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. 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.
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