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Laser Diodes

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Acronym: LD

Definition: semiconductor lasers with a current-carrying p–n junction as the gain medium

German: Laserdioden

Category: lasers

How to cite the article; suggest additional literature

Laser diodes 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.

Diode lasers are lasers based on one or several laser diodes.

Most semiconductor lasers are based on laser diodes, but there are also some types of semiconductor lasers not requiring a diode structure and thus not belong to the category of diode lasers. In particular, there are quantum cascade lasers and optically pumped semiconductor lasers. The latter can be made of undoped semiconductor materials which cannot conduct significant electrical currents.

Types of Laser Diodes

edge-emitting laser diode

Figure 1: Schematic setup of an edge-emitting low-power laser diode. The waveguide and the output beam emerging at one edge of the wafer die are shown, but not the electrode structures.

Most laser diodes (LDs) are 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:

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.

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, pumping solid-state lasers
InGaAs / GaAs 900–1100 nm pumping EDFAs and other fiber amplifiers; high-power VECSELs
InGaAsP / InP 1000–1650 nm optical fiber communications

Table 1: Emission wavelengths of various common types of laser diodes.

Note that there some laser diodes operating outside the spectral regions given in the table. For example, InGaN lasers may be optimized for longer emission wavelengths, reaching the green spectral region, although typically with lower performance. Besides, there are e.g. lead salt diodes for generating mid infrared light.

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 optical 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, as the resonance frequencies react less to temperature changes than the optical gain does. 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.

Voltage–Current Characteristics

Laser diodes have voltage–current characteristics like other diodes. A substantial current flows only above for certain critical voltage, which depends on the used material system. (The critical voltage is roughly proportional to the bandgap energy of the material and also depends somewhat on the device temperature.) Above the critical voltage, the current rises rapidly with increasing voltage.

voltage-current characteristics of a laser diode

Figure 2: Current vs. applied voltage for an 808-nm laser diode. For a current of 1.2 A, as needed for the nominal output power of 1 W, the required voltage is roughly 1.8 V. (For comparison, the photon energy for 808 nm is 1.53 eV.) Note that this curve is shifted to the right for increasing device temperature; one then obtains a higher current for the same voltage.

A laser diodes is normally not operated by applying a fixed voltage, because the flowing current could then very sensitively depend on that voltage, and could also be substantially affected by the device temperature. There could even be a catastrophic runaway effect: a high current could lead to a strong temperature rise, which could further increase the current, finally destroying the diode. Therefore, in practice one usually uses a laser diode driver which stabilizes a certain current; this means that it automatically adjusts the voltage such that the desired current is obtained. Alternatively, one uses a constant power mode, where the drive current is automatically adjusted for achieving the desired output power.

Note that the current rather than the voltage determines the rate with which carriers are injected into the laser diode. Therefore, there is a strong relation between the flowing current and the emitted optical power. There is essentially no output power below a certain threshold current, and above the laser threshold the output power grows roughly in proportion to the current minus the threshold current.

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 laser diodes 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.

The highest power conversion efficiency is typically achieved not for the highest output power, but for a somewhat reduced output power, because the required voltage is then lower.

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:

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.

Low-power laser diodes generate the largest revenues of all laser types – mainly due to applications in communications and data storage. High-power laser diodes have far lower sales numbers and volumes, and are used mainly for displays (with fast growth), medical and military applications. Direct use of high-power laser diodes for material processing his a small volume so far, but exhibits rapid growth.

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)

(Suggest additional literature!)

See also: diode lasers, semiconductor lasers, distributed feedback lasers, distributed Bragg reflector lasers, broad area laser diodes, diode bars, diode stacks, external-cavity diode lasers, surface-emitting semiconductor lasers, laser diode modules, fiber-coupled diode lasers, beam shapers, direct diode lasers, laser diode drivers
and other articles in the category lasers

In the RP Photonics Buyer's Guide, 134 suppliers for laser diodes are listed.


Dr. R. Paschotta

This encyclopedia is authored by Dr. RĂ¼diger 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) and software could become very valuable for your business!

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