Name: characterizing far-infrared laser emissions and the measurement of their frequencies
Uploaded: 2015-12-18T15:00:00
Description: Watch this Scientific Journal Video about Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies at JoVE.com

This work was supported in part by the Washington Space Grant Consortium under Award NNX10AK64H.

1. Planning of Experiments
<ol>
	<li>Conduct a survey of the literature to assess prior work performed using the laser medium of interest, which for this experiment is CH2F2. Identify all known laser emissions along with all information about the lines such as their wavelength and frequency. Several surveys of known laser emissions are available13,31&#8211;37.</li>
	<li>Compile all spectroscopic investigations of the molecule used as the laser medium with a focus o...

There are several critical steps within the protocol that require some additional discussion. When measuring the far-infrared laser wavelength, as outlined in step 2.5.3, it is important to ensure the same mode of the far-infrared laser emission is being used. Multiple modes of a far-infrared laser wavelength (i.e., TEM00, TEM01, etc.) can be generated within the laser cavity and thus it is important to identify the appropriate adjacent cavity modes being used to measure the wavelength...

The measurement of far-infrared laser frequencies was first performed by Hocker and co-workers in 1967. They measured the frequencies for the 311 and 337 &#956;m emissions from the direct-discharge hydrogen cyanide laser by mixing them with high order harmonics of a microwave signal in a silicon diode1. To measure higher frequencies, a chain of lasers and harmonic mixing devices were used to generate the laser harmonics2. Eventually two stabilized carbon dioxide (CO2) lasers were chosen to synthesize the necessary difference frequencies3,4. Today, far-infrared laser frequencies up to 4 THz can be measured with this technique using only the first harmonic of the difference frequency generated by two stabilized CO2 reference lasers. Higher frequency laser emissions can also be measured using the second harmonic, such as the 9 THz laser emissions from the methanol isotopologues CHD2OH and CH318OH.5,6 Over the years, the accurate measurement of laser frequencies has impacted a number of scientific experiments7,8 and permitted the adoption of a new definition of the meter by the General Conference of Weights and Measures in Paris in 1983.9&#8211;11
Heterodyne techniques, such as those described, have been immensely beneficial in the measurement of far-infrared laser frequencies generated by optically pumped molecular lasers. Since the discovery of the optically pumped molecular laser by Chang and Bridges12, thousands of optically pumped far-infrared laser emissions have been generated with a variety of laser media. For example, difluoromethane (CH2F2) and its isotopologues generate over 250 laser emissions when optically pumped by a CO2 laser. Their wavelengths range from approximately 95.6 to 1714.1 &#956;m.13&#8211;15 Nearly 75% of these laser emissions have had their frequencies measured while several have been spectroscopically assigned16&#8211;18.
These lasers, and their accurately measured frequencies, have played a crucial role in the advancement of high-resolution spectroscopy. They provide important information for infrared spectral studies of the laser gases. Often these laser frequencies are used to verify the analysis of infrared and far-infrared spectra because they provide connections among the excited vibrational state levels that are often directly inaccessible from absorption spectra19. They also serve as the primary radiation source for studies investigating transient, short-lived free radicals with the laser magnetic resonance technique20. With this extremely sensitive technique, rotational and ro-vibrational Zeeman spectra in paramagnetic atoms, molecules, and molecular ions can be recorded and analyzed along with the ability to investigate the reaction rates used to create these free radicals.
