gain recovery in a quantum dot semiconductor optical

Gain recovery in a quantum dot semiconductor

optical amplifier and corresponding pattern

effects in amplified optical signals at 1.5 µm

J. Park,1 Y. D. Jang,

1 J. S. Baek,

1 N. J. Kim,

1 K. J. Yee,

1 H. Lee,

1 D. Lee,

1,* S. H. Pyun,

2

W. G. Jeong,2 and J. Kim

3

1Department of Physics, Chungnam National University, Daejeon 305-764, Korea 2Department of Materials Engineering, Sungkyunkwan University, Suwon 440-746, Korea

3Department of Information Display, Kyung Hee University, Seoul 130-701, Korea *[email protected]

Abstract: Fast gain recovery observed in quantum-dot semiconductor-

optical-amplifiers (QDSOAs) is useful for amplifying high-speed optical

signals. The small but finite slow recovery component can deteriorate the

signal amplification due to the accumulation of gain saturation during 10

Gb/s operation. A study of the gain recoveries and pattern effects in signals

amplified using a 1.5 µm InAs/InGaAsP QDSOA reveals that the gain

recovery is always fast, and pattern-effect-free amplification is observed at

the ground state. However, at the excited state, the slow component

increases with the current, and significant pattern effects are observed.

Simulations of the pattern effects agreed with the observed experimental

trends.

©2012 Optical Society of America

OCIS codes: (230.5590) Quantum-well, -wire and -dot devices; (300.6530) Spectroscopy,

ultrafast; (250.5980) Semiconductor optical amplifiers; (160.4760) Optical properties.

References and links

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2. J. Park, N. J. Kim, Y. D. Jang, E. G. Lee, J. M. Lee, J. S. Baek, J. H. Kim, H. S. Lee, K. J. Yee, D. Lee, S. H.

Pyun, W. G. Jeong, and J. Kim, “Gain dynamics of an InAs/InGaAsP quantum dot semiconductor optical

amplifier operating at 1.5 µm,” Appl. Phys. Lett. 98(1), 011107 (2011).

3. P. Borri, W. Langbein, J. M. Hvam, F. Heinrichsdorff, M.-H. Mao, and D. Bimberg, “Ultrafast gain dynamics in

InAs-InGaAs quantum-dot amplifiers,” IEEE Photon. Technol. Lett. 12(6), 594–596 (2000).

4. S. Schneider, P. Borri, W. Langbein, U. Woggon, R. L. Sellin, D. Ouyang, and D. Bimberg, “Excited-state gain

dynamics in InGaAs quantum-dot amplifiers,” IEEE Photon. Technol. Lett. 17(10), 2014–2016 (2005).

5. M. Sugawara, N. Hatori, M. Ishida, H. Ebe, Y. Arakawa, T. Akiyama, K. Otsubo, T. Yamamoto, and Y. Nakata,

“Recent progress in self-assembled quantum-dot optical devices for optical telecommunication: temperature-

insensitive 10 Gb s−1 directly modulated lasers and 40 Gb s−1 signal-regenerative amplifiers,” J. Phys. D Appl.

Phys. 38(13), 2126–2134 (2005).

6. T. W. Berg, S. Bischoff, I. Magnusdottir, and J. Mork, “Ultrafast gain recovery and modulation limitations in

self-assembled quantum-dot devices,” IEEE Photon. Technol. Lett. 13(6), 541–543 (2001).

7. S. Dommers, V. V. Temnov, U. Woggon, J. Gomis, J. Martinez-Pastor, M. Laemmlin, and D. Bimberg,

“Complete ground state gain recovery after ultrashort double pulses in quantum dot based semiconductor optical

amplifier,” Appl. Phys. Lett. 90(3), 033508 (2007).

