Harmonic Mode-locking Operation Research Articles

O-band femtosecond mode-locked laser by using Ni-MOF in praseodymium-doped fluoride fiber

This work reports an ultrafast mode-locked fiber laser at the less investigated region of the O-band using a nickel metal-organic framework (Ni-MOF), which works as a saturable absorber (SA). Ni-MOF was prepared using the high-power laser-induced method. A stable mode-locked laser of 450 fs pulse duration was obtained. The repetition rate and signal-to-noise ratio (SNR) were 0.408 MHz and 52.5 dB, while the lasers operate at a wavelength of 1298.1 nm. At the pump power of 88 mW, the mode-locked laser was characterized to have an average output power of 0.69 mW, corresponding to a pulse energy of 1.69 nJ and a pulse peak power of 3.53 kW. Additionally, harmonic mode-locked laser operation was also achieved above the pump power of 88 mW. With the harmonic mode-locked laser operation, the pulse repetition rate increased to 2.45 MHz when the pump power was 125.7 mW. To the author's knowledge, this is the first report of novel MOF material used as an SA in praseodymium-doped fluoride fiber (PDFF) laser.

Passive harmonic mode-locking in Tm–Ho doped fiber laser with MoS2–ZnO saturable absorber deposited on the arc-shaped fiber

This work reports a harmonic mode-locked Tm–Ho doped fiber laser (THDFL) using newly fabricated MoS2–ZnO as a saturable absorber (SA). It generates harmonic mode-locking (HML) operation with a center wavelength of 1911.4 nm and a 3-dB bandwidth of 3.02 nm. The fundamental repetition rate and pulse width were 14.1 MHz and 2.05 ps, respectively. Based on the radio frequency (RF) spectrum, the observed signal-to-noise (SNR) of the fundamental HML pulse was 67.22 dB. Increasing the pump power with fine-tuning of the polarisation controller (PC), higher order HML pulses were also observed using the MoS2–ZnO SA. The highest stable HML frequency was 300 MHz, observed at the 21st HML. The central wavelength and 3-dB bandwidth of the HML pulse were 1915.2 nm and 2.29 nm, respectively. The 21st HML pulses demonstrated a SNR of 40.09 dB. No supermode noise was observed in the RF spectrum, indicating that the 21st HML pulses were relatively stable. With industry demand, this high repetition rate HML with MoS2–ZnO SA opens new avenues for comprehension and manipulation of electronic phenomena, paving the way for future innovations in ultrafast lasers.

Fundamental frequency and second harmonic mode-locked operations in an erbium-doped fiber laser based on Cr2Sn2Te6 saturable absorbers

With the rapid development of ultrafast photonics, the need for optical modulators is expanding. In particular, the optical modulator with a high damage threshold and the ability to generate multiple soliton states. This has prompted an ongoing search for optical modulators based on two-dimensional (2D) materials. Therefore, in this work, a novel 2D material Cr2Sn2Te6 (CSnT) with modulation depth of 6.17% and saturation intensity of 15.97 MW/cm2 is prepared as saturable absorber (SA). The nonlinear optical absorption characteristics and pulse modulation applications of CSnT are showed in the Erbium-doped fiber laser (EDFL). The fundamental frequency mode-locked operation and the second harmonic mode-locked operation are realized simultaneously in the ring cavity. Moreover, CSnT SA has a high damage threshold and high stability, the damage threshold reaches 467 mW and the signal-to-noise ratio (SNR) is higher than 70 dB. The experimental findings demonstrate that the CSnT-based saturable absorber possesses remarkable nonlinear optical properties, indicating its considerable potential in the realm of ultrafast photonics.

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.

Multiple soliton mode-locking operations of a Holmium-doped fiber laser based on nonlinear polarization rotation

Multiple soliton mode-locking operations are realized in a Holmium-doped (Ho-doped) fiber laser based on nonlinear polarization rotation (NPR) technique. The fundamental mode-locking (FML) operation with a repetition rate of 19.91 MHz and a pulse duration of 974 fs is realized at 2056.1 nm. Bunched solitons operation with up to 7 soliton pulses is firstly implemented in the Ho-doped fiber laser by increasing the pump power. The Ho-doped fiber laser can also evolve into the harmonic mode-locking (HML) operation at ∼ 2056 nm by finely adjusting the intra-cavity parameters. The pulse duration is 965 fs at the highest repetition rate of 139.19 MHz, which is believed to be the first report on the femtosecond HML operation in the Ho-doped fiber lasers.

