Low-Power Optical Injection into Suppressed Longitudinal Modes for Wideband Optical Frequency Comb Generation in Gain-Switched Discrete Mode Lasers

Discrete-mode lasers (DMLs) provide single-mode emission, narrow linewidth, and compatibility with photonic integration, making them promising sources of optical frequency combs (OFCs). We demonstrate the generation of single- and dual-OFCs in a gain-switched (GS) DML through low-power continuous-wave optical injection (OI) into suppressed longitudinal modes <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathrm{q}_{\mathrm{i}})$</tex>, spanning a tuning range exceeding 4 THz. The DML, operating at 15°C, biased at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{I}_{\text{bias}}=30\text{mA}(\mathrm{I}_{\text{th}}=12.3\text{mA})$</tex>, and emitting at 1546.025 nm, operates in a single-mode regime with adjacent longitudinal modes suppressed by over 60 dB and spaced by 1.244 nm (156.14 GHz), emitting around 2 mW of power. The lasing wavelength corresponds to the central mode <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathrm{q}=0)$</tex>. Subsequently the DML is gain-switched at either 5 GHz or 2 GHz with a modulation amplitude of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{V}_{\text{RF}}=1.5\mathrm{V}$</tex>. Finally, we inject powers of 3 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{W}$</tex> or 30 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{W}$</tex> at specific <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{q}_{\mathrm{i}}$</tex> modes ranging from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{q}_{\mathrm{i}}=-14$</tex> to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{q}_{\mathrm{i}}=14$</tex>. The setup is shown in [1]. Under these conditions, we can generate single- or dual-OFCs with significant carrier-to-noise ratio (CNR), reduced linewidth, and low noise floor.