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Anal. Chem. 1982, 5 4 , 1026-1028
Time-of-Flight Optical Spectrometry with Fiber Optic Waveguides Wllllam B. Whltten Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
Fiber optic waveguides provide tlme-domaln spectral dlsperslon In a novel absorption spectrometer. The comblnatlon of a superbroadened pulse from a mode-locked Nd-glass laser as a light source and a SI avalanche dlode detector permlt an absorptlon spectrum to be determined wlth a slngle pulse of the source.
It has been shown in previous papers that multimode graded index optical fibers can be used to obtain time-of-flight dispersion for absorption spectrometry ( I , 2). The spectral components of a light pulse are detected at different times as they emerge from a long optical fiber because of the wavelength dependence of the group velocity. To achieve nanosecond time resolution which is required for spectral resolution of the order of nanometers, the prototype instruments used a time-correlated single photon counting techinique. The repetition rate of the spark source was only a few kilohertz, however, so that a typical determination would require several hours. There was also no provision for synchronizing the source to an external stimulus to exploit the time resolution capabilities of the system. The purpose of this article is to describe a dual beam fiber optic time-of-flight spectrometer with which an absorption spectrum can be obtained from a single pulse of the source. A complete dual beam spectral determination requires less than 3 p s with the present setup. Since most of this time is delay within the fiber, sequential determinations could be made less than 200 ns apart. The time resolution of the apparatus is determined by the duration of the source, about 10 ps.
EXPERIMENTAL SECTION A block diagram of the spectrometer is shown in Figure 1. The dispersive elements of the instrument are retained from the previous single photon version (2) but the source and detector portions have been changed substantially. The light source requirements for single pulse operation are formidable. The same number of photons which was generated over a several hour period in the photon counting experiments must now be emitted in less than a nanosecond. Similarly, the number of detected photons per resolution element should be about the same as before, lo4 for 1% precision, but now the nanosecond collection time requires a linear rather than a digital detector. The light source selected for the single pulse spectrometer is the continuum generated when an intense pulse from a modelocked Nd-glass laser is focused into a cell containing cC14( 3 , 4 ) . Construction details follow closely the specifications given by Mano and Shapiro in their patent (5)and are shown schematically in Figure 2. The laser gain medium is a Kigre Q-88 phosphate glass rod, 1.27 cm in diameter with Brewster angle faces. The rod is pumped by a Korad K-1 flash head. The laser cavity is formed by a 10-m radius 100% reflecting mirror at the rear and a 50% plane mirror at the front. A dye cell placed at Brewster's angle in the cavity and filled with a solution of Eastman 9860 Q-switch dye in 1,2-dichloroethane permits passively mode-locked operation. Laser action is restricted to a few transverse modes by an iris placed within the cavity. The mode-locking dye solution rate. is pumped through the cell at a 0.05 mL The laser output is a train of pulses, each approximately 10 ps in duration and separated by the 8-115 round trip time of the cavity. The pulses have a wavelength of 1.054 ym and energies
which vary from 1to 10 mJ, depending on the dye concentration. For single pulse operation, one pulse is selected from the train by an electrooptic switch. The normally horizontally polarized laser output is mometarily rotated 90° by a Pockels cell so that it can pass through a vertical polarizer to the experiment. The electrical pulse for the Pockels cell is obtained from a transmission line 65 cm in length and charged to 14.5 kV. The transmission line is discharged through the 50-0 Pockels cell by a laser-triggered spark gap. When the switched-out laser pulse is focused into a 10-cm cell of CC14,a superbroadened continuum is produced (3) which has a time duration comparable to the 10-ps laser pulse (6). Observations with a vidicon spectrometer show that the spectral range of the continuum extends from 450 nm to the infrared. The continuum is collected by a collimating lens and separated from the residual laser excitation by a dichroic mirror and a CS 1-75 filter. The spectral character of the continuum varies somewhat from shot to shot so a dual beam technique which uses the same source pulse for sample and reference excitation is essential. The light is directed by a beam splitter through sample and reference cuvettes, and the respective beams are focused into the appropriate optical fibers by microscope objective lenses. The dispersing fibers are from a Corning Corguide 5050 fiber optic waveguide, with nominal lengths of 525 and 550 m for the reference and sample fibers. The Corguide 5050 designation corresponds to a nominal attenuation of 5 dB km-' at 820 nm and a bandwidth of 500 MHz km-l at 900 nm. The two fibers are wound in a single layer on a 30 cm diameter drum as described previously (I). Both were cut from a single 1100-m fiber and are therefore well matched in attenuation and dispersion. The matching of the two fibers is not critical, however, because spectra obtained from running blank vs. blank and separate wavelength calibrations for the two fibers permit any necessary corrections to be made. The output ends of the two fibers are imaged by a microscope objective onto a Texas Instruments TIED 56 avalanche diode biased at 135 V (7). The diode output is monitored with a Tektronix 7104 oscilloscope. The oscilloscope is operated in the delayed sweep mode so that a 200-11s time interval can be studied in detail after the approximately 2.6 ys time for the longer wavelengths to pass through the shorter fiber. The oscilloscope is triggered by a signal from a second photodiode which detects a portion of the 1.05-ym pulse switched from the laser train.
RESULTS AND DISCUSSION A typical response curve with no samples or reference cell present is shown in Figure 3. The origin corresponds to a time delay of 2.6 p s . The two major features represent the wavelength components of the source pulse after being delayed and dispersed by the reference and sample fibers. CS 1-75 and CS 4-70 filters have been placed before the beam splitter to attenuate the long wavelength light in both beams. A wavelength calibration scale obtained by making a series of measurements with interference filters in front of the beam splitter is also provided. The difference in intensities of the outputs of the two fibers can probably be attributed to different efficiencies of coupling the continuum into the fibers. The curves show some structure with a pronounced minimum at 725 nm. This wavelength corresponds to that at which an absorption maximum is commonly observed in optical fibers (8). Similar structure at longer wavelengths can be observed more clearly when the CS 4-70 filter is removed. The absorption bands are attributed to overtone bands of OH-radical impurities in the silica fiber core (9).
0 1982 American Chemical Society 0003-2700/82/0354-1026$01.25/0
ANALYTICAL. CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
1027
CCI,
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DIODE BLANK
5-14 1 I . 525-rn
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I
I
Figure 1. Block diagram of the dual beam fiber optlc time-of-flight spectrometer.
I
I
i
I
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2 ns div-’ Figure 4. Oscllloscope trace of spectrometer response when a 700-nm Interference filter is placed in the incident beam.
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---
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Figure 2. Block dlagram of the mode-locked Nd-glass laser and single pulse selection apparatus.
m
r m
900 700 600 500 SAMPLE WAVELENGTH l n m l
900 700600 500 REFERENCE WAVELENGTH (nml
Flgure 5. Oscilloscope trace with didymium glass in the sample beam, reference empty. Inset shows wavelength scale in nanometers. Tiime scale is 20 ns division-’.
r r - 900 700600 500 REFERENCE WAVELENGTH (nml
-
900 700 600 500 SAMPLE WAVELENGTH (nml
Figure 3. Oscilloscope trace of detector signal vs. time WW no sample or reference present. The time scale is 20 ns division-’. The inset gives a wavelength scale in nanometers for the spectral components obtained from measurements with a series of interference filters.
An example of the system response when a 700-nm interference filter is placed in the continuum is shown in Figure 4. Since the duration of the source is negligible, the 1.1-ns width should be a convolution of the detector and oscilloscope response, the material dispersion due to the spectral width of the filter, and the modal dispersion which arises because different opitcal modes within the fiber have slightly different transit times. The detector and oscilloscope responses are measured by observing a portion of the mode-locked laser train. A half-width of 0.7 ns is obtained. The material dispersion is about 0.12 ns nm-l at 700 nm (2) so that the 8-nm bandwidth of the filter would give a 1.0-ns contribution to the pulse width. The signal bandwidth of the fiber is specified to be 500 MHz km-l at 900 nm, which would contribute 0.35 nm in a 500-m fiber. The root mean square of these three quantities is 1.3 ns, in good agreement with the measured width. Single pulse absorbance measurements were made with a piece of didymium glass in the sample beam. Again the long wavelengths were attenuated with CS 1-75 and 4-70 filters.
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Figure 6. Absorption spectrum of didymium glass sample calculated from data of Figure 5 (squares) and measured on conventional spectrophotometer (curve).
An oscilloscope trace showing light passing through the reference and sample fibers is shown in Figure 5. Structure due to the fiber absorbance is again observed in the reference signal which passed through the short fiber and now additiorial mimima due to absorbance peaks in the didymium glass can be seen in the light which passed through the sample fiber. An absorption spectrum calculated at approximately 1-ns intervals from the sample and reference traces is shown in Figure 6 along with the spectrum of the didymium glalss measured with a Cary 14 spectrophotometer. The two spectra are in good qualitative agreement. The customary way to determine absorbance vs. wavelength on a dual beam instrument is to make separate determina-
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
tions, one of sample vs. blank and one of blank vs. blank to determine the base line. The absorbance spectrum of the sample should be the difference between the two. With the present apparatus, the detector noise is substantial and also the coupling efficiency for focusing the two beams into the respective fibers (60 pm core diameter) is sensitive to sample alignment. Accordingly, the blank vs. blank determination was not used. Instead, the calculated spectrum was shifted by 0.2 in absorbance to compensate for the difference in coupling into the two fibers. The difference in absorbance of the two fibers themselves is therefore not taken into account. This difference may be responsible for the apparent increase in sample absorbance near 500 nm. With more accurate data, such as might be obtained by signal averaging, the blank vs. blank determination should probably be used to correct for differences in attenuation vs. wavelength for the two fibers. The calculated absorbance is not as intense as that measured for the same sample with the conventional spectrophotometer. The loss in integrated intensity is probably due to the inability of the detector-oscilloscope combination to follow the rapidly varying optical signal. An estimate of the demands on the detector can be made from the conventionally obtained spectrum and the dispersion of the fiber. The rate of change of the absorbance can be expressed dA _ _ -d
dt
dt
Io I
ldl I dt
log - = -log e- -
For a Gaussian impulse response with full width at halfmaximum, T , the maximum rate of change of absorbance is approximately log e
(e) \
UL
/max
N-
7
For our detector with T = 0.7 ps, the maximum rate is 0.6 ns-'. The calculated points in Figure 6 are about 1 ns apart so there are portions of the spectrum where the detector would have to respond nearly twice as fast to accurately follow the absorbance. The spacing of the calculated points demonstrates how the variation of dispersion with wavelength affects the spectral resolution with other responses fixed. While fiber attenuation is increasing rapidly as the wavelength is shortened, the dispersion also increases. Thus, fiber length could perhaps be reduced to obtain higher transmission without sacrificing spectral resolution a t the shorter wavelengths. With short fibers, modal dispersion is also less important so that step index fibers with pure silica cores might become useful for time-of-flight spectrometry in the ultraviolet region. The previous results show that single pulse absorption spectrometry with fiber optic waveguides can be performed with useful spectral resolution. There are a number of ways
in which the quality of the single pulse time-of-flight spectra can be improved, however. We are currently trying to enhance the signal-to-noise ratio by increasing the source intensity and improving the rejection of undesired pulsed in the laser train. Fibers and detectors of higher bandwidths than those used here are already commercially available so that somewhat higher time resolution is certainly possible. A second generation instrument would probably use a transient digitizer to measure and store the signal from the detector. Commercial digital circuitry is a t present too slow to be effective for this application. The 10-ps time resolution has not been demonstrated by the present experiments. However, a large number of spectral measurements on this time scale have been made by other investigators with similar laser-generated continuum sources but with spatial rather than time domain dispersion (IO). The time-of-flight technique offers the capability of extremely rapid spectral acquisition-a few nanoseconds per resolution element. This rate is 2 to 3 orders of magnitude faster than for optical multichannel analyzers or diode arrays which are presently available. The spectrum of a single event is obtained with the present instrument within 2.8 p s of the flash. The detector is busy for only 200 ns, however, so that repetitive spectra could be obtained a t a 5-MHz rate with up to 13 spectra pipelined within the fibers. Shorter fibers (reduced dispersion) or restrictions on spectral range could allow even faster repetition rates. This feature might be attractive when a repetitive continuum source of subnanosecond duration is already available, for example, a synchrotron light source.
-
ACKNOWLEDGMENT We wish to thank J. M. Ramsey for many helpful discussions.
LITERATURE CITED (1) Whitten, W. B.; Ross, H. H. Anal. Chem. 1979, 57, 417-419. (2) Whitten, W. B.;Ross, H. H. Anal. Chem. 1980, 52, 101-104. (3) Aifano, R. R.; Shapiro, S. L. Chem. Phys. Left. 1971, 8 , 631-633. (4) Magde, D.;Bushaw, 8. A.; Wlndsor, M. W. Chem. Phys. Left. 1974, 28, 263-269. ( 5 ) Aifano, R. R.; Shaplro, S. L. U S . Patent No. 3 782 828. (6) Busch, G. E.; Jones, R. P.; Rentzepis, P. M. Chem. Phys. Lett. 1973, 18, 178-185. (7) Ramsey, J. M.; Hieftje, G. M.; Haugen, G. R. Appl. Opt. 1979, 18,
1913-1920. (8) Keck, D. 8.;Love, R. E. "Applied Optics and Optical Engineering"; Kingsiake, R., Thompson, B. J., Eds.; Academic Press: New York, 1980. 19) Keck. D. 8.: Maurer. R. D.;Schultz, P. C. ADD/. . . Phvs. . Lett. 1979, 22, 307-309. (IO) "Ultrashort Light Pulses"; Shapiro, S. L, Ed.; Springer Verlag: New York, 1977.
RECEIVED for review December 18, 1981. Accepted February 23, 1982. Research sponsored by the Office of Energy Research, U S . Department of Energy, under Contract W7405-eng-26 with the Union Carbide Corp.
