Laser frequency stabilization2018-06-29
Stabilizing the absolute frequency of a laser is central in many applications such as gas sensing or calibration of optical spectrum analyzers. Absolute frequency stabilization relies on the spectroscopic lines. Here we stabilize a telecom DFB laser on a HCN standard cell.
Using a precision low-noise laser diode controller such as the Koheron CTL100 laser controller produces a narrow linewidth emission with good short term stability. However, the absolute frequency of the laser is not precisely known and will drift in the long run (hours, days ...).
To overcome this issue, the laser frequency can be referenced to a spectroscopic line. An atomic or molecular transition is only weakly influenced by the ambient conditions so drifts are drastically reduced. Absolute frequency knowledge is achieved using lines that are traceable to a metrological standard. For example some HCN gas cells under standard pressures provides NIST-traceable absorption lines. These lines are used for wavelength calibrations in the telecom C-Band. Here we show how to stabilize a laser diode on such a line.
The stabilization setup is:
A DFB telecom laser at 1550 nm is controlled with a Koheron CTL100-B-400 controller. The laser output is separated into two paths: one passing through the spectroscopic cell and the other is a reference beam. Detecting both beams using a Koheron PD100B-DC balanced photodector provides a spectroscopic absorption measurement immune from laser intensity noise.
The detector output feeds the Koheron PI200-T laser servo controller input. By adjusting the servo controller setpoint, we subtract an offset to the absorption profile. We obtain an error signal allowing us to lock on the side of an absorption line.
The fast output of the laser servo controller controls the laser diode current, whereas the slow output controls its temperature.
A waveform generator connected to the setpoint modulation input of the servo controller performs arbitrary laser frequency modulations.
Before closing the loop, we send a modulation ramp on the laser controller's DC input (blue) and observe the photodetector output (yellow) and the error signal (green).
The error signal is the absorption line displaced by an offset that can be tuned using the setpoint potentiometer of the servo controller.
For increased sensitivity, one may consider saturating the photodetector outside the absorption line to maximize the signal in the region of interest of the line.
Laser diode control
We tune the current and / or the temperature of the laser to correct its frequency. Here, we use the DC input of the laser controller to control the diode current. The DC input provides three modulation ranges: high, medium and low. On one side, the medium modulation range provides a higher correction depth. However, the broadband noise injected to the laser current would be bigger than with the low modulation range. On the other side, the low range will inject minimal noise but with reduced dynamic range.
To combine the best of both modulation ranges, we connect the fast output of the servo controller on the DC modulation of the laser controller (set on low range). The slow output is connected to the temperature modulation. Thanks to the slow integrator between the fast and the slow outputs, the servo controller adjusts the laser temperature to maintain the DC current modulation input centered on zero. We thus benefit from the large bandwidth and low noise of the current modulation while having a large tuning range thanks to the temperature modulation.
Tuning the lock
To tune the lock gains, we send a square wave on the setpoint modulation input and optimize the step response. Here we use a 100 kHz square wave and observe the result for several fast integrator gains. The blue signal is the setpoint modulation input, the green one is the error signal and the yellow one is the photodetector output.
- Gain in position 1 (Minimum gain). The rise time is too slow to reach a steady state and the error signal does not return to zero. The bandwidth is below 100 kHz.
- Gain in position 5. A steady state is reached after some overshoot and the error signal returns to zero. The rise time is 220 ns, that is a bandwidth of 1.6 MHz.
- Gain in position 6. Gain is too high: signals oscillate.
Arbitrary frequency modulation
Once the laser is frequency-locked on the fringe side, we can produce arbitrary frequency modulations using the setpoint modulation input of the servo controller.
For example, thanks to the high bandwidth of 1.6 MHz, we can produce high quality ramps at 10 kHz: