Low FSR Mode-Locked Laser Based on InP-Si₃N₄ Hybrid Integration

Extending hybrid integrated diode lasers to multi-frequency oscillation

“…In that case, the output is not based on ultrashort pulses but ideally features minimum intensity variations, i.e., quasi-continuous light exhibiting a comb spectrum. There have recently been reports of a new mode-locking concept in hybrid lasers based on narrowband feedback without saturable absorber, which seem to fall into this category [57][58][59][60][61]. This includes own work, which formed the first experimental report of this type of modelocking based on narrowband feedback [57], presented in Chapter 4.…”

“…Our study was followed by a detailed experimental analysis at lower repetition rates around 2.2 GHz [58] while a full physical explanation of the working principle based on numerical modeling was given in [61]. Recently, with another type of integrated, narrow-feedback laser indeed much lower repetition rates have been observed, down to 360 MHz [59]. This value is well below the named limit of about 1 GHz, although the authors mention that mode-locking may have been incomplete due to the presence of groups of independently locked modes.…”

“…An overview of these publications is given in Table 5.2. In [59], a very low repetition rate of 360 MHz was realized, which corresponds to roundtrip times longer than 2.75 ns, i.e., the roundtrip time exceeds the upper state lifetime, but a comb was still generated. We note that with bulk feedback based on a fiber Bragg grating, an even lower repetition rate of 255 MHz [60] was realized.…”

“…Based on the observation of surprisingly narrow mode-spacings [59,60], generating frequency combs using a narrowband feedback filter is of promise for realizing sub-GHz repetition rates. A first important question, which we investigate numerically in Section 5.2, is whether this approach can be extended to even lower repetition rates.…”

“…Inspecting recent results for instability, that is, Ref. [59] reporting the lowest repetition rate (360 MHz) with chip-based fundamental diode laser mode-locking, the authors suspect that there are indeed groups of modes that undergo mutually independent mode-locking. Perhaps this is a consequence of the used intracavity filter, a SiN Bragg waveguide, being relatively broadband (about 2.3 nm at −3 dB).…”