Abstract:
A multiple phase wavelength locker employs an etalon with multiple steps, the steps providing optical cavities having different optical lengths for use with multiple photodetectors, such that a resonance position of each etalon step is offset by a fraction of a resonance period. The stepped etalon can be employed to track the exact wavelength of a laser in a wavelength division multiplexing (WDM) system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of co-pending and commonly-assigned U.S. provisional patent application Serial No. 60/364,246, filed Mar. 14, 2002, by Torsten Wipiejewski, and entitled “THREE PHASE WAVELENGTH LOCKER,” which application is incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to wavelength lockers for lasers, and more particularly, to a multiple phase wavelength locker.  
           [0004]    2. Description of the Related Art  
           [0005]    For widely tunable lasers, wavelength lockers usually employ one etalon and two photodetectors. FIG. 1 illustrates the structure of an optoelectronic device  100  having a laser  102 , lens  104 , isolator  106  and a wavelength locker including two taps  108 ,  110 , an etalon  112 , two photodetectors  114 ,  116 , an output beam  118  and a thermoelectric cooler (TEC)  120 . The etalon  112  and associated photodetector  116  are used to track the wavelength of the output beam  118  and provide feedback to a control circuit (not shown). The other photodetector  114  provides a reference signal.  
           [0006]    The wavelength of the output beam  118  is controlled in such a way that it is aligned with a frequency channel of the ITU (International Telecommunication Union) grid. The frequency spacing of the etalon  112  needs to be the same as the ITU grid. For most applications this is 50 GHz.  
           [0007]    Thus, the etalon  112  thickness has to be made with extremely high precision to achieve the desired wavelength (optical frequency) spacing. At the wavelength of interest, around 1550 nm, the correlation between optical frequency spacing and wavelength difference is Δf(GHz)=Δλ(pm)/8. The necessary precision makes the etalon  112  expensive.  
           [0008]    The etalon  112  resonance wavelength also needs to be aligned to the channels of the ITU grid with an offset. This requires a high precision mounting step or a way of tuning the etalon  112  after the mounting procedure. In either case, the cost of the device  100  is increased.  
           [0009]    U.S. Pat. No. 6,323,987 to Rinaudo et al. describes a different approach of a stepped etalon with the transmission through the etalon being offset by the different length of the etalon in the various positions. In Rinaudo, the transmission through the etalon depends on the position at the device. Each position exhibits almost identical transmission versus wavelength curves, but according to the different position on the etalon, the transmission curves show a phase offset. The wavelength can be determined by comparing the transmission intensity for the three etalon positions.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention discloses a monolithically integrated wavelength detection device employing a plurality of photodetectors and an etalon with a plurality of steps, the steps providing optical cavities having different optical lengths for use with the photodetectors, wherein a resonance wavelength of each cavity is spectrally offset from adjacent cavities, resulting in a spectral response of a signal from each of the photodetectors with a phase difference according to a fraction of the wavelength resonance period, and resulting in a detectable slope of a spectral response for all wavelengths. A thickness difference between adjacent steps corresponds to a wavelength offset of different resonance peaks. A sum of signals resulting from the steps of the etalon comprises a reference signal. A wavelength change can be detected independent of an absolute wavelength position of the etalon. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Referring now to the drawings in which like reference numbers represent corresponding parts throughout:  
         [0012]    [0012]FIG. 1 illustrates the structure of an optoelectronic device;  
         [0013]    [0013]FIG. 2 illustrates the structure of an optoelectronic device according to an embodiment of the invention;  
         [0014]    [0014]FIG. 3 is a graph illustrating the absorption and the corresponding relative photocurrent response in a multiple phase wavelength locker according to the preferred embodiment of the invention; and  
         [0015]    [0015]FIG. 4 illustrates the structure of an optoelectronic device according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    In the following description, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, a preferred embodiment of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.  
         [0017]    [0017]FIG. 2 illustrates the structure of an optoelectronic device according to the present invention, comprising an etalon  200  with three monolithically integrated pin-photodiodes  202 ,  204 ,  206 . The pin-photodiodes  202 ,  204 ,  206  incorporate an i-InGaAs absorbing layer  208  fabricated on top of an n-InP substrate  210  and sandwiched by a p-InP cladding layer  212 . The pin-photodiodes  202 ,  204 ,  206  may include another heavily p-type doped contact layer  214  on their top surface. An electrical connection is made by a metal electrode  216  on top of the contact layer  214  and a common backside electrode  218  on the substrate  210 .  
         [0018]    Laser output  220 ,  222 ,  226  reach the pin-photodiodes  202 ,  204 ,  206  through the substrate  210 , wherein the substrate  210  forms, together with the top surface of the pin-photodiodes  202 ,  204 ,  206 , the etalon  200 . The laser output  220 ,  222 ,  226  is absorbed in the undoped absorption layer  208  of the pin-photodiodes  202 ,  204 ,  206 . The amount of absorption is wavelength dependent according to the etalon  200  resonance. The etalon  200  thickness is identical for every integrated pin-photodiode  202 ,  204 ,  206 , except for a small thickness difference created by etching of the topmost cladding layer  212 .  
         [0019]    The thickness differences in the topmost cladding layer  212  provide three steps in the etalon  200  for use with the plurality of pin-photodiodes  202 ,  204 ,  206 . The three steps provide optical cavities having different optical lengths for use with the three pin-photodiodes  202 ,  204 ,  206 , wherein a resonance wavelength of each cavity is spectrally offset from adjacent cavities, resulting in a spectral response of a signal from each of the pin-photodiode  202 ,  204 ,  206  with a phase difference according to a fraction of the wavelength resonance period, and resulting in a detectable slope of a spectral response for all wavelengths. A thickness difference between adjacent steps corresponds to a wavelength offset of different resonance peaks. A sum of signals resulting from the steps of the etalon  200  comprises a reference signal. A wavelength change can be detected independent of an absolute wavelength position of the etalon  200 .  
         [0020]    [0020]FIG. 3 is a graph illustrating the absorption and the corresponding relative photocurrent response in the structure of FIG. 2. The absorption is proportional to the photocurrent signals detected by each of the photodiodes  202 ,  204 ,  206 , and is represented by phases  300 ,  302 ,  304 , respectively. The photocurrent signals exhibit strong resonances for certain wavelengths in a periodic way. Due to the small etalon  200  thickness difference at the steps, the position of the photocurrent signals&#39; maxima is different for the three photodiodes  202 ,  204 ,  206 , as represented by the phases  300 ,  302 ,  304 . The difference of the photocurrent signals can be used to precisely determine the wavelength of the incoming light.  
         [0021]    The photocurrent response in FIG. 3 is calculated for the case that the reflectivity of the substrate  210  back side is 10% and the top of the cladding layer  212  is 90%. Both surfaces  210  and  212  may contain additional coating layers to create the desired reflectivity values. The coating on the substrate  208  back side acts as a partial anti-reflection (AR) coating, while the coating on the top of the cladding surface  212  is a highly reflective coating. The coating on the top of the cladding surface  212  could also be a metal coating to achieve the high reflectivity, wherein no light passes through the device in the case of a metal coating.  
         [0022]    The absorbing layer  208  is 200 nm thick, with an absorption coefficient of 10,000 cm −1 . The thickness of the absorbing layer  208  could be changed to optimize the photocurrent response without changing the principle of the multiple phase wavelength locker.  
         [0023]    The highest photocurrent can be obtained when the total absorption is adjusted such that the reflectivity of the back side of the substrate  210  decreased by the amount of total absorption in the etalon  200  equals the reflectivity of the top of the cladding layer  212 . In this particular case, the thickness of the absorption layer  212  would be 550 nm. However, for the application as wavelength detection device, the photocurrent response might be adjusted according to other parameters, such as the wavelength dependence of the photocurrent signal.  
