Showing papers by "Li Li published in 2007"

Heat generation and thermal lensing in end-pumped Tm,Ho: YLF laser crystals

Abstract: Thermal lensing in an end-pumped Tm,Ho:YLF laser crystal has been investigated with a theoretical model of continuous-wave quasi-three-level laser. Considering energy transfer upconversion (ETU) and ground-state reabsorption, the rate equations are given. The influence of ETU on the fractional thermal loading is calculated, and the results show that the latter critically depends on the pump-to-mode size ratio. The temperature distributions of a Tm,Ho : YLF crystal for different pump powers have been analysed. The output power and the focal length of the thermal lens as a function of pump power are obtained in experiment. The experimental results are compared with the theoretical results, and the experimental results show that the theoretical results are reasonable.

Microring resonator-coupled Mach-Zehnder interferometer as trigger pulse generator, optical differentiator, and integrator

Abstract: Microring resonator-coupled Mach-Zehnder interferometer (MRCMZI) is attractive for its applications in the field of all-optical switch, with low threshold, fast response speed and compact scale suitable for integration. The paper provides analytically and numerically investigation on optical pulse transmission characteristics of MRCMZI. The analytical formulations of dynamic process are derived in an easy-understood method of light beam tracing. These formulations can degenerate gradually to steady-state response when incident pulse becomes quasi-continuous wave (CW) or adequately broad with duration much more than cavity lifetime. For the coupled-microresonator system, several interesting signal processing tasks, such as optical differentiation, integration, pulse compression and trigger pulse generation, can be realized. Because of the complementarity between two output ports of MZI, the optical pulse differentiation and integration operations can be carried out synchronously at two output ports through optical interference between the reference light of direct-bus and the modulated light of coupled-microresonator bus. The interesting functions related to optical pulse shaping have potential applications in future all-optical communication.

Tunable bistability and asymmetric line shape in ring cavity-coupled Michelson interferometer

Abstract: A novel configuration of ring cavity-coupled Michelson interferometer is proposed to create sharp asymmetric multiple-resonance line shape, in which a ring cavity is side-coupled to one arm and a phase shifter is introduced into the other arm for static phase compensation. Such asymmetric line shape allows the tuning of the system between zero and complete transmission, with a phase offset much narrower than the full width of the cavity resonance itself. As tuning between resonance peak and notch of such asymmetric profile, optical transmission becomes much more sensitive to the round-trip phase shift of ring cavity than that in the case of symmetric Lorentzian line shape. By cooperating Kerr nonlinearity and cavity feedback, novel hysteresis loops and intrinsic bistability are achievable by adjusting incident power. The shapes of hysteresis curves associated with asymmetric resonance line shape are different from those arising from symmetric line shape. By adjusting the static phase compensation of phase shifter, tunable hysteresis loop and asymmetric multiple-resonance transmission can be easy performed. The simply constructed device is a good reference for sensitive optical switch, filter and sensor.

Intrinsic optical bistability in Tm-doped laser crystal pumped at 648nm avalanche wavelength

Abstract: Intrinsic optical bistability and dynamic hysteresis phenomenon, induced by nonlinear excitation and energy level coupling, are predicted theoretically in single Tm-doped laser crystal pumped at 648nm avalanche wavelength. Taking into account the dominant energy transfer processes including excited state absorption, cross relaxation, energy transfer up-conversion and decay of the metastable level Tm 3+ 3 F 4 , the analytical formula of avalanche threshold condition are deduced in the steady-state approximation. Based on the theory of microscopic coupled rate equations, intrinsic optical bistability and influence of system parameters on hysteresis loop are studied numerically in detail by using a four-order Runge-Kutta technique. The numerical results predict that intrinsic optical bistability of near 2 micrometer fluorescence emission relevant to Tm 3 F 4 → 3 H 6 transition is achievable experimentally in single Tm-doped laser crystal. For realization of low threshold optical bistability and notable bistable hysteresis loop, it is beneficial to properly increase Tm 3+ -doped concentration and suppress energy transfer up-conversion. Furthermore, the interesting results obtained numerically show that laser-induced thermal effect is not the indispensable factor for occurrences of intrinsic optical bistability in rear-earth-doped crystal. Cooperation of nonliear excitation and energy level coupling can essentially lead to intrinsic optical bistability in rear-earth-doped system.

Theoretical and experimental investigation of thermal effect in end-pumped Tm,Ho:YLF lasers

Abstract: Thermal lensing in an end-pumped Tm,Ho:YLF laser crystal has been investigated with a theoretical model of continuous-wave (CW) quasi-three-level laser. Under considering energy transfer up-conversion (ETU) and ground state re-absorption (GSA), the rate equations are given. The influence of ETU on fractional thermal loading is calculated, and the results show that fractional thermal loading critically depends on the pump-to-mode size ratio. The temperature distributions of a Tm,Ho:YLF crystal under different pump powers have been analyzed. The output power and the focal length of the thermal lens as a function of pump power are obtained in experiment. The experimental results are compared with theoretical results, and the experimental results show that the theoretical results are reasonable.

