NASA Reduced Gravity Student Flight Opportunities Program 2002 Competition A Study of Optical Properties of ZBLAN Microspheres Produced in Microgravity II Serena Eley*—Flyer, Senior, Physics, AUG01 (flyer) Dirk Englund*—Flyer, Senior, Physics, AUG01 (flyer) Wook Hwang—Flyer, Senior, Physics Joseph Jewell*—Flyer, Sophomore, Aeronautics, AUG01 (flyer) Contact: Serena Eley, [email protected], (626) 395-1530 Advisers: Dr. Lute Maleki, JPL Senior Research Scientist. [email protected], (818) 354-3688 Dr. Steven Frautschi, Professor of Physics at Caltech. [email protected], (626) 395-6689 California Institute of Technology 1200 East California Boulevard Pasadena, California 91125 (626) 395-6811
Transcript
NASA Reduced Gravity Student Flight Opportunities Program2002 Competition
First Choice: August 8, 2002Second Choice: March 21, 2002Third Choice: July 18, 2002
3
Abstract
The goal of this experiment is to investigate the transmission andlasing properties of ZBLAN microspheres produced in microgravity. Tothis end, we propose to fabricate microspheres from doped and undopedZBLAN as well as silica-based glass aboard the KC-135, so that we maylater compare their optical properties.
ZBLAN, a member of the heavy-metal fluoride family of glasses, promisesto play an prominant role in future fiber-optic technologies. ZBLAN fibershave the potential to transmit light with losses close the the theoreticalminimum allowed by matter—much lower than could be achieve withsilica-based fibers. Furthermore, ZBLAN has a wider optical transmis-sion window, which also comprises a larger region of near-IR light, whereit matters most.
This fluoride-based glass is not commercially used because, when it isfabricated in a gravity environment, tiny crystals form, thus destroying itstransmittance properties. If produced in a microgravity environment, onthe other hand, crystallization can be sharply reduced, allowing ZBLANto solidify into the amorphous form that gives this glass its outstandingtransmission properties.
The optical transmission properties of ZBLAN can be deduced fromthe longevity of whispering gallery modes induced in microspheres of theglass. Because of ZBLAN’s low attenuation, these microspheres might infact produce the resonator with the highest quality factor produced todate.
In addition to their high quality factors (Q’s), ZBLAN microspheres,when doped with erbium, are capable of lasing by up-conversion of infraredlight[6]. We hope that with higher-Q microspheres, we will be able toachieve higher conversion rates as well as narrower linewidths.
1 Introduction
The advent of fiber optics revolutionized the communications industry. Greatlyimproved band widths allow for unprecedented data transmission rates. How-ever, conventional silica-based optics suffer from serious limitations. They areonly capable of carrying information within a small range of the electromagneticspectrum and, even within this range, exhibit a relatively high signal attenua-tion. Seeking to discover a less restricted medium to replace current silica-basedmaterials, researchers have fabricated glasses in the heavy metal fluoride fam-ily. The most promising of these glasses is ZBLAN, named for its zirconium,barium, lanthanum, aluminum, and sodium components. ZBLAN can transmitlight with losses close to the absolute theoretical minimum if properly produced.Unfortunately, when fabricated in the presence of gravity, tiny crystals formwhich destroy ZBLAN’s potential transmittance properties. Dr. Dennis Tuckerof NASA’s Marshall Space Center has demonstrated that, in a microgravityenvironment, this crystallization does not occur.
Light resonances in microspheres produced in microgravity could be ex-tremely long-lived since the material attenuation coefficient is predicted to be
4
far lower than for any microspheres fabricated to date. This could result inunprecedented quality factors (Q’s). Since the Q value is related to the res-onator’s absorptiono coefficient, knowledge of the effects gravity on Q can beused to infer crystallization effects during the solidifying stage of ZBLAN.
In addition to the higher Q values, we believe that erbium-doped ZBLANmicrospheres can be used to create improved low threshold green lasers, whichhave a variety of applications in cavity quantum electrodynamics (cavity QED),communications, data storage, and medicine.
Part of our experiment will involve the quantitative comparison of the qual-ity factor of ZBLAN microspheres fabricated in normal earth gravity versus inmicrogravity aboard the KC-135. It highlights a novel approach to character-izing ZBLAN based on cavity QED techniques refined from previous research.The fabrication procedure is derived from a process developed by Dr. LuteMaleki and Dr. Vladimir Iltchenko of NASA’s Jet Propulsion Laboratory, whoare both advising our group in its efforts. In 1998, Dr. Dennis Tucker showedthat only very little crystallization occurs in ZBLAN fibers fabricated in a mi-crogravity environment in a previous experiment aboard the KC-135. We havebeen in correspondence with Dr. Tucker, whose experiences with the rather dif-ficult process of producing ZBLAN in microgravity have been very helpful. Wecouldn’t have asked a better person, since Dr. Tucker is involved in designing anexperiment to test ZBLAN fibers produced on the International Space Station.
The other part of the experiment will focus on how lower crystallization andabsorption in ZBLAN microspheres affects their lasing properties. That this ideais feasable was proven by recent research by Serge Haroche of the LaboratoireKastler Brossel, which demonstrated low-threshold green lasing with erbium-doped ZBLAN microspheres. We hope that with the high-Q, Er-doped ZBLANmicrospheres produced in microgravity will improve this groups results in termsof up-conversion efficiency. A higher-Q resonator would also yield a narrowerlinewidths. Such microsphere lasers, which produce light in the green frequencyrange, would have a variety of applications in cavity quantum electrodynamics(cavity QED), communications, data storage, and medicine.
