A method for speeding up the gain recovery time of a semiconductor optical amplifier (SOA) using continuous wave (CW) assist light is examined as a method for improving the response speed and reducing the pattern effect in an all‐optical‐wavelength conversion method using the cross‐gain modulation (XGM) of SOAs. The assist light is input at a transparent wavelength of the SOA gain curve and can effectively and rapidly recover the carrier while suppressing the drop in the gain of the SOA. However, since the SOA transparent wavelength is changed by the carrier density and distribution in the SOA, the optimum condition is believed to greatly affect the SOA operating conditions such as the incident light wavelength, intensity, and propagation direction. In this paper, we theoretically and experimentally examined how the wavelength and the propagation direction of the assist light affect the response speed of the SOA. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 88(5): 10–20, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20131