IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 54, NO. 3, JUNE 2005
1079
A Spectroscopically Resolved Photo- and
Electroluminescence Microscopy Technique for
the Study of High-Power and High-Brightness
Laser Diodes
Stephen Bull, Student Member, IEEE, Alexander V. Andrianov, Ian Harrison, Michael Dorin, Robert B. Kerr,
John Noto, and Eric C. Larkins, Member, IEEE
Abstract—A novel and advanced characterization technique
is described for performing optical studies of the luminescence
properties of materials. It was developed for the investigation of
semiconductor materials, including semiconductor laser diodes
and photonic integrated circuits. A quantitatively calibrated,
spatially and spectrally resolved imaging technique is described,
which is based upon the technologies of photoluminescence microscopy and photoluminescence spectroscopy. The principles of
the spectroscopically resolved photo- and electroluminescence microscopy techniques are outlined in this paper, and the developed
instrument is described in detail. Design calculations used to select
and set up the experimental apparatus are presented, and results
are found to compare well with this analysis. Various experimental
measurements are used to demonstrate the performance of the
new instrument. The study of strain and defects in high-power
laser diodes is presented as one of the more challenging applications of the new technique. The results presented demonstrate
the ability of this technique to image photoluminescence shifts
occurring in the substrate of packaged laser bars, enabling the
investigation of strain, defects and their evolution with aging.
Other applications of the technique include the spectroscopic
measurement of near- and far-field patterns and virtual sources
of laser diodes, investigations of spectral hole burning and optical
scattering processes in lasers and photonic integrated circuits,
and studies of organic LEDs. In the future, applications are also
envisaged in medicine and the biological sciences.
Index Terms—Electroluminescence microscopy, high-brightness laser diodes, high-power laser diodes, photoluminescence
microscopy, quantitative imaging, spectroscopic imaging.
I. INTRODUCTION
D
URING the last few years, great progress has been made in
the development of high-power and high-brightness laser
diodes [1]–[6]. These devices find application in a wide range
Manuscript received January 23, 2004; revised December 19, 2004. This
work was supported by the European Commission through the IST projects
POWERPACK (IST-2000-29447) and ULTRABRIGHT (IST-1999-10356).
S. Bull, I. Harrison, and E. C. Larkins are with the School of Electrical and
Electronic Engineering, University of Nottingham, Nottingham NG7 2RD, U.K.
(e-mail: steve.bull@nottingham.ac.uk).
A. V. Andrianov is with the School of Electrical and Electronic Engineering,
University of Nottingham, Nottingham NG7 2RD, U.K. and also with the A.F.
Ioffe Physical Technical Institute, St. Petersburg 194021, Russia.
M. Dorin, R. B. Kerr, and J. Noto are with Scientific Solutions, Inc., North
Chelmsford, MA 01863 USA.
Digital Object Identifier 10.1109/TIM.2005.847219
of fields, including manufacturing and materials processing [7],
[8], medicine [9], [10], and telecommunications [11], [12]. They
are also used as pump lasers for solid-state lasers [13], [14], fiber
amplifiers, and fiber lasers [15], [16]. The three major considerations for laser diodes in any application are output power, beam
quality, and reliability. However, reliability and output power
are closely linked, so that the study of degradation processes
becomes increasingly important as the output power continues
to increase. The degradation of high-power laser diodes is influenced by many factors. One significant factor affecting device
reliability is the packaging-induced mechanical stress caused
during the soldering of a laser bar to a heatsink [17]–[19]. This
process can introduce an inhomogeneous strain profile across a
device [20], which plays a role in defect formation. There are
indications that degradation rates have a nonlinear dependence
upon packaging induced strain. Indeed, the existence of a strain
threshold for defect creation has been observed [21]. There is
also a second, lower packaging-induced strain threshold, above
which a significant increase in the degradation rate is observed
during operation [22].
A wide range of experimental techniques exists for characterizing the degradation of laser diodes. Microphotoluminescence ( -PL) has been used extensively to measure and study
packaging-induced strain [20], [23] and PL microscopy (PLM)
has often been used for observing and identifying extended defects [24]–[26]. Photocurrent (PC) spectroscopy [21], [23], [27],
micro-Raman [21], [23], and electroluminescence (EL) studies
combined with a fast Fourier transform (FFT) analysis [28]–[30]
have also been used to study the influences of strain and defects.
In photoluminescence, above bandgap excitation light creates electron-hole pairs, which are excited from their equilibrium states. As the electron and hole distributions relax back
toward equilibrium, some of the excess energy is emitted as
light. PLM images this luminescence and is a proven technique
for the study of material defects and device degradation processes. The power of the PLM technique for the study of defects
comes from the intrinsic sensitivity of optical processes to the
presence of defects. The PLM technique is nondestructive, offers nearly diffraction limited spatial resolution, and has a very
high sensitivity to nonradiative defects. Furthermore, it often requires no sample preparation (e.g., when used to observe facet
defects). Electroluminescence microscopy (ELM) complements
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PLM and uses very low currents. ELM is also nondestructive because the currents used are insufficient to cause defect generation. Furthermore, measurements at different excitation or emission wavelengths can often provide information on the nature of
the defects [31].
The development of a spectroscopically resolved PLM/ELM
system (S-PLM/S-ELM) is reported and has been realized as
an extension to our custom-built optical materials evaluation
system (OMES). A more-detailed description of this multifunction instrument can be found elsewhere [22], [24], [28], [31].
