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Very low input voltage cascaded travelling wave electroabsorption modulator (CTWEAM) for more than 100 Gbps
Optics Communications 297 (2013) 43–47
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Very low input voltage cascaded travelling wave electroabsorption modulator (CTWEAM) for more than 100 Gbps A.B.M. Hamidul Islam, Urban Westergren n School of Information and Communication Technology, KTH Royal Institute of Technology, SE-16440 Kista, Sweden
a r t i c l e i n f o
abstract
Article history: Received 14 June 2012 Received in revised form 7 January 2013 Accepted 8 January 2013 Available online 4 February 2013
This paper presents the large signal modeling and simulation of two segmented travelling wave electroabsorption modulators (TWEAM) as a cascaded TWEAM (CTWEAM) structure. It will be demonstrated that a mushroom shaped mesa structure with low series resistance (0.69 O-mm) will have 3 dB bandwidth more than sufficient for 100 Gbps operation. For a CTWEAM device with 990 mm active segment length, 1.1 V bias voltage and only 0.4 V peak-to-peak (Vp–p) modulating voltage has a bandwidth of 110 GHz, extinction ratio 44 dB and flat frequency response. This device would be an attractive candidate for short distance optical fiber communication as well as long distance telecommunication at 0.4 Vp–p and 1 Vp–p input RF signal, respectively. & 2013 Elsevier B.V. All rights reserved.
Keywords: Electroabsorption modulator Travelling wave 100 Gbps
1. Introduction Nowadays, the multiple quantum well (MQW) electroabsorption modulator (EAM) is attractive due to its high speed, large extinction ratio and low driving voltage, which is based on quantum-confined stark effect (QCSE) [1–3]. The compact size EAM can be monolithically integrated with external distributed feedback (DFB) laser source [1,4]. For high speed operation lumped EAM is short in size, but suffers from lack of modulation efficiency. To increase the modulation efficiency, it is necessary to use a longer device structure that increases junction capacitance and reduces the bandwidth, i.e., bandwidth is limited by RC time constant. Travelling wave EAM (TWEAM) has solved this problem by distributing the capacitance along the length of the device by proper design of the transmission line which increases the length of the device and also the modulation efficiency without sacrificing bandwidth. The speed of this modulator is limited by microwave attenuation loss due to mismatch between propagation velocity of microwave signal and group velocity of optical signal along the length of the device. This problem has been solved by proper design of the device into the segmented TWEAM [5]. EAM monolithically integrated with a distributed feedback (DFB) laser has become a common high-speed transmitter in optical networks. Compact size, excellent performance and reasonable cost have been achieved as a result of research. Monolithic integration has been important to make it possible to combine the advantages of continuous wave (CW) light from a laser with the modulation
n
Corresponding author. E-mail address: [email protected] (U. Westergren).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.01.028
properties offered by an external modulator. Breakthroughs in externally modulated lasers (EML) have been reported for 10 Gbps (Giga bit per second) and 40 Gbps [1–3,6] transmitters, and more recently also for 100 Gbps [5,7]. In order to reach bandwidths of the order of 100 GHz with low electrical reflection TWEAM was proposed [5–7] and demonstrated with record low 2 Vp–p driving voltage for 10 dB optical extinction for 230 mm total active length [8] and 2.5 Vp–p for 180 mm length with improved impedance matching [9]. The segmented structures were used to overcome the limit imposed by the RC constant of long modulators. It is based on transformation of the low characteristic impedance of active segments, which are suitable for high speeds, into a higher input impedance which makes it possible to achieve low electrical reflections throughout the bandwidth. These devices have been demonstrated to be useful in electrical time-division multiplexed (ETDM) transmitters with on-off-keying (OOK) up to 100 Gbps [10]. The bit rate has been demonstrated to be extendable to beyond 100 Gbps making the devices suitable for 100 Gbps Ethernet transmission [7]. Results for other 100 Gbps components such as multiplexer (MUX), driver amplifier (TWA), receiver, clock and data recovery circuit (CDR), and demultiplexer (DeMUX) are reported elsewhere as indicated in [7]. In this paper, we have studied the large signal modeling using the circuit simulator SPICE of segmented CTWEAM performance and optimization. The principle of using a cascaded structure with two TWEAMs in series reduces the voltage swing for each of the two electrical input contacts compared to the single input contact of one TWEAM, thus allowing the use of drive electronics with relatively small breakdown voltage. In Section 2, we describe the detailed design of CTWEAM. Section 3 describes the effect of modulating signal, biasing voltage and input optical power on
44
A.B.M.H. Islam, U. Westergren / Optics Communications 297 (2013) 43–47
device performances such as bandwidth, extinction ratio and overshooting. Next, we show the device optimization by changing the active segment and also microstrip line ratio that changes the device total effective impedance to reduce microwave return loss.
2. Cascaded modulator A segmented cascaded TWEAM (CTWEAM) consists of two segmented TWEAMs which is shown in Fig. 1. The cross section has a mushroom shaped mesa structure with lower vertical series resistance 0.69 O-mm compared to 1.25 O-mm previous designs [5]. Each segmented TWEAM [5,8,11] consists of two active (first (1a) and second (2a)) segments as shown in Fig. 1(a) and three passive (first (0t), second (1t) and third (2t) microstrip) transmission lines as shown in Fig. 1(b) and data for different CTWEAM designs are given
in Table 1. In this design, second microstrip (1t) length of first modulator is kept equal or at least 50 mm greater than the length of first active segment (1a) line of second modulator. If this microstrip length is less than the first active segment length of second modulator then the cascading modulator design will not be possible. Similarly, second microstrip (1t) length of second modulator is kept equal to or at least 50 mm greater than the length of second active segment (2a) line of first modulator. This design is more complex than a single segmented TWEAM, but it provides more controls of bandwidth, extinction ratio, overshooting and also undershooting to improve performance of the device. By varying its active and microstrip segment lengths, we can tune the CTWEAM to operate as we choose.
3. Results and discussion All component values of each segmented TWEAM are the same as those used in previous designs [5] except the vertical series resistance, the active segment lengths, microstrip line lengths and the load impedance (25 O). In the following, a number of designs are studied in order to investigate which parameters have impact on the performance and how an optimum design can be reached for operation at 100 Gbps. A general design goal of a modulating voltage swing of 0.4 Vp–p will be used for each of the two electrical inputs to the cascaded TWEAM in order to allow use of silicon-based drive electronics with some margin to what has been demonstrated at 100 Gbps [12]. 3.1. Effect of modulating signal Fig. 2 shows the 3 dB bandwidth of CTWEAM at different modulating voltages. The 3 dB bandwidth increases with increasing modulating signal voltage because of the nonlinear relation between Table 1 Design data for different CTWEAM [lengths are in mm]. Design Total active length La,tot
1A 1B 1C
Fig. 1. (a) Non-cascaded modulator and (b) Cascaded modulator 1A lateral layout of the chip with active segments in red, passive waveguides in blue, and microstrips in black. (c) Vertical cross section of a segmented TWEAM (lengths are in mm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
990 990 990
Active length of second modulator
Microstrip lengths of first modulator
Active length of first modulator
Microstrip lengths of second modulator
L0t
L1a
L2a
L20t L21t L22t L21a
L22a
100 220 245
100 160 50 220 200 270 100 100 100 350 50 245
560 110 250
L1t
L2t
200 270 100 110 100 160 50 560 100 350 50 250
Fig. 2. Effect of modulating signal on 3 dB bandwidth of different CTWEAM.
A.B.M.H. Islam, U. Westergren / Optics Communications 297 (2013) 43–47
modulating voltage and the 3 dB bandwidth of different modulator designs due to the intrinsic nonlinearity of the device performance. At modulating voltage 0.4 Vp–p, the device has 110 Gbps (Giga bit per second) and the eye the diagram at this voltage is shown in Fig. 3, which is comparable with previous work [5] and [8] 100 Gbps (at 3 Vp–p) and 100 Gbps (at 2.0 Vp–p), respectively. Extinction ratio: It is the ratio of optical power at pon label to optical power at poff label. It is calculated by 10logðpon =poff Þ, (dB). Overshooting: It is the ratio of peak amplitude of the optical power, ppeak to the optical power level, pon. It is calculated by 10logðppeak =pon Þ, (dB). Bandwidth: The bandwidth Df of a RC circuit can be determined by measuring its 10% to 90% of the transition time, tr (in ps) in an eye diagram. It is calculated by 0:35=tr , (GHz) [13]. Fig. 4 shows the effect on extinction ratio when changing the modulating signal for different designs of cascaded modulators, with results comparable to previous works [5,8]. When the modulating signal is increased linearly the extinction ratio is also increased but nonlinearly. This is due to the nonlinearity of the device absorption as a function of the junction voltage. Fig. 5 shows effect of reflection factor (S11 parameter) of microwave signal at different frequencies for CTWEAM modulator 1A which has about 10 dB reflection factor at 100 GHz which is an improved value compared to previous works [5] and [8] where the reflection becomes larger than 10 dB above 75 GHz and 85 GHz, respectively. Other CTWEAM designs 1B and 1C have about 10 dB reflection factor at 55 GHz and 65 GHz, respectively. In real life system, below 10 dB reflection factor is normally acceptable in electronics for data transmission.
