We report on a metrological-grade mid-IR source with a 10–14 short-term instability for high-precision spectroscopy. Our source is based on the combination of a quantum cascade laser and a coherent radiation obtained by difference-frequency generation in an orientation-patterned gallium phosphide (OP-GaP) crystal. The pump and signal lasers are locked to an optical frequency comb referenced to the primary frequency standard via an optical fiber link. We demonstrate the robustness of the apparatus by measuring a vibrational transition around 6 μm on a metastable state of CO molecuels with 11 digits of precision.
Orientation-patterned gallium phosphide (OP-GaP) crystals are used here for the first time for the generation of continuous-wave coherent mid-infrared radiation around 5.85 μm by difference frequency generation (DFG) of a Nd:YAG laser at 1064 nm and a diode-laser at 1301 nm. We provide the first characterization of the linear, thermo-optic and nonlinear properties of OP-GaP in a DFG configuration, and we derive an effective nonlinear coefficient deff = 17 pm/V (3) for first-order quasi-phase-matched OP-GaP. This novel nonlinear material can be used to referenced the mid-IR light to a frequency standard by locking the pump and signal laser to a near-IR optical frequency comb.
We present a widely-tunable, singly-resonant optical parametric oscillator, emitting more than 1 W in the region
between 2.7 and 4.2 μm. Two configurations have been studied in order to improve the frequency stability and
the linewidth of the OPO emission. First, we stabilized the signal frequency to a high-finesse Fabry-Perot cavity.
Then, we locked both pump and signal frequency to the frequency comb generated by a NIR fs mode-locked
fibre laser, linked to the caesium primary standard. With this last configuration we carried out saturation
spectroscopy of several transitions belonging to the ν1 rovibrational band of CH3I, resolving their electronic
quadrupole hyperfine structure, and determining the absolute frequency of the hyperfine components with a
50-kHz-uncertainty. An upper limit for the idler linewidth has been estimated as 200 kHz FWHM.
LiInSe2 is one of the few (in the meanwhile 6) non-oxide nonlinear crystals whose band-gap (2.86 eV) and transparency
enabled in the past nanosecond optical parametric oscillation in the mid-IR without two-photon absorption for a pump
wavelength of 1064 nm. However, the first such demonstration was limited to the 3.34-3.82 μm spectral range with a
maximum idler energy of 92 μJ at 3.457 μm for a repetition rate of 10 Hz. Now we achieved broadly tunable operation,
from 4.7 to 8.7 μm, reaching maximum idler pulse energy of 282 μJ at 6.514 μm, at a repetition rate of 100 Hz
(~28 mW of average power).
LiInSe2 is one of the few (in the meanwhile 6) non-oxide nonlinear crystals whose band-gap (2.86 eV) and transparency
enabled in the past nanosecond optical parametric oscillation in the mid-IR without two-photon absorption for a pump
wavelength of 1064 nm. However, the first such demonstration was limited to the 3.34-3.82 μm spectral range with a
maximum idler energy of 92 μJ at 3.457 μm for a repetition rate of 10 Hz. Now we achieved broadly tunable operation,
from 4.65 to 7.5 μm, with a single crystal, reaching maximum idler pulse energy of 282 μJ at 6.514 μm, at a repetition
rate of 100 Hz (~28 mW of average power).
We report standardized absorption and scattering losses measurements of the nonlinear crystals LiInSe2 and LiInS2 in IR range by high average power 1064 nm radiation and tunable radiation of optical parametric oscillator (OPO) based on a
periodically poled lithium niobate (PPLN) pumped by a diode-pumped, Q-switched TEM00 mode Nd:YVO4 laser
operated at 1064 nm.
We report standardized absorption and scattering losses measurements of the nonlinear crystals LiInSe2 and LiInS2 in IR
range by high average power 1064 nm radiation and tunable radiation of optical parametric oscillator (OPO) based on a
periodically poled lithium niobate (PPLN) pumped by a diode-pumped, Q-switched TEM00 mode Nd:YVO4 laser
operated at 1064 nm.
Bulk single crystals up to 20 mm in diameter and 40 mm long for LiInS2 and up to 10 mm, 20mm, respectively, for LiInSe2 have been grown. Their color changed from colorless to rose for the first one and from yellow to dark red for the other. All crystals have wurtzite-type lattice, lattice parameters were determined. A band gap was found to be 3.72 and 3.57 eV for LiInS2 and 3.02, 2.86 eV for LiInSe2 at 80 and 300K respectively. Color variations are due to point defects, first of all to interstitial sulfur, resulting in additional wide absorption bands in the shortwave part of transparency range. For LiInS2 the SHG phase matching conditions were found to be similar for samples of different color and some difference from Boyd's predictions of 1973 was shown.
We present the optical frequency measurements of the 2S-8S/D and 2S-12D two-photon transitions in hydrogen and deuterium. From an analysis taking into account these results and the very accurate measurements of the 1S-2S transition, we show that the optical frequency measurements have superseded the microwave determination of the 2S Lamb shift and we deduce optimized values for the Rydberg constant and for the 1S and 2S Lamb shifts. We report also the recent development of our 1S-3S experiment.
