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Direct modulation at >25 Gbps is achieved with 1310 nm wavelength wafer fused VCSELs by adjusting the strain in the quantum well active region and the cavity photon lifetime. 25 + Gbps large signal modulation with 10−12 BER at 1310 nm across 10 km of standard single mode fiber is demonstrated.
© 2016 Optical Society of America
1. Introduction
The huge expansion in data traffic during the last few years is now facing severe limitations due to its exponentially increasing energy demand. Long wavelength (LW) vertical cavity surface emitting lasers (VCSELs) are here of strategic importance providing low cost and low power consumption sources for optical interconnects in data centers [1,2], where power dissipation is a major problem, and local-area optical fiber networks, spanning transmission distances of 100 m to 10 km.
At present there exist several competing technologies for fabricating LW VCSELs, in particular emitting at 1300 nm [1,3–6]. Among those, the wafer-fusion approach has the advantage of allowing to combine high quality InP-based active regions with high reflectivity, low absorption and high thermal conductivity GaAs-based distributed Bragg reflectors (DBRs). 1300 nm VCSELs fabricated with this technology present record single mode (SM) power due to better thermal management [7], high-yield wavelength setting based on nm-scale cavity engineering [8], demonstrated reliability [9], and compatibility with conventional GaAs-based VCSEL manufacturing [10]. Moreover, wafer-fused VCSELs have been employed in coarse wavelength division multiplexing (CWDM) (1270, 1290, 1310 and 1330 nm) modules for 40 Gbps (4 × 10 Gbps) low-power consumption optical links [11]. All these results were based on active InAlGaAs/InP quantum well (QW) regions with 1% compressive strain in the QWs. Further advancement of this technology towards 100 Gbps (4 × 25 Gbps) CWDM optical links or even higher bandwidth capacities requires reaching large signal modulation at 25 + Gbps, presenting a challenge for LW-VCSELs.
In terms of continuous wave (CW) parameters such as single mode power LW VCSELs already show better performance than short wavelength (< 1 µm) VCSELs, but they are still lagging behind in terms of modulation bandwidth [12–15]. It has been shown that InP-based 1300 nm VCSELs can operate at bandwidth in excess of 15 GHz [16]. Yet, the maximum reported modulation bandwidth of wafer-fused VCSELs reached so far was in the range of 7 – 8 GHz [17]. Larger modulation bandwidth in VCSELs can be reached by increasing the differential gain provided by the active region [18], which in turn can be increased by larger strain in the QWs [19,20]. In addition, decreasing the cavity photon life time by optimization of the VCSEL output coupling may further improve the modulation characteristics [12, 13, 17, 20].
Here, we show that by increasing the strain in the InAlGaAs/InP QWs to 1.6% and reducing the cavity photon lifetime by adjusting the number of pairs in the DBRs, it is possible to significantly improve both the static and the dynamic performance of wafer fused VCSLEs emitting near 1310 nm. We report large signal modulation and data transmission over 10 km at 25 Gbps using single mode fiber (SMF) without compromising the high single mode output power characteristics of the devices.
2. VCSEL structure and CW characteristics
The wafer fused VCSELs [7–11] comprise an InP based active layer sandwiched between two undoped GaAs/AlGaAs DBRs. The active region is grown by metalorganic vapor phase epitaxy (MOVPE) and contains several InAlGaAs/InP, compressively strained QWs and a buried tunnel junction (TJ) for current and optical confinement. The undoped bottom and top DBRs of the base devices contain 35 and 21 pairs, respectively. Processed VCSELs are top emitting and employ double intra-cavity contacts. A 1.5 µm thick layer of Si3N4 is used as a passivation layer.
For the investigations presented here, devices with 1.3 and 1.6% compressively strained InAlGaAs/InP QWs, tunnel junction diameters of 6 µm, and different numbers of top DBR pairs have been fabricated and tested. Static characteristics were measured at 20 and 70°C. Statistical analysis shows small changes in output power (P), threshold current (Ith) and slope efficiency (Seff) (Table 1) with increasing QW strain from 1.3 to 1.6% at the same output coupling (21 pairs top DBR). In order to test the impact of the output coupling on the static characteristics, two batches were processed from the same double-fused wafer (1.6% strain). The first batch incorporated devices with 21 top DBR pairs, and in the second one three top-DBR pairs were selectively removed by wet etching [17] preceding the final device processing. Further reduction of the top DBR reflectivity results in inferior static characteristics.
Table 1. Measured static parameters at 20 and 70°C for two VCSEL structures having different top DBRs.
Light-current-voltage (LIV) measurements of these batches show significant improvement in P and Seff with small penalty in Ith in the temperature range of 20 to 70°C. Figure 1 presents several light-current (LI) characteristics measured at 20 and 70°C of two VCSEL structures with different top DBRs. in the values of Ith, P at 10 mA and Seff at 20 and 70° C are summarized in Table 1.
Fig. 1 LI characteristics at 20°C (a) and 70°C (b) for typical 1310 nm VCSELs with 1.6% QW compressive strain and 21 and 18 top DBR pairs.
Most of these devices operate in a single mode with side-mode suppression-ratios (SMSR) of more than 30 dB up to currents of 8 – 10 mA, corresponding to more than 1.8 and 3 mW SM output power in the 20 – 70° C temperature range for devices with 21 and 18 top-DBR pairs. Figure 2(a) presents the emission spectrum of a typical VCSEL with 18 top-DBR pairs at 20°C and 10 mA drive current. Increasing the output coupling has also a large impact on the wall-plug-efficiency (WPE). Devices with 1.6% strain and 18 pairs of the top DBR exhibit the highest WPE of ~32%. Figure 2(b) shows the WPE versus current for these devices at 20° and 70°C. At 10 mA, the WPEs are 22% at 20°C and 15% at 70°C, respectively.
Fig. 2 Emission spectrum measured at 20 °C and 10 mA driving current (a) and wall plug efficiency (WPE) at 20 and 70°C for VCSELs with 1.6% QW compressive strain and 18 pairs of the top DBR (b).
The very good performance of the wafer-fused VCSELs incorporating 1.6% strained QWs high yield demonstrates that increasing the QW strain to this level does not compromise the quality of the double-fused VCSEL wafer.
3. Small-signal modulation
We performed small-signal modulation (S21) measurements of the VCSELs for different QW compressive strain levels and top-DBR pair numbers in order to assess the impact of differential optical gain and cavity photon lifetime on the dynamic characteristics. The S21 characteristics were measured with an HP 20-GHz network analyzer model N5230A and the VCSELs under test were contacted directly using a 25-GHz bandwidth probe 40-GS-150P. The emitted beam was coupled to a 50-GHz bandwidth photodetector via a SMF. A bias-T was used to combine the RF and DC current source for driving the VCSEL.
One important parameter extracted from the S21 responses is the −3dB bandwidth (f3dB), which allows an educated guess of the maximum achievable bit rate for data transmission. For devices with 21 pairs of the top DBR, the maximum f3dB increases from 8 to 9.5 GHz by increasing the QW strain from 1.3 to 1.6%. Largest values are obtained for drive currents of 5 – 6 mA (Fig. 3(a)). The D factor [21] calculated for the same reflectivity of the top DBR (21 pairs) increases from 2.5 to 4.3 GHz(mA) –1/2 by increasing the QW strain from 1 to 1.6% (Fig. 3(b)).
Fig. 3 f3dB at different bias currents extracted from S21 curves at 20°C for two VCSELs with the same top DBR with 21 pairs and 1.3 or 1.6% compressively strained QWs (a) and D-factor versus QWs strain at 20°C (b).
By removing three pairs from the top DBR of the devices with 1.6% strain, f3dB increases up to 11.5 GHz, but the peak of f3dB occurs now at higher currents (Fig. 4(a)). In order to evaluate the influence of temperature on modulation bandwidth, S21 measurements have been performed at different heat-sink temperatures. At 10 mA bias current the bandwidth is above 11 GHz up to 60°C (Fig. 4(b)), indicating large data transmission potential still at increased temperatures.
Fig. 4 f3dB versus current at 20°C for two VCSELs with the same QW strain (1.6%) and different output coupling, corresponding to 21 and 18 pairs of the top DBR (a) and f3dB versus temperature at 10 mA bias for devices with 1.6% strain in QWs top DBR of 18 pairs (b).
4. Large-signal modulation
In order to determine the large-signal modulation response of the wafer-fused VCSELs (1.6% strain in QWs and top DBR 18 pairs), we performed data transmission measurements at various bit rates and heat-sink temperatures. A pseudorandom binary bit pattern sequence (PRBS) with standard non-return to zero (NRZ) modulation scheme using a 27–1 bit word-length was generated by a 12100B bit pattern generator from SHF, followed by an 8 dB amplifier to compensate for the losses of the electrical wires and to provide larger modulation amplitudes. After amplification, the bit sequence was fed into the VCSELs using a 65-GHz SHF bias-T and a 40 GHz-rated coplanar SG probe. We use simple VCSEL-to-fiber coupling with angled lensed, standard single-mode optical fiber or multimode fiber pigtails. Our back-to-back (B2B) configuration consists of a 5 m long fiber and a variable optical attenuator for both determining the bit error ratio (BER) and measuring the eye diagrams. Eye diagrams and bit error ratio measurements were studied using a 70 GHz Agilent sampling oscilloscope and a SHF 11100B error analyzer. We used a New Focus receiver (1484-A-50) with an InGaAs diode having a bandwidth of ~22 GHz for recording the eye diagrams and the BERs.
To prove energy-efficient optical data transmission, all experiments where performed without using O-band optical amplifiers for data rates from 25 to 35 Gbps in B2B configuration and over 10 km of SMF, in the temperature range of 20 to 70°C. Figure 5 shows 25 Gbps open eye diagrams at 20 and 70°C in B2B configuration at current bias of 12 and 9 mA respectively. At 20°C, measurements have been performed at data rates of 25, 30 and 35 Gbps in B2B configuration (Fig. 6(a)) and after 10 km of SMF (Fig. 6(b)). At 35 Gbps, the eye is still open after 10 km of SMF. The energy to data rate ratio per km is here as low as 85 fJ/bit, which still presents an upper limit, since a 0.5 dB attenuator was used in front of the receiver. To our knowledge this is the lowest value yet reported for this bit rate.
Fig. 6 Open eye diagrams at 20°C in B2B configuration and 11 mA (a) and after 10 km of SMF and 12 mA (b) for data rates 25, 30 and 35 Gbps.
BER curves measured at 25 Gbps in B2B and after 10 km of SMF are plotted as a function of the received optical power in Fig. 7. Error-free operation with BER smaller than 1 × 10−12 was achieved for received optical power of –8.5 dBm.
Fig. 7 BER versus received power at 25 Gbps in B2B configuration and after 10 km of SMF at 20 °C using wafers with 1.6% QW strain.
5. Conclusion
By increasing the compressive strain in the InP/InAlGaAs QWs and reducing the photon lifetime by DBR pair removal, the static and dynamic properties of wafer-fused VCSELs emitting near 1310 nm show large improvements. Using 1.6% strain and removing three pairs of the top DBR, the –3dB bandwidth increased to 12 GHz. Error-free data transmission at 25 Gbps over 10 km of standard SMF at 20°C is demonstrated. The energy to data rate ratio per km at 35 Gbit/s at 10 km is as low as 85 fJ/bit. The way for producing low power consumption parallel or CWDM modules for 4 × 25 Gbps optical links for data centers and other local O-band area network applications is now open.
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