Lab Report Fiber Optics PDF

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Laboratory Activity: Fiber Optics and Optical Power Measurements J. A. D. Bautista A. A. M. Castillo Abstract— This laboratory report will discuss the characteristics of optical fibers, specifically, the single-mode fiber (S MF) and the multi-mode fiber (MMF). The report will go into the power measu...

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Laboratory Activity: Fiber Optics and Optical Power Measurements J. A. D. Bautista

A. A. M. Castillo Abstract— This laboratory report will discuss the characteristics of optical fibers, specifically, the single-mode fiber (S MF) and the multi-mode fiber (MMF). The report will go into the power measurements of both types of fibers and will also observe the power outputs for the mechanical splicing combinations using the two fibers. Furthermore, the characteristics of a 50/50 fiber coupler is also observed and discussed. Fig. 1. Optical Fiber Structure

I. CONCEPT AND THEORY In 1854 a British physicist by the name o f John Tyndall discovered that light could be bent around a corner through a curved spout of running water. In this experiment he permitted water to spout from a tube, the light on reaching the limit ing surface of air and water was totally reflected and seemed to be washed downwards by the descending liquid [1] . Tyndall discovered the idea of total internal reflection (TIR) and it is fro m this concept where optical fiber co mmun ication is built on. Like any other fo rm o f co mmun ication, fiber optic communicat ion is co mposed of three elements, a light source which acts as the sender of informat ion, a fiber media which acts as the transmission mediu m, and a light detector for the receiving end [2]. Most light sources emit light with wavelengths of 1300n m and 1550n m since these are the points when the least attenuation is experienced, as will be discussed in depth later. For this activity, the focus is on the transmission mediu m known as the optical fiber.

There are basically two types of fibers: stepped index and graded index. Graded index fibers has a h igh index of refract ion at the center of the fiber and exh ibits a gradual decrease of the index as one moves away from the center. On the other hand, step-index fibers have an abrupt and distinct difference between the fiber core and cladding. The graded index fiber and the step index fiber are illustrated in Fig. 2 and Fig. 3, respectively.

Fig. 2. Multi-mode Graded index fiber.

Optical Fibers Fig 3. Multi-mode Stepped index fiber.

Optical fibers are the actual media that guides the ligh t [2]. The fibers can either be made of glass or plastic, but glass fibers are more preferred because they exh ibit less attenuation. The typical fiber structure is usually made up of a core center where the light actually propagates in; a cladding of lower index of refract ion that allows the light to undergo TIR and propagate down the fiber; and the buffer coating which serves as protection for the other parts of the fiber. A typical structure for an optical fiber is shown in Fig. 1.

The stepped index fiber is further classified into two types: the single mode and the multi-mode fiber. The mu lti-mode stepped index fiber has, mult iple paths for the light to travel, as shown in Fig. 2 and Fig. 3 while the single mode fiber only allows a single light ray to propagate as shown in Fig. 4 [2].

Fig. 4. Single Mode Fiber

Refractive Index and Total Internal Reflection Optical fiber communication relies on the concept of Total Internal Reflection (TIR) for light to properly propagate down the media to its destination. TIR is achieved when light goes from a medium of higher refractive index to a lower refractive index and the angle of the reflected beam exceeds 90 degrees from the normal of the interfaces. This property is governed by Snell’s law given below, and Fig. 5. illustrates the concept of TIR.

where n 1 and n 2 are refractive indexes of material 1 and material 2, while θ 1 and θ 2 are angles of the incident ray and the reflected ray, respectively, with respect to the normal of the interface.

attenuation of silica fiber is shown in Fig. 7. As shown in the graph, three “windows” are identified as ideal wavelengths for light sources. Nowadays, the 1300n m and 1550n m windows are co mmonly in use. These are the points where the attenuation of silica is at a local minima [3]. The most significant factors contributing to the attenuation are Rayleigh scattering and material absorption. Material absorption occurs as a result of the imperfect ion and impurities in the fiber. The most common impurity is the hydroxyl (OH-) mo lecule, which remains as a residue fro m manufacturing of the fiber [4]. The absorbed light particles are lost to the impurities thus causing a loss in power. Rayleigh scattering is the result of elastic collisions between the light wave and the silica mo lecules in the fiber [4].When th elastic collisions occer, the light scattered in all directions. If the scattered light continues to propagate down the fiber, no attenuation occurs but there is also the chance that the scattered light is unable to continue down the fiber.

Splicing Two optical fiber splicing methods are available for permanent jo ining of two optical fibers. The optic cable fusion splicing with an insertion loss of less than 0.1db is imple mented using a special equip ment called fusion splicer. The other type is mechanical splicing with an insertion loss of less than 0.5d B. Mechanical splicing uses a small mechanical splice, that precisely aligns two bare fibers and secures them mechanically [7]. Mechanical splicing is the splicing method mentioned in this activity.

