US20140334830A1 - System and Method for Generating A Frequency Modulated Linear Laser Waveform - Google Patents
System and Method for Generating A Frequency Modulated Linear Laser Waveform Download PDFInfo
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- US20140334830A1 US20140334830A1 US13/527,638 US201213527638A US2014334830A1 US 20140334830 A1 US20140334830 A1 US 20140334830A1 US 201213527638 A US201213527638 A US 201213527638A US 2014334830 A1 US2014334830 A1 US 2014334830A1
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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
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
- H04B10/548—Phase or frequency modulation
- H04B10/556—Digital modulation, e.g. differential phase shift keying [DPSK] or frequency shift keying [FSK]
- H04B10/5563—Digital frequency modulation
-
- 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
- H04B10/516—Details of coding or modulation
- H04B10/548—Phase or frequency modulation
-
- 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
-
- 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
-
- 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
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2575—Radio-over-fibre, e.g. radio frequency signal modulated onto an optical carrier
Definitions
- a frequency modulated laser waveform generating system is also provided.
- the frequency modulated laser waveform generating system includes a single frequency laser that produces a laser output signal.
- the laser output signal includes a single frequency having a narrow linewidth defining a sine wave centered at ⁇ o .
- An electro-optical modulator is coupled to the laser and receives the laser output signal from the laser.
- An electric ramp generator is coupled to the electro-optical modulator.
- the laser generating system 20 includes a single output frequency laser 36 that produces a laser output signal having a single frequency.
- the laser output signal includes a very pure spectrum with an ultra-narrow linewidth.
- the linewidth of the laser output signal is close to a perfect sine wave, and is centered at ⁇ o , shown at 60 in FIG. 3 .
- the laser output signal ⁇ o 60 is unmodulated.
- An electro-optical modulator 38 is coupled to the single output frequency laser 36 .
- the electro-optical modulator 38 receives the laser output signal from the single output frequency laser 36 , and modulates the frequency of the laser output signal.
- the electro-optical modulator 38 includes an electro-optical crystal that changes properties of the laser output signal when an electric field is applied.
Abstract
Description
- The invention described herein was made in part by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without payment of any royalties thereon or therefore.
- The invention generally relates to a system and a method of generating a frequency modulated linear laser waveform.
- Frequency modulation of a single frequency laser with a liner triangular waveform is of great importance to many laser remote sensing and interferometry applications. An application of a linear frequency modulation may include a laser radar (e.g. light detection and ranging (LIDAR)) system for target ranging and three-dimensional profiling of surfaces. Linear frequency modulation of a continuous wave laser beam may enable such a LIDAR system to make precise target distance measurements. However, the frequency modulation must be nearly perfectly linear with time to provide high resolution and accuracy.
- A system for generating a frequency modulated laser waveform is provided. The system includes a laser that is configured for producing a laser output signal. The laser output signal includes a single frequency and a linewidth centered at ωn. An electro-optical modulator is coupled to the laser. The electro-optical modulator receives the laser output signal from the laser. An electric ramp generator is coupled to the electro-optical modulator. The electric ramp generator generates a ramp having a frequency chirp with a bandwidth B over a time period T, and a modulation offset frequency ωm. The ramp controls the electro-optical modulator to modulate the laser output signal to define a modulated waveform. A band-pass optical filter is coupled to the electro-optical modulator. The band-pass optical filter receives the modulated waveform and filters out harmonic frequencies created by the electro-optical modulate during modulation of the laser output signal. The band-pass optical filter includes a bandwidth BW that is greater than the modulation bandwidth B and less than the modulation offset frequency ωm, and is centered at the frequency of the laser output signal ωo plus the modulation offset frequency ωm.
