US20100067607A1 - All-optical balanced detection system - Google Patents
All-optical balanced detection system Download PDFInfo
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- US20100067607A1 US20100067607A1 US12/284,195 US28419508A US2010067607A1 US 20100067607 A1 US20100067607 A1 US 20100067607A1 US 28419508 A US28419508 A US 28419508A US 2010067607 A1 US2010067607 A1 US 2010067607A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/69—Electrical arrangements in the receiver
- H04B10/697—Arrangements for reducing noise and distortion
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/67—Optical arrangements in the receiver
- H04B10/676—Optical arrangements in the receiver for all-optical demodulation of the input optical signal
- H04B10/677—Optical arrangements in the receiver for all-optical demodulation of the input optical signal for differentially modulated signal, e.g. DPSK signals
Definitions
- the present invention relates to a sampling arrangement particularly well-suited for analysis of high speed data signals and, more particularly, to a sampling arrangement with two or more coupled sampling gates.
- Digital sampling is a technique used to visualize a time-varying waveform by capturing quasi-instantaneous snapshots of the waveform via, for example, a sampling gate.
- the gate is “opened” and “closed” by narrow pulses (strobes) in a pulse train that exhibit a well-defined repetitive behavior such that ultimately all parts of the waveform are sampled.
- the sampling implementation can either be real-time or equivalent-time, where real-time sampling refers to the case where the sampling rate is higher than twice the highest frequency content of the waveform under test (Nyquist sampling), while equivalent-time sampling uses an arbitrarily low sampling rate.
- equivalent-time sampling requires the measured waveform to be repetitive (in order to provide accurate signal reconstruction)—a fundamental limitation when compared to real-time sampling.
- the present invention is independent of the sampling rate, and hence, can be either real-time or equivalent-time sampling.
- phase modulated signals have already been employed in commercial systems, such as differential phase shift keying (DPSK) and differential quaternary phase shift keying (DQPSK).
- DPSK differential phase shift keying
- DQPSK differential quaternary phase shift keying
- the data is encoded as the relative phase shift between consecutive symbols.
- DPSK modulation schemes for example, a ⁇ phase shift between bits represents a logical “1” and a zero phase shift represents a logical “0”.
- DQPSK modulation each symbol contains two bits of information by allowing four different relative phase changes between consecutive bits (e.g., 0, ⁇ /2, ⁇ and 3 ⁇ /2).
- FIG. 1 is used to further clarify the concept of phase-encoded modulation formats such as phase-shift keying (PSK), differential phase-shift keying (DPSK), and QPSK and DQPSK as defined above.
- PSK phase-shift keying
- DPSK differential phase-shift keying
- QPSK and DQPSK QPSK and DQPSK as defined above.
- the optical phase and amplitude of the data signal are visualized in constellation diagrams showing the optical field amplitude as the radial distance from origin R and the optical field phase as the angle ⁇ .
- the logical marks (ones) and spaces (zeros) are represented as either absolute phase and amplitude levels (for PSK and QPSK formats, FIGS.
- each symbol contains, as shown, two bits of information. Therefore, four different logical phase and amplitude combinations are used to represent the “symbols” in either of these modulation format types.
- the amplitude of the data signal is constant for each of these phase-encoded modulation techniques. Hence, if only the power of the incoming signal is “detected” using a conventional photodetector-based o/e conversion device, the phase information will be lost. To extract the phase information, the signal needs to be mixed with an optical reference signal which converts the phase information into amplitude information.
- delay interferometers such as Mach-Zehnder interferometers (MZIs), Michelson interferometers, or the like, are commonly used in which the signal itself serves as reference after being delayed one (or more) bit periods.
- MZIs Mach-Zehnder interferometers
- Michelson interferometers or the like
- an independent reference signal is necessary to extract the phase information from each bit.
- the DI is an interferometric structure where the incoming optical waveform is equally split up in two paths and one path is delayed relative to the second path before recombining the two paths.
- the relative delay is coarsely set equal to an integer number of bit slots (most commonly one bit slot) and finely tuned to match a particular relative phase delay of the optical carrier. For example, in the DPSK case, the relative delay is a multiple of ⁇ in order to effectively translate the relative phase shifts between the symbols into a binary amplitude modulated signal.
- the DI has two output ports—a constructive interference port and a deconstructive interference port (the ‘destructive’ port outputting the complementary data of the ‘constructive’ port). In order to optimize a DPSK receiver in terms of signal sensitivity, both outputs from the DI are detected by a balanced detector structure.
- the signal is first evenly split so as to applied as “equal power” inputs into two separate DIs with different relative optical phase delays (+ ⁇ /4+n* ⁇ and ⁇ /4+m* ⁇ , where n and m are integers) and each DI pair of outputs is thereafter detected by a balanced detector structure.
- a balanced detector structure By properly choosing the relative phase delays, two bits of information per symbol can be separated and represented as one bit per balanced detector output.
- the amplitude modulated output from each balanced detector is thereafter sampled (for example, digital sampling) in order to visualize each bit's corresponding eye-diagram.
- balanced detection and electrical sampling suffers from two major limitations: (1) limited measurement bandwidth (currently ⁇ 50 GHz); and (2) significant impedance mismatch, resulting in distortion in the measured waveform.
- high speed signal characterization (10 GSymbols/s, 40 GSymbols/s or higher), these effects can influence the measurement results to such an extent that the measured waveform is dominated by the measurement system impulse response, which is unacceptable when needing to recover such high speed data signals.
- the present invention relates to a sampling arrangement particularly well-suited for analysis of high speed data signals and, more particularly, to a sampling arrangement comprising two or more coupled sampling gates for recovering information from modulated input signals.
- a sampling arrangement utilizes two (or more) separate sampling gates controlled by the same strobe frequency, f s , to acquire samples from two (or more) input signals.
- the lengths of the paths to the sampling gates are adjusted by tunable (or fixed) delay lines so as to enable precise, time-overlapped sampling of all input signals.
- suitable delay line arrangements include a “fixed” delay line, a “set-and-forget” delay line and a “tunable” delay line.
- the delay lines are used to ensure that the time delay from the output of each DI to the two corresponding sampling gates are equal to within a fraction of the temporal resolution of the sampling gates.
- every pair of samples originating from the two outputs of each DI originates from the same time “slice” of the waveform under test.
- the acquired pairs of samples are then, after detection and analog-to-digital (A/D) conversion, combined in software to samples representing balanced detection of the sample pairs.
- A/D analog-to-digital
- the two sampling gates are used for more than one input signal pair, such as in the case for a DQPSK signal where after demodulation by two DIs the two output signal pairs are measured in order to present the eye-diagram of each bit in the 2 bits/Symbol DQPSK signal.
- the DI output pairs can be measured by the sampling gates by switching in a predetermined fashion.
- Another embodiment of the present invention includes a sampling gate to sample an external reference clock, which can be used to establish the time base for the acquired samples from the signal under test.
- FIG. 1 illustrates the modulation principle of four different phase encoded modulation formats visualized in constellation diagrams containing information about both the amplitude and phase of the optical field;
- FIG. 2 illustrates a prior art arrangement for demodulating DPSK signals
- FIG. 3 shows an embodiment of the present invention for performing sampling in a demodulated DPSK signal, and also illustrates measured eye-diagrams of a demodulated 40 GSymbol/s DPSK signal using this embodiment of the present invention
- FIG. 4 shows the embodiment of the present invention from FIG. 3 with a typical signal DI demodulation setup in front of the four input ports of the present invention
- FIG. 5 illustrates a timing condition associated with the arrangement of FIG. 4 ;
- FIG. 6 illustrates an embodiment of the present invention using an external reference clock to synchronize the acquired samples.
- the DPSK signal is demodulated using a delay interferometer (DI) 10 having a relative delay difference between the two interferometer arms.
- DI 10 is shown as comprising a first signal path 12 and a second path 14 .
- An incoming modulated DPSK signal passes through a splitter 16 such that an even power level of signal is directed into paths 12 and 14 .
- Second path 14 includes a delay element 18 , represented as a fixed amount of delay (25 ps in this example) and a variable amount of delay (shown as ⁇ ).
- the phase shift is selected such that a delay of an integer number of bits (generally a single bit) is obtained.
- the original and the phase-shifted versions of the DPSK-encoded signal are thereafter recombined in a signal combiner 20 and split along two separate output paths 22 and 24 .
- the output signals along paths 22 and 24 comprise half of the power of the combined original/phase-shifted signals.
- the phase information in the DPSK signal is converted into two amplitude modulated signal, a first “constructive interference” signal with power P c along a first output path 22 , and a second “destructive interference” signal (exhibiting the complementary information) at power P d along a second output path 24
- the present invention utilizes a sampling technique to individually measure the waveform on each DI output, in a manner to be described in detail hereinbelow.
- a software-embedded algorithm is then used to combine the samples in a manner which emulates the operation of an ideal balanced detector, performing the operation P c ⁇ P d , to create a sampled output waveform as shown in eye diagram 34 of FIG. 3 .
- DPSK such as DQPSK
- similar reasoning applies but instead of having only two output signals after a single DI, there can be more DI's each having two output signals which can be taken care of by embodiments of the present invention to be described below.
- the sampling of the two DI output signals is performed in the optical domain, so as to completely remove the influence of the bandwidth limitations inherent in optical-electronic conversion and provide a final result which can be very close to the targeted ideal result P c ⁇ P d .
- the sampling technique of the present invention is not limited to the optical domain; electrical sampling techniques may be used in suitable applications (for example, lower speed applications).
- FIG. 3 shows an embodiment of the present invention which utilizes the same incoming DPSK-encoded signal and demodulating arrangement including DI 10 as described above in association with FIG. 2 .
- the embodiment of FIG. 3 may also be utilized if only one bit in a two-bits-per-symbol DQPSK signal is sampled.
- a sampling arrangement 40 formed in accordance with the present invention is used in place of prior art o/e conversion arrangements to more accurately recover the data from the phase-encoded incoming signal.
- the “constructive” signal propagating along first output path 22 is shown as applied as an input to first signal port Al of arrangement 40 .
- the “destructive” signal propagating along second output path 24 is applied as an input to second signal port A 2 .
- the technique of the present invention can be scaled to support a larger number of input ports, as will be discussed in detail below.
- the input signals can be either optical or electrical.
- the present invention is a combination of performing sampling of pairs of input signals in hardware and using software algorithms to combine the created samples into a single output corresponding to balanced detection of the input signal pairs.
- such a result is shown as the simultaneous sampling of A 1 and A 2 and subsequent reconstruction of a sampled version of the power of A 1 -A 2 .
- a pair of sampling gates 42 and 44 which are opened and closed by a common strobe source 46 representing a sampling frequency f s .
- the sampling can be performed either in the electrical domain or in the optical domain, depending on the domain of the signals arriving at ports A 1 and A 2 .
- optical sampling is the preferred embodiment in order to eliminate all high speed electronics and o/e conversion.
- Optical sampling gates 42 and 44 may comprise any one of a wide variety of implementations using different nonlinear optical processes to create the gating functionality.
- Exemplary suitable components include, but are not limited to, four-wave mixing in fiber, sum-frequency generation in optical crystals and cross-phase modulation in fiber or semiconductor optical amplifiers.
- strobe source 46 is illustrated as a single element, it is to be understood that separate strobe sources, having the same sampling frequency f s may also be used, with each separate strobe source used to control a separate gate.
- a key design parameter for the present invention is to facilitate alignment of the sampling times of gates 42 and 44 via strobe source 46 such that the two parts of the signal are synchronously sampled in order for combination in the software to be accurate.
- a delay line 48 is disposed at first input port Al and is used to adjust the distance (or time delay) from the input A 1 to sampling gate 42 , thereby adjusting the sampling time of gate 42 relative to the sampling time of gate 44 .
- FIG. 5 will describe an example which highlights the condition for adjusting delay line 48 .
- the operation of delay line 48 can be either adjustable or fixed, depending on the measurement application.
- the output samples from the sampling gates 42 and 44 are digitized by analog-to-digital converters (A/D) 50 and 52 , respectively, and subsequently fed into a software processing and signal visualization system 54 .
- A/D analog-to-digital converters
- the main functionality of system 54 related to the present invention is to combine the acquired sample pairs for each measurement in order to provide balanced detection functionality.
- the software can be used to visualize each measured input signal pair as the corresponding balanced detected signal.
- Eye diagram 28 of FIG. 3 is the sampled output associated with the constructive port
- eye diagram 30 is the sampled output associated with the destructive port
- eye diagram 34 is the resultant DPSK sampled information eye diagram, where each of these diagrams was created by system 54 .
- An alternative embodiment of the present invention allows for detection of the output samples from the sampling gates 42 and 44 using low bandwidth balanced receivers in order to perform the balanced detection in the hardware before digitizing the samples in an A/D converter.
- the present invention is independent of the particular method utilized to time stamp each sample.
- the technique of the present invention has been found to work for both real-time sampling and equivalent-time sampling, irrespective of the time-base design used for equivalent-time sampling.
- FIG. 4 illustrates an embodiment suitable for measurement of, for example, QPSK or DQPSK signals, which requires the generation of two sample pairs for proper demodulation.
- the incoming phase-encoded signal is split along signal paths 12 and 14 by a power splitter 16 .
- the portion of the signal propagating along signal path 14 is thereafter applied as an input to a first DI 10 - 1
- the remaining portion propagating along signal path 12 is applied as an input to a second DI 10 - 2 , as shown in FIG. 4 .
- Each DI 10 includes a separate delay element 18 , illustrated as delay element 18 - 1 (associated with DI 10 - 1 ) and delay element 18 - 2 (associated with DI 10 - 2 ).
- Delay elements 18 - 1 and 18 - 2 are shown as exhibiting the appropriate bit delays TS and phase relations ⁇ 1 and ⁇ 2 for demodulation of the input signal.
- a set of four output signals have been created, a first signal pair A 1 and A 2 from DI 10 - 1 (similar to the embodiment of FIG. 3 , as discussed above) and a second signal pair B 1 and B 2 , from DI 10 - 2 .
- switches 56 and 58 are positioned at the entrance ports of arrangement 40 , in front of sampling gates 42 and 44 , in order to facilitate alternating sampling of the input signal pairs from DI's 10 - 1 and 10 - 2 , that is, first A 1 , A 2 and then B 1 , B 2 .
- a second delay line 48 - 2 associated with input B 1 is included in the arrangement to provide the same synchronization activity as delay line 48 defined above and discussed in detail below.
- FIG. 4 also points out that o/e conversion of the signals can be performed before the sampling takes place.
- a photodiode or other o/e conversion element is disposed along each signal path, and is collectively illustrated as conversion component 70 in FIG. 4 .
- sampling gates 42 and 44 will comprise electronic sampling gates.
- the positioning of o/e conversion component 70 is flexible and can be either directly after DIs 10 - 1 and 10 - 2 , or at any other point in front of sampling gates 42 and 44 .
- delay lines 48 - 1 and 48 - 2 , as well as switches 56 and 58 can be either electrical or optical.
- FIG. 5 illustrates the critical timing required between sampling gates 42 and 44 in order to sample each part of the generated signal pairs at the correct matching times.
- FIG. 5 is an extracted part of the embodiment of FIG. 4 .
- the signal is not split up until point A, corresponding to the output of the DI 10 - 1 . From this point on, it is critical that the time difference from point A until each part of the signal is sampled very close to equal in order to generate samples originating from the same time in the original signal. This is a condition in order to be able to combine the two samples in the software and emulate the balanced detection.
- the timing condition can be expressed using the notations in FIG. 5 as
- delay line 48 - 1 plays a critical role to facilitate the fulfillment of this timing condition, in particular since the lengths of the conventional output fiber connectors of a delay interferometer is normally beyond the control of the constructor of the demodulating system present invention.
- delay line 48 may be omitted, in particular for low bandwidth sampling gate solutions with high ⁇ .
- the timing condition applies to all input signal pairs within the system of the present invention (e.g., in the arrangement of FIG. 4 , a similar condition applies from point B to sampling gates 42 and 44 ).
- FIG. 6 illustrates an embodiment of the present invention where an external reference signal source 60 is used to supply a gating control signal for the system, where the frequency f c of reference clock signal C is directly related to the frequency of the demodulated signals appearing at ports A 1 , A 2 , B 1 and B 2 .
- the reference clock output signal C from source 60 is sampled by a separate sampling gate 62 , using the same strobe source 46 .
- the generated clock samples are then digitized by an A/ID converter 64 and applied as an input to software processing system 54 .
- the timebase of the external clock can be determined by the embedded software algorithms. Since the frequency f c of the external clock is directly related to the frequency of the input signal bit rate, the time-base of the external clock can be directly transferred to the recovered output signal signal.
Abstract
Description
- The present invention relates to a sampling arrangement particularly well-suited for analysis of high speed data signals and, more particularly, to a sampling arrangement with two or more coupled sampling gates.
- Digital sampling is a technique used to visualize a time-varying waveform by capturing quasi-instantaneous snapshots of the waveform via, for example, a sampling gate. The gate is “opened” and “closed” by narrow pulses (strobes) in a pulse train that exhibit a well-defined repetitive behavior such that ultimately all parts of the waveform are sampled. The sampling implementation can either be real-time or equivalent-time, where real-time sampling refers to the case where the sampling rate is higher than twice the highest frequency content of the waveform under test (Nyquist sampling), while equivalent-time sampling uses an arbitrarily low sampling rate. However, equivalent-time sampling requires the measured waveform to be repetitive (in order to provide accurate signal reconstruction)—a fundamental limitation when compared to real-time sampling. The present invention is independent of the sampling rate, and hence, can be either real-time or equivalent-time sampling.
- The recent advances in the field of optical communication with new, more complex, data modulation formats as a key technology has created a need for optical waveform characterization tools which are capable of extracting more information from the waveform than simply its power as a function of time.
- In particular, many different modulation formats have been developed which use modulation of the phase of the optical carrier to encode the data to be transmitted. A few types of phase modulated signals have already been employed in commercial systems, such as differential phase shift keying (DPSK) and differential quaternary phase shift keying (DQPSK). For these differential modulation formats the data is encoded as the relative phase shift between consecutive symbols. In DPSK modulation schemes, for example, a π phase shift between bits represents a logical “1” and a zero phase shift represents a logical “0”. For DQPSK modulation, each symbol contains two bits of information by allowing four different relative phase changes between consecutive bits (e.g., 0, π/2, π and 3π/2).
-
FIG. 1 is used to further clarify the concept of phase-encoded modulation formats such as phase-shift keying (PSK), differential phase-shift keying (DPSK), and QPSK and DQPSK as defined above. For each type of modulation, the optical phase and amplitude of the data signal are visualized in constellation diagrams showing the optical field amplitude as the radial distance from origin R and the optical field phase as the angle φ. InFIG. 1 , the logical marks (ones) and spaces (zeros) are represented as either absolute phase and amplitude levels (for PSK and QPSK formats,FIGS. 1( a) and (b), respectively), or as phase and amplitude transitions for the differentially-coded phase and amplitude levels (for DPSK and DQPSK formats,FIGS. 1( c) and (d), respectively). For D/QPSK each symbol contains, as shown, two bits of information. Therefore, four different logical phase and amplitude combinations are used to represent the “symbols” in either of these modulation format types. - It is to be noted that the amplitude of the data signal is constant for each of these phase-encoded modulation techniques. Hence, if only the power of the incoming signal is “detected” using a conventional photodetector-based o/e conversion device, the phase information will be lost. To extract the phase information, the signal needs to be mixed with an optical reference signal which converts the phase information into amplitude information. For differentially-modulated signals, delay interferometers (DIs), such as Mach-Zehnder interferometers (MZIs), Michelson interferometers, or the like, are commonly used in which the signal itself serves as reference after being delayed one (or more) bit periods. For absolute phase encoded signals (e.g. PSK or QPSK), an independent reference signal is necessary to extract the phase information from each bit.
- The DI is an interferometric structure where the incoming optical waveform is equally split up in two paths and one path is delayed relative to the second path before recombining the two paths. The relative delay is coarsely set equal to an integer number of bit slots (most commonly one bit slot) and finely tuned to match a particular relative phase delay of the optical carrier. For example, in the DPSK case, the relative delay is a multiple of π in order to effectively translate the relative phase shifts between the symbols into a binary amplitude modulated signal. The DI has two output ports—a constructive interference port and a deconstructive interference port (the ‘destructive’ port outputting the complementary data of the ‘constructive’ port). In order to optimize a DPSK receiver in terms of signal sensitivity, both outputs from the DI are detected by a balanced detector structure.
- In order to recover the data embedded in an incoming DQPSK signal, the signal is first evenly split so as to applied as “equal power” inputs into two separate DIs with different relative optical phase delays (+π/4+n*π and −π/4+m*π, where n and m are integers) and each DI pair of outputs is thereafter detected by a balanced detector structure. By properly choosing the relative phase delays, two bits of information per symbol can be separated and represented as one bit per balanced detector output. The amplitude modulated output from each balanced detector is thereafter sampled (for example, digital sampling) in order to visualize each bit's corresponding eye-diagram.
- A major concern when using balanced detection for optical to electrical (o/e) conversion followed by electrical digital sampling is the influence of the measurement system on the measured waveform, which is known to introduce measurement error. In particular, balanced detection and electrical sampling suffers from two major limitations: (1) limited measurement bandwidth (currently <50 GHz); and (2) significant impedance mismatch, resulting in distortion in the measured waveform. For high speed signal characterization (10 GSymbols/s, 40 GSymbols/s or higher), these effects can influence the measurement results to such an extent that the measured waveform is dominated by the measurement system impulse response, which is unacceptable when needing to recover such high speed data signals.
- Thus, a need remains in the art for an arrangement capable of characterizing (visualizing) high symbol rate optical signals without being hampered by the measurement system bandwidth or the distortion due to o/e conversion and related impedance matching issues.
- The needs remaining in the prior art are addressed by the present invention, which relates to a sampling arrangement particularly well-suited for analysis of high speed data signals and, more particularly, to a sampling arrangement comprising two or more coupled sampling gates for recovering information from modulated input signals.
- In accordance with the present invention, a sampling arrangement utilizes two (or more) separate sampling gates controlled by the same strobe frequency, fs, to acquire samples from two (or more) input signals. The lengths of the paths to the sampling gates are adjusted by tunable (or fixed) delay lines so as to enable precise, time-overlapped sampling of all input signals. Examples of suitable delay line arrangements include a “fixed” delay line, a “set-and-forget” delay line and a “tunable” delay line.
- In particular, for the application of measuring the output signal pairs of one or more delay interferometers (DIs), as in the case of DPSK and DQPSK signals, the delay lines are used to ensure that the time delay from the output of each DI to the two corresponding sampling gates are equal to within a fraction of the temporal resolution of the sampling gates. Hence, every pair of samples originating from the two outputs of each DI originates from the same time “slice” of the waveform under test. The acquired pairs of samples are then, after detection and analog-to-digital (A/D) conversion, combined in software to samples representing balanced detection of the sample pairs. With this scheme, the need to perform balanced detection in hardware is avoided. In particular, when using optical sampling gates, the sampling gate bandwidth (temporal resolution) can be extremely high and impedance mismatch is no longer an issue, since the sampling takes place in the optical domain.
- In one embodiment of the present invention, the two sampling gates are used for more than one input signal pair, such as in the case for a DQPSK signal where after demodulation by two DIs the two output signal pairs are measured in order to present the eye-diagram of each bit in the 2 bits/Symbol DQPSK signal. By including, for example, optical switches before the two sampling gates, the DI output pairs can be measured by the sampling gates by switching in a predetermined fashion.
- Another embodiment of the present invention includes a sampling gate to sample an external reference clock, which can be used to establish the time base for the acquired samples from the signal under test.
- Other and further aspects and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
- Referring now to the drawings,
-
FIG. 1 illustrates the modulation principle of four different phase encoded modulation formats visualized in constellation diagrams containing information about both the amplitude and phase of the optical field; -
FIG. 2 illustrates a prior art arrangement for demodulating DPSK signals; -
FIG. 3 shows an embodiment of the present invention for performing sampling in a demodulated DPSK signal, and also illustrates measured eye-diagrams of a demodulated 40 GSymbol/s DPSK signal using this embodiment of the present invention; -
FIG. 4 shows the embodiment of the present invention fromFIG. 3 with a typical signal DI demodulation setup in front of the four input ports of the present invention; -
FIG. 5 illustrates a timing condition associated with the arrangement ofFIG. 4 ; and -
FIG. 6 illustrates an embodiment of the present invention using an external reference clock to synchronize the acquired samples. - Prior to describing the details of the exemplary sampling arrangement of the present invention, a prior art arrangement for demodulating DPSK-encoded signals will be reviewed with reference to
FIG. 2 . The DPSK signal is demodulated using a delay interferometer (DI) 10 having a relative delay difference between the two interferometer arms.DI 10 is shown as comprising afirst signal path 12 and asecond path 14. An incoming modulated DPSK signal passes through asplitter 16 such that an even power level of signal is directed intopaths Second path 14 includes adelay element 18, represented as a fixed amount of delay (25 ps in this example) and a variable amount of delay (shown as Δφ). The phase shift is selected such that a delay of an integer number of bits (generally a single bit) is obtained. The original and the phase-shifted versions of the DPSK-encoded signal are thereafter recombined in asignal combiner 20 and split along twoseparate output paths splitter 16, the output signals alongpaths - At the two outputs from
DI 10, the phase information in the DPSK signal is converted into two amplitude modulated signal, a first “constructive interference” signal with power Pc along afirst output path 22, and a second “destructive interference” signal (exhibiting the complementary information) at power Pd along asecond output path 24 - With traditional techniques, these two output signals would be applied as inputs to a balanced opto-electronic detector, which would subtract the one signal from the other and convert the difference into the electronic domain, ideally providing an electrical signal representing Pc−Pd. In the prior art arrangement of
FIG. 2 , a pair of photodiodes 21 and 23 are used to provide this opto-electronic conversion. However, such o/e conversion techniques are limited in bandwidth and quality of impulse response. As a result, the electrical signal created after detection does not represent the ideal case, in particular for high speed signals. - In contrast, the present invention utilizes a sampling technique to individually measure the waveform on each DI output, in a manner to be described in detail hereinbelow. A software-embedded algorithm is then used to combine the samples in a manner which emulates the operation of an ideal balanced detector, performing the operation Pc−Pd, to create a sampled output waveform as shown in eye diagram 34 of
FIG. 3 . For input signals other than DPSK (such as DQPSK), similar reasoning applies but instead of having only two output signals after a single DI, there can be more DI's each having two output signals which can be taken care of by embodiments of the present invention to be described below. - In a preferred embodiment of the present invention, the sampling of the two DI output signals is performed in the optical domain, so as to completely remove the influence of the bandwidth limitations inherent in optical-electronic conversion and provide a final result which can be very close to the targeted ideal result Pc−Pd. However, the sampling technique of the present invention is not limited to the optical domain; electrical sampling techniques may be used in suitable applications (for example, lower speed applications).
-
FIG. 3 shows an embodiment of the present invention which utilizes the same incoming DPSK-encoded signal and demodulatingarrangement including DI 10 as described above in association withFIG. 2 . The embodiment ofFIG. 3 may also be utilized if only one bit in a two-bits-per-symbol DQPSK signal is sampled. As will be described in detail below, asampling arrangement 40 formed in accordance with the present invention is used in place of prior art o/e conversion arrangements to more accurately recover the data from the phase-encoded incoming signal. The “constructive” signal propagating alongfirst output path 22 is shown as applied as an input to first signal port Al ofarrangement 40. Similarly, the “destructive” signal propagating alongsecond output path 24 is applied as an input to second signal port A2. - It is to be understood that the technique of the present invention can be scaled to support a larger number of input ports, as will be discussed in detail below. Moreover, the input signals can be either optical or electrical. In its most general form, the present invention is a combination of performing sampling of pairs of input signals in hardware and using software algorithms to combine the created samples into a single output corresponding to balanced detection of the input signal pairs.
- Referring back to the particular embodiment of
FIG. 3 , such a result is shown as the simultaneous sampling of A1 and A2 and subsequent reconstruction of a sampled version of the power of A1-A2. At the core of the present invention is a pair ofsampling gates common strobe source 46 representing a sampling frequency fs. The sampling can be performed either in the electrical domain or in the optical domain, depending on the domain of the signals arriving at ports A1 and A2. However, to fully take advantage of the capability of the present invention, optical sampling is the preferred embodiment in order to eliminate all high speed electronics and o/e conversion. By digitally sampling the output waveform from the balanced detector structure, the corresponding electrical eye-diagram, showing logical binary amplitude levels corresponding to the phase transitions in the DPSK signal, can be visualized. -
Optical sampling gates strobe source 46 is illustrated as a single element, it is to be understood that separate strobe sources, having the same sampling frequency fs may also be used, with each separate strobe source used to control a separate gate. - A key design parameter for the present invention is to facilitate alignment of the sampling times of
gates strobe source 46 such that the two parts of the signal are synchronously sampled in order for combination in the software to be accurate. Adelay line 48 is disposed at first input port Al and is used to adjust the distance (or time delay) from the input A1 to samplinggate 42, thereby adjusting the sampling time ofgate 42 relative to the sampling time ofgate 44.FIG. 5 will describe an example which highlights the condition for adjustingdelay line 48. In general, the operation ofdelay line 48 can be either adjustable or fixed, depending on the measurement application. - The output samples from the
sampling gates signal visualization system 54. The main functionality ofsystem 54 related to the present invention is to combine the acquired sample pairs for each measurement in order to provide balanced detection functionality. Furthermore, the software can be used to visualize each measured input signal pair as the corresponding balanced detected signal. Eye diagram 28 ofFIG. 3 is the sampled output associated with the constructive port, eye diagram 30 is the sampled output associated with the destructive port and, most importantly, eye diagram 34 is the resultant DPSK sampled information eye diagram, where each of these diagrams was created bysystem 54. - An alternative embodiment of the present invention allows for detection of the output samples from the
sampling gates - It is to be understood that the present invention is independent of the particular method utilized to time stamp each sample. In particular, the technique of the present invention has been found to work for both real-time sampling and equivalent-time sampling, irrespective of the time-base design used for equivalent-time sampling.
-
FIG. 4 illustrates an embodiment suitable for measurement of, for example, QPSK or DQPSK signals, which requires the generation of two sample pairs for proper demodulation. As before, the incoming phase-encoded signal is split alongsignal paths power splitter 16. In this case, however, the portion of the signal propagating alongsignal path 14 is thereafter applied as an input to a first DI 10-1, and the remaining portion propagating alongsignal path 12 is applied as an input to a second DI 10-2, as shown inFIG. 4 . EachDI 10 includes aseparate delay element 18, illustrated as delay element 18-1 (associated with DI 10-1) and delay element 18-2 (associated with DI 10-2). Delay elements 18-1 and 18-2 are shown as exhibiting the appropriate bit delays TS and phase relations Δφ1 and Δφ2 for demodulation of the input signal. In particular, for the case of DQPSK the phase relations can be, for example, Δφ1=+π/4 and Δφ2=−π/4, in order to separate each bit in the 2 bits/Symbol DQPSK data signal. By flipping theswitches - In this case, a set of four output signals have been created, a first signal pair A1 and A2 from DI 10-1 (similar to the embodiment of
FIG. 3 , as discussed above) and a second signal pair B1 and B2, from DI 10-2. In order to most efficiently utilize the elements ofsampling arrangement 40, switches 56 and 58 are positioned at the entrance ports ofarrangement 40, in front ofsampling gates delay line 48 defined above and discussed in detail below. -
FIG. 4 also points out that o/e conversion of the signals can be performed before the sampling takes place. In this case, a photodiode or other o/e conversion element is disposed along each signal path, and is collectively illustrated asconversion component 70 inFIG. 4 . In this case, samplinggates e conversion component 70 is flexible and can be either directly after DIs 10-1 and 10-2, or at any other point in front ofsampling gates switches -
FIG. 5 illustrates the critical timing required betweensampling gates FIG. 5 is an extracted part of the embodiment ofFIG. 4 . InFIG. 5 , the signal is not split up until point A, corresponding to the output of the DI 10-1. From this point on, it is critical that the time difference from point A until each part of the signal is sampled very close to equal in order to generate samples originating from the same time in the original signal. This is a condition in order to be able to combine the two samples in the software and emulate the balanced detection. - The timing condition can be expressed using the notations in
FIG. 5 as |(Tc−tc)−(Td−td)|<Δτ, where Tc is the propagation time for the “constructive” signal from point A tosampling gate 42, tc is the propagation time for the sampling strobe pulse fromstrobe source 46 to samplinggate 42, Td is the propagation time for the “destructive” signal from point A tosampling gate 44, td is the propagation time for the sampling strobe pulse fromstrobe source 46 to samplinggate 44, and Δτ represents the temporal resolution ofsampling gates sampling arrangement 40,delay line 48 may be omitted, in particular for low bandwidth sampling gate solutions with high Δτ. The timing condition applies to all input signal pairs within the system of the present invention (e.g., in the arrangement ofFIG. 4 , a similar condition applies from point B to samplinggates 42 and 44). - It has been pointed out that the present invention is independent on the time-base design used to synchronize the acquired samples into a replica of the original signal. However, it should be noted that the present invention is compatible with U.S. Pat. No. 7,327,302, issued to M. Westlund et al. on Feb. 5, 2008, assigned to the assignee of this application and hereby incorporated by reference.
FIG. 6 illustrates an embodiment of the present invention where an externalreference signal source 60 is used to supply a gating control signal for the system, where the frequency fc of reference clock signal C is directly related to the frequency of the demodulated signals appearing at ports A1, A2, B1 and B2. As shown, the reference clock output signal C fromsource 60 is sampled by aseparate sampling gate 62, using thesame strobe source 46. The generated clock samples are then digitized by an A/ID converter 64 and applied as an input tosoftware processing system 54. With this input information, the timebase of the external clock can be determined by the embedded software algorithms. Since the frequency fc of the external clock is directly related to the frequency of the input signal bit rate, the time-base of the external clock can be directly transferred to the recovered output signal signal. - It is to be understood that other advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the claims appended hereto.
Claims (17)
Priority Applications (5)
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US12/284,195 US20100067607A1 (en) | 2008-09-18 | 2008-09-18 | All-optical balanced detection system |
CN200980135355.9A CN102150384B (en) | 2008-09-18 | 2009-09-17 | All-optical balanced detection system |
US13/119,220 US9325428B2 (en) | 2008-09-18 | 2009-09-17 | Sampling-based balanced detection system |
PCT/US2009/057236 WO2010033654A2 (en) | 2008-09-18 | 2009-09-17 | All-optical balanced detection system |
EP09815161.6A EP2351263B1 (en) | 2008-09-18 | 2009-09-17 | Sampling-based balanced detection system |
Applications Claiming Priority (1)
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US12/284,195 US20100067607A1 (en) | 2008-09-18 | 2008-09-18 | All-optical balanced detection system |
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US13/119,220 Continuation US9325428B2 (en) | 2008-09-18 | 2009-09-17 | Sampling-based balanced detection system |
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Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20120134667A1 (en) * | 2009-02-23 | 2012-05-31 | Exfo Inc. | All-Optical, Phase Sensitive Optical Signal Sampling |
US10749600B2 (en) * | 2018-04-12 | 2020-08-18 | The Boeing Company | Systems and methods for single optical fiber data transmission |
