Picosecond pulses from a mid-infrared interband cascade laser

Johannes Hillbrand1,∗, Maximilian Beiser1, Aaron Maxwell Andrews1,2, Hermann
Detz1,3, Robert Weih4, Anne Schade5, Sven Höfling5,6, Gottfried Strasser1,2,
Benedikt Schwarz1,+

1Institute of Solid State Electronics, TU Wien, Gusshausstraße 25-25a, 1040
Vienna, Austria
2Center for Micro- and Nanostructures, TU Wien, Gusshausstraße 25-25a, 1040
Vienna, Austria
3CEITEC, Brno University of Technology, Brno, Czech Republic
4Nanoplus Nanosystems and Technologies GmbH, 97218 Gerbrunn, Germany
5Technische Physik, Physikalisches Institut, University Würzburg, Würzburg,
Germany
6SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews,
KY16 9SS, United Kingdom
∗e-mail<EMAIL_ADDRESS>
+e-mail<EMAIL_ADDRESS>
The generation of mid-infrared pulses in monolithic and electrically pumped
devices is of great interest for mobile spectroscopic instruments. The gain
dynamics of interband cascade lasers (ICL) are promising for mode-locked
operation at low threshold currents. Here, we present conclusive evidence for
the generation of picosecond pulses in ICLs via active mode-locking. At small
modulation power, the ICL operates in a linearly chirped frequency comb regime
characterized by strong frequency modulation. Upon increasing the modulation
amplitude, the chirp decreases until broad pulses are formed. Careful tuning
of the modulation frequency minimizes the remaining chirp and leads to the
generation of 3.2 ps pulses.

Optical frequency combs (OFC) operating in the mid-infrared (MIR) spectral
range are a powerful spectroscopic tool 1. Measuring the molecular fingerprint
in the MIR region allows to identify chemical species and to determine their
concentration. MIR frequency comb sources based on the non-linear conversion
of near-infrared mode-locked lasers 2, 3 and microresonators 4 have reached a
high level of maturity featuring octave spanning spectra 5. Semiconductor
laser OFCs 6 are advantageous in applications requiring compactness and low
power consumption. Quantum cascade laser (QCL) frequency combs 7, 8 are among
the most investigated technologies. However, gain bandwidth and dispersion 9,
10 limit their spectral bandwidth on the order of 100 cm-1. One way to
overcome this issue is spectral broadening in an external non-linear fiber or
waveguide. Recent results 11 revealed, however, that the temporal output of
QCLs is strongly chirped accompanied by the suppression of amplitude
modulation. In fact, the ultrafast gain dynamics of QCLs are believed to be
highly unfavorable for the formation of light pulses 12. Previous attempts of
mode-locking in monolithic QCLs were limited to cryogenic temperatures and low
peak powers 13. This makes non-linear techniques for spectral broadening very
inefficient. First results to enter the mid-infrared from shorter wavelengths
were demonstrated using passively mode-locked GaSb-based type-I cascade diode
lasers with 10 ps pulse duration around 3.25 µm.

Figure 1: a: Scanning electron microscope picture of the modulation section of
the ICL. The modulation section (left) and a ground contact (right) are
optimized for RF injection via RF tips. b: Light-Current-Voltage (L-I-V)
characteristics of the ICL at room temperature for a homogeneously biased
laser (blue line) as well as for 2.5 V absorber bias with (dotted red line)
and without (solid red line) RF modulation at $f_{rep}\approx 10.15\,$GHz. The
RF power is 31 dBm.

Type-II interband cascade lasers (ICL) are an interesting alternative that
already cover a major part of the mid-infrared up to
6$\,\mathrm{\SIUnitSymbolMicro}\mathrm{m}$ 14, 15, 16. They combine the
carrier injection and extraction scheme of QCLs with the advantages of an
interband lasing transition. Hence, the upper-state lifetime of the optical
transition in ICLs is significantly longer than the cavity round-trip time.
This enables low dissipation operation and has important consequences for
mode-locking of ICLs. While the short upper-state lifetime of QCLs prevents
the formation of short pulses, this issue is not present in ICLs. Furthermore,
the ICL active material can be switched to absorption at the laser wavelength,
which was shown by using them as photodetector at zero-bias 17. Together with
the fast carrier injection scheme, this allows the realization of efficient
high-speed modulators with cut-off frequencies of several gigahertz 18. Hence,
ICLs exhibit all required properties for efficient active mode-locking via
modulation of the gain at the cavity round-trip frequency $f_{rep}$ 19. Recent
efforts aimed at the generation of OFCs via passive mode-locking of ICLs.
However, the experimental results did not show the formation of pulses 20.
Instead, such passive ICL frequency combs are characterized by a continuous
output intensity with a strong frequency modulation 18, similarly to what was
found in QCLs 8, 11.

In this letter, we report on the generation of picosecond pulses in two-
section Fabry-Pérot ICLs. The dry etched laser ridges are
$6\,\mathrm{\SIUnitSymbolMicro}\mathrm{m}$ wide and split into a
$3520\,\mathrm{\SIUnitSymbolMicro}\mathrm{m}$ long gain section and a
$480\,\mathrm{\SIUnitSymbolMicro}\mathrm{m}$ long modulation section (Fig.
1a). The modulation section was designed to minimize parasitic capacitance and
allow efficient RF injection via coplanar RF tips. A
$1.5\,\mathrm{\SIUnitSymbolMicro}\mathrm{m}$ thick Si3N4 passivation layer was
used for the modulation section while keeping its top contact area as small as
possible. The passivation layer of the gain section is thinner
($250\,\mathrm{n}\mathrm{m}$) in order to improve the thermal performance of
the laser. The back facet of the device was high-reflection coated using Si3N4
and gold, while the front facet was left uncoated. The active region is
comprised of 6 stages and operates at
3.85$\,\mathrm{\SIUnitSymbolMicro}\mathrm{m}$ (2600 cm-1). At room
temperature, the ICL emits up to $4.2\,\mathrm{m}\mathrm{W}$ of optical power
in continuous wave operation when both sections are biased homogeneously (Fig.
1b). When the bias of the modulation section is set to 2.5 V additional loss
is added to the cavity causing the threshold current density to increase and
the maximum output power decreases to $1.9\,\mathrm{m}\mathrm{W}$. The
injection of an RF signal at $f_{rep}$ into the modulation section reduces the
threshold by about 20%, showing that the laser is strongly influenced by the
active modulation.

Figure 2: a: SWIFTS characterization of the ICL at -2 dBm injected RF power.
The gain section is operated at 770 A/cm2 and the modulation section at 2.5 V.
blue line: intensity spectrum. red line: SWIFTS spectrum. blue dots: SWIFTS
amplitudes expected for full phase-coherence of the ICL comb. green dots:
intermodal difference phases of adjancent comb lines. The right part shows a
zoom-in on the center burst of the intensity and SWIFTS interferograms. b:
reconstructed intensity and instantaneous wavenumber of the ICL frequency
comb.

The characterization of the temporal output intensity of mid-infrared
semiconductor lasers is challenging. Due to the high repetition rate and the
relatively low average power, the peak power is expected to be too low for
established non-linear pulse characterization techniques 21. Instead, we
employ a linear phase-sensitive autocorrelation technique called ’SWIFTS’ 22.
This method uses a Fourier transform infrared (FTIR) spectrometer and a fast
quantum well infrared photodetector (QWIP) to measure the amplitudes and
phases of the beatings between adjacent laser modes (details can be found in
Ref. 22). In this way, SWIFTS allows the reconstruction of both the temporal
intensity and instantaneous frequency of the ICL frequency comb. This method
is not restricted to mode-locked operation and is valid for arbitrary periodic
signals. Furthermore, it should be noted that recent experiments with a
passively mode-locked quantum dot laser proved that SWIFTS and conventional
intensity autocorrelation in a non-linear crystal are able to retrieve the
same pulse width 23. At 2.5 V bias of the modulation section, the laser
generates a narrow beatnote at the cavity round-trip frequency, which can be
extracted directly from the laser current. This beatnote results from the
beating of adjacent cavity modes. Its narrow linewidth on the kHz level
indicates that the cavity modes are phase-locked. In order to provide a stable
reference for SWIFTS, we inject a weak RF signal at -2 dBm into the modulation
section. Previous experiments showed that such a weak modulation is able to
lock the frequency of the beatnote and leaves the spectral phases of the free-
running OFC unchanged 24. The SWIFTS analysis of the ICL in this state is
displayed in Fig. 2a. The intensity spectrum spans over roughly 28 cm-1 and
consists of several lobes. The SWIFTS spectrum is commensurate with the values
expected for full phase-coherence (blue dots in Fig. 2a) over the entire span
of the spectrum, which proves frequency comb operation. The intermodal
difference phases $\Delta\phi$ retrieved from the SWIFTS data decrease
linearly over a range of exactly 2$\pi$. This particular frequency comb state
was found in QCLs operating at 8$\,\mathrm{\SIUnitSymbolMicro}\mathrm{m}$ 11,
ICLs at 4$\,\mathrm{\SIUnitSymbolMicro}\mathrm{m}$ 18 and quantum dot lasers
at 1.25$\,\mathrm{\SIUnitSymbolMicro}\mathrm{m}$ 23 and appears to be
universal in semiconductor laser OFCs. Recent theoretical work attributes its
origin to the interplay of dispersion and the Kerr effect in lasers with
spatial hole burning 25. Both SWIFTS interferograms (Fig. 2a right) have a
local minimum at zero-path difference, which indicates the suppression of
amplitude modulation 8. Indeed, the reconstructed intensity (Fig. 2b top) does
not show isolated pulses. In contrast, the instantaneous wavenumber is
strongly modulated and linearly chirps through the entire spectrum within a
cavity round-trip period.

Figure 3: SWIFTS analysis of the ICL frequency comb for 6 dBm (a), 18 dBm (c),
30 dBm (e) and 36 dBm (g) injected power. The wavenumber in the top right
corner indicates the bandwidth of the spectrum. The gain section was operated
at 800 A/cm2 and the modulation section bias was 2.5 V. The GDD is deduced
from the slope of the intermodal difference phases (red dotted line). b,d,f,h:
The corresponding reconstructed temporal intensity and instantaneous
wavenumber. The shaded grey area indicates the fraction of phase range and of
the cavity round-trip period, respectively, which is occupied by the pulses.

In the following, we will investigate the influence of an increased modulation
strength on the ICL frequency comb dynamics. Fig. 3 shows the detailed SWIFTS
analysis of the ICL for injection power levels from 6 dBm to 36 dBm. The
modulation frequency is altered slightly around $f_{rep}$ to account for a
small detuning of the laser round-trip frequency with temperature due to the
high injection power. At 6 dBm (Fig. 3a), the ICL still operates in a similar
OFC state as in Fig. 2a and the reconstructed time signal (Fig. 3b) does not
show isolated pulses. When the injected power is further increased to 18 dBm
(Fig. 3c), the spectral bandwidth grows to 33 cm-1 and the multiple lobes of
the spectrum start to disappear. Interestingly, the SWIFTS spectrum shows that
the ICL is not fully phase-locked in this transition state to the actively
mode-locked regime. The intermodal difference phases still decrease linearly,
but only cover the range of 0.53$\cdot 2\pi$. Since $\Delta\phi$ is directly
proportional to the group delay with 2$\pi$ corresponding to one cavity round-
trip, this means that the ICL emits pulses with roughly 0.53$\cdot
2\pi$/2$\pi\approx$53% duty cycle. Indeed, the reconstructed time signal (Fig.
3d) shows broad and linearly chirped pulses. At 30 dBm, the spectrum consists
of a single lobe spanning over 36 cm-1 (Fig. 3e). The slope of $\Delta\phi$
decreases, which corresponds to a decrease of the group delay dispersion (GDD)
to -5.4 ps2 at 30 dBm compared to -19 ps2 at 6 dBm. The reconstructed time
signal (Fig. 3f) shows a train of isolated pulses with 27 ps full width at
half maximum (FWHM). When the injected power is increased by another 6 dB,
only a minor change in the shape of the spectrum and the GDD is observed (Fig.
3g).

Figure 4: a: SWIFTS characterization for 920 A/cm2 gain section current
density and 3 V modulation section bias. The modulation power is 32 dBm and
the modulation frequency is roughly 15 MHz higher than $f_{rep}$. In contrast
to the weakly modulated ICL (Fig. 2a), both SWIFTS interferograms now have
local maxima at zero delay of the FTIR mirrors. This is an indicator for
strong amplitude modulation of the laser intensity. b: Reconstructed time
domain signal. The inset shows a zoom on a single pulse, revealing a FWHM of
3.2 ps.

Detuning the modulation frequency away from the round-trip frequency $f_{rep}$
of the free-running laser is another knob to influence the temporal output of
the laser. Fig. 4a shows the SWIFTS characterization at 32 dBm injected power,
while the modulation frequency is 15 MHz higher than $f_{rep}$. The spectrum
consists of a single lobe spanning over 45 $cm^{-1}$ and is fully coherent.
Indeed, the intermodal difference phases do not show a linear chirp as in Figs
3a-h and occupy the phase range of only 0.13$\cdot 2\pi$. The reconstructed
time signal (Fig. 4b) displays much shorter pulses than in Fig. 3g. A zoom on
a single pulse reveals a FWHM of 3.2 ps. The peak power is 114 mW, which is an
enhancement of more than 40 with respect to the average power of 2.7 mW and
proves that the energy can be efficiently stored by the gain medium over a
round-trip. The inset in Fig. 4b shows that the pulse is not transformation-
limited with several smaller pulses arriving shortly after the first intense
pulse. This is because the intermodal difference phases in Fig. 4a do not
synchronize perfectly. In conclusion, our results show that ICLs provide all
features necessary to generate MIR pulses within a single semiconductor laser
ridge. The free-running or weakly modulated ICL frequency comb operates in a
regime characterized by a strong linear chirp and suppression of amplitude
modulation, which was also found in QCLs and quantum dot lasers. Upon
increasing the modulation amplitude, the laser spectrum broadens accompanied
by the formation of broad and chirped pulses. By carefully tuning the
modulation frequency away from the round-trip frequency of the free-running
laser, the pulse duration decreases to 3.2 ps with a peak power enhancement of
over 40. This illustrates the large potential of ICLs as a compact source for
mid-infrared pulses. From the spectral bandwidth one can assume that further
optimization of the laser dispersion, modulation section length or adding
additional segments will enable the generation of subpicosecond pulses 26.

This work was supported by the Austrian Science Fund (FWF) within the projects
”NanoPlas” (P28914-N27), ”Building Solids for Function” (Project W1243),
”NextLite” (F4909-N23), as well as by the Austrian Research Promotion Agency
through the ERA-Net Photonic Sensing program, project ”ATMO-SENSE” (FFG:
861581). J.H. was supported by the ’Hochschuljubiläumsstiftung’ of the city of
Vienna. H.D. was supported by the ESF project CZ.02.2.69/0.0/
0.0/16_027/0008371. A.M.A was supported by the projects COMTERA - FFG 849614
and AFOSR FA9550-17-1-0340.

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