Abstract:
Both a system and method for optically powering a network component, such as the transponder of a picocell, is provided. The system includes a vertical cavity surface emitting laser (VCSEL) for processing an input signal, a remotely-located optical power source, and an optical fiber for conducting optical power from the source to the VCSEL. The VCSEL may be electrically biased from current generated by an optical-electro converter coupled to the fiber, or directly optically biased from light from the optical power source. A bias tee is connected between an input signal and an input of the VCSEL such that the VCSEL generates a modulated optical signal. The system may be the transponder of a picocell system where the VCSEL generates an optical uplink signal conducted to a head-end circuit via the same or a separate optical fiber.

Description:
FIELD OF THE INVENTION 
   This invention generally relates to a system and method for optically powering a remotely located component of an optical network, and is specifically concerned with the use of a photo detector array to convert optical power delivered from a remotely located source into electrical power to bias a vertical cavity surface emitting laser (VCSEL) to process an input signal. 
   BACKGROUND OF THE INVENTION 
   Optical networks are presently in use in many buildings. Despite their inherently high bandwidth capacity relative to systems based on electrical cables, such networks must often be expanded to keep up with the ever increasing bandwidth demands of improved computer and telecommunications systems. Network expansion typically involves the addition of network components that are remotely located from the central processing unit of the network. In particular, the growth of wireless communication has increased the demand for wireless interfaces with existing office and building-sized optical networks. Around these wireless interfaces, picocells or so-called hot spots are typically created for high-speed wireless data communication. Such interfaces can take the form of transponders that serve as remote antennas which in turn are connected to a central head-end. 
   Electrical power is necessary to operate the opto-electronic semiconductor devices present in the added network components, whether they take the form of transponders or some other remotely-added equipment. As optical fibers cannot transmit electrical power, some other means for providing electricity to the added components is necessary. The simplest way would be to add additional electrical power lines to the network. However, the remote location of the added components often makes the addition of such power lines difficult and expensive. Another solution might be the use of cable that combines both optical fibers and an electrical power line. While such a solution would be less expensive than the separate installation of electrical power lines, it would still necessitate the addition of electrical cables. 
   Passive picocell designs have been proposed using electro-absorption modulators (EAMs). In some of these designs, the EAM is biased by an electrical signal. This can be derived from light received from a remote source of optical power via an optical fiber that is converted into electrical power. Unfortunately, EAMs are relatively expensive, being manufactured in small numbers. Worse yet, EAMs require the use of single mode optical fiber in order to function, whereas most small scale, short distance optical networks use multimode fiber in order to reduce overall system cost. Hence, such a solution again requires (in most cases) the replacement of at least some optical fiber of the system, and is disadvantageously expensive due to the cost of EAMs. 
   Clearly, what is needed is a system to add new components, such as transponders, to an existing optical network which would not require the separate installation of electrical power lines, or the replacement of any of the existing optical fiber. Ideally, such a system would be relatively easy and inexpensive to implement, and completely compatible with the existing network infrastructure. Finally, such a system should also be capable of implementing whole new building-sized networks utilizing inexpensive multi-mode optical fiber. 
   SUMMARY OF THE INVENTION 
   The invention is a system for remotely optically powering a network component that avoids all the aforementioned shortcomings associated with the prior art. To this end, the system of the invention generally comprises a circuit including a VCSEL that is biased by power received from a remotely located optical power source via an optical fiber. In one embodiment, the biasing light received from the remote power source is coupled to an opto-electrical converter such that the VCSEL is biased by electrical power. The opto-electrical converter may be a photodetector array, or a photodiode. In another embodiment, the biasing light is coupled directly to the active region of the VCSEL in order to directly bias it. In either embodiment, the circuit includes a bias tee or other component for modulating the optical output of the VCSEL in accordance with an input signal. 
   The circuit may comprise the transponder of a picocell system, wherein the VCSEL converts an electrical input signal into an optical uplink signal that is conducted to a head-end circuit via either the power transmitting optical fiber, or a separate optical fiber. The transponder may include a radio frequency circuit that converts a radio signal into an input signal that is used to modulate the biasing current of the VCSEL generated by the optical-electro converter so that the optical output of the VCSEL is modulated into the uplink signal. The head-end circuit may also include a radio frequency circuit that converts a digital baseband signal into a radio frequency signal which is then converted into an optical downlink signal. The head-end circuit may further include an optical power source, such as a light emitting diode (LED) or erbium doped fiber amplifier (EDFA) or laser diode to generate the biasing power. The head-end circuit may simultaneously conduct optical power and the optical downlink signal via the same or different optical fibers in parallel to the transponder. The head-end circuit may further have an opto-electrical converter for converting the optical uplink signal into an electrical uplink signal, and the radio frequency circuit of the head-end circuit may also operate to convert the electrical uplink signal received from the transponder into a digital baseband signal. 
