https://hal.archives-ouvertes.fr/hal-00140068/document

On thermal effects in solid state lasers: the case of ytterbium-doped materials

http://digital.csic.es/bitstream/10261/161210/4/FinalManuscriptPSLIPSS.pdf

Influence of substrate and film thickness on polymer LIPSS formation

A review of theoretical and experimental studies of thermal effects in solid-state lasers is presented, with a special focus on diode-pumped ytterbium-doped materials. A large part of this review provides however general information applicable to any kind of solid-state laser. Our aim here is not to make a list of the techniques that have been used to minimize thermal effects, but instead to give an overview of the theoretical aspects underneath, and give a state-of-the-art of the tools at the disposal of the laser scientist to measure thermal effects. After a presentation of some general properties of Yb-doped materials, we address the issue of evaluating the temperature map in Yb-doped laser crystals, both theoretically and experimentally. This is the first step before studying the complex problem of thermal lensing (part III). We will focus on some newly discussed aspects, like the definition of the thermo-optic coefficient: we will highlight some misleading interpretations of thermal lensing experiments due to the use of the dn/dT parameter in a context where it is not relevant. Part IV will be devoted to a state-of-the-art of experimental techniques used to measure thermal lensing. Eventually, in part V, we will give some concrete examples in Yb-doped materials, where their peculiarities will be pointed out.

A Mach–Zehnder interferometric system for measuring the refractive indices of uniaxial crystals

Compared with conventional high-end optical systems, application of freeform optics offers many advantages. Their widespread use, however, is held back by the lack of a suitable measurement method.The NANOMEFOS project aims at realizing a universal freeform measurement machine to fill that void.The principle of operation of this machine requires a novel sensor for surface distance measurement, the development and realization of which is the objective of the work presented in this thesis. The sensor must enable non-contact, absolute distance measurement of surfaces with reflectivities from 3.5% to 99% over 5 mm range, with 1 nm resolution and a 2s measurement uncertainty of 10 nm for surfaces perpendicular to the measurement direction and 35 nm for surfaces with tilts up to 5°. To meet these requirements, a dual-stage design is proposed: a primary measurement system tracks the surface under test by translating its object lens, while the secondary measurement system measures the displacement of this object lens. After an assessment of various measurement principles through comparison of characteristics inherent to their principle of operation and the possibilities for adaptation, the differential confocal measurement has been selected as the primary measurement method. Interferometry is used as secondary measurement method. To allow for correction of tilt dependent error through calibration, a third measurement system has been added, which measures through which part of the aperture the light returns. An analytical model of the differential confocal measurement principle has been derived to enable optimization. To gain experience with differential confocal measurement, a demonstrator has been built, which has resulted in insights and design rules for prototype development. The models show satisfactory agreement with the experimental results generated using the demonstrator, thus building confidence that the models can be applied as design and optimization tools. Various properties that characterize the performance of a differential confocal measurement system have been identified. Their dependence on the design parameters has been studied through simulations based on the models. The results of this study are applied to optimize the sensor for use in NANOMEFOS. An optical system has been designed in which the interferometer and the differential confocal systems are integrated in a compact design. The optical path of the differential confocal system has been folded using prisms and mirrors so that it can be realized within the allotted volume envelope. For the same reason, many components are adapted from commercially available parts or are custom made. An optomechanical and mechatronic design has been made around the optical system. A custom focusing unit has been designed that comprises a guidance mechanism and actuator to enable tracking of the surface. To achieve a low measurement uncertainty, it aims at accurate motion, high bandwidth and low dissipation. The lateral position of the guidance reproduces within 20 nm and from the frequency response, it is expected that a control bandwidth of at least 800 Hz can be realized. Power dissipation depends on the form of the freeform surface and is a few mW for most expected trajectories. Partly custom electronics are used for signal processing, and to drive the laser and the focusing unit. Control strategies for interferometer nulling, focus locking and surface tracking have been developed, implemented and tested. Various tests have been performed on the system to evaluate the performance. Calibrations must be carried out to achieve the required measurement uncertainty. One calibration is based on a new method to measure tilt dependency of distance sensors. The sensor realized has 5 mm measurement range, -2.5 µm to 1.5 µm tracking range, sub-nanometer resolution, and a small-signal bandwidth of 150 kHz. Using the test results, the 2s measurement uncertainty after calibration is estimated to be 4.2 nm for measurement of rotationally symmetric surfaces, 21 nm for measurement of medium freeform surfaces and 34 nm for measurement of heavily freeform surfaces. To test the performance of the machine with the sensor integrated, measurements of a tilted flat have been carried out. In these measurements, a tilted flat serves as a reference freeform with known surface form. The measurements demonstrate the reduction of tilt dependent error using the new calibration method. A tilt robust, single point distance sensor with millimeter range and nanometer uncertainty has been developed, realized and tested. It is installed in the freeform measurement machine for which it has been developed and is currently used for the measurement of optical surfaces.

An optical distance sensor : tilt robust differential confocal measurement with mm range and nm uncertainty

We present and discuss a technique to measure concurrently the temperature dependence of quartz refractive indices in the thermal range 25–300° C. The magnitude of both indices is found to decrease slightly with increasing temperature in the thermal range considered.

Measurement of the temperature dependence of quartz refractive indices : a technique

We present and discuss a Fabry-Perot type interferometric method that permits the concurrent measurement of the thermo-optic coefficients of quartz retarders in the thermal range 25-300 degrees C. The magnitude of both the coefficients is found to increase slightly with increasing temperature.

https://iopscience.iop.org/article/10.1088/0963-9659/3/3/001/pdf

Interferometric method for concurrent measurement of the thermo-optic coefficients of quartz retarders

Heterodyne interferometric measurement of the thermooptic coefficients of quartz retardation plates

We present and discuss a technique to measure concurrently the temperature dependence of quartz refractive indices in the thermal range 25 degree(s)C - 300 degree(s)C. The magnitude of both indices is found to decrease slightly with increasing temperature in the thermal range considered.

Heterodyne interferometric measurement of the temperature coefficient of birefringence of quartz retardation plates

A heterodyne detection interferometer is described that can be efficiently used to control the state of polarization (SOP) and the phaseshift induced by polarizing optical devices. The basic equations are derived. The technique is used to measure the SOP of linear retarders.

https://iopscience.iop.org/article/10.1088/0963-9659/2/1/001/pdf

G. Carbonara

Papers

Measurement of the temperature dependence of quartz refractive indices : a technique

Interferometric method for concurrent measurement of the thermo-optic coefficients of quartz retarders

Heterodyne interferometric measurement of the thermooptic coefficients of quartz retardation plates

Heterodyne interferometric measurement of the temperature coefficient of birefringence of quartz retardation plates

Heterodyne interferometric characterization of polarizing optical devices