In this work, an optically pumped molecular laser, shown in Figure 1, has been used to generate far-infrared laser radiation from difluoromethane. This system consists of a continuous wave (cw) CO2 pump laser and a far-infrared laser cavity. A mirror internal to the far-infrared laser cavity redirects the CO2 laser radiation down the polished copper tube, undergoing twenty six reflections before terminating at the end of the cavity, scattering any remaining pump radiation. Therefore the far-infrared laser medium is excited using a transverse pumping geometry. To generate laser action, several variables are adjusted, some simultaneously, and all are subsequently optimized once laser radiation is observed.
In this experiment, far-infrared laser radiation is monitored by a metal-insulator-metal (MIM) point contact diode detector. The MIM diode detector has been used for laser frequency measurements since 1969.21&#8211;23 In laser frequency measurements, the MIM diode detector is a harmonic mixer between two or more radiation sources incident on the diode. The MIM diode detector consists of a sharpened Tungsten wire contacting an optically polished Nickel base24. The Nickel base has a naturally occurring thin oxide layer that is the insulating layer.
Once a laser emission was detected, its wavelength, polarization, strength, and optimized operating pressure were recorded while its frequency was measured using the three-laser heterodyne technique25&#8211;27 following the method originally described in Ref. 4. Figure 2 shows the optically pumped molecular laser with two additional cw CO2 reference lasers having independent frequency stabilization systems that utilize the Lamb dip in the 4.3 &#956;m fluorescence signal from an external, low pressure reference cell28. This manuscript outlines the process used to search for far-infrared laser emissions as well as the method for estimating their wavelength and in accurately determining their frequency. Specifics regarding the three-laser heterodyne technique as well as the various components and operating parameters of the system can be found in Supplemental Table A along with references 4, 25&#8211;27, 29, and 30.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>Vacuum pump</td>
			<td>Leybold</td>
			<td>Trivac D4A</td>
			<td>HE-175 oil; Quantity = 3</td>
		</tr>
		<tr>
			<td>Vacuum pump</td>
			<td>Leybold</td>
			<td>Trivac D8B or D16B</td>
			<td>Fomblin Fluid; Quantity = 1 of each</td>
		</tr>
		<tr>
			<td>Vacuum pump</td>
			<td>Leybold</td>
			<td>Trivac D25B</td>
			<td>HE-175 oil; Quantity = 1</td>
		</tr>
		<tr>
			<td>Optical chopper with controller</td>
			<td>Stanford Research Systems</td>
			<td>SR540</td>
			<td></td>
		</tr>
		<tr>
			<td>Lock-in amplifier</td>
			<td>Stanford Research Systems</td>
			<td>SR830</td>
			<td></td>
		</tr>
		<tr>
			<td>Spectrum analyzer</td>
			<td>Agilent</td>
			<td>E4407B</td>
			<td>ESA-E Series, 9 kHz to 26.5 GHz Spectrum Analyzer</td>
		</tr>
		<tr>
			<td>Amplifier&#160;</td>
			<td>Miteq</td>
			<td>AFS-44</td>
			<td>Provides amplification of signals between 2 and 18 GHz. The amplifier is powered by a Hewlett Packard triple output DC power supply, model E3630A.</td>
		</tr>
		<tr>
			<td>Amplifier&#160;</td>
			<td>Avantek</td>
			<td>AWL-1200B</td>
			<td>Provides amplification of signals less than 1.2 GHz.</td>
		</tr>
		<tr>
			<td>Power supply</td>
			<td>Hewlett Packard</td>
			<td>E3630A</td>
			<td>Low voltage DC power supply for amplifier.</td>
		</tr>
		<tr>
			<td>Power supply</td>
			<td>Glassman</td>
			<td>KL Series</td>
			<td>High voltage power supply for the CO2 lasers; Quantity = 2; negative polarity</td>
		</tr>
		<tr>
			<td>Power supply</td>
			<td>Fluke</td>
			<td>412B</td>
			<td>High voltage power supply used with the NIST Asymmetric HV Amp</td>
		</tr>
		<tr>
			<td>Detector</td>
			<td>Judson Infrared Inc</td>
			<td>J10D</td>
			<td>For fluorescence cell; Quantity = 2</td>
		</tr>
		<tr>
			<td>CO2 laser spectrum analyzer</td>
			<td>Optical Engineering&#160;</td>
			<td>16-A</td>
			<td>Currently sold by Macken Instruments Inc.</td>
		</tr>
		<tr>
			<td>Thermal imaging plates with UV light</td>
