Traveling-wave Electroabsorption Modulators Research Articles

New Impedance Matching Scheme for 60 GHz Band Electro-Absorption Modulator Modules

This letter proposes a new impedance matching scheme of a traveling wave electro-absorption modulator (TWEAM) module for a 60 GHz band radio-over-fiber (ROF) link. A microstrip bandpass filter (BPF) was used to achieve impedance matching at the 60 GHz band, and termination resistance was carefully designed to obtain an input impedance close to 50 Ω. Also, a bias circuit for the device was designed in the module. The measured return loss and frequency response show that the modulator module observes the characteristics of a filter without the need of a further tuning process.

Enhanced performance in traveling-wave electroabsorption Modulators based on undercut-etching the active-region

A novel traveling-wave electroabsorption modulator (TWEAM) based on undercut-etching-active-region waveguide (UEAW) is proposed and demonstrated. By selectively wet-etching the InGaAsP from InP to reduce InGaAsP active region, the waveguide can reduce parasitic capacitance with high conductivity in n-InP and p-InP cladding layers. In comparison with conventional ridge-waveguide (RW) TWEAM, 3 dB lower optical-insertion-loss, at least 6 dB higher in radio-frequency-link gain (dc to 40 GHz), and faster electrooptical response (3-dB bandwidth of 25 GHz at 50 /spl Omega/-termination for UEAW and 15 GHz for RW) are obtained in UEAW-TWEAM. A 10-Gbs/s operation with low swing voltage of 0.6 V has been achieved in UEAW-TWEAM, a 3.2-dB enhancement over RW-TWEAM. It indicates the tradeoff in designing electroabsorption modulators can be greatly released.

Analog Performance of Multiple-quantum-well Electroabsorption Modulator

Linearity is an important property of optical devices in analog communication systems. In this paper, we study the 3rd-order intermodulation distortion (IMD3) of an InP/InGaAsP multiple-quantum-well (MQW) traveling-wave type electroabsorption modulator (TW-EAM). We observe that dips in the measured IMD3 results are different from those by general transfer curve of electroabsorption modulators (EAMs). In order to verify it, we analyse the phenomena by using the absorption coefficients according to a wavelength and a bias voltage. Thus, we can see that the dips result from the quantum-confined Stark effect (QCSE). We propose the method to enhance linearity of MQW-EAMs by using this effect in addition to the E/O response

Optical label swapping using payload envelope detection circuits

Optical label swapping of 10-Gb/s nonreturn-to-zero (NRZ) labels attached to 40-Gb/s return-to-zero payloads is demonstrated using payload envelope detection (PED) circuits based on 40-Gb/s clock recovery with nanosecond locking time. The circuits generate a digital envelope signal representing the payload location without having to process the 40-Gb/s data. The low-frequency PED signal, which is generated from the recovered 40-GHz packet clock by a radio-frequency mixer, can be utilized to erase/rewrite the label through traveling-wave electroabsorption modulators. This approach does not require active timing control to erase the label. Nearly penalty-free rewriting of a new 10-Gb/s NRZ label was demonstrated.

Compact optical 3R regeneration using a traveling-wave electroabsorption modulator

We propose and demonstrate a novel reamplification, retiming, and reshaping regeneration approach that utilizes the photocurrent signal and the nonlinear electrooptical transfer function of a traveling-wave electroabsorption modulator in an electrical ring oscillator. All the required functionalities such as clock recovery, pulse generation, and nonlinear decision are implemented with the same device, leading to a very compact configuration. In addition, wavelength conversion and electrical signal monitoring can be realized at the same time. With a degraded 10-Gb/s return-to-zero input, 1.0 dB of negative power penalty and 50% timing jitter reduction are measured after regeneration.

Temperature-dependent effects in high-speed travelling-wave electroabsorption modulators

The temperature-dependent effects in a segmented travelling-wave electroabsorption modulator are demonstrated and analysed. Optimum operation voltages with the highest modulation efficiency at different temperatures are identified. This can ensure the modulator working at 50 Gbit/s with RF extinction ratio >8.4 dB between 10–50°C at λ=1540 nm.

