Longer Wavelength GaAs-Based VCSELs for Extended-Reach Optical Interconnects

Data centers of today are increasing in size and are built to accommodate strong traffic demands while providing sustainably by having clients sharing resources under one roof. Their massive scale puts pressure on the server network topology and has incited a need for data transmission links that are energy efficient and capable of operation at high bit rates with reach up to a few kilometers. Optical interconnects (OIs) offer large bandwidth and low attenuation at long distances, and are therefore suitable for this task. The most commonly used OIs, with 850 nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) and multi-mode fiber (MMF), have a 25 Gb/s reach that is limited to a few hundred meters. However, the fiber chromatic dispersion and attenuation that limit the OI reach can be reduced significantly by increasing the wavelength of this very same technology. The upper limit of the GaAs-based VCSEL technology, with strained InGaAs quantum wells (QWs), is about 1100 nm.With further improved OI performance, new hyperscale data center topologies can be realized and explored. This will lead to a larger number of possible solutions in traffic engineering as well as for power management. 1060 nm VCSELs could soon open up for lane rates of 100+ Gb/s over distances up to 2 km and help reach the Tb/s link speed aim of data center OI standards, in which capacity is built up mainly by employing multiple parallel lanes, increasing symbol rate by going from binary to four-level pulse amplitude modulation (PAM-4), and optimizing with electrical mitigation techniques such as digital signal processing.In this work we show that 1060 nm GaAs VCSELs are suitable light sources for long-reach OIs by first demonstrating their overall stable performance and capability of error-free data transmission up to 50 Gb/s back-to-back and 25 Gb/s over 1 km of MMF. With PAM-4, we show 100 Gb/s error-free capability over 100 m of MMF, suitable for wavelength division multiplexed OIs that can transmit data at several wavelengths from 850 to 1060 nm over the same fiber channel. We also assemble single-mode 1060 nm VCSEL and single-mode fiber links and demonstrate 50 Gb/s error-free transmission over 1 km using pre-emphasis and 40 Gb/s over 2 km without the use of any electrical mitigation techniques. These results stem from careful VCSEL design, including strained InGaAs QWs with GaAsP barriers, doped AlGaAs distributed Bragg reflectors, a short optical cavity and multiple oxide layers. In addition, we show that the fabrication of such a device poses no increase in complexity and can be realized using standard processing techniques

22 Gb/s error-free data transmission beyond 1 km of multi-mode fiber using 850 nm VCSELs

The first error-free data transmission beyond 1 km of multi-mode fiber at bit-rates exceeding 20 Gb/s is demonstrated using a high modulation bandwidth, quasi-single mode (SMSR similar to 20 dB) 850 nm VCSEL. A VCSEL with small similar to 3 mu m aperture shows quasi-single mode operation with a narrow spectral width. The top mirror reflectivity of the VCSEL is optimized for high speed and high output power by shallow etching. A combination of narrow spectral width and high optical power reduces the effects of fiber dispersion and fiber and connector losses and enables such a long transmission distance at high bit-rates

1060 nm VCSELs for long-reach optical interconnects

Reach extension of high capacity optical interconnects based on vertical-cavity surface-emitting lasers (VCSELs) and multimode fibers (MMFs), as needed for large-scale data centers, would benefit from high-speed GaAs-based VCSELs at 1060 nm. At this wavelength, the chromatic dispersion and attenuation of the optical fiber are much reduced in comparison with 850 nm. We present single and multimode 1060 nm VCSELs based on designs derived partly from our high-speed 850 nm VCSEL designs. The single-mode VCSEL, with a modulation bandwidth exceeding 22 GHz, supports back-to-back data rates up to 50 Gbps at 25 \ub0C and 40 Gbps at 85 \ub0C under binary NRZ (OOK) modulation. Using mode-selective launch, we demonstrate error-free 25 Gbps transmission over 1000 m of 1060 nm optimized MMF. Higher data rates and/or longer distances will be possible with equalization, forward-error-correction, and/or multilevel modulation

