In many high-tech systems, accuracy and predictability are key to the performance of the equipment. PhotonFirst has extensive experience in measuring strain, temperature, shape and pressure in high-tech equipment. Below a number of applications is described, to give you a first idea:
- High resolution strain measurement
- High resolution temperature measurement
- High dynamic strain range and -resolution with fiber laser sensor
- Multi-parameter sensing
- Shape reconstruction for structural deformation
High resolution strain measurements
ASML, the worlds largest and most advanced manufacturer of lithography equipment, requires high resolution strain sensing in their lithography machine in order to correct for nanometer vibration of the mechanics. A scanning narrow-linewidth laser, aligned to a pi-shift FBG, is inserted into an on-chip unbalanced Mach-Zehnder interferometer (MZI). The MZI functions as a wavelength tracker and can measure the optical power simultaneous; from this information a spectrum can be generated, and the FBG wavelength can be determined with high accuracy.
Even though the scanning laser is a narrow line-width laser, implying a low wavelength noise during modulation, the linewidth ‘noise’ can cause a white noise floor when looking into non-wavelength corrected algorithms. With the PhotonFirst interrogator these miniscule deviations can be measured and compensated for in the strain/temperature measurements. Strain values up to 0.2 nε can be measured, depicted by the power spectral density of the strain noise.
High resolution temperature measurements
ASML required high resolution (mK) temperature mapping within the lithography machine. As a research topic, ASML was investigating the feasibility of a single fiber alternative to 100 electrical wires to accommodate 50 NTCs in a small surface area. To achieve high resolution interrogation the complete assessment of every used component was required. Items considering FBG parameters (FWHM, reflectivity), SLD-noise, electronics noise, and others, enabled a well-educated estimate of the final performance of the system.
The single fiber proved itself a more than suitable alternative for NTCs:
- All 50 sensors can spectrally be resolved and therefore measured with 28 [kHz] sampling.
- .High resolution sensitivity was achieved: a FBG strain resolution (std) up to 25 ne with 28 [kHz] sampling. Due to the fabrication quality of the FBGs there is an envelope in the reflectivity, consequently the same form of envelope is seen in the signal-to-noise of the FBG central wavelength.
- The NTC reference measurements showed that the FBGs have very comparable temperature response curves. And even tend to have a shorter response time.
High dynamic strain range and -resolution with fiber laser sensor
High resolution strain measurements is relevant for high-tech applications like lithography machines, electron microscopes, hydrophone applications. Using a fiber laser as sensor enables highly sensitive (vibration) measurements. An active laser within a fiber is used as a sensor. The laser is a narrow linewidth FBG cavity within an erbium doped fiber. The erbium is stimulated with an external pump source and the cavity starts to lase on its central wavelength. When an external stimulation is present, like strain, the laser cavity changes accordingly. By measuring the output wavelength (shift) with the PhotonFirst interrogator the strain is measured with high precision. At PhotonFirst a noise floor of 0.46 nε 3σ with 200 kHz sampling over 0.01 sec was demonstrated, which is equivalent to 0.5 pε/√Hz. The noise performance of the fiber laser remains the same under applied strain of ~150 με and is expected to remain over larger strain ranges. The high frequency noise is mainly limited by the dark noise / shot noise of the electronics, and within the low frequencies the thermal behavior of the fiber-laser sensor is measured (1/f ). In other configurations the fiber laser sensor was stimulated with an acoustic signal.
A Fiber Bragg Grating (FBG) is a periodic modulation of the refractive index along a single mode fiber core. The periodicity results in reflection of light waves that match the periodic spacing in wavelength, while other wavelengths are transmitted unperturbed. Due to environmental changes (temperature, strain, pressure etc.) the refractive index and grating period are influenced which result in a wavelength shift of the reflected peak. Precise monitoring of the spectral peak positions can thus be used for sensing. In many applications it is desirable to distinguish between physical contributions to the Bragg shift, e.g. temperature and strain, or temperature and pressure. Different approaches can be taken towards separation, such as the use of an additional FBGs in strain-free condition for temperature correction. However, multiparameter sensing is also achievable in single FBG sensors, by use of polarization maintaining (PM) fibers. PM fibers have a stress-induced birefringence, resulting in splitting of a waveguide mode into two orthogonal waveguide modes. An FBG written in a PM fiber, allows separate interrogation of the FBG peak in the two polarization axes (fast and slow). The birefringence results into FBG peak splitting as the refractive index in both polarization differs by Δn. As a function of temperature or strain, the peaks shift differently, allowing for separation of variables.
Another version of multi-parameter sensing involves the separation between pressure and temperature. Instead of applying stress rods like in PM fibers, airy holes can be used in the fiber. As a function of pressure, a differential peak shift will occur, unlike with temperature.
Shape reconstruction for structural deformation
FBG sensing with integrated photonics can well be used for Structural Health Monitoring for various structures based on the temperature-compensated strain levels. By taking the advantage of the high sample rate of the PhotonFirst interrogator systems, with given structural information and the measured strain level, it becomes possible to capture, analysis, and reconstruct the deformed shape of the monitored structure in high-speed (52 μs/sample). PhotonFirst have been involved in many projects and experiments where stain measurements were used to obtain shape and vibration information in e.g. large buildings, airplane wings, helicopter blades, vibration on composite panels, etc. Based on the measured strain level and given structure information, we can apply Frenet-Serret formulas to reconstruct the shape of the deformed structure. In addition, by arranging FBG strain sensors as a Rosette strain gage, the bending and torsional deformation of the structure can be monitored simultaneously.
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