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Core Optical Assembly#

Basic Raman optical assembly overview

Build the basic optical assembly for the DIYraman setup in a backscattering configuration. This module provides the minimum optical path you need to record your first Raman spectra.

The design shown here is built around a surplus B&W Tek spectrometer module, which is commonly available on the used market for around 200€. Other spectrometers with a similar input geometry / SMA905-connector can also be adapted.

For cost reasons, this setup uses a low-cost 532 nm green laser pointer together with a dichroic mirror (cut-on 550 nm) and a longpass filter (cut-on 550 nm). You can find more information in the respective Parts and Materials sections below or in the BOM. Because the laser operates at 532 nm and the filters begin transmitting at 550 nm, the usable Raman signal will be in the Stokes region above roughly 600 cm⁻¹.

Full vs. Basic Optical Assembly

The Full-Build incorporates the DIY-Linear-Stage along with two stepper motors to motorise the kinematic laser mount for convenience.
The basic setup on this page is intentionally simpler and sufficient to capture your first Raman signals.

Sub-Assemblies#

Sample assembly overview Laser assembly overview
Sample Assembly

Holds the cuvette and microscope objective on two guide rods.		 Laser Assembly

Mounts the excitation laser in a Thorlabs KM100 kinematic mirror mount.
Focusing assembly overview Dichroic assembly overview
Focusing Assembly

Filters and focuses the collected signal light towards the fibre / spectrometer input.		 Dichroic Assembly

Holds the dichroic mirror that separates laser light from Raman signal.

Sample Assembly#

Exploded view of the sample assembly

The sample assembly positions the cuvette in front of the microscope objective on a pair of guide rods. Before printing and assembling the parts, make sure that your sourced components (cuvette, rods, objective) match the dimensions of the printed holders.

Parts and Materials#

Visit the complete Bill of Materials

Everything you will need, both printed parts and sourced parts, along with their cost and supplier, can be viewed in the dedicated BOM!

Sourced parts#

Qty Name Model / Spec. Notes
1x Quartz Cuvette Any Matching the dimensions of Kuvettecase_Bottom and Kuvettecase_Top.
1x Microscope objective Any, Leitz PL Fluotar 20x/0.45 D EPI, oo/0 Any, 20x, inf.- corrected, WD > Cuvette-Thickness (alternative to printed part: CP42/M)
2x Guide Rod ⌀ 6 mm, 100 mm Any, ER Assembly Rods for 30 mm Cage Systems i.e. ER4 (4" ~ 100 mm) Ideally for optical applications (tight tolerances)
2x M3 screw + nut

Printed parts#

Qty Part name (.stl) Print preview
1x Defaultholder Print orientation of Defaultholder
2x Rodholder Print orientation of Rodholder
1x Sample-Stage Print orientation of Sample-Stage
1x Kuvettecase_Top Print orientation of Kuvettecase_Top
1x Kuvettecase_Bottom Print orientation of Kuvettecase_Bottom
1x Objective-Mount_Insert Print orientation of Objective-Mount_Insert

Before you print

Make sure your sourced parts are compatible with the printed parts:

  • Are the rods the correct diameter for the rod holders?
  • Does your cuvette fit into the Kuvettecase_Bottom and Kuvettecase_Top?
  • Does your microscope objective fit into the Objective-Mount_Insert?

Quick compatibility checklist

  • Cuvette → Kuvettecase_Bottom / Kuvettecase_Top → Sample-Stage
  • Microscope objective → Objective-Mount_Insert → Defaultholder
  • Rods → Rodholder → Sample-Stage → Objective-Mount_Insert → Defaultholder

Once you have confirmed that your parts match, you can print the components and validate their fit using the steps below.

Step 1 – Check rod fit#

Affected parts

  • Defaultholder
  • Rodholder (2×)
  • Sample-Stage
  • Objective-Mount_Insert

To ensure the printed parts slide smoothly but without play on the rods, test-fit a rod through each opening in the parts listed above.

Checking rod fit in the sample assembly

  • The rod should pass through with a noticeable but reasonable amount of force. A snug fit minimises wobble and keeps the optical axis stable.
  • If the rod does not fit at all, lightly sand the edges of the holes and try again.
  • If sanding is not enough, slightly increase the hole compensation (or scale the holes) in your slicer settings and re-print the affected parts.

