Abstract:
In a coarse wavelength division multiplexing (CWDM) optical transmission system, a distributed feedback (DFB) laser is tuned so that the peak reflection of the grating overlaps with the gain range of the DFB laser. The diffraction grating is tuned so that the peak is positioned on the long wavelength end of the gain spectrum at a selected temperature. The optical transmission system operates in an environment having a wide temperature range (i.e., about −40° C. to about 85° C.). Heat is applied to the laser and as the laser temperature increases, the gain range overtakes the grating peak. When the gain range and the grating peak overlap at increased laser temperature, laser output is improved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit under 35 U.S.C. §119 of U.S. provisional patent application No. 60/700,703 filed on Jul. 18, 2005, and PCT application number PCT/US2006/027534 filed on Jul. 18, 2006, the disclosures of which are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    In recent years there has been a rapid increase in demand for data communications capacity. Traditional users of data communications, including business and government computer networks, have formed an expanding market. In addition, new applications such as digital television, digital telephony and consumer use of computer networks have emerged and grown. Responding to and encouraging this growing demand, advances have been made in electronic and optical communications technologies. 
         [0003]    Optical communication devices and networks offer important advantages over other communication systems. Among these advantages are high-bandwidth, imperviousness to a electrical noise, and resistance of the transmission media to electrochemical corrosion. The basic elements of an optical communication system are a light source device that can be modulated to produce a modulated optical signal, a receiver device that responds to the modulated optical signal, and a transmission medium. The light source device can be, for example, a solid-state laser diode. 
         [0004]    Common wavelengths of operation for such a light source device are, for example, 850, 1300 or 1550 nm. A typical receiver device includes, for example, a PIN-type photodiode or an avalanche photodiode device (APD). Generally, the transmission medium is an optical waveguide, such as, for example, a glass optical fiber. 
         [0005]    The unprecedented capacity of optical fiber makes it an ideal medium for the transport of high-bandwidth signals. In order to recoup installation and maintenance costs, however, it is important to optimize installed capacity. In one approach to achieving increased transmission capacity, a fiber cable typically consists of a number of individual fibers. Four, 12, 24, 40 or more fibers are often included in an optical cable. Such an arrangement allows multiple signals to be transmitted in parallel, providing a system that is, in effect, spatially multiplexed. 
         [0006]    Effective data capacity can also be increased by time division multiplexing (TDM) and wavelength division multiplexing (WDM), both of which allow the transmission of multiple data signals over a single fiber. In TDM, portions of two or more signals are chronologically interleaved such that, a portion of a first signal is transmitted during a first time interval and a portion of a second signal is subsequently transmitted during a second time interval. By repeating this process to produce a series of signal portions in subsequent time intervals, transmission of the two or more signals can be achieved. 
         [0007]    In WDM, each optical signal is transmitted on a different optical wavelength. This is analogous to, for example, the free space transmission of multiple radio channels on different frequencies. It is well known to separate light into different bands of wavelength (or color) using an optical prism or a plurality of optical filters. Typically in WDM systems, data signals are carried on non-visible wavelengths. These signals are demultiplexed at a receiving device using optical filters. Although the optical signals used in a WDM system may not include visible light, it is common to refer to various carrier wavelengths as “colors.” Accordingly a number of different wavelengths, taken together, is referred to as “a set of colors.” A WDM “channel” is a signal running on a unique wavelength. Each WDM channel on a fiber is substantially independent of the other channels with regard to bit rates and data transmission protocol. Accordingly, a mixture of serial data interface Serial Digital Interface (SDI), High-Definition Serial Digital Interface (HD-SDI), Synchronous Digital Hierarchy/Synchronous Optical NETwork (SDH/SONET), Gigabit Ethernet and Fast Ethernet can easily be used to transport data signals on the same optical fiber using WDM. 
         [0008]    There are two types of WDM, dense WDM (DWDM) and coarse WDM (CWDM). In DWDM, incoming optical signals are assigned to specific wavelengths within a designated frequency band, the 1550 nm region. The wavelength spacings are, for example, 0.8 nm or 1.6 nm. The signals are then multiplexed onto one fiber. DWDM enables multiple video, audio, and data channels to be transmitted over one fiber while maintaining system performance and enhancing transport systems. 
         [0009]    DWDM is well-suited to demanding applications such as high-volume digital video. DWDM typically uses temperature-stabilized lasers in order to fix the center wavelength and narrowband filters, providing many densely spaced channels. Typical channel spacing for broadcast-class DWDM equipment is 100 GHz corresponding to a channel spacing of approximately 0.8 nm, thereby avoiding the need for wavelength lockers. The wavelengths used are specified in ITU-T Recommendation G.694.1. 
