Small-signal Modulation Research Articles

High frequency modeling for quantum-well laser diodes

High frequency modeling of quantum-well (QW) laser diodes for optoelectronic integrated circuit (OEIC) design is discussed in this paper. Modeling of the intrinsic device and the extrinsic components is discussed by accounting for important physical effects at both dc and high frequency. The concepts of equivalent circuits representing both intrinsic and extrinsic components in a QW laser diode are analyzed to obtain a physics-based high frequency model. The model is based on the physical rate equations, and is versatile in that it permits both small- and large-signal simulations to be performed. Several procedures of the high frequency model parameter extraction are also discussed. Emphasis here is placed on validating the model via a comparison of simulated results with measured data of the small-signal modulation response, obtained over a wide range of optical output powers.

Frequency response of large aperture oxide-confined 850 nm vertical cavity surface emitting lasers

Small and large signal modulation measurements are carried out for 850 nm vertical cavity surface emitting lasers (VCSELs). The resonance frequency, damping factor, parasitic frequency, and D-factor are extracted. Small signal modulation bandwidths larger than 20 GHz are measured. At larger currents the frequency response becomes partially limited by the parasitics and damping. Our results indicate that by increasing the parasitic frequency, the optical 3 dB bandwidth may be extended to ∼25 GHz. A decrease in the damping should enable VCSEL bandwidths of 30 GHz for current densities not exceeding ∼10 kA/cm2 and ultimately error-free optical links at up to 40 Gbit/s.

Extraction of semiconductor microchip differential gain by use of optically pumped semiconductor laser

The small-signal modulation response of a vertical external cavity surface emitting laser is analyzed to determine its resonance frequency in relation to photon density, allowing nondestructive extraction of characteristic parameters of chips, such as internal loss and differential gain.

Improvement of modulation bandwidth in multisection RSOA for colorless WDM-PON

We demonstrated two-section reflective semiconductor optical amplifier (RSOA) with dramatic improvement of small-signal modulation bandwidth above 10 GHz as colorless source for wavelength division multiplexed-passive optical network (WDM-PON). The device provides the fiber-to-fiber gain of 22.8 dB, 3-dB amplified spontaneous emission (ASE) bandwidth of 30 nm, and ripple of 1.5 dB. Good performance at 2.5 Gbps was obtained with an extinction ratio of 8 dB and a power penalty of 2 dB at a 10(-9) bit error rate (BER) up to 20 km transmission.

Influence of Residual Facet Reflection on the Eye-Diagram Performance of High-Speed Electroabsorption Modulated Lasers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The influence of residual facet reflection on the small-signal modulation response and the large-signal eye-diagram performance of 40 Gb/s electroabsorption modulated lasers (EMLs) is studied. Measurement and simulation show that the eye-diagram performance is not only limited by the modulation bandwidth of the device, but also significantly degraded by the low frequency resonances in the frequency response due to residual facet reflection. Clear eye-opening has been demonstrated by optimizing the anti-reflection (AR) coating of the EA modulator facet. </para>

General Method for Calculating the Response and Noise Spectra of Active Fabry–Perot Semiconductor Waveguides With External Optical Injection

We present a theoretical method for calculating small-signal modulation responses and noise spectra of active Fabry-Perot semiconductor waveguides with external light injection. Small-signal responses due to either a modulation of the pump current or due to an optical amplitude or phase modulation of the input field can be calculated. Both responses and noise spectra are given through semianalytical expressions taking into account the longitudinal extent and finite end-facet reflectivities of the active device. Different examples of responses and spectra are presented for semiconductor optical amplifiers and an injection-locked laser. We also demonstrate the applicability of the method to analyze slow and fast light effects in semiconductor waveguides. Finite reflectivities of the facets are found to influence the phase changes of the injected microwave-modulated light.

Modeling of the Transient Characteristics of Heterojunction Bipolar Transistor Lasers

A charge control model for the transient analysis of the bipolar transistor laser in forward active mode is developed to describe the dynamics of electron, photon and charge densities. From three coupled-rate equations, analytical expressions are obtained for the threshold base current density, the steady-state electron and photon densities, and small-signal frequency response. We find that the threshold current density decreases with increasing spontaneous emission lifetime of electrons in the quantum well as well as a linear dependence of the photon density on the base current. By optimizing the model parameters in their physical range, we show that a -3-dB bandwidth of 55 GHz can be achieved for small-signal modulation of the laser. By numerically solving the rate equations, we investigate the -3-dB bandwidth under large-signal modulation to show that it remains nearly unchanged from the small-signal case. We further perform analysis of laser switching and find that the turn-on time is reduced by increasing the base current or decreasing the spontaneous emission lifetime of electrons in the quantum well.

Modulation Bandwidth and Linewidth Enhancement Factor of High-Speed 1.55-$\mu$m Quantum-Dash Lasers

InAs-InGaAlAs-InP quantum-dash lasers have been fabricated showing continuous-wave operation up to 100degC with a characteristic temperature of 88 K between 25degC and 85degC and output powers above 27 mW at room temperature (RT). The small-signal modulation bandwidth of 10 GHz at RT amounts still to 4 GHz at 85degC. The linewidth enhancement factor above threshold is evaluated by means of the frequency-modulation/amplitude-modulation method in dependence on modulation frequency and drive current, exhibiting a value of 2.5 slightly above threshold.

