WO2001008331A1

WO2001008331A1 - Automatic optical wavelength control

Info

Publication number: WO2001008331A1
Application number: PCT/US2000/019982
Authority: WO
Inventors: Lee Zipin
Original assignee: Luxn, Inc.
Priority date: 1999-07-21
Filing date: 2000-07-20
Publication date: 2001-02-01

Abstract

An apparatus for optimizing the power received by a receiver at one location in a communications system, wherein the power is sent through a network by a laser transmitter remotely located from the receiver. The system includes a transmitter with a temperature control device (30) for varying the temperature of a laser source (22). A transmitter microcontroller (32) responds to a directive from a system controller (16) for setting the temperature and thereby the wavelength of the laser source. The receiver receives the laser signal transmitted through an optical communications network. The receiver bandpass filter (38) allows passage of a specific range of wavelengths. The output from the bandpass filter is detected and converted to a digital signal that is processed by a receiver microcontroller (44). The system controller outputs data to the transmitter microcontroller to set the temperature and thereby wavelength of the laser. The system controller determinesthe data by executing an algorithm that directs the transmitter microcontroller to cause the temperature controller to vary the laser temperature and wavelength.

Description

Specification

AUTOMATIC OPTICAL WAVELENGTH CONTROL

The application claims priority from U.S. Provisional Application Serial Number 60/144,190 filed July 21, 1999.

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to optical communications systems, and more particularly to a method and apparatus for controlling the transmitted laser wavelength using a feedback signal from a systems receiver to a transmitter portion of the system.

