We present a cost-effective 25-Gb/s electro-absorption modulator integrated laser (EML) transmitter optical sub-assembly (TOSA) using all-in-one flexible printed circuit board (FPCB) wiring and a metal optical bench (MOB). For a low cost and high bandwidth TOSA, internal and external wirings and feed-through of the TOSA to transmit radio-frequency (RF) signal are configured all-in-one using the FPCB. The FPCB is extended from an exterior of the TOSA package up to an EML chip inside the package through the slit formed on a rear sidewall of the package and die-bonded on the MOB. The EML TOSA shows a modulated output power of more than 3.5 dBm and a clear eye pattern with a dynamic extinction ratio of ~8.4 dB at a data rate of 25.78 Gb/s.
© 2013 Optical Society of America
With the tremendous growth of data traffic due to the prevalence of the fiber-to-the-home (FTTH) and social network service, the demand for high speed data communication is incessantly increasing. The standardization of 100G Ethernet was completed in 2010 , and the adoption of 100G Ethernet transceiver starts to grow. The most important requirement of light transceivers in the data communication is cost reduction without sacrifices of their performance. Especially at a higher data rate of over 25 Gb/s, transmitter optical sub-assembly (TOSA) becomes major portion of transceiver manufacturing cost.
For light sources, direct modulation lasers (DMLs) are emerging as good candidates due to their simple operation and low power consumption, but electro-absorption modulator integrated lasers (EMLs) are still considered more suitable for the use of high-speed applications beyond 25 Gb/s and for long-distance transmission.
Since the distributed feedback laser diode (DFB-LD) and electro-absorption modulator (EAM) sections concurrently operate, heat dissipation efficiency is important in EML-based TOSA packages. Also for 25-Gb/s operation of the EML, high frequency transmission structure of the TOSA package should be carefully designed. From these viewpoints, a mini-flat (or legacy butterfly type) package compatible to the XMD-MSA is preferred for the EML compared to other types of package [2–5]. Generally, EML TOSAs with a structure of the mini-flat package consist of the ceramic submount (i.e., internal wiring), ceramic feed-through, and outer flexible printed circuit board (FPCB; i.e., external wiring) for high-speed signal transmission. The ceramic submount can include radio-frequency (RF) transmission lines with via holes on ground patterns and a 50-ohm thin film resistor for impedance matching of the EML. The ceramic feed-through is often used for RF signal transmission and provides hermeticity in the mini-flat packages. However, due to the low brazing productivity of the ceramic feed-through and the difficulty of forming the thin film resistor and the via holes on the ceramic submount, packaging cost of the mini-flat type EML TOSA has been inevitably high. Besides, high frequency characteristics of the TOSA can also be degraded by multiple wire bonding and soldering around the ceramic feed-through.
On the other hand, for the realization of a low cost TOSA, TO (transistor outline)-CAN type (or coaxial type) TOSAs based on the DML and EML have tried [6–8]. The DML-based TO-CAN type TOSA is very attractive and can be a proper choice for cost reduction due to the characteristics of low power consumption of the DML. TO-CAN type structure could also be applied to EML TOSAs for cost effectiveness . However, there can be two problems for the TO-CAN type EML TOSA: one is decrease of the heat dissipation efficiency caused by low thermal conductivity of a TO-CAN material; the other is deterioration of the high frequency characteristics at a speed of over 25 Gb/s due to large inductance of lead pins around TO-stems and long bondwires. For the TO-CAN type TOSAs, getting over the problems of the heat dissipation efficiency and high frequency characteristics is still very challenging.
As another low cost TOSA, an uncooled VCSEL transmitter fabricated on a FPCB was reported , but its structure did not use an optical bench and a thermo-electric cooler (TEC). Our research group also reported a cost-effective 100G (4 × 25G) TOSA consisting of the arrayed waveguide grating wavelength division multiplexer (AWG-WDM), silicon EML carrier, and integrated FPCB .
In this paper, we propose an EML TOSA with low cost and high performance characteristics operating at 25 Gb/s which uses all-in-one FPCB wiring and a metal optical bench (MOB). Not only to eliminate the expensive ceramic submount and feed-through which are commonly used for mini-flat EML TOSA packaging, but also to enhance frequency characteristics of the TOSA, entire signal paths are configured by the all-in-one FPCB wiring (where “all-in-one” means that the ceramic submount, ceramic feed-through, and outer FPCB are combined to one FPCB). The FPCB is inserted from exterior of the TOSA through the slit formed on a rear sidewall of the TOSA, reaching up to the EML chip. The MOB is employed for optical axis alignment, components positioning, and heat dissipation of the EML. A cheap surface mountable device (SMD) type 50-ohm resistor is installed in electrically parallel with the EML for impedance matching. By the replacement of the high cost packaging subsidiaries with the low cost FPCB, MOB, and SMD resistor, we demonstrate a cost-effective EML TOSA with good high frequency characteristics.
2. Structure of proposed EML TOSA
According to the standardization of 100-Gb/s Ethernet, EML light sources operate at four single-mode wavelengths around 1300 nm with a specified channel spacing of local area network wavelength division multiplexing (LAN-WDM). An EML working at 1300-nm band is carefully designed to have characteristics of high optical output power, high extinction ratio (ER), and high 3-dB modulation bandwidth. DFB-LD and EAM sections are butt-jointed to optimize each separate section which consists of compressively strained InGaAsP/InGaAsP multiple quantum wells, and a passive waveguide is inserted between two sections for electrical isolation. N-type InP substrate is employed to obtain high optical output power, and quarter wavelength shifted gratings are formed using e-beam lithography for stable single-mode operation of the DFB-LD. To avoid optical back-reflection from a front facet of the EAM to the DFB-LD, an EAM waveguide is tilted by an angle of 7° in a longitudinal direction, and the front facet of the EAM is anti-reflection (AR)-coated.
For broadband frequency characteristics of EMLs [11–13] and efficient RF wiring in the case of extension to a multi-channel structure, an EML structure with traveling-wave type electrodes is adopted instead of that with lumped type electrodes. Grounded coplanar waveguide (GCPW) and microstrip line (MSL) electrodes are used for input/output ports and other region of the EML, respectively, to transmit RF signal. To minimize the effect of the N-InP substrate on RF signal and its RF propagation loss, bottom ground metal is applied on etched bottom surface of EAM, and then thick benzocyclobutene (BCB) layer is coated. To keep equi-potential state between top and bottom ground metals, top ground metals at input and output ports are electrically connected to bottom ground ones through via holes.
A schematic diagram of our proposed EML TOSA module is illustrated in Figs. 1(a) and 1(b). The conventional mini-flat EML TOSAs compatible to the XMD MSA are generally composed of a ceramic submount as internal wiring, a ceramic feed-through as connection, and an outer FPCB as external wiring for RF signal transmission. However, our EML TOSA has distinct features such as the all-in-one FPCB, MOB, and SMD type 50-ohm impedance matching resistor. To lower manufacturing cost and to improve frequency characteristics of the conventional TOSAs, entire DC and RF wirings are configured using all-in-one FPCB, which functions as the internal and external wirings and the signal feed-through at the same time. As shown in Fig. 1(a), the FPCB is extended from an exterior of the TOSA package to an EML chip inside the package and die-bonded on the MOB. The use of the all-in-one FPCB leads to elimination of the high-cost ceramic feed-through and reduction of total wire bonding and soldering steps. As a result, high frequency characteristics of the TOSA can be improved. The slit depicted in Fig. 1(a), which is formed on a rear sidewall of the package to insert the FPCB, is sealed using elastic epoxy.
As shown in Fig. 1(b), the RF transmission line of the all-in-one FPCB has a bent GCPW structure with multiple via holes formed on top and bottom ground patterns of the GCPW electrode. The FPCB, which is relatively easy to fabricate with low cost, was designed to work at a speed of over 25 Gb/s by carefully selecting material with a low dielectric constant of 3.4 and a low dissipation factor of ~0.002. RF return losses (S11/S22) and insertion losses (S21/S12) of the FPCB were calculated to be less than −25 dB (@20 GHz) and more than −0.3 dB (@20 GHz), respectively, where the FPCB with a length of 14 mm and the epoxy-sealed slit region were considered except the wire bonding, EML chip, and 50-ohm matching resistor in the simulation.
The MOB is made of copper-tungsten (CuW) with an excellent thermal conductivity of ~180 W/(mK) and thus can efficiently dissipate the heat emitted from the EML. The heat is spread to the MOB and then gets out to the package by a thermo-electric cooler (TEC). Also the MOB is processed to have steps, as shown in Figs. 1(a) and 1(b), so that it is suitable for aligning optical axis and positioning such components as the EML, monitoring photodiode (MPD), aspherical lens, and thermistor. The single aspherical lens is designed to have a magnification factor of 4.25 for efficient coupling of the light from the EML into an 8°-angled fiber ferrule through a sapphire window and an isolator, as depicted in Fig. 1(a). The EML is inclined by an angle of ~27° to the MOB in a longitudinal direction considering the 8°-angle of the fiber ferrule. The SMD type 50-ohm matching resistor which is easily obtainable and cheap in comparison with a thin film resistor is bonded on the FPCB in electrically parallel with the EML. A capacitor is placed on the FPCB to bypass the DC power supply noise. Figure 1(b) shows wire bonding configuration of the TOSA including pin arrangement. One terminal of the FPCB is wire-bonded to optical and electrical devices on the MOB, and the other terminal is soldered to the PCB.
3. Fabrication and performance
EML chips with a structure of the traveling-wave electrode were fabricated by optimizing each semiconductor fabrication process such as the dry etching, grating formation, BCB coating, metal deposition, and so on. As can be seen in Fig. 2, the EML chip consists of DFB-LD and EAM sections. The DFB-LD and EAM waveguides have a structure of shallow ridge and deep ridge, respectively. Since the DFB-LD operates at DC current bias, it has a large pad size, while ground-signal-ground (GSG) EAM pads are relatively small and controlled to have 50-ohm impedance for low RF reflection. The EAM was formed to have 10 quantum wells, a modulator length of 120 μm, and a detuning of ~60 nm for an EAM absorption peak from a lasing wavelength of the DFB-LD.
Figures 3(a) and 3(b) show measured optical characteristics of the fabricated EML chip, whose front and rear facets were AR-coated. Figure 3(a) shows the light-current (LI) curve of the EML as a function of injection current at both temperatures of 25°C and 40°C. The LI-curves show the smooth and linear increase of the optical output power without any kink and saturation up to 200 mA. This means that the DFB-LD is in a single-mode state, and this also shows good grating fabrication and butt-joint coupling between the EAM and the DFB-LD. The threshold currents at 25°C and 40°C are around 22 mA and 26 mA, respectively. The continuous-wave (CW) output powers are ~13.5 mW and ~12 mW (@100 mA) at both temperatures, respectively. From this result, it is expected that semi-cooled operation is possible for the fabricated EML. Figure 3(b) shows static extinction ratio (ER) at temperatures of 25°C and 40°C for variations in the EAM bias voltage applied. A lasing wavelength was about 1294.8 nm at a bias current of 100 mA for the DFB-LD and a temperature of 25°C. The EML shows static attenuation values of ~16 dB and ~18.5 dB at temperatures of 25°C and 40°C, respectively, with a bias voltage of around −3.5 V applied. The relatively low static ER at a low voltage of around −2 V may be attributed to a large detuning of ~60 nm for an EAM absorption peak from the lasing wavelength. The ER is expected to be further improved by reducing the detuning or by increasing the number of quantum wells of the EAM.
We measured the small signal electro-optic (E/O) response of the fabricated EML chip with a 65-GHz vector network analyzer (VNA), a 50-GHz photodetector, and RF probes. From the measurement, a 3-dB modulation bandwidth of ~33 GHz was achieved at a bias voltage of −2 V.
All-in-one FPCB was also fabricated using polyimide type flexible copper-clad-laminate (FCCL), and then the S-parameters of the FPCB itself were evaluated. S11 and S21 of the FPCB were measured to be less than −22 dB and more than −1 dB at 20 GHz, respectively. These values are good enough for the FPCB to be used for 25-Gb/s operation.
Figures 4(a) and 4(b) show a picture of a fabricated EML TOSA module. An EML chip was first die-bonded on the step-processed MOB made of CuW by using solder paste (with a thermal conductivity of ~50 W/(mK)) at a temperature of 170°C for efficient heat dissipation. To monitor output power of the EML, an MPD was placed at a rear side of the EML chip. A single aspherical lens with tombstone shape was laser-welded on the MOB to couple the light from the EML to an 8°-slanted fiber ferrule. The lens can convert high numerical aperture (NA; i.e., ~0.5) of an EAM waveguide to low NA (~0.12) of a single-mode fiber. A TEC and the MOB were carefully installed in a TOSA package compatible to XMD-MSA, as shown in Fig. 4(a). The all-in-one FPCB was inserted into the package through the slit, and it was bonded on the MOB by using silver paste. A SMD type 50-ohm resistor with a size of 400 μm × 200 μm was bonded on the FPCB, being in electrically parallel with the EML. To minimize impedance mismatch due to the high inductance of wire bonding, two bondwires with a diameter of 25.4 μm were wedge-bonded at input and output GSG pads of the EML. A receptacle including an isolator was actively aligned with the EML through the lens and sapphire window, and it was laser-welded on one surface of the TOSA package, as shown in Fig. 4(b). Finally the slit was sealed using elastic epoxy, and the cover lid was seam-sealed on the TOSA. The fabricated EML TOSA has a compact size of 5.3 mm × 5.4 mm × 8.4 mm, excluding receptacle and FPCB parts.
Figure 5 shows the output lasing spectra of the packaged EML TOSA module at temperatures of 25°C and 40°C, with the DFB-LD operating at a bias current (Ib) of 100 mA. Lasing wavelengths are 1294.8 nm and 1296.2 nm at temperatures of 25°C and 40°C, respectively. From the spectra, it is confirmed that stable single-mode lasing owing to the quarter wavelength shifted grating is secured for both temperatures, with the spectra exhibiting a side mode suppression ratio (SMSR) of over 46 dB. The CW optical powers of the TOSA were ~6.5 dBm and ~5.6 dBm at temperatures of 25°C and 40°C, with the DFB-LD operating at a current of 100 mA. Considering that the output power of the EML chip was about 13.5 mW (~11.3 dBm) at a temperature of 25°C and an insertion loss of the isolator integrated in the receptacle was ~0.5 dB, a coupling loss between the EML chip and the receptacle is estimated to be about 4.3 dB. This coupling loss seems to be a little bit high, inferred to come from amounts of shift occurring in laser-welding of the receptacle and the lens. For various samples, the TOSA output powers were mostly ranged from 6 − 8.5 dBm at the same current and temperature conditions. If we consider output power deviation of the EML chips, the EML-receptacle coupling loss is in between 2 − 4.5 dB, and it is expected to show a further uniform value from optimization of a laser-welding condition.
Figures 6(a) and 6(b) show the RF return losses (S11) and E/O responses of the EML TOSA module for variations in the EAM bias voltage (Vb) when the DFB-LD operated at 100 mA. The TOSA was measured by using the similar set-up configured for the EML chip. That is, the FPCB of the TOSA was directly probed by a GSG RF probe with a pitch of 750 μm. As shown in Fig. 6(a), the frequency representing a return loss of below −10 dB is about 26 GHz (i.e., S11 = −12.5 dB@20 GHz), and this return loss exceeds the requirement for 25-Gb/s modulation. E/O response curves exhibit a 3-dB modulation bandwidth of ~30 GHz at a bias voltage of −2 V, as shown in Fig. 6(b), and the 3-dB bandwidth increases gradually as the bias voltage decreases up to −2.5 V. No severe fluctuation is observed near the range of 3 − 5 GHz in the E/O responses, whose fluctuation often originates from the optical back-reflection at a front facet of the EML. This means that the 7°-tilting of the EAM waveguide and the AR coating at the front facet of the EML are very effective. Also any RF resonance peak due to the inductances of bondwires near the EML is not observed.
Figures 7(a) and 7(b) show measured eye patterns of electrical input signal itself and opto-electrical output signal of the EML TOSA, respectively, with the pulse patterns of 25.78-Gb/s non-return to zero (NRZ), 231-1 pseudo-random bit sequence (PRBS). After the electrical input signal of 500 mV, as shown in Fig. 7(a), from a pulse pattern generator (PPG; Textronix 26-Gb/s BSA260C) is amplified to ~3.5 Vpp by a 43-Gb/s RF amplifier, a modulation driving voltage (Vd) of 3.5 Vpp and a bias voltage of −2 V were applied to the EML TOSA through a bias-Tee. The modulated optical output power was over 3.5 dBm. Then the optical output of the TOSA was converted to an electrical output signal by using a 50-GHz photodetector, and the signal was measured by using a broadband oscilloscope (Agilent 86100C Infiniium DCA-J with a bandwidth of 65 GHz). As shown in Fig. 7(b), the EML TOSA shows very clear eye pattern, and a dynamic ER is as large as 8.4 dB for back-to-back transmission at a temperature of 25°C, Ib = 100 mA, Vb = −2 V, and Vd = 3.5 Vpp. Figure 8 shows measured the bit error rate (BER) characteristics of the EML TOSA for back-to-back and after 15-km transmission through a standard single mode fiber. The BER curves were obtained by using a 43-Gb/s photoreceiver (U2T MPRV1331A) and a BER tester (BERT; Textronix 26-Gb/s BSA260C) with the same TOSA driving condition. Almost no power penalty can be found after the 15-km transmission, and error-free transmission is achieved, as shown in Fig. 8. From these results, we showed that a cost-effective EML TOSA can be realized by utilizing the all-in-one FPCB and MOB, and we also demonstrated experimentally that our EML TOSA can exhibit good 25-Gb/s modulation characteristics.
We measured the TEC power consumption of the fabricated EML TOSA. The TEC power consumption values for the EML TOSA at MOB temperatures of 25°C, 40°C, and 45°C (read at a thermistor on the MOB) were less than 3 W, 0.9 W, and 0.6 W, respectively, at the TOSA case temperature of 75°C. It is believed that our TOSA can operate at a TEC power consumption of below ~0.9 W at MOB temperature ranges between 40°C and 45°C.
Basically, the fabricated EML TOSA is non-hermetically sealed around the slit region, so that external moisture may be penetrated into the TOSA through the slit. However, if an amount of penetrated moisture is very small and properly blocked before influencing on the EML, the TOSA will be able to operate normally. Therefore, the getter which can absorb the penetrated moisture was inserted into the TOSA, and then the cover lid was seam-sealed on the TOSA. To verify reliability for the EML TOSA, two TOSA modules were prepared by carefully epoxy-sealing the slit region, inserting the getter into the TOSA, and seam-sealing cover lids. The sealed EML TOSA revealed a leak of 2.4 × 10−8 atm∙cc/sec at a fine leak test with a He bombing condition of 45 psi and 2 hours. This value did not satisfy a leak of 1 × 10−8 atm∙cc/sec, but it indicates that the moisture penetration into the TOSA can be very low. The 8585 damp-heat test was performed in a thermal chamber for total 2000 hours at 85°C temperature and 85% relative humidity, and variations of optical power for two modules were less than ~0.5 dB for 2000 hours. Even though two samples were tested, this result shows that the EML TOSAs were not damaged by any probable moisture penetration and operated normally, and it also shows a feasibility that our non-hermetically sealed EML TOSA is capable of passing the tough damp heat test. In the near future, we will confirm the result from the same test for more TOSA modules.
A cost-effective EML TOSA operating at 25 Gb/s was successfully realized, which uses the all-in-one FPCB wiring and MOB. To reduce packaging cost and to enhance high frequency characteristics of conventional mini-flat TOSAs, entire electrical wirings were configured using all-in-one FPCB. The MOB was used to dissipate the heat emitted from the EML chip, align optical axis, and position optical components for the TOSA. A SMD type matching resistor was installed around the EML chip for impedance matching. The use of the all-in-one FPCB, MOB, and SMD resistor could lead to the realization of the low cost EML TOSA. Furthermore, the all-in-one FPCB could contribute to the improvement of frequency characteristics of the TOSA through reduction of wire bonding and soldering steps in comparison with conventional mini-flat TOSAs. As a result, the proposed EML TOSA showed a 3-dB modulation bandwidth of ~30 GHz, a modulated output power of over 3.5 dBm, and a clear 25.78-Gb/s optical waveform with a dynamic ER of ~8.4 dB. From these results, we concluded that our EML TOSA is capable of operating at a data rate of 25 Gb/s, and it can be used as a low cost light source for 100-Gb/s Ethernet transceivers.
This work was supported by the IT R&D program of MKE/KEIT [10035425, 100G Optical Transceivers for Ethernet], Republic of Korea.
References and links
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