The coevals of extremist short pulsation with high repeat rate attracts the VECSEL in the field of optical technology such has high velocity optical webs and high informations processing. Invent of VECSEL gives much attractive force towards it, because it gives precise characteristic compared to the other techniques while it is used in the imagination technique. But it has some issues like alliance of the external pit and optical beam yoke, since the VECSEL is activated utilizing optical pumping. Apart from these issues, this method gives distinguishable belongingss those are non comparable with the other surface breathing optical masers. In this literature reappraisal of VECSEL, the rule of operation, distinguishable features, public presentation parametric quantities and challenges with an application is discussed and the up to day of the month work done besides included.
A VECSEL is a semiconducting material optical maser which has a surface breathing Active part addition, and a optical maser resonating chamber which is pumped either electrically or optically. The semiconducting material addition bit contains an Active part with several Quantum Wells over a distributed Bragg reflector ( DBR ) . The optical maser resonating chamber is completed by the external pit mirror which is separated from the active part by few centimeters apart to acquire the end product in a low divergency, round, diffraction limited beam of good quality. To acquire good quality beam and optical maser manner size, the separation between the external mirror and the active part is varied, and the aggregation efficiency of the system is improved by utilizing a curving mirror in the external pit mirror. Output power scaling to multiwatt scope is achieved by increasing the optical maser manner country at high input pump beginning. The thickness of the semiconducting material construction scales about few microns and it is mounted on a substrate. A heat sink is attached to the substrate to cut down the thermic gradient of the system. Extra constituents like saturable absorber for passively Mode-locked surface-emitting optical masers, Optical Filter for single-channel frequence end product or wavelength tuning, Optical Clocking and Non-linear crystal for intracavity frequence duplicating.
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The electrical pump method injects charge bearers which limits the end product power to 1W and the useable active country. In the instance of optical pumping, unvarying distribution of the pump power over the active country is applied which eliminates the free charge bearer injection into the undoped active part. Hence we get a decrease in the optical loss and extra heat dissipation.
perpendicular external pit surface-emitting optical maser ( VECSEL )
Fig 1: VECSEL apparatus
Interband passage occurs in the active part stuff during optical pumping which has wide pump soaking up set and no issue on the wavelength stableness of the pump optical maser. The soaking up of the optical pump beam is depending on the stuff used in the active part. Carriers generated during the optical pumping in the barrier are trapped by the Quantum Wellss. Carrier concentration and the system temperature affect the Quantum Efficiency, Bandwidth addition, Differential addition and Wavelength addition. Due to heat dissipation, the DBR mirror is much affected by the pump power degree and topographic point size than the active part. This system has Bandwidth addition of few 10s of nanometres and has broad applications like coevals of green optical maser rectifying tube for bantam projectors, spectrometry and extremist short pulsation coevals. The sub subdivision of this reappraisal deals about the device rule, fiction of VECSEL, and the extremist short pulsation coevals utilizing Passive Mode lock technique.
VECSEL Gain Structure:
The VECSEL addition construction consists of a Bragg Mirror and an Active addition part with several quantum Wellss, fabricated in a semiconducting material utilizing an epitaxial growing procedure. The pick of stuff for the fiction of VECSEL construction should be lattice matched. The 27-period Bragg mirror utilizing AlAs/GaAs stack is grown epitaxial on the substrate. The active part is grown over the DBR, in the GaAs active part 6 quantum Wellss are formed utilizing InGaAs beds which are compressively strained due to higher lattice changeless compared to the GaAs with a spacing of ?/2 by GaAs barrier beds of overall thickness 7?/2. The quantum Wellss are formed precisely at the anti nodes of the standing moving ridge generated in the optical maser system.
Fig 2: ( a ) Layers of VECSEL – active part and DBR ( B ) Variation of E-field w.r.t place ( Z )
The Entire length of the active part and the DBR is about 1µm. The window bed is to the full crystalline, which allows the optical signal to make the external pit mirror and the capping bed is to forestall bearer from spreading to semiconductor surface where the charge bearers combine non-radiatively.
Principle of Operation:
The VECSEL active country is optical pumped utilizing a rectifying tube optical maser beginning. The optical yoke efficiency is high and most of the incident beam is absorbed by the active part which consequences in the Interband passage within the barriers. After absorbing the incident pump power charge bearers is created and are confined in the Quantum Well. The quantum good has a high refractive index compared to the barrier bed which enhances the charge parturiency in this part by holding lower bandgap energy. The passage of the charge bearers in the quantum good gives a higher order exponential addition on the order of 6000/cm [ 3 ] . For a thin active bed ( 10nm ) gives out a higher addition in the surface breathing construction compared to the border emitting construction. Thus the addition is confined inside the active part and it ‘s expressed as a map of parturiency factor, ‘? ‘
Where E ( Zi ) – Electric Field magnitude in the ith quantum good.
From Fig 2 ( B ) we can detect the electric field strength of the standing moving ridge confined in the active part. The VECSEL has a strong spectral filtering introduced by the longitudinal parturiency factor which controls the device public presentation. By seting the optical bomber pit developed by the DBR and the air interface, the spectral dependance of the longitudinal parturiency factor is controlled [ 1 ] . The tight control of the longitudinal parturiency factor is obtained by doing a crisp resonance in the optical bomber pit which improves the addition of the system at a high budget of cost. As the active part is pumped optically, it starts heating inherently increases the optical thickness about 0.1nm/K which reduces the resonance extremum and addition of the system scales down.
High temperature dependance in the addition of the VECSEL system ; the addition profile displacement as the temperature addition to longer wavelength at a rate of 0.3nm/K. However the effectual addition lessenings with addition in temperature.
Fig 3: Conventional diagram of a VECSEL
The optical pump power is incident on the active part ( Gain Structure ) of the VECSEL construction additions, the active part temperature additions, the addition lessenings and the population inversion occurs at high passage to counterbalance the decrease in addition. This increases the power dissipation and cut off the optical maser operation. In the instance of surface breathing laser the spectral filtering consequence on the longitudinal parturiency factor reduces the thermic tally off and controls the device public presentation. The external resonating chamber is fold with level or curving mirrors and extra semiconducting material saturable absorber mirrors ( SESAMs ) . The end product coupling defines a TEM00 pit manner with the topographic point size equivalent to the incident beam. Addition of SESAMs in the resonating chamber pit is to acquire a inactive manner lockup.
Fig 4: Output Spectrum of VECSEL
The addition is altered by altering the pump beginning and during the ON-time the addition exceeds the loss co-efficient and the optical maser visible radiation is produced with high end product mean power. Therefore we get high norm end product power with good beam quality.
High runing Power:
In order to acquire high end product power and high beam quality, the input pump power demand to be raised. Design 1: Kuznetsov M et all developed a system with 13 InGaAs quantum Wellss in the GaAs substrate and the emanation wavelength about 980nm. This device architecture is pumped with a optical maser rectifying tube pump with power 3W at operating wavelength 808nm. Output coupling has transmittal efficiency about 4 % and the beam end product power is 0.69W in the TEM11 manner, 0.52 W in TEM00 manner and 0.37 W coupled to a single-mode fibre [ 7 ] .
Design 2: Holm et all developed a system with the lattice matched AlGaAs/GaAs stuff with a lesser emanation wavelength 870nm shows a lower differential addition and T0 compared the labored InGaAS/GaAs construction but the end product power is lower ( 0.15W ) .
These two designs have a resonating sub-cavity formed by the active part during operation which causes an addition in effectual addition with the add-on of spectral filtrating consequence. Using anti-resonant short sub-cavity in the active part we can cut down the filtering consequence which affects the public presentation of the device.
The end product power features of the Design 1 VECSEL device is analyzed by reaching the GaAs substrate with a peltier chilling system by a thermally conductive paste. The device is power scaled upto 1.5W utilizing a fibre coupled laser rectifying tube with an emanation wavelength of 830nm and topographic point size 90µm incident on the active part of the VECSEL device and it is operated in the TEM00 manner [ 5 ] .
Fig 5: End product power spectrum of an InGaAs/GaAs VECSEL pumped with 1.5W optical maser rectifying tube
From the graph it states that at a peltier scene of 0 & A ; deg ; C the optical maser end product power reaches upto 400mW and axial rotation over suddenly. For 60 & A ; deg ; C, the optical maser end product power reaches upto 190mW and axial rotation over easy. It shows that the functionality of the device is limited by the temperature dependance of the quantum good non on the longitudinal parturiency factor. The sweetening of the end product is done by back reflecting the unabsorbed pump power through the well in the DBR.