   The transponder may also include an AC coupling circuit connected to the electrical power generated by the opto-electrical converter for separating the downlink signal from the biasing current and conducting it to the radio frequency circuit of the transponder for transmission. Finally, the head-end circuit and the transponder of the picocell may include electrical circulators or radio frequency (RF) switches between the inputs and outputs of their respective radio frequency circuits to allow non-interfering processing of downlink and uplink signals. 
   The use of VCSELs biased by electrical power generated by remotely-located optical power sources provides a number of advantages over the prior art. Unlike EAMs, VCSELs are low cost and compatible with both multi-mode and single mode optical fibers. They are easily operated by the amount of power that typically can be transmitted through optical fibers. Additionally, VCSELs may be fabricated to operate within any of the wavelengths currently in use in optical networks, including the 0.85, 1.3, and 1.55 micrometer wavelengths used in high speed networks. Finally, the conversion of optical power provides clean, spike-free electrical power that is immune from electromagnetic and radio frequency interference, thereby enhancing the reliability of the resulting network. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a first embodiment of the system of the invention; 
       FIG. 2  is a graph illustrating how the current-voltage characteristics of a VCSEL and of a photodetector array may be matched in order to maximize the photonic output of the VCSEL; 
       FIG. 3  illustrates how the modulation bandwidths of a VCSEL increases with different biasing currents; 
       FIG. 4  is an “eye” diagram of the photonic output of a 1.30 micrometer VCSEL biased in accordance with the schematic diagram of  FIG. 1 , illustrating in particular the sharp definition between digital ones and zeros at a data rate of 2.4 gigabits per second; 
       FIG. 5A  is a second embodiment of the system of the invention illustrating how the photodetector array of the circuit of  FIG. 1  may be eliminated, and how the VCSEL may be biased directly by optical power; 
       FIG. 5B  is a graph illustrating the optical output of the VCSEL as milliwatts of intensity versus milliwatts of incoming laser pump power; 
       FIG. 6  is a schematic diagram of a first embodiment of a passive picocell system employing the circuitry of the invention, and 
       FIG. 7  is a second embodiment of a passive picocell system of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference now to  FIG. 1 , wherein like numbers designate like components throughout all the several figures, the system  1  of the invention includes a remote component  3  of a network (not shown) that processes an input signal. The remote component  3  includes input signal source  4  connected to a circuit  5  that includes a VCSEL  6 , a photodetector array  8  and a bias tee  10 . A DC transformer  12  may also be included in the circuit  5  to adjust the voltage versus amperage of the power generated by the photodetector array  8  in order to maximize the photonic output of the VCSEL  6 . Accordingly, the use of such a component is generally not preferred. 
   The system  1  further includes a remote optical power source  15  which in the example of  FIG. 1  is formed by a laser light source  17  having an output connected to an optical fiber  19 . Fiber  19  in turn is optically coupled to the photodetector array  8 . The optical power source  17  may be formed from any one of a number of commercially-available components, such as a laser diode, an erbium-doped fiber amplifier (EDFA) or a light emitting diode, so long as it is capable of delivering at least three and preferably six or more milliwatts of power through the optical fiber  19  to the photodetector array  8 . 
   In operation, laser light generated by the laser light generator  17  is transmitted to the photodetector array  8  via the optical fiber  19 , as previously indicated. The photodetector array  8  in turn generates a DC current which is sourced to the VCSEL  6  in order to bias it. The biasing current is modulated by the action of the modulated input signal on the inductor and capacitor (not shown) included within the bias tee  10 . By providing the modulated bias current to the VCSEL,  6 , the VCSEL&#39;s optical output signal is likewise modulated. 
     FIG. 2  illustrates the importance of adjusting the current-voltage of the biasing current in order to maximize the light output of the VCSEL. The rising graph illustrates the current-voltage characteristics of, for example, a typical 1.3 micrometer VCSEL. The upper graph illustrates combinations of current and voltage that can be generated by a typical photodetector array formed by, for example, six indium-gallium-arsenide detectors connected in series. Optical output of the VCSEL is maximized at the intersection of these two graphs, indicated by the arrow. Accordingly, the optical output of the VCSEL is maximized at a current of approximately 2.8 milliamps at approximately two volts. The photodetector array  8  may be adjusted to provide such a combination of current and voltage by either the addition or subtraction of different types of photodetectors, or by the use of a DC transformer  12  as is illustrated  FIG. 1 . However, the use of such DC transformers is generally not preferred, as some of the power generated by the photodetector array would have to be used to power this component. Such DC transformers  12  are commercially available semiconductor components which, by themselves, form no part of the instant invention. 
