The Earth 2.0 (ET) space mission has entered its phase B study in China. It seeks to understand how frequently habitable Earth-like planets orbit solar-type stars (Earth 2.0s), the formation and evolution of terrestrial-like planets, and the origin of free-floating planets. The final design of ET includes six 28 cm diameter transit telescope systems, each with a field of view of 550 square degrees, and one 35 cm diameter microlensing telescope with a field of view of 4 square degrees. In transit mode, ET will continuously monitor over 2 million FGKM dwarfs in the original Kepler field and its neighboring fields for four years. Simultaneously, in microlensing mode, it will observe over 30 million I < 20.5 stars in the Galactic bulge direction. Simulations indicate that ET mission could identify approximately 40,000 new planets, including about 4,000 terrestrial-like planets across a wide range of orbital periods and in the interstellar space, ~1000 microlensing planets, ~10 Earth 2.0s and around 25 free-floating Earth mass planets. Coordinated observations with ground-based KMTNet telescopes will enable the measurement of masses for ~300 microlensing planets, helping determine the mass distribution functions of free-floating planets and cold planets. ET will operate from the Earth-Sun L2 halo orbit with a designed lifetime exceeding 4 years. The phase B study involves detailed design and engineering development of the transit and microlensing telescopes. Updates on this mission study are reported.
We present a temperature influence (in range from 78 up to 300 K) on the spectroscopic and laser properties of Tm:SrF2 crystal doped with 2 at. % of Tm3+. The sample was grown using the temperature gradient technique in shape of a single-crystal fiber (d= 2 mm, l = 5 mm) with plane-parallel face-polished without any AR coating. The Tm:SrF2 crystal was mounted in a temperature-controlled copper holder of the liquid nitrogen cryostat. The measured absorption and emission spectra remained broad even at low temperature. The fluorescence lifetime was fitted with a double exponential function, and the measured lifetime changed significantly with temperature decrease. The 147 mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (T < 95 % @ 763 nm, HR @ 1750-2100 nm) placed inside the cryostat and a curved output coupler (r=150 mm, R=97.5 % @ 1750-2100 nm) placed outside the cryostat. For longitudinal pumping, a fiber coupled laser diode was used. The diode was operating in the pulse regime (5 ms pulse length, 10 Hz repetition rate) at wavelength 763 nm. At room temperature, the laser emission was achieved at 1949 nm with a high 38 % slope efficiency. With a temperature decrease, the slope efficiency increased, and the laser threshold decreased, and the laser output wavelength shifted.
Fluoride crystals (CaF2, SrF2) doped with neodymium Nd3+ and codoped with non-active ions such as Y3+, La3+, or Gd3+ for breaking clusters of active ions, have become interesting active media for diode-pumped mode-locked lasers. They posses broad emission spectra and longer fluorescence lifetime in comparison with widely used crystals such as Nd:YAG or Nd:YVO4. The crystal fibers grown by several groups in recent years might be attractive for high power operation and possibility of beam guiding. We investigated Nd,Gd:CaF2 cylinders 10 mm long with diameter of 1.9 mm end pumped by 790 nm LD. In free running regime the wavelength tunability of 41 nm (1040-1081nm) was achieved. In continuously passively mode-locked regime using SESAM the pulses 1.3 ps long at 1065 nm with 2 nm spectral width were generated. The average output power was 200 mW for pump power of 1.6 W. Comparison with operation of standard bulk Nd,Gd:CaF2 active media is also presented.
We present the influence of optically inactive Gd3+ ions codoping on the spectroscopic and laser properties of a novel fluoride Tm,Gd:SrF2 crystal. Eight crystals in shape of single-fiber (r = 2 mm, l = 5 mm) with different low Gd3+ (0, 0.5, 1, 2 at.%) and Tm3+ (2 and 3 at. %) concentrations were tested. The addition of an inactive ion Gd3+ had an influence on the absorption spectra, especially on the value of the absorption coefficient in the 0.76 μm pumping band. The fluorescence spectra excited at 763 nm have been influenced only slightly promoting lower wavelength peaks of 3F4→3H6 transition. The 2 μm fluorescence decay time was measured. The decay curves exhibited double exponential behavior, and two components of lifetime were estimated. While the faster component of about 5.5 ms increased with the Gd3+ addition, the slower component of about 19.7 ms was not affected. The Tm,Gd:SrF2 quasi-cw laser was tested. The pumping diode emitting at a wavelength of 763 nm was operating in the pulse regime (5 ms pulse duration, 10 Hz repetition rate). A 148 mm long hemispherical resonator formed by a flat pump mirror and a curved (r = 150 mm) output coupler was used. The Gd3+ addition had not a strong effect on the laser performance and the high slope efficiency up to 57 % with maximum output power amplitude up to 5.5 W at 1949 nm was achieved with all samples. For wavelength tuning, a MgF2 birefringent filter was used and the wide tunability range of up to 199 nm (1807 – 2006 nm) was reached.
An efficient soliton mode-locking femtosecond Yb3 + : Y2SiO5 (Yb:YSO) laser at 1059 nm assisted with semiconductor saturable absorber mirror has been successfully realized and demonstrated. Nearly Fourier transform-limited pulse duration as short as 236 fs has been achieved with a pulse repetition frequency of 79.6 MHz and an average output power of 903 mW, corresponding to maximum pulse energy of 11.34 nJ and pulse peak power of 48 kW, respectively. The dependence of pulse duration on the intracavity pulse energy in soliton mode-locking regime has been further investigated and discussed, which is fitted by the soliton mode-locking theory and indicates the nonlinear refractive index n2 of the Yb:YSO crystal to be around 2.1 to 4.8 × 10 − 16 cm2 / W.
The effect of doping concentration of Nd3+ and co-doping Y3+ on the spectroscopic properties are investigated systematically. Due to the particular clustering effect, the quench effect was demonstrated in lightly doped NdxGd0.03Sr0.97-xF2.03+x (x=0.0005,0.0015,0.0065,0.01) crystals. For a 3% Gd:SrF2 crystal, the fluorescence lifetime at 1054 nm decrease from 380.9 to 159.8 μs by doping Nd3+ from 0.15 at.% to 1 %at.%, while the emission cross section decreases to 4.12 × 10−20cm2 at 1054 nm. However, the absorb cross section were increased when the concentration of Nd3+ increase from 0.5 % to 0.65 %. Thus, there is an optimum doping concentration of Nd3+. According to the research, the optimum doping concentration of Nd3+ is 0.15 % in 3% Gd:SrF2 crystals.
Nd:Ca1-xYxF2+x(x=0%,2%,5%,10%) crystals were grown by Temperature gradient technique. The emission spectrum and absorption spectrum were tested. The emission cross section, emission bandwidth and absorption cross section were calculated. The influence of codoping ions on the spectral performance was compared and analyzed. These crystals have better spectral parameters than Nd:CaF2 crystals. By increasing the concentration of Y3+, spectral properties could be optimized. Among these crystals, the crystal with x=10% has the longest fluorescence lifetime (283.5 μs). The crystal with x=5% has the largest emission cross section(2.90× 10-20 cm2) and the largest absorption cross section(2.18× 10-20 cm2).
The spectroscopic properties of the 1.0 at.%Pr:Ca0.97R0.02F2.03(R=Y, La, Gd) crystals are investigated. X-diffraction and room temperature absorption spectra have been registered and analyzed. The emission spectra and decay curves of the crystals were obtained at room temperature. The photoluminescence intensity in the visible region is significantly enhanced by co-doping R3+ ions in Pr:CaF2 crystal. The different effects among the R3+ (Y3+, La3+ and Gd3+) regulating ions on the crystals were observed and compared. Pr:Ca0.97La0.02F2.03 and Pr:Ca0.97Y0.02F2.03 crystals have substantially strong emission at blue and orange region, while the Pr:Ca0.97Gd0.02F2.03 crystal is more suitable for the red emission emitting.
