Optical and electrical investigations of vertical-cavity surface-emitting lasers (VCSEL) with a monolithically integrated electro-optical modulator (EOM) allow for a detailed physical understanding of this complex compound cavity laser system. The EOM VCSEL light output is investigated to identify optimal working points. An electro-optic resonance feature triggered by the quantum confined Stark effect is used to modulate individual VCSEL modes by more than 20 dB with an extremely small EOM voltage change of less than 100 mV. Spectral mode analysis reveals modulation of higher order modes and very low wavelength chirp of < 0.5 nm. Dynamic experiments and simulation predict an intrinsic bandwidth of the EOM VCSEL exceeding 50 GHz.
©2012 Optical Society of America
Presently a technology transition from copper based computer interconnects to much lower power and higher speed optical interconnects is gaining momentum [1–3]. Vertical-cavity surface-emitting lasers (VCSEL), being low-cost but high speed and high efficiency devices, are implemented in large quantities into optical interconnects, both inside computers and between computers and peripherals up to distances of 100 m. VCSELs are approaching data rates close to 50 Gb/s  and record-high energy efficiencies below 100 fJ/bit at slightly lower rates . Further increases of data rates beyond 50 Gb/s, which will be asked for in a few years time, demand equivalent increases of the cut-off frequency, being difficult to reach with directly modulated VCSELs. Their cut-off frequency increases linearly with a quadratic increase of current density inside the VCSEL. Consequently, conventional high speed VCSELs suffer from accelerated device degradation caused by the very high but necessary drive currents . While external modulation schemes used for today’s ultrahigh-speed, long-distance interconnects based on edge emitting lasers provide higher modulation speed, a low-cost implementation into vertical device concepts is rather unexplored. Novel and conceptually completely different concepts propose the monolithic integration of a modulator section on top of a continuous wave (CW) driven VCSEL to indirectly modulate the laser [5–9]. Thereby, life time issues associated with high-speed operation of directly modulated VCSELs requiring extremely-high current densities are eliminated and existing low cost VCSEL processing technologies are preserved. Such an integrated concept can be realized by employing an electro-optic modulator (EOM). For external modulators the EOM concept was initially demonstrated in 1988 using an 82 fold GaAs quantum well (QW) active medium . High modulation bandwidth of more than 20 GHz was demonstrated 20 years later [8, 9]. These monolithic EOM VCSEL devices essentially consist of two Fabry-Perot cavities tuned to have similar resonance wavelengths. The required field strength to achieve a given modulation amplitude depends on the fine-tuning of both resonances. As we will show in this work, the full potential of monolithic EOM VCSELs can be realized by operating the two cavity modes in resonance. Around this resonance extinction ratios larger than 20 dB are achieved for individual optical modes at ultra-small EOM voltage changes of less than 0.1 V. S-parameter analyses of multi-mode devices operating at this resonance suggest an intrinsic bandwidth of more than 50 GHz.
2. EOM-VCSEL device concept
The device concept of the present EOM VCSEL aims at the monolithic integration of a modulator with a VCSEL to benefit from the established fabrication and processing technologies and match the industry-relevant 850 nm wavelength range. Initial results of this device were published in [6, 7]. Figure 1 shows schematics of the device depicting a third electric middle-contact. Compared to a conventional VCSEL this additional contact electrically divides the device into two sections: at the basis a VCSEL and on top the EOM section realized as a second cavity within the VCSEL top DBR. All DBRs consist of Al0.9Ga0.1As and Al0.15Ga0.85As λ/4 layers employing graded interfaces with doubled doping levels to facilitate charge carrier transport within the device.
2.1 VCSEL section
A conventional design with a GaAs MQW gain-medium with Al0.2Ga0.8As barriers is chosen for the VCSEL section, similar to that previously published by us [3, 5]. Adjacent to the active cavity within the first DBR pair an AlAs layer enclosed by AlGaAs-gradings is introduced for post-growth oxidation of a crack-free aluminum-oxide current-aperture using a novel oxidation technology developed by us . The bottom high-reflectivity AlGaAs DBR is n-doped while the intermediate DBR between active-cavity and EOM-cavity is p-doped. Within this p-doped DBR a lattice matched InGaP layer adjacent to a p+ -doped GaAs contact layer serves as etch-stop during processing of the middle-contact. Active QW spectral position is confirmed to be at 835 nm wavelength by photoluminescence characterization while InGaP lattice matching is verified by X-ray diffraction measurements.
