1 Fundamentals

An optical cavity is a structure designed to trap light by repeated reflection between boundaries with high reflectivity. By storing electromagnetic energy for a finite time, the cavity selects certain wavelengths and spatial field patterns while reducing others. This selective behavior makes cavities useful for controlling laser emission, enhancing nonlinear processes, and increasing sensitivity in measurement systems.

1.1 Definition and function

In the broadest sense, an optical cavity is any arrangement that confines light so that it circulates or bounces back and forth. The simplest version uses two mirrors facing one another, but other geometries may rely on curved mirrors, dielectric interfaces, or whispering-gallery confinement. The main function is to create a resonant environment in which only specific optical fields persist efficiently.

1.2 Resonance in optical cavities

Resonance occurs when the phase accumulated by light during a round trip matches the cavity geometry. Under resonant conditions, waves reinforce themselves rather than cancel, so the optical field builds up inside the cavity. This enhancement depends on wavelength, cavity length, and reflection properties.

1.2.1 Constructive interference

Constructive interference happens when successive passes of a light wave arrive in phase. The reflected field then adds coherently to the existing field in the cavity, increasing intensity. This principle underlies the sharp transmission peaks seen in resonant optical devices.

1.2.2 Standing-wave formation

In many cavities, especially those with two opposing mirrors, the forward and backward propagating waves combine to form standing waves. These patterns contain nodes and antinodes that remain fixed in space. The allowed standing-wave shapes are determined by the cavity dimensions and boundary conditions.

1.3 Boundary conditions

Boundary conditions specify how the electromagnetic field behaves at the cavity surfaces. For ideal mirrors, the field must satisfy reflection rules that enforce phase relationships and spatial constraints. These conditions determine which resonant modes are allowed and how strongly each mode is supported.

1.4 Light confinement mechanisms

Light may be confined by metallic reflection, dielectric multilayers, total internal reflection, or geometric circulation. In high-quality cavities, confinement is strong enough that photons make many round trips before escaping. The better the confinement, the narrower the resonance and the longer the storage time.

2 Cavity types

Optical cavities appear in several common forms, each optimized for a different balance of alignment, mode structure, and confinement. Some are simple and widely used in lasers, while others are specialized for integrated optics or microscale experiments.

2.1 Fabry–Pérot cavity

A Fabry–Pérot cavity consists of two parallel mirrors separated by a fixed distance. It is one of the most widely used resonator geometries because it is simple and highly tunable. Its transmission spectrum contains a series of sharp resonances separated by the free spectral range.

2.2 Ring cavity

A ring cavity guides light around a closed loop, usually using several mirrors or a waveguide path. Because the circulating beam travels in one direction, ring cavities are useful in systems where spatial separation of input and output beams is advantageous. They are common in laser experiments and nonlinear optics.

2.3 Confocal cavity

A confocal cavity uses mirrors whose radii of curvature and spacing create a special focusing condition. This geometry supports a distinctive mode structure with increased degeneracy among some resonances. It is often studied because it combines practical alignment behavior with clear theoretical properties.

2.4 Microcavity

A microcavity is a cavity with dimensions on the scale of micrometers or smaller. Its small size can produce strong field confinement and large interaction strengths between light and emitters. Microcavities are important in integrated photonics, semiconductor optics, and quantum devices.

A whispering-gallery-mode resonator confines light by repeated total internal reflection near the edge of a curved structure. The name comes from the acoustical analogy in circular spaces, where waves travel along the perimeter. These resonators can achieve very high quality factors and are often made from microscopic discs, spheres, or rings.

3 Key properties

The behavior of an optical cavity is described by several related parameters. These quantities determine how sharply the cavity resonates, how long it stores energy, and how strongly it interacts with external light.

3.1 Resonant frequencies

Resonant frequencies are the specific frequencies at which the cavity supports standing or circulating modes. They depend on cavity length, refractive index, and mirror phase shifts. Only light near these frequencies is efficiently stored or transmitted.

