Abstract:
A dynamic range extender for optical transmitters comprises a bipolar distortion compensator for increasing drive signal gain as the absolute level of an input signal increases beyond a selected input voltage threshold, a signal coupler for dividing the input signal into complementary signals, a unipolar distortion compensator for increasing drive signal gain of each complementary signal beyond a selected forward current threshold, a signal clipper for pre-clipping each complementary signal below a selected clipping threshold, and complementary driver outputs to drive each of a pair of laser diodes in a complementary push-pull arrangement. The pre-clipping prevents the laser diodes from being driven below their threshold current level, and the distortion compensation suppresses second and third order harmonic distortion when the complementary signals generated by the laser diodes are combined by differential photodiodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part application under 37 CFR 1.53 of patent application “HIGH DYNAMIC RANGE FIBER OPTIC LINK”, Ser. No. 09/071,220 filed on May 1, 1998, now abandoned. 
    
    
     LICENSING INFORMATION 
     The invention described below is assigned to the United States Government and is available for licensing commercially. Technical and licensing inquiries may be directed to Harvey Fendelman, Legal Counsel For Patents, Space and Naval Warfare Systems Center, San Diego D0012, 53510 Silvergate Avenue, San Diego, Calif. 92152-5765; telephone (619)553-3818; fax (619)553-3821. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to linear amplifiers and particularly to fiber optic transmission links in which it is desirable to minimize linear distortion in high power RF signals. 
     Multi-channel broadband fiber optic transmission links using current-modulated laser diodes are limited in dynamic range by even and odd harmonic distortion products generated in response to frequency mixing products of the modulation signal. In particular, even and odd harmonic distortion limits multi-octave bandwidth performance, and third-order distortion limits sub-octave bandwidth performance. These non-linearities worsen as RF power to the laser diode is increased. As RF power to the laser diode is increased, clipping occurs when the laser diode is driven below the laser threshold current. Sub-threshold characteristics of laser diodes severely limit the useful dynamic range of fiber optic transmission links. 
     An optical signal transmission arrangement to reduce even harmonic distortion of a light emitting diode is described in U.S. Pat. No. 4,393,518 issued to Briley on Jul. 12, 1983. Briley divides an electrical input signal into a positive and a negative portion with respect to a selected reference level. These divided signals are transmitted independently through two optical transmitters and received by differential photodiodes to recover the original input signal. While this arrangement may reduce even harmonic frequency distortion, the light emitting diodes are operated only above the biasing point. Laser diode signal transmission systems, on the other hand, typically modulate the laser diodes above and below the biasing point. 
     Nazarthy et al., U.S. Pat. No. 5,253,309, issued on Oct. 12, 1993, discloses modulated optical transmission systems using two optical fibers to reduce second harmonic distortion, but does not compensate for signal clipping introduced by the electro-optical modulator&#39;s nonlinear transfer function. 
     Piehler et al., U.S. Pat. No. 5,940,196, issued on Aug. 17, 1999, discloses an optical transmission system that combines multiple signals having different wavelengths and identical modulation to increase signal-to-noise ratio, but does not compensate for signal clipping introduced by the electro-optical modulator&#39;s nonlinear transfer function. 
     SUMMARY OF THE INVENTION 
     A dynamic range extender for optical transmitters of the present invention comprises a bipolar distortion compensator for increasing drive signal gain as the absolute level of an input signal increases beyond a selected input voltage threshold, a signal coupler for dividing the input signal into complementary signals, a unipolar distortion compensator for increasing drive signal gain of each complementary signal beyond a selected forward current threshold, a signal clipper for pre-clipping each complementary signal below a selected clipping threshold, and complementary driver outputs to drive each of a pair of laser diodes in a complementary push-pull arrangement. The pre-clipping prevents the laser diodes from being driven below their threshold current level, and the distortion compensation suppresses second and third order harmonic distortion when the complementary signals generated by the laser diodes are combined by differential photodiodes. 
     An advantage of the dynamic range extender for optical transmitters of the-present invention is that high fidelity RF signals may be generated from laser diodes at high optical power levels. 
