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This work evaluates the performance of the Complex Master Slave (CMS) method, that processes the spectra at the interferometer output of a spectral domain interferometry device without involving Fourier transforms (FT) after data acquisition. Reliability and performance of CMS are compared side by side with the conventional method based on FT, phase calibration with dispersion compensation (PCDC). We demonstrate that both methods provide similar results in terms of resolution and sensitivity drop-off. The mathematical operations required to produce CMS results are highly parallelizable, allowing real-time, simultaneous delivery of data from several points of different optical path differences in the interferometer, not possible via PCDC.

Progress will be presented in adapting supercontinuum sources to a variety of applications with emphasis on signal processing procedures. These are customised to alleviate noise and take full advantage of the large bandwidth and large power spectral density of modern supercontinuum sources.

Supercontinuum (SC) light source is certainly one of the best option for ultra-high resolution optical coherence tomography (UHR-OCT). Over the last few years several demonstrations have been done for each commonly used wavelength range [1-2-3]. Nowadays, SC dedicated to UHR-OCT is a mature technology with turn-key commercially available system [4]. The new challenge to answer for SC source is the cost reduction one. In this study, we demonstrate that a Q-switched based SC (QS-SC) could be an alternative to the current state of the art SC based on a Mode-Locked laser (ML-SC). This QS-SC, whose cost is less than 15 % of the ML-SC, offers similar possibilities in terms of bandwidth, beam quality and optical density within the OCT band [5]. We demonstrate the usefulness of such a source by direct comparison with the ML-SC source commonly used. Our study includes a comparison of the pulse to pulse stability of both sources over the OCT wavelength range, where it is shown that the QS-SC is much more stable than the ML-SC. Also, a noise analysis conducted from the OCT point of view shows that the source repetition rate is a key parameter for any SC based OCT system. A comparison of images acquired from biological and non-biological samples is performed with emphasis on their contrast. Our conclusion is that a QS-SC can be used a useful source for UHR-OCT if compromise can be done in terms of speed of the detection unit. Finally, our study has been done at a central wavelength of 1270 nm, however the ultra-broad flat spectrum of the QS-SC makes it an interesting source for the 800 nm or visible range OCT too, opening the door for low-cost multi-band or multi-modal OCT. REFERENCES 1. K. Bizheva, B. Tan, B. MacLelan, O. Kralj, M. Hajialamdari, D. Hileeto, and L. Sorbara, “Sub-micrometer axial resolution OCT for in-vivo imaging of the cellular structure of healthy and keratoconic human corneas,” Biomed. Opt. Express 8, 800-812 (2017). 2. W. Yuan, J. Mavadia-Shukla, J. Xi, W. Liang, X. Yu, S. Yu, and X. Li, "Optimal operational conditions for supercontinuum-based ultrahigh-resolution endoscopic OCT imaging," Opt. Lett. 41, 250-253 (2016). 3. C. Cheung, J. Daniel, M. Tokurakawa, W. Clarkson, and H. Liang, "High resolution Fourier domain optical coherence tomography in the 2 μm wavelength range using a broadband supercontinuum source," Opt. Express 23, 1992-2001 (2015). 4. http://www.nktphotonics.com/lasers-fibers/product/superk-oct-broadband-sources-optical-coherence-tomography/ 5. http://www.nktphotonics.com/lasers-fibers/product/superk-compact-supercontinuum-lasers/

A detailed assessment of embryo development would assist biologists with selecting the most suitable embryos for transfer leading to higher pregnancy rates. Currently, only low resolution microscopy is employed to perform this assessment. Although this method delivers some information on the embryo surface morphology, no specific details are shown related to its inner structure. Using a Master-Slave Swept-Source Optical Coherence Tomography (SS-OCT), images of bovine embryos from day 7 after fertilization were collected from different depths. The dynamic changes inside the embryos were examined, in detail and in real-time from several depths. To prove our ability to characterize the morphology, a single embryo was imaged over 26 hours. The embryo was deprived of its life support environment, leading to its death. Over this period, clear morphological changes were observed.

Spectroscopic spectral domain interferometry and spectroscopic optical coherence tomography combine depth information with spectrally-resolved localised absorption data. These additional data can improve diagnostics by giving access to functional information of the investigated sample. One possible application is measuring oxygenation levels at the retina for earlier detection of several eye diseases. Here measurements with different hollow glass tube phantoms are shown to measure the impact of a superficial absorbing layer on the precision of reconstructed attenuation spectra of a deeper layer. Measurements show that a superficial absorber has no impact on the reconstructed absorption spectrum of the deeper absorber. Even when diluting the concentration of the deeper absorber so far that an incorrect absorption maximum is obtained, still no influence of the superficially placed absorber is identified.

Ultrahigh resolution optical coherence tomography (UHR-OCT) is enabled by using a broad band source. Simultaneously, this makes the OCT image more sensitive to dispersion mismatch in the interferometer. In spectral domain OCT, dispersion left uncompensated in the interferometer and detector non-linearities lead together to an unknown chirp of the detected interferogram. One method to compensate for the chirp is to perform a pixel-wavenumber calibration versus phase that requires numerical extraction of the phase. Typically a Hilbert transform algorithm is employed to extract the optical phase versus wavenumber for calibration and dispersion compensation. In this work we demonstrate UHR-OCT at 1300 nm using a Super continuum source and highlight the resolution constraints in using the Hilbert transform algorithm when extracting the optical phase for calibration and dispersion compensation. We demonstrate that the constraints cannot be explained purely by the numerical errors in the data processing module utilizing the Hilbert transform but must be dictated by broadening mechanisms originating from the experimentally obtained interferograms.

In ultra-high resolution (UHR-) optical coherence tomography (OCT) group velocity dispersion (GVD) must be corrected for in order to approach the theoretical resolution limit. One approach promises not only compensation, but complete annihilation of even order dispersion effects, and that at all sample depths. This approach has hitherto been demonstrated with an experimentally demanding 'balanced detection' configuration based on using two detectors. We demonstrate intensity correlation (IC) OCT using a conventional spectral domain (SD) UHR-OCT system with a single detector. IC-SD-OCT configurations exhibit cross term ghost images and a reduced axial range, half of that of conventional SD-OCT. We demonstrate that both shortcomings can be removed by applying a novel generic artefact reduction algorithm and using analytic interferograms. We show the superiority of IC-SD-OCT compared to conventional SD-OCT by showing how IC-SD-OCT is able to image spatial structures behind a strongly dispersive silicon wafer. Finally, we question the resolution enhancement of square root 2 that IC-SD-OCT is often believed to have compared to SD-OCT. We show that this is simply the effect of squaring the reflectivity profile as a natural result of processing the product of two intensity spectra instead of a single spectrum., Comment: 11 pages, 6 figures

International audience; A general theoretical framework is described to obtain the advantages and the drawbacks of two novel Fourier Domain Optical Coherence Tomography (OCT) methods denoted as Master/Slave Interferometry (MSI) and its extension denoted as Complex Master/Slave Interferometry (CMSI). Instead of linearizing the digital data representing the channeled spectrum before a Fourier transform can be applied to it (as in OCT standard methods), channeled spectrum is decomposed on the basis of local oscillations. This replaces the need for linearization, generally time consuming, before any calculation of the depth profile in the range of interest. In this model two functions, g and h, are introduced. The function g describes the modulation chirp of the channeled spectrum signal due to nonlinearities in the decoding process from wavenumber to time. The function h describes the dispersion in the interferometer. The utilization of these two functions brings two major improvements to previous implementations of the MSI method. The paper details the steps to obtain the functions g and h, and represents the CMSI in a matrix formulation that enables to implement easily this method in LabVIEW by using parallel programming with multi-cores.© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Corneal B-scan images and signal-to-noise ratio measurements using ultra-high resolution Spectral Domain Optical Coherence Tomography (SD-OCT) are reported. A comparison of results is obtained using a Ti:Sa laser and a supercontinuum optical source, is performed. Beside some differences in the SNR, the images are strikingly similar.

Optical coherence tomography (OCT) is a non-invasive imaging technique with many applications and widespread use in ophthalmology [1]. The axial resolution in OCT is inversely proportional to the bandwidth of the optical source used, but the improved axial resolution comes at the price of more significant effects of dispersion stemming from the imaging system and the imaged medium. In recent years, spectral intensity (SI) OCT has been shown, as a classical realisation of quantum OCT, to remove even orders of dispersion intrinsically [2, 3]. One major drawback of SI OCT is however halving of the imaging range which is crucial in spectrometer based OCT which is limited by the spectral resolution, and hence number of pixels of the spectrometer. In this work we demonstrate SI OCT with the full imaging range.

We investigate the polarization of supercontinuum generated in nominally non-birefringent silica photonic crystal fibers over the entire spectrum of the source (450-2400 nm). We demonstrate that the degree of polarization varies over the spectrum but that some parts of the spectrum show stable polarization extinction ratios (PER) of over 10 dB. We experimentally demonstrate how the spectrally resolved polarization develops with increasing power and along the length of the nonlinear fiber. The experimental results are compared to numerical simulations of coupled polarization states mimicking the experimental conditions. Subsequently, a single-shot pulse-to-pulse polarization dependent relative intensity noise (PD-RIN) was measured and the noise characteristics were analyzed using long-tailed and rogue wave statistics. To do this, we used a range of 10 nm narrow bandpass filters (BPF) between 550 nm to 2200 nm, and fast photo detectors, to record 800 consecutive pulses. Peaks from these pulses are first extracted, then distribution of their pulse height histogram (PHH) is constructed. Analysis using higher-order moments about the mean (variance, skewness and kurtosis) showed that: (1) around the pump wavelength of 1064nm, the PD-RIN is lowest, PHH exhibits a Gaussian distribution, and higher order moments are zero, (2) further away from pump, PD-RIN increases in parabolic fashion, PHH follows a left-skewed long-tailed Gamma distribution, and higher-order moments increase. Spectrally, the difference of the PD-RIN in the two orthogonal axes increases with PER.

Supercontinuum (SC) light is a well-established technology, which finds applications in several domains ranging from chemistry to material science and imaging systems [1-2]. More specifically, its ultra-wide optical bandwidth and high average power make it an ideal tool for Optical Coherence Tomography (OCT). Over the last 5 years, numerous examples have demonstrated its high potential [3-4] in this context. However, SC light sources present pulse-to-pulse intensity variation that can limit the performance of any OCT system [5] by degrading their signal to noise ratio (SNR). To this goal, we have studied and compared the noise of several SC light sources and evaluated how their noise properties affect the performance of Ultra-High Resolution OCT (UHR-OCT) at 1300 nm. We have measured several SC light sources with different parameters (pulse length, energy, seed repetition rate, etc.). We illustrate the different noise measurements and their impact on a state of the art UHR-OCT system producing images of skin. The sensitivity of the system was higher than 95 dB, with an axial resolution below 4μm.

In this Letter, we investigate the possibility of using a commercially available Q-switch-pumped supercontinuum (QS-SC) source, operating in the kilohertz regime, for ultra-high resolution optical coherence tomography (UHR-OCT) in the 1300 nm region. The QS-SC source proves to be more intrinsically stable from pulse to pulse than a mode-locked-based SC (ML-SC) source while, at the same time, is less expensive. However, its pumping rate is lower than that used in ML-SC sources. Therefore, we investigate here specific conditions to make such a source usable for OCT. We compare images acquired with the QS-SC source and with a current state-of-the-art SC source used for imaging. We show that comparable visual contrast obtained with the two technologies is achievable by increasing the readout time of the camera to include a sufficient number of QS-SC pulses.

Within the last decade, 3D printing has moved from a costly approach of building mechanical items to the present state-of-the-art phase where access to 3D printers is now common, both in industry and in private places. The plastic printers are the most common type of 3D printers providing prints that are light, robust and of lower cost. The robustness of the structure printed is only maintained if each layer printed is properly fused to its previously printed layers. In situations where the printed component has to accomplish a key mechanical role there is a need to characterize its mechanical strength. This may only be revealed by in-depth testing in order to discover unwanted air-gaps in the structure. Optical coherence tomography (OCT) is an in-depth imaging method, that is sensitive to variations in the refractive index and therefore can resolve with high resolution translucid samples. We report on volume imaging of a 3D printed block made with 100% PLA fill. By employing ultrahigh resolution OCT (UHR-OCT) we show that some parts of the PLA volume reveal highly scattering interfaces which likely correspond to transitions from one layer to another. In doing so, we document that UHR-OCT can act as a powerful tool that can be used in detecting fractures between layers stemming from insufficient fusion between printed structure layers. UHR-OCT can therefore serve as an useful assessment method of quality of 3D prints.

In this report we applied the principle of Master-Slave Interferometry (MSI) to an Optical Coherence Tomography (OCT) employing a Super-Continuum (SC) light source. A-scans and B-scan images of biological and non-biological sample are presented in order to demonstrate similar performance with the images obtained with the resampled Fourier Transform (FT) based OCT technique. Dispersion tolerance of MSI method is demonstrated as a constant axial resolution over the depth range even though dispersion is left uncompenstaed in the system.

Spectroscopic optical coherence tomography (SOCT) combines the imaging capability of optical coherence tomography with spectroscopic absorption information. SOCT requires a large bandwidth combined with a broadband spectrometer, due to the processing of the measured data, which includes dividing the spectrum in spectral bands. Both, spectral and axial resolution of SOCT depend on the spectral width of each window. A supercontinuum source with its broad spectrum allows a sufficient number of windows combined with a reasonable axial resolution, which depends on the application. Here a SOCT system is used in the visible spectral range from 480 to 730 nm by combining a supercontinuum light source, a Michelson interferometer and a commercial available broadband spectrometer. This wavelength range is chosen because it covers a range of useful absorbers, including that of human proteins. The system is tested with a laser dye rhodamine B for calibration and verification. Rhodamine B has an absorption peak at around 542 nm, which resembles the absorption spectrum of several proteins in the globin group. The results show that the absorption spectrum of rhodamine B can be reconstructed with sufficient accuracy and demonstrate that varying spectroscopic information can be retrieved from different depths.

Ytterbium-doped double-clad photonic crystal fibres (PCF) are ideal candidates for amplification of diffraction limited pulses to energies of several mJ. The combination of large mode field diameter (MFD), large pump area and high numerical aperture allows for low cost multimode diode pumping of short fiber lengths with high pump absorption and high thresholds for detrimental non-linear effects. We present results from ns-pulsed Q-switched laser and MOPA systems with pulse energies exceeding 1mJ and peak powers up to 17kW, by using a PCF with MFD of 24mm. The results are scalable with the PCF MFD to much higher pulse energies. Recent reports on truly single mode PCF with MFD up to 100mm indicates in this way, that pulse energies exceeding 10mJ could be reached.

