R85-956642

(NASA-CE-178799) INVESTIGATIO N Of N86-24S57

BREADBOAR D 12flPEEA10RE PROFILIN G SYS'I^fl FCB

SSHE PUE1 PREEUiNE E DIAGNOSTIC S final

Technical Report, Oct. 1S83 - flay 1985 Unclas

(Dnzted Techacloyies Eeseaich Center) U4 p G3/35 42920

_g^^^^^^^^^^^^^^^^___^^^^_^^^^^^^^^^^^^^___^.^^^_____^_^^_^^^^^^^__^^^^^^^^

INVESTIGATION OF BREADBOARD TEMPERATURE

PROFILING SYSTEM FOR SSME FUEL PREBURNER

DIAGNOSTICS

Final Technical Report

J.A. Shirley

January, 1986

NASA Marshall Space Flight Center

Contract NAS8-34655

October 1983 — May 1985

UNITED

TECHNOLOGIES

RESEARCH

CENTER

Easi Hartford, Connecticut 06108

85-5-47-33
R85-956642

Investigation of Breadboard Temperature

Profiling System for SSME Fuel Preburner

Diagnostics

Final Technical Report

Contract NAS8-34655

J. A. Shirley

January 1986

Prepared for

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

United Technologies Research Center

East Hartford, Connecticut 06108
R85-956642

Investigation of Breadboard Temperature

Profiling System for SSME Fuel Preburner

Diagnostics

TABLE OF CONTENTS

SUMMARY

INTRODUCTION

FIBER OPTIC RAMAN THERMOMETRY

Optical Fiber Tests . 4

Optical Head Tests 10

Simulated Raman Measurements ... 15

Comparison to Predicted Signal Rates 28

Discussion of Results 33

CONCLUSIONS AND RECOMMENDATIONS 38

REFERENCES 39

I TECHNOLOGIES
R85-956642

Investigation of Breadboard Temperature Profiling System

for SSME Fuel Preburner Diagnostics

SUMMARY

Results of an experimental investigation performed by United Technologies

Research Center (UTRC) under Contract NAS8-34655, sponsored by the George C.

Marshall Space Flight Center to determine the feasibility of temperature

profiling in the space shuttle main engine (SSME) fuel preburner are presented. In this application it is desired to measure temperature in the preburner

combustor with a remote, non-intrusive optical technique. A previously reported conceptual design study identified a system using spontaneous Raman

backscattering excited by a remotely-located argon ion laser propagated to the

SSME through a small diameter optical fiber as the best approach. The results

of measurements with a breadboard Raman system are described in this report.

The hot, high pressure atmosphere of the SSME fuel preburner is simulated by a

static, electrically heated high pressure optical cell containing molecular

hydrogen. Experiments have included: tests of multimode optical fibers for

transmission of argon ion laser radiation at 488 nm; tests of a breadboard

version of the optical head used to couple the laser probe into the combustor

and collect backscattered Raman-shifted radiation; and Raman tests of the

entire system with the high pressure cell. Initial measurements, observing

Raman scattering from pure rotational transitions were not completely successful, because of interferences from stimulated Raman radiation generated by the

high power laser radiation in the optical fiber which is subsequently Rayleigh

scattered in the test cell. Measurements of vibrational Raman scattering were

successful, and the results of experiments undertaken up to temperatures of

1100 K and pressures to 4500 psia are reported. Temperature precision and

accuracies down to 3 percent have been demonstrated with good spatial resolution « .04 cm3).
R85-956642

INTRODUCTION

There is no capability at present to measure gas temperatures in the hot

gas path of the space shuttle main engine (SSME), except by the insertion of

thermocouples through existing access ports. Thermocouples may perturb the

flow yielding inaccurate results and furthermore are generally slow to respond.

Future engine developments and improvements to the present engine will most

likely require accurate means to measure hot gas temperatures in critical SSME

system components with high spatial and temporal resolution. The objective of

this investigation is to develop a nonperturbing optical diagnostic technique

for measuring gas temperature profiles in the SSME fuel preburner. This

report describes the results of a successful experimental investigation of

breadboard hardware for SSME thermometry.

A previous report (Ref. 1) presented the conclusions of a conceptual

design study aimed at the development of a nonintrusive, optical means of

temperature measurement in the SSME fuel preburner. In that study spontaneous

Raman backscattering measurements of hot molecular hydrogen spectra, implemented through optical fibers, was selected as the best technical approach. Key

to the selection of this approach was the limited optical access in the existing SSME preburner. Only two transducer ports are available on the existing

engine and they are not diametrically opposed. For this reason a single port

technique is favored. Spontaneous Raman can be used with this type of access.

Furthermore at the extremely high pressures and moderate temperatures existing

in the SSME, Raman measurements should be realizable using an optical fiber to

convey the laser radiation to the preburner to excite the Raman spectrum, and

a second fiber to collect the Raman-shifted radiation and transport it to

spectroscopic detection apparatus. The laser and detection apparatus can

thereby be located remotely in a protected environment. At the same time the

optical fiber provides a relatively protected means of transporting radiation

to and from the engine.

In the breadboard hardware development investigations, reported herein,

Raman scattering has been measured using optical fibers and a medium power

argon-ion laser. An electrically-heated, high pressure optical cell has been

used to simulate the high pressure molecular hydrogen conditions existing in

the SSME fuel preburner.

The application of moderate power argon-ion (approximately 3-10 Watts)

radiation through multimode core fibers has been experimentally investigated.

A breadboard version of the optics to focus the laser into the probe volume,

to collect Raman scattered radiation and to focus it into another fiber, has

been developed. A spatial resolution of 10 mm at working distances up to 270
R85-956642

mm has been demonstrated. The experiments to be described include measurements of Raman scattering from pure rotational transitions, and Q-branch

transitions. Rotational scattering measurements were attempted first because

calculations predict higher intensities for rotational transitions as opposed

to vibrational-rotational transitions. Interferences from stimulated Raman

scattering generated in the laser transmitting fiber and from grating scattering, however, were found to preclude such measurements. Vibrational

Q-branch measurements overcame both of these limitations. Molecular hydrogen

gas temperatures in a heated optical cell were successfully measured. The

measurements show, however, that power losses are more severe than anticipated

in the transmitter fiber for low order spatial propagation modes in the fiber.

Low order modes are needed to obtain good measurement spatial resolution and

to couple the collected Raman signal efficiently into the receiver fiber leading to the detection apparatus. This likely means that the laser will need to

be located near the preburner (less than 10 meters) to limit power losses.

The detection apparatus can still be located remotely, say 40 meters away,

from the rocket engine. These measurements indicate that it should be feasible to measure temperatures at about 5 percent precision, with spatial resolution of 1 cm and 10 millisecond temporal resolution.

In the next section of this report the details of the measurement technique are very briefly reviewed. The results of fiber transmission tests are

then described, followed by a description of tests of the optical head which

couples laser radiation into the preburner and collects Raman backscattered

radiation. The results of measurements in the high pressure optical cell are

presented next. These results are compared to the predicted performance

needed to meet overall measurement goals. Conclusions and recommendations for

future investigations are described. References are contained in the last

section.
R85-956642

FIBER OPTIC RAMAN THERMOMETRY

This section describes experiments undertaken to develop fiber optic

Raman diagnostics for thermometry of the fuel preburner of the space shuttle

main engine. The results should be generally applicable where gas densities

are high and molecular hydrogen is a dominant species.

The principles upon which Raman thermometry are based are described in

the conceptual design report (Ref. 1) prepared under the previous study contract. Reference should be made to that report for the theoretical background

for fiber optic Raman thermometry (FORT). The general concept of FORT diagnostics is illustrated in Fig. 1. The system is conceived as composed of a

high power continuous wave laser, an optical head mounted directly to the preburner, and a spectrograph/optical multichannel detector (OMD) combination.

Optical fibers permit the location of the laser and the detection apparatus at

a safe distance from the engine. The optical head serves to focus the laser

light into the preburner, defines the probe volume, and collects the Ramanshifted light which is backscattered and focuses it into the receiver fiber.

This fiber connects to the spectrograph where the radiation is dispersed and

then detected with a multichannel detector. It is the distribution of radiation in the Raman spectrum which determines the gas temperature, as described

in Reference 1.

This section begins with a description of tests of the power handling

capabilities of commercial optical fibers. Relatively high power radiation is

required to be incident on the probe volume to achieve the measurement objectives. Following that discussion is a description of tests to determine the

spatial resolution of the optical head. This is an important parameter of the

diagnostic system and also is important to characterize in a backscattering

system working with a small aperture. After that Raman measurements with the

breadboard FORT system are described.. In these measurements, conditions in

the preburner are simulated by a high pressure, electrically-heated optical

cell. Measurements of both pure rotational Raman scattering and vibrationalrotational Q-branch scattering are reported.

Optical Fiber Tests

The transmitter fiber is one of the most critical elements in the FORT

diagnostic system. This fiber must transmit the laser radiation without

unacceptable attenuation and must not severely degrade the laser beam quality.

The importance of the power handling capability of the transmitter optical fiber was pointed out in the conceptual design report (Ref. 1). There the

development followed the analysis of Smith (Ref. 2), who studied the theoreti-
R85-956642 FIG. 1

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R85-956642

cal power handling capacity of single mode fibers. He found that stimulated

Raman and stimulated Brillouin scattering limit the practical power capacity

of small core, low-loss fibers at high radiation intensities. Following

Smith's analysis, the critical radiation intensity for laser wavelengths of

interest, was estimated in Reference 1 to be approximately 40 kW/cm per

transverse mode in the fiber. The critical intensity is the intensity at

which nonlinear conversion becomes important, i.e. comparable to the transmitted power. Tests were therefore undertaken to verify these predictions as

applied to multimode fibers with core diameters of 50 to 100 microns.

Optical fibers possess excellent transmission characteristics in the near

infrared region of the spectrum, having attenuations as low as 0.2 dB/km.

Raman scattering is stronger for short wavelengths however, so that operation

is desired away from the best transmission windows of the fiber. Attenuation

in the fiber generally follows an inverse fourth power relation with wavelength because the dominant loss mechanism at visible wavelengths is Rayleigh

scattering. According to this scaling, good fiber attenuation factors in the

operating range of the argon ion lasers being considered for this application

would be on the order of 20 dB/km.

The transmission of several commercially available multimode fibers has

been measured. The ends of the fibers used in these tests were cleaved,

rather than lapped and polished, to avoid possible problems with polishing

materials or epoxy remaining on or near the ends of the fiber. The results

for fibers with core diameters near 100 microns and 50 microns are shown in

Figs. 2 and 3, respectively. The size of the laser focal spot size on the end

of the fiber was found to be important. Focusing the laser too tightly produced excessively high intensities at the end of the fiber, resulting in failure usually by melting of the output end of the fiber. Fiber failures were

experienced using lenses with focal lengths less than about 15 mm, for which

focal spot diameters are estimated to be less than 10 microns. Typically the

transmission through fibers about 40 meters long was around 50 percent. Accounting for Fresnel reflection losses at the ends of the fiber, the lowest

fiber attenuation measured was 49 dB/km for fibers with core diameters near

100 microns, and 32 dB/km for fibers near 50 microns. One 50 micron fiber exhibited nonlinear behavior, perhaps due to Brillouin scattering. Evidence of

stimulated Raman scattering in the transmitter fiber was found in the Raman

measurements and is discussed in the rotational Raman measurements section.

The results of the tests for the transmission of 488 nm argon ion laser

radiation measured with a number of commercial multi-mode core optical fibers

are summarized in Table 1.

