WO2018163152A1 - A device and a method for generating optical chirp signals for use in a coherent detection system - Google Patents

A device and a method for generating optical chirp signals for use in a coherent detection system Download PDF

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Publication number
WO2018163152A1
WO2018163152A1 PCT/IL2018/050202 IL2018050202W WO2018163152A1 WO 2018163152 A1 WO2018163152 A1 WO 2018163152A1 IL 2018050202 W IL2018050202 W IL 2018050202W WO 2018163152 A1 WO2018163152 A1 WO 2018163152A1
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WO
WIPO (PCT)
Prior art keywords
coherent electromagnetic
electromagnetic signal
signal
frequency
change
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Application number
PCT/IL2018/050202
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French (fr)
Inventor
Dan Alon
Itamar GURMAN
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Oryx Vision Ltd.
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Publication date
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Publication of WO2018163152A1 publication Critical patent/WO2018163152A1/en

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Classifications

    • 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
    • H04B10/50—Transmitters
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
    • G01S17/06—Systems determining position data of a target
    • G01S17/08—Systems determining position data of a target for measuring distance only
    • G01S17/32—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
    • G01S17/34—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/491—Details of non-pulse systems
    • G01S7/4911—Transmitters

Definitions

  • the present disclosure generally relates to the field of telecommunication systems. More particularly, the present disclosure relates to systems implementing coherent detection.
  • coherent detection in telecommunication systems is considered to yield a performance with an improved Signal-To- Noise ratios (SNRs), when compared to other detection methods. Specifically, in cases where modulation is phase or frequency, coherent detection may improve Signal to Noise ratio dramatically.
  • SNRs Signal-To- Noise ratios
  • Chirp is a commonly known signal modulation technique in which the frequency increases (up-chirp) or decreases (down- chirp) with time.
  • the use of coherent detection in conjunction with the use of chirp modulation is widely adapted in radar systems.
  • the basics of this technique are known in the art ever since the early 1950s, and in particularly, many publications were concerned with strategies of ensuring that mode hopping is prevented .
  • US 2960914 describes an electro-optical light shutter adapted to transmit light within a predetermined portion of the spectrum for determinable intervals of time in response to low- amplitude electrical signals
  • US3166673 discloses devices for modulating an output beam of radiation produced by stimulated emission.
  • US3243722 relates to controlling the frequency of masers (including lasers), as in radar applications where extremely stable frequency is required or communication applications where frequency modulation is desired.
  • US 3354407 describes a method and a mechanism for modulating a laser beam.
  • US 3435372 relates to method for modulating or grating lasers with focused acoustic waves.
  • US 3464027 relates to gating or modulation the output of high gain lasers by propagating a time-varying refractive index perturbation having a wave length approximately equal to the beam width of the laser beam internally within the laser medium.
  • US 3492063 discloses a system for providing deflection of a light beam, operating in conjunction with strategically positioned optical lenses and a beam inverter, redirects an incident light beam through substantially the same volume electro-optical material a plurality of times so as to provide cumulative deflection.
  • US 3506334 describes devices through which time-coherent light is passed and which act to select or vary certain angular relationships between light beams entering and leaving the device .
  • US 4733253 describes an apparatus and method for detecting and eliminating laser diode mode hoping.
  • US 4737798 discloses a method and apparatus for sensing and controlling laser diode mode hopping.
  • US 5896193 describes a method and apparatus of generating a test signal for broadcast to a plurality of locations for testing optical devices is disclosed, by which a tunable laser sweeps from a lower wavelength to a higher upper wavelength providing a test station with a variable input signal.
  • US 20140105239 discloses a method for reducing mode hopping in a laser cavity.
  • CN 103326238 discloses a method for suppressing automatic mode hopping of a tunable laser.
  • EP 2451033 discloses a continuous mod-hop-free Grating- Tuned External Cavity Laser (GTECL) .
  • FIG. 1 demonstrates a typical coherent RADAR block diagram of a prior art system.
  • Generating frequency sweeps in light sources may be implementing by following any one of different methods.
  • Several prior art examples include:
  • Tx optical transmitted
  • Rx reflected
  • LO local oscillator
  • the chirp modulation itself is required to be a wideband modulation in order to facilitate acceptable detection resolution. For example, if a linear FM modulation is utilized, a longer frequency sweep in each pulse length will result in a better detection resolution.
  • this contradiction is typically considered as an instability in the light source.
  • abrupt frequency hops may occur. Such frequency hops are caused by excitation of different resonance modes ⁇ lines in the illuminator. Given the fact that each mode ⁇ line may have a different response to the modulation control signal, the resulting frequency sweep will exhibit a non-uniform modulation.
  • Frequency hops have a decremental effect on the coherent detection system. These hops are a fundamental property of the illumination source.
  • the present invention seeks therefore to provide a solution to utilize a technique that coexists with the frequency hops by maintaining the desired frequency gradient across multiple modes.
