Week 8

1. Week 8 Optical fiber communications Bahria University

2. Fiber Connectors • Demountable fiber connectors are more difficult to achieve than optical fiber splices. • This is because they must maintain similar tolerance requirements to splices in order to couple light between fibers efficiently, but they must accomplish it in a removable fashion. • Also, the connector design must allow for repeated connection and disconnectionwithout problems of fiber alignment, which may lead to degradation in the performance of the transmission line at the joint. Bahria University

3. Conti… • There are a large number of demountable single-fiber connectors, both commercially available and under development, which have insertion losses in the range 0.2 to 3 dB. • Fiber connectors may be separated into two broad categories: • Butt-jointed connectors • Expanded beam connectors Bahria University

4. Conti…. • Butt-jointed connectors rely upon alignment of the two prepared fiber ends in close proximity (butted) to each other so that the fiber core axes coincide. • Expanded beam connectors utilize interposed optics at the joint (i.e. lenses) in order to expand the beam from the transmitting fiber end before reducing it again to a size compatible with the receiving fiber end. • Butt jointed connectors are the most widely used connector type and a substantial number have been reported, Now we will discuss the both connectors types one by one. Bahria University

5. Types of the Butt Joint Connectors • There are various types of the Butt joint connectors, we will discuss some of them in detail that are as follows: • Cylindrical Ferrule connectors • Biconical Ferrule connectors • Double Eccentric connectors • Duplex and multiple fiber connectors Bahria University

6. Cylindrical Ferrule connector • The basic ferrule connector (sometimes referred to as a concentric sleeve connector), which is perhaps the simplest optical fiber connector design shown in the figure below: Bahria University

7. Conti…. • The two fibers to be connected are permanently bonded (with epoxy resin) in metal plugs known as ferrules which have an accurately drilled central hole in their end faces where the stripped (of buffer coating) fiber is located. • Within the connector two ferrules are placed in an alignment sleeve which, ( Each ferrule is for one fiber) using accurately machined components, allows the fiber ends to be butt jointed. The ferrules are held in place via a retaining mechanism. Bahria University

8. Alignment Issue • The fiber alignment accuracy of the basic ferrule connector is largely dependent upon the ferrule hole into which the fiber is inserted. • The ceramic materials possess outstanding thermal, mechanical and chemical resistance characteristics in comparison with metals and plastics. • In addition, unlike metal and plastic components, the ceramic ferrule material is harder than the optical fiber and is therefore unaffected by the grinding and polishing process. Bahria University

9. Average Losses • Typical average losses for multimode graded index fiber (i.e. core/cladding: 50/125 μm) and single-mode fiber (i.e. core/cladding: 9/125 μm) with the precision ceramic ferrule connector are 0.2 and 0.3 dB respectively. Bahria University

10. Duplex and multiple fiber connector • A number of duplex fiber connector designs were developed in order to provide two-way communication, but few have found widespread use to date. • The media interface connector plug shown in Figure below was part of a duplex fiber connector which was developed to meet the American National Standards Institute (ANSI) specification for use within optical fiber LANs. • This connector plug will mate directly with connectorized optical LAN components (i.e. transmitters and receivers) Bahria University

11. Multifiber connection • Multiple-fiber connection is obviously advantageous when interconnecting a large number of fibers. The cylindrical ferrule connectors can be assembled in housings to form multiple-fiber configurations. • In addition to assembling a number of single-fiber connectors to form a multiple-fiber connector, other examples of multiple-fiber connectors have been explored. • Silicon chip arrays were suggested for the jointing of fiber ribbon cable, which utilizes V-grooved silicon chips. Bahria University

12. Conti…. • In this connector, ribbon fibers were mounted and bonded into the V-grooves in order to form a plug together with precision metal guiding rods and coil springs. • Connection is then accomplished by butt jointing the two pairs of guiding rods in the slitted sleeves located in the adaptor. Bahria University

13. Commercial used Connectors • Connectors are for both single mode and multimode fibers. • Multimode optical connectors are generally used within data communications (i.e. LANs), transport (i.e. automobiles and aircraft) and with specific test instruments, while single-mode fiber connectors are employed extensively in optical fiber telecommunication systems. Bahria University

14. Conti…. Bahria University

15. Expanded Beam Connectors • An alternative to connection via direct butt joints between optical fibers is offered by the principle of the expanded beam. • Fiber connection utilizing this principle is illustrated in Figure below, which shows a connector consisting of two lenses for collimating and refocusing the light from one fiber into the other. • The use of these interposed optics makes the achievement of lateral alignment much less critical than with a butt-jointed fiber connector. Bahria University

