Name: Author Spotlight: Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&amp;#233;rot Etalon
Uploaded: 2023-02-03T15:00:00
Duration: 1 min 10 s
Description: Watch this Scientific Journal Video about Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-Pérot Etalon at JoVE.com

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&amp;#233;rot Etalon for Application in Trace-Gas Spectroscopy

<ol>
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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. -. 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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 border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<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, &#216;1.8 mm, 0.23 Pitch, 8&#176;, 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&#160;</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, &#177;5&#176;, 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, &#216;1.8 mm, 8&#176;, 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 &#216;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&#160;</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>&#216;1/2&#34; UVFS Broadband Precision Window, Uncoated, t = 3 mm</td>
			<td>Thorlabs</td>
			<td>WG40530</td>
			<td>Spacers</td>
		</tr>
		<tr>
			<td>&#216;1/2&#34; Broadband Dielectric Mirror, 1280 - 1600 nm</td>
			<td>Thorlabs</td>
			<td>BB05-E04</td>
			<td>Mirror</td>
		</tr>
		<tr>
			<td>&#216;1/2&#34; 70:30 (R:T) UVFS Plate Beamsplitter, Coating: 1.2 - 1.6 &#181;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.

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

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

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>

Watch this Scientific Journal Video about Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-Pérot Etalon at JoVE.com

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.

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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Etalon Assembly and Fiber-Etalon Alignment

To begin, place the 3D-printed cell on the table with the etalon pit facing upward. Insert an O-ring into the etalon pit and press it slightly into the designated groove. Place the beam splitter with the reflective surface facing upward onto the O-ring in the etalon pit.Using a tweezer, carefully place the two spacers onto the beam splitter to generate a clear aperture for the gas and excitation laser, which enters the air cavity via the through-hole running from one side of the cell to the other. Align the mirror on top of the spacers with the reflective side facing down so that the beam splitter, spacers, and mirror are aligned concentrically. Take the 3D-printed etalon cap and put the O-rings into the designated grooves.Align the cap to the rectangular groove of the cell, lift the cell, and apply pressure on the cap to fix the spacers in place while simultaneously inserting four M4 screws through the designated holes from the back side. Mount the screws with four M4 nuts on the front side and tighten them until the pressure from the cap is enough to hold the spacers in place and the O-rings are sufficiently compressed. For fiber-etalon alignment, mount the pigtailed ferrule and the GRIN lens system with the ferrule clamp and ensure that the translation stage in the Z direction is moved to its maximum height.Align the 3D-printed cell underneath this system, fixing its position at a height slightly below the GRIN lens, pointing directly to the center of the opening. Apply one or two drops of adhesive on the front end of the GRIN lens with a pipette. Lower down the translation stage in the Z direction until contact with the anti-reflection coated surface of the beam splitter is insured.Continue to lower the GRIN lens until sufficient pressure is applied and the springs are under enough tension. Turn on the modulated laser and the oscilloscope. Ensure the oscilloscope has the highest possible resolution when starting the alignment process.Then, set the time resolution so that the two to three periods of the modulation are visible. To start the alignment process, ensure that the GRIN lens points normally on the beam splitter surface. Step by step, deflect the first goniometric stage slightly and then move the other goniometric stage around the zero position.If no change is observed on the oscilloscope, deflect the first goniometric stage slightly more and repeat this iterative process until the triangular modulation becomes visible on the oscilloscope. Once a strong back-reflection is observed, adjust the oscilloscope's resolution and ensure the peak of the etalon's reflectance function sits centrally on the triangular modulation slopes. Tune the etalon's peak by changing the temperature of the laser until the peak is centered on the slope.With slight movements of the goniometric stages, try to maximize the peak strength while simultaneously maximizing the peak-to-peak ratio of the triangular modulation. When the alignment is finished, mount the UV lamp close to the GRIN lens mounted at a 45-degree angle. Next, cure the adhesive applied on the front end of the GRIN lens.After 5 to 10 minutes, turn off the UV lamp and apply more adhesive around the GRIN lens without touching it. Expose the adhesive to UV light for another five to 10 minutes. Repeat this step until the opening of the cell is completely filled with a homogenous layer of adhesive and perform the final cure for at least one hour.The representative image shows a good and worse alignment. The better the alignment, the higher the peak-to-peak ratio of the triangular modulation and the more the reflectance peak approaches zero.

Etalon Characterization

To evaluate the produced etalon, use the fiber optic setup and a measurement system capable of temperature-tuning the laser step-wise with a sufficient data-logging rate. To obtain measurements for calculating the theoretical free spectral range or FSR, perform a temperature sweep corresponding to at least two FSRs by increasing the temperature step-wise and letting the thermoelectric cooler settle for two to three seconds before measuring for another two to three seconds each time. After performing a temperature sweep corresponding to a wavelength sweep of the laser, process the measurement data with any numerical calculation program.Use any signal processing library with an integrated peak finder. The distance between two subsequent peaks represents FSR. Evaluate the width of the peak at its half height to calculate the full width at half maximum.Convert the temperature into wavelength by using the temperature-tuning coefficient of the laser. Calculate the full width at half maximum and FSR from the measurements. Finally, calculate the finesse of the fabricated Fabry-Perot etalons.This study resulted in the fabrication of Fabry-Perot etalons with a well-defined reflectance function. A comparison of the measured and calculated metrics of the fabricated Fabry-Perot etalons, etalon demonstrated that the measured finesse and full width at half maximum were comparable to the calculated values of ideal Fabry-Perot etalons. Photothermal interferometry measurements of water vapor and ambient air are shown here.The signal was extracted by means of a fast Fourier transform and compared to the background signal with the excitation laser turned off.