Theoretical and Experimental Study of High-Speed Small-Signal Cross-Gain Modulation of Quantum-Dot Semiconductor Optical Amplifiers

We numerically and experimentally investigate the high-speed small-signal cross-gain modulation (XGM) characteristics of a quantum-dot (QD) semiconductor optical amplifier (SOA). From a p-doped QD SOA operating at 1.3 mum, high-speed small-signal XGM responses up to 40 GHz are measured from low to high injection currents and improve at high injection currents. In the numerical model, we set up about six hundred coupled rate equations, where the carrier dynamics of QD electron and hole states are considered separately and the enhanced hole occupation due to p-type doping is included. The high-speed small-signal XGM spectra are calculated at various modulation frequencies and pump-probe detunings. We identify how the two separate XGM mechanisms of total carrier density depletion (TCDD) at low injection current and spectral hole burning (SHB) at high injection current affect the high-speed small-signal XGM behavior. From the measured and calculated results, we show that high-speed small-signal XGM responses of QD SOAs can be improved by injecting more carriers to the QD excited states, which enhances high-speed XGM induced by SHB rather than by TCDD.

40-Gb/s Low Chirp Electroabsorption Modulator Integrated With DFB Laser

A 40-Gb/s monolithically integrated transmitter containing an InGaAsP multiple-quantum-well electroabsorption modulator (EAM) with lumped electrode and a distributed-feedback semiconductor laser is demonstrated. Superior characteristics are exhibited for the device, such as low threshold current of 20 mA, over 40-dB sidemode suppression ratio at 1550 nm, and more than 30-dB DC extinction ratio when coupled into a single-mode fiber. By adopting a deep ridge waveguide and planar electrode structures combined with buried benzocyclobutene, the capacitance of the EAM is reduced to 0.18 pF and the small-signal modulation bandwidth exceeds 33 GHz. Negative chirp operation is also realized when the bias voltage is beyond 1.6 V.

Polarization-Resolved Modulation Response of Single-Transverse-Mode Vertical-Cavity Surface-Emitting Lasers

The small-signal modulation response of vertical-cavity surface-emitting lasers (VCSELs) is studied numerically, based on the spin-flip model. A detailed characterization of the influence of various parameters, such as the dichroism, the birefringence, the spin-flip rate and the noise level is done. The analysis is performed in regions of the parameter space where the operating conditions, in the absence of modulation, are such that there is either stable single linear polarization, or bistability of two orthogonal linear polarizations or polarization instability. In the instability region the intensities of the orthogonal polarizations display anticorrelated self-sustained oscillations that result in a complex small-signal modulation response, exhibiting multiple resonance peaks with frequencies that vary with the injection current and the spin-flip rate. In contrast, in parameter regions where there is stable single polarization the modulation response of the total intensity is as that of a single-mode laser, exhibiting a conventional single-resonance peak, and is almost not influenced by the value of the spin-flip rate. Polarization mode-hopping results in a distortion of the polarization-resolved modulation response at low frequencies.

Analytical Modeling of the Transistor Laser

We derive analytic expressions for the small-signal modulation response of the transistor laser (TL) operating in the common-emitter (CE) and common-base (CB) configurations. We compare the performance (current gain and small-signal modulation bandwidth) of the TL in these two modes of operation. The CE operation results in a small-signal modulation response with the same relaxation oscillation limitations as conventional lasers. The CB configuration shows a bandwidth enhancement due to a bandwidth equalization together with a suppression of the relaxation oscillations. Finally, we show that the small-signal responses of the CB and CE can be approximated by a third-order transfer function.

High-Speed Small-Signal Cross-Gain Modulation in Quantum-Dot Semiconductor Optical Amplifiers at 1.3 $\mu$m

Wavelength conversion using cross-gain modulation (XGM) in quantum-dot (QD) semiconductor optical amplifiers (SOAs) is investigated. Small-signal measurements reveal that the XGM bandwidth as well as the conversion efficiency strongly depends on the bias current. Thus, it is possible to tune the XGM by increasing the current from a low efficiency with a 10-GHz bandwidth to a very efficient one with bandwidths well exceeding 40 GHz. Two different saturation mechanisms are responsible for this pronounced influence of the bias current: 1) total carrier depletion that leads to a slow broadband cross-gain saturation and 2) spectral hole burning that causes spectrally narrow-band high-speed XGM. With increasing current, the saturation by depleting the carrier reservoir, which feeds the QDs, is minimized, and therefore, spectral hole burning becomes more dominant. Large-signal wavelength conversion experiments using 50 ps pulses indicate that efficient high-speed XGM is feasible for pump and probe signal detuning up to 10 nm. With increasing detuning, larger pulse broadening and a decreasing efficiency are observed, consistent with the small-signal results. The results on the QD SOAs are compared to conventional quantum-well devices.

