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A new type of semiconductor quantum well (QW) for high-speed all optical wavelength converter (AOWC) is proposed and demonstrated in this work. Based on InGaAsP (well)/InGaAlAs (barrier) multiple QW, large electron band offset ratio relative to heavy hole can be attained to shorten sweep rate of photocarrier driven by electric field, realizing high-speed efficient AOWC through cross absorption modulation (XAM). By such QWs, an optical waveguide with high-speed electrode connection is fabricated. A −3dB bandwidth of 38 GHz with 8V bias in time-varying photocurrent and all optical response is observed. The corresponding sweep time is less than 10ps, consistent with calculated tunneling rate of QW and thus confirming high sweep rate through field-driven tunneling processing. All-optical conversion with error-free 40Gb/s data transmission and −11dB of conversion efficiency in system performance is also attained in this device, suggesting that such AOWC has potential for 100Gb/s application.
©2011 Optical Society of America
1. Introduction
All-optical signal processing has brought a great impact on developing the next generation of high-capacity optical-fiber related products and networks, drawing lots of attention on the related studies. One of the key technologies mostly relies on developing high-speed all-optical wavelength converters (AOWC). There have been several schemes proposed and demonstrated. Using silica fiber or silicon-based material, optical nonlinearity can realize sub-picosecond response, but long interaction length and high-power pump source are needed due to low conversion coefficient, restricting overall conversion efficiency and thus the response speed (~10ps) by the fiber dispersion [1,2]. Semiconductor devices and its related integration module are other potential candidates. Semiconductor optical amplifier (SOA) for either optical-electrical-optical (OEO) or cross gain modulation (XGM) can realize not only optical gain but also AOWC performance, bringing all optical processing into photonic integration technology. However, slow recovery time in XGM and simultaneous cross phase modulation (XPM) from quantum well (QW) is found to be in the order of >50ps, causing pattern dependence at high bit-rate data modulation [3,4]. Up to 40Gb/s speed has been realized through optical delay-lined interference, another dummy pump light, or nonlinear polarization rotation of SOA for fastening recovery mechanism (12ps) [5–8]. However, additional or complicated structure is generally required, such as interferometer, assisted light source, and polarization controller. The performance by SOA is still imposed by the fundament material issues. Owing to quantum-confined Stark effect (QCSE) in quantum confined structure, cross absorption modulation (XAM) as well as XPM can enable efficient AOWC processing. Rather than the forward bias on SOA (p-i-n structure), strong electric field by reverse bias offers faster recovery mechanism so as to the order of 10ps magnitude can be obtained. Several approaches have been proposed [9–14]. State-of-the-art 100Gb/s using XPM performance has been attained, but complicated setups, e.g. DI and polarization control, are still needed [13]. In such operation schemes, high-speed all-optical processing devices mostly depend on material properties. On the other hand, the efficient of AOWC can be boosted by quantum-confined structure; however, speed is simultaneously reduced by photocarrier swept-out mechanism because of band offset, especially in the behavior of heavy hole. In order to attain above 40Gb/s or even 100Gb/s of AOWC for next generation network, QW design for fast carrier dynamics and efficient modulation then becomes quite important. In this work, a novel QW structure, high band-offset ratio material InGaAsP (P-Q, P quaternary)/InAlGaAs (Al-Q, Al quaternary), has been proposed to fasten carrier sweep process across QW. Through the applied electric field, recovery time shorter than 8ps of XAM with −12dB conversion efficiency has been demonstrated from a single waveguide device of AOWC. A successful 40Gb/s of data transmission is also demonstrated, indicating such simple scheme can be potentially applied to 100Gb/s in all optical processing regime.
2. Device design and fabrication
Once the pump light is absorbed in QW region, XAM can be operated by blue shift of QW from the generated photocarriers, such as charge screening, band filled effect, or exciton bleaching. The probe light can thus be modulated through QCSE of QW. Therefore, the speed limitation based on XAM in QW will be mostly relied on carrier sweep-out behavior. High sweep rate of photocarrier can set up high screening electric field to drive probe light by QCSE, where three major processing are thermion emission, recombination, and tunneling processing. When the QW device is operated under high reverse bias, due to the low overlap in electron- and hole- wave functions, the probability of recombination can be neglected. Also, by comparing with field-enhanced tunneling processing, thermal emission effect can be neglected due to lower statistical distribution of hot carrier. Hence, the swept-out rate by tunneling processing out of multiple QWs (MQWs) is assumed as the main operation mechanism to recover absorbing state after high pump power excitation [15,16]. Considering a single quantum well case, carrier swept time () through tunneling processing can be represented by Eq. (1) [17-18]:
is the band gap difference between well and barrier. and are band offsets of conduction and valence band respectively. Lw and Lb are widths of quantum well and barrier, where the effective mass is denoted as mi and mbi (i = e or h are for electron or hole), andis the nth sub-band energy level. F is the electric field in well. Although high external electric field could fasten carrier tunneling processing, conduction electron confinement will then be reduced to deteriorate QCSE. Therefore, band offset ratio and carrier effective mass of conduction and valance bands are main design issues for overall all optical performance. In general semiconductors of bandgaps λ = 1300nm or 1500nm, effective mass of valance-band heavy hole (~0.1 mo) is around one order of magnitude larger than conduction-band electron. Heavy-hole dynamics is thus the key issue responsible for slow carrier sweep rate. Although valance band offset () could be reduced through band gap engineering, the lowered conduction band offset () will inevitably reduce optical modulation efficiency due to reduced QCSE by light conduction-band electron mass. Therefore, as a point view of material design, large band offset ratio () becomes one of the main parameters to get high-speed efficiency AOWC. Large could have advantages of allowing high applied electric field to fasten carrier processing while still maintaining significant QCSE [18,19].Heterogeneous MQWs, P-Q/P-Q and Al-Q/Al-Q, are two common material structures operated at optical fiber communication (1300nm and 1500nm), where the corresponding band offset ratio of electron to heavy hole is around 0.3/0.7 and 0.5/0.5 respectively [20]. Due to the higher Al-Q band gap line-up factor than P-Q in electron affinity parameter, P-Q/Al-Q materials can thus be taken to further enlarge band offset ratio to 0.6/0.4, thus increasing MQW design tolerance. In order to further design P-Q/Al-Q MQW for fastening carrier sweep-out time, a model based on transfer matrix method is used to extract carrier tunneling processing. The strain compensation for polarization independence material with 8 wells of In0.54Ga0.46As0.91P0.09 (tensile strain, 8nm) and 9 barriers of In0.56Al0.16Ga0.28As (compressive strain, 9nm) are used. The corresponding band offsets for electron, heavy hole, and light hole are 67meV, 46meV, and 83meV. Figure 1 plots the carrier swept-out time (life time) against electric field. Because of large effective mass in heavy hole, the sweep processing shows at least one order of magnitude slower than electron and light hole, leading to the dominated mechanism during high-speed modulation and also consistent with the argument in the previous section. Through P-Q/Al-Q MQWs to address band offset () ratio, both electron- and hole- swept-out process can be reduced to less than 10ps for high-speed AOWC.
Based on the results of Fig. 1, strain-compensated P-Q/Al-Q MQWs is grown as active region by a MOCVD system, sandwiched by top p-InP and bottom n-InP layers for AOWC waveguide. Underneath, a passive waveguide is grown for integrating top active waveguide, defining optical spot-size converter (SSC), which is for efficient coupling from a single-mode fiber to the small core of AOWC waveguide. Figure 2 shows the fabricated waveguide core. Whole processing is defined by selective undercut-etching active region [21]. The finished AOWC waveguide and passive waveguide are 2.5μm and 8μm respectively. The length of EAM region is 100μm. N- and p- metal ohmic contact for biasing AOWC are deposited by evaporating Ni/AuGe/Au and Ti/Au metals. In the two ends of optical waveguide of AOWC, two coplanar waveguides (CPW) are fabricated as the connection lines for receiving high-speed modulated photocurrent as well as biasing AOWC.
Fig. 2 (left) device picture and (right) the schematic plot of measurement setup. RX is photoreceiver.
3. Experimental results and discussion
In characterizing AOWC, small-signal optical-to-electrical (OE, photocurrent) measurement, is first performed to exam carrier swept rate in MQWs. The measurement setup is shown in the right side of Fig. 2. A time-modulated optical pump power is obtained by a 40Gb/s Mach-Zender modulator (MZM, JDSU 40Gb/s) with a broadband Vector Network Analyzer (VNA). The modulated signal is then amplified by an EDFA and coupled into AOWC for exciting photocurrent. The pump light is TE-polarized centered at 1530nm. After the light is converted into photocarriers, as shown in Fig. 2, CPW line is used as current collector. By VNA, the final OE response is extracted. Left side of Fig. 3 shows the normalized OE response. As biasing from 4 to 8 V, −3dB bandwidth drop increases from 10GHz to 38GHz, suggesting the electrical-field enhancement of carrier swept processing. Above 7V, the response is saturated, mainly limited by MZM bandwidth. As photocarrier are swept out of active region, the generated photocurrent will flow out and propagate in the waveguide, may affecting the overall frequency OE response due to the electrodes and circuits connected to AOWC. In order to check this point, VNA is also used to exam electrical properties of waveguide (or S-parameter). The −3dB drop in electrical transmission (S21) is 50GHz which is larger than OE bandwidth, further confirming the speed of received photocarrier is modulator-limited. For further testing high-speed properties of cross optical absorption through carrier swept process, a 1555nm probe light (Fig. 2) is sent to AOWC with 1545nm pump light using co-directional scheme with pump light, performing OO response. After filtering out pump light, the modulated probe light responses is sent into VNA for analysis. As shown in the right side of Fig. 3, the response exhibits the same behavior with OE response for biasing at voltages of higher than 7V; indicating all-optical response based on cross absorption is mainly attributed to high-speed carrier sweep processing. Using distributive photocurrent model, theoretical OE response plotted with dash curve in Fig. 3 can be simulated to fit the experimental results, where the fitted response time is 8ps. Furthermore, the insert of Fig. 3 plots the DC all optical conversion efficiency. −12dB DC conversion efficiency at bias of 8V with 8dB extinction ratio is obtained in all optical modulation. Large QCSE still exists in such high electric field, implying that large electron confinement in conduction band. By accompanying with OE and OO conversion, it indicates that large band offset ratio using P-Q/Al-Q MQW could be operated at 10ps regime of response time.
Fig. 3 (left) different-bias OE response with frequency. The dash curve is simulated curve. (right) OO response.
In order to further test system performance of AOWC by P-Q/Al-Q MQW, a large signal with non-return-to-zero (NRZ) 40Gb/s pattern is used. The experiment follows the setup of small-signal OO conversion, except that an EDFA and an electrical amplifier (Picosecond 5882 40Gb/s Amplifier) are placed behind filter and receiver (RX, U2T XPVD 2120R). Device insertion loss is −12.5dB, including coupling loss and propagation loss. The input average optical power of pump signal and DC probe power was set as 3dBm and 0dBm. A pseudo random bit string (PRBS) with pattern length of 231-1 with bit rate of 40Gb/s is used for digital pulse train. The back-to-back and converted eyes of AOWC are measured and plotted in Fig. 4 , where back-to-back eye is directly measured after MZM with amplification. As shown, eye pattern exhibits the similar behaviors with back-to-back eye pattern. Extracted from eye pattern, AC conversion efficiency is −11dB. And optical signal-to-noise ratios (OSNR, Q value) are extracted as 5.9 and 5.5 for pump and converted signal. And the AC extinction ratios of 5dB (pump) and 5.6dB (converted signal) are observed, where the enhancement is mainly from the nonlinear transfer function of XAM (inserted plot of Fig. 3). The noise from EDFA and electrical amplifier could limit AC extinction ratio and OSNR performance. No error floor of bit error rate test (BERT) and less than 0.5dB loss budge in 10-9 BER are observed in high-speed data transmission. 40Gb/s transmission without pattern dependence in such AOWC is realized, indicating material structure of P-Q/Al-Q MQW can have capability to get speed as well as efficient XAM in system performance.
Fig. 4 (left) eye diagram of pump signal and converted signal, (right) the corresponding bit error rate.
4. Conclusion
In this work, a high band-offset ratio of quantum well, namely InGaAsP/InGaAlAs (P-Q/Al-Q), is first employed as high-speed all optical wavelength converter (AOWC) through cross absorption modulation (XAM) effect. Using P-Q and Al-Q as well and barrier material, high band-offset ratio of electron to hole can be realized to enhance the field-driven tunneling rate of heavy hole for at high electric field while still sustaining large quantum confine Stark effect (QCSE) in electron. Using high-speed microwave coplanar waveguides (CPW) as connection lines, an optical waveguide based on P-Q/Al-Q material is fabricated as a high-speed AOWC. By driving voltage of 8V, −3dB bandwidth 38GHz of photocurrent is directly detected from CPW line of AOWC, limited by modulator. Same bandwidth of all optical conversion is also observed, confirming high-speed sweep processing of photocarrier as well as cross-absorption modulation can be realized in such QW. By fitting photocarrier response through distributive photocurrent model, less than 10ps response of all-optical conversion is suggested. A successful 40Gb/s data transmission is also demonstrated in this device.
Acknowledgments
The authors would like to thank the financial supports from the National Science Council, Taiwan (NSC99-2221-E-110-029-MY3) and “Aim for the Top University Plan Taiwan” (97C030133). The wafer and high-speed instrument supports from Land Mark Optoelectronics Corporation and Professor H. Taga in NSYSU are also of great help.
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