         [0010]    CWDM is a method of combining multiple optical signals in the 1550 nm band at a lower density than DWDM. The wavelength spacing in CWDM is typically 10 to 20 nm. The CWDM wavelengths are standardized in ITU-T Recommendation G.694.2. The Recommendation provides optical parameter values for physical layer interfaces of CWDM applications with up to 16 channels. CWDM typically has a lower cost per channel than DWDM. CWDM offers a cost-effective alternative where, for example, a few more channels in a short fiber span is desired. CWDM conventionally uses non-stabilized lasers in combination with broadband filters. CWDM transmitters generally have lower power consumption than DWDM transmitters. 
         [0011]    Lasers operate by oscillating a flux of photons within a lasing medium. Energy is added to the lasing medium to promote electrons out of their atomic ground states and into elevated electrons states. This addition of energy is referred to as “pumping” the laser. To be effective, this pumping must produce a higher density of electrons in the elevated electron state than in a lower electrons state; a condition referred to as “inversion” of the medium. When a first photon passes through an inverted lasing medium, there is a finite probability that it will initiate an electron state transition from a higher energy state to a lower energy state. This electron state transition produces a second photon traveling in the same direction and in phase with the first photon. 
         [0012]    The two parallel photons are referred to as “coherent photons” or “coherent light.” The tendency of the first photon to produce additional photons is referred to as “stimulated emission.” The result is an “amplification” or “gain” of the stimulating photon within the lasing medium. 
         [0013]    As coherent photons pass through the pumped lasing medium some photons are absorbed by the atoms of the medium. This absorption of photons counteracts the gain of the medium to reduce the overall photon flux. When the gain of the medium is high enough, in comparison to the absorption level of the medium, so that more photons are produced than absorbed, lasing (i.e., amplification of initial photons) begins. The condition at which lasing begins is referred to as a “lasing threshold.” 
         [0014]    The probability that a stimulating photon will stimulate an electron transition, and associated emission of a further coherent photon, is related to the distance that the particular photon travels through the lasing medium. In order to increase the effective length of travel of the stimulating photon, the stimulating photon is reflected back and forth (i.e., oscillated) within the lasing medium. 
         [0015]    One way of oscillating photons within a lasing medium is to provide planar reflective surfaces (mirrors) disposed in substantially parallel spaced relation to one another at opposite ends of the lasing medium. This arrangement is referred to as a Fabry-Perot resonator. The efficiency of the Fabry-Perot resonator is limited by the reflectivity of the reflective surfaces. In addition, Fabry Perot resonators produce multiple wavelengths corresponding to optical standing waves, or modes, determined by the geometry of the resonator. 
         [0016]    Typically, WDM systems use distributed feedback (DFB) laser diodes as a light source. DFB laser diodes are also referred to as single longitudinal-mode laser (SLML) diodes. In a distributed feedback laser, a series of interfaces between regions of differing refractive index provide multiple opportunities to reflect passing photons. As will be understood by one of ordinary skill in the art, variations in the refractive index of the lasing material can be achieved by corresponding variations in material composition and/or variations in the boundary geometry of the lasing medium. 
         [0017]    Light of a particular wavelength can be efficiently reflected when the distances between interfaces corresponds to a multiple of one quarter of the light&#39;s wavelength. As a result, DFB lasers can be arranged to efficiently generate and amplify light of a particular wavelength (i.e., a narrow band of wavelengths) while producing substantially no other wavelengths of light. This monochromatic light production is desirable in optical communications, because substantially monochromatic light is correspondingly free of chromatic dispersion when transmitted through an optical fiber. 
         [0018]    Heating of the lasing medium causes thermal expansion, with a corresponding increase in distance between refractive index interfaces. Conversely, cooling of the medium causes thermal contraction of the lasing medium and a reduction in the distance between refractive index interfaces. Because the wavelength of light produced by a DFB laser is related to the spacing between the refractive index interfaces, the laser can be tuned (i.e., its color controlled) by controlling the temperature of the lasing medium. 
         [0019]    The gain of the lasing medium is also related to the wavelength of light being amplified and the temperature of the lasing medium. As the temperature of the lasing medium changes, the wavelength of peak amplification also changes. In typical lasing media, gain decreases substantially monotonically with increasing temperature over an operational temperature range. 
