Browsing by Subject "Fractional Fourier transforms"

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Open Access
Digital computation of the fractional Fourier transform
(Institute of Electrical and Electronics Engineers, 1996-09) Özaktaş, Haldun M.; Arıkan, Orhan; Kutay, M. A.; Bozdağı, G.
An algorithm for efficient and accurate computation of the fractional Fourier transform is given. For signals with time-bandwidth product N, the presented algorithm computes the fractional transform in O(NlogN) time. A definition for the discrete fractional Fourier transform that emerges from our analysis is also discussed.
Open Access
Digital Fourier optics
(Optical Society of America, 1996-03-10) Özaktaş, Haldun M.; Miller, D. A. B.
Analog Fourier optical processing systems can perform important classes of signal processing operations in parallel, but suffer from limited accuracy. Digital–optical equivalents of such systems could be built that share many features of the analog systems while allowing greater accuracy. We show that the digital equivalent of any system consisting of an arbitrary number of lenses, filters, spatial light modulators, and sections of free space can be constructed. There are many possible applications for such systems as well as many alternative technologies for constructing them; this paper stresses the potential of free-space interconnected active-device-plane-based optoelectronic architectures as a digital signal processing environment. Implementation of the active-device planes through hybridization of optoelectronic components with silicon electronics should allow the realization of systems whose performance exceeds that of purely electronic systems.
Open Access
Fast and accurate algorithm for the computation of complex linear canonical transforms
(Optical Society of America, 2010-08-05) Koç A.; Özaktaş, Haldun M.; Hesselink, L.
A fast and accurate algorithm is developed for the numerical computation of the family of complex linear canonical transforms (CLCTs), which represent the input-output relationship of complex quadratic-phase systems. Allowing the linear canonical transform parameters to be complex numbers makes it possible to represent paraxial optical systems that involve complex parameters. These include lossy systems such as Gaussian apertures, Gaussian ducts, or complex graded-index media, as well as lossless thin lenses and sections of free space and any arbitrary combinations of them. Complex-ordered fractional Fourier transforms (CFRTs) are a special case of CLCTs, and therefore a fast and accurate algorithm to compute CFRTs is included as a special case of the presented algorithm. The algorithm is based on decomposition of an arbitrary CLCT matrix into real and complex chirp multiplications and Fourier transforms. The samples of the output are obtained from the samples of the input in ∼N log N time, where N is the number of input samples. A space-bandwidth product tracking formalism is developed to ensure that the number of samples is information-theoretically sufficient to reconstruct the continuous transform, but not unnecessarily redundant.
Open Access
Fast computation of the ambiguity function and the Wigner distribution on arbitrary line segments
(IEEE, 2001) Özdemir, A. K.; Arıkan, Orhan
By using the fractional Fourier transformation of the time-domain signals, closed-form expressions for the projections of their auto or cross ambiguity functions are derived. Based on a similar formulation for the projections of the auto and cross Wigner distributions and the well known two-dimensional (2-D) Fourier transformation relationship between the ambiguity and Wigner domains, closed-form expressions are obtained for the slices of both the Wigner distribution and the ambiguity function. By using discretization of the obtained analytical expressions, efficient algorithms are proposed to compute uniformly spaced samples of the Wigner distribution and the ambiguity function located on arbitrary line segments. With repeated use of the proposed algorithms, samples in the Wigner or ambiguity domains can be computed on non-Cartesian sampling grids, such as polar grids.
Open Access
Fractional Fourier domains
(Elsevier BV, 1995-09) Özaktaş, Haldun M.; Aytür, O.
It is customary to define the time-frequency plane such that time and frequency are mutually orthogonal coordinates. Representations of a signal in these domains are related by the Fourier transform. We consider a continuum of “fractional” domains making arbitrary angles with the time and frequency domains. Representations in these domains are related by the fractional Fourier transform. We derive transformation, commutation, and uncertainty relations among coordinate multiplication, differentiation, translation, and phase shift operators between domains making arbitrary angles with each other. These results have a simple geometric interpretation in time-frequency space.
Open Access
The fractional fourier transform
(IEEE, 2001) Özaktas, Haldun M.; Kutay, M. A.
