There is always possibility that you need to stabilize a laser but do not have an appropriate modulator like AOM or EOM in hand. The dichroic atomic vapor laser lock(DAVLL) technique provides a really simple and robust modulation-free way to do that.
Although there is no modulation in DAVLL, the error signal of it is still resemble to the FM spectroscopy and the prevailing Pound-Drever-Hall(PDH) locking technique with a dispersive shape, which provide a stable zero-crossing point for locking. The dispersive error signal comes from the differential measurement of the Zeeman split levels.
The apparatus lack the complex of difficult alignment of the general modulators. Since the polarization in this technique is really important, good quality polarization beam splitter(PBS) with correct coating should be used.
The photo of the apparatus.
The coil and the heating strip.
With the pump beam(brown) blocked, and only observing the signal PD, we get the typical doppler broadened absorption signal.
With the pump beam unblocked, we can clearly see the hyperfine features in the saturated absorption (doppler free) signal from signal PD.
When we turn on the balance mode, observing signal PD with reference PD subtracted, we can see the dispersive shaped DAVLL error signal. We can also easily recognize the hyperfine transitions as well as crossovers from this snapshot.
And the last step is locking. The residue noise of after locking is shown below. This apparatus achieves sub MHz frequency noise according to a WS-7 wavemeter and will not easily get out of lock(as long as the laser is not drifting too far away).
Thanks Ben and Loic for kindly helping me debugging the system.