<title>Ultrafast Nd fiber lasers</title>

Abstract

ABSTRACT Femtosecond pulse generation based on Kerr-type and carrier-type modelocking of Nd3+ doped fiber lasers isdiscussed. Special emphasis is be payed to the startup conditions and the methods used to initiate the modelockingprocess.Keywords: femtosecond phenomena, Nd fiber laser, ultrashort pulses, mode-locking, selfstarting 1 INTRODUCTION Since rare earth ions were first incorporated in single mode fibers,1 a large number of applications have beendemonstrated resulting in the establishment of rare earth doped fibers as one of the major tools in fiber optics.These developments have largely been stimmulated by the use of Er amplifieres within telecommunications. In paralell it has been realized that rare earth doped fibers offer an attractive medium for the generation of short optical pulses by employing active or passive modelocking techniqes. The first results were published by Phillipset al.,2 who employed active modelocking to obtain 20 ps pulses in a Nd doped fiber. Sub-pico-second pulses werefirst generated by Fermann et a.l. using an active-passive modelocking technique, i.e. using short lengths of fiberand intra-cavity dispersion adjustment. Sub-100 fs pulses were first obtained by employing pure (non-selfstarting)passive modelocking.4 In this pulse stability is ensured by passive amplitude modulation instead of active pulsemodulation. This system was optimized by simplifying the passive amplitude modulation mechanism5 and pulsewidths as short as 34 Is were obtained, modelocking the full inhomogeneous bandwidth ofa glass laser for the firsttime. The first selfstarting passively modelocked Nd fiber lasers was subsequently demonstrated using multiplequantum well (MQW) satrurable absorbers.6'7 These developments made obvious that rare earth doped fibersare serious competitors to conventional bulk femto-second oscillators such as colliding pulse modelocked (CPM)dye lasers,8 passively modelocked color-center lasers9 or even the Ti:sapphire laser.'° The attractions of passivelymodelocked fiber laser oscillators are: 1) monolithic all-solid state fiber laser designs are possible, 2) pump powerrequirements are low allowing for directly diode pumped systems, and 3) wavelengths ranging from the blue side