     Another advantage is that RF optical power levels of currently available laser diodes may be extended beyond their linear operating range while maintaining low harmonic distortion. 
     Still another advantage is that present invention may substantially increase the number of communication channels in parallel optical channel applications. 
     Yet another advantage is that the transmission range of a communications signal may be extended without sacrificing fidelity, reducing the number of repeaters and amplifiers required for multiple subscriber reception. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram of a dynamic range extender for optical transmitters of the present invention for a fiber optic transmission link. 
     FIG. 2 is a set of exemplary current waveforms for the dynamic range extender of the present invention. 
     FIG. 3 illustrates transmission of complementary RF optical power signals on separate optical fibers. 
     FIG. 4 illustrates transmission of complementary RF optical power signals on a single optical fiber using a wavelength division multiplexer. 
     FIG. 5 is a schematic of an exemplary complementary RF power optical receiver. 
     FIG. 6 is a block diagram of an optical dynamic range extender of the present invention. 
     FIG. 7 is an exemplary schematic for a bipolar distortion compensator. 
     FIG. 8 is an exemplary schematic for a signal coupler. 
     FIG. 9 is an exemplary schematic for a unipolar distortion compensator. 
     FIG. 10 is an exemplary schematic for a pre-clipper of the present invention. 
     FIG. 11 is an exemplary schematic for an optical device driver. 
     FIG. 12 is an alternative schematic for an optical device driver with a differential amplifier. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 is a flow diagram  10  of a dynamic range extender for optical transmitters of the present invention for transmitting RF optical power over a fiber optic transmission link. At step  102 , the instantaneous current level of an RF input signal is compared to a selected input voltage threshold. If the absolute value of the input signal current exceeds the input voltage threshold, an optional bipolar distortion compensation may be applied at step  104  to boost the signal gain with current, compensating in advance for the nonlinear distortion due to clipping. Complementary signals, i.e., non-inverted and inverted signals, are generated from the compensated input signal at step  106 . Each complementary signal level is compared to a selected forward current threshold at step  108 . An optional unipolar distortion compensation may be applied at step  110  to further compensate in advance for nonlinear distortion in each complementary signal. At step  112 , each complementary signal level is compared to a clipping threshold. When the reverse current of either complementary signal level exceeds the clipping threshold, the complementary signal is clipped at the clipping threshold. The complementary clipped signals are then biased at step  116  for driving RF power optical devices to generate complementary RF optical energy over the fiberoptic link substantially free of distortion at each cable drop and fanout station. The intended meaning of the term “clipper” with respect to the present invention is limited to a device that outputs a signal level substantially proportional to an input signal for input signal levels that are greater than a selected critical value and a constant signal level for input signal levels that are less than or equal to the critical value. It is important to note that while the distortion compensators of the present invention may compensate for non-linearity of an optical transmitter&#39;s transfer function at current levels within the optical transmitter&#39;s operating range, they specifically compensate for distortion typically produced by clipping at current levels below the optical transmitter&#39;s operating range. As a result, the linear RF-optical power output capability of the laser diodes is extended to the laser diode peak power rating, surpassing the present range between the laser threshold current and the upper end of the linear portion of the transfer function. 
     FIG. 2 is a set of exemplary current waveforms  20  that show the operation of the dynamic range extender on a sinusoidal RF input signal  202 . When the absolute value of input current voltage  202  exceeds an input current threshold I 1 , a bipolar distortion compensation is applied to boost the current level with increasing current as shown in plot  204 . Plots  204  and  206  illustrate the complementary signals generated in step  106 . 
     When the forward signal current of either complementary signal exceeds a forward current threshold I 2 , a unipolar distortion compensation is applied to boost the current level of each complementary signal with increasing forward current as shown in plots  208  and  210 . 
     When the reverse current of the compensated complementary signals exceeds a clipping threshold I C , each compensated complementary signal is pre-clipped to generate complementary clipped signals as shown in plots  212  and  214 . A laser bias I B =I T +I C  is then added to bias the RF optical power devices to their operating current level as shown in plots  216  and  218 . Applying the biased complementary clipped signals to the RF optical power devices generates optical signals that are modulated above the laser threshold current. When the optical signals are detected and differentially combined, the original input signal waveform is reproduced. 
