- _ NASA-CR-19! 338

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\ 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
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 nm

Laser Diode

Figure 2 Liquid Atomic Fractions as a Function of Solid

Composition for LPE Growth of GaxInl_xASyPl_y

Lattice 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 nm

Laser Diode

13

Figure 7 Temperature Dependance of L/I curves for a 1083 --- 15

nm Laser

Figure 8 Spectrum of 1083 nm Laser that can be stabilized -- 16

at He Absorption Lines

Figure 9 Coherent Length measurement of a 1083 nm Single --- 17

Mode Laser

Figure i0 Oxygen Free Copper Heat Sink For 1083 nm

Laser 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 Temperature

Stabilization 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 Characteristics

of Laser 1.083-19 for He Absorption Lines

Figure 22 Temperature and Current Tuning Characteristics

of 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)._

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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

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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.

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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:

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FOR LPE GROWTH

MATCHED TO 11001

OF Ga x Inl_xASyPl_y

lnP AT 635°C.

6

LATTICE

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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 pcontact. 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 farfield 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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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
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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

II

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Figure i0 Oxygen Free Copper Heat Sink For 1083 nm Laser Diode

19
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PHOTODETECTOR (_ ATHOI_ [_

PHOTODETECTOR ANODE

LASER ANODE

LASER CATHODE

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RF INPUT

Ground

Figure 11 Hermetically Sealed TO-8 Package

20
Pin ÷ 1 0

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Pin + 1

SEE NOTE 1_

Pin ÷_

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Pin ÷3

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Pin ÷4 Pin ÷5

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Pin ÷ 6

Pin ÷ 0

(-)0

Pin + 8

Pin ÷7

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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.
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Temperature Contro//ed Laser Diode Mounl

Figure 14 Housing for Two Stage Temperature Controlled Laser

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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

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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
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Figure 17 Diagram of He Cell and RF source

30
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32
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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
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34
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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37
_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).

, i:

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39