Name: a silicon-tipped fiber-optic sensing platform with high resolution and fast response
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Description: Watch this Scientific Journal Video about A Silicon-tipped Fiber-optic Sensing Platform with High Resolution and Fast Response at JoVE.com

This work was supported by U.S. Naval Research Laboratory (Nos. N0017315P0376, N0017315P3755); U.S. Office of Naval Research (Nos. N000141410139, N000141410456); U.S. Department of Energy (Nos. DE-SC0018273, DE-AC02-09CH11466, DE-AC05-00OR22725).

1. Fabrication of Low-Finesse Sensors
<ol>
	<li>Fabricate the silicon pillars. Pattern a piece of 200-&#181;m-thick double-side-polished (DSP) silicon wafer into standalone silicon pillars (Figure 2a), using standard micro-electro-mechanical system (MEMS) fabrication facilitates. 
	NOTE: The patterned wafer is bonded on another larger silicon wafer using a thin layer of photoresist. The bonding force of the photoresist is strong enough to hold the pillars upright, but also weak enough to detach from the substrate for later steps.</li>
	<li>Prepare the lead-in fiber. Strip off the plastic coating of the distal end of a single-mode optical fiber. Clean the stripped section using a lens tissue dipped with alcohol. Cleave the cleaned fiber using an optical fiber cleaver.</li>
	<li>Apply a thin layer of UV-curable glue on the end-face of the cleaved lead-in fiber (Figure 2b). Put a little drop of UV-curable glue on a piece of glass slide. Thin the glue layer by spin-coating or manually swinging the glass slide. Transfer the glue layer to the fiber end by pressing the end face of the lead-in fiber against the glass slide.</li>
	<li>Attach a silicon pillar to the fiber end. Align the lead-in fiber with one of the silicon pillars, meanwhile monitor the real-time reflection spectrum of the silicon FPI using a spectrometer. Use a UV lamp to cure the glue when a satisfactory spectrum is observed (Figure 2c). 
	NOTE: In general, the curing process takes around 10 to 15 minutes.</li>
	<li>Detach the sensor from the substrate. After the UV glue is fully cured, lift up the lead-in fiber along with the silicon pillar detached from the substrate (Figure 2d). 
	NOTE: Some residual photoresist is remained on the top surface of the silicon pillar (Figure 2e). For most cases, the photoresist residual does not affect the function of the sensor. If needed, the photoresist layer can be removed by alcohol.</li>
	<li>Examine the fabricated sensor head. Use a microscope to examine the geometry of the fabricated sensor head. A typical image of a sensor successfully fabricated is seen in Figure 2f.</li>
</ol>
2. Fabrication of High-Finesse Sensors
<ol>
	<li>Coat both sides of a silicon wafer with high-reflectivity mirrors. Coat one side of a 75-&#181;m-thick double-side-polished silicon wafer with a 150 nm thick gold layer using a sputtering coating machine, and coat the other side with a high-reflectivity (HR) dielectric mirror. 
	NOTE: The dielectric HR coating was done by an outside company; reflectivity of this coating was tested to be no less than 98% by the company. However, detailed materials and structure of the coating are unknown due to the proprietary protection by the company, see the Table of Materials for more information.</li>
	<li>Prepare the collimated lead-in fiber. Splice a short section of graded-index multi-mode fiber (GI-MMF) with a single-mode fiber, and then, under an optical microscope, cleave the GI-MMF with a quarter of the period of the light trajectory within the MMF left to form a fiber collimator (Figure 3a). 
	NOTE: The GI-MMF is used to expand the modal field diameter so that a spectrum with a better visibility can be obtained4,16. The length of the GI-MMF, which is around 250 &#181;m in this work, is exactly one quarter of the period of the ray trajectory.</li>
	<li>Attach a fragmented double-side coated silicon to the lead-in fiber. Assemble a high-finesse sensor by following the similar steps of attaching a silicon pillar to the fiber end for fabricating low-finesse sensors (steps 1.3 &#8211; 1.5). 
	NOTE: The side with the dielectric coating will be attached to the collimator to let in the coming light (Figure 3b, 3 c). In this case, the previous silicon pillar is replaced with a silicon fragment, which was not patterned. In the future, the patterned silicon wafer will be coated with the high-reflectivity mirrors, so that the sensors are more uniform and easier for fabrication. The difference in the fabrication steps of 1.3-1.5 is that a reflection spectra notch with proper visibility should be obtained first before the glue was transferred to the end face of the collimator.</li>
	<li>Polish the irregularly-shaped silicon fragment into a circular shape using a fiber polishing machine.</li>
	<li>Examine the fabricated sensor head. Use a microscope to examine the sensor head to make sure a desirable circular shape is achieved (Figure 3d).</li>
</ol>
3. Signal Demodulation for Low-Finesse Si-FOSP
NOTE: The system used for demodulating the low-finesse Si-FOSP is shown in Figure 4a. The following detailed steps help set up the system and perform the data processing.
<ol>
	<li>Connect a C-band broadband source to port 1 of an optical circulator.</li>
	<li>Splice port 2 of the optical circulator with the lead-in fiber of a low-finesse sensor.</li>
	<li>Connect port 3 of the optical circulator to a high-speed spectrometer which communicates with a computer for data storage.</li>
	<li>Check the spectrum of the sensor to make sure the system works properly. See the typical spectrum shown in Figure 4b.</li>
</ol>
4. Signal Demodulation for High-Finesse Si-FOSP
NOTE: The system used for demodulating the high-finesse Si-FOSP is shown in Figure 5a. The following detailed steps help set up the system and do the data post-processing.
<ol>
	<li>Sweep a tunable DFB laser using a current controller. 
	NOTE: The peak-to-peak sweeping voltage, which varies for different lasers and controllers, should be large enough to cover the spectrum notch.</li>
	<li>Connect the output of the tunable laser to port 1 of an optical circulator.</li>
	<li>Splice port 2 of the optical circulator to a high-finesse sensor.</li>
	<li>Connect port 3 of the optical circulator to a photodetector.</li>
	<li>Use a data acquisition device to read the output of the photodetector, which is stored by a computer.</li>
	<li>Check the spectrum of the sensor to make sure the system works properly. See a typical frame of spectrum shown in Figure 5b. Find the valley position using a polynomial curve fitting.</li>
</ol>

