Abstract:
Methods for testing optical components, such as laser diodes or light emitting diodes, that are manufactured for use in optical transmitters or transceivers. The testing methods are performed using a true RMS conversion circuit in a test apparatus. The testing methods are performed by receiving an optical signal with AC and DC components from the optical component that is to be tested. The optical signal is converted to an electrical signal, and the AC and DC components of the electrical signal are separated. An RMS circuit of the test apparatus converts the AC component of the electrical signal to a DC function of the RMS value of the AC component of the electrical signal. This testing method can be used to determine whether optical components are suitable for use by customers or in products.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 11/103,766, filed Apr. 12, 2005, which claims the benefit of U.S. Provisional Application Ser. No. 60/599,259, filed Aug. 4, 2004. The foregoing patent applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. The Field of the Invention  
         [0003]     The invention generally relates to fiber-optic test equipment. More specifically, the invention relates to test equipment for testing optical transmitters and receivers used in fiber-optic communications.  
         [0004]     2. Description of the Related Art  
         [0005]     Fiber-optic networking can be used to communicate in modern high-speed networks. To transmit data on a fiber-optic network, the data must be converted from an electronic signal to an optical signal. This conversion may be done, for example, by using a transmitter or transmitting optical subassembly (TOSA). The transmitters and TOSAs often include light generating devices such as a laser or light emitting diode (LED). The light generating device is modulated according to digital data to produce a modulated optical signal.  
         [0006]     When optical signals are received, those optical signals must generally be converted to an electronic signal. This is often accomplished using a receiver or a receiver optical subassembly (ROSA). Receivers and ROSAs generally include a photo sensitive device such as a photodiode connected to a transimpedance amplifier (TIA). When an optical signal impinges the photo sensitive device, a modulated current is induced in the photo sensitive device. This current can be converted by the TIA to an electronic signal usable by digital devices on a network.  
         [0007]     Manufacturers of ROSAs and TOSAs typically perform various performance testing on the ROSAs and TOSAs before they are delivered to distributors and end customers. This performance testing can be used to detect defects or to sort components into groups of different rated values.  
         [0008]     More particularly, testing directed towards the ROSA may include testing the responsivity of the ROSA to a modulated optical signal, testing the amount of current produced for a given amount of optical signal and so forth. Testing responsivity includes comparing a modulated optical signal input into the ROSA to an AC electrical signal produced by the ROSA as a result of receiving the AC optical signal.  
         [0009]     Testing may be performed on the TOSA to characterize operating characteristics of the TOSA. One test that may be performed includes plotting the amount of optical energy produced by the TOSA as a function of the amount of current used to drive the TOSA. Another test includes measuring the amount of noise produced by the TOSA.  
         [0010]     Many of these tests have conventionally been performed using expensive high-frequency test equipment. For example, some tests use a high frequency communications analyzer costing in the tens of thousands of dollars. Further, many of these test devices are general-purpose test devices. As such, these devices require excessive amounts of human interaction to perform the test result in and increase test times for each component. When each and every component manufactured is tested, this requires an inordinate amount of manpower and equipment to process testing of the components quickly.  
         [0011]     Additionally, testing is often not repeatable from part to part. This is due to the changing nature of cables and the like associated with general purpose test equipment.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     An embodiment of the invention includes a method of testing optical components. The method includes receiving an optical signal from an optical source. The optical signal including AC and DC components. The optical signal is converted to an electrical signal. The AC and DC components of the electrical signal are separated. The AC component of the electrical signal is converted to a function of the RMS value of the AC component of the electrical signal. The function of the RMS value is provided to a data acquisition system.  
         [0013]     The testing methods allow for AC and DC testing to be completed on optical components using a single inexpensive test board. This can conserve resources as only a single test fixture is used to perform both sets of tests.  
         [0014]     These and other advantages and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0015]     In order that the manner in which the above-recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above 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:  
         [0016]      FIG. 1  illustrates a block diagram overview of a test apparatus for testing optical components;  
         [0017]      FIG. 2  illustrates a schematic drawing showing the general construction of a test apparatus for optical components; and  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Referring now to  FIG. 1 , an embodiment of the invention is illustrated as a test apparatus  100  that provides for monitoring of AC and DC signals produced by a transmitter. The apparatus  100  shown in  FIG. 1  may be used to test transmitter devices. In one example, the transmitter device may be a TOSA or a transmitter which includes a laser diode or light emitting diode (LED). The apparatus shown in  FIG. 1  includes a detector device  102 . The detector device  102  may be for example a detector that includes a photodiode coupled to a transimpedance amplifier with a differential output. The detector device  102  will be discussed in more detail with reference to  FIG. 2  described herein. The detector device  102  outputs a positive output signal at a positive output signal terminal  104  and a negative output signal at a negative output terminal  106 . The positive output signal is fed into a positive input port  108  of an RMS detector board  110 . Similarly the negative output signal is input to a negative input port  112  of the RMS detector board  110 .  
         [0019]     The RMS detector board  110  includes circuitry for filtering DC components of the output signals from the AC components. The DC components of the signal are output at detector board DC output ports  114 ,  116 . The DC output signals may be received by a data acquisition system  118  for use in characterizing the properties of a transmitter.  
         [0020]     The AC portions of the signals are converted to a function of the RMS value of the AC signals which is then fed to a RMS output port  120 . The function of the RMS value of the AC signal may be received by the data acquisition system  118  and used in characterizing various characteristics of the transmitter.  
         [0021]     An alternate embodiment of the invention may be used to test receiver devices by using a transmitter fixed as a portion of the test apparatus  100 . In this example, the detector device  102  may be removable such that testing can be performed on numerous detector devices or receivers produced by a manufacturer.  
         [0022]     Referring now to  FIG. 2 , a circuit diagram illustrates various features of one embodiment.  FIG. 2  illustrates a transmitter source  250 . The transmitter source includes an AC supply  252  and a DC supply  254 . The DC supply  254  may be used to bias a LED or laser diode  256 .  
         [0023]     The AC supply  252  may be used to modulate the LED or laser diode  256  with data or for other reasons. In one embodiment, the AC supply  252  is configured to operate at between 1 KHz and 200 MHz. Other embodiments of the invention may allow for frequencies up to 2.5 GHz and beyond.  
         [0024]     When the test apparatus  200  shown in  FIG. 2  is used for testing laser diodes and LEDs, the transmitter source may be configured to couple to a transmitter such as an LED or laser diode. This configuration may include an appropriate test fixture that allows for quick removal and replacement of transmitters in the test fixture. The LED or laser diode  256  may be optically coupled to a receiver  102 . The optical coupling shown in  FIG. 2  includes a path through a patch cord  258 . This allows the optical signal produced by the LED or laser diode  256  to be transmitted to the receiver  102 .  
         [0025]     The receiver  102  includes a photodiode  260  and a transimpedance amplifier  262 . The photodiode  260  converts optical signals received from the patch cord  258  to a small electrical current through the photodiode  260 . The transimpedance amplifier  262  converts the small current through the photodiode  260  into a higher power differential electrical signal that is output as a differential signal on a positive output  264  and a negative output  268 . The differential signal includes a positive differential signal  270  and a negative differential signal  272 . The positive differential signal is fed to a first impedance matching network  274 . The negative differential signal is sent to a second impedance matching network  276 .  
         [0026]     The first and second impedance matching networks  274 ,  276  may be configured to match the line characteristics from the output of the transimpedance amplifier  262 . In one embodiment, the impedance matching networks  274 ,  276  are fabricated on a printed circuit board that includes various paths for receiving different values of components such as capacitors, resistors and inductors. Thus a printed circuit board can be customized for a particular test by stuffing the board with appropriately chosen components. Embodiments of the invention contemplate the use of several different kinds of matching networks. For example and not by way of limitation, a matching network may include specially designed printed circuit board traces that have a particular capacitance, inductance and/or resistance. The matching networks may include fixed components such that the matching networks are fixed for a particular application or use. The matching networks may include switched components such that the matching network may be used for a plurality of different applications with minimal reconfiguration. The matching networks may comprise a variable filter for even further flexibility in designing tests apparatus. In one example the matching networks may include a digital signal processor (DSP) that functions as a filter. In some embodiments the first and second impedance matching networks  274 ,  276  are designed with similar or complementary printed circuit board layouts and components. This helps to ensure that the positive differential signal  270  and the negative differential signal  272  remain in phase with respect to each other.  
         [0027]     The positive differential signal passes through the first impedance matching network  274  to a first filter  278 . The first filter  278  separates AC and DC signals from the positive differential signal  270 . The DC portions of the positive differential signal  270  are fed to a DC output  114 . The negative differential signal  272  follows a similar path through the second impedance matching network  276  to a second filter  280  where the DC portion of the negative differential signal  272  is output at a DC output  116 . The first and second filters  278 ,  280  may be in one example bias tees.  
         [0028]     The AC output from the first and second filters  278 ,  280  is fed into an amplifier  282 . The amplifier  282  in one embodiment is a high frequency amplifier with a wide bandwidth, low noise and other desirable characteristics. One example of an amplifier that may be used is the AD8129 available from Analog Devices. This particular amplifier functions at frequencies up to 250 MHz. The amplifier  282  is the differential amplifier that compares the positive AC signal and the negative AC signal and produces a difference of the two AC signals. This difference of the two AC signal is fed to a true RMS converter  284 .  
         [0029]     The true RMS converter  284  converts the difference of the AC signals to a function of the RMS value of the difference of the AC signals. In one embodiment the true RMS converter  284  maybe part number AD8361 from analog devices. This particular true RMS converter outputs a signal that is generally 7.5 times the value of the RMS value of the difference of the AC signals. The function of the RMS value of the difference of the AC signals may vary slightly from the 7.5 value depending on the configuration of the true RMS converter  284 . Alternate functions maybe readily obtained from the data sheet for this device which is available from analog devices on their website. As mentioned previously, the present embodiment shown is designed for operation between 1 KHz and 200 MHz. However, other embodiments may be designed to function up to 2.5 gigahertz and beyond. When embodiments are designed for frequencies above 200 MHz, typically the embodiments will be designed as a band pass filter for a range of frequencies so as to obtain the best results from of the true RMS converter  284 . For example, it may be desirable to bias the true RMS converter  284  such that lower frequencies are less usable when the circuit is designed for higher frequencies. When constructing circuits for use above 200 MHz, an alternate amplifier  282  may be used that has a bandwidth suitable for use above 200 MHz.  
         [0030]     In constructing the test apparatus  200  it is desirable to construct the apparatus using a printed circuit board layout for certain portions of the circuits. It may also be desirable to ensure that traces on the printed circuit board are matched for positive and negative signal paths. If the traces are not matched for positive and negative signal paths, positive and negative signals may vary slightly in their phase from each other resulting an erroneous readings from the RMS converter  284 .  
         [0031]     The apparatuses described herein should be calibrated when used as test equipment. However, using the components described herein as test apparatuses has been shown to be very accurate. Thus an automated calibration may be used where calibration equipment used in the automated calibration is calibrated so as to verify the accuracy of the automated calibration. In one embodiment, a buffer circuit connected to the output of the true RMS converter  284  may be used to compensate for a circuit that is out of calibration. The buffer circuit may be an amplifier circuit this about unity (i.e. a gain of 1), but adjustable slightly up or down to compensate for any inaccuracies.  
         [0032]     The terms high and low frequency, as described herein should be considered relative terms rather than applying to a specific standard of frequencies. Thus high-frequency as used herein is used to describe communications that use relatively high modulation rates as opposed to a specific range of frequencies as is used in some areas of the electronic and communication arts.  
         [0033]     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. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.