Digital electronic synchronization of ultrafast lasers

A method of synchronizing the output of first and second lasers and delivering respectively first and second passively modelocked laser pulse trains from the lasers such that each laser pulse train has the same pulse repetition frequency and the pulse trains have a predetermined phase relationship at a target, the first and second lasers including respectively first and second laser resonators at least one of which includes one or more resonator-length adjusting devices, the method comprising the steps of:

• directing a portion of the first and second laser pulse trains onto respectively first and second photodetectors;
• digitizing the output of said photodetectors to provide first and second digital electronic pulse trains corresponding respectively to the first and second laser pulse trains, said first digital electronic pulse train having the same pulse repetition frequency as the first laser pulse train, said second digital electronic pulse train having the same pulse repetition frequency as the second laser pulse train, and pulses in said first and second digital electronic pulse trains having the same phase relationship as the phase relationship of corresponding pulses in said first and second laser pulse train portions at said photodetectors;
• providing first and second flip-flop circuits cooperative with a logic AND-gate;
• triggering said first flip-flop circuit to a high logic state with a pulse in said first electronic pulse train;
• triggering said second flip-flop circuit to a high logic state with a pulse in said second electronic pulse train;
• connecting said flip-flop circuits to said AND-gate such that when both flip-flops are in said high logic state, said AND-gate is asserted and said flip-flop circuits are reset to a low logic state, the first of said flip-flop circuits to be triggered thereby remaining at said high state until the other thereof is triggered, thereby delivering third and fourth electronic pulse trains from an output port of respectively said first and second flip-flop circuits with a pulse in one thereof having a duration longer than a corresponding pulse in the other thereof if there is a phase difference between the triggering pulses, both of said corresponding pulses having the same duration if there is no phase difference between the triggering pulses;
• generating from said third electronic and fourth electronic pulse trains respectively first and second analog voltage signals, said analog voltage signals being proportional to the duration of the corresponding electronic pulses in the pulse train;
• subtracting said first analog voltage signal from second analog voltage signal, thereby generating a third analog voltage signal, said third analog voltage being proportional to the phase difference between corresponding pulses in said laser pulse trains at said photodetectors;
• adding a fourth analog voltage signal to said third analog voltage signal to generate a fifth analog voltage signal, said fourth analog voltage signal corresponding to the predetermined phase relationship of the laser pulse trains at the target;
• and operating one or more of said one or more resonator-length-adjusting devices responsive to said fifth analog voltage until said fifth analog voltage signal is minimized.

A method of synchronizing the output of first and second lasers and delivering respectively first and second passively modelocked laser pulse trains from the lasers such that each laser pulse train has the same pulse repetition frequency and the pulse trains have a predetermined phase relationship at a target, the first and second lasers including respectively first and second laser resonators at least one of which includes one or more resonator-length adjusting devices, the method comprising the steps of:

• (a) directing a portion of the first and second laser pulse trains onto respectively first and second photodetectors;
• (b) digitizing the output of said photodetectors to provide first and second digital electronic pulse trains corresponding respectively to the first and second laser pulse trains, said first digital electronic pulse train having the same pulse repetition frequency as the first laser pulse train, said second digital electronic pulse train having the same pulse repetition frequency as the second laser pulse train, and pulses in said first and second digital electronic pulse trains having the same phase relationship as the phase relationship of corresponding pulses in said first and second laser pulse train portions at said photodetectors;
• (c) generating third and fourth electronic pulse trains from said first and second digital electronic pulse trains, pulses in said third electronic pulse train having different duration from corresponding pulses said fourth electronic pulse train when the phase of said corresponding pulses is different;
• (d) generating from said third electronic and fourth electronic pulse trains respectively first and second analog voltage signals, said analog voltage signals being proportional to the duration of the corresponding electronic pulses in the pulse train;
• (e) subtracting said first analog voltage signal from second analog voltage signal, thereby generating a third analog voltage signal, said third analog voltage being proportional to the phase difference between corresponding pulses in said laser pulse trains at said photodetectors;
• (f) operating one or more of said one or more resonator-length-adjusting devices responsive to said third analog voltage until said third analog voltage is minimized, whereby said laser pulse train portions have the same frequency and are in phase at said photodetectors;
• (g) following step (f) adding a fourth analog voltage signal to said third analog voltage signal to generate a fifth analog voltage signal, said fourth analog voltage signal corresponding to the predetermined phase relationship of the laser pulse trains at the target;
• and
• (h) operating one or more of said one or more resonator-length-adjusting devices responsive to said fifth analog voltage until said fifth analog voltage is minimized, whereby the laser pulse trains have the same frequency and have the predetermined phase relationship at the target.

A method of synchronizing the output of first and second lasers and delivering respectively first and second passively modelocked laser pulse trains from the lasers such that each laser pulse train has the same pulse repetition frequency and the pulse trains have a predetermined phase relationship at a target, the first and second lasers including respectively first and second laser resonators at least one of which includes one or more resonator-length adjusting devices, the method comprising the steps of:

• (a) directing a portion of the first and second laser pulse trains onto respectively first and second photodetectors;
• (b) digitizing the output of said photodetectors to provide first and second digital electronic pulse trains corresponding respectively to the first and second laser pulse trains, said first digital electronic pulse train having the same pulse repetition frequency as the first laser pulse train, said second digital electronic pulse train having the same pulse repetition frequency as the second laser pulse train, and pulses in said first and second digital electronic pulse trains having the same phase relationship as the phase relationship of corresponding pulses in said first and second laser pulse train portions at said photodetectors;
• (c) generating third and fourth electronic pulse trains from said first and second digital electronic pulse trains, pulses in said third electronic pulse train having different duration from corresponding pulses in said fourth electronic pulse train when the phase of said corresponding pulses is different;
• (d) generating from said third electronic and fourth electronic pulse trains respectively first and second analog voltage signals, said analog voltage signals being proportional to the duration of the corresponding electronic pulses in the pulse train;
• (e) subtracting said first analog voltage signal from second analog voltage signal, thereby generating a third analog voltage signal, said third analog voltage being proportional to the phase difference between corresponding pulses in said laser pulse trains at said photodetectors;
• (f) adding a fourth analog voltage signal to said third analog voltage signal to generate a fifth analog voltage signal;
• (h) following step (f) operating one or more of said one or more resonator-length-adjusting devices responsive to said fifth analog voltage until said fifth analog voltage signal is minimized, whereby the laser pulse trains have at least the same frequency;
• and
• (g) if following step (h) the laser pulse trains do not have the phase relationship at the target, varying said fourth voltage and repeating step (h) until the laser pulse trains have the predetermined phase relationship at the target.