junction capacitor. Which in turn to reduce the shunt resistance and also the RC time constant that helps to increase the bandwidth at high optical input power. But, the bandwidth of modulator design 1C decreases very little at high input optical power due to saturation effect of photogenerated current which is caused by screening effect [15] and also it suffers from higher return loss before near 50 GHz. The photogenerated current nonlinearly depends on input optical power and junction voltage [15]. As the input optical power increases from 10 mW to 50 mW, the extinction ratio of all CTWEAMs increase more than 9% but the overshooting increases more than 100% to distort the output signal.
3.3. Effect of biasing voltage Fig. 7 shows the effect of biasing voltage on bandwidth of different cascading modulators at constant modulating voltage 0.4 Vp–p and 14 mW input optical power. When the biasing voltage increases from 0.5 V to 2.5 V, the bandwidth of cascading modulator 1A, 1B and 1C increases 16%, 17% and 7%, respectively. If the biasing voltage is increasing then electron and hole wave functions are moving in opposite direction which causes to increase the absorption and also the absorption spectrum. As the biasing voltage is increased, the voltage drop across the mesa p-doped resistor is also increased by the photogenerated current
3.2. Effect of input optical power Fig. 6 shows the effect of input optical power for different cascading modulators where other parameters are kept unchanged. When the input optical power changes discretely from 10 mW to 50 mW, the bandwidth of the modulator 1A and 1B increase from 109 Gbps to 128 Gbps and 66 Gbps to 77 Gbps, respectively. The active segment of the modulator can be modeled as a mesa resistance series connection with a junction capacitor [14]. This junction capacitor is parallel connected with a shunt resistor which is equal to the change in junction voltage divided by the change in photogenerated current [14]. At low frequency, the capacitor is an open circuit and the total voltage drops between these two resistors, which in turn does not decrease the RC time constant that helps to increase bandwidth. But, at high frequency the mesa resistance and the shunt resistance are parallel with the junction capacitor which helps to decrease the total RC time constant that causes an increase in the bandwidth. As the optical power increases below saturated optical power, the bandwidth is increased due to an increase of photogenerated current which causes large voltage drop in the p-doped resistance and decreases the voltage drop across the
45
Fig. 4. Effect of modulating signal on extinction ratio.
Fig. 3. Eye diagram at input modulating voltage 0.4 Vp–p for cascading modulator 1A.
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Fig. 5. Effect of reflection factor (S11 parameter).
Fig. 8. Effect of active segment ratio Ri on 3 dB bandwidth for different designs. Fig. 6. Effect of input optical power on bandwidth.
Fig. 9. Effect of microstrip length ratio Rji on 3 dB bandwidth for different designs. Fig. 7. Effect of biasing voltage on bandwidth.
3.4. Effect of active segment ratio acting as a reduced value of the shunt resistor that is parallel with the junction capacitor [14]. This reduces the RC time constant and increases the bandwidth.
