A resonance fiber-optic gyro (RFOG) with quadrature error reducer comprising: a laser assembly including: a master laser, a clockwise slave laser, a CW optical phase lock loop (OPLL) to control the CW slave laser, the CW(OPLL) coupled to receive an output of the master laser, a counter clockwise slave laser, and a CCW optical phase lock loop (OPLL) to control the CCW slave laser, the CCW OPLL coupled to receive an output of the master laser; a fiber resonator assembly including: a first circulator having a first input coupled to receive a first output of the laser assembly, a second circulator having a first input coupled to receive a second output of the laser assembly, and a fiber coil coupled between first outputs of the first and second circulators (132 and 134); a resonance tracking loop to generate a resonance frequency signal, an output of the resonance tracking loop coupled to a OPLL mixer (112 or 122) in one of the CCW OPLL and the CW OPLL of the laser assembly; characterised in that the resonance fiber-optic gyro further comprises a quadrature error reducer circuit coupled to receive a second output of one of the first circulator and the second circulator (132 or 134), the quadrature error reducer circuit including: an amplitude control loop to generate a common modulation signal, an output of the amplitude control loop coupled to a common phase modulator in the laser assembly; and a second harmonic phase control loop to selectively adjust a phase of a second harmonic modulation signal in the amplitude control loop at startup. a laser assembly including: a master laser, a clockwise slave laser, a CW optical phase lock loop (OPLL) to control the CW slave laser, the CW(OPLL) coupled to receive an output of the master laser, a counter clockwise slave laser, and a CCW optical phase lock loop (OPLL) to control the CCW slave laser, the CCW OPLL coupled to receive an output of the master laser; a master laser, a clockwise slave laser, a CW optical phase lock loop (OPLL) to control the CW slave laser, the CW(OPLL) coupled to receive an output of the master laser, a counter clockwise slave laser, and a CCW optical phase lock loop (OPLL) to control the CCW slave laser, the CCW OPLL coupled to receive an output of the master laser; a fiber resonator assembly including: a first circulator having a first input coupled to receive a first output of the laser assembly, a second circulator having a first input coupled to receive a second output of the laser assembly, and a fiber coil coupled between first outputs of the first and second circulators (132 and 134); a first circulator having a first input coupled to receive a first output of the laser assembly, a second circulator having a first input coupled to receive a second output of the laser assembly, and a fiber coil coupled between first outputs of the first and second circulators (132 and 134); a resonance tracking loop to generate a resonance frequency signal, an output of the resonance tracking loop coupled to a OPLL mixer (112 or 122) in one of the CCW OPLL and the CW OPLL of the laser assembly; characterised in that the resonance fiber-optic gyro further comprises a quadrature error reducer circuit coupled to receive a second output of one of the first circulator and the second circulator (132 or 134), the quadrature error reducer circuit including: an amplitude control loop to generate a common modulation signal, an output of the amplitude control loop coupled to a common phase modulator in the laser assembly; and a second harmonic phase control loop to selectively adjust a phase of a second harmonic modulation signal in the amplitude control loop at startup. an amplitude control loop to generate a common modulation signal, an output of the amplitude control loop coupled to a common phase modulator in the laser assembly; and a second harmonic phase control loop to selectively adjust a phase of a second harmonic modulation signal in the amplitude control loop at startup.