Abstract:
A method for compensating for wavelength drift in a fiber-optic laser transmitter includes 1) controlling a temperature within the optoelectronic assembly at a defined level; 2) driving the optoelectronic assembly to emit light, wherein the emitted light has a wavelength that is within a channel of operation, the channel of operation including a range of wavelengths centered around a channel center wavelength; 3) accessing from memory within the optoelectronic assembly a control value associated with the temperature of the optoelectronic assembly at defined points within an operational lifetime of the optoelectronic assembly; and 4) recalculating the defined level by reference to the control value, whereby a wavelength of the optoelectronic assembly is maintained within the channel of operation despite an expected drift of wavelength.

Description:
[0001]    This application claims priority to, and hereby incorporates by reference, U.S. Provisional Patent Application 60/425,031, filed Nov. 8, 2002. 
     
    
     
       BRIEF DESCRIPTION OF THE INVENTION  
         [0002]    The present invention relates generally to optoelectronic components. More particularly, the present invention relates to method for compensating for wavelength drift in a fiber-optic laser transmitter.  
         BACKGROUND  
         [0003]    The proliferation of communication technologies creates every day increases in demand for data transfer channels. Optical networks are a highly-reliable and efficient way to satisfy this demand. As a result, there is a desire to achieve higher data throughput in existing optical networks. A current means for satisfying this desire is the use of Dense Wave Division Multiplexing (DWDM). As shown in FIG. 1, DWDM data from a plurality of sources is converted into optical signals  2  with different wavelengths by a plurality of optoelectronic transceivers  4 . After being multiplexed by an optical multiplexer/demultiplexer  6 , optical signals  2  may pass through a single optical cable  8  simultaneously, which greatly increases network throughput.  
           [0004]    There are several standards for a signal traveling through an optical network. These standards specify acceptable wavelengths of a signal (channel) and the distance or spacing between neighboring channels. There exists a need, therefore, for optoelectronic transceivers capable of operating on specific wavelengths. Currently, the most popular standards are 200 GHz (gigahertz) spacing, which is equivalent to 1.6 nm (nanometers) spacing between neighboring channels, 100 GHz, equivalent to 0.8 nm spacing, 50 GHz, equivalent to 0.4 nm spacing, and 25 GHz equivalent to 0.2 nm spacing between channels. The specific wavelengths (i.e., channels) acceptable for data transfer in an optical network are proscribed by the International Telecommunications Union (ITU).  
           [0005]    Optical amplifiers, used to increase the strength of an optical signal before it enters an optical network, typically have an optimal operational wavelength range. For modern Ebrium-Doped Fiber Amplifiers (EDFA) the typical operational wavelength range is 1523 to 1565 nm. If the network is using a 200 GHz standard for channel spacing, the number of available channels is 22. For 100 GHz standard the number of channels is 45; for 50 GHz—90 channels; for 25 GHz standard—180 channels.  
           [0006]    [0006]FIG. 2 shows a schematic representation of wavelength intervals when the channel spacing standard is 100 GHz. The distance between neighboring channel centers  10  is 0.8 nm. For a signal to stay within the allowed pass band  14  its wavelength must be within 0.1 nm of the center of the specified channel. Operation outside the allowable allowed pass band  14  results in high attenuation of the transmitted signal, and in extreme cases, potential cross-talk with an adjacent channel.  
           [0007]    The wavelength emitted by the laser shifts as the laser emitter ages. In order to calculate how much the laser emitter wavelength can shift before it starts encroaching on a neighboring channel, several parameters of laser emitter calibration must be taken into account.  
           [0008]    When calculating the allowable pass band of a laser emitter, an allowance must be made for an initial setup tolerance  16  (FIG. 3) and temperature control tolerance  18 . For example, for a part in which the initial wavelength is targeted at the center channel, and with a set-up tolerance of +/−10 pm (picometers), and a temperature control tolerance of +/−20 pm, for a combined set-up and temperature control tolerance of +/−30 pm. Based on these tolerances and a 100 pm maximum total wavelength offset tolerance, the allowable wavelength aging is +/−70 pm over the life of the part.  
           [0009]    There are several factors determining the wavelength of a signal produced by traditional laser sources. These factors include current density, temperature of the laser emitter, as well as specific inherent characteristics of the laser emitter. The relationship between the temperature of the laser emitter and the wavelength produced is typically around 0.1 nm/° C. for Distributed Feedback (“DFB”) sources that are commonly used in DWDM applications. This means that if the laser emitter temperature is increased by 10° C., the wavelength of the emitted light will shift about +1 nm.  
           [0010]    Since the wavelength produced by a transceiver at a specified laser emitter temperature and current density differs from one laser emitter to the other, the optoelectronic transceivers are initially calibrated before being installed in an optical network. The calibration includes monitoring the wavelength of optical signals produced by the laser emitter while varying its temperature as well as other operating conditions, and then storing calibration information in the memory of a microprocessor. It also includes receiving analog signals from sensors in the optoelectronic device and converting the analog signals into digital values, which are also stored in the memory. As a result the device generates control signals based on the digital values in the microprocessor to control the temperature of the laser emitter. The method of calibrating an optoelectronic transceiver is described in detail in a U.S. patent application entitled “Control Circuit for Optoelectronic Module With Integrated Temperature Control,” identified by Ser. No. 10/101,248, and filed on Mar. 18, 2002, which is incorporated herein by reference.  
           [0011]    For performance and reliability reasons, it is desirable to operate a laser emitter at a temperature between 15° C. and 50° C. There are several factors limiting the acceptable range of operating temperatures. First, a laser emitter ages more rapidly when operated at temperatures above 50° C., and may cause reliability concerns at typical end of life conditions (20-25 years). The quantum efficiency of the laser emitter decreases with age and, therefore, forces the transceiver to operate at higher currents in order to provide a fixed optical power, which further accelerates the aging of the laser emitter. In addition, temperature performance characteristics of the device used to control the laser temperature determine the lower limit of the available range of temperatures. A well-designed thermal system using a single-stage thermoelectric cooler (TEC) as a temperature control device can typically provide up to 40° C. cooling. Since the standard maximum operating temperature of a transceiver is 70° C., the 40° C. cooling capability of the TEC means that the effective operating range of the laser emitter in the transceiver is restricted to temperatures between 30° C. and 50° C.  
