Write a python application monitoring lasers!

Life is short, use python!

There are bunches of frameworks to support a graphical user interface (GUI) on your python application. I choose to use wxPython because I was familiar with that. I embed the graph generated by matplotlib into the GUI to give realtime updating of the data.

This application uses a HighFinesse wavelength meter monitoring a Rubidium locked laser and a commercial He-Ne laser. It can automatically record data into a file and calculate the deviation. A round-robin buffer is used to show short-term and long-term data.





Playing with tapered amplifier (TA)


Tapered amplifier, is a device with name resembling its design. It is an laser amplifier(or a gain chip). Its gain area is fabricated to be a tapered shape. The tapered shape help guide the light to a larger output facet, which prevent gain saturation as well as optical damage to the facet.

A drawing of a typical tapered amplifier chip is presented on the left. A drawing of the assembled C-mount tapered amplifier is on the right.


Tapered Amplifier System

When there is no input seed light, a powered tapered amplifier will still emit florescence in both direction. The output florescence (also known as ASM) from the two facet is very different, thus can be used to determine the direction of the chip. Using ASM as a guide light, we can easily align the seed light into the input facet. It is important that you may not use high current as well as high seed power when adjusting seed light coupling. Tapered amplifier will be killed when pumped with high current without proper seeding. Misaligned seed light may also destroy the input facet of tapered amplifier.

When the seed light is partially aligned, you can see laser coming out at the center section of the output ASM. With the help of a laser power meter, I can optimize the amplified laser to maximum. Now, it is safe to turn up the current! I can easily get over 450mW single frequency laser after tapered amplifier.

The tapered amplifier system is shown below.


The seed light comes from a standard ECDL amplified by a injection locking laser.


Since tapered amplifier has a very bad output spatial mode, I have to use several cylindrical lens to shape the beam. In order to prevent retro-reflected laser damaging tapered amplifier(because the reflected laser will be also amplified and collimated to the small input facet which may kill the coating), a good optical isolator is put at the output of the system. Lenses between the tapered amplifier and the isolator are also slightly misaligned to reduce reflection.

AOM Sideband Generation

For experiment needed, I need to generate a sideband(frequency shift a portion of laser) at several hundred MHz. I use an AOM to do that.


Finally, I get around 70mW pure linear polarized output at the output of PM fiber with equal power in the two frequencies. I also set up a scanning FP-cavity beside by system to monitor the system ensuring single frequency output. The yellow channel is the seed light, the green channel is the AOM output. You can clearly see the sideband generated by the AOM 100MHz apart.


Lock laser to an atomic transition without modulation

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.

Laser Locked!

Finally lock my homemade diode laser onto the super stable cavity!

It is not that easy to do this because the cavity is a ULE cavity with finesse as high as 300,000. The line width of this little monster is as low as 5kHz, which is far below the typical ECDL bandwidth. (So the PDH dispersive signal we get from the oscilloscope is actually representing the line width of.our unlocked laser!) We also need to fine tune the loop filter and stabilize our modulators to get a stable lock.

The laser table. See our homemade ECDL (can be hermetic but we have not do that yet).ecdl_photo

The locking table. Mode matching, cavity, vacuum, etc. You can see the strong transmission light through the cavity after lock in the small monitor on the upper right corner of the photo.cavity_photo

Our homemade high speed photo detector. Powered by a LEMO connector with very soft silica cable. The shielding is done by CNC the copper box, which works really good that no interference from the modulator can be seen at the PD output in the spectrum analyzer down to -90dBm.pd_photo

Our all-homemade electronics!elec_photo

In order to further determine the actual line width, we need to setup another set of laser and measure the beat notes between the two lasers.

Our preliminary scheme is to collect the beat notes with a high speed photodetector and down covert it to kilohertz region using a mixer. The final result will be readout by a dynamic analyzer (SR785) and a frequency counter. The signal generator, dynamic analyzer and the frequency counter will all be synchronized to a rubidium frequency standard.

P.S. Our GIANT SR785 just arrived!