The Lock Assist is a unique feature of the Laser Lock Box instrument that allows users to simply select a set point for the closed-loop control of their system, corresponding to a particular zero-crossing point on the error signal. While this instrument is designed for the purpose of locking a laser, the principle can be applied to similar control loop applications with a zero-crossing error signal.

Computation of error signal

In its most common configuration, such as when used for Pound-Drever-Hall locking, the Laser Lock Box uses a modulation output and demodulator on the input to form the error signal. In this case, the error signal checks for the condition when the light is completely inside the cavity, and nothing is reflected from it. More technically, it measures the frequency difference between the laser and the resonance of the cavity, by interrogating the  cavity with the modulation signal. Alternatively, the demodulation can be bypassed and the error signal computed by other means, e.g. using the Phasemeter or Time and Frequency Analyzer in Multi-instrument Mode to do offset-frequency locking.

While Lock Assist can be used regardless of the source of the error signal, it's most commonly used when using the demodulation scheme. In this case, the values around which the output is modulated is set by a ramp, slowly sweeping the frequency of the laser through a range that's expected to include the resonant frequency of the cavity. Lock Assist helps in setting the parameters of this sweep to view the region where the error signal indicates the cavity resonance lies, selecting the point on the scan that corresponds to the specific resonance to which you want to lock, and using this value as the set point of the loop once the scan is disabled.

How to use Lock Assist

Lock Assist is engaged from the “Laser” icon in the top-right of the Oscilloscope view. When the lock assist is engaged, the oscilloscope automatically puts a probe after the offset correction as shown in the figure by Probe A. The horizontal axis of the Oscilloscope view is changed from units of time to units of volts, representing the range of the scan signal. Selecting the error zero crossing point will pick the scan offset at that point, and by default engage the fast controller control loop at that set point.

When the lock assist is selected, the trigger settings and time base settings are not used as the view is set to synchronize with the scan signal. Changing these settings will disable the lock assist and go back to normal observation mode. Other signal cannot be used as trigger in lock assist while Scan signal can be used as trigger at all times.

Increasing visibility of lock points

To increase the visibility of the cross points, the acquisition settings can be varied by changing from ‘Normal’ to ‘Precision’ or by increasing the averaging on the signal. These settings will improve the signal to noise ratio by getting rid of high-frequency noise. Typical scan rates are at frequencies less than 20 Hz, and therefore setting the averaging or putting in precision mode would maintain the slow signal dynamics. Another option is to utilize the low-pass filters in the error computation path. Setting the frequency of the filter to a low value can filter noise but will fundamentally limit your control bandwidth. This is mostly never changed but can be setup at low values (>1kHz) for initial lock acquisition and then gradually be increased.

Additionally, the waveform can be zoomed to have a closer look at a particular cross-point. In lock assist, zooming in in vertical direction (amplitude), allows the error signal to be steeper, and allows the lock assist to recognize the zero-crossing point better. Zooming in horizontal direction (timebase), reduces the amplitude of the actuator modulation signal and narrows down the region in which the cross-over point can be found. For two or more zero-crossing events in the error signal, zoom-in also allows the lock box to engage the controllers to any particular cross-point.

Applying offsets to the setup

If the zero-crossing point is near the end of the scan signal, scrolling the Oscilloscope view left and right changes the offsets on the scan signal to position the zero-cross point near the zero of the scan signals. Sometimes this is not desirable; for example, in optical cavity locking, it might be preferable to move the relative location of the scan and zero-crossing by moving the laser frequency through the thermal offset. This setting can be changed manually through the offset in the slow controller path in the Laser lock box (if that is connected to the slow/thermal actuator of the laser).

Once the lock assist is turned on in optimal conditions, the zero-crossing point can be selected, and the control loop can be engaged. 

If you are facing difficulties in using the lock assist on the Laser Lock Box, contact us at support@liquidinstruments.com.