Laser radiation in the wavelength range around 2 μm is required for its specific properties - it is very suitable for medical
applications, remote sensing, or pumping of optical parametric oscillators to generate ultrafast pulses in the mid-IR
region further exploited in nonlinear optics. Crystals as YLF, YAG, LLF, and GdVO4 doped by holmium were already
investigated and found suitable for the tunable laser generation around 2.1 mμ. Only a few works are devoted to the laser
operation of holmium-doped fluorides as CaF2. In this work, pulsed and continuous-wave laser operation of a modified-
Bridgman-grown Ho:CaF2 active crystal at room temperature is reported. A commercial 50 W 1940 nm Tm-fiber laser
was used to pump a laser oscillator based on a novel 10 mm long 0.5 at.% Ho:CaF2 active crystal placed in the Peltiercooled
holder. In the pulsed regime (10 ms, 10 Hz), the laser slope efficiency of 53 % with respect to the absorbed pump
power was achieved. The laser generated at the central wavelength of ~2085 nm with the maximum mean output power
of 365 mW corresponding to the power amplitude of 3.65 W. In the continuous wave regime, the maximum output
power was 1.11 W with the slope efficiency of 41 % with respect to the absorbed pump power. To our best knowledge
this is the first demonstration of this laser active material operating in the CW regime at room temperature. The tuning
range over 60 nm from 2034 to 2094 nm was achieved using a birefringent filter showing the possibility to develop
a mode-locked laser system generating pulses in the sub-picosecond range.
Fluoride-type crystals (CaF2, SrF2) doped with neodymium Nd3+ and codoped with buffer ions for breaking clusters of active ions and increasing fluorescence efficiency, present interesting alternative as laser active media for the diode-pumped mode-locked lasers. In comparison with widely used materials as Nd:YAG or Nd:YVO4, they have broad emission spectra as well as longer fluorescence lifetime, in comparison with Nd:glass, SrF2 and CaF2 have better thermal conductivity. In spite of the fact, that this thermal conductivity decreases with Nd3+ doping concentration, these crystals are alternative for the Nd:glass in subpicosecond mode-locked laser systems. In this paper we review the basic results reported recently on these active materials and in the second part we present our results achieved in low power diode pumped passively mode locked lasers with Nd,La:CaF2 and Nd,Y:SrF2 crystals. The pulses as short as 258 fs at wavelength of 1057 nm were obtained in the first case, while 5 ps long pulses at 1065 nm were generated from the second laser system.
A polycrystalline ceramic based on an Nd,Y:SrF2 single crystal was successfully fabricated and its laser performance was experimentally investigated. We obtained dual-wavelength continuous-wave operation at the wavelengths of 1050.5 and 1058.0 nm. The maximum output power and slope efficiency were 750 mW and 31.5%, respectively. In the passively Q-switched operation, the shortest pulse with a 169-ns duration was also obtained, and the corresponding maximum repetition rate and single pulse energy were 7.3 kHz and 19.2 μJ, respectively.
The uniform-quality, large-area, monolayer graphene saturable absorber (SA) with sandwich structure was fabricated, tested, and successfully applied for the generation of diode-pumped Yb:Y2SiO5 mode-locked laser. Without extra negative dispersion elements, the shortest pulse with duration of ∼883 fs was obtained at 1042.6 nm with an output power of ∼1 W. These promising experimental results suggested that the low-cost, high-quality graphene SA could potentially be employed in practical, high-power, ultrafast mode-locking laser systems.
Fluoride-type crystals (CaF2, SrF2) doped with neodymium Nd3+ present interesting alternative as a laser active media for the diode-pumped mode-locked laser systems mainly because of their broad emission spectra as well as longer fluorescence lifetime in comparison with well-known materials as Nd:YAG or Nd:YVO4. In comparison with Nd:glass active material, SrF2 and CaF2 have better thermal conductivity. In spite of the thermal conductivity decreases with doping concentration, these crystal might be interesting alternative for the Nd:glass mode-locked laser systems. In this contribution we present the first results of the Nd,Y:SrF2 mode-locked laser diode-pumped at 796nm. Mode-locking operation using SESAM was successfully achieved in the pulsed pumping regime (pulse-duration 1.5 ms, frequency 10 Hz) with the overall average output power of 2.3 mW (corresponding to the power amplitude of 153 mW) in one output beam at the wavelength of ~1055 nm. The actual pulse-duration was 87 ps.
