Digital data storage technology nowadays gets enormous change and makes physicists, materials scientists and electrical engineers to keep on updating with the materials requirements for media recording.Â The development in high density data storage technology needs efficient and short wavelength laser diode to focus the laser spot with much greater precision. This in turn allows data to get packed more tightly and the data can be stored in lesser space and hence storing more data on the disc. Blue and violetÂ semiconductor laser diodes are used nowadays for high density optical recording. In this literature review, the fundamentals of laser diode, its working principle, its current trend, factors affecting the growth of laser diode technology and the future direction have been analyzed.
Lasers are monochromatic, meaning they have only one frequency. The acronym for Laser is "Light Amplification by Stimulated Emission of Radiation". Lasing function requires many photons of light with same frequency must travel in the same direction, so as to interfere with each other light, which in turn increase the light's amplitude. 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.
2. Definition - Laser Diode
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A laser diode, also known as anÂ injection laserÂ orÂ diode laser, is a semiconductorÂ device which produces coherent radiation (i.e. the waves produced are at the same phase and frequency) in infraredÂ (IR) or visible spectrum whenÂ currentÂ passes through it. Laser diodes are used inÂ optical fiberÂ systems, compact disc (CD) players,Â laser printers, remote-control devices, intrusion detectionÂ systems, etc. The role of laser diode in optical recording systems is of greater interest since world is in need of high density optical disc.
Small size and weight: The laser diode normally measures lesser than one mm and weighs just a fraction of gram, thus making it perfect for use in many portable electronic equipments.
Low current,Â voltage, and power requirements:Â Most laser diodes need only a few mW of power at very low DC voltage and several mA. Hence, it can be operated even using small battery power supplies.
Low intensity:Â A laser diode cannot be used particularly for spectacular purposes like burning holes in a metal, bringing down satellites or so due to its low intensity.
Wide-angle beam:Â The laser beam from laser diode have wide angle such that it produces a "cone" instead of a "pencil" of infrared or visible light, although this "cone" can be collimated easily by using convex lenses.
3. 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.
4. Laser Diode - Working Principle
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 asÂ phononsÂ (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. The basic structure is of laser diode is shown in Fig. 1.
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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 anÂ alloyÂ ofÂ aluminiumÂ and 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.
Fig.1. Basic structure of a laser diode
Laser light is produced in a similar way to a phonon or photon creation in conventional p-n 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.
In a conventional laser, light emitted from that of atoms is pumped continuously between twoÂ mirrors 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 aÂ Fabry-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 inÂ Continuous WaveÂ (CW) mode orÂ PulsedÂ mode. In Continuous Wave lasersÂ atoms are pumped continuously by creating a steady state population inversion and hence the continuous output. InÂ pulsed mode operation,Â 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.
5. 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.
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.
5.1. Recording Process:
In the recording process (as shown in Fig. 2), a collection of input data stream of digital information is first encoded and then modulated using encoder and modulator respectively to induce drive signal for a laser source. The laser source (usually laser diode) emits a light beam which is then directed and focused towards the storage medium through illumination optics in order to precise the laser beam. When the recording medium moves under the scanning spot, the laser light energy just heats up the small localized region onto the medium with respect to the data to be recorded. The storage medium usually changes its reflective properties with the influence of heat. The modulation of light beam with respect to the input data stream makes a circular track of data marks in recording medium since the medium rotates. After each revolution of circular track, the scan spots path is changed in radius to allow data to be stored in another track.
Fig.2. Recording process of an optical medium
5.2. Readout Process
In readout process of the medium (Fig. 3), 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 splitterÂ into the illumination optics, in which the beam is focused into the medium. When the read data pass under the scan spot, the light reflected gets modulated. Then the modulated light is obtained by the illumination optics and diverted by the beam splitter towards the servo/data optics, which then converge the incoming light onto the detectors. The detectors function is to convert light modulation into a current modulation. Then that current signal is amplified and decoded back in order to produce the output data stream similar to data input stream while recording process.
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Fig.3. Readout process of an optical medium
6. Growth of Laser diode and optical disc:
At past, optical discs were only used to store computer software and music. TheÂ laser discÂ format stores only analog video signals, but commercially lost mainly due to its non-re-recordability and high cost.
Most of the first-generation disc devices used an infrared laser reading head. Here an infrared laser diode is used for reading and writing. The size of the laser spot is directly proportional to its wavelength so wavelength can be the limiting factor for information density. The infrared range is not much shorter wavelength range and it is beyond the long-wavelength part of the visible light spectrum. So it supports only less density of data to store than any visible light colours like red, blue and violet. One such example for high-density data storage in this generation achieved using infrared laser was 700MB of net user data for 12Â cm CD.
Second-generation optical discs were used for storing large amounts of data, which also includes quality digital video. These types of discs are mostly read and write with a visible red light laser. The shorter wavelength of the laser and higherÂ numerical apertureÂ allows narrow light beam allowing smaller pits and lands in the disc. In DVD format, this laser diode allows 4.7GB of data storage on a standard 12Â cm, single-sided and single-layer disc.
Third-generation optical discs are under research and development, which was created forÂ high-definition videoÂ storage and supports higher data storage capacities with the use of shorter wavelength visible light laser diodes. The Blu-ray disc normally uses blue-violet lasers producing smaller pits and lands so as to provide higher storage capacity of data per layer. Blu-ray disc can store up to 25Â GB per single layered and 50Â GB per dual layered one.
6.4. Next generation
The following formats are expected to go beyond the current third-generation discs and it is believed that they have the ability to hold more than 1TB of data: Holographic Versatile Disc, Layer-Selection-Type Recordable Optical Disc (LS-R) and Protein-coated disc. These formats too need shorter wavelength lasers but these techniques uses different architectures to store data precisely. So the currently available laser diodes are good enough to get the above mentioned data storage. Even though if the wavelength of laser diode gets shorter and shorter it is sure that a very high capacity of data can be stored in a single disc.
7. Types of Laser diodes:
Double heterostructure laser
Quantum well laser
Quantum cascade laser
Distributed feedback laser
Vertical cavity surface emitting laser (VCSEL)
Vertical external cavity surface emitting laser (VECSEL)
Separate confinement heterostructure laser
External cavity diode laser
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 (simple QW laser) to reduce the production cost and size of the optical pickup head.
8. 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 Ã- 400 Î¼m2) 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.
Fig.4. Schematic of a compact optical disk readout head using a VCSEL integrated with an intracavity absorber
Fig.4 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.
9. A step to shorter wavelength laser diode:
Short wavelength laser diode gives us a solution to focus the laser beam spot with higher precision. So the data to be stored can be packed tightly and hence data gets stored in very less space, thus producing high density recording onto the disc. Blue and violetÂ semiconductor laser diodes are used nowadays for high density optical recording. Blue lasers usually operate at 400Â nmÂ and but the operation ranges between 360 and 480Â nm. The popular 405Â nm laser diode currently using in Blu-ray is not actually blue in colour, but it appears as violet to our eyes. The blue semiconductor lasers were first developed in late 1990s and at that time, blue lasers seems to be large and requires expensiveÂ gas laserÂ instruments. Moreover it rely on population inversionÂ in rare gases and hence needed very large currents and strong cooling systems. The prior developmentÂ of many research groups invented a lot of laser diodes and developed commercially suitable compact blue-violetÂ semiconductor laser diodes.
10. Quantum well laser diodes for shorter wavelength:
The double heterostructure laser is also a waveguide for electron waves and not just for light waves. It has been realized that there will be discrete levels in the potential well. The simple calculation proves that if the active layer of the double heterostructure is as thin as several tens of nm, the levels of electron would split into tens of meV and such a structure is now we called as aÂ quantum well. A typical quantum well structure of laser is shown in Fig. 6. The multiple quantum well structure is used to improve the overlap of the gain region of the laser diode and also for getting shorter wavelength.
(a) Single quantum well (SQW) (b) Multiple quantum well (MQW)
Fig.5. Quantum well lasers
11. Blu-ray Discs need shorter wavelength laser diode:
Any discs normally stores audio and video information inÂ pits. Pits are spiral grooves which run from the centre of the disc to the edges (as shown in Fig.6). AÂ laserÂ reads the other side of these pits - theÂ bumpsÂ - in order to play movie or program stored on a DVD or any type disc. The pits must be packed closely so as to store more data onto the disc. So smaller the pits and the bumps, laser diode used for reading the data must be more precise.
Unlike current DVDs, which use aÂ red laserÂ to read and write data, Blu-ray uses aÂ blue laserÂ (which is where the format gets its name). A blue laser has aÂ shorter wavelengthÂ (405 nm) than that of a red laser (650 nm). The smaller laser beam from laser diode focuses precisely, causing it to read the recorded information in the form of pits (0.15 Âµm). This pit size is greater than twice as the pit size of DVD. Also, Blu-ray disc has track pitchÂ reduced from 0.74 Âµm toÂ 0.32 Âµm. Smaller the pits, smaller should be the beam and hence getting shorter track pitch (as shown in Fig.7) together enable a single-layer Blu-ray disc to hold more than 25 GB of information (which is about 5 times of the amount of information stored on a single DVD).
Fig.6. DVD Vs. Blu-ray disc construction
Fig.7. CD Vs. DVD Vs. Blu-ray Writing
12. Blue Laser Diode:
In past, it was seen that developing blue laser diode was an impossible thing due to the criticality in crystallizing the semiconductor which was essential to produce such a light. Such type of semiconductor which has capability in producing blue or blue-violet laser light would have to be manufactured by the use of elements present at the high end of periodic table which is really a complex process to crystallize since they have strong bonding characteristics. This seems to be a great challenge at past.
The development in the field of large capacity recording and high speed recording for such advanced Blu-ray Disc systems have been upcoming using blue-violet laser diodes. So blue-violet laser diodes with high output power is required in order to achieve these types of 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.
12.2. InGaN-Based Blue-Violet Laser Diodes:
The GaN based blue-violet laser diodes development with a shorter wavelength of 405 nm gives us a great interest in using it in optical storage systems for optical source. These devices are commercially available now, even though, the demand for multilayer disk systems and faster recording require high power laser diode. Fig.8 shows the schematic structure of InGaN-based blue-violet laser diodes. The epitaxial layers were normally developed by metal organic chemical vapour deposition. The structure consists of an n-type GaN waveguide layer, n-type AlGaN cladding layer, p-type GaN evaporation preventing layer, a p-type GaN waveguide layer, a p-type AlGaN cladding layer, an InGaN three QW active region, and a p-type GaN contact layer.
Fig.8. Schematic structure of InGaN-Based Blue-Violet Laser Diodes
The use of innerstripe laser diode structure is the solution to reduce the junction temperature. In innerstripe laser diode, AlN is used as an electrical insulator instead of SiO2 which is used as an insulator in conventional ridge waveguide laser diode. In such conventional laser diode, p-type cladding and guide layers are covered by SiO2 which actually have poor thermal conductivity. In a conventional ridge waveguide laser diode, a low refractive index dielectric (usually SiO2) is used in order to provide the contrast of refractive index for the purpose of optical confinement. Also it is used to isolate the bond pads electrically. But, heat flow in SiO2 is very poor because of its low thermal conductivity. So an insulator with good thermal properties is required to increase laser diode performance. So SiO2 is replaced with AlN which has been found that it decreases the self heating of active region and high power performance gets increased. The AlN has high thermal conductivity depending on its crystal quality.
Fig.9. (a) input current vs optical output; (b)Pulse oscillation waveform
Fig.9 sShows the typical characteristics for InGaN-Based Blue-Violet Laser Diodes. It has been shown that continuous oscillation is achieved at a threshold current of 36mA and the value for pulsed operation found to be 28mA. The wavelength for pulsed oscillation for different input current is shown in which 50mA input current results in 401.35nm as wavelength. Thus from these characteristics it has been shown that shorter wavelength requites high output power.
12.3. A step forward to 342-nm ultraviolet laser diode:
The use of semiconductor laser diodes that emits shorter wavelength UV light is realized for a more number of applications including high density data storage. Nitride materials of Group III are one of the most promising material for fabricating such type of shorter wavelength devices.
Here an AlGaN multiple-quantum-well laser diode (Fig. 10) is described which emits light at 342 nm, shortest wavelength not ever reported until for an laser diode driven electrically. Ultraviolet laser diodes and light-emitting diodes (LEDs) with GaN, AlGaInN or AlGaN as active layers can emit shorter than 365 nm wavelength of light. This is because GaN is having large bandgap energy of 3.4 eV. For UV LEDs, operation of such nitride-based structure has been evaluated already in deep UV wavelengths which are short as 210 nm. The steps in developing shorter wavelength UV Laser diodes driven electrically have been limited in recent years. Several research groups have reported UV Laser diodes with AlGaInN/AlGaN (in the form: well/barrier), AlGaInN/AlGaInN, AlGaN/AlGaInN, or GaN/AlGaN QWs which are grown on sapphire (GaN and SiC) substrates. But still the laser emission wavelength ranges only from 343 to 365 nm. For laser diodes it is difficult to move the lasing wavelength to the shorter UV region, since because it requires thicker layers, more complex in structures, and low dislocation density to achieve both electrical and optical confinement for lasing and also for high emission efficiency.
An AlGaN layers (usually used as cladding layers for achieving optical confinement) growth along with high AlN mole fractions seems to be very difficult to achieve due to the issues with poor crystalline quality. For high value of AlN mole fractions or for thicker layers, due to the tensile strain, the epitaxial AlGaN layers which are grown on substrates like as sapphire (GaN & SiC) results in crack formation and dislocations. So AlGaN materials with high crystalline quality and high AlN mole fractions (i.e. with crack formation and low dislocation densities) are essential for the fabrication of best performance devices. For blue-violet laser diodes and LEDs based on GaInN active layers, the efficiency of emission is improved by including indium clusters, which permits the capture of both holes and electrons in the localized centre.
Fig.10. AlGaN MQW UV laser diode
An device structure of UV laser diode made using AlGaN layer is shown in Fig.10. The room-temperature emission spectra of a UV-Laser diode with 900 mm-long cavity which is operating below and above the threshold values is shown in Fig. 11a and b. It has been shown (Fig. 11a) that the peak of the spontaneous emission moves to a shorter wavelength region and the width going to be narrower just by increasing the injection current. A broad lasing emission of peak wavelength 342.7 nm is observed and also a full-width at half-maximum (FWHM) as 0.9 nm is observed at a current 415 mA (Fig. 11b). The increase in the injection current makes the peak of the emission to move slightly towards shorter wavelength region and the width going to be narrower. Finally, a sharper lasing emission at 342.3 nm wavelength with full-width at half-maximum (FWHM) as 0.3 nm is obtained with that of 512mA current.
Fig.11. Emission spectra of the AlGaN MQW UV-Laser diode for injection currents
(a) below threshold and (b) above threshold
Fig.12. Voltage/current and Light output/current characteristic of AlGaN MQW UV-Laser diode operating at a 342.3 nm wavelength
As shown in Fig. 12, it is seen that above the threshold value of injecting current the output power increases spontaneously providing a high output power lasing with shorter wavelength.
12.4. Indium-free success of laser diode for high density optical storage:
Until today, LEDs based on GaN quantum wells with Al or In can cover the spectral range of red (wavelengths > 600 nm) to deep-UV (wavelengths typically 210 nm). But in the case of laser diodes due to tighter material requirements for lasing, are difficult to fabricate than that of LED and are confined to narrow spectral range of blue (wavelength of 488 nm) to near-UV (wavelength of 340 nm). For high-density optical data storage, wavelength somewhere between 250 nm and 300 nm is to be achieved for next generation disks following the Blu-ray one (uses 405-nm lasers) and this can be achieved even with AlGaN quantum-well laser diodes itself with some structure changes.
However, involvement in widening the materials lasing spectral region is being slow, but still grand efforts has been taken to improve the quality of the material. For example, Nichia Corporation's blue laser diodes are change the lasing wavelength of about 4 nm per year.
Researchers at Hamamatsu Photonics in Japan reported an electrically pumped AlGaN blue laser diode with 342 nm which is the shortest wavelength achieved until. The record wavelength before is 343 nm and 342 nm achievement is not so much the previous records, but the fact is, the laser diode as shown in fig. is an indium-free structure.
Fig.13. An image of visible photoluminescence from Hamamatsu's UV laser diode.
Inset picture- close up of emission region.
The usual blue laser diode in Blu-ray disk equipment with 405 nm wavelength is a double heterostructure strip waveguide design as explained before with one to three InGaN quantum wells for the purpose of carrier confinement, a GaN waveguide layers and AlGaN cladding layers. In order to achieve a short wavelengths beyond the 3.4eV band edge of GaN, the best solution is switching into indium-free AlGaN quantum wells. And this should be made from AlGaN waveguide and cladding layers but with different proportion of aluminium content. But there is a problem with this! Layers with large content of aluminium results in crack due to the tensile strain, since because AlN has the small lattice constant of the group III nitrides.
Finally it has been concluded that, it is very hard to analyze degradation problems for short wavelength laser diodes, since high aluminium content may lead to degradation. At this shorter UV wavelength, the photon energy is high which would result in photo-enhanced facet or bulk degradation and catastrophic optical damage. The material quality with low dislocation density substrates may be the critical factor for the production of successful shorter wavelength laser diodes in the future.
Thus the essential need for shorter wavelength laser diode and its different possible designs at current and future trend have been studied. It has been seen that a 342.3 nm laser diode have been created until today and also the possibility of reducing the wavelength further have also been analyzed. It have be noted down that fabrication and design of UV-Laser diodes seems to be more complex when compared to the LEDs and also the laser diode requires higher quality materials to be used with it for getting high efficiency and shorter wavelength. It has been seen that the use of AlN as an electrical insulator instead of SiO2 in InGaN blue-violet laser diode helps to reduce the self-heating of diode to provide high-power performance. Also an indium free laser diode has been analyzed which helps in the understanding of future direction of shorter wavelength laser diode. And it is realized that if the wavelength of laser diode gets shorter and shorter, a very high capacity of data can be stored in a single disc.