We report high frequency (20-100 GHz range) optical field intensity oscillations in laterally-coupled-cavity verticalcavity surface-emitting lasers with several different techniques. The oscillation frequency is defined by the photon energy splitting of the coupled states. The resonance effect is stable in an extended current range and can enable modulation frequency resonances at higher frequencies as compared to the conventional relaxation oscillation frequency of the laser. This paves a way towards high-speed data transmission solutions at data rates beyond ~200 Gb/s with the advantage of better laser stability, as the resonance observed can reach high frequencies even at low current densities. A ~75 GHz intensity modulation between optical modes of a coupled-cavity VCSEL array was first reported by the authors in a two-aperture configuration in 2023 applying optical excitation [1]. Studies of 4- and 10-element coupled VCSEL arrays give further insight into the effects observed. New 3D numerical simulations and electrical modulation techniques have been applied to address the specific nature of the photon-photon resonance studies.
KEYWORDS: Vertical cavity surface emitting lasers, Resistance, Patents, Near field optics, Near field, Multimode fibers, Fiber lasers, Data transmission
Applying coherent arrays of muti-aperture lasers was proposed to improve data transmission over multimode fiber. We propose a novel compact a coherent and incoherent multi-aperture VCSEL array design in which multiple single-mode VCSEL apertures are electrically driven in parallel. Such approach allows a high output power as in standard multimode devices but shows significantly reduced spectral width not exceeding 0.2nm as well as high speed performance exceeding 25GHz with current density ~20kA/cm2 and beam divergence of 22O (1/e2). Moreover, we study the application of such devices for IM/DD 100Gbit/s PAM-4 and 50Gbit/s OOK.
VCSEL arrays can play an important role in the increasing the data throughput of VCSEL-based optical interconnects both due to the need to increase the channel density and due to new emerging technologies like optical wireless. In this work we show the progress in the development of high-speed VCSEL arrays suitable for multicore fiber transmission leading to an increase of the total throughput through single fiber to 600 Gbps. We also discuss a novel type of compact VCSEL mini-arrays capable of high-speed modulation and coherent emission at the same time. Photon-photon resonance and coherent effects can help increase the resonant frequency and the bandwidth of the VCSELs and enable devices capable of 100 GHz operation.
Strain-induced birefringence in GaAs-based oxide-confined VCSELs (Vertical-Cavity Surface-Emitting Laser) can split the optical modes into orthogonally polarized components. A polarization switching at very high frequencies can occur between these components, which is of great interest for optical communication systems of the future. In this study, we focus our investigation on the frequency characteristics of the polarization switching between the optical modes, which is caused by polarization self-modulation (PSM) in fiber-coupled systems. Moreover, we analyze the PSM that is originating in different optical modes of the VCSEL and compare multi-mode and single-mode VCSELs.
We report on vertical cavity surface emitting lasers (VCSELs) having a -3dB modulation bandwidth above 30 GHz and a narrow spectrum down to single mode (SM) operation. The 850 nm and 910 nm SM VCSELs in combination with the IN5612 VCSEL driver from Inphi Corporation allowed to reach 106 Gb/s PAM4 with the TDECQ values of only 1.5 dB. For the multimode VCSELs, TDECQ of ~2.6 dB were achieved in combination with the same driver chip. VCSELs with the reduced spectral width allow to cover transmission distance over multimode fiber reaching 1.0-2.5 km at 50 Gbaud. Furthermore, reduction of the aperture size to a certain limit allows to reach ultimate modulation bandwidths at the same current density as applied in the large aperture VCSELs but at lower total currents and thus much lower current-induced overheating. The latter enables a significant improvement in the reliability of the devices and stimulates further research in novel types of VCSEL-based devices.
