Compact dual-wavelength digital holography using VCSEL technology (Conference Presentation)

The high-speed topography of both specular reflective and diffuse reflective surfaces with one single instrument represents a major challenge in optical metrology. Surface metrology techniques exist, which can cope with both types of surfaces, such as white light interference microscopy (WLI) and confocal microscopy (CM). However, the measurement is very time-consuming ranging from multiple seconds to minutes depending on the field of view and resolution. This is due to the fact, that WLI and CM are highly informative, revealing all surface parameters such as roughness, waviness, and shape. Structured illumination, on the other hand, recovers the shape only, but is restricted to the inspection of rough scattering surfaces. Moreover, a projection device, often implemented via a spatial light modulator, and a certain angle between projection and observation direction are necessary to enable the application of triangulation. In that manner, it is not only limited by the type of surface but also is quite bulky and heavy. Dual wavelength holography can overcome the aforementioned shortcomings. It can be applied on rough scattering and specular reflective surfaces. In addition, the measurement range and the axial resolution can be adjusted via the choice of the dual wavelength pairs. Moreover, a flexible and light weight setup can be enabled, without the necessity to employ imaging lenses due to the recovery of complex information (amplitude and phase) and numerically refocusing the recorded wavefield to the object plane. In the last years, Vertical Cavity Surface Emitting Lasers (VCSEL) have attained more and more attraction. Commonly they are used in the form of multimode VCSEL arrays and have for instance been applied for face recognition in smartphones. However, single mode VCSELs do likewise exist. They offer excellent coherent properties, with a coherence length of a few tens of cm, an isotropic angular radiation profile and dimensions < mm, allowing a very compact and lightweight setup arrangement. Moreover, the wavelength can be tuned within a few nm via changing the current applied to the VCSEL or the temperature, which makes them extremely attractive for dual wavelength optical metrology. The only downside of single mode VCSEL is the limited power of not more than a few mW, which restricts the investigated field of view to a few mms. In this paper, we describe the application of VCSEL for the dual wavelength digital holography. The surfaces investigated range from diffuse scattering to specular reflective. At first, the spectral response of the VCSEL is determined in a calibration process. A wavemeter with sub-Angstrom spectral resolution has been employed to investigated stability and repeatability of the wavelength. For the sake of a compact and lightweight setup and in order to take advantage of the little power of the VCSEL, an off-axis arrangement has been chosen. The shape measurements are taking on referenced surfaces. The measurement system, the evaluation of the data and an error analysis based on the referenced surfaces will be shown and discussed in this paper.