94 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 31, NO. 1, JANUARY 1, 2019
Facet Cooling in High-Power
InGaAs/AlGaAs Lasers
Seval Arslan , Sinan Gündoğdu , Abdullah Demir, and Atilla Aydınlı
Abstract— Several factors limit the reliable output power of window approach, the facet is separated from the current
a semiconductor laser under CW operation, such as carrier injection window by placing the pump electrode away from
leakage, thermal effects, and catastrophic optical mirror damage the facet. Facet temperature was shown to be lower using a
(COMD). Ever higher operating powers may be possible if the
COMD can be avoided. Despite exotic facet engineering and 30 μm compared to 0 μm long current blocking region due
progress in non-absorbing mirrors, the temperature rise at the to reduced current density near facet [8]. However, the length
facets puts a strain on the long-term reliability of these diodes. of the unpumped current blocking window is limited due to
Although thermoelectrically isolating the heat source away from the optical absorption in this region degrading the quantum
the facets with non-injected windows helps lower the facet efficiency of the lasers [9]. Alternatively, ultra-high vacuum
temperature, data suggests the farther the heat source is from
the facets, the lower the temperature. In this letter, we show that cleaving and passivation of the facets eliminates the surface
longer non-injected sections lead to cooler windows and biasing states and leads to higher COMD thresholds at the cost of
this section to transparency eliminates the optical loss. We report higher complexity and expense of fabrication [10]–[12]. So far,
on the facet temperature reduction that reaches below the bulk these approaches have achieved effective results increasing Tc.
temperature in high power InGaAs/AlGaAs lasers under QCW However, none of these studies have demonstrated laser facet
operation with electrically isolated and biased windows. Acting
as transparent optical interconnects, biased sections connect the temperature lower than its bulk or cavity. Although eliminating
active cavity to the facets. This approach can be applied to a the surface recombination and optical absorption is critical for
wide range of semiconductor lasers to improve device reliability the reduction of mirror temperature as targeted in the previous
as well as enabling the monolithic integration of lasers in photonic methods, self-heat load contributes greatly and is the dominant
integrated circuits. source of facet heating, but it has not been addressed before,
Index Terms— Semiconductor lasers, diode lasers, high power which is targeted by the method reported in this letter.
lasers, catastrophic optical damage, reliability. In this work, facet temperature is reduced using a new,
I. I biased window approach in InGaAs/AlGaAs high power laserNTRODUCTION
diodes without degrading output power. To reduce the facet
DESPITE record-high output powers [1] and electro- temperature, the laser chip is monolithically separated into twooptical power conversion efficiencies [2], catastrophic regions by dividing the top electrode to lasing and transparent
optical damage (COD) has been known and recognized as window sections. This configuration allows keeping the high
one of the limitations of semiconductor lasers since their self-heat load of the lasing region away from the heat sensitive
invention [3]. This type of heating induced device failure is output facet. The transparent window, next to the facet, oper-
a key to both device performance and reliability [4]. Many ating at very low current produces negligible heat. We used
successful methods were reported in the literature to prevent thermoreflectance spectroscopy with a resolution below 1K
facet mirror reaching the critical temperature of T = 120-
◦ c [2], [3], [13]–[18]. The temperature distribution on the facet160 C and causing COD at the output mirror facet (COMD) under operating current is determined by a full-scale image of
[3]. Non-absorbing mirrors (NAMs) close to the facets and/or the object plane. The facet temperature of a 5.00 mm long laser
non-injecting current windows have been implemented to was compared with lasers of the same length that employed
reduce optical absorption and facet heating [5], [6]. NAMs are 0.75 mm and 1.00 mm long biased windows. The output power
produced by quantum well-intermixing close to the mirrors of the laser with biased windows while at transparency was
[2]. However, this method is a complex process with tight brought to the same level as the output power of the control
fabrication tolerances [7]. In non-injected current blocking lasers. Facet temperature was observed to decrease by 40% in
Manuscript received August 14, 2018; revised November 13, 2018; accepted the waveguide region. We note that the reduced facet temper-
November 26, 2018. Date of publication November 30, 2018; date of current atures are also below the bulk temperature of the 5.00 mm
version December 24, 2018. This work was supported by Ermaksan A.S. long control laser demonstrating a cooling effect on laser
(Corresponding author: Atilla Aydınlı.)
