Born of necessity of application, the Vertical Cavity Surface Emitting Laser (VCSEL) is now found in nearly all optical networking systems based on standards such as the IEEE 802.3z and ANSI X3.t11. Reliability continues to be the hallmark of the technology, and the volume manufacturing aspects are now realized. While VCSEls satisfying optical networking standards continue to provide the highest volume applications, the advantages of the technology are beginning to enable novel optical equipment. This paper explores development of VCSELs at wavelengths from 650 to 850nm, and the commercial applications of these devices in both the data communications and optical sensing arenas. VCSELs operating at longer wavelengths are also being developed, but are not at a stage of commercialization to be discussed in this forum.

Accumulation of all advantageous properties VCSELs are famous for, like low power consumption, circular low divergent beam profile, high modulation bandwidth, and scalability of monolithic arrangements, results in two-dimensional (2D) VCSEL arrays that appear as key components to reach highest aggregate bandwidths of tomorrow's parallel optical transceivers. We report on 2D VCSEL arrays, substrate emitting although operating at 850 nm and prepared for flip-chip bonding, that are well suited for the customer's needs in terms of speed, power consumption, reliability and compact integration. Based on advanced technology, our arrays target the requirements of transceivers in the OC-192 VSR and 10 Gigabit Ethernet arena. In this paper we present the basic technology, static and dynamic device characteristics as well as reliability data for a 4x12 850 nm bottom-emitting VCSEL array. A13

As the bit rates of high-speed data transmission increase, the dynamic properties of the VCSEL are becoming more important. High photon density is of great importance for high modulation efficiency and low noise. We will present a schematic of the Zarlink 850nm 10Gb/s selectively oxidized VCSEL design and results from small and large signal modulation. The modulation characteristics were improved by utilizing InGaAs quantum wells, which have better gain characteristics than GaAs wells. 10Gb/s transmission over 300m of OFS-Fitel LaserWave^tm 300 Fiber for a 12micrometers aperture multimode VCSEL at 80 degree(s)C will be demonstrated. Bit error rate (BER) measurement with the corresponding power penalty will also be presented.

This paper reviews the recent advances made in monolithic GaAs based, directly modulated, 1.3micrometers VCSEL array technology. Such VCSEL arrays are poised to begin occupying a large telecommunications application space. We present data demonstrating 1.3 micrometers VCSELs having ~ 1mW optical power across a wide temperature range of 10 to 90 degree(s)C while operating with low voltages of less than 2.5V. The data includes performance on typical 8 and 12 element arrays at the die level as well in the module. We also present very encouraging preliminary reliability results.

An Emcore D180 metalorganic chemical vapor deposition (MOCVD) system was used to develop 1.3 micrometers InGaAsN/GaAs vertical cavity surface emitting lasers (VCSELs). The 1.3 micrometers VCSEL consists of double GaInNAs/GaAs quantum well active region and 1(lambda) cavity with DBRs consisting of alternating layers of GaAs/AlGaAs to obtain a large difference in index of refraction. Wavelengths ranging from 1.275 to 1.31 micrometers have been investigated. The room temperature peak power measured to date is approximately 1 mW, with a slope efficiency of 0.13 mW/mA and a threshold current of 1.5 mA.

We demonstrated highly strained GaInAs/GaAs QW VCSELs emitting at 1.16 micrometers . The fabricated device shows the record low threshold current density and high efficiency in 1.1-1.2 micrometers wavelength range. The VCSEL structure was monolithically grown on a (100) n-type GaAs substrate by a low-pressure metalorganic vapor phase epitaxy (MOVPE). The active region consists of triple 8 nm thick Ga_0.64In_0.36As TQWs separated by 25 nm GaAs barrier layers. The compressive strain of QWs is 2.3%. The threshold current is 3 mA for a 10micrometers ~10micrometers oxide device, corresponding to a threshold current density of 3 kA/cm². We achieved the maximum output power of over 2 mW and a slope efficiency of 0.3 W/A at 25 degree(s)C, which are the record data for 1.2 micrometers band GaInAs VCSELs. The maximum CW operating temperature is 85 degree(s)C. The threshold current is almost constant in the temperature range of 20-70 degree(s)C which results from appropriate wavelength matching between gain peak and lasing mode. The temperature dependence of the lasing wavelength is 0.07 nm/K. We present the details of temperature characteristics of the fabricated VCSEL and discuss a possibility of uncooled GaInAs/GaAs VCSELs for high speed LANs.

Secondary pulsations are an example of diffusive turn-off transients that can limit the performance of VCSELs in optical communication systems. Secondary pulsations are firstly analysed by using a model where a modal expansion of the electric field is performed. The maximum power of the secondary pulsations and the time at which they appear fluctuate when the spontaneous emission noise is present. A linear relation between the two previous quantities for each individual turn-off event is found. In the single-mode regime, the averaged maximum power during turn-off transients increases when increasing the injection current. However, in the multi-mode regime, the strength of secondary pulsations decreases when increasing the current. Secondary pulsations are also analysed by using a spatio-temporal description of the VCSEL dynamics, where the modal profiles are determined from the distribution of injected carriers and the thermal lens. This model also incorporates polarization effects and a frequency-dependent susceptibility. In this model, the carrier-induced refractive index changes increase the strength of secondary pulsations as compared to that obtained with the modal expansion. It is also shown that the use of ring-shaped electrical contact enhances the strength of secondary pulsations, while it decreases when multi-transverse mode operation is present.

It has been proposed that the temperature of the active layer in VCSELs could be inferred from the wavelength shift of the laser line. However, in VCSELs the laser emission wavelength is primarily selected by the cavity resonance, instead of the material gain peak. Hence the shift of the laser emission only provides an estimate of the temperature averaged over the whole cavity volume. We present a non-invasive microprobe technique for the temperature mapping of operating VCSELs, based on the analysis of the spontaneous electroluminescence emission transmitted through the DBR mirrors. While the sample is temperature stabilized and held onto a xy piezo stage, it is scanned across with an optical microscope (achieving ~2 um spatial resolution). The signal is spectrally resolved and analyzed by a CCD. By comparing the spectra taken under cw and pulsed current injection, the temperature contribution to the emission lineshape can be extracted straightforwardly. We demonstrate the capability of the proposed technique by mapping the temperature rise of a broad area proton implanted oxide VCSEL. Our results clearly demonstrate that the temperature rise is not uniform across the device cross-section, in contrast to the uniform temperature distribution measured by the laser wavelength shift method.

We report on the operational characteristics of hybrid proton implanted/selectively oxidized vertical-cavity surface-emitting lasers (VCSELs) in the 980 nm region. We investigate the output and spectral characteristics of different sized implant/oxide aperture VCSELs. Important operational variables such as current/voltage threshold, output power, efficiency and transverse operating wavelengths are extracted from experiments. A clear correlation is deduced between the sizes of implant and oxide apertures and the output power and lasing wavelength. Moreover, precise choice of implant/oxide dimensions yields a single transverse mode and high output power VCSEL. Finally, we simulate these different sized VCSELs with a vector optical solver (Illinois-VCSEL optical solver) based on a numerical mode-matching method developed by Seurin and Chuang. We successfully compare the experimental spectra with theoretical results and find very good agreement.

The Near-Field Scanning Optical Microscope (NSOM) is a tool that combines the spatial resolution of scanning probe microscopy with optical characterization techniques. Using this technique, we have generated high-resolution spatial intensity maps of the output from vertical-cavity surface-emitting lasers (VCSELs) in the near-field region of the facet as a function of operating current. The VCSELs studied were proton implanted, gain guided devices designed to operate at ~850nm. Optical signals that have been spatially imaged include total intensity, the spectrally resolved intensity of individual transverse modes, and the derivative of intensity with respect of operating current. Deviations from expected mode patterns in the devices have been qualitatively linked to unacceptable levels of noise in operating lasers. These deviations can be observed at operating currents below the actual onset of unacceptable noise. We have also found that derivative spectroscopy can be used to sensitively detect the cutoff points of transverse modes. Using the spatial intensity profile at the cutoff point of an allowed mode, a first approximation to the index of refraction profile can be made that is in good agreement with prior work. A series of index profile estimates from the cutoff points of a VCSEL can provide information on the evolution of the index profile and the thermal lens as the power is ramped up.

