Design Analysis of a High-Speed Directly Modulated Laser with Push-Pull Silicon Ring Modulators

Silicon Photonics; Ring Modulator Transmitters

An exact solution to the steady-state dynamics of a microring modulator with simultaneous index and loss modulation is derived. We experimentally validate the theory by demonstrating the generation of an optical frequency comb from a carrier depletion-based ring modulator on silicon.

We demonstrate both the amplitude and bi-phase modulation of ultrawideband (UWB) monocycle pulses using silicon microring modulator. The integrated silicon ring modulator generates the monocycle pulse train and modulates it simultaneously. By applying different driving voltages, the amplitude and phase-modulated UWB signal with a data rate of 2.5 Gb/s are achieved. The ring modulator can be thermally tuned for a free spectral range with a ~50-mW power consumption.

We have developed a technique for laser tuning at rates of 100 kHz or more using a pair of acousto-optic modulators. In addition to all-electronic wavelength control, the same modulators also can provide electronically variable Q-switching, cavity length and power stabilization, chirp and linewidth control, and variable output coupling, all at rates far beyond what is possible with conventional mechanically tuned components. Tuning rates of 70 kHz have been demonstrated on a radio-frequency-pumped CO2 laser, with random access to over 50 laser lines spanning a 17% range in wavelength and with wavelength discrimination better than 1 part in 1000. A compact tuner and Q-switch has been deployed in a 5-10-kHz pulsed lidar system. The modulators each operate at a fixed Bragg angle, with the acoustic frequency determining the selected wavelength. This arrangement doubles the wavelength resolution without introducing an undesirable frequency shift.

High-speed directly modulated distributed feedback (DFB) laser is crucial for high-speed optical communication systems. The modulation bandwidth and frequency chirp of such devices are primarily affected by the differential gain and linewidth enhancement factor (LEF) of the quantum wells (QWs) in the active region. In this work, the optical gain properties of long wavelength AlGaInAs-InP strained quantum wells (QWs) with different crystal orientations are numerically investigated using the multi-bands k.p theory, which considers both valence-band anisotropic and nonparabolicity. Compared to the QW laser grown on a conventional (001) substrate, a higher differential gain, and smaller LEF are observed for the laser grown on the (110) substrate with the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\rm{(\bar{1}10)}}$</tex-math></inline-formula> plane as the mirror facet. This is primarily due to the reduced effective mass of the valence band. The dynamic characteristics of (001)-and (110)-oriented DFB laser is theoretically studied using the one-dimensional traveling wave model (1D TWM) at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\rm{25}}\;^\circ \text{C}$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\rm{95}}\;^\circ \text{C}$</tex-math></inline-formula>. The simulation results show that the lower chirp and wider modulation bandwidth can be achieved for the QW laser grown on (110) substrate.

We demonstrate new power-efficient techniques for generating high-fidelity waveforms from directly modulated lasers using periodic high-contrast spectral and temporal windowing. Real-time optical M-FSK communication experiments demonstrate flexible and scalable transceiver designs with record sensitivity.

Silicon ring modulators with interdigitated diodes demonstrate 7.5 dB extinction ratios at 3 dB insertion loss for 1 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> , a 4.5 dB improvement over the lateral diode design, and can operate up to 10 Gb/s.

We have made two-beam smart-pixel optical receivers using a hybrid attachment of GaAs-AlGaAs multiple quantum-well (MQW) pin devices to foundry-fabricated 0.8-μm linewidth CMOS circuits. Results from a repeater in which receiver output is coupled to a transmitter circuit driving a differential pair of MQW modulators are reported. When tested with high-contrast, directly-modulated laser diodes, an optical energy of 26 fJ (-21.5 dBm) in each beam is required to obtain a bit error rate of 1×10/sup -9/ at 622 Mb/s, and operation at this error rate is observed to 1 Gb/s. The described receiver (one of several we have made) has three amplification stages, with the first being of the transimpedance type. The reported receiver fits easily within a 45×25 μm area, and the entire repeater circuit draws about 2 mA from a 5-V power supply, with the transmitter accounting for about 20 percent of the total.