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An U.S. patent (No. 9995628 B1) has been issued to protect the related technologies.

The choice of the size (length and diameter) of the silicon FPI is made upon the tradeoff between requirements on the resolution and speed. In general, a smaller size provides a higher speed but also reduces the resolution2. A short length is advantageous for obtaining a higher speed, but it is not superior for obtaining a high resolution due to the expanded FWHM of the reflection notches. Using HR coatings to reduce the FWHM can help improve the resolution, but it will limit the dynamic range due to the signal demodulation using laser scanning. A smaller diameter increases the speed, but the diameter should be larger than the modal field diameter of the lead-in fiber so that a good spectrum can be achieved. It is, however, also found that a silicon diameter larger than that of the fiber helps improve the sensitivity for bolometry due to the reduced conduction heat loss to the fiber4.Therefore, the choice of the sensor size is highly dependent on the specific applications.
Although we only demonstrate the very basic structures, fabrication protocols, and signal demodulation systems for the Si-FOSP, there are various techniques that can fit it into other applications or further improve the performance. For example, instead of using UV-curable glue to attach the sensor, a fusion splicing technique can be applied to elevate the operation temperature above 1,000 &#176;C28.&#160;With such a high operation temperature, innovative types of photonic devices can be made, such as micro-heaters, infrared emitters, and bubble generators. Another example is the self-temperature compensated gas pressure sensing using the wavelength difference when the heating laser is turned on and off11. Furthermore, through the development of novel peak recognition techniques29,30, temperature measurement over extended dynamic range can be realized.

