Figure 1

(Color online) (a) Setup for Doppler-free saturation spectroscopy. Light from the laser passes through a polarizing beam splitter (PBS). Then the polarization is rotated by a Faraday rotator $\left(r\right)$, so that it leaves through the second PBS port after passing through the vapor cell. The cell contains isotopically enriched rubidium at room temperature and is magnetically shielded by a single layer of $\mu$-metal. The spectrometer consists of a $5\times$ beam expander, a 2400 lines/mm grating, and a 70 cm focusing lens. A single mode can be isolated with a $125\mu \text{m}$ slit. (b) The 10 GHz modes imaged on a charge-coupled device in the focal plane are spaced by about $130\mu \text{m}$. (c) Hyperfine structure of the $D_2$ line in ${}^{87}\text{R}\text{b}$. (d) Saturation spectrum of the rubidium $D_2$ line with transitions from the $5S_{1/2}$, $F=2$ ground state to the excited states $5P_{3/2}$, $F^{\ast }=1,2,3$ and associated crossover resonances. The first derivative, which will be used for locking, shows clearly the six expected peaks.Reuse & Permissions

Figure 2

(Color online) (a) The repetition rate of the 10 GHz comb is locked to a 10 GHz synthesizer by photodetecting a small amount of light from the laser output. The feedback goes to an acousto-optic modulator in the pump beam. With the spectrometer, the saturation signal is obtained by lock-in technique and used to lock one comb mode to an atomic transition. (b) The offset frequency of the 1 GHz comb is measured with a $2f\text{−}3f$ interferometer and locked with an AOM in the pump beam to a synthesizer. The repetition rate is measured with a photodiode and also locked to a synthesizer. To ensure accurate measurements and definitive tests of the comb stability, all synthesizers are referenced to a stable 10 MHz signal from a hydrogen maser. (c) Light from both stabilized combs is combined in fibers. For the broadband 1 GHz comb a 10 nm bandpass (BP) filter at 800 nm is used to isolate the overlapping regions of the spectrum. (d) The beat signal between both combs is detected on a photodetector. The obtained microwave signal is filtered and amplified so that its frequency can be counted.Reuse & Permissions

Figure 3

(Color online) (a) Frequency domain picture of the interference between modes of the 10 GHz laser and the 1 GHz laser. Beat signals (horizontal lines) between all modes of the locked combs occur. For purposes of clarity, only some beat signals are indicated. (b) The corresponding microwave spectrum shows the 1 GHz repetition rate plus higher harmonics, the $10\mathrm{th}$ harmonic of the 1 GHz repetition rate together with the 10 GHz repetition rate, and the beat signalsReuse & Permissions

Figure 4

(a) Measurement of the beat signal ${\nu }_{b2}$ between the 10 GHz and the 1 GHz comb over 4 h. The graph gives the offset from the average value of 837.1856 MHz. (b) Corresponding Allan deviation with error bars for the frequency measurement.Reuse & Permissions