  • high Q illuminators e.g. high Q lasers
  • an optical transmitter configured to transmit coherent electromagnetic signals being chirp optical signals, each having a plurality of frequency changes that occur over time, wherein the optical transmitter comprising:
  • an illumination resonant source configured to generate at least one coherent electromagnetic signal
  • a processor (or any other applicable decision making device) configured to generate a modified modulation control signal upon occurrence of a frequency change at the at least one coherent electromagnetic signal, wherein the modified modulation control signal is adjusted to enable modulating the at least one coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the at least one coherent electromagnetic signal frequency over time (df/dt), as that value was prior to the occurrence of the frequency change at the at least one coherent electromagnetic signal;
  • a modulator configured to modulate the at least one coherent electromagnetic signal
  • a transmitter configured to transmit the at least one coherent electromagnetic signal in its modulated form.
  • chirp as used herein throughout the description and claims is used to denote a signal having a frequency that changes with time, for example any modulated signal utilizing frequency sweeps, irrespective of the type of sweep that is used.
  • the type of sweep may be a linear frequency sweep, a non ⁇ linear frequency sweep, a chirp-up, a chirp-down, etc.
  • the processor is further configured to determine characteristics of the illumination resonant source mode used for generating the at least one coherent electromagnetic signal.
  • the optical transmitter further comprising a feedback loop that is configured to continuously monitor a rate of the frequency change at the at least one coherent electromagnetic signal and to adjust the modified modulation control signal to enable modulating said at least one coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the at least one coherent electromagnetic signal frequency over time (df/dt), as that value was prior to the occurrence of the frequency change at the at least one coherent electromagnetic signal.
  • the optical transmitter further comprising an arrangement configured to provide a closed loop feedback that is fast enough to correct the rate of chirping without the need to first identify the event.
  • the feedback loop arrangement continuously monitors the rate of chirping and is adapted to correct for mode hopping events without detecting their occurrence.
  • the optical transmitter further comprising a detector configured to identify a frequency change at the at least one coherent electromagnetic signal generated (occurrence of a hopping event, being a mode or a line hopping event) and to initiate an indication upon occurrence of such a change.
  • the processor is further configured to receive from the detector that indication and to generate the modified modulation control signal in response to receipt of the indication.
  • the modified modulation control signal generated upon receiving from the detector said indication is used for adjusting a rate of change of the frequency of the at least one electromagnetic signal.
  • the processor is further configured to generate the modified modulation control signal by adjusting properties of the modulation control signal to match characteristics of the illumination resonant source mode currently in use by the illumination resonant source.
  • the modified modulation control signal is a member selected from among a group that consists of: an analog voltage provided to a piezo electric linear actuator, a current pulse width modulation control signal, a digital control signal provided to a step motor driver, a periodic signal delivered to an acousto-optic modulator, and a signal derived from changes occurring due to using substances that vary their dielectric properties and/or their physical length as a function of their respective temperature .
  • the modulator is configured to modulate the at least one coherent electromagnetic signal following the occurrence of the frequency change, in accordance with the modified modulation control signal generated by the processor.
  • the detector is configured to identify a frequency change at the at least one coherent electromagnetic signal by implementing at least one of the following options:
  • the illumination resonant source is a stabilized illumination source configured to sweep through a plurality of modes/lines in a predetermined schedule, thereby all frequency changes at the at least one electromagnetic signal, occur at pre-determined points of time.
  • a method for transmitting a coherent electromagnetic signal being a chirp optical signal having a plurality of frequency changes that occur over time, the method comprising: providing a coherent electromagnetic signal;
  • a modified modulation control signal that is adjusted in accordance with one or more changes associated with the frequency change, wherein the adjustment is adapted to enable modulating the coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the electromagnetic signal frequency over time (df/dt), as that value was prior to detecting the change in the frequency of the coherent electromagnetic signal has occurred; continue modulating the coherent electromagnetic signal in accordance with the adjusted modulation control signal; and
  • the method further comprising a step of determining characteristics of the illumination resonant source mode used for generating the coherent electromagnetic signal.
  • the method further comprising a step of monitoring the modulated coherent electromagnetic signal in order to detect if there is a frequency change (mode hopping) at the modulated coherent electromagnetic signal being monitored.
  • the modified modulation control signal is used for adjusting a rate of change of the frequency of the coherent electromagnetic signal.
  • the modified modulation control signal is generated by adjusting properties of the modified modulation control signal to match characteristics of the illumination resonant source mode that is currently in use for generating the coherent electromagnetic signal.
  • detecting a frequency change at the coherent electromagnetic signal is carried out by implementing at least one of the following options :
  • the coherent electromagnetic signal is generated by a stabilized illumination source configured to sweep through a plurality of modes/lines in a predetermined schedule, thereby all frequency changes at the electromagnetic signal, occur at pre-determined points of time.
  • the method further comprising a step of continuously monitoring a rate of the frequency change at the at least one coherent electromagnetic signal and adjusting the modified modulation control signal to enable modulating said at least one coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the at least one coherent electromagnetic signal frequency over time (df/dt), as that value was prior to the occurrence of the frequency change at the at least one coherent electromagnetic signal.
  • FIG . 1 demonstrates a typical coherent RADAR block diagram of a prior art system
  • FIG . 2 exemplifies three linear chirp signals
  • FIG . 3 illustrates an equivalent schematic representation in a form of an electric circuit of a half wave rectifier functionality
  • FIG . 4 illustrates a time domain signal obtained from a chirped signal generated by a control signal
  • FIG . 5 exemplifies a corrected time domain signal derived from a chirped signal generated by another control signal
  • FIG . 6 demonstrates a comparison between the FFT of the time domain signal shown in FIG. 4, and the FFT of the corrected time domain signal shown in FIG. 5 DESCRIPTION OF EXEMPLARY EMBODIMENTS
  • information may be encoded by applying different chirp modulation techniques while using the derivative df/dt (1 st or higher orders) of the chirp signal rather than by applying the absolute value of the frequency itself. For this reason, it becomes possible to maintain the same frequency derivative while operating at different modes.
  • the proposed solution provided by the present invention allows the illuminator to hop modes while still maintaining the desired frequency derivative, for example while using linear frequency sweep, non-linear sweep, chirp up, chirp down, etc.
  • FIG. 2 exemplifies three linear chirp signals.
  • Signals A and C are signals that are known in prior art systems.
  • Signal A is an 'ideal' linear chirp signal operating in a single mode.
  • Signal A is characterized by a single gradient and a single interception point.
  • Signal C on the other hand is a typical liner chirp signal suffering from mode hopping.
  • each of the three modes of signal C is characterized by having a different frequency gradient (df/dt) and a different interception point.
  • Signal B exhibits the same frequency hops as signal C, however, the modulation signal is manipulated to ensure that a constant gradient (df/dt) is maintained along the entire signal.
  • both signals A and B are useful in the sense that they will return identical results following signal processing.
  • Signal C on the other hand, will result in corrupted results.
  • Prior art solutions focus on techniques that aim at eliminating mode hopping. In other words, they provide methods that ensure that only signals of type A above are generated.
  • the proposed solution of the present invention allows coherent detection systems to operate while applying a plurality of modes/lines within a single modulated signal.
  • Chirp modulation is typically generated using some form of a control signal.
  • the control signal value influences the modulated signal frequency thereby creating the desired frequency sweep.
  • the control signal may be for example an analog voltage provided to a piezo electric linear actuator.
  • PWM pulse width modulation
  • TEC thermoelectric cooler
  • TEC thermoelectric cooler
  • a digital control signal provided to a step motor driver.
  • a periodic signal delivered to an acousto-optic modulator is typically generated using some form of a control signal.
  • control signal properties need to match the specific characteristics of the illumination resonant source mode that is in use. If this mode is constant (i.e. there is no mode hopping) a single control signal may be used. However, once mode hopping occurs, the control signal properties need to be adjusted to maintain the same value for the parameter df/dt.
  • Identification of the occurrence of a mode hopping event may be carried out by implementing any one of a variety of methods. For example, one or more of the following methods may be implemented:
  • Monitoring signal power - mode hopping is detected upon measuring an abrupt change in power.
  • Monitoring signal frequency - mode hopping is detected upon measuring an abrupt change in frequency.
  • Monitoring illumination source driver - mode hopping is detected upon measuring an abrupt change in the illuminator accepted power.
  • Monitoring detected signal - mode hopping is detected upon measuring corrupted values in the detected signal known to be the result of mode hopping.
  • the modulation control signal is adjusted so as to maintain the desired constant value for the parameter df/dt.
  • a simple lookup table strategy can be adopted in order to match the control signal properties to the respective modulation mode.
  • An alternative approach may be adopted by developing an analytic or empiric formula by which the control signal's properties are derived from results of a set of measurements that were carried out.
  • FIG. 3 illustrates an embodiment of a method for carrying out the solution provided by the present invention.
  • the method exemplified by the embodiment illustrated in FIG. 3 begins by modulating signal utilizing a frequency sweep (step 300) by a chirp generator.
  • the signal is monitored (step 310) in order to detect if there is a mode hop in the signal being monitored (step 320) .
  • a processor then adjusts the properties of the control signal in order to accommodate the new mode (step 340) and returns the adjusted properties to the chirp generator.
  • FIG. 4 illustrates a time domain signal obtained from a chirped signal generated by the control signal 410.
  • mode hoping events 420
  • these hoping events divide the signal into four distinct portions, namely, 430, 440, 450 and 460.
  • each of these four portions has its own unique amplitude, frequency and starting phase.
  • the signal portion designated 450 is the desired mode of operation.
  • FIG. 5 exemplifies a corrected time domain signal (500) derived from a chirped signal generated by control signal 510, having two non-linearity points designated by 520 and 530, which correspond to mode hopping events 540 and 550.
  • the resulting time domain signal thus obtained (500) may be characterized as a signal having a substantially uniform frequency throughout the entire signal, whereas phase and amplitude jumps are not corrected in this example.
  • curve 600 illustrates the FFT of the time domain signal 400 shown in FIG. 4
  • curve 610 illustrates the FFT of the corrected time domain signal 500 shown in FIG. 5.
  • the correction algorithm applied was sufficient to substantially improve the spectral cleanliness of the signal.
  • step 350 the signal is monitored and a check is carried out to determine whether the end of the signal has been reached. If the end of the signal is reached the current modulation is terminated (step 360) . Otherwise, the modulation of the signal by utilizing a frequency sweep by the chirp generator is continued (step 300) .
  • the scope of the present invention also applies for well stabilized illumination sources that sweep through a plurality of modes/lines in a predetermined fashion.
  • the mode hoping events can be made predictable, without the need to monitor the signal, by implementing any prior art technique that is known per se for stabilizing the illumination source.
  • the modulation signal may already include the adjustments required to compensate for the predicted mode hopping events.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Electromagnetism (AREA)
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Abstract

An optical transmitter is provided which is configured to transmit coherent electromagnetic signals being chirp optical signals, having a plurality of frequency changes that occur over time. The optical transmitter comprises: an illumination resonant source configured to generate a coherent electromagnetic signal; a processor configured to generate a modified modulation control signal upon occurrence of a frequency change at the coherent electromagnetic signal, wherein the modified modulation control signal is adjusted to enable modulating the coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the coherent electromagnetic signal frequency over time (df/dt), as that value was prior to the occurrence of the frequency change; a modulator configured to modulate the coherent electromagnetic signal; and a transmitter configured to transmit the coherent electromagnetic signal in its modulated form, towards a coherent detector.

Description

A DEVICE AND A METHOD FOR GENERATING OPTICAL CHIRP SIGNALS FOR USE IN
A COHERENT DETECTION SYSTEM
TECHNICAL FIELD
The present disclosure generally relates to the field of telecommunication systems. More particularly, the present disclosure relates to systems implementing coherent detection.
BACKGROUND
The use of coherent detection in telecommunication systems is considered to yield a performance with an improved Signal-To- Noise ratios (SNRs), when compared to other detection methods. Specifically, in cases where modulation is phase or frequency, coherent detection may improve Signal to Noise ratio dramatically.
Chirp is a commonly known signal modulation technique in which the frequency increases (up-chirp) or decreases (down- chirp) with time. The use of coherent detection in conjunction with the use of chirp modulation is widely adapted in radar systems. The basics of this technique are known in the art ever since the early 1950s, and in particularly, many publications were concerned with strategies of ensuring that mode hopping is prevented .
Following is a partial list of publications that relate to this subject:
US 2960914 describes an electro-optical light shutter adapted to transmit light within a predetermined portion of the spectrum for determinable intervals of time in response to low- amplitude electrical signals
US3166673 discloses devices for modulating an output beam of radiation produced by stimulated emission. US3243722 relates to controlling the frequency of masers (including lasers), as in radar applications where extremely stable frequency is required or communication applications where frequency modulation is desired.
US 3354407 describes a method and a mechanism for modulating a laser beam.
US 3435372 relates to method for modulating or grating lasers with focused acoustic waves.
US 3464027 relates to gating or modulation the output of high gain lasers by propagating a time-varying refractive index perturbation having a wave length approximately equal to the beam width of the laser beam internally within the laser medium.
US 3492063 discloses a system for providing deflection of a light beam, operating in conjunction with strategically positioned optical lenses and a beam inverter, redirects an incident light beam through substantially the same volume electro-optical material a plurality of times so as to provide cumulative deflection.
US 3506334 describes devices through which time-coherent light is passed and which act to select or vary certain angular relationships between light beams entering and leaving the device .
US 4733253 describes an apparatus and method for detecting and eliminating laser diode mode hoping.
US 4737798 discloses a method and apparatus for sensing and controlling laser diode mode hopping.
US 4937833 describes analog frequency modulation of an optical laser.
US 5896193 describes a method and apparatus of generating a test signal for broadcast to a plurality of locations for testing optical devices is disclosed, by which a tunable laser sweeps from a lower wavelength to a higher upper wavelength providing a test station with a variable input signal.
US 20140105239 discloses a method for reducing mode hopping in a laser cavity.
CN 103326238 discloses a method for suppressing automatic mode hopping of a tunable laser.
EP 2451033 discloses a continuous mod-hop-free Grating- Tuned External Cavity Laser (GTECL) .
The use of chirped coherent detection can be extended from Radio Frequencies (RF) to optical frequencies by utilizing light sources, such as lasers, to generate the chirped signal. FIG. 1 demonstrates a typical coherent RADAR block diagram of a prior art system.
Generating frequency sweeps in light sources, such as lasers, may be implementing by following any one of different methods. Several prior art examples include:
1. Changing the physical length by means of electromechanical devices such as actuators (including piezoelectric), motors , etc.;
2. Changing the electric length by using substances that vary their dielectric properties (index of diffraction) as a function of the applied field (typically electric) ;
3. Changing the electric length by use of substances that vary their dielectric properties (index of diffraction) and/or their physical length as a function of their respective temperature .
To create a useful optical transmitted (Tx) signal, a long coherent distance is required in order to ensure that the reflected (Rx) signal remains coherent (in phase) with the local oscillator (LO) signal.
In addition, the chirp modulation itself is required to be a wideband modulation in order to facilitate acceptable detection resolution. For example, if a linear FM modulation is utilized, a longer frequency sweep in each pulse length will result in a better detection resolution.
Unfortunately, as any person skilled in the art would appreciate, these two requirements (i.e. a long coherent distance and a wideband modulation) are contradictory requirements. A long coherent distance is equivalent to high Q light sources, whereas wideband frequency sweeps are equivalent to low Q structures.
In practical terms, this contradiction is typically considered as an instability in the light source. As the frequency sweep increases beyond the range defined by the Q factor of the illuminator, abrupt frequency hops may occur. Such frequency hops are caused by excitation of different resonance modes\lines in the illuminator. Given the fact that each mode\line may have a different response to the modulation control signal, the resulting frequency sweep will exhibit a non-uniform modulation.
Frequency hops have a decremental effect on the coherent detection system. These hops are a fundamental property of the illumination source. The present invention seeks therefore to provide a solution to utilize a technique that coexists with the frequency hops by maintaining the desired frequency gradient across multiple modes.
SUMMARY
The disclosure may be summarized by referring to the appended claims .
It is an object of the present disclosure to provide a novel method for obtaining long frequency sweeps in high Q illuminators (e.g. high Q lasers) . It is another object of the disclosure to enable coherent detection systems to operate while applying a plurality of modes/lines within a single modulated signal.
Other objects of the present disclosure will become apparent from the following description.
According to a first embodiment of the present disclosure, there is provided an optical transmitter configured to transmit coherent electromagnetic signals being chirp optical signals, each having a plurality of frequency changes that occur over time, wherein the optical transmitter comprising:
an illumination resonant source configured to generate at least one coherent electromagnetic signal;
a processor (or any other applicable decision making device) configured to generate a modified modulation control signal upon occurrence of a frequency change at the at least one coherent electromagnetic signal, wherein the modified modulation control signal is adjusted to enable modulating the at least one coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the at least one coherent electromagnetic signal frequency over time (df/dt), as that value was prior to the occurrence of the frequency change at the at least one coherent electromagnetic signal;
a modulator configured to modulate the at least one coherent electromagnetic signal; and
a transmitter configured to transmit the at least one coherent electromagnetic signal in its modulated form.
The term "chirp" as used herein throughout the description and claims is used to denote a signal having a frequency that changes with time, for example any modulated signal utilizing frequency sweeps, irrespective of the type of sweep that is used. The type of sweep may be a linear frequency sweep, a non¬ linear frequency sweep, a chirp-up, a chirp-down, etc. According to another embodiment, the processor is further configured to determine characteristics of the illumination resonant source mode used for generating the at least one coherent electromagnetic signal.
By still another embodiment, the optical transmitter further comprising a feedback loop that is configured to continuously monitor a rate of the frequency change at the at least one coherent electromagnetic signal and to adjust the modified modulation control signal to enable modulating said at least one coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the at least one coherent electromagnetic signal frequency over time (df/dt), as that value was prior to the occurrence of the frequency change at the at least one coherent electromagnetic signal. Thus, according to this embodiment, the optical transmitter further comprising an arrangement configured to provide a closed loop feedback that is fast enough to correct the rate of chirping without the need to first identify the event. In other words, the feedback loop arrangement continuously monitors the rate of chirping and is adapted to correct for mode hopping events without detecting their occurrence.
In accordance with another embodiment, the optical transmitter further comprising a detector configured to identify a frequency change at the at least one coherent electromagnetic signal generated (occurrence of a hopping event, being a mode or a line hopping event) and to initiate an indication upon occurrence of such a change.
By yet another embodiment, the processor is further configured to receive from the detector that indication and to generate the modified modulation control signal in response to receipt of the indication.
According to still another embodiment, the modified modulation control signal generated upon receiving from the detector said indication, is used for adjusting a rate of change of the frequency of the at least one electromagnetic signal.
In accordance with another embodiment, the processor is further configured to generate the modified modulation control signal by adjusting properties of the modulation control signal to match characteristics of the illumination resonant source mode currently in use by the illumination resonant source.
According to a further embodiment, the modified modulation control signal is a member selected from among a group that consists of: an analog voltage provided to a piezo electric linear actuator, a current pulse width modulation control signal, a digital control signal provided to a step motor driver, a periodic signal delivered to an acousto-optic modulator, and a signal derived from changes occurring due to using substances that vary their dielectric properties and/or their physical length as a function of their respective temperature .
In accordance with yet another embodiment, the modulator is configured to modulate the at least one coherent electromagnetic signal following the occurrence of the frequency change, in accordance with the modified modulation control signal generated by the processor.
By still another embodiment, the detector is configured to identify a frequency change at the at least one coherent electromagnetic signal by implementing at least one of the following options:
a. monitoring the at least one coherent electromagnetic signal power, and detecting a mode hopping upon measuring an abrupt change in the at least one coherent electromagnetic signal power;
b. monitoring the at least one coherent electromagnetic signal phase, and detecting a mode hopping by detecting changes in the at least one coherent electromagnetic signal phase and/or a derivative thereof;
c. monitoring a driver of the illumination resonant source, and detecting a mode hopping upon measuring an abrupt change in a driver performance such as the illuminator accepted power; and
d. monitoring the at least one coherent electromagnetic beam properties, and detecting a mode hopping upon measuring an abrupt change in at least one coherent electromagnetic beam property such as a special mode.
According to another embodiment, the illumination resonant source is a stabilized illumination source configured to sweep through a plurality of modes/lines in a predetermined schedule, thereby all frequency changes at the at least one electromagnetic signal, occur at pre-determined points of time.
According to another aspect of the present invention there is provided a method for transmitting a coherent electromagnetic signal being a chirp optical signal having a plurality of frequency changes that occur over time, the method comprising: providing a coherent electromagnetic signal;
modulating the coherent electromagnetic signal while utilizing a frequency sweep;
upon occurrence of a frequency change in the coherent electromagnetic signal, generating a modified modulation control signal that is adjusted in accordance with one or more changes associated with the frequency change, wherein the adjustment is adapted to enable modulating the coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the electromagnetic signal frequency over time (df/dt), as that value was prior to detecting the change in the frequency of the coherent electromagnetic signal has occurred; continue modulating the coherent electromagnetic signal in accordance with the adjusted modulation control signal; and
transmitting the coherent electromagnetic signal.
According to another embodiment of this aspect of the invention, the method further comprising a step of determining characteristics of the illumination resonant source mode used for generating the coherent electromagnetic signal.
In accordance with another embodiment, the method further comprising a step of monitoring the modulated coherent electromagnetic signal in order to detect if there is a frequency change (mode hopping) at the modulated coherent electromagnetic signal being monitored.
By yet another embodiment of this aspect, the modified modulation control signal is used for adjusting a rate of change of the frequency of the coherent electromagnetic signal.
According to still another embodiment of this aspect of the invention, the modified modulation control signal is generated by adjusting properties of the modified modulation control signal to match characteristics of the illumination resonant source mode that is currently in use for generating the coherent electromagnetic signal.
In accordance with a further embodiment, detecting a frequency change at the coherent electromagnetic signal is carried out by implementing at least one of the following options :
a. monitoring the coherent electromagnetic signal power, and detecting a mode hopping upon measuring an abrupt change in the coherent electromagnetic signal power;
b. monitoring the coherent electromagnetic signal phase, and detecting a mode hopping by detecting changes in the at least one coherent electromagnetic signal phase and/or a derivative thereof; c. monitoring a driver of the illumination resonant source, and detecting a mode hopping upon measuring an abrupt change in the driver's performance such as the illuminator accepted power; and
d. monitoring the at least one coherent electromagnetic beam properties, and detecting a mode hopping upon measuring an abrupt change in at least one coherent electromagnetic beam property such as special mode .
According to yet another embodiment, the coherent electromagnetic signal is generated by a stabilized illumination source configured to sweep through a plurality of modes/lines in a predetermined schedule, thereby all frequency changes at the electromagnetic signal, occur at pre-determined points of time.
By still another embodiment, the method further comprising a step of continuously monitoring a rate of the frequency change at the at least one coherent electromagnetic signal and adjusting the modified modulation control signal to enable modulating said at least one coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the at least one coherent electromagnetic signal frequency over time (df/dt), as that value was prior to the occurrence of the frequency change at the at least one coherent electromagnetic signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate several embodiments of the disclosure and, together with the description, serve to explain the principles of the embodiments disclosed herein. FIG . 1 demonstrates a typical coherent RADAR block diagram of a prior art system;
FIG . 2 exemplifies three linear chirp signals;
FIG . 3 illustrates an equivalent schematic representation in a form of an electric circuit of a half wave rectifier functionality;
FIG . 4 illustrates a time domain signal obtained from a chirped signal generated by a control signal;
FIG . 5 exemplifies a corrected time domain signal derived from a chirped signal generated by another control signal; and
FIG . 6 demonstrates a comparison between the FFT of the time domain signal shown in FIG. 4, and the FFT of the corrected time domain signal shown in FIG. 5 DESCRIPTION OF EXEMPLARY EMBODIMENTS
Some of the specific details and values in the following detailed description refer to certain examples of the disclosure. However, this description is provided only by way of example and is not intended to limit the scope of the invention in any way. As will be appreciated by those skilled in the art, the claimed method and device may be implemented by using other methods that are known in the art per se. In addition, the described embodiments comprise different steps, not all of which are required in all embodiments of the invention. The scope of the invention can be summarized by referring to the appended claims .
According to the solution provided by the present disclosure, information may be encoded by applying different chirp modulation techniques while using the derivative df/dt (1st or higher orders) of the chirp signal rather than by applying the absolute value of the frequency itself. For this reason, it becomes possible to maintain the same frequency derivative while operating at different modes. In other words, the proposed solution provided by the present invention allows the illuminator to hop modes while still maintaining the desired frequency derivative, for example while using linear frequency sweep, non-linear sweep, chirp up, chirp down, etc.
FIG. 2 exemplifies three linear chirp signals. Signals A and C are signals that are known in prior art systems. Signal A is an 'ideal' linear chirp signal operating in a single mode. Signal A is characterized by a single gradient and a single interception point. Signal C on the other hand is a typical liner chirp signal suffering from mode hopping. As may be seen from the FIG., each of the three modes of signal C is characterized by having a different frequency gradient (df/dt) and a different interception point.
The solution provided by the present invention, enables generation of signal B. Signal B exhibits the same frequency hops as signal C, however, the modulation signal is manipulated to ensure that a constant gradient (df/dt) is maintained along the entire signal. In coherent detection systems utilizing frequency sweep modulation techniques, both signals A and B are useful in the sense that they will return identical results following signal processing. Signal C, on the other hand, will result in corrupted results.
Prior art solutions focus on techniques that aim at eliminating mode hopping. In other words, they provide methods that ensure that only signals of type A above are generated. The proposed solution of the present invention allows coherent detection systems to operate while applying a plurality of modes/lines within a single modulated signal.
