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Lasers are monochromatic, meaning they have only one frequency. Laser is an acronym for "Light Amplification by Stimulated Emission of Radiation". For a laser to function, many photons of light of the same frequency must all travel in the same direction, causing them to constructively interfere with each other, which increases the amplitude of the light. In the past, lasers were difficult to make and maintain. Today, lasers come in packages as simple as a single diode and they are used as laser sources in optical recording. The laser used in CDs has wavelength of 780 nm, 650 nm tin DVDs and 405 nm in Blu-rays discs. The shorter the wavelength gets, the higher recording density becomes.
What is Laser Diode?
A laser diode, also known as aninjection laserordiode laser, is a semiconductordevice that produces coherent radiation (in which the waves are all at the same frequency and phase) in the visible orinfrared(IR) spectrum whencurrentpasses through it. Laser diodes are used inoptical fibersystems, compact disc (CD) players,laser printers, remote-control devices, intrusion detectionsystems, etc. Small size and weight: A typical laser diode measures less than one millimeter across and weighs a fraction of a gram, making it ideal for use in portable electronic equipment.
Low current, voltage, and power requirements: Most laser diodes require only a few milliwatts of power at 3 to 12 volts DC and several milliamperes. Therefore, they can operate using small battery power supplies.
Low intensity: A laser diode cannot be used for spectacular purposes such as burning holes in metal, bringing down satellites, or blinding aircraft pilots. Nevertheless, its coherent output results in high efficiency and ease ofmodulationfor communications and control applications.
Wide-angle beam: A laser diode produces a "cone" rather than a "pencil" of visible light or IR, although this "cone" can be collimated using convex lenses.
Root of Semiconductor laser diode
The idea for semiconductor laser was proposed early in 1957. Soon after the construction of the fundamental theory of lasers by Schawlow and Townes in 1958, followed by the experimental verifications of laser oscillation in a ruby laser and a He-Ne laser in 1960, the pioneering work on semiconductor lasers was performed. In 1962, pulse oscillation at a low temperature in the first semiconductor laser, a GaAs laser, was observed. In 1970, continuous oscillation at room temperature was accomplished. Since then, remarkable development has been made by the great efforts in different areas of science and technology. Nowadays, semiconductor lasers have been employed practically as one of the most important optoelectronic devices and are widely used in a variety of applications in many areas.
In an ordinary p-n diode, electric current flows only in one direction and the electrons move towards the barrier and pass through it to combine with holes present on the other side and gives out energy asphonons(due to lattice vibrations) that disappear in the silicon itself. As in case of Light Emitting Diode (LED) or Laser Diode (LD), the same process as that in p-n diode takes place, except that, here the energy is given out as photons (i.e. packets of light) and not as phonons.
In a laser diode, the emission of photons is made to be pure and powerful. To get emission of photon, silicon as used in p-n diode has to be replaced by a different material, especially analloyofaluminiumand gallium arsenide or indium gallium arsenide phosphide or so. Electrons which are injected into the diode combined with holes to produce current but some of the excess energy is converted into photons. These photons generated again interact with incoming electrons producing more and more photons and so achieving a self-perpetuating process called resonance. This continuous process of electron to photon is called as stimulated emission process.
Laser light is produced in a similar way to a phonon or photon creation in conventional pn diode or LED. In laser diode one end of the diode is polished so that photons created gets reflected and emerge from it. The other end of the diode is left roughened to confine the light outwards.
In a conventional laser, light emitted from that of atoms is pumped continuously between twomirrors to get the concentrated light beam. In semiconductor laser diode, the same type of process happens but in a different way. Here the photons bounce back and forth in p-n junction i.e. between p-type and n-type semiconductor and this junction is known as aFabry-Perot resonant cavity. Thus the laser light gets amplified which emerges out from the polished end in a beam just parallel to that of the junction.
Lasers can be normally operated inContinuous Wave(CW) mode orPulsedmode. In Continuous Wave lasersatoms are pumped continuously by creating a steady state population inversion and hence the continuous output. Inpulsed mode operation,the laser light output varies with respect to time, with typical ON and OFF period. Light output ceases while reaching off period which makes population inversion to become depleted, and hence continued pumping is required to restore the population inversion for ON period so as to produce laser output. Most of the laser diodes operate in CW mode with the red and near infrared (NIR) range of wavelength. A variety of wavelengths (including blue (as used in Blu-Ray Disc players), and power options can be obtained by modifying the construction and material types used.
Role of Laser diode in Optical Data Storage:
It is essential to know the entire process of optical data storage to understand the role of laser diode in it.
Process of Optical Data Storage:
Optical data storage process involves storing information in a medium so that, when a light beam scans the medium, the reflected light from the medium can be used to recover the information. Storage media are of different types and many types of systems are used to scan data.
In the recording process (Fig. 1), an input stream of digital information is encoded and modulated using encoder and modulator respectively to induce drive signal for a laser source. The laser source (laser diode) emits an intense light beam that is directed and focused into the storage medium with illumination optics. As the medium moves under the scanning spot, energy from the intense scan spot is absorbed, and heats up the small localized region. The storage medium changes its reflective properties under the influence of the heat. Since the light beam is modulated in correspondence to the input data stream, a circular track of data marks is formed as the medium rotates. After every revolution, the path of the scan spot is changed slightly in radius to allow another track to be written.
The laser is used at a constant output power level so that light will not heat the medium beyond its thermal writing threshold. The laser beam (laser diode as source) is directed through a beam splitterinto the illumination optics, in which the beam is focused into the medium. When the data to be read pass under the scan spot, the reflected light is modulated. The modulated light is collected by the illumination optics and directed by the beam splitter to the servo and data optics, which converge the light onto detectors. The detectors convert the light modulation into current modulation. Then the current signal is amplified and decoded to produce the output data stream.
Growth of Laser diode and optical disc:
Initially, optical discs were used to store music and computer software. Thelaser discformat stored analog video signals, but commercially lost to theVHSvideotape cassette, due mainly to its high cost and non-re-recordability; other first-generation disc formats were designed only to store digital data and were not initially capable of use as a video medium.
Most first-generation disc devices had an infrared laser reading head. The minimum size of the laser spot is proportional to its wavelength, thus wavelength is a limiting factor against great information density, too little data can be stored so. The infrared range is beyond the long-wavelength end of the visible light spectrum, so, supports less density than any visible light colour. One example of high-density data storage capacity, achieved with an infrared laser, is 700MB of net user data for a 12cm compact disc.
Second-generation optical discs were for storing great amounts of data, including broadcast-quality digital video. Such discs usually are read with a visible-light laser (usually red); the shorter wavelength and greaternumerical apertureallow a narrower light beam, permitting smaller pits and lands in the disc. In the DVD format, this allows 4.7GB storage on a standard 12cm, single-sided, single-layer disc; alternately, smaller media, such as theMiniDiscand theDataPlayformats, can have capacity comparable to that of the larger, standard compact 12cm disc.
Third-generation optical discs are in development, meant for distributinghigh-definition videoand support greater data storage capacities, accomplished with short-wavelength visible-light lasers and greater numerical apertures. The Blu-ray disc uses blue-violet lasers and focusing optics of greater aperture, for use with discs with smaller pits and lands, thereby greater data storage capacity per layer. In practice, the effective multimedia presentation capacity is improved with enhanced videodata compressioncodecssuch asH.264, andVC-1.
The following formats go beyond the current third-generation discs and have the potential to hold more than one terabyte (1TB) of data: Holographic Versatile Disc, Layer-Selection-Type Recordable Optical Disk (LS-R) and Protein-coated disc.
Types of Laser diodes:
- Double heterostructure lasers
- Quantum well lasers
- Quantum cascade lasers
- Separate confinement heterostructure lasers
- Distributed feedback lasers
- VCSELs - Vertical cavity surface emitting laser
- VECSELs - Vertical external cavity surface emitting laser
- External cavity diode lasers
Amongst the types indicated above, Quantum Well (QW) laser diodes are used at most for shorter wavelengths and higher efficiency. The different structures of Quantum well laser diodes are: single quantum well (SQW), multiple quantum well (MQW), and graded-index separate-confinement heterostructure (GRINSCH). The main advantages of a quantum well laser diode are more efficient current-to-light conversion, better confinement of the output beam, and the potential to radiate a variety of wavelengths. Also the promising development in Vertical cavity surface emitting laser makes its desire to replace currently using Fabry-Perot lasers to reduce the production cost and size of the optical pickup head.
Vertical Cavity Surface Emitting Lasers
Vertical cavity surface emitting lasers (VCSELs) are made by sandwiching a light emitting layer (i.e., a thin semiconductor of high optical gain such as quantum wells) between two highly reflective mirrors. The mirrors can be dielectric multilayered or epitaxial growth mirrors of distributed Bragg reflectors (DBRs) with reflectivity greater than 99.9%. Light is emitted normally from the surface of the mirrors. The simple testing procedure is one of the merits of VCSELs even though the epitaxial growth of DBRs is required. This is because VCSELs allow manufacturers to carry out on-wafer testing prior to dicing and packaging so that the production cost is much lower than that of facet emitting lasers. In addition, the compact size of VCSELs (typically 400 - 400 µm2) yields more devices per wafer than do facet emitting lasers. Hence, these unique characteristics of VCSELs allow manufacturing of low-cost semiconductor lasers in large quantities. Narrow beam divergence, low power consumption, high modulation bandwidth, and easy polarization control are the other advantages of VCSELs over facet emitting lasers.
In optical disk readout systems such as CD and DVD, the low-cost Fabry-Perot semiconductor laser (i.e., facet emitting laser) is usually used as the optical source. A separate external photodetector is also used in the readout system to monitor the light reflected from the optical disk. However, the optical beam emitting from the Fabry-Perot laser facets is highly asymmetric so that the allowable information density of the optical disk has to be maximized using precise design of optical lenses. Therefore, it is desired to replace Fabry-Perot lasers by VCSELs to reduce the production cost and size of the optical pickup head. This is possible because the light emitted from VCSELs is a highly symmetric circular beam and a photodetector can be integrated monolithically with VCSELs so that a low-cost and compact size optical pickup head can be realized. Figure 1.2 shows a proposed VCSEL with an intracavity QW absorber to realize the integrated optical disk readout head. In this case, the CW optical beam emitting from the VCSEL is tightly focused onto the optical disk, and the reflected beam directly penetrates into the VCSEL cavity. The reflected signal from the optical disk is accurately measured by using the intracavity absorber, which is a photodetector under reverse bias. The major advantages of integrating VCSELs with an intracavity absorber as the optical pickup head are
- The circular output beam of VCSELs maximizes the data storage density of the optical disk.
- The single longitudinal mode behaviour of VCSELs eliminates the intermodal noise that can be found in Fabry-Perot lasers.
- The possibility of constructing 2D arrays, which can create novel optical pickup head for parallel readout to increase the information bandwidth of the readout systems.
Hence, it is believed that the use of VCSELs in optical pickup systems will reduce the production cost but increase the storage density of the optical disk even though the VCSEL has the same lasing wavelength as the Fabry-Perot laser used in the original system.
A step to shorter wavelength laser diode:
Short wavelength laser diode makes it possible to focus the laser spot with greater precision. This allows data to be packed more tightly and stored in less space, so it's possible to fit more data on the disc. Blue and violetsemiconductor laser diodes are used nowadays for high density optical recording. Blue lasers usually operate at 400nmbut in general the operation of these devices has been demonstrated between 360 and 480nm. The most popular 405nm laser is not in fact blue, but appears to the eye as violet, a color for which a human eye has a very limited sensitivity. Until the late 1990s, when blue semiconductor lasers were developed, blue lasers were large and expensivegas laserinstruments which relied on population inversionin rare gas mixtures and needed high currents and strong cooling. The prior developmentof many research groups made a series of inventions and developed commercially viable blue and violetsemiconductor lasers.
Blu-ray Discs need shorter wavelength laser diode:
Discs store digitally encoded video and audio information inpits- spiral grooves that run from the center of the disc to its edges (as shown in Fig. 3). Alaserreads the other side of these pits - thebumps- to play the movie or program that is stored on the DVD. The more data that is contained on a disc, the smaller and more closely packed the pits must be. The smaller the pits (and therefore the bumps), the more precise the reading laser diode must be.
Unlike current DVDs, which use ared laserto read and write data, Blu-ray uses ablue laser(which is where the format gets its name). A blue laser has ashorter wavelength(405 nanometers) than a red laser (650 nanometers). The smaller beam focuses more precisely, enabling it to read information recorded in pits that are only0.15 microns(µm) (1 micron = 10-6meters) long -- this is more than twice as small as the pits on a DVD. Plus, Blu-ray has reduced thetrack pitchfrom 0.74 microns to0.32 microns. The smaller pits, smaller beam and shorter track pitch (as shown in Fig. 4) together enable a single-layer Blu-ray disc to hold more than 25 GB of information - about five times the amount of information that can be stored on a DVD.
Blue Laser Diode:
The development of a blue or blue-violet laser was seen an impossible task because of the difficulty of crystallizing the compound semiconductor needed to produce this light. A semiconductor capable of emitting blue or blue-violet light would need to be manufactured from elements at the high end of the periodic table because elements at the high end of the periodic table are difficult to crystallize because of their strong bonding characteristics. This was a great challenge.
Development of high speed recording and large capacity recording for Blu-ray Disc (BD) systems using blue-violet laser diodes have been progressing. High-power blue-violet laser diodes are strongly required to achieve these systems.
For optical recording usually GaN-based Blue-Violet Laser Diodes are used. A commercial product with 405nm wavelength has already been available in the market. The research is going on in reducing the wavelength further to increase the storage density.
InGaN-Based Blue-Violet Laser Diodes:
The development of GaN-based blue violet laser diodes (LD) with a wavelength of _405 nm has attracted great interest, due to their use as light sources in optical storage systems. Although these devices are now commercially available, the continuous demand for high speed recording and multilayer disk systems will require the development of LDs with higher output power.
For high power laser diodes, efficient heat dissipation is critical as self-heating leads to lower slope efficiency and reduced device lifetime. The LD junction temperature has been shown to strongly influence the degradation rate of the device. Consequently, the high resisitivity of p-GaN and p-AlGaN results in Joule heating in the p cladding layer of nitrides laser diodes. Besides reducing the operating voltage and threshold current to reduce dissipated power, various methods have been used to improve the thermal properties. It has been observed that a slower degradation rate during lifetime testing with epi-down bonding, due to more efficient heat dissipation from the resistive p-GaN layers. It has been found that use of an innerstripe LD structure to reduce junction temperature. Simulation data have demonstrated more efficient heat dissipation through the crystalline structures of inner-stripe LDs, compared to a conventional ridge waveguide LD in which the p-layers are covered by SiO2 with its fairly poor thermal conductivity of 1W/mK. In this paper, we propose and demonstrate a nitride ridge waveguide LD using AlN as the electrical insulator. We report on the method for fabricating such LDs, and compare their device performance with conventional ridge waveguide LDs. In a conventional ridge waveguide structure, hereafter referred to as SiO2-LD, a low refractive index dielectric is typically used to provide the refractive index contrast for optical confinement and to electrically isolate the bond pads. SiO2 is widely used for this purpose. However, heat flow through SiO2 is poor due to its low thermal conductivity of 1W/mK. Therefore, an insulator with better thermal properties would be desirable to increase LD performance. Here, we investigate whether replacing SiO2 with AlN may reduce self-heating of the active region and increase high-power performance. The electrical properties of undoped AlN approach those of an insulator, and it has the potential advantages of high thermal conductivity of up to 280 W/mK, depending on its crystal quality.
A step forward to 342-nm ultraviolet laser diode:
The realization of semiconductor laser diodes and light-emitting diodes that emit short-wavelength ultraviolet light is of considerable interest for a number of applications including chemical/biochemical analysis, high-density data storage and material processing. Group III nitride materials are one of the most promising candidates for fabricating such devices. Here we describe an AlGaN multiple-quantum-well laser diode that emits light at 342 nm, the shortest wavelength ever reported for an electrically driven laser diode. To fabricate the laser, a low-dislocation-density AlGaN layer with an AlN mole fraction of 0.3 was grown on a sapphire substrate using a hetero facetcontrolled epitaxial lateral overgrowth (hetero-FACELO) method1-3. An AlGaN multiple-quantum-well structure was then grown on the high-quality AlGaN layer. Lasing at a wavelength of 342.3 nm was observed under pulsed current mode at room temperature. Ultraviolet laser diodes (UV-LDs) and light-emitting diodes (LEDs) with GaN, AlGaN or AlGaInN active layers can emit light at a wavelength shorter than 365 nm due to GaN having a large bandgap energy of 3.4 eV. Operation of such nitride-based UVLEDs has already been demonstrated in the deep UV at wavelengths as short as 210 nm (ref. 4). However, progress in developing short-wavelength electrically driven UV-LDs has been limited in recent years despite demonstrations of lasing and stimulated emission in the deep UV region from AlGaN and AlN layer under optically pumping5,6. Several groups have reported UV-LDs with AlGaInN/AlGaN (well/barrier), AlGaN/AlGaInN, AlGaInN/AlGaInN or GaN/AlGaN quantum wells grown on sapphire, GaN and SiC substrates3,7-10. However, the laser emission wavelengths reported span only from 343 to 365 nm, and detailed characteristics have only been reported for LDs operating at wavelengths of 350.9 nm or longer3,7-9. For LDs it is difficult to shift the lasing wavelength towards the shorter UV region, because they require a more complex structure, thicker layers and lower dislocation density than LEDs in order to achieve suitable optical and electrical confinement for lasing as well as high emission efficiency. The growth of AlGaN layers with high AlN mole fractions, which are typically used as cladding layers to achieve optical and electrical confinement, is very difficult because of issues with poor crystalline quality11. As a result of tensile strain, epitaxial AlGaN layers grown on substrates such as sapphire, SiC and GaN suffer from dislocations and crack formation, in particular at higher AlN mole fractions or for thicker layers. AlGaN materials with high AlN mole fractions and having high crystalline quality (low dislocation density and crack-free) are necessary for the fabrication of high-performance devices. For blue-violet LDs and LEDs based on GaInN active layers, emission efficiency is improved by the presence of indium-rich clusters, which allows the capture of electrons and holes in localized centres12,13. Previously reported UV-LDs include small amounts of indium in all the active layers, with the exception of the GaN/AlGaN active layer. Indium-free AlGaN active layers have never been used in any previous UV-LD. However, the inclusion of indium in the active layers inhibits emission at shorter UV wavelengths because of the very small bandgap of the InN and its sensitive growth condition. We believe that it is possible to further shift the emission wavelength to shorter UV wavelengths by increasing the AlN mole fraction in the active layers. The lack of indium in the AlGaN active layers increases the probability of non-radiative recombination in the active layers14,15. From this point of view, it is important to grow the AlGaN layer with a reduced number of dislocations acting as non-radiative centres and demonstrate operation of UV-LDs in which the multiple quantum wells (MQWs) consist of AlGaN wells and barriers without the assistance of indium. Here we report the first detailed characterization of an indium free AlGaN MQW UV-LD fabricated on a sapphire substrate. First, a low-dislocation-density AlGaN layer was fabricated by metalorganic vapour phase epitaxy (MOVPE). Al0.3Ga0.7N layers, even with relatively high AlN mole fractions of 0.3, were successfully grown by the hetero-FACELO method1-3. In order to evaluate the quality of the AlGaN layer, photoluminescence (PL) measurements from MQWs on the Al0.3Ga0.7N layer (sample A) and an Al0.2Ga0.8N layer (sample B) were performed. Sample B consists of the layers that have been used to previously fabricate 355-nm UV-LDs3 and a low dislocation density with an average dark spot density of 3.9 _ 108 cm22 has already been determined by cathodoluminescence measurements. The PL intensity of sample A was 16% lower than that of sample B. This result indicates that the crystalline quality of sample A is almost equivalent to that of sample B, and that the Al0.3Ga0.7N layer has a low dislocation density and is suitable for producing a UV laser structure. Next, we fabricated UV-LDs on the Al0.3Ga0.7N layer. The device structure is illustrated in Fig. 1. The design provides suitable optical confinement with a theoretically calculated factor of 0.8 in the waveguide as a result of the low refractive index of the Al0.3Ga0.7N cladding layers. Figure 2a,b shows a series of room-temperature spontaneous and lasing spectra of a UV-LD with a 900 mm-long cavity operating below and above threshold, respectively. These spectra were measured using a calibrated spectrometer with a resolution of 0.3 nm and the LD was driven in pulsed-current mode with a pulse duration of 10 ns and a repetition frequency of 5 kHz. Spontaneous emission can be observed at a peak wavelength of approximately 345 nm with a full-width at half-maximum (FWHM) of only 6 nm at a current of 185 mA. This very narrow spectrum of the spontaneous emission can be interpreted in terms of a low fluctuation of the well width and a homogeneous composition in the AlGaN MQWs. The peak of the spontaneous emission shifts to a shorter wavelength and the width becomes narrower by increasing the injection current. The relatively broad lasing emission with a peak wavelength of 342.7 nm and a FWHM of 0.9 nm can be observed at a current of 415 mA, as shown in Fig. 2b. On increasing the injection current the peak of the emission slightly shifts to a shorter wavelength and the width becomes narrower.
The light output/current and the voltage/current (V-I) characteristics of the same device. The device exhibits clear nonlinear behaviour in the light output/current characteristic. The threshold current of _390 mA corresponds to a threshold current density of 8.7 kA cm22. The measured pulse output power from one side of the facets is close to 16 mW, as measured by a silicon photodiode (Hamamatsu S1337-1010BQ). The differential external quantum efficiency (DEQE) for the output from both facets was estimated to be 8.2%. These output characteristics are comparable with previous reports on UV-LDs lasing at longer wavelengths3,7,8. From the V-I characteristics, it can be seen that the operation voltage is 25 V at the threshold. The differential series resistance around the threshold is estimated to be _32 V from the slope of the V-I curve. The device exhibits rather high resistive characteristics compared to previously reported UV-LDs3,8,9. Conductivity tends to decrease with increasing AlN mole fraction due to an increase in donor and acceptor ionization energies, which lowers the carrier concentration, and also as a result of the degradation in crystalline quality, which decreases carrier mobility20-22. It would appear that the high resistivity of this device is mainly attributable to the low carrier concentration and mobility of each AlGaN cladding and guiding layer, as well as the low conductivities of the p- and n-contacts. A next step towards developing a higherperformance device for continuous-wave operation would be achieved through having a smaller device resistance. It should be noted that the design and fabrication of UV-LDs is much more complex than that for LEDs, and it also requires the use of higher-quality materials. To confine a sufficient number of carriers in the MQWs so as to invert the electron population, we used AlGaN cladding layers with a sufficiently high AlN mole fraction. The low refractive index of the cladding layer also leads to substantial optical confinement in the waveguide. Nonradiative recombination results in an increase of lasing threshold or even failure of the population inversion. In all previous successful demonstrations of UV-LDs, several growth techniques were performed to reduce dislocation density in the layers3,7-10. The effects of using an AlGaN layer with a high AlN mole fraction of 0.3 and a low dislocation density brought about by the hetero-FACELO method appear to be critical for achievement of laser operation.
Indium-free laser diode:
Today, LEDs based on (Al,In)GaN quantum wells cover the spectral range from the red (wavelengths greater than 600 nm) to deep-UV wavelengths (210 nm). In contrast, laser diodes are more difficult to fabricate, as a result of the tighter material requirements for lasing, and are confined to a much narrower spectral range from blue (488 nm) to near-UV (340 nm) wavelengths. For high-density optical data storage, the next-generation disks following Blu-ray (based on 405-nm lasers) are expected to operate with a wavelength somewhere between 250 nm and 300 nm, which, in principle, can be achieved with AlGaN quantum-well laser diodes. However, progress in widening the material system's spectral region of lasing has been slow, despite grand efforts to improve the material quality. For example, Nichia's blue laser diodes have been inching forward with a change in the lasing wavelength of 4 nm per year report an electrically pumped AlGaN laser diode at 342 nm - the shortest wavelength achieved so far. However, it is not so much the record wavelength that is most notable - 343 nm has been reached before - but the fact that the laser diode is an indium-free structure. The usual 405-nm laser diode in Blu-ray disk equipment is a double heterostructure strip waveguide design with one to three InGaN quantum wells in the gain region for carrier confinement, a GaN waveguide for light confinement, and AlGaN cladding layers.
The usual 405-nm laser diode in Blu-ray disk equipment is a double heterostructure strip waveguide design with one to three InGaN quantum wells in the gain region for carrier confinement, a GaN waveguide for light confinement, and AlGaN cladding layers. To achieve shorter wavelengths beyond the GaN band edge at 3.4 eV the obvious solution is to switch to indium-free AlGaN quantum wells, with the waveguide and cladding made from AlGaN but with different aluminium content.