Pulsed Operation of VCSELs for High Peak Powers

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1 Application Note AN-2138 Pulsed Operation of VCSELs for High Peak Powers INTRODUCTION There are a number of reasons one might drive multimode VCSELs in a pulsed mode (pulsed in this document will mean no more than 10% duty cycle). Among these are extension of battery life, noise immunity through synchronous detection, and the need for high peak powers. Advanced Optical Components (AOC) VCSELs have been designed for relatively low power applications. Despite this fact, impressive peak powers can be achieved when the driving current pulse is short enough and repeats infrequently. The desirable beam characteristics of VCSELs can be exploited in applications that meet those two conditions. The constraints on pulse width and duty cycle arise from both reliability and eye-safety considerations, as will be discussed below. While AOC offers this general application assistance, it is up to the customer to assure that any particular design meets performance and safety goals. Pulsing the current through a VCSEL reduces the junction temperature of the device, and can thereby increase the amount of peak power available in an application. One characteristic of VCSELs somewhat limits pulsed operation, however. In LEDs and edge emitting lasers, where the series resistance is quite low, the voltage across the device is almost completely dropped at the light-emitting junction, but in a VCSEL a substantial drop occurs in the distributed series resistance. This inherent series resistance comes from the distributed Bragg reflector that must be used to form the laser. A typical multimode VCSEL has series resistance of about 25 ohms, and some may be as high as 40 ohms. (Single-mode VCSELs have even higher resistance.) If the pulsed application requires a 200-mA drive current, the voltage drop across the VCSEL could be = 8 V (actually about 9.5 V, because the diode drop must be added to the series resistance). Even in a more typical situation, a greater than 5 V compliance may be required. Fundamentally, the increased peak power available in short pulses is due to reduced junction heating. There is simply not enough time for the junction to reach thermal equilibrium. Within broad limits, the shorter the pulse, the higher the available power. Duty cycle controls the junction temperature at the beginning of the pulse; pulse width controls the junction temperature at the end of the pulse. In this note, we describe the effects of pulsewidth and duty cycle on power emitted from a VCSEL. We describe a simple relationship for a suggested maximum forward current as a function of both the duty cycle and pulsewidth. Under pulsed operation, the VCSEL slope efficiency remains at its low-current dc value; hence the attainable power is obtained by multiplying the VCSEL slope efficiency by the maximum forward current. Rev A Finisar Corporation AN-2138 Page 1 of 7

2 DRIVE CONDITION EFFECTS ON PERFORMANCE The pulse width effect is illustrated in Figure 1. Even with an on-time as long as 1µs, the maximum peak power has tripled from its dc value (the maximum peak power is defined as the value at "rollover," where any additional increase in current will actually decrease the emitted power). One should not design applications dependent on the power available at rollover, however, due to adverse reliability and controllability effects. A reasonable rule of thumb is to operate at no more than half of rollover power. Using this guideline, the safely-available peak power at 100 ns on-time is more than ten times the dc value, and even higher peaks would be attainable with shorter pulses or lower duty cycles. Figure 1 Figure 2 Figure 2 shows the duty cycle effects for a fixed on-time of 200ns. Using the half-rollover criterion above, it is obvious that for any duty cycle 8% or less, the safe operating power is about the same (there is a catch, however, as described in the reliability section below). Shorter pulses and lower duty cycles than shown are acceptable, and will result in even higher attainable peak powers, but operation with longer pulses and higher duty cycles is subject to reliability constraints. Rev A Finisar Corporation AN-2138 Page 2 of 7

3 THERMAL MODEL The temperature rise of a VCSEL under pulsed operation is what controls the achievable power and the reliability. While the detailed analysis of the thermal properties is quite complex, simple formulas can be used to describe the junction temperature for a given pulse current amplitude, pulse width and duty cycle. This equations can be solved iteratively and used to determine the maximum current pulse for a given junction temperature rise. The equations are given below, with typical values for both inputs and results summarized in Table 1. Details of this type of calculation can be found in Zhao, Y. G., et al., "Transient temperature response of vertical-cavity surface-emitting semiconductor lasers," IEEE Journal of Quantum Electronics, vol. 31, pp , Table 1. Parameters and values used to describe the thermal properties of a VCSEL Rev A Finisar Corporation AN-2138 Page 3 of 7

4 The heating of the VCSEL during a pulse is shown schematically in figure 3. The current pulse starts the heating process, which continues until the pulse is turned off. As depicted, for short pulses, the thermal rise of the junction can be significantly less than predicted by the DC parameters. This is the basic principle behind achieving higher peak operating powers from a VCSEL. The shorter the current pulse, the less heating, and therefore higher optical power outputs. Figure 3. Schematic of the dependence of junction temperature during a current pulse RELIABILITY CONSIDERATIONS The reliability of AOC VCSELs under dc operation is described in Oxide Isolated VCSEL Reliability Summary, and 850 nm VCSEL Products Optoelectronics Reliability Study. These application notes are applicable for typical data communication applications as well, where the duty cycle is 50% and the pulse width is significantly shorter than the thermal time constant. Pulsed operation as described in this application note is 10% or lower duty cycle, and with widely varying pulse widths, so the reliability must be computed differently. While there is relatively little reliability data available for the pulsed applications, we have done enough testing to suggest that long-lived applications are possible, due primarily to the reduction in peak junction temperature-resulting from short-pulsed operation, and to the reduction in average junction temperature-resulting from low duty cycle operation. In addition, low duty cycle operation leads to very long calendar life. Calendar life is the time equipment can operate continuously, and is equal to the calculated dc operation life, multiplied by the reciprocal of the duty cycle. That is, for a 1% duty cycle, the calendar reliability increases a hundred-fold over the dc time. Finally, acceptable lifetime is defined differently for each possible application; in some cases even a few hours of calendar lifetime is adequate. Rev A Finisar Corporation AN-2138 Page 4 of 7

5 Figure 4. Maximum peak current allowed as a function of pulsewidth and duty-cycle It would be impossible to treat every possible combination of duty cycle, frequency, and peak current for reliability. The general behavior can be described, however, for three different operating regimes, categorized by frequency. The first regime is for pulse repetition frequencies greater than 400 khz, where the on-time is much less than the thermal time constant. The second is for frequencies between 400 khz and 40 khz, where the on-time and the time constant are comparable. The third is slower than 40 khz, where the on-time is much greater than the time constant. In the first regime, the junction temperature during the pulse is nearly equal to the temperature calculated for dc operation at the average current. That is, the reliability can be estimated as that for this lower temperature, but for the peak current. An example of predicted calendar reliability in this regime is given in Figure 5. Figure 5. Reliability estimates for ambient temperature of 40 C and 100ns pulses Rev A Finisar Corporation AN-2138 Page 5 of 7

6 ADVANCED OPTICAL COMPONENTS Finisar s ADVANCED OPTICAL COMPONENTS division was formed through strategic acquisition of key optical component suppliers. The company has led the industry in high volume Vertical Cavity Surface Emitting Laser (VCSEL) and associated detector technology since VCSELs have become the primary laser source for optical data communication, and are rapidly expanding into a wide variety of sensor applications. VCSELs superior reliability, low drive current, high coupled power, narrow and circularly symmetric beam and versatile packaging options (including arrays) are enabling solutions not possible with other optical technologies. ADVANCED OPTICAL COMPONENTS is also a key supplier of Fabrey-Perot (FP) and Distributed Feedback (DFB) Lasers, and Optical Isolators (OI) for use in single mode fiber data and telecommunications networks LOCATION Allen, TX - Business unit headquarters, VCSEL wafer growth, wafer fabrication and TO package assembly. Fremont, CA Wafer growth and fabrication of 1310 to 1550nm FP and DFB lasers. Shanghai, PRC Optical passives assembly, including optical isolators and splitters. SALES AND SERVICE Finisar s ADVANCED OPTICAL COMPONENTS division serves its customers through a worldwide network of sales offices and distributors. For application assistance, current specifications, pricing or name of the nearest Authorized Distributor, contact a nearby sales office or call the number listed below. AOC CAPABILITIES ADVANCED OPTICAL COMPONENTS advanced capabilities include: 1, 2, 4, 8, and 10Gbps serial VCSEL solutions 1, 2, 4, 8, and 10Gbps serial SW DETECTOR solutions VCSEL and detector arrays 1, 2, 4, 8, and 10Gbps FP and DFB solutions at 1310 and 1550nm 1, 2, 4, 8, and 10Gbps serial LW DETECTOR solutions Optical Isolators from 1260 to 1600nm range Laser packaging in TO46, TO56, and Optical subassemblies with SC, LC, and MU interfaces for communication networks VCSELs operating at 670nm, 780nm, 980nm, and 1310nm in development Sensor packages include surface mount, various plastics, chip on board, chipscale packages, etc. Custom packaging options Rev A Finisar Corporation AN-2138 Page 6 of 7

7 Contact Information Finisar Corporation 1389 Moffett Park Drive Sunnyvale, CA USA Phone: +1 (408) Website: Rev A Finisar Corporation AN-2138 Page 7 of 7