JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 6, MARCH 15, 2008 685
Electro-Absorption Modulator Integrated Lasers With
Enhanced Signal Injection Efficiency
Cheng Guan Lim, Member, IEEE
Abstract—By incorporating a lumped-element impedance-matching network on the laser diode carrier/submount, the signal injection efficiency of electro-absorption modulator integrated lasers is significantly improved through the reduction of input return loss. Over a frequency range spanning from 5.8 to 10 GHz, an improvement of approximately 12% to 37% in signal injection efficiency is observed. To avoid increasing the component-count, a unique implementation of the lumped element impedance matching network is proposed.
Index Terms—Electro-absorption modulator integrated lasers, impedance matching, input return loss.
I. INTRODUCTION
T
HE ADVENT of optical fiber communications to ac-commodate the ever-increasing demand on channel transfer-rate and transmission capacity has led to perpetual interest in dense wavelength division multiplexing (DWDM) [1]. 10-Gb/s optical fiber communication systems have been deployed in parts of the communication infrastructure to pro-vide a higher channel transfer-rate and larger transmission capacity. In the past, Mach–Zehnder modulators (MZM) have been used in sectors where the required transmission rate and range are beyond the capability of directly modulated laser diodes [2], [3]. Since the introduction of the relatively cheaper electro-absorption modulator (EAM) [4] and electro-absorp-tion modulator integrated lasers (EML) [5], these devices have replaced the MZM in some of the segments where EAMs and EMLs are applicable [6]–[8]. Besides costing less than the MZM and can be monolithically integrated with a dis-tributed-feedback (DFB) laser diode (LD), the EAM has the advantage of operating at a significantly lower voltage [9]. Nevertheless, one tedious problem associated with EAMs and EMLs is the large impedance mismatch between these devices and the signal source. Typical 10-Gb/s EMLs exhibit an input return loss of more than 8 dB and 10 dB for frequencies above approximately 5 and 8 GHz, and a 3 dB modulation bandwidth ranging from 8 to 11 GHz. Although some of these EMLs have sufficient bandwidth to meet the specified bandwidth (i.e., more than 10 GHz), their input return loss falls short of the specified value (i.e., less than 10 dB for frequency up to more than 10 GHz). To-date,Manuscript received July 30, 2007; revised November 17, 2007.
The author was with the Department of Photonics/Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan, R.O.C. He is now with the Ultrafast Photonic Devices Laboratory, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-568, Japan (e-mail: c.g.lim@ieee.org).
Digital Object Identifier 10.1109/JLT.2007.915275
two methods of designing EML modules to meet the required specifications have been proposed [10], [11]. To reduce the input return loss, Ishimura et al. [10] have proposed to use a current-compensation technique to control the impedance seen by the signal source, whereas Lim et al. [11] have proposed a reactive impedance matching technique to trim the reactive part of the impedance seen by the signal source. The unfavorable aspect of these two methods is that the device capacitance of the EAM section of the EML must be reduced to not more than approximately 0.4 pF; causing optical performance to be compromised. The tradeoff between a low device capacitance and a good optical performance (e.g., extinction ratio, chirp, optical output power) has made it difficult to achieve the radio-frequency (RF) and optical performance specifications si-multaneously. A typical 10-Gb/s EML design with an optimum optical performance has a device capacitance of approximately 0.6 pF for the EAM section. Although a difference of 0.2 pF is small in the general domain, it is very significant in the high-frequency (HF) domain. In fact, this is a large value (a difference of approximately 33%) considering the physical size of the EAM, and its impact on the performance of the EAM is significant. To relief the above problem, a method (similar to that proposed in [12]) to improve the signal injection efficiency and optical performance of 10-Gb/s EMLs will be proposed in this paper. In contrast to [12], a systematic lumped-element impedance-matching network (L-network) design procedure is followed and implementation method and design equations will be presented in this paper. Moreover, the “ ” design proposed in [12] functions more like staggered-step impedance trans-former or open-circuit stubs which consist of two impedance discontinuities that cause signal reflections and result in signal distortion and degraded jitter. Furthermore, using two discrete sections to introduce the required capacitance and inductance restrict the implementation to bit-rates lower than 10 Gb/s because parasitic inductance and capacitance are introduced by these sections. In this paper, a unique implementation to show how this impedance-matching technique that originally used lumped-elements can be applied to higher frequencies in excess of that demonstrated in [12]. This unique implementation that uses only one microstrip transmission-line section instead of two (as in [12]) to implement the L-network is expected to have better signal quality and jitter.
