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Probing the thermal effects that limit the performance of vertical-cavity surface-emitting lasers (VCSELs) reveals an intricate interplay between carrier transport, recombination mechanisms, thermal conduction, and optical features. An understanding of this interplay requires an accurate yet computationally-efficient multiphysical approach at the microscopic level. We address this task by characterizing oxide-confined multimode 850 nm VCSELs over a wide temperature range, and by simulating them with VENUS, our in-house multiphysics simulator. The agreement with the experimental results in the whole explored temperature range provides physical insight into the mechanisms that determine VCSEL rollover and turn off, and demonstrates the predictive capabilities of the model. A complete set of model parameters determined through this combined experiment-simulation approach is presented.

The properties of vertical-cavity surface-emitting lasers (VCSELs) are investigated by means of a multiphysical Vcsel Electro-opto-thermal NUmerical Simulator (VENUS). VENUS includes a three-dimensional vectorial electromagnetic code, a description of the quantum well optical response, a heat equation solver, and a quantum-corrected drift-diffusion simulator. The proposed suite includes coupling mechanisms often overlooked in VCSEL simulation and allows to reassess the impact of parameters, which can be critically dependent on implementation details inaccessible in commercial codes. The agreement with experimental results holds the promise of the application of this framework to the computer-aided design of innovative VCSEL concepts.

Spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) have proven to be a highly promising device technology for high-speed optical communication systems. In spin-lasers, the polarization state of the laser emission can be controlled by the carrier spin state exploiting the transfer of angular momentum between carriers and photons. The resonance frequency of the polarization dynamics can be increased by inducing birefringence into the resonator. Here we discuss the role of the photon lifetime and show results on the influence of this parameter on the static polarization behavior.

Using an optical transmission electroabsorption modulator (T-EAM) in combination with a common image sensor and a high-power illumination source allows to build a three-dimensional (3D) imaging system based on the indirect time-of-ight (iTOF) principle. In contrast to most available iTOF systems that use specialized image sensors, in this case the EAM is doing the required demodulation at the receiver in the optical domain. A first 3D imaging setup was built and investigations on the performance of the system are reported. A high-power laser source was characterized in order to design and fabricate matched large-area T-EAMs. The fabricated modulators show extinction ratios of about 6 dB and bandwidths of up to 37MHz at 1mm2 size. 3D images taken with the prototype camera consisting of the T-EAM, the laser source, and a Raspberry Pi camera show promising results with a depth inaccuracy below 5 cm at a distance of 1 m.

Vertical-cavity surface-emitting lasers (VCSELs) are widely used in optical data communication mainly in data centers for short-haul transmissions. However, their intensity modulation resonance frequency does not exceed 40 GHz which also limits the achievable modulation bandwidth and data rate. In contrast, spin-VCSELs can overcome these bandwidth limitations by modulating spin and polarization instead of current and intensity. In spin-VCSELs, the birefringence determines the resonance frequency of the polarization dynamics as well as the modulation bandwidth. We control the birefringence and thus the polarization dynamics via the elasto-optic effect by mechanically or thermally induced strain providing polarization oscillation frequencies up to more than 200 GHz. Detailed analysis shows that spin-VCSELs offer polarization dynamics with good signal strength even when operating close to threshold and at high temperatures. Here, we analyze devices with integrated surface gratings. VCSELs with different grating periods as well as mesa diameters and resulting different oxide apertures were investigated. © 2021 SPIE.

A preliminary study of the dynamic behaviour of a GaAs/AlGaAs 850 nm VCSEL is presented, with the focus on the small-signal analysis and in particular on the optical amplitude modulation response. Simulations are performed with our in-house quantum-corrected one-dimensional drift-diffusion code D1ANA, updated to perform the AC analysis. The-3 dB cutoff frequency of the device is extracted as a function of VCSEL bias current and compared with experimental results.

Since todays Internet traffic is more and more concentrated in hyperscale datacenters,1 new concepts for shortrange optical communication systems with high modulation bandwidth, high temperature stability, and low energy consumption are urgently needed. Birefringent spin-lasers, in particular spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs), are a novel type of ultrafast laser devices which promise to serve as ultrafast transmitters for the next generation of optical communication systems. While current-driven intensitymodulated VCSELs are state-of-the-art laser devices for short-range communication, their modulation bandwidth is limited to values below 40 GHz.2, 3 Recently, we were able to demonstrate that modulating carrier spin and light polarization in spin-VCSELs instead of carrier density and light intensity in conventional devices enables ultrafast polarization dynamics and a modulation bandwidth of more than 200 GHz.4 This high modulation bandwidth was achieved by increasing the resonance frequency of the coupled carrier spin-photon system by implementing high values of birefringence to the cavity of 850 nm GaAs/GaAlAs VCSELs. Here, we show experimental results for the intensity and polarization dynamics in highly birefringent spin-VCSELs as a function of bias current, birefringence, and temperature and demonstrate the capability of spin-VCSELs for ultralow energy consumption and high temperature stability. Furthermore, we present first results on polarization dynamics in 1.3 μm VCSELs for potential long-range communication systems and discuss novel concepts for future integrated and electrically pumped devices.

Data centers play an important role in the ongoing demand for higher data rates. Here intensity-modulated vertical-cavity surface-emitting lasers (VCSELs) are the emitters of choice. The intensity dynamics resonance is currently limited to around 30 GHz. Using the much faster polarization dynamics in VCSELs can be a promising alternative. The polarization dynamics resonance is mainly determined by the frequency difference of the two orthogonal linearly polarized modes, which is defined by the birefringence. We have experimentally investigated mechanisms for birefringence manipulation from large-scale down to on-chip solutions. Polarization oscillations with frequencies in excess of 200 GHz have been observed.

Vertical-cavity surface-emitting lasers (VCSELs) are commonly used in optical data communication mainly for short-haul transmissions in data centers. Spin-VCSELs can be a promising solution in order to overcome the bandwidth limitations of conventional VCSELs by utilizing the spin and polarization instead of current and intensity. Recently, their polarization dynamics have been enhanced to resonance frequencies of more than 200 GHz by implementing a large amount of birefringence into the laser cavity. For future applications onchip solutions to control the birefringence are preferred. For this purpose, a keyhole-shaped mesa-structure on standard wafer material for an 850nm oxide-confined AlGaAs-VCSEL is used. A variable heating current is driven into the semiconductor ridge connected to the mesa at a constant pump current. This creates an asymmetrical heat gradient. Here we investigate the polarization behaviour in a spin-VCSEL with thermally induced birefringence. We analyze the hysteresis in the heating and pump current of the sample to identify optimized working points near the polarization switching points.

In single-mode vertical-cavity surface-emitting lasers (VCSELs) the frequency difference between the two orthogonal modes, which is defined by the birefringence present in the cavity, is the key factor to enable ultrafast polarization dynamics in spin-lasers. This could be a promising alternative to overcome the bandwidth limitations in short-haul data transmission. Therefore, controlling the birefringence is indispensable to utilize the full potential of the polarization dynamics. Splittings of around 100GHz were realized with an on-chip approach by integration of a surface grating in an oxide-confined AlGaAs-based VCSEL. In this paper we present further details of the parameter search process using a three-dimensional vectorial optical VCSEL electro-magnetics (VELM) model. We also show the geometrical properties of the processed grating structure.

The indirect time-of-flight principle is one possibility to build a three-dimensional (3D) camera system. Available products based on this principle mostly use special CMOS sensors for demodulation of the optical signal at the receiver. This special CMOS chip can be replaced by a standard image sensor in combination with a quantum well electroabsorption modulator. In this case, the modulator heavily influences the 3D camera performance. Especially the characteristics of large-area devices are of major interest. Transmission electroaborption modulators with sizes in the square millimeter range have been fabricated for operating wavelengths of 850 and 940 nm. While the 850nm devices were realized as non-resonant structures, for 940nm devices a resonant design was developed to overcome the limitation in the number of quantum wells. Investigations of the static and dynamic behavior show extinction ratios up to 2.5 dB and corner frequencies up to 30 MHz. A single-point distance measurement setup demonstrates the high potential of the devices for the 3D application.

On our way to develop a comprehensive simulator of vertical-cavity surface-emitting lasers including carrier transport, optical modes, light-matter interaction and heat conduction, we address some critical multiscale aspects of the adopted coupling strategy. The quantum corrections to the semiclassical carrier transport framework are the bridge from nanometer to micrometer scales. In this paper, such corrections are shown to be fundamental in view of predicting the mode competition ruling VCSEL operation. Nevertheless, they can generate unforeseen features such as swirling electron flows in the active region, which are discussed here in detail. The simulation approach has been finally tested through a successful comparison with a large set of experimental results.

Refractometry has a variety of applications in chemistry, biotechnology, or food industry. A number of sensor systems are available to analyze the refractive index of liquid solutions or even biological tissue. We present a novel technique that aims at recording local distributions of refractive indices. The approach relies on the phase shift upon total internal reflection (TIR) and uses an extremely short Fabry-Perot microcavity of less than 3 μm effective length as the key element to precisely measure refractive indices at less than 100 μm spatial resolution.

We present a concept for optically controlled current confinement in vertical-cavity surface-emitting lasers (VCSELs) based on the monolithic integration of a phototransistor. Omitting the usual oxide aperture improves the manufacturability and prevents built-in strain near the active zone. Measured continuous-wave operation characteristics of fabricated devices show hysteresis loops in the current–voltage and light–current curves or a negative differential resistance region. Moreover, the optoelectronic influence of the integrated phototransistor is compared with a regular VCSEL. Requirements for the switch-on point of the laser are defined and explained.

The lasing operation of vertical-cavity surface-emitting lasers (VCSELs) results from the interaction of diverse physical mechanisms. For this reason, a reliable model must solve the coupled carrier transport, thermal and optical problems. With the aim of removing the present lack of established simulation tools, in this work we present our in-house electro-opto-thermal VCSEL numerical simulator VENUS. After a description of its four constituents, a validation with experimental results obtained at different temperatures is presented, demonstrating the predictive capabilities of VENUS.

Increasing the birefringence splitting in single-mode vertical-cavity surface-emitting lasers (VCSELs) enables high-speed polarisation dynamics which can be the basis to overcome the current bandwidth limitations in short-haul data transmission. The authors observe large birefringence splittings of up to 98 GHz in an oxide-confined AlGaAs-based VCSEL with a tailored integrated surface grating. Since surface gratings are routinely used in VCSEL production, there is a great potential of this technique to realise spin-VCSELs for ultrafast optical communication.

The appeal of lasers can be attributed to both their ubiquitous applications and their role as model systems for elucidating nonequilibrium and cooperative phenomena. Introducing novel concepts in lasers thus has a potential for both applied and fundamental implications. Here we experimentally demonstrate that the coupling between carrier spin and light polarization in common semiconductor lasers can enable room-temperature modulation frequencies above 200 GHz, exceeding by nearly an order of magnitude the best conventional semiconductor lasers. Surprisingly, this ultrafast operation relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both commonly viewed as detrimental in spintronics and conventional lasers. Our results overcome the key speed limitations of conventional directly modulated lasers and offer a prospect for the next generation of low-energy ultrafast optical communication., 10 pages, 4 figures

For short-haul optical interconnects, state-of-the-art technology are vertical-cavity surface-emitting lasers (VCSELs). To transmit data, direct current modulation is used. The corresponding intensity modulation resonance frequency is determined by design and material parameters of the laser and therefore practically limited to a few tens of GHz. To overcome this limitation, an alternative approach is the utilization of spin-VCSELs. In this case, the information carrier is no longer represented by the intensity, but instead by the polarization. The polarization can be controlled by the carrier spin. The birefringence in the cavity has the strongest impact on the polarization modulation resonance frequency. This can be explained by the generation of resonant polarization oscillations in the circular polarization degree in a spin-VCSEL. The circular polarization is composed of the two orthogonal linearly polarized cavity modes. The electromagnetic fields emitted from the two modes are coupled in phase by birefringence and in amplitude by dichroism. However, dependent on the birefringence in the cavity, their frequencies may differ. Spin pumping, i.e., circularly polarized optical pumping pulses, causes the fact that both modes become active. This results in an oscillation of the circular polarization degree of the emitted light, representing the polarization dynamics resonance frequency of the spin-VCSEL device. We demonstrate that the birefringence can be manipulated in actual VCSEL devices over a broad tuning range. Employing this parameter tuning, we demonstrate a polarization dynamics resonance frequency of 89 GHz, which is much faster than currently obtained intensity dynamics resonance frequencies. Not only the maximum frequency, but also the amplitude of the polarization effects should be optimized. An important factor for the amplitude damping is the dichroism, which represents the difference in the gain of the two orthogonal modes. We investigate the influence of birefringence on dichroism and the polarization oscillation amplitude.

We have devised an oxide- and regrowth-free approach for current confinement in vertical-cavity surface-emitting lasers (VCSELs) which uses the photons in the cavity to optically define a current path through the device. For this purpose, an optical switch, implemented as a phototransistor (PT), is epitaxially integrated into the cavity. The PT layers become locally conductive where the highest photon density is reached in the resonator and establish the current aperture. By introducing additional photons with an external laser beam into the resonator, we show here that this current aperture can be manipulated. We demonstrate the possibility of redefining the location of the current aperture and discuss the consequences on the light-current characteristics of the devices.

Polarization dynamics in vertical-cavity surface-emitting lasers (VCSELs) are much faster than their intensity-driven counterparts and can be a potential approach to overcome the bandwidth limitation in short-distance data transmission. The birefringence splitting B as the frequency difference between the two fundamental polarization modes is an important factor determining the polarization dynamics in spin-VCSELs. Although B can be strongly influenced by mechanical bending, for later applications an on-chip solution for birefringence tuning is favored. With an electrically driven asymmetric heating device we have demonstrated a thermally induced tuning range of ΔB = 45GHz. The maximum achievable birefringence tuning was not limited by the laser but by material parameters and the fabrication process. In this paper we present an optimized design for thermally induced birefringence tuning and additional possibilities to increase the efficiency of the mechanism.

Vertical-cavity surface-emitting lasers (VCSELs) are used for short-haul optical data transmission with increasing bit rates. The optimization involves both enhanced device designs and the use of higher-order modulation formats. In order to improve the modulation bandwidth substantially, the presented work employs spin-pumped VCSELs (spin-VCSELs) and their polarization dynamics instead of relying on intensity-modulated devices. In spin-VCSELs, the polarization state of the emitted light is controllable via spin injection. By optical spin pumping a single-mode VCSEL is forced to emit light composed of both orthogonal linearly polarized fundamental modes. The frequencies of these two modes differ slightly by a value determined by the cavity birefringence. As a result, the circular polarization degree oscillates with their beat frequency, i.e., with the birefringence-induced mode splitting. We used this phenomenon to show so-called polarization oscillations, which are generated by pulsed spin injection. Their frequency represents the polarization dynamics resonance frequency and can be tuned over a wide range via the birefringence, nearly independent from any other laser parameter. In previous work we demonstrated a maximum birefringence-induced mode splitting of more than 250 GHz. In this work, compared to previous publications, we show an almost doubled polarization oscillation frequency of more than 80 GHz. Furthermore, we discuss concepts to achieve even higher values far above 100 GHz.

The birefringence splitting B, which is the frequency difference between the two fundamental linear polarization modes in vertical-cavity surface-emitting lasers (VCSELs), is the key parameter determining the polarization dynamics of spin-VCSELs that can be much faster than the intensity dynamics. For easy handling and control, electrical tuning of B is favored. This was realized in an integrated chip by thermally induced strain via asymmetric heating with a birefringence tuning range of 45 GHz. In this paper we present our work on VCSEL structures mounted on piezoelectric transducers for strain generation. Furthermore we show a combination of both techniques, namely VCSELs with piezo-thermal birefringence tunability.

Vertical-cavity surface-emitting laser diodes (VCSELs) provide extraordinary properties like sub-mA threshold current, multi-GHz modulation capability, or relative intensity noise close to the quantum limit. VCSELs have become a preferred light source for optical sensor technologies and imaging systems. The purpose is to review some recent developments with special attention to mass market employments and to reveal future applications.

While the high-frequency performance of conventional lasers is limited by the coupled carrier-photon dynamics, spin-polarized lasers have a high potential to overcome this limitation and to push the direct modulation bandwidth beyond 100 GHz. The key is to utilize the ultrafast polarization dynamics in spin-polarized vertical cavity surface-emitting lasers (spin-VCSELs) which is decoupled from the intensity dynamics and its fundamental limitations. The polarization dynamics in such devices, characterized by the polarization oscillation resonance frequency, is mainly determined by the amount of birefringence in the cavity. Using an approach for manipulating the birefringence via mechanical strain we were able to increase the polarization dynamics to resonance frequencies of more than 40 GHz. Up to now these values are only limited by the setup to induce birefringence and do not reflect any fundamental limitations. Taking our record results for the birefringence-induced mode splitting of more than 250 GHz into account, the concept has the potential to provide polarization modulation in spin-VCSELs with modulation frequencies far beyond 100 GHz. This makes them ideal devices for next-generation fast optical interconnects. In this paper we present experimental results for ultrafast polarization dynamics up to 50 GHz and compare them to numerical simulations.

Vertical-cavity surface-emitting lasers (VCSELs) are the transmitters of choice in high-speed optical interconnects. Further progress beyond present 25 Gb/s devices is hampered by the limited modulation bandwidth of not more than 30 GHz. For higher data rates one thus has to resort to higher-order modulation formats [1] with substantially increased system complexity. Polarization modulation of VCSELs is a potential alternative to intensity modulation. In this technique the birefringence splitting B, namely the frequency difference between the two fundamental polarization modes, is a key parameter. By optical spin injection a birefringent VCSEL can be excited to oscillations in the degree of circular polarization, where the oscillation frequency is close to B [2]. In this work we show a large tuning range of B in a VCSEL achieved via anisotropic strain caused by an integrated heater.

Current confinement in vertical-cavity surface-emitting lasers (VCSELs) is nowadays realized most commonly with buried Al x O y oxide apertures very close to the active region [1]. A novel oxide-free and regrowth-free approach uses the generated spontaneous light in the laser resonator in association with an epitaxially integrated phototransistor (PT) for an optical definition of the current path through the device [2]. Initially, the PT, which is configured as an optical switch, acts as an isolating barrier layer. Only leakage current generates some spontaneous emission in the active zone, which is partly absorbed in a thin InGaAs quantum well (α-QW) embedded between the base and the collector layers. The resulting photocurrent takes the role of a base current and turns parts of the PT layers conductive after exceeding a certain threshold base current, depending on the PT's current gain and the layer structure of the VCSEL. Hence, the current-voltage curve shows a negative differential resistance (NDR) region when the PT opens and the resistance decreases. Once opened, the current aperture remains stable in diameter, thus lasing starts after reaching the threshold current density. Without an oxide layer, potentially problematic strain near the active zone is reduced. Moreover, the manufacture is less complex. Ultimately, the PT-VCSEL is intended to exhibit improved reliability and efficiency compared to present oxidized VCSELs.

Vertical-cavity surface-emitting lasers (VCSELs) are widely used in short-range optical data transmission and optical sensing. Tailored heating and infrared illumination with large two-dimensional parallel-driven VCSEL arrays are gaining more and more attention. In the latter fields as well as in high-speed datacom the laser performance is limited by internal heating at high operating currents or elevated ambient temperatures, resulting in a saturation of the achievable output power and a reduction of the device lifetime. The average internal temperature Ti in the laser cavity is a key parameter to assess new VCSEL designs for applications involving high thermal stress. A versatile experimental method to determine T i should be easy to employ in standard test environments. The increase of the internal temperature above the ambient level Ta is commonly quantified by the thermal resistance R Th = ΔT i / ΔP diss , which relates the temperature change ΔT i to a change of dissipated power ΔP diss . It is well known that the emission wavelength λ can be used as an indicator for Ti, because the optical cavity length depends on Ti and directly affects λ. In the conventional experimental approach, the coefficient C = dλ/dT i is either assumed to be constant [1], which introduces an error due to its material composition dependence, or C is (correctly) determined from Th based on easily measurable light-current-voltage (LIV) curves for different Ta and associated optical spectra at a reduced number of operation points [3]. No empirical parameter is needed and nonlinearities due to temperature-dependent thermal conductivities are automatically included. Both single-mode and multi-mode devices can be analyzed by monitoring the wavelength shifts of fixed transverse modes. Here we apply this method to determine the maximum lasing temperature of various VCSELs and show that it is a figure of merit for temperature stability.

Using the elasto-optic effect, increase of the frequency difference between the two orthogonally polarised modes, the so-called birefringence splitting, in standard single-mode oxide-confined AlGaAs-based vertical-cavity surface-emitting lasers is achieved to values beyond 250 GHz. A large birefringence is required for the generation of ultra-fast polarisation oscillations for potential future high-speed communication applications.

Recently, we reported the first successful fabrication of a marker-free microparticle detector with a unique working principle: when a particle is introduced into the cavity of an electrically pumped vertical-external-cavity surface-emitting laser, both the change of electrical operation as well as optical output parameters can be directly monitored, which enables phenotyping of properties, such as particle volume and refractive index. In the present contribution, we investigate this mechanism by numerical modeling of the wave-optical characteristics of the sensor.

