Michael Mei

Optical sampling by laser cavity tuning

Most time-resolved optical experiments rely either on external mechanical delay lines or on two synchronized femtosecond lasers to achieve a defined temporal delay between two optical pulses. Here, we present a new method which does not require any external delay lines and uses only a single femtosecond laser. It is based on the cross-correlation of an optical pulse with a subsequent pulse from the same laser. Temporal delay between these two pulses is achieved by varying the repetition rate of the laser. We validate the new scheme by a comparison with a cross-correlation measurement carried out with a conventional mechanical delay line.

Imaging ex vivo and in vitro brain morphology in animal models with ultrahigh resolution optical coherence tomography

The feasibility of ultrahigh resolution optical coherence tomography (UHR OCT) to image ex vivo and in vitro brain tissue morphology on a scale from single neuron cells to a whole animal brain was investigated using a number of animal models. Sub-2-microm axial resolution OCT in biological tissue was achieved at different central wavelengths by separately interfacing two state-of-the-art broad bandwidth light sources (titanium:sapphire, Ti:Al2O3 laser, lambdac=800 nm, Deltalambda=260 nm, Pout=50 mW and a fiber laser light source, lambdac=1350 nm, Deltalambda=470 nm, Pout=4 mW) to free-space or fiber-based OCT systems, designed for optimal performance in the appropriate wavelength regions. The ability of sub-2-microm axial resolution OCT to visualize intracellular morphology was demonstrated by imaging living ganglion cells in cultures. The feasibility of UHR OCT to image the globular structure of an entire animal brain as well as to resolve fine morphological features at various depths in it was tested by imaging a fixed honeybee brain. Possible degradation of OCT axial resolution with depth in optically dense brain tissue was examined by depositing microspheres through the blood stream to various depths in the brain of a living rabbit. It was determined that in the 1100 to 1600-nm wavelength range, OCT axial resolution was well preserved, even at depths greater than 500 microm, and permitted distinct visualization of microspheres 15 microm in diameter. In addition, the OCT image penetration depth and the scattering properties of gray and white brain matter were evaluated in tissue samples from the visual cortex of a fixed monkey brain.

Low Phase Noise 250 MHz Repetition Rate Fiber fs Laser for Frequency Comb Applications

Er3+ -doped fiber-lasers efficiently generate frequency-combs for optical frequency metrology. We have for the first time resolved the two main concerns against fiber lasers vs the well established Ti:Sa technology by increasing the repetition rate to 250 MHz and decreasing the carrier envelope offset phase noise well below 1 rad.

THz Time-Domain Spectrometer Based on LT-InGaAs Photoconductive Antennas Exited by a 1.55 μm Fibre Laser

We present a THz time-domain spectrometer based on a 1.55 mum fibre laser and LT- InGaAs/InAlAs MQW photoconductive antennas. We discuss the system stability and present first spectroscopic data taken with the system.

THz Photoconductive Antennas for 1.55 µm Telecom Wavelengths

We present low temperature grown InGaAs multiple-quantum well structures which are successfully applied as transmitters and receivers in THz time domain systems. Operation at telecom wavelengths of 1.55µm with a femtosecond fiber laser is demonstrated.

Imaging brain morphology with ultrahigh-resolution optical coherence tomography

The morphology of healthy and pathological human brain tissue, as well as the brain structural organization of various animal models has been imaged in-vitro using ultrahigh resolution optical coherence tomography (UHR OCT). Micrometer-scale OCT resolution (< 2 μm axial resolution) was achieved at different central wavelengths by interfacing three state-of-the-art broad bandwidth light sources (Ti:Al2O3, λc = 790 nm, Δλ = 260 nm and Pout = 50 mW; PCF based laser, λc = 1150 nm, Δλ = 350 nm and Pout = 2 W; Fiber laser based light source, λc = 1350 nm, Δλ = 470 nm and Pout = 4 mW) to a modular free-space OCT system, utilizing a dynamic focusing and designed for optimal performance in the appropriate wavelength regions. Images acquired from a fixed honeybee brain demonstrated the ability of UHR OCT to image the globular structure of the brain, some fine morphological details such as the nerve fiber bundles connecting the medulla (visual center) to the honeybee eyes, and the interfaces between different tissue layers in the medulla. Tomograms of various human neuropathologies demonstrated the feasibility of UHR OCT to visualize morphological details such as small (~20 μm) calcifications typical for fibrous meningioma, and enlarged nuclei of cancer cells (~10-15 μm) characteristic for many other neuropathologies. In addition UHR OCT was used to image cellular morphology in living ganglion cells.

Ultrafast Lasers and Frequency Combs for Industrial Applications

We have developed compact ultrafast laser sources for a wide variety of applications, including quality control and optical frequency combs for precision measurements. Fields ranging from astronomical observatories to spaceflights, optical clocks or long distance links are served with this technology.

Rapid scan mode with THz OSCAT spectrometer

We present OSCAT laser systems as a general tool for pump and probe experiments. The current development offers unlimited temporal scanning range using the intracavity motor and a rapid scan mode with a range of 100 ps.

THz time domain spectroscopy based on OSCAT scheme

We present an all fiber THz spectrometer based on a novel scanning technique without any external moveable delay stage. The optical delay between the pump and probe pulses and thus the scanning range is controlled via the repetition rate of the laser source.

Ultrahigh-resolution optical coherence tomography in the visible and 1300-nm wavelength region

Ultrahigh axial resolution OCT is demonstrated in human cells and other human biopsies for two fiber broadened femtosecond light sources, achieving 0.5μm axial resolution in the visible and 1.4μm in the in the 1300nm wavelength region.