KEYWORDS: Vertical cavity surface emitting lasers, Polarization, Birefringence, Near field optics, Switching, Near field, Multimode fibers, Data transmission, Receivers, Modulation
We report high-frequency polarization self‐modulation (PSM) in high speed vertical-cavity surface emitting lasers (VCSELs) connected to the stress-induced birefringence in oxide-confined aperture VCSELs. Polarization oscillations up to 45 GHz were captured. We analyze the far and the near field of the device and show how the fiber-coupling conditions induce optical feedback, affect emission properties of the device and influence the polarization switching phenomenon. In conditions where the PSM was suppressed, we demonstrate NRZ high-speed multi-mode fiber data transmission up to 90 Gbit/s.
New applications in sensing, automotive and on-board applications require vertical-cavity surface-emitting lasers (VCSELs) operating at high data rates up to very high ambient temperatures. We study temperature stability of the 850 nm Quantum-Dot (QD) VCSELs and benchmark them to Quantum-Well (QW) VCSELs of similar design.
QD VCSELs enable extension of the temperature stability and demonstrate threshold currents below 1 mA for operation range from 30°C to 200°C. The role of gain to cavity detuning is discussed in details. 25 Gbit/s NRZ multi-mode fiber transmission with QD VCSELs is realized at temperatures up to 180°C. Pulsed operation of QD VCSELs with 8 μm oxide aperture diameter is studied at temperatures from 30°C to 125°C and 1 W peak power is realized on 100 ns pulses at room temperature.
Optical VCSEL-based links operating in on-off keying (OOK) modulation represent a robust energy-efficient solution for short-reach optical interconnects in datacenters. We report on the optical and electronic elements of such link and their integration into the transmitter (TR) and receiver (RX) assemblies. A single channel transceiver link capable of 40-56 Gbit/s OOK transmission over multimode fiber at record energy-efficiency of ~4.5 pJ/bit is demonstrated. VCSEL driver and receiver transimpedance amplifier (TIA) circuits capable of generating 80-100 Gbit/s error-free signals are characterized on a special test-board assembly. Real-time 56 Gbit/s transmission experiments of the complete link are done, resulting in bit-error ratios (BER) below standard Forward Error Correction (FEC) levels without equalization or signal processing.
Novel lasing modes in a vertical-cavity surface-emitting laser (VCSEL)-type structure based on an antiwaveguding cavity are studied. Such a VCSEL cavity has an effective refractive index in the cavity region lower than the average index of the distributed Bragg reflectors (DBRs). Such device in a stripe geometry does not support in–plane waveguiding mode, and all modes with a high Q-factor are exclusively VCSEL-like modes with similar near field profile in the vertical direction. A GaAlAs–based VCSEL structure studied contains a resonant cavity with multiple GaInAs quantum wells as an active region. The VCSEL structure is processed as an edge-emitting laser with cleaved facets and top contact representing a non–alloyed metal grid. Rectangular-shaped ~400x400 µm pieces are cleaved with perpendicular facets. The contact grid region has a total width of ~70 μm. 7 μm–wide metal stripes serve as non–alloyed metal contact and form periodic rectangular openings having a size of 10x40 μm. Surface emission through the windows on top of the chip is measured at temperatures from 90 to 380 K. Three different types of modes are observed. The longest wavelength mode (mode A) is a VCSEL–like mode at ~854 nm emitting normal to the surface with a full width at half maximum (FWHM) of the far field ~10°. Accordingly the lasing wavelength demonstrates a thermal shift of the wavelength of 0.06 nm/K. Mode B is at shorter wavelengths of ~840 nm at room temperature, emitting light at two symmetric lobes at tilt angles ~40° with respect to the normal to the surface in the directions parallel to the stripe. The emission wavelength of this mode shifts at a rate 0.22 nm/K according to the GaAs bandgap shift. The angle of mode B with respect to the normal reduces as the wavelength approaches the vertical cavity etalon wavelength and this mode finally merges with the VCSEL mode. Mode B hops between different lateral modes of the VCSEL forming a dense spectrum due to significant longitudinal cavity length, and the thermal shift of its wavelength is governed by the shift of the gain spectrum. The most interesting observation is Mode C, which shifts at a rate 0.06 nm/K and has a spectral width of ~1 nm. Mode C matches the wavelength of the critical angle for total internal reflection for light impinging from semiconductor chip on semiconductor/air interface and propagates essentially as an in–plane mode. According to modeling data we conclude that the lasing mode represents a coupled state between the TM–polarized surface–trapped optical mode and the VCSEL cavity mode. The resulting mode has an extended near field zone and low propagation losses. The intensity of the mode drastically enhances once is appears at resonance with Mode B. A clear threshold is revealed in the L–I curves of all modes and there is a strong competition of the lasing mechanisms once the gain maximum is scanned over the related wavelength range by temperature change.
The development of advanced OM5 wideband multimode fiber (WBMMF) allowing high modal bandwidth in the spectral range 840-950 nm motivates research in vertical-cavity-surface-emitting-lasers (VCSELs) at wavelengths beyond the previously accepted for short reach communications. Thus, short wavelength division multiplexing (SWDM) solutions can be implemented as a strategy to satisfy the increasing demand of data rate in datacenter environments. As an alternative solution to 850 nm parallel links, four wavelengths with 30 nm separation between 850 nm and 940 nm can be multiplexed on a single OM5-MMF, so the number of fibers deployed is reduced by a factor of four. In this paper high speed transmission is studied for VCSELs in the 850 nm – 950 nm range. The devices had a modulating bandwidth of ~26-28 GHz. 50 Gb/s non-return-to-zero (NRZ) operation is demonstrated at each wavelength without preemphasis and equalization, with bit-error-rate (BER) below 7% forward error correction (FEC) threshold. Furthermore, the use of single-mode VCSELs (SM-VCSELs) as a way to mitigate the effects of chromatic dispersions in order to extend the maximum transmission distance over OM5 is explored. Analysis of loss as a function of wavelength in OM5 fiber is also performed. Significant decrease is observed, from 2.2 dB/km to less than 1.7 dB/km at 910 nm wavelength of the VCSEL.
New applications in industrial, automotive and datacom applications require vertical-cavity surface-emitting lasers (VCSELs) operating at very high ambient temperatures at ultrahigh speed. We discuss issues related to high temperature performance of the VCSELs including temperature response and spectral properties. The influence of the gain-to-cavity wavelength detuning on temperature performance and spectral width of the VCSELs is discussed. Performance of the oxide-confined 850 nm VCSELs with increased temperature stability capable of operating at bit rates up to 25 Gbit/s at heat sink temperature of 150°C and 35Gbit/s at 130°C. Furthermore, opposite to previous studies of VCSELs with large gain-to-cavity detuning, which demonstrated strongly increased spectral width and a strong redistribution of the mode intensities upon current increase. VCSELs demonstrated in this work show good reproducibility of a narrow spectrum in a wide range of currents and temperatures. Such performance strongly improves the transmission distance over multi-mode fiber and can reduce mode partition noise during high speed operation.
In this paper we propose and evaluate performance of the higher order mode filter for 850 nm multi-mode fiber transmission. First the operation principles of the filters are presented and then experimental validation of manufactured optical components is made in an optical transmission system. Excellent operation in the 850 nm transmission experiments up to 54 Gbit/s is shown.
Design of the oxide–confined vertical cavity surface emitting laser (VCSEL) with enhanced engineered lateral leakage of high–order transverse optical modes is studied by three–dimensional optical modeling to evaluate the robustness of the leakage selection approach with respect to thermal effects. Both Joule heat and heat generated by the free carrier absorption of the optical mode in the doped semiconductor layers and their impact on the refractive index profile are considered. We show that for typical regimes of the VCSEL design and operation absorption–induced heat exceeds by several times the Joule heat while the shape of the generated heated domains strongly differ. Modeling shows that well defined spectral separation between the transverse optical modes persists upon increase in injection current. Further, upon increase in current the lateral extension of the fundamental mode decreases and the mode shrinks towards the center of the VCSEL structure thus reducing the lateral leakage and increasing the mode lifetime, whereas similar effect for high–order transverse modes is much weaker. Thus the preferred conditions for the lasing of the fundamental mode persist and even improve upon current increase. At high currents the fundamental mode becomes favorable at all aperture diameters, also for those where the cold cavity approximation predicts preference for the excited mode lasing.