Compared to conventional vertical-cavity surface-emitting lasers (VCSELs), spin-pumped VCSELs offer the possibility of polarization control and fast polarization dynamics. It has been demonstrated that oscillations in the circular polarization degree can be excited. In short, the frequency of these polarization oscillations is determined by the frequency splitting between the two orthogonal linearly polarized cavity modes and therefore by the cavity birefringence. The polarization oscillation frequency is the resonance frequency of the VCSEL’s polarization dynamics and can be compared to the conventional resonance frequency for intensity modulation. We have demonstrated polarization oscillations up to 44 GHz, exceeding the direct intensity resonance frequency of the investigated devices by far. As the polarization oscillation frequency can be increased by increasing the cavity birefringence and a VCSEL cavity birefringence of more than 250 GHz has been demonstrated, using polarization dynamics is a possible way of substantially increasing the modulation speeds of VCSELs. This is for instance interesting for high-bandwith short-haul optical interconnects. The experimental results associated with the polarization oscillation effects can be simulated by the widely used spin-flip model. In this work we focus on the amplitude of the polarization oscillations. Previous publications have shown a decrease with increasing oscillation frequency. Here, we show amplitude dependencies on several system parameters like the photon and carrier lifetimes as well as pumping conditions. Based on this, we investigate how to increase the polarization oscillation amplitude, since a significant amplitude is necessary for, e.g., data transmission applications.

Spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) have a high potential to overcome limitations of conventional purely charge-based lasers. Probably the most important feature of such spin-lasers lies in their ultrafast spin and polarization dynamics which are decoupled from the intensity dynamics and their limitations. This yields the potential to modulate the polarization state of spin-VCSELs with frequencies far above the barriers known for the intensity modulation dynamics of conventional VCSELs. Such a quality makes them ideal devices for fast optical interconnects. While in conventional devices relaxation oscillations provide insights in the intensity dynamics and modulation bandwidth, in spin-VCSELs oscillations in the circular polarization degree are an ideal measure for investigating the dynamics of the coupled spin-photon system. These polarization oscillations (POs) can be generated using pulsed spin injection and have been proven to be much faster than intensity dynamics in the devices. Their frequency is mainly dependent on the birefringence in the cavities and can be increased by adding mechanical strain. Using an approach for manipulating the birefringence via mechanical strain we demonstrated tunable POs with frequencies up to 44 GHz, recently. Taking our results for strain-induced birefringence splitting of more than 250 GHz into account, the concept has the potential to overcome conventional limitations and to provide polarization modulation in VCSELs with bit rates beyond 100 Gbit=s. In this paper we investigate numerically the in uence of the spin decay rate on the PO amplitude and frequency in order to investigate potential limitations for future ultrafast polarization modulation schemes.

We present a novel approach to determine the thermal resistance and the internal temperature of vertical-cavity surface-emitting lasers (VCSELs) based on easily accessible laser parameters. The described method does not use any empirical parameters or pulsed measurements that are often mentioned in literature. We explain how to determine the thermal resistance and show the computation of the internal temperature for any operation point. Furthermore the data evaluation can be used for characteristic parameter extraction that enables us to establish an isothermal and temperature-dependent modeling of the VCSEL operation curves.

We have devised a novel oxide-free and regrowth-free approach for optically controlled current confinement in vertical-cavity surface-emitting lasers (VCSELs). This is realized with a monolithically integrated phototransistor (PT), which is configured as an optical switch and embedded between the two Bragg reflectors. We have fabricated functional PT-VCSELs by one-step epitaxial growth plus metal deposition with different top contact sizes. We present light–current–voltage characteristics of the lasers as well as a simple theoretical model explaining the occurrence of a distinct turn-on point and clarify epitaxial design requirements to reach strong optically controlled current confinement.

Controlling the coupled spin-photon dynamics in vertical-cavity surface-emitting lasers (VCSELs) is an attractive opportunity to overcome the limitations of conventional, purely charge based semiconductor lasers. Such spin-controlled VCSELs (spin-VCSELs) offer several advantages, like reduced threshold, spin amplification and polarization control. Furthermore the coupling between carrier spin and light polarization bears the potential for ultrafast polarization dynamics. By injecting spin-polarized carriers, the complex polarization dynamics can be controlled and utilized for high-speed applications. Polarization oscillations as resonance oscillations of the coupled spin- photon system can be generated using pulsed spin injection, which can be much faster than the intensity dynamics in conventional devices. We already demonstrated that the oscillations can be switched in a controlled manner. These controllable polarization dynamics can be used for ultrafast polarization-based optical data communication. The polarization oscillation frequency and therefore the possible data transmission rate is assumed to be mainly determined by the birefringence-induced mode-splitting. This provides a direct tool to increase the polarization dynamics toward higher frequencies by adding a high amount of birefringence to the VCSEL structure. Using this technique, we could recently demonstrate experimentally a birefringence splitting of more than 250 GHz using mechanical strain. Here, we employ the well-known spin-flip model to investigate the tuning of the polarization oscillation frequency. The changing mechanical strain is represented by a linear birefringence sweep to values up to 80πGHz. The wide tuning range presented enables us to generate polarization oscillation frequencies exceeding the conventional intensity modulation frequency in the simulated device by far, mainly dependent on the birefringence in the cavity only.

Spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) provide novel opportunities to overcome several limitations of conventional, purely charge-based semiconductor lasers. Presumably the highest potential lies in the spin-VCSEL's capability for ultrafast spin and polarization dynamics which can be significantly faster than the intensity dynamics in conventional devices. By injecting spin-polarized carriers, these coupled spin-photon dynamics can be controlled and utilized for high-speed applications. While relaxation oscillations provide insights in the speed and direct modulation bandwidth of conventional devices, resonance oscillations in the circular polarization degree step in for the spin and polarization dynamics in spin-VCSELs. These polarization oscillations can be generated using pulsed spin injection and achieve much higher frequencies than the conventional intensity relaxation oscillations in these devices. Furthermore polarization oscillations can be switched on and off and it is possible to generate short polarization pulses, which may represent an information unit in polarization-based optical communication. The frequency of polarization oscillations is mainly determined by the birefringence-induced mode splitting between both orthogonal linearly polarized laser modes. Thus the polarization modulation bandwidth of spin-VCSELs can be increased by adding a high amount of birefringence to the cavity, for example by incorporating mechanical strain. Using this technique, we could demonstrate tunable polarization oscillations from 10 to 40 GHz in AlGaAs-based 850nm VCSELs recently. Furthermore a birefringence-induced mode splitting of more than 250 GHz could be demonstrated experimentally. Provided that this potential for ultrafast dynamics can be fully exploited, birefringent spin-VCSELs are ideal devices for fast short-haul optical interconnects. In this paper we review our recent progress on polarization dynamics of birefringent spin-VCSELs and investigate numerically how ultrafast polarization oscillations can be utilized for data communication using simulations based on the spin-flip model.

Using the elasto-optic effect we increase the frequency difference between the two orthogonally polarized modes, the so-called birefringence splitting, in standard single-mode oxide-confined GaAs-based vertical-cavity surface-emitting lasers (VCSELs). The birefringence may play an important role in the realization of ultrafast polarization modulation for high-speed data transmission. For practical implementation it is necessary to miniaturize the strain-inducing mechanism for birefringence tuning in VCSELs. The goal is the realization of integrated structures on the VCSEL chip. In this paper we discuss our work on miniaturized bending devices as the next step in achieving extremely high birefringence splitting. Furthermore measurements with integrated hotspot structures on VCSEL chips were made to reach much smaller scales for birefringence fine-tuning.

Spin-VCSELs offer numerous advantages over conventional lasers like reduced threshold, spin amplification and ultrafast polarization dynamics. The latter have the potential to generate polarization modulation frequencies far above the conventional intensity relaxation oscillation frequency of one and the same device and thus can be an interesting basis for ultrafast optical data transmission. We have shown that fast polarization oscillations can be generated by pulsed spin injection. Furthermore the oscillation frequency can be tuned via modification of the VCSEL’s cavity strain. Using this technique, oscillation frequencies with a tuning range from nearly zero up to 40 GHz can be demonstrated. In the device under study, this is more than six times the intensity relaxation oscillation frequency, which is nearly independent of the strain. Now we demonstrate the influence of the strain-induced birefringence splitting on the oscillation frequency. We find that the polarization oscillation frequency is directly corresponding to the birefringence splitting. The reason is that the polarization oscillates according to the beating frequency of the two orthogonal linearly polarized cavity modes in the VCSEL. In the case of spin-pumping, those two modes form the circular polarization output of the laser by superposition. Their frequencies are shifted by birefringence manipulation and form the basis of birefringence splitting. The measurement results are compared with simulations employing the spin-flip model. Our results show that high-frequency polarization oscillations can not only be generated with the help of external strain but with high birefringence splitting in general.

We report the monolithic integration, fabrication, and electrooptical properties of AlGaAs-GaAs-based transceiver (TRx) chips for 850 nm wavelength optical links with data rates of multiple Gbit/s. Using a single butt-coupled multimode fiber (MMF), low-cost bidirectional communication in half- and even full-duplex mode is demonstrated. Two design concepts are presented, based on a vertical-cavity surface-emitting laser (VCSEL) and a monolithically integrated p-doped-intrinsic-n-doped (PIN) or metal-semiconductor-metal (MSM) photodetector. Whereas the VCSEL-PIN photodiode (PD) chips are used for high-speed bidirectional data transmission over 62.5 and 50 μm core diameter MMFs, MSM TRx chips are employed for 100 or 200 μm large-area fibers. Such a monolithic transceiver design based on a well-established material system and avoiding the use of external fiber coupling optics is well suited for inexpensive and compact optical interconnects over distances of a few hundred meters. Standard MMF networks can thus be upgraded using high-speed VCSEL-PIN transceiver chips which are capable to handle data rates of up to 10 Gbit/s.

We present the epitaxial growth and processing as well as measurement results of vertical-cavity surface-emitting lasers (VCSELs) monolithically integrated with photodiodes for high-speed bidirectional data transmission over multimode fibers. For this purpose, metal–semiconductor–metal and PIN photodiodes have been investigated. Reflectivity measurements of the as-grown structure reveal the expected emission wavelength of the laser buried underneath the photodiode layers. Digital data transmission up to 6 Gbit/s is demonstrated.

We present polarization-resolved intensity noise investigations of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with an integrated small-area surface grating. We demonstrate that the reduction of the complex polarization and transverse mode behavior via the introduced high dichroism of the grating relief lead to an improved, quite simple noise behavior. The observed noise behavior is in good qualitative agreement with a two-mode noise model.

In this paper, we have developed new design of the injector for quantum cascade structures based on the effective mass superlattice. The transmission spectra of the conventional injector and effective-mass injector have been compared. Advantage of the effective-mass injector is more uniform spectrum of pass-band. This feature allows to improve controllability of the quantum cascade laser, to increase stability of its operation and to decrease sensitivity to noise in applied voltage bias. Also, we have computed geometrical parameters for effective-mass injector which is appropriate to provide inverted population in existing quantum-cascade lasers. Such injector should be characterized by 15–25 numbers of layers with widths of layers up to 2 nm.

Vertical-cavity surface-emitting lasers (VCSELs) are used for oxygen monitoring via tunable diode laser spectroscopy at 760 nm wavelength. For the desired application, novel polarization-stable laser diodes based on AlGaAs were developed. We present measurements of the pressure-broadening coefficients of the electric dipole forbidden oxygen A-band b(exp 1) Sigma+(exp 9) --> X(exp 3) Sigma+(exp 9) transition at 760 nm. The time the pressure-broadening coefficients were determined with a temperature tuned VCSEL. Generally temperature tuning has the disadvantage of frequent mode-hops, but the advantage of a wider tuning range in comparison to current tuning. Because of special techniques of polarization stabilization with a combination of a dielectric surface grating and a surface relief the VCSELs have a mode hop-free tuning range of more than 7 nm and a sidemode suppression of more than 30 dB. This provides a low cost laser diode system with a wide tuning range, which enables the possibility of simultaneous measurement of temperature, pressure and oxygen concentration in air, high pressure measurements and also a higher accuracy of oxygen concentration measurements.

Vertical-cavity surface-emitting lasers (VCSELs) showing continuous-wave operation with emission wavelengths ranging between 717 and 790 nm have been fabricated. The samples are grown by solid source molecular beam epitaxy on GaAs substrates and are entirely based on the AlGaAs material system. A maximum single-mode output power of 2.0 mW is obtained at 754 nm wavelength by using the inverted surface relief technique.

We present inverted surface relief vertical-cavity surface-emitting lasers (VCSELs) showing a side-mode suppression ratio above 30 dB up to maximum optical output powers of 6.3 mW and differential quantum efficiencies exceeding 90%. An evaluation of 160 relief devices on a sample incorporating graded layers yields 157 lasers with single-mode output powers of at least 2.9 mW, proving the high effectiveness of the technique despite the comparatively simple manufacture.

Vertical-cavity surface-emitting lasers (VCSELs) with a monolithically integrated grating in the outcoupling aperture have proven to generate a stable and predictable polarization of the emitted light. This paper provides further insight into the polarization anisotropies of grating-controlled highly multimode VCSELs by direct experimental measurements of crucial vecto- rial characteristics, i.e., modal dichroism and birefringence. This is achieved by a careful analysis of linewidths and peak positions of the dominant and the suppressed polarizations of the emission spectra recorded close to threshold. These experimental results allow us for the first time to compare the observed dichroism and birefringence directly with simulations performed with a fully vectorial, three-dimensional model. Large dichroisms exceeding 30 GHz, even for higher transverse modes, were found to agree very well with the simulations. Such high dichroisms clearly indicate the excellent capability of integrated surface gratings for reliable polarization control, even under extreme operation conditions.