Mode-locking Regime Research Articles

Generation of wavelength-switchable nanosecond mode-locked pulses in an Erbium/Ytterbium co-doped fiber laser cavity

This article experimentally demonstrates the generation of mode-locked (ML) pulses, which can operate either at 1568 nm or 1608 nm in an Erbium/Ytterbium co-doped fiber laser cavity (EYDF). The wavelength switching was achieved by adjusting the cavity loss. Depending on the cavity length and losses, the laser could operate in either mode-locked (ML) or Q-switched (QS) regimes. We explored the characteristics of the QS and ML pulses, based on the pump power, cavity length and loss. The laser produced stable QS pulses with a repetition rate of 22.73 kHz and a width of 12.2 μs at a pump power of 3.1 W at 1608 nm, for a 30 m long cavity. The laser produced 520 ns ML pulses at 1568 nm with a repetition rate of 877 kHz, average output power of 2.15 mW, and energy of 2.45 nJ with an additional single-mode fiber (SMF) length 200 m at the same pump power. The laser could switch to 1608 nm wavelength when the cavity losses were adjusted.

Transient behaviors in a spectrum-tailored all-PM NALM mode-locked fiber laser.

We demonstrate an all-polarization-maintaining (PM) dispersion-managed mode-locked ytterbium-doped fiber oscillator based on a nonlinear amplifying loop mirror (NALM). Experimentally, three mode-locking regimes with distinct tailored spectra are achieved with carefully designed cavity setup and adjusted pump power. Using the dispersive Fourier transformation (DFT) technique, the real-time transition dynamics among these regimes are observed as the pump power decreases. It is revealed that the intracavity gain does not decrease continuously during the relaxation period of the gain fiber. Instead, it decreases first and then increases, ultimately leading to the overdriving of NALM. This is attributed to the increase in the population inversion rate of gain fiber under the remaining pump. The dynamic evolution of intracavity gain and non-monotonic transmission properties of NALM collectively induce chaotic pulsation during the transformation. This work not only provides a new perspective for the design and development of novel spectrum-tailored laser sources, but also deepens the understanding of the transient dynamics of ultrafast fiber lasers.

Time localized tilted beams in nearly-degenerate laser cavities.

We show that nearly-degenerate Vertical External-Cavity Surface-Emitting Lasers may emit a set of tilted beams of individually addressable mode-locked pulses. These time localized beams feature a Gaussian profile and they are emitted in pairs with opposite transverse k-vector. Because they are phase locked, their interference leads to a non homothetic pattern in the near-field emission of the laser. In the simplest situation, when a single pair is emitted, this is a stripe pattern. Our analysis discloses the role of third order (spherical) aberrations of the cavity in stabilizing this spatio-temporal mode-locked regime and in selecting the value of the transverse k-vector.

Multistability manipulation by reinforcement learning algorithm inside mode-locked fiber laser

Abstract Fiber mode-locked lasers are nonlinear optical systems that provide ultrashort pulses at high repetition rates. However, adjusting the cavity parameters is often a challenging task due to the intrinsic multistability of a laser system. Depending on the adjustment of the cavity parameters, the optical output may vary significantly, including Q-switching, single and multipulse, and harmonic mode-locked regimes. In this study, we demonstrate an experimental implementation of the Soft Actor–Critic algorithm for generating a harmonic mode-locked regime inside a state-of-the-art fiber laser with an ion-gated nanotube saturable absorber. The algorithm employs nontrivial strategies to achieve a guaranteed harmonic mode-locked regime with the highest order by effectively managing the pumping power of a laser system and the nonlinear transmission of a nanotube absorber. Our results demonstrate a robust and feasible machine-learning–based approach toward an automatic system for adjusting nonlinear optical systems with the presence of multistability phenomena.

Tunable and mode-locked Tm,Ho:GdScO3 laser.

A novel, to the best of our knowledge, Tm,Ho:GdScO3 crystal grown using the Czochralski method was investigated for its polarized spectroscopic properties and laser performance in both tunable continuous-wave (CW) and mode-locked regimes. The crystal's multisite structure (Gd3+/Sc3+ site) and Tm3+/Ho3+ dopants contributed to spectral broadening, enabling a tunable laser operation from 1914 to 2125 nm (with a broad range of 215 nm). Additionally, a pulse duration of 72 fs was achieved for E || b polarization. These results demonstrate the potential of the Tm,Ho:GdScO3 perovskite crystal as a promising gain material for ultrafast lasers operating around 2 µm.

Topological lasing demonstrated in the mode-locked regime

Topological lasing demonstrated in the mode-locked regime

Heterogeneous tunable III-V-on-silicon-nitride mode-locked laser emitting wide optical spectra

We demonstrate a III-V-on-silicon-nitride mode-locked laser through the heterogeneous integration of a semiconductor optical amplifier on a passive silicon-nitride cavity using the technique of micro-transfer printing. In the initial phase of our study, we focus on optimizing the lasing wavelength to be centered at 1550 nm. This optimization is achieved by conducting experiments with 27 mode-locked lasers, each incorporating optical amplifiers featuring distinct multiple-quantum-well photoluminescence values. Subsequently we present a comprehensive study investigating the behavior of the mode-locking regime when the electrical driving parameters are varied. Specifically, we explore the impact of the gain voltage and saturable absorber current on the locking stability of a tunable mode-locked laser. By manipulating these parameters, we demonstrate the precise control of the optical spectrum across a wide range of wavelengths spanning from 1530 to 1580 nm. Furthermore, we implement an optimization approach based on a Monte Carlo analysis aimed at enhancing the mode overlap within the gain region. This adjustment enables the achievement of a laser emitting a 23-nm-wide spectrum while maintaining a defined 10 dB bandwidth for a pulse repetition rate of 3 GHz.

Observation of Replica Symmetry Breaking in Standard Mode-Locked Fiber Laser.

We report the first experimental demonstration of the replica symmetry breaking (RSB) phenomenon in a fiber laser system supporting standard mode-locking (SML) regime. Though theoretically predicted, this photonic glassy phase remained experimentally undisclosed so far. We employ an ytterbium-based mode-locked fiber laser with a very rich phase diagram. Two phase transitions are observed separating three different regimes: cw, quasi-mode-locking (QML), and SML. The regimes are intrinsically related to the distinct dynamics of intensity fluctuations in the laser spectra. We set the connection between the RSB glassy phase with frustrated modes and onset of L-shaped intensity distributions in the QML regime, which impact directly the replica overlap measure.

Sub-60-fs mode-locked Tm,Ho:CaYLuAlO4 laser at 2.05 µm

Tm,Ho:CaYLuAlO4 (Tm,Ho:CALYLO) crystal has wide emission spectra both for π-polarization and σ-polarization, showing significant potential for the generation of ultrashort pulses. Here, a widely tunable and passively mode-locked laser operation based on Tm,Ho:CALYLO crystal under two polarizations was demonstrated for what we believe to be the first time ever. For π-polarization, a maximum output power of 1.52 W and a tuning range of 255.3 nm were achieved in the continuous wave (CW) regime. In the mode-locked regime, a pulse duration of 68 fs and an average output power of 228 mW were achieved upon GaSb-based semiconductor saturable absorber mirror (SESAM). As for σ-polarization, a broader tuning range of 267.1 nm was realized, leading to the shorter pulse duration of 58 fs at 79.7 MHz repetition rate.

Continuous-wave and SESAM mode-locked operation of a Yb:YSr3(PO4)3 laser.

We report on the continuous-wave (CW) and, for what we believe to be the first time, passively mode-locked (ML) laser operation of an Yb3+-doped YSr3(PO4)3 crystal. Utilizing a 976-nm spatially single-mode, fiber-coupled laser diode as pump source, the Yb:YSr3(PO4)3 laser delivers a maximum CW output power of 333 mW at 1045.8 nm with an optical efficiency of 55.7% and a slope efficiency of 60.9%. Employing a quartz-based Lyot filter, an impressive wavelength tuning range of 97 nm at the zero level was achieved in the CW regime, spanning from 1007 nm to 1104 nm. In the ML regime, incorporating a commercially available semiconductor saturable absorber mirror (SESAM) to initiate and maintain soliton-like pulse shaping, the Yb:YSr3(PO4)3 laser generated pulses as short as 61 fs at 1062.7 nm, with an average output power of 38 mW at a repetition rate of ∼66.7 MHz.

Sub-100-fs Kerr-lens mode-locked Yb:Lu2O3 laser with more than 60% optical efficiency.

Yb-doped sesquioxides represent one of the most excellent laser crystals applying for high-power ultrafast lasers owing to their very high thermal conductivities and broadband emission spectra. Pumped by a high-brightness Yb-fiber laser at 976 nm, the Yb:Lu2O3 laser delivers a maximum output power that amounts to 3.55 W in the continuous-wave regime with an optical efficiency of 75%. In the mode-locked regime, 90-fs pulses were generated via soft-aperture Kerr-lens mode-locking at 1080.6 nm with an average output power of 2.85 W, which corresponds to an optical efficiency of 60.3% and a slope efficiency of 68.8%. Average output power of the mode-locked Yb:Lu2O3 laser can be further scaled to 3.05 W at the expense of the pulse duration (178 fs), which corresponds to an optical efficiency as high as 64.5%. To the best of our knowledge, it is the highest optical efficiency ever reported from any solid-state Kerr-lens mode-locked Yb lasers.

