PROJECT REPORT COUPLING OF LIGHT THROUGH FIBER PHY 564 SUBMITTED BY: GAGANDEEP KAUR ( )

Transcription

1 PROJECT REPORT COUPLING OF LIGHT THROUGH FIBER PHY 564 SUBMITTED BY: GAGANDEEP KAUR ( ) 1

2 INTRODUCTION: An optical fiber (or fiber) is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including sensors and fiber lasers. Figure 1: A bundle of optical fibers Light is kept in the core of the optical fiber by total internal reflection. This causes the fiber to act as a waveguide. Fibers which support many propagation paths or transverse modes are called multimode fibers (MMF), while those which can only support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a larger core diameter, and are used for shortdistance communication links and for applications where high power must be transmitted. Singlemode fibers are used for most communication links longer than 550 meters (1,800 ft). 2

3 Figure 2: A TOSLINK fiber optic audio cable being illuminated on one end Single-mode fiber: Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or mono-mode fiber. The behavior of larger-core multi-mode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation (hence the name). The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes. Figure 3: The structure of a typical single-mode fiber. 1. Core: 8 µm diameter 2. Cladding: 125 µm dia. 3. Buffer: 250 µm dia. 4. Jacket: 400 µm dia. 3

4 Multi-Mode Fiber: Multimode fiber has higher "light-gathering" capacity than single-mode optical fiber. In practical terms, the larger core size simplifies connections and also allows the use of lower-cost electronics such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers (VCSELs) which operate at the 850 nm wavelength (single-mode fibers used in telecommunications operate at 1310 or 1550 nm and require more expensive laser sources. Single mode fibers exist for nearly all visible wavelengths of light). However, compared to single-mode fibers, the limit on speed times distance is lower. Because multi-mode fiber has a larger core-size than single-mode fiber, it supports more than one propagation mode, hence it is limited by modal dispersion, while single mode is not. Also, because of their larger core size, multi-mode fibers have higher numerical apertures which means they are better at collecting light than single-mode fibers. Due to the modal dispersion in the fiber, multi-mode fiber has higher pulse spreading rates than single mode fiber, limiting multi-mode fiber s information transmission capacity. Single-mode fibers are most often used in high-precision scientific research because the allowance of only one propagation mode of the light makes the light easier to focus properly. Jacket color is sometimes used to distinguish multi-mode cables from single-mode, with the former being orange and the latter yellow. A wide range of colors are commonly seen, however, so jacket color cannot always be relied upon to distinguish types of cable. TYPES of Multi-mode fibers: Multi-mode fibers are described by their core and cladding diameters. Thus, 62.5/125 µm multimode fiber has a core size of 62.5 micrometres (µm) and a cladding diameter of 125 µm. In addition, multi-mode fibers are described using a system of classification determined by the ISO standard OM1, OM2, and OM3 which is based on the bandwidth of the multi-mode fiber. Tasks performed in Dr. La Rosa s Lab: 1. Coupling the laser light through a fiber: a) Mounting the equipment in a proper manner on the optical table: This was done using the base post holders as shown below: Figure 4: Pedestal-Base Post Holders 4

5 b) The most challenging task while coupling the laser light through the fiber was the alignment of the laser and the other equipment. One needs to be very patient and critical about the exact alignment without which it becomes impossible to focus and let pass the laser through the fiber. Figure 5: Overview of Setup Figure 6: Input Fiber Optic Mount 5

6 Figure 7: Output Fiber Optic Mount b) Joining Lengths of Optical Fiber: Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of the fibers must be carefully cleaved, and then spliced together either mechanically or by fusing them together with an electric arc. Special connectors are used to make removable connections. Figure 8: Twisted Fiber Optics Coupler The coupler works by bringing two fiber cores into interaction distance with each other. Physically, this means that the fibers are twisted into each other. Light from the core leaks into the cladding of the fiber. This light then transfers partially into the cladding of the other core due to proximity. This leaked light can similarly leak into the second core. Thus the light is transferred from one core to the other over a certain pair length. This process is cyclic, i.e. it is possible for this light to leak back into the original core. 6

7 Applications of couplers: This coupler is used extensively in the telecommunications industry as a means of transmitting a reduced noise signal (Common Mode Rejection). This application will become even more widespread as fiber is used in last mile home broadband communications. Other applications of this device include high sensitivity inferometers. ATTENUATION IN OPTICAL FIBERS: 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 usually use units of db/km through the medium due to the relatively high quality of transparency of modern optical transmission media. The medium is typically usually a fiber of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a digital signal across large distances. Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal. Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption. Light scattering: The propagation of light through the core an optical fiber is based on total internal reflection of the light wave. Rough and irregular surfaces, even at the molecular level of the glass, can cause light rays to be reflected in many random directions. We refer to this type of reflection as diffuse reflection, and it is typically characterized by wide variety of reflection angles. Most of the objects that you see with the naked eye are visible due to diffuse reflection. Another term commonly used for this type of reflection is light scattering. Light scattering from the surfaces of objects is our primary mechanism of physical observation. Figure 9: Specular Reflection 7

8 Figure 10: Diffuse Reflection Light scattering depends on the wavelength of the light being scattered. Thus, limits to spatial scales of visibility arise, depending on the frequency of the incident light wave and the physical dimension (or spatial scale) of the scattering center, which is typically in the form of some specific microstructural feature. Since visible light has a wavelength of the order of one micron (one millionth of a meter) scattering centers will have dimensions on a similar spatial scale. Thus, attenuation results from the incoherent scattering of light at internal surfaces and interfaces. In (poly) crystalline materials such as metals and ceramics, in addition to pores, most of the internal surfaces or interfaces are in the form of grain boundaries that separate tiny regions of crystalline order. It has recently been shown that when the size of the scattering center (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent. This phenomenon has given rise to the production of transparent ceramic materials. Similarly, the scattering of light in optical quality glass fiber is caused by molecular level irregularities (compositional fluctuations) in the glass structure. Indeed, one emerging school of thought is that a glass is simply the limiting case of a polycrystalline solid. Within this framework, "domains" exhibiting various degrees of short-range order become the building blocks of metals and alloys, as well as glasses and ceramics. Distributed both between and within these domains are micro structural defects which will provide the most ideal locations for the occurrence of light scattering. This same phenomenon is seen as one of the limiting factors in the transparency of IR missile domes. UV-Vis-IR absorption: In addition to light scattering, attenuation or signal loss can also occur due to selective absorption of specific wavelengths, in a manner similar to that responsible for the appearance of color. Primary material considerations include both electrons and molecules as follows: 1) At the electronic level, it depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific wavelength or frequency in the ultraviolet (UV) or visible ranges. This is what gives rise to color. 2) At the atomic or molecular level, it depends on the frequencies of atomic or molecular vibrations or chemical bonds, how close-packed its atoms or molecules are, and whether or not the atoms or 8

9 molecules exhibit long-range order. These factors will determine the capacity of the material transmitting longer wavelengths in the infrared (IR), far IR, radio and microwave ranges. The design of any optically transparent device requires the selection of materials based upon knowledge of its properties and limitations. The lattice absorption characteristics observed at the lower frequency regions (mid IR to far-infrared wavelength range) define the long-wavelength transparency limit of the material. They are the result of the interactive coupling between the motions of thermally induced vibrations of the constituent atoms and molecules of the solid lattice and the incident light wave radiation. Hence, all materials are bounded by limiting regions of absorption caused by atomic and molecular vibrations (bond-stretching) in the far-infrared (>10 µm). Figure 11: Normal modes of vibration in a crystalline solid. Thus, multi-phonon absorption occurs when two or more phonons simultaneously interact to produce electric dipole moments with which the incident radiation may couple. These dipoles can absorb energy from the incident radiation, reaching a maximum coupling with the radiation when the frequency is equal to the fundamental vibrational mode of the molecular dipole (e.g. Si-O bond) in the far-infrared, or one of its harmonics. The selective absorption of infrared (IR) light by a particular material occurs because the selected frequency of the light wave matches the frequency (or an integral multiple of the frequency) at which the particles of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies (or portions of the spectrum) of infrared (IR) light. Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural resonant frequencies of vibration of the objects. When IR light of these frequencies strike an object, the energy is either reflected or transmitted. 9

10 EQUIPMENT USED: COUPLERS: Figure 12 Figure 13 illustrates the design of a basic fiber optic coupler. A basic fiber optic coupler has N input ports and M output ports. N and M typically range from 1 to 64. The number of input ports and output ports vary depending on the intended application for the coupler. Types of fiber optic couplers include optical splitters, optical combiners, X couplers, star couplers, and tree couplers. Figure 13: Basic passive fiber optic coupler design An optical splitter is a passive device that splits the optical power carried by a single input fiber into two output fibers. Figure 14 illustrates the transfer of optical power in an optical splitter. The input optical power is normally split evenly between the two output fibers. This type of optical splitter is known as a Y-coupler. Figure 14: Optical splitter 10

11 An optical combiner is a passive device that combines the optical power carried by two input fibers into a single output fiber. Figure 15 illustrates the transfer of optical power in an optical combiner. Figure 15: Optical combiner Star and tree couplers: These are multiport couplers that have more than two input or two output ports. A star coupler is a passive device that distributes optical power from more than two input ports among several output ports. Figure 16 shows the multiple input and output ports of a star coupler. A tree coupler is a passive device that splits the optical power from one input fiber to more than two output fibers. A tree coupler may also be used to combine the optical power from more than two input fibers into a single output fiber. Figure 17 illustrates each type of tree coupler. Star and tree couplers distribute the input power uniformly among the output fibers. Figure 16: Star coupler. Figure 17: (1 X M) and (N X 1) tree coupler designs. 11

12 Fiber optic couplers should prevent the transfer of optical power from one input fiber to another input fiber. Directional couplers are fiber optic couplers that prevent this transfer of power between input fibers. Many fiber optic couplers are also symmetrical. A symmetrical coupler transmits the same amount of power through the coupler when the input and output fibers are reversed. Passive fiber optic coupler fabrication techniques can be complex and difficult to understand. Some fiber optic coupler fabrication involves beam splitting using microlenses or graded-refractive-index (GRIN) rods and beam splitters or optical mixers. These beamsplitter devices divide the optical beam into two or more separated beams. Fabrication of fiber optic couplers may also involve twisting, fusing, and tapering together two or more optical fibers. This type of fiber optic coupler is a fused biconical taper coupler. Fused biconical taper couplers use the radiative coupling of light from the input fiber to the output fibers in the tapered region to accomplish beam splitting. Figure 18 illustrates the fabrication process of a fused biconical taper coupler. Figure 18: Fabrication of a fused biconical taper coupler (star coupler). CLEAVER: It s very important to cleave the fiber after stripping off the cladding. Cleaving helps remove the rough edge of the fiber. By making the edge smooth, more light could be coupled in the fiber. 12

13 Figure 19: The Fitel S315 Single Fiber Field Cleaver is designed for cleaving fiber in the field quickly and easily. Figure 20: Sumitomo Fiber Optic Cleavers Can be used as either a mass or single fiber cleaver. Utilizes an automatic anvil drop for fewer required steps and better cleave consistency. Prevents double scoring of the fibers. Has superior blade height and rotational adjustment. Is available with automatic fiber scrap collection. Can be operated with one hand. 13

14 STRIPPER: It is used to get rid of the fiber coating (cladding). Connector: Figure 21:TO-S-N175-C- Ripley No-Nik Buffer Stripper Figure 22: FIBER OPTICS CONNECTOR Lock-in amplifier: A lock-in amplifier (also known as a phase-sensitive detector) is a type of amplifier that can extract a signal with a known carrier wave from extremely noisy environment (S/N ratio can be as low as -60 db or even less). It is essentially a homodyne with an extremely low pass filter (making it very narrow band). Lock-in amplifiers use mixing, through a frequency mixer, to convert the signal's phase and amplitude to a DC actually a time-varying low-frequency voltage signal. 14

15 Figure 23 Figure 24: SR7265 DSP Lock-In Amplifier 15

16 Conclusion: It was a great experience of successfully coupling laser light to SMF-28 fiber (same fiber used in WDM). Learned how to use all the equipment shown above. I wish to apply the same fundamentals in other commercial applications relating to Electrical Engineering and Optical Communication. Also, looking forward to attend the Laser Safety Training. Refrences:

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