optical power to be used to benefit linear dynamic range. Theoretically, 20 dB of excess optical power, attenuated through low biasing, can result in an increase in linear dynamic range of 15 dB     

High-power ultralow-chirp 10-Gb/s electroabsorption modulator integrated laser with ultrashort photocarrier lifetime

A high-power, ultralow-chirp electroabsorption modulator (EAM) integrated with a distributed-feedback laser diode (EML) having ultrashort lifetime of photogenerated holes in the EAM quantum-well (QW) structure is reported for the first time. A shallow QW structure having a small valence band offset to enhance the sweepout of photogenerated holes was employed as EAM absorption layer. The measured hole lifetimes were 7-11 ps, and the measured frequency chirp (/spl alpha/-parameter) was low or negative at low EAM reverse bias voltages even under high optical output power conditions. Successful 10-Gb/s 80-km normal-dispersion single-mode fiber transmission (chromatic dispersion D=1600 ps/nm) and the record average fiber optical output power (P/sub f/) of +5.3 dBm were achieved at 25/spl deg/C. In addition, semicooled operation of EML at enhanced bit rates has been demonstrated for application in small-form-factor protocol-agnostic optical transceivers. A 10.7-Gb/s 1600-ps/nm transmission was achieved at 45/spl deg/C and P/sub f/=+3.0 dBm.     

Electric field switching in a resonant tunneling diodeelectroabsorption modulator

The basic mechanism underlying electric field switching produced by a resonant tunneling diode (RTD) is analyzed and the theory compared with experimental results; agreement to within 12% is achieved. The electroabsorption modulator (EAM) device potential of this effect is explored in an optical waveguide configuration. It is shown that a RTD-EAM can provide significant absorption coefficient change, via the Franz-Keldysh effect, at appropriate optical communication wavelengths around 1550 nm and can achieve up to 28-dB optical modulation in a 200-μm active length device. The advantage of the RTD-EAM over the conventional reverse-biased p-n junction EAM, is that the RTD-EAM has, in essence, an integrated electronic amplifier and, therefore, requires considerably less switching power     

All-optical frequency conversion in a traveling-wave semiconductorlaser amplifier

All-optical frequency conversion over the entire gain spectrum of a traveling-wave semiconductor laser amplifier is analyzed by numerical solution of a nonlinear wave equation system. The wavelength dependence of the gain coefficient g, the linewidth enhancement factor α, the differential gain dg/dN, and the gain saturation effect are contained in the model. The method yields a high conversion efficiency and a converted signal output power up to 10 dBm is obtainable. It is shown that the input signal power can vary by three orders of magnitude with nearly no degradation of the conversion efficiency. By means of the input powers, the conversion efficiency can be maximized. The dependence of the conversion efficiency is analyzed for fixed input powers. Simultaneous conversion of an optical data signal to several wavelengths is analyzed. The requirements for the output filter are outlined     

dBm average optical output power operation of small-chirp 40-gbps electroabsorption Modulator with tensile-strained asymmetric quantum-well absorption Layer

A small-chirp 40-Gbps electroabsorption modulator (EAM) with high optical output power capability has been developed for the first time. An optimized tensile-strained asymmetric quantum-well structure is employed as the absorption layer of the EAM so that small chirp and reduction of the lifetime of the photogenerated holes for high optical output power tolerance is obtained. Deteriorations of frequency response and chirp due to carrier pileup under high optical output power conditions were prevented by enhancing carrier sweepout, which was experimentally confirmed as a hole lifetime as short as 35 ps under high optical output power conditions. As a result, good frequency response (bandwidth > 30 GHz) and small chirp (/spl alpha/<1) were obtained under the condition of the zero bias voltage and +4.5 dBm continuous-wave (CW) optical output power (P/sub out,CW/). Clear eye opening and high dynamic extinction ratio under 40-Gbps non-return-to-zero modulation persisted to a high average output power (P/sub out,ave/) condition of P/sub out,ave/=+1.0 dBm.     

