Wavelength division multiplexed confocal microscopy using supercontinuum

We investigate both theoretically and experimentally wavelength division multiplexed confocal imaging by using white light supercontinuum. We show that with the optimized pinhole diameter an axial resolution of 0.75 μm and detection efficiency of 80% can be achieved. In addition, we applied the axial WDM confocal system to 3D surface measurement and the result agreed well with that measured by commercially available surface profilometer. The measured sensitivity of the system is 3.25 nm. Finally, we demonstrated lateral confocal imaging by using supercontinuum. An effective lateral scanning range of 130 μm was obtained.     

In vivo imaging of dynamic biological specimen by real-time single-shot full-field optical coherence tomography

We demonstrate the feasibility of a compact single-shot full-field time domain optical coherence tomography (OCT) for imaging dynamic biological sample in real-time. The system is based on a Linnik type polarization Michelson interferometer and a four-quadrature phase-stepper optics, which can simultaneously capture four quadraturely phase-stepped interferograms on a single CCD. Using a superluminescent diode as light source with center wavelength of 842 nm and spectral width of 16.2 nm, the system yields an axial resolution of 19.8 μm, and covers a field of view of 280 × 320 μm² (220 × 250 pixels) with a transverse resolution of 4.4 μm by using a 10× microscope objective (0.3 NA). Three-dimensional OCT images of biological samples such as an onion slice and a diaptomus were obtained without any image averaging or pixel binning. In addition, in vivo depth resolved dynamic imaging was demonstrated to show the beating internal structure of a diaptomus with a fame rate of 5 fps.     

Modeling of arrayed waveguide grating for wavelength interrogation application

We have proposed and discussed a design of arrayed waveguide grating (AWG) for the application of wavelength interrogation. The spectral responses of a silica-based 16 channel AWG with channel spacing 1.6 nm have been simulated when different receiver waveguide spacing are used. It was found that the 3-dB bandwidth is reduced about 50% as the receiver waveguide spacing increasing from 20 μm to 30 μm. The effect of bandwidth of the spectral response on wavelength resolution of AWG based interrogator has been estimated and discussed.     

A combined multiple-SLED broadband light source at 1300 nm for high resolution optical coherence tomography

We demonstrate a compact, inexpensive, and reliable fiber-coupled light source with broad bandwidth and sufficient power at 1300 nm for high resolution optical coherence tomography (OCT) imaging in real-time applications. By combining four superluminescent diodes (SLEDs) with different central wavelengths, the light source has a bandwidth of 145 nm centered at 1325 nm with over 10 mW of power. OCT images of an excised stage 30 embryonic chick heart acquired with our combined SLED light source (<5 μm axial resolution in tissue) are compared with images obtained with a single SLED source (∼10 μm axial resolution in tissue). The high resolution OCT system with the combined SLED light source provides better image quality (smaller speckle noise) and a greater ability to observe fine structures in the embryonic heart.     

Optical design of a full-pupil field,non-contact and high resolution corneal curved objective lens

It has been a challenge to overcome the corneal curvature radius to design a full-pupil field, non-contact and high resolution corneal curved objective lens, which covers the cornea full-pupil field and has the ability to resolve corneal cells. In this paper, we report an optical design of a full-pupil field, non-contact corneal curved objective lens for high resolution cornea imaging. The advantages of this lens are that it has a wide field of view (FOV) with the corneal curved image surface, maintains the beam normal incidence, as well as non-contact lens imaging, and offers a cell-level lateral resolution of cornea structure. The analysis of optimization shows that the system achieves diffraction limit in a circular FOV of 4 mm diameter covering the full-pupil zone. The theoretical lateral resolution is about 2.5 μm with an image space NA of 0.16, which is sufficient to resolve corneal cells of 7 μm diameter, and the working distance is larger than 15 mm which is enough for a non-contact objective lens. So the optical design is effectively and efficiently meeting the demand of specifications.     

The ridge waveguide fabrication with periodically poled MgO-doped lithium niobate for green laser

Ridge waveguides were fabricated using an external field, a precision lapping machine and neutron loop discharge (NLD) in magnesium-oxide-doped lithium niobate. The measured quasi-phase-matching (QPM) wavelength of the second-harmonic generation (SHG) in a 30 mm long periodically poled magnesium-doped lithium niobate (PPMgLN) ridge waveguide which has a domain period of 6.8 μm is about 532 nm. A fabricated periodically poled magnesium-doped lithium niobate ridge waveguide was duty cycle of 51.9 ± 2.83% and demonstrated second-harmonic generation. By using this periodically poled magnesium-doped lithium niobate ridge waveguide, highly effective, low-cost optical devices with high power or short wavelength can be achieved.     

Experimental generation of high-contrast Talbot images with an ultrashort laser pulse

A femtosecond Ti:sapphire laser oscillator emitting pulses with 800 nm central wavelength, 10.9 fs pulse width, and 75 MHz repetition rate, combined with a dispersion-compensated diffractive system, was used to implement a large-area, high-contrast, broadband optical interference technique based on the Talbot effect. Chromatic artifacts associated with the huge spectrum of the optical source (approximately 150 nm) are compensated for with an air-separated hybrid diffractive-refractive lens doublet. The spatial resolution of the chromatically compensated Talbot images under femtosecond illumination is nearly identical to that achieved under continuous wave monochromatic illumination. Furthermore, the temporal width of the signal at the Talbot planes is limited by the group-delay-dispersion coefficient which is shown to be small. High-contrast one-dimensional periodic structures of 96.1 μm spacing generated by Talbot diffractometry are experimentally demonstrated.     

Improvement of optical sensing performances of a double-slot-waveguide-based ring resonator sensor on silicon-on-insulator platform

We demonstrate a label-free photonic biosensor with double slots based on micro-ring resonator. The footprint is less than 25 μm × 15 μm. Finite-difference time-domain (FDTD) method is used to analyze the influence of several key parameters on the performance of the double-slots micro-ring resonators. An asymmetric structure is considered for the ring waveguide in order to improve the sensor's bending efficiency. Our numerical analysis shows that the sensitivity of double-slot micro-ring resonator sensor with the radius of 5 μm reaches a value of 708 nm/RIU. The quality factor of 580 and the free spectral range (FSR) of 33 nm are achieved.     

Optical probe using eccentric optics for optical coherence tomography

We propose and demonstrate an OCT optical probe using eccentric optics. This probe enabled both forward imaging and side imaging by dividing a circular scanning area into two semicircular scanning areas using an external motor to rotate the flexible tube. The outer diameter of the probe was 2.6 mm, and its rigid portion length was 10 mm. The lateral resolution was 23 μm, and the eccentric radius was 1.1 mm. The circumferential length in scanning was 6.9 mm, and the working distance was 5 mm. OCT images of 1.5 mm × 6.9 mm (in tissue, axial × circumference), including forward image and side image, were measured with the axial resolution of 19 μm in air and a frame rate of one frame per second. The epidermis, dermis, and sweat gland of in vivo human ventral finger tips were observed.     

Investigation of a biofunctional polymeric coating deposited onto silicon microcantilevers

The paper deals with an appealing route to activate silicon microcantilevers (90, 110 and 130 μm long, 35 μm wide and 2 μm thick) for specific binding of biochemical species. The method consists in coating the underivatized microcantilevers with a biofunctional copolymer (based on N,N-dimethylacrylamide bearing silanating moieties) that was developed for low-density microarray assays on microscope glass slides. Coating deposition was obtained by dip-coating and its microstructure investigated by analyzing the resonance frequency values of bare and coated microcantilevers, by SEM and SFM imaging, SFM tip-scratch tests and XRR experiments. Results indicate that the coating is 2.5 nm thick and has a density of 1.22 g/cm³. The coating surface is nanostructured, displaying nanoblobs, which are from few up to 20 nm wide and, on average, 1.6 nm high. The diameter of the biggest nanoblobs is of the same order of magnitude of the gyration radius of the copolymer chains, suggesting that nanoblobs may identify individual macromolecules.     

Design and analysis of a vertical directional coupler between a three-dimensional plasmonic slot waveguide and a silicon waveguide

The design of a vertical directional coupler between a three-dimensional plasmonic slot waveguide and a silicon waveguide is theoretically investigated in detail. It consists of two steps: the design of isolated plasmonic slot waveguide and silicon waveguide and the determination of the gap between the two waveguides and the length of a coupling region. The designed structure transfers 70.8% of the power carried by the silicon waveguide mode to the plasmonic slot waveguide mode when the gap is 150 nm and the coupling length is 2.14 μm. The wavelength dependence of our vertical directional coupler is also studied. The analysis shows that the amount of the transferred power changes slightly over a very wide wavelength range between 1.40 μm and 1.61 μm. Moreover, if we employ the fabrication technology for silicon photonics, it is quite tolerant to the variation of the length of its coupling section. Finally, the vertical directional coupler is considered for a polarizer.     

