Diode laser communications module production test system
Overview: With the rapid popularization of the Internet, Giga-level network communication bandwidth, as well as the wider use of ATM / Sonet, general telephone communications and other related products manufacturing industry continues to develop, the use of WDM (Wavelength Division Multiplexed) technology, high-capacity broadband access system The industry is becoming the mainstream in the near future. Such access can use the system to avoid duplication in the installation of new lines of communication on the basis of the increase of the existing fiber-optic communications bandwidth transmission lines.

WDM technology will allow different wavelengths of light signals through optical fiber all the way to carry out transmission to become a reality. As the system requirements are small, low-power, the laser diode (Laser Diodes) has become the indispensable core of the system components. In the WDM system, at a certain distance, optical signals are erbium-doped fiber amplifier (EDFA: Erbium Doped Fiber Amplifiers) to enlarge. Some companies, such as Lucent Technologies has been the further development of this technology has become a Terabit-capacity Dense and Ultra-Dense WDM systems.

In essence, the laser diode (LD) is a positive, if the current incentives under the condition of semiconductor devices. From the top of its wavelength 1550nm (infrared) to the minimum 750nm (green area), usually the output power from a few milliwatts to several watts range. Their work can be in the form of pulse (pulse) can be sustained (continuous wave). Laser diodes are extremely sensitive to changes in temperature --- the temperature a few degrees Celsius could lead to a "transition mode" (modehopping) or the output of optical wavelength step.

At present, optical communication systems in large-scale use of the two laser diodes: FP (Fabry-Perot) and the DFB (Distributed Feedback). The main difference between the two in the output characteristics of the different light. FP lasers can produce contains a number of discrete wavelengths of light, while the DFB lasers were issued with a nominal wavelength of light. Laser DFB usually a reflection of sorting device (reflection gratings) in addition to the elimination of other than nominal wavelength light waves.

Due to technical requirements of WDM with a variety of different wavelengths of light signal transmission at the same time, so all the WDM system using DFB laser. The FP lasers are used for most of that corresponds to a path of a fiber-optic transceivers (transceiver) systems, such as Local Area Networds (LANs), Fiber To The Curb (FTTC) and Fiber To The Home (FTTH).

Laser diodes are usually in the other components together in a package inside the module, this module normally includes a laser diode (LD), an LED backlight (BD), used to control the LD optical power output, a temperature controller (TEC) , Will be used to maintain the temperature at 25, as well as a module used to monitor the temperature of the thermistor (Thermistor). Using Keithley

Test:

As mentioned above, along with broadband access technology, laser diode demand is growing. Therefore, today's laser diode manufacturers, made on the following issues: the laser diode production and the product itself, the ever-increasing complexity of the case, how to ensure that the product's cost-effective test equipment and test accuracy. In fact, due to the laser diode module with the production of value-added products as well as the assembly process is a growing process, such as on a diode due to the backlight (Back facet photo-diode) failure and damage to the integrity of the module for maintenance costs will be far more In the assembly before the full power of the diode tests. Therefore, in order to reduce the cost of the test, a flexible high-speed (High-speed flexible) test solution is the best choice.
A typical DFB laser diode module testing process normally required to complete the project the following test:

¡ñ laser diode forward voltage (Laser diode forward voltage)

¡ñ turning point test (Kink test) / linearity test (Slope efficiency)

¡ñ current threshold (Threshold current)

¡ñ backlight current (Back facet current)

¡ñ Power (Optical output power)

¡ñ LED backlight voltage drop (Back facet voltage drop)

¡ñ covertly backlight LED current (Back facet dark current)

Top 5 parameters of the test is the most common, can be called a test of LIV scanner in the process of full results. This rapid and relatively low cost of the DC test earlier in the process of testing to identify components of the failure, thus the high price of those non-DC test equipment to the next test program more effective to play its role.

Forward voltage test (Forward Voltage Test)

Forward voltage examinations used to test laser diode (LD) of the positive features. Test, usually measured to the requirements of being a scanning laser diode current (IF), at the same time test of its forward voltage drop. Some components require high-power scanning 2 ~ 3A (usually as a step to 1mA) current, and most of the components required for the current scan no more than 1A. Every step of the scan is usually required at the time a few milliseconds or so. Test voltage range for the typical 0 ~ 10V (with a resolution of micro-level V).

Current threshold test

The so-called threshold current refers to the beginning of light-emitting laser diode when the current value of positive incentives. The current value can be calculated through the light output of the second-order differential to be the maximum. Figure III gives the definition of the above diagram. The top of the curve is a laser diode scan forward to the current characteristics of the light output. That is one of the curves in the middle-order differential curve. The bottom is the second-order differential curve, the peak point gives the current threshold.

Light test

Light test used to test the laser diode optical output power of small, usually with the value of the power of the current incentive increases, with the general mW or W said. The principle test usually both AC and DC. Based on the principle of the exchange is usually used to test optical power meter. And based on the principle of the direct current test is usually used as follows: a reverse bias of the photodiode (reverse-biased photodiode) placed in the laser diode under test issued by the light output, and then use a little table-an (Picoammeter) or electrometer (Electrometer) test on the photodiode arising from the current size, good for last in advance through the system software to calculate the actual value of optical power. In the process, the photodiode current induced on the typical value is usually 0 ~ 3mA, the minimum requirements of resolution 100nA. In the actual test, based on the principle of the direct current test method than the principle of exchange based on the way to test speed. LED backlight (Back facet monitor diode) test

The test used to detect when the laser diode optical power output increases, the LED backlight (reverse bias) in response to the situation. Its typical induced current measurement range is 0 ~ 100mA, resolution 100nA. Test equipment commonly used a little table-an (Picoa mmeter) or electrometer (Electrometers). Turning point test (Kink Test) / linearity test (Slope Efficiency)

The tests measured as a test of the laser diode current positive incentives (IF) and the optical power output laser diode (L) the relationship between the curves of linear mixed. In theory, when the laser diode in the work of the rated range, L and the IF should be strictly linear relationship, this is the case, one of the first order differential should be a similar level in a straight line. If the first-order differential curve there is a clear inflection point (Kink), or the lack of smooth curves, we believe that the defective laser diode. In other words, when the laser diode turning point occurred in the current incentive points, its optical power output and current value of the incentive will not be a linear relationship between the ratio. At the same time, L vs. IF second-order differential curve of the maximum value is measured in the laser diode current threshold value.

Testing requirements:

¡ñ throughput (Throughput) date - by the throughput of LIV scanner points, and a complete scan of the time-consuming process, usually about 10 seconds, so if we can reduce the time to scan the entire matter of seconds, the laser diode The manufacturer is no doubt mean a huge income. For example, Lucent Technologies, a well-known in the laser diode test equipment company profits so that when the 100-point test equipment to do the scanning LIV test, test VF, IF, IBD and L, time-consuming 10 to 15 seconds.

¡ñ resolutions / sensitivity (Reso lution / Sensitivity) --- scan at the beginning of a positive test voltage LED backlighting and the dark current (Dark current) tests are small-signal test (Low level measu rement). The current and voltage test of the minimum requirements of resolution nA in class and class mV. Keithley Instruments provided by the test program can be covered by the indicators.

¡ñ dynamic range / flexibility (Dyn amic Range of Operations / Flexi bility) --- in the actual testing, because of the LD module itself as the structure is bound to a user's request, therefore request the appropriate test equipment must be wide Dynamic range and flexibility to meet the specifications of different products and different forms of packaging. Keithley Instruments, Inc. owned by the 7000 series maximum program-controlled switching technology to meet the requirements of the.

¡ñ current input and output capability (High Current Capability) --- With the continuous expansion of the size of the network, LD module's power is also on the rise, so that the optical communication equipment amplification in line 22 to lengthen the distance between. This relatively new, high-power devices require testing equipment 3A with the input and output capacity. Keithley Instruments, the 2420 digital form is the source of its 2400 series of high-current models, 3A with the input and output capabilities, which allows users to be able to maintain the original test platform will remain basically unchanged under the prerequisite of the realization of high-current test Function, thereby greatly reducing the cost of user testing.