Abstract:
An optical transmitter for coarse wavelength division multiplexed (CWDM) optical communication systems uses a conventional laser (e.g. laser diode) and in addition a heater element is provided thermally coupled to the laser. A thermal sensor and associated control circuit drive the heater so as to control the power consumed by the heater to assure that the laser&#39;s temperature is not lower than a predetermined minimum working temperature. When the sensed laser temperature is above this predetermined minimum temperature, the control circuit turns off the heater. The total operating range of the transmitter in terms of ambient temperature is thus extended beyond its inherent operating range by the maximum laser temperature rise created by the heater. This allows a CWDM optical transmitter with the heater and control circuitry to be used in outdoor applications where a wide ambient temperature range is required.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to communications and more specifically to optical communications and more specifically to laser transmitters used in coarse wavelength division multiplexed optical communications systems.  
       BACKGROUND  
       [0002]     Optical communications are well-known; this field typically involves transmitting light (optical) signals over optical fiber. A typical application is, for instance, a cable television system, but optical communications are also suitable for telephony and data communications. Optical communications typically use a technology called wavelength division multiplexing (WDM) wherein a number of separate optical links, each with its own optical wavelength, are multiplexed into one light stream transmitted on a single optical fiber. Such WDM systems utilize wavelength specific transmitters, multiplexers and (near the receiver) demultiplexers, the multiplexers and demultiplexers including wavelength specific optical filters. One form of WDM called dense wavelength division multiplexing (DWDM) involves transmitting signals of many tightly spaced wavelengths on the same fiber and allows use of optical amplifiers. DWDM is especially useful for long haul systems due to the possible use of optical amplification. DWDM transmitters have a typical bit rate of up to 10 gigabits per second. DWDM transmitters usually require the use of cooling for the laser in the optical transmitter. Typically the laser is thermally coupled to a thermoelectric cooler (TEC) which can actively heat and cool the laser. The TEC is typically located inside the laser&#39;s package. There is also present sophisticated control circuitry intended to maintain the laser temperature at a constant predetermined temperature such that its wavelength is not affected by changes in external (ambient) temperature. The required TEC components, the associated control circuitry, and their calibration substantially increase cost of the resulting DWDM transmitter.  
         [0003]     Therefore, the communications industry developed coarse wavelength division multiplexing (CWDM) transmitters which also allow use of multiple wavelength transmissions on the same optical fiber. CWDM is generally a lower cost alternative to DWDM and is especially useful for shorter haul (less than 80 kilometer) optical transport. Typically, due to a smaller possible number of wavelengths, CWDM systems have a much lower bit rate capacity. Additionally, the cost of the CWDM transmitter is substantially lower than that of a DWDM transmitter. CWDM transmitters also typically use significantly less electric power and thereby exhaust significantly less heat than do DWDM systems. The chief difference is that in a CWDM system, the wavelength separation between each wavelength transmitted on the single optical fiber is significantly greater than in a DWDM system, by approximately a factor of 12.5 to 50. Another way to characterize the difference between DWDM and CWDM is that DWDM typically has 0.4, 0.8, or 1.6 nanometer wavelength spacing between channels whereas CWDM has a 20 nanometer wavelength spacing between channels. Hence, while DWDM systems multiplex a larger number of individual wavelength channels onto one fiber by providing relatively small separations between each channel, CWDM systems have significantly greater interchannel spacing and carry fewer channels. The ITU (International Telecommunications Union) has defined standards for CWDM to allow operation over a limited laser temperature range. To define the inter-channel spacing of 20 nanometers (nm) in conjunction with currently available optical filters, the ITU allows a maximum pass band window of approximately 14 nm wavelength, to which the laser output wavelength must correspond. A laser&#39;s output wavelength at room temperature (25° C.) is dependent on its intrinsic wavelength accuracy which is normally accurate to approximately ±2 nm for high grade lasers and ±3 nm for lower grade lasers. In addition, the laser wavelength changes with temperature due to a well understood physical phenomena, resulting in the wavelength drifting about 0.1 nm for every 1° C. change in laser temperature. The directly modulated lasers used in both CWDM and DWDM systems are typically distributed feedback lasers of the well known type which are commercially available from a number of vendors. The direct modulation applies the information signal to be carried, which is for instance that of a television channel, to directly modulate the laser&#39;s optical signal (light beam).  
