Since new modulation schemes have become a great center of interest, Mach-Zehnder fiber fused interferometers (MZI) became necessary to ensure the direct detection (DD) from phase modulated signals, like DPSK (Differential Phase Shift Keying) or DQPSK. As these modulation schemes are very interesting in optical transmissions due to their capabilities, MZIs cannot be overlooked in the future optical links. In this paper, we review the base principles for using MZI as DPSK demodulators, and the major characteristics associated to these devices. In order to evaluate the minimal characteristics required for efficient modulation format conversion, we propose some numerical simulation results. Based on these results, we depicted a new fabrication process for the realization of MZIs by adiabatic stretching of one of the interferometer's arm. This is carried out by a CO2 laser, and the fabrication major points that must be respected are discussed. After review the MZI thermal dependency mechanism, the last part presents the experimental results obtained for the quantification of MZI temperature control accuracy needed for its utilization as an 43 Gbit/s PSBT (Phase Shaped Binary Transmission) encoder.
A simple and handy technique to produce optical fiber components such as Bragg gratings has been used. A CO2 laser beam, focused on a single mode fiber, can change its optical properties locally, allowing therefore the creation of a Bragg grating inside the fiber itself. In the present study we investigate the effect of this laser beam on the fiber and show how optical fiber components can be produced. When exposing a single-mode optical fiber to a high-power CO2 laser beam a small device is developed insdie this fiber. Inside this micro-structure, the optical power is exchanged between the core and the cladding modes. This device may exhibit either a very selective rejection behavior similar to Bragg grating or an oscillating behavior like tapered fiber. The coupled mode theory combined with a mathmatical algorithm can be used to study the propagation of the modes involved in the fiber.
In order to generate an amplitude modulated optical wave up to 275 GHz, we present an heterodyne technique using two 1.55 μm DFB lasers controlled in temperature by PC. Experimental set-up is described. Theoretical and experimental studies of the lasers spectrum are presented. Beating signal is observed in a 60 GHz bandwidth photodiode, frequency and power stability study is reported. Our final aim is the bandwidth measurement of photoreceivers and optical passive components for optical communications.
In the state of polarization principle, a component is cut in a lot of trunks. Each trunk is characterized by vectors linked to local PDL and PMD values and by the Pauli's matrix. This vectorial approach is used to determine the complete characteristics of an optical assembly as a function of PDL and PMD vectors of each trunk. We particularly put in evidence the output PDL vector evolution versus the wave time constant, (e.g. versus the frequency). A study of the PDL maximum and minimum evolution (position and value) is also achieved. This study shows the posituons of the PDL maximum and minimum values are constant for the highest wave time constant values. This point out that the PDL maximum (minimum) value is obtained when the PDL and PMD vectors of each trunk are parallel (anti-parallel respectively). An evolution of this values and positions is proposed for the lowest wave time constant values. For a sufficiently high frequency, it is possible to demonstrate the assembly had an almost null PDL.
Optical components based on geometrical and refractive index variations could be studied by coupled mode theory. For that we introduce a new coupling coefficient which takes into account not only the geometrical variations, but also the core refractive index variations along the propagation axis for the coupled mode theory. The results show that the coupling coefficient between modes can be separated in a sum of two coefficients. The first one is the classic coefficient which takes into account the radius variation along axis. The second one is reported in order to study the index variation. We examine with more details the effect of the core dopants diffusion due to the heating during the fabrication process. The concatenation of a biconical tapers and long period grating is presented.
Local mode coupling theory is used for the study of the tapered fibres with a variation of the core index along the propagation axis. An application to the diffusion of the fibre dopants during the realization by melting with a CO2 laser is done. The results show that the coupling coefficient between modes can be separated in a sum of two coefficients. The first one is the classic coefficient which takes into account the core radius variation along axis. The second one is reported in order to analyse the index core variation.
