High frequency modulation and (quasi)single-sideband emission of mid-infrared ringand ridge quantum cascade lasers

BORISLAV HINKOV,1,* JAKOB HAYDEN,2 ROLF SZEDLAK,1

PEDRO MARTIN-MATEOS,3 BORJA JEREZ,3 PABLO ACEDO,3

GOTTFRIED STRASSER,1 AND BERNHARD LENDL2

1Institute of Solid Stated Electronics, Technische Universität Wien, 1040 Vienna, Austria2Institute of Chemical Technologies and Analytics, Technische Universität Wien, 1060 Vienna, Austria3Electronics Technology Department, Universidad Carlos III de Madrid, 28911 Leganés, Madrid, Spain*borislav.hinkov@tuwien.ac.at

Abstract: We investigate the high frequency modulation characteristics of mid-infraredsurface-emitting ring and edge-emitting ridge quantum cascade lasers (QCLs). In particular, adetailed comparison between circular ring devices and ridge-QCLs from the same laser material,which have a linear waveguide in a "Fabry-Pérot (FP) type" cavity, reveals distinct similaritiesand differences. Both device types are single-mode emitting, based on either 2nd- (ring-QCL) or1st -order (ridge-QCL) distributed feedback (DFB) gratings with an emission wavelength around7.56 µm. Their modulation characteristics are investigated in the frequency-domain using anoptical frequency-to-amplitude conversion technique based on the ro-vibrational absorptions ofCH4. We observe that the amplitude of frequency tuning ∆f over intensity modulation index mas function of the modulation frequency behaves similarly for both types of devices, while thering-QCLs typically show higher values. The frequency-to-intensity modulation (FM-IM) phaseshift shows a decrease starting from ∼72 at a modulation frequency of 800 kHz to about 0 at160 MHz. In addition, we also observe a quasi single-sideband (qSSB) regime for modulationfrequencies above 100 MHz, which is identified by a vanishing -1st -order sideband for bothdevices. This special FM-state can be observed in DFB QCLs and is in strong contrast to thebehavior of regular DFB diode lasers, which do not achieve any significant sideband suppression.By analyzing these important high frequency characteristics of ring-QCLs and comparing themto ridge DFB-QCLs, it shows the potential of intersubband devices for applications in e.g. novelspectroscopic techniques and highly-integrated and high-bitrate free-space data communication.In addition, the obtained results close an existing gap in literature for high frequency modulationcharacteristics of QCLs.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distributionof this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

1. Introduction

The absorption measurement and sensitive detection of various molecules in the mid-IR spectralrange have gathered much attention in recent years. Especially gases held responsible forgreenhouse-processes and global warming are of great interest. This includes e.g. carbon dioxide(CO2), nitrous oxide (N2O) or methane (CH4), which make up to 98% of the global greenhousegas emission (IPCC 2014 [1]). They have, together with many other molecules, their fundamentalabsorptions in the mid-IR spectral range.In this context, laser-based optical detection of gaseous molecules has many advantages

such as: fast response time and rapid measurements in the microsecond range [2], low limitsof detection (LOD) up to the parts-per-trillion range [3], the determination of isotope ratios(e.g. for CO2 [4]), the analysis of minute-amounts of probe-samples in the microliter-range [5],high gas selectivity [5] with no requirements for sample pre-treatment [5] and the realization

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#356108 https://doi.org/10.1364/OE.27.014716 Journal © 2019 Received 26 Dec 2018; revised 8 Mar 2019; accepted 5 Apr 2019; published 7 May 2019

of small-footprint and portable sensor-configurations [6]. For achieving this, various tracegas sensors have recently been demonstrated. This includes dual-comb spectroscopy in themid-IR [2], QEPAS [7, 8], multipass gas measurements [6] and various other concepts [3–5].Besides those detection schemes, another class of sensors has recently emerged, based on

dispersion spectroscopy measurements. It combines features like baseline- and normalization-free (immune to source-power fluctuations) characteristics with a linear output-dependency ongas concentration and constant sensitivities. Typical examples are CLaDS [9], HPSDS [10]or most recently 2f-wavelength modulation Fabry-Pérot photothermal interferometry (2f-WMFP-PTI) [11], probing the changed dispersion in a FP cavity.

