OBEL: The effect of absorption and dispersion in ultrahigh-resolution OCT

One of the most exciting OCT developments in recent years has been the deployment of broadband pulsed Ti:Al₂O₃ lasers and other sources, allowing modern OCT devices to achieve axial resolutions of about 1μm. The very mechanism by which these high resolutions are attained, the broad source bandwidth, has increased the susceptibility of the modality to the deleterious effects of frequency-dependent sample absorption and dispersion. At standard OCT resolutions, it is usually sufficient to compensate for second-order (group-velocity) dispersion, which can be achieved by inserting a dispersion-compensating element into the reference arm, or employing a novel technique such as utilising grating tilt in a rapid scanning frequency-domain optical delay line. When broad bandwidths are used, higher orders of dispersion have a significant effect on the signal, which cannot be compensated so easily. Our recent study¹ has demonstrated the effect of water (the major constituent of biological media) dispersion and absorption on distorting and broadening the axial point-spread function (and thereby degrading the resolution) of ultrahigh-resolution OCT systems. Both effects were found to be most significant for wavelengths above ~1μm. As an example, for an OCT source which gives rise to a 1-μm full-width-at-half-maximum (FWHM) resolution, and propagation through a 1-mm water cell, if up to third-order dispersion compensation is applied, then the optimal source centre wavelength is 0.8μm, and the effective resolution is 1.5μm. The incorporation of additional bandwidth would yield no effective resolution improvement, due to uncompensated high-order dispersion and long-wavelength absorption.

Some of the results of the study are illustrated in the following figures, extracted from the paper. In Fig. 1, the displayed envelope broadening factors (EBFs) are quantitative measures (based on the root-mean-squared width of the square of the curve) of the resolution degradation in each case. A number of observations can be made from the curves. It is clear that even dispersion correction up to the third order is insufficient to completely mitigate broadening effects. Additionally, when merely second-order dispersion correction is applied, the structure of the envelope is characteristic of the (now predominant) odd-order dispersion - a slowly-decaying tail with prominent fringes. Finally, the ability of absorption to actually mitigate the effects of dispersion is clearly evidenced in the right columns of the figure.

Figure 1. Plots of interferograms due to the effects of absorption and disperion. The source resolution (SR) is 1μm. The source centre wavelengths are indicated for each column, and the single-pass propagation distance in water is 1mm. The top row shows plots of the undistorted source power spectral density (red) and the absorption-distorted power spectral density (black). The centre row shows the normalised interferogram (point-spread function) envelopes under different dispersion compensation conditions (no compensation, only 2nd-order compensation, both 2nd and 3rd order compensation, and full compensation, as given in the legend). The bottom row is similar to the centre row, except that absorption effects are ignored.

Fig. 2 displays plots of the effective resolution (ER, the product of source resolution and envelope broadening factor) versus centre wavelength for a range of different source resolutions and dispersion compensation conditions. The propagation distance is again 1mm. If full dispersion compensation is applied, 1μm ER is attainable with a 1μm source resolution (SR) only at centre wavelengths below ~0.9μm, and even increasing the bandwidth will not improve upon the ER at higher wavelengths. If merely second-order dispersion compensation is utilised, then the maximum attainable ER, for any SR, is 1.8μm, at a centre wavelength of 0.55μm. (The required SR is 1 μm). If an SR greater than 1μm is used (1.5μm or 2μm), then an ER of less than 2.5 μm is attainable up to centre wavelengths of ~0.85μm, but ultrahigh SRs of 1μm or less give poor results at these centre wavelengths. This situation is significantly improved if third-order dispersion compensation is incorporated into the system. Under this condition, an SR of 1μm gives consistent results below 0.9-μm centre wavelength, attaining a minimum ER of 1.5μm at centre wavelength 0.8μm. If no dispersion compensation is applied at all, the ER is minimized at around 1 -- 1.1 μm for all SRs. The minimum ER (3.6μm) occurs at a centre wavelength of 1.0μm, for a 2-μm SR. At all centre wavelengths, an SR of 2 μm or greater is required to attain the minimum ER at that wavelength, due to the additional uncompensated dispersion introduced with broader-bandwidth sources.

Figure 2. Plots of effective resolution (ER) vs. centre wavelength. The dispersion-compensation conditions for each plot are given, and the single-pass propagation distance is 1mm. Multiple curves indicate different source resolutions (SRs), with the legend in the lower-left panel applicable to all plots.

This analysis showed that if an axial resolution target approaching 1μm is to be attained in OCT imaging, then it is necessary to compensate for multiple dispersion orders, no matter how great the source bandwidth. Moreover, the source centre wavelength should not be located in the vicinity of a sample absorption peak, so that the entirety of the spectrum is utilised in generating the signal.