Method of creating difference frequency laser pulses by difference frequency generation, comprising the steps of - providing ultrashort laser pulses having a spectral bandwidth corresponding to a Fourier limit of below 20 fs and simultaneously containing first spectral components and second spectral components having larger frequencies than the first spectral components, and - driving a DFG process by the ultrashort laser pulses in a first optically non-linear crystal, wherein the DF laser pulses are created in the first optically non-linear crystal by difference frequencies between the first and second spectral components, resp., said difference frequencies comprising third spectral components being lower in frequency than both of the first and second spectral components, wherein the DF laser pulses have a spectral width of at least 500 nm, characterized by the steps of - providing a first femtosecond enhancement cavity with first resonator mirrors (M 11 to M 14) spanning a first beam path, wherein the first optically non-linear crystal is placed in the first beam path of the first enhancement cavity, wherein the first femtosecond enhancement cavity is adapted for a coherent superposition of the ultrashort laser pulses, - input coupling and coherently adding of the ultrashort laser pulses in the first femtosecond enhancement cavity, wherein at least one circulating ultrashort laser pulse is created in the first femtosecond enhancement cavity, which drives the DFG process in the first optically non-linear crystal for generating the DF laser pulses, and the first femtosecond enhancement cavity is adapted for recycling the at least one ultrashort laser pulse passing through the first optically non-linear crystal, and - output coupling of the DF laser pulses from the first enhancement cavity, wherein - the DF laser pulses have a centre wavelength in a range of 4 μm to 20 μm. - providing ultrashort laser pulses having a spectral bandwidth corresponding to a Fourier limit of below 20 fs and simultaneously containing first spectral components and second spectral components having larger frequencies than the first spectral components, and - driving a DFG process by the ultrashort laser pulses in a first optically non-linear crystal, wherein the DF laser pulses are created in the first optically non-linear crystal by difference frequencies between the first and second spectral components, resp., said difference frequencies comprising third spectral components being lower in frequency than both of the first and second spectral components, wherein the DF laser pulses have a spectral width of at least 500 nm, - providing a first femtosecond enhancement cavity with first resonator mirrors (M 11 to M 14) spanning a first beam path, wherein the first optically non-linear crystal is placed in the first beam path of the first enhancement cavity, wherein the first femtosecond enhancement cavity is adapted for a coherent superposition of the ultrashort laser pulses, - input coupling and coherently adding of the ultrashort laser pulses in the first femtosecond enhancement cavity, wherein at least one circulating ultrashort laser pulse is created in the first femtosecond enhancement cavity, which drives the DFG process in the first optically non-linear crystal for generating the DF laser pulses, and the first femtosecond enhancement cavity is adapted for recycling the at least one ultrashort laser pulse passing through the first optically non-linear crystal, and - output coupling of the DF laser pulses from the first enhancement cavity, wherein - the DF laser pulses have a centre wavelength in a range of 4 μm to 20 μm.