II. DESIGN OF THEIMPEDANCE-MATCHINGELEMENT
Fig. 1(a) shows the schematic drawing of a typical design of an EML, and Fig. 1(b) shows the structure details of the EAM section. Based on this device structure of the EML, an equiva-lent electrical circuit model of the EML was derived [Fig. 1(c)]
Fig. 1. Schematic diagram of the (a) electro-absorption modulator integrated laser, (b) device structure of the electro-absorption modulator section, and (c) equivalent electrical circuit model of the EML.
where the laser diode is represented by a - - diode. Using the dimensions of a typical EAM design, each of the model parameters was calculated and extracted from and re-sponses. The extracted values for the device resistance (R1), dif-ferential resistance (R2), isolation resistance (R3), and capaci-tance of the absorption layer (C1), intrinsic layers (C2 and C4), iron-doped layers (C3 and C5) , and polyimide layer (C6) are 5 , 300 , 20 k , 0.42 pF, 0.732 pF, 0.082 pF, and 0.085 pF, respectively.
On a module level, the equivalent electrical circuit model of an EML module is shown in Fig. 2(a) where the signal source impedance is represented by a 50- termination, the signal feeder on the seven-pin butterfly package for soldering the signal lead of the RF connector is represented by a co-planar waveguide transmission-line CPW1, the signal feeder on the LD carrier/submount where the EML is bonded is represented by a microstrip transmission-line TL1, R4 acts as a termina-tion resistor to pull the impedance of the EAM close to the
Fig. 2. (a) Equivalent electrical circuit model of the EML module. (b) Input return loss response. (c) Modulation response of the EML module.
characteristic impedance of the signal source, the equivalent capacitance of C1, C2, C3, C4, and C5 as in Fig. 1(c) is rep-resented by C7, the bonding-wire that connects the signal line of CPW1 to TL1, the ground plane of CPW1 to the ground plane of TL1, the signal line of TL1 to the bond-pad of the EAM, the bond-pad of the EAM to the termination resistor are represented by L1, L2, L3, and L4, respectively. The value of L1, L2, L3, and L4 used in this work are 0.3, 0.3, 0.5, and 0.5 nH, respectively. These are very reasonable and practical values to consider. Typically, CPW1 and TL1 are designed to have a characteristic impedance of 50 . Taking all the above into consideration, the input return loss response and modulation response obtained from the well-established RF simulator (Advanced Design System) developed by Agilent Technologies are shown in Fig. 2(b) and (c), respectively. It
LIM: ELECTRO-ABSORPTION MODULATOR INTEGRATED LASERS WITH ENHANCED SIGNAL INJECTION EFFICIENCY 687
Fig. 3. (a) Portion of the equivalent electrical circuit of the EML module that has been treated as the load. (b) Input return loss response. (c) Smith chart of the load.
was observed that the input return loss is greater than 10 dB for frequencies above approximately 5 GHz. Although this design has a modulation bandwidth of approximately 12 GHz, its signal injection efficiency and signal quality are not good due to substantially high input reflection.
To improve the input return loss response of the EMLs, the L-network appears to be a very suitable candidate among the various impedance-matching techniques as it can achieve a good impedance-matching between a complex load and a microwave system [12]. To design an appropriate L-network, the load [Fig. 3(a)] impedance was noted (i.e., ) at the frequency which the input return loss response crossed the 10 dB mark (i.e., 6.5 GHz) and inserted into the following equations:
(1) (2)
Fig. 4. Schematic diagram showing the incorporation of the L-network (L5 and C8) before the portion of the equivalent electrical circuit of the EML module that has been considered as the load.
where and are the reactance of the capacitor and inductor respectively, and are the real and imaginary parts of the load impedance, and is the characteristic impedance of the transmission-line which the load is to be matched to. As a re-sult, the value of L5 and C8 (Fig. 4) were found to be 0.633 nH and 0.466 pF, respectively. As the value of the inductance and capacitance of the L-network are non-standard values, this adds on to the additional cost. To implement the L-network with no additional cost to the original cost, it is proposed here to imple-ment the L-network using transmission-lines whereby no ad-ditional concern over reliability issue due to increased compo-nent-count is introduced. Here, the microstrip transmission-line has been used for illustration. The shunt capacitance and in-ductance of the microstrip transmission-line L-network are calculated as follows:
(3) (4) where is the permittivity of free-space, the relative per-mittivity of the substrate, the surface area of the transmis-sion-line, the thickness of the substrate, the propagation con-stant, the length of the transmission-line, and the frequency of concern. The shunt capacitance and series inductance of var-ious combinations of length and width of the microstrip trans-mission-line L-network were calculated and the one that gives the desired capacitance and inductance was used [Fig. 5(a)].
