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Abstract
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.
© 2017 Optical Society of America
1. Introduction
Ultrafast laser sources near 2 µm have attracted wide interest for their potential applications in molecule spectroscopy, multi-photon fluorescence microscopy, IR supercontinuum generation. They can also be used as pump sources of optical parametric oscillators (OPOs) for 3-10 μm ultrafast laser generation [1–4]. In the recent years, ultrafast mode-locked lasers near 2 µm wavelength have been reported in Tm-doped or Tm,Ho-codoped garnets [5, 6], tungstate [7, 8], sesquioxide [9, 10], fluoride [11] and aluminate [12].
Calcium fluoride (CaF2) has been proved to be an excellent laser host material with good optical and thermal properties. In the cube lattice structure of CaF2, the F- ions locate at the corners of the cube, while Ca2+ ions are at the center of the cube. Only half of these cubes are occupied by Ca2+, and the other half are vacant. While Tm3+ and Y3+ ions are doped in CaF2, the Tm3+ ions, Y3+ ions and the compensation ions F- randomly inhabit the vacancies of cubic lattices, resulting in disordered lattice field. The disordered structure of Tm,Y:CaF2 causes the ground state splitting and spectral broadening [13, 14], which is beneficial to dual-wavelength mode locking. Such dual-wavelength mode-locked lasers can be used to generate THz radiation by difference frequency generation (DFG) [15]. The generated THz frequency is equal to the frequency difference of the dual-wavelength pulses. The dual-wavelength pulses in these lasers are automatically temporally synchronous and spatially overlapped [16–20]. Compared to 1-μm and 1.5-μm lasers, the dual-wavelength laser at 2 μm has a higher quantum efficiency for THz generation.
Since Tm-doped CaF2 crystal has a low absorption cross section, a high Tm-doping is generally desirable for efficient absorbing of pumping light. Moreover, high Tm-doping can cause cross-relaxation process and result in high quantum efficiency [21]. However, too high Tm doping will increase the probability of non-radiative deexcitation due to clustering effect and result in laser quenching [22]. Tm-Tm clusters will also cause the decrease of absorption cross sections [23]. So far, the laser performance of Tm:CaF2 crystal with 2% doping has been reported with an output power of 130 mW [22], and the laser of Tm:CaF2 ceramic with 4% doping only emitted an output power of 60 mW with a slope efficiency of 5.5% [24]. Recently, a CW output power of 453 mW with a slope efficiency of 21% from a Tm,Y:CaF2 laser was reported [25]. By comparison of spectroscopic characteristics and laser performances of Tm:CaF2 and Tm,Y:CaF2 grown in our laboratory, we find codoping of Y3+ ions in Tm:CaF2 can alleviate cluster quenching effect and improve the laser performance.
In this paper, the spectroscopic characteristics, CW and mode-locking laser performances of Tm,Y:CaF2 crystal were systematically studied. A CW output power of 586 mW and a laser slope efficiency of 26% were realized. The Tm,Y:CaF2 laser shows a wavelength redshift phenomenon as the pump power increases. Benefitting from spectral splitting of disordered crystal, both single-wavelength and dual-wavelength mode-locking was experimentally realized. To our knowledge, it is the first time to demonstrate the mode-locked laser of Tm,Y:CaF2 disordered crystal. .
2. Experimental results and discussion
2.1 Spectroscopic characteristics
The 3 at.% Tm, 3 at.% Y:CaF2 crystal was grown by temperature gradient technique [25, 26]. A polished Tm,Y:CaF2 plate with 2 mm in thickness was used for spectroscopy measurement. For comparison, a 3 at.% Tm:CaF2 sample was also tested. The absorption spectrum was measured using a UV/VIS/NIR spectrophotometer (Jasco V-570). The fluorescence was excited by a CW laser at 808 nm. The fluorescence spectrum was recorded using a time-resolved fluorimeter (FLS980) with grating blazed at 1820 nm. All the measurements were conducted at room temperature.
The absorption spectra of Tm:CaF2 and Tm,Y:CaF2 are shown in Fig. 1(a). The absorption cross section of Tm,Y:CaF2 at 767 nm is 0.45 × 10−20 cm−2, which is larger than that of Tm:CaF2. The larger absorption cross section of Tm,Y:CaF2 at 767 nm can be attributed to the stronger crystal lattice field caused by Y3+ ions. The codoping of Y3+ helps to break Tm-Tm clusters and form Tm-Y clusters. While the absorption cross section of Tm,Y:CaF2 at 792 nm is 0.12 × 10−20 cm−2, which is smaller than that of Tm:CaF2 (0.22 × 10−20 cm−2). Y3+ ions separate the clustered Tm3+ ions in the lattice to an appropriate distance, resulting in the increased probability of cross relaxation of the neighboring Tm3+ ions. The Tm3+ ions in the ground-state energy level 3H6 can be pumped to the higher level 3F4 by cross relaxation process in a quite short time, leading to the decrease of Tm3+ amount at 3H6 level and smaller absorption cross sections at 792 nm.
