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Widely tunable telecom MEMS-VCSEL for
terahertz photomixing
MOHAMMAD TANVIR HAIDAR,1,*SASCHA PREU,2SUJOY PAUL,1CHRISTIAN GIERL,1JULIJAN CESAR,1
ALI EMSIA,1AND FRANKO KÜPPERS1
1Institute for Microwave Engineering and Photonics, Technische Universität Darmstadt, Merckstr. 25, 64283 Darmstadt, Germany
2Terahertz Systems Technology, Department of Electrical Engineering and Information Technology, Technische Universität Darmstadt,
Merckstr. 25, 64283 Darmstadt, Germany
*Corresponding author: haidar@imp.tu‑darmstadt.de
Received 8 June 2015; revised 2 August 2015; accepted 31 August 2015; posted 1 September 2015 (Doc. ID 242294); published 23 September 2015
We report frequency-tunable terahertz (THz) generation
with a photomixer driven by an ultra-broadband tunable
micro-electro-mechanical system vertical-cavity surface-
emitting laser (MEMS-VCSEL) and a fixed-wavelength
VCSEL, as well as a tunable MEMS-VCSEL mixed with a
distributed feedback (DFB) diode. A total frequency span of
3.4 THz is covered in direct detection mode and 3.23 THz
in the homodyne mode. The tuning range is solely limited
by the dynamic range of the photomixers and the Schottky
diode/photoconductor used in the experiment. © 2015
Optical Society of America
OCIS codes: (140.7260) Vertical cavity surface emitting lasers;
(040.2840) Heterodyne; (040.2235) Far infrared or terahertz.
http://dx.doi.org/10.1364/OL.40.004428
For various applications, such as security, information and com-
munication technology, and earth and space science, wide tun-
ability of continuous-wave (CW) terahertz (THz) systems is
required [1,2]. Photomixing of two CW heterodyned laser
beams is frequently used for such applications [3,4]. For THz
generation, at the front end, an ultra-fast photodiode down-
converts the optical beat note between two lasers at frequencies
ν1and ν2ν1νTHz into an AC current oscillating at νTHz
[5]. This AC current is then radiated by an appropriately de-
signed antenna. The unprecedented advantage of photomixing
systems is their inherently large tuning range: at 1550 nm
(850 nm), one THz corresponds to a detuning of the two lasers
by only about 8 nm (2.4 nm) [i.e., by 0.52% (0.28%)], achiev-
able with many laser concepts. The large availability of optical
components around 1550 nm facilitates inexpensive generation
of THz compared with established 850 nm systems. Telecom-
wavelength-compatible THz photomixers [3,6,7] represent an
ideal optical-to-radio frequency (RF) converter for this purpose.
In order to fully exploit the frequency coverage of the photo-
mixers, lasers with large tuning ranges of at least 3 THz,
potentially 5 THz, are required [3]. The most common, com-
pact choices are distributed-feedback (DFB) laser diodes.
However, telecom DFB diodes are typically tunable by not more
than 0.6 THz, requiring at least three diodes to cover a frequency
span of 3 THz [8]. Larger spans would require even more diodes.
Further, the system has to be reconfigured when another set of
diode pairs is used, aggravating performing a large frequency
scan. An alternative is grating tuned sources [9]. Due to the re-
quirement of an external grating, these lasers are fairly bulky,
with little potential for chip-sized integration.
We reported for the first time, to our best knowledge,
vertical-cavity surface-emitting laser (VCSEL)-based wide
tunable CW THz-signal generation with photomixer up to
690 GHz in which the large tunability was achieved by a micro-
electro-mechanical system (MEMS)-VCSEL [10]. In this
Letter, we report VCSEL-based ultra-wideband tunable THz-
signal generation with a photomixer from −1.67 to 1.73 THz
(a total frequency span of 3.4 THz). A second homodyne setup
based on the MEMS-VCSEL and a DFB diode covers a fre-
quency span of 3.23 THz. In contrast to the conventional edge-
emitting lasers (EELs), VCSELs have vertical cavities and emit
light perpendicular to the wafer surface. Its few-micrometers
short optical cavity attains a large free spectral range (FSR),
which in turn enables single-mode emission. A large mode-hop
free tuning range is engineered by incorporating a MEMS
movable distributed-Bragg-reflector (DBR) mirror on top of
the half-VCSEL structure. Therefore, a MEMS-VCSEL-based
photonic system can cover the whole THz range with a single
pair of compact, chip-sized semiconductor lasers.