III. SIMULATIONRESULTS ANDEVALUATION
Consequently, results showed that an input return loss of below 10 dB for frequencies below 9.5 GHz and a modulation bandwidth of 11 GHz have been achieved [Fig. 5(b) and (c)]. As the input return loss still falls short of the required value, the capacitance of the absorption layer was reduced, and it was found that the device capacitance of the EAM section must not be more than approximately 0.5 pF in order for this approach to meet both specifications. With the device capacitance of the EAM section reduced, the input return loss of the load was noted to cross the 10 dB mark at 7.7 GHz [Fig. 6(a)], and the load impedance at this frequency was noted to be [Fig. 6(b)]. With this impedance, the inductance and capacitance of the L-network were calculated
Fig. 5. (a) TL2 is the microstrip transmission-line implementation of the L-net-work shown in Fig. 4. The (b) input return loss response and (c) modulation responses of the 10 Gb/s EML module incorporating the microstrip transmis-sion-line implementation of the L-network.
to be 0.478 nH and 0.393 pF, respectively. The shunt capaci-tance and series induccapaci-tance of various combinations of length and width of the microstrip transmission-line L-network were calculated and the one that gives the desired capacitance and inductance was used. Consequently, results showed that the input return loss stays below 10 dB up to a frequency of more than 10 GHz and the modulation bandwidth is nearly 12 GHz [Fig. 7(a) and (b)]. Compared to the typical design shown in Fig. 2(a), this design has significantly improved the signal injection efficiency of the EML from 60%–68% to 80%–97% over a frequency range spanning from 5.8 to 10 GHz. In comparison to the techniques proposed by Ishimura et al. [10] and Lim et al. [11], the method proposed in this work has the advantage of not having to reduce the device capacitance of the EAM drastically by approximately 33% which greatly
Fig. 6. (a) Input return loss response and (b) Smith chart of the load for the case whereby the device capacitance of the EAM section of the EML is reduced to approximately 0.5 pF.
affects the optical performance of the EAM. Although the design proposed in this work also resorts to reducing the device capacitance of the EAM (from approximately 0.6 to 0.5 pF), the amount (i.e., 14%) is relatively small compared to 33%. The improvement of the proposed design is more than twofold, hence relieving the compromise made on the optical perfor-mance by a factor of more than two. Considering the difficulty in achieving good signal injection efficiency and good optical performance simultaneously, this is indeed a significant and additional improvement.
To understand the mechanism responsible for the above im-provement, the input impedance response of the proposed de-sign [Fig. 5(a)] as well as that of the original dede-sign [Fig. 2(a) with device capacitance reduced to approximately 0.5 pF] are plotted as shown in Fig. 8. With the incorporation of the L-net-work, it can be observed from Fig. 8(a) that the real part of the input impedance has been controlled to within 39 to 55 which would otherwise vary from 43 to 110 over a range of frequency spanning from 0.1 to 9 GHz. From Fig. 8(b), it can be observed that for a frequency range spanning from 0.1 to 6.8 GHz, the improvement in signal injection efficiency is due to reduced inductance. From approximately 6 to 9 GHz, the imaginary part of the input impedance of the original design de-creases rapidly from approximately and becomes capac-itive at around 7.3 GHz. As for the proposed design, the imag-inary part of the input impedance decreases gradually to close to the purely resistive mark but does not become capacitive. For
LIM: ELECTRO-ABSORPTION MODULATOR INTEGRATED LASERS WITH ENHANCED SIGNAL INJECTION EFFICIENCY 689
Fig. 7. (a) Input return loss response and (b) modulation responses of the 10-Gb/s EML module incorporating the microstrip transmission-line imple-mentation of the L-network. The device capacitance of the EAM section was reduced from approximately 0.6 to 0.5 pF.
the proposed design, the imaginary part of the input impedance is controlled to within to instead of within around to . In a nutshell, the improvement of the signal injection efficiency of the EMLs is due to the well-controlled input impedance brought about by the uniquely implemented L-network. With this method, EMLs can be designed to yield enhanced signal injection efficiency and better optical perfor-mance than before.