Fiber-optic sensors (FOSs) have been the focus for many researchers due to its unique properties, such as its small size, its low cost, its light weight, and its immunity to electromagnetic interference (EMI)1. These FOSs have found wide applications in many areas such as environmental monitoring, ocean surveillance, oil exploration, and industrial process among others. When it comes to the temperature-related sensing, the traditional FOSs are not superior in terms of resolution and speed for the cases where measurement of minute and fast temperature variations is desirable. These limitations stem from the optical and thermal properties of the fused silica material on which many traditional FOSs are based. On one hand, the thermo-optic coefficient (TOC) and thermal expansion coefficient (TEC) of silica are 1.28x10-5 RIU/&#176;C and 5.5x10-7 m/(m&#183;&#176;C), respectively; these values lead to a temperature sensitivity of only about 13 pm/&#176;C around the wavelength of 1550 nm. On the other hand, the thermal diffusivity, which is a measure of the speed of temperature change in response to thermal energy exchange, is only 1.4x10-6 m2/s for silica; this value is not superior for improving the speed of silica-based FOSs.
The fiber-optic sensing platform (FOSP) reported in this article breaks the above limitations of fused silica-based FOSs. The new FOSP utilizes crystalline silicon as the key sensing material, which forms a high-quality Fabry-Perot interferometer (FPI) on the end of the fiber, here referred to as silicon-tipped FOSP (Si-FOSP). Figure 1 shows the schematic and operational principle of the sensor head, which is the core of the Si-FOSP. The sensor head essentially consists of a silicon FPI, whose reflection spectrum features a series of periodic fringes. Destructive interference occurs when the OPL satisfies 2nL=N&#955;, where n and L are the refractive index and length of the silicon FP cavity, respectively, and N is an integer that is the order of the fringe notch. Therefore, positions of the interference fringes are responsive to the OPL of the silicon cavity. Depending on the specific applications, the silicon FPI can be made into two types: low-finesse FPI and high-finesse FPI. The low-finesse FPI has a low reflectivity for both ends of the silicon cavity, while the high-finesse FPI has a high reflectivity for both ends of the silicon cavity. The reflectivities of silicon-air and silicon-fiber interfaces are roughly 30% and 18%, thus the sole silicon FPI shown in Figure 1a is essentially a low-finesse FPI. By coating a thin high-reflectivity (HR) layer on both ends, a high-finesse silicon FPI is formed (Figure 1b). Reflectivity of the HR coating (either dielectric or gold) can be as high as 98%. For both types of Si-FOSP, both n and L increase when temperature increases. Thus, by monitoring the fringe shift, the temperature variation can be deduced. Note that for the same amount of wavelength shift, the high-finesse FPI gives a better discrimination due to the much narrower fringe notch (Figure 1c). While the high-finesse Si-FOSP has better resolution, the low-finesse Si-FOSP has a larger dynamic range. Therefore, the choice between these two versions depends on the requirements of a specific application. Furthermore, due to the large difference in full width at half maximum (FWHM) of the low-finesse and high-finesse silicon FPIs, their signal demodulation methods are different. For example, the theoretical FWHM of 1.5 nm is reduced by about 50 times to only 30 pm when both ends of the sole silicon FPI are coated with a 98% HR layer. Therefore, for the low-finesse Si-FOSP, a high-speed spectrometer would suffice for the data collection and processing, while a scanning laser should be used to demodulate the high-finesse Si-FOSP due to the much narrower FWHM that cannot be resolved well by the spectrometer. The two demodulation methods will be explained in the protocol.
The silicon material chosen here is superior for temperature sensing in terms of resolution. As a comparison, the TOC and TEC of silicon are 1.5x10-4 RIU/&#176;C and 2.55x10-6 m/(m&#8729;&#176;C), respectively, leading to a temperature sensitivity of around 84.6 pm/&#176;C which is about 6.5 times higher than that of all silica-based FOSs2. &#160;In addition to this much higher sensitivity, we have demonstrated an average wavelength tracking method to reduce the noise level and thus improve the resolution for a low-finesse sensor, leading to a temperature resolution of 6x10-4 &#176;C 2, in comparison to the resolution of 0.2 &#176;C for an all silica-based FOS3. The resolution is further improved to be 1.2x10-4 &#176;C for a high-finesse version4. &#160;The silicon material is also superior for sensing in terms of speed. As a comparison, the thermal diffusivity of silicon is 8.8x10-5 m2/s, which is more than 60 times higher than that of silica2. &#160;Combined with a small footprint (e.g., 80 &#181;m diameter, 200 &#181;m thickness), the response time of 0.51 ms for a silicon FOS has been demonstrated2, &#160;in comparison to the 16 ms of a micro-silica-fiber coupler tip temperature sensor5. &#160;Although some research work related to temperature measurement using very thin silicon film as the sensing material has been reported by other groups6,7,8,9, &#160;none of them possesses the performance of our sensors in terms of either resolution or speed. For example, the sensor with a resolution of only 0.12 &#176;C and a long response time of 1 s was reported.7 A better temperature resolution of 0.064 &#176;C has been reported10; &#160;however, the speed is limited by the relatively bulky sensor head. What makes the Si-FOSP unique lies in the new fabrication method and data processing algorithm.
Besides the above advantages for temperature sensing, the Si-FOSP can also be developed into a variety of temperature-related sensors aiming at measuring different parameters, such as gas pressure11, air or water flow12,13,14 , and radiation4,15. &#160;This article presents a detailed description of the sensor fabrication and signal demodulation protocols along with three representative applications and their results.

