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Ultrabroad-band Raman light: Linear detuning & gain suppression

RD Kelly, James M. Christian, Graham S. McDonald

Published Jun 19, 2013• 0 citations
OpenAlex

Abstract

Ultrabroad-band multifrequency Raman generation is a (laser-driven) resonant-symmetric two-colour pumping technique for exciting polychromatic light beams that are characterized by potentially very wide “top-hat” temporal bandwidths [1]. The spectra associated with such beams may have a width of the same order as the pump frequency, and contain scores of discrete Raman lines of comparable energy [2,3]. Here, we will report findings from the first systematic analysis addressing the interplay between finite material dispersion and detuning (where the frequency difference between the two pump waves no longer exactly matches the Raman transition). Of principal concern is how the dispersion-detuning feedback loop influences bandwidth generation. For simplicity, a continuous-wave model [4] is considered so that the input pulses driving the medium excitation are long compared to the characteristic dephasing time of the polarization wave (and transient dynamics are thus neglected). A host of new results will be unveiled (see Fig. 1), a particular highlight of which is the recent identification of a unique parameter regime wherein bandwidth growth may be strongly enhanced. Fully-numerical calculations are also compared against a generalized three-wave gain-suppression analysis [2] that can accommodate linear detuning. References [1] G. S. McDonald, G. H. C. New, L. L. Losev, A. P. Lutsenko, and M. J. Shaw, Opt. Lett. 19, 1400 (1994). [2] G. S. McDonald, G. H. C. New, L. L. Losev, and A. P. Lutsenko, J. Phys. B: At. Mol. Opt. Phys. 30, L719 (1997). [3] G. S. McDonald, G. H. C. New, Y. M. Chan, L. L. Losev, and A. P. Lutsenko, J. Mod. Opt. 45, 1099 (1998). [4] K. S. Syed, G. S. McDonald, and G. H. C. New, J. Opt. Soc. Am. B 17, 1366 (2000).