The optical properties of LiInS2 suggested it as a promising material for generation of coherent radiation in the mid-IR region. Before investigating such capabilities its optical and mechanical properties have to be characterized precisely, and especially their evolution with temperature. Sufficiently large and suitably oriented crystals of good optical quality were studied. We first deduced the transparency range of these samples, as well as the frequencies of the optical phonons. We observed a phase- matched second-harmonic generation, using a nanosecond-OPO in the range 2.4 - 2.6 microns as the pump source and estimated a first value of the type-II nonlinearity deff(XY) equals 7.4 pm/V. The thermal expansion, thermo-optic, piezoelectric and electro-optic coefficients were determined along the three principal directions of polarization from -20 degrees Celsius up to + 120 degrees Celsius by means of original interferometric methods. A so-called Fabry-Perot Thermal Scanning (FPTS) interferometric method has been developed to measure accurately the electro-optic coefficients. For LiInS2 the values of ri3 were found to be of the same order of magnitude as its piezoelectric coefficients, but around one order of magnitude smaller than the electro-optic coefficients of the well known KTiOPO4.
Nd-doped LiInS2 single crystals were grown using the Bridgeman technique and their optical properties studied. An intense emission with a dominant line at 1077 nm and groups of lines centered at 600, 760, 820 and 890 nm in absorption/luminescence excitation spectra are related to Nd3+. Doping results in a considerable increase of broad band absorption in the 400 - 460 nm range and a blue emission line at 440 nm at 80 K. Both are associated with transitions between InLi level and valence band.
The walkoff-compensated (WOC) twin-crystal device enables to reduce the aperture effects in critically phase-matched parametric generation. In that device, instead of using a monolithic bulk crystal of length 21c, two identically-cut crystals of length lc are disposed in tandem such that the relative sign of the walkoff angle (rho) in the first and second items are opposite, while keeping unchanged the relative sign of their non-linear effective susceptibilities through a proper orientation of their optic axes. Applied to KTiOPO4 (KTP) for type-II (oeo) second-harmonic generation (SHG) of a color center laser, the 2 X lc twin- crystal device leads to a conversion efficiency 3.5 times higher than that of a 21c single bulk crystal. The same WOC technique has been applied successfully in the type-I (ooe) SHG of a CO2 laser using an AgGaSe2 (AGSE) device. The extension of the WOC method to a periodical structure composed of N twin-device stacks is considered. We report the first demonstration of the SHG efficiency enhancement of such a structure, which is made of four KTP plate optically contacted in the WOC configuration.
Raymond Felder, D. Touahri, Ouali Acef, L. Hilico, Jean-Jacques Zondy, Andre Clairon, Beatrice de Beauvoir, Francois Biraben, Lucile Julien, Francois Nez, Yves Millerioux
The absolute frequency measurement of each hyperfine component of the 5S3/2 and 5S5/2 levels in rubidium was done at ENS more than one year ago using Ti-Sa lasers. We built two devices based on diode lasers to study some metrological properties. We measure the frequency differences between hyperfine components of the 5S5/2 level and we calculate the corresponding hyperfine constants. We also measure the frequency interval between the 5S3/2 and 5S5/2 levels using a Schottky diode. The measured stability in terms of Allan variance is 3*10-13t-1/2 up to 2000 s. The light shift is investigated and the difference between our two systems is 1.7 kHz. The repeatability of one system is better than 10-12 and will allow the absolute frequency measurement at this level via the LPTF frequency synthesis chain.
Jean-Jacques Zondy, D. Touahri, Ouali Acef, L. Hilico, M. Abed, Andre Clairon, Yves Millerioux, Raymond Felder, Beatrice de Beauvoir, Francois Nez, Francois Biraben, Lucile Julien
A frequency chain, derived from the one used to measure the absolute frequency ((nu) $= 473 THz) of the He-Ne/I2 optical standard, is currently being implemented in order to measure the frequency of a diode laser stabilized on the two-photon transition of rubidium vapor. The measurement scheme is based on the comparison of the frequency of this near-IR potential secondary standard to the 13th harmonic frequency of the R(12)-CO2/OsO4 LPTF secondary standard at (nu) equals 29.096 THz. Recent results on the frequency synthesis are reported, enabling the testing of long-term stability of this Rb-locked system with respect to the IR reference standard.
We have built an optical frequency synthesis chain starting from our laboratory-measured reference at 29 THz (CO2/OsO4 laser) to measure with a more than 10 fold improved accuracy the frequency of a He-Ne laser locked on a hyperfine transition of Iodine. The result of the measurement of the laser locked on the 'f' component of (127)R11-5 transition is (nu) equals 473 612 353 586.9 +/- 5 kHz. ((Delta) (nu) /(nu) equals 1. 10-11). The uncertainty found is only limited by the reproducibility of that standard laser. Our chain is potentially capable of measuring optical frequencies from the visible to near IR range with an accuracy level of 10-12, which is presently limited by our reference accuracy.
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