The work presented here was conducted in the framework of the FFG funded project &#34;Green Sensing&#34; and the NATO SPS program &#34;Photonic Nano Particle Sensors for Detecting CBRN events&#34;. The work was also supported by TU Graz Open Access Publishing Fund.

Fabrication of a Fabry-P&#233;rot Etalon for Application in Trace-Gas Spectroscopy

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J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Factors+affecting+the+performance+of+micromachined+sensors+based+on+Fabry-Perot+interferometry.">Factors affecting the performance of micromachined sensors based on Fabry-Perot interferometry.</a> Journal of Micromechanics and Microengineering. 15 (9), 1770-1776 (2005).</li><li>Rees, D., Fuller-Rowell, T. J., Lyons, A., Killeen, T. L., Hays, P. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stable+and+rugged+etalon+for+the+Dynamics+Explorer+Fabry-P&#233;rot+interferometer.+1:+Design+and+construction.">Stable and rugged etalon for the Dynamics Explorer Fabry-P&#233;rot interferometer. 1: Design and construction.</a> Applied Optics. 21 (21), 3896-3902 (1982).</li><li>Killeen, T. L., Hays, P. B., Kennedy, B. C., Rees, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stable+and+rugged+etalon+for+the+Dynamics+Explorer+Fabry-P&#233;rot+interferometer.+2:+Performance.">Stable and rugged etalon for the Dynamics Explorer Fabry-P&#233;rot interferometer. 2: Performance.</a> Applied Optics. 21 (21), 3903-3912 (1982).</li><li>Marques, D. M., Guggenheim, J. A., Munro, P. R. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysing+the+impact+of+non-parallelism+in+Fabry-P&#233;rot+etalons+through+optical+modelling.">Analysing the impact of non-parallelism in Fabry-P&#233;rot etalons through optical modelling.</a> Optics Express. 29 (14), 21603-21614 (2021).</li><li>Marques, D. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modelling+Fabry-P&#233;rot+etalons+illuminated+by+focussed+beams.">Modelling Fabry-P&#233;rot etalons illuminated by focussed beams.</a> Optics Express. 28 (5), 7691-7706 (2020).</li><li>Islam, M. R., Ali, M. M., Lai, M. -. H., Lim, K. -. S., Ahmad, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chronology+of+Fabry-Perot+interferometer+fiber-optic+sensors+and+their+applications:+A+review.">Chronology of Fabry-Perot interferometer fiber-optic sensors and their applications: A review.</a> MDPI Sensors. 14 (4), 7451-7488 (2014).</li><li>Preisser, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-optical+highly+sensitive+akinetic+sensor+for+ultrasound+detection+and+photoacoustic+imaging.">All-optical highly sensitive akinetic sensor for ultrasound detection and photoacoustic imaging.</a> Biomedical Optics Express. 7 (10), 4171-4186 (2016).</li><li>Chen, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Micro-fiber-optic+acoustic+sensor+based+on+high-Q+resonance+effect+using+Fabry-P&#233;rot+etalon.">Micro-fiber-optic acoustic sensor based on high-Q resonance effect using Fabry-P&#233;rot etalon.</a> Optics Express. 29 (11), 16447-16454 (2021).</li><li>Bialkowski, S. E., Astrath, N. G. C., Proskurnin, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Photothermal Spectroscopy Methods. , (2019).</li><li>Campillo, A. J., Petuchowski, S. J., Davis, C. C., Lin, H. -. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabry-P&#233;rot+photothermal+trace+detection.">Fabry-P&#233;rot photothermal trace detection.