Abstract:
Apparatus and methods are provided for measuring a selected optical behavior of a tunable opto-electric device by using the electrical characteristics of the opto-electronic device. The benefit of the present invention is the elimination or reduction in complexity of optical wavelength reference hardware that is currently required for wavelength referencing and locking. Accordingly, the present invention reduces the cost and complexity of the optical packaging of tunable opto-electronic telecommunication components. Furthermore, the present invention also significantly simplifies optical and electronic design of system level products with tunable opto-electronic devices.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION  
       [0001]    This patent application claims benefit of pending prior U.S. Provisional Patent Application Serial No.  60 / 348 , 230 , filed Oct. 19, 2001 for ELECTRONIC METHOD AND APPARATUS FOR MEASURING OPTICAL WAVELENGTH AND LOCKING TO A SET OPTICAL WAVELENGTH OF FABRY-PEROT TUNABLE CAVITY OPTO-ELECTRONIC DEVICES, which patent application is hereby incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to semiconductor optoelectronic devices in general and, more particularly, to wavelength tunable surface emitting semiconductor lasers and wavelength tunable filters.  
         BACKGROUND OF THE INVENTION  
         [0003]    Tunable wavelength Fabry-Perot based filters and VCSELs find a variety of applications in WDM telecommunications equipment. The wavelength tunability of these devices originates from the Fabry-Perot cavity gap change under the influence of a control signal. The tuning curve function describing the relationship between the control signal and the wavelength of the filter passband or the lasing wavelength is usually highly dependent on environmental conditions and changes with time due to polarization effects in the structure. In a first case, which involves WDM systems where the transmission channel wavelength has to comply with ITU grid, it is very important to precisely lock the lasing wavelength of a tunable laser to a particular wavelength value on the grid. In a second case, which involves tunable add/drop applications, the tunable laser must also be locked to a particular ITU channel. In a third case, which involves scanning applications, it is important to know the passband wavelength at a given moment in time to identify the ITU channel that the system is sampling. The first two cases are referred to as wavelength locking. The third case is known as wavelength referencing.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with an aspect of the present invention, a method and apparatus are provided for using the electrical characteristics of an opto-electronic device to measure the optical behavior of that device. The benefit of the present invention is the elimination or significant reduction of optical wavelength reference hardware that is currently required for wavelength referencing and locking. Accordingly, the present invention reduces the cost and complexity of the optical packaging of tunable opto-electronic telecommunication components. Furthermore, the present invention also significantly simplifies optical and electronic design of system level products with tunable opto-electronic devices. It is important to note that optical and electrical wavelength locking and referencing techniques do not mutually exclude each other and can be combined in many different ways to compliment each other.  
           [0005]    The above and other features of the invention, including various novel details of constructions and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and apparatus embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    These and other objects and features of the present invention will be more fully disclosed by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:  
         [0007]    [0007]FIG. 1 is a schematic side sectional view of a tunable Fabry-Perot filter;  
         [0008]    [0008]FIG. 2 is a schematic side sectional view of a tunable VCSEL;  
         [0009]    [0009]FIG. 3 is a schematic diagram of a first novel circuit implementing a method and apparatus for optical tunable device capacitance measurement in accordance with the present invention; and  
         [0010]    [0010]FIG. 4 is a schematic diagram of a second novel circuit implementing a method and apparatus for optical tunable device capacitance measurement in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]    In pending prior U.S. patent application Ser. No. 10/136,057, filed Apr. 29, 2002 by Parviz Tayebati et al. for MICROELECTROMECHANICALLY TUNABLE, CONFOCAL, VERTICAL CAVITY SURFACE EMITTING LASER AND FABRY-PEROT FILTER (Attorney&#39;s Docket No. CORE-33 CON 2), and in pending prior U.S. patent application Ser. No. 09/543,318, filed Apr. 5, 2000 by Peidong Wang et al. for SINGLE MODE OPERATION OF MICROELECTROMECHANICALLY TUNABLE, HALF-SYMMETRIC, VERTICAL CAVITY SURFACE EMITTING LASERS (Attorney&#39;s Docket No. CORE-53 CON), and in pending prior U.S. patent application Ser. No. 09/750,434, filed Dec. 28, 2000 by Peidong Wang et al. for TUNABLE FABRY-PEROT FILTER AND TUNABLE VERTICAL CAVITY SURFACE EMITTING LASER (Attorney&#39;s Docket No. CORE-67), there are disclosed tunable Fabry-Perot filters and tunable vertical cavity surface emitting lasers (VCSEL&#39;s). The three aforementioned patent applications are hereby incorporated herein by reference.  
