Simple Methods for stabilization of diode lasers.
We have developed two different methods for simple and convenient stabilization of diode laser systems that are used in our work — and have potentially broad applicability. Both schemes use Doppler-broadened (vapor cell) atomic samples. These techniques remove long-term drifts, producing residual noise below the 1 MHz level for time scales from 10 msec to 100 sec. These methods are described in the following Review of Scientific Instruments articles:
Faraday Rotation scheme
This method is particularly useful for locking to the side of atomic transition (as we would require for our differential phase shift spectroscopy method). We have a optical polarimeter capable of detecting microradian-level optical rotation changes (similar to the design used for atomic parity violation experiments at UW Seattle). An atomic vapor cell (thallium in our case) is placed between the polarizer and analyzer of the system. Applying an AC magnetic field to a Faraday glass allows sinusoidal polarization modulation, and lock-in detection is used to obtain high-precision optical rotation signals. By applying very small (few Gauss) magnetic fields to the atomic sample, we can obtain very high signal-to-noise Faraday rotation lineshapes, at which point we feedback the polarimeter signal to the laser PZT to hold the output polarization fixed.
This scheme offers the ability to lock over a wide range of frequencies on either side of an atomic resonance. In our application, we study a ‘forbidden’ M1/E2 transition, demonstrating that this scheme is well-suited to non-E1 transitions, where saturated absorption schemes are not convenient.
AOM-based multiple-beam locking scheme
Charles Cao ’09 with laser locking apparatus, including AOM, supplementary vapor cell oven, and signal processing electronics
We developed this scheme for our most recent indium work where, again, we require long-term frequency stability at the 1 MHz level, and appreciate the convenience of lock point tunable over nearly 1000 MHz. Again, we use a Doppler-broadened heated cell (indium in this case. Our blue laser produces efficient second-order diffraction in an AOM tuned to 200 MHz. By double-passing, we obtain a large number of frequency-shifted beams, in particular, two shifted by 800 MHz (roughly one-half of the Doppler-broadened profile). These beams, first intensity-matched, are sent together through a heated indium cell, and the laser is tuned near the desired hyperfine resonance of the 410 nm line. Two frequency shifted absorption dips result. We use the resulting differential transmission signal as an error signal, which is processed and fed back to the laser PZT.
A further improvement to this scheme, offering better common mode noise rejection, is to polarization-tag and then analyze the two laser beam frequency components, so that they truly co-propagate through the cell. This simple and robust method greatly facilitated our recent indium two-step spectroscopy experiment.