Single-mode Vertical-cavity Surface-emitting Lasers Research Articles

Polarization dynamics induced by parallel optical injection in a single-mode VCSEL.

We report an experimental study of the polarization nonlinear dynamics in a 1550nm single-mode vertical-cavity surface-emitting laser (VCSEL) subject to parallel optical injection. Experimentally measured stability maps identifying regions of different nonlinear dynamics for various values of bias current are reported. We show that VCSELs with more than a 35dB polarization mode suppression ratio can have rich nonlinear dynamics in both linear polarizations, including periodic and chaotic behaviors appearing simultaneously in both polarization modes.

Application of nanophotonics to the next generation of surface-emitting lasers

Abstract Novel trends and concepts in the design and fabrication of vertical cavity surface-emitting lasers (VCSELs) and their integration in optical networks and implementation in integrated photonics applications are discussed. To serve these goals and match the growing bandwidth demands, significant changes are to be implemented in the device design. New lateral leakage-mediated single-mode VCSELs, including both devices confined by oxide layers and those confined by alloy-intermixed regions, are likely to be good candidates for light sources for the data networks of the future. An overview of the records in VCSEL transmission distances and transmission speeds is discussed in this context.

Analysis of the polarization of single-mode vertical-cavity surface-emitting lasers subject to parallel optical injection

We investigate experimentally and theoretically the polarization switching found in single-transverse-mode vertical-cavity surface-emitting lasers (VCSELs) when subject to parallel optical injection. Our analysis focuses on a recently observed state in which injection locking of the parallel polarization and excitation of the free-running orthogonal polarization of the VCSEL are obtained. A simple nonlinear dependence between the power of both linear polarizations and the frequency detuning is found. Also, the total power emitted by the VCSEL is constant and independent of the injected optical power and the frequency detuning. We check these results experimentally for a variety of frequency detunings and bias currents applied to the device. We report experimental and theoretical stability maps in the injected power-frequency detuning plane for different bias currents, identifying the regions in which the state is observed. A simple analytical expression that describes the map boundary for large and negative frequency detunings is obtained. This provides a simple method to extract the linear dichroism of the device.

Effective 100 Gb/s IM/DD 850-nm Multi- and Single-Mode VCSEL Transmission Through OM4 MMF

To cope with the ever increasing data traffic demands in modern data centers, new approaches and technologies must be explored. Short range optical data links play a key role in this scenario, enabling very high speed data rate links. Recently, great research efforts are being made to improve the performance of vertical-cavity surface-emitting lasers (VCSELs) based transmission links, which constitute a cost-effective solution desirable for massive deployments. In this paper, we experimentally demonstrate intensity-modulation direct-detection transmissions with a data rate of 107.5 Gb/s over 10 m of OM4 multimode fiber (MMF) using a multimode VCSEL at 850 nm, and up to 100 m of OM4 MMF using a single-mode VCSEL at 850 nm. Measured bit error rates were below 7% overhead forward error correction limit of 3.8e−03, thus, achieving an effective bit rate of 100.5 Gb/s. These successful transmissions were achieved by means of the multiband approach of carrierless amplitude phase modulation.

Modeling and optimization of single-mode vertical cavity surface emitting lasers

Vertical cavity surface emitting laser (VCSEL) plays a vital role in optical network. The present investigation reports the performance comparison of the modeling of single-mode VCSELs at room temperature for continuous wave operation. VCSEL for the study consists of InGaAsP-based cavity or active region sandwiched between GaAs/AlGaAs top mirror and GaAs/AlAs bottom mirrors with the aim of increasing the power conversion efficiency (PCE), lasing power, and decreasing the threshold current. It is observed that VCSELs with lower diameter are most suitable to achieve energy-efficient operation. The PCE obtained is ∼50% for the proposed single-mode VCSELs. The proposed VCSELs are suitable for short-reach optical interconnects such as chip-to-chip and board-to-board communication in high-performance computers.

