Fiveable
Fiveable
Fiveable
Fiveable

🔬Modern Optics

🔬modern optics review

9.1 Fourier transforms in optics

6 min readLast Updated on July 22, 2024

Fourier transforms are the backbone of modern optics, allowing us to break down complex signals into their frequency components. This powerful tool lets us analyze and manipulate optical systems with ease, from calculating diffraction patterns to characterizing imaging systems.

The beauty of Fourier transforms lies in their ability to switch between spatial and frequency domains seamlessly. This duality opens up a world of possibilities for optical signal processing, enabling techniques like spatial filtering and correlation that are crucial in many applications.

Fourier Transforms in Optics

Mathematical foundation of Fourier transforms

Top images from around the web for Mathematical foundation of Fourier transforms
Top images from around the web for Mathematical foundation of Fourier transforms
  • Decompose a function into its constituent frequencies enables analysis and manipulation of complex signals
    • Continuous Fourier transform F(u)=f(x)ei2πuxdxF(u) = \int_{-\infty}^{\infty} f(x) e^{-i2\pi ux} dx expresses a function as a sum of complex exponentials with different frequencies
    • Inverse Fourier transform f(x)=F(u)ei2πuxduf(x) = \int_{-\infty}^{\infty} F(u) e^{i2\pi ux} du reconstructs the original function from its frequency components
  • Relate the spatial and frequency domains in optics allows for powerful analysis and processing techniques
    • Spatial domain represents physical space where optical fields and images exist (real space)
    • Frequency domain represents the spatial frequencies present in the optical field or image (reciprocal space)
  • Analyze and manipulate optical systems using Fourier transforms enables efficient design and optimization
    • Diffraction patterns can be calculated by taking the Fourier transform of the aperture function (Fraunhofer diffraction)
    • Imaging systems can be characterized by their transfer function, which is the Fourier transform of the point spread function (PSF)
    • Optical signal processing techniques, such as spatial filtering and correlation, rely on Fourier transform properties

Spatial vs frequency domains

  • Related by Fourier transforms allows for seamless conversion between the two representations
    • Forward Fourier transform converts from spatial to frequency domain, revealing the spatial frequency content
    • Inverse Fourier transform converts from frequency to spatial domain, reconstructing the original optical field or image
  • Affect each other's properties, enabling powerful analysis and manipulation techniques
    • Scaling in the spatial domain causes inverse scaling in the frequency domain (larger objects have lower spatial frequencies)
    • Shifting in the spatial domain introduces a linear phase factor in the frequency domain (spatial shifts do not affect frequency content)
    • Convolution in the spatial domain corresponds to multiplication in the frequency domain (useful for analyzing imaging systems)
    • Multiplication in the spatial domain corresponds to convolution in the frequency domain (enables spatial filtering techniques)

Properties for diffraction and imaging

  • Calculate diffraction patterns using Fourier transforms simplifies the analysis of complex apertures
    • Fraunhofer diffraction occurs in the far-field, where the diffraction pattern is the Fourier transform of the aperture function (slit, circular aperture)
    • Fresnel diffraction occurs in the near-field, where the diffraction pattern is the Fourier transform of the product of the aperture function and a quadratic phase factor (zone plate)
  • Analyze imaging systems using Fourier transforms characterizes their performance and limitations
    • Coherent imaging systems have an image that is the convolution of the object with the PSF, which is the Fourier transform of the pupil function (holography)
    • Incoherent imaging systems have an image that is the convolution of the object intensity with the squared modulus of the PSF (photography)
  • Characterize the imaging system in the frequency domain using the optical transfer function (OTF)
    • OTF is the Fourier transform of the PSF and represents the system's ability to transfer spatial frequencies
    • Modulation transfer function (MTF) is the magnitude of the OTF and quantifies the contrast reduction at different spatial frequencies (resolution limit)

Physical interpretation of Fourier transforms

  • Represents the angular spectrum of plane waves, revealing the directional composition of the optical field
    • Each point in the frequency domain corresponds to a plane wave with a specific spatial frequency and direction (wave vector)
    • Amplitude and phase of each plane wave component are given by the complex value at the corresponding frequency domain point (Fourier spectrum)
  • Represents the spatial frequency content, characterizing the level of detail and sharpness in the image
    • Low spatial frequencies correspond to smooth variations and overall structure (background, large objects)
    • High spatial frequencies correspond to fine details and sharp edges (textures, small features)
  • Helps understand the optical system's behavior, guiding the design and optimization process
    • Bandwidth of the system determines the range of spatial frequencies that can be transmitted or captured (resolution limit)
    • Filtering in the frequency domain can enhance or suppress specific spatial frequencies (image processing, super-resolution)

Fourier Transform Properties and Applications

Mathematical foundation of Fourier transforms

  • Enable efficient analysis and manipulation of optical systems by exploiting their mathematical properties
    • Linearity F[af(x)+bg(x)]=aF(u)+bG(u)\mathcal{F}[af(x) + bg(x)] = aF(u) + bG(u) allows for superposition and scaling of Fourier transforms
    • Scaling F[f(ax)]=1aF(ua)\mathcal{F}[f(ax)] = \frac{1}{|a|}F(\frac{u}{a}) relates spatial scaling to frequency scaling (magnification, demagnification)
    • Shifting F[f(xx0)]=ei2πux0F(u)\mathcal{F}[f(x-x_0)] = e^{-i2\pi ux_0}F(u) introduces a linear phase factor in the frequency domain (spatial translation)
    • Convolution F[f(x)g(x)]=F(u)G(u)\mathcal{F}[f(x) * g(x)] = F(u)G(u) simplifies the analysis of linear systems (imaging, filtering)
    • Multiplication F[f(x)g(x)]=F(u)G(u)\mathcal{F}[f(x)g(x)] = F(u) * G(u) enables spatial filtering techniques (apodization, modulation)
  • Relate energy in spatial and frequency domains through Parseval's theorem, ensuring conservation of energy
    • f(x)2dx=F(u)2du\int_{-\infty}^{\infty} |f(x)|^2 dx = \int_{-\infty}^{\infty} |F(u)|^2 du states that the total energy is preserved under Fourier transform
  • Lead to the discrete Fourier transform (DFT) for sampled and discretized signals
    • DFT F[k]=n=0N1f[n]ei2πkn/NF[k] = \sum_{n=0}^{N-1} f[n] e^{-i2\pi kn/N} computes the Fourier transform of a discrete signal
    • Inverse DFT f[n]=1Nk=0N1F[k]ei2πkn/Nf[n] = \frac{1}{N} \sum_{k=0}^{N-1} F[k] e^{i2\pi kn/N} reconstructs the original discrete signal
    • Fast Fourier transform (FFT) algorithms efficiently compute the DFT (Cooley-Tukey algorithm)

Properties for diffraction and imaging

  • Simplify the analysis of linear shift-invariant (LSI) systems, which are common in optics
    • LSI systems are characterized by their impulse response or transfer function (space-invariant PSF)
    • Output of an LSI system is the convolution of the input with the impulse response (image formation)
    • Transfer function is the Fourier transform of the impulse response (OTF)
  • Exploit Fourier transform properties for optical signal processing, enabling advanced manipulation techniques
    • Spatial filtering manipulates the frequency content of an optical field or image (Fourier plane filtering)
      1. Low-pass filtering attenuates high spatial frequencies, reducing noise and smoothing the image (anti-aliasing)
      2. High-pass filtering attenuates low spatial frequencies, enhancing edges and fine details (sharpening)
      3. Band-pass filtering selects a specific range of spatial frequencies (contrast enhancement, feature extraction)
    • Correlation and pattern recognition detect the presence and location of a specific pattern in an image
      • Correlation is performed by multiplying the Fourier transforms of the image and the pattern, followed by an inverse Fourier transform (matched filtering)

Physical interpretation of Fourier transforms

  • Provide insights into the propagation of optical fields, enabling efficient simulation and analysis
    • Angular spectrum method propagates optical fields using Fourier transforms (Fourier optics)
      1. Field at any plane is the inverse Fourier transform of the product of the angular spectrum and a propagation phase factor
    • Fresnel and Fraunhofer diffraction can be interpreted in terms of angular spectrum propagation (near-field and far-field)
  • Reveal the information content and limitations of an imaging system, guiding the design and optimization process
    • Spatial resolution is determined by the bandwidth of the imaging system (Fourier space coverage)
      • Abbe's resolution limit states that the minimum resolvable distance is inversely proportional to the numerical aperture (diffraction limit)
    • Sampling theorem relates the spatial resolution to the sampling rate in the frequency domain (Nyquist rate)
      • Nyquist rate is the minimum sampling rate required to avoid aliasing, which is twice the highest spatial frequency present in the image (undersampling, oversampling)


© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.