Performance optimisation of a holographic Fourier domain diffuse correlation spectroscopy instrument

Abstract

We have previously demonstrated a novel interferometric multispeckle Fourier domain diffuse correlation spectroscopy system that makes use of holographic camera-based detection, and which is capable of making in vivo pulsatile flow measurements. In this work, we report on a systematic characterisation of the signal-to-noise ratio performance of our system. This includes demonstration and elimination of laser mode hopping, and correction for the instrument’s modulation transfer function to ensure faithful reconstruction of measured intensity profiles. We also demonstrate a singular value decomposition approach to ensure that spatiotemporally correlated experimental noise sources do not limit optimal signal-to-noise ratio performance. Finally, we present a novel multispeckle denoising algorithm that allows our instrument to achieve a signal-to-noise ratio gain that is equal to the square root of the number of detected speckles, whilst detecting up to ∼1290 speckles in parallel. The signal-to-noise ratio gain of 36 that we report is a significant step toward mitigating the trade-off that exists between signal-to-noise ratio and imaging depth in diffuse correlation spectroscopy.

Validation of the Openwater wearable optical system: cerebral hemodynamic monitoring during a breath-hold maneuver

“…To address this challenge, other methods have been proposed to increase SNR of intensity or electric field autocorrelation function measurements 20 , 51 . These methods include the use of highly parallelized homodyne single photon detection, 52 – 56 heterodyne interferometric detection, 57 – 60 heterodyne holographic detection, 61 , 62 source light with wavelength beyond the water peak, 63 – 66 and high coherence pulsed sources 67 – 73 The Openwater system does not use high-SNR temporal autocorrelation function measurements to derive blood flow.…”

Comparing the performance potential of speckle contrast optical spectroscopy and diffuse correlation spectroscopy for cerebral blood flow monitoring using Monte Carlo simulations in realistic head geometries

“…For SCOS, the implementation model was based on 850 nm operation with a CMOS camera with pixel number, read noise, and quantum efficiency properties based on aggregated camera properties for commercially available cameras 35 . Although great success has recently been shown in the application of interferometric methods to both SCOS and DCS, 14 , 15 , 36 – 41 we restricted the analysis in this work to strictly homodyne methods. The photon flux at each simulated source–detector separation was estimated from the diffuse reflectance output from MMC and scaling the results based on estimates from previous studies 42 .…”

Interferometric diffuse optics: recent advances and future outlook

“… 44 This trend is likely to continue. For instance, the 2D CMOS camera used in Fourier domain DCS 13 , 47 only costs hundreds of dollars, which is dozens of times less expensive than the source. State-of-the-art iDWS 36 has used similar light sources as state-of-the-art DCS/DWS 44 to date.…”

“…Finally, although iDWS at 1064 nm can also reduce the laser source cost, 64 InGaAs sensors unfortunately are expensive. Though not strictly a limitation, we project that lowering the light source cost, perhaps in conjunction with noise mitigation strategies, 47 will be important to enabling new applications in the coming years.…”

“…Others have proposed signal processing strategies to deal with noise from mode hopping. 47 Despite the benefits of a large reference count, several works have operated in a regime in which the reference count is comparable to the sample count. 48 Whether this is a necessary consequence of using detectors with a low saturation level at short S-C separation, or whether there are actually advantages to working in this regime, remains unclear.…”