Abstract:
A wavelength locker for monitoring the wavelength drift of a laser uses a pair of detectors for detecting a power component of the laser beam and a wavelength component of the laser beam, respectively. Various positionings of the power detector and/or variations to the collimating lens provide a compact arrangement with fewer components.

Description:
FIELD OF THE INVENTION  
         [0001]    Embodiments of the present invention are directed to wavelength lockers and, more particularly, embodiments of the present invention are directed to more compact wavelength lockers conserving valuable package space.  
         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 for each channel. The fiber can simultaneously carry multiple channels in this manner. At a receiving end, these 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 carried by a fiber 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. This period is well within the expected service life of modern laser diodes. Hence, this wavelength drift is unacceptable as a given channel may drift and interfere with adjacent channels causing cross talk. To remedy this situation 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 to adjust various parameters, such as temperature or drive current, of the laser diode 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.  
           [0005]    [0005]FIGS. 1A and 1B show a type of conventional wavelength locker configuration. A laser  6  produces a laser beam centered about a set frequency or wavelength. The laser  6  emits a light beam from both a front facet  15  and a back facet  13 . The actual modulated light carrying the data channel emerges from the front facet  16 , which is coupled to an optical fiber (not shown). The beam  12  that emerges from the back facet  13  is used for monitoring purposes since it has the same wavelength as the beam emerging from the front facet  15 . The monitored beam  12  passes through a lens  8 . A beam splitter  10  splits a monitored beam  12  into two beams. The first beam  14  passes through the splitter  10  and is received by a first detector  16 , hereinafter referred to as the power monitor detector  16 . The second beam  20  is deflected and passes through a wavelength filter (etalon)  22  after which it is received by a second detector  24 , hereinafter referred to as the filter detector  24 .  
           [0006]    In operation, the detectors  16  and  24 , which may be for example, photodiode or optoelectrical detectors, output an electric signal based on the optical input of the received beam. The first detector  16  receives the first beam  14  and outputs a signal that is a function of the monitored beam&#39;s  12  power. The second detector  24  receives the second beam  20  and outputs a signal that is a function of both the monitored beam&#39;s  12  power as well as its wavelength. Thus, by mathematically operating on these signals as output by the detectors,  16  and  24 , the wavelength of the monitored laser beam  12  can be determined and compared to the set frequency to determine any wavelength drift of the laser&#39;s  6  output.  
           [0007]    The above configuration includes a beam splitter  10  as well as a filter  22  and second detector  24 , positioned perpendicular to the optical axis of the monitored beam  12 . Thus, this arrangement takes up an undesirably large amount of space in an optical device package.  
           [0008]    [0008]FIG. 2 shows an alternate wavelength locker configuration that uses a “stacked” arrangement of detectors. As shown, the filter detector  26  and the power monitor detector  27  are stacked one on top of the other with a filter  28  placed in front of the filter detector  26 . A collimated beam  29  strikes both of the detectors,  26  and  27  with the lower portion of the beam  29  first passing through the filter  28  prior to striking the filter detector  26 . Unfortunately, in this configuration the center portion of the collimated beam  29  where the power of the beam is the highest is not used. Thus, this configuration is not as sensitive to detect small changes in the beam as is desired.  
           [0009]    Since optoelectronics packaging is one of the most difficult and costly operations in the manufacturing process, designers are always striving for simpler more compact cost effective arrangements and solutions.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The following is a brief description of the drawings, wherein like numerals indicate like elements throughout:  
         [0011]    [0011]FIG. 1A is a plan view of conventional configuration for a wavelength locker;  
         [0012]    [0012]FIG. 1B is a block diagram of the wavelength locker shown in FIG. 1A;  
         [0013]    [0013]FIG. 2 is a block diagram of a conventional stacked detector wavelength locker;  
         [0014]    [0014]FIG. 3 is a block diagram of a wavelength locker according to one embodiment of the invention;  
         [0015]    [0015]FIG. 4 is a plan view of wavelength locker according to one embodiment of the invention;  
         [0016]    [0016]FIG. 5 is a diagram plotting the filter response for various placements of the power monitor detector;  
         [0017]    [0017]FIG. 6 is a block diagram of a wavelength locker according to another embodiment of the invention;  
         [0018]    FIGS.  7 A- 7 B show plan views of yet another embodiment of the invention using the GRIN lens as both a collimator and a beam splitter; and  
         [0019]    [0019]FIG. 8 is a ray tracing diagram showing the operation of the GRIN lens of FIGS.  7 A- 7 B.  
     
    
     DETAILED DESCRIPTION  
       [0020]    One embodiment of the present invention is shown in FIG. 3. Here, a back facet  30  of a laser diode  32  outputs a monitored beam  34 . The monitored beam  34  passes through a lens  36  to produce a collimated beam  38 . The collimated beam  38  passes through a filter (etalon)  40  and thereafter the now collimated, filtered beam  42  falls on a filter detector  46  which outputs a signal indicating the power of the beam  34  as well as the wavelength of the beam being output by the laser diode  32 .  
         [0021]    Unlike the conventional examples shown in FIGS. 1A and 1B, no beam splitter is used. Instead, the second detector  48  is placed directly in the path of the monitored beam in front of the lens  36 . The signals output by the detectors,  46  and  48 , can be mathematically operated on to determine the wavelength of the monitored beam  34 . Two cases for possible placement of the second detector are shown in FIG. 3. In the first case (case 1), the power monitor detector  48  is centered in the path of the monitored beam  34  about 10 μm behind the laser  32 . In the second case (case 2) the power monitor detector  48 ′ is placed about 30 μm behind the laser and offset to one side by about 10 μm.  
