Apparatus and method for measuring chirps in fiber optic transmitters

Abstract

A method and apparatus for monitoring chirps in a modulated light beam. The modulated light is passed through a variable dispersion element or "VDE", and light from the variable dispersion element is detected to produce a signal. While controlling the VDE to vary the applied dispersion, a parameter representative of the intensity of the modulation in the light after passing through the VDE, such as the peak intensity of the modulated light, is measured. The chirp is determined as a function of a variation in the parameter with dispersion, such as as a function of dispersion at which the parameter is at a minimum value. The method may be applied to an optical beam modulated by a common digital transmitter used in an optical communication system.

Description

[0001] Optical communication systems are used to send information most typically in digital form. For example, optical communication systems are used to transmit data between computers in a data center for cloud computing. A typical optical communication system includes a transmitter, which includes an emitter and a modulator. The emitter and the modulator may include an emitter, such as a diode or a laser, that can be internally controlled to modulate the light output according to a drive signal applied to the emitter. Alternatively, the emitter and the modulator may include an emitter arranged to emit a constant light beam and a separate modulation device for changing one or more characteristics of the emitted light (such as the amplitude or phase of the light), such as an electro-absorption modulator or an electro-optic modulator. The transmitter also includes a conversion circuit for receiving data in electronic digital form and controlling the emitter and the modulator to emit light modulated according to the data.

[0002] One form of modulation commonly used in optical communication systems is called pulse amplitude modulation or "PAM." In pulse amplitude modulation, light is modulated to one of several discrete amplitudes during successive short intervals. The duration of each interval may be on the order of one-tenth or one-hundredth of a nanosecond. For example, in "4-PAM" modulation, four discrete amplitudes are employed; the amplitude during each interval represents two bits of digital information. The communication system also includes an optical fiber connected to receive light output from the transmitter's emitter and modulator. The receiver includes a photodetector connected to receive the light from the optical fiber and to generate an electrical signal representing the light. A signal processing circuit receives the electrical signal from the photodetector and converts the signal back into digital data represented by the signal. For example, where the transmitter uses PAM, the signal processing circuit may determine the amplitude of the electrical signal during each interval.

[0003] The optical fiber is typically a "single-mode" fiber adapted to conduct the light from the emitter and the modulator along a single path. In single-mode transmission, the time delay of transmission through the fiber is equal to the length of the fiber divided by the speed of light in the fiber. However, the speed of light in an optical fiber generally varies with the wavelength of the light. This phenomenon is called "optical dispersion". The modulation applied by the emitter inherently causes some variation in wavelength.

[0004] In addition, the transmitting element of the transmitter may introduce additional changes in wavelength over time. The change in wavelength over time is called "chirp". Chirp is usually characterized by the chirp coefficient alpha (α). If the wavelength gradually decreases over time, α has a positive sign. If the wavelength gradually increases over time, α has a negative sign. The magnitude of α indicates the rate of increase or decrease.

[0005] The interaction between chirp and optical dispersion can introduce distortion into the transmitted signal. For example, if the wavelength gradually changes during the transmission of a light pulse, optical dispersion will cause the light that makes up different parts of the pulse to propagate at different speeds. If the light emitted at the beginning of the pulse propagates faster than the light emitted at the end of the pulse, the pulse of light that reaches the receiver will have a longer duration than the pulse emitted by the transmitter. For example, in a PAM modulation scheme, the light emitted by the transmitter during one interval can arrive at the receiver at the same time as the light emitted by the transmitter during the next interval. This effectively reduces the distinction between different light levels emitted during various intervals in the PAM modulation scheme. As data transmission speeds increase, the adverse effects of distortion caused by chirp become more significant.

[0006] In one method of measuring chirp, a network analyzer is connected directly to an optical emitter so that the analyzer can apply an analog signal (such as a pure sinusoidal waveform modulating signal) directly to the emitter. The emitter is connected to a photodetector via an optical fiber with known optical dispersion. The photodetector is connected to the network analyzer so that the photodetector feeds an analog response signal back to the network analyzer. The network analyzer gradually varies or sweeps the frequency of the modulating signal applied to the emitter and records the amplitude of the response signal at each modulation frequency. Plotting the amplitude of the response signal versus the modulation frequency produces a pattern with distinct, sharp minima or "nulls" at specific modulation frequencies. The frequencies at which these nulls occur vary with the chirp of the emitter, so that the chirp can be calculated based on the frequencies at which the nulls occur.

[0007] There are several serious disadvantages to the frequency sweeping method. A typical transmitter used in an optical communication system includes a light emitter and a modulator packaged together with a conversion circuit (e.g., in a single chip or small module). It is very difficult or impossible to apply an analog electronic signal from a network analyzer to the light emitter. It is usually not possible to apply a modulated signal with a varying modulation frequency through the conversion circuit of the transmitter. The conversion circuit is usually arranged to operate at a single fixed clock rate or at several selectable clock rates.

[0008] In another chirp measurement method, the light output from the emitter is directed into an optical instrument called a Fabray-Perot resonator, allowing the chirp to be measured directly. This method can be used to measure the chirp when the emitter is actuated by the transmitter's conversion circuit. However, the cost, complexity, and lack of portability of the instrumentation required for this method make this method impractical for use in a data center environment. Summary of the invention

[0009] One aspect of the present disclosure provides a method for measuring chirp. The method according to this aspect includes actuating an emitter to emit light and modulating the emitted light, and also includes directing the modulated light through a variable dispersion element and measuring a parameter representing the intensity of the modulation of the light after passing through the variable dispersion element. For example, the parameter can be the peak intensity of the modulated light. The method includes varying the dispersion applied by the variable dispersion element. The method also includes determining one or more dispersions that produce one or more minimums, maximums, or minimums and maximums of the measured parameters, and determining the emitter chirp based on these dispersions. The light can be amplitude modulated at a fixed modulation frequency, and the modulation can include non-sinusoidal modulation, such as modulation to different amplitudes in a set of discrete amplitudes in a sequence, so that a series of discrete amplitudes are reproduced at the modulation frequency. As discussed further below, this type of modulation can be easily formed by actuating a conversion circuit of a commonly used emitter. Therefore, the chirp of an emitter incorporating a conversion circuit as well as an emitter and a modulator can be easily measured without accessing internal components within the emitter. Methods according to some examples may be applied to general testing, such as when testing transmitters used in data centers.

[0010] Another aspect of the present disclosure provides an apparatus for measuring chirp in modulated light. The apparatus according to this aspect includes: a variable dispersion element adapted to receive modulated light and apply variable dispersion to the modulated light; a photodetector connected to receive the modulated light and provide a signal representing the modulation; and a signal processing circuit that operates to measure a parameter representing the intensity of the modulation in the signal. The apparatus may also include: an emitter and a modulator connected to an input of the variable dispersion element; and a modulation signal source connected to the emitter and the modulator. The modulation signal source may be adapted to supply a set of discrete values ​​in a recurring sequence to the emitter and the modulator so that the modulated light includes a series of recurring discrete amplitudes. For example, the emitter and the modulator may be elements of the emitter. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a schematic diagram depicting an apparatus according to one aspect of the present disclosure.

[0012] Figure 2is a schematic diagram of an example modulator.

[0013] Figure 3 It is a depiction Figure 2 A graph of the chirp of the modulator shown in .

[0014] Figure 4 , Figure 5 , Figure 6 is a graph depicting the variation of the parameter with dispersion in a simulation of one measurement under different operating conditions.

[0015] Figure 7 is depicted using 0.9V pi Plot of the predicted results of chirp measurements in simulation for a large sinusoidal modulated signal.

[0016] Figure 8 is a graph depicting an example of a non-sinusoidal modulated signal.

[0017] Fig. 9 is depicted in use Figure 8 Plot of the predicted results of chirp measurement in the simulation of the modulated signal.

[0018] Fig.10 is a schematic diagram depicting an apparatus according to another aspect of the present disclosure.

[0019] Fig.11 is a flow chart illustrating an example method according to aspects of the present disclosure. DETAILED DESCRIPTION

[0020] Figure 1 An apparatus that can be used for a method of measuring chirp according to the present disclosure is shown. The apparatus includes a signal generator 114 connected to an emitter and a modulator 112, which is a device under test or "DUT". In this example, the emitter and modulator are arranged to modulate the amplitude of the emitted light according to a modulation signal from the signal generator, and the signal generator is arranged to emit a sinusoidal signal at a fixed frequency. The frequency is of the same order of magnitude as the frequency of the modulation signal that will be applied to the DUT in service. For example, where the DUT will operate at GHz frequencies, the modulation signal used for chirp measurement may also be in the GHz range. The light output by the DUT is connected to the optical input of a variable dispersion optical element ("VDE") 118. The VDE is adapted to apply a known dispersion value to the light passing through the VDE and to vary the known dispersion in response to user input.

[0021] The VDE is typically used as a tunable dispersion compensator and may include elements such as micro-electromechanical systems, Gires–Tournois etalons, fiber Bragg gratings, wavelength grating routers, virtual imaging phased arrays, and fiber arrays with various dispersions. A photodetector 122 is connected to the output of the VDE 118. The photodetector generates an analog electrical signal corresponding to the amplitude of the light from the VDE. A signal processor 124 measures a parameter, such as the peak intensity of the modulated light. The measured parameter represents the signal intensity of the modulation in the light reaching the photodetector 122.

[0022] In operation during a method according to one example of the present disclosure, the modulated signal is maintained at a constant frequency while VDE is adjusted to vary the dispersion. The measured signal strength at the modulation frequency is plotted versus the dispersion. The plot of the measured signal strength versus dispersion shows clear maxima and minima at specific dispersion values, as shown below: Figures 4 to 6 The chirp parameter alpha (α) can be calculated based on: the dispersion value where the minimum occurs; the value where the maximum occurs; or both. Most typically, α is calculated based on the value where the minimum occurs as follows:

[0023] The value of α can be derived using the following equation based on the known modulation signal frequency f, the minimum VDE dispersion D and its corresponding minimum position count u (0, 1, 2…), the optical wavelength λ, and the speed of light constant c in vacuum. An example formula for deriving the chirp parameter α can be as follows:

[0024]

[0025] Figure 2 An emitter and modulator 212 used as a device under test (DUT) to obtain results using this method is shown. The emitter and modulator use a continuous emitter 201 coupled to a secondary point based Mach-Zender modulator 203. The modulator 203 includes a beam splitter that splits an incident light beam from the emitter 201 into a first light beam 205 and a second light beam 207.

[0026] The beam splitter is controllable so as to vary the portion of the optical power of the incident beam that is directed into the first beam 205. The ratio of the power directed into the first beam to the power of the incident beam is referred to herein as the "splitting ratio". The first beam is transmitted along a first single-mode optical waveguide 209, which is formed of a material that exhibits a substantial electro-optic effect (i.e., a refractive index that varies with an applied voltage). A modulation signal is applied as a voltage to an electrode 211 near the first waveguide 209. Therefore, the refractive index and, therefore, the speed of light in the first waveguide varies with the modulation signal. The second beam 207 is transmitted through a second waveguide 213, which is identical to the first waveguide, but which is not affected by the modulation signal.

[0027] The two beams pass from the waveguide into the combiner 215 to form an outgoing beam 217. When the voltage on the electrode 211 is zero, the two beams 205 and 207 take the same time to reach the combiner 215 and are therefore in phase with each other and constructively reinforce each other so that the outgoing beam 217 has substantially the same power as the incident beam from the emitter 201. When the voltage on the electrodes increases, the first beam is delayed relative to the second beam so that the two beams are out of phase when they reach the combiner so that the beams begin to interfere destructively and the power of the output beam decreases. Therefore, the power of the output beam varies with the modulation signal. This modulation process introduces chirp into the output beam. The chirp coefficient α and other properties of the emitted beam, such as the variation of its intensity over time, are calculated using known methods. The calculated chirp coefficient varies with the splitting ratio of the modulator, such as Figure 3 shown.

