Good coupling efficiency requires precise positioning of the fiber to center the core in the focused laser beam. For multimode fibers, with their large cores, optical fiber positioners can achieve good coupling efficiency. Single-mode fibers require more elaborate couplers with submicron positioning resolution, like the ULTRAlign and 562F stainless steel positioners F-915 and F-1015 fiber optic couplers. These are also useful with Multi-mode fibers when maximum coupling efficiency is required.

SC — the SC connector is becoming increasingly popular in single-mode fiber optic telecom and analog CATV, field deployed links. The high-precision, ceramic ferrule construction is optimal for aligning single-mode optical fibers. The connectors' outer square profile combined with its push-pull coupling mechanism, allow for greater connector packaging density in instruments and patch panels. The keyed outer body prevents rotational sensitivity and fiber endface damage. Multimode versions of this connector are also available. The typical insertion loss of the SC connector is around 0.3 dB.

Moreover, Landsberg and Mandelstam did not at first publish their results of scattering at a shifted frequency; instead they gave an oral presentation at a conference in Moscow in April 1928 based on their measurements, which were taken in February of that year. By the time they submitted their results in May 1928 and published them in July, 16 papers had been published on the Raman effect, many by Raman and his colleagues.

In his explanation of the new phenomenon, Raman showed that the frequency shift is a characteristic of the molecule comprising the scattering medium; it is independent of the frequency of the incident light. This differentiated the Raman effect from fluorescence, which strongly depends on the frequency of the incident light. There are notable exceptions, of course: Brillouin scattering and Raman scattering coupled to acoustic waves in a condensed medium (acoustic-optical effects in crystals). Raman spectra of molecules differ from infrared spectra in their selection rules and their polarization characteristics; however, the measured frequency shifts of the Raman lines correspond to the infrared frequencies of the scattering molecules.

In fact, some of the Nobel nominations for the 1930 award included other scientists in recognition of the Raman effect. One nomination went jointly to Raman and Heisenberg, who further developed Smekal’s concepts and contributed to a quantum theory of dispersion by atoms. Two others recognized Raman and R.W. Wood, the American scientist who confirmed Raman’s experiments. Another was for Raman, Landsberg and Mandelstam.

In their 1925 paper, Kramers and Heisenberg used the Bohr correspondence principle and extended Smekal’s previous work on incoherent scattering. They stated the possibility of the converse process: An atom in an excited state collides with a photon, and, following the collision, the atom shifts to the lower energy state, while the scattered photon is shifted in frequency to higher energy; i.e., a red incident light is scattered as a blue light. The scientists postulated that irradiating an atom with monochromatic light results in the atom radiating coherent spherical waves (Rayleigh scattering) and also incoherent spherical waves (Raman scattering) whose frequencies are combinations of the incident frequency and frequencies that correspond to possible transitions to other stationary states.

Optical fibers are circular dielectric wave-guides that can transport optical energy and information. They have a central core surrounded by a concentric cladding with slightly lower (by ≈ 1%) refractive index. Optical fibers are typically made of silica with index-modifying dopants such as GeO2. A protective coating of one or two layers of cushioning material (such as acrylate) is used to reduce cross talk between adjacent fibers and the loss-increasing microbending that occurs when fibers are pressed against rough surfaces.

Scattering can couple energy from guided to radiation modes, causing loss of energy from the fiber. There are unavoidable Rayleigh scattering losses from small-scale index fluctuations frozen into the fiber when it solidifies. This produces attenuation proportional to l/λ4. Irregularities in core diameter and geometry or changes in fiber axis direction also cause scattering. Any process that imposes dimensional irregularities — such as microbending — increases scattering and hence attenuation.

Light scattering was a popular research area in physics laboratories worldwide in the 1920s. The topic was under investigation by Lord Rayleigh in England, Jean Cabannes in Paris, Robert W. Wood in New York, and Grigory Landsberg and Leonid Mandelstam in the Institute of Physics in Moscow.