S. Arslan and S. Gündoğdu are with the Physics Department, Bilkent facet. Reduction of large facet temperature is by 30% below
University, 06800 Ankara, Turkey (e-mail: sevalsaritas67@gmail.com). the bulk temperature of the control laser. Our results indicate
A. Demir is with the UNAM-National Nanotechnology Research Cen- that biased window approach should lead to increased COMD
ter, Institute of Materials Science and Nanotechnology, Bilkent University,
06800 Ankara, Turkey (e-mail: abdullah.demir@unam.bilkent.edu.tr.) threshold level and reliability of the semiconductor lasers.
A. Aydınlı is with the Electrical and Electronics Engineering Department,
Uludağ University, 16059 Bursa, Turkey. II. EXPERIMENTAL
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org. The layer structure of the GaAs-based high-power laser
Digital Object Identifier 10.1109/LPT.2018.2884465 emitting around 915 nm is composed of an n-GaAs sub-
1041-1135 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
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ARSLAN et al.: FACET COOLING IN HIGH-POWER InGaAs/AlGaAs LASERS 95
strate, 3000 nm n-AlGaAs cladding, 500 nm n-AlGaAs
waveguide, 8 nm InGaAs QW, 500 nm p-AlGaAs waveguide,
1000 nm p-AlGaAs cladding and finally 100 nm GaAs cap
layer [19]. In this study, 100 μm wide broad area high
power laser diodes with different lasing cavity and window
lengths were used to compare the performance of the lasers
and their facet temperatures. The main difference between
the designs under investigation is the presence of an electri-
cally isolated window region. The control lasers have cavity
lengths of 4.00 and 5.00 mm, whereas coupled cavities are
4.25+0.75 mm and 4.00+1.00 mm, where 0.75 and 1.00 mm
are the window lengths. Ridge waveguides were formed on
the same wafer by wet etching, a dielectric layer (Si3N4) was
deposited as the electrical insulator, and p-contact windows
were opened on top of the waveguide regions. Ti/Pt/Au and
AuGe/Ni/Au were deposited for p- and n- metals, respectively,
and alloyed to obtain ohmic contacts. 300 nm deep trenches Fig. 1. Front facet output power at various injection currents for standard
4.00 mm and 5.00 mm long lasers and 5.00 mm long laser with 1.00 mm long
for electrical isolation between the laser and window regions window region (window current is at 170 mA). The schematic illustration of
were opened after the formation of the waveguide. Trench the two different laser designs is also shown. Inset shows the threshold current
depth was limited to 300 nm to conserve the mode profile and density versus inverse cavity length line graph.
not to introduce optical losses. The resistance between the
laser and window regions was measured as 69 . Fabricated
lasers were cleaved and mounted epi-up on a copper block. threshold currents of the lasers were determined for each bias
Facet temperatures of the diode lasers were measured current. To illustrate the performance of the biased window,
by CMOS based-thermoreflectance method using the ‘four- we compare the L-I characteristics of the 5.00 mm long
bucket’ technique [10]. For thermoreflectance measurements, laser that has a 1.00 mm window section biased at I2 =
a LED at 450 nm is used as the probe light and directed onto 170 mA, with standard 4.00 and 5.00 mm lasers, in Fig. 1.
