laser diode collimators (original) (raw)

Definition: devices for collimating the output of laser diodes

Categories: general optics, laser devices and laser physics

• lenses
  • laser diode collimators
  • achromatic lenses
  • aspheric aspheric
  • ball lenses
  • condensers
  • cylindrical lenses
  • diffractive lenses
  • f–theta lenses
  • fiber lenses
  • field lenses
  • Fresnel lenses
  • gradient-index lenses
  • Kerr lens
  • loupes
  • magnifying glasses
  • microlens arrays
  • microlenses
  • objective lenses
  • ocular lenses
  • rod lenses
  • scanning lenses
  • telecentric lenses
  • zoom lenses
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Related: beam collimatorscollimated beamslaser diodes

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Contents

What are Laser Diode Collimators?

Laser diodes usually emit strongly diverging light, essentially because the emitting areas are normally quite small. For many applications, it is necessary to first transform the radiation into a collimated beam, using some kind of beam collimator. Because the emission properties of different types of laser diodes can be quite different, different types of laser diode collimators have been developed and are described in the following section.

In principle, a collimator can simply be a lens, placed at a distance of its focal length from the laser diode. However, there are various practical complications which require additional thoughts.

For most laser diodes, the beam divergence is so substantial that it is advantageous to use aspheric lenses for beam collimation to avoid significant spherical aberrations of the collimated beam. (For good performance, the numerical aperture of the lens should be significantly larger than that of the laser diode — for example, by a factor of two.) On the other hand, the emission bandwidth is usually quite small, so that achromatic optics are not required in most cases.

Collimators for Single-emitter Laser Diodes

Most available laser diode collimators are designed for use with low-power single-emitter edge-emitting laser diodes. For example, such devices are required in laser pointers.

Figure 1: Simple laser diode collimator.

We first consider only one direction of the emission (horizontal or vertical). The distance between the laser diode output facet and the collimation lens needs to be identical to the focal length of the lens because otherwise the beam after the lens would be convergent or divergent. (We assume that the focus is close to the laser diode's output facet, which is often the case.) It is then simple in principle to calculate the beam diameter after the lens for one of the directions: it is the full divergence angle times the focal length. However, the output beam divergence of laser diodes is often specified in terms of a full width at half maximum (FWHM), whereas for collimated laser beams one usually specifies the ($1/e^2$) beam radius or diameter. Typically, the latter is larger by a factor of ≈1.7, which however somewhat depends on the shape of the intensity profile.

Single-emitter laser diodes usually exhibit substantially different divergence in two orthogonal directions. In typical cases, one has about 10° divergence (full width at half maximum) in the “slow” direction and ≈30° in the “fast” direction. On the other hand, the focus positions are often very close together (very small astigmatism) at least for so-called index-guided laser diodes, so that an ordinary lens (not a cylindrical one) placed at an appropriate distance from the laser diode output can simultaneously collimate light in both directions. Only, the beam radius in the two directions will be substantially different; it may for example be about three times larger in the “fast” direction.

In many cases, it is required to transform the beam profile into a circular one; this is often accomplished by applying an anamorphic prism pair, which may either be used separately or integrated into a laser diode collimator device. Alternatively, one may use two cylindrical lenses, each one having a focal length which is appropriate for obtaining the desired beam radius in its direction. Another technical option is a cylindrical microlens mounted with high precision close to the laser diode facet for transforming the beam into a circular one.

For gain-guided laser diodes (typically those with higher output powers), there is the additional problem of astigmatism: the focus positions for the two directions do not exactly coincide. Although their distance is small compared with the length of the laser diode chip, it is substantial when compared with the Rayleigh length. A consequence of that is that slightly different lens positions (distances to the output facet of the laser diode) would be required for perfect collimation in the two directions. Although an anamorphic prism pair can still be used for producing an approximately circular beam, the astigmatism cannot be corrected that way. A pair of cylindrical lenses, which can be independently adjusted, can be a good solution.

Strictly speaking, the used cylindrical lenses often have acylindrical surfaces, i.e., some deviations from the cylindrical shape to minimize aberrations (see the article on aspheric optics).

For the typically moderate output powers, plastic optics are usually sufficient, particularly when a high optical performance is not required. However, glass lenses with higher quality are also often used. For particularly short focal lengths, one often uses ball lenses or half-ball lenses.

For certain applications, one uses only a cylindrical lens, which collimates the output beam only in one direction. Such devices are sometimes called line generators.

Collimators for Broad-area Emitters, Diode Bars and Diode Stacks

The emitting region of a broad-area laser diode is much larger in the direction along the wafer surface (often called the horizontal direction) than in the other one; this kind of laser design is chosen to obtain substantially higher output powers. The beam divergence in that “slow” direction is significantly smaller than in the fast direction, but by far not as small as it would be for a diffraction-limited beam: the emission is highly multimode in that direction. Therefore, the beam quality in that direction is far worse. A consequence of that is that while the beam diameter after a simple collimation lens is significantly smaller in the “slow” direction, the residual beam divergence in that direction is much larger.

