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  • 1.
    Calil Kores, Cristine
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Ismail, Nur
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Geskus, Dimitri
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Dijkstra, M.
    Bernhardi, Edward H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Temperature dependence of the resonance line of optically pumped distributed-feedback lasers2018In: Optics InfoBase Conference Papers, OSA - The Optical Society , 2018Conference paper (Refereed)
    Abstract [en]

    We characterize experimentally and theoretically a distributed-feedback laser resonator subject to a thermal chirp. The total accumulated phase shift determines the resonance wavelength. The reflectivities (outcoupling losses) at the resonance wavelength govern the resonance linewidth.

  • 2.
    Calil Kores, Cristine
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Department of Applied Physics, School of Engineering Sciences, Roslagstullsbacken 21, 10691 Stockholm, Sweden .
    Ismail, Nur
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Geskus, Dimitri
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Dijkstra, Meindert
    Bernhardi, Edward
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Visiting scientist .
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Advanced Technology Institute, Department of Electrical and Electronic Engineering, University of Surrey, Guildford GU2 7XH, UK.
    Temperature dependence of the spectral characteristics of distributed-feedback resonators2018In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 26, no 4, p. 4892-4905Article in journal (Refereed)
    Abstract [en]

    We characterize the spectral response of a distributed-feedback resonator when subject to a thermal chirp. An Al2O3 rib waveguide with a corrugated surface Bragg grating inscribed into its SiO2 top cladding is experimentally investigated. We induce a near-to-linear temperature gradient along the resonator, leading to a similar variation of the grating period, and characterize its spectral response in terms of wavelength and linewidth of the resonance peak. Simulations are carried out, showing good agreement with the experimental results and indicating that the wavelength of the resonance peak is a result only of the total accumulated phase shift. For any chirp profile we are able to calculate the reflectivities at the resonance wavelength, and this information largely explains how the linewidth of the resonance changes. This result shows that the increase in linewidth is governed by the increase of the resonator outcoupling losses. 

  • 3.
    Carrasco, Irene
    et al.
    Univ Surrey, Adv Technol Inst, Dept Elect & Elect Engn, Guildford GU2 7XH, Surrey, England..
    Agazzi, Laura
    Univ Twente, MESA Inst, Integrated Opt Micro Syst, POB 217, NL-7500 AE Enschede, Netherlands..
    Loiko, Pavel
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. Univ Surrey, Adv Technol Inst, Dept Elect & Elect Engn, Guildford GU2 7XH, Surrey, England.;Univ Twente, MESA Inst, Integrated Opt Micro Syst, POB 217, NL-7500 AE Enschede, Netherlands.
    Energy-Transfer Processes Among Non-Homogeneously Distributed Rare-Earth Ions and Impact on Amplification and Lasing2018In: 20th International Conference on Transparent Optical Networks, ICTON / [ed] Jaworski, M Marciniak, M, Institute of Electrical and Electronics Engineers (IEEE), 2018, article id 8473493Conference paper (Refereed)
    Abstract [en]

    Energy-transfer processes such as energy-transfer upconversion are often detrimental to the performance of rareearth- doped amplifiers and lasers on the typical luminescence transitions in the near-infrared spectral region between 1-2 mu m. In order to quantify the influence of these interionic processes on amplification and lasing, not only luminescence decay curves have to be measured, but also the population dynamics of the electronic level scheme need to be modeled. The usually encountered non-homogeneous ion distributions complicate the situation. Here we present a stochastic model of energy-transfer processes that takes a statistical ion distribution into account. The influence of energy-transfer upconversion and cross-relaxation on amplification and lasing on the 1.06 mm transition in Nd3+, the 1.53 mm transition in Er3+, or the 1.84 mm transition in Tm3+ under these conditions is investigated.

  • 4. Chang, Lantian
    et al.
    Dijkstra, Meindert
    Ismail, Nur
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    de Ridder, Rene M.
    Worhoff, Kerstin
    Subramaniam, Vinod
    Kanger, Johannes S.
    Waveguide-coupled micro-ball lens array suitable for mass fabrication2015In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 23, no 17, p. 22414-22423Article in journal (Refereed)
    Abstract [en]

    We demonstrate a fabrication procedure for the direct integration of micro-ball lenses on planar integrated optical channel waveguide chips with the aim to reduce the divergence of light that arises from the waveguide in both horizontal and vertical directions. Fabrication of the lenses is based on photoresist reflow which is a procedure that allows for the use of photolithography for careful alignment of the lenses with respect to the waveguides and enables mass production. We present in detail the design and fabrication procedures. Optical characterization of the fabricated micro-ball lenses demonstrates a good performance in terms of beam-size reduction and beam shape. The beam half divergence angle of 1544 nm light is reduced from 12.4 degrees to 1.85 degrees.

  • 5. Chang, Lantian
    et al.
    Weiss, Nicolas
    van Leeuwen, Ton G.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Optics and Photonics, OFO.
    de Ridder, Rene M.
    Worhoff, Kerstin
    Subramaniam, Vinod
    Kanger, Johannes S.
    Chip based common-path optical coherence tomography system with an on-chip microlens and multi-reference suppression algorithm2016In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 24, no 12, p. 12635-12650Article in journal (Refereed)
    Abstract [en]

    We demonstrate an integrated optical probe including an on-chip microlens for a common-path swept-source optical coherence tomography system. This common-path design uses the end facet of the silicon oxynitride waveguide as the reference plane, thus eliminating the need of a space-consuming and dispersive on-chip loop reference arm, thereby obviating the need for dispersion compensation. The on-chip micro-ball lens eliminates the need of external optical elements for coupling the light between the chip and the sample. The use of this lens leads to a signal enhancement up to 37 dB compared to the chip without a lens. The light source, the common-path arm and the detector are connected by a symmetric Y junction having a wavelength independent splitting ratio (50/50) over a much larger bandwidth than can be obtained with a directional coupler. The signal-to-noise ratio of the system was measured to be 71 dB with 2.6 mW of power on a mirror sample at a distance of 0.3 mm from the waveguide end facet. Cross-sectional OCT images of a layered optical phantom sample are demonstrated with our system. A method, based on an extended Fourier-domain OCT model, for suppressing ghost images caused by additional parasitic reference planes is experimentally demonstrated.

  • 6. Eichhorn, Marc
    et al.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Spectroscopic Foundations of Lasers: Spontaneous Emission Into a Resonator Mode2015In: IEEE Journal of Selected Topics in Quantum Electronics, ISSN 1077-260X, E-ISSN 1558-4542, Vol. 21, no 1, p. 9000216-Article in journal (Refereed)
    Abstract [en]

    We review the physics underlying the process of spontaneous emission, with a special focus on spontaneous emission into a resonator mode. We define the mode volume, verify the fundamental modal dimensions, present the spectral mode profile, the coherence time, the Q-factor, the Fuchtbauer-Ladenburg equation, and the Purcell factor, and discuss their influence on different types of lasers. We obtain the relation between peak emission cross section, radiative lifetime, and emission linewidth. By interpreting spontaneous emission as stimulated emission driven by vacuum fluctuations, we derive the spontaneous-emission rate into a resonator mode and establish physical expressions for the fractions of spontaneous emission and total decay from the upper laser level into this mode. Furthermore, we discuss coupling of the atomic system with the coherent field inside a lasing resonator mode, resulting in the formation of a Mollow triplet, and demonstrate that it leads to a reduction of the spontaneous-emission rate by a factor of 2.

  • 7. Feng, Xue
    et al.
    Liu, Zhicheng
    Michel, Jurgen
    Ning, Cun-Zheng
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Tong, Limin
    Yin, Leijun
    Comment on "High Gain Submicrometer Optical Amplifier at Near-Infrared Communication Band"2016In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 117, no 21, article id 219701Article in journal (Other academic)
  • 8.
    Geskus, Dimitri
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Yong, Y. S.
    Bernhardi, E. H.
    Agazzi, L.
    García-Blanco, S. M.
    Aravazhi, S.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Concentration dependence of optical gain in Yb 3+ -doped potassium double tungstate channel waveguides2019In: Proceedings 2015 European Conference on Lasers and Electro-Optics - European Quantum Electronics Conference, CLEO/Europe-EQEC 2015, Optical Society of America, 2019Conference paper (Refereed)
  • 9.
    Geskus, Dimitri
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Yong, Y. S.
    Bernhardi, E. H.
    Agazzi, L.
    García-Blanco, S. M.
    Aravazhi, S.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Concentration dependence of optical gain in Yb3+-doped potassium double tungstate channel waveguides2014In: Optics InfoBase Conference Papers, OSA - The Optical Society , 2014Conference paper (Refereed)
  • 10.
    Ismail, Nur
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Calil Kores, Cristine
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Optics and Photonics, OFO.
    Geskus, Dimitri
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    The Fabry-Pérot resonator: Spectral line shapes, generic and related Airy distributions, linewidths, finesses, and performance at low or frequency-dependent reflectivity2016In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 24, no 15, p. 16366-16389Article in journal (Refereed)
    Abstract [en]

    We systematically characterize the Fabry-Pérot resonator. We derive the generic Airy distribution of a Fabry-Pérot resonator, which equals the internal resonance enhancement factor, and show that all related Airy distributions are obtained by simple scaling factors. We analyze the textbook approaches to the Fabry-Pérot resonator and point out various misconceptions. We verify that the sum of the mode profiles of all longitudinal modes is the fundamental physical function that characterizes the Fabry-Pérot resonator and generates the Airy distribution. Consequently, the resonator losses are quantified by the linewidths of the underlying Lorentzian lines and not by the measured Airy linewidth. Therefore, we introduce the Lorentzian finesse which provides the spectral resolution of the Lorentzian lines, whereas the usually considered Airy finesse only quantifies the performance of the Fabry-Pérot resonator as a scanning spectrometer. We also point out that the concepts of linewidth and finesse of the Airy distribution of a Fabry-Pérot resonator break down at low reflectivity. Furthermore, we show that a Fabry-Pérot resonator has no cut-off resonance wavelength. Finally, we investigate the influence of frequency-dependent mirror reflectivities, allowing for the direct calculation of its deformed mode profiles.

  • 11.
    Ismail, Nur
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Kores, Cristine Calil
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Geskus, Dimitri
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Mode profiles and Airy distributions of Fabry-Perot resonators with frequency-dependent mirror reflectivity2017In: Laser Resonators, Microresonators, and Beam Control XIX / [ed] Kudryashov, AV Paxton, AH Ilchenko, VS, SPIE - International Society for Optical Engineering, 2017, Vol. 10090, article id UNSP 1009011-1Conference paper (Refereed)
    Abstract [en]

    We thoroughly investigate the Fabry-Perot resonator, avoid approximations, and derive its generic Airy distribution, equaling the internal resonance enhancement, and all related Airy distributions, such as the commonly known transmission. We verify that the sum of the mode profiles of all longitudinal modes is the fundamental physical function characterizing the Fabry-Perot resonator and generating the Airy distributions. We investigate the influence of frequency-dependent mirror reflectivities on the mode profiles and the resulting Airy distributions. The mode profiles then deviate from simple Lorentzian lines and exhibit peaks that are not located at resonant frequencies. Our simple, yet accurate analysis greatly facilitates the characterization of Fabry-Perot resonators with strongly frequency-dependent mirror reflectivities.

  • 12.
    Kores, Cristine C.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Geskus, Dimitri
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Ismail, Nur
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Dijkstra, Meindert
    Bernhardi, Edward H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    The linewidth of distributed feedback resonators: the combined effect of thermally induced chirp and gain narrowing2017In: Laser Resonators, Microresonators, and Beam Control XIX / [ed] Kudryashov, AV Paxton, AH Ilchenko, VS, SPIE - International Society for Optical Engineering, 2017, article id UNSP 1009014-1Conference paper (Refereed)
    Abstract [en]

    Distributed-feedback (DFB) laser resonators are widely recognized for their advantage of generating laser emission with extremely narrow linewidth. Our investigation concerns ytterbium-doped amorphous Al2O3 channel waveguides with a corrugated homogeneous Bragg grating inscribed into its SiO2 top cladding, in which lambda/4 phase-shift provides a resonance and allows for laser emission with a linewidth as narrow as a few kHz. Pump absorption imposes a thermal chirp of the grating period, which has implications for the spectral characteristics of the resonator. Thermal effects on the spectral response of a DFB passive resonator were investigated via simulations using Coupled Mode Theory by considering (i) a constant deviation of the grating period or (ii) a chirp with a linear profile. We report an increase of the resonance linewidth up to 15%. This result is due to two factors, namely changes of the grating reflectivity at the resonance frequency up to 2.4% and of the shift of resonance frequency up to 61 pm due to an accumulated phase shift imposed on the grating by the chirp profile. The linewidth decrease due to gain is on the order of 106, which is a much larger value. Nevertheless, according to the Schawlow-Townes equation the linewidth increase of the passive resonator due to a thermal chirp quadratically increases the laser linewidth.

