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Optica Publishing Group
  • Quantum Electronics and Laser Science Conference
  • OSA Technical Digest (Optica Publishing Group, 1999),
  • paper QThI2

Mode partition noise and chaos in a microchip Nd:YVO4 multimode laser

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Abstract

With a reflective grating, the laser modes can be partitioned and noise level will be enhanced. This generic noise-enhanced feature is named as mode partition noise, which has a quantum mechanical origin, in which the noise level of modal output is larger than that of total output. Here, we report a unique manifestation caused by the interplay between noise and chaos in a free-running multimode Nd: YVO4 microchip laser, mainly, as chaos appears, mode partition noise could be smaller than total output noise. Without chaos, common characteristics of mode-partition noise remains. Since common RIN (or SNR) is not a good quality to discriminate the noise level of non-constant signal, we develop different characterization methodology such that the noise level and chaos can be simultaneously recognized.1 The basic idea is that the dimension of chaotic attractor is usually small and noise is infinite-dimensional. With a phase space reconstruction on the measured time series,2 noise will dominate the redundant dimensions by which noise level can be distinguished. Practically, as power is measured, we have a time series. With phase space reconstruction, one can derive a trajectory matrix. Here d should be large enough and we utilize a singular value decomposition (SVD) method to deduce the message of dimension, namely the normalized SVD eigenvalues which are rearranged in descending order. For chaotic signal, the SVD eigenvalue spectrum displays two segments when the dimension d is large enough. The first segment presents an exponential decay, which mainly reflects the nature of chaos. The second segment is a line (noise floor) which is approximately horizontal. On the other hand, for periodic and quasiperiodic signals the first segment will be replaced by a sudden dropping. As noise strength is increased, the floor will be lifted while the first segment remains unchanged. Meanwhile, both uniform and Gaussian white noises show a flat SVD spectrum. The magnitude of eigenvalue at the noise floor indicates the strength of noise embedding in time series. As shown in Fig. 1, the typical SVD spectra for relaxation oscillations and detector noise are presented. In contrast, as shown in Fig. 2 the SVD spectrum for chaos where the order of noise floors has been reversed. It remains a serious theoretical concern about the SVD spectrum relation between relaxation oscillation and chaos in terms of the modal outputs and the total output. We employ a typical multimode laser model with inhomogeneous broadening to explore this issue and the result is shown in Fig. 3. One can see that as chaos occurs, the common feature of mode partition noise has been modified and total output noise can be larger than modal output noise.

© 1999 Optical Society of America

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