Patent Publication Number: US-2004042742-A1

Title: Thermally equalized optical module

Description:
FIELD OF THE INVENTION  
       [0001] This invention generally relates to optical devices for use in optical communication networks and in particular to thermally insulated optical modules.  
       BACKGROUND OF THE INVENTION  
       [0002] Optical communication networks have gained widespread acceptance over the past few decades. With the advent of optical fiber, communication signals are transmitted as light propagating along a fiber supporting total internal reflection of the light propagating therein. Many communication systems rely on optical communications because they are less susceptible to noise induced by external sources and are capable of supporting very high speed carrier signals and increased bandwidth. In modern optical networks Wavelength-Division-Multiplexing (WDM) is applied for the simultaneous transmission of many different communication channels at different wavelengths using a single optical waveguide. Typically, the communication channels are provided within a 1530-1565 nanometer (nm) range, and are separated by multiples of 100 Giga Hertz (GHz), i.e. approximately 0.8 nm. Major issues in WDM optical communication networks are the signal quality of each channel and the relative accuracy of the channels, i.e. the wavelength setting for each channel is accurate to within the tolerances set by the International Telecommunications Union (ITU) WDM grid.  
       [0003] A major problem affecting the signal quality and the relative channel accuracy is the sensitivity of the optical elements of the communication network to temperature changes resulting in changes in physical dimensions of and/or physical stresses within the optical elements. For example, in WDM optical communication components expansion, contraction or bending of an optical element due to temperature changes of even less than 1° C. is capable of substantially degrading the optical performance of the network.  
       [0004] In order to reduce signal degradation, temperature sensitive optical elements are assembled in optical modules having the optical element in thermal contact with a temperature regulation system utilizing, for example, a Peltier element and packaged in a sealed container. Such modules are disclosed in the prior art, for example, in U.S. Pat. No. 5,845,031 issued to Aoki in Dec. 1, 1998, U.S. Pat. No. 5,919,383 issued to Beguin et al. in Jul. 6, 1999, U.S. Pat. No. 5,994,679 issued to DeVeau et al. in Nov. 30, 1999, and U.S. Pat. No. 6,114,673 issued to Brewer et al. in Sep. 5, 2000, which are incorporated herein by reference.  
       [0005] However, these prior art modules result in an environment contained in the container having substantial thermal gradients and, furthermore, greatly differing thermal gradients depending on outside conditions. These thermal gradients result in a different temperature at different locations within an optical element disposed in the container. For example, the bottom part of the optical element is attached to a heating element and has a temperature of 60° C. whereas the top of the optical element has a temperature of 59° C. resulting in physical stresses causing substantial signal degradation.  
       [0006] It is an object of the invention to substantially reduce the thermal gradient within and immediately about an optical element disposed in a thermally insulated container.  
       [0007] It is yet further an object of the invention to provide an optical module having a plurality of optical elements disposed therein wherein thermal gradients caused by thermal energy emitting optical elements disposed therein are substantially reduced.  
       SUMMARY OF THE INVENTION  
       [0008] In accordance with the present invention there is provided an optical module comprising:  
       [0009] at least an optical element;  
       [0010] a heat source/sink thermally coupled with at least one of the at least an optical element for sourcing/sinking heat there to/from for temperature regulating the at least one optical element;  
       [0011] a thermally insulating packaging forming an enclosure surrounding the at least an optical element, the thermally insulating packaging for providing a first thermally controlled environment within the enclosure; and,  
       [0012] a thermally conductive structure disposed within the enclosure, the thermally conductive structure being thermally coupled to the heat source/sink for being temperature regulated thereby, the thermally conductive structure outlining a space surrounding at least one of the at least one optical element for providing together with the heat source/sink a second thermally controlled environment therein, the second thermally controlled environment for providing a lower temperature gradient across the at least one optical element than absent the thermally conductive structure.  
       [0013] In accordance with the present invention there is further provided an optical module comprising:  
       [0014] a plurality of optical elements wherein, in use, at least one of the plurality of optical elements is emitting thermal energy;  
       [0015] a thermally insulating packaging forming an enclosure surrounding the plurality of optical elements, the thermally insulating packaging for providing a first thermally controlled environment within the enclosure; and,  
       [0016] a thermally conductive structure disposed within the enclosure, the thermally conductive structure being thermally coupled to a heat sink for being temperature regulated thereby, the thermally conductive structure outlining a space surrounding at least one optical element for substantially absorbing the thermal energy emitted.  
       [0017] In accordance with the present invention there is yet further provided an optical sub module comprising:  
       [0018] at least an optical element;  
       [0019] a thermo coupler thermally coupled with the at least an optical element for sourcing/sinking heat there to/from for temperature regulating the at least one optical element, the thermo coupler for being coupled at a predetermined location to a holding structure of a thermally insulating package, the thermally insulating package forming an enclosure surrounding the thermo coupler and the at least an optical element for providing a first thermally controlled environment therein; and,  
       [0020] a thermally conductive structure thermally coupled to the thermo coupler for being temperature regulated thereby, the thermally conductive structure outlining a space surrounding the at least an optical element for providing together with the thermo coupler a second thermally controlled environment within the enclosure. 
     
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
     [0021] Exemplary embodiments of the invention will now be described in conjunction with the following drawings, in which:  
     [0022]FIG. 1 a  is a simplified cross sectional view of an optical module according to the invention;  
     [0023]FIG. 1 b  is a simplified cross sectional view of another embodiment of an optical module according to the invention;  
     [0024]FIGS. 2 a  to  2   e  are simplified perspective views of different embodiments of a thermally conductive structure according to the invention;  
     [0025]FIG. 3 is a simplified cross sectional view of an optical module according to the invention illustrating placement of a plurality of optical elements and thermally conductive structures in a top view;  
     [0026]FIG. 4 a  is a simplified cross sectional view of an optical module illustrating placement of a thermal energy emitting optical element in combination with other optical elements according to the prior art in a top view;  
     [0027]FIGS. 4 b  to  4   d  illustrate different embodiments of placement of a thermal energy emitting optical element and thermally shielding of other optical elements according to the invention in a top view; and,  
     [0028]FIG. 5 is a simplified cross sectional view of yet another embodiment of an optical module according to the invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0029] The present invention provides a thermally conductive structure in thermal contact with a base and surrounding an optical element disposed within a thermally insulated packaging. Due to the dual temperature shielding a temperature gradient within a space surrounding the optical element and, therefore, across the optical element is substantially lower than absent the thermally conductive structure. Since the optical element is within an environment determined based on dual temperature shielding thereof, adjusting of the temperature at the optical element itself is more easily, accurately, and repeatably performable. For a large range of temperatures outside of the thermal insulating packaging the temperature within the enclosure is adjustable with a known thermal gradient therein. The thermally conductive structure is for sufficiently reducing this thermal gradient within a portion of the thermally insulating packaging and, in particular, within the space surrounding the optical element.  
     [0030] Optical modules are designed to provide a controlled environment for the protection of sensitive optical elements. A major aspect in the design of optical modules is the adjustment of the operational temperature of the optical elements. Referring to FIG. 1 a , an optical module  100  according to the invention is shown. The optical module  100  comprises an optical element  104 . A heat source/sink  102  is thermally coupled with the optical element  104  for sourcing/sinking heat to/from the optical element  104 . The heat source/sink  102  provides temperature regulation of the optical element  104  in order to keep its operating temperature within predetermined limits. Depending on the normal operating temperature of the optical element  104  and an ambient temperature of an environment where the optical module is placed the heat source/sink  102  is a heat source such as, for example, a resistive heating element or a Peltier thermoelectric device. Alternatively, the heat source/sink is a heat sink and, for example, the Peltier thermoelectric device is operated as such. Preferably, the optical element  104  is coupled to the heat source/sink  102  using a thermally conductive material in order to provide good thermal coupling. The optical element  104  is contained within a thermally conductive packaging  106 . The thermally conductive packaging  106  forms an enclosure  107  surrounding the optical element  104  and provides an approximately isothermal environment within the enclosure  107 . Packaging is commercially available and generally comprises an outside wall and air within the package. Alternatively, a package includes an inside wall  108 , an outside wall  112 , and an intermediate layer  110  disposed therebetween. In one embodiment, shown in FIG. 1 a , the heat source/sink  102  is abutted to the thermally conductive packaging  106  forming together with the thermally conductive packaging  106 . Optionally, the inside wall  108  is also made of a thermally conductive material and thermally coupled to the heat source/sink  102  making the inside wall  108  a portion of the heat source/sink. In another embodiment  200  the heat source/sink  102  is disposed within a thermally conductive packaging  120  as shown in FIG. 1 b . Optionally, the heat source/sink  102  is thermally insulated from the packaging  120  using, for example, a thermally insulating medium  122 .  
     [0031] The optical modules  100 ,  200  further comprise a thermally conductive structure  114 . The thermally conductive structure  114  is thermally coupled to the heat source sink  102  for being temperature regulated thereby. The thermally conductive structure  114  outlines a space  115  surrounding the optical element  104  and provides together with the heat source/sink  102  a second thermally controlled environment therein. The second thermally controlled environment provides a lower temperature gradient across the optical element  104  than absent the thermally conductive structure  114 .  