Description of the Prior Art Optical communications systems rely on semiconductor lasers to provide a stable carrier. The laser's output power, an optical signal, is modulated by the (analog or digital) signal that the communication system conveys. State-of-the-art laser technology provides devices that generate power at a specified wavelength (e.g., 1549.32 nanometer [n ]) for a specified operating temperature. In general, the actual wavelength varies with temperature, typically in the order of 0.1 nm per degree C. Therefore, to maintain a specified wavelength, the laser's operating temperature must be regulated. Lasers intended for operation at specific wavelengths are constructed as modules. A thermally coherent package includes a laser die, a thermistor, and a thermoelectric cooler (TEC), as illustrated in Figure 1. In the figure, the TEC is shown as a thermal mass, and is an integral part of a substrate for the laser die and the thermistor. The laser die, a piece of semiconductor crystal, dissipates a few tens or hundreds of milliwatts of power during normal operation, which is enough to heat the die substantially if no corrective action were taken. The thermistor's electrical resistance varies inversely to the substrate's temperature, following a manufacturer-specified curve. The thermistor is mounted close to the laser die within the package, so that the thermistor's resistance indicates the laser die's temperature. The TEC acts as a heat pump, moving heat from the laser die to the module package, to be dissipated in the system containing the package. The TEC's efficiency in cooling the laser die varies with applied TEC voltage. If the voltage polarity is reversed, the TEC will heat the die instead of cool it. Fig 2 shows a circuit designed as a simple temperature regulator that sets and maintains the laser's operating temperature. Figure 2 shows the interaction of the thermistor and the TEC (thermally connected with each other and with the laser die) using the operational amplifier (op amp) in a feedback configuration. Potentiometer Rl provides a voltage reference (as some fraction of the positive supply voltage, depending on the potentiometer's setting) to the op amp's negative input (N). This reference value is fixed for a given potentiometer setting, and is independent of other circuit conditions. The op amp outputs a voltage to the TEC that is proportional to the difference between its input voltages (P-N). The op amp's gain factor is very high, and typically in the order of 10⁶. In practical circuits such as the one illustrated, the op amp's output reaches its limits with only microvolts of difference between P and N. Thermistor TH and resistor R2 also divide the supply voltage according to the ratio of their resistance. The result is applied to the op amp's positive input (P). At higher temperatures, TH's resistance decreases and voltage P rises. The op amp drives the TEC such that positive drive causes the TEC to cool the thermistor (and the laser die). If the thermistor's temperature is such that the voltage P is above the reference voltage N, indicating that the module's temperature is too high, the op amp drives the TEC to cool the module and drive P to nearly equal N. If P were to fall below N, the op amp's output polarity would change, and the TEC would heat the module, again bringing P back to N. The system's equilibrium is established when the TEC is cooling a warm laser (or heating a cooled module, due perhaps to a cold operating environment) such that the thermistor's resistance works with R2 to create a voltage nearly equal to the reference, determined by Rl . The circuit shown in Figure 2 illustrates the basic operation of a local feedback system, but it does not account for thermal inertia, or for frequency and power limitations of the op amp. Practical circuits are more complex, and include frequency compensation to assure loop stability and limit overshoot, and a power stage to manage the high current required by the TEC. The circuit uses potentiometer Rl to set the reference voltage, which determines the laser module's equilibrium temperature. Another source for this reference voltage is the output of a digital-to-analog converter (D/A) set by a microcontroller (μC), as illustrated in Figure 3. The op amp still compares the voltage determined by the thermistor with the reference , and drives the TEC to maintain the corresponding module temperature. The output voltage of the D/A indicates the desired temperature. With this scheme, the microcontroller can change the laser's temperature, and corresponding wavelength, based on software control. Basic temperature control assumes that the laser's wavelength is correct and stable when the laser module's temperature is stable at a specified temperature. However, this mechanism does not monitor or measure the laser's wavelength. Instead, the module's temperature is used to indicate the wavelength. For a given temperature, the laser's wavelength will change as the laser ages and can change as other circuit conditions change (e.g., drive current). Further, communication systems require a match between the laser's wavelength and that of the optical filter at the receiver. The specifications of this filter also change with temperature (although it is generally less sensitive to temperature variations), and different receivers (different filters) will have different passband characteristics. The basic temperature controller described in references to Figs 1-3 does not account for these factors. Fig 4 shows a prior art system using a basic temperature control technique for wavelength monitoring and control at the laser (transmitter) end of the communications channel. This enhancement uses additional components and subsystems to evaluate and control the laser's wavelength directly, rather than by temperature alone (indirectly). The TC block, between the microcontroller and the laser module, encapsulates the op amp-based temperature control subsystem previously illustrated (Figure 3). The optical output from the laser feeds a splitter. As shown, 95% of the laser's optical power is transferred to the optical fiber (and on to the remote receiver). The splitter routes the remaining 5% to an optical bandpass filter (BP). This component passes optical energy best at the minimum loss point of its pass band and attenuates optical energy at other wavelengths, as the curve suggests. The filter has a center wavelength (minimum loss point) equal to the laser's nominal wavelength (and the desired wavelength for this optical channel). The output of the filter feeds a photodetector (PD) block, which provides a voltage proportional to the strength of the optical signal provided by the filter. This voltage is digitized with a analog-to-digital (A/D) block, so that the microcontroller can monitor the signal level. The system as illustrated shows a clear closed-loop feedback path. The microcontroller, executing a software algorithm, reads the level of the optical signal from the photodetector and sets the reference level of the temperature control subsystem, the algorithm varies the reference level to optimize the optical signal level, indicating that the laser's wavelength matches the optical filter's peak wavelength. With such a software algorithm, the system can set the proper wavelength for an arbitrarily chosen laser within a specified tolerance of the specified nominal wavelength. The system can also maintain the wavelength within the system as the laser die ages. This enhancement offers a significant performance gain over the basic temperature control technique. However, the enhancement is expensive. Each transmitter (laser module) needs the optical splitter, bandpass filter, and receive level blocks. The optical splitter diverts some link power to serve in this wavelength control function, reducing the optical power available to carry data to the remote receiver. Further, the method is (in a larger system sense) open loop, in that it offers control according to a local reference. It does not account for the characteristics of the receive filter at the remote end of the fiber link.