2.2 EOM section
The EOM section starts on top of the middle-contact layer with the remaining part of the p-DBR. To employ the quantum-confined Stark effect for light output modulation an undoped cavity is formed by a GaAs MQW stack with Al0.2Ga0.8As barriers. These EOM QWs are spectrally shifted towards shorter wavelengths with respect to the active VCSEL. On top of the EOM-cavity a Si-doped 10x DBR with a GaAs cap layer protecting against oxidation finalizes the structure. Spectral positioning of emission wavelength and EOM QWs aims at a minimized absorption within the EOM element while maintaining a sufficient refractive index modulation referred to as region II in . Consequently EOM QWs are tuned to a higher energy with respect to the active QWs of the VCSEL section. The EOM principles of the VCSEL output are described as follows. According to the Kramers-Kronig relation, the change of the complex refractive index of the EOM-QW section by applying an electric field is due to both the spectral shift of the QW absorption peak as well as the change in oscillator strength of the excitonic absorption . Thus, both the absorption strength as well as the EOM-cavity resonance-wavelength are changed. If the VCSEL-cavity resonance wavelength is close to the EOM-cavity resonance wavelength the latter becomes significant. Our EOM-VCSEL design is targeted at working under resonance condition. Details on the EOM medium operating principle and on other possible designs of EOM VCSELs are given in .
Prior to the final prototype, test samples are grown and investigated to tune and optimize all parts of the device individually. DBR stop band reflectivity and cavity spectral position are determined by surface reflectivity measurements and compared well with transfer matrix simulations. A single growth run is performed to grow the entire monolithic EOM VCSEL at 680°C using metal-organic vapor phase epitaxy. Temperature is solely lowered to 595°C for the deposition of the lattice matched InGaP etch-stop layer which is covered with 5 nm of GaAs prior to heating up again to resume DBR growth. A horizontal flow Aixtron 200/4 reactor equipped with TMIn, TMGa, TMAl, TBP and arsine precursors is used yielding excellent uniformity on 2-inch GaAs:Si (1 0 0) substrates for the whole structure comprising close to 400 layers. Optical reflectance and reflectance anisotropy are recorded in situ by an optical sensor to monitor DBR growth stability and process cycles. Employing standard lithography and dry etching techniques devices are processed with varying mesa diameters from 25 to 36 μm and 45 to 56 μm for EOM and VCSEL sections, respectively . Selective oxidation of the Al-rich aperture layer is realized using optimized conditions as described in . Three ohmic contacts are realized for device operation, bottom and middle contact for the VCSEL part, middle and top contact for the EOM part.
3. Static characteristics
Dynamic and static characteristics are investigated at room temperature (RT) with operational devices mounted on a copper heat-sink as described in . For all measurements the VCSEL section is driven at constant current above lasing threshold to enable CW emission. Modulation of the optical output is realized solely by applying a reverse EOM-voltage. The current flow across the EOM section is monitored simultaneously as a measure for the photo-absorption of laser emission within the modulator. Fundamental laser characteristics and the corresponding EOM photocurrent of a multi-mode device with 28 μm EOM mesa-diameter are given in Fig. 2 . Data show a constant lasing threshold independent of the applied reverse EOM bias indicating good optical modulator isolation from the VCSEL section.
Optical output-power and EOM photocurrent increase linearly with the VCSEL drive current up to about 3.3 mA. With increasing EOM reverse bias, however, the absorption increases and the output power decreases. Within this current range, modulation of the VCSEL output is thus primarily due to change of absorption strength in the EOM section. Upon further increase of VCSEL current, the optical output power is decreasing, likely limited by thermal rollover. Within a current range of 5 to 6.5 mA, the VCSEL output power increases again but strong superlinearly. Up to the narrow output peak maximum at resonance the photocurrent increases linearly and then drops sharply. Upon further increased current both values finally decrease. The second optical output power increase cannot be explained by a change of absorption within the EOM section. Instead, we attribute this feature to resonance matching of the two coupled cavities within the device. For devices sharing a similar cavity design, a resonant output power behavior is typical . Here, we analyze dynamics (section 4.2) but first we derive a qualitative explanation for the observed resonance peak.
For larger EOM reverse bias, the energy of the exciton absorption peak is decreased but the refractive index change can be positive or negative thus the resonance wavelength of the EOM cavity is altered depending on device properties [5, 10, 12]. Our EOM cavity resonance at zero bias is set to a slightly longer wavelength with regard to the VCSEL resonance and the energetic separation between both cavity resonances is increased by a larger reverse bias. This increase is counter-balanced by increasing the VCSEL current. The intrinsic temperature and subsequently the effective refractive index of the VCSEL section are both increasing due to heating. The resonance wavelength shift of both cavities depends on their spectral resonance positions controlled (amongst others) by the corresponding effective refractive indices . The refractive index change of the EOM MQW due to the quantum-confined Stark-effect is rather limited (Δn ≈ 0.01) . Thus resonance modulation requires precise matching of the cavity resonances and definition of the correct operating point of the device. For our structure, the separation of the resonances is designed and experimentally observed to be only 1 nm. Reflection spectra of test samples containing the individual cavity structures and the final EOM VCSEL together with fits using the transfer matrix method are used.
4. Resonance analysis
To distinguish between absorption related and index of refraction induced resonance effects on signal modulation we analyze now in more detail the output power and the photocurrent within the 1-3 V EOM bias range as a function of the VCSEL drive current (see Fig. 3 ). Additionally this figure includes the sum of both values (the EOM photocurrent is converted to the corresponding optical output power at the wavelength of operation) giving the total output power Ptot of the VCSEL section. Ptot shows an almost flat behavior for 5 mA (black curve, bottom viewgraph). Thus, the modulation of the light output as seen in the center of the figure is solely driven by absorption within the EOM section and a rather large EOM voltage sweep of 1.5 V is necessary to realize 3 dB optical intensity modulation. At higher VCSEL drive currents of 6.0 and 6.2 mA (red and green curves) a sudden increase of the output power is revealed. Upon a voltage change of less than 100 mV Ptot almost doubles and only a fraction of this increase is due to absorption in the EOM section. This step like increase of the total VCSEL output power directly proves the onset of coupling between the two cavities: the EOM resonance wavelength is varying as a function of voltage. This steep increase of optical output power at resonance represents a very efficient modulation of the device. We call this electro-optic resonance modulation (ERM).
4.1 Spectrally resolved EO resonance modulation
For our mesa diameter of 28 µm and current aperture diameter of 5 µm the device shows four significant transverse modes. The impact of the ERM on individual modes is analyzed using an optical spectrum analyzer.
Figure 4 shows the fundamental mode (#0) at 856.2 nm and three higher order modes numbered (#1-3) at shorter wavelengths for bias values between 2.6 and 3.2 V. Between 2.9 V and 3.0 V the intensities of the 1st and the 2nd modes are increasing by 20 dB and 27 dB, respectively. The wavelength of all modes shifts by less than 0.5 nm upon this bias change involving ERM.
4.2 Intrinsic performance
Previous large-signal experiments on similar devices revealed a limited modulation bandwidth of 3 GHz . Here we present a small-signal analysis to investigate the origin of this bandwidth limitation. Opposite to conventional current modulated devices the small-signal modulation transfer function of photon lifetime τp modulating devices decreases with 1/ω instead of 1/ω2 . For this investigation the device is biased ensuring resonance conditions. Direct measurements of the small-signal modulation bandwidth (S21) identify tight limitations with a sharp drop at the beginning of the frequency range. As this is usually an indication of a parasitic limit, we conducted a more detailed investigation of the device impedance (S11). The results of this investigation are presented in Fig. 5 .
The EOM-section is modeled as reverse-biased PIN-diode and the parasitic device values can be extracted with high accuracy. We split the EOM-section into the diode capacitance of the space-charge regions of the p- and n-side (which can be modeled as one lumped element Cdiode) and an additional capacitance CQW from the EOM QWs only. Each of these capacitors has a leakage resistance Rdiode and RQW, respectively. Additionally we found a spreading resistance Rspread caused by the surrounding cladding layers, a pad capacitance Cpad and a non-perfect contact resistance Rcontact. The response from the parasitic equivalent circuit perfectly describes the measured S11-values across the entire (setup-limited) frequency range from 0 to 40 GHz. Changes of device values are also consistent with changes of the EOM-voltage. The lumped element values for different EOM-voltages are given in Table 1 .
For increasing voltage, the space-charge capacitance Cdiode decreases consistently with the increase of the depleted region. As expected the corresponding leakage resistance Rdiode becomes significantly larger. The outer parasitic network is practically invariant to changes in voltage, whereas the EOM-section itself varies its impedance significantly. With increasing photo-current (compare Fig. 3) the leakage resistance RQW decreases, while the capacitance CQW becomes larger due to the variation of the refractive index and photo-absorption induced heating. From this equivalent circuit model we can extract the parasitic response of the EOM-section which is clearly limiting the high-speed performance of our prototype device. Extraction of the parasitic response by means of fitting from the S21-curve yields very similar results.
The parasitic limited performance of the device is also in agreement with our estimates based on device geometries and doping levels. To determine the intrinsic device speed with resonance conditions the parasitic response is deconvoluted from the measured frequency response. The intrinsic device speed can be fitted to Eq. (1) as described by van Eisden et al. . This fit yields a photon lifetime τp of 4.1 ps, a resonance-frequency ω0 of 2π·27 GHz and a damping coefficient γ of 1.5 1011 s−1 at driving conditions of 6.2 mA and 2.8 V. The result is depicted in Fig. 6 , predicting an intrinsic device bandwidth of 56 ± 5 GHz.
Such a bandwidth together with a large overshoot is typical for intra-cavity loss modulated lasers [9, 15]. A well-tailored parasitic response is needed to flatten out the characteristic overshoot in the device response without limiting the bandwidth. Therefore, a detailed knowledge about the parasitic network of the chip as presented in Fig. 5 is crucial. Due to similar confinement factors the resonance frequencies of common laser diode designs tend to saturate at some 20-30 GHz limiting the device bandwidths to some 40 Gbps. Optically isolated external modulators can overcome these limitations at the expense of a more complex system layout connected with higher cost and energy consumption. Intra-cavity loss-modulated devices, especially EOM-VCSELs, can thus overcome the bandwidth limitation of directly modulated laser diodes at the low manufacturing costs of a conventional VCSEL.
Future EOM VCSEL generations will reduce the large parasitics of this prototype by employing smaller EOM mesa diameters and additional oxide apertures for the EOM part. Additionally, efficient modulation could necessitate single mode operation. The modulation depth of the EOM cavity can be improved by employing a larger numbers of QWs, which are precisely tuned to minimize absorption while maintaining sufficient refractive index modulation. Additionally high bandwidth device processing and mounting is required to reduce the external parasitics.
Modulation characteristics of a monolithic EOM VCSEL are analyzed and absorption and refractive index changes are separately investigated in detail. We find strong electro-optic resonance modulation (ERM), where the light output of the device is modulated by changing the coupling between the two integrated cavities. This allows for ultra-low voltage modulation with a swing of less than 100 mV for optical amplitude changes up to 27 dB. Determination of the equivalent circuit diagram of the parasitics and deconvolution allows to determine a record high intrinsic bandwidth of the EOM VCSEL of 56 GHz. Thus data rates of close to 100 Gbit/s seem to be feasible with technologically improved devices.
We acknowledge funding by the Deutsche Forschungsgemeinschaft (CRC 787), the EU FP7 program under Agreement No. 224211, the state of Berlin (100 x 100 Optics) and VI-Systems GmbH, Berlin. We would like to thank Dr. V. A. Shchukin, Prof. N. N. Ledentsov and Prof. U. W. Pohl for helpful discussions.
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