Claire Luo

Broadband IR supercontinuum generation using single crystal sapphire fibers.

In this paper, an investigation on broadband IR supercontinuum generation in single crystal sapphire fibers is presented. It is experimentally demonstrated that broadband IR supercontinuum spectrum (up to 3.2microm) can be achieved by launching ultra-short femtosecond laser pulses into single crystal sapphire fiber with a dimension 115microm in diameter and 5cm in length, which covers both the near IR spectral region and the lower end of the mid-IR spectral range. Furthermore, the mechanism of supercontinuum generation in single crystal sapphire fibers is briefly addressed. When the fiber length is shorter than the dispersion length, the self-phase modulation dominates the broadening effect. In this case, the broad supercontinuum spectrum with a smooth profile can be obtained. However, when the fiber length is longer than the dispersion length, the soliton-related dynamics accompanied by the self-phase modulation dominates the broadening effect. There are discrete spikes in the spectrum (corresponding to different order solitons). The above assumption of supercontinuum generation mechanism is quantitatively modeled by the computer simulation program and verified by the experimental results. Thus, one can adjust the spectral profile by properly choosing the length of the sapphire fibers. The broad IR spectral nature of this supercontinuum source can be very useful in a variety of applications such as broadband LADAR, remote sensing, and multi-spectrum free space communications.

Surface enhanced Raman spectroscopy by interfered femtosecond laser created nanostructures

A type of surface enhanced Raman spectroscopy (SERS) by interfered femtosecond laser created nanostructures on Cu metal is presented. It is found out that finer and more uniform nanostructures (with an average feature size 100 nm or smaller) can be created on Cu metal by interfered femtosecond illumination with a phase mask. Significantly enhanced Raman signal (with an enhancement factor around 863) can be realized by using the nanostructured Cu substrate created by the interfered femtosecond laser illumination. The experimentally measured enhancement factor agrees relatively well with the theoretical analyses. Since the nanostructures can be inscribed in real time and at remote locations by the femtosecond laser inscription, the proposed SERS can be particularly useful for the standoff detection of chemicals.

Analysis of terahertz generation via nanostructure enhanced plasmonic excitations

In this paper, we conduct a quantitative study on the physical mechanism of electrons dynamics near the nanostructured metal film surfaces, as well as the efficiency of generated terahertz radiation associated with different types of nanostructures. The simulation results show that although the oscillating motion of emitted electrons outside the metal surface may affect the terahertz generation efficiency to some extent, this efficiency is predominantly determined by the electric field magnitude inside the metal film associated with nanostructure enhanced plasmonic excitations. Due to the field enhancement effect of the nanostructure, an appropriately designed nanostructured surface could greatly enhance the strength of generated terahertz signal via the increased nonlinear interactions between the light and the nanostructures.

Broadband supercontinuum generation covering UV to mid-IR region by using three pumping sources in single crystal sapphire fiber.

In this paper, we demonstrate that the the bandwidth of the supercontinuum spectrum generated in a large mode area sapphire fiber can be enhanced by employing triple pumping sources. Three pumping sources with wavelengths of 784 nm, 1290 nm, and 2000 nm are launched into a single crystal sapphire fiber that is 5 cm in length and has a core diameter of 115 microm. The nonlinear interactions due to self-phase modulation and four-wave mixing form a broadband supercontinuum that covers the UV, visible, near-IR and lower mid-IR regions. Furthermore, we explore the possibility of generating a broadband supercontinuum expanding from the UV to far-IR region by increasing the number of pumping sources with wavelengths in the mid- and far-IR.

