# Multi-type quantum well semiconductor membrane external-cavity surface-
emitting lasers (MECSELs) for widely tunable continuous wave operation

Patrik Rajala<EMAIL_ADDRESS>Philipp Tatar-Mathes  Optoelectronics
Research Centre (ORC), Physics Unit / Photonics, Faculty of Engineering and
Natural Science, Tampere University, Korkeakoulunkatu 3, 33720 Tampere,
Finland  Hoy-My Phung  Optoelectronics Research Centre (ORC), Physics Unit /
Photonics, Faculty of Engineering and Natural Science, Tampere University,
Korkeakoulunkatu 3, 33720 Tampere, Finland  Robert Bosch GmbH, Robert-Bosch-
Campus 1, 71272 Renningen, Germany Jesse Koskinen  Optoelectronics Research
Centre (ORC), Physics Unit / Photonics, Faculty of Engineering and Natural
Science, Tampere University, Korkeakoulunkatu 3, 33720 Tampere, Finland  Sanna
Ranta  Optoelectronics Research Centre (ORC), Physics Unit / Photonics,
Faculty of Engineering and Natural Science, Tampere University,
Korkeakoulunkatu 3, 33720 Tampere, Finland  Mircea Guina  Optoelectronics
Research Centre (ORC), Physics Unit / Photonics, Faculty of Engineering and
Natural Science, Tampere University, Korkeakoulunkatu 3, 33720 Tampere,
Finland  Hermann Kahle<EMAIL_ADDRESS>Optoelectronics
Research Centre (ORC), Physics Unit / Photonics, Faculty of Engineering and
Natural Science, Tampere University, Korkeakoulunkatu 3, 33720 Tampere,
Finland  Institute for Photonic Quantum Systems (PhoQS), Center for
Optoelectronics and Photonics Paderborn, and Department of Physics, Paderborn
University, Warburger Straße 100, 33098 Paderborn, Germany

###### Abstract

Membrane external-cavity surface-emitting lasers (MECSELs) are at the
forefront of pushing the performance limits of vertically emitting
semiconductor lasers. Their simple idea of using just a very thin (hundreds of
nanometers to few microns) gain membrane opens up new possibilities through
uniform double side optical pumping and superior heat extraction from the
active area. Moreover, these advantages of MECSELs enable more complex band
gap engineering possibilities for the active region by the introduction of
multiple types of quantum wells (QWs) to a single laser gain structure. In
this paper, we present a new design strategy for laser gain structures with
several types of QWs. The aim is to achieve broadband gain with relatively
high power operation and potentially a flat spectral tuning range. The
emphasis in our design is on ensuring sufficient gain over a wide wavelength
range, having uniform pump absorption, and restricted carrier mobility between
the different quantum wells during laser operation. A full-width half-maximum
tuning range of $>$ 70 nm ($>$ 21.7 THz) with more than 125 mW of power
through the entire tuning range at room temperature is demonstrated.

MECSEL, VECSEL, semiconductor laser, quantum wells, thermal management, active
region design

> During the last decades many application fields ranging from atomic and
> molecular physics[1] to spectroscopy[2, 3] and from broadband sensors to
> optical telecommunication systems[4, 5] have shown an increasing demand for
> widely tunable lasers[6]. Due to their compactness, low costs, and
> relatively high efficiencies, semiconductor lasers are in general favoured
> over other laser systems. When it comes to highly tunable lasers, titanium-
> sapphire lasers (Ti:sapph.) are the gold standard. Their ability to produce
> a flat emission power spectrum on wavelengths ranging roughly from 650 nm to
> 1100 nm is superior to any semiconductor laser [7, 8]. However, many
> application fields mentioned above do not require this whole tuning range or
> the power level that Ti:sapph. provides. At the same time, they would
> benefit strongly from the unique properties of semiconductor lasers,
> particularly the compactness, ease of manufacturing and lower costs, if
> these could be achieved while maintaining a relatively flat spectral tuning
> range. Membrane external-cavity surface-emitting lasers (MECSELs)[9, 10] are
> able to answer this need by enabling the use of advanced gain structures
> with multi-type quantum well (QW) designs, as it will be demonstrated in
> this paper.

