All-optical balanced detection system

Abstract

A two gate sampling system designed to perform sampled balanced detection of one or more input signal pairs. The present invention performs simultaneous sampling of both signals in each signal pair followed by digitization and combination of the sample pairs using software. By first sampling the signals and then combine the sampled into the corresponding balanced detected signal it is possible to avoid the bandwidth limitations and impedance problems introduced by traditional balanced detectors and electrical oscilloscopes. In particular for optical sampling gates very high bandwidth sampling gates can be designed without any impedance issues and hence almost perfect balanced detection reconstruction can be performed for very high speed signals. Balanced detection is becoming more and more important as the new phase modulated optical data signals are introduced to the market, such as e.g. PSK, DPSK, QPSK and DQPSK. The present invention is well suited for analysis of these new type of signals.

1 . A sampling arrangement for performing balanced detection of a pair of input data signals, the sampling arrangement comprising
 at least one pair of sampling gates, a first sampling gate of said pair responsive to a first input signal of the pair of input signals and a second sampling gate of said pair responsive to a second input signal of said pair of input signals, each sampling gate creating a stream of samples representative of the respective input signal applied thereto;   a strobe source for controlling the at least one pair of sampling gates to open and close at a predetermined sampling frequency f s ;   an adjustable delay line disposed at the input of one sample gate of said at least one pair of sampling gates for minimizing differences in path lengths between each input signal and each sampling gate; and   a processor configured to perform balanced detection of the sampled pair of input signals to generate as an output a sampled balanced detected version of said input signal pair.   
   
   
       2 . A sampling arrangement as defined in  claim 1  wherein the at least one pair of sampling gates comprises a pair optical sampling gates. 
   
   
       3 . A sampling arrangement as defined in  claim 1  wherein the at least one pair of sampling gates comprises a pair of electrical sampling gates. 
   
   
       4 . A sampling arrangement as defined in  claim 1  wherein the strobe source comprises an optical component. 
   
   
       5 . A sampling arrangement as defined in  claim 1  wherein the strobe source comprises an electrical component. 
   
   
       6 . A sampling arrangement as defined in  claim 1  wherein the strobe source comprises a pair of strobe elements operating at the same frequency, each separate strobe element coupled to control a separate gate of the at least one pair of sampling gates. 
   
   
       7 . A sampling arrangement as defined in  claim 1  wherein the sampled pair of signals is represented by optical signals and the balanced detection of each sampled pair is performed by
 using a optical-to-electrical balanced receiver having two optical inputs, S 1  and S 2 , and an electrical output corresponding to S 1 -S 2 ;   an A/D converter coupled to the output of the balanced receiver to convert an analog set samples into a digital representation thereof.   
   
   
       8 . A sampling arrangement as defined in  claim 1  wherein the sampled pair of signals is represented by electrical signals and the balanced detection of each sampled pair is performed by
 using an electrical circuit having two electrical inputs, S 1  and S 2 , and an electrical output corresponding to S 1 -S 2 ;   an A/D converter coupled to the output of the balanced receiver to convert an analog set samples into a digital representation thereof.   
   
   
       9 . A sampling arrangement as defined in  claim 1  wherein the sampled pair of signals is represented by optical signals and the balanced detection of each sampled pair is performed by
 detecting the first and second component of each sampled pair with separate optical-to-electrical receivers;   each component of the sampled pair are coupled to an A/D converter to convert the analog sets of samples into digital representations thereof;   the digitally represented pairs of sampled outputs are then subtracted on a sample by sample basis to form the reconstructed balanced detected signal.   
   
   
       10 . A sampling arrangement as defined in  claim 1  wherein the adjustable delay line is selected from the group consisting of: fixed delay, set-and-forget delay or tunable delay. 
   
   
       11 . A sampling arrangement as defined in  claim 10  wherein the adjustable delay line is selected from the group consisting of: optical delay lines and electrical delay lines. 
   
   
       12 . A sampling arrangement as defined in  claim 1  wherein
 a single pair of optical sampling gates is responsive to a single pair of optical input signals to form a sampled pair of signals;   a single optical strobe source controls the single pair of optical sampling gates at a predetermined frequency;   an adjustable optical delay line is used to control the input signal pair paths differences to each sampling gate; and   the output sampled signals from the single pair of optical sampling gates are converted to electrical signals using separate optical-to-electrical receivers followed by A/D conversion of each signal, wherein the digitally represented signals are then subtracted to form the balanced detected version of the input signal pair.   
   
   
       13 . A sampling arrangement as defined in  claim 1  wherein
 two pairs of optical sampling gates are responsive to two pairs of optical input signals to form two sampled pairs of signals;   a single optical strobe source controls the two pairs of optical sampling gates at a predetermined frequency;   adjustable optical delay lines are used to control each input signal pairs paths differences to each sampling gate; and   the output sampled signals after each pair of sampling gates are converted to electrical signals using separate optical-to-electrical receivers followed by A/D conversion of each signal wherein the digitally represented signals are then subtracted to form balanced detected versions of both input signal pairs.   
   
   
       14 . A sampling arrangement as defined in  claim 1  wherein
 a single pair of optical sampling gates is responsive to at least two pairs of optical input signals to form sampled pairs of signals;   a single optical strobe source controls the at least two pairs of optical sampling gates at a predetermined frequency, the sampling arrangement further comprising:   optical switches disposed in the input signal path before each sampling gate associated with two or more inputs and a single output and controlled such that the optical switches select, in a predetermined order, which pair of input signals is sent to the sampling gates; and   adjustable optical delays disposed along each input signal pair to control the input signal pair paths differences to each sampling gate, wherein   the output sampled signals after the two sampling gates are converted to electrical signals using separate optical-to-electrical receivers followed by A/D conversion of each signal such that the digitally represented signals are then subtracted to form the balanced detected versions of the input signal pairs.   
   
   
       15 . A sampling arrangement as defined in  claim 1  wherein the arrangement further comprises
 at least one additional sampling gate responsive to at least one input reference clock signals to form sampled reference clock signals wherein the at least one additional sampling gate is controlled by the same strobe source as the sampling gates responsive to the input signal pairs; wherein   the output sampled reference clocks after the sampling gates are converted to electrical signals using separate optical-to-electrical receivers followed by A/D conversion of each clock such that the digitally represented clock signals are used to create time-bases for the sampled signals to correctly position each sample in the reconstructed balanced detected versions of the input signal pairs.   
   
   
       16 . A sampling arrangement as defined in  claim 1  wherein the arrangement further comprises
 a demodulation method for optical signals containing data encoded in the optical phase coupled to the input of the sampling arrangement, wherein after demodulation the input optical signal is split up in a number of input signal pairs containing the optical phase information converted into the form of amplitude modulation.   
   
   
       17 . A sampling arrangement as defined in  claim 16  wherein the demodulation method is designed to demodulate modulation formats selected from the group consisting differential phase shift keying (DPSK), differential quadrature phase shift keying (DQPSK), phase shift keying (PSK), quadrature phase shift keying (QPSK) or differential eight level phase shift keying (D8PSK)