4 Experiment O1: Physical Characteristics of Optical Fibers Jasmine Roberts, Columbia Roberts, Columbia University
Aim: Aim: The purpose of this lab is to observe the physical characteristics of optical fibers based on length of the cables and wavelength of light that propagates through the cable. Methods: Methods: In the first part of the lab a PIN rotation mechanism was used to plot intensity versus angle of the red LED emission that propagates through cables of different lengths. The angle attained is the angle of rotation to the normal. This is done for fiber optic cable lengths of .1, .5, 1, 2, 4, and .15 meters. Using these plots the numerical aperture will be found. In the second part of the lab we observe the absorption and diffusion of two different wavelengths across different fiber optic cable lengths. An LED was attached on one end and the photo detector on the other end of the fiber optic cable. The voltage used in this part was obtained from a point where the device was not in saturation. The plot made was done to plot V out versus optic cable length. The cable lengths used for this piece were .1, .5, 1, 2, 4, 6, and 8 meter fibers. This process was done twice, first using a red LED, and then using an infrared LED. Using these two plots we will obtain the attenuation coefficient and injection loss of each LED as a parameter dependent on the cable length. Results: Results: Numerical Aperture, Attenuation coefficient, and injection loss will be obtained from the data. The voltage used in this part was obtained from a point where the device was not in saturation. Numerical aperture is said to be ninety percent of our plot of the rotational angle versus intensity. The numerical aperture represents the range of angles that a system can accept or emit light. Of our numerical apertures obtained, the .1 meter fiber optic cable had the highest. The attenuation coefficient and injection loss were achieved from our plots of cable length vs. output voltage. The voltage used in this part was obtained from a point where the device was not in saturation. The plot made shows V out versus optic cable length. The plot shows that as the cable length gets longer the output voltage decreases. The plot is transformed to fit a logarithmic scale, the slope and y-intercept are obtained which represents the attenuation coefficient and the injection loss respectively. General Terms: Fiber Terms: Fiber optics, Optical Systems Additional Key Words and P hrases: numerical hrases: numerical aperture, attenuation coefficient, injection loss
1. INTRODUCTION 1.1. Fiber Geometry
Optical fibers are a kind of waveguide, which are usually made of some type of glass, are in contrast to other waveguides, fairly flexible. The most commonly used glass is silica due to its favorable properties, particularly its potential for extremely low propagation losses (the importance of which will be discussed in the report). In laser optics, fibers have a core with a refractive index which is somewhat higher than the surrounding medium. Typical core diameters range from 4 to 8 µ m for single-mode fibers used for communications and from 200 to 1000 µ m for large-core fibers used in power transmission applications. The transition of the optical parameters from the core to the cladding can be discontinuous (step-index fiber) or smooth (grade-index fiber). The
Fiber Optic Numerica Numericall Aperture Aperture Geometry Geometry Fig. 1. Fiber
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Numerical Aperture Aperture Measur Measuremen ementt Method Method Fig. 2. Numerical
index contrast between core and cladding determines the numerical aperture of the fiber. 2. NUMERICAL APERTURE
Numerical aperture, aperture, NA, is defined as the sine of half the angle of a fiber’s light acceptance cone. All modes of light entering the fiber at angles less than that which correspond to the NA, will be bound or confined to the core of the fiber. The larger the NA of a fiber, the larger the light acceptance cone. NA is determined from the measurements of the far-field optical power distribution exiting from a two-meter length of fiber. A narrow emission pattern is more desirable for an LED in a fiber optic communication system, because material dispersion in the fibers cause different optical wavelengths to travel at different speeds. Thus, a narrower emission pattern would be less lossy. If the NA of the LED was much larger than the equilibrium value of the NA in the fiber, the incident angle of some of the light would be under the critical angle of the fiber, causing this light to be transmitted through the optical interface in the fiber and lost to cladding. This would be undesirable in a situation requiring minimal loss. An LED with a wide emission pattern would be useful when the use of filters to display different colors was desired. An LED with a wide emission pattern would be useful for a demonstration of dispersion. By holding up a glass prism to an LED with a broad spectrum, one could take note of the separate optical wavelengths that would appear. 2.1. Measurement Description R This measurement method is adopted from a Corning method.[3] The measurement consists of launching a narrow spectral band of light into the fiber and measuring the far-field intensity distribution emitted from the end of the fiber with the indicated optics. The detector and detector aperture are stepped linearly across the far-field distribution and the intensity, at a distance y from the axis of the spatial field pattern, is measured. The detector and aperture are moved linearly across a transformed spatial representation of the far field and not radially across the direct far-field. Therefore, the
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Numerical Apertur Aperture e Calculati Calculation on Results Results Fig. 3. Numerical
angular intensity is corrected by the following equation: I (θ) = E (y)cos(θ )
(1)
y = distance from axis of spatial-field pattern and is related to the emission angle ( θ ) by y = fsin(θ ), where f is the focal length of the transform lens E (y ) = irradiance at a distance y from the axis of the spatial-field pattern (θ) = angle with respect to axis of the fiber The data is normalized to the peak angular intensity and plotted versus emission angle ( θ). The spatial distance between the 5 % shall be noted and the sine of the half angle between the points represents the numerical aperture. θ1 and θ 2 were determined by linear interpolation. x = x 1 +
y − y1 (x2 − x1 ) y2 − y1
(2)
Materials Materials with broad absorption absorption are used to relax the spectral spectral requirements requirements on diode-pumped laser systems, and to minimize the need to regulate temperature. While is retain retainss the advant advantage agess of a semico semicondu nducto ctorr light light emitte emitterr, the spectr spectrum um can be tailor tailored ed or tuned to a variety of illumination applications e.g. applications e.g. a fluorescent microscope. microscope . If the PIN detector were on the rotator and the fiber were stationary, the results would be the same. This is because when the fiber is rotating, the distance from the fiber to the detector remains the same; the only difference is the angle, and this would be maintained if the detector were on the rotator. The difference between the expected value for the polished fiber and the measured unpolished value can be explained by the fact that an unpolished fiber causes incident light to the fiber to be diffused, losing some light as a result. 3. LINEAR ATTENUATION
Attenuation, Attenuation, in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance travelled through a transmission medium. Attenuation coefficients in fiber optics use units of db/m through the Solid State, Microwave, & Fiber Optics Lab, Vol. 1, No. 4, Article 4, Publication date: October 2013.
