Tech Guide

Temporal Coherence


Figure 1: The amplitude of a single frequency wave as a function of time t (red) and a copy of the same wave delayed by Ï„(green). The coherence time of the wave is infinite since it is perfectly correlated with itself for all delays Ï„.

Figure 2: The amplitude of a wave whose phase drifts significantly in time Ï„c as a function of time t (red) and a copy of the same wave delayed by 2Ï„c(green). At any particular time t the wave can interfere perfectly with its delayed copy. But, since half the time the red and green waves are in phase and half the time out of phase, when averaged over t any interference disappears at this delay.
Temporal coherence is the measure of the average correlation between the value of a wave and itself delayed by Ï„, at any pair of times. Temporal coherence tells us how monochromatic a source is. In other words, it characterizes how well a wave can interfere with itself at a different time. The delay over which the phase or amplitude wanders by a significant amount (and hence the correlation decreases by significant amount) is defined as the coherence time Ï„c. At Ï„=0 the degree of coherence is perfect whereas it drops significantly by delay Ï„c. The coherence length Lc is defined as the distance the wave travels in time Ï„c.

One should be careful not to confuse the coherence time with the time duration of the signal, nor the coherence length with the coherence area (see below).

The relationship between coherence time and bandwidth
It can be shown that the faster a wave decorrelates (and hence the smaller τc is) the larger the range of frequencies Δf the wave contains. Thus there is a tradeoff:

au_c Delta f approx 1.
Formally, this follows from the convolution theorem in mathematics, which relates the Fourier transform of the power spectrum (the intensity of each frequency) to its autocorrelation.

Examples of temporal coherence
We consider four examples of temporal coherence.

A wave containing only a single frequency (monochromatic) is perfectly correlated at all times according to the above relation. (See Figure 1)
Conversely, a wave whose phase drifts quickly will have a short coherence time. (See Figure 2)
Similarly, pulses (wave packets) of waves, which naturally have a broad range of frequencies, also have a short coherence time since the amplitude of the wave changes quickly. (See Figure 3)
Finally, white light, which has a very broad range of frequencies, is a wave which varies quickly in both amplitude and phase. Since it consequently has a very short coherence time (just 10 periods or so), it is often called incoherent.
Monochromatic sources are usually lasers; such high monochromaticity implies long coherence lengths (up to hundreds of meters). For example, a stabilized and monomode helium–neon laser can easily produce light with coherence lengths of 300 m.[7] Not all lasers are monochromatic, however (e.g. for a mode-locked Ti-sapphire laser, Δλ ≈ 2 nm - 70 nm). LEDs are characterized by Δλ ≈ 50 nm, and tungsten filament lights exhibit Δλ ≈ 600 nm, so these sources have shorter coherence times than the most monochromatic lasers.

Holography requires light with a long coherence time. In contrast, optical coherence tomography uses light with a short coherence time.

Measurement of temporal coherence

Figure 3: The amplitude of a wavepacket whose amplitude changes significantly in time Ï„c (red) and a copy of the same wave delayed by 2Ï„c(green) plotted as a function of time t. At any particular time the red and green waves are uncorrelated; one oscillates while the other is constant and so there will be no interference at this delay. Another way of looking at this is the wavepackets are not overlapped in time and so at any particular time there is only one nonzero field so no interference can occur.

Figure 4: The time-averaged intensity (blue) detected at the output of an interferometer plotted as a function of delay Ï„ for the example waves in Figures 2 and 3. As the delay is changed by half a period, the interference switches between constructive and destructive. The black lines indicate the interference envelope, which gives the degree of coherence. Although the waves in Figures 2 and 3 have different time durations, they have the same coherence time.
In optics, temporal coherence is measured in an interferometer such as the Michelson interferometer or Mach–Zehnder interferometer. In these devices, a wave is combined with a copy of itself that is delayed by time τ. A detector measures the time-averaged intensity of the light exiting the interferometer. The resulting interference visibility (e.g. see Figure 4) gives the temporal coherence at delay τ. Since for most natural light sources, the coherence time is much shorter than the time resolution of any detector, the detector itself does the time averaging. Consider the example shown in Figure 3. At a fixed delay, here 2τc, an infinitely fast detector would measure an intensity that fluctuates significantly over a time t equal to τc. In this case, to find the temporal coherence at 2τc, one would manually time-average the intensity.

