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...

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 border="0" cellpadding="0" cellspacing="0" style="width:1056px;" width="1055">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">200 Proof Pure Ethanol</td>
			<td style="width:213px;">Koptec</td>
			<td style="width:201px;">V1001</td>
			<td style="width:391px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">5 Channels Duplex CWDM</td>
			<td style="width:213px;">Fiber Store</td>
			<td style="width:201px;">5MDD-ABS-FSCWDM</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Butterfly Laser Diode Mounts</td>
			<td style="width:213px;">Tholabs</td>
			<td style="width:201px;">LM14S2</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">CastAway CTD</td>
			<td style="width:213px;">Yellow Springs Instrument</td>
			<td style="width:201px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">CTD</td>
			<td style="width:213px;">Seabird</td>
			<td style="width:201px;">SBE 19plus</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Current Meter</td>
			<td style="width:213px;">Nortek</td>
			<td style="width:201px;">Vector</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Data Acquisition Device</td>
			<td style="width:213px;">National Instruments</td>
			<td style="width:201px;">NIUSB4366</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Digital Oscilloscope</td>
			<td style="width:213px;">RIGOL</td>
			<td style="width:201px;">DS1204B</td>
			<td>200 MHz 2 GSa/s</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Diode Laser</td>
			<td style="width:213px;">Thorlabs</td>
			<td style="width:201px;">LM9LP</td>
			<td>Wavelength: 632 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Fixed BNC Terminator Kit</td>
			<td style="width:213px;">Thorlabs</td>
			<td style="width:201px;">FTK01</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Function Waveform Generator&#160;</td>
			<td style="width:213px;">RIGOL</td>
			<td style="width:201px;">DG4162</td>
			<td>160 MHz 500 GSa/s</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">High Precision Cleaver</td>
			<td style="width:213px;">Fujikura</td>
			<td style="width:201px;">CT-32</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">High Reflection Dielectric Coating</td>
			<td style="width:213px;">Evaporated Coating INC (ECI)</td>
			<td style="width:201px;"></td>
			<td>Materials and structure of the coating are unknown</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">I-MON 512 Spectrometer</td>
			<td style="width:213px;">Ibsen Phtonics</td>
			<td style="width:201px;">P/N: 1257110</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">InGaAs Biased Detector</td>
			<td style="width:213px;">Tholabs</td>
			<td style="width:201px;">DET01CFC</td>
			<td>FC/PC output:0-10V; Quantity: 2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:22px;width:251px;">Laser Diode</td>
			<td style="width:213px;">Qphotonic</td>
			<td style="width:201px;">QFLD-405-20S</td>
			<td>Wavelength: 405 nm</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:23px;width:251px;">Laser Diode Current Controller</td>
			<td style="width:213px;">Tholabs</td>
			<td style="width:201px;">LDC 210C</td>
			<td>1 A and 100 mA range&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Laser Diode Temperature Controller</td>
			<td style="width:213px;">Tholabs</td>
			<td style="width:201px;">TEC 200C</td>
			<td>Quantity: 2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Latex Examination Gloves</td>
			<td style="width:213px;">HCS</td>
			<td style="width:201px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Micro Slides</td>
			<td style="width:213px;">Corning Incorporated</td>
			<td style="width:201px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Narrow Linewidth DFB Laser</td>
			<td style="width:213px;">Eblana</td>
			<td style="width:201px;">EP1550-NLW-B06-100FM</td>
			<td>Wavelength:1550 nm</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:251px;">Optical Fiber Fusion Splicer</td>
			<td style="width:213px;">Sumitomo electric industries, LTD</td>
			<td style="width:201px;">3822-2</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Optical Microscope and Monitor</td>
			<td style="width:213px;">Ikegami Tsushinki Company</td>
			<td style="width:201px;">PM-127</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Optical Spectrum Analyzer</td>
			<td style="width:213px;">Yokogawa</td>
			<td style="width:201px;">AQ6370C</td>
			<td>wavelength range: 600-1700 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Polish Machine</td>
			<td style="width:213px;">ULTRA TEC</td>
			<td style="width:201px;">41076</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Post-mountable Irises</td>
			<td style="width:213px;">Thorlabs</td>
			<td style="width:201px;"></td>
			<td>Quantity: 2</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Pump Laser</td>
			<td style="width:213px;">Gooch and Housego</td>
			<td style="width:201px;">0400-0974-SM</td>
			<td>Wavelength: 980 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Si Amplified Photodetector</td>
			<td style="width:213px;">Thorlabs</td>
			<td style="width:201px;">PDA36A</td>
			<td>Wavelength: 350-1100 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Silicon wafer</td>
			<td style="width:213px;">University Wafer</td>
			<td style="width:201px;"></td>
			<td>thickness: 10 &#181;m, 200 &#181;m, 75 &#181;m, 40 &#181;m</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Single mode fiber&#160;</td>
			<td style="width:213px;">Corning</td>
			<td style="width:201px;">SMF-28</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:251px;">Single Mode Fused&#160; Fiber Coupler</td>
			<td style="width:213px;">Thorlabs</td>
			<td style="width:201px;"></td>
			<td>Wavelength: 1550 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">SM 125 interogrator</td>
			<td style="width:213px;">Micron Optics</td>
			<td style="width:201px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Submersible Aquarium Pump</td>
			<td style="width:213px;">Songlong</td>
			<td style="width:201px;">SL-403</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Superluminscent LED</td>
			<td style="width:213px;">Denselight Semiconductors</td>
			<td style="width:201px;">DL-BP1-1501A</td>
			<td>wavelength range:1510-1590 nm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Syringe Pump</td>
			<td style="width:213px;">Cole Parmer</td>
			<td style="width:201px;">74905-02</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">Travel Translation Stage</td>
			<td style="width:213px;">Thorlabs</td>
			<td style="width:201px;">LT1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">UV curable glue</td>
			<td style="width:213px;">Epoxy Technology</td>
			<td style="width:201px;">PB109077</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:251px;">UVGL-15 Compact UV Lmap</td>
			<td style="width:213px;">UVP</td>
			<td style="width:201px;">P/N:95-0017-09</td>
			<td>254/365 nm</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:251px;">Variable Optical Attenuators</td>
			<td style="width:213px;">Tholabs</td>
			<td style="width:201px;">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 images<sup class="xref...