Chirp modulation is typically generated using some form of a control signal. The control signal value influences the modulated signal frequency thereby creating the desired frequency sweep. The control signal may be for example an analog voltage provided to a piezo electric linear actuator. Another example is a PWM (pulse width modulation) current control signal used to control a TEC (thermoelectric cooler) to heat or cool a medium. Yet another example may be a digital control signal provided to a step motor driver. Yet another example is a periodic signal delivered to an acousto-optic modulator.
By setting the value of the control signal, one may adjust the rate of change of the signal frequency. To obtain the desired waveform, the control signal properties need to match the specific characteristics of the illumination resonant source mode that is in use. If this mode is constant (i.e. there is no mode hopping) a single control signal may be used. However, once mode hopping occurs, the control signal properties need to be adjusted to maintain the same value for the parameter df/dt.
Identification of the occurrence of a mode hopping event may be carried out by implementing any one of a variety of methods. For example, one or more of the following methods may be implemented:
1. Monitoring signal power - mode hopping is detected upon measuring an abrupt change in power.
2. Monitoring signal frequency - mode hopping is detected upon measuring an abrupt change in frequency.
3. Monitoring illumination source driver - mode hopping is detected upon measuring an abrupt change in the illuminator accepted power.
4. Monitoring detected signal - mode hopping is detected upon measuring corrupted values in the detected signal known to be the result of mode hopping.
Once a mode hop is detected, the modulation control signal is adjusted so as to maintain the desired constant value for the parameter df/dt. Now, because each resonant cavity is characterized by a finite set of possible modes, a simple lookup table strategy can be adopted in order to match the control signal properties to the respective modulation mode.
An alternative approach may be adopted by developing an analytic or empiric formula by which the control signal's properties are derived from results of a set of measurements that were carried out.
FIG. 3 illustrates an embodiment of a method for carrying out the solution provided by the present invention.
The method exemplified by the embodiment illustrated in FIG. 3 begins by modulating signal utilizing a frequency sweep (step 300) by a chirp generator.
The signal is monitored (step 310) in order to detect if there is a mode hop in the signal being monitored (step 320) .
If in the affirmative, the new mode properties are identified (step 330), a processor then adjusts the properties of the control signal in order to accommodate the new mode (step 340) and returns the adjusted properties to the chirp generator. The following figures demonstrate the above described steps.
FIG. 4 illustrates a time domain signal obtained from a chirped signal generated by the control signal 410. In this FIG. there are three mode hoping events (420) as identified abrupt jumps in amplitude, phase and frequency. Consequently, these hoping events divide the signal into four distinct portions, namely, 430, 440, 450 and 460. As may be noted, each of these four portions has its own unique amplitude, frequency and starting phase. The signal portion designated 450 is the desired mode of operation.
FIG. 5 exemplifies a corrected time domain signal (500) derived from a chirped signal generated by control signal 510, having two non-linearity points designated by 520 and 530, which correspond to mode hopping events 540 and 550.
The resulting time domain signal thus obtained (500) may be characterized as a signal having a substantially uniform frequency throughout the entire signal, whereas phase and amplitude jumps are not corrected in this example.
In FIG. 6 curve 600 illustrates the FFT of the time domain signal 400 shown in FIG. 4, whereas curve 610 illustrates the FFT of the corrected time domain signal 500 shown in FIG. 5. As may be observed from comparing the two curves shown this FIG., the correction algorithm applied was sufficient to substantially improve the spectral cleanliness of the signal.
Reverting now to FIG. 3, if no mode hop is detected, the signal is monitored and a check is carried out to determine whether the end of the signal has been reached (step 350) . If the end of the signal is reached the current modulation is terminated (step 360) . Otherwise, the modulation of the signal by utilizing a frequency sweep by the chirp generator is continued (step 300) .
The scope of the present invention also applies for well stabilized illumination sources that sweep through a plurality of modes/lines in a predetermined fashion. In other words, the mode hoping events can be made predictable, without the need to monitor the signal, by implementing any prior art technique that is known per se for stabilizing the illumination source. As will be appreciated by those skilled in the art, in these cases, there is no need to monitor the laser signal nor to adjust the modulation signal with a feedback loop. Instead, the modulation signal may already include the adjustments required to compensate for the predicted mode hopping events.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

WHAT IS CLAIMED IS:
1. An optical transmitter configured to transmit coherent electromagnetic signals being chirp optical signals, each having a plurality of frequency changes that occur over time, wherein the optical transmitter comprising:
an illumination resonant source configured to generate at least one coherent electromagnetic signal;
a processor configured to generate a modified modulation control signal upon occurrence of a frequency change at the at least one coherent electromagnetic signal, wherein said modified modulation control signal is adjusted to enable modulating said at least one coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the at least one coherent electromagnetic signal frequency over time (df/dt), as that value was prior to the occurrence of the frequency change at the at least one coherent electromagnetic signal;
a modulator configured to modulate the at least one coherent electromagnetic signal; and
a transmitter configured to transmit the at least one coherent electromagnetic signal in its modulated form.
2. The optical transmitter of claim 1, wherein said processor is further configured to determine characteristics of the illumination resonant source mode used for generating said at least one coherent electromagnetic signal.
3. The optical transmitter of claim 1, further comprising a feedback loop that is configured to continuously monitor a rate of the frequency change at the at least one coherent electromagnetic signal and to adjust said modified modulation control signal to enable modulating said at least one coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the at least one coherent electromagnetic signal frequency over time (df/dt), as that value was prior to the occurrence of the frequency change at the at least one coherent electromagnetic signal.
4. The optical transmitter of claim 1, further comprising a detector configured to identify a hopping event at the at least one coherent electromagnetic signal generated and to initiate an indication upon occurrence of such a change.
5. The optical transmitter of claim 4, wherein the processor is further configured to receive from the detector said indication and to generate the modified modulation control signal in response to receipt of said indication.
6. The optical transmitter of claim 5, wherein the modified modulation control signal generated upon receiving from the detector said indication, is used for adjusting a rate of change of the frequency of the at least one electromagnetic signal.
7. The optical transmitter of claim 1, wherein the processor is further configured to generate said modified modulation control signal by adjusting properties of said modulation control signal to match characteristics of the illumination resonant source mode currently in use by the illumination resonant source.
8. The optical transmitter of claim 1, wherein said modified modulation control signal is a member selected from among a group that consists of: an analog voltage provided to a piezo electric linear actuator, a current pulse width modulation control signal, a digital control signal provided to a step motor driver, a periodic signal delivered to an acousto-optic modulator, and a signal derived from changes occurring due to using substances that vary their dielectric properties and/or their physical length as a function of their respective temperature .
9. The optical transmitter of claim 1, wherein said modulator is configured to modulate the at least one coherent electromagnetic signal following the occurrence of said frequency change, in accordance with the modified modulation control signal generated by the processor.
10. The optical transmitter of claim 4, wherein the detector is configured to identify a frequency change at the at least one coherent electromagnetic signal by implementing at least one of the following options:
a. monitoring said at least one coherent electromagnetic signal power, and detecting a mode hopping upon measuring an abrupt change in the at least one coherent electromagnetic signal power;
b. monitoring said at least one coherent electromagnetic signal phase, and detecting a mode hopping by detecting changes in the at least one coherent electromagnetic signal phase and/or a derivative thereof;
c. monitoring a driver of the illumination resonant source, and detecting a mode hopping upon measuring an abrupt change in said driver performance; and
d. monitoring the at least one coherent electromagnetic beam properties, and detecting a mode hopping upon measuring an abrupt change in at least one coherent electromagnetic beam property such as special mode .
11. The optical transmitter of claim 4, wherein the illumination resonant source is a stabilized illumination source configured to sweep through a plurality of modes/lines in a predetermined schedule, thereby all frequency changes at the at least one electromagnetic signal , occur at pre-determined points of time.
12. A method for transmitting a coherent electromagnetic signal being a chirp optical signal having a plurality of frequency changes that occur over time, said method comprising:
providing a coherent electromagnetic signal;
modulating said coherent electromagnetic signal while utilizing a frequency sweep;
upon occurrence of a frequency change in the coherent electromagnetic signal, generating a modified modulation control signal that is adjusted in accordance with one or more changes associated with said frequency change, wherein said adjustment is adapted to enable modulating said coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the electromagnetic signal frequency over time (df/dt), as that value was prior to detecting the change in the frequency of the coherent electromagnetic signal has occurred; continue modulating said coherent electromagnetic signal in accordance with the adjusted modulation control signal; and
transmitting the coherent electromagnetic signal.
13. The method of claim 12, further comprising a step of determining characteristics of the illumination resonant source mode used for generating said coherent electromagnetic signal.
14. The method of claim 12, further comprising a step of monitoring the modulated coherent electromagnetic at the modulated coherent electromagnetic signal being monitored.
15. The method of claim 12, wherein the modified modulation control signal is used for adjusting a rate of change of the frequency of the coherent electromagnetic signal.
16. The method of claim 12, wherein said modified modulation control signal is generated by adjusting properties of said modified modulation control signal to match characteristics of the illumination resonant source mode that is currently in use for generating said coherent electromagnetic signal.
17. The optical transmitter of claim 12, wherein detecting a frequency change at the coherent electromagnetic signal is carried out by implementing at least one of the following options :
a. monitoring said coherent electromagnetic signal power, and detecting a mode hopping upon measuring an abrupt change in the coherent electromagnetic signal power;
b. monitoring said coherent electromagnetic signal phase, and detecting a mode hopping by detecting changes in the at least one coherent electromagnetic signal phase and/or a derivative thereof; and
c. monitoring a driver of the illumination resonant source, and detecting a mode hopping upon measuring an abrupt change in said driver performance.
18. The method of claim 12, wherein said coherent electromagnetic signal is generated by a stabilized illumination source configured to sweep through a plurality of modes/lines in a predetermined schedule, thereby all frequency changes at the electromagnetic signal, occur at pre-determined points of time.
19. The method of claim 12, further comprising a step of continuously monitoring a rate of the frequency change at the at least one coherent electromagnetic signal and adjusting said modified modulation control signal to enable modulating said at least one coherent electromagnetic signal while maintaining a value of a parameter associated with a change of the at least one coherent electromagnetic signal frequency over time (df/dt), as that value was prior to the occurrence of the frequency change at the at least one coherent electromagnetic signal.
PCT/IL2018/050202 2017-03-08 2018-02-21 A device and a method for generating optical chirp signals for use in a coherent detection system WO2018163152A1 (en)

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