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- _ NASA-CR-19! 338
i / °i
\ FINAL REPORT
Contract Number:
SBIR Phase II
2/v-3 "
NAST-lO34 /353/(
Program /9. ¢5
1-083 Micron Tunable CW Semiconductor Laser
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submitted by:
General Optronics Corp.
20lsen Ave., Edison, NJ 08820
to
NASA Resident Office
JET Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, California 91109
.=
June 28, 1991
https://ntrs.nasa.gov/search.jsp?R=19930007500 2020-08-01T05:56:20+00:00Z
Final Report
Contract Number: NAS7-I034
1.083 Micron Tunable CW Semiconductor Laser
Table of Contents
2.
3.
4.
Q
6.
7.
8.
9.
i0.
ii.
12.
Objective of the Program. ............................. 1
Summary ............................................... 2
Introduction .......................................... 3
Device structure and Fabrication Procedures ........... 4
4.1 Device Structure and Crystal Growth ............... 4
4.2 Device Fabrication ................................ 8
Device Characteristics ................................ 12
Development of a Hermetically Sealed Package .......... 15
Temperature and Power Stabilization ................... 19
Temperature and Current Tuning of Laser Diode to He lines 27
Life-Test ............................................. 33
Delivery of Prototype Units ........................... 34
Conclusions ........................................... 38
References ............................................ 39
Figure Captions
Figure i Structure of Single Longitudinal Mode 1080 nmLaser Diode
Figure 2 Liquid Atomic Fractions as a Function of SolidComposition for LPE Growth of GaxInl_xASyPl_yLattice Matched to [i00] InP at 635°C.
Figure 3 SEM Photograph of CNS Laser Structure
Figure 4 L/I and V/I curve of a 1083 nm Laser Diode ii
Figure 5 Spectrum of a 1.083 Micron Laser Diode 12
Figure 6 Far Field of the Light Output from a 1083 nmLaser Diode
13
Figure 7 Temperature Dependance of L/I curves for a 1083 --- 15nm Laser
Figure 8 Spectrum of 1083 nm Laser that can be stabilized -- 16at He Absorption Lines
Figure 9 Coherent Length measurement of a 1083 nm Single --- 17Mode Laser
Figure i0 Oxygen Free Copper Heat Sink For 1083 nmLaser Diode
19
Figure ii Hermetically Sealed TO-8 Package 2O
Figure 12 Circuit Diagram of a Laser in TO-8 Package 21
Figure 13 Circuit Diagram for Power and TemperatureStabilization of a Laser Diode
24
Figure 14 Housing for Two Stage Temperature Controlled
Laser
25
Figure 15 Temperature Stability of a Two Stage thermo
. -electric cooler controlled Laser Package
26
Figure 16 Energy Diagrams of HeII Absorption Lines 29
Figure 17 Diagram of He Cell and RF source 3O
Figure 18 Temperature and Current Tuning Characteristics --- 31
of Laser 1.083-10 for He Absorption Lines
Figure 19 Temperature and Current Tuning Characteristics --- 32
.... of Laser 1.083-15 for He Absorption Lines
Figure 20 Life-Test Data of 1083 nm Laser Diodes
Figure 21 Temperature and Current Tuning Characteristicsof Laser 1.083-19 for He Absorption Lines
Figure 22 Temperature and Current Tuning Characteristicsof Laser 1.083-23 for He Absorption Lines
34
36
37
I. Ob]gctive 9_ the Proqram
This program was initiated in response to NASA 86-1 SBIR
solicitation subtopic 08.10. The statement of work from the
solicitation is as follows:
08.10 Subtopic: Tunable CW Laser for the 1.08 Micron Reqion
A tunable CW laser is desired to produce light equivalent to
the helium spectral line at 1.08 microns. This laser will serve as
an optical pumping source for He3 and He4 atoms used in space
magnetometers. This light source can be fabricated either as a
semiconductor laser diode or a pumped solid state laser. Continuous
output power of greater than i0 mW is desired. Semiconductor lasers
:can be thermally tuned, but must be capable of locking onto the
helium resonance lines. Solid state lasers must have efficient
_pumping sources suitable for space configuration. Additional
requirements: space magnetometer applications will include low
mass(<0.5 kg.), low power consumption (<0.75 W), and high
stability/ reliability for long missions (5-10 years)._
\
\\
1
2. Summary
Continuous wave operation of a 1.08 micron InGaAsP/ InP double
hetrostructure semiconductor laser has been successfully achieved
utilizing a channelled substrate narrow stripe (CNS) structure. The
fabrication process utilizes one-step liquid phase epitaxy. The
project goals of I0 mW continuous output power and 1.083 micron
emission wavelength have been achieved. The lasers have a typical
threshold current around 80 mA and over i0 mW output power at an
operating temperature of 20 degree C. The emission wavelength is
tunable with wide temperature and current ranges to match the HeII
absorption lines D0(I0829.08 _), DI(I0830.25 _) and D2(I0830.34 _).
