Slow light delay based on all-optical wavelength conversion using four-wave mixing (FWM) and dispersion medium is experimentally studied. Firstly, all-optical wavelength conversion with band-width of 28.4nm (from 1549.3nm to 1520.9nm) by FWM in 1500m dispersion flattened highly nonlinear fiber (DF-HNLF) is realized. Then, the maximum delay of 12.0ns is experimentally achieved by transmitting the original signal and wavelength-converted signal in 3.88 km dispersion compensating fiber (DCF). The experimental results are in good agreement with the principles.
Slow light delay based on all-optical wavelength conversion using four-wave mixing (FWM) and dispersion medium is experimentally studied. Firstly, all-optical wavelength conversion with band-width of 28.4nm (from 1549.3nm to 1520.9nm) by FWM in 1500m dispersion flattened highly nonlinear fiber (DF-HNLF) is realized. Then, the maximum delay of 12.0ns is experimentally achieved by transmitting the original signal and wavelength-converted signal in 3.88 km dispersion compensating fiber (DCF). The experimental results are in good agreement with the principles.
Slow light based on four-wave mixing (FWM) wavelength conversion and dispersion is experimentally studied. The FWM bandwidth of the highly nonlinear fiber is measured to be 40 nm, which is also the slow light tunable bandwidth. A 34 ns delay of 500 MHz sine signal is achieved in standard single mode fiber, and 198 ns delay of short pulses with 100 ps width is achieved in dispersion compensation fiber (DCF). An advancement of 209 ns for the 100 ps short pulses is also achieved in the DCF. The method to increase the slow light delay is discussed, and large delay up to microseconds is expected when wideband FWM wavelength conversion and large dispersion fibers are used. The expected large delay will help us realize high efficiency fiber delay lines and all-fiber buffers.
We compared experimentally the properties of optically controlled tunable time delay based on a Raman-assisted optical parametric amplifier (OPA) with different gain media of normal dispersion shifted fibers (DSF) and highly nonlinear fibers (HNLF). The Raman-assisted OPA using the DSF has previously been demonstrated to obtain large light delay for one single pulse. Position optimization of pump wavelength according to zero dispersion wavelength of the fibers results in narrow band gain spectrum through a parametric process. Stimulated Raman scattering further modifies the shape of the gain spectrum. The modified gain spectrum with sharper transitions leads to a large group index change so that large time delay can be achieved. In the paper, we experimentally demonstrated propagating 10Gbps return-to-zero (RZ) optical packets through slow-light fiber delay line based on the Raman-assisted OPA with above 100ps time delay. We also experimentally compared the gain, the bandwidth and the time delay versus variable pump powers for the Raman-assisted OPA by using 1km-long DSF and 1km-long HNLF as the gain medium, respectively. While smaller time delay was obtained in the slow-light scheme using HNLF due to its wider gain spectrum, lower pump power threshold was found compared to that using DSF. In the experiment, we found that the slow-light scheme has THz gain bandwidth and indicates that slow light could be achieved at higher bit rates with further optimized system parameters.
Slow light study is discussed in various methods of generation, materials and approaches. In this study, the control of group velocity, Vg and group velocity dispersion (GVD) of a pulse propagating in optical fiber is simulated and demonstrated. The slow light generation, which is closely related to Vg of a light pulse for this study is based on stimulated Brillouin scattering (SBS), the most efficient approach as reported by previous researchers. By using Matlab R2011b software, the slow light generation is simulated for the slow light structure in the form of hollow conical shape resonator, focusing on the geometrical parameters. The simulation work is extended to the control of Vg and GVD, to investigate the influences of refractive index of the materials and wave number of the light pulses. While most of slow light researches neglect the effect of dispersion, this study explicitly includes the second term of the wave number’s expansion to represent GVD. Therefore, the experimental work investigates the creation of a strongly dispersive material with large chirp and small group delay via SBS in a highlynonlinear optical fiber as opposed to previous SBS experiments which mainly focus on the creation of a slow light material with minimum dispersion and chirp. To demonstrate the control of Vg and GVD, a two-frequency pump field which consists of two distributed feedback lasers with the relative frequency separation nearly twice the Brillouin frequency and a slow light medium in the form of a 2-km high nonlinear single mode optical fiber are employed. For input field with slowly varying amplitude, the experiment obtained output pulses with large GVD and decreased intensity. All-optical control of the GVD is demonstrated by measuring the linear frequency chirp impressed on a 28-nanosecond-duration optical pulse and it is tunable over the range of ± 7.8 ns2 m-1. In addition, the maximum observed value of GVD is 109 times larger than that obtained in a typical single-mode silica optical fiber. The simulation results for Gaussian pulses as input signal are in good agreement with the experimental ones, and therefore the control of Vg and GVD for different shapes of input pulses using the developed system can be predicted.
