Overview

Raman scattering, also known as the Raman effect, is a quantum optical phenomenon in which light undergoes inelastic scattering when interacting with matter. In this process, a small fraction of incident photons exchange energy with molecular vibrations, represented by quantized lattice vibrations called phonons. As a result, the scattered photons emerge with slightly shifted energies and wavelengths. The probability of this event is extremely low—on the order of one photon in ten million—since most photons are scattered elastically via Rayleigh scattering. Despite its weakness, the Raman effect is highly significant because the frequency shifts correspond directly to the vibrational energy levels of molecules, providing a unique spectroscopic “fingerprint.” This principle underpins Raman spectroscopy, now widely applied in chemistry, physics, biology, and materials science for molecular identification and structural analysis.

At higher intensities, Raman scattering evolves into stimulated Raman scattering (SRS), where photon–phonon interactions become coherent and strongly amplify the scattered light. This nonlinear mechanism forms the basis of Raman fiber amplifiers (RFAs) in optical telecommunications. By injecting pump light at specific wavelengths into transmission fibers, RFAs provide distributed gain along the fiber, compensating losses and enabling long-haul and submarine communication systems. Within this article, the technical and scientific foundations of Raman scattering will be discussed, with particular emphasis on its role in telecommunications.

Brief history

The theoretical concept of inelastic light scattering was first proposed by Adolf Smekal in 1923 [1], but it was not experimentally confirmed until 1928, when C.V. Raman and K.S. Krishnan in Calcutta [2–4], and independently Landsberg and Mandelstam in Moscow [5], observed frequency-shifted scattered light. Using filtered sunlight, Raman’s group detected faint new spectral lines, a discovery that earned him the 1930 Nobel Prize and became known as the Raman effect.

Decades later, the advent of low-loss optical fibers in the 1970s opened a new frontier for Raman research. Stimulated Raman scattering in fibers, first demonstrated by Stolen and Ippen in 1973 [6], was initially regarded as a nonlinear limitation. By the mid-1990s, however, it was redefined as a practical amplification mechanism. Distributed Raman amplifiers (DRAs), which exploit the transmission fiber itself as the gain medium by injecting pump light, became crucial for extending the reach and capacity of long-haul and submarine communication systems. Today, Raman amplification remains a fundamental component of high-capacity optical networks.

Technical insights into Raman scattering

Raman scattering is one of the two primary forms of inelastic light scattering in optical media, the other being Brillouin scattering, which arises from acoustic vibrations. In Raman scattering, incident photons interact with molecular vibrations and exchange energy with phonons. Depending on the vibrational state of the medium, the scattered photon can emerge at lower energy (Stokes scattering) or at higher energy (anti-Stokes scattering), see Figure 1. Under ordinary illumination, this effect is weak and random, but when driven by high optical power it enters the regime of stimulated Raman scattering.

SRS is of particular importance in fiber-optic telecommunications, where it enables Raman fiber amplification. In this process, a strong pump laser transfers energy coherently to a weaker data-carrying signal within the same fiber. Because the Raman gain is distributed along the transmission span, this method compensates fiber losses gradually rather than at discrete points, reducing noise accumulation and improving overall system performance. Unlike erbium-doped fiber amplifiers (EDFAs), which operate only in fixed spectral bands, Raman amplification can be tuned by adjusting the pump wavelength. For example, a pump near 1450 nm efficiently amplifies signals around 1550 nm, the lowest-loss transmission window in silica fibers. This spectral flexibility makes Raman amplifiers indispensable for extending system capacity across multiple wavelength bands. As a result, Raman amplification has become a cornerstone technology in long-haul and submarine optical networks, sustaining the ever-increasing demand for global data traffic.

Fundamental and principles

The Raman effect can be understood by examining how light interacts with the vibrational dynamics of molecules. When electromagnetic radiation encounters matter, most photons scatter elastically (Rayleigh scattering), leaving the molecule unchanged in its vibrational state. In contrast, Raman scattering arises when the interaction leads to an exchange of energy between the photon and molecular vibrations. The molecule is briefly promoted to a virtual state—a short-lived, non-quantized state allowed by quantum mechanics—before re-emitting a photon. If the molecule gains vibrational energy, the scattered photon is red-shifted (Stokes scattering). If the molecule loses vibrational energy, the photon is blue-shifted (anti-Stokes scattering). These shifts correspond directly to vibrational energy levels and are commonly represented in Jablonski diagrams, see Figure 2.

