Long-wavelength VCSELs Research Articles

Advanced Packaging Technology for Novel 1-dimensional and 2-dimensional VCSEL Arrays

Abstract In this paper we present an optimized manufacturing technique for special long-wavelength 1-dimensional and 2-dimensional Vertical Cavity Surface Emitting Laser Diode (VCSEL) arrays with focus on die bonding and a special wire bonding process as well as additional possibilities to make the manufacturing more productive and to increase the product quality, reliability, and life time. VCSEL arrays have a very broad application potential. Objective of this paper is the development of an assembly technology for long-wavelength VCSEL arrays with high positioning accuracy for automated production with high total yield, using gold-based conductive glue (because silver migration is a concern) securing high yield and extremely high reliability and lifetime. Due to special customer requirements, a final thickness of the conductive glue of 35 micron with low standard deviation is necessary. For highest reliability, gold wire bonding of the top side contact of the VCSEL to a silicon substrate with gold metallization has been a customer requirement. With the described technique we develop and produce customer specific products with dedicated wavelength, performance, and packaging options. Bases on our findings, very flexible and scalable solutions are possible, matching many different applications. Finally, we will present an overview of our results on the physical and electro-optical characterization of the VCSEL devices. This yielded a very productive manufacturing technique, meeting the requirements mentioned above. Special design features of the VCSEL ensure that the relatively high thermal resistance of the cured conductive glue layer does not impair the electro-optical properties or the lifetime of the VCSELs. For the interconnection of the top side contact we used gold wire with 20 microns diameter, using ball-wedge, Stand-Off Stich Bond process with special loop geometry, required by the customer. Due to the properties of the semiconductor material of the VCSELs a low bonding temperature is required, enabled by our special wire bonding process. Gold wire bonding delivered excellent results, by far exceeding the customer specification. The optical spectrum of the VCSELs and other measurements indicate that the assembly processes do not harm the laser diodes or their electro-optical properties. All customer requirements for the 1-dimensional and 2-dimensional VCSEL arrays have been met. The electro-optical and burn-in data furnished proof of the quality of the engineered assembly technology. The technologies developed for low current as well as high current laser arrays will enable new devices for a huge amount of new applications because of the novel manufacturing processes as well as innovative packaging of the VCSEL arrays.

Phase noise modeling and characterization of a self-started VCSEL-based harmonic frequency generator (SVHFG)

This article presents the experimental characterization of the first self-started closed-loop system for the generation of microwave harmonics using a long-wavelength VCSEL modulated in a gain-switched regime. A 15-GHz microwave signal is generated with a phase noise of −104 dBc/Hz at 10 kHz from the carrier when the fundamental frequency is f0=2.5 GHz. The side-mode suppression ratio (SMSR) for all the microwave frequencies generated is higher than 30 dB. We propose a predictive model to estimate the phase noise of the fundamental oscillating frequency, considering the noise contributions of each loop component. Several simulation scenarios are included to demonstrate the significant influence of the electrical power, the laser frequency noise, and the optical fiber dispersion over the phase noise performance. We also present the carrier-to-noise ratio deterioration of the microwave harmonics distributed through an optical fiber link.

1.3‐µm buried‐heterostructure VCSELs with GaAs/AlGaAs metamorphic DBRs grown by MOCVD

The authors report buried-heterostructure long-wavelength VCSELs with GaAs/AlGaAs metamorphic DBRs, which has a potential for low thermal resistance and operation in the single transverse mode with a large emission area. In spite of their being upside-up and unmounted, they achieve CW operation at 17°C, and VCSELs with a 15-µm mesa exhibit single-mode operation.