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 ...

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Research

JoVE Journal

Engineering

7.0K Views.  Michigan State University. 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.

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

Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators 1,2. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials.
Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides 3, asymmetric split-ring resonators 4, and photonic band gap structures integrated into parallel-plate waveguides 5. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc.
The sensor design we use here is based on a simple parallel-plate waveguide 6,7. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE1) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove 6,8. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index 9.
Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves 10. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.

The procedure for implementing a refractive index sensor for terahertz frequencies based on a grooved parallel-plate waveguide geometry is described here. The method yields a measurement of the refractive index of a small volume of liquid through monitoring of the shift in the resonant frequency of the waveguide structure

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic ...

High-resolution, High-speed, Three-dimensional Video Imaging with Digital Fringe Projection Techniques

Digital fringe projection (DFP) techniques provide dense 3D measurements of dynamically changing surfaces.&#160;Like the human eyes and brain, DFP uses triangulation between matching points in two views of the same scene at different angles to compute depth.&#160;However, unlike a stereo-based method, DFP uses a digital video projector to replace one of the cameras1. The projector rapidly projects a known sinusoidal pattern onto the subject, and the surface of the subject distorts these patterns in the camera&#8217;s field of view. Three distorted patterns (fringe images) from the camera can be used to compute the depth using triangulation.
Unlike other 3D measurement methods, DFP techniques lead to systems that tend to be faster, lower in equipment cost, more flexible, and easier to develop.&#160;DFP systems can also achieve the same measurement resolution as the camera.&#160;For this reason, DFP and other digital structured light techniques have recently been the focus of intense research (as summarized in1-5). Taking advantage of DFP, the graphics processing unit, and optimized algorithms, we have developed a system capable of 30&#160;Hz 3D video data acquisition, reconstruction, and display for over 300,000 measurement points per frame6,7.&#160;Binary defocusing DFP methods can achieve even greater speeds8.
Diverse applications can benefit from DFP techniques. Our collaborators have used our systems for facial function analysis9, facial animation10, cardiac mechanics studies11, and fluid surface measurements, but many other potential applications exist.&#160;This video will teach the fundamentals of DFP techniques and illustrate the design and operation of a binary defocusing DFP system.

This video describes the fundamentals of digital fringe projection techniques, which provide dense 3D measurements of dynamically changing surfaces.&#160;It also demonstrates the design and operation of a high-speed binary defocusing system based on these techniques.

Digital fringe projection (DFP) techniques provide dense 3D measurements of dynamically changing surfaces.&#160;Like the human eyes and brain, DFP uses ...

Construction of a High Resolution Microscope with Conventional and Holographic Optical Trapping Capabilities

High resolution microscope systems with optical traps allow for precise manipulation of various refractive objects, such as dielectric beads 1 or cellular organelles 2,3, as well as for high spatial and temporal resolution readout of their position relative to the center of the trap. The system described herein has one such "traditional" trap operating at 980 nm. It additionally provides a second optical trapping system that uses a commercially available holographic package to simultaneously create and manipulate complex trapping patterns in the field of view of the microscope 4,5 at a wavelength of 1,064 nm. The combination of the two systems allows for the manipulation of multiple refractive objects at the same time while simultaneously conducting high speed and high resolution measurements of motion and force production at nanometer and piconewton scale.