8. N. J. Kim, J. M. Oh, M. D. Kim, D. Lee, S. H. Pyun, W. G. Jeong, and J. W. Jang, “Gain characteristics of

InAs/InGaAsP quantum dot semiconductor optical amplifiers at 1.5 µm,” Appl. Phys. Lett. 90(24), 241108

(2007).

9. K. J. Han, D. W. Jang, J. H. Kim, C. K. Min, T. H. Joo, Y. S. Lim, D. Lee, and K. J. Yee, “Synchronously

pumped optical parametric oscillator based on periodically poled MgO-doped lithium niobate,” Opt. Express

16(8), 5299–5304 (2008).

10. J. Kim, C. Meuer, D. Bimberg, and G. Eisenstein, “Numerical simulation of temporal and spectral variation of

gain and phase recovery in quantum-dot semiconductor optical amplifiers,” IEEE J. Quantum Electron. 46(3),

405–413 (2010).

11. K. L. Hall, G. Lenz, A. M. Darwish, and E. P. Ippen, “Subpicosecond gain and index nonlinearities in InGaAsP

diode lasers,” J. Opt. Commun. 111(5-6), 589–612 (1994).

#158473 - $15.00 USD Received 23 Nov 2011; revised 24 Feb 2012; accepted 27 Feb 2012; published 2 Mar 2012(C) 2012 OSA 12 March 2012 / Vol. 20, No. 6 / OPTICS EXPRESS 6215

12. Y. D. Jang, N. J. Kim, H. Lee, D. Lee, S. H. Pyun, W. G. Jeong, J. W. Jang, D. K. Oh, and J. S. Kim, J. “Effects

of band-offset on the carrier lifetime in InAs quantum dots on InP substrates,” J. Appl. Phys. 101(9), 096103

(2007).

A semiconductor optical amplifier (SOA) is potentially very useful for optical signal

amplification and signal processing [1]. However, conventional SOAs made from bulk

materials or quantum wells are not adequate for amplifying high-speed optical signals due to

the pattern effects caused by the slow gain recovery (~100 ps) [2]. In contrast,

In(Ga)As/GaAs quantum dot (QD) SOAs operating below 1.3 µm have shown very fast gain

recovery (~1 ps), which is expected of zero-dimensional QDs [3, 4]. A QDSOA was used to

cleanly amplify a signal without producing pattern effects at 10 and 40 Gb/s at 1.55 µm [5].

Fast gain recovery without slow components was observed at the ground state (GS) of an 1.5

µm InAs/InGaAsP QDSOA, whereas fast gain recovery with non-negligible slow components

was reported at the excited state (ES) [2].

In practical applications, such as amplification of 10 or 40 Gb/s signals in non-return-to-

zero sequence (NRZ), gain saturation accumulates due to long pulses (the minimum pulse

width: 100 ps at 10 Gb/s or 25 ps at 40 Gb/s), which can be considered as the sum of many

pulses that are not separated by intervals. In this case, the slow gain recovery component

contributes significantly to the shape of the amplified signals. Berg et al. described

calculations of the gain changes in a QDSOA caused by high-repetition short pulse trains, and

they predicted the development of serious distortion in the amplified signals, which originated

from slow wetting layer dynamics [6]. Dommers et al. reported gain recovery after short

double pulses in an InGaAs/GaAs QDSOA; the residual gain changes at a delay of 5 ps were

not negligible at low currents and became more serious at the ES [7]. However, the effects of

gain recovery on long pulses, such as those used in telecommunications systems, have not

been studied in relation to gain recovery, which is very important for systems designed using

QDSOAs.

In this work, we investigated the gain dynamics of an InAs/InGaAsP QDSOA operated at

1.5 µm and the corresponding pattern effects in the amplified optical signals at 10 Gb/s. The

bias current was selected to be typical of operating condition for an in-line amplifier or

booster amplifier, and the input power was selected to yield similar saturation levels at

different wavelengths as a means for developing a fair comparison. We found that the slow

gain recovery component, although small, was critical for signal distortion in a long pulse due

to the accumulation of gain saturation. The slow component mainly contributed to pattern

effects observed in 10 Gb/s amplified signals at the ES.

