Abstract:
Passive thermal stabilization of a wavelength monitor includes shielding a wavelength dependent filter of the wavelength monitor from ambient cavity temperatures with portions extending from a submount supporting detectors for the wavelength monitor. The portions and the submount may constitute a single piece, or may be multiple pieces. The shield covers at least one surface of the wavelength dependent filter. The shield may include electrical interconnections for the detectors. A temperature detector may be provided on the submount or the shield portions. The submount or shield portions may be modeled and/or designed to have a section that tracks the thermal response of the wavelength dependent filter and the temperature detector may be mounted on that section.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to passive thermal stabilization of a wavelength monitor, i.e., making the wavelength monitor less sensitive to variations in ambient cavity temperatures, without feeding back this information to the light source being monitored itself. In particular, the present invention is directed to providing a shield and/or a temperature control for a wavelength dependent filter of a wavelength monitor. 
     BACKGROUND OF THE RELATED ART 
     In telecommunications, it is often desirable to stabilize light emitting devices against drift in emission wavelength. These drifts can occur due to current fluctuations, age, and temperature. The current supplied to the light emitting devices can be controlled using wavelength lockers, including wavelength dependent elements such as a Fabry-Perot etalon, to determine an output wavelength. However, typically these wavelength dependent elements are also sensitive to thermal variations. Thus, unless the temperature of the wavelength dependent element is controlled, the characteristics of the wavelength locker vary significantly in accordance with the temperature and the wavelength cannot be adequately stabilized. 
     Current solutions to controlling the temperature in wavelength lockers include active thermal compensation and material index compensation. Active thermal compensation typically involves detecting the temperature and using the detected temperature to control the wavelength or to adjust the temperature of the light emitting device itself using a heater/cooler. This solution requires more active elements and increased processing. Material index compensation typically involves using optical elements fabricated of different materials having equal and opposite index-temperature coefficients or of materials having little or no net change in index-temperature coefficient. However, these materials are wavelength dependent, so they are not reliable over a large wavelength range. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is therefore directed to a passive thermal stabilization of a wavelength monitor which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art. 
     It is an object of the present invention to minimize the effect of ambient cavity temperatures on the wavelength monitor. It is a further object of the present invention to effectively control the temperature of the wavelength monitor. 
     At least one of the above and other objects may be realized by providing an optical sub-assembly for use with a wavelength monitor including a submount on which two detectors are to be provided, a wavelength dependent filter mounted adjacent to one of the two detectors, and a shield portion extending from the submount over the wavelength dependent filter. 
     The submount and the shield portion extending therefrom may a single piece. The shield portion may extend over at least two surfaces of the wavelength dependent interference filter. The wavelength dependent filter may be an etalon. The optical sub-assembly may include a substrate on which the submount is mounted. The wavelength dependent filter may be mounted on the substrate. The shield portion may extend in an optical path of a detector, the shield portion having a hole in the optical path of the detector. The shield portion may be coextensive with an edge of the submount. The optical sub-assembly may include electrical interconnection tracks for the detectors, the electrical interconnection tracks extending along the shield portion. 
     The optical sub-assembly may include a mimicking element having thermal characteristics matched to that of the wavelength dependent filter. A temperature detector may be on the mimicking element. A thermal adjuster adjusting the temperature of the wavelength dependent filter controlled by the temperature detector may be included. The wavelength dependent filter may be mounted directly on the thermal adjuster. The optical sub-assembly may include a substrate, with the wavelength dependent filter is mounted on one side of the substrate and the thermal adjuster mounted on an opposite side of the substrate. 
     The optical sub-assembly may include a temperature detector mounted on one of the submount and the shield portion. A thermal adjuster adjusting the temperature of the wavelength dependent filter controlled by the temperature detector may be included. The temperature detector may be mounted on a section of one of the submount and the shield portion that most closely matches the thermal response of the wavelength dependent filter. 
