JEOS:RP - Journal of the European Optical Society Rapid publications
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Comparison of closed loop and sensorless adaptive optics in widefield optical microscopy
Get PDFWe report on a closed loop widefield adaptive optics, optical microscopy system in which the feedback signal is provided by backscattered light from the sample acting as a guide star. The improvement in imaging performance is compared to an adaptive optics system controlled via an image optimisation routine commonly described as sensorless adaptive optics. The samples viewed were imaged without fluorescence to ensure that photobleaching and other potential variations did not affect the comparisons in system performance though the method is equally applicable for fluorescence microscopy. The closed loop system is self-optimising for different areas of the sample, using a common reference wavefront, with the accuracy of the loop being limited by variation across the sub-aperture images induced by guide star elongation. Optimisation using an image sharpness metric gives slightly sharper images but takes significantly longer. We thus believe that both wavefront sensor based closed loop AO and metric based optimisation have a role to play in AO for microscopy and that the method of backscattered light as a guide star has a great potential in the application of AO, particularly to optical coherence tomography
Spherical refractive correction with an electro-optical liquid lens in a double-pass system
Get PDFIn this study we show a novel double-pass configuration to correct the spherical refractive error by means of an electro-optical liquid lens.The proposed method enables spherical correction in the -12 D to +7 D range without movable parts using an electro-optical liquid lens.We have measured the optical performance of the spherical corrector in terms of power, pupil size and optical quality verifying that it fitsthe requirements to be applied to a double-pass system. We have also evaluated the performance of the proposed method in patients bycomparison with a conventional double-pass system using a Badal optometer
Effective and flexible modeling approach to investigate various 3D Talbot carpets from a spatial finite mask
Get PDFWe present an effective modeling approach for a fast calculation of the Talbot carpet from an initially 2-dimensional mask pattern. The introduced numerical algorithm is based on a modified angular-spectrum method, in which it is possible to consider the border effects of the Talbot region from a mask with a finite aperture. The Bluestein’s fast Fourier transform (FFT) algorithm is applied to speed up the calculation. This approach allows as well to decouple the sampling points in the real space and the spatial frequency domain so that both parameters can be chosen independently. As a result an extended three-dimensional Talbot-carpet can be calculated with a minimized number of numerical steps and computation time, but still with high accuracy. The algorithm was applied to various 2-dimensional mask patterns and illumination setups. The influence of specific mask patterns to the resulting field intensity distribution is discussed
Compensation of hologram distortion by controlling defocus component in reference beam wavefront for angle multiplexed holograms
Get PDFHolographic memory has the potential to function as a recording system with a large capacity and high data-transfer-rate. Photopolymer materials are typically used as a write-once recording medium. When holograms are recorded on this medium, they can distort due to shrinkage or expansion of the materials, which degrades the reconstructed image and causes a higher bit error rate (bER) of the reproduced data. We propose optically compensating for hologram distortion by controlling aberration components in the reference beam wavefront while reproducing data, thereby improving the reproduced data quality. First, we investigated the relation between each aberration component of the reference beam and the signal to noise ratio (SNR) of the reproduced data using numerical simulation and found that horizontal tilt and the defocus component affect the SNR. Next, we experimentally evaluated the reproduced data by controlling the defocus component in the reference beam and found that the bER of the reproduced data could be decreased by controlling the defocus center with respect to the hologram position and phase modulation depth of the defocus component. Then, we investigated a practical control method of the defocus component using an evaluation value similar to the definition of the SNR for actual data reproduction from holograms. Using a defocus controlled wavefront enabled us to decrease the bER from 3.54 x 10^-3 with a plane wave to 3.14 x 10^-4. We also investigated how to reduce the bERs of reproduced data in angle multiplexed holograms. By using a defocus controlled wavefront to compensate for hologram distortion on the 40th data page in 80-page angle multiplexed holograms, the bERs of all pages could be decreased to less than 1x10^-3. We showed that controlling the defocus component is an effective way to compensate for hologram distortion and to decrease the bER of reproduced data in holographic memory
The photonic wheel - demonstration of a state of light with purely transverse angular momentum
