In this paper, the existing test algorithm is described in detail based on the basic principle of QSFP+ optical module, from the hardware equipment required for the test to the specific process of the test algorithm, and the principle and process of the dichotomy test modulated optical transmitter are analyzed and discussed in detail.
Table of Contents
Tx Optical Power and Extinction Ratio Modulation of the QSFP+ Module
Discuss the vertical cavity surface emitting laser VCSEL (Vertical Cavity Surface Emitting Laser) based on gallium arsenide semiconductor material, and its optical power shows the trend in the figure below when driven by current. The blue and yellow curves come from two adjacent VCSEL chips on the same wafer, and there are significant differences between them, as shown in Figure 1. The difference between lasers in the same batch is its inherent characteristic, which is the reason why each laser of the QSFP+ module needs to be individually modulated for optical power and extinction ratio.
Fig. 1 VCSEL current-optical power curve
When driving the laser with an alternating current of NRZ binary code, it can be expressed as Figure 2.
Fig. 2 Drive VCSE
The communication requirements of different distances and the characteristics of photodiodes with different characteristics determine the requirements for the optical power of the transmitter, which generally presents a specific range, such as 0.5mW to 1mW; Lower limit, such as 3dB.
At the same time, in order to ensure the performance of the relevant parameters of the eye diagram of the optical transmitter, the jitter and the eye diagram mask. The input current working range of the laser should be far away from the laser threshold current, that is, gradually increase the input laser current, and the laser will start to emit light when the current is greater than the threshold (at this time, the optical gain in the laser resonator is greater than the loss). Coupled with the oversaturation of VCSEL-type lasers when the current is too large, the optical power drops.
Specific to specific QSFP+ module products, the performance indicators that QSFPTEK promises to customers are that the optical power is between -6dBm and 0dBm, and the extinction ratio is greater than 3dB. In order to meet this requirement, considering that the optical power and extinction ratio are in fact the larger the better, and the optical power at the weak light time of the “0” level should be far enough away from the threshold, the typical modulation can be expressed as the following figure ( Various losses in the process of coupling light into the fiber by 3dB). The driving current oscillates from 3mA to 12mA, the output optical power oscillates from 1mW to 4mW, the average optical power is 2.5mW, and the 3dB coupling loss is 1.25mW, and the extinction ratio is 10*log(4/1) = 6dB.
In the actual production of a large number of products, because the driving current-optical power characteristic curve of the VCSEL cannot be measured in advance, and this curve is usually not a straight line, the method used is to adjust the direct current component (DC) and alternating current component (AC) of the driving current in turn. method to approach the target value, as shown in Figure 3. The DC defined by this driver chip determines the part that expresses the “1” level for the laser output drive current, and the AC determines the difference between the pull-down and the “0” level based on the height of the “1” level. For example, when DC=12mA, AC=9mA, the output current oscillates from 3mA to 12mA. In fact, the maximum DC of this driver chip is limited to 12.7mA.
Fig.3 LASER optical power tuning
Modulation step 1, DC modulation:
Set AC=0, start the binary search from DC=7mA, the goal is to make the optical power reach 4mW, the initial step size of +/-0.04mW is set to 2mA, and each measurement takes 3 seconds. After 6 times of measurement, the predetermined value range is finally reached, and the corresponding DC is 9.75mA. As shown in Figure 3.
Modulation step 2, AC modulation:
Set DC=9.75mA, start binary search from AC=3mA, the goal is to make the optical power expressing “0” level reach 1mW, the initial step size of +/-0.04mW is set to 2mA, and each measurement time takes 3 seconds. After 6 times of measurement, it finally reaches the predetermined value range, the corresponding AC is 6.5mA, and the driving current corresponding to the “0” level is 9.75-6.5 = 3.25mA. That is, the input current oscillates between 3.25mA and 9.75mA, which can make the output average optical power reach (4.03+1.04)/2 = 2.535mW, and the extinction ratio can reach 10*log(4.03/1.04) = 5.88dB.
The modulation process took a total of 36 seconds. As shown in Figure 4.
Fig.4 LASER AC tuning
QSFP+ Module Transmitter and Receiver Eye Diagram Test
The main parameters considered for the eye diagram (as shown in Figure 5) can be divided into two categories: amplitude and time. The amplitude category includes light intensity and extinction ratio; the time category focuses on jitter, rise and fall times.
Fig.5 Parameters in eye diagram
The test of the optical signal eye diagram mainly uses the time-sharing oscilloscope DCA86100 series produced by Agilent, especially for the QSFP+ module with 4 optical channels. The traditional test method is to measure the eye diagram of the 4 channels in turn. A common eye diagram test time is 10 seconds, and it takes a total of 80 seconds to test the optical and electrical eye diagrams of the entire QSFP+ module.
QSFP+ Module Receiver Sensitivity Test
The typical bit error rate requirement for the physical layer is lower than 1E-12. For a 10Gbps optical module, it means that the number of bit errors within 100 seconds should not exceed one. The measurement method is generally as shown in the figure below. Pseudo random binary sequence coding (PRBS, Pseudo Random Binary Sequences) is used to simulate random data with a known code pattern. After passing through the object to be tested, it returns to the error detector. 0 or 1) to do the detection.
Fig.5 BER test functional block diagram
The relationship between the bit error rate at the receiving end of the optical module and the received light intensity is usually as shown in the figure below. The weaker the light, the higher the bit error rate. The following figure is an example. When the optical power reaches -9dBm, the bit error rate is 1E-12, which just meets the requirements.
Fig.6 Rx sensitivity test
This paper introduces the test flow of the QSFPTEK QSFP+ module’s transmit optical power, extinction ratio, eye diagram and receive sensitivity in detail. All QSFPTEK optical modules have undergone such strict testing processes, and currently the 40G QSFP+ SR4, 40G QSFP+ LR4, ER4 and other QSFP+ modules are on sale. For details, please contact [email protected].