Abstract:
This invention disclosure describes the application of a polarization insensitive acoustically-tuned optical filter used in a multichannel WDM system to equalize variations in the power level of the WDM channels. The invention also describes a simple means for providing a low frequency control system which enables the equalizer to determine the signal levels of N optical carriers prior to equalizing the signals.

Description:
FIELD OF THE INVENTION 
     The present invention relates to equalization of wave-length-dependent optical signals, and more particularly to equalization of wavelength-dependent optical signals using a polarization-independent acoustically tuned optical filter. 
     BACKGROUND OF THE INVENTION 
     It is known that long optical fiber transmission links for telecommunications can be built using cascaded chains of optical amplifiers. Erbium doped optical fiber amplifiers are particularly well-suited for implementing these long distance transmission systems due to their excellent performance characteristics and ease of fabrication. 
     However, multiplexed optical-signals utilizing wave-length division multiplexed (WDM) systems and erbium doped optical amplifiers exhibit a variation in signal gain that is a function of the individual wave-lengths. Moreover, utilizing cascaded optical amplifiers to compensate for attenuation over the transmission link only exaggerates the variation in signal gain for the separate wavelengths. For example, a 10 channel WDM system with a 1 nm channel spacing could easily having a gain variation over the 10 nm signal band of from 1 to 3 dB after amplification. The total gain variation is increased by the product of the number of cascaded amplifiers, and thus will certainly be much larger. While a 1 to 3 dB gain variation may be acceptable for short amplifiers chains, with 10 or more cascaded amplifiers the resulting 10 to 10 dB gain variation is not likely to be acceptable. 
     Large variation in component signal levels of a multiplexed signal over the wavelength spectrum complicates the design and performance of optical receivers and -detectors, and thus it is advantageous to equalize variations in signal level for any wavelength-dependent elements in the optical transmission path, particularly wavelength-dependent gain due to amplification. 
     OBJECTS OF THE INVENTION 
     Accordingly it is a primary object of this invention to obviate the above noted and other disadvantages of the prior art. 
     It is a further object of this invention to control the optical signal level of a optical signal composed of a plurality of differing wavelengths. 
     It is a yet further object of this invention to provide for uniform wavelength amplification of an optical signal composed of a plurality of differing wavelengths. 
     It is a still further object of this invention to provide for automatic adjustment of an optical signal composed of multiple wavelengths. 
     SUMMARY OF THE INVENTION 
     The above and other objects and advantages are achieved in one aspect of the invention by including a polarization-independent acoustically tuned optical filter (PIATOF) after a set of cascaded optical amplifiers to produce a uniform signal level for each associated wavelength of the input optical signal. 
     Multiple optical signals at differing wavelengths are combined by a wavelength division multiplexor and passed through a series of optical amplifiers. The output signal from the cascaded amplifiers is input to a PIATOF. A PIATOF is a two port output device, and the output of port one of he PIATOF is tapped and the tapped signal is supplied to a demultiplexer to separate the input signal according to wavelength. The resultant output signals of the, demultiplexer are input to a control circuitry. The control circuitry compares the output signal levels of the PIATOF for each wavelength and determines a proper RF power signal to be input at the control electrode of the PIATOF so that the signal level for each wavelength of the output signal at port one of the PIATOF is uniform after the amplification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a fiber optic communication link including a DFB laser and an optical amplifier. 
     FIG. 2 is an illustration of a fiber optic communication system in accordance with the instant invention and including multiple light sources emitting different wavelengths with a polarization independent acoustically-tuned optical filter for equalizing the output signal of an amplifier. 
     FIG. 3 illustrates another embodiment of the invention wherein a plurality of digital signals are combined with a low frequency reference carrier, amplified, and input to a polarization independent acoustically-tuned optical filter which equalizes the signal level of the individual signals. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, wherein is depicted an optical fiber  110  coupled to a lightwave source  120  such as a DFB laser with optical intensity L at wavelength X. Optical Fiber  110  is coupled to an optical amplifier  130 , resulting in an optical gain and an output optical intensity of L A . The gain, K=L A /L, of optical amplifier  130  is a function of the wavelength λ of the input lightwave. Thus if multiple wavelengths are combined in an input lightwave source and amplified, the gain for each wavelength after amplification will not be uniform, but rather dependent on the input wavelength. 
     Referring now to FIG. 2, wherein one embodiment of the instant invention is depicted. N lightwave sources  210  transmit separate lightwave with wavelengths λ i . i=1, . . . ,n, which are input to a wavelength division multiplexor  220  (WDM). The signals are combined in WDM  220  and transmitted on fiber optic cable  110  to optical amplifier  130 . The optical input signal to amplifier  130  is designated as L in , and as stated above is the combination of the individual lightwaves at wavelengths λ i . After amplification by amplifier  130 , the resultant optical signal L out , which is the sum of the individual amplified lightwaves at wavelengths λ i  does not exemplify a uniform optical signal at the individual wavelengths. The output signal L out  is tapped at tap  230  and a portion of L out  is input into a 1×N demultiplexer  240  to isolate each separate lightwave at wavelength λ i . The intensity of the optical signal from the output of the demultiplexer is designated as L i  for each wavelength λ i . The untapped optical signal is input into a polarization independent acoustically-tuned optical filter  250  (PIATOF) which functions as a multi-channel splitter and equalizer. Polarization independent acoustically-tuned optical filters using wavelength division multiplexing are described by D. A. Smith et al. in “Integrated-optic Acoustically Tunable Filters for WDm Networks” IEEE JSAC, Vol. 8 pgs. 1151-1159, 1990, and D. A. Smith et al. in “Integrated-optic Acoustically Tunable Filters: Devices and Applications”, Optical Fiber Conference (OFC&#39;91), San Diego, Feb. 18-22, 1991, p. 142, both of which are incorporated by reference into this application. 
     PIATOF  250  has one input port, two output ports, and a control electrode for determining the distribution of the input optical signal between the two ports. For the N WDM (λ 1 , λ 2 , . . . λ n ) wavelengths of L out  which are input to the PIATOF  250 , each of the signals can be directed to either of the two output ports by applying an RF signal at frequency i  to the control electrode of the PIATOF. The frequency f i  is the corresponding frequency for wavelength λ i . After applying RF power P i  at frequency f i , all the optical signal on channel i at wavelength λ appears at port  2  of the PIATOF. Power P i  is determined emperically as it depends on construction of the PIATOF. Applying RF power X i P i  at frequency f i , the optical signal levels corresponding to an initial lightwave intensity L i  appear at the respective ports of the PIATOF. 
     