Characterization and performance of the femtosecond fluorescence up-conversion microscope

The characterization and performance of the femtosecond fluorescence up-conversion microscope is reported in this paper. This new fluorescence microscope is a combination of the frequency up-conversion technique and a confocal optical configuration, which simultaneously achieves femtosecond time and nanometer space resolution. The femtosecond time resolution was evaluated by measuring the rise up of time-resolved fluorescence from a dye molecule, and it was 520 fs and 460 fs with 100× (N.A.=1.3) and 40× (N.A.=0.75) objective lenses, respectively. The best transverse (XY) resolution was 0.34 m with the 100× objective lens for 400 nm excitation. An axial (Z) resolution as high as 1.1 m was obtained for 600 nm fluorescence detection with a 50 m pinhole and a 100× objective lens. The axial resolution was remarkably improved compared with ordinary confocal microscopes owing to the up-conversion process, which requires spatial overlap between the tightly focused gate and the fluorescence beams. Femtosecond time-resolved fluorescence measurements were performed for micro-meter sized particles in liquids, fluorescent beads and C519/toluene micro droplets, by using the laser trapping technique. The high potential of the fluorescence up-conversion microscope was demonstrated. PACS 78.47.+p; 87.64.-t; 82.53.-k     

基于旋转扫描探头的OCT内窥成像系统设计

为了同时获取样品的表面和深度信息,研究光学相干层析的成像原理,建立了基于光学相干层析技术的内窥系统,实现了旋转扫描成像,系统的工作波长为1 310 nm,工作带宽为80 nm.理论推导及计算机仿真得到了系统信噪比与干涉仪的分光比、反射率之间的关系并分析了理论分辨率和探测深度.提出外径为5 mm的内窥镜扫描探头,聚焦距离为12 mm,数值孔径NA为0.47,折射率分布常量A=0.218 7.利用微型电机驱动直角棱镜实现扫描,旋转速度为25 rpm,旋转一周得到640个采样点.采用多层盖玻片和洋葱表皮作为样品进行实验分析,得到了盖玻片和洋葱的图像,横向分辨率和纵向分辨率分别为10 μm和15 μm.结果表明,设计的光学相干层析内窥系统能够用于旋转扫描成像,获取更多的组织信息.     

Er-Yb co-doped glass waveguide amplifiers using ion exchange and field-assisted annealing

Er³⁺-Yb³⁺ co-doped waveguide amplifiers fabricated using thermal two-step ion-exchange are demonstrated. K⁺-Na⁺ ion-exchange process was first carried out in pure KNO₃ molten bath, and then field-assisted annealing (FAA) was used to make the buried waveguides. The effective buried depth is estimated to be ∼3.4 μm for the buried FAA waveguides. With the use of cut-back method, the fiber-to-guide coupling loss of ∼4.38 dB, the waveguide loss of ∼2.27 dB/cm, and Er³⁺ absorption loss ∼5.7 dB were measured for a ∼1.24-cm-long waveguide. Peak relative gain of ∼7.0 dB is obtained for a ∼1.24-cm-long waveguide. The potential for the fabrication of compact optical amplifiers operating in the range of 1520-1580 nm is also demonstrated.     

Semi-elliptical dielectric-loaded surface plasmon-polariton waveguides

Semi-elliptical dielectric-loaded surface plasmon-polariton waveguide (DLSPPW) is proposed. The mode effective index, field confinement, and propagation length of the fundamental mode supported by it are calculated at the telecom wavelength λ = 1.55 μm for different dimensions of a polymer ridge (with the refractive index of 1.535) placed on a gold film surface. The waveguide structure is found to exhibit 23% increase of the propagation length while showing similar confinement as compared to conventional rectangular DLSPPW when ridge thickness t < 450 nm (ridge width w = 600 nm) and ridge width w < 320 nm (ridge thickness t = 600 nm).     

Second harmonic generation by femtosecond Yb-doped fiber laser source based on PPKTP waveguide fabricated by femtosecond laser direct writing

The frequency doubling of femtosecond pulses from an Yb-doped fiber laser source was demonstrated in a PPKTP waveguide fabricated by femtosecond laser direct writing. The PPKTP waveguide contains a fixed period of 8.9 μm and the feomtosecond fundamental pulses have a central wavelength of 1044 nm. A maximum SHG power of 406 mW was produced, yielding a conversion efficiency of 5.6%. Numerical simulations were carried out to investigate the property of frequency doubling for femtosecond pulses. The results show that the SHG process proceeds even the quasi-phase-matching (QPM) condition is not well satisfied, which is significantly different from that of “long” pulses or CW light and is accorded with the experimental results.     

Magnetic layered structure for the production of polarized neutron microbeams

A neutron waveguide is a three-layer structure in which a guiding layer with low optical potential is placed between two cladding layers with high optical potential. Under proper operation conditions, the neutron density is resonantly enhanced inside the guiding layer. In our experimental scheme, the neutron beam enters through the surface of the top layer at glancing angle and goes out from the edge of the guiding layer, with an angular distribution corresponding to Fraunhofer diffraction from a narrow slit. The incident neutron beam is relatively wide (0.1 mm) and highly collimated (0.01°). The outgoing sub-micron beam is extremely narrow at the outlet (0.1 μm) and more divergent (0.1°). So far only the production of unpolarized sub-micron neutron beams has been reported. Here we present first experiments on polarized sub-micron neutron beams. For these studies a polarized incident beam was used and two types of magnetic waveguides were investigated: a polarizing magnetic waveguide Fe(20 nm)/Cu(140 nm)/Fe(50 nm)//glass and a non-polarizing magnetic waveguide Py(10 nm)/Al(140 nm)/Py(50 nm)//glass (Py is permalloy). The waveguide samples were characterized by polarized neutron reflectometry.     

Non-destructive quantification of pharmaceutical tablet coatings using terahertz pulsed imaging and optical coherence tomography

Optical coherence tomography (OCT) and terahertz pulsed imaging (TPI) are two powerful techniques allowing high quality cross-sectional images from within scattering media to be obtained non-destructively. In this paper, we report experimental results of using OCT and TPI for quantitatively characterizing pharmaceutical tablet coatings in the thickness range of 10-140 μm. We found that the spectral OCT system developed in-house has an axial resolution of 0.9 μm, and is capable of quantifying very thin coatings in the range of 10-60 μm. The upper limit of 60 μm within the tablet coating and core is owed to the strong scattering of OCT light, which has relatively short wavelengths in the range of 0.5-1.0 μm. On the other hand, TPI utilizes terahertz radiation that has substantially long wavelengths in the range of hundreds of microns, and thus is less prone to the scattering problem. Consequently TPI has been demonstrated to be able to quantify thicker coatings in the range of 40-140 μm and beyond. We concluded that OCT and TPI are two complementary analytical techniques for non-destructive and quantitative characterization of pharmaceutical tablet coatings.     

Numerical study of sub-wavelength plasmonic waveguide

The authors present an analysis of a plasmonic waveguide, simulated using a two-dimensional finite-difference time-domain technique. With the surface structures located on the surface of the metal, the device is able to confine and guide light waves in a sub-wavelength scale. And two waveguides can be placed within 150 nm (∼6% of the incident wavelength) that will helpful for the optoelectronic integration. Within the 20 μm simulation region, it is found that the intensity of the guided light at the interface is roughly two to four times the peak intensity of the incident light, and the propagation length can reach approximately 40 μm at the wavelength of 2.44 μm.     

Laser-generated surface acoustic waves in a ring-shaped waveguide resonator

Surface acoustic wave (SAW) waveguide resonator is formed by a ring-shaped strip of copper 10 μm wide and ∼130 μm in diameter embedded into a 0.8 μm thick layer of silica on a silicon wafer. SAWs are excited at one side of the copper ring by a short laser pulse focused into a spatially periodic pattern and detected via diffraction of the probe laser beam overlapped with the excitation spot. SAW wavepackets with central frequency 460 MHz travel around the ring and are detected each time they make a full circle and pass trough the probe spot. Potential applications of ring resonators for SAWs are discussed.