A modular optical honeycomb breadboard realized with 3D-printable building bricks and industrial aluminum extrusions

Graphical abstract


a b s t r a c t
Optical breadboards with honeycomb structure provide a solid surface with mounting hole grids for building optical assemblies, sub-systems and experiments in the fields of quantum-optics and photonics. Performance criteria are the ability to resist bending under load (stiffness) and the ability to dissipate induced vibrations to the board (damping). The hardware presented in this paper deals with the possibility of assembling optical breadboards using 3D-printed building bricks with honeycomb structure, so-called 'breadboard bricks', and industrial aluminum extrusions, so-called 'breadboard profiles'. With this do-it-yourself approach, it is possible to make changes to the breadboard, such as making an opening, changing its shape or increasing the mounting surface whenever needed. Furthermore, the breadboard is automatically compatible with industrially relevant mechanical design platforms. Aluminum extrusions and the PLA thermoplastic filament provide mechanical stiffness and damping, respectively. Further characteristics are low costs and a modular design. All this makes it especially suited for agile prototyping of (laser) optical assemblies in many engineering processes.

Hardware in context
Optical breadboards are the backbone for high-precision laser-and optics-related experiments. They permit the secure attachment of optical fixtures and related devices to their top surface. Optical breadboards feature a large stiffness to resist bending under load and the ability to dissipate induced horizontal and vertical vibrations on the board's surface. Thus, the alignment of optical elements remains stable over time.
printed 'breadboard brick' modules with honeycomb structure of PLA thermoplastic filament are combined with industrial aluminum extrusions. This provides damping by the mechanical properties of PLA, while the mechanical stiffness is due to the combination of structures vertical (honeycomb) and horizontal (grooves) to the breadboard surface. In addition, a nut and groove system is used for mounting optical elements following the idea of optical benches. This makes the board suitable for the assembly of optical setups in inline geometry, i.e. setups with optics aligned along a single axis -but is not restricted to this case.
As a result, our optical breadboard is suited for high-precision optical experiments, such as interferometric setups that require stability in the range of fractions of the wavelength over the duration of seconds up to minutes. Due to its lowcost, low-weight, scalability, reusability, ease of integration in industrial environment and professional features, the optical breadboard is of interest for small-to-medium enterprises (SMEs), start-ups, industry and cutting-edge research, but also for private individuals who have little space available or who do not have fixed installation locations.
As original approach, unit cells of the same size of 12 x 12 cm 2 are used to assemble the optical breadboard. Compared to state-of-the-art optical breadboards, benches and tables, the hardware presented here is characterized by its modular and scalable nature, which not only allows the low-cost realization of individual sizes (m x n; m,n 2 N) and geometries (Lshaped, U-shaped, O-shaped,. . .), but also the possibility to make easy customizations 'on the fly' in context of mounting and/or adjustment of optical experiments. The design of the 'BBBs' addresses three essential aspects that are required for a successful implementation of a 3D-printable optical breadboard: (I) A mechanical stable internal structure which secures stiffness and counteracts the potential deformability of the PLA thermoplastic filament. (II) A reliable connection between the 'breadboard bricks' and the 'BB profiles' which is easily applicable and provides a stable and rigid surface. (III) Mounting hole rails allowing for a pre-aligned assembly of all kinds of mechanical and opto-mechanical components.
In the context of 3D-printed structures, the attachment of additional components (e.g. mounts for optics) to the printed structure by utilizing screws is of major importance. Typically, threaded holes are realized by the use of nuts, which are inserted into extra pockets in the base structure, or by the insertion of threaded metal sleeves (melt/bond). The alternative approach pursued in this article can be understood as a further development of the first solution. By combining special T-nuts ('item Industrietechnik GmbH') together with appropriate grooves in the 3D-printed structure, a variable pre- aligned optics mounting system can be achieved. Remarkably, the grooves are designed such that different thread diameters (M3 or M4) can be used. The connection technology between the 3D-printed 'BBBs' and the structure-stabilizing 'BB profiles' is of fundamental importance. The connection must feature a high mechanical stability, an ease of accessibility as well as a quick & resealable operation. These requirements are met by using a cam lock connection. This type of connector is able to convert an applied torque into a translation rotated by less then 180°which allows for the connection between the 'BBBs' and aluminum construction profiles to be parallel to the plane of the optical breadboard. It is accessible from the front side. In addition, the maximum force is achieved within half a rotation, which also fulfills the aspect of quick operation.
The use of PLA thermoplastic filament in combination with aluminum offers further advantages: Due to the low kilogram price of PLA (Prusament PLA: 24.99 €/kg) and aluminum extrusions, the presented hardware is considerably cheaper than conventional systems. It also results in a significant weight reduction. Far more important is, that PLA has ideal damping properties due to its elasticity module. Furthermore, we would like to add, that the ubiquitous availability of the aluminum extrusions as engineering and construction tool allows for a direct connection of the optical breadboard with other industrial constructions.
This hardware allows the user to build optical breadboards of different size and shapes with an emphasis on convertability, remountability and scalability. This hardware is compatible with established construction and state-of-the-art building kit systems. This hardware opens optical breadboards to the broad public, small-and-medium-sized enterprises (SMEs) and start-ups. This hardware allows for an easy and fast adjustment of optical components 'on the fly' by combining the individual advantages of 1D optical benches and 2D optical breadboards. This hardware is much cheaper than comparable established solutions due to its reusability and the use of 3D-printing technology in combination with industrial aluminum extrusions.

