Abstract:
An optical test system for use in testing fiber optical transmission systems and components comprises a modular construction in a single housing. The modular construction includes a plurality of laser source channels, a MUX, additional optical conditioning devices both upstream and downstream of the MUX, and a master controller for operating the system components in synchrony. The integral housing, together with the modular construction, provides increased structural density that occupies in many instances at least seventy percent less space than is required for other optical test systems. The use of a master controller to drive the system affords expanded system functionality.

Description:
RELATED APPLICATIONS 
     This application claims benefit of priority to U.S. Provisional Application Ser. No. 60/273,147, filed Mar. 2, 2001, and U.S. Provisional Application Ser. No. 60/301,993, filed Jun. 29, 2001, each of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of The Invention 
     The invention pertains to the field of optical source equipment and related methods for use in fiber optic communications. More specifically, the optical source equipment includes an optical source bank that is used, for example, in testing optical amplifiers and wavelength division multiplexing (WDM) systems. 
     2. Statement of the Problem 
     Rapid advances in WDM or dense wavelength multiplexing (DWDM) provide cost-effective increases in the capacity of fiber-optic data transmission systems through the use of multiple wavelengths of light. DWDM is a higher-capacity version of WDM. WDM systems support the multiplexing of up to four channels or wavelengths on a single fiber. Commercially available DWDM systems support up to 40 wavelengths or channels, and this capacity is steadily increasing. Data transmission capacity is also increased by time division multiplexing (TDM) rates in which a plurality of separate data signals are transmitted on the same line by interleaving a piece of each signal one after another in time. Maximum transmission capacity is determined as a trade-off between the DWDM channel count and the maximum supported TE)M switching rate. For example, a system operating on 40 channels at OC-48 with TDM might operate at a net throughput of about 100 Gbps. Future systems operating on the OC-192 protocol at 40 channels might have a net throughput of 400 Gbps, and a future system operating on 100 channels might have a net throughput of one terabit per second. 
     While these future systems are not yet practical, continuing advances in DWDM and TDM technology are expected to expand maximum net throughput rates over the next several years. 
     The advances in net throughput rates require corresponding advances in fiber-optic system components, which drive advances in test components, test equipment and signal amplifiers. For example, erbium-doped fiber amplifiers (EDFA) are used in DWDM systems to eliminate or minimize the use of regenerative repeaters, and can be used as in-line repeating amplifiers, transmitter booster amplifiers, and receiver pre-amplifiers. EDFA devices comprise a section of glass fiber, which is doped with erbium. The erbium-doped fiber amplifies laser light transmitted through the doped section of fiber. EDFA technology has been used, by way of example, to support a mix of four 2.5 Gbps digital video streams in delivery of an 80 channel AM cable television network signal over a 100 km distance using one EDFA at the output node and one at midspan. 
     Optical test equipment for use in testing system components for fiber-optic transmissions is continually outdated in the face of rapid capacity advances. Traditional DWDM test equipment uses an eight or sixteen channel multiplexer where, for example, eight channels may be allocated to a mainframe. Attempts to provide additional sources, e.g., more than 40 laser diodes that are each linked with a corresponding channel, source modulation electronics, attenuator, polarization control, and error injection devices produce unwieldy agglomerated test systems that are connected with a patchwork of optical cables. The test systems grow to occupy large amounts of space, and test measurement errors may be induced, for example, by movements in the optical cables that interconnect the respective devices. Depreciation and use of optical test equipment may comprise a substantial percentage, e.g., twenty or thirty percent, of the total manufacturing costs of optical system components. 
     A problematic side-effect of continually increasing capacity in optical data transmission systems is that increased amounts of space are required to accommodate the associated increases in volume of test equipment. For example, such equipment is regularly supported by racks. Each rack, according to industry standards, typically has dimensions of approximately seventy-two inches in height, twenty-seven inches in depth, and nineteen inches in width, which corresponds to a rack volume of about 21.4 cubic feet. 
     To date, no manufacturer has been able to offer an optical source array in the confines of a single integrated box containing more than about seventeen laser sources within the box. Within boxes formed to meet these restrictions the number of laser sources are primarily limited by the size of the electronic and optical assemblies required, and by heat dissipation requirements. 
     Many commercial systems do not use all of the available rack space because, for example, the housing may be shallower or shorter than the corresponding depth or height of the rack. One typical system housing a total of eight laser source cards has dimensions of five and one-quarter inches in height, seventeen and three-quarters inches in width, and seventeen inches of depth, for a total volume of 0.92 cubic feet. The eight laser sources or channels in a single box of this volume produces a density of approximately 8.7 laser sources per cubic foot inside the box, and approximately 1.5 laser sources per vertical inch in the rack. These numbers represent about the maximum density that can be obtained within a single box due to the size of the electronic and optical assemblies required, and to excessive heat dissipation from electro-optical components in the box. The addition of special cooling systems other than blowers for air convection systems is practically not done because it adds significant cost and maintenance complexity to these systems. 
     SOLUTION 
     The present invention overcomes the problems that are outlined above by providing an improved optical test equipment having modular components that reside in a single housing for a dense system that occupies less rack space than prior systems and is comprised of modular components that may be selectively and programmably upgraded over time to accommodate increasing throughput and bandwidth. These advantages are facilitated by a densifted way of coupling laser source cards with channel option cards, together with use of improved theromoelectric coaling systems and laser pulse modulation systems. 
     Optical test systems produced in accordance with the concepts described below may, for example, have exceptionally high densities of laser source cards, all under the influence of a master controller and a shared interface that is located internal to the housing and operable for control of the laser source channels. The master controller is compatible with information and command exchange protocols for the transmission and receipt of information. For example, the densities may exceed eight, ten, twelve, fourteen, sixteen, eighteen, and even twenty sources per cubic foot of space inside a single box. 
     The principles described herein may be applied to greatly improve optical test system densities where, for example, specific embodiments could include eighteen sources within a single housing adapted to fit within a conventional test equipment rack to provide a volumetric density of greater than 9.5 laser source channels per cubic foot, and a vertical density of at least 2 laser source channels per vertical inch in a standard equipment rack having a width of nineteen inches. Another specific embodiment actually constructed included an array of fifty channels internal to the housing, to provide a volumetric density of greater than 9.5 laser source channels per cubic foot and at least 2.2 laser sources per vertical inch in a standard nineteen-inch-wide equipment rack. A third specific embodiment actually constructed included one hundred of the laser source channels internal to the housing to provide a volumetric density of greater than 15 laser source channels per cubic foot and a vertical density of at least 4.5 laser source channels per vertical inch in a standard nineteen-inch-wide equipment rack. These densities are unattainable in the prior art and constitute a significant advance. 
     More than forty-eight, one-hundred, or even two hundred such sources may be supported in a single equipment rack according to the principles described herein, for example, one-hundred laser source cards are housed on a standard test equipment rack in a standard box having dimensions of twenty-one inches in height, seventeen and three-quarters inches in width, and twenty-five and one-half inches in depth. 
     The optical test system, according to one aspect of the improvements described below, relates to a modular construction of the individual channels, i.e., the system housing contains a plurality of modular laser source channels. At least one of the laser source channels includes a modular laser source card having a first electrical bus and a first optical coupling. A modular channel option card contains a second electrical bus that is compatible for mating engagement with the first electrical bus and a second optical coupling compatible for mating engagement with the first optical coupling. A mounting system within the housing, such as a rail guide, sequentially deploys the laser source card and the channel option card to form a completed modular channel that may assume any geometry, but is preferably oriented in a straight line to optimize density of the channel array. There may be any number of such channels provided in the array, such as 48, 100, or 200 such channels, which have heretofore not been controllable as a single array. 
     The channel option card contains an optical conditioning device, such as a shutter, a variable optical attenuator, a polarization controller, a polarization scrambler, a power monitor, and a wavelength reference device. These devices may be placed in communication with a master controller for feedback and quality control of optical test operations. 
     Additional optical conditioning devices may optionally be provided within the same housing to provide further densification, and all devices in the housing may be driven by a single master controller to permit expanded test functionality and synchronization. For example, a modular multiplexer (MUX) may be located internal to the housing to provide a combined comb of laser wavelengths from the channel array. Additional modular optical conditioning devices or components may be placed internal to the housing in an optical pathway downstream from the MUX to provide optical conditioning on the combined comb. The functionality of the additional optical conditioning devices downstream of the MUX may be the same as or different from that of the functionality of the channel-specific optical conditioning device on the channel option card. In addition, the downstream devices may include other options, such as service channel WDM signal processing relating to the combined comb. 
     In a particularly preferred option, the channel option card includes a laser power monitor that is configured to measure laser power output from the laser source card. The laser power monitor is configured to provide laser power measurement information to the controller, where the laser power measurement information represents the total laser power output from the laser source card including a laser source and associated electronics and optics. The controller is configured to adjust electrical power input to the laser source card and control laser power output based upon the power measurement information. This technique differs from prior systems that adjust laser power input based upon gain at the laser chip alone, and ignore the effects of other electronics and optics on the laser source card. 
     A further aspect of the optical test system, according to preferred instrumentalities of the invention, includes the provision of optical connectors configured to provide laser source input to the plurality of channels when the optical connectors are coupled with an external laser source. A plurality of such systems can be coupled to multiply the total number of channels that are combined in a single MUX; this permits, for example, two of such systems to be coupled in a manner that doubles the number of channels passing through the MUX of the second system. Yet another aspect of the optical test system, according to preferred instrumentalities, includes the provision of a programmable modulation controller that resides within the same housing and is capable of generating a plurality of waveforms. The modulation controller includes a plurality of function generators that, for example, generate waveforms including at least two members of the group conaisting of square waves, sawtooth waves, and sine waves. In particularly preferred instances, the modulation controller discharges these waveforms into a shared rail system connected to each channel. The laser source card in each channel is programmably configurable to operate a switch accepting a selected one of the waveforms from the rail system. The laser source card may include a gain block that is programmably configurable to adjust an amplitude of the selected waveform, e.g., by attenuation or amplification, and additional switching mechanisms in the laser source card may selectively bypass the gain block. 
