Figure 6: Cross section of photonic band gap cladding PCF [3] . ..... honeycomb of air and silica, with holes that refuse entry to light of certain wavelengths. The.
OPTICAL COMMUNICATIONS Report on:
PHOTONIC CRYSTAL FIBER Submitted to:Prof. Dr.-Ing. G. Wenke
Submitted by: Puneeth Jubba Honnaiah
5007761
Shridhar Reddy
5007751
Srujana Patel
5007710
Suhitha Dandu
5007756
PHOTONIC CRYSTAL FIBER Contents 1
Introduction.............................................................................................................................. 3
2
From Conventional Fibers to PCF ........................................................................................... 3
3
Photonics crystal fibers and its advantages ............................................................................. 4
4
Types of photonic crystal fibers .............................................................................................. 5
5
Fabrication process .................................................................................................................. 7
6
Photonic crystal fibers in the market ..................................................................................... 10
7
Applications of PCF .............................................................................................................. 11
8
Future scope of PCFs ............................................................................................................. 14
9
Conclusion ............................................................................................................................. 16
10
References ............................................................................................................................. 17
List of Figures Figure 1: Dispersion characteristics of PCF and conventional fibers [2] ....................................... 4 Figure 2: Birefringence characteristics of PCF and conventional fibers [2]................................... 4 Figure 3: Bending losses of PCF and single mode fiber [2] ........................................................... 5 Figure 4: a) Cross section of triangular cladding PCF b) Schematic of an index guided PCF [3] 6 Figure 5: Schematic of a photonic band gap PCF [3] ..................................................................... 7 Figure 6: Cross section of photonic band gap cladding PCF [3] .................................................... 7 Figure 7: Schematic of the PCF fabrication process [5] ................................................................. 8 Figure 8: Cross-section of PCF at different stages. (a) PCF preform (b) PCF cane (c) PCF of 20μm diameter (d) Microstructure around PCF core [5] .............................................................. 10 Figure 9: Schematic diagram of a laser Guide Star [9] ................................................................. 12 Figure 10: Image of laser light irradiation from telescope [9] ...................................................... 12 Figure 11 Schematic diagram of cross-sectional view of all silica double cladding fiber [9] ...... 14
List of Tables Table 1: Specification of Photonic Crystal fiber ........................................................................... 12 Table 2: Specifications of high non-linear PCF ............................................................................ 13 Table 3: Specifications of double cladding fiber ......................................................................... 14
ONT REPORT
Page 2
PHOTONIC CRYSTAL FIBER 1 Introduction Optical fiber is a solid thread comprising a core and a cladding of constant refractive index difference. Light is guided in the core due to the phenomenon of total internal reflection and also due to the high refraction property of light, which occurs as a result of the difference between the refractive indexes of the core and cladding [1]. This refracted light bears much higher loss during propagation over extended distances, and thus requires repeaters and amplifiers for extended distance communications. Photonic crystal fibers (PCFs) are established as an alternative fiber technology. PCF is a kind of optical fiber that uses photonic crystals to form the cladding around the core, that is, with a periodic arrangement of low-index material in a background with higher refractive index. Photonic crystal is a low-loss periodic dielectric medium constructed using a periodic array of microscopic air holes that run along the entire fiber length [1]. Light is trapped in the core, providing a much better wave guide to photons than standard fiber optics. PCFs are more similar to conventional optical fibers as light is confined in a solid core by exploiting the modified total internal reflection mechanism. In fact, there is a positive refractive index difference between the core region and the photonic crystal cladding, where the air-hole presence causes a lower average refractive index. As a result, PCFs display unique optical properties.
2 From Conventional Fibers to PCF Optical fibers transmit information in the form of short optical pulses over long distances at exceptionally high speeds. This technology has been developed at an incredible rate to being key components of the sophisticated global telecommunication network. However Optical fibers are not confined only to telecommunication networks. Modern optical fibers represent a careful trade-off between optical losses, optical nonlinearity, group velocity dispersion, and polarization effects. To overcome these effects PCFs came into existence. The interest of researchers has been attracted by the ability to structure materials on the scale of the optical wavelength, a fraction of micrometers or less, in order to develop new optical medium, known as photonic crystals. Photonic crystals rely on a regular morphological microstructure, incorporated into the material, which radically alters its optical properties, hence providing great dispersion, polarization characteristics and extremely small bending losses. Photonic crystal fibers are optical fibers that contain an array of roughly wavelength sized holes running along the fiber axis which extended the possibilities of fiber optic technology. More than a decade after the inception of the concept, PCF is now a proven technology which is competing with conventional fibers in many applications.
