Uploaded June 2026 | Updated September 2026, 3 hours ago
How small a spot can a laser be focused? When looking at ray diagrams, it can look like the rays always converge to the same point, making an infinitely small spot. In reality, how well a laser beam can be collimated or focused is limited by diffraction.
Not all lasers will focus with the same performance. Inside a laser cavity, light can oscillate in many different patterns, which all combine into the beam observed coming from the laser. These oscillation patterns are called “spatial modes”, “transverse modes”, “TEM modes”, or “LP modes” if in an optical fiber. Lasers containing more than one spatial mode are called "multimode" lasers. Only "single-mode" lasers, which consist solely of the fundamental TEM00 or LP01 mode, can achieve a diffraction-limited spot when focused.
The beams from these single-mode lasers are called Gaussian beams, where their intensity profiles follow Gaussian curves along each axis. The intensity of the beam’s profile tapers off gradually, which can make the beam's width harder to define. As a result, the diameter or width of a Gaussian beam is typically defined by where their intensity falls to 1/ e2 of their max intensity.
With aberration-free optics, a Gaussian beam’s diffraction-limited spot size depends on its wavelength, its incident 1/ e2 beam diameter, and the effective focal length of the focusing optic.
For many objective lenses, focal length may not always be specified in its data sheet, and numerical aperture can be used instead. This assumes that the incident 1/ e2 beam diameter matches the max pupil diameter specification of the objective lens.
Smaller focused spots can be achieved using shorter wavelengths. Similarly smaller focused spots are achieved by increasing the incident collimated beam diameter or using a shorter effective focal length optic.
In practice, not all elements may achieve this diffraction limit. Optical designs may limit performance in high numerical aperture systems. They can also feature defects due to the manufacturing process used, which can distort or aberrate the beam. This can prevent the beam from focusing to the diffraction-limited spot size and can deform the beam at the focus.
When an optical system’s performance does not limit the performance, it’s referred to as “diffraction limited.”
How small a spot can a laser be focused? When looking at ray diagrams, it can look like the rays always converge to the same point, making an infinitely small spot. In reality, how well a laser beam can be collimated or focused is limited by diffraction.
Not all lasers will focus with the same performance. Inside a laser cavity, light can oscillate in many different patterns, which all combine into the beam observed coming from the laser. These oscillation patterns are called “spatial modes”, “transverse modes”, “TEM modes”, or “LP modes” if in an optical fiber. Lasers containing more than one spatial mode are called "multimode" lasers. Only "single-mode" lasers, which consist solely of the fundamental TEM00 or LP01 mode, can achieve a diffraction-limited spot when focused.
The beams from these single-mode lasers are called Gaussian beams, where their intensity profiles follow Gaussian curves along each axis. The intensity of the beam’s profile tapers off gradually, which can make the beam's width harder to define. As a result, the diameter or width of a Gaussian beam is typically defined by where their intensity falls to 1/ e2 of their max intensity.
With aberration-free optics, a Gaussian beam’s diffraction-limited spot size depends on its wavelength, its incident 1/ e2 beam diameter, and the effective focal length of the focusing optic.
For many objective lenses, focal length may not always be specified in its data sheet, and numerical aperture can be used instead. This assumes that the incident 1/ e2 beam diameter matches the max pupil diameter specification of the objective lens.
Smaller focused spots can be achieved using shorter wavelengths. Similarly smaller focused spots are achieved by increasing the incident collimated beam diameter or using a shorter effective focal length optic.
In practice, not all elements may achieve this diffraction limit. Optical designs may limit performance in high numerical aperture systems. They can also feature defects due to the manufacturing process used, which can distort or aberrate the beam. This can prevent the beam from focusing to the diffraction-limited spot size and can deform the beam at the focus.
When an optical system’s performance does not limit the performance, it’s referred to as “diffraction limited.”
