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The Review of Laser Engineering

Enhanced Absorption Phenomena in Negative Nonlinear Absorption Effects.

Yoshinobu Maeda, Seiichi IKEDA, Masatoshi Migitaka

Published Jan 1, 1996• 0 citations
Publisher

Abstract

A negative input-output characteristic was obtained in an erbium doped yttrium aluminum garnet crystal which has multiple-level energy structure: absorption can occur between a ground level and an excited level (4I15/2-4I9/2) and between two other excited levels (4I13/2-2H11/2) in Er3+. According as modulation degrees of the incident laser decreased, the transmitted laser waveform was distorted and eventually inverted against the incident one in case that the modulation degree was less than 0.5. It is suggested that an enhanced absorption phenomenon takes place since the excited state absorption from 4I13/2 to 2H11/2 and that a 4S3/2 level plays an important role. Because the time-dependence of this phenomenon matches a life time of the 4S3/2 level. By considering multiple excited state absorption in detail, active functions such as an optical signal inverter could be derived from the negative nonlinear absorption effect.

DOI
10.2184/lsj.24.1197

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A negative input-output characteristic was obtained in an erbium doped yttrium aluminum garnet crystal which has multiple-level energy structure: absorption can occur between a ground level and an excited level (4I15/2-4I9/2) and between two other excited levels (4I13/2-2H11/2) in Er3+. According as modulation degrees of the incident laser decreased, the transmitted laser waveform was distorted and eventually inverted against the incident one in case that the modulation degree was less than 0.5. It is suggested that an enhanced absorption phenomenon takes place since the excited state absorption from 4I13/2 to 2H11/2 and that a 4S3/2 level plays an important role. Because the time-dependence of this phenomenon matches a life time of the 4S3/2 level. By considering multiple excited state absorption in detail, active functions such as an optical signal inverter could be derived from the negative nonlinear absorption effect.
A negative input-output characteristic was obtained in an erbium doped yttrium aluminum garnet crystal which has multiple-level energy structure: absorption can occur between a ground level and an excited level (4I15/2-4I9/2) and between two other excited levels (4I13/2-2H11/2) in Er3+. It was caused at the wavelength between 786.6 and 788.4nm by the variation of incident laser intensity in the intensity range of 60 nW/cm2 to 100 W/cm2. It is considered that an enhanced absorption occurs due to the excited state absorption from 4I13/2 to 2H11/2. Because its spectra match the wavelength dependence of the incident laser. In addition, optical signal inverter phenomenon was observed using a laser diode modulated with 10 MHz. By considering multiple excited state absorption in detail, active functions such as an optical signal inverter could be derived from the negative nonlinear absorption effect.
A negative input-output characteristic was obtained in an erbium-doped yttrium aluminum garnet crystal which has multiple-level energy structure: absorption can occur between a ground level and an excited level (4I−4I) and between two other excited levels (4I−2H) in Er3+. It occurred at a wavelength between 786.6 and 788.4 nm by variation of the incident laser intensity in the intensity range of 60 nW/cm2 to 100 W/cm2. It is considered that an enhanced absorption occurs due to the excited-state absorption from 4I to 2H, its spectra matching the wavelength dependence of the incident laser In addition, optical signal inverter phenomenon was observed using a laser diode modulated at 10 MHz. By considering multiple excited-state absorption in detail, active functions such as an optical signal inverter could be derived from the negative nonlinear absorption effect. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(4): 74–80, 1997
The dependence of the negative nonlinear absorption effect on modulation degree of an incident laser was investigated in an erbium doped yttrium aluminum garnet crystal which has a five-level system: absorption can occur between a ground level and an excited level (4I15/2-4I9/2) and between two other excited levels (4I13/2-2H11/2) in Er3+. According as modulation degrees of the incident laser decreased, the transmitted laser waveform was inverted against the incident one for incident modulation degree under 90% and sample lengths greater than 2mm. By calculating rate equations based on an analytical model for the five-level system, it was confirmed that the enhanced absorption was caused by stimulated emission from 4S3/2 level to 4I13/2 level and close agreement between observed and calculated values was obtained in the relation between transmitted modulation degree and incident one.
There has been considerable interest in all-optical devices for achieving the high-speed and parallel processing. Up to the present the amount of absorption in any absorption process has been considered to be proportional to the incident light intensity until in some cases saturation occurs [1]. Recently we observed experimentally negative nonlinear absorption (NNA) phenomena which cause transmission to decrease as the laser intensity increase and confirmed that the transmitted laser intensity with the variation of incident laser showed negative nonlinear (i. e. Maeda's effect) [2, 3]. In this report we demonstrate that the net quantity of light absorbed in an erbium doped yttrium aluminum garnet crystal can be a negative function of the incident laser intensity. The absorbing material has a four-level energy structure: absorption can occur between a ground level and an excited level and between two other excited levels. The input-output characteristic has a negative region due to the absorption between excited states.
Negative nonlinear absorption, which causes the transmission to decrease as the laser intensity increases, was observed in a 16 at. % Er:YAG crystal using a laser diode operating at 787 and 784 nm. This effect for 787 nm was strong because 787 nm was the center of an absorption peak that is related to the excited-state absorption from 4I13/2 to 2H11/2. It is suggested that these phenomena take place because the absorption cross section from 4I13/2 to 2H11/2 is larger than that from 4I15/2 to 4I9/2 using a double-absorption model.
Dependence of the negative nonlinear absorption effect on both modulation frequency and sample temperature was investigated in an erbium yttrium aluminum garnet crystal. The phase of transmitted laser was opposite to that of the incident laser at modulation frequencies lower than 1 MHz. However, the transmitted laser waveform was asymmetrical above 5 MHz and the time-dependence characteristics agreed with the lifetime of the 4S3/2 level in Er3+. At temperatures lower than 25 K, enhanced absorption was weakened because the number of phonons decreased due to low temperature. It is suggested that the negative nonlinear absorption effect occurred due to enhanced absorption in three excited states. © 1998 Scripta Technica. Electron Comm Jpn Pt 2, 80(6): 43–48, 1997