OCT imaging with temporal dispersion induced intense and short coherence laser source

Highlights

• Depth resolution can be increased in OCT by introducing temporal dispersion in probed beam.

• Photonic bandgap available in cholesteric liquid crystal is used to introduce this temporal dispersion.

• The coherence length is observed to be reduced by 730 times in a multimode He–Ne laser.

Abstract

Lower coherence length and higher intensity are two indispensable requirements on the light source for high resolution and large penetration depth OCT imaging. While tremendous interest is being paid on engineering various laser sources to enlarge their bandwidth and hence lowering the coherence length, here we demonstrate another approach by employing strong temporal dispersion onto the existing laser source. Cholesteric liquid crystal (CLC) cells with suitable dispersive slope at the edge of 1-D organic photonic band gap have been designed to provide maximum reduction in coherence volume while maintaining the intensity higher than 50%. As an example, the coherence length of a multimode He–Ne laser is reduced by more than 730 times.

Introduction

Optical coherence tomography (OCT) is a rapidly developing imaging modality, which provides non-invasive cross-sectional images through weakly scattering, semitransparent biological and non-biological media with micrometer-scale resolution [1], [2], [3], [4]. Currently, there are three important criteria in typical OCT measurements: the maximum depth of imaging, the speed of measurement, and the speckle appearance in imaging. In general, under the laser safety limit, use of an intense laser source enables high penetration depth of imaging through a scattering or absorptive media and improves the signal to noise ratio (SNR) of the OCT imaging. But, one of the signature properties possessed in a conventional laser source is the larger coherence volume [5], [6], [7], [8], [9], which has disadvantages in promotion of speckles. The speckle is generated when an imaging sample imparts a range of random path length differences over a highly coherent optical source. Therefore, in order to circumvent the problem of the speckle appearance, the coherence volume of a laser source has to be reduced. Although reduction in spatial coherence of the laser effectively reduces the coherence volume, while we are talking about OCT, the temporal coherence length of the laser source has to be very low indeed, because, standard OCT images are synthesized from low (temporal-) coherence interferometry (LCI) and the signals are obtained by the so-called depth-or A-scan. The complex degree of the temporal-coherence defines longitudinal or depth point spread function, whose full width at half maximum (FWHM) defines the depth resolution in OCT [3]. For Gaussian light, the depth resolution is

$$∆z=\frac{c{\tau }_c}{2}=\frac{2\mathit{ln}2}{\pi }\frac{{{\lambda }_0}^2}{∆\lambda }=\frac{l_c}{2}\dots$$

where ${\tau }_c$ is the FWHM coherence time, $l_c$ is the FWHM coherence length, ${\lambda }_0$ is the center wavelength, and $∆\lambda$ is the spectral bandwidth. From Eq. (1), it is clear that the higher depth resolution in OCT demands larger bandwidth or lower temporal coherence of the light source.

There is an intensive research going on engineering the larger bandwidth as well as higher intensity for the laser source [10], [11], [12], [13]. First implementations of the OCT principle used superluminescent laser diodes (SLDs) at ${\lambda }_0=830\mathit{nm}.$ These diodes yield coherence lengths in the order of 10-µm range but, strive with the lower beam power in 10 mW range. Clivaz et al. [3], used the fluorescence light from a Ti-sapphire crystal pumped by an argon laser operating at the central wavelength of ${\lambda }_0=780\mathit{nm}$ and a beam power of 4.8 µW has been obtained with a depth resolution of 1.9 µm. Schmitt et al. [5] used two light emitting diodes at peak wavelengths of 1240 nm and 1300 nm to synthesize a source with a short coherence length. However, along with the progress in different synthetic engineering techniques towards the reduction of the coherent length, it could be an interesting approach to reduce it further by employing an additional highly temporal dispersive medium. In our previous work [14], we have shown the strong temporal dispersion of the photonic bandgap available in a CLC. Depending on the thickness (L) of the dispersive medium, the coherence length $\left(l_c\right)$ of the multimode He-Ne laser source has been shown (in our previous work) to be reduced from 22 cm to 5 cm, hence the effective coherence volume gets reduced and facilitates to have speckle free high throughput image of a biological sample [14]. In this communication, our prime objective is to illustrate another extension of our temporal dispersive medium to further reduce the value of $l_c$ of the same source to 300 µm and demonstrate a series of OCT images. We believe that this easily employable additional dispersive medium can reduce the $l_c$ value beyond the intrinsic value of $l_c$ of any source. This generic approach could be interesting for the OCT field.