Get PDFIn classical mechanics, a system may possess angular momentum which can be either transverse (e.g. in a spinning wheel) or longitudinal(e.g. for a spiraling seed falling from a tree) with respect to the direction of motion. However, for light, a typical massless wave system,the situation is less versatile. Photons are well-known to exhibit intrinsic angular momentum which is longitudinal only: the spin angularmomentum defining the polarization and the orbital angular momentum associated with a spiraling phase front. Here we show that itis possible to generate a novel state of the light field that contains purely transverse angular momentum, the analogue of a spinningmechanical wheel. We realize this state by tight focusing of a polarization tailored light beam and measure it using an optical nano-probingtechnique. Such a novel state of the light field can find applications in optical tweezers and spanners where it allows for additionalrotational degree of freedom not achievable in single-beam configurations so far
Intensity noise in ultra-high frequency modulated semiconductor laser with strong feedback and its influence on noise figure of RoF links
Get PDFWe characterize the intensity noise associated with the ultra-high frequency direct modulation of semiconductor lasers under strong optical feedback over a millimeter-wave frequency passband between 54.5 and 56.5 GHz. Enhancement of the modulation response over this millimeter wave band due to optical feedback is clearly shown by time-delay rate equation analysis fully involving the strong optical feedback effect. The contribution of the relative intensity noise of the laser to the noise figure of an ultra-high speed radio over fiber link is evaluated. We show that subjecting the laser diode to strong feedback improves the noise figure of a 55.8-GHz radio over fiber link nearly by 20 dB in the regime of small-signal modulation and 10 dB under large-signal modulation
Observing cross-sectional images and averaged optical patterns of photonic crystal fibers using partially incoherent laser light
Get PDFWe couple the coherent laser light and the partially incoherent laser light into the photonic crystal fibers, respectively, and compare the observed fiber cross-sectional images. Lower coherence light can indeed help us obtain higher-quality and uniform cross-sectional images on the fiber output endfaces. We have observed rather unique and clear averaged optical mode patterns within the microstructures of a silica-air index-guiding solid-core photonic crystal fiber, which might be due to the stress-induced refractive-index inhomogeneity formed therein during the fiber manufacturing process. The experimental results are also compared with those images taken by other light sources or imaging systems. Besides, we successfully achieve capturing consecutively the various near-field averaged optical patterns of the solidcore photonic crystal fiber using this single-wavelength partially incoherent laser light imaging system
Linear phase retrieval for real-time adaptive optics
Get PDFWe developed a fast phase retrieval algorithm that is suitable for real-time applications such as adaptive optics. The phase retrieval model is developed by linearising the pupil function in the approximation of small aberrations and is valid for low-NA focused field. The linear model in conjunction with a particular choice for the position of the single out-of-focus measurement plane and an efficient control algorithm, significantly reduces the computation time for phase retrieval. The experimental results demonstrate the validity of the described approach for fast correction of aberrations
Spectral switching control of ultrafast pulses in dual core photonic crystal fibre
Get PDFThe work presented in this paper is a study of an all-optical narrow-band switch in extended spectral area by dual core photonic crystal fibre expressing nonlinear coupler performance. The investigation is focused on the nonlinear propagation of femtosecond pulses in the near infrared spectral region at up to 50 kW peak power which induces spectral broadening through almost two octaves. The mutual effect of nonlinear spectral transformation and field redistribution between the two fibre cores is analyzed by both theoretical and experimental approaches. The simulation of the nonlinear propagation is based on coupled generalized nonlinear Schrödinger equations. A modified numerical model utilizing split-step Fourier method was adapted for dual core fibres. The complex experimental study was accomplished for various input settings such as polarization, intensity and selective coupling into each core and the selective detection of spectra from each core. The presented work encompasses promising results obtained regarding a spectral intensity switch between the two output channels by input intensity or polarization change in the S-band of optical communication systems
Dynamics of a chain of optically coupled micro droplets
Get PDFWe study a chain of fluid droplets excited by two incoherent laser beams. Such structured object is merely an array of spherical lenses, that can guide a TEMpq optical mode. Taking into account the optical forces exerted by two counterpropagating beams, we show that the droplets can be trapped and the chain auto-organizes in the optical potential. The model takes into account the possible coalescence of several droplets, and shows that the droplet size can increase before they become trapped at stable postitions. For some input beam parameters (beam waist size and position), we have observed dynamic trapping : the droplets experience collective oscillation. Meanwhile, the beam shape evolves periodically in time