Yao-Hui Chen

Manipulation of Random Lasing Action From Dye-Doped Liquid Crystals Infilling Two-Dimensional Confinement Single Core Capillary

The experimental detail of a random lasing action from dye-doped nematic liquid crystals (LCs) inside single-core capillaries with a core diameter below 50 μm was investigated. The resonant characteristics, including the number of emission spikes, the full width at half maximum (FWHM), and the estimated Q-factor, were shown to depend on the core diameter of a capillary. In contrast with a capillary with a larger core diameter and having various emission spikes, only three emission spikes were excited from a capillary with a 10-μm core diameter, owing to the smaller effective area of the pump beam. However, the decrease in the amplitude of emission spikes and the broadening of the linewidth accompanying the higher lasing threshold from a capillary with a 50-μm core diameter are attributed to the decrease in the pump fluence and the increase in the scattering loss, respectively. In this paper, a random laser (RL) with the shortest FWHM emission peak of about 0.47 nm and the highest Q-factor of about 1268 was generated from the capillary with a 20-μm core diameter. By means of temperature adjustment, the emission spectra of the RL that is related to the birefringence and alignment ordering of the LC molecules inside a capillary with a 20-μm core diameter can be effectively altered. Our experiments show that the RL, revealing adjustable output emission spectra, can be a promising device in using remote sensing applications.

Multiwavelength Laser With Adjustable Ultranarrow Wavelength Spacing

This paper investigates the use of a dual wavelength laser source, a frequency shifter, an erbium-doped fiber amplifier, and an optical filter to compose a multiwavelength fiber laser (MWFL) with an ultranarrow wavelength spacing. The frequency shifter can be achieved by modulating an optical carrier and an electrical sinusoidal signal in an optical single-sideband modulation format. The detuning range of the frequency shifting can be controlled by the applied electrical sinusoidal signal, and an MWFL with ultranarrow wavelength spacing can be achieved. Experimental results prove that 144 stable optical carriers can be generated simultaneously with a 10-GHz wavelength spacing, > 15-dB optical signal-to-noise ratio, and <; 0.83-dB maximum peak power variation, if a dual wavelength laser source is fed into the MWFL and the frequency of the electrical sinusoidal signal is set to 10 GHz. Results prove that the ability of the proposed scheme practically and efficiently assist the progress of optical systems.

Investigation of noise-like pulses from a net normal Yb-doped fiber laser based on a nonlinear polarization rotation mechanism.

We investigated the characteristics of noise-like pulses (NLPs) from a net normal dispersion Yb-doped fiber laser (YDFL) by using the grating pairs (GPs) inside the laser cavity as a dispersion compensation element. Without the insertion of the slit inside the laser cavity, the operation of the YDFL is at an NLP state with the double-scale intensity autocorrelation trace once the mode-locked pulses are generated. Through the dispersion delay line outside the laser cavity, the substantial temporal compression of the NLPs has been demonstrated. After inserting the slit between the GPs as a bandpass filter, the operation state of the YDFL can be switched between the NLPs and the dissipated solitons by means of a pump power. Besides, the NLPs can also transit to the bound solitons as the YDFL is operated within long and short wavelength regimes through the spatial shift of the slit.

Investigation of laser speckle noise suppression by using polymer-stabilized liquid crystals within twisted nematic cell

We propose a robust method to suppress laser speckle using a polymer-stabilized liquid crystal (PSLC) device with high initial transmittance. With applied voltage, a large modulation depth has been produced through light scattering because of the refractive index mismatch between the rotated nematic liquid crystals and the polymer networks. By using PSLCs with 5 wt % monomer, a speckle noise reduction rate of approximately 54.7% can be achieved with an applied voltage of 6 V. The lowest speckle contrast of approximately 0.025 with relatively high discrimination from the projected AF image has also been demonstrated through a wedge PSLC cell.