A temperature and power stabilization electronics circuit has also
been design to control the output power and temperature of the
laser to within 0.02°C. This temperature control is required for
the lasing wavelength to stay within the Doppler width of the
absorption lines. Temperature stability of within 0.005°C has been
demonstrated over a few weeks period. Operation life-test of the
lasers has also been performed at 30°C. No failure or degradation
has been observed over 3000 hours of life-test. Four operating
units, which can be tuned to all the absorption lines, have been
delivered to JPL Magnetometer Laboratory.
k\
\\
2
3. Introduction
A Vector Helium Magnetometer (VHM) developed by Jet Propulsion
Laboratory in the mid 1970's [i] has impressive sensitivity of 0.01
nT (10 -7 gauss) [2]. Many such VHM's have since been utilized
successfully in various space missions. The VHM enables scientists
to accurately measure the interplanetary magnetic fields and the
detailed structure of the magnetosphere of various planets. An RF
powered helium discharge tube is used as the light source for
pumping the helium magnetometer. Recent advances in semiconductor
lasers offer an opportunity to further improve the sensitivity and
miniaturize the Vector Helium Magnetometer. The use of a
semiconductor laser for pumping such helium magnetometers is
particularly advantageous because of their small size, low voltage
operation, low power consumption and high reliability.
It is well known that because the composition of the
quaternary active layer of an InGaAsP/InP laser can be adjusted,
The lasing wavelength of the laser can range from 1 micron to 1.7
micron. However, for wavelength shorter than i.i _m Continuous
Wave (CW) room temperature operation become much more difficult,
because the potential and refractive index differences between
cladding layer and the active layer are small.
In this report we describe the development of the quaternary
1.083 _m wavelength InGaAsP/InP laser which has achieved results
better than the original goals.
\
\\
4. Device Structure and Fabrication Procedures
4.1 Device Structure and Crystal Growth
The structure of a quaternary InGaAsP/InP CNS laser is shown
schematically in Fiqure i. The CNS laser has been grown using a
one-step liquid phase epitaxy (LPE). Before LPE growth, Cd
diffusion is performed on the n-type InP substrate to facilitate a
reverse junction for eliminating leakage current outside the
channels. Cd instead of Zn is used because Cd has been shown to
diffuse much more slowly and does not further diffuse into the LPE
layers during the crystal growth. The Cd diffusion is performed at
600 degree C using a mixture of InP powder, Cd3P 4 and red
phosphorus as the source. It requires 12 minutes to obtain 0.5 _m
diffusion depth. The use of phosphorus provides sufficient vapor
pressure to prevent InP surface decomposition. Channels with 4.5 _m
width and 1.5 _m depth were etched through the diffusion layer
before the LPE growth. The chemical etchant used is a BPK solution
(2HBr : 4H3PO 4 : IN K2Cr207 ) and the etching time is 30 seconds at
5 degree C.
By using the phase diagram of GaxInl_xASyPl_y and the
conditions for lattice matching to InP, melt compositions can be
obtained properly for growing an active layer having an emission
wavelength between 1 micron and 1.7 micron. Use of the phase
diagram as shown in Fiqure 2 togetherwith an equation relating the
band-gap energy Eg(y) and the composition:
4
C
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(5"1
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EC
o
O
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121 _
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Figure 2.
I I I I
IXAs
©
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[]
E]
3.0
2.0
1.0
0 0.2 0.4 0.6 0.8 1.O
y ISOLID COMPOSITION!
,_,o0,o^TOM,C_R^CT,O._X'.,X'.X_AS A FUNCTION OF SOLID COMPOSITION y,
FOR LPE GROWTH
MATCHED TO 11001
OF Ga x Inl_xASyPl_y
lnP AT 635°C.
6
LATTICE
!
O
)K
_©
Eg(y) : 1.35 - 0.72 y + 0.12 y2 (eV)
enabled us to find the fractions of As, P and Ga in the Indium
solution for proper growth of the quaternary compound having an
emission wavelength at 1.08 _m. The corresponding solid composition
is obtained from the above equation and Vegard's Law for GaxInl_ x
ASyPl_y lattice matched to InP which is given by:
a(x,y) = 0.1894y - 0.4184x + 0.013xy + 5.8696
where a(x,y) = a (InP) = 5.8696 _.
Thus, the solid composition for the 1.083 #m active layer is
Ga0.16Ino.84As0.35P0.65-
Before placing the substrate into the crystal growth furnace,
the substrate was cleaned with Acetone, Methanol and TCE. Also, it
was etched with 5H2SO 4 : 1 H202 : 1 H20 solution for one minute.
The melts are baked in the Indium solution for one hour each at
high (656 degree C ) and Low (646 degree C) temperature. The
temperature is then decreased with a small ramp at a cooling rate
of 0.4 degree C/min. Crystal growth starts at 635 degree C. Four
J
LPE layers are grown which consist of: 0.2 _m Sn doped InP buffer
layer (i min), 0.15 _m undoped InGaAsP active layer (I0 sec), 1.0
_m Zn doped InP confinement layer (I0 min) and 1.0 _m Zn doped
InGaAsP contact layer (5 min). Fiqure3 shows the scanning electron
microscope photograph of the cross section of this laser.
7
Figure 3. SEM Photograph of CNS Laser
4.2 Device Fabrication
After the wafer is grown, the wafer is first processed with p-
contact. This is done in three sequential steps: Zn skin diffusion,
AuZn metal deposition and Ti-Pt thin layer sputtering. The wafer is
then lapped in the n side to a thickness of between 75 an i00 _m.
The N-contact is applied by evaporation of AuGe alloy. Both P and
N contacts are then alloyed at 410 degree C. to form a good ohmic
contact. Electroplated gold stripes 4 to 5 Nm wide and 3 _m height
are formed on the P surface to serve as masks for proton
implantation. The implantation voltage is adjusted according to the
thickness of the top two LPE layers. Since the performance of such
InGaAsP/InP lasers are more sensitive to temperature, efficient
heat extraction is important. Hence, a gold-plated heat sink of 12
_m thick is applied to each laser chip to help spread the heat into
a larger heat sink. The wafer is cleaved into chips of the size 250
_m X 250 _m. The chips are then mounted P-side down on the heat
sink.
5. Device Characteristics
Under phase I program, continuous wave operation of a 1.08
micron InGaAsP/ InP double hetrostructure semiconductor laser has
been successfully achieved.
The threshold current for most of the devices is between 75 mA
and 90 mA at I0 degree C. The output power exceeded I0 mW with 200
mA of driving current. A typical light output power versus drive
current curve at i0 degree C is shown in Fiqure 4. As the
temperature is increased to 20 degree C., the threshold current
increases to I00 mA and the maximum output power drops to 4 mW.
The peak wavelength of the lasers that have been tested is in
the range of 1.081 micron to 1.084 micron. Fiqure 5 shows the
spectra distribution of the laser which was measured at 5 mW output
power and i0 degree C temperature. The wavelength can be changed
continuously with different drive currents and temperatures.
The far field patterns for directions parallel and
perpendicular to the junction plan are shown in Figure 6. The far-
field patterns are measured at ii0 mA drive current with output
power of 5 mW at I0 degree C ambient temperature. The curve shows
very good fundamental transverse mode operation. The angles of half
intensity for parallel and perpendicular directions are 22 degrees
and 38 degrees respectively.
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13
In phase II we have perform the optimization by adjusting the
compositions and growth temperature to obtain good lattice matching
and strong photoluminescent intensity. The resulting laser can
operate at a higher temperature with better characteristics. I0 mW
output power at 30°C operation can be obtained. Fiqure 7 shows the
temperature dependence of the light versus current curve of a 1.083
_m laser. The characteristic temperature T O is about 40 degrees K.
