Applications of Semiconductor lasers

Applications of Semiconductor optic masersAbstract- Semiconductor opthalmic masers postulate the potential to meet the demands of next generation utmost speed opthalmic ne dickensrk applications and also have great impact on various other technology domains. Its embarrassed cost, easy wavelength tune ability, baseborn military force consumption and pure turnout wanton it ideal for optical intercourse applications. This paper is a review of the merits, demerits, current applications, commercial availability and future directions of semiconducting material optical masers in optical communion. Comparative analysis of these semiconductor lasers with respect to various parameters also been conducted.Keywords Semiconductor lasers, Fabry-Perot laser, Distributed feedback (DFB) laser, External Cavity Diode Lasers (ECDL), Multi Quantum Well (MQW) laser, Vertical Cavity Surface Emitting Lasers (VCSEL)i.IntroductionThe semiconductor lasers were discovered in 1962 by Robert Hall and his team members 1, 2. With the advent of this technology patents and articles started to publish on this technology but at that time this technology was not mature ample to realize the dreams of the scientists, engineers and physicist. But with liftment in the field and the enabling technologies soon made it possible to produce inexpensive commercially available semiconductor laser. The origination of semiconductor lasers not only revolutionizes the optical communication but it has many applications in other domains also.Semiconductor lasers atomic number 18 one of the popular optical communication wild source for data transmission. They are supposed to be the laser of the future, because of their compactness in size, easy integration, much output military unit, optical pumps for solid-state lasers (primary light source i.e laser diode pumps another solid state lasers) and their rapid improvement.Semiconductor lasers are essentially diodes which produces coherent ligh t by dint of the process of aroused emission. They uses semiconductor as a gain medium. The gain medium is handle by an external source, which is electrical in nature in case of semiconductor lasers 3.As they are basically p-n junctions, so they are compact and can be fabricated on large scale by the use of advance semiconductor IC technology. Semiconductor lasers are very efficient in converting electrical power into optical power 10.In section.II basic types of semiconductor lasers are described in terms of their basic working procedure, wagess, disadvantages, commercial availability future directions and applications. Section.III produces the comparison of semiconductor lasers with respect to various parameters and section.IV contains the conclusion.II. Types of semiconductor lasersFol kickoffing are the basic types of semiconductor lasersFabry Perot (FP) LasersDistributed Feedback (DFB) lasersMulti Quantum Well (MQW) laserExternal Cavity Diode Lasers (ECDL)Vertical Cavity Sur face Emitting Lasers (VCSEL)A. Fabry-Perot (FP) lasersIn FP lasers, reflects create the right condition for the lasing to occur. oscillate cavity is formed when two mirrors are put in front of each other. The bouncing of light will take place between these two mirrors, the distance between these two mirrors are adjusted in much(prenominal) a way that this distance is the integral six-fold of half wavelength, only is such scenario light will reinforce itself. Wavelengths that are not resonant they undergo destructive interference and deflect. One of the two mirrors is fully reflective and the second mirror concedes very small amount of light to pass through 4.Figure.1.a Fabry Perot Laser conceptual structure, Ref 4Figure.1.b Fabry Perot Laser structure, Ref www.scholar.lib.vt.eduA.1. AdvantagesFP laser gives comb of (Amplified Spontaneous firing off) ASE distributor point akin in frequency 5.FP laser can be used for transmission of data with multi elan optical fiber 6.FP lase r also have uniform intensity over Erbium Doped Fiber Amplifier (EDFA) 5.Perfect for optical links where long term power and wavelength stability is required.A.2. DisadvantagesHigh dispersion in high speed and long transmission system because the unearthly comprehensiveness can span as much as 5 nm 6.At 2.5Gbps and higher the wavelength center of FP laser goes out of wavelength tolerance.Not suitable for long distances.With the improvement in VCSELs technology, applications of 1310 nm and 1550 nm applications are moving from FP laser to VCSEL.A.3. approaching directions and ApplicationsIncrease in bandlargeness up to 15 GHz (the running value) by reducing abject frequency roll off parasitic effects of slash circuits by injection locking 7.FP lasers can generate indivi treble longitudinal mode laser 8.FP lasers external optical modulator for optical access network 9. ghostly width of FP laser allows transmission to distances of 40 Km at 1.7 Gbps.5FP offers Corse wavelength div ision multiplexing (CWDM) with channels at 1.3m and 1.5m simultaneously on a single fiber 5.FP lasers offer benefits in LAN that use one wavelength per fiber 6.FP laser can be used as multi wavelength source of Amplified Spontaneous Emission (ASE) for spectrum sliced 5.The 1550 nm FP laser can support Synchronous Optical Network (SONET) 6.A.4. Commercial availability of FP lasersMultiple Application Platform (MAP) FP laser 9 is FP laser source with key features like single mode or multi mode output, congenital modulation, LAN extensions for instrumentation (LXI) compliant etc. This device has weight of nearly of half kilo gram is capable of operating on different optical fibers such as Flexcor and SMF-28 10 with operational temperature compass of 10C to 40C. This product is commercially available.B. Distributed feedback (DFB) lasersIn FP laser there is feedback of light from the mirrors, this light feedback can be provided in distributed manner by series of closely place mirrors /reflectors (means there is a periodic variation in the width of the cavity). in that respect is a corrugated section in the cavity the incident light passes through section causes series of reflections. If the corrugation period is the integral multiple of half wavelength (Bragg condition) the resulting transmitted wave will add in phase. This mechanism suppresses other longitudinal modes and allow only single longitudinal mode whose wavelength is equal to twice the corrugation period. 11Figure.2. DFB laser structure 4B.1. AdvantagesDFB lasers provides better wavelength stability than cleaved end face laser 12The line width of DFB is less than that of reflective end face lasers 12.Low chirp 13Better wavelength selectivity.DFB laser is used in high performance system because of its narrow spectral output width 6.B.2. DisadvantagesThe amplitude of the standing wave is fixed because of gratingsEmitted wavelength near threshold current cant be controlled 14Wavelength chirp is associat ed with the spatial burning of the pallbearer holes.Some values of rare facet phase give unacceptable performance.B.3.Future directions and applications of Distributed feedback (DFB) lasersAll optical flip-flop is one of the building block for fast optical packet switching as it temporary stores the header information while the payload is routed to the correct output port. Single distributed feedback (DFB) laser diode based, all optical flip flop can be used to serve this purpose. Experimental results shows that DFB lasers can provide optical memory in an economical way and at the same time shows fast switching of optical packets (as low as 45 ps can be achieved). Using DFB lasers as an all optical flip flop 40 Gbps can be switched. 15Used in DWDM system where tune ability of signals are required.Used in the sensing of thorough narrow line width of the signal.B.4. Commercial availability of Distributed Feedback laserThe production of Quantum dot pure Green laser 16 is possible by using DFB laser technology. DFB laser gives high reliability for optical communication to form quantum dot crystal with a wavelength of 1064nm 16. There is significant amount of reduction in power consumption for converting 1064nm quantum dot laser from electricity to light by the use of DFB laser.C. Multi Quantum well (MQW) lasersIn Quantum well lasers the active region is very narrow which causes quantum project to occur. The wavelength which emits is dependent on the width of the active region rather than the band gap. This feature gives shorter wavelength than conventional lasers 17.Figure.3. Cavity structure of MQW laser 4C.1. AdvantagesEnhanced gain ascribable to quantum well structure and gain co-efficient is usually 3 times or more than single layer FP laser 13Produce shorter oscillation wavelengthSmaller linewidth sweetening factor (), at larger linewidth enhancement factor the laser instability is enhanced 13.Small optical lossesBetter confinement of laser actions becau se of multiple active regions.Low threshold currentHigh modulation speedLow temperature dependencyC.2. DisadvantagesThe internal structure of MQW laser is designed such that it is more susceptible to back reflections.C.3. Future directions and ApplicationsModulating retro reflector combines optical retro reflector and optical modulator (MQW lasers).Strained quantum well technology can become the hollow out technology for high performance semiconductor device by expanding the flexibility of band engineering 18.Optical repeaters, as it gives saturated output power to virtually 3dBm.C.4.Commercial availability of MQW lasersSANYO GaAlAs index channelise laser diodes with MQW structure, these are infrared powerful lasers source 19 of maximum power up to 200mW and wavelength range of 782nm to 830nm is commercially available.SONY GaAlAs index guided laser diodes with MQW structure these are infrared powerful lasers source 19 gives optical power range from 90mW up to 60W. This huge optic al output power gives high brightness.D. External Cavity Diode Lasers (ECDL)ECDL can also be used to achieve the single longitudinal mode wavelength. ECDL is a semiconductor based laser with one end of the laser diode has anti reflective coating and laser resonator is completed with collimating mirror and external mirror.As they are tunable lasers so for wavelength selection they use diffraction gratings. End mirror contains collimating lens and diffraction grating. The anti reflective coated end of the diode gets feedback from the diffracted beam. Tuning of the emitted wavelength is done through the rotation of the diffracting grating. There is some more variation for this setup. 11, 20Figure.4. EDCL setup Ref www.clu-in.orgD.1.AdvantagesSide mode suppression ratio (SMSR) is better than -40 dB and has narrow intrinsic line widthPulse repetition rate can easily be selected 21Filter can be inserted for the fixing of emission wavelength in mode locked diode laser 21 etc.D.2. Disadvant agesThe direction of output beam changes by rotating the diffraction grating which not suitable for many optical communication applications.In fixed direction of the output beam, the zero order reflection of the beam bounced by the mirror is lost.D.3.Future directions and applicationsTunable continuous wave THz radiation can be generated by the use of dual mode ECDL from 250 MHz to several THz 22Without mode hopping wavelength tuning over 40 nm around 1550 nm is expected by new pattern of EDCL with all dielectric thin film Fabry Perot filters. 23Atomic clock ECDL is used in the optical system of the atomic clock it is used for the cooling and the spying of clock transition. 24Mode locked ECDL are mostly used in optical communication (DWDM) for data transmission. 21Testing and measurement in optical fiber communicationNon-linear frequency conversion.D.4. Commercial availability of ECDLTEC-100 and TEC-120 25 External cavity diode Littrow laser system gives optical output power up to 200mW and running range (mod-hop free) up to 30GHz.DLX-100 26 External cavity tunable diode gives output power up to 1W and mod-hop free tuning up to 15GHz.E. Vertical Cavity Surface Emitting Lasers (VCSEL)VCSEL achieves single longitudinal mode operation in different manner. The active region is placed between two highly reflective surfaces/mirrors. These two reflective surfaces have alternated high and low refractive index. The reflectivity of the surfaces is between 99.5-99.9% that is why light oscillates perpendicularly through the layers and emits from the top or the bottom of the device. 27E.1. AdvantagesIt has high wavelength stabilityIt is less sensitive to temperature (can race reliably up to 80 C)Less refrigeration is required for VCSEL as it operation do not emit much heat.It provides high powered per unit area ( up to 1200W/cm2)It emits circular beam as a transmitter which leads to small optical loss.Figure.5. VCSEL structure 4E.2. DisadvantagesBecause of high mirror reflectivity in the VCSEL internal structure lowers the optical output power.They emit low output power (in mW) because of their small active medium volume.InP based VCSEL suffers from having low conduction band offset, low conduction band offset means low confinements of electron which results improper temperature stability of active material gain.To produce longer wavelength in VCSEL, mechanism of reduplicate fusion is used. This mechanism increases the cost as it is complicated procedure 28.E.2. Future directions and applications of Vertical External Cavity Surface-Emitting Laser (VECSEL) 20, 32There is an issue of optical power supply for Si-photonics transceiver which are being developed for high density optical interconnect on parallel processors can ideally solved by VECSEL as the emits in 1330 nm and 1550 nm. 29High speed modulation up to 25 Gb/sec can be achieved by using VCSEL at low current of 7.4 KA/cm2. 30Fast modulation frequency (in GHz) made its use in optical fiber communication as transmitterIdeal for local and subway system area networks as they produce low output power.Threshold less laser 31 etcE.3. Commercial availability of (VCSEL)VCSEL technology has flourished and gives grand range of products with ultra low noise, narrow line width, high optical output power (up to 1kW) and high speed modulation (speed 5GHz) 27.For high optical output power up to 1KW, a module for cooling of this system is used and fiber is pigtailed for high brightness VCSEL array.VCSELs are commercially available in blue, green and UV lasers 27.iii. Comparison of semiconductor lasersIn this section DFB, ECDL, MQW and VCSEL are compared with respect to different parameters and their suitability as optical transmitter in different optical networks.ParametersTuning agent railroad siding power ( in dBm)Tuning speedModulation speedLasersDFBTemperature13Slow (m-sec)Fast (multi-GHz)MQWQuantum confined stark effect7Fast(Fast(multi-GHz)VCSELMEMsOptically pumped 6Electrica lly pumped -3Fast(-sec)Fast ( few GHz)ECDLPeizo transducer13Slow(m-sec)Slow (Table.1 Comparison of semiconductor lasersTable.1 shows relative analysis of semiconductor lasers. Each semiconductor laser has got competitive advantage with respect to different parameter.Table.2 gives a comprehensive view of the semiconductor lasers and their application in a specific network type as an optical transmitter.Semiconductor LaserNetwork typeFP laser compact to medium range (Local Metro) NetworksDFB LaserLong haul networkMWQ LaserShort to medium range (Local Metro) NetworksECDLLong haul networkVCSELShort to medium range (Local Metro) NetworksTable.2 Semiconductor lasers with their network type supportLasersMax spectral output widthFP5 nmVCSEL0.5 nmDFB0.1 nmTable.3 Spectral width comparison of short range lasersSpectral output width measure the range of the wavelengths produce by a laser. From table.3 it is evident that DFB laser has the narrowest spectral output width, which makes DFB lase r the best choice for high performance short range optical transmission systems.iv. ConclusionThis paper gives a review on the semiconductor lasers, their working, applications, commercial availability and future directions. Relative analysis of these lasers shows that they have competitive advantage in one or other laser parameters and this analysis also shows that which semiconductor laser is appropriate to which type of optical network. Semiconductor lasers have wide applications in optical fiber communication. Lots of exciting research is going on in this field and there is still room of improvements.