SUMMARY It is therefore an object of this present invention to provide an improved method and apparatus for controlling the wavelength of a laser transmitter. It is a further object of the present invention to provide a method and apparatus for controlling a laser transmitter wavelength that compensates for variations and changes in component characteristics. It is another object of the present invention to provide a method and apparatus for controlling the wavelength of a laser transmitter in communications systems that compensates for temperature changes at the laser and at a system receiver. It is an object of the present invention to provide a method and apparatus for controlling the wavelength of a laser transmitter in a communications system that dynamically compensates for different receiver filters when the transmitter connects to different receivers. Briefly, a preferred embodiment of the present invention includes an apparatus for optimizing the power received by a receiver at one location in a communications system, wherein the power is sent through a network by a laser transmitter remotely located from the receiver. A typical optical receiver has an optical bandpass filter and the function of the invention is to provide an automatic and on-going adjustment of the laser wavelength so as to place the wavelength at the minimum loss point of the system, approximately coinciding with the minimum loss point of the filter. The system includes a transmitter with a temperature control device for varying the temperature of a laser source. A transmitter microcontroller responds to a directive from a system controller for setting the temperature and thereby the wavelength of the laser source. The receiver, remotely located from the transmitter, receives the laser signal transmitted through an optical communications network. The receiver bandpass filter allows passage of a specific range of wavelengths. The laser signal output from the bandpass filter is detected and converted to a digital signal that is processed by a receiver microcontroller for use by a system controller. The system controller outputs directive data to the transmitter microcontroller for the purpose of directing the transmitter microcontroller to set the temperature and thereby wavelength of the laser. The directive data can be sent to the transmitter by any available communications method, such as a modem connection to a PSTN, etc. The system controller determines/influences the directive data by executing an automatic wavelength control algorithm that directs the transmitter microcontroller to cause the temperature controller to vary the laser temperature and thereby wavelength. The system controller then evaluates data indicating the received laser power. The data is passed to the system controller from the receiver microcontroller. The system controller continually adjusts the laser temperature to search for and set an optimum laser wavelength for achieving a maximum power detected at the receiver. An advantage of the present invention is that it provides a system for dynamically adjusting a laser transmitter wavelength for achieving optimum receiver detected power. A further advantage of the present invention is that it automatically adjusts a laser transmitter wavelength to compensate for component aging. A still further advantage of the present invention is that in providing wavelength adjustment, it allows the use of inexpensive laser sources of lesser wavelength accuracy. Another advantage of the present invention is that it dynamically compensates for different receive filters when a transmitter connects to different receivers.

