Experiment - External-Cavity Diode Lasers (Assembly)
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Last updated
Last updated
Assembling the external-cavity diode laser is the most mechanically tricky aspect of the entire experiment and will require some patience. The most critical aspect of the external cavity laser is achieving feedback mode, such that the light reflected off the grating is coupled back into the diode gain chip. This enables the narrowing of the linewidth and the tunability of the laser. The procedure for this is discussed in detail below.
Optomechanics
Thorlabs LDM21 Laser Diode Mount
Thorlabs KM05S Grating Mount
≥5/8'' 10-32 Bolt
Electronics
Accessories
ESD Protection Bracelet (Strongly Recommended)
Thorlabs TO-Can Opener (Recommended)
Optical Power Meter
Unscrew the faceplate of the LDM21 diode mount and the diode mounting bracket, according to page 4 of the LDM21 manual. The pinout for the Sharp diode and corresponding socket pinout are shown below.
It is optimal to wear an ESD bracelet while handling a laser diode to prevent burning it out.
Decapping: It can be helpful to 'decap' the laser diode using a can-opener. This means removing the front facet of the laser diode, which has a partially reflective mirror in it. This removes the internal cavity and turns the diode into a simple LED rather than a laser. 'Lasing' will only be re-established when you align the grating correctly, i.e. are in feedback mode, and this is a great way to ensure you have achieved feedback. However, this is optional.
Mounting and Collimation: Pick up the diode carefully by its collar, taking care not to touch the front optical surface. Place it in the LDM21 mount and replace the clamping bracket and the front plate.
Turn the current limit of your diode driver all the way down, and then plug in your diode driver and temperature controller to the LDM21. Put on laser safety glasses. Slowly increase the current limit until the diode turns on; this is the current threshold.
Now, insert the C230TMB-D into the S1TM09 mount and tighten it all the way down. Insert the S1TM09 into the internal SM1 threads on the LDM21 mount. You should mount the LDM21 on a table and point it at an object far away. Rotate the S1TM09 until the laser beam is collimated; you can find a guide on the details in this video. The beam width should be constant over a range of several centimeters.
Tighten all the adjustment bolts on the KM05S mount. Unpackage the piezo and pry the two parts of the KM05S apart slightly. Insert the piezo in between the plates under the horizontal adjustment side; this side will have a flat inset (the other side has a v-groove).
Put on gloves now. If you get oil from your hands on the diffraction grating, it is no longer usable. Unpackage the grating and slide it into the KM05S mount, and tighten the set-screw. Place the 10mm spacer into the slot of the CPMA3 bracket, and the KM05S on top of the spacer at roughly a 45° angle. Then, slide the 10-32 bolt in from beneath and screw it into the KM05S mount until very tight. The KM05S should not slide around within the bracket.
Screw two ER cage rods into the LDM21 holes on the bottom. The bottom of the LDM21 is defined by the fast axis of the diode (the wider dimension of the beam); the fast axis should be horizontal (i.e. parallel to your table). Slide the CP30 bracket and CPMA3 bracket on together as shown in the CAD.
With an alignment tool placed just in front of the collimator, you should be able to see light reflected back onto the alignment tool. This is the back-reflected light from the grating. Using the two knobs on the KM05S, align the back-reflected light into the collimating lens. If your diode is decapped, you will see lasing once feedback is established. At this point, you have aligned your ECDL.
