Abstract: An actual optical field spectrum is mixed down to baseband and can be observed directly to measure the optical field spectrum of an optical signal based on a local oscillator approach. This local oscillator approach is achieved without adding an additional local oscillator. A single optical source, such as a laser, is controlled by a modulation source for selectively modulating the optical signal produced by the optical source. Accordingly, the optical signal produced by the optical source alternates between two states, namely, an unmodulated state and a modulated state which carries the optical field spectrum of interest. Preferably, the optical signal produced by the optical source is routed to an optical conduit in parallel with an optical delay line. Although the two states of the optical signal produced by the optical source occur sequentially in time, they are mixed together concurrently after being channeled through the parallel optical circuit comprising the optical conduit and the optical delay line.