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ADN2872 Arkusz danych(PDF) 10 Page - Analog Devices |
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ADN2872 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 20 page ADN2872 Rev. 0 | Page 10 of 20 THEORY OF OPERATION Laser diodes have a current-in to light-out transfer function, as shown in Figure 23. Two key characteristics of this transfer function are the threshold current, ITH, and slope in the linear region beyond the threshold current, referred to as slope efficiency, LI. P1 PAV P0 ITH CURRENT PAV = ΔP ΔI ER = P1 P0 2 P1 + P0 LI = ΔP ΔI Figure 23. Laser Transfer Function DUAL-LOOP CONTROL Typically, laser threshold current and slope efficiency are both functions of temperature. For FP and DFB type lasers, the threshold current increases and the slope efficiency decreases with increasing temperature. In addition, these parameters vary as the laser ages. To maintain a constant optical average power and a constant optical extinction ratio over temperature and laser lifetime, it is necessary to vary the applied electrical bias current and modulation current to compensate for the laser changing LI characteristics. Single-loop compensation schemes use the average monitor photodiode (MPD) current to measure and maintain the average optical output power over temperature and laser aging. The ADN2872 is a dual-loop device, implementing both this primary average power control loop and a secondary control loop, which maintains a constant optical extinction ratio. The dual-loop control of the average power and extinction ratio implemented in the ADN2872 can be used successfully with both lasers that maintain good linearity of LI transfer characteris- tics over temperature, and with those that exhibit increasing nonlinearity of the LI characteristics over temperature. Dual Loop The ADN2872 uses a proprietary patented method to control both average power and extinction ratio. The ADN2872 is constantly sending a test signal on the modulation current signal and reading the resulting change in the MPD current as a means of detecting the slope of the laser in real time. This information is used in a servo to control the ER of the laser, which is done in a time-multiplexed manner at a low frequency, typically 80 Hz. Figure 24 shows the dual-loop control implementation on the ADN2872. ERSET MPD INPUT IPA PAVSET IEX Φ1 Φ2 Φ2 Φ2 OPTICAL COUPLING BIAS SHA MOD SHA MOD CURRENT Gm 1.2V VBGAP 100 2 BIAS CURRENT VCC HIGH SPEED SWITCH Figure 24. Dual-Loop Control of Average Power and Extinction Ratio A dual loop is made up of an average power control loop (APCL) and the extinction ratio control loop (ERCL), which are separated into two time states. During Time Φ1, the APC loop is operating, and during Time Φ2, the ER loop is operating. Average Power Control Loop The APCL compensates for changes in the laser diode (LD), ITH and LI, by varying IBIAS. APC control is performed by measuring the MPD current, IMPD. This current is bandwidth limited by the MPD. This is not a problem because the APCL must be low frequency and the APCL must respond to the average current from the MPD. The APCL compares IMPD × RPAVSET to the BGAP voltage, VBGAP. If IMPD falls, the bias current is increased until IMPD × RPAVSET equals VBGAP. Conversely, if the IMPD increases, IBIAS is decreased. Modulation Control Loop The ERCL measures the slope efficiency, LI, of the laser diode by monitoring the IMPD changes. During the ERCL, IMPD is temporarily increased by ΔIMOD. The ratio between IMPD and ΔIMOD is a fixed ratio of 50:1, but during startup, this ratio is increased to decrease settling time. During ERCL, switching in ΔIMOD causes a temporary increase in average optical power, ΔPAV. However, the APC loop is disabled during ERCL, and the increase is kept small enough so as not to disturb the optical eye. When ΔIMOD is switched into the laser circuit, an equal current, IEX, is switched into the PAVSET resis- tor. The user sets the value of IEX; this is the ERSET setpoint. If ΔIMPD is too small, the control loop knows that LI has decreased, and increases IMPD and, therefore, ΔIMOD accordingly until ΔIMPD is equal to IEX. The previous control cycle status of the IBIAS and IMOD settings are stored on the hold capacitors, PAVCAP and ERCAP. The ERCL is constantly measuring the actual LI curve; it compen- sates for the effects of temperature and for changes in the LI curve due to laser aging. Therefore, the laser can be calibrated once at 25°C so that it can then automatically control the laser over temperature. This eliminates the expensive and time consuming temperature calibration of a laser. |
Podobny numer części - ADN2872 |
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Podobny opis - ADN2872 |
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