Continuous-Wave and Mode-Locked Operation of an In-Band Pumped Tm,Ho,Lu:CaGdAlO4 Laser

We investigate in-band pumping of a Tm,Ho,Lu:CaGdAlO4 laser using a Raman-shifted Er-fiber laser (1678 nm), in the continuous-wave (CW) and mode-locked (ML) regimes. A maximum output power of 524 mW is obtained in the CW regime with a 5% output coupler at an absorbed pump power of 2.04 W, corresponding to a slope efficiency of 27.9%. A maximum CW wavelength tuning range of 160 nm at the zero level, from 1984 to 2144 nm, is obtained with a 0.2% output coupler. In the ML regime, pumping with 5.5 W (unpolarized), the average output power (0.2% output coupler) reaches 148 mW at a repetition rate of ~96 MHz. The output spectrum is centered at 2071.5 nm with a FWHM of 21.5 nm (σ-polarization). The pulse duration amounts to 218 fs (time-bandwidth product equal to 0.327).

Mode-locking of a Tm,Ho:CALYGO laser delivering 50 fs pulses at 2.08 µm.

We study the polarization-dependent laser performance of a novel, to the best of our knowledge, "mixed" Tm,Ho:CaYGdAlO4 crystal in the continuous-wave (CW) and mode-locked regimes. Both in terms of the CW tunability range (261 nm) and the minimum pulse duration (50 fs at 2078 nm, spectral width of 95 nm) in the mode-locked regime, σ-polarization is superior. With extended inhomogeneous spectral broadening due to structural and compositional disorder, Tm,Ho:CaYGdAlO4 is promising for few-optical-cycle pulse generation around 2 µm.

Wavelength tunable actively mode-locked Tm3+ doped fiber laser with GHz repetition rate

Using a phase modulator to synchronize the longitudinal modes in an all-fiber cavity, we demonstrate a wavelength tunable (1955–2045 nm) diode pumped actively mode-locked Thulium doped fiber laser with tunable repetition rate up to 10 GHz with the shortest measured pulse width of 42 ps and average output power of 5 mW. We have performed a detailed analysis of the stable mode-locking regimes, including the self mode-locking condition that challenges the harmonic mode-locked operation. We demonstrate a supermode suppression ratio of > 50 dB across the entire wavelength band of operation while maintaining a pulse width and timing jitter of < 50 ps and < 1 ps, respectively.

Spatiotemporally reconfigurable light in degenerate laser cavities

We show that a III-V semiconductor vertical external-cavity surface-emitting laser (VECSEL) can be engineered to generate light with a customizable spatiotemporal structure. Temporal control is achieved through the emission of temporal localized structures (TLSs), a particular mode-locking regime that allows individual addressing of the pulses traveling back and forth in the cavity. The spatial profile control relies on a degenerate external cavity, and it is implemented due to an absorptive mask deposited onto the gain mirror that limits the positive net gain within two circular spots in the transverse section of the VECSEL. We show that each spot emits spatially uncorrelated TLSs. Hence, the spatiotemporal structure of the light emitted can be shaped by individually addressing the pulses emitted by each spot. Because the maximum number of pulses circulating in the cavity and the number of positive net-gain spots in the VECSEL can be increased straightforwardly, this result is a proof of concept of a laser platform capable of handling light states of scalable complexity. We discuss applications to three-dimensional all-optical buffers and to multiplexing of frequency combs that share the same laser cavity.

Generation of dual and quad-optical frequency combs in the injected radiation free mode-locked frequency-shifted feedback laser