A brief introduction to the fractional Fourier transform and its properties is given. Its relation to phase-space representations (time- or space-frequency representations) and the concept of fractional Fourier domains are discussed. An overview of applications which have so far received interest are given and some potential application areas remaining to be explored are noted.
Open Access
Fractional Fourier transforms and their optical implementation. II
(Optical Society of America, 1993) Özaktaş, Haldun M.; Mendlovic, D.
The derivation of a linear transform kernel for fractional Fourier transforms is presented. Discussed in direct relation to fractal Fourier transforms are spatial resolution and the space-bandwidth product for propagation in graded-index media. Results show how fractional Fourier transforms can be made the basis of generalized spatial filtering systems.
Open Access
Fundamental structure of Fresnel diffraction: Longitudinal uniformity with respect to fractional Fourier order
(Optical Society of America, 2011-12-24) Özaktaş, Haldun M.; Arik, S. O.; Coşkun, T.
Fresnel integrals corresponding to different distances can be interpreted as scaled fractional Fourier transformations observed on spherical reference surfaces. Transverse samples can be taken on these surfaces with separation that increases with propagation distance. Here, we are concerned with the separation of the spherical reference surfaces along the longitudinal direction. We show that these surfaces should be equally spaced with respect to the fractional Fourier transform order, rather than being equally spaced with respect to the distance of propagation along the optical axis. The spacing should be of the order of the reciprocal of the space-bandwidth product of the signals. The space-dependent longitudinal and transverse spacings define a grid that reflects the structure of Fresnel diffraction.
Open Access
Generalization of time-frequency signal representations to joint fractional Fourier domains
(IEEE, 2005-09) Durak, L.; Özdemir, A. K.; Arıkan, Orhan; Song, I.
The 2-D signal representations of variables rather than time and frequency have been proposed based on either Hermitian or unitary operators. As an alternative to the theoretical derivations based on operators, we propose a joint fractional domain signal representation (JFSR) based on an intuitive understanding from a time-frequency distribution constructing a 2-D function which designates the joint time and frequency content of signals. The JFSR of a signal is so designed that its projections on to the defining joint fractional Fourier domains give the modulus square of the fractional Fourier transform of the signal at the corresponding orders. We derive properties of the JFSR including its relations to quadratic time-frequency representations and fractional Fourier transformations. We present a fast algorithm to compute radial slices of the JFSR.
Open Access
Generalized filtering configurations with applications in digital and optical signal and image processing
(1999) Kutay, Mehmet Alper
In this thesis, we first give a brief summary of the fractional Fourier transform which is the generalization of the ordinary Fourier transform, discuss its importance in optical and digital signal processing and its relation to time-frequency representations. We then introduce the concept of filtering circuits in fractional Fourier domains. This concept unifies the multi-stage (repeated) and multi-channel (parallel) filtering configurations which are in turn generalizations of single domain filtering in fractional Fourier domains. We show that these filtering configurations allow a cost-accuracy tradeoff by adjusting the number of stages or channels. We then consider the application of these configurations to three important problems, namely system synthesis, signal synthesis, and signal recovery, in optical and digital signal processing. In the system and signal synthesis problems, we try to synthesize a desired system characterized by its kernel, or a desired signal characterized by its second order statistics by using fractional Fourier domain filtering circuits. In the signal recovery problem, we try to recover or estimate a desired signal from its degraded version. In all of the examples we give, significant improvements in performance are obtained with respect to single domain filtering methods with only modest increases in optical or digital implementation costs. Similarly, when the proposed method is compared with the direct implementation of general linear systems, we see that significant computational savings are obtained with acceptable decreases in performance.
Open Access
Image processing with the fractional Fourier transform: synthesis, compression and perspective projections
(2000) Yetik, I Şamil
In this work, first we give a summary of the fractional Fourier transform including its definition, important properties, generalization to two-dimensions and its discrete counterpart. After that, we repeat the concept of filtering in the fractional Fourier domains and give multi-stage and multi-channel filtering configurations. Due to the nonlinear nature of the problem, the transform orders in fractional Fourier domain filtering configurations have usually not been optimized but chosen uniformly up to date. We discuss the optimization of orders in the multi-channel filtering configuration. In the next part of this thesis, we discuss the application of fractional Fourier transform based filtering configurations to image representation and compression. Next, we introduce the fractional Fourier domain decomposition for continuous signals and systems. In the last part, we analyse perspective projections in the space-frequency plane and show that under certain conditions they can be approximately modeled in terms of the fractional Fourier transform.
