  The role of various magnetic inter-chain couplings has been investigated
recently by numerical methods in doped frustrated quantum spin chains. A
non-magnetic dopant introduced in a gapped spin chain releases a free spin-1/2
soliton. The formation of a local magnetic moment has been analyzed in term of
soliton confinement. A four-spin coupling which might originate from cyclic
exchange is shown to produce such a confinement. Dopants on different chains
experience an effective space-extended non-frustrating pairwise spin
interaction. This effective interaction between impurity-spins is long-ranged
and therefore is expected to play a crucial role in the mecanism of
antiferromagnetic (AF) long-range ordering (LRO) observed in spin-Peierls (SP)
compounds such as CuGeO$_3$ doped with non-magnetic impurities.