The system described herein employs a traditional optical trap as well as an independent holographic optical trapping line, capable of creating and manipulating multiple traps. This allows for the creation of complex geometric arrangements of refractive particles while also permitting simultaneous high-speed, high-resolution measurements of the activity of biological enzymes.

High resolution microscope systems with optical traps allow for precise manipulation of various refractive objects, such as dielectric beads 1 or cellular ...

Synthesis and Microdiffraction at Extreme Pressures and Temperatures

High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary interiors, design materials with novel properties, understand the mechanical behavior of materials exposed to very high stresses as in explosions or impacts. Synthesis and structural analysis of materials at extreme conditions of pressure and temperature entails remarkable technical challenges. In the laser heated diamond anvil cell (LH-DAC), very high pressure is generated between the tips of two opposing diamond anvils forced against each other; focused infrared laser beams, shined through the diamonds, allow to reach very high temperatures on samples absorbing the laser radiation. When the LH-DAC is installed in a synchrotron beamline that provides extremely brilliant x-ray radiation, the structure of materials under extreme conditions can be probed in situ. LH-DAC samples, although very small, can show highly variable grain size, phase and chemical composition. In order to obtain the high resolution structural analysis and the most comprehensive characterization of a sample, we collect diffraction data in 2D grids and combine powder, single crystal and multigrain diffraction techniques. Representative results obtained in the synthesis of a new iron oxide, Fe4O5 1 will be shown.

The laser heated diamond anvil cell combined with synchrotron micro-diffraction techniques allows researchers to explore the nature and properties of new phases of matter at extreme pressure and temperature (PT) conditions. Heterogeneous samples can be characterized in situ under high pressure by 2D mapping and combined powder, single-crystal and multigrain diffraction approaches.

High pressure compounds and polymorphs are investigated for a broad range of purposes such as determine structures and processes of deep planetary ...

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we demonstrate an optical spectroscopy method that is capable of resolving spectral features beyond that of the spin fine structure and homogeneous linewidth of single quantum dots (QDs) using a standard, easy-to-use spectrometer setup. This method incorporates both laser and photoluminescence spectroscopy, combining the advantage of laser line-width limited resolution with multi-channel photoluminescence detection. Such a scheme allows for considerable improvement of resolution over that of a common single-stage spectrometer. The method uses phonons to assist in the measurement of the photoluminescence of a single quantum dot after resonant excitation of its ground state transition. The phonon&#39;s energy difference allows one to separate and filter out the laser light exciting the quantum dot. An advantageous feature of this method is its straight forward integration into standard spectroscopy setups, which are accessible to most researchers.

The manuscript describes a method of phonon-assisted quasi-resonant fluorescence spectroscopy that incorporates both laser-limited resolution and photoluminescence (PL) spectroscopy. This method utilizes optical phonons to provide linewidth-limited resolution spectra of atom-like semiconductor structures in the energy domain. The method is also easily realized with a single spectrometer optical spectroscopy setup.

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we ...

High-resolution Thermal Micro-imaging Using Europium Chelate Luminescent Coatings

Micro-electronic devices often undergo significant self-heating when biased to their typical operating conditions. This paper describes a convenient optical micro-imaging technique which can be used to map and quantify such behavior. Europium thenoyltrifluoroacetonate (EuTFC) has a 612 nm luminescence line whose activation efficiency drops strongly with increasing temperature, due to T-dependent interactions between the Eu3+ ion and the organic chelating compound. This material may be readily coated on to a sample surface by thermal sublimation in vacuum. When the coating is excited with ultraviolet light (337 nm) an optical micro-image of the 612 nm luminescent response can be converted directly into a map of the sample surface temperature. This technique offers spatial resolution limited only by the microscope optics (about 1 micron) and time resolution limited by the speed of the camera employed. It offers the additional advantages of only requiring comparatively simple and non-specialized equipment, and giving a quantitative probe of sample temperature.

Europium thenoyltrifluoroacetonate (EuTFC) has an optical luminescence line at 612 nm, whose activation efficiency decreases strongly with temperature. If a sample coated with a thin film of this material is micro-imaged, the 612 nm luminescent response intensity may be converted into a direct map of sample surface temperature.

Micro-electronic devices often undergo significant self-heating when biased to their typical operating conditions. This paper describes a convenient ...

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 ...