The results of an optoelectronic system—frequency-shifted feedback (FSF) laser experimental examination are presented. The considered FSF laser is seeded only with optical amplifier spontaneous emission (ASE) and operates in the mode-locked regime, whereby the output radiation is sequence of short pulses with a repetition rate determined by the delay time in its optical feedback circuit. In the frequency domain, the spectrum of such a pulse sequence is an optical frequency comb (OFC). These OFCs we call initial. We consider the possibility of tunable acousto-optic (AO) dual and quad-comb frequency spacing downconversion in the FSF laser seeded with ASE and operating in the mode-locked regime. The examined system applies a single frequency shifting loop with single AO tunable filter as the frequency shifter that is fed with several radio frequency signals simultaneously. The initial OFCs with frequency spacing of about 6.5 MHz may be obtained in the wide spectral range and their width, envelope shape and position in the optical spectrum may be tuned. The dual-combs are obtained with a pair of initial OFCs aroused by two various ultrasound waves in the acousto-optic tunable filter (AOTF). The dual-combs frequency spacing is determined by the frequency difference of the signals applied to the AOTF piezoelectric transducer and can be tuned simply. The quad-combs are obtained with three initial OFCs, forming a pair of dual-combs, appearing when three ultrasound frequencies feed the AOTF transducer. The quad-combs frequency spacing is defined by the difference between the frequency spacing of dual-combs. Quad-combs with more than 5000 spectral lines and tunable frequency spacing are observed. The successive frequency downconversion gives the possibility to reduce the OFC frequency spacing form several MHz for initial OFC to tens of kHz for quad-combs.Graphical abstract

Transparent nanopaper for ultrashort pulse generation in the near-infrared region.

Transparent nanopaper (T-paper) can be applied in the field of electromagnetic shielding materials, antistatic materials, composite conductive materials, electric pool materials, super capacitors, and thermal management systems. However, this kind of T-paper has not been employed in ultrafast photonics yet. For the first time, to our knowledge, transparent electrical nanopaper is used in fiber lasers, different from the conventional pulsed fiber laser, which operates in the Q-switched regime under low pump power and then in the mode-locked regime under high pump power. Mode-locking is achieved first with a pulse duration of 550fs under low pump power (166mW). When further increasing the pump power up to 198mW, the proposed fiber laser can be converted from a mode-locked to Q-switched state, which is a result of the two-photon absorption effect. The proposed fiber laser based on T-paper can be potentially applied in optical tomography, metrology, spectroscopy, micro-machining technology, and biomedical diagnostics.

Transient dynamics in mode-locked all-PM Er-doped fiber laser with NALM

In this study, we utilized the Time-Stretch Dispersive Fourier Transform (TS-DFT) method to explore rapid transient states in both time and frequency domains of an all-polarization-maintaining dispersion-managed figure-eight mode-locked laser with Nonlinear Amplifying Loop Mirror (NALM). We detail the different stages of build-up dynamics at varying pump powers, encompassing the Q-switching (QS) stage, transient single pulse formation, pulsation beating patterns, bound states, and the final transition to a single pulse state. Our findings reveal that the mode-locked bound state regime emerges from a single pulse stage following QS, different from previously reported directly resulting from QS pulses or relaxation oscillations. In a novel contribution to the field, we have experimentally demonstrated the transient single pulse stage of build-up dynamics and compared it with the fundamental mode-locked single pulse.

Analysing the temporal amplitude and average output power distribution in the mode-locked regions of an erbium-doped figure-eight fibre laser (theoretical and experimental results)

The noise like pulses (NLPs) are ultrashort light pulses that present very complex dynamics, they are a particular case of the multiple light pulses generated by the partial mode-locked regime. These ultrashort light pulses exhibit very complex behaviour in amplitude and width, in the temporal domain and subsequently in the frequency domain. In this paper we present a theoretical and experimental analysis of temporal amplitude distribution of NLPs within the mode-locked regions of an erbium-doped figure-eight Fibber laser. The mode-locked regions were studied at different settings of the four polarization controllers (HWR1, QWR1) in the ring section and in the nonlinear optical loop mirror (HWR2, QWR2, NOLM) of the fibre laser cavity. We found a wide variety of values of the temporal amplitude of NLPs. The experimental results of the temporal amplitude are compared with simulations generated with the theoretical model based on the theory of polarization of light and the theoretical model of the NOLM. Also, we discovered experimentally that the theoretical model performs well in predicting the complex dynamics of the temporal distribution of amplitude. Finally, we experimentally discovered that the average power output distribution of the cavity has a behaviour which is close to proportional to the temporal amplitude distribution. We analyse NLPs with a carrier wavelength of 1562 nm with typical duration of ns and a repetition rate of 0.9 MHz.

Temporal Cavity Soliton Interaction in Passively Mode-Locked Semiconductor Lasers

Weak interactions of temporal cavity solitons resulting from gain saturation and recovery in a delay differential model of a long cavity semiconductor laser were studied numerically and analytically using an asymptotic approach. This paper shows that in addition to the usual soliton repulsion leading to a harmonic mode-locking regime, soliton attraction is also possible in a laser with a nonzero linewidth enhancement factor. It is shown numerically that this attraction can lead either to pulse merging or to pulse bound-state formation.