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Module ‐ ECDL with Isolation Baseplate

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UMass Ion Trappers edited this page Feb 7, 2025 · 19 revisions

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Extended Cavity Diode Laser

Extended Cavity Diode Lasers (ECDL) use feedback from an external diffraction grating to form an external cavity, narrowing and controlling the laser wavelength by adjusting the relative diffraction angle back to the diode.

laser safety warning

Warning : Lasers can be dangerous if you do not wear proper eye protection or have no laser safety training. Building a laser is especially dangerous given the temptation to 'look' at the diode as you're testing it. Do not attempt to build a laser unless you have completed a laser safety course and have the proper eye protection. (https://en.wikipedia.org/wiki/Laser_safety)

ECDL components:

  1. Laser diode (Nichia, TopGaN etc.)
  2. Collimation lens assembly (C610TMG-A lens f = 4mm, S05TM09 adapter, SM05L05 1/2inch lens tube, LMR05)
  3. Diffraction grating (GH13-36U) with parametrized Littrow angle
  4. PZT (PA4HKW) for tuning the length of the external cavity (and angle) controlling the wavelength
  5. Mirror (ME05S-P01) for parallel output
  6. TEC for temperature control (TECH4 with Thorlabs TED200C temperature controller)
  7. 3D printed enclosure (see optomech.laser_box for detailed design)
  8. Brewster window to create nearly 'air-tight' enclosure (BW2002 window and BW20M window mount)
  9. Laser safety glasses (https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=762)

Tools needed:

  1. Spanner wrenches to adjust the collimation of the laser.
  2. 5/64 ball driver to adjust the feedback of the laser.

Bolts Needed:

Function Bolt Size
Connecting grating mount to KM100PM 3/32"
KM100PM to the Upper Al Plate 8/32"
Lower Al plate to Optical Table 1/4"

The Thorlabs KM100PM is used to align the diffraction grating feedback, providing coarse wavelength tuning. The custom and dynamic 3D printed cover box is needed for stable operation.

Optical Isolator baseplate (with part number):

  • Input Mirrors: A pair of mirrors used for alignment of the beam. (BB05-E02)
  • Cylindrical Lenses: A pair of cylindrical lenses for proper collimation of the beam. (LJ1821L1-A and LK1085L1-A)
  • Optical Isolator: An isolator placed in the output path for optical isolation, preventing back-reflections into the ECDL. (IO-3D-405-PBS)

This design ensures that the laser beam is properly aligned, collimated, and isolated to maintain laser stability.

Implementation

The grating mount angle can be customized for any wavelength depending on the diffraction grating. We use 420nm laser diodes and GH13-36U diffraction gratings (3600 lines / mm) for feedback.

# Littrow angle calculation
wavelength = 422e-6   #wavelength in mm (not meters!)
grating_pitch_d = 1/3600   # Lines per mm (part = GH13-36U)
littrow_angle = np.arcsin(wavelength/(2*grating_pitch_d))*180/np.pi

The stl and step files for this grating mount are here:

Laser enclosure

We also enclose the laser within a 3D printed box for isolation from the environment, reducing thermal and acoustic noise. The box is designed to accept a Brewster window mount (BW20M) directly to the front to make it nearly air-tight.

ECDL with a cover box real ECDL with a cover box

ECDL isolator baseplate

ECDL isolator plate real ECDL isolator plate

Stability improvement with 'air-tight' cover using Brewster window:

ECDL air-tight stability

Detailed Step-by-step Instructions to build the laser:

1. To allow temperature feedback for the laser we'll use two aluminum plates, both ¼ inches thick. The lower plate will thermally sink to the optical table and is 64mm by 64mm with no holes. The upper plate holds the laser is 43mm by 43mm with 8-32 threading. A TEC (ThorLab parts number-TECH4) will be sandwiched between them, attached with thermally conductive epoxy.

Figure 1: a) Upper plate, lower plate and TEC. b) Plates and TEC stacked together.

  1. We designed a grating mount and 3D printed it. You can create your own grating mount from PyOpticL. You’ll need to input the Littrow angle into your code, then run and check the design on FreeCAD and print it. On the grating mount, we will attach three components – PZT, Grating, and parallel mirror. In our case, we are using PA4HKW as PZT, GH13-24V as grating for visible light, ME05S-P01 as the mirror. All the components are glued to the mount using optical adhesive.

Figure 2: a) 3D printed grating mount. (b) grating, mirror and PZT glued to the mount.

You must be careful maintaining the orientation of the grating and not to add any glue to the sides of the PZT.

Check Thorlabs instructions if you need.

Grating Tutorial: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=9026

PZT Tutorial: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=5030

  1. We use Km100PM as our main mount. From the KM100PM take the stage off. To check the thermal stability, we need to epoxy a thermistor (NTC 10K OHM) to the mount using a thermally conductive adhesive (8329TFF).

Figure 3: (a) KM100PM mount with stage. (b) KM100PM without the stage. (c) The mount with thermistor glued.

  1. Next, we combine our mount for housing the lens that will help to collimate the laser diode. We are using lens mount - SMR05, Lens tube – SM05L05, Lens adapter - S05TM09, Lens - C610TMD-B. The figure below shows how to combine them.

Figure 4: (a) Parts for lens. (b) Combination of SM05L05, S05TM09, C610TMD-B. (c) Combined with the lens mount SMR05.

  1. In this part, we will mount the laser diode. We use laser diode adapter S05LM56 for housing the laser diode. Then, an ESD protection and stress relief cable will be connected with the laser diode. In our case, we are using a laser diode with type "A" so we'll use a SR9A-DB9 cable from Thorlabs. You should be very careful handling the laser diode because it is ESD sensitive.

  2. Now, we need to combine all the bits together. First, attach the KM100PM mount on the Al plate stack (Figure 6). Then attach the grating mount (Figure 7).

Figure 6: KM100Pm mounted on the Al stack

Figure 7: (a) Side view with grating mount. (b) Top view with grating mount attached.

Before attaching the lens and laser diode mount to the main mount, we have to make sure the beam is collimated. The lens mount is screwed with the vertical leg of KM100PM. Below is the figure with lens mount.

Figure 8: (a) Side view with the lens mount. (b) Top view with the lens mount.

Electrical Connections:

  1. You have to solder the PZT, TEC and Thermistor. The PZT will be connected to a PZT controller MDT693B.
  2. The Thermistor and the TEC will be connected to Koheron temperature controller, CTL200-0. If you don’t have Koheron temperature controller, they can be connected to laser diode current controller LDC205C.

Figure 9: Koheron Temperature controller with a homemade cage.

  1. The SR9A-DB9 cable will be connected to the current controller - LDC205C.

Congratulations!! You just built a laser.

Please always wear proper eye safety goggles at all times when working with laser diodes. It is recommended you complete a laser safety course before attempting any work with a laser.

Additionally, please contact us if you have any questions, as we will update this tutorial based on your feedback.

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