Download High-Power Diode Lasers under External Optical Feedback

High-Power Diode Lasers under External Optical Feedback
Britta Leonhäusera, Heiko Kissel*a, Jens W. Tommb, Martin Hempelb, Andreas Ungera,
and Jens Biesenbacha
a
DILAS Diodenlaser GmbH, Galileo-Galilei-Str. 10, D-55219 Mainz, Germany;
b
Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie,
Max-Born-Str. 2A, D-12489 Berlin, Germany
ABSTRACT
We carried out a comprehensive study on single emitters with different antireflection (AR) coatings in the wavelength
range between 780nm and 976nm, which have been exposed to optical feedback to investigate the reversible and
irreversible impacts caused by back-reflected light. By observing the near-field pattern while varying the probe current,
we got information about the influence on filamentation and on peak-power densities with and without external optical
feedback. For GaAs-based laser diodes, the energy gap of GaAs makes a distinction at a wavelength of about 870nm. For
shorter wavelengths, e.g. at 808nm, a substantial part of the feedback light is absorbed by the substrate and GaAs cap
layers very close to the front facet leading to a significant heating of the outcoupling facet. For longer wavelengths, e.g.
976nm, this energy intrusion is not a local one at the front facet, but rather spreads along the whole cavity length.
Keywords: broad area laser, single emitter, optical feedback, GaAs based laser diodes, degradation
1. INTRODUCTION
The general function of diode lasers is based on optical feedback. Photons emitted from the active region are backreflected by the AR and high-reflection (HR) mirrors into the gain medium. In this way, optical feedback is realized and
finally controls the process of stimulated emission. This type of desired internal optical feedback can be tailored (e.g. by
proper choice of facet reflectivities and device geometries such as waveguide architecture, stripe width or cavity length).
External optical feedback can be intentionally implemented, e.g., by external gratings and mirrors widely used for
stabilization and controlled tuning of the emission wavelength or for improving the brightness of device emission [1,2].
In order to enhance the impact of such external optical elements to the gain medium, the internal feedback is often
reduced by using smaller reflectivities of the outcoupling facet (RAR < 1%).
Unintentional external feedback can occur from optical elements in fiber-coupled modules like micro-lenses or fiber
ends, from fiber combiners as well as by radiation from other sources for instance in multi-module configurations.
Within the last few years, unintentional external feedback has become an important issue in many high-power diode laser
applications. It modifies the laser parameters in many ways, e.g. reducing the threshold current, influencing the optical
output power or changing the emission kinetics. In addition to such reversible modifications of external optical feedback
to the device properties, high output powers accounting for very high power densities at the outcoupling facet of broad
area lasers (BALs) can lead to irreversible impacts such as accelerated gradual and sudden degradation including
catastrophic optical (mirror) damage [CO(M)D] [3-6].
The optical near-field is an important parameter with high practical relevance, because its properties determine the
coupling efficiency of a high-power BAL into a fiber. We will discuss the impact of optical feedback to filamentation
amplitudes of single emitters with different reflectivity of the front facet coatings at several wavelengths in the spectral
range between 780nm and 980nm. For GaAs-based laser diodes, different energy reabsorption scenarios in case of
external feedback can be expected for wavelengths above and below about 870nm due to the energy gap of GaAs.
*[email protected]; phone +49-6131-9226-227; fax +49-6131-9226-255; www.dilas.com
2. EXPERIMENTAL SETUP
In our experiments, we used conduction-cooled single emitters at wavelengths between 780nm and 976nm with a pcontact width of 100µm and a cavity length of 4mm from several suppliers. They were cleaved from mini-bars used in
DILAS modules [7] for an easier investigation of external optical feedback influence on the near-field distribution and
the emission spectra compared to mini-bars with five emitters. The single emitters were mounted with AuSn on ceramic
base plates.
Figure 1. Experimental setup for external optical feedback: (1) feedback beam and (2) diagnostic beam paths.
The experimental setup for the investigation of single emitters under defined external optical feedback is shown in Fig. 1.
For better understanding, the laser beam is represented by a green line.
The investigated laser diodes are conduction cooled by a Peltier element which is mounted on a water cooled copper
block. The base plate temperature of the diodes is fixed to 20°C. The emitted laser beam is collimated by an aspheric
plano-convex-lens. It passes through an uncoated wedge prism where it is split into several attenuated beams due to
multiple reflections inside the prism. The diagnostic beam (2) is used to observe the near-field pattern and the spectral
behavior with the help of two CCD cameras. The main beam (1) generates a defined optical feedback.
The main beam (1) passes a beam-splitter cube (BSC). The reflected beam is used to measure the optical output power of
the laser diode using a calibrated optical power meter. The transmitted beam runs through a thin film polarizer; its angle
was adjusted to control the feedback strength. Finally, the beam is reflected by a dielectric mirror back into the laser
diode. The reflected beam from the BSC (not shown in Fig. 1) can be used to measure and control the feedback intensity.
The diagnostic beam (2) is split by a 50:50 beam splitter. The transmitted beam is imaged onto the near-field camera
with an achromatic lens to yield an image of the diode facet. The collimation associated with the imaging optics forms a
telecentric optical system. The reflected beam illuminates a grating via a tilted mirror and a lens-system expanding the
beam. The grating splits the beam into its spectral parts. Afterwards the beam is imaged onto a second camera using a
plano-convex-lens. Due to mirrors in the optical path to the spectrometer, the laser beam is rotated by 90 degrees along
the axis of propagation. Thus, the y-direction of the camera depicts the slow-axis near-field pattern and the x-direction
shows the spectrum.
The optical feedback is adjusted by observing the beam position with a converter card and the changes in the spectrum
using the spectral camera. First, one reflects the feedback beam near the emitted beam by tilting the reflecting mirror.
This can be controlled with a converter card. With further tilting of the reflecting mirror, the beam is slid back into the
diode. Now, changes in the emission spectrum can be observed. Because of the divergence and the spot width of the
beam, there is a wide range where the spectrum of the diode responds. The best adjustment is reached, when the
spectrum shows broadening and redshift (for small feedback strengths) and in addition to that the beat-frequencies of the
internal and the external resonator modes as spikes on the spectrum (see right part of Fig. 2).
3. IMPACT OF EXTERNAL OPTICAL FEEDBACK
An easy way to realize optical feedback is simply placing a semitransparent mirror in front of a diode laser’s outcoupling
facet. The mirror reflects a certain part of the emitted light back into the laser cavity as schematically shown in Fig. 2.
Figure 2. Schematic drawing of a simple setup for external optical feedback illustrating the impact on a laser diode (left part).
Emission spectrum of a 976 nm single emitter showing mode beats due to optical feedback from an external cavity (right part).
The driving current of a diode laser flows directly through its active zone. In case of external feedback, a certain part of
the back-reflected laser radiation is reabsorbed by the laser diode leading not only to several perturbations of the laser
operation without feedback, but also to changes of the supplied electrical power via a modification of the charge carrier
density, i.e. the power supply and the delivered electrical energy are included in the feedback loop and cannot be easily
neglected when considering effects of external optical feedback.
3.1 Reversible impact of external optical feedback to output parameters
Several publications report on changes of the laser emission parameters under external optical feedback.
A reduction of the laser threshold by 20 percent caused by the presence of external feedback has been reported by
Takiguchi et al. [5] for 808 nm emitting devices. Hempel et al. [8] reported an emission power increase by 2.3 percent
for other 808 nm emitting devices, which have originally been designed as low-cost pump sources. This change has been
reversibly achieved when imaging about 25 percent of the output power back into itself. The threshold reduction is
caused by a higher gain due to more photons within the active region [10].
Chuang et al. [9] analyzed linewidth enhancement factors versus feedback conditions for devices emitting at a
wavelength of 1.5µm. The value was found to increase with the feedback strength in the moderate feedback region.
Thus, the Henry-factor can’t be treated constant, e.g. for modelling purposes, if external feedback comes into play.
Another feedback induced reversible impact to the emission properties of the diode affects the emission spectrum. Lang
and Kobayashi [10] observed different spectral behaviors depending on resonator length, driving current and feedback
strength. The emission spectrum shows a red or blue shift as well as broadening or narrowing. This happens due to the
alternated electron density within the laser resonator causing different local refractive indices and therefore different
optical path lengths. Furthermore, the beat-frequency of the internal and external cavity can be observed as spikes on the
emission spectrum [see e.g. right part of Fig. 2].
There is also a substantial impact of external feedback to the emission kinetics [10-13]. Although this topic is interesting
from the scientific point of view, we will focus here to the time-integrated situation, i.e. to continuous wave operation.
Depending on the external resonator length, the laser diode shows different emission behaviors. For a resonator length
with the mirror positioned at a distance less than the length corresponding to the laser relaxation oscillation frequency
(short resonator), so-called pulse package oscillations can be observed [11]. For a longer resonator, different regimes of
feedback occur. This starts with narrowing or broadening of the emission line, continues with mode hopping and the
appearance of satellite modes and finally ends up in the coherence collapse [17].
The optical near-field is a device parameter of utmost high practical relevance, because the near-field properties
determine the coupling efficiency of a high-power BAL into a fiber. For undisturbed operation, one may expect a top-hat
shaped distribution. In fact, the near-field shows an intensity distribution with pronounced maxima and minima. This is
due to nonlinear effects in the laser medium, such as nonlinear ohmic heating or nonlinear electron distribution as
explained in Ref. [14]. Traveling through the medium, the wave front comes across regions with higher or lower
refractive indices. There, the phase velocity is lowered or accelerated, which finally leads to a non-uniform intensity
distribution, the so-called filamentation.
During and after fiber coupling, the fiber end produces unintentional optical feedback to the diode laser. The electrical
field of the back reflected light takes effect on the electric field within the laser cavity. This influences the charged
carrier density which varies the refractive index. As shown by Lang and Kobayashi [10], the optical length of the internal
cavity changes with the refractive index and therefore the spectral behavior and the near-field pattern is modified.
In the following, we discuss the impact of optical feedback to devices with different front facet coatings, i.e. devices with
different susceptibilities to external optical feedback.
Figure 3. Near-field pattern without feedback (blue lines) and with 4% external feedback (red lines) for BALs
emitting at 976nm (a) with RAR = 1% and (b) with RAR = 4%.
Figure 3 shows the normalized near-field pattern without and with 4% external feedback at an injection current of 8A for
diodes emitting at 976nm with two different AR coatings on the front facet (a) with RAR = 1% and (b) with RAR = 4%. For
RAR = 1%, the near-field distribution shows two broad filaments besides the center of the emitter. In case of external
feedback, the near-field of the low AR coated diode becomes multistable due to a modification of the local carrier
density in the active region and its strong influence on the refractive index [10]. Thus, one of the two initial filaments is
increased dramatically and can cause CO(M)D at the front facet because of the resulting high peak power density on a
small front facet area [see also Fig. 4(a)]. Compared to the low AR device, the near-field pattern of the diode coated with
RAR = 4% shows more filaments that are less pronounced. There are only small changes in the near-field distribution
when external feedback is applied without significant changes in the power density on the front facet.
(a)
(b)
Figure 4. False color illustration of the near field distribution vs. injection current for a BAL emitting at 976nm (a) with RAR = 1%
and (b) with RAR = 5%, both operated with 4% external optical feedback.
For a better understanding, the modification of the near-field distribution or the development of filaments, respectively,
with increasing injection current can be displayed in false color diagrams. Such an illustration makes the importance of
the peak-power density more obvious. Figure 4 visualizes the development of the near-field distribution of a BAL
emitting at 976nm with 4% external feedback – (a) with RAR = 1% and (b) with RAR = 5%. For the BAL with RAR = 1%,
only one filament with a PPD of 12.59 MW/cm2 is clearly pronounced at I = 15A. At the same injection current, the
BAL with RAR = 5% shows a more homogenous near-field distribution with a significantly smaller PPD of
9.67 MW/cm2.
Figure 5 quantifies the reversible changes of the near-field filamentation of high-power BALs under external feedback
for devices with different reflectivities of the front facet coating. While Fig. 5(a) explains the way how the filamentation
is quantified, Fig. 5(b) shows the changes of the filamentation amplitude in dependence on the front facet reflectivity
with and without presence of external optical feedback.
We use the term filamentation amplitude as a measure for the non-uniformity of the optical near-field. It is calculated
from the near-field intensity within the central region of the emitter stripe. The limits of the region are set by the
locations (left and right), where the first derivative of the near-field intensity equals zero. This criterion makes sure that
the filamentation amplitude is not interfered by the falling intensity at the edges of the emitter stripe; this approach is
illustrated in Fig. 5(a). The value of filamentation amplitude itself is determined as the standard deviation of the nearfield amplitude values divided by the average. Thus, a value of zero indicates an ideal top-hat-shaped near-field pattern,
while increasing values point to increasing non-uniformity. Certainly, this approach involves a loss of information and
will fail, in particular, if near-field distributions substantially change their shape.
Figure 5. (a) Near-field pattern (black) and first derivative (red) along the aperture of a BAL with RAR = 4% emitting at 976 nm.
The region that has been used for the determination of the filamentation amplitude is gray shaded.
(b) Filamentation amplitude vs. RAR. Red circles refer to 976 nm emitting devices, blue rhombs to 915nm
devices and green triangles to 780nm devices while yellow squares indicate data from 808 nm emitting
devices [8]. Open symbols show the situation without, full ones with external feedback.
The data from the 976 nm emitting devices shown in Fig. 5(b) indicate a substantial enhancement of the filamentation
amplitude under external feedback for RAR < 4%. For RAR ≥ 4%, the effect of external feedback on the near-field seems to
be rather small. This holds for devices emitting at all considered wavelengths. The absolute values of the filamentation
amplitudes for the investigated device sets differ up to a factor of 2. This comparison, however, doesn’t allow for any
general statements, because the device sets differ in much more parameters than the wavelength. The common feature,
however, is that for RAR ≥ 4% the filamentation amplitudes are not substantially affected by the external feedback (except
for BALs emitting at 780nm). But the influence of external optical feedback on the filamentation amplitude decreases
with increasing AR reflectivity for all considered wavelengths.
Except for the fiber coupling efficiency in modules, low filamentation amplitudes are important for another reason:
Device degradation is often a local effect, i.e. it begins at a particular location, which is either pre-damaged or
experiences an extra load in terms of current or photon density resulting in local heating or strains. Larger filamentation
amplitudes are an expression for a more inhomogeneous photon density distribution, and it has been even experimentally
proven that irreversible degradation starts exactly in the most pronounced filaments [15, 16].
3.2 Irreversible impact of external optical feedback to output parameters
3.2.1 Gradual degradation
Hempel et al. analyzed the early stages of gradual degradation on BALs emitting at 808 nm [8]. A power loss of
0.346 W·kh-1 is observed within the first 200 h of accelerated aging for a reference device without feedback. A device
from the same batch being operated under external feedback (~ 25%) degraded gradually by 0.796 W·kh-1. That means,
external optical feedback more than doubled the degradation rate. Optical inspection of the outcoupling facet didn’t
reveal any signatures that could explain the observed power loss. A detailed analysis of the devices using micro-PL,
micro-Raman und cathodoluminescence (CL) spectroscopy revealed that the gradual degradation is caused by alterations
underneath the outcoupling facet in a depth of about 5 to 20 μm (counted from the facet along the laser axis). Clear
signatures of point defect creation have been detected by Raman spectroscopy within the cladding layers. These layers
are also the ones with the highest Al-content. Dehomogenization of the waveguide CL points to alterations also in this
layer. At the same time, CL and PL mapping proved that the quantum-well, i.e. the place with the highest optical load
where the conversion from electrical to photon energy actually takes place, is not affected by any perceptible aging
effects. This has been the most surprising result of this particular study on irreversible effects of BAL operation under
external optical feedback.
We have started a similar study on BALs emitting at 976nm. First CL investigations of gradually degraded BALs that
were operated under certain external feedback strength didn’t show enhanced point defect creation in any layers close to
the front facet. This indicates a locally and spatially different absorption behavior of GaAs BALs emitting at 808nm and
976nm.
Due to the divergent emission of BALs, only a certain part of back-reflected laser light is coupled into the thin
waveguide layers and the quantum well at the position of the electrically pumped emitter stripe; its exact fraction is
indeed unknown. This part of external feedback is responsible for different possible modifications (independent on the
emission wavelength) of laser emission parameters as already described in Sec. 3.1.
Figure 6. Schematic drawing illustrating the absorption behavior for GaAs-based BALs in different wavelength regions.
In the following, we will discuss, what happens to the fraction of external optical feedback not coupled back into the
active region of a BAL. The key parameter for answering this question is the emission wavelength. All BALs considered
in this work and nearly all other semiconductor lasers in the spectral range between 630 nm and 1.100 nm are based on
heavily n-doped GaAs substrates. Thus, the optical properties (namely the band gap) of the substrate material make the
distinction. That means, we have to distinguish between BALs emitting below or above ~ 870 nm (corresponding to the
energy gap of GaAs at room temperature).
Figure 6 illustrates the different absorption behavior for these two groups. For shorter wavelengths, such as the widely
used 808nm, a substantial part of feedback photon energy will be absorbed by the GaAs substrate, buffer and cap layers
very close to the front facet. The absorption coefficient  is in the range of several thousand cm-1 (slightly below
870 nm) and up to 10.000 cm-1 for significantly shorter wavelengths (e.g. 808 nm). This is well in line with experimental
findings from Hempel et al. [6,8].
For longer wavelengths (e.g. 976 nm), the absorption coefficient of heavily n-doped GaAs substrates is much smaller,
 ~ 5 cm-1 [19]. For devices with cavity lengths of 4 mm considered in this paper, the effective optical path length will
be 8 mm due to reflection at the HR coating on the back facet. Thus, more than 98% of the feedback radiation is
absorbed in the mentioned GaAs layers. In contrast to BALs with an emission wavelength below ~ 870 nm, the intrusion
of feedback photon energy is not a local one (close to the front facet), but rather spreads along the entire cavity. This
leads to a weaker and more homogeneous heating of the whole device structure.
3.2.2 Catastrophic degradation [CO(M)D]
Gradual degradation can be considered as one precursor for sudden degradation. Indeed, many accelerated long-term
aging tests result in sudden degradation events, among them CO(M)D. But there is another and more direct route to the
occurrence of CO(M)Ds as well. It has been already addressed in section 3.1 and it becomes clear if one considers
Fig. 5(b) for the area of small front facet reflectivities. When a device is already operated at high optical output powers,
an increase of the filamentation amplitude might cause CO(M)D even at a defect-free device not operated before. It is
important to note that the CO(M)D threshold must be exceeded only in one single filament for a few nanoseconds, and
this position will serve as initial point for further defect propagation (see e.g. Ref. [18]), which could lead to a complete
device failure. The often used term CO(M)D threshold is better described by the local (peak) power density leading to a
CO(M)D failure of a pristine device. Testing a few devices of one particular batch, one can find a batch-specific
CO(M)D threshold, except for instant failures due to initial device defects.
Takiguchi et al. [5] reported the results of an experimental and theoretical study on the COMD behavior of BALs
emitting at 808 nm operated under external optical feedback. Selected results of this study are reproduced in Fig. 7(b).
The authors increased stepwise the external feedback rate Rext for a given operation current in order to approach
smoothly towards the COMD threshold. They defined the parameter Rext as the power ratio between the corresponding
powers of back-reflected feedback light and laser output, both measured at the front facet.
Figure 7. (a) Experimentally determined COMD diagram for BALs with RAR=1% emitting at 976 nm. Red squares mark COMD
events for given currents and feedback strengths Rext. The blue line is the result of modelling calculations
according to Ref. [5]. (b) Calculated COMD diagram showing Rext vs. front facet reflectivity R [5]. The plot shows
the parameter ranges, where secure operation is possible (bottom left) and impossible (gray shaded).
Figure 3 in Ref. [5] shows that the devices tend to CO(M)D failures at high optical output powers even at small feedback
rates Rext. The BALs investigated in our study shows the same behavior [see Fig. 7(a) for BALs with RAR = 1% emitting
at 976nm). In other words, the feedback strength must be kept small in order to protect the devices against CO(M)D
failures. These investigations led to a model, which considers the front-facet area as the main absorptive (and therefore
heavily loaded) region, and the power level there as a result of the constructive interference between light fields
controlled by the internal and external feedback loops. The main result of Takiguchi et al. is depicted in Fig. 7(b), where
an optimum front facet reflectivity is revealed at R ~ 7% (see arrow) for the investigated BALs, at which the CO(M)D
threshold is shifted towards highest external optical feedback rates Rext. At the optimum R-value (i.e. optimum
reflectivity of the AR coating), the BALs are most tolerant against external feedback in terms of sensitivity to CO(M)D
formation. The main message is: There is an optimal front facet reflectivity, where devices show the weakest
susceptibility to CO(M)D induced by external optical feedback.
This general statement of Takiguchi et al. [5] agrees well with our experimental finding shown in Fig. 5(b) and provides
a direct link between filamentation and CO(M)D threshold. A draft of this link was already given in the preceding
section, when stating that increased filamentation amplitudes will result in higher local power densities (at constant
output power). Thus, the open circles in Fig. 5(b) are considered to resemble that part of Fig. 7(b) for small R-values left
of the optimum front facet reflectivity that is indicated by the arrow. This assumption also provides the link between
reversible and irreversible impacts of external optical feedback. Moreover, this would subsequently determine the
mechanism, that caused the COMDs reported in Ref. [5], namely filamentation changes rather than gradual degradation.
Figure 8. (a) Measured near-field pattern together with a corresponding top-hat distribution containing 95% of the optical output
power (B0 – near-field width, HF – filament height). (b) Peak power density versus RAR for an injection current of I = 10A
and Rext = 4%. Red circles refer to 976nm emitting devices, blue rhombs to 915nm devices and green triangles to 780nm
devices. Open symbols show the situation without, full ones with external optical feedback.
Figure 8(a) shows a typical near-field pattern from which the peak power density of the strongest filament can be derived
(see e.g. Ref. [15]). We integrate the output power within the near-field distribution over the whole emitter width which
equals 100%. The top hat distribution for 95% output power defines the width B 0 and the height H0 of a top-hat shaped
near-field distribution. The height of the strongest filament determines the superelevation in relation to H0. The peak
power density (PPD) is then given by
.
HF is the height of the strongest filament and HE the emitter height. The power density of the most intensive filament is
important because of the probability of CO(M)D occurrence. In Ref. [15] we have already shown a correlation between
COMD locations on the outcoupling facet and the position of strong filaments in the near-field pattern. Therefore we
observed the PPDs for several devices with different AR coatings; they are shown in Fig. 8(b). As expected, the PPD
with external optical feedback decreases with increasing reflectivity of the AR coating on the outcoupling facet for all
considered BALs in the wavelength range between 780nm and 976nm. With other words, an enhanced AR reflectivity
helps to avoid CO(M)Ds at the front facet occurring due to PPDs exceeding the device and batch specific CO(M)D level.
4. CHALLENGES
Although external optical feedback represents a current issue of high practical relevance, there are only a few
experimental studies published so far. Even our discussion in Sec. 3 has rather qualitative character, because of a number
of experimental handicaps and indeterminacies shortly discussed in the following:

There is no accepted standard setup for the implementation of external feedback to BALs for investigations of
feedback-induced modifications of laser parameters as well as for aging experiments. The Rext-values for the
feedback strength given in this paper for our experiments described above indicates the whole amount of backreflected laser radiation.

If external optical feedback leads to lowering the CO(M)D threshold due to an increase in the filamentation
amplitude (and local power density as well), this process will be very fast, i.e. it proceeds on a ns-timescale, as
shown in COD experiments in absence of external feedback. Although the resolution of optical near-field and
COD kinetics on this timescale is feasible [20], the addition of an controlled external feedback loop into such a
streak-camera setup is rather challenging.

As already discussed in Sec. 3.2.1, it is even unknown, what fraction of the external feedback is really coupled
back into the active layers (waveguide and quantum well). Typically, optical feedback is adjusted by observing
the beam position (for IR wavelengths using a converter card, the optical output power and the modification of
the emission spectrum (shift of the spectral position, line width, mode beats). But there is no practical solution
for the quantification of the feedback photon energy to the active region.
5. CONCLUSIONS
Within the last few years, the effects related to unintentional external optical feedback have become an important issue in
many high-power diode laser applications.
Besides reversible impacts to the device properties such as modifications of the laser threshold, the emission spectra and
kinetics as well as near-field distribution, we have discussed on irreversible impacts leading to gradual and catastrophic
degradation of BALs.
There are different routes towards CO(M)D under external feedback operation. On the one hand, enhanced gradual
degradation causes a subsequent reduction of the CO(M)D threshold, on the other hand, external optical feedback leads
to an enhancement of the filamentation amplitude or local power density on the outcoupling facet, respectively.
We found, that an increasing AR reflectivity leads to a more uniform near-field distribution with an increasing number of
filaments. Simultaneously, the influence of external optical feedback on the near-field pattern decreases significantly.
Thus, the CO(M)D threshold is improved due to smaller peak-power densities at the same optical output power. With
other words, enhanced AR reflectivities of RAR ≥ 4% helps to avoid CO(M)Ds in case of external feedback.
In dependence on the emission wavelength of GaAs-based BALs, there are two different energy absorption scenarios. In
BALs emitting at  < 870 nm (corresponding to the energy gap of GaAs at room temperature), strong absorption of the
feedback photon energy and subsequent heating takes place very close to the front facet, whereas for  > 870 nm the
feedback energy intrusion rather spreads along the entire cavity.
There is no standard setup for optical investigations and aging experiments under external feedback. The quantification
of the optical feedback fraction really coupled back into the active layers of diode lasers is an unsolved problem.
The number of published experimental studies on the impact of external optical feedback to BAL degradation is rather
small, and systematic studies are urgently required.
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