2 Technical Background
This section will give a brief introduction to the subject areas relevant to thegroup’s project. It also includes short descriptions of the experimental tech-niques to be used.
2.1 Material Properties of ZBLAN
The heavy-metal fluoride glasses (HMFGs) have potential use in telecommu-nications fibers, owing to their relatively low optical losses, especially in theinfrared range. ZBLAN has the potential for an attenuation coefficient of .001decibels per kilometer (dB/km), but is currently limited by crystallization ef-fects to 10 dB/km, much poorer than the standard 0.2 dB/km for silica-based
5
Figure 1: Comparison of ZBLAN and silica-based fibers attenuation.
fiber optics. Refer to figure 1 for a graphical comparison.
The most studied HMFG is the ZBLAN group, containing fluorides of zir-conium, barium, lanthanum, aluminum, and sodium [1]. The most commonlyused form is 53ZrF4-20BaF2-4LaF3-3AlF3-20NaF. ZBLAN is the most stablefluorozirconate glass with respect to crystallization. ZBLAN can be cooled asquickly as 1 K/sec and experience only partial crystallization, but the result-ing 10 dB/km attenuation coefficient still limits the applications for long fibers.This crystallization is thought to be a result of ZBLAN being more of a mixturethan a compound. Like variously sized marbles in a jar, the different compo-nents will arrange themselves based on weights when gravity is present. Themicroscopic crystals act as mirrors, reflecting coupled light and thus reducingthe quality factor Q. The resulting large attenuation prevents ZBLAN frombeing a reasonable medium from which to make microspheres. If it is producedin microgravity, crystallization is virtually absent, as is apparent from Fig. 2
Zirconium and barium fluorides provide the main structure for ZBLAN.While aluminum, lanthanum, and sodium make ZBLAN more thermally stable(less susceptible to temperature flux), they do not have a large effect on theprimary structural arrangement, ZrF2−
6 bipyramidal molecules linked with Ba2+
ions (Figure 3).Doping ZBLAN with a rare-earth element is necessary for our second goal,
investigating the lasing properties of ZBLAN. In this form, the rare-earth el-ement replaces the central zirconium atom in the basic bipyramidal structure.Another chemical property of ZBLAN of importance to us is the glass transi-tion temperature. At 543 K [3], this value is far less than that of silica-basedglasses. This means that different temperature considerations must be made
6
Figure 2: ZBLAN is strikingly clearer when solidfied in 0 · g (left) than in 1 · g(right). [13]
Figure 3: Molecular Structure of ZBLAN [11]
when forming ZBLAN and silica-based spheres using the same setup apparatus.
2.2 Microspheres
Optical resonators are indispensable tools to researchers in many areas of physics.In fact, in recent years, a growing field has emerged in physics dedicated pre-cisely to these optical resonators. Known as cavity quantum electrodynamics(QED), it involves the study of atom-quantized field interactions confined to asmall region of space. [10]
In their search for better resonators, many physicists have turned micro-spheres (Figure 4). Typically between 100 to 1000 microns in diameter, suchresonators have losses significantly lower than other kinds of resonators. If lightof the proper frequency is introduced into the glass sphere, it will travel justwithin the surface boundary due to total internal reflection. The modes pro-duced in this way are known as Whispering-Gallery Modes (WGM’s) and arethe best means of achieving low loss and small mode volumes inside an opticalcavity.
These resonators have a range of technological and scientific applications.
7
Figure 4: Image of ZBLAN microsphere during production in ORIONICS FW-304 fiber splicer.
They promise to be extremely useful in future optical communication devices,interferometric sensing, high resolution spectroscopy, lasers, and in combinationwith many other photonic devices. [5]
2.2.1 Fabricating Microspheres
The fabrication process involves bringing the glass to its melting point and thenallowing it to cool. While the glass is liquid, surface tension causes it to beadup into a spherical shape. Common methods of melting a fiber tip involvethe use of a fusion splicer, a laser, or an oxygen-hydrogen torch [4]. For ourpurposes, the fusion splicer is the best choice, since the behavior of flames in 0gadds uncertainty, the laser re-alignment could be extremely problematic duringflight, and the splicer is simply a durable road warrior.
2.2.2 Quality Factor Q
The fraction of light lost during each cycle around the sphere is commonly mea-sured by the quality factor Q. Since Q is inversely related to the fractionalintensity loss per round trip, a larger Q indicates lower absorption. Thus, mea-suring Q allows us to compare the absorption of ZBLAN samples produced in 0gand 1g, as well as of silica-based glass in the two gravity conditions. We expectthe quality factor to be significantly higher for ZBLAN microspheres producedin microgravity than for microspheres with gravity present; for silica-based mi-crospheres, on the other hand, we expect no such marked difference.
Unlike for silica-based microspheres, the limitations on the Q factor forZBLAN spheres is unknown. As shown by [9], surface scattering ultimatelylimits the Q of a silica-based microsphere. Thus, if the ZBLAN or silica-basedmicrospheres have such low internal absorption that they broach this limit, wewill be comparing surface scattering losses, which in itself will be very inter-esting. If this occurs, we will still be able to compare the material absorption
8
coefficients by moving to wavelength where where internal losses outweigh sur-face scattering losses.
To compare absorption based on measurements of Q, one needs to reduce theother factors affecting Q. Research on silica-based microsphere resonators hasshown that two other appreciable factors are the loss due to surface scatteringand the loss due to optical absorption by water in the surface of sphere [9].
Surface scattering losses depend mostly on two microsphere properties: sur-face inhomogeneities, which cause diffusive scattering and surface curvature,which causes a greater fraction of light to be refracted to the outside of thesphere. Thus, surface scattering can be reduced by cleaning the surface1 and/orincreasing the sphere’s radius. Microspheres produced in microgravity are morespherical. Thus, surface inhomogeneities are reduced, and Q is larger.
