Laser facility such as the Megajoule Laser dedicated to laser-matter interaction including inertial fusion need pre-amplifier modules (PAM) which must respect a high beam quality. The current PAM use Nd:Phosphate material to work at 1053 nm with a repetition rate of 1shot/5min limited by a low thermal diffusion. However, it would be interesting to increase the shot rate for alignment or diagnostic purposes. Therefore, we propose to change this amplification material by crystal Nd:Lu:CaF2 with a thermal diffusion ten times higher in a new amplification architecture scheme in view of achieving a repetition rate of 10Hz. However, this material must fulfill the beam specifications to be integrated in the actual amplification chain. We report here a characterization of the thermal induced effects under a diode pump energy density of 13J/cm2. We begin by studying the spatially resolved induced birefringence with a cross polarizer-analyzer setup and then we measure the wave-front variations along two perpendicular polarizations. As the thermal elevation implies stress and then birefringence, we use an IR camera to study the surface thermal diffusion of the samples. Finally we reconstruct the stress pattern of our samples by simulating the global setup with COMSOL software which includes the thermal and mechanic Multiphysics interaction. This model allows us first to compare with experimental results and then to entirely simulate the mechanical behavior of this new material. These results obtained for the regenerative amplifier would enable us to study a new architecture scheme like disks multi-pass amplifier.
A review of our work on all-optical gas sensors is presented with an emphasis on the development of both new infrared (IR) sources and IR to visible converters. Many radicals spectroscopic signatures associated to gases of interest are in the 2.5 -15 μm spectral range (4000-350 cm-1). This spectral domain matches rare-earth ions emissions when embedded into chalcogenide glasses which are well- known for having low phonon energies. We present here results concerning the development of IR sources and IR to visible converters based on rare earth doped chalcogenide fibers. The development of all-optical gas sensors in the 3 to 5 µm spectral range is described showing IR signal conversion into visible light using specific excited state absorption mechanisms in rare earth doped materials. This wavelength conversion enables the use of silica fibers to transport the “gas” signal over large distances considerably increasing the scope of possible applications. An example of all-optical sensor using this photon conversion is presented in the case of CO2 detection. The implementation of this type of sensor for different gases such as methane is finally discussed. This all-optical sensor can be typically used over a kilometer range, with sensitivity around hundreds of ppm with cost effective detection heads, making this tool suitable for field operations. Finally, the photon conversion at the heart of this all-optical sensor is discussed as a general mean to detect infrared radiations avoiding the use of infrared detectors for a large span of applications.
Luminescence properties of Pr3+ and Dy3+ doped GaGeSbSe(S) vitreous systems have been studied. The synthesis process to obtain homogeneous glasses has been determined and fibers have been successfully drawn from the produced preforms and characterized. Fibers show a mid-IR luminescence matching with the CO2 absorption band at 4.3 μm and can be used in an environmental monitoring sensor for the CO2 underground storage. The luminescence and glasses properties have been investigated on bulk samples and fibers in order to improve the efficiency of an optical CO2 sensor prototype operating from high to low concentration, down to the ppm level.
Chalcogenide glass fibers are matchless devices to collect mi-infrared signal. Depending on the spectroscopic strategy, different kind of optical fibers have been developed during the past 10 years. The first fibers have been fabricated from selenide glasses to implement Fiber Evanescent Wave Spectroscopy (FEWS). It is an efficient way to collect optical spectra in situ, in real time and even, in the future, in vivo. Thanks to selenide glass fibers, it is possible to record such spectra on the mid-infrared range from 2 to 11 μm. This working window gives access to the fundamental vibration band of most of biological molecules and numerous multi-disciplinary works have been led in biology and medicine.
New glasses, only based on tellurium, have been recently developed, initially in the frame of the Darwin mission led by the European Space Agency (ESA). These glasses transmit light further toward the farinfrared and permit to reach the absorption band of CO2 located at 15 μm as requested by the ESA. Moreover, these telluride glass fiber are also very interesting for FEWS and medical application. Indeed, they give access to the mid-infrared signal of biomolecules beyond 11 μm, where classical selenide glass fibers are blind. Alternatively, in order to fight against global warning, some optical fibers have been developed for the monitoring of the CO2 stored into geological storage area underground. These fibers were doped with Dy3+ which emits a broad fluorescent band embedding the CO2 absorption band at 4.3 μm. thus, these fibers are used both to transmit light and as secondary sources in the mid-infrared.
