Monolithic solid-state lasers for spaceflight

Microchip lasers are the most compact (~1mm3) and the simplest diode pumped solid state lasers The mirrors of the optical cavity are directly deposited on the polished faces of a thin (-1mm) laser material. They are fabricated using collective fabrication processes allowing a low cost mass production. A microchip laser has a typical size of 1×1×0.5 mm3. It is very reliable and simple to use. The microchip laser is a kind of optical transformer which transforms a poor quality laser diode beam to a diffraction limited TEM00 and single frequency laser beam Moreover, by active or passive Q-switching , very short pulses (-0.4 to 2 ns) with very high peak power (0.5 to 50 kW) can be obtained. Standard wavelengths are: 1μm or 1.3μm with Nd doped materials such as YAG, YVO4, LMA, etc.; 1.5μm with Er and Yb co-doped phosphate glass; 2μm with Tm doped materials. The laser threshold is generally low, i.e as low as 9mW for 1.5μm microchip laser with a slope efficiency of 38%. Passively Q-switched Nd:YAG microchip laser can be fabricated using a novel process. A thin film (~30-50μm) of Cr4+:YAG, which is a saturable absorber at 1064nm, is epitaxially grown on 25mm diameter and 0.5 to 1mm thick Nd:YAG subtrates. The absorption coefficient of the epitaxial thin film saturable absorber is ~20-25 cm-1, about five times higher than the bulk Cr4+:YAG. The saturable absorption is adjusted by polishing the thin film. This process has the advantage to keep the monolithic structure of the microchip laser without any optical interface between the active and passive materials, and allows a great flexibility for the design of Q-switched lasers. Typical pulse energies are between 0.5 to 5μJ using low power diode pumps, and the pulse width is 0.4 to 2ns. Repetition rates as high 100kHz have been obtained. The pulse energy, width and peak power are independant of the pump power, which acts only on the pulse repetition rate which increases linearly with the diode pump power. The plane-parallel Fabry-Perot cavity of the microchip laser is optically stabilized by thermal effects due to the heating by the pump power. The laser mode is also defined by thermal effects. In order to avoid this problem, stable plano-concave optical cavities are fabricated using photolithography and ion beam etching technologies such as used currently in microelectronics. Small microlenses with 100-200μm in diameter and 1μm thick are directly fabricated with a collective process on laser materials such as Nd:YAG or Er,Yb:glass. The microlense acts as a concave micromirror for the microchip laser cavity, with a radius of curvature of few mm, so that the cavity is optically stable and the laser mode is well defined. The laser threshold is drastically reduced, i.e. less than 2mW incident pump power in CW mode Moreover, this technology allows also the fabrication of micro-optical components, such as microlenses on silica substrates, which can be used for the fabrication of optical microsystems including microchip lasers. For exemple such microlenses are used for the fiber coupling of a stable cavity microchip laser. The low cost, the re lability, the excellent beam quality and the high peak power of the microchip lasers are very useful for several industrial applications such as: time of flight range finding; micro-marking of materials; compact green laser for alignment; injection locking; etc.

The ability to perceive speech is related to the listener's ability to differentiate among frequencies (i.e., spectral resolution). Cochlear implant (CI) users exhibit variable speech-perception and spectral-resolution abilities, which can be attributed in part to the extent of electrode interactions at the periphery (i.e., spatial selectivity). However, electrophysiological measures of peripheral spatial selectivity have not been found to correlate with speech perception. The purpose of this study was to evaluate auditory processing at the periphery and cortex using both simple and spectrally complex stimuli to better understand the stages of neural processing underlying speech perception. The hypotheses were that (1) by more completely characterizing peripheral excitation patterns than in previous studies, significant correlations with measures of spectral selectivity and speech perception would be observed, (2) adding information about processing at a level central to the auditory nerve would account for additional variability in speech perception, and (3) responses elicited with spectrally complex stimuli would be more strongly correlated with speech perception than responses elicited with spectrally simple stimuli. Eleven adult CI users participated. Three experimental processor programs (MAPs) were created to vary the likelihood of electrode interactions within each participant. For each MAP, a subset of 7 of 22 intracochlear electrodes was activated: adjacent (MAP 1), every other (MAP 2), or every third (MAP 3). Peripheral spatial selectivity was assessed using the electrically evoked compound action potential (ECAP) to obtain channel-interaction functions for all activated electrodes (13 functions total). Central processing was assessed by eliciting the auditory change complex with both spatial (electrode pairs) and spectral (rippled noise) stimulus changes. Speech-perception measures included vowel discrimination and the Bamford-Kowal-Bench Speech-in-Noise test. Spatial and spectral selectivity and speech perception were expected to be poorest with MAP 1 (closest electrode spacing) and best with MAP 3 (widest electrode spacing). Relationships among the electrophysiological and speech-perception measures were evaluated using mixed-model and simple linear regression analyses. All electrophysiological measures were significantly correlated with each other and with speech scores for the mixed-model analysis, which takes into account multiple measures per person (i.e., experimental MAPs). The ECAP measures were the best predictor. In the simple linear regression analysis on MAP 3 data, only the cortical measures were significantly correlated with speech scores; spectral auditory change complex amplitude was the strongest predictor. The results suggest that both peripheral and central electrophysiological measures of spatial and spectral selectivity provide valuable information about speech perception. Clinically, it is often desirable to optimize performance for individual CI users. These results suggest that ECAP measures may be most useful for within-subject applications when multiple measures are performed to make decisions about processor options. They also suggest that if the goal is to compare performance across individuals based on a single measure, then processing central to the auditory nerve (specifically, cortical measures of discriminability) should be considered.