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Emission Spectra Spectra for fibers fibers (a) FL RED (b) (b) 0.5m (c) 1m 1m (d) 2m Fig. 4. Emission Solid State, Microwave, & Fiber Optics Lab, Vol. 1, No. 4, Article 4, Publication date: October 2013.
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Emission Spectra Spectra for (a) 4.0m and (b) unpolished unpolished fibers fibers & Output Output v. v. Fiber Length for plotted plotted for a Fig. 5. Emission RED LED on (c) Normal and (d) Logarithmic Scales
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v. Fiber Length for plotted for for an Infrared LED on (c) Normal and (d) (d) Logarithmic Scales Fig. 6. Output v.
medium due to the relatively high quality of transparency of modern optical transmission media. Attenuation can be quantifies by the following equation: Attenuation ( dB ) = 10 × log10
Input Intensity (W ) Output I ntensity ntensity (W )
(3)
3.1. Measurement Description
Insertion or Injection Loss. In Loss. In any fiber optic interconnection, some loss occurs. Insertion loss for a connector or splice is the difference in power that you see when inserting the device into the system. For example, taking a length of fiber and measuring the optical power through the fiber. Note the values. The difference between the first reading (P 1 ) and the second ( P 2 ) is the insertion loss, or the loss of optical power that occurs when you insert the connector into the line. This is measured as: I L(dB ) = 10 log 10 ( P 1 P 2
P 2 ) P 1
(4)
V can can be obta obtain ined ed by the the rati ratio o of V . Since Since Power Power = (Volt (Voltage age)( )(Cur Curren rent), t), curren currentt is conconin
out
) α ( V V ) /bfFrom the slope and y-intercept, stant throughout the circuit, therefore ( P P the attenuation coefficient and injection loss were determined to be -.2677 and 4.6231 2
in
1
out
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respectively for the red LED. The attenuation coefficient and injection loss were determined to be -.3414 and 2.0354 respectively for the infrared LED. The two attenuation versus distance curves can be assumed to be somewhat accurate. There could be other differences in the fibers contributing to the obtained values. For example, some of the fibers may have lost some of the incident light because they were not polished as well as others. Impurities and inhomogeneities within the fibers could also be causes of attenuation. The 940 nm measurements show much more attenuation as length increases. The main source of fiber loss at 940 nm is due to absorbing elements, including metal ions and O H − ions. This is also a major source of loss at 660 nm, but diffusion losses due to Rayleigh’s law also have a significant effect. 4. CONCLUSION
— Numerical Numerical aperture aperture (NA) represents represents the range of angles that a light can be taken and emitted. — The F.1 F.1 meter cable had the highest numerical aperture for the system. — The unpoli unpolishe shed d fiber fiber optic optic cable cable had the lowest lowest numeri numerical cal apertu aperture re,, despit despite e the length of the cable being the second smallest of the group (.15 meters). This shows that whether the cable is unpolished or polished, it plays a greater role than the actual length of the cable. This happens because the unpolished end will cause great dispersion of the light, which means numerical will be drastically reduced. — The The outp output ut volt voltag age e decr decrea ease sess in a syst system em as the the leng length th of the the opti optica call cabl cable e gets gets long longer er.. — The slope and y-intercept y-intercept are obtained obtained which represents represents the attenuatio attenuation n coefficient coefficient and the — The red LED has a larger larger attenuation attenuation coefficient coefficient and injection loss compared compared to the Infrared LED. 5. REFERENCES [1]
Z. Valy Vardeny ardeny. Telecom Telecommunic munication ations: s: A Boost for Fibre Fibre Optics, Optics, Nature Nature 416, 489491, 2002. [2] ”Fibre Optics”. Bell College. Archived from the original on 2006-02-24. [3] Numerical Aperture Measurement Method http://course.ee.ust.hk/elec http://course.ee.ust.hk/elec342/readings/corningnum 342/readings/corningnumericalaperturemeasureme ericalaperturemeasurement.pdf nt.pdf [4] Introduction to Optical Fibers, dB, Attenuation and Measurements. http://www.cisco.com/ http://www.cisco.com/en/US/tech/tk en/US/tech/tk482/tk876/technolog 482/tk876/technologiestechnote09186a008011b40 iestechnote09186a008011b406.shtml 6.shtml [5] Hecht, Eugene. Optics. Pearson Education. San Francisco, 2002.
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