Water absorption related losses have been documented by several researchgroups. [9] has shown that the Q decreases for several minutes after the micro-sphere is produced and has linked this observation to water being absorbed inthe surface. In 0g, convection currents in the air are eliminated. Since thesecurrents, which continually supply more water to the hot sphere, would be elim-inated, water absorption can be reduced by producing the spheres in an inertgas, such as N2. Water absorption losses have been shown to play a smaller rolefor higher-frequency modes; in particular, for the mode near 800 nm that will bestudied in this experiment, these losses should be insignificant. Furthermore,the losses should be smaller than those reported by [9] since this experimentuses a fiber splicer instead of the hydrogen-oxygen torch used by [9].
It is important to note that these studies all focus exclusively on silica-based microspheres. For the purposes of studying ZBLAN, referencing this pastresearch is useful only to indicate important variables to consider to optimizethe quality of the spheres. The chemistry of ZBLAN is quite different from thatof silica-based compounds.
Though preventing water absorption is not crucial when forming silica spheres,since, as discussed, the Q not limited by this variable, it is of dire importance tominimize water absorption during ZBLAN microsphere fabrication. Adsorbedwater will react with ZBLAN causing the sphere to appear milky. (We suspectthat this is an oxidization reaction, but need to further research this.) Thismilkiness significantly reduces the potentially ideal properties of the ZBLANproduct.
Procedures to remove water introduced to the fiber surface long before spherefabrication need to be considered. The option of heating the ZBLAN just enoughto evaporate adsorbed water, yet not to the melting point, has been eliminated.Doing such could potentially initiate a chemical reaction between the water andcomponents in the ZBLAN. Vacuum pumping fibers stored in an appropriatecontainer appears to be the best method of removing pre-adsorbed water.
Another reaction to avoid is the incorporation of oxide into the ZBLAN glassduring melting. Using an acid HCl solution of ZrOCl2 to etch the ZBLAN fibercauses an oxide layer to form on the surface, which only becomes problematic
1This is commonly done using fluoride plasma etching.
9
if the glass must be melted. In conclusion, it will be extremely difficult for usin an non-industrial lab to declad ZBLAN fibers using methods that will surelysufficiently prevent water absorption and/or the formation of an oxide layeron the fiber surface. A commonly used technique, which we may employ, forsignificantly reducing these effects is plasma etching using an ionized fluoridegas. Such etching takes place at nearly room temperature and requires the useof a plasma etcher, an apparatus that looks like a microwave oven.
Water absorption and oxide layer formation during decladding procedures,in addition to fabrication, was briefly mentioned above. Decladding the ZBLANfiber is necessary because melting a cladded fiber tip will not result in a homoge-neously mixed sphere. The resulting groupings of index gradients will increaselosses the same way that crystallization does so. ZBLAN fiber is extremely brit-tle, thus is difficult to handle even when cladded. Despite this, the difficultiescreated by cladding the fibers outweigh those experienced by purchasing fibercore with a mechanically removable plastic jacketing.
Another major variable to consider during the sphere fabrication processis the melting temperature. If the temperature of the fiber splicer arc is toohigh, selective vaporization, also referred to as breakdown, can occur. Giventhat ZBLAN is a mixture and not a compound, all components should not beexpected to react similarly and simultaneously to a given applied temperature.Adjusting the electrode separation directly affects the temperature of the arc,thus we will set an exact distance such that the sphere melts at a reasonablerate (given 20 second intervals of 0g), yet breakdown will not occur. Sincefiber splicers are designed for silica-based fibers, which have significantly highermelting points, preventing breakdown will be a serious consideration. A furtherconcern during the sphere production is that the temperature gradient acrossthe melting fiber tip must be minimized to avoid partial partial and un-evenheating.
In the case that the Q for the ZBLAN microspheres produced in microgravityis limited by surface scattering losses, this result alone would be interesting. Onewould still observe a difference in the Q for microspheres produced in 1g and0g. The absorption coefficient for ZBLAN that is produced in 1g is 10dB/km;that for silica-based is a minimum of 0.2dB/km. For silica, the maximum Qlimited by material absorption at for modes with 800 nm wavelengths is slightlylower than the Q limited by surface losses. If a similar relationship is truefor ZBLAN, as is a reasonable assumption, the difference between the Q’s forZBLAN produced in 0g and 1g (which correspond to Q limited by surface lossesand Q limited by material losses, respectively), would still be appreciable.
2.2.3 Measuring Q
To determine Q, one needs to measure the intensity of the output from thecavity as a function of the specified driving frequency. This is commonly doneby optically coupling the microsphere to the input (a tunable single-mode laser)and the output (an optical spectrum analyzer).
Q can be measured in two ways. First, from the transient response, one
10
Figure 5: An ideally matched fiber taper-microphere system, [8].
observes how quickly the output decays after the input of a short impulse. Thesecond way involves analyzing the steady-state response. The resonator is driven(optically pumped) at a specific frequency and the output is measured. Fromthe resulting resonance curve, the Q can be derived.
Optical coupling is a difficult procedure that requires high precision. Themethods most suited for our interests are the fiber tapering and prism tech-niques. In the first, a signal is sent through a tapered fiber. Near the taperedpart of the strand, the light leaves the fiber, and a part of it enters the micro-sphere, traveling along the inner circumference. The light then exits the sphereand enters a waveguide. The signal from the waveguide is processed and outputonto an oscilloscope. The Q for the sphere and tapered fiber system at the peakof the resonance curve (which approximates a Lorentzian function) equals twicethe Q of the microsphere alone.