Figure 8 shows a typical spectrum, the side mode suppressing has
improved significantly. A coherent length measurement as shown in
fiqure 9 indicate that the coherent length is longer than 30 cm.
This mean that the line width is less than one GHz or the Doppler
width of the absorption lines.
14
o
C) O0
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00 o
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17
6. Development of a Hermetically Sealed Packaqe
The lasers are mounted on standard General Optronics' heat
sinks as shown in Fiqure I0. These heat sinks are made of oxygen
free copper. Next to diamond, oxygen free copper has the highest
thermal conductivity. With evaporated indium for die bonding, we
can obtain very low thermal resistance of 25 degrees C per watt
which gives good laser reliability.
A hermetically sealed TO-8 package as shown in Fiqure Ii are
used for housing the laser. A detector, a thermistor and a thermal
electric cooler are also installed in the package for feedback
stabilization of the temperature and optical power. The pin
assignments as well as dimensions are also shown in the figure.
The equivalent circuit for the TO-8 laser package is shown in
Fiqure 12. The laser in this package can be directly modulated up
to 2 GHz.
18
0.014"
I0.0361
II0.021
10.0531
II0.190
I0.4831
0.094
10.2391
CHIP
POSITIVE
NEGATIVE
0.O45"10.1141
LEAD
CERAMICSTANDOFF
LEAD
II
0.100
10.2541
Y
0.220"
10.5601
I ! in cm
Figure i0 Oxygen Free Copper Heat Sink For 1083 nm Laser Diode
19
•_---- .BOO--
rr--.437
t /..... __
. 040
TYP. REF,
\I
/
1HEIGHT
.250
PIN CONFIGURATION
COOLER (÷)
THERMI8TOR
1
2
,
G
6
7
9
IQ
O(_ BIAS INPUT
PHOTODETECTOR (_ ATHOI_ [_
PHOTODETECTOR ANODE
LASER ANODE
LASER CATHODE
. COOLER (-)RF INPUT
Ground
Figure 11 Hermetically Sealed TO-8 Package
20
Pin ÷ 1 0
COOLERPin + 1
SEE NOTE 1_
Pin ÷_
12uHPin ÷3
Pd,&
Pin ÷4 Pin ÷5
LASER
-
)Pin ÷ 6
Pin ÷ 0
(-)0Pin + 8
Pin ÷7
©
Figure 12 circuit diagram of a Laser in TO-8 Package
21
7. Temperature and Power Stabilization
The lasing wavelength of the semiconductor laser can change
substantially with changing output power or heat sink temperature.
The wavelength change as a function of temperature is typically 1
Angstron/degree C. Therefore, in order to operate at helium
absorption wavelength of 1.083 microns the heat sink temperature
and the optical power should be stabilized. With the attached
detector in the backside of the laser and with a thermal electric
cooler and a thermistor, electronic circuits can be designed to
stabilize the temperature to within 0.02 degrees C and output power
to within 1%. The electronic diagram of the temperature and power
control circuit is shown in fiqure 13. With this circuit the
temperature of the laser can be stabilized to 0.01°C in the range
of -15°C to 30°C, the laser current can be from 0 to 250 mA. If
higher current is required the power transistor Q1 should be
replaced with another transistor with higher current rating.
A two stage temperature control system has been build to test
how well we can control the laser temperature. Fiqure 14 shows the
housing of the two stages temperature controlled 1083 nm laser
diode. IXL model LDM-4412 laser diode mount was utilized. Fic[ure 15
shows the data taken for two stages temperature controlled system.
Fiqure 15a shows the variation of room temperature which spans
3.1°C over a period of 4000 minutes. With one stage thermo-electric
cooler control the laser plate temperature variation (Figure 15b)
is within 0.0116°C during the same period. The laser, Which sit on
this plate has another thermo-electric cooler for temperature
22
stabilization. The variation of laser temperature [Figure 15c_ was
less than 4.12 x 10 -3 degrees C over 4000 minutes.
