**3.2 Experimental results for multi-wavelength DHM (MWDHM) with and without spatial heterodyning (MWDHM-SH)**

In this section, we show an example of using the MWDH technique using a microscopy setup similar to the single wavelength DHM shown in **Figure 4**. Thus, the technique would be abbreviated as (MWDHM). A series of micro-scale objects have been custom fabricated for this experiment as shown in **Figure 16(a)** and **(b)**. **Figure 17(a)** shows a MWDHM setup with achromatic optics and can be operated in either the "one-shot" or "two-shot" Michelson configuration. **Figure 17(b)** is a photograph of the object consisting of 4 bars of photoresist (See element in red

*(a) MWDHM Michelson recording configuration. (b) Photograph of the object. (c) Amplitude reconstruction of the* λ*<sup>1</sup> hologram only. (d) Wrapped phase difference between* λ*<sup>1</sup> and* λ*<sup>2</sup> reconstructions. (e) Unwrapped phase showing MO curvature. (f) the flattened topogram after removal of the curvature.*

circle in **Figure 16(a)**), each 50 μm wide, on a silicon wafer substrate. **Figure 17(c)** is the intensity reconstruction of the *λ*<sup>1</sup> hologram only. **Figure 17(d)** shows the wrapped phase difference between *λ*<sup>1</sup> and *λ*<sup>2</sup> reconstructions. **Figure 17(e)** shows the unwrapped phase (Ref. [24]) with the residual MO quadratic phase curvature and **Figure 17(f)** is the 3D topogram after removal of the MO phase using Eq. (13). The relevant reconstruction parameters are: *d* = 22.7 cm, *λ*<sup>1</sup> = 632.8 nm, *λ*<sup>2</sup> = 488.0 nm, Λ = 2.13 μm, Δ*x* = 6.7 μm, and *N* = 1024, and *M* = 2.75. Note that the phase rings are due to a slight mismatch in collimation between the *λ<sup>1</sup>* and *λ<sup>2</sup>* beams, which causes circularly symmetric phase beating since at least one wavefront is not well collimated. This situation arises often in physical lab setups in which both beams are coaxially aligned and filtered using the same pinhole prior to using a single collimation lens. Chromatic dispersion will prevent both wavelengths from being collimated simultaneously, unless an achromatic lens is used.

Here we show an example using the MWDHM technique with the microscopy setup of **Figure 17(a)**, operated in the spatial heterodyne configuration (MWDHM-SH). The object is a set of 3 rectangular photoresist bars, each 75 μm wide, on a silicon wafer (See element in black circle in **Figure 16(a)**). In this case, the object is simultaneously illuminated by two wavelengths at normal incidence and only a single composite hologram is recorded by the CCD (i.e. "one-shot"). The single hologram is reconstructed twice, one at each fundamental wavelength, and the block-match algorithm is used to align the images prior to phase subtraction. **Figure 18(a)** shows the intensity reconstruction of the *λ*<sup>1</sup> hologram only, with the region of interest circled, **Figure 18(b)** shows the wrapped phase difference between *λ*<sup>1</sup> and *λ*<sup>2</sup> reconstructions after block matching and phase subtraction,

#### **Figure 18.**

*(a) The intensity reconstruction of the* λ*<sup>1</sup> hologram only, with the region of interest circled. (b) the wrapped phase difference between* λ*<sup>1</sup> and* λ*<sup>2</sup> reconstructions after block matching and phase subtraction. (c) the unwrapped phase with the residual MO quadratic phase curvature, and (d) the 3D topogram after removal of the MO phase, and correction of phase errors.*

*Latest Advances in Single and Multiwavelength Digital Holography and Holographic Microscopy DOI: http://dx.doi.org/10.5772/intechopen.94382*

while **Figure 18(c)** shows the unwrapped phase with the residual MO quadratic phase curvature, and **Figure 18(d)** is the 3D topogram after removal of the MO phase and correction of phase errors. The relevant reconstruction parameters are: *d* = 23 cm, *λ*<sup>1</sup> = 632.8 nm, *λ*<sup>2</sup> = 488 nm, Λ = 2.13 μm, Δ*x* = 6.7 μm, *N* = 1024, and *M* = 2.75.