</a> Applied Physics Letters. 41 (4), 327-329 (1982).</li><li>Breitegger, P., Lang, B., Bergmann, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intensity+modulated+photothermal+measurements+of+NO2+with+a+compact+fiber-coupled+Fabry-P&#233;rot+interferometer.">Intensity modulated photothermal measurements of NO2 with a compact fiber-coupled Fabry-P&#233;rot interferometer.</a> MDPI.Sensors. 19 (15), 1424 (2019).</li><li>Waclawek, J. P., Kristament, C., Moser, H., Lendl, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Balanced-detection+interferometric+cavity-assisted+photothermal+spectroscopy.">Balanced-detection interferometric cavity-assisted photothermal spectroscopy.</a> Optics Express. 27 (9), 12183-12195 (2019).</li><li>Pevec, S., Donlagic, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Miniature+all-fiber+Fabry-P&#233;rot+sensor+for+simultaneous+measurement+of+pressure+and+temperature.">Miniature all-fiber Fabry-P&#233;rot sensor for simultaneous measurement of pressure and temperature.</a> Applied Optics. 51 (19), 4536-4541 (2012).</li><li>Dudzik, G., Krzempek, K., Abramski, K., Wysocki, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solid-state+laser+intra-cavity+photothermal+gas+sensor.">Solid-state laser intra-cavity photothermal gas sensor.</a> Sensors and Actuators B: Chemical. 328, 129072 (2021).</li><li>Chen, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Micro-fiber-optic+acoustic+sensor+based+on+high-Q+resonance+effect+using+Fabry-P&#233;rot+etalon.">Micro-fiber-optic acoustic sensor based on high-Q resonance effect using Fabry-P&#233;rot etalon.</a> Optics Express. 29 (11), 16447-16454 (2021).</li><li>H&#228;lg, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+silicon+pressure+sensor+with+a+low-cost+contactless+interferometric+optical+readout.">A silicon pressure sensor with a low-cost contactless interferometric optical readout.</a> Sensors and Actuators A: Physical. 30 (3), 225-230 (1992).</li><li>Fischer, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+microphone+hears+ultrasound.">Optical microphone hears ultrasound.</a> Nature Photonics. 10 (6), 356-358 (2016).</li><li>Waclawek, J. P., Bauer, V. C., Moser, H., Lendl, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=2f-wavelength+modulation+Fabry-P&#233;rot+photothermal+interferometry.">2f-wavelength modulation Fabry-P&#233;rot photothermal interferometry.</a> Optics Express. 24 (25), 28958-28967 (2016).</li><li>Chen, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Acoustic+performance+study+of+fiber-optic+acoustic+sensors+based+on+Fabry-P&#233;rot+etalons+with+different+Q+factors.">Acoustic performance study of fiber-optic acoustic sensors based on Fabry-P&#233;rot etalons with different Q factors.</a> MDPI Micromachines. 13 (1), 118 (2022).</li><li>Meyendorf, N., Ida, N., Singh, R., Vran, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Handbook of Nondestructive Evaluation 4.0. , (2022).</li><li>Kim, Y. J., Celliers, P. M., Eggert, J. H., Lazicki, A., Millot, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interferometric+measurements+of+refractive+index+and+dispersion+at+high+pressure.">Interferometric measurements of refractive index and dispersion at high pressure.</a> Scientific Reports. 11, 5610 (2021).</li><li>Stollberger, W. 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There are no conflicts of interest.

