Appl. Phys. B 66, 19–26 (1998) Applied Physics B Lasers and Optics  Springer-Verlag 1998 Theoretical and experimental analysis of intensity noise in a codoped erbium–ytterbium glass laser S. Taccheo 1 , P. Laporta 1 , O. Svelto 1 , G. De Geronimo 2 1 Centro di Elettronica Quantistica e Strumentazione Elettronica del CNR, Dipartimento di Fisica, Politecnico di Milano, P.zza Leonardo da Vinci, 20133 Milano, Italy (Fax: +39-2/2399-6126, E-mail: laporta@axp7000.cdc.polimi.it) 2 Dipartimento di Elettronica e Informazione, Politecnico di Milano, P.zza Leonardo da Vinci 32, 20133 Milano, Italy Received: 6 January 1997/Revised version: 10 April 1997 Abstract. A comprehensive analysis of the mechanisms in- ducing intensity noise in a single-frequency, diode-pumped erbium–ytterbium glass laser is presented. Owing to the en- ergy transfer between the codopant and the active material, the Er:Yb laser shows a markedly reduced sensitivity to pump power fluctuations, and hence its intensity noise spectrum is mainly determined by the fluctuations in the cavity losses. The experimental measurements confirm the results of the theoretical analysis. PACS: 42.55R; 42.60B; 4280 Laser-intensity stability is of great interest for system design in metrology, sensor applications, and optical communica- tions. With particular reference to the latter application, this requirement is of the upmost importance for analog transmis- sion in cable television optical systems, where any spurious modulation of the carrier has to be avoided [1, 2]. The amplitude or intensity noise of a laser can be con- veniently described in terms of the relative intensity noise (RIN), defined as the Fourier transform of the intensity fluctu- ations autocorrelation function C pp (τ) = hδI(t)δI(t + τ)i/ ¯ I 2 , where the angle brackets denote the ensemble average, ¯ I is the average value, and δI = I − ¯ I represents a fluctuation; hence RIN(ν) = ∞ Z −∞ C pp (τ) exp(−i2πντ) dτ is the spectral density function S δI / ¯ I 2 of the intensity fluc- tuations and represents, at a given frequency ν, the ratio between the power of the intensity noise, in a 1-Hz band- width,and the square of the average optical intensity. In solid-state lasers, different noise sources may be individuat- ed and contribute to the RIN spectrum. The technical noise due to mechanical vibrations, acoustical noise, and thermal noise dominates at low frequencies. A second major contri- bution arises from the noise of the pump source. Intensity modulation and wavelength fluctuations in the pump light from diode lasers, due for instance to mode competition, can easily increase, in a typical Nd:YAG laser, the noise spec- tral output by as much as 10 to 20 dB. Finally, the shot noi generally produces a white uniform noise floor at all freque cies; however, if the pump is below the quantum noise limi i.e. if the pump is squeezed, the free-running laser produce a squeezed output at nearly dc frequencies. This effect has been observed in diode lasers pumped with sub-Poissonian currents [3] and has also recently been described for solid- state lasers [4]. Whatever the origin of the perturbation is, the phenomenon of relaxation oscillations, due to a nonline interaction between the population inversion and the photo density in the active material, is always present and results in a RIN enhancement in a relatively narrow frequency in- tervalaround the relaxation oscillation frequency. In most solid-state lasers, this frequency is located between a few tens of kilohertz to about one megahertz. The resonant re- laxation oscillation is present for all pump noise levels and even if the pump source is squeezed. This is because the re onant relaxation oscillation is driven by vacuum fluctuation and hence is present in the intensity noise spectrum irresp tive of the magnitude of the other noise sources [4]. In the lastfew years, the intensity noise of nonplanar monolith- ic Nd:YAG lasers has been investigated by several authors with the aim of quietening the laser by active feedback con trol systems [5–7], and the dominant noise source affecting the output laser intensity stability was found to be ascrib- able to pump source fluctuations [7]. A few measurements and attempts to reduce the intensity noise of linear [8, 9] a ring [10] Er:Yb codoped fibre lasers have also recently bee reported. In this work, we focus our attention on and experimenta ly investigate the mechanisms through which external nois sources induce intensity noise in a codoped bulk Er:Yb:glas laser. As a result of the energy transfer process between the ytterbium ions, which absorb the pump light, and the erbiu ions,which are responsible for laser action, this solid-state laser has a different sensitivity to perturbation of cavity pa rameters compared to, e.g., Nd:YAG and Er:glass lasers. In particular, we show that codoping lowers the influence of the pump power fluctuations on the RIN, which therefore becomes dominated by fluctuations in the cavity loss. The experimental measurements carried out on a bulk erbium–