Families Citing this family (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US8478132B1 (en) * | 2010-06-21 | 2013-07-02 | Rockwell Collins, Inc. | Systems and methods for reducing mechanical sensitivity of phase sensitive optical signals |
EP2709295A1 (en) * | 2012-09-14 | 2014-03-19 | Alcatel Lucent | Visualisation of an optical signal by linear optical sampling |
US9772353B2 (en) * | 2013-06-04 | 2017-09-26 | Tektronix, Inc. | Equivalent-time sampling technique for non-coherently modulated signals |
CN107171736B (en) * | 2017-05-18 | 2019-08-02 | 杭州电子科技大学 | Full light samples device |
CN109039462B (en) * | 2018-07-19 | 2020-04-03 | 中国科学院西安光学精密机械研究所 | Multi-modulation format compatible high-speed laser signal phase-lock-free receiving system and method |
CN111917485A (en) * | 2020-08-10 | 2020-11-10 | 武汉普赛斯电子技术有限公司 | Intensity modulation optical signal eye pattern measuring device and method based on linear light sampling |
Citations (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4000417A (en) * | 1975-08-25 | 1976-12-28 | Honeywell Inc. | Scanning microscope system with automatic cell find and autofocus |
US5481542A (en) * | 1993-11-10 | 1996-01-02 | Scientific-Atlanta, Inc. | Interactive information services control system |
US5631553A (en) * | 1993-05-31 | 1997-05-20 | Universite Du Quebec A Trois-Rivieres | High precision RF vector analysis system based on synchronous sampling |
US6564160B2 (en) * | 2001-06-22 | 2003-05-13 | Agilent Technologies, Inc. | Random sampling with phase measurement |
US20050185255A1 (en) * | 2004-02-19 | 2005-08-25 | Doerr Christopher R. | Linear optical sampling method and apparatus |
US20070188363A1 (en) * | 2006-02-10 | 2007-08-16 | Picosolve, Inc. | Equivalent time asynchronous sampling arrangement |
Family Cites Families (15)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4755742A (en) | 1986-04-30 | 1988-07-05 | Tektronix, Inc. | Dual channel time domain reflectometer |
US4751468A (en) | 1986-05-01 | 1988-06-14 | Tektronix, Inc. | Tracking sample and hold phase detector |
GB2320635A (en) * | 1996-12-19 | 1998-06-24 | Northern Telecom Ltd | Optical timing detection using an interferometer |
EP0854379B1 (en) * | 1996-12-19 | 2010-11-03 | Nortel Networks Limited | Interferometer for all-optical timing recovery |
JP3422913B2 (en) * | 1997-09-19 | 2003-07-07 | アンリツ株式会社 | Optical sampling waveform measuring device |
US6396606B1 (en) * | 1998-12-24 | 2002-05-28 | Worldcom, Inc. | Method of avoiding excessive polarization mode dispersion in an optical communications link |
US6822743B2 (en) * | 2001-03-07 | 2004-11-23 | Paul Trinh | Integrated-optic channel monitoring |
GB2385144B (en) * | 2002-01-23 | 2006-02-08 | Marconi Optical Components Ltd | Optical signal demodulators |
DE102005041368A1 (en) * | 2005-08-31 | 2007-03-01 | Siemens Ag | Demodulation method for optical differential phase shift keying (DPSK) binary signals, involves demodulating phase-modulated and amplitude-modulated optical DPSK binary signal and evaluating phase and amplitude data of demodulated signal |
WO2008080636A1 (en) * | 2007-01-04 | 2008-07-10 | Telefonaktiebolaget Lm Ericsson (Publ) | Improvements in or relating to the sampling of optical signals |
JP4883813B2 (en) | 2007-01-15 | 2012-02-22 | アンリツ株式会社 | Optical signal monitoring apparatus and method |
US20080240736A1 (en) * | 2007-03-28 | 2008-10-02 | Nec Laboratories America, Inc. | Inter-Symbol Interference-Suppressed Colorless DPSK Demodulation |
WO2009068324A1 (en) | 2007-11-28 | 2009-06-04 | Telefonaktiebolaget Lm Ericsson (Publ) | Optical sampling |
US8055141B2 (en) * | 2007-12-17 | 2011-11-08 | Alcatel Lucent | Balanced optical signal processor |
US8027588B2 (en) * | 2008-02-05 | 2011-09-27 | University Of Central Florida Research Foundation, Inc. | Systems and methods for optical carrier recovery |
-
2008
- 2008-09-18 US US12/284,195 patent/US20100067607A1/en not_active Abandoned
-
2009
- 2009-09-17 US US13/119,220 patent/US9325428B2/en not_active Expired - Fee Related
- 2009-09-17 CN CN200980135355.9A patent/CN102150384B/en not_active Expired - Fee Related
- 2009-09-17 EP EP09815161.6A patent/EP2351263B1/en not_active Not-in-force
- 2009-09-17 WO PCT/US2009/057236 patent/WO2010033654A2/en active Application Filing
Patent Citations (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4000417A (en) * | 1975-08-25 | 1976-12-28 | Honeywell Inc. | Scanning microscope system with automatic cell find and autofocus |
US5631553A (en) * | 1993-05-31 | 1997-05-20 | Universite Du Quebec A Trois-Rivieres | High precision RF vector analysis system based on synchronous sampling |
US5481542A (en) * | 1993-11-10 | 1996-01-02 | Scientific-Atlanta, Inc. | Interactive information services control system |
US6564160B2 (en) * | 2001-06-22 | 2003-05-13 | Agilent Technologies, Inc. | Random sampling with phase measurement |
US6756775B2 (en) * | 2001-06-22 | 2004-06-29 | Agilent Technologies, Inc. | Quasi-periodic optical sampling |
US20050185255A1 (en) * | 2004-02-19 | 2005-08-25 | Doerr Christopher R. | Linear optical sampling method and apparatus |
US20070188363A1 (en) * | 2006-02-10 | 2007-08-16 | Picosolve, Inc. | Equivalent time asynchronous sampling arrangement |
Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20120134667A1 (en) * | 2009-02-23 | 2012-05-31 | Exfo Inc. | All-Optical, Phase Sensitive Optical Signal Sampling |
US8768180B2 (en) * | 2009-02-23 | 2014-07-01 | Exfo, Inc. | All-optical, phase sensitive optical signal sampling |
US10749600B2 (en) * | 2018-04-12 | 2020-08-18 | The Boeing Company | Systems and methods for single optical fiber data transmission |
Also Published As
Publication number | Publication date |
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CN102150384B (en) | 2014-08-20 |
US20110182573A1 (en) | 2011-07-28 |
WO2010033654A2 (en) | 2010-03-25 |
US9325428B2 (en) | 2016-04-26 |
WO2010033654A3 (en) | 2010-06-03 |
EP2351263A4 (en) | 2014-12-10 |
EP2351263B1 (en) | 2016-04-06 |
EP2351263A2 (en) | 2011-08-03 |
CN102150384A (en) | 2011-08-10 |
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