In the prism method, a dialectic surface of higher refractive index is posi-tioned close to the sphere. Radiation is the coupled into such a mode by opticaltunneling through the gap between the microsphere and a prism. [6]
2.3 Lasing Properties
2.3.1 Green Lasing
When doped with erbium, these microspheres can be used to create a greenmicrolaser. Due to its very low threshold, small size, and ease of fabrication, thistype of laser promises applications in telecommunications, optical data storage,medicine, and display technology.
11
2.3.2 Erbium energy levels
When pumped with a 801 nm diode laser, the trivalent erbium ion is excitedto a lasing level, which can undergo population inversion to create a 550 nm(green) laser. This process requires two-photons for successful up-conversion(Figure 6).
The lasing level decays at different rates for silica and ZBLAN [7]. Thelifetime of this state in ZBLAN greatly exceeds the lifetime for silica, prohibitinggreen lasing in silica at reasonable pump powers. However, we should be ableto observe green lasing from erbium-doped ZBLAN microspheres.
2.3.3 Laser qualities
A set-up based on [6] will be used to measure the laser’s properties. First,collimation of the light output from the microsphere to a silicon photodiode willbe achieved by the use of a high aperature lens. Power absorption measurementswill be made by the photodiode. Second, a monochromator connected to a photomultiplier will observe the green radiation.
With this set-up, the intensity of green radiation can be compared to theabsorbed pump power. Multiple research groups ([6], [12], [14]) have noticedan “apprenticeship” effect, where the intensity of green radiation and the ab-sorbed pump power do not correlate. These irregularities, however, disappearafter a warm-up period of a few minutes. With more spherical and less crys-tallized microspheres, as are those produced in microgravity, the apprenticeshipeffect might be reduced. Furthermore, a systematic difference in the appren-ticeship effect might lead to a better understanding of this poorly understoodphenomenon.
By adjusting the wavelength of the pump laser to match the resonance of theWGM, a threshold of 30 microwatts was obtained in erbium-doped ZBLAN [6],which is more than 100 times lower than the previous lowest obtained from anymaterial. We hope to improve this threshold with our microgravity-fabricatedZBLAN microspheres.
2.4 Test Objectives
In our experiment, we are most interested in the lasing and transmittance prop-erties of the ZBLAN microspheres. To determine the transmittance properties,we will rely mainly on measuring the quality factor Q (see Sec. 2.2.2). If weare in the regime where the Q is limited by the absorption of the glass, thenthe Q value can be used to calculate the attenuation coefficient of the ZBLANproduced. Such regimes can always be achieved by choosing the appropriatelaser frequency, since ZBLAN’s absorption properties are frequency dependent.These measurements will yield an absorption spectrum, which we will compareto that for silica-based glass. If Q is surface-scattering limited, we can learnabout the scattering properties. The surface-scattering losses, too, will be com-pared to those for silica-based glass produced under identical conditions. We
12
Figure 6: Energy level diagram of Er3+ in doped ZBLAN [6]
expect to find much higher values for Q for ZBLAN spheres produced in 0g thanin 1g.
To investigate the lasing properties, we observe the absorbed pumped powerand the output intensity of green radiation (Sec. 2.3.1). As mentioned above,the lasing threshold is predicted to be lower for ZBLAN spheres produced in 0g.
2.4.1 Experimental Goals—Summary
1. At JPL, fabricate doped and undoped ZBLAN and silica-based micro-spheres of various sizes in 1 · g using the FW-304 fiber splicer.
2. In microgravity, fabricate Er-doped and undoped ZBLAN and silica-basedmicrospheres, again using the FW-304 fiber splicer.
3. Back at JPL, test the effects of size, doping, and gravity on the whispher-ing gallery modes in the microspheres to determine transmittance andcrystallization, which entails Q factor measurements.
4. Also at JPL, observe green lasing in erbium-doped microspheres producedunder the various gravitational conditions.
13
2.5 Test Description
The immediate goal of our experimental procedure while on the KC-135 is toproduce microspheres of doped and undoped ZBLAN glass as well as of un-doped silica-based glass in 0g conditions. Making a sphere involves insertinga 15cm long ZBLAN fiber into one end of a fiber splicer. The splicer createsa high voltage between two electrodes separated by approximately 1cm. Theair between the electrodes breaks down, thus the resulting plasma carrying thecurrent. This small voltage arc is hot enough to melt the extremely thin fibersat a fairly rapid rate, so that a small sphere beads up at the end of the fiber.Producing a sphere that’s 100µm takes about 5 seconds.
Due to the water absorption and oxide layer issues previously addressed,sphere fabrication will be performed in an inert environment. Thus our setupwill be contained within a glovebox filled with N2, He, or Ar and mounted ona sturdy board. Using an appropriate measurement apparatus, we will insurethat the inevitable water inside the glovebox remains less than 1ppm. This valuewas suggested by Dr. Refik Kortan of Bell-Labs, who has successfully pulledZBLAN fibers in a 1ppm of water environment without ending up with a milkyproduct.