Design files
The design file required for 3D-printing the 'breadboard brick' is directly available with the article in form of a technical drawing (see Fig. 1) and uploaded as a '.stl' file to a corresponding repository.
'breadboard brick': Detail drawing used for 3D-printing of the 'breadboard brick' module. Here, the given prices are summed-up totals from the bill of materials. As shown in Fig. 2

Build instructions
This chapter provides and focuses on general and detailed step-by-step instructions for the construction of a 3 x 3 optical breadboard suitable for small scale optical experiments. In an additionally provided section, design decisions and possible alternatives are discussed. A supplemental video showing the building concept and the assembly of the 3 x 3 optical breadboard is available through the following URL: https://nbn-resolving.org/urn:nbn:de:gbv:700-202012143905.

General tips
In this short section general tips on the building and 3D-printing aspects are provided. The main focus is on 3D-printing and the required accuracy of fit. In addition, aspects of planning are addressed in advance.
As a first step the desired size (m x n) of the optical bench should be estimated to calculate the required amount of 'breadboard bricks'. Bear in mind that depending on the printer settings, the printing of one 'BBB' may take up to 15 hours. To improve the 3D-printing speed a lower z-resolution (= larger layer thickness) might be useful depending on the desired quality. We note that a lower z-resolution may lead to a distortion of overhanging structures. The 'breadboard bricks' used in this work were printed with a 200 lm resolution.
It is advised to print the 'breadboard brick' in a vertical position to eliminate the necessity for additional support structures. To prevent the 'breadboard brick' from falling over while printing the application of an intermediate bonding agent, such as glue stick is adviced. (This tip only holds true for a vertical printing position.) As tool for building the breadboard we suggest a 'Philips' screwdriver size #4.

Preparing the 'breadboard brick' and the 'breadboard profile'
This section gives a guideline for the assembly of the 'breadboard brick' with the 'breadboard profile'. It describes the main rules to get a stable connection which is the prerequisite for the build-up of an optical breadboard (shown in the subsection thereafter). Fig. 3 schematically sketches the connection concept.

Assembling a 3 x 3 optical breadboard
In this section the assembly of an optical breadboard is demonstrated. Exemplarily an optical breadboard size of 3 x 3 was chosen since it covers all necessary steps for any type of shape and size. The build-up of other sizes and/or shapes can be performed in a similar way.

