Commercial OTDRs usually provide >90 dB dynamic range to allow the measurement of long fibers with high losses.Īnother practical issue is that one has to consider the Fresnel reflection at the fiber input terminal. Theoretically the dynamic range requirement is determined by the roundtrip fiber attenuation over the total length L, which is Δ P dB = 2 α dB L. In the example shown in Figure 4.3.5, there is 50 dB reduction in the received optical power level over 100 km single-mode fiber. Therefore tradeoffs have to be made between the resolution and the SNR.ĭynamic range of the optical receiver in an OTDR is also an important issue. For example, a pulse width of 100 ns in an OTDR will provide a spatial resolution of approximately 10 m in the fiber. In fact, the spatial resolution of an OTDR is proportional to the pulse width τ by R resolution = 0. However, wide pulse width corresponds to lower spatial resolution in the OTDR measurement. Both of these factors may contribute to increasing the receiver SNR. A wide optical pulse carries high optical energy for a fixed-peak power level, and at the same time the required receiver electrical bandwidth is also relatively low. Spatial resolution is another concern for an OTDR. However, the price paid for this sensitivity improvement are increased complexity as well as the requirements of a high-quality single-frequency laser diode and the frequency shifter for the local oscillator. Compared to the direct detection case discussed at the end of Section 4.3.2 in which the minimum detectable power level was –63 dBm, the OTDR with coherent detection has more than three orders of magnitude sensitivity improvement over its direct detection counterpart. Based on Equation 2.7.16, the required signal optical power is approximately –95 dBm to achieve SNR = 1. AM: amplitude modulator, AO: acousto-optic frequency modulator, PC: polarization controller, PD: photodetector, BPF: RF bandpass filter, ADC: analog-to-digital converter.Īs an example, assume that the photodiode responsivity is ℜ = 1 A / W and the receiver bandwidth is 1 MHz. Block diagram of an OTDR with coherent detection. Generally, in coherent detection the power of the local oscillator is strong enough such that the receiver SNR is mainly limited by the shot noise of the local oscillator.įigure 4.3.8. An RF bandpass filter with the central frequency at ω IF is used to select the intermediate frequency component. A polarization controller is used in the local oscillator branch to match the state of polarization and maximize the coherent detection efficiency. The backscattered optical signal from the fiber is combined with the frequency-shifted local oscillator through a fiber coupler and detected by a wideband photodiode. An acousto-optic (AO) modulator is used to shift the optical frequency of the local oscillator by ω IF, which determines the intermediate frequency (IF) of the coherent heterodyne detection. The other part is used as the local oscillator for coherent detection. One of them is used as the stimulating signal that is modulated into a pulse train by an external modulator and sent into the fiber under test. The input-referred rms noise of the TIA can be obtained as 〈 i amp 2〉 = 〈 v amp 2〉/ Z TIA 2.įigure 4.3.8 shows the block diagram of a coherent OTDR, where the output from a single-frequency laser diode is split into two parts. Due to the noise contribution of the amplifier, an output noise power〈 v amp 2〉 is measured when the input signal current is turned off. For example, assume a TIA is used as the preamplifier with the transimpedance gain Z TIA = v out/ i in, where i in is the input signal current and v out is the output signal voltage. This preamplifier noise is commonly expressed as an "input-referred rms noise" 〈 i amp 2〉 so that it can be used together with 〈 i th 2〉, 〈 i sh 2〉, and 〈 i dk 2〉. Noise generated by the preamplifier also has to be considered in the receiver SNR analysis. 4.2.4 and 4.2.5 to amplify the photocurrent signal and convert it into an electrical voltage. In an optical receiver, an electrical preamplifier has to be used immediately after the photodiode as shown in Figs. For an electric bandwidth B of the receiver, the mean-square noise current, or equivalently the dark current noise power is 〈 i dk 2〉 = σ dk 2 B.
![dark noise avalanche vs photodiode dark noise avalanche vs photodiode](https://image2.slideserve.com/4845533/reverse-bias-scan-l.jpg)
Where I D is the dark current of the photodiode which depends on the structure of the pn junction, the doping levels of the material, and the temperature of the photodiode.