10-Gb/s modulation speed and transmission over 10-km SM fiber with BER < 10−11 up to 100°C temperature are achieved with optimized wafer-fused GaAs/AlGaAs-InP/InAlGaAs VCSELs incorporating re-grown tunnel junction. These VCSELs operate in the 1310-nm waveband and emit more than 1-mW single mode power in the full temperature range.
© 2009 OSA
Vertical cavity surface emitting lasers (VCSELs) operating in the 1310-nm waveband at multi-Gb/s speeds and high temperatures are highly desirable for applications in local area fiber networks. Such devices would offer reduced power consumption, uncooled operation and flexible multi-wavelength configurations, all of which are essential features for the rapidly expanding, dense optical communication infrastructure. Significant progress has been achieved in recent years with such long wavelength VCSELs, using several device technologies . In particular, 1-2-mW single mode (SM) power and 10Gb/s modulation up to 85°C have been reported for such devices [2–8]. In addition, error-free transmission over 10-km at room temperature for both 1.3 and 1.5-μm wavelengths VCSELs [3,4,6] and at 85°C for 1.3-µm VCSELs [2,3,7] have been demonstrated.
Among the successful relevant VCSEL technologies, the wafer-fused approach provides particularly high SM power due to efficient heat dissipation  and ease of construction of multi-wavelength VCSELs, e.g., for application in coarse wavelength division multiplexing (CWDM) transmitters . In this paper we report 10-Gb/s operation and transmission with wafer-fused VCSELs emitting in the 1310-nm band with improved design. Devices with 6-μm apertures show single fundamental mode operation in the 2-10 mA current range with an output power exceeding 1-mW up to 100°C. Small signal modulation curves show > 6-GHz bandwidths for currents above 5-mA. Large signal modulation at 10-Gbps shows clear open eye diagrams up to 100°C. In addition, the 10-Gb/s transmission experiments in back-to-back configuration and through a 10-km SM fiber with bit error rate (BER) better than 10−11 at both room temperature and 100°C, and with power penalties less than 0.5 and 1dB, respectively, were demonstrated. The obtained results have the potential to reduce both the power consumption and cost in datacom and GPON transceivers.
2. VCSEL structure and fabrication
Figure 1 shows a schematic cross-section of the device. The VCSEL comprises an InAlGaAs/InP-based quantum well (QW) active cavity and a re-grown tunnel junction (TJ) with 6-μm diameter aperture, fused on both sides to AlGaAs/GaAs un-doped distributed Bragg reflectors (DBRs), as described in [4,9,10]. Both the InP-based active cavity and the GaAs-based DBRs are grown by low pressure metal-organic vapor phase epitaxy (LP MOVPE).
The growth of the active cavity was performed in two steps. In the first step, the n-InP current spreading layer, the InAlGaAs QWs, and the p-n and tunnel junction layers were grown. The TJ aperture was then defined using photolithography and wet chemical etching. In the second step, the over-growth of the patterned TJ mesa was performed. The re-grown TJ serves at the same time as a source of holes injection and introduces a refractive index profile that supports the optical cavity modes. The current flow is confined to the TJ disc by a reverse biased p-n junction. In the present work, the VCSEL structure is designed for reaching high-speed modulation characteristics by optimizing the QW active structure, the gain-cavity wavelength off-set and by reducing the electrical parasitics. First, the number of QWs was increased to six for obtaining higher differential gain. Second, the off-set between of the peak in the photoluminescence (PL) spectrum at room temperature and the VCSEL cavity mode was set at ~50-55-nm for high temperature operation. Third, low index polyimide passivation was used under the bond pads in order to decrease the parasitic bond pad capacitance. Finally, the area of the reverse biased p-n junction was reduced to decrease the parasitic capacitance.
3. Static device characteristics
The single-mode operation of VCSELs depends on the device design and on the size and lateral index control introduced by the re-grown tunnel junction [9–11]. Increasing the size of optical aperture is the common way to increase the maximum output power, but with increasing it - higher second transversal modes appears. Figure 2 depicts the light-current-voltage characteristics of a nominal device with 6-μm TJ aperture, measured in the temperature range of 20-100°C. The maximum output power at room temperature is 3-mW and as high as 1-mW at 100°C. The threshold current is ~1 mA and is almost constant up to 70°C, which reflects the device optimization for high temperature operation. At ~60°C the threshold exhibits a minimum of ~0.9-mA, which reflects the ~50-nm wavelength off-set between the room-temperature PL peak and cavity mode at 20°C. The maximum output power, limited by the thermal roll-over, decreases linearly with temperature and reaches a value close to 1-mW at 100°C.
The room temperature and 100°C emission spectra of the wafer fused VCSEL, measured at different diode currents, are shown in Fig. 3 . The room temperature spectra are single mode at currents up to 10 mA, with side-mode suppression ratio (SMSR) in excess of 40 dB (Fig. 3a). At 12-mA the SMSR is lower than 20-dB. With increasing temperature, the SMSR remains higher than 40-dB for higher currents; at 100°C, such high SMSR is maintained up to 16-mA, at which thermal role over takes place. Thus, with this VCSEL design single mode operation with more than 1-mW output power is obtained for the entire temperature range of 20-100°C.
4. Modulation characteristics and transmission experiments
The relative intensity noise (RIN) spectra of the 1310-nm band VCSELs at 20°C and at 85°C, measured using the HP 71400C Lightwave Signal Analyzer system, are presented in Fig. 4 . A resonance frequency of 10-GHz is reached at a bias current of 10-mA for both test temperatures. The extracted value of the modulation current efficiency factor (MCEF) from the RIN plots yields the values of 3.8-GHz/mA1/2 (T = 20°C) and 3.5-GHz/ mA1/2 (T = 85°C). One can see that at elevated temperatures the MCEF value is just slightly lower compared to 20°C. This is an indication of well-optimized design of the VCSEL structure, with good modulation potential within a large temperature range.
Small signal modulation characteristics (s21) of the 1310-nm band VCSELs were measured on-wafer using a probe station with high frequency (HF) probes and data acquisition set-up based on the signal analyzer system Agilent 8702E with 6-GHz test bandwidth. In Fig. 5a and 5b, the s21 spectra at both 20°C and 85°C temperature in the 2-10-mA current range are plotted. The 3-dB cut-off is higher than 6-GHz for currents above 5-mA at both temperatures. The extracted values for MCEF using the s21 plots yields 3.34-GHz/mA½ (T = 20°C) and 3.18-GHz/mA½ (T = 85°C), respectively. These values are only slightly lower than those extracted from the RIN spectra. This discrepancy is related to the fact that the RIN-based data indicate the intrinsic modulation capabilities of the VCSEL and are not influenced by external parasitic factors (capacitance, resistance, etc), which is the case for S21 measurements. Based on the s21 measurements, one can predict the modulation capabilities of the VCSEL device at 10-Gb/s data rates for bias currents higher than 5-mA, for which the 3-dB modulation bandwidth exceeds 6-GHz.
Large signal modulation tests of the 1310 nm band VCSELs were performed on- wafer using a probe station with HF contacting probes and the 12.5 Gb/s Agilent N4903A J-BERT. As expected from the RIN and s21 measurements, the devices show good large signal modulation response as well. The tests, performed at 10.3125 Gb/s data rates (10G Base Ethernet standard) with a Pseudo Random Binary Sequence (PRBS) of “231-1” input signal and electrical modulation amplitude extinction ratio in excess of 5-dB, show open eye diagrams for “Back to Back” (B2B) transmissions (3 m of SMF patch-cord), which conforms to the mask tests with 20% margins at 20°C and 100°C as well (see insets in Fig. 6a and 6b). For a bias current of 8 ÷ 9 mA the rise/fall time values are within the 35 ÷ 40-ps range and are compatible with 10-Gb/s operation.
The transmission tests of optical data at 10-Gb/s through 10-km of single mode fiber (SMF) line have been performed at both room temperature and up to 100°C. The VCSEL emission was coupled into SM fiber patch-cord with a special mini-coupling lens termination w/o optical isolator. Next, the patch-cord was connected to the 10-km SMF line via standard FC/PC adapter. The tests were performed for VCSEL device temperatures of 20°C and 100°C. As depicted in Fig. 6 (a) and (b), the transmission through 10-km of SMF shows open eye diagrams with applied 20% margins test mask. A few violations of the mask margins are rather related to the receiver sensitivity than to the VCSEL modulation performances and transmission phenomena through the fiber. Modulation voltage was 0.7-Vpp and bias current was 9-mA. A BER better than 10−11 was measured for such tests at both room temperature and 100°C with power penalties of less than 0.5 and 1-dB, respectively.
By design optimization of the device structure of wafer-fused 1310-nm VCSELs we demonstrate an improvement of the high-speed performance maintaining high output single mode power in excess of 1-mW throughout the 20-100°C temperature range. Modulation bandwidth exceeding 6-GHz at bias currents > 5-mA and 10-Gb/s modulation up to 100°C have been achieved. 10-Gb/s transmission, back-to-back and through the 10-km SM fiber, with BER better than 10−11 at both 20°C and 100°C and with power penalties below 0.5 and 1-dB, respectively, was demonstrated.
This work was partly supported by the Swiss innovation promotion agency (CTI) and by the European Commission via the project MOSEL.
References and links
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