In this paper, we present two approaches to generating and distributing FCC-regulated UltraWideBand (UWB) pulse
signals in the optical domain, based on optical phase modulation. In the first approach, an electrical Gaussian pulse train
is applied to modulate the phase of an optical carrier using an electrooptic phase modulator. A 25-km single-mode fiber
link is then used to realize PM-IM conversion which has a frequency response equivalent to a microwave bandpass
filter. When the Gaussian pulse train is distributed over the 25-km fiber, the Gaussian pulses are then shaped into
doublet pulses at the receiver front-end. Therefore, the UWB pulses are not generated but also distributed over optical
fiber. In the second approach, instead of using an electrooptic phase modulator, optical phase modulation is
implemented in the optical domain based on cross phase modulation in a nonlinear fiber. The PM-IM conversion is then
achieved by use of a fiber Bragg grating that serves as a frequency discriminator. Electrical monocycle and doublet
pulses are obtained at the output of the photodetector. Experimental results for both approaches are presented and
discussed. The use of the second configuration to implement pulse on-off, polarity and shape modulation in the optical
domain is also discussed.
In this paper, optical phase modulation to intensity modulation by the use of a fiber Bragg grating (FBG) based frequency discriminator is proposed and experimentally demonstrated. In the proposed approach, the optical carrier frequency is placed at the quadrature point of the positive or negative slope of the reflection response of the FBG. The phase modulated light reflected from the two opposite slopes will have a π phase difference, which makes bipolar operation possible in an all-optical microwave signal processor or an optical code division multiple-access system. Both the frequency and phase responses of the FBG are taken into account to build a theoretical model in a frequency domain. Phase modulation to intensity modulation conversion based on a Gaussian apodized FBG is experimentally implemented. The results confirm the theoretical analysis.
In this paper, we propose a novel approach to implementing unipolar-bipolar phase-time encoding/decoding for optical code division multiple access (CDMA) networks. In the proposed approach, an electrooptic phase modulator and two fiber Bragg grating (FBG) arrays are employed to perform En/De coding. At the transmitter, a low-bit-rate data sequence modulates the optical phase and is then mapped to a high-bit-rate optical sequence via the encoder FBG array in a unipolar way. At the receiver, an identical FBG array that functions as a matched filter is used. Bipolar decoding is achieved by locating the optical carriers on either the positive or the negative slopes of the reflection responses of the decoder FBG array. The proposed encoding/decoding scheme is equivalent to a sequence inversion keyed direct sequence CDMA, which can provide an improved performance compared with the conventional incoherent scheme using optical orthogonal codes. In addition, compared with bipolar decoder applying balanced detection, this approach has a simpler architecture. A proof-of-principle experiment is demonstrated.
In this paper an all-optical signal processor that performs both microwave mixing and bandpass filtering in a radio-over-fiber link is proposed and demonstrated. The frequency mixing is achieved by applying a local oscillator frequency and a BPSK modulated subcarrier to an electrooptic phase modulator. The mixed signals at the output of the electrooptic phase modulator are then fed to a single mode fiber link, which acts as a dispersive device for bandpass filtering and distributes the mixed signal to a remote site. The combination of the phase modulator, a multiwavelength laser source and the SMF link forms an all-optical microwave bandpass filter to suppress the levels of unwanted mixing products. A subcarrier frequency up-conversion from 3.25 GHz and 3.5 GHz to 11.7 GHz performed over a 25 km fiber link is experimentally demonstrated, in which BPSK modulation formats with data rates of 172 Mb/s and 344 Mb/s are applied. Eye diagrams are measured at the receiver end after demodulation, demonstrating a good up-conversion is achieved.
In this paper, we propose a novel approach to realizing all-optical microwave filtering using a broadband light source and fiber Bragg gratings (FBGs) with identical reflectivities. The filter tapping coefficients are determined by the spectrum profile of the broadband optical source. Since the FBGs have identical reflectivities, the characteristics of the FBGs with high uniformity is possible, which simplifies implementation and reduces implementation error. In addition, the spectrum profile of the broadband source can be controlled using an optical comb filter. Experiments are carried out and the results agree well with the theoretical analysis.
Different architectures for all-optical microwave filtering using fiber Bragg gratings (FBGs) have been proposed in the last few years. Most of the research efforts have been focused on the basic structures and synthesis of wideband FBG-based all-optical microwave filters with possible reconfigurability and tunability, where all FBGs are assumed to have identical time delay for all the frequency components of the incident light. However, when the filter is proposed to work at very high frequency range, the dispersion effects induced by the FBGs have to be addressed. In this paper, a theoretical model is established to study the dispersion effects in the FBG-based all-optical microwave filter. Simulations and experiments are carried out. The results show that the effective coefficients of the filter are not constant, but a function of the modulating frequency, which will degrade the performance of the all-optical microwave filters and limit the highest operating frequency. To reduce the dispersion effects, we propose to use properly-apodized short FBGs with relatively flat reflection spectra and group delay responses.
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