Design decisions and possible alternatives
The choice of industrial aluminum extrusions was deliberate in the realization of the optical breadboard. Reasoning behind this decision is the resulting stiffness which was carefully considered against the limited reusability as well as the low material price of aluminum, itself. It should be emphasized at this point that 'item Industrietechnik GmbH' is only one possible manufacturer of aluminum extrusions. Alternatively, 'Bosch Rexroth AG' and other manufacturers can be used, also. In this case, the feather of the 'BBB' may then have to be slightly adjusted depending on the profiles' shape. Although we cannot recommend this procedure, it is possible to use only single (12 cm long) 'BB profile' with threaded holes at both ends. Theoretically, these 'BB profiles' can then be connected to each other with a set screw, but practically the misalignment that occurs through twisting turns out to be a major issue.
An extraordinary aspect that has not yet been fully addressed is the incorporation of the existing product catalog from, e.g. 'item Industrietechnik GmbH' or 'Bosch Rexroth AG'. While this work primarily focuses on aluminum extrusions in the context of an optical breadboard, it is also possible to use all further available design and construction tools of the respective  companies. For example, necessary components to create a subframe for incorporating the optical breadboard in an industrial environment are directly available at the online store of 'item Industrietechnik GmbH'. There are no limits to the potential mechanical constructions.
To create a rigid foundation with a minimum of flexibility a honeycomb structure was directly embedded into the design of the 'breadboard brick'. An approximate density of 30% was chosen, which serves as a well trade-off between the stability and the amount of material required. For the specific dimensions a lower landwidth limit of 1.5 mm and at least 3.0 mm for the outer rims was chosen. By aiming for a periodical structure along the long axis we ended up with the specific dimensions shown in Fig. 1. Material wise there are two alternatives worth mentioning, PETG and ABS. The primary differences, apart from the printing properties, are a different elasticity and temperature resistance. Depending on the final application it might be advisable to choose the filament which suits the individual requirements best. In this work we chose PLA primarily because of its ease to print in a prototyping context and its outstanding damping features.
The reasoning behind the special T-nuts can be boiled down to two aspects. First, these nuts have a spring-loaded steel ball, which fixes them sufficiently in the groove but at the same time makes them easy to move and handle. Secondly, there are different threaded options (M3/M4/M5) for the same physical size of T-nut, which offers a great versatility for mounting components onto the 'breadboard brick'. A budget alternative is to use ordinary nuts but have in mind that this may require a modification of the grooves of the 'breadboard brick'. In addition, this type of fastening offers a long lifespan compared to threads cut directly into the 'breadboard bricks'. This way, repeated repositioning and fixing of optics is not a concern and reflects the intended case of use.

Operation instructions
Since the scalable optical breadboard presented in this work primarily serves as a platform for optical setups/experiments the instructions for operation is foremost dependent on the corresponding setup. Nevertheless, a few general operation instructions can be summarized in this chapter to provide the user with an optimal experience. In addition, laser safety issues must be addressed in the context of optical experiments made available to the general public.

General operation instructions
It is considered good practice to plan the desired layout of the optical experiment beforehand to derive the required dimensions of the optical breadboard. While planning have in mind that it is generally faster to add a new row of 'breadboard bricks' compared to swapping out the 'BB profiles' for longer ones. For an easy alignment of the beam path it is advisable to direct the laser light along the inline geometry whenever possible. The modularity of the whole system allows the user to mount and pre-align related components, e.g. for building a telescope or a spatial frequency filter onto a subset of 'breadboard bricks' even before the final assembly. Avoid over-tightening of bolts since the 3D-printed components are prone to deform or break especially when utilizing low infill levels. A maximum of 1.0 Nm of torque is advisable when mounting optics in the rails of the 'breadboard bricks' (valid for PLA filament). The use of an enclosure for the optical breadboard is strongly recommended and can be easily realized by utilizing additional 'breadboard bricks' or sufficient sheet material available from 'item Industrietechnik GmbH'.

