# Wavefront shaping techniques in complex media

- Details
- Category: Phase measurement
- Published on Thursday, 10 October 2013 22:16

## Off-axis holography [tutorial]

Off-axis holography is a popular technique to reconstruct a hologram. It allows retrieving the amplitude and the phase of a field pattern by measuring only one image with a digital camera. It relies on an intereferometric setup with a non-zero angle between the reference beam and the signal beam and requires to numerically filter the spatial frequencies.

This technique was initially presented in [1]. The schematic of the experimental setup is shown in Fig. 1. The idea is to make interfere the unknown "signal" wavefront of complex field with a plane wave of amplitude with a given angle between the direction of propagation of the two beams. We assume that and are in the same state of polarization.

**Figure 1.** Schematic of the setup.

The intensity measured by the camera reads:

The three terms correspond to signals centered around the spatial frequencies 0, and . If the spatial frequency is larger than the maximal spatial frequency in the signal to reconstruct, the Fourier transform of the measured intensity pattern shows three components corresponding orders 0, +1 and -1 that do not overlap. We note that, since is constant, the order -1 is directly proportional to the complex field we want to retrieve but simply shifted by the spatial frequency . The idea is then to numerically select the order -1 in the Fourier domain and center the signal around the zero frequency. The inverse Fourier transform then directly gives access to the complex image . The steps of this process are illustrated in Fig. 2 and Fig. 3.

The following images were obtained with the Matlab code attached to this article and used to simulate an off-axis holography measurement (the file is available at the bottom of this page, the code includes comments). For the sake of simplicity, in this example, we try to recover an image with a flat amplitude and with a random phase pattern, similar to the phase of speckle pattern [Fig 2.a.]. After interference with the tilted plane wave [Fig 2.b.], we calculate the intensity pattern as it would be measured on the camera [Fig 2.c.].

**Figure 2.** Phase wavefront to measure (a), tilted plane wave reference (b) and resulting interference pattern.

We then calculate the Fourier transform of this intensity pattern [Fig 3.a.]. We note that the angle of the tilted plane wave has to be high enough so that order 0 and -1 do not overlap. We then select only the -1 order [Fig 3.b.] and shift it around the zero frequency [Fig 3.b.].

**Figure 3.** Numerical spatial frequency filtering. (a) Complete Fourier transform of the intensity pattern. (b) Fourier transform filtered to keep only the -1 order. (c) Signal shifted around the zero spatial frequency.

The reconstructed complex field is obtained by calculating the inverse Fourier transform of the last step. We compare in Fig 4. the phase of the in initial image and the phase of the recovered one.

**Figure 4.** Comparison between the actual phase pattern (a) and the reconstructed one (b).

This technique allows to perform a one shot measurement of the complex field for one state of polarization at the cost of a loss of resolution due to the need to resolve the fringes inside one speckle grain. Such a technique can be extended to obtain a full image for both polarizations by using two reference beams with orthogonal polarizations with tilt angles and in two different directions.

[1] E. Cuche et al., Opt. Express, 39, (2000)

## Comments

12Hi Sanjeev,

Did you actually measure a decorrelation curve with intensity only (no reference) and compared it with decorrelation of the reconstructed field with the intereferences?

When you say your laser is stable, you mean the intensity is stable? If you have interferometric instabilities or phase fluctuations, the interference signal will fluctuate.

Try to remove the scattering medium and measure the interference signal (i.e. a simple Mach Zehnder), if the intensity signal varies too much, that means you do not have good interferometric stability.

Quoting Sanjeev:

That is not relevant. You need to quantify the decorrelation in time. Do not trust the visual aspect.

Sebastien

The +2 and -2 order maybe are due to the overexpousre in your experiment. Taking overexposure as truncation, it will introduce higher order frequency

Hi Sanjiv.

Is it in experiment or in simulation.

If it is with experimental data, first try your code in simulation (with a code similar to the one I share).

If the results change over time in experiment, maybe your system is not stable (decorrelation of the medium, stability of the laser, camera, etc...). To test that, just take consecutive images (without Fourier treatment) and check their correlation to the first image.

Best,

Sebastien

Each time I reconstruct the hologram the complex field I reconstruct is different at different time. Why is this happening ? How can I reduce this error ?

https://ekbil-my.sharepoint.com/:i:/g/personal/sanjeev_a_ekb_co_il/EbytTozXzO5AmqJ-RmnjO-cB2Ram1VIBvFxlqQlbn65qSg?e=4Cnstf

Could you let me know what error I am doing in experiment ?

Quoting Sébastien Popoff:

Everything is in the equation for the intensity.

Quoting Jitender:

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