2.1. Step-by-step guide

LIR: a step-by-step guide 

Prerequisites:

We assume that you

• are familiar with using the controls of your laser (for instance DCC110, SC110)
• have a reference signal onto which you want to stabilize ("lock") your laser.
• the laser operates in single mode and is tunable over a sufficiently wide range. (see below)
• are a bit familiar with the theory behind the operation of an LIR (see also the manual pages)
• have an oscilloscope capable of measuring voltages up to 200V and, optionally, being capable of operating in x-y mode.
• have at least a basic understanding of the operating principle of the lock-in technique.

Thanks to its configurability the LIR can easily be adopted to a multitude of applications. It is beyond the scope of this tutorial to treat all of them. We therefore focus on the most common task, namely the stabilization of a tunable diode laser to the top of a resonance fringe in atomic/molecular spectroscopy. Furthermore we assume that for best ease-of-use (direct backplane-communication between the modules) the LIR110 is used together with SC110 as a driver for the piezo-electric actuator (PZT) and DCC110 as a current driver.

7.3.1.0 Diode Laser, Cell/FPI and Detector Preparation

Adjust the driving current of the Diode Laser Head DL 100 in order to find your signal structure of interest nicely situated within the SC 110 scan range using the SC 110 trigger as trigger source. Make sure that the laser operates single mode within the full frequency scanning range by adjusting the current (and maybe even the feed forward parameter of your Scan Control SC 110). Use the SC 110 offset and the SC110 amplitude potentiometer in order to zoom into one resonance. With the SC 110 trigger delay potentiometer the resonance can be pulled into the middle of the oscilloscope display. An appropriate scan frequency is 50 Hz.

7.3.1.1 External and internal connections

We recommend to adapt the set-up shown in Figure 9.5. Usually when the modules are delivered to the customer, the backplane connections are preset by the factory as shown above.
1. Feed the (amplified) spectroscopy signal into the input connector (9) of the LIR110. The Input Amplifier is preset (default) to operate with voltage sources (e.g. pre-amplified photo diodes). If the customer wants to connect a photodiode directly, please read Paragraph 4.3.2.2.
2. Make sure the SC110 HV output is connected to the PZT of your laser. Feed this signal also to channel 1 (x-channel) of your oscilloscope (Attention! make sure that your oscilloscope can take high voltages! The SC110 provides up 150V).
3. Connect the monitor output BNC-connector (28) to channel 2 (y-channel) of your oscilloscope.
4. Connect the trigger output of the SC110 to the trigger input of your oscilloscope and trigger on this signal.

7.3.1.2. Preparations

5. Switch the Monitor Output Selector (27) to "input amplitude"
6. Set the Modulation Source INT./EXT: switch (5) to "INT."
7. Select AC1 on the Coupling Mode Selector Switch (10)
8. Make sure that your oscilloscope is set to time-dependent mode (not x-y-mode)
9. Set the Modulation ON/OFF switch (21) to OFF
If you now scan your laser (frequency typically 50 Hz) across the desired resonance, the usual spectroscopy signal should appear. You can adjust the amplitude of the signal with the Input Amplifier Gain BCD-Switch (11).

7.3.1.3. Settinging the modulation frequency

In order to obtain a LIR signal that is proportional to the 1:st derivative of this signal one needs to modulate the photo detector signal.
10. Temporarily reconnect the BNC cable from (28) to the Modulation Output BNCConnector (3) in order to monitor the modulation signal on the oscilloscope. (You also need to temporarily trigger on channel 2).
11. With the Frequency Coarse BCD-switch (4) and the Frequency Fine Potentiometer (1) adjust the frequency to a appropriate value for your task, e.g. 800Hz.
12. Reconnect the BNC cable from (3) to (28) (and use the external trigger source from the SC110 again).
13. Set the Modulation ON/OFF switch (21) to ON
14.You should now see your spectroscopy signal modulated with the frequency you chose above. Try playing with the Amplitude Potentiometer (2) to adjust the amount of modulation. Finally set the modulation amplitude to "zero".
15. Set the Modulation ON/OFF switch (21) to position OFF.

If your oscilloscope is capable of operating in x-y mode we recommend to make use of this feature since it allows better visual feedback. In order to make use of this feature the following steps are necessary:

• Switch your oscilloscope to x-y mode and make sure that both channels are set to DC-coupling mode. The scan-ramp signal (from the SC110) on the x-axis corresponds to the laser frequency/wavelength, while the y-axis corresponds to the photo detector signal. To test this you can temporarily switch off the scan on the SC110 by setting the trigger switch to "ext." (see item 7 on page 5, part VIII, DL100 manual). You will see a spot on the oscilloscope. By slowly turning the offset knob on the SC you can make the spot "walk" the absorption signal.
• Depending on the settings of the SC110 you might run into trouble with resolving the x-axis well due to the high voltage coming from the SC110. In this case we recommend to pick up the low-voltage version of the scan-signal. i.e the feed-forward signal, from the backplane, for instance with the DCB110 card. Attention: Great take should be taken when attempting to extract
  signals from the backplane, since the backplane carries voltages up to 280V!
• On the SC110 set the trigger switch to "int." in order to start scanning again
• in the following steps that are specific to operation in x-y will be marked with the
  suffix "xy", eg "27xy"

