Characterisation and stabilisation of 2 µm external cavity diode lasers for gravitational wave detection

We present characterisation and stabilisation results for three prototype tuneable external cavity diode lasers (ECDLs), low-cost compact laser sources operational at 1925 – 1955 nm central wavelength with a 150 nm tuning range. The ECDL free-running relative intensity noise shows good performance in the range of 10 −6 / Hz up to 1 kHz and below 10 −7 / Hz at higher frequencies. The frequency noise drops as 1/ f up to 1 kHz and, as seen in other diode laser sources, flattens off at a level of 40 Hz/ Hz at higher frequencies. Signal injections onto the laser diode current and the angle of the Littrow grating in the laser housing allow modulation of the intensity and frequency of the laser light. Transfer functions were taken of these feedback mechanisms to characterise laser performance and stabilisation. We present measurements of stabilised intensity noise and stabilised frequency noise for a 2 µm ECDL. The relative intensity noise between 10 Hz – 50 kHz is reduced to a level of 2 × 10 −8 / Hz . Frequency noise suppression was achieved by Pound-Drever-Hall locking of the laser to a 10 cm rigid cavity, giving a noise reduction across 1 Hz – 100 kHz. From 10 Hz – 1 kHz, there is a 200× suppression to below 10 Hz/ Hz .