When sanding printed parts

Fine plastic dust can be harmful if inhaled. Work in a well-ventilated area, wear a dust mask, and wipe the parts with a damp cloth after sanding to remove any remaining dust.

Step 2 – Install the microscope objective#

  1. Slide the Objective-Mount_Insert into the Defaultholder and confirm a smooth, wobble-free fit.
  2. Insert your microscope objective into the Objective-Mount_Insert.
  3. Insert two M3 screws and two nuts into the clamp mechanism on the side of the mount, as shown in the illustration.

Installing the microscope objective in the holder

  1. Make sure the objective sits straight and level in the mount.
  2. Gently tighten both M3 screws until the objective is held firmly.
  3. Check from the side that the objective remains aligned parallel to the rods (its optical axis should be parallel to the rod direction).

Checking the axial alignment of the microscope objective

Do not overtighten

Tightening the screws too much can deform the printed part or the objective barrel. Tighten only until the objective no longer moves if you tap it lightly.

Step 3 – Check the cuvette holder#

  1. Place your cuvette into the Kuvettecase_Bottom. Confirm that the contents of the cuvette are visible through the circular opening in the bottom part.

Checking cuvette fit in the bottom holder

  1. Slide the Kuvettecase_Top over the Kuvettecase_Bottom with the cuvette inside. The top should sit tightly and close with some friction.
  2. If the Kuvettecase_Top is too tight, lightly sand the inside surfaces. Afterwards, wipe off dust with a damp cloth.
  3. If it still does not fit, slightly scale the Kuvettecase_Top in your slicer (for example, to 101 % in X/Y/Z) and re-print.
  4. The cover is optional but recommended to reduce spills and minimise stray light entering from above.

Step 4 – Assemble the sample sub-assembly#

  1. Place both Rodholder parts upright on your workspace and push both rods through the first Rodholder.
  2. Slide the Defaultholder (with mounted objective and insert) and then the Sample-Stage onto both rods, as shown in the graphic.

Assembling the sample sub-assembly

  1. Push the second Rodholder onto the rods to lock the parts in place.
  2. Place the Kuvettecase_Bottom with the cuvette on the Sample-Stage so that the opening faces the microscope objective. It should sit loosely but stably on the stage.

Sample sub-assembly – assembled drawing

Laser Assembly#

Exploded view of the laser assembly

The laser assembly holds a cylindrical laser module in a Thorlabs KM100 kinematic mount using a printed adapter and heat-set threaded inserts.

Parts and Materials#

Visit the complete Bill of Materials

Everything you will need, both printed parts and sourced parts, along with their cost and supplier, can be viewed in the dedicated BOM!

Sourced parts#

Qty Name Model / Spec. Notes
1x Laser 532nm Any, 532 nm DPSS, 30mW - 70mW, (⌀ 18 mm) Cylindrical laser module, 18 mm body.
1x Kinematic Mount Thorlabs KM100 Kinematic mirror mount.
2-3x M4 heat-set insert
1x M3 heat-set insert

Printed parts#

Qty Part name (.stl) Print preview
1x Laser_KM100-Insert Print orientation of Laser_KM100-Insert
1x Laser_KM100-Mount Print orientation of Laser_KM100-Mount

Before you print

Check that the laser body diameter matches the Laser_KM100-Insert.

Step 1 – Prepare the printed KM100 mount (heat-set inserts)#

  1. Insert an M4 brass heat-set insert (slim side first) into the centre opening on the bottom of the Laser_KM100-Mount.
  2. Using a hot soldering iron with a flat tip, gently press on the insert until the surrounding plastic softens and the insert sinks into the part.
  3. Stop when the insert is slightly below the plastic surface. The KM100 will sit on this face, so it should remain flat.

Melting heat-set inserts into the KM100 base mount

  1. Repeat the process for the side holes in the mount where additional M4 inserts are required (see illustration).
  2. Allow the part to cool completely before mounting any hardware.

Tools for heat-set inserts

A temperature-controlled soldering iron with a flat tip works well. Dedicated heat-set insert tips are inexpensive and make this step easier.

Step 2 – Prepare the laser insert (Laser_KM100-Insert)#

  1. Heat-set the M3 insert into the small side hole of the Laser_KM100-Insert.
  2. Make sure the insert does not protrude into the inner bore where the laser will sit. A slight protrusion on the outside is fine.