IV. DEVICEIMPLEMENTATION
Fig. 9 shows a possible implementation of the proposed EML carrier/submount designs using microstrip transmission-line with ground plane. The coplanar ground plane of which the EML is bonded is connected to the bottom ground plane using vias. On one side of the EML are the conventional 50-microstrip transmission-line and the uniquely implemented L-network [represented by TL2 in Fig. 5(a)]. On the other side of the EML consists of two 50- microstrip transmission-lines that act as bond-pads on both ends of the 50- thin-film termi-nation resistor and a direct-current (dc) bias feed-line.
The value of L1 and L2 should be kept as low as possible to minimize their deteriorating effects on module performance. Hence, the length of the EML carrier/submount was determined to minimize the value of L1 and L2. To achieve reasonably good precision for the value of L3 and L4, the repetitive accu-racy of the automatic wire-bonder must be carefully evaluated. The choice of bonding-wire would then determine the distance
Fig. 8. Input impedance response of the EML module incorporating a reduced device capacitance of the EAM section (approximately 0.5 pF) and the mi-crostrip transmission-line implementation of the L-network. (a) Real part and (b) imaginary part. The responses for the proposed design are shown in solid-line, whereas the responses in dashed-line belong to that of the typical design shown in Fig. 2(a).
Fig. 9. Possible implementation of the proposed EML carrier/submount de-signs using microstrip transmission-line with ground plane.
from the microstrip transmission-line L-network and the 50-thin-film termination resistor to the EML. As a result, the total length of the signal feeder was 3.4 mm. For the responses shown in Fig. 5 and a 0.3-mm-thick HF substrate having a relative di-electric constant of 8.7, the length and width of the microstrip transmission-line L-network are 1.641 and 0.861 mm, respec-tively. Generally, the EAM section of EMLs for 10-Gb/s appli-cations is 150 to 200 m long. The continuous-wave DFB LD section does not affect the HF performance of the EML because the isolation resistance between the EAM section and DFB LD section is typically 20 k . Hence, the length of the DFB LD section, design of the dc bias feed-line, and the bonding-wire connecting them have no effect on the HF performance of the EML. Nevertheless, the choice of the length of the DFB LD
Fig. 10. Input return loss responses and modulation responses of the 10-Gb/s EML module incorporating the microstrip transmission-line implementation of the L-network for the case where microstrip transmission-line fabrication has a precision of620 m. (a) 020 m and (b) + 20 m. The device capacitance of the EAM section was 0.45 pF.
section determines the maximum optical signal power and the optical signal extinction ratio. Typically, the length of the DFB LD section of 10-Gb/s EMLs is 300 to 500 m.
The robustness of this implementation to fabrication preci-sion was evaluated based on the tolerance of L1, L2, L3, L4 , and the microstrip transmission-line (i.e., the conventional 50-microstrip transmission-line and the proposed 50-microstrip trans-mission-line L-network). In practice, the value of L1 and L2 can be as low as 0.1 nH. However, a value of 0.2 nH have been con-sidered in the evaluation.
V. TOLERANCEANALYSIS
Through a series of simulations, it was found that the pro-posed implementation can tolerate a fabrication precision of 20 m for the microstrip transmission-line and 0.1 nH for L3 and L4. Fig. 10(a) shows the input return loss and modula-tion responses when the microstrip transmission-line is 20 m short of design value. When L3 and L4 have an inductance that is 0.1 nH lower than the nominal value of 0.5 nH, it was observed from the input return loss responses that the 10 dB cross-over frequency was extended from 10.7 to 11.5 GHz and the peak of the hump at around 6 GHz was raised from 13 dB to 11 dB.
On the other hand, the 10 dB cross-over frequency was re-duced to 10.4 GHz and the peak of the hump at around 6 GHz was lowered to 15 dB. In either case, the 3 dB bandwidth is in excess of 10 GHz (i.e., more than 13 GHz for the former and more than 12 GHz for the latter). The input return loss and modulation responses for the case where the microstrip trans-mission-line is 20 m in excess of design value are shown in Fig. 10(b). As can be noted from the input return loss responses, the 10 dB cross-over frequency extends to 11 GHz and the peak of the hump at around 6 GHz rises to 11 dB when L3 and L4 have an inductance of 0.4 nH. For the case where L3 and L4 have an inductance of 0.6 nH, the input return loss response showed that the input return loss rises above 10 dB at 10 GHz and has a peak of 15 dB at around 6 GHz. The 3 dB band-width for the former is more than 13 GHz and more than 12 GHz for the latter. It should be noted that the device capacitance of the EAM section of the EML used in the above harsh evalua-tion was 0.45 pF. The above evaluaevalua-tion is a harsh one because microstrip transmission-line fabrication has a higher precision than that considered here. In actual fact, microstrip transmis-sion-lines can be fabricated very precisely through calibration. Also, by using the appropriate bonding-wires and knowing the capability of the automatic wire-bonder the nominal values of
LIM: ELECTRO-ABSORPTION MODULATOR INTEGRATED LASERS WITH ENHANCED SIGNAL INJECTION EFFICIENCY 691
L1, L2, L3, and L4 can be achieved. It should be mentioned that the value of L1 and L2 that produced the results shown in Fig. 7 was 0.3 nH. Hence, some tolerances have already been incorpo-rated in that design because the value of L1 and L2 can be as low as 0.1 nH in practice. Therefore, it should be possible to meet the specifications with an EML having an EAM section with a device capacitance of 0.5 pF.