<table><tbody><tr><td>200 Proof Pure Ethanol</td><td>Koptec</td><td>V1001</td><td></td></tr><tr><td>5 Channels Duplex CWDM</td><td>Fiber Store</td><td>5MDD-ABS-FSCWDM</td><td></td></tr><tr><td>Butterfly Laser Diode Mounts</td><td>Tholabs</td><td>LM14S2</td><td></td></tr><tr><td>CastAway CTD</td><td>Yellow Springs Instrument</td><td></td><td></td></tr><tr><td>CTD</td><td>Seabird</td><td>SBE 19plus</td><td></td></tr><tr><td>Current Meter</td><td>Nortek</td><td>Vector</td><td></td></tr><tr><td>Data Acquisition Device</td><td>National Instruments</td><td>NIUSB4366</td><td></td></tr><tr><td>Digital Oscilloscope</td><td>RIGOL</td><td>DS1204B</td><td>200 MHz 2 GSa/s</td></tr><tr><td>Diode Laser</td><td>Thorlabs</td><td>LM9LP</td><td>Wavelength: 632 nm</td></tr><tr><td>Fixed BNC Terminator Kit</td><td>Thorlabs</td><td>FTK01</td><td></td></tr><tr><td>Function Waveform Generator&amp;nbsp;</td><td>RIGOL</td><td>DG4162</td><td>160 MHz 500 GSa/s</td></tr><tr><td>High Precision Cleaver</td><td>Fujikura</td><td>CT-32</td><td></td></tr><tr><td>High Reflection Dielectric Coating</td><td>Evaporated Coating INC (ECI)</td><td></td><td>Materials and structure of the coating are unknown</td></tr><tr><td>I-MON 512 Spectrometer</td><td>Ibsen Phtonics</td><td>P/N: 1257110</td><td></td></tr><tr><td>InGaAs Biased Detector</td><td>Tholabs</td><td>DET01CFC</td><td>FC/PC output:0-10V; Quantity: 2</td></tr><tr><td>Laser Diode</td><td>Qphotonic</td><td>QFLD-405-20S</td><td>Wavelength: 405 nm</td></tr><tr><td>Laser Diode Current Controller</td><td>Tholabs</td><td>LDC 210C</td><td>1 A and 100 mA range&amp;nbsp;</td></tr><tr><td>Laser Diode Temperature Controller</td><td>Tholabs</td><td>TEC 200C</td><td>Quantity: 2</td></tr><tr><td>Latex Examination Gloves</td><td>HCS</td><td></td><td></td></tr><tr><td>Micro Slides</td><td>Corning Incorporated</td><td></td><td></td></tr><tr><td>Narrow Linewidth DFB Laser</td><td>Eblana</td><td>EP1550-NLW-B06-100FM</td><td>Wavelength:1550 nm</td></tr><tr><td>Optical Fiber Fusion Splicer</td><td>Sumitomo electric industries, LTD</td><td>3822-2</td><td></td></tr><tr><td>Optical Microscope and Monitor</td><td>Ikegami Tsushinki Company</td><td>PM-127</td><td></td></tr><tr><td>Optical Spectrum Analyzer</td><td>Yokogawa</td><td>AQ6370C</td><td>wavelength range: 600-1700 nm</td></tr><tr><td>Polish Machine</td><td>ULTRA TEC</td><td>41076</td><td></td></tr><tr><td>Post-mountable Irises</td><td>Thorlabs</td><td></td><td>Quantity: 2</td></tr><tr><td>Pump Laser</td><td>Gooch and Housego</td><td>0400-0974-SM</td><td>Wavelength: 980 nm</td></tr><tr><td>Si Amplified Photodetector</td><td>Thorlabs</td><td>PDA36A</td><td>Wavelength: 350-1100 nm</td></tr><tr><td>Silicon wafer</td><td>University Wafer</td><td></td><td>thickness: 10 &amp;micro;m, 200 &amp;micro;m, 75 &amp;micro;m, 40 &amp;micro;m</td></tr><tr><td>Single mode fiber&amp;nbsp;</td><td>Corning</td><td>SMF-28</td><td></td></tr><tr><td>Single Mode Fused&amp;nbsp; Fiber Coupler</td><td>Thorlabs</td><td></td><td>Wavelength: 1550 nm</td></tr><tr><td>SM 125 interogrator</td><td>Micron Optics</td><td></td><td></td></tr><tr><td>Submersible Aquarium Pump</td><td>Songlong</td><td>SL-403</td><td></td></tr><tr><td>Superluminscent LED</td><td>Denselight Semiconductors</td><td>DL-BP1-1501A</td><td>wavelength range:1510-1590 nm</td></tr><tr><td>Syringe Pump</td><td>Cole Parmer</td><td>74905-02</td><td></td></tr><tr><td>Travel Translation Stage</td><td>Thorlabs</td><td>LT1</td><td></td></tr><tr><td>UV curable glue</td><td>Epoxy Technology</td><td>PB109077</td><td></td></tr><tr><td>UVGL-15 Compact UV Lmap</td><td>UVP</td><td>P/N:95-0017-09</td><td>254/365 nm</td></tr><tr><td>Variable Optical Attenuators</td><td>Tholabs</td><td>M-VA/00016951 P/N: VOA50-APC</td><td></td></tr></tbody></table>

a silicon-tipped fiber-optic sensing platform with high resolution and fast response

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This FOSP relies on a silicon Fabry-Perot interferometer (FPI) attached to the fiber end, referred to as Si-FOSP in this work. The Si-FOSP generates an interferogram determined by the optical path length (OPL) of the silicon cavity. Measurand alters the OPL and thus shifts the interferogram. Due to the unique optical and thermal properties of the silicon material, this Si-FOSP exhibits an advantageous performance in terms of sensitivity and speed. Furthermore, the mature silicon fabrication industry endows the Si-FOSP with excellent reproducibility and low cost toward practical applications. Depending on the specific applications, either a low-finesse or high-finesse version will be utilized, and two different data demodulation methods will be adopted accordingly. Detailed protocols for fabricating both versions of the Si-FOSP will be provided. Three representative applications and their according results will be shown. The first one is a prototype underwater thermometer for profiling the ocean thermoclines, the second one is a flow meter to measure flow speed in the ocean, and the last one is a bolometer used for monitoring exhaust radiation from magnetically confined high-temperature plasma.