[0028] In the simulation, the above reference Figure 1 The use of projectiles and the use of Figure 2 The modulator as a result of the measurement method of the DUT is predicted and compared with the calculated chirp coefficient. The first simulation uses a 0.1V applied to the modulator pi = A relatively small sinusoidal modulation signal is biased at the quadrature point. Under these conditions, the intensity of the output beam varies almost exactly sinusoidally. At a splitting ratio of 0.15, the simulation indicates that the peak intensity of the output beam, and therefore the peak intensity measured by the signal processing circuit 124, will be as follows: Figure 4 The predicted minimum values ​​of the peak intensity occur at dispersions of about +100 ps / nm and about -900 ps / nm, with the maximum being between these dispersion values. The chirp coefficient calculated from these values ​​is 2.76, exactly equal to the chirp coefficient calculated for a splitting ratio of 0.15. At a splitting ratio of 0.5, the predicted variation of the peak intensity with dispersion is Figure 5 , where the predicted minimums are at about -450 ps / nm and +450 ps / nm. These values ​​yield a chirp factor of 0, again exactly the same as Figure 3 At a splitting ratio of 0.85, the predicted minimum values ​​of the peak intensity occur at dispersions of about -100 ps / nm and +900 ps / nm, as shown in Figure 6 These values ​​correspond to a chirp factor of -2.76, again exactly equal to Figure 3 The calculated chirp coefficient of the modulator is shown.

[0029] A second simulation is performed for a larger sinusoidal modulation signal sufficient to vary the intensity of the output beam between about 1.0 times and about 0.15 times the full intensity. With this large modulation signal, the intensity of the output beam from the modulator varies as a slightly distorted sinusoid. With this slightly non-sinusoidal modulation, the pattern of variation of the peak intensity with respect to the dispersion (i.e., the predicted value of the measurement that will be obtained) is similar to that in the first simulation, but not exactly the same. Here again, the dispersion value associated with the minimum in the predicted peak intensity is used to determine the predicted value of the measured chirp coefficient. The results are shown in Figure 2. Figure 7 In this figure, the calculated chirp coefficient of the modulator at various splitting ratios is shown by the dashed line 225; this is consistent with Figure 3 The predicted value of the chirp coefficient obtained from the measured minimum in the peak intensity is indicated by the solid line curve 227. These results indicate that the non-sinusoidal modulation of the beam intensity will introduce some measurement errors. Despite these errors, the chirp coefficient obtained from the measured minimum in the peak intensity still provides a reasonably accurate value of the chirp coefficient.

[0030] The third simulation is to use 0.1V pi The obvious non-sinusoidal modulation signal is carried out, such as Figure 8 The signal consists of a repeating 4-PAM sequence of four discrete voltages. Figure 8 In the simulation, these voltages are represented by horizontal lines. There are brief transitions between successive intervals, as represented by sloping lines. In this simulation, the sequence is repeated with a fundamental modulation frequency of 13.25 GHz. The results are shown in Fig. 9 In this figure, the calculated chirp coefficient of the modulator at various splitting ratios is shown by the dashed line 229; this is consistent with Figure 3 The predicted value of the chirp coefficient that would be obtained from the measured minimum in the peak intensity is indicated by the solid line curve 231. Here again, the results indicate that the non-sinusoidal modulation of the beam intensity will introduce some measurement errors. Despite these errors, the chirp coefficient obtained from the measured minimum in the peak intensity will still provide a measure of the chirp coefficient that is useful for some purposes (e.g., as a quality assurance check to determine whether the emitter has a reasonable range of chirp values).

