it is suggested in prior literatures that using polarization switching should increase the nonlinearity tolerance of optical communication signals . if the two neighboring symbols are located at orthogonal polarization states , the inter - symbol interference between the two neighboring symbols does not generate coherent beating , and thus suffers from reduced nonlinear distortions . the modification to current system structure is relatively small and it can be readily applied in different direction - detection systems . in this invention record , we focus on rz - dqpsk format , which is popular for 40 gb / s - based long - haul optical communication systems . compared to ook format , the differential precoding and decoding in dqpsk / dpsk would require additional arrangement . we systematically describe the design of transmitter and receiver in this proposed multi - symbol polarization - switching scheme . introducing polarization switching by several symbols is proven capable of mitigating the inter - symbol interference in the rz - dqpsk system while reducing the requirements on the polarization switching speed at the transmitter side . the diagram of fig1 shows that the schematic of the polarization - switched dqpsk system operating at 50 gbits / s . the output of the laser ( 101 ) is modulated by a dqpsk modulator , which is driven by the data block ( 103 ) that is specially designed for multi - symbol polarization - switched dqpsk signals . in subsequent , the generated dqpsk signals are polarized using a polarization modulator ( 104 ). its modulation speed actually depends on how often the polarization states vary . after transmitting over the optical fiber link , the received polarization - switched dqpsk signals are differentially detected by the receiver ( 105 ). the detail structure for a 2 - symbol polarization - switching dqpsk is illustrated in fig2 . original data bits ( 201 ) are first de - multiplexed into several parallel , low - speed data blocks ( 202 ). in fig2 , a 2 - symbol polarization - switching 50 gb / s dqpsk is used for illustration . low - speed dqpsk differential precoders ( 203 ) are then employed for encoding the bits in each data block . subsequently , a multiplexer ( 204 ) is to combine the four inphase and quadrature inputs , respectively , into higher bit rate ( i out and q out ) at 25 gb / s . the outputs of multiplexer ( 204 ) drive the optical modulator to generate the dqpsk signals . as a result , the modulation speed of the polarization modulator ( 104 ) is also reduced to 6 . 25 ghz , instead of 12 . 5 ghz for 1 - symbol polarization - switching scheme . the 2 - symbol polarization - switched dqpsk signals are plotted in fig3 , where the polarization states change from x - polarization to y - polarization by every two symbols . it can be observed that the output pulses ( 301 , 302 ) are in the same polarization state while the other two pulses ( 303 and 304 ) have the orthogonal state . the purpose of this bits manipulation in fig2 ensures that differential detection receiver ( 105 ) would correctly recover the encoded bits ( 201 ). block 105 of fig1 is further depicted elaborately in fig4 . as for these differential - detection receivers , the beating is possible only if the polarization states of current symbol and its delayed symbols are the same . the received polarization - switched signals are split into two branches , and each is composed of one delay ( 401 ), a phase shifter ( 402 ) and a pair of balanced detector ( 403 ). to make sure the coherent beating , the delay time in block 401 has been increased to a 4 symbol duration from a 1 symbol duration , and then the demodulated signals are further de - multiplexed into 4 branches for data management . the block diagram of fig5 presents key aspects of the inventive details discussed above . the inventive multi - symbol polarization switching for differential - detection optical systems entails both modulation and demodulation aspects . the modulation entails bits manipulation for multi - symbol switching 103 and a low speed polarization modulator 104 . the demodulation aspect entails a time delay for polarization - switched dqpsk signals 401 . a significant aspect of the present invention includes the feature that the polarization states change every m symbols in differential - detection optical systems , thereby reducing the requirement of polarization modulator bandwidth at the transmitter side . another significance is configuration of the differential precoder and differential detector to adapt the practical transceiver to the multi - symbol schemes . the low - speed components are sufficient for generating the multi - symbol polarization - switched differential - encoded signals . in addition , this particular configuration of precoding enables the differential receiver correctly recovering the transmitted data . from the foregoing , it can be appreciated that with the invention , instead of alternating the polarization state symbol - by - symbol , the polarization state varies every several symbols , resulting in reduced bandwidth requirements on polarization switching modulator and its electrical driver . according to our simulated 50 gb / s rz - dqpsk results , the performance of 2 - symbol polarization - switching case has a quite similar performance as one - symbol polarization - switching scheme . in other words , the invention has the following merits : less complexity , lower cost and high tolerance to fiber nonlinearity as single - symbol polarization switching transmitter . in addition , the design of the transmitter and can be applied in other differential - detection formats , such as dpsk and d8psk . the foregoing is to be understood as being in every respect illustrative and exemplary , but not restrictive , and the scope of the invention disclosed herein is not to be determined from the detailed description , but rather from the claims as interpreted according to the full breadth permitted by the patent laws . it is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that those skilled in the art may implement various modifications without departing from the scope and spirit of the invention . those skilled in the art could implement various other feature combinations without departing from the scope and spirit of the invention .