High-Speed PAM4-Based Optical SDM Interconnects With Directly Modulated Long-Wavelength VCSEL

This paper reports the demonstration of high-speed PAM-4 transmission using a 1.5-{\mu}m single-mode vertical cavity surface emitting laser (SM-VCSEL) over multicore fiber with 7 cores over different distances. We have successfully generated up to 70 Gbaud 4-level pulse amplitude modulation (PAM-4) signals with a VCSEL in optical back-to-back, and transmitted 50 Gbaud PAM-4 signals over both 1-km dispersion-uncompensated and 10-km dispersion-compensated in each core, enabling a total data throughput of 700 Gbps over the 7-core fiber. Moreover, 56 Gbaud PAM-4 over 1-km has also been shown, whereby unfortunately not all cores provide the required 3.8 $\times$ 10 $^{-3}$ bit error rate (BER) for the 7% overhead-hard decision forward error correction (7% OH HDFEC). The limited bandwidth of the VCSEL and the adverse chromatic dispersion of the fiber are suppressed with pre-equalization based on accurate end-to-end channel characterizations. With a digital post-equalization, BER performance below the 7% OH-HDFEC limit is achieved over all cores. The demonstrated results show a great potential to realize high-capacity and compact short-reach optical interconnects for data centers.

The Electron Stopper Layer Effect on Long-Wavelength VCSEL with AsSb-Based DBR Temperature Distribution

The Electron Stopper Layer Effect on Long-Wavelength VCSEL with AsSb-Based DBR Temperature Distribution

Performance Analysis of 40-Gb/s Transmission Based on Directly Modulated High-Speed 1530-nm VCSEL

We propose and experimentally demonstrate the use of a novel-design long-wavelength VCSEL with high direct-modulation speed to implement 40-Gb/s transmission suitable for optical interconnects and short-reach scenarios. The single-mode operation, together with the use of a common optical bandpass filter, enables a fiber reach in the kilometer range. The VCSEL is characterized and directly modulated at 40 Gb/s with a power consumption of 18.5 mW. Bit-error-rate (BER) performance has been analyzed for a pseudorandom bit sequence (PRBS) of length $2^{7}-1$ as commonly done in most of similar studies, as well as for increasing the PRBS lengths up to $2^{31}-1$ to understand the system limitations when applying more realistic data, so as to induce slower dynamics physical effects such as thermal effects. BER values below common forward error correction threshold are measured without an optical amplification up to 1 km of a single-mode fiber. In particular, error-free operation ( $\text {BER} with 1.6 dB of penalty with respect to the back-to-back case was measured for the PRBS length of $2^{7}-1$ .

Very high-capacity short-reach VCSEL systems exploiting multicarrier intensity modulation and direct detection.

Multicarrier intensity modulation of a bandwidth-limited long-wavelength VCSEL is exploited combined to direct detection to achieve very high capacity simple systems for short-reach applications. Tailored FDM subcarriers modulation and allocation allow to match the non-uniform frequency response of the system induced by the direct modulation and detection of the FDM signal and by the uncompensated SSMF propagation, overcoming the VCSEL bandwidth limitations. A whole transported throughput ranging from 34 Gb/s to 25 Gb/s from few hundreds meters to 20 km of SSMF propagation is experimentally demonstrated even by employing a 5-GHz band VCSEL source.

Polarization mode control of long-wavelength VCSELs by intracavity patterning.

Polarization mode control is enhanced in wafer-fused vertical-cavity surface-emitting lasers emitting at 1310 nm wavelength by etching two symmetrically arranged arcs above the gain structure within the laser cavity. The intracavity patterning introduces birefringence and dichroism, which discriminates between the two polarization states of the fundamental transverse modes. We find that the cavity modifications define the polarization angle at threshold with respect to the crystal axes, and increase the gain anisotropy and birefringence on average, leading to an increase in the polarization switching current. Experimental measurements are explained using the spin-flip model of VCSEL polarization dynamics.

Optical Injection and Lasing Dynamics in Long-Wavelength VCSELs With Intracavity Patterning

We present the static and optical injection response of wafer-fused long-wavelength VCSELs with and without intracavity patterning. It is found that the extra optical cavity patterning increases the gain anisotropy between orthogonal polarization modes, increasing the polarization stability, and influencing the dynamic response of the device. Dynamics features are summarized on maps in the injection power-master/slave detuning plane, and good agreement is obtained with the spin-flip model of VCSEL polarization modes.

Heterogeneously integrated long-wavelength VCSEL using silicon high contrast grating on an SOI substrate.

