Optical waveguide monitoring

Details Optical waveguide monitoring Optical waveguide monitoring

2002-03-05

2004-11-09

Nguyen, Tu T. (Department: 2877)

Optics: measuring and testing

For optical fiber or waveguide inspection

Reexamination Certificate

active

06816243

ABSTRACT:

TECHNICAL FIELD
This invention relates to optical waveguides, and more particularly to monitoring optical waveguides.
BACKGROUND
Optical waveguides guide optical signals to propagate along a preferred path or paths. Accordingly, they can be used to carry optical signal information between different locations and thus they form the basis of optical telecommunication networks. The most prevalent type of optical waveguide is an optical fiber based on index guiding. Such fibers include a core region extending along a waveguide axis and a cladding region surrounding the core about the waveguide axis and having a refractive index less than that of the core region. Because of the index-contrast, optical rays propagating substantially along the waveguide axis in the higher-index core can undergo total internal reflection (TIR) from the core-cladding interface. As a result, the optical fiber guides one or more modes of electromagnetic (EM) radiation to propagate in the core along the waveguide axis. The number of such guided modes increases with core diameter. Notably, the index-guiding mechanism precludes the presence of any cladding modes lying below the lowest-frequency guided mode. Almost all index-guided optical fibers in use commercially are silica-based in which one or both of the core and cladding are doped with impurities to produce the index contrast and generate the core-cladding interface. For example, commonly used silica optical fibers have indices of about 1.45 and index contrasts of up to about 2-3% for wavelengths in the range of 1.5 microns.
Another type of waveguide fiber, one that is not based on TIR index-guiding, is a Bragg fiber, which includes multiple dielectric layers surrounding a core about a waveguide axis. The multiple layers form a cylindrical mirror that confines light to the core over a range of frequencies. The multiple layers form what is known as a photonic crystal, and the Bragg fiber is an example of a photonic crystal fiber.
An important characteristic of an optical waveguide is the transmission loss, or attenuation, of the waveguide. Transmission loss can be described as a logarithmic relationship between the optical output power and the optical input power in a waveguide system. It is a measure of the decay of signal strength, or loss of light power, that occurs as light pulses propagate through the length of a waveguide. Transmission loss can be caused by several intrinsic and extrinsic factors. In optical fibers, for example, intrinsic factors include scattering and absorption. Extrinsic causes of attenuation include cable-manufacturing stresses, environmental effects, and physical bends in the fiber.
In optical fibers the primary mechanism for transmission loss is Rayleigh scattering in the solid fiber core. Any structural and physical defect, such as voids in the core and/or cladding, or fiber eccentricity, significantly enhance light scattering and/or increase the fiber transmission loss. In addition to the increased transmission loss, structural flaws in the fiber can also lead to reduced mechanical strength and consequently a higher probability of fiber failure in the field. Therefore, it is important to detect the presence of defects in the fiber during the fiber manufacturing process. A number of methods have been developed to monitor the quality of the fiber in real time during fiber drawing.
One such method involves measuring the diameter of an optical fiber and detecting the presence of holes or voids in the fiber. The fiber is transversely illuminated with monochromatic light and an interference pattern is produced in the far field due to the superposition of light reflected from the fiber surface and light refracted through the fiber. The interference fringe pattern depends on the fiber core and cladding diameters and their respective refractive indexes. The number of interference fringes is related to the fiber diameter. However, the presence of a hole results in missing fringes. Thus changes in the interference pattern can be used to detect holes in the fiber.
Another method, utilizing the far field interference pattern created by illuminating an optical fiber with monochromatic light, determines a spatial frequency spectrum from the interference pattern using a Fast Fourier transform. The spatial frequency spectrum contains a frequency component corresponding to the fiber diameter. If there are voids or holes present in the fiber, the spatial frequency spectrum will contain additional components. Detecting these additional components and observing their behavior over time enables determining the extent of the fiber defects as well as their growth with time. In addition, the total power of the interference pattern is measured, which is affected by the size of a hole. Analysis of the total power in the interference pattern together with the intensity of the components in the spatial frequency spectrum compensates for fluctuations in the light source intensity and enables determining void size.
SUMMARY
The invention features techniques for monitoring the quality (e.g., optical and mechanical properties, including the presence of defects) in optical waveguides (e.g., photonic crystal fibers). The inventors have recognized that the spectral composition of light reflected from the side (e.g., light incident on the outside of the waveguide non-parallel to the waveguide axis, such as having an angle of incidence from about −85 degrees to +85 degrees) of certain optical waveguides (e.g., photonic crystal fibers) depends on the structure and composition of the waveguide. Hence, by monitoring the spectral composition of light reflected from the side of the waveguide and comparing the measured spectrum to a reference spectrum, one can evaluate the quality of the fiber.
In a first aspect, the invention features a method for monitoring the quality of a photonic crystal fiber. The method includes directing test light toward a side of a photonic crystal fiber and detecting measurement light emerging from the photonic crystal fiber in response to the test light. The method also includes monitoring the quality of the photonic crystal fiber based on the measurement light.
Embodiments may include one or more of the following. The emerging light can include reflected light. Monitoring the quality of the photonic crystal fiber can include determining a measurement spectrum of the measurement light. The measurement spectrum can be related to the bandgap of the photonic crystal fiber. Monitoring the quality of the photonic crystal fiber further can include determining an error signal that is based on a function of the measurement spectrum. The function of the measurement spectrum can also be a function of a reference spectrum (e.g., an empirically or theoretically determined reference spectrum). The function can be related to a difference (e.g., a weighted difference) between the measurement spectrum and the reference spectrum.
The method can further include drawing a photonic crystal fiber preform into the photonic crystal fiber while the measurement light is detected. Moreover, the method can include adjusting draw parameters based on the photonic crystal fiber quality.
The photonic crystal fiber can be a Bragg fiber. The photonic crystal fiber can be designed to guide light having a wavelength between 1.2 microns and 1.7 microns, or a wavelength between 0.7 microns and 1.0 microns.
The measurement light can be detected over a range of angles. The detection of measurement light can include collecting the measurement light with light collecting optics.
Monitoring the quality of the photonic crystal fiber can include detecting structural and/or compositional defects in the photonic crystal fiber. The detection of structural and/or compositional defects is based on a spectrum of the measurement light. Monitoring the quality of the photonic crystal fiber can also include detecting differences between a measurement spectrum based on the measurement light and a reference spectrum.
Directing the test light can include directing (e.g., simultaneo

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