Diode-pumped mode-locked femtosecond Tm:CLNGG disordered crystal laser

Diode-pumped SESAM mode-locked Tm:Sc2SiO5 laser

We report a diode-pumped passively mode-locked Tm:Sc₂SiO₅ (Tm:SSO) laser for the first time, to the best of our knowledge. The stable continuous-wave (CW) mode-locking is achieved with a semiconductor saturable absorber mirror (SESAM). Operating at the eye-safe wavelength of 1967.7 nm, the pulsed laser delivers a pulse duration of 16.5 ps with an average output power of 207 mW. At a fundamental repetition frequency of 81 MHz, the signal-to-noise ratio is as high as 70 dB. These results demonstrate the great potential of Tm:SSO crystal for ultrashort pulse generation.

Ultrafast Fiber Lasers with Low-Dimensional Saturable Absorbers: Status and Prospects

Wide-spectral saturable absorption (SA) in low-dimensional (LD) nanomaterials such as zero-, one-, and two-dimensional materials has been proven experimentally with outstanding results, including low saturation intensity, deep modulation depth, and fast carrier recovery time. LD nanomaterials can therefore be used as SAs for mode-locking or Q-switching to generate ultrafast fiber laser pulses with a high repetition rate and short duration in the visible, near-infrared, and mid-infrared wavelength regions. Here, we review the recent development of emerging LD nanomaterials as SAs for ultrafast mode-locked fiber laser applications in different dispersion regimes such as anomalous and normal dispersion regimes of the laser cavity operating in the near-infrared region, especially at ~1550 nm. The preparation methods, nonlinear optical properties of LD SAs, and various integration schemes for incorporating LD SAs into fiber laser systems are introduced. In addition to these, externally (electrically or optically) controlled pulsed fiber laser behavior and other characteristics of various LD SAs are summarized. Finally, the perspectives and challenges facing LD SA-based mode-locked ultrafast fiber lasers are highlighted.

Passively Q-switched Tm:CaLu0.1Gd0.9AlO4 laser at 2 µm with hematite nanosheets as the saturable absorber

We report the first passive Q-switching operation at 1.95 µm utilizing the disordered Tm:CaLu_0.1Gd_0.9AlO₄ (Tm:CLGA) crystal and the hematite (α-Fe₂O₃) nanosheets as the saturable absorber. The nonlinear saturable absorption properties of the hematite nanosheets were investigated by the conventional Z-scan technology. The modulation depth of 14.3% with the low saturation intensity of 205 kW/cm2 was obtained, indicating that the hematite could be a suitable saturable absorber for the mid-infrared pulse generation. Using the disordered Tm:CLGA crystal as the gain medium, the passive Q-switching operation could be realized with the hematite nanosheets as the saturable absorber, producing the shortest pulse duration of 402 ns with a repetition rate of 76 kHz. The experimental results convinced us that the hematite nanosheets could be of great interest in the optical pulse generation in the mid-infrared region.

67-fs pulse generation from a mode-locked Tm,Ho:CLNGG laser at 2083 nm

We report on a mode-locked Tm,Ho:CLNGG laser emitting in the 2 µm spectral range using single-walled carbon nanotubes (SWCNTs) as a saturable absorber (SA). Pulses with duration of 98 fs are generated at 99.28 MHz repetition rate with an average output power of 123 mW, yielding a pulse energy of 1.24 nJ. Using a 0.5% output coupling, pulses as short as 67 fs, i.e., 10 optical cycles, are produced after extracavity compression with a 3-mm-thick ZnS plate.

Sub-80 fs mode-locked Tm,Ho-codoped disordered garnet crystal oscillator operating at 2081 nm

We demonstrate a mode-locked (ML) femtosecond laser based on the disordered garnet crystal Tm,Ho:CNGG. Employing a single-walled carbon nanotube saturable absorber, pulses as short as 83 and 76 fs at 2081 nm are achieved without and with external compression, respectively. The latter represents, to the best of our knowledge, the shortest pulse duration obtained from any Ho-doped or Tm,Ho-codoped laser. The average power amounts to 67 mW at a repetition rate of 102 MHz. By analyzing the soliton ML regime, the nonlinear refractive index of Tm,Ho:CNGG is estimated to be ∼1.1×10^-19  m²/W.

