In free-space optical (FSO) communications, signal beams suffer from atmospheric turbulence, transmission loss, and pointing errors. With different channel conditions, there exist different optimal divergence angles for the beams. Therefore, a high-performance adjustable collimator for the transmitter is an important component in an FSO system. An adjustable, high-sensitivity, optical collimating scheme is proposed. In the proposed scheme, a graded-index fiber is coated with palladium film and placed in a hydrogen environment and the graded-index fiber can be extended precisely while the palladium atoms react with the hydrogen molecules. Consequently, with different hydrogen pressures, the output beams present different divergence angles. Theoretical analysis and numerical calculation show that the output divergence angle can be precisely adjusted with the system design.