         [0022]    [0022]FIG. 4 shows a set up for testing the impact of a detector between the lens  36  and the laser  32  on the etalon  40  to measure the etalon response. As shown, the set up comprises a laser diode  32  mounted on a substrate  31 . The power monitor detector  48  is also mounted on the substrate behind the laser diode  32 . A collimating lens  32  collimates the light from the laser  32  which is then filtered by filter  40  and is detected by the filter detector  46 .  
         [0023]    Measurements were taken with the power monitor photodiode  48  placed at two different locations as discussed above. For the first measurement, the power monitor photodiode  40  was placed approximately 10 μm behind the laser diode  32 . For the second measurement, the power monitor photodiode  40  was placed approximately 30 μm behind and 10 μm to the side of the laser diode  32 . In both cases, sufficient light was collected by filter detector  46  for the wavelength locker to operate within acceptable specifications. For the disclosed embodiments a minimum signal strength of 20 μA output by the filter detector  36  is required for effective wavelength locking. In the first case, the light collected produced a 136 μA signal output from the filter detector  46 . In the second case, a 72 μA signal was produced from the collected light by the filter detector  46 . Both, well within the acceptable range.  
         [0024]    In addition to signal strength, the extinction ratio (ER) is also a factor that needs to be considered. When positioning the power monitor detector  40  in the direct path of the monitored laser beam it blocks some of the light that would otherwise pass through the etalon  40  and reach the filter detector  46 . The extinction ratio (ER) is a measure of the effectiveness of the etalon filter for wavelength locking. The extinction ratio is defined as:  
         [0025]    ER=(Maximum filter detector current)/(minimum filter detector current). The minimum ER specification for the disclosed embodiments is 3 dB.  
         [0026]    As shown in FIG. 5, without the power monitor detector  48  partially blocking the path of the laser, the measured ER was 4.9 dB. With the detector 10 μm behind the laser, the measured ER was 4.3 dB. Finally, with the detector 30 μm behind the laser 32 and 10 μm to the side of the laser  32  a higher ER of 5.3 dB was measured. These measurements are shown in FIG. 5 which again demonstrates that a sufficient ER measurement can be obtained. In particular, it is noted that there is no appreciable change in etalon response as the power monitor detector  48  is repositioned between the etalon  40  and the laser  32 .  
         [0027]    This embodiment of the invention eliminates the need for a beam splitter as well as reduces the overall footprint of the wavelength locker saving package space. Of course, the examples offered show the power monitor detector  48  in two alternate positions; however, it is understood by those skilled in the art that the power detector  48  could be anywhere within the area of the beam  34  so long as sufficient light can be gathered by the detectors  40  and  46 . For example, the power detector may be positioned 5-15 μm behind the laser 32 and 20-40 μm to the side of the laser  32 .  
         [0028]    [0028]FIG. 6 shows another embodiment of the invention that uses a lens having an angled, polished face to split the monitored beam between the two detectors. As shown, the back facet  60  of a laser diode  62  outputs a monitored beam  64  which is collimated through a micro-gradient index (GRIN) lens  65 . The end face  66  of the GRIN  65  is angled at 45 degrees and is coated with a broadband partially reflective coating. Of course other angles may be appropriate such as in a range between 30-60 degrees. The GRIN lens  65  used in this fashion permits the use of a single element as both a collimator and a splitter.  
         [0029]    The splitting ratio can be selected by the appropriate selection of the coating material. For example, a coating may be selected to provide for 30% transmission and 70% reflection of passing light. A thin film filter  67  filters the reflected beam. The power monitor detector  68  gives a signal (signal  1 ) proportional to power only and the filter detector  69  gives a signal (signal  2 ) that is a function of wavelength and power. As before, by mathematically operating on these two signals, as with controller  61 , the wavelength of the monitored beam  64  can be determined.  
         [0030]    Alternatively, the filter  67  can be omitted and instead, a thin film filter  65  can be applied directly on the GRIN end face  66 . In this case, both detectors,  67  and  68 , produce a signal having a function of wavelength since filtered light reaches both detectors. In this case, the sum of the two signals can be used to monitor the power of the laser. Further, in this alternate arrangement, the difference of the two detector signals has twice the slope vs. wavelength compare the case when the filter  67  is used, effectively enhancing the wavelength locker sensitivity.  
         [0031]    FIGS.  7 A-B show yet another embodiment of the present invention similar to the embodiment shown in FIG. 4. A laser  70  is mounted on a sub-mount  71  on a substrate  72 . A monitored beam from the back facet of the laser  70  is collimated with a GRIN lens  73 . A thin film reflective coating filter  74  is placed on the far end of the GRIN lens  73  that allows a portion of the monitored beam to pass through. As shown in FIG. 7A the portion of the monitored beam that passes through is filtered by a filter  75  and then passes to the filter detector  76 . In FIG. 7B, the filter  75  is replaced by a thin film filter  75 ′ also coating the GRIN lens  73  However, unlike the previous embodiments, the power detector  77  is placed adjacent to the laser  70  since a second portion of the monitored light is reflected back through the GRIN lens by the thin film reflective coating  74 .  
         [0032]    This is better shown in FIG. 8. The GRIN lens  73  collects the monitored light  78  from the laser diode  70 . The GRIN lens  73  collimates the light. The partially reflective coating  74  applied on the end face of the GRIN reflects a portion of the light back  79  while allowing another portion of the light to pass  80 . The light reflected back is focused on the power detector  77  located near the laser  77 . In this configuration the GRIN lens  73  acts as both a lens and beam splitter. This wavelength locker can be tuned simply by moving the lens  73  in translation or rotate the assembly containing the filter  75 .  
         [0033]    Several 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.