Raman Scattering Spectrum

2.2.1 Raman Scattering Spectrum

Raman spectroscopy is a spectroscopic technique used to observe vibrational, rotational and other low-frequency modes in a system [25]. It relies on inelastic scattering or Raman scattering of monochromatic light, usually from a laser in the visible, near infrared or near ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. Typically, a sample is illuminated with a laser beam.

Electromagnetic radiation from the illuminated spot is collected with a lens and sent through a monochromator. Elastic scattered radiation at the wavelength corresponding to the laser line (Rayleigh scattering) is filtered out, while the rest of the collected light is dispersed onto a detector by either a notch filter or a band pass filter.

Here, we measure the crystalline state of the Si-cored fiber by using Raman spectroscopy.

Fig. 2-9(a) shows the Raman spectra of the single-crystal Si, Si-cored fibers in core diameter of 100 μm with or without silica cladding were measured using a commercial micro-Raman microscope (Protrustech, BWII) equipped with a monochromator having a focal length of 75 cm. The excitation laser in 532 nm wavelength was focused on the Si core through the transparent silica cladding when the Raman spectrum were taken. It’s worth noting that the Raman shift happened after the silica cladding was removed in Fig.

2-9(a). The Raman peaks of the reference single-crystal Si wafer and the Si-cored fiber are located at 520.85 cm^-1 and 519.5 cm^-1, respectively. It is apparent that the peak position (519.5 cm^-1) of the transverse optical (TO) mode of the Si-cored fiber is close to

that of single-crystal Si (520.85 cm^-1). After the silica cladding was removed to unveil the Si core, the TO mode shifted to the wavenumber of 520.85 cm^-1, which is coincident with that of the reference crystal Si. It has been reported that the Raman peak of single-crystal Si when subjected tensile stress would shift to shorter wavenumbers [26].

Therefore the amount of wavenumber short-shift, about 1.4 cm⁻¹, in the Si-core fiber indicates that the Si core is under a tensile stress. As shown in Fig. 2-9(b), the tensile stress in the Si core may originate from the difference of coefficient of thermal expansion (CTE) between silica and Si [26]. The CTEs of Si and fused silica are about 2.5 and 0.5 (10^-6/K), respectively [27, 28]. During the drawing process, the melting Si firstly conformed to the shape of the tube at 1650 ^oC, and then bonded to the fused silica tube when it began to solidify [29, 30]. Owing to the CTE difference, the Si being solidified would shrink more than that of the fused silica tube during cooling. Since the Si strongly adhered to its surrounding fused silica tube, the tube would exert a tensile stress to prevent the solidifying Si core from shrinking, therefore causing the Raman peak shifted to 519.5 cm^-1. After the silica cladding was removed from the Si-cored fiber, the tensile stress in the Si core was then released and the Raman peak returned to the one identical to the single-crystal Si wafer’s (520.85 cm^-1). In order to investigate the uniformity of crystalline state of Si-cored fiber, a series of Raman spectrum were measured over 40 cm long at an interval of 2 cm, the results are shown in Fig. 2-10. The red curve measured from a single-crystal Si wafer is used as reference and the black curves are the Raman spectrum taken every 2 cm intervals, showing the same peak position (519 cm^-1) and almost the same bandwidth. This result indicates that the crystallinity of the Si core was uniform throughout the fiber.

(a)

(b)

Fig. 2-9 (a) Micro-Raman spectra of reference single-crystal Si, Si-cored fibers with and without silica cladding. (b) A schematic illustration shows the tensile stress being exerted on a solidifying Si core caused by the CTE differences between silica and Si core, and the release of such stress when the silica cladding is being removed.

Fig. 2-10 The Raman spectra measured at an interval of 2 cm over its 40 cm length where the reference spectrum in red is of single-crystal Si wafer.

2.2.2 High-Resolution Transmission Electron Microscope Image

High-resolution transmission electron microscopy (HRTEM) is an imaging mode of the transmission electron microscope (TEM) that allows for direct imaging of the atomic structure of the sample. HRTEM is a powerful tool to study properties of materials on the atomic scale, such as semiconductors, metals, nanoparticles and sp²-bonded carbon.

A Si-cored fiber was sectioned by using a focused ion beam and its cross-section was examined by using HR-TEM (JEOL) operated at 200 keV. As shown in Fig. 2-11(a), uniform and periodical crystal lattices are clearly seen. No obvious defects such as dislocations, vacancies or grain boundaries were found, indicating that the defect free Si core was indeed of single-crystal phase. The calculated lattice plane spacing is 0.192 nm.