Dynamic control of the speed of a light signal, based on stimulated Brillouin scattering in optical fibers, was theoretically studied and also experimentally demonstrated as the core object of this thesis. To date, slow light based on stimulated Brillouin scattering has shown the unmatched flexibility to offer an efficient timing tool for the development of all-optical future router. Nevertheless, the seeming perfect Brillouin slow light suffered from three major obstacles: naturally narrow signal bandwidth, strong change of signal amplitude, and significant signal distortion. The essential contribution of this work has been mostly dedicated to resolve all those impairments so as to make Brillouin slow light a completely operating all-optical delay line for practical applications. Actually, high capability of tailoring the spectral distribution of the effective Brillouin resonance makes possible to resolve partially or completely all those problems. First of all, a broadband spectral window was passively obtained in between two Brillouin gain/loss resonances by simply appending two segments of fibers showing different Brillouin frequency shifts. The global Brillouin gain of the concatenated fibers manifests a gain/loss doublet resonance showing a broad window in between gain/loss peaks. In practice, this configuration has a crucial advantage that it removes the need of the pump modulation, generally used to create a polychromatic pump source. Therefore, a broadband Brillouin slow and fast light was simple realized with a reduced distortion. Secondly, the signal amplification or attenuation associated to the signal delay was completely compensated by superposing Brillouin gain and loss resonances with identical depth but different width. As a result, the Brillouin gain led to effectively a spectral hole in the center of the broadband absorption and opened a transparent window while the sharp change in the refractive index was preserve. This way it makes possible to realize zero-gain Brillouin slow light. This configuration was also exploited to produce Brillouin fast light with a total absence of signal loss, simply by swapping the spectral position of the two pumps. At last, a signal was continuously delayed through a Brillouin fiber delay line without any distortion. Due to the strong induced dispersion, pulse broadening is a major difficulty in all slow light systems and it is impossible to compensate such broadening using a linear system. Therefore, a conventional Brillouin slow light system was combined with a nonlinear optical fiber loop mirror that gives a nonlinear quadratic transmission. Using this configuration, the inevitable pulse broadening was completely compensated at the output of the loop and a signal was delayed up to one symbol without any distortion. Brillouin slow light systems were further studied in the spectral domain. For a given Brillouin resonance the spectrum of a light pulse was optimized to better match the Brillouin bandwidth. When the time envelope of a pulse was properly shaped, it was clearly observed that the spectral width of the pulse became minimized while preserving the pulse duration. This way the maximum time delay through Brillouin slow light could be enhanced for a fixed pulse width. Brillouin fast light was even realized in total absence of any pump source, which is a plain requirement for the generation of Brillouin slow or fast light. This self-generated delay line, key contribution of this thesis, relies on both spontaneous and stimulated Brillouin scattering in optical fibers. In this implementation, a light signal was strongly boosted above the so-called Brillouin threshold, so that most power of the signal was transferred to a backward propagating wave, namely the generation of amplified spontaneous Stokes wave. Since the center frequency of the intense Stokes wave is below the signal frequency by exactly Brillouin shift of the fiber used, this wave led to a Brillouin loss band centered at the signal frequency. Consequently, the signal experienced fast light propagation and the propagation velocity of the signal was self-controlled, simply by varying the signal input power. This technique has many practical advantages such as its high simplicity of the configuration and an invariant signal power in the output of this delay line. Additionally, this system self-adapts the signal bandwidth as the spectrum of the amplified Stokes wave matches the spectral distribution of the signal. An alternative method to generate all-optical delay line was proposed instead of slow light. This scheme makes use of the combination of wavelength conversion and group velocity dispersion. This type of delay line was mainly aimed at improving the storage capability of delaying element. The wavelength of a signal was simply and efficiently converted at a desired wavelength using cross gain modulation in semiconductor optical amplifiers. Then the converted signal was delivered to a high dispersive medium and arrived at the end of the medium with relative time delay due to the group velocity dispersion. A fractional delay of 140 was continuously produced through this delay line for a signal with a duration of 100 ps, preserving signal bandwidth and wavelength. The effect of slow light on linear interactions between light and matter was experimentally investigated to clarify the current scientific argument regarding slow light-enhanced Beer-Lambert absorption. It was predicted that real slowing of the light group velocity could enhance the molecular linear absorptions so as to improve the sensitivity of this type of sensing. However, the experimental results unambiguously show that material slow light (slow light in traveling wave media) does not enhance the Beer-Lambert absorption.
All-optical tunable slow light technology has promising applications in all-optical network and optical information process. Tunable slow light based on stimulated Brillouin scattering is experimentally investigated, using highly nonlinear microstructured fiber which is designed and drawn by ourselves. The experiment setup is composed of single pump and a single-stage delay. When the pump power reaches 162.6 mW, a maximum delay of 76 ns, equal to 0.76 pulse width, is achieved in the highly nonlinear microstructured fiber of 120 m in length. In addition, by adjusting the pump power, the tunable slow light can be realized. This scheme of slow light has advantages such as large delay, being all-optical tunable and compatible with optical communications systems.