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Investigation of Nonlinear Optical Organic Glass Waveguides and their Applications
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MSc. Edgars Nitišs (ISSP UL)

In the last couple of decades photonics related research and industry has experienced a rapid growth. According to Photonics21 (the European Technology Platform for photonics) Multiannual Strategic Roadmap 2014 – 2020 [1], in the nearest future photonics will have the main role in the development of measurement technology and informatics. It ought to overcome limitations of electronics in computers through optical computing, provide cheap and effective solutions for optical communications and sensing etc. The field of information technology (IT) has been benefiting from the development of optical communication technology greatly since the first demonstration of low loss (20 dB/km at 546 nm) optical fiber 1979 by Kapron, Keck and Maurer [2]. The optical communications industry has increased exponentially ever since. According to report provided by Cisco, currently the optical communication networks ensure data transmission rates at approximately 1.6 Exabytes per month mainly due to upload and download of visual content such as videos and images [3]. The same report states that the actual demand for data transmission rate will exceed 120 Exabytes per month in year 2017. To meet these requirements, new dedicated data transmission solutions will be required.

In the device level the growth in data transmission rate may be obtained through increasing the speed and efficiency of individual electro-optically (EO) active elements of the network. One of such elements, that determines the data rate, is the EO modulator. It uses electrical signal to modulate the light intensity or phase. Such modulation can carry information over optical fibers both in long-haul and short-haul communication networks. For the EO modulation of light intensity or phase an optically intrinsic material is necessary. A current standard is to use lithium niobate (LiNbO3) as the (NLO) material in the EO modulators [4]. LiNbO3 is a nonlinear crystalline material with rather low EO coefficients and high dielectric constants. These, along with absorption coefficient, are the main material parameters that determine the operational speed of the modulator. The fact that EO coefficients of the material are rather low, some requirements towards the device must be set. Firstly, the device must be made longer to obtain considerable EO modulation at low voltages and, secondly, it must work in a travelling-wave regime. Unfortunately, at high frequencies i) the high dielectric constants of the LiNbO3 cause the mismatch between the optical wave and modulating wave to appear and ii) dielectric and electrode losses begin to dominate the drop in the optical response. Due to this, the LiNbO3 based devices have the potential to operate only up to 100 GHz [5]. A recent trend is to consider organic NLO materials for application as active medium in the EO modulators. The NLO active organic materials are particularly interesting due to their multiple advantageous properties such as low cost, low dielectric constants, high nonlinearity (up to 300 pm/V) and others [6]. Due to these characteristics of the NLO polymers, a considerable effort has been devoted towards development of organic or hybrid waveguide EO modulators, which implement EO organic material as the waveguide core thus increasing the EO efficiency of the device [7]. In most cases a polymer EO modulator can be used for operation in the visible spectral range. Already in 2010 one of industry leaders GigOptix presented a commercially available polymer EO modulator capable of operating at 100 Gb/s. Unfortunately, the EO polymers still possess insufficient thermal stability which limits their application in the optical communication networks. Thus considerable effort is devoted towards research and characterization of new NLO organic materials and development of new type of EO modulator designs.

In these theses the author concentrates on tackling the relevant issues in

  1. preparation of poled NLO active organic materials;
  2. implementation of methods for linear and non-linear optical characterization of NLO organic materials;
  3. development of a new type of CMOS compatible polymer/organic glass EO modulator.

The NLO organic materials must possess large second-order nonlinear coefficients which can be obtained by electric poling with corona discharge [8]. For maximal possible NLO efficiency one must achieve the highest polar order of dopant molecules in the system maintaining the chromophore structure, concentration and optical properties of thin films. For polymer poling purposes the corona triode device is very attractive because one can have good control of the ion source and the poling field. The author reports on investigation of the polymer film morphology modifications during their corona poling for fabrication of NLO active materials. It is demonstrated that at certain poling conditions surface and spatial inhomogeneities in the poled area of the sample appear. Densities of the inhomogeinities depend on the strength of the poling field, the sample temperature during the poling, and the pre-poling conditions. Optimization of the poling conditions directed toward avoiding surface modifications increases the overall observable effective nonlinearity of the sample up to 10 times. To investigate, understand, and eventually explain the formation of the spatial and surface structure inhomogeinites in the poled material optical, second harmonic, and scanning electron microscope measurements, as well as the conductivity measurements of the thin films were used.

After NLO organic material poling, it requires full characterization of its linear and non-linear optical properties. For the characterization of linear properties of the material, spectral reflectometry and Kramers-Kronig transformation based techniques are implemented. The nonlinearity of the organic material is characterized by means of EO coefficients which for thin film samples can be measured by various techniques [9]. Multiple techniques that employ the measurement of thin film EO coefficients have been developed, however, the most widely used include the Fabry-Perrot technique, transmission polarimetric techniques, the Mach – Zehnder interferometric (MZI) technique, Teng – Man (TM) technique and the attenuated total reflectance (ATR) technique [10]. All these techniques employ the modulation depth based measurements. The fact that no one technique has been established as standard for estimation of thin film EO coefficients clearly indicates that not all of the measurements and respective data interpretation is truly straight forward. In this contribution the experimentally retrieved EO coefficients of thin films obtained by three mentioned techniques – MZI, TM and ATR – of the polymer film under investigation will be presented. The author will demonstrate that both EO coefficients (r13 and r33) of poled organic thin films can be determined by applying Abelès matrix formalism to the interpretation of experimental MZI and TM data. Such approach takes into account the thickness variations and multiple internal reflection effect in the thin film, both of which may contribute greatly to the experimentally retrieved values.

The work is concluded by presenting a new type patent-pending Silicon-on-insulator (SOI) - polymer modulator design. One of the great advantages of the proposed structure is the simplicity of preparation since it could be integrated with the cost effective and mature CMOS technology. By choosing optimal structure parameters and material our proposed SOI polymer modulator could operate both in the 800 to 900 nm and in 1260 to 1675 nm communication wavelength ranges. We demonstrate the preparation steps of the SOI – polymer EO modulator as well as results obtained by numeric modeling, based on which we will outline the advantages and drawbacks of the design.


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