As the FPE fabricated following the protocol given here is optimized for a specific application, possible adaptations and critical steps are explained in this chapter. First of all, the FPE and the measurement cell are designed for PTI measurements. Therefore, a gas inlet and outlet, as well as a channel for the excitation laser, which is perpendicular to the probe laser, are added to the cell. All the openings of the cell are either made air-tight via O-rings and/or covered via UVFS windows to allow laser propagation. If used differently, the cell, as given in Supplementary Coding File 1, can be redesigned and adapted to the specific application. The threading in step 1.4 is done post printing. The threads could also be 3D-printed, but as these tend to wear out fast, only holes with the appropriate core hole diameter are printed, and these are threaded afterward.
The choice of material for the spacers in step 2.1 is crucial. The parallelism of the spacers determines the parallelism of the etalon mirrors and, hence, influences the finesse7. A &#189; inch UVFS precision window, as provided in the Table of Materials, with a parallelism of &#8804;5 arcsecs and a surface flatness of &#955;/10 over the clear aperture was used in this study. The coefficient of thermal expansion of UVFS is 0.55 x 10&#8722;6/&#176;C. The temperature stability can be further increased by using, for example, Zerodur5 spacers, with a coefficient of thermal expansion lower than&#160;0.1 x 10&#8722;6/&#176;C; however, this has the disadvantage of higher costs.
The FPE is formed by one fully reflecting mirror, as well as a beamsplitter. The beamsplitter has one 70% reflecting surface, as well as an anti-reflective-coated back side. This enables the coupling of the light in and out of the etalon. Additionally, the beamsplitter&#39;s substrate features one wedged side to prevent unwanted etalon effects. The back side of the mirror is roughened for the same reasons.
In step 5.1, the optoelectronic setup for tracking the alignment process is described. All the fibers used are standard SMF-28 fibers with FC/APC connectors. Due to the designated application for PTI, a balanced photodetector was readily available in this study, but this is not necessary in general. A conventional photodetector can be used instead; in this case, using a 1 x 2 coupler is obsolete. These changes do not affect the other components of the setup, as presented in Figure 5. The triangular current modulation of the probe laser, as described in step 5.4, corresponds to a wavelength sweep. A current range sufficient to sweep over at least one reflectance peak of the FPE has to be chosen. Therefore, one FSR can serve as a rule of thumb. Calculations for the FSR of an ideal FPE can be found in the introduction section. Together with the current tuning coefficient (nm/mA) of the laser, given in the respective manual, the current range covering one FSR can be calculated. As an example, the laser used in this work had a current tuning coefficient of 0.003 nm/mA and emitted at a wavelength of 1,550 nm. The expected FSR of an ideal FPE with 3 mm mirror spacing, d, is approximately 0.4 nm. This gives a current tuning range of 133 mA.
In this work, the modulation frequency was set to 100 Hz for convenient display at the oscilloscope. As the desired current tuning range is rather large, a fixed-fiber attenuator can be used to remain within the power limits of the used detector. The attenuator can be mounted directly after the isolator.
The UV-curing adhesive used in step 6 and step 7 is transparent to laser light and has a refractive index of 1.56. The alignment process, as described in step 7.1,&#160;is dependent on the available photodetector. The balanced detector used in this setup generates a negative voltage &#34;Signal&#34; output. For reasons of generality, a positive voltage output is assumed for the description of step 7.10 and in Figure 6. For a well-aligned etalon, the reflectance peak will go toward zero, while the triangular function will increase its peak-to-peak ratio.
For the etalon characterization in step 8.1, numerical calculation software is used (see Table of Materials). The measured voltage for each temperature step is averaged and plotted, as shown in Figure 7. To convert the temperature steps into wavelength steps, the temperature tuning coefficient of the probe laser is used. Signal analysis libraries have integrated peak-finding algorithms, which can be used for that purpose. As the data analysis strongly depends on the data format, no code is provided here, but it can be made available by the corresponding author upon request.
A possible limitation of the fabrication technique presented here is the thermal and mechanical stability in changing environments. As the scope of this instructional paper is the low-cost prototyping of FPEs for laboratory applications, no tests concerning mechanical and temperature stability are given here. If the FPE is used for mobile applications or in changing environments, additional measures have to be taken in order to mechanically stabilize the fiber-GRIN lens system relative to the etalon.
A new method to fabricate and characterize an FPE is demonstrated here with standard optical components available in every photonic laboratory. The presented FPE has a finesse of approximately 15 and a sensitivity sufficient for detecting approximately 5 ppmV of water vapor. Besides the presented application for PTI, this FPE could be used in applications such as building optical microphones20, which are commonly applied in the field of non-destructive testing23, refractive index measurements24,25, or hygrometers26, just to name a few.

In its most basic form, an FPE consists of two plane-parallel partially reflecting mirror surfaces1. In the following explanations, when referring to mirrors, the optical substrate and the reflective coating are addressed as one. In most applications, the mirrors used feature one wedged surface2 to prevent unwanted etalon effects. Figure 1 illustrates the formation of the interference pattern of an air-spaced etalon (Figure 1A), as well as the reflectance function for different mirror reflectivities (Figure 1B).
The light enters the cavity through one mirror, undergoes multiple reflections, and leaves the cavity by reflection as well as transmission. As this article focuses on the fabrication of an FPE operated in reflectance, the further explanations refer to reflection specifically. The waves leaving the cavity interfere, depending on the phase difference, q = 4&#960;nd/&#955;. Here, n is the refractive index inside the cavity, d is the mirror spacing, and &#955; is the wavelength of the interferometer&#39;s light source, here called the probe laser. A minimum reflectance occurs when the optical path difference matches the integer multiple of the wavelength, <img alt="Equation 2" src="/files/ftp_upload/65174/65174eq02.jpg" style="margin: auto;" />. The finesse of an ideal plane-parallel etalon is determined by the mirror reflectivities R1 and R2 only3:
<img alt="Equation 3" src="/files/ftp_upload/65174/65174eq03.jpg" style="margin: auto;" />
However, a real etalon is subject to many losses, which degrade the theoretically achievable finesse4,5,6. Deviation of the mirror parallelism7, non-normal incidence of the laser beam, beam shape8, mirror surface impurities, and scattering, among others, lead to a reduction in the finesse. The characteristic interference pattern can be described by the Airy function1:
<img alt="Equation 4" src="/files/ftp_upload/65174/65174eq04.jpg" style="margin: auto;" />
The full width at half maximum (FWHM), as well as the free spectral range (FSR) of the reflectance function, can be calculated as follows:
<img alt="Equation 5" src="/files/ftp_upload/65174/65174eq05.jpg" style="margin: auto;" />
<img alt="Equation 6" src="/files/ftp_upload/65174/65174eq06.jpg" style="margin: auto;" />
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/65174/65174fig01.jpg" /> 
Figure 1: Fabry-P&#233;rot interferometer theory. (A) A schematic depiction of the multi-beam interference for an air-spaced etalon with wedged windows. A plane wave, E0, enters the cavity under a certain angle, &#966;, through an anti-reflection (AR)-coated surface and subsequently undergoes multiple reflections between the highly reflecting (high R) surfaces spaced at a distance, d. With each reflection, part of the light is out-coupled of the etalon either in transmission or reflection, where it interferes with the other waves. (B) The reflectance function of an ideal Fabry-P&#233;rot etalon for different mirror reflectivities (y-axis). <a href="https://www.jove.com/files/ftp_upload/65174/65174fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
FPEs can be found in a wide range of applications9,10,11. In the case presented here, the FPE is used in a photothermal interferometry (PTI) setup. In PTI, small density and, hence, refractive index changes, induced by the periodic excitation followed by the fast thermalization of a target gas via a second laser, are measured interferometrically12. The amount of heat and, thus, the magnitude of the refractive index change are proportional to the gas concentration. When measuring the intensity of the reflectance function of the FPE at its steepest point (operation point), these refractive index changes shift the reflectance function, thereby altering the measured intensity. As the reflectance function can be assumed to be linear in the region around the operation point, the measured signal is then proportional to the gas concentration. The sensor&#39;s sensitivity is determined by the slope of the reflectance function and is, therefore, proportional to the finesse. PTI, in combination with FPEs, has proven to be a sensitive and selective method to detect trace amounts of gases and aerosols13,14,15,16,17,18. In the past, many sensors for pressure and acoustic measurements relied on the use of moveable parts, like membranes, substituting the second mirror of the FPE19. Deflections of the membrane lead to a change in the mirror distance and, thus, the optical path length. These instruments have the disadvantage of being prone to mechanical vibrations. In recent years, the development of optical microphones using solid FPEs has reached a commercial level20. By abstaining from the use of moveable parts, the measurand changed from distance to the refractive index inside the Fabry-P&#233;rot cavity, thus increasing the ruggedness of the sensors significantly.
Commercially available air-spaced FPEs cost beyond what is acceptable for prototyping and testing, as well as high-volume production instrument integration. Most scientific publications constructing and using such FPEs discuss the topic of fabrication only minimally21,22. In most cases, specific equipment and machines (e.g., clean rooms, coating facilities, etc.) are necessary; for example, for fully-fiber-integrated FPEs, special micromachining equipment is necessary. To reduce the manufacturing costs and enable the testing of multiple different FPE configurations to enhance their suitability for PTI setups, a new fabrication method was developed, which is described in detail in the following protocol. By using only commercially available, standard bulk-optic and telecom fiber-optic components, the manufacturing costs could be reduced to less than &#8364;400 euros. Every facility working with standard photonic equipment should be able to reproduce our fabrication scheme and adapt it to their applications.

<table><tbody><tr><td>Auto-Balanced Photoreceiver Nirvana</td><td>New Focus, Inc.</td><td>2017</td><td>Balanced Photodetector</td></tr><tr><td>Benchtop laser diode/TEC controller, 1A/96 W</td><td>Thorlabs</td><td>ITC4001</td><td></td></tr><tr><td>Butterfly laser diode mount</td><td>Thorlabs</td><td>LM14S2</td><td></td></tr><tr><td>Clamping fork</td><td>Thorlabs</td><td>CF175</td><td></td></tr><tr><td>compactRIO</td><td>National Instruments</td><td></td><td>For data aquisition</td></tr><tr><td>Dust remover</td><td>RS Components</td><td>168-1644</td><td></td></tr><tr><td>FC/APC to FC/APC Single L-Bracket Mating Sleeve</td><td>Thorlabs</td><td>ADAFCB3</td><td>Multiple needed</td></tr><tr><td>Fiber cleaning fluid</td><td>Thorlabs</td><td>RCS3</td><td></td></tr><tr><td>Fiber optic SM circulator</td><td>AFW technologies</td><td>CIR-3-15-L-1-2</td><td></td></tr><tr><td>Fiber optic SM coupler 1 x 2, 90/10</td><td>AFW technologies</td><td>FOBC-1-15-10-L-1-S-2</td><td>Only if balanced photodetector is used</td></tr><tr><td>Fiber optic SM isolator</td><td>AFW technologies</td><td>ISOD-15-L-1-2</td><td></td></tr><tr><td>Fiber optic storage reels</td><td>Thorlabs</td><td>FSR1</td><td>Multiple needed</td></tr><tr><td>Fixed fiber optic attenuator</td><td>Thorlabs</td><td>FA15T-APC</td><td>Different attenuation levels used</td></tr><tr><td>GRIN/Ferrule Sleeve, 1.818 mm Internal Diameter, 10 mm Length, Borosilicate Glass</td><td>Thorlabs</td><td>51-2800-1800</td><td>Fiber-GRIN-lens system</td></tr><tr><td>GRIN Lens, &amp;Oslash;1.8 mm, 0.23 Pitch, 8&amp;deg;, 1560 nm Design Wavelength, AR Coated: 1250 - 1650 nm</td><td>Thorlabs</td><td>GRIN2315A</td><td>Fiber-GRIN-lens system</td></tr><tr><td>Handheld UV-LED lamp</td><td>RS Components</td><td>220-6819</td><td>Lamp for curing the adhesive&amp;nbsp;</td></tr><tr><td>High precision stage and base</td><td>Newport</td><td>9062-X-M</td><td>Three nedded</td></tr><tr><td>Hose conector</td><td>RS Components</td><td></td><td>M5 threaded</td></tr><tr><td>Large Goniometer, 44.5 mm Distance to Point of Rotation, &amp;plusmn;5&amp;deg;, Metric</td><td>Thorlabs</td><td>GNL18/M</td><td>Two needed</td></tr><tr><td>L-Bracket Mating Sleeve</td><td>Thorlabs</td><td>ADAFCB3</td><td></td></tr><tr><td>Magnetic button clamps</td><td>Thorlabs</td><td>BM075</td><td>Multiple needed</td></tr><tr><td>Micrometer screw</td><td>Newport</td><td>9355</td><td>Three nedded</td></tr><tr><td>MIL-A-3920 Optical Adhesive with Resiliency, 1 oz.</td><td>Thorlabs</td><td>NOA61</td><td>UV-curing adhesive</td></tr><tr><td>Mounting Base, 50 mm x 75 mm x 10 mm</td><td>Thorlabs</td><td>BA2/M</td><td></td></tr><tr><td>O-Rings</td><td>Haberkorn</td><td></td><td>Sizes given in text</td></tr><tr><td>Passive component fiber tray</td><td>Thorlabs</td><td>BFCT</td><td>Multiple needed</td></tr><tr><td>Pedestal base adapter</td><td>Thorlabs</td><td>BE1</td><td></td></tr><tr><td>Pigtailed Ferrule, &amp;Oslash;1.8 mm, 8&amp;deg;, FC/APC, AR Coated: 1310/1550 nm</td><td>Thorlabs</td><td>SMPF0115-APC</td><td>Fiber-GRIN-lens system</td></tr><tr><td>Post holder</td><td>Thorlabs</td><td>PH30/M</td><td></td></tr><tr><td>Post-Mountable &amp;Oslash;2.5 mm Ferrule Clamp, M4 Tap</td><td>Thorlabs</td><td>FCM/M</td><td></td></tr><tr><td>Python</td><td>Python</td><td>3.9</td><td>Numerical data analysis software</td></tr><tr><td>Right-angled-bracket</td><td>Newport</td><td>9062-A-M</td><td></td></tr><tr><td>Self-centering lens mount</td><td>Thorlabs</td><td>SCL03</td><td></td></tr><tr><td>Silberschnitt 3001</td><td>Bohle&amp;nbsp;</td><td>3001</td><td>Glas cutter set</td></tr><tr><td>SM1-threaded standard cage plate</td><td>Thorlabs</td><td>CP33/M</td><td></td></tr><tr><td>UV-curing device</td><td>Formlabs</td><td>Form Cure</td><td></td></tr><tr><td>1550 nm 20 mW butterfly DFB laser diode</td><td>AeroDiode</td><td>1550LD-5-0-0-2</td><td></td></tr><tr><td>3D-printer</td><td>Formlabs</td><td>3+</td><td></td></tr><tr><td>&amp;Oslash;1/2&quot; UVFS Broadband Precision Window, Uncoated, t = 3 mm</td><td>Thorlabs</td><td>WG40530</td><td>Spacers</td></tr><tr><td>&amp;Oslash;1/2&quot; Broadband Dielectric Mirror, 1280 - 1600 nm</td><td>Thorlabs</td><td>BB05-E04</td><td>Mirror</td></tr><tr><td>&amp;Oslash;1/2&quot; 70:30 (R:T) UVFS Plate Beamsplitter, Coating: 1.2 - 1.6 &amp;micro;m, t = 3 mm</td><td>Thorlabs</td><td>BST06</td><td>Beamsplitter</td></tr></tbody></table>

fabrication of a low-cost, fiber-coupled, and air-spaced fabry-p&#233;rot etalon

Fabry-P&#233;rot etalons (FPE) have found their way into many applications. In fields such as spectroscopy, telecommunications, and astronomy, FPEs are used for their high sensitivity as well as their exceptional filtering capability. However, air-spaced etalons with high finesse are usually built by specialized facilities. Their production requires a clean room, special glass handling, and coating machinery, meaning commercially available FPEs are sold for a high price. In this article, a new and cost-effective method to fabricate fiber-coupled FPEs with standard photonic laboratory equipment is presented. The protocol should serve as a step-by-step guide for the construction and characterization of these FPEs. We hope this&#160;will enable researchers to conduct fast and cost-effective prototyping of FPEs for various fields of application. The FPE, as presented here, is used for spectroscopic applications. As shown in the representative results section via proof of principle measurements of water vapor in ambient air, this FPE has a finesse of 15, which is sufficient for the photothermal detection of trace concentrations of gases.

As can be seen in Figure 7, an FPE with a well-defined reflectance function could be fabricated.
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/65174/65174fig07.jpg" /> 
Figure 7: Measured reflectance function of the finished FPE. A temperature sweep, corresponding to a wavelength sweep of the laser, was performed to measure the reflectance function of the FPE. This is used to evaluate metrics like the full width at half maximum (FWHM) and the free spectral range (FSR) of the fabricated device. Relative reflectance refers to the relative proportion of light being back-reflected into the fiber after passing the FPE. <a href="https://www.jove.com/files/ftp_upload/65174/65174fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The measured metrics of the FPE are listed in Table 1 and compared to the calculated values of an ideal etalon with the same specifications. The formulas for an ideal FPE can be found in the introduction section.
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td></td>
			<td>Measured</td>
			<td>Ideal FPE</td>
		</tr>
		<tr>
			<td>Finesse</td>
			<td>12.8</td>
			<td>17.1</td>
		</tr>
		<tr>
			<td>FWHM</td>
			<td>0.0268 nm</td>
			<td>0.0234 nm</td>
		</tr>
		<tr>
			<td>FSR</td>
			<td>0.3441 nm</td>
			<td>0.4004 nm</td>
		</tr>
		<tr>
			<td>Sensitivity</td>
			<td>14 1/nm</td>
			<td>21 1/nm</td>
		</tr>
	</tbody>
</table>
Table 1: Comparison of the measured and calculated metrics of the fabricated FPE etalon.
To validate the aptitude for a designated application, the FPE is used for PTI measurements of water vapor in ambient air. Therefore, an excitation laser with a wavelength of 1,364 nm is guided into the cell perpendicularly to the probe laser. Both lasers intersect inside the FPE. The excitation laser is modulated sinusoidally with a frequency of 125 Hz. By stabilizing the probe laser on the steepest slope of the FPE, via constant current, the highest sensitivity of the sensor is achieved. For water vapor measurements, the cell is operated with open windows and exposed to ambient air with a concentration of 13,762 ppmV, as measured by a reference device (temperature = 21.4 &#176;C, pressure = 979.9 hPa, relative humidity = 52.2%). The signal is extracted by means of a fast Fourier transform (FFT) and compared to the background signal with the excitation laser turned off, as shown in Figure 8. A signal-to-noise ratio of more than 7,000 can be obtained, corresponding to a detection limit of approximately 5 ppmV (3&#963;).
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/65174/65174fig08.jpg" /> 
Figure 8: PTI measurements of water vapor in ambient air. In black, the FFT signal of a measurement with 125 Hz laser excitation is shown. In blue, the background signal without excitation is depicted. The inset shows the measured peak at 125 Hz in more detail. <a href="https://www.jove.com/files/ftp_upload/65174/65174fig08large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Supplementary Coding File 1: Measurement_cell.SLDPRT. CAD file for the measurement cell. The cell can be adapted to the requirements of the specific application and subsequently 3D-printed. <a href="https://www.jove.com/files/ftp_upload/65174/measurement_cell.zip" target="_blank">Please click here to download this File.</a>
Supplementary Coding File 2: cap_etalon.SLDPRT. CAD file for fixing the etalon inside the measurement cell. <a href="https://www.jove.com/files/ftp_upload/65174/cap_etalon.zip" target="_blank">Please click here to download this File.</a>
Supplementary Coding File 3: cap_window.SLDPRT. CAD file for fixing the laser windows onto the measurement cell. <a href="https://www.jove.com/files/ftp_upload/65174/cap_window.zip" target="_blank">Please click here to download this File.</a>

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Author Spotlight: Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&#233;rot Etalon  

This protocol describes the construction of a low-cost, discrete, fiber-coupled, and air-spaced Fabry-Perot etalon with various applications, such as in trace-gas spectroscopy. The fabrication is possible in any facility with standard optical laboratory equipment available.

Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&#233;rot Etalon

Fabry-P&#233;rot etalons (FPE) have found their way into many applications. In fields such as spectroscopy, telecommunications, and astronomy, FPEs are ...

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5.1K Views.  Technical University of Graz. This protocol describes the construction of a low-cost, discrete, fiber-coupled, and air-spaced Fabry-Perot etalon with various applications, such as in trace-gas spectroscopy. The fabrication is possible in any facility with standard optical laboratory equipment available.

Video: Author Spotlight: Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-Pérot Etalon  

Fabrication, Densification, and Replica Molding of 3D Carbon Nanotube Microstructures

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip devices, and their applications. In particular, capabilities for cost-effective fabrication of polymer microstructures were transformed by the advent of soft lithography and other micromolding techniques 1, 2, and this led a revolution in applications of microfabrication to biomedical engineering and biology. Nevertheless, it remains challenging to fabricate microstructures with well-defined nanoscale surface textures, and to fabricate arbitrary 3D shapes at the micro-scale. Robustness of master molds and maintenance of shape integrity is especially important to achieve high fidelity replication of complex structures and preserving their nanoscale surface texture. The combination of hierarchical textures, and heterogeneous shapes, is a profound challenge to existing microfabrication methods that largely rely upon top-down etching using fixed mask templates. On the other hand, the bottom-up synthesis of nanostructures such as nanotubes and nanowires can offer new capabilities to microfabrication, in particular by taking advantage of the collective self-organization of nanostructures, and local control of their growth behavior with respect to microfabricated patterns. 
Our goal is to introduce vertically aligned carbon nanotubes (CNTs), which we refer to as CNT "forests", as a new microfabrication material. We present details of a suite of related methods recently developed by our group: fabrication of CNT forest microstructures by thermal CVD from lithographically patterned catalyst thin films; self-directed elastocapillary densification of CNT microstructures; and replica molding of polymer microstructures using CNT composite master molds. In particular, our work shows that self-directed capillary densification ("capillary forming"), which is performed by condensation of a solvent onto the substrate with CNT microstructures, significantly increases the packing density of CNTs. This process enables directed transformation of vertical CNT microstructures into straight, inclined, and twisted shapes, which have robust mechanical properties exceeding those of typical microfabrication polymers. This in turn enables formation of nanocomposite CNT master molds by capillary-driven infiltration of polymers. The replica structures exhibit the anisotropic nanoscale texture of the aligned CNTs, and can have walls with sub-micron thickness and aspect ratios exceeding 50:1. Integration of CNT microstructures in fabrication offers further opportunity to exploit the electrical and thermal properties of CNTs, and diverse capabilities for chemical and biochemical functionalization 3.

We present methods for fabrication of patterned microstructures of vertically aligned carbon nanotubes (CNTs), and their use as master molds for production of polymer microstructures with organized nanoscale surface texture. The CNT forests are densified by condensation of solvent onto the substrate, which significantly increases their packing density and enables self-directed formation of 3D shapes.

The introduction of new materials and processes to microfabrication has, in large part, enabled many important advances in microsystems, lab-on-a-chip ...

Fabricating Metamaterials Using the Fiber Drawing Method

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves 2. This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks 3, negative refractive index materials 4, and lenses that resolve objects below the diffraction limit 5. However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size 6-7. Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response 8. We combine the stack-and-draw technique used to produce microstructured polymer optical fiber 9 with the Taylor-wire process 10, using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 &deg;C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.

Metamaterials at terahertz frequencies offer unique opportunities, but are challenging to fabricate in bulk. We adapt the fabrication procedure for microstructured polymer optical fibers to inexpensively fabricate metamaterials potentially on an industrial scale. We produce polymethylmethacrylate fibers containing ~10 &mu;m diameter indium wires separated by ~100 &mu;m, which exhibit a terahertz plasmonic response.

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe ...

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices 1-3. Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

We describe the use of pulsed laser deposition (PLD), photolithography and wire-bonding techniques to create micrometer scale complex oxides devices. The PLD is utilized to grow epitaxial thin films. Photolithography and wire-bonding techniques are introduced to create practical devices for measurement purposes.

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from ...