The purpose of this paper is to present a detailed characterization of a dual-wavelength VCSEL - the BiVCSEL. This device consists of two active optical cavities, which share a coupling mirror and can be independently electrically pumped. We present the output powers for the two emitted wavelengths (short - (lambda) _S, long - (lambda) _L versus the currents in the two cavities (I_top, I_bot). These ((lambda) _S, (lambda) _L)-(I_top, I_bot) maps identify the different regimes of operation of the BiVCSEL: emission at only one wavelength (either short or long) and dual-wavelength emission, each domain being delimitated by the threshold curves. These curves are passing through a single point, which identify the dual-emission threshold (I^th_top, I^th_bot). The apparition of a parasitic lasing mode due to the oxide apertures will be also presented as well as the competition between this mode and the designed lasing modes of the structure.

Vertical cavity semiconductor optical amplifiers (VCSOAs) are attractive devices for use in coherent optical amplification, especially where 2-D amplifier arrays are required. However, the coherence preservation quality of a VCSOA depends strongly on the bias condition, resonant wavelength mismatch, and the optical input power level. We characterize the coherence degree of a VCSOA as a function of these parameters by measuring interference fringe visibility with an interferometer. The dominant factors influencing the contrast of the fringes are the ratio of coherent, stimulated emission photons to amplified spontaneous emission (ASE) photons, and the spectral distortion of the amplified signal. Mostly, the overall gain and the saturation characteristic of the amplifier determine the ratio of stimulated emission to ASE. The spectral distortion of the signal is due to the narrow gain window of the VCSOA, but the effect significantly degrades the visibility only for relatively large wavelength mismatch from the gain peak. Analytic expressions may be used to identify the optimal bias current and optical input power to maximize the amplifier gain and visibility of the interference.

Demonstration of a concept for a single-frequency, grating-outcoupled surface-emitting (GSE) semiconductor laser is reported at wavelengths of 1310 and 1550 nm with output powers exceeding 2 mW. First-order distributed Bragg reflector (DBR) gratings are used for feedback and a second-order grating provides surface emission. The device has a 6 x 10 micrometers outcoupling aperture that approximately matches the spot size of a single mode fiber. This architecture allows probe-testing at the wafer level similar to vertical cavity surface emitting lasers (VCSELs). These initial GSE lasers have demonstrated pulsed threshold currents of 42 mA at 1288 nm and 49 mA at 1552 nm with ~ 40 dB side-mode suppression ratios (SMSR).

Aiming at various blooming demands of optical communication, AXT provides high performance and highly reliable VCSELs, including implant, oxide, single mode and array devices. The push toward higher modulation rates, from 1 to 10 Gbps, has led to advances in VCSEL design and production techniques. The reliability of VCSEL devices is of prime importance for all new design and is an integral part of the development process. In this paper, we will overview the development activities of high speed VCSELs at AXT and the corresponding device characterization and reliability results.

Reliability of the oxide confined VCSEL used in the Gore nLIGHTEN^TM parallel optic interconnect is discussed. The Gore reliability program for oxide confined devices has been active for approximately five years. The excellent long term reliability results have been obtained through an approach centered upon fundamental reliability research. The details of the device lifetime measurements and projections are presented along with some specific examples of projects from our reliability research experiences.

In the past two years, Agilent Technologies has had a unique opportunity to study the reliability of VCSELs from most major manufacturers. We report on our methodology for qualifying prospective VCSEL suppliers, and briefly discuss our findings. The expected use environment for VCSELs is covered, along with VCSEL reliability limitations with existing technology. Differences between maverick and wearout failures are discussed, and examples of each are shown; VCSEL failure analysis is also briefly touched on. Finally, recent challenges in using oxide VCSELs in non-hermetic packaging, and 10 Gb/s operation, are briefly covered.

In this paper, we describe 850 nm oxide VCSEL array technologies being developed at Emcore Optical Devices. We demonstrate the excellent performance, uniformity and reliability of oxide VCSEL arrays operating at 2.5Gb/s per channel which are entering into high volume production. Due to the ever-increasing demand for bandwidth by high-bit-rate data communications links, VCSELs operating at even higher bandwidths are needed. We discuss the development of oxide VCSELs capable of transmitting 10 Gb/s for application in the 10 gigabit Ethernet and other emerging high-aggregate bandwidth standards.

Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited for applications requiring high-density 2-dimensional arrays of lasers, such as massively parallel interconnects or imaging applications. We have successfully fabricated 64x64 arrays containing alternating rows of selectively-oxidized 850 nm VCSELs and resonant-cavity photodetectors (RCPDs) on semi-insulating GaAs. In order to reduce the input/output pin count, we employed a matrix addressable architecture, where all the VCSELs (or RCPDs) in each row are connected by a common metal trace at the base of their mesas. The columns are connected by metal traces that bridge from mesa top to mesa top, connecting every other row (i.e., only VCSELs or only RCPDs). The pitch of devices in the array is 55 microns, and total resistance contributed by the long (up to 3.5 mm) row and column traces is below 50 ohms. The epitaxial design, fabrication and performance of these arrays is discussed.

A coherently coupled vertical cavity surface emitting laser (VCSEL) array, based on injection locking, was realized with master and slave lasers being integrated on the same monolithic chip. The radiation of the emitters is collimated by a micro lens array. The mechanical stability is achieved by a proprietary optical module, consisting of the laser array, micro lens array, heat sink with Peltier element and a pin grid array for individual electric connections. For frequency and phase adjustment the individual operating currents have to be tuned with an accuracy of better than 0.1 %. This could be realized with a laser-trimmed array of micro resistors on a chip comparable in size to the laser chip. With 21 slave lasers nearly 90 % of the theoretically possible increase of the peak power density could be achieved. The coherent superposition is stable for hundreds of hours of operation. The small size of the micro resistors allows their integration on the laser chip thus reducing the number of connectors to only two - independent of the number of lasers.

Cold-cavity modal behavior of a 2-D (4x4 square lattice geometry) antiguided vertical cavity surface emitting laser (VCSEL) array is studied by the means of an effective-index model and fiber-mode approximation. The calculations show that the 2-D array can operate under a resonant condition, provided that a resonance in both of the 1-D directions is satisfied. Although out-of-phase and adjacent modes will compete with the in-phase mode around its resonant position, our simulation shows that, with the introduction of inter-element loss, the in-phase mode can be favored to lase for a wide range of inter-element width, s, around its resonant position. The effective-index model is shown to be in qualitative agreement with a more comprehensive (exact) 3-D beam-propagation-based simulation, which takes into account the actual layered structure. The 2-D antiguides are constructed from shifting the cavity resonance between the element and inter-element regions and fabricated by chemically selective etching and two-step MOCVD growth. While both diffraction-limited resonant in-phase and out-of-phase modes are observed from top-emitting arrays, a 2-D bottom-emitting structure is adopted to improve heat removal. Preliminary results of 40 mW pulsed and 10 mW CW powers have been obtained from the junction up and down arrays respectively.

A novel, fast simulation tool for transient response is developed to study jitter and noise caused by lateral cavity interactions in VCSEL arrays. The cavity mode profiles, obtained from a paraxial eigenmode analysis, are used to derive fast 1-D rate equations that implement gain confinement, edge clipping, wide angle scattering and diffraction (self-interference) losses. These equations are augmented by lateral coupling terms describing the interactions among nearest neighbor cavities. Slow time scale coupling describes interactions of phase-shifted cavities via mutually induced electric polarization, cross-hole burning and cross-cavity gain due to optical fringe-field interactions. The tool is used to study cavity cross-talk, lateral bit pattern error effects, and the possibility of excitation of long range modulations over the array. Conclusions relating VCSEL packing density to BER, bit suppression by neighboring cavities, and array phase locking are given.

At Avalon Photonics Ltd. high-performance multimode VCSELs and VCSEL arrays are developed and fabricated for applications in low-cost fiber-optic communication links. We report on state-of-the-art oxide-confined 850 nm VCSELs for current-generation parallel optical link modules with data-rates up to 3.125 Gbit/s per channel. The high performance and high reliability of these devices is reviewed. Moreover, 10 Gbit/s VCSELs are developed for the next-generation 10-Gigabit Ethernet standard (10-GbE). Transmission over 600 m high-bandwidth multimode fiber at 10 Gbit/s is demonstrated. Mainly due to their low noise level and high linearity, these high-performance devices are also well suited for transparent fiber-optic links using subcarrier multiplexed modulation schemes in the low GHz range. Spurious-free dynamic ranges larger than 100 dBHz^2/3 were measured, which is sufficient for important applications like cable television distribution and remote antenna addressing in mobile phone systems.