We report an electrically pumped hybrid cavity AlGaInAs-silicon long-wavelength VCSEL using a high contrast grating (HCG) reflector on a silicon-on-insulator (SOI) substrate. The VCSEL operates at silicon transparent wavelengths ~1.57 μm with >1 mW CW power outcoupled from the semiconductor DBR, and single-mode operation up to 65 °C. The thermal resistance of our device is measured to be 1.46 K/mW. We demonstrate >2.5 GHz 3-dB direct modulation bandwidth, and show error-free transmission over 2.5 km single mode fiber under 5 Gb/s direct modulation. We show a theoretical design of SOI-HCG serving both as a VCSEL reflector as well as waveguide coupler for an in-plane SOI waveguide, facilitating integration of VCSEL with in-plane silicon photonic circuits. The novel HCG-VCSEL design, which employs scalable flip-chip eutectic bonding, may enable low cost light sources for integrated optical links.

Inverted-relief cavity used in VCSELs to suppress higher-order transverse modes

One of the most acute VCSEL disadvantages is an excitation of higher-order transverse modes in devices emitting more intense radiation. The paper presents results of simulation of VCSEL performance with the aid of the inverted-relief cavity used to suppress those modes. This suppression method is very efficient and relatively simple from technological point of view, so it is worth to be recommended. Full Text: PDF References C. Jung, R. Jäger, M. Grabherr, P. Schnitzer, R. Michalzik, B. Weigl, S. Müller, K.J. Ebeling, "4.8 mW singlemode oxide confined top-surface emitting vertical-cavity laser diodes", Electron. Lett. 33, 1790 (1997). CrossRef W. Nakwaski, "VCSEL structures used to suppress higher-order transverse modes", Opto-Electr. Rev. 19, 119 (2011). CrossRef R. Sarzała, T. Czyszanowski, M. Wasiak, M. Dems, Ł. Piskorski, W. Nakwaski, K. Panajotov, "Numerical Self-Consistent Analysis of VCSELs", Adv. in Opt. Techn. (2012). CrossRef H. Martinsson, J.A. Vukušič, A. Larsson, "Single-mode power dependence on surface relief size for mode-stabilized oxide-confined vertical-cavity surface-emitting lasers", IEEE Photon. Technol. Lett. 12, 1129 (2000). CrossRef H.J. Unold, S.W.Z. Mahmoud, R. Jäger, M. Grabherr, R. Michalzik, K.J. Ebeling, "Large-area single-mode VCSELs and the self-aligned surface relief", IEEE J. Sel. Top. Quantum Electron. 7, 386 (2001). CrossRef D. Supper, J. Pfeiffer, M. Ilzhöfer, Ch. Giuliani, Ch. Degen, A.P. Lima, A. Ramakrishnan, G. Ebbinghaus, G. Steinle, "Long-wavelength VCSEL with integrated relief for control of singlemode emission", Opt. Comm. 267, 447 (2006). CrossRef

Simulation of a Single-Mode Tunnel-Junction-Based Long-Wavelength VCSEL

We investigate the physics of an internal device for a high-performance, vertical-cavity surface-emitting laser operating at 1.305 μm. Experimental results are analyzed using as the simulation software a photonic-integrated-circuit simulator in 3D (PICS3D), which is a state-of-the-art 3D simulator for surface- and edge-emitting laser diodes, semiconductor optical amplifiers, and other similar active waveguide devices. The 2D/3D semiconductor equations are coupled to the optical modes in both lateral and longitudinal directions. Optical properties such as the quantum well/wire/dot optical gain and spontaneous emission rates are computed self-consistently. Careful adjustments of material parameters led to an excellent agreement between simulation and measurements. Simulation results show that the maximum output power is limited by electron leakage from quantum wells.

Spatial-Mode Discrimination in Guided and Antiguided Arrays of Long-Wavelength VCSELs

Three means of optical confinement imposed on InAlGaAs/InP 1.3 μm VCSEL arrays are investigated with self-consistent numerical model of laser operation. Laterally patterned tunnel junction (TJ), in-build guiding realized with air-gap patterning, and antiguiding schemes are investigated and optimized to achieve single-mode operation. The analysis shows that mode discrimination in laterally patterned TJ is very responsive to the injected current, the air-gap patterning reduces influence of the working conditions and supports multimode operation, and finally, antiguiding schemes provide single-mode operation for prescribed geometrical design.

Performance Analysis of Vertical and Horizontal Transmitter Array Modules Using Short- and Long-Wavelength VCSELS for Optical Interconnects

For short- and long-wavelength vertical surface-emitting lasers (VCSELs), performances of optical transmitter array modules with vertically and horizontally packaged structures are analyzed for application to on-board optical interconnection. 850 and 1310 nm wavelengths are selected for the short- and long-wavelength light sources. For each wavelength, top- and bottom-emitting VCSELs are used to compare the performances of four different module structures: bottom-emitting types of 850 and 1310 nm for the vertical modules, and top-emitting types of 850 and 1310 nm for the horizontal modules. The horizontal transmitter modules are packaged using the conventional wire-bonding technology for the interconnect between VCSEL and driver chips while the vertical modules are packaged using the flip-chip-bonding technology. Signal crosstalk is investigated using 1 $\,\times\,$ 4 VCSEL array chips, which are driven using the same Si-CMOS circuits designed for 5 Gb/s operation. The vertical modules show lower crosstalk than the horizontal modules. When the gap distance between the VCSEL aperture and the waveguide input port increases up to 500 $\mu{\rm m}$ for each module, the crosstalk increases in a range ${-}{58}$ to ${-}{\rm 37}~{\rm dB}$ . The 3-dB bandwidth performances are sensitively degraded in a range 4.6–2.0 GHz when the gap distance increases. In the crosstalk, the 1310-nm vertical module shows the best performance, while in the 3-dB bandwidth, the 850-nm horizontal module shows the best performance. The four modules shows clear eye openings at 2.5 Gb/s with bit error rate ${ and can be used for on-board optical interconnections.

NEAR FIELD AND FAR FIELD EFFECTS IN THE TAGUCHI-OPTIMIZED DESIGN OF AN InP/GaAs-BASED DOUBLE WAFER-FUSED MQW LONG-WAVELENGTH VERTICAL-CAVITY SURFACE-EMITTING LASER

Long-wavelength VCSELs (LW-VCSEL) operating in the 1.55 μm wavelength regime offer the advantages of low dispersion and optical loss in fiber optic transmission systems which are crucial in increasing data transmission speed and reducing implementation cost of fiber-to-the-home (FTTH) access networks. LW-VCSELs are attractive light sources because they offer unique features such as low power consumption, narrow beam divergence and ease of fabrication for two-dimensional arrays. This paper compares the near field and far field effects of the numerically investigated LW-VCSEL for various design parameters of the device. The optical intensity profile far from the device surface, in the Fraunhofer region, is important for the optical coupling of the laser with other optical components. The near field pattern is obtained from the structure output whereas the far-field pattern is essentially a two-dimensional fast Fourier Transform (FFT) of the near-field pattern. Design parameters such as the number of wells in the multi-quantum-well (MQW) region, the thickness of the MQW and the effect of using Taguchi's orthogonal array method to optimize the device design parameters on the near/far field patterns are evaluated in this paper. We have successfully increased the peak lasing power from an initial 4.84 mW to 12.38 mW at a bias voltage of 2 V and optical wavelength of 1.55 μm using Taguchi's orthogonal array. As a result of the Taguchi optimization and fine tuning, the device threshold current is found to increase along with a slight decrease in the modulation speed due to increased device widths.

Electrical Modeling of Long-Wavelength VCSELs for Intrinsic Parameters Extraction

We present an efficient method to model the small- signal modulation response of a long-wavelength VCSEL chip using an equivalent electrical circuit. This circuit serves two distinct purposes. Based on T-matrix formalism, it is used to remove the parasitics contribution originating from the electrical access of the chip in order to obtain the optical cavity intrinsic frequency response as defined by the rate equations. The same circuit is also used to extract the intrinsic cavity parameters since every circuit element represents a physical optical cavity entity. The extraction of reliable intrinsic parameters requires that the circuit element values be representative of the device under test. To achieve this, we have developed a new methodology based on static and dynamic measurements such as the S-parameters and the turn-on delay time. In accordance with this procedure, each element of the cavity is fixed without numerical optimization. The good agreement between measured and simulated curves confirm the validity of the technique used.

