Reliable temperature measurements of hot structures of re-entry vehicles is one of the main challenges associated with atmospheric heating. During re-entry, several minutes at hypersonic velocities results in severe aerothermal loads and a resultant temperature increase of more than 1000°C. In contrast to single-point measurements provided by thermocouples, optical fiber sensors allow temperature measurement at multiple positions along the fiber line. Key challenges of this technique include packaging, integration and extraction of the temperature contribution from a signal that is also influenced by strain effects.
MPBC developed optical fiber sensors for such temperatures with special packaging optimizing between protective capability and fast thermal conductivity. The fiber sensors were initially calibrated with thermocouples using a standard oven, then by means of a test in the DLR arc-heated wind tunnel L3K at a material temperature of 1000°C.
To well monitor the fast heat fluxes in reentry two special ruggedized interrogation modules (Interrogator) was developed with a data acquisition at 100 Hz and 3.5 kHz. The Interrogators have large memory capacity to save data during 1 hour, and a USB memory stick as back up. The Interrogators was validated for vacuum, thermal cycling and vibrations, being completely functional during the tests. The vibrations tests were successful even an accurate sweeping part the piezo of the tunable Fabry-Perot Interferometer was sweeping during the vibrations in X, Y and Z-axis. DLR integrated both 100 Hz and 3.5 kHz into the hypersonic flight experiment ATEK for measuring temperature distribution of the motor case of the second stage motor and hybrid module structure, respectively. The 3.5 KHz Interrogator was integrated in the hybrid module, which was part of the launcher block equipped with a parachute. The second stage performed the return flight without parachute and allowed testing the impact resistance of the new DLR’s data acquisition system and some measurement techniques.
The ATEK flight experiment was successfully launched on 13th July 2019 from the launch site Esrange in Kiruna. The second stage and the payload reached an apogee of approx. 240 km and continued the descent without any thrust and landed approx. 500 seconds after the take-off at a distance of approx. 67 km from the launch site. The Health Monitoring System allowed the measurement of aerothermal and mechanical loads on the hybrid payload structure and the motor case along the complete flight. Part of the data has been transmitted during flight to ground via telemetry at a low sampling rate of several Hertz. In addition, several impact-resistant data acquisition units could acquire the data at a high sampling rate of several Kilohertz and stored it onboard. The housing of interrogator and memory stick box was nearly completely undamaged. All four fiber optic connectors were still attached. MPB will recuperate and evaluate its functionality. The 100Hz Interrogator, without protection, impacted the ground with a velocity of about 95 m/s and was damaged due to the impact
Atmospheric reentry transition is produced at hypersonic velocity and is accompanied by a sharp excessive heat load for a few minutes, on the exposed materials, leading to a temperature increase of more than 1000°C. MPBC developed optical fiber sensors for such temperatures with special packaging optimizing between protective capability and fast thermal conductivity. The fiber sensors were calibrated with thermocouples first using standard oven, then with stationary plasma at Von Karman Institute (Belgium) followed by a test within a wind tunnel (1000°C, 8 Mach number) at DLR-Cologne.
Silicon nanocrystals (Si-nc) embedded in silica exhibit intense visible photoluminescence (PL) at room temperature.
However, under continuous wavelength (CW) laser excitation at 405 nm, the Si-nc PL intensity decreases with time,
approximately with two decay constants. The fast decay component is unchanged by repetitive laser exposures, it is
related to the local sample heating induced by the laser. The slower time constant corresponds to a permanent decrease
of the PL emission. This photodeterioration strongly affects the precision of optical gain measurements using VSL
(Variable Stripe Length) or P&P (Pump and Probe) techniques, hindering the development of Si-nc technology for
photonics applications. In this context, a procedure that would restore the PL intensity of Si-nc samples or minimize this
deterioration is highly desirable. UVC light (254 nm) irradiation of samples followed by an annealing at different
temperatures for 1 h under nitrogen flux increases the PL emission of Si-nc embedded in silica that have been previously
exposed to a CW laser pumping. Although this procedure does not prevent the decrease of the PL intensity associated
with the increase of sample temperature under CW pumping (the fast decay component), it contributes significantly to
reduce the permanent deterioration of the PL intensity. This procedure can also be applied to non-irradiated samples. The
PL emission collected from treated samples was studied as a function of laser irradiation time, and compared to that of
non-treated samples. The resistance to degradation of light-emitting silicon nanocrystals can be increased by UVC
irradiation followed by annealing at an optimal temperature of 400 °C under nitrogen environment. Following this
treatment, a reliable optical gain measurement can be performed once the local heating has been stabilized (the fast
decay component).
The variable stripe length (VSL) is a convenient method for the measurement of optical gain. However, several inherent
experimental constraints such as pump beam non-uniformity, diffraction from the movable cache and sample edges, and
gain saturation challenge its proper implementation. A modified VSL configuration, which addresses these constraints,
has been developed and implemented for gain measurements in SiO2 structures containing silicon nanocrystals. A
microprocessor based acquisition of several control parameters provides reliable and reproducible optical gain
measurements.
Recently, it has been shown that the photoluminescence (PL) spectrum emitted by silicon nanocrystals (Si-nc) can be modulated by means of light interference effects, when the Si-nc are produced by the implantation of Si ions in a SiO2 film grown on Si substrate (SiO2/Si). Optical interference must be considered for both the pump laser and the light emitted by the Silicon nanocrystals. In this study, strong variations of the PL spectrum intensity are observed as a function of the SiO2 thickness so that a PL intensity up to three times greater than the one recorded from Si-nc embedded in fused silica has been observed. A Fresnel equation solver [1, 2] has been developed and used to model the emission spectrum of Si-nc in these structures. This model determines the normalized depth profile of emitting centers using the measured luminescence spectra of a series of samples covering a range of SiO2 thicknesses, providing a powerful tool for the study of the Si-nc luminescence mechanism by comparing the shape of the emitter depth profile to those of Si-nc and implanted Si+ depth distributions.
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.