Description

Menlo Systems optical frequency combs are the only combs on the market with the patented ultra-low-noise (ULN) figure9^® oscillator design (US9705279B2, EP3041093B1). This special design enables our combs to perform “spectral purity transfer”:
The superior spectral purity of Menlo Systems’ optical reference systems is copied to every comb line.
This includes all frequency extensions, reaching from 500-2000 nm and even Mid-IR comb light at 3.1 µm.

 

1. Setup for measuring sub-Hz linewidth transfer

 

A Menlo Systems ultra-low-noise frequency comb is stabilized to an optical reference system (ORS) operating at 1550 nm. The light of a second sub-Hz linewidth laser is superimposed with comb light at 729 nm, producing a beatsignal with a linewidth of 0.3 Hz.

This measurement proves sub-Hz linewidth transfer from 1550 nm to 729 nm which corresponds to a frequency distance of 210 THz.

If you are thinking about purchasing a frequency comb: Ask for phase noise data at your specific target wavelength(s)!

Phase noise is the most meaningful measure for most comb applications. The phase noise of any proper frequency comb is low close to the optical reference which is used to stabilize the comb. But only Menlo Systems’ ultra-low-noise figure9^® oscillators are capable of transferring the spectral purity of the optical reference to all comb lines!

 

2. Why transfer oscillator schemes are not suitable to qualify frequency combs

 

A frequency comb can be used as a so called transfer oscillator. This technique links different optical frequencies using a frequency comb as a transfer oscillator and specialized high-end electronics (see Ref 1 for a more detailed description). This application can be used for example in comparing optical clocks that have vastly different wavelengths.

Let us call the optical frequencies of interest ν_y and ν_z. The corresponding comb mode numbers and resulting beat signal frequencies are called m_y, m_z and Δ_y, Δ_z.

In terms of the combs repetition rate (ν_rep) and carrier envelope offset frequency (ν_ceo), the optical frequencies of interest are given by:

ν_y = m_y ⋅ ν_rep + ν_ceo + Δ_y

ν_z = m_z ⋅ ν_rep + ν_ceo + Δ_z

The frequency difference is expressed as: ν_y- ν_z = (m_y - m_z ) ν_rep + (Δ_y - Δ_z) and will include comb induced noise via ν_rep and the noise of the beat signals Δ_y, Δ_z.

The transfer oscillator scheme is designed to reject frequency comb induced noise. This is done by mixing both beat signals with ν_ceo and multiplying one of the resulting signals with the exact ratio of the comb mode numbers (m_y / m_z). A third mixer is used to generate the “projected beat signal” ν_c which is given by:

ν_C = ν_A - ν_B = ν_y - (m_y / m_z)ν_z

This rf signal is independent of the frequency comb’s repetition rate and ceo-frequency (up to a certain bandwidth, see Ref. 1 for further details). Obviously, a scheme which rejects frequency comb induced noise, is not suitable for comb noise analysis or comb-comb comparisons.

Benkler et.al. (see Ref. 2 and Ref. 3) demonstrated frequency transfer at the 10^-21 stability level. To measure such stabilities, Benkler et. al. used two different frequency combs in the transfer oscillator scheme and compared the projected beat signals. The reference comb was a Menlo Systems FC1500-ULN. The impressive result of a relative frequency stability on the 10^-21 level proves that frequency combs, used as transfer oscillators, are excellent tools to compare two different optical frequencies. But since the projected beat signals do not include comb induced noise, a measurement in the transfer oscillator scheme cannot provide any information about the stability or phase noise of the frequency comb itself.

 

Ref. 1: Telle, H. R., Lipphardt, B. & Stenger, J. (2002): Kerr-lens, mode-locked lasers as transfer oscillators for optical frequency measurements. Applied Physics B: Lasers and Optics. https://link.springer.com/article/10.1007/s003400100735

Ref. 2: Benkler, E., Lipphardt, B., Puppe, T., Wilk, R., Rohde, F., Sterr, U., End-to-end topology for fiber comb based optical frequency transfer at the 10^-21 level. Optics Express 2019, Vol. 27, Issue: 25. https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-25-36886

Ref. 3: https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-10-15023