Performance the effect of variation the wavelength of a
high power diode laser an end- pumped laser system.
Jassim Mohammed Jassim,
Babylon University, Babylon University
Yaseen Hasan Kadhim
Ahmed Kadem Kodeary
College of Science for Woman, College of Science
Abstract
The present work is a theoretical and experimental study on the temperature effect on the power
output of an end- pumped Nd:YAG laser system, using a (GaAs) laser diode as a pumping source . The
heating effect due to the current injection through the semiconductor laser causes a shift (200-235 mA) in
the threshold current for temperature variation (5-35 Co) ,These variation ,also ,led to a spectral shift ( 800809 nm) in the diode laser output, that is , a (0.25-0.3 nm/ Co) . The consequences of these changes on the
Nd:YAG laser output power was an increase in the system power ( 0.62- 8.5 mW) for the temperature
changes (5-35 Co) .The experimental results fairly high agreement with the theoretical calculations .
‫الخالصه‬
‫تممف مما اممحث ثراسممن ريث ممج يبيةممج ت يةا ممج رديث ممج رتممير دي ريِم دج ثرسمميثيا قدم ثراممديا ثرنظيِممج زمما زابةزممج ر م ي ثرسظرممج ثر ممداج‬
‫ تام ا أن ثرتمير ي ثرسميثي ثرامظت زما حاما ثرت مظي قامي‬. ) ‫ثرمضنةخ بشكل طمةرا بةث م ج ر م ي ثرمدثةةر زيةانمظ ثركمظر ةم‬, ‫ثرادةم ةم ةظك‬
‫( أن امحث ثرتغ مي أةضمظ‬5-35 Co) ‫( رتغ مي ريِم دج ثرسميثيا زما‬200-235 mA) ‫مظي ثرتتامج زما‬
‫ر ي أشاظه ثرمةصال تتساب ت ح م‬
‫ما ت د‬
‫ يتمظج امحث ثرتغ مي قدم دمديا رمي‬. (0.25-0.3 nm/ Co) ‫ باسماج‬, ‫ ( ما رمي ر م ي ثرمدثةةر‬800-809 nm) ‫أرّى إر ت ح م ط فما زما‬
‫ ثراتمظج ثرت يةا مج تت مظب‬. (5-35 Co) ‫ ( بتغ ي ريِج ثرسميثيا زما‬0.62- 8.5 mW) ‫ر ي ثرادةم ةم ةظك ِد زةظرا ا دديا ثرمابةزج زا‬
. ‫إر حد كا ي زع ثرسسظبظ ثرابيةج‬
1- Introduction
Laser diodes have long operating lifetimes, on the order of ten thousand hours,
provided they are handled with care and operated with proper driver circuits. A number
of reasons including current surge, electrostatic discharge, and temperature surge
contribute to the reduction of a laser diode's lifetime or to its complete failure. The
temperature to characterizes the sensitivity of the laser's threshold current to temperature,
while the temperature characterizes the sensitivity of the laser's differential slope
efficiency to temperature. Thermal characterization also gives insight into device
functioning and points the way toward better laser design energy transitions are coupled
to lattice vibrational modes, device performance is often closely tied to device heating.
For semiconductor laser diodes in particular such critical parameters as wavelength,
threshold current, quantum efficiency, and device lifetime are all strongly dependent on
temperature [Yi, 1995].
The subject of heat generation and transport is thus significant in the design of active
devices; a thorough understanding of device heating requires the examination both of
internal heat exchange and of external heat flow between a device and its environment.
Several effects can lead to severe heating in diode lasers. due to current flow through
resistive layers, lasers are plagued by processes such as radiation absorption and nonradiative recombination (through which carriers emit heat rather than light)[Menzel
,1995]. Increased temperature leads to a larger spread in energy of the carrier distribution
in the active region, consequently decreasing device efficiency due to both increased
leakage of carriers out of the quantum well and decreased carrier density at the lasing
optical transition. In addition, higher operating temperature can cause the wavelength of
the output light to drift and can decrease device lifetime [Choi and Turner , 1997].