Strain Sensing Based on Multiscale Composite Materials Reinforced with Graphene Nanoplatelets

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are proposed to create the electrical network in this work: (a) the incorporation of the nanoplatelets into the epoxy matrix and (b) the coating of the glass fabric with a sizing filled with the same nanoplatelets. Both types of multiscale composite materials, with an in-plane electrical conductivity of ~10-3 S/m, showed an exponential growth of the electrical resistance as the strain increases due to distancing between adjacent functionalized graphene nanoplatelets and contact loss between overlying ones. The sensitivity of the materials analyzed during this research, using the described procedures, has been shown to be higher than commercially available strain gauges. The proposed procedures for self-sensing of the structural composite material would facilitate the structural health monitoring of components in difficult to access emplacements such as offshore wind power farms. Although the sensitivity of the multiscale composite materials was considerably higher than the sensitivity of metallic foils used as strain gauges, the value reached with NH2 functionalized graphene nanoplatelets coated fabrics was nearly an order of magnitude superior. This result elucidated their potential to be used as smart fabrics to monitor human movements such as bending of fingers or knees. By using the proposed method, the smart fabric could immediately detect the bending and recover instantly. This fact permits precise monitoring of the time of bending as well as the degree of bending.

The integration of conductive nanoparticles, such as graphene nanoplatelets, into glass fiber composite materials creates an intrinsic electrical network susceptible to strain. Here, different methods to obtain strain sensors based on the addition of graphene nanoplatelets into the epoxy matrix or as a coating on glass fabrics are proposed.

The electrical response of NH2-functionalized graphene nanoplatelets composite materials under strain was studied. Two different manufacturing methods are ...

A High Performance Impedance-based Platform for Evaporation Rate Detection

This paper describes the method of a novel impedance-based platform for the detection of the evaporation rate. The model compound hyaluronic acid was employed here for demonstration purposes. Multiple evaporation tests on the model compound as a humectant with various concentrations in solutions were conducted for comparison purposes. A conventional weight loss approach is known as the most straightforward, but time-consuming, measurement technique for evaporation rate detection. Yet, a clear disadvantage is that a large volume of sample is required and multiple sample tests cannot be conducted at the same time. For the first time in literature, an electrical impedance sensing chip is successfully applied to a real-time evaporation investigation in a time sharing, continuous and automatic manner. Moreover, as little as 0.5 ml of test samples is required in this impedance-based apparatus, and a large impedance variation is demonstrated among various dilute solutions. The proposed high-sensitivity and fast-response impedance sensing system is found to outperform a conventional weight loss approach in terms of evaporation rate detection.

This paper presents an impedance-based apparatus for evaporation rate detection of solutions. It offers clear advantages over a conventional weight loss approach: a fast response, high-sensitivity detection, a small sample requirement, multiple sample measurements, and easy disassembly for cleaning and reuse purposes.

This paper describes the method of a novel impedance-based platform for the detection of the evaporation rate. The model compound hyaluronic acid was ...

Method for Recording Broadband High Resolution Emission Spectra of Laboratory Lightning Arcs

Lightning is one of the most common and destructive forces in nature and has long been studied using spectroscopic techniques, first with traditional camera film methods and then digital camera technology, from which several important characteristics have been derived. However, such work has always been limited due to the inherently random and non-repeatable nature of natural lightning events in the field. Recent developments in lightning test facilities now allow the reproducible generation of lightning arcs within controlled laboratory environments, providing a test bed for the development of new sensors and diagnostic techniques to understand lightning mechanisms better. One such technique is a spectroscopic system using digital camera technology capable of identifying the chemical elements with which the lightning arc interacts, with these data then being used to derive further characteristics. In this paper, the spectroscopic system is used to obtain the emission spectrum from a 100 kA peak, 100 &#181;s duration lightning arc generated across a pair of hemispherical tungsten electrodes separated by a small air gap. To maintain a spectral resolution of less than 1 nm, several individual spectra were recorded across discrete wavelength ranges, averaged, stitched, and corrected to produce a final composite spectrum in the 450 nm (blue light) to 890 nm (near infrared light) range. Characteristic peaks within the data were then compared to an established publicly available database to identify the chemical element interactions. This method is readily applicable to a variety of other light emitting events, such as fast electrical discharges, partial discharges, and sparking in electrical equipment, apparatus, and systems.

Emission spectroscopy techniques have traditionally been used to analyze inherently random lightning arcs occurring in nature. In this paper, a method developed to obtain the emission spectroscopy from reproducible lightning arcs generated within a laboratory environment is described.

Lightning is one of the most common and destructive forces in nature and has long been studied using spectroscopic techniques, first with traditional ...