High-speed Modulation Characteristics Research Articles

A tutorial on laser-based lighting and visible light communications: device and technology [Invited

This tutorial focuses on devices and technologies that are part of laser-based visible light communication (VLC) systems. Laser-based VLC systems have advantages over their light-emitting-diode-based counterparts, including having high transmission speed and long transmission distance. We summarize terminologies related to laser-based solid-state lighting and VLC, and further review the advances in device design and performance. The high-speed modulation characteristics of laser diodes and superluminescent diodes and the on-chip integration of optoelectronic components in the visible color regime, such as the high-speed integrated photodetector, are introduced. The modulation technology for laser-based white light communication systems and the challenges for future development are then discussed.

32-Gbps single silicon microring resonator-loaded Mach–Zehnder modulator

Design and high-speed modulation of a compact and low-voltage silicon microring resonator-loaded Mach–Zehnder modulator (Si MRR-MZM) with a lateral P–N junction are discussed. The driving voltage or the size of a Si MZM is expected to be significantly reduced owing to an enhanced phase shift in an MRR. Modulation characteristics of the MRR-MZM are analyzed on the basis of different device parameters, such as the coupling efficiency K and round-trip length of an MRR. Si MRR-MZMs with different device parameters are fabricated using a complementary metal–oxide–semiconductor (CMOS)-compatible process and their high-speed modulation characteristics are compared. A half-wave voltage of 3.4 V is demonstrated in an MRR-MZM with K = 0.12 and a round-trip length of 128 µm, and the product of the half-wave voltage and the length of a phase shifter (VπL) was decreased to 0.036 V cm. The 3 dB bandwidth of an MRR-MZM with K = 0.21 and a round-trip length of 180 µm is measured to be approximately 16 GHz, and nonreturn-to-zero (NRZ) modulation up to 32 Gbps is successfully demonstrated for an operation voltage of 4.0 V.

12.5 Gbit/s discretely tunable InP-on-silicon filtered feedback laser with sub-nanosecond wavelength switching times.

A heterogeneously integrated InP-on-silicon fast tunable filtered feedback laser is demonstrated. The laser device consists of a main Fabry-Pérot cavity connected to an integrated arrayed waveguide grating of which the outputs form external cavities in which semiconductor optical amplifiers can be switched to provide single-mode operation and tunability. The laser can operate at four different wavelengths whereby switching between each wavelength channel is done within one nanosecond. For each wavelength channel 12.5 Gbit/s NRZ-OOK direct modulation is demonstrated. The combination of fast wavelength switching with straightforward wavelength control and high-speed direct modulation characteristics make the demonstrated laser structure very attractive for use in optical packet or burst switching systems.

Relative intensity noise in high-speed hybrid square-rectangular lasers

Relative intensity noise (RIN) and high-speed modulation characteristics are investigated for an AlGaInAs/InP hybrid square-rectangular laser (HSRL) with square side length, rectangular length, and width of 15,300, and 2 μm, respectively. Single-mode operation with side-mode suppression larger than 40 dB has been realized for the HSRL over wide variation of the injection currents. In addition, the HSRL exhibits a 3 dB modulation bandwidth of 15.5 GHz, and an RIN nearly approaches standard quantum shot-noise limit 2hv/P=−164 dB/Hz at high bias currents due to the strong mode selection of the square microcavity. With the increase of the DC bias current of the Fabry–Perot section, significantly enhanced modulation bandwidth and decreased RIN are observed. Furthermore, intrinsic parameters such as resonance frequency, damping factor, and modified Schawlow–Townes linewidth are extracted from the noise spectra.

Mode and modulation characteristics for microsquare lasers with a vertex output waveguide

The mode and high-speed modulation characteristics are investigated for a microsquare laser with a side length of 16 μm and a 2-μm-wide output waveguide connected to one vertex. The longitudinal and transverse mode characteristics are analyzed by numerical simulation and light ray model, and compared with the lasing spectra for the microsquare laser. Up to the fifth transverse mode is observed clearly from the lasing spectra. Single mode operation with the side mode suppression ratio of 41 dB is realized at the injection current of 24 mA, and the maximum output power of 0.53 (0.18) mW coupled into the multiple (single) mode fiber is obtained at the current of 35 mA, for the microsquare laser at the temperature of 288 K. Furthermore, a flat small-signal modulation response is reached with the 3-dB bandwidth of 16.2 GHz and the resonant peak of 3.6 dB at the bias current of 34 mA. The K-factor of 0.22 ns is obtained by fitting the damping factor versus the resonant frequency, which implies a maximum intrinsic 3-dB bandwidth of 40 GHz.

Thermal and high speed modulation characteristics for AlGaInAs/InP microdisk lasers.

Thermal and high speed modulation characteristics are investigated for a unidirectional-emission microdisk laser with a radius of 7 μm surrounded by BCB-cladding layer, with a threshold current of 1.5 mA at the temperature of 287 K. The lasing spectra under different widths of pulsed current are measured to characterize the temperature rise during the pulse period, and the thermal distribution in the microdisk laser is simulated by the finite-element modeling technique. A temperature rise of 25 K is estimated for the microdisk laser biased at 20 mA. Furthermore, small signal modulation response with 3dB bandwidth up to 20 GHz is obtained for the microdisk laser at the biasing current of 18 mA, and eye-diagrams at the modulation bit rates of 20, 25, and 30 GHz are also measured at the temperature of 287 K.

High-Speed Direct-Modulated Unidirectional Emission Square Microlasers

High-speed modulation characteristics are investigated for a 20-μm-side-length AlGaInAs/InP square resonator microlaser with a 1.5-μm-wide midpoint output waveguide. Single-mode operation with a maximum output power of 0.34 mW and a side mode suppression ration of 42 dB are realized for the microlaser at a temperature of 288 K. A 3-dB bandwidth of 14.9 GHz is obtained, and the eye diagrams at 15 and 20 Gb/s are demonstrated for the microlaser with a bias current of 30 mA. Furthermore, the bit error rate of 2 × 10−8 is obtained at 8 Gb/s for a square microlaser with a 3-dB bandwidth of 10 GHz. In addition, different output patterns are observed for different lasing modes, which agree with the finite-difference time-domain simulation results.

Investigations on high speed directly modulated microdisk lasers accounting for radial carrier hole burning

High-speed modulation characteristics are investigated for microdisk lasers theoretically and experimentally. In rate equation analysis, the microdisk resonator is radially divided into two regions under uniform carrier density approximation in each region. The injection current profile, carrier spatial hole burning, and diffusion are accounted for in the evaluation of small-signal modulation curves and the simulation of large-signal responses. The numerical results indicate that a wide mode field pattern in radial direction has merit for high-speed modulation, which is expected for coupled modes in the microdisk lasers connected with an output waveguide. For a 15-μm-radius microdisk laser connected with a 2-μm-wide output waveguide, the measured small-signal response curves with a low-frequency roll-off are well in agreement with the simulated result at a 2-μm radial width for the mode intensity distribution. The resonant frequencies of 7.2, 5.9, and 3.9 GHz are obtained at the temperatures of 287, 298, and 312 K from the small-signal response curves, and clear eye diagrams at 12.5 Gb/s with an extinction ratio of 6.1 dB are observed for the microdisk laser at the biasing current of 38 mA and 287 K.

Fabrication and modulation characteristics of 1.3 µm p-doped InAs quantum dot vertical cavity surface emitting lasers

We present the fabrication of 1.3 µm waveband p-doped InAs quantum dot (QD) vertical cavity surface emitting lasers (VCSELs) with an extremely simple process. The continuous-wave saturated output power of 1.1 mW with a lasing wavelength of 1280 nm is obtained at room temperature. The high-speed modulation characteristics of p-doped QD VCSELs of two different oxide aperture sizes are investigated and compared. The maximum 3 dB modulation bandwidth of 2.5 GHz can be achieved at a bias current of 7 mA for a p-doped QD VCSEL with an oxide aperture size of 10 µm in the small signal frequency response measurements. The crucial factors for the 3 dB bandwidth limitation are discussed according to the parameters' extraction from frequency response.

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.

High-Speed Frequency Modulated DBR Lasers for Long-Reach Transmission

We present a novel high-speed transmitter consisting of a frequency modulated DBR laser and optical filters. The refractive index modulation in the phase control region of the DBR laser allows high-speed frequency modulation. The generated frequency modulated signal is converted to an intensity modulated signal using the edge of the optical filter pass band. We present theoretical simulations of high-speed modulation characteristics and extension of transmission reach. With the proposed transmitter, we review the experimental demonstration of 180-km transmission of a 10-Gb/s signal with a tuning range of 27nm and 60-km transmission of a 20-Gb/s signal.

