Introduction to Special Issue on “Advances in 3OM: Opto-Mechatronics, Opto-Mechanics, and Optical Metrology”

1. Introduction

The 3OM concept was introduced in 2008 and combines three complementary domains: opto-mechatronics, opto-mechanics, and optical metrology.

Opto-Mechatronics was introduced in 2005 by Prof. Hyungsuck Cho in a seminal book with the same name [1]. It combines optics and mechatronics, the latter of which is a blend of precision mechanics, electronics, control, and automation, as well as IT. This multidisciplinary approach is beneficial in the engineering and design of complex systems, adding value to each of the component fields. A good example can be seen in the form of laser scanning techniques [2,3], which involve, for example, galvanometer-based [4,5,6], polygon mirror [7,8,9], or Risley prisms [10,11,12,13] and can be designed for a wide range of applications, from commercial to industrial and high-end purposes. These include applications in areas as diverse as biomedical imaging [14] and remote sensing [15].

Opto-Mechanics addresses positioning issues and errors in optical components [16,17] and tolerances of both mechanical and optical parts, as well as methods for tackling these aspects. Researchers in this field strive to fill the gap between the high (theoretical) precision and requirements of optical design and the practical capabilities that are inherent in mechanical technologies [2]. Another aspect that is often overlooked is related to the kinematics and dynamics of systems with optical moving parts (i.e., mirrors, lenses, and prisms). This imposes finite element analyses (FEAs), especially of fast-moving (for example, rotational) parts, in order to control structural integrity issues, as well as deformation levels [3,9]. FEAs and various other mechanical analyses are necessary, even for systems with non-moving or slow-moving (but large and heavy) optical parts (and mounts), for example, with refractive elements (e.g., Risley prisms [18]) or segmented mirrors [19]. These aspects bridge opto-mechanics and opto-mechatronics. This relationship is strengthened by the necessity for control and automation (as included in mechatronics) in (precise positioning and displacements in) systems with moving components [20,21,22].

Optical Metrology comprises a wide area of applications that are related to both domains above, especially opto-mechatronics. It includes domains that range from industrial measurements to imaging, with the latter being used both in industry, with non-destructive testing (NDT), and in the biomedical field, with a wide range of techniques, for example, optical coherence tomography (OCT) [23,24,25], confocal microscopy [26], or photoacoustics.

3OM also enhances laser manufacturing, which, in turn, implies robotics, machine vision, and machine learning, the latter with AI algorithms. Besides biomedical imaging, it is closely connected to various laser techniques in medicine. Finally, its field of interest extends to include remote sensing, as well as security and defense.

Regarding devices and specific technologies, we must highlight the rapidly evolving field of micro-electro-mechanical systems (MEMSs), which has applications in all the above fields [24,27,28,29,30].

2. An Overview of the Articles in This Special Issue

In the work described in Contribution 1, the reflectivity of a cholesteric liquid crystal was numerically investigated for an anisotropic defect layer inside. The optical phenomena were modeled for different parameters of the system, as well as adjusting the external applied electric field. The possibilities of controlling the reflection spectrum were obtained.

The authors of Contribution 2 studied a shock wave that may be induced with a high-power laser pulse. A theoretical model of laser material processing was developed, with experimental validation via piezo-resistive methods. The field of laser micromachining was also approached in Contribution 3 for laser drilling. Optical detection achieved a >95% accuracy for tens-of-micrometer-deep holes with a micrometer diameter. Contribution 4 focuses on optical vortices (i.e., ultra-intense laser pulses with helical phases), assessing their field distributions.

The authors of Contribution 5 approached an opto-mechatronic device, introducing and demonstrating a low-cost 3D-printed 1-DOF laser scanner. Its parameters were determined and validated experimentally, offering an alternative to the more expensive (and most common) galvanometer scanners.

Optical metrology was targeted for Contribution 6, with the Digital Image Correlation (DIC) method used for displacement- and stress–strain-invariant deformation measurements. A reverse retrieval strategy was also developed. The DIC measurements allowed for robust and efficient displacement invariance measurement, with an average accuracy of 0.1% observed for the stress–strain results.

A system development is proposed in Contribution 7, with a broadband mode coupler for multimode OCT in the O-band (1.26–1.36 μm). Key design parameters were studied. Reflected signals from the sample were separated using the same fiber device before interfering with the reference light, which had not been previously possible. The proposed fiber device is expected to represent a key component in efficiently achieving multimode OCT operation with better signal collection efficiency and improved penetration depth for deep tissue imaging.

Contribution 8 explores high-performance focal plane arrays and their fields of application, including remote sensing, astronomical, and surveillance instruments. Whereas in the analysis of an instrument performance analysis, it is assumed that the image of a point source is at the center of a detector pixel, in reality, it may fall at any position in the detector pixel. Pointing errors and jitter may lead to errors of up to 20%. Critical factors that impact the performance estimate include the optical centroid efficiency (OCE) and the ensquared energy (i.e., the energy on the rectangular detector pixel (EOD)). Simulations were performed for imaging with and without a generalized rectangular central obscuration. Contribution 10 represents the second part of this study, analyzing the performance of the OCE vs. the EOD. The three Seidel primary aberrations of an optical component (i.e., spherical, coma, and astigmatism, plus defocus) were considered. The study concluded that the choice of the larger pixel might be advantageous for low-aberration instruments in dynamic and unpredictable environments. Thus, for pixels larger than a certain threshold, a small pixel shows better performance in the face of jitter, misalignment, and other environmental conditions.

The authors of Contribution 9 utilized computer simulations to approach microstructure observations using speckle interferometry. This study demonstrates that the separation of two close points is not impossible when coherent light is used. The condition examined comprised different light phases between the two points. This discussion on the resolution of microstructure observations based on speckle interferometry led to a new interpretation of the Rayleigh criterion in super-resolution techniques.

3. Conclusions

In summary, 3OM research is multidisciplinary, bridging several optical, mechanical, and electrical engineering fields. The ten contributions to this Special Issue represent an enticing taster of the many areas of interest involving 3OM. Two of the works were prepared for the second edition of the International Conference Advances in 3OM [31]. During the publishing process, the third edition of this Conference was under development [32], and, linked to it, the second edition of this Special Issue of Photonics was launched [33]. Another Special Issue, this time of Sensors, also addresses a 3OM topic of high interest: laser scanning and its various applications [34]. We hope that these Special Issues offer readers an insight into these fields and inspire scholars to contribute to advancing our knowledge on these topics for the benefit of humankind.