A method of creating a local oscillator light beam LO for a phase-resolved spectroscopy measurement comprises the steps of providing a first measuring light beam (1) and a second measuring light beam (2) being aligned to each other, creating the local oscillator light beam LO by an optical non-linear interaction of a first portion (1A) of the first measuring light beam (1) and a first portion (2A) of the second measuring light beam (2) in an optical nonlinear medium (20), and superimposing the local oscillator light beam LO, a second portion (1B) of the first measuring light beam (1) and a second portion (2B) of the second measuring light beam (2) with a predetermined mutual phase relationship, for providing a sample light beam (3) for the phase-resolved spectroscopy measurement. The local oscillator light beam LO and the second portions (1B, 2B) of the first and second measuring light beams (1, 2) are superimposed with a displaced Sagnac interferometer (10).

A new spectral measurement technique is provided which enables measurement even if the light to be measured exists for a very short period. In one embodiment, a broadband pulsed light wave whose wavelength shifts temporally and continuously in a pulse interferes with a light wave to be measured. The intensity at each wavelength of the light wave to be measured is obtained using a Fourier transform of the output signal from a detector that has detected the intensity of the wave resulting from the interference. A laser beam from a laser source is converted to a supercontinuum wave by a nonlinear optical element, and a pulse extension element extends pulses of the supercontinuum wave, thus generating the broadband pulsed light wave.

An optical device includes: a second light source configured to emit laser light having a wavelength different from that of measurement light emitted from a first light source; a light splitter; a first mirror configured to add a first modulation signal to the measurement light and add a displacement signal to the laser light; a second mirror configured to reflect a second beam of the measurement light; a first color separator configured to separate the second beam of the measurement light from a second laser light beam based on a difference in wavelength; an optical modulator driven based on a drive signal and configured to add a second modulation signal to the second laser light beam after the separation; a first photodetector configured to receive the measurement light including a sample-derived signal and the first modulation signal; and a second photodetector configured to receive the laser light including the displacement signal and the second modulation signal.