The exponential gain of two-beam coupling in InP:Fe crystals under dc fields is a function of the pump beam intensity at a given temperature. We propose two methods to extend the sample volume working under optimum conditions. The first method is based on a negative thermal gradient and the second uses an auxiliary incoherent beam. Theoretical simulations predict an integrated gain of 15 cm-1 and 10 cm-1, respectively (for lambda = 1.06-mu-m, LAMBDA = 5-mu-m and E0 = 10 kV/cm) in thick crystals (L = .5 mm). Preliminary experimental results confirm that high gain performances can be achieved by using these methods.