The proliferation of high-bandwidth applications has given rise to a growing interest, between network providers, on upgrading networks to deliver broadband services to homes and small businesses. There has to be a good balance between the total cost of the infrastructures and the services that can be offered to the end users, as they are very sensitive to equipment outlay, requiring the use of low-cost optical components. Coarse Wavelength Division Multiplexing (CWDM) is an ideal solution to the tradeoff between cost and capacity (Thiele, 2007). This technology uses all or part of the 1270 to 1610 nm wavelength fiber range with an optical channel separation of about 20 nm. This channel separation allows the use of low-cost, uncooled, Directly Modulated Lasers (DML). The main advantages of these devices associated with uncooled operation are (Nebeling, 2007); No integration of TEC and cooler required, less complexity for control electronics, reduced power consumption, only laser diode current required, lower device cost. Otherwise, the direct modulation of the laser current leads to a modulation of the carrier density giving rise to a chirp frequency. This results in a broadened linewidth and a laser wavelength drift (Henry, 1982; Linke, 1985). Since wavelength chirp was recognized several years ago, (Koch, 1988; Hinton, 1993) many papers have addressed the causes and implications of chirp on the optical system performance (Cartledge, 1989; Hakki, 1992; Horche, 2008). However, a large part of the research has focused on fiber transmission properties. The idea of these studies is, generally, based on considerations regarding the interaction of the laser frequency chirp with the fiber dispersion. It is known that if the chirp parameter (-factor) is positive, as is always the case for directly modulated lasers, then the frequency components of the leading edge of the pulse will be blue-shifted and the trailing edge red-shifted. If the pulses are transmitted over a negative dispersion fiber where the blue wavelength is slower than the red, to some extent, pulse compression and significant transmission performance improvement is expected (Morgado & Cartaxo, 2001; Tomkos et al., 2001). To counteract the effect of chromatic dispersion, the system can be moved to a non-zero negative dispersion-shifted fiber, but this method has some problems that are difficult to resolve when it comes to a CWDM metropolitan or access network; The traffic must be interrupted. This method may only be used for upgrading long wavelength channels where

Modulation characteristics of single-longitudinal-mode semiconductor lasers are sufficiently well understood to permit meaningful modeling of high-bit-rate dispersive optical fiber communications systems. Semiconductor laser modulation dynamics are briefly reviewed with an emphasis on the interdependence of wavelength chirping and damping of relaxation oscillations. Beginning with the basic equations governing the laser optical field, computer simulations are described which evaluate the system 'eye closure' resulting from chirping and related laser modulation constraints. The effects of laser structure, parasitics and intentional transmitter and receiver current filtering are explored. Tolerances for deviation from the zero dispersion wavelength are estimated for bit rates up to 16 Gb/s. The analysis indicates that proper choice of linear transmitter and receiver filtering can significantly reduce degradation resulting from inherently nonlinear chirping-induced dispersive distortion. >

Summary form only given, as follows. Novel structures and mechanisms for high-speed modulation of semiconductor lasers are proposed and analyzed. The modulator consists of an anti-resonant Fabry-Perot cavity constituted by deep etched subwavelength trenches, which acts as a rear reflector of the laser. The change of the absorption coefficient in the modulator results in a change in the Q-factor of the laser, and consequently the lasing threshold and output power. An example structure and simulation results for a distributed feedback laser is presented. The monolithically integrated Q-modulated laser (QML) has advantages of high speed, high extinction ratio, low wavelength chirp and no insertion loss, and it does not require bandgap engineering of the quantum well material as in conventional electroabsorption modulated laser (EML).

We present an accurate, easy-to-use large-signal SPICE circuit model for depletion-type silicon ring modulators (Si RMs). Our model includes both the electrical and optical characteristics of the Si RM and consists of circuit elements whose values change depending on modulation voltages. The accuracy of our model is confirmed by comparing the SPICE simulation results of 25 Gb/s non-return-to-zero (NRZ) modulation with the measurement. The model is used for performance optimization of monolithically integrated Si photonic NRZ and pulse-amplitude-modulation 4 transmitters in the standard SPICE circuit design environment.

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We study the process variability impact of silicon ring modulators on resonance wavelength, which is a key parameter to design WDM communication systems based on those devices.

A silicon dual-ring modulator designed for chirp tuning in an intensity-modulated system is described and its performance is modelled. Previous experimental work using this device geometry partially demonstrates the advantages of the dual-ring approach. However, we provide here the first comprehensive theoretical treatment from which optimal operation parameters can be deduced. The device consists of two, over-coupled micro-ring resonators independently coupled to a MachZehnder interferometer and driven by a push-pull signal. Utilizing the interference effect provided by the Mach-Zehnder geometry, the device produces a large modulation depth and improved linearity, compared to single ring geometries, provided that the appropriate resonance detuning between the two micro-ring resonators and the correct phase condition are met. The differential drive configuration generates opposing signs of dispersion from the two rings leading to an adjustable modulation chirp that can be tuned into the negative or positive regime, or can be fixed at an essentially zero-chirp condition. System-level simulation is reported to validate the chirp tuning for a non-return-to-zero signal at a modulation rate of 28 Gb/s.