Spatial coherence

In some systems, such as water waves or optics, wave-like states can extend over one or two dimensions. Spatial coherence describes the ability for two points in space, x1 and x2, in the extent of a wave to interfere, when averaged over time. More precisely, the spatial coherence is the cross-correlation between two points in a wave for all times. If a wave has only 1 value of amplitude over an infinite length, it is perfectly spatially coherent. The range of separation between the two points over which there is significant interference is called the coherence area, Ac. This is the relevant type of coherence for the Young's double-slit interferometer. It is also used in optical imaging systems and particularly in various types of astronomy telescopes. Sometimes people also use "spatial coherence" to refer to the visibility when a wave-like state is combined with a spatially shifted copy of itself.

Examples of spatial coherence
Spatial coherence

Figure 5: A plane wave with an infinite coherence length.



Figure 6: A wave with a varying profile (wavefront) and infinite coherence length.



Figure 7: A wave with a varying profile (wavefront) and finite coherence length.



Figure 8: A wave with finite coherence area is incident on a pinhole (small aperture). The wave will diffract out of the pinhole. Far from the pinhole the emerging spherical wavefronts are approximately flat. The coherence area is now infinite while the coherence length is unchanged.



Figure 9: A wave with infinite coherence area is combined with a spatially shifted copy of itself. Some sections in the wave interfere constructively and some will interfere destructively. Averaging over these sections, a detector with length D will measure reduced interference visibility. For example a misaligned Mach–Zehnder interferometer will do this.

Consider a tungsten light-bulb filament. Different points in the filament emit light independently and have no fixed phase-relationship. In detail, at any point in time the profile of the emitted light is going to be distorted. The profile will change randomly over the coherence time au_c. Since for a white-light source such as a light-bulb au_c is small, the filament is considered a spatially incoherent source. In contrast, a radio antenna array, has large spatial coherence because antennas at opposite ends of the array emit with a fixed phase-relationship. Light waves produced by a laser often have high temporal and spatial coherence (though the degree of coherence depends strongly on the exact properties of the laser). Spatial coherence of laser beams also manifests itself as speckle patterns and diffraction fringes seen at the edges of shadow.

Holography requires temporally and spatially coherent light. Its inventor, Dennis Gabor, produced successful holograms more than ten years before lasers were invented. To produce coherent light he passed the monochromatic light from an emission line of a mercury-vapor lamp through a pinhole spatial filter.

In February 2011, Dr Andrew Truscott, leader of a research team at the ARC Centre of Excellence for Quantum-Atom Optics at Australian National University in Canberra, Australian Capital Territory, showed that helium atoms cooled to near absolute zero / Bose–Einstein condensate state, can be made to flow and behave as a coherent beam as occurs in a laser.[8][9]

Spectral coherence


Figure 10: Waves of different frequencies (i.e. colors) interfere to form a pulse if they are coherent.

Figure 11: Spectrally incoherent light interferes to form continuous light with a randomly varying phase and amplitude
Waves of different frequencies (in light these are different colours) can interfere to form a pulse if they have a fixed relative phase-relationship (see Fourier transform). Conversely, if waves of different frequencies are not coherent, then, when combined, they create a wave that is continuous in time (e.g. white light or white noise). The temporal duration of the pulse Delta t is limited by the spectral bandwidth of the light Delta f according to:

Delta fDelta t ge 1,
which follows from the properties of the Fourier transform and results in Küpfmüller's uncertainty principle (for quantum particles it also results in the Heisenberg uncertainty principle).

If the phase depends linearly on the frequency (i.e. heta (f) propto f) then the pulse will have the minimum time duration for its bandwidth (a transform-limited pulse), otherwise it is chirped (see dispersion).

Measurement of spectral coherence
Measurement of the spectral coherence of light requires a nonlinear optical interferometer, such as an intensity optical correlator, frequency-resolved optical gating (FROG), or spectral phase interferometry for direct electric-field reconstruction (SPIDER).

Polarization coherence

Light also has a polarization, which is the direction in which the electric field oscillates. Unpolarized light is composed of incoherent light waves with random polarization angles. The electric field of the unpolarized light wanders in every direction and changes in phase over the coherence time of the two light waves. An absorbing polarizer rotated to any angle will always transmit half the incident intensity when averaged over time.

If the electric field wanders by a smaller amount the light will be partially polarized so that at some angle, the polarizer will transmit more than half the intensity. If a wave is combined with an orthogonally polarized copy of itself delayed by less than the coherence time, partially polarized light is created.

The polarization of a light beam is represented by a vector in the Poincaré sphere. For polarized light the end of the vector lies on the surface of the sphere, whereas the vector has zero length for unpolarized light. The vector for partially polarized light lies within the sphere

Applications

Holography
Coherent superpositions of optical wave fields include holography. Holographic objects are used frequently in daily life in bank notes and credit cards.

Non-optical wave fields
Further applications concern the coherent superposition of non-optical wave fields. In quantum mechanics for example one considers a probability field, which is related to the wave function psi (mathbf r) (interpretation: density of the probability amplitude). Here the applications concern, among others, the future technologies of quantum computing and the already available technology of quantum cryptography. Additionally the problems of the following subchapter are treated.

Quantum coherence

In quantum mechanics, all objects have wave-like properties (see de Broglie waves). For instance, in Young's double-slit experiment electrons can be used in the place of light waves. Each electron's wave-function goes through both slits, and hence has two separate split-beams that contribute to the intensity pattern on a screen. According to standard wave theory (Fresnel, Huygens) these two contributions give rise to an intensity pattern of bright bands due to constructive interference, interlaced with dark bands due to destructive interference, on a downstream screen. (Each split-beam, by itself, generates a diffraction pattern with less noticeable, more widely spaced dark and light bands.) This ability to interfere and diffract is related to coherence (classical or quantum) of the wave. The association of an electron with a wave is unique to quantum theory.

When the incident beam is represented by a quantum pure state, the split beams downstream of the two slits are represented as a superposition of the pure states representing each split beam. (This has nothing to do with two particles or Bell's inequalities relevant to an entangled state: a 2-body state, a kind of coherence between two 1-body states.) The quantum description of imperfectly coherent paths is called a mixed state. A perfectly coherent state has a density matrix (also called the "statistical operator") that is a projection onto the pure coherent state, while a mixed state is described by a classical probability distribution for the pure states that make up the mixture.

Large-scale (macroscopic) quantum coherence leads to novel phenomena, the so-called macroscopic quantum phenomena. For instance, the laser, superconductivity and superfluidity are examples of highly coherent quantum systems whose effects are evident at the macroscopic scale. The macroscopic quantum coherence (Off-Diagonal Long-Range Order, ODLRO) [Penrose & Onsager (1957), C. N. Yang (1962)] for laser light, and superfluidity, is related to first-order (1-body) coherence/ODLRO, while superconductivity is related to second-order coherence/ODLRO. (For fermions, such as electrons, only even orders of coherence/ODLRO are possible.) Superfluidity in liquid He4 is related to a partial Bose–Einstein condensate. Here, the condensate portion is described by a multiply-occupied single-particle state. [e.g., Cummings & Johnston (1966)]

Regarding the occurrence of quantum coherence at a macroscopic level, it is interesting to note that the classical electromagnetic field exhibits macroscopic quantum coherence. The most obvious example is the carrier signal for radio and TV. They satisfy Glauber's quantum description of coherence.

See also

Atomic coherence
Coherence length
Coherent state
Laser linewidth
Measurement in quantum mechanics
Measurement problem
Optical heterodyne detection
Quantum decoherence
Quantum Zeno effect
ReferencesEdit

Rolf G. Winter; Aephraim M. Steinberg (2008). "Coherence". AccessScience. McGraw-Hill.
M.Born; E. Wolf (1999). Principles of Optics (7th ed.). Cambridge University Press. ISBN 978-0-521-64222-4.
Loudon, Rodney (2000). The Quantum Theory of Light. Oxford University Press. ISBN 0-19-850177-3.
Leonard Mandel; Emil Wolf (1995). Optical Coherence and Quantum Optics. Cambridge University Press. ISBN 0-521-41711-2.
Arvind Marathay (1982). Elements of Optical Coherence Theory. John Wiley & Sons. ISBN 0-471-56789-2.
Peng, J.-L.; Liu, T.-A.; Shu, R.-H. (2008). "Optical frequency counter based on two mode-locked fiber laser combs". Applied Physics B 92 (4): 513. Bibcode:2008ApPhB..92..513P. doi:10.1007/s00340-008-3111-6.
Saleh, Teich. Fundamentals of Photonics. Wiley.
Hodgman, S. S.; Dall, R. G.; Manning, A. G.; Baldwin, K. G. H.; Truscott, A. G. (2011). "Direct Measurement of Long-Range Third-Order Coherence in Bose-Einstein Condensates". Science 331 (6020): 1046–1049. Bibcode:2011Sci...331.1046H. doi:10.1126/science.1198481. PMID 21350171.
Pincock, S. (25 February 2011). "Cool laser makes atoms march in time". ABC Science. ABC News Online. Retrieved 2011-03-02.
External linksEdit

"Dr. SkySkull". "Optics basics: Coherence". Skulls in the Stars.