Fabricating half of the spheres will take place on the KC-135 and a matchingbatch will be made in a lab at NASA’s Jet Propulsion Laboratory, in order tocompare spheres made in 1g conditions to those produced in microgravity. Alltesting and data analysis will be done in laboratories at the California Instituteof Technology and NASA’s Jet Propulsion Lab. The following is a generaloutline of our post-flight procedure and data analysis plan:
2.5.1 Pre-flight Fiber Preparation
We have two major options for pre-flight ZBLAN fiber preparation. One isto use a procedure similar to that used last year, which involved basic cuttingfiber lengths, stripping, and cleaving procedures. In order to prevent the samewater absorption problems experienced, we would fluoro plasma etch the fibersand then place them into glass vacuum tubes. The other option is to purchaseprecut, prestripped, precleaved, pre-vacuum packaged ZBLAN fibers from Bell-Labs since we know that they produce the fibers in an appropriately regulatedenvironment, a fact that we do not know about our previous fiber suppliers. Inthis case, we would purchase only enough prepackaged fibers for the 0g ZBLANspheres. We would still utilize plasma etching for the 1g spheres, since thesewill be produced at JPL, where we will have more leeway for trial-and-error.Additionally, we will prepare the silica-based fibers using the same technique aswe used last year. Our silica-based spheres turned out nicely spherical, lackingthe slight droop and minute disfigurement present when produced in a 1g en-vironment. This will certainly be even more evident when we measure the Qfactor of these spheres next week.
14
2.5.2 Experiment Description Overview
1. Part I: Fabricating Microspheres
2. Microsphere fabrication (KC-135):
(a) In portable glove box filled with argon, backfill vacuum containers
(b) Make appropriate adjustments to electrode separation of fiber splicerto make it suitable for the different types of glasses used.
(c) Make 20 microspheres using splicer, in an inert environment in glovebox.
3. Part II: Testing Undoped ZBLAN and Silica-based Microspheres
(a) Excite resonant modes in spheres using single-mode tapering tech-nique (see Fig. 5).
(b) Observe output intensity using photodetector coupled to microsphereresonator
(c) Observe transient response:
i. Excite resonator with short pulseii. Observe how the osciallating signal decays in timeiii. Determine quality factor Q and absorption coefficients δi.iv. Repeat for different wavelengths in IR region to get absorption
spectrum.
(d) Observe steady-state response
i. Determine Qii. Repeat for different wavelengths in IR region to get absorption
spectrum.iii. Compare with transient response measurements.
(e) Determine surface-scattering losses, if possible.
i. Use pumping beam at frequency f0 where absorption effects areminimized in spheres.
ii. Repeat for different frequency in near f0.iii. Analyze if Q limited by surface scattering or internal absorption.
(f) Compare Q values obtained for ZBLAN microspheres produce in 0 ·g and 1 · g to those obtained for silica-based microspheres undercorresponding conditions.
(g) We expect the following results:
i. Higher Q for microspheres produced in 0 · g than for those pro-duced in 1 · g
ii. Higher Q for ZBLAN microspheres than for silica-based micro-spheres
15
iii. Higher conduction frequency range for ZBLAN microspheres
4. Part III: Testing Doped ZBLAN Microspheres
(a) Couple microsphere to silicon-diode and take three measurements:i. Pump-power from near-IR laser absorbed in microsphereii. Q of pump modeiii. Intensity of green up-conversion radiation using a spectrum an-
alyzer(b) We expect the following results:
i. Higher Q for microspheres produce in 0g than for those producedin 1g.
ii. ZBLAN microspheres exhibit more efficient up-conversion thanthat shown in silica-based ones in previously published results
iii. Higher Q for ZBLAN microspheres than for silica-based onesfrom Part I
iv. Lower lasing threshold for ZBLAN microspheres produced in 0gthan for those produced in 1g and than for silica-based ones inpreviously published results as well as our own 1g results
2.6 Justification of Follow-Up Flight
A combination of our previous experience and further research helped us toformulate a new, strict procedure for both pre-flight fiber preparation and in-flight fiber fabrication. Elements of this procedure are discussed above, in thesections analyzing methods of preventing water absorption. The goals of thisexperiment coincide with those of last year’s version. Water absorption effectsfor ZBLAN lack a strong research background, unlike those for silica-basedglasses. Our first experiment allowed us to understand the sensitivity of ZBLANto water absorption. The use of a mere glove bag, versus the newly proposedglovebox with a precise humidity regulator, did not adequately prevent waterabsorption, resulting in visibly milky ZBLAN microspheres. The milkiness,though not detrimental to the experiment, results in reduced optical properties.
We are currently still in the process of analyzing the data from our sum-mer flight. After mastering coupling and analyzing procedures on test silicaspheres, we will complete data analysis on our spheres that were fabricatedon the KC-135. In about two weeks, we will have numerical results, that is,Q factors for our ZBLAN and silica-based spheres. It is clear, though, thatthe ZBLAN spheres’ Q factors will be significantly lower than their potentialdue to this milkiness. INO’s (a ZBLAN fiber distributor in Quebec) researchand development department informed us that, after re-solidifying ZBLAN, weimmediately can determine whether our results will reflect ZBLAN’s amazingoptical properties based on the clear presence or lack of milkiness. Now thatwe know exactly how to prevent this milkiness and have time to raise adequatefunds and construct a setup with a strictly controlled environment, we havehigh expectations of a follow-up flight.
2. Serena Eley, Dirk Englund, and Joseph Jewell participated in the Summerof 2001 flights. Wook Hwang has no previous experience on the KC-135.
3.2 Experiment Description/Background
For a detailed discussion of the experiment description and background, pleaserefer to Sec. 2 (Sec. 2.5 in particular.) Here we repeat the main points:
• We are researching the effects of gravity on the solidification of ZBLANmicrospheres.
• In our experiment, we will produce microspheres of two different typesof glasses, ZBLAN and silica-based, doped and undoped, under differentgravity conditions. Afterwards, we will test the optical properties of thesemicrospheres in laboratories at Caltech and JPL.
• Note that the laser applications (Sec. 2.3) will not require us to use anylaser equipment at JSC, since all analysis will be done at Caltech and JPL.
• The proposed experiment has not yet been done before.
