Abstract:
A laser transmitter bias circuit for a laser diode transmitter, for use in a optical transmission system, e.g. in commercial CATV systems. The laser transmitter bias circuit reduces power consumption and heat dissipation by eliminating the conventional need for a distinct constant DC current supply for biasing the laser diode. Radio frequency (RF) circuitry, e.g., a radio frequency amplifier (e.g., a Hybrid Amplifier), connected in series to the laser diode supplies both a DC bias-current (I B ) and an RF drive-current (I P ) through the laser diode. The DC bias current through the laser diode in turn powers (and/or biases) the radio frequency amplifier and, optionally, other radio frequency circuitry. An optional diode-bypass current (I BP ) path may be connected in parallel with the laser diode, and in series with the radio frequency amplifier to control bias current.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates generally to a semiconductor laser drive circuit, more particularly, to a semiconductor laser drive of use in an optical transmitter, having immediate applications in commercial CATV systems. 
     2. Related Art 
     Commercial terrestrial Cable Television (CATV) systems typically utilize optical links (e.g., fiber-optic cable, with RF transmitter-receiver pairs) to carry radio frequency (RF) digital signals over long distances. The light-producing component is typically a semiconductor laser diode operating in forward bias mode. 
     The forward bias of the semiconductor laser diode is maintained by a DC current generally referred to as the bias current (I B ). A semiconductor laser emits light when a diode current (I D ) flowing through it exceeds a threshold value (I TH ), the value of which may vary from diode to diode, and may vary in a particular diode during operation due to operational conditions such as temperature. Generally, the bias current I B  is controlled so as to establish and maintain the relation I B ≈I TH . Radio frequency (RF) signals expressed as a RF modulated electrical current (I P ) are superimposed upon the DC bias current (I B, ) through the laser diode to obtain modulated light signals that will carry the RF information long distances (e.g., through a fiber optic cable physically and optically coupled to the laser diode). The current flowing through the laser diode (I D ) during operation is therefore equal to the sum of the currents I P  plus I B . The amperage of bias current I B  can be larger or smaller than the maximum amplitude of RF modulated current I P . 
     FIG. 1 is derived from FIG. 1 of U.S. Pat. No. 5,563,898, issued to Ikeuchi, (incorporated herein by reference except for the parenthetical references to “ground” in FIGS. 11 and 13 thereof) and is representative of the related art&#39;s approach to supplying bias current (I B ) and RF modulated electrical current (I P ) to the laser diode  11 . The related art generally teaches that a distinct drive-current (I P ) supply unit  12  (powered by the full supply voltage V DD ) plus a distinct bias-current (I B ) supply unit  13  (connected between GND and the laser diode  11 ), are both necessary to provide and regulate the current I D  (e.g., I D =I P +I B ) through the laser diode  11 . The related art generally teaches that the such a distinct bias-current (I B ) supply unit  13  should be connected in parallel with the drive-current (I P ) supply unit  12  between a terminal (Node 2 ) of the laser diode  11  and ground. Thus, in the related art, the electrical power that is consumed in the drive-current (I P ) supply unit  12  to provide bias current (I B ) to the laser diode  11  is essentially dissipated as waste heat, and does not power the operation of the drive-current (I P ) supply unit  12 . Therefore, the semiconductor laser drive circuits of the related art are not as power-efficient as the present invention discloses that a semiconductor laser drive circuit can be. There is a need for a power efficient method and apparatus for transmitting RF signals through an optical medium. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of the semiconductor laser drive circuits of the related art. The disclosed invention has applications in cable television (CATV) distribution networks as well as in many other applications involving fiber-optic transmission of RF signals. RF digital signals that may be transmitted using the inventive semiconductor laser drive circuits and methods may include QAM (e.g., M-ary Quadrature Amplitude Modulated), Baseband, QPSK and other types. 
     The present invention exploits the observation that a typical semiconductor laser diode, such as laser diode  11  shown in FIG. 1, typically operates with a significant bias current (I B ) but with a forward bias voltage drop (V FB ) across its terminals (i.e., measured between Node  1  and Node  2 ) that is very small compared to supply voltage V DD  needed to operate RF components such as a drive-current supply unit (e.g., an RF Amplifier). The present invention further exploits a discovery that such RF components generally can operate at a reduced supply voltage V D  that is less than the typical full supply voltage V DD  (e.g., Vdd=24 volts), and that V D  may be equal to V DD  minus V FB . Accordingly, the invention provides semiconductor laser drive circuits wherein the DC bias current (I B, ) through the laser diode  11  is used to power the RF components of the laser drive circuits (e.g., in  12 ) rather than being dissipated as heat (e.g., in  13 ), and thus eliminating the need for a distinct bias-current (I B ) supply unit  13  as was required in the related art circuit shown in FIG.  1 . 
     In a first general aspect, the present invention provides an apparatus comprising: a laser diode having a DC bias-current (IB) and a radio frequency (RF) modulated RF drive-current (IP); and a drive-current supply unit adapted to pass the DC bias-current (IB) and the RF drive-current (IP) through said laser diode, wherein said drive-current supply unit includes at least an output stage of a radio frequency (RF) amplifier (e.g., a Hybrid) that passes the RF drive-current (IP) and at least a portion of the DC bias-current (IB). 
     In a second, more particular, aspect the present invention provides an apparatus comprising: a laser diode having a DC bias-current (IB) and an RF drive-current (IP); and a push-pull (Hybrid) RF amplifier connected in series with the laser diode between the supply voltage (VDD) and ground, wherein the push-pull (Hybrid) RF amplifier passes the RF drive-current (IP) and at least a portion of the DC bias-current (IB). 
     In a third general aspect, the present invention provides an apparatus comprising: a laser diode having a DC bias-current (IB) and an RF drive-current (IP); a drive-current supply unit for passing the DC bias-current (IB) and the RF drive-current (IP) through said laser diode, wherein said drive-current supply unit includes at least an output stage of a radio frequency (RF) amplifier that passes the RF drive-current (IP) and at least a portion of the DC bias-current (IB); and a diode-bypass current (IBP) path, wherein the diode-bypass current (IBP) path is connected in parallel with the laser diode, and the diode-bypass current (IBP) path is connected in series with the drive-current supply unit. 
     In a fourth general aspect, the present invention provides a semiconductor laser drive circuit for driving a laser diode having a DC bias-current (IB) and an RF drive-current (IP), comprising: circuitry for passing the DC bias-current (IB) through said laser diode, wherein said circuitry includes at least an output stage of a radio frequency (RF) amplifier that passes at least a portion of the DC bias-current (IB), 
     In a fifth general aspect, the present invention provides a semiconductor laser drive circuit for passing a DC bias-current (IB) and an RF drive-current (IP) through a laser diode, the circuit comprising radio frequency (RF) circuitry adapted to RF modulate the RF drive-current (IP) and further adapted to pass the DC bias-current (IB). 
     In a sixth general aspect, the present invention provides an apparatus comprising a laser diode having a DC bias-current (IB) and an RF drive-current (IP); and radio frequency (RF) circuitry that passes at least a portion of the DC bias-current (IB) and the RF drive-current (IP). 
     In a seventh general aspect, the present invention provides an optical transmission system comprising: an optical signal transmitter for transmitting RF signals, the transmitter including a laser transmitter bias circuit; an optical signal receiver; an optical link medium being operatively connected between the optical signal transmitter and the optical signal receiver; wherein the laser transmitter bias circuit includes: a laser diode having a DC bias-current (IB) and an RF drive-current (IP); a drive-current supply unit adapted to pass the DC bias-current (IB) and the RF drive-current (IP) through said laser diode, wherein said drive-current supply unit includes at least an output stage of a radio frequency (RF) amplifier that passes the RF drive-current (IP) and at least a portion of the DC bias-current (IB). 
     In a eighth general aspect, the present invention provides a method for communicating radio frequency (RF) informational signals through an optical link medium, said method comprising: providing an optical signal transmitter for transmitting RF signals, the transmitter including a laser transmitter bias circuit, wherein the laser transmitter bias circuit includes: a laser diode having a DC bias-current (IB) and a radio frequency (RF) modulated RF drive-current (IP); and a drive-current supply unit adapted to pass the DC bias-current (IB) and the RF drive-current (IP) through said laser diode, wherein said drive-current supply unit includes at least an output stage of a radio frequency (RF) amplifier that passes the RF drive-current (IP) and at least a portion of the DC bias-current (IB). 
     The foregoing and other features and advantages of the invention will be apparent from the following more particular description of embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and inventive aspects of the present invention will become more apparent upon reading the following detailed description, claims, and drawings, of which the following is a brief description. 
     FIG. 1 is block diagram depicting an overview of a typical topology of the semiconductor laser drive circuits of the related art; 
     FIG. 2 is a block diagram depicting a general topology of a simple RF laser drive circuit wherein the laser diode bias current is provided by and powers RF circuitry connected in series with the laser diode, in accordance with embodiments of the invention; 
     FIG. 3 is a block diagram depicting a general topology as in FIG. 2, wherein current controller is added to regulate the current through the laser diode and into the RF circuitry connected in series with the laser diode; 
     FIG. 4 is a circuit diagram depicting a simple RF laser drive circuit wherein a laser diode-bypassing current controller is added to regulate the current through the laser diode and into the RF amplifier connected in series with the laser diode, in accordance with embodiments of the invention; 
     FIG. 5 is a circuit diagram depicting a RF laser drive circuit wherein the RF amplifier includes a push-pull amplifier connected in series with the laser diode, in accordance with embodiments of the invention; 
     FIG. 6 is a circuit diagram depicting a RF laser drive circuit wherein the RF amplifier includes an asymmetric push-pull amplifier connected in series with the laser diode, in accordance with embodiments of the invention; 
     FIG. 7 is a block diagram depicting a RF digital signal transmission system including a transmitter including a RF laser drive circuit having a topology as in FIG. 2 in accordance with embodiments of the present invention. 
    
    
     It should be noted that the same element numbers are assigned to components having the same, or approximately the same functions and structural features. Thus, elements in different figures and labeled with the same element number may be identical, or substantially similar in composition, structure and/or function, and where the function of such element has been explained, there is no necessity for repeated explanation thereof in the detailed description. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As already noted, FIG. 1 is block diagram depicting an overview of a typical topology of the semiconductor laser drive circuits of the related art. During operation, the bulk of the current I D  passing through the semiconductor laser diode  11  will be conducted into and through the distinct bias-current supply unit ( 13 ) and will be therein dissipated as waste heat. 
     FIG. 2 is a block diagram depicting a general topology of a RF laser drive circuit  100  wherein the laser diode&#39;s ( 111 ) bias current (I B ) is provided by and powers at least the output stage of an RF amplifier  122  (and/or other RF circuitry  120 ) connected in series with the laser diode ( 111 ), in accordance with embodiments of the invention. The RF circuitry  120  may generally comprise RF circuits ordinarily present in the RF stages of a RF laser transmitter. The term “RF circuitry” as used in the claims will not include a DC current path between the laser diode  111  and ground GND that is provided solely to pass DC bias current I B  nor a portion thereof. The diode  111  (referred to generally herein as a laser diode) may be identical to the laser diode  11  employed in the related art as shown in FIG. 1, or any laser diode now or ever known to persons skilled in the art. Alternatively, the “laser” diode  111  may be a simple light emitting diode (LED), or diode  111  may be a plurality of parallel-connected (i.e., connected in parallel between Node 1  and Node 2 ) LEDS and/or laser diodes (e.g., outputting various lightwave frequencies, polarizations, and/or phases). The radio frequency signal RF is supplied to the RF circuitry  120  and used therein to modulate the drive-current I P , and may be transmitted (as the RF modulated light) through an inventive RF digital signal transmission system (see, e.g., FIG.  7 ). The radio frequency signal RF may be and/or include QAM (e.g., M-ary Quadrature Amplitude Modulated), Baseband, QPSK and other known types of signals. The radio frequency signal RF, although nominally, “radio frequency” may be presented to the nominal RF circuitry  120  of the circuit  100  at lower than “radio” frequencies, (e.g., at audio frequencies or at much higher frequencies) as individual device (e.g., diode, capacitor, transistor) characteristics permit. The RF circuitry  120  to be supplied by the laser diode&#39;s ( 111 ) bias current (I B ) at V D  on Node 2  may include an RF amplifier circuit  122  or at least the RF output stage thereof. The RF circuitry nominally labeled  120  in FIG. 2 may be and represent any one of the specific and alternative embodiments of RF circuitry depicted in subsequent Figures, e.g.,  120 B,  120 C,  120 D or of many other specific alternative embodiments that would be known by persons skilled in the art (e.g., the circuit  12  in FIG. 2 in the Ikeuchi patent, when supplied by V D  rather than V DD ). 
     In the topology of FIG. 2, the laser diode&#39;s ( 111 ) DC bias current (I B ≈I TH ) is the same as the current powering the RF circuitry  120  (which includes circuits that function as a drive-current supply unit and circuits that function as a bias-current supply unit) within the RF laser drive circuit  100 . RF circuitry  120  passes both the RF drive-current I P  and the DC bias current (I B ). At any given supply voltage V DD , the power consumed (and heat dissipated) by laser diode  111  (e.g., laser diode  11 ) plus RF circuitry  120  operating in the topology of circuit  100  will generally be less than (e.g., sometimes half) the power that would be consumed to control and power the laser diode  11  plus the RF circuitry (e.g, within drive current supply unit  12 ) operating in the topology of FIG.  1 . The power consumption of the entire RF laser drive circuit  100  will be independent of the laser diode&#39;s ( 111 ) current (I B ) requirements. Specifically, in various embodiments of the invention, the optical power output P O  of laser diode ( 111 ) during operation, will have no significant effect upon the total power consumption of (and heat dissipated in) the RF laser drive circuit  100 . 
     FIG. 3 is a block diagram depicting a general topology of an RF laser drive circuit  100 A similar to circuit  100  in FIG. 2, wherein a current controller  130  is added (in series and/or in parallel with the laser diode  111 ) to regulate the DC current I B  flowing through the laser diode  111  and into the RF circuitry  120  (e.g., including RF amplifier  122 ) connected in series with the laser diode  111 . The laser diode-bypassing current controller  130  functions to regulate the current  1   B  through the laser diode  111  and/or to supply necessary current to power the RF circuitry  120 . The current controller  130  may be implemented with one or a plurality of transistors (as in supply unit  13  in FIG. 2 of the Ikeuchi patent) or variable resisters, potentiometers, etc. connected in series, and/or in parallel with the laser diode  111 . In many cases (and by design), the DC current required by the RF circuitry  120  will be greater than the bias current (I B ≈I TH ) of the laser diode  111 . Thus, in many cases the current controller  130  will be implemented as a parallel laser-bypass current (I BP ) path (e.g., having controllable resistance) connected parallel to the laser diode  111  between Node 1  and Node  2 . The laser-bypass current (I BP ) combines with the bias current (I B ≈I TH ) through the laser diode  111  at Node 2  and enters and powers the RF circuitry  120 . Thus, the RF circuitry  120  passes the RF drive-current I P  and the DC bias current (I B ) and DC diode bypass-current (I BP ). The composite DC current I B +I BP  supplies the current required to power the RF circuitry  120 . The addition of the laser-bypassing current (I BP ) path in parallel with the laser diode  111  does not reduce the power efficiency of the circuit  100 A (e.g., versus the power efficiency of circuit  100  as implemented without such a bypass current) because all the current I AMP  supplied to the RF circuitry  120  is usefully employed to power the RF circuitry  120 , rather than dissipated as waste heat. 
     FIG. 4 is a circuit diagram depicting a simple RF laser drive circuit  100 B having the laser diode bypass topology depicted in FIG.  3  and RF circuitry  120 B that includes a very simple RF Amplifier  122 B. RF Amplifier  122 B includes a single transistor T 1  connected between Node 2  of the diode  111  and GND, (e.g., a bipolar transistor T 1  that may be biased to a quiescent state with current I AMP  supplied from Node 2  at reduced supply voltage V D . The RF Amplifier  122 B amplifies the radio frequency voltage signal RF and transforms that signal into RF modulated current (I P ) which is used in the forward biased laser diode  111  to create RF modulated light. The current I AMP  includes the laser bias current (I B ) plus any laser-bypassing current (I BP ) passing through the current controller  130 B). Thus, the RF circuitry  120 B (e.g., comprised of single transistor) passes the RF drive-current I P  and the DC bias current (I B ) and any DC diode bypass-current (I BP ). 
     The laser diode-bypassing current (I BP ) passing through current controller  130 B can be controlled externally by applying a voltage signal to bypass control terminal  135 . A lowpass filter (i.e., a filter that passes DC current, but which blocks RF signals), e.g., LOWPASS 1  (including inductor L 2  and capacitor C 2 ), may be provided within the laser-diode bypassing current (I BP ) path (i.e, within current controller  130 B), to direct the RF modulated current (I P ) that is controlled by the RF circuitry  120  (including  120 B,  120 C, and  120 D as shown in FIGS. 4,  5 , and  6  respectively) exclusively through the laser diode  111 , rather than through the current controller  130 B. A capacitor C 1  may be provided at or near Node 1  (i.e., on the opposite side of the laser diode  111  away from the RF circuitry  120 B) in order to operate as a RF ground, to facilitate passage of the RF modulated current (I P ) through the laser diode  111 . In alternative embodiments of the invention, the single-transistor RF amplifier may be replaced with any conventional amplifier circuit that would have the appropriate DC and RF characteristics between Node 1  and GND, such as for example a Darlington transistor set, and other multiple-transistor amplifiers, and the push-pull amplifier circuits of FIGS. 5 and 6. 
     FIG. 5 is a circuit diagram depicting a RF laser drive circuit  100 C wherein the RF circuitry  120 C includes a push-pull amplifier (also referred to in the art as a Hybrid amplifier), in accordance with embodiments of the invention. The push pull (Hybrid) amplifier, as will be well understood by persons skilled in the art, operates in conjunction with a RF phase splitter  126  to transform the signal RF into RF modulated current (I P ) superimposed upon a DC bias current (which passes alternately through transistors Q 1  and Q 2 ) that will be equal to I BP  plus I B  (or less than I BP  plus I B  if some current supplied from Node 2  at reduced supply voltage V D  is simultaneously used to power the RF phase splitter or other RF components within RF circuitry  120 C). The push pull amplifier includes two transistors (Q 1  and Q 2 ) which may be bipolar transistors biased to the quiescent point by the current I AMP  supplied from Node 2  at reduced supply voltage V D . The push pull amplifier within RF circuitry  120 C may be alternatively implemented with any other type of switching device, such as metal oxide semiconductor field effect transistor (MOSFET) switches that may be substituted for bipolar transistors (Q 1  and Q 2 ). The push pull amplifier within RF circuitry  120 C further includes three magnetically-coupled coils ( 127 - 1  and  127 - 2  and  128 ) connected to form a transformer that has a centertapped dual primary windings (i.e., coils  127 - 1  and  127 - 2 ) and a secondary winding  128  that are magnetically coupled through transformer core Tcore. Each of the dual primary windings  127 - 1  and  127 - 2  is RF-grounded by capacitor C 5  connected at the centertap of the transformer. The secondary winding  128  is electrically coupled to the laser diode  111 , generates RF modulated current I P , and is RF-grounded by capacitor C 6 . Lowpass filter LOWPASS  3  (comprising inductor coil L 5 ) prevents the RF modulated current IP generated by the magnetically coupled secondary winding  128  from being shorted (i.e., RF-grounded) through capacitor C 5 , while permitting DC current (e.g., I B  and I BP ) to flow throw the transformer into the RF circuitry  120 C. 
     Just as in the single-transistor RF amplifier  122 B of FIG. 4, the push pull amplifier within RF circuitry  120 C amplifies the radio frequency voltage signal RF and transforms that signal into RF modulated drive-current (I P ) which is used in the forward biased laser diode  111  to create RF modulated light. The DC component of current I AMP  includes the laser bias current (I B ) plus any laser-bypassing current (I BP ) passing through the current controller  130 B, and passes to ground through RF circuitry  120 C. Thus, the RF circuitry  120 C passes the RF drive-current I P  and the DC bias current (I B ) and any DC diode bypass-current (I BP ). The push-pull amplifier output stage and its RF active components (Q 1 , Q 2 ,  127 - 1 ,  127 - 2 ) included within the RF circuitry  120 C pass the RF drive-current I P  and at least a portion of each of the DC bias current (I B ) and any DC diode bypass-current (I BP ). 
     If the RF circuitry  120 C supplied by Node 2  at reduced supply voltage V D  includes circuits (such as the RF phase splitter  126  within RF circuitry  120 C) other than the push pull amplifier output stage (Q 1 , Q 2 ,  127 - 1 ,  127 - 2 , and R 1 ), then a portion of the DC component of current I AMP  from Node 2  may be diverted to supply such circuits (e.g., RF phase splitter  126 ) through a lowpass filter e.g., LOWPASS 2  (including inductor L 4  and capacitor C 4 ). The lowpass filter (e.g, LOWPASS 2 ) within current controller RF circuitry, will direct the RF modulated drive-current (I P ) that is controlled by the push pull amplifier output stage (Q 1 , Q 2 ,  127 - 1 ,  127 - 2 , and R 1 ), exclusively through the laser diode  111 , rather than through the other circuits within RF circuitry  120 C. In some alternative embodiments, it may be possible to supply DC power to such other circuits within RF circuitry  120 C by tapping the DC currents I B  and I BP  from Node 5  (the centertap of the transformer RF-grounded by C 5 ) or from Node 6  (between secondary coil  128  and capacitor C 6 ), instead of by providing a parallel DC current tap/path through LOWPASS 2 . 
     FIG. 6 is a circuit diagram depicting a RF laser drive circuit  100 D wherein the RF circuitry  120 D includes an asymmetric push-pull (Hybrid) amplifier connected in series with the laser diode  111 , in accordance with embodiments of the invention. The asymmetry of the push-pull amplifier enables it to generate an RF modulated current I P  without a distinct separately RC-grounded secondary winding as in FIG.  5 . The magnetically coupled coils ( 127 - 1 A,  127 -  1 B, and  127 - 2 ) have an impedance ratio of 1:N. The impedance ratio N (where N is a positive number) may be manipulated to match the push-pull amplifier&#39;s impedance to the laser diode&#39;s impedance. Typically, N is between 1 and 4 to match a 35-75 ohm push-pull amplifier impedance to a 5-50 ohm laser impendence. Just as in the symmetrical push-pull amplifier design (e.g.,  120 C of FIG. 5) the asymmetrical push-pull amplifier output stage included within the RF circuitry  120 D passes the RF drive-current I P  and at least a portion of each of the DC bias current (I B ) and any DC diode bypass-current (I BP ). 
     FIG. 7 is a block diagram depicting a RF digital signal transmission system  200  including a transmitter Tx including a RF laser drive circuit  100  having either a general topology as in FIG. 2 or as in FIG. 3 in accordance with embodiments of the present invention, and further including receiver Rx. The RF digital signal transmission system  200  is for transmission of informational signals (i.e. informational signals carried into the system  200  as RF electronic signals RFin, and out of the system as RF electronic signals RFout) through an optical link  130  (e.g., fiber optic cable) that operatively connects the transmitter Tx to the receiver Rx. The system  200  includes an external conductor  210  for carrying in radio frequency (RF) digital information signals as electronic signals RFin. The system  200  also includes an external conductor  211  for carrying out the radio frequency (RF) digital informational signals as electronic signals RFout. 
     Embodiments of the present invention have been disclosed. A person of ordinary skill in the art would realize, however, that certain modifications would come within the teachings of this invention. Therefore, the following claims should be studied to determine the true scope and content of the invention.