While all mentioned detection schemes share that they rely on high-performance mid-IR lasers,among them the dispersion techniques are sensitive to the modulation characteristics of thesource. This includes the frequency and intensity modulation, FM and IM, respectively, andalso the modulation cut-off frequency (defined as the 3-dB cut-off) and the sideband ratio (SR).Especially the latter is a very interesting figure of merit in singlemode emitting devices which,for some special FM-state, can even lead to the full suppression of one sideband-order (i.e. +1stor -1st order). This is called the single-sideband (SSB) regime. More details on this specificstate can be found in: [12, 13]. References on possible spectroscopic or telecommunicationapplications are [14] and [15].A type of coherent emission source that addresses all those characteristics is the quantum

cascade lasers (QCLs) [16]. QCLs are semiconductor lasers emitting in the mid-infrared toTHz spectral region [17–23], which can e.g. be used for various applications like molecularspectroscopy and medical diagnostics [5], food screening [24], security applications [25, 26] andoptical free-space communications [27]. Focusing on the recent advancements in dispersionspectroscopy and related techniques, the modulation capabilities of QCLs are of great interest. Pre-vious work has shown that, in contrast to diode lasers which show a limiting relaxation-oscillationpeak in their modulation response at relatively low frequencies (typically low GHz-range [28]),QCLs are capable of direct high-frequency current-modulation [12, 13, 29–31]. This is a di-rect consequence of their ultrafast intersubband transitions (∼low picosecond range) and theinterplay with the cavity photon lifetime on the nanosecond timescale. Nowadays, optimizedlow-capacitance mid-IR emitting DFB-QCLs reach modulation frequencies of up to 23.5 GHzand 26.5 GHz, in their optical and electrical response, respectively [13]. But until now, onlylinear edge-emitting ridge-waveguide devices have been analyzed in such experiments, leavingout surface-emitting ridge- and ring-QCL geometries. The latter show significant potentialfor array-integration [32,33], farfield-emission, -collimation and -direction [34–38], as well aspolarization control of the emitted radiation in ring-devices [34] and most recently presentedalso in monolithic integration of ring-cavity on-chip laser-detector schemes [39, 40].By, for the first time, investigating and comparing the modulation-characteristics of current-

modulated mid-IR surface emitting ring- and edge emitting ridge-DFB QCLs from the same gainmaterial, this work is hence contributing to closing the existing gap in literature. As it was shownin literature [13], changing the mode coupling mechanism by implementing a 1st -order DFBgrating to a FP-cavity leads to a completely different modulation response, by showing a strong,unexpected, resonance-feature around the roundtrip frequency of the cavity. When comparingsuch ridge DFB-QCLs with typical 2nd-order ring-DFB QCLs, which, in contrast to linear cavitydevices, support counter-propagating whispering gallery modes [41], their modulation responseneeds to be investigated in detail.The high frequency modulation properties of lasers are the product of their electrical and

optical response. While the electrical characterization is typically done using a rectificationtechnique ( [13, 29, 42]), the optical response of DFB lasers can be analyzed by direct opticalsidebands measurements with an FTIR (for modulation frequencies in the GHz-range) [13],by beating measurements on a fast detector of the modulated laser under investigation with a

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reference DFB laser that is not modulated [13], or by using an optical frequency-to-amplitudeconversion technique and by analyzing the obtained signal amplitude and phase with a lock-inamplifier. We chose the latter technique to directly investigate the FM-IM response of our devices.Those measurements give novel insight into the dynamical properties and the suitability

of ring-QCLs for dispersion spectroscopy and other related fields of application, where fastmodulation capabilities and high integration-levels, i.e. small and compact laser-sources/sensors,are highly beneficial.

Fig. 1. (a) Experimental setup using the frequency-to-amplitude conversion technique (redarrows indicate the direction of electrical current, the green square highlights the bias-teewhich is used for all measurements above 2 MHz). (b) Singlemode laser spectra takenin continuous-wave mode of the ring-QCL at 100 K (black line: 150 mA, red line: 180mA). The region of the molecular absorption which is used for the frequency-to-amplitudeconversion is marked in gray (1322.04 cm−1 to 1322.2 cm−1).