In extreme cases, strong absorption at a laser's output facet can cause a thermal runaway
effect which ends in the catastrophic failure and melting of the device. Thermal
management is thus a critical issue in the performance of semiconductor the emission
wavelength will shift an average of 0.11 nm/°C and the threshold current will change an
average of 0.2 mA/°C. In addition, the operating lifetime drops by a factor of two for
every 25°C rise in operating temperature. Fortunately, thermoelectric coolers devices
provide a simple, reliable solution to precise temperature control in many applications of
optoelectronic devices. These solid-state devices can heat or cool small thermal loads to
more than 60°C from ambient and achieve temperature stabilities of better than 0.001°C
[Piprek, 2003, et al Sze and Kwok, 2007].
2- Cooling Semiconductor Laser
Laser diodes are purchased not as bare semiconductor chips, but mounted in
housings that allow easy handling and mounting to heat sinks for heat removal They also
incorporate electrical leads attached to the chip[Yang and Cory ,2007]. Often the housing
is similar to a transistor housing or other integrated circuit package. They usually have
provision for direct attachment to a heat sink. A typical view of a packaged
semiconductor laser. The laser must be mounted intimately to the heat sink, to minimize
thermal resistance. It must be remembered that the case temperature, which is what is
easily measured, is always lower than the junction temperature, which is what is
important. The manufacturers of laser diodes have developed structures with low thermal
resistance, to keep this difference small. The thermal resistance is defined as the change
in junction temperature per unit change in power. Low values of this parameter are
desirable. The thermal resistance of the diode is often in the range from 10 to 50 °C /watt.
Some manufacturers specify the value of the thermal resistance of their diodes. The
power dissipated through heat in the active region of a semiconductor laser diode is
removed by the mechanisms of: [Li and Liu,2008]
A. Convection: Convective Heat Transfer Naturally occurring air currents cause thermal
transfer between the heated (or cooled) device and the ambient surrounding air. The rate
of transfer depends on the geometry of the load and its surroundings. This heat-transfer
mechanism can become a limiting factor when the device must be cooled (or heated) far
from ambient .[ Li and Liu ,2008,et al Pipe ,2004 ]
B. Conduction: Conductive Heat Transfer Any material connecting the thermal load to
another surface provides a path for heat conduction. In most applications of
thermoelectric coolers, the thermoelectric module itself is the largest conductive path.
the effect of this path is generally accounted for in the performance data provided by the
thermoelectric cooler manufacturer. However, the thermal conductivity of other paths, if
any are present, must be included in calculating the total heat load. [ Li and Liu ,2008,et
al Pipe ,2004 ].
C. Radiation: Radiative Heat Transfer Radiative heat transfer varies as the power of the
temperature difference between the thermal load and its surroundings.
Figure 1: The power dissipated through heat in the active region of a semiconductor laser diode is
removed by the mechanisms of convection, conduction, and radiation. [Pipe ,2004 ].
The materials used are usually in Cooling Semiconductor Laser, Thermoelectric coolers
are commercially available and represent small, compact devices, which require no water
inlets or other fluids. They are very suitable for providing localized cooling for devices
like laser diodes. [Yang and Cory ,2007 ,et al Headley and Agrawal , 2005 ]
2-1 Heat sink
The heat sink may be simply a flat piece of metal, like copper or aluminum. A sheet with
dimensions of 2 * 20 * 30 mm is adequate for dissipating about 200 milliwatt of
electrical power. heat-sink structures are also available, often with fins to help dissipate
the heat .The lifetime of the laser is inversely proportional to the operating temperature
and hence a proper heat sink, if used to mount the laser, will ensure its longevity. We
have already described the harmful effects of excess temperature, both on laser
performance and on lifetime [Bogatin and Potter, 1997, et al Lundstrom, 2000, et al
Mark, 2004].
To dissipate the heat generated by the electrical current, good heat sinking is essential.
Heat Sink Conceptually, the role of the heat sink is simple: to provide a constanttemperature surface, usually near room temperature. In some applications a large block of
aluminum works well [Tsukiji and Yoshida, 2001].
However, when the heat to be dissipated exceeds about a watt, then fins, forced air or
even fluid cooling are called for. A 4" x 4" x 2" aluminum block at ambient temperature
will heat up only about 5°C in 10 minutes with 10 W being pumped into it. But over a
period of an hour, it will continue to heat to about 23°C above ambient. In contrast, a
typical finned heat sink of roughly the same size will heat only about 12°C above
ambient. [ Johnson, 2003]
2-2 Peltier cooler
In 1834 Jean C. Peltier observed that, by passing an electric current through a
junction of dissimilar metals, heat could be created or absorbed at the junction, depending
on the direction of current flow [Pipe, 2004]. Thermoelectric coolers employ the socalled Peltier effect. When a current passes through the junction of two different
conducting materials, the junction will be heated or cooled. Reversing the direction of the
current flow changes the effect from heating to cooling or vice versa. The traditional
Peltier cooler relies upon heat exchange that occurs as carriers move between different
average energy levels on either side of a semiconductor/ metal interface. An energy level
difference also occurs across a semiconductor/ semiconductor interface at which the
material composition changes (heterojunction). The materials used are usually
semiconductors [Johnson, 2003]. Thermoelectric coolers are commercially available and
represent small, compact devices, which require no water inlets or other fluids. They are
very suitable for providing localized cooling for devices like laser diodes. Many
manufacturers offer heat sinks, thermoelectric coolers, and thermoelectric cooler drive
circuitry that are compatible with commercial laser models [Rohwer and Srivastava,
2003].
Figure2: Illustration and band structure of (a,c) conventional Peltier cooler and (b,d) p-n junction diode.
Note that the direction of current to achieve the same cooling profile is different in the two cases. [Pipe,
2004]
3- Calculation the Effect variation of temperature on the threshold
current, and wavelength on output of laser diode
3-1-Calculation the Effect of temperature on the threshold current
Electric current: For a diode to operate as a laser, gain should exceed loss. Above the
threshold should be: Threshold current - the current at which gain equals loss . Power
characteristics of semiconductor laser diodes depend strongly on temperature. The
threshold current increases with temperature due to [Ozgur and Demir, 2009]:
*Diffusion of electrons and holes from the active layer.
.
*Enhancement in the non-radiative recombination.
.
Due to large currents for the heavy injection and small active-region size, an operating laser
diode can be strongly overheated. Important aim of laser-diode design - to reduce the
threshold current (and therefore to fight overheating) this Cooling is essential [Suematsu
and Iga, 2008].
The threshold current depends on temperature in an exponential fashion, according to the
equation:
It T2   It T1 exp T2  T1  / To ............(1)
where It(T2) and It(T1) are the threshold currents at temperatures T1 and T2 respectively,
and T0 is a scale factor with a typical value around 150 K. As an example, if T1 = 0 C°
and T2 = 30 C°, the threshold current will be about 22% higher at the higher temperature.
[Ozgur and Demir, 2009]
Figure 3: Light output vs. laser bias current, showing the threshold current [Menzel, 1995].
equation(2) for account the output power for laser diode.
P  Z I  I th ...................(2)
where P the output power for laser diode ,Z constant 1.21 W/A , I the current of supply
for laser diode ,Ith the threshold current for laser diode.[ Ifflander, 2001]
3-2-Calculation the Effect of temperature on shifted wavelength
The wavelength of a diode laser depends on the amount of energy released in the
form of a photon when an electron and a hole combine [Nakamura, 2000 ,et al Kaufmann
and Kunzer,2002 ]. In the argot of semiconductor physics, this is the semiconductor's
bandgap energy.Control of the temperature is also necessary to stabilize the wavelength
of laser operation. GaAs lasers shift their wavelength of operation at a rate around 0.250.3 nm/C°, with the wavelength increasing as the temperature increases [Nasu and
Mukaihara, 2005]. Thus if a specified operating wavelength is required, the temperature
of the diode must be accurately controlled. Wavelength tuning the emission wavelength
of a semiconductor laser can be also varied by varying the diode current or heat-sink
temperature, or by applying a magnetic field bias current level changes the emission
wavelength because the injected carrier concentration changes the refractive index of the
cavity, and it also shifts the peak photon energy according to Eq. By changing the heatsink temperature from10 to 120 K, the emission wavelength can be varied from 16 pm to
9 pm. Applying hydrostatic pressure to a laser diode can provide a broad tuning range.
Pressure affects emission wavelength through its effects on:
(1) The energy gap, (2) The cavity length, and (3) The refractive index.
The bandgap varies linearly with hydrostatic pressure for some binary compounds
[Kaufmann and Kunzer, 2002]. Diode lasers can also be tuned by a magnetic field. For
semiconductors with large effective mass anisotropy, the magnetic energy levels depend
on the orientation of the applied magnetic field with respect to the crystal axis [Nasu and
Mukaihara, 2005]. Both conduction and valence bands have their energies quantized into
Landau levels.
T   O  K  (TJ  300 K )..........(3)
K   Temperaturecoefficientofthewavelength,
O  Wavelengthat 300K .
Control of the temperature is also necessary to stabilize the wavelength of laser operation.
GaAs lasers shift their wavelength of operation at a rate around 0.25-0.3 nm/C°, with the
wavelength increasing as the temperature increases. Thus if a specified operating
wavelength is required, the temperature of the diode must be accurately controlled
[Ifflander, 2001].
4- Calculation the Effect of wavelength on output of end- pumped laser
system
The efficiency of a solid state laser is determined by three key parameters: the pump
semiconductor laser diode electrical – optical conversion efficiency,  D , the coupling or
transfer efficiency of the pump light into the active medium, T , and the optical –
optical conversion efficiency of the active gain media,  optopt . These parameters can
be further broken down to give the overall efficiency as [Koechner, 1994]:
   DTopt  opt   DTabs SQ B ST ASE E R .....(4)
where T ’ is the optical efficiency of coupling the pump light,  abs is the absorption
efficiency of the gain media,  S is the stokes efficiency or ratio of the input pump
photon energy to output photon energy,  Q is the quantum efficiency or fraction of
pump photons reaching the upper laser level,  B is the spatial beam overlap of the
resonator modes with the upper state
inversion,  ST is the storage or depletion
efficiency,  ASE represents the loss due to amplified spontaneous emission which is the
reciprocal of the depopulation rate of the upper laser level,  E is the fraction of
absorbed energy extracted and  R is the resonator loss including reflective and scattering
losses. Sometimes the efficiencies are grouped as the transfer efficiency, T , upper-state
lifetime efficiency U   S Q and extraction efficiency under Q-switched operation,
 eq   ST ASE E . The goal of this research is to explore the experimental realization of
the optimized design and determine its impact on the wall-plug efficiency of a flight laser
transmitter. The variation of the output wavelength for diode laser if used to pumped
different active material, is shown in figure (4), [Albers, 2009].
As shown Figure(4), Matching between the laser diode emission of wavelength( 808nm)
and the absorption line Nd:YAG crystal [Azcárate andCodnia ,1999].
Figure 4: Matching between the laser diode emission and the 808nm Nd:YAG absorption line [Azcárate
andCodnia ,1999].
5- Measurements and Results:
5-1 Operating temperature of the threshold current for laser diode
The current and output of laser diode are measured at different temperatures (T)
of the laser diode, power meter type (Gentec-E ) and Thermometer Type(Micos), as
shown in the set-up illustrated figure ( 5 ).
Beam shaping optics
Figure 5: Experiment is used for measuring the threshold current for laser diode.
The threshold current of laser diode is variation between (200-235 mA) when a
temperature variation between (5- 35 Co) , showed in figure (6). The increasing in the
threshold current with temperature is caused by diffusion of electrons and holes from the
active layer, and enhancement in the non- radiative recombination.
235
the threshold current (mA)
230
225
220
215
210
205
200
195
0
5
10
15
20
25
30
35
40
temperature(C)
Figure 6: Variation of the threshold current for laser diode in function of the temperature.
We present experimental results deduced from output power of laser diode measurements
in function of the threshold current and temperature, used above set up. Figures ( 7,8,9,
11,12 and 13) show the evolution of the output power of laser diode , which measured on
the threshold current between (200-235 mA) and temperature between ( 5- 35Co)
respectively. The comparison of measurements shows a good agreement between
measurements and the theoretical models. As experimental results and theoretical results
are proximate because value of constant (Z) on equation(2) , where value varies with
varied temperatures.
EXPERIMENTAL
THEORETICAL
EXPERIMENTAL
THEORETICAL
350
300
300
250
250
POWER(mW)
POWER(mW)
350
200
150
200
150
100
100
50
50
0
0
0
0
100
200
300
400
100
500
200
300
400
500
CURRENT(mA)
CURRENT(mA)
Figure7: Variation of the Light output in function
of the threshold current at temperature 5 °C .
Comparison to theoretical result.
Figure8: Variation of the Light output in function
of the threshold current at temperature 10 °C .
Comparison to theoretical result.
THEORETICAL
350
250
POWER(mW)
300
200
POWER(mW)
EXPERIMENTAL
THEORETICAL
EXPERIMENTAL
300
150
100
250
200
150
100
50
50
0
0
0
100
200
300
400
0
500
100
CURRENT(mA)
Figure9: Variation of the Light output in function
of the threshold current at temperature 15 °C .
Comparison to theoretical result .
200
300
CURRENT(mA)
400
Figure10: Variation of the Light output in function
of the threshold current at temperature 20 °C
Comparison to theoretical result.
300
250
250
200
200
POWER(mW)
POWER(mW)
EXPERIMENTAL
THEORETICAL
300
150
100
500
EXPERIMENTAL
THEORETICAL
150
100
50
50
0
0
0
100
200
300
400
500
CURRENT(mA)
Figure11: Variation of the Light output in function
of the threshold current at temperature 25 °C .
Comparison to theoretical result .
0
100
200
300
400
500
CURRENT(mA)
Figure12: Variation of the Light output in function
the threshold current at temperature 30 °C
Comparison to theoretical result.
EXPERIMENTAL
THEORETICAL
300
250
POWER(mW)
200
150
100
50
0
0
100
200
300
400
500
CURENT(mA)
Figure13: Variation of the Light output in function of the threshold current at temperature 35 °C.
Comparison to theoretical result.
5-2 Operating temperature of laser Diode
The wavelength (  )of laser diode was measured at different temperatures (T) of
the laser diode at constant output optical power (450 mW),by using Monochrometer
(range measuring of 1-900 nm ), has a resolution of (1nm) , power meter type (Gentec-E )
and Thermometer Type( Micos ), as shown in the set-up illustrated figure ( 14 )
Beam shaping optics
Figure 14: Shows how experiment is used for measure the effecting of temperature on the
wavelength shifting of Laser diode.
Figure(15) shown the influence of temperatures on wavelength for laser diode ,
comparison experimental study with theoretical study effect varies temperatures on
shifting wavelength of semiconductor laser.
EXPERIMENTAL
THEORETICAL
810
809
WAVELENGTH(nm)
808
807
806
805
804
803
802
801
800
0
5
10
15
20
25
30
35
40
TEMPERATURE(C)
Figure15: Variation of the shifted wavelength of laser diode as a function of the temperature;
Comparison to theoretical result.
An experimental study for effect varies temperatures on shifting wavelength of
Semiconductor Laser, experimental results of wavelength varies range (800nm-809nm),
change temperature from (5-35°C),a rate of change (0.25-0.3 nm/°C) .
The empirical, theoretical results for effect varies temperatures on shifting wavelength of
Semiconductor Laser which are matching about 95%.
5-3 Operating temperature of end- pumped laser system
The output power (40 mw) of end- pumped laser system was measured at
different temperatures (T) of the laser diode at constant output optical power (450mw),
by using power meter type (Gentec-E) and Thermometer Type (Micos), as shown in the
set-up illustrated figure (16)
Beam shaping optics
Nd:YAG crystal
mirrors
Figure 16: Experiment is used for measuring operating temperature of end- pumped laser system.
Found the threshold current of Nd:YAG laser of end- pumped laser system at
temperatures (31°C) is (315 mA).Shown in figure (17).
10
9
outpot power (mW)
8
7
6
5
4
3
2
1
0
0
100
200
300
400
500
current (mA)
Figure 17 : Variation of the output of end- pumped laser system
in function of the threshold current at temperature 31 °C .
Output Power (mW)
Found the output power of end- pumped laser system is variation between (1.5-8 mW)
at wavelength (1064 nm) when a temperature variation between (5-35°C).Shown in
figure (18).
9
8.5
8
7.5
7
6.5
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0
5
10
15
20
25
30
35
40
45
Temperatures(C)
Figure18: Variation of the output of end- pumped laser system in function of the temperature .
Conclusions:
We concluded the output power is highly sensitive to the diode output
wavelength. As the pump center wavelength is adjusted between the absorption peak at
808 nm at temperature diode pumped 31 °C and the local minimum at 804 nm at
temperature diode pumped 15 °C , the output power drops by 50% .As the diode output
wavelength continues to increase , it eventually matches the absorption line at 808 nm
and the 1064 nm output power partially recovers.
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