High temperature sensing using higher-order-mode rejected sapphire-crystal fiber gratings

In this paper, we report the fabrication of higher-order-mode rejected fiber Bragg gratings (FBGs) in sapphire crystal fiber using infrared (IR) femtosecond laser illumination. The grating is tested in high temperature furnace up to 1600 degree Celsius. As sapphire fiber is only available as highly multimode fiber, a scheme to filter out higher order modes in favor for the fundamental mode is theoretically evaluated and experimentally demonstrated. The approach is to use an ultra thin sapphire crystal fiber (60 micron in diameter) to decrease the number of modes. The small diameter fiber also enables bending the fiber to certain radius which is carefully chosen to provide low loss for the fundamental mode LP01 and high loss for the other high-order modes. After bending, less-than-2-nm resonant peak bandwidth is achieved. The grating spectrum is improved, and higher resolution sensing measurement can be achieved. This mode filtering method is very easy to implement. Furthermore, the sapphire fiber is sealed with hi-purity alumina ceramic cement inside a flexible high temperature titanium tube, and the highly flexible titanium tube offers a robust packaging to sapphire fiber. Our high temperature sapphire grating sensor is very promising in extremely high temperature sensing application.

Innovative long period gratings: principles and applications

In this paper, some recent advances in long period gratings, including (1) widely tunable single resonant band long period gratings, (2) long period gratings fabricated in single crystal sapphire fibers, and (3) long period gratings fabricated in photonic crystal fibers are presented. The applications of these novel long period grating to next generation ultra high capacity optics communication networks, high temperature distributed fiber optic sensors, and highly sensitive biosensors are discussed. Quantitative analyses and preliminary experimental results are provided.

Single chip super broadband InGaN/GaN LED enabled by nanostructured substrate

A new type of LED, single chip super broadband InGaN/GaN LED is presented in this paper. The LED is composed of an InGaN/GaN quantum well layer deposited on the nanostructured sapphire substrate, inscribed by femtosecond laser ablation. The super broadband emission is enabled due to the large variation of indium composition in a small local area so that different wavelengths can be emitted over a small area and the summation of these different emission wavelengths forms super broadband emission, which covers the entire visible spectral range. The result of this paper represents a major technological advance in white light LED lighting because it enables single chip white LED lighting without the need of phosphor down converter that can significantly improve the efficiency without the Stokes loss and reduce the cost.

Ultra-fast speed, low grating lobe optical beam steering using unequally spaced phased array technique

In this paper, we present a low grating lobe optical beam steering technique using unequally spaced phased array, in which the required spacings among phase elements are quantitatively analyzed so that the grating lobe can be minimized by the destructive interference from these unequally spaced phase elements. The large grating lobe is one of the major drawbacks of optical phased array technology, which limits the light efficiency and quality of the light beam. Thus, the low grating lobe technique presented in this paper could substantially improve the light efficiency and the quality of light beam, which may play an important role in a variety of applications such as fast speed ladar beam steering, large size high resolution display, and wide bandwidth free space optics communications.

Mid-IR supercontinuum generation and applications: a review

In this paper, a review on mid-IR supercontinuum generation (SCG) and its applications is presented. First, the physical mechanism of the supercontinuum generation in IR crystal fiber is introduced. Second, the recent progress on IR single crystal fiber, in particular ultrathin core double cladding IR single crystal fiber is described. Third, the transmission characteristics of mid-IR crystal fiber is illustrated. Fourth, the mid-IR supercontinuum generation in IR single crystal fiber is presented. Finally, the application of IR supercontinuum for smart target recognition is illustrated

Super broadband ultrafast waveguide switches based on dynamic waveguiding effect

In this paper, a new type of waveguide switch-field induced dynamic optical waveguide switch is presented. The switching mechanism is based on electric-field induced dynamic waveguiding effect in nanodisordered potassium tantalate niobate (KTN) crystals. By applying an electric field at different locations, different waveguide paths are created, which result in different output locations. The major advantages of this unique optical switch are broad bandwidth, covering the entire 1300 nm – 1600 nm fiber optic communication window, and ultrafast switching speed (on the order of nanosecond), which can be very useful for next generation optical networks such as the one used in data center networks.