There are many performance data that specify a complex unit like MOG. We have concentrated on ergonomics, but always with the ultimate laser linewidth in mind. Our lasers are limited by the optical characteristics of the external cavity; MOG noise is inconsequential. Our normal cavities are short (20mm), the grating feedback is small (10%), and the diodes are not AR coated; the linewidth is typically around 1MHz. By doubling the cavity length, we have demonstrated linewidths of 325kHz fwhm (full width at half max) or 115kHz rms, measured over tens of seconds. Linewidths below 50kHz will certainly be possible with MOG if the external cavity feedback is strong.

Current noise

The curve above shows the diode injection current noise (or lack thereof). Measured at 100mA into four 1N4148 diodes (effective resistance about 3.5 ohms). Black trace is short-circuit, blue is with current on. Unfortunately we cannot distinguish the traces; the current noise level is below -140dBm (at 10Hz RBW).

Frequency noise

Here we show typical feedback error signal noise spectra with a MOGunit.

Linewidth

The figure below shows the beatnote frequency with two lasers locked to transitions in Rb87, with different hyperfine ground states separated by 6.8GHz. One MOG was locked to a saturated absorption peak, the other to an electromagnetically induced transparency (EIT) using FM sideband locking (essentially Pound-Drever-Hall). The beatnote width is 4kHz for a 22.5s average, and less than 1kHz for a single sweep of 0.2s. The work has recently been published in Applied Physics Letters.

Scan range

The scan range available depends very much on the mechanical and optical design of your laser. Using a standard laser diode for cd-rom writing applications (Sharp GH0781JA2C or Roithner Lasertechnik ADL-78901TX), without special coatings, and a low-efficiency 1800l/mm grating, scans of about 1GHz are typical in a Littrow configuration. Increased scan range is possible with higher dispersion and/or higher feedback gratings, or using a Littman-Metcalf arrangement.

We can instead ramp the diode injection current and cavity length (piezo) together. With standard uncoated diode, low-feedback grating and Littrow configuration, we can scan the entire 780nm rubidium hyperfine structure for both naturally occurring isotopes, nearly 10GHz. The figure below shows the saturated absorption and MOG error signal with a standard MOGbox.