端面抽运Tm, Ho:YLF激光器热转换系数及热透镜效应的研究

Abstract: 根据激光二极管纵向抽运Tm,Ho:YLF激光器的能级跃迁和能量传递过程，在考虑能量传递上转换和基态重吸收的前提下，建立了准三能级速率方程理论模型.给出了Tm,Ho:YLF激光器的热转换系数的解析表达式，讨论了抽运光斑半径与激光束腰半径的比值对热转换系数的影响.实验上得到了热焦距随抽运功率的变化关系，并将其与理论结果进行了对比，发现符合较好.

Bistable chromatic switching in 648 nm laser pumped Tm-doped crystal

Abstract: Intrinsic optical bistability is of great interest and has potential applications in all-optical communication. In the present work, we have extended occurrences of bistable chromatic switching to a new rare-earth-doped system, namely single Tm-doped laser crystal pumped at 648 nm avalanche wavelength. Intrinsic bistability and chromatic switching of fluorescence emission are theoretically predicted, based on the rate equation theory. By using a four-order Runge-Kutta technique, chromatic switching spectra and bistable hysteresis loop are studied numerically in detail. The numerical results show that intrinsic chromatic switching of visible-infrared spectra is achievable experimentally corresponding to the transitions of Tm 1D2→3H6 (357 nm), 1G4→3H6 (469 nm), 3H4→3H6 (787 nm), 3F4→3H6 (1716 nm), as pump power increases far beyond avalanche threshold from low power level. Furthermore, we have found that the chromatic switching appears bistable in the emission spectra for pump intensity in the vicinity of avalanche threshold. The bistable hysteresis loops of luminescence are also demonstrated numerically for the dominant emission band. Intrinsic bistable chromatic switching in Tm-doped crystal is mediated by resonant excited-state absorption up-conversion.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Investigation of thermal effects in longitudinally diode-pumped Tm,Ho:YLF lasers

Abstract: Under considering energy transfer up-conversion (ETU) and ground state re-absorption (GSA), the rate equations of the Tm,Ho:YLF laser are given. The influence of ETU on fractional thermal loading is calculated for the continuous wave and Q-switched Tm,Ho:YLF lasers and the results show that the fractional thermal loading critically depends on the pump-to-mode size ratio. Furthermore, the fractional thermal loading depends on the pulse repetition frequency for a Q-switched laser. The temperature distributions of a Tm,Ho:YLF crystal under different pump powers have been analyzed. The thermal focal length as a function of pump power is calculated.

Influence of energy-transfer up-conversion on diode-end-pumped Q-switched Tm,Ho:YLF lasers

Abstract: The influence of energy-transfer up-conversion (ETU) on diode-end-pumped actively Q-switched Tm,Ho:YLF lasers is investigated by the rate equation analysis. The theoretical results show that the energy-transfer up-conversion reduces not only pulse energy but also effective upper level life. The practical example of the diode-end-pumped Q-switched Tm,Ho:YLF laser has been used to verify the present model.

All-optical switch with low threshold over a wide wavelength range by use of a Mach-Zehnder racetrack resonator

Abstract: This paper presents a theoretical investigation of an all-optical switch by using a Mach–Zehnder racetrack resonator. The switch can realize nonlinear optical switching between steady ON/OFF states at about 200 microwatt low threshold over a wide wavelength range. The finesse of the racetrack resonator is adjustable by controlling the coupling ratio of the Mach–Zehnder coupler. Some undesirable effects are discussed. Attenuation, group delay and partial coherence result in a switching power increase and an extinction ratio reduction. Optical bistability and instability do not limit the practical applications of the device in optical switching. A desired tradeoff between bandwidth/response time and switching power can be obtained by adjusting the finesse. Picosecond ultrafast optical switching is predicted.

Optical bistability in an end-pumped Tm,Ho:YLF laser at room temperature

Abstract: We demonstrate strong optical bistability in 2μm continuous wave Tm,Ho:YLF laser pumped by a 792nm laser diode near room temperature. The bistable region is as much as 100mW wide at 283K and can be controlled by the temperature of the laser crystal. The influence of crystal temperature on the characteristics of optical bistability is obtained. The influence of the pump-to-mode ratio on the bistable characteristics of the laser is also discussed.

Resonator-enhanced low power all-optical switch with a nonlinear ratio-variable coupler

Abstract: One of the major challenges in realizing practical all-optical switching devices is the need for strong and fast material nonlinearity. The use of resonance-enhanced nonlinear effects in optical cavities has been explored as a promising way to lower the switching power. This paper provides numerically investigation of resonator-enhanced Mach-Zehnder interferometer all-optical switch by employing a nonlinear ratio-variable coupler with the merits of easy fabrication and excellent nonlinear optical controllability. This performs nonlinear optical switching with advantages of hundreds tW low switching threshold, steady switch ON/OFF states and a wide wavelength range by combining the high nonlinearity of ring and adjustable coupling ratio of RVC. Resonator finesse is controllable by incident power with potential applications in controllable photonic devices. Some undesirable effects involving attenuation, group delay and partial coherence lead to increase of switching power and low extinction ratio. Optical bistability and instability do not limit practical applications in optical switch. A desired tradeoff between response time and switching power can be achieved by controlling finesse. Picosecond ultrafast all-optical switch is predicted.