To find the optimum active segment length for device best performance, we have varied one of the four active segment
A.B.M.H. Islam, U. Westergren / Optics Communications 297 (2013) 43–47
47
Table 2 Performance data of all the cascaded modulators. CTWEAM design
1A 1B 1C
Modulating voltage ¼ 0.4 V(p–p) BW (GHz)
ER (dB)
Overshooting (dB)
Jitter (fs)
BW (GHz)
ER (dB)
Overshooting (dB)
Jitter (fs)
110 65 101
4.0 4.2 4.1
0.20 0.00 0.63
90 290 260
119 77 115
10 11 11
0.47 0.16 1.56
80 260 230
lengths while the other three active segment lengths remain equal. The ratio, Ri (i¼ 1–4) in percentage, is the ith active segment length over the total active segment length of CTWEAM as shown in Fig. 8 for 3 dB bandwidth variation. We all the time keep (equal or at least 50 mm) the length of second microstrip (1t) line of first modulator greater than the length of first active segment (1a) of second modulator and similarly, the length (equal or at least 50 mm) of second microstrip (1t) line of second modulator greater than the length of second active (2a) segment of the first modulator, otherwise the cascaded design will not be possible. The ratio can be written as Ri ð%Þ ¼
length of ith active segment of CTWEAM 100% total active segment length of CTWEAM
If the ratio R1 increases, then the bandwidth decreases more and also the overshooting increases (signal distortion). Ratio R2 has no effect on extinction ratio, very little effect on changing the bandwidth but it has big effect on overshooting. If the ratio R3 changes then the bandwidth and overshooting increase more than 30%. Ratio R4 has also big effect on changing the bandwidth, extinction ratio and also overshooting. The 3 dB bandwidth increases or decreases as the active segment length ratio increases which changes the total effective impedance and thus the microwave return loss. By selecting suitable active length ratio of R1, R3 and R4, we can tune the device for optimum performance. 3.5. Effect of microstrip length ratio To find the optimum microstrip length, we have varied one of three passive transmission line (microstrip) lengths of jth (j¼1, 2) modulator by keeping total passive transmission line (microstrip) length of jth (j ¼1, 2) modulator constant and also the microstrip length ratio of other two microstrip line lengths of the same modulator have been kept equal to the corresponding original ratio (as shown in Table 1) and also the total microstrip line length of other modulator has been kept the same as in original design. The ratio, Ri (i¼0, 1 and 2) in percentage, is the ith microstrip length of jth (j¼1, 2) modulator over the total microstrip length of the same modulator. This can be written as Rjið%Þ ¼
Modulating voltage ¼ 1 V(p–p)
length of ith microstrip line of jth ðj ¼ 1,2Þ modulator 100% total microstrip line lenght of jth ðj ¼ 1,2Þ modulator
It has been seen by varying all the six microstrip ratios, only microstrip ratio R10, R12 and R21 has significant effect on 3 dB bandwidth as shown in Fig. 9 and no significant effect on extinction ratio and overshooting. By selecting appropriate microstrip length ratios, the CTWEAM could be tuned for optimum performance.
After careful investigations of all the designing parameters, we have designed three types of cascading modulators and their performance data are shown in Table 2.
4. Conclusions A bandwidth of 110 GHz, extinction ratio 4.0 dB and overshooting 0.20 dB have been obtained for cascaded modulator 1A. It is the best design among all the cascaded modulators for real life application, because the proposed design gives a tradeoff between bandwidth, extinction ratio and overshooting. A comparison with simulations of a single TWEAM using the same model as above shows that in order to have 4100 GHz bandwidth, more than 0.6 Vp–p modulating voltage would be required for 4 dB optical extinction ratio, making the use of silicon-based electronics more questionable. All the results that have been obtained from this research work could be used to design and fabrication cascaded modulators for practical applications.
