a tunable passively mode-locked laser comprising a Fabry-Perot laser resonator that produces laser pulses and a laser control mechanism that controls an optical path length of the Fabry-Perot laser resonator to tune a pulse repetition frequency of the laser pulses;

an optical detector to receive at least a portion of the laser pulses from the laser and to produce electronic pulses in response to the received laser pulses; and

a feedback control circuit that extracts a monitor signal whose spectral range centers at a harmonic frequency of the pulse repetition frequency from the electronic pulses and processes the monitor signal to produce an error signal indicating a deviation between the harmonic frequency of the pulse repetition frequency of the laser pulses from either a reference harmonic frequency of a desired pulse repetition frequency in a reference signal, or a reference frequency that is offset from the reference harmonic frequency, wherein the feedback control circuit applies the error signal to the laser control mechanism which, in response, tunes the pulse repetition frequency of the laser pulses to reduce the deviation,

wherein the feedback control circuit comprises:

a signal filter that filters the electronic pulses in frequency and extracts signal spectral components around the harmonic frequency of the pulse repetition frequency within a predetermined bandwidth to produce the monitor signal, wherein the signal filter rejects signals at the pulse repetition frequency;

a signal amplifier that amplifies the monitor signal to output an amplified monitor signal at the harmonic frequency of the pulse repetition frequency;

a signal mixer that is directly connected to the signal amplifier and mixes the amplified monitor signal at the harmonic frequency of the pulse repetition frequency and the reference signal to produce the error signal; and

a signal integrator that integrates an output from the signal mixer and to produce the error signal,

wherein the Fabry-Perot laser resonator comprises:

first and second optical reflectors separated from each other to reflect the laser pulses therebetween;

an optical gain medium located in an optical path between the first and second optical reflectors;

an optical element located in an optical path between the first and second optical reflectors to lock phases of different longitudinal resonator modes to produce the laser pulses; and

optical fiber forming at least part of an optical path between the first and second optical reflectors, and comprising a doped section as the optical gain medium; and

wherein the laser control mechanism comprises:

a variable optical delay element in an optical path of the Fabry-Perot laser resonator to coarsely adjust the optical path length of the Fabry-Perot laser resonator; and

a piezo electrical actuator in an optical path of the Fabry-Perot laser resonator to finely adjust the optical path length of the Fabry-Perot laser resonator.