Self-organized quantum dots for 1.3 µm photonic devices M. Laemmlin*a, G. Fiola, C. Meuera, M. Kuntza, F. Hopfera, N.N. Ledentsova, A.R. Kovshb, and D.

Bimberga aInstitut fuer Festkoerperphysik, Technische Universitaet Berlin, Hardenbergstr. 36, 10623 Berlin,

Germany bNL Nanosemiconductor GmbH, Konrad-Adenauer-Allee 11, 44263 Dortmund, Germany

ABSTRACT Nanotechnology is a driver for novel opto-electronic devices and systems. Nanosemiconductors like quantum dots allow controlled variation of fundamental electronic and optical properties by changing the size and shape of the nanostructures. This applies directly to self-organized quantum dots which find a versatile use in many kinds of photonic devices. Wavelength tunability, decreased laser threshold, scalability of gain by stacking quantum dot layers, low linewidth enhancement factor and temperature stability are consequences of three-dimensional carrier confinement in semiconductor quantum dots. Directly modulated lasers using quantum dots offer further advantages like strongly damped relaxation oscillations yielding low patterning effects in digital data transmission. Quantum dot mode-locked lasers feature a broad gain spectrum leading to ultra-short pulses with sub-ps width and a low alpha factor for low-chirp. Thereby, optical comb generators for the future 100G Ethernet are feasible. Semiconductor optical amplifiers based on quantum dots show advantages as compared to classical ones: broad bandwidth due to the inhomogeneous quantum dot size distribution, ultrafast gain recovery for high-speed amplification and small patterning in optical data transmission. We present our most recent results on temperature stable 10 Gb/s, 23°-70°C direct modulation of lasers, ultrafast 80 GHz and short 710 fs optical pulse combs with mode-locked lasers and semiconductor optical amplifiers showing ultrafast amplification of these optical combs as well as error-free 40 Gb/s data modulation, all based on a quantum dot gain medium.

Keywords: Quantum Dots, Directly modulated laser, Mode-locked laser, Semiconductor optical amplifier

1. INTRODUCTION

Today, quantum dots (QD) form the basis of novel generations of optoelectronic devices like edge and surface emitting lasers and amplifiers [1-3]. QDs have a large potential also for future quantum cryptographic and communication systems. Besides gaining a deeper insight into the exciting physics of nanostructured semiconductors, the use of GaAs-based QDs in diode lasers and amplifiers at data- and telecom wavelengths has been demonstrated to yield a large number of decisive advantages for lightwave communication systems, both from point of view of performance and of cost. Amongst them are:

• Lasing wavelengths in the 1.3 µm spectral range, both for edge and surface emitters using GaAs substrates [4, 5]. 1.5 µm emission wavelength of GaAs based edge emitting lasers using metamorphic buffers [6]

• Very low transparency current density (<6 A/cm2 per QD sheet) and internal losses (~1.5 cm-1), high internal quantum efficiency of 98% for a triple sheet QD-laser at 1.15 µm. 12 W output power, equivalent to a power density of 18.2 MW/cm2, for a 6-fold MOCVD grown stack. In lifetime tests at 1.0 W, 1.5 W and 50°C heat sink temperature no aging of these lasers within 3000 h could be observed [7, 8]

• Stability enhancement by 23 dB for external optical feedback at 1.3 µm [9, 10] • Large tuning range of > 200 nm [11] • Improved radiation hardness and suppressed facet overheating, increasing the COMD level [12, 13]. • Complete suppression of filamentation for the transverse ground mode up to stripe widths of 9 µm at 1.3 µm

leading to strongly increased coupling efficiency into fibers [14].

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• Deep mesa lasers with superb index guided performance down to very narrow stripe widths (1 µm) and completely spherical far field [15] are opening new opportunities for cost-efficient photonic crystal, distributed feedback applications and low-cost coupling to fibers.

• 12 GHz modulation bandwidth at room temperature [16] • 10 Gb/s error-free data modulation (error rate 10-12) obtained at –2 dBm receiver power, 1.3 µm emission

wavelength [17, 18] • Passive mode-locking in the range of 5 to 80 GHz at wavelengths around 1.3 µm [15, 19] with pulse width

below 1 ps. Hybrid mode-locking [15, 20] at frequencies up to 40 GHz yields a significant improvement of the pulse timing jitter and enables external synchronization.

• 40 Gb/s error-free amplification (error rate 10-11) with a fiber net gain of 8 dB, 1.3 µm emission wavelength • 1.2 mW/ 2 mW output power and a slope efficiency of 64 % at 300 K for VCSELs with fully oxidized

/semiconductor mirrors at 1.3 µm [5, 21]. Several epitaxial improvements were proposed and partially realized to achieve the abovementioned results, i.e. growth of InGaAs/GaAs QDs on template layers [22], overgrowth of QDs with quantum well layers [23], stacking of QDs [24], close stacking of QDs leading to vertical coupling of the QD layers [25], defect reduction techniques [26], introduction of strain relaxation layers [27], p-doping of the GaAs barrier layers [28], and tunnel injection of carriers into the QDs through a thin barrier layer [29]. In the following part of this paper we present in detail the high-speed properties of QD-based directly modulated and mode-locked lasers and amplifiers.

2. DIRECTLY MODULATED QUANTUM DOT LASERS

Directly modulated laser diodes are the key component of fiber-based datacom, converting digital electrical signals into digital optical signals at a rate of 10 Gb/s or more. Main issues of research are the improvement of modulation speed, reduction of power consumption, reduction of temperature sensitivity and the simplification of processing and mounting of the laser diodes. As presented in the previous section, QD lasers comprise several advantages making them ideal devices for fiber-based datacom.

2.1. Device structure

The Al0.35Ga0.65As/GaAs laser structures incorporating a tenfold stack of InGaAs quantum dots emitting at 1.3 µm wavelength were grown by molecular beam epitaxy [30]. The wafers were then processed into ridge waveguide (RW) structures with stripe widths from 1 to 4 µm by dry etching through the active layer to suppress current spreading and provide strong index guiding of the optical mode [31]. A 1000 µm long, 1 µm width RW diode with 95 % HR coating on the rear facet and backside n-contact was mounted in a fiber-optic module comprising a temperature controlled heat sink, a microwave port with an integrated impedance-matching bias network, and a single-mode fiber (SMF) pigtail. At room temperature, the QD laser module had a threshold current density of 270 A/cm2, emission wavelength of 1280 nm and a small-signal modulation bandwidth of about 7 GHz. Temperature dependent measurements were done with a device of 500 µm length, 4 µm ridge width and HR-coated rear facet.

2.2 Eye pattern and bit-error rate measurements on QD laser module

Eye pattern measurements were carried out back-to-back with the QD module biased at 5-7 times the threshold current density and a non-return-to-zero (NRZ) pseudo-random binary sequence (PRBS, word length of 215-1) with 2.5 Vp-p amplitude (12 dBm). The average output power into fiber was 1-3 mW. The inset of Fig. 1a shows clearly open 10 Gb/s eye patterns, with a signal-to-noise (S/N) ratio of 6.8, an extinction ratio of 4.9 dB and a peak-to-peak timing jitter of

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-14 -12 -10 -8 -6 -4 -2 0

Receiver power [dBm] Receiver optical power [dBm]

30 ps. Open eye patterns were observed up to 12 Gb/s. Due to the strong damping of relaxation oscillations in QD lasers, all eye patterns show very little overshoot. BER measurements were carried out at data rates of 8, 10, 11 and 12 Gb/s, keeping the eye pattern measurement settings. We inserted a semiconductor optical amplifier between laser and BER tester to compensate for optical losses due to a low laser-to-fiber coupling efficiency of 10 %. Fig. 1a shows the BER measurements for the QD laser module. Both for 8 and 10 Gb/s, we achieve error free operation (BER < 10-11) at –4.5 dBm and -2 dBm receiver power, respectively. No error floor could be detected. There is a considerable power penalty of 2.5 dB when moving from 8 to 10 Gb/s data rate in agreement with the moderate bandwidth of 7 GHz.

Fig. 1 BER measurement of QD laser module at 8 Gb/s and 10 Gb/s (a) and at 10 Gb/s for different temperatures (b), inset shows the corresponding eye patterns. The BER curve at 8 Gb/s follows a straight line whereas the data for 10 Gb/s show a curvature that is unexpected. A possible reason for this effect might be a saturation of the RF-amplifier used to amplify the electrical signal at the BER tester.

2.3. Temperature-dependent bit-error rate measurements

One of the main advantages of QD carrier confinement is the decreased temperature sensitivity of the threshold current and quantum efficiency. Cost-effective packaging of QD lasers is only feasible if this temperature insensitivity can also be demonstrated for the dynamic properties of QD lasers. While temperatures below room temperature generally improve the dynamic behavior, high temperatures might lead to roll-over of modulation speed. Preliminary eye pattern measurements at elevated temperatures have shown qualitatively that 10 Gb/s digital modulation seems possible up to 70°C without current adjustment [32]. However, low-error data modulation (BER < 10-9) cannot be judged by eye pattern measurements. Recently, we carried out the first BER measurements at 10 Gb/s and elevated temperatures (50 and 70°C) without adjustment of any of the driving parameters (bias current, RF power, bit rate). For room temperature, we achieved a BER below 10-12 only limited by measurement time, comparable to the results depicted in Fig. 1a. Figure 1b indicates a temperature dependent error floor for the BER, thus limiting the BER at 70°C to values of about 10-9. The error floor is due to the decrease of modulation bandwidth at higher temperatures and could be completely eliminated by an increase of the modulation bandwidth of the QD lasers by a few GHz. The small power penalty of less than 2 dB associated with the increase in temperature indicates the decreased temperature sensitivity of the threshold current and output power.

3. MODE-LOCKED QUANTUM DOT LASERS Mode-locking of monolithic semiconductor lasers is an efficient method to generate a regular pattern of short optical pulses at high repetition rate and narrow spectral width with a small footprint device. The application of mode-locked lasers in optical datacom often requires time or wavelength multiplexing of the optical pulses. Therefore the pulses should be short, have a narrow spectral width (ideally close to the Fourier limit), high peak power and a low timing jitter.

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QD lasers offer a broad gain spectrum (> 50 nm) leading to ultra-short pulses with sub-ps width, feasibility of long cavities (~ 1 cm) for low repetition rate applications [15, 33] and a low α-factor [34] for low-chirp, Fourier-limited pulses. At the same time, the mode-locked QD lasers comprise the advantages mentioned in the previous sections.

3.1. Device structure

Quantum dot devices for mode-locking were grown and processed similar to the laser structures described in the previous section, with fifteen layers of QDs for maximum gain and shortest cavities. The samples for mode-locking were processed into two-section devices by defining a metallization and a contact layer gap of 20 µm between the sections yielding an insulation resistance above 10 kΩ. The length ratio between reverse bias section and gain section was set to 1:9. The devices were mounted p-side up on a copper heat sink and were electrically connected with a double probe head. The samples with 4 µm ridge width had lengths between 2000 and 500 µm corresponding to round trip frequencies of 20 to 80 GHz. All samples with length below 1 mm were HR coated (95 % reflectivity) on the absorber section facet in order to ensure ground state lasing and enhance pulse auto-collision effects in the absorber section. All measurements were carried out at room temperature (297 K) and continuous wave.

3.2. Passive mode-locking

The 500 µm long laser was passively mode-locked at currents between 10 and 60 mA and reverse bias voltages between 0 and -10 V. The mode-locking frequency was 80 GHz, which is to our knowledge the highest ML frequency achieved for quantum dot lasers. Fig. 2a shows the corresponding autocorrelation trace, along with the full temporal range including the neighboring cross-correlation peaks. Fig. 2b depicts the reverse bias vs. current scan of the autocorrelation traces and the corresponding FWHM pulse widths. With increasing reverse bias, the onset of lasing shifts to larger currents, due to the increasing absorption within the waveguide. The onset of lasing occurred abruptly as mode-locking, we observed no transition region. With increasing current at constant reverse bias, the pulses became broader, until we observed a cw offset, i.e. incomplete mode-locking. At even higher currents, we observed a transition region with all kinds of complex pulse patterns until all intensity fluctuations flattened out to cw lasing. The minimum pulse width we achieved with this device was 1.5 ps. The corresponding spectrum yields a time-bandwidth product of 1.7, which is well above the Fourier transform limit of 0.32 for sech2-shaped pulses, indicating the large potential to further reduce the pulse width, e.g. by optimization of the passive section.

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Fig. 2 Autocorrelation trace of a passively mode-locked quantum dot laser at 1.3µm and 80 GHz repetition rate. The side peaks correspond to the cross-correlation of two successive pulses, while the middle peak presents the autocorrelation of a pulse (a). Field scan of autocorrelation traces with color-coded FWHM pulse widths of a 80 GHz passively mode-locked QD laser. Three regimes of operation can be distinguished (b).

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Similar pulse and Fourier product characteristics were found for other devices at frequencies of 20 and 40 GHz. The smallest pulse width to period ratios we achieved at frequencies between 20 and 80 GHz were in the range between 2 and 10 %. A first hint on the intrinsic limitations of mode locking in QD lasers is given by the increasing displacement of the center wavelength of the saturable absorber and the center wavelength of the lasing emission that we observed for high currents and large reverse bias voltages. The red shift of the absorber wavelength is possibly due to the quantum confined Stark effect, while the blue shift of the gain spectrum is due to filling of QDs emitting at shorter wavelength at higher pump currents. Both effects lead to an intrinsic limitation of the spectral overlap of both sections and might be responsible for the increase of pulse widths for larger reverse bias voltages (see Fig. 2b) and the large time-bandwidth product. Besides a small pulse width a low pulse jitter is desirable for the application of QD mode-locked lasers as optical clocks. We investigated the timing jitter of passively mode-locked QD lasers with repetition rates up to 40 GHz by means of optical cross-correlation measurements and sideband noise measurements using a 50 GHz optical detector and a 50 GHz spectrum analyzer. Comparison of autocorrelation and cross correlation (e.g. Fig. 2a) allowed us to estimate the uncorrelated jitter to be less than 1 ps. However, the main jitter contribution is expected to be correlated jitter, i.e. jitter depending on the temporal position of hundreds or thousands of preceding pulses. Sideband noise measurements of a 40 GHz passively mode-locked QD laser yielding a root mean square (RMS) timing jitter of 5 to 15 ps showed the dominating influence of correlated jitter. The timing jitter decreases with the gain current.

3.3. Hybrid mode-locking

Most applications of mode-locked lasers require synchronization with an external electrical signal. This can be done by applying a radio-frequency (RF) signal to one of the laser sections, typically with a frequency in the vicinity of the cavity round trip frequency. We found that the application of RF power to the absorber section was much more efficient than modulation of the gain section. The large series resistance of the reverse biased absorber section caused a doubling of the RF voltage amplitude and a large modulation of the reverse bias voltage, while the small series resistance of the forward biased gain section lead to a current modulation which was strongly damped by the RC bandwidth (~7 GHz). Hybrid mode-locking of QD lasers yielded small improvement of the pulse width. The shortest deconvoluted pulse width obtained for the 20 GHz device, best fitted by a sech2 shaped pulse, was 710 fs (Fig. 5), as compared to 900 fs for the passively mode-locked device. The improvement of hybrid mode-locking for the 40 GHz device was only marginal (a few percent). The locking range, i.e. the frequency range were the pulse repetition frequency locks to the RF signal, was found to be 90 MHz and 7 MHz for 20 GHz and 40 GHz repetition frequency, respectively.

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Fig. 3 Field scan of autocorrelation traces with color-coded FWHM pulse widths of a 40 GHz mode-locked QD laser (left). Pulse width and RMS jitter dependence on gain current for a 40 GHz hybrid mode-locked QD laser. The jitter decreases with increasing pulse width (right). The timing jitter however decreased from over 10 ps for passively mode-locked operation to values between 0.5 and 2 ps for the 40 GHz device when applying 14 dBm of RF power to the absorber section. Fig. 3 shows the dependence of

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pulse width and jitter on the gain current at a constant absorber voltage of -7 V. The RMS jitter was calculated from the sideband noise measurement between 1 kHz and 1 GHz. We observe a trade-off behavior, i.e. shortest pulse width corresponds to largest jitter and vice versa. The smallest jitter observed was 400 fs, which lies already in the range of state-of-the-art monolithic quantum well devices [35]. We checked the significance of the RMS jitter values measured by sideband noise integration by comparing to time domain oscilloscope measurements. A 70 GHz bandwidth oscilloscope with a 40 GHz precision time base and a 50 GHz optical detector was used to analyze the 40 GHz pulse trace from the mode-locked QD laser. Oscilloscope jitter values differed by less than 10 % from the values found by sideband noise integration.

4. QUANTUM DOT AMPLIFIER MODULES For optical networks, optical amplifiers are of largest importance serving as boosters, in-line or pre-amplifiers. They also perform regenerative (2R), wavelength-conversion or switching tasks for optical signal processing. QD amplifiers offer large advantages as compared to classical ones: broad bandwidth due to QD size distribution, ultra-fast gain recovery (~100 fs [36]) for high-speed amplification, reduced chirp due to a low α-factor [34] enabling fast switching and intrinsic strongly damped relaxation oscillations [37] yielding low patterning effects.

4.1. Device structure

Quantum dot amplifier structures were grown and processed similar to the laser structures, with either ten or fifteen layers of QDs. The cavity facets were tilted by 7° and anti-reflection coated with a residual reflectivity of 10-3 in order to prevent the formation of longitudinal modes. The tenfold stacked samples with 4 µm width and 2 mm length were mounted in a fiber-coupled module comprising two SMF ports, a temperature controlled heat sink and a DC supply. Due to large fiber coupling losses of about 12 dB the maximum fiber net gain of the module was 10 dB, corresponding to 22 dB chip gain at a current density of 600 A/cm2. The -3 dB saturation output power of the amplifier module was +2.5 dBm for the same drive current. The gain maximum was centered at a wavelength of 1290 nm.

4.2. Eye pattern and bit-error rate measurements

Bit patterned optical input for the amplifier was generated using a tunable external cavity laser (ECL) combined with an electro-optical modulator and a bit pattern generator. The eye pattern measurements were carried out with the QD amplifier module biased at 400-600 A/cm2, with an optical NRZ PRBS with a word length of 231-1 and -5.8 dBm average optical power. The average optical output power from the QD amplifier module was +2.5 dBm. The amplifier output was measured with a fast photodetector using a semiconductor optical amplifier as optical pre-amplifier.

Fig. 4 BER measurement of QD amplifier module at 40 Gb/s for different drive currents; the inset shows the eye pattern for 120 mA drive current.

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Clearly open eye patterns were observed at a bit rate of 40 Gb/s (inset of Fig. 4), yielding an extinction ratio of 6 dB, a S/N ratio of 7.7 and a RMS timing jitter of 1.4 ps. Bit error rate measurements were performed by varying the optical input power at the preamplifier and for two different drive currents. Error-free modulation (BER < 10-10) was achieved for 120 mA drive current, only limited by measurement time. For lower drive current, a power penalty of 2 dB and an error floor at BER < 10-9 were found.

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Fig. 5 (left) Autocorrelation measurement of 20 GHz, 710 fs pulse amplification in QD SOA; we compare both the input and output signal. (right) 80 GHz auto- and cross correlation of input and output signals of the QD amplifier. Amplification of ultra-short pulses with sub-picosecond width was checked using the QD ML laser at 20 GHz presented in the previous section. The output from the ML laser was fiber-coupled to a SOA of the same QD gain material. 710 fs pulses in a 20 GHz pulse train were amplified with a fiber net gain of 8 dB by the 4 mm long QD SOA. The maximum chip gain of this device was 26 dB. No degradation of the pulse width by the SOA performance could be observed as demonstrated in Fig. 5, left comparing the autocorrelation traces of the input and the output. At 80 GHz repetition frequency the pulse width of 1.9 ps was broadened by 15 % after amplification (Fig. 5, right).

5. SUMMARY For InAs-GaAs based quantum dot lasers emitting at 1300 nm digital modulation with bit error rates below 10-11 at -2 dBm receiver power were demonstrated. Error rates of 10-9 or better were realized between 23°C and 70°C without current adjustment. Passive and hybrid mode-locked QD lasers generate optical pulses with repetition frequencies up to 80 GHz and 40 GHz, respectively, with an ultra-short minimum pulse length of 710 fs at 20 GHz. The minimum root mean square jitter of 40 GHz hybrid mode-locked pulses was found to be 400 fs with a corresponding pulse width of 4 ps. Quantum dot based 1.3 µm fiber-coupled amplifier modules showed 40 Gb/s operation, respectively, with bit error rates below 10-10. Amplifier chip gain up to 26 dB has been demonstrated. Ultrafast 80 GHz and short 710 fs optical pulse combs from mode-locked lasers were amplified with 8 dB net gain and without significant pulse broadening. These achievements became possible due to systematic development of self-organized growth technologies and demonstrate that QD device performance is now comparable or even superior to quantum well devices. Acknowledgements We are indebted to S. Ferber and C. Schubert from the Fraunhofer Heinrich-Hertz-Institut, Berlin for assistance with the bit-error rate measurements, as well as A. Umbach and A. Steffan from u2t photonics AG, Berlin for packaging of the QD devices. Parts of this work were supported by BMBF, DFG (Sfb296), EU-IST-027638 STP (TRIUMPH), Zukunftsfond Berlin (TOB, ProFit-Monopic), and SANDiE Network of Excellence of the European Commission No. NoE-NMP-CT-2004-500101.

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