Two measurement techniques for topological measurements are currently applied at the group of Photonics and Terahertz Technology: photorefractive and digital Holography.
- Basic Principles of Holography
- Photorefractive Holography
- Digital Holography
- Digital holographic Microscopy
- Dual-wavelength scanning
- Overview of Resolution of holographic Techniques
Basic Principles of Holography:
The basic principle of holography is the superposition of two coherent beams creating an interference pattern, the so called hologram. If one of the beams has been reflected by an object, it is called object beam and contains the entire topographic information of the object. If this beam is superimposed with a plane reference beam onto a holographic material, a hologram is created in which the information of the object can be stored (Figure 1a). This information can be retrieved by again shining the reference beam onto the holographic material so that the object beam is recreated (Figure 1b).
Figure 1a: Writing a hologram, O = object beam, R= reference beam.
Figure 1b: Retrieving a hologram, RO = reconstructed object beam, R = reference beam
Using a photorefractive material as holographic material is called photorefractive holography. When a broadband light source is deployed, the entire depth information can be saved for instance in a photorefractive crystal with only one shot [1,2]. An evaluation over all spectral components, which is similar to optical coherence tomography, enables the recovery of the depth information. The lateral resolution is defined by the size of the pixels of the camera and optics used optionally and lies in the µm range. It is however restricted to the diffraction limit. The axial resolution is dependant on the amount of the spectral components of the light source and is currently about 100 µm. The photographic image and the measured depth information of a sample are given in figure 2a and 2b.
Figure 2a: Photo of a sample
Figure 2b: Depth information of the sample
In contrast to photorefractive holography, there is no holographic material recording the hologram. A CCD- or CMOS Camera is instead used to save the interference pattern. A principle digital holography set-up is shown in Figure 3a.
Figure 3a: Principle set-up of digital holography , O = object beam, R= reference beam
Amplitude and phase of the object beam are then numerically reconstructed in order to gain the object information. The lateral resolution is defined by the size of the pixels of the camera and optics used optionally and lies in the µm range. The axial resolution is in the range of 10 nm. A reconstruction of bond pads of a MOEMS structure (Figure 3b) is given in Figure 3c.
Figure 3b: bond pads
Figure 3c: reconstructed bond pads
Digital holographic Microscopy:
When expanding the digital holographic set-up by a microscope objective, both lateral and axial resolution can be increased. Furthermore, numerical amplitude and phase filters can be applied to the recorded data, so that filter effects like dark field microscopy can be carried out without adding further mechanical components to the set-up. A typical digital holographic microscopy set-up is shown in Figure 4.
Figure 4: Digital holographic microscopy, O = object beam, R = reference beam, MO = microscope objectiv
The amplitude and the phase reconstruction of a USAF test chart can be seen in Figure 5a and 5b respectively.
Figure 5a: amplitude reconstruction
Figure 5b: phase reconstruction
As the topographic structure of an object is encoded in the phase information, it can be numerically unwrapped. A three dimensional representation of the USAF test chart is shown in Figure 6. Differences in height between 10 and 400 nm can be reconstructed. Laterally, the resolution is restricted to the diffraction limit and is smaller than 2 µm.
Figure 6: 3-dimensional reconstruction of a USAF test chart
Samples characterized by bigger differences in height, can be measured with the help of dualwavelength scanning. This technique uses two holograms at two different wavelengths. The phase difference of these two holograms is calculated representing a higher synthetic wavelength with which higher structures can be reconstructed. Figure 7 shows the reconstruction of 200 µm high metal steps, which was calculated with two holograms at wavelengths of 833 and 834 nm.
Figure 7: 3-dimensional reconstruction of metal steps with dual-wavelength scanning
Overview of Resolution of holographic Techniques:
|Technique||Imaging area||Lateral resolution||Axial resolution (minimun)||Axial resolution (Maximum)|
|Photorefractive Holography||up to 25 mm²||dependant on size of pixel||200 µm||1,5 mm|
|Digital Holography||up to 25 mm²||dependant on size of pixel||30 nm||420 nm|
|Digital holographic Microscopy||10 mm² to 0,1 mm²||up to 1µm||10 nm||420 nm|
|Dual-wavelength scanning||up to 25 mm²||up to 1 µm||450 nm to 3,5 µm||9 µm to 70 µm|
-  Koukourakis, N., Kasseck, C., Rytz, D., Gerhardt, N. C., & Hofmann, M. R. (2009). Single-shot holography for depth resolved three dimensional imaging. Optics Express, 17(23), 21015-21029.
-  D. Grosse, N. Koukourakis, N. C. Gerhardt, T. Schlauch, J. C. Balzer, A. Klehr, G. Erbert, G. Tränkle, and M. R. Hofmann, “Single-shot holography with colliding pulse mode-locked lasers as light source,” in Proceedings of the International Quantum Electronics Conference and Conference on Lasers and Electro-Optics Pacific Rim 2011, (Optical Society of America, 2011), paper C835.