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Optics Letters

Tunable pulse advancement and delay by frequency-chirped stimulated Raman gain with optical nanofiber

Yun Qi, Wei Jin, Hoi Lut Ho

Published Dec 8, 2020• 1 citation
Publisher

Abstract

We demonstrate a novel method to optically tune the pulse advancement and delay based on stimulated Raman gain in hydrogen. With a frequency-chirped pump, the generated signal pulse is selectively amplified at the leading or trailing edge of the pump pulse, depending on whether the frequency difference between the pump and the signal beam is blue or red detuned from the Raman transition, which results in advancement or delay of the signal peak. Different from the method of slow/fast light, where advancement and delay are accompanied with power loss and gain, respectively, for a single resonance, both the advancement and delay are realized in the gain region for the method here. With a piece of 48-mm-long optical nanofiber in hydrogen, the time-shift for a signal peak ranging from 3.7 to −3.7 ns is achieved in a Raman-generated pulse with width of .

DOI
10.1364/ol.409127

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We demonstrate a novel method to optically tune the pulse advancement and delay based on stimulated Raman gain in hydrogen. With a frequency-chirped pump, the generated signal pulse is selectively amplified at the leading or trailing edge of the pump pulse, depending on whether the frequency difference between the pump and the signal beam is blue or red detuned from the Raman transition, which results in advancement or delay of the signal peak. Different from the method of slow/fast light, where advancement and delay are accompanied with power loss and gain, respectively, for a single resonance, both the advancement and delay are realized in the gain region for the method here. With a piece of 48-mm-long optical nanofiber in hydrogen, the time-shift for a signal peak ranging from 3.7 to −3.7 ns is achieved in a Raman-generated pulse with width of .
We report the first demonstration of group delay tuning with stimulated Raman scattering-induced dispersion in a hydrogen-filled hollow-core optical fiber.A pump laser induces a sharp refractive index change near the S 0 (0) Raman transition of hydrogen molecules, enabling the control of the group velocity of signal pulses around the Stokes wavelength.Experiments with an 80-m-long hollow-core fiber filled with 2.5 bar hydrogen achieved continuous tuning of the pulse delay up to 1.42 ns by varying the Raman amplification from 0 to 10 dB.The tunable pulse delay is realized by changing the pump power as well as the hydrogen pressure.This work provides a new technique for controlling the pulse propagation in optical fibers with high flexibility.
Slow and fast light (SFL) in nonlinear media has attracted considerable interest for several years and has been observed in plenty of configurations. It generally requires a sharp spectral resonance which in turn results in a strong change of the group velocity at the resonance, thus leading to optical delaying or advancement. SFL has also been observed in optical fibres by taking advantage of the parametric gain, stimulated Brillouin or Raman scattering, leading to the development of all-optical tunable fiber-based delay lines at telecommunication wavelengths. But the implications of SFL go well beyond pulse delaying or advancement. As the group and phase velocities have to be distinguished one from another in a slow-light medium, this can have other important implications, in particular on the pulse walk-off or on the frequency chirp. We recently demonstrated in a Raman medium with normal dispersion that pulse walk-off between the pump and the signal can be cancelled out despite the large frequency detuning, leading to an increasing interaction length and enhancement Raman conversion efficiency.
We have demonstrated that stimulated Brillouin scattering can be used to generate all-optical slow-light pulse delays of greater than a pulse length for pulses as short as 16 ns in a single-mode fiber. Since the induced delay is generated for wavelengths detuned from the pump field by the Brillouin frequency, tuning near an electronic resonance of the material is not required and thus delays can be induced at telecommunication wavelengths. This represents a significant improvement over previous demonstrations of slow light in solids and is an important step towards developing an all-optical tunable delay line for telecommunications. In addition, these results strongly suggest that analogous delays can be achieved using stimulated Raman scattering at telecommunication data rates.
We investigate the behavior of a system for generating ``fast-light'' pulses in which a bichromatic Raman pumping beam is used to generate optical gain at two frequencies and a region of anomalous dispersion between them. It is expected that increasing the gain will increase the pulse advancement. However, as the gain increases, the pumping field becomes increasingly distorted, effectively limiting the pulse advancement. We observe as much as 12% of the input pump power converted to orthogonal polarization, broadening of the initially bichromatic pump field (25 MHz initial frequency separation) to more than 2.5 GHz, and a temporal collapse of the pump beam into an erratic train of sub-500-ps pulses. The instability is attributed to the combined effects of the cross modulation instability and stimulated Raman scattering. Extreme distortion of an injected pulse that should (absent the instability) experience an advancement of 21% of its width is observed. We conclude that the fast-light pulse advancement is limited to just a few percent of the pulse width using this pulse advancement technique. The limitation imposed by the instability is important because careful study of the information velocity in fast-light pulses requires that pulse advancement be large enough to distinguish the velocities of different pulse features. Possible methods for achieving pulse advancement by avoiding the distortion caused by the instability are discussed.
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.
The nonlinear process of stimulated Raman scattering is important for silicon photonics as it enables optical amplification and lasing. To understand the dynamics of silicon Raman amplifiers (SRAs), a numerical approach is generally employed, even though it provides little insight into the contribution of different SRA parameters to the signal amplification process. In this paper, we solve the coupled pump-signal equations analytically under realistic conditions, and derive an exact formula for the envelope of a signal pulse when picosecond optical pulses are amplified inside a SRA pumped by a continuous-wave laser beam. Our solution is valid for an arbitrary pulse shape and fully accounts for the Raman gain-dispersion effects, including temporal broadening and group-velocity reduction (a slow-light effect). It can be applied to any pumping scenario and leads to a simple analytic expression for the maximum optical delay produced by the Raman dispersion in a unidirectionally pumped SRA. We employ our analytical formulation to study the evolution of optical pulses with Gaussian, exponential, and Lorentzian shapes. The ability of a Gaussian pulse to maintain its shape through the amplifier makes it possible to realize soliton-like propagation of chirped Gaussian pulses in SRAs. We obtain analytical expressions for the required linear chirp and temporal width of a soliton-like pulse in terms of the net signal gain and the Raman-dispersion parameter. Our results are useful for optimizing the performance of SRAs and for engineering controllable signal delays.