A novel design for high brightness planar technology light-emitting diodes (LEDs) and LED on-wafer arrays on absorbing substrates is proposed. The design integrates features of passive dielectric cavity deposited on top of an oxide– semiconductor distributed Bragg reflector (DBR), the p–n junction with a light emitting region is introduced into the top semiconductor λ/4 DBR period. A multilayer dielectric structure containing a cavity layer and dielectric DBRs is further processed by etching into a micrometer–scale pattern. An oxide–confined aperture is further amended for current and light confinement. We study the impact of the placement of the active region into the maximum or minimum of the optical field intensity and study an impact of the active region positioning on light extraction efficiency. We also study an etching profile composed of symmetric rings in the etched passive cavity over the light emitting area. The bottom semiconductor is an AlGaAs–AlAs multilayer DBR selectively oxidized with the conversion of the AlAs layers into AlOx to increase the stopband width preventing the light from entering the semiconductor substrate. The approach allows to achieve very high light extraction efficiency in a narrow vertical angle keeping the reasonable thermal and current conductivity properties. As an example, a micro-LED structure has been modeled with AlGaAs-AlAs or AlGaAs-AlOx DBRs and an active region based on InGaAlP quantum well(s) emitting in the orange spectral range at ~610 nm. A passive dielectric SiO2 cavity is confined by dielectric Ta2O5/SiO2 and AlGaAs-AlOx DBRs. Cylindrically–symmetric structures with multiple ring patterns are modeled. It is demonstrated that the extraction coefficient of light to the air can be increased from 1.3% up to above 90% in a narrow vertical angle (full width at half maximum (FWHM) below 20°). For very small oxide–confined apertures ~100nm the narrowing of the FWHM for light extraction can be reduced down to 5°. Consequently high efficiency high brightness arrays of micro-LEDs becomes possible. For single emitters the approach is particularly interesting for oscillator strength engineering allowing high speed data transmission and for single photonics applying single quantum dot (QD) emitters and allowing >90% coupling of the emission into single mode fiber. We also note that for longer wavelength (~1300nm) QDs the thickness of the layers and surface patterns significantly increase allowing greatly reduced processing tolerances and applying further simplifications due to the possibility of using high contrast GaAs-AlOx DBRs.
In this paper we present the results of relative intensity noise (RIN) measurements for single- and multi mode 850 nm vertical cavity surface emitting lasers (VCSEL). The method applied for the RIN measurements is based on an electrical spectrum measurement of a biased and unmodulated laser. The conducted measurements show that the RIN values of around 150 dB/Hz can be expected from MM and SM VCSELs.
Modern high speed multimode fibers exhibit very low modal dispersion but suffer from the chromatic dispersion in glass, which is rather high at 850nm. To increase the transmission capacity on multimode fibers tested two approaches are applied and evaluated. On the transmitter side we designed and fabricated novel types of Vertical Cavity Surface Emitting Lasers (VCSELs) with spectrally ultra-narrow lasing emission both through achieving simple transverse mode operation and the negligible chirp. For increasing the transmission efficiency high-order modulation formats capable to increase the transmission capacity per single transmitter are applied. We report on development of very fast optical VCSEL and photodiodes within the European Project ADDAPT and show our recent results on high speed transmission over multi-mode fiber using 850nm VCSELs and GaAs PIN photodiodes applying different modulation formats. We show that the transmission distance and the transmission capacity of a single lane can be improved by applying high speed single mode VCSELs which are modulated with 4-PAM, 8-PAM and DMT modulation.
In this paper, we evaluate the transmission throughput/range limits for multi mode and single mode 850 nm vertical cavity surface emitting lasers. Transmission experiments in the various configurations like fibre length are performed. We utilize the most basic modulation format as amplitude shift keying without any form of digital signal pre and post processing. Operation up to 50 Gbit/s below 7% FEC limit was achieved for both multi and single mode VCSEL. Experiments showed that SM VCSEL outperforms MM VCSEL in both fields: transmission distances and high speed performance operating error free at 25 Gbit/s up to 200 meters and achieving two orders of magnitude lower BER at 50 Gbit/s (3.7*10-5 for SM comparing to 3.1*10 -3 for MM).
Oxide–confined apertures in vertical cavity surface emitting laser (VCSEL) can be engineered such that they promote
leakage of the transverse optical modes from the non– oxidized core region to the selectively oxidized periphery of the
device. The reason of the leakage is that the VCSEL modes in the core can be coupled to tilted modes in the periphery if
the orthogonality between the core mode and the modes at the periphery is broken by the oxidation–induced optical field
redistribution. Three–dimensional modeling of a practical VCSEL design reveals i) significantly stronger leakage losses
for high–order transverse modes than that of the fundamental one as high–order modes have a higher field intensity close
to the oxide layers and ii) narrow peaks in the far–field profile generated by the leaky component of the optical modes.
Experimental 850–nm GaAlAs leaky VCSELs produced in the modeled design demonstrate i) single–mode lasing with
the aperture diameters up to 5μm with side mode suppression ratio >20dB at the current density of 10kA/cm2; and ii)
narrow peaks tilted at 37 degrees with respect to the vertical axis in excellent agreement with the modeling data and
confirming the leaky nature of the modes and the proposed mechanism of mode selection. The results indicate that in–
plane coupling of VCSELs, VCSELs and p–i–n photodiodes, VCSEL and delay lines is possible allowing novel photonic
integrated circuits. We show that the approach enables design of oxide apertures, air–gap apertures, devices created by
impurity–induced intermixing or any combinations of such designs through quantitative evaluation of the leaky
emission.
Discrete Multitone Transmission (DMT) transmission over standard multimode fiber (MMF) using high-speed
single (SM) and multimode (MM) Vertical-Cavity Surface-Emitting Lasers (VCSELs) is studied. Transmission speed in
the range of 72Gbps to 82Gbps over 300m -100m distances of OM4 fiber is realized, respectively, at Bit-Error-Ratio
(BER) <5e-3 and the received optical power of only -5dBm. Such BER condition requires only 7% overhead for the
conversion to error-free operation using single Bose-Chaudhuri-Hocquenghem forward error correction (BCH-FEC)
coding and decoding. SM VCSEL is demonstrated to provide a much higher data transmission capacity over MMF. For
100m MMF transmission SM VCSEL allows 82Gbps as compared to MM VCSEL resulting in only 34Gbps at the same
power (-5dBm). Furthermore, MM VCSEL link at 0dBm is still restricted at 100m distance by 63Gbps while SM
VCSEL can exceed 100Gbps at such power levels. We believe that with further improvement in SM VCSELs and fiber
coupling >100Gbps data transmission over >300m MMF distances at the BER levels matching the industry standards
will become possible.
Existing optical networks are driven by dynamic user and application demands but operate statically at their maximum performance. Thus, optical links do not offer much adaptability and are not very energy-efficient. In this paper a novel approach of implementing performance and power adaptivity from system down to optical device, electrical circuit and transistor level is proposed. Depending on the actual data load, the number of activated link paths and individual device parameters like bandwidth, clock rate, modulation format and gain are adapted to enable lowering the components supply power. This enables flexible energy-efficient optical transmission links which pave the way for massive reductions of CO2 emission and operating costs in data center and high performance computing applications. Within the FP7 research project Adaptive Data and Power Aware Transceivers for Optical Communications (ADDAPT) dynamic high-speed energy-efficient transceiver subsystems are developed for short-range optical interconnects taking up new adaptive technologies and methods. The research of eight partners from industry, research and education spanning seven European countries includes the investigation of several adaptive control types and algorithms, the development of a full transceiver system, the design and fabrication of optical components and integrated circuits as well as the development of high-speed, low loss packaging solutions. This paper describes and discusses the idea of ADDAPT and provides an overview about the latest research results in this field.