An InAs/InGaAsP QDSOA was fabricated in a ridge waveguide structure 3 µm in width

and with a 7° tilt. Both facets of the 2 mm long QDSOA were antireflection-coated, and the

QDSOA was fiber-pigtailed. The active medium consisted of 7 stacks of InAs/InGaAsP QDs

with round dome shapes [8]. The chip gain at 1494 nm (ES) was 17.4 dB under an operating

current of 300 mA, and the gain at 1535 nm (GS) was 7.2 dB. The insertion loss per facet was

approximately 8.5 dB. The transverse electric (TE) gain was dominant in the QDSOA due to

the small aspect ratios of the QDs.

Gain recovery was measured using a pump–probe method. In this method, a strong pump

pulse depleted the carriers through stimulated emission, and a weak probe pulse (x1/10)

examined the changes caused by the strong pump signal at the same wavelength. We used a

heterodyne pump–probe technique to detect only the weak probe signal in a waveguide

structure in which spatial separation between the pump and probe signals was not possible [2].

Figure 1 shows a schematic diagram of the experimental setup. Femtosecond pulses from an

optical parametric oscillator had a pulse width of 260 fs, a repetition rate of 80 MHz, and

were tunable across the entire gain band [9]. A dispersion compensating fiber was inserted to

minimize pulse broadening by compensating for the dispersion caused by the pigtail fibers of

the acousto-optic modulators and the polarization controllers.


Fig. 1. A heterodyned pump–probe setup. OPO: optical parametric oscillator, PBS: polarization

beam splitter, PC: polarization controller, AOM: fiber pigtailed acousto-optic modulator, DCF:

dispersion compensating fiber.

In a QDSOA, the depleted states at the GS are quickly filled by carriers from the upper

discrete states, and the depleted discrete states are sequentially filled by carriers from the

upper-lying continuum states, such as the wetting layer or the barrier. Relaxation from

discrete states is rapid, and the processes involving continuum states are relatively slow in the

QDSOAs [2]. A gain changes in the energy position are expected to include contributions

from any perturbed states, with amplitudes that depend on the energy separation and line

shape (typically Lorentzian) [10].

Fig. 2. Gain recovery curves. Operation current dependence at the (a) GS and (b) ES. Note the

significant increase in the slow components at the ES as the current increased.

Figure 2 shows the measured raw gain recovery curves at the GS and ES. To recapitulate

typical operational situations for system applications, we examined the dependence of the

gain recovery characteristics on the bias current around 400 mA. Because the QDSOA

yielded two quite different gains at the two states, the measurements were performed with a 3


dB gain saturation input power at 300 mA for both states. Femtosecond pump pulses were

used to determine the input pump power that yielded a 3 dB gain saturation. The gain

saturation was 3.4 dB at 300 mA and 4.0 dB at 500 mA for the GS. For the ES, the values

were 1.6, 3.0, 4.0, and 4.9 dB at 200, 300, 400, and 500 mA, respectively. The gain dynamics

were dominated by the fast component in both states. The slow recovery component was

almost negligible at both currents at the GS. In contrast, the slow component at the ES was

not negligible at 300 mA and increased significantly at higher currents.

Due to the finite 260 fs pulse width, which was on the order of the fast gain recovery time,

the actual gain dynamics were convoluted with the pulse employed in the measured curves.

The actual recovery curves were deconvoluted from the measured recovery curves using the

measured pump–probe autocorrelation function [11]. Fitting was performed with two fast

components related to discrete energy states, and two slow components related to continuum

states [2].

Fig. 3. The extracted actual gain recovery curves. Operating current dependence at the (a) GS

and (b) ES.

Figure 3 shows the actual gain recovery curves (impulse response functions) extracted

from fits which include effects of two photon absorption, coherent artifacts, and a finite pulse

width [11]. At the GS, the gain dynamics at 500 mA were almost the same as those at 300

mA, with negligible slow components, as shown in Fig. 3(a). The fast recovery time was ~0.7

ps, and the very small slow recovery time was ~300 ps. The resulting 90–10% recovery time

was 4 ps at 500 mA. This result predicts excellent amplification of 10 and 40 Gb/s optical

signals without pattern effects under most conditions.

Figure 3(b) shows the actual gain recovery curves at the ES for various given operation

currents. The time constants were almost the same as those at the GS. However, in contrast to

the behavior observed at the GS, the slow component was not negligible, rather, it increased

significantly as the current increased. At 500 mA, the slow component increased significantly

relative to that at 300 mA. Because the slow component accumulated during the long 100 ps

pulse, the observation of a stronger slow component at a higher current predicted the

development of larger pattern effects during amplification of 10 or 40 Gb/s optical signals.

Considering that the relatively high gain in the QDSOAs typically comes from ES


contributions [5, 8], this predicted effect may cause serious problems during signal

amplification using InAs/InGaAsP QDSOAs.

To confirm the predictions for the ES and GS, we measured eye diagrams for 10 Gb/s

amplified signals (NRZ pseudo-random-binary-sequence (PRBS) 231

-1) under various

conditions. For the GS, the 1535 nm laser light from a tunable laser was modulated by a

Mach–Zehnder modulator and amplified with an erbium-doped fiber amplifier (EDFA), as

shown in Fig. 4. The signal passed through a polarization controller before directing the

transverse electric (TE) polarized light into the QDSOA. The output signal was then amplified

using another EDFA. The light passed through an attenuator that equalized the input power to

the sampling oscilloscope (digital communication analyzer), which permitted a fair

comparison of the eye patterns. Because no usable amplifiers are available at the ES, the 1494

nm signal was obtained by injection-locking a 1480 nm pump laser diode using a fiber-Bragg-

grating that reflected at 1494 nm. The laser displayed a clean lasing peak at 1494 nm with an

output power exceeding 10 mW. The gain at the ES was relatively high for the QDSOA, so no

further amplification was necessary for the examination of the eye patterns.

Fig. 4. The setup for measuring eye diagrams and observing the pattern effects in the amplified

signals.

Fig. 5. Eye diagrams measured at 1535 nm (GS) for the (a) back-to-back, bias current of (b)

300 mA, (c) 500 mA, and (d) 600 mA.

Figure 5 shows the eye diagrams of 10 Gb/s signals at the GS at different applied currents.

A large input power of 9 dBm was selected to give reasonable gain saturations: 0.82, 1.05,

and 1.08 dB at 300, 500, and 600 mA, respectively. The small gain saturation is likely due to

the fast gain recovery in the QDSOA at the GS. We did not observe degradation of the eye

patterns from the back-to-back signals, even at the highest currents. Clean amplification

without pattern effects at the GS was consistent with the fast gain dynamics at all currents.


Fig. 6. Eye diagrams measured at 1494 nm (ES). (a) Back to back, (b) bias current of 200 mA,

(c) 300 mA, (d) 400 mA, (e) 500 mA, and (f) 600 mA.

Figure 6 shows the eye diagrams of 10 Gb/s signals measured at the ES (1494 nm). The

input power was –2.5 dBm, and the gain saturations were 0.35, 0.8, 1.1, 1.6, and 1.9 dB at

200, 300, 400, 500, and 600 mA, respectively. At a fixed input power, the gain saturation

increased with the current but the absolute gain remained high, providing more carriers at a

higher current. Figure 6 clearly shows the pattern effects at the ES, even at 300 mA. The

effects became severe at higher bias currents. The observed pattern effects were consistent

with the results of the gain dynamics described earlier. The pattern effects call for caution

when using QDSOAs for amplification at the ES.

The eye patterns measured at the ES were consistent with the dependence of the gain

dynamics on the applied current; however they did not agree with the mechanism reported for

an In(Ga)As/GaAs QDSOA [4]. The fast gain recovery at the GS was typically explained in

terms of fast carrier relaxation from the ES, and the fast gain recovery at the ES was typically

explained in terms of the fast relaxation from the upper discrete states, which requires

available carriers in the upper states. In an In(Ga)As/GaAs QDSOA, the gain recovery at the

ES was relatively slow at low currents and became fast at higher currents, which attested to

the role of the available carriers in the higher states [4]. In contrast, the opposite result was

obtained in the InAs/InGaAsP QDSOA tested here: the slow component increased

significantly with an increase in the carrier density at higher energy states and at higher

currents. This discrepancy is likely due to the proximity of the continuum states to the ES in

the InAs/InGaAsP QD system [12]. The gain change in each state included additional

contributions from other energy states through the lineshape function, which is typically

Lorentzian. In an In(Ga)As/GaAs QD system, the GS and ES are located well below the

wetting layer, so the recovery in the depleted state provides the dominant contribution without

significant contributions from other states. Consequently, if the number of carriers in the

upper state is sufficient to fill the depleted state (at higher current), the gain recovery is

expected to be rapid. On the other hand, the wetting layer level is relatively close to the

depleted state (ES) in InAs/InGaAsP QDSOA and can introduce noticeable effects into the

gain recovery.

Ideally, gain changes can be calculated for pulses of any duration and shape by

convoluting the pulse with the corresponding impulse response function obtained from the

pump–probe measurements (Fig. 3). We calculated amplified optical pulses and compared

these with the measured eye diagrams. The fast gain recovery components on the order of 1 ps

did not contribute to distortion in the optical pulses at 10 or 40 Gb/s; however, the slow


components were the main source of the pattern effects (signal distortion) because the gain

decreased continuously over the pulse duration due to the accumulation of gain saturation.

A comparison of the gain changes in quasi-CW pulses (10 Gb/s signals) at the GS or at the

ES was not straightforward because the slow gain recovery components were the main

sources of signal distortion under quasi-CW conditions; however, the slow gain components

were quite different at the GS and ES. We selected two situations: a fixed magnitude for the

fast components, 0.2 dB, or a fixed average gain saturation for the quasi-CW signals, 1 dB,

under continuous operation. Gain saturation was controlled using the power of input signal,

which changed the gain saturation through convolution with the impulse response function.

The output signal was calculated by multiplying the input signal with the gain at each time

point. Because the impulse response function obtained at a high gain saturation of 3 dB is

likely to provide stronger slow recovery components than an impulse response function

obtained at a low gain saturation, the calculated results provide an upper limit for signal

distortion and the measured pulses are expected to be less distorted.

Fig. 7. Calculated normalized pulse patterns (10 Gb/s) for a given fast gain saturation (0.2 dB):

(a) back-to-back pulse trains, (b) calculated pulse pattern at the ES, and (c) calculated pulse

pattern at the GS.

Pulse patterns of a fixed fast gain recovery component, 0.2 dB, were compared for the 10

Gb/s optical signals in both the ES and GS. The shape of the 10 Gb/s signal used for the

calculation was the same as that used for the measured input pulse. A PRBS (231

−1) pulse

train (~2.15 billion pulses) was generated to simulate the experimental condition. The output


pulse patterns were calculated through convolution for 40 sets of 200 pulses which were

sampled every 50 million. The forty sets of amplified pulse trains were then overlaid in Fig. 7,

in which pulses of each set are plotted in the same color. Because of the slow gain recovery

component, the overall gain decreased after the start of the pulse trains and retained an

average value during continuous operation. The gain at each time depended on the number of

missing pulses prior to an incoming pulse. A PRBS (231

−1) pulse train can have consecutive

“1” or “0” of longer than 26 pulse length. However, the time constant of the slow recovery

component was 300 ps and so gain almost reaches the saturation point after 10 consecutive

pulses. The calculated maximum (minimum) pulse power after 26 consecutive “0” (“1”) was

1 (0.032), close to that of the sampled sets, 0.93 (0.036). For display purpose, the data

corresponding to a 3000 ps window were selected after 2000 ps from the start of the pulse

train. A pulse pattern provides more information about the fluctuations and can be used to

reconstruct an eye pattern by displacing and overlapping the pulses.

The average gain saturation was ~8 dB at the ES and 0.5 dB at the GS for a given 0.2 dB

fast gain saturation. The very large differences were due to differences in the magnitude of the

slow component, which accumulated throughout the pulse duration, even though it was small

at any given impulse. Accordingly, pulse distortion was very large at the ES and minor at the

GS, as shown in Fig. 7. Considering the importance of the slow component in this calculation,

the significantly increased pattern effects at high currents in the measured eye diagram, as

shown in Fig. 6, should arise from the more prominent slow components at higher currents, as

observed in the gain recovery curves (Fig. 3).

Fig. 8. Calculated gain charges due to pulse trains at the ES (a) and at the GS (b) for a given

average gain saturation under quasi-CW operation.

Figure 8 shows the calculated gain change at 10 Gb/s for a given average gain saturation

in both states, 1 dB, under quasi-CW operation. The fast component was 0.44 dB at the GS

and 0.025 dB at the ES for this case. The average gain saturation reached a quasi-equilibrium

state by the 1000 ps time point. Different colors represent different sets of pulse trains. Note

that the fluctuations in the gain change were more pronounced for the ES than for the GS,

even though the average saturated gain was the same.


Figure 9 shows the calculated pulse patterns at 10 Gb/s under quasi-CW operation

conditions for the given average gain saturation. More pronounced power fluctuations were

observed for the ES, although the average gain saturation was the same. This was due to the

fact that the contribution of the fast recovery component, which did not contribute to signal

distortion, was much larger at the GS; therefore, the gain fluctuations were smaller than at the

ES, in which most gain saturation arose from the accumulated slow component. Although the

pulse power used to calculate the gain dynamics and CW optical signal powers was different,

the trend was readily apparent.

Fig. 9. Calculated amplified pulse trains at the ES (a) and at the GS (b) for a given average gain

saturation under quasi-CW operation.

The measured and calculated results indicated that amplification at the GS can provide

high saturation output powers because it is difficult to saturate the gain in a long pulse due to

the predominance of the fast gain recovery component. Additionally, negligible pattern effects

were observed under normal operation conditions. On the other hand, amplification at the ES

may lead to troublesome pattern effects under some operational conditions. The present

findings suggest utilization of the GS gain in the InAs/InGaAsP QDSOA for high-speed

amplification and a long device to provide sufficient gain at the GS.

In summary, we examined the gain dynamics and pattern effects for 10 Gb/s optical

signals under various applied currents from an InAs/InGaAsP QDSOA operating around 1.5

µm. At the GS, no pattern effects were observed at any applied current. This was consistent

with the measured gain recovery curves, which recovered rapidly and included almost no

slow recovery components; however, at the ES, the pattern effects were not negligible at low

currents, and they became more significant at higher currents. The results agreed well with the

measured gain recovery behavior: the slow gain recovery component was not negligible at

low currents, and it became more significant as the applied current increased. Simulations of

the amplified optical signals using the obtained impulse response functions agreed well with

the trends observed in the measured eye diagrams. These results suggested that the


InAs/InGaAsP QDSOAs should be operated at the GS to provide clean amplification of high-

speed optical signals.

Acknowledgments

The authors acknowledge the support of the National Research Foundation of Korea (Grant

No. 2009-0081380 and No. R11-2008-053-01001-0 through QMMRC) and the KICOS (Grant

No. M60605000007-6A0500-00710).


gain recovery in a quantum dot semiconductor optical

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