1. Introduction

Vertical-cavity surface-emitting lasers (VCSELs) enable low-cost and low-power transmitters, and the advantages come from a simple fabrication process, ease of mass production, low threshold current and low operating current. Owing to these advantages, VCSELs are developing into an ideal light source for short distance optical interconnects and local area networks (LANs). In particular, ultra low threshold VCSELs are expected to be used as transmitters for portable equipments where power consumption should be minimized. However, unlike other semiconductor laser devices, the temperature dependence of VCSELs′ threshold current is very complicated showing approximate quadratic temperature dependence [1]–[2]. Owing to this complex behavior of the threshold current, a complicated control mechanism is necessary to operate the transmitter over a wide temperature range. As a result, the driving circuit becomes more complex, and therefore one of the VCSELs′ advantages may be lost. To design a low cost and low consumption-power VCSEL transmitter operating reliably over a wide temperature range, the temperature dependence of the threshold current of VCSELs should be improved so that a simple driving circuit may be used. Simultaneously, a high-yield passive-alignment module is desired.

TTC-Agilent Technologies have developed InGaAlAs ridge Fabry-Perot lasers operating at 10-Gb/s up to 110 °C with constant current swing [3]. Also, a ridge waveguide 1.3-μm GaInNAs laser with high temperature stability enabled uncooled 2.5-Gb/s operation in the temperature range from 25 to 100 °C with a constant modulation voltage [4]. However, the bias current should be controlled in accordance with temperature change and it has not been reported whether the lasers operate at temperature below 0 °C. Also, the threshold current of these lasers are much greater than that of typical oxide VCSELs. This makes these lasers difficult to be used in mobile equipments where low power-consumption is a matter of primary interest. In this paper, we have investigated the possibility of fixed-current operation of GaAs/AlGaAs VCSEL transmitters over a wide temperature range. Experimental result has shown that 1.25-Gb/s operation at fixed “on” and “off” current is attainable over a wide temperature range from -20 to 120 °C with a passively aligned module that employs a VCSEL with d ² I_th/dT ² as low as 0.114 μA/°C².

2. Experiment and result

GaAs/AlGaAs-based VCSELs show smaller temperature dependence of the threshold current than those with other material system [1] so that the devices may be the best answer to wide temperature range operation. Therefore, we have grown epitaxial wafers for GaAs/AlGaAs 850-nm oxide VCSELs using Metal-Organic Chemical Vapor Deposition (MOCVD) technique and fabricated top emission VCSELs for data transmission experiment. The basic structure of our VCSELs consists of three undoped GaAs quantum-wells which are sandwiched by graded composition Al_0.15Ga_0.85As/Al_0.92Ga_0.08As p-DBR mirror layers of 23.5 pairs and n-DBR mirror layers of 39 pairs. The p-type and n-type DBR mirror layers are uniformly doped with carbon by 2×10¹⁸ cm^-3 and silicon by 2×10¹⁸ cm^-3, respectively.

 

Fig. 1. Passively aligned bidirectional module: (a) photograph and (b) schematic diagram.

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Also, on the top of the p-DBR layer, a 55-nm-thick highly doped (> 3×10¹⁹ cm^-3) p-type GaAs layer is deposited for low resistance Ohmic contacts. Used contact metals are Cr/Ni/Au and AuGe/Ni/Au for p- and n-type Ohmic contacts, respectively. Polyimid is used under the contact metal in order to reduce the parasitic capacitance. However, package parasitic capacitance is measured to be 1.2 pF while the chip capacitance is 0.5 pF.

Figure 1 shows the passively aligned bidirectional VCSEL transceiver module used for the data transmission experiment. This bidirectional transceiver module is commercially available from Phothena Optics Corporation. The passive alignment between the VCSEL chip and multi-mode fiber (MMF) is essential for cost reduction. This Lego block style assembly is useful for alignment-free structures with compact size, and does not require capital equipment. Also, this type of module enables full duplex link using a single fiber line since the injected signals having different wavelengths are separated by a 45° filter as can be seen in Fig. 1(b). Since our primary concern is the performance of the transmitter, the 45° filter was removed from the module in this study.

 

Fig. 2. Setup for the data transmission experiment.

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Figure 2 shows the experimental setup for the transmission experiment, which consists of a pulse pattern generator (Anritsu MP1763C), a driving circuit, the VCSEL transmitter module within a temperature controlled oven, 1 m-long MMF, photoreceiver (HP 83487A) and an oscilloscope (Aglient 86100A).

 

Fig. 3. Temperature dependence of the static characteristics of a VCSEL chip in the temperature range from -20 to 120 °C with a 20 °C step: (a) L–I and V–I characteristics and (b) temperature dependence of the threshold current.

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Figure 3 shows the temperature dependence of the static characteristics of a VCSEL chip in the temperature range from -20 to 120 °C. Measured L–I and V–I curves as a function of temperature with a 20 °C step are shown in Fig. 3(a). The optical output power linearly increases with the operating current at first, and it becomes saturated by self-heating effect with the saturation current decreasing with temperature rise. The average slope efficiency between the threshold current and 4 mA is 0.46 W/A at -20 °C and decreases to 0.23 W/A at 120 °C showing gradual decrease with temperature rise in between. The differential resistance, dV/dI, at 4mA also decreases from 75.2 Ω at -20 °C to 58.5 Ω at 120 °C showing smaller temperature dependence compared to that of the slope efficiency. The temperature dependence of the threshold current in the temperature range from -20 to 120 °C is shown in Fig. 3(b). The threshold current shows a nearly minimum value of 0.79 mA at 50 °C, and increases to 1.32 mA at -20 °C and 1.37 mA at 120 °C showing nearly quadratic dependence on temperature. From the curve-fitting, we extracted the minimum threshold current, I_th,min, of 0.77 mA at 50.3 °C and the temperature coefficient, d ² I_th/dT ², of 0.114 μA/°C². The temperature coefficient derived here is smaller than the previously reported value of the 850 nm-wavelength oxide VCSEL [1].

 

Fig. 4. Filtered eye diagrams measured in the temperature range from -20 to 120 °C with a 20 °C step with fixed “on” and “off” current of 4.6 mA and 1.0 mA, respectively. The time scale is 200 ps/div., the vertical scale is 100 μW/div., the bit rate is 1.25-Gb/s and non-return-to-zero pseudorandom bit sequence of 2⁷-1 is used.

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In Fig. 4, we have shown eye diagrams obtained from our transmitter module to confirm that the fixed current operation in the temperature range from -20 to 120 °C is possible with the VCSEL transmitter. The bit rate is 1.25-Gb/s and used are non-return-to-zero (NRZ) pseudorandom bit sequence of 2⁷-1 and a fourth-order Bessel-Thompson filter with cutoff frequency of 1.0 GHz at the receiver. We have fixed “on” and “off” current at 4.6 mA and 1.0 mA, respectively. The “off” current has been adjusted to approximately 0.2 mA higher than the minimum threshold current, I_th,min, for high speed direct modulation.

As shown in Figs. 4(a) and 4(b), at low temperature from -20 to 0 °C, the timing jitter gradually increases as temperature falls. According to the approximately quadratic behavior of the threshold current change with temperature, the threshold current at this temperature range is higher than the fixed “off” current level. Therefore, the timing jitter and turn-on delay time increase due to pattern effect and spontaneous emission noise [5]. Since the overshoot increases as “off” level carrier density decreases compared with the threshold carrier density, “1” level intensity noise at low temperature is greater than that at 20 °C although the fourth-order Bessel-Thompson filter has been used at the receiver. Even though the timing jitter and “1” level intensity noise are present at -20 and 0 °C, the Gigabit Ethernet specification is satisfied.

In the temperature range from 20 to 100 °C clear eyes are observed. Since the variation of the threshold current in this temperature range is small as shown in Fig. 3(b), the eye diagrams in this temperature range are similar except for the reduction of “1” level intensity at 100 °C. Also, the extinction ratio (ER) exceeds 9 dB over the temperature range from -20 to 120 °C. However, the ER becomes smaller than 9 dB when the “on” current is lower than 4.6 mA or “off” current is higher than 1.0 mA. In addition, the fall time of 262 ps at -20 °C reaches to 364 ps at 20 °C. The greater fall time at 20 °C is due to the secondary optical output pulsation by the carriers supplied from the periphery of the active area [6]. At 120 °C, the timing jitter increases again by the same reason as that at -20 °C and 0 °C. The overshoot is negligible at temperature higher than 20 °C and it leads to low intensity noise of “1” level. Owing to decreased output power at temperature above 100 °C, the signal-to-noise ratio (SNR) starts to decrease at 100 °C. As previously mentioned, the package parasitic capacitance is 1.2 pF, which is about 2 times larger than that of a typical pig-tailed module. Computer simulation using a modified large-signal equivalent-circuit model based on [7]–[8] shows that rise and fall times decreases by 30.1 ps and 59.8 ps, respectively, and clearer eyes are attainable if we decrease the parasitic capacitance by half.

Clear open eyes were observed over the temperature range from -20 to 120 °C with fixed “on” and “off” current of 4.6 mA and 1.0 mA, respectively. If we set the “off” current 0.1 to 0.2 mA above the minimum threshold current I_th,min and the “on” current high enough to meet the ER > 9 dB, fixed current operation is possible over a wide temperature range at 1.25-Gb/s. The fixed current operation will lead to cost reduction of VCSEL transmitter modules with a simple driving circuit. Of course, the temperature range of fixed current operation becomes wider as d ² I_th/dT ² decreases.

 

Fig. 5. Eye diagrams measured with “off” current of 0.6 mA and 0.8 mA with fixed “on” current of 4.6 mA at -20 °C and 120 °C.

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Figure 5 shows the eye diagrams measured at -20 °C and 120 °C with fixed “on” current of 4.6 mA and varying “off” current of 0.6 and 0.8 mA. At -20 °C, a relatively clear eye diagram is observed with “off” current of 0.8 mA while a marginal eye is observed with the decreased “off” current of 0.6 mA. The overshoot increases as “off” current decreases to 0.6 mA at -20 °C because the difference between the threshold current and “off” current increases. Therefore, “1” level intensity noise of the filtered eye diagram increases. The stronger overshoot is ascribed to the larger difference between “off” current and the threshold current. Because of the intensity and timing noise, Gigabit Ethernet specification cannot be satisfied with “off” current of 0.6 mA. However, Figs. 5(c) and 5(d) show clear eye diagrams at both “off” currents at 120 °C. As previously mentioned, the ER at 20 °C cannot exceed 9 dB when “off” current is set above 1 mA or “on” current is set below 4.6 mA. Therefore, we may define “off” current range of 1.25-Gb/s fixed-current transmission as 0.8 to 1.0 mA when “on” current is 4.6 mA. The “off” current range widens as “on” current increases above 4.6 mA as long as the output power saturation by self-heating does not occur at the “on” current.

According to the results of [5], the ratio of the “on” current to threshold current, I_on/I_th, should be greater than 2.1 at 1.25-Gb/s. Experimentally obtained value is found to be greater than the theoretical result of 2.1. Reflection noise owing to the passive alignment and waveform distortion arising from turn-on/turn-off relaxation oscillation and timing jitter cause the greater I_on/I_th. Closer observation of Fig. 4 reveals that the reason for the most serious degradation of eye diagrams differs depending on temperature. At -20 °C and 0 °C, most detrimental factors are overshoot and timing jitter, and decreasing the temperature dependence of the threshold current is a solution to the problem. In the temperature range from 0 to 100 °C, “off” current is above or near the threshold, and the secondary pulsations [6] are most detrimental. At 120 °C, low “1” level output power and consequent low SNR as well as timing jitter are limiting factors, and the solution is decreasing the temperature dependence of VCSELs' threshold current and slope efficiency.

3. Conclusion

Using GaAs/AlGaAs-based 850-nm oxide VCSELs with the minimum threshold current of 0.79 mA at 50 °C and small temperature coefficient d ² I_th/dT ² of 0.114 μA/°C², we have fabricated passively aligned transmitter modules for low-power operation and low cost. The transmitter module have satisfied the Gigabit Ethernet specification in the temperature range from -20 to 120 °C with the fixed “on” and “off” current, which paves the way to low-cost and low-power-consumption transmitter modules with simple driving circuits and passive alignment techniques. The smallest ratio of “on” current to the threshold current I_on/I_th is 3.36 that is larger than the theoretical value of 2.1. The major reasons for the larger I_on/I_th ratio and degraded eye diagrams are different for different operation temperature. At temperature below 0 °C, overshoot and timing jitter are most detrimental. In the temperature range from 20 to 100 °C the secondary pulsations deteriorate the eye diagram most significantly while timing jitter and small SNR have most adverse effects at 120 °C. Therefore, further reduction of the temperature dependence of VCSEL characteristics and elimination of secondary pulsations necessitate further development efforts.