Some broad-area emitters are sold with a fast axis collimation lens (FAC lens) fixed to the output facet. This is a lens with relatively high numerical aperture which collimates the output only in that “fast” direction, so that it is no longer necessary to mount additional optical parts very close to the diode. The same technique can be applied to diode bars, e.g. packages containing multiple broad emitters in an array. A single cylindrical (or in fact slightly acylindrical) fast axis collimation lens (sometimes called a fiber lens) may be sufficient for all emitters. Problems can occur when such an emitter array exhibits a too large “smile”.

For diode stacks, one often uses an array of cylindrical microlenses, each one working for one of the contained bars. In a larger distance, one may have a single cylindrical lens for collimating the beam in the slow direction (SAC = slow axis collimation lens).

Collimators for VCSELs and VCSEL Arrays

It is relatively easy to collimate the output of a vertical cavity surface-emitting laser (VCSEL), since the output beam is usually circular and exhibits a quite moderate beam divergence. A simple spherical lens with moderate numerical aperture is normally sufficient for that purpose. If the desired beam radius is relatively small, a microlens may be employed.

For collimating the output of VCSEL arrays, containing many VCSELs in a 2D pattern, one may use microlens arrays.

Mechanical Aspects

Laser diode beam collimators do not only consist of optical elements, but also contain mechanical parts for fixing the position of the collimation lens and any other optics, and also the laser diode. Often, they are made such that laser diodes with a certain housing (e.g., TO can for low-power emitters) can conveniently be fixed to the collimator, e.g. using a threaded retaining ring. Subsequently, the collimator often needs to be attached to other parts, e.g. a kinematic mount.

Some laser diode collimators are adjustable, e.g. such that by rotating a cap (possibly equipped with a scale) one can fine-adjust the focus via the distance between laser diode facet and collimation lens.

A high mechanical stability can be important, for example because significant beam pointing fluctuations may result from temperature changes.

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 diode collimator?

A laser diode collimator is an optical device, typically containing one or more lenses, used to transform the highly divergent light from a laser diode into a parallel collimated beam.

Why are aspheric lenses often used for laser diode collimation?

The strong beam divergence from a laser diode requires a lens with a high numerical aperture. Aspheric lenses are used to avoid the significant spherical aberrations that would occur with standard spherical lenses under these conditions.

How can the elliptical beam from a laser diode be made circular?

What is astigmatism in a laser diode and how is it corrected?

Astigmatism means that the focus positions for the fast and slow axes of the diode's emission do not coincide. This can be corrected with a pair of cylindrical lenses, where the position of each lens is adjusted independently.

What are fast and slow axis collimation lenses?

For high-power diode bars, a fast axis collimation (FAC) lens is a cylindrical microlens that collimates the highly divergent light of the 'fast axis'. A slow axis collimation (SAC) lens is a larger cylindrical lens used to collimate the less divergent 'slow axis'.

Is it difficult to collimate the beam of a VCSEL?

No, collimating a VCSEL output is relatively easy because its beam is usually circular and has only a moderate beam divergence, so a simple spherical lens is often sufficient.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 17 suppliers for laser diode collimators. Among them:

⚙ hardware

A wide range of laser accessories is available, including fiber-optic beam collimators and focusers and various kinds of beam expanders. The laser diode collimators are easy to integrate and are typically used with fiber coupled lasers and pigtailed receptacles in communications and data transfer.

⚙ hardware

Shanghai Optics provide many different types of standard collimating lenses, including aspheric and achromatic lenses for many different light sources such as highly divergent laser diodes. Our standard collimating lenses can convert divergent laser beams to well-collimated laser beams that enter beam expanders for interferometry, laser material processing and laser scanning applications.

We also provide custom collimating lenses for projecting a source at infinity for infinite conjugate testing of optical systems. The collimating lenses can consist of several optical elements. The selection of optical materials and optical configuration depends on the entrance pupil diameter, wavelength, focal length, and field of view of the optical system under test.

⚙ hardware

Avantier offers a range of standard collimating lenses, including aspheric and achromatic lenses suitable for highly divergent laser diodes. These lenses convert divergent laser beams to well-collimated beams for use in laser scanning applications, laser material processing, and interferometry with beam expanders. We also provide custom collimating lenses with multiple optical elements that project a source at infinity for infinite conjugate testing. Selection of optical materials and configuration is based on various parameters like entrance pupil diameter, wavelength, focal length, and field of view.

⚙ hardware

PowerPhotonic manufactures a large range of high-power diode laser optics for diode bar and diode stack correction applications. Our slow-axis-collimator (SAC) optics minimize the slow-axis divergence of diode laser bars and stacks. Our SACs can also be designed with built-in smile correction. For Fast Axis Collimation, we can provide fast axis collimator arrays, with either fixed or matched pitch. Additionally, we have a range of high power diode correction optics that can be customised as a result of wavefront analysis.

⚙ hardware

Schäfter+Kirchhoff offer various laser diode collimators. You can select them easily using our product configurator. These high quality lasers can e.g. be used for machine vision applications.

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