         [0020]    The power in a DFB laser&#39;s main spectral peak, also referred to as the “main mode” determines the power produced by the laser. Peak amplitude generally ranges from 10 mW to 50 mW and can be more. Ideally, the main spectral peak contains all the power produced by the laser. In a non-ideal laser, the laser signal includes side peaks, also referred to as “side modes,” that contain some power. A measure used to describe the amount of power in the main mode versus the amount of power in the side modes is the side-mode suppression ratio (SMSR). A DFB laser&#39;s SMSR describes the amplitude difference between the main mode and the largest side mode in decibels. 
         [0021]    A typical SMSR value is greater than 30 dB, indicating that most of the power resides in the main mode. The more power residing in the main mode of the laser, generally the higher the SMSR value of the laser. Another measurement useful in the DFB laser is the mode offset which is the wavelength separation between the main mode and the largest side mode. This is typically 1 nm. 
       SUMMARY 
       [0022]    The inventors have discovered that it is advantageous to provide a laser device, including a lasing medium, a reflective portion and a heater device. The heater device is thermally coupled to the lasing medium and/or the reflective portion. The heater device is adapted to warm the lasing medium and/or the reflective portion when a temperature falls below a threshold value. A thermal gain characteristic of the lasing medium and/or a thermal characteristic of the reflective portion are selected to optimize operation of the laser device over a temperature range above the threshold value. Accordingly, in one embodiment of the present invention a lasing device is operated with temperature control over a first operational temperature interval and without temperature control over a second operational temperature interval. 
         [0023]    In one embodiment, the heater device is adapted to bring the lasing medium and/or the reflective portion only to a lower operational threshold of the laser device. This is unlike the temperature stabilized lasers used in DWDM systems, which are maintained within a narrow band of temperatures about an operational optimum temperature by a temperature control system. 
         [0024]    According to the present invention, it is anticipated that the laser device will operate over a relatively broad range of temperatures, as compared with a DWDM system, and the characteristics of the lasing medium and reflective portion respectively are optimized in anticipation of this change in thermal conditions. 
         [0025]    Although thermal variation in the light wavelengths produced by CWDM lasers is typically not a significant concern when such lasers operate at room temperature of 25° C. Such thermal variation does it come a problem when DFB lasers are required to operate over a very wide temperature range such as, for example, −40° C. to 85° C. 
         [0026]    Temperature changes affect DFB lasers, according to the invention, in three ways. First the material gain amplitude changes. The material gain decreases as temperature increases and increases when temperature decreases. Second, the material gain peak shifts towards the longer wavelengths as temperature increases with a temperature coefficient of about 1 nm/° C. Third, the grating reflection spectrum peaks shift towards longer wavelength as the temperature increases, with a temperature coefficient of about 0.1 nm/° C. 
         [0027]    The International Telecommunication Union (ITU) specification for CWDM systems specifies that wavelength drift not exceed 13 nm. Conventional DFB lasers can, under ideal conditions, come close to meeting ITU specifications. Ideal conditions, however, are not always possible. 
         [0028]    The problems of meeting ITU specification are solved by the present invention of a coarse wavelength division multiplexing (CWDM) optical transmission system, and a distributed feedback (DFB) laser that is tuned so that the peak reflection of the grating overlaps with the gain range of the DFB laser over a particular temperature range. The diffraction grating is tuned so that the reflectance spectrum peak is advantageously placed on the gain spectrum curve with regard to the gain at a specified temperature. When the peak reflection spectrum and the gain range spectrum coincide, laser output is maximized. The optical transmission system operates in an environment having a wide temperature range (i.e., −40° C. to 80° C.). At the low end of the temperature range, the grating peak and the gain range are separated. The grating peak and the gain range shift at different rates as temperature changes. As heat is applied and the laser temperature increases, the gain range overtakes the grating peak and the two spectra overlap. When the spectra overlap at increased laser temperature, laser output is improved over laser output at low temperatures. The optical transmission system of the present invention reduces the range of temperatures over which the laser operates and also reduces wavelength drift such that the system meets requirements of the CWDM standard as set out in ITU G.695. 
         [0029]    In a coarse wavelength division multiplexing (CWDM) optical transmission system, a distributed feedback (DFB) laser is tuned so that the peak reflection of the grating overlaps with the gain range of the DFB laser. Specifically, the diffraction grating is tuned so that the peak is advantageously placed on the wavelength spectrum with regard to the gain at a specified temperature. When the peak reflection spectrum and the gain range spectrum coincide, laser output is maximized. The optical transmission system operates in an environment having a wide temperature range (i.e., −40° C. to 80° C.). At the low end of the temperature range, the grating peak and the gain range are separated. 
         [0030]    The grating peak and the gain range shift at different rates as temperature changes. As heat is applied and the laser temperature increases, the gain range overtakes the grating peak and the two spectra overlap. When the spectra overlap, at increased laser temperature, laser output is improved over laser output at low temperatures. The optical transmission system of the present invention reduces the range of temperatures over which the laser operates and also reduces wavelength drift such that the system meets requirements of the CWDM standard as set out in ITU G.695. 
         [0031]    The present invention together with the above and other advantages may best be understood from the following detailed description of the embodiments of the invention illustrated in the drawings, wherein: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  shows, in graphical frequency domain representation, a portion of an exemplary optical spectrum used for wavelength division multiplexing (WDM); 
           [0033]      FIG. 2  shows, in cutaway perspective view, a portion of a distributed feedback laser device according to one embodiment of the invention; 
           [0034]      FIG. 3  shows a graphical representation of gain and reflectance curves for a laser device according to one embodiment of the invention; 
           [0035]      FIG. 4  shows a graphical representation of gain and reflectance curves in relation to temperature variation for a laser device according to one embodiment of the invention; 
           [0036]      FIG. 5  shows a graphical representation of gain and reflectance curves for a laser device according to one embodiment of the invention; 
           [0037]      FIG. 6  shows a graphical representation of gain and reflectance curves for a laser device according to one embodiment of the invention; 
           [0038]      FIG. 7  shows a graphical representation of gain and reflectance curves for a laser device according to one embodiment of the invention; 
           [0039]      FIG. 8  shows, in block diagram form, a laser device according to one embodiment of the invention; 
           [0040]      FIG. 9  shows, in block diagram form, a laser device according to one embodiment of the invention; 
           [0041]      FIG. 10  shows, in block diagram form, a laser device according to one embodiment of the invention; 
           [0042]      FIG. 11  shows, in block diagram form, a laser device according to one embodiment of the invention; 
           [0043]      FIG. 12  shows, in block diagram form, a laser device according to one embodiment of the invention; 
           [0044]      FIG. 13  shows, in flow diagram form, a method of operating a laser device according to one embodiment of the invention; and 
           [0045]      FIG. 14  shows, in block diagram form, and optical communication system including a laser device according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]      FIG. 1  shows, in graphical frequency domain form, CWDM and DWDM channels distributed across a wavelength spectrum. The difference in bandwidth usage between CWDM and DWDM is readily apparent. In the exemplary system of  FIG. 1 , the CWDM channels are spaced 20 nm apart to accommodate a drift of laser wavelengths. A maximum of 13 nm from peak to peak is desirable. CWDM in this example fits a maximum of eight channels in the wavelength range from 1470 nm to 1510 nm. 
         [0047]    In DWDM, the channels are spaced approximately 0.8 nm apart. Temperature stabilization of the laser enables DWDM channels to be grouped closer together than in CWDM channels. For clarity of presentation,  FIG. 1  shows approximately only one third of the DWDM channels possible in the wavelength range between approximately 1525 nm to 1630 nm. 
         [0048]      FIG. 2  shows, in cutaway perspective view, a portion of a dual heterojunction laser device  100  according to one embodiment of the invention. The device  100  includes a substrate region  102 . According to various embodiments of the invention, the substrate region may be formed of any appropriate material including, for example, a semiconductor material such as gallium aluminum arsenide or indium gallium arsenide phosphide. The substrate region  102  includes a first lower surface  103 . 
         [0049]    According to one embodiment of the invention, a first junction  104  is disposed within the substrate region  102 . The first junction  104  defines an interface between a first region  106  of substrate  102  having a first atomic doping characteristic and a second region  108  having a second atomic doping characteristic. According to one embodiment of the invention, the first junction  104  is disposed in a substantially planar arrangement. Also according to one embodiment of the invention, the first junction  104  is disposed in substantially planar spaced relation to the first surface  103 . 
         [0050]    A second junction  110  is disposed within the substrate  102  in substantially planar spaced relation with respect to the first junction  104 . The second junction  110  is disposed between the second region  108  and a third region  112  of substrate  102 . Like the first junction  104 , the second junction  110  defines an interface between regions of different atomic doping characteristics. One of ordinary skill in the art will understand that the first and second junctions may be formed by, for example thermal diffusion and/or ion implantation. In addition, a doping profile may be selected for each junction according to the requirements of a particular application, and implemented according to the routine knowledge of one of skill in the art. 