Once the optical fiber is terminated with a particular connector, the connector endface preparation will determine what the connector return loss, also known as back reflection, will be. The back reflection is the ratio between the light propagating through the connector in the forward direction and the light reflected back into the light source by the connector surface. Minimizing back reflection is of great importance in high-speed and analog fiber optic links, utilizing narrow line width sources such as DFB lasers, which are prone to mode hopping and fluctuations in their output.

At the age of 60, Raman then formed the Raman Research Institute (supported with his own funds and donations that he raised). He also remained a professor, as well as the President of the Indian Academy of Sciences in Bangalore, until his death in 1970.

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Raman used a small Adam Hilger spectroscope for his initial studies, and he detected the spectrum of the scattered light using photography. Since the intensity of the frequency-shifted light was extremely weak, long exposure times were required to record the spectra.

The outer sheath of fiber cables can be removed using electrical cable stripping tools, and scissors or a razor blade can trim the Kevlar strength member. However, the fiber coating must be very carefully removed to avoid damaging the fiber — surface flaws and scratches are the cause of most fiber failures. The coating can be removed using our Fiber Optic Strippers.

For greater environmental protection, fibers are commonly incorporated into cables. Typical cables have a polyethylene sheath that encases the fiber within a strength member such as steel or Kevlar strands.

Barry R. Masters OSA Fellow, SPIE Fellow, is with the department of biological engineering, MIT, Cambridge, Mass., U.S.A.

The Austrian physicist Adolf Smekal provided the theoretical basis for inelastic light scattering in 1923. This type of scattering was also implied in the dispersion theory of Hendrik A. Kramers and Werner Heisenberg (1925). Smekal proposed that photons could be scattered inelastically by vibrational transitions of molecules (Die Naturwissenschaften, 11, 1923). He assumed the quantum nature of light, used Bohr’s Correspondence Principle, and predicted that the scattered monochromatic light would consist of the original wavelength together with longer and shorter wavelengths. Smekal showed that the shift in frequency between the incident and scattered light corresponds to the energy difference between two states of the molecule.

Bandwidth of an optical fiber determines the data rate. The mechanism that limits a fiber's bandwidth is known as dispersion. Dispersion is the spreading of the optical pulses as they travel down the fiber. The result is that pulses then begin to spread into one another and the symbols become indistinguishable. There are two main categories of dispersion, intermodal and intramodal.

As for Landsberg and Mandelstam, they had published their results after Raman’s were in print. In addition, their paper cited previous works by Raman; although these corresponded to articles that had been published prior to Raman’s March 1928 Nature article detailing his discovery, these references perhaps confused the Nobel Committee and led them to believe that the Russians’ work did not represent an independent and simultaneous discovery.

In 1929, the Faraday Society of London held a special symposium dedicated to the Raman effect. That same year, Raman was knighted by the British government in India. The following year, he was given the Hughes Medal by the Royal Society. Also in 1930, Raman received the Nobel Prize in Physics for his “‘investigations on the scattering of light and the effect named after him.”

By 1925, Raman had observed the frequency-shifted scattered light in more than 50 liquids and, by 1927, he had noticed that the scattered light was polarized. He described the phenomenon—which he called modified scattering—in a paper in Nature. Later it would be called the Raman effect.

The index of refraction varies depending upon wavelength. Therefore, different wavelengths will travel down an optical fiber at different velocities. This is known as Chromatic Dispersion.

Fiber optictechnology

But it was his trip home that would lead Raman to change history. During his sea voyage, he observed the blue opalescence of the Mediterranean and wondered about the origin of this beautiful phenomenon. Raman was aware of Lord Rayleigh’s explanation—that the color of the sea was due to the reflection of the blue sky—but he did not accept it. So, with a polarizing Nicol quartz prism that he carried in his pocket, he proceeded to demonstrate Rayleigh’s explanation to be false; he quenched the surface reflection of the sky on the sea surface, and noted that the blue color of the sea was unattenuated. With a diffraction grating, he showed that the maximum spectral intensity was different for the blue sky and the blue sea.

The main challenge Raman faced in his experimental work was posed by the extremely weak intensity of the scattered light. In his early studies, Raman used a heliostat—a mechanically driven mirror that tracked the motion of the sun to provide a light source. Eventually, however, he came to realize that the sunlight was not sufficiently intense on its own. Thus, in 1927, he acquired a 7-in. refracting telescope, which he used in combination with a short-focus lens to condense the sunlight into a narrow beam.

SPC and UPC Polish — in the Super PC (SPC) and Ultra PC (UPC) polish, an extended polishing cycle enhances the surface quality of the connector, resulting in back reflections of -40 to -55 dB and < -55dB, respectively. These polish types are used in high-speed, digital fiber optic transmission systems.

Fiber opticinternet

Qualitatively, NA is a measure of the light gathering ability of a fiber. It also indicates how easy it is to couple light into a fiber.

There are two broad classifications of modes: radiation modes and guided modes. Radiation modes carry energy out of the core; the energy is quickly dissipated. Guided modes are confined to the core, and propagate energy along the fiber, transporting information and power. If the fiber core is large enough, it can support many simultaneous guided modes. Each guided mode has its own distinct velocity and can be further decomposed into orthogonal linearly polarized components. Any field distribution within the fiber can be expressed as a combination of the modes. The two lowest-order guided modes of a circularly symmetrical fiber — designated LP01 and LP11 — are illustrated in Figure 1.

In the near infrared and visible regions, the small absorption losses of pure silica are due to tails of absorption bands in the far infrared and ultraviolet. Impurities — notably water in the form of hydroxyl ions — are much more dominant causes of absorption in commercial fibers. Recent improvements in fiber purity have reduced attenuation losses. State-of-the-art systems can have attenuation on the order of 0.1 dB/km.

Many multimode fiber experiments are sensitive to the distribution of power among the fiber's modes. This is determined by the launching optics, fiber perturbations, and the fiber's length. Mode scrambling is a technique that distributes the optical power in a fiber among all the guided modes. Mode filtering simulates the effects of kilometer lengths of fiber by attenuating higher-order fiber modes.

One scrambling technique is to splice a length of graded-index fiber between two pieces of step-index fiber — this ensures that the downstream fiber's core is overfilled regardless of launch conditions. Mode filtering can be achieved by wrapping a fiber several times around a finger-sized mandrel; bending sheds the high-order modes.

Some light is invariably launched into a fiber's cladding. Though cladding modes dissipate rapidly with fiber length, they can interfere with measurements. For example, the output of a single-mode fiber will not have a Gaussian distribution if light is propagating in the cladding. You can remove cladding modes by stripping a length of fiber coating and immersing the bare fiber in an index matching fluid such as glycerin.

Still, many Austrian, German and Russian physicists felt strongly that credit should be shared. They refused to adopt the name “the Raman effect,” and referred instead to “combination scattering” or “the Smekal-Mandelstam-Raman scattering.” In 1931, K.W.F. Kohlrausch, an Austrian physicist, gave his book a title that recognized both Smekal and Raman: Der Smekal-Raman Effekt.

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Kramers and Heisenberg further developed Smekal’s concepts and published their quantum theory of the dispersion by atoms in 1925. They showed that the frequency-shifted light was incoherent, and they introduced the concept of a “virtual state.” Later, they realized that Smekal’s note contained an important concept: The Raman effect corresponded to a transition between two discrete levels and all forms of excitation. Subsequently, many types of Raman effects were observed in solids.

Following in the footsteps of Albert Einstein and Marian Smoluchowski, Mandelstam developed a theory of light scattering at an interface that varied by fluctuations. At the same time, Peter Debye published his theory about the specific heats of solids using concepts about propagating elastic waves in solids. By the time Mandelstam made the connection between the Fourier components in his theory and Debye’s elastic waves, it was too late; Leon Brillouin, working independently in France, had already published a theoretical paper explaining that scattered light could be shifted in frequency.

By 1917, Raman had had enough of his double life. He quit his government position and devoted himself fully to science. He accepted a full-time professorship—the endowed Pailt Chair of Physics—at Calcutta University, where he remained for 15 years.

Fiber fiber opticcable

End surface quality is one of the most important factors affecting fiber connector and splice losses. Quality endfaces can be obtained by polishing or by using a fiber cleaver. Polishing is employed in connector terminations when the fiber is secured in a ferrule by epoxy. The following describes the popular connectors and their endface preparation styles.

Who inventedfiberoptics

One way to achieve both scrambling and filtering is to introduce microbending to cause rapid coupling between all fiber modes and attenuation of high-order modes. One approach is to place a stripped section of fiber in a box filled with lead shot. A more precise way is to use Newport'. FM-1 Mode Scrambler. This specially designed tool uses a calibrated mechanism to introduce microbending for mode scrambling and filtering.

In 1933, Raman became director and professor at the Indian Institute of Science (IIS) at Bangalore. The next year, he established the Indian Academy of Sciences. Over the following decade, he published more than 30 papers in the Proceedings of the Indian Association for the Cultivation of Science, Nature, Philosophical Magazine and Physical Review. In 1937, he quit his position following disputes with some staff and members of the Council of the IIS.

Optical fibre diagram

Back home in India, Raman and his students observed the frequency shift of scattered light. They knew the phenomena was not Rayleigh scattering, since that type of scattering did not produce a frequency shift; however, they needed to exclude the possibility that minute traces of fluorescence were causing the shift. To do this, they purified the liquids multiple times. When the phenomenon remained, they concluded that it was not due to fluorescence.

This principle implies that a pulse with a wider FWHM will spread more than a pulse with a narrower FWHM. Dispersion limits both the bandwidth and the distance that information can be supported. This is why for long communications links it is desirable to use a laser with a very narrow line width. Distributed Feedback (DFB) lasers are popular for communications because they have a single longitudinal mode with a very narrow line width.

Raman was both a prolific investigator and a skilled communicator. By the late 1920s, he was achieving recognition for his work on the Raman effect—due in part to his tireless efforts to demonstrate and distribute his results. After his first publication of the Raman spectra in the March 16, 1928, Indian Journal of Physics, Raman mailed 2,000 reprints to scientists in the United States, Canada, France, Germany and Russia. In this way, Raman consolidated his priority and credit for the discovery. Shortly afterwards, the Raman effect was confirmed by some of the world’s most authoritative physicists in the field of light scattering and optics in France, Canada, Germany, the United States and Italy.

The Numerical Aperture (NA) of a fiber is defined as the sine of the largest angle an incident ray can have for total internal reflectance in the core. Rays launched outside the angle specified by a fiber's NA will excite radiation modes of the fiber. A higher core index, with respect to the cladding, means larger NA. However, increasing NA causes higher scattering loss from greater concentrations of dopant. A fiber's NA can be determined by measuring the divergence angle of the light cone it emits when all its modes are excited.

Chandrasekhara Venkata Raman was born in 1888 in a village in southern India. As a child, Raman was precocious, curious and highly intelligent. His father was a college lecturer in mathematics, physics and physical geography, so the young Raman had immediate access to a wealth of scientific volumes. By the age of 13, he had read Helmholtz’s Popular Lectures on Scientific Subjects.

In the much weaker processes of Raman and Brillouin scattering, however, the internal energy of the scatterer changes. Brillouin scattering refers to the transfer of energy to acoustic modes of vibration in the material; it differs from the optical modes that are excited in Raman scattering. These are inelastic scattering processes, in which the photon’s energy and frequency are changed. A photon is absorbed, raising the molecule to a higher energy state; then a photon is emitted and the molecule moves into a different energy state (vibrational or rotational) from the initial one.

Raman used a violet filter to isolate a band of violet light incident on a sample liquid. At 90 degrees to the incident light, he placed another violet glass filter. This enabled him to observe violet light scattered from the sample, which represented normal Rayleigh scattering. When he replaced the second filter with a green one, however, the Rayleigh-scattered light was blocked but there was still some green light visible, demonstrating a second form of scattering.