High-Speed, Low-Current-Density 850 nm VCSELs

We report on the design, fabrication, and evaluation of large-aperture, oxide-confined 850 nm vertical cavity surface emitting lasers (VCSELs) with high modulation bandwidth at low current densities. We also compare the use of InGaAs and GaAs quantum wells (QWs) in the active region. Both VCSELs reach an output power of 9 mW at room temperature, with a thermal resistance of 1.9deg C/mW. The use of InGaAs QWs improves the high-speed performance and enables a small-signal modulation bandwidth of 20 GHz at 25degC and 15 GHz at 85degC. At a constant bias current density of only 11 kA/cm2, we generate open eyes under large-signal modulation at bit rates up to 25 Gbit/s at 85degC and 30 Gbit/s at 55degC.

Monolithically Integrated 100 GHz DFB-TWEAM

A monolithically integrated distributed feedback (DFB) laser and traveling-wave electro-absorption modulator (TWEAM) with ges 100 GHz -3 dBe bandwidth suitable for Non-return-to-zero (NRZ) operation with on-off keying (OOK) is presented. The steady-state, small-signal modulation response, microwave reflection, chirp characteristic, and both data operation and transmission were investigated. The DFB-TWEAM was found to be an attractive candidate for future short distance communication in high bitrates systems.

Common-emitter and common-base small-signal operation of the transistor laser

We derive analytic expressions for the transistor laser in the common-emitter and common-base configuration and compare the performance of the transistor in these two modes of operation. We show that the common-base operation results in a wide-band, small-signal modulation response. This effect is due to reduced carrier lifetime in the base. The bandwidth equalization and the suppression of the relaxation oscillation frequency are shown. A bandwidth of 48 GHz is predicted for a vertical cavity laser biased at 10Ith.

40 GHz small-signal cross-gain modulation in 1.3 μm quantum dot semiconductor optical amplifiers

Small-signal cross-gain modulation of quantum dot based semiconductor optical amplifiers (QD SOAs), having a dot-in-a-well structure, is presented, demonstrating superiority for ultrahigh bit rate wavelength conversion. Optimization of the QD SOA high speed characteristics via bias current and optical pump power is presented and a small-signal 3 dB bandwidth exceeding 40 GHz is demonstrated. The p-doped samples investigated here enable small-signal wavelength conversion within a range of 30 nm, limited mainly by the gain bandwidth.

Fabrication and modulation characteristics of InGaAsP/InGaAsP MQW DCPBH lasers at λ∼1.57 μm

InGaAsP/InGaAsP multiple-quantum-well (MQW) double-channel planar-buried heterostructure (DCPBH) lasers emitting at λ∼ 1.57 μm were fabricated by optimizing the epitaxial growth with material characterization. At 25 °C (85 °C), a 1.8-μm-wide and 300-μm-long antireflectivity/high reflectivity coated laser exhibits a threshold current of 8 mA (23 mA) and a slope efficiency of 0.34 mW/mA (0.24 mW/mA) in continuous-wave mode (cw) as a result of the optimized thickness of the p-InP filling layer in the PBH structure with p-n-p-n current blocking layers. The maximum cw output power was approximately 20 mW at 25 °C, which was reduced to 17 mW at 85 °C. The peak wavelength varied from 1572 nm at 25 °C to 1602 nm at 100 °C for a fixed output power of 5 mW, indicating a temperature coefficient of ∼ 0.4 nm/K. The resonance frequency in the small-signal modulation response of devices depends on the etching depth of the U-shaped double channel; it increases from 0.4 GHz without channel etching to 4.3 GHz with 7-μm-thick etching. The full-width at half maximum values in the horizontal and vertical far-field patterns were approximately 24.5° (25.2°) and 29.4° (30.1°), respectively, at 25 °C (85 °C).

Turn-on dynamics and modulation response in semiconductor quantum dot lasers

We show that the dynamic response of electrically pumped semiconductor quantum dot lasers can be quantitatively understood by including the strongly nonlinear character of electron-electron scattering processes. The numerical simulations presented here combine a microscopic approach used for calculating the nonradiative scattering rates with a rate equation model used for modeling the complex dynamic turn-on behavior. Simulated turn-on delay, relaxation oscillation frequency, small-signal modulation response, and eye patterns of the quantum dot laser are presented and compared with experimental results at an emission wavelength of 1300 nm. The strong damping of the relaxation oscillations is attributed to an anomalous mechanism which involves Auger capture, including the mixed electron-hole process from the wetting layer (WL) into quantum dot states, and relies upon the pump current dependent ratio of WL electron and hole densities.

Dynamic properties of InAs∕InP (311)B quantum dot Fabry–Perot lasers emitting at 1.52μm

Dynamic properties of truly three-dimensional-confined InAs∕InP quantum dot (QD) lasers obtained by molecular beam epitaxy growth on a (311)B oriented substrate are reported. The relative intensity noise and small signal modulation bandwidth experiments evidence maximum relaxation frequency of 3.8GHz with a clear relaxation oscillation peak, indicating less damping than InAs∕GaAs QD lasers. The Henry factor amounts to ∼1.8 below threshold and increases to ∼6 above threshold, which is attributed to band filling of the thick wetting layer.