The simplified cross-sectional schematic of the MEMS-
VCSEL is given in Fig. 1. The wavelength-tunable MEMS-
VCSEL is comprised of two main functioning parts. The first
part is a half-VCSEL structure containing a fixed-plane bottom-
DBR mirror, the active gain medium (quantum wells), and a
current confining buried-tunnel junction (BTJ). The active
VCSEL structure is encapsulated with a low dielectric-constant
material benzocyclobutene (BCB). BCB contributes in reducing
device parasitic effects, thus making MEMS-VCSEL suitable for
high-speed applications. The second part consists of a MEMS
movable DBR mirror, which is deposited via low-temperature,
plasma-enhanced chemical vapor deposition (PECVD). The
mirror consists of 11.5 dielectric SiNx/SiOx layer pairs. This
mirror, at first, is deposited on top of the half-VCSEL structure
4428 Vol. 40, No. 19 / October 1 2015 / Optics Letters Letter
0146-9592/15/194428-04$15/0$15.00 © 2015 Optical Society of America
using a Ni-sacrificial layer. Later, the sacrificial layer is etched
away and the intrinsic stress gradient implemented in the re-
leased DBR pushes it upward. It results in a concave DBR struc-
ture that, along with the half-VCSEL, incorporates a tunable air
gap. The BTJ confines the current flow through the quantum
wells in order to maximize the overlap of the electrical and the
optical fields. Two heat- and current-spreading layers embed the
quantum wells and the BTJ for a homogeneous electrical pump-
ing of the active region (minimal current crowding), as well as
for optimal heat transport from the quantum wells to the sub-
strate. The electro-thermal actuation is enabled by the top elec-
trode evaporated on the DBR. A small circular opening in the
electrode positioned around the center of the MEMS-DBR cou-
ples out the VCSEL light. For a detailed description of device
structure and fabrication, refer to [11].
Wavelength tuning is achieved by electro-thermal actuation
of the MEMS-DBR. A driving current, Itune, is applied through
the electrode of the MEMS-DBR, causing heat, Pheat, which
thermally expands the DBR. The thermal expansion modulates
the air gap, Lair, resulting in tuning of the resonator length and,
respectively, the resonance wavelength, λ,byΔλ∝ΔLair. Since
the heating power is Pheat I2
tune ·RMEMS, where RMEMS is the
electrical resistance of the heating electrode on top of the
MEMS-DBR, the change in emission wavelength is Δλ∝
ΔLair ∝Pheat ∝I2
tune. The optical emission spectra of the tun-
able VCSEL, used for the THz experiment, have been measured
for different thermal heating powers as illustrated in Fig. 2. The
tuning range is given as the black envelope, obtained by the peak
and hold option of the optical spectrum analyzer (OSA) of the
fundamental laser peak while tuning. A mode-hop free CW-
tuning of 70 nm (corresponding to 8.8 THz) from λ
1510 nm to λ1580 nm is demonstrated at a device temper-
ature of 25°C. In particular, the envelope of the peak emission
power of the VCSEL does not vary considerably in the wave-
length range of 1530–1565 nm. This wavelength range is of
particular interest due to the fact that erbium-doped-fiber-
amplifier (EDFA) can be used in this region. Several single emis-
sion spectra at different heating powers (induced by Itune) are
shown for better identification of the tuning characteristics of
the MEMS-VCSEL. An even larger tunability of 102 nm (ap-
prox. 12.5 THz) of a nonhigh-speed MEMS-VCSEL, not used
in this THz experiment, has already been reported [12].
It is therefore possible to cover the whole THz frequency range
with one tunable VCSEL and a fixed-wavelength laser. The
highly single-mode emission characteristics of the MEMS-
VCSEL is observed with a side-mode suppression-ratio (SMSR)
greater than 55 dB (with respect to the higher-order transverse
mode) throughout the operation. This is particularly advanta-
geous for THz photomixing. A further key feature of lasers, not
only for THz applications, is the linewidth. We measure the
MEMS-VCSEL linewidth using the delayed self-heterodyne
technique (DSH). The DSH beat-signals (captured by an elec-
trical spectrum analyzer) at varying VCSEL bias currents (Itune is
set to 0 mA) have Voigt profiles. The linewidth of the MEMS-
VCSEL can be estimated from the full-width at half-maximum
(FWHM) of the Voigt-fitted beat-signal data. Because of the
autocorrelation technique from DSH, linewidths at different
bias currents are then calculated as FWHM∕
ffiffiffi
2
p[13]. The re-
sulting MEMS-VCSEL linewidth versus inverse optical power is
presented in Fig. 3. A minimum linewidth of 46 MHz is
calculated at 26 mA VCSEL bias current. The intrinsic MEMS-
VCSEL linewidth of 21 MHz at the same bias current is calcu-
lated from the Lorentzian part of the Voigt-fitted beat-signal
data. Therefore, the Gaussian linewidth broadening most prob-
ably results from Brownian motion of the MEMS-DBR.
As the MEMS-DBR is suspended over the half-VCSEL
structure, it is not immune to external disturbances such as vi-
brations. As a result, wavelength fluctuations can be observed,
necessitating stabilization techniques. We developed a control
circuit that stabilizes the wavelength of the MEMS-VCSEL.
The heart of the control circuit is a commercially available tele-
com wavelength-locker (WL) module. A fraction of the total la-
ser power is given as input to the WL. Inside the WL, the input
optical power is split into two branches. One is fed to an etalon,
followed by a p-i-n photodiode (PD). Fabry–Perot oscillations
in the etalon lead to frequency-periodic oscillations of the trans-
mission with a FSR of 0.8 nm or 100 GHz for this case. The
corresponding PD current, Ietalon, is then a strong function of
the wavelength of the laser. The other branch directly goes to
a reference p-i-n PD, measuring a reference current, Iref ,inorder
Fig. 1. Cross-sectional schematic of MEMS-VCSEL.
Fig. 2. Emission spectra for different tuning currents (electro-
thermal tuning). A single emission spectrum of the VCSEL lasing
at 1510 nm is continuously tuned up to 1580 nm. The corresponding
peak emission envelope (black) defines the tuning range of 70 nm,
while the FSR is 80 nm.
Letter Vol. 40, No. 19 / October 1 2015 / Optics Letters 4429
to monitor input power fluctuations. The wavelengthis locked to
the rising edge of one peak of the etalon’s transmission function
where Ietalon Iref . The control circuit generates a normalized
error/feedback voltage after a logarithmic transimpedance ampli-
fier as Verr 1V·logIetalon∕Iref . This feedback signal is
added to the main tuning current of the MEMS-DBR. Any fluc-
tuation of the wavelength of the VCSEL gives rise to counter-
acting Verr, nulling the fluctuations. In order to alter the
locking wavelength, the tuning current is increased without
any feedback being added. A circuitry is incorporated that can
sense the oncoming lock points by evaluating the periodic Verr.
Once a lock point is detected while tuning, only then Verr is
added so that the MEMS-VCSEL wavelength is stabilized.
Depending on the FSR of the etalon, sweeps and locking in steps
of 0.8 nm are enabled for this tuning and locking circuit. The
whole locking and tuning are accomplished automatically using
a microcontroller-enabled stepping from one lock point to an-
other. In Fig. 4, 28 nm tuning from λ1534.92 nm to λ
1562.17 nm, as used in the experiment, is illustrated. Higher
tuning resolution can be achieved by using a WL with lower
FSR. We observe wavelength drifts of 20 pm (2.5 GHz)
during 6 h of operation of the MEMS-VCSEL. Temperature-un-
stabilized WL-module causes these drifts. Other than that, the
low-frequency movements of the MEMS-DBR (thermal/
mechanical influences) are very well compensated by the WL-
circuit. Very fast and high-frequency mechanical movements
in the range of 500 MHz, however, cannot be compensated
by the current WL circuit. We, therefore, do not exclude the
possibilities of frequency perturbations of the MEMS-VCSEL
during THz-measurement. An improved WL circuit that covers
the mechanical bandwidth of the MEMS-DBR movements can
ensure the wavelength stability to subpicometer accuracy under
all conditions. A packaged MEMS-VCSEL, in contrast to its on-
wafer counterpart, would show much better natural stability;
however, one still needs a fast WL circuit to ensure robust
operation during sensitive measurements.
Figure 5shows the experimental photomixing setup. The
MEMS-VCSEL is biased and temperature stabilized at
26 mA and 22°C, respectively. The maximum power coupled
to the fiber at this bias current is 0.66 mW. First, 1% power
of the on-wafer MEMS-VCSEL is tapped out for WL control
and sweep circuit. Because we require sufficient power for THz-
photomixing, the VCSELs signals are amplified by two separate
amplifiers. An EDFA (ATx Telecom Systems, Model: 1.5-AMP)
and a semiconductor optical amplifier (THORLABS, Model:
SOA 1117) are used after the MEMS-VCSEL and fixed-
wavelength VCSEL, respectively. We use tunable band-pass
filters (TBF) after each amplifier to remove the amplified spon-
taneous emission (ASE) noise to get a clean THz signal. The
TBF used after the fixed-wavelength VCSEL is positioned at its
emission wavelength. An appropriate low-cost fixed-wavelength
filter can be used alternatively. The 3 dB bandwidths of both of
the filters are 200 pm (25 GHz) during the experiment. As the
first-stage amplifiers and TBFs are not polarization maintaining
(PM), two polarization controllers are implemented prior to the
PM-photomixing setup. After filtering, the VCSEL outputs are
combined by a 50/50 coupler and further amplified by a PM-
EDFA (PriTel, Model: PMFA-35-S-IO) to reach an output
power of 22 mW each. Finally, the combined output signal
pumps a p-i-n photomixer (WIN-PD from Heinrich Hertz
Institute/TOPTICA Photonics) for THz generation. For the
homodyne measurement, the fixed-wavelength VCSEL, sub-
sequent amplifier, and TBF are replaced by a DFB laser as illus-
trated in Fig. 5. On the detection side, either a zero-bias Schottky
Fig. 3. MEMS-VCSEL linewidth over the inverse optical power.
Power-current (L–I) curve of the VCSEL (inset).
Fig. 4. 0.8 nm spaced electro-thermal tuning by WL circuit from
λ1534.92 nm up to λ1562.17 nm. This corresponds to a dif-
ference-frequency tuning from −1.67 to 1.73 THz with respect to
the fixed-wavelength VCSEL at λ1548.18 nm.
Fig. 5. (I) Tunable THz photomixing setup using a MEMS-
VCSEL and a fixed-wavelength VCSEL for direct power detection.
(II) Photomixing of the MEMS-VCSEL with a DFB laser (relevant
changes are highlighted in gray) for homodyne detection.
4430 Vol. 40, No. 19 / October 1 2015 / Optics Letters Letter
diode (direct power detector) or a photoconductor (homodyne
field detector) is used. A commercial CW TOPTICA setup is
used for lock-in detection with 600 ms integration time of
the received THz photocurrent for the Schottky diode measure-
ment and only 30 ms for homodyne detection. The p-i-n photo-
mixer source is bias-modulated for lock-in detection. The THz
setup can follow a frequency tuning up to 1.75 THz, which is
limited by the dynamic range of the photomixer and Schottky
diode/photoconductor used in the setup.
To demonstrate the large tuning range of the MEMS-
VCSEL for direct detection THz-setup, we sweep the differ-
ence frequency from −1.67 to −0.066 THz and from 0.03
to 1.73 THz with respect to the fixed-wavelength VCSEL.
The detected THz signal is plotted in Fig. 6. The roll-off of
the detected THz-signal is due to the roll-off of the p-i-n emit-
ter ∼ν−2−ν−4[4] and the roll-off of the Schottky diode
∼ν−2[14] that are used for detection. The further tuning
capability of this setup is, therefore, solely limited by the
dynamic range of the THz emitter/receiver system. To demon-
strate continuous tunability and frequency coverage for the ho-
modyne THz setup, the MEMS-VCSEL is locked to a certain
wavelength while the DFB laser is scanned over its tuning range
of 1559.43–1554.99 nm in 100 MHz steps, corresponding to a
total tuning range of 550 GHz. To cover a larger frequency
range, the MEMS-VCSEL is stepped to another lock point (al-
tered by 400 GHz) followed by a repeated DFB laser scanning.
In this procedure, the whole THz bandwidth of the photomix-
ing setup can be covered. To demonstrate the large tunability of
this system, we again measure negative and positive difference
frequencies. At first, the MEMS-VCSEL is locked to
1544.52 nm. Optical beating of the MEMS-VCSEL with the
longest DFB laser wavelength of 1559.43 nm generates a THz
frequency of −1.84 THz. Subsequently, longer wavelengths for
the MEMS-VCSEL of up to 1569.59 nm cover a difference
frequency of 1.78 THz with respect to the shortest DFB laser
wavelength of 1554.99 nm. However, the noise floor of the
THz system is reached during these frequency windows.
The detected THz-signal is plotted in Fig. 6where different
frequency windows by MEMS-VCSEL stepping are illustrated
with the color gradients. A total span of 3.23 THz is covered,
with a peak dynamic range around 100 GHz of 72 dB at an
integration time of only 30 ms. Several water-vapor absorption
lines at 555, 751, 986, 1095, 1112, 1161,1206, and
1226 GHz can be identified in Fig. 6. The absorption line
at 1408 GHz can also be seen; however, this is heavily influ-
enced by the system noise.
In summary, we demonstrate a MEMS-VCSEL-based ultra-
wideband tunable THz system for driving a THz photomixing
setup at unprecedented bandwidth. A purely VCSELs-based
system is demonstrated in which the tunable MEMS-VCSEL
is stepped through 100 GHz spaced lock points with respect
to the fixed-wavelength VCSEL covering a frequency span of
3.4 THz. We set up a second system where the fixed-wavelength
VCSEL is replaced by a DFB diode that is scanned in 100 MHz
steps over its tuning range in each frequency window, which
is selected by appropriate choice of the lock points of the
MEMS-VCSEL. This setup covers a frequency span of
3.23 THz. For further higher resolution, the DFB diode could
even be tuned on a 1 MHz level. The tuning range is limited by
the dynamic range of the THz system, not by the laser system.
With improved THz emitter and detector components, the tun-
ing range can be further extended to cover the whole THz range.
Funding. Hessian LOEWE (Sensors towards Terahertz
program).
Acknowledgment. The authors would like to thank
VERTILAS for fabricating the half-VCSEL.
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Fig. 6. Direct detection with a Schottky diode: THz signal against
the difference frequencies from −1.67 to −0.066 THz (triangles)
and from 0.03 to 1.73 THz (squares). Homodyne detection
with a photoconductor: power-amplitude spectrum from −1.61 to
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Letter Vol. 40, No. 19 / October 1 2015 / Optics Letters 4431
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