,,z
Z
0
L_- ,"-'1,""1-,'-I..el
-13O3
OJ1,4
,q
ID..,
E'-I
3:0
13.1
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tl:f-,-..I
-,--I:::::1o itl
-,--I e,...Iu o
{""1l--I
14
-,.--IF.I.,
@
24
Temperature Contro//ed Laser Diode Mounl
Figure 14 Housing for Two Stage Temperature Controlled Laser
25
L
-C.
1.
C'_
E.
.Z
_c _ _qsq
i i .,o?7 17 _5
26
24 L-_11
0 788 1538 2304 3072 3840
Time (Minutes)
tI0. I ), 1t811
L.A$1[J Ti..AT[ I
20 48655 - -L.)
20 ....
0 7C8 1538 2304 3072 3840
Time (Minutes)
- 6 8 _5_"j09IO. I:,. lq19
L_$(! I
tl oT. , 111_.6[-0
DAT* rILl NIO,
t) -68 5 .......
_ -68 -
-6.8 }9134 - I !
-6.8 toter4
0 Z( 8 1536 2304 3072 3840
Time (Minutes}
Figure 15 Temperature Stability of a Two Stage thermo-electric
cooler controlled Laser Package
26
8. Temperature and Current Tuninq of Laser Diode to He lines
The energy levels involved in optical pumping for He
magnetometer is shown in Fiqure 16. The main absorption lines are
D0(I0829.08 _), D](I0830.25 _) and D2(I0830.34 _). The separation
between D O and D 1 line is 1.17 _, while the separation of D 1 and D 2
lines is only 0.09 _. The Doppler width is about .05 _, therefore,
D 1 and D 2 lines can not be separated, we will call the merged lines
DI2 line. The DI2 line is a much stronger absorption line not only
because it consist of two lines, but also because each of the two
lines has multiple degeneracy. The tuning of the diode laser
wavelength to the He absorption lines are performed by focusing the
laser output into a He cell as shown in Fiqure 17, which was supply
by JPL. For a fixed laser drive current the temperature of the
laser can be changed so that the lasing wave length will coincide
with the He absorption lines. When this occurs the He cell will
have strong florescence at 1.083 _m wavelength. This phenomena can
be easily observed using a silicon CCD TV camera. Fiqure 18 shows
a typical tuning characteristics of a 1083 nm laser we can obtain
resonance absorption on both D O and DI2 lines. We can observe
florescent for temperature ranging from 13.8°C to 18.3°C and drive
current from 170 mA to 260 mA. The electric power required for the
laser is typically 0.4 Watts. For drive current increase of i0 mA,
the temperature has to be lower by 0.33°C. To shift the wavelength
from D O line to the longer wavelength DI2 line the temperature has
to be increased by 1.4 degrees C. Which corresponds to peak
wavelength shift of .87 A/degrees c. Fiqure 19 Shows the tuning
27
f
curve of another laser diode that was housed in a two stage
temperature controlled package and delivered to JUL. For this laser
diode the operation temperature has to lower at about 7 to 12
degrees C. It requires 1.35 degrees C temperature change to shift
from D O line to DI2 line. The temperature dependance of the peak
wave length is about 0.9 _ / degrees C.
28
f
i
? _',.) ---f- --_ ...... O
/ ,o.'Jg_,,-,'i
00 '. , +i
.'..°,--.-,.....o:'
/e //--+,23SI--_ L_--O (.%IETASTASL[}
i/ _ -I
I
/
/
/
/
/
i 6 × IO 5 cm- I
/
/
/
/
/
/
Energy levels of helium involved in optical
pumping.
DO : i0829.08 A (9234.395 cm-l) 2.767775xi0(14)
D1 . 10830.25 k (9233.397 cm-l) 2.767476xi0(14)
D2 : 10830.34 A (9233.320 cm-l) 2.767453xi0(14)
/,,Do_er Half-Width = -- =
m 1.6(-19)'4.9.38(6)
29.9 GHz
2.3 GHz
_=.73 CHz (300 K).84 CHz (400 K)
Line Width :
C
,_P ,x_ .,k = 25.s78 G.z (1 ;,)o./
1 GHz = .039 A
at _=1.083 um
Figure 16 Energy Diagrams of HeII Absorption Lines
29
/
f
.i L
HIll1I- (I__ tt - T--_J'-"_ ,_
OI
I@NIT ION
POWER
12VDC% R1?