Open Access
Kesirli fourier dönüşümü genliklerinden karmaşık sinyallerin geri kazanımı
(IEEE, 2004-04) Ertosun, M. Günhan; Atlı, Haluk; Özaktaş, Haldun M.; Barshan, Billur
Bu makalede kesirli Fourier dönüşümü genlikleri kullanılarak karmaşık sinyallerin evrelerinin bulunması üzerinde durulmuştur. Bu aynı zamanda optik eksende enine boyuna rastgele iki yerde yapılan genlik ölçümlerinden evre bilgisinin bulunmasına karşılık gelmektedir. İteratif algoritmanın yakınsaklığı, gürültü ve ölçüm hatalarının etkisi ve bunların dönüşümün kesir değerine olan bağlılığı incelenmiştir. Genel olarak, kesir değerinin ünitere yakın olduğu durumlarda, sıfıra yakın olduğu durumlara göre daha iyi sonuçlar elde edilmiştir. Buna göre, en iyi sonuçları elde etmek için, iki ölçüm düzlemi arasındaki kesir değeri ünitere olabildiğince yakın seçilmelidir.
Open Access
Kesirli fourier dönüşümünün zaman bölgesinde sonlu farklar yöntemine uygulanması
(IEEE, 2010-04) Sayın, I.; Arıkan F.; Arıkan, Orhan
Bilgisayarların hız ve belleklerinin gelişmesi ile birlikte elektromanyetik problemlerin çözümünde saysal yöntemler sıkça kullanılmaya başlanmış ve bu konuda çok sayda araştırma yapılmıştır. Saysal Elektromanyetik yöntemleri genel olarak zaman ve frekans tabanlı yöntemler olarak sınıflandırılabilir. Zaman tabanlı yöntemler geçici tepkilerin ve geniş bantlı problemlerin incelenmesinde kullanışlı olurken, frekans tabanlı yöntemler durağan hal tepkilerin ve dar bantlı problemlerin incelenmesinde en iyi çözümü vermektedir. Her iki yaklaşımın da avantajlarını ön plana çıkarabilecek bir yöntem geliştirilebileceği düşünülmektedir. Uzayda ve/veya zamanda Kesirli Fourier Dönüşümü uygulanarak bazı durumlarda hesaplama karmaşıklığı azaltılabilir. Kesirli Fourier Dönüşümü, sürekli Fourier Dönüşümünün genelleştirilmiş halidir. Son yıllarda bu konu üzerinde çeşitli çalışmalar yapılmakta ve uygulama alanları genişlemektedir. Genel olarak, sinyal işleme ve gürültü süzme gibi alanlarda kullanılmaktadır. Bu çalmada Kesirli Fourier Dönüşümü, ilk kez Maxwell denklemlerine zaman bölgesinde uygulanmış ve elde edilen diferansiyel denklemler sonlu farklar yaklaşımı ile ayrık hale getirilmiştir. Elde edilen ayrık sonlu fark denklemlerinin çözümü için öneriler sunulmuştur.
Open Access
Linear algebraic analysis of fractional Fourier domain interpolation
(IEEE, 2009) Öktem, Figen S.; Özaktaş, Haldun M.
In this work, we present a novel linear algebraic approach to certain signal interpolation problems involving the fractional Fourier transform. These problems arise in wave propagation, but the proposed approach to these can also be applicable to other areas. We see this interpolation problem as the problem of determining the unknown signal values from the given samples within some tolerable error. We formulate the problem as a linear system of equations and use the condition number as a measure of redundant information in given samples. By analyzing the effect of the number of known samples and their distributions on the condition number with simulation examples, we aim to investigate the redundancy and information relations between the given data.
