Fabrication and Testing of Photonic Thermometers

Photonic thermometry can help transition the field of temperature metrology from resistance-based legacy technology to frequency-based measurements. The main advantage of this technique is that we can leverage advances in frequency metrology to make more precise measurements whilst fabricating devices that overcome the physical limitations of current technology such as size and sensitivity to mechanical shock and environmental changes. The implication of this technique extend toward changing how we analyze in the United States and the world.

An ultra stable photonic thermometer will reduce the need for frequent calibrations. Future development of optomechanical temperature standards can put the calibration standard in the user hands eliminating the need for specialized calibration facilities. Generally, individuals new to this method will struggle because it is an intersection of two divergent fields both of whom have their own language, technology requirements and specialized protocols for maximizing outcome.

Any new user must learn to adapt and adjust to sometimes competing requirements. To begin, clean an SOI wafer as described in the accompanying text protocol. Coat the wafer with 20 to 50 milliliters MA-N 2405 negative tone photoresist.

Spin it at 4, 000 RPM for 60 seconds and then transfer the wafer to a hot plate and bake it at 90 degrees Celsius for 15 minutes. After baking, expose the photoresist to the pattern shown here using a standard E-beam lithography setup. Then place the wafer into MIF 319 developer solution and incubate it for 60 seconds.

Transfer the developed wafer into water and rinse it for an additional 60 seconds. Next, perform an ICP RIE etch of the 220 nanometer thick silicon layer to remove the unprotected silicon. Dissolve the resist mask in pure acetone for one hour, followed by an isopropanol rinse.

Then rinse the wafer for 60 seconds in deionized water and dry the wafer using nitrogen. Now place the wafer back into the spin coater. Deposit a one nanometer thick protective polymer film top layer on the wafer.

Finally, dice the wafer with a wafer dicing saw into 20 millimeter by 20 millimeter small and easy to handle chips. Place the photonic chip on the six-axis stage and orient the chip so that the on-chip input and output ports are aligned with the V groove array. Then turn on vacuum suction through on-stage integrated vacuum pumping port to hold the chip in place.

Use the top view digital camera to locate and place the photonic devices of interest in the center of the six-axis stage. Now position the V groove array holder arm close to the chip and use vacuum suction through an integrated pumping port to hold the array in place. Using the side view digital cameras as a visual feedback, position the fiber array above the on-chip gradient couplers and raise the stage to bring the photonic chip to within 10 micrometers of the fiber array's bottom edge.

The edge of the V groove fiber array should be roughly aligned within 50 to 100 micron accuracy relative to on-chip alignment marks. This procedure brings the optical fiber within a relative proximity of the corresponding gradient couplers. Once the chip has been roughly aligned, activate the automated search for the six-axis stage.

This algorithm searches for the maximum transmission of broadband light through the chip's input and output ports. It should take no longer than 20 to 30 seconds. Once the optimal alignment is achieved, check device viability before proceeding with bonding.

Using a program like LabVIEW, control the on-stage integrated module to thermally cycle the chip's temperature while recording the spectral response. Analyze the recorded the spectra from the laser spectrometer to verify the temperature sensitivity of the device. Slowly lower the array down to the chip's surface while viewing the side view digital camera.

Next, carefully position the epoxy-filled syringe in close vicinity to the fiber array's edge using another XYZ micron precision stage. Once in position, dispense a single micro droplet of epoxy. While the epoxy hardens, periodically run the automated alignment routine to prevent drift-induced loss of signal.

After the epoxy cures, test the photonic chip performance and light coupling efficiency by recording the transmission spectra at different temperatures. Light coupling efficiency typically increases after the bonding process. Use about one milligram of thermal grease to thermally couple the fiber-bonded photonic chip to a small copper cylinder.

Then gently lower the chip copper cylinder down a glass tube. Once in position, back fill the glass tube with argon gas and seal it using a rubber cork. Then place the packaged photonic thermometer into a metrology temperature dry well stable to within one micro Kelvin.

Using the custom-built computer program, set the settling time for 20 to 30 minutes, the number of thermal cycles to at least three, and the temperature step size to between one and five Celsius. Also, set the number of consecutive scans to at least five and the laser power in the nanowatt to microwatt range. As shown here, the ring resonator transmission spectra shows a narrow dip in transmission corresponding to the resonance condition at each temperature.

The resonance fringe shifts to longer wavelengths as temperature is increased from 20 Celsius to 105 Celsius in five degree Celsius increments. Our preliminary analysis of the thermal cycling experiment suggests that the humidity-induced changes in the epoxy are likely the largest driver of hysteresis in packaged photonic thermometers and that unpackaged devices do not show any significant hysteresis. Additionally, the hysteresis in the packaged device can be ameliorated by using hydrophobic epoxy as a desiccant to the glass tube before sealing a tighter seal around the rubber cork glass junction.

While attempting this procedure, it is important to remember to minimize any chemical contamination of the samples such as moisture in the packaged tube since it will severely degrade the measurement accuracy. Following this procedure, other methods like laser locking or dual frequency comb spectroscopy can be performed in order to answer additional questions about the long-term stability of these devices, impulse loading, and characterization of thermal physical properties. After its development, this technique has paved the way for researchers in the field of metrology to explore precision metrology for other physical and chemical quantities such as pressure, vacuum and trace gas analysis in embedded systems.

Don't forget that working with harsh chemicals in a clean room and bright light sources such as lasers in the lab can be extremely hazardous and precautions such as the use of personal protective equipments such as laser goggles should always be taken while performing this procedure.