         [0051]    As shown in the illustrated embodiment, a further interface  114  of the substrate  102  defines a corrugated curve interface disposed in close proximity to, and generally parallel to, the second junction  110 . According to one embodiment, this further corrugated curve interface  114  corresponds to an upper surface of the substrate  102 . A further region  116  is disposed adjacent to the corrugated curve interface  114 . According to one embodiment of the invention, the corrugated curve interface  114  includes a substantially periodic region so as to define length of periodicity (or wavelength)  132  of the corrugation. 
         [0052]    According to various embodiments, this region  116  includes a further portion of the substrate  102 . In one alternative embodiment, the region  116  includes an epitxial layer disposed adjacent to interface  114 . According to one embodiment, region  116  includes an upper surface  118 . Also, in one embodiment, upper surface  118  includes a substantially planar region disposed in substantially parallel space relation with respect to surface  103 . 
         [0053]    In one embodiment, the substrate material  102  includes a first end surface  120  and a second and surface  122 . The first end surface  120  is disposed in substantially parallel space relation to second end surface  122 , and both first and second and surfaces  120 ,  122  are disposed substantially perpendicular to junctions  104  and  110 . According to one embodiment of the invention, a corresponding layer  124 ,  126  of material is disposed adjacent to surfaces  120 ,  122 . 
         [0054]    In various embodiments of the invention, surfaces  103  and  118  are switchingly coupled to respective sources of electrical potential so that an electrical current  128  can be caused to flow between surfaces  118  and  103  through the substrate material  102 . Accordingly, various electrical terminals and or electrical connectors, such as, for example, metallic and/or semiconductor traces may be electrically coupled to surfaces  118  and  103  respectively. 
         [0055]    In operation, region  108  of laser device  100  forms an active region for light emission and amplification. Electrical current  128  serves to pump a portion of the atoms of region  108  and place those atoms into an inversion state, whereupon those atoms are subject to stimulated emission of photons by interaction with other for fans passing through the active region. The proximity of corrugated surface  114  to junction  110  causes a variation in optical index of refraction of region  108  periodically along a longitudinal axis  130 . According to one embodiment of the invention, this variation in optical index of refraction varies periodically with a length of periodicity corresponding to the length of periodicity  132  of the corrugated interface  114 . This periodic variation in index of refraction effects a reflection of photons of one or more selected frequencies along longitudinal axis  130 . Repeated reflections (i.e., oscillation) of these photons provides an extended effective length of photon travel within region  108  so as to allow efficient stimulation of additional photon production, and amplification of light within region  108 . 
         [0056]    As would be understood by one of ordinary skill in the art, the gain characteristic of region  108  depends on a variety of factors including chemical and crystallographic characteristics of the material of substrate  102  within region  108 . In addition, the gain of region  108  varies as a function of temperature and as a function of a wavelength of light passing through the region. In like fashion, as discussed above, optical reflectivity within region  108  varies as a function of the light passing through the region. 
         [0057]      FIG. 3  shows, in graphical form, a representation  200  of optical gain and reflectivity within region  108 . Curve  202  shows optical gain as a function of light wavelength. In various embodiment of the invention, curve  202  may take a variety of forms including the form of a normal bell curve. A maximum  204  of curve  202  corresponds to a light wavelength lambda zero of maximum gain. 
         [0058]    Reflectance as a function of optical wavelength within region  108  is shown by a grating reflectance curve  206 . The grating reflectance curve  206  varies as a function of optical wavelength and exhibits at least one local maximum  208 . The stimulation of photons within the active region  208  proceeds as a function of gain and reflectance. Accordingly, where the gain curve  202  and reflectance curve  206  coincide to produce a maximum stimulation value  210  the production of stimulated photons is at a maximum. When this production of stimulated photons exceeds a lasing threshold, the device  100  begin producing laser light. 
         [0059]      FIG. 4  shows, in graphical form, an optical gain curve  305  showing variation in optical gain of region  108  with respect to a variation in temperature of region  108 . This variation is indicated by broken line  300 . Also indicated is a relative variation in the relative distance between gain and reflectivity response peaks as a function of temperature. 
         [0060]    As shown, according to one embodiment of the invention, a first value  304  of gain  302  is relatively high at a temperature of about −40° C. Optical gain  300  decreases substantially monotonically to a second lower value  324  at a temperature of about 85° C. At a temperature of about 0° C., optical gain  300  has an intermediate value  326 . One of skill in the art will appreciate that the variation of gain  300  with temperature may be affected by a variety of factors that may be controlled by the designer. Accordingly, in various embodiments, curve  300  may be nonlinear and may be nonmonotonic. 
