Abstract:
Power control for a laser is performed on at least a per-packet basis, rather than a per-pulse basis, and that end-of-life detection may similarly be performed. This is achieved by accumulating the current generated by a photodiode in response to the light signal generated by the laser, subtracting therefrom a preset threshold current which is similarly modulated in response to the data signal used to drive the laser, and comparing the resulting difference to the value prior to having begun accumulating and subtracting. The result of the comparison, which may be filtered, is used to control the driver of the laser or as an indicator, e.g., for use in end-of-life detection.

Description:
TECHNICAL FIELD 
     This invention relates to the art of controlling generated signals, and more particularly, to controlling the output power of a laser used for burst mode transmission in a passive optical network (PON). 
     BACKGROUND OF THE INVENTION 
     In prior art passive optical networks using a bursty signal format power control for the laser requires high speed circuitry, because the power of each light pulse is measured and controlled. Such high speed circuitry is expensive, and consumes substantial power. 
     SUMMARY OF THE INVENTION 
     We have recognized that when using a bursty signal format power control for the laser may be performed on at least a per-packet basis, rather than a per-pulse basis, and that end-of-life detection may similarly be performed. This is achieved by accumulating the current generated by a photodiode in response to the light signal generated by the laser, subtracting therefrom a preset threshold current which is similarly modulated in response to the data signal used to drive the laser, and comparing the resulting difference to the value prior to having begun accumulating and subtracting. The result of the comparison, which may be filtered, is used to control the driver of the laser. 
     Computationally, this is similar to integrating the current generated by the photodiode in response to the signal generated by the laser; dividing by the number of bits which drove the laser to generate light and further dividing by the length of one bit, so as to find the average peak magnitude of the bits over a packet; and comparing this average peak magnitude with a preset threshold which is set to the current level which would be generated by the photodiode when the laser is supplying the desired power level for typical use, e.g., as specified by a user. Advantageously, because the average peak magnitude over a packet is used, slower components may be employed, reducing both cost and power consumption. 
     Additionally, by using a different threshold current, e.g., one-half of the current level which would be generated by the photodiode when the laser is supplying the desired power level for typical use, end-of-life detection for the laser may be achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing: 
     FIG. 1 shows an exemplary power control for a source in accordance with the principles of the invention. 
    
    
     DETAILED DESCRIPTION 
     The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intend intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that performs the same function, regardless of structure. 
     Thus, for example, it will be appreciated by those skilled in the art that the block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. The functions of the various elements shown in the FIGs., including functional blocks labeled as “processors” may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, read-only memory (ROM) for storing software, random access memory (RAM), and non-volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the FIGS. are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementor as more specifically understood from the context. 
     In the claims hereof any element expressed as a means for performing a specified function is intended to encompass any way of performing that function including, for example, a) a combination of circuit elements which performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function. The invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. Applicant thus regards any means which can provide those functionalities as equivalent as those shown herein. 
     FIG. 1 shows an exemplary power control for a source in accordance with the principles of the invention. More specifically, shown in FIG. 1 are a) current source  103 , b) precharge voltage source  105 , c) threshold current source  107 , d) comparator  109 , e) switches  111  and  113 , f) controllable gain driver  119 , and g) up/down counter  121 . 
     Current source  103  generates current pulses. When used to control laser power, such as for use in a passive optical network (PON), source  103  may be made up of photodiode  117  which is optically coupled to laser  115 . The current signal output by photodiode  117  is directly proportional to the output power supplied by laser  115 . Photodiode  117  is considerably more stable than laser  115  in terms of their respective responses to temperature and age variations. As a result, changes in the signal supplied as an output from photodiode  117  reflect, essentially, changes in the output of laser  115 . The pulses supplied from source  103  are grouped in packets, which may be supplied in a bursty manner. 
     Capacitor  101 , which is shown explicitly as part of source  103 , may be a parasitic capacitance of photodiode  117 . Alternatively, an explicit capacitor may be used, or an explicit capacitor may be used in conjunction with the parasitic capacitance of photodiode  117 . 
     In operation, prior to the beginning of a packet, e.g., during an interpacket quiescent interval and/or during a preceding packet that is not used for power measurement, reset switch  113  is closed. This precharges capacitor  101  to the voltage of precharge voltage source  105 . Note that the length of time that reset switch  113  is closed should be sufficient to accurately precharge capacitor  101  to the voltage of precharge voltage source  105 . Such precharging may be achieved more rapidly if switch  113  and voltage source  105  have a low on-resistance. Thereafter, reset switch  113  is opened. 
     The packet is then transmitted, e.g., by supplying data bits to controllable gain driver  119 , which in turn drives laser  115 . The power generated for any bit for which laser  115  supplies a pulse of light is a function of the power supplied by controllable gain driver  119  and the ability of laser  115  to turn that power into light, which is influenced by factors such as its composition, age, and temperature. The amplitude of the output signal of controllable gain driver  119 , e.g., the gain of controllable gain driver  119 , is controllable, e.g., digitally by specifying various gains using binary inputs, or via an analog control arrangement (not shown). 
     During the period of transmission, switch  111  closes each time a pulse of light is generated by laser  115  for the duration light is emitted. This causes current pulses to be injected from threshold current source  107  into capacitor  101 . The magnitude of the injected current pulses is the threshold level against which the output of source  103  is being compared. The values selected for such threshold level will be described more fully hereinbelow. Preferably, the parasitic capacitance of current source  107  is made as small as possible in order to minimize charge injection from its parasitic capacitance. 
     Note that in order to inject the right amount of charge into capacitor  101  it is important that the duration for which switch  111  is closed substantially matches the duration of the current generated by photodiode  117  in response to light generated by laser  115 . If pulse-width distortions occur in driver  119 , laser  115 , or photodiode  117 , they can either be precompensated for before the data is supplied to driver  119  or else the signal controlling switch  111  must be distorted in substantially the same distortion caused by driver  119 , laser  115 , and photodiode  117 . Also, switch  111  as well as switch  113  must be designed to minimize unwanted charge injection into capacitor  101 , e.g., from the switches controlling signal, in order to obtain accurate operation of the peak comparator. This may be achieved by implementing switches  111  and  113  using small, symmetric CMOS switches. 
     During the transmission of the packet there is integration by capacitor  101  of the output current of photodiode  117  and the current pulses injected from threshold current source  107 . The injected current pulses from threshold current source  107  and the output of photodiode  117  are arranged to combine in a subtractive manner. As a result, if the injected current from threshold current source  107  and the peak current output of photodiode  117  are not substantially identical, the voltage on capacitor  101  may change, i.e., it will either increase or decrease. 
     At the end of the packet transmission, the resulting voltage on capacitor  101  is compared against the voltage of precharge voltage source  105  by comparator  109 , with the result of the comparison being indicated by the output of comparator  109 . Note that comparator  109  may be a clocked comparator, so that is uses no power except when it is instructed by the clock to perform the comparison. For use in power control, the output of comparator  109  may be filtered, e.g., by up/down counter  121 , and the filtered output is supplied to controllable gain driver  119 , which controls the magnitude of the signal driving laser  115 . 
     The power level is only changed in between packets, so that an essentially constant power level is employed to driver laser  115  during the packet. 
     When performing power control, current source  107  is set to the peak current level which would be generated by photodiode  117  when laser  115  is supplying the desired power level for typical use, as specified by a user. 
     Similarly, for use in performing end-of-life detection threshold current source  107  is set to the peak current level which would be generated by photodiode  117  when laser  115  is nearing the end of its useful life as specified by the user, e.g., one half or one third of the power level desired for typical use. 
     Other levels for threshold current source  107  may be used for other user specified functions, e.g., to provide an early end of life warning, or other function desired by the user. Those of ordinary skill in the art will be able to develop such functions and specify appropriate thresholds therefor. 
     The particular thresholds used may be multiplexed on a per-packet basis, and the output of comparator  109  demultiplexed accordingly, so that for different packets a different function may be performed.