We report on sub-100-fs pulse generation from a passively mode-locked laser based on a novel disordered crystal,
lanthanum calcium lithium niobium gallium garnet (LCLNGG) codoped with thulium (Tm3+) and holmium (Ho3+) ions.
In the continuous-wave regime, the Tm,Ho:LCLNGG laser generated a maximum output power of 350 mW at
2080.5 nm with a slope efficiency of 23.8%. By using a Lyot filter, the laser wavelength was continuously tuned over a
broad range of ~210 nm (1904.1 – 2114.1 nm). Soliton mode-locking was initiated and stabilized by a transmission-type
single-walled carbon nanotube saturable absorber. Pulses as short as 63 fs were obtained at a central wavelength of
2072.7 nm with an average output power of 63 mW at a pulse repetition rate of ~102.5 MHz.
A comparative study of three disordered calcium niobium gallium garnet (CNGG)-type crystals codoped with Tm3+ and Ho3+ ions is performed: (i) without host modifiers (CNGG), (ii) with Li+ cations added (CLNGG), and (iii) with Li+ and La3+ cations added (LCLNGG), all grown by the Czochralski method. The crystals exhibit inhomogeneously broadened luminescence bands extending beyond 2.1 μm. A diode-pumped Tm,Ho:LCLNGG laser generates 562 mW at 2082 nm with a slope efficiency of 17.4% and a laser threshold of 0.46 W. A continuous wavelength tuning between 1904.1 and 2121.1 nm (tuning range: 217 nm) is achieved with this new garnet compound. The Tm,Ho:LCLNGG crystal is promising for generation of ultrashort pulses from mode-locked lasers emitting above 2 μm.
We report on efficient mid-IR difference-frequency generation (DFG) at ~8 μm in orientation-patterned GaAs (OPGaAs), by mixing the signal and idler fields inside a nanosecond, singly-resonant, periodically-poled MgO-doped LiNbO3 optical parametric oscillator (OPO). The temperature and spectral acceptance bandwidths as well as the DFG output performance are compared for two OP-GaAs samples with different lengths. Temperature tuning of the DFG is studied by implementing a transversely chirped Volume Bragg Grating (VBG) as one of the OPO cavity mirrors for the signal wave. The maximum DFG average power amounts to 215 mW at 8.15 μm for a pulse repetition rate of 35 kHz. The corresponding overall optical conversion efficiency from 1 to 8 μm is ~1.1%.
Tm,Ho co-doped disordered calcium niobium gallium garnet (CNGG) crystals are investigated as a novel gain medium for mode-locked lasers near 2 μm. With a GaSb-based semiconductor saturable absorber mirror (SESAM) and chirped mirrors for dispersion compensation such a laser is mode-locked at a repetition rate of 89.3 MHz. For a 5% output coupler, a maximum average output power of 157 mW is obtained with a pulse duration of 170 fs (28-nm broad spectrum centered at 2.075 μm, leading to a time-bandwidth product of 0.331). With a 0.5% output coupler, 73-fs pulses are generated at 2.061 μm with a spectral width of 62 nm (time-bandwidth product of 0.320) and an average output power of 36 mW.
Mode-locked lasers emitting ultrashort pulses in the 2-μm spectral range at high (100-MHz) repetition rates offer unique opportunities for time-resolved molecular spectroscopy and are interesting as pump/seed sources for parametric frequency down-conversion and as seeders of ultrafast regenerative laser amplifiers. Passively mode-locked lasers based on Tm3+- and Ho3+-doped bulk solid-state materials have been under development for about a decade. In 2009 we demonstrated the first steady-state operation of such a Tm:KLu(WO4)2 laser using a single-walled carbon nanotube (SWCNT) saturable absorber (SA), generating 10-ps pulses at 1.95 μm. In 2012 this laser produced 141-fs pulses at 2.037 μm. More recently, the study of numerous active media with different SAs resulted in the generation of sub-100-fs (sub-10-optical-cycle) pulses. Materials with broad and smooth spectral gain profile were selected, naturally emitting above 2 μm to avoid water vapor absorption/dispersion effects, including anisotropic materials, strong crystal-field distortion in hosts that do not contain rare-earths, crystals with structural or compositional (i.e. mixed compounds) disorder that exhibit inhomogeneous line broadening, mixed laser ceramics, and Tm,Ho-codoping of ordered and disordered crystals and ceramics. A broad absorption band in semiconducting SWCNTs spans from 1.6 to 2.1-μm whereas the absorption of graphene extends into the mid-IR and scales for multilayers, increasing the modulation depth. Compared to GaSb-based semiconductor SA mirrors (SESAMs), the carbon nanostructures exhibit broader spectral response and can be fabricated by simpler and inexpensive techniques. Chirped mirrors were implemented for groupvelocity dispersion compensation, to generate the shortest pulses, down to 52 fs at 2.015 μm.
We demonstrate the laser performance of a gadolinium scandium gallium garnet (GSGG) single crystal with 10 at.% Yb3 + -doping concentration. In the case of continuous-wave operation, the laser wavelength was blueshifted in the range from 1067.1 to 1027.2 nm with increasing the transmission of the output coupler from 0.5% to 30%. The maximum output power produced was 3.2 W with 3% output transmission. By employing a Cr4 + : YAG crystal as the saturable absorber, a stable Q-switched laser beam with 21-ns pulse duration and 38-μJ single-pulse energy was achieved at a 20-kHz repetition rate. This laser crystal should be a promising candidate for nanosecond pulse generation especially in harsh environments, such as outer space, due to its wide absorption and emission spectral bandwidths and strong radiation resistance.
We report on a high-power, continuously tunable Tm:CaGdAlO4 (Tm:CALGO) laser with a volume Bragg grating (VBG) as a wavelength selector and a homemade ∼1.7-μm Raman fiber laser as a pump source. Taking the advantages of high diffraction efficiency of VBG, low quantum loss of in-band pumping, and smooth and flat gain spectrum of Tm:CALGO, >5-W output power was achieved in the range of 1945.6 to 1993.7 nm, within a total tuning range from 1919.7 to 1993.7 nm, i.e., 74 nm. The beam quality M2 factors at 5-W output power around 1959.5 nm were 1.31 and 1.32 in the x- and y-directions, respectively.
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