References [1] H. Fukano, T. Yamanaka, M. Tamura, Y. Kondo, Journal of Lightwave Technology 24 (2006) 2219. [2] J.-W. Shi, A.-C. Shiao, C.-C. Chu, Y.-S. Wu, IEEE Photonics Technology Letters 19 (2007) 345. [3] Y. Luo, B. Xiong, J. Wang, P. Cai, C. Sun, Japanese Journal of Applied Physics 45 (2006) L1071. [4] M. Chacinski, U. Westergren, B. Stoltz, L. Thyle´n, R. Schatz, S. Hammerfeldt, Journal of Lightwave Technology 27 (2009) 3410. [5] R. Lewe´n, S. Irmscher, U. Westergren, L. Thyle´n, U. Eriksson, Journal of Lightwave Technology 22 (2004) 172. ¨ ¨ [6] L. Thyle´n, U. Westergren, P. Holmstrom, P. Janes, Optical Fiber Telecommunications VA: Components and Subsystems of Recent Developments in High Speed Optical Modulators, Elsevier, Amsterdam, 2008, pp183. [7] /www.hecto.euS. [8] M. Chacinski, U. Westergren, B. Wille´n, B. Stoltz, L. Thyle´n, IEEE Electron Device Letters 29 (2008) 1014. [9] M. Chacinski, U. Westergren, B. Stoltz, L. Thyle´n, IEEE Electron Device Letters 29 (2008) 1312. [10] M. Chacinski, U. Westergren, B. Stoltz, R. Driad, R.E. Makon, V. Hurm, A.G. Steffan, IEEE Journal on Selected Topics in Quantum Electronics 16 (2010) 1321. [11] M. Chacinski, U. Westergren, L. Thyle´n, R. Schatz, B. Stoltz, Optical Fiber Communication JThA 32 (2008). [12] M. Meghelli, IEEE Journal of Solid-State Circuits 39 (2004) 2403. [13] G.P. Agrawal, Fiber-Optic Communication Systems, third ed., John Wiley & Sons, Inc., New York, 2002. [14] G.L. Li, C.K. Sun, S.A. Pappert, W.X. Chen, P.K.L. Yu, IEEE Transactions on Microwave Theory and Techniques 47 (1999) 1177. [15] F. Cappelluti, G. Ghione, IEEE Transactions on Microwave Theory and Techniques 51 (2003) 1096.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Very low input voltage cascaded travelling wave electroabsorption modulator (CTWEAM) for more than 100 Gbps A.B.M. Hamidul Islam, Urban Westergren n School of Information and Communication Technology, KTH Royal Institute of Technology, SE-16440 Kista, Sweden
a r t i c l e i n f o
abstract
Article history: Received 14 June 2012 Received in revised form 7 January 2013 Accepted 8 January 2013 Available online 4 February 2013
This paper presents the large signal modeling and simulation of two segmented travelling wave electroabsorption modulators (TWEAM) as a cascaded TWEAM (CTWEAM) structure. It will be demonstrated that a mushroom shaped mesa structure with low series resistance (0.69 O-mm) will have 3 dB bandwidth more than sufficient for 100 Gbps operation. For a CTWEAM device with 990 mm active segment length, 1.1 V bias voltage and only 0.4 V peak-to-peak (Vp–p) modulating voltage has a bandwidth of 110 GHz, extinction ratio 44 dB and flat frequency response. This device would be an attractive candidate for short distance optical fiber communication as well as long distance telecommunication at 0.4 Vp–p and 1 Vp–p input RF signal, respectively. & 2013 Elsevier B.V. All rights reserved.
Keywords: Electroabsorption modulator Travelling wave 100 Gbps
1. Introduction Nowadays, the multiple quantum well (MQW) electroabsorption modulator (EAM) is attractive due to its high speed, large extinction ratio and low driving voltage, which is based on quantum-confined stark effect (QCSE) [1–3]. The compact size EAM can be monolithically integrated with external distributed feedback (DFB) laser source [1,4]. For high speed operation lumped EAM is short in size, but suffers from lack of modulation efficiency. To increase the modulation efficiency, it is necessary to use a longer device structure that increases junction capacitance and reduces the bandwidth, i.e., bandwidth is limited by RC time constant. Travelling wave EAM (TWEAM) has solved this problem by distributing the capacitance along the length of the device by proper design of the transmission line which increases the length of the device and also the modulation efficiency without sacrificing bandwidth. The speed of this modulator is limited by microwave attenuation loss due to mismatch between propagation velocity of microwave signal and group velocity of optical signal along the length of the device. This problem has been solved by proper design of the device into the segmented TWEAM [5]. EAM monolithically integrated with a distributed feedback (DFB) laser has become a common high-speed transmitter in optical networks. Compact size, excellent performance and reasonable cost have been achieved as a result of research. Monolithic integration has been important to make it possible to combine the advantages of continuous wave (CW) light from a laser with the modulation
n
Corresponding author. E-mail address: [email protected] (U. Westergren).