         [0061]    In the illustrated embodiment, the peak value of reflectance curve  306  is shown as a function of temperature by broken line  303 . In the illustrated embodiment, the reflectance is shown to be substantially linear and invariant with temperature. One of skill in the art will appreciate, however, that the reflectance  303  may exhibit other characteristics including linear and nonlinear variation with temperature. 
         [0062]    Also shown in  FIG. 4  is a broken line  328 . Line  328  indicates a lasing threshold for a laser device according to an exemplary embodiment of the invention. As illustrated, the lasing threshold  328  is substantially linear and invariant with temperature. One of skill in the art will appreciate, however, that a variety of other lasing characteristics are possible within the scope of the present invention. The lasing characteristic line  328  indicates the threshold at which lasing will occur when the gain characteristic and reflectance characteristic of region  108  are each sufficiently high for a particular optical wavelength. 
         [0063]      FIG. 5  shows a further illustration of a gain curve  305  and a reflectance curve  306 , both shown with respect to wavelength, according to one embodiment of the invention. The curves of  FIG. 4  are shown at a temperature of, for example, about −40° C. As illustrated, a maximum  402  of gain curve  305  occurs at a first wavelength  404 . A maximum  406  of reflectance curve  306  occurs at a substantially different second wavelength  408 . Because of a scalar difference  410  between these maxima  402 ,  404  a region of overlap  412  between the gain curve  305  and the reflectance curve  306  has a relatively small area, and a maximum of this coincident region is substantially below the lasing threshold  328 . Accordingly, for an active region  108  (as shown in  FIG. 1 ) having the characteristic curves shown in  FIG. 4 , lasing cannot be effectively and reliably achieved at about 40° C. 
         [0064]      FIG. 6  shows another illustration of a gain curve  305  and a reflectance curve  306 , both shown as a function of wavelength, according to one embodiment of the invention. The curves of  FIG. 6  are shown at a temperature of, for example, about 0° C. At this temperature, the maximum  502  of gain curve  305  occurs at a third wavelength  504 . The maximum  506  of reflectance curve  306  occurs at a fourth wavelength  508 . Wavelength  504  is greater than wavelength  404 , and wavelength  508  is greater than wavelength  408 . Because gain varies more strongly as a function of temperature than reflectance, however, scalar difference  510  is smaller than scalar difference  410 . The result is an increased overlap  512  between curves  305  and  306  at a temperature of about 0° C. as compared with the corresponding overlap  412  at a temperature of about −40° C., and a coincidence of curves  305  and  306  at approximately the lasing threshold  328 . It should be noted that the increased overlap  512  and elevated coincidence of curves  305  and  306  occurs despite the fact that an absolute value of the gain maximum  502  (at about 0° C.) is less than the absolute gain maximum  402  (at about −40° C.). 
         [0065]      FIG. 7  shows still another illustration of gain curve  305  and reflectance curve  306  as a function of wavelength according to one embodiment of the invention. The curves of  FIG. 7  are shown at a temperature of, for example, about 85° C. At this temperature, the maximum  602  of gain curve  305  occurs at a fifth wavelength  604 . The maximum  606  of reflectance curve  306  occurs at a sixth wavelength  608 . It should be noted that wavelengths  604  and  608  are higher than wavelengths  504  and  508  respectively. As is apparent from  FIG. 7 , wavelengths  604  and  606  are, in this exemplary embodiment, substantially equal to one another at a temperature of about 85° C. Accordingly, a scalar difference  610  between wavelengths  604  and  608  is approximately 0. The result is that gain curve  305  and reflectance curve  306  overlap with an area  612 , and coincide at a value substantially equal to the lasing threshold  328 . This coincidence occurs despite the fact that maximum the  602  of gain curve  305  is again substantially lower than the maximum  502  of the gain curve, as exhibited at a temperature of approximately 0° C. 
         [0066]    Referring again to  FIG. 4 , one sees that by proper selection of device characteristics, one of skill in the art can prepare a laser device adapted to produce light above the lasing threshold  328  between a temperature about 0° C. and about 85° C., for example. This is achieved by maintaining proper overlap of the gain curve  305  and reflectance curve  306  to produce a lasing result  350  within the indicated temperature range. Having recognized that proper selection of parameters could produce this result, the inventors have further discovered that improved lasing can be achieved by using a temperature control device to broaden the operative range of a laser device. 
         [0067]      FIG. 8  shows, in block diagram form, a laser system  700  including a temperature control device according to one embodiment of the invention. The laser system  700  includes a laser device  702 . According to one embodiment, the laser device includes a DFW laser device similar, for example, to that shown in  FIG. 1 . A temperature sensor device  704  is thermally coupled to the laser device  702 . According to one embodiment of the invention, the temperature sensor device  704  is disposed directly adjacent to the laser device  702 . In another embodiment of the invention, a thermally conductive medium such as, for example, a thermally conductive grease is disposed between the laser device  702  in the temperature sensor device  704 . 
         [0068]    The temperature sensor device  704  is signalingly coupled to a control device  706  by way of a signaling medium  708 . According to one embodiment of the invention, the signaling medium  708  is an electrically conductive medium, such as, for example, a metallic trace. According to one embodiment of the invention, the signaling medium  708  is electrically coupled to the temperature sensor device  704  and to a signal input of the temperature control device  706 . 
         [0069]    According to one embodiment of the invention, the temperature control device  706  includes a control output  710  that is coupled to a corresponding input of a heater device  712  by means of, for example, an electrical conductor  714 . The temperature control device  706  also includes a power input  716 . The power input  716  is electrically coupled through a power conductor  718  to a source of heater power  720 . 
         [0070]    According to one embodiment of the invention, the heater device  712  is thermally coupled to the laser device  702 . In one embodiment of the invention, the heater device  712  is integrally formed with the heater device  702 . In another embodiment of the invention, the heater device  712  includes a discrete heater device that is disposed adjacent to the laser device  702 . 
         [0071]    In operation, the temperature sensor device  704  detects a temperature of the laser device  702 . Responsive to this detected temperature, the temperature sensor device  704  dispatches a signal over the signaling medium  708  to the temperature control device  706 . The temperature control device  706  receives the signal and, depending on a state of the signal, controls a state of output  710 . In a first state, output  710  transfers power, such as electrical power received from power supply  720 , to heater device  712  thereby activating, or leaving active, heater device  712 . In a second state, output  710  does not transfer power to heater device  712 , thereby deactivating, or leaving inactive, heater device  712 . 
         [0072]    According to one embodiment of the invention, the control device  706  activates the heater device  712  when a temperature detected by the temperature sensor device  704  is equal to or less than a threshold temperature. According to one embodiment of the invention, the threshold temperature is approximately 0° C. In one embodiment of the invention, activation of the heater device is adapted to raise a temperature of the laser device  702  from a first temperature of about −40° C. to a second temperature of about 0° C. 
         [0073]      FIG. 9  shows, in block diagram form, a further embodiment of the invention including a laser device  752  disposed within a case  753 . According to one embodiment of the invention, a heater device  762  is thermally coupled to the laser device  752 . According to one embodiment, a control device  756  is disposed within the case  753  and is operatively coupled to the heating device  762  to control a heating state of the heating device. In one aspect of the illustrated embodiment, a temperature sensor device  754  is thermally coupled to the case  753  to detect a temperature of the case. 
         [0074]    According to one embodiment of the invention, the temperature sensor device  754  is operatively coupled to the temperature control device  756  to control an operative state of the temperature control device  756 . In one embodiment, a power supply  770  is disposed within the case  753 . In another embodiment of the invention, the case  753  includes an integrated circuit device, and both the laser device  752  and the temperature sensor device  762  are mutually formed on a common integrated circuit device substrate. In still another embodiment of the invention, the laser device  752 , the heater device  762 , the temperature control device  756 , and the temperature sensor device  754  are all usually formed on a common integrated circuit device substrate. In still another embodiment of the invention, the temperature sensor device  754 , and the control device  756  are mutually formed on a common substrate of an integrated circuit device which is operatively coupled to a discretely formed laser device  752 . 
         [0075]      FIG. 10  shows another embodiment of the invention  800  in which a plurality of laser devices  802  are mutually formed on a common integrated circuit substrate  801 . According to the illustrated embodiment, a respective plurality of heater devices  812  are also mutually formed on the common integrated circuit substrate  801 , along with a single sensor device  804  and a single temperature control device  806 . According to one embodiment of the invention, each heater device of the plurality of heater devices  812  is maintained in a common operative state by single temperature control device  806 . The operative state of the single temperature control device is, for example, controlled by the single sensor device  804 . The single sensor device  804  is adapted to sense a temperature of, for example, the common integrated circuit substrate  801 . 
