Mobile CARS instrument for combustion and plasma diagnostics

Download Mobile CARS instrument for combustion and plasma diagnostics

Post on 03-Oct-2016




0 download

Embed Size (px)


<ul><li><p>Mobile CARS instrument for combustion andplasma diagnostics</p><p>Torger J. Anderson, Gregory M. Dobbs, and Alan C. Eckbreth</p><p>A compact and easily transportable coherent anti-Stokes Raman spectroscopy system for combustion andplasma diagnostics is described. The instrument is readily adaptable to a wide variety of test environmentsand experiments. The system is capable of withstanding high noise and vibration levels and is remotelyoperable to protect the operator and delicate components from high noise levels or hazardous environments.It is intended for single-pulse temperature and concentration measurements in practical combustion systems,such as gas turbines, diesel engines, and plasma process applications. The system is operational, and initialmeasurement demonstrations in a supersonic combusting flow are described.</p><p>1. IntroductionCoherent anti-Stokes Raman spectroscopy (CARS)</p><p>is a nonlinear optical technique for gas temperatureand species concentration measurements which hasconsiderable demonstrated potential for analysis ofpractical combustion and plasma systems.lA Requir-ing no physical probe, it is nonintrusive and capable ofhigh spatial resolution and accurate measurements formost common combustion species. The use of short-pulse lasers makes the measurements virtually instan-taneous with respect to the process being observed.</p><p>Due to the complexity of the optical system, mostCARS systems to date have been laboratory systemsset up on large optical tables analyzing combustionsystems integrated in the CARS experiment. A fewinstruments have been tailored to test facilities formeasurements in practical combustion devices,4 5 but,because of their bulk and lack of flexibility, their usehas been mostly restricted to the facilities in whichthey were first installed. In contrast, plumbed sys-tems have been constructed in which a laser or CARSsystem has been centrally located with tubes to directthe laser beams to the test site.6 7 With such systems,the addition of new facilities is complex, and CARScapability is not provided at sites remote from thecentrally located laser.</p><p>To overcome these limitations, we developed a mo-bile CARS instrument which is compact and flexibleenough to be accommodated by most test facilities andcombustion experiments. The system offers a remoteoperation capability for installations in hazardous orhigh noise environments. Although compact, it has</p><p>The authors are with United Technologies Research Center, EastHartford, Connecticut 06108.</p><p>Received 7 June 1986.0003-6935/86/224076-10$02.00/0. 1986 Optical Society of America.</p><p>configurational flexibility to meet future needs anddevelopments. In its first field application, the mobileCARS system demonstrated measurements in a super-sonic combusting flow. Some results of this demon-stration are included. Future developments, includ-ing a near-real-time presentation of temperature andspecies concentration measurements, will also be de-scribed.II. CARS Overview</p><p>The theory of CARS and its application as a com-bustion diagnostic are well explained in severalreviews.1"4 '8 3 As illustrated in Fig. 1, incident laserbeams at frequencies o and (2 (termed the pump andStokes, respectively) interact through the third-ordernonlinear susceptibility of the medium X(3) to generatean oscillating polarization and thus coherent radiationat frequency Wo3 = 2 - W2. When the frequencydifference (l - WO2) is close to the frequency x, of aRaman resonance of a species, the magnitude of theCARS radiation at (3 is resonantly enhanced and re-sults in a signature uniquely characteristic of that mo-lecular species. Measurements of medium propertiesare performed from the shape of the spectral signatureand/or intensity of the CARS radiation.</p><p>CARS spectra can be acquired in two basic ways, asshown in Fig. 1. The scanned approach uses a spec-trally narrow Stokes beam to generate one small sec-tion of the CARS spectrum. The Stokes beam isscanned across the Raman frequency range to generatea high resolution CARS spectrum. This method re-quires a considerable amount of time and, as such,cannot be used where fluctuations in the medium areoccurring. The broadband method overcomes thisproblem by employing a Stokes source that has a broadspectrum, typically 150-200 cm-'. This allows theentire CARS spectrum to be generated simultaneouslyproviding virtually instantaneous measurements whenshort pulse lasers are employed. This method is pre-ferred in examining practical combustion systems</p><p>4076 APPLIED OPTICS / Vol. 25, No. 22 / 15 November 1986</p></li><li><p>* Phase matching</p><p>-I kk 2- k3</p><p>Ikil = niwi /c</p><p>I -ABroadband</p><p>A\ I -AW2 WI 'J3</p><p>Fig. 1. Coherent anti-Stokes Raman spectroscopy.</p><p>where turbulence and temperature fluctuations arealmost always present.