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In research and engineering, short laser pulses are fundamental for metrology and communication. The generation of pulses by passive mode-locking is especially desirable due to the compact setup dimensions, without the need for active modulation requiring dedicated external circuitry. However, well-established models do not cover regular self-pulsing in gain media that recover faster than the cavity round trip time. For quantum cascade lasers (QCLs), this marked a significant limitation in their operation, as they exhibit picosecond gain dynamics associated with intersubband transitions. We present a model that gives detailed insights into the pulse dynamics of the first passively mode-locked QCL that was recently demonstrated. The presence of an incoherent saturable absorber, exemplarily realized by multilayer graphene distributed along the cavity, drives the laser into a pulsed state by exhibiting a similarly fast recovery time as the gain medium. This previously unstudied state of laser operation reveals a remarkable response of the gain medium on unevenly distributed intracavity intensity. We show that in presence of strong spatial hole burning in the laser gain medium, the pulse stabilizes itself by suppressing counter-propagating light and getting shortened again at the cavity facets. Finally, we study the robustness of passive mode-locking with respect to the saturable absorber properties and identify strategies for generating even shorter pulses. The obtained results may also have implications for other nanostructured mode-locked laser sources, for example, based on quantum dots.

We demonstrate absorber-free passive and hybrid mode-locking at sub-GHz repetition rates, using a hybrid integrated extended cavity diode laser operating near 1550 nm. The laser is based on InP as a gain medium and a Si3N4 waveguide feedback circuit. Absorber-free Fourier domain mode-locking with ≈15 comb lines at around 0.2 mW total power is achieved with repetition rates around 500 MHz, using three highly frequency-selective micro-ring resonators that extend the on-chip cavity length to 0.6 m. To stabilize the repetition rate, hybrid mode-locking is demonstrated by weak RF modulation of the diode current. The RF injection reduces the Lorentzian linewidth component from 8.9 kHz to a detection-limited value of around 300 mHz. To measure the locking range of the repetition rate, the injected RF frequency is tuned with regard to the passive mode-locking frequency and the injected RF power is varied. The locking range increases approximately as a square-root function of the injected RF power. At 1 mW injection, a wide locking range of about 80 MHz is obtained. We also observe the laser maintaining stable mode-locking when the DC diode pump current is increased from 40 mA to 190 mA, provided that the cavity length is maintained constant with thermo-refractive tuning.

Amorphous-Ge (α-Ge) or free-standing nanoparticles (NPs) synthesized via hydrogen-free plasma-enhanced chemical vapor deposition (PECVD) were applied as transmissive or reflective saturable absorbers, respectively, for starting up passively mode-locked erbium-doped fiber lasers (EDFLs). Under a threshold pumping power of 41 mW for mode-locking the EDFL, the transmissive α-Ge film could serve as a saturable absorber with a modulation depth of 52–58%, self-starting EDFL pulsation with a pulsewidth of approximately 700 fs. Under a high power of 155 mW, the pulsewidth of the EDFL mode-locked by the 15 s-grown α-Ge was suppressed to 290 fs, with a corresponding spectral linewidth of 8.95 nm due to the soliton compression induced by intra-cavity self-phase modulation. The Ge-NP-on-Au (Ge-NP/Au) films could also serve as a reflective-type saturable absorber to passively mode-lock the EDFL with a broadened pulsewidth of 3.7–3.9 ps under a high-gain operation with 250 mW pumping power. The reflection-type Ge-NP/Au film was an imperfect mode-locker, owing to their strong surface-scattered deflection in the near-infrared wavelength region. From the abovementioned results, both ultra-thin α-Ge film and free-standing Ge NP exhibit potential as transmissive and reflective saturable absorbers, respectively, for ultrafast fiber lasers.

The generation of stable trains of ultrashort (femtosecond to picosecond), terahertz-frequency radiation pulses with large instantaneous intensities is an underlying requirement for the investigation of light–matter interactions for metrology and ultrahigh-speed communications. In solid-state electrically pumped lasers, the primary route to generate short pulses is through passive mode-locking; however, this has not yet been achieved in the terahertz range, defining one of the longest standing goals over the past two decades. In fact, the realization of passive mode-locking has long been assumed to be inherently hindered by the fast recovery times associated with the intersubband gain of terahertz lasers. Here we demonstrate a self-starting miniaturized short pulse terahertz laser, exploiting an original device architecture that includes the surface patterning of multilayer-graphene saturable absorbers distributed along the entire cavity of a double-metal semiconductor 2.30–3.55 THz wire laser. Self-starting pulsed emission with 4.0-ps-long pulses is demonstrated in a compact, all-electronic, all-passive and inexpensive configuration.A passively mode-locked quantum cascade laser (QCL) is developed by employing a heterogeneous gain medium and integrating graphene saturable absorbers along the entire QCL waveguide. Self-starting optical pulses of 4.0 ps are electrically generated in the 2.30–3.55 THz frequency range.

In this paper, passive mode-locking in Ti:sapphire laser with a coherent absorber cells with rubidium vapor placed in the cavity is demonstrated experimentally. For the best of our knowledge these experiments are the first time experimental demonstration of passive mode-locking based on self-induced transparency regime in the coherent absorber. Up to now, such a regime has not been observed experimentally and was predicted only theoretically.

Despite the continuing progress in integrated optical frequency comb technology 1 , compact sources of short, bright pulses in the mid-infrared wavelength range from 3 to 12 μm so far remain beyond reach. The state-of-the-art ultrafast pulse emitters in the mid-infrared are complex, bulky and inefficient systems based on the downconversion of near-infrared or visible pulsed laser sources. Here we show a purely DC-driven semiconductor laser chip that generates 1-ps solitons at the centre wavelength of 8.3 μm at GHz repetition rates. The soliton generation scheme is akin to that of passive nonlinear Kerr resonators 2 . It relies on a fast bistability in active nonlinear laser resonators, unlike traditional passive mode-locking, which relies on saturable absorbers 3 , or active mode-locking by gain modulation in semiconductor lasers 4 . Monolithic integration of all components—drive laser, active ring resonator, coupler and pump filter—enables turnkey generation of bright solitons that remain robust for hours of continuous operation without active stabilization. Such devices can be readily produced at industrial laser foundries using standard fabrication protocols. Our work unifies the physics of active and passive microresonator frequency combs while simultaneously establishing a technology for nonlinear integrated photonics in the mid-infrared 5 . New fully integrated semiconductor laser architectures are shown to be able to generate bright and background-free picosecond solitons at GHz repetition rates in the mid-infrared range.

We report the generation of a stable, broadband frequency comb, covering more than 10 THz, using a dispersion fiber Fabry-Perot resonator with a high quality factor of 69 millions. This platform ensures robust and easy integration into photonic devices via FC/PC connectors, and feature quality factors comparable to those of microresonators. We demonstrate a passive mode-locking phenomenon induced by the coherent interaction of the Kerr effect and Brillouin scattering, which generates a frequency comb with a repetition rate exceeding the free spectral range of the cavity. This parametric process modulates the continuous wave (CW) pump and can then be transformed into a train of almost square-wave pulses thanks to the generation of switching waves. Our results are supported by advanced numerical simulations, and theoretical derivations that include the Brillouin effect in the Fabry-Perot configuration. The very high stable feature of this optical frequency comb lying in the GHz range is critical to several applications ranging from telecommunication, spectroscopy and advanced microwave generation. Optical frequency combs are key tools in spectroscopy and telecom. Here, authors report a stable and broadband comb (>10 THz) from a high-Q fiber Fabry-Perot resonator via Kerr-Brillouin passive mode-locking. This easily integrable platform ensures state-of-the-art photonic performance.

It is believed that passive mode locking is virtually impossible in quantum cascade lasers (QCLs) because of too fast carrier relaxation time. Here, we revisit this possibility and theoretically show that stable mode locking and pulse durations in the few cycle regime at terahertz (THz) frequencies are possible in suitably engineered bound-to-continuum QCLs. We achieve this by utilizing a multi-section cavity geometry with alternating gain and absorber sections. The critical ingredients are the very strong coupling of the absorber to both field and environment as well as a fast absorber carrier recovery dynamics. Under these conditions, even if the gain relaxation time is several times faster than the cavity round trip time, generation of few-cycle pulses is feasible. We investigate three different approaches for ultrashort pulse generation via THz quantum cascade lasers, namely passive, hybrid and colliding pulse mode locking.

We present a theoretical model and numerical simulation results of the dynamics of a monolithic edge-emitting mode-locked tapered quantum well laser. The comprehensive simulation employs a ( 2 + 1 ) dimensional traveling wave model and incorporates models for lateral current spreading and carrier diffusion, while also addressing thermal effects in the laser cavity. Our investigation of the pulse generation by passive mode-locking demonstrates good agreement with experimental observations. Furthermore, we perform a study of the intra-cavity propagation dynamics of the laser, which provides valuable insights for a better understanding of the laser’s behavior.

This work demonstrates the generation of near-infrared supercontinuum (SC) light using a mode-locked erbium-doped fiber laser (MLEDFL) incorporating polyaniline (PANI) as a saturable absorber (SA). The organic PANI-based SA is fabricated in the form of a thin film by embedding PANI compounds into a polymer host and is integrated into the laser cavity to achieve stable passive mode-locking. The laser delivers soliton pulses centered at 1565.6 nm with a pulse duration of 0.92 ps. These amplified picosecond pulses are subsequently launched into a highly nonlinear photonic crystal fiber (PCF) with a dispersion of 1.44 ps/(km·nm) at the pump wavelength, resulting in broadband SC generation. At a pump power of 24 dBm, the system produces a broad optical spectrum extending from 1380 nm to beyond 1750 nm, with an output power exceeding −30 dBm. To the best of our knowledge, this represents the first demonstration of near-infrared SC generation seeded by PANI-SA mode-locked pulses in a PCF.