           [0012]    Finally, persons skilled in the art recognize that the wavelength of a laser diode varies during its operational lifetime. As a result, steps need to be taken in order to ensure that the wavelength does not drift outside of a selected channel during this operational lifetime. Prior art techniques for preventing this drift include the use of wavelockers, which are expensive and of questionable reliability.  
         SUMMARY  
         [0013]    A method of operating an optoelectronic assembly includes 1) controlling a temperature within the optoelectronic assembly at a defined level; 2) driving the optoelectronic assembly to emit light, wherein the emitted light has a wavelength that is within a channel of operation, the channel of operation including a range of wavelengths centered around a channel center wavelength; 3) accessing from memory within the optoelectronic assembly a control value associated with the temperature of the optoelectronic assembly at defined points within an operational lifetime of the optoelectronic assembly; and 4) recalculating the defined level by reference to the control value, whereby a wavelength of the optoelectronic assembly is maintained within the channel of operation despite an expected drift of wavelength. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a schematic representation of a multi-channel optical network.  
         [0015]    [0015]FIG. 2 is a schematic representation of a wavelengths in multi-channel 100 GHz standard.  
         [0016]    [0016]FIG. 3 is a schematic representation of a laser emitter, pass band calculation.  
         [0017]    [0017]FIG. 4 is a block diagram of an embodiment of an optoelectronic transceiver.  
         [0018]    [0018]FIG. 5 is a block diagram illustrating an embodiment of circuitry for controlling the temperature of a laser emitter.  
         [0019]    [0019]FIG. 6 is a block diagram depicting a portion of a circuit implementing the microprocessor of FIG. 5.  
         [0020]    [0020]FIG. 7 is a flowchart depicting process steps for controlling the temperature of a laser emitter.  
         [0021]    [0021]FIG. 8 is a diagram of a setup and tuning system.  
         [0022]    [0022]FIG. 9 is a flowchart depicting process steps for calibrating a laser emitter.  
         [0023]    [0023]FIG. 10 is a diagram of a channel lookup table.  
         [0024]    [0024]FIGS. 11A and 11B illustrate plots of power and current in a laser over time.  
         [0025]    [0025]FIGS. 12A and 12B illustrate wavelength drift lookup tables.  
         [0026]    [0026]FIGS. 13A and 13B illustrate target wavelength adjustments.  
         [0027]    [0027]FIGS. 14A and 14B illustrate processing steps for adjusting a target temperature for a laser. 
     
    
     DESCRIPTION OF EMBODIMENTS  
       [0028]    A number of embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described. It will be appreciated that in the development of any such embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0029]    [0029]FIG. 4 shows a schematic representation of a fiber optic transceiver  100 . Transceiver  100  contains at a minimum transmit and receiver circuit paths and power  97  and ground connections  96 . Further, transceiver  100  includes a Receiver Optical Subassembly (ROSA)  102 , which contains a mechanical fiber receptacle and coupling optics, as well as a photo diode and a pre-amplifier (preamp) circuit. ROSA  102  is in turn connected to a post-amplifier (postamp) integrated circuit  104 , the function of which is to take relatively small signals from ROSA  102  and amplify and limit them to create a uniform amplitude digital electronic output, which is connected to outside circuitry via the RX+ and RX− pins  95 . The postamp circuit  104  provides a digital output signal known as Signal Detect or Loss of Signal indicating the presence or absence of suitably strong optical input. All the components of the transceiver  100  are preferably located in a protective housing  30 , except for connectors that may protrude from the housing.  
         [0030]    Suitable housings, including metallic, plastic, potting box and other housing structures are well known in the art. In one embodiment, the protective housing  30  are as follows: width, 3 cm or less; length, 6.5 cm or less, and height, 1.2 cm or less. A GBIC standard (SFF-8053 GBIC standard version 5.5) requires the dimensions of a module housing to be approximately 3 cm×6.5 cm×1.2 cm. Thus, the protective housing  30  of this embodiment meets the form factor requirements of the GBIC standard. In another embodiment, the physical dimensions of the module housing are: width, 0.54 inches or less; length, 2.24 inches or less; and height, 0.34 inches or less. The SFP MSA (Small Form Factor Pluggable Multisource Agreement) requires the dimensions of a compliant module housing to be approximately 0.54″×2.24″×0.34″. Thus, the module housing in that embodiment meets the form factor requirements of the SFP standard. Note that the present invention is not limited to the form factor requirements described above. A person of ordinary skill in the art having the benefit of this disclosure will appreciate that the present invention is adaptable to various existing or yet to be determined form factors, some of which can be smaller than the ones identified here.  
         [0031]    The transmit circuitry of transceiver  100  consists of a Transmitter Optical Subassembly (TOSA)  106  and a laser driver integrated circuit  108 , with signal inputs obtained from the TX+ and TX− pins  90 . TOSA  106  contains a mechanical fiber receptacle and coupling optics, as well as a thermo-electric cooler (TEC) and a laser diode or LED. The laser driver circuit  108  provides AC drive and DC bias current to the laser. The signal inputs for the driver are obtained from I/O pins (not shown) of transceiver  100 . In other embodiments, the TEC is external to the TOSA  106 . In yet other embodiments, the TEC is integrated within a laser transistor-outline (TO) package.  
         [0032]    In addition, the optoelectronic transceiver  100  includes a thermoelectric cooler (TEC) driver  116  and additional circuitry that is not shown for controlling the temperature of the TOSA  106 . An embodiment of the TEC driver  116  and the additional circuitry is described in greater detail below in connection with FIG. 5.  
         [0033]    Also shown in FIG. 4 is microprocessor  200 , which may comprise two or more chips, configured for controlling the operations of the transceiver  100 . Suitable microprocessors include the PIC16F873A, PIC16F870 and PIC16F871 8-bit CMOS FLASH microcontrollers manufactured by Microchip Technology, Inc. Microprocessor  200  is coupled to provide control signals to the post-amplifier  104  and laser driver  108 , and these components and the ROSA  102  and TOSA  106  provide feedback signals back to the microprocessor  200 . For example, microprocessor  200  provides signals (e.g., bias and amplitude control signals) to control the DC bias current level and AC modulation level of laser driver circuit  108  (which thereby controls the extinction ratio (ER) of the optical output signal), while post-amplifier circuit  104  provides a Signal Detect output to microprocessor  200  to indicate the presence or absence of a suitably strong optical input.  
