Dispersion measurements of a 1.3¼m quantum dot ... measurements of a 1.3¼m quantum dot...

Dispersion measurements of a 1.3¼m quantum dot ... measurements of a 1.3¼m quantum dot semiconductor optical amplifier over ... on a 1.3 m quantum dot semiconductor optical ...
Dispersion measurements of a 1.3¼m quantum dot ... measurements of a 1.3¼m quantum dot semiconductor optical amplifier over ... on a 1.3 m quantum dot semiconductor optical ...
Dispersion measurements of a 1.3¼m quantum dot ... measurements of a 1.3¼m quantum dot semiconductor optical amplifier over ... on a 1.3 m quantum dot semiconductor optical ...
Dispersion measurements of a 1.3¼m quantum dot ... measurements of a 1.3¼m quantum dot semiconductor optical amplifier over ... on a 1.3 m quantum dot semiconductor optical ...
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  • Dispersion measurements of a 1.3m quantum dot semiconductor opticalamplifier over 120 nm of spectral bandwidthM. Bagnell, J. Davila-Rodriguez, A. Ardey, and P. J. Delfyett Citation: Appl. Phys. Lett. 96, 211907 (2010); doi: 10.1063/1.3430742 View online: http://dx.doi.org/10.1063/1.3430742 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v96/i21 Published by the American Institute of Physics. Related ArticlesPhase and coherence extraction from a phased vertical cavity laser array Appl. Phys. Lett. 101, 031116 (2012) Direct generation of optical vortex pulses Appl. Phys. Lett. 101, 031113 (2012) Self-organized InAs/InGaAsP quantum dot tube lasers Appl. Phys. Lett. 101, 031104 (2012) Directional whispering gallery mode emission from Limaon-shaped electrically pumped quantum dot micropillarlasers Appl. Phys. Lett. 101, 021116 (2012) Semiconductor lasers driven by self-sustained chaotic electronic oscillators and applications to optical chaoscryptography Chaos 22, 033108 (2012) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

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  • Dispersion measurements of a 1.3 m quantum dot semiconductor opticalamplifier over 120 nm of spectral bandwidth

    M. Bagnell,a J. Davila-Rodriguez, A. Ardey, and P. J. Delfyettb

    CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA

    Received 2 March 2010; accepted 27 April 2010; published online 27 May 2010

    Group delay and higher order dispersion measurements are conducted on a 1.3 m quantum dotsemiconductor optical amplifier at various injection currents. White-light spectral interferometry isperformed, along with a wavelet transform to recover the group delay. The group delay, groupvelocity dispersion, and higher order dispersion terms are quantified. The measurement spans bothground state and first excited state transitions, ranging from 1200 to 1320 nm. The group velocitydispersion, 2, is found to be 6.310

    3 fs2 7.6 fs/nm at an injection current of 500 mA. 2010American Institute of Physics. doi:10.1063/1.3430742

    Semiconductor quantum dot materials have gained wide-spread attention for use as sources in mode-locked lasers.Large gain bandwidth, wide variability of linewidth enhance-ment factor, low chirp, relatively low sensitivity to tempera-ture fluctuation, and high modulation bandwidth have beencited as advantages over quantum well devices.1 Their largegain bandwidth, which can be greater than 15 THz, is espe-cially attractive for the generation of broad mode-locked la-ser spectra, with the potential for transform limited pulsewidths of less than 100 fs. This capability is attractive formany applications including high speed communication andsignal processing.2

    Semiconductor optical amplifier SOA gain is con-trolled by current injection, which also results in a change inthe carrier concentration in the active region. The complexindex of refraction of a medium is known to be dependent onthe carrier concentration in the medium. Therefore, changingthe gain in a laser cavity changes the mode spacing of amode-locked laser. As the SOA gain is often one of the keyelements determining both the static and dynamic dispersionof a mode-locked laser, it is important to have a clear under-standing of the dependence of the dispersion and its higherorder terms on injection current. This information can beused in determining the proper dispersion compensationtechniques for broad bandwidth mode-locked laser sources.

