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You can buy laser diodes from:
- DILAS delivers mounted bars and configured packages with wavelengths from 640 nm to 2500 nm and either CW or QCW operation mode.
- JENOPTIK Laser GmbH, manufacturer of high quality laser diodes emitting at standard wavelengths of 808, 915, 938 and 976 nm, and output powers up to 1 kW and more.
Ask RP Photonics for advice on various aspects of laser diodes and their use e.g. for pumping solid-state lasers.
Acronym: LD
Definition: semiconductor lasers with a current-carrying p–n junction as the gain medium
Laser diodes (= diode lasers) are electrically pumped semiconductor lasers in which the gain is generated by an electrical current flowing through a p–n junction or (more frequently) a p–i–n structure. In such a heterostructure, electrons and holes can recombine, releasing the energy portions as photons. This process can be spontaneous, but can also be stimulated by incident photons, in effect leading to optical amplification, and with optical feedback in a laser resonator to laser oscillation. The article on semiconductor lasers describes more in detail how the laser amplification process in a semiconductor works.
Most semiconductor lasers are diode lasers, but there are also optically pumped semiconductor lasers which do not require a diode structure and thus do not belong to the category of diode lasers.
Types of Laser Diodes
Laser diodes are normally built as edge-emitting lasers, where the laser resonator is formed by coated or uncoated end facets (cleaved edges) of the semiconductor wafer. They are often based on a double heterostructure, which restricts the generated carriers to a narrow region and at the same time serves as a waveguide for the optical field (double confinement). The current flow is restricted to the same region, sometimes using isolating barriers. Such arrangements lead to a relatively low threshold pump power and high efficiency. The active region is usually quite thin – often so thin that it acts as a quantum well. In some cases, quantum dots are used.
Some modern kinds of LDs are of the surface-emitting type (see below), where the emission direction is perpendicular to the wafer surface, and the gain is provided by multiple quantum wells.
There are very different kinds of LDs, operating in very different regimes of optical output power, wavelength, bandwidth, and other properties:
- Small edge-emitting LDs generate between a few milliwatts and up to roughly half a watt of output power in a beam with high beam quality. The output may be emitted into free space or coupled into a single-mode fiber. Such lasers can be designed to be either index guiding (with a waveguide structure guiding the laser light within the LD) or gain guiding (where the beam profile is kept narrow via preferential amplification on the beam axis).
- Small LDs made as distributed feedback lasers (DFB lasers) or distributed Bragg reflector lasers (DBR lasers) with short resonators can achieve single-frequency operation, sometimes combined with wavelength tunability.
- External cavity diode lasers contain a laser diode as the gain medium of a longer laser resonator, completed with additional optical elements such as laser mirrors or a diffraction grating. They are often wavelength-tunable and exhibit a small emission linewidth.
- Broad area laser diodes (also often called broad stripe laser diodes, wide stripe lasers, or high brightness diode lasers) generate up to a few watts of output power. The beam quality is significantly poorer than that of lower-power LDs, but better than that of diode bars (see below). Tapered broad-area lasers can exhibit an improved beam quality and brightness.
- Slab-coupled optical waveguide lasers (SCOWLs), containing a multi-quantum well gain region in a relatively large waveguide, can generate a watt-level output in a diffraction-limited beam with a nearly circular profile.
- High-power diode bars contain an array of broad-area emitters, generating tens of watts with poor beam quality. Despite the higher power, the brightness is lower than that of a broad area LD.
- High-power stacked diode bars (→ diode stacks) are stacks of multiple diode bars for the generation of extremely high powers of hundreds or thousands of watts.
- Monolithic surface-emitting semiconductor lasers (VCSELs) typically generate a few milliwatts with high beam quality. There are also external-cavity versions of such lasers (VECSELs) which can generate much higher powers with still excellent beam quality.
Laser diodes may emit a beam into free space, but many LDs are also available in fiber-coupled form. The latter makes it particularly convenient to use them, e.g., as pump sources for fiber lasers and fiber amplifiers.
Nearly all electrically pumped semiconductor lasers are laser diodes; quantum cascade lasers are an exception. Other semiconductor lasers rely on optical pumping and therefore do not require a p–n junction; they can be made of undoped semiconductor materials.
Emission Wavelengths
The emission wavelength of a laser diode is essentially determined by the bandgap of the laser-active semiconductor material: the photon energy is close to the bandgap energy. In quantum well lasers, there is also some influence of the quantum well thickness. A variety of semiconductor materials makes it possible to cover wide spectral regions. In particular, there are many ternary and quaternary semiconductor compounds, where the bandgap energy can be adjusted in a wide range simply by varying the composition details. For example, an increased aluminum content (increased x) in AlxGa1−xAs causes an increase in the bandgap energy and thus a shorter emission wavelength. Table 1 gives an overview on typical material systems.
| Laser diode material (active region / substrate) |
Typical emission wavelengths | Typical application |
|---|---|---|
| InGaN / GaN, SiC | 380, 405, 450, 470 nm | data storage |
| AlGaInP / GaAs | 635, 650, 670 nm | laser pointers, DVD players |
| AlGaAs / GaAs | 720–850 nm | CD players, laser printers |
| InGaAs / GaAs | 900–1100 nm | pumping EDFAs; high-power VECSELs |
| InGaAsP / InP | 1000–1650 nm | optical fiber communications |
Table 1: Emission wavelengths of various types of laser diodes.
Most laser diodes emit in the near-infrared spectral region, but others can emit visible (particularly red or blue) light or mid-infrared light.
Emission Bandwidth and Wavelength Tuning
Most LDs emit a beam with an optical bandwidth of a few nanometers. This bandwidth results from the simultaneous oscillation of multiple longitudinal (and possibly transverse) resonator modes (multimode laser diodes). Some other kinds of LDs, particularly distributed feedback lasers, operate on a single resonator mode (→ single-frequency operation), so that the emission bandwidth is much narrower, typically with a linewidth in the megahertz region. Further linewidth narrowing is possible with external cavities and particularly with narrowband optical feedback from a reference cavity (→ stabilization of lasers).
The emission wavelength (center of the spectrum) of multimode LDs is usually temperature sensitive, typically with an increase of ∼ 0.3 nm per 1 K temperature rise, resulting from the temperature dependence of the gain maximum. (The temperature influences the thermal population distributions in the valence and conduction band.) For that reason, the junction temperature of LDs for diode pumping of solid-state bulk lasers has to be stabilized, if the absorption bandwidth of the laser crystal is narrow (e.g. only a few nanometers wide). It is also possible to tune the emission wavelength via the junction temperature.
Single-mode diodes can have a significantly smaller temperature coefficient of the emission wavelength. For applications in scanning spectroscopy, the wavelength is sometimes scanned by operating the laser intermittently. The temperature then rises during each current pulse and causes the optical frequency to fall. The wavelength of external-cavity lasers can also be tuned, e.g. by rotating the diffraction grating in the laser cavity.
Power Conversion Efficiency
Diode lasers can reach high electrical-to-optical efficiencies – typically of the order of 50%, sometimes even above 60%. (There are development programs on the way to push efficiencies of high-power LDs above 70%.) The efficiency is usually limited by factors such as the electrical resistance, carrier leakage, scattering, absorption (particularly in doped regions), and spontaneous emission. Particularly high efficiencies are achieved with laser diodes emitting e.g. around 940–980 nm (as used e.g. for pumping ytterbium-doped high-power fiber devices), whereas 808-nm diodes are somewhat less efficient.
Beam Quality and Beam Shaping
Some low-power LDs can emit beams with relatively high beam quality (even though the high beam divergence requires some care to preserve that during collimation). Most higher-power LDs, however, exhibit a relatively poor beam quality, combined with other non-favorable properties, such as a large beam divergence, high asymmetry of beam radius and beam quality between two perpendicular directions, and astigmatism. It is not always trivial to find the best design for beam shaping optics, being compact, easy to manufacture and align, preserving the beam quality and avoiding interference fringes, removing astigmatism, having low losses, etc. Typical parts of such diode laser beam shaping optics are collimating lenses (spherical or cylindrical), apertures, and anamorphic prisms.
Beam Combining
As the light emitted by a laser diode is linearly polarized, it is possible to combine the outputs of two diodes with a polarizing beam splitter, so that an unpolarized beam with twice the power of a single diode but the same beam quality can be obtained (polarization multiplexing). Alternatively, it is possible to combine the beams of LDs with slightly different wavelengths using dichroic mirrors (→ spectral beam combining). More systematic approaches of beam combining allow combining a larger numbers of emitters with a good output beam quality.
Pulse Generation
Although the most common mode of operation of LDs is continuous-wave operation, many LDs can also generate optical pulses. In most cases, the principle of pulse generation is gain switching, i.e. modulating the optical gain by switching the pump current. Small diodes can also be mode-locked for generating picosecond or even femtosecond pulses. Mode-locked laser diodes can be external-cavity devices or monolithic, in the latter cases often containing different sections operated with different current.
Noise Properties
Different types of diodes have very different noise properties. The intensity noise is typically close to quantum-limited only well above the relaxation oscillation frequency, which is very high (often several gigahertz). However, some low-power LDs operated at cryogenic temperatures have been demonstrated to exhibit even significant amplitude squeezing, i.e., intensity noise well below the shot noise limit. In all semiconductor lasers, intensity noise is generally coupled to phase noise, making these noise properties strongly correlated.
As mentioned above, linewidth values are very different. Multimode LDs exhibit a lot of excess noise associated with mode hops. Noise in different modes can be strongly anti-correlated, so that the intensity noise in single modes can be much stronger than the noise of the combined power. This has the important consequence that the intensity noise can be increased when the beam e.g. of a diode bar is truncated at an aperture or spectrally filtered.
The diode driver can also contribute a lot to the laser noise, because even very fast current fluctuations can be transformed into intensity and phase fluctuations of the generated light.
Device Lifetime
When operated under proper conditions, diode lasers can be very reliable during lifetimes of tens of thousands of hours. However, much shorter lifetimes can result from a number of factors, such as operation at too high temperatures (e.g. caused by insufficient cooling) and current or voltage spikes, e.g. from electrostatic discharge or ill-designed laser drivers.
There are different failure modes, including catastrophic optical damage (COD) (with complete device destruction within milliseconds or less) and steady degradation. Apart from the operation conditions, various design factors strongly influence the lifetime. For example, designs with aluminum-free active regions have been found to have superior reliability and lifetimes, and certain coatings (or just additional semiconductor layers) on optical surface can also be very helpful. The details of some advanced diode designs have not been disclosed by manufacturers in order to maintain a competitive advantage.
In order to improve device lifetimes, LDs are often operated at reduced current levels (and thus output powers). Moderate power reductions can at the same time increase the wall-plug efficiency due to the lower junction voltage, whereas stronger reductions reduce the efficiency.
Applications
Laser diodes are used in a very wide range of applications. The following list gives some important examples:
- Low-power single-mode LDs with high beam quality are used for data recording and reading on CD-ROMs, DVDs, Blu-ray Discs, and holographic data storage media. Such lasers can operate in different spectral domains from the infrared to the blue and violet region, with the shorter wavelengths allowing for higher recording densities.
- Single-mode LDs are widely used in optical fiber communications, particularly in data transmitters. In some cases, the data modulation is done directly via the drive current.
- Single-mode LDs are also applied in spectroscopy with very compact low-power measurement devices.
- Small red laser diodes (→ red lasers) are used as laser pointers.
- Distance measurements are often done with modulated low-power diode lasers. Similar lasers are used in laser printers, scanners and bar code readers.
- Broad area laser diodes, diode bars and diode stacks are often used for diode pumping of solid-state lasers. Fiber-coupled broad area LDs also serve as pump sources of fiber amplifiers.
- Some kinds of surgery (e.g. treatment of enlarged prostates) and dermatological therapies can be done with radiation from diode bars.
- High-power diode stacks are directly used in material processing in cases where a high beam quality is not required, e.g. for surface hardening, welding and soldering. Compared with other high-power lasers, they are simpler and have a much better wall-plug efficiency.
In terms of sales volumes, the applications in optical data storage and telecommunications are very dominating. The third most important application, which is pumping of solid-state lasers, already has sales volumes which are nearly an order of magnitude lower than the previously mentioned sectors.
Related Devices
LDs are often used in the form of laser diode modules, containing a variety of additional components e.g. for beam shaping and cooling and protection of the LD, and wavelength conversion.
A semiconductor optical amplifier (SOA) has a setup which is similar to an LD, but the end reflections are suppressed. Without an input signal, such a device can act as a superluminescent diode (SLD), generating light via amplified spontaneous emission. The optical spectrum is then smooth and normally much broader.
Light-emitting diodes (LEDs) use the same mechanism of generating photons as LDs, but they usually do not exhibit significant optical amplification (laser gain). So-called resonant-cavity LEDs do exploit some degree of stimulated emission, and are in this sense intermediate between ordinary LEDs and SLDs.
Bibliography
| [1] | R. N. Hall et al., “Coherent light emission from GaAs junctions”, Phys. Rev. Lett. 9 (9), 366 (1962) |
| [2] | N. Holonyak and S. F. Bevacqua, “Coherent (visible) light emission from Ga(AS1−xPx) junctions”, Appl. Phys. Lett. 1, 82 (1962) |
| [3] | J. F. Butler et al., “Properties of the PbSe diode laser”, IEEE J. Quantum Electron. 1 (1), 4 (1965) |
| [4] | C. A. Wang and S. H. Groves, “New materials for diode laser pumping of solid-state lasers”, IEEE J. Quantum Electron. 28 (4), 942 (1992) |
| [5] | P. J. Delfyett et al., “High-power ultrafast laser diodes”, IEEE J. Quantum Electron. 28 (10), 2203 (1992) |
| [6] | J. G. Endriz et al., “High power diode laser arrays”, IEEE J. Quantum Electron. 28 (4), 952 (1992) |
| [7] | J. V. Moloney et al., “Quantum design of semiconductor active materials: laser and amplifier applications”, Laser & Photon. Rev. 1 (1), 24 (2007) |
| [8] | W. W. Chow and S. W. Koch, Semiconductor-Laser Fundamentals, Springer, Berlin (1999) |
| [9] | L. A. Coldren and S. W. Corzine, Diode Lasers and Photonic Integrated Circuits, John Wiley & Sons, New York (1995) |
See also: semiconductor lasers, external-cavity diode lasers, distributed feedback lasers, distributed Bragg reflector lasers, broad area laser diodes, diode bars, diode stacks, surface-emitting semiconductor lasers, laser diode modules, fiber-coupled diode lasers, beam shapers
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!
The Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book. Maybe you would enjoy reading it also in that form! The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company.
You may order the print version via Wiley-VCH.
