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.