Jeroen Kalkman

Ultralow-threshold erbium-implanted toroidal microlaser on silicon

We present an erbium-doped microlaser on silicon operating at a wavelength of 1.5 mum that operates at a launched pump threshold as low as 4.5 muW. The 40 mum diameter toroidal microresonator is made using a combination of erbium ion implantation, photolithography, wet and dry etching, and laser annealing, using a thermally grown SiO2 film on a Si substrate as a starting material. The microlaser, doped with an average Er concentration of 2x10^(19) cm(-3), is pumped at 1480 nm using an evanescently coupled tapered optical fiber. Cavity quality factors as high as 3.9x10^(7) are achieved, corresponding to a modal loss of 0.007 dB/cm, and single-mode lasing is observed.

Demonstration of an erbium doped microdisk laser on a silicon chip

Using ion beam implantation of erbium a silica micro-disk laser on silicon chip is demonstrated. A clear transition of spontaneous emission to stimulated emission is observed, with threshold powers of less than 40 micro-Watts.

Quantitative comparison of the OCT imaging depth at 1300 nm and 1600 nm

One of the present challenges in optical coherence tomography (OCT) is the visualization of deeper structural morphology in biological tissues. Owing to a reduced scattering, a larger imaging depth can be achieved by using longer wavelengths. In this work, we analyze the OCT imaging depth at wavelengths around 1300 nm and 1600 nm by comparing the scattering coefficient and OCT imaging depth for a range of Intralipid concentrations at constant water content. We observe an enhanced OCT imaging depth for 1600 nm compared to 1300 nm for Intralipid concentrations larger than 4 vol.%. For higher Intralipid concentrations, the imaging depth enhancement reaches 30%. The ratio of scattering coefficients at the two wavelengths is constant over a large range of scattering coefficients and corresponds to a scattering power of 2.8 ± 0.1. Based on our results we expect for biological tissues an increase of the OCT imaging depth at 1600 nm compared to 1300 nm for samples with high scattering power and low water content.

Coupling of Er ions to surface plasmons on Ag

Er3+ ions located 100 nm beneath the surface of silica glass show an enhanced photoluminescence decay rate when the glass is covered with Ag. Correcting for concentration quenching effects, the decay rate is enhanced by 70%, compared to the case without Ag. The data are in agreement with a model that takes into account variations in local density of states and excitation of surface plasmons and lossy surface waves, resulting in direct evidence for the efficient generation of surface plasmons by excited Er3+ ions. Using the model, optimum conditions for coupling to surface plasmons are derived, which can be used to enhance the emission rate and quantum efficiency of a wide range of Er-doped materials

Integrated system for combined Raman spectroscopy-spectral domain optical coherence tomography

Raman spectroscopy (RS) and optical coherence tomography (OCT) are powerful tools for optical analysis of tissues with mutually complementary strengths and limitations. OCT excels at visualizing tissue microstructure but lacks molecular specificity, while RS can relay tissue biochemical composition but typically cannot relate microstructure. Previous implementations of combined RS-OCT have utilized a common sample arm while maintaining independent RS and OCT detection arms. We present the design and application of an integrated RS-OCT instrument with a common detection arm for both RS and OCT. The detector is a spectrograph capable of sequential detection of the 855-nm OCT signal and the Raman scatter generated by a 785-nm source. The capabilities of the instrument are demonstrated ex vivo in the calvaria and retina of rodents, as well as in vivo in human skin.

Multiple and dependent scattering effects in Doppler optical coherence tomography

Doppler optical coherence tomography (OCT) is a technique to image tissue morphology and to measure flow in turbid media. In its most basic form, it is based on single (Mie) scattering. However, for highly scattering and dense media multiple and concentration dependent scattering can occur. For Intralipid solutions with varying scattering strength, the effect of multiple and dependent scattering on the OCT signal attenuation and Doppler flow is investigated. We observe a non-linear increase in the OCT signal attenuation rate and an increasingly more distorted Doppler OCT flow profile with increasing Intralipid concentration. The Doppler OCT attenuation and flow measurements are compared to Monte Carlo simulations and good agreement is observed. Based on this comparison, we determine that the single scattering attenuation coefficient micros is 15% higher than the measured OCT signal attenuation rate. This effect and the distortion of the measured flow profile are caused by multiple scattering. The non-linear behavior of the single scattering attenuation coefficient with Intralipid concentration is attributed to concentration dependent scattering.