High-speed digital modulation characteristics of oxide-confined vertical-cavity surface-emitting lasers-numerical simulations consistent with experimental results

The vertical-cavity surface-emitting laser (VCSEL) is a preferred light source for short-distance high-speed fiber-optic communication links. We simulate the digital modulation behavior of typical oxide-confined VCSELs under realistic working conditions with a comprehensive model that includes the detailed geometry when calculating the optical fields and that accurately accounts for the dynamic effects of carrier density and temperature on the modal distributions. The intrinsic output characteristics of single- and multimode VCSELs were studied as functions of bias and modulation depth under a 2/sup 7/-1 pseudorandom bit sequence current modulation at 2 and 10 Gb/s. The data were used to create numerical eye diagrams that show, e.g., the significant impact of the bit pattern history and the noise on the timing jitter in both single- and multimode VCSELs. For the single-mode VCSEL, the choice of the bias current and modulation depth was less critical due to its higher damping of the relaxation oscillations. The simulated VCSELs were fabricated and experimentally evaluated. The measured eye diagrams showed the same characteristic features as those in the simulations.

High-speed modulation and switching characteristics of In(Ga)As-Al(Ga)As self-organized quantum-dot lasers

The dynamic characteristics, and in particular the modulation bandwidth, of high-speed semiconductor lasers are determined by intrinsic factors and extrinsic parameters. In particular, carrier transport through the heterostructure and thermalization, or quantum capture in the gain region, tend to play an important role. We have made a detailed study of carrier relaxation and quantum capture phenomena in In(Ga)As-Al(Ga)As self-organized quantum dots (QD's) and single-mode lasers incorporating such dots in the gain region through a variety of measurements. The modulation bandwidth of QD lasers is limited to 5-6 GHz at room temperature and increases to /spl sim/30 GHz only upon lowering the temperature to 100 K. This behavior is explained by considering electron-hole scattering as the dominant mechanisms of electron relaxation in QD's and the scattering rate seems to decrease with increase of temperature. The switching of the emission wavelength, from the ground state to an excited state, has been studied in coupled cavity devices. It is found that the switching speed is determined intrinsically by the relaxation time of carriers into the QD states. Fast switching from the ground to the excited state transition is observed. The electrooptic coefficients in the dots have been measured and linear coefficient /spl tau/=2.58/spl times/10/sup -11/ m/V. The characteristics of electrooptic modulators (EOM's) are also described.

Transmission experiments using oxide confined vertical cavity surface emitting lasers

The high speed modulation characteristics and performance in optical fibre transmission links of oxide confined vertical cavity surface emitting lasers are reported. The lasers have bandwidths of 13 GHz. Error-free transmission is obtained for both subcarrier modulated multichannel transmission and directly modulated digital transmission at 8 Gbit/s.

New hybrid integrated laser diode-drivers using microsolder bump bonding: SPICE simulation of high-speed modulation characteristics

This paper proposes two types of new hybrid integrated laser diode (LD)-drivers that use microsolder bump bonding instead of conventional wire bonding. In one, an LD and a driver are flip-chip bonded to each other; in the other, an LD and a driver are flip-chip bonded onto a substrate. Their performances are compared to those of a monolithic LD-driver and a conventional hybrid one using wire bonding by a simulation program with integrated circuit emphasis (SPICE) with particular emphasis on high-speed LD modulation. The nonreturn-to-zero (NRZ) eye patterns modulated at signal speeds up to 30 Gb/s by the new hybrid integrated LD-drivers were hardly inferior to those by the monolithic LD-driver, whereas those by conventional hybrid ones were greatly degraded over 10 Gb/s. The new hybrid integrated LD-drivers are a feasible alternative to monolithic ones for high-speed optical transmitters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-speed modulation characteristics of helium-implanted zinc-diffused vertical cavity surface emitting lasers

The high-speed modulation characteristics of helium-implanted zinc-diffused vertical cavity surface emitting semiconductor lasers are measured. Devices with a nominal active region diameter of 10 and 20 mu m exhibit a move-out rate of 4.5-6.5 GHz/mW/sup 1/2/ and a K factor of 0.65-0.85 ns. Maximum modulation frequency is limited by ohmic heating to approximately 5.5 GHz for 10- mu m diameter lasers and to 2.7 GHz for 20- mu m diameter lasers. A rolloff in the response below the relaxation oscillation frequency is observed and is explained well by a diffusion capacitance model.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-speed modulation characteristics of a GaSb/AlGaSb multiquantum-well laser diode

The high-speed modulation characteristics of a GaSb/AIGaSb multiquantum-well laser diode (λ = 1.66μm) are investigated. The longitudinal mode spectra over 1GHz modulation with quasi-single-mode and small frequency ‘chirping’ were confirmed.