Phase-noise and shot-noise limited operations of low coherence optical time domain reflectometry

Abstract

This letter shows theoretically and experimentally that Fresnel end reflection of the waveguide under test degrades the sensitivity of low coherence optical time domain reflectometry (OTDR). Optical mixing of end reflection and reference light in the OTDR produces the phase noise in proportion to end reflectivity. With the balanced detection technique, the excess photon noise is subdued and the phase noise becomes the dominant source of sensitivity degradation. At 3.2% end reflection and 300 μA mean photocurrent, the noise floor due to the phase noise is −138 dB/Hz. By reducing the fiber end reflection with matching oil, a shot-noise limited sensitivity of −140 dB at a 3 Hz bandwidth has been demonstrated at submillimeter resolution.

Theoretical and practical considerations on detection performance of time domain, Fourier domain, and swept source optical coherence tomography

“…This is a classical noise in excess of photon shot noise and can be on the order of 19 dB. 25,26 An important source of photon excess noise is second-order correlations or photon bunching, often referred to as Brown-Twiss correlations. 27 Since the arrival times of bunched photons in a matched dual detector OCT system will occur almost simultaneously, this technique can be used to remove significant amounts of excess noise.…”

Signal-to-noise ratio in an OCT/confocal system and penetration depth in OCT

“…Taking into account that the reference signal is larger than all the other terms in (7), the mean square current fluctuation, > ∆ < 2 p I of the photodetector current can be approximated 4,9,10 as: in (8a) is the equivalent current noise receiver and includes both the thermal and amplifier noise. e is the electron charge, W the electrical bandwidth, Π represents the polarization degree of the light and ∆ν the effective optical line-width 3,4 . Considering that R at is the reflectivity at the air-tissue interface, R fer cumulates the reflectivity of the fiber end in the object arm and Rayleigh backscattering noise, σ is the efficiency of light re-injected into the system from the longitudinal scanner 9,11 and γ the coupler cross efficiency of the first directional coupler, the photocurrent due to the airtissue interface, background, the fiber end reflection and reference beam are respectively:…”

“…While the statistics of the optical source plays little role in the confocal channel, it influences considerably the S/N ratio in the OCT channel. Low coherence optical sources, like superluminiscent diodes (SLD), Xenon lamps or superfluorescent fiber sources used in OCT are thermal sources described by the Bose-Einstein photon-counting statistics 3,4 . By comparison, an ideal single mode laser is described by the Poisson statistics and gives only shot noise.…”

“…A couple of papers recently addressed some of the aspects connected to noise in confocal 1,2 and OCT systems [2][3][4][5][6][7][8][9][10] . In confocal systems, the strength of the signal is traded off for better depth resolution.…”

Simultaneous en-face imaging of two layers in the human retina by low-coherence reflectometry

“…A morecomplex configuration can be designed that uses an extra coupler in each interferometer and replaces the f lat mirrors with corner-cube mirrors. In this way a balanced photodetection technique 8 can be implemented, with the advantage that the first term in an equation similar to Eq. (2) for balanced detection prevails (shot-noise-limited regime), and consequently superior signal -noise ratios are obtained.…”