Open Access
Linear canonical transforms, degrees of freedom, and sampling in optical signals and systems
(IEEE, 2014) Özaktaş, Haldun M.; Öktem, F. S.
We study the degrees of freedom of optical systems and signals based on space-frequency (phase-space) analysis. At the heart of this study is the relationship of the linear canonical transform domains to the space-frequency plane. Based on this relationship, we discuss how to explicitly quantify the degrees of freedom of first-order optical systems with multiple apertures, and give conditions for lossless transfer. Moreover, we focus on the degrees of freedom of signals in relation to the space-frequency support and provide a sub-Nyquist sampling approach to represent signals with arbitrary space-frequency support. Implications for simulating optical systems are also discussed.
Open Access
A new approach to time-frequency localized signal design
(IEEE, 2002) Özdemir, Ahmet Kemal; Aydın, Zafer; Arıkan, Orhan
A novel approach is presented for the design of signals in Wigner Domain. In this method, the desired signal features in the time-frequency domain are specified in two stages. First the user specifies the spine curve around which the energy of the desired signal is distributed in the time-frequency plane. Then, the user specifies the spread of the desired signal energy around the spine. In addition to this fundamentally new way of defining the time-frequency features of the desired signal, the actual synthesis of the signal is performed in a warped fractional Fourier transform approach [1]. After obtaining the designed signal, it is transformed back to the original time domain providing the final result of the new signal synthesis technique. In contrast to the conventional algorithms, the proposed method provides very good results even if the inner cross-term structure of the desired signal is not specified.
Open Access
Non-orthogonal domains in phase space of quantum optics and their relation to fractional Fourier transforms
(Elsevier BV * North-Holland, 1995-10-15) Aytür, O.; Özaktaş, Haldun M.
It is customary to define a phase space such that position and momentum are mutually orthogonal coordinates. Associated with these coordinates, or domains, are the position and momentum operators. Representations of the state vector in these coordinates are related by the Fourier transformation. We consider a continuum of "fractional" domains making arbitrary angles with the position and momentum domains. Representations in these domains are related by the fractional Fourier transformation. We derive transformation, commutation, and uncertainty relations between coordinate multiplication, differentiation, translation, and phase shift operators making arbitrary angles with each other. These results have a simple geometric interpretation in phase space and applications in quantum optics.
Open Access
Nonseperable two-dimensional fractional Fourier transform
(Optical Society of America, 1998) Sahin, A.; Kutay, M. A.; Özaktaş, Haldun M.
Previous generalizations of the fractional Fourier transform to two dimensions assumed separable kernels. We present a nonseparable definition for the two-dimensional fractional Fourier transform that includes the separable definition as a special case. Its digital and optical implementations are presented. The usefulness of the nonseparable transform is justified with an image-restoration example.
Open Access
Relationships among ray optical, Gaussian beam, and fractional Fourier transform descriptions of first-order optical systems
(Elsevier BV * North-Holland, 1997-11-01) Özaktaş, Haldun M.; Erden, M. F.
Although wave optics is the standard method of analyzing systems composed of a sequence of lenses separated by arbitrary distances, it is often easier and more intuitive to ascertain the function and properties of such systems by tracing a few rays through them. Determining the location, magnification or scale factor, and field curvature associated with images and Fourier transforms by tracing only two rays is a common skill. In this paper we show how the transform order, scale factor, and field curvature can be determined in a similar manner for the fractional Fourier transform, Our purpose is to develop the understanding and skill necessary to recognize fractional Fourier transforms and their parameters by visually examining ray traces. We also determine the differential equations governing the propagation of the order, scale, and curvature, and show how these parameters are related to the parameters of a Gaussian beam.
Open Access
Repeated fractional Fourier domain filtering is equivalent to repeated time and frequency domain filtering
(Elsevier BV, 1996-10) Özaktaş, Haldun M.
Any system consisting of a sequence of multiplicative filters inserted between several fractional Fourier transform stages, is equivalent to a system composed of an appropriately chosen sequence of multiplicative filters inserted between appropriately scaled ordinary Fourier transform stages. Thus every operation that can be accomplished by repeated filtering in fractional Fourier domains can also be accomplished by repeated filtering alternately in the ordinary time and frequency domains.