0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.01.028
properties offered by an external modulator. Breakthroughs in externally modulated lasers (EML) have been reported for 10 Gbps (Giga bit per second) and 40 Gbps [1–3,6] transmitters, and more recently also for 100 Gbps [5,7]. In order to reach bandwidths of the order of 100 GHz with low electrical reflection TWEAM was proposed [5–7] and demonstrated with record low 2 Vp–p driving voltage for 10 dB optical extinction for 230 mm total active length [8] and 2.5 Vp–p for 180 mm length with improved impedance matching [9]. The segmented structures were used to overcome the limit imposed by the RC constant of long modulators. It is based on transformation of the low characteristic impedance of active segments, which are suitable for high speeds, into a higher input impedance which makes it possible to achieve low electrical reflections throughout the bandwidth. These devices have been demonstrated to be useful in electrical time-division multiplexed (ETDM) transmitters with on-off-keying (OOK) up to 100 Gbps [10]. The bit rate has been demonstrated to be extendable to beyond 100 Gbps making the devices suitable for 100 Gbps Ethernet transmission [7]. Results for other 100 Gbps components such as multiplexer (MUX), driver amplifier (TWA), receiver, clock and data recovery circuit (CDR), and demultiplexer (DeMUX) are reported elsewhere as indicated in [7]. In this paper, we have studied the large signal modeling using the circuit simulator SPICE of segmented CTWEAM performance and optimization. The principle of using a cascaded structure with two TWEAMs in series reduces the voltage swing for each of the two electrical input contacts compared to the single input contact of one TWEAM, thus allowing the use of drive electronics with relatively small breakdown voltage. In Section 2, we describe the detailed design of CTWEAM. Section 3 describes the effect of modulating signal, biasing voltage and input optical power on
44
A.B.M.H. Islam, U. Westergren / Optics Communications 297 (2013) 43–47
device performances such as bandwidth, extinction ratio and overshooting. Next, we show the device optimization by changing the active segment and also microstrip line ratio that changes the device total effective impedance to reduce microwave return loss.
2. Cascaded modulator A segmented cascaded TWEAM (CTWEAM) consists of two segmented TWEAMs which is shown in Fig. 1. The cross section has a mushroom shaped mesa structure with lower vertical series resistance 0.69 O-mm compared to 1.25 O-mm previous designs [5]. Each segmented TWEAM [5,8,11] consists of two active (first (1a) and second (2a)) segments as shown in Fig. 1(a) and three passive (first (0t), second (1t) and third (2t) microstrip) transmission lines as shown in Fig. 1(b) and data for different CTWEAM designs are given
in Table 1. In this design, second microstrip (1t) length of first modulator is kept equal or at least 50 mm greater than the length of first active segment (1a) line of second modulator. If this microstrip length is less than the first active segment length of second modulator then the cascading modulator design will not be possible. Similarly, second microstrip (1t) length of second modulator is kept equal to or at least 50 mm greater than the length of second active segment (2a) line of first modulator. This design is more complex than a single segmented TWEAM, but it provides more controls of bandwidth, extinction ratio, overshooting and also undershooting to improve performance of the device. By varying its active and microstrip segment lengths, we can tune the CTWEAM to operate as we choose.
3. Results and discussion All component values of each segmented TWEAM are the same as those used in previous designs [5] except the vertical series resistance, the active segment lengths, microstrip line lengths and the load impedance (25 O). In the following, a number of designs are studied in order to investigate which parameters have impact on the performance and how an optimum design can be reached for operation at 100 Gbps. A general design goal of a modulating voltage swing of 0.4 Vp–p will be used for each of the two electrical inputs to the cascaded TWEAM in order to allow use of silicon-based drive electronics with some margin to what has been demonstrated at 100 Gbps [12]. 3.1. Effect of modulating signal Fig. 2 shows the 3 dB bandwidth of CTWEAM at different modulating voltages. The 3 dB bandwidth increases with increasing modulating signal voltage because of the nonlinear relation between Table 1 Design data for different CTWEAM [lengths are in mm]. Design Total active length La,tot
1A 1B 1C
Fig. 1. (a) Non-cascaded modulator and (b) Cascaded modulator 1A lateral layout of the chip with active segments in red, passive waveguides in blue, and microstrips in black. (c) Vertical cross section of a segmented TWEAM (lengths are in mm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
990 990 990
Active length of second modulator
Microstrip lengths of first modulator
Active length of first modulator
Microstrip lengths of second modulator
L0t
L1a
L2a
L20t L21t L22t L21a
L22a
100 220 245
100 160 50 220 200 270 100 100 100 350 50 245
560 110 250
L1t
L2t
200 270 100 110 100 160 50 560 100 350 50 250
Fig. 2. Effect of modulating signal on 3 dB bandwidth of different CTWEAM.
