An optical oscillator 10 combines optical signals L1′ and L2 to generate an optical signal L3. The optical signal L1′ includes light waves W1′ and W2′ having frequencies spaced apart by a frequency difference Δf1. The optical signal L2 includes light waves W3 and W4 having frequencies spaced apart by a frequency difference Δf2. The optical oscillator 10 separates the optical signal L3 into optical signals L4 and L5, wherein the optical signal L4 includes the light waves W1′ and W3 and the optical signal L5 includes the light waves W2′ and W4. The optical oscillator 10 compares the frequency differences Δf1 and Δf2 based on frequency difference Δf3 between the light waves W1′ and W3 included in the optical signal L4 and frequency difference Δf4 between the light waves W2′ and W4 included in the optical signal L5.