Optimization of Packaging Design of TWEAM Module for Digital and Analog Applications

Packaging technologies for a broadband and narrowband modulator with a traveling wave electroabsorption modulator (TWEAM) device were developed. In developing a broadband modulator, the effects of the device and packaging designs on the broadband performance were investigated. The optimized designs were obtained through a simulation with the result that we developed a broadband modulator with a 3 dB bandwidth of 38 GHz in the electrical-to-optical (E/O) response, an electrical return loss of less than −10 dB at up to 26 GHz, an rms jitter of 1.832 ps, and an extinction ratio of 5.38 dB in a 40 Gbps non-return to zero (NRZ) eye diagram. For analog application, the effect of the RF termination scheme on the fractional bandwidth was studied. The microstrip line with a double stub as a matching circuit and a laser trimming process were used to obtain an S11 of −34.58 dB at 40 GHz and 2.9 GHz bandwidth of less than −15 dB.

40-Gb/s Optical Clock Recovery Using a Compact Traveling-Wave Electroabsorption Modulator-Based Ring Oscillator

Compact optical clock recovery is demonstrated by utilizing filtered and amplified photocurrent in a traveling-wave electroabsorption modulator. The 40-GHz optical clock was recovered from a 40-Gb/s optical time-division multiplexed signal and transferred to a new clock wavelength. The recovered optical clock has 500-fs timing jitter and 8-ps pulsewidth.

Nonlinear Properties of Traveling-Wave Electroabsorption Modulator

We report the nonlinear properties of traveling-wave electroabsorption modulators experimentally and theoretically. The saturation characteristics of the dc transfer curve give rise to the superlinear, linear, and sublinear properties of output signal levels against the input signal levels. The dynamic behavior of the transmission curves for a wide range of the input radio-frequency signal levels can be described using the dc transfer curve and the fast Fourier transform algorithm.

Simultaneous 160-Gb/s Demultiplexing and Clock Recovery by Utilizing Microwave Harmonic Frequencies in a Traveling-Wave Electroabsorption Modulator

A traveling-wave electroabsorption modulator (TW-EAM) is used to realize three simultaneous functions: demultiplexing, detection, and pulse generation. These coexisting functions are achieved by utilizing microwave harmonic frequencies and independent wavelengths in the TW-EAM. When combined with a phase-locked-loop, these functions enable simultaneous optical demultiplexing, electrical clock recovery, and optical clock generation at line-rates of 40 and 160-Gb/s.

Slope Efficiency and Dynamic Range of Traveling-Wave Multiple-Quantum-Well Electroabsorption Modulators

High-performance InGaAsP-InP multiple-quantum-well traveling-wave electroabsorption modulators (EAMs) have been developed for analog optical links. A high slope efficiency of >4/V at 1.55 μm for a 300-μm-long EAM is 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.

Photocurrent-Assisted Wavelength (PAW) Conversion With Electrical Monitoring Capability Using a Traveling-Wave Electroabsorption Modulator

A new mechanism of cross-absorption modulation is proposed and experimentally demonstrated to assist wavelength conversion using a traveling-wave electroabsorption modulator (TW-EAM). The photocurrent signal generated by the pump propagates along the TW electrodes and changes the absorption of the waveguide, which imprints data to the probe. The photocurrent signal can also be received by an external electronic circuit to provide monitoring capability. This photocurrent-assisted mechanism does not rely on the saturation of absorption and has the potential to reduce the high pumping power required by EAM-based wavelength converters. Using 2.5-Gb/s nonreturn-to-zero data, the conversion range can cover 30 nm in C-band and the lowest power penalty is 0.5 dB.

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 π/ 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 xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> 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.

Performance of traveling-wave electroabsorption modulators depending on microwave properties of waveguides calculated using the FDTD method

We present a method to analyze electrical and optical characteristics of traveling-wave electroabsorption modulators (TW-EAMs) using the finite-difference time-domain method. We consider the interaction between electromagnetic fields and optical powers in waveguides in time domain to model electrical and optical behaviors of TW-EAMs. The effects of microwave properties of waveguides on modulation response and output optical power of TW-EAMs are analyzed by the proposed method. The characteristic impedance of TW-EAMs is more important than microwave index to obtain large modulation bandwidth of TW-EAMs when impedance matching techniques are not used. However, effective refractive index match between microwave and lightwave becomes important as the waveguide length increases. The microwave property closely related to the extinction ratio of output optical powers is the microwave loss. When impedance match is achieved by low-impedance termination, the velocity matching between microwave and lightwave becomes important.