Fig. 1 (a) Absorption spectra and (b) fluorescence spectra of 3at.% Tm:CaF2 and 3at.% Tm, 3at.% Y:CaF2 crystals at 300 K.
The fluorescence spectra of Tm:CaF2 and Tm,Y:CaF2 are shown in Fig. 1(b). The emission intensity of Tm,Y:CaF2 at 1820 nm is 3.4 times as high as that of Tm:CaF2 crystal. As discussed above, Y3+ ions break Tm3+ clusters, which also helps to increase the transition oscillation strength of 3F4→3H6. Besides, we measured the emission lifetimes. The emission lifetimes are 8.15 ms for Tm,Y:CaF2 and 6.16 ms for Tm:CaF2, indicating that Y3+ ions as the buffer ions increase the emission lifetime.
2.2 CW laser performance
A five-mirror X-folded resonator was adopted in the laser experiment (Fig. 2). The laser cavity mirrors M1, M2, and M3 had the same radius of curvature (ROC) of −100 mm, and were all highly reflectively coated for laser wavelength and anti-reflectively coated for pumping wavelength. The Tm,Y:CaF2 sample for laser experiment was cut with a Brewster angle and had a size of 4 × 4 × 9 mm3. The Tm,Y:CaF2 sample was wrapped by indium foil and mounted in a copper block, which was cooled by circulated water at 13°C. With a single-emitter laser diode pumping at 790 nm, The Tm,Y:CaF2 sample absorbed 49% of the incident pump power. The focused pump spot size was measured to be 25 × 86 µm2, and the laser mode radius was calculated to be ~35µm in the crystal.
Fig. 2 Experimental setup of Tm,Y:CaF2 laser. L1, L2: convex lenses with the same focal length of 100 mm; M1, M2 and M3: concave mirrors with the same radius of curvature (ROC) of −100 mm; OC: output coupler.
A plano-plano high-reflectivity mirror was used as an end mirror for CW laser operation (Fig. 1). The CW laser performance was investigated with different output couplers (0.5%, 2% and 5%). For the 5% and 2% couplers, the CW laser displayed a similar performance. The laser threshold was 0.23 W for the 5% coupler and 0.18 W for 2% coupler. The maximum output power was 586 mW with a slope efficiency of 26.0% for 5% coupler, and 583 mW with a slope efficiency of 25.3% for 2% coupler (Fig. 3(a)). The CW laser did not saturate, and higher output power was limited by the available pump power. The laser output spectra were centered at around 1922 nm (Fig. 3(b)). For comparison, Tm:CaF2 laser in the same condition had a maximum CW output power of 150 mW with a slope efficiency of 19% by using 5% coupler.
Fig. 3 CW laser performance of Tm,Y:CaF2. (a) output power versus absorbed pump power with 0.5%, 2% and 5% couplers. (b) CW laser spectra for 0.5%, 2% and 5% couplers.
For the 0.5% coupler, the CW laser showed a different performance. The laser slope efficiency was only 17%, and a slight output power saturation was observed beyond 1.67 W absorbed pump power (Fig. 3). In the experiment, the wavelength redshift from 1932 nm to 1963 nm was observed as the pump power increased, as shown in Fig. 4. For the 0.5% coupler, since there was a high optical power density in the cavity, the laser reabsorption became stronger, which should be responsible for the wavelength redshift [27]. Limited by the Tm,Y:CaF2 gain bandwidth, the wavelength redshift up to 1963 nm was achieved under the pump power of 1.5 W (Fig. 4). The output power saturation is related to the laser reabsorption and wavelength redshift. The reabsorption increases with the intracavity laser intensity. Initially, as the laser intensity increases, the laser wavelength has a redshift to reduce the reabsorption amount and remain the laser slope efficiency. When the wavelength redshift is up to the largest wavelength of 1963 nm, the increased intracavity intensity only induces stronger laser reabsorption, which results in decrease of the slope efficiency and causes power saturation.
Fig. 4 CW laser spectra vary with absorbed pump power for 0.5% output coupler. Blue line is the spectrum for 0.5W absorbed pump power, green line is for 0.86 W, red line is for 1.5 W and violet line is for 2.46 W.
2.3 single-wavelength mode locking
In mode-locking operation, a wedged mirror with 2% transmission was used as output coupler. With a semiconductor saturable absorber mirror (SESAM, SAM-1920-2-30ps-4) as mode locker (Fig. 2), stable CW mode locking was established when the absorbed pump power was over 1.66 W. The used SESAM had a modulation depth of 1.2%, linear loss of 0.8% and recovery time of 30 ps. The mode-locked pulse trains were detected by a high-speed mid-infrared detector (EOT, ET-5000) and recorded by a 500-MHz oscilloscope (Tektronix, DPO3054), which are shown in Fig. 5(a) and 5(b). The mode-locked pulses had a period of 10 ns, which corresponded to the cavity length of ~1.5 m. We measured the radio-frequency (RF) spectrum of the mode-locked pulses with a 6-GHz radio-frequency spectrometer (AV4037B), which is shown in Fig. 5(c) and 5(d). The RF spectrum shows a signal-to-noise ratio as high as 68 dB, indicating clean CW mode-locking without Q-switching instability. The mode-locked pulses had a repetition rate of 99.45 MHz according to the RF spectrum (Fig. 5(c)).
Fig. 5 (a) and (b) are mode-locked pulse trains in the time scale of 1 ms/div and 20 ns/div, respectively; (c) and (d) are fundamental and harmonic radio-frequency spectra of the mode-locked pulses.
In CW mode locking operation, the average output power linearly increased with the absorbed pump power, and a maximum average output power of 114 mW was obtained under an absorbed pump power of 2.46 W (Fig. 6(a)). The higher average output power was only limited by the available pump power. With a commercial autocorrelator (APE, PulseCheck150), autocorrelation trace of the mode-locked pulses was recorded (Fig. 6(b)). The mode-locked pulses had a pulse duration of 22 ps, assuming a Gaussian pulse shape. The long mode-locked pulses were attributed to the long recovery time of the used SESAM and strong vapor absorption near 1.9 µm. The mode-locking spectrum is shown in the inset of Fig. 6(b). The spectrum is centered at 1887 nm with a FWHM bandwidth of 2.8 nm.
Fig. 6 (a) Average output power versus absorbed pump power (blue line: linear fitting); (b) autocorrelation trace of the mode-locked pulses. Inset is the mode-locking optical spectrum.
2.4 dual-wavelength synchronous mode locking
Multi-wavelength emission is a typical feature of disordered crystals, which benefits to generating multi-wavelength mode-locked pulses [16–20]. By translating M2 mirror, the dual-wavelength oscillation was realized. Then, by optimizing the end mirrors, stable dual-wavelength mode locking was established. In dual-wavelength mode locking operation, the average output power linearly increased with the pump power. The maximum average output power was 78 mW under the absorbed pump power of 2.46 W (Fig. 7(a)). The RF spectrum in Fig. 7(b) shows a signal-to-noise ratio of 60 dB and a pulse repetition rate of 104.65 MHz in the dual-wavelength mode-locking operation. The dual-wavelength mode-locking was long-term stable and no apparent wavelength competition existed benefitting from multiple independent emission centers in Tm,Y:CaF2 disordered crystal. The mode-locked pulse spectrum is shown in Fig. 8(a), which presents two spectral peaks at 1880.7 nm and 1889.0 nm. And the intensities of the two wavelengths are quite different, which derives from gain difference of various emission centers in Tm,Y:CaF2 disordered crystal. The autocorrelation trace of the mode-locked pulses is shown in Fig. 8(b), in which optical beating between the two wavelengths is clearly observed. The optical beating indicates the two dual-wavelength pulses were temporally synchronous. The pulse synchronization is attributed to saturable absorption of SESAM. It is easily understood, two synchronous pulses mean higher pulse energy fluence on the SESAM, which results in a smaller saturable loss.
Fig. 7 (a) Average output power versus absorbed pump power and (b) radio-frequency spectrum in dual-wavelength mode-locking operation.
Fig. 8 The optical spectrum (a) and autocorrelation trace (b) of dual-wavelength synchronously mode-locked pulses. Inset in Fig. 7(b) is the zoomed beating pulses.
For optical beating, frequency difference (Δν) and beating pulse period (T) should satisfy the relation Δν × T = 1. In our case, the frequency difference of the dual wavelengths is ~0.7 THz, which well matches with the beating pulse period of 1.4 ps (Inset in Fig. 7(b)). With such synchronous dual-wavelength pulses, coherent THz radiation can be achieved by difference frequency generation.
3. Conclusion
The spectroscopic characteristics, CW and mode locking laser performances of a novel Tm,Y:CaF2 disordered crystal were studied. A maximum output power of 586 mW with a slope efficiency of 26% was obtained in the CW laser. The wavelength redshift in the laser was observed as the pump power increased. For mode-locking operation, the laser could operate in single-wavelength mode locking or dual-wavelength synchronous mode locking. In single-wavelength mode-locking operation, the laser generated mode-locked pulses with pulse duration of 22 ps, average output power of 114 mW and repetition rate of 99.45 MHz at 1887 nm. Benefitting from spectral splitting of the disordered crystal, the laser could also emit synchronous dual-wavelength pulses at 1880.7 nm and 1889.0 nm with pulse duration of 43 ps, which may be used for coherent THz radiation generation. These results indicate that Tm,Y:CaF2 disordered crystal is a promising gain medium for multi-wavelength CW and mode-locked lasers near 2 µm wavelength.
Funding
National Basic Research Program of China (2013CBA01505); the Shanghai Excellent Academic Leader Project (15XD1502100); National Natural Science Foundation of China (NSFC) (61675130, 11421064, 61422511).
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