Optical signal inversion phenomenon for 1.53 μm laser in an erbium-doped lithiumniobateYoshinobu Maeda, Hirotoshi Nagata, Taizou Nakashima, Junichiro Ichikawa, and Kaoru Higuma Citation: Applied Physics Letters 75, 4067 (1999); doi: 10.1063/1.125538 View online: http://dx.doi.org/10.1063/1.125538 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/75/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Er 3+ -doped strontium lithium bismuth borate glasses for broadband 1.5 μm emission - optical properties AIP Conf. Proc. 1512, 550 (2013); 10.1063/1.4791155 Spectroscopic and upconversion properties of erbium-doped potassium lithium tantalate niobate crystals under800 nm femtosecond laser excitation J. Appl. Phys. 108, 043520 (2010); 10.1063/1.3475511 Highly optical damage resistant crystal: Zirconium-oxide-doped lithium niobate Appl. Phys. Lett. 91, 081908 (2007); 10.1063/1.2773742 Periodically poled structures in doped lithium niobate crystals J. Appl. Phys. 92, 1544 (2002); 10.1063/1.1490619 All-optical inverter operating at 1.53 μm laser in erbium yttrium aluminum garnets Appl. Phys. Lett. 74, 1651 (1999); 10.1063/1.123643

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.113.111.210 On: Mon, 22 Dec 2014 05:40:15

Optical signal inversion phenomenon for 1.53 mm laser in an erbium-dopedlithium niobate

Yoshinobu Maedaa)

Department of Information and Control Engineering, Toyota Technological Institute, 2-12-1 Hisakata,Tempaku, Nagoya 468-8511, Japan

Hirotoshi Nagata, Taizou Nakashima, Junichiro Ichikawa, and Kaoru HigumaOptoelectronics Research Division, New Technology Research Laboratories, Sumitomo Osaka CementCompany, Ltd, 585 Toyotomi-cho, Funabashi, Chiba 274-8601, Japan

~Received 2 June 1999; accepted for publication 4 November 1999!

Modulation characteristics of a negative nonlinear absorption effect were investigated in 3 wt %erbium-doped lithium niobate using a 1530 nm laser diode. With a decreasing incident modulationdegree, a reversed-phase waveform was obtained in the transmitted laser at modulation degreessmaller than 85% for a sample length of 14 mm. The reversed-phase transmitted waveforms wereobserved at modulation frequencies from 1 kHz to 10 GHz. The negative nonlinear absorption effectfor 1.5 mm can be explained by considering an enhanced absorption model for a four-level systemof the Er31 ion. © 1999 American Institute of Physics.@S0003-6951~99!04352-1#

In optical networks, special attention has been paid toall-optical devices for use in high-speed and parallel process-ing. For example, the third order nonlinear optical effect isused to obtain optical switching devices which might even-tually be used in an optical computer. However, becausethese devices need a high energy laser source, above theorder of kW/cm2, realization of such devices is very difficult.A development of digital optoelectronic circuits has alsobeen hampered by the lack of an optical inverter which canwork as NOR and NAND logic gates.1 In this regard, Maedafound recently negative nonlinear absorption~NNA! effectby Er-doped garnets2–8 and Er-doped glasses,9,10 which maybe applied to achieve an all-optical inverter.

The NNA effect differs from the saturation of opticalabsorption, because the effect is observed at a laser intensityof 60 nW/cm2 to 100 W/cm2.4 In this letter, modulation char-acteristics of the NNA effect were investigated for 1.53mmlight. As the material, Er-doped lithium niobate (Er:LiNbO3)crystal was used because it has a potential application toelectro-optical waveguide devices for optical networks. TheLiNbO3 based optical modulators are already in service inglobal fiber communication systems.11 Concerning Er-dopedLiNbO3 application of them to waveguide lasers and ampli-fiers has been proposed.12,13 The Er-doped LiNbO3 all-optical inverter may provide a novel way to develop 1.5mmoptical networks.

The laser diode consists of a single-facet antireflection-coated Fabry–Perot laser diode, mounted on a temperature-controlled heat sink. The desired wavelength is selected bytuning the angle of the high-diffraction-efficiency gratingused as an external mirror in the cavity. The typical spectrallinewidth is 1 MHz. The optical modulation intensities ofsinusoidal waves were obtained by direct optical modulation~Sumitomo Osaka Cement Co., Ltd!. The modulation degreeof the incident laser intensity is defined asM51003(I max

2Imin)/(Imax1Imin) ~%!, where I max and I min represent the

maximum and minimum intensities of the incident laser, re-spectively. This also can be explained using the contrast ratioK5I max/Imin , sinceM5K21/K11. The intensity of the in-cident laser was typically about 1 mW. Parallel laser beamwas inserted into the Er-doped LiNbO3 crystal normal to itsc axis. The sample surface was polished but not coated byantireflection films. The sample lengthL along the opticalpath was 14 mm. This single crystal sample had been spe-cially grown from the melt containing 3 wt % of Er andsliced without a poling treatment~ELAN Ltd., St. Peters-burg!. The incident and transmitted laser intensities weremeasured using photodetectors and a digitizing oscilloscope.