There are several ways to cut down the thermic electric resistance in the device like MOVPE epitaxial grown of DBR and active bed and bonded with the diamond heat sink [ 7 ] . An effectual method to extinguish the heat from the active part is by puting an uncoated sapphire window on top of the active addition medium. This method enhances the input pump power with less thermic blowout in the device. An alternate stuff for the heat distributing home base is Silicon Carbide ( SiC ) which has a thermic conduction greater than the sapphire stuff. The promotion of the chilling system in the device consequences in the invent of micro chip VECSEL which has higher thermic electric resistance tolerance.
Operational Wavelength of VECSEL:
Assorted wavelength the VECSEL can be operated and with the stuff systems used for the device construction.
The GaAs substrate is chosen for the close IR operating government with GaAs/AlAs Bragg Reflector. The stuff incorporated in the quantum well is a strong map of the operating wavelength. For a device operation wavelength at 850nm, lattice matched GaAs/AlGaAs quantum Wellss used and at 1000nm labored InGaAs/GaAs quantum Wellss used. Using an InGaP quantum Wellss with AlGaInP DBR construction gives an end product power of 200mW at emanation wavelength of 660nm under Ar ( green ) -ion optical maser pumping.
In the 1.5µm operating government InP substrates are used and the DBR mirror stack has higher thickness compared to the GaAs substrate construction due to the long operating wavelength and low refractile index contrast. Absorption and scattering loss are non negligible and the reflector efficiency is reduced. An optimal design with GaInAsP quantum good fabricated with the GaAs/AlAs DBR to get the better of the above said issues. This design gives a low electrical electric resistance and high coefficient of reflection, but it ‘s hard for fabrication. Using a GaSb substrate the device can be fabricated to run in 2 to 2.5µm government which is used in the atmospheric detection of pollutants like CH4 and CO.
The optical maser device addition is a strong map of bearer denseness and it is expressed as,
g=g0ln ( N/N0 )
As the soaking up of the barrier stuff is increased, which cause more bearer flow in the quantum good and increases the charge parturiency consequences in the higher addition.
Lasing threshold status is defined as R1R2Tlossexp ( 2?gthNwLw ) = 1
Where R1 and R2 the pit mirror coefficient of reflection ‘s, Tloss is the transmittal factor due to round-trip pit loss, gth is the threshold stuff addition, Nw is the figure of QW ‘s in the addition medium, and Lw is the QW thickness.
The threshold pump power is dependent on the figure of quantum Wellss. For a little figure of quantum wells the threshold pump power additions suddenly and for more quantum wells it increases easy.
Output optical maser power addition w.r.t the figure of quantum Wellss and lessening in the coefficient of reflection.
Fig 6: Threshold pump power and end product power of input pump beginning
Laser end product power is depend on the input pump power and cut downing the coefficient of reflection of the external mirror at higher optical maser threshold.
Fig 7: Output power of VECSEL
In order to acquire high addition, low VT and high end product power we need to utilize a extremely lattice matched active addition component in the VECSEL. The semiconducting material stuff strained InGaAs Quantum wells gives promising consequences but it induces the strain in the device which will impact the construction stableness, dependability and public presentation factor. By adding a tensile strained GaAsP strain counterbalancing bed we can extinguish the issues. The repose curve of tensile strain, bandwidth w.r.t GaAsP composing is shown below,
Fig 8: Tensile strain and bandgap wavelength response of the VECSEL V. Concentration of GaAsP
In the VECSEL device dissipates heat during high power operation and it should be removed in order to keep the public presentation of the system. Mounting the device with the heat sink like diamond, In and sapphire heat distributing stuff can cut down the heat. Most of the heat is from the DBR while absorping the optical pump beam and the bottom substrate contributes on the effectual heat dissipation [ 9 ] . By mounting the DBR straight on the heat sink or by cut downing the figure of mirror bed we can well cut down the heat. Due to the addition in the temperature the spectral displacement occurs, reduces the bandwidth, PL displacement to longer wavelength and additions refractile index. The end product response curve is shown below,
Fig 9: Thermal Impedance of the VECSEL construction Vs bit thickness
Assortment of wavelength is generated expeditiously utilizing the mature semiconducting material stuff.
Low threshold electromotive force ( VT ) , High end product power, High selectivity and public presentation efficiency is achieved by Band Gap technology in the active addition part – quantum good.
Broad bandwidth is achieved by tuning the design parametric quantity.
Operating wavelength of the optical maser and the pumping beginning can be selected by the design parametric quantities.
High handiness of wide pump bandwidth ( 40nm ) adds an efficient soaking up of the input pump beam.
Very short pump soaking up length gives a good soaking up and good convergence between the optical maser manners and the input pump beam.
Wide wavelength from UV to IR scope can be generated with the bing semiconducting material stuffs.
No p-n junction is implemented in the construction, so easy in batch production and increased dependability of the device.
Since optically pumped no I2R loss.
High end product power coevals is possible in the broad operating wavelength government and has broad application like extremist short pulsation, intra pit doubling and spectrometry.
Requires an external pump and external pit to accomplish high end product power and requires a stable mechanical design of the optical maser fiction.
Size of the VECSEL optical maser is bigger compared to the conventional optical maser system.
In the semiconducting material quantum good surface breathing optical maser has a wide and homogenous addition and a broad choice of the centre wavelength creates new avenues in the country of extremist short pulsation, spectrometry and end product wavelength tunable map.
Intra Cavity laser soaking up spectrometry
Intra Cavity Doubling
Ultra Short Pulse Generation
VECSEL as a Passive Mode locking Surface Emitting Laser to bring forth extremist short pulsation
The wide bandwidth of VESEL end product gives more involvement in coevals of the extremist short pulsation utilizing a inactive manner lock technique [ 1 ] . Integrating an intracavity semiconducting material saturable absorber mirrors ( SESAMs ) in the external pit of the surface breathing semiconducting material optical maser we can passively mode lock to bring forth an extremist short pulsation. The device is grown epitaxially with an active part consists of InGaAs/GaAs strained quantum Wellss on a DBR and it is pumped optically utilizing a high brightness optical maser rectifying tube light beginning.
In the synchronal pumping of the optical maser beginning, with Nd: YAG and Ti sapphire optical masers gives a low repeat rate about 100MHz. Therefore in VECSEL the uninterrupted pumping of the optical maser beginning, the optical maser pit is long with unit of ammunition trip frequence about 168MHz and the manner locked occurred at 336MHz ( Second Harmonic Oscillation ) . Because of the longer round trip clip the Amplified self-generated emanation reduces the addition and the end product power is reduced at the same time and the pulse continuance is increased. Using SESAMs in the external pit of VECSEL we can passively mode lock to bring forth an effectual extremist short pulsation [ 2 ] . In this reappraisal I have discussed about a VESEL construction which generated the end product beam with these parametric quantities, pulsations of 22ps FWHM at 1030nm with repeat rate about 4.4 GHz.
The optically wired VECSEL gives an end product beam with high norm end product power and round diffraction limited with good quality [ 2 ] . The end product power scaled up to few Wattss by increasing the input pump beginning and the optical maser manner in operation. VECSEL system is ab initio pumped utilizing an optical pump beginning – high brightness rectifying tube optical maser. Semiconductor saturable absorbers mirror ( SESAMs ) in the external pit is mounted to acquire a manner lock. The repeat rate of the end product beam plays an of import function in high-energy natural philosophies, optical clocking, and A/D convertors. The Neodymium: YVO has been passively mode locked with pulse repeat rates about 30GHz, pulses – 6.8ps and FWHM at 1064nm. To acquire extremist short pulsation in Nd-doped glass in the scope of femto second, due to the big fluence value reduces the coefficient of reflection and the addition impregnation is big which consequences in the Q-switching instability. In order to acquire low addition impregnation with repeat rate of several GHz with no Q-switching, the semiconducting material optical masers with quantum Wellss are applied for the inactive manner locked. Each quantum good in the active part has a addition as a map of its well breadth. The end product power from the manner locked laser rectifying tube is in the order of few 10s of factory Wattss. VECSEL has a curious behavior scaling end product power to higher value made much attending towards it.
As shown in the Figure 5, ab initio the VECSEL device is pumped with optical maser rectifying tube pump breathing wavelength 810nm and end product power 1.6W continuously over an active country of 90×90 µm2. The device absorbs 60 % of the incident pump beginning. The optically aroused substrate and the SESAM formed the terminal mirror of the resonating pit. An end product coupling with a radius of curvature 10 millimeter with transmittal efficiency 0.4 % of the generated optical maser wavelength gives an end product TEM00 pit manner ( little manner ) with the topographic point size equivalent to the incident beam. If the incident pump beginning gives a fixed topographic point size in the addition medium, the by changing the length of the pit we can alter the manner size in the saturable absorber keeping fixed topographic point size in the addition medium.
Fig 10: Mode-locked diode-pumped VECSEL pit with SESAM
The active addition construction is grown epitaxial utilizing MOCVD procedure with an array of 12 InGaAs compressively strained quantum Wellss between the GaAsP tensile strained barriers and the thickness of the stack is varied to run into the net strain to nothing. The spacing between the quantum Wellss are maintained ?/2 by GaAs barrier beds. Below the quantum Wellss, the Bragg mirror is grown epitaxial 27 jumping beds of AlAs and AlGaAs. On top of the quantum good a window bed of about 450nm is patterned to do the bearers off from the surface. Finally a cresting bed of 10nm GaAs is grown to finish the VECSEL device construction.
A die country of about 5mm square is diced from the wafer, lapped and polished to cut down the thickness of the GaAs substrate to about 200µm. This device has a capableness of bring forthing more heat, so by mounting the dice which is soldered with Cu and mounted on the In which acts as a heat sink. Analyzing the photoluminescence spectrum of this device shows a extremum at 980nm and lasing occurs in the wavelength scope 1000nm to 1040nm as a map of the temperature fluence in the device by pump beginning and the alteration in optical thickness across the wafer. A mirror with an incorporated saturable absorber is SESAM processed by the semiconducting material engineering used for the coevals of extremist short pulsation by inactive manner lock. The SESAM contains GaAs substrate on top of it 25 layer braces of GaAs/AlAs Bragg mirror is grown. The stuff used for the Bragg mirror has higher bandgap energy to avoid soaking up. A individual quantum well of thickness 20nm utilizing InGaAs with a low delicacy anti resonating pit ?/2 is grown at low temperature by Molecular beam epitaxy ( MBE ) [ 2 ] . To acquire a high transition depth the thickness of saturable absorber is increased.
The deepness of incursion of the optical strength in the SESAM is a strong map of the location of the saturable absorber stuff. The cardinal parametric quantities of SESAM are transition deepness, impregnation fluence, recovery clip and non-saturable losingss.
The strength loss of the SESAM is 1.3 % with a recovery clip 4ps and the 130fs fast constituent decoloring response. The length of the active bed is 28mm and the SESAM length is 6mm. Due to short length of the SESAM ( 40 times lesser than active bed ) the pit manner is tightly focused. This make the absorber impregnation pulse shorter than the addition impregnation pulse consequences in fast soaking up impregnation and all right pulsation defining.
refractile index profile and strength distribution in a SESAM
Fig 11 ( a ) : Structure of a typical SESAM Fig 11 ( B ) : SESAM – Optical Intensity Profile
The saturable absorber is a quantum good as shown in the figure 6. Alternatively of a quantum good, if a quantum point is fabricated on the DBR mirror at high temperature for about 14 beds with a denseness of 5E10 points per bed gives a wide soaking up profile and uniformity in the spectral response [ 5 ] . In this manner the saturable loss is reduced down to 1 % compared to the quantum good saturable absorber. Hence the impregnation fluence magnitude is less and the end product response of 13ps from the InGaAs/GaAs VECSEL emanation at 1030nm. The slow temperature MBE procedure of the saturable absorber in the SESAM consequences in the addition of the saturable loss due to fast charge trap and soaking up recovery.
Once the optical pump with a power of 1.4W is incident on the active bed of the VECSEL construction, the pump beginning is absorbed by the quantum good. Then the charges excited consequences in the population inversion.
Fig 12: Excitement and relaxation of bearers in a semiconducting material
excitement and relaxation of a SESAM
The Bottom DBR and the external mirror signifier the pit which enhances the end product beam strength and quality with end product power of 21.6mW – high norm end product power, the pulsations of 22ps FWHM at 1030nm with repeat rate about 4.4 GHz is obtained. Light of the pump beginning on the active part increases the temperature of the device and degrades the addition of the system. The end product emanation optical maser wavelength is a map of temperature induced by the pump beginning and the fluctuation in the optical thickness in the device. Thus addition in the pump beginning power reduces the end product power due to heat dissipation. Several ways explored to scale the end product power to higher value with better thermic budget. In the InGaAs/GaAs VECSEL emanation at 963nm delivers an mean end product power of 200mW with close transform limited pulse breadth FWHM 3.2ps continuance [ 3 ] . Heat dissipation rose due to the addition in input pump power, but the heat dissipation is reduced by puting an uncoated sapphire window on the active bed which spreads out the heat in the surface. In this manner we can raise the input pump power to 7.4W from an 805nm optical maser rectifying tube without any lose to the addition of the system. From a optical maser operating at 952nm an norm end product power of 950mW with repeat rate of 6GHz and pulse breadth of 15ps is produced. The optical maser is pumped with an input pump beginning power 15.8W [ 4 ] . At the maximal input pump power, the norm end product power achieved is 2W with a pulse breadth of 15.3ps and an optical bandwidth of 1nm.
The end product beam from the VECSEL optical maser in analyzed utilizing a fast rectifying tube monitoring system with a 50-GHz photodiode and a 26-GHz amplifier and spectrum analyser which gives a wireless frequence spectrum contains cardinal extremum of the pit unit of ammunition trip repeat rate about 4.43GHz. From the curve it states that the VECSEL end product is stable with no Q-switching and no side band up to the degree of -70dBc.
Fig 13. Radio-frequency spectrum of the VECSEL power end product demoing mode locking without Q-switching instabilities at a repeat rate of 4.43 GHz.
The pulse autocorrelation is carried for the end product beam with a best tantrum which gives a FWHM pulse continuance about 22ps.
Fig 14. Autocorrelation hint of the mode-locked pulsations
The bandwidth of 0.25nm wavelength is shown in the optical spectrum of the VECSEL. The wave form is consistent with no spectral scope due to the etalon formed by the DBR and the back surface of the GaAs wafer which is soldered with Cu and mounted on the In. The net saturable soaking up loss is about 1 % and it gives an asymmetric end product response.
Fig 15: Bandwidth spectrum of VECSEL
In the uninterrupted manner operation the consequence of etalon displacements the spectral bandwidth and affects the coefficient of reflection of the DBR mirror. The clip to bandwidth merchandise of the pulsations shows a strong stage transition effects and it is 1.5 times greater than the transform bound is due to the effects of the scattering and impregnation in the addition construction and the SESAM.
The quantum Wellss have a higher differential addition which saturates the addition. This introduces the phase-modulation and it besides shapes the tail of the pulsation. Refractive index of the active part is a strong dependant of bearer denseness in that part [ 6 ] . Numeric extension theoretical account by R. Paschotta et al trades about the saturable loss and addition, Phase alterations, scattering and bandwidth of the device. A negative stage displacement is formed due to the transform limited end product pulsation which is compensated by the positive stage displacement scattering. As seen in the figure 8, the rate of non-linear stage alteration is non unvarying in the caput and tail border of the pulse consequences in asymmetry behavior.
VECSEL with an optical pumping mechanism gives a high end product power with less end product loss. The optical pumping gives unvarying bearer distribution over a big country with no electrical loss. Because of this benefit this devices finds broad application like intra pit doubling, extremist short pulsation coevals and spectrometry. Using passively manner locked surface breathing optical maser system we can bring forth the pulsation in the order of femto 2nd with high norm end product power of several Wattss. Several thermic electric resistance decrease techniques are found to better the public presentation of the optical maser system. In current engineering it is possible to bring forth an end product power of 3.3W for a heat sink temperature 20 & A ; deg ; C in the GaSb dual-chip VECSEL at an emanation wavelength of 2.25µm. An end product power of 3W was achieved in CW operation at 2.0?m emanation wavelength for a heat-sink temperature of 20 & A ; deg ; C, and up to 6W was obtained when the sample was thermoelectrically cooled to ?15 & A ; deg ; C. In pulsed operation ( 200ns pulse length ) , over 21W of on-time end product power at room temperature was measured. The optical quantum efficiency of the devices reached really high values of 45 % at room temperature and 55 % at ?15 & A ; deg ; C heat-sink temperature [ Ref: Fraunhofer Institute, Germany, the Institute of Photonics, Glasgow and LISA Laser Products, Germany ] .