     FIG. 3 illustrates transmission of complementary RF optical power signals  302  and  304  over an optical link  30  on separate optical fibers  306  and  308  at wavelengths λ 1  and λ 2  respectively, where λ 1  may be equal to λ 2 . 
     FIG. 4 illustrates an alternate arrangement of an optical fiber link  40  that multiplexes RF optical power signals  302  and  304  on a single optical fiber  404  using a wavelength division multiplexer (WDM) coupler  402 , where λ 1  and λ 2  are unequal. RF optical power signals  302  and  304  are transmitted through optical fiber  404  and demultiplexed with a WDM coupler  406  at the output end of optical fiber  404 . 
     FIG. 5 is a schematic of an exemplary complementary RF power optical receiver  50 . Complementary RF optical power signals  302  and  304  are converted to an electrical signal representative of the input signal by optical receivers such as photodiodes  502  and  504 . RF chokes  506  provide a DC path for providing a bias voltage to photodiodes  502  and  504 , and bypass capacitors  508  provide a low impedance ground return for RF electrical output  510  generated by photodiodes  502  and  504  in response to complementary RF power signals  302  and  304 . In this arrangement, photodiodes  502  and  504  are connected in series with opposing polarity to positive and negative supply voltages as shown. When photodiode  502  conducts, output  510  swings positive. When photodiode  504  conducts, output  510  swings negative. Because output  510  represents the difference of the linear portions of optical outputs  302  and  304 , output  510  is substantially free of clipping effects due to high drive power. Alternatively, a wideband operational amplifier may be connected to -the-outputs of photodiodes in a common polarity configuration to generate a difference signal similar to output  510 . 
     FIG. 6 is a block diagram  60  for an optical dynamic range extender of the present invention. An RF input signal  603  is produced by source  602 . A bipolar distortion compensator  604  applies a bipolar distortion compensation to RF input signal  603 . An RF signal coupler  606  inputs compensated RF signal  605  and outputs an inverted compensated signal  607  and a non-inverted compensated signal  609 . Unipolar distortion compensators  608  and  610  input inverted and non-inverted signals  607  and  609  and apply a unipolar distortion compensation to the forward current portions of inverted and non-inverted signals  607  and  609  respectively. Pre-clippers  612  and  614  input compensated inverted and non-inverted signals  607  and  609  and output complementary clipped signals  615  and  617  to optical device driver  616 . Optical device driver  616  converts the electrical current of complementary clipped signals  615  and  617  to complementary optical power outputs  302  and  304  at wavelengths λ 1  and λ 2  respectively. In alternative embodiments, bipolar distortion compensator  604  and/or unipolar distortion compensators  608  and  610  may be omitted for specific applications. 
     FIG. 7 is an exemplary schematic  70  for bipolar distortion compensator  604 . Diodes  702  and  704  are connected in parallel with opposing polarity and respectively reverse biased by voltage sources  706  and  708 . Diodes  702  and  704  may be, for example, Schottky diodes such as Hewlett-Packard part no. HSMS-286. RF chokes  710 ,  712 , and  714  provide a DC return for the bias current while presenting a high impedance to the RF signal at input  603  and output  605 . Bypass capacitors  716  and  718  provide a low impedance return for stray RF signals. A resistor  720 , typically about 15Ω, provides an upper limit to the impedance between input  603  and output  605  at input RF current levels that are less than I 1  as shown in FIG.  2 B. Voltage sources  706  and  708  establish a reverse bias for diodes  702  and  704 . As the level of the input RF signal current increases beyond I 1 , one of diodes  702  and  704  becomes increasingly conductive. The increased conductivity results in a corresponding increase in current, providing the bipolar distortion compensation shown in plot  204  of FIG.  2 . Additional Schottky diodes may be connected in parallel to increase the distortion compensation. 
     FIG. 8 is an exemplary schematic  80  for signal coupler  606 . A balun  802  transforms compensated signal  605  from a single-ended or unbalanced signal to complementary signals  803  and  805 . Balun  802  may be, for example, a suitable length of coaxial cable. A matching impedance network  804  matches the output impedance of balun  802  to load resistors  806  and  808 . Complementary compensated signals  607  and  609  are output with respect to input signal return  811  as represented in plots  204  and  206  of FIG.  2 . Alternatives for signal coupler  606  include commercially available wideband hybrid couplers with 0° and 180° output ports and wideband RF differential amplifiers, not shown. 
     FIG. 9 is an exemplary schematic  90  for unipolar distortion compensators  608  and  610 . Schottky diodes  902  and  904  are connected in parallel and are reverse biased by a DC voltage source  906 . RF chokes  908  and  910  provide a low impedance circuit for the bias current while presenting a high impedance to the RF signal at input  607 / 609  and output  611 / 613 . A resistor  910 , typically about 15Ω, provides an upper limit to the impedance between input  607 / 609  and output  611 / 613  for input current levels that are less than I 2  as shown in FIG. 2B determined by the diode threshold voltage and voltage source  906 . As the level of the input RF signal current increases, diodes  902  and  904  become increasingly conductive. The increased conductivity results in a corresponding increase in current, providing the unipolar distortion compensation shown in plots  208  and  210  of FIG.  2 . Additional Schottky diodes may be connected in parallel to increase the distortion compensation. 
     FIG. 10 is an exemplary schematic  1000  for pre-clippers  612  and  614 . A transmission line  1002  matches the impedance of complementary compensated signal  611 / 613  to Schottky diodes  1004 . Schottky diodes  1004  are reverse biased at the clipping threshold by voltage source  1006 . RF choke  1008  provides a low impedance circuit for the bias voltage, while bypass capacitor  1010  maintains a constant voltage at the clipping threshold when Schottky diodes  1004  are forward biased by complementary compensated signal  611 / 613 . Additional pre-clipping stages  1020  may be connected to complementary clipped outputs  615 / 617  if desired to increase the clipping performance. 
     FIG. 11 is an exemplary schematic  1100  for optical device driver  616 . Bypass capacitors  1102  and  1104  block the clipping bias from clipped complementary outputs  615 / 617 , while resistors  1106  and  1108  may be used to linearize the drive current for laser diodes that do not incorporate internal resistors. A typical value for resistors  1106  and  1108  is 15 Ohms. RF chokes  1110  and  1112  provide a low impedance path for DC voltage sources  1114  and  1116 . DC voltage sources  1114  and  1116  generate an operating current bias for laser diodes  1118  and  1120 . Laser diodes  1118  and  1120  produce complementary optical energy signals  302  and  304  at wavelengths λ 1  and λ 2  respectively corresponding to electrical waveforms  216  and  218  shown in FIG.  2 B. 
     FIG. 12 is an alternative schematic  1200  for optical device driver  616 . Biased, clipped complementary outputs  615 / 617  are differentially amplified by transistors  1202  and  1204  to drive laser diodes  1206  and  1208 . RF chokes  1210  and  1212  provide a DC path for forward biasing laser diodes  1206  and  1208 . The voltage from the plus and minus voltage sources and the values of resistors  1214 ,  1216 , and  1218  determine the operating current range and limit the forward drive current through transistors  1202  and  1204  and may be selected so that differential transistors  1202  and  1204  perform the pre-clipping function. 
     In addition to avoiding direct signal clipping effects, the clipping compensator of the present invention avoids second-order intermodulation products by driving substantially identical optical transmitters with complementary signals. Subtracting the restored electrical signals from the modulated optical energy has been found to suppress second order distortion of the modulation signal introduced by the nonlinear transfer curves of laser diodes. 
     Alternatively, the diode polarities and the corresponding biasing source polarities may be reversed from that shown in the figures to practice the present invention. 
     Modifications and variations of the present invention may be made within the scope of the following claims to practice the invention otherwise than as described in the examples above.