We report on a 116 Gb/s on-off keying (OOK), four pulse amplitude modulation (PAM) and 105-Gb/s 8-PAM optical transmitter using an InP-based integrated and packaged externally modulated laser for high-speed optical interconnects with up to 30 dB static extinction ratio and over 100-GHz 3-dB bandwidth with 2 dB ripple. In addition, we study the tradeoff between power penalty and equalizer length to foresee transmission distances with standard single mode fiber.

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100 GHz Externally Modulated Laser for Optical Interconnects

We experimentally demonstrate C-band 2 × 56 Gb/s pulse-amplitude modulation (PAM)-4 signal transmission over 100 km standard single-mode fiber (SSMF) using 18 GHz direct-modulated lasers (DMLs) and direct detection, without inline optical amplifier. A delay interferometer (DI) at the transmitter side is used to extend the transmission reach from 40 to 100 km. A digital Volterra filter at the receiver side is used to mitigate the nonlinear distortions. We obtain an average bit error ratio (BER) of 1.5 × 10(-3) for 2 × 56 Gb/s PAM-4 signal after 100 km SSMF transmission at the optimal input power, which is below the 7% forward error correction (FEC) threshold (3.8 × 10(-3)).

Transmission of 2 × 56 Gb/s PAM-4 signal over 100 km SSMF using 18 GHz DMLs

With the continuously growing popularity of cloud services, the traffic volume inside the data centers is dramatically increasing. As a result, a scalable and efficient infrastructure for data center networks (DCNs) is required. The current optical DCNs using either individual fibers or fiber ribbons are costly, bulky, hard to manage, and not scalable. Spatial division multiplexing (SDM) based on multicore or multimode (few-mode) fibers is recognized as a promising technology to increase the spatial efficiency for optical DCNs, which opens a new way towards high capacity and scalability. This tutorial provides an overview of the components, transmission options, and interconnect architectures for SDM-based DCNs, as well as potential technical challenges and future directions. It also covers the co-existence of SDM and other multiplexing techniques, such as wavelength-division multiplexing and flexible spectrum multiplexing, in optical DCNs.

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Enabling Technologies for Optical Data Center Networks: Spatial Division Multiplexing

We demonstrate the transmission of 51.56-Gb/s on-off keying (OOK) signals generated by using a 1.55-μm directly modulated laser (DML) over 15-km long standard single-mode fiber. In this experiment, a duobinary electrical equalizer based on a finite-impulse-response filter is used at the receiver to increase the dispersion-limited transmission distance. We evaluate the performances of the 51.56-Gb/s OOK signals with respect to the transmission distance by using the frequency response analysis of the proposed system. This result is used to explain why it is effective to utilize the duobinary equalization (instead of binary equalization) for increasing the transmission distance.

Transmission of 51.56-Gb/s OOK signal using 1.55-μm directly modulated laser and duobinary electrical equalizer.

We demonstrate 84-Gb/s four-level pulse amplitude modulation (PAM-4) over up to 1.6-km standard single mode fiber using a 20-GHz single mode short cavity vertical cavity surface emitting laser diode at a transmission wavelength of 1525 nm. Different equalizer approaches including a common feedforward equalizer, a nonlinear Volterra equalizer (NLVE), a maximum likelihood sequence estimator (MLSE) and their combinations are evaluated working either as an equalizer for a standard PAM-4 or a partial response PAM-4 signal with seven levels. It is demonstrated that a standard FFE is not enough for a transmission distance of >0.6 km, while the use of an NLVE or FFE + MLSE is able to improve the transmission distance towards 1 km. The use of partial-response PAM-4 FFE in combination with a short memory MLSE is able to efficiently equalize the bandwidth limitations, showing more than 10-times BER improvement compared to standard NLVE or FFE + MLSE at a transmission distance of 1.6 km. Using a partial-response NLVE instead of an PR-FFE further performance improvement is achieved, resulting in BERs below the KP4 FEC-threshold with a BER-limit of 2E-4 after 1.6-km transmission distance, allowing error free operation.

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Experimental Demonstration of 84 Gb/s PAM-4 Over up to 1.6 km SSMF Using a 20-GHz VCSEL at 1525 nm

We demonstrate a 56 Gb/s four-level pulse-amplitude modulation (PAM-4) transmission using direct detection and a long-wavelength 18-GHz bandwidth vertical-cavity surface-emitting laser as directly modulated light source for short-reach inter- and intra-connects in datacenters and short-reach networks. Error-free transmission over 2 km at 7% hard-decision forward-error correction threshold is achieved by applying powerful equalization schemes at the receiver side. Three equalization schemes, i.e., a maximum likelihood estimation (MLSE), a feed-forward equalizer (FFE), and a combination of the FFE and the MLSE are thoroughly investigated, and the performance comparison between them is carried out.

Directly PAM-4 Modulated 1530-nm VCSEL Enabling 56 Gb/s/ $\lambda $ Data-Center Interconnects

Semiconductor lasers have been rapidly evolving to meet the demands of next-generation optical networks. This imposes much more stringent requirements on the laser reliability, which are dominated by degradation mechanisms (e.g., sudden degradation) limiting the semiconductor laser lifetime. Physics-based approaches are often used to characterize the degradation behavior analytically, yet explicit domain knowledge and accurate mathematical models are required. Building such models can be very challenging due to a lack of a full understanding of the complex physical processes inducing the degradation under various operating conditions. To overcome the aforementioned limitations, we propose a new data-driven approach, extracting useful insights from the operational monitored data to predict the degradation trend without requiring any specific knowledge or using any physical model. The proposed approach is based on an unsupervised technique, a conditional variational autoencoder, and validated using vertical-cavity surface-emitting laser (VCSEL) and tunable edge emitting laser reliability data. The experimental results confirm that our model (i) achieves a good degradation prediction and generalization performance by yielding an F1 score of 95.3%, (ii) outperforms several baseline ML based anomaly detection techniques, and (iii) helps to shorten the aging tests by early predicting the failed devices before the end of the test and thereby saving costs.

Degradation Prediction of Semiconductor Lasers Using Conditional Variational Autoencoder

At IMAPS2021, we reported on 1-dimensional VCSEL arrays and the packaging technology developed for these arrays, but this time we present the manufacturing workflow and look at the challenges presented by the packaging of special long-wavelength 2-dimensional vertical-cavity surface-emitting laser (VCSEL) arrays in various dimensions. We will show that available packaging technologies for epoxy application by silk screening, die- and wire bonding can be adapted for the unique properties of the 2D VCSEL arrays, delivering high accuracy for production with great overall yield. Proof of the excellent reliability and lifetime will be given through mechanical and electro-optical testing results. The technologies developed for 2D laser arrays will enable new devices for various new applications like high channel count interconnects for data communication and high power VCSEL solutions for 3D sensing. The perfected wire bonding process secured a homogeneous monometallic structure of the wire bonds, a reduced, well defined bond loop height, with sufficient clearance of the bonding wire above the VCSEL surface. Properly set-up manufacturing procedures for the packaging of the VCSEL arrays are necessary to achieve high performance and reliability of the products presented. Processing of large 2D-VCSEL arrays will be part of the paper. 2D arrays have enabled VCSEL technologies to address a wide range of new laser-based sensing applications. VCSEL products vary from laser chips with only a few emitters for proximity sensors up to several thousands of emitters for LiDAR systems. Applications include systems for the automotive, industrial, medical and consumer markets. There is a strong and growing market opportunity for long wavelengths VCSELs, e.g.  based on InP material enabling single mode and multi-mode 2D-VCSEL arrays from 1.3 to beyond 2 µm. For 3D sensing systems and free-space optical communication, long wavelength 2D-VCSEL arrays offer improved eye safety and further benefits. The long wavelength VCSELs discussed in this paper can be deployed in a wide range of sensing and optical communication applications.

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New Packaging Technology for 2-dimensional VCSEL Arrays and Their Electro-Optical Performance and Applications

In this paper we present the manufacturing workflow and look at the challenges presented by the packaging of special long-wavelength 1- and 2-dimensional Vertical Cavity Surface Emitting Laser Diodes (VCSEL) arrays in the dimension of 230 by 1200μm2 for the 1-D and 1 by 2 mm2 for the 2-D arrays. The focus rests on the development of the major processes used for the assembly of these diodes and their contribution to the reliability of the final product. We will show that available packaging technologies for epoxy application by silk screening, die- and wire bonding can be adapted for the unique properties of the VCSELs, delivering high accuracy for automated production with great overall yield. Proof of the excellent reliability and lifetime will be given through mechanical and electro-optical testing results. The technologies developed for laser arrays will enable new devices for various new applications like high channel count interconnects for data communication and high power VCSEL solutions for 3D sensing.

R. Hohenleitner

Papers

Directly PAM-4 Modulated 1530-nm VCSEL Enabling 56 Gb/s/ $\lambda $ Data-Center Interconnects

Degradation Prediction of Semiconductor Lasers Using Conditional Variational Autoencoder

New Packaging Technology for 2-dimensional VCSEL Arrays and Their Electro-Optical Performance and Applications

New Packaging Technology for Disruptive 1- and 2- Dimensional VCSEL Arrays and Their Electro- Optical Performance and Applications