Fig. 1. (a) Schematic of InAs/GaAs fourth-order CPML design (total cavity length of 1580 μm) with four gain sections (360 μm for each section) and three equally spaced saturable absorbers (50 μm for each SA). (b) Cross-section SEM image of a fabricated device with a 45° tilted angle. Inset: magnified SEM image on electrical isolation trench region.

Fig. 2. (a) Temperature-dependent continuous-wave light current (L-I) characteristics of fourth-order QD-CPML from 20°C to 100°C under varied reverse bias voltages from 0 to −5  V. The kinks in L-I curves at high VSA are induced from non-linear saturation effect of SAs. (b) Optical spectral evolutions with temperature increased from 20°C to 100°C. The operating current and reverse bias voltage at each temperature are slightly adjusted to achieve flat-top comb spectra.

Fig. 3. Fourth-order QD-CPML. (a) Optical spectrum 3 dB bandwidth mapping as a function of SA reverse bias voltage ranging from 0 to 5 V with 0.5 V step and injection current of gain sections varying from 50 to 250 mA with 10 mA step. (b) A precisely swept map of comb line numbers within 3 dB optical bandwidth as a function of SA reverse bias voltage varying from 3 to 4.5 V with 0.1 V step and gain section injection current changing from 210 to 250 mA with 1 mA step [the mapping area corresponds to the red rectangular zone in (a)]. (c) Pulse width and (d) time–bandwidth product (TBP) mapping as a function of SA reverse bias voltage ranging from 0 to 5 V with 1 V step and gain section injection current ranging from 50 to 250 mA with 10 mA step. (e) Optical spectra and (f) pulse AC trace evolutions with injection current from 50 to 250 mA with 10 mA step at −3  V reverse bias voltage.

Fig. 4. (a) Optical spectrum of flat-top QD-CPML under optimized bias condition of Ig=224  mA and VSA=−3.8  V. (b) Optical linewidth of the frequency noise spectra from 18 filtered comb lines. (c) Relative intensity noise (RIN) of 18 filtered individual comb lines and the whole laser. (d) Wavelength stability of 18 comb lines over 10 min and a single comb line wavelength offset over 20 h (inset). (b), (c), and (d) are characterized at the same set of operating conditions as in (a).

Fig. 5. (a) Experimental setup used to measure B2B NRZ and PAM-4 transmission characteristics of QD-CPML, including ISO, optical isolator; OBPF, optical bandpass filter; PDFA, praseodymium-doped fiber amplifier; PC, polarization controller; AWG, arbitrary waveform generator; RF Amp, RF amplifier; MZM, Mach–Zehnder modulator; OSC, optical sampling oscilloscope. (b) 70 Gbit/s NRZ and (c) 40 GBaud PAM-4 optical eye diagram using comb line at 1321.28 nm.

Fig. 6. (a) Combined optical spectra of fourth-order QD-CPML for extended optical bandwidth under the temperatures of 15°C, 25°C, 51°C, and 63°C (purple line: Ig=195  mA, VSA=−2.8  V; blue line: Ig=217  mA, VSA=−5  V; green line: Ig=240  mA, VSA=−3.3  V; red line: Ig=260  mA, VSA=−2.8  V). The maximum channel counts can reach 60 comb lines within 6 dB optical bandwidth. (b) Combined optical spectra of all 60 channels with filtering of each channel via the OBPF.

Table 1. Comparison of Mode-Locked Comb Laser on Various Material Platforms and Structures