  • 13.
    Kores, Cristine Calil
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Geskus, Dimitri
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Ismail, Nur
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Dijkstra, M.
    Bernhardi, E. H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Characterization of Ultranarrow-Linewidth Distributed-Feedback Resonators Below Laser Threshold2016Conference paper (Refereed)
  • 14.
    Kores, Cristine Calil
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Ismail, N.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Geskus, D.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Dijkstra, M.
    Univ Twente, MESA Inst, Opt Sci, NL-7500 AE Enschede, Netherlands..
    Bernhardi, E. H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. Univ Surrey, Adv Technol Inst, Guildford GU2 7XH, Surrey, England..
    Temperature Dependence of Spectral Characteristics of Distributed-Feedback Resonators2018In: FIBER LASERS AND GLASS PHOTONICS: MATERIALS THROUGH APPLICATIONS / [ed] Taccheo, S Mackenzie, JI Ferrari, M, SPIE-INT SOC OPTICAL ENGINEERING , 2018, article id 106830BConference paper (Refereed)
    Abstract [en]

    The spectral response of a distributed-feedback resonator with a thermal chirp is investigated. An Al2O3 channel waveguide with a surface Bragg grating inscribed into its SiO2 top cladding is studied. A linear temperature gradient along the resonator leads to a corresponding variation of the grating period. We characterize its spectral response with respect to wavelength and linewidth changes of the resonance peak. Simulated results show good agreement with the experimental data, indicating that the resonance wavelength is determined by the total accumulated phase shift. The calculated grating reflectivities at the resonance wavelength largely explain the observed changes of the resonance linewidth. This agreement demonstrates that the linewidth increase is caused by the increase of resonator outcoupling losses.

  • 15.
    Kores, Cristine Calil
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Ismail, Nur
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Bernhardi, E. H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Distributed phase shift and lasing wavelength in distributed-feedback resonators2019Conference paper (Refereed)
  • 16.
    Kores, Cristine Calil
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Ismail, Nur
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Bernhardi, E. H.
    KTH.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. Univ. of Surrey (United Kingdom) .
    Lasing wavelength in dielectric distributed-feedback lasers with a distributed phase shift2019Conference paper (Refereed)
    Abstract [en]

    Distributed-feedback waveguide lasers based on Bragg-grating resonators generate ultranarrow-linewidth emission. Oscillation at the center of the reflection band ensures maximum reflectivity, hence minimum laser linewidth. The required μ/2 phase shift is often introduced by a distributed change in effective refractive index, e.g. by adiabatically widening the waveguide. Despite careful design and fabrication, the experimentally observed resonance wavelength deviates, thereby placing the resonance and laser emission at a position with lower reflectivity inside the reflection band. This effect is usually incorrectly attributed to fabrication errors. Here we show theoretically and experimentally that the decay of light intensity during propagation from the phase-shift center into both sides of the Bragg grating due to (i) reflection by the periodic grating and (ii) the adiabatic refractive-index change causes an incomplete accumulation of designed phase shift, thereby systematically shifting the resonance to a shorter wavelength. Calculations are performed based on the characteristic-matrix approach. Experimental studies are carried out in a distributed-feedback channel-waveguide resonator in amorphous Al2O3 on silicon with a distributed phase shift introduced by adiabatic widening of the waveguide according to a sin2 function. Calculations and experiments show good agreement. Considering in the design the overlap integral between distributed phase shift and light intensity provides the desired performance.

  • 17.
    Kores, Cristine Calil
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Ismail, Nur
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Bernhardi, E. H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. Univ. of Surrey (United Kingdom).
    Spectral behavior of integrated distributed-feedback resonators utilizing a distributed phase shift2019Conference paper (Refereed)
    Abstract [en]

    Bragg-grating-based distributed-feedback waveguide resonators, with a discrete phase shift introduced inside the Bragg grating, exhibit within their grating reflection band a Lorentzian-shaped resonance line with an ultranarrow linewidth. If the phase shift is π/2, the resonance is located at the center of the reflection band, i.e., at the Bragg wavelength, where the grating reflectivity is maximum, hence the resonance linewidth is minimum. Alternatively, the required π/2 phase shift is often introduced by a distributed change in effective refractive index, e.g. by adiabatically widening the waveguide. Despite careful design and fabrication, the experimentally observed resonance wavelength deviates from the designed one. Besides deviations owing to fabrication errors, a fundamental, systematic shift towards shorter wavelengths occurs. We show theoretically and experimentally that the decay of light intensity during propagation from the phase-shift center into both sides of the Bragg grating due to (i) reflection by the periodic grating and (ii) the adiabatic refractive-index change causes an incomplete accumulation of designed phase shift by the oscillating light, thereby systematically shifting the resonance to a shorter wavelength. Calculations are performed based on the characteristic-matrix approach. Experimental studies are carried out in distributed-feedback channel-waveguide resonators in an amorphous aluminum oxide thin film on silicon with a distributed phase shift introduced by adiabatic widening of the waveguide according to a sin2 function. Calculations and experiments show good agreement. Considering in the design the overlap integral between distributed phase shift and light intensity provides a performance that is much closer to the desired value.

  • 18.
    Kores, Cristine Calil
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Ismail, Nur
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Bernhardi, Edward H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Accumulation of distributed phase shift in distributed-feedback lasers2018In: Optics InfoBase Conference Papers, OSA - The Optical Society , 2018Conference paper (Refereed)
    Abstract [en]

    In distributed-feedback lasers, a distributed phase shift of /2, seemingly placing the resonance at the Bragg wavelength, is not accumulated because of light confinement and asymmetry, leading to a systematic resonance shift to shorter wavelengths.

  • 19.
    Kores, Cristine Calil
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Ismail, Nur
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Bernhardi, Edward H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Laurell, Fredrik
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Accumulation of Distributed Phase Shift in Distributed-Feedback Resonators2019In: IEEE Photonics Journal, ISSN 1097-5764, E-ISSN 1943-0655, Vol. 11, no 1, article id 1500109Article in journal (Refereed)
    Abstract [en]

    Distributed-feedback waveguide lasers based on Bragg-grating resonators generate ultranarrow-linewidth emission. Oscillation at the center of the reflection band ensures maximum reflectivity, hence minimum linewidth. The required pi/2 phase shift is often introduced by a distributed change in effective refractive index, e.g., by widening the waveguide. Despite careful design and fabrication, the experimentally observed resonance wavelength deviates from the designed wavelength. Even when thermally induced chirp or fabrication errors are negligible, this deviation is still present. Here, we show theoretically and experimentally that this deviation is of fundamental nature. The decay of light intensity during propagation from the phase-shift center into both sides of the Bragg grating due to reflection by the periodic grating and the refractive index change causes an incomplete accumulation of designed phase shift, thereby systematically shifting the resonance to a shorter wavelength. Considering the overlap integral between the distributed phase shift and light intensity in the design provides the desired performance.