         [0024]    The thickness adjustment of the etalon  200  can be performed more easily on the top of the cladding layer  212  rather than the back side of the substrate  210 . Wet or dry etching techniques may be employed. Anodic oxidation followed by selective wet etching has been demonstrated earlier to be an adequate technique. See, for example, the publication by T. Wipiejewski et al., entitled “Multiple Wavelength Vertical Cavity Surface Emitting Laser Diode Arrays,” IEEE Photon. Tech. Lett., 1996, which publication is incorporated by reference herein.  
         [0025]    The thickness of the etalon  200  needs to be adjusted according to the different refractive index of the p-InP substrate  210  as compared to glass. In this embodiment, the etalon  200  has a total thickness, through the p-InP cladding layer  212 , of approximately 946 μm for a channel spacing of 50 GHz, while the thickness difference between adjacent steps is 81 nm, which corresponds to a one third or 120° wavelength offset of the different resonance peaks.  
         [0026]    A temperature sensor  226  can also be monolithically integrated in the substrate  210 . The temperature sensor  226  could be, for example, a simple pn-junction where there is a voltage change with temperature at a constant current. The signal from the temperature sensor  226  can be used to extend the accuracy of the wavelength locker over a wider temperature range.  
         [0027]    [0027]FIG. 4 illustrates the structure of an optoelectronic device  400  according to an another embodiment of the present invention, including a laser  402 , lens  404 , isolator  406 , output beam  408 , and a wavelength detection device, namely a multiple phase wavelength locker, comprised of lens  410 , polarizer  412 , quarter-wave plate  414 , etalon  416  and photodetectors  418 ,  420 ,  422 .  
         [0028]    The additional lens  410  focuses the light and distributes it across three paths in the wavelength locker device. The polarizer  412  and quarter-wave plate  414  are employed to reduce back reflections of these signals. For every wavelength, there is a variation in at least two of the signals, as described in FIG. 3. Thus, a wavelength change can be detected independent of the absolute wavelength position of the etalon  416 . Therefore, the total thickness of the etalon  416  is not critical. Also, no alignment to the ITU grid is necessary.  
         [0029]    The plurality of steps in the etalon  416  provide optical cavities having different optical lengths for use with the photodetectors  418 ,  420 ,  422 , wherein a resonance wavelength of each cavity is spectrally offset from adjacent cavities, resulting in a spectral response of a signal from each of the photodetectors  418 ,  420 ,  422  with a phase difference according to a fraction of the wavelength resonance period, and resulting in a detectable slope of a spectral response for all wavelengths. A thickness difference between adjacent steps corresponds to a wavelength offset of different resonance peaks. A sum of signals resulting from the steps of the etalon  416  comprises a reference signal. A wavelength change can be detected independent of an absolute wavelength position of the etalon  416 .  
         [0030]    Since the wavelength locker signal is basically independent of the wavelength, the locking range can also be extended. For example, instead of setting the locking range corresponding to the ITU channel spacing, it can also be set to a spacing of three channels (150 GHz). The wavelength locker can then detect the wavelength, even if the output beam  408  of the laser  402  is closer to a neighboring channel. This approach might relax the reliability requirements for tunable lasers  402 .  
         [0031]    Moreover, the wavelength locker is configured for back facet monitoring of the laser  402 , although front facet monitoring may be employed as well. For front facet monitoring, the polarizer  412  and the quarter-wave plate  414  may not be necessary.  
       Conclusion  
       [0032]    This concludes the description of the preferred embodiment of the invention. The following describes some alternative embodiments for accomplishing the present invention.  
         [0033]    For example, different configurations and different numbers of etalons, steps and photodetectors other than those explicitly described herein could be used without departing from the scope of the present invention. In addition, different materials and different constructions or fabrications of the etalon and photodetectors other than those explicitly described herein could be used without departing from the scope of the present invention.  
         [0034]    The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.