         [0034]    Importantly, the bias current level and the AC modulation level both affect the optical output wavelength of transceiver  100 . Persons skilled in the art recognize that increases in the bias current and, to a lesser extent, increases in the AC modulation can increase the temperature of the active region of a laser chip. More specifically, as the bias current and AC modulation increase, so does the power dissipation of the laser chip. And as the power dissipated in the laser chip increases, so does the temperature of the laser chip, which has a fixed thermal resistance. This is true even though the temperature at the base of the laser chip is typically controlled by the TEC.  
         [0035]    Temperature and/or other physical conditions of various components of transceiver  100  may be acquired using sensors that are coupled to microprocessor  200 . In some embodiments, conditions of the optical links may also be acquired using the sensors.  
         [0036]    In addition to, and sometimes in conjunction with these control functions, there are a number of other tasks that may be handled by microprocessor  200 . These tasks include, but are not necessarily limited to, the following:  
         [0037]    Setup functions. These generally relate to the required adjustments made on a part-to-part basis in the factory to allow for variations in component characteristics such as laser diode threshold current.  
         [0038]    Identification. This refers to the storage of an identity code within a general purpose memory (e.g., an EEPROM). Additional information, such as sub-component revisions and factory test data, may also be stored within the general purpose memory for purposes of identification.  
         [0039]    Eye safety and general fault detection. These functions are used to identify abnormal and potentially unsafe operating parameters and to report these to the host device and/or perform laser shutdown, as appropriate. Sensors may be used to identify such abnormal or potentially unsafe operating parameters.  
         [0040]    Receiver input optical power measurement. This function is used to measure the input optical power and a report of this measurement may be stored in the memory.  
         [0041]    Laser diode drive current. This function is used to set the output optical power level of the laser diode.  
         [0042]    Laser diode temperature monitoring and control. In one embodiment, a temperature controller (e.g., a thermal-electric cooler (TEC)) is disposed in or near TOSA  106  for controlling the temperature of the laser emitter therein. In this embodiment, microprocessor  200  is responsible for providing control signals to the temperature controller.  
         [0043]    Note that transceiver  100  has a serial interface  202  for communicating with a host device. As used herein, a host device refers to a link card to which a transceiver is attached and/or a host system computer to which a transceiver provides an optical connection. Host systems may be computer systems, network attached storage (NAS) devices, storage area network (SAN) devices, optoelectronic routers, as well as other types of host systems and devices.  
         [0044]    In some embodiments the optoelectronic transceiver  100  includes an integrated circuit controller that may perform some of the functions listed above. For example, an integrated circuit controller performs the tasks of identification and eye safety and general fault detection, while the microprocessor provides control signals to the temperature controller and also may perform other tasks.  
         [0045]    Further, the optoelectronic transceiver may also include the TX disable  91  and TX fault  92  pins described in the GBIC (Gigabit Interface Converter) standard. In the GBIC standard (SFF-8053), the TX disable pin  91  allows the transmitter to be shut off by the host device, while the TX fault pin  92  is an indicator to the host device of some fault condition existing in the laser or associated laser driver circuit.  
         [0046]    [0046]FIG. 5 illustrates a portion of temperature control circuitry  101  of the transceiver  100 . The temperature control circuitry  101  is coupled to a TOSA  106 . In some embodiments, TOSA  106  includes a laser assembly  112  (e.g., a laser transistor outline package), which in turn includes a laser emitter (e.g., an edge emitting laser diode) that is activated when a positive bias current, I laser bias , is applied across its p-n junction. Also shown in FIG. 5 are a laser temperature sensor  110  and a thermoelectric cooler (TEC)  114  coupled to the laser assembly  112 . In some other embodiments, the laser temperature sensor  110  and/or the TEC  114  are integrated within the laser assembly  112 . In yet other embodiments, the laser temperature sensor  110  and/or the TEC  114  are external to the TOSA  106 .  
         [0047]    In some embodiments, the laser temperature sensor  110  is a thermistor. Any other device suitable for measuring the temperature of the laser diode may also be used. The laser temperature sensor  110  generates a signal (V TL ) that varies as a function of the temperature of the laser diode. As described above, and as is well known to those skilled in the art, the wavelength of optical signals generated by a laser diode varies as a function of the temperature of the laser diode. Accordingly, in other embodiments, a sensor that measures the wavelength of the optical signals directly may be substituted for the laser temperature sensor  110 . In still other embodiments, a device measuring an operating condition of the laser diode that varies as a function of the temperature of the laser diode is used instead of the laser temperature sensor  110 .  
         [0048]    With reference still to FIG. 5, laser driver circuitry  108  supplies both AC drive power and a positive DC bias current I laser bias  to the laser assembly  112  to activate the laser emitter and to set the AC modulation of the laser assembly. The microprocessor  200  controls this aspect of the laser driver circuitry  108  via the bias control signal and the amplitude control signal. The laser driver circuitry  108  also transmits a voltage V(I laser bias ), which is proportional to the I laser bias , so that the microprocessor  200  may indirectly monitor the actual value of I laser bias , which may vary due to operating conditions such as temperature. In some embodiments, the microprocessor  200  monitors a signal from a back facet photodiode (also called a monitor photodiode) instead of (or in addition to) the voltage V(I laser bias ). In some embodiments the microprocessor  200  uses the monitored signal(s) to determine an adjustment to the DC bias current I laser bias .  
         [0049]    An additional input is provided to the microprocessor  200  by an ambient temperature sensor  120 , which measures the ambient temperature surrounding the TOSA  106  and generates a signal (V TA ) for the microprocessor  200  that varies as a function of the ambient temperature. Although a laser temperature sensor  110  is preferably placed in the proximity of a laser emitter, the temperature reading from the laser temperature sensor  110  generally differs from the actual temperature of the laser emitter because the laser temperature sensor  110  is physically separated from the laser emitter. As a consequence, the temperature reading from the laser temperature sensor  110  and its signal V TL  vary as a function of the outside temperature. By receiving the ambient temperature signal V TA , the microprocessor  200  is able to compensate for the effect of the ambient temperature on the temperature reading from the laser temperature sensor.  
         [0050]    In addition to the V(I laser bias ), V TL  and V TA  signals, the microprocessor  200  receives inputs from a host device  220  through serial interface circuitry  202  (FIG. 6). In some embodiments, using the information collected from the host device, the laser driver circuitry  108  and the ambient temperature sensor  120 , the microprocessor  200  generates an analog TEC Command signal to set the temperature of the laser emitter in the laser assembly  112 . In particular, the microprocessor  200  generates the TEC Command signal based on inputs of V(I laser bias ) from the laser driver circuitry  108 , V TL  from the laser temperature sensor, V TA  from the ambient temperature sensor  120 , and calibrated values previously stored within the microprocessor  200  during the calibration of the optoelectronic transceiver  100 .  
         [0051]    The TEC Command signal is provided to the TEC driver circuitry  116 . The TEC driver circuitry  116  is configured to generate an output signal V TEC  to drive the TEC  114  in accordance with the TEC Command signal.  
         [0052]    [0052]FIG. 6 is a logical block diagram illustrating a portion of a circuit implementing the microprocessor  200 . The microprocessor  200  includes serial interface circuitry  202  coupled to host device interface input/output lines. In some embodiments, the serial interface circuitry  202  operates in accordance with the two wire serial interface standard that is also used in the GBIC (Gigabit Interface Converter) and SFP (Small Form Factor Pluggable) standards; however, other serial interfaces could equally well be used in alternate embodiments. In yet other embodiments, a multiple-pin interface could be used in place of a serial interface. The interface circuitry  202  is used for setup and querying of the microprocessor  200 , and enables access to the optoelectronic transceiver  100  by a host device  220  connected thereto.  
         [0053]    The microprocessor  200  may also include one or more volatile and/or nonvolatile memory devices, such as a general purpose EEPROM (electrically erasable and programmable read only memory) device  204 , as shown in FIG. 4. Tables and parameters may be set up using the EEPROM device  204  by writing values to predefined memory locations in the memory devices, and various output values may be output by reading from predetermined memory locations in the memory devices. Included in the EEPROM device  204  are one or more lookup tables, which may be used to assign values to control outputs as a function of inputs provided by various sensors.  
         [0054]    Also as shown in FIG. 6, the microprocessor  200  includes analog to digital circuitry (A/D)  206  for receiving analog signals from other parts of the optoelectronic transceiver  100  and converting the analog signals to digital values, which may be processed by the digital control logic  208 . The control logic  208  is configured to receive digital values from the A/D  206  as well as lookup tables, from the EEPROM  204  and from the host device  220  through the serial interface  202 . In addition, the control logic  208  is configured to write selected digital values to predefined memory locations in the EEPROM  204  and output digital values to host devices when polled through the serial interface circuitry  202 . Furthermore the control logic  208  is configured to determine the TEC Command signal using the methodology described above. In one embodiment, the control logic  208  is implemented by software instructions executable by the microprocessor  200 . In this embodiment, the methodology and/or mathematical formula used to determine the TEC Command signal can be updated and modified without having to replace the microprocessor  200 .  
         [0055]    Lastly, as illustrated in FIG. 6, digital to analog output circuitry (D/A)  210  is provided to receive digital values from the control logic  208  and convert them into analog signals to regulate other parts of the optoelectronic transceiver  100 .  
         [0056]    As described in detail above, the wavelength intervals of a channel spacing standard at 100 GHz is 0.8 nm. In order to operate at two channels, therefore, the transceiver  100  must be able to adjust the wavelength output by at least 0.8 nm. Similarly, to operate at three channels, the transceiver  100  must be able to adjust the wavelength output by at least 1.6 nm. The degree to which the wavelength output must be adjusted continues in this fashion for each additional channel. But as indicated above, the channel limit of a 100 GHz channel spacing standard is 45. The channel spacing, and thus the ability of the transceiver  100  to adjust the wavelength output varies proportionally with the channel spacing standard.  
         [0057]    In order to control the wavelength output of the transceiver  100 , the temperature of laser emitters is adjusted as described in detail below. And as indicated above, the relationship between the temperature of the laser emitter and the wavelength produced is typically around 0.1 nm/° C. This means that if the wavelength output of the transceiver  100  must be adjusted by, for example, is 0.8 nm, the laser emitter temperature must be adjusted by approximately 8° C. Similarly, if the wavelength output of the transceiver  100  must be adjusted by, for example, is 1.6 nm (to support 3 channel selectability), the laser emitter temperature must be adjusted by approximately 16° C.  
         [0058]    [0058]FIG. 7 is a flowchart for controlling the temperature of laser emitters using a microprocessor  200 . In step  702  control firmware and initial settings are downloaded from a host device, such as a computer, preferably through serial interface circuitry  202 . The control signals include data relating to laser aging and the effect of ambient temperatures on the wavelength of optical signals from a laser emitter, and they may be transmitted to the microprocessor  200  in the optoelectronic transceiver  100  during calibration of the optoelectronic transceiver, as described below. The control commands and signals are stored in the EEPROM  204  (FIG. 6) in step  704 . The receipt and storage of control commands and signals in steps  702  and  704  may be accomplished prior to operation of the laser emitter, or while the laser emitter is operating. During operation of the laser emitter, analog signals representing a variety of operating conditions of the laser emitter, including its temperature, the voltage corresponding to the laser bias current, and the ambient temperature surrounding the laser emitter, are generated and received by a microprocessor  200  (FIG. 6) in step  706 . The analog signals are converted to digital values in step  708  and stored in the EEPROM  204  of the microprocessor  200  in step  710 . Lastly, in step  712  the microprocessor  200  generates control signals for the temperature control mechanism, which preferably includes a TEC  114  and a TEC driver  116 , based on the control signals and digital values that have been stored in the EEPROM  204  of the microprocessor  200  during the preceding steps.  
         [0059]    [0059]FIG. 8 is a logical block diagram illustrating a system for setup and tuning of an optoelectronic assembly. As described above, in applications using Dense Wavelength Division Multiplexing (DWDM), laser emitters must be tuned to transmit optical signals having wavelengths that correspond to ITU channels. The spacing of the ITU channels for DWDM at 100 GHz is 0.8 nm±0.1 nm, at 200 GHz is 1.6 nm+0.2 nm, and at 50 GHz is 0.4 nm±0.05 nm. Laser diodes that are commercially available generally include specification data on the wavelength of optical signals the laser diodes emit while operating at room temperature. However, it is desirable to operate laser diodes used in optoelectronic assemblies above the ambient temperature.  
         [0060]    Generally, operating laser diodes above the ambient temperature allows TECs to function more efficiently. More specifically, TECs are more efficient when heating than cooling because the thermoelectric effect and resistive heating are working together when the TECs are heating the laser diodes, rather than opposing one another as is the case when the TECs are cooling the laser diodes. Efficiency is of particular importance in pluggable transceiver applications, where the available power, and thus the ability of TECs to function, is limited to specified levels. But operating laser diodes at high temperatures may shorten the useful life of the laser diodes.  
         [0061]    It is therefore preferable for many applications to tune a laser diode by adjusting the TEC Command signal so that the laser diode emits optical signals that fall within a desired ITU channel wavelength for a selected DWDM frequency when the operating temperature of the laser diode is as high as possible, but not more than 50° C.  
         [0062]    With reference to FIG. 8, an optoelectronic transceiver  100  is coupled to transmit optical signals to a wave meter  802 . The wave meter  802  measures the wavelength of the optical signals and provides the wavelength to a computer  804  with test software. The computer  804  sends signals to the optoelectronic transceiver  100  through a serial interface using the two wire serial interface standard to adjust the temperature of the laser emitter until the target wavelength for one or more ITU channels is reached as described in detail below.  
         [0063]    [0063]FIG. 9 includes a flowchart for calibrating an optoelectronic transceiver  100 . In preferred embodiments, the transceiver  100  is calibrated to operate within one or more ITU channels. In a first step, the computer  804  sets a target wavelength (step  901 ). This may be accomplished, for instance, by heating the optoelectronic transceiver to 50° C., decreasing the temperature until a first ITU channel is found, and then setting the target wavelength to a wavelength with that ITU channel. While the target wavelength lies within an ITU channel, it is not necessarily at the center of the channel. In some embodiments, the target wavelength is lower than the center wavelength to allow for red-shifting that occurs when lasers age as described in more detail below.  
         [0064]    And as indicated below, the process of calibrating the transceiver  100  to operate within, for example, two or more channels typically proceeds from the highest channel to the lowest channel. So when step  901  is executed for the first time (in the context of two or more channels), the target wavelength typically corresponds to the highest channel. The calibration process then steps through the channels sequentially as steps  901 - 918  are executed for each channel. If the laser is calibrated for operation in only one channel, steps  901 - 918  may be executed just one time.  
         [0065]    The computer  804  then commands the microprocessor  200  to set the temperature of the laser emitter in the laser assembly  112  (via the TEC Command signal) (step  902 ). The first time step  902  is executed, the temperature is preferably set to T setup  or 50° C., which is the preferred maximum operating temperature of the laser assembly  112 . During subsequent executions of step  902 , however, the temperature is set differently. As described below in connection with step  916 , a final temperature for a given channel is selected. When calibrating the next channel, the temperature set in step  902  is preferably this final temperature offset by a predefined amount. For example, if the channel spacing 0.8 nm, this predefined offset may be 7 or 8° C. (i.e., the temperature set in step  902  would be approximately 7 or 8° C. less than the final temperature for the previously calibrated channel).  
         [0066]    Additionally, the computer  804  may communicate with the microprocessor  200  through the serial interface  202 . The computer  804  may also set I laser bias  and the AC modulation to default values.  
         [0067]    The computer  804  then checks the wavelength of the optical signals via the wave meter  802  (step  904 ). If the measured wavelength is not approximately equal to the target wavelength (step  906 -No), the computer  804  adjusts the temperature of the laser emitter in the laser assembly  112  (step  908 ). Preferably, the measured wavelength is not approximately equal to the target wavelength until they are within 10 pm of each other. The direction of the adjustment depends upon whether the measured wavelength is greater than or less than the target wavelength. Preferably, the first adjustment is a reduction since the temperature must be less than or equal to T setup . Further, the adjustment of the temperature in step  908  represents a coarse adjustment such that it preferably corresponds to a 3-10 pm adjustment of the optical signal&#39;s wavelength (depending on the resolution of the D/A  210  and the configuration of the transceiver generally). The goal of steps  904 - 908  is to get the wavelengths to approximately match, not exactly match so the amount of the reduction in step  908  does not have to be very fine. The computer  904  then repeats steps  904 - 908  until the measured wavelength is approximately equal to the target wavelength.  
         [0068]    Once the measured wavelength is approximately equal to the target wavelength (step  906 -Yes), the computer  804  adjusts the DC bias and the AC modulation current to achieve the operational target values for laser power and extinction ratio (step  910 ). In some embodiments, this step may be accomplished using a digital communications analyzer or other external equipment to measure the laser power and extinction ratio while the DC bias and AC modulation current are adjusted. The precise operational target values may vary from one embodiment to the next. In an alternate embodiment, step  910  can be skipped during the calibration of channels other than the first channel, if the channels are sufficiently close that the DC bias and AC modulation levels for the first channel are also suitable for use with those other channels.  
         [0069]    The computer  804  then checks the wavelength of the optical signals via the wave meter  802  (step  912 ). If the measured wavelength is not equal to the target wavelength (step  914 -No), the computer  804  adjusts the temperature of the laser emitter in the laser assembly  112  (step  916 ). Typically, the measured wavelength is “equal” to the target wavelength once it is within 1-5 pm of the target wavelength. Again, the direction of the adjustment depends upon whether the measured wavelength is greater than or less than the target wavelength. Further, the amount of the temperature reduction in step  916  is preferably smaller than the amount of the reduction in step  908 . For example, the adjustment in step  916  may correspond to a 1-3 pm adjustment of the optical signal&#39;s wavelength (depending on the resolution of the D/A  210  and the configuration of the transceiver generally).  
         [0070]    When the measured wavelength is determined to be equal to (or within a predefined margin of) the target wavelength (step  914 -Yes), the computer  804  stores values corresponding to (or representing) the temperature of the laser emitter, the DC bias current I laser bias , and the AC modulation in a channel lookup table  1000  (FIG. 10) maintained by the EEPROM  204  of the microprocessor  200  (step  918 ). FIG. 10 illustrates an exemplary channel lookup table  1000 , which includes a channel designation and corresponding values for the TEC temperature of the laser emitter, the DC bias current I laser bias , and the AC modulation.  
