Ultra-high Resolution Software-defined Optical Filter Design And Its Applications

As a fundamental optical signal processing technique,optical filtering is widely used in optical communications and microwave photonics.It is an essential module enabling the pass or the suppression of an optical signal or an electrical signal modulated on the light.It can also transform the spectra or the waveforms of the optical signal in the frequency domain or in the time domain.With the continuous development in the field of optical communications and microwave photonics,there have been higher requirements for the filter resolution and precision.The high-resolution optical filter will be an important functional device to promote the all-optical flexible networking and the development of microwave photonics.It will also spawn new frontier interdisciplines.The higher reconfigurable resolution,wider central wavelength tunability and higher filter rejection ratio are always the development trends.However,compared with the microwave signal,the light wave has a very high frequency,which makes it more difficult to control.At present,the control precision of optical filters with bandwidths in GHz range is still not high enough,and it hinders the further development of high-precision optical signal processing.Stimulated Brillouin scattering(SBS)is a fundamental optical nonlinear effect in the optical fiber.Its natural linewidth of only 10 to 30 MHz provides very high frequency selectivity and is ideal for high-resolution optical signal processing.In this dissertation,we use SBS effect to realize an ultra-high-resolution software-defined optical filter with high precision.We also propose filter optimizations in terms of the response reconfigurability,central wavelength tunability,polarization sensitivity,filter rejection ratio and so on.The main contents and achievements of this dissertation are fourfold: First,we propose a method to digitally control the Brillouin pump spectrum,which dramatically increases the control precision and resolution of the Brillouin gain.Second,we propose ultra-high-resolution software-defined arbitrary-shaped optical filters and microwave photonic filters and take several measures to improve the filter parameters and performance.Third,we apply the proposed filter to two typical applications: the reconfigurable optical add and drop multiplexer(ROADM)and the microwave pulse shaping through photonics.The filter performs well and shows great superiority in these two applications.Four,we further promote the miniaturization and instrumentation of this technique.Specifically,this dissertation includes:1.We propose a method for precise and digital control of the Brillouin gain spectrum.Using stimulated Brillouin scattering as an optical filter has been proposed for more than 30 years.However,the control precision of the Brillouin gain spectrum was quite limited previously.In this dissertation,we propose a method to digitally design the Brillouin pump with high resolution by using an electrical digital to analog converter(DAC)or an arbitrary waveform generator.The high-precision electrical waveform generated by the DAC modulates the Brillouin pump wave in an IQ modulator with carrier-suppressed single-sideband modulation so that the Brillouin pump can be fully configured with very high resolution.In order to overcome the non-ideal factors such as the nonlinearity of the optical and electrical components,we propose a feedback correction technique.The generated pump waveform is iterated multiple times according to the measured Brillouin gain spectrum until the designed shape has been achieved.The high-resolution feedback adjustment of the Brillouin gain is the core contribution of this dissertation.2.We propose a series of filter optimization measures and achieve polarization-insensitive rectangular optical filter with high rejection ratio.In this dissertation,we optimize several filter parameters to significantly improve the filter performance.In order to increase the rejection ratio of the filter,we adopt a cascaded multi-stage Brillouin amplification structure.The Brillouin pump is then used in a more efficient way,and the filter rejection ratio exceeds 40 dB.We employ a single-tone frequency-swept pump to solve the polarization dependent issue of the Brillouin filter.By using a delayed orthogonal swept pump,the filter is polarization insensitive.We finally realize high-precision reconfigurable rectangular optical filter with 50 MHz-3 GHz bandwidth.The 20-dB rectangular shape factor reaches 1.056,which is the best value reported for the narrowband rectangular optical filter.3.We analyze the noise performance of the broadband Brillouin filter via experiments and simulation.Different from traditional passive optical filters,the SBS-based active filter will amplify the in-band signal instead of suppressing the out-of-band signal.The amplification will induce spontaneous emission as noises in the signal and degrade the signal quality.In this dissertation,we analyze the noise induced by the broadband SBS gain and attempt to optimize the noise performance by configuring the filter parameters carefully.We investigate into the effect of different parameters on the noise performance such as the pump broadening methods,the pump and signal power level,the polarization states of the signal,the fiber length and so on.The experiments and simulation results provide some ideas for reducing the noise introduced by the filter,which can further improve the practicality of the filter.4.We demonstrate an ultra-selective flexible reconfigurable optical add and drop multiplexer based on high-resolution reconfigurable rectangular Brillouin filters.The next generation all-optical network requires a high-resolution optical switching with high flexibility,which brings challenges to optical filtering techniques.In this dissertation,we use the proposed reconfigurable narrowband rectangular filter to realize an ultra-selective flexible ROADM and demonstrate the add and drop function of a polarization division multiplexed orthogonal frequency division multiplexed(OFDM)signal.The OFDM signal with a bandwidth of 2 GHz is well selected or rejected with 25 dB amplification or rejection,and the bandgap between two adjacent sub-bands can be set as narrow as 300 MHz,thanks to the quasi-ideal rectangular filter response.The experiment proves that the proposed rectangular filter is capable of filtering the quadrature phase shift keying signal with high fidelity,showing the significant advantages of this scheme.5.We realize software-defined arbitrary-shaped microwave photonic filters with high precision and demonstrate its application in microwave photonics.The microwave photonic filter has higher requirements for the filter precision and flexibility.In this dissertation,by using specially designed pump waveform with high-precision control,we realize software-defined arbitrary-shaped microwave photonic filters.The central frequency of the filter can be tuned with a resolution of 1 MHz.For the first time,the filter response can be precisely defined by software with 15 MHz resolution.We present filters with different responses such as truncated Gaussian,Gaussian,super-Gaussian,triangular shapes and so on.We further demonstrate the microwave pulse shaping using these highly controllable filters.By passing through different filters,the pulse shape has been changed accordingly and precisely.The high agreement between theoretical and experimental results proves once again the extremely high control precision of the filter.6.We propose a low-cost directly-modulated pump approach to promote the practicability and the instrumentation of the filterIn order to further achieve the miniaturization and instrumentation of the Brillouin filter,we propose an alternative approach to obtain gigahertz-wide arbitrary-shaped filters using a cost-effective directly modulated laser(DML)and a DAC with low sampling rate.We manage to precisely adjust the optical spectrum of the DML thereby controlling the Brillouin filter response arbitrarily for the first time.Compared with the previous approach adopting complex IQ modulator,the DML approach has much simpler structure and lower cost while still keeping similar flexibility and performance,which paves the road to realizing compact filters and integrated instruments.Meanwhile,we further design a graphical user interface for handy control of the filter,providing software support for the instrumentation.In this dissertation,we propose a software-defined ultra-high resolution optical filter based on SBS effect in the optical fiber.It aims at improving the control precision and the flexibility of narrowband optical filters in GHz and sub-GHz range.For the first time to the best of our knowledge,the filter shape,bandwidth,and central frequency can all be precisely defined by software with MHz resolution.The goal of this dissertation is to provide a narrowband optical filter with multiple functions and excellent performance as well as high practicability,which can be used in a wide range of applications.In this dissertation,we still call it an optical “filter”.However,its connotation has been far beyond a traditional filter which can only be used for signal selection.In fact,it is more like an optical signal processing platform with high resolution and high precision.The unprecedented multi-dimensional flexibility offers new possibilities to process optical signals and microwave signals in the optical domain.This solution proposed here will play an essential and irreplaceable role in the field of optical communications and microwave photonics.