7.3.1.4. Obtaining the error signal

16. Using the offset and amplitude potentiometers on SC select a clearly visible absorption fringe.
17. Reduce the scanning frequency on the SC to about 3 Hz. This is necessary because we are going to apply a low-pass filter to obtain the DC-component of the LIR signal, but we still want to be able to see the laser scanning. For this purpose set the Low-Pass BCD Switch (13) initially to position "4" (cut-off at 16Hz).
18. Adjust the phase to 0° by setting both the phase coarse BCD-switch (7) and the phase fine potentiometer (8) to position "0".
19. Set the Monitor Output Selector (27) to "lock-in-output"
20. Slowly increase the modulation amplitude (2) until you start seeing something that looks approximately like the first derivative (dispersion signal) of the spectral line you selected in step 16. The signal can be optimized in many ways:

7.3.1.5. Lock-in signal optimization

21. In order to optimize the phase between the reference signal and the photodiode signal, turn the Phase fine Potentiometer (8) until the error signal vanishes (i.e. the signal is more or less flat)
22. Turn the Phase Coarse BCD-switch (6) one notch. The dispersion signal reappears optimized. Turning the switch two more notches (180°) will inverse the error signal.
23.If your signal appears noisy (if for instance one can still see remainders of the 800Hz modulation) turn the Low-Pass BCD Switch (13) clockwise. This will cause a drop in signal amplitude which can be compensated by increasing the input gain (11)
24. The error signal is also proportional to the modulation amplitude. Increasing the modulation amplitude might further help to optimize the signal. A too large amplitude, however, will wash out the error signal, because it will effectively result in taking the average of the spectral line.

Note: The lock-in measurement bandwidth is adjusted by the Low Pass (13) cut-off frequency. That itself has to be chosen according to the modulation Frequency (1,4) and the spectral width (FWHM) of the respective resonance or absorption signal on which the customer wants to stabilize the laser frequency. As an example, the bandwidth needed to stabilize a laser properly onto a Doppler broadened absorption signal (several 100 MHz for Rb) is much lower than the bandwidth needed to stabilize the same laser onto its Doppler free (about 10 MHz for Rb) absorption signal or onto a high finesse Fabry-Perot Interferometer. Hence both the Frequency (1,4) setting as well as the Low Pass (13) filter setting are strongly dependent on the individual locking task, mainly on the input amplitude, the input noise amplitude, the input noise spectrum and the resonance width.

7.3.1.6. Locking with the internal PID regulator

The basic operation principle of a proportional, integraI and differential (PID) regulator loop is a vast topic and can not be the subject of this manual. Please refer to the standard literature for a more general description. Here we can give only very simple recipes for using the PID regulator for the task at hand.

The internal PID inside the LIR is set to lock onto ground (GND) . Thus, in order to lock on top of a fringe, one needs to center the error signal around ground.
25. Turn the P, I and D potentiometers (15-17) all the way counter-clockwise.
26. Set the Regulator output gain potentiometer (20) and the PID output Range potentiometer (22)to position "5".
27. Make sure you know where the ground level of your oscilloscope is
26. Adjust the lock-in-offset potentiometer (12) so that the dispersion line is centered symmetrically around zero (flat parts on ground level).
27. Switch off the scan on the SC110 by setting the trigger switch to "ext." You should now see a constant DC signal (make sure you are in DC-coupling mode).
27xy:.Switch off the scan on the SC110 by setting the trigger switch to "ext." You should now see a spot. (make sure you are in DC-coupling mode).
28. Manually scan the laser using the offset knob on the SC. Position the line (spot for xy-mode) on the point where the slope of the error signal crosses the ground level. Then move slightly away from ground in order to be able to see the effect of locking (see step 30).
29. Set the regulator ON/OFF switch (19)to ON. If all P,I and D parameters are set to zero, nothing should happen.
30. Slowly increase the I contribution (16).

a) If the line on the oscilloscope moves towards ground, continue until it starts to oscillate vertically. Decrease the I contribution a bit until the oscillation vanishes.
b) If the spot quickly moves away from ground and then back towards it (i.e the laser changes its frequency) the error signal has the wrong sign. In this case, switch the regulator OFF (19) and the scan ON. Turn the Phase Coarse BCDswitch (6) two notches to invert the error signal. return to step 26.

30xy Slowly increase the I contribution (16).

a) If the spot on the oscilloscope moves towards ground, continue until it starts to oscillate vertically. Decrease the I contribution a bit until the oscillation vanishes.
b) If the spot jumps horizontally (i.e the laser changes its frequency) the error signal has the wrong sign. In this case, switch the regulator OFF (19) and the scan ON. Turn the Phase Coarse BCD-switch (6) two notches to invert the error signal. return to step 26.

31. Then an increase of the P-contribution (15) follows. As a last step, the Dcontribution (17) will be set to a position where it is possible to increase the Pand the I-contribution once more.

Is the error signal locked to ground when you, for instance, knock on the optical table? Congratulations! You have successfully mastered the first hurdle and established a stable feedback loop. Now it is time to play around with the many paramters in order to obtain a configuration that is optimized for your particular application.