Measurements were made of the focal properties of the radiation exiting

the fiber. In these tests, the argon ion laser was focused into the 50 micron
R85-956642

FIBER TRANSMISSION TESTS

ARGON-ION LASER (488 nm)

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R85-956642 FIG. 3

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.85-5-47-15
R85-956642

Table 1

Summary of Multi-Mode Fiber Transmission Tests

488 nm Argon-ion Radiation

Fiber Supplier-No. Core Dia Clad Dia Length Trans. Atten Objectmicrons micron m dB/km ive

Remarks

001

001

001

001

002

003

004

Newport

Newport

Newport

Newport

Newport

F-MLD

F-MLD

F-MLD

F-MLD

F-MSD

Times Fiber

General Fiber

103

103

103

103

51

50

100

140

140

140

140

125

126

140

50

50

48.5

48.5

50

41

40

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49

59

32

39

63

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output fail

8W in

4.2W out

4.1W out

3.9W out

nonlinear

nonlinear

005 Corning 1515 63 130 60 .46 45

input fail

5X 3 W out

low power transmission and attenuation, max power out = 0.35 W
R85-956642

fiber with a 25 mm focal length objective and the radiation from the output of

the fiber was collimated with a 15 mm objective and focused with a 267 mm

focal length plano-convex lens. The resulting beam waist diameter was determined by scanning a knife blade through the beam at the focus (Ref. 3). Power

was measured with a thermopile detector located after the knife-blade. The

total power of the beam (0.3 W) was kept low to prevent burning the edge of

the blade. The waist diameter was determined to be 570 microns. The spot

size at the fiber exit face is estimated from the magnification of the lenses

to be 32 microns. In tests with a 100 micron core fiber the beam was found to

be about 40 microns in diameter at the end of the fiber.

An estimate can be made of the degradation in laser beam quality as a

result of propagation through the fiber. The beam at the exit of the laser

was measured to be 1.4 times diffraction limited for a T£MQQ mode. With a

beam diameter measured at the end of the fiber of 40 microns, along with a

measured divergence of 0.18 radians, the beam is calculated to be 12 times

diffraction limited for a T£MQQ mode. This divergence, which is the full

angle, compares with the 0.4 radian full acceptance angle of the fiber.

Therefore the beam quality is one-eighth the beam quality of the laser. These

estimates are important because they determine how tightly the laser radiation

can be focused by the optical head in the measurement volume and therefore

affect the measurement spatial resolution.

The results of the tests with multimode optical fibers indicate that up

to 4 Watts of raw power can be transmitted through the fiber. This is the

power level used in signal magnitude estimates in the conceptual design study.

Later tests to be described indicate that all of this power is not usable, because it cannot be focused within the collection volume that the receiver

fiber views. When focusing into the fiber is adjusted to obtain the smallest

focal diameter at the probe volume, the fiber transmission is much lower,

approximately 25% or less.

Optical Head Tests

The function of the optical head is twofold. First, the optics must collimate light from the transmitter fiber and focus it at the measurement point

in the preburner. Second, the optics must collect backscattered Raman-shifted

radiation and focus it into the receiver fiber for transmission to the spectrograph/detector. The design of the optical head determines the measurement

volume.

The layout of the optical head design is shown in Fig. 4. The beam from

the laser is collimated, and then directed along the principal optical axis of

the head by a small prism. An intermediate lens brings the radiation to a

10
R85-956642 FIG. 4

BREADBOARD OPTICAL HEAD DESIGN

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COLLECTING

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OPTICAL FIBER

POSITIONERS

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FIBER

RECEIVER

FIBER

85-5-47-25

11
R85-956642

real focus and, finally, the focusing lens projects it into the measurement

volume (through a high pressure window). The focusing lens collects the

backscattered light and focuses it at the intermediate focus. The intermediate lens recollimates the collected light, whence it skims past the prism and

is focused onto the receiver fiber. An absorbing glass filter is used to block

the unshifted, scattered laser light. By moving either the intermediate lens

or the focusing lens along the axis, the focal point of the laser can be

changed so that different points across the diameter of the preburner can be

probed.

The optical head is forced by constraints of the preburner to operate at

large f-numbers when viewing across the combustor. This increases the extent

of the measurement volume along the axis, which determines the spatial resolution. It is desired to keep the resolution as small as possible, i.e. 0.5 to

1.0 cm. In a high f-number system, the on-axis rays contribute greatly to the

sampling length. If the central rays are blocked, as suggested by Eckbreth

and Davis (Ref. 4), the resolution can be improved. The small prism provides

some obscuration. Additional obscuration can be provided by an opaque disk

centered on the optical axis.

The spatial resolution of the optical head has been measured by simulating Raman scattering in the measurement volume by a thin cell (2 mm) containing a dye that fluoresces in the incident laser light. The cell is translated along the axis to map out the spatial extent of the optical response. For

these measurements the laser is chopped and the fluorescence radiation is

detected by a photomultiplier tube and phase-sensitive tuned amplifier.

Figure 5 shows the results of the measurement of a series of fluorescence

profiles. These data show the effect of different obscuration diameters from

8 mm to 11 mm. The maximum obscuration possible depends on the distance at

which the system is focused. It is approximately 11.4 mm in this design of

the optical head. An appropriately located 1 cm diameter aperture was used to

simulate the limiting aperture of the transducer port.

