All-Optical Balanced Detection System

• US 20110182573A1
• Filed: 09/17/2009
• Published: 07/28/2011
• Est. Priority Date: 09/18/2008
• Status: Active Grant

• Current Assignee: EXFO Incorporated (11172239 Canada, Inc.)
• Original Assignee: EXFO Incorporated (11172239 Canada, Inc.)
• Inventors: Westlund, Mathias, Skold, Mats, Andrekson, Peter, Sunnerud, Henrik
• Granted Patent: US 9,325,428 B2
• Time in Patent Office: Days
• US Class Current: 398/25
• CPC Class Codes: H04B 10/677 for differentially modulated signal, e.g. DPSK signals
• CPC Class Codes: H04B 10/697 Arrangements for reducing noise and distortion

Abstract

A two-gate sampling system has been 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 combining 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 types of signals.

Claims

1. A sampling arrangement for performing balanced detection of a pair of input data signals apportioned from an original signal at a common point, 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 operable to provide sampling for controlling the at least one pair of sampling gates to open and close at a predetermined sampling frequency f_s;

at least one adjustable delay means associated with said at least one pair of sampling gates configured to ensure that said at least one pair of sampling gates generate samples originating from the same time interval in said original signal; 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 adjustable delay means comprises at least one adjustable delay line for minimizing differences in path lengths between each input signal and each sampling gate.

3. A sampling arrangement as defined in claim 1, wherein the pair of input data signals is encoded on an optical carrier signal, the efficiency with which the stream of samples is created by each sampling gate therefore being dependent upon and controlled by the state of polarization (SOP) of said optical carrier signal.

4. A sampling arrangement as defined in claim 3 wherein the at least one pair of sampling gates comprises sampling gates responsive to an optical sampling signal with the strobe source being operable to provide the optical sampling signal.

5. A sampling arrangement as defined in claim 1 wherein the at least one pair of sampling gates comprises sampling gates responsive to an electrical sampling signal, the strobe source being operable to provide the electrical sampling signal.

6. A sampling arrangement as defined in claim 1 wherein the at least one pair of sampling gates comprises sampling gates responsive to an optical sampling signal, the strobe source being operable to provide the optical sampling signal and the state of polarization (SOP) of each sampling signal being disposed to provide relatively high sampling efficiency at each sampling gate.

7. A sampling arrangement as defined in claim 1, wherein the input signal is electrical and the at one pair of sampling gates are electrical and the strobe source generates an electrical sampling signal which is thereafter applied as an input to the sampling gates.

8. 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 at least one respective sampling gate.

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

using a optical-to-electrical balanced receiver having two optical inputs, S1 and S2, and an electrical output corresponding to S1-S2;

an A/D converter coupled to the output of the balanced receiver to convert an analog set samples into a digital representation thereof.

10. 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, S1 and S2, and an electrical output corresponding to S1-S2;

an A/D converter coupled to the output of the balanced receiver to convert an analog set samples into a digital representation thereof.

11. 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.

12. A sampling arrangement as defined in claim 1 wherein the adjustable delay means is selected from the group consisting of:

fixed delay means, set-and-forget delay means or tunable delay means.

13. A sampling arrangement as defined in claim 11 wherein the adjustable delay means is selected from the group consisting of:

optical delay means and electrical delay means.

14. 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 predetermined frequency;

an adjustable optical delay means is disposed in either or both of the input signal pair and the sampling signal pair 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.

15. 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 means 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.

16. 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 delay means 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.

17. 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.

18. 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.

19. A sampling arrangement as defined in claim 17 wherein the demodulation method is designed to demodulate modulation formats selected from the group comprising 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).