     FIG. 3  illustrates how the modulation bandwidth increases as the biasing current is increased to the VCSEL. When the biasing current is at one milliamp, the response is reasonably positive up through approximately 6.0 gigahertz, but then falls off sharply below minus 3 decibels after that. Two milliamps substantially improves performance, with the frequency increasing to approximately 8.0 gigahertz, before the response falls off to under minus 3 decibels. The use of 2.8 milliamps and 3 milliamps allows the VCSEL to operate at a frequency of 8.6 gigahertz before the response falls off to minus 3 decibels. In view of the almost identical curve traced by 2.8 milliamps and 3 milliamps, the graph of  FIG. 3  indicates that a current of 2.8 milliamps is optimal in order to obtain a maximum modulation bandwidth from a VCSEL as additional a mounts of current do not result in any significant bandwidth increase. 
     FIG. 4  illustrates the ability of the VCSEL  6  to transmit unambiguous and error free digital information when operated within the system  1  illustrated in  FIG. 1 . Specifically, this diagram illustrates how a VCSEL operating within the  FIG. 1  configuration at a biasing current of 2.8 milliamps and approximately 2.0 volts can generate error free data transmission at a rate of 2.4 gigabits per second. This is a particularly impressive result when one considers that other types of semiconductor edge-emitting lasers would require approximately six or seven times as much power (i.e., between 30 and 40 milliwatts) in order to transmit data error free at the same rate as that illustrated in  FIG. 4 . 
     FIG. 5A  illustrates a second embodiment  20  of the system of the invention. In this embodiment, the further detector array  8  has been eliminated, and the VCSEL  6  is directly optically pumped by the output of the laser light generator  17  of the optical power source  15 . Such optical pumping may be accomplished if light of a shorter wavelength is directed into the active region of the VCSEL  6 . For example, if the VCSEL  6  generates 1.30 micrometer laser light when biased, then the light generated by the pumping laser  17  should have a wavelength of approximately 1.2 micrometers or less. 
     FIG. 5B  illustrates that the relative power efficiencies between electrical and optical pumping are substantially the same. For example, if the pump power is 4.0 milliwatts, then the resulting intensity of the 1.3 micrometer laser light from the VCSEL  6  would be approximately 0.80 milliwatts. While not specifically shown in the drawings, the optical output of the VCSEL  6  might easily be separated from the optical pump power transmitted through the optical fiber  19  by the combination of a filter or a Bragg grating and optical circulator or fiber coupler. Several other techniques for separating the optical downlink signal from the power signal exist. The embodiment of the system  1  of  FIG. 5A  is advantageously simpler and less expensive, requiring only that the laser light generator  17  produce pumping light at a shorter wavelength than the optical output of the VCSEL  6 . 
     FIG. 6  schematically illustrates a picocell system  21  that embodies the invention. The system  21  generally comprises a transponder  22  remotely connected to a head-end circuit  23  via at least one optical fiber. 
   The transponder  22  both receives and transmits data from a radio frequency transceiver source  24  via antenna  25 . Antenna  25  is connected to a circulator  27  which functions to prevent interference between uplink and downlink signals during the operation of the system  21 . Such circulators are formed from a ferrite material and include three terminals that allow radio frequency to flow between any two adjacent ports in one direction only. Such devices are known in the prior art and per se form no part of the present invention. One port of the circulator  27  is connected to a photodetector  31  by way an AC coupling  29   a,  while the other port of this component is connected to a VCSEL  33  via AC coupling  29   b.  The VCSEL  33  is optically coupled to the head-end circuit  23  via optical fiber  34  which is preferably a multi-mode fiber. A bias current conductor  35  conducts biasing current generated by the photodetector  31  to the VCSEL  33 . 
   The head-end circuit  23  includes a source  37  of light which may be a laser, an EDFA or an LED. Source  37  is remotely connected to the photodetector  31  of the transponder  22  via optical fiber  39 . Head-end circuit  23  further includes a photodetector  41  whose input is remotely connected to the output of the VCSEL of the transponder  22  via the previously mentioned optical fiber  34 . Both the source  37  of light and the photodetector  41  are connected to two of the three ports of another circulator  43 . The third port of the circulator  43  is connected to an antenna  47 . The antenna  47  both receives and transmits data from the radio frequency transceiver  48 . 
   In operation, the transmission of downlink and uplink signals between the transponder  22  and head end circuit  23  often occurs in a time-duplex fashion. When a downlink signal is to be transmitted, the radio transmission  48  converts a digital base band downlink signal into a radio frequency signal, which is transmitted to the antenna  47 . The circulator  43  conducts the radio frequency signal to an input of the source  37  of laser light in order to modulate its optical output and create an optical downlink signal. The optical downlink signal is transmitted via optical fiber  39  to the photodetector  31  of the transponder  22 , which in turn converts the optical downlink signal into a radio frequency downlink signal. The radio frequency downlink signal is in turn conducted to the circulator  27  via AC coupling  29   a,  where it is transmitted through the antenna  25  to the radio frequency transceiver  24 . In this manner, the downlink signal from radio frequency transmission  48  is transmitted to the radio frequency transceiver  24 . 
   When the picocell system  21  is used to transmit an uplink signal from the radio frequency transceiver  24  to transceiver  48 , the source  37  of laser light transmits a biasing signal via optical fiber  39  to the photodetector  31  in order to generate a biasing current. The biasing current is in turn conducted into the VCSEL  33  via conductor  35 . At the same time, radio frequency transceiver  24  transmits an uplink signal which is received by the antenna  25  and conducted into the AC coupling  29  via circulator  27 . The time-varying electrical charge generated in the AC coupling  29   b  from the uplink radio frequency signal is applied to the input of the VCSEL  33  such that its optical output is modulated in accordance with the radio frequency uplink signal. The resulting optical uplink signal is in turn transmitted to the photodetector  41  of the head-end via the optical fiber  34 , and from thence to the antenna  47  via circulator  43  to the radio frequency transceiver  48 . 
   While the picocell system  21  is illustrated as having two optical fibers  34  and  39  for uplink and downlink signals, respectively, it may also employ only a single optical fiber  48  (indicated in phantom) which is bifurcated at either end to connect with the photodetector  34  and VCSEL of the transponder and the laser light source  37  and photodetector  41  of the head-end circuit  23 . Additionally, while the operation of the picocell system as has been described in time-duplex terms, this same architecture could also operate via frequency duplex where uplink and downlink signals are transmitted simultaneously on different frequency bands. 
     FIG. 7  illustrates a second embodiment  50  of a picocell system of the invention likewise including a transponder  52 , and a head-end circuit  54 . At the transponder end of this system  50 , a radio frequency transceiver  56  formed from the combination of a laptop computer  57  in a wifi card  58  transmits a downlink radio signal. The transponder  52  includes an antenna  60  connected to a circulator  62  whose remaining two ports are respectively connected to a bias-tee circuit  62  which in turn is connected to the input of the VCSEL  63 , and a photodiode  65 . The optical output of the VCSEL  63  is coupled to an uplink optical fiber  64 . The optical input of the photodiode  65  is coupled to a downlink optical fiber  66 . The transponder  52  further includes a photodetector array  67  whose input is coupled to a bias optical fiber  68 , and whose electrical output is connected to the bias-tee  62  via conductor  69 . 
   Turning next to the head-end circuit  54  of the system  50 , circuit  54  includes a photo receiver  71  coupled to the other end of the uplink optical fiber  64 , and an optical transmitter in the form of a DFB-ld coupled to the downlink optical fiber  66 . An erbium doped fiber amplifier (EDFA)  74  is coupled to the input of the bias current optical fiber  68  in order to power the photodetector array  67  of the transponder. Both the output of the photo receiver  71  and the input of the optical transmitter  72  are connected to two of the three ports of a circulator  63 . The remaining port is connected to the output of a radio frequency transmitter  76  formed by the combination of a computer  77 , and a wifi card  79 . 
   In operation, the wifi card  79  of the radio frequency transceiver  76  was based on the IEEE 802.11 b/g standard. Thus, the frequency of the downlink data rate was 2.4 GHz. The data rate could be read out on the laptop  57  of the radio frequency transceiver  56  or measured from the time file transmission took between the computer and the laptop or vice versa. 
   While the invention has been described with reference to several preferred embodiments, many variations and modifications of these embodiments will become apparent to those skilled in the art. For example, the circulator  43  described with reference to the  FIG. 6  embodiment may be eliminated in alternate designs. The antenna interfaces shown on the head-end circuits  23  and  54  of the  FIGS. 6 and 7  embodiments may also be eliminated, and these circuits  23  and  54  may be directly wired to a backbone network such as the internet or a corporate intranet. All such variations and modifications are intended to fall within the scope of the invention, which is limited only by the language of the claims and equivalents thereto.