High Speed Optical Links Using CAP Modulation and Novel Equalisation Techniques

High speed optical links suffer from inter-symbol-interference (ISI) due to their limited bandwidth. Equalisation is typically used to mitigate ISI and therefore improve the link capacity. This dissertation explores novel equalisation techniques for carrierless amplitude and phase (CAP) modulation based optical communication systems including OM4 based and plastic optical fibre (POF) based links. An 850 nm VCSEL based OM4 link using CAP-16 scheme is studied. For the first time, the CAP equaliser, is proposed to mitigate both crosstalk channel interference (CCI) and ISI in the link at the receiver side. Performance comparisons are studied between the CAP-16 scheme using CAP equaliser and a conventional equaliser, pulse amplitude modulation (PAM-4) scheme, and discrete multitone (DMT) scheme. CAP based data transmission of 112 Gb/s is achieved over 150 m OM4 fibre with this novel equaliser, while the conventional equaliser can only support over 1 m OM4 fibre and fails to recover the signals at the same data rate. In addition, this novel equaliser provides a 1.2 dB and 1.7 dB improvement in receiver sensitivity over PAM-4 and DMT schemes, respectively, at 112 Gb/s over 100 m OM4 fibre. A novel pre-CAP-equaliser solving CCI at the transmitter side is also proposed. Data transmission of 56 Gb/s over 100 m OM4 fibre is reported experimentally with an improvement of 0.7 dB in receiver sensitivity compared to using the CAP equaliser at the receiver side. A simulation study shows a 2 dB improvement in receiver sensitivity at 112 Gb/s over 100 m OM4 fibre. Furthermore, an artificial neural network (ANN) equaliser in conjunction with the CAP equaliser structure is explored in a VCSEL based OM4 fibre link in order to further mitigate the nonlinear impairments. For 112 Gb/s data transmission over 100 m OM4 fibre, a 2.4 dB improvement of receiver sensitivity is achieved compared to the CAP equaliser. In addition to the electrical equalisers, a monolithically integrated silicon optical equaliser consisting of three taps is used for 50 Gb/s data transmission. After 10 km standard single mode fibre (SSMF), error free eye diagrams at the receiver are demonstrated. A μLED based POF link based on an APD receiver is also investigated with the CAP equaliser at the receiver side. Data transmission rates of 4 Gb/s over 25 m and 5 Gb/s over 10 m POF links are demonstrated with this equaliser while the conventional equaliser can only support 4 Gb/s over 10 m and fails to recover the signals for 5 Gb/s data transmission

Comparison of Intersymbol Interference Power Penalties for OOK and 4-PAM in Short-Range Optical Links

We present results of experimental and theoretical investigations of intersymbol interference in 4-PAM transmission in short-range optical communications links based on the power penalty. A test link comprised of a directly modulated 850 nm VCSEL with up to 200 m of multimode fiber and direct detection was used. The link bandwidth was below 10 GHz and the maximum achieved data rate with 4-PAM was 44 Gbps over 100 m of fiber. In the same case and at similar sensitivity, only 32 Gbps could be achieved with OOK. If typical forward error correction could be applied, the sensitivity of the 4-PAM system was improved by up to 4 dB, reaching -10 dBm at 25 Gbps

Quasi-Single Mode VCSELs for Longer-Reach Optical Interconnects

The vertical-cavity surface-emitting laser (VCSEL) is the standard light source in optical interconnects connecting racks in supercomputers and datacenters hosting services such as online data sharing, storage and processing. VCSELs have numerous advantages such as low power consumption, fast direct modulation at low currents, circular output beam for efficient fiber coupling and low-cost fabrication. However, today's commercial optical interconnects, operating at around 10 Gbit/s over up to 300 m of multimode fiber, have insufficient speed and reach for future datacenters. The goal of this work has been to extend the maximum reach of GaAs-based 850 nm VCSEL-based optical interconnects. Recently developed high-speed VCSELs featuring strained InGaAs quantum wells and multiple oxide layers are highly transverse multimode with large root-mean-square (RMS) spectral widths around 1 nm. This leads to problems with chromatic and modal fiber dispersion, limiting the maximum reach to around 100 m at 25 Gbit/s. By using quasi-single mode VCSELs with a side-mode suppression ratio of ~15-20 dB and RMS spectral widths around 0.3 nm, the reduced dispersive effects enable extended reach at high data rates. Two different techniques to reduce the spectral width are investigated. By using a small oxide aperture, the number of modes guided by the VCSEL waveguide is significantly reduced. A 3 um oxide aperture VCSEL was used to transmit 22 Gbit/s over 1100 m of OM4 fiber. However, small aperture devices have high resistance and relatively low output power. The spectral width of larger aperture VCSELs can be reduced by etching a shallow surface relief that induces a mode selective loss to suppress higher order modes. In effect, the etched feature acts as an integrated mode filter. A 6 um oxide aperture surface relief VCSEL enabled transmission at 20 Gbit/s over 2000 m of fiber, setting a new bit-rate-distance product record for directly modulated 850 nm VCSEL links. Benefits and drawbacks of both methods are discussed and compared

1060 nm GaAs VCSELs for Extended Reach Optical Interconnects in Data Centers

The data centers of today are increasing in size and are built to accommodate strong data traffic demands while providing sustainably by having clients sharing resources under one roof. Their massive scale puts pressure on the server network topology and has incited a need for data transmission links that are energy efficient and capable to operate at high bit rates with reach up to a few kilometers. Optical interconnects (OIs) offer large bandwidth and low attenuation at long distances, and are therefore suitable for this task. The most commonly used OIs, with 850 nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) and multi-mode fiber (MMF), have a 25 Gb/s reach that is limited to a few hundred meters. However, the fiber chromatic dispersion and attenuation that limit the OI reach can be reduced significantly by increasing the wavelength of this very same technology. The upper limit of the GaAs-based VCSEL technology, with strained InGaAs quantum wells (QWs), is about 1100 nm.With further improved OI performance, new hyperscale data center topologies can be realized and explored. This will lead to many more possible solutions in traffic engineering as well as for power management. 1060 nm VCSELs could open up for lane rates of 10, 25 and possibly 50 Gb/s over distances up to 2 km and help reach the Tb/s link speed aim of the Ethernet standard.In this work we show that the 1060 nm GaAs VCSEL is a suitable light source for long-reach OIs by demonstrating its overall stable performance and capability of error-free data transmission up to 50 Gb/s back-to-back and 25 Gb/s over 1 km of MMF. These results stem from careful VCSEL design, including strained InGaAs QWs with GaAsP barriers, doped AlGaAs distributed Bragg reflectors, a short op-tical cavity and multiple oxide layers. We also show that the fabrication of such a device poses no increase in complexity and can be realized using standard processing techniques

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

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

Multilevel Modulation and Transmission in VCSEL-based Short-range Fiber Optic Links

As the demand for ever higher throughput short-range optical links is growing, research and industry associations have shown increased interest in multilevel modulation formats, such as the four leveled pulse amplitude modulation, referred to as 4-PAM. As on-off keying (OOK) persists to be the choice for low latency applications, for example high performance computing, datacenter operators see 4-PAM as the next format to succeed current OOK-based optical interconnects. Throughput can be increased in many ways: parallel links can be deployed, multicore fibers can be used or more efficient modulation formats with digital signal processing is an alternative. Therefore, to improve link data rates, the introduction of new modulation formats and pre-emphasis are primarily considered in this thesis. In a bandwidth-limited link, turning towards spectrally efficient formats is one of the methods to\ua0 overcome the bandwidth requirements of OOK. Such are the considerations when opting for 3-PAM or 4-PAM schemes. Both require lower bandwidth than OOK and are potential candidates in such scenarios. 4-PAM provides double spectral efficiency and double data rate at the same symbol rate as on-off keying, but, as with any technology transition, new challenges emerge, such as a higher SNR requirement, a lower tolerance to VCSEL nonlinearities and skewing of the signal in the time domain. 3-PAM could potentially be an in-between solution, as it requires 33% less bandwidth than OOK and is less sensitive to VCSEL dynamics which could impair the transmission. A study is presented where 3-PAM has outperformed both OOK and 4-PAM in the same link. Detailed investigation of legacy 25G class VCSELs has shown that devices with moderate damping are suitable for the transition to 4-PAM. The pre-emphasis of signals is a powerful tool to increase link bandwidth at the cost of modulation amplitude. This has been investigated in this thesis for on-offkeying and has shown 9% and 27% increase in bit rate for error-free operation with two pre-emphasis approaches. Similarly, pre-emphasis of a 4-PAM electrical signals has enabled 71.8 Gbps transmission back-to-back with lightweight forward error correction and 94 Gbps net data rate was achieved with the same pre-emphasis and post-processing using an offline least-mean-square equalizer