- A frequency modulated laser waveform generating system is also provided. The frequency modulated laser waveform generating system includes a single frequency laser that produces a laser output signal. The laser output signal includes a single frequency having a narrow linewidth defining a sine wave centered at ωo. An electro-optical modulator is coupled to the laser and receives the laser output signal from the laser. An electric ramp generator is coupled to the electro-optical modulator. The electric ramp generator generates a linear ramp having a frequency chirp with a bandwidth B between the range of 300 MHz and 3.0 GHz over a time period T between the range of 0.01 msec and 0.20 msec, a modulation offset frequency ωm between the range of 5.0 GHz and 30 GHz, and a deviation less than or equal to 71 Khz. The linear ramp controls the electro-optical modulator to modulate the laser output signal to define a modulated waveform. A band-pass optical filter is coupled to the electro-optical modulator. The band-pass optical filter receives the modulated waveform and filters out harmonic frequencies created by the electro-optical modulator during modulation of the laser output signal to define a pure filtered modulated waveform. The band-pass optical filter includes a bandwidth BW that is greater than the modulation bandwidth B and less than the modulation offset frequency ωm, and is centered at the frequency of the laser output signal ωo plus the modulation offset frequency ωm. An optical circulator interconnects the electric ramp generator and the band-pass optical filter. The optical circulator includes an input that receives the modulated waveform from the electric ramp generator, an output/input that sends and receives the modulated waveform to and from the band-pass optical filter, and an output leg that transmits the pure filtered modulated waveform.
- A method of generating a laser frequency modulated waveform is also provided. The method includes generating a laser output signal having an ultra narrow linewidth defining a sine wave. A linear ramp having a frequency chirp with a bandwidth B over a time period T, and a modulation offset frequency ωm is generated. The laser output signal from the single frequency laser is modulated with an electro-optical modulator driven by the linear ramp to define a modulated waveform. The modulated waveform from the electro-optical modulator is filtered with a band-pass optical filter to remove harmonic frequencies in the modulated waveform created by the electro-optical modulator during modulation of the laser output signal. The band-pass optical filter produces a pure filtered modulated waveform.
- Accordingly, the output from the frequency modulated laser waveform generating system is a pure linear triangular frequency modulated waveform that is suitable for use in remote laser sensing applications, such as a LIDAR system, as well as other applications that may require a highly linear frequency modulated laser beam.
-
FIG. 1 is a schematic diagram of a system for generating a frequency modulated laser waveform. -
FIG. 2 is a graph of a linear triangular frequency modulated waveform of a laser beam. -
FIG. 3 is graph showing bandwidth over which the linear triangular frequency modulated waveform is filtered and the harmonic frequencies filtered out of the linear triangular frequency modulated waveform. - Referring to the Figures, wherein like numerals indicate like parts throughout the several views, laser generating system is generally shown at 20 in
FIG. 1 . Referring toFIG. 2 , thelaser generating system 20 produces a frequency modulated linear laser waveform generally shown at 22. WithinFIG. 2 , time is shown along ahorizontal axis 24, and the frequency of the modulated linear waveform is shown along avertical axis 26. The modulated linear waveform produced by thelaser generating system 20 and shown inFIG. 2 is suitable for use in, but is not limited to, a laser radar (e.g. LIDAR) system for target ranging and three-dimensional profiling of surfaces. As shown inFIG. 2 , the modulated linear waveform includes and/or defines a triangular or sawtooth shape. The modulated linear waveform is shown being transmitted at 28, and is shown being received at 30 upon reflection off of a target. The modulated linear waveform is delayed between transmission and receipt by a light round trip time (t). The delay between the received modulated linear waveform and the transmitted modulated linear waveform causes the received modulated linear waveform to mix with the transmitted modulated linear waveform, thereby generating an interference signal. The frequency of the interference signal is equal to the difference between frequency of the transmitted modulated linear waveform and the frequency of the received modulated linear waveform. The frequency of the interference signal is directly proportional to a distance to the target. The frequency of the interference signal shifts due to the Doppler effect when the target and/or thelaser generating system 20 moves relative to the other during time t. The relative velocity between thelaser generating system 20 and the target may be determined by the frequency of the interference signal during “up chirp” periods, generally indicated at 32, and during “down chirp” periods, generally indicated at 34. In order for this targeting technique to work accurately, the frequency modulated laser waveform must be highly linear. - Referring to
FIGS. 1 and 3 , thelaser generating system 20 includes a singleoutput frequency laser 36 that produces a laser output signal having a single frequency. The laser output signal includes a very pure spectrum with an ultra-narrow linewidth. The linewidth of the laser output signal is close to a perfect sine wave, and is centered at ωo, shown at 60 inFIG. 3 . The laseroutput signal ω o 60 is unmodulated. - An electro-
optical modulator 38 is coupled to the singleoutput frequency laser 36. The electro-optical modulator 38 receives the laser output signal from the singleoutput frequency laser 36, and modulates the frequency of the laser output signal. The electro-optical modulator 38 includes an electro-optical crystal that changes properties of the laser output signal when an electric field is applied. - An
electric ramp generator 40 is coupled to the electro-optical modulator 38 and drives the electro-optical modulator 38 with a pre-defined modulation waveform to define a modulated waveform. For example, theelectric ramp generator 40 may drive the electro-optical modulator 38 with a linear frequency modulation waveform. However, it should be appreciated that the pre-defined modulation waveform may differ from the linear frequency modulation waveform described herein. Theelectric ramp generator 40 includes a voltage controlledoscillator 42 controlled by a digital micro-controller 44. The digital micro-controller 44 generates a pre-defined ramp, which is provided to the voltage controlledoscillator 42 to create the pre-defined modulation waveform. For example, the digital micro-controller 44 may generate a nearly perfect linear ramp to create the linear frequency modulation waveform. - The
electric ramp generator 40 controls the electro-optical modulator 38 to modulate the laser output signal to define the modulated waveform. The linear ramp output from the digital micro-controller 44 includes a deviation from a perfect ramp that is less than or equal to 71 Khz, or less than 0.001%. The linear ramp includes a frequency chirp with a bandwidth B over a time period T, and a modulation offset frequency ωm that is offset from Direct Current, i.e.,ω o 60. Preferably, the modulation offset frequency ωm of the linear ramp is between the range of 5 GHz and 30 GHz, and more preferably is equal to 6.0 GHz, and the bandwidth B of the linear ramp is preferably between the range of 300 MHz and 3.0 GHz, and is more preferably equal to 600 MHz over a time period T between the range of 0.01 msec and 0.20 msec, and more preferably equal to 0.15 msec. However, it should be appreciated that the values of the modulation offset frequency ωm, the bandwidth B of the linear ramp, and the time period T may differ from the preferred values described above. As shown inFIG. 3 , the modulated waveform includes afirst order sideband 62, asecond order sideband 64, athird order sideband 66, etc. - A band-pass
optical filter 46 is coupled to the electro-optical modulator 38. Anoptical circulator 48 interconnects theelectric ramp generator 40 and the band-passoptical filter 46. Theoptical circulator 48 includes aninput leg 50 that receives the modulated waveform from theelectric ramp generator 40, an output/input 52 that sends the modulated waveform to the band-passoptical filter 46 and receives the reflected modulated waveform from the band-passoptical filter 46, and an output leg 54 that transmits the pure filtered modulated waveform returned from the band-passoptical filter 46 to the target. - The very narrow band-pass
