We experimentally demonstrate signal gain for picosecond signals due to an orthogonally polarized pump beam in an AlGaAs channel waveguide at a wavelength of 1.55 mu m, below half the band gap. We show as much as 40% signal gain, or depletion, depending on the polarization states of the pump and the probe. The results are in good agreement with theory

Aitchison, J. S.

Kang, Jin U.

Stege,am, G I.

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University of Central Florida STARS Faculty Bibliography 1990s Faculty Bibliography 1-1-1995 Signal Gain Due To A Polarization Coupling In An Algaas Channel Wave-Guide J. S. Aitchison Jin U. Kang University of Central Florida G I. Stege,am University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib1990 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 1990s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Aitchison, J. S.; Kang, Jin U.; and Stege,am, G I., "Signal Gain Due To A Polarization Coupling In An Algaas Channel Wave-Guide" (1995). Faculty Bibliography 1990s. 1261. https://stars.library.ucf.edu/facultybib1990/1261 Appl. Phys. Lett. 67, 2456 (1995); https://doi.org/10.1063/1.114606 67, 2456© 1995 American Institute of Physics.Signal gain due to a polarization coupling inan AlGaAs channel waveguideCite as: Appl. Phys. Lett. 67, 2456 (1995); https://doi.org/10.1063/1.114606Submitted: 26 July 1995 . Accepted: 21 August 1995 . Published Online: 05 August 1998J. S. Aitchison, Jin U. Kang, and G. I. StegemanARTICLES YOU MAY BE INTERESTED INThe third-order nonlinear optical coefficients of Si, Ge, and Si1−xGex in the midwave andlongwave infraredJournal of Applied Physics 110, 011301 (2011); https://doi.org/10.1063/1.3592270Signal gain due to a polarization coupling in an AlGaAs channel waveguideJ. S. AitchisonDepartment of Electronics and Electrical Engineering, Glasgow University, Glasgow G12 8LT, ScotlandJin U. Kang and G. I. StegemanCREOL, University of Central Florida, Orlando, Florida 32826~Received 26 July 1995; accepted for publication 21 August 1995!We experimentally demonstrate signal gain for picosecond signals due to an orthogonally polarizedpump beam in an AlGaAs channel waveguide at a wavelength of 1.55mm, below half the band gap.We show as much as 40% signal gain, or depletion, depending on the polarization states of the pumpand the probe. The results are in good agreement with theory. ©1995 American Institute ofPhysics.Interest in all-optical switching has resulted in a greatdeal of attention being focused on the nonlinearity of semi-conductors in the half-band-gap spectral region. The idealmaterial must have a large nonlinear refraction coefficient,n2, small linear,a0, and nonlinear,a2, loss coefficients, at awavelength suitable for optical communications. AlGaAswaveguides have proven ideal for such all-optical switchingapplications due to the ability to tune the half band gap intothe 1.55mm low-loss telecommunications window.1 How-ever, one of the valid criticisms that has been raised is thelack of gain in this material system that makes fanout impos-sible. In this letter we demonstrate that polarization couplingcan be used to obtain signal gain at the expense of a highintensity pump beam.Nonlinear coupling between orthogonally polarizedbeams has been previously studied for application in signalamplification and all-optical switching. Both photorefractivematerials2 and nonlinear optical fibers3–6 have been investi-gated. However, the slow response time of the photorefrac-tive process makes it unsuitable for operation with shortpulses in a high repetition rate system. The ultrafast nonlin-ear response in weakly birefringent fibers has been used todemonstrate polarization instabilities, all-optical switching,and weak signal controlled switching.4–6 However, the lownonlinearity of silica glass means that fibers of 1–10 m arerequired. Although it is possible to have polarization cou-pling between two orthogonally polarized beams in millime-ter long channel waveguide due to the ultrafast Kerr nonlin-earity, the material requirements, i.e., large nonlinearity andlow absorptive losses are not easy to satisfy. To date, polar-ization switching that involves the equal excitation of thetwo eigenmodes has been demonstrated in AlGaAs channelwaveguides below half the band gap.7 In this letter, we ex-perimentally show strong energy coupling between two or-thogonally polarized beams resulting in weak signal gain inan AlGaAs channel waveguide using the off-resonant bound-electronic Kerr nonlinearity at 1.55mm. This intensity de-pendent coupling between two orthogonally polarized beamsis demonstrated in a typical pump/probe geometry and theexperimental results show good agreement with theory.The polarization coupling occurs due to the nonlinearpolarization grating induced by two orthogonally polarizedbeams. In order to obtain efficient coupling between twoorthogonally polarized beams, two conditions must be satis-fied, namely, the difference between the propagation con-stants must be less than or of order of the nonlinear~intensity-dependent! coupling constant and a sample lengthmust be of order of the beat length, or longer. In the case ofphotorefractive materials, the polarization coupling has beendemonstrated in cubic crystals such as GaAs, InP, and CdTewhere the material birefringence is zero.2 AlGaAs is alsocubic and therefore has no linear birefringence. However,when used in waveguide structures, an induced waveguidebirefringence occurs. The waveguide birefringence can becontrolled by properly designing the waveguides and henceit can be made small. Although the waveguide used in thisexperiment is not optimized to minimize the birefringence,nevertheless the birefringence was small enough to demon-strate the desired effects. We estimated the difference be-tween the mode indices of TE~transverse electric! and TM~transverse magnetic! modes to be about 0.0003, giving abeat length of 5 mm.The experiment was performed with a NaCl color centerlaser~CCL! operating at a wavelength of 1.55mm and pro-ducing 5 ps pulses at a repetition rate of 76 MHz. Usingpicosecond, rather than subpicosecond, pulses avoids theproblems of crosstalk between orthogonally polarized sig-nals, due to the Raman effect.8 A 1.74 cm long, strip-loadedAlGaAs channel waveguide was used for this experiment.The high index, guiding region is made of Al0.18Ga0.82As andthe upper and lower cladding regions of Al0.24Ga0.76As.Waveguides were formed using standard optical lithographyand reactive ion etching.The governing equations for polarization coupling be-tween two orthogonally polarized beams are shown below:dEedz52a21 ik0n2,eF uEeu2Ee1DuEmu2Ee1D2Em2Ee* exp~22iDkz!G ,dEmdz52a21 ik0n2,mF uEmu2Em1DuEeu2Em1D2Ee2Em* exp~2iDkz!G ,2456 Appl. Phys. Lett. 67 (17), 23 October 1995 0003-6951/95/67(17)/2456/3/$6.00 © 1995 American Institute of PhysicswhereEe andEm are the electric fields for the TE and TMmodes, respectively,a is the linear loss coefficient, which isabout 0.1 cm21, k0 is the propagation constant in thevacuum,n2,e andn2,m are the nonlinear refractive index co-efficient for the TE and TM fields, respectively,D is the ratiobetween the cross and self phase modulations, andDk is thewave-vector mismatch defined ask0(ne2nm), wherene andnm are the effective indices for TE and TM fields, respec-tively. The last term in the equation, which is typically neg-ligible in most linear waveguide problems due to the largewave-vector mismatch, provides the energy coupling be-tween the orthogonally polarized fields.Experiments were performed using the pump-probe ge-ometry shown in Fig. 1. The pump and the signal beam weregenerated using a half-waveplate and polarizing beam split-ter ~PBS!; the two beams were then recombined using a sec-ond PBS and end-fire coupled into the channel waveguideusing a microscope objective. At the output a second micro-scope objective was used to collect the light and a polarizerwas used to block the pump beam. Using a TM pump, weobserved an increase in the TE-polarized output signal due topolarization coupling. This is shown in Fig. 2, where theoutput signal intensity is plotted as a function of pump in-tensity. Note that as pump power is increased, the signaloutput increases and shows gain of as much as 40%. How-ever, when the polarization of the pump and the signal wasreversed, depletion of the signal beam was observed. This isshown in Fig. 3.The theoretical curves based on the reported values areshown in Fig. 4 for both the gain and depletion cases.9 Theparameters based on the reported values were varied withinthe experimental uncertainty. We used values ofn2,e andn2,m of 1.5310213 cm2/W and 1.3 10213 cm2/W (ne2nm) of 0.000314, andD of 0.92. The solid and dotted linesrepresent the TE signal with a TM pump and the TM signalwith a TE pump, respectively. The predicted gain for a TE-polarized signal and depletion for a TM-polarized signal isconsistent with the experimental results.The case considered above occurs when the initial rela-tive phase difference between the two beams is zero. Whenthe phase difference isp/2, the process is reversed so thatgain occurs for a TM signal with a TE pump, and depletionfor a TE signal with a TM pump. This was verified experi-mentally.In conclusion, we demonstrated ultrafast signal gain dueto polarization coupling in AlGaAs channel waveguides for1.55mm picosecond pulses. For a waveguide with a smallerwave-vector mismatch, a larger gain is expected. Such a de-FIG. 1. Schematic diagram of experimental setup showing the color centerlaser~CCL!, half-wave plates~l/2!, polarizing beam splitters~PBS!, outputpolarizer~P!, and the detectors~Det!.FIG. 2. TE signal output intensity as a function of TM pump input intensity.FIG. 3. TM signal output intensity as a function of TE pump input intensity.FIG. 4. Theoretical curves showing both gain and depletion of signal as afunction of pump input intensity. Solid and dotted lines represent TE signalfor TM pump and TM signal for TE pump, respectively.2457Appl. Phys. Lett., Vol. 67, No. 17, 23 October 1995 Aitchison et al.vice can play an important role in AlGaAs applications be-cause it can be used in combination with a switching deviceto implement fanout.The work in the UK was supported by the SERC throughthe nonlinear optics initiative, and at CREOL was sponsoredby NSF.1Reviewed in G. I. Stegeman and A. Miller,Photonic Switching, edited byJ. Midwinter ~Academic, Orlando, 1992!, Vol. I, p. 81.2For example, J. C. Fabre, J. M. C. Jonathan, and G. Roosen, J. Opt. Soc.Am. B 5, 1730~1988!.3H. G. Winful, Opt. Lett.11, 33 ~1986!.4S. Trillo, S. Wabnitz, R. H. Stolen, G. Assanto, C. T. Seaton, and G. I.Stegeman, Appl. Phys. Lett.49, 1224~1986!.5S. Trillo, S. Wabnitz, W. C. Banyai, N. Finlayson, C. T. Seaton, G. I.Stegeman, and R. H. Stolen, IEEE J. Quantum Electron.25, 104 ~1988!.6C. G. Krautschik, G. I. Stegeman, and R. H. Stolen, Appl. Phys. Lett.61,1751 ~1992!.7C. C. Yang, A. Villeneuve, G. I. Stegeman, C.-H. Lin, and H.-H. Lin, Opt.Lett. 18, 1487~1993!.8Y. H. Kao, M. N. Islam, J. M. Saylor, R. E. Slusher, and W. S. Hobson,‘‘Raman effect in AlGaAs waveguides for all-optical applications,’’Digestof QELS’95 ~Optical Society of America, Washington, DC, 1995!, pp.115–116.9D. C. Hutchings, J. S. Aitchison, B. S. Wherrett, G. T. Kennedy, and W.Sibbett, Opt. Lett.20, 991 ~1995!.2458 Appl. Phys. Lett., Vol. 67, No. 17, 23 October 1995 Aitchison et al.

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Signal Gain Due To A Polarization Coupling In An Algaas Channel Wave-Guide

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