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Fiber-coupled Diode Lasers

Author: the photonics expert (RP)

Definition: diode laser devices where the generated light is coupled into an optical fiber

Alternative term: pigtailed diode lasers

More general term: diode lasers

Categories: article belongs to category fiber optics and waveguides fiber optics and waveguides, article belongs to category laser devices and laser physics laser devices and laser physics

DOI: 10.61835/jks   Cite the article: BibTex plain textHTML   Link to this page   LinkedIn

For many applications, it is convenient to couple the output of a laser diode into an optical fiber to deliver the light to the place where it is needed. Fiber-coupled (also called fiber-integrated or fiber-pigtailed) diode lasers have several advantages:

  • The light exiting the fiber has a circular and smooth (homogenized) intensity profile and a symmetric beam quality, which is in many cases very convenient. For example, less sophisticated optics are required for generating a circular pump spot for an end-pumped solid-state laser.
  • It becomes possible to remove the laser diodes together with their cooling arrangements e.g. from a solid-state laser head, which can then be more compact, and more space is left for other parts at this place.
  • Defective fiber-coupled diode lasers can easily be replaced without changing the alignment of the device where the light is used.
  • Fiber-coupled devices can be easily combined with other fiber-optic components.
photograph of a fiber-coupled diode laser
Figure 1: Photograph of fiber-coupled diode lasers from Coherent, having different kinds of fiber-optic cables. The image was kindly provided by Coherent.

Types of Fiber-coupled Diode Lasers

Many diode lasers are thus sold in fiber-coupled form, with robust fiber coupling optics (e.g., a permanent laser-welded fiber attachment) built into the laser package. The used fibers and techniques differ very much for different diode lasers:

The simplest case is that of a VCSEL (vertical cavity surface-emitting laser), which usually emits a beam with high beam quality, moderate beam divergence, no astigmatism, and a circular intensity profile. A simple spherical lens is sufficient for imaging the emitting spot to the core of a single-mode fiber. The coupling efficiency can be of the order of 70–80%. It is also possible to couple directly (butt couple) the fiber to the emitting surface of the VCSEL.

Small edge-emitting laser diodes also emit in a single spatial mode, thus in principle also allowing efficient coupling to a single-mode fiber. However, the coupling efficiency can be significantly degraded by the ellipticity of the beam, if a simple spherical lens is used. Also, the beam divergence is relatively high in at least one direction, requiring a lens with relatively high numerical aperture. Another problem is astigmatism of the diode output, particularly for gain-guided diodes; this may be compensated with an additional weak cylindrical lens. With output powers of up to a few hundred milliwatts, fiber-coupled gain-guided LDs can be used for, e.g., pumping erbium-doped fiber amplifiers.

fiber-coupled low-power laser diode
Figure 2: Schematic setup of a simple fiber-coupled low-power edge-emitting laser diode. A spherical lens (or possibly a lens doublet) is used to image the laser diode facet to the fiber core. Beam ellipticity and astigmatism can degrade the coupling efficiency.

Broad area laser diodes are spatially multimode in the long direction of the emitter. If a circular beam is simply shaped with a cylindrical lens (e.g. a fiber lens, see Figure 3) and then launched into a multimode fiber, a lot of the brightness (radiance) will be lost because the high beam quality in the fast axis direction can not be utilized. A power of e.g. 1 W can be launched into a multimode fiber with a 50-μm core diameter and a numerical aperture (NA) of 0.12. This is sufficient e.g. for pumping a low-power bulk laser, e.g. a microchip laser. Even a launched power of 10 W is possible.

fiber-coupled broad-area laser diode
Figure 3: Schematic setup of a simple fiber-coupled broad-area laser diode. A fiber lens is used to collimate the beam in the fast axis direction.

An improved technique for broad-area lasers is based on reshaping the beam for a symmetrized beam quality (and not only symmetrized beam radius) before launching. That allows for a higher brightness.

For diode bars (diode arrays), the problem of asymmetric beam quality is even more severe. Here, the outputs of individual emitters may be coupled into separate fibers of a fiber bundle. The fibers are arranged in a linear array on the side of the diode bar, but as a circular array on the output end. Alternatively, some kind of beam shaper for symmetrizing the beam quality may be used before launching into a single multimode fiber. This can be done e.g. with a two-mirror beam shaper or with some micro-optic elements. It is possible e.g. to couple 30 W into a fiber with 200-μm (or even 100-μm) core diameter and an NA of 0.22. Such a device can be used for, e.g., pumping an Nd:YAG or Nd:YVO4 laser with roughly 15 W of output power.

For diode stacks, fibers with still larger core diameters are used. It is possible e.g. to couple hundreds of watts (or even several kilowatts) of optical power into a fiber with a 600-μm core diameter and NA = 0.22.

Drawbacks of Fiber Coupling

Some potential disadvantages of fiber-coupled diode lasers, compared with free-space emitting lasers, are:

  • The cost is higher. This may be offset, however, by the savings from simpler beam processing and delivery.
  • The output power is slightly reduced, and more importantly the brightness (radiance). The loss of brightness can be substantial (more than an order of magnitude) or rather small, depending on the technique of fiber coupling. In some cases, this may not matter, but in other cases it introduces significant challenges e.g. for the design of a diode-pumped bulk laser or a high-power fiber laser.
  • In most cases (particularly with multimode fibers), the fiber is not polarization-maintaining. The fiber output will then normally be partially polarized, and the polarization state can change when the fiber is moved or the temperature changes. This can cause substantial stability problems of diode-pumped solid-state lasers when the pump absorption is polarization-dependent (as in, e.g., Nd:YVO4).

One may also not obtain a fiber-coupled laser diode product for each optical wavelength.

Beam Quality of the Fiber Output

The beam quality of the fiber output is often not specified; in many cases, only the fiber core diameter and the numerical aperture (NA) are known, and a step-index multimode fiber is assumed. There is no equation to calculate exactly the beam quality in that case because it depends on the distribution of optical power over the fiber modes, and this distribution itself depends on the launching conditions. However, the beam quality M2 factor can be roughly estimated, assuming that the power is well distributed over the modes, so that the numerical aperture represents a reasonable (perhaps slightly too high) estimate for the actual beam divergence. This leads to the equation

$${M^2} \approx \frac{{\pi \: r_\rm{co}}}{\lambda }{\rm{NA}}$$

where <$r_\rm{co}$> is the fiber core radius (i.e., half the core diameter). If the light is launched mostly in lower-order guided fiber modes, the beam quality can also be substantially better, but may then be degraded by strong bending of the fiber.

More to Learn

Encyclopedia articles:

Suppliers

The RP Photonics Buyer's Guide contains 84 suppliers for fiber-coupled diode lasers. Among them:

AeroDIODE

fiber-coupled diode lasers

SHIPS TODAY: AeroDIODE offers fiber-coupled laser diodes emitting between 520 nm and 1650 nm as stock items or associated with a CW or pulsed laser diode driver. They are compatible with our high speed nanosecond pulsed drivers or high power CW drivers with air cooling for the multimode high power laser diode versions.

The single mode laser diodes (either Fabry–Pérot or DFB laser diode) can reach high power in nanosecond pulse regime up to 500 mW. Most turn-key diode & driver solutions are optimized for single-shot to CW performances with pulse width lengths down to 1 ns. The laser diode precision pulses are generated internally by an on-board pulse generator, or on demand from an external TTL signal. Many multimode versions are available with CW emission up to 300 W in a 200-µm core multimode fiber or up to 250 W in a 135-µm core fiber or 160 W in a 105 µm core fiber.

See also our tutorial on fiber-coupled laser diodes.

Focuslight Technologies

fiber-coupled diode lasers

Focuslight Technologies produces a wide range of high power diode lasers, including fiber-coupled modules, based on diode bars or on single emitters. Output powers between 15 W and 500 W and various emission wavelengths such as 808 nm, 980 nm are available.

Lumics

fiber-coupled diode lasers

The Lumics fiber-coupled diode lasers are perfect for OEM integrators developing advanced end-user laser systems, featuring single emitters with exceptional longevity. Lumics offers single-mode 14-pin BTF lasers for high-precision industrial applications and multi-mode diode laser modules in various sizes, customizable to meet specific customer needs. The multi-mode, multi-emitter packages are designed for detachable fibers with core diameters ranging from 105 µm to 1000 µm. Users can specify wavelengths, power levels, and fiber core diameters to ensure an optimal diode laser match for their applications.

Monocrom

fiber-coupled diode lasers

Monocrom has developed highly efficient fiber-coupled diode lasers. The Monocrom solder-free laser bar Clamping™ technique avoids any “smile” effect and thus allows for an optimum coupling efficiency.

We also provide wavelength-stabilized laser modules with a grating for wavelength locking.

For the most demanding applications of high brightness (radiance), we offer fiber-coupled high power diode laser systems capable of delivering 1 kW continuous-wave power.

Bright Solutions

fiber-coupled diode lasers

Bright Solutions offers the BDL and BFD – fiber-coupled diode laser modules:

  • up to 200 W cw (or 400 W quasi-cw) in a 200-μm core fiber
  • flat top beam profile
  • integrated TE cooler, current and temperature controller
  • optional fiber sensor
  • pulsed models are available

They are used e.g. for pumping of solid-state and fiber lasers, material processing or illumination.

Sacher Lasertechnik

fiber-coupled diode lasers

Sacher Lasertechnik offers various types of fiber-pigtailed laser diodes, including both DFB and DBR lasers.

Sheaumann Laser

fiber-coupled diode lasers

Sheaumann’s fiber-coupled modules are well-suited for applications where a circular, homogenized beam is desired, such as those utilizing DPSS end-pumping or compact ones requiring simple optics. Using proprietary diode fabrication and fiber preparation/alignment processes to achieve a low-noise output, Sheaumann offers modules with a high beam quality ideal for such applications as spectroscopy and free-space optical communications. These unique technologies also reduce speckle, and when paired with an appropriate connector, modules can be used as high-performance pointing devices. Let Sheaumann custom design your module, from diode to fiber to package, to work optimally in your system and outperform your competition.

Schäfter + Kirchhoff

fiber-coupled diode lasers

The fiber-coupled laser diode beam sources of type 51nano-S have reduced power noise (< 0.1% RMS (< 1 MHz)), reduced coherence length (≈ 300 µm) and a low speckle contrast as a result of the internal RF modulation. They are available with wavelengths from 405 nm to 1550 nm.

CSRayzer Optical Technology

CSRayzer provides many kinds of fiber coupled diode lasers, e.g. for fiber amplifier pumping, but also pulsed DFB laser modules with narrow linewidth.

TOPTICA Photonics

fiber-coupled diode lasers

While all of Toptica's diode lasers can be combined with Toptica's versatile fiber launch FiberDock™, we also offer dedicated, permanently fiber-coupled systems.

The iBeam-smart-PT as well as the Multi-Laser Engine iChrome xLE cover the range from 375–785 nm and provide a single-mode polarization maintaining fiber delivery.

Toptica's proprietary COOL technology enables drop-shipment and ensures constant output powers even under strongly varying ambient conditions.

Lumibird

fiber-coupled diode lasers

Lumibird manufactures various fiber-coupled laser diodes modules with compact design, high peak power up to 600 W and high coupling efficiency. Fibers are field replaceable.

Eblana Photonics

fiber-coupled diode lasers

Eblana Photonics' core product offerings are based on our patented Discrete-Mode (DFB like) manufacturing technology, which delivers industry leading performance with fully scalable, consistent production and integration capability.

Eblana’s products are used extensively for trace gas sensing and environmental monitoring in the near- and mid-IR.

In addition, the inherent low linewidth performance of the DM platform in comparison to typical DFB lasers is uniquely suited to distributed fibre sensing and LIDAR applications, as well as scientific and metrology projects.

Available in a variety of industry standard and custom package types, Eblana can also help you realise any custom wavelength requirements with a proven track record of success.

AMS Technologies

fiber-coupled diode lasers

AMS Techno­logies provides fiber-coupled diode lasers in various formats, ranging from single laser diodes to modules to complete diode laser systems:

Advanced Photonic Sciences

fiber-coupled diode lasers

APS now offers 445 nm (blue) fiber-coupled laser modules with varying powers of 50 W, 100 W, 150 W and 200 W. With the exception of the 50 W power module, all units are self-contained, internally-cooled, and designed for mounting. These units, equipped with a SMA connector at the fiber output, can be used for various applications such as illumination, cutting, engraving, and more.

CNI Laser

fiber-coupled diode lasers

All series of CNI laser modules can be fiber-coupled with a choice of different fibers such as single-mode, polarization-maintaining or multimode. Even beam fiber coupling (with a flat beam profile) is also available.

Edmund Optics

fiber-coupled diode lasers

Edmund Optics offers various fiber-coupled diode lasers, including visible and infrared emitters with high brightness fiber output, frequency-stabilized laser diodes, class IV high-power devices, Coherent® Diamond FLQ-Series fiber lasers and Coherent®high performance OBIS™ LX/LS fiber-pigtailed laser systems.

RPMC Lasers

fiber-coupled diode lasers

Serving North America, RPMC Lasers offers one of the widest wavelength selections of fiber-coupled diode lasers, available in wavelengths from UV through SWIR. You can filter our entire laser diode catalog for MM fiber, SM fiber, and PM fiber, depending on your needs. These products include multimode single emitters, multi-emitter fiber-coupled modules, single-mode laser diodes, laser diode bars, and laser diode stacks, available in many different fiber-coupled packaging, SM/MM/PM fibers, fiber diameter, and wavelength stabilized and narrow linewidth options. We also offer a large selection of various types of fiber-coupled laser diode modules. Standard and custom options available. Let RPMC help you find the right laser today!

QPC Lasers

fiber-coupled diode lasers

QPC Lasers products range from sub-watt single-mode PM fiber coupled diodes for LIDAR and communications to multi-mode fiber-coupled modules with outputs in the hundreds of watts for medical, materials processing and pumping applications. Optional features include Brightlock monolithically spectrally stabilized diodes for unmatched linewidth and spectral control.

Frankfurt Laser Company

fiber-coupled diode lasers

Frankfurt Laser Company offers fiber-coupled single-mode and multimode laser diodes – also including high-power diodes. Both cooled and uncooled packages are available.

Questions and Comments from Users

2021-05-04

Can the output beam of a multimode fiber-coupled laser (200–300 μm diameter) also be treated as a Gaussian beam with waist etc?

The author's answer:

What you get after the fiber is certainly not a Gaussian beam, but rather an incoherent superposition of contributions from many different fiber modes. However, if the intensity profile is not too far from Gaussian, you may still treat it with Gaussian beam propagation, just taking into account the much increased M2 factor.

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