Fig. 1. (Color online) A typical fiber-optic communication network for the core, metro and access network scenarios, where the IM/DD links are addressed in the metroedge and intra-/inter-data center networks. CO: center office; RN: remote node; DCI: datacenter interconnects. © [2020] IEEE. Reprinted, with permission, from Ref. [4].

Fig. 2. (Color online) A schematic diagram of the IM/DD system based on DML. DSP: digital signal processing; DAC: digital-to-analog convertor; LDD: laser diode driver; DML: directly modulated laser; SMF: single mode fiber; MMF: multi-mode fiber; ADC: analog-to-digital convertor.

Fig. 3. Model used in the rate equation analysis of semiconductor lasers. Copyright © 2012 John Wiley & Sons, Inc. Reprinted, with permission, from Ref. [ 20].

Fig. 4. (Color online) The sketch of the modulation transfer function for increasing values of relaxation resonance frequency (normalized to ). Including relationships between the peak frequency , the resonance frequency , and the 3-dB down cutoff frequency .

Fig. 5. (Color online) Schematics of different types of coupled-cavity lasers. (a) Two-section DBR laser. © [1998] IEEE. Reprinted, with permission, from Ref. [41]. (b) Passive feedback laser. © (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Reprinted, with permission, from Ref. [35]. (c) DFB+R laser. Reprinted with permission from Ref. [17] © The Optical Society. (d) DR laser. © [2017] IEEE. Reprinted, with permission, from Ref. [38]. HR: high-reflection coating, 3%: 3%-reflection coating, AR: anti-reflection coating.

Fig. 6. (Color online) (a) Example of the detuned loading and PPR in a two-section DBR laser: round trip gain (blue curve) and phase (red dashed curve) function at the DBR threshold. The squared red marker represents the lasing mode; the blue markers indicate nonlasing cavity modes. The green asterisks on the reflectivity curve represent the modes locations in the maximum detuned loading condition. © [2013] IEEE. Reprinted, with permission, from Ref. [42]. (b) Example of the detuned loading in a DFB+R laser: in-cavity etalon profile for DFB+R with 3% coating (red), passive feedback laser (PFL) with HR coating (black), and the stopband of the DFB section (blue). Reprinted with the permission from the authors of Ref. [40].

Fig. 7. (Color online) (a) Measured lasing spectrum at 27 mA with using PPR. (b) Measured small-signal responses of the laser at various bias currents, with using PPR. (c) Measured lasing spectrum at 27 mA without using PPR. (d) Measured small-signal responses of the laser at various bias currents, without using PPR. The laser has a 50-μm-long active section, and the response of –3 dB is marked by a dashed horizontal grey line. Reprinted by permission from Springer Nature, Nature Photonics^[16], 2021.

Table 1. High-speed optical interface standards.

Table 2. Optical modules for 5G fronthaul.

Table 3. The meaning of the symbols in the rate equations.

Table 4. Reported stare-of-the-art works of DMLs.

Table 5. Reported state-of-the-art works with beyond 200 Gb/s per channel IM/DD transmissions.