We describe a global method for the generation of an amplitude modulated optical wave in the range of telecommunication frequencies. This method is based on the optical heterodyning of two DFB lasers. With the assumption of longitudinal multimode lasers, we study the influence of the laser ray width on the spectral purity of the signal generated by the photodiode. We describe the experimental set-up which permits to obtain a tunability of the source up to 275 GHz. We present the results obtained in reception by beat of two identical lasers in a large bandwidth photodiode. A study of the power and frequency stability of the generated signal in the quadratic receiver is carried out for several hours. An experimental fusion set-up by CO2 laser has been developed in our laboratory to ensure the realisation of fibre optical components. Studying these components behaviour under fast optical wave variation is a new approach which brings some useful information regarding component bandwidth. This property enables the validation of components for high rate transmissions. The lasers beating set-up is successfully used to test tapered fibres. The optical component is placed at the coupler output. Tapered fibre bandwidth is measured by an optical wave modulation frequency sweeping in the 0-40 GHz range.
We describe a method for generating an amplitude modulated optical wave in the range of telecommunication frequencies. The principle is based on optical heterodying of two free DFB lasers. Frequency modulation of the optical wave resulting from beat is equal to the difference of the two laser wave frequencies. Control in temperature of laser sources ensure their wavelength stability. We theoretically study and also simulate the influence of the spectral width of the laser rays on spectral purity of the generated signal in a quadratic receiver. We then describe the experimental set up which permits to obtain a tunability of the source up to 275 GHz. We present the results obtained in reception by beat of two identical lasers in a 60 Ghz bandwidth photodiode. The signal generated in quadratic receiver is in terms of spectral width (FWHM) of about 37 MHz for a power level of - 35 dBm. Study of power and frequency stability of the generated signal in photodiode is carried out. This work permits to test large bandwidth photodiodes, it would also allow to characterize in bandwidth passive optical components.
Hemispherical microlenses have been fabricated at the end of a single-mode optical fiber by a new two-steps method which uses only a continuous CO2 laser. In the first step, the fiber is heated by the laser and stretched until its split resulting in two symmetrical tapers. In the second step, we form the microlenses by laser melting the taper ends. Parameters of heating and stretching are automated and optimized to ensure the reproducibility. The microlens characterization, including the focal length and beam waist measurements, has been performed. It shows the interest of the microlens-fiber to several applications.
Many possibilities are offered to analyze the field in tapered fibers. We have used in this work the bpm method and the coupled-mode theory for a conical fiber. In the second case, it is shown by a length-scale criterion that some geometries should provide a good efficiency. The comparison between the quasi 3D numerical simulation and the local mode method is also performed. The complete resolution procedure is then dependent on certain essential approximations and simplifying hypothesis.
Many possibilities are offered to compute field. We have used in this work the bpm method and the local mode method for a conical fiber. In the second case, it is shown by a length-scale criterion that some geometry should provide a good efficiency. We examine the fiber end and show that an appropriate lens form makes it possible to adapt the power distribution between the optical fiber and the photodiode. Some taper shapes are suggested and the model accuracy is discussed according to the nature of the decrease criterion. We obtain in this case an assessment along this guide that is optimized. The comparison of this results with the quasi 3D numerical simulation of the guided optical propagation is also performed.
In this paper, we describe a sensor configuration based on optical coupling between two optical fibers submitted simultaneously to the same constraint. Theoretical and experimental results are reported on photoelasticity and the evanescent field of these coupled fibers due to geometrical deformations. Our purpose was to build a time multiplexed sensor network by coupling short pulses. This arrangement exhibits high sensitivity and resolution without any drawbacks of the classical OTDR.
This sensor is based on the optical coupling between two optical fibers, submitted simultaneously to the same constraint. Due to the geometrical deformations, photoelasticity, and evanescent field this coupling was studied theoretically and experimentally, and used to build a time multiplexed sensor network by coupling short pulses. This arrangement has high sensitivity and resolution, without some drawbacks of the classical OTDR.
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