The new spectroscopically resolved techniques bring together
PLM imaging and PL spectroscopy, resulting in, for the first
time, a high-quality imaging technique that is quantitatively calibrated and both spectrally and spatially resolved. A primary application of the S-PLM technique, and the main one presented
in this paper, is to produce two-dimensional (2-D) strain profiles
of the mirror facet of laser diodes. This provides useful information related to the strain fields surrounding defects and their
influence on defect formation and propagation. Both strain and
defects in the substrates of high-power laser diodes have a severe effect on device performance and reliability. The S-PLM
technique is demonstrated by imaging PL shifts in the substrate
of packaged laser bars [32], [33]. This application is particularly demanding, because of the weak PL signal intensity and
) wavelengths shifts involved. However, the
the small (
technique is equally applicable to electroluminescence studies
(where the signal strength is typically greater), including the
spectroscopic measurement of near- and far-field patterns and
the virtual sources of laser diodes. These investigations will be
useful in the development of high-brightness laser diodes with
superior beam quality for medical and fiber coupling applications [34], [35]. Nonlinear optical effects, such as spectral hole
burning, can also be studied with S-ELM. These effects are important, since they play a significant role in telecommunications
and the emerging technologies of photonic integrated circuits
[36]–[38]. The applications of the technique could also extend
to the study of organic LEDs [39], [40] and diverse measurements in the fields of medicine and the biological sciences including, for example, fluorescence microscopy [41], [42].
Section II describes the experimental setup, while Section III
presents the calculations used to design the system. Section IV
addresses the issues of system calibration, while Section V describes the specialized image processing software required for
specific applications. Section VI presents a range of results,
which demonstrate the performance of the new instrument.
II. EXPERIMENTAL SETUP
The PLM/ELM setup in the OMES system allows the direct acquisition of PLM/ELM images of the sample surface by
using a Wright Instruments camera with a 1024 1024 pixel
Thomson THX31156 CCD detector array. A range of optical
(UV, 275.4 nm,
excitation sources are available including
302.5 nm and visible, 488.0 nm, 514.5 nm), He-Ne (632.8 nm)
and tunable Ti:Sapphire (675 to 1050 nm) lasers. A microscope
objective and a set of lenses collect the light and focus the image
of the sample onto the CCD array. To improve the signal-tonoise ratio (SNR), the CCD chip is cooled with liquid nitrogen.
Fig. 1.
Schematic layout of the system.
For a wavelength of 0.86
(approximate peak GaAs PL waveand is
length), the spatial resolution of the system is
limited by the numerical aperture of the microscope objective
used. The other lenses used in the system determine the field of
view. All of the equipment is connected to a central computer by
IEEE 488, RS-232 data links and by digital and analog signals,
as shown in Fig. 1. Experiments are then controlled by software
written in LabVIEW.
PL spectroscopy uses a monochromator to spectrally separate the response at different wavelengths. However, spectroscopically resolved high-quality imaging requires a different
approach. To enable spectroscopic imaging measurements to
be performed, two tunable liquid crystal Fabry-Pérot (LCFP)
etalons from Scientific Solutions, Inc. were, therefore, inserted
into the system in series.
A LCFP is similar to a classic air-gap Fabry-Pérot interferometer, except that the resonant cavity is filled with liquid crystal
(LC). Changing the index of refraction of the LC by the application of a low-voltage electric field is effectively equivalent to
changing the optical path length between the mirrors of a classic
FP, and allows for solid-state spectral tuning [43], [44]. The
dual-LCFP system used here has a spectral width of
at a wavelength of 0.86
and was chosen because of its stability over long periods of time and simple control. Other tunable FP filters require the use of piezoelectric actuators and so
suffer from drift and hysteresis problems. To overcome these
problems, they require the use of complex and expensive control systems.
A LCFP etalon with the required spectral resolution has
multiple transmission resonances across our passband. For this
system, it was therefore necessary to place two LCFP etalons
in series to expand the spacing between adjacent transmission
resonances, known as the free spectral range (FSR), while maintaining the required spectral resolution. The system response
of the chosen LCFP filters (at a constant voltage) is shown in
Fig. 2. This shows that there are 12 transmission resonances
over the operating range of the filter (700 to 1100 nm), with
the FSR varying from 20 to 45 nm, showing clearly the need
for the order-sorting bandpass filters. The chosen Fabry-Pérot
BULL et al.: HIGH-POWER AND HIGH-BRIGHTNESS LASER DIODES
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Fig. 2. Modeled response of the LCFP etalons.
etalons have a finesse of 12. Since LC devices are sensitive to
polarization, the tunable function of the filter is only possible in
one polarization direction. A polarizer is used to block out the
nontunable polarization state. Since weak PL signals are being
imaged, the collection efficiency of the system is very important
and has been carefully optimized. This optimization required
a detailed calculation of the expected PL signal intensity per
pixel at the CCD camera. Further details of this calculation are
presented in Section III.
To make an automated system, a filter positioner/holder for
order-sorting bandpass filters controlled by a stepper motor was
designed and incorporated into the system. The holder is designed to hold six standard 25-mm-diameter bandpass filters.
Initial tests with this holder indicated that images taken with
different order-sorting filters were sometimes shifted in posi(for a field of view of
). This
tion by up to 20
was caused, in part, by the misalignment of the individual filters
with respect to the optical axis. The automated holder was then
individual adjustment of each
modified to allow angular
. Using this configuration, the
filter with a resolution of
image shifts were reduced but could not be entirely eliminated,
because of the slightly misaligned (i.e., nonparallel) faces of the
filters. To correct for this effect, the complete filter holder was
moved to a point in the optical path where the beam was slightly
divergent rather than parallel. This allowed the image shifts to be
eliminated by careful alignment of the individual filters. Further
details on the order-sorting filters are presented in the section on
system calibration.
