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.     

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.     

Distributed Brillouin fiber sensing based on spectrum line fitting and wavelet packet denoising

A novel signal processing method is proposed to improve the spatial resolution, frequency resolution and dynamic characteristics of BOTDR. For the BOTDR system with 50 ns pump pulse, by using spectrum line fitting technology based on Levenberg-Marquardt (LM) algorithm, the spatial resolution is improved from 5 m to 5 cm. Combination of LM fitting algorithm, a large frequency scanning interval is adopted without sacrificing measurement accuracy of the BOTDR system. It reduces the number of sampling points of Brillouin spectrum significantly. So, the fitting speed is improved greatly. This is the first time using a large scan interval to increase the spectrum line fitting speed. To improve the fitting speed, the difference between the reference and measured spectrum is used to estimate the variation of Brillouin frequency shift. The measured amplitude of Brillouin spectrum is used to estimate the width of region strain occurred. Finally, by using wavelet packet denoising technology, the spectra containing noise are fitted successfully.     

Continuous-tunable 4.144-4.851 μm twin-MgO:PPLN cascaded optical parametric generator pumped by a 1.342 μm Nd:GdVO4 laser

An all-solid-state mid-infrared optical parametric generator with wide tunability by using multi-grating periodically poled 5 mol.-% MgO-doped lithium niobate (MgO:PPLN) is reported. The pump source is a diode-pumped Q-switched Nd:GdVO₄ laser operated at 1.342 μm with pulse width of 150 ns and repetition rate of 50 kHz. To extend the interaction length, two identical multi-grating MgO:PPLN crystals have been cascaded in the OPG system. When the incident pump average power is 10 W, the obtained maximum idler output power is 340 mW at 4.144 μm. Compared with only using one multi-grating MgO:PPLN crystal, the obtained idler output power increases by 20.1%. 4.144-4.851 μm continuous-tunable idler output is obtained with six grating periods from 29 to 31.5 μm and temperature from 40 to 200 °C. To our knowledge, this is the first time to use 1.342 μm laser as the pump source of OPG.     

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.     

Quadrature fringes wide-field optical coherence tomography and its applications to biological tissues

We propose and demonstrate quadrature fringes wide-field optical coherence tomography (QF WF OCT) to expand an optical Hilbert transformation to two-dimensions. This OCT simultaneously measures two quadrature interference images using a single InGaAs CCD camera to obtain en face OCT images. The axial and lateral resolutions are measured at 29 μm in air and 70 μm limited by a pixel size of camera using a superluminescent diode with a wavelength of 1.3 μm as the light source; the system sensitivity is determined to be −90 dB. The area of the en face OCT images is 4.0 mm × 4.0 mm (160 × 160  pixels). The OCT images are measured axially with steps of 10 μm. The en face OCT images of a in vivo human fingertip and a in situ rat brain are three-dimensionally measured up to the depth of about 3 mm with some degradations of a lateral resolution.     

Endoscopic optical probe with a PZT actuator and MR tracker

A miniaturized optical probe with a magnetic-resistance (MR) position tracker and piezoelectric-transduce (PZT) mirror system was developed for endoscopic optical imaging. All of the optical components such as collimation and focal lenses, reflection mirror, PZT linear actuator and MR sensor were wholly packaged in a single scanning probe with a volume of 3.57 cm³. This endoscopic probe has the advantages of having a small volume, extended stroke length (4.5 mm), high scanning speed (18.2 mm/s), efficient recoupling ratio (78.3%) and high spatial resolution (11.7 μm) compared to conventional endoscopes. Consequently, it showed the potential for improving the endoscopic imaging system and utilizing the image-guided robotic surgery system.     

Research on the optical properties of collodion film

This paper centers on the deposition process and optical properties of collodion film. Collodion film was prepared on the double side polished silicon and k9 optical glass using the sol–gel method. The studying results have showed four characteristics of collodion film. First of all, the thickness of collodion film decreases with increasing the revolution speed. Secondly, the refractive index of collodion film changes from 1.529306 to 1.500128, which accords with the normal dispersion. Thirdly, the transmittance of collodion film is higher in the visible wavelength range 380–760 nm and its average transmittance is 91.9%. At last, the absorption property is very well in the infrared region. The infrared absorption coefficient is greater than 0.69/μm in range of 3–5 μm, and it is up to 1.433528/μm in 8–14 μm because of its many strong infrared absorption peaks. In addition, the absorption characteristics have been analyzed in detail.     

Digital Micro-mirror Device-based broadband optical image sensor for robust imaging applications

To the best of the authors' knowledge, presented for the first time is the design of a robust broadband optical image sensor using a Digital Micro-mirror Device (DMD). Electronic focus control of the imaging lens and full programmability of the spatial sampling aperture shape, size, and location on the DMD plane that mechanically scans the incident incoherent optical irradiance distribution lead to imaging smartness. Dual port single-point photo-detection design provides imaging operation robustness to the global light irradiance variations such as via environmental effects, e.g., moving clouds. As the Texas Instruments (TI) DMD can provide light modulation over 400 nm to 2500 nm wavelengths, visible, Near Infrared (NIR), and Short-Wave Infrared (SWIR) bands can be simultaneously processed to generate three independent band images via three point photo-detectors. A proof-of-concept experiment in the SWIR band at 1580 nm is conducted using an incoherent heart-shaped target that is sampled using the DMD imager set for a 68.4 μm side square moving pinhole. A 60 × 60 pixel image from the proposed imager produces a 0.94 cross-correlation peak when compared to an optically attenuated heart shape image produced by a near 9 μm pixel size phosphor coated Charge Coupled Device (CCD) imager. Using the dual-detection method, robust 633 nm visible light imaging of an Air Force (AF) Chart figure is successfully demonstrated for 3 Hz global light fluctuation. Applications for the proposed imager include optical sensing in the fields of astronomy, defense, medicine, and security.     

Long range surface plasmon polariton waveguides at 1.31 and 1.55 μm wavelengths

We investigate characteristics of gold metal strip waveguides based on long range surface plasmon polaritons (LRSPPs) along thin metal strips embedded in a polymer for practical applications at the telecommunication wavelengths of 1.31 and 1.55 μm. Guiding properties of the gold strip waveguides are theoretically and experimentally evaluated with the limited thickness and width up to ∼20 nm and ∼10 μm, respectively. The lowest propagation loss of ∼1.4 dB/cm is obtained with a 14.5-nm-thick and 2-μm-wide gold strip at 1.55 μm. With a single-mode fiber, the lowest coupling loss of ∼0.4 dB/facet is achieved with a 14.5-nm-thick and 5-μm-wide gold strip at 1.55 μm. The lowest insertion losses are obtained 8-9 dB with 1.5 cm-long gold strips of a limited thickness and width at both the wavelengths. We demonstrate a 10 Gbps optical signal transmission via the LRSPP waveguide with a 14 nm-thick, 2.5 μm-wide, and 4 cm-long gold strip. These LRSPP waveguides have potential applications for optical interconnects and communications.     

Two-dimensional dopant profiling of silicon with submicron resolution using near field optics on silicon/electrolyte contacts

We demonstrate the possibility to use near field optics to perform two-dimensional dopant profiling on silicon surface, with deep submicron spatial resolution. The sample surface is contacted by an aqueous electrolyte giving a reverse biased junction that is illuminated by a subwavelength optical source, in near filed conditions. A staircase calibration structure was used with several boron-doped layers with either 4 μm or 0.4 μm thickness and doping between 10¹⁷ and 10²⁰ at/cm³. Measurements were performed on the sample cross section. It is shown that photocurrent surface mapping shows up the doped areas with a lateral resolution better than 100 nm.     

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.     

Two-dimensional optical micro-scanner on silicon technology

A two-dimensional optical micro-scanner, which main components are two mobile flat and a concave micro-mirrors, is designed such that, all optical components can be fabricated on the same substratum. The optical parameters, which physical dimensions are between 50 and 500 μm, are obtained within the geometrical optics. The optical performance is evaluated by means of the MTF and Rayleigh resolution criteria, given 80% of modulation for a frequency of 8 cycles/mm with a Gaussian source, the resolution limit is 30 μm.     

Femtosecond and nanosecond laser damage thresholds of doped and undoped triazenepolymer thin films

The influence of pulse duration on the laser-induced damage in undoped or infrared-absorbing-dye doped thin triazenepolymer films on glass substrates has been investigated for single, near-infrared (800 nm) Ti:sapphire laser pulses with durations ranging from 130 fs up to 540 fs and complementarily for infrared (1064 nm) Nd:YAG ns-laser single-pulse irradiation. The triazenepolymer material has been developed for high resolution ablation with irradiation at 308 nm. Post-irradiation optical microscopy observations have been used to determine quantitatively the threshold fluence for permanent laser damage. In contrast to our previous studies on a triazenepolymer with different composition [J. Bonse, S.M. Wiggins, J. Solis, T. Lippert, Appl. Surf. Sci. 247 (2005) 440], a significant dependence of the damage threshold on the pulse duration is found in the sub-picosecond regime with values ranging from ∼500 mJ/cm² (130 fs) up to ∼1500 mJ/cm² (540 fs). Other parameters such as the film thickness (50 nm and 1.1 μm samples) or the doping level show no significant influence on the material behavior upon irradiation. The results for fs- and ns-laser pulse irradiation are compared and analyzed in terms of existent ablation models.     

Neurosensory tissue morphology with intraoperative optical fiber probe in aqueous medium

In this work, the author has demonstrated cross-sectional imaging of a retina of an ex vivo fish sample using a common path frequency domain optical coherence tomography at 0.8 μm range. It has been introduced that an integrated surgical hypodermic needle fiber probe can stabilize the flexible glass optical fiber and provides a close proximity to the specimen for intraoperative image guiding. In addition, the light source characteristics were matched to the common path interferometer while operating in the aqueous medium (saline solution), in order to mimic the in vivo condition, in that it shows greater bandwidth and shorter center wavelength for larger input current or output power to sustain the appropriate level of coherence reference peak by the partial reflection at the glass fiber probe interface.     

Demonstration of a no-moving-parts axial scanning confocal microscope using liquid crystal optics

For the first time, to the best of authors’ knowledge, a no-moving-parts axial scanning confocal microscope (ASCM) system is designed and demonstrated using a combination of a large diameter liquid crystal (LC) lens and a classical microscope objective lens. By electrically controlling the 5 mm diameter LC lens, the 633 nm wavelength focal spot is moved continuously over a 48 μm range with a measured 3-dB axial resolution of 3.1 μm using a 0.65 numerical aperture (NA) micro-objective lens. The ASCM is successfully used to image an Indium Phosphide (InP) twin square optical waveguide sample with a 10.2 μm waveguide pitch and 2.3 μm height and width. Using fine analog electrical control of the LC lens, a super-fine sub-wavelength axial resolution of 270 nm is demonstrated. The proposed ASCM can be useful in various precision three-dimensional (3D) imaging and profiling applications.     

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.     

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.     

Design of midwave infrared athermalization optical system with a large focal plane array

Based on the most advanced staring focal plane array which had a format of 640 × 480 and the pixel pitch of 15 μm, a set of all-sphere midwave infrared ahermalization optical system was designed. The working wavelength was in 3–5 μm, the full field of view was 8.58°, the relative aperture was 1/2, the efficient focal length (EFL) was 80 m. The opticalsystem consisted of four lenses with three kinds of material – Ge, ZnSe and Si. All surfaces were sphere, which was easier to process test, making the cost inexpensive, and it could avoid using diffractive surface and aspheric surface. The image quality of the system approaches the diffraction limit in the temperature range −60 °C-180 °C. The design results proved that, the high resolution midwave infrared optical system had compact structure, small volume, high resolution and excellent image quality, meeting the design requirements, so that it could be used for photoelectric detection and tracking system.     

High repetition rate mid-infrared generation with singly resonant optical parametric oscillator using multi-grating periodically poled MgO:LiNbO3

A high repetition rate mid-infrared singly resonant optical parametric oscillator (OPO) using MgO-doped multi-grating periodically poled LiNbO₃ (MgO:PPLN) is demonstrated. A 1064 nm Q-switched Nd:YVO₄ laser at 10 kHz repetition rate and pulse width of 17.8 ns was used to pump the OPO. The period of the quasi-phase matched (QPM) grating in the multi-grating MgO:PPLN chip varied from 25.5 to 31.5 μm in steps of 0.5 μm. This corresponds to the generation of a signal beam from 1.37 to 1.64 μm and an idler beam from 3.0 to 4.8 μm, respectively. A maximum signal power of 250 mW and idler power of 140 mW has been obtained with an input pump beam of power 1.92 W, for a grating period of 30.5 μm. A maximum optic-optic conversion efficiency of 20% and 7.4% in the idler has been observed. It has been observed that the output power increases as the period of the grating increases.