Jin U. Kang

Sagnac loop interferometer based on polarization maintaining photonic crystal fiber with reduced temperature sensitivity

A Sagnac loop interferometer based on polarization-maintaining photonic crystal fiber was built and analyzed. Mainly the temperature dependence of the Sagnac loop filter function was measured and analyzed. By measuring the filtering function of the Sagnac loop as a function of the temperature over 200 degrees C, we deduced an unambiguous temperature dependent birefringence coefficient, d n/dT = 2.0 x 10-9 /K. Over the full temperature swing, the maximum peak shifts was less than 10% of the filter period. For comparison, a standard Sagnac loop was built with the exact same length and experimental condition, where we deduced d n/dT = 7.0 x 10-8 /K.

Norepinephrine Controls Astroglial Responsiveness to Local Circuit Activity

Astrocytes perform crucial supportive functions, including neurotransmitter clearance, ion buffering, and metabolite delivery. They can also influence blood flow and neuronal activity by releasing gliotransmitters in response to intracellular Ca(2+) transients. However, little is known about how astrocytes are engaged during different behaviors in vivo. Here we demonstrate that norepinephrine primes astrocytes to detect changes in cortical network activity. We show in mice that locomotion triggers simultaneous activation of astrocyte networks in multiple brain regions. This global stimulation of astrocytes was inhibited by alpha-adrenoceptor antagonists and abolished by depletion of norepinephrine from the brain. Although astrocytes in visual cortex of awake mice were rarely engaged when neurons were activated by light stimulation alone, pairing norepinephrine release with light stimulation markedly enhanced astrocyte Ca(2+) signaling. Our findings indicate that norepinephrine shifts the gain of astrocyte networks according to behavioral state, enabling astrocytes to respond to local changes in neuronal activity.

The nonlinear optical properties of AlGaAs at the half band gap

We report experimental values for the nonlinear optical coefficients of AlGaAs, in the half-band-gap spectral region. The dispersion of the nonlinear refractive-index coefficient, n/sub 2/, is measured for both TE- and TM-polarized light. We observe n/sub 2/(TE)>n/sub 2/(TM) and a ratio of cross-phase modulation to self-phase modulation (TE) of /spl sim/0.95, as predicted from band structure calculations. The spectral dependence of the two- and three-photon absorption coefficients are also measured. Finally, the implications for all-optical switching and spatial soliton propagation are discussed.

Performance of a time- and wavelength-interleaved photonic sampler for analog-digital conversion

We experimentally demonstrate the sampling of microwave signals using a novel time- and wavelength-interleaved pulse train derived from a mode-locked fiber laser. Experimental results are presented indicating a modulator limited bandwidth of 18 GHz and a laser relative intensity noise limited effective number of bits of /spl sim/7 when tested for use in a hybrid photonic analog-digital converter architecture.

Multiwavelength Raman fiber-ring laser based on tunable cascaded long-period fiber gratings

Multiwavelength Raman fiber lasers using intracavity tunable cascaded long-period fiber gratings (LPFGs) are discussed. The application of the cascaded LPFGs as a multichannel fiber filter for the multiple-wavelength generation is proposed and experimentally demonstrated. The characteristics of multiwavelength fiber-ring lasers can be controlled by changing the physical parameters of cascaded LPFGs such as the separation distance between the gratings, grating length, and number of gratings. The multiwavelength Raman fiber-ring laser with nine wavelength-division-multiplexing channels with 100-GHz spacing and 19 channels with 50-GHz spacing has been achieved by varying the physical parameters of cascaded LPFGs.

Simultaneous measurement of temperature and strain using dual long-period fiber gratings with controlled temperature and strain sensitivities

Unambiguous simultaneous measurement of strain and temperature based on dual long-period fiber gratings by controlling their thermal and strain sensitivities is proposed and experimentally demonstrated. The difference in the wavelength peak shift and the separation with the variation of strain and temperature allows discrimination between the strain and temperature effects, respectively.

Real-time 4D signal processing and visualization using graphics processing unit on a regular nonlinear-k Fourier-domain OCT system.

We realized graphics processing unit (GPU) based real-time 4D (3D + time) signal processing and visualization on a regular Fourier-domain optical coherence tomography (FD-OCT) system with a nonlinear k-space spectrometer. An ultra-high speed linear spline interpolation (LSI) method for λ-to-k spectral re-sampling is implemented in the GPU architecture, which gives average interpolation speeds of >3,000,000 line/s for 1024-pixel OCT (1024-OCT) and >1,400,000 line/s for 2048-pixel OCT (2048-OCT). The complete FD-OCT signal processing including λ-to-k spectral re-sampling, fast Fourier transform (FFT) and post-FFT processing have all been implemented on a GPU. The maximum complete A-scan processing speeds are investigated to be 680,000 line/s for 1024-OCT and 320,000 line/s for 2048-OCT, which correspond to 1GByte processing bandwidth. In our experiment, a 2048-pixel CMOS camera running up to 70 kHz is used as an acquisition device. Therefore the actual imaging speed is camera-limited to 128,000 line/s for 1024-OCT or 70,000 line/s for 2048-OCT. 3D Data sets are continuously acquired in real time at 1024-OCT mode, immediately processed and visualized as high as 10 volumes/second (12,500 A-scans/volume) by either en face slice extraction or ray-casting based volume rendering from 3D texture mapped in graphics memory. For standard FD-OCT systems, a GPU is the only additional hardware needed to realize this improvement and no optical modification is needed. This technique is highly cost-effective and can be easily integrated into most ultrahigh speed FD-OCT systems to overcome the 3D data processing and visualization bottlenecks.

C-band wavelength-swept single-longitudinalmode erbium-doped fiber ring laser.

A wavelength-swept single-longitudinal-mode erbium-doped fiber ring laser capable of operating at sweeping frequency in the order of a few kHz is designed and demonstrated by using a fiber Fabry-Perot tunable filter and a Sagnac loop incorporated with a 3.5-meter unpumped erbium-doped fiber. The laser operates in continuous-wave (CW) mode and can sweep approximately 45 nm over the entire C-band (1520nm-1570nm) window with linewidth less than 0.7 kHz. The optimum wavelength sweeping frequency in order to achieve the best output power stability was found to be approximately20Hz with sweeping-induced power fluctuation of only 0.1%.

Highly stable tunable dual-wavelength Q-switched fiber laser for DIAL applications

We demonstrate a stable dual-wavelength Q-switched fiber laser for applications in differential absorption Lidar with tunability in absolute wavelength over several nanometers and discrete tuning in wavelength spacing from 1.1 to 3.3 nm. Single longitudinal-mode operation is also achieved by employing Sagnac loop saturable absorber filter. The delayed self homodyne linewidth measurement of the dual-wavelength continuous-wave laser shows a very narrow spectral linewidth and frequency jitter (<40 kHz). The advantages of tunable wavelengths, narrow linewidth, temperature insensitivity, and high stability in an efficient, cost effective, rugged, compact and light system make it a promising technology for airborne Lidar systems.

Linearized Mach-Zehnder intensity modulator

A novel configuration of electrooptic intensity modulator based on a Mach-Zehnder interferometer with ring resonators coupled to each arm is proposed and analyzed theoretically. It is shown that this configuration offers a large improvement in modulation linearity with no requirements for complicated and precise electrical control.