     It is especially preferred that the master controller be compatible with information and command exchange protocols for the transmission and receipt of information to and from external test system components. By way of example, these protocols may include at least one protocol selected from the group consisting of RS-232, GPIB, Ethernet, and telephony networking protocols (e.g., for modem dial-up connections). By way if example, a modem networking protocol may be used for troubleshooting operations on the optical test system. 
     It is especially preferred that the master controller be compatible with information and command exchange protocols for the transmission and receipt of information to and from external test system components. By way of example, these protocols may include at least one protocol selected form the group consisting of RS-232, GPIB, Ethernet, and telephony networking protocols (e.g., for modem dial-up connections). By way of example, a modem networking protocol may be used for troubleshooting operations on the optical test system. 
     While the concepts of the optical test system that are described above emphasize the importance of providing modular system components in a single, compact, and dense housing, it is also permitted to connect the optical test system with other devices that are external to the system, and these external devices may even include additional optical test components or network controllers. For example, an optical power measurement module internal to the housing may be configured to accept optical input from test system components external to the housing. The transfer of information between the internal and external systems may be governed by a network controller. 
     Other aspects of the optical test system, according to preferred instrumentalities, include the provision of a data input device, such as a disk drive or Internet connection, that is capable of reprogramming the optical test system with factory software upgrades. 
     In preferred embodiments, the aforementioned modulation controller of the optical test system is programmably configured for mutual interaction with the laser source card. The laser source card contains a programmable configurable switch for use in accepting a selected one of the waveforms as drive input for the laser source. 
     In especially preferred embodiments, the modulation controller includes a plurality of waveform function output lines, and a corresponding plurality of function generators allocated to each of the waveform function output lines. Each function generator is capable of generating a waveform output on its corresponding waveform function output line. A number of waveform input connectors are allocated to selected ones of the waveform function output lines. Each waveform input connector is capable of receiving waveform input from an external function generator when an external function generator is connected to the waveform input connector. Each waveform input connector is also capable of providing the waveform input as output comprising an external waveform output. A corresponding number of programmably configurable waveform selection switches in the modulation controller are capable of selecting inputs between the generated waveform output and the external waveform output. Each switch of the corresponding number of switches is allocated to one of the selected ones of the waveform input lines, which preferably and collectively form a shared rail system coupled in common with each of the laser source cards. 
     To provide compatible interaction between the laser source card and the modulation controller in these especially preferred embodiments, each of the laser source cards includes a programmably configurable rail selection switch capable of switching between the shared rail systems to provide selected waveform outputs as drive input for the corresponding laser source. 
     The modulation controller may include additional rail systems, such as a coherence rail system in communication with each of the laser source cards. The coherence rail system includes a coherence control function generator capable of generating a coherence control waveform output and a programmably controllable coherence rail switch capable of selecting between the coherence control waveform output and a ground. Accordingly, the programmably configurable rail selection switch in each laser source card is capable of selecting between the coherence rail system and the shared rail systems to provide drive input for the laser source. 
     Another rail system that the modulation controller optionally provides may comprise a digital modulation rail system including a digital modulation function generator capable of generating a digital waveform output, and a programmably controllable digital modulation switch capable of selecting between the coherence control waveform output and a ground. In this case, each laser source card may include a second switch that is capable of selecting between the digital modulation rail system and a ground. The second switch on the laser source card may provide a bypass of the gain block that feeds the digital waveform output directly to the laser source. 
     A particular advantage of the modular construction that is described above is that it enables methods of manufacture and use that are not enabled by prior systems. For example, the optical test system may be produced by a method including the steps of preconfiguring the optical test system for use in test operations at a manufacturing facility prior to shipping the optical test system to a customer. The optical test system is fully ready for the performance of test operations when the customer receives the optical test system as an integrated package. 
     The method of making the optical test system may, for example, comprise the steps of inserting into the housing a plurality of modular laser source channels, that include a laser source card having a first electrical bus and a first optical coupling, and placing into the housing a modular MUX in communication with each laser source channel. Additional steps may include inserting a modular optical conditioning device internal to the housing in an optical pathway downstream from the MUX, and programmably configuring the optical conditioning device for integrated system use as a device selected from the group consisting of a shutter, a variable optical attenuator, a polarization controller, a polarization scrambler, a power monitor, and a wavelength reference device. 
     Where the optical test system includes a master controller internal to the housing, the step of programmably configuring may include a step of programmably configuring the master controller. Furthermore, the method may include adding a modular channel option card, or an array of such cards, to the laser source channel. The system may be calibrated at the manufacturing facility, and it may be constructed by specially trained technicians to incorporate all of the requirements of a complete test system using materials, such as special laser diodes, that are either provided by or specified by the customer. This type of special construction greatly reduces setup downtime upon arrival of the completed test system. Similarly, the integrated modular optical test system permits greater system accuracy and control at expanded test operations according to a method of optical testing. Where, for example, the optical test systems includes a plurality of laser source channels, a MUX, additional optical components, and a master controller all confined in a single housing, a method of use may include energizing the laser source channels to provide test waveforms, and combining the waveforms through use of the MUX to provide a combined comb. The method of use may further include a step of operating the optical test system through use of a unified set of program commands directed from the master controller. Additionally, the method may include a step of troubleshooting system problems through use of a telecommunications linkage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram depicting an integrated modular optical test system that includes a modular laser source channel array and internal MUX that operate according to the principles described above; 
         FIG. 2  depicts an optional channel configuration for the optical test system, wherein a fiber bundle provides input to the MUX; 
         FIG. 3  depicts another optional channel configuration for the optical test system, wherein an optical fiber bypasses the MUX; 
         FIG. 4  depicts yet another optional channel configuration for the optical test system, wherein a laser source card couples with a corresponding optical coupling and an electrical bus; 
         FIG. 5  is a circuit schematic depicting a modulation controller and connections to an array of laser source cards that are used for waveform switching operations that provide selected waveforms to drive a plurality of laser sources from a common rail system; 
         FIG. 6  is a process flow diagram illustrating a method of making the optical test system; and 
         FIG. 7  is a block functional diagram illustrating aspects of firmware that are used to program the optical test system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic block diagram of an optical test system  100  illustrating, by way of example, a modular structure that operates according to preferred principles of the invention. 
     An optical source array  102  is comprised of a plurality of individual channels, such as channel  103 , which each contain a corresponding plurality of elements. The optical source array contains a total of N such channels, where N may be, for example, 100 or 200 channels as needed for test purposes. The optical source array, as depicted in  FIG. 1 , consumes less power and occupies a smaller footprint than prior devices. An additional advantage is that the array may be selectively configured to meet the demands of specific test purposes and need not be provided with too many channels. Additional channels may be selectively added or removed to meet future demands. 
     The individual channels of the optical source array  102  are modularly constructed to meet the needs of specific test situations. By way of example, in the optical source array  102 , a laser source module bank  104  includes a plurality of individual laser source module cards, e.g., card  106  including a laser diode or any other type of optical telecommunications laser source. An example of a commercially available laser source module is the 515 module available from ILX Lightwave of Boulder, Colo. A modulation switch circuitry bank  108 , e.g., comprising individual switch circuitry  110 , permits selective laser modulation according to permitted system modulation functions, such as sine wave, square wave, triangular or sawtooth wave, and rectangular wave function modulations, for each laser source module card as will be described in more detail below. A thermal pulse modulation control bank  112 , e.g., formed of thermoelectric cooler/heater (TEC) circuitry  114 , compensates for temperature variances in the individual laser diodes of the laser source module array  104  to provide a stable laser output. In each channel, the laser source module cards, such as card  106 , preferably include the switch circuitry  110  and the TEC circuitry  114  as integral components; however, the switch circuitry  110  and the TEC circuitry  114  may be provided as separate modular cards with compatible plug-in connectors. 
     A channel option array  116  comprising individual channel option cards, such as card  118 , may be selectively added using commercially available components to provide shutter control for each laser, a variable optical attenuator, a polarization controller, a polarization scrambler, a power monitor, and a wavelength reference. These devices may be used individually, selectively combined in series, or not used at all, depending upon test needs. 
     In cases where the channel option card  118  is a power monitor card, it is preferred to use a tap coupler, e.g., a 99%/1% coupler where power measurement is made on the 1% tap. Prior power monitor devices monitor current at a laser chip on the laser source card and use this measurement to stabilize the power output of the laser. Prior techniques are, therefore, only sensitive to effects on the laser that can affect the power stability. These techniques are insensitive to power changes that derive from changes in the other optical and circuitry elements that are connected to the laser. Placing a power monitor downstream of the laser in the position of card  118  advantageously permits monitoring and/or selective adjustment of laser power output based upon the total channel laser power output. 
     Where, for example, the channel option card  118  is a polarization controller or polarization scrambler, the card operates upon polarized light from the laser source card  106  to align polarization in a controlled manner to optimize external modulation power and to control polarization dependent dispersion and polarization-dependent loss. A polarization scrambler generates all states of polarization in a certain time interval, which averages out polarization-dependent effects. By way of example, a commercially available device that can be used as both a polarization alignment device and a polarization scrambler is, by way of example, the Polarite II model PCS-3X-PC/APC-7 which is available from General Photonics. 
     Where, for example, the channel option card  118  is a wavelength reference, or wavelength lock an optical filter and power meter provide feedback that measure and stabilize the laser frequency from the laser source card  106 . The feedback signal is derived using the intensity or phase of light that is reflected from or transmitted through the filter. 