To conclude, original microstructurated fibers have also been used for mid-infrared sensing. They exhibit a nice sensitivity compared to classical chalcogenide glass fibers.
Chalcogenide glasses are a matchless material as far as mid-infrared (IR) applications are concerned. They transmit light typically from 2 to 12 μm and even as far as 20 μm depending on their composition, and numerous glass compositions can be designed for optical fibers. One of the most promising applications of these fibers consists in implementing fiber evanescent wave spectroscopy, which enables detection of the mid-IR signature of most biomolecules. The principles of fiber evanescent wave spectroscopy are recalled together with the benefit of using selenide glass to carry out this spectroscopy. Then, two large-scale studies in recent years in medicine and food safety are exposed. To conclude, the future strategy is presented. It focuses on the development of rare earth-doped fibers used as mid-IR sources on one hand and tellurium-based glasses to shift the limit of detection toward longer wavelength on the other hand.
High optical quality rare-earth-doped LiYF4 (YLF) epitaxial layers were grown on pure YLF substrates by liquid phase epitaxy (LPE). Thulium, praseodymium and ytterbium YLF crystalline waveguides co-doped with gadolinium and/or lutetium were obtained. Spectroscopic and optical characterization of these rare-earth doped waveguides are reported. Internal propagation losses as low as 0.11 dB/cm were measured on the Tm:YLF waveguide and the overall spectroscopic characteristics of the epitaxial layers were found to be comparable to bulk crystals. Laser operation was achieved at 1.87 μm in the Tm3+ doped YLF planar waveguide with a very good efficiency of 76% with respect to the pump power. Lasing was also demonstrated in a Pr3+ doped YLF waveguide in the red and orange regions and in a Yb3+:YLF planar waveguide at 1020 nm and 994 nm.
We report on the study of praseodymium doped ZBLA channel waveguides obtained by ionic exchange. Fluorozirconate
glasses synthesis, photolithography process and ionic exchange are studied. Chemical characterizations are performed on
waveguides, as well as refractive index profile. Spectroscopic measurement and emission cross section calculation are
also reported. These results show the ability of rare earth doped fluoride glasses channel waveguide to serve as active
materials for integrated solid state laser sources.
The Nd3+-doped Silicon Rich Silicon Oxide (SRSO) layers were elaborated by reactive magnetron cosputtering.
We report on refractive index measurements of Nd3+-doped SRSO layers obtained by both m-lines method
and reflectance spectroscopy. From these measurements, the Si volume fraction and also the Nd3+-doped SRSO index
dispersion were deduced by using the Bruggeman model. At 1.06 μm, work wavelength, Nd3+-doped SRSO refractive
index was equal to 1.543 corresponding to a Si volume fraction of 6.5%.
Optical losses measurements were performed on these waveguides at different wavelengths and were about 0.3 dB/cm at
1.55 μm and 1 dB/cm at 1.06 μm. Measurements are confirmed by theoretical models showing that the losses are
essentially attributed to surface scattering.
Guided fluorescence by top pumping at 488 nm on planar waveguides was studied as a function of the distance
between the excitation area and the output of the waveguide and also as a function of the pump power. The guided
fluorescence at 945 and 1100 nm was observed until 4mm of the output of the waveguide and, of course, decreased when
the excitation area moved away from the output. The fluorescence intensity increased linearly for low pump power and
this linear increasing of the guided fluorescence of Nd3+ excited by a non resonant excitation at 488 nm confirms the
strong coupling between the Si- nanoparticles and rare earth ions.
Downconversion is investigated as a promising way to enhance silicon solar cells efficiency. The efficiency of the
downconversion process is investigated for the (Pr3+, Yb3+) codoping in two fluoride hosts: KY3F10 and CaF2. Strong
near-infrared emission from ytterbium ions after excitation of praseodymium ions at 440 nm is observed in both KY3F10
and CaF2 as a result of the efficient energy transfer from praseodymium to ytterbium. In particular, very high Pr3+ to Yb3+ energy transfer efficiencies (ETE) are achieved for low Yb3+ and Pr3+ concentrations (ETE=97% in CaF2:0.5%Pr3+-
1%Yb3+) in CaF2 in comparison with KY3F10. A low Yb3+ concentration offers the advantage to limit the Yb3+
concentration quenching which is observed in other hosts, where the Yb3+ concentration has to be larger to achieve a
high ETE for solar cell applications.
Many industrial and scientific applications need ultra-short and energetic pulses. Diode-pumped systems based on
ytterbium-doped crystals have a huge interest thanks to their good thermal and spectroscopic properties. Among them,
Yb:CaF2, shows very promising results for short pulse generation, and its long fluorescence lifetime, 2.4 ms, indicates a
high energy storage capacity.
We present a diode-pumped regenerative amplifier based on an Yb:CaF2 crystal optimized to produce short pulses for
various repetition rates ranging from 100 Hz to 10 kHz. The experiment is performed with a 2.6-% Yb doped 5-mm-long
CaF2 crystal grown by using the Bridgman technique and used at Brewster angle. To optimize the injection pulse
spectrum in terms of bandwidth and maximum gain, the seed pulses are generated by a broadband Yb:CALGO oscillator
centered at 1043 nm with a FWHM bandwidth of 15 nm at a repetition rate of 27 MHz. The pulses are then stretched to
260 ps with a transmission grating. The shortest pulse duration generated is 178-fs, and the corresponding energy is
1.4 mJ before compression (620 μJ after), at a repetition rate of 500 Hz for 16 W of pump power. The bandwidth is 10
nm centered at 1040 nm. At 10 kHz repetition rate, 1.4 W of average power before compression is obtained,
corresponding to an optical-optical efficiency of 10%. We also noticed that the pulse duration tends to increase above 1
kHz, reaching 400 fs at 10 kHz.
Many industrial and scientific applications need ultra-short pulses with high average power. Diode-pumped systems
based on ytterbium-doped crystals have a huge interest thanks to their good thermal and spectroscopic properties. Among
them, Yb:CALGO and Yb:CaF2, hold exceptional positions exhibiting a very atypical combination of ultrabroad
bandwidth and high thermal conductivity, therefore very promising for short pulse and high power applications.
In this paper we present an overview of the results obtained with these two crystals. First, we detail the origin of this
exceptional gathering of their broad emission bands and good thermal properties. Second, we present the results obtained
in femtosecond regime with these two crystals including a discussion on the actual limitations of Yb-doped ultrafast solid-state lasers.
Cryogenic cooling is a very interesting and promising apparatus for high power lasers, especially with Yb-doped
materials. In fact, it is now well known that operating this type of laser materials at cryogenic temperatures such as 77K
(liquid nitrogen temperature) positively affects their performance, especially at high power levels, because of increased
thermal conductivities and absorption and emission cross sections. We present a high-power diode-pumped Yb:CaF2 laser operating at cryogenic temperature (77 K). A laser output power of 97 W at 1034 nm was extracted for a pump
power of 245 W. The corresponding global extraction efficiency (versus absorbed pump power) is 65%. The laser small
signal gain was found equal to 3.1. The laser wavelength could be tuned between 990 and 1052 nm with peaks which
well correspond to the structure of the gain cross section spectra registered at low temperature.
We report on a diode-pumped regenerative amplifier based on Yb:CaF2 material, delivering pulses up to 1.8mJ pulse
energy at a repetition rate of 100Hz. The crystal is pumped at the zero line at 978 nm with a 10W continuous wave (CW)
fiber coupled laser diode. The pulses have a spectral bandwidth of 16nm centered at 1040 nm, which indicates a good
potential for millijoule range sub 100fs pulse duration after compression. It is also a good candidate for seeding higher
energy diode-pumped ytterbium lasers.
