Matthias Pospiech and Sha Liu University of Hannover, Germany Page Updated: October 22, In the further proceedings we are going to take a closer look at different techniques of constructing a laser diode. The focus thereby is on single mode laser diodes. Single mode waves are preferred in most cases. The realm of multimode lasers are high power lasers, where single mode operating is not of importance. Compared with broad-area lasers, where the entire laser chip is excited, the threshold current of lasers with stripe geometry is reduced roughly proportional to the area of contact.
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Matthias Pospiech and Sha Liu University of Hannover, Germany Page Updated: October 22, In the further proceedings we are going to take a closer look at different techniques of constructing a laser diode. The focus thereby is on single mode laser diodes. Single mode waves are preferred in most cases. The realm of multimode lasers are high power lasers, where single mode operating is not of importance.
Compared with broad-area lasers, where the entire laser chip is excited, the threshold current of lasers with stripe geometry is reduced roughly proportional to the area of contact.
We differ mainly between two different types of structures. In case that the current injection is restricted to a small region along the junction plane these are termed gain-guided. Devices incorporating a built-in refractive index variation in the lateral direction are termed index-guided lasers. The active region is planar and continuous. Lasing however occurs only in a limited region of the active layer beneath the stripe contact where high density of current flows.
This horizontal confinement of the em—wave propagating through the active region is thereby accomplished by the small refractive index variation produced by the current generated population inversion. If the em—wave spreads in the horizontal plane outside of the horizontal dimensions of the stripe, it will be absorbed by the unexcited region of the active layer.
In the vertical directions the lower refractive indices of the surrounding layers reflects the em—wave back into the active region. The mode control is necessary for improving the em—wave current linearity and the modulation response of lasers. The active region is thereby surrounded by materials with lower refractive indices in both the vertical y and lateral x transverse directions — the active region is buried in lower refractive indices layers e.
InP on all sides. For this reason, these lasers are called buried-heterostructure lasers. As a result, the lasing characteristics of buried-heterostructure lasers are primarily determined by the rectangular waveguide that confines the mode inside the buried region. The transverse dimensions of the active region and the index discontinuities are chosen so that only the lowest order transverse modes can propagate in the waveguide.
Another important feature of this laser is the confinement of the injected carriers to the active region. They are i the buried heterostructure that is also called etched-mesa buried-heterostructure EMBH to distinguish it from other BH lasers. There are other structures which are easier in fabrication, but these are not discussed here. Index-Guided Structures Historically, gain-guided devices based on the stripe geometry were developed first in view of their ease of fabrication.
However gain-guided lasers exhibit higher threshold currents than index-guided lasers and have other undesirable characteristics that become worse as the laser wavelength increases. In spite of these difficulties in fabrication, their superior performance characteristics — low threshold current, stable fundamental mode operation, and good high speed modulation characteristics — make them a prime candidate for high-performance applications.
This lead to lasing of higher modes and allows for mode jumps. So the question is, how wavelength selection can be achieved. Shorter optical cavities are not practical since it is difficult to handle very small chips. Another possible method is to insert an optical feedback in the device to eliminate other frequencies.
Periodic gratings incorporated within the lasers waveguide can be utilized as a means of optical feedback. DFB and DBR lasers oscillate in a single-longitudinal mode even under high-speed modulation, in contrast to Fabry-Perot lasers, which exhibit multiple-longitudinal mode oscillation when pulsed rapidly. The grating thereby consist of corrugations with a periodic structure. They are used because of their frequency selectivity of single axial mode operation. The period of grating is chosen as half of the average optical wavelength, which leads to a constructive interference between the reflected beams.
Significant reflections can also occur in harmonics frequencies of the medium. The corrugations are typically etched on the surface of the waveguide, and these are refilled with a different index material during a second growth. The concept of the grating is that many reflections can add up to a large net reflection. At the Bragg frequency the reflections from each discontinuity add up exactly in phase.
As the frequency is deviated from the Bragg condition, the reflections from discontinuities further into the grating return with progressively larger phase mismatch.
Figure 18 shows a schematic of such a laser with one grating mirror. Besides the single frequency property provided by the frequency-selective grating mirrors, this laser can include wide tunability. Since the refractive index depends on the carrier density this can be exploited to vary the refractive index electro optically on the sections by separate electrodes.
The potential tunability of DBR Lasers is one of the main reasons why they are of great importance. As indicated in Figure 18, there are usually three sections, one active, one passive, and the passive grating. The first provides the gain, the second allows independent mode phase control and the grating is a mode selective filter.
By applying a current or voltage to the sections the refractive index changes, shifting the axial modes of the cavity. A distributed feedback laser DFB also uses grating mirrors, but the grating is included in the gain region.
Reflections from the ends are suppressed by antireflection coatings. Thus, it is possible to make a laser from a single grating, although it is desirable to have at least a fraction of a wavelength shift near the center to facilitate lasing at the Bragg frequency. The idea behind this concept is, apart from the wavelength selectivity, to improve the quality of the laser, as the active length is a quarter-wavelength long. This applies for no shift in the gratings, where the cavity can be taken to be anywhere within the DFB, since all periods look the same.
The modes of this laser are placed symmetrically around the Bragg frequency. However only the modes with lowest losses will lase. With symmetrical gain profile around the Bragg frequency, this means that two modes are resonant. To suppress one mode we need to apply additional perturbation reflections, such as from uncoated cleaves at the end. An alternative method is to introducing an extra quarter wavelength element in the grating.
This quarter wave shifted grating is shown in figure This however is more difficult to fabricate. The DBR is widely tunable, but relatively complex since a lot of structure must be created along the surface of the wafer. Both lasers however work in single mode. Their limitation in the output power mainly arises form the leakage current, which increases with an increase in the applied current. The stripes are closely spaced so that the radiation from neighboring stripes is coupled to form a coherent mode of the entire array to produce high power laser diodes.
These laser diodes operate in multimode, in contrary to those described so far. The array modes, often referred to as supermodes, are phaselocked combinations of the individual stripe modes and characterized by the phase relationship between the optical fields supported by adjacent stripes.
It therefore suggests itself for pumping of solid state lasers, because the wavelength range from to nm can be covered with appropriate semiconductor materials. Through stacking of these bars one can build up two-dimensional arrays. With such devices powers up to a few kW have already been achieved. Older laser diodes, called edge-emitting diodes, emit coherent em—waves parallel to the boundaries between the semiconductor layers. And because of its short cavity length it emits only one mode.
As a result, in a single pass of the cavity, a photon has a small chance of triggering a stimulated emission event at low carrier densities.
Such a high reflectivity can not be achieved by the use of metallic mirrors. DBRs as described on page These are formed by laying down alternating layers of semiconductor or dielectric materials with a difference in refractive index. Semiconductor materials used for DBRs have a small difference in refractive index therefore many periods are required. Since the DBR layers also carry the current in the device, more layers increase the resistance of the device therefore dissipation of heat may become a problem.
There are a number of different types of lasers, which generally differ in the way in which high-reflectivity mirrors and current confinement is achieved. One simple method is to etch a pillar down to the active layer. These are termed mesa2 etched structure. Etched mesas are typically a few micrometer in diameter which allows fabrication of a large number of lasers on a single substrate. The large difference in refractive index between the air and device material also act to guide the em—wave emitted.
However, for practical use a suitable bonding scheme is required. A polyamide can be used for filling up the region around the etched mesas, to allow a practical bonding scheme. Another problem with this type of structure are the loss of carriers due to surface recombination at the sidewalls and poor dissipation of heat from the laser cavity.
Another technique for current confinement is ion implantation. This scheme reduces current spreading, producing a region of high gain at the center of the opening where laser action takes place. Since the substrate is absorbing in these cases, em—waves are emitted from the top.
It can also be emitted from the bottom by using a structure in which the substrate near the emitting region has been etched away.
Optoelektronik by Ebeling
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DSCA HANDBOOK PDF
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