We report a mid-infrared (mid-IR or 3.5 to 5.0  μm) laser source for testing jamming code effectiveness against heat-seeking missiles in open-field conditions. We developed an optical parametric oscillator (OPO) based on a ZnGeP₂ (ZGP) crystal for converting a 2.1-μm pump laser into mid-IR. The pump laser is an acousto-optically Q-switched Ho:YAG laser pumped by a continuous-wave (cw) Tm:fiber laser. The maximum possible average mid-IR power obtained is 6.5 W at a pump power of 20 W with a 33% power conversion efficiency. The pump pulses resulting from the Q-switched operation of the Ho:YAG laser have a pulse width of 30 ns (FWHM) at a pulse repetition frequency (PRF) of 50 kHz. We characterized the output beam in terms of its output power and wavelength spectrum. The output power of the pump laser can be modulated in a pattern of on/off pulses with a smaller frequency than the PRF. The frequency and the duty cycle of these on/off pulses can be adjusted. Identical on/off pulse modulation appears on the mid-IR output beam after the OPO. It is possible to create jamming patterns and codes in mid-IR band using this feature. We have presented examples of on/off modulation of the OPO output at arbitrary pattern frequencies and duty cycles.