The performance of WDM networks is highly dependent on the wavelength selective components within the network. When using Bragg gratings as wavelength selective elements they have to fulfill a number of criteria such as low loss, high reflectivity, nearly square filter function and high background rejection. Highly photosensitive germanium doped silica-on-silicon planar waveguides have been produced by plasma enhanced chemical vapor deposition (PECVD). Total insertion losses (including fiber to waveguide coupling losses) down to 1 dB for a 5.1 cm long waveguide have been measured at 1541 nm. A germanium content of 8.6 mol% in the core combined with the PECVD process insures high photosensitivity towards 193 nm. Twenty mm long Bragg gratings have been induced by illuminating the waveguides with 193 nm light through a phase mask. The induced gratings show up to 99.98 percent reflectivity and a background rejection better than 22 dB at plus or minus 1 nm from the Bragg wavelength. No hydrogen loading or any other kind of sensitization was necessary to produce these gratings. The spectral flatness (3 dB reflection bandwidth: 0.55 nm and 1 dB reflection bandwidth: 0.47 nm) around the Bragg wavelength and the high background rejection make these gratings well suited building blocks for multi-wavelength network components.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A three-layer silica structure was grown on a 4-inch silicon wafer using plasma enhanced chemical vapor deposition. The 2.5 micrometers thick core layer is surrounded by 12 micrometers thick buffer and cladding layers. An aluminum layer, deposited on the cladding glass and patterned with the desired waveguide structures, serves as a mask for the UV exposure. An excimer laser operating on ArF giving an energy of 60 mJ/pulse was used to expose the sample to the total fluence of 4800 J/cm2 of 193 nm light. The rise in refractive index is estimated to be around 3 - 10-3 without hydrogen loading or any other kind of sensitization. The Y-splitters were evaluated on an automatic alignment setup using a semiconductor laser operating at 1542 nm. The splitting ratio was measured to be 1:1 at 1542 nm and 1:0.95 at 1310 nm. We measured the loss in the splitter to be 5 dB (fiber to fiberneglecting the splitting). The minimum coupling loss from fiber to waveguide was theoretically calculated to be 1.1 dB, leaving the propagation loss to be smaller than 2.8 dB/cm. The same value was measured for straight waveguides next to the couplers, indicating no measurable excess loss induced by the Y-splitters.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have produced UV-written, erbium-doped glass waveguides. A planar, three-layer structure (buffer/erbium-doped core/cladding) is exposed to 193 nm excimer laser light through an aluminum mask.

Bragg gratings have been written with 193 nm UV-light through a phase mask in low-loss germanium-doped buried waveguides. Without any kind of sensitisation, reflectivities of up to 99.98% and background rejection better than 22 dB have been obtained for 20 mm long Bragg gratings.

New results in erbium-doped planar waveguide amplifiers and lasers are presented. The waveguides are produced in silica-on-silicon technology using plasma-enhanced chemical vapor deposition. Waveguide propagation losses outside the erbium absorption band are improved to values below 0.05 dB/cm for a core–cladding index step of around 1.4%. Amplifier arrays show a net gain of more than 12 dB over the entire C-band.

Supercontinuum (SC) sources are of great interest for many applications due to their ultra-broad optical bandwidth, good beam quality and high power spectral density [1]. In particular, the high average power over large bandwidths makes SC light sources excellent candidates for ultra-high resolution optical coherence tomography (UHR-OCT) [2-5]. However, conventional SC sources suffer from high pulse-to-pulse intensity fluctuations as a result of the noise-sensitive nonlinear effects involved in the SC generation process [6-9]. This intensity noise from the SC source can limit the performance of OCT, resulting in a reduced signal-to-noise ratio (SNR) [10-12]. Much work has been done to reduce the noise of the SC sources for instance with fiber tapers [7,8] or increasing the repetition rate of the pump laser for averaging in the spectrometer [10,12]. An alternative approach is to use all-normal dispersion (ANDi) fibers [13,14] to generate SC light from well-known coherent nonlinear processes [15-17]. In fact, reduction of SC noise using ANDi fibers compared to anomalous dispersion SC pumped by sub-picosecond pulses has been recently demonstrated [18], but a cladding mode was used to stabilize the ANDi SC. In this work, we characterize the noise performance of a femtosecond pumped ANDi based SC and a commercial SC source in an UHR-OCT system at 1300 nm. We show that the ANDi based SC presents exceptional noise propertiescompared to a commercial source. An improvement of ~5 dB in SNR is measured in the UHR-OCT system, and the noise behavior resembles that of a superluminiscent diode. This preliminary study is a step forward towards development of an ultra-low noise SC source at 1300 nm for ultra-high resolution OCT.