# Optical OFDM Error Floor Estimation by Means of OTDR Enhanced by Front-End Optical Preamplifier
## 1 Introduction  
Considering test and measurement equipment used during installation and maintenance of contemporary lightwave transmission systems, optical time-domain reflectometer (OTDR) is still an outstanding fault detection and localization tool enabling simple and integral characterisation of a variety of refractive and reflective events on a fiber link, such as splices, connectors and breaks, with according measurements of insertion/return loss [1; 2; 3; 4], Figure 1.  
Basically, OTDR transmits a high-power laser light pulse into the fiber, and detects reflections from various structural irregularities along the fiber (backscatter), describing them in the power-loss-versus-distance format, i.e., expressing the time axis in distance units by inherently taking into account the speed of light in the fiber [5; 6; 7; 8]. The main functional OTDR entities are: microprocessor, pulse trigger and generator, laser diode, optical directional coupler (ODC), detector, analog-to-digital converter (ADC) and display, Figure 1.  
The test is initiated by the microprocessor executing the according instructions and sending the control signals to the pulse generator to trigger the laser. The outgoing series of laser light pulses transverses the ODC on their way into the fiber under test, whereas the reverse backscatter incoming signal is channeled by the ODC towards the detector--most often the avalanche photodiode (APD). Further on, the received signal proceeds through the ADC to the microprocessor for final analysis and display, which specifically includes statistical averaging to improve the signal-to-noise ratio (SNR), and thus enhance theOTDR trace by displaying average power of a number of sampling points, rather than the instantaneous values, Figure 2.  
Actually, the small-amplitude backscatter signal returning to the OTDR includes Rayleigh scattering and Fresnel reflections as well. The Rayleigh scattering enables calculating the fiber attenuation as a function of distance, which is represented by the constantly falling part of an OTDR trace, whereas Fresnel reflection causes a strong reflection back, and so enables detection of physical events along the fiber link identifiable by spikes in OTDR trace due to connectors, mechanical splices, fiber breaks or opened connectors, Figure 3.  
Figure 1: Optical time-domain reflectometer (OTDR) architecture and trace. Figure 2: Typical OTDR trace. Non-reflective events-fusion splices and bending, and are visible as discrete drops in backscatter level, as these cause loss, but no reflection, whereas mechanical splices, connectors and cracks cause both reflection and loss. The events' signatures are presented in Figure 3.  
Specifically, the OTDR dynamic range, defined as the difference (in dBs) between the incoming reflected backscatter signal level at the OTDR front end and the noise floor level at the far end of the fiber, also taking into account attenuation introduced by connectors, splices and splitters, determines the far-end events visibility and measurement accuracy, especially for a long-fiber. Consequently, the dynamic range should be about 5 to 8 dB larger than the maximal fiber loss measurable by the OTDR [1]. Moreover, considering the dynamic range from the bottom up perspective, the low limit of the OTDR dynamic range, i.e., the noise floor is commonly referred to as the signal value for which the SNR equals unity, observed with the longest pulse (i.e., the strongest) and the three-minutes long averaging [9]. On the other hand, in R&D, production, installation and advanced maintenance of fiber transmission systems, their end-to-end performance is mostly expressed by the bit-error-rate (BER), where specifically the residual BER (BER floor), is the ultimate irreducible lower-limit performance metrics for high-SNR conditions, when time dispersion (and consequent inter-symbol interference (ISI) remains the dominant error generating mechanism), and it is measured out-of-service, using the pseudo-random binary sequence (PRBS) generator and receiver.