The diffraction pattern from the selected sample area showed a 6-fold symmetry pattern, which is typically observed in the [110] Si (Fig. 2-11(b)). The results from the HR-TEM characterization are consistent with those of the Raman spectroscopy, both indicating that the Si core forms a single-crystal structure after optimizing the fabrication parameters.

(a)

(b)

Fig. 2-11 (a) HR-TEM image of the cross-sectional area of a single-crystal Si-cored fiber.

(b) Diffraction pattern of [110] Si core.

2.3 Optical Characteristics of Silicon-Cored Fibers

In addition to fabrication process and material analysis, the most important property is optical transmission loss owing to its fiber structure. Here, we measured the transmission loss of the Si-cored fibers by using the standard cut-back method. In telecommunications, a cut-back method is a destructive technique for determining certain optical fiber transmission characteristics, such as attenuation and bandwidth. The measurement technique consists of performing the desired measurements on a long length of the fiber under test, cutting the fiber under test at a point near the launching end, repeating the measurements on the short length of fiber, and subtracting the results obtained on the short length to determine the results for the residual long length. The transmission loss can be quantitatively expressed as a total attenuation α between two arbitrary points X and Y on the same fiber is

α(dB) = 10 ∗ log₁₀(𝑃_𝑥 𝑃_𝑦)

Px is the power output at point X. Py is the power output at point Y. Point X is assumed to be closer to the optical source than point Y. The attenuation coefficient or attenuation rate α is given by α(dB/km) = A/L. Here L is the distance between points X and Y. The benefit of this technique is that it allows measurement of the fiber characteristics without introducing errors due to variation in the launch conditions. For example, the coupling efficiency of the light source is kept consistent between the initial and the cutback measurements.

Notable some common contributors for transmission loss may be cracks, grain boundaries (poly-crystallinity), surface roughness, longitudinal perturbations, and impurity [32]. The cut-back method was adopted for the measurement in the wavelength

method, the factor of coupling loss between light source and fiber is neglected. Therefore, accuracies of the length and launching condition are the main factors affecting accuracy of the measurement [33]. Light was launched from a tunable laser (LUNA Phoenix 1400) through a lead-in single-mode fiber into a Si-cored fiber. The optical coupling was monitored by using a charge-coupled device to ensure that the light was properly delivered to the Si-cored fiber. The transmitted light was then collected by a power meter.

As shown in Fig. 2-13(a) and (b), the measured transmission losses with a Si-cored fiber (core/cladding diameter: 130/813 μm) is ~0.3 to 0.4 dB/cm and another Si-cored fiber with smaller diameter (core/cladding diameter: 22.5/209 μm) is ~0.6 to 0.9 dB/cm in the wavelength regime from 1520 to 1560 nm. The error bar is resulted from the flatness on the end face of fiber during the cut-back process and can be further improved. Besides, we listed and summarized other group fabricating Si-cored fiber with different method all over the world in Table. 1. It is worth noting that the measured transmission loss of Si-cored fiber in our work is the smallest loss after the comparison with other groups’ results.

Fig. 2-12 The experiment setup of transmission loss measurement. SMF is the commercial single-mode fiber. Two cameras were used to real-time monitor the coupling condition in side, top view between a SMF and a Si-cored fiber. A photodetector was used to collect the transmitted light.

(a)

(b)

Fig. 2-13 (a) The measured transmission losses of a Si-cored fiber (core/cladding diameter:

130/813 μm) and (b) is another Si-cored fiber with smaller size (core/cladding diameter:

Source

Table. 1 Comparison between the reported Si-cored fibers and this work in terms of source materials, fabrication methods, crystallinity of fiber cores, transmission losses and core diameters.

2.4 Summary

In this chapter, cored fibers are introduced. We demonstrate that single-crystal Si-cored fibers can be obtained by using a rapidly vertical drawing method. The use of low-cost polycrystalline Si powders as the preform material is an economic way of fabricating high quality meter-long single-crystal Si-cored fiber. By optimizing the processing temperature at around 1650 ^oC, the soften silica cladding could still provide substantial confinement on the molten Si core during the drawing process. At the drawing speed of ca. 10 m/min, the rapid and dramatic reduction in the cross-sectional area generates a strong stress and spatial confinement on the Si core, making single crystalline possible.

The crystallinity transformation from polycrystalline powders into single-crystal phase has been investigated and confirmed by using both the micro-Raman spectroscopy and HR-TEM. According to the HR-TEM images and micro-Raman spectra, the Si core possesses outstanding properties, including single crystallinity, high purity and high uniformity throughout the meters long fiber. The transmission losses is approximately 0.3 to 0.9 dB/cm of the Si-cored fibers in the optical communication regime are much lower than those reported previously.

Chapter 3 Fabrication and Optical Characteristics of Silicon-Cored Tapered Fibers

3.1 Introduction to Silicon-Cored Tapered Fibers

With the size of modern electronics and photonics devices shrinking, the importance of micro- and nano-scale waveguides is gradually enhanced. Micro- and nano-scale waveguides have played an integral role in the recent success of Si photonics forming the basis of a number of compact optoelectronic devices [36]. In addition to possibility of integration, these micro- and nano-scale waveguides can also directly be used as a spot-size converter to connect with other Si photonics devices [37]. Due to the relationship between the optical nonlinearity and cross-section area of waveguide, the waveguide with smaller dimension can extremely enhance the nonlinearities, such as Kerr effect [38], especially for these materials with high refractive index, like Si, chalcogenide glass and so on. The number of guided modes is also mainly dependent on the size of waveguide, it is an effective method for decreasing the numbers of guided modes by shrinking the size of waveguide. Moreover, the simulated calculations in the number of guided modes with different dimension of Si-cored fiber are shown as follows. Some researches regarding to Si tapered waveguide have been reported. A three-dimensional Si taper have been fabricated by using a glass mask with ultraviolet lithography, dry etching on a silicon-on-insulator substrate, whose process details are shown in Fig. 3-1 [39]. Another report on tapered Si optical fiber have been made in 2008, in which a Si-cored fiber was tapered by using a fusion splicer. Smaller starting core diameters around 5.6μm, 2.7μm and 1.3μm were demonstrated. Due to the short moving range of the fusion splicer, so the

diameter of 1.3μm, and the cross-section view of microscope image is shown in Fig. 3-2.

However, it is a little regretful that no measurement of transmission loss is shown in this paper [36].

In order to extend the potential applications of Si-cored fibers, we try to reduce the dimension of the Si-cored fiber by using a similar drawing method presented previously in our group for fabricating the silica microfiber [40].

Fig. 3-1 (Top)The schematic of the fabrication process for 3-dimensional Si taper structure. (Bottom)The schematic of the propagation direction of input and output light [39].

Fig. 3-2 Microscope images of the longitudinal taper profiles for starting fiber core diameters of (a) 5.6μm, (b) 2.7μm, and (c) 1.3μm. The 70μm scale bar is applicable for all images [36].

3.2 Fabrication of Silicon-Cored Tapered Fibers

3.2.1 Setup of Miniaturized Fiber Drawing Tower

Fiber drawing tower is the equipment which makes the optical fiber [41-43]. The whole system includes a furnace, a drawing mechanism, a coating and curing process, a diameter measuring and feedback control, and finally an optical fiber take-up machine.

All these subsystems have to be integrated with parameters settled carefully and then the optical fiber can be manufactured in ultra-long length without broken. The temperature of the furnace, for instance, should be controlled in order to preserve a stable neck-down shape [44]. The gas flow was purged into the furnace to minimize the defect, which can also adjust the temperature profile and the viscosity of glass [45]. After leaving the furnace, the fiber was cooled before coating to prevent meniscus effect and formation of bubble. The coating machine should be pressurized to fit the requirement of high speed drawing [46]. The diameter and tension of optical fiber can be measured in situ and feedback to control the stability of fiber drawing [47, 48].

A microfiber have be made by drawing an optical fiber preform down to a microfiber in a diameter of few micrometers in our previous work. In the past work for making silica microfiber, the preform used in the experiment was a single-mode optical fiber in core/cladding diameter of 8.2μm/125μm [49]. The fiber preform with both ends being fastened to a fiber feeder and a drawing wheel respectively passed through a hot zone where the silica material became viscous liquid. Both the feeder and the wheel moved in the same direction at different speeds but with the desired constant ratio. By controlling the feed velocity (V_f) into the hot zone to be much slower than the drawing velocity (V_d),

therefore, the resultant diameter reduction D D_{f d} can be related to the speed ratio

d f

V V as

V_f D²_f  V_d D_d² (2.1)

where D_f and D_d are the diameters of fiber preform and MNOW, respectively.

Here we propose a modified miniature fiber drawing tower (MMFDT).The MMFDT is a small version of a traditional fiber drawing tower. The MMFDT comprises only a heat source, drawing mechanisms and several sets of regulators. The other subsystems such as feeding mechanisms and diameter measurement are neglected. Fig. 3-3 shows the experimental setup of MMFDT. The whole system was below 50 cm in height. We adapted the hydrogen oxygen flame to produce a Si-cored tapered fiber whose diameter smoothly varied from a few hundreds of μm to several μm. The highest temperature of hydrogen oxygen flame could reach up to 3000 ℃, which was sufficient to soften the silica cladding and further beyond the melting temperature of Si (1414 ℃) to make Si liquefied. However, an accompanying problem was caused by the fact that the fiber preform was heated from the lateral side and the tapered fiber would be readily bent if the tensile strength was not balanced at both ends. Hence we need to control the hydrogen oxygen flame, drawing stage simultaneously to prevent the flame contacting the microfibers after the drawing stops.

Some parameters have to be taken care to further improve the yield of tapered fiber drawing including the distance between the flame and the fiber preform, the speed of drawing, and the gas flow rate of hydrogen oxygen flame. The hydrogen oxygen flame could be divided into light blue inner cone and colorless outer cone. The temperature of

1400 ℃, which was insufficient to liquefy Si core inside cladding. Therefore, we adopted the light blue inner cone of hydrogen oxygen flame to drawing Si-cored tapered fiber. In addition, the control of gas flow rate of hydrogen oxygen flame was extremely critical.

We controlled the gas flow rate by two sets of regulators. Fig. 3-4 shows the photograph of two sets of regulators whose flow rates were 2500 c.c / min and 100 c.c / min, respectively. The right one with flow rate 100 c.c / min was used for fiber drawing, and the left one, 2500c.c/min, was used for gas releasing. During the drawing process, we set the gas flow rate at the level of 70 c.c / min. At this level, the shape of flame was similar to a sphere, which made the flame stable enough for smooth fiber drawing.

The drawing speeds not only decided the diameter of microfibers but also determined the success of drawing conditions. If too slow for the drawing speed, the tapered fiber would be bent in the waist section or the not continuous Si core happened. Such bending in the waist section could induce a large optical loss for transmission. Besides, if the heat of flame was not high enough to soften the silica cladding instantly, such a high drag force would break the fiber preform.

Fig. 3–4 Two regulators used to control the gas flow rates of hydrogen oxygen flame.

3.2.2 Fabrication Process of Silicon-Cored Tapered Fibers

To demonstrate the potential of Si-cored tapered fiber, we taper the Si-cored fiber mentioned before by using a hydrogen oxygen flame as heat source and the modified minimized fiber drawing tower. We choose Si-cored fibers in the diameter of cladding/core ranging from 300/30 μm to 100/10 μm for tapering. Before the tapering, we have to cleave the both end-facets of Si-cored fiber by using a fiber cleaver for silica single mode fiber in order to couple light into Si-cored tapered fiber. After a Si-cored fiber was fixed on the stage, check the gas flow be stable, make sure the flame shape like a sphere, then tapering can be started.

A Si-cored fiber fixed on the transition stage was moved into the center flame region, after a few seconds for heating, the Si core region would emitted extremely strong white light indicating the quite high temperature and the solid Si have transformed into the liquefy Si. The strong emission picture is shown in Fig. 3-5. When the Si-cored fiber emitted strong white light, the transition stage was moving downward for tapering the Si-cored fiber into tapered fiber. We had to instantly move the transition stage when the white light from Si-core emitted. Otherwise, the Si-cored fiber would be twisted due to the liquefy Si core could not provide a force to support the original shape. A Si-cored fiber in an original core/cladding diameter of 20/115 μm was chosen for making Si-cored tapered fiber. The 20 μm core diameter was tapered to 2.6 μm, and 115 μm cladding diameter was tapered to 13 μm in the waist section as shown in Fig. 3-6(a). Fig. 3-6(b) shows a series of microscope images of a Si-cored tapered fiber from one end to the other end. The total taper-to-taper distance is ~ 2 cm, which is determined by the moving distance of the transition stage. Such tapering length is far beyond the previously reported

untapered fiber to the taper waist. In addition to tapering with this smaller core dimension (core diameter ~ 20 μm), we also tried to fabricate Si-cored tapered fiber with larger

untapered fiber to the taper waist. In addition to tapering with this smaller core dimension (core diameter ~ 20 μm), we also tried to fabricate Si-cored tapered fiber with larger