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Anal. Chem. 1987, 5 9 , 1780-1783
Wavelength Division Multiplexer for Fiber Optic Sensor Readout Ming-Ren S. Fuh and Lloyd W. Burgess* Center for Process Analytical Chemistry, Department of Chemistry, BG-IO, University of Washington, Seattle, Washington 98195
Optical wavelength division multipiexers/demultiplexers (WDM) have been used in telecommunications to Increase the Information capaclty of single flber optk systems. From a spectroscopic point of Mew, a WDM Is a monochromator wlth fiber optk Inputs and outputs. The stability, ruggedness, and compactness of these d e v b make them very attractive for use in on-ilne chemkal process monltorlng. 01 the many WDW designs, the use of a dispersive element offers the advantage of a hlgh channel capadty, whlle avddlng currulative losses. A GRIN rod-prlsm grating approach was used to construct an inexpensive device with 11 input/output flbers coverlng a range of 90 nm. Coupled wlth a 35siement self-scanned photodlode array, the whde system is a compact spectrometer with multichannel analysls capaMUty. The performance of thts system operated ln several spectroscopic modes using various fiber configurations Is described.
Fiber optics is recognized as having great potential in a number of chemical sensing applications. Important uses include remote sensing, on-line measurement, and real-time monitoring for such diverse systems as chemical process streams, environmental wells, and in vivo medical diagnostics. In many sensing implementations, optical fibers carry the analytical information in the form of a modulated absorbance, luminescence, or scattered light signal from the sampling site to a conventional spectroscopic instrument. The instrumentation used to analyze this informaton may be placed in a central location where the environment can be carefully controlled. This is a particularly viable approach where multiplexing a number of probes and/or continuous operation justifies the use of a general purpose instrument. However, for some applications, such as biomedical monitoring, this approach is impractical or even impossible. The alternative is to design a rugged, inexpensive, and simple fiber-based instrument that can be used to accomplish on-site measurements. We report here on an integrated optical device that is applicable for fiber optic sensor readouts in such a system. Wavelength division multiplexer/demultiplexertechnology (WDM) has been used in telecommunication to increase the information capacity and fault isolation properties of optical fiber systems. The multiplexer is used to combine different wavelength signals to form a composite signal for transmission over a single fiber or to separate the composite signal into individual channels for detection. There are many different approaches to wavelength division multiplexing, and an excellent description of WDM technology is given in reference 1. One design that may be particularly suited to fiber-based chemical sensing consists of a dispersing element and a graded refractive index (GRIN) rod lens in a fully integrated device. The dispersing element containing a WDM has an advantage in that it can provide a high channel capacity, while avoiding the cumulative losses of dichroic mirror filter devices.
EXPERIMENTAL SECTION The basic configuration of this type of WDM, first reported by Tomlinson (2), is shown in Figure 1. It consists of a quar0003-2700/87/0359-1780$01.50/0
ter-pitch GRIN rod lens, which is used as a collimating and focusing element, a prism with a grating replicated on the hypotenuse, and a number of input/output fibers. The entire device is cemented together in a solid assembly. Multiwavelength light enters the device from an input fiber, where the diverging beam is collimated by the GRIN rod lens. The light strikes the Littrow mounted reflection grating and is dispersed according to wavelength. The GRIN rod lens refocuses the dispersed beam onto the different optical fiber outputs. A design formula for predicting the output beam position has been developed by Kobayashi and Seki (3). For most practical cases the channel spacing, S, can be obtained by using the relationship S = noa1/2ADcos 8
(1)
where no and all2 are the refractive index on the rod axis and the quadratic gradient constant, respectively, A is the grating groove spacing, D is the center-to-center fiber spacing, and 8 is the grating angle with respect to the lens face. Characterization. A WDM based on this design was constructed for us by PTR Optics, Waltham, MA. Specifications were selected to provide a prototype that could be tested in multiple sensing modes and configurations. The device was built on a 5 mm diameter GRIN lens having a 0.1 mm gradient constant and an axial refractive index of 1.55. An aluminum-coated 1200 groove/mm grating replica on a BK-7 glass wedge was cemented to one end of the lens. The angle of the wedge, 13'55', was the blaze angle of the grating. All input/output channels were identical and consisted of Spectran 820 step index fibers having a 105 pm core, 125 pm o.d., and a numerical aperture of 0.24. These were fixed at the other end of the GRIN lens in a sideby-side array. The fibers were aligned such that one channel was centered at the focal point for 488-nm light and the remaining 10 fibers at 10-nm spacing up to 588 nm.
RESULTS AND DISCUSSION Normalized transmission spectra for this demultiplexer were obtained by using the experimental setup shown in Figure 2. Channel zero (the fiber centered at 588 nm) was used as the input fiber, and the output a t each remaining fiber was scanned to determine its band-pass characteristics. As Figure 3 shows, the WDM outputs exhibit bandwidths ranging from 8 to 12 nm at half maximum. Bandwidth and spectral overlap between adjacent fiber channels decrease in the longer wavelength region. Since the device is bidirectional, each optical fiber can be used as either a n input or an output. Because the fibers are positioned along the lens radius, the spectral region covered by the device will depend on which fiber is used as the polychromatic input. The center wavelength as a function of input channel is shown in Table I. Out-of-band radiation appearing in a fiber channel can be a problem, particularly if the device is to be used to monitor fluorescence in the presence of relatively intense excitation light. The stray light component in each fiber channel was observed by use of an argon ion laser operating at 488 nm as a light source and channel zero as the input channel. Each output fiber was positioned at the monochromator input and the intensity at 488 nm measured relative to that of the 488 nm output channel. The out-of-band light component for each is listed in Table 11. The large signal in channel 10 (498 nm) is due to the cross talk with the adjacent 488-nm channel. As would be expected, the amount of stray light decreased with 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
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~
Table I. Channel Center Wavelength Using Various Inputs output wavelength, nm
channel no. 0
white
1 2 3 4 5 6 7 8 9
578 568 558 548 538 528 518 508 498 488
10
578
white 558 548 538 528 518 508 498 488 478
568 558
558 548 538
white
white
538 528 518 508 498 488 478 468
518 508 498 488 478 468 458
548 538 528 518
white 498 488 478 468 458 448
538 528 518 508 498
white 478 468 458 448 438
528 518 508 498 488 478
white 458 448 438 428
518 508 498 488 478 468 458
508 498 488 478 468 458 448 438
white 438 428 418
white 418 408
498 488 478 468 458 448 438 428 418
white
488 478 468 458 448 438 428 418 408 398
398
white
Table 11. Stray Light Components of Each Channel
-ING
FIBER
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10
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wavelength, nm
ratio
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0.000 30 0.000 29 0.000 65 0.000 43 0.000 47 0.000 39 0.000 84 0.002 03 0.11600 1.00000
-35.2 -35.4 -31.9 -33.7 -33.3 -34.1 -30.8 -26.9 -9.4
dB = 10 log (out/out,88)
Flgure 1. Wavelength division multiplexer (WDM).
A channel 0 W COUPLER
channel no.
510 540 570 WAVELENGTH (NM)
Flgure 3. Normalized transmission spectra of WDM.
600
increasing separation from this output. Over-filling the numerical aperture of the input fiber resulted in a significant increase in stray light appearing a t the outputs. Launch conditions that matched the input aperture were obtained by winding several meters of Spectran 820 fiber into a 5-cm coil to fill the fiber modes. Light was launched into this intermediate length of fiber and then stripped of cladding modes before coupling it to the demultiplexer input. Applications. The WDM was tested in several configurations and modes that are frequently used in chemical sensing applications. Though originally optimized for use in the near-IR region, this WDM readily lends itself for use in the visible region and down to about 350 nm. In this spectral region, it can be used for interface optics and monochromator in one package or as a fiber optic coupled source for a specific wavelength of radiation. To facilitate multiwavelength detection of signals dispersed by the WDM, the fiber output was positioned over a Hamamatsu 35 element photodiode array operated in the charge storage mode. Fibers were held in place by sandwiching them in an aluminum block containing V-grooves milled on 3-mm centers. This positioned each fiber at a diode center with two “dark” diodes between successive outputs. Sequential switching of the scan circuitry (Hamamatsu C2333) delivers the diode signal to a sample-and-hold amplifier, which is then fed to the analog-to-digital (A/D) converter on an IBM PC/XT. This experimental setup is shown in Figure 4. Absorbance Measurement. Absorbance measurements using fiber optic probes typically required two optical fibers (or bundles), one for the source radiation, and a second to collect light transmitted by the sample. A single fiber is generally not employed because of the large stray light component that would result from back surface reflection at the fiber endface. The two waveguides can be oriented a t 180’ or at Oo if a diffuse or specular reflector is used to direct light back in the direction of the source. The source input fiber of the WDM can be used to collect the transmitted light. The signal at each detector element in the diode array is then a function of sample absorption in each channel band-pass.
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
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LENS
ABSORBANCE
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channel 0
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Figure 4. Experimental setup for WDM absorbance measurement. a
OUTPUTS
FLUORESCENCE CELL
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DIODEARRAY
Figure 6. Experimental setup for WDM fluorescence measurement.
I
3 0 7 -
i
25,
b
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CENTER WAVELENGTH (NM) 498 518 538 5" 570 9 7 ' ' - : ' ''T ~
DIODE NUMBER Flgure 7. Fluorescence spectra of 1 X M fluorescein disodium salt in pH 6.5 (X) and 5.0 (0)0.1 M citric acid buffers.
*
I
S-D A = -log B-D
where S is the sample signal, D the dark signal, and B a blank reading. The detection limit at three times the standard deviation of the dark signal is 0.002 absorbance units a t 578 nm. Luminescence Measurement. Luminescence is also a sensing mode that is frequently utilized in fiber optic chemical analysis. Two basic configurations are employed in these types of measurement. One can use two fibers (or bundles) adjacent to one another (Oo configuration) as in absorbance measurements or, alternatively, the same fiber can be used to transmit the light in both directions. This is possible because the signal is now shifted in frequency from the exciting light. In fact, the single-fiber configuration is desirable, resulting in a probe of very small volume, which exhibits efficient collection of emitted radiation due to the complete overlap of the excitation and observation volumes at the fiber tip. However, ancillary optics are needed to separate the exciting light from the returning signal. An example of a two-fiber fluorescent measurement using the 0' configuration is shown in Figure 6. Figure 7 shows M the change in relative fluorescence intensity for fluorescein disodium salt (Aldrich Chemical Co., Inc.) in 0.1 M citric acid buffer a t pH 6.5 and 5.0, using approximately 0.2 mW of laser power. The signals are 10 scans each with a 0.77-s integration time. Some blooming onto adjacent "dark diodes from channels having the higher relative signal intensities is evident in this raw data plot. Variations in endface
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
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1.or
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4
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Figure 9, Response of fluorescence pH probe. INTEGRATOR PH SDJSOR
Figure 8. Experimental setup for double-pass WDM fluorescence measurements.
preparation of the fibers over individual diodes are responsible for the ragged shape of the emission envelope. This is not observed in the absorption spectrum because it is a ratio. In a single-fiber fluorescence measurement, the WDM can be used as a double-pass device that simplifies the optical system configuration. Figure 8 illustrates this concept in a simple arrangement with two observation channels. The WDM was constructed so that the short wavelength output channel was centered at 488 nm. Launching this light into the output will result in i t emerging a t the normal input channel zero, which is connected to a probe in the chemical system. Fluorescence excited a t the probe is collected into the channel 0 fiber, propagates back through the WDM, and is dispersed and focused to individual output fibers as we described previously. Additional rejection of the excitation radiation now becomes necessary. This can be accomplished by using a band-pass filter in front of the detector at each output. Since the ability of the filter to reject this light is a function of the incident angle, the stray radiation reaching the detector can be minimized by collimating each fiber output with a quarter-pitch GRIN lens in front of the filter. The probe used in this example consists of a porous glass bead with which fluorescein isothiocyanate has been covalently bonded (4). A photodiode with 540 f 5 nm band-pass filter (EG&G, DF-540) operated in the photovoltaic mode (5) is used as the fluorescence detector. A second diode with a 488 f 5 nm filter (EG&GDF488) was used to monitor laser output. This is placed a t channel 9 (498 nm output channel) and takes advantage of
adjacent channel crosstalk. The low intensity fluorescence signal is sent to a gated integrator (Evans 4130A). Laser intensity is monitored during the gating period (7.2 s) and the two outputs are collected with a 12-bit A/D converter and an IBM P C / X T computer. The response of this probe to pH is shown in Figure 9 as the ratio of intensity a t these two wavelengths. With this detection scheme, a sensitivity of approximately 0.1 pH units was obtained in the region between pH 3 and 7.
CONCLUSION It is clear that this device has a potential for great utility in fiber optic chemical sensing systems. Attractive features of the WDM include its simplicity, ruggedness, versatility, and low cost. We have demonstrated that a WDM can be used in both absorbance and luminescence measurement modes. The fiber outputs can be easily interfaced to a photodiode array to provide multiwavelength detection. The integrated structure of the device eliminates problems associated with exposure of the optics to the environment in on-site systems. In addition, the optical component count is reduced and alignment simplified relative to other fiber optic instrumental interfaces. This device may be designed and fabricated for use in a variety of sensing applications by specifying resolution, spectral coverage, channel count, etc., becoming a base on which to construct very compact field instrumentation for use with fiber probes. LITERATURE CITED (1) Spencer, J. L. Proc. SPIE 1983, 403, 117. (2) Tomlinson, W.; Aumiller, G. Appl. Phys. Lett. 1977, 31, 169. (3) Kobayashl. K.; Seki, M., I€€€ J. Quantum Nectron 1980, QE-16, 11. (4) Fuh, M. S.; Burgess, L. W.; Hirschfeld, T.; Christian, G. D. ACS Northwest Regional Meeting, Portland, OR, June 16-18, 1986. (5) EG&G Electro-Optics, Silicon Photovoltaic Detectors and Detector/Amplifier Combinations Application Notes, 1985.
RECEIVED for review December 18,1986. Accepted March 31, 1987.