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     

Amplified spontaneous emission and gain characteristics ofFabry-Perot and traveling wave type semiconductor laser amplifiers

Semiconductor laser amplifiers are investigated with respect to amplified spontaneous emission power and gain characteristics. The influence of the spontaneous emission coefficient, input power, and facet reflectivity on amplifier saturation characteristics is analyzed. It is shown that a small β, zero reflectance device has advantageous properties in terms of high gain and output power at the 3-dB compression point. The low reflectance contributes to a slow saturation, whereas a low β means a larger optical model and hence lower intensities and less saturation for a given output power or gain     

All-optical wavelength conversion by EAM with shifted bandpass filter for high bit-rate networks

We report an original optical wavelength converter for transparent network and high-speed transmission. Based on an electroabsorption modulator (EAM) in cross-absorption modulation configuration, a red-shifted center wavelength filtering of the converted signal decreases the impact of the limited EAM response time. This original and simple device demonstrates wavelength conversion at 40 Gb/s, leading to a reduction of 4.7 dB on the bit-error rate (BER) penalty compared to the case of a centered filter. An absolute 1.8-dB BER power penalty, low switching energy (500 fJ/data pulse) and small applied DC bias are reported.     

High-power high-efficiency 0.98-μm wavelengthInGaAs-(In)GaAs(P)-InGaP broadened waveguide lasers grown by gas-sourcemolecular beam epitaxy

We describe the design and experimental results for high-power, high-efficiency, low threshold current, 0.98-μm wavelength, broadened waveguide (BW) aluminum-free InGaAs-(In)GaAs(P)-InGaP lasers. The decrease in the internal losses with an increase in the width of the waveguide layer for a separate-confinement heterostructure multiple-quantum-well (SCW-MQW) structure is attributed to lower free-carrier absorption due to the reduced overlap of the optical mode with the highly doped cladding regions. The BW lasers grown with both InGaAsP and GaAs waveguides show lower internal losses and similar threshold currents than those designed for an optimum optical confinement factor within the QW region. We report a record-low internal loss of 1.8±0.2 cm^-2 for (In)GaAs(P)-InGaP lasers grown by gas-source molecular beam epitaxy (GSMBE). The temperature dependence of internal loss suggests that optical loss from free-carrier absorption in the waveguide dominates at T>40°C, while near room temperature, the residual loss is attributed to scattering and free-carrier absorption in the QW's. The relative insensitivity of internal loss near room temperature has enabled the use of a simplified InGaAs-GaAs-InGaP BW structure to achieve very high CW and quasi-CW (QCW) power operation. We report the highest CW output power of 6.8 W for a GaAs-InGaP laser, and the highest quasi-continuous output power of 13.3 W measured for a single 100-μm-wide aperture, 0.8-0.98-μm wavelength Al-free laser diode grown by GSMBE     

Wavelength switching and routing using absorption and resonance

A resonator side coupled to a pair of waveguides can switch an optical signal between two ports by means of absorption. The absorptive mechanism is used to suppress the resonant power transfer, rather than to promote loss. Thus, the input signal only suffers small attenuation, provided that the mode volumes of the resonators can be made small enough. Multiply-coupled resonators lead to improved crosstalk performance for both the ON and OFF switched states. The performance of such devices are analyzed analytically, and universal switching curves are derived     

Analysis of large-signal dynamic characteristics of 10-Gb/s tunable distributed Bragg reflector lasers integrated with electroabsorption modulator and semiconductor optical amplifier based on the time-dependent transfer matrix method

We investigate large-signal dynamic characteristics of tunable distributed Bragg reflector (DBR) lasers with different structures. Because of high chirp and complex structures of tunable DBR lasers, it is difficult to accurately analyze large signal chirp and output pulse shapes with analytical models. Therefore, we apply a numerical model based on the time-dependent transfer matrix method to tunable DBR lasers integrated with electroabsorption modulator (EAM) and semiconductor optical amplifier (SOA). Using this model, we find a suitable /spl alpha/-parameter range in the Bragg grating section (/spl alpha//sub Bragg/-parameter) that produces the tolerable wavelength chirp while maintaining a proper tuning range. To employ the tunable lasers in wavelength division multiplexed networks and improve flexibility of the networks, the tunable lasers should have high output power and low wavelength chirp. According to our simulation results, the EAM section had better be located in between SOA and DBR laser sections to obtain high output power and low wavelength chirp.     

