An ultrafast mode-locked laser comprising: a mode-locked laser source configured to output laser pulses having a repetition rate; a synchronous electro-optic modulator, SEOM,, comprising: a quarter wave plate; a first polarizing beam splitter or polarizer; an electro-optic modulator, EOM,; circuitry configured to drive the SEOM with a drive waveform, the drive waveform being a phase-coherent sinusoidal waveform at a frequency equal to half of the repetition rate of the mode-locked laser source; and a second polarizing beam splitter; wherein the SEOM is arranged to direct the laser pulses from the laser source through the quarter wave plate, the first polarizing beam splitter, the EOM, and then to the second polarizing beam splitter; wherein the second polarizing beam splitter is configured to split the light pulses from the EOM into a first pulse train in a first direction having a first polarization state and a second pulse train in a second direction having a second polarization state; wherein the ultrafast mode-locked laser further comprises one or more optical elements configured to feed the second pulse train back to the SEOM. a mode-locked laser source configured to output laser pulses having a repetition rate; a synchronous electro-optic modulator, SEOM,, comprising: a quarter wave plate; a first polarizing beam splitter or polarizer; an electro-optic modulator, EOM,; circuitry configured to drive the SEOM with a drive waveform, the drive waveform being a phase-coherent sinusoidal waveform at a frequency equal to half of the repetition rate of the mode-locked laser source; and a second polarizing beam splitter; wherein the SEOM is arranged to direct the laser pulses from the laser source through the quarter wave plate, the first polarizing beam splitter, the EOM, and then to the second polarizing beam splitter; wherein the second polarizing beam splitter is configured to split the light pulses from the EOM into a first pulse train in a first direction having a first polarization state and a second pulse train in a second direction having a second polarization state; wherein the ultrafast mode-locked laser further comprises one or more optical elements configured to feed the second pulse train back to the SEOM. a quarter wave plate; a first polarizing beam splitter or polarizer; an electro-optic modulator, EOM,; circuitry configured to drive the SEOM with a drive waveform, the drive waveform being a phase-coherent sinusoidal waveform at a frequency equal to half of the repetition rate of the mode-locked laser source; and a second polarizing beam splitter; wherein the SEOM is arranged to direct the laser pulses from the laser source through the quarter wave plate, the first polarizing beam splitter, the EOM, and then to the second polarizing beam splitter; wherein the second polarizing beam splitter is configured to split the light pulses from the EOM into a first pulse train in a first direction having a first polarization state and a second pulse train in a second direction having a second polarization state; wherein the ultrafast mode-locked laser further comprises one or more optical elements configured to feed the second pulse train back to the SEOM.