ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
417
Fiber Optic Waveguides for Time-of-Flight Optical Spectrometry W.
B. Whitten"
and
H. H. Ross
Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
1 km in length impractical for spectral measurements in this wavelength region. Consequently, time resolution of better than 1 ns is required for adequate (-1 nm) wavelength separation over most of the visible spectral region. Single photon time-correlation techniques provide a convenient way to achieve the desired time resolution as long as the samples t o be studied are stable under multiple flashes of weak light. T h e exciting light is attenuated so that, after traversing the fiber, sample, etc., a t most a single photon is detected per flash. The time delay between the flash and the detection of the photon is measured and stored digitally. The transmission spectrum in the time domain is then merely the histogram of photons counted vs. their transit time. Some advantages of the method are t h a t the exciting flash can be of low intensity and the timing signals are converted as early as possible into digital logic levels for high timing precision. T h e principal disadvantage is t h a t a large number of flashes is required to accurately define a complete spectrum, since the standard deviation of a given channel count will be equal to the square root of the number of counts in the channel.
Transit lime and pulse broadening within an 1100-m optical fiber have been studied with single photon time-correlation techniques at a number of wavelengths. The results confirm that a fiber optic waveguide can be used as the dispersing element in a time-of-flight optical spectrometer.
In conventional optical spectrometry, information about the intensity of polychromatic light as a function of wavelength is obtained parametrically via a spatial coordinate. This coordinate might be the distance light is deflected by a prism or grating, or the displacement of an interferometer reflector, for example. A spectrum covering a range of wavelengths is measured by translating the light beam or detector, or with a position-sensitive detector such as a photographic plate, sensor array, etc. Recently, Orofino and Unterleitner ( I ) proposed that the variation of photon velocity with wavelength in a fiber optic waveguide could be used to provide spectroscopic dispersion in the time domain. With time instead of distance as t h e spectral parameter, spectra could be measured with a single detector in a rigid system. Experiments by Franks e t al. (2, 3) on the time-dispersion of Cerenkov light pulses in optical fibers showed t h a t useful spectral resolution could be obtained with commercially available fibers. T h e purpose of this paper is to show how single photon time-correlation techniques can be used in conjunction with a fiber optic waveguide for optical timeof-flight spectrometry. For a well designed multimode graded index optical fiber ( 4 ) ,the transit time, 7,of a pulse of monochromatic light is chiefly determined by t h e photon group velocity, ug, of the central core material T
= L/U,
EXPERIMENTAL The optical fiber studied in the present investigation was a Corning Glass Works Model 5050 fiber with a nominal attenuation at 820 nm of 5 dB km-' and a bandwidth of 500 mHz at 900 nm. The 1119-m fiber was wound from the shipping spool in a single layer on a Lucite drum 30 cm in diameter. The drum had a movable plate in the circumference so that the winding tension could be relieved. For mechanical protection, the drum and fiber were placed inside an aluminum box with the fiber ends brought out through opaque Teflon sleeving. The fiber ends and sleeving were secured to 0.3-cm diameter phenolic rods with heatshrinkable plastic tubing for convenience of manipulation. The instrumentation was assembled from commercially available components. Light pulses from an Ortec Model 9352 nanosecond light pulser were, after suitable filtering, focused by a 20X microscope objective onto one end of the fiber. A timing signal from the pulser was delayed for 5.4 1 s by a digital delay generator and applied to the start input of a time to pulse height converter. Photons emerging from the fiber were detected by an RCA C31034 photomultiplier in a cooled housing (-30 "C). The photomultiplier output was converted to a timing signal by a fast discriminator and applied to the stop input of the time to pulse height converter, whose output was stored in a pulse height analyzer. In this way, the time difference between the delayed trigger and detected photon is converted into a proportional voltage and these voltage pulses are sorted into a histogram of the number of photons vs. time. It is essential that at most a single photon is detected per flash with the present experimental setup because the first photon detected after the flash stops the time to pulse height converter. If more than one photon per flash were detected, the resulting spectrum would be biased in favor of shorter time delays. This problem can be avoided by attenuating the light with neutral density filters until the average number of photons counted during a scan of the time to pulse height converter is 0.05. At this rate, the probability of two photons arriving within the same scan is about 0.001 (2% error). The time calibration of the time to pulse height converter-pulse height analyzer combination was performed with the crystalcontrolled digital time delay generator and checked by inserting an additional cable of known length between the discriminator and the stop input of the time to pulse height converter. Interference filter bandwidths were determined on a Cary 14