the laser facet through a 0.40 numerical aperture 20× micro- In this study, we characterized uncoated and epi-up lasers
scope objective. Lasers were mounted on a copper baseplate to simplify the observation of the facet cooling. The power
for testing at room temperature and they are tested up to 5A conversion efficiency of the lasers is around 52%, which is
injection current (I1). They were biased by LDX-3600 diode mainly limited by uncoated and epi-up mounted operation.
driver under QCW current injection with 20 ms pulse duration The threshold currents of the 4.00 mm, 5.00 mm long lasers
and 20% duty cycle. Lasers with biased windows were tested and that of the 4.00+1.00 mm long laser biased at I2 =
under various window bias currents (I2) of 0, 50, 100, 150, 170 mA, are 482 mA, 568 mA and 408 mA, respectively.
170 mA while lasing with current I1. The temperature mod- We observe that the threshold current of the laser decreases
ulated back-reflected light is collected with CMOS camera as a function of window bias as the biased window section
triggered by a diode driver and analyzed by a computer. The of the structure is brought to transparency compensating for
relation between the relative change in reflectivity and the the loss. Increasing the bias current increases the spontaneous
temperature change is (given as)[10], [14], [20]. emission, hence contributing to the gain in the pump section.
R 1 ∂R Therefore, the threshold current of the 4.00+1.00 mm long= T = κT (1) laser is smaller than that of the 4.00 mm long conventional
R R ∂T
laser for 170 mA bias current. Furthermore, considering that
where κ , is the thermoreflectance coefficient that depends threshold current is an exponential function of temperature,
on the sample material and probe light wavelength [10]. the colder laser (4.00+1.00 mm long) has a lower threshold
We calculated the thermoreflectance constant of GaAs sub-
± × − current. We measured the optical power at different windowstrate at 450 nm as (3.2 0.6) 10 4 by modulating the laser bias currents. The output power of the 4.00 and 5.00 mm long
temperature using a Newport 3700 temperature controller. lasers and that of the laser with window biased at 170 mA are
Since the aim is to compare facet temperature changes between 2.06 W, 2.15 W and 2.02 W at 5 A pump current, respectively.
a 5.00 mm long high-power laser with 0.75 and 1.00 mm long The transparency and threshold currents of the window are
biased windows and a 5.00 mm long control laser, possible calculated using the basic laser rate equations [21]. Threshold
differences between κ’s of GaAs and AlGaAs layers that current density in the limit of infinite cavity length, J 0th , and
make up the laser are not critical. Temperature resolution
± ◦ tan θ are found from the intercept and slope of the thresholdduring the measurements was 0.1 C with a spatial resolution
± current density versus inverse cavity length line graph (insetof 0.7 μm. of Fig.1), respectively, as 77±7 A/cm2 and 18±3 A/cm,
the internal loss, αi , is calculated as 0.39 cm−1 (not shown).
III. RESULTS AND DISCUSSION Hence, the transparency and threshold currents for 1.00 mm
All samples were characterized by measuring L-I-V long window are estimated as 76± 7 mA (indicated in Fig.3)
curves under CW for each biased window current (I2) and and 257± 28 mA, respectively.
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96 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 31, NO. 1, JANUARY 1, 2019
Fig. 2. (a) Temperature map of 4.00+1.00 mm long laser at I1 = 5.00 A and I2 = 0.17 A and (b) Temperature line scans for 5.00 mm, 4.25+0.75 mm and
4.00+1.00 mm long lasers taken at the active region through the direction perpendicular to surface.
Fig.2 shows the facet temperature measurement results at in Fig. 2b, are indicative of the epitaxial growth interface).