< TG-27_I
OUTPUT
-- MODULATORI-1ETER8.4 VDC
GROUND
F I - I 5 [i f [:1 I - 12 '
I "' I (]}(]}l[If
L1 - 2'2uH R - ::-?_,ohm
CELL PI;.ES_UR[ ,8 TORR
TO Z7
I-IFGFLITABA
He Cell and RF source
•JVA 1-19-90
TD-27
Lt2:::_CLS.4 VDC
Ii i_ I"I,]DULAT OR
/J.-" TOP VIEW
Figure 17 Diagram of He Cell and RF source
30
CCWF-0
n--
m
F--
0
n
00I I
0
0
0
0
c0
C_v
0_1
t
0
(1)C
um
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o
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r,.) m
_ -,.-i-,-i
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f
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<I,CO
32
x
/
9. Life-Test
Life-test of the 1.083 _m lasers have been performed using
the automatic power control (ABC) mode of operation. A total of
eight lasers have been life-tested at 30 degrees C for over 3000
hours. Fiqure 20 shows the data for life-test of the lasers. After
operation of 3000 hours no laser has failed. The data shows similar
trend as that of other quaternary GaInAsP/InP lasers operating at
1.3 _m or 1.55 _m wavelength region. Since the component material
are the same we can expect the life time to be similar to that of
1.3 #m or 1.5 _m lasers which has been established to be over 106
hours ( > i00 years). However, to establish a reliable life-time
for the 1.083 _m laser, longer and accelerated life test should be
performed.
33
7
F-00LLIO
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i0. Delivery of Prototype Units
Two shipments of devices have been made for the contract. The
first shipment were made on January 19, 1989. Which included two
devices with the tuning characteristics as shown in figure 18 and
figure 19. The serial number of the devices are JPL-IO and JPL-15.
The later device have been housed in a two stage thermo-electric
cooler temperature controlled housing as shown in figure _14. A
preliminary test at JPL and Polatomics shows that the laser is very
sensitive to feed back reflections, which produces unwanted noise.
The second shipment included two additional devices in TO-8
packages with removable caps. Which were manufactured and shipped
to JPL magnetometer Laboratory on May I, 1991. The serial number
for the two devices on second shipment are 1.083-19 and 1.083-23.
Their tuning characteristics curves are shown in fiqure 21 and
fiqure 22 respectively.
35
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_l. Conclusions
A InGaAsP/InP semiconductor laser with emission wavelength at
].08 _m has been successfully developed by using a channelled
substrate narrow stripe structure. CW operation with an output
power of over i0 mW has been achieved. The lasers have a typical
threshold current around 80 mA and over i0 mW output power at an
operating temperature of 20 degree C. The emission wavelength is
tunable with wide temperature and current ranges to match the HeII
absorption lines D0(I0829.08 _), Di(i0830.25 _) and D2(I0830.34 _).
A temperature and power stabilization electronics circuit has also
been design to control the output power and temperature of the
laser to within 0.02°C. This temperature control is required for
the lasing wavelength to stay within the Doppler width of the
absorption lines. Temperature stability of within 0.005°C has been
demonstrated over a few weeks period. Operation life-test of the
lasers has also been performed at 30°C. No failure or degradation
has been observed over 3000 hours of life-test. Four operating
units, which can be tuned to all the absorption lines, have been
delivered to JPL Magnetometer Laboratory.
The project goals as set by the NASA SBIR program have been
completely and successfully accomplished.
The Project was carried out with Dr. C. S. Wang as principal
investigator, and Mr. Jan-Shin Chen, Mr Ken-Gen Lu and Mr. Keng
Ouyang as co-principal investigator.
38
] 2 . t_I orQ_nces
:!<;
[1] E. J. Smith, B. V. Connor, and G. J. Foster, Jr., "Measuring
the Magnetic Fields of Jupiter and the Outer Solar System"
[EEE Trans. Magn. Mag-ll, 962 (1975).
[2] E. J. Smith, L. Davis Jr., D. J. Jones, P. J. Coleman Jr.,
D. S. Colburn, P. Dyal, and C. P. Sonett. " Saturn's
Magnetosphere and Its Interaction With the Solar Wind"
J. Geophys. Res, 85, 5655 (1980).
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