     [0032] Referring to FIGS. 2 a  to  2   e  various embodiments of the thermally conductive structure  114  according to the invention are shown. Advancing from FIG. 2 a  to FIG. 2 e  temperature control within space  115  is improved. FIG. 2 a  shows the simplest embodiment of the secondary thermally conductive structure  114  being made of a plurality of U-bent wires or rods  230  surrounding the optical element  104 . In the embodiment shown in FIG. 2 b  the wires are replaced by a U-shaped cover made of a wire mesh  232 . Replacing the wire mesh with a sheet material  234 , as shown in FIG. 2 c , substantially increases the surface area for heat conduction thus improving temperature control within the space  115 . Referring to FIG. 2 d , the container  240  replaces the cover incorporated in the FIGS. 2 a  to  2   c . The container  240 , together with the hear source/sink  102 , provides a nearly complete enclosure for the optical element  104 , however the container  240  does not provide a sealed environment. As shown, the container  240  includes an optically transparent region  239 . The optically transparent region permits optical communication between the optical component  104  (not shown) and an optical waveguide outside the container  240 . In the embodiment shown in FIG. 2 e , the cover is again replaced with a container  240 . The container  240  includes an opening  238  for allowing propagation of a light beam therethrough. As is evident, there are numerous methods for thermally coupling the thermally conductive structure  114  to the heat source/sink  102  such as inserting a portion of the thermally conductive structure  114  into a slot or groove disposed in the heat source/sink  102  at a predetermined location, or affixing the thermally conductive structure  114  to the heat source/sink  102  using an adhesive.  
     [0033] Preferably, the thermally conductive structure  114  is made of a material having high thermal conductivity, for example, metals such as Al or CuMo alloy. The thickness of the metal or the diameter of the wires used is dimensioned large enough to provide sufficient thermal conductivity within the thermally conductive structure. Further preferably, the thermally conductive structure  114  is designed to have sufficient thermal conductivity ensuring the structure to be approximately isothermal during normal operation of the optical element  104 .  
     [0034] The thermally conductive structure  114  provides together with the heat source/sink  102  in the space  115  surrounding the optical element  104  a second thermally controlled environment. Due to the dual temperature shielding, a temperature gradient within the space  115  and, therefore, across the optical element  104 , for example, between points A and B in FIG. 1 a , is substantially lower than absent the thermally conductive structure  114 . Since the optical element  104  is within an environment determined based on dual temperature shielding thereof, adjusting of the temperature at the optical element  104  itself is more easily, accurately, and repeatably performable. For a large range of temperatures outside of the thermal insulating packaging  106  the temperature within the enclosure  107  is adjustable with a known thermal gradient therein. The thermally conductive structure  114  is for sufficiently reducing this thermal gradient within the enclosure  107  and, in particular, within the space  115 . Thus, whereas prior art devices result in greatly differing thermal gradients depending on outside conditions, the present invention improves an operating range for an optical module without requiring that the optical element  104  supports extremely varied thermal gradients across. Therefore, the optical module according to the invention allows use of highly thermally sensitive optical elements for a large range of outside temperatures.  
     [0035] Referring to FIG. 3, an optical module  300  according to the invention is shown. The optical module  300  includes a plurality of optical elements, for example, elements  302 ,  304 ,  306 ,  308 , and  310  as shown in FIG. 3. Using prior art thermal insulation techniques requires a thermally insulating packaging meeting the most stringent requirements for protecting the most thermally sensitive optical element of the plurality of optical elements often resulting in a high cost package. As shown in FIG. 3, the present invention provides an apparatus for individual temperature adjustment of each optical element depending on their thermal sensitivity. For example, the optical elements  302  and  306  are less thermally sensitive and, therefore, need not have a thermal conductive structure disposed thereabout. The optical elements  308  and  310  are more thermally sensitive and are provided with, for example, a thermally protective structure  312  such as shown, for example in FIG. 2 c . The optical element  304 , for example, is highly thermally sensitive and is provided with a sealed thermally conductive structure  314 . As shown in FIG. 3, it is possible to nest a plurality of optical elements having an approximately same thermal sensitivity in groups under one thermally conductive structure so long as none of the elements generates substantial amounts of heat. The optical module  300  is highly advantageous by allowing optional temperature isolation at each optical element individually in accordance with its thermal sensitivity due to the plural temperature shielding. Therefore, adjustment of the operational temperature of each individual optical element is more easily, accurately, and repeatably performable.  
