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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-05-13

(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. In this heterojunction, electrons and holes can combine, 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. This arrangement leads to a lower threshold pump power and better efficiency, compared with earlier diode designs. The active region also often contains quantum wells or quantum dots. Some modern kinds of LDs are of the surface-emitting type (see below), where the emission direction is perpendicular to the wafer surface.

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, generating between a few milliwatts and up to roughly half a watt of output power in a beam with high beam quality, sometimes with coupling into single-mode fibers. 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 rather short resonators can achieve single-frequency operation.
• 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), generating 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.
• Surface-emitting semiconductor lasers (VCSELs) typically generate a few milliwatts with high beam quality. However, there are also external cavity versions of such lasers (VECSELs) which can generate much higher powers with still excellent beam quality.

Most types of laser diodes are available either with emission into free space on 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.

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.

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 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 Combination

As the light emitted by a laser diode is linearly polarized, it is possible to combine the outputs of two diodes with a polarizing beamsplitter, 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. More systematic approaches of beam combining allow combining a larger numbers of emitters with a good output beam quality.

Output Spectrum and Wavelength Tuning

Most LDs are emitting 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.

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 have 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.

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.
• 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 often significantly 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

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

Category: lasers

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!