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Lasers & Sources

Optical-fiber random lasers

The first erbium/germanium co-doped single-mode fiber laser with randomly spaced Bragg gratings could enable applications in photonic devices or circuits.
10 July 2009, SPIE Newsroom. DOI: 10.1117/2.1200906.1688

Fifteen years passed between Letokhov's pioneering theoretical work1 and the first observation of laser action in amplifying scattering media.2 Since then, several systems combining multiple scattering and optical gain to produce lasing have been developed and investigated, including neodymium-glass powders, dye titanium dioxide solutions, zinc oxide nanoclusters, and π-conjugated polymer films.3

Unlike for conventional lasers, where it is considered detrimental, scattering plays a positive role in random lasers. It increases the path length or dwell time of light in the active medium. One of the characteristic drawbacks of these lasers (in their 3D configurations) is their inefficient pumping, which is due to scattering of the pump radiation in the random medium. As a consequence, random lasers tend to be small and have low power.

We recently fabricated and investigated the properties of 1D random lasers based on erbium/germanium (Er/Ge) co-doped single-mode fibers with randomly spaced Bragg gratings4 (see Figure 1). The random grating array forms a complex cavity along the fiber that can have a high quality factor. A unique feature of this optically pumped random laser is that the pump does not interact with the disorder. It can excite the full length of the fiber, thus improving the system's efficiency.

We used commercial single-mode Er/Ge co-doped fibers produced by INO (Quebec, Canada) at 1535nm. Erbium is the active element while germanium doping improves the fiber's photosensitivity. We fabricated the Bragg gratings by exposing the fiber to UV light (244nm) from a frequency-doubled argon-ion laser using a mask with a spatial period of 1059.8nm. We created several approximately 5mm-long gratings within the fiber, with spacings (Di) between 4.2 and 5.8mm. Spectral measurements showed that every Bragg grating has a narrow (full width at half maximum ≈ 0.17nm) reflection spectrum centered at 1535.5nm, with a maximum reflectivity of approximately 7–8%. Variations in mask alignment, recording exposure, and fiber tension caused offsets in these values, resulting in a bandwidth of roughly 0.27nm for the entire array.

Figure 1. Schematic of the 1D random-laser arrangement. R: Reflection. Di: Random distance between gratings. T: Transmission.

We achieved laser action by end-pumping the structured fibers with 980nm radiation from a semiconductor laser. Figure 2 shows output emission spectra of a random laser with a cavity formed by seven gratings, measured at different levels of relatively low pump intensities. The photoluminescence spectra (black and red) correspond to pump powers of 7.0 and 7.5mW, respectively, below the laser threshold. A narrow lasing line at approximately 1535.5nm is observed when a threshold power of 10mW is reached.

Figure 2. Laser emission spectra using seven gratings measured at pump levels below (black and red) and above (blue) the lasing threshold.

Emission spectra obtained with an experimental random laser are shown in Figure 3 for different numbers of gratings in the array. For the largest number of gratings used, the extended laser cavity had a total length of 150cm. As the number of gratings increases, more and more cavities are formed and more lines are observed in the emission spectra (see the upper curves of Figure 3). The vertical lines mark the spectral positions of certain emission lines, showing that new cavities formed in the disordered system do not destroy the existing cavities. The cavity positions and sizes are defined by the particular setup of the chain of gratings. This also determines the system's spectral signature. For pump powers between 20 and 40mW, the peak positions remain unchanged, but the intensity of each peak changes as a function of input power. Fluctuations in the relative powers among the spectral modes are observed even at fixed pump powers.

Figure 3. Emission spectra of random lasers with 7, 14, 20, and 31 gratings, for two values of the pump power, both well above the lasing threshold. The continuous red and blue curves are for pump powers of 20 and 40mW, respectively. The vertical lines mark the spectral positions of certain emission lines. To facilitate visualization, some curves have been displaced vertically.

To understand the behavior of our 1D random laser, we must consider the transmission properties of the optical fiber with the chain of Bragg gratings when it is not pumped (see Figure 1 for the relevant geometry). As the number of gratings increases, the array becomes more and more reflective. The average transmission decays exponentially with the number of gratings. The rate of decay is characterized by the localization number, Nξ. Note that our system is not statistically stationary. It is therefore more appropriate to discuss the localization in terms of the number of gratings rather than a length, ξ=NξDi〉. In Figure 3, Nξ ≈ 6. For a grating number greater than Nξ, the disordered system operates in the localized regime. In this situation and for a typical realization, the system's transmission is exponentially small. However, an important feature of systems consisting of a 1D lossless chain of scatterers is that they can be nearly transparent at some specific (resonant) wavelengths, which is associated with the presence of an intense and strongly confined field inside the cavity. The strong spectral peak of the laser emission coincides with a highly transparent system state. A localized spatial mode is present near the center of the system, and the random arrays to the right and left form the distributed cavity.

Because of the low transmission in the localization regime, photons require a long time to escape from the cavity. The lasing threshold depends exponentially on the system length and an exponentially small gain is sufficient to initiate lasing oscillations. The observed intensity fluctuations of the spectral lines could be caused by Kerr-type nonlinearities. Despite the relatively low emission power, the field inside the cavity can be much stronger because of the confined mode structure. These effects are important in distributed Bragg-reflector fiber lasers longer than 20cm.5 The change in refractive index in a given section shifts the cavity's resonance frequency and also affects the realization of the random system.

Since lasing frequencies are determined by the particular realization of the disorder, these lasers could lead to applications in secure communications, because the lasing spectrum provides a ‘fingerprint’ of the source. The simplicity of their fabrication and the possibility to specify the bandwidth could facilitate their use as active elements in photonic devices or circuits. We next plan to construct lasers with statistically stationary disorder and, using multimode fibers, study the properties of random lasers with a small number of propagation channels.

This research was supported in part by the Mexican National Council for Science and Technology (Consejo Nacional de Ciencia y Tecnología: CONACYT) through grant 47700-F, and by joint award UCM-42127 from the University of California Institute for Mexico and the United States (UC MEXUS) and CONACYT.

Elena I. Chaikina, Noemí Lizárraga, Eugenio Méndez
Optics Department
Ensenada Center for Scientific Research and Higher Education (CICESE)  
Ensenada, Mexico

Elena Chaikina received her PhD on the basis of a study of the physics of semiconductors and dielectrics from the Polytechnic Institute of Leningrad in 1984. She worked at the Ioffe Physical-Technical Institute in St. Petersburg until 1994. Since 1995, she has been a researcher in the Optics Department of her current research institute. Her interests include scattering of electromagnetic waves, localization phenomena, and random lasers.

Noemí Lizárraga received her MSc degree from CICESE and is currently pursuing a PhD. Her thesis work focuses on the dynamics of random fiber lasers.

Eugenio Méndez obtained a BSc in physics from the Universidad Autónoma Metropolitana (Mexico), and MSc and PhD degrees in applied optics from Imperial College, London (UK). He has been with the Optics Department of CICESE since 1987. He is co-author of more than 90 papers in peer-reviewed journals, a member of the Mexican Sistema Nacional de Investigadores, and a fellow of the Optical Society of America. His research interests are in the general areas of statistical optics and optics of biological systems.