The NULLTIMATE project developed and realized three concepts of achromatic phase shifters for nulling interferometry.
One of the concepts is based on dispersive plates made of three materials which where fully
characterized regarding their refractive index and thermo-optic behavior between 100K and 330 K. The other
two concepts are based on mirror optics, one of which uses the phase shift of π when crossing a focus, the
other the reversal of electric fields at reflection. An optical bench has been set up to test and characterize these
phase shifters at wavelengths 2 − 2.4 μm with the option of changing to the 10 μm domain. We summarize the
development of the achromatic phase shifters and report on the current status of the test bench.
The temperature-dependent dispersion of the refractive index of optical materials is analyzed by introducing an
appropriate formulation of what we call their "normalized
thermo-optic coefficients" (NTOC). These parameters are
obtained experimentally by performing accurate interferometric measurements of both thermal expansion and changes in
optical thickness of parallelepipedic shaped samples of millimetric size which are submitted to a linear ramp of
temperature. Changes in optical thickness are recorded as function of temperature at a few discrete laser wavelengths;
they are expressed as power series of temperature and the use of a simple vectorial formalism allows then to determine
the thermal behavior of the dispersion law over the whole transparency range of the material. The validity and reliability
of the proposed method has been confirmed by modeling the temperature dependence of the type II second harmonic generation 1.064 μm - 0.532 μm in a KTiOPO4 single crystal, recorded up to 120°C.
The Darwin/TPF mission aims at detecting directly extra solar
planets. It is based on the nulling interferometry, concept proposed
by Bracewell in 1978, and developed since 1995 in several European and
American laboratories. One of the key optical devices for this
technique is the achromatic phase shifter (APS). This optical
component is designed to produce a π phase shift over the whole
Darwin spectral range (i.e. 6-18 μm), and will be experimentally
tested on the NULLTIMATE consortium nulling test bench (Labèque et
al). Three different concepts of APS are being simulated: dispersive plates focus crossing and field reversal. In this paper, we show how thermal, mechanical and optical models are merged into a single robust model, allowing a global numerical simulation of the optical component performances. We show how these simulations help us to optimizing the design and present results of the numerical model.
A proper implementation of electro-optic materials in laser systems requires an accurate knowledge of their electro-optic coefficients, along with their temperature dependence which could be of importance at high power level. A new technique has been developed for this purpose, which takes advantage of the thermodynamic equivalence of two
intensive parameters, namely the temperature and applied electric field. A suitably oriented parallelepipedic shaped sample is exposed to a laser beam and acts as a Fabry-Perot interferometer which is submitted to a linear ramp of temperature. The interference pattern is observed by reflection and the shift of interference fringes generated by the thermo-optic effect is detected through amplitude modulation of the light beam and recorded as a function of
temperature. We then switch from amplitude- to phase- modulation by applying a suitable electric field to the crystal: the signal features now the derivative of the thickness fringes generated by the electro-optic effect. The thermo- and electro-optic coefficients are obtained from the fringe shift recorded respectively through amplitude- and phase- modulated operating modes. The study of both KTiOPO4 and LiInS2 single crystals is given as an example to illustrate the so-called Fabry-Perot Thermal Scanning Interferometric (FPTSI) method.
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.
The thermo-optic coefficient (delta) n divided by (delta) T of CaF2, ZnS and Ge single crystals have been measured in the infrared from 20 degrees C to 100 degrees C. The laser interferometric method employed allows a determination of (delta) n divided by (delta) T with an accuracy close to 10-6K-1 in the case of nonabsorbing materials. For Ge the uncertainty is increased by a factor of 3 and is mainly due to its increasing absorption coefficient with temperature. The behavior of ZnS was examined at 1.06 micrometers and 10.6 micrometers laser radiations; CaF2 and Ge were investigated respectively at 1.06 micrometers and 10.6 micrometers.
The pyroelectric effect exhibited by two nonlinear optical materials is used to measure the residual absorption in their transparency window. The polarization behavior of potassium titanyl phosphate KTiOPO4 and thallium arsenic selenide Tl3AlSe3 are first described to determine the value of their pyroelectric coefficient. The pyrospectroscopic technique is then applied for absorption coefficient measurements at discrete laser wavelengths with a relative uncertainty estimated around 10 percent.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.