Laser diode communication module production test system detailed explanation

　　With the rapid popularization of the Internet, the widespread application of Giga-level bandwidth network communications, and the continuous development of related communication products such as ATM/Sonet and general telephone manufacturing, broadband and large-capacity access systems using WDM (Wavelength Division Multiplexed) technology are gradually becoming the mainstream development trend in the industry. Using this access system can greatly increase the transmission bandwidth of existing optical fiber communication lines while avoiding the repeated installation of new communication lines.

　　The application of WDM technology makes it possible to transmit optical signals of different wavelengths through one optical fiber. Since the system requires small size and low power consumption, laser diodes have become an indispensable core component of the system. In the WDM system, the optical signal is amplified by erbium doped fiber amplifiers (EDFA) at a certain distance. Some companies, such as Lucent Technologies, have further developed this technology into a Dense and Ultra-Dense WDM system with a terabit capacity.

　　Essentially, a laser diode (LD) is a semiconductor light-emitting device that is excited by a forward current. Its wavelength ranges from 1550nm (infrared region) to 750nm (green region), and its output power usually ranges from a few milliwatts to several watts. Its operating mode can be pulsed or continuous. Laser diodes are extremely sensitive to temperature changes - a temperature change of a few degrees Celsius may cause "mode hopping" or a step in the output wavelength of the light.

　　At present, there are two types of laser diodes widely used in optical communication systems: FP (Fabry-Perot) and DFB (Distributed Feedback). The difference between the two is mainly reflected in the different output light characteristics. FP lasers can produce light containing several discrete wavelengths, while DFB lasers emit light with a rated wavelength. Usually, there is a reflection grating in the DFB laser to eliminate other light waves except the rated wavelength.

　　Since WDM technology requires the simultaneous transmission of optical signals with multiple different wavelengths, DFB lasers are used in all current WDM systems. FP lasers are mostly used in systems where one optical fiber channel corresponds to one transceiver, such as Local Area NetWords (LANs), Fiber To The Curb (FTTC) and Fiber To The Home (FTTH).

　　Laser diodes are usually packaged together with other components in a module. Such a module usually includes a laser diode (LD), a backlight diode (BD) to monitor the output optical power of the LD, a temperature controller (TEC) to maintain the operating temperature at 25, and a thermistor to monitor the module temperature.

Test Introduction:

　　As mentioned above, with the development of broadband access technology, the demand for laser diodes is growing. Therefore, for today's laser diode manufacturers, the following question is raised: How to ensure the high cost-effectiveness and test accuracy of product testing equipment when the output of laser diodes and the complexity of the products themselves are increasing. In fact, since the added value of laser diode modules is a process that continues to increase with the production and assembly process, for example, the cost of repairing a complete module damaged by the failure of the back facet photo-diode will be much greater than the cost of conducting a complete electrical test on the diode before assembly. Therefore, in order to reduce the cost of testing, a high-speed flexible testing solution is undoubtedly the best choice.

　　A typical DFB laser diode module test process usually needs to complete the following tests:

　　Laser diode forward voltage

　　●Kink test/Slope efficiency test

　　●Threshold current

　　●Back facet current

　　Optical output power

　　●Back facet voltage drop

　　Back facet dark current

　　The first five parameters are the most common and all results can be obtained in one test procedure called a LIV scan. This fast and relatively low-cost DC test can identify failed components early in the test program, allowing more expensive non-DC test equipment to be used more effectively later in the test program.

　　Forward Voltage Test

　　The forward voltage test is used to check the forward characteristics of the laser diode (LD). During the test, it is usually required to scan a forward current (IF) for the laser diode being tested, and test its forward voltage drop at the same time. Some high-power components require a current scan of 2 to 3A (usually in steps of 1mA), while most components require a scan current of no more than 1A. The scan time for each step is usually required to be controlled within a few milliseconds. The typical voltage test range is 0 to 10V (with a resolution of microvolts).

　　Threshold current test

　　The so-called threshold current refers to the forward excitation current value when the laser diode starts to emit light. This current value can be obtained by calculating the maximum value of the second-order differential of the output light intensity. Figure 3 shows a schematic diagram of the above definition. The top curve is the light output characteristic when the forward current is scanned for the laser diode. The middle curve is its first-order differential curve. The bottom is its second-order differential curve, in which the peak point gives the position of the threshold current.

　　Light intensity test

　　The light intensity test is used to check the optical output power of the laser diode. This power value usually increases with the increase of the excitation current and is generally expressed in mW or W. There are usually two test principles: AC and DC. Tests based on the AC principle usually require an optical power meter. Tests based on the DC principle usually use the following method: place a reverse-biased photodiode at the output end of the laser diode under test, and then use a picoammeter or electrometer to test the current generated on the photodiode. Finally, the actual optical power value is calculated through the pre-programmed system software. In this process, the typical value of the induced current on the photodiode is usually 0 to 3mA, requiring a minimum resolution of 100nA. In the actual test process, the test method based on the DC principle is faster than the test method based on the AC principle. Back facet monitor diode test

　　This test is used to detect the response of the backlight diode (reverse bias) when the output optical power of the laser diode increases. The typical measurement range of the induced current is 0-100mA, with a resolution of 100nA. The test equipment usually uses a picoammeter or electrometer. Kink Test/Slope Efficiency

　　This test is used to check the linearity of the relationship curve between the forward excitation current (IF) of the laser diode under test and the output optical power (L) of the laser diode. Theoretically, when the laser diode operates within the rated range, L and IF should have a strictly linear relationship, so that its first-order differential should be an approximately horizontal straight line. If there is an obvious inflection point (Kink) on the first-order differential curve, or the curve is not smooth enough, then we believe that the laser diode is defective. In other words, when the laser diode operates at the excitation current point where the inflection point appears, its output optical power must not be linearly proportional to the excitation current value. At the same time, the maximum value of the second-order differential of the L vs. IF curve is the threshold current value of the laser diode under test. [page]

Testing requirements:

　　●Throughput: Throughput is affected by the number of LIV scanning points, and a complete scanning process usually takes about 10 seconds. Therefore, if the entire scanning time can be reduced by a few seconds, it will undoubtedly mean huge benefits for laser diode manufacturers. For example, when the famous Lucent Technology uses the test equipment of Li Instrument Company to perform a 100-point LIV scanning test on laser diodes, it takes 10 to 15 seconds to test VF, IF, IBD and L.

　 ● Resolution/Sensitivity --- The forward voltage test at the beginning of the scan and the backlight diode dark current test are both low level tests. The minimum resolution requirements for current and voltage tests are at nA and mV levels respectively. The test solutions provided by Keithley Instruments can cover this indicator.

　 ● Dynamic Range of Operations/Flexi bility --- In actual testing, since the structure of the LD module itself is bound to vary with the user's requirements, the corresponding test equipment must have a wide dynamic range and great flexibility to adapt to products of different specifications and different packaging forms. Keithley Instruments' 7000 series programmable switch technology meets this requirement to the greatest extent.

　 ●High Current Capability --- As the scale of the network continues to expand, the power of the LD module is also increasing, so that the distance between the amplifiers in the optical communication line can be extended. This relatively new, high-power device requires the test equipment to have an input and output capability of 3A. Keithley Instruments' 2420 digital source meter is a high-current model in its 2400 series products, with an input and output capability of 3A, which allows users to achieve high-current testing functions while keeping the original test platform basically unchanged, thereby greatly reducing the user's testing costs.

　　Temperature Stability: The intensity and wavelength of the output optical signal of the LD are extremely sensitive to changes in ambient temperature. Therefore, the stability of the actual temperature relative to the set temperature point has a decisive influence on the quality of the test data. Keithley Instruments' latest product, the 2510 TEC temperature controller, is designed specifically for applications in this field. It has a resolution of 0.001°C and a stability of ±0.005°C, representing the leading level of this indicator in the current market.

　　Low Test System Noise - The above-mentioned kink test has very strict requirements on system noise. If the noise of the test system is too high, the user will have to smooth its first-order reciprocal curve before further analysis. Due to the low noise characteristics of Keithley Instruments products, users do not need to do smoothing when using the test system provided by the company, thereby improving efficiency.

　　Reliability and technical services (Reliability/Local Support) - Any failure of the test equipment may cause the entire production line to stop operating. Therefore, for users, it is undoubtedly critical to choose a supplier with a good reputation for product reliability. At the same time, it is very important whether the supplier can provide timely, fast and complete pre-sales and after-sales technical services. The high quality of Keithley Instruments products has a very high reputation internationally, and the strong technical support capabilities of Keithley Instruments Beijing Office enable domestic users to have confidence in local technical services.

Typical test system configuration:

　　The system consists of the following equipment: one 2400 source meter, two 6517A electrometers, and a computer equipped with a GPIB interface card. (If the LD module to be tested requires an excitation current greater than 1A but less than 3A, the user can replace the 2400 with a 2420 to form a test system.) Among them, the 2400 (or 2420) is used to scan an excitation current for the LD and synchronize with the 6517A through a trigger device (Trigger Link Cable and Trigger Link Adapter box). The computer programs the instrument through the GPIB bus and analyzes and calculates the collected data.

　　Each device in the system has a built-in data storage device. The system can first store the test results in the built-in storage device. With the above-mentioned trigger device (Trigger Link), the system can perform LIV scanning tests independently from the computer and GPIB bus, so that the system's test speed is kept at the level of hardware triggering, rather than the relatively slow software triggering. When the entire test is completed, the data in the storage device can be read into the computer through the GPIB bus for subsequent analysis and processing.

System equipment list:

basic configuration:

　　●2400/2420 digital source meter; (provides forward bias for laser diode)

　　●6517A electrometer; (measures output optical power)

　　●Model 6517A electrometer; (measures BFMD) ●3 GPIB cables;

　　●One GPIB interface card;

　　●8502 trigger adapter;

　　●Two 8503 DIN-to-BNC trigger cables;

　　●8501-1 trigger line diagram Six laser diodes LIV scanning test extended system configuration diagram Extended configuration:

　　●2400 source meter used to bias the Modulator;

　　A 2000-type digital multimeter for measuring voltage or resistance;

　　●7002 switch controller;

　　●7053 high current switch card;

　　●7012 4X10 matrix switch card;

　　●2361 trigger controller;

　　●2510 TEC Temperature Controller

　　Of course, a desktop computer or industrial computer with equivalent configuration and a set of testing software are also essential.

　　Note: For some photodiodes, users can use the 2400 instead of the 6517A for current testing.

　　Testing process:

　　1. Place the LD module to be tested in the test fixture, and then initialize the system through the computer;

　　2. The computer configures the bias applied to the LD laser (Laser), LD backlight tube (BFMD) and external photodiode (external PD);

　　3. Turn on the output of the instrument and put the 6517A into standby mode;

　　4.2400 trigger, scanning begins;

　　5. When the 2400 scans the laser forward current from 0 to 1A in steps of 10mA, the 2400 will automatically send a trigger signal to the 6517A through the hardware trigger device inside the system when the scanning current of each step is stable. Then the 2400 stores the excitation current value and the measured forward voltage value of the step into the memory;

　　6.6517A receives the trigger from 2400 and measures the current flowing through BFMD and PD. After completion, it automatically gives 2400 a trigger signal and stores the measurement results in the memory.

　　7. After receiving the trigger signal from 6517A, 2400 will automatically increase the scan current value to the next step;

　　8. The same triggering process is repeated until the entire scanning process is completed;

　　9. When the entire scan is completed, 2400 sends a test end trigger signal (EOT) to the main control computer. After receiving the signal, the computer shuts off the output of the instrument, reads the measurement results from the instrument's memory, and then performs subsequent processing.