  • 20.
    Kores, Cristine Calil
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Laser Physics.
    Ismail, Nur
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Geskus, Dimitri
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Dijkstra, M.
    Bernhardi, Edward H.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Frequency and linewidth dependence of distributed-feedback resonators on thermal chirp2017In: 2017 19th International Conference on Transparent Optical Networks (ICTON), IEEE Computer Society, 2017, article id 8025100Conference paper (Refereed)
    Abstract [en]

    We present an analysis of the spectral characteristics of distributed-feedback (DFB) laser resonators with thermally chirped distributed mirrors. Such DFB resonators have the interesting capability of producing a linewidth as narrow as a few kHz. The investigated devices are ytterbium-doped amorphous Al2O3 channel waveguides with a periodic Bragg grating inscribed into its SiO2 top cladding. The resonance in the spectral response of the resonator results from a distributed λ/4 phase-shift produced by increasing the waveguide width. Its frequency is determined by the period of the Bragg grating, whereas its linewidth is determined by the intrinsic losses and the outcoupling losses of the resonator. When such a device is optically pumped to achieve gain and eventually lasing, the grating period becomes thermally chirped, thereby influencing the spectral characteristics of the resonator. We investigate experimentally and via simulations the frequency and linewidth of the resonance in the presence of a thermally induced linear chirp on the grating profile. Experiments and simulations show good quantitative agreement.

  • 21.
    Kores, Cristine Calil
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics.
    Ismail, Nur
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Geskus, Dimitri
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Dijkstra, M.
    Bernhardi, Edward H.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Spectral broadening in distributed-feedback resonators due to thermal chirp2018In: Optics InfoBase Conference Papers, Optical Society of America, 2018Conference paper (Refereed)
    Abstract [en]

    We investigate the spectral response of an unpumped distributed-feedback waveguide resonator, subject to thermal chirp. Experimental and simulated results indicate that such chirp causes an increase in outcoupling losses and, therefore, in the resonance linewidth.

  • 22.
    Loiko, P.
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Pollnau, M.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. Univ Surrey, Adv Technol Inst, Guildford GU2 7XH, Surrey, England..
    Stochastic Model of Energy-Transfer Processes Among Rare-Earth Ions2018In: FIBER LASERS AND GLASS PHOTONICS: MATERIALS THROUGH APPLICATIONS / [ed] Taccheo, S Mackenzie, JI Ferrari, M, SPIE-INT SOC OPTICAL ENGINEERING , 2018, article id 106831XConference paper (Refereed)
    Abstract [en]

    Energy-transfer processes strongly affect the performance of lanthanide-doped photonic devices. In this work, we introduce a simple stochastic model of energy-transfer processes and successfully apply it to the example of cross-relaxation (CR) and energy-transfer upconversion (ETU) in amorphous Al2O3:Tm3+ waveguides on silicon intended for lasers operating at similar to 2 mu m. The stochastic model is based on the rate-equation formalism and considers two spectroscopically distinct ion classes, namely single ions and ions with neighbours (pairs and clusters), with the corresponding ion fractions being dependent on the doping concentration. We prove that a more accurate description of the luminescence properties of amorphous Al2O3:Tm3+ is obtained when accounting for the presence of these distinct ion classes. Based on the developed model, we derive microscopic CR and ETU parameters of C-CR = 5.83 x 10(-38) cm(6)s(-1), C-ETU1 = 0.93 x 10(-40) cm(6)s(-1), and C-ETU2 = 7.81 x 10(-40) cm(6)s(-1), and determine the laser quantum efficiency eta(q) of excitation of Tm3+ ions in the upper laser level. For the maximum Tm3+ concentration of 5.0 x 10(20) cm(-3) studied experimentally in this investigation, eta(q) reaches 1.73. Furthermore, the transition cross-sections at the pump and laser wavelengths are determined. For the H-3(6) -> F-3(4) transition, the maximum stimulated-emission cross-section is sigma(e) = 0.47 x 10(-20) cm(2) at 1808 nm.

  • 23.
    Loiko, Pavel
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Ismail, Nur
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Bradley, Jonathan D. B.
    Götelid, Mats
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Refractive-index variation with rare-earth incorporation in amorphous Al2O3 thin films2017In: Journal of Non-Crystalline Solids, ISSN 0022-3093, E-ISSN 1873-4812, Vol. 476, p. 95-99Article in journal (Refereed)
    Abstract [en]

    Rare-earth-doped amorphous aluminum oxide (Al2O3;RE3+) thin films are attractive materials for near-IR amplifiers and lasers that can be integrated with silicon-on-insulator waveguides or deposited onto CMOS-fabricated integrated optical structures. We characterize reactively co-sputtered Al2O3;Tm3+ films on silicon by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The refractive index is systematically studied for different Tm3+ concentrations. The resulting increase of refractive index is explained by analyzing the mechanism of incorporating Tm3+ ions into the amorphous Al2O3 network. Sellmeier dispersion formulas are presented.

  • 24.
    Loiko, Pavel
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Stochastic Model of Energy-Transfer Processes Among Rare-Earth Ions. Example of Al2O3:Tm3+2016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 46, p. 26480-26489Article in journal (Refereed)
    Abstract [en]

    Energy-transfer processes strongly affect the performance of lanthanide-doped photonic devices. In this work, we introduce a simple stochastic model of energy-transfer processes and successfully apply it to the example of cross-relaxation (CR) and energy-transfer upconversion (ETU) in amorphous Al2O3:Tm3+ waveguides on silicon intended for lasers operating at similar to 2 mu m. The stochastic model is based on the rate-equation formalism and considers two spectroscopically distinct ion classes, namely single ions and ions with neighbors (pairs and clusters), with the corresponding ion fractions being dependent on the doping concentration. We prove that a more accurate description of the luminescence properties of amorphous Al2O3:Tm3+ is obtained when accounting for the presence of these distinct ion classes. Based on the developed model, we derive microscopic CR and ETU parameters of C-CR = 5.83 X 10(-38) cm(6) s(-1), C-ETU1 = 0.93 x 10(-40) cm(6) s(-1), and C-Eru2 = 7.81 x 10(-40) cm(6) s(-1), and determine the laser quantum efficiency eta(q) of excitation of Tm3+ ions in the upper laser level. For the maximum Tm3+ concentration of 5.0 X 10(20) cm(-3) studied experimentally in this investigation, eta(q) reaches 1.73. Furthermore, the transition cross sections at the pump and laser wavelengths are determined. For the H-3(6) -> F-3(4) transition, the maximum stimulated-emission cross section is sigma(e) = 0.47 X 10(-20) cm(2) at 1808 nm.

  • 25.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Efficient dielectric waveguide lasers2016In: International Conference on Transparent Optical Networks, Institute of Electrical and Electronics Engineers (IEEE), 2016Conference paper (Refereed)
    Abstract [en]

    In the past decade, rare-earth-doped and also transition-metal-doped dielectric waveguides have emerged as highly efficient laser devices. This paper reviews the results in Ti:Sapphire, polymer, sesquioxides, amorphous aluminium oxide, and potassium double tungstates that have recently been obtained in our research group, partly in collaboration with other research groups.