IN THE DRAWING Fig. 1 is a diagram illustrating a prior art laser source with a thermoelectric cooler; Fig. 2 illustrates a laser temperature regulation circuit; Fig. 3 is a circuit diagram of a circuit for controlling a thermoelectric cooler to control a laser temperature; Fig. 4 is a block diagram of a prior art system for controlling the wavelength of a laser transmitter; Fig. 5 is a block diagram of the network feedback control system for optimizing received laser power according to the present invention; Fig. 6 is a graph illustrating the characteristic of an optical receiver band pass filter; Fig. 7 illustrates a network for use in the feedback loop of the system of the present invention; Fig. 8 is a flow chart illustrating the algorithm used to find an optimum laser wavelength; and Fig. 9 is a block diagram illustrating feedback to tune a receiver filter for optimizing detected laser signal power.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention is illustrated as a block diagram in Fig. 5 of the drawing. A system 10 includes transmitter apparatus 12, receiver apparatus 14 and a system controller 16. These elements of the invention function in the system 10, which also includes an optical communications network 18, and a feedback communications system 20. The system 20 may be included as part of the optical network 18, or may be a separate network system of any type for data transmission. The transmitter 12 includes a laser source 22 with an output 24 to the optical network 18, and a data input 26. The source 22 has a thermal connection 28 to a temperature control device 30 for cooling and heating the source to a desired temperature. A transmitter microcontroller 32 has a connection 34 to the temperature control device, and a connection 36 to the communications network 20 for receiving direct data from the system controller 16. The receiver 14 includes an optical bandpass filter 38, a photo detector 40, an analog- to-digital converter (A/D) 42, and a receiver microcontroller 44. In operation, the transmitter 12 is initially set to transmit a laser signal within the passband of the filter 38 as closely as initial data/knowledge will allow. The output 46 of the bandpass filter is fed to the photo detector 40 and converted to an analog electrical signal at 48, which provides input to A/D converter 42. The output at 50 of the A D converter is input to the receiver microcontroller 44 which stores the data indicative of the power level of the laser signal output from the bandpass filter. The system controller 16 receives data from the receiver microcontroller 44, indicated in Fig. 5 by line 52, and executes an automatic wavelength algorithm for transmission of directive data through the feedback system 20 to the transmitter microcontroller 32. The directive data includes instruction to change the temperature of the laser in a specified direction. After a pre-set time interval, the controller 16 again evaluates the laser power output from the bandpass filter 38. If the power is increased, another directive is sent instructing the transmitter microcontroller to change the temperature again in the same direction as initially directed. The power is again evaluated, and this process continues until the power is reduced. At this point the system controller instructs the transmitter microcontroller to again change the temperature, but this time in the reverse direction. This process continues, resulting in the temperature dithering closely around the temperature that results in the wavelength of the laser being closely set to a value that coincides with the minimum loss point of the system, which is generally near the minimum loss point of the bandpass filter 38. An optical network such as 18 has a very large bandwidth and is capable of transmission of large quantities of data. On the other hand, the data required to be sent from the system controller 16 to the receiver microcontroller 32 is relatively small and infrequent, and therefore the communications system 20 can be a relatively low bandwidth communication system. The system 20 can therefor be selected from various network systems known to those skilled in the art. for example, it can be a modem connection to a PSTN. Alternatively, it can include an optical transmitter located at the receiver 14 location and use a channel on the optical network 18. An optical receiver would then be required at the transmitter 12 location for reception of the directive data for the receiver microcontroller 32. Figure 6 illustrates the receive bandpass filter's response to input power (P_R* VS. the optical signal's wavelength, λ) within its passband. As shown, the filter significantly passes a range of wavelengths (from λι_ow to λ_nj). At a particular wavelength (λ_mi_d) the filter response is optimum (P_{RX, opt})- The goal of the AWC algorithm is to set the transmitting laser's temperature such that the optical signal level detected at the receiver is optimized. Fig. 7 illustrates a communications system 20 including modems 51 and 53 for connection to a PSTN network 55. The system controller 16 receives data from receiver microcontroller 44 and inputs it to the system 20 at 54. The system 20 outputs the data at 36 to the transmitter microcontroller 32. Alternatively, the system controller can be located near the transmitter 12, and in such an arrangement the positions of the controller 16 and system 20 in Fig 5 would be reversed. The system controller execution will now be described in further detail. In an actual system implementation, the AWC algorithm may reside on either the transmitter's microcontroller or on the receiver's, or on an additional computing subsystem (as represented by the System Controller 16). In other words, the system controller 16 can be located at the receiver end, and can alternatively also be incorporated within the receiver microcontroller. The system controller 16 could instead be located at the transmitter end of the system, and can alternatively be incorporated within the transmitter microcontroller 32. As a further alternative, it can be a separate controller 16 as indicated in Fig. 5. The distribution of computing resources is not important to proper algorithm execution, as long as the block executing the algorithm can set the reference temperature of the laser module at the transmitter, and can monitor the optical power level at the receiver. The AWC algorithm is implemented in a combination of software, communications and hardware input and output. The system controller software executes a wavelength dithering function that changes the laser's wavelength by varying its temperature. The software monitors the optical power lever at the receiver, and notes the changes in receive level as the laser's temperature changes. In this way, the software "learns" the characteristics of the receive bandpass filter's curve. It initially sets the laser's temperature that places the laser wavelength near the middle of the band pass filter curve to optimize received power level. However, the AWC algorithm is adaptive. The laser's temperature continues to (slowly) vary (dither) around the optimum setting, and the setting designated as "optimum" is continuously recalculated. The technique operates continuously to keep the laser tuned to the minimum loss point of the system, automatically compensating for environmental temperature changes, component aging, and other system variations. Explaining the operation now in more detail, when the receiver does not recognize a signal from the transmitter, the transmitter must set a