PATENT ABSTRACT
A low-noise current source driver for a laser diode load is achieved by means of a current-regulated supply connected across the load, and a shunt regulator. The shunt regulator comprises a shunting element, a current sensing element for sensing current conducted through the load, and an error amplifier responsive to a difference between the current sensed by the current sensing element and a first reference current. The current regulator is designed to respond to a signal a signal representative of a second reference current to produce an appropriate corresponding output current. The shunting element is connected across the power supply and load, and is controlled by the error amplifier to conduct all current from the current regulated supply in excess of the first reference current. The second reference current is greater than the first reference current by an amount sufficient to ensure that noise and ripple currents cannot cause the output of the current-regulated supply to drop below the first reference current.

PATENT DESCRIPTION
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to laser power supplies and, more specifically to low-noise power supplies for laser diodes. 
     BACKGROUND OF THE INVENTION 
     In semiconductor lasers, particularly CW-operated laser diodes (Continuous Wave, or continuous mode), power supply induced noise currents manifest themselves as corresponding instabilities in output level and wavelength. Accordingly, CW laser diodes typically require an accurate, low-noise current source to achieve high stability. Due to the high power levels often required of laser power supplies, it is common practice to use switch-mode power supplies to maximize efficiency. However, it is well-known that such switching power supplies generate considerable noise and high output ripple as compared to “quieter” but less efficient linear supplies. 
     To overcome this problem, a linear pass element connected as a current driver is usually employed in series with “raw” power supply output and the laser diode load. An example of such an arrangement  100  is shown in FIG. 1. A voltage regulated “raw” or “bulk” power supply  102  provides power for a load comprising one or more laser diodes  104  (e.g., an array or diodes). Typically, the power supply  102  is a switching power supply. The output of the power supply  102  is smoothed by a capacitor  106 . A ground-referenced current source  108  comprising a linear pass element  110 , a current sensing element  112  and an error amplifier  114  controls the amount of current conducted through the diode load. The linear pass element  110 , typically a FET (field-effect transistor), conducts current from the power supply  102  through the laser diode(s)  104  into the grounded current sensing element  112 . A voltage develops across the current sensing element  112  in proportion to the amount of current being conducted through the laser diodes  104 . The error amplifier  114  compares the sensed current to a control voltage that indicates the desired laser diode current and adjusts the current conducted by the linear pass element  110  accordingly to maintain constant current at the desired level. The filtering effect of the capacitor  106 , in combination with the ripple and noise rejection of the linear current source  108 , improves overall stability and minimizes power supply induced noise. 
     In operation, with the current source  108  conducting current through the laser diode(s)  104 , energy is drawn from the capacitor  106  through the diodes, as a result of which the voltage on the capacitor falls. Therefore, the current source has to have sufficient compliance to continue to maintain current regulation as the “raw” supply voltage falls. For good efficiency, a low voltage loss across the current source is desired, but this requires a large and bulky capacitor to minimize voltage “droop”. 
     The disadvantages of such an implementation include: 
     a) The power dissipated in the linear pass element  110  may be considerable, resulting in substantial heat generation and consequent inefficiency. Heat sinking and cooling may be required, resulting in a large, expensive, inefficient system. 
     b) All of the laser diode current flows through the linear pass element  110 , requiring a high-current device with commensurate size and cost penalties. 
     c) Laser diodes are presently very expensive. If the series pass element  110  were to fail to a short-circuit condition, then the voltage stored on the capacitor  106  would be applied directly across the laser diode(s)  104 , resulting in unregulated current flow, potentially producing excessive light output and possible diode damage. 
     Another example of a series-connected linear pass element being used to regulate current conducted through laser diode load is disclosed in U.S. Pat. No. 5,287,372 (“ORTIZ”), incorporated in its entirety by reference herein. ORTIZ discloses a zero-current, switched, full wave quasi-resonant converter that provides a current to directly drive the laser diode. Referring to FIG. 2 of ORTIZ, a linear pass element  24  (Q 1 ) is connected in series with the laser diode load  31  and is used to regulate the current conducted therethrough. The laser diode driver circuit described in ORTIZ suffers from the disadvantages described hereinabove with respect to the current driver circuit arrangement of FIG.  1 . 
     BRIEF DESCRIPTION (SUMMARY) OF THE INVENTION 
     It therefore is a general object of the present invention to provide an improved technique for driving laser diodes. 
     It is a further object of the present invention to create a smaller, less expensive, low-noise current driver for laser diodes without the efficiency loss of a series-connected linear pass element. 
     It is a further object of the present invention to create a low-noise current driver for laser diodes that can employ less expensive, lower-current devices while maintaining good load regulation. 
     According to the invention, a low-noise current source driver for a laser diode load comprises a current-regulated supply connected across the load, and a shunt regulator. The shunt regulator comprises a shunting element, a current sensing element for sensing current conducted through the load, and an error amplifier responsive to a difference between the current sensed by the current sensing element and a signal representative of a first reference current. The current regulator is designed to respond to a signal representative of a second reference current to produce an appropriate corresponding output current. The shunting element is connected across the power supply and load, and is controlled by the error amplifier to conduct all current from the current regulated supply in excess of the first reference current. The second reference current is greater than the first reference current. The shunting element may be a field-effect transistor (FET) or a bipolar transistor. The current sensing element may be a small-value resistor or a Hall-effect device. 
     Generally speaking, the second reference current is always greater than the first reference current by an amount sufficient to ensure that ripple and noise currents cannot cause the current-regulated supply output to dip below the first reference current. This is accomplished in one of three ways: 
     the second reference current is made greater than the first reference current by a fixed amount; 
     the second reference current is made greater than the first reference current by a fixed proportion (e.g., percentage); or 
     the second reference current is made greater than the first reference current by an amount equal to the sum of a fixed proportion of the first reference current and a fixed amount. 
     Other objects, features and advantages of the invention will become apparent in light of the following description thereof. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference will be made in detail to preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The drawings are intended to be illustrative, not limiting. Although the invention will be described in the context of these preferred embodiments, it should be understood that it is not intended to limit the spirit and scope of the invention to these particular embodiments. 
     Often, similar elements throughout the drawings may be referred to by similar references numerals. For example, the element  199  in a figure (or embodiment) may be similar or analogous in many respects to an element  199 A in another figure (or embodiment). Such a relationship, if any, between similar elements in different figures or embodiments will become apparent throughout the specification, including, if applicable, in the claims and abstract. In some cases, similar elements may be referred to with similar numbers in a single drawing. For example, a plurality of elements  199  may be referred to as  199 A,  199 B,  199 B, etc. 
    
    
     The structure, operation, and advantages of the present preferred embodiment of the invention will become further apparent upon consideration of the following description taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram of a prior-art current driver for laser diodes; 
     FIG. 2 is a schematic diagram of a current driver for laser diodes, according to the invention; 
     FIGS. 3A and 3B are graphs illustrating one aspect of the current driver, according to the invention; 
     FIG. 4A is a block diagram demonstrating a technique for generation of an offset demand signal, according to the invention; 
     FIG. 4B is a schematic diagram of a circuit realization of the block diagram of FIG. 4A, according to the invention; 
     FIG. 5A is a block diagram demonstrating another technique for generation of an offset demand signal, according to the invention; 
     FIG. 5B is a schematic diagram of a circuit realization of the block diagram of FIG. 5A, according to the invention; 
     FIG. 6A is a block diagram demonstrating another technique for generation of an offset demand signal, according to the invention; and 
     FIG. 6B is a schematic diagram of a circuit realization of the block diagram of FIG. 6A, according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As a general proposition, the present inventive technique provides an efficient low-noise current source driver for laser diodes by “shunting” noise currents around the load rather than by attempting to “block” noise currents from passing through the load using a series-connected pass element. According to the invention generally, a current source laser diode driver comprises a “bulk” current supply set to provide slightly more current than is required by a laser diode load and a shunting element such as an FET connected across the laser diode load. An error amplifier measures the current in the laser diode load, and controls the shunting element to “shunt” any load current in excess of the required load current. 
     FIG. 2 is a schematic diagram of a low-noise current source laser diode driver  200 , according to the invention. A current-regulated power supply  202  (contrast  102 ) supplies current to a load comprising one or more laser diodes  204  (compare  104 ). A shunt regulator  208  comprising a shunting element  210  (which may be an FET; compare  110 ), a current sensing element  212  (compare  112 ) and an error amplifier  214  (compare  114 ) is connected across (around) the laser diodes  204 , as shown. The error amplifier  214  measures the difference in current between a desired current (“Demand”) through the laser diodes  204  and the current passing through the laser diodes, as measured by the current sensing element  210 . The current sensing element  212  is suitably a small-value resistor or a Hall-effect sensing device. The current-regulated power supply  202  is set to provide slightly more current than what is required by the laser diode load  204 . This is accomplished by providing a reference signal (“Demand+Δ”) to power supply  202  that exceeds the desired load current (“Demand”) by a small amount “Δ”. The amount of current “Δ” in excess of the required current is determined such that it slightly exceeds the amount of ripple and current noise present in the output of the current-regulated power supply  202 . By shunting essentially any and all current in excess of the required load current (“Demand”), the laser diodes  204  are provided with clean, substantially noise-free current at the required level. 
     The current-regulated power supply  202  may be implemented using any of a wide variety of different circuit topologies. Typically, however, it is implemented by controlling the duty cycle of one or more power switching elements according to an error signal derived from the difference between the desired output current (“Demand+Δ”, in this case) and actual output current. Typically, output current pulses from such a switching (switch-mode) current supply are smoothed by a low-pass filtering element such as a capacitor. 
     The connections between of the elements in FIG. 2 are as illustrated. The power supply  202  has an output which supplies current to one of two terminals of the laser diode load  204 . The other terminal of the laser diode load  204  connects to ground via the current sensing element  212 , and to an input of the error amplifier  214 . The other input of the error amplifier  214  receives the signal indicative of desired current (“Demand”). The output of the error amplifier  214  is provided to the gate of the shunting element (FET)  210 . The source and drain of the FET  210  are connected between the output of the power supply  202  and ground. The power supply  202  has an input for receiving a signal indicative of the reference signal (“Demand+Δ”). 
     The operation of current source driver  200  of the present invention is illustrated in FIGS. 3A and 3B. 
     FIG. 3A is a graph  300 A showing the current output  302  of the current-regulated power supply  202  and the signal  304  at the output of the error amplifier  214 . The output current  302  includes noise and ripple currents that cause its actual current output to deviate from its desired output current (“Demand+Δ” indicated by a dashed line). Note that the amount “Δ” by which the current output  302  of the power supply  202  is set to exceed the desired load current (“Demand”) is selected such that the minimum excursions of the current output  302 , including noise, will not dip below the desired load current. That is, “Δ” is chosen to be at least as great as, preferably just greater than, the anticipated magnitude of the noise and ripple present in the current output  302  of the current-regulated power supply  202 . 
     The signal  304  is generally representative of the “excess” load current (i.e., current in excess of the required current (“Demand”) as measured by the error amplifier  214  and is used to drive the shunting element  210  to conduct (divert) said “excess” load current around the laser diodes  204 . 
     FIG. 3B is a graph  300 B showing the current  306  conducted (shunted) through the shunting element  210  under control of the error amplifier  214  and the load current  308  through the laser diodes  204 . By shunting all of the excess current through the shunting element  210 , the load current  308  through the laser diodes  204  is accurately controlled to the desired level (“Demand”) with minimal noise. 
     Note that for proper operation, the regulated current output of the power supply must always be maintained (slightly) greater than the desired current in the diode load. The gain of the error amplifier  214  may be enhanced at high frequencies to cancel out any high frequency noise current in the diode load current. 
     The advantages of this approach include: 
     a) A high current series pass element is not required. 
     b) The efficiency is high because the switching power supply drives the load directly. 
     c) The ripple and noise is “skimmed” from the power supply output current and is bypassed around the laser diode load. Only “smooth” current flows into the diode load. Only noise and ripple currents (plus a small margin) are conducted by the shunting element. 
     d) The diode driver is more reliable due to the elimination of the high power series element along with its related heat. 
     e) The power supply can be designed to limit the maximum current (and therefore, the maximum power) into the diode load. In the worst case, if the shunting element were to fail to an open-circuit condition, power into the laser diode load would still be maintained at non-damaging levels by the current supply. If the shunting element were to fail to a short-circuit condition, this would not normally cause damage to the laser diodes. 
     Optionally, when the required load current (Demand) is set to zero, the power supply can be commanded to zero as well, but the pass element can be turned on slightly to absorb any slight noise current output from the power supply and prevent it from being conducted through the load. Those of ordinary skill in the art will understand that this is readily accomplished by setting (Demand+Δ) equal to zero such that the reference input to the error amplifier (Demand) is slightly negative. In this condition, the error amplifier will cause the shunt element to absorb any and all noise and/or leakage current from the power supply output, preventing it from being conducted through the laser diode load. Those of ordinary skill in the art will also understand that there are alternative methods of accomplishing essentially the same result. 
     Three general approaches to controlling the output of the current-regulated power supply are now described: 
     1) The power supply can be commanded (controlled) to provide an output current that is a fixed amount “Δ” greater than the desired load current. A benefit of this approach is its simplicity. This approach is shown and described hereinbelow with respect to FIGS. 4A and 4B. 
     2) The power supply can be commanded to provide an output current that is a greater than the desired load current by a fixed portion “α” of the desired load current. A benefit of this approach is its efficiency. Switching noise and ripple tend to increase roughly in proportion to the current output setting, so this technique tends to maintain the output current of the power supply at the lowest possible setting, thereby minimizing the amount of current that must be conducted by the shunting element. This approach is shown and described hereinbelow with respect to FIGS. 5A and 5B. 
     3) The power supply can be commanded to provide an output current that is the sum of a fixed amount “Δ” greater than the desired load current and a fixed portion “α” of the desired load current. A benefit of this approach is combined efficiency and reliability. This approach is shown and described hereinbelow with respect to FIGS. 6A and 6B. 
     FIG. 4A is a block diagram  400 A of a circuit for generating a controlling signal for the power supply. A signal representative of the desired load current (“Demand”) is presented at a first input  422  of a summing element  420 . A signal representative of an offset amount “Δ” is presented at a second input  424  of the summing element  420 . The summing element  420  produces an output signal  426  representative of the sum of the two signals at its inputs  422  and  424 . 
     FIG. 4B is a schematic diagram of a circuit realization  400 B generally equivalent to the block diagram of FIG.  4 A. An operational amplifier  440  has a first input resistor  442  and a second input resistor  444  connected to a positive input (“+”) thereof. A signal representative of the desired load current (“Demand”) is provided to the operational amplifier  440  via the first input resistor  442  and a signal representative of an offset amount “Δ” is provided via the second input resistor  444 . A first feedback network resistor  446  is connected between an output of the operational amplifier  440  and a negative input (“−”) thereof. A second feedback network resistor  448  is connected between the negative input (“−”) and ground. In this configuration, assuming all equal-valued resistors (“R”), a signal at the output of the operational amplifier is representative of the sum of the two input signals (“Demand+Δ”). 
     FIG. 5A is a block diagram  500 A of another circuit for generating a controlling signal for the power supply. A signal representative of the desired load current (“Demand”) is presented at a first input  532  of a scaling element  530 . A scale factor (“1+α”) is applied via a second input  534  of the scaling element  530 . The scaling element  530  produces an output signal  536  representative of the desired load current multiplied by the scale factor (“Demand(1+α)”). 
     FIG. 5B is a schematic diagram of a circuit realization  500 B generally equivalent to the block diagram of FIG.  5 A. An operational amplifier  540  has a signal representative of the desired load current (“Demand”) connected to a positive input (“+”) thereof. A first feedback network resistor  546  (“αR”) is connected between an output of the operational amplifier  440  and a negative input (“−”) thereof. A second feedback network resistor  548  (“R”) is connected between the negative input (“−”) and ground. In this configuration, with resistor values “R” and “αR” as shown, a signal at the output of the operational amplifier is representative of the desired load current multiplied by the scale factor (1+α), i.e., (“Demand(1+α)”). 
     FIG. 6A is a block diagram  600 A of another circuit for generating a controlling signal for the power supply. A signal representative of the desired load current (“Demand”) is presented at a first input  632  of a scaling element  630 . A scale factor (“1+α”) is applied via a second input  634  of the scaling element  630 . The scaling element  630  produces an output signal representative of the desired load current multiplied by the scale factor (“Demand(1+α)”), which is in turn connected to a first input  622  of a summing element  620 . A signal representative of an offset amount “Δ” is presented at a second input  624  of the summing element  620 . The summing element  620  produces an output signal  626  representative of the sum of the two signals at its inputs  622  and  624 , or (“Demand(1+α)+Δ”). 
     FIG. 6B is a schematic diagram of a circuit realization  600 B generally equivalent to the block diagram of FIG.  6 A. An operational amplifier  640  has a first input resistor  442  (“RA”) and a second input resistor  644  (“RB”) connected to a positive input (“+”) thereof. A signal representative of the desired load current (“Demand”) is provided to the operational amplifier  640  via the first input resistor  642  and a signal representative of an offset amount “Bias” is provided via the second input resistor  644 . A first feedback network resistor  646  (“RC”) is connected between an output of the operational amplifier  640  and a negative input (“−”) thereof. A second feedback network resistor  648  (“RD”) is connected between the negative input (“−”) and ground. In this configuration, assuming resistor values “RA”, “RB”, “RC” and “RD” as shown signal at the output of the operational amplifier is represented by the expression below:          O                 u                 t                 p                 u                 t     =           D                 e                 m                 a                 n                   d   ·   R                   B     +     B                 i                 a                   s   ·   R                   A           R                 A     +     R                 B              (     1   +       R                 C       R                 D         )                              
     Converting to the equivalent notation used in FIG.  6 A:        α   =         R                   B        (       R                 C     +     R                 D       )             (       R                 A     +     R                 B       )        R                 D       -   1             Δ   =     B                 i                 a                   s   ·       R                   A        (       R                 C     +     R                 D       )             (       R                 A     +     R                 B       )        R                 D                                  
     Those of ordinary skill in the art will understand that there are many other ways to generate the signal (“Demand+Δ”) that controls the output of the current-regulated power supply, including the use of virtual ground summing stages. Those of ordinary skill in the art will also recognize that suitable current-regulated power supplies can be designed to be responsive to many different types of controlling signal, e.g., a control voltage or a controlling current. 
     The present inventive technique provides a combination of good efficiency, low noise, lower-cost components, and high reliability. 
     Although the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character—it being understood that only preferred embodiments have been shown and described, and that all changes and modifications that come within the spirit of the invention are desired to be protected. Undoubtedly, many other “variations” on the “themes” set forth hereinabove will occur to one having ordinary skill in the art to which the present invention most nearly pertains, and such variations are intended to be within the scope of the invention, as disclosed herein.