Small-signal Modulation Bandwidth Research Articles

Improved performance of InGaAs/GaAs microdisk lasers epi-side down bonded onto a silicon board.

We study the impact of improved heat removal on the performance of InGaAs/GaAs microdisk lasers epi-side down bonded onto a silicon substrate. Unlike the initial characteristics of microlasers on a GaAs substrate, the former's bonding results in a decrease in thermal resistance by a factor of 2.3 (1.8) in microdisks with a diameter of 19 (31) µm, attributed to a thinner layered structure between the active region and the substrate and the better thermal conductivity of Si than GaAs. Bonded microdisk lasers show a 2.4-3.4-fold higher maximum output power, up to 21.7 mW, and an approximately 20% reduction in the threshold current. A record high 3 dB small-signal modulation bandwidth of 7.9 GHz for InGaAs/GaAs microdisk lasers is achieved.

Impact of oxide aperture diameter on optical output power, spectral emission, and bandwidth for 980 nm VCSELs

We produce experimental 980 nm vertical cavity surface emitting lasers (VCSELs) with a wide range of oxide aperture diameters (ø) from ∼2.5 to 15 µm on wafers designed to minimize the epitaxial growth and VCSEL design complexity. The structures are grown in batches of 12, 3-inch diameter wafers in a production metal-organic vapor phase epitaxy machine. We characterize the top emitting VCSELs at room temperature (∼25 °C) – grouped into unit cells with 16 rows and 15 columns—using an automated (university-built) wafer mapping system, resulting in two-dimensional colorized maps of several performance attributes of interest including optical output power, threshold current, and maximum power conversion efficiency. By etching part of the topmost layer of the upper distributed Bragg reflector to decrease the VCSEL optical cavity photon lifetime, we boost the small signal modulation bandwidth (f3dB). The room temperature maximum f3dB is ∼30 GHz for VCSELs with ø ∼3 µm and ∼20 GHz for VCSELs with ø ∼15 µm.

30 GHz bandwidth temperature stable 980 nm vertical-cavity surface-emitting lasers with AlAs/GaAs bottom distributed Bragg reflectors for optical data communication

We present a 980 nm vertical-cavity surface-emitting laser (VCSEL) design which achieves 32 GHz small-signal modulation bandwidth (f3db) at 15 °C and record-high 27 GHz at 85 °C. Our devices utilize binary AlAs/GaAs bottom distributed Bragg reflector material layers to improve thermal conductance. We extract key VCSEL figures-of-merit from static optical output power-current-voltage (LIV), spectral emission, and high frequency dynamic measurements and observe highly temperature stable performance for these parameters.

Power, Bandwidth, and Efficiency of Single VCSELs and Small VCSEL Arrays

Single-emitter vertical-cavity surface-emitting lasers (VCSELs) and multiple-emitter VCSEL arrays designed for emission at 980 nm, and with large oxide aperture diameters ( φ ) from ∼10.5 to 33.5 μm and with φ of ∼7.5 μm that are configured in three-emitter and seven-emitter electrically parallel triangular and hexagonal arrays, respectively, have record bandwidths and optical output powers for VCSELs with such large cumulative emitting areas. We demonstrate room temperature (RT) maximum small-signal modulation bandwidths ( f 3dBmax) of 26.6 to 18.6 GHz with corresponding continuous wave (CW) optical output powers ( L ) of 16.2 and 47 mW for VCSELs with φ ∼13.5 and 33.5 μm, respectively. For parallel 980 nm triple and septuple arrays with φ ∼7.5 μm for each VCSEL, we demonstrate RT f 3dBmax of 25.5 and 24.8 GHz with corresponding CW L of 22.7 and 50.1 mW, respectively. We perform a comparative analysis of the RT results, which consist of standard static light output power–current–voltage ( LIV ) characteristics to determine the typical VCSEL figures-of-merit including wall plug efficiency (WPE), LI slope efficiency, spectral emission versus current ( I ), and small-signal frequency response curves. We examine the tradeoffs in optical output power, small-signal modulation bandwidth, and WPE for the various VCSEL designs, and find that while the combination of f 3dBmax and L are record values for single VCSELs and VCSEL arrays, these parameters fall off as the cumulative VCSEL array area increases to emit higher overall optical power.

Impact of distributed Bragg reflector on carrier and photon dynamics in GaN-based surface emitting diodes manifested by ultrafast transient absorption spectroscopy

Small signal modulation bandwidth characterized by relaxation oscillations (ROs) of GaN-based vertical cavity surface emitting laser diodes are of great importance in high-speed applications. In this work, we investigate the impact of distributed Bragg reflectors (DBRs) on ROs by developing a simulation framework, which can accommodate the reflected photons self-consistently in the rate equation model. Carrier and photon dynamics in the active region and at the transmitting end have been evaluated for a typical GaN/InGaN multiple quantum well heterostructure having a dielectric DBR. Temporal broadening in photon dynamics is observed due to the phase difference. Interestingly, an enhanced radiative recombination rate is also noticed due to the increased confinement, which leads to a reduced turn-on delay. Transient absorption spectroscopy has been performed to obtain the decay rates of captured carriers in the well and it turns out be in harmony with the simulation results. Photoluminescence spectroscopy also validates the increased radiative recombination.

Small-Signal Analysis of High-Performance VCSELs

In this paper, a comprehensive model to describe the small-signal modulation response of ultra-high performance single- and multi-mode vertical-cavity surface-emitting lasers (VCSELs), with modulation bandwidths exceeding 30 GHz, is presented. Traditionally, utmost simplified dynamic models are used to extract dynamic figures of merit from single-mode edge-emitting lasers. These methods are later on also applied to evaluate the dynamic performance of VCSELs, even though these devices have a very different geometrical layout and modal confinement. However, to understand the dynamic performance of high-speed VCSELs, a model supporting the transverse and longitudinal mode profile, and the driving current inhomogeneity in the active region, is needed. Therefore, multi-mode VCSEL rate equations are established here. Moreover, to access the dynamic figures of merit of these devices, a comprehensive analytical fitting function based on our carrier reservoir splitting approach is derived. Thus, because of the high carrier and photon densities inside these optimized VCSELs, the common carrier reservoir splits up as a result of numerous effects such as mode competition, carrier diffusion and spatial hole burning. These and other effects have a tremendous impact on the small signal modulation response shape and bandwidth, and also on the current distribution profile in the carrier reservoirs. Compared with our recently reported work, this novel model presented includes the effects of gain compression and inhomogeneous current injection between the different lasing modes. Consequently, it is found that the further tuning of our multi-mode VCSEL dynamic model, to include these effects, yields a more physical and consistent figures of merit of high-performance VCSELs.

Enhancement of the MQW-RSOA's Small-Signal Modulation Bandwidth by Inductive Peaking

We present a small-signal model of the multiple quantum well (MQW) reflective semiconductor optical amplifier (RSOA) with the accompanying parasitic circuit, followed by a numerical simulation of its modulation response. We analyze the corresponding $-$ 3 dB bandwidth's dependence on the bias current, optical power of the input signal, and the RSOA's active region length. Material, electrical and optical parameters, and the overall design of MQW RSOA are derived bottom-up from the fundamental principles. It is shown that the modulation response, resulting from either intrinsic or parasitic-like model (including transport effects), usually provides high $-3$ dB bandwidth, which is clamped by the chip's parasitics, leading to a relatively poor external modulation bandwidth. This issue can be overcome by an advanced design of the RSOA structure, as the one proposed in this paper and with the optimized bonding, which may improve the external modulation bandwidth significantly through the inductive peaking effect (IPE). Moreover, IPE's efficiency increases if the intrinsic or parasitic-like modulation bandwidth is in the proximity of the parasitic circuit resonant frequency.

Impact of damping on high speed 850 nm VCSEL performance * * Project supported by the National Natural Science Foundation of China (Nos. 61335004, 61675046, 61505003), the Open Fund of IPOC2017B011 (BUPT).

High speed VCSELs are important optical devices in short-reach optical communication links and interconnects because of their low cost and high modulation speeds. In this paper, the impact of damping on the their static and dynamic characteristics is analyzed and demonstrated. Through the shallow corrosion of the top layer DBR, the VCSELs with different damping is designed and fabricated. With the increase of the surface etch depth from 0 to ~55 nm for 9 μm oxide-aperture VCSEL, the K factor related with the damping is reduced from 0.31 to 0.23 ns−1. When the etch depth of the VCSEL with 9 μm oxide-aperture is decreased to ~25 nm, output power is increased from 4.03 to 4.70 mW and small signal modulation bandwidth is also increased from 15.46 to 16.37 GHz. It shows that there is a tradeoff between damping and differential gain for improving modulation speed.

采用BCB平整技术的高速850 nm垂直面发射激光器

Vertical-cavity surface-emitting lasers(VCSELs) are widely used in short-reached optical interconnects and data communication links because of their low energy consumption and high modulation speed. Capacitance, as the parasitic parameters, affects the modulation bandwidth. In this paper, parasitic capacitance of VCSELs is reduced by using a low-k benzocyclobutene(BCB) planarization technique. The detail BCB planarization technique has been discussed with optimal process parameters, which is useful for high-speed VCSEL fabrication. The small signal modulation bandwidth of the low-k BCB planarization VCSEL with 7 μm oxide aperture has been achieved to 15.2 GHz.

AlGaInAs EML having high extinction ratios fabricated by identical epitaxial layer technique

AlGaInAs electroabsorption-modulated lasers (EMLs) fabricated by identical epitaxial layer technique are demonstrated. The EML device shows an infinite characteristic temperature when the temperature ranges from 20 oC to 30 oC. The integrated modulator has static extinction ratios of larger than 20 dB at a reverse bias voltage of −2 V. The small signal modulation bandwidth of the modulator is larger than 11 GHz. At 10 Gb/s data modulation, the dynamic extinction ratio is about 9.5 dB in a back to back test configuration. Because only a simple fabrication procedure is needed, our EMLs are promising low cost light sources for optical fiber transmission applications.

Red and Near-Infrared III-Nitride Quantum Dot Lasers

InGaN/GaN self-organized quantum dots and similar dots in GaN nanowires are formed by strain relaxation and the luminescence from these nanostructures extends to longer wavelengths than generally obtained with quantum wells. We have exploited this advantage by incorporating these nanostructures in the gain region of edge-emitting diode lasers. Here, we describe the characteristics of 650 nm self-organized quantum dot lasers epitaxially grown on GaN and 1.3 μm dot-in-nanowire array lasers grown on (001) Si by molecular beam epitaxy. The devices are characterized by relatively low threshold currents, excellent temperature stability (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> > 200 K), and differential gain ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-16</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The highest measured small-signal modulation bandwidth of the dot-in-nanowire laser is 3.3 GHz. The experiments have been complemented by theoretical modeling of the self-organized quantum dots and lasers made with them. These new classes of devices open up new opportunities in applications such as displays, optical data storage, heads-up displays in automobiles, plastic fiber communication, and silicon photonics.

Mode and Lasing Characteristics for Hybrid Square-Rectangular Lasers

Whispering-gallery mode (WGM) microlasers are potential light sources for monolithic photonic integration circuits and optical interconnects. In this study, stable single-mode hybrid square-rectangular lasers (HSRLs) are reported, based on mode coupling between WGM and Fabry–Perot (FP) modes. Mode selection mechanisms are explained based on numerical results of the resonant reflection of the square microcavity and energy distributions of coupled mode in the two cavities, and the reduction of effective linewidth enhancement factor is expected as two cavities are modulated individually. HSRLs with different sizes are fabricated by contact photolithography and dry-etching techniques. For a hybrid laser with a square-side length a = 15 μm and the FP cavity length and width of L = 300 μm and d = 2 μm, stable single-mode operation with a side-mode suppression ratio up to 47 dB is obtained, and a small-signal modulation bandwidth of 13.3 GHz is realized by modulating the current of the FP cavity. The dynamic characteristics are further investigated for the hybrid laser. Furthermore, threshold currents and tunable behaviors are compared for HSRLs with a = 10, 15, and 20 μm and d = 1.5 and 2 μm.

Heat dissipation effect on modulation bandwidth of high-speed 850-nm VCSELs

Vertical-cavity surface-emitting lasers (VCSELs) have been widely used in short-reach optical communication links and interconnects because of their high modulation speed and low energy consumption. In this paper, the effect of heat dissipation on the modulation bandwidth of high-speed 850-nm VCSELs has been investigated. With increasing bottom distributed Bragg reflector (DBR) diameter from 34 μm to 46 μm, optical output power increased as well as the −3 dB modulation bandwidth. The maximum small-signal modulation bandwidth of 16.2 GHz was achieved with an aperture of 5 μm. Thermal analysis showed that the heat dissipation of the bottom DBR diameter, which determined the differential gain of the quantum wells, has a strong effect on the modulation bandwidth.

Analysis of the photon–photon resonance influence on the direct modulation bandwidth of dual-longitudinal-mode distributed feedback lasers

The paper explores the possibilities to extend the direct modulation bandwidth in dual-longitudinal-mode distributed feedback lasers by exploiting the photon–photon resonance induced by the interaction of the two modes in the laser cavity. The effects on the direct amplitude modulation and on the direct modulation of the difference frequency between the two modes are analyzed using simulation and experimental results. When the photon–photon resonance, which occurs at the difference frequency between the two modes, is properly placed at a higher frequency than the carrier-photon resonance, the small-signal amplitude modulation (AM) bandwidth of the laser can be significantly increased. However, both simulations and experiments point out that a high small-signal AM bandwidth does not lead to a high large-signal AM bandwidth if the small-signal modulation response has significant variations across the modulation bandwidth. The paper shows that a high large-signal AM bandwidth is obtained when the two modes are significantly unbalanced, whereas a high-bandwidth difference frequency modulation can be best detected when the two modes are balanced and the DC bias is properly chosen.

EML Array fabricated by SAG technique monolithically integrated with a buried ridge AWG multiplexer

We report the fabrication of a ten channel electroabsorption modulated DFB laser (EML) array. Different emission wavelengths of the laser array are obtained by selective area growth (SAG) technique, which is also used for the integration of electroabsorption modulators (EAM) with the lasers. An arrayed waveguide grating (AWG) combiner is integrated monolithically with the laser array by butt-joint regrowth (BJR) technique. A buried ridge waveguide structure is adopted for the AWG combiner. A self aligned fabrication procedure is adopted for the fabrication of the waveguide structure of the device to eliminate the misalignment between the laser active waveguide and the passive waveguide. A Ti thin film heater is integrated for each laser in the array. With the help of the heaters, ten laser emissions with 1.8nm channel spacing are obtained. The integrated EAM has a larger than 11dB static extinction ratios and larger than 8GHz small signal modulation bandwidths. The light power collected in the output waveguide of the AWG is larger than −13dBm for each wavelength.

Temperature-Insensitive High-Speed Directly Modulated 1.55-&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\mu \text{m}$ &lt;/tex-math&gt; &lt;/inline-formula&gt; Quantum Dot Lasers

Modulation properties and temperature stability of short cavity ridge waveguide lasers based on high-quality InAs quantum dots exhibiting a total modal gain of ~90 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> are reported. The 338-μm-long lasers show a threshold current of 20 mA at room temperature and an output powers of up to 36 mW. A maximum small signal modulation bandwidth of 15 GHz was obtained at 14 °C, which degrades to 13 GHz at 60 °C and 8 GHz at 80 °C. Digital modulation at 25 Gb/s between 15 °C and 50 °C was obtained with clear open eyes under constant drive conditions (dc and ac). The maximum data rates of 32 and 35 Gb/s were obtained for 338- and 230-μm-long lasers, respectively, at 14 °C.

Impact of Photon Lifetime on the Temperature Stability of 50 Gb/s 980 nm VCSELs

An increase of the small-signal modulation bandwidth to more than 25 GHz that is stable over a wide temperature range from 25 °C to 85 °C is reported upon systematic, joint variations of the photon lifetime and the oxide-aperture diameter for 980-nm vertical-cavity surface-emitting lasers. Error-free back-to-back data transmission at 50 Gb/s from 25 °C to 75 °C for completely unchanged driving conditions is reported, leading to an improved energy efficiency of the laser.

A 1.3-μm four-channel directly modulated laser array fabricated by SAG-Upper-SCH technology

A monolithically integrated four-channel directly modulated laser (DML) array working at the 1.3-μm band is demonstrated. The laser was manufactured by using the techniques of selective area growth (SAG) of the upper separate confinement heterostructure (Upper-SCH) and modified butt-joint method. The fabricated device showed stable single mode operation with the side mode suppression ratio (SMSR) >35dB, and high wavelength accuracy with the deviations from the linear fitted values <±0.03nm for all channels. Furthermore, small signal modulation bandwidth >7GHz was obtained, which may be suitable for 40 GbE applications in the 1.3-μm band.

Single-Mode High-Speed 1.5-μm VCSELs

Single-mode 1.5-μm InP-based vertical-cavity surface-emitting lasers (VCSELs) with a 1.5-λ-long semiconductor cavity and two dielectric distributed Bragg reflectors (DBRs) are presented. The electrical, thermal, and optical characteristics are studied as a function of tunnel junction diameter and for different temperatures ranging from –10 up to 65 °C. Small-signal modulation bandwidths in excess of 21 GHz at room temperature are demonstrated for a DC power consumption below 10 mW. In this paper, the superior dynamic characteristics of these VCSELs are shown by demonstrating operation at data rates up to 50 Gb/s with bit-error-ratio slightly above 10 –12 in back-to-back configuration by nonreturn-to-zero modulation and without any equalization. Neither forward error correction nor digital signal processing was required.

20-Gb/s Modulation of Silicon-Integrated Short-Wavelength Hybrid-Cavity VCSELs

We investigate the dynamics of silicon-integrated 850-nm-wavelength hybrid-cavity vertical-cavity surface-emitting lasers (VCSELs). The VCSELs consist of a GaAs-based half-VCSEL attached to a dielectric distributed Bragg reflector on a silicon substrate using ultra-thin divinylsiloxane-bis-benzocyclobutene adhesive bonding. A 5- $\mu \text{m}$ oxide aperture diameter VCSEL, with a small signal modulation bandwidth of 11 GHz, supports data transmission at bit rates up to 20 Gb/s. The modulation bandwidth and the large signal modulation characteristics are found to be impaired by the high thermal impedance.