A Passive Broadband Impedance Equalizer for Improving the Input Return Loss of Electro-Absorption Modulators

Based on the context of an electro-absorption modulator integrated laser and the industry's standard 7-pin package having a RF connector, a complete module design that incorporates a simple yet effective broadband impedance equalizer to improve the input return loss of electro-absorption modulators (EAM) is proposed. The passive broadband impedance equalizer effectively reduces the input return loss of the EAM from between -9.84 and -5.26 dB to between -11.5 and -9.35 dB over a frequency range from 4.4 to 12.2 GHz without having to add a resistor in series with the EAM or reducing the EAM absorption-layer capacitance from the value that was optimized for optimal optical performance.     

Chirp Control of an Electroabsorption Modulator to be used for Regeneration and Wavelength Conversion at 40 Gbit/s in All-Optical Networking

Chirp control to produce low or negative values of chirp at the output of an electroabsorption modulator (EAM) is an important mechanism for reducing the signal degradation due to chromatic dispersion in high-speed transmission over standard single-mode fibre. An analytical model for the chirp performance of an EAM capable of optical regeneration and simultaneous wavelength conversion operating at 40 Gbit/s is derived. A chirp control approach is identified using this model by exploring the tradeoff between the α-parameter describing the chirp factor (based on the nonlinear absorption coefficients) and bias voltage requirements of an EAM. In particular, an optimum range of bias voltage is determined to ensure reduced chirp operation when a two-tone signal (i.e., comprised of bias and modulating voltages) is applied to the EAM. It is also demonstrated for large signal operation at 40 Gbit/s that the optimum range of reverse bias voltage is between 0 and 2 V to obtain low values for the chirp factor (between +1 and −2) in order to facilitate the necessary chirp control in all-optical networking. In addition, it is identified that at 40 Gbit/s higher positive values of the second- and third- order nonlinear coefficients of chirp must be avoided when operating at reverse bias voltages less than 1 V.     

An analysis of the large-signal characteristics of AlGaAs/GaAsheterojunction bipolar transistors

The large-signal characteristics of AlGaAs/GaAs heterojunction bipolar transistors are reported. A harmonic balance analysis technique is used for their analysis. This is based on equivalent circuit extractions using approximate physical equations for constraining the fitted solutions and for describing certain circuit element value bias trends. Class A and Class AB large signal behavior was measured and modeled satisfactorily. Power saturation is shown to occur due to the input signal entering the cutoff or the saturation region of the HBT operations. This is illustrated by time-dependent current/voltage waveforms and the power dependence of large-signal equivalent circuit elements. Depending on device bias and matching conditions the main courses of nonlinearities in device output may be caused by the nonlinearities in transconductance, input conductance, and base-collector capacitance     

Influence of the confinement factor on the wavelength-dependentoutput properties of a tapered traveling-wave semiconductor amplifier

We analyze the influence of the confinement factor on the output properties of a tapered traveling-wave semiconductor amplifier as a function of the wavelength of the input field and current injected into the amplifier. The interaction of the optical field with the amplifying medium is described by means of a susceptibility function taking into account both the gain and the carrier-induced antiguiding effect, while the beam propagation method is used to describe the evolution of the field along the amplifier. We find that, for the same input power, devices with lower optical confinement factors allow for the same output optical power with better beam quality and higher saturation power than devices with higher optical confinement factor, in agreement with the available experimental results     

Analysis of basic four-wave mixing characteristics in asemiconductor optical amplifier by the finite-difference beampropagation method

We have numerically analyzed nondegenerate four-wave mixing (FWM) among short optical pulses in a semiconductor optical amplifier (SOA) by the finite-difference beam propagation method (FD-BPM). We used the nonlinear propagation equation taking into account gain spectrum dynamic gain saturation which depends on carrier depression, carrier heating, and spectral hole-burning, group velocity dispersion, self-phase modulation, and two-photon absorption. To analyze FWM in an SOA, the evolution in time and spectral domain of two input optical pulses with different frequencies during propagation was calculated. From this simulation, it has become clear that the method me used here is a very useful technique for simulating FWM characteristics in SOA's. We also found that the wavelength dependence of the gain is crucial if the detuning is larger than 1 THz