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

In the scope of my PhD thesis at the Institute of Electrical Measurement and Sensor Systems, I'm working on evaluating the miniaturization potential of photothermal gas and aerosol sensors. A lot of effort is put into making photonic sensors either more robust and versatile or low cost and miniaturized, but it's quite challenging to combine these goals with necessary selectivity and sensitivity. The main advantage of our protocol stems from the easy accessibility of the method.

Without having the need for a clean room or special machines, our protocol offers the chance for cost-effective prototyping with standard photonic equipment. We hope that our protocol enables researchers fast and easy prototyping of multiple etalon configurations in a time and cost-effective way. We will continue working on miniaturizing photonic aerosol and gas sensors.

We will focus on new sensing principles by exploiting the interactions between structured light and structured matter. 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 backside. 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 the 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 oscilloscopes 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 5 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. To evaluate the produced etalon, use the fiber optic setup and a measurement system capable of temperature tuning the laser stepwise 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 stepwise 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 in 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.