Strong spectral-mode-selection behavior has been observed in a three-mirror solid-state resonator that was constructed by colinearly aligning an aspheric feedback mirror with a standing-wave two-mirror resonator. This spectral mode selection is measured to be much stronger than that predicted by the frequency selection theory of simple Fabry-Perot cavities. Our study shows that the spatial modes and their associated eigenvalues vary at different oscillation wavelengths in a complex extended-cavity resonator. It is the interplay between spatial modes and spectral modes that induces the strong spectral-mode selection

We present a numerical calculation of the heat transport in a Bragg mirror configuration made of materials that do not obey Fourier’s law of heat conduction. The Bragg mirror is made of materials that are described by the Cattaneo-Vernotte equation. By analyzing the Cattaneo-Vernotte equation’s solutions, we define the thermal wave surface impedance to design highly reflective thermal Bragg mirrors. Even for mirrors with a few layers, very high reflectance is achieved (>90%). The Bragg mirror configuration is also a system that makes evident the wave-like nature of the solution of the Cattaneo-Vernotte equation by showing frequency pass-bands that are absent if the materials obey the usual Fourier’s law.

We report on the fabrication and investigation of pyramidal GaAs micro-cavities on top of a Bragg mirror. A finite-difference time-domain simulation supports the experimentally found optical mode structure for such a cavity shape.

The dependence of the parameters of the Gaussian mode in laser resonators on the properties of the medium in the cavity is studied. Experimental verification of the theoretical results is presented. It is found that the modes in a high-gain laser may differ widely from the usual free space resonator results. Also, resonator configurations which in free space are unstable may, with a suitable medium, support low-loss Gaussian modes.

The absorption cross-section of the Ho:YLF crystal is close to the emission cross-section of the Tm:YAG laser. According to the passive Q-switching theory, a giant laser pulse cannot be generated from the Ho:YLF Q-switched Tm:YAG laser system unless an internal focusing lens is utilized. In a previous work we experimentally demonstrated that passive Q-switching of the 2017-nm, flashlamp pumped Tm,Cr:YAG laser with a Ho:YLF saturable absorber could be obtained with an internal focusing lens. In this paper, we theoretically investigate the optical performance of the Ho:YLF Q-switched Tm:YAG laser system by solving the coupled rate equations. The simulation results indicate that the results obtained numerically are in good agreement with that obtained experimentally. Moreover, we study the passive Q-switching performance of the Ho:YLF Q-switched Tm:YAG laser system as functions of the reflectivity of the output coupler, the initial population of the saturable absorber ground state, the laser pumping rate, and the loss inside the laser resonator. On the other hand, assuming that a polarizer is utilized inside the laser cavity, we explore the passive Q-switching performance of the Ho:YLF Q-switched Tm:YAG laser system when the polarization of the laser light is along different direction between the two saturable absorber principal axes. Effect of the relative position between the saturable absorber and the output coupler is also investigated.

Procedure to estimate resonator losses and beam quality factors of unstable resonators with phase-unifying output couplers are derived. We discover that resonator losses have linear relations with the central values of reflection coefficient of the output couplers and the beam quality factors can be improved when phase-unifying designs are applied to the output couplers.

For achieving highest peak powers in a solid state laser (SSL) system, significant energy output and short pulses are necessary. For mode-locked lasers, it is well-known from the Fourier theorem that the largest gain bandwidths produce the narrowest pulse-widths; thus are transform limited. For an inhomogeneously broadened line width of a laser medium, if the intensity of pulses follow a Gaussian function, then the resulting mode-locked pulse will have a Gaussian shape with the emission bandwidth/pulse duration relationship of pulse ≥ 0.4402/c. Thus, for high peak power SSL systems, laser designers incorporate gain materials capable of broad emission bandwidths. Available energy outputs from a phosphate glass host doped with rare-earth ions are unparalleled. Unfortunately, the emission bandwidths achievable from glass based gain materials are typically many factors smaller when compared to the Ti:Sapphire crystal. In order to overcome this limitation, a hybrid "mixed" laser glass amplifier – OPCPA approach was developed. The Texas petawatt laser that is currently in operation at the University of Texas-Austin and producing high peak powers uses this hybrid architecture. In this mixed-glass laser design, a phosphate and a silicate glass is used in series to achieve a broader bandwidth required before compression. Though proven, this technology is still insufficient for the future compact petawatt and exawatt systems capable of producing high energies and shorter pulse durations. New glasses with bandwidths that are two and three times larger than what is now available from glass hosts is needed if there is to be an alternative to Ti:Sapphire for laser designers. In this paper, we present new materials that may meet the necessary characteristics and demonstrate the laser and emission characteristics these through the internal and external studies.

Graphene grown by thermal decomposition of a two-inch 6H silicon carbide (SiC) wafers surface was used to modulate a large energy pulse laser. Because of its saturable absorbing properties, graphene was used as a passive Q-switcher, and because of its high refractive index the SiC substrate was used as an output coupler. Together they formed a setup where the passively Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal laser was realized with the pulse energy of 159.2 nJ. Our results illustrate the feasibility of using graphene as an inexpensive Q-switcher for solid-state lasers and its promising applications in integrated optics.

: Current microscale semiconductor laser resonators may attain small modal volumes but require structures that are large compared to the wavelength, such as photonic crystals [11] or Bragg mirrors [12]. Metallic coatings provide stronger confinement of light and consequently higher device-packing density and therefore have been proposed for reducing the size of semiconductor nanowire lasers [13, 14]. The drawback of using metals, however, is their high dissipation losses at optical frequencies. Lasing in three-dimensional (3D) subwavelength metal-coated cavities operating at cryogenic temperatures to reduce the dissipation losses in the metal and increase the gain in the semiconductor has been reported [8]. We show that the losses in metal coated gain waveguides, as well as 3D laser resonators, can be significantly reduced by introducing a low index shield layer between the gain medium and the metal.

A rate equation analysis of the passive Q-switching process will be used to develop a design criterion that enables single-frequency, short pulse duration lasers to be built. The analysis will show that the difference in build-up time between any two longitudinal modes of the laser resonator should be comparable to or greater than the laser pulse duration to ensure single frequency operation. In most resonators with lengths <10cm, there is sufficient difference in the build-up times of competing modes to ensure single-frequency operation without the use of intracavity mode-selection elements. For example, a Nd:YAG laser with F/sub 2//sup -/:LiF saturable absorbers ranging in small-signal transmission from 0. 12 to 0.55 was observed to always be single-frequency when operated with a resonator length of 10cm and always be multimode with a 30-cm long resonator.

Ultrafast laser systems with pulse durations in the femto-and picosecond regime have proven their potential in several material processing applications. So far, a lot of these applications have been limited by the costly and difficult operation of the laser systems. We report on a new generation of powerful ultra-compact laser oscillators combining ’cavity - dumping’ with the proven techniques of direct diode-pumping and passive semiconductor saturable absorber mirror (SESAM) modelocking. Without using complex amplifier schemes these lasers generate femto-and picosecond short pulses with energies in the µJ range at tunable repetition rates up to 1 MHz. We demonstrate the use of the laser for fabrication of photonic devices in bulk glass. The combination of high pulse energies at high repetition rates together with a short pulse duration and good beam quality opens up new dimensions in high speed micro processing.Ultrafast laser systems with pulse durations in the femto-and picosecond regime have proven their potential in several material processing applications. So far, a lot of these applications have been limited by the costly and difficult operation of the laser systems. We report on a new generation of powerful ultra-compact laser oscillators combining ’cavity - dumping’ with the proven techniques of direct diode-pumping and passive semiconductor saturable absorber mirror (SESAM) modelocking. Without using complex amplifier schemes these lasers generate femto-and picosecond short pulses with energies in the µJ range at tunable repetition rates up to 1 MHz. We demonstrate the use of the laser for fabrication of photonic devices in bulk glass. The combination of high pulse energies at high repetition rates together with a short pulse duration and good beam quality opens up new dimensions in high speed micro processing.

Two technologies of all-silicon on-chip spectrometers based on the Fabry-Perot interferometer principle are studied and under development for a target wavelength of 158µm ([CII]). We are developing these spectroscopic capabilities with the objective of including them in polarimetric detector arrays cooled at 50mK. The first solution is a tunable cavity Fabry-Perot with silicon mirrors driven by cryogenic piezoelectric motors with a sub-micron step size. Each mirror is a dielectric Bragg structure made of quarter-wave layers of silicon and air providing a high reflectivity without metal losses. The theoretical performance of a Fabry-Perot resonator with such Bragg mirrors has been confirmed by measurement in a low temperature FTS: the finesse of this interferometer is more than twice that of a traditional Fabry-Perot. The second solution is a fixed Fabry-Perot array with a silicon microstructured cavity, which allows having different optical indices in different areas. The cavity is made of deep-etched silicon microstructures whose section is adapted to obtain the adequate optical index. Therefore, multiple wavelengths around 158µm, distributed on the array, are transmitted by this Fabry-Perot. The mirrors of this spectrometer are metallic capacitive grids designed to be highly reflective at the targeted wavelength, easy to manufacture with reduced metal losses. The simulations show high performances in resolution, close to the Bragg mirrors Fabry-Perot. The first prototypes of this solution have been manufactured by the CEA/LETI and will be soon measured in the cryogenic facilities in Saclay.

Replacing silver by aluminum in solar mirrors by improving solar reflectance with dielectric top layers