Jinendra Kumar Ranka

Visible continuum generation in air–silica microstructure optical fibers with anomalous dispersion at 800 nm

We demonstrate experimentally for what is to our knowledge the first time that air–silica microstructure optical fibers can exhibit anomalous dispersion at visible wavelengths. We exploit this feature to generate an optical continuum 550 THz in width, extending from the violet to the infrared, by propagating pulses of 100-fs duration and kilowatt peak powers through a microstructure fiber near the zero-dispersion wavelength.

Ultrahigh-resolution optical coherence tomography using continuum generation in an air–silica microstructure optical fiber

We demonstrate ultrahigh-resolution optical coherence tomography (OCT) using continuum generation in an air-silica microstructure fiber as a low-coherence light source. A broadband OCT system was developed and imaging was performed with a bandwidth of 370 nm at a 1.3-mu;m center wavelength. Longitudinal resolutions of 2.5 microm in air and ~2 microm in tissue were achieved. Ultrahigh-resolution imaging in biological tissue in vivo was demonstrated.

Optical properties of high-delta air–silica microstructure optical fibers

We analyze the waveguide properties of microstructure optical fibers consisting of a silica core surrounded by a single ring of large air holes. Although the fibers can support numerous transverse spatial modes, coupling between these modes even in the presence of large perturbations is prevented for small core dimensions, owing to a large wave-vector mismatch between the lowest-order modes. The result is an optical fiber that can appear single mode with propagation properties that can be achieved only in multimode waveguides.

Direct RF to optical frequency measurements with a femtosecond laser comb

By spanning an optical octave (>300 THz) with a broadened femtosecond laser frequency comb, we directly measure optical frequency standards at 1064/532 nm, 633 nm and 778 nm in terms of the microwave frequency that controls the comb spacing.

Measuring the intensity and phase of ultrabroadband continuum

Summary form only given.We have developed a technique for measuring the continuum generated using microstructure optical fiber and have shown that the continuum has well-behaved spectral phase in a /spl sim/25-nm-bandwidth region near 850 nm. Our technique is based on the Temporal Analysis by Dispersing a Pair Of Light E-fields (TADPOLE) and FROG techniques. Our laser is capable of 20-fs pulses with /spl sim/100-nm bandwidths, so about a dozen straightforward measurements could yield the spectral phase for the entire continuum, whether uncompressed or compressed to a single cycle, without the need for extremely thin crystals.

High-resolution optical coherence tomography using self-adaptive FFT and array detection

We developed a novel optical coherence tomographic (OCT) system which utilized broadband continuum generation for high axial resolution and a high numeric-aperture (N.A.) Objective for high lateral resolution (<5 micrometers ). The optimal focusing point was dynamically compensated during axial scanning so that it can be kept at the same position as the point that has an equal optical path length as that in the reference arm. This gives us uniform focusing size (<5 mum) at different depths. A new self-adaptive fast Fourier transform (FFT) algorithm was developed to digitally demodulate the interference fringes. The system employed a four-channel detector array for speckle reduction that significantly improved the images signal-to-noise ratio.

Long-term locking of the carrier-envelope phase in a 15-fs Ti:S laser using a self-referencing frequency domain technique

Summary form only given. Shortly after generation of optical pulses approached the few cycle regime, interest was sparked in locking the relative phase between the carrier frequency and the pulse amplitude envelope. In general the relative phase in successive pulses generated by mode-locked lasers is not constant due to a difference between the group and phase velocities inside the cavity. To date nearly all techniques of phase control of femtosecond (fs) pulses have utilized time domain methods. However, these techniques have not utilized active feedback and rapid dephasing occurs due pulse energy fluctuations inside the cavity. Using a self-referencing technique we have successfully stabilized this offset frequency, and thus the relative carrier-envelope phase, to an uncertainty as low as 10-5 radians in one second. Our experimental setup is a Kerr-lens mode-locked Ti:sapphire laser which generates an output pulse of roughly 15 fs with an average power of 300 mW.

Ultrahigh-resolution OCT using continuum generation in an air-silica microstructure optical fiber

We demonstrate ultrahigh resolution optical coherence tomography using the continuum generation in an air-silica microstructure fiber. A broadband OCT system was developed, supporting a bandwidth of 370 nm at 1.3 micrometers center wavelength. We achieved longitudinal resolutions of 2.5 micrometers in air, or ~2 micrometers in tissue. This is to our knowledge with the highest longitudinal OCT resolution demonstrated at this wavelength range and the first application of this new light source for OCT. We will also describe the application of this technique for imaging biological tissue in vivo.

Precise control of the pulse-to-pulse carrier-envelope phase in a modelocked laser

Using a coherent frequency domain technique, we lock the relative pulse-to-pulse carrier-envelope phase of a 15-fs pulse produced by a mode-locked Ti:Sapphire laser to various rational integer fractions of 21π.