OPTICAL SYSTEM

The requisite laser beams were obtained using only two independant diode laser systems. Both systems were based on high power Spectra Diode lasers frequency stabilised by an external grating cavity [9] [online], , with reference to the saturated absorbtion spectrum of cæsium. The output in zeroth order of the grating was about 10 mW, which was sufficient for the repumping beams. To obtain more power for the main cooling beams the light was amplified to 100 mW by injection locking another diode, and a further increase in power could realily be achieved by using the same master to control several slave lasers whilst still maintaining the long term reliability required for a clock. The master laser was computer controled so that its frequency could either be locked at a fixed offset from the F=4 $\rightarrow$ F=5 saturated absorption line or moved in less than one 1 ms to any nearby frequency.

Figure 3 shows the schematic of the lasers and the associated optics. The light from the slave laser is double-passed through acousto-optic modulators to enable a frequency shift to be introduced between vertical beams and to shut off all the beams quickly. The AOMs shift the frequency of the light, relative to the laser frequency, by 2$\times$80 MHz in the horizontal beams (H) or 2 $\times$(80$\pm \delta$) for the vertical beams (V1,V2). In order that these output beams have a small detuning below the required transition we used another AOM at a frequency of 2 $\times$ 75 MHz before the absorption cell. The frequency of this AOM is dithered to allow phase sensitive detection of the saturated absorption spectrum in the cell. Part of the repumping laser beam is mixed with the horizontal beam (H) and some was transmitted separately (HR) to the clock for $\pi$-polarized beam required for the second scheme described in section 3. Optical fibres for the vertical beams eliminate possible movements from the misalignment of the back reflection through the AOMs and ensure good beams quality.

Figure 3: Laser Systems: AOM: Acousto-Optic Modulator, Cs: Cæsium Cell, ML: Master Laser, P: Photodetector, RL: Repumper Laser, S: Shutter, SL: Slave Laser, OI: Optical Isolator, $\lambda$/2, $\lambda$/4: Retardation plates