Safety precautions
If not smoothed, the sharp edges of the 'BB profiles' pose a risk of injury. The usual safety regulations for working with laser light and electronics apply. In case of any uncertainty it is strongly recommended to look up the related safety regulations elsewhere. Always handle laser light with care! Reflected and scattered light can easily damage or blind your eyes! It may also cause skin incineration and can light up flammable materials.

Validation and characterization
In this chapter different representative geometric shapes are build and presented, to give a better impression of the possible designs and applications. Furthermore, the validation of the hardware with respect to its stiffness, high precision and damping is performed by means of the fringe stability of a Michelson-Morley interferometer.  (2 x 2, 3 x 3, 4 x 4), an L, O, U or X shape can easily be realized by utilizing only a few 'BB profiles' of different lengths. The derived shapes are transferable to larger dimensions and can be freely combined with each other to create customized solutions. As simple example, a T shape is perfectly suited for coupling several laser sources into an experiment, whereby each source can be mounted in one arm of the T shape and switched between via a flip mirror. In one of its simplest forms, a 4 x 1 shape, its resemblance and functionality to the original 'Leybold GmbH' optical bench is uncanny.

Possible geometrical shapes
Key feature is the balance between the stiffness through incorporating continuous industrial aluminum extrusions and the agility achieved along one dimension through the chosen quick connecting mechanism. This agility results from the possibility to disassemble the entire system back into its components within minutes and to rebuild it into a new shape or, in the most simple case, to extend it.

Interferometer
We have build a Michelson-Morley interferometer on a 3 x 3 base to validate the stiffness and damping properties of our optical breadboard. Although the achieved stiffness is highly dependent on the used environment and the smallest vibrations may negatively impact the interference pattern, we chose a normal desk in one of our office rooms for the validation process to create a level playing field. Therefore, the shown results should be easily reproducible at home, e.g. in ones basement. A basic design was used for the layout, consisting of two protected silver mirrors ('Thorlabs GmbH', type: PF10-03-P01), a 50/50 beamsplitter plate ('Thorlabs GmbH', type: BSW10R), a plano-convex lens ('Thorlabs GmbH', type: LA1608-ML, f = 75 mm) and a 650 nm laser diode ('Picotronic GmbH', type: DB1650-1-3-FA-F3400, P 6 1 mW) as depicted in Fig. 5.
The optics are mounted onto the breadboard by mirror mounts from 'Liop-Tec GmbH', type: SR100-HS-100L-2-YE, and the standard 1" system from 'Thorlabs GmbH' consisting of a base plate (type: BA1), a 20 mm postholder (type: PH20) and a 20 mm post (type: TR20) were used. The final assembly and the accomplished interference pattern are shown in Fig. 6. The depicted concentric fringes can be interpreted as two curved wavefronts interfering with a small deviation in the relative path lengths of each arms. Noteworthy is the high contrast achieved, indicating great stability of the fringe pattern and sufficient precision of adjustment in the order of % 0:2lm. Thereby, both, a good mechanical stiffness and good damping properties are validated.  Fig. 7 further provides the relative intensity variation of the interference pattern's center measured through a 1.5 mm pinhole as a function of time over a period of five hours. For this measurement, we highlight, that the breadboard was placed onto a standard desk in one of our offices, i.e. within an environmental condition of ambient temperature, ambient air pressure and in presence of common building vibrations (2nd floor). Furthermore, a standard, not-temperature-stabilized, battery-driven laser system at a wavelength of k ¼ 650 nm was used. Data acquisition with a commercial laser power meter ('Coherent Inc.', type: LabMax-TOP + OP-2 VIS) started immediately after final adjustment of the interferometer. We assume that these conditions best correspond to the anticipated field of application of the presented breadboard: agile prototyping in the field of optics & photonics. As a result, already after a few minutes a quasi-stable state was reached (Fig. 7, left), that is characterized by a phase shift over time of 2.2 mrad/s, corresponding to k/50 per minute. In this time range, the amplitude is determined with an RMS noise amplitude of about 5%, that is in the limit of the detector's noise of the power meter. On the long term, the phase stability has further improved (Fig. 7, right) validating the high quality of this breadboard.