##  I Introduction

Leinonen et al.[11, 12] have demonstrated a vertical external-cavity surface-
emitting laser (VECSEL) capable of operating on two different wavelengths at
the same time owing to the use of two different kinds of QWs in a single laser
gain structure, which proves that the operation of a surface-emitting laser
employing a relatively complex QW gain structure is certainly feasible. Wider
tunability and operation on two different wavelengths with the aid of
utilizing non-linear optical processes, namely second-harmonic generation
(SHG) and sum-frequency generation (SFG), has been demonstrated more recently
as well [13]. Other strategies to achieve wide tuning ranges with VECSELs have
included a multi chip cavity configuration[14], the use of an inserted
blade[15], a two-mode resonant microcavity[16], careful gain element
reflectance engineering and optimization of the laser gain element[17, 18] as
well as the use of quantum dots (QDs) for quantum confinement[19]. However,
the VECSEL technology used in all of these previous demonstrations imposes
certain limits on continuous tuning throughout the wavelength range enabled by
the QWs or QDs. The main limitation arises from the monolithically integrated
distributed Bragg reflector (DBR), which locks the phase of the standing wave
of the optical field in a VECSEL’s gain region during laser operation, as can
be seen in Fig. 1.

Figure 1: (a) A simplified exemplary VECSEL gain chip for operation at 1 µm
wavelength with simulated E-field intensity. The antinodes of the standing
wave of the optical field match with the QWs and resonant operation is
present. (b) The exact same structure with simulated E-field intensity at 960
nm. The standing wave of the optical field no longer overlaps with the QWs and
the laser emission would stop as no field enhancement could be created. (c)
The same active region, but now in a MECSEL configuration with the external
cavity mirror about 500 $\lambda$ away from the active region membrane. The
laser “finds” the best possible overlap between the standing wave of the
optical field and the quantum wells, even though the match is no longer
perfect.

Figure 1a demonstrates how the standing wave of the optical field in a VECSEL
on the designed operation wavelength, which in our example is 1 µm, overlaps
well with the QWs to create field enhancement. Figure 1b in turn demonstrates
how changing the operation wavelength away from the design wavelength
(demonstrated in Fig. 1a shifts the overlap between the standing wave of the
optical field and the gain providing QWs completely out of phase, because the
DBR locks the phase of the optical field at its interface. In MECSELs, this
limitation shown in Fig. 1b is removed [20], as both mirrors are external and
the phase of the optical field on the edges of the laser membrane is not pre-
determined, but rather depends on the current state of the external mirrors
and the entire cavity. This gives the laser the ability to “find” the best
possible overlap between the standing wave of the optical field and the gain
providing QWs in any situation and provide enough gain for laser operation
even on a wavelength that is far from the designed operation wavelength, as is
illustrated in Fig. 1c. The chosen wavelength range in this paper is around 1
µm, because several notable benchmarks for wide tuning range exist in this
wavelength range and because the material system utilized is very well
developed and thus does not introduce any extra variables. Figure 2 summarizes
the development of tuning bandwidths in the 1 µm wavelength range by showing
the advances made with VECSELs and MECSELs and how the work presented in this
paper compares to them. The comparison here is made by considering the full-
width half-maximum (FWHM) of the emission spectrum, as that is what is in the
end significant, if we are to eventually have a constant power over the tuning
range resulting in a flat emission spectrum. It is worth noting that the
VECSELs seen in Fig. 2 are explicitly designed for wide tuning, while the
MECSELs are not (due to their relative novelty in the field), which already
shows the potential and inherent advantage when it comes to tuning range, as
demonstrated with Fig. 1. We can also see from Fig. 2 that when it comes to
FWHM of the tuning range, our design utilizing multiple types of QWs is a
highly effective strategy that does not rely on complicated processing,
coatings or pumping setups, but just optimized epitaxial design of the gain
structure.

[[15]][[16]][[17]][[18]][[21]][[20]][[19]]This work

Figure 2: Overview over the FWHM of a selection of VECSELs (black squares) and
MECSELs (full orange rhombuses) with relatively wide full tuning ranges around
1000 nm emission wavelength, plotted over their central emission wavelength of
their FWHM tuning ranges. Both, quantum well (QW) and quantum dot (QD) based
laser active structures are included.

##  II Design and structure simulation

While the addition of multiple types of QWs to a MECSEL gain structure does
not significantly complicate the epitaxial growth or the processing of the
MECSEL, there are few important additional steps in the design phase that need
to be taken into account. In the first demonstration of a multi-type QW
surface-emitting laser by Leinonen et al.[11], there were two main design
ideas in this dual-wavelength VECSEL: (1) separating the different kinds of
QWs from each other with electron blocking layers (EBLs) to prevent most of
the excited charge carriers from diffusing extensively towards the lowest
energy states inside the structure, i.e. the QWs of the longer emission
wavelength, and (2) matching the standing wave of the electric field during
operation in a way that ensures that when the laser is operated on the shorter
wavelength, QWs corresponding to that emission wavelength are in the nodes of
the electric field and the other QWs are in the antinodes, and vice versa for
the longer wavelength operation. Moving from dual-wavelength operation to
continuous broadband tuning operation with a narrow wavelength range at a
time, design point (1) is still valid, as the charge carriers will otherwise
always diffuse mostly towards the energy states of the longer wavelength QWs.
This would significantly reduce the tuning range available, which was
effectively demonstrated by the photoluminescence (PL) measurement results by
Leinonen et al.[11]. Point (2) is not as straightforward for the cases of
continuous tuning and using MECSEL technology instead of VECSELs. It is clear
that on the shorter wavelength end it is beneficial to have clear matching of
antinodes on the shorter emission wavelength QWs and matching of nodes on the
longer emission wavelength QWs to minimize the re-absorption by the long
wavelength QWs during operation. However, the opposite way this should not
play a role, as the QWs of shorter emission wavelength are not capable of
absorbing photons emitted by the QWs of longer emission wavelength. The
absence of a phase-locking DBR allows the laser to choose the suitable phase
on whatever wavelength it is currently operating (see also Tatar-Mathes et
al.[22]). Thus it is more important to just have a good matching of antinodes
on the QWs of the longer emission wavelength, when we are operating the MECSEL
on the longer side of the wavelength range.
In addition to the aspects mentioned, the choice of QW materials, their number
both per group and in total, and the consideration of pump power distribution
inside the structure during operation, are of high importance. For the choice
of QW materials, we must consider the tunability of a single type of QW. For
VECSELs consisting of a single type of QW, a wide, full tuning range in this
wavelength range has been demonstrated for example by Borgentun et al.[18, 17]
with 32 nm and 43 nm, respectively, and by Broda et al.[16] with 70 nm
(without temperature tuning). For MECSELs the latest record values were
published by Yang et al. [20, 9] and Priante et al. [23] within the 1.0 µm to
1.2 µm range. As seen from these results, the tuning range is not evenly
distributed around the center wavelength, but follows the typical inverted
parabolic behaviour, which reassembles the spectral shape of the gain above
laser threshold (if the cavity mirrors possess a flat reflection
characteristic). Thus the choice of QW materials is a trade-off between
maximum tuning range and constant emission power everywhere within, when using
multiple QW types in a single structure. In order to receive a relatively flat
emission power distribution throughout the tuning range, based on previous
experiments, a separation of 60 nm in nominal emission wavelengths was chosen.
Suitable nominal wavelengths in the chosen 1 µm wavelength range were then 950
nm and 1010 nm. These were achieved by using 7 nm thick InGaAs QWs with an In
fraction of 13.0 % and 20.5 %, respectively. In addition, GaAs barriers, GaAsP
strain compensation layers, AlAs EBLs (transparent to both the pump and
emission wavelengths), and lattice matched GaInP window layers were used in
the structure. The number of QWs per group that can be used, is almost
entirely dictated by the amount of local strain that the structure can
withstand and thus an amount of two QWs per QW group was chosen in our design.
The pump power distribution during operation was taken into account when
deciding the number and distribution of the chosen QWs inside the structure.
QWs of the shorter emission wavelength (950 nm) have 37 % less quantum
confinement for electrons and 33 % for holes compared to the 1010 nm QWs. Thus
they provide less small-signal gain. In order to have a (more) homogeneous
emission characteristic throughout the tuning range, more of the short
wavelength QWs must be utilized than of those of longer nominal emission
wavelength. Based on the simulations by Phung et al.[24], we were able to
derive that $\sim$ 2 µm is a good value of total thickness for the structure,
so that we are still able to pump all QWs sufficiently with single-side
pumping (SSP) under the given conditions. The structure otherwise was designed
to be double-side pumped (DSP) (i.e. the distribution of QWs), but for
comparison we wanted the structure to be able to operate with SSP as well, to
be able to compare the changes between the different pumping regimes. Thus we
decided to grow the 2 µm thick structure, which allows us to use eleven QW
groups of two QWs per group, when we take into account how much spacer,
barrier, charge carrier blocking, and strain compensation layers are also
needed. In order to match the $\sim$ 35 % difference in quantum confinement
between the two types of QWs as closely as possible, a distribution of seven
short and four long wavelength QW packages was chosen.
When the amounts and specific materials of the layers as well as the total
thickness of the structure are decided, the only thing left to do is to place
all of them inside the structure accordingly. Achieving a more homogeneous
pump power distribution during DSP, the QW distribution was chosen to be
symmetric when looking from the middle of the structure. Due to antinode and
node matching the structure itself in detail is not completely symmetric as
the placements and thicknesses of the spacer layers were adjusted last in the
design process. This was done by simultaneously matching antinodes on QWs of
the shorter wavelength and nodes on the QWs of the longer wavelength on the
shorter target wavelength of 950 nm. The same was done for the antinodes on
the QWs of the longer wavelength on the longer target wavelength of 1010 nm.
Figures 3a and 3b show the simulated standing wave of the electric field
intensity inside the structure during operation on wavelengths of 953 nm and
1009 nm, respectively. The structure was designed using the SimuLase® software
and the optical field simulations were made using the transfer matrix method
in the expected lasing temperature of 350 K. In addition to the node and
antinode matching described above, Figs. 3a and 3b show the ordering of the QW
groups in five sections divided by the EBLs mentioned at the beginning of this
section.

(a)

(b)

Figure 3:  The gain membrane’s architecture is shown. (a) Refractive index
(violet solid line) and the simulated standing wave E-field intensity (solid
orange line) of the short wavelength (nominally 953 nm) resonance are plotted
over the thickness of the gain membrane. (b) Refractive index (violet solid
line) and the simulated standing wave E-field intensity (solid red line) of
the long wavelength (nominally 1009 nm) resonance are plotted over the
thickness of the gain membrane.

This kind of order and distribution was chosen as a balance between keeping
the structure simple enough to have good antinode and node matching on
different wavelengths without thickening the structure needlessly, as well as
having a rather balanced amount of pump light reaching each of the QW group
sections in both SSP and DSP situations. As a consequence of this trade-off,
three antinodes (at distances $\sim$ 425 nm, 850 nm, and 1350 nm, see Figs. 3a
and 3b) could not be brought in overlap with any QW pair as the space was
needed for the EBLs. Based on the Phung et al.[24] simulation, we could
estimate that 62 % of the pump light is absorbed in the shorter wavelength QW
group sections and 33 % in the longer wavelength QW group sections in the
structure depicted in Figs. 3a and 3b. Considering the seven to four relation
in the number of QW groups, this is a very good balance in terms of QWs
receiving an even amount of pump light, when finally applying DSP.
The sample was mounted in the lasing setup shown in Fig. 5 in the same
orientation as it is depicted in Figs. 3a and 3b. Therefore, when applying
SSP, a slight benefit for right side pumping is likely, as the barriers around
the QWs are thicker on the left side of the structure. This might compensate
the smaller pump power density on the left side of the gain membrane, when
pumping from the right. A more homogeneous pumping condition for all QWs would
be the consequence, which in turn would lead to better performance. This is
further discussed in the characterization results in section "Broadband
wavelength tuning".

###  II.1 Photoluminescence characterization

The structure was grown by molecular beam epitaxy with a VG V80 machine. While
the growth parameters of all of the semiconductor materials used in this
structure are known very well, the usage of more than one kind of QW in a
single structure requires some uncommon calibration methods. The method of
choice applied here was to first grow PL samples for each kind of QW
separately, fine tune their material composition, and then combine them in the
final structure.

Figure 4: The PL signal at room temperature of the final broadband laser
structure (violet) compared to the 950 nm QW calibration sample’s (orange) and
1010 nm QW calibration sample’s (red) PL. Measurement was done with an RPM
2000 system using a 785 nm continuous wave excitation laser. The intensities
are normalized to the 950 nm QW calibration sample’s intensity.

The PL calibration samples contain only one barrier and one QW group with two
QWs of the same kind in it. Figure 4 shows the measured PL curves of both of
the PL calibration samples as well as the final structure’s PL curve. There we
can see that the PL signal of the final structure is very much as expected
based on the PL signals from the two separate QW calibration samples. The peak
PL emission wavelengths are very close to what was desired, as we expect the
gain maximum to be 15 to 25 nm red-shifted during laser operation. A much
higher intensity of the PL signal of the final device was also expected, as
there are a total of eleven QW groups compared to the one of the calibration
samples. If we look at the calibration samples’ PL intensity compared to each
other, we can also see the difference in quantum confinement mentioned in the
last chapter, as the 1010 nm QWs emit an almost twice as strong PL signal.
However, this effect has been quite well balanced with the QW distribution, as
in the final device the relative difference between the PL signal from the two
peak wavelengths is much smaller and actually the signal from the shorter
wavelength QWs is slightly better. This also allows us to assume that the EBLs
are working as intended and prevent the longer wavelength QWs from attracting
the vast majority of all charge carriers, when comparing this result to the PL
results shown by Leinonen et al. [11].

##  III Laser characterization

In the following, the characterization setup and measurements are described in
detail. All presented results in this section have been performed at the same
heat sink temperature of 20∘C. Also, the same laser cavity mirrors and the
same cavity configuration have been used for all measurements.

###  III.1 Experimentation setup

A schematic illustration of the characterization setup is shown in Fig. 5. The
V-cavity consisted of a plane outcoupling mirror M3 with a reflectivity of
$R_{\text{M3}}=\left(99.5\pm 0.3\right)$ %, and two curved high-reflecting
mirrors M1 and M2, which both had the same reflectivity of $R_{\text{M1,
M2}}>99.9\,\%$ and the same radius of curvature of $r_{\text{M1,
M2}}=300\,\text{mm}$. The mirror distances of M1 and M2 to the gain membrane
sandwich were adjusted to $L_{1}=291\,\text{mm}$ and $L_{2}=293\,\text{mm}$.
M3 was positioned under an opening angle of 11∘ between $L_{2}$ and $L_{3}$.
The distance between M2 and M3 was $L_{3}\,=\,291\,\text{mm}$. For tuning, a 1
mm thick birefringent filter was used, inserted in the $L_{3}$ cavity arm in
an orientation to support parallel polarization. The free spectral range of
the birefringent filter at 988 nm was calculated to be 111.2 nm. The
calculated cavity mode diameter on the gain membrane was about 250 µm using
the ray transfer method for a Gaussian TEM00 beam.

Figure 5: Experimental setup of the MECSEL employing the broadband gain
structure and a V-cavity. By using 90∘ off-axis parabolic mirrors with a high-
reflection protected gold coating, the pump beam is focused down to a nearly
circular pump spot onto the laser-active membrane as illustrated.

Two identical 808 nm LIMO diode lasers (only two construction numbers apart
from each other) coupled into identical multi-mode fibers
$\left(\mathrm{MHP200L02~{}from~{}THORLABS\textsuperscript{\textregistered}}\right)$
with a 200 µm core diameter and a numerical aperture of 0.22 were used as a
pump source. The fiber output was collimated by a $f\,=\,100\,\text{mm}$ anti-
reflection coated plano-convex lens, and focused onto a spot size of about 330
µm in diameter by a 90∘ off-axis parabolic mirror
$\left(\mathrm{MPD249H-M01~{}from~{}THORLABS\textsuperscript{\textregistered}}\right)$
with a protected gold reflection coating ($R_{808\,\text{nm}}>96\,\%$) and a
reflected focal length of 101.6 mm. Thus, the ratio between the cavity mode
and pump spot diameter was about 0.76, which is within the suggested optimum
from 0.65 to 0.82 derived by Laurain et al.[25] via simulations. The parabolic
mirror used in these experiments has a diameter of 50.8 mm as well as a hole
with a diameter of 3 mm that does not cut the intra cavity laser mode. The
collimated pump beam irradiated on the parabolic mirror covered almost the
whole area of this mirror. For the pump beam, the losses caused by this 3 mm
hole were investigated and were below 1 %, and therefore negligible.
Nevertheless, the incident pump power $P_{\mathrm{pump}}$ was determined via
measuring the power reflected by the parabolic mirror. Therefore, all
reflection losses as well as the losses introduced by the centered 3-mm-
through-hole and the fiber coupling losses were taken into account. With an
angle of incidence of the pump laser, that ranges from almost 0∘ to less than
15∘, this pump approach enables a nearly circular pump spot with a sagittal to
tangential diameter relation of $D_{\text{p, sag}}$/$D_{\text{p,
tan}}\,>\,0.96$ in the focus. The MECSEL gain membrane sandwiched between
transparent SiC heat spreaders, in the following called MECSEL gain element,
was mounted between water/glycol cooled copper heat sinks (not drawn in Fig.
5) with an aperture diameter of 1.5 mm. The opening angle of the aperture is
60∘ to provide enough space for the incident pump laser (details can be found
elsewhere [26]).
The thermal resistance $R_{\mathrm{th}}$ represents one of the most important
parameters of MECSELs, it allows to compare and to classify results. In order
to calculate $R_{\mathrm{th}}$, one needs to determine reflected pump power
$P_{\mathrm{refl}}$, transmitted pump power $P_{\mathrm{trans}}$, and the
absorbed pump power $P_{\mathrm{abs}}$. A power transfer measurement has to be
performed to receive the wavelength shift $\Delta\lambda$/$\Delta
P_{\mathrm{abs}}$ and the corresponding output power $P_{\mathrm{out}}$. Also,
a temperature tuning measurement is essential to determine the thermal shift
$\Delta\lambda$/$\Delta T_{\mathrm{hs}}$. In Fig. 6a it is plotted how the
fractions of $P_{\mathrm{pump}}$ in percent are distributed when interacting
with the gain sandwich. $P_{\mathrm{refl}}$ was calculated. For 808 nm the
refractive index of SiC is $n_{\mathrm{SiC}}$ = 2.60 [27]. For angles of
incident between 0∘ and 15∘ the reflectivity of the SiC-air interface changes
only in the range of 10-5, therefore a constant reflection of 19.2 % can be
assumed for unpolarized light. The linear fit to the transmitted power values
revealed a transmission of $P_{\mathrm{trans}}$ = 3.9 %. Now,
$P_{\mathrm{abs}}$ can be calculated: $P_{\mathrm{abs}}$ = $P_{\mathrm{pump}}$
\- $P_{\mathrm{refl}}$ \- $P_{\mathrm{trans}}$ = 76.3 %. The wavelength shift
$\Delta\lambda$/$P_{\mathrm{diss}}=\left(0.90\pm 0.03\right)$ nm/W was
extracted from spectra taken during the power transfer measurement plotted in
the inset in Fig. 6a. Then it was plotted over the dissipated power
$P_{\mathrm{diss}}$ in Fig. 6b. $P_{\mathrm{diss}}$ was calculated as follows:
$P_{\mathrm{diss}}$ = $P_{\mathrm{abs}}$ \- $P_{\mathrm{out}}$, where
$P_{\mathrm{out}}$ is the output power measured. Due to the relatively high
reflectivity of the used outcoupler of $R_{\mathrm{M3,\,out}}=\left(99.5\pm
0.3\right)\%$ high power emission was not expected. We see a linear increase
in output power starting at the threshold of $P_{\mathrm{th}}=1.63$ W.

(a)

(b)

Figure 6: (a) Distribution of incident pump power $P_{\mathrm{pump}}$ to
reflected, transmitted, and absorbed power. (inset) Power transfer measurement
under DSP condition to determine the spectral shift $\Delta\lambda$ per
absorbed pump power $P_{\mathrm{abs}}$. (b) The power depending spectral shift
plotted here was determined during the power transfer measurement plotted in
the inset in Fig. 6a.

A little kink in the linear behaviour can be seen at about eleven watts of
absorbed pump power $P_{\mathrm{abs}}$. This might origin from a polarization
jump, as no polarization maintaining element like a birefringent filter is
implemented - the laser is operating freely. Besides the change in the slope
efficiency, no change or deviation from the typical linear behaviour in the
spectral shift (see Fig. 6b) is visible. The setup configuration for this
measurement was the same as depicted in Fig. 5. No birefringent filter or any
other optical intra cavity element was used for the power transfer
measurement. The pump power was increased symmetrically in the double-side
configuration. Finally, the thermal shift of the laser emission
$\Delta\lambda$/$\Delta T_{\mathrm{hs}}$ was measured under constant incident
pumping of $P_{\mathrm{pump}}$ = 6 W (3 W from each side). For this
measurement the heat sink temperature $T_{\mathrm{hs}}$ was changed gradually
from 5∘C to 23∘C by changing the water/glycol cooling temperature. The
temperature difference between chiller and heat sink was determined to be in
the range of 0.1∘C and is therefore negligible. The thermal wavelength shift
of $\Delta\lambda$/$\Delta T_{\mathrm{hs}}=\left(0.24\pm 0.01\right)$ nm/K was
determined via a linear fit to the measured data which are plotted in Fig. 6b.
This allows to calculate the thermal resistance $R_{\mathrm{th}}$ of the
MECSEL gain element including error propagation after [28] by dividing the
spectral shift per $P_{\mathrm{diss}}$ by the spectral shift per change of
$T_{\mathrm{hs}}$ to $R_{\mathrm{th}}=\left(3.75\pm 0.28\right)$ K/W.
Considering the use of SiC heat spreaders with a thickness of $\sim$ 350 µm
each, this result meets the expected range [24] and is similar to a previous
result in Phung et al.[29].

###  III.2 Broadband wavelength tuning

For tuning measurements an incident pump power of $P_{\mathrm{pump}}$ = 18.0 W
was chosen, because our previous experiments revealed the largest tuning range
under SSP at this pump power. Higher pump powers lead to a smaller tuning
range and worse absolute output power values due to occurring thermal rollover
under this SSP condition. The outer heat spreader facets of the gain element
were anti-reflection coated for the wavelengths of the laser as well as the
pump to reduce cavity losses. The spectra were recorded with an ANDO AQ-6315A™
optical spectrum analyzer (OSA) with 0.05 nm resolution. The results of SSP
measurements are shown in Figs. 7a and 7b. As described in section “Design and
structure simulation”, we can see a slightly better performance in both output
power and tuning range for the SSP case from the right side (Fig. 7b) due to a
beneficial barrier width distribution (similar to the structures described for
example by Phung et al. [30] and Baumgärtner et al.[31]). Additionally, the
output power did not drop below 50 % of $P_{\mathrm{max}}$ between the
performance peaks of the two different QW kinds and a FWHM of 70 nm was
reached maintaining an output power of $>$ 125 mW. In the long wavelength
range around 1005 nm the output power is similar to the short wavelength
around 960 nm. For the left side pumping (Fig. 7a) the long wavelength area is
about 25 % inferior in terms of output power. Although this comparison was not
specifically intended, it clearly shows that the QW and barrier width
distribution should be matched with the absorption behaviour of the applied
pump light in future structure designs.

(a)

(b)

Figure 7:  A set of exemplary spectra (orange $\rightarrow$ red curves) to
depict wavelength tuning are shown in these two plots. The intensity of the
spectra was normalized to the correspondingly measured output power, which was
plotted over wavelength. 18 W of pump power were irradiated (a) only from the
left, and (b) only from the right side.

In order to obtain a maximum full tuning range, the total incident pump power
$P_{\mathrm{pump}}$ was increased to 24 W and divided equally to 12 W per side
for the case of DSP. Beforehand experiments (see Fig. 6a) revealed best output
power performance and the largest tuning range under DSP condition at this
pump power configuration. As can be seen in Fig. 8 a wider tuning range of
86.2 nm or 26.5 THz was achieved compared to the single-side pumping
measurements, where $\sim$ 80 nm were reached. However, since the peak output
powers received at the 965 nm and 1015 nm rose significantly when applying
DSP, the FWHM tuning range is no longer as wide, as the output power in the
middle of the spectrum is just under half of the peak power. If, however, we
look at the tuning range at which 125 mW of output power (which was roughly
the FWHM output power level in the SSP case) is maintained, we have an
outstanding tuning range of 80 nm or 25 THz reaching from 940 nm to 1020 nm.

Fig. 9c$\downarrow$

Fig. 9b

$\downarrow$Fig. 9a

$\downarrow$

Figure 8:  A set of exemplary spectra (orange $\rightarrow$ red curves) to
depict wavelength tuning are shown in this plot. The intensity of the spectra
was normalized to the correspondingly measured output power, which was plotted
over wavelength. Pump power was applied under symmetric DSP condition. The PL
(grey curve with filled area underneath) of the corresponding broadband gain
structure from Fig. 4 was plotted for comparison with the tuning behaviour of
the MECSEL.

The PL curve of the corresponding broadband gain structure from Fig. 4 was
plotted again in Fig. 8 for comparison with the laser tuning. It is further
noteworthy how accurately the MECSEL tuning behaviour follows the PL
characteristic in a relative intensity comparison taking the spectral red
shift and typical inverted parabolic shape of the corresponding QWs’ gain into
account.

###  III.3 Beam profiles

It is a built-in feature of MECSELs to possess the same excellent beam quality
properties (M2 < 1.1 and a TEM00 Gaussian transverse mode profile [10, 29]) as
VECSELs[32, 33]. This is enabled by satisfying the geometrical condition of
having the squared gain region thickness $L^{2}$ much smaller than the laser
mode area $s$ ($L^{2}$ $\ll$ $s$) inside the laser cavity[34]. In this work
the relation is 1 : 12 272 with $L^{2}$ = 4 µm2 and $s$ = 49 087 µm2. The
external mirrors provide full mode control as the gain region’s impact on beam
distortion is present[29] but minimal, compared to classical solid-state
lasers. Therefore, the behaviour of the transverse mode shape during the
tuning measurements is an important characteristic to prove undisturbed
operation of the laser. Beam profiles were recorded every second time a
spectrum was taken.

(a)(b)(c)

Figure 9:  Beam profiles taken during the tuning measurement plotted in Fig.
8. Three examples have been chosen at (a) low ($\sim$ 945 nm), (b) medium
($\sim$ 995 nm), and (c) high ($\sim$ 1015 nm) output power.

In Fig. 9 three exemplary beam profiles for low, medium, and high output power
were plotted. In the plot of Fig. 8 it is indicated to which spectrum the
plotted beam profiles of Fig. 9 corresponds. Small deformation of the beam
profile can be seen in Figs. 9b and 9c, but the fundamental Gaussian TEM00
character remains intact. During beam profile recording and taking spectra
with the OSA, the laser cavity was not realigned and all parameters were kept
constant, solely the birefringent filter was rotated. This shows that the
excellent beam properties typical to MECSELs were maintained with the novel
multi-type QW design.

##  IV Conclusions

The widest tuning of a vertically emitting semiconductor laser, 26.5 THz (86.2
nm), emitting around 1 µm, was shown. Of greater importance is the FWHM tuning
of 70 nm (and 80 nm tuning range with steady 125 mW of output power), which is
effectively double to previously reported results in the 1 µm wavelength
range. This result shows the tremendous capability of MECSELs utilizing
multiple types of QWs for significantly increasing the tuning range of
vertically emitting semiconductor lasers, while maintaining the typical
excellent beam quality (M2 < 1.1[10]) and high power (> 125 mW) throughout the
whole tuning range. In our design process four relevant strategy points were
recognized: 1) separation of different types of QW areas inside the active
region from each with EBLs to maintain sufficient carrier concentration in all
QWs, 2) ensuring maximum overlap with the standing wave of the optical field
on each of the QW types on their designed emission wavelengths, while
minimizing the overlap with other QW types, if they are capable of absorbing
the photons emitted, 3) choosing correct QW materials and particularly the
nominal emission wavelength between them, 4) designing the structure as a
whole so that each of the QW types are pumped equally.

This paper has only scratched the surface and shown the applicability when it
comes to the potential of multi-type QW MECSELs. The advances that can be made
from this point onwards, are two-fold. First of all, more QW types could be
utilized to either increase the emission power in the middle of the tuning
range if more power throughout the tuning range is needed, or to widen the
tuning range by choosing three or more QW types accordingly. Secondly, and
more importantly, the total thickness of the MECSEL could be at least doubled
to truly take advantage of the MECSELs’ double-side pumping capabilities, when
we keep in mind that the structure shown in this paper could still be properly
pumped from one side only. The doubling of the total thickness allows for much
more QW groups to be utilized, which in turn can then be turned into either
higher emission power or wider tuning range. In the DSP pumping regime, two
pump lasers operating on different wavelengths could also be used to cover a
wider pumping wavelength range and avoid absorption in unwanted regions of the
active area. Finally, stacking of more than one MECSEL with additional heat
spreaders in-between instead of complete monolithic structures is a great
option for future widely tunable multi-type QW MECSELs.

###### Acknowledgements.

The authors would like to thank Antti Härkönen for the initial, passionate
scientific discussions on this project, and gratefully acknowledge the Academy
of Finland (No. 326455), the Academy of Finland PREIN Flagship Programme (No.
320165), the Horizon 2020 Marie Skłodowska-Curie Actions - NetLaS (No.
860807), the Magnus Ehrnrooth Foundation, and the Finnish Foundation for
Technology Promotion for the financial support.

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