     [0036] Another advantage of the present invention is the capability of combining thermal energy emitting optical components such as a laser diode with thermal sensitive optical elements in one optical module. Referring to FIG. 4 a  a combination of a thermal energy emitting optical component with optical elements  404 , and  406  in one module according to the prior art is shown. As is evident, the optical element  406  in proximity of the thermal energy emitting optical element  402  is affected by the presence of the heat source as indicated by the isothermals—dashed lines—surrounding the thermal energy emitting optical element  402 . As shown in FIG. 4 a  the optical element  406  is located in an area having a substantial thermal gradient—temperature change normal to the isothermals—resulting in a thermal gradient across the optical element  406 . Known solutions to this problem include using a fan to provide forced convection inside the optical module in order to equalize the temperature field, thus reducing the thermal gradient and thermal isolation of different elements within different packages disposed in a spaced relation. These options, in general, results in large optical modules in order to provide enough space for the placement of thermally sensitive optical elements and severely restricts the design of optical circuits placed in the optical module. The size limitations of optical modules, reliability aspects and cost constraints often render these solutions prohibitive.  
     [0037] This problem is easily solved by the present invention. For example, providing the thermally sensitive optical element  406  with a thermally conductive structure  408  substantially reduces the thermal gradient induced by the thermal energy emitting optical element  402  within a space  407  surrounding the optical element  406 , as shown in FIG. 4 b . For a highly thermal sensitive optical element a sealed thermally conductive structure substantially blocking the thermal energy emitted from the optical element  402  is preferred. In another embodiment shown in FIG. 4 c  the thermal energy emitting optical element  402  is provided with a thermally conductive structure  410  for substantially absorbing the emitted thermal energy and thus for protecting the optical element  406 . Optionally, the embodiments shown in FIGS. 4 b  and  4   c  are combined to provide maximum thermal protection of the optical element  406  as shown in FIG. 4 d.    
     [0038] Referring to FIG. 5, yet another embodiment  500  of an optical module according to the invention is shown. Here, a thermal insulating packaging  510  is provided with a holding structure having openings or insertion slots  514  at predetermined locations for insertion of optical sub modules  502 . The optical sub modules  502  comprise at least an optical element  504 . A thermo coupler  506  is thermally coupled with the at least an optical element  504  for sourcing/sinking heat there to/from for temperature regulating the at least an optical element  504 . A thermally conductive structure  508  is thermally coupled to the thermo coupler  506  for being temperature regulated thereby. The thermally conductive structure  508  provides a space  507  surrounding the at least an optical element  504  for providing together with the thermo coupler a thermally controlled environment surrounding the at least an optical element  504 . The thermo coupler  506  is a thermally conductive coupler for conducting heat to/from an active heat source/sink  516  such as a Peltier thermoelectric device disposed in the thermally insulating packaging  510 . Alternatively, the thermo coupler  506  is an active heat source/sink having electrical contacts to be mated with their counterparts disposed in the thermally insulating packaging  510 , not shown. The holding structure  512  is made of a thermally conductive material or, alternatively, of a thermally insulating material depending on design considerations. For example, having a holding structure  512  made of a thermally insulating material allows insertion of optical sub modules having heat sources/sinks operated at different temperatures, thus increasing design flexibility. The optical module  500  according to the invention allows easy assembly of the same using prefabricated sub modules. This is highly advantageous by enabling, for example, a technician to assemble the optical module  500  on site according to design considerations of an optical network. In a preferred embodiment the optical sub modules  502  are sealed.  
     [0039] Of course, though in the preferred embodiments the thermally conductive structure is disposed on a base having active heating/cooling thereof, this is not necessary. In an embodiment, the thermally conductive structure is disposed on a thermally conductive base to provide increased heat conduction about the optical element in order to maintain the optical element in an environment with little or no temperature gradients therein. Thus, active temperature control is not required for the invention. Of course, even a thermally conductive base material acts as a source/sink when used in accordance with the invention though it is distinguishable from an active heat source/sink.  
     [0040] The term “highly thermally conductive” as used herein and in the claims that follow refers to a material that conducts heat well such as Al or Cu. Typically, materials such as air, fiberglass, glass and so forth are not highly conductive and it is anticipated that some forms of ceramic material may in fact be highly thermally conductive and others may be more insulating than thermally conductive. The term insulating material refers to a material that insulates between two thermal regions even though that material is nominally thermally conductive in nature.  
     [0041] Numerous other embodiments of the invention will be apparent to persons skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.