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Vertical cavity surface emitting lasers (VCSELs) are prepared by standard epitaxial crystal growth techniques on GaAs substrates. The VCSELs include AlGaAs distributed Bragg reflectors (DBRs) and selectively oxidized AlAs or AlGaAs current confinement/optical wave-guiding layers. An additional one-lambda-thick selectively oxidized AlAs layer is placed beneath the VCSEL and used as a sacrificial layer. The entire VCSEL disk, geometrically defined by first etching a mesa down into the GaAs substrate, is separated from the substrate by selectively removing the sacrificial Al-oxide layer by wet chemical etching. The lift-off VCSEL disks designed for emission at 980 nanometers have a typical diameter of 10 to 50 micrometers and a typical thickness of 7 to 8 micrometers. In this paper I present the design, fabrication, and potential applications of lift-off VCSEL disks.

To control oxidation process precisely is important to fabricate small-aperture oxide-VCSELs with high reproducibility and uniformity. A real-time observation of the oxidation through the CCD camera was previously reported. However, it is difficult to observe the oxidation rate of a production-scale VCSEL wafer in a chamber by IR microscope. To measure simply and to control precisely the depth of AlAs-oxidation, the oxidation was in-situ monitored via optical probing technique for AlAs lateral oxidation (OPTALO). The substrates of the 780-nm oxide VCSELs were prepared with dry-etched 20-micron stripes filled over 7mm square area. The trenches of the stripes grooved periodically were 10 micron wide and 5 micron deep, which enables AlAs layer to be oxidized laterally. An optical fiber was placed over the substrate to illuminate using a halogen lamp and to collect the reflection to a spectrometer. Due to the spectral difference between the oxidized and non-oxidized area, the reflectivity linearly changes during the oxidation according with the ratio of the oxidation and non-oxidation area. The side-lobes of the spectra were averaged as the OPTALO signal, then the transients of the signals showed good agreement with transients of the depth of AlAs lateral oxidation.

We compare various approaches aiming at large-area high-power single-mode oxidized VCSELs. Stable and reproducible single-mode emission with SMSR (side-mode suppression ratio) greater than 30dB and output powers well above 5mW are reported for the long monolithic cavity and self-aligned shallow surface etching approaches, both of which are suitable for commercial production. Additionally, Photonic Crystal Surface-Emitting Lasers (PCSELs) are introduced, which enable advanced mode control by novel transverse optical guiding techniques.

We report on optical components for parallel transmit and receive module, operating at 850nm, designed for short haul optical multimode fiber networks. The component is realized by flip-chip bonding of arrayed optoelectronic devices, i.e., VCSEL and PIN detector array, onto ultra-thin silicon- on-sapphire (UTSi) substrate, which is optically transparent and electrically insulated. Flip-chipped assemblies provide several advantages over conventional wire bond techniques, such as extremely low interconnection parasitics that enable high data rates at low power. Using UTSi technology further improves performance by minimizing crosstalk through its insulating substrate while providing the means for a reliable, low cost optical assembly directly onto the substrate. In addition, applying UTSi technology to optical modules allows a higher degree of functional integration within the module. The insulating substrate provides excellent isolation between mixed signal circuitry, enabling the integration of high performance transmitters, receivers and other sensitive analog circuits with digital circuitry on the same substrate. Furthermore, the integration of VCSEL and photodetector array with UTSi circuits for parallel optical interconnects yields several packaging advantages, such as parallelism, scalability, compactness and simplicity.

Ultrafast modulation of semiconductor lasers at a rate higher than the relaxation-oscillation limited frequency is important for millimeter wave photonics and many other applications. Edge-emitters of compound cavities and other multi-section devices have been employed for such generation in the past. We demonstrate in this paper two methods of modulating VCSELs in the frequency range between 20GHz and 130GHz through a detailed numerical simulation. The first method employs two coupled VCSELs and high frequency oscillation is provided by inter-VCSEL coupling, while the second method utilizes multi-transverse mode beating in a large VCSEL. We show that the mode beating is greatly enhanced by collecting laser output from part of the output facet, providing a relatively easy laser modulation at a frequency larger than 100GHz.

We present an experimental and rate-equation based theoretical study of the current-driven polarization modulation properties of VCSELs. In such lasers a high-contrast polarization flip is often observed at a particular value of the pump current. When modulating the current around the polarization switching value, we measure the critical modulation amplitude necessary to force synchronized back-and-forward polarization flips, as a function of the modulation frequency. This yields the polarization modulation frequency response. For a proton-implanted VCSEL the shape of the measured response curve is characterized by time constants that are very long compared with the usual time scales of laser dynamics (such as photon and carrier lifetimes), and compatible with the measured thermal relaxation time. Indeed, both the polarization modulation and the thermal frequency response curves show a cut-off frequency of about 90kHz, independent of the particular value of the switching current. In the frequency response curve of an air-post VCSEL one clearly sees remnants of the thermal influence on the switching. However, one cannot say that a thermal cut-off inhibits polarization switching above a certain modulation frequency. Notwithstanding the difference in impact of thermal effects depending on the type of device under study, our results indicate that it is necessary to incorporate a temperature-dependent variable in realistic models describing the dynamical polarization properties of VCSELs.

A comprehensive beam propagation method (BPM) for the modeling of oxide-confined visible emitting (665nm) vertical-cavity surface-emitting lasers (VCSELs) aimed at polymer optical fibre (POF) communications is presented. In this model all the major physical processes, including the current density distribution, the self-heating effect of the devices, the carrier lateral diffusion in quantum wells, and the optical field modes are considered self-consistently. Using the model, the current flow, carrier diffusion in the active quantum wells, and temperature distribution are calculated. The model indicates severe current crowding around the edge of the oxide aperture within the VCSEL. Using a simple function to describe the variation of optical gain with temperature and wavelength, the threshold properties, transverse modes and the optical output current-light characteristics are calculated. The simulation results are compare favorably with measurements.

The optically pumped semiconductor thin-disk laser with external-cavity (OPS-TDL) is a new type of semiconductor laser structure with the capability of achieving high output power while retaining good beam quality. We demonstrate the first AlGaInP-based red light emitting OPS-TDL structure. The device has been pumped optically with an argon-laser at 514~nm. The device has an epitaxial backside mirror and a multiple quantum well active region, consisting of strained InGaP quantum wells arranged in several groups as a periodic gain structure. A peak single-mode output power of more than 200mW at 660nm has been obtained in pulsed operation. Various designs for the active layer have been investigated.

We have performed an extensive experimental study of the high-speed digital modulation characteristics of BCB-planarized oxide confined 850 nm VCSELs. In particular, we have compared the performance of single- and multimode VCSELs intended for high capacity free space optical interconnects. The digital modulation characteristics were evaluated by recording eye diagrams from 2 to 12 Gbit/s and measuring bit-error-rates (BER) at 10 Gbit/s. The single-mode VCSELs produce open and symmetric eyes at all bit rates and this behavior is maintained under large variations in bias current and modulation depth. For the multimode VCSELs, symmetric eyes can only be achieved under certain bias and modulation conditions. Both VCSELs allow for error free transmission at 10 Gbit/s. The receiver sensitivity at a BER of 1^-9 under optimum bias and modulation conditions was -12.9 and -13.4 dBm for the single-mode and multimode VCSELs, respectively. The single-mode VCSELs produce near Gaussian beams ideal for free space optical interconnects where well defined and stable beams are needed for high efficiency and low cross-talk. The multimode VCSELs, on the other hand, have beam characteristics that are unpredictable and dependent on bias current, and are therefore less favorable for free space optical interconnects.

It is well known that vertical-cavity surface-emitting lasers (VCSELs) can abruptly switch between two orthogonal linear polarization states if the current is changed. The impact of externally induced in-plane anisotropic strain on this switching was experimentally demonstrated in proton-implanted devices. In this contribution we present a further and thorough experimental investigation of the polarization behavior of different types of VCSELs (proton-implanted, air-post and oxide-confined), under varying strain conditions. We first measure the influence of the strain on the orientation of the axes of the linear polarization states. These axes can be rotated from the crystallographic direction [110] over [100] to [110]. At the same time, we monitor the exact birefringence. From the combination of these two measurements the amount of residual strain in these devices is deduced. Applying strain not only changes the frequency splitting between the two modes (due to birefringence) and their orientation, but also lifts the degeneracy in the gain of the polarization modes. We therefore also measure the gain difference (dichroism) as a function of the applied strain, via the mode suppression ratio and the optical spectrum. Due to the effect on both the birefringence and the dichroism, strain also changes the position of the polarization switching point as a function of current and can lead to the observation of double (consecutive) polarization switching. All this experimental evidence will help to build up a better understanding of the physics of polarization switching in VCSELs.