     Where, for example, the channel option card  118  is a shutter, the shutter mechanism, such as a mechanically actuated fiber switch in a V-groove mount, is preferably used to disrupt or transmit laser emissions from the laser source card  106  without having to change the current at the laser. This ability avoids the necessity of deenergizing and reenergizing the laser, which requires a long settling time to stabilize laser emissions upon reenergization. By way of example, commercially available shutter devices include the model FOSW 1-1-L-PC-L-1 shutter from ChynOptics, which has a 50 ms switching time. 
     Where, for example, the channel option card  118  is a variable optical attenuator, such as the OZ Optics model DD-100-11-1550-9/125-S-40-3D3S-1-0.5-485:1-6-MC/SPI, the attenuator is used to reduce the intensity of light in the channel  103  to much lower and stable power levels than the laser source card  106  can achieve alone with a reduction in current. The individual channel attenuator reduces the power level of the channel for whatever level is needed for the combined comb using one device before the comb is delivered to a system under test. 
     Each channel in the optical source array  102  shares a common electrical backplane  120  and a common optical backplane  122 , which respectively provide compatible electrical or optical couplings that mate with corresponding couplings on the individual channels. The specific manner of connectivity is not critical, so long as the connectors provide the optical and electrical pathways that are required for module compatibility with the optical test system  100 . 
     An optional but preferred multiplexer (MUX)  124  combines the individual channel emissions from the optical source array  102  to provide a combined comb including the combined emissions. For example, a commercially available MUX is the model AWG-NG-48x1-100G-1.5—FC/APC from SDL PIRI. The creation of a wavelength comb within a single instrument advantageously facilitates operations on the combined comb within the test system  100 , as opposed to prior techniques requiring a separate device that occupies an additional footprint. Comb operations are, accordingly, simplified and expanded, as a single programmable controller is enabled to direct these functions in a more versatile manner than could be obtained from separate devices. An additional advantage is that fiber management and integrity is controlled within the enclosure of test system  100 , which reduces set-up time and the risk of fiber damage. 
     The optical pathway proceeds from the multiplexer  124  to a series of optional modular service channel WDM processors  126  and  128 , which are coupled with corresponding service channel sources  130  and  132  for conventional data transmission signal processing, e.g., for WDM-TDM handshake recognition relating to endpoint interpretation of the channels in the combined comb. 
     A beam splitter  134 , e.g., a 99%/1% splitter, provides light from the combined comb to an auto-calibration device  136 , which includes an optical filter and power meter that provide feedback for measurement and stabilization of the laser frequency. The feedback signal is derived using the intensity or phase of light that is reflected from or transmitted through the filter at emission wavelengths corresponding to the design wavelengths for the channels of laser source array  104 . Power control of individual laser source cards in the laser source array  104  may, thus, be regulated after MUX processing to form a combined comb. 
     An optional variable optical attenuator  138 , such as the OZ Optics model DD-100-11-1550-9/125-S-40-3D3S-1-0.5-485:1-6-MC/SPI, reduces the intensity of light in the combined comb downstream of MUX  124  to much lower and stable power levels than the laser source array  104  can achieve alone with a reduction in laser diode current. The individual channel attenuator reduces the power level of the channel for whatever level is needed for the combined comb using one device before the comb is delivered to a system under test. 
     A polarization controller or polarization scrambler  140  operates upon the combined comb downstream of MUX  124  to align polarization in a controlled manner to optimize external modulation power and to control polarization dependent dispersion and polarization-dependent loss. A polarization scrambler generates all states of polarization in a certain time interval, which averages out polarization-dependent effects and identifies minimum and maximum transmission orientations. By way of example, a commercially available device that can be used as both a polarization alignment device and a polarization scrambler is the Polarite II model PCS-3X-PC/APC-7, which is available from General Photonics. 
     A splitter  142  divides the optical pathway for the combined comb into a polarized output segment leading to polarizer  144  and a non-polarized segment  146 . The segment leading to polarizer  144  is in optical communication with an optical power measurement module  148 , which monitors the power output in the combined comb at different polarization states. Optical connectors  150  and  152  are present to receive optical input from other sources external to the optical test system  100 , such as a system power monitor  154  or a general-purpose power monitor  156 . 
     The non-polarized segment  146  is advanced by a splitter  158  or a series of such splitters leading to an output panel  160  including a plurality of optical connectors  162  and  164 . The panel  160  may be provided on the front or rear of the optical test system  100 , or two or more such panels  160  may be present on both the front and rear or the sides. 
