Propagation-induced transition from slow to fast light in highly doped erbium fibers Oscar G. Calderón, * Sonia Melle, M. A. Antón, F. Carreño, and Francisco Arrieta-Yañez Escuela Universitaria de Óptica, Universidad Complutense de Madrid, C/ Arcos de Jalón s/n, 28037 Madrid, Spain E. Cabrera-Granado Department of Physics, Duke University, Durham, North Carolina 27708, USA Received 4 April 2008; revised manuscript received 16 June 2008; published 10 November 2008 We analyze the propagation regime of an amplitude-modulated 1536 nm signal when traveling along a highly doped erbium fiber pumped at 977 nm as a function of the fiber length. A propagation-induced transition from superluminal to subluminal propagation takes place along the fiber length which allows a change in regime solely based upon increasing the signal modulation frequency. This peculiar behavior is due to the interplay between pump absorption and pump-power broadening of the spectral hole induced by coherent population oscillations. The effect of ion density on this frequency-dependent regime change has been investigated. DOI: 10.1103/PhysRevA.78.053812 PACS numbers: 42.65.-k, 42.50.Gy I. INTRODUCTION Controlling the speed of light in solid state materials at room temperature is a task that has received recent attention due to its potential applications in telecommunications 1. Bigelow et al. 2 carried out the first experiment concerning slow light propagation at room temperature in solids based on coherent population oscillations CPOs. They reported a reduction of the speed of light in a 7.25-cm-long ruby rod down to 57 m / s by producing a hole as narrow as 37 Hz half width at half maximum in the absorption spectrum. This hole is created by the periodic modulation of the ground state population at the beat frequency between the probe field and the control field propagating along the sample. The hole linewidth is proportional to the inverse of the relaxation life- time of the excited level 3. In addition, Bigelow et al. 4 observed both superluminal and ultraslow light propagation in an alexandrite crystal arising from CPO involving chro- mium ions either in inversion or mirror sites within the crys- tal lattice. They measured group velocities as slow as 91 ms -1 to as fast as -800 ms -1 . They also analyzed the propagation of smooth and discontinuous pulses through the abovementioned materials 5. They found that a discontinu- ous jump within a pulse propagates at the usual phase veloc- ity of light whereas the smooth portion of the pulse propa- gates at the group velocity. Room-temperature slow light via CPO has also been observed in semiconductor structures, such as VCSEL’s 6 and quantum dots 7, devices in which bandwidths as large as 2–3 GHz were obtained. Öham et al. 8 showed the possibility of controlling simultaneously the delay and the amplitude of optical signals at 10 GHz by combining sections of slow and fast light propagation in an integrated semiconductor device. Subluminal and superlumi- nal propagation in other solids at room temperature has also been achieved such as in photorefractive materials based on the dispersive phase coupling effect in nonlinear wave mix- ing processes 9,10, and in a Kerr medium due to the strong highly dispersive coupling between different frequency com- ponents of the light pulse 11. Much research has been focused on the control of the speed of light in optical fibers since these devices would be compatible with fiber-optic communication systems. Song et al. 12 and Okawachi et al. 13 demonstrated slow and fast light in optical fibers for the first time. The underlying mechanism is known as stimulated Brillouin scattering, which consists in the interaction of two propagating waves, a pump wave and a Stokes wave, which generates an acoustic wave at the frequency difference of the pump and the Stokes fields. The slow light resonance can be placed at the desired wavelength by changing the frequency of the pump field. A related process, stimulated Raman scattering, has also been used in optical fibers to demonstrate an ultrafast all-optical controllable delay 14. A modification of group velocity by CPO has been reported in an erbium doped fiber EDF by Schweinsberg et al. 15, where an amplitude-modulated 1550 nm signal copropagates with a 980 nm pump signal. They used a 13 m-long EDF with Er ion density of 1.78  10 24 m -3 90 ppm wt.. They observed a change from sub- luminal to superluminal propagation upon increasing pump power. They obtained a maximum fractional delay of 0.089 and a maximum fractional advancement of 0.124. By using the same experimental system, fast light pulse propagation has been studied in more detail in Refs. 16,17. The effect of ion density on slow light propagation enabled by CPO has been experimentally addressed for highly doped erbium fi- bers 18,19. It was found that ultra-high ion concentration can simultaneously increase the fractional delay and the bandwidth of the signals that can propagate through the fi- bers without noticeable distortion. In this work, we analyze the subluminal and superluminal propagation of amplitude-modulated signals through highly doped erbium fibers pumped with a 977 nm laser. Due to the strong depletion of the pump field along the fiber, the amplitude-modulated signal changes from being amplified to being absorbed when propagating through it. Thus, a propagation-induced change from superluminal to sublumi- nal could take place along the fiber. Our aim is to study this question by analyzing the fractional advancement achieved * oscargc@opt.ucm.es; URL:http://www.ucm.es/info/laserlab PHYSICAL REVIEW A 78, 053812 2008 1050-2947/2008/785/0538128 ©2008 The American Physical Society 053812-1