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.
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.
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.
We report for the first time on wafer-fused InGaAs-InP/AlGaAs-GaAs 1550 nm vertical-cavity surface-emitting lasers (VCSELs) incorporating a InAlGaAs/InP MQW active region with re-grown tunnel junction sandwiched between top and bottom undoped AlGaAs/GaAs distributed Bragg reflectors (DBRs) all grown by molecular beam epitaxy. InP-based active region includes seven compressively strained quantum wells (2.8 nm) optimized to provide high differential gain. Devices with this active region demonstrate lasing threshold current < 2.5 mA and output optical power > 2 mW in the temperature range of 10-70°C. The wall-plug efficiency (WPE) value-reaches 20 %. Lasing spectra show single mode CW operation with a longitudinal side mode suppression ratio (SMSR) up to 45 dB at > 2 mW output power. Small signal modulation response measurements show a 3-dB modulation bandwidth of ~ 9 GHz at pump current of 10 mA and a D-factor value of 3 GHz/(mA)1/2. Open-eye diagram at 30 Gb/s of standard NRZ is demonstrated. Achieved CW and modulation performance is quite sufficient for fiber to the home (FTTH) applications where very large volumes of low-cost lasers are required.
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).
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.
High order modulation of 850nm bare Vertical Cavity Surface Emitting Lasers (VCSELs) has been studied. In particular, four-level pulse amplitude modulation (PAM-4) has been investigated in order to reach data transmission rates up to 25 GBaud. A probe station has been designed and built as a part of the bare VCSELs high speed test bed and static characterization bench. The PAM-4 quality evaluation methods have been developed.
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