ONT REPORT
Page 3
PHOTONIC CRYSTAL FIBER 3 Photonics crystal fibers and its advantages Due to the huge variety of air-hole arrangements, PCFs offer a wide possibility to control the refractive index contrast between the core and the photonic crystal cladding and, as a consequence, novel and unique optical properties. Since PCFs provide new or improved features, beyond what conventional optical fibers offer, it has a comparatively better dispersion, Polarization, Birefringence, Non-linearity and small bending losses. Dispersion: In PCFs, the dispersion can be controlled and tailored with unprecedented freedom. Due to this property the zero dispersion wavelength fibers could be formed in wide spectral range, that is, from visible to Infrared wavelength. In fact, due to the high refractive index difference and the flexibility of changing air-hole sizes and patterns, a much broader range of dispersion behaviours can be obtained with PCFs than with standard fibers.
Figure 1: Dispersion characteristics of PCF and conventional fibers [2]
Birefringence: Birefringent fibers, where the two orthogonally polarized modes carried in a fiber propagate at different rates, are used to maintain polarization states in optical devices and subsystems. The modes become birefringent if the core microstructure is deliberately made twofold symmetric. By slightly changing the air-hole geometry, it is possible to produce levels of birefringence that exceed the performance of conventional birefringent fiber by an order of magnitude.
Figure 2: Birefringence characteristics of PCF and conventional fibers [2]
ONT REPORT
Page 4
PHOTONIC CRYSTAL FIBER Non-Linearity: An attractive property of solid-core PCFs is that effective index contrasts much higher than in conventional optical fibers can be obtained by making large air-holes, or by reducing the core dimension, so that the light is forced into the silica core. In this way a strong confinement of the guided-mode can be reached, thus leading to enhanced nonlinear effects, due to the high field intensity in the core. Bending losses: Conventional fibers suffer additional loss if bent more tightly than a certain critical radius. For wavelengths longer than a certain value, that is the long-wavelength bend loss edge, all guidance is effectively lost. PCFs with larger relative air-hole diameters are less sensitive to bending loss.
Figure 3: Bending losses of PCF and single mode fiber [2]
4 Types of photonic crystal fibers PCFs can be divided into two main categories depending on the physical mechanism that provides light guidance. One type of PCF operates by the Modified-Total-Internal- Reflection (M-TIR) principle and has a solid core surrounded by a periodic lattice. The other type of PCF operates based on the Photonic BandGap (PBG) effect. PBG PCFs possess a periodic cladding in which only some frequencies are allowed to propagate. 1. PCF operates by the Modified-Total-Internal- Reflection(Index guiding PFC): Similar to conventional fibers, index guiding PCFs transport light through a solid core by total internal reflection. The micro structured air-filled region in PCFs effectively lowers the index of the cladding – effectively creating a step-index optical fiber. This principle is called as the Modified-Total-Internal- Reflection (M-TIR). The fiber behaves in many ways like standard step-index fibers, but it has a number of advantages. Index guiding PCFs are made of undoped silica that provides very low losses, sustains high powers and temperature levels, and may withstand nuclear radiation. A typical cross section of an index guided PCF is shown in the figure 4. The PCF consists of a triangular lattice of air holes where the core is defined by a “missing” air hole. The pitch is labeled Λ, and measures the period of the hole structure (the distance between the centers of neighboring air holes). The hole size is labeled d, and measures the diameter of the holes. ONT REPORT
Page 5
PHOTONIC CRYSTAL FIBER
Figure 4: a) Cross section of triangular cladding PCF b) Schematic of an index guided PCF [3]
The biggest advantage of this PCF is by varying the size and location of the cladding holes and/or the core, the fiber transmission spectrum, mode shape, nonlinearity, dispersion, air filling fraction and birefringence can be tuned to reach values that are not achievable with conventional optical fibers. It is easy to incorporate more than one core into the photonic crystal cladding, allowing one to form arrays of coupled or independent waveguide [4]. In solid core PCFs, as in all TIR fibers, the vast majority of light propagates in the glass. The flexibility in design can be explored to achieve endlessly single mode at all wavelengths and large mode area at shorter wavelengths. 2. Photonic Band gap Fibers that have periodic micro structured elements and a core of low index material (e.g. Hollow core): These fibers have a new guiding mechanism where light is guided based on photonic band gap of the cladding region. It allows the guidance in a low index region i.e. hollow core such that the majority of the power is propagated through the hollow core. According to photonic band gap phenomena if the frequency of the external light matches the band-gap frequency, the light gets trapped in the hole and thus is guided throughout the length of the fiber. Therefore there is no need of having a greater refractive index of the core.