  • 26.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Rare-earth-doped lasers on a silicon chip2014In: Optics InfoBase Conference Papers, OSA - The Optical Society , 2014Conference paper (Refereed)
  • 27.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Rare-earth-doped lasers on a silicon chip2019In: Proceedings 2015 European Conference on Lasers and Electro-Optics - European Quantum Electronics Conference, CLEO/Europe-EQEC 2015, Optical Society of America (OSA) , 2019Conference paper (Refereed)
  • 28.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Rare-Earth-Ion-Doped Channel Waveguide Lasers on Silicon2015In: IEEE Journal of Selected Topics in Quantum Electronics, ISSN 1077-260X, E-ISSN 1558-4542, Vol. 21, no 1, p. 1602512-Article in journal (Refereed)
    Abstract [en]

    This paper reviews the recent developments in rare-earth-ion-doped channel waveguide lasers. Optical gain in rare-earth-ion-doped waveguides has been increased by two orders of magnitude to similar to 1000 dB/cm and waveguide lasers with extremely high slope efficiencies and output powers exceeding the Watt level have been demonstrated. Of particular interest in integrated optics is the recent integration of rare-earth-ion-doped channel waveguide lasers in amorphous materials directly deposited on a silicon substrate. Remarkable performance with respect to slope efficiency, output power, and laser linewidth has been achieved.

  • 29.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Rare-earth-ion-doped waveguide lasers on a silicon chip2015In: Optical Components and Materials XII, SPIE - International Society for Optical Engineering, 2015, Vol. 9359, p. 935910-, article id 935910Conference paper (Refereed)
    Abstract [en]

    Rare-earth-ion-doped materials are of high interest as amplifiers and lasers in integrated optics. Their longer excited-state lifetimes and the weaker refractive-index change accompanied with rare-earth-ion excitation compared to electron-hole pairs in III-V semiconductors provide spatially and temporally stable optical gain, allowing for high-speed amplification and narrow-linewidth lasers. Amorphous Al2O3 deposited onto thermally oxidized silicon wafers offers the advantage of integration with silicon photonics and electronics. Layer deposition by RF reactive co-sputtering and micro-structuring by chlorine-based reactive-ion etching provide low-loss channel waveguides. With erbium doping, we improved the gain to 2 dB/cm at 1533 nm and a gain bandwidth of 80 nm. The gain is limited by migration-accelerated energy-transfer upconversion and a fast quenching process. Since stimulated emission is even faster than this quenching process, lasers are only affected in terms of their threshold, allowing us to demonstrate diode-pumped micro-ring, distributed-feedback (DFB), and distributed-Bragg-reflector (DBR) lasers in Al2O3: Er3+ and Al2O3:Yb3+ on a silicon chip. Surface-relief Bragg gratings were patterned by laser-interference lithography. Monolithic DFB and DBR cavities with Q-factors of 1.35x10(6) were realized. In an Er-doped DFB laser, single-longitudinal-mode operation at 1545 nm was achieved with a linewidth of 1.7 kHz, corresponding to a laser Q-factor of 1.14x1011. Yb-doped DFB and DBR lasers were demonstrated at 1020 nm with output powers of 55 mW and a slope efficiency of 67% versus launched pump power. A dual-phase-shift, dual-wavelength laser was achieved and a stable microwave signal at similar to 15 GHz was created via the heterodyne photo-detection of the two laser wavelengths.

  • 30.
    Pollnau, Markus
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Eichhorn, M.
    Emission cross section, füchtbauer-ladenburg equation, and purcell factor2017In: Nano-Optics: Principles Enabling Basic Research and Applications, Springer Netherlands, 2017, p. 387-404Conference paper (Refereed)
    Abstract [en]

    We review the physics underlying the process of spontaneous emission into a resonator mode. We verify the fundamental modal dimensions, present the spectral mode profile, the Füchtbauer-Ladenburg equation, and the Purcell factor. Furthermore, we obtain the relation between peak emission cross section, radiative lifetime, and emission linewidth.

  • 31.
    Pollnau, Markus
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Eichhorn, M.
    Laser eigenvalue, coherence time, Q-factor, and linewidth2015In: CLEO: Applications and Technology, Optical Society of America, 2015Conference paper (Refereed)
    Abstract [en]

    We generalize the theory of continuous-wave lasers by considering spontaneous emission in the photon rate equation. We relate the laser coherence time, Q-factor, and linewidth to the passive-resonator parameters, thereby unifying resonator and laser theory.

  • 32.
    Pollnau, Markus
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Eichhorn, M.
    The schawlow-townes linewidth - A threefold approximation2014In: Optics InfoBase Conference Papers, OSA - The Optical Society , 2014Conference paper (Refereed)
  • 33.
    Pollnau, Markus
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. University of Twente, Netherlands.
    Hammer, M.
    Dongre, C.
    Hoekstra, H. J. W. M.
    Combined microfluidic-optical DNA analysis with single-base-pair sizing capability2016In: Biomedical Optics Express, ISSN 2156-7085, E-ISSN 2156-7085, Vol. 7, no 12, p. 5201-5207, article id 275075Article in journal (Refereed)
    Abstract [en]

    DNA sequencing by microchip capillary electrophoresis (CE) enables cheap, highspeed analysis of low reagent volumes. One of its potential applications is the identification of genomic deletions or insertions associated with genetic illnesses. Detecting single base-pair insertions or deletions from DNA fragments in the diagnostically relevant size range of 150−1000 base-pairs requires a variance of σ2 < 10−3. In a microfluidic chip post-processed by femtosecond-laser writing of an optical waveguide we CE-separated 12 blue-labeled and 23 red-labeled DNA fragments in size. Each set was excited by either of two lasers powermodulated at different frequencies, their fluorescence detected by a photomultiplier, and blue and red signals distinguished by Fourier analysis. We tested different calibration strategies. Choice of the fluorescent label as well as the applied fit function strongly influence the obtained variance, whereas fluctuations between two consecutive experiments are less detrimental in a laboratory environment. We demonstrate a variance of σ2 ≈4 × 10−4, lower than required for the detection of single base-pair insertion or deletion in an optofluidic chip.

  • 34.
    Pollnau, Markus
    et al.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Univ Twente, Netherlands.
    Hammer, Manfred
    Dongre, Chaitanya
    Hoekstra, Hugo J. W. M.
    DNA separation and fluorescent detection in an optofluidic chip with sub-base-pair resolution2015In: Microfluidics, BioMEMS, and Medical Microsystems XIII, SPIE - International Society for Optical Engineering, 2015, Vol. 9320, article id 93200JConference paper (Refereed)
    Abstract [en]

    DNA sequencing in a lab-on-a-chip aims at providing cheap, high-speed analysis of low reagent volumes to, e.g., identify genomic deletions or insertions associated with genetic illnesses. Detecting single base-pair insertions/deletions from DNA fragments in the diagnostically relevant range of 150-1000 base-pairs requires a sizing accuracy of S < 10(-3). Here we demonstrate S = 4x10(-4). A microfluidic chip was post-processed by femtosecond-laser writing of an optical waveguide. 12 blue-labeled and 23 red-labeled DNA fragments were separated in size by capillary electrophoresis, each set excited by either of two lasers power-modulated at different frequencies, their fluorescence detected by a photomultiplier, and blue/red signals distinguished by Fourier analysis. Different calibration strategies were tested: a) use either set of DNA molecules as reference to calibrate the set-up and identify the base-pair sizes of the other set in the same flow experiment, thereby eliminating variations in temperature, wall-coating and sieving-gel conditions, and actuation voltages; b) use the same molecular set as reference and sample with the same fluorescence label, flown in consecutive experiments; c) perform cross-experiments based on different molecular sets with different labels, flown in consecutive experiments. From the results we conclude: Applying quadratic instead of linear fit functions improves the calibration accuracy. Blue-labeled molecules are separated with higher accuracy. The influence of dye label is higher than fluctuations between two experiments. Choosing a single, suitable dye label combined with reference calibration and sample investigation in consecutive experiments results in S = 4x10(-4), enabling detection of single base-pair insertion/deletion in a lab-on-a-chip.

  • 35.
    Pollnau, Markus
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. Department of Electrical and Electronic Engineering, Advanced Technology Institute, University of Surrey, Guildford, United Kingdom.
    Ismail, Nur
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Novel aspects of the Fabry-Pérot resonator2018In: Advanced Study Institute on NATO ASI on Quantum Nano-Photonics, Springer, 2018, p. 277-295Chapter in book (Refereed)
    Abstract [en]

    We systematically characterize the Fabry-Pérot resonator. We derive the generic Airy distribution of a Fabry-Pérot resonator, which equals the internal resonance enhancement factor, and show that all related Airy distributions are obtained by simple scaling factors. We verify that the sum of the mode profiles of all longitudinal modes generates the Airy distribution. Consequently, the resonator losses are quantified by the linewidths of the underlying Lorentzian lines and not by the measured Airy linewidth. We introduce the Lorentzian finesse which provides the spectral resolution of the Lorentzian lines, whereas the usually considered Airy finesse quantifies the performance of the Fabry-Pérot resonator as a scanning spectrometer. © Springer Nature B.V. 2018.

  • 36. van Dalfsen, K.
    et al.
    Aravazhi, S.
    Grivas, C.
    Garcia-Blanco, S. M.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. University of Twente, Netherlands .
    On the efficiency of Tm-doped 2-mu m lasers2015In: Solid State Lasers XXIV: Technology and Devices, SPIE - International Society for Optical Engineering, 2015, Vol. 9342, article id 93421UConference paper (Refereed)
    Abstract [en]

    A potassium double tungstate layer with the composition KY0.40Gd0.29Lu0.23Tm0.08(WO4)(2) was grown onto a pure KY(WO4)(2) substrate by liquid-phase epitaxy, microstructured by standard lithography and Ar-ion etching, and overgrown by a pure KY(WO4)(2) layer. The end-facets were polished. Laser experiments were performed on these buried, ridge-type channel waveguides in a resonator with one butt-coupled mirror and Fresnel reflection from the other endfacet, resulting in a high output-coupling degree of 89%, compared to intrinsic round-trip losses of only 2%. By pumping with a Ti:Sapphire laser at 794 nm, 1.6 W of output power at 1.84 mu m with a maximum slope efficiency of similar to 80% was obtained. To the best of our knowledge, this result represents the most efficient 2-mu m channel waveguide laser to date. We determined the optimum Tm3+ concentration in double tungstate channel waveguides to be at least 8at.% for efficient lasing. The theoretical limit of the slope efficiency depends on the Stokes efficiency which here is 43.2%, the outcoupling efficiency which here is 99%, and the pump quantum efficiency. The pump quantum efficiency of a 2-mu m Tm3+ laser pumped around 800 nm hinges on the efficiency of its cross-relaxation process. By fitting the macroscopic cross-relaxation parameter which linearly depends on the Tm3+ concentration to concentration-dependent luminescencedecay data, calculating the overall decay rate of the pump level, and deriving the concentration-dependent pump quantum efficiency, we obtain a theoretical limit for the slope efficiency of 83% for the chosen Tm3+ concentration. The experimental slope efficiency of similar to 80% closely approaches this limit.

  • 37. Vazquez-Cordova, S. A.
    et al.
    Bernhardi, E. H.
    Worhoff, K.
    Herek, J. L.
    Garcia-Blanco, S. M.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Univ Twente, Integrated Opt MicroSyst Grp, MESA Inst Nanotechnol, Netherlands.
    Spiral amplifiers in a-Al2O3:Er on a silicon chip with 20 dB internal net gain2015In: INTEGRATED OPTICS: DEVICES, MATERIALS, AND TECHNOLOGIES XIX, 2015, Vol. 9365, article id 93650MConference paper (Refereed)
    Abstract [en]

    Spiral-waveguide amplifiers in erbium-doped amorphous aluminum oxide are fabricated by RF reactive co-sputtering of 1-mu m-thick layers onto a thermally-oxidized silicon wafer and chlorine-based reactive ion etching. The samples are overgrown by a SiO2 cladding. Spirals with several lengths ranging from 13 cm to 42 cm and four different erbium concentrations between 0.5-3.0x10(20) cm(-3) are experimentally characterized. A maximum internal net gain of 20 dB in the small-signal-gain regime is measured at the peak emission wavelength of 1532 nm for two sample configurations with waveguide lengths of 13 cm and 24 cm and erbium concentrations of 2x10(20) cm(-3) and 1x10(20) cm(-3), respectively. The obtained gain improves previous results by van den Hoven et al. in this host material by a factor of 9. Gain saturation as a result of increasing signal power is investigated. Positive net gain is measured in the saturated-gain regime up to similar to 100 mu W of signal power, but extension to the mW regime seems feasible. The experimental results are compared to a rate-equation model that takes into account migration-accelerated energy-transfer upconversion (ETU) and a fast quenching process affecting a fraction of the erbium ions. Without these two detrimental processes, several tens of dB/cm of internal net gain per unit length would be achievable. Whereas ETU limits the gain per unit length to 8 dB/cm, the fast quenching process further reduces it to 2 dB/cm. The fast quenching process strongly deteriorates the amplifier performance of the Al2O3:Er3+ waveguide amplifiers. This effect is accentuated for concentrations higher than 2x10(20) cm(-3).

