Polarisation sensitive OCT

Catherine Chin

Polarisation sensitive optical coherence tomography (PS-OCT) is a functional extension to intensity based light imaging system by utilising additional information carried by polarised light to provide enhance contrast. In traditional imaging, when only backscattered intensity is collected and analysed, information related to depth and layered structure are neglected.

In a typical microscopy system, researchers in the field worked to improve axial resolution and penetration depth, hoping to pick up sub-micron features not currently visible with most commercial OCT systems. Most research target at expanding the broadband capabilities of such systems by using super continuum light sources. At the same time, intense research has been going on to develop new techniques to extract additional information otherwise not discoverable with intensity based imaging.

PS-OCT on the other hand measures depth resolved phase retardation of incident optical light and the altering state of light reflected from the target sample as a result of birefringence. Birefringence is the optical property of a material having a refractive index that depends on the polarisation state and propagation direction of light. When a light enters a material of birefringence, the incident light can be decomposed into two orthogonal components with varying refractive indices, commonly known as parallel (co-) and perpendicular (cross-) optical axes, thus polarisation dependent phase velocity.

Birefringence occurs in many biological materials such as tissue structures, eyes and blood vessels. Several structures in the eye (e.g. cornea, retinal nerve fibre layer, retinal pigment epithelium) alter the polarization state of the light and show varying degree of birefringence. Therefore a tissue specific contrast not observable in conventional OCT system can be seen clearly in PS-OCT images.

PS-OCT imaging is of huge clinical importance. Applications of PS-OCT have seen interest in many new areas of clinical science, such as:

• Dental decay
• Retina imaging / ocular disease
• Collagen assessment
• Skin thermal damage diagnosis

The PS-OCT system implemented in Kent and NKT is simple. It’s a proof of working concept. Further enhancement such as high resolution imaging can be optimised. In this system, a broadband super continuum white light source (BSCWL) from NKT Photonics is used. The source emits a broadband spectrum between 450 nm and 2400 nm. An additional accessory such as spectrum splitter can be used to transmit a range of wavelength of interest into the PS-OCT system. For example, for skin imaging, wavelengths around 1300 nm are predominately used.

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