       
         OUTPIATOF 1 =L i cos 2 (X i π/2)  
       
     
     
       
         OUTPIATOF 2 =L i sin 2 (X i π/2)  
       
     
     Accordingly the optical signal level appearing at port  1  can be independently controlled by applying a specified set of RF power levels determined by a set of parameters (X 1 . . . X n ) at frequencies f i  corresponding to the wavelengths λ i . 
     Continuing to refer to FIG. 2, PIATOF  250  is used as a means for equalizing the power levels of the optical signals resulting from amplifier  130 . After signal L out  is demultiplexed into the signals L i , each signal L i  is input to a bank of n photodetectors  260  to determine the signal levels of the L i  and is input to Control System  280  to compare the respective levels. Control System  280  determines the coefficients X i  for the respective wavelengths λ i  so as to equalize the output signal from PIATOF  250 . RF power X i P i  at frequency f i  for each i=1, . . . , n is combined an input to the PIATOF on the control port of the device. The resultant output of the PIATOF displays a uniform signal for each wavelength λ i . 
     A further embodiment of the instant invention is shown in FIG.  3 . Multiple transmitting DFB lasers  310  carrying conventional digital data (D 1 , . . . , D N ) are modulated by low frequency control signals (ω i =1 k to 10 k) with a small modulation depth from m=0.01 to 0.05. Each transmitting laser  310  is modulated by a separate control frequency (ω 1 , . . . , ω N ). After combining the modulated input signals at WDM multiplexor  320 , the combined signal is passed through a series of fiber amplifier  330 . After the cascaded amplifiers  330 , a PIATOF  340  is installed in the transmission path. A 10 dB optical tap  345  is installed on output one of the PIATOF, and the tapped optical signal is provided to a single photodiode at photodetector  350 . Photodetector  350  converts the tapped optical signal to an electrical signal, and demodulation circuit  360  demultiplexes the signal into the signals C i  at the input frequencies ω i . Each signal C i  is input to Control System  370  to compare the respective levels. Control System  370  determines the coefficients X i  for the respective frequencies ω i  so as to equalize the output signal from the PIATOF  340 . Applying RF power X i P i  at RF source  390  at frequency ω i  for each i=1, . . . ,n, the signals are combined and input to the PIATOF at the control port one of the PIATOF is attenuated by the factor cos 2 (X i π/2). By adjusting the coefficient X i  for frequency ω i  for frequency ω i  of the RF power at the control port, the output signal is equalized. Control circuitry  370  continuously monitors the modulated signal at the frequencies (ω i , . . . . ,ω N ) providing dynamic equalization of gain/loss due to elements in the network. 
     While there has been shown and described what is at present considered the preferred embodiment of the invention it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.