default laser wavelength to initiate end- to-end operation. The system controller instructs the transmitter to set its laser temperature to an appropriate initial value. Depending on the implementation, this default value may be (a) one determined at the factory to provide the channel's nominal wavelength; (b) the last value used before end-to-end communication was lost (if any); or (c) a value calculated based on factory configuration, the most recent value used, and one offered by the receiver. Choice (a) is used if the transmitter is new or has had little use, and assumes that it is connecting to a receiver with nominal filter characteristics. Choices (b) and (c) allow for laser aging with associated wavelength drift; (b) is appropriate if the transmitter would be establishing communication with the same receiver, while (c) is appropriate for environments in which the transmitter may be connecting with a different receiver. The initial laser temperature setting allows the transmitter and receiver to establish communication. AWC then begins dithering the temperature to optimize the received signal strength. The system controller instructs the transmitter to change its laser temperature by, perhaps, one degree in a particular direction (warmer or cooler). It waits for the laser's temperature to reach equilibrium at the new setting, and then notes the reported signal strength from the receiver. Thermal equilibrium may be achieved in a few seconds, but the system may wait tens of seconds. AWC adapts to aging and other slow phenomena. It is not necessary to dither quickly. Further, slow adjustments prevent any disruption of the data signal. If the signal is stronger, the system controller notes the new optimum, and requests another temperature change in the same direction. If the signal has degraded, the result is noted and the temperature is changed in the opposite direction. During dithering, the system controller maintains the laser's temperature within a configured range to assure optimal laser performance and lifetime parameters. The algorithm may also consider and incorporate filter flatness characteristics. The optical filter 38 may have numerous local peaks and dips within the passband. These are generally insignificant to communications channel performance (a fraction of a decibel). With these filters, AWC will respond to significant changes in received optical power (perhaps 0.5 to 1 dB) before changing the direction of its temperature adaptation. This characteristic prevents the algorithm from myopically mistaking a local dip for the edge of the filter' s passband. Optical systems that employ add/drop connections may require that a laser signal traverse several filters before detection at the receiver. The filter 38 in such a case represents the total system filter/loss characteristic. The AWC algorithm is suited to signal paths incorporating two or more cascaded filters. As described, the algorithm provides the optimum laser wavelength for any optical path. Referring now to Fig. 8 of the drawing, a flow chart is displayed illustrating a dithering sequence with correction for temperature excursions outside an acceptable range of operation of the laser. Other algorithms, with and without correction for temperature extremes will be apparent to those skilled in the art and these are included in the spirit of the present invention. For example, instead of changing incremental direction when an out of range temperature excursion is encountered, the algorithm could reset the temperature to the initial value, or the algorithm could simply reverse course to the last acceptable temperature, etc. Upon start-up an initial temperature is set (block 58) as discussed in detail above. After waiting a set time period, or upon waiting until the rate of temperature change is below a pre- defined value, the power data at the receiver is recorded (block 60). The temperature is then incremented by a pre-set value in a first direction (block 62). The temperature is then checked whether it is or is not in the acceptable range of operation (block 64). If the temperature is acceptable 66, the power data is recorded (block 68). The power level data is compared with the last data recorded (block 70). If the new power data shows that the detected power has increased 72, the temperature is incremented again in the same direction (62), etc. If the power data indicates that the detected power has not increased (73), the direction of increment is changed (block 74) and the temperature is incremented in the new direction (62), etc. Referring to block 64, if the temperature is not in the acceptable range 76, the direction of incrementing is changed (block 74) and the temperature is incremented in the changed direction (block 62), etc. Various alternative algorithms will be apparent to those skilled in the art upon reading this disclosure, and they are included in the spirit of the present invention. For example, if the loop from block 64 to 74 to 62 to 64 is continually repeated, the algorithm could direct the temperature to be reset to the original value (block 58). The flowchart of Fig. 8 summarizes the operation of the AWC algorithm. The algorithm accounts for ripple within the filter passband and also specifies exception handling for two cases wherein: (1) the system controller loses supervisory contact with either the transmitter or the receiver; and (2) the receiver reports to the SC a loss of signal from the transmitter. These cases may indicate that the receiver module in a system has been pulled from its chassis, or otherwise deactivate; or that the transmitter is being redeployed to a different receiver via a new fiber route. Block 58 of Fig. 8 refers to algorithm initialization. The algorithm begins when the system (or parts of it) begin operation following the application of power (i.e., cold start). The algorithm also restarts here following a supervisory communication failure (due, perhaps, to an operator removing a transmitter or receiver module). (Note that a particular system implementation may be required to assure that the transmitter's laser is off, for safety reasons, until supervisory control is established and operational parameters are set. This description assumes that the transmitter disables the laser during exception conditions.) The initialization block 58 establishes supervisory contact between the SC and the transmitter and receiver. The laser is enabled after the supervisory paths are established. Following the initialization block 58, and when the receiver reports to the SC that it has lost the transmitter's laser signal (i.e., warm start), the algorithm (block 60) establishes default levels for the following algorithm variables: a. T (laser temperature): the initial operating temperature for the laser. As described earlier, T may be set according to a factory-set calibration value; the most recent value (if the system determines that the same transmitter and receiver were previously paired and operational, but the receiver lost the transmit laser signal due to a transient fault, e.g. a fiber break); or a value calculated from the default value, the most recent value, and one offered by the receiver. b. D (temperature delta): the amount to change the temperature during each execution of the dither loop. c. M (filter ripple margin): a measure of the peak-to-peak filter ripple power. If the receive signal changes by more than M, the change is due to the edge of the filter rather than the ripple within the filter's passband. d. P (peak receive power) is initialized to zero. This variable will store the highest power level reported by the receiver. During algorithm steady state, P is the optimum receive power and indicates the optimum laser temperature. The algorithm dither loop 62 comprises the remainder of the flowchart. The loop steps are described as follows: a. According to block 64, the laser temperature T is adjusted by a dither value D. The transmitter sets the laser temperature to this new value of T. The algorithm pauses appropriately to allow the laser to stabilize at the new temperature. (Initially this may take some time, as T may differ significantly from room temperature. During loop operation, when T changes by D, assumed to be a small amount, less time would be required.) b. According to block 66, the current power level from the receiver is requested. Either the receiver or the SC may average several readings over a short time to minimize the effects of receive noise and other perturbations and provide a more reliable power level indication. R is set to this value. c. Block 68 describes comparing R with P, where P equals the received power at the previous temperature "T" prior to incrementing the temperature by D as described for block 64. If the new level R is higher than P (70), then the system declares a new peak receive power P by copying the value of R to P (block 72). The loop is repeated; the dither value D is unchanged, so the next loop iteration begins by setting the laser's temperature further in the direction that just found this newest peak value. If R does not exceed P (74), the algorithm asks if the power is now less than P by the value of filter ripple margin M (block 76). If so (78), the laser's wavelength has changed more than the filter's passband ripple would allow, so the dither value D is reversed (block 80). The next iteration of the loop will change the laser's temperature in the opposite direction, closer to the value at which the value P was found. If R is between P, and (P - M) indicated by the "NO" responses 82, a higher peak power level P may still be discovered. In this case the algorithm leaves the dither value D unchanged and trys another loop iteration. At any point during the operation of the dither loop 62 the algorithm may return to cold start (58), if the SC loses supervisory communication with either transmitter or receiver; or to warm start (60), if the receiver reports loss of laser signal from the transmitter. The AWC algorithm described above works by moving the laser's temperature in small increments around an established optimal point of operation. The amount of deviation around the optimal point is controlled by the parameter M, based on the receive filter's passband ripple characteristics. This definition of the parameter "M" is according the preferred embodiment of the present invention. Other definitions or values for M are also included in the spirit of the present invention, for example if the filter has a smooth characteristic without ripple, or in further example if other system variations influence the system ripple value. The algorithm may exhibit better peak tracking behavior (at the expense of additional complexity) if the value of M is changed at different states of operation. In particular, when the algorithm determines that it has found a "global" peak receive value and the corresponding optimal laser temperature (using the initial value of M already defined), it may reduce the value of M. In this "peak tracking" mode the system would deviate less from the peak as it dithers. The operation still tracks the peak as it changes due to aging, etc., but maintains a tighter lock on it. The exception conditions (warm and cold starts) still bring the algorithm to the original mode of operation when the peak is lost. A further variation of this theme would specify that the system revert to the original mode periodically (e.g., once a day) to assure that the optimal point is still globally optimal. The AWC algorithm can be applied to systems that use lasers tuned by non-thermal means (e.g., electrical or electromechanical). The algorithm is unchanged in these systems, but the laser's temperature is maintained at a prescribed (constant) operational temperature (e.g., via a system such as that illustrated in Figure 2) while the laser's wavelength is changed via an electrical signal or other appropriate means. AWC also applies to systems that use a fixed-wavelength laser and a tunable receive filter. The goal is the same as described here: keep the transmit wavelength matched to the receive filter passband and adapt to aging and other system changes. The algorithm would be applied in the same way, but the receive filter's center wavelength would be altered (dithered) rather than the laser's wavelength. As described, AWC allows an optical transmitter to work with different receivers (i.e., different receive filters) and adapt to the characteristics of each. The optical components industry provides filters with center wavelengths specified according to specifications provided by the International Telecommunications Union (ITU). The particular channel specifications, referred to as the ITU grid, call for channels spaced 0.4 nm apart. With temperature control, a laser can be moved among adjacent ITU channels via a temperature change of about four degrees Celsius. Under normal conditions, AWC allows a transmitter using such a laser to adapt to receive filters that span as many as eight or more ITU channels. In communication systems using wavelength-division multiplexing (WDM) that put multiple laser signals of different wavelengths on the same fiber link, AWC allows a single transmitter (with a laser designed for operation on one ITU channel) to serve multiple channels. This reduces manufacturing costs and the costs to the communications customer associated with keeping spare transmitter modules, since the need for multiple module variants is reduced. Referring again to Fig. 5 of the drawing, the above mentioned alternative embodiment using electrical or mechanical tuning methods is simply illustrated by replacing the thermocooler 30 with an electrical or mechanical laser wavelength tuning apparatus 30. Such tuning apparatus is known to those skilled in the art, and a detailed description need not be given in order to reproduce the present invention. Fig. 9 illustrates a system arrangement as described briefly above wherein a receiver filter 84 is tuned by an optical filter tuning apparatus 86 to optimize the signal from an untuned laser source 88, as detected by a detector 90. The A D 92 converts the output from the detector 90, providing a digital power indication to a controller 94. The controller 94 provides an algorithm similar to that described in reference to Fig 8 for incrementing the filter 84 center frequency by directing the tuning apparatus 86. The network system 96 of Fig 9 is not needed for the basic algorithm performance, but is included in Fig 9 as an alternative embodiment, since some communication between the transmitter location microcontroller 98 and the system controller 94 is useful. For example, it may be beneficial for the receiver portion 100 to know when the transmitter portion 102 is going to start transmission, and on what wavelength. The communication data, again, may not be of large quantity, and can be sent through any type of network 96. Although the present invention has been described above in terms of a specific embodiment, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications that fall within the true spirit and scope of the invention. What is claimed is:

Claims

1. An automatic wavelength control system comprising: (a) a laser transmitter for transmission of a laser signal over an optical communications network, said transmitter including (i) a laser source; (ii) a wavelength control apparatus for adjusting a wavelength of said laser signal; (b) a laser receiver for receiving said laser signal including (i) a bandpass filter for selecting a range of laser wavelengths to be received; (ii) a detection apparatus for detecting a laser signal power level and for providing a corresponding digital power data indication of said power level; (c) a controller system including (i) a first microcontroller apparatus for recording said power data; (ii) a second microcontroller apparatus responsive to directive data received through a feedback communications network for directing said wavelength control apparatus for setting a wavelength of said laser signal; and (iii) a system controller apparatus responsive to said power data to execute an algorithm for dithering the wavelength of said laser source and outputting said directive data for transmission through said feedback network to said second microcontroller for adjusting a wavelength of said laser signal to optimize said power level.

2. A system as recited in Claim 1 wherein said feedback communications network includes a channel included in said optical communications network.

3. A system as recited in Claim 1 wherein said feedback communications network includes a channel included in a modem connected PSTN.

4. A system as recited in claim 1 wherein said wavelength control apparatus is configured for adjusting a temperature of said laser source, whereby said wavelength is adjusted by adjusting said temperature.

5. A system as recited in claim 1 wherein said wavelength control apparatus is configured for adjusting an electrical property relating to said laser source for adjusting said wavelength.

6. A system as recited in claim 1 wherein said wavelength control apparatus is configured for adjusting a mechanical property relating to said laser source for adjusting said wavelength.

7. A system as recited in claim 1 wherein said algorithm directs steps including

(a) setting said laser signal to an initial wavelength;

(b) recording an initial digital power data indication corresponding to said initial wavelength;

(c) incrementing said wavelength to an incremented wavelength;

(d) comparing said initial power data with a second power data indication of a power level corresponding to said incremented wavelength; and

(e) repeating steps (c) and (d) to find an optimum wavelength to receive a maximum signal power level.

8. An automatic system for maximizing a received laser signal comprising: (a) a laser transmitter for transmission of a laser signal over an optical communications network, said transmitter including a laser source; (b) a laser receiver for receiving said laser signal including (i) a bandpass filter for selecting a range of laser wavelengths to be received; (ii) a detection apparatus for detecting a laser signal power level and for providing a corresponding digital power data indication of said output; (iii) a filter tuning apparatus for adjusting said wavelength to be received; (c) a controller system including (i) a first microcontroller apparatus for recording said power data; (ii) a second microcontroller apparatus responsive to directive data received through a feedback communications network for directing said filter tuning apparatus to set said range of laser wavelengths to be received; and (iii) a system controller apparatus responsive to said power data to execute an algorithm for directing said tuning apparatus to dither said range of wavelengths to optimize said power level.

9. A system as recited in claim 8 wherein said algorithm directs steps including

(a) setting said range of wavelength to an initial range;

(b) recording an initial digital power data;

(c) incrementing said range to an incremented range;

(d) comparing said initial power data with a second power data subsequent to said incrementing; and

(e) repeating steps (c) and (d) to find an optimum range of wavelengths of said filter to receive a maximum signal power level.