     At least one of the above and other objects may be realized by providing an optical sub-assembly for use with a wavelength monitor including a wavelength dependent filter, a mimicking element having thermal characteristics matched to that of the wavelength dependent interference filter and subject to similar thermal conditions as the wavelength dependent filter, a temperature detector on the mimicking element, and a thermal adjuster controlled by the temperature detector, the thermal adjuster adjusting a temperature of the wavelength dependent filter. 
     These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, aspects and advantages will be described with reference to the drawings, in which: 
     FIG. 1A is a schematic side view of an optical sub-assembly of the present invention; 
     FIG. 1B is a schematic bottom view of the optical sub-assembly of FIG. 1A; 
     FIG. 1C is a schematic elevational perspective bottom view of the optical sub-assembly of FIG. 1A; 
     FIG. 1D is a schematic elevational perspective side view of the optical sub-assembly of FIG. 1A; 
     FIG. 1E is a schematic elevational perspective top view of the optical sub-assembly of FIG. 1A; 
     FIG. 1F is a side view of the optical sub-assembly of FIG. 1A mounted on a substrate; 
     FIG. 2A is a schematic bottom view of a second optical subassembly of the present invention; 
     FIG. 2B is a schematic elevational perspective bottom view of the optical sub-assembly of FIG. 2A; 
     FIG. 3 is a schematic side view of a third optical sub-assembly of the present invention; and 
     FIG. 4 is a schematic side view of a fourth optical sub-assembly of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention will be described in detail through preferred embodiments with reference to accompanying drawings. However, the present invention is not limited to the following embodiments but may be implemented in various types. The preferred embodiments are only provided to make the disclosure of the invention complete and make one having an ordinary skill in the art know the scope of the invention. The thicknesses of various layers and regions are emphasized for clarity in accompanying drawings. Throughout the drawings, the same reference numerals denote the same elements. 
     Conventional component based assembly techniques for wavelength lockers result in all the components being exposed to the atmospheric temperature of the cavity. In accordance with the present invention, shielding and/or more accurate temperature control of the wavelength dependent filter can result in improved thermal performance. The signals detected in a wavelength monitor including an optical sub-assembly in accordance with the present invention may be used to control a light source being monitored in accordance with known techniques. 
     FIGS. 1A-1F illustrate various views of a first optical sub-assembly  10  of the present invention. As seen in the side view of FIG. 1A, the optical sub-assembly  10  includes a carrier  12 , electrical interconnection tracks  14  and detectors  16 . The detectors  16  are provided on a submount portion  12   a  of the carrier  12 . FIG. 1B illustrates a bottom view of the optical sub-assembly  10 . As can be seen therein, the carrier  12  includes a shelf portion  12   b  extending out from a top edge of the submount portion  12   a  and a stepped portion  12   c  extending from part of a bottom edge of the submount portion  12   a . FIG. 1C more clearly illustrates the relationship between the different portions  12   a - 12   c  of the carrier  12 . FIG. 1D is a perspective side view of a side opposite that shown in FIG.  1 A. As can be seen therein, the carrier  12  has holes  18  allowing passage of light through the carrier  12  to the detectors  16 . FIG. 1E is a perspective top view further showing the electrical interconnection tracks  14  for the detectors  16 . As shown therein, the electrical interconnections  14  are continued on the shelf portion  12   b.    
     As can be seen in outline form in FIG. 1B, a wavelength dependent filter  20 , e.g., an etalon, a notch filter, a dielectric thin film stack, etc., is to be mounted next to the stepped portion  12   c . FIG. 1F illustrates the optical sub-assembly  10  with the wavelength dependent filter  20  mounted on a substrate  22 . 
     The wavelength dependent filter  20  may be attached to the carrier  12  or to the substrate  22 . The substrate  22  may also support optical elements for directing light from a light source to the detectors  16  and/or the light source itself. As can be seen in FIGS. 1A-1F, the shelf portion  12   b  and the stepped portion  12   c  help to shield the wavelength dependent filter  20  from the ambient package environment. 
     For serving as a temperature shield, the carrier  12  needs to be thermally conductive. For serving as a submount for the detectors  16  and for providing the interconnection tracks  14 , the carrier  12  needs to be electrically insulating. The carrier  12  may be made of ceramic, alumina, or aluminum nitride. If the carrier  12  is a single piece, the thermally conductivity thereof may be enhanced by providing highly conductive material on portions that will not interfere with the functioning of the detectors  16 . If the carrier is more than one piece, different materials may be used for the different portions. For example, if the eletrical interconnection tracks  14  are provided on the stepped portion  12   c , the shelf portion  12   b  could be highly conductive. However, the shelf portion  12   c  provides more room for the electrical interconnection tracks  14 . 
     FIGS. 2A-2B illustrate a second embodiment of the optical sub-assembly of the present invention providing further shielding to the wavelength dependent filter. As shown therein, portions  12   d  and  12   e  of the carrier  12  extend along the shelf portion  12   b , further protecting the wavelength dependent filter  20  from the ambient package environment. When the carrier  12  is mounted with a substrate, as in FIG. 1F, the wavelength dependent filter  20  will be enclosed. 
     A third embodiment of the optical sub-assembly of the present invention is shown in FIG.  3 . As shown in the side view of FIG. 3, the carrier  12  includes the submount portion  12   a  for detectors and the shelf portion  12   b , which both provides electrical interconnection tracks for the detector and acts as a shield for the wavelength dependent filter  20 , as in the previous embodiments. Here the extension, i.e., the shelf portion  12   b , from the submount  12   a  only extends along one surface of the wavelength dependent filter  20 . This is in contrast to the first embodiment in which extensions from the submount  12   a  extended along two sides of the wavelength dependent filter  20  and to the second embodiment in which extensions from the submount  12   a  extended along three sides of the wavelength dependent filter  20 . 
     By using any of the above shielding configurations, the thermal gradient through the wavelength dependent filter  20  can be dramatically limited. This thermal stability may be improved by providing a temperature detector  42  somewhere on the carrier  12 , as shown in FIG.  3 . The temperature is then provided to a thermal adjuster  44 , e.g., a thermoelectric cooler, which adjusts the temperature of the substrate  22  on which the wavelength dependent filter  20  is mounted. Alternatively, the wavelength dependent filter  20  may be mounted directly on the thermal adjuster  44 . For more accurate temperature control, the carrier  12  may be modeled and/or designed such that the detector  42  may be mounted on a region of the carrier  12  which most closely reflects the thermal response of the wavelength dependent filter  20 . 
     A fourth embodiment of the optical sub-assembly of the present invention is shown in FIG.  4 . Here, rather than a physical shield, a mimicking element  40  is provided, here on the substrate  22 , in a similar thermal environment as the wavelength dependent filter  20 . The thermal gradient across the mimicking element  40  is matched to that of the wavelength dependent filter  20 . The temperature detector  42 , such as a thermistor, is mounted on the mimicking element  40 . Since the temperature detector  42  is mounted directly on the mimicking element  40 , rather than just near the wavelength dependent filter  20 , accurate monitoring of the wavelength dependent filter&#39;s temperature can be realized. Rather than feeding back the temperature information along with the data from the detectors  16  to control the light source, the temperature of the wavelength dependent filter itself is stabilized. As shown herein, this thermal stability is achieved using the thermal adjuster  44  for adjusting the temperature of the substrate  22  on which the wavelength dependent filter  20  is mounted. 
     Alternatively, the wavelength dependent filter  20  may be mounted directly on the thermal control  44 . The thermal adjuster  44  is controlled by the output of the temperature detector  42 . Without the shielding of the previous embodiments, the temperature of the wavelength dependent filter  20  can still vary, e.g., by 5°, while with the shielding the temperature variations may be less than 1°, making adjustment of the temperature easier. 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the present invention is not limited thereto. For example, the shielding does not have to be integral with the carrier, but may be a separate piece attached thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility without undue experimentation. Thus, the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.