Abstract:
A general method is given for screening laser diodes for electrostatic discharge, (ESD), damage. The laser diode may be selectively isolated from the laser driver so that a current-voltage (I-V), curve can be taken and then compared to curves taken previously on the same laser diode to ascertain the possibility of ESD damage. Presumably the initial I-V curve will be representative of the characteristics of that particular laser in the undamaged state. Such an initial curve may be supplied by the manufacturer and may be a curve specific to a particular laser diode. Comparison with a standard curve is not sufficient to determine ESD damage in the early stages of failure. Some embodiments focus on isolating the laser diode from the laser driver, storing the information locally in the transceiver, and providing some analysis resulting in flagging laser diodes showing changes that are indicative of ESD damage.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/444,486 filed Feb. 3, 2003 which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. The Field of the Invention 
   The present invention relates to screening vertical cavity surface emitting lasers (VCSELs) for quality control purposes. More specifically, the invention relates to methods and apparatus for testing VCSELs that have been installed in transceiver modules in which damage to the laser may have taken place either during the installation into the transceiver module or some time after installation into the transceiver module. 
   2. Description of the Related Art 
   In the field of data transmission, one method of efficiently transporting data is through the use of optical fibers. Digital data is propagated through a fiber optic cable using light emitting diodes or lasers. Light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electromagnetic interferences that would otherwise interfere with electrical signals. Light signals are more secure because they do not allow portions of the signal to escape from the fiber optic cable as can occur with electrical signals in wire-based systems. While there may be an evanescent field that enables one to siphon some portion of the light off the fiber by bending the fiber such that it is possible to tap fiber communications without breaking the fiber, it is in general much more difficult than for electrical communications. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on copper wire. 
   To accomplish communication in a fiber optic network, one component that is used is a transceiver module. A transceiver module has an optical input port and an optical output port. The optical input port is typically connected to a photodiode. The photodiode is connected to control circuitry within the transceiver module such that the combination of the photodiode and control circuitry can be used to monitor optical data received from the fiber optic network. 
   The optical output port typically is connected to a laser or a laser diode. The laser is also connected to the control circuitry. By modulating the signals to the light emitting diode or laser diode, digital optical data can be propagated from the transceiver module onto the fiber-optic network. The transceiver module also typically includes components for converting optical signals to electrical signals and electrical signals to optical signals so that electrical components in the network can communicate with the optical portions of the network. 
   In an 850 nanometer fiber-optic transceiver module, a particular laser known as a vertical cavity surface emitting laser (VCSEL) is often used. The emitting area of a VCSEL is defined by where the current flows through the quantum well region. Since the VCSEL is created as a uniform planar structure there is nothing initially to determine where the current will go. The two most common methods of solving this problem are: 1) disrupting the lattice structure by particle implantation (usually hydrogen, i.e., protons), which causes the current to preferentially flow through the non-disrupted region defined by the negative image of the implantation mask and 2) blocking the current by creating a dielectric oxide layer in the region surrounding the emitting cavity, commonly known as the oxide confinement technique. In recent times, the technique of oxide confinement has been used to create a variety of oxide laser diodes known as oxide defined vertical cavity surface emitting lasers more commonly known as oxide VCSELs. While the oxide lasers diodes exhibit some desirable characteristics, they also have the unfortunate drawback of being very susceptible to electrostatic discharge (ESD) damage. This is easy to predict since the lattice disruption confinement technique disrupts the layers all the way through the VCSEL structure, while the oxide confinement technique only creates a very thin dielectric layer. For any given applied voltage, the electric field strength will be proportional to the thickness of the dielectric region, so the thin oxide layer will have a much higher field across it and sustain damage at a lower voltage than the thick implanted layers. Whereas most typical electronics have an ESD resistance of least 500 V and often much higher, the oxide VCSEL is susceptible to ESD damage at voltages as low as 200 V or less. Another difficulty with ESD damage to oxide VCSELs is that such damage is often latent, meaning that it is not immediately detectable simply by observing the initial performance of the oxide VCSEL. However, ESD damage causes the oxide VCSEL to quickly degrade. 
   One method of screening oxide VCSELs to verify that they have not been damaged by an ESD is by examining the current/voltage characteristics of the oxide VCSEL. These characteristics are often shown in an I-V graph. By comparing the current/voltage characteristics of oxide VCSELs that are known to be operable and undamaged with the oxide VCSEL under test, one may be able to make a determination as to whether the oxide VCSEL under test has been damaged by ESD. Often, however, because the change in the current/voltage characteristics may be slight when ESD damage has taken place on an oxide VCSEL, the testing of the oxide VCSEL may need to be performed over time to ensure that there is no change in the current/voltage characteristics of the particular oxide VCSEL. In other words, it may not be sufficient to compare the current/voltage characteristics of the oxide VCSEL under test with known current/voltage characteristics, because oxide VCSELs have inherent variation in their current/voltage characteristics. Electrostatic discharge damage to an oxide VCSEL may appear to be simply a device variation. 
   While presently it is possible to use such testing to evaluate the condition of the oxide VCSEL prior to the oxide VCSEL being installed in another component such as a transceiver module, once the oxide VCSEL has been installed in another component testing can be much more difficult. Further exacerbating the problems with testing oxide VCSELs is the fact that installation of the oxide VCSEL into other components may actually be the cause of the ESD that damages the oxide VCSEL. It would therefore be beneficial if testing of the oxide VCSEL could be performed after the oxide VCSEL has been installed in another component or after the other component has been in service for a period of time. 
   BRIEF SUMMARY OF THE INVENTION 
   One embodiment of the invention includes a transceiver module incorporating a laser diode. The transceiver module further includes selectable switches for disconnecting the laser diode from other circuitry in the transceiver. External test pins are connected to the laser diode such that when the laser diode is disconnected from other circuitry in the transceiver module, testing can be done on the laser diode to detect electrostatic discharge damage. 
   Another embodiment of the invention includes a transceiver module that includes a laser diode coupled through a laser driver to a microprocessor. Memory is also connected to the microprocessor for storing reference operating characteristics. The microprocessor is able to collect periodic operating characteristics of the laser diode. These periodic operating characteristics can be compared to the reference operating characteristics to detect damage to the laser diode. 
   Another embodiment of the invention includes a laser diode coupled through a laser driver to a microprocessor. Memory is connected to the microprocessor. The microprocessor is able to collect periodic operating characteristics at different times and store the periodic operating characteristics in the memory. Subsequently, operating characteristics taken at different times can be compared to detect damage to the laser diode. 
   Other embodiments of the invention include methods for manufacturing transceivers. One method includes connecting a laser diode to a laser driver. The laser driver is connected to a microprocessor. Memory is connected to the microprocessor. Reference operating characteristics for the laser diode are stored in the memory. The microprocessor is configured, such as through programming, to collect periodic operating characteristics and to compare those operating characteristics to the reference operating characteristics stored in the memory. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In order that the manner in which advantages and features of the invention are obtained, a description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
       FIG. 1  illustrates the voltage/current characteristics of a typical oxide VCSEL shown using an I-V graph; 
       FIG. 2  illustrates a transceiver module that has external contact points for screening an oxide VCSEL installed in a transceiver module; and 
       FIG. 3  illustrates a transceiver module wherein the control circuitry within the transceiver module is configured to perform a self-screening operation on the oxide VCSEL. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention provide apparatus for testing a laser diode including oxide VCSELs, which have been installed in transceivers, over time to determine if damage has occurred to the laser diode. One embodiment of the invention allows for the laser diode to be separated from other circuitry such that test equipment can be attached to the laser diode. An operating characteristic such as current/voltage characteristics or current versus optical power characteristics of the laser diode can be measured and recorded for comparison with subsequent measurements or a reference measurement. In another embodiment of the invention, a laser driver in the transceiver includes circuitry for measuring current/voltage, current versus power and/or other characteristics of the laser diode. The measured characteristics may be stored in memory on the transceiver for comparison, with subsequent measurements or a reference measurement. The comparison may be accomplished by using a microprocessor. While the following discussion references oxide VCSELs, the invention is not limited only to oxide VCSELs. Other types of laser diodes may also be used. 
   An exemplary I-V graph is shown in  FIG. 1  and generally designated as  100 . I-V graph  100  is a voltage plot along the horizontal axis, with the corresponding current plotted on the vertical axis. The oxide VCSEL is screened by monitoring how the current through the oxide VCSEL reacts as a bias voltage across the oxide VCSEL is increased. For example, in a forward bias test, the voltage across the oxide VCSEL is gradually increased. The current through the oxide VCSEL is negligible until the voltage across the laser diode reaches a point known as the cut-in voltage  102 . This point is generally difficult to determine accurately, but the log-linear slope after the forward the forward bias knee  104  is easy to measure accurately. Extrapolating this slope back to the horizontal voltage axis gives a value usually referred to as the threshold voltage  110 . Threshold voltage is a more reliable and more easily determined number for comparison than cut-in voltage  102 . The current through the laser diode then increases exponentially in a manner as illustrated by a forward bias knee  104 . By monitoring such characteristics as the cut-in voltage  102  and the forward threshold voltage  110  and comparing these characteristics either with oxide VCSEL reference characteristics that are known to be good or with the reference characteristics of the oxide VCSEL under test when it was known to be good, the condition of the oxide VCSEL can be ascertained. Similar testing can be done by applying a reverse bias voltage to the oxide VCSEL. The reverse bias testing is similar to the forward bias testing except that the breakdown voltage  106 , reverse bias knee  108  and/or a reverse threshold voltage are monitored to determine the condition of the oxide VCSEL. 
     FIG. 2  shows an optical fiber transceiver module  200  that has an optical input port  202  and an optical output port  204 . The optical input port  202  provides an interface point to a fiber optic network for a photodiode  206  that is installed in the transceiver module  200 . The photodiode  206  may be connected to control circuitry, such as a microprocessor  208 . The photodiode  206  receives optical signals representing data existing on a fiber optic network. This data can then be fed to the microprocessor  208 , which is connected to a communications connector  210 . The communications connector  210  is configured to connect to a communications bus on an electronic component that communicates with the optical components of the fiber optic network. In one embodiment of the invention the communications bus is an I 2 C bus. 
   The optical output port  204  is connected to a laser diode  212 . The laser diode  212  is connected to biasing circuitry  214  that may be modulated for modulating the output of the laser diode  212 . The modulated output of the laser diode  212  can be propagated through the optical output port  204  and onto the fiber optic network. The biasing circuitry  214  is connected to and controlled by a laser driver  216 . The laser driver  216  is configured so that it is able to source the current needed to appropriately drive the laser diode  212 . The laser driver  216  is connected to the microprocessor  208 . The microprocessor  208  sends small signal values to the laser driver  216  to modulate the laser diode  212 . The microprocessor  208  is connected to the communications connector  210  such that an electronic component connected to the transceiver module  200  through the communications connector  210  may propagate digital data on to the fiber optic network. 
   In one embodiment of the invention, test connections are made to the anode and cathode of the laser diode  212 . The test connections are routed to external test pins  218  that may be, for example, pogo style pins. The test pins  218  are mounted such that they do not contact the metallic body of the transceiver module  200 . The transceiver module  200  also has selectable switches  220  placed between the laser driver  216  and the laser diode  212 , or within the laser driver  216  itself, for removing the laser diode  212  from other circuitry within the transceiver module  200 . When the selectable switches  220  are in the open position, as shown in  FIG. 2 , the test pins  218  can be connected to external testing equipment to measure the current/voltage characteristics of the laser diode  212 . In this way, testing of the laser diode  212  can be effectuated after the laser diode  212  has been installed in the transceiver module  200 . While, in this example, the screening of the laser diode  212  makes use of the current/voltage characteristics, other tests may also be performed to measure other characteristics of the laser diode  212  that indicate ESD damage. The present embodiment provides a way to isolate the laser diode  212  from the other components in the transceiver module  200  such that appropriate screening can be performed. 
   Referring now to  FIG. 3 , an alternate embodiment of the present invention is shown.  FIG. 3  generally shows a transceiver module  300 . As in  FIG. 2 , the transceiver module  300  has an optical input port  302  and an optical output port  304 . A photodiode  306  is installed in the transceiver module  300  in a manner similar to that described for the photodiode  206  of  FIG. 2 . A laser diode  312  is installed and configured in the transceiver module  300  in a fashion similar to that of the laser diode  212  of  FIG. 2 . The laser driver  316  is connected to the laser diode  312  such that it is able to bias the laser diode  312  through two alternate paths. The first path is through the biasing circuitry  314 , which path is used in the ordinary course of propagating digital data onto the fiber optic network. In a laser diode screening mode, the second path through selectable switches  320 , which in one embodiment of the invention may be within the laser driver  316 , can be used to periodically monitor current/voltage characteristics of the laser diode  312  by applying a varying voltage across the laser diode  312  and measuring the current through the laser diode  312 . Periodic as used herein does not require the monitoring to be done at any specified interval, but rather that the monitoring is done repeatedly at different times. The monitoring may be done, for example, when a transceiver is powered up or at some other convenient time. 
   In the present embodiment, a microprocessor  308  is configured to collect the periodic current/voltage characteristics of the laser diode  312 . This collected information can be compared to reference calibration characteristics stored in computer memory such as an electronically erasable programmable read only memory (EEPROM)  322  that is connected to the microprocessor  308 . The reference characteristics on the EEPROM  322  may be data collected about the particular laser diode  312  before it was installed into the transceiver module  300  and known to be in an undamaged condition. For example prior to installation, in one embodiment of the invention, reference I-V graphs may be generated for the laser diode  312  at three different temperatures. The results of these measurements may be stored as quadratic spline coefficients. Using quadratic spline coefficients minimizes the amount of data that needs to be stored on the EEPROM  322 . Further, using quadratic spline coefficients allows the reference I-V graph to be easily interpolated. For example, I-V graphs at other temperatures for the laser diode  312  may be interpolated using the generated quadratic spline coefficients. Interpolated I-V graphs for any temperature between two extremes of the previously generated quadratic spline coefficients can then be generated. Additionally, testing at different temperatures allows ESD damage that behaves differently at different temperatures to be detected. Notably, ESD damage to an oxide layer may leave areas where tunneling causes significant conduction. Tunneling is temperature sensitive, such that comparing results at different temperatures may help to reveal ESD damage that might otherwise remain latent over a period of time. 
   The microprocessor  308  can be configured, such as by executing a program, to periodically collect and compare the current/voltage characteristics or to collect and compare the characteristics on the occurrence of some event, such as start-up. Alternately, an electronic component connected to the transceiver module  300  through a communications connector  310  may issue a command to the microprocessor  380  to collect and compare the current/voltage characteristics of the laser diode  312 . The microprocessor  308  may be further configured to compare successive characteristics taken at different times to evaluate the current/voltage characteristics of the laser diode  212  over time. 
   In one embodiment of the invention, the periodic current/voltage characteristics of the laser diode  212  may be recorded as cubic splines. The slope of the forward biased laser region (i.e. the region that includes the sloped line after the forward bias knee  104  in  FIG. 1 ) and the threshold voltage (i.e. threshold voltage  110  in  FIG. 1 ) can be generated and recorded from the cubic splines. These two pieces of information may be stored and compared over time to detect ESD damage by noting abnormal changes in threshold, efficiency, slope etc. 
   Additionally, while measuring the current characteristics, optical power from the laser diode could be measured from both a fiber coupled to the optical output port  304  and from an internal monitor diode. This would help to generate near perfect calibration of the monitor diode over temperature and power. For example, in one embodiment, when there is a suspected problem with a transceiver module, an end user may measure and compare optical power from the laser diode versus current as compared with previous measurements, such as a reference measurement taken prior to the installation of the laser diode in a transceiver. Periodic measurements may also be compared with previous periodic measurements to detect deterioration of a laser diode. Part of the reason for calibrating the laser monitor diode is to facilitate in-situ data collection. By using the monitor diode, the laser in a given TR can be tested without removing it from the system and without removing any of the optical or electronic connections. 
   If the microprocessor  308  detects patterns that tend to show damage to the laser diode  312 , the microprocessor can set a general Tx fault flag. When a general Tx fault flag is set, the electronic component connected to the transceiver module  300  is notified that a problem exists with the transceiver module  300 . In one embodiment, this general Tx fault flag is a data object stored in a memory map of the microprocessor  308  corresponding to an available memory cell according to the I 2 C communications protocol. When the electronic component becomes aware of the problem existing on the transceiver module  300 , a polling routine may be performed to isolate the specific problems existing with the laser diode  312 . 
   Alternately, the transceiver module  300  may notify the electronic component connected to the transceiver module  300  of a specific problem with the laser diode  312 . In this manner, the laser diode  312  can be screened after the laser diode  312  has been installed in the transceiver module  300  and can continuously be screened throughout the service life of the transceiver module  300 . 
   In an alternative embodiment of the invention, the laser driver  316  or an analog to digital converter on the microprocessor  308  can be used to determine the voltage applied to the laser diode  312  and the resulting current flow for determining the current/voltage characteristics of the laser diode  312 . After a one time calibration of the laser driver  316 , the current/voltage characteristics can be measured by sweeping each section of an I-V curve with the laser driver  316  while using the microprocessor  308  to control the DC bias on the laser. In this embodiment, the switches  320  are not used. Advantageously, this embodiment helps to eliminate various bit error rate challenges that may be caused by additional circuit board traces, electronic hardware or switching components. 
   While the present invention illustrates using the current/voltage characteristics and current versus power characteristics of the laser diode  312  to detect ESD damage, other operating characteristics may be tested as well using the apparatus described herein. The present invention may also be useful for detecting damage to other types of lasers including but not limited to Fabry-Perot lasers and distributed feed back (DFB) lasers. The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive.