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METHOD FOR DEFOCUS CORRECTION

IN OPTICAL COHERENCE MICROSCOPY

Dmitry Lyakin1, Anton Sdobnov2, Vladimir Ryabukho2,1

1Institute of Precision Mechanics and Control, RAS, Russia

2Saratov State University, Russia

SARATOV FALL MEETING – SFM’2015International Symposium “Optics and Biophotonics – III”September 22 – 25, 2015Saratov, Russia

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CONTENT OF PRESENTATION

• Optical Coherence Microscopy

• Defocus Problem

• Known Methods for Defocus Correction

• Main Idea of Presented Method

• Experimental Setup

• How Does It Work

• Experimental Results

• Conclusions

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OPTICAL COHERENCE MICROSCOPY (OCM)

Single-point OCM

Full-field OCM

OCM = Optical Coherence Tomography (OCT) +

+ High-Numerical Aperture (High-NA) Optics

[1] J.A.Izatt, M.R.Hee, G.M.Owen, E.A.Swanson, J.G.Fujimoto, Optical coherence microscopy in scattering media // Optics Letters,1994, Vol.19, No.8, p.590-592.

[2] A. Dubois, L. Vabre, A.-C. Boccara, E. Beaurepaire, High-resolution full-field optical coherence tomography with a Linnik microscope // Applied Optics, 2002, Vol.41, No.4, P.805-812.

•Low temporal coherence (broadband) light•High spatial coherence (point) source•Point photodetector•En-face imaging by point-by-point object scanning

•Low temporal coherence (broadband) light•Low spatial coherence (extended) source•Area camera as photodetector•En-face imaging by single z-scan

Fig.1. Fiber single-point optical coherence microscope (from [1]).

Fig.2. Linnik-type full-field optical coherence microscope (from [2]).

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DEFOCUS PROBLEM

Optical Coherence Tomography (OCT) Signal

Confocal Microscopy (CM) Signal

groupOCT ndz

d – geometrical thickness; ngroup≈n(λ0) - λ0δn/δλ – group refractive index

n

ndz im

CMpar

n=n(λ0) – phase refractive index of object medium; nim=nim(λ0) – phase refractive index of immersion

Fig.3. Degradation of the signal of an Otical Coherence Microscope from the depth of the two layer object (microscope cover glass + air gap between glass and metal mirror) at increasing NA

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KNOWN METHODS FOR DEFOCUS CORRECTION

Extending Depth of Focus

Numerical Correction

J. Ojeda-Castaneda, L.R. Berriel-Valdos, Zone plate for arbitrarily high focal depth // Applied Optics, 1990, Vol.29, No.7, P.994-997. E.R. Dowski, Jr., W.T. Cathey, Extended depth of field through wave-front coding // Applied Optics, 1995, Vol.34, No.11, P.1859-1866.Z. H. Ding, H. W. Ren, Y. H. Zhao, J. S. Nelson, and Z. P. Chen, High-resolution optical coherence tomography over a large depth range with an axicon lens // Optics Letters. 2002, Vol.27, No.4, P.243–245. R.A. Leitgeb, M. Villiger, A.H. Bachmann, L. Steinmann, T. Lasser, Extended focus depth for Fourier domain optical coherence microscopy // Optics Letters, 2006, Vol.31, No.16, P.2450-2452.K. S. Lee and J. P. Rolland, Bessel beam spectral-domain high-resolution optical coherence tomography with micro-optic axicon providing extended focusing range // Optics Letters, 2008, Vol.33, No.15, P.1696–1698. A. Zlotnik, Y. Abraham, L. Liraz, I. Abdulhalim, Z. Zalevsky, Improved extended depth of focus full field spectral domain Optical Coherence Tomography // Optics Communications, 2010, Vol.283, P.4963-4968.

S. Labiau, G. David, S. Gigan, A.C. Boccara, Defocus test and defocus correction in full-field optical coherence tomography // Optics Letters, 2009, Vol.34, No.10, p.1576-1578.A.A. Grebenyuk, V.P. Ryabukho, Numerical correction of coherence gate in full-field swept-source interference microscopy // Optics Letters, 2012, Vol.37, P.2529-2531.

Mechanical Adjustment of Optical Elements

J.M. Schmitt, S.L. Lee, K.M. Yung, An optical coherence microscope with enhanced resolving power in thick tissue // Optics Communications, 1997, Vol.142, P.203–207. A. Dubois, G. Moneron, C. Boccara, Thermal-light full-field optical coherence tomography in the 1.2 μm wavelength region // Optics Communications, 2006, Vol.266, Iss.2, P.738-743.J. Binding, J. Ben Arous, J.-F. Léger, S. Gigan, C. Boccara, L. Bourdieu, Brain refractive index measured in vivo with high-NA defocus-corrected full-field OCT and consequences for two-photon microscopy // Optics Express, 2011, Vol.19, No.6, P.4833-4847.

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Fig.4. Adjustment of axial position of microscope objective in object arm of an optical coherence microscope to overlap coherence and focal planes (gates) (from[3])

[3] A. Dubois, G. Moneron, C. Boccara, Thermal-light full-field optical coherence tomography in the 1.2 μm wavelength region // Optics Communications, 2006, Vol.266, Iss.2, P.738-743.

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MAIN IDEA OF PRESENTED METHOD

USAGE OF ILLUMINATING INTERFEROMETER AS LIGHT SOURCE FOR AN OPTICAL COHERENCE MICROSCOPE

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EXPERIMENTAL SETUP

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HOW DOES IT WORK

Interference peak (2) from the rear plane of a single-layer object in the signal of an interference microscope arising at

CMOCTcomp zzz

and posses its maximal value when the shift Δz1 of the mirror M1 of illuminating interferometer from its zero path length position becomes equal to the value δzcomp

CMzz

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EXPERIMENTAL RESULTS

Fig.5. Experimental results obtaned at single-laer object measurementSLD: λ0=0.831 μm, Δλ=16 nmMicroscope objective lenses: LOMO® SHP-OPA-20-50, 20x, NA=0.50, ∞/0Object: Menzel-Gläser® microscope cover slip #1 (Shott ® D263 glass), d=145±1 μm, n(λ0)=1.517, nim(λ0)=1

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CONCLUSIONS

THANK YOU

FOR YOUR ATTENTION!!!

We have presented the method for correction of defocus in an optical coherence microscope caused by mismatch of the refractive index of a layered object and immersion and leading to degradation of the interference signal from the depth of the object. This method is based on the usage of an illuminating low-coherence interferometer as a light source for the optical coherence microscope. We have demonstrated experimentally that we are able to compensate this index-induced defocus creating corresponding optical path difference in arms of illuminating low-coherence interferometer. This method can be applied for defocus correction in commercially available interference microscopes which construction does not allow moving optical parts within microscope, for example such as Mirau microscopes. This method can also be used to simultaneous determination of geometrical thickness and refractive index of an object becouse the two quantities δzCM and δzcomp are obtained experimentally.