16 WEDNESDAY MORNING

glass particles synthesized with vapor-phase reac- tion of raw materials and is successively consoli- dated into a transparent preform by zone melting. Such a simple process has the advantage of long large-diameter preform fabrication.

In the early stages of VAD development, the process was thought to have some bearing on re- fractive-index profile formation, in OH-ion reduction, in single-mode fiber fabrication, and in geometrical uniformity of core diameter along fiber length. These technical problems have finally been worked out.

The refractive-index profile in the VAD preforms was thought to be formed by mutual overlapping of different gas-phase constituents in the flame and/or on the preform, but the actual phenomenon of profile formation indicates that the index profile is deter- mined by the deposition properties of SiOn-GeOZ particles synthesized with flame hydrolysis. The measured 6-dB bandwidth of graded-index multi- mode fibers has been improved to reach above 6 GHz . km at 1.3 p m by controlling the temperature distribution on the porous preform during deposition. Many of the production fibers have bandwidths in excess of 1 GHz . km.

Another concern was the difficulty of minimization of the contaminated OH-ion because the process uses the oxyhydrogen flame for reaction. However, the introduction of the dehydration technique using SG€Iz as a dehydration reagent leads to the success in OH-ion reduction from -30 ppm to <I ppm in 1978. The OH concentration in fibers drawn from the dehydrated preform has been decreased in 1980 to a lower level of < I ppb.

VAD fabrication of single-mode fibers requires a different technique from that of multimode fibers. It has become possible to produce long low-loss single-mode fibers by using the torch specially de- signed for core porous preform deposition. The length of single-mode fiber drawn from one preform has reached to over 100 km, and the loss has re- duced to -0.2 dB/km at I .55 pm, whose geomet- rical uniformity has also been measured to be within + I % .

Considering the research and development of future fiber transmission systems, fiber cost re- duction will become of prime importance, especially for subscriber loop application. Lower fiber pro- duction costs will be achieved by using low-cost raw materials, increasing the deposition speed, syn- thesizing thoroughly core and cladding excluding jacketing silica tube, and high speed fiber drawing.

It has been confirmed that the dehydration pro- cess in VAD allows the use of low purity starting materials, for example, SiCI4 containing SiHCI3 or SiH2CI2, without additional transmission loss. At present, VAD glass deposition speed is typically 30-100 glh, which is equivalent to 6-20 km/h for fiber length with jacketing process. However, this speed can go to >300 g/h by improving the depo- sition conditions and process construction in pro- duction. (Invited paper, 25 min)

1. T. Izawa, S. Kobayashi, S. Sudo, and F. Hanawa, in Jechnical Digest, first International Confer- ence /OOC(IECE, Tokyo, 1977). p. 375.

WC3 Ultraviolet-excited fluorescence in optical fibers and preforms

HERMAN M. PRESBY, Bell Laboratories, Crawford Hill Laboratory, Holmdel, N.J. 07733.

A new and nondestructive method is described to measure the refractive-index profile of Ge02-doped optical fibers and preforms within seconds and with accuracy comparable with the best existing tech- niques. The method is based on the fact that ger- maniumdoped optical fibers and preforms fluoresce at a wavelength near 420 nm when excited by UV

radiation below 350 nm. The GeO2-absorption spectrum, measured by utilizing a preform slab- sample illuminated with a deuterium lamp coupled to a grating monochromator, exhibits a relatively broad absorption curve centered at 240 nm with a tail extending to nearly 380 nm. The proportionality of fluorescent intensity to dopant concentration and hence to the index difference has been established for the GeOz-SiOZ binary system by measurements made on preforms with the arrangement shown in Fig. 1.

A He-Cd laser with an output of 15 mW at a wavelength of 325 nm is used as the UV exciting source: 325 nm is well into the tail of the GeOp ab- sorption band. Hence very little of the UV is ab- sorbed leading to uniform excitation of the preform. The output of the laser is focused onto the preform by a 20-cm focal-length UV lens. Thus a very thin beam, very much less than the core diameter, is used to probe the core. The preform is held on a translational stage so that it can be moved relative to the video-camera which detects the fluorescent radiation. The preform is first positioned with the aid of an auxiliary collimated light source so that the camera is focused on the center of the core. The laser beam is then adjusted by the focusing lens to pass through the center of the preform. A com- puter-controlled video digitizer is utilized to address specified regions of the video field and collect the intensity of the fluorescing core, which is then di- rectly plotted. A display monitor is utilized to view the preform for alignment and for setting the digi- tization limits. Calibration is achieved by first viewing the fluorescent intensity of a known preform standard.

The profiles of binary GeO2-SiOp preforms ob- tained in this manner almost instantly are in excellent agreement with the corresponding profiles obtained by both slab-interferometry and the focusing method. Examples of two single-mode preforms are shown in Figs. 2 and 3. A photograph of the monitor, shown at the top, displays the digitized intensity profile and the fluorescing core region. The plotted intensity profile is shown below the photograph.

Fluorescence profiling is thus a powerful yet simple way to obtain the index distributions of pure GeOZ-SiOn preforms. Its versatility is enhanced by the fact that the profile is not obtained 2s an inte- grated effect, but the profiling beam uniquely defines the diameter across which the profile is measured. Thus circular symmetry is not assumed, and the preform can be rotated and probed along a different diameter to detect nonuniformities. The technique described can also be applied directly to fibers with the use of a microscope instead of a camera lens to make the observations. (13 min)

WC4 Optical attenuation in IR fibers

JAMES A. HARRINGTON and ARLIE G. STANDLEE, Hughes Research Laboratories, Malibu, Calif. 90265.

Fiber optical waveguides have been fabricated which have the unique capability of transmitting the long wavelength radiation required in certain sensor, long distance communication, and power delivery systems. These polycrystalline fibers, made from alkali and thallium halides, were prepared using hot extrusion techniques. The fiber losses, however, are much greater than the projected losses for the crystalline fiber materials. To determine the source of our IR fiber losses, we have measured the atten- uation due to scattering and absorption between 4 and 10 p m in KRS-5 (TlBrl) fibers.

The total attenuation coefficient aT is the.sum of the contributions due to scattering as and to ab- sorption O!A. To obtain U T for the fiber, we mea- sured the insertion loss using HF/DF chemical, CO, and COP laser sources. The differential and inte-

grated light-scattering contribution U S was mea- sured by using a 2.5-cm diam diffuse gold-coated integrating sphere. The absorptive losses were then obtained from U A = OIT - U S .

The results of our measurements at 10.6 p m on 500-pm diam KRS-5 fiber indicate, in general, that scattering and absorptive losses are approximately equal. As an example, we found CYT = 0.57 dB/m, U S = 0.27 dB/m, and 014 = 0.30 dB/m for one 1.35-m long fiber. Figure 1 shows the differential light scattered from this fiber. From these data we see that the scattered-light intensity increases markedly at the output end of the fiber. We feel this excess scattering results from light coupled into higher-order modes by roughness of the fiber end. Some improvement in the overall scattered light flux was obtained by using a nonstandard end finish (see Fig. 1). At 5 fim, we have found the total attenuation to be at least twice as large as it is at 10.6 pm.

The effect of fiber grain size and bend radius on transmission has also been studied. Our smallest average grain size fiber (3 pm) shows a reduction in transmission as the bend radius decreases. For 250-pm diam fiber, 1.7 m long, radius of 8 cm re- duces transmission by loyo, while at 2 qn the change in transmission is 40%. Below bend radii of 10 cm this change is permanent as the fiber has been plastically deformed. In general, bending causes increased scattering losses most likely re- sulting from separation of grain boundaries.

( 13 min)

Wednesday MORNING

10June 1981 WD

LINCOLN ROOM

8:30 AM Environmental Measurements: 1

S. H. Melfi, Presider

WDI Need for remote-sensing systems of the atmosphere

JAMES D. LAWRENCE, JR., NASA Langley Re- search Center, Hampton, Va. 23665.

Recent concern over the potential impact of man's activities on the atmosphere has focused increased attention by the public and scientific community on issues related to atmospheric chemistry and dynamics. It was quickly realized that much information available is incomplete and qualitative and is lacking a broad foundation based on fundamental understanding of atmospheric pro- cesses. As a result, advancement of our knowledge in atmospheric sciences through intensive research is a prerequisite for the evaluation of human activity on the environment. A critical element of this re- search is an effective atmospheric observation program.

This paper will briefly review the chemical cycles, trace specie, budgets, and dynamic processes in the atmosphere. The need for observations of critical species and processes will be summarized. The status of current and evolving remote-sensing sys- tems for atmospheric studies will be discussed, and the potential contributions from these systems will be highlighted. Special meniion will be made of remote-sensing techniques utilizing lasers.

(Invited paper, 25 min)

WEDNESDAY MORNING

DIGITIZER

17

WC3 Fig. 1. Arrangement.

WC3 Fig. 2. Single-mode preform, example 1.

WC3 Fig. 3. Single-mode preform, example 2.

1 .o 1 I I

0.9 - DIFFERENTIAL LIGHT SCATTERING - KRS-5 FIBER

2 0.8 - (500pm DIA., 10.6flm)

0.4 Ai

0 20 40 60 80 100 120 140

POSITION ALONG FIBER. em

WC4 Fig. 1. Light scattering in KRS-5 fiber at 10.6 pm. Solid curve is for a fiber end polished with conventional polishing powders and techniques, while the dashed curve represents an alternative end finish.