laser threshold (original) (raw)

Author: the photonics expert (RP)

Definition: an operation condition of a laser where laser emission just starts to occur

Category: article belongs to category laser devices and laser physics laser devices and laser physics

laser physics

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DOI: 10.61835/f7d Cite the article: BibTex BibLaTex plain text HTML Link to this page! LinkedIn

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What is a Laser Threshold?

The threshold of a laser is the state where the small-signal gain just equals the resonator losses, so that laser emission can just begin. This is the case for a certain pump power (the threshold pump power), or (for electrically pumped lasers) a certain threshold current. Significant power output, good power efficiency and stable, low-noise performance require operation well above the threshold. Typically, one operates a laser between 3 and 10 times above threshold.

threshold power and slope efficiency

Figure 1: Output versus input power for an optically pumped laser. The threshold pump power is 5 W, and the slope efficiency is 50%.

The “edge” occurring at the threshold is very slightly rounded due to the influence of amplified spontaneous emission.

The threshold pump power can often be quite easily calculated based on the requirement that the small-signal round-trip gain reaches the level of the round-trip power losses at the laser wavelength. See the article on threshold pump power for an example.

A low threshold power requires low resonator losses and a high gain efficiency, the latter being achieved e.g. by using a small laser mode area in an efficient gain medium with limited emission bandwidth. The optimization of the laser output power for a given pump power usually involves a compromise between high slope efficiency and low laser threshold. However, an overall optimization of laser performance may have to take into account additional aspects, such as the achievable pulse duration in a Q-switched laser, avoiding Q-switching instabilities in a mode-locked laser, or minimizing thermal effects. The question of which value is most appropriate for the threshold pump power is one of the issues of laser design.

Even for operation below the laser threshold, the gain medium emits luminescence (for optically pumped lasers, this can be called fluorescence). Above threshold (in continuous-wave operation), the intensity level of the luminescence is typically clamped to values close to that at the laser threshold (→ gain clamping). Just below threshold, a laser already emits some power which results from amplified spontaneous emission and has a bandwidth which is large compared with the above-threshold laser emission, but small compared with the regular luminescence bandwidth.

The fundamental origin of the laser threshold is the power loss via luminescence into a large number of spatial modes (propagating in all directions). Under certain circumstances, it is possible to obtain a thresholdless laser by suppressing the luminescence with a microcavity. That can be realized, for example, based on the principle of photonic band gaps in photonic crystals.

Most lasers are operated with a pump power which is just a few times the threshold pump power, i.e., the pump parameter is often order of 3–10. This is often a good regime, since a lower threshold could not significantly increase the efficiency but could introduce problems related to, e.g., too high intracavity intensities.

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 a laser threshold?

The threshold of a laser is the operating point where the optical small-signal gain just equals the losses of the laser resonator. Laser emission begins for a pump power above the corresponding threshold pump power.

What is the physical origin of the laser threshold?

The fundamental origin of the laser threshold is the power loss from luminescence (spontaneous emission) into a large number of spatial modes. Lasing starts only when stimulated emission into the laser mode is strong enough to overcome all resonator losses.

How can a low laser threshold be achieved?

A low threshold power requires minimizing resonator losses and maximizing the gain efficiency. High gain efficiency can be achieved by using an efficient gain medium with a small laser mode area and a limited emission bandwidth.

Why are most lasers operated well above their threshold?

Lasers are typically operated at 3 to 10 times their threshold power to achieve significant power output, good power efficiency, and stable, low-noise performance.

What happens just below the laser threshold?

Just below threshold, a laser emits some power from amplified spontaneous emission (ASE). This emission has a bandwidth which is broader than the laser light above threshold but narrower than the regular luminescence bandwidth.

Can a laser operate without a threshold?

Yes, under certain circumstances, it is possible to build a thresholdless laser. This is achieved by suppressing spontaneous emission into unwanted modes, for example by using a microcavity based on a photonic crystal.

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