         [0076]    According to one aspect of the invention, when a temperature of the substrate  801  falls below a threshold temperature such as, for example, about 0° C., every heater of the plurality of teachers  812  is activated. In one embodiment of the invention, power for the plurality of heaters  812  is applied by an external power supply  814 , 
         [0077]      FIG. 11  shows still another embodiment of the invention. According to the arrangement  850  of  FIG. 11 , a plurality of laser devices  852  is disposed on a common substrate  851  such as, for example, an integrated circuit device substrate. Also mutually disposed on the common substrate is a temperature sensor device  854  and a temperature control device  856 , as well as a common heater device  852 . The common heater device  852  is thermally coupled to two or more of the plurality of laser devices  852 . 
         [0078]    When activated, the common heater device  862  is adapted to elevate a temperature of the two or more of the plurality of laser devices  852 . According to one embodiment of the invention, the common heater device  862  is thermally coupled to the two or more laser devices  852  by conductive heating through, for example, the common substrate  851 . In another embodiment of the invention, the common heater device  862  is thermally coupled to the two or more laser devices  852  by, e.g., thermal radiation received by the two or more laser devices  852 . In still another embodiment of the invention, the common heater device  862  is thermally coupled to the plurality of laser devices by convective heating through, for example, a fluid medium such as a gas, or a liquid. According to one embodiment of the invention, power to activate the heating device  862  is supplied by an external power supply  870 . 
         [0079]      FIG. 12  shows another embodiment of the invention  900  in which a plurality of laser devices  902  are mutually disposed on a common integrated circuit substrate  901 . A common heater device  912  is also mutually disposed on the common integrated circuit substrate  901 , along with a temperature control device  906 . According to one embodiment of the invention, a temperature sensor device  904  is thermally coupled to one (e.g.,  907 ) of the plurality of laser devices  902 . The temperature sensor device  904  is signalingly coupled to the temperature control device  906 , which is in turn operatively coupled to the heater device  912 . According to one aspect of the invention, the temperature control device  906  controls an activation state of the heater device  912  according to a localized temperature of the one laser device  907  as detected by the temperature sensor device  904 . The heater device  912  is mutually thermally coupled to two or more of the plurality of laser devices  902 . Accordingly, a temperature of two or more of the plurality of laser devices  902  is controlled according to a temperature detected at one of the plurality of laser devices  902 . 
         [0080]    In another embodiment of the invention, the temperature control device  906  receives a temperature control signal from a plurality of temperature sensor devices. For example, in one embodiment of the invention, the temperature control device  906  receives a first temperature signal from a first temperature sensor device thermally coupled to a laser device  902 , and a second temperature signal from a second temperature sensor device  920  thermally coupled to substrate  901 . According to one embodiment of the invention, power to activate the temperature sensor device  906  and/or the heater device  912  is received from an external power supply  922 . 
         [0081]      FIG. 13  shows, in flow diagram form, a method  950  for operating a temperature control device for a laser device according to one embodiment of the invention. As illustrated, the method includes detecting a temperature  952 . Based on a value of the detected temperature, a decision  954  is made. If the detected temperature is less than a threshold temperature, such as for example 0° C., then a heater device is activated  956  (or left activated). If the detected temperature is greater than the threshold temperature, then the heater device is not activated  958  (or deactivated). Thereafter, either instantaneously or at some time interval, the process is repeated  960  beginning with detecting a temperature  952 . 
         [0082]      FIG. 14  shows, in block diagram form, a portion of an exemplary telecommunication system  980  according to one embodiment of the present invention. The telecommunication system  980  includes a plurality of transmitter devices  982  having a respective plurality of optical outputs  984 . Each of the plurality of optical outputs  984  is signalingly coupled to a respective input of an optical multiplexer  986 . An output of the optical multiplexer  986  is coupled to an input of a communication medium such as, for example, an optical fiber  988 . An output of the optical fiber  988  is coupled to an input of an optical demultiplexer  990 . A plurality of outputs of the optical demultiplexer  990  are coupled to respective inputs of the respective plurality of receiver devices  992 . At least one of the plurality of receiver devices  992  is adapted to operate under temperature control over a first operative temperature range, and adapted to operate without temperature control over a second operative temperature range. In one exemplary embodiment, at least one of the plurality of receiver devices  992  includes a receiver device as illustrated in one or more of  FIGS. 1-13  of the present application. 
         [0083]    While the invention has been described in detail in connection with the exemplary embodiments, it should be understood that the invention is not limited to the above disclosed embodiment. Rather, the invention can be modified to incorporate any number of variations, alternations, substitutions, or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not limited by the foregoing description or drawings, but is only limited by the scope of the appended claims.