Abstract:
A system that transmits amplitude modulated data in a wavelength-encoded format and then uses a wavelength-sensitive receiver to convert the received optical signal back to the original amplitude modulated data. This system enables transmission of optical signals that are less sensitive to attenuation and attenuation changes. This system is applicable to data in digital, multilevel, or analog formats.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to fiber optic modulation and coding methods and more particularly it relates to a wavelength-sensitive receiver for converting wavelength changes to amplitude changes in an optical transmission system which uses a wavelength-encoded modulation format. 
     2. Background of the Invention 
     Fiber optic systems typically use amplitude modulation (AM) to encode data on an optical carrier signal. In this encoding method, the amplitude of the carrier waveform is modified according to the information signal that it is transporting. The receiver simply detects the intensity of the light that hits the photodetector and then uses a decision circuit to decode the signal back to the digital data (in the case of digital modulation). Any fiber optic system will have attenuation due to the losses of splices and connectors, and within the optical fiber itself. Due to these losses, a fiber optic receiver must be able to tolerate amplitude variations in the fiber optic link. This is the dynamic range of the receiver. 
     Another way to encode digital data on a fiber optic link is to code a “0” bit on one frequency and a “1” bit on another frequency. This binary format is referred to as frequency-shift keying. However, since this is the optical domain this could also be referred to as the wavelength-shift keying where the subcarrier wavelength determines the logical state. By encoding the data in the wavelength rather than in the intensity of the light, the sensitivity of the fiber optic link to attenuation is reduced. The receiver must be able to detect changes in wavelength rather than changes in intensity to decode the information and prevent contamination of the optical signal due to light intensity losses. 
     One problem with the use of wavelength-shift keying is that photodetectors, which are used to convert optical signals to an electrical output, respond to the intensity of light independent of wavelength. 
     SUMMARY OF THE INVENTION 
     It is a primary purpose of the present invention to facilitate wavelength-shift keying among modulation formats, by use of a wavelength-sensitive receiver which converts wavelength changes to amplitude changes. This is accomplished in one embodiment of the invention by employing a fiber optic receiver for a link that has the information coded in the wavelength domain. In order to convert the wavelength information, a photodetector in the wavelength-sensitive receiver is preceded by a device such as a Fabry-Perot (FP) filter that has a transmission function with the appropriate shape for the wavelengths of interest. Due to the reflective properties of the cavity within the FP filter, the optical transmission through the filter will have variations in intensity versus input wavelength. Since these properties can be predicted, the transmission output of the FP filter can be designed to yield a certain intensity at the “0” bit wavelength, for example, and another intensity at the “1” bit wavelength. After passing through the FP filter, the light intensity is detected using a conventional photodetector that then converts the light to an electrical signal to be used by a decision circuit or for any other appropriate purpose. In one embodiment the combination of a photodetector preceded by a FP filter is the wavelength-sensitive receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The objects, advantages and features of the present invention will become more apparent to those skilled in the art from the following detailed description, when read in conjunction with the accompanying drawing, wherein: 
         FIG. 1  schematically shows the functional elements of an exemplary fiber optic data system, with signal representations, embodying the present invention; 
         FIG. 2  shows an exemplary waveform as the transmission characteristics of a particular FP filter; 
         FIG. 3  is an exemplary FP filter that may be employed in an embodiment of the invention; 
         FIGS. 4A through 4D  show signals at different locations in the  FIG. 1  embodiment; 
         FIGS. 5A through 5D  are representative graphs similar to  FIGS. 4A through 4D , demonstrating use of the invention for converting multiple wavelengths to multiple intensities; 