TaSe2 nanosheets for harmonic mode-locked fiber laser

• It is the first time to achieved a harmonic mode-locking fiber laser based on TaSe 2 . • 128th harmonic mode-locked pulses at 908.8 MHz repetition rate are stably generated. • The work provides a new solution for generating high repetition frequency pulses. Passively harmonic mode locking of an erbium-doped fiber laser based on TaSe 2 nanosheets saturable absorber (SA) is demonstrated. The TaSe 2 nanosheets are prepared by the liquid phase exfoliation technology and then deposited onto side surface of a D-shaped fiber to form a fiber-compatible TaSe 2 SA. The balanced twin-detector method is utilized to measure nonlinear absorption characterization of the fabricated SA, which gives saturable absorption, nonsaturable absorption, and saturation intensity as 5.49%, 24.6%, and 89.2 MW/cm 2 , respectively. Then, TaSe 2 SA is inserted into a passively mode-locked erbium-doped fiber laser, and the fiber laser is demonstrated with a fundamental frequency of 7.1 MHz. By adjusting the polarization controller and pump power, harmonic mode-locking operation is realized. As the pump power increases to 380 mw, 128th harmonic mode-locked pulses at 908.8 MHz repetition rate are stably generated. The results confirm that TaSe 2 is indeed a very desirable photonic material, which can be used in photonics, fiber optic communication, laser material processing and light modulators.

Highly Efficient Multiple Watt Gain-Switched 1.7 μm All-Fiber Laser Pumped by 1.6 μm Harmonic Dissipative Soliton Resonance Pulses

Here, we firstly utilize a newly explored 1.6 &#x03BC;m harmonic dissipative soliton resonance (DSR) pulses as a pump source to realize a highly efficient gain-switched thulium fiber laser at 1719.52 nm. The 1.6 &#x03BC;m DSR pulses with a fundamental repetition rate of 360 kHz were produced from a nonlinear amplifying loop mirror (NALM) ring cavity, which directly output high power mode-locking pulses due to freedom of soliton-area limitation and pulse breaking. The harmonic mode-locking (HML) operation was achieved by properly adjusting the polarization controllers (PCs). The thulium fiber laser cavity constructed by a Fiber Bragg Grating (FBG) pair was resonantly (in-band) pumped by the 1.6 &#x03BC;m harmonic DSR pulses. Taking the merit of the strong absorption of Tm^3&#x002B; at 1.6 &#x03BC;m and the laser cavity having a low insert loss, the 1.7 &#x03BC;m gain-switched fiber laser with the maximum repetition rate of 2.16 MHz, corresponding to the average power of 2.826 W and the pulse energy of 1.33 &#x03BC;J, was obtained at a high slope efficiency of about 71.9&#x0025;. The maximum output power could exceed 3 W with a high slope efficiency of over 70&#x0025;, which is the record in 1.7 &#x03BC;m all-fiber pulsed thulium lasers.

Oxygen vacancy engineering of MOF-derived ZnO/Co3O4 nanocomposites for high harmonic mode-locking operation

In this contribution, MOF-derived ZnO/Co3O4 composites were fabricated and applied to realize harmonic mode-locking operation in an Er-doped fiber laser, producing the highest repetition rate of 1.46 GHz and a pulse duration of ∼1.5 ps.

Self-mode-locking and self-phase modulation in Tm3+-doped double clad fiber laser for pulse peak power enhancement and multi-wavelength generation

A dual frequency mode-locked Tm3+-doped double clad fiber laser (TDCFL) is proposed by combining self-mode-locking with self-phase modulation (SPM) effect. The SPM effect stimulates the 24th harmonic mode-locking operation. The unpumped gain fiber-based self-mode-locking effect initiates the fundamental mode-locked operation. This fundamental mode-locked pulse acts as a modulation envelope of the 24th harmonic one. In this situation, the maximum pulse peak power and the largest pulse energy improve greatly. Its maximum pulse peak power is at least 311 times higher than the fundamental mode-locked one. And the difference of pulse energy is ~7.8 times. Furthermore, this dual frequency mode-locked TDCFL generates multi-wavelength light under high pump power. These results could offer an exciting new prospect for obtaining simple, compact and high performance multi-wavelength mode-locked fiber lasers.