[0031] In the simulations discussed above, the peak intensity of the beam was used as a measure of the signal strength at the modulation frequency. Figure 7 and Fig. 9 The measurement errors in question are caused by spurious signal components at higher frequencies than the modulation frequency, which are incorporated into the modulated beam when the beam is modulated by a non-sinusoidal signal. These components can be suppressed by filtering the intensity signal from the photodetector or at the DUT using an analog or digital filter to remove components with frequencies higher than the modulation frequency before measuring the peak intensity.

[0032] As discussed above, the method discussed above can be used to measure chirp in a light beam modulated by a non-sinusoidal modulation signal having a constant modulation frequency. In a method according to another example, Fig.10 As schematically depicted in FIG. 1 , the conversion circuit is arranged to encode digital data into four signal levels (A) to (D), as described above in conjunction with Figure 8 For example, the conversion circuit may be arranged to encode the data as follows:

[0033] Digital data (binary) like Figure 8 Signal levels shown 00 A 01 B 10 C 11 D

[0034] The variable dispersion element 118, the photodetector 122 and the signal processing circuit are as described above with reference to Figure 1 The modulation signal is generated by repeatedly providing the digital data sequence 00, 10, 11, 01 to the conversion circuit of the transmitter so that during consecutive intervals, the modulation signal levels A, C, D, B are repeated during consecutive intervals. This enables the transmitter and modulator 122 to provide the above reference Figure 8 The modulated output light in question. The modulation frequency is one-fourth of the clock rate of the emitter. In other words, the period of the modulated signal is equal to four signal intervals. Here again, the dispersion imposed by the VDE 118 is varied, and a parameter, such as peak intensity, representing the intensity of the modulation at the modulation frequency in the light reaching the photodetector 122 is measured. Here again, the chirp of the emitter and modulator is determined from the pattern of variations of the parameter, such as the dispersion associated with a minimum in the parameter.

[0035] Because the method can thus measure chirp in a transmitter using a digital signal applied to a digital signal input of the transmitter, the method can be readily used to measure chirp without the difficult or impossible task of disassembling the transmitter to route an analog test signal to the emitter and modulator. Thus, the method greatly simplifies chirp measurements, particularly in environments such as data centers and quality assurance laboratories.

[0036] In a variation of this method, the transmitter may include a software or hardware feature that causes the transmitter to enter a chirp test mode in response to a command. In the test mode, the conversion circuit applies a repeating sequence of modulation signal levels discussed above without any digital data being input to the conversion circuit.

[0037] References Fig.10 The method discussed uses a 4-PAM transmitter to provide a repeating sequence of modulation levels. The number of modulation levels may vary. For example, an 8-PAM transmitter may use a repeating sequence of 8 modulation levels. In this case, the modulated signal more closely resembles a sinusoid, so that errors caused by stray high-frequency components in the modulated signal are reduced. In another variation, where the transmitter is arranged to enter a test mode, the conversion circuit may use more signal levels during the test mode than in normal operation. For example, a conversion circuit that normally uses 4 levels in a 4-PAM scheme may use eight levels during the test mode.

[0038] Fig.11 An example method of measuring chirp according to aspects of the present disclosure is shown. Although the operations are described in a particular order, it should be understood that the operations may be performed in a different order or simultaneously. In addition, operations may be added or omitted.

[0039] In block 1110, the emitter emits light according to a modulation signal generated by a signal generator, and the modulator modulates the emitted light. In block 1120, the modulated light passes through the VDE, and the signal processor measures a parameter representing the modulation intensity of the light after the light has passed through the VDE. In block 1130, the VDE is adapted to apply a known dispersion to the light passing through the VDE, and to vary the known dispersion in response to user input. In block 1140, by analyzing a plot of the measured signal strength versus dispersion, distinct minima and maxima may be determined at particular dispersion values. In block 1150, the emitter chirp may be determined based on the dispersion value corresponding to the minimum value, while the maximum value indicates other related values.