         FIG. 6  is a diagram of an embodiment of the invention where the optical signal contains analog information, and showing representative waveforms at various locations. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides, in several embodiments, a method and an apparatus for converting optical wavelength changes to intensity changes in a fiber optic receiver for a link that has the information coded in the wavelength domain. An example of information coded in the wavelength domain is a binary format where a “0” bit is transmitted on one wavelength and a “1” bit is transmitted on another wavelength. A receiver comprises a wavelength-sensitive element such as a FP filter and a photodetector in order to decode the “zeros” and “ones.” The wavelength-sensitive element may be realized by using a FP filter. The optical transmission through the FP filter will have variations in intensity versus input wavelength. An exemplary FP filter is illustrated in  FIG. 3  and is detailed herein below. 
       FIG. 1  illustrates a binary signal that is encoded, converted and decoded according to the invention. At its input, tunable laser  11  maps digital input  10  to corresponding wavelengths  11 A. In this example, there is a digital “1” pulse at bit location  10 A and a digital zero “pulse” at bit location  10 B. These are converted to respective wavelengths λ 1  and λ 0  ( 11 A) by tunable laser  11 . The signal travels through fiber optic cable or link  12  where any attenuation losses do not affect the wavelength-encoded data. The signal reaches wavelength-sensitive receiver  13 A which, in this embodiment, comprises FP filter  13  and photodetector  14 . Light exiting the FP filter has two intensities corresponding to λ 0  and λ 1  as shown by the graph of  FIG. 2 . Photodetector  14  then converts the optical signal to an electrical output based on its intensity. Decision circuit  15  is employed to decode and recover the digital data. It may comprise, for example, a continuously acting voltage comparator and a clocked flip-flop or a sample-and-hold circuit. The actual means for realizing this circuit is well within the skill of those in this technical field and need not be detailed here. If the received voltage from the photodetector is below a predetermined threshold, the decision circuit produces a “0” bit at its output. Alternatively, if the voltage exceeds the threshold, a “1” bit is produced. 
     A Fabry-Perot filter is a known device that consists of a cavity formed by two partially transmitting mirrors placed in parallel with each other, as shown in  FIG. 3 . It may be more specifically referred to as a Fabry-Perot interferometer (FPI) or FP etalon. A simple FP filter typically consists of two mirrors and a precisely fixed air gap. Incident light enters the FP cavity  33  through optical fiber  27  and collimating lens  31 . Once inside the cavity, the light beam undergoes multiple reflections between mirrors  32  and  34  so that it can interfere with itself many times. Eventually some of the light is transmitted out of the other end of the cavity through focusing lens  35  and optical fiber  36 , for example. There are certain wavelengths of light that become stronger while undergoing multiple reflections due to constructive interference. Other wavelengths will experience destructive interference. The end result can be illustrated as a plot of optical transmission intensity through the cavity vs. wavelength that has multiple peaks and valleys (see  FIGS. 2 ,  4 B and  5 B). The FP filter performance is exhibited by the sharpness and separation of the peaks. That waveform is determined by the air gap, mirror separation and reflectivity, as is known to those skilled in the art. A critical aspect of this invention is that certain wavelengths of light can be converted to particular amplitudes using the combination of a FP filter and a photodetector. The FP filter behavior can be predicted using known design parameters such as mirror separation and reflectivity. Thus the FP characteristics are selectable for the purpose at hand. 
       FIGS. 2 ,  4 B and  5 B display a plot of the transmission intensity vs. input wavelength for a particular filter.  FIGS. 2 and 4B  also display how the plot may be used for binary operation. For use in the invention, the minimum and maximum wavelengths are chosen as the carrier wavelengths of the signals (λ 0  and λ 1 ). The FP filter may be custom designed to have a minimum at the “0” bit wavelength (λ 0 ) and a maximum at the “1” bit wavelength (λ 1 ). 
       FIGS. 4A through 4D  provide waveform details of the  FIG. 1  embodiment. Specifically in the example, a signal from laser  11  has an initial input power P IN, LINK  and is transmitted in two pulses,  41  and  42  respectively, on one of two wavelengths (λ 0  and λ 1 ) as in  FIG. 4A . A retum-to-zero (RZ) data format is shown where an inactive period follows each signal. It is additionally contemplated that a non-return-to-zero (NRZ) data format could also be implemented. 
     Pulses  41  and  42  are applied to FP filter  13 , which has characteristic waveform  43  shown in  FIG. 4B . The optical power at the output of the FP filter, P OUT , is a function of the input power, P IN, RX  and the transmission intensity curve T(λ) of  FIG. 4B , where:
 