Highly stable MXene (V2CTx)-based harmonic pulse generation

Abstract MXene as a novel two-dimensional (2D) material exhibits a lot of advantages in nonlinear optics. However, the common MXene, Ti3C2Tx and Ti2CTx nanosheets, easily suffer from degradation under ambient conditions, greatly limiting their practical applications. Here, we demonstrated one of MXene compounds, V2CTx, which has a strong modulation depth (nearly 50%), can serve as an excellent saturable absorber (SA) in passively mode-locked (PML) fiber lasers. More importantly, 206th harmonic order has been successfully generated in Er-doped mode-locked fiber laser, exhibiting maximum repetition rate of 1.01 GHz and pulse duration of 940 fs, which to the best of our knowledge, is the highest harmonic mode-locked fiber laser from the MXene SA so far. In addition, the high harmonic order mode-locked operation can maintain at least 24 h without any noticeable change, suggesting MXene V2CTx nanosheets have excellent stability in this mode-locked fiber laser. It is anticipated that the present work can pave the way to new design for MXene-based heterostructures for high-performance harmonic mode-locked lasers.

Sub-nanosecond multiple-wavelength harmonic mode-locked Tm-Ho co-doped fiber laser

We demonstrate an all-fiber sub-nanosecond passively mode-locked Tm-Ho co-doped fiber laser. With 1.1-km single-mode fiber (SMF) inserted into the cavity, the laser system can generate 183.4-kHz and 720-ps pulses based on a GaAs saturable absorber (SA). The output spectrum is presented with three wavelengths, which are at 1858.5, 1861.3, and 1864.2 nm, respectively. In addition, with the adjustment of the polarization controller and pump power, the multiple-pulse operation, such as the harmonic mode-locking operation, can also be observed. Higher order of harmonic mode locking is promising but limited by the damage threshold of the SA.

Stabilization of passive harmonic mode locking in a fiber ring laser

We propose the model of a harmonically mode-locked soliton fiber ring laser based on the nonlinear polarization rotation taking into account the gain depletion and recovery effects. It is shown that a specific timing jitter could arise in such lasers, since the pulses in the cavity are not strongly identical. To suppress the jitter and stabilize the harmonic mode-locking operation, a method using a small frequency shift followed by the laser radiation filtering is described. The performed numerical simulation shows that the proposed method is able to provide extremely stable harmonic mode locking in a soliton fiber ring laser.

Saturable absorber based on a single mode fiber - graded index fiber - single mode fiber structure with inner micro-cavity.

An Er-doped mode-locked fiber laser with a saturable absorber based on single mode - graded index multimode - single mode fiber (SMF-GIMF-SMF) with inner micro-cavity is demonstrated. The modulation depth of the saturable absorber was measured to be 1.9% when the SMF-GIMF-SMF structure is bent to a certain state. Such a simple saturable absorber enables the mode-locking operation in a ring Er-doped fiber laser and ultrafast pulses with pulse energy of 0.026 nJ and pulse width of 528 fs at the fundamental repetition rate of 14.34 MHz can be generated. In addition, the harmonic mode-locking operation can also be achieved.

Quasi-regenerative mode locking in a compact all-polarisation-maintaining-fibre laser

A novel technique of mode locking in erbium-doped all-polarisation-maintaining-fibre laser has been developed and preliminary investigated. The proposed quasi-regenerative technique combines the advantages of conventional active mode locking (when an intracavity modulator is driven by an independent RF oscillator) and regenerative mode locking (when a modulator is driven by an intermode beat signal from the laser itself). This scheme is based on intracavity intensity modulation driven by an RF oscillator being phase-locked to the actual intermode frequency of the laser. It features also possibilities of operation at multiple frequencies and harmonic mode-locking operation.

Femtosecond Passively Er-Doped Mode-Locked Fiber Laser With WS2 Solution Saturable Absorber

This study presents the femtosecond fundamental and harmonic mode-locked Er-doped fiber laser with WS2 solution saturable absorber (SSA). The SA is fabricated based on a D-shaped fiber (DF) embedded in WS2 nanosheets solution. Such WS2 solution has virtues of good antioxidant capacity, excellent scattering resistance, high heat dissipation, and high damage threshold. This kind of SA shows a modulation depth of 11%, a saturable intensity ${\rm I_{sat}}$ of 5 MW/cm2, and nonsaturable loss of 18%. By employing DF-WS2 SSA, a stable mode-locked fiber laser is achieved with repetition rate of 10.2 MHz and pulse duration of 660 fs. At the pump power of 350 mW, 460.7-MHz repetition rate harmonic mode-locking (HML) operation is also obtained, which corresponds to 45th harmonics of the fundamental cavity repetition rate. The pulse duration is 710 fs and signal-to-noise ratio is 66 dB, showing the excellent performance in HML fiber laser with SA. The results indicate that DF-WS2 solution can work as a potential SA for ultrafast nonlinear optics.

