Modulation system and methods for optical source bank

ABSTRACT

A modulation control system is used in fiber optic data transmission and fiber optic test equipment. A modulation controller includes a plurality of function generators that are linked by a switching mechanics, to a rail system that drives a plurality of laser source cards. The switching mechanism for each rail is capable of driving the rail at the function-generated waveform or switching to an external source waveform. The laser source cards are intelligent in the sense that they can be programmably controlled by switching to select waveforms from a selected one of the rails.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

RELATED APPLICATIONS

This application claims benefit of priority to U.S. Provisional Application Ser. No. 60/273,142 filed Mar. 2, 2001, and U.S. Provisional Application Ser. No. 60/302,048, filed Jun. 29, 2001, each of which is hereby incorporated by reference.

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 is 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 illustrating, by way of example, a modular structure that operates according to preferred principles of the invention.

An optical source array is comprised of a plurality of individual channels, such as channel , 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, or 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 are modularly constructed to meet the needs of specific test situations. By way of example, in the optical source array , a laser source module bank includes a plurality of individual laser source module cards, e.g., card including a laser diode or any other type of optical telecommunications laser source. An example of a commercially available laser source module is the module available from ILX Lightwave of Boulder, Colo. A modulation switch circuitry bank , e.g., comprising individual switch circuitry , 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 , e.g., formed of thermal element control (TEC) circuitry , compensates for temperature variances in the individual laser diodes of the laser source module array to provide a stable laser output. In each channel, the laser source module cards, such as card , preferably include the switch circuitry and the thermal element control (TEC) circuitry as integral components, however, the switch circuitry and the thermal element control (TEC) circuitry may be provided as separate modular cards with compatible plug-in connectors.

A channel option array comprising individual channel option cards, such as card , 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 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, however, 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 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 is a polarization controller or polarization scrambler, the card operates upon polarized light from the laser source card 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, commercially available device that can be used as both a polarization alignment device and a polarization scrambler is, by way of example, the model PCS-3X-PC/APC-7 which is available from General Photonics.

Where, for example, the channel option card 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 . The feedback signal is derived using the intensity or phase of light that is reflected from or transmitted to the filter.

Where, for example, the channel option card 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 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 Lightwave Link, which has a 50 ms switching time.

Where, for example, the channel option card 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 to much lower and stable power levels than the laser source card 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 system under test combined comb using one device before the comb is delivered to a system under test.

Each channel in the optical source array shares a common optical backplane and a common electrical backplane , 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 .

An optional but preferred multiplexer (MUX) combines the individual channel emissions from the optical source array to provide a combined comb including the combined emissions. For example, a commercially available MUX is the model AWG-NG-48×1-100G-1.5—FC/APC from SDI PIRI. The creation of a wavelength comb within a single instrument advantageously facilitates operations on the combined comb within the test system , 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 amore 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 , which reduces set-up time and the risk of fiber damage.

The optical pathway proceeds from the multplexer to a series of optional modular service channel WDM processors and , which are coupled with corresponding service channel sources and 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 , e.g., a 99%/1% splitter, provides light from the combined comb to an auto-calibration device , 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 to the filter at emission wavelengths corresponding to the design wavelengths for the channels of laser source array . Power control of individual laser source cards in the laser source array may, thus, be regulated after MUX processing to form a combined comb.

An optional 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, operates on the combined comb downstream of MUX reduce the intensity of light in the combined comb to much lower and stable power levels than the laser source array 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 system under test .combined comb using one device before the comb is delivered to a system under test.

A polarization controller or polarization scrambler operates upon the combined comb downstream of MUX 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 device that can be used as both a polarization alignment device and a polarization scrambler is the model PCS-3X-PC/APC-7, which is available from General Photonics.

A splitter divides the optical pathway for the combined comb into a polarized output segment leading to polarizer and a non-polarized segment . The segment leading to polarizer is in optical communication with an optical power measurement module , which monitors the power output in the combined comb at different polarization states. Optical connectors and are present to receive optical input from other sources external to the optical test system , such as a system power monitor or a general-purpose power monitor .

The non-polarized segment is advanced by a splitter or a series of such splitters leading to an output panel including a plurality of optical connectors and . The panel may be provided on the front or rear of the optical test system , or two or more such panels 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 , and the discussion of electronics has until now not included a discussion of the control circuitry. A master control circuit 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 during normal system control operations in the intended environment of use. For example, the master control circuit 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 is provided to drive laser source emissions from the laser source array according to standard WDM-TDM practices. The modulation control module may also be incorporated as part of the master control circuit . The modulation control module is provided with a plurality of connectors, such as connector , for use in coupling with an external modulation input source . These connectors may be optical or electrical connectors, and the number of connectors corresponds to the number of modulation options in the optical source array . Thus, the external modulation input source may be configured to drive modulation of the optical source array in a manner that is not provided for by the electronics in the modulation control module .

The electronics on modulation control module include a function generator that accepts instructions from the master control circuit to drive individual elements (e.g., laser source card ) of the laser source bank in a predetermined manner that is compatible with conventional WDM-TDM 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 , and individual channels are intelligent in the sense that they are programmed by instructions from the master control circuit to accept one of the provided waveforms to energize the laser.