Coherent acoustic phonon oscillation accompanied with backward acoustic pulse below exciton resonance in a ZnO epifilm on oxide-buffered Si(1 1 1)

Unlike coherent acoustic phonons (CAPs) generated from heat induced thermal stress by the coated Au film, we demonstrated the oscillation from c-ZnO epitaxial film on oxide buffered Si through a degenerate pump–probe technique. As the excited photon energy was set below the exciton resonance, the electronic stress that resulted from defect resonance was used to induce acoustic wave. The damped oscillation revealed a superposition of a high frequency and long decay CAP signal with a backward propagating acoustic pulse which was generated by the absorption of the penetrated pump beam at the Si surface and selected by the ZnO layer as the acoustic resonator.

Room temperature excitonic dynamics of non-polar a-plane ZnO epifilms

Pump polarization dependent carrier dynamics, particularly excitonic dynamics, of non-polar a-plane zinc oxide (ZnO) epifilms with two different thicknesses were investigated using time resolved measurements. Unlike the electron and hole dynamics through the above-bandgap excitation, transient differential reflectance (TDR) traces revealed similar trends under two orthogonal pump polarization conditions relative to the c-axis (Epu⊥c and Epu∥c) of a-ZnO around near-exciton-resonance excitation. By means of a band diagram, the bandgap renormalization (BGR) effect can be reasonably explained by the screening of the Coulomb potential energy due to the accumulation of relaxed free carriers that were initially excited through the absorption of two cascaded pump photons via the excitonic level, a process known as two photon absorption (TPA). Thus, the modulation depths of the TPA around zero time delay, due to simultaneous absorption of one pump and one probe photon via the excitonic level, increased linearly wi...

Bound States of Dispersion-Managed Solitons From Single-Mode Yb-Doped Fiber Laser at Net-Normal Dispersion

The bound states of multiple dispersion-managed solitons (DMSs) were experimentally investigated from a net-normal-dispersion passive mode-locked Yb-doped fiber laser based on the nonlinear polarization rotation mechanism through the proper adjustment of the waveplate at certain pump power. The long-term operation of the bound double DMSs has been experimentally verified and reveals the possible application in the fields of optical communication. In addition, we observed multiple bound solitons and the bound states of the multiple solitons whose intensity autocorrelation traces were similar to that operating at single and double bound soliton states. By means of the grating pairs at the outside laser cavity for pulse compression, the bound states of multiple solitons below 1 ps can be experimentally retrieved. The observed results in this paper illustrate that the passive mode-locked fiber laser around zero cavity dispersion can provide the platform to explore the fundamental physics of soliton dynamics.

Electrical manipulation of dye-doped liquid crystal random laser within photonic crystal fiber

Random lasers (RLs) have been extremely studied and attracted a great attention over several decades [1] owing to its unique characteristic that can be used in speckle noise suppression [2], biomedical diagnosis [3] or submicron optical lithography. Unlike conventional laser, RLs are produced by the multiple light scattering within disordering scattering material [4] that reveals the characteristics of multiple emission spikes on the broad spectrum. Because of the intrinsic birefringence, LCs are superior candidate for the random laser generation that have been reported inside the capillary tube and manipulated by the temperature [5]. In this work, we demonstrated the electric manipulation of dye-doped liquid crystals (DD-LCs) inside photonic crystal fiber (PCF).

Temperature tuning of lasing emission from dye-doped liquid crystal at intermediate twisted phase

Temperature tuning of lasing emission from dye-doped cholesteric liquid crystal (CLC) at intermediate twisted phase has been demonstrated in this work. With heavily doping of 42.5% chiral molecules into the nematic liquid crystals, the shifts of photonic bandgap versus temperature is obviously as thermal controlling of the sample below the certain value. By the differential scanning calorimetr measuremet, we demonstrate the phase transition from the CLC to the smectic phase when the temperature is lowered to be about 15°C. Between CLC and smectic phase, the liquid crystal mixtures are operated at intermediate twisted phase that can be used the temperature related refractive mirror. After pump by the Q-switched Nd:YAG laser, the lasing emission from this dye doped LC mixtures has been demonstrated whose emission wavelength can be tuned from 566 to 637 nm with 1.4°C variation.

Temperature dependent color cone lasing in cholesteric liquid crystal

We investigate the color cone lasing behavior from dye-doped cholesteric liquid crystal (CLC) and find the central wavelength as well as intensity contrast of long and short emission peaks will change as temperature varies.