  • 38. Vazquez-Cordova, S. A.
    et al.
    Yong, Y. -S
    Herek, J. L.
    Garcia-Blanco, S. M.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. University of Twente, P.O. Netherlands.
    Optical gain around 1.5 μm in erbium-doped waveguide amplifiers2015In: 2015 17th International Conference on Transparent Optical Networks (ICTON), IEEE Computer Society, 2015, Vol. 2015, article id 7193702Conference paper (Refereed)
    Abstract [en]

    The development in the field of erbium-doped integrated waveguide amplifiers for the generation of broadband and high-speed optical amplification around 1.5 - 1.6 μm is discussed and current and future potential applications are explored. In erbium-doped amorphous aluminium oxide on a silicon wafer, an internal net gain per unit length of 2.0 dB/cm at the wavelength of 1533 nm and internal net gain over a bandwidth of 80 nm has been demonstrated. Spiral-waveguide amplifiers of different lengths and erbium concentrations have been studied and an internal net gain of 20 dB in the small-signal-gain regime has been achieved. Currently, a highly promising crystalline host material, potassium double tungstate, which provides high emission cross sections to rare-earth ions and has generated an internal net gain per unit length of > 1000 dB/cm when exploiting the ytterbium transition at 1 μm, is doped with erbium ions and tested for amplification around 1.5 - 1.6 μm.

  • 39.
    Vazquez-Cordova, Sergio A.
    et al.
    Univ Twente, MESA Inst, Opt Sci, POB 217, NL-7500 AE Enschede, Netherlands..
    Aravazhi, Shanmugam
    Univ Twente, MESA Inst, Integrated Opt Microsyst, POB 217, NL-7500 AE Enschede, Netherlands..
    Grivas, Christos
    Univ Southampton, Sch Phys & Astron, Southampton SO17 1BJ, Hants, England..
    Yong, Yean-Sheng
    Univ Twente, MESA Inst, Opt Sci, POB 217, NL-7500 AE Enschede, Netherlands..
    Garcia-Blanco, Sonia M.
    Univ Twente, MESA Inst, Opt Sci, POB 217, NL-7500 AE Enschede, Netherlands..
    Herek, Jennifer L.
    Univ Twente, MESA Inst, Opt Sci, POB 217, NL-7500 AE Enschede, Netherlands..
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. Univ Twente, MESA Inst, Integrated Opt Microsyst, POB 217, NL-7500 AE Enschede, Netherlands..
    High gain in erbium-doped channel waveguides2017In: 2017 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE & EUROPEAN QUANTUM ELECTRONICS CONFERENCE (CLEO/EUROPE-EQEC), IEEE , 2017Conference paper (Refereed)
  • 40. Vazquez-Cordova, Sergio A.
    et al.
    Dijkstra, Meindert
    Bernhardi, Edward H.
    Ay, Feridun
    Worhoff, Kerstin
    Herek, Jennifer L.
    Garcia-Blanco, Sonia M.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. University of Twente, Netherlands .
    Erbium-doped spiral amplifiers with 20 dB of net gain on silicon2014In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 22, no 21, p. 25993-26004Article in journal (Refereed)
    Abstract [en]

    Spiral-waveguide amplifiers in erbium-doped aluminum oxide on a silicon wafer are fabricated and characterized. Spirals of several lengths and four different erbium concentrations are studied experimentally and theoretically. A maximum internal net gain of 20 dB in the small-signal-gain regime is measured at the peak emission wavelength of 1532 nm for two sample configurations with waveguide lengths of 12.9 cm and 24.4 cm and concentrations of 1.92 x 10(20) cm(-3) and 0.95 x 10(20) cm(-3), respectively. The noise figures of these samples are reported. Gain saturation as a result of increasing signal power and the temperature dependence of gain are studied.

  • 41. Vázquez-Córdova, S. A.
    et al.
    Aravazhi, S.
    Grivas, C.
    Heuer, A. M.
    Kränkel, C.
    Yong, Y. -S
    García-Blanco, S. M.
    Herek, J. L.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics. University of Twente, Netherlands.
    Spectroscopy of erbium-doped potassium double tungstate waveguides2017In: Integrated Optics: Devices, Materials, and  Technologies XXI, SPIE - International Society for Optical Engineering, 2017, Vol. 10106, article id 1010604Conference paper (Refereed)
    Abstract [en]

    We report the spectroscopy of crystalline waveguide amplifiers operating in the telecom C-band. Thin films of erbiumdoped gadolinium lutetium potassium double tungstate, KGdxLuyEr1-x-y (WO4)2, are grown by liquid-phase epitaxy onto undoped potassium yttrium double tungstate (KYW) substrates and micro-structured by Ar+-beam etching. Channel waveguides with erbium concentrations between 0.45-6.35 × 1020 cm-3 are characterized. The transition cross-sections of interest are estimated. The effect of energy-transfer up-conversion (ETU) is experimentally investigated. Microscopic and macroscopic ETU parameters are extracted from a simultaneous analysis of 20 decay curves of luminescence on the transition 4I13/2 → 4I15/2. The correlation between ETU and the doping concentration is studied. Pump excited-state absorption (ESA) on the transition 4I11/2 → 4F7/2 is investigated via a direct ESA measurement using a double-modulation pump-probe technique. The effect of ESA is studied for different pump wavelengths. The pump wavelength of 984.5 nm is found to be favorable for the complete range of erbium concentrations.

  • 42. Vázquez-Córdova, S. A.
    et al.
    Aravazhi, S.
    Grivas, C.
    Yong, Y. -S
    García-Blanco, S. M.
    Herek, J. L.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    High gain in erbium-doped channel waveguides2017In: Optics InfoBase Conference Papers, OSA - The Optical Society , 2017Conference paper (Refereed)
  • 43. Yong, Y. -S
    et al.
    Aravazhi, S.
    Vázquez-Córdova, S. A.
    Carvajal, J. J.
    Díaz, F.
    Herek, J. L.
    García-Blanco, S. M.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. University of Twente, Netherlands.
    Direct confocal lifetime measurements on rare-earth-doped media exhibiting radiation trapping2017In: Optical Materials Express, ISSN 2159-3930, E-ISSN 2159-3930, Vol. 7, no 2, p. 527-532Article in journal (Refereed)
    Abstract [en]

    Radiation trapping occurs in rare-earth-doped active media with strong spectral overlap of luminescence and ground-state absorption. It is demonstrated experimentally that a confocal measurement mitigates the influence of radiation trapping on the measured luminescence lifetime, hence allowing for direct extraction of the lifetime from the measured decay curves. The radiation trapping effect is largely suppressed by probing a small sample volume and rejecting the photons reemitted from the unpumped region. This non-destructive measurement method is applied to ytterbium (Yb3+) activated potassium double tungstate crystalline layers with Yb3+ concentrations ranging from 1.2 at.% up to 76 at.% (~8 × 1019 - 5 × 1021 cm-3). The measured lifetime values are comparable to the results reported for Yb3+-doped potassium double tungstate powder diluted in liquid.