Melting the M3 insert into the laser adapter

Step 3 – Mount the KM100 in the printed base#

  1. Place the KM100 on top of the Laser_KM100-Mount so that the central hole in the KM100 aligns with the centre M4 insert.
  2. The rotation of the KM100 is not critical at this point; the printed mount supports multiple orientations.
  3. Use one of the supplied M4 screws to fasten the KM100 to the mount. Tighten only until the mount no longer moves freely.
  4. Use a second screw in one of the side slots if desired to further stabilise the mount.

Mounting the KM100 on the printed base

Step 4 – Insert the printed laser adapter#

  1. Check that the small nylon set screw on the KM100 (perpendicular to the front opening) is loosened so that it does not block the aperture.
  2. From the front of the KM100, push the Laser_KM100-Insert into the circular opening. It should seat firmly but without excessive force.
  3. Apply light pressure with both thumbs until the front face of the printed insert is flush with the inner reference ring of the KM100.
  4. Lightly tighten the nylon set screw on the KM100 to hold the printed insert in place.

Step 5 – Insert the laser module#

  1. From the back of the mount, slide the laser module into the Laser_KM100-Insert that is now fixed inside the KM100.

Installing the laser module into the adapter

  • The exact insertion depth is not critical. Ideally, the laser front face is close to flush with the front of the Laser_KM100-Insert.
  • Insert an M3 screw into the M3 heat-set insert in the side of the Laser_KM100-Insert and gently tighten it until the laser is held securely.

Laser assembly – assembled drawing

Avoid crushing the laser body

Do not overtighten the M3 clamping screw. Tighten only until the laser can no longer rotate or slide when you try to move it by hand.

Focusing Assembly#

Exploded view of the focusing assembly

The focusing assembly combines the longpass filter, focusing lens and (optionally) an SMA905 fibre adapter on a pair of rods. This module filters and focuses the Raman signal into the spectrometer input.

Parts and Materials#

Visit the complete Bill of Materials

Everything you will need, both printed parts and sourced parts, along with their cost and supplier, can be viewed in the dedicated BOM!

Sourced parts#

Qty Name Model / Spec. Notes
1x Longpass filter (550 nm) Thorlabs FELH0550
1x Focusing lens Thorlabs AC127-019-A - f = 19 mm, Ø1/2" Achromatic Doublet (mounted in CP32/M)
2x Guide Rod ⌀ 6 mm, 100 mm Any, ER Assembly Rods for 30 mm Cage Systems i.e. ER4 (4" ~ 100 mm) Ideally for optical applications (tight tolerances)
6x M3 heat-set insert
6x M3 screws

Printed parts#

Qty Part name (.stl) Print preview
3x Defaultholder Print orientation of Defaultholder
2x Rodholder Print orientation of Rodholder
1x SMA905_Insert Print orientation of SMA905_Insert

Before you print

Confirm that the longpass filter and focusing lens diameters match the openings in the printed Defaultholder parts.

Step 1 – Check rod fit (again)#

The rod fit procedure is identical to Step 1 – Check rod fit in the sample assembly:

  • Test-fit the rods through all Rodholder parts and through the bores in each Defaultholder that will sit on the rods.
  • Adjust with light sanding or slicer hole compensation if necessary.

Step 2 – Prepare the holders (heat-set inserts)#

  1. Identify the clamping points in the Defaultholder parts where screws will press on the longpass filter, focusing lens and SMA905 insert.
  2. Heat-set M3 inserts in all corresponding holes so that the inserts sit slightly below the plastic surface.
  3. Let the parts cool fully before inserting any optics.

Handle optics only after plastic has cooled

Hot plastic can warp or damage optical coatings. Always complete all heating/soldering steps before inserting filters or lenses.

Step 3 – Assemble the focusing module#

  1. Insert both rods into the first Rodholder.
  2. Slide the three Defaultholder blocks onto the rods in the following order (from the sample side towards the spectrometer side):
  3. Defaultholder with the SMA905_Insert and connected SMA905 fibre (if used).
  4. Defaultholder holding the focusing lens.
  5. Defaultholder holding the longpass filter.
  6. Push the rods through the second Rodholder to lock the sequence of holders in place.

Focusing assembly – assembled drawing

Orientation of optics

Make sure you keep track of which side of the longpass filter and focusing lens should face the incoming light, according to their datasheets. If in doubt, mark the correct side with a small sticker on the edge before mounting.

Dichroic Assembly#

Exploded view of the dichroic assembly

The dichroic assembly mounts a Thorlabs dichroic mirror in a KM100 kinematic mount, held by a printed base that will later sit on the main baseplate. The dichroic separates the excitation laser from the Raman signal.

Parts and Materials#

Visit the complete Bill of Materials

Everything you will need, both printed parts and sourced parts, along with their cost and supplier, can be viewed in the dedicated BOM!

Sourced parts#

Qty Name Model / Spec. Notes
1x Kinematic Mount Thorlabs KM100 Kinematic mirror mount.
1x Dichroic mirror (550 nm) Thorlabs DMLP550
1x M4 heat-set insert

Printed parts#

Qty Part name (.stl) Print preview
1x Dichroic_KM100-Mount Print orientation of Dichroic_KM100-Mount

Step 1 – Prepare the printed KM100 mount#

  1. Insert the M4 heat-set insert into the central hole of the Dichroic_KM100-Mount.
  2. Using a hot soldering iron, press the insert straight into the opening until it sits slightly below the plastic surface.
  3. Position the KM100 on the Dichroic_KM100-Mount so that the bottom screw hole aligns with the M4 insert.
  4. Use one of the provided M4 screws to fasten the KM100 to the mount until it no longer moves freely, but can still be rotated with gentle force.

Rotation during alignment

Later, on the baseplate, the KM100 will be rotated to 45° with respect to the incoming laser beam. You will slightly loosen this screw again to set that angle.

Step 2 – Install the dichroic mirror#

Handle optics carefully

Work in a clean area and wear powder-free nitrile gloves. Avoid touching the coated surfaces and never wipe them with dry cloths.

Spectral performance of the Thorlabs DMLP550 dichroic mirror at 45° AOI
Source: Thorlabs – “Longpass Dichroic Mirrors/Beamsplitters”

  1. Review the Thorlabs handling guide for high-performance optics:
    Thorlabs – Optics Handling and Care.
  2. The dichroic mirror is directional: one side carries the dichroic coating (reflecting the laser, transmitting the Raman signal), the other side carries an anti-reflection (AR) coating.

Orientation of the dichroic coating on the DMLP550
Source: Thorlabs – “Longpass Dichroic Mirrors/Beamsplitters”

  1. On the Thorlabs DMLP550, the dichroic side is indicated by a small engraved arrow pointing towards the coated surface. This arrow is etched on the outer edge and can be hard to see in dim light.
  2. With clean powder-free nitrile gloves, carefully hold the dichroic by its outer edge and locate the engraved arrow.
  3. Insert the dichroic mirror into the KM100 so that:
  4. The dichroic (arrow-marked) side faces the incoming laser beam (front side).
  5. The AR-coated side faces towards the KM100 adjustment knobs (back side).
  6. Gently tighten the nylon set screw on the KM100 until the dichroic sits evenly and flush in the mount.

Dichroic assembly – assembled drawing

Baseplate Mounting#

Basic assembly mounted on the baseplate

In this final step you mount all sub-assemblies onto the baseplate to form the complete basic Raman optical assembly.

Parts and Materials#

Visit the complete Bill of Materials

Everything you will need, both printed parts and sourced parts, along with their cost and supplier, can be viewed in the dedicated BOM!

Sourced parts#

Qty Name Notes
1x Sample Assembly
1x Laser Assembly
1x Focusing Assembly
1x Dichroic Assembly
14x M4 screw, 16 mm For fastening sub-assemblies to the baseplate.
6x M4 nut For additional clamping where required.

Printed parts#

Qty Part name (.stl) Print preview Notes
1x Baseplate_Small Print orientation of Baseplate_Small 250 × 140 × 7 mm

Step 1 – Mount the sub-assemblies onto the baseplate#

Assembly steps for mounting all sub-assemblies

  1. Mount the Sample Assembly using four M4 screws from the bottom of the baseplate.
  2. Mount the Laser Assembly using four M4 screws from the bottom and four M4 nuts from above to tighten.
  3. Mount the Focusing Assembly using four M4 screws from the bottom.
  4. Mount the Dichroic Assembly using two (or four) M4 screws from the bottom and the corresponding M4 nuts from above.

Fine-tune the dichroic assembly

The dichroic assembly as a whole is designed to move in X/Y direction to allow for positional adjustments. You may want to re-position the assembly within the bounds of its mount later on, so do not overtighten screws and nuts at this stage.

Sub-assemblies mounted on the baseplate – line art

Continue reading#