3.2 Free spectral range

The free spectral range is the frequency spacing between adjacent resonances. It is set primarily by the round-trip optical path length. Larger cavities generally have a smaller free spectral range, while shorter cavities have more widely separated resonances.

3.3 Quality factor

The quality factor, or Q factor, measures how long a cavity stores energy relative to the energy lost per cycle. A high-Q cavity has narrow resonance linewidths and low dissipation. This property is especially important in spectroscopy and quantum optics.

3.4 Finesse

Finesse describes how sharply a cavity distinguishes one resonance from the next. It is related to mirror reflectivity and internal losses. A cavity with high finesse produces narrow peaks and strong selectivity for resonant frequencies.

3.5 Mode volume

Mode volume is a measure of how spatially concentrated the electromagnetic field is within the cavity. Smaller mode volume means the field is confined to a smaller region, often increasing the strength of light-matter interaction. This quantity is central in microcavity and cavity-QED studies.

3.6 Cavity lifetime

Cavity lifetime is the average time light remains stored before escaping or being absorbed. It depends on reflectivity, scattering, absorption, and output coupling. Long lifetimes allow more circulations and greater spectral selectivity.

4 Optical modes

An optical cavity supports discrete field distributions called modes. These modes describe how the electric and magnetic fields vary in space and how they evolve in time.

4.1 Longitudinal modes

Longitudinal modes are determined by the field variation along the propagation direction. In a simple cavity, they correspond to wavelengths that fit an integer number of half-wavelengths into the optical length. They are labeled by mode order and are separated by the free spectral range.

4.2 Transverse modes

Transverse modes describe the field pattern across the direction of propagation. They depend on mirror curvature, cavity geometry, and diffraction effects. Different transverse modes can share the same longitudinal order while having distinct spatial shapes.

4.2.1 Gaussian modes

Gaussian modes are common low-order transverse modes with smooth intensity profiles. The fundamental Gaussian mode is often the most efficient for coupling into and out of a cavity. It is also preferred in laser systems because of its simple beam shape.

4.2.2 Higher-order modes

Higher-order modes show additional nodes and more complex spatial structure. They may arise when the cavity is deliberately excited away from the fundamental mode or when alignment is imperfect. Although sometimes undesirable, they are useful in certain mode-matching and sensing applications.

4.3 Polarization effects

Polarization affects how the cavity interacts with the electric field orientation. Some cavities treat orthogonal polarizations similarly, while others introduce splitting due to mirror coatings, birefringence, or geometry. Polarization control is important in precision experiments and polarization-sensitive devices.

4.4 Mode coupling

Mode coupling occurs when energy transfers between different cavity modes. It may be caused by imperfections, nonlinear effects, thermal changes, or intentional perturbations. Coupling can broaden resonances, shift frequencies, or create hybrid field distributions.

5 Cavity design and parameters

Designing an optical cavity requires balancing confinement, stability, spectral selectivity, and practical losses. Small changes in geometry or mirror properties can significantly alter the resonator response.

5.1 Mirror reflectivity

Mirror reflectivity strongly influences how many round trips the light can make. Higher reflectivity usually improves storage time and resonance sharpness, but it may also reduce output coupling. In many devices, mirror reflectivity is chosen to optimize the intended use rather than maximize confinement alone.

5.2 Cavity length

Cavity length sets the optical path over which phase accumulation occurs. It directly affects the resonant frequencies and the spacing between them. Short cavities support wider mode spacing, while long cavities support more closely spaced resonances.

5.3 Curvature of mirrors

Mirror curvature shapes the internal beam and helps determine whether the cavity can support stable modes. Curved mirrors can focus light back into the resonator axis, improving confinement of Gaussian beams. The curvature must be matched to the cavity length for stable operation.

5.4 Stability criteria

Stability criteria describe whether a beam remains confined after many round trips. A stable cavity returns rays or wave packets to bounded trajectories rather than letting them diverge. These criteria are essential in cavity design, especially for resonators with curved mirrors.

5.5 Loss mechanisms

Losses reduce cavity performance by removing energy from the resonant field. They include transmission through mirrors, absorption in materials, scattering from surface roughness, diffraction, and coupling to unwanted modes. Minimizing loss is key to achieving high Q and strong resonance contrast.

6 Interaction with light and matter

Because an optical cavity concentrates electromagnetic energy in space and time, it can dramatically alter how light interacts with atoms, molecules, quantum dots, and other emitters. This altered interaction is the basis for many advanced optical experiments.

6.1 Cavity enhancement effects

Cavity enhancement refers to the increase in effective interaction strength caused by multiple passes of light through the same region. As a result, absorption, emission, and nonlinear conversion can become much more efficient. Enhancement depends on finesse, mode volume, and spectral matching.

6.2 Purcell effect

The Purcell effect is the modification of spontaneous emission by the cavity environment. When an emitter is placed in a resonant cavity, its emission rate into the cavity mode may increase relative to free space. This effect is strongest when the emitter frequency matches the cavity resonance and the mode volume is small.

6.3 Strong coupling regime

The strong coupling regime occurs when the interaction between light and matter exceeds the relevant loss rates. In this case, energy can be exchanged coherently between an emitter and the cavity field. The resulting dynamics are often described as coupled light-matter excitations.

6.4 Cavity quantum electrodynamics

Cavity quantum electrodynamics studies quantum interactions between individual emitters and cavity modes. It explores how confinement alters emission, coherence, and quantum state evolution. The field is central to many schemes for controlling single photons and quantum bits.

7 Applications

Optical cavities are used across science and engineering because they provide frequency selectivity, field buildup, and controlled coupling to matter. Their versatility makes them foundational components in many optical instruments.

7.1 Lasers

In lasers, the cavity provides feedback that selects the lasing modes and supports stimulated emission. It helps determine output wavelength, beam quality, and threshold behavior. The cavity is often the defining element that distinguishes a laser from an amplifier.

7.2 Optical filters

Cavities can serve as narrowband optical filters that transmit only selected frequencies. Their resonance peaks make them useful in wavelength discrimination, signal processing, and instrumentation. Filter behavior can be tuned by changing cavity spacing or refractive index.

7.3 Frequency combs

Optical cavities are used in the generation and stabilization of frequency combs. They can enhance nonlinear processes or act as references that support evenly spaced spectral lines. Stable resonator properties are crucial for maintaining comb coherence.

7.4 Spectroscopy

In spectroscopy, cavities increase the effective path length of light through a sample or amplify weak spectral signals. This makes it easier to detect small absorption features and weak emission lines. High-finesse cavities are especially valuable for trace analysis.

7.5 Sensors and metrology

Cavity-based sensors detect tiny changes in length, refractive index, temperature, or pressure by monitoring resonance shifts. In metrology, cavities provide highly stable frequency references and precision comparison standards. Their sensitivity makes them useful in interferometric measurement systems.

7.6 Quantum information experiments

Cavities are used to mediate interactions between photons and quantum systems such as atoms, ions, and solid-state emitters. They help generate single photons, control quantum states, and improve readout efficiency. These capabilities support many experimental platforms in quantum information science.

Optical cavities are closely connected to other structures that guide, interfere with, or confine light. Understanding these related concepts helps place cavities within the broader field of photonics.

8.1 Optical resonators

Optical resonators are the general class of devices that store light by repeated reflection or circulation. An optical cavity is a common type of resonator, and the terms are often used nearly interchangeably. Resonator theory covers both spatial and spectral mode behavior.

8.2 Interferometers

Interferometers split and recombine light to measure phase differences and small optical changes. Some interferometric devices incorporate cavities to sharpen their response or enhance sensitivity. The connection lies in the shared reliance on coherent interference.

8.3 Photonic crystals

Photonic crystals control light using periodic variations in refractive index. Defects in these structures can form cavity-like localized modes. They are often used to confine light at very small scales.

8.4 Waveguides

Waveguides direct light along a predetermined path, often by total internal reflection or index contrast. When arranged in closed loops or combined with reflectors, they can function as cavity elements. Waveguide-based cavities are important in integrated photonics.