     The foregoing discussion has focused primarily upon the optical pathway within the optical test system  100 , and the discussion of electronics has until now not included a discussion of the control circuitry. A master control circuit  166  includes a central processing unit, magnetic or optical data storage, random access memory, and program logic, as required to interact with other system components of the optical test system  100  during normal system control operations in the intended environment of use. For example, the master control circuit  166  may comprise a conventional motherboard for a personal computer, as well as any other circuitry and data storage devices that are commonly used with computers. A modulation control module  168  is provided to drive laser source emissions from the laser source array  106  according to standard test practices. The modulation control module  168  may also be incorporated as part of the master control circuit  166 . The modulation control module  168  is provided with a plurality of N connectors, such as connector  170 , for use in coupling with an external modulation input source  172 . These connectors may be optical or electrical connectors, and the number of connectors corresponds to the number of waveform modulation options in the optical source array  102 . Thus, the external modulation input source  172  may be configured to drive modulation of the optical source array  102  in a manner that is not provided for by the electronics in the modulation control module  168 . 
     The electronics on modulation control module  168  include a function generator that accepts instructions from the master control circuit  166  to drive individual elements (e.g., laser source card  106 ) of the laser source bank  104  in a predetermined manner that is compatible with conventional test practices. This function generator may be switched to an OFF mode to accept external inputs. 
     In an ON mode, the function generator provides sine waves, triangular or sawtooth waves, square waves, and any other wave form that is known or useful to those skilled in the art. The modulation depths are selectively adjustable from 0 to 100%. The modulation control module preferably provides signals comprising a plurality of these waveforms to each channel in the optical source array  102 , and individual channels are intelligent in the sense that they are programmed by instructions from the master control circuit  166  to accept one of the provided waveforms to energize the laser. 
     An optical or magnetic disk drive  174 , such as a Zip drive, is used to provide software upgrades to the master control circuit  166 , as well as to log the performance of optical test system  100 . These functions may also be accomplished using a modem or network connection to an appropriate server, e.g., an Internet server, or other suitable terminus. 
     A front panel display  176 , e.g., a 10-inch color liquid crystal display or plasma display panel, provides a graphical user interface showing all of the source channels in the optical source array  102 , their emission power levels, and the emission wavelengths. An intuitive command set is provided for interaction with the master control circuit  166  to allow rapid modifications to the system setup. Single source commands are provided to adjust the properties of individual lasers on each channel. Comb commands are provided to adjust the properties of the complete comb. Modulation functions are provided to adjust the operation of the modulation control module  168 . 
     The optical test system is compliant with any number of data transmission protocols that are commonly used in networking and optical test systems. External interfaces  178  exist for connections to other devices that use these protocols, such as RS-232, GPIB, and Ethernet. Furthermore, these interfaces preferably include a modem connection for either an internal or external modem, which interfaces with the manufacturer of optical test system  100  for trouble shooting purposes. The modem may also provide real-time test measurement data summaries to remote locations or a telephony network. 
     Except for those components that are specifically noted above as being external to the optical test system  100 , all of the system components that are described above are preferably internal to a single box  180 , and are provided as modular cards or boards that may easily be replaced or renewed on a component by component basis. This feature provides an extremely compact modular system that occupies a small footprint and can be upgraded for small incremental costs over a period of many years. 
     External optical and electrical systems can also be provided for use in combination with the optical test system  100 . For example, each channel in the optical source array  102  is preferably provided with an optical connector, such as connector  182 , that accepts a fiber optic coupling for connection with an additional optical source system, such as an external microwave modulation system  184 , which may, for example, be an optical test mainframe. In this manner, additional sources may be combined into the comb that is processed through MUX  124 . 
     Similarly, external optical devices may be provided downstream of the optical test system  100 , e.g., a generic device  186 , with power measurements being obtainable at any point from the downstream pathway by a simple tap, such as tap  188 , for feedback to the optical power measurement module  148  through one of connectors  150  or  152 . Further splitters, such as 2×2 splitter  190  and 1×1 splitter  192  may be used as needed to branch the optical pathway to other equipment  194 , which may include measurement systems such as power meters and the like, or it may branch to open system architecture or networks. Other pathway branches, for example, lead to test equipment, which may include 1×N switches for the testing of, for example, erbium doped fiber amplifiers (EDFA) or other DWDM system components. 
       FIGS. 2–4  provide additional detail concerning optional configurations for use of the individual channels, e.g., channel  103 , of the optical source array  102  In  FIGS. 2–4 , like numbering has been retained with respect to identical elements that are shown in the  FIG. 1  schematic diagram, however, the configurations differ in a user-determined manner. 
       FIG. 2  depicts the optical source array being fed by a laser source channel  103 . A first pathway segment  202  within channel  103  feeds laser output to optical coupling  182 , which resides in the electrical/optical backplane  122 . In turn, a second fiber optic cable  204  feeds MUX  124 , which also accepts a fiber optic bundle  206  to provide a combined comb output to fiber optic cable  208 . In turn, fiber optic cable  208  feeds optical coupler  162  in the output panel array  160 . In this embodiment, the optical elements  126  through  158  are not required and may be either absent in optical test system  100  or bypassed by a patch cable. The pathway segment  202  may include one or more channel option cards, such as a shutter, variable optical attenuator, polarization controller, polarization scrambler, power monitor or wavelength reference card, as described in the context of channel option bank  118  in  FIG. 1 . 
