High resolution optical coherent-channel analyzer using balanced-coherent detection and temperature-tuned DFB laser as local oscillator




Isaac, Rejoy

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The rapid increase in demand for bandwidth in optical networks over the last two decades has led to the development of wavelength division multiplexing where multiple channels are transmitted simultaneously at different wavelengths over a single optical-fiber to maximize the usage of the bandwidth available in fiber. Increasing demand for bandwidth has led to narrower channel spacing and the use of advanced modulation schemes that are more spectrally efficient than traditional on-off keying techniques [1]. Nonlinearities and dispersion effects in fiber accumulate over a long distance and can adversely affect the quality of a channel. Hence the ability to measure detailed features of the optical spectrum is crucial to study the performance of a communication link. A conventional optical spectrum analyzer (OSA) based on a diffraction grating has a wide wavelength or frequency scanning range, but suffers from poor frequency resolution. The narrowest resolution bandwidth reported for a grating based OSA is - 0.06nm or 7.5GHz at 1550nm [1]. Various spectral features of interest, such as the transmission spectrum of a laser and modulation spectrum of a channel require sub-picometer resolution, which cannot be achieved by conventional methods using a diffraction grating. High resolution spectrum analyzers (HRSAs) have been built based on heterodyne detection where a portion of the optical spectrum is converted to radio-frequency (RF) with DC corresponding to the local-oscillator (LO) central frequency [3-6]. This is a common technique used in RF-spectrum analyzers. The resolution bandwidth is determined by the electrical bandwidth of the optical receiver. The lowest resolution bandwidth reported is of the order of tens of MHz [6]. Widespread implementation of these instruments however, has been limited owing to their cost and size, one of the major factors being the external cavity (ECT) lasers used as the local-oscillator source in such instruments. We have built a coherent-channel analyzer (CCA) based on balanced coherent detection using a commercial distributed-feedback (DFB) laser as the LO. The use of a DFB laser for the CCA has the potential of reducing the cost of the instrument by at least one-tenth of the price of an HRSA. In this thesis we describe the working of the CCA. We provide an end-to-end system model, analyze the resolution and sensitivity performance of the system, and demonstrate a frequency resolution of 100MHz over the DFB tuning range of 200GHz with a sensitivity of -95dBm. The CCA provides a practical, cost and size effective alternative to the HRSA at the cost of tunability.



bandwidth, optical networks