A.B.M.H. Islam, U. Westergren / Optics Communications 297 (2013) 43–47
modulating voltage and the 3 dB bandwidth of different modulator designs due to the intrinsic nonlinearity of the device performance. At modulating voltage 0.4 Vp–p, the device has 110 Gbps (Giga bit per second) and the eye the diagram at this voltage is shown in Fig. 3, which is comparable with previous work [5] and [8] 100 Gbps (at 3 Vp–p) and 100 Gbps (at 2.0 Vp–p), respectively. Extinction ratio: It is the ratio of optical power at pon label to optical power at poff label. It is calculated by 10logðpon =poff Þ, (dB). Overshooting: It is the ratio of peak amplitude of the optical power, ppeak to the optical power level, pon. It is calculated by 10logðppeak =pon Þ, (dB). Bandwidth: The bandwidth Df of a RC circuit can be determined by measuring its 10% to 90% of the transition time, tr (in ps) in an eye diagram. It is calculated by 0:35=tr , (GHz) [13]. Fig. 4 shows the effect on extinction ratio when changing the modulating signal for different designs of cascaded modulators, with results comparable to previous works [5,8]. When the modulating signal is increased linearly the extinction ratio is also increased but nonlinearly. This is due to the nonlinearity of the device absorption as a function of the junction voltage. Fig. 5 shows effect of reflection factor (S11 parameter) of microwave signal at different frequencies for CTWEAM modulator 1A which has about 10 dB reflection factor at 100 GHz which is an improved value compared to previous works [5] and [8] where the reflection becomes larger than 10 dB above 75 GHz and 85 GHz, respectively. Other CTWEAM designs 1B and 1C have about 10 dB reflection factor at 55 GHz and 65 GHz, respectively. In real life system, below 10 dB reflection factor is normally acceptable in electronics for data transmission.
junction capacitor. Which in turn to reduce the shunt resistance and also the RC time constant that helps to increase the bandwidth at high optical input power. But, the bandwidth of modulator design 1C decreases very little at high input optical power due to saturation effect of photogenerated current which is caused by screening effect [15] and also it suffers from higher return loss before near 50 GHz. The photogenerated current nonlinearly depends on input optical power and junction voltage [15]. As the input optical power increases from 10 mW to 50 mW, the extinction ratio of all CTWEAMs increase more than 9% but the overshooting increases more than 100% to distort the output signal.
3.3. Effect of biasing voltage Fig. 7 shows the effect of biasing voltage on bandwidth of different cascading modulators at constant modulating voltage 0.4 Vp–p and 14 mW input optical power. When the biasing voltage increases from 0.5 V to 2.5 V, the bandwidth of cascading modulator 1A, 1B and 1C increases 16%, 17% and 7%, respectively. If the biasing voltage is increasing then electron and hole wave functions are moving in opposite direction which causes to increase the absorption and also the absorption spectrum. As the biasing voltage is increased, the voltage drop across the mesa p-doped resistor is also increased by the photogenerated current
3.2. Effect of input optical power Fig. 6 shows the effect of input optical power for different cascading modulators where other parameters are kept unchanged. When the input optical power changes discretely from 10 mW to 50 mW, the bandwidth of the modulator 1A and 1B increase from 109 Gbps to 128 Gbps and 66 Gbps to 77 Gbps, respectively. The active segment of the modulator can be modeled as a mesa resistance series connection with a junction capacitor [14]. This junction capacitor is parallel connected with a shunt resistor which is equal to the change in junction voltage divided by the change in photogenerated current [14]. At low frequency, the capacitor is an open circuit and the total voltage drops between these two resistors, which in turn does not decrease the RC time constant that helps to increase bandwidth. But, at high frequency the mesa resistance and the shunt resistance are parallel with the junction capacitor which helps to decrease the total RC time constant that causes an increase in the bandwidth. As the optical power increases below saturated optical power, the bandwidth is increased due to an increase of photogenerated current which causes large voltage drop in the p-doped resistance and decreases the voltage drop across the
45
Fig. 4. Effect of modulating signal on extinction ratio.
Fig. 3. Eye diagram at input modulating voltage 0.4 Vp–p for cascading modulator 1A.
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Fig. 5. Effect of reflection factor (S11 parameter).
Fig. 8. Effect of active segment ratio Ri on 3 dB bandwidth for different designs. Fig. 6. Effect of input optical power on bandwidth.
Fig. 9. Effect of microstrip length ratio Rji on 3 dB bandwidth for different designs. Fig. 7. Effect of biasing voltage on bandwidth.
3.4. Effect of active segment ratio acting as a reduced value of the shunt resistor that is parallel with the junction capacitor [14]. This reduces the RC time constant and increases the bandwidth.
To find the optimum active segment length for device best performance, we have varied one of the four active segment
A.B.M.H. Islam, U. Westergren / Optics Communications 297 (2013) 43–47
47
Table 2 Performance data of all the cascaded modulators. CTWEAM design
1A 1B 1C
Modulating voltage ¼ 0.4 V(p–p) BW (GHz)
ER (dB)
Overshooting (dB)
Jitter (fs)
BW (GHz)
ER (dB)
Overshooting (dB)
Jitter (fs)
110 65 101
4.0 4.2 4.1
0.20 0.00 0.63
90 290 260
119 77 115
10 11 11
0.47 0.16 1.56
80 260 230
lengths while the other three active segment lengths remain equal. The ratio, Ri (i¼ 1–4) in percentage, is the ith active segment length over the total active segment length of CTWEAM as shown in Fig. 8 for 3 dB bandwidth variation. We all the time keep (equal or at least 50 mm) the length of second microstrip (1t) line of first modulator greater than the length of first active segment (1a) of second modulator and similarly, the length (equal or at least 50 mm) of second microstrip (1t) line of second modulator greater than the length of second active (2a) segment of the first modulator, otherwise the cascaded design will not be possible. The ratio can be written as Ri ð%Þ ¼
length of ith active segment of CTWEAM 100% total active segment length of CTWEAM
If the ratio R1 increases, then the bandwidth decreases more and also the overshooting increases (signal distortion). Ratio R2 has no effect on extinction ratio, very little effect on changing the bandwidth but it has big effect on overshooting. If the ratio R3 changes then the bandwidth and overshooting increase more than 30%. Ratio R4 has also big effect on changing the bandwidth, extinction ratio and also overshooting. The 3 dB bandwidth increases or decreases as the active segment length ratio increases which changes the total effective impedance and thus the microwave return loss. By selecting suitable active length ratio of R1, R3 and R4, we can tune the device for optimum performance. 3.5. Effect of microstrip length ratio To find the optimum microstrip length, we have varied one of three passive transmission line (microstrip) lengths of jth (j¼1, 2) modulator by keeping total passive transmission line (microstrip) length of jth (j ¼1, 2) modulator constant and also the microstrip length ratio of other two microstrip line lengths of the same modulator have been kept equal to the corresponding original ratio (as shown in Table 1) and also the total microstrip line length of other modulator has been kept the same as in original design. The ratio, Ri (i¼0, 1 and 2) in percentage, is the ith microstrip length of jth (j¼1, 2) modulator over the total microstrip length of the same modulator. This can be written as Rjið%Þ ¼
Modulating voltage ¼ 1 V(p–p)
length of ith microstrip line of jth ðj ¼ 1,2Þ modulator 100% total microstrip line lenght of jth ðj ¼ 1,2Þ modulator
It has been seen by varying all the six microstrip ratios, only microstrip ratio R10, R12 and R21 has significant effect on 3 dB bandwidth as shown in Fig. 9 and no significant effect on extinction ratio and overshooting. By selecting appropriate microstrip length ratios, the CTWEAM could be tuned for optimum performance.
After careful investigations of all the designing parameters, we have designed three types of cascading modulators and their performance data are shown in Table 2.
4. Conclusions A bandwidth of 110 GHz, extinction ratio 4.0 dB and overshooting 0.20 dB have been obtained for cascaded modulator 1A. It is the best design among all the cascaded modulators for real life application, because the proposed design gives a tradeoff between bandwidth, extinction ratio and overshooting. A comparison with simulations of a single TWEAM using the same model as above shows that in order to have 4100 GHz bandwidth, more than 0.6 Vp–p modulating voltage would be required for 4 dB optical extinction ratio, making the use of silicon-based electronics more questionable. All the results that have been obtained from this research work could be used to design and fabrication cascaded modulators for practical applications.
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