Hertz-level frequency comparisons between diverse color lasers without a frequency comb

We present a simple yet powerful technique to measure and stabilize the relative frequency noise between two lasers emitting at vastly different wavelengths. The noise of each laser is extracted simultaneously by a frequency discriminator built around an unstabilized Mach–Zehnder fiber interferometer. Our protocol ensures that the instability of the interferometer is canceled and yields a direct measure of the relative noise between the lasers. As a demonstration, we measure the noise of a 895 nm diode laser against a reference laser located hundreds of nm away at 1561 nm. We also demonstrate the ability to stabilize the two lasers with a control bandwidth of 100 kHz using a Red Pitaya and reach a sensitivity of 1Hz2/Hz limited by detector noise. We independently verify the performance using a commercial frequency comb. This approach stands as a simple and cheap alternative to frequency combs to transport frequency stability across large spectral intervals or to characterize the noise of arbitrary color sources.