Driven primarily by cloud service and data-center applications, short-reach optical communication has become a key market segment and growing research area in recent years. Short-reach systems are characterized by direct detection-based receiver configurations and other low-cost and small form factor components that induce transmission impairments unforeseen in their coherent counterparts. Innovative signaling and digital signal processing (DSP) play a pivotal role in enabling these components to realize their ultimate potentials and meet data rate requirements in cost-effective manners. This paper presents an overview of recent DSP developments for short-reach communications systems and discusses future trends.

Digital Signal Processing for Short-Reach Optical Communications: A Review of Current Technologies and Future Trends

Advanced modulation formats combined with digital signal processing and direct detection is a promising way to realize high capacity, low cost and power efficient short reach optical transmission system. In this paper, we present a detailed investigation on the performance of three advanced modulation formats for 100 Gb/s short reach transmission system. They are PAM-4, CAP-16 and DMT. The detailed digital signal processing required for each modulation format is presented. Comprehensive simulations are carried out to evaluate the performance of each modulation format in terms of received optical power, transmitter bandwidth, relative intensity noise and thermal noise. The performance of each modulation format is also experimentally studied. To the best of our knowledge, we report the first demonstration of a 112 Gb/s transmission over 10km of SSMF employing single band CAP-16 with EML. Finally, a comparison of computational complexity of DSP for the three formats is presented.

Experimental study of PAM-4, CAP-16, and DMT for 100 Gb/s Short Reach Optical Transmission Systems

Short range optical data links are experiencing bandwidth limitations making it very challenging to cope with the growing data transmission capacity demands. Parallel optics appears as a valid short-term solution. It is, however, not a viable solution in the long-term because of its complex optical packaging. Therefore, increasing effort is now put into the possibility of exploiting higher order modulation formats with increased spectral efficiency and reduced optical transceiver complexity. As these type of links are based on intensity modulation and direct detection, modulation formats relying on optical coherent detection can not be straight forwardly employed. As an alternative and more viable solution, this paper proposes the use of carrierless amplitude phase (CAP) in a novel multiband approach (MultiCAP) that achieves record spectral efficiency, increases tolerance towards dispersion and bandwidth limitations, and reduces the complexity of the transceiver. We report on numerical simulations and experimental demonstrations with capacity beyond 100 Gb/s transmission using a single externally modulated laser. In addition, an extensive comparison with conventional CAP is also provided. The reported experiment uses MultiCAP to achieve 102.4 Gb/s transmission, corresponding to a data payload of 95.2 Gb/s error free transmission by using a 7% forward error correction code. The signal is successfully recovered after 15 km of standard single mode fiber in a system limited by a 3 dB bandwidth of 14 GHz.

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Multiband Carrierless Amplitude Phase Modulation for High Capacity Optical Data Links

The vertical-cavity surface-emitting laser (VCSEL) has become a light source of great importance for industrial and consumer applications. This includes communication and sensing in particular, where dynamics and optical mode behavior are key performance characteristics. This tutorial treats relevant VCSEL basics, performance requirements and recent progress toward higher speed, higher single-mode power, and polarization control.

Advances in VCSELs for Communication and Sensing

In the paper, we review our work on heterogeneous III-V-on-silicon photonic components and circuits for applications in optical communication and sensing. We elaborate on the integration strategy and describe a broad range of devices realized on this platform covering a wavelength range from 850 nm to 3.85 μm.

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III-V-on-Silicon Photonic Devices for Optical Communication and Sensing

For the first time a vertical-cavity surface-emitting laser (VCSEL) with a single-mode wavelength-tuning over 102 nm in the range of 1550 nm is demonstrated. The fiber-coupled optical output power has a maximum of 3.5 mW and is > 2 mW over the entire tuning range. The sidemode suppression ratios are > 45 dB. The wavelength tuning is achieved with the micro-electro mechanical actuation of a mirror membrane fabricated with surface micro-machining for on-wafer mass production. The mirror membrane consists of low cost dielectric materials (SiOx/SiNy) deposited with low temperature (< 100°C) Plasma Enhanced Chemical Vapor Deposition (PECVD).

Surface micromachined tunable 1.55 μm-VCSEL with 102 nm continuous single-mode tuning.

Current short-range optical interconnects capacity is moving from 100 to 400 Gb/s and beyond. Direct modulation of several laser sources is used to minimize bandwidth limitations of current optical and electrical components. This total capacity is provided either by wavelength division multiplexing or parallel optics; it is important to investigate on the ultimate transmission capabilities of each laser source to facilitate current capacity standards and allow for future demands. High-speed four-level pulse amplitude modulation at 25 GBd of a 1.5 μ m vertical-cavity surface-emitting laser (VCSEL) is presented in this paper. The 20 GHz 3 dB-bandwidth laser is, at the time of submission, the largest bandwidth of a 1.5 μ m VCSEL ever reported. Forward error correction (FEC) is implemented to achieve transmission over 100 m virtually error free after FEC decoding. Line rate of 100 Gb/s is achieved by emulation polarization multiplexing using 50 Gb/s signal obtained from a single VCSEL.

High-Speed 1550 nm VCSEL Data Transmission Link Employing 25 GBd 4-PAM Modulation and Hard Decision Forward Error Correction

100 Gb/s optical fiber transmission link with a single 1.5 um VCSEL has been experimentally demonstrated using 4-level pulse amplitude modulation

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100 Gb/s single VCSEL data transmission link

Room-temperature lasing at 2.55 μm is reported for InP-based GaInAs/GaAsSb type-II quantum well lasers in pulsed mode up to 42 °C. This record long-wavelength lasing has become feasible by implementing compressive strain in both materials and a carrier confinement design using an AlAsSb/AlGaInAs electron/hole blocking layer. The device concept appears promising for extending the wavelength range further towards 3 μm.

Type-II InP-based lasers emitting at 2.55 μm

Type-II InP-based light sources provide a promising concept for mid-infrared lasers. These have recently made huge progress, as the first electrically and optically pumped lasers could be demonstrated beyond the wavelength limit for type-I InP-based lasers (~2.3 μm). In this paper, we introduce the material system and device concepts, and report the latest achievements, such as electrically pumped lasing operation up to a wavelength of 2.6 μm in pulsed mode, continuous-wave resonant-cavity light-emitting diode operation up to a wavelength of 3.3 μm at 20-80 °C and photoluminescence even up to 3.9 μm.

T. Gruendl

Papers

Surface micromachined tunable 1.55 μm-VCSEL with 102 nm continuous single-mode tuning.

High-Speed 1550 nm VCSEL Data Transmission Link Employing 25 GBd 4-PAM Modulation and Hard Decision Forward Error Correction

100 Gb/s single VCSEL data transmission link

Type-II InP-based lasers emitting at 2.55 μm

InP-Based Type-II Quantum-Well Lasers and LEDs