The number and type of vibrational modes depend on molecular structure. For a molecule containing N atoms, there are 3N−6 fundamental vibrational modes (3N−5 for linear molecules). As shown for a triatomic molecule in Figure 3, vibrational motions are generally grouped into stretching (valence) modes, involving changes in bond length, and bending (deformation) modes, involving variations in bond angle. For diatomic molecules, the vibrational frequency (ν̃) can be obtained from quantum mechanical considerations of the atomic masses m₁, m₂, the speed of light c and the bond force constant f. It is given by:

A crucial requirement for Raman activity is a change in molecular polarizability—that is, a change in how easily the electron cloud of the molecule can be distorted by an external electric field. This differs fundamentally from infrared absorption, where the selection rule requires a change in dipole moment. As a result, Raman and IR spectroscopy are often complementary: symmetric bonds such as O=O or C–C are strongly Raman-active but weak in IR, while polar bonds such as C=O or O–H are prominent in IR but weak in Raman.

Experimentally, Stokes scattering dominates under ambient conditions because most molecules reside in their ground vibrational state. Anti-Stokes scattering requires molecules to be thermally populated in excited vibrational states, which is relatively rare at room temperature. Consequently, anti-Stokes lines are weaker but temperature-dependent. This principle is exploited in distributed temperature sensing (DTS) in optical fibers, where the ratio of anti-Stokes to Stokes backscatter provides a direct measure of local fiber temperature. Such techniques illustrate how Raman scattering extends beyond spectroscopy into practical industrial sensing.

Another important feature of Raman scattering is that the frequency shift depends only on the vibrational energy of the molecule, not the excitation wavelength. A C–C stretch, for example, may consistently appear around 1000 cm^−¹ regardless of whether the excitation is a green (532 nm) or red (633 nm) laser. This wavelength-independence underpins Raman’s reliability as an identification technique across different experimental setups.

Finally, it is worth noting that Raman scattering is inherently a weak process—only about one in ten million incident photons undergo inelastic scattering. Yet with the development of lasers and sensitive detectors, the effect has been harnessed not only for fundamental spectroscopy but also for industrial monitoring, environmental sensing, biomedical diagnostics, and, most importantly, optical telecommunications, where stimulated Raman scattering forms the basis of distributed fiber amplification.

Applications of the Raman effect

Ever since its discovery, the Raman effect has found a vast array of applications across science and engineering. Here we outline some of the major application areas:

Material science: The most widespread use of Raman scattering is in Raman spectroscopy, a non-destructive method for chemical and structural analysis. A laser illuminates the sample and the scattered light is analyzed, with the positions of Raman peaks revealing molecular vibrational frequencies that serve as unique fingerprints of materials. Raman spectroscopy is applied broadly—from identifying chemical compounds and minerals to evaluating semiconductor crystal quality. It complements infrared spectroscopy by accessing vibrational modes that IR cannot detect. Conventional Raman microscopy, however, is limited by the diffraction limit of light, restricting spatial resolution to a few hundred nanometers. To overcome this, tip-enhanced Raman spectroscopy (TERS) has emerged, combining scanning probe microscopy with Raman analysis. Recently, Bakhtbidar et al. [7–9] demonstrated TERS imaging with ~10 nm spatial resolution and exceptionally high sensitivity, enabling detection of sub-monolayer (0.1 nm) carbonate adsorption on material surfaces. This breakthrough highlights Raman spectroscopy as a powerful nanoscale characterization technique, opening new avenues for surface chemistry and materials science.

Industrial and security sensing: Raman scattering also underpins a variety of sensing technologies. In distributed fiber-optic sensing, backscattered Raman signals are used to measure temperature or strain along kilometers of fiber. Such distributed temperature sensing systems are deployed in oil wells, power grids, pipelines, and infrastructure monitoring. In atmospheric studies, Raman LIDAR employs laser pulses and detects Raman-shifted returns from molecules such as N₂, O₂, and H₂O, enabling remote profiling of gas concentrations and water vapor. Security and defense sectors utilize Raman techniques for standoff detection of explosives and hazardous substances, allowing safe identification at a distance with high-power lasers and collection optics.

Photonics and lasers: Raman scattering is leveraged to create new light sources. In optics, a Raman laser uses a gain medium (like a crystal, fiber, or even a gas) where an intense pump laser is partly converted to a shifted wavelength by stimulated Raman scattering, resulting in laser output at a new wavelength. For example, Raman fiber lasers can generate wavelengths that are not accessible by conventional laser transitions. If you have a high-power 1064 nm Nd:YAG laser, by putting a Raman-active crystal or a fiber, you can get a 1st Stokes at ~1115 nm, and further Stokes shifts if desired. This is used to produce lasers in spectral regions where good traditional lasers don't exist. In telecommunications, as we’ll detail next, Raman amplifiers exploit the effect to boost optical signals. Additionally, Raman scattering has some niche applications like Raman cooling of atoms (an advanced technique in atomic physics to cool atoms using anti-Stokes scattering to remove vibrational energy) and has been observed to play a role even in astrophysical contexts (e.g., certain spectral lines in astronomy are attributed to Raman scattering in interstellar media).

Telecommunications: A crucial modern application is in fiber-optic telecom networks, where stimulated Raman scattering in silica fiber is used to amplify signals and extend the reach of optical communication systems. This is discussed in its own section below, given its importance in that specific field.

Raman scattering in telecommunications

In modern optical telecommunications, the Raman effect plays a central role in extending transmission distances and improving system performance. Its primary application is in Raman fiber amplifiers, which provide distributed gain along the transmission span rather than discrete, module-based amplification. The principle is straightforward: a high-power optical pump in the 1420–1490 nm range transfers part of its energy to the data-carrying signal via stimulated Raman scattering. The pump can be launched in the same direction as the signal (co-propagating), in the opposite direction (counter-propagating), or both. In this way, the transmission fiber itself becomes the gain medium.

Raman amplification offers several key advantages over conventional erbium-doped fiber amplifiers. First, it improves the optical signal-to-noise ratio (OSNR) by amplifying the signal continuously along the fiber. Since the signal spends less distance at low power, it is less susceptible to noise and nonlinear impairments. Second, Raman gain is spectrally flexible. Unlike EDFAs, which are limited to fixed bands (C- and L-bands), Raman amplification can be tuned to virtually any wavelength band by selecting appropriate pump wavelengths. Multi-wavelength pumping enables a broad and flat gain profile, allowing uniform amplification across entire WDM (wavelength-division multiplexing) grids, including extensions into the S-band. Third, distributed Raman pre-amplification reduces nonlinear penalties such as self-phase modulation and four-wave mixing by “lifting” channel powers gradually along the span rather than applying a large boost at the transmitter.

Technically, distributed Raman amplification achieves gains of ~0.2 dB/km, which over tens of kilometers amounts to 10–15 dB of net gain. In practice, usable Raman gain is limited to about 15–20 dB due to double Rayleigh scattering and pump-induced noise. As a result, Raman is typically combined with EDFAs in hybrid amplifier designs. In such systems, Raman provides distributed pre-amplification, while EDFAs supply higher lumped gain per span. Discrete Raman amplifiers—standalone devices containing dedicated fiber spools—have also been demonstrated, especially for bands inaccessible to erbium, though distributed Raman in transmission fiber remains the industry standard.

The deployment of Raman amplifiers has been made possible by advances in high-power pump lasers. Reliable diodes delivering 500 mW to 1 W around 1450 nm became commercially available in the early 2000s. Today, multi-pump configurations are common, combining outputs at several wavelengths to produce broad, flat gain profiles. Careful engineering is required to manage safety (due to high pump power in the fiber), nonlinear interactions, and back-reflections, but these challenges have been effectively addressed in modern networks. Our forthcoming publication on Raman amplification will provide new insights into its implementation and optimization in modern communication systems.

Conclusion

Raman scattering is a fundamental light–matter interaction in which photons exchange energy with molecular vibrations, producing scattered light at shifted wavelengths. Though inherently weak, this process has had a profound impact across science and technology. From its discovery by C.V. Raman in 1928—recognized with the Nobel Prize in Physics—it has evolved from a quantum curiosity into a cornerstone of modern optics.

The effect arises from changes in molecular polarizability and the creation of virtual states, giving rise to Stokes and anti-Stokes scattering. Stokes signals dominate under ambient conditions, while anti-Stokes intensities provide valuable temperature-dependent information. These principles underpin Raman spectroscopy, which today is widely used in chemistry, biology, materials science, and industrial monitoring for its ability to provide molecular fingerprints without destroying the sample.

Beyond spectroscopy, Raman scattering enables powerful sensing and imaging techniques and plays a pivotal role in telecommunications. Stimulated Raman scattering in optical fibers forms the basis of Raman fiber amplifiers, which provide distributed, low-noise gain and extend transmission distances in long-haul and submarine networks. The history of Raman scattering illustrates how fundamental physics can transform global infrastructure. Ongoing advances in both spectroscopy and fiber-based amplification ensure that the Raman effect will remain essential in scientific research and in sustaining the growth of high-capacity communication systems.