22-Gb/s Long Wavelength VCSELs

1.55-microm vertical cavity surface-emitting low-parasitic lasers show open eyes up to 22-Gb/s modulation speed. Uncooled error-free operation over a wide temperature range up to 85 degrees C under constant bias conditions is demonstrated at 12.5-Gb/s data rate. At these fixed bias conditions the laser characteristics are practically invariant with temperature. These are the highest data-rates reported from a long-wavelength VCSEL structure to date.

InP-Based Long-Wavelength VCSELs and VCSEL Arrays

Long-wavelength, InP-based, vertical-cavity surface-emitting lasers with buried tunnel junction (LW-BTJ-VCSELs) and VCSEL arrays are presented. Emitting at the telecommunications wavelengths around 1.3 and 1.55 mum, these devices show high modulation bandwidth in excess of 10 GHz over a wide temperature range. Small-signal equivalent circuits for the parasitic and intrinsic response are given. Various data-transmission experiments are performed with data rates up to 12.5 Gb/s over different fiber channels. 1-D LW-BTJ-VCSEL arrays can be used in wavelength-division multiplexing systems for high bandwidth per fiber channel. 2-D high-speed arrays with 64 individually addressable channels are presented that are particularly suited for optical ultrabroadband interconnects. Furthermore, large-scale arrays are investigated for high-power applications. In a detailed investigation, we derive scaling rules considering the influence of aperture, array size, and pitch on laser power. At room temperature, optical output powers in the watt regime have been realized at 1.55 mum. Divergence angles are as low as 13deg-17deg full-width at half-maximum. Wall plug efficiencies in excess of 20% over a wide current range and high-temperature operation up to 80degC are obtained.

Diffraction loss in long-wavelength buried tunnel junction VCSELs analyzed with a hybrid coupled-cavity transfer-matrix model

Intra-cavity diffraction in VCSELs is a loss mechanism that potentially can cause a significant decrease in efficiency and a rise in the threshold current, particularly in cavities with small lateral features with a high index contrast. One such VCSEL type is the 2.3 microm GaSb-based buried tunnel junction (BTJ) VCSEL studied in this work, where the BTJ induced topology of the top layers gives rise to excess loss through diffraction. Diffraction loss is difficult to measure, and also the numerical estimation must be done with care because of the non-axial propagation of the diffracted fields. We present a simulation method with spatially varying dimensionality, such that the field is three-dimensional (3D) in the entire cavity, whereas the material structure of the cavity is modelled in 3D near the BTJ and the layers with a varying topology, but elsewhere is assumed to be 1D like in a regular DBR structure. We find that the diffraction loss displays a non-monotonic behaviour as a function of the BTJ diameter, but as expected it rapidly increases below a certain diameter of the BTJ and may even become the dominant cause of loss in some device designs. We also show that the diffraction loss can be much reduced if the layers above the BTJ can be deposited such that the surface profile becomes smoother with increasing distance from the BTJ.

Gas temperature measurements using widely tunable long-wavelength VCSEL

A method for gas temperature measurements with a widely tunable laser diode is presented. The method involves rapidly switching the laser frequency between two distantly spaced absorption lines chosen for optical thermometry. Direct absorption spectroscopy using a single-mode VCSEL was employed to probe the R10 and R22 lines of the 2ν1+2ν20+ν3 combination band of CO2 near 6355.9 and 6363.7 cm-1 sequentially. A specially designed 0.5-m cryogenic gas cell was filled with 10 mbar CO2 at room temperature and cooled to 150 K with liquid N2. The VCSEL was modulated with a 10-kHz ramp superimposed on a 1-kHz square waveform to scan two 0.04 cm-1 intervals sequentially. The gas temperatures obtained with the VCSEL in the 150–300 K range are in a good agreement with those derived from gas pressure ratios. The maximum relative error of temperature measurements using the VCSEL was ± 3%. A compact VCSEL-based sensor can be developed for gas temperature and concentration measurements in the Martian atmosphere. The method proposed can be used for many applications including in situ monitoring of combustion processes.