Novel output states in the erbium-doped fiber laser near zero dispersion with semiconductor saturable absorber mirror

we report the observation of operation states near zero dispersion in the erbium-doped fiber laser with the semiconductor saturable absorber mirror (SESAM). The Q-switching, Q-switching mode-locking (QML), continuous wave mode-locking (CML), harmonic mode-locking (HML), double-pulse and multiple-pulse operation states can be manipulated by adjusting the polarization controller with semiconductor saturable absorber mirror. The output 3dB spectral bandwidth and pulse width of operation states are also measured and analyzed. It is shown that continuous wave mode-locked state is stable and without spectral sidebands. Therefore, the proposed scheme will be useful and feasible in the optical telecommunication system.

Harmonic mode-locking and wavelength-tunable Q-switching operation in the graphene–Bi2Te3heterostructure saturable absorber-based fiber laser

We have investigated the broadband saturable absorption property of graphene–Bi2Te3 heterostructures and demonstrated their applications for stable harmonic mode-locking operation in a Yb-doped fiber laser and wavelength-tunable Q-switching operation in an Er-doped fiber laser. The modulation depth of a graphene–Bi2Te3 heterostructure saturable absorber (G-Bi2Te3-SA) is dependent on the coverage of Bi2Te3 on the graphene. By using 15%-Bi2Te3-covered G-Bi2Te3-SA with a modulation depth of 23.28% and saturable intensity of 3.32 MW/cm2, the harmonic mode-locked Yb-doped fiber laser outputs the mode-locked pulses with a pulse duration down to 189.94 ps, spectral bandwidth of 3.5 nm, and repetition rate of 79.13 MHz (21st order of the fundamental frequency). After inserting the G-Bi2Te3-SA with 85% coverage of Bi2Te3 on graphene into Er-doped fiber laser cavity, whose modulation depth and saturable intensity are about 40.79% and 12.48 MW/cm2, respectively, the wavelength-tunable Q-switched pulse with tunable wavelength range over 13.2 nm has been obtained by adjusting the intracavity fiber filter. These results suggest that the graphene–Bi2Te3 heterostructure could serve as a high nonlinear photonic device for practical applications.

Graphene Saturable Absorber Based on Slightly Tapered Fiber With Inner Air-Cavity

We experimentally demonstrate a passively mode-locked fiber laser operation by using a saturable absorber based on graphene deposited on slightly tapered optical fiber with inner air-cavity. Such a saturable absorber enables a strong light–graphene interaction owing to the evanescent field of the excited cladding mode, and as a result, stable passively mode-locked laser pulses with pulsewidth of 674 fs have been generated, and in addition, a harmonic mode-locking operation can also be achieved.

Dual-wavelength rectangular pulse erbium-doped fiber laser based on topological insulator saturable absorber

We reported on the generation of the dual-wavelength rectangular pulse in an erbium-doped fiber laser (EDFL) with a topological insulator saturable absorber. The rectangular pulse could be stably initiated with pulse width from 13.62 to 25.16 ns and fundamental repetition rate of 3.54 MHz by properly adjusting the pump power and the polarization state. In addition, we verified that the pulse shape of the dual-wavelength rectangular pulse can be affected by the total net cavity dispersion in the fiber laser. Furthermore, by properly rotating the polarization controllers, the harmonic mode-locking operation of the dual-wavelength rectangular pulse was also obtained. The dual-wavelength rectangular pulse EDFL would benefit some potential applications, such as spectroscopy, biomedicine, and sensing research.

Harmonically mode-locked Er-doped fiber laser based on a Sb2Te3 topological insulator saturable absorber

In this letter we present for the first time, to the best of our knowledge, a harmonically mode-locked Er-doped fiber laser with antimony telluride (Sb2Te3) topological insulator material used as a saturable absorber (SA). The SA was prepared via mechanical exfoliation of the bulk material. The 80 nm thick Sb2Te3 layers transferred onto fiber ferrule entirely cover the fiber core. The Er-doped fiber mode-locked laser based on such SA generated optical pulses was centered at 1558 nm with 1.9 ps duration and a fundamental repetition rate of 3.75 MHz. Increasing the pump power results in stable harmonic mode-locked operation up to the 81st harmonic at 304 MHz repetition frequency. The laser was capable of generating optical solitons with 2.2 ps duration. The number of generated harmonics could be tuned only by changing the pump power injected into the laser cavity.