         [0071]    If there is an additional channel for calibration (step  920 -Yes), steps  901 - 918  are executed for the additional channel. If not, the calibration process terminates.  
         [0072]    During the operation of the transceiver  100 , the microprocessor  200  may receive commands through the serial interface  202  to select one of the channels for which the calibration steps described above have been executed. In still other embodiments, a specific channel is selected just once, in which case the transceiver  100  is then semi-permanently configured to operate at the selected channel. In either case, the microprocessor  200  uses a channel identifier preferably included with the commands to look up corresponding temperature, the DC bias current I laser bias , and AC modulation values and configures the transceiver  100  accordingly.  
         [0073]    In some embodiments, a transceiver controller (not illustrated) is used to perform some of the functions otherwise performed by the microprocessor  200 . For example, a transceiver controller may be used to look up values in tables and outputting these values through one or more digital to analog converters. Accordingly, the lookup table  100  (or portions of the lookup table  100 ) may also be accessible to or stored by the transceiver controller so that it may output some control signals while the microprocessor  200  outputs other control signals.  
         [0074]    Attention now turns to an embodiment in which characteristics of lasers that change over time are compensated for in one or more ways.  
         [0075]    [0075]FIG. 11A includes a conceptual representation of a typical plot of the optical power level, P 1 , and I laser bias  of the laser diode in a transceiver  100  configured such that I laser bias  is constant over a period of time. P 1  is represented by a solid line and I laser bias  is represented by a dashed line. As shown, P 1  decreases in a weak exponential fashion. The end of life (“EOL”) of the laser diode, which may be twenty five years, is marked in the plot and corresponds to a decay in P 1  that is typically less than one half.  
         [0076]    [0076]FIG. 11B includes a conceptual representation of a typical plot of P 1  and I laser bias  of the laser diode in a transceiver  100  configured such that P 1  is constant over a period of time. Again, P 1  is represented by a solid line and I laser bias  is represented by a dashed line. As shown, P 1  remains constant until the EOL, at which point P 1  typically declines in a nearly linear fashion. I laser bias  approximately doubles over time to maintain P 1  constant until the EOL, at which point the laser diode can no longer increase I laser bias  to keep P 1  constant. As stated above, the temperature of the laser, T 1 , increases with I laser bias , which causes the wavelength of the laser to increase (i.e., a red shift). Further, since the temperature sensor of the TEC is typically remote from the laser emitter, the temperature of the laser emitter will increase, even when the temperature sensed by the temperature sensor of the TEC is well controlled. The present invention, in part, compensates for the change in the wavelength resulting from the change in laser emitter temperature due to increases in I laser bias .  
         [0077]    Additionally, persons skilled in the art know that the change in T 1  (ΔT 1 ) is equal to the change in P 1 (ΔP 1 ) times the thermal resistance of the laser diode, R 1  (i.e., that ΔT 1 =ΔP 1  * R 1 ) and that ΔP 1  is equal to the change in I laser bias  (ΔI laser bias ) times the change in laser voltage V laser  (i.e., ΔP 1 =ΔI laser bias  * ΔV laser ). Changes in laser voltage (ΔV laser ) caused by changes in laser current (ΔI laser bias ) are typically very small, where the series resistance of the laser is small, which is the case in preferred embodiments. Therefore, the effect on laser voltage is small and can be practically ignored. The result is that the equations above can be simplified as follows: ΔP 1  αΔI laser bias  and ΔT 1 αΔI laser bias  * R 1 . Thermal resistance is a constant for a given device, and therefore the change in laser temperature is roughly proportional to the change in laser current (ΔT 1 αΔI laser bias ).  
         [0078]    Since changes in laser wavelength are proportional to changes in laser temperature, in can be inferred from the above equation that the change in laser wavelength (Δλ) is also proportional to the change in bias current (ΔλαΔI laser bias ). Thus, in this first relationship, changes in I laser bias  are proportional to changes in the wavelength of the laser diode because the wavelength increases as I laser bias  increases. Another way to state this is that the wavelength of light output by a laser diode can be represented as linear function of I laser bias . It is noted that even if the relationship between wavelength and I laser bias  is not strictly linear, this relationship can be treated as being linear in the small region of operation that is relevant to operation of the laser.  
         [0079]    As stated above, if the laser emitter temperature is increased by 10° C., the wavelength increases by about 1 nm. The precise nature of this first relationship may be determined for a given laser diode or class of laser diodes by experimentation or calculation. A determination of this relationship is a way of characterizing the behavior of the laser diode.  
         [0080]    Another way of characterizing a laser diode is by reference to the laser diode&#39;s index of refraction. Laser diodes include a laser cavity that may be characterized by an index of refraction, n 1 , which may increases over time. Further, persons skilled in the art recognize that 1) the wavelength of the laser diode is inversely proportional to (i.e., an inverse function of) the index of refraction (i.e., λ α 1/n 1 ); 2) that n 1  is proportional to (i.e., is a linear function of) the current density of the laser diode, I d , times T 1  (i.e., n 1  α I d  * T 1 ); and 3) that I d  is equal to the area of the laser junction of the laser diode, A 1j  times I laser bias  (i.e., I d =A 1j  * I laser bias ). Combining these relationships as follows: Δλα1/(I d  * T 1 )Δλα1/(A 1j  * I laser bias  * T 1 )→Δλα1/I laser bias , where T 1  is a function of I laser bias  and A 1j  which may be fixed and repeatable for laser diodes or a class of laser diodes. In other words, the result of combining these relationships is that the changes in the wavelength of the laser diode are also inversely proportional to I laser bias . So in this second relationship, as n 1 , and thus I laser bias , increases, the wavelength of the laser diode decreases (i.e., a blue shift). The precise nature of this second relationship may be determined for a given laser diode by experimentation and/or calculation.  
         [0081]    A complicating factor is that the strength of the first and second relationships may independently vary over time and by laser diode type. Thus the extent to which the first and second relationships offset one another (e.g., the extent to which one of the relationships dominates) may vary over time and by laser diode type or class. But even this complicating factor may be quantified through experimentation and/or calculation.  
         [0082]    By quantifying these relationships, the laser controller can be configured to compensate for predicted wave length shifts, by making adjustments to the TEC temperature set point, such that the laser diode may be able operate within a narrow wavelength tolerance for extended times (e.g., 20 or 25 years, as is typically required for telecommunications applications).  
         [0083]    A result of quantifying the first and/or second relationship. may be a lookup table with entries for 1) time, temperature, and laser bias; 2) time and temperature; or 3) temperature and I laser bias . FIG. 12A illustrates an exemplary lookup table  1200  with columns for time, temperature, and I laser bias . The temporal entries may correspond to, for example, a counter value maintained within the microprocessor  200  that is updated periodically during the operation of the laser diode. Reference may be made to such a lookup table  120  by the microprocessor  200  during the operation of the laser diode to adjust the target temperature of the TEC  114  as needed. For example, the microprocessor  200  may be configured to reference this lookup table  120  periodically during the operation of the laser diode, locate a row with a temporal value that corresponds to the counter value, and adjust the target temperature of the TEC  114  by reference to the temperature value in this row. Alternatively, the microprocessor  200  may be configured to monitor the value of I laser bias  and reference this lookup table  120  periodically or whenever this value changes by a predetermined amount. The microprocessor  200  may locate a row with an I laser bias  value that corresponds to the measured value of I laser bias  and adjust the target temperature of the TEC  114  by reference to the temperature value in this row. Further, the microprocessor  200  may select a row that most closely matches a given temporal or I laser bias  value or interpolate a target temperature by reference to two or more rows in a lookup table  1210 ,  1200  that most closely matches a given temporal or I laser bias  value.  
         [0084]    In other embodiments, instead of using a lookup table, the microprocessor  200  implements wavelength compensation using a computation that is a function of a measured operating parameter of the laser diode, such as the bias current. For instance, the compensation can be a computed function of a ratio, Δλ/ΔI laser bias , determined either during calibration of the laser diode or determined for all laser diodes of a particular type. For instance, if it determined during calibration that wavelength change by 5 pm when bias current changes by 1 mA (Δλ/ΔI laser bias =5 pm/mA), then the microprocessor can be programmed to increase the TEC control (to increase the amount of cooling) by a predefined increment when the bias current increases by 1 mA, where each increment in the TEC control corresponds to a wavelength change of 5 pm.  
         [0085]    Another characterization of the laser diode may include a determination that the wavelength of the laser diode will shift higher or lower during a given period of time. Similarly, it may be determined that the wavelength of the laser diode will shift higher or lower by a specific amount during a given period of time. In either of these cases, it may not be known how much the shift will be at any specific time or value of I laser bias . In such cases, it may only be possible to shift the initial target wavelength by a certain amount to compensate for a wavelength drift with an expected direction or direction and magnitude.  
         [0086]    Still another result of such activity may be a characterization of the laser diode that allows for an initial adjustment of the target wavelength in combination with an ongoing adjustment of the target temperature of the TEC  114  by reference to temporal or I laser bias  values during the operation of the laser diode. The on going adjustment may be a single adjustment of the target temperature during the operational lifetime of the laser diode, or may comprise a sequence of such adjustments over the device&#39;s operational lifetime.  
         [0087]    Yet another result of quantifying these relationships may be the creation of a linear or non-linear equation or function that conforms to a time/temperature relationship or I laser bias /temperature relationship of a given laser diode. In this case, a time or I laser bias  value is input to a given equation or function to calculate a corresponding target temperature of the TEC  114 .  
         [0088]    And as noted above, a laser diode may be calibrated for operation within two or more channels. The characterization of a laser diode may, therefore, be extended to account for two or more channels. It is possible that the wavelength drift of a laser diode may be different for each channel. FIG. 12B illustrates an exemplary lookup table  1210  that is similar to the first lookup table  1200 , but extended to include data for a second channel.  
         [0089]    In a first embodiment of the present invention, the target wavelength for a given ITU channel is offset above or below the channel center  10  to account for expected wavelength drift over time. If a blue shift is expected, the target wave length may be set above the channel center  10 . Conversely, if a red shift is expected, the target wavelength may be set below the channel center  10 . This allows for a greater amount of wavelength drift than would be possible if the target wavelength were set to the channel center.  
         [0090]    For example, if by reference to the first and second relationships, it is determined that a given laser diode will experience a blue shift of 90 pm (i.e., a “90 pm blue shift”) during its useful life (or its defined operational lifetime, such as 20 or 25 years), the target wavelength is set to a wavelength higher than the channel center  10 . With respect to FIG. 3, it is stated above that 30 pm of tolerance is required for the initial setup and temperature control such that 70 pm of wavelength drift from the channel center  10  in either direction is possible without breaching a 200 pm channel. Without the use of the present invention, therefore, this particular laser diode could not previously be used to operate within a 200 pm channel (without the use of, for example, a wavelocker) since the expected 90 pm blue shift exceeds the 70 pm of allowable drift from the channel center  10 .  
         [0091]    To compensate, the target wavelength for this channel and this particular laser diode is set, for example, 40 pm above the channel center  10  (i.e., set to the channel center  10  wavelength plus a 40 pm offset). First, the 40 pm offset plus the 30 pm of required tolerance does not exceed the 100 pm of channel space above the channel center  10  (i.e., there is 30 pm of available channel space above the target wavelength even when allowing for the 30 pm of required tolerance). Second, by offsetting the expected 90 pm blue shift by the 40 pm offset, the wavelength of the exemplary laser diode will drift to approximately 50 pm below the channel center  10  wavelength during its useful life. Adding the 30 pm of required tolerance to this expected 50 pm shift below the channel center  10  produces a possible blue shift below the channel center  10  of up to 80 pm, which is within the 100 pm of channel space below the channel center  10  (i.e., there is 20 pm of available channel space). The offset of the target wavelength in this example is illustrated in FIG. 13A.  
         [0092]    [0092]FIG. 13B illustrates another example in which it is determined that a given laser diode will experience a red shift of 90 pm (i.e., a “90 pm red shift”) during its useful life. To compensate, the target wavelength is set below a channel center  10  wavelength. Again, it is stated above with respect to FIG. 3 that 30 pm of tolerance is required for the initial setup and temperature control such that 70 pm of wavelength drift from the channel center  10  in either direction is possible without breaching a 200 pm channel. Without the use of the present invention (or, for example, a wavelocker), therefore, this particular laser diode could not previously be used to operate within a 200 pm channel since the expected 90 pm red shift exceeds the 70 pm of allowable drift from the channel center  10 .  
         [0093]    To compensate, the target wavelength for this channel and this particular laser diode is set, for example, 50 pm below the channel center  10  (i.e., set to the channel center  10  wavelength minus a 50 pm offset). First, the 50 pm offset plus the 30 pm of required tolerance does not exceed the 100 pm of channel space below the channel center  10  (i.e., there is 20 pm of available channel space below the target wavelength even when allowing for the 30 pm of required tolerance). Second, by offsetting the expected 90 pm red shift by the 50 pm offset, the wavelength of the exemplary laser diode will drift to approximately 40 pm above the channel center  10  wavelength during its useful life. Adding the 30 pm of required tolerance to this expected 40 pm shift above the channel center  10  produces a possible red shift above the channel center  10  of up to 70 pm, which is within the 100 pm of channel space above the channel center  10  (i.e., there is 30 pm of available channel space).  
         [0094]    The embodiments of the present invention described above in connection with FIGS. 13A and 13B may be used to select a target wavelength as indicated. This target wavelength may then be used in connection with the processing steps described above in connection with FIG. 9 to calculate initial values for temperature and I laser bias . In a preferred embodiment, the target wavelength is offset from the channel center wavelength by a wavelength offset of at least 30 pm and less than 70 pm. In other embodiments, the target wavelength is offset from the channel center wavelength by a wavelength offset of at least 40 pm and less than 80 pm. In yet another embodiment the target wavelength is offset from the channel center wavelength by a wavelength offset of at least 50 pm. Further, in the preferred embodiment, the wavelength control tolerance (described above with reference to FIG. 3) is about 30 pm, but in other embodiments, the wavelength control tolerance may be as small as 10 pm or as large as 35 pm.  
         [0095]    Turning to another embodiment, FIG. 14A illustrates a temperature control flowchart  1400 . Briefly, this flowchart illustrates a process in which the microprocessor  200  polls I laser bias  to obtain an index into, for example, the wavelength drift lookup table  1200  and adjust target temperature of the TEC  114  accordingly. This embodiment—and other embodiments described below—may be used after executing the processing steps described above in connection with FIG. 9 to compensate for a predicted wavelength drift (e.g., to adjust the temperature value(s) stored in one or more executions of step  918 ).  
         [0096]    In a first step, the microprocessor  200  obtains the value of I laser bias  (step  1402 ). As described above, the laser driver supplies a voltage V(I laser bias ) that is proportional to I laser bias  to the microprocessor  200 . The microprocessor  200  may, therefore, use this voltage to calculate the current value of I laser bias . Alternatively, the wavelength drift lookup table  1200  is indexed by values of V(I laser bias ) instead of laser bias to eliminate the need for the microprocessor to calculate laser bias from V(I laser bias )  
         [0097]    The microprocessor  200  then scans a lookup table (e.g., wavelength drift lookup table  1200 ) to locate a corresponding entry (step  1404 ). As described above, the corresponding entry may be the entry with an exact match of the current value of I laser bias , the entry that includes the closest match of the current value of I laser bias , or a set of entries that enable the interpolation of a target temperature as described above.  
         [0098]    Once the microprocessor  200  locates a corresponding entry (or set of entries) and extracts or calculates a target temperature of the TEC  114  from that entry, the microprocessor  200  adjusts the target temperature of the TEC  114  (step  1406 ). This preferably includes the microprocessor  200  adjusting the value of the TEC Command signal such that the TEC  114  maintains the laser diode at the new target temperature until the target temperature of the TEC  114  is recalculated or otherwise reset.  
         [0099]    The microprocessor  200  eventually returns to step  1402  to repeat the process. In some embodiments, the microprocessor  200  continuously monitors I laser bias  and executes steps  1404  and  1406 , as described above, each time an accumulated change in the value of I laser bias  reaches a predefined percentage amount. For example, each time I laser bias  changes by 1% since the last execution of step  1406  (since the beginning of the laser diode&#39;s operation), the microprocessor  200  executes steps  1404  and  1406 . Alternately, steps  1404  and  1406  may be performed each time the laser diode is powered on, or each time the laser diode&#39;s internal cumulative operation counter increases by a predefined amount (e.g., a predefined number of hours of operation, such as or 512 hours, 1024 hours, or 50,000 hours of operation) which is preferably at least 500 hours and less than 100,000 hours.  
         [0100]    [0100]FIG. 14B illustrates a temperature control flowchart  1410  consistent with another embodiment of the present invention. Generally, this flowchart illustrates a process in which the microprocessor  200  recalculates the target temperature of the TEC  114  each time a predefined period of time passes.  
         [0101]    In a first step, the microprocessor  200  increments a counter maintained in, for example, the EEPROM device  204  (step  1412 ). This step is only executed after the passage of a predefined period of time (e.g., 512 hours of operation, or more generally, a value between 500 and 100,000 hours). The microprocessor  200  then scans a lookup table (e.g., wavelength drift lookup table  1200 ) to locate an entry corresponding to the value of the counter incremented in step  1412  (step  1414 ). Once the microprocessor  200  locates a corresponding entry and extracts a target temperature of the TEC  114  from that entry, the microprocessor  200  adjusts the target temperature of the TEC  114  (step  1416 ). Like step  1406  above, this step preferably includes the microprocessor  200  adjusting the value of the TEC Command signal such that the TEC  114  maintains the laser diode at the new target temperature until step  1416  is re-executed. The microprocessor  200  then returns to step  1412  after the passage of the predefined period of time.  
         [0102]    In other embodiments, the use of a target wavelength offset, as illustrated by FIGS. 12A and 12B, and periodic or intermittent adjustments of the target temperature of the TEC  114  are combined. This may be necessary, for example, if the microprocessor  200  and/or the TEC  114  cannot keep the wavelength of the laser diode within a given channel by means of target wavelength offset or the periodic or intermittent adjustments of the target temperature of the TEC  114  alone. In these embodiments, the target wavelength is set by reference to an estimated drift of the laser diode&#39;s wavelength (e.g., to set the target wavelength used in steps  901 - 918  described above) and then the target temperature of the TEC  114  is intermittently adjusted as described above.  
         [0103]    The foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Various modifications may occur to those skilled in the art having the benefit of this disclosure without departing from the inventive concepts described herein. Accordingly, it is the claims, not merely the foregoing illustration, that are intended to define the exclusive rights of the invention.