Self-consistent time-domain large-signal model of high-speed traveling-wave electroabsorption modulators

A new self-consistent large-signal model for traveling-wave electroabsorption modulators (TW EAMs) is presented. A time-domain finite-difference approach is exploited to carry out a fully coupled analysis of the nonlinear distributed interaction between the microwave and optical fields in the device. RF and optical nonlinearities and saturation effects are taken into account, as well as the influence on the microwave electrode propagation parameters of the nonuniform distribution of the optical power along the traveling direction. The model is applied to the analysis of a InGaAsP/InP TW EAM in small- and large-signal operation. Its performance in terms of bandwidth, linearity, and chirp are investigated as examples of application. The technique is validated in small-signal low optical-power condition through a comparison with the results of a small-signal frequency-domain approach.

Microwave CAD circuit modeling of traveling-wave electroabsorption modulator

Conformal-mapping techniques are used to derive an effective and advanced calculation model for the microwave properties of a light-intensity traveling-wave electroabsorption modulator, resulting in a quasi-TEM computer-aided-design circuit model. For a given device geometry, the model provides the transmission-line (TML) parameters' characteristic impedance and waveguide (WG) propagation constant. Closed-form expressions are derived for the reactive circuit elements. The calculation model includes both the effect of metal conduction loss, as well as losses due to induced currents in the semiconductor substrate. We treat both the buried heterostructure, as well as the ridge-mesa optical WG implementations. The model is validated with full-wave simulations and compared to measured TML parameters.

Nonlinearity of traveling-wave electroabsorption modulator according to microwave characteristics

In this paper, we introduce a model to analyze the linearity of multiple-quantum-well (MQW) traveling-wave electroabsorption modulator (TW-EAM). The device length, microwave loss (ML), velocity mismatch (VM) between lightwaves and the microwaves, and internal reflection (IR) due to impedance mismatch have effect on the linearity of a TW-EAM. ML has little effect on the intermodulation distortions (IMDs). VM also has little influence on IMD3 (third-order IMD), but severely affect IMD2 (second-order IMD), which become as high as 30 dB for the input frequency of 100 GHz, the device length of 0.8 mm, and VM of 20%. IR has different nonlinear characteristics according to the wavelength of the input frequency and the device length. As a result, it is known that IMDs improve when the device length equal to a quarter of the wavelength of the input signal and that the device length of 0.4 mm is appropriate for the analog fiber-optic communication with the frequency range of 50 GHz.

InP-InGaAsP high-speed traveling-wave electroabsorption modulators with integrated termination resistors

Traveling-wave electroabsorption modulators for operation at 1.55 /spl mu/m have been designed and fabricated. Devices of different lengths were characterized. Modulators with integrated termination resistors showed wide modulation bandwidths and excellent bandwidth-length products. A bandwidth of 43 GHz was measured for a 450-/spl mu/m-long device, which corresponds to a 19.3-GHz/spl middot/mm bandwidth length product. For a device length of 250 /spl mu/m, a bandwidth of 67 GHz is extrapolated from measurements up to 45 GHz.

High extinction ratio and saturation power traveling-wave electroabsorption modulator

An InGaAsP multiquantum-well traveling-wave electroabsorption modulator is demonstrated with high extinction ratio and modulation efficiency. By designing a strain-compensated quantum-well active region with traveling-wave design, high saturation power (>14 dBm) for >20-GHz high-speed performance (1.5 dB drop at 20 GHz) is achieved. Due to high modulation efficiency (>30 dBN for 0 to 1 V 40-dB extinction ratio in 2 V), error free 10-Gb/s operation with 1 V/sub p-p/ driving voltage is obtained. By comparing codirections and counterdirections of optical and microwave interactions, pulse generation at 40 GHz shows that the traveling-wave performance has an advantage for short pulses with high-power output, where pulsewidth as short as 4.5 ps is obtained in this kind of device.

160 Gbit∕s error-free demultiplexing by ultrafast optical gate monolithically integrating photodiode and electroabsorption modulator

A 320 Gbit/s error-free demultiplexing operation with a receiver sensitivity of −18 dBm using a monolithic optical gate integrating a uni-travelling-carrier photodiode and a travelling-wave electroabsorption modulator has been demonstrated. High on/off ratios of 14 dB for adjacent channels and 28 dB for the others have also been achieved.