</p><p>For efficient signal generation, the incident laserbeams must be so aligned that the three-wave mixingprocess is properly phased. Phase matching ensuresthat the CARS signal generated at one spatial locationwill be in phase with the signals generated elsewhereand that constructive interference occurs. If phasematching is not satisfied, the CARS signals generatedat various spatial locations will destructively interfere,and no net signal will be produced. In gases, phasematching is easily satisfied by a collinear arrangementof the laser beams. In many diagnostic circumstances,however, collinear phase matching leads to poor spa-tial resolution because the CARS radiation undergoesan integrative growth process. As a result, a phasematching geometry must be selected based on trade-offs between signal strength, spatial resolution, andthe ability to filter the CARS signal from the laserbeams. A CARS system to be used in a variety ofexperiments must be capable of easily employing anumber of varied phase matching geometries.13-15</p><p>A high power laser is required to provide the sourceof optical energy for a system which generates CARSfor diagnostic use. A Nd:YAG laser is generally usedbecause of its ability to generate a very high peakpower with Q-switching. Frequency doubling pro-duces a beam in the visible region near the CARSspectral range to provide the CARS pump beam. Adye laser generates the Stokes beam which can betuned to the appropriate wavelength for CARS to beobserved from the species of interest. This laser ispumped by a portion of the frequency-doubledNd:YAG beam. An optical system combines thepump and Stokes beams in the desired phase matchinggeometry, focuses them at the measurement point, andrecollimates them along with the CARS beam. It alsoseparates the relatively weak CARS signal from thehigh power laser beams and directs the signal to aspectrograph for analysis. Measurements are oftenrequired in experiments conducted in hazardous ornoisy environments, and, as such, the system must becapable of being operated remotely. Because a largeamount of data is acquired at a high rate, a computer</p><p>system is required to store it for future analysis.These basic considerations provide the perspectivearound which our mobile CARS system was designed.</p><p>Ill. General DesignThe mobile CARS instrument consists of three basic</p><p>subsystems which were derived from an earlier instru-ment. 4 A cart-mounted transmitter contains aNd:YAG laser and the associated optics necessary toalign the system and generate the laser beams. Thelaser power supply, because of its bulk, remains in aseparate frame and links the transmitter to its controlsthrough a single umbilical. A receiver separates theCARS signal from unwanted light, utilizes the residuallaser beams to generate a reference CARS signal forconcentration measurements, and focuses the CARSsignal into an optical fiber for transmission to the thirdsubsystem, the data acquisition and control rack. Thelatter contains a spectrograph to analyze the CARSsignal and a data acquisition computer to store CARSspectra on magnetic tape. It also contains remotecontrols for the laser and actuators in the transmitterand receiver which are adjusted to maximize the CARSsignal. A separate and larger computer is currentlyused to analyze the CARS spectral information tomake temperature and species concentration measure-ments.</p><p>Figure 2 illustrates the basic design of the system. ACARS application in a typical combustion tunnel facil-ity with a separate control room is represented. Eventhough CARS is a spatially precise measurement tech-nique, its utilization requires line-of-sight optical ac-cess through the tunnel. A transmitter is positionedto direct the laser beams through a window and focusthem at the measurement point. With the necessaryoptics and positioners in the transmitter and on eitherside of the tunnel, the measurement point can bemoved throughout the combustion experiment limitedonly by optical access. A receiver accepts the laser andCARS beams emerging from the tunnel and sends theCARS signal via optical fiber to the data acquisitionand control rack located in the control room.IV. Transmitter</p><p>The transmitter is constructed on a low slung three-wheeled cart containing rigid metal frames supportingtwo optical pallets. The 60- X 90-cm upper pallet isrigidly supported 18 cm above the 60- X 120-cm lowerpallet on a rigid aluminum frame and is hinged toprovide access to items on the lower pallet. The entirecart is enclosed in an aluminum box with hinged coversto eliminate stray laser beams and protect the opticalcomponents.</p><p>The optical system is based on a 20-Hz Nd:YAGlaser (Quanta Ray DCR-2A) mounted on the loweroptical pallet. The laser head was repackaged to effecta more compact transmitter. The electronics, con-trols, and structural frame of the laser head were re-moved leaving only the optical rail to be installed inthis subsystem. The laser electronics were installed inthe top of the laser power supply, and the control panel</p><p>15 November 1986 / Vol. 25, No. 22 / APPLIED OPTICS 4077</p><p>Approach - W3 AR</p><p>Stokes</p><p>PumPs 1i</p><p>* Energy level diagramWi (03</p><p>WI ii</p><p>* SpectrumScanned</p></li><li><p>Fig. 2. Typical application proposed for the mobile CARS system;measurements in a combustion tunnel. The transmitter (left) di-rects and focuses the laser beams through an optical window in thetunnel into the combusting flow. The beams propagate along withthe CARS beam through a window on the other side of the tunnelinto the receiver (center). Here the CARS beam is focused onto anoptical fiber for transmission to the remotely located instrument andcontrol subsystem (background). The transmitter and receiver arelinked to the instrument rack by a single umbilical which runs</p><p>through the laser power supply (right).</p><p>was incorporated in a rack-mounted box in the instru-ment and control subsystem. The laser power supply,as provided by the manufacturer, is a separate casteredcart 56 cm wide X 120 cm high X 84 cm long. It wasmodified to accommodate the laser head electronicsand two umbilicals, one to the transmitter/receiverand the other to the instrument and control subsys-tem.</p><p>To make more efficient use of space on the lowerpallet, the optical rail in the laser head was shortenedby 30 cm by removing the end containing the harmonicgenerator. As a consequence of laser repackaging, thebeam height above the pallet was reduced from theoriginal 18 to 10 cm. This allows a reduction in overalltransmitter height and provides better optical stabil-ity in vibratory environments.</p><p>Figure 3 shows the mobile system optical layoutincluding the two transmitter pallets. The 1.06-,gmoutput of the laser is folded 1800 by two mirrors allow-ing the harmonic generator to be located behind theoptical rail. The optics directing the high intensitybeams require hard coatings capable of withstandingup to 1 GW/cm 2 for the 10-ns pulse of the Nd:YAGlaser. The optical components used in the mobilesystem are described in Table I.</p><p>A harmonic generator frequency doubles the 1.06-,um laser beam with an efficiency of -30%. A harmon-ic beam splitter transmits most of the residual 1.06-,Mmbeam and reflects the 532-nm primary green beam to aPellin-Broca prism which disperses any collinear 1.06-,um radiation into a trap. The residual 1.06-Am beamtransmitted through the beam splitter enters a secondharmonic generator where a secondary green beam isproduced. Because the polarization purity of the 1.06-,um beam is reduced by the first frequency-doublingprocess, the second harmonic generator is less effi-cient. Nevertheless, by generating a secondary green</p><p>Fig. 3. Optical schematic of the mobile CARS transmitter andreceiver. Component labels: BS, beam splitter; D, dichroic; DC,dye cell; F, filter; FO, fiber optic; HG, harmonic generator; L, lens; M,mirror; P, prism; PB, Pellin Broca prism; RC, reference cell. Num-bers following the labels correspond to components listed in Table I.</p><p>beam, the overall system efficiency is improved. Thesecondary beam is also turned with a dichroic allowingthe residual 1.06-Mm beam to be trapped. After thedichroic, any collinear 1.06-,um radiation which re-mains with the green beam is absorbed in a piece of 3-mm thick KG3 IR absorbing glass. The secondary532-nm beam is used only for dye laser pumping, andas such absorption-induced lensing in the absorbingglass is not a major concern. Note that this approachis not employed with the primary 532-nm beam toensure that no phase distortion occurs. Finally, thetwo green beams are turned and brought to a periscopefor transmission to the upper optical pallet.</p><p>The secondary green beam is directed and focusedinto a dye cell in a laser cavity to generate the Stokesbeam used for CARS. The dye cell is an 8- X 8-cmnylon block with a 5-cm diam window on each side. A10-mm channel runs horizontally through the center ofthe block and ends in 12.7-mm tube fittings on eitherend. Plastic tubing connects the cell to a pump and a1.8-liter reservoir to provide a continuously flowingdye mixture to the cell. This prevents heating of thedye and blooming of the dye laser beam. The second-ary green beam is focused into one side of the dye cellchannel to pump the Stokes beam oscillator. Theprimary green beam is routed around the upper pallet,and a beam splitter directs 30% of it into the other endof the dye cell channel to amplify the Stokes beamwhich was folded back through the dye cell by a rooftopprism. The cell is oriented at the Brewster angle withrespect to the dye laser axis to reduce reflection lossesat the window surfaces and to maximize the degree ofpolarization generating a horizontally polarizedStokes beam. To preserve the polarization purity,care is taken when mounting the windows to avoidstress-induced birefringence.</p><p>Five such dye cells are mounted on a translationstage and can be individually translated into the lasercavity as desired. Dye mixtures in the cells are con-</p><p>4078 APPLIED OPTICS / Vol. 25, No. 22 / 15 November 1986</p></li><li><p>Table 1. Optical Components Used in the Mobile CARS System(see Fig. 3)</p><p>Item Description Specifications Notes Item Description Specifications Notes</p><p>BS3 Reference cell CARSbeam splitter</p><p>D1 532 -nm/1.06-,amDichroic beam splitter</p><p>D2 BOXCARS pump/Stokes coupl...</p></li></ul>