The temperature and field dependence of reversible magnetization have been measured on a YBa$_2$Cu$_3$O$_{7-delta}$ single crystal at six different doping concentrations. It is found that the data above 2 T can be described by the scaling law based on the GL-LLL (lowest Landau level approach based on Ginzburg-Landau theory) critical fluctuation theory yielding the values of the slope of upper critical field $-mathrm{d}H_{mathrm{c2}}(T)/mathrm{d}T$ near $T_mathrm{c}$. This set of values is self-consistent with that obtained in doing the universal scaling for the six samples. Based on a simple Ginzburg-Landau approach, we determined the doping dependence of the coherence length $xi$ which behaves in a similar way as that determined from $xi= hbar v_mathrm{F}/E_mathrm{sc}$ with $E_mathrm{sc}$ the superconducting energy scale. Our results may suggest a growing coherence length towards more underdoping.
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