Abstract:
Numerous features may be incorporated into a wavelength locker to reduce the noise inherent therein. These features may be used in any combination thereof. These features include avoiding the use of reflectors, using a diffractive splitter which outputs evanescent beams for diffractive orders greater than one, using anti-reflective coatings, using an opaque material with through holes for the light, and designing the wavelength locker to be used at a tilt

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application Serial No. 60/325,543 entitled “Reduced Noise Wavelength Locker Module” filed on Oct. 1, 2001, the entire contents of which are hereby incorporated by for all purposes. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention is directed to a wavelength locker/monitor, more particularly to a wavelength locker/monitor which reduces noise.  
         BACKGROUND OF THE INVENTION  
         [0003]    A wavelength locker disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 09/543,760 entitled “An Etalon, a Wavelength Monitor/locker Using The Etalon and Associated Methods” filed on Apr. 5, 2000, the entire contents of which are hereby incorporated by reference for all purposes, is shown in FIG. 1. A wavelength locker module  1100  includes an optics block  105 , a pair of photodetectors  122 ,  124  and a mount  126  for housing the photodetectors  122 ,  124 . The optics block  105  includes a splitter diffractive element  110 , two reflectors  112 ,  114 , and two correcting diffractive optical elements  116 ,  118 . An etalon  120  may be mounted to the optics block  105  or otherwise positioned between the optics block and one of the photodetectors  122 ,  124 .  
           [0004]    The splitter diffractive element  110  receives an input beam  130  and splits two beams  132 ,  134  off of the input beam  130 . A through beam  136  continues on in the optical system if the wavelength locker  100  is used to monitor light output from a front facet of a light source. Typically, the split beams  132 ,  134  are the +/−1 diffractive order beams. As shown in this particular configuration, the beam  132  serves as a reference beam and the beam  134  serves as the filter beam. The reflectors  112 ,  114  reflect the reference beam  132  and the filter beam  134 , respectively, to the corrector diffractive elements  116 ,  118 , respectively. The reference beam  132  travels to the reference photodetector  122 . The filter beam  134  travels through the etalon  120  to the filter photodetector  124 .  
           [0005]    The use of the reflectors to fold the reference and filter beams allows the separation of reference and filtered beams to be large enough that the detectors may be packaged in the optical system without having to manufacture diffractives with extremely small feature size and more than two phase levels. However, these reflectors also give rise to noise in the reference and filter signals, e.g., the reflected signal of the etalon in the reference side and the etalon effect due to the parallel surfaces of the optics block  105 . Further, the reflectors potentially clip the through beam  136 , reducing the power therein and adding noise.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is therefore directed to a wavelength monitor/locker which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.  
           [0007]    It is an object of the present invention to provide various features which aid in reducing noise in a wavelength locker. These features may be used in any combination in a wavelength locker. These features include avoiding the use of reflectors, using a diffractive splitter which outputs evanescent beams for diffractive orders greater than one, using anti-reflective coatings, using an opaque material with through holes for the light, and designing the wavelength locker to be used at a tilt.  
           [0008]    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  
       [0009]    The foregoing and other objects, aspects and advantages will be described with reference to the drawings, in which:  
         [0010]    [0010]FIG. 1 is a schematic side view of an embodiment of an integrated wavelength locker;  
         [0011]    [0011]FIG. 2 is a schematic side view of an embodiment of an integrated wavelength locker of the present invention; and  
         [0012]    [0012]FIG. 3 is a schematic side view of another embodiment of the integrated wavelength locker of the present invention. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0013]    In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known devices and methods are omitted so as not to obscure the description of the present invention with unnecessary details.  
         [0014]    A wavelength locker configuration incorporating several features for noise reduction is shown in FIG. 2. Here, a “Y” wavelength locker  200 , i.e., the light path of the input, filter and reference beams forms a “Y” is shown. The “Y” wavelength locker  200  does not use reflectors, thereby eliminating one source of noise present in the configuration of FIG. 1. The “Y” wavelength locker  200  includes an optics block  205 , a pair of photodetectors  222 ,  224  and a mount  226  for housing the photodetectors. The optics block  205  includes a splitter diffractive element  210  and two correcting diffractive optical elements  216 ,  218 . An etalon  220  maybe mounted to the optics block  205  or otherwise positioned between the optics block  205  and one of the photodetectors  222 ,  224 .  
         [0015]    The splitter diffractive element  210  receives an input beam  130  and splits two beams  132 ,  134  off of the input beam  130 . The through beam  136  continues on in the optical system if the wavelength locker  200  is used to monitor light output from a front facet of a light source. Typically, the split beams  132 ,  134  are the +/−1 diffractive order beams. As shown in this particular configuration, the beam  132  serves as a reference beam and the beam  134  serves as the filter beam. The reference beam  132  and the filter beam  134  propagate to the corrector diffractive elements  216 ,  218 , respectively. The reference beam  132  travels to the reference photodetector  222 . The filter beam  134  travels through the etalon  220  to the filter photodetector  224 .  