2. Device design, experimental setup and electrical characterization

The devices used in this work are based on a typical state-of-the-art two-phonon resonance designwith an In0.53Ga0.47As/In0.52Al0.48As active region grown on low-doped InP substrate. Theactive region is comprised of 35 periods, sandwiched between two 500 nm thick InGaAs spacers(Si-doped, n = 5x1016 cm−3) and designed for lasing around λ ∼ 7.5 µm. The top cladding consistsof two subsequent InAlAs layers (1st : 1500 nm, Si-doped: n = 1x1017 cm−3; 2nd: 800 nm,Si-doped: n = 2x1017 cm−3), covered by a highly doped InGaAs contact- (350 nm, Si-doped, n =8x1018 cm−3) and an InGaAs capping-layer (100 nm, Si-doped, n = 1x1020 cm−3). More detailson the device design can be found in [43,44]. For achieving singlemode emission, a first-orderDFB grating is implemented to the edge-emitting ridge-devices and a second-order DFB gratingto the ring-QCLs for surface/substrate-emission. Typical ring- as well as ridge-devices areinvestigated and compared to each other in the following. They are driven in continuous-wave(CW) mode at liquid nitrogen temperatures between 80 K and 100 K at two distinct bias currents,one close to lasing-threshold and the second one at ∼20-25% higher drive currents, while thecurrent modulation ranges from 800 kHz to 160 MHz (limited by the used function generator"Rigol DG4062") with modulation-amplitudes of a few milliamperes. More precisely, forfrequencies ranging from 800 kHz to 2 MHz, the laser current is modulated via the currentdriver (Wavelength Electronics "QCL OEM300") without using a bias-tee. For such kind ofmeasurements, the current modulation amplitude IAC,0 is set via the analogue input of the currentdriver to 1 mA. For frequencies above 2 MHz, a bias-tee is used, by connecting its AC-portdirectly to the signal generator output (see green box in Fig. 1(a)). The measurements were

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Fig. 2. (a) Typical absorption- (black) and dispersion-spectra (red) of CH4. The spectralregion for frequency-to-amplitude conversion is again marked in gray. (b) & (c) Modulationspectra as recorded via the lock-in amplifier for a modulation frequency of fmod = 150 MHz:(b) amplitude-trace and (c) phase-signal.

repeated with two modulation amplitudes of 50 mVpp and 500 mVpp, respectively, set directlyat the signal generator. We used this measurement to analyze the impact of the modulation depthon the obtained modulation characteristics. Subsequently, the data was analyzed independentlyand compared, yielding identical results, which shows that there is no distinguishable impact ofthe modulation amplitudes in our experiments (i.e. the measurements are performed in the socalled small signal regime; for more details see section 3.).

The resulting modulation characteristics of the lasers are analyzed in the presented experimentsconcerning their relative frequency-to-intensity (or amplitude) modulation as well as the relativephase between frequency and amplitude modulation response, both as function of modulationfrequency. To measure the frequency tuning of the devices, we use a frequency-to-amplitudeconversion technique, as shown e.g. in [45] using CO2. A detailed theoretical description of thedata analysis follows in the next chapter.

In the present experiment we use the absorption lines of CH4 (instead of CO2) at low pressure(30 mbar, diluted to about 1%V to result in a peak absorption of α · L = 0.5 for a 10 cm absorptioncell; see also Fig. 2(a) for the corresponding spectra of CH4), which are centered around1316.8 cm−1 and 1322.12 cm−1, respectively. Consequently, any induced frequency modulationis converted into a directly detectable intensity modulation through those CH4-absorptions(wavelength-dependent portion of the laser radiation is absorbed). To illustrate this techniqueFig. 1(a) shows the used experimental setup including the frequency-to-amplitude conversiontechnique. More details can also be found in [10]. In addition Fig. 1(b) shows typical emissionspectra for the ring-QCL at 100 K and drive currents of 150 mA (black trace) and 180 mA (redtrace), respectively. Typical absorption- and dispersion-spectra of CH4 are given in Fig. 2(a).The spectral region of frequency-to-amplitude conversion is marked in gray in both figures. Totune both devices under investigation to the CH4 absorption lines (i.e. the marked gray areain Fig. 1(b)), while driving them at fixed drive currents, their temperatures had to be adaptedaccordingly: ∼100 K for the ring-QCL and ∼80 K for the ridge-QCL, respectively.The analysis of the modulation characteristics is performed in the frequency-domain. For

the investigated modulation frequencies between 800 kHz and 160 MHz, the detector signal isdirectly fed into a lock-in amplifier which analyzes the amplitude and phase of the signal at themodulation frequency. Since the lock-in amplifier’s bandwidth is limited to 250 kHz, the detectorsignal is down-mixed using a reference signal at a frequency which is 100 kHz lower than themodulation frequency of the laser itself (see Fig. 1(a) and [10]). After low-pass filtering, theresulting down-converted signal is analyzed at the difference frequency of 100 kHz. The output

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Fig. 3. (a) Amplitude of frequency tuning ∆f over intensity modulation index m as functionof modulation frequency (800 kHz to 160 MHz) for the ring and ridge laser at differentlaser driving conditions. Ring laser (Jth = 110 mA): 148 mA and 185 mA, ridge laser(Jth = 231 mA): 245 mA and 297 mA. (b) FM-IM phase shift within the modulation range(800 kHz to 160 MHz) of the two QCLs (ring and ridge) for the same different laser drivingconditions as shown in (a).

of the lock-in amplifier is continuously recorded, while the center frequency of the modulatedlaser is slowly scanned across the molecular resonance (1 Hz, averaging of 19 scans), resulting inspectral traces as shown in Fig. 2(b) and (c) (red line).

3. Frequency domain analysis

The theoretical model for fitting the spectral data recorded through the Lock-in amplifier wasadopted from the one described by Hangauer et al. [12]. It is based on the calculation of thecomplex laser spectrum under modulation-conditions, which is composed of a set of discrete,equally spaced lines (given by the modulation frequency), located around the laser centerfrequency. Neglecting scaling factors, the spectrum is analytically calculated from the intensitymodulation index m = IAC,0

IDC−Ithr (IAC,0 is the amplitude of the modulation current IAC , IDC

stands for the DC bias current and Ithr represents the threshold current of the respective laser),which is much smaller than 1 in the so-called small signal regime (IAC,0 IDC - Ithr ), thefrequency modulation index β = ∆ f

fmod(∆f = IAC,0 · γ, instantaneous frequency tuning amplitude

with the current tuning coefficient γ) and the frequency- to intensity-modulation (FM-IM)phase shift φFM−IM [46]. Next, each line is multiplied with the complex transfer functionT( fcenter + n ∗ fmod) given by the Faddeeva function (cp. Fig. 2(a)). It models the response, i.e.attenuation and phase change, of the gas and corresponds to the transmission spectrum of the gassample including anomalous dispersion. The resulting signal is then detected by a square-lawdetector which allows extracting the frequency component at the modulation frequency fmod.Each spectral line is multiplied by the complex conjugate of its neighbor and all products aresummed up. This procedure is repeated for each center frequency, leading to an amplitude- andphase-spectrum, corresponding to the measured spectra.The model was fitted to each complex spectrum adjusting the local variables (φIM (phase

of IM), v0 (laser center frequency), γ, φFM−IM and S1 (scaling factor)). To remove the smalluncertainty in the scaling of the wavelength axis, the well-known spacing between the twoabsorption lines centered around 1322.12 cm−1 is used, to include the stepsize of the wavenumberaxis as fitting parameter and consequently to adapt, i.e. slightly stretch/compress, the wavelengthaxis.

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Fig. 4. Sideband ratio |E−1 |/|E+1 | of the minus first to the plus first order sideband of thecorresponding electrical fields within the investigated modulation range (200 kHz to 160MHz) for both lasers at different driving conditions (same as used before).

4. Results and data analysis

The frequency tuning amplitude of both devices is analyzed first. For removing scaling effectswith the modulation current IAC , Fig. 3(a) shows the frequency tuning amplitude ∆f normalizedto the current modulation index m, as function of the modulation frequency between 800 kHzand 160 MHz. The IM index normalized FM behavior is consistently slightly larger in thering-devices, while the overall trend is similar in both types of devices and also in good agreementwith literature [12]. The larger frequency tuning originates partly from the higher drive currentabove threshold (J − Jth) of the ring-devices which was, together with the temperature, needed totune the wavelength of the rings to the absorption line of CH4. The contribution from geometricaldifferences between both devices (e.g. different waveguide-width, longer ring resonator etc.)which might especially impact the FM behavior through the refractive-index tuning in this highfrequency tuning regime and which impacts the whispering gallery modes in the ring cavitydifferently than the DFB-modes in the ridge device, have to be analyzed in more detail in futurework.

Figure 3(b) completes the data by showing the corresponding FM-IM phase shift in the samemodulation frequency range. We observe the expected decay in this curve for both types ofdevices, due to the operation of the devices in the electronic tuning regime with its spectralblue-shift and its FM-IM phase of zero. This also corresponds well with recent findings fromliterature [12] and can be understood in more detail by the transition from a temperature drivenand therefore red-shifting regime (low modulation frequencies in the kHz-range, FM-IM phase of180) to a regime driven by the blue-shifting charge carrier density mechanism (high modulationfrequencies, FM-IM phase 0) [12].Finally, Fig. 4 shows the so-called sideband-ratio (SR) of the devices, i.e. the ratio between

the amplitudes of the two first-order frequency components of the modulated lasers. In goodagreement with the findings in [13] and [12], both, ring- and ridge-QCL, can operate in the quasisingle-sideband (qSSB) modulation regime. The single-sideband (SR) regime is characterizedby a vanishing sideband of one order (positive or negative side), while the other one (negativeor positive side, respectively) can still be observed. A precursor of this is the qSSB regime,where the SR reaches small values, typically below 0.1, or even side-mode suppression ratios of

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above 15 dB for one of the two first-order sideband modes. For our devices this qSSB regimeis achieved for high modulation frequencies above ∼100 MHz together with low laser drivingcurrents (-1st order sideband). Following the observed trend, we expect that the SR is furtherdecreasing for even higher modulation frequencies above 160 MHz up to the GHz range, thatare currently not possible with our setup. Such predictions for the GHz-modulation behaviorinclude the analysis of a possible SSB resonance around the cavity roundtrip frequency of theDFB-QCLs (between 35 GHz and 40 GHz for our ring devices), comparable to resonancesobtained in ridge-DFB QCLs [13], and are the topic of future investigations.In addition to its dependency on the device properties (e.g. cavity geometry like ring- vs

ridge-device), the drive conditions of the devices play an important role for the operation inthe (q)SSB regime. This includes the DC driving current of the lasers, since the IM-index mdepends on the DC optical output power of the devices [12]. In good agreement with literature,Fig. 4 shows lower SRs when driving the QCLs at lower driving currents [12]. We observe thisbehavior for both, ring- and ridge-DFB devices.Such a SSB/qSSB regime is an interesting and unique feature, since it allows the selective

tuning, including switching off and on, of a specific spectral (laser) emission feature. Classicaldiode lasers do not show such a behavior [12], which is e.g. beneficial for high-speed laserspectroscopy [14] or telecommunication links [15].

In conclusion we investigated the high frequency modulation capabilities of typical singlemodesurface-emitting ring-QCLs in the range of 800 kHz to 160 MHz. In comparison to ridge-QCLsfrom the same gain material, a frequency-domain analysis shows similar behavior with slightlyhigher values for ∆ f /m and the FM-IM phase shift in the ring-devices. At the same time the qSSBregime can be achieved with the surface-emitting ring- and the edge-emitting ridge-QCLs. Thismakes them, together with their use in array-integration, their low divergence farfield emission,their excellent collimation and direction capabilities and polarization control of their opticalemission, very good candidates for spectroscopic measurements and high-bitrate free-space datacommunication using highly integrated systems.Further work will include a device-analysis for even higher modulation frequencies up to

the GHz-range. To allow experiments at such high modulation frequencies, first of all a fasterfunction generator is needed. In addition, typical state-of-the-art experiments also use speciallydesigned RF-waveguides such as 50-Ω impedance matched coplanar waveguides or microstriplines to reduce the parasitic capacitance to the low picofarad-range, as e.g. shown in [13, 29].To obtain a further reduction in the parasitic inductance, which is e.g. introduced by usingbond-wires (even when only a few millimeter long), such RF-cavities are typically combinedwith special multi-contact RF-probes.

Funding

Austrian Science Fund (FWF) (F49-P09, M2485-N34); Austria Research Promotion Agency(FFG) (4241316)

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