    A Taylor expansion of the propagation constant, , pro-vides a set of coefficients which is used to quantify the dis-persive properties of the SOA. is related to the spectralphase by the following:

    =

    L=

    /c nL

    , 1

    where is the spectral phase, L is the length of themedium, and n is the frequency dependent refractive index.Taking the first derivative yields the following:

    d/d =d/d

    L. 2

    This is the group delay over the length of the device, thequantity measured in this work. The expansion of is

    = 0 + 1 0 +1

    2!2 02 +

    1

    3!3 03

    + . . . , 3

    and its derivative is

    d

    d= 1 + 2 0 +

    1

    2!3 02 + . . . , 4

    where the first term is constant in frequency, representing thepath length difference of the interferometer arms. By fittingthe acquired group delay curve to Eq. 4, the group velocitydispersion, 2, and higher order dispersion terms are deter-mined.

    White-light interferometry has been used for dispersioncharacterization of a wide variety of materials.35 The tech-nique uses a Michelson or MachZehnder style interferom-eter with a broadband light source Fig. 1. The light is di-vided into a reference beam and another which passesthrough the device under test. The beams are recombined andthe spectral phase is determined by the interference of thebeams. Interference can be measured either temporally, witha movable mirror and single photodetector or spectrally, bykeeping the relative path length difference static, and spa-tially separating and detecting spectral components with aspectrometer. The static version, used here, has no movingparts and is preferable under the condition that a spectrom-eter with sufficient resolution is available.

    The interference spectrum, shown in Fig. 2, has sinu-soidal fringes which vary in periodicity due to the differentphase delays arising from a frequency dependent refractive

    aElectronic mail: mbagnell@creol.ucf.edu.bElectronic mail: delfyett@creol.ucf.edu.

    FIG. 1. Color online MachZehnder style, spectral interferometer experi-mental setup; SOA, semiconductor optical amplifier; DUT, device undertest; BS, beam splitter; L, coupling lens; M, mirror; VOD, variable opticaldelay; and OSA, optical spectrum analyzer.

    APPLIED PHYSICS LETTERS 96, 211907 2010

    0003-6951/2010/9621/211907/3/$30.00 2010 American Institute of Physics96, 211907-1

    Downloaded 20 Jul 2012 to 132.170.95.49. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

    http://dx.doi.org/10.1063/1.3430742http://dx.doi.org/10.1063/1.3430742http://dx.doi.org/10.1063/1.3430742

  • index. By finding the local periodicity at each point in theinterferogram, the group delay of the device under test isdetermined.6

    Traditionally, the Fourier transform technique is used torecover the spectral phase of the signal and numerical differ-entiation is then required to calculate the group delay. Thisprocess often requires smoothing functions to reduce the am-plification of noise in the data. Numerical differentiation andsmoothing functions can be avoided by the use of the wave-let transform WT. As shown by Deng et al.,7 a WT willextract the group delay directly from the interferogram.While this method requires more processing time than theFourier transform method, it is handled readily by currentdesktop computers.

    To calculate the WT, the overlap integral of the interfer-ence spectrum and a user-defined wavelet function with aknown periodicity is calculated. The wavelet period is variedto find the period for which the overlap integral is maxi-mized. This is done for each measured frequency in the in-terferogram. The result is the local periodicity at each pointin the interferogram, i.e., the group delay. Figure 3 shows theWT result, a contour plot where the maximum at each wave-length value corresponds to the group delay value for thatwavelength.

    Interference spectra are acquired using a MachZehnderinterferometer. This configuration, shown in Fig. 1, is chosenfor ease of alignment and to measure the group delay on asingle pass through the sample. The light source is a 3 mmSOA fabricated from the same wafer as the 2 mm SOA undertest.

    Dispersion characteristics are measured for a ten layerInAs/GaAs quantum dot SOA with a 4 m wide waveguide.The uncoated facets were fabric