 P   OUT   =T (λ) P   IN, RX ;
 
 P   IN, RX   =a P   IN, LINK ;
         where a=attenuation factor of the fiber optic cable (LINK).       

     Substituting values for λ 0  and λ 1  into T(λ) yields P min  and P max  as in  FIG. 4C , represented as follows:
 
 P   min   =T (λ 0 ) P   IN, RX   =T   min   P   IN, RX ;
 
 P   max   =T (λ 1 ) P   IN, RX   =T   max   P   IN, RX .
 
     The optical signals out of the filter are converted to electrical signals by photodetector  14 . The photodetector converts the optical power to minimum and maximum currents, I min  and I max , as illustrated in  FIG. 4D , and according to the following equations:
 
I min =           P min ;
 
I max =         P max .
       where I=current out of photodetector;             =responsivity of the photodetector; and   P=optical power.       

       FIG. 5B  is a graph of optical transmission intensity vs. wavelength, T(λ), similar to  FIGS. 2 and 4B . This figure displays use of the linear portion of the graph for converting multiple wavelengths to intensities. Thus, the same FP filter can be used in a system where multiple wavelengths are used to transmit encoded data as illustrated by  FIGS. 5A through 5D  (multilevel modulation format), the description of which closely follows that of  FIGS. 4A through 4D . The minimum and maximum wavelengths of the linear portion of  FIG. 5B  are represented by λ Lmin  and λ Lmax  respectively. The wavelength difference between successive signals used to send data is represented by Δλ, in  FIG. 5A . In theory, Δλ could be infinitely small, which is desired for the embodiment of the invention comprising analog data as shown in  FIG. 6 . Therefore, all wavelengths within the range λ Lmin  to λ Lmax  are available to be mapped from all the amplitude variations that exist in an analog signal (see  FIG. 6 ). “Mapping” may also be described as assigning corresponding values similar to a mathematical function. 
     As before, the pulses shown in  FIG. 5A  are applied to the FP filter, which has the characteristic waveform of  FIG. 5B .  FIG. 5C  shows the corresponding optical power out of the filter, which is applied to the photodetector. The corresponding electrical signals from the photodetector are shown in  FIG. 5D . 
     Using wavelengths within the linear portion of the FP filter provides a one-to-one correspondence between wavelength and amplitude of the signal out of the photodetector. Normally, a FP filter is designed to have sharp peaks (resonances) centered around a particular wavelength of interest. However, the FP filter used in the present invention is custom designed so that peaks and valleys of the transmission plot are more gradual and a near one-to-one correspondence is achieved. As a result, the output intensity of the FP filter is most responsive to changes in wavelength and no two wavelengths will result in the same intensity for the range of wavelengths of interest. 
     In addition to discrete data levels (digital or multilevel), the invention is useful for optical transmission of analog modulated data. In this configuration, the invention could be described as similar to an AM signal converted to an FM signal for transmission and subsequently converted back to an AM signal in order to be decoded at the receiver. 
     In accordance with  FIG. 6 , AM data provides control signal  55  to tune (select wavelength) a linear, continuously tunable laser  61 . The input signal to laser  61  is shown as curve  55  as a plot of signal Intensity Ix versus time. Hence, laser  61  is designed to be tuned according to changes in amplitude of an input control signal. The laser converts the AM data to FM data  56  where the frequency is alternatively the wavelength of the light rather than an electrical subcarrier frequency as is often done in the electrical domain. Signal  56  is plotted as wavelength versus time. After transmission through fiber optic link  62 , the optical signal passes through FP filter  63  designed to operate over the wavelengths of the tunable laser, its characteristic waveform being shown as curve  57 . The FP filter should be designed to have a linear optical transmission intensity versus incoming wavelength (as in  FIG. 5B ). The FM data is then converted back to AM data since the intensity of the light at the output of the FP filter will be directly proportional to the wavelength of light. This is represented by curve  58  in a plot of power or intensity versus time. After passing through the FP filter, the light is detected by a photodetector, amplified, and offset to achieve the appropriate voltage levels. The current out of the photodetector is represented in curve  59 . Receiver  64 , which includes the photodetector, amplifiers, and offset circuitry, should also be linear over the range of incoming optical signal levels. Note that a decision circuit is not included in the receiver for a system transmitting analog data. In order to maximize the signal quality, the tunable laser and FP filter should be designed to utilize the same wavelength range, and the linear portion of the transmission intensity versus wavelength characteristic should be fully utilized. The photodetector may be any suitable device which is responsive to light intensities to provide a useful output, as one skilled in this technical field might decide to use. 
     The terms “coded” and “decoded” as used herein are generally taken to mean the same as modulated and demodulated. 
     While the invention has been illustrated and described by means of specific embodiments, it is to be understood that numerous changes and modifications may be made therein without departing from the intent and scope of the invention as defined in the appended claims and equivalents thereto.