Extensive studies of the output characteristics of single quantum well lead-telluride lasers developed at the General Motors Research Laboratories were carried out. Threshold currents, output powers and line structures were measured as a function of temperature. Very low-current lasing thresholds, record high operating temperatures and over 30% tuning ranges were achieved. Excellent reproducibilities, good far-field patterns and reasonable linewidths (approx. 500 kHz) were found

Bielinski, J. W.

Freed, C.

English

NASA Technical Reports Server

nMassachusetts Institute of Technology
Lincoln Laboratory
P.O. Box 73
Lexington, Massachusetts 02173
LEAD SALT DIODE LASERS AND DEVELOPMENT
OF TUNABLE SOLID STATE LASERS FOR REMOTE SENSING
SEMI-ANNUAL PROGRESS REPORT
July 1984
Prepared for NASA under Contract NAG-1-164.
https://ntrs.nasa.gov/search.jsp?R=19840021138 2020-03-20T22:34:36+00:00Z
LEAD SALT DIODE LASERS AND DEVELOPMENT
OF TUNABLE SOLID STATE LASERS FOR REMOTE SENSING
SEMI-ANNUAL PROGRESS REPORT
Prepared for NASA under Contract NAG-1-164
by
C. Freed and J. W. Bielinski
ABSTRACT
Extensive studies of the output characteristics of single quantum well
lead-telluride lasers developed at the General Motors Research Laboratories
were carried out. Threshold currents, output powers and line structures
were measured as a function of temperature. Very low-current lasing
thresholds, record high operating temperatures and over 30% tuning ranges
were achieved. Excellent reproducibilities, good far-field patterns and
reasonable linewidths (~500 kHz) were found.
Introduction
During the last several months, extensive studies of the output
characteristics of lead-telluride quantum-well diode lasers were carried
out. These lasers were developed and fabricated at the General Motors
Research Laboratories (GMR) and loaned to MIT Lincoln Laboratory (MIT/LL)
at no cost to the program, in the expectation that the results will
mutually benefit all participants.
The multiplicity of data obtained both at GMR and MIT/LL clearly
indicated that the lead-telluride quantum-well lasers achieved major
improvements in tuning range, open-loop frequency reproducibility and mode
stability, as well as significantly lower-current lasing thresholds and
record high operating temperatures.
Brief Description of the PbTe Quantum-Well Diode Lasers
Molecular beam epitaxial (MBE) growth of Pbi_xEuxSeyTej_y
lattice-matched to PbTe substrates has been used to fabricate double
heterojunction diode lasers, including the single quantum-well, large opti-
cal cavity (LOG) devices to be discussed in this report; PbEuSeTe is a new
material recently developed at the General Motors Research Laboratories,
which can be used to cover the 2.6 to 6.6 urn wavelength range.U~3) Mesa
stripe geometry diode lasers were fabricated(^) with stripe widths from
16 to 22 ym and the cleaved cavity lengths were typically 325 to 450 urn
long. The PbTe quantum-well width, Lz, was varied in the sequence 300,
o
600, 1200 and 2500 A in a series of otherwise similar growths. Strong
o
quantum effects were observed for Lz jC 1200 A, and the shift in laser
emission energy was in approximate agreement with that calculated from a
finite square well potential. Dale Partin reported in greater detail on
the design and construction of these single quantum well PbEuSeTe diode
lasers.(5) This report will concentrate on some of the observed output
characteristics.
Operating Temperatures
Lasing was achieved at record high temperatures, 174K aod 281K under
cw and pulsed (-1 ys) operation, respectively.(5) These temperatures
are well within thermoelectric cooling range.
Threshold Currents and Power Outputs
Lasing was achieved also at remarkably low threshold currents. At 13K
o
the cw threshold current was only 2.1 ma for a 600 A single quantum well
laser(6) (less than the biasing current for many infrared detectors).
Figure 1 shows the cw threshold current and corresponding current
density as a function of temperature for MBE-4, which was a single, 300A
quantum-well diode laser. At low temperatures lasing began at very low
threshold currents on transitions between n=l states. The increasing
steepness of the threshold current at higher temperatures was probably due
to the fact that at the higher temperatures a greater fraction of the
electrons goes into pumping transitions between n=2 states. Notice that
for n=2 state transitions there was a decrease in threshold current as one
increased the operating temperature. Eventually a temperature was
reached(5) (under pulsed conditions) at which the n=2 threshold current
approximately equalled the n=l threshold. Above this temperature the n=2
threshold current increased rapidly and the n=l transition was no longer
observed. This behavior of theshold currents was typical for other quantum
well lasers too, and not just a unique characteristic of MBE-4.
Figure 2 shows the single ended output power of MBE-4, as a function
of current. The maximum (multiline) output power of about 5 mw was obtain-
ed in the vicinity of 60K. The various colors indicate the power at
several different temperatures.
It should be noted that 550 ma diode current corresponded to laser
operation at 50 to 75 times threshold current between 77 and 20K, respect-
ively. Occasionally at least we used cw current drives up to about 1A
without catastrophic failure or deterioration.
Experimental Arrangements
Figures 3 and 4 show block diagrams of the experimental setups used to
perform high-resolution measurements of the output spectra, linewidths,
tuning rates, frequency reproducibility, and spatial characteristics of
mode patterns.
The diode laser radiation was combined on a beamsplitter with that of
a grating-tunable stable CO reference laser. The beatnote of the two
lasers was detected and very high resolution measurements of linewidths
and/or tuning rates were achieved by using a spectrum analyzer. In order
to facilitate some of the measurements (e.g., linewidths and tuning rates),
the diode lasers could also be frequency-offset-locked as described
elsewhere(7) and illustrated in Fig. 4. A portion of the radiation was
also deflected to a grating spectrometer in order to determine the spacings
of longitudinal modes and lasing transitions.
Far Field Radiation Patterns
Detailed observation on the spatial characteristics of output radia-
tion impinging on a scatter plate was accomplished and recorded with a very
sensitive and sophisticated TV-compatible infrared scanning camera and
video recorder. This system, together with a rotatable K-mirror enabled us
to obtain intensity distributions, along any arbitrarily chosen axis, as
well as three-dimensional intensity distributions and equal intensity
contour lines. We found that the far field radiation patterns remained
remarkably unchanged for any value of current drive; this is demonstrated
by Fig. 5 which shows intensity distributions of MBE-4 at 77K for nine
current levels beginning at lasing threshold and up to nearly 50 times the
current at threshold. After reaching full scale deflection on the most
sensitive detection scale, the laser current drive level was adjusted to
produce full scale deflections using successive steps of the scope display
attenuator.
The spotsize measured on the scatter plate had a diameter of about 1.5
mm. A 2" diameter 2" focal length Germanium meniscus lens was used to
focus the laser radiation onto the scatter plate which was placed 12 ft.
from the collimator lens.
The magnified spot at the highest current drive (~46 times thresh-
old) is shown in the top left corner of Fig. 5 together with the cross
section applicable to the nine intensity distributions of Fig. 5.
The black lines in Fig. 6 show the lasing transitions at 100K operat-
o
ing temperature of MBE-2 which was also a 300A single quantum-well diode
laser. Lasing threshold was reached at about 100 mA and the laser started
operating on a transition between n=l states in the conduction and valence
bands. Transitions between the n=2 states required higher currents as was
clearly demonstrated in Fig. 6. Also notice that at 100K, for this laser
at least, there was a wide current range between n=l and n=2 transitions at
which no lasing at all occurred and the power output was zero.
The blue lines in Fig. 6 show the transitions at 90K operating temper-
ature. Notice that at 90K lasing can occur simultaneously on transitions
between n=l and n=2 states, and there is no drop out of laser power as was
observed at 100K.
The red lines in Fig. 6 show the lasing transitions at 77K. A careful
inspection of the line structure at 77K will indicate the presence of
longitudinal modes in the two transitions just above and below 1950
cm-1. Since the typical spacing between adjacent longitudinal modes was
in the vicinity of 2 1/2 orr1, a family of longitudinal modes within the
same transition must appear visually very closely spaced because the verti-
cal scale spans 1000 cm'1 on Figures 5-7. At 77K the longitudinal modes
appeared consecutively as the current was increased or decreased in a mono-
tonic fashion. Longitudinal modes may also occur simultaneously,
especially at lower temperatures. Also note that nearly 6 cm-1 continu-
ous frequency tuning with current was possible (e.g. in the transition at
1900 on-1).
Notice, however, that there were transitions both in the n=l and n=2
group, which were spaced more than 10-15 cm-1 apart. At this time at
least, the exact cause of these spacings is not known. It has been
suggested(B) that the transition degeneracy may be due to strains in the
crystal. Indeed, the transition structures became more complex at lower
temperatures where more strains were likely to occur. As an example, Fig.
7 shows the transitions of MBE-2 at 60, 40, and 20K, respectively. Note
that all the transitions shown in Fig. 7 operate between n=l states of the
conduction and valence • bands. The lasing threshold current for the n=2
transitions at these low temperatures would have to exceed the 550 ma
current limit used in this set of measurements. Notice the closely spaced
appearance of the set of longitudinal modes in the 1820 cm'1 transition
at 60K in the vicinity of 310 ma current drive.
The color coded lines of Fig. 8 show the line structure of the other
300A single quantum-well diode laser, MBE-4, at 110, 100, 77 and 20K,
respectively. Figure 8 also illustrates the points just discussed; the
occurence of n=l and n=2 state transitions at the higher operating tempera-
tures, and the absence of n=2 transitions and increased complexity of the
line structure at 20K. Also notice, that by tuning both with temperature
and current one can cover more than 700 orr1, i.e., over 30% tuning range
with a single quantum-well diode laser.
Tuning Rates and Frequency Reproduclbllity
Two tuning rates were applicable both as a function of current and as
a function of temperature. A relatively fast tuning rate could be
discerned by drawing an average slope across the line structure. About an
order of magnitude smaller tuning rates were applicable by tuning within a
mode. The tuning rates within a mode were determined by measuring the
change in the beat frequency against a fixed CO or against a second diode
laser reference line, as either the current or the temperature was changed
by a small amount.
Heterodyne beats against about 60 different CO laser transitions
between 5.1 and 6.0 urn were obtained. Since the quantum-well laser
transitions between the n=2 states occurred at wavelengths shorter than 4.7
ym, heterodyning against a CO laser was no longer possible and a second
quantum-well diode laser was used as a reference. It must be emphasized
that in spite of the complex appearance of the line structures, the repro-
ducibility of lasing frequencies was quite remarkable in comparison with
any other conventional lead-salt diode laser we previously encountered.
The beatnotes were reproducible week after week (for at least several
months) by simply adjusting the currents and temperatures to the previously
recorded values.
Table I shows typical tuning rates in GHz/ma as a function of either
current at a fixed temperature, or as a function of temperature at a fixed
current. The highest tuning rates were produced by temperature changes.
As previously mentioned, the tuning rates within a mode were about an order
of magnitude smaller than across transitions. Smaller tuning rates were
o
caused by current changes. Notice, however, that in the 300A quantum-well
lasers even the smallest tuning rate, which occurred within a mode, was
about 700 MHz/ma, i.e., 700 kHz/ya. Thus the diode laser current supply
must have very low noise and ripple, much less than 1 ya peak, in order to
avoid spurious frequency modulations.
Measurements of Linewidths
The high current-tuning rate (~1 MHz/ya) encountered during these
experiments necessitated the design and construction of a new, ultra low
noise power supply. In the narrow-band mode (used for manual tuning) the
noise was not measurable with any of the equipment available to us. We
estimated that the combined (power supply plus measuring devices) peak 60
Hz ripple did not exceed 0.3 uv; however, in the wide-band mode (used for
external modulation and frequency-locked operation) a small, but clearly
observable white noise (without any spectral features) was observed.
Figure 9 shows spectrum analyzer displays of two beatnotes, one for
MBE-2 at 20K and the other for MBE-4 at 77K. The vertical scales are
logarithmic with a 10 db/cm calibration; the horizontal frequency scale is
10 MHz/cm. The dotted lines are computer generated Lorentzian profiles
with 500 kHz full widths at half maximum (FWHM) power. Flat, white noise
spectra 40 and 4fi db below the respective Lorentzian peaks were also added
to the computer generated dotted profiles in order to correct for the noise
floor of the measuring system. Note that both pictures were taken with the
diode lasers operating in a He-compressor driven closed-cycle refrigerator;
however, since the diode lasers were frequency-offset-locked to the CO
laser, the beatnotes appeared very stationary and stable for any length of
time, regardless of compressor vibrations.
The approximately 500 kHz FWHM beatnotes shown in Fig. 9 are (to the
best of our knowledge) the narrowest linewidths obtained from lead-salt
tunable diode lasers operating in a commercial (CTI) closed cycle refriger-
ator and with a commercial (LAI) temperature controller, but using power
supplies and feedback circuits designed and constructed at Lincoln
Laboratory. However, the 500 kHz linewidths shown in Fig. 9 are probably
an order of magnitude broader than estimated from the Schawlow-Townes
formula (which equated nsp(l+a2)il)(9). Undoubtedly, there was at
least some contribution to the linewidths, shown in Fig. 9, which was due
to (white) noise of the power supply (which was used in the wide-band mode
in order to provide frequency-locked operation). However, there were at
least preliminary indications that most of the broadening was due to
adjacent longitudinal modes. Further clarification of this very important
problem area will have to depend on the results to be obtained in the next
phase of our investigations.
Conclusions
Experimental evidence has shown that the PbTe quantum-well diode
lasers achieved:
• very low lasing thresholds (2.1 ma at 13K)
• record high operating temperatures (17AK cw, 281K pulsed)
• over 700 cm-1 (>30%) tuning with a single laser
• excellent reproducibility
• good far-field patterns
• reasonable linewidths (-500 kHz)
Although the measurements answered many questions, puzzling results
also remain. Perhaps foremost of these relate to the detailed physics
which determine the spacings of the various transitions. Further investi-
gation of linewidths is also necessary. It is believed, however, that even
the partial results outlined above indicate that quantum-well structures
achieved a real break-through in the state of the art of lead-salt diode
lasers.
References
1. D. L. Partin, SPIE Conference on Tunable Diode Lasers, San Diego, CA,
Vol. 438, 25 August 1983.
2. D. L. Partin, Appl. Phys. Lett. _43, 996 (1983).
3. D. L. Partin and C. M. Thrush, to be published.
4. D. L. Partin, R. F. Majkowski and C. M. Thrush, J. Appl. Phys. 55,
678 (1984). ~~
5. D. L. Partin, "Single Quantum Well Lead-Europium-Selenide-Telluride
Diode Laser," Device Research Conference, Santa Barbara, Ca, (1984),
to be published in Appl. Phys. Lett.
6. D. L. Partin, private communication.
7. C. Freed, J. W. Bielinski, and W. Lo, Programmable, Secondary
Frequency Standard Based Infrared Synthesizer Using Tunable Lead-Salt
Diode Lasers, SPIE Proc. 438, 119 (1983).
8. B. Lax, private communication.
9. C. Freed, J. W. Bielinski, and W. Lo, Fundamental Linewidth in
Solitary, Ultranarrow Output PbSi_xSex Diode Lasers, Appl. Phys.
Lett. 43, 629 (1983).
TYPICAL TUNING RATES IN GHz/mA AS FUNCTIONS OF
DIODE CURRENT AND TEMPERATURE
LASER DIODE
MBE-2
(300 A)
MBE-4
(300 A)
MBE-5
(600 A)
f(l); T = 77 K
WITHIN MODE
0.70 - 1.5
1.25 (n = 2)
0.75 - 1.1
1.67 (n = 2)
0.25
ACROSS TRANSITIONS
8.9
12.4
3.5
f(T)
WITHIN MODE
16.6
(200 mA; 77 K)
13.6
(67 mA; 77 K)
ACROSS TRANSITIONS
121
(250 mA; 20-77 K)
126
(250 mA; 20-77 K)
OVER 700 cm"1 (~2 /mi) TUNING WITH A SINGLE LASER
NEARLY 6 cm 1 CONTINUOUS TUNING WITH CURRENT IN A SINGLE MODE
TABLE I
141432 N
THRESHOLD CURRENT (mA)
W
O
O
O
CO
o
o
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m o
2
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DO
m
S o
o
Ol
o I I I I I 1 I I
(O
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o>01 (O01
o>01
o
(0
Ol
o
THRESHOLD CURRENT DENSITY (A/cm2)
m
CO
I
O
o
c
m
z
H
D
m
Z
CO
H
O
T1
00
m
SINGLE-ENDED OUTPUT POWER OF MBE-4
600
141425 N A.BC
BLOCK DIAGRAM FOR THE HIGH RESOLUTION TESTING
OF THE OUTPUT SPECTRUM, TUNING RATE, AND SPATIAL
CHARACTERISTICS OF LEAD-SALT DIODE LASERS
ULTRA-STABLE
LOW-NOISE
POWER SUPPLY ROTATABLE
K-MIRRORS
BEAM
COLLIMATOR
LEAD-SALT
DIODE LASER
STABLE CO
OR CO2 LASER
l&K-MIRROR'/l
FLIP-MIRROR
TELESCOPE
TV-COMPATIBLE
INFRARED SCANNING
CAMERA AND VIDEO
RECORDER
BEAM-
SPLITTER
FAST HgCdTe
DETECTOR
(0-90 GHz)
GRATING
SPECTROMETER
SPECTRUM ANALYZER
(RF and Microwave)
136312-N
I
CURRENT
SUPPLY
TUNABLE
DIODE LASER
(TDL)
SERVO
AMPLIFIER •*-
LINE-CENTER
STABILIZED
CO2 OR CO
REFERENCE
LASER
BEAMSPLITTERS
FREQUENCY
DISCRIMINATOR
MICROWAVE
SYNTHESIZER
0-18 GHz
FAST HgCdTe
DETECTOR
GRATING
SPECTROMETER
FREQUENCY-LOCKED DIODE LASER
OUTPUT FOR PRECISELY TUNABLE
HIGH RESOLUTION SPECTROSOPY
''TDL = ^REFERENCE ± "SYNTHESIZER ± ft F COUNTER
BLOCK DIAGRAM OF A PRECISE, CONTINUOUSLY
TUNABLE INFRARED FREQUENCY SYNTHESIZER
124207 N 01
FOCUSED FAR FIELD PATTERN OF MBE-4
SPOTSIZE =* 1.5 mm; MAGNIFICATION - 70
538.8 mA; = 46.1 X THRESHOLD
11.7 mA; = 1 X THRESHOLD 13.2 mA; = 1.13 X THRESHOLD 14.72 mA; =* 1.26 X THRESHOLD
16.4 mA; = 1.4 X THRESHOLD 18 mA; = 1.54 X THRESHOLD 25.8 mA; =• 2.21 X THRESHOLD
83.9 mA; = 7.17 X THRESHOLD 278.6 mA; = 23.8 X THRESHOLD 538.8 mA; = 46.1 X THRESHOLD
LASIIMG TRANSITIONS OF MBE-2
250CT
2400
2300
^
£ 2200
£
Q 2100
Z
m
g 2000
UJ
QC
"- 1900
DC
UJ
1800
1700
1600
1500,
I I I I I
I I
100 K
90 K
77 K
4.0
4.5
5.0
X
I-
O
Z111_J
LU
5.5
6.0
6.5
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
I (mA)
LASING TRANSITIONS OF MBE-2
2500'
2400
2300
^
E 2200
u
O 2100
z
UJ
g 2000
HI
oc
"• 1900
QC
UJ
C/5
2 1800
1700
1600
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I I I I I I I I I I I I I I
—
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— -" — O f\ L/
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5.0 2
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<
5.5
6.0
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50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
I (mA)
Ml428 NAB
LASING TRANSITIONS OF MBE-4
2500"
2400
2300
'L 2200
u
0 210°
z
LU
2 2000o
ff
\±. 1900
oc111
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I I I I I I I I I I I I I I
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— ._ • ' — ^^
I I I I I I I I I I I I I I
0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
I (mA)
r~
8
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4.5
£*""
=1
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/n
5.0 §
ILI
_J
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<
'
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5.5 ^
6.0
6.5
H
141427 N A B("
BEAT-NOTE SPECTRA OF 300 A SINGLE QUANTUM-WELL
LASERS
(10 kHz Filter; Lorentzian Fits)
MBE-2
10dB
MBE-4
46 dB
10 MHz
v =*
1.298 mW
0.317 mA
1714.49 cm'1
PT « 0.916 mW
I =* 0.094 mA
V =* 1865.01 cm'1


Lead salt diode lasers and development of tunable solid state lasers for remote sensing

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