In describing the experimental system, it is appropriate to
compare S-PLM with the conventional -PL method since they
represent competing techniques for obtaining similar information. In the S-PLM technique, points in a 2-D plane
are measured in parallel at a single wavelength, . Spectroscopic information is obtained by repeating this procedure for
different wavelengths. Similar measurements can also be performed using -PL. However, assuming a CCD detector array
point.
is used, -PL measures in parallel for a single
Spatial information is obtained by performing this procedure
for a 2-D raster of points across the surface of the sample. From
the data, information such as peak wavelength can be extracted
by using fitting functions. A -PL line scan with 1000 points
of a standard 1-cm laser bar)
(i.e., 1 PL spectrum per
takes
. Using -PL to cover an area equivalent
to one S-PLM image with a similar spatial resolution to that
achievable by S-PLM would require a measurement time in
excess of 10 hours. In comparison, an S-PLM measurement
. While
with a spectral resolution of 0.6 nm would take
the S-PLM measurement is significantly quicker, the spectral
resolution is less than that of -PL. A further advantage of the
S-PLM system is that it is a high-quality imaging system, while
-PL is only a topographic technique. Furthermore, features of
interest can be identified in the image and selected for detailed
spectral analysis. Identifying and selecting a specific feature
or defect for -PL spectroscopy is much more difficult. From
these arguments it is clear that S-PLM and -PL are complementary techniques. Indeed, our current experimental setup can
also be used to perform -PL measurements if a higher spectral
resolution is required. The excitation spot is easily positioned
on the site of interest on the sample by using the standard PLM
imaging system [24].
III. SYSTEM DESIGN
As part of the system design, a detailed quantitative analysis
of the system was carried out to do as follows:
1) calibrate the instrument thereby permitting quantitative
comparisons between experimental and simulated results;
2) investigate the influence of the various components on the
global performance of the instrument and ensure that they
are optimized, considering in particular the imaging and
spectral resolutions and the transmission efficiency of the
system; and
3) design an appropriate tunable filter for the targeted
applications.
For the analysis of the system the PL signal intensity per pixel
at the CCD camera must be found by considering each component of the system in turn. The number of electrons per pixel at
the CCD camera is given by
(1)
where
is the quantum efficiency of the CCD detector
is the area of the CCD imaging array,
is the
chip,
is the sample area imaged onto
area of one CCD pixel,
is the transmission efficiency of the opthe CCD array,
tical system,
is the exposure time,
is the horizontal
is the vertical binning
binning factor of the CCD pixels,
is the total photon density colfactor of the CCD pixels,
lected by a lens with a given numerical aperture (NA), and the
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the internal quantum efficiency of the semiconductor material
and can be expressed by
(3)
Fig. 3. Geometry of the incident excitation upon the sample surface.
final factor in (1) represents the maximum PL efficiency taking
into account the width of the tunable filter.
For the system developed here, the transmission of three
lenses (including a quartz window on the CCD camera), the
imaging objective, a narrow bandpass filter (order-sorting
filter), the tunable bandpass filter (two LCFP etalons and a polarizer) and two low-pass filters to eliminate scattered light from
the laser excitation source (the first filter is located before the
tunable filter, while the second is attached to the camera head)
. Using transhave been included in the calculation of
mission values measured/quoted for each of the components,
. The main reasons for this low transmission
efficiency are the transmission values of each tunable filter
etalon, which are approximately 45%, respectively, and 20%
for the two etalons in series with the order-sorting bandpass
filter. In addition, the tunable filter etalons are polarized, resulting in a further reduction factor of two for unpolarized light.
[
], the excitation density
In order to calculate
must be considered and it should be noted that in our PLM setup,
the excitation beam is usually incident at an angle to the sample
surface. Fig. 3 shows the geometry of the excitation upon the
sample surface. In calculating the excitation density beneath the
surface, the power density of the excitation laser and the angle it
makes with the sample surface are considered along with factors
representing the transmission of light at the air-semiconductor
interface and the absorption of the excitation light as it passes
through the semiconductor material. The excitation power density at a distance beneath the surface is therefore given by
(2)
where
is the power transmission coefficient of light
is the refractive
through the semiconductor surface,
index of the semiconductor at the excitation wavelength
and
is the absorption coefficient of the excitation beam
in the semiconductor. The total photon density of PL light
beneath the semiconductor surface,
generated at a distance
[
], (emitted in all directions) is then determined. The photon density of the excitation source is found
from the excitation power density [see (2)] and the photon
energy. This is translated into the photon density of PL through
where
is the photon energy of the excitation source,
is the internal quantum efficiency of the semiconductor material
(i.e., the number of luminescence photons generated/number of
excitation photons absorbed, which must be determined experimentally) and is the elementary electronic charge. Finally,
, the total photon density emitted from a point below
the surface and can be collected by a lens of
is determined. The fraction of the PL power collected by a lens
with a given NA can be calculated by integrating the fraction of
the light emitted from a point in a cone defined by
and
as given by
(4)
If the PL emission is homogeneous on the scale of the absorption
depth of the excitation beam, it is clear from translational symmetry arguments, that the volume density of light emitted from
a small volume located at a point beneath the surface within
is equal to the volume denthe collection cone defined by
plane passing through a
sity of light generated in the
. A term is included
point on the surface with an angle
in (4) to account for the reabsorption of the PL light by the semiis subsequently integrated
conductor material. The value of
to 0, in order to include the light generated at different
from
depths below the surface. The geometry of this scenario is illusis given by (5):
trated in Fig. 4. The final expression for
(5)
where
is the refractive index of the semiconductor at
the wavelength of the peak PL,
, and
is the absorption
coefficient of the PL signal in the semiconductor.
The final quantity required in (1) is the maximum PL efficiency of the system. For the purposes of this discussion, we
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TABLE I
VALUES OF THE COEFFICIENTS USED IN (1) TO (7). THESE QUANTITIES ARE
APPROPRIATE FOR PL INVESTIGATIONS OF A GAAS-BASED DEVICE USING
Ar LASER EXCITATION
Fig. 4. Geometry of the collection of photons by a lens of numerical aperture
NA = sin(
).
assume Maxwell-Boltzmann statistics and bulk energy density
of states so that the PL spectrum is described by
(6)
, thus the
The maximum PL signal occurs for
maximum PL efficiency for a bandpass filter is given by
(7)
where
is the spectral width of the tunable filter at
and is the Naperian base. Equations (5) and (7) give the two
main terms required for the calculation of the number of electrons per pixel on the CCD camera as given by (1). Using these
expressions, calculations were carried out to determine the expected signal strengths with the various filter designs available.
The various quantities used in these calculations are summarized in Table I. These quantities are appropriate for PL inveslaser (488.0 nm)
tigations of a GaAs-based device using
excitation. Once the chosen filter was installed and initial test
experiments had been performed, the design calculations were
used to enable the experimental and theoretical intensities to be
compared. The results of these test measurements are presented
in Section VI.
As described, the detailed calculations also allowed the determination of the limiting elements in the system, which have
the largest impact upon the signal strength. This provided an
insight into how the system could be optimized. For example,
,
by replacing the current microscope objective (
) with a new part with
,
the signal strength at the CCD camera could be increased by a
(assuming that the objective had the same transfactor of
mission and no other system parameters were changed). Most
NIR objectives with a high NA have either a short working
). One objective was
distance or low transmission (
,
in the desired
found with
spectral range and a working distance of 3.4 mm, which would
increase in the signal strength at the
give a factor of
CCD camera compared to the current objective. However, the
objective with the larger numerical aperture has a smaller field
of view (
), which is significantly less than
275
. Thus,
that achievable with the current lens (275
the imaging of laser diode bar facets would take considerably
longer as four times as many images would need to be taken to
image the same area (albeit with improved spatial resolution).
This illustrates the difficulties associated with the optimization
of the imaging system.
The design calculations were also used to determine the design specifications for the custom LCFP filter pair to achieve
the desired system performance for the targeted applications. In
selecting suitable etalons, five main aspects of the system performance were considered: (1) the available signal intensity; (2)
the required spectral resolution; (3) the required spatial resolution; (4) the FSR between the transmission orders of the LCFP
etalons relative to the passband widths of the order-sorting filters; and (5) the sample area to be imaged. The main tradeoffs in the selection were between the spectral resolution, FSR,
and transmission efficiency. For example, LCFP etalon configurations are available with spectral resolutions as small as
0.0002 nm. However, the transmission of such a filter would
be unsuitable for the imaging of weak PL signals. A spectral
was selected for the filter because the calwidth of
culations indicated that the transmission of the system with this
filter would be sufficient for high-quality images to be produced
using practical exposure times. The results in Section VI show
that an appropriate filter was indeed selected.
IV. SYSTEM CALIBRATION
Two independent types of calibration are required for this
system. First, the intensities of all of the monochromatic images
need to be corrected for the spectral response of the system, in
particular that of the order-sorting filters. These filters have a
10-nm passband and are separated by 10 nm, which results in
dips in the system response of 20%–40% at the crossover points
between the filters. Second, all of the images need to be corrected for the slight (primarily radial), spatial nonuniformity in
from center to edge).
the collection efficiency (typically
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The spectral response of the system was measured using a
halogen lamp with a known emission spectrum. The beam from
the lamp was focussed and reflected off a mirror mounted on
the sample positioner down the optical axis. A series of images
at intervals of 0.5 nm were then taken using the CCD camera
at specific wavelengths determined by the tunable filter. This
procedure eliminates the need for an external spectrometer and
also calibrates for the spectral sensitivity of the CCD camera and
not just that of the optics. To determine the spectral response of
the system, the integral intensity over all pixels of each image
is found. This is then corrected for the spectrum of the lamp
and normalized. This method gives an excellent SNR, since the
SNR is proportional to the square root of the number of pixels.
All pixels of an image are then multiplied by the normalization
factor for the wavelength at which the image was taken.
The spatial nonuniformity in collection efficiency across individual images was characterized by using a small point light
diameter pinhole) mounted
source (an IR LED behind a 5on the six-axis positioner. Images were taken for a 2-D matrix of
positions, which covered the size of the image. From the points
in these images where the maximum intensity occurs, a 2-D plot
can be made of how the collection efficiency varies across the
image. This data is then used to correct the spatial nonuniformity in collection efficiency for all images. Software has been
developed to automate both of the calibration procedures and
image corrections.
Fig. 5. Example PLM and S-PLM images of dark line defects.
TABLE II
COMPARISON OF THE CALCULATED AND MEASURED SIGNAL INTENSITIES FOR
THE PLM AND S-PLM IMAGES SHOWN IN FIG. 5. THE MEASUREMENT
CONDITIONS FOR EACH IMAGE ARE ALSO SHOWN
V. IMAGE PROCESSING SOFTWARE
Specialized software is required to perform the image analysis needed for a specific application. An individual image is
comprised of over a million pixels. In order to produce a full
2-D strain profile, the software must first extract the local luminescence spectrum from each pixel in the image (or those in
the user defined region of interest). Then, it must fit a theoretical curve to the spectrum associated with each pixel, in order
to determine parameters such as the peak wavelength and the
luminescence linewidth. These extracted parameters can then
be plotted as 2-D topograms, giving a visual representation of
their variation across the imaged region. In the results presented
in Section VI, the theoretical functions used for the fitting are
quadratic and cubic polynomials.
The software is currently being expanded to include a larger
range of fitting functions suitable for a variety of applications, including theoretical spectral lineshapes for spontaneous
and stimulated emission. Both undoped and doped, bulk and
quantum well (QW) material will be considered. Details
of these theoretical functions will be presented in a future
publication.
VI. DEMONSTRATION OF INSTRUMENT PERFORMANCE
Initial measurements were made on a degraded laser diode
bar and example PLM and S-PLM images are shown in Fig. 5.
The measurement conditions for each image are summarized in
Table II, together with the theoretical pixel intensities for each
image found from the calculations presented in Section III. All
of the expected
of the experimental results lie within
value. For spectroscopically resolved images where only the
low-resolution etalon was used, higher signal strengths than expected were achieved. This could be caused by the assumption
of equal transmission coefficients for both etalons. The results
have relatively large error bars because of the difficulty in accurately determining the power density and angle of incidence of
the excitation beam. As aforementioned, the latter has a strong
influence on the signal strength.
S-PLM was used to investigate the V-defect seen on the facet
of the laser bar in Fig. 5. A series of 125 images were taken at
0.5 nm intervals from 833.0 nm to 895.0 nm with exposure times
and a
of 30 seconds, a power excitation density of
200
field of view. The luminescence imaged here
200
is spontaneous emission and previous measurements have confirmed that it is randomly polarized. Measurements have therefore been made in only one direction of polarization. However,
for measurements that are polarization sensitive, the etalons can
be rotated and two measurements made. To produce a 2-D topogram showing the variation of the peak PL wavelength, it is
necessary to extract the PL spectra, fit the data and accurately
determine the peak PL wavelengths for a large number of pixels
). Fig. 6(a) shows an example
within an image (
S-PLM image (taken at a wavelength of 860 nm), while Fig. 6(b)
shows how the peak PL wavelength varies across the laser facet.
by
This region represents a sample area of
and the peak PL wavelength of each pixel was extracted after
fitting a quadratic polynomial to the raw PL spectra. (The active
layer has been excluded from this topogram because of the difvariference in its emission wavelength.) In Fig. 6(b),
ation in the peak PL wavelength is observed around the area of
the dark line V-defect. The magnitude of this shift is due to the
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Fig. 6. (a) S-PLM image of the mirror facet of a high-power laser bar taken
at 860 nm. (b) S-PLM 2D topogram showing the variation of the peak PL
wavelength over the laser facet.
presence of the defect and not to strain alone. Relatively large
) strain-induced variations in the luminescence wave(
length are still observed in the region just to the left of the defect.
The shifts in the PL peak wavelength from different regions of
the bar have been compared with those observed by -PL measurements and good agreement is obtained [32], [33].
As discussed in the introduction, this experimental system
can also be used for performing spectroscopically resolved
electroluminescence measurements (S-ELM). Many opportunities exist here and as a first demonstration of this capability,
S-ELM has been used to confirm some observations in previous
work [45]. In this paper, images of spontaneous emission were
taken of a 975-nm high-brightness tapered laser structure with a
specially fabricated windowed backside contact. As the experimental system is qualitatively calibrated, these images of the
spontaneous emission observed through the windowed contact
can provide a great deal of information (with appropriate data
processing) of the processes occurring inside of the laser cavity.
In addition, from the images themselves, various features of
interest can be observed, including defects. An example of an
ELM image of the spontaneous emission through the windowed
contact of a tapered laser is shown in Fig. 7(a). Two areas of the
cavity are shown: the wide end of the tapered section (i.e., at
the emitting facet) and the point at which the ridge waveguide
and tapered sections of the device meet. Full details of the
device structure can be found elsewhere [45]. In these images,
the nature of the features marked “A” and “B” was previously
Fig. 7. (a) ELM images of the spontaneous emission imaged through the
windowed backside contact of a tapered laser diode. (b) Typical stimulated
emission spectrum extracted from S-ELM images of a point in the region
marked “A”.
unclear, but were thought to be caused by scattered laser light
at the output facet and the beam spoiler trenches, respectively.
By taking a series of S-ELM images at these points and plotting
spectra from the bright regions in the images, the spectroscopic
analysis is able to confirm the previous hypotheses about
regions “A” and “B.” A typical example of a spectrum plotted
from a point in region “A” is shown in Fig. 7(b) and in this
figure we observe a typical stimulated emission spectrum with
its peak at the lasing wavelength (975 nm).
VII. CONCLUSION
We have developed a new spectroscopically resolved photoand electroluminescence microscopy (S-PLM/S-ELM) technique, which is suitable for a wide range of applications in
electronics/photonics and the biological sciences. The new
spectroscopically resolved techniques bring together PLM
imaging and PL spectroscopy resulting in, for the first time, a
high-quality imaging technique that is quantitatively calibrated
and both spectrally and spatially resolved. The system development has been discussed and detailed design calculations for
the system presented. The performance of the system compares
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favorably with these design calculations. Important calibration and image processing issues have been described and
demonstrate the complexity of the system and the challenges
overcome during its development.
The study of strain and defects in high-power laser diodes
is presented as one of the more challenging applications of the
)
new technique, and the results show that small shifts (
in the peak photoluminescence wavelength can be observed in
regions with defects and/or different soldering-induced strain.
In the example presented, luminescence shifts as large as 15 nm
in the region around the defect have been observed, while further away from the defect site, PL shifts up to 5 nm have been
observed. These PL shifts require further spectral analysis but
may be caused by a significant disturbance in the strain field in
the regions adjacent to the defect or to changes in the shape of
the PL spectrum at these points. These results show that S-PLM
is a sensitive and useful tool for studying defects and strain fields
in compound semiconductor devices, as well as their evolution
during aging and their contribution to device degradation processes. Additionally, the capability of the system to perform
S-ELM imaging has also been demonstrated by a simple example of the spectroscopic analysis of the spontaneous emission from inside the cavity of a high-brightness tapered laser
diode with a windowed backside contact. The use of S-ELM in
this type of measurement allows the direct observation of important effects inside the cavity (e.g., spatial hole burning, carrier/thermal lensing, optical pumping, electrical over-pumping
and filamentation). Data from investigations of this type will
provide important information needed for the development of
the next generation of laser design and simulation tools. Further
applications of the new system are also envisaged in studies of
photonic integrated circuits, organic LED’s, and in the fields of
medicine and the biological sciences.
ACKNOWLEDGMENT
The contributions of S.R.A. Dods, J. Morgan, and R. Xia in
the construction and development of the OMES system are also
acknowledged.
REFERENCES
[1] A. Knauer, F. Bugge, G. Erbert, H. Wenzel, K. Vogel, U. Zeimer, and
M. Weyers, “Optimization of GaAsP/AlGaAs-based QW laser structures for high power 800 nm operation,” J. Electron. Mater., vol. 29, pp.
53–56, 2000.
[2] S. Sujecki, L. Borruel, J. G. Wykes, T. M. Benson, P. Sewell, I. Esquivias, P. Moreno, D. Rodriguez, and E. C. Larkins, “Design of high
power pump lasers for C band EDFA amplifiers,” in Proc. 4th Int. Conf.
Transparent Optical Networks ICTON’02, Warsaw, Poland, pp. 55–58.
[3] G. Erbert, G. Beister, R. Hülsewede, A. Knauer, W. Pitroff, J. Sebastian,
H. Wenzel, M. Weyers, and G. Tränkle, “High-power highly reliable
Al-free 940 nm diode lasers,” IEEE J. Sel. Topics Quantum Electron.,
vol. 7, no. 2, pp. 143–148, Mar.-Apr. 2001.
[4] J. Sebastian, G. Beister, F. Bugge, F. Buhrandt, G. Erbert, H. G. Hänsel,
R. Hülsewede, A. Knauer, W. Pitroff, R. Staske, M. Schröder, H. Wenzel,
M. Weyers, and G. Tränkle, “High-power 810 nm GaAsP-AlGaAs diode
lasers with narrow beam divergence,” IEEE J. Sel. Topics Quantum Electron., vol. 7, no. 2, pp. 334–349, Mar.-Apr. 2001.
[5] G. W. Yang, R. J. Hwn, Z. T. Xu, and X. Y. Ma, “Design consideration
and performance of high-power and high-brightness InGaAs-InGaAsPAlGaAs quantum well diode lasers ( = 0:98 m),” IEEE J. Sel. Topics
Quantum Electron., vol. 6, no. 4, pp. 577–586, Jul.-Aug. 2000.
[6] A. Al-Muhanna, L. J. Mawst, D. Botez, D. Z. Garbuzov, R. U. Martinelli,
and J. C. Connolly, “High-power (> 10 W ) continuous-wave operation
from 100 m aperture 0.97 m emitting Al-free diode lasers,” Appl.
Phys. Lett., vol. 73, pp. 1182–1184, 1998.
[7] H.-G. Treusch, A. Ovtchinnikov, X. He, M. Kanskar, J. Mott, and
S. Yang, “High-brightness semiconductor laser sources for materials
processing: Stacking, beam shaping, and bars,” IEEE J. Sel. Topics
Quantum Electron., vol. 6, no. 4, pp. 601–614, Jul.-Aug 2000.
[8] W. Schulz and R. Poprawe, “Manufacturing with novel high-power
diode lasers,” IEEE J. Sel. Topics Quantum Electron., vol. 6, no. 4, pp.
696–705, Jul.-Aug 2000.
[9] “Special issue on lasers in medicine and biology,” IEEE J. Sel. Topics
Quantum Electron., vol. 7, p. 873, 2001.
[10] “Special issue on lasers in medicine and biology,” IEEE J. Sel. Topics
Quantum Electron., vol. 5, p. 893, 1999.
[11] G. P. Agrawal, Fiber-Optic Communication Systems. New York:
Wiley, 1997.
[12] A. Yariv, Optical Electronics in Modern Communications. New York:
Oxford University Press, 1997.
[13] K. Shigihara, Y. Nagai, S. Karakida, A. Takami, Y. Kokubo, H. Matsubara, and S. Kakimoto, “High-power operation of broad-area laser
diodes with GaAs and AlGaAs single quantum wells for Nd:YAG laser
pumping,” IEEE J. Quantum Electron., vol. 27, no. 6, pp. 1537–1543,
Jun. 1991.
[14] G. T. Maker and A. I. Ferguson, “Frequency-modulation mode-locking
and Q-switching of diode-laser-pumped Nd-YLF laser,” Electron. Lett.,
vol. 25, pp. 1025–1026, 1989.
[15] H. Po, J. D. Cao, B. M. Laliberte, R. A. Minns, R. F. Robinson, B. H.
Rockney, R. R. Tricca, and Y. H. Zhang, “High power Nd-doped single
transverse mode fiber laser,” Electron. Lett., vol. 29, pp. 1500–1501,
1993.
[16] E. Rochat, K. Haroud, and R. Dandliker, “High-power Nd-doped fiber
amplifier for coherent intersatellite links,” IEEE J. Quantum Electron.,
vol. 35, no. 10, pp. 1419–1423, Oct. 1999.
[17] M. Fukuda, Reliability and Degradation of Semiconductor Lasers and
LEDs. Boston, MA: Artech, 1991.
[18] A. Jakubowicz, “Material and fabrication-related limitations to highpower operation of GaAs/AlGaAs and InGaAs/AlGaAs laser diodes,”
Mater. Sci. Eng., vol. B44, pp. 359–363, 1997.
[19] E. W. Kretuz, N. Wiedmann, J. Jandeleit, D. Hoffmann, P. Loosen,
and R. Poprawe, “Reliability and degradation mechanisms of
InGa(Al)As/GaAs DQW high-power diode lasers,” J. Cryst. Growth,
vol. 210, pp. 313–317, 2000.
[20] E. Martin, J. P. Landesman, J. P. Hirtz, and A. Fily, “Micro-photoluminescence mapping of packaging-induced stress distribution in highpower AlGaAs laser diodes,” Appl. Phys. Lett., vol. 75, pp. 2521–2523,
1999.
[21] J. W. Tomm, A. Gerhardt, T. Elsaesser, D. Lorenzen, and P. Hennig,
“Simultaneous quantification of strain and defects in high-power diode
laser devices,” Appl. Phys. Lett., vol. 81, pp. 3269–3271, 2002.
[22] R. Xia, E. C. Larkins, I. Harrison, S. R. A. Dods, A. V. Andrianov,
J. Morgan, and J. P. Landesman, “Mounting-induced strain threshold
for the degradation of high-power AlGaAs laser bars,” Photon. Technol.
Lett., vol. 14, pp. 893–895, 2002.
[23] J. W. Tomm, A. Gerhardt, R. Müller, V. Malyarchuk, Y. Saint-Marie, P.
Galtier, J. Nagle, and J. P. Landesman, “Spatially resolved spectroscopic
strain measurements on high-power laser diode bars,” J. Appl. Phys., vol.
93, pp. 1354–1362, 2003.
[24] A. V. Andrianov, S. R. A. Dods, J. Morgan, J. W. Orton, T. M. Benson,
I. Harrison, E. C. Larkins, F. X. Daiminger, E. Vassilakis, and J. P. Hirtz,
“Optical and photoelectric study of mirror facets in degraded high power
AlGaAs 808 nm laser diodes,” J. Appl. Phys., vol. 87, pp. 3227–3233,
2000.
[25] M. Baeumler, J. L. Weyher, S. Müller, W. Jantz, R. Stibal, G. Herrmann,
J. Luft, K. Sporrer, and W. Späth, “Investiagtion of degraded laser diodes
by chemical preparation and luminescence microscopy,” in Inst. Phys.
Conf. Ser., vol. 160, 1997, pp. 467–470.
[26] C. Frigeri, J. L. Weyher, M. Baeumler, S. Müller, W. Jantz, J. Luft,
G. Herrmann, and W. Spath, “Cross-sectional TEM study of degraded
high power diode lasers,” in Inst. Phys. Conf. Ser., vol. 160, 1997, pp.
483–486.
[27] J. W. Tomm, A. Gerhardt, R. Müller, M. L. Biermann, J. P. Holland, D.
Lorenzen, and E. Kaulfersch, “Quantitative strain analysis in AlGaAsbased devices,” Appl. Phys. Lett., vol. 82, pp. 4193–4195, 2003.
[28] R. Xia, A. V. Andrianov, S. Bull, I. Harrison, J. P. Landesman, and E.
C. Larkins, “Micro-electroluminescence spectroscopy investigation of
mounting-induced strain and defects on high-power GaAs/AlGaAs laser
diodes,” Opt. Quantum Electron., vol. 35, pp. 1099–1106, 2003.
BULL et al.: HIGH-POWER AND HIGH-BRIGHTNESS LASER DIODES
[29] A. Klehr, G. Beister, G. Erbert, A. Klein, J. Maege, I. Rechenberg, J.
Sebastian, H. Wenzel, and G. Tränkle, “Defect recognition via longitudinal mode analysis of high power fundamental mode and broad area
edge emitting laser diodes,” J. Appl. Phys., vol. 90, pp. 43–47, 2001.
[30] P. J. Bream, S. Bull, R. Xia, A. V. Andrianov, I. Harrison, and E. C.
Larkins, “The influence of defects on the emission spectra of high
power laser diodes,” in Europhysics Conf. Abstracts, vol. 27E, CC2M,
Conf. Lasers and Electro-Optics (CLEO) Europe, Munich, Germany,
Jun. 2003.
[31] R. Xia, H. Xu, I. Harrison, B. Beaument, A. V. Andrianov, S. R. A.
Dods, J. Morgan, and E. C. Larkins, “Spectrally resolved electroluminescence microscopy and -electroluminescence investigations of GaNbased LEDs,” J. Cryst. Growth, vol. 230, pp. 467–472, 2001.
[32] S. Bull, A. V. Andrianov, I. Harrison, and E. C. Larkins, “Development of a spectroscopically resolved photoluminescence microscopy
technique for studying strain and defects in high power laser diodes,”
in Europhysics Conf. Abstracts, vol. 27E, CC4-04-MON, Conf. Lasers
and Electro-Optics (CLEO) Europe, Munich, Germany, Jun. 2003.
, “The study of strain and defects in high power laser diodes by
[33]
spectroscopically resolved photoluminescence microscopy,” Eur. Phys.
J. – Appl. Phys., vol. 27, pp. 469–473, 2004.
[34] O. P. Gough, U. Buckley, B. Corbett, and J. D. Lambkin, “Low-divergence laser structures for cost-effective fiber coupling applications,”
IEEE J. Sel. Topics Quantum Electron., vol. 6, no. 4, pp. 571–576,
Jul.-Aug. 2000.
[35] J. M. Verdiell, M. Ziari, and D. F. Welch, “Low-loss coupling of 980
nm GaAs laser to cleaved singlemode fiber,” Electron. Lett., vol. 32, pp.
1817–1818, 1996.
[36] A. Uskov, J. Mørk, and J. Mark, “Wave mixing in semiconductor laser
amplifiers due to carrier heating and spectral-hole burning,” IEEE J.
Quantum Electon., vol. 30, no. 8, pp. 1769–1781, Aug. 1994.
[37] R. Nietzke, P. Panknin, W. Elsässer, and E. O. Göbel, “Four-wave mixing
in GaAs/AlGaAs semiconductor lasers,” IEEE J. Quantum Electon., vol.
25, no. 6, pp. 1399–1406, Jun. 1989.
[38] C.-Y. Tsai and C.-Y. Tsai, “Carrier density depinning above threshold
in semiconductor lasers: Effects of carrier heating and spectral hole
burning,” IEE Proc. Optoelectron., vol. 144, pp. 209–212, 1997.
[39] C. W. Tang and S. A. Van Slyke, “Organic electroluminescent diodes,”
Appl. Phys. Lett., vol. 51, pp. 913–915, 1987.
[40] J. H. Bourroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K.
Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, “Light emitting
diodes based on conjugated polymers,” Nature, vol. 347, pp. 539–541,
1990.
[41] K. A. Christensen, K. A. Bradley, M. D. Morris, and R. V. Morrison,
“Raman imaging using a tunable dual-stage liquid-crystal Fabry-Perot
interferometer,” Appl. Spectrosc., vol. 49, pp. 1120–1125, 1995.
[42] H. R. Morris, C. C. Hoyt, and P. J. Treado, “Imaging spectrometers for
fluorescence and Raman microscopy – Acousto-optic and liquid crystal
tunable filters,” Appl. Spectrosc., vol. 48, pp. 857–866, 1994.
[43] W. J. Schneller, J. Noto, R. B. Kerr, and R. A. Doe, “Liquid crystal
Fabry-Perot etalons,” in Proc. SPIE, vol. 3355, 1998, pp. 877–883.
[44] J. Noto, Y. Betremieux, R. B. Kerr, H. Zhang, M. J. Taylor, S. Watchon,
and M. Dorin, “Tandem-etalon tunable filter for NIR spectral imaging,”
in Proc. SPIE, vol. 5157, 2003, pp. 178–189.
[45] S. Bull, J. G. Wykes, A. V. Andrianov, J. J. Lim, L. Borruel, S. Sujecki, S. C. Auzanneau, M. Calligaro, M. Krakowski, I. Esquivias, and
E. C. Larkins, “Imaging of spontaneous emission from 980 nm tapered
lasers with windowed n-contacts,” Eur. Phys. J.– Appl. Phys., vol. 27,
pp. 455–459, 2004.
[46] THX31156 CCD Data Sheet and Calibration Data Provided by Wright
Instruments.
[47] M. R. Brozel and G. E. Stillman, Properties of Gallium Arsenide, 3rd
ed. London, U.K.: INSPEC IEE, 1996, pp. 207–213.
Stephen Bull (S’01) received the M.Eng. degree
in electronic engineering with German (first class
honors) and the Ph.D. degree from the University
of Nottingham in 2001 and 2004, respectively.
His Ph.D. research was in the characterization and
degradation of high-power and high-brightness semiconductor lasers, using spectroscopically resolved
imaging techniques.
His current research interests include the numerical modeling of laser diodes, spectroscopic imaging
techniques for the characterization of laser diodes and
the validation of simulation tools and the degradation dynamics of high-power
laser bars.
1087
Alexander V. Andrianov received the M.Sc. degree
in optoelectronics with honors from Leningrad Electrotechnical Institute, Russia, in 1979 and the Ph.D.
degree in physics of semiconductors from Ioffe Physical Technical Institute, Russia, in 1983.
He is currently a Senior Research Fellow with
Ioffe Physical Technical Institute, St Petersburg,
where his research interests include the optics of
low-dimensional systems, nonequilibrium phenomena in semiconductors and semiconductor
devices, and wide bandgap materials. He was a
Research Associate with the University of Nottingham from 1996 to 1998
and again in 2002. During this time, his research activities included the
establishment of a multifunctional optical laboratory for the characterization
semiconductor devices, photo- and electroluminescence studies of high-power
laser diodes and investigations of the optical properties of the GaAs:N and
GaN:As material systems.
Ian Harrison received the B.Sc. degree from the
University of Nottingham, Nottingham, U.K., and
the Ph.D. degree from the University of Manchester,
Manchester, U.K.
Since 1987, he has been with the University of Nottingham. He recently spent a year with the University of California, Santa Barbara, where he worked on
high-speed digital circuits and devices. He has published extensively on the electrical and optical properties of semiconductors devices and materials.
Dr. Harrison received the Royal Academy of Engineering’s Engineering Foresight Award in 2001.
Michael Dorin received the B.S. degree in astronomy from Boston University, Boston, MA, in
1994.
He has been an Applications Engineer, as well as
the Director of Sales and Marketing, at Scientific Solutions, Inc. since May of 2001. He has known and
worked with Dr. Noto and Dr. Kerr since 1990.
Robert B. Kerr received the B.S. degree in physics
from Ohio University, Athens, OH, in 1979, the M.S.
and Ph.D. degrees, both in atmospheric science, from
The University of Michigan, in 1981 and 1986, respectively.
He joined Scientific Solutions, Inc. (SSI) in 1997,
serving originally as Director of Research, and since
2000, as Chief Executive Officer. He was a member
of the technical staff at the Aerospace Corporation,
El Segundo, CA, in 1987. He was a Professor of
Astronomy at Boston University, Boston, MA, from
1988 to 1997.
Dr. Kerr is currently serving his second term on the Arecibo Observatory
Scientific Advisory Committee.
1088
IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 54, NO. 3, JUNE 2005
John Noto received the B.S. degree in physics and astronomy from the University of Rochester, Rochester,
NY, the M.S. degree in physics from Tufts University, and the Ph.D. degree in astronomy/space physics
from Boston University, Boston, MA.
He is the President and Co-Founder of Scientific
Solutions, Inc. (SSI), as well as the inventor of the
Liquid Crystal Fabry-Perot interferometer as developed by SSI. He has more than 10 years of experience in optical research and development, focused on
narrow-band spectroscopy and interferometry.
Eric Larkins (S’92–M’89) received the B.S. degree
in electrical engineering from Cornell University,
Ithaca, NY, in 1980 and the M.S. and Ph.D. degrees,
both in electrical engineering from Stanford University in 1985 and 1991, respectively. He pursued
the Ph.D. degree research on light-emitting heterostructure thyristors and molecular-beam epitaxy,
receiving full support from 1985 as a Kodak Fellow.
He joined the School of Electrical and Electronic
Engineering, University of Nottingham, U.K., as
Lecturer in 1994 and was appointed Professor of
Optoelectronics in 2002. From 1991 to 1994, he was a visiting scientist with
the Fraunhofer Institute for Applied Solid State Physics, Freiburg, Germany,
where he worked on high-speed laser diodes, optical modulators, and MSM and
intersubband photodetectors. His current research interests include high-power
laser diodes, functional photonics, and new optical materials.