Return Loss Measurement in the Presence of Variable Insertion Loss Using Optical Frequency Domain Reflectometry

The high spatial resolution and high sensitivity inherent to optical frequency domain reflectometery enables precise measurements of distributed insertion loss and return loss events. The ability to compensate return loss for variable insertion loss greatly adds to the accuracy and practicality of measurements. Further, the capability of measuring the Rayleigh backscatter internal to the instrument provides a stable power calibration artifact. Introduction Excessive system return loss (RL) negatively impacts source stability and contributes to loss in high-speed fiber optic telecommunication systems. In order to meet system performance goals, precise measurement of return loss in individual components as well as in installed networks is required, especially when temporary mechanical connections are used. Most current return loss measurements are made using optical continuous wave reflectometry (OCWR) and optical time domain reflectometry (OTDR) [1]. One of the primary error sources for RL measurements using these devices is the error induced by variable insertion loss at the connection to the test equipment optical interface. While this error source may be minimized by splicing the device under test (DUT) to the instrument, this procedure is not practical in high volume manufacturing environments or for installed networks. Both OTDR and optical frequency domain reflectometery (OFDR) are well suited for characterizing networks with some degree of spatial resolution. Both techniques typically have enough sensitivity to monitor the fiber Rayleigh backscatter level which can, in turn, be used to measure distributed loss and gain [2,3]. Typically OTDRs lack sufficient spatial resolution to be useful at the component and module level where one might be interested in, for example, locating a spurious reflection among a concatenation of several components each with multiple elements. OFDR is a tunable laser-based frequency domain technique that has several distinct advantages over time domain and low coherence techniques when the optical systems under test are several tens of meters in length [4,5]. These advantages include sub-millimeter resolution measurements over a few hundred meters of optical length, high sensitivity, and high dynamic range. The capability of measuring localized insertion loss using OFDR presents a unique opportunity to provide consistent measurements of device RL even in the presence of variable connector loss, even for short lead lengths. Further, the lack of a dead zone and high sensitivity allows our OFDR-based instrument to calibrate return power levels to the Rayleigh backscatter level of fiber within the instrument. This onboard calibration capability provides a highly stable and reproducible reference for RL measurements. This paper outlines the methodology used to establish a value for the scatter in optical fiber, and how this Rayleigh scatter level is used to maintain consistent reflection measurements. Measurement Apparatus The optical network used to implement OFDR is shown in Fig. 1. Light from a tunable laser source is split into measurement and reference optical paths. In the measurement path, the light is further split by a 50/50 coupler. A third coupler is used to recombine the light from the measurement path with the light from the reference path. After recombination, the light is split by a polarization beam splitter. Interference is detected at two PIN photodiodes that are connected via amplification circuitry to a data acquisition card. This polarization diverse detection scheme ensures that an interference signal will be present on at least one of the detectors irrespective of the polarization state of the field reflected from the device under test (DUT). Not shown in Fig. 1 is an auxiliary interferometer used to monitor phase error during laser tuning. This technique is called triggered acquisition and is commonly used in OFDR systems to remove laser tuning errors from the data [4]. Also not shown is a portion of the network wherein a Hydrogen Cyanide gas-cell is used to monitor the instantaneous wavelength of the scanning laser. The network shown in Fig. 1 is used to measure reflected power as a function of wavelength. The back-reflected power as a function of length is obtained via the Fourier transform of the raw data (see reference [6] for details). The maximum measurable length for this instrument is determined by the sampling resolution in the optical Fig. 1. Optical network used to perform polarization diverse measurements of Rayleigh backscatter. frequency domain which is in turn determined by the physical delay difference of the auxiliary interferometer used for data triggering. In this paper, the instrument used had a maximum scan range of 30 m with ~20 μm spatial resolution. To calibrate the measured back-reflection to an absolute RL, the response of a set of polished flat end face connectors was recorded. The expected value of the RL can be calculated using the Fresnel equation: