Abstract

We have passively Q- switched and mode locked a Neodymium doped fiber laser with a semiconductor saturable absorber minor (SESAM). The fiber used has an annular shaped active region (M-profile fiber) Fig.1a, possessing the advantage that the pump light may be completely absorbed over a short length. Butt coupling a SESAM at one of the fiber ends is therefore possible because there is no pump light left leading to an unwanted saturation of the absorber. We have made a modal analysis in the scalar approximation comparing the absorption of the M-profile fiber and a standard double clad one provided high power diode lasers are used for optical pumping. Figs.1b give examples of LP_{m p} modes of a M-profile fiber. In the calculations the following parameters were used: pump wavelength λ= 8o3 nm, Δ_{corc-first cladding} = 0.003 and Δ_{first-second cladding} = 0.04; 1)M-profile fiber: 5 µm ring width and 85 µm ring centre diameter; 2) double clad fiber: 100 µm pump core diameter, 6.5 µm doped core diameter. For the M-profile fiber not only the low order modes i.e. LP₀₁ interact strongly with the doped region but higher order modes do as well. For the double-clad fiber it is only the field of the very low order modes (here LP_{0 1},) have an overlap with the active material whereas all higher order modes have almost nothing. Based upon the theoretical findings we have realised a set-up where the SESAM is directly attached to the fiber just as the incoupling mirror. Such an optical scheme has not been realised up till now as we believe. The SESAM was polished at the back side possessing a reflectivity of about 98%. The fiber length was 200 cm, the amount of pump radiation leaving the fiber was less than 5%. The measured pulse width using a fast detection electronics ( risetime 60 ps ) is shown in Fig. 1c. For a certain range of pump powers we have obtained Q-switch pulses with a very low jitter (< 1% ). Locking a large bandwidth could not be achieved being partially due to the SESAM and to the multimode behaviour ( azimuthal direction ) of laser.

© 1998 IEEE