Let’s take a step back and explore elastic scattering. If the scattered photon has the same energy as the incident one, but a different direction of propagation, the result is elastic scattering. Examples include Rayleigh scattering (with particles much smaller than wavelength of light) or Mie scattering (particles of a size similar to the wavelength of light). In Rayleigh scattering, energy is conserved, and its intensity is proportional to the fourth power of the incident frequency. The oscillating electric field induces dipoles in the material that radiate the light; this occurs in the plane perpendicular to the dipole and also perpendicular to the electric field vector of the incident light.

In a culturally anomolous and brazen act, when Raman was 18, he arranged his own marriage to Lokasundari (later called Lady Raman), a 13-year-old woman from Madras. The two then moved to Calcutta, where Raman accepted a position in the Indian Finance Department. During the next ten years—from 1907 to 1917—he struggled to balance his well-paying government job with his drive to be a scientist.

The mode field diameter is now given to provide easier matching of lens to optical fiber for a Gaussian beam. A high numerical aperture lens must collimate the diverging output beam of a laser diode. Newport's F-L Series Diode Laser Focusing Lenses, are AR-coated for high transmittance at popular laser diode wavelengths and — with numerical apertures up to 0.5 — are useful for collimating or focusing.

PC Polish — the Physical Contact (PC) polish results in a slightly curved connector surface, forcing the fiber ends of mating connector pairs into physical contact with each other. This eliminates the fiber-to-air interface, there by resulting in back reflections of -30 to -40 dB. The PC polish is the most popular connector endface preparation, used in most applications.

Intramodal Dispersion, sometimes called material dispersion, is a result of material properties of optical fiber and applies to both single-mode and multimode fibers. There are two distinct types of intramodal dispersion: chromatic dispersion and polarization-mode dispersion. As its name implies, intermodal dispersion is a phenomenon between different modes in an optical fiber. Therefore this category of dispersion only applies to mulitmode fiber. Since all the different propagating modes have different group velocities, the time it takes each mode to travel a fixed distance is also different. Therefore as an optical pulse travels down a multimode fiber, the pulses begin to spread, until they eventually spread into one another. This effect limits both the bandwidth of multimode fiber as well as the distance it can transport data.

When light is launched into a fiber, the modes are excited to varying degrees depending on the conditions of the launch — input cone angle, spot size, axial centration and the like. The distribution of energy among the modes evolves with distance as energy is exchanged between them. In particular, energy can be coupled from guided to radiation modes by perturbations such as microbending and twisting of the fiber — increasing the attenuation.

Perhaps most interestingly, Raman used his own dark-adapted eyes as photodetectors. Only after he had observed the frequency shift with his eyes and a direct-vision spectroscope did he repeat the observation with a mercury arc lamp and a Hilger baby quartz spectrograph. Surprising as it may seem, the human eye can detect single photons over a high dynamic range.

For our F-SV fiber, for which V = 2, the Gaussian width is approximately 28% larger than the core diameter, so the light should be focused to a spot size 1.28 times the core diameter at the fiber surface. For a Gaussian laser beam, the required beam diameter D incident upon focusing lens of focal length f to produce a focused spot of diameter w is D = 4λf/( πw). Given the laser beam waist and divergence, it's easy to determine the distance needed between the focusing lens and the laser to expand the beam to the required diameter.

The Stokes and the anti-Stokes lines are equally displaced from the Rayleigh line; the Stokes line has the higher intensity. In condensed matter, Raman scattering is described quantum mechanically by the exchange of a phonon (quanta of mechanical energy) between a photon and the non-propagating modes of excitation of the condensed matter.

SMA — due to its stainless steel structure and low-precision threaded fiber locking mechanism, this connector is used mainly in applications requiring the coupling of high-power laser beams into large-core multimode fibers. Typical applications include laser beam delivery systems in medical, bio-medical, and industrial applications. The typical insertion loss of an SMA connector is greater than 1 dB.

One of the requirements of that position was to obtain training abroad in order to achieve parity with foreign professionals. Confident in his genius, Raman claimed that he did not need any foreign training; on the contrary, he was prepared to train those from other countries. Moreover, he argued, he had already earned a prestigious international reputation in physics due to his publications. Since Raman refused to budge, the University had no choice but to waive this requirement in order to secure the rising star. In 1924, Raman was elected a Fellow of the Royal Society. It is as if he knew he was destined for greatness. Indeed, in 1925, when Raman was attempting to obtain funds to purchase a spectroscope, he told his benefactor: “If I have it, I think I can get a Nobel Prize for India.”

Fiber Cleaving is the fastest way to achieve a mirror-flat fiber end — it takes only seconds. The basic principle involves placing the fiber under tension, scribing with a diamond or carbide blade perpendicular to the axis, and then pulling the fiber apart to produce a clean break. Our F-BK3 and FK11 fiber optic cleavers make the process especially quick and easy. It is wise to inspect fiber ends after polishing or cleaving.

The characteristics of the focused beam must match the fiber parameters for good coupling efficiency. For multimode fibers this is straightforward. General guidelines are:

Port Configuration: Number of input ports x number of output ports. e.g. 2 x 2 Coupling Ratio: The ratio of the power at an output port to the launched power expressed in dB. e.g. -10log (P2/P1). Isolation: The ratio of the power at an output port in the transmitted wavelength band to that in the extinguished wavelength band, expressed in dB. Directivity: The ratio of the power returned to any other input port to the launched power, expressed in dB. e.g.-10log (P4/P1). Bandwidth: The range of operating wavelengths over which performance parameters are specified. Excess Loss: The ratio of the total power at all output ports to the launched power, expressed in dB. e.g. -10log [(P2+P3)/P1]. Uniformity: The difference between maximum and minimum insertion losses. Extinction Ratio: The ratio of the residual power in an extinguished polarization state to the transmitted power, expressed in dB. Return Loss: The ratio of the power returned to the input port to the launched power, expressed in dB. e.g.-10log (P5/P1). Polarization-Dependent Loss (PDL): The maximum (peak-to-peak) variation in insertion loss as the input polarization varies, expressed in dB.

Meanwhile, Landsberg and Mandelstam were studying the theories of the specific heats of solids and the published works of Einstein and Debye. They investigated Brillouin scattering from large samples of quartz. Their light source was a mercury arc lamp with a filter to narrow the bandwidth of the excitation light. They placed a spectrograph at 90 degrees to the incident light. While the scattering effect from liquids was strong, the similar effect from quartz was extremely weak and necessitated a 15 hour exposure time. These Russian physicists independently rediscovered the Raman effect in crystalline quartz and calcite; their work was published in 1928.

Uses of optical fibre Physics

But the Nobel Committee decided the award should go to Raman alone, and the rest is history. Raman is the only India Nobel laureate whose award in physics was based on work completed in India. He was a great man known for his driving ambition and passion for science. A few days before his death on November 21, 1970, Raman spoke these words, “Science can only flower out when there is an internal urge. It cannot thrive under external pressure.” A tree grows where Raman died.

APC Polish — the Angled PC (APC) polish, adds an 8 degree angle to the connector endface. Back reflections of <-60 dB can routinely be accomplished with this polish.

FC — the FC has become the connector of choice for single-mode fibers and is mainly used in fiber-optic instruments, SM fiber optic components, and in high-speed fiber optic communication links. This high-precision, ceramic ferrule connector is equipped with an anti-rotation key, reducing fiber endface damage and rotational alignment sensitivity of the fiber. The key is also used for repeatable alignment of fibers in the optimal, minimal-loss position. Multimode versions of this connector are also available. The typical insertion loss of the FC connector is around 0.3 dB. Drilled-out, metallic FC connectors, having insertion losses of >1 dB, are being used with Newport's large-core (>140 µm) fibers.

The Normalized Frequency Parameter of a fiber, also called the V number, is a useful specification. Many fiber parameters can be expressed in terms of V, such as: the number of modes at a given wavelength, mode cut off conditions, and propagation constants. For example, the number of guided modes in a step index multimode fiber is given by V2/2, and a step index fiber becomes single-mode for a given wavelength when V<2.405. Mathematically, V=2 π·NA·a/λ where “a” is the fiber core radius.

ST — the ST connector is used extensively both in the field and in indoor fiber optic LAN applications. Its high-precision, ceramic ferrule allows its use with both multimode and single-mode fibers. The bayonet style, keyed coupling mechanism featuring push and turn locking of the connector, prevents over tightening and damaging of the fiber end. The insertion loss of the ST connector is less than 0.5 dB, with typical values of 0.3 dB being routinely achieved. Drilled-out, metallic ST connectors, with insertion losses of >1 dB, are used with Newport's large-core (>140 µm) fibers.

What is optical fibre in Physics

Raman was deeply interested in music and acoustics. While in college, he read the scientific papers of Lord Rayleigh and his treatise on sound as well as the English translation of Helmholtz’s The Sensations of Tone. This initiated Raman’s later interest in the physics of drums and stringed instruments such as the violin. He used fine-chalk powder and photography to investigate the vibrational nodes of drums; the white chalk remained only at the nodes of the vibrating membrane.

How dofiberoptics transmit data

In 1921, Raman had traveled to Europe from his home in Calcutta to attend the Congress of Universities of the British Empire at Oxford. While he was there, he conducted some acoustic research on the central gallery of St. Paul’s Cathedral in London. He also met with three outstanding British physicists: Joseph J. Thompson, Ernest Rutherford and William H. Bragg. He lectured to the Physical Society on his research in acoustics and optics.

First, Smekal’s work was not widely known at the time that Raman had conducted his scattering experiments. A letter summarizing Smekal’s findings was published in Die Naturwissenschaften, but it was not abstracted and most likely had not been seen by Raman and his colleagues.

Light power propagating in a fiber decays exponentially with length due to absorption and scattering losses. Attenuation is the single most important factor determining the cost of fiber optic telecommunication systems, as it determines spacing of repeaters needed to maintain acceptable signal levels.

Not everyone agreed that Raman deserved full credit for discovering the Raman effect. After all, Smekal had provided the theoretical basis for light scattering in 1923, and Landsberg and Mandelstam had simultaneously discovered the Raman effect on solid quartz in 1928. Why was the Nobel given only to Raman?

The following year, he created an even more powerful light source by using highly monochromatic light from a mercury arc lamp together with a large-aperture condenser and cobalt-glass filter. Sometimes he replaced the glass filters with liquid ones.

To maximize coupling into a single-mode fiber, you must match the incident field distribution to that of the fiber mode. For example, the mode profile of the HE11 mode of a step index fiber can be approximated by a Gaussian distribution with a 1/e width w given by:

When he wasn’t at the Finance Department, he was conducting experiments at the Indian Association for the Cultivation of Sciences (IACS) in Calcutta. The IACS had been formed along the pattern of the Royal Institution in London. Its journal Proceedings was renamed the Indian Journal of Physics in 1926. Raman’s early works become known to an international audience when he published his research in the journals Nature, Philosophical Magazine and the Physical Review.

Polarization Mode Dispersion (PMD) is actually another form of material dispersion. Single-mode fiber supports a mode, which consists of two orthogonal polarization modes. Ideally, the core of an optical fiber is perfectly circular. However, the fact that in reality, the core is not perfectly circular, and mechanical stresses such as bending introduce birefringency in the fiber, causes one of the orthogonal polarization-modes to travel faster than the other, hence causing dispersion of the optical pulse.

During this voyage, Raman sent two papers to the journal Nature positing that the color of the sea was due to light scattering by the water molecules—a phenomenon he called molecular diffraction. Thus began Raman’s new research obsession: the molecular basis of light scattering.

Since the core has a higher index of refraction than the cladding, light will be confined to the core if the angular condition for total internal reflectance is met. The fiber geometry and composition determine the discrete set of electromagnetic fields, or fiber modes, which can propagate in the fiber.