# Integrated Components and Solutions for High-Speed Short-Reach Data Transmission
## 5 Discussion  
It is well-known that short-reach transmission systems are extremely sensitive to cost of components due to the enormous scale of deployments in intra- and inter-data center optical interconnects. Thus, optical transmitters and receivers at low-cost are required in these applications. To select cost-effective transmitter and receiver components more equitably, the technology and component options based on different transmission distances have been summarized in Figure 16. Note that the cost of components usually includes material cost, yield rate, packaging cost, test cost, design cost, and so on. Most of these costs are difficult to predict in the actual production process, thus it is hard to calculate the accurate cost of components. In this paper, the relative costs of various components are discussed. With the development of low-cost coherent detection and monolithic integration technologies, VCSEL, DML, EML, MZM, IQ modulator, and monolithic integrated transmitter have been the major options to construct the transmitter of high-speed short-reach transmission systems. Here, we compare the supported wavelength \(\lambda\), supported fiber type, transmission distance, bandwidth, frequency chirp, footprint, and relative cost of different transmitter components as depicted in Table 6. The transmission signal in C-band would be greatly affected by CD effect. In IM/DD transmission systems, since the transmission signal is often a double sideband, the interaction between square law detection in single-end PD and CD would produce fiber power fading, which may cause severe ISI contributions to their adjacent symbols. In addition, since the electrical signals of direct modulation components such as VCSEL and DML are directly applied to their laser cavities, these components whose responses were caused by transient chirp and adiabatic chirp would show a higher frequency chirp than EML caused by transient chirp. While, for external modulation components such as MZM, IQ modulator and monolithic integrated transmitter, the frequency chirp does not exist. The interplay between the frequency chirp and CD would induce severe nonlinear distortions and lead to serious degradation of transmission performance. To deal with these problems, DML is applied in mostly <40-km O-band high-speed transmission systems as summarized in Tables 2 and 6. In addition, the application of VCSEL is mostly used in hundreds of meters of 850-nm few-/multi-mode transmission systems so that the influence of CD can be weakened to a certain extent. It is generally believed that the combination of MM-VCSEL and MMF has been recognized as a highly effective solution for <300-m short-reach optical interconnects. Figure 15: (**a**) Experimental setup for 800-Gbit/s transmission measurements; (**b**) optical spectra showing a single -95-GHz wide channel with eight digital subcarriers (from SC1 to SC8); (**c**) all received eight subcarrier probabilistic shaping 64-QAM constellations; DAC: digital to analog converter; Table 1. the first subcarrier; DSP: digital signal processing; MZMD: Mach Zehnder modulator driver; PIC: photonic integrated circuit; TIA: trans-impedance amplifier; (From [27] © 2021 IEEE). Certainly, the SM-VCSEL combining MMF or SMF system can support a few kilometers transmission distance. On the other hand, the bandwidth is another important factor in the selection of transmitter components. As depicted in Table 5, the bandwidth of monolithic integrated transmitter is usually greater than 40-GHz. Generally, despite many efforts made to achieve beyond 100-Gbit/s per lane signal transmission, the common bandwidth of VCSEL is around 25-GHz, as shown in Table 1, while that of DML is less than 30-GHz in Table 2. To break the DML bandwidth ceiling of around 30 GHz, special physical effects including detuned-loading and photon-photon resonance have been introduced to enhance the laser response in the high frequency region, and the state-of-art bandwidth of DML can be up to 100-GHz [47].