Enhancement of Chaos Bandwidth in VCSELs Induced by Simultaneous Orthogonal Optical Injection and Optical Feedback

In this paper, we present an experimental study of bandwidths of broadband chaotic signals in a single-mode vertical-cavity surface-emitting laser (VCSEL) subject to orthogonal optical injection and optical feedback simultaneously. Bandwidths of the two linear polarizations and total power are measured for different values of frequency detuning and bias currents. When the VCSEL is subject to optical feedback only, both linear polarizations of the VCSEL are excited showing chaotic dynamics. Under simultaneous optical feedback and optical injection for small bias currents, we demonstrate that the orthogonal polarization can give a much larger value of the chaotic bandwidth than the total emitted power in agreement with Perez et al.

Polarization characteristics of 850-nm vertical-cavity surface-emitting lasers with intracavity contacts and a rhomboidal oxide current aperture

The polarization characteristics of 850-nm vertical-cavity surface-emitting lasers (VCSELs) with intracavity contacts and a rhomboidal oxide current aperture are studied. It is found that radiation polarization is always directed along the minor diagonal of the rhomboidal aperture (along the [\(\overline 1 \) 10] direction) for all single-mode VCSELs. The numerical simulation of carrier transport does not reveal the significant anisotropy of carrier injection to the active region. Furthermore, an analysis of the spatial distribution of the fundamental mode for two orthogonal polarizations within an effective waveguide model shows close transverse optical-confinement factors. Optical loss anisotropy in the asymmetric microcavity and/or gain anisotropy in the strained active region are the most likely mechanisms responsible for fixing the polarization.

Design Considerations for Single-Mode Vertical-Cavity Surface-Emitting Lasers With Impurity-Induced Intermixing

We report on the modeling of 850-nm (Al,Ga)As/GaAs vertical cavity surface-emitting lasers (VCSELs), where the electrical and optical confinement is realized by impurity-induced intermixing. The 3-D simulations of electromagnetic fields are performed by the finite-element method using full vector Maxwell’s equations. An example of a transverse junction VCSEL with Zn-induced intermixing of the areas surrounding the quantum well region is compared with the case of a VCSEL, where the optical modes are confined by selective oxidation of the Al-rich GaAlAs aperture layers. We show that Zn-induced intermixing results in a high efficiency of the leakage of the VCSEL aperture-confined modes into the surrounding intermixed area. Furthermore, there is a strong mode selectivity through the leakage loss between the fundamental and excited transverse modes of the optical cavity. This enables potential realization of a single transverse mode VCSEL at the aperture diameters of 10 $\mu \text{m}$ or above and consequently the realization of high-power single-mode VCSELs. Optimum mode selectivity is realized once the impurity intermixing profile proceeds down to the depth of about 2.6 $\mu \text{m}$ into the bottom distributed Bragg reflector below the quantum well region. Leakage of the optical modes into the intermixed area is accompanied by the evolution of narrow tilted lobes in the far-field emission spectra (at $\sim 60^{\circ }$ tilt angles for the design considered). This effect is similar to the one previously observed in oxide-confined leaky VCSELs and originates from the laterally propagating optical modes outside the non-intermixed aperture region. Where necessary, by applying such phenomenon, a phase-coupled array can be configured. We believe that such single-mode VCSELs may become extremely important in 3-D sensing, printing, and laser illumination. Furthermore, LIDARs with an option of beam steering without moving parts can be realized in 2-D phase-coupled arrays.

Mechanism of the polarization control in intracavity- contacted VCSEL with rhomboidal oxide current aperture

The possible mechanisms of the polarization control in single-mode intracavity- contacted vertical-cavity surface-emitting lasers (IC-VCSELs) with the rhomboidal selectively- oxidized current aperture were investigated. It was found that the lasing emission polarization of all single-mode VCSELs is fixed along the minor diagonal of the rhomboidal-shape aperture (the [110] direction). Numerical modelling of carrier transport did not reveal any sufficient injection anisotropy in the laser active region, while the transverse optical confinement factors calculated for the fundamental mode with two orthogonal polarizations are identical. Optical loss anisotropy and/or gain anisotropy are the most likely mechanisms of inducing the polarization fixation.

Single-Mode 850-nm VCSELs for 54-Gb/s ON–OFF Keying Transmission Over 1-km Multi-Mode Fiber

By combing Zn-diffusion and oxide-relief apertures with strong detuning (>20 nm) in our demonstrated short-cavity ( $\lambda $ /2) 850-nm vertical-cavity surface-emitting lasers (VCSELs), wide electrical-to-optical bandwidth (29–24 GHz), low-differential resistance ( $\sim 100~\Omega )$ , and (quasi) single-mode (SM) with reasonable output power ( $\sim 1.4$ mW) performances can be simultaneously achieved. Error-free ON–OFF keying transmission at 54-Gb/s data rate through 1-km OM4 multi-mode fiber can be achieved by using highly SM device with forward error correction and decision feedback equalization techniques. As compared with the reference device with a larger oxide-relief aperture and a multi-mode performance, the SM device exhibits lower bit-error rate ( $1\times {10}^{-5} $ versus $1\times {10}^{-2})$ at 54 Gb/s. This result indicates that modal dispersion plays more important role in transmission than that of output power does. We benchmark these results to an industrial 50-Gb/s SM VCSEL. It shows a higher bit-error-rate value $\sim 3.5\times {10}^{-3}$ versus $\sim 1.4\times {10}^{-4}$ under the same received optical power.

Harmonic distortion dependent on optical feedback, temperature and injection current in a vertical cavity surface emitting laser

In this paper, selective optical feedback is used to investigate the nonlinearity behaviours of a vertical cavity surface emitting laser (VCSEL) with the modulation signal. A single mode VCSEL with both parallel and orthogonal optical feedback (OF) signals modulated at 1 MHz frequency over a range of modulation depth is investigated. We also investigate the nonlinear characteristics of the orthogonal polarization modes XP and YP of the VCSEL by changing the injection current and temperature. The results show an enhancement in the harmonic distortions (HDs) of both XP and YP modes with parallel OF, and the total suppression of HDs with orthogonal OF. We show that for the VCSEL with orthogonal OF, the second and third harmonic components of the XP and YP modes decrease and reach the noise floor level of the output power spectrum. Additionally, peaks of second and third harmonic components change radically when varying the bias current and temperature. The results reveal that orthogonal OF can be employed as a new tool to improve the linear dynamic range and to control the nonlinear characteristics of the VCSEL, thus making these devices a promising optical source in present and future optical communication applications.

Direct Evidence of the Leaky Emission in Oxide-Confined Vertical Cavity Lasers

We report the observation of narrow tilted lobes in the far-field emission pattern of leaky oxide-confined vertical-cavity surface-emitting lasers (VCSELs). The VCSEL cavity is surrounded by two selectively oxidized aperture layers, which are intentionally designed to produce a high lateral leakage of the high-order transverse modes of the vertical cavity. The device operates in the fundamental transverse mode at oxide aperture diameters below $5~\mu \text{m}$ and currents up to 4 and 5 mA. At higher currents or larger aperture diameters, an additional high-order transverse optical mode evolves. This mode is revealed by the appearance of a shorter wavelength emission line in the electroluminescence spectra of the device, resulting in multi-mode lasing and in the related changes in the near- and far-field patterns. In the far-field pattern, the evolution of the high-order mode is revealed by the evolution of the two overlapping emission lobes at moderate (~5°) tilt angles with respect to the normal to the surface and by the appearance of a multi-spot or a ring pattern in the CCD camera images. Most importantly, the appearance of this high-order transverse mode is accompanied by an observation of much narrower lobes in the far-field pattern observed at significantly larger tilt angles (~35°). The tilt angle and the narrow angular width of these emission lobes in the far-field spectrum are in agreement with those calculated in the 3-D cold cavity modeling of the optical modes of the device. These narrow tilted lobes revealed in this paper are the fingerprints of the leakage effect in specially designed oxide-confined VCSELs with their intensity being proportional to the optical power leaving the aperture region in the direction parallel to the surface and propagating into the oxidized region. The effect can be applied for engineering of single-mode VCSELs, coherently-coupled 2-D VCSEL arrays, laterally integrated VCSEL-photodetector chips, and VCSELs integrated to slow light waveguides, for coupled optical gates for optical computers and other types of photonic integrated circuits.

Very High Bit-Rate Distance Product Using High-Power Single-Mode 850-nm VCSEL With Discrete Multitone Modulation Formats Through OM4 Multimode Fiber

In order to investigate the tradeoff between optical spectral width and modulation speed of 850-nm Zn-diffusion vertical-cavity surface-emitting laser (VCSEL) and its influence on the performance of discrete multitone (DMT) modulation, two kinds of high-speed VCSEL structures with different cavity lengths (λ/2 and 3λ/2) are studied. By shortening the cavity length to λ/2, allocating the oxide layer in the standing-wave peak, and performing a Zn-diffusion aperture in our VCSEL structure, stable dual mode in the output optical spectra across the full range of bias currents with good high-speed performance (∼16-GHz bandwidth) can be achieved. Compared with its multimode reference, it shows far less roll-off with regard to the maximum data rate versus transmission distance over OM4 multimode fibers under forward error correction (FEC) threshold (BER $ ). On the other hand, for the 3λ/2 VCSEL structure, by using the same Zn-diffusion conditions as those of dual-mode counterpart, highly single-mode operation (side-mode suppression ratio> 35 dB) with high available power can be achieved over the full range of bias currents. Although such device shows a smaller 3-dB electrical-to-optical bandwidth (12 versus 16 GHz) than that of the dual-mode one, it exhibits a superior transmission performance by use of DMT modulation format. A record high bit-rate distance product (107.6 Gb/s·km) at nearly 50-Gb/s transmission under FEC threshold (BER $ ) through 2.2-km OM4 fibers has been successfully demonstrated by the use of single-mode VCSEL with optimized structures. In addition, error-free (BER $ ) transmission at 20.3 Gb/s with bit-rate distance product of 44.66 Gb/s·km has also been demonstrated.

Polarization stable low threshold current single fundamental mode VCSELs

The characteristics of polarization stable microstructure vertical-cavity surface-emitting lasers (PS-MS-VCSELs) with low threshold current and single fundamental mode (SFM) operation were theoretically and experimentally investigated. Elliptical air hole photonic crystal (EPC) structure was incorporated in the top mirror of the MS-VCSELs to realize single fundamental mode operation. By controlling the mode loss difference among the two orthogonal modes, the fundamental mode and other high order modes of the MS-VCSELs, through suitable oxide aperture shape and EPC parameters, a high performance PS-MS-VCSEL is achieved with output power of 1.6 mW, low threshold current of 0.8 mA as well as more than 20 dB orthogonal polarization suppression ratio (OPSR).

Advances in Polymer-Based Optical MEMS Fabrication for VCSEL Beam Shaping

In this paper, we discuss the design, fabrication, and characterization of electrothermally actuated polymer-based microoptical electromechanical microsystems (MOEMS) for active microoptics in vertical cavity surface emitting laser (VCSEL) devices. We describe in particular the principle of a SU8-based MOEMS designed for single-mode VCSEL beam active focusing. The ultimate objective is the realization of parallel compact optical scanners for sensing applications using collective and low-cost technologies. After discussing the advantages of the epoxy resist SU-8 for fabricating an integrated movable lens on active optical devices, we present our latest advances in technology for ensuring precise MOEMS fabrication on small III–V samples and for achieving accurate alignment of lenses on suspended circular membranes. Finally, we present our first results on the beam focusing of multimode VCSELs, which demonstrate the feasibility of our approach and could provide new insights in the MEMS-VCSEL field.

Enhancement of Chaotic Signal Bandwidth in VCSELs Induced by Polarized Optical Injection

In this paper, we present a theoretical study of bandwidths of broadband chaotic signals generated by a single-mode vertical-cavity surface-emitting laser subjected to optical injection and feedback. Both parallel and orthogonal optical injections are considered. Bandwidths of the linearly polarized and total signals are analyzed for different values of the linear dichroism and frequency of the injected field. For the case of parallel optical injection, our results are in agreement with recent experimental results of Hong et al. Both linear polarizations are excited when orthogonal optical injection is considered. In this case, the dependence of the orthogonal polarization bandwidth on the frequency detuning is similar to that found in the case of parallel injection. We also predict that the selection of the orthogonal polarization can give a much larger value of the chaotic bandwidth than that obtained in the total emitted power.

Single-mode temperature and polarisation-stable high-speed 850nm vertical cavity surface emitting lasers

A new intracavity-contacted design to realize temperature and polarization-stable high-speed single-mode 850 nm vertical cavity surface emitting lasers (VCSELs) grown by molecular-beam epitaxy is proposed. Temperature dependences of static and dynamic characteristics of the 4.5 pm oxide aperture InGaAlAs VCSEL were investigated in detail. Due to optimal gain-cavity detuning and enhanced carrier localization in the active region the threshold current remains below 0.75 mA for the temperature range within 20-90°C, while the output power exceeds 1 mW up to 90°C. Single-mode operation with side-mode suppression ratio higher than 30 dB and orthogonal polarization suppression ratio more than 18 dB was obtained in the whole current and temperature operation range. Device demonstrates serial resistance less than 250 Ohm, which is rather low for any type of single-mode short- wavelength VCSELs. VCSEL demonstrates temperature robust high-speed operation with modulation bandwidth higher than 13 GHz in the entire temperature range of 20-90°C. Despite high resonance frequency the high-speed performance of developed VCSELs was limited by the cut-off frequency of the parasitic low pass filter created by device resistances and capacitances. The proposed design is promising for single-mode high-speed VCSEL applications in a wide spectral range.

Single-Mode Vertical-Cavity Surface-Emitting Lasers Array With Zn-Diffusion Aperture for High-Power, Single-Spot, and Narrow Divergence Angle Performance

A single-mode vertical-cavity surface-emitting laser (VCSEL) array at 850 nm with excellent performance in terms of high output power, single-lobe far-field, and narrow divergence angle has been demonstrated. By use of the Zn-diffusion process with proper sizes of oxide current-confined and Zn-diffusion optical apertures, each unit of VCSEL in the demonstrated array is highly single-mode (side-mode suppression ratio 30 dB) with a narrow far-field divergence angle (~5°) and an extremely high maximum single-mode output power (~7.1 mW). Due to the excellent single-mode performance of each VCSEL unit, the 10 × 10 array exhibits a single-lobe and nearly circular symmetric far-field pattern, very narrow divergence angle (~4°), and output power as high as around 187.4 mW. Furthermore, its bias-dependent output optical spectra measured in different positions of array shows very high similarity. This result indicates that the excellent uniformity of single-mode performance of each VCSEL unit in the array. The far-field pattern simulation results indicate that although the coherence in our array is low, its output beam quality is as high as that of a single-mode unit VCSEL, which exhibits a near diffraction limited M square factor (M 2 ≈ 2).

Direct generation of 2-ps blue pulses from gain-switched InGaN VCSEL assessed by up-conversion technique.

Ultra-short pulses in blue region generated from compact and low-cost semiconductor lasers have attracted much attention for a wide variety of applications. Nitride-based vertical-cavity surface-emitting lasers (VCSELs), having intrinsic high material gain and short cavities, favor the generation of ultra-short blue pulses via a simple gain-switching technique. In this study, we fabricated a single-mode InGaN VCSEL consisting of 10-period InGaN/GaN quantum wells (QWs). The output pulses were evaluated accurately with an up-conversion measurement system having time resolution of 0.12 ps. We demonstrated that ultra-short blue pulses, as short as 2.2 ps at 3.4 K and 4.0 ps at room temperature, were generated from the gain-switched InGaN VCSEL via impulsive optical pumping, without any post-processing. The gain-switched pulses we obtained should greatly promote the development of ultra-short blue pulse generation. In addition, this successful assessment demonstrates the up-conversion technique's usefulness for characterizing ultra-short blue pulses from semiconductor lasers.

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