78 fs SWCNT-SA mode-locked Tm:CLNGG disordered garnet crystal laser at 2017 nm

A passively mode-locked Tm:CLNGG laser using single-walled carbon nanotubes (SWCNT) as a saturable absorber (SA) is demonstrated at 2017 nm. Pulses as short as 78 fs are generated at an 86 MHz repetition rate with an average output power of 54 mW. By increasing the output coupling from 0.5% to 1.5%, a higher power of 100 mW is achieved for slightly longer pulses with a duration of 105 fs at 1996 nm.

Diode-pumped Tm:KY(WO4)2 laser passively modelocked with a GaSb-SESAM

We present the first diode-pumped modelocked thulium (Tm³⁺) laser based on a double-tungstate crystalline gain material. The solid-state laser consists of a Tm:KY(WO₄)₂ crystal as gain medium and a GaInSb/GaSb quantum well saturable absorber for self-starting passive mode locking. The laser is pumped by a multi-mode fiber-coupled laser diode at a wavelength of 793 nm. An average output power of 202 mW is achieved at a center wavelength of 2032 nm. Pulses with duration of 3 ps are generated at a repetition rate of 139.6 MHz. We also report on the first noise evaluation of a modelocked solid-state laser operating in the 2-µm wavelength range. We measured a timing jitter of sub-100 fs and a relative intensity noise of only 0.04% (frequency range from 500 Hz to 1 MHz).

Spectroscopic characteristics, continuous-wave and mode-locking laser performances of Tm,Y:CaF2 disordered crystal

The spectroscopic characteristics, continuous-wave (CW) and mode-locking laser performances of Tm,Y:CaF₂ disordered crystal were studied. A maximum CW output power of 586 mW was obtained with a slope efficiency of 26%. The Tm,Y:CaF₂ mode-locked laser could operate in two states: single-wavelength mode locking or dual-wavelength synchronous mode locking. The single-wavelength mode-locked laser generated pulses with pulse duration of 22 ps, repetition rate of 99 MHz, and pulse energy of 1.15 nJ at 1887 nm. Alternatively, the laser could also be mode-locked simultaneously at 1880.7 nm and 1889.0 nm wavelengths. The beating modulation in autocorrelation trace shows that the dual-wavelength pulses were temporally synchronous.

Theoretical and experimental studies for high-repetition-rate disordered crystal lasers with harmonic self-mode locking

A harmonically self-mode-locked Nd:Sr3Y2/(BO3)4 disordered crystal laser with subpicosecond pulse duration is demonstrated. We exploit the damped harmonic oscillator model to numerically verify that the mode spacing of the laser cavity can be modified to be the harmonics of the free spectral range of the Fabry-Perot cavity when the optical length of the laser cavity is close to a commensurate ratio of the optical length of the Fabry-Perot cavity. In experiment, the Fabry-Perot cavity can be formed by the pump facet of the disordered crystal and the front mirror. A 110 GHz single-pulse harmonically mode-locked pulse train with pulse duration of 857 fs is experimentally achieved under optical lengths of 27.19 and 4.08 mm for the laser cavity and Fabry-Perot cavity respectively, corresponding to a fractional number of 20/3. A maximum output power of 162 mW is obtained at an incident pump power of 3.1 W.

Dual-wavelength synchronous operation of a mode-locked 2-μm Tm:CaYAlO4 laser

We experimentally demonstrated dual-wavelength synchronous operation of a high-power passively mode-locked 2-μm Tm:CaYAlO4 (Tm:CYA) disordered crystal laser with semiconductor saturable absorber mirror (SESAM) as mode locker. The mode-locked laser emitted an average output power as high as 830 mW with pulse duration of 35.3 ps and repetition rate of 145.4 MHz. The mode-locking dual wavelengths were centered at 1958.9 nm and 1960.6 nm, respectively. Autocorrelation trace clearly shows beating pulses with pulse width of 3.5 ps and repetition rate of 0.13 THz.

Tm,Ho:KLu(WO₄)₂ laser mode-locked near 2 μm by single-walled carbon nanotubes

We demonstrate passive mode-locking of a Tm,Ho-codoped crystalline laser operating on the Ho³⁺-ion transition ⁵I₇→⁵I₈ near 2 µm using a single-walled carbon nanotube saturable absorber. The Tm,Ho:KLu(WO₄)₂ laser emits nearly transform-limited pulses with duration of 2.8 ps at a repetition rate of 91 MHz. The output power amounts to 97 mW.

Wavelength-versatile graphene-gold film saturable absorber mirror for ultra-broadband mode-locking of bulk lasers

An ultra-broadband graphene-gold film saturable absorber mirror (GG-SAM) with a spectral coverage exceeding 1300 nm is experimentally demonstrated for mode-locking of bulk solid-state lasers. Owing to the p-type doping effect caused by graphene-gold film interaction, the graphene on gold-film substrate shows a remarkably lower light absorption relative to pristine graphene, which is very helpful to achieve continuous-wave mode-locking in low-gain bulk lasers. Using the GG-SAM sample, stable mode-locking is realized in a Yb:YCOB bulk laser near 1 μm, a Tm:CLNGG bulk laser near 2 μm and a Cr:ZnSe bulk laser near 2.4 μm. The saturable absorption is characterised at an intermediate wavelength of 1.56 μm by pump-probe measurements. The as-fabricated GG-SAM with ultra-broad bandwidth, ultrafast recovery time, low absorption, and low cost has great potential as a universal saturable absorber mirror for mode-locking of various bulk lasers with unprecedented spectral coverage.

90-fs diode-pumped Yb:CLNGG laser mode-locked using single-walled carbon nanotube saturable absorber

A diode-pumped Yb:CLNGG laser is mode-locked with a single-walled carbon nanotube saturable absorber (SWCNT-SA) for the first time. Pulse durations as short as 90 fs are obtained at ~1049 nm with 0.4% output coupler, the shortest pulses to our knowledge for a diode-pumped 1-µm laser applying SWCNTs as saturable absorber. Using 3% output coupler, the maximum average output power reached 90 mW at a repetition frequency of 83 MHz.

Mode-locking of 2 μm Tm,Ho:YAG laser with GaInAs and GaSb-based SESAMs

We have investigated passive mode-locking of Tm,Ho:YAG lasers with GaInAs- and GaSb-based semiconductor saturable absorber mirrors (SESAMs). With a GaInAs-based SESAM, stable dual-wavelength mode-locking operation was achieved at 2091 nm and 2097 nm, generating pulses with duration of 56.9 ps and a maximum output power of 285 mW. By using the GaSb-based SESAMs, we could generate mode-locked pulses as short as 21.3 ps at 2091 nm with a maximum output power of 63 mW. We attribute the shorter pulse duration obtained with the GaSb SESAMs to the ultrafast recovery time of the absorption and higher nonlinearity compared to standard GaInAs SESAMs.

Mode-locked Tm,Ho:YAP laser around 2.1 μm

A passively mode-locked Tm,Ho:YAP laser around 2.1 μm wavelength employing a semiconductor saturable absorber mirror is demonstrated. Stable continuous wave mode-locking operation was achieved at variable center wavelengths of 2036.5 nm, 2064.5 nm, 2095.5 nm, 2103.5 nm, and 2130 nm, respectively. Pulses as short as 40.4 ps were obtained at 2064.5 nm with a spectral FWHM of 0.5 nm at output powers of 132 mW and a repetition rate around 107 MHz. A maximum output power of 238 mW was obtained at 2130 nm with a pulse duration of 66 ps.

Graphene mode-locked femtosecond laser at 2 μm wavelength

We experimentally demonstrated a passively mode-locked femtosecond laser by using a graphene-based saturable absorber mirror (graphene SAM) in the spectral region of 2 μm. The graphene SAM was fabricated by transferring chemical-vapor-deposited, high-quality, and large-area graphene on a highly reflective plane mirror. Stable mode-locked laser pulses as short as 729 fs were obtained with a repetition rate of 98.7 MHz and an average output power of 60.2 mW at 2018 nm.