Abstract:
An optical transmitter is disclosed having a temperature stabilization system for an optical filter for maintaining constant the frequency response of the filter. The filter is mounted within a housing having a substantially higher thermal conductivity. The housing may include a copper-tungsten alloy and extend along the optical axis of the filter. The housing is in thermal contact with a thermo-electric cooler (TEC) and a temperature sensor. The TEC and temperature sensor are electrically coupled to a controller which adjusts the temperature of the TEC according to the output of the temperature sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/899,229, filed Feb. 2, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     This invention has applications in high speed transmitters deployed in optical fiber-based communications systems. 
     2. The Relevant Technology 
     Laser transmitters have recently been developed in which a laser, such as a distributed feedback (DFB) laser, is directly modulated to produce adiabatically chirped pulses. The pulses are passed through an optical discriminator or ‘optical spectrum reshaper’ (OSR) that converts the adiabatically chirped pulses into pulses having an increased amplitude modulation and extinction ratio. In some systems, the OSR also performs a pulse shaping function. 
     In such systems, it is important that the laser frequency be aligned with respect to the transmission spectrum of the OSR. This is generally implemented by a control loop that compares the average optical power before and after the OSR component. The control loop maintains the DFB laser wavelength at a calibrated set point by continuously adjusting the DFB laser temperature via a thermoelectric cooler (TEC). 
     In some transmitters, the output of the laser and the amount of light reflected back from the OSR are measured to evaluate alignment of the laser frequency with respect to the OSR. It is therefore important that the frequency response of the OSR be maintained constant in order to provide an accurate reference for controlling the frequency of the laser. 
     In view of the foregoing it would be an advancement in the art to provide a system and method for stabilizing the frequency response of an OSR. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect of the invention, a transmitter includes a directly modulated laser optically coupled to a filter assembly including an optical filter having a transmission edge. The optical filter is disposed within a housing formed of a material having substantially greater thermal conductivity than the optical filter. A temperature modulator and temperature sensor are in thermal contact with the housing and are electrically coupled to a controller that adjusts the temperature of the temperature modulator according to an output of the temperature sensor in order to maintain the transmission edge of the filter proximate a predetermined frequency. 
     In another aspect of the invention, the housing includes a copper-tungsten alloy that extends along the optical axis of the filter leaving opposing ends exposed. The housing may include plates adhered to the filter by means of a compliant adhesive, such as an ultraviolet cured adhesive. Each plate may be secured to adjacent plates by means of solder. 
     In another aspect of the invention, the temperature sensor is mounted to the housing at a midpoint between a first surface contacting the temperature modulator and a second surface opposite the first surface. 
     In another aspect of the invention, a photodiode is positioned to receive optical signals reflected from the optical filter. A locking circuit is coupled to the photodiode and the laser and controls the laser according to the output of the photodiode 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  is a top plan view of a transmitter module in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic block diagram of a transmitter module in accordance with an embodiment of the present invention; 
         FIG. 3  is a front elevation view of a temperature stabilizing system for an optical filter in accordance with an embodiment of the present invention; 
         FIG. 4  is an isometric view of a housing suitable for use in the temperature stabilizing system of  FIG. 3 ; 
         FIG. 5A  is an isometric view of an alternative embodiment of a housing suitable for use in a temperature stabilizing system for an optical filter in accordance with an embodiment of the present invention; 
         FIG. 5B  is a front elevation view of the housing of  FIG. 5A ; 
         FIG. 6A  is an isometric view of another alternative embodiment of a housing suitable for use in a temperature stabilizing system for an optical filter in accordance with an embodiment of the present invention; and 
         FIG. 6B  is a front elevation view of the housing of  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a transmitter module  10  includes a laser  12 , such as a distributed feedback (DFB) laser. A collimating lens  14  is used to direct a collimated beam from the laser  12  along an optical axis  16 . The beam may pass through an isolator  18  and a small fraction (typically 5%) is re-directed to a photodiode  20  by a tap beam splitter  22 . The tap beam splitter  22  may be produced by depositing an anti-reflection coating on one side of a small piece of polished glass and a second controlled-reflection coating on the opposite side. 
     In one embodiment of the invention, the portion of the collimated beam passing through the beam splitter  22  is incident on an optical spectrum reshaper (OSR)  24  positioned on the optical axis  16 . The OSR  24  may be embodied as one or more filters, including, but not limited to, a single cavity filter, coupled multi-cavity (CMC) filter, a thin film coupled multi-cavity filter, a periodic multi-cavity etalon, a fiber Bragg grating, a ring resonator filter, or any other optical element having a wavelength-dependent loss. The OSR  24  may also comprise a fiber, a Gire-Tournois interferometer, or some other element with chromatic dispersion. The OSR  24  may be fabricated as a solid optical element or may include gas-filled gaps, such as an OSR  24  embodied as a periodic multi-cavity etalon. In such embodiments, xenon, or other gas may be present in the gas-filled gaps. 
     In other embodiments, the OSR  24  is formed of a dielectric thin film. In particular time division multiplexing (TDM) applications that require lower cost and complexity may benefit from the use of a dielectric thin film OSR  24 . However, dielectric thin film OSR  24  may still, in some module configurations, require thermal management as described hereinbelow. 
     The spectral response of the OSR  24  may be similar to a Fabry-Perot cavity in which non-transmitted light is reflected. Therefore, depending on the location of the lasing wavelength relative to the passband of the OSR  24 , a portion of the incident optical beam will be transmitted while a residual portion of the incident beam is reflected. The reflected portion of the beam passes back through the tap beam splitter  22  and a portion of the power, such as about 5%, is diverted onto a second photodetector  26 , as shown in  FIG. 1 . 
     In transmitters configured as described hereinabove, it is important to maintain spectral alignment of the wavelength of the laser  12  with respect to the OSR  24 . In operation, the laser  12  may be biased to generate a base frequency signal and is modulated according to a data signal to generate adiabatically chirped pulses that include frequency excursions away from the base frequency, such as up to a peak frequency. The OSR  24  preferably includes a passband having a high slope spectral response, or “transmission edge” near, preferably between, the base and peak frequencies in order to convert at least a portion of the frequency modulation of the adiabatically chirped pulses to amplitude modulation and to increase the extinction ratio of the output of the OSR  24  by attenuating the base frequency. 
     Referring to  FIG. 2 , while still referring to  FIG. 1 , the frequency alignment between the laser  12  and the OSR  24  is generally implemented by a controller  28  that compares the average optical power before and after the OSR  24 . For example, the ratio of the photo currents produced by photodetectors  20 ,  26  may be used to “lock” the relative spectral positions of the laser  12  with respect to the response of the OSR  24 . During calibration, the optimal set point for the laser wavelength relative to the OSR spectral response is determined. During operation, the control loop then acts to maintain the laser wavelength at this calibrated set point by continuously adjusting the laser temperature via a thermoelectric cooler (TEC)  30  to which it is coupled in response to the currents produced by the photodetectors  20 ,  26 . For example, if the DFB lasing wavelength changes, the ratio of the photodiode signals provides an error signature allowing the controller  28  coupled to the TEC  30  to re-adjust the DFB temperature to maintain the correct wavelength. 
     Use of the OSR  24  to provide wavelength locking advantageously saves space within the module  10 , which is important for optical layout design in a miniaturized transmitter module  10 . The OSR  24  also provides a sharper spectral slope as compared to prior wavelength locking etalons. The OSR  24  provides these advantages while also serving as an optical discriminator enhancing the amplitude modulation and extinction ratio of the transmitter, and eliminating the need for an additional component for providing the wavelength locking functionality. Double-function of the OSR  24  is an important aspect of the above described transmitter  10  and is compatible with the implementation of a TOSA in an XFP transceiver. 
     The OSR  24  may be angled with respect to an optical axis  16  of the beam incident on the OSR  24 . For example, an angle of from 0.5 to two degrees from normal may be used. The angle of the OSR  24  relative to the optical axis  16  may be used to fine-tune the spectral response of the OSR  24  and also minimize back-reflection along the optical path. The spectral response may also be tuned by changing the temperature of the OSR  24 . One or both of the temperature and angle may be adjusted to cause the spectral response of the OSR to coincide with the ITU grid. 
     The output of the OSR  24  may be focused by a lens  32  and coupled to a standard optical fiber  34 , such as a standard single-mode optical fiber. A second optical isolator component may optionally be added between the lens  32  and the fiber  34 . 
     Transmitters as described above enable a 10 Gb/s directly modulated laser operating in the C &amp; L band to transmit information over a &gt;200 km fiber length without the need for dispersion compensation. This is a breakthrough achievement when compared with transmission distances of &lt;20 km for standard directly modulated laser transmitters. Such transmitters may be deployed in both time division multiplexing (TDM) and dense wavelength division multiplexing (DWDM) optical links. 
