1. Introduction

Monolithic photonic integration circuit (MPIC) has increasingly become important as one of the key technologies in optical fiber networks [1], such as fiber-to-the-home (FTTH), passive optical network (PON), because it offers compact multi-functional optical elements in a chip with broadband, efficiency, and low cost. In order to keep up the rapid growth of communication capacitance in optical fiber network, high-speed optical link with low transmission loss is thus one of the main requirements for applications of MPIC. Among MPICs, integration of electroabsorption modulators (EAM) and semiconductor optical amplifiers (SOA) have attracted many interests and been widely used for broadband operations with low insertion loss, high-extinction ratio, and low-driving voltage [2–5]. It also has been shown that SOA devices have not only been shown with capability of boosting output optical power in MPIC, but also been used for other functional usages, such as, optical switch, wavelength conversion [6–8]. Therefore, how to achieve broadband and efficient MPIC of SOA/EAM is thus an important topic for high-speed optical links. In EAMs performances, the modulation is mostly limited on microwave- and optical- properties of waveguide. Integrating SOAs into EAM waveguide, the EAM optical transmission loss can be compensated. However, the broadband properties still relies on the physical limitations of single EAM waveguide [9], therefore restricting the high-speed modulation efficiency. In order to pursue MPIC of EAM/SOA for high-speed optical links in the future, it is thus necessary to overcome the EAM waveguide limit.

Some physical limitations have been known to be posed on the design and fabrication of EAM [9–13]. P-i-n material is a typical layer structure used as EAM- and SOA- waveguide material. As multiple-quantum wells (M.Q.W.s) are inserted into the intrinsic region (i-layer), the strong confined electric field can thus be formed to attain low D.C. driving voltage operation; however, the high loaded capacitance is simultaneously created. Therefore, poor microwave propagation properties are formed at the high-speed regime, e.g. low characteristic impedance (~20Ω) and high microwave loss, limiting high-speed performance of EAM [9–13]. Several methods have been proposed to enhance electrical properties for good electrical-optical (EO) link. The waveguide downscaling on active region or increasing active region thickness are used for reducing impedance mismatching [10,11]. Adapting a low impedance (<50Ω) in the load line, the microwave reflection can be reduced, but the high electrical reflection is still on the source line [12]. Reducing the cladding layer impedance with small active region is used for improving microwave transmission [13]. The termination and phase-match of waveguide can be benefited from integrating cascaded multi-sections of EAMs and high-impedance transmission lines (HITL) [14,15] to release the burden of impedance design. All such kinds of methods are based on improving EAM microwave properties; however, the price of electrical-optical (EO) link is still on high optical insertion loss of waveguide, for example, high scattering loss from small waveguides, band-to-band intrinsic loss, and optical coupling loss, restricting device design and fabrication. The trade-off of design on bandwidth, input electrical driving power, and output optical power is still an issue to solve. In this work, for further boosting the EO link limitation of EAM, a new scheme monolithic integration, called cascaded-integration (CI), is proposed and demonstrated. Monolithically cascading segmented EAMs with the segmented SOAs in an optical waveguide, the optical transmission can be enhanced by SOA gain, while the driving voltage at high-speed regime can be reduced by interconnecting EAMs with HITLs to reducing impedance mismatch. Thus, the improvement of high-speed optical link can be expected.

2. Device design and fabrication

Figure 1 shows CI of EAMs and SOAs. The optical waveguide consists of segmented EAMs periodically connected by segmented SOAs, forming a distributive optical processing of re-modulation and re-amplification. Between the segmented EAMs, a microwave strip line made by low dielectric constant material (PMGI, Polymethylglutarimide) serving as a HITL bypasses the segmented SOAs for high-speed electrical connection lines. Therefore, by combining high-impedance HITL (~70 Ω) and low-impedance EAM transmission line (~20Ω), the overall microwave transmission line can be made to enhance impedance match with 50Ω instrument. In high-speed transmission design, an equivalent circuit model is used to calculate the high frequency response of p-i-n and HITL waveguides [13,14,16]. In order to get more insight into the overall electrical- and also electrical-to-optical (EO) transmission, two structures are calculated for comparison: a single section (SS) EAM (reference) and CI EAMs (three sections EAMs). The total EAM waveguide length of 300μm with i-layer thickness of 250 nm is kept the same for both structures, while varying the length of HITL. The HITL is defined by microwave strip line with metal width of 6μm and PMGI thickness of 10μm. The simulated results against frequency are shown in Fig. 2 , where a parameter γ is defined as ratio of HITL length to EAM length. Due to highly loaded capacitance in p-i-n structure, a single section of EAM has a low characteristic impedance of 16Ω, causing a high reflection (S₁₁) and low transmission (S₂₁) in high frequency regime. Once the segmented EAMs and segmented HITLs are subsequently connected, the overall microwave properties can be significantly enhanced, resulting in better EO response. In the point view of design, the CI gives one more freedom as compared with SS, i.e. the parameter of HITL. As shown in Fig. 2b, the higher γ is set, the better high-speed performance is obtained. The further advantage shows that the CI integration of EAMs and SOAs in a waveguide, not only the optical signal can be amplified by the SOA, but also the low-voltage driving operation from the thin active region of EAM waveguide can be expected, allowing higher tolerance in high-speed design.

 

Fig. 1 Schematic diagram of the cascaded-integration (CI) EAMs and SOAs, where the HITLs connect with segmented EAMs and bypass segmented SOAs.

Download Full Size | PDF

 

Fig. 2 (a)Simulated microwave S-parameters and (b)EO response of the device under different HITL lengths, where the parameter γ is defined as the ratio of HITL length to EAM length and the total EAM is 300μm long.

Download Full Size | PDF

The p-i-n epilayers are grown in a metal organic chemical vapor (MOCVD) system. The active region is composed by strain-compensated InGaAsP-based multiple quantum wells (MQWs, 6 wells and 7 barriers) with separate confinement heterostructure (SCH), sandwiched by cladding layers of p-InP (top) and n-InP (bottom). Identical layer structure is used for EAM and SOA waveguides. Two type devices are designed and fabricated for comparison: (A) CI, three sections of EAMs (100μm length for each) and two sections of SOAs (200μm length for each), and (B) SS EAM and SOA. The total EAM and SOA lengths for both types are set as 300μm and 400μm respectively, where the optical gains for both types are also kept the same. The electrical isolation between EAMs and SOAs is made by H⁺-ion implantation into a 40-μm wide region. In order to reduce the low optical reflection on the cleaved facets, the waveguides aligned by 7° off from the cleaved facet are fabricated. The top p-InP ridge is first defined by HBr-based and then HCl-based etching solutions. An H₂O₂-base solution is then utilized to selectively undercut etch the MQWs from InP material [13]. The active region width of finished waveguide is 3.7μm wide. The metallization of Ti/Pt/Au and Ni/AuGe/Ni/Au are deposited as p- and n-type contacts through an e-beam evaporation system. Two sections of HITL with 380μm length each are bypassing SOAs by depositing 2μm thick and 6μm wide Au on the top of 10μm thick PMGI, forming high-impedance microwave strip line. The PMGI is also for planarization and final coplanar waveguide line bridging.

3. Measurement and results

TE-polarized light of 0dBm power at wavelength 1600nm is used to test D.C. fiber-to-fiber optical transmission measurement. Two tapered fibers and a tunable optical filter were used to enhance coupling efficiency and filter out the amplified spontaneous emission (ASE). Figure 3 shows the measured results of CI and SS structure. Measuring both structures, same DC modulation depth with the bias (from 0V to 5V) at different SOA injection currents (set at 0 and 60mA) is observed. The optical gain is 7dB for both structures. In comparison with SS, the waveguide length of CI is longer because of multi-sections ion-implantation regions (for isolation), which will cause higher propagation loss in waveguide. However, the extra loss due to fabrication and excitation wavelength is less than 1dB for all applied bias and current, indicating the optical loss effect deteriorating modulation efficient is not significant. High propagation loss due to the absorption in isolation region can be reduced by increasing the M.Q.W. transition energy using bandgap engineering technique, for example, M.Q.W. intermixing, and re-growth. As high as modulation extinction ratios of higher than 20dB in 2V (at 60mA, from 2.5V to 4.5V) are obtained from the set wavelengths, suggesting that the high modulation extinction ratios of thin Q.W.s (6 wells) can still be attained by the long waveguide. Using optical spectrum analyzer to check SOA gain, same optical gains is obtained, indicating no significant degradation from SOA spontaneous emission in modulation depth is found for different applied pump currents.

 

Fig. 3 Optical transmission of 1600nm wavelength with EAM reverses bias for CI and SS, where SOA injection current is set as 0mA and 60mA. The insertion loss of SS is lower than CI’s by less than 1 dB, indicating the optical loss in isolation regions is low.

Download Full Size | PDF

In order to investigate high-speed characterization, S-parameter (microwave transmission and reflection) of the device is measured from a high-speed Vector-Network-Analyzer (VNA) for analyzing the electrical propagation properties of waveguide. The extracted characteristic impedance of SS EAM is 15Ω, which is mainly resulted from high waveguide intrinsic capacitance. Thereby, a 50Ω instrument probing leads to high electrical reflection (S11) of higher than −5dB at high frequency regime of above 5GHz. Once cascading HITLs into segmented EAMs to form CI structure, lower than −12dB of S₁₁ is found from D.C. to 30GHz. As a result, a higher electrical −3dB bandwidth is measured in CI (20GHz) than in SS (12GHz). As from HITL, a characteristic impedance of 66Ω is obtained, which is designed as the total length as 4 times of EAM’s, thus bringing up the equivalent impedance of CI to 31Ω. An equivalent circuit model is used to examined and also applied to fit device S-parameters based on the processed structure. The calculation results (line) plotted in Fig. 4 are quite consistent with the experiment results, further confirming the better microwave performance of CI structure.

 

Fig. 4 The measured (dot) and calculated (line) microwave S-parameters for CI and SS EAM and SOA.

Download Full Size | PDF

As characterizing high-speed optical link, a Vector-Network-Analyzer (VNA) with broadband photodetector is used to test high-speed EO response. The optical wavelength is set as 1600nm. The input and output electrical load lines of both structures are terminated by a 50Ω instrument and a 50Ω resistor for testing high-speed optical link. Figure 5 plots the EO conversion efficiency against with frequency, where the −3dB bandwidths of SS and CI structures are 13GHz and >30GHz respectively. Better EO response at high frequency regime is observed in CI, while the responses of both structures in the low frequency regime (D.C. to 10GHz) exhibit the same behavior. As seen in the insert of Fig. 5, the same responses with frequency are observed for different pump current of 0mA and 60mA, suggesting the spontaneous emission of SOAs did not affect the overall high speed optical link. A RF-link gain of 13.5dB is found, verifying the D.C. optical gain measurement. Using the extracted microwave properties of the waveguide by equivalent circuit model [13,14,16], the high-speed EO responses of both structures are also calculated to compare as well as fit the measured results. The simulated results plotted in line curves are quite in agreement with experimental results, indicating the lower microwave reflection at high frequency modulation in CI redeem the better high-speed EO conversion than SS. In this work, the bandwidth limitation relies on material structure and also waveguide size. The SS EAM is so called the conventional traveling wave EAM. Although several works have shown SS EAM with bandwidth exceeding 30GHz [10–13], applying CI scheme with that material and waveguide structure can further enhance the high-speed modulation by improving impedance match and lower the high-speed electrical driving power.

 

Fig. 5 The measured (dot) and calculated (line) EO-response for CI and SS, where the −3dB bandwidths of SS and CI are 13GHz and >30GHz respectively. Above 17 GHz enhancement of −3 dB bandwidth from CI is obtained. The inset shows that RF link gain of 13.5dB is obtained from CI as the SOA injection current is 60mA.

Download Full Size | PDF

4. Conclusion

A new monolithic integration method named as cascaded-integration (CI) for enhancing high-speed optical link is proposed and demonstrated. High-speed electroabsorption modulators (EAMs) and semiconductor optical amplifiers (SOAs) are taken as device structures of CI. CI is defined by cascading segmented EAMs and segmented SOAs, allowing periodic high-impedance transmission lines (HITLs) to interconnect EAMs. Therefore, high impedance mismatch due to single EAM waveguide can therefore be greatly reduced from HITL, simultaneously bringing out high electrical- and optical- transmission of device. In comparison with the conventional integration scheme, namely single section (SS) SOA-integrated EAM, lower microwave reflection in CI at high-speed regime (5GHz~30GHz) is observed in CI. CI exhibits less than −12dB electrical reflection, better than SS’s (higher than −5dB for frequency higher than 5GHz). Thereby, the higher microwave transmission with same optical transmission and gain can bring up high-speed electro-optical link. An –3dB bandwidth electrical-to-optical (EO) response of greater than 30GHz is measured in CI, better than 13GHz in SS, suggesting that CI can offer the advantages in high-speed optical modulation. Due to long EAM waveguides of total 300μm, high extinction ratio of >20 dB in 2V of swing voltage and 7-dB D.C. optical gain under 60mA current injection are observed in CI, leading to RF link gain of 13.5 dB. Thereby, the distributive effects by CI during the long waveguide interaction, the advantages of high EO response with broadband operation can be taken in high-speed optoelectronic devices, reducing the trade-off issue between speed and output optical-link efficient.