VI. CONCLUSION
To substantially subside the unfavorable effects (i.e., reduced extinction ratio, compromised chirp performance, and lower op-tical output power) of reducing the device capacitance of the EAM section of the EML to improve the signal injection ef-ficiency of the EML, this work has proposed to incorporate a lumped-element impedance-matching network (L-network) on the LD carrier. To avoid additional cost and reliability issue due to increased component-count, a unique implementation of the L-network was proposed and illustrated. With the L-net-work incorporated and the device capacitance of the EAM re-duced to 0.5 pF (a reduction of about 14% as oppose to about 33% in the case of other methods), the signal injection effi-ciency was found to increase from approximately 60%–68% to 80%–97% (an improvement of approximately 12%–37%) over a frequency range spanning from 5.8 to 10 GHz. This was achieved at the expense of a reduction in modulation bandwidth from approximately 13 to 12 GHz (a mere 7.69% for the case where the device capacitance of the EAM is reduced to 0.5 pF). There are several approaches to apply this impedance-matching technique to higher bit-rates. One of them is to incorporate the driver into the package whereby the EML carrier/submount be-comes shorter. If, however, the same package is to be used, HF substrates with a higher dielectric constant should be used. Otherwise, appropriate packages that reduce the length of the EML carrier/submount should be used. Basically, the size of the EML carrier/submount must be reduced in order for this impedance-matching network to support higher data-rates.
ACKNOWLEDGMENT
The author would like to thank the Chips Implementation Center (CIC) for the loan of the Advanced Design System—a popular and well-established commercial HF computer-aided-design package.
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Cheng Guan Lim (S’94–A’97–M’00) received the
B.Eng. (honors) degree in electronic and electrical engineering in 1997 and the Ph.D. degree at the Institute of Microwaves and Photonics, School of Electronic and Electrical Engineering, University of Leeds, Leeds, U.K., in 2001.
From 2001 to 2003, he was a Research Engineer at the Yokohama Research and Development Lab-oratories, Furukawa Electric Company, Yokohama, Japan, working on the development of semiconductor laser modules for digital and analog telecommuni-cation applitelecommuni-cations. In 2003, he joined Agilent Technologies, Singapore, as a Senior Engineer focusing on optoelectronic device manufacturing. From 2004 to 2005, he was a Senior Research Scientist at the Institute for Infocomm search, Singapore (a member of the Agency for Science, Technology, and Re-search, Singapore, and a National University of Singapore affiliated research institute) dealing with radio-over-fiber technologies. He was an Assistant Pro-fessor at the Department of Photonics/Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C. Currently, he is a Researcher at the National Institute of Advanced Industrial Science and Tech-nology, Tsukuba, Japan, where he conducts research on intersubband transi-tions in InGaAs/AlAs/AlGaAsSb-coupled double-quantum-wells. His prime re-search interests include the investigation of the fundamental physics of advanced nanostructures and the optical and electronic properties of advanced novel mate-rials through simulation and experimentation. His secondary research interests are photonic packaging, RF photonics, and photonic integrated circuits. He has one granted U.S. patent, written one invention disclosure, and published several international refereed journal and conference papers.
Dr. Lim has served as a Steering Committee Member for the Inaugural IEEE International Workshop on Antenna Technology 2005 (Singapore), and as a Technical Program Committee member for the 2005 Symposium on Technology Fusion of Optoelectronics and Communications-International Conference on Photonics (Taiwan, R.O.C.). In 1995, he was awarded the Crab Tree Prize for outstanding academic achievement, the Hewlett-Packard Prize in 1997 for the recognition as the best student in High-Frequency Engineering, and the F. W. Carter Prize in 2001 for presenting the best Ph.D. dissertation. His biograph-ical profile was selected for inclusion in the 25th Silver Anniversary Edition of