     For reliable performance over module lifetime and case temperature extremes, several optoelectronic packaging techniques may be employed, in particular to facilitate DWDM implementation of the above-described technology. The DWDM version, and others, of the above described technology may advantageously use an OSR  24  that is temperature controlled to maintain good optical performance and also provide a wavelength locking function. The physical size and high-performance optical specifications of the OSR  24  make a strong demand on the thermal management to achieve the desired optical performance over all environmental conditions. 
     As for most solid-etalon wavelength locker configurations, the temperature of the OSR  24  is preferably tightly controlled to maintain accurate calibration of the spectral response for locking purposes. A typical transmission-etalon type locker is also dependent on minimal change in insertion loss over life and temp. However, the filter slope of the OSR  24  is preferably higher than standard locker etalon and thus can help compensate for residual changes in insertion loss and thereby keep the locking accuracy budget acceptable. 
     Active temperature control of the OSR  24  can lead to varying thermal gradients across the material constituting the OSR  24  as the module case temperature changes. This is due to several contributions, including thermal conductivity of the OSR material, thermal conductivity of the surrounding internal module environment (including Nitrogen, Argon, or Xenon gas), initial calibration conditions, and magnitude of the case temperature variation. Thermal gradients across the OSR  24  cause “averaging” of a range of spectral responses resulting in an increase in insertion loss and reduction in effective spectral slope. These parameters are of particular concern for transmission performance as well as wavelength stability of the module. 
     Thermal management of the OSR  24  may be improved by encasing the OSR  24  within a hermetically sealed housing  38  filled with an inert gas such as Xenon. The low thermal conductivity of Xenon gas reduces the thermal gradients experienced by the OSR  24  and the result is improved wavelength locking accuracy and OSR performance over case temperature variation. 
     A preferred design of the OSR  24  would also use a high thermal conductivity material such as Silicon or Sapphire. This would greatly enhance temperature uniformity within the OSR material, although there are drawbacks in terms of wavelength sensitivity. There are also manufacturing tolerance issues that, to date, have prevented successful implementation of the OSR  24  using these materials. In the absence of a high thermal conductivity material, the OSR  24  may be fabricated from a high refractive index optical glass such as LaSFN9 material. Fused silica and other standard polishing glasses may also be used. LaSFN9 (and optical glasses in general) exhibit low thermal conductivity. 
     Referring to  FIGS. 3 and 4 , an OSR  24  in accordance with the invention includes an isothermal housing  38 . Good performance of the isothermal housing  38  is important due to sensitivity of the OSR  24  to thermal gradients. The isothermal housing  38  surrounds the OSR  24 , leaving ends  40   a ,  40   b  exposed, such that a beam may pass through the OSR  24  along the optical axis  16 . 
     The isothermal housing  38  preferably has a much higher thermal conductivity than the OSR  24 . For example, the housing  38  may be formed of a copper-tungsten alloy (CuW) or aluminum nitride (AlN). The use of a material having high thermal conductivity, such as a CuW alloy, enhances temperature uniformity across the actual OSR  24 . The temperature of the OSR  24  may be maintained to within 0.05° C. accuracy to provide very accurate wavelength stability. The housing  38  preferably has a coefficient of thermal expansion substantially equal that of the OSR  24 . For example, a CuW housing is well suited for encasing an OSR  24  formed of LaSFN9. 
     In the illustrated embodiment, the housing  38  includes plates  42   a - 42   d  secured to sides of the OSR  24 . The plates  42   a - 42   d  are preferably secured to the sides of the OSR  24  by means of a compliant adhesive  44 , such as an ultraviolet cured epoxy. The compliant adhesive  44  may advantageously accommodate differences in the coefficient of thermal expansion of the housing  38  and OSR  24 . In an alternative embodiment, no adhesive  44  is used. In such embodiments, the OSR  24  is preferably in close contact with the plates  42   a - 42   d . However, angle differences between sides of the OSR  24  and the plates  42   a - 42   d  may result in air gaps that may be filled with whatever gas is present in the transmitter  10 , such as xenon. 
     Edges of adjacent plates  42   a - 42   d  may be joined to one another by means of solder beads  46 , such as a lead-tin alloy, in order to enhance the equalization of temperature at the corners of the housing  38 . Alternatively, a highly thermally conductive adhesive may be used such as a silver epoxy. 
     The housing  38  may mount to a thermoelectric cooler (TEC)  48 . In the illustrated embodiment, only one TEC  48  is used. In other embodiments, more than one TEC  48 , each engaging one of the plates  42   a - 42   d , may be used. A temperature sensor  50  is in thermal contact with the housing  38 . The TEC  48  and temperature sensor  50  are electrically coupled to a controller  52  that controls the temperature of the TEC  48  according to the output of the temperature sensor  50 . In some embodiments, the temperature sensor  50  is located at a distance  54  midway between the TEC  48  and the top of the housing  38  in order to provide more accurate feedback regarding the average temperature of the housing  38 . The temperature sensor  50  may also be located at about midpoint of the length of the housing  44  as illustrated. In an alternative embodiment, the TEC  48  is replaced with a heater element in thermal contact with the housing  38  and electrically coupled to the controller  52 . Inasmuch as the heater element provides temperature stabilization by heating alone, the OSR  24  in such embodiments is preferably stabilized at a temperature above the maximum module case temperature range of the transmitter module  10 . 
     The photodetectors  20 ,  26  may also be disposed to reduce temperature induced variation. In some embodiments, the photodetectors  20 ,  26  are embodied as InGaAs photodiodes and are preferably located in close physical proximity to one another, as shown in the module layout of  FIG. 1 . This produces a similar thermal environment for the two photodiodes  20 ,  26  under all conditions and is also compatible with transitioning the optical layout design into a miniaturized TOSA package. Similarly, the common tap splitter  22  (where a “reflection-mode” configuration is adopted with the OSR spectral response) helps to cancel residual interference and subcavities that could modify “effective” lock ratio over life and/or temperature. In addition, the photodetectors  20 ,  26  and tap beam splitter  22  may all be located on a common temperature controlled substrate to reduce sensitivity to case temperature variation. For example, the photodetectors  20 ,  26  and tap beam splitter  22  may be coupled to the same TEC  48  as the OSR  24 . 
     Referring to  FIGS. 5A and 5B , in an alternative embodiment, the isothermal housing  38  is formed of angled plates  56   a ,  56   b  each having two legs  58   a ,  58   b  bearing surfaces  60   a ,  60   b , respectively, that are positionable adjacent surfaces of the OSR  24 . The use of angled plates  56   a ,  56   b  reduces manufacturing costs by eliminating two solder joints as compared with the embodiment of  FIG. 4 . 
     As in the embodiments above, the angled plates  56   a ,  56   b  are preferably formed of a material having high thermal conductivity such as CuW or AlN. In the illustrated embodiment, a channel  62  is formed at the intersection of the surfaces  60   a ,  60   b . The channel  62  receives a corner of the OSR  24  and may serve to loosen tolerances that would be required to form an intersection of the surfaces  60   a ,  60   b  that exactly matched the corner of the OSR  24 . 
     The legs  58   a ,  58   b  of the angled plate  56   a  are larger than the legs  58   a ,  58   b , of the angled plate  56   b , such that the other angled plate  56   b  can be readily nested against the angled plate  56   a . As in the above embodiments, the angled plates  56   a ,  56   b  may secure to the OSR  24  by means of an adhesive  44 , such as a UV cured epoxy. The angled plates  56   a ,  56   b  may be secured to one another by solder or by an adhesive, such as a silver epoxy. 
     In the illustrated embodiment, the leg  58   a  of the angled plate  56   a  extends beyond the angled plate  56   b  of the assembled housing  38 . The larger leg  58   a  preferably secures to a substrate such as the TEC  48 . Its increased length may facilitate securement to the TEC  48  due to a larger area available for bearing an adhesive. The larger area of the leg  58   a  may also facilitate a higher rate of heat transfer with the TEC  48 . 
     Referring to  FIGS. 6A and 6B , in another alternative embodiment, the housing  38  includes a U-shaped member  64  having surfaces  66   a - 66   c  for engaging surfaces of the OSR  24 . A top plate  68  secures across the U-shaped member  64  such that the top plate  68  and U-shaped member  64  completely encircle the OSR  24 . The top plate  68  and U-shaped member may include a material having high thermal conductivity such as CuW or AlN. The top plate  68  is secured to the U-shaped member  64  by means of solder  46  or silver epoxy. 
     The OSR  24  may secure to one or both of the top plate  68  and U-shaped member  64  by means of an adhesive  44 , such as a UV cured epoxy. In some embodiments, a channel may be formed at the intersections of the surfaces  66   a  and  66   b  and the surfaces  66   b  and  66   c  to receive the corners of the OSR  24 , as in the embodiment of  FIGS. 5A and 5B . In the illustrated embodiment, no such channels are formed such that a small gap exists between the surfaces of the OSR  24  and the surfaces  66   a - 66   c . The gap may be filled with a gas such as xenon or may be filled with an adhesive, such as a UV cured epoxy. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.