KEYWORDS: Vertical cavity surface emitting lasers, Oxides, Near field optics, Near field, Refractive index, 3D modeling, Waveguides, Reflectivity, Resistance, Semiconductors
Oxide–confined vertical cavity surface emitting lasers (VCSEL) are inherently leaky structures, despite the fact that the oxidized periphery region surrounding the all–semiconductor core has a lower refractive index. The reason is that the VCSEL modes in the non–oxidized core region can be coupled to tilted modes in the selectively oxidized periphery as the orthogonality between the core mode and the modes at the periphery is broken by the oxidation–induced optical field redistribution. Engineered VCSEL designs show that the overlap between the VCSEL mode of the core and the tilted mode in the periphery can reach >30% resulting in significant leakage. Three–dimensional modeling confirms that the leakage losses are much stronger for high order transverse modes which have a higher field intensity close to the oxidized region. Single mode lasing in the fundamental mode can thus proceed up to large aperture diameters. A 850–nm GaAlAs leaky VCSEL based on this concept is designed, modeled and fabricated, showing single–mode lasing with aperture diameters up to 5 μm. Side mode suppression ratio >20dB is realized at the current density of 10kA/cm2 in devices with the series resistance of 90 Ω.
We manufacture and compare parallel optical transceiver and receiver assemblies on test boards for parallel data transmission over multimode fiber using single mode (SM) and multimode (MM) vertical-cavity surface-emitting laser (VCSEL) arrays. VCSELs, GaAs PIN photodetector arrays, commercially-available 12 channel VCSEL driver arrays and 12 channel limiting amplifier arrays were assembled into multi-channel transceiver and receiver assemblies on testboards designed to operate up to 16 channels and coupled to multimode fiber ribbon through industrial connectors. MM VCSEL arrays easily allow 25 Gb/s error-free data transmission over 100m of OM4 fiber with only a minor penalty in the sensitivity (0.5 dB). As opposite increasing the distance to 150-200 m causes a strong increase in the noise level making the error free transmission at 200 m impossible. Using of single mode SM VCSEL arrays allows error-free 25 Gbit/s NRZ PRBS 215-1 transmission over 1 km distances over OM4 fiber and above 600 m over OM3 fiber. In a different set of experiments PAM4 transmission up to 50 Gbit/s using SM VCSEL arrays is studied.
We address demands and challenges for GaAs–based Vertical–Cavity Surface–Emitting Lasers (VCSEL) in data communication. High speed modulation (~50Gb/s) at a high reliability can be realized with a proper VCSEL design providing a high differential gain. In cases where extreme temperatures are required electrooptic modulation in duo– cavity VCSELs can be applied as the modulation speed and the differential gain are decoupled. Single mode operation of VCSELs is necessary to counteract the chromatic dispersion of glass fibers and extend distances to above 1 km while using standard multimode fibers. Oxide layer engineering or using of photonic crystals can be applied. Parallel error–free 25Gb/s transmission over OM3 and OM4 multimode fiber (~0.5 and 1 km, respectively) is realized in large aperture oxide–engineered VCSEL arrays. Passive cavity VCSELs with gain medium placed in the bottom DBR and the upper part made of dielectric materials a complete temperature insensitivity of the emission wavelength can be realized. Engineering of the oxide aperture region enables near field vertical cavity lasers. Such devices can operate in a high– order transverse mode with an effective mode angle beyond the angle of the total internal reflection at the semiconductor–air interface. Near filed coupling to optical fibers and waveguides becomes possible in this case.
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