			<td>Optical Engineering&#160;</td>
			<td></td>
			<td>Primarily used for aligning the CO2 reference lasers. Currently sold by Macken Instruments Inc.</td>
		</tr>
		<tr>
			<td>Resistors</td>
			<td>Ohmite&#160;</td>
			<td>L225J100K</td>
			<td>100 kW, 225 W. Between 4 to 6 resistors are used in each ballast system. Each CO2 laser has its own ballast system. Fans are used to cool the resistors.</td>
		</tr>
		<tr>
			<td>HV relay, SPDT</td>
			<td>CII Technologies</td>
			<td>H-17</td>
			<td>Quantity = 3; one for each CO2 laser</td>
		</tr>
		<tr>
			<td>Amplifier&#160;</td>
			<td>Princeton Applied Research</td>
			<td>PAR 113</td>
			<td>Used with fluorescence cell; Quantity = 2</td>
		</tr>
		<tr>
			<td>Oscilloscope</td>
			<td>Tektronix</td>
			<td>2235A</td>
			<td>Similar models are also used; Quantity = 2</td>
		</tr>
		<tr>
			<td>Oscilloscope/Differential amplifier</td>
			<td>Tektronix</td>
			<td>7903 oscilloscope with 7A22 differential amplifier</td>
			<td></td>
		</tr>
		<tr>
			<td>Power meter with sensor</td>
			<td>Coherent</td>
			<td>200</td>
			<td>For use below 10 W.&#160; This is the power meter shown in Figure 2.</td>
		</tr>
		<tr>
			<td>Power meter with sensor</td>
			<td>Scientech, Inc</td>
			<td>Vector S310</td>
			<td>For use below 30 W</td>
		</tr>
		<tr>
			<td>Multimeter</td>
			<td>Fluke</td>
			<td>73III</td>
			<td>Similar models are also used; Quantity = 3</td>
		</tr>
		<tr>
			<td>Data acquisition</td>
			<td>National Instruments</td>
			<td>NI cDAQ 9174 chassis with NI 9223 input module</td>
			<td>Uses LabVIEW software</td>
		</tr>
		<tr>
			<td>Simichrome polish</td>
			<td>Happich GmbH</td>
			<td></td>
			<td>Polish for the Nickel base used in the MIM diode detector. Although the Nickel base can be used immediately after polishing, a 12 hour lead time is typically recommended.</td>
		</tr>
		<tr>
			<td>Pressure gauge</td>
			<td>Wallace and Tiernan</td>
			<td>61C-1D-0050</td>
			<td>Series 300; for CO2 laser; Quantity = 3</td>
		</tr>
		<tr>
			<td>Pressure gauge with controller</td>
			<td>Granville Phillips</td>
			<td>Series 375</td>
			<td>For far-infrared laser</td>
		</tr>
		<tr>
			<td>Zirconium Oxide felt</td>
			<td>Zircar Zirconia</td>
			<td>ZYF felt</td>
			<td>Used as a beam stop</td>
		</tr>
		<tr>
			<td>Zirconium Oxide board</td>
			<td>Zircar Zirconia</td>
			<td>ZYZ-3 board</td>
			<td>Used as a beam stop; Quantity = 4</td>
		</tr>
		<tr>
			<td>Teflon sheet</td>
			<td>Scientific Commodities, Inc</td>
			<td>BB96312-1248</td>
			<td>1/32 inch thick; used for the far-infrared laser output window</td>
		</tr>
		<tr>
			<td>Polypropylene</td>
			<td>C-Line sheet protectors</td>
			<td>61003</td>
			<td>used for the far-infrared laser output window</td>
		</tr>
		<tr>
			<td>Vacuum grease</td>
			<td>Apiezon</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Power supply</td>
			<td>Kepco</td>
			<td>NTC 2000</td>
			<td>PZT power supply</td>
		</tr>
		<tr>
			<td>PZT tube</td>
			<td>Morgan Advanced Materials</td>
			<td></td>
			<td>1 inch length, 1 inch outer diameter, 0.062 inch thickness, reverse polarity (positive voltage on outside); Quantity = 3</td>
		</tr>
		<tr>
			<td>ZnSe (AR coated)</td>
			<td>II-VI Inc</td>
			<td></td>
			<td>CO2 laser window (Quantity = 3), lens, and beam splitter (Quantity 3)</td>
		</tr>
		<tr>
			<td>NaCl window</td>
			<td>Edmond Optics</td>
			<td></td>
			<td>Quantity = 1</td>
		</tr>
		<tr>
			<td>CaF window</td>
			<td>Edmond Optics</td>
			<td></td>
			<td>Quantity = 2</td>
		</tr>
		<tr>
			<td>Laser mirrors and gratings</td>
			<td>Hyperfine, Inc</td>
			<td></td>
			<td>Gold-coated; includes positioning mirrors</td>
		</tr>
		<tr>
			<td>Glass laser tubes and reference cells</td>
			<td>Allen Scientific Glass</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>MIM diode detector</td>
			<td>Custom Microwave, Inc</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Other</td>
			<td></td>
			<td></td>
			<td>Other materials include magnetic bases, base plates, base clamps, XYZ translation stage, etc.</td>
		</tr>
	</tbody>
</table>

characterizing far-infrared laser emissions and the measurement of their frequencies

The generation and subsequent measurement of far-infrared radiation has found numerous applications in high-resolution spectroscopy, radio astronomy, and Terahertz imaging. For about 45 years, the generation of coherent, far-infrared radiation has been accomplished using the optically pumped molecular laser. Once far-infrared laser radiation is detected, the frequencies of these laser emissions are measured using a three-laser heterodyne technique. With this technique, the unknown frequency from the optically pumped molecular laser is mixed with the difference frequency between two stabilized, infrared reference frequencies. These reference frequencies are generated by independent carbon dioxide lasers, each stabilized using the fluorescence signal from an external, low pressure reference cell. The resulting beat between the known and unknown laser frequencies is monitored by a metal-insulator-metal point contact diode detector whose output is observed on a spectrum analyzer. The beat frequency between these laser emissions is subsequently measured and combined with the known reference frequencies to extrapolate the unknown far-infrared laser frequency. The resulting one-sigma fractional uncertainty for laser frequencies measured with this technique is &#177; 5 parts in 107. Accurately determining the frequency of far-infrared laser emissions is critical as they are often used as a reference for other measurements, as in the high-resolution spectroscopic investigations of free radicals using laser magnetic resonance. As part of this investigation, difluoromethane, CH2F2, was used as the far-infrared laser medium. In all, eight far-infrared laser frequencies were measured for the first time with frequencies ranging from 0.359 to 1.273 THz. Three of these laser emissions were discovered during this investigation and are reported with their optimal operating pressure, polarization with respect to the CO2 pump laser, and strength.

As mentioned, the frequency reported for a far-infrared laser emission is an average of at least twelve measurements performed with at least two different sets of CO2 reference laser lines. Table 2 outlines the data recorded for the 235.5 &#956;m laser emission when using the 9P04 CO2 pump laser. For this far-infrared laser emission, fourteen individual measurements of the beat frequency were recorded. The first set of measurements were recorded while using the 9R1...

Watch this Scientific Journal Video about Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies at JoVE.com

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

We describe the generation of far-infrared radiation using an optically pumped molecular laser along with the measurement of their frequencies with heterodyne techniques. The experimental system and techniques are demonstrated using difluoromethane (CH2F2) as the laser medium whose results include three new laser emissions and eight measured laser frequencies.

The generation and subsequent measurement of far-infrared radiation has found numerous applications in high-resolution spectroscopy, radio astronomy, and ...

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Three-dimensional (3D) printing as a type of additive manufacturing shows continuing increase in application and consumer popularity. The fused filament ...

Solution Blow Spinning of Polymeric Nano-Composite Fibers for Personal Protective Equipment

Light-weight, protective armor systems typically consist of high modulus (&#62;109 MPa) and high-strength polymeric fibers held in place with an elastic resin material (binder) to form a non-woven, unidirectional laminate. While significant efforts have focused on improving the mechanical properties of the high-strength fibers, little work has been undertaken to improve the properties of the binder materials. To improve the performance of these elastomeric polymer binders, a relatively new and simple fabrication process, known as solution blow spinning, was used. This technique is capable of producing sheets or webs of fibers with average diameters ranging from the nanoscale to the microscale. To achieve this, a solution blow spinning (SBS) apparatus has been designed and built in the laboratory to fabricate non-woven fiber mats from polymer elastomer solutions.
In this study, a commonly used binder material, a styrene-butadiene-styrene block-co-polymer dissolved in tetrahydrofuran, was used to produce nanocomposite fiber mats by adding metallic nanoparticles (NPs), such as iron oxide NPs, that were encapsulated with silicon oil and thus incorporated in the fibers formed via the SBS process. The protocol described in this work will discuss the effects of the various critical parameters involved in the SBS process, including the polymer molar mass, the selection of the thermodynamically appropriate solvent, the polymer concentration in solution, and the carrier gas pressure to assist others in performing similar experiments, as well as provide guidance to optimize the configuration of the experimental setup. The structural integrity and morphology of the resultant non-woven fiber mats were examined using scanning electron microscopy (SEM) and elemental X-ray analysis via energy-dispersive X-ray spectroscopy (EDS). The goal of this study is to evaluate the effects of the various experimental parameters and material selections to optimize the structure and morphology of the SBS fiber mats.

The primary goal of this study is to describe a protocol to prepare polymeric fiber mats with consistent morphology via solution blow spinning (SBS). We aim to use SBS to develop novel, tunable, flexible polymeric fiber nanocomposites for various applications, including protective materials, by incorporating nanoparticles in a polymer-elastomer matrix.

Light-weight, protective armor systems typically consist of high modulus (&#62;109 MPa) and high-strength polymeric fibers held in place with an elastic ...

Fabrication of a Fabry-P&amp;#233;rot Etalon for Application in Trace-Gas Spectroscopy

Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&#233;rot Etalon

Fabry-P&#233;rot etalons (FPE) have found their way into many applications. In fields such as spectroscopy, telecommunications, and astronomy, FPEs are used for their high sensitivity as well as their exceptional filtering capability. However, air-spaced etalons with high finesse are usually built by specialized facilities. Their production requires a clean room, special glass handling, and coating machinery, meaning commercially available FPEs are sold for a high price. In this article, a new and cost-effective method to fabricate fiber-coupled FPEs with standard photonic laboratory equipment is presented. The protocol should serve as a step-by-step guide for the construction and characterization of these FPEs. We hope this&#160;will enable researchers to conduct fast and cost-effective prototyping of FPEs for various fields of application. The FPE, as presented here, is used for spectroscopic applications. As shown in the representative results section via proof of principle measurements of water vapor in ambient air, this FPE has a finesse of 15, which is sufficient for the photothermal detection of trace concentrations of gases.

Author Spotlight: Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&amp;#233;rot Etalon  

This protocol describes the construction of a low-cost, discrete, fiber-coupled, and air-spaced Fabry-Perot etalon with various applications, such as in trace-gas spectroscopy. The fabrication is possible in any facility with standard optical laboratory equipment available.

Fabry-P&#233;rot etalons (FPE) have found their way into many applications. In fields such as spectroscopy, telecommunications, and astronomy, FPEs are ...