Patent Publication Number: US-2004057477-A1

Title: Wavelength locking device

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001] The present invention is directed, in general, to an optoelectronic device and, more specifically, a wavelength locking device having an optical filter and a low reflector, a method of manufacture therefor, and an optical communications system including the same.  
       BACKGROUND OF THE INVENTION  
       [0002] Electromagnetic radiation sources, such as lasers, used in optical communication systems, have stringent requirements. For instance, the wavelength locking range of a laser is an important parameter to control and stabilize. For many laser systems, including diode, solid-state, organic dye and gas lasers, the gain profile can be much wider than the axial-mode spacing of the laser cavity. Consequently, the laser may oscillate over an undesirably broad spectrum of multiple axial modes. Moreover, in certain applications, using semiconductor diode lasers for example, changes in environmental temperature, or operating current variations, may cause the laser to become unlocked or to lock at an undesired wavelength of light.  
       [0003] A number of techniques have been developed to reduce the spectral width of the axial modes. One well-known means of stabilizing the locking range involves coupling an external grated waveguide, such as a fiber Bragg grating (FBG), to a laser at its output facet. Fabry-Perot (F-P) lasers, for example, may have a broadband low reflectivity (LR) coating on the output facet that governs the wavelength where maximum internal reflectivity of the laser occurs, the so-called chip wavelength. Grated waveguides, such as FBGs, have their own narrow wavelength of maximum reflectivity, the so-called grating wavelength. When a FBG is coupled to the output end facet of a laser, the FBG may thus provide a narrow wavelength of optical feedback to the laser. So long as the chip and the grating wavelengths are substantially similar, the feedback can stimulate radiation thereby causing the laser to emit light, or lase, at the feedback wavelength of the grating, instead of the chip wavelength.  
       [0004] Such external grating stabilized laser packages however, remain problematic. They may still be relatively sensitive to temperature variations, for example, about 10 picometers per degree centigrade (pm/° C.). Moreover, because the LR coating may be sensitive to temperature, the chip wavelength may shift significantly away from the grating wavelength, causing the laser to lase at the chip wavelength. Under such circumstances the chip laser is said to be outside of the locking range of the grating waveguide. This may necessitate additional expenditures for active temperature stabilization. Moreover, the reflectivity and band shape for a grating, such as a FBG, are difficult to adjust. There are also additional expenses associated with producing an external grating, which may be fragile and difficult to fabricate. Finally, an external grating stabilized laser package may not be as compact as desired for certain semiconductor and telecommunication applications.  
       [0005] Previous efforts to resolve this problem have not lead to entirely satisfactory solutions. For example, the locking range of a FBG-stabilized laser may be increased by increasing the maximum reflectivity of the FBG, but at the cost of reduced output power. A grating internal to the laser chip, such as a diffraction grating, may be used to form a distributed feed back (DFB) laser to facilitate stabilization of the lasing wavelength, instead of an external FBG. However, such DFB lasers may have a greater temperature dependent shift than the temperature dependence of a laser coupled to an external grated waveguide. Moreover, DFB lasers are expensive to produce due to the added complexity of the design.  
       [0006] Accordingly, what is needed in the art is a compact wavelength locking device that does not experience previously encountered drawbacks.  
       SUMMARY OF THE INVENTION  
       [0007] To address the above-discussed deficiencies of the prior art, the present invention provides a wavelength locking device. The device comprises a low reflector and an optical filter. The optical filter may be located between the low reflector and an input end of the wavelength locking device. The optical filter and low reflector cooperate to lock an oscillation wavelength of a radiation source to a wavelength substantially determined by the optical filter.  
       [0008] In another embodiment, the present invention provides a method of manufacturing a wavelength locking device having the above-described properties. The method comprises attaching a low reflector to a substrate and attaching an optical filter to the substrate between the low reflector and an input end of the wavelength locking device.  
       [0009] The foregoing has outlined preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010] Understanding the invention may be facilitated from the following detailed description and accompanying FIGUREs. In accordance with the standard practice in the optoelectronic industry, various features may not be drawn to scale. Rather, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
     [0011]FIG. 1 illustrates a cross-sectional view of a wavelength locking device, which has been constructed in accordance with the principles of the present invention;  
     [0012]FIG. 2 illustrates a cross-sectional view of an alternative embodiment of a wavelength locking device;  
     [0013]FIG. 3 illustrates a cross-sectional view of another alternative embodiment of a wavelength locking device;  
     [0014]FIG. 4 illustrates a cross-sectional view of yet another embodiment of a wavelength locking device;  
     [0015]FIG. 5 illustrates a cross-sectional view of still another embodiment of a wavelength locking device;  
     [0016]FIG. 6 illustrates, by flow diagram, a method of manufacturing a wavelength locking device according to the present invention; and  
     [0017]FIG. 7 illustrates an optical communication system, which may form one environment where a wavelength locking device, similar to that shown in FIG. 1, may be included.  
    
    
     DETAILED DESCRIPTION  
     [0018] The present invention recognizes that the deficiencies associated with the use of grating-based locks can be avoided by replacing such gratings with a wavelength locking device comprising an optical filter and reflector. FIG. 1, illustrates a cross-sectional view of one embodiment of such a wavelength locking device  100 . An optical filter  105  and low reflector  110  may be attached to any conventional substrate  115  conducive with the intended application, for example, a glass substrate for semiconductor or telecommunication applications. As illustrated, the optical filter  105  is located between the low reflector  110  and an input end  120  of the device  100 . The optical filter  105  and low reflector  110  cooperate to lock an oscillation wavelength of a radiation source  125  to a wavelength substantially determined by the optical filter  105 . The source  125  may be coupled to the device  100  via a connector  130 , such as an optical fiber. In other embodiments, however, the source  125  could be integrated into the device  100  or other devices depicted herein. In certain preferred embodiments, the device  100  may further include one or more collimators  135 ,  140 , located, respectively, between the optical filter  105  and the input end  120 , and between the low reflector  110  and an output end  145  of the device. In certain preferred embodiments, the collimators  135 ,  140  comprise conventionally made laser collimator lens.  
     [0019] For purposes of the present invention, a radiation source  125  is defined as any device capable of emitting coherent electromagnetic energy. For example, in certain preferred embodiments, the radiation may be an optical wave comprising coherent light emitted by an optical laser source, such as a semiconductor laser. The optical wave may thus comprise a wavelength or band of wavelengths of light that oscillate at a particular frequency or band of frequencies characteristic of the radiation source. As noted above, the wavelength locker device of the present invention, such as device  100 , may function to lock the oscillation wavelength of the radiation source to a narrow band of wavelengthssubstantially determined by the characteristics of the optical filter  105 .  
     [0020] The term optical filter  105  as used herein refers to any material that allows only a targeted band of wavelengths of radiation to be transmitted or pass through the material, or only a band-pass of wavelengths to be reflected by the material. Preferably, the band or band-pass wavelength of the optical filter  105  has a low temperature dependence, as represented by a low temperature coefficient (i.e., the change in the center of the band or band-pass wavelength per unit change temperature). For example, in certain preferred embodiments, the temperature coefficient is less than about 10 pm/° C., and more preferably less than about 2 pm/° C., and even more preferably less than about 1 pm/° C.  
     [0021] In certain preferred embodiments, the optical filter  105  may include one or more thin film optical filters. The thin film optical filter may be comprised, for example, of alternating layers of two or more dielectric materials on a substrate, such as polished glass. Each thin film filter may thus transmit a certain band of wavelengths and reflect or absorb at all other wavelengths of radiation. In certain preferred embodiments, any number of thin film filters may be combined to form a more complex filter, such as a Wavelength Division Multiplexing (WDM) type filter. The fabrication of thin film optical filters using conventional thin film deposition techniques are well known to those of ordinary skill in the art. Commercial suppliers of such thin film filters include: Deposition Sciences Inc., (Santa Rosa, Calif.); Irdian Spectral Technologies Inc. (Ottawa, Canada); or Corning NetOptix Inc. (Marlborough, Mass.).  
     [0022] In certain advantageous embodiments, the optical filter  105  includes at least one surface  150  oriented at an angle  155  substantially non-perpendicular to an optical path  160  from the input end  120 . Preferably, the angle  155  is sufficient to cause wavelengths of radiation not passed by the filter  105  to be reflected out of the field of view of the input end  120 , thus avoiding feedback at these wavelengths. For example, in certain preferred embodiments, the angle  155  is less than about 88 degrees or greater than about 92 degrees. An additional advantage of orienting the filter  105  to such angles  155  is that the band-pass of the filter  105  is changed to a shorter wavelength. This provides an additional means of tuning the filter&#39;s  105  performance to optimize it for a particular application.  
     [0023] The term low reflector  110  as used herein refers to any material capable of reflecting a portion of light (i.e., at least about 0.1% reflectance) received from the optical filter  105 , and transmitting the remaining portion to the output end  145 . The extent of low reflectance may be tailored to be any amount desired for particular applications. In particular, low reflectance is important to achieve optimal levels of output power and performance of the source  125 . Thus, the low reflector  110 , in this, and any other embodiments described herein, is not a mirror. The term mirror as used herein refers to a surface that reflects substantially all the light (e.g., greater than about 90%) that it receives, and does not transmit light. In contrast, the low reflector  110  has a reflection coefficient of less than about 10 percent, and more preferably less than about 6, and transmits substantially all of the balance of light  145  that is not reflected. Moreover, the low reflector  110  preferably has a reflectance and transmittance that is spectrally flat. For example, the change in reflectance and transmittance is less than about 1% and more preferably less than about 0.1%, over a band width of at least about 1 nm, and more preferably at least about 10 nm.  
     [0024] The reflective coatings may be comprised of any conventional materials well known to those of ordinary skill in the art. The low reflector  110 , for example, may comprise a glass plate or similar surface that has a desired amount of reflective coating on the surface  165  facing the optical filter  105 . In certain preferred embodiments, the opposite side  170  of the low reflector  110  may include an anti-reflective coating comprised of any conventional materials well known to those of ordinary skill in the art.  
     [0025] In certain advantageous embodiments, the low reflector  110  is oriented at an angle  175  that is substantially perpendicular to an optical path  180  from the optical filter  105 . The angle  175  is preferably sufficient to allow reflected radiation to be directed back to the source  125 , as depicted by the leftward pointing arrows in FIG. 1 and subsequent figures, thus providing feedback only at wavelengths passed by the filter  105 . For example, in certain preferred embodiments, the angle  175  is between about 88 and about 92 degrees, and more preferably between about 89 and about 91 degrees, and even more preferably between about 89.94 degrees and about 90.06 degrees.  
     [0026]FIG. 2 illustrates an alternative embodiment of the wavelength locking device  200  that folds the optical path and thereby produces a smaller device package  200 . The device components may include an analogous optical filter  205 , low reflector  210 , substrate  215 , input end  220 , collimators  235 ,  240 , output end  245 , and other above-described components similar to those depicted in FIG. 1. The optical filter&#39;s surface  250 , however, is oriented at an angle  252  that is substantially perpendicular to the optical path  260  from the input end  220  to the optical filter  205 . Additionally, the angle of orientation  255  of a reflective surface  257  in the filer  205  is configured so as to reflect radiation of the wavelengths defined by the optical filter  205  along an optical path  280 . For example, the angle  255  may be between about 43 and about 47 degrees, and more preferably between about 44 degrees and about 46 degrees. In certain preferred embodiments, the optical filter  205  may comprise a band-pass filter, for example. In such a device  200 , the low reflector  210  is preferably oriented at an angle  275  that is substantially perpendicular to the optical path  280  from the optical filter  205 .  
     [0027]FIG. 3 illustrates another alternative embodiment of the wavelength locking device  300 . Again, certain device components, including the substrate  315 , input end  320 , collimators  335 ,  340  and output end  345  are similar to those described above, with the exception that the optical filter  105  and low reflector  110  components shown in FIG. 1, form at least a portion of a monolithic, (i.e., single unit) component  385 . Specifically, a first surface  350  of the monolithic component  385  comprises the optical filter and a second surface  365  of the monolithic component  385  comprises the low reflector. Analogous to that described for device  100 , in certain preferred embodiments, the first surface  350  is oriented at a first angle  355  non-perpendicular to a first optical path  360  from the device&#39;s  300  input end  320 . Additionally, the second surface  365  is oriented at a second angle  375  that is substantially perpendicular to a second optical path  390  from the first surface  350 .  
     [0028]FIG. 4 illustrates yet another alternative embodiment of the wavelength locking device  400 . Similar to the device depicted in FIG. 1, the device  400  may include an optical filter  405 , low reflector  410 , substrate  415 , collimators  435 ,  440 , output end  445 , and other above-described embodiments. In certain embodiments analogous to the device  300  shown in FIG. 3, the device  400  may include a monolith component (not shown), similar to that described for device  300 , instead of the separate optical filter  405  and low reflector  410  components. In addition, the device  400  includes a radiation source  425  attached to the substrate  415  that provides input to the optical filter  405  along an optical path  460 . The device  400  may further include a first collimator  435 , attached to the substrate  415  between the optical filter  405  and source  425 . The device  400  may also include a second collimator  440  attached to the substrate  415  between the low reflector  410  and the output end  445  of the device  400 . In certain preferred embodiments, the second collimator  440  may comprise a fiber collimator.  
     [0029]FIG. 5 illustrates still another alternative embodiment of the wavelength locking device  500 . Similar to the above described devices, the device  500  may include an optical filter  505 , low reflector  510 , substrate  515 , collimators  535 ,  540 , output end  545 , and other above-described embodiments, such as an alternative monolith component as discussed above. In addition, the device  500  includes a radiation source  525  attached to the substrate  515  and providing input to the optical filter  505 . The device  500  may further include a first collimator  535  attached to the substrate  515  between the optical filter  505  and the source  525 . A second collimator  540  may also be attached between the source  525  and the output end  545  of the device  500 . In certain preferred embodiments, the collimators  535 ,  545  may comprise laser collimators. In yet other embodiments, the device  500  may further include a fiber collimator  595  attached to the substrate  515 , between the second collimator  540  and the output end  545 .  
     [0030]FIG. 6, illustrates, by flow diagram, another aspect of the present invention, a method  600  of manufacturing a wavelength locking device, similar to the devices illustrated in FIGS.  1 - 5 . The method  600 , may comprise providing a substrate in step  605 . This may be followed by a step  610  of attaching an optical filter to the substrate and a step  615  of attaching a low reflector to the substrate. Step  615  further includes attaching the optical filter between the low reflector and an input end of the wavelength locking device. As discussed above, the low reflector and optical filter cooperate to lock an oscillation wavelength of a radiation source to a wavelength substantially determined by the optical filter.  
     [0031] Attaching the optical filter  610  may further include a step  620  of orienting a surface of the optical filter to an angle substantially non-perpendicular to a first optical path from the input end. Alternatively, for the manufacture of devices analogous to that depicted in FIG. 2, the optical filter may be oriented in a step  625  an angle so as to reflect radiation only of certain wavelengths defined by an optical filter along an optical path that is substantially perpendicular to the optical path from the input end to the optical filter. Attaching the low reflector  615  may further include a step  630  of orienting a surface of the low reflector to an angle substantially perpendicular to an optical path from the optical filter.  
     [0032] In certain alternative embodiments, the steps  610  and  615  of attaching the low reflector and optical filter, respectively, may be achieved by a single step  635  of attaching a monolithic component to the substrate. As depicted in FIG. 3, for such devices  300 , a first surface of the monolithic component comprises the optical filter and a second surface of the monolithic component comprises the low reflector. The step  635  of attaching the monolithic component may further include a step  640  of orienting a first and second surface of the monolithic component to angles substantially non-perpendicular and perpendicular to a first and second optical path, respectively, analogous to steps  620  and  625  described above.  
     [0033] Yet other alternative embodiments, such as the manufacture of devices shown in FIGS. 4 and 5 for example, may further include a step  645  of attaching a radiation source to the substrate. Certain embodiments, such as the manufacture of the device  400  shown in FIG. 4 for example, may further include a step  650  of attaching a first collimator to the substrate between the optical filter (OF) and the source. These embodiments may also include a step  655  of attaching a second collimator to the substrate between the low reflector (PR) and an output end of the wavelength locking device. And, in certain preferred embodiments, step  655  may comprise attaching a fiber collimator.  
     [0034] Alternative advantageous embodiments, such as the manufacture of the device  500  shown in FIG. 5 for example, may further include a step  660  of attaching a first collimator to the substrate between the source and the optical filter. Such embodiments may further include a step  665  of attaching a second collimator between the source and an output end of the wavelength locking device. These embodiments may further include a step  670  of attaching a fiber collimator between the second collimator (C) and the output end.  
     [0035]FIG. 7, illustrated yet another embodiment of the present invention, an optical communication system  700 , which may form one environment where a wavelength locking device  705 , such as the device  100  shown in FIG. 1 for example, may be included. Analogous to the device shown in FIG. 1, and as described in detail above, the wavelength locking device  705 , includes a low reflector  105  and optical filter  110  and other optional components as discussed above and illustrated in FIG. 1. However, all other alternative and preferred embodiments described in the context of the device  100 , shown in FIG. 1, or other devices discussed above may be also applied to the device  705  incorporated into the optical communication system  700 .  
     [0036] The optical communication system  700  may include a radiation source  710  and an optical waveguide  715  that couples the radiation source  710  to an input end  120  (illustrated in FIG. 1) of the wavelength locking device  705 . The radiation source  710 , may comprise a number of different devices, however, in an exemplary embodiment the source device  710  comprises an optical signal source, such as a semiconductor diode laser. Such devices may include group III-V based device, for example an indium phosphide/indium gallium arsenide phosphide (InP/InGaAsP) based device, a gallium arsenide (GaAs) based device, an aluminum gallium arsenide (AlGaAs) based device, or another group III-V based device. Even though the present invention is discussed in the context of a group III-V based device, it should be understood that the present invention is not limited to group III-V compounds and that other compounds located outside groups III-V may be used.  
     [0037] The system  700  may also include an optical waveguide  720  coupling the wavelength locking device  705  to a receiver  725 . The optical communication system  700 , however, is not limited to merely the devices previously mentioned. For example, the optical communication system  700  may further include various photodetectors, optical combiners and optical amplifiers configured in a fashion well known to those of ordinary skill in the art.  
     [0038] Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention.