Coherent Optical Techniques
The temporal coherence properties of electromagnetic radiation can provide a powerful means to probe variations in optical pathlength. or to distinguish photons that have originated from different sources. These properties are exploited in metrology applications, where differences in optical pathlength can be detected via their impact on the interference pattern formed by mixing a signal wave and a reference wave. This essentially constitutes a homodyne or heterodyne measurement (depending on specific implementation) and coherent detection provides very high sensitivity detection in the presence of background “noise” from sources of incoherent radiation. Coherent detection has been utilised for optical communications and is widely exploited in biophotonics to measure weak reflected or backscattered light signals.
Measurements of the relative phase of a signal relative to a reference beam can enable the optical path length to be detrmined to a precision that can be much smaller than the wavelength of light. This interferometric measurement approach is used a wide range of optical measurements, e.g. to measure vibrations or to monitor fluctuations in the size of a cell to provide information about physiological processes. Interferometry can be extended to quantitative phase imaging1, where 2-D detectors record the interference pattern in each pixel. The resulting “phase maps” can provide wide-field surface profilometry, e.g. of optical substrates, and can be utilised in tomographic techniques to provide high resolution 3-D images of individual cells.
For imaging and metrology on a larger scale, low coherence interferometry can be used for depth resolved measurements where the detection of interference (i.e. measurement of fringe modulation depth) effectively provides the detected signal. In a manner analogous to SONAR ranging or depth-resolved imaging can be implemented by reflecting (or back-scattering) low coherence light off a sample in one arm of an interferometer and the returning radiation can be interfered with light from the reference arm. Fringes will only be visible when the two interferometer arms are matched to within the coherence length of the light being used. Low coherence interferometry can be implemented by raster scanning a single beam or using wide-field illumination and detection – in which case it is often described as holography. Low coherence interferometry is widely used in biophotonics for imaging and in this context an important technique is optical coherence tomography (OCT)2, which has become a standard ophthalmic technique for depth-resolved imaging in the eye. The coherent range-gated detection of reflected or singly back-scattered photons also provides a powerful means to acquire diffraction-limited ballistic light images in optically scattering media such as biological tissue3 and this is a very active field of research with many biomedical applications4. We have worked extensively on wide-field techniques utilising low coherence interferometry, including photorefractive holography5 and single-shot full-field OCT6. Today we are utilising OCT as a depth –ranging tool to stabilise image acquisition for handheld multiphoton microscope7 that we are developing for clinical applications.
The spatial coherence properties of EM radiation are also being exploited, e.g. in techniques designed to overcome the distortion of images in biological tissue through aberration and scattering by measuring the phase profiles of EM waves emerging from tissue and correcting these using adaptive optics techniques. “Wavefront sensing” and adaptive optics have been a major research topic in the Photonics Group, initially for applications in astronomy to counter the effects of air turbulence in the atmosphere and more recently for application to retinal imaging and microscopy.
The programmable manipulation of wavefronts is the basis of adaptive optics and is also increasingly being used for a range of techniques sometimes described as compressive sensing, whereby a complex illumination pattern enables a detector to acquire images with more information that would be possible in the case of uniform illumination. One instance of this would be the “single pixel camera”8, where a sequence of different illumination patterns would be applied and the light detected such that a series of single pixel measurements can provide an image. In more sophisticated approaches, a spatial light modulator or other adaptive optics device can be utilised to illuminate a sample in a turbid medium with a sequence of light fields of different intensity and phase patterns and the detected light signals can be used to recover an image that would be less distorted than that obtained by direct detection9.
2 Huang, D., Swanson, E. A., Lin, C. P., Schuman, J. S., Stinson, W. G., Chang, W., Hee, M.R., Flotte, T., Gregory, K., Puliafito, C.A., & Fujimoto, J. G.. (1991).. Science, 254 (1991), 1178; http://www.jstor.org/stable/2879328
3 C. Dunsby and Paul M. W. French, Invited Topical review, J. Phys. D: Appl. Phys. 36 No 14 (21 July 2003) R207-R227
4 Optical Coherence Tomography, Technology and Applications, Editors: W. Drexler & J. G. Fujimoto (2008, Springer Berlin Heidelberg, ISBN: 978-3-540-77550-8)
5 S. C. W. Hyde, R. Jones, N. P. Barry, J. C. Dainty and P. M. W. French, Opt Lett, 20 (1995) 2330
6 C. Dunsby, Y. Gu, P. M. W. French, Opt. Express, 11 (2003) 105-115
7 B. Sherlock, S. Warren, J. Stone, M. Neil, C. Paterson, J. Knight, P. French and C. Dunsby, Biomed. Opt. Exp. 6 (2015) 1876
8 M. F. Duarte, M. A. Davenport, D. Takhar, J. N. Laska, T. Sun, K. F. Kelly, and R. G. Baraniuk, IEEE Signal Process. Mag. 25 (2008), 83
9 I. Vellekoop & C. M. Aegerter, Opt. Lett. 35 (2010) 1245