Si-FOSP as an underwater thermometer for profiling ocean thermoclines 
Recent oceanographic research has demonstrated that the blurring of underwater imaging stems not only from turbidity in contaminated waters but also from temperature microstructures in clean ocean17,18. The latter effect has been the focus of many oceanographers, aiming to find an effective way to rectify the blurred images19, to better understand and improve optical communication in the water, as well as to develop means of quantifying turbulence in the ocean20,21. The Si-FOSP used as a temperature sensor has been demonstrated to outperform its current counterpart for measuring the swift temperature variations of water turbulence22. In this application, the low-finesse sensor shown in Figure 1a along with the signal demodulation system in Figure 4a is used. Given the superior performance of the Si-FOSP temperature sensor, it has been developed into a patented underwater instrument23 (Figure 6a), which is intended to characterize the thermoclines of open waters. This sub-section presents the results of a field testing (Figure 6b) on the Flint Creek Reservoir in Mississippi, USA.
Figure 6c shows a measured thermocline of the Flint Creek Reservoir on September 13th, 2016. The blue curve was obtained by the Si-FOSP temperature sensor, while the red and black curves were obtained by two reference commercial CTDs (oceanography instruments for measuring conductivity, temperature, and depth of seawater). Obviously, the Si-FOSP temperature sensor agrees with the reference sensors, but with more details of the temperature structures (see the inset of Figure 6c) that may give a bunch of extra information. The more informative data collected by the Si-FOSP temperature sensor is expected to impact many branches of oceanographic research.
Si-FOSP as a large-dynamic-range flow sensor 
Measurement of gas or liquid flows is pivotal to various academic and industrial sectors, which may provide important information to oceanography, weather research, process controls, transportation, and environment monitoring. Representative results of the Si-FOSP working as a flow sensor will be demonstrated. A low-finesse Si-FOSP is used for this application. However, since this flow sensor needs the sensing head to be actively heated by another laser, the system used is slightly different from that shown in Figure 4a. Specifically, an extra heating laser is used to activate the sensing head, and a detailed description of the system for flow measurement has been reported12,13,14.
Figure 7a shows the Si-FOSP flow sensor situated in a water tank, with a side-by-side comparison to a commercial flow sensor. Obviously, readout of the fiber sensor generally agrees with that of the commercial flow sensor, as shown in Figure 7b; however, the Si-FOSP flow sensor exhibits a much clearer response when the water flows calm down, as illustrated by the close-up view in Figure 7b.
Si-FOSP as an EMI-immune bolometer for high-temperature plasma physics 
Scientists investigating high-temperature plasma physics in tokamaks are trying to convert the exhaust power of magnetic confinement fusion reactors into photon emission to mitigate the heat flux impinged onto the plasma facing components24. Figure 8a shows the interior of a tokamak25. The photon emission is typically measured by a bolometer. While resistive and infrared video bolometers have achieved a noise equivalent power density (NEPD) of 0.2 W/m2 and 0.23 W/m2, respectively, in a laboratory environment26,27, they are vulnerable to the harsh environment associated with the high-temperature plasma. The Si-FOSP reported in this work stands out as a promising alternative to the existing bolometers. To obtain a resolution as high as possible, the high-finesse version shown in Figure 1b will be used. Also, slightly different from the single-channel demodulation system shown in Figure 5a, a two-channel system will be used to compensate for the drift of the laser by using another dummy reference4,15.
Figure 8b gives the measured results of one Si-FOSP bolometer in a laboratory environment, in comparison with another resistive bolometer. Our Si-FOSP bolometer has a NEPD of 0.27 W/m2 which is close to those of the electronic counterparts26,27. Noting that the Si-FOSP bolometer has inherent resistance to the EMI typically found in high-temperature plasma physics, it is expected to hold great promises toward practical applications in tokamaks.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59026/59026fig1.jpg" /> 
Figure 1: Schematics showing the low-finesse (a) and high-finesse (b) Si-FOSP. (c) Simulated reflection spectra of the two versions of Si-FOSPs with a 75 &#181;m thick silicon cavity. The minute shift of the spectrum (from solid to dashed curves) is much better discriminated by the high-finesse sensor. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59026/59026fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59026/59026fig2.jpg" /> 
Figure 2: Fabrication of low-finesse Si-FOSPs. (a)-(e) Schematic fabrication steps and (f) image of a fabricated sensor head compared with a human hair. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59026/59026fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59026/59026fig3.jpg" /> 
Figure 3: Fabrication of high-finesse Si-FOSPs. (a)-(c) Schematic fabrication steps and (d) image of one fabricated sensor. Inset in (d) shows the top view of the sensor head. GI-MMF, graded-index multi-mode fiber; HR, high-reflectivity. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59026/59026fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59026/59026fig4.jpg" /> 
Figure 4: (a) Schematic system of the demodulation system and (b) one typical frame of reflection spectrum for a low-finesse Si-FOSP. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59026/59026fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/59026/59026fig5.jpg" /> 
Figure 5: (a) Schematic system of the demodulation system and (b) one typical frame of scanned spectrum for a high-finesse Si-FOSP. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59026/59026fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/59026/59026fig6.jpg" /> 
Figure 6: Representative results as an underwater thermometer. (a) Image and (b) field deployment of the prototyped sensor instrument. (c) Measured thermocline of Flint Creek Reservoir, Mississippi, USA, on September 13th, 2016. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59026/59026fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/59026/59026fig7.jpg" /> 
Figure 7: Representative results as a flow sensor. (a) Image of the flow testing arrangement and (b) comparison between the measured flow field by the Si-FOSP and that of a commercial flow sensor. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59026/59026fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/59026/59026fig8.jpg" /> 
Figure 8: Representative results as a bolometer for high-temperature plasma research. (a) Image of the inner high-temperature plasmas space in a tokamak25 and (b) measured results in a laboratory environment. This figure is adopted and modified from Wikimedia Commons. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/59026/59026fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about A Silicon-tipped Fiber-optic Sensing Platform with High Resolution and Fast Response at JoVE.com

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

This work reports an innovative silicon-tipped fiber-optic sensing platform (Si-FOSP) for high-resolution and fast-response measurement of a variety of physical parameters, such as temperature, flow, and radiation. Applications of this Si-FOSP span from oceanographic research, mechanical industry, to fusion energy research.