         [0012]    Looking now at FIG. 1, there is shown a tunable Fabry-Perot filter  5 . Filter  5  generally comprises a substrate  10 , a bottom mirror  15  mounted to the top of substrate  10 , a bottom electrode  20  mounted to the top of bottom mirror  15 , a thin membrane support  25  atop bottom electrode  20 , a top electrode  30  fixed to the underside of thin membrane support  25 , a reinforcer  35  fixed to the outside perimeter of thin membrane support  25 , and a confocal top mirror  40  set atop thin membrane support  25 , with an air cavity  45  being formed between bottom mirror  15  and top mirror  40 .  
         [0013]    As a result of this construction, a Fabry-Perot filter is effectively created between top mirror  40  and bottom mirror  15 . Furthermore, by applying an appropriate voltage across top electrode  30  and bottom electrode  20 , the position of top mirror  40  can be changed relative to bottom mirror  15 , whereby to change the length of the Fabry-Perot cavity, and hence tune Fabry-Perot filter  5 .  
         [0014]    Correspondingly, and looking next at FIG. 2, a tunable vertical cavity surface emitting laser (VCSEL)  50  can be constructed by positioning a gain region (or “active region”)  55  between bottom mirror  15  and bottom electrode  20 . As a result, when gain region  55  is appropriately stimulated, e.g., by optical pumping, lasing can be established between top mirror  40  and bottom mirror  15 . Furthermore, by applying an appropriate voltage across top electrode  30  and bottom electrode  20 , the position of top mirror  40  can be changed relative to bottom mirror  15 , whereby to change the length of the laser&#39;s resonant cavity, and hence tune VCSEL  50 .  
         [0015]    Tunable Fabry-Perot filters and tunable VCSEL&#39;s of the type disclosed above are advantageous, since they can be quickly and easily tuned to a desired wavelength by simply changing the voltage applied across the top electrode and the bottom electrode.  
         [0016]    However, it has also been found that tunable Fabry-Perot filters and tunable VCSEL&#39;s of the type disclosed above have performance characteristics which can vary slightly from unit to unit. In addition, it has also been found that the performance characteristics of any given unit can vary slightly in accordance with its age, temperature, etc. Accordingly, it is generally not possible to precisely predict in advance the exact voltage which must be applied to a particular device in order to tune that device to a specific wavelength. This can present an issue in some applications, particularly telecommunications applications, where the devices may need to be tuned to precise, known wavelengths (e.g., to the ITU WDM grid).  
         [0017]    The present invention is based on the fact that the gap of Fabry-Perot cavity determining the working wavelength of a tunable device can be found from the electric capacitance of the cavity using the following formulae: 
           C ( x ):=(ε 0   ·ε·A )/ x   
         [0018]    where ε 0 =dielectric constant of vacuum;  
         [0019]    ε=dielectric constant of the substance filling the cavity;  
         [0020]    A=cavity cross-section; and  
         [0021]    x=cavity gap.  
         [0022]    The cavity wavelength can be found in the first approximation as follows: 
         λ:=x/n, 
         [0023]    where n=Fabry-Perot cavity order.  
         [0024]    It is important to note that using an AC based capacitance measurement will allow a reduction of the influence of polarization of various layers of device structure of filter  5  or VCSEL  50  on the result of the gap measurement between bottom and top electrodes  20  and  30 . Accordingly, the accuracy with which the wavelength is defined is increased.  
         [0025]    A convenient method to measure the capacitance between bottom and top electrodes  20  and  30  is to create an LC (where L is inductance and C is capacitance of the cavity) circuit or an RC (where R is resistance) circuit and characterize the electric frequency response of this circuit. Since the value of L or R is known, the value of C can be found. Placing the inductor L or resistor R inside a thermally stabilized and hermetically sealed telecommunication package makes the inductance and resistance values more stable, thereby improving the accuracy of capacitance (and thus wavelength) measurement.  
         [0026]    In the present invention, two preferred methods, and apparatus associated with each of the methods, are shown hereinbelow to implement the above capacitance measurement.  
         [0027]    Referring to FIG. 3, there is shown a circuit  60  implementing a first method for optical tunable device capacitance measurement, which involves providing an electrical frequency generator  65 . The output signal from electrical frequency generator  65  is provided to the RC or LC circuit  60  that contains the Fabry-Perot optical tunable device  70  and electrical element  75 . The response of circuit  60  to the input frequency signal is detected with an electric measurement circuit  80 . Fabry-Perot optical tunable device  70  and electrical element  75  are located inside the optical package  85 . A tuning circuit  95  generates a tuning signal that controls the wavelength of Fabry-Perot optical tunable device  70 .  
         [0028]    The first method involves providing an electrical frequency generator  65 , feeding its signal to the RC or LC circuit  60  (containing the Fabry-Perot optical tunable device  70 ) and detecting the reaction of circuit  60  to the input frequency of electrical frequency generator  65 , and wherein the optical wavelength is determined therefrom using the formula described above.  
         [0029]    A second method involves construction of an electrical oscillator where the electrical capacitance of the Fabry-Perot optical tunable device is a part of a frequency setting circuit. Measuring the frequency of the generated signal allows determination of the capacitance of the optical tunable device and, in turn, this capacitance is used to determine optical wavelength generated by the optical tunable device.  
         [0030]    Looking now at FIG. 4, there is shown one implementation of the second capacitance measurement method that involves construction of an electrical oscillator  100 . A frequency setting circuit  105  includes the electrical capacitance of the Fabry-Perot optical tunable device  110  in concert with the electrical component  115 . Measuring the frequency of the generated signal by the electric frequency measurement circuit  120  allows determination of the capacitance of the optical device and thus determines the optical wavelength. Tuning circuit  125  generates a tuning signal that controls the wavelength of Fabry-Perot optical tunable device  110 .  
         [0031]    Still looking at FIG. 4, in a preferred embodiment of the invention, electrical Oscillator  100 , Fabry-Perot optoelectronic device  110 , and electrical component  115  are disposed inside the optical componenet package  130  so as to provide a strictly controlled environment. This environment, in turn, improves the accuracy of the capacitance measurement.  
         [0032]    Both the methods and apparatus described above can be used in optical wavelength locking as well as wavelength referencing applications. In wavelength referencing applications, the optical wavelength is measured in accordance with either of the two methods described above and may be used as a direct measurement.  
         [0033]    In wavelength locking applications, the electrical signal generated by one of the two methods representing the optical wavelength is used as a feedback signal in an electrical closed loop control circuit. For instance, an optical tunable device may be locked to a certain optical wavelength by defining a corresponding electrical signal frequency. An oscillator circuit, including the electrical capacitance of the Fabry-Perot optical tunable device as part of a frequency setting circuit, can be considered a controlled variable frequency oscillator. The frequency of this oscillator can be locked to a stable reference frequency using a Phase-locked Loop (PLL) thus locking the working optical wavelength of a tunable Fabry-Perot optical filter or VCSEL to a certain wavelength value. And this frequency can be an optical wavelength value corresponding to a channel on the ITU grid.  
         [0034]    It will be recognized that many configurations similar to those described above can be designed using different values, combinations and architectures which will yield the same results as the claimed invention. Thus, while this invention has been particularly shown and described with references to preferred embodiments herein, it is understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims.