# Synopsis: Inside and out of lasers

Is the intracavity field of a laser necessarily a coherent state?

Coherent states are superposition states of the eigenstates of a simple harmonic oscillator, and in some sense they are the “most classical-like” quantum states of the electromagnetic field. For typical laser configurations consisting of a $100%$ reflecting mirror and a partially reflecting mirror, it is not difficult to show that if the laser is to emit a field in a coherent state, the internal cavity field also needs to be in a coherent state. In this picture, the role of the partially transmitting mirror is simply to attenuate the amplitude of the coherent state as it exits the laser cavity.

However, as described in a paper appearing in Physical Review A, David Pegg of Griffith University in Brisbane, Australia, shows that this description, while useful, can mask rich and interesting physics. Pegg assumes that the intracavity field is described by a large mean photon number and a narrow photon number distribution. Since the uncertainty principle limits how well the precise number of photons in the field and their respective phases can be known, the phase of the field is poorly defined. However, Pegg demonstrates that the role of the transmitting mirror is to entangle the output field with the input field, thereby creating a well-defined phase difference between them. The end result is that the partially transmitting optic can create phase-coherent laser light, regardless of the state of the intracavity field.

The work sheds new light on the role of the output coupler in a laser cavity and may be applicable to a wide range of quantum optics experiments that involve optical cavity fields and their intrinsic phase coherence. – Frank Narducci

Correction (15 June 2009): Paragraph 1, sentence 1, “Coherent states are eigenstates of a simple harmonic oscillator…” changed to “Coherent states are superposition states of the eigenstates of a simple harmonic oscillator…”

More Features »

### Announcements

More Announcements »

Optics

## Previous Synopsis

Materials Science

## Next Synopsis

Quantum Information

## Related Articles

Nonlinear Dynamics

### Focus: Image—Cooperating Lasers Make Topological Defects

A circle of interacting lasers is a new model system for exploring topological defects, disordered structures that show up in a wide variety of seemingly unrelated systems. Read More »

Optics

### Viewpoint: Inducing Transparency with a Magnetic Field

A magnetic field applied to an atomic sample in an optical cavity generates optical transparency that could be used to enhance the frequency stability of lasers. Read More »

Photonics

### Focus: Image—Honeycomb Diffraction

Predictions of diffraction patterns for honeycomb photonic crystals were part of a comprehensive study of these structures that may be useful in nanoscale photonic devices. Read More »