optical filter 46 receives the modulated waveform and filters out other harmonic frequencies created by the electro-optical modulator 38 during modulation of the laser output signal. The harmonic frequencies are generally shown in the regions labeled byreference numeral 56. The band-passoptical filter 46 may include but is not limited to a fiber optic-based reflective filter that reflects the pure filtered modulated waveform back to theoptical circulator 48, whereupon theoptical circulator 48 directs the pure filtered modulated waveform to the output leg 54 of theoptical circulator 48. The band-passoptical filter 46 includes a bandwidth BW that is greater than the bandwidth B of the linear ramp and less than the modulation offset frequency ωm. The bandwidth BW of the band-passoptical filter 46 is centered at the frequency of the laser output signal ωo plus the modulation offset frequency ωm, i.e., (ωo+ωm). The band-passoptical filter 46 removes the harmonic frequencies, i.e., filters the harmonic frequencies (generally indicated by regions 56) that are outside of the bandwidth BW of the band-passoptical filter 46. Therefore, the modulated waveform output from theoptical circulator 48 includes a pure linear frequency modulated waveform suited for transmission to the target. - As shown in
FIG. 3 , the band-passoptical filter 46 is centered at thefirst order sideband 62 at ωo+ωm, away from Direct Current, i.e.,ω o 60. However, the band-passoptical filter 46 may alternatively be positioned at thesecond order sideband 64, thethird order sideband 66, or some other higher order sideband. Positioning the band-passoptical filter 46 at a higher order sideband, i.e., thesecond order sideband 64 or higher, multiplies the modulation bandwidth B. For example, if the modulation bandwidth B is equal to 600 MHz when the band-passoptical filter 46 is centered at thefirst order sideband 62, then the modulation bandwidth B will equal 1200 MHz when the band-passoptical filter 46 is centered at thesecond order sideband 64, and will equal 1800 MHz when centered at thethird order sideband 66. This allows for bandwidth multiplication of the liner ramp just by selecting the position of the band-passoptical filter 46 on the desired sideband. - A method of generating frequency modulated laser waveform is also provided. The method includes generating the laser output signal. The laser output signal is generated from the single
output frequency laser 36 described above. As noted above, the laser output signal includes an ultra narrow linewidth defining a nearly perfect sine wave. - A pure linear ramp is generated with an
electric ramp generator 40. The linear ramp is used to drive the electro-optical modulator 38 to modulate the frequency of the laser output signal. The linear ramp includes a frequency chirp with a bandwidth B over a time period T, and a modulation offset frequency ωm. As noted above, the modulation offset frequency ωm of the linear ramp is preferably between the range of 5 GHz and 30 GHz, the bandwidth B of the linear ramp is preferably between the range of 300 MHz and 3.0 GHz over a time period T between the range of 0.01 msec and 0.20 msec, and the linear ramp may include a deviation from a perfect ramp of less than or equal to 71 Khz, or less than 0.001%. - The laser output signal from the single
output frequency laser 36 is modulated with the electro-optical modulator 38 to define the modulated waveform. As described above, the modulated waveform preferably includes linear triangular modulated waveform. The modulated waveform is then transmitted from the electro-optical modulator 38 to the band-passoptical filter 46 through anoptical circulator 48. - The modulated waveform from the electro-
optical modulator 38 is filtered with the band-passoptical filter 46 to remove harmonic frequencies in the modulated waveform that are created by the electro-optical modulator 38 during modulation of the laser output signal. Filtering out the harmonic frequencies from the modulated waveform produces a pure filtered modulated waveform. The pure filtered modulated waveform is reflected from the band-passoptical filter 46 back to theoptical circulator 48; whereupon theoptical circulator 48 may transmit the pure filtered modulated waveform to the target. - While some embodiments of the invention have been herein illustrated, shown and described, it is to be appreciated that various changes, rearrangements and modifications may be made therein, without departing from the scope of the invention as defined by the appended claims. It is intended that the specific embodiments and configurations are disclosed for practicing the invention, and should not be interpreted as limitations on the scope of the invention as defined by the appended claims and it is to be appreciated that various changes, rearrangements and modifications may be made therein, without departing from the scope of the invention as defined by the appended claims.
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US13/527,638 US8897654B1 (en) | 2012-06-20 | 2012-06-20 | System and method for generating a frequency modulated linear laser waveform |
US14/550,028 US9712250B2 (en) | 2012-06-20 | 2014-11-21 | System and method for generating a frequency modulated linear laser waveform |
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US9712250B2 (en) | 2017-07-18 |
US8897654B1 (en) | 2014-11-25 |
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