Spectral domain optical coherence tomography imaging with an integrated optics spectrometer

We designed and fabricated an arrayed-waveguide grating (AWG) in silicon oxynitride as a spectrometer for spectral domain optical coherence tomography (SD-OCT). The AWG has a footprint of only 3.0 cm × 2.5 cm, operates at a center wavelength of 1300 nm, and has 78 nm free spectral range. OCT measurements are performed that demonstrate imaging up to a maximum depth of 1 mm with an axial resolution of 19 μm, both in agreement with the AWG design parameters. Using the AWG spectrometer combined with a fiber-based SD-OCT system, we demonstrate cross-sectional OCT imaging of a multilayered scattering phantom.

Determination of the scattering anisotropy with optical coherence tomography

In this work we demonstrate measurements with optical coherence tomography (OCT) of the scattering phase function in the backward direction and the scattering anisotropy parameter g. Measurements of the OCT attenuation coefficient and the backscattering amplitude are performed on calibrated polystyrene microspheres with a time-domain OCT system. From these measurements the phase function in the backward direction is determined. The measurements are described by the single scattering model and match Mie calculations very well. Measurements on Intralipid demonstrate the ability to determine the g of polydisperse samples and, for Intralipid, g = 0.35 ± 0.03 is measured, which is well in agreement with g from literature. These measurements are validated using the Intralipid particle size distribution determined from TEM measurements. Measurements of g and the scattering phase function in the backward direction can be used to monitor changes in backscattering, which can indicate morphological changes of the sample or act as contrast enhancement mechanism.

Toward Spectral-Domain Optical Coherence Tomography on a Chip

We present experimental results of a spectral-domain optical coherence tomography system based on an integrated optical spectrometer. A 195-channel arrayed-waveguide-grating (AWG) spectrometer with 0.4-nm channel spacing centered at 1300 nm and a 125-channel AWG with 0.16-nm channel spacing centered at 800 nm have been fabricated in silicon oxynitride waveguide technology. Interferometric distance measurements have been performed by launching light from a broadband source into a free-space Michelson interferometer, with its output coupled into the AWG. A maximum imaging depth of 1 mm and axial resolution of 25 and 20 μm in air are demonstrated for the 800- and 1300-nm ranges, respectively.

Heartbeat-Induced Axial Motion Artifacts in Optical Coherence Tomography Measurements of the Retina

PURPOSE To investigate the cause of axial eye motion artifacts that occur in optical coherence tomography (OCT) imaging of the retina. Understanding the cause of these motions can lead to improved OCT image quality and therefore better diagnoses. METHODS Twenty-seven measurements were performed on 5 subjects. Spectral domain OCT images at the macula were collected over periods up to 30 seconds. The axial shift of every average A-scan was calculated with respect to the previous average A-scan by calculating the cross-correlation. The frequency spectrum of the calculated shifts versus time was determined. The heart rate was determined from blood pressure measurements at the finger using an optical blood pressure detector. The fundamental frequency and higher order harmonics of the axial OCT shift were compared with the frequency spectrum of blood pressure data. In addition, simultaneous registration of the movement of the cornea and the retina was performed with a dual reference arm OCT setup, and movements of the head were also analyzed. RESULTS A correlation of 0.90 was found between the fundamental frequency in the axial OCT shift and the heart rate. Cornea and retina move simultaneously in the axial direction. The entire head moves with the same amplitude as the retina. CONCLUSIONS Axial motion artifacts during OCT volume scanning of the retina are caused by movements of the whole head induced by the heartbeat.