       FIG. 3  shows a configuration that is identical with respect to  FIG. 2 , except the MUX  124  is absent in  FIG. 3 . A bypass cable  300  travels directly to the connector  162  in the output panel array  160 . 
       FIG. 4  depicts insertion of a laser source module card  106  into channel  103  The laser source module card  106  contains a printed circuit board  400 , which has compatible electronics for modulation of a laser source  402 , such as a laser diode  404 , which is coupled with fiber optic cable  406  leading to a plug-in optical connector  408 . The printed circuit board  400  also has an electrical bus  410 , such as a PCI bus. The optical coupler  408  and the bus  410  mate with corresponding receptacles  412  and  414  in the electrical/optical backplane  122  for the transmission of electrical and optical signals. The channel option cards (not depicted in  FIG. 4 ), such as channel option card  118  shown in  FIG. 1 , may be daisy-chained with the laser source card  106  by opposite pairs of electrical buses and optical connectors that are identical to the optical coupler  408 , the bus  410 , and receptacles  412  and  414   
       FIG. 5  is a circuit diagram that schematically depicts switching logic in the electronics of the modulation control module  168  and a plurality of laser source module cards, such as card  106 , which are denoted in  FIG. 5  as cards  106 A,  106 B, and  106 C. 
     Within the modulation control module  168 , there exist a plurality of function generators  500 ,  502 ,  504 , and  506 . The function generators  500  and  502  are program configurable to generate any type of waveform, such as a sine, square, or triangular waveform, that may be useful in the optical test system  100 . The function generators  500  and  502  are normally programmed to generate signals that differ from one another as, for example, sine waves and square waves. Function generator  504  provides what is commonly known as a coherence control sine wave, which creates a 1 GHz line width in the laser emission. Function generator  506  is a digital function generator that switches between full on and full off. The amplitude of signal modulation provided from the respective function generators is +/−1V for a peak-to-peak amplitude of 2V. A number of external source inputs from the external source input array  170  (see  FIG. 1 ) provide an option for bypassing the function generators  500 - 506 , except the function generator  504  does not necessarily need a bypass. Thus, programmably controlled switches  514 ,  516  and  518  permit a user to configure each channel of the optical source array  102  (see  FIG. 1 ) for use with an integral function generator or an external waveform source. The coherence control signal may be turned on or off by switching to ground. A digital modulation output jack  520  is provided to permit synchronization of signal timing between multiple interconnected optical test systems  100 . 
     Output from the various waveform sources travels on dedicated lines or rails, such as line  522  for modulation  1 , line  524  for modulation  2 , line  526  for coherence, and  528  for TTL digital switching. Each of laser source cards  106 A to  106 C is connected to the lines  522 – 528  in an identical manner, which is described herein in reference to card  106 A. A programmably controlled switch contains three terminals  532 ,  534  and  536 , which are respectively connected to corresponding lines  522 ,  524  and  526 . A switching terminal is selectively connected to one of the three terminals  532 – 536  for modulation of a laser diode  540  according to the waveform on the corresponding one of lines  522 – 526 . As shown in the case of card  106 A, this connection is between terminal  536  and line  526 . A programmably controlled gain block  542  is used to attenuate the amplitude of the waveform to a peak-to-peak value less than the 2V peak-to peak value on the corresponding one of lines  522 – 526 . A second switch  544  is used to modulate the laser diode  540  at full amplitude using digital switching with bypass of the gain block  542 . As shown in the case of card  106 A, switch  544  is set to enable digital switching. Alternatively, switch  544  could be switched to ground, in which case modulation from terminal  536  and line  526  would be enabled. 
       FIG. 6  depicts a flow chart that represents the various setup configuration options for optical test system  100 . Except as noted below, this setup is performed at the factory prior to delivery of the optical test system  100  to the customer. In step  600 , the system is provided as an empty box that requires various modules to be installed according to a customer&#39;s test needs. A decision is made in step  602  as to the source of laser diodes for the laser source bank  106 . This decision has three options. The customer can supply precalibrated diodes for factory installation in step  604 . The factory can install a recommended array, e.g., for specific DWDM test applications, in step  606 , or nothing is installed so that the customer can install the diodes in step  608 . 
     A decision is made in step  610  whether to install a small signal modulation option. If yes, then in step  612  a modulation control module  168  is installed that permits modulation to signal depths ranging from 0 to 100%. If no, the modulation control module  168  is installed without this option. These options may be provided as an add-on board or chip in a standard modulation control module. Similarly, a decision is made in step  614  whether to install a large signal modulation and coherence control option. If yes, then a modulation control module  168  is installed in step  616  with these options enabled. If no, the modulation control module  168  is installed without this option. 
     A decision is made in step  618  whether polarization control should be installed. If yes, then channel option card, such as card  118 , is installed to provide this function in step  620 . 
     This process is repeated for all of the optional modular system components that are shown in  FIG. 1 . For example, a decision is made in step  622  whether a source shutter will be installed as one of the channel options for card  118 . If yes, then the shutter option card is added to one or more channels of the optical source array  102  in step  624 . This pattern may be repeated for the MUX  124 , as well as the optical system components  128  through  158 , according to customer specifications. Accordingly, it is seen that the modularity of optical test system  100  permits factory setup and calibration in a manner that could not be obtained from prior test systems, which typically are assembled at the test site by the customer and may include components from different manufacturers that may have numerous incompatibilities. The factory setup also eliminates test measurement errors due to erroneous, poor or non-optimized installation of system components, and the inclusion of modules within a single system permits on-line factory diagnostics through use of a modem in the external interfaces  178 . Furthermore, soft patches for system upgrades and new drivers may be added to the modular system through use of the data storage drive  174 . 
       FIG. 7  depicts a block diagram of preferred firmware functionality within optical test system  100 . This functionality is executed by program instructions in the master control circuit  166 . Block  702  lists various input/output functions. The system accepts outside commands and functions, for example, as directed by a controller for the external microwave modulation source for synchronization of the optical source array  102 . Block  702  maintains a display on the front panel display  176  denoting the current comb or pipeline status for each channel. Block  702  also interprets or fields keystrokes from the front-panel display and updates the display according to these keystrokes. Block  702  also controls the GPIB controller, e.g., a TNT 4882 controller, and the RS 232 transceiver, as described in connection with the external interfaces  178 . Block  702  also parses SCPI commands from a host computer, which may be part of the master control circuit  166  or a remote processor on a network. Block  702  also maintains the current data structure of the optical test system  100 . 
     Block  704  represents source and measurement functionality. Block  704  interprets tokenized commands that are passed from block  702  by translating these commands into corresponding source code and measurement commands. Block  704  manages the status of the laser sources in each channel of the optical source array  102 . Block  704  also creates and controls a measurement module wherein the output of the respective channels may be coordinated with measurements obtained from the channel output, such as power measurements obtained from the channel option bank  116  or test measurements obtained from the test equipment  196  when such measurements can be allocated back to a particular channel or source. Block  704  additionally controls other devices in the optical pathway of optical test system  100 , such as polarization controllers or scramblers. Block  704  maintains the data structure of each channel, as needed for channel configuration and passes instrument configuration information to block  702 . 
     Block  706  pertains to control functions for the laser source cards, such as card  106 , which each may be provided with their own processor. Block  706  interacts with block  704  by receiving byte-level source code and transmitting source status information. Block  706  uses the byte-level source to control energization of the individual laser diodes or DFB&#39;s. Block  706  set the power and wavelength of the laser emission according to limits that are within predetermined system specifications. Block  706  maintains calibration tables for each laser source, and it drives a source-level polarization controller, as will be explained in more detail below. 
     Block  708  pertains to measurement processor functions, which may include calibrated optical power measurements at system taps, such as may be measured through the optical power measurement module  148  or the channel option bank  116 . Block  708  controls the MUX  124 , which may from time to time require servo adjustment of optical components in the case of a tunable MUX. Block  708  maintains calibration tables for photodiodes that are used in obtaining optical measurements. Block  708  autoranges and adjusts measurements for a chosen photodiode. 
     Due to the densification of the channel source array  102 , as made possible by the concepts discussed above, temperatures within the housing may rise in comparison with respect to less dense sources. The increased temperatures are associated with temperature dependant effects upon the system electronics. These effects could ostensibly result in test measurement variances unless the system electronics accommodate these variances. Accordingly, a new method of cooling the laser diodes has been devised, which includes a switch-mode bi-directional control of laser diode temperature. 
     The circuitry is preferably located in the laser source cards, such as laser source card  106 , and includes a thermoelectric cooler/heater device having a Peltier junction between a first junction conductor and a second junction conductor. A switch-mode DC power supply is electrically connected to the first junction conductor and the second junction conductor for flowing DC electrical current across the Peltier junction. The Peltier junction operates as a heater when current flows in one direction and a cooler when current flows; in the other direction. The thermoelectric controller is heat-conductively connected to the laser diode on the laser source card. The master controller  166  or other control logic integral to the laser source card uses measurements from a temperature sensor to switch the DC current direction in maintaining the temperature of the laser diode within a preferred operating range. Additional information concerning the thermoelectronic controller may be found in provisional application Ser. No. 60/272997 filed as a provisional application on Mar. 2, 2001, which is hereby incorporated by reference to the same extent as though fully repeated herein. 
     The foregoing discussion is intended to illustrate the concepts of the invention by way of example with emphasis upon the preferred embodiments and instrumentalities. Accordingly, the disclosed embodiments and instrumentalities are not exhaustive of all options or mannerisms for practicing the disclosed principles of the invention. The inventors hereby state their intention to rely upon the Doctrine of Equivalents in protecting the full scope and spirit of the invention.