Abstract:
A coherent lightwave demodulator is disclosed which is insensitive to the polarization state of the incoming message signal. The demodulator includes a pair of optical hybrids (aligned to the polarization state of the local oscillator) and a pair of optical balanced receivers. A squaring arrangement, for example a delay demodulator, responsive to the output from the pair of optical balanced receivers eliminates the polarization-dependent component of the message signal and allows the recovery of the message signal regardless of the polarization state of the incoming message signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a polarization insensitive coherent lightwave detector and, more particularly, to a coherent lightwave detection system where only the polarization state of the local oscillator must be known to recover information from the transmitted optical signal. The polarization state of the transmitted signal is irrelevant to achieving recovery. 
     2. Description of the Prior Art 
     Coherent lightwave detection systems are becoming increasingly prevalent in many lightwave communication arrangements. In many such systems, conventional heterodyne/homodyne techniques are used to recover the transmitted signal. These techniques require that the polarization state of the local oscillator be accurately aligned to the polarization state of the message signal to assure accurate recovery of the message. This limitation is considered to be a serious drawback to the advance of coherent lightwave detectors. One prior art arrangement which addresses this polarization problem is described in the article entitled &#34;Demodulation of Optical DPSK Using In-Phase and Quadrature Detection&#34; by T. G. Hodgkinson et al. appearing in Electronic Letters, Vol. 21, No. 19, September 1985, at pp. 867-77, In this arrangement, a 90° optical hybrid is used to achieve the in-phase and quadrature detection, where the local oscillator is connector to a first input of the hybrid via a first polarization controlling member and the transmitted DPSK signal is connected to the remaining input of the hybrid via a second polarization controlling member. However, since the polarization state of both signals are subject to drift, the polarization controllers must be monitored to insure correct operation of the demodulator. 
     An alternative coherent detection system, described in U.S. Pat. No. 4,506,388 issued to M. Monerie et al on Mar. 19, 1985, although capable of being used with a message signal of random polarization, still requires that the polarization state of the local oscillator match that of the message signal. 
     A need remains in the prior art, therefore, for a coherent detection system which is truly independent of the polarization state of the transmitted message signal. 
     SUMMARY OF THE INVENTION 
     The need remaining in the prior art is addressed by the present invention which relates to a polarization insensitive coherent lightwave detection systems and, more particularly, to a coherent lightwave detection system where only the polarization of the local oscillator must be known to recover information from the transmitted optical signal. The polarization state of the transmitted signal is irrelevant to achieving recovery. 
     It is an aspect of the present invention to perform a &#34;squaring operation&#34; in order to remove the message signal&#39;s polarization state from the final recovered data signal, where delay demodulators may be used to perform this squaring operation. 
     Another aspect of the present invention is to utilize a balanced design including a pair of optical hybrids and a pair of balanced detectors to maintain the advantages of prior art balanced arrangements, including common-mode rejection and maximization of optical signal power. 
     Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole FIGURE illustrates a double balanced DPSK coherent detection system according to the present invention which is insensitive to the polarization state of the DPSK message signal. 
    
    
     DETAILED DESCRIPTION 
     A coherent lightwave detection system which is insensitive to, or independent of, the polarization state of the received message signal is illustrated in the FIGURE. Referring to the FIGURE, received message signal E R  and local oscillator E L  are first applied as separate inputs to a 3 dB optical coupler 10. Received signal E R  is presumed to be a DPSK signal for the purpose of the present discussion, and can be expressed as ##EQU1## where M(t) represents the DPSK modulation signal, ω R  is the carrier frequency, and θ R  (t) is the phase noise associated with the carrier. Similarly, local oscillator signal E L  can be expressed as ##EQU2## where ω L  is the carrier frequency and θ L  (t) is the phase noise associated with the carrier. Since the polarization state of the local oscillator is known, coupler 10 may be designed to evenly split the power, P L , of the local oscillator. Therefore, the polarization states of the two signals can be represented as 
     
         P.sub.L =1/2P.sub.L H+1/2P.sub.L V,                        (3) 
    
     and 
     
         P.sub.R =β.sup.2 P.sub.R H+(1-β.sup.2)P.sub.R V. (4) 
    
     Therefore, the output signals E 1  (t) and E 2  (t) from 3 dB coupler 10 can be expressed as ##EQU3## where Φ t  and Φ r  are the phase shift components introduced by coupler 10 due to the nature of the 3 dB coupler and conservation of energy, Φ t  -Φ r  =π/2. 
     Output signals E 1  (t) and E 2  (t) subsequently travel along a separate pair of signal paths, where E 1  (t) is applied as an input to a first 90° hybrid component 12 and E 2  (t) is applied as an input to a second 90° hybrid component 14. Hybrids 12 and 14 can also be considered as polarization-selective beam splitters which will split the input signal into &#34;horizontal&#34; and &#34;vertical&#34; components. Therefore, in accordance with equations (3)-(6), the four outputs from hybrids 12 and 14 will be: ##EQU4## where δ is defined as the phase shift introduced by hybrid components 12 and 14. 
     The &#34;vertical&#34; outputs E 1V  (t) and E 2V  (t) are subsequently applied as a pair of inputs to a first balanced receiver 16. In particular, for the arrangement shown in the FIGURE, balanced receiver 16 comprises a pair of photodiodes 18 and 20, each photodiode responsive to a separate one of the signals E 1V  (t) and E 2V  (t). For example, photodiode 18 is responsive to signal E 1V  (t) and photodiode 20 is responsive to signal E 2V  (t). Thus, the lightwave signal is transformed into a current, where the pair of currents are applied as an input to an amplifying component 22. In a similar fashion, the horizontal components E 1H  (t) and E 2H  (t) are applied as separate inputs to a second balanced receiver 24. The photocurrent outputs from receivers 20 and 24 can be expressed as 
     
         i.sub.v (t)=C[E.sub.2V.sup.2 (t)-E.sub.IV.sup.2 (t)],      (12) 
    
     and 
     
         i.sub.h (t)=C[E.sub.1H.sup.2 (t)-E.sub.2H.sup.2 (t)],      (13) 
    
     where C is the known constant ηe/hω. Referring to equations (8) and (9), it can be shown that ##EQU5## where Φ(t)=θ L  (t)+θ R  (t)+Φ t  -Φ r  =θ L  (t)-θ R  (t)+π/2. The term ω IF  is defined as ω R  -ω L , since the difference between the two optical frequencies will result in a frequency in the IF region. Subtracting E 2H   2  (t) from E 1H   2  (t) to solve for i h  (t), the DC components will cancel out, and ##EQU6## In a similar manner it can be shown that ##EQU7## At this point, both i h  (t) and i v  (t) still include β terms and as such are considered as polarization dependent. This polarization dependence is removed in accordance with the present invention by a squaring operation, in this case represented as a delay demodulator 30. In operation, output photocurrent i v  (t) from balanced receiver 16 is applied to a first delay component 32 of demodulator 30, where component 32 forms a version of i v  (t) which is delayed by a predetermined time interval T and multiplies i v  (t) by i v  (t-T), where this multiplicative product is denoted D v  (t). For this case, D v  (t) can be expressed as ##EQU8## where ΔΦ=Φ(t)-Φ(t-T). Similarly, output photocurrent i h  (t) is applied to a second delay component 34 to form a second delayed signal D h  (t), where D h  (t) can be expressed as ##EQU9## The final recovered data signal D(t) can therefore be obtained by adding together the components D v  (t) and D h  (t), as illustrated by the summing unit 36. In accordance with equations (18) and (19), therefore, D(t) can be expressed as ##EQU10## which in accordance with the teachings of the present invention is independent of the polarization parameter β of received message signal E R  (t).