Optical Reflection Measurements Lecture-3

Optical Reflection Measurements Total Return Loss Technique A total return-loss measurement results in a single number that represents the total fractional power that is reflected from a test device. In order to make an accurate total return loss measurement, all unwanted powers reflected back into the power meter should be measured and taken into account. When a test device contains multiple reflections, a total return-loss measurement can give inconsistent results.

Optical Reflection Measurements Basic Concepts for Spatially Resolved Reflectometry Two point spatial resolution refers to the minimum distance between two reflectors that can still be resolved by the measurement system. This value is approximately equal to the FWHM of a single reflection on a reflectometry trace. The term single-point spatial resolution refers to the accuracy for determining the location of a single reflector.

Basic Concepts for Spatially Resolved Reflectometry As spatial resolution is decreased, the effects of fiber dispersion must eventually be considered. This problem arises since the accompanying increase in spectral width causes spreading of the probe signal as it travel along the fiber. The effects of dispersion depend on the spectral content of the probe signal combined with the length and dispersion characteristics of the fiber. Optical Reflection Measurements

Basic Concepts for Spatially Resolved Reflectometry The measurement of Rayleigh backscatter (RBS) is a very powerful method for characterizing fiber optic links. Conventional OTDRs use RBS to determine the location of fiber breaks, the fiber attenuation coefficient, splice loss, and various other link characteristics. The success of using RBS in long- and midterm resolution applications has spurred a strong desire that high resolution reflectometry techniques be also capable of measuring RBS.

Optical Reflection Measurements Basic Concepts for Spatially Resolved Reflectometry Two difficulties arise when attempting to measure RBS with high spatial resolution. First, the strength of the RBS signal becomes very small as the spatial resolution is decreased. Second, the measured RBS amplitude becomes very noisy because of coherent interface effects. This noisiness is sometimes referred to as coherent speckle.

Basic Concepts for Spatially Resolved Reflectometry Optical reflectometry can be classified into direct-detection and coherent-detection. Coherent techniques more complex to implement but it has some advantages like larger dynamic ranges, increased signal sensitivity, and the possibility for dispersion cancellation. In case of direct detection, only the reflected signal is incident on the detector. Optical Reflection Measurements

Optical Low Coherence Reflectometry OLCR is based on a coherent detection scheme that is often referred to as white-light interferometry. It uses broad spectral width optical sources with short coherence length. Compared to other high resolution reflectometry techniques, OLCR currently offers substantial advantages in both theoretical performance and practical implementation.

Optical Reflection Measurements Optical Time-domain Reflectometry One advantages of time-domain techniques is that measurement can be performed over large distance range. After a pulse is injected into the test fiber, return signals need only be collected as a function of time. Due to poor sensitivity, high-resolution direct-detection OTDR techniques will be limited to applications involving high reflectivity signal.

Optical Reflection Measurements Optical Time-domain Reflectometry The poor sensitivity associated with the high speed photodiodes can be greatly improved by using a photon counting detection scheme. Photon counting can be performed using photo-multiplier tubes or gain quenched avalanche photodiodes (APD). Practical methods for photon counting at wavelengths longer than about 1 micrometer are difficult to achieve.