mode competition (original) (raw)

Author: the photonics expert (RP)

Definition: the phenomenon that different resonator modes experience laser amplification in the same gain medium, leading to cross-saturation effects

Categories: optical resonators, laser devices and laser physics, physical foundations

• modes

  • guided modes
  • cladding modes
  • tunelling modes = leaky modes
  • resonator modes
  • Hermite–Gaussian modes
  • Laguerre-Gaussian modes
  • LP modes
  • higher-order modes
  • mode competition
  • mode coupling
  • mode hopping
  • mode matching
  • mode radius
  • V-number
  • (more topics)

• laser physics

  • cooperative lasing
  • gain efficiency
  • in-band pumping
  • gain narrowing
  • gain saturation
  • Kuizenga–Siegman theory
  • laser dynamics
  • laser gain media
  • laser transitions
  • laser threshold
  • lasing without inversion
  • linewidth enhancement factor
  • lower-state lifetime
  • McCumber theory
  • metastable states
  • mode competition
  • mode hopping
  • modes of laser operation
  • multiphonon transitions
  • non-radiative transitions
  • optical pumping
  • output coupling efficiency
  • parasitic lasing
  • population inversion
  • pulse generation
  • radiation-balanced lasers
  • radiative lifetime
  • rate equation modeling
  • reciprocity method
  • relaxation oscillations
  • single-frequency operation
  • single-mode operation
  • slope efficiency
  • spatial hole burning
  • spiking
  • Stark level manifolds
  • stimulated emission
  • threshold pump power
  • thresholdless lasers
  • transition cross-sections
  • twisted-mode technique
  • ultrafast laser physics
  • upconversion
  • upper-state lifetime
  • wavelength tuning
  • (more topics)

Related: resonator modesmode couplinggain saturationspatial hole burningMode Competition - Increased or Decreased by Spatial Hole Burning?

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DOI: 10.61835/twz Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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What is Mode Competition in Lasers?

A frequently used (and very valuable) concept in laser physics is to describe the light circulating in an optical resonator in terms of resonator modes. The light in each mode can then be characterized by a few parameters, e.g. electric field amplitude (or total power) and optical phase, apart from optical frequency and polarization. When only a few modes participate e.g. in the lasing process, the overall number of parameters is small, and various physical phenomena can be described on that simple basis.

The modes of a laser resonator all experience optical amplification in the same gain medium, e.g. a laser crystal, in which they spatially overlap to a significant extent. This leads to the phenomenon of mode competition or gain competition. Unfortunately, the terms are not precisely defined, and different meanings appear to be used in the literature:

• the fact that different modes experience amplification in the same gain medium, and that this leads to cross-saturation effects, where stimulated emission by one mode causes gain saturation not only for itself (self-saturation), but also for the other modes
• the phenomenon that the power distribution over several modes is unstable

Of course, the last-mentioned phenomenon can be a consequence of the former condition.

In the case of the former meaning of the term, mode competition can in principle be quantified: it is complete if the intensity distributions of the modes are identical, so that self- and cross-saturation are equally strong (assuming that the emission cross-sections are also identical). Mode competition is reduced if the overlap of the intensity distributions is incomplete. Some examples:

• In a single-mode fiber, only longitudinal modes are involved. (Differences in mode diameter are often negligible.) At least for unidirectional propagation, there is then complete mode competition.
• In a linear resonator, each resonator mode forms a standing-wave pattern in the gain medium, which differs for modes with different frequencies. Therefore, if one mode acquires a large power and strongly saturates its gain, another mode may see a reduced amount of gain saturation, as its standing-wave pattern has its anti-nodes at different positions (→ spatial hole burning).
• If a laser resonator contains a dispersive prism, modes with different optical frequencies may obtain a relative transverse offset in the gain medium.

In a situation with strong mode competition (in the sense of strong overlap of mode intensity distributions), which may occur, e.g., in a unidirectional ring laser, a single mode may be excited in the steady state (→ single-mode operation, single-frequency lasers): the mode with the highest net gain will saturate the gain so that it exactly balances its losses, and any other mode will then experience a negative net gain, which causes its power to fade away.

Situations with reduced mode competition can be more complicated: A strong lasing mode cannot saturate the gain for the others to a level which prevents them from lasing. One then has a situation with complicated nonlinear dynamics, often dominated by cross-saturation effects, but possibly also influenced even by tiny mode coupling effects. Depending on various details, a stable distribution of optical power over several modes may occur, or (more frequently) an unstable power distribution, which causes additional laser noise.

The discussion has shown that strong mode overlap can lead to a stable situation. Depending on the definition of terms, this may be considered as a situation with strong mode competition (because of the strong overlap), or on the contrary as a situation with weak competition (as the situation is stable, not exhibiting explicit signs of competition).

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is mode competition in a laser?

Mode competition occurs when different resonator modes are amplified in the same gain medium. This causes cross-saturation effects, where the amplification of one mode reduces the optical gain available for other modes with which it spatially overlaps.

How does spatial hole burning affect mode competition?

In a linear laser resonator, a strong lasing mode saturates the gain mostly at the locations of its intensity maxima (anti-nodes). Other modes with different standing-wave patterns can still experience gain in other locations. This spatial hole burning effect reduces mode competition, often allowing for multimode operation.

Can strong mode competition be beneficial?

Yes. In situations with strong mode competition, such as in a unidirectional ring laser, the mode with the highest net gain can suppress all other modes. It saturates the gain to a level where only it can sustain lasing, leading to stable single-mode operation.

Bibliography

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