However, in all practical cases, the laser light is not truly monochromatic. A truly monochromatic wave requires a wave train of infinite duration. The spectral emission line from which it originates does have a finite width, because of the Doppler effect of the moving atoms or molecules from which it comes. Compared to the ordinary sources of light, the range of frequency (line width) of the laser is extremely small. This range is called line width or bandwidth.
Why the laser light is monochromatic? Following are the factors responsible for making the laser beam monochromatic:
- Laser light consists of essentially one wavelength, having its origin in stimulated emission from one set of atomic energy levels. This is possible because laser transition, in principle, involves well-defined energy levels.
- EM wave of frequency n = (E2 - E1) only can be amplified, n has a certain range which is called line width. This line width is decided by various broadening factors such as Doppler effect of moving atoms and molecules.
- The generation of laser is such that the laser cavity forms a resonant system and laser oscillation is sustained only at the resonant frequencies of the cavity. This leads to the further narrowing of the laser line width. So laser light is usually very pure in wavelength, we say it has the property of monochromatic.
Monochromatic output, or high frequency stability, is of great importance for lasers being used in interferometric measurements since the wavelength is the measure of length and distance and must be known with extreme precision, at least one part in a million, and it must remain constant with time. The same holds true for lasers used in chemical and many other scientific analytical applications. Both these techniques are important in quality control and inspection. For these applications, frequency stabilized 632.8 nm HeNe laser (a frequency of approximately 473 THz) with a 1 MHz bandwidth are commercially available.
Another important laser: the Nd:YAG laser used in most laser designators, generates an output beam at 1.064 microns, with a typical bandwidth of 0.00045 microns, an amazingly narrow line width of 0.04 percent of the central wavelength. This spectrally pure output is critical for a multitude of applications, including remote sensing for specific chemical constituents and high signal-to-noise ratio (SNR) communications.
This property of monochromoticity has excellent applications in high-resolution spectroscopy to observe specific transitions in a molecule. A practical application is the separation of isotopes in the nuclear industry where the fissionable isotope of Uranium, 235U, is separated from the non-fissionable one 238U by exploiting the minute difference in their energy levels.
In the case of spontaneous emission, the emission is natural where as in the case of stimulated emission, it is induced or stimulated. Further there is no amplification in the case of spontaneous emission as well as no phase relationship between emitted photons, as it happens in the case of stimulated emission. But one has to remember that under normal conditions, there is far more atoms in the lower level than in the upper level and as such absorption dominates stimulated emission. In order to reverse this trend, there must be much more atoms in the upper level than in the lower level. This specific condition is called population inversion and is essential for stimulated emission to be in a predominant position for generation of laser. In the case of laser, the stimulated emission process is responsible for the emission of photons and amplification. Since the emitted photons have a definite phase relationship with each other, coherent output is produced. i.e. the atoms emit photons in phase with the incoming stimulating photons and emitted waves adds to the incoming waves, generating brighter output. Addition is due to the relative phase relationship. Photons of ordinary light also come from atoms without any phase relationship with each other and are not coherent. Therefore, laser is called a coherent light source where as an ordinary light is called an incoherent light source.
To sum up, the two conditions necessary for laser action are population inversion and stimulated emission. Inside a laser, the stimulated emission occurs in a resonant cavity with mirrors at both ends. Thus by repeating this process of interaction of photon with excited atoms many times, one can produce a highly coherent beam of light. Since a common stimulus triggers the emission events, which provide the amplified light, the emitted photons are "in step" and have a definite phase relation to each other. These emitted photons having a definite phase relation to each other, generates coherent output, i.e. the atoms emit photons in phase with the incoming stimulating photons and emitted waves add to the incoming waves, generating brighter output. Addition is due to the relative phase relationship. Photons of ordinary light also come from atoms, but independent of each other and without any phase relationship with each other and are not coherent. Therefore, laser is called a coherent light source where as an ordinary light is called an incoherent source of light. The concept of coherence can be well understood from the following figure.
There are two types of coherence - spatial and temporal.
Correlation between the waves at one place at different times, or along the path of a beam at a single instant, are effectively the same thing, and are called "temporal coherence". Correlation between different places (but not along the path) is called "spatial coherence".
To understand coherence, let us take two points on a wave front, at time equal to zero. There will be a certain phase difference between these two points and if it remains same even after lapse of a period of time, then the electromagnetic wave (em) has perfect coherence between the two points. In case, the phase difference remains same for any two points anywhere on the wave front, then we say that the electromagnetic wave has perfect spatial coherence, where as if this is true only for a specific area, then the electromagnetic wave is said to have only partial spatial coherence. Spatial coherence is related to directionality and uniphase wave fronts.
Now let us consider a single point on the wave front. There will be a phase difference between time, t = 0 and t = d t of the electromagnetic wave. If this phase difference remains same for any value of d t, then we say that the em wave has perfect temporal coherence. But if this is only for a specific value of d t, then the em wave has partial temporal coherence.
It may be understood that these two types of coherence are independent of each other. i.e. an em wave with partial temporal coherence can have perfect spatial coherence.
- Coherence is a property of waves that indicates the ability of the waves to interfere with each other. Two waves that are coherent can be combined to produce an unmoving distribution of constructive and destructive interference (a visible interference pattern) depending on the relative phase of the waves at their meeting point. Waves that are incoherent, when combined, produce rapidly moving areas of constructive and destructive interference and therefore do not produce a visible interference pattern.
- Another way of saying the same thing is that coherence is a measure of the ability of a light source to produce high contrast interference fringes when the light is interfered with itself in an interferometer. High coherence means high fringe visibility with excellent contrast, (i.e., good black and white fringes, or black and whatever color the light is); low coherence means washed-out fringes, and zero coherence means no fringes.
- One of the ways of understanding coherence is to predict it. Suppose one can take a snapshot of the waves, and then take another snapshot at a later time. If the two snapshots look almost identical, even with a long time interval, then we have a high degree of coherence.
- A wave can also be coherent with itself, a property known as temporal coherence. If a wave is combined with a delayed copy of itself, the duration of the delay over which it produces visible interference is known as the coherence time of the wave, Δtc. From this, a corresponding coherence length lc, can be estimated as :
lc = Δtcwhere c is the speed of the light wave.
Coherence time (Δtc) relates to the finite bandwidth of the source and in general, it is proportional to the bandwidthΔtcα1/Δnwhere Δ n is the frequency bandwidth. The temporal coherence comes from the monochromaticity of the laser beam. The narrower the line width Δλ or Δ n of the light source, the better is its temporal coherence.
- If the laser supports the oscillation of multiple longitudinal modes but no higher-order transverse modes, it means that the laser output has finite temporal coherence but perfect spatial coherence. The longitudinal modes in a laser are equally spaced in frequency by c/2L where c is the velocity of light and L is the effective length of the laser resonator. The number of longitudinal modes determines the coherence length of the laser. The relation between the coherence length lc, the longitudinal mode spacing c/2L and the number of modes N (N>!) is
N = [c/2 lc ] / c/2 L = L/ lc
- Small coherence lengths are obtained with lasers, which support oscillation over a very wide spectral bandwidth.
- There are certain gas lasers which have very long coherence length of tens of meters, while other lasers, especially Diode lasers have coherence length of the order of millimeters.
- Typically for a commercially available He-Ne laser with 632.8 nm wave-length and 0.01 nm spectral bandwidth, the coherence length is about 4cm or at the most of the order of the length of its resonator because of the presence of many longitudinal modes.
- A frequency stabilized 632.8 nm HeNe laser (a frequency of approximately 473 THz) with a 1 MHz bandwidth would have a coherence length of about 300 meters. In this case, the coherence length is much longer than the length of the cavity because only a single longitudinal is forced to be active in it at any given time.
- An 800 nm laser diode with a 1 nm spectral width would have a coherence length of about 0.64 mm.
- A 600 nm LED with a spectral width of 60 nm would have a coherence length of around 6 μm.
- He-Ne lasers are also much more spatially coherent than LEDs. LEDs generally have a very short spatial coherence length, typically only a couple of wavelengths.
- In holography, the temporal coherence length determines the maximum depth of the object in a reflection hologram, and the spatial coherence length determines the lateral size. Holography, which is based on interference between light beams, long coherence length enables taking holograms of large bodies, which require greater depth of field. Both the light reflected from the near part of the body, and the light reflected from the far part of the body, will still be coherent with the reference beam.
- Spatial coherence refers to how spherical the wave front is. Does every portion of the wave front appear to have exactly the same center of curvature?
- The requirement for high temporal coherence is in coherent, or heterodyne detection. In these systems, energy reflected off the target is mixed with energy from the original laser to create a fringe pattern. The photons are supposed to maintain the fixed phase relationship for the time needed to hit the target and return in order to have proper contrast in the fringe pattern.
- One of the important applications includes Doppler velocity measurements of the target through the measurements of the frequency shift because of the moving targets. The frequency shift from the target-reflected energy is a function of the target velocity. However, if the frequency of the laser itself is shifting (because of poor coherence) during the time of flight, this creates a broadening or an error in the frequency of the returned beam that limits how accurately one can measure the Doppler velocity.
- Spatial coherence is high for sphere waves and plane waves, and is related to the size of the light source. A point source emits spatially coherent light, while the light from a finite source has lower coherence. Spatial coherence can be increased with a spatial filter; a very small pinhole preceded by a condenser lens. The spatial coherence of light will increase as it travels away from the source and becomes more like a sphere or plane wave. Light from distant stars, though far from monochromatic, has extremely high spatial coherence.
- Coherence length is defined as the length over which energy in two separate waves remains constant. With respect to the laser, it is the greatest distance between two arms of an interferometric system for which sufficient interferometric effects can be observed.
- Using Michelson Interferometer, one can estimate the coherence length by measuring the maximum path difference between the two beams, which still show the interference pattern.
- Since the temporal coherence is a measure of the ability of the radiation to perform interference, as a result of differences in path lengths between the two beams, it is thus important in interferometry and holography.
- Ordinary light is not coherent because it comes from independent atoms, which emit on time scales of about 10-8 seconds. There is a degree of coherence in sources like the mercury green line and some other useful spectral sources, but their coherence does not approach that of a laser.
The high degree of collimation arises from the fact that the cavity of the laser has very nearly parallel front and back mirrors, which constrain the final laser beam to a path, which is perpendicular to those mirrors. Collimation refers to the degree to which the beam remains parallel with distance. A perfectly collimated beam would have parallel sides and would never expand at all. Its divergence angle would be exactly zero. Diffraction plays an important role in determining the size of laser spot that can be projected at a given distance. The oscillation of the beam in the resonator cavity produces a narrow beam that subsequently diverges at some angle depending on the resonator design, the size of the output aperture, and resulting diffraction effects on the beam. These diffraction effects usually referred as a beam-spreading effect are a result of the light waves passing through a small opening. These diffraction phenomena impose a limit on the minimum diameter of a light point after passing through an optical system. For a laser, the beam emerging from the output mirror can be thought of as the opening or aperture, and the diffraction effects on the beam by the mirror will limit the minimum divergence and spot size of the beam. For beams in TEM 00 mode, diffraction is usually the limiting factor in beam divergence.
In fact one can say that, divergence angle describes the directionality of the laser. For a perfect spatially coherent laser beam, the diffraction limited divergence angle θ is given by,
Using the above equations, and assuming K or 2 x (1 / π) as unity, let us calculate the minimum divergence (full angle) that can be theoretically achievable for the most well known lasers, i.e. Nd:YAG (l = 1.06 mm with 3mm diameter) and He-Ne laser (l = 0.6328 mm with 1mm beam diameter). The divergence angles are 0.353 milli-rad or 0.02014° and 0.6328 mrad or 0.03607° respectively. Compare this with the divergence of the light from a torchlight (20° or more) and the high directionality of laser beams becomes quite obvious. As the spatial coherence becomes partial or the degree of coherence reduces, the divergence increases accordingly and for calculating the divergence, the diameter of the beam D is to be replaced with the coherence area in the above-mentioned equation.