(1)
n
n dX where L is the length of the fiber, c and X are the speed and wavelength of light in vacuum, and n is the wavelength-dependent index of refraction. T h e spectral dispersion of the fiber, commonly called material dispersion in contrast to modal dispersion, is determined by the variation of 7 with wavelength
dT _
dX
XL d 2 n c dX2
(3)
T h e data of Malitson ( 5 ) for the index of refraction of fused silica can be used to estimate the spectral resolution that can be obtained for a fiber of given length. From Malitsons's equation for n vs. A, the group dispersion from Equation 3 was found to range from 6.6 X lo-" s nm-' km-' a t 900 nm t o 1.1 X s nm-' km-' a t 400 nm. T h e transit time for 400-nm photons in 1 km of fused silica was calculated to be 163 ns longer t h a n t h a t of 900-nm photons. Practical limits on the length of a fiber for spectrometric measurements are set by the attenuation. While fiber attenuation is continually being reduced by improvements in purity and fabrication techniques, high values of attenuation in the blue region of the spectrum make fibers of much over 0003-2700/79/035 1-0417501 O O / O
C
1979 American Chemical Society
418
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979 0 0 n 0
0 0 N
In
0
0 (u 0
w Izo 2 s 0 0
t
0
no
150
200
250
TIME (ns)
Figure 2. Combined transmission spectrum of five interference filters with central wavelengths from left to right of 900, 700, 600, 500, and 436 nm
s 0
0 v) 0
50
I
I
t
4 I
0
6
a
1 x3
I
I
I
12
14
16
18
TIME (ns) Figure 1. Time response of 1-m fiber at a number of wavelengths: 436 nm (+) 590 nm (A), 800 nm (X), 900 nm (0)
Spectrophotometer. To ensure that the photomultiplier was counting single photon events, the linearity of counting rate with light intensity was checked with neutral density filters. A series of ten interference filters with nominal bandwidths at 50% transmission from 5 to 10 nm and with central wavelengths ranging from 900 to 436 nm were placed between the source and microscope objective lens to determine the transit time and pulse broadening of the fiber as a function of wavelength. Measurements of photon counts vs. delay time were made for both the 1119-m fiber and for a I-m fiber so that the time response of the measuring system could be separated from the combined system/fiber response. The absorption spectrum of a solution of KMnO, in 1 N H2S04 (0.2 g L-') was measured from 450 t o 700 nm. The sample was placed in a quartz cuvette with 0.5-cm path length and run alternately with a l N H2S04blank. -4Corning CS 4-70 and two CS 1-75 filters were placed behind the sample to remove the red and near infrared light. If this weakly absorbed light were allowed to pass through t o the detector, the flash intensity, and hence the counting rate, for visible region photons would have to be much lower to avoid pulse pileup. For these measurements, the flashlamp was run with O2 instead of air t o obtain somewhat higher time resolution at longer wavelengths. The absorbance was calculated as the logarithm of the ratio of blank counts to sample counts, the correction for dark counts being negligible. The wavelengths were obtained by linear interpolation between the ten transit times obtained with the interference filters. For comparison, the absorption spectrum of the same sample was measured with the Cary 14 spectrophotometer.
RESULTS AND DISCUSSION T h e response of the measuring system with the 1-m fiber a t various wavelengths is shown in Figure 1. There is a marked increase in the width a t half maximum of the response on going to the longer wavelengths, from 1.8 ns a t 436 nm to 3.7 ns at 900 nm. These results represent the combined time resolution of the flash, photomultiplier, and electronics without the long fiber optic waveguide. The contribution to the timing error of the electronics alone can be determined by measuring the response a t high light intensity. Under these conditions. the stop pulse will be due to a large number of photons arriving nearly simultaneously at the photomultiplier and all coming
from the initial portion of the flash. T h e time spread found in this way was less than 0.3 ns. T h e timing error of the RCA C31034 photomultiplier has been studied by Reisse et al. (6). They obtained a timing resolution of 0.6 ns with 660-nm light. T h e timing error of the photomultiplier would be expected to decrease with increasing wavelength because the electrons from the photocathode would have less spread in kinetic energy ( 7 , 8). Since the various independent contributions to the timing error should add quadratically, by far the largest contribution to the width of the time response must come from the spark source. When the 1-m fiber is replaced by the 1119-m fiber, the counting peaks for the various interference filters are displaced in time because of the dispersion of the fiber. T h e combined time domain spectra of five interference filters are shown in Figure 2. It can be seen that the spectral dispersion is much more pronounced in the short wavelength region. The transit times for the different wavelengths are within a few percent of those calculated for fused silica. T h e transmission spectra in the time domain of the ten interference filters studied are shown in more detail in Figures 3a and 3b. T h e time scales are all measured relative to the transit time of peak intensity. There is a considerable variation in widths and shape among the various peaks with prominent shoulders in some cases. As discussed below, the shapes of the output peaks may not be entirely due t o the fiber. I t should be possible to deconvolute the output peaks to remove the broadening due t o the flash duration, photomultiplier response, and electronics (determined with the 1-m fiber), and due to the material dispersion of the fiber. Unfortunately, the spectrum of the spark source is not perfectly flat with wavelength but shows some sharp lines. If such a line has a wavelength within or near the interference filter passband, this spectral peak passing through the interference filter may be wider or narrower than t h e transmission spectrum of the filter itself. T h e shoulders on the 800-nm peak, for example, do not appear when oxygen is used instead of air in the spark source. T h e 436-nm peak is distorted by a spectral line close to the filter transmission maximum. This distortion does not appear a t all wavelengths, however. At 700 nm. the istrumental broadening and material dispersion can be subtracted off quadratically, giving a value for modal dispersion of 1.4 ns. This value is not far from what would be expected based on the manufacturer's specification of 500 MHz km-' for the bandwidth a t 900 nm. T h e absorption spectrum of the K M n 0 4 sample is shown in Figure 4. It can be seen that the principal features of the spectrum are reproduced very well by the time-of-flight method, although the resolution is not as high as that for the
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
419
0 0 0 n
Lc
t
z 3
0 00 0 N 0
I
I
I
I
-5
0
5
0
-m
I
+
to
TIME (ns) Figure 3a. Transmission spectra in time domain of interference filters: 436 nm (+), 590 nm ( A ) , 800 nm (X), 850 nm (V),900 nm (0)
400
450
500
550
600
650
700
WAVELENGTH (nm) Figure 4. Absorption spectrum of KMnO, in 1 N H,SO,. The crosses represent time-of-flight data while the solid curve is the spectrum measured with the conventional spectrophotometer
0
+
I
L.-m
I -5
I
++
+-
I
I
1
0
5
$3
TIME (ns) Figure 3b. Transmission spectra in time domain of interference tilters: 460 nm (V),500 nm ( A ) , 540 nm (X), 600 nm (+), 700 nm ( 0 )
Cary Id. T h e random noise in the spectral data is much more pronounced in t h e blue region where the total number of counts is low. Because of the increased attenuation of the fiber in this region, the counting rate was 100 times less than a t 600 nm. Thus. for Poisson counting statistics, the relative fluctuations should be 10 times larger a t the shorter wavelengths. Such a variation is qualitatiwly borne out in Figure 4. b'e ha\,e shown that in addition to providing a cmvenient uay t u study light pulse propzgatioii in fiber waveguides. single
photon time correlation techniques in conjunction with a n optical fiber can be used for absorption spectrometry. T h e method could be used t o study transient phenomena on a nanosecond time scale. For example, if the source were synchronized t o a n external stimulus (an actinic laser flash or orienting electric field pulse, for example), it would be possible t o obtain transmission spectra as a function of tinie by varying the delay of the flash with respect to the stimulus. There are analytical applications as well, particularly where time resolution is important, such as nionitoring flow spectra, and in situations where a rugged system with no moving parts is desired. T h e method seems particularly suitable for studying the spectra of fast discharges. One obstacle which must be overcome is the high attenuation of the commercially available fibers a t the shorter wavelengths. T h e problem niay be alleviated by improving the overall time resolution of the system and reducing the fiber length. Because the attenuation of the fiber increases exponentially with length while spectral resolution is linear, the useful wavelength region could then be extended toward the near ultraviolet.
LITERATURE CITED (1) T. A . Orofino and F. C . Unterleitner, Appl. Opt.. 15. 1907 (1976).
(2) L . A . Franks, M. A . Nelson, and T. J. Davies, Appl. Phys. Lett.. 27, 205 (1975). (3) L. A. Franks, M. A. Nelson, T. J. Davies, P. Lyons, and J. Golob. J . Appl. Phys.. 48, 3639 (1977). (4) R . Olshansky and D. 6 .Keck, App/. O p t , 15, 483 (1976). ( 5 ) I . H. Malitson. J . Opt. SOC.A m . , 55. 1205 (1965). (6) R . Reisse, R. Creecy, and S. K. Poultney, Rev Sci. Instrum., 44, 1666 (1973). (7) R . Nathan and C. H. B. Mee, Phys. Status Soldi A , 2 , 67 (1970). (8) B. Sipp. J. A . Miehe, and R. Lopez-Delgado, Opt. Comrnun., 16, 202 (1976).
RECEIVED for review October 11. 1978. Accepted November 30.1978. Oak Ridge National Laboratory is operated for the U.S. Department of Energy by Lniun Carbide Corp. under ('ontract LV-7405-elig-26. '['his article was supported by the Basic Energy Sciences.