A reflective graphene saturable absorber mirror (SAM) was successfully fabricated by chemical vapor deposition technology. A stable diode-pumped passively mode-locked Yb3+:Sc2SiO5 laser using a graphene SAM as a saturable absorber was accomplished for the first time. The measured average output power amounts to 351 mW under the absorbed pump power of 12.5 W. Without prisms compensating for dispersion, the minimum pulse duration of 7 ps with a repetition rate of 97 MHz has been obtained at the central wavelength of 1063 nm. The corresponding peak power and the maximum pulse energy were 516 W and 3.6 nJ, respectively.
In this contribution we present spectroscopic and laser properties of TGT (temperature gradient technique) grown
Nd,Y:SrF2 crystals with neodymium concentration of 0.4, 0.65 and 0.8 at.%. The absorption cross-section, fluorescence spectra and fluorescence decay time were measured. For the laser experiments, the noncoated crystal samples 3.5 or 5 mm thick were pumped by a 796 nm laser diode matching the Nd:SrF2 absorption peak. Several output couplers with
reflectivity ranging from 70 to 98 % at the generated wavelength were tested. In the pulsed pumping regime (pulseduration
2 ms, frequency 10 Hz), the maximum average output power of 75 mW was obtained with the slope efficiency
as high as 48 % and the optical-to-optical efficiency of 42 % with respect to the absorbed pump power. The output beam
spatial profile was nearly Gaussian in both axes, oscillations started at the wavelength of 1057 nm. At higher pumping
levels, the second emission line at 1050 nm appears corresponding to our fluorescence measurements. Wavelength
tuning using birefringent filter from 1048 to 1070 nm is probably given by crystal-field splitting of the 4F3/2 manifold in Nd3+. True-CW laser operation was also successfully obtained at lower pumping level with the maximum output power of 90 mW using output coupler reflectivity of 98 %.
Efficient diode-pumped Yb:LuY2SiO5 laser mode locked by single walled carbon nanotube saturable absorber is reported for the first time. Under the absorbed pump power of 12.83 W, continuous wave mode-locked pulses were generated with the maximum average output power of 1.50 W. We obtained pulses as short as 5.2 ps around a center wavelength of 1058.6 nm. The peak power and the single pulse energy of the mode-locked laser were up to 2.9 kW and 15.0 nJ, respectively.
Diode-pumped soliton and non-soliton mode-locked Yb:(Gd1-xYx) 2SiO5 (x=0.5) lasers have been demonstrated together
for the first time to the author's knowledge. For the non-soliton mode locking, output power could achieve ~1.2 W, and
pulse width was about 20ps. For the soliton mode-locked operation, the pulse width was 1.4ps at the wavelength of
1056nm and 375fs at the wavelength of 1042nm, with a pair of SF10 prisms as the negative dispersion elements. The
repetition rate was 48 MHz. The critical pulse energy in the soliton-mode locked operation against the Q-switched mode
locking was much lower than the value in non-soliton mode-locked operation.
We reports on a diode-pumped passively mode-locked Yb:SSO laser with a SESAM. Pulses
duration as short as ~2 ps with a repetition rate of 53 MHz were generated. The output power
achieved ~1.9 W at a pump power of 11.5 W.
A new alloyed crystal, Yb:LYSO, has been grown by the Czochralski method in our institute for the first time, and its
effective diode-pumped cw tunable laser action was demonstrated. The alloyed crystal retains excellent laser properties
of LSO with reduced growth cost, as well as the favorable growth properties of YSO. With a 5-at.% Yb:LYSO sample,
we achieved 2.84 W output power at 1085 nm and a slope efficiency of 63.5%. And its laser wavelength could be tuned
over a range broader than 80nm, from 1030nm to 1111 nm. This is the broadest tunable range achieved for Yb:LYSO
laser, as far as we know.
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