Consider the size (diameter) of a collimated beam as it propagates as shown in the figure. It can be seen that the diameter increases. This increase of the beam size is due to the beam divergence and the same is measured in milliradians (mrad). It is either measured as full angle (measure of increase in diameter) or as half angle (measure of increase in radius). For example, the diameter of a beam of 1mrad full angle divergence, after propagation of 1Km, will be 1m (Physical optics). For small angle, the divergence can be approximated as the ratio of the beam diameter to the distance from the laser aperture.
The solid angle corresponding to one millirad is:
B = 10-3W/(0.785 x 10-6)(0.8 x 10-6) = 1.6 x 109 Watts/m2-steradian The radiance of a milliwatt helium neon laser is far greater than 106 Watts/m2-steradian, that of the sun which emits more than 1026 W.
This is a unique advantage for many of the laser applications in various areas.
The integer N cannot be an arbitrary number. It is limited by the fluorescence curve and only the modes for which the gain of laser of the laser medium G (λ) > 1 would be supported.
The above equation can be rewritten as:
Assuming a cavity of length 50 cm, it gives us the possible number of modes as 159 x 104 and the separation between two modes as 300 MHz. However, if the laser bandwidth is of the order of 2.5 GHz, it can support only 6 longitudinal modes.
Some important points related to longitudinal modes:
- Modes governed by the axial dimensions of the resonant cavity are called longitudinal modes. The longitudinal modes are formed when the two waves with the same frequency and amplitude are moving against each other.
- The to and fro movement of the electromagnetic radiation is controlled by the laser cavity end mirrors and only the waves with nodes at both ends are sustained or allowed, which means that the cavity length should be an integral multiple of the half wavelengths. Thus the cavity length and the refractive index of the laser medium determine the frequencies that are allowed inside the cavity.
- The other important aspect is that the frequencies are spaced at equal intervals.
- In applications where power is more important as in most high power applications for material processing or medical surgery, multimode lasers can be used. As such the laser is being used as a mean for transferring the energy on to the target. Thus there is not much importance for the longitudinal laser modes. However, for applications involving interference such as holography or interferrometric measurements, and in applications related to spectroscopic and photochemical, where single well-defined wavelength is required, single mode lasers are very critical.
- Increase in the cavity length increases the number of possible laser modes under the fluorescence curve. However, it reduces the frequency gap between the adjacent modes. This leads to that a single mode laser can be made by reducing the length of the cavity, such that only one longitudinal mode will remain under the fluorescence curve with GL>1.
- The multiple longitudinal mode structure gives rise to a power fluctuation phenomenon termed mode sweeping. All unstabilized helium neon lasers exhibit this effect, which is due to thermal instability causing variation in the cavity length. As the cavity length changes, there is a small change in mode spacing which is typically 10 kHz or less under normal conditions.
As explained earlier that the amplitude of a light beam is increased in a laser by multiple passes of coherent light waves through the active medium. The process is accomplished by an active medium placed between a pair of mirrors that act as a feedback mechanism. During each round trip between the mirrors, the light waves are amplified by the active medium and reduced by internal losses and laser output. A number of different combinations of mirrors, such as plane and curved, have been utilized in practical laser. Some of them are shown in the figure.
Most common form of structure is a stable resonator, which concentrate light along the laser axis, extracting energy efficiently from that region, but not from the outer regions far from the axis. This cavity will then have a set of nearly loss less resonant modes, which will have the form of very nearly perfect Hermite-gaussian or Laguerre-gaussian mathematical functions. The lowest-order mode will have an essentially ideal gaussian profile with a certain spot size, which depends only on the spacing and radii of the mirrors and the wavelength of the light and not on the mirror diameter, which is assumed to be very large typically four to five times of the beam size. This spot size, called the "gaussian spot size" and can be estimated by a simple formula in terms of the cavity length L, the end mirror radii r1 and r2, and the wavelength. The beam thus it produces has an intensity peak in the center, and a Gaussian drop in intensity with increasing distance from the axis. The fundamental TEM00 mode is only one of many transverse modes that satisfy the round-trip propagation criteria.
For most applications for example like holography, the TEM00 mode is considered most desirable, but multi-mode beams can often deliver more power, though with a poorer beam quality, and may be acceptable in applications where power is the main criterion.
The laser can be forced to lase in a single TEM00 mode by simply putting a pinhole with proper diameter between the two mirrors. The pinhole diameter should be equal to the diameter of the lower mode as this would allow only this mode to pass through the pinhole, and all higher modes will be attenuated. Since radiation inside the optical cavity undergoes multiple passes, only the basic mode will be amplified, and appear in the output.
It was thus felt that to recognize, quantify and determine the beam propagation characteristics, a figure of merit would be very necessary and useful. Therefore, the concept of a dimensionless beam propagation parameter, M2 was developed in 1970 for all types of lasers. M2 is a quantitative measure of the quality of the laser beam and according to ISO standard 11146, it is defined as the beam parameter product (BPP) divided by λ / π. The beam divergence, as discussed earlier, is
(Beam parameter product (BPP) is the product of a laser beam's divergence angle and the diameter of the beam at its narrowest point (the "beam waist"). Its units are mm mrad.
M2 can also be defined in the following manner:
- The ratio of the BPP of an actual beam to that of an ideal Gaussian beam at the same wavelength. This parameter is a wavelength-independent measure of beam quality.
- It is the ratio of the divergence of the real beam to that of a theoretical diffraction-limited beam of the same waist size with a Gaussian beam profile.
For any laser beam, the product of the beam radius (w0R) and the far-field divergence (θ) is a constant, and the ratio,
There are some other important points related to M2:
- The value of M2 is always greater than or equal to 1 and ranges from 1 for a diffraction-limited TEM00 laser beam, to several hundred for a distorted, poor quality beam.
- M2 = 1 only occurs for single-mode TEM00 Gaussian beams.
- Helium neon lasers typically have an M2 factor that is less than 1.1. For ion lasers, the M2 factor is typically between 1.1 and 1.3. Collimated TEM00 diode laser beams usually have an M2 ranging from 1.1 to 1.7. For high-energy multimode lasers, the M2 factor can be as high as 3 or 4. In all cases, the M2 factor, which varies significantly, affects the characteristics of a laser beam and cannot be neglected in optical designs.
- Though carefully/optimally designed lasers can achieve the M2 ~ 1, high power lasers have very much higher M2 value of 100 or even more. Thermal distortions in the active gain media, use of poor optical quality of components, diffraction effects at apertures etc. are the main reasons for the reduction of beam quality. Beam quality also gets affected adversely when the lasers work at higher cavity modes. A pump source with uniform intensity distribution, an optimized cavity design with least sensitivity to thermal lensing, high optical quality components and a gain medium least prone to thermal disturbance are a pre-requisite to a laser system for the generation of high beam quality output.
- John Ambroseo : Coherent Laser - THE PHOTONICS DESIGN AND APPLICATIONS HANDBOOK 2001
- LEOT tutorial on lasers
- Coherence-free definition
- Definition of a laser
- Melles Griot - FAQ's
- Melles Griot Guide Book, The Practical Applications of Light, Tutorial on Laser Beam Measurement
- Molecular expressions Microscopy Primer Physics of light and color - Introduction to lasers
- Quantum properties of light
- Rami Arieli The Laser Adventure
- Sam's Laser FAQ - Items of Interest
- WHY LASERS
- Encyclopedia of Laser Physics and Technology
- E. Siegman, New developments in laser resonators, Proc. SPIE, 1224, 2 (1990)
- E. Siegman, Defining, measuring and optimizing laser beam quality, Proc.SPIE, 1868, 2 (1993)
- ISO standard 11146