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Ultrabroad-band multifrequency Raman generation is a (laser-driven) resonant-symmetric two-colour pumping technique for exciting polychromatic light beams that are characterized by potentially very wide “top-hat” temporal bandwidths [1]. The spectra associated with such beams may have a width of the same order as the pump frequency, and contain scores of discrete Raman lines of comparable energy [2,3]. Here, we will report findings from the first systematic analysis addressing the interplay between finite material dispersion and detuning (where the frequency difference between the two pump waves no longer exactly matches the Raman transition). Of principal concern is how the dispersion-detuning feedback loop influences bandwidth generation. For simplicity, a continuous-wave model [4] is considered so that the input pulses driving the medium excitation are long compared to the characteristic dephasing time of the polarization wave (and transient dynamics are thus neglected). A host of new results will be unveiled (see Fig. 1), a particular highlight of which is the recent identification of a unique parameter regime wherein bandwidth growth may be strongly enhanced. Fully-numerical calculations are also compared against a generalized three-wave gain-suppression analysis [2] that can accommodate linear detuning.
References
[1] G. S. McDonald, G. H. C. New, L. L. Losev, A. P. Lutsenko, and M. J. Shaw, Opt. Lett. 19, 1400 (1994).
[2] G. S. McDonald, G. H. C. New, L. L. Losev, and A. P. Lutsenko, J. Phys. B: At. Mol. Opt. Phys. 30, L719 (1997).
[3] G. S. McDonald, G. H. C. New, Y. M. Chan, L. L. Losev, and A. P. Lutsenko, J. Mod. Opt. 45, 1099 (1998).
[4] K. S. Syed, G. S. McDonald, and G. H. C. New, J. Opt. Soc. Am. B 17, 1366 (2000).
Throughout the past few decades, science has progressed towards the ability to probe many extremely fast processes and a large amount of research has been aimed at the area of few-femtosecond pulse generation. This thesis describes the generation of coherent broadband radiation through two-colour pumping of molecular hydrogen confined to a unidirectional ring cavity, and the subsequent synthesis of high peak power and few-femtosecond pulses. A set of normalised semi-classical field equations are derived in Bloch form describing the process of ultra-broadband multi-frequency Raman generation or UMRG, and a 3-wave gain suppression analysis is derived from a subset of the plane wave UMRG field equations which describes gain suppression within the ring cavity in terms of both medium and cavity parameters. The gain suppression analysis is further generalised to include finite levels of linear two-photon frequency detuning of the pump beams. Simulations of the plane wave ultra-broadband multi-frequency Raman (UMRG) equations show that a broad frequency spectrum of mutually coherent sideband can be generated. The inverse Fourier transform of spectra generated in this way yields a train of high power near Fourier limited pulses in the time domain which can range from a few-femtoseconds in duration to tens of attoseconds with repetition rates equal to the Raman transition frequency. Pulses synthesised in this way are limited only by the level of medium dispersion, the reflection bandwidth of the chosen coupling mirror and the chosen Raman medium. Simulations of the transverse UMRG equations within the ring cavity geometry have shown ring cavity enhanced UMRG to be resilient to transverse effects such as finite beam width, beam diffraction and the transverse beam separation of the applied pump beams.
High order Raman generation has received considerable attention as a possible method for generating ultrashort pulses. A large number of Raman orders can be generated when the Raman-active medium is pumped by two laser pulses that have a frequency separation equal to the Raman transition frequency. High order Raman generation has been studied in the different temporal regimes, namely: adiabatic, where the pump pulses are much longer than the coherence time of the transition; transient, where the pulse duration is comparable to the coherence time; and impulsive, where the bandwidth of the ultrashort pulse is wider than the transition frequency. To date, almost all of the work has been concerned with generating as broad a spectrum as possible, but we are interested in studying the spectra of the individual orders when pumped in the transient regime. We concentrate on looking at extra peaks that are generated when the Raman medium is pumped with linearly chirped pulses. The extra peaks are generated on the low frequency side of the Raman orders. We discuss how linear Raman scattering from two-photon dressed states can lead to the generation of these extra peaks.
Subfemtosecond pulses can be generated in a Raman generator by using electromagnetically induced transparency if ultrabroad Raman sidebands are generated and then propagate in such a way that the generated sidebands at higher frequencies are temporally advanced with regard to those at lower frequencies. However, it was recently demonstrated by experiments [1] that above-resonance Raman excitations ensured the establishment of anti-phased Raman coherence for pulse compression but only very limited number of Raman sidebands could be generated due to Raman self-defocusing [2], while broadband sidebands could be generated under below-resonance Raman excitations, which supported no pulse compression in the same Raman media. In order to establish anti-phased Raman coherence and simultaneously overcome Raman self-defocusing to benefit efficient generation of broadband sidebands, we suggest in this report a new technique to generate sub-fs pulse train in a Raman generator driven by three sufficiently intense single-mode laser beams whose carrier frequencies are tuned off the Raman resonance with two different two-photon detunings. The Raman interaction establishes a molecular modulation through nonadibatic processes. The simulation on the basis of the fundamental rotational Raman transitions in H2 shows that sub-fs pulse can be generated by above-resonance or below-resonance Raman excitations, or even Raman excitations with opposite detunings, as is shown in Fig.1. This opens a new way for experiments to observe enhanced generation of broadband Raman sidebands, together with compression of the phase-locked Raman sidebands into sub-fs pulse trains in the same Raman media
The exact resonance of pump and Stokes with the Raman transition is essential for successful generation of Raman solitons.1 Even slight detuning (~1 % of the Raman linewidth) leads to rapid soliton decay. Viewed in frequency space, this is due to an asymmetric gain profile for the injected Stokes seed. Through the frequency dependence of gain and Raman dispersion, this leads to the rapid growth of an out-of-phase component of the Stokes beam, which does not participate in the transient reversal of pump depletion. The asymmetric gain profile can be compensated by suitably modifying the total phase shift of the injected seed, resulting in an enhancement of those frequency components that see lower gain. We show that almost complete soliton stability can be achieved this way.
The authors have extended the theory of stimulated Raman scattering to include the effects of laser bandwidths, in both the transient and steady-state regimes. The case of two interacting laser beams, a pump laser and a probe (Stokes) laser, is treated. Using the phase-diffusion model for laser bandwidth, the authors demonstrate that in the absence of dispersion, the forward Raman gain is essentially independent of the laser bandwidths in the high-gain limit, while in the low-gain limit the gain coefficient is inversely proportional to the sum of the bandwidths. It is further shown that when the pump-laser bandwidth is much larger than the linewidth of the Raman medium, the stimulated Stokes output assumes the same spectrum as the pump laser in the high-gain limit. A possible interpretation of these results is discussed assuming a "phase-locking" of the Stokes phase to the fluctuations in the pump-laser phase, due to the nonlinear interaction of the two beams through the Raman medium.
We investigate the influence of pump bandwidth and pump intensity noise on the quantum-noise properties of lasers based on a Raman atomic-gain medium, which recently have been predicted to emit narrow-bandwidth, sub-shot-noise light. We show that, using finite-bandwidth pump light, the predicted limit to the laser bandwidth can still be well below the value given by the Shawlow-Townes formula and much smaller than the input bandwidth. The laser intensity noise is shown to be relatively insensitive to input phase noise as long as the Rabi frequency on the lasing transition is large compared to the bandwidth. On the other hand, even small amounts of pump amplitude noise tend to increase strongly the laser spectral bandwidth, especially in the high-intensity limit. Including dynamic pump-noise reduction through multilevel atomic cycling in our model leads to a Mandel Q parameter value even below Q=-1/2, together with a significant enhancement of the output intensity-noise squeezing.