A series of measurements like that shown in Fig. 5 have been carried out

to study the effect of observation distance on spatial resolution. The results are shown for one obscuration diameter in Fig. 6. For these tests the

distance from the intermediate lens to the final lens is changed to focus at

different distances from the limiting aperture. The solid curve shows the

spatial resolution calculated from a simple relationship derived in Ref. 4:

' 2f(A + B)

Do (1)

Here A is the collecting aperture (the receiver fiber core diameter), B is the

diameter of the laser spot in the focal volume, f is the distance of the focal

12
R85-956642 FIG. 5

EFFECT OF OBSCURATION ON SCATTERING PROFILE

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DISTANCE FROM APERTURE, mm

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85-5-47-17
R85-956642 FIG. 6

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SUMMARY OF SPATIAL RESOLUTION TESTS

OBSCURATION DIAMETER — 11 mm

60

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FOCAL DISTANCE FROM APERTURE, mm

85—5—47-19

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R85-956642

point from the focusing lens, and DQ is the diameter of the obscuration

disk. The diameter of the laser spot was measured. It can be related to the

diameter of the spot at the exit of the transmitter fiber through the magnification of the lenses. Equation (1) is based on simple geometrical concepts

within the framework of paraxial ray optics. It should not be expected to

give precise results in every case, but is useful for estimating resolution

with obscured systems.

The effect of obscuration on the spatial resolution and collected dye

signal maximum is show in Fig. 7. It is clear from this data that the use of

larger obscuration diameters to achieve a given spatial resolution incurs a

substantial penalty in the collected signal.

It should be pointed out that the optical head used in these tests is a

relatively simple design. Simple off-the-shelf plano-convex lenses were used

throughout. The design could be improved by the use of lenses optimized to

work in this configuration. For example the final lens focusing into the

probe volume works at finite conjugates, and a piano convex lens does not give

the best results in this case. Furthermore the lens focusing into the receiver fiber functions at an f-number of about 4. This is about the f-number

where spherical aberration effects become important. Indeed, in later Raman

tests, this particular lens was replaced with a doublet (corrected at the ends

of the visible spectrum for chromatic and spherical aberration) and spatial

resolutions of 0.5 to 1 cm at a working distance of 275 mm were observed with

the 8 mm square obscuration of the laser turning prism. This is nearly an

order of magnitude improvement over the results displayed in Fig. 6 and highlights the importance of proper lens design. Thus, with proper lens designs,

spatial resolutions of 1 cm are readily achieved with an estimated 66 percent

loss in the signal incurred.

An important point is that many of the optical performance parameters,

such as spatial resolution and collection efficiency, are determined by how

tightly the laser is focused into the probe volume. This is determined largely by the laser beam quality as it exits from the transmitter fiber.

Simulated Raman Measurements

The results of measurements of hydrogen Raman spectra using the fiber

system in conjunction with the optical head are described in this section.

The results of unsuccessful rotational Raman measurements are described first,

followed by a discussion of the successful vibrational Raman measurements.

The breadboard FORT diagnostic apparatus is shown in the photograph in

Fig. 8. The nominal 3 Watt argon-ion laser is shown to the right in the

15
R85-956642 FIG. 7

EFFECT OF OBSCURATION ON RESOLUTION AND SIGNAL MAXIMUM

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85-5-47-21
ORIGINAL PAGE !3

OF POOR QUALITY FIG. 8

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17 85-5-47-29
R85-956642

background, focusing into the transmitter fiber. Rayleigh scattering in the

transmitter fiber makes it appear to glow. The breadboarded optical head is

in the foreground to the right. The large cylinder is the high pressure cell.

The receiver fiber can be seen at the optical head, held in the fixture with

two micrometer heads, and also at the top center of the photo. The fiber connects to a spectrograph located at the end of the 40 meter fiber in another

laboratory. A safety container vessel drops over the cell and optical head

for heated, high pressure experiments. The optical head is shown in a closeup photograph in Fig. 9. The intermediate focal point is seen clearly.

Preburner measurements were simulated by heating pure hydrogen in a

static high pressure optical cell. Figure 10 shows a cross section through

the cell. The cell has 12.7 mm diameter bore, with a central heated region

165 mm long, between windows 335 mm apart. The windows are 25.4 mm thick,

25.4 mm diameter sapphire. Cracking with quartz windows was experienced well

below the 6000 psi design pressure. The cell is heated by an internally wound

molybdenum heater. The heater is controlled by a set-point temperature controller connected to a thermocouple attached to the center of the heated inner

liner.

Sapphire and quartz rods 110 mm long were inserted in the cell with one

end of each near the inner surface of the end windows. This was necessary to

prevent thermal buoyancy effects from severely bending the optical beam.

Without the rods, the Raman signal dropped rapidly with heating of the cell

just 50 C above the ambient temperature.

Rotational Raman Measurements

Rotational Raman scattering is calculated to be about 70 percent stronger

than vibrational-rotational Raman scattering in the conceptual design report

(Ref. 1). Accordingly tests to observe the pure rotational spectrum were

undertaken first.

The rotational S-branch of hydrogen was observed from room temperature

hydrogen contained in the cell at 1000 psia as shown in Fig. 11. This and

other spectra were obtained by measuring the spectrum on an optical multichannel detector with the laser turned on, and subtracting from this spectrum,

channel by channel, another spectrum obtained with the laser blocked. The

locations of the hydrogen rotational transitions were determined from a wavelength calibration provided by observing the Raman spectrum of liquid toluene

placed in a cuvette in the probe volume of the optical system. A broad peak

centered at 460 cm and a strong background level are present in the

rotational Raman spectrum.

The broad peak is believed to arise from stimulated Raman radiation

generated in the transmitter fiber, which is then Rayleigh scattered in the

18
ORIGINAL PAGE ft

OF POOR QUALITY

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R85-95664

2 FIG

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R85-956642 FIG. 11

FORT HYDROGEN ROTATIONAL RAMAN SPECTRUM

40,000-

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30,000-

25,000-

20.000-

15,000-

10,000

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P= 1000 psia

T = 294 K

I I I I I 7 I I I

0 50 100 150 200 250 300 350 400 450 500

CHANNEL NUMBER (2.4 cm ~

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85-5-47-3

21
R85-956642

probe volume of the high pressure cell. The evidence for this assertion is

shown in Fig. 12. This figure shows a comparison of two spectra: one using a

fiber 40 meters in length to transmit the laser probe radiation and one using

a short 1.3 meter fiber. The broad peak nearly disappears in the spectrum

with the short fiber. The broad peak was completely absent in tests conducted

with no transmitter fiber at all. It should be noted that the Raman shift of

the broad peak corresponds closely to the shift reported for SiOo — 467

cm" (Ref. 5). Comparing the integrated intensity of the broad peak to the

hydrogen Raman spectrum and accounting for the Rayleigh scattering cross section of hydrogen (Ref. 6) the stimulated Raman power appears to be about 2 to

3 percent of the 488 nm laser line, assuming all the collected SiC^ Raman

shifted radiation is Rayleigh scattered (there may be some contribution from

reflections from optical components).

A large background is present in the rotational Raman spectra in Figures

11 and 12. A comparison with a simulated spectrum assuming the background

arises solely from Rayleigh scattering shows the background is much too high

for this to be the case. The spectrometer was tuned to larger Raman shifts on

the Stokes side of the spectrum, that is to longer wavelength where the background steadily dropped to smaller levels. It is believed that the large

background is due to grating scatter of the laser radiation in the spectrograph (Ref. 7). Reflections from optical surfaces contribute to the large

amount of laser light which enters into the receiver fiber. Except for the

collecting lens (see Fig. 4), all the optics in the breadboard optical hardware are uncoated. Anti-reflection coatings would decrease the reflectivity

per surface from 4% to less than 0.25%, typically.

There are several strategies for dealing with the spurious broad peak and

the large background. The stimulated Raman radiation exiting the transmitter

fiber can be blocked with a filter. A short pass dielectric filter was found

to be effective, but results in an insertion loss. Using a 90° dispersing

prism in the optical path, after the laser radiation from the transmitter

fiber is recollimated, was less effective, probably due to the limited dispersion of the prism glass. The only way of effectively dealing with grating

scatter is to employ a double spectrograph. A better solution for both problems is to observe vibrational Raman scattering instead.

Vibrational Raman Measurements

The spectrograph used to measure rotational Raman spectra was designed

for coupling to a fiber, having an f-number of 2.1. This spectrograph does

not focus satisfactorily at the wavelength of l^ vibrational scattering

however and, therefore, could not be used. An available spectrograph with a

limited speed (f/6.7) was used. Overfilling by the radiation from the receiver fiber is expected in this instrument.

22
R85-956642 FIG. 12

EFFECT OF TRANSMITTER FIBER LENGTH ON

SPECTRUM IN ROTATIONAL RAMAN REGION

500

CHANNEL NUMBER (2.4 cm ~

 1/CHAN)

23

85-5-47-1
R85-956642

Vibrational Raman spectra from room temperature to 1100 K have been

recorded at pressures from 1000 to more than 4000 psia. For heated runs the

cell is initially pressurized and sealed off. The pressure rises in this

constant volume case, but is not linear with temperature because of the small

volume of unheated tubing outside the cell.

Figure 13 shows the measured spectrum of room temperature hydrogen at a

pressure of 2000 psia. The points shown as small squares are the measurements, while the calculated spectrum is shown as a solid line. The intensities are calculated from the integrated intensities of Ref. 1 with a Gaussian

lineshape model. The rationale for this shape is that the focal intensity

distribution refocused after leaving the fiber is usually well approximated by

a Gaussian function. The full width at half maxmimum is determined by comparing calculations at a number of widths to experimental spectra. The calculations show a change in the degree of resolution of the Q(0) and Q(l) lines

from which the spectral resolution is determined quite satisfactorily. The

agreement with the calculations shown in Fig. 13 is quite good. The only

adjustments required, are to scale the calculations to the peak height of Q(l)

and to adjust the location of this peak. The spectral dispersion is independently determined from a neon emission lamp spectrum. To obtain a spectrum,

the optical multichannel detector is scanned for a set number of frames continuously reading the tube and accumulating the signal with the laser blocked;

this procedure samples "dark" current and the natural background radiation.

This spectrum is saved and the tube is reread for the same number of frames

with the laser unblocked. The dark spectrum then is subtracted from the laser

induced spectrum. Typically the dark count rate is 1000 signal counts per

second. The baseline signal level far away from the Q-branch lines is 16

counts per second. The standard deviation of this baseline signal is about 3

counts per second. Therefore the baseline probably is mostly due to residual

grating scatter of the laser line rather than imperfect background subtraction. Again, anti-reflection coating all the elements in the optical head

should substantially reduce this background.

The spectrum at a moderate cell temperature, 650K, is shown in Fig. 14.

The recorded temperature is the temperature of a tungsten-5% rhenium/tungsten-26% rhenium thermocouple inserted into the center of the cell under the

heater element. Again the agreement with the calculations is good.

The Q-branch spectrum obtained with an indicated cell temperature of

1000K is shown in Fig. 15. The pressure in this case (4420 psia) is near the

expected full scale hydrogen pressure. The nominal operating pressure in the

fuel preburner is 5500 psia and the hydrogen mole fraction is about 0.89,

therefore the hydrogen partial pressure is approximately 4900 psia. The final

pressure in the heated cell is determined by the initial pressure and the

24
R85-956642 FIG. 13

HYDROGEN Q-BRANCH RAMAN SPECTRUM

3.0

2.5 -

CO

2.0 H

LLJ

CL

CO

z

<

<

1.5 -

1.0 -

0.5 -

1 =29-5 K

P = 2000 psia

3.0 cm ~

 ] (FWHM:

V 'I 1* T—r- -1 1 r

250' 260 270. 280 290 300 310 320 330 340 350

CHANNEL NUMBER (0.95 cm ~i /CHAN)

85-5-47-5

25
R85-956642

FIG. 14

HYDROGEN Q-BRANCH RAMAN SPECTRUM

3.0-

LU

o

LU

CL

<

cc

2.5

2.0-1

1-5H

1.0-

0.5-

T = 650 K

P = 372'J osia

3.0 cm~ ' (FWHM)

220 230 240 250 260 270 280 290 300 310 320 330

CHANNEL NUMBER (0.95 cm ~

 1/CHAN)

B5-5-56-7

26
R85-956642

FIG. 15

HYDROGEN Q-BRANCH RAMAN SPECTRUM

2.0'

CO.

z

LU

'

cr

O

LU

CL

CO

<

DC

1.5-

1.0-

0.5-

T = 1000 K

P = 4420 psia

3.0cm~1 (FWHM)

i

r^ r i i i i i ^ \ i

120 140 160 180 200 220 240 260 280 300 320 340

CHANNEL NUMBER (0.95 cm ~

 1/CHAN)

85-5-47-23

27
R85-956642

temperature rise after the cell is sealed off before heating. The final cell

pressure is be determined empirically, due to heat losses discussed previously.

The agreement with calculations is very good.

The vibrational Raman spectra have been analyzed to determine the temperature from the distribution of intensities. Representative results are shown

in Figures 16, 17 and 18. The measured Q-branch transitions are integrated

across the individual lines and normalized by the weighting factor, Wj and

the system spectral responsivity. The weighting factor can be derived from

Equation (14) of Reference 1 by rewriting the vibrational population as a

Boltzmann factor times a function of J, the rotational quantum number. The

weighting factor then is just the remaining function of J. The figures show

as a solid line the dependence for the measured cell temperature. The dotted

curve is a line least-squares fit to the experimental data. The temperature

uncertainties reflect the uncertainty in the fit to the line (Ref. 8) and are

seen to be less than 10 percent at the signal rates of these experiments.

Vibrational Raman spectra of hydrogen have been measured at a number of

different combinations of temperature and pressure with the fiber optic

system. The results of data reduction of a series of measurements with an

indicated cell temperature of 1000 K are summarized in Table 2. The cell

pressure in this series is 4420 psia. The temperature uncertainties determined from the quality of the least squares fit to the spectra are shown as

well. It may be argued from this data that the temperature uncertainty overestimates the errors, although more data would needed to definitively examine this point. It should be noted that, at present, the least squares analysis weights all the transitions equally. The proportional uncertainty of the

high J transitions is greater, however so they are weighted perhaps too heavily. This aspect of the data reduction should be examined more closely in

future investigations. There are other approaches to the data reduction, such

as fitting the data point by point to spectra calculated with the known system

spectral response. This method probably is slower, but perhaps not significantly depending on the computational power available.

Comparison to Predicted Signal Rates

The measured signal rates were compared to predicted signal rates to

assess the overall performance of the system and identify any problem areas.

To do this the absolute spectral response of the optical system was measured.

A tungsten strip filament lamp was placed at the focal point in the probe

volume. All system elements were the same with the exception that the cell

window was not used. The effect of this window should be negligible. The

lamp was heated to a relatively low temperature to avoid saturating the opti28
R85-956642

FIG. 16

H2 RAMAN TEMPERATURE ANALYSIS

ID

o

o

cr

O

500 1000

ROTATIONAL ENERGY, cm

-1

29

85-5-56-9
R85-956642 FIG. 17

H2 RAMAN TEMPERATURE ANALYSIS

10'

o

o

DC

O

LU

10'

P = 3720 psia

T = 662±45 K

T = 650 K

_J I I I I I l I I L l I I I I I I I

500 1000 1500 2000

ROTATIONAL ENERGY, cm

-1

85-5-56-8

30
R85-956642 FIG. 18

H2 RAMAN TEMPERATURE ANALYSIS

1.0

\

o

o

o

UJ

<

QC

O

LU

10

-2

10

-3

T =1000 K

P = 4420 psia

= 10G5±32K . \

J I

1000 2000

ROTATIONAL ENERGY, cm ~

 1

3000

31 85-5-56-11
R85-956642

Table 2

Results of Raman Measurements at 1000K Cell Temperature

Measured Temperature

Run Temperature, K Uncertainty, K

1

2

3

4

5

6

7

8

9

1005

1005

1047

1020

1021

1085

1179

840

925

32

69

66

78

111

86

133

86

51

32
R85-956642

cal multichannel detector. The lamp temperature, measured with a disappearing

filament pyrometer, was 1540 K. The spectral intensity of the lamp was calculated from the analytic expressions of Pon and Hessler (Ref. 9) for the emissivity of tungsten as a function of temperature and wavelength. This calibration gives the overall system responsivity and includes: Fresnel reflection

losses at all uncoated optical surfaces, transmission of the absorptive glass

filter, geometrical blockage by the obscuration of the laser turning prism,

transmission through the receiver fiber, grating efficiency, losses due to

overfilling the spectrograph and the response of the optical multichannel

detector. These factors correspond to the factors T!Q and e of equation 30

in Reference 1. The measured system response is 4.1 x 10 counts/photon.

If this is compared to estimates of the above factors, it is indicated that

the grating is overfilled by a factor of 4.2, which corresponds to the beam

expanding at f/3.3 into the f/6.7 spectrometer as it leaves the receiver

fiber. This is very reasonable.

To compare predicted signal estimates with the measured Raman signal

rates, the laser power and the spatial resolution were measured and the collection solid angle was estimated. The spatial resolution was determined by

translating a 2 mm thick dye cell through the focal volume and observing the

signal on the optical multichannel detector. The resolution measured was 1 cm

and was less than measured previously as described in the section on the

optical head. This possibly may be due to greater chromatic aberration effects in the former measurements, which should not affect the OMD measurements

because the signal is dispersed. Another possible reason for the improved

spatial resolution is that a higher quality lens was used in the Raman tests,

leading to sharper focusing. The collection solid angle was estimated from

the geometrical aperture of the the high pressure cell window, knowing the

location of the focal point in the cell. The incident laser power was 0.5

Watts. More will be said about the incident laser power level in the discussion of the results.

The Raman spectrum measured at room temperature and 2000 psia is compared

to the signal count rates calculated from the above system parameters in

Figure 19. The measured signal count rates are integrated across the individual Q-branch components. The line fit to the data indicates that the two

agree within a factor of 1.7. This is considered to be quite good. The only

factor that has not been explicitly considered is that due to not focusing the

collected Raman shifted radiation completely into the acceptance region of the

receiver fiber. This is not accounted for in the spectral lamp calibration

because the image of the filament overfills the fiber.

Discussion of Results

The laser transmission is not as high for the Raman tests as the 45

percent transmission measured in the fiber transmission tests and shown in

33
R85-956642 FIG. 19

COMPARISON OF CALCULATED AND MEASURED COUNT RATES

o

0

(fj

LU

CC

O

o

Q

LL)

§

O

Q

P=2000

T = 29-CK

1.7x

O(0)

O O (2i

O Q (31

0(1)

_i i i i i i i

10°

MEASURED COUNT RATE, sec

-1

85-5-56-10

34
R85-956642

Figures 2 and 3. In those measurements the focusing was adjusted to obtain

maximum transmission through the fiber. Careful adjustment of the focus was

found to be necessary in later tests to obtain the smallest spot size at the

probe volume, however. This results in the laser radiation propagating in the

lowest order modes in the fiber. With the microscope objective used for the

Raman tests, the laser beam is focused to approximately 19 microns at the

entrance to the fiber. With an incident power of 3.3 Watts, which was the

maximum available from the medium frame laser used, the intensity is 1.2

MW/cm . In Reference 1 the critical intensity for nonlinear conversion

processes was estimated to be about 40 kW/cm per transverse mode. Evidence

for stimulated Raman scattering is shown in Figure 10. The threshold for

stimulated Brillouin scattering is known to be lower than for Raman scattering, so Brillouin scattering is expected to affect the fiber transmission as

well. Brillouin scattering occurs in the backward direction and requires

special equipment to observe. The low transmission through the fiber when it

is adjusted to produce the lowest order modes should not be surprising.

Focusing into the fiber was adjusted while viewing the dye fluorescence signal. An optimum was found close to the condition giving low order fiber

propagation modes. This suggests that it is necessary to have a small laser

spot size to couple efficiently into the receiver fiber. Modes that do not

focus well, overfill the receiver fiber entrance and are not collected.

The results of the measurements can be extended to reassess the performance of a fully developed optical system for measurements with a higher power

laser and a spectrograph more closely matched to the receiver fiber. If a

faster spectrograph and an intensified diode array are used, a collection

efficiency of .066 and a detector response of .07 counts/photon is expected.

This is a factor of 11 higher collection efficiency than obtained here.

Taking the usable laser power as 2 Watts and the collection solid angle of

.003 steradians, along with the design objectives of 0.01 second temporal

resolution and, 1 cm spatial resolution, the average Raman signal count per

transition is calculated to be 430 photons collected. This is expected to

result in a temperature uncertainty of 70 K or 7 percent at 1000 K. If the

optical speed of the system is increased to make more precise temperature measurements, then the f-number must be decreased to about f/5.7 to get a 25 K

temperature precision. This modification should be considered for the test

bed engine hardware.

The measurements and calculations are summarized in Table 3. The first

two columns compare the measurements and calculations of Figure 19. The last

two columns compare the predictions from the conceptual design report and the

currently projected performance. The measurements represent a 30 second average. The combination of the collection and quantum efficiencies are the result of the tungsten lamp calibration. The total count represents the total

35
R85-956642

Table 3

Comparison of Raman Signal Estimates

Quantity

Present Results

Measured Calculation

Est. Performance

Ref. 1 Projection

0.5

0.0004

0.003

16322

1

30

0.5 4

0.

0.0004

0.

0.003 0.

2

2

002

16322 19600

1 1

30 .01

2

0.066

0.07

0.003

16322

1

.01

3(-9)* 5(-10) 3(-10)

96k 167k 2500 430

Refers to Q(l) line at 300 K and 2000 psia. The other Raman

strengths correspond to an average transition at 1000 K and

5500 psia.

36
R85-956642

for the strongest Q(l) transition. The results in the first columns fall

within the factor of 1.7 of each other as in Fig. 19. The projections from

the conceptual design report are summarized in the third column, while the

current projections of performance are shown in the last column. It should

be noted that the projections in Reference 1 were for rotational Raman scattering, while the rest are for vibrational Q-branch scattering. The laser

power, incident on the focal volume, has been scaled down, in recognition of

the limited fiber transmission of modes which can be tightly focused. The

quantum efficiency of the multichannel detector has been reduced because of

the fall off of the detector efficiency in the red portion of the spectrum.

An omission in the conceptual design report has been corrected. Optical

multichannel detector efficiency is defined such that two detected photoelectrons are required for each registered count. The collection efficiency has

been decreased by the obscuration factor which was neglected in the conceptual

design reported, as stated therein. The Raman intensity 1^ in the conceptual design was for one of the weaker transitions, while the projection in the

last column is for an average transition.

37
R85-956642

CONCLUSIONS AND RECOMMENDATIONS

The feasibility of measuring temperature profiles in the SSME preburner

has been investigated experimentally. The gas conditions existing in the

preburner have been simulated by high pressure hydrogen contained in a heated

optical cell. Experimental measurements have been made with available

equipment, which, with care, can be extended to predict performance of a

system designed for actual diagnostics. In particular the system can be

scaled to higher laser power, a larger spectrometer aperture and a more

sensitive detector.

One of the conclusions of this study is that attenuation of low order

spatial modes in the optical fiber limits the laser intensity that can be

realized at the laser focal spot in the probe volume. The probe volume is

determined ultimately by the volume that can be coupled into the receiver

fiber. Nonlinear scattering processes are probably responsible for the high

attenuation of low order spatial modes.

The spatial resolution objective of 1 cm appears to be achievable at

working distances of 100 to 300 mm, with collection solid angles of .003

steradians (not accounting for blockage).

It appears that a 5 % temperature resolution is feasible, keeping the

original spatial and temporal resolution goals of 1 cm and 10 milliseconds.

If a larger optical aperture can be provided, improved precision may be

possible. Other trade-offs are possible. If the temporal resolution is

relaxed to 0.1 sec, temperature precision close to 1 percent should be

achievable.

It is recommended that a larger optical window be sought (f/6) and that

it be located in the preburner as near the turbine entrance as practical.

Placement of the laser close to the engine in an acoustic enclosure with

suitable cooling provided is recommended. This is to minimize the length of

the transmitter fiber. The design of the optical head should be considered in

detail to ensure that spherical aberration and chromatic aberration do not

limit working at the larger optical apertures. Finally it is recommended that

a design objective of 5 percent temperature precision be considered for applications where high temporal response is desired.

38
R85-956642

REFERENCES

1. J. A. Shirley: Investigation of the Feasibility of Temperature Profiling

Optical Diagnostics in the SSME Fuel Preburner. Final Technical Report

prepared under NASA/Marshall Contract NAS8-34774, 1983;

2. R. G. Smith: "Optical Power Handling Capacity of Low Loss Optical Fibers

as Determined by Stimulated Raman and Brillouin Scattering." Appl.

Optics, Vol. 11, pp. 2409-2494, 1972.

3. Y. Susaki and A. Tachibana: "Measurement of the ym Sized Radius of

Gaussian Laser Beam Using the Scanning Knife-Blade Technique." Appl.

Optics Vol. 14, pp. 2809-2810, 1975.

4. A. C. Eckbreth and J. W. Davis: "Spatial Resolution Enhancement in

Coaxial Light Scattering Geometries." Appl. Optics, Vol. 16, pp. 804-806,

1977.

5. W. D. Johnston, Jr., I. P. Kaminov and J. G. Bergman, Jr.: "Stimulated

Raman Gain Coefficient for LiNb03, Ba2NaNb5015, and Other Materials."

Appl. Phys. Lett., Vol. 13, pp. 190-193, 1968.

6. R. R. Rudder and D. R. Bach: "Rayleigh Scattering of Ruby-Laser Light by

Neutral Gases." J. Opt. Soc. Amer., Vol. 58, pp. 1260-1266, 1968.

7. R. 0. Kagel: "Raman Spectroscopy" in Handbook of Spectroscopy, Vol. II,

J. W. Robinson, Ed., CRC Press, 1974, p. 111.

8. E. D. Crow, F. A. Davis and M. W. Maxfield: Statistics Manual. Dover, New

York, 1960, p. 160.

9. R. M. Pon and J. P. Hessler: "Spectral Emissivity of Tungsten: Analytic

Expressions for the 340-nm to 2.6-ym Spectral Region." Appl. Optics,

Vol. 23, pp. 975-976, 1984.

39
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4. PERFORMING ORGANIZATION REPORT NUMBER(S)

R85-956642

5. MONITORING ORGANIZATION REPORT NUMBER(S)

6a. NAME OF PERFORMING ORGANIZATION

United Technologies

Research Center

6b. OFFICE SYMBOL

(If applicable)

UTRC

7a. NAME OF MONITORING ORGANIZATION

George C. Marshall Space Flight Center

Marshall Space Flight Center

6c. ADDRESS (Gty, State, and ZIP Code)

East Hartford, Connecticut 06108

7b ADDRESS (City, State, and ZIP Code)

Huntsville, Alabama 35812

8a. NAME OF FUNDING/SPONSORING

ORGANIZATION

George C. Marshall Space Flight

Center

8b. OFFICE SYMBOL

(If applicable)

9. PROCUREMENT INSTRUMENT IDENTIFICATION NUMBER

NAS8-34655

8c. ADDRESS (City. State, and ZIP Code)

Huntsville, Alabama 35812

10 SOURCE OF FUNDING NUMBERS

PROGRAM

ELEMENT NO.

PROJECT

NO.

TASK

NO.

WORK UNIT

ACCESSION NO.

11. TITLE (Include Security Classification)

Investigation of Breadboard Temperature Profiling System for SSME Fuel Preburner

Diagnostics

12. PERSONAL AUTHOR(S)

Shirley, John Allen

13a. TYPE OF REPORT

Final Technical

13b. TIME COVERED

FROM 10/83 TO 5/85

14. DATE OF REPORT (Year, Month, Day) 115 PAGE COUNT

1986, January, 17 39

16. SUPPLEMENTARY NOTATION

17. COSATI CODES

FIELD GROUP SUB-GROUP

18 SUBJECT TERMS (Continue on reverse if necessary and identify by block number)

RAMAN OPTICAL FIBER

HYDROGEN

LASER SPF.f.TKf)SrnPY __^

19. ABSTRACT (Continue on reverse if necessary and identify by block number)

The feasibility of measuring temperatures in the space shuttle main engine (SSME) fuel

preburner using spontaneous Raman scattering from molecular hydrogen has been studied

experimentally. Laser radiation is transmitted to the preburner through a multimode

optical fiber. Backscattered Raman-shifted light is collected and focused into a second

fiber which connects to a remotely-located spectrograph and a multichannel optical

detector. Optics collimate and focus laser light from the transmitter fiber defining the

probe volume, which is approximated by a cylinder 8 mm long, and 0.3 mm in diameter. The

high pressure (5500 psia), high temperature (1000K) preburner environment has been

simulated by a heated pressure cell having windows and. containing pure hydrogen. Temperatures determined by the distribution of Q-branch ro-vibrational transitions demonstrate

precision and accuracy of 3 percent. The measurements indicate that preburner temperatures

can be determined with 5 percent accuracy with spatial resolution less than 1 cm and

resolution nf 10 milliseconds at the nominal preburner operating conditions.

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DD FORM 1473, 84 MAR 83 APR edition may be used until exhausted.

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