A Silicon-tipped Fiber-optic Sensing Platform with High Resolution and Fast Response

This method produces an innovative, adaptable fiber-optic sensing platform. Development of the platform was originally driven by the des-ign to create an underwater thermometer for turbulence characterization of ocean waters. Advantages of this platform include high sensitivity, faster response and small size, along with the excellent manufacturability due to the use of the well-established MEMS fabrication techniques.

It can be used for many temperature-related measurements, such as temperature measurements for turbulence characterization, liquid and gas flow measurements, and the radiation from high-temperature plasma in some fusion. Fabricate the sensor on a bench with a spectrometer. The first step is to fabricate silicon pillars on a silicon wafer.

This wafer has stand-alone pillars ready for use in sensors. An overview of the pillars is in this schematic. They're patterned from a 200 micrometer thick, double side polished silicon wafer, using standard micro-electromechanical system fabrication methods.

Photoresist is on the tops of each pillar and the substrate. Prepare the lead-in fiber by stripping off the plastic coating of the optical fiber. Use a lens tissue dipped in alcohol to clean the stripped section.

Take the cleaned fiber to an optical fiber cleaver to cleave it. Next, acquire UV curable glue and a glass slide. Put a little drop of the UV curable glue on the glass slide.

Then, manually swing or spin-coat the slide to distribute the glue. The glue will be in a thin layer on the surface. Get the cleaved lead-in fiber and press its end face against the slide to transfer the glue.

Attach the opposite end of the fiber to a sensor interrogator for monitoring the reflection spectrum. Then, work with the silicon pillars and the fiber's cleaved end. Have the wafer with the pillars on a translation stage that moves in the horizontal plane.

Fix the fiber to a linear stage that moves vertically. Adjust the stages to align the fiber with one of the pillars, while using the real-time reflection spectrum as feedback. This reflection spectrum is an example of one that suggests the alignment is satisfactory.

Place the fiber in contact with the pillar to attach them, once the spectrum is satisfactory. Once the pillar and fiber are attached, cure the bond under a UV lamp. When curing is complete, lift the fiber with the vertical translation stage to detach it and the silicon pillar from the substrate.

Inspect the sensor head under a microscope to examine its geometry. This is a typical successfully fabricated sensor. Gather the materials to fabricate a high-finesse sensor.

This includes fragments from a double-side polished silicon wafer, with a sputtered gold layer on one side, seen as yellow. The other side has a high-reflectivity, dielectric mirror coating, seen as blue. Next, prepare the collimated lead in fiber by splicing a short section of graded-index multi-mode fiber with a single-mode fiber.

Cleave the multi-mode fiber. As depicted in this schematic, form a fiber collimator by cleaving the graded-index multi-mode fiber to be a quarter of the period of the light trajectory. Now, on a glass slide, place a small drop of UV curable glue.

After thinning the glue by manually swinging or spin-coating the slide, press the graded-index multi-mode fiber end against the slide to transfer glue. Connect the other end of the fiber to a sensor interrogator for monitoring the reflection spectrum. Next, position a fragment of the wafer on the horizontal translation stage.

Have the dielectric side pointing upward. Place the prepared fiber in the vertical translation stage and move it toward the fragment to attach the two pieces. Comparative with the low finesse fabric paired infrared merging sensors, fabrication of high finesse sensors, has a most stringent requirement as a optical alignment of the leading fiber with the silicon element.

Place the fiber and attached wafer fragments, under a UV lamp to cure. This is an example of the assembly after curing, when it is ready for the next steps. Before proceeding, polish the fragment into a disk-like shape.

Examine the sensor head under a microscope, to make sure it has the desired shape. Incorporate the completed low finesse device into a demodulation system. The system is straight forward, and involves only a few elements.

A spectrometer and a computer. This is the setup, in schematic form. There is a broadband source, with output, through an optical fiber.

The fiber goes to port one of an optical circulator. The optical fiber from port two of the circulator, is spliced to the lead-in fiber, of the low finesse sensor. Connect port three of the circulator, to a high speed spectrometer.

Use a computer connected to the spectrometer for data storage. Check the spectrum of the sensor, to make sure the system works properly. This spectrum is typical.

Prepare a demodulation system with the high finesse sensor. The setup is only slightly more complicated than the low finessed demodulation system. Despite this, the setup still involves only a few elements.

Use a distributed feedback laser, connected to a current controller. Connect the laser output via optical fiber to port one of an optical circulator The fiber from port two of the circulator, is spliced to the high finesse sensor. Connect port three of the optical circulator, to a photo detector.

Data from the photo detector, goes to a data acquisition device, and into a computer. Check the spectrum of the sensor, to make sure the system works properly, and yields a typical spectrum. A low finesse system sensor, designed to measure thermoclines in open water, collected the field testing data in blue.

The red and black curves are measurements made with reference instruments currently available on the market. A closer look at the data, suggests that a low finesse sensor system provides more detail. The data in red, are from a low finesse sensor setup, as a flow sensor situated in a water tank.

The data in black, are from a reference commercial flow sensor. The two generally agree. However when water is calm, the low finesse sensor exhibits a much clearer response.

A high finesse sensor is promising as a robust high resolution bolometer, for measuring photon emission in plasmas. These results compare the high finesse sensor, with a resistive bolometer. Keep in mind that a sensor made of base UV glue are not intended for applications above 100 degrees Celsius, due to the reduced stability of the epoxy at high temperatures.

Attaching the leading fiber and acidic im-pe-der with fusion splicing, may lead to a sensor platform of raising temperature about 1, 000 degrees Celsius;enabling other exciting applications in high temperature environments. Examples of high temperature applications include macro heaters, infrared emitters, and temperature monitoring in inches at the power plants. While using the UV lamp and the lasers, make sure you are wearing a lab coat and a laser safety goggles to protect your skin and eyes.