I1 = 5 A for 5.00 mm, 4.25+0.75 mm (with I2 = 150 mA) As this is a comparative study, and all layers are the same in all
and 4.00+1.00 mm (with I2 = 170 mA) lasers. Fig.2a shows samples, changes in facet temperature for 4.00+1.00 mm and
the temperature distribution map of 4.00 mm+1.00 mm laser 4.25+0.75 mm long lasers at waveguide region with respect to
and Fig. 2b shows vertical temperature scans (from the top that of 5.00 mm long laser is not expected to be significantly
of the epitaxial region to GaAs substrate) for 5.00 mm, affected assuming the same constant for all layers that of GaAs
4.25+0.75 mm and 4.00+1.00 mm long lasers, respectively. as (i. e. (3.2±0.6)×10−4). The spatial resolution of ±0.7 μm
As seen in Fig. 2a, active region has the highest temperature allows us to characterize the temperature changes in the
and heat spreads vertically towards the copper heat sink as waveguide region including the active region. We find that the
well as laterally through the electroplated gold heat spreader. facet temperature of the waveguide region in 4.00+1.00 mm
In Fig. 2a, we note that the right-hand side of the mesa is laser is reduced by 40% demonstrating that a biased window
cooler than the left-hand side due to its larger spacing from at transparency leads to cooler windows.
the edge of the chip compared to that of the left. In Fig. 2b, In Fig.3, we compare the temperature rise and front facet
temperature rise along the facet vertical from the top of the output power of the 5.00 mm long conventional laser and
epitaxial region to GaAs substrate is plotted as a function of 4.00+1.00 mm long lasers as a function of window bias
distance from the epitaxial surface. The average temperature current (I2) at different pump currents (I1). As expected, facet
is determined along the 100 μm long width of the ridge facet. temperature increases with increasing I1. The thermal resis-
For 4.00+1.00 mm long lasers, facet temperature fluctuations tance of 5.00 mm long laser is determined as 12.8 K/W using
(Tav = 54 ± 0.4 ◦C) are nearly homogenous while 5.00 mm the wavelength shift of Fabry-Perot modes. Hence, the bulk
long lasers show clear filamentation (Tav = 79±3.0 ◦C) temperatures of 5.00 mm long laser at I1 of 1A, 3A and 5A are
(not shown). In fact, the temperature inside the filaments 12.4, 27.8, and 49.7 ◦C, respectively. The facet temperatures
in 5.00 mm long lasers reaches up to 150 ◦C while the of this laser at the waveguide region for the same currents
temperature in the 4.00+1.00 mm long laser rises only up were measured as 14.7, 33.7, and 58.7 ◦C, respectively. The
to 65 ◦C at most. Both the magnitude of the temperature facet temperatures are higher than the bulk temperatures as
fluctuations as well as the comparison of the hottest regions expected, due to the non-radiative recombination at the laser
show that 4.00+1.00 mm long biased window lasers operat- facet. We measured the facet temperatures of 4.00+1.00 mm
ing at similar optical output power have much cooler facet long lasers at the waveguide region as a function of I2 for
than that of the 5.00 mm long control lasers. One can also different I1 values. For higher I1, the temperature of the
safely conclude that biased window approach increases the 4.00+1.00 mm long laser increases correlated with dissipated
threshold for filament formation. The waveguide region has power. There is a slight temperature increase at window bias
the highest temperature and the temperature falls towards the current of I2 = 50 mA. This is due to Joule heating of the
GaAs substrate as seen in the temperature map. Sharp changes window since the window is not transparent at this bias cur-
in the vertical scans are mainly due to the differences in the rent. I2 = 50 mA is below the calculated window transparency
reflectivities of materials that make up the epitaxial layer, and current level of 76 mA and thus generates 69 mW of dissipated
consequently are indicative of different values of thermore- power corresponding to 0.9 ◦C temperature rise in agreement
flectance. Thus, sharp peaks delineate the transition between with the experimental data. Temperature decreases slightly
the growth layers (e.g., the peaks on the GaAs substrate side after reaching I2 = 100 mA, which is above the transparency
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ARSLAN et al.: FACET COOLING IN HIGH-POWER InGaAs/AlGaAs LASERS 97
sections integrated with a laser cavity can be applied in laser
bars and in DFB laser integrating monolithic photonics circuits
to improve their performance and long-term reliability.
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