Due to remarkable properties of the chalcogenide glasses (Chgs), especially sulphide glasses, amorphous chalcogenide films should play a motivating role in the development of integrated planar optical circuits and their components. This paper describes the fabrication and properties of optical waveguides of undoped and erbium doped
sulphide films obtained by RF magnetron sputtering and laser ablation (PLD). The deposition parameters were adjusted to obtain, from sulphide glass targets with a careful control of their purity, layers with appropriate compositional, morphological, structural characteristics and optical properties. A transmission loss of 0.8 dB/cm can be obtained for rib waveguides produced by dry etching under CF4 plasma (4-300 μm wide, 5.5 μm film thickness, 1.5 μm etched thickness). The photo-luminescence of erbium doped Ge20Ga5Sb10S65 films were clearly observed in the n-IR and mid-IR spectral domain. The study of their decay lifetime with a well adapted annealing treatment controlling the roughness variation reached value of the bulk counterpart. Amplification tests were carried out leading to a complete characterisation of the Erbium doped waveguide. Gain on/off of 4.4 dB (3.4 dB/cm) were achieved for a signal at 1.54
μm in multiple modes sulphide:Er waveguides. The first demonstration of photoluminescence in mid-IR in an Er3+- doped Chg waveguide could potentially be employed to produce sources or amplifiers operating in the mid-IR.
The presentation will give the state of the art and the results of the last advances obtained in the growth, the
characterization and the implementation of three kinds of rare-earth doped halide laser crystals: Pr3+ doped fluorides,
Yb3+ doped CaF2 and its isotypes, and Er3+ and Pr3+ doped chloride and bromide KPb2Cl5 and Tl3PbBr5.
Due to remarkable properties of the chalcogenide glasses, especially sulphide glasses, amorphous chalcogenide films
should play a motivating role in the development of integrated planar optical circuits and their components. This paper
describes the fabrication and properties of optical waveguides of pure and rare earth doped sulphide glass films prepared
by two complementary techniques: RF magnetron sputtering and pulsed laser deposition (PLD). The deposition
parameters were adjusted to obtain, from sulphide glass targets with a careful control of their purity, layers with
appropriate compositional, morphological, structural characteristics and optical properties. These films have been
characterized by micro-Raman spectroscopy, atomic force microscopy (AFM), X-ray diffraction technique (XRD) and
scanning electron microscopy (SEM) coupled with energy dispersive X-ray measurements (EDX). Their optical
properties were measured thanks to m-lines prism coupling and near field methods. Rib waveguides were produced by
dry etching under CF4, CHF3 and SF6 atmosphere. The photo-luminescence of rare earth doped GeGaSbS films were
clearly observed in the n-IR spectral domain and the study of their decay lifetime will be presented. First tests were
carried out to functionalise the films with the aim of using them as sensor.
Results of diode-pumped cw and fs laser operation of an Yb3+:CaF2 single crystal are reported. With a 5-at.-% Yb3+-doped sample we obtained 5.8 W output power at 1053 nm, for 15 W of incident pump power at 980 nm. In passively mode-locked diode-pumped regime, using a Brewster-cut, 5-at.-% Yb3+-doped sample and prisms for dispersion compensation, the oscillator provided pulses as short as 150 fs, with 880 mW of average power and up to 1.4 W average output power, with pulse duration of 220 fs, centred at 1049 nm. The laser wavelength could be tuned from 1018 nm to 1072 nm in cw regime and from 1040 nm to 1053 nm in mode-locked regime. Using chirped mirrors for dispersion compensation, we obtained up to 1.74 W of average power, with pulse duration of 230 fs. For all these reasons, Yb:CaF2 crystal is showing the great potential as active medium for high average power femtosecond oscillators and as amplifier medium for femtosecond pulses.
We present here the first CW high power laser operation obtained under diode-pumping with an Yb3+:CaF2 crystal. This crystal exhibits good thermo-optical properties and can easily be grown in bulk crystals or in thin films. A maximum power of 5.8 W in a diffraction limited beam has been obtained with a 5% ytterbium-doped crystal of 4 mm-long. Moreover, the laser wavelength has been tuned over 54 nm, between 1018 and 1072 nm, and the double-pass small-signal gain has been measured to be more than 1.8, showing the great potential of Yb3+:CaF2 as a gain media for ultra-short pulses operation or as amplifier.
By using laser selective excitation and low temperature time-resolved spectroscopy techniques, we have been able to experimentally identified the ion centers of tetragonal, trigonal and cubic symmetries in a low concentrated crystal as 0.03%Yb3+:CaF2. This low temperature study was then completed by an analysis of the room temperature spectroscopic properties and of the laser potential of more concentrated Yb3+ doped CaF2 single crystals grown in our laboratory. A laser slope efficiency of 50% with respect to the absorbed 920 nm pump power was obtained, and the laser wavelength could be tuned between 1000 to 1060 nm.