         [0016]    The splitter diffractive element  210  may be designed to be binary with periods small enough that the higher orders than +1 and −1 are evanescent. The corrector diffractives  216 ,  218  are located on the other face of the optics block  205  and may be manufactured using any known technique. The corrector diffractives are to maintain a normal or quasi-normal beam with respect to the etalon and to limit the size of the module  200 .  
         [0017]    An anti-reflective (AR) structure  240  may be provided in the path of the through beam  136  to alleviate reflection from the output surface of the wavelength locker  200 . The AR structure  240  may have a period smaller than the working wavelengths. The AR structure  240  may be formed using a patterning technique to avoid the location of the corrector diffractives  216 ,  218 . Then, the AR structure  240  is protected during the formation of the diffractive corrector elements.  
         [0018]    Further, the mount  226  supporting the photodetectors  222 ,  224  may be a ceramic chip carrier with the photodetectors  216 ,  218  flip chip mounted thereto. The mount  226  may be provided with through holes  242 ,  244 ,  246  for the through beam  136  and the reference and filter beams  132 ,  134 , respectively. These through holes  242 ,  244 ,  246  diminish the risk of stray light hitting the photodetectors  222 ,  224 . Further, the use of flip chip bonding reduces the height of the module  200 , as well as facilitating the use of the through holes.  
         [0019]    Additionally, anti-reflective features  242  may be provided on the optics block  205  on the same surface as the splitter  210  to further reduce an etalon effect on the reference signal  134 . Since the splitter  210  is a high frequency grating, thus having very small features, using an AR coating on this surface will adversely affect the grating. Therefore, the anti-reflective feature may be a binary diffractive at 90 degrees from the splitter so that light is scattered out of the plane. Further, by varying the etch depth and/or the duty cycle of the grating for the splitter  210  in a known fashion, the splitter  210  may be designed to further act as an AR feature.  
         [0020]    Additionally or alternatively, to reduce the etalon effect from the optics block  205  on the reference signal  132  and/or the filter signal  134 , the wavelength locker  200  may be designed to be used at a tilt. The corrector diffractive elements  216 ,  218  are designed using periods that are slightly smaller or larger than for normal incidence such that the actual etalon has a normal beam. The corrector diffractive element  218  may also correct for an etalon with a free spectral range smaller than a desired value by more than 0.1 GHz by adjusting the incident angle of the beam on the etalon  220 . The tilt will further help reduce back reflection problems. The tilt may be introduced such that the angle for the reference beam  132  from the splitter  210  becomes larger with respect to normal, i.e., the reference beam  132  is shallower on the diffractive  216 , so that more power is provided to the filter beam  134 .  
         [0021]    The embodiments shown in FIGS. 1 and 2 have assumed that the wavelength locker is to monitor a beam from a front facet of a light source, i.e., an application beam is to pass therethrough. In FIG. 3, a wavelength locker  300  for use with light output from a back facet of a light source, or if no through beam is to be provided, is shown. The wavelength locker  300  includes an optics block  305  and a pair of photodetectors  322 ,  324 . The optics block  305  includes a splitter diffractive element  310  and two correcting diffractive optical elements  316 ,  318 . Since no beam is to pass through the wavelength locker  300 , an etalon  320  may be formed using two etalon coatings  350  provided on a substrate  352  mounted to the optics block  305  or otherwise positioned between the optics block  305  and one of the photodetectors  322 ,  324 . The path to the reference detector  322  may include AR coatings  340  to help reduce noise. The mount for the photodetectors  322 ,  324  may be either configuration previously shown.  
         [0022]    Again, the splitter diffractive element  310  receives an input beam  330  and splits two beams  332 ,  334  off of the input beam  330 . Typically, the split beams  332 ,  334  are the +/−1 diffractive order beams. The zero order beam is very small relative to the diffracted beams. As shown in FIG. 3, the light to be monitored is diverging, so the splitter diffractive element  310  may also collimate the input beam  330 . As shown in this particular configuration, the beam  332  serves as a reference beam and the beam  334  serves as the filter beam. The reference beam  332  and the filter beam  334  propagate to the corrector diffractive elements  316 ,  318 , respectively. The reference beam  332  travels to the reference photodetector  322 . The filter beam  334  travels through the etalon  320  to the filter photodetector  324 . The detectors  322 ,  324  may be mounted in either configuration shown in FIGS. 1 and 2.  
         [0023]    While specific embodiments for reducing noise in a wavelength locker have been illustrated, it is to be understood that many of the elements used for reducing noise may be used in any of the above configurations. For example, while the reflectors cannot be eliminated from the configuration in FIG. 1, the splitter  110  could be designed to be evanescent for orders higher than +/−1, an AR coating could be provided at the output of the through beam  136 , and/or the photodetectors could be mounted on a ceramic carrier having through holes for each beam. Further, the configuration in FIG. 1 could be tilted to reduce noise, although the reflectors could still provide a path for unwanted signals.  
         [0024]    It will be obvious that the invention may be varied in a plurality of ways, such as the use of different noise reducing features in various combinations. Such variations are not to be regarded as a departure from the scope of the invention.