3.3 Equipment Description
Our test setup includes the following items:
1. Orionics Fiber Splicer
(a) DESCRIPTION
i. FW 304ii. CASE - Molded fiberglass, ”O”-ring sealed, dust and watertight
with cover closed and latchediii. ARC - High frequency AC, tungsten electrodes
A. Frequency: 20 kHz - 60 kHzB. Voltage: 1.2 kV (reaches maximum of 60 kHz when first
activated)C. Temperature: just hot enough to melt glass
iv. POWER - 12 VDC (from power supply, see below)v. BATTERY - Internal battery removedvi. DURABILITY - Withstands 1ft drop onto concrete floor w/o
internal damage with cover closed
17
Figure 7: Sketch of experimental set-up.
18
Figure 8: The preferred position of our setup is to have the short side of ourrectangular mounting board perpendicular with the airplane side walls.
(b) DIMENSIONS: 15.5” x 15” x 5”
(c) WEIGHT: 28 lbs
2. Glove Box
(a) Description:
i. Made from Plexiglassii. Includes 2 pairs of glovesiii. Includes 1 equipment access holeiv. Filled with helium or argon before take-off.
(b) Dimensions and Weight:
i. 37” x 27 ” x 18 ”ii. entrance: 18.0 ” x 16”iii. wall thickness: 0.5 ”iv. weight: 19 lbs
3. Power Supply (Tripp Lite PR7)
(a) DESCRIPTION
i. A/D conversionii. 115VAV (60 Hz) to 13.8 VDC, 2.8 amps max, duplex GFCI outlet
(b) DIMENSIONS: 6” x 8” x 4”
(c) WEIGHT: 7 lbs
4. ZBLAN undoped fibers
19
(a) QUANTITY: 20 per flight
(b) DESCRIPTION
i. Bell-Labs
(c) DIMENSIONS:
i. LENGTH - 15cmii. no cladding
(d) WEIGHT: Negligible
5. ZBLAN doped fibers
(a) QUANTITY: 20 per flight
(b) DESCRIPTION
i. Bell-Labs (?) or Infrared Fiber Systems
(c) DIMENSIONS:
i. LENGTH - 15cmii. no cladding
(d) WEIGHT: Negligible
6. Silica-based fibers
(a) QUANTITY: 20 per flight
(b) DIMENSIONS:
i. LENGTH - 15cm
(c) WEIGHT: Negligible
7. Fiber Outbox
(a) QUANTITY: 3
(b) DESCRIPTION
i. Styrofoam strip on which finished fibers will be mounted is tapedinside. The tape can hold 1 lb per 4” and we used 5”.
ii. Plastic Tupperware container w/standard click-on lid
(c) DIMENSIONS: 5” x 5” x 2.5”
(d) WEIGHT: 0.1 lbs
8. Fiber Inbox
(a) DESCRIPTION
i. Fibers stored in between 2 strips of styrofoam that are tapedinside. The tape can hold 1 lb per 4” and we used 5”.
ii. Plastic Tupperware container w/standard click-on lid
(b) DIMENSIONS: 10” x 6” x 2.5”
20
(c) WEIGHT: 0.3 lbs
9. Mounting Board
(a) DESCRIPTION
i. Made from Alii. Perpendicular ”supports” screwed on to restrict motion of splicer
and power supply
(b) DIMENSIONS: 48.0” x 25.5” x 0.5”
(c) WEIGHT: 26 lbs
10. Surge Protector
(a) DESCRIPTION
i. 6 outletsii. 4ft pwr cordiii. 330 V cutoff, 15 amp cutoff
(b) DIMENSIONS: 10” x 1.5” x 1”
(c) WEIGHT: 0.5 lbs
11. Multimeter
(a) DESCRIPTION: Fluke 87 True RMS
(b) DIMENSIONS: 8” x 4” x 2”
(c) WEIGHT: 0.5 lbs
12. Vacuum Tubes
(a) contain fibers
Contents of ToolboxItem Qty Description Weight
Wire Cutters 1 standard negligibleTweezers 2 pairs standard negligibleScotch Tape 1 roll to tape down fibers inside outbox negligible
• Overall Assembly Weight: 90 lbs
• Overall Assembly Dimensions: 48.0” x 25.5” x 5.75”
• Layout of equipment during flight: The exact position of the ensemble isnot important. It is preferable, though, that the 25.5” side be situatedperpendicular to the aircraft wall. The board, to which all of the equip-ment is bolted and/or strapped, will be bolted to the 20” x 20” grid ofholes in the aircraft floor at exactly 6 points.
21
Figure 9: Jewell uses the Fiber Splicer ORIONICS FW-304 to produce a ZBLANmicrosphere.
3.4 Structural Design
All devices are securely mounted (Fig. 7), in the following way:
Item Dimensions (LxWxH) Weight ConstraintOrionics Fiber Splicer, FW 304 15.5” x 15” x 5” 28 lbs Strapped boardPower Supply 6” x 8” x 4” 7 lbs Strapped to boardFiber Out-boxes (with fibers) 5” x 5” x 2.5” 0.1 lbs Velcroed & strapped to boardFiber Inbox (with fibers) 10” x 6” x 2.5” 0.3 lbs Velcroed & strapped to boardMounting Board 48.0” x 25.5” x 0.75” 26 lbs Bolted to aircraft floorSurge Protector 10” x 1.5” x 1” 0.5 lbs strapped to boardGlovebox 37” x 27 ” x 18 ” 19 lbs bolted to boardVacuum Container bolted to boardMultimeter 8” x 4” x 2” 0.5 lbs bolted to board
The splicer, fiber in-boxes and out-boxes and a tool box will be containedinside the glove box.
Overall Assembly Weight: 90 lbs
3.5 Electrical Systems
3.5.1 Electrical Components
1. All wiring and electrical devices
(a) Electrical Devices
22
23
i. Fiber SplicerA. Fuse protectedB. Draws <2.8 amps when arc is active
ii. Power SupplyA. A/D conversion: 115VAC (60 Hz) to 13.8 VDC, 2.8 amps
A. 6 outlets (plastic inlets will be placed in the unused outlets)B. Power shutdown at 15 amps, up to 1875 W, 330 V cutoff; 60
HzC. The master ”kill switch” is located on the surge protector.
iv. Multimeter(b) Wiring
i. From 115 VAC, 60 Hz, Single Phase aircraft outlet to surge pro-tector
ii. From surge protector to power supplyiii. From power supply to fiber spliceriv. Multimeter connected in series with splicer and power supply
(c) Each power cord from an aircraft power distribution paneli. 115 VAC, 60 Hz, Single phase, standard household outlet
(d) Aircraft outlets used, and voltage and current draw on each outleti. Only one aircraft outlet will be used - 115 VAC, 60 Hz, Single
Phase, standard household plug.ii. The fiber splicer will draw about 2.8 amps, which is 14 % of the
outlet rated supply current. For the surge protector, power shut-down occurs at 15 amps (75 % of the rated supply current), thusour experiment will not draw more than 80% of the maximumoutlet current of 20 amps.
(e) The surge protector limits the current. For additional safety, boththe fusion splicer and the power supply are fuse protected.
2. The surge protector switch is easily accessible and acts as a master ”killswitch.”
3. Note that the air-vents of all electrical equipment will be covered with airfilters to prevent ingestion of airborne debris.
3.5.2 Load Table
Power Source Details Load Analysis
Name: Power Cord 1 Surge Protector (shut-off at 330 V, at 15 amps)Voltage: 115 VAC, 60 Hz Power Supply (fuse limited to 7.0 amps)Wire Gauge: 12 Fiber Splicer (fuse protected) - 2.8 AmpsMax Outlet Current: 20 Amps Total Current Draw: 2.8 Amps
24
3.6 Pressure/Vacuum System
The ZBLAN fibers will be taken on the plane in a steel container which is underweak vacuum. This inbox will be bolted, inside the glove box, to the mountingboard. Once the glove box is filled with helium, the ZBLAN fiber in-box willbe opened to ensure that the fibers never come into contact with air. All ofthis will be done before take-off. The vacuum sealed in-box poses absolutely nohazards.
3.7 Laser System
We will not require a laser system until we evaluate the microspheres back atCaltech/JPL.
3.8 Crew Assistance Requirements
We will not require special assistance from the crew.
3.9 Institutional Review Board
N/A
3.10 Hazard Analysis
• High voltage is applied to the arc area during fiber fusion; contact tothe fusion area should be avoided, since an electrical shock can occur.However, the area is protected from the operator.
• The fusion splicer should not be operated with the base plate removedfrom the case, since exposed wiring creates a shock hazard.
• Misuse of the fiber fusion splicer in the presence of flammable gas ormaterials may cause an explosion. Our experiment involves no flammablematerials.
• Preventive Measures
1. Fuses are implemented in both the DC power supplies and the in-strument. Should a power short or surge occur, main power will beimmediately cut off.
2. The surge protector will prevent any possibly unforeseen problemsthat might occur with current spikes.
Tools Needed During Flight (kept in tool bag)Item Qty Description Weight
Wire Cutter 1 standard negligibleTweezers 2 pairs standard negligibleScotch Tape 1 roll to tape down fibers inside outbox negligible
The team will provide all of these tools. We will keep all ground tools in atool box or bag labeled ”Caltech” or ”CIT” in black marker on masking tape.We have reduced the number of tools actually necessary in flight to the bareminimum, requiring only lab tweezers, and wire cutters.
3.12 Ground Support Requirements
1. We will require ground power – 115 VAC, 60 Hz, for a standard householdthree-prong outlet.
2. A small table on which to setup our equipment is requested.
3.13 Hazardous Materials
The materials used aboard the KC-135 for this experiment are not toxic, corro-sive, explosive, or flammable.
3.14 Procedures
Refer to Sec. 2.5
4 Outreach
In the case that we are accepted to participate in the NASA KC-135 2002program, one of our priorities will be to inform the community about the ex-periences that the RGSFOP program offers its participants. Group memberswho participated last year believe this to have been one of the best experi-ences of their lives. This gives us strong motivation and a feeling of personalresponsibility to share our experience with others.
26
4.1 Objective
With the construction of the International Space Station, microgravity will soonbecome an available natural resource for both science and industry. If recog-nized and utilized, science under microgravity could lead to ground-breakingtechnological innovations over the next twenty years. It is therefore critical thatwe, as participants in the KC-135 program, spread the word about how projectslike ours might exploit microgravity to achieve advances in science that couldone day revolutionize the communications industry. In doing this we will helpextend the scientific vision to the children of today, who will become the adultsof tomorrow.
4.2 Activities
In order to achieve this goal, we will address student audiences in several lo-cal high schools. In a classroom setting, we plan to describe the developmentand results of our experiment, our individual perceptions of microgravity, andthe opportunities available to students in the field of science. The focus of ourefforts will be to reach a cross-section of high-school students and encouragecareers in science. Our targeted schools include large minority populations thatare traditionally underrepresented in science as well as schools whose studentscommonly enter into careers in science. We have contacted and received enthu-siastic responses from the four schools listed below. At the current time, we arestill finalizing plans with these four schools and are contacting others.
• Garfield High School
– 5101 E. Sixth St
– Los Angeles, 90022
– 323-268-9361
– Current Principal: Norma Danyo
• La Canada High School
– 4463 Oak Grove Ave
– La Canada
– 818-952-4200
– Current Assistant Principal, Curriculum: Lindi Arthur
• San Marino High School
– 2701 Huntington Drive
– San Marino 91102
– 626-299-7020
– Current Principal: Loren Kleinrock
27
• Roosevelt High School
– 456 S. Matthews Street– Los Angeles, 90033– 323-268-7204– Current Principal: Henry Ronquillo
Also, we would like to present at the Compton Community College (1111East Artesia Boulevard, Compton, California 90221-5393) and still need to finda contact there.
At these schools we will present our project to physics classes. Refer to theattached powerpoint file for a sample of our presentation. (Please note that allpictures included in the presentation will actually be those from our 2002 flight,if we are granted a renewal flight. This presentation has actually specificallybeen created for our 2002 outreach.) We also intend to describe the potentialof microgravity as a resource for discovering new science and technology. Wewill stress the significance of the International Space Station as a portal to thisnew domain of science, a field that ultimately has the potential to revolutionizemany aspects of modern society.
We will follow up with a second visit in the spring where we will discuss theresults of our data analysis as well as an overview of college science programsand research opportunities. Each member of our team will contribute duringthis talk by describing his or her experiences in the scientific environment froma personal perspective.
The final step in our outreach program is to set up an exhibit in a ma-jor science museum in San Francisco. The two possibilities are the CaliforniaAcademy of Sciences and the Exploratorium. This exhibit will contain an arrayof posters, video clips, and a demonstration of our experiment. We will do thisfor one week during our spring break in 2002.
4.3 Website
To supplement our outreach program we have designed a technical website,kc135.caltech.edu, which contains complete information on the nature of ourexperiment (including theoretical background and future prospects). We willincorporate links to NASA sites for the KC-135.
5 Administrative
5.1 Letter of Endorsement from Caltech
See Appendix.
5.2 Statement of Supervising Faculty
See Appendix.
28
5.3 Project Budget
5.3.1 Expenses
Fibers from Bell-LabsUndoped ZBLAN Fibers $3000.00ErF3 Doped ZBLAN Fibers $3000.00Glovebox $1500.00Travel Expenses:Airfare ($350/person) $1400.00Hotel $2000.00Food ($25/person/day) $1375.00Local Transportation $ 600.00Class III Physicals $ 350.00Mailing Reimbursement for Proposals $35.00Copying Reimbursement for Proposals $45.00Postal Costs of Equipment to Johnson Space Center (UPS) $58.00Total $11,363.00
5.3.2 Funding
We expect partial funding from JPL for our equipment and materials expenses.Additionally, we are also exploring grants from the National Science Foundation,the Mellon Foundation, the JPL Education Office, and various industry sources.
5.4 Institutional Review Board
No animal subjects will be used in this experiment.
5.5 Parental Consent Form
All team members are over 18, so no parental consent forms are required.
6 Appendix
The following pages contain:
• Participant Information Forms
• Photocopies of Picture ID’s
• Statement of Current Undergraduate Status
• Statement of Supervising Faculty
• Letter of Endorsement from Caltech
29
References
[1] 1998 Photonics Directory: The Photonics Dictionary. Flourozirconate (ZBLAN)http:ioapc13.epfl.chPhotonicsDOCSwcd00024wcd024b3.htm
[2] Cai, Ming. personal home page, URL: http://www.its.caltech.edu/ mingcai/, Mar2001.
[3] Crichton, et al. Viscosity Temperature Dependence and Crystallization of HeavyMetal Fluoride Glasses. MRS Symposia Procedings. 1986. Volume 88, pp.115-8
[4] Gorodetsky ML and Ilchenko VS, Laser Physics 2, 1004 (1992).
[5] Griffel G, Serpenguezel A, and Arnold A, 1995 IEEE Symposium on FrequencyControl (Institute of Electrical and Electronics Engineers, New York, 1995).
[6] von Klitzing W et al. Very low threshold green lasing in microspheres by up-conversion of IR photons, J.Opt.B: Quantum Semiclass. Opt. 2 (2000), pp 204-206.
[7] Layne CB, Lowdermilk WH, and Weber MJ Multiphonon relaxation of rare-earthions in oxide glasses, Phys. Rev. B, 1997, 16, (1), pp 10-20
[8] Ming Cai, Oscar Painter, and Kerry Vahala, Observation of Critical Coupling ina Fiber Taper to a Silica-Microsphere Whispering-Gallery Mode System, Phys.Rev. Lett, Vol. 85(1), 74 (2000).
[9] Vernooy DW, Ilchenko VS, Mabuchi H, Streed EW, and Kimble HJ, High-Qmeasurements of fused-silica microspheres in the near infrared, Optics Letters,Vol. 23(4), 247 (1998).
[10] Mossberg Research Group, Cavity Quantum Electrodynamics, URL:http://opticb.uoregon.edu/ mosswww/cqed/cqed.html, Mar 2001.
[11] Murtagh, M.T. Compositional investigation of YB(3+)-doped glasses for laser-induced fluorescent cooling applications. Rutgers the State University of New Jer-sey - New Brunswick, 1999.
[12] Piehler D and Eden JG 11.7 mW green InGaAs-laser-pumped erbium fibre laser.Electron. Lett. 1994, 33, (23), pp 1958-1960
[13] Tucker, Dennis.ZBLAN Research Takes Step Forward. June 3, 1997.http://www.ssl.msfc.nasa.gov/msl1/themes/accomplishments.htm
[14] Whitley TJ et al. Upconversion pumped green lasing in erbium doped fluorozir-conate fibre. Electron. Lett. 1991, 27, (20), pp 1785-1786.