Fig. 5. T otal Internal Reflection inside the Optical Fiber.

Optical Power in Watts and dBM In optical co mmunicat ion, optical power measures the rate at which photons arrive at a detector, it is a measure of energy transfer per time and has a unit of Watts [5]. The power level is too wide to be exp ressed on a linear scale. Thus, the logarith mic scale known as decibel (dB) is used to express in optical co mmunicat ions [4]. The decibel does not give a magnitude of power, but it is a rat io of the output power to the input power, both in Watts, as expressed by, dB = 10log(Pout /Pin )

(1)

The power level related to 1mW is noted as dBm and the power level related to 1µW is noted as dBµ. The dBm and dBµ equations are given as follows,

Fiber coupler A fiber coupler is an optical fiber device with one or more input fibers and one or several output fibers. Light fro m an input fiber can appear at one or more outputs, with the power distribution potentially depending on the Wavelength and polarization [6]. II. M ET HODOLOGY The activity calls for following safety guides for eye safety as well as proper handling of fiber optic cable. A paraphrased list of the guidelines is given below.

dBm = 10log(P/1mW)

(2)

dBµ = 10log(P/1µW)

(3)

Attenuation The material most used in optical fibers is silica (SiO2) [3]. Silica fiber exh ibit d ifferent attenuation rates given different wavelengths for the source input. A graph of the spectral

Eye Safety  Do not directly shine visib le and infrared radiation into your eyes.  Turn off power source during manipulat ion and concatenation of optical fiber.  No bare fiber will be handled in this lab to eliminate danger of serious eye injury due to microscopic glass particles.

Proper fiber handling 

maintain optical quality and cleanliness of the fiber endfaces and instrument connector interfaces.

 

Wash hands in soap before the activity. Use a lint-free tissue and residue free isopropanol for cleaning optical surfaces . Allow 15 seconds for surfaces to dry before mating. Always cap fiber end, bulkheads and mating sleeves to percent contamination of optically clean surfaces.

For the first part of the activity, the value of the optical output power measured at the opposite end of the SM optical fiber is read in dBm and it is converted to mW by isolating P in (2). The equation for dBm to mW is given by, (4)

 

List of Materials and Equipment        

 

single mode fiber multimode fiber optical power meter optical source FC connector FC bulkhead infrared sensor fiber mating sleeves

  

2x2 fier coupler semiconductor grade isopropyl alcohol lint-free tissue bulkhead caps fiber connector caps

To observe the different optical power behavior with the SM and MM fiber several measurements are taken. In Part I of the activity, the SM fiber is coupled with a 1.3 micron light, then measurements of the optical power at the opposite end of the fiber are taken. Next, the SM fiber is coupled with a mat ing sleeve into a MM fiber, then optical power is measured at the end of the MM fiber. A similar procedure is done in Part II of the activity for the MM fiber, except that this time, it is the SM fiber that is coupled with the MM fiber, and the power output is measured fro m the end of the SM fiber. For the Part III of the activity, a 50/50 also called a 3 d B fiber coupler is used and output power is measured fro m the remain ing three ends. The characteristics and parameters of the fiber coupler is then analyzed based on the observed data. . III. RESULT S AND DISCUSSION T ABLE I VALUES FOR P ART I OF THE ACTIVITY

Fiber Type Single Mode SMF-MMF splice

Power in dBm -7.94

-8.75

Power (mW) 0.160mW 0.133mW

Computations from dBm to mW in Table I is as follows, (5)

= 0.160mW = 0.133mW

A comparison of the values of the output power of the SMF alone to when it was coupled with the MMF via the mating sleeve, gives the observation that the addition of the MMF also added further attenuation or loss in power. The loss in power can be computed by simply, subtracting the dBm value of the SMF alone from the power of the SM-MMF power loss, given by, (

(6)

)

This additional loss could have been brought about by connector losses (caused by the mating sleeve). But the more probable cause would be the fact that because there is a longer fiber, the light travels a longer d istance, thus, being more prone to Rayleigh scattering or absorption losses. T ABLE III VALUES FOR P ART II OF THE ACTIVITY

Fiber Type Multi-mode

Power in dBm

-7.69

Power in mW 0.170mW

MMF-SMF splice

-10.16

0.096mW

Part II of the activ ity is essentially similar to the procedures done for Part I, except that this time the MMF was used and a MMF-SMF splice was created. Measure power values for Part II of the activity is shown in Table II. Co mputations using (4) for converting dBm to mW for Part II are as follows,

= 0.170mW = 0.096mW

For the additional loss of the MMF-SMF splice, the loss can be calculated using (6) as in Part I, (

Thus the experimental ratio is 46/ 54 (Port 3/ Port 4), instead of 50/50.

)

IV. A NSWERS T O QUEST IONS

The additional 2.47dB loss can again be attributed to loss caused by the mating sleeve or the Rayleigh scattering and absorption because of the added length of the fiber,

1.

SMF-28 is manufactured to the most demanding specifications in the industry and is widely used in the transmission of voice, data and/or video. It has a core diameter o f 8.2u m a nu merical aperture of 0.14 and a refractive difference of 0.36% [9].

T ABLE IIIII VALUES FOR P ART III OF THE ACTIVITY

Port

Power in dBm

Port 2 Port 3 Port 4

-23.26 -9.97 -11.48

Power in mW 0.004 0.101 0.071

Power in µW 4 101 71

For the values in Part III, the measure power are converted into mw and µW using a version (2) and (3). Co mputations are as follows,

What is the core diameter of SMF-28 optical fiber?

2.

What is the conventional color of singlemode fiber? The fiber's jacket color is at times used to differentiate mu lti-mode fibers (orange) fro m single-mode (yellow) fibers [10].

3.

Assuming 100% coupling efficiency of power into the optical power meter, how much optical power is lost in the SMF-MMF mechanical splice? As shown in (6), a loss of 0.81 dB is added when the SMF-MMF splice was made. This translates to an additional 18% loss.

Port 2 = 0.004mW = 4µW

4.

Port 3 = 0.101mW

Assuming 100% coupling efficiency of power intothe optical power meter, how much optical power is lost in the MMF-SMF mechanical splice? Similar to question 3, the additional loss of the MMFSMF splice was already co mputed in the discussion and the results are about -2.47 d B wh ich translates to an additional 43% loss.

= 101µW Port 4 5.

= 0.071mW = 71µW

Based on the values of the power fro m Port 3 and Port 4, it can be seen that the theoretical coupling ratio of 50/50 is not followed, instead, the experimental coupling rat io is computed by, (

)

(

)

The measured output powers at 3 and 4 are consistent with what launched input power(at port 1)? No, they do not add up, the sum of Port 3 and Port o f are less than the input power. This is because of the loss incurred by the ray as it p ropagated down the fiber coupler.

6.

Given your data, what is the coupling ratio of the device? As computed in the discussion, the experimental coupling ratio is 46/54 (Port 3/ Port 4), instead of 50/50.

7.

Assuming a 4% reflection off the glass-air interface at port 3 and 4, estimate the power that should be measured at port 2 (state in both dBm and mW or µ W). Explain why how this is consistent with your measured value, and if there is no discrepancy, hypothesize

reasons for such by identifying possible other sources of loss in path.

[9] http://www.photonics.byu.edu/FiberOpticConnectors .parts/images/smf28.pdf

The computation for the experimental power at port 2 given a 4% reflection is given by, Power at Port 2 = (Power at Port 3(µW) + Power at Port 4(µW)) x 0.04

[10] www.tech-faq.com/multi-mode-fiber.ht m

Fro m the formu la the experimental power for Port 2 is 6.88 µW or .006mW o r -22.22 dB, which are values greater than the experimental value meaning loss is also experienced by the reflected beam that enters port 2. V. CONCLUSION Optical fibers are essential for optical co mmunicat ion. It is important to understand the characteristics of the fiber especially with how power is los t as light propagates down the fiber. With an understanding of the attenuation characteristics of the fiber, an efficient commun ication system can be realized. VI. REFERENCES [1] Allan, W. B., Fiber Optics: Theory and Practice, (Plenum Press, NewYork, 1973). [2] http://www.openoptogenetics.org/images/f/fb/Funda mentals_of_Fiber_Optics.pdf. [3] http://lib.tkk.fi/Diss/2006/isbn9512282658/ isbn9512 282658.pdf. [4] http://books.google.com/books?id=5LMp7y xfeDA C &pg=PA53&dq=optical+power&h l=en&ei=u weCTe SDNc3Ccdb IAD&sa=X&oi=book_result&ct=result&resnum=2& ved=0 DUQ6AEwAQ#v=onepage&q=optical%20power&f =false [5] http://books.google.com/books?id=hw1PFAr2L0s C& pg=PA237&dq=optical+power+definition&hl=en&ei =7weCTd3xCIO3cL65xaM D&sa=X&oi=book_resul t&ct=result&resnum=2&ved=0CDYQ6A EwA Q#v= onepage&q=optical%20power%20definition&f=false [6] http://www.timbercon.com/Fiber-Opt ic-Coupler.ht ml [7] http://www.fiberoptics4sale.co m/Merchant2/fiber optic-splicing-tutorial.php [8] http://www.scribd.co m/doc/3942245/ Optical-fiberStructures...