Figure 1 shows the result of an experiment in whichM isvaried for a sample length of 14 mm with the incident laser

a!Electronic mail: s6tmae@toyota-ti.ac.jp

FIG. 1. ~a!–~e! Changes in the transmitted laser intensity forM values of92%, 90%, 88%, 85%, and 60%, respectively.~f! Change in the incidentlaser intensity atM560%. The vertical axes are scaled in arbitrary units.

APPLIED PHYSICS LETTERS VOLUME 75, NUMBER 26 27 DECEMBER 1999

40670003-6951/99/75(26)/4067/3/$15.00 © 1999 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.113.111.210 On: Mon, 22 Dec 2014 05:40:15

intensity modulated at 1 kHz. Figures 1~a!–1~e! show thechanges in the transmitted laser intensity forM values of92%, 90%, 88%, 85%, and 60%, respectively (K524, 19,15.7, 12.3, and 4!. Figure 1~f! shows the change in the inci-dent laser intensity atM560%. The vertical axes are scaledin arbitrary units. AsM gradually decreases, the transmittedwaveform changes, as shown in Figs. 1~a!–1~c!, and areversed-phase waveform is obtained as shown in Figs. 1~d!and 1~e!.

The modulation frequency dependence of the NNA ef-fect was investigated when the incident laser was modulatedat modulation frequencies from 0.1 MHz to 1 GHz. Figures2~a!–2~e! show the changes in the transmitted laser intensityat modulation frequencies of 0.1, 1, 10, 100 MHz, and 1GHz, respectively, and Fig. 2~f! shows the change in theincident laser intensity at a modulation frequency of 1 GHz.The vertical axes are scaled in arbitrary units. The transmit-ted waveforms change into the opposite phase of the incidentwaveform at modulation frequencies from 0.1 MHz to 1GHz, as shown in Figs. 2~a!–2~e!.

The NNA effect was investigated with the incident lasermodulated at 10 GHz. Figures 3~a! and 3~b! show thechanges in the transmitted and incident laser intensities, re-spectively. The vertical axes are scaled in arbitrary units. Thetransmitted waveforms change into the phase opposite that ofthe incident waveform at modulation frequency of 10 GHz.

Figure 4 shows the transmitted spectra of the Er-dopedLiNbO3 sample at room temperature which has an absorptionpeak with the center at 1531 nm corresponding to both the

4I 15/2–4I 13/2 and the 4I 9/2–

4I 13/2 transitions of Er31.14,15

Similar spectra were also reported on Er-doped Ti:LiNbO3

waveguides, exhibiting a sharp absorption at 1532 nm.16

In order to explain the above experimental results, it isnecessary to consider an enhanced absorption process, inwhich the amount of absorption increases due to the increasein incident laser intensity. Figure 5 shows the energy-leveldiagram of Er31. The occurrence of ground-state absorption4I 15/2–

4I 13/2(E1–E2) and excited-state absorption4I 13/2–

4I 9/2 (E2–E4) has been confirmed when photons at1530 nm are incident on the Er31 doped crystals andglasses.14–16 In the present experiment, the 1530 nm laser isirradiated onto the sample and electrons in the groundE1(4I 15/2) level are excited to the excitedE2(4I 13/2) levelwith an absorption cross section ofs1 . In the upconversionprocess, the electrons in theE2(4I 13/2) level absorb incidentphotons again and are excited to theE4(4I 9/2) level with anabsorption cross section ofs2 . Most of the electrons that areexcited to theE4 level relax nonradiatively to theE3(4I 11/2)level at rate ofA43. The transition from theE3(4I 11/2) levelto the E2(4I 13/2) level may lead to stimulated emission inaddition to spontaneous emission. Enhanced absorption canbe intensified if this stimulated emission process occurs, be-cause the light generated by stimulated emission will inducethe transition of electrons from theE3 (4I 11/2) level to theE2(4I 13/2) level at a transition rateB32, thereby contributingto the absorption of incident photons.3,5

It is reported that the stimulated emission between4I 11/2

and 4I 13/2 excited states of Er31 has been produced at roomtemperature in many host materials heavily doped with theseions. Using 1.53mm erbium glass laser radiation, Pollacket al. stimulated the4I 11/2 and 4I 13/2 laser emission pumped

FIG. 2. ~a!–~e! Changes in the transmitted laser intensity at modulationfrequencies of 0.1, 1, 10, 100 MHz, and 1 GHz, respectively.~f! Change inthe incident laser intensity at a modulation frequency of 1 GHz. The verticalaxes are scaled in arbitrary units.

FIG. 3. ~a! and~b! Changes in the transmitted and incident laser intensitiesat modulation frequency of 10 GHz, respectively. The vertical axes arescaled in arbitrary units.

FIG. 4. Transmitted spectra of the Er-doped LiNbO3 sample at room tem-perature which has an absorption peak with the center at 1531 nm.

FIG. 5. Schematic energy-level diagram of the Er31 ion and an enhancedabsorption model.

4068 Appl. Phys. Lett., Vol. 75, No. 26, 27 December 1999 Maeda et al.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.113.111.210 On: Mon, 22 Dec 2014 05:40:15

solely by the upconversion process in Er31 doped crystals.14

They suggested that the most efficient upconversion can beobtained through the mechanism in which energy transfer(Wet) occurs in systems with high concentration of activeions. These are systems in which absorption and emission donot take place within the same center. Energy transfer insuch systems occurs without charge transport through Cou-lomb interaction, e.g., dipole–dipole. As a result, the excita-tion energy migrates from one ion to its neighbor before it isable to radiate a quantum of energy. Subsequently, electronscascade nonradiatively from the shortlived4I 9/2 to the second4I 11/2 state inverting its population with respect to the first4I 13/2 excited state.

In a previous paper,5 the NNA effect was shown to bedependent on the modulation degree in the Er:YAG crystalusing a 788 nm laser. With an increasing modulation degree,the transmitted waveform cannot change to the oppositephase of the incident waveform. Therefore, a bias componentof the incident laser is necessary to produce the NNA effectsufficiently. For a wavelength of 788 nm, the reversed-phasewaveform was obtained only at modulation degrees smallerthan 30%. In this study, the NNA effect was obtained at themuch larger modulation degree of approximately 85% usinga 1530 nm laser. However, the transmitted modulation de-gree is calculated to be approximately 70% (K55.6) for theincident modulation degreeM585% (K512.3). The outputconstant ratio must be greater than or equal to the input onein order to actually have a useful inverter.

The negative nonlinear absorption effect, which may beapplied to the achievement of an all-optical inverter, wasobtained in Er:LiNbO3 using a 1530 nm laser diode. How-

ever, an inverter practical for logic functions must be, first ofall, cascadable. That means that the output of one invertermust be able to serve as the input of another inverter. On theother hand, an erbium-doped optical amplifier can providethe needed gain for cascadability. Therefore, in the future,the all-optical inverter may be composed of a device derivedfrom the NNA effect and an erbium-doped amplifier.

This study is carried out as part of ‘‘Ground ResearchAnnouncement for Space Utilization’’ promoted by NASDAand the Japan Space Forum.

1F. R. Beyette, S. A. Feld, X. An, K. M. Geib, M. J. Hafich, G. Y. Rob-inson, and C. W. Wilmsen, Electron. Lett.27, 497 ~1991!.

2Y. Maeda, J. Appl. Phys.72, 3835~1992!.3Y. Maeda and M. Migitaka, Appl. Phys. Lett.70, 4 ~1997!.4Y. Maeda, S. Ikeda, T. Sakakibara, and M. Migitaka, Trans. Inst. Electr.Eng. Jpn., Part D117, 10 ~1997!.

5Y. Maeda, J. Appl. Phys.83, 1187~1998!.6Y. Maeda, Appl. Phys. Lett.72, 395 ~1998!.7Y. Maeda and T. Yamada, J. Appl. Phys.83, 7436~1998!.8Y. Maeda, Appl. Phys. Lett.74, 1651~1999!.9Y. Maeda, T. Sakakibara, and M. Migitaka, Jpn. J. Appl. Phys., Part 236,L283 ~1997!.

10Y. Maeda, A. Konishi, H. Hashima, H. Wakabayashi, and T. Yamada,Jpn. J. Appl. Phys., Part 237, 4842~1998!.

11H. Nagata and J. Ichikawa, Opt. Eng.34, 3284~1995!.12J. Aim, J. A. Aust, and N. A. Sanford, Appl. Phys. Lett.69, 3785~1996!.13C. Huang, L. McCaughan, and D. M. Gill, J. Lightwave Technol.12, 803

~1994!.14S. A. Pollack, D. B. Chang, M. Birnbaum, and M. Kokta, J. Appl. Phys.

70, 7227~1988!.15J. Koetke and G. Huber, Appl. Phys. B: Lasers Opt.61, 151 ~1995!.16N. N. Puscas, D. M. Grobnic, I. M. Popescu, M. Guidi, D. Scarano, G.

Perrone, and I. Montrosset, Opt. Eng.35, 1311~1996!.

4069Appl. Phys. Lett., Vol. 75, No. 26, 27 December 1999 Maeda et al.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

130.113.111.210 On: Mon, 22 Dec 2014 05:40:15