In this article, we introduce an innovative and practically promising fiber-optic sensing platform (FOSP) that we proposed and demonstrated recently. This ...

a-silicon-tipped-fiber-optic-sensing-platform-with-high-resolution

Nanyang Technological University

Research

JoVE Journal

Engineering

7.0K Views.  Michigan State University. Deze methode produceert een innovatief, aanpasbaar glasvezelstensingplatform. De ontwikkeling van het platform werd oorspronkelijk gedreven door de des-ign om een onderwaterthermometer voor turbulentiekarakterisering van oceaanwateren tot stand te brengen. Voordelen van dit platform zijn hoge gevoeligheid, snellere respons en kleine omvang, samen met de uitstekende maakbaarheid als gevolg van het gebruik van de gevestigde MEMS fabricagetechnieken.Het kan worden gebruikt voor vele tempe...

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

High-resolution Fiber-optic Microendoscopy for in situ Cellular Imaging

Many biological and clinical studies require the longitudinal study and analysis of morphology and function with cellular level resolution. Traditionally, multiple experiments are run in parallel, with individual samples removed from the study at sequential time points for evaluation by light microscopy. Several intravital techniques have been developed, with confocal, multiphoton, and second harmonic microscopy all demonstrating their ability to be used for imaging in situ 1. With these systems, however, the required infrastructure is complex and expensive, involving scanning laser systems and complex light sources. Here we present a protocol for the design and assembly of a high-resolution microendoscope which can be built in a day using off-the-shelf components for under US$5,000. The platform offers flexibility in terms of image resolution, field-of-view, and operating wavelength, and we describe how these parameters can be easily modified to meet the specific needs of the end user.
We and others have explored the use of the high-resolution microendoscope (HRME) in in vitro cell culture 2-5, in excised 6 and living animal tissues 2,5, and in human tissues in vivo 2,7. Users have reported the use of several different fluorescent contrast agents, including proflavine 2-4, benzoporphyrin-derivative monoacid ring A (BPD-MA) 5, and fluoroscein 6,7, all of which have received full, or investigational approval from the FDA for use in human subjects. High-resolution microendoscopy, in the form described here, may appeal to a wide range of researchers working in the basic and clinical sciences. The technique offers an effective and economical approach which complements traditional benchtop microscopy, by enabling the user to perform high-resolution, longitudinal imaging in situ.

High-resolution Fiber-optic Microendoscopy for in situ Cellular Imaging

In many biological and clinical situations it is advantageous to study cellular processes as they evolve in their native microenvironment. Here we describe the assembly and use of a low-cost fiber-optic microscope which can provide real time imaging in cell culture, animal studies, and clinical patient studies.

Many biological and clinical studies require the longitudinal study and analysis of morphology and function with cellular level resolution.  ...

Bioengineering

Implementation of a Reference Interferometer for Nanodetection

A thermally and mechanically stabilized fiber interferometer suited for examining ultra-high quality factor microcavities is fashioned. After assessing its free spectral range (FSR), the module is put in parallel with a fiber taper-microcavity system and then calibrated through isolating and eliminating random shifts in the laser frequency (i.e. laser jitter noise). To realize the taper-microcavity junction and to maximize the optical power that is transferred to the resonator, a single-mode optical fiber waveguide is pulled. Solutions containing polystyrene nanobeads are then prepared and flown to the microcavity in order to demonstrate the system&#8217;s ability to sense binding to the surface of the microcavity. Data is post-processed via adaptive curve fitting, which allows for high-resolution measurements of the quality factor as well as the plotting of time-dependent parameters, such as resonant wavelength and split frequency shifts. By carefully inspecting steps in the time-domain response and shifting in the frequency-domain response, this instrument can quantify discrete binding events.

A reference interferometer technique, which is designed to remove undesirable laser jitter noise for nanodetection, is utilized for probing an ultra-high quality factor microcavity. Instructions for assembly, setup, and data acquisition are provided, alongside the measurement process for specifying the cavity quality factor.

A thermally and mechanically stabilized fiber interferometer suited for examining ultra-high quality factor microcavities is fashioned. After assessing ...