16. Conti… • A similar configuration has been used for single-mode fiber connection in which the lenses have a 2.5 mm diameter but with antireflection-coated lenses, average losses around 0.7 dB were obtained using single-mode. Bahria University

17. Alternative Arrangement • Figure below shows an expanded beam connector which employs a molded spherical Lens . • The fiber is positioned approximately at the focal length of the lens in order to obtain a collimated beam and hence minimize lens-to-lens longitudinal misalignment effects. • A lens alignment sleeve is used to minimize the effects of angular misalignment which, together with a ferrule, grommet, spring and external housing, provides the complete connector structure. • The repeatability of this relatively straightforward lens design was found to be good, incurring losses of around 0.7 dB. Bahria University

18. Design Problem On White Board Bahria University

19. GRIN Rod Lenses • An alternative lens geometry to facilitate efficient beam expansion and collimation within expanded beam connectors is that of the graded index (GRIN) rod lens. Arrangement: • The GRIN-rod lens, which arose from developments on graded index fiber waveguides, comprises a cylindrical glass rod typically 0.2 to 2 mm in diameter which exhibits a parabolic refractive index profile with a maximum at the axis similar to graded index fiber. • Light propagation through the lens is determined by the lens dimensions and, because refractive index is a wavelength-dependent parameter, by the wavelength of the light. Bahria University

20. Conti…. • The GRIN-rod lens can produce a collimated output beam with a divergent angle α of between 1° and 5° from a light source situated on, or near to, the opposite lens face, as illustrated in Figure below: • Conversely, it can focus an incoming light beam through a small area located at the center of the opposite lens face. Bahria University

21. Conti…. • Ray propagation through the GRIN-rod lens medium is approximately governed by the paraxial ray equation: • Furthermore, the refractive index at r from the optical axis in a gradient index medium, may be expressed as: • By using both equations: ( Assuming n = n1 ) Bahria University

22. Source Coupling • Coupling from the light source to the fiber can be very inefficient, the coupling efficiency is defined as: ή=Pf/Ps • Where Pf is the power in the fiber and Ps is the power emitted by the source. • Several mechanism contribute to the inefficiency produced due to the source coupling: • Reflection Loss • Area mismatch loss • Numerical Aperture loss Bahria University

23. Reflection Loss • If an air gap exists between the emitting surface and the fiber then power is reflected at the boundary, that is called the Fresnel reflection we have already discussed. • If the index matching fluid has been filled between the fiber and source or if the fiber and source has close enough (in contact) then this loss will disappear. • 4 % of the light will be reflected back towards the source of light. Bahria University

24. Area Mismatch loss • If the source area is larger than the fiber core then some of the power will lost. • The reduction in the efficiency is the ratio of the core area to the source area (Ac/As). • If the source is smaller than the core then this loss will disappear. Bahria University

25. NA Loss • As we know that light incident at the angles larger than the waveguide’s acceptance angle is not efficiently transmitted. • When coupling light from the source into the fiber, the losses owing to this effect can be very significant. • The efficiency for this phenomenon is as follows: • As the NA of the fiber is higher this loss will be lower. Like for NA= 0.24 this loss will be 12.4 while for the NA=0.48 this will be 6.4 dB. Bahria University

26. Coupling Efficiency of the Graded Index Fiber • Coupling efficiency of the GRIN fiber is lesser as compare to the SI fiber, because here source is selected according to the NA of the core axis that is larger as compare ( moving to cladding interface) other axis. • Therefore Power is less efficiently coupled into GRIN than into SI fiber, the coupling efficiency will be: • This equation holds if the LED’s emitting surface has the same size as the fiber core. Bahria University

27. Conti…. • But if the emitting surface is smaller, then most of the light will be collected over fiber axis where NA is larger, and over all collection efficiency will be increased then efficiency will be: • Where ae, radius of the emitting surface and af is the radius of the fiber core. • Edge emitting and LASER diodes can improve the coupling efficiency if these are used with the Lenses. • As we know the coupling inefficiency in multimode fibers is the wide angular distribution( spread) of the light the best way to reduce this angular spread is to use Lens. Bahria University

28. Summary • The efficiency of the couplers depends on the radiation pattern and the NA of the fiber. • Surface emitting LEDs has loss more than 12 dB but that loss can be improved by using lens. • Laser diodes and the edge emitting LEDs has narrower radiation pattern than the surface emitting LEDs. Bahria University