An optical or magnetic disk drive , such as a Zip drive, is used to provide software upgrades to the master control circuit , as well as to log the performance of optical test system . 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 , e.g., a 10″ color liquid crystal display or plasma display panel, provides a graphical user interface showing all of the source channels in the optical source array , their emission power levels, and the emission wavelengths. An intuitive command set is provided for interaction with the master control circuit 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 provides to adjust the properties of the complete comb. Modulation functions are provided to adjust the operation of the modulation control module .

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 exist for connections to other devices that use these protocols, such as RS-, 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 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 , all of the system components that have previously been described are preferably internal to a single box , 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 . For example, each channel in the optical source array is preferably provided with an optical connector, such as connector , that accepts a fiber optic coupling for connection with an additional optical source system, such as an external microwave modulation system , 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 .

Similarly, external optical devices may be provided downstream of the optical test system , e.g., a generic device , with power measurements being obtainable at any point from the downstream pathway by a simple tap, such as tap , for feedback to the optical power measurement module through one of connectors or . Further splitters, such as 2×2 splitter and 1×1 splitter may be used as needed to branch the optical pathway to other equipment , 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 , of the optical source array 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 . A first pathway segment within channel feeds laser output to optical coupling , which resides in the electrical/optical backplane . In turn, a second fiber optic cable feeds MUX , which also accepts a fiber optic bundle to provide a combined comb output to fiber optic cable . In turn, fiber optic cable feeds optical coupler in the output panel array . In this embodiment, the optical elements through are not required and may be either absent in optical test system or bypassed by a patch cable. The pathway segment 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 in FIG. .

FIG. 3 shows a configuration that is identical with respect to FIG. 2, except the MUX is absent in FIG. 3. A bypass cable travels directly to the connector in the output panel array .

FIG. 4 depicts insertion of a laser source module card into channel . The laser source module card contains a printed circuit board , which has compatible electronics for modulation of a laser source , such as a laser emitting diode , which is coupled with fiber optic cable leading to a plug-in optical connector . The printed circuit board also has an electrical bus , such as a PCI buss. The optical coupler and the bus mate with corresponding receptacles and in the electrical/optical backplane for the transmission of electrical and optical signals. The channel option cards (not depicted in FIG. ), such as channel option card shown in FIG. 1, may be daisy-chained with the laser source card by opposite pairs of electrical buses and optical connectors that are identical to the optical coupler , the bus , and receptacles and

FIG. 5 is a circuit diagram that schematically depicts switching logic in the electronics of the modulation control module and a plurality of laser source module cards, such as card , which are denoted in FIG. 5 as cards A, B, and C.

Within the modulation control module , there exist a plurality of function generators , , , and . The function generators and 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 . The function generators and are normally programmed to generate signals that differ from one another as, for example, sine waves and square waves. Function generator provides what is commonly known as a coherence control sine wave, which incorporates a I GHz line width in the laser emission. Function generator 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 (see FIG. 1) provide an option for bypassing the function generators -, except the function generator does not necessarily need a bypass. Thus, programmably controlled switches , and permit a user to configure each channel of the optical source array (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 is provided to permit synchronization of signal timing between multiple interconnected optical test systems .

Output from the various waveform sources travels on dedicated lines or rails, such as line for modulation , line for modulation , line for coherence, and for TTL digital switching. Each of laser source cards A to C is connected to the lines - in an identical manner, which is described herein in reference to card A. A programmably controlled switch contains three terminals , and , which are respectively connected to corresponding lines , and . A switching terminal is selectively connected to one of the three terminals - for modulation of a laser diode according to the waveform on the corresponding one of lines -. As shown in the case of card A, this connection is between terminal and line . A programmably controlled gain block 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 -. A second switch is used to modulate the laser diode at full amplitude using digital switching with bypass of the gain block . As shown in the case of card A, switch is set to enable digital switching, which deactivates modulation from switch . Alternatively, switch could be switched to ground, in which case modulation from terminal and line would be enabled.

FIG. 6 depicts a flow chart that represents the various setup configuration options for optical test system . Except as noted below, this setup is performed at the factory prior to delivery of the optical test system to the customer. In step , the system is provided as an empty box that requires various modules to be installed according to a customer's test needs. A decision is made in step as to the source of laser diodes for the laser source bank . This decision has three options. The customer can supply precalibrated diodes for factory installation in step . The factory can install a recommended array, e.g., for specific DWDM test applications, in step , or nothing is installed so that the customer can install the diodes in step .

A decision is made in step whether to install a small signal modulation option. If yes, then in step a modulation control module is installed that permits modulation to signal depths ranging from 0 to 100%. If no, the modulation control module 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 whether to install a large signal modulation and coherence control option. If yes, then a modulation control module is installed in step with these options enabled. If no, the modulation control module is installed without this option.

A decision is made in step whether polarization control should be installed. If yes, then channel option card, such as card , is installed to provide this function in step .

This process is repeated for all of the optional modular system components that are shown in FIG. . For example, a decision is made in step whether a source shutter will be installed as one of the channel options for card . If yes, then the shutter option card is added to one or more channels of the optical source array in step . This pattern may be repeated for the MUX , as well as the optical system components through , according to customer specifications. Accordingly, it is seen that the modularity of optical test system 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 . Furthermore, soft patches for system upgrades and new drivers may be added to the modular system through use of the data storage drive .

FIG. 7 depicts a block diagram of preferred firmware functionality within optical test system . This functionality is executed by program instructions in the master control circuit . Block 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 . Block maintains a display on the front panel display denoting the current comb or pipeline status for each channel. Block also interprets or fields keystrokes from the front-panel display and updates the display according to these keystrokes. Block also controls the GPIB controller, e.g., a TNT controller, and the RS transceiver, as described in connection with the external interfaces . Block also parses SCPI commands from a host computer, which may be part of the master control circuit or a remote processor on a network. Block also maintains the current data structure of the optical test system .

Block represents source and measurement functionality. Block interprets tokenized commands that are passed from block by translating these commands into corresponding source code and measurement commands. Block manages the status of the laser sources in each channel of the optical source array . Block 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 or test measurements obtained from the test equipment when such measurements can be allocated back to a particular channel or source. Block additionally controls other devices in the optical pathway of optical test system , such as polarization controllers or scramblers. Block maintains the data structure of each channel, as needed for channel configuration and passes instrument configuration information to block .

Block pertains to control functions for the laser source cards, such as card , which each may be provided with their own processor. Block interacts with block by receiving byte-level source code and transmitting source status information. Block uses the byte-level source to control energization of the individual laser diodes or DFB's. Block set the power and wavelength of the laser emission according to limits that are within predetermined system specifications. Block maintains calibration tables for each laser source, and it drives a source-level polarization controller, as will be explained in more detail below.

Block 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 or the channel option bank . Block controls the MUX , which may from time to time require servo adjustment of optical components in the case of a tunable MUX. Block maintains calibration tables for photodiodes that are used in obtaining optical measurements. Block autoranges and adjusts measurements for a chosen photodiode.

Due to the densification of the channel source array , 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 , and includes a thermoelectric controller 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 thermoelectric coupler 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 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 serial No. 60/272,997 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.

CLAIMS

1. A source modulation system for use in optical data transmission systems and optical data test components, comprising: a plurality of laser source channels each including a laser source card having a laser source that generates laser output with adjustable signal modulation depth, the laser source channels including a programmably controllable rail selection switch for use in switching between selected rail lines to provide a selected laser source drive input corresponding to a selected rail line; a modulation controller including a plurality of function generators that are each capable of generating waveforms for use at the laser source channels; and a shared rail system connecting the modulation controller with each of the laser source channels, the shared rail system including a number of rail lines each connecting the modulation controller with the laser source cards, the rail lines being equal in number to the function generators on the modulation controller.

2. The source modulation system of claim 3, wherein the modulation controller includes a number of waveform input connectors allocated to selected ones of the rail lines, each waveform input connector being capable of receiving waveform input from an external function generator when an external function generator is connected to the waveform input connector and providing the waveform input as output comprising an external waveform output, and a corresponding number of programmably configurable waveform selection switches capable of selecting inputs between the corresponding function generator output and the external waveform output, each of the corresponding number of switches being allocated to one of the selected ones of the rail lines.

3. The source modulation system of claim 1, further including a coherence rail system in communication with each of the laser source cards, the coherence rail system having 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.

4. The source modulation system of claim 3, wherein the programmably controllable rail selection switch in at least one 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.

5. The source modulation system of claim 3, wherein the modulation controller includes 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.

6. The source modulation system of claim 5, wherein at least one laser source card includes a second switch capable of selecting between the digital modulation rail system and ground.

7. The source modulation system of claim 6, wherein the laser source card includes a gain block for adjusting an amplitude of waveforms from the shared rail system to adjust the modulation depth of the laser output, and wherein the second switch provides a bypass of the gain block.

8. The source modulation system of claim 1, wherein the function generators are operable to produce on the number of rail lines waveforms selected to include at least two members of the group consisting of square waves, sawtooth waves, and sine waves.

9. The source modulation system of claim 1, wherein at least one of the laser source cards contains a programmably configurable switch for use in accepting a selected one of the waveforms as drive input for the laser source.

10. The source modulation system of claim 9, wherein the laser source card includes a gain block that is programmably configurable to adjust an amplitude of the selected waveform, to adjust the modulation depth of the laser output.

11. The source modulation system of claim 10, wherein the gain block operates by attenuating the amplitude of the waveform.

12. The source modulation system of claim 10, wherein the laser source card includes a bypass mechanism that is programmably configurable to bypass the gain block.

13. The source modulation system of claim 1, comprising a mechanism for accepting external sources to drive modulation input for each of the laser source channels.

14. The source modulation system of claim 13, wherein the mechanism comprises signal input connectors having a one-to one relationship with the number of channels.

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