  • 44. Yong, Y. S.
    et al.
    Aravazhi, S.
    Vázquez-Córdova, S. A.
    García-Blanco, S. M.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    1050 dB/cm Gain in a 57.5at.% Yb-doped KGd(WO4)2 thin film at 981 nm2014In: Optics InfoBase Conference Papers, OSA - The Optical Society , 2014Conference paper (Refereed)
  • 45. Yong, Y. -S
    et al.
    Aravazhi, S.
    Vázquez-Córdova, S. A.
    Herek, J. L.
    García-Blanco, S. M.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics.
    Temperature dependence of transition cross sections in rare-earth-doped laser materials2017In: Optics InfoBase Conference Papers, OSA - The Optical Society , 2017Conference paper (Refereed)
  • 46. Yong, Y. -S
    et al.
    Aravazhi, S.
    Vázquez-Córdova, S. A.
    Herek, J. L.
    García-Blanco, S. M.
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics. University of Twente, Netherlands.
    Temperature-dependent absorption and gain of ytterbium-doped potassium double tungstates for chip-scale amplifiers and lasers2017In: Integrated Optics: Devices, Materials, and  Technologies XXI, SPIE - International Society for Optical Engineering, 2017, Vol. 10106, article id 1010606Conference paper (Refereed)
    Abstract [en]

    Ytterbium-doped potassium rare-earth double tungstate thin films are excellent candidates for highly efficient waveguide lasers, as well as high-gain waveguide amplifiers, with a record-high optical gain per unit length of 935 dB/cm recently demonstrated. However, the spectroscopic properties of these highly ytterbium-doped thin films and, in particular, their temperature dependence are not well investigated. These characteristics are required for the understanding of the behavior of the fabricated optical devices and crucial for further device optimization. We experimentally determined the absorption cross-sections for a potassium ytterbium gadolinium double tungstate, KYb0.57Gd0.43(WO4)2, thin film grown lattice matched onto an undoped KY(WO4)2 substrate. At room temperature, the peak cross-section value at 981 nm and the overall absorption spectrum are very similar to those of Yb-doped bulk potassium double tungstate crystals, although Yb is now the dominating rare-earth content. The temperature-dependent study shows a significant decrease of the absorption cross-section values at 933 nm and 981 nm with increasing temperature. We verify theoretically that this is due to the temperature dependence of fractional populations in the individual Stark levels of the absorbing crystal-field multiplet, in combination with the linewidth broadening with increasing temperature. Further investigations suggest that the broadening of absorption linewidth at 981 nm originates in the intra-manifold relaxation between the two lowest Stark levels of the ground state. Finally, the implications of the spectroscopic findings on the operating characteristics of waveguide amplifiers are investigated. Amplifiers operating at 80 °C are expected to exhibit only 67% of the maximum theoretical gain at room temperature.

  • 47. Yong, Yean-Sheng
    et al.
    Aravazhi, Shanmugam
    Vazquez-Cordova, Sergio A.
    Carjaval, Joan J.
    Diaz, Francesc
    Herek, Jennifer L.
    Garcia-Blanco, Sonia M.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Temperature-dependent absorption and emission of potassium double tungstates with high ytterbium content2016In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 24, no 23, p. 26825-26837Article in journal (Refereed)
    Abstract [en]

    We study the spectroscopic properties of thin films of potassium ytterbium gadolinium double tungstates, KYb0.57Gd0.43(WO4)(2), and potassium ytterbium lutetium double tungstates, KYb0.76Lu0.24(WO4)(2), specifically at the central absorption line near 981 nm wavelength, which is important for amplifiers and lasers. The absorption cross-section of both thin films is found to be similar to those of bulk potassium rare-earth double tungstates, suggesting that the crystalline layers retain their spectroscopic properties albeit having >50 at.% Yb3+ concentration. The influence of sample temperature is investigated and found to substantially affect the measured absorption cross-section. Since amplifiers and lasers typically operate above room temperature due to pump-induced heating, the temperature dependence of the peak-absorption cross-section of the KYb0.57Gd0.43(WO4)(2) is evaluated for the sample being heated from 20 degrees C to 170 degrees C, resulting in a measured reduction of peak-absorption cross-section at the transitions near 933 nm and 981 nm by similar to 40% and similar to 52%, respectively. It is shown that two effects, the change of Stark-level population and linewidth broadening due to intra-manifold relaxation induced by temperature-dependent electronphonon interaction, contribute to the observed behavior. The effective emission cross-sections versus temperature have been calculated. Luminescence-decay measurements show no significant dependence of the luminescence lifetime on temperature.

  • 48.
    Yong, Yean-Sheng
    et al.
    Univ Twente, MESA Inst Nanotechnol, Opt Sci Grp, POB 217, NL-7500 AE Enschede, Netherlands..
    Aravazhi, Shanmugam
    Univ Twente, MESA Inst Nanotechnol, Integrated Opt Microsyst Grp, POB 217, NL-7500 AE Enschede, Netherlands..
    Vazquez-Cordova, Sergio A.
    Univ Twente, MESA Inst Nanotechnol, Opt Sci Grp, POB 217, NL-7500 AE Enschede, Netherlands..
    Herek, Jennifer L.
    Univ Twente, MESA Inst Nanotechnol, Opt Sci Grp, POB 217, NL-7500 AE Enschede, Netherlands..
    Garcia-Blanco, Sonia M.
    Univ Twente, MESA Inst Nanotechnol, Opt Sci Grp, POB 217, NL-7500 AE Enschede, Netherlands..
    Pollnau, Markus
    KTH, School of Engineering Sciences (SCI), Applied Physics, Materials and Nanophysics. Univ Twente, MESA Inst Nanotechnol, Integrated Opt Microsyst Grp, POB 217, NL-7500 AE Enschede, Netherlands..
    Temperature Dependence of Transition Cross Sections in Rare-earth-doped Laser Materials2017In: 2017 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE & EUROPEAN QUANTUM ELECTRONICS CONFERENCE (CLEO/EUROPE-EQEC), IEEE , 2017Conference paper (Refereed)
  • 49. Yong, Y.S.
    et al.
    Aravazhi, S.
    Vázquez-Córdova, S.A.
    García-Blanco, S.M.
    Pollnau, Markus
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics. Integrated Optical MicroSystems Group, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, Enschede, Netherlands.
    Highly Yb-doped KGd(WO4)2 thin-film amplifier2015In: CLEO: Science and Innovations 2015, Optical Society of America, 2015Conference paper (Refereed)
    Abstract [en]

    We report record-high small-signal gain of 1050 dB/cm at 981 nm wavelength in a KGd<inf>0.425</inf>Yb<inf>0.575</inf>(WO<inf>4</inf>)<inf>2</inf> thin film. The sensitivity of gain to the shift of beam-focus position, which is critical under non-waveguiding conditions, is investigated.

1 - 49 of 49
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