Fiber Optic Distributed Sensors for High-resolution Temperature Field Mapping

The reliability of computational fluid dynamics (CFD) codes is checked by comparing simulations with experimental data. A typical data set consists chiefly of velocity and temperature readings, both ideally having high spatial and temporal resolution to facilitate rigorous code validation. While high resolution velocity data is readily obtained through optical measurement techniques such as particle image velocimetry, it has proven difficult to obtain temperature data with similar resolution. Traditional sensors such as thermocouples cannot fill this role, but the recent development of distributed sensing based on Rayleigh scattering and swept-wave interferometry offers resolution suitable for CFD code validation work. Thousands of temperature measurements can be generated along a single thin optical fiber at hundreds of Hertz. Sensors function over large temperature ranges and within opaque fluids where optical techniques are unsuitable. But this type of sensor is sensitive to strain and humidity as well as temperature and so accuracy is affected by handling, vibration, and shifts in relative humidity. Such behavior is quite unlike traditional sensors and so unconventional installation and operating procedures are necessary to ensure accurate measurements. This paper demonstrates implementation of a Rayleigh scattering-type distributed temperature sensor in a thermal mixing experiment involving two air jets at 25 and 45 &#176;C. We present criteria to guide selection of optical fiber for the sensor and describe installation setup for a jet mixing experiment. We illustrate sensor baselining, which links readings to an absolute temperature standard, and discuss practical issues such as errors due to flow-induced vibration. This material can aid those interested in temperature measurements having high data density and bandwidth for fluid dynamics experiments and similar applications. We highlight pitfalls specific to these sensors for consideration in experiment design and operation.

We demonstrate use of a fiber optic distributed sensor for mapping the temperature field of mixing air jets. The Rayleigh scattering-based sensor generates thousands of data points along a single fiber to provide exceptional spatial resolution that is unattainable with traditional sensors such as thermocouples.

The reliability of computational fluid dynamics (CFD) codes is checked by comparing simulations with experimental data. A typical data set consists ...

Optical Detection of E. coli Bacteria by Mesoporous Silicon Biosensors

A label-free optical biosensor based on a nanostructured porous Si is designed for rapid capture and detection of Escherichia coli K12 bacteria, as a model microorganism. The biosensor relies on direct binding of the target bacteria cells onto its surface, while no pretreatment (e.g.&#160;by cell lysis) of the studied sample is required. A mesoporous Si thin film is used as the optical transducer element of the biosensor. Under white light illumination, the porous layer displays well-resolved Fabry-P&#233;rot fringe patterns in its reflectivity spectrum. Applying a fast Fourier transform (FFT) to reflectivity data results in a single peak. Changes in the intensity of the FFT peak are monitored. Thus, target bacteria capture onto the biosensor surface, through antibody-antigen interactions, induces measurable changes in the intensity of the FFT peaks, allowing for a &#39;real time&#39; observation of bacteria attachment.
The mesoporous Si film, fabricated by an electrochemical anodization process, is conjugated with monoclonal antibodies, specific to the target bacteria. The immobilization, immunoactivity and specificity of the antibodies are confirmed by fluorescent labeling experiments. Once the biosensor is exposed to the target bacteria, the cells are directly captured onto the antibody-modified porous Si surface. These specific capturing events result in intensity changes in the thin-film optical interference spectrum of the biosensor. We demonstrate that these biosensors can detect relatively low bacteria concentrations (detection limit of 104 cells/ml) in less than an hour.

Optical Detection of E. coli Bacteria by Mesoporous Silicon Biosensors

A label-free optical biosensor for rapid bacteria detection is introduced. The biosensor is based on a nanostructured porous Si, which is designed to directly capture the target bacteria cells onto its surface. We use monoclonal antibodies, immobilized onto the porous transducer, as the capture probes. Our studies demonstrate the applicability of these biosensors for the detection of low bacterial concentrations within minutes with no prior sample processing (such as cell lysis).

A label-free optical biosensor based on a nanostructured porous Si is designed for rapid capture and detection of Escherichia coli K12 bacteria, as a ...

Wideband Optical Detector of Ultrasound for Medical Imaging Applications

Optical sensors of ultrasound are a promising alternative to piezoelectric techniques, as has been recently demonstrated in the field of optoacoustic imaging. In medical applications, one of the major limitations of optical sensing technology is its susceptibility to environmental conditions, e.g. changes in pressure and temperature, which may saturate the detection. Additionally, the clinical environment often imposes stringent limits on the size and robustness of the sensor. In this work, the combination of pulse interferometry and fiber-based optical sensing is demonstrated for ultrasound detection. Pulse interferometry enables robust performance of the readout system in the presence of rapid variations in the environmental conditions, whereas the use of all-fiber technology leads to a mechanically flexible sensing element compatible with highly demanding medical applications such as intravascular imaging. In order to achieve a short sensor length, a pi-phase-shifted fiber Bragg grating is used, which acts as a resonator trapping light over an effective length of 350 &#181;m. To enable high bandwidth, the sensor is used for sideway detection of ultrasound, which is highly beneficial in circumferential imaging geometries such as intravascular imaging. An optoacoustic imaging setup is used to determine the response of the sensor for acoustic point sources at different positions.

Optical detection of ultrasound is impractical in many imaging scenarios because it often requires stable environmental conditions. We demonstrate an optical technique for ultrasound sensing in volatile environments with miniaturization and sensitivity levels appropriate for optoacoustic imaging in restrictive scenarios, e.g. intravascular applications.

Optical sensors of ultrasound are a promising alternative to piezoelectric techniques, as has been recently demonstrated in the field of optoacoustic ...

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit ...

A Random-displacement Measurement by Combining a Magnetic Scale and Two Fiber Bragg Gratings

Long-distance displacement measurements using optical fibers have always been a challenge in both basic research and industrial production. We developed and characterized a temperature-independent fiber Bragg grating (FBG)-based random-displacement sensor that adopts a magnetic scale as a novel transferring mechanism. By detecting shifts of two FBG center wavelengths, a full-range measurement can be obtained with a magnetic scale. For identification of the clockwise and counterclockwise rotation direction of the motor (in fact, the direction of movement of the object to be tested), there is a sinusoidal relationship between the displacement and the center wavelength shift of the FBG; as the anticlockwise rotation alternates, the center wavelength shift of the second FBG detector shows a leading phase difference of around 90&#176; (+90&#176;). As the clockwise rotation alternates, the center wavelength shift of the second FBG displays a lagging phase difference of around 90&#176; (-90&#176;). At the same time, the two FBG-based sensors are temperature independent. If there is some need for a remote monitor without any electromagnetic interference, this striking approach makes them a useful tool for determining the random displacement. This methodology is appropriate for industrial production. As the structure of the whole system is relatively simple, this displacement sensor can be used in commercial production. In addition to it being a displacement sensor, it can be used to measure other parameters, such as velocity and acceleration.

A protocol to create a full-range linear displacement sensor, combining two packaged fiber Bragg grating detectors with a magnetic scale, is presented.

Long-distance displacement measurements using optical fibers have always been a challenge in both basic research and industrial production. We developed ...

Rapid Repetition Rate Fluctuation Measurement of Soliton Crystals in a Microresonator

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the nonlinearity in a propagation Kerr medium. The development of dissipative Kerr solitons (DKSs) in high-Q microcavities drives a novel, compact, chip-scale soliton source. When DKSs serve as femtosecond pulses, the repetition rate fluctuation can be applied to ultrahigh precision metrology, high-speed optical sampling, and optical clocks, etc. In this paper, the rapid repetition rate fluctuation of soliton crystals (SCs), a special state of DKSs where particle-like solitons are tightly packed and fully occupy a resonator, is measured based on the well-known delayed self-heterodyne method. The SCs are generated using a thermal-controlled method. The pump is a frequency fixed laser with a linewidth of 100 Hz. The integral time in frequency fluctuation measurements is controlled by the length of the delay fiber. For a SC with a single vacancy, the repetition rate fluctuations are ~53.24 Hz within 10 &#181;s and ~509.32 Hz within 125 &#181;s, respectively.

Here, we present a protocol to generate soliton crystals in a butterfly-packaged micro-ring resonator using a thermal tuned method. Further, the repetition rate fluctuations of a soliton crystal with a single vacancy are measured using a delayed self-heterodyne method.

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the ...

Multicolor Fluorescence Detection for Droplet Microfluidics Using Optical Fibers

Fluorescence assays are the most common readouts used in droplet microfluidics due to their bright signals and fast time response. Applications such as multiplex assays, enzyme evolution, and molecular biology enhanced cell sorting require the detection of two or more colors of fluorescence. Standard multicolor detection systems that couple free space lasers to epifluorescence microscopes are bulky, expensive, and difficult to maintain. In this paper, we describe a scheme to perform multicolor detection by exciting discrete regions of a microfluidic channel with lasers coupled to optical fibers. Emitted light is collected by an optical fiber coupled to a single photodetector. Because the excitation occurs at different spatial locations, the identity of emitted light can be encoded as a temporal shift, eliminating the need for more complicated light filtering schemes. The system has been used to detect droplet populations containing four unique combinations of dyes and to detect sub-nanomolar concentrations of fluorescein.

Multicolor fluorescence detection in droplet microfluidics typically involves bulky and complex epifluorescence microscope-based detection systems. Here we describe a compact and modular multicolor detection scheme that utilizes an array of optical fibers to temporally encode multicolor data collected by a single photodetector.

Fluorescence assays are the most common readouts used in droplet microfluidics due to their bright signals and fast time response. Applications such as ...

Optimizing Brillouin Optical Time-Domain Analyzers Based on Gain Spectrum Engineering

Demonstrated is a unique method for sensing performance enhancement in Brillouin optical time-domain analyzers (BOTDA). A Brillouin gain spectrum (BGS) is superimposed with two symmetric Brillouin loss spectra (BLS). This leads to a complex engineered spectrum shape that is more resistant to the sensing system noise. Instead of only one pump and probe interaction as in the conventional BOTDA setup, three optical probe waves are exploited, with one probe located in the BGS and the other two symmetrically in the BLS. Due to the resistance and insensitivity of the engineered spectrum shape to the noise, the sensing performance is enhanced by 60% and the measurand resolution is doubled.

A protocol for Brillouin optical time-domain analyzers based on gain spectrum engineering is presented. Enhancements in the sensing performance, including sensing range and measurand resolution are achieved and the excess Brillouin intensity noise is studied. The protocol introduces a new way to enhance distributed Brillouin sensing performance.

Demonstrated is a unique method for sensing performance enhancement in Brillouin optical time-domain analyzers (BOTDA). A Brillouin gain spectrum (BGS) is ...

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

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