Abstract:
Wavelength lockers, which monitor the wavelength drift of a laser&#39;s beam due to aging, are tunable to adjust for manufacturing misalignments between its various optical components. Tuning is accomplished by adjusting the position of a collimating lens using a flexure. Adjusting the lens position changes the angle of incidence of a monitored beam relative to a fixed filter and detector and avoids the challenging manufacturing task of rotating the filter and detector to achieve the same result.

Description:
FIELD OF THE INVENTION  
         [0001]    Embodiments of the present invention are directed to wavelength lockers and, more particularly, to arrangements and methods for tuning wavelength lockers.  
         BACKGROUND INFORMATION  
         [0002]    Wavelength division multiplexing (WDM) is a technique used to transmit multiple channels of data simultaneously over the same optic fiber. At a transmitter end, different data channels are modulated using light having different wavelengths or, colors if you will, for each channel. The fiber can simultaneously carry multiple channels in this manner. At a receiving end, these multiplex channels are easily separated prior to demodulation using appropriate wavelength filtering techniques.  
           [0003]    The need to transmit greater amounts of data over a fiber has led to so-called Dense Wavelength Division Multiplexing (DWDM). DWDM involves packing additional channels into a given bandwidth space. The resultant narrower spacing between adjacent channels in DWDM systems demands precision wavelength accuracy from the transmitting laser diodes.  
           [0004]    Unfortunately, as laser diodes age, they are known to exhibit a wavelength drift of up to 0.15 nm from their set frequency over about a fifteen year period. In a DWDM system this wavelength drift is unacceptable as a given channel may drift and interfere with adjacent channels causing cross talk. Thus, most laser transmitters use what is commonly referred to in the art as a wavelength locker to measure drift frequency vs. set frequency. This information can be fed back to a controller such that, various parameters, such as temperature or drive current, of the laser diode can be adjusted to compensate for the effects of aging and keep the diode laser operating at its set frequency. Most laser transmitters with an integrated wavelength locker use either an etalon or thin film filter to measure the laser wavelength variation. In order to work accurately, it is important that the locker be tuned by precision alignment with the collimated beam being monitored. A typical wavelength locker requires a placement accuracy of the etalon and collimated beam within better than +/−0.5 degree. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    The following is a brief description of the drawings, wherein like numerals indicate like elements throughout:  
         [0006]    [0006]FIG. 1 is a plan view of a wavelength locker according to one embodiment of the invention;  
         [0007]    [0007]FIG. 2 is a block diagram showing a simplified view of the wavelength locker of FIG. 1;  
         [0008]    [0008]FIG. 3 is a graph plotting output current from the wavelength locker detectors against frequency;  
         [0009]    [0009]FIG. 4 is a graph showing peak transmission frequency verses beam angle of incidence on the etalon; and  
         [0010]    [0010]FIG. 5 is a block diagram showing a flexure used to adjust the height of a collimating lens. 
     
    
     DETAILED DESCRIPTION  
       [0011]    Referring now to the drawings and, more particularly to FIG. 1, one embodiment of the present invention shows a wavelength locker package having a quasi-planar substrate  2  having a positioning floor  4 . First and second detectors,  10  and  12 , respectively, are attached to the floor  4 . A laser  14 , produces a laser beam centered about a set frequency or wavelength. The laser  14  emits a light beam from both a front facet  16  and a back facet  18 . The actual modulated light carrying the data channel emerges from the front facet  16 , which is coupled to an optical fiber  20 . The beam  22  that emerges from the back facet  18  and is used for monitoring purposes. A lens  28 , such as a graded index (GRIN) lens, is used to collimate the beam  22 .  
         [0012]    These components require precise alignment and present many challenges during manufacture when actually attaching the various devices to the positioning floor  4 . In high performance opto-electronic packages, such as these, critical optical elements require more precise placement than can be obtained with the combination of floor height control and two-dimensional pick and place.  
         [0013]    Thus as shown, the fiber  20  is mounted using a miniature flexure  30  which allows for a small amount of vertical adjustment. In one embodiment, the flexure  30  is made of thin spring steel that has been etched or stamped, then bent in a press. The flexure  30  may comprise of two or more legs  32  which rest on the substrate surface or positioning floor  4 . In one embodiment, the legs are joined by a bridge  34  that supports or clamps the fiber  20 . When the bridge  34  is translated in the y direction, opposite legs  32  give elastically in opposite x directions.  
         [0014]    The flexure  30  may be designed so that in its natural or non-flexed state, the optical axis of the optical component attached to the bridge rests slightly above the optical plane of the package. Final adjustment of the height is obtained by applying pressure to the flexure  30 , thereby lowering the bridge  34  height. Dragging the flexure  30  in the plane parallel to the plane of the substrate may be used to correct the lateral position. When adequate alignment is reached, the legs  32  are permanently attached to the floor  4  or substrate  2 . The attachment may be by, for example, laser welding, soldering, or adhesive bonding.  
         [0015]    In another refinement of the flexure design, the flexure  30  has more than two legs. The first pair of legs is attached to the frame after coarse optical alignment. The flexure is then finely realigned, using the residual flexibility left after the two first legs are attached. When the optimum position is reached, the remaining legs are attached. Various flexure designs are described in U.S. Pat. Nos. 6,207,950 and 6,227,724.  
         [0016]    Still referring to FIG. 1, in operation a collimated beam  22  emerges from lens  28  and thereafter encounters a splitter  24  that splits the beam  22  into two additional beams. The first beam is shown reflected at an angle normal to the original beam  22 . This reflected beam passes through an etalon (filter)  26  and then falls on the first detector  10 . The second beam passes straight through the splitter  24  and falls upon the second detector  12 .  
         [0017]    The portion of the beam that traverses the etalon  26  is a function of both the beam&#39;s power and the wavelength of the beam. The portion of the beam that passes directly from the splitter  24  to the second detector  12  is a function of the beam&#39;s power. Thus, by subtracting these two components the wavelength of the beam currently being output can be determined and compared to the set frequency to determine any drift of the laser&#39;s output.  
         [0018]    [0018]FIG. 2 shows a simplified block diagram of the wavelength locker according to one embodiment of the invention. As will be appreciated by those skilled in the art, this type of wavelength locker is very sensitive to alignment with the collimated beam  22  and raises many packaging concerns. The response of an etalon  26  verses the beam angle of incidence on etalon is:  
       v   =       v   0       cos        (     θ   n     )                               
 
         [0019]    Where:  
         [0020]    υ peak transmission frequency shift;  
         [0021]    υ 0  is the peak frequency at normal incidence;  
         [0022]    θ is the angle of incidence in air; and  
         [0023]    n the index of refraction of the etalon (e.g., for fused silica, n=1.44 around 1550 nm).  
         [0024]    As shown in FIG. 2, when the angle θ is zero degrees as shown, υ=υ 0 , since the cosine divisor of the above equation is 1 when θ is zero. As shown by dotted line  31 , any deviation from a zero degree angle of incidence and the response of the etalon will be skewed and the output waveform shifted out of phase.  
         [0025]    Referring to FIG. 3, there is shown a graph plotting current, I 1  and I 2 , output from the detectors,  10  and  12 , respectively, against the frequency (i.e., 1/wavelength) of the laser beam. As illustrated, the output from detector  10  (I 1 ) comprises a frequency component and a power component. The output from detector  12  (I 2 ) comprises the laser&#39;s output beam power component and is therefore illustrated as a horizontal line. The output I 1  from detector  10  comprises both a power component and a wavelength component and is therefore illustrated as a waveform shifted in the positive direction along the y axis.  
         [0026]    In operation, the wavelength locker monitors changes in wavelength or frequency. However, as illustrated in FIG. 3, this is sometimes difficult when monitored channel falls on either a peak or a trough of the frequency waveform. Thus, for example when monitoring ITU (International Telecommunication Union) channel A, if the angle of incidence θ is at a point where the output waveform I 1  is at a peak, it is difficult to monitor since the slope dl/dv=0. Hence, to more accurately monitor the frequency, tuning the angle of incidence θ is desirable to change the phase of the waveform from I 1  to I 1 ′. This allows the wavelength locker to monitor a more suitable portion of the waveform  33  where small changes in dl/dv can be more readily detected and providing a more accurate reading.  
         [0027]    However, as illustrated in FIG. 4, only a very small change in the angle of incidence is warranted. FIG. 4 shows a graph plotting frequency shift against the beam angle of incidence. When the angle of incidence θ is 0 degrees, the frequency shift is 0. As is further illustrated in this example, the measured wavelength currently being output by the laser  14  is very sensitive to the angle of incident. As shown, a shift in the angle of incident of less that 1 degree results in a 10 GHz measured frequency shift. This skew is exponential in nature as a rotation of the etalon  26  and detector  10  to the collimated beam of as little as 2 degrees causes a measured frequency shift of greater than 50 GHz. As those skilled in the art will appreciate, in order to shift the waveform I 1  to I 1 ′ as shown in FIG. 3, only slight changes in the angle of incidence is need.  
         [0028]    One way of changing the angle of incidence θ is by controlling the rotation of the etalon  26  and first detector  10  relative to the beam. That is, when mounting these items during the manufacturing process, care must be taken to ensure that they are positioned so that the angle is precisely rotated to the desired angle from zero (i.e., so that the portion of the beam reflected from the splitter  24 ′ is slightly rotated from the normal to the etalon  26  and detector  10 ). However, in practice this is sometimes difficult to achieve and some fine tuning of the wavelength locker components may be necessary after the various components are fixed in place relative to the substrate.  
         [0029]    According to one embodiment of the invention, it has been discovered that rather than rotating the detector  10  and etalon  26 , fine tuning of the wavelength locker can be achieved simply by adjusting the position of the flexure  36 , and thus the position of the lens  28  (i.e., in the x and/or y-directions). That is, translating the lens  28  up and down and side to side relative to the substrate  2  has the same effect as rotating the detector  10  and etalon  26  relative to the beam reflected from the splitter  24  and thus changes the angle of incidence. However, unlike rotating the detector and etalon which would require additional machining components, fine tuning of the angle of incidence can be achieved by using the machines already developed for flexure coupling.  
         [0030]    Referring again to any of FIGS. 1, 4, or  5 , in one embodiment of the present invention, a lens mount  36  is used that allows the lens to be easily adjusted in the x or y-directions. In one embodiment, the lens mount comprises a flexure  36 . Just as the first flexure  30  used to mount the fiber  20 , a lens flexure  36  is used to mount the lens  28  relative to the substrate  2 .  
         [0031]    [0031]FIG. 5 shows a front view looking into the lens  28 . As above, the lens flexure  36  may be designed so that in its natural or non-flexed state, the optical axis of the lens  28  mounted thereto rests slightly above the optical plane of the package. Final adjustment of the height is obtained by applying pressure to the flexure  36 , therefore lowering the lens  28  height. Lateral positioning of the lens  28  is achieved by translating the legs  38  of the flexure in the x-directions When adequate alignment is reached, the legs  38  are permanently attached to the floor or substrate  2  (not shown in FIG. 5). In this case, alignment is reached when the angle of incidence, θ, between the beam reflected from the splitter  24  and the etalon  26  and detector  10  is shifted to a desired position as illustrated in FIG. 3. The attachment may be by, for example, laser welding, soldering, adhesive bonding or any other suitable method.  
         [0032]    Embodiments of the present invention are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.