Fundamentals of Laser Diode Amplifiers
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Above the critical voltage, the current rises rapidly with increasing 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. 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. 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. 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.
Types of Laser Diodes
Typical parts of such diode laser beam shaping optics are collimating lenses spherical or cylindrical , apertures and anamorphic prism pairs. 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.
More systematic approaches of beam combining allow combining a larger numbers of emitters with a good output beam quality. Although the most common mode of operation of LDs is continuous-wave operation , many LDs can also generate optical pulses.
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In most cases, the principle of pulse generation is gain switching , i. Small diodes can also be mode-locked for generating picosecond or even femtosecond pulses.
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Mode-locked laser diodes can be external-cavity devices or monolithic, in the latter cases often containing different sections operated with different current. 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. 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. 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. 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.
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. 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. LDs are often used in the form of laser diode modules , containing a variety of additional components e. A semiconductor optical amplifier SOA has a setup which is similar to an LD, but the end reflections are suppressed.
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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. Among them:. Suggest additional literature! See also: diode lasers , semiconductor lasers , distributed feedback lasers , distributed Bragg reflector lasers , broad area laser diodes , diode bars , diode stacks , high brightness laser diodes , 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 categories optoelectronics , lasers.
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Sorry, we don't have an article for that keyword! The waveguide and the output beam emerging at one edge of the wafer die are shown, but not the electrode structures. 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 or wide stripe 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. High brightness laser diodes are laser diodes which are optimized for a particularly high radiance brightness.
Different technologies may be used, and such lasers are available on quite different power levels. 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. Albert S. Propagation Engineering in Wireless Communications. Abdollah Ghasemi. Stefan M.
Buy Fundamentals of Laser Diode Amplifiers on giuliettasprint.konfer.eu ✓ FREE SHIPPING on qualified orders. The major advantages of the basic static design of laser diode amplifiers; Systematic analysis of signal and noise properties, including gain, bandwidth and.
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