[0040] Receivers commonly used in digital communication circuits combine a photodetector and a signal processing circuit. In the case where the receiver's signal processing circuit is capable of capturing a parameter representative of the modulation intensity, such as a peak intensity value, the receiver can be used as a photodetector and a signal processing circuit during the test method. In this case, the method can be performed without any special test equipment other than a variable dispersion element.

[0041] In the method discussed above, only a single modulation frequency is employed. The ability to test without varying the modulation frequency greatly simplifies testing and enables testing of devices that operate at a fixed clock rate, such as digital transmitters. However, in another variation of the method, more than one modulation frequency may be used. For example, if the transmitter is operable at multiple clock rates, the chirp may be determined at each clock rate.

[0042] In the examples discussed above, the light was amplitude modulated. The methods of the present disclosure may be applied to the measurement of chirp in systems where the light is phase modulated or frequency modulated, or some combination of such modulations. In these cases, the chirp test mode may require configuring the DUT to be amplitude modulated, or adjusting the chirp calculation formula accordingly.

[0043] Because these and other variations and combinations of the above-described features may be used, the foregoing description should be considered illustrative rather than limiting of the scope of the disclosure.

Claims

1. A method for measuring chirp, characterized in that: include: actuating the emitter to emit light and modulating the emitted light; directing said modulated light through a variable dispersion element and measuring a parameter representative of the intensity of said modulation in said light after passing through said variable dispersion element; varying the dispersion applied by the variable dispersion element; as well as determining one or more dispersions that produce one or more minima, maxima, or minima and maxima of the measured parameter; as well as The chirp of the emitter is determined based on the dispersion.

2. The method according to claim 1, characterized in that Wherein modulating the emitted light comprises: performing amplitude modulation on the emitted light at a modulation frequency.

3. The method according to claim 2, characterized in that Wherein the parameter representing the intensity of the modulation is the peak intensity of the modulated light.

4. The method according to claim 3, characterized in that Wherein measuring the parameter representing the intensity of the modulation comprises: directing the light from the variable dispersion element to a photodetector such that the photodetector produces an intensity of the light after passing through the variable dispersion element; and measuring a peak value of the signal.

5. The method according to claim 4, characterized in that Also includes: The signal from the photodetector is filtered to remove components of the signal at frequencies above the modulation frequency.

6. The method according to claim 2, characterized in that The amplitude modulating the emitted light comprises: modulating the amplitude of the emitted light to different amplitudes in a set of discrete amplitudes in a sequence, so that a series of discrete amplitudes are reproduced at the modulation frequency.

7. The method according to claim 6, characterized in that Wherein the emitter is part of a transmitter, the transmitter includes conversion circuitry that operates to accept digital data and modulate the light from the emitter at discrete levels.

8. The method according to claim 7, characterized in that Wherein modulating the light from the emitter comprises actuating the conversion circuit to supply a modulation signal having a series of discrete values ​​during consecutive intervals and repeating the series of values ​​at the modulation frequency.

9. The method according to claim 8, characterized in that Wherein actuating the conversion circuit comprises: supplying digital data to the conversion circuit, the digital data comprising a series of repeated digital values.

10. The method according to claim 6, characterized in that The set of discrete amplitudes consists of four amplitudes.

11. A device for measuring chirp in modulated light, characterized in that include: a variable dispersion element adapted to receive the modulated light and to apply variable dispersion to the modulated light; a photodetector connected to receive the modulated light and to provide a signal representative of the modulation; as well as Signal processing circuitry operative to measure a parameter representative of the strength of the modulation in the signal.

12. The device according to claim 11, characterized in that Also included is an emitter and a modulator connected to the variable dispersion element.

13. The device according to claim 12, characterized in that Also included is a modulation signal source connected to the emitter and the modulator.

14. The device according to claim 13, characterized in that Wherein the modulation signal source operates to modulate light from the emitter and modulator to a series of discrete values ​​that reproduce at a modulation frequency.

15. The device according to claim 13, characterized in that Wherein the emitter and modulator and the modulation signal source are elements of a transmitter operative to receive a digital signal comprising a series of digital values ​​and to modulate the emitted light according to the digital values.

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