Fabrication and Characterization of Disordered Polymer Optical Fibers for Transverse Anderson Localization of Light

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed polymer optical fiber is composed of 80,000 strands of poly (methyl methacrylate) (PMMA) and polystyrene (PS) that are randomly mixed and drawn into a square cross section optical fiber with a side width of 250 &mu;m. Initially, each strand is 200 &mu;m in diameter and 8-inches long. During the mixing process of the original fiber strands, the fibers cross over each other; however, a large draw ratio guarantees that the refractive index profile is invariant along the length of the fiber for several tens of centimeters. The large refractive index difference of 0.1 between the disordered sites results in a small localized beam radius that is comparable to the beam radius of conventional optical fibers. The input light is launched from a standard single mode optical fiber using the butt-coupling method and the near-field output beam from the disordered fiber is imaged using a 40X objective and a CCD camera. The output beam diameter agrees well with the expected results from the numerical simulations. The disordered optical fiber presented in this work is the first device-level implementation of 2D Anderson localization, and can potentially be used for image transport and short-haul optical communication systems.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. This microstructured fiber can transport a small localized beam with a radius that is comparable to the beam radius of conventional optical fibers.

We develop and characterize a disordered polymer optical fiber that uses transverse Anderson localization as a novel waveguiding mechanism. The developed ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve the solar conversion efficiency of a concentrated photovoltaic system. The proposed system will also lower the solar cell cost in the concentrated photovoltaic system by replacing the expensive tandem solar cells with the cost-effective single junction solar cells. The structures and the parameters of high contrast gratings for the dispersive elements were numerically optimized. The large-area fabrication of high contrast gratings was experimentally demonstrated using nanoimprint lithography and dry etching. The quality of grating material and the performance of the fabricated device were both experimentally characterized. By analyzing the measurement results, the possible side effects from the fabrication processes are discussed and several methods that have the potential to improve the fabrication processes are proposed, which can help to increase the optical efficiency of the fabricated devices.

The fabrication of high contrast gratings as the parallel spectrum splitting dispersive element in a concentrated photovoltaic system is demonstrated. Fabrication processes including nanoimprint lithography, TiO2 sputtering and reactive ion etching are described. Reflectance measurement results are used to characterize the optical performance.

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

Fabrication of an Optical Cell Dryer for the Spectroscopic Analysis Cells

Optical cells, which are experimental instruments, are small, square tubes sealed on one side. A sample is placed in this tube, and a measurement is performed with a spectroscope. The materials used for optical cells generally include quartz glass or plastic, but expensive quartz glass is reused by removing substances, other than liquids, to be analyzed that adhere to the interior of the container. In such a case, the optical cells are washed with water or ethanol and dried. Then, the next sample is added and measured. Optical cells are dried naturally or with a manual hairdryer. However, drying takes time, which makes it one of the factors that increase the experiment time. In this study, the objective is to drastically reduce the drying time with a dedicated automatic dryer that can dry multiple optical cells at once. To realize this, a circuit was designed for a microcomputer, and the hardware using it was independently designed and manufactured.

A protocol for fabricating a device for simultaneously drying multiple optical cells is presented.

Optical cells, which are experimental instruments, are small, square tubes sealed on one side. A sample is placed in this tube, and a measurement is ...

Low-cost Custom Fabrication and Mode-locked Operation of an All-normal-dispersion Femtosecond Fiber Laser for Multiphoton Microscopy

A protocol is presented to build a custom low-cost yet high-performance femtosecond (fs) fiber laser. This all-normal-dispersion (ANDi) ytterbium-doped fiber laser is built completely using commercially available parts, including $8,000 in fiber optic and pump laser components, plus $4,800 in standard optical components and extra-cavity accessories. Researchers new to fiber optic device fabrication may also consider investing in basic fiber splicing and laser pulse characterization equipment (~$63,000). Important for optimal laser operation, methods to verify true versus apparent (partial or noise-like) mode-locked performance are presented. This system achieves 70 fs pulse duration with a center wavelength of approximately 1,070 nm and a pulse repetition rate of 31 MHz. This fiber laser exhibits the peak performance that may be obtained for an easily assembled fiber laser system, which makes this design ideal for research laboratories aiming to develop compact and portable fs laser technologies that enable new implementations of clinical multiphoton microscopy and fs surgery.

A method is presented to build a custom low-cost, mode-locked femtosecond fiber laser for potential applications in multiphoton microscopy, endoscopy, and photomedicine. This laser is built using commercially available parts and basic splicing techniques.

A protocol is presented to build a custom low-cost yet high-performance femtosecond (fs) fiber laser. This all-normal-dispersion (ANDi) ytterbium-doped ...

Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-Pérot Etalon

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