This kind of air core band gap fiber can have a high threshold power for non linearity effects and a high damage threshold. They have extreme dispersion values for pulse compression or dispersion compensation and Fresnel reflections are not induced at open fiber ends. The light is guided only in a narrow wavelength region of 100-200nm width and is helpful in pulse compression with high optical intensities due to higher percentage of power propagation in hollow core. It is possible to guide light at wavelengths where the transparency of the glass ONT REPORT
Page 6
PHOTONIC CRYSTAL FIBER material is relatively poor. Since major part of the mode is confined in air, the fibers are insensitive to radiation which makes them suitable for use in radiation hazardous environments.
Figure 5: Schematic of a photonic band gap PCF [3]
Figure 6: Cross section of photonic band gap cladding PCF [3]
5 Fabrication process The fabrication process of PCF preform is distinct from the conventional optical fiber preform due to air hole structure of the PCF. PCF are produced by number of different methods which generally involves stacking a number of capillary silica tubes with specific length and diameter and rods into a larger diameter glass tube or jacket tube as a preform and then drawing the PCF. Some examples of the fabrication process are stack-and-draw, extrusion, sol-gel casting, injection molding and drilling. Drilling process can be used to produce polymer and soft-glass micro structured fibers over a broad range of geometries and can also be applied to different optical materials. The major drawback is the duration of time it takes to produce multi-featured structure and reduction in perform yield with increased complexity and increased surface roughness. Extrusion process can be used for low softening points such as soft glasses and polymers. Since the holes are not restricted to hexagonal or circular structure, a diverse range of cladding ONT REPORT
Page 7
PHOTONIC CRYSTAL FIBER structures can be fabricated. It can also be reproduced thus allowing the production of complex fiber geometries in a single step. It also involves an additional process which leads to the formation of crystals (and hence increased fiber loss) for some glass types. The most common method used is stack and draw designed by J. C. Knight, which is relatively fast, clean, low-cost, and flexible. The process of stack and draw is illustrated in figure 7.
Figure 7: Schematic of the PCF fabrication process [5]
i.
Capillaries Fabrication
The first step is to fabricate capillaries from undoped high grade silica preform tubes which are drawn to outer diameter of 1-2 mm. It will be difficult to keep individual capillary in its respective lattice position if the capillary size is below 1 mm. On the other hand, larger diameter leads to lesser rings of air –holes which in turn affects the PCF guiding characteristics. The temperature of the furnace has to be maintained between silica softening and melting points around 2100° C. To control the size of capillaries, preform feeding rate and tractor drawing speed are set according to simplified mass conservation law ∙
=
∙
where Af, Ad are cross-sectional area of preform and capillary respectively, Vf is feeding speed while Vd is drawing speed [5]. ii.
Stacking
To avoid PCF scattering loss and reduction in mechanical strength, the surface has to be cleaned using cleaning agents and the inner wall by ultrasonic bath. To achieve high pressure inside the capillaries during fiber drawing for preventing holes from collapsing, one end of the capillaries are sealed using high temperature Butanol-fuel hand torch. Hexagonal metal jigs are used to ONT REPORT
Page 8
PHOTONIC CRYSTAL FIBER stack the capillaries into triangular lattice arrangement. The capillaries are cut into short lengths with all sealed ends in one direction. These are inserted into a suitable jacketing tube, and voids at six hexagonal sides are filled with packing rods of different sizes [5]. The center capillary is replaced with a rod of the same size as the other capillaries, creating the core of PCF. A metal stop is attached to the end of preform where capillaries end are open. iii.
Drawing of PCF
The PCF preform is connected to a glass handle using furnace of the fiber drawing tower which requires two preforms to be slightly touch each other at the furnace heating zone with lower preform held fixed at its position. To fuse two preforms, increase the furnace temperature gradually to silica melting point 2100° C and then bring it down below 1600° C. Subsequently, the PCF preform is annealed and heat polished in furnace, with high temperature (~1700ºC) and simultaneous fast repeating vertical movement to slightly fuse the jacketing tube with capillaries inside [5]. The Drawing process involves two stages, caning and fiber pulling. In the first stage, PCF preform is drawn to some 1- 2mm canes. Initial feeding rate and drawing speed are set according to mass conservation law. The temperature of furnace is set in the range of 1900ºC - 2000ºC because the temperatures above 2000ºC will result in collapse of air-holes whereas temperatures lower than 1900ºC may lead to random cracks. Interstitial holes are closed by applying vacuum inside the preform. Meanwhile, temperature of furnace is adjusted carefully in steps of 5ºC depending on the observed cross-sectional surface of samples. In the second stage, one end of PCF cane is sealed using hand torch and inserted into a suitable silica cladding tube. It is crucial to heat the PCF cane in the furnace at 1600ºC for a few quick passes, aiming to create homogenous temperature environment in all air-holes, so that the fabricated PCF has uniform microstructure along the fiber [5]. Drawing of cane to fiber is similar to the first stage, with judicious control of drawing speed, temperature and pressure. Vacuum is applied inside the cladding tube so as atmospheric pressure will force the gap between PCF cane and cladding tube to close in furnace. Temperature of furnace is increased steadily to minimize the interstitial holes or otherwise if the holes are intended. When the process is stable, polymer coating is applied to the fabricated PCF using dip-coating and UV curing equipment and subsequently coated PCF is wind into a large spool using Capstan winder. The cross-section of PCF from preform to fiber is shown in figure 8. The microstructure of fabricated PCF is observed using Field Emission Scanning Electron Microscope (FESEM).
ONT REPORT
Page 9
PHOTONIC CRYSTAL FIBER
Figure 8: Cross-section section of PCF at different stages. (a) PCF preform (b) PCF cane (c) PCF of 20μm diameter (d) Microstructure around PCF core [5]
6 Photonic crystal fibers in the market From prototypes to commercial products, PCFs have been in market from past few decades. Many multinational fibre manufacturing companies have started marketing PCFs and are quite successful. Companies such as THORLABS, NKT photonics and RP photonics offer PCFs with different commercial names. Their price varies from fourteen thousand to twenty thousand Euros per meter. The marketing strategy is application driven. More precisely the specific PCFs are available for desirable applications [6]. NKT Photonics offers PM-1550 1550-01 which is a Polarization-Maintaining (PM) photonic crystal fibers that incorporate a non-circular circular core in combination with the large refractive index step between air and glass; this creates strong form birefringence [8]. The result can be a shorter beat length that reduces the bend-induced induced coupling between polarization states compared with conventional PM fibers, and a much reduced thermal sensitivity of birefringence. The temperature coefficient of birefringence of these fibers is up to 30 times less than that of other leading stress-birefringent fibers. These fibers are sold based on the overall optical specifications and not the physical structure. Thorlab’s offers LMA-PM-5, LMA LMA-PM-10, LMA-PM-15, a selection of Endlessly Single Mode (ESM), Large-Mode-Area (LMA), PM Photonic Crystal Fibers (PCFs). A conventional single ONT REPORT
Page 10
PHOTONIC CRYSTAL FIBER mode fiber is actually multimode for wavelengths shorter than the second-mode cutoff wavelength, limiting the useful operating wavelength range in many applications. In contrast, Crystal Fibre's endlessly single mode PCFs are truly single mode at all wavelengths for which fused silica is transparent. In practice, the useful operating wavelength range is limited only by bend loss. Although the cladding possesses six-fold symmetry, the mode profile is very similar to the quasi-Gaussian fundamental mode of a conventional, axially symmetric, step-index fiber, resulting in a form overlap that is >90%. Unlike conventional fibers, these fibers are fabricated from a single material: undoped, high-purity, fused silica glass. The PM performance is achieved via stress rod applied birefringence. The combination of material and very large mode area enables high power levels to be transmitted through the fiber without material damage or the adverse effects caused by the fiber's nonlinear properties. These fibers are also sold based on the overall optical specifications and not the physical structure [8]. NKT Photonics' polarization-maintaining (PM) highly nonlinear photonic crystal fibers which are available as NL-PM-750, guide light in a small solid silica core, surrounded by a microstructure cladding formed by a periodic arrangement of air holes in the silica. The optical properties of the core closely resemble those of a slightly elliptical rod of glass suspended in air; this results in a strong confinement of the light, a large nonlinear coefficient, and a substantial splitting of the effective indices of the polarization modes. The zero-dispersion (ZD) wavelength has been chosen for use with Ti-Sapphire laser sources, but the dispersion is also anomalous at the fundamental Neodymium wavelength (1060 nm). These fibers are also sold based on the overall optical specifications and not the physical structure.
7 Applications of PCF 1. Laser Guide Star: Index-guiding PCFs are the same as ordinary optical fibers in that both are based on the principle of TIR to guide waves. One of the major differences between them is that the effective refractive index of the PCF claddings varies greatly with the wavelength. Because of this feature, PCFs perform single mode operation regardless of the length of the wavelength or the size of the core area (ESM-operation). ESM occurs when the effective refractive index of the crystalline structured claddings varies significantly with the wavelength [9]. For high accuracy astronomical observations, it is necessary to correct atmospheric turbulence. To measure the wavefront of an image, natural guide stars have been used as reference objects for adaptive optics corrections. A narrow-line laser emitting at a sodium resonance line wavelength is used to create a yellow artificial star in the