         [0004]     Present  FIG. 1  shows a CWDM optical communications system of the type well known in the field. It includes in this case just two optical transmitters  10  and  12  although typically more transmitters would be present in an actual system, there being one transmitter per channel (wavelength). Each optical transmitter  10  and  12  includes a laser outputting an optical signal. The conventional CWDM multiplexer/filter  14  includes a set of optical filters each of which is a pass band filter and passes one particular relatively narrow pass band, typically as described above having a 20 nm wavelength spacing between channels and each channel having a 14 nm bandwith. The multiplexer/filter  14  thus includes a number of corresponding optical filters of the type well known in the field and which are commercially available. A device  14  with a single such filter is also referred to as an optical add/drop multiplexer (OADM). The multiplexer/filter  14  is connected by a span of optical fiber  18 , typically up to 80 kilometers long, to, at the receiver end, CWDM demultiplexer/filter  22  which essentially contains the same type of filter components as the multiplexer/filter  14 . In this case the demultiplexer  22  separates (filters) the optical signal into two distinct wavelengths each of which is applied respectively to receivers  26 ,  28 . In this case transmitter  10  transmits a signal to be detected by receiver  26  and transmitter  12  transmits a signal to be detected by receiver  28 .  
         [0005]      FIG. 2  shows the 14 nm pass band typical of CWDM systems as defined by the ITU. As shown, the optical signal occupies the 14 nm pass band having both the minimum laser wavelength and a maximum laser wavelength with a nominal central laser wavelength. Typically the nominal laser wave length is in the range of 1270 to 1610 nm. As shown, the maximum laser wave length at 25° C. is separated by 6 nm wavelength from the minimum laser wavelength at 25° C., where 25° C. represents (nominal) room temperature. There is substantial optical filter attenuation both above and below the 14 nm pass band.  
         [0006]     Hence the 14 nm optical filter window shown in  FIG. 2  typically allows for a 100° C. range of operation for the above-mentioned type high grade lasers, and a 80° C. range of operation for low grade lasers. In both cases, that operating temperature range is sufficient for most indoor transmitter operation conditions, where the laser transmitter is located within a building. Hence, in such indoor applications, the required transmitter ambient temperature range is typically 0° to 50° C. which leads to a slightly wider laser temperature operating range of approximately 0° to 70° C. However, most outdoor applications, as is typical in cable television, require transmitter operation over a wider temperature range. This is because outdoor transmitters are exposed to extreme winter cold and extreme summer heat, especially when they are in the sun. This is especially a problem in North America with its wide temperature ranges. Note that in more temperate climates such as in Western Europe, a narrower outdoor operating temperature range is more common. However, in North America, a typical temperature operating range for a laser transmitter in an outdoor environment is approximately −40 to −85° C. This includes approximately 25° C. of heating caused by the laser operation itself plus heating due to the sun. Thus, presently outdoor transmitters using a CWDM laser can only be used in situations in which the expected temperature operating range is relatively narrow, such as Western Europe or Japan and hence are not suitable for North America.  
         [0007]     At the present time, it is not possible to reliably use CWDM transmitters in outdoor installations in places such as North America, Eastern Europe, or Russia having wide annual temperature extremes. Of course, it would be desirable to make CWDM technology available in such areas due to its relatively low cost.  
       SUMMARY  
       [0008]     In accordance with this disclosure, an optical (laser) transmitter suitable for use in a CWDM system has its effective operating temperature extended so as to make it suitable for use in outdoor environments having a very large temperature range such as for instance −40° to 85° C. This is done by relatively inexpensive modifications to a conventional CWDM transmitter and so the resulting transmitter is still substantially less expensive than a DWDM transmitter. This is done by heating the laser, using in one version a low cost heater mounted external to the conventional laser package. A heat sink is mounted to the laser package and an electrical power consuming device (heater) is also mounted to the heat sink. No electric (active) cooling need be provided. A thermal sensor is also mounted to the heat sink. A control circuit is electrically connected between the heater and the thermal sensor such that it controls the power consumed by the heater. This assures that the laser operating temperature is never lower than a predetermined minimum temperature. When the laser temperature is detected as being above this predetermined minimum temperature, the control circuit turns the heater off. Of course, when the laser temperature is below the predetermined minimum temperature, the control circuit turns on the heater and provides sufficient current thereto so as to achieve the predetermined minimum temperature. Hence, the total operating temperature range of the transmitter is extended beyond the inherent 80° C. or 100° C. range of respectively low grade or high grade lasers as mentioned above, due to the maximum laser temperature rise provided by the heater. This advantageously allows use of the transmitter in an outdoor environment over a greater temperature range, as extended by the amount of heating provided by the heater.  
         [0009]     The term “laser” here also refers to a laser diode. Such devices are commercially available in a conventional housing with a plurality of external electrical connectors (pins). The package is usually all or partly metal, and so is thermally conductive. While in one embodiment the heater is co-mounted to a heat sink (thermally conductive member) with the packaged laser, this is not limiting, and the heater may be located inside the laser package.  
         [0010]     Also provided in one version is in a “cold start” control circuit to make sure that the optical transmitter when first powered up rapidly achieves the predetermined minimum temperature while avoiding undesirable temperature fluctuations during laser steady state operation. This feature is used primarily when the optical transmitter is being serviced or adjusted and the laser is thereby powered down and must be re-started, or when a power failure has interrupted the operation of the transmitter. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a CWDM optical communication system both of the type known in the art and in which improvements in accordance with this disclosure may be present;  
         [0012]      FIG. 2  shows the 14 nm pass band of a typical CWDM optical signal;  
         [0013]      FIG. 3  shows a block diagram of an optical transmitter in accordance with this disclosure;  
         [0014]      FIG. 4  shows an optical pass band similar to that of  FIG. 2  but as extended in accordance with this disclosure.  
         [0015]      FIG. 5  shows detail of the control circuit of the  FIG. 3  optical transmitter. 
     
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 3  shows an optical transmitter  30  in accordance with this disclosure. This is intended to be used as a replacement for each of optical transmitters  10 ,  12  in a system such as that of  FIG. 1 . In most respects, optical (laser) transmitter  30  is a conventional CWDM transmitter as described above. Optical transmitter  30  may be part of an optical transceiver also including a conventional receiver section (not shown). The remainder of the system of  FIG. 1  when used with optical transmitter  30  of  FIG. 3  is conventional; no special components are needed at the receiver end. However, as mentioned below, there may be some associated changes in the design parameters of the multiplexer/filter  14  and demultiplexer/filter  22  of the  FIG. 1  system.  
         [0017]     The  FIG. 3  transmitter  30  includes a conventional distributed feedback (DFB) directly modulated laser  36  of the type well known in the field. Also provided conventionally is a power supply and other auxiliary circuitry (not shown) of the type standard in optical transmitters. Conventional laser  36  (in most cases, the packaged laser) is mounted on a heat sink  38  which is a thermally conductive structure. Heat sink  38  may be conventionally associated with a circuit board or similar mounting for carrying the conventional circuitry associated with a laser  36 . Also mounted on heat sink  38  is a suitable conventional thermal sensor  42 . This particular configuration is not the only one suitable; however, thermal sensor  42  is in suitable thermal contact with laser  36  so as to sense the operating temperature of laser  36 . Also thermally associated with laser  36  is a heater element  44 . As shown, heater  44  is mounted on the heat sink but again this particular configuration is only illustrative.  
         [0018]     Heater  44  is for instance a standard type resistance heater, or in another version a field effect transistor (FET) of the type normally referred to as a power transistor which sinks a relatively large amount of electric current and hence generates a significant amount of heat. An advantage of using a field affect transistor is that it is easily controlled by a gate current and hence the control circuitry associated therewith is relatively simple. In this case, the control circuit  50  is shown connected via a feedback path (conductor)  48  to the thermal sensor  42  and by a control path  52  to the heater  44 . As indicated, if the heater  44  includes a field effect transistor, path  52  carries a control (gate) voltage to control the field effect transistor in heater  44 . The FET also has a voltage source supply (not shown) coupled to its source/drain terminals. Hence in  FIG. 3 , the heater  44  is external to the package of laser  36 , and no active cooling function needs be provided.  
         [0019]     Control circuit  50  in one embodiment is an analog circuit of the type well known in the electrical engineering field for controlling a heater in response to a sensed temperature. In another embodiment, control circuit  50  is embodied in a suitably programmed microprocessor or a microcontroller and associated driver circuits.  
         [0020]     As explained above, in operation control circuit  50  operates to effectively extend the range of ambient operating temperature of laser  36 . The control circuit  50  is such that it controls the power consumed by the heater  44  to assure that the operating temperature of the laser is not lower than a predetermined minimum working temperature. When the laser temperature is sensed by sensor  42  is above the predetermined minimum temperature, control circuit  50  turns off heater  44 , that is does not supply any electric power thereto. Otherwise, heater  44  is sourced with suitable power (current) via a control signal on control line  52  so as to maintain the laser temperature to at least the predetermined working temperature.  
         [0021]     The resulting effect on the light beam output from laser  36  is shown in  FIG. 4  which corresponds to  FIG. 2 . As shown in the left hand portion of  FIG. 4  there is a heating zone, in this case over a temperature range of 45° C. during which the heater  44  is in operation. The heating zone is such that the wavelength of the light beam output by laser  36  is within the 14 nm pass band. Thus, the minimum output laser wavelength allowed is on the right hand portion of the heating zone. As seen from  FIG. 4 , either the fundamental (room temperature) laser wavelength, or the center wavelength of the optical filter associated with the laser, is shifted (compared to  FIG. 2 ) such that the nominal wavelength of the laser operating at 25° C., for instance, is not at the center of the optical filter pass band. For instance, the filter in the CWDM multiplexed/filter  14  outputs a pass band that is shifted in terms of wavelength compared to that of a conventional system such that the filter is suitable for a conventional laser  36  operating at temperatures in the range 5° to 85° C. As shown in  FIG. 4 , the control circuit  50  is set so that the laser temperature is always above 5° C. and hence a 45° C. temperature rise from the heater  44  ensures that the optical transmitter operates at as low as −40° C. ambient temperature. Hence there is typically a different pass band for the optical filter in the CWDM multiplexer/filter associated with the  FIG. 3  optical transmitter than with an unheated conventional CWDM optical transmitter. However, since these optical filters are commercially available with a large variety of pass bands, providing a slightly different pass band for the optical filter is easily accomplished technically. Note that transmitter  30  and multiplexer  14  in certain embodiments need not be separate devices but may be combined into one apparatus.  
         [0022]     An additional feature in one embodiment is provided in control circuit  50  as shown in greater detail in  FIG. 5 . The input line  48  carrying the signal from the thermal sensor  42  carries this signal to control circuit  50  whereas the output signal, which is the control signal for the heater  44 , is shown on line  52  similar to  FIG. 3 . The internal circuitry of control circuit  50  includes three elements, the first of which is a fast control loop  54 , the second of which is a slow control loop  56 , and the third of which is a current limiter circuit  60 . Each of these is conventional and in one embodiment they are embodied in a set of analog components, each of circuits  54 ,  56  and  60  including an operational amplifier with conventional associated resistors and capacitors. The values of the resistors and capacitors depend on the particular characteristics of the heater  44 , in terms of how much heat it needs to produce, which of course depends on the operating characteristics of laser  36  and on the desired operating temperature range.  
         [0023]     This particular control circuit  50  thereby has additional complexity, referred to above, which provides a solution to the “cold start” problem. This is a problem identified by the present inventors. This problem involves the time that elapses from the time the transmitter is turned on, that is powered up, at a cold temperature until the control circuit  50  can bring the temperature of the laser  36  up to the required predetermined minimum working temperature. This time is referred to here as a cold start duration. During the cold start duration, the transmitter  30  is not operational since it will not be transmitting an optical signal within the desired pass band of  FIG. 4 . The present inventors have recognized a trade-off between the maximum power consumed by the heater  44 , the thermal mass of the laser  36  and the heater  44 , and the cold start duration. The present inventors have also recognized that while a fast acting control process can shorten the cold start duration, this may also cause undesirable temperature fluctuations during later steady state operation. Of course, the transmitter of  FIG. 3  is only occasionally powered up; in normal operation, it is in its steady state operation mode. A cold start typically occurs when the system is first installed or after it has been shut down either due to power failure or for maintenance or adjustment.  
         [0024]     The stability of temperature during steady state operation has been identified as important. Hence the control circuit  50  of  FIG. 5  determines when rapid heating is required during cold start and when a steady state condition is reached such that the desired short cold start duration does not impede temperature stability during the steady state operation. Control circuit  50  (see  FIG. 5 ) includes two control loops, the fast acting control loop  54  and the slow acting control loop  56  each including an operational amplifier. Each loop  54 ,  56  receives as an input signal the same temperature sensing signal on line  48 . However, the values of associated resistors and capacitors of the fast loop  54  and slow loop  56  are different, so that the R,C values for the slow acting control loop are relatively much higher. The slow loop  56  is associated with the steady state mode operation of the device and sets the heater control signal on line  52  when the temperature sensor  42  indicates that the temperature is not too far from the desired temperature. In contrast, the fast loop  54  provides the heater control signal on line  52  during the cold start duration mode and causes a greater amount of current to be sourced to heater  44 , but only during the cold start duration.  
         [0025]     The  FIG. 5  control circuit is not required in all embodiments and a simpler control circuit (with only one operating mode) is used if the cold start duration is not considered to be a problem in any particular system. Also provided in  FIG. 5  is a conventional current limiter circuit  60  which conventionally includes an operational amplifier and several diodes to make sure that the amount of current supplied on line  52  does not exceed some predetermined maximum value.  
         [0026]     This disclosure is illustrative and not limiting; further modifications will be apparent to those skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims.