WO2020150537A1

WO2020150537A1 - Pulsed optical transmitter with improved pulse shape and reduced frequency chirp

Info

Publication number: WO2020150537A1
Application number: PCT/US2020/013967
Authority: WO
Inventors: Jiaxi Kan; Bouchaib Hraimel
Original assignee: O-Net Communications (Usa) Inc.
Priority date: 2019-01-16
Filing date: 2020-01-16
Publication date: 2020-07-23
Also published as: EP3912235A4; CN113632328A; JP2022518434A; EP3912235A1; KR20210126604A

Abstract

Designs of optical transmitters that generate laser pulses with improved optical waveform shape in generated laser pulses and provide compensation for the optical chirp of the optical transmitters. Such pulsed optical transmitters can be used in various applications, including light detection and ranging systems (LiDAR) and other optical sensing systems such as optical time-domain reflectometer (OTDR) systems, and optical communication systems.

Description

PULSED OPTICAL TRANSMITTER WITH IMPROVED PULSE SHAPE

AND REDUCED FREQUENCY CHIRP

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This patent document claims priority to and benefits of U.S. Provisional Application No. 62/793,069 entitled“PULSED OPTICAL TRANSMITTER WITH IMPROVED PULSE SHAPE AND REDUCED FREQUENCY CHIRP” filed by Applicant O-Net Communications (USA) Inc. on January 16, 2019.

TECHNICAL FIELD

[0002] This patent document provides designs of optical transmitters that generate laser pulses for various applications using laser pulses.

SUMMARY

[0003] This patent document provides designs of optical transmitters that generate laser pulses with improved optical waveform shape in generated laser pulses and provide

compensation for the optical chirp in generated laser pulses. In addition, the disclosed designs for optical transmitters can used to maintain a constant lateral beam width of the laser output. Such pulsed optical transmitters can be used in various applications, including light detection and ranging systems (LiDAR) and other optical sensing systems such as optical time-domain reflectometer (OTDR) systems, and optical communication systems.

[0004] In one aspect, the disclosed technology can be implemented to provide a pulsed optical transmitter that includes a laser to generate a laser pulse in response to an electrical laser control pulsed signal; a laser driver circuit coupled to the laser to apply the electrical laser control pulsed signal to the laser for generating the laser pulse; a first electrical pulse generator to generate a first electrical pulse signal, the first electrical pulse signal having a first pulse amplitude and a first pulse width in time; a second electrical pulse generator to generate a second electrical pulse signal which has a second pulse amplitude less than the first pulse amplitude and sufficiently high to cause the laser drive circuit to trigger lasing operation in the laser, a second pulse width in time greater than the first pulse width, a leading pulse edge ahead of a leading pulse edge of the first electrical pulse signal; and a signal mixer coupled to the first and second electrical pulse generators to receive the and combine the first and second electrical pulse signals to produce a laser driver control pulse signal, the signal mixer further coupled to the laser driver circuit to apply the laser driver control pulse signal to the laser driver circuit to cause the laser driver circuit to produce the electrical laser control pulsed signal.

[0005] In another aspect, the disclosed technology can be implemented to provide a pulsed optical transmitter to include a laser to generate a laser pulse in response to an electrical laser control pulsed signal; a laser driver circuit coupled to the laser to apply the electrical laser control pulsed signal to the laser for generating the laser pulse; a first electrical pulse generator to generate a first electrical pulse signal based on a clock signal, the first electrical pulse signal having a first pulse amplitude and a first pulse width in time; a second electrical pulse generator to generate a second electrical pulse signal based on the same clock signal as the first electrical pulse generator, the second electrical pulse generator structured to render second electrical pulse signal to have a second pulse amplitude less than the first pulse amplitude|[jKi][BA2] and a second pulse width in time greater than the first pulse width; and a signal mixer coupled to the first and second electrical pulse generators to receive the and combine the first and second electrical pulse signals to produce a laser driver control pulse signal, the signal mixer further coupled to the laser driver circuit to apply the laser driver control pulse signal to the laser driver circuit to cause the laser driver circuit to produce the electrical laser control pulsed signal.

[0006] In another aspect, the disclosed technology can be implemented to provide a method for operating a pulsed optical transmitter to generate laser pulses. This method can include operating, and applying a common clock signal to, a first electrical pulse generator to generate a first electrical pulse signal having a first pulse amplitude and a first pulse width in time and a second electrical pulse generator to generate a second electrical pulse signal having a second pulse amplitude less than the first pulse amplitude K3]:[BA4] and a second pulse width in time greater than the first pulse width; turning off the first and second electrical pulse generators together after generation of the first and second electrical pulse signals; combining the first and second electrical pulse signals to produce a laser driver control pulse signal; and applying the laser driver control pulse signal to a laser diode to generate a laser pulse.

[0007] In yet anther aspect, the disclosed technology can be implemented to provide a method for operating a pulsed optical transmitter to generate laser pulses. This method includes operating a first electrical pulse generator to generate a first electrical pulse signal having a first pulse amplitude and a first pulse width in time and a second electrical pulse generator to generate a second electrical pulse signal having a second pulse amplitude less than the first pulse amplitude and a second pulse width in time greater than the first pulse width; turning off the first and second electrical pulse generators together after generation of the first and second electrical pulse signals; combining the first and second electrical pulse signals to produce a laser driver control pulse signal; applying the laser driver control pulse signal to a laser diode to generate a laser pulse; and applying a leading edge and a front portion of the second electrical pulse signal to produce the laser driver control pule signal to drive the laser diode to lase without the first electrical pulse signal while delaying the first electrical pulse signal in time to a later time to combine the first and the second electrical pulse signals to drive the laser diode to produce a laser pulse in response to the delayed first electrical pulse signal.

[0008] Those and other implementations are described in greater detail in the drawings, the description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 shows an example of a pulsed laser diode with driving circuitry.

[0010] FIG. 2 includes FIGS. 2A-2D and shows an example of the optical chirp and waveform distortion in the pulsed laser diode and its driving circuitry in FIG. 1.

[0011] FIG. 3 A shows an example of a pulsed optical transmitter architecture including two pulse generators to reduce optical chirp and waveform distortion.

[0012] FIG. 3B further shows the relationship between the operation pulse and pre-pulse of the two pulse generators in FIG. 3 A and the tuning of the pre-pulse relative to the operation pulse.

[0013] FIG. 4 includes FIGS. 4A-4D and shows an example for operating the two pulse generators in FIG. 3 A to provide optical chirp compensation in the laser output of a laser diode with driving circuitry based on the example in FIGS. 3A and 3B.

DETAILED DESCRIPTION

[0014] This patent document provides designs of optical transmitters that generate laser pulses for various applications using laser pulses.

[0015] FIG. 1 shows an example of a typical pulsed laser transmitter used in various applications. In this example, a laser diode (LD) 110 is coupled to a driving circuit that includes a clock circuit 102, a pulse generator circuit 104 and a laser diode driver (LDD). The driving circuit is designed to be a current pulse generator that drives the laser diode 110 to produce laser pulses. This type of pulse modulated diode lasers tends to experience or exhibit undesired large distortions, undesired large chirp in frequency, and undesired broad width of the output laser beam |duringjjK5] the laser current transition period when the driving current changes from a low current below the threshold current (when the laser diode is operated in the stimulation mode generating simulated emission in response to the driving current) to a larger current above threshold current (when the laser diode is operated in the lasing mode the amplification of the simulated emission exceeds the optical loss). When the driving current is a low current below the threshold current, the laser diode is operated in the stimulation mode generating simulated emission but the presence of spontaneous emission at this low current below the threshold current causes the output light of the laser beam to have both a broad spectrum and a broad lateral beam width due to the lack of the lasing operation in the laser diode. In this current transition period, the optical signal or the laser output from the laser diode can be distorted from its original electric signal waveform, the transmitted or generated optical frequencies in|[jK6]i[BA7] the laser output from the laser diode can exhibit a significant change in form of a frequency chirp. In addition, the lateral beam width of the laser diode output changes from a broad beam width when the driving current is below the threshold current to a narrow and directional beam width when the driving current is above the threshold current and causes the laser diode to operate in the lasing mode.

[0016] FIG. 2 shows the operation of the laser transmitter in FIG. 1. FIG. 2 includes FIGS. 2A, 2B, 2C and 2D. FIG. 2A shows the single pulse generator 104 in FIG. 1 generates an electrical pulse signal with a desired pulse amplitude, pulse shape and pulse duration that is used to drive the laser diode 110 to generate a laser pulse that is, ideally, in the identical or nearly identical pulse shape and pulse duration. However, due to the transition caused by the leading edge of the electrical pulse signal for driving the laser diode 110, the optical frequencies of the generated optical pulse contain an undesired optical chirp in frequency. As a result, the optical pulse generated by the laser diode 110 is distorted from the original electric signal waveform, the temporal shape of the generated optical pulse is also distorted. This optical chirp in frequency of the laser diode output can result in distortions in the optical receiver output signal dependent on the optical path and optical receiver performance. [0017] FIG. 2B shows an example of the dependence of the optical power output of the laser diode with respect to the amplitude of the driving current pulse applied to the laser diode in FIG. 1 based on the single pulse generate 104 design. The leading edge of the driving current pulse (ILD) increases from a low value below the amplitude of the lasing current threshold ([THRESHOLD) as shown in the initial time during which the laser diode is in the spontaneous emission mode emitting light with a broad beam width and a broad spectral range with an output optical power less than PTHRESHOLD. Subsequently, the amplitude the driving current pulse (ILD) continues to increase above the amplitude of the lasing current threshold ([THRESHOLD), the light emission in the laser diode changes from the spontaneous emission mode to the lasing mode in which the laser diode emits laser light with a narrow beam width and a narrower spectral range with an output optical power greater than PTHRESHOLD. FIG. 2C shows the optical intensity of the laser diode output as a function of time showing the laser pulse shape in the time domain. FIG.2D further illustrates the change in the optical spectral range of the laser diode output over time when being driven by the single current pulse showing a broad initial spectral range when the laser diode is in the spontaneous emission mode and a narrower spectral range when the laser diode is in the lasing mode.

[0018] This patent document discloses new pulsed laser diode transmitters with driving circuitry that can be used to reduce the above optical output waveform distortion, optical |chirp|_[jK8] and the output beam width. The disclosed driving circuitry for the laser diode is designed to generate two driving current pulses to a laser diode: a normal pulse current and a pre-pulse current, which are added together to generate the driving current pulse for driving the laser diode. The pre-pulse signal can be used to force the laser diode to change from the stimulation mode to the lasing mode at a low optical power level. This is achieved by a combination of several techniques. First, the amplitude of the pre-pulse signal is generated to generate a driving current amplitude just above the lasing current threshold for the laser diode to operate in the lasing mode but at a level not much higher than the threshold. Second, the pre pulse signal has a pulse duration longer than the desired pulse duration for the operation pulse to generate a desired laser pulse by the laser diode. Third, the leading edge of the pre-pulse signal triggered before the leading edge of the operation pulse in time so that the pre-pulse signal is used to drive the laser diode to transition from the spontaneous emission to laser emission before the arrival of the leading edge of the operation pulse so that, by the time when the operation pulse is used to generate the current to drive the laser diode, the laser diode is already operated in the laser mode and the operation pulse is to increase the driving current to the laser diode in a pulse shape and a pulse duration that are specifically designed for a desired laser pulse output. Fourth, the tailing edge of the pre-pulse signal and the trailing edge of the operation pulse are sent to coincide in time so both pulse signals are turned off at the same time. As a result of this use of a combination of a pre-pulse signal and a normal pulse signal in driving the laser diode, the large optical distortion and optical chirp would also occur at a lower power level. Therefore, these large optical |di storti on|p _K9] , frequency chirp and large beam width produced by the transition period by the pre-pulse signal would produce a small impact to an optical transmission system using laser diodes that are driven in this manner. Various laser transmission systems, such as OTDR and LIDAR systems, can benefit from the disclosed laser transmitter technology.

[0019] FIG. 3 A shows an example of a pulsed optical transmitter architecture including two pulse generators to reduce undesired optical jchirpjpKio], undesired waveform distortion and undesired broad beam width. This optical transmitter 300 in the example in FIG. 3A includes a clock generator 302 to generate a clock signal and two pulse generators, Pulse Gen 1 (304) and Pulse Gen 2 (303), which are electrically coupled to the clock generator302 to receive the same clock signal from the clock generator 302 and to generate first and second electrical pulse signals based on that same clock signal. A signal mixer 305 is coupled to receive the first and second electrical pulse signals from Pulse Gen 1 (304) and Pulse Gen 2 (303) to add the two pulse signals to each other to produce a control pulse signal to a downstream laser diode driver (LDD) circuit 306. The laser diode driver (LDD) circuit 306 is coupled to receive the LDD control pulse signal from the signal mixer 305 and to produce a LDD driver signal in response to the received LDD control pulse signal. A laser diode (LD) 310 is coupled to the LDD circuit 306 and is energized by the LDD driver signal to produce laser pulses as the optical output of the optical transmitter 300.

[0020] The pulse generated by the first generator 304 (Pulse Gen 1) is an operation pulse signal that has a sufficiently high amplitude to operate the laser diode 310 above its lasing threshold. The second generator 303 (Pulse Gen 2) generates a lower amplitude pulse signal that is wider than the operation pulse signal in time, e.g., in the front time. Both Pulse Gen 1 (304) and Pulse Gen 2 (303) can be controlled or operated to turn off pulse signals at the same edge time. This operation of the Both Pulse Gen 1 (304) and Pulse Gen 2 (303) can reduce a background noise in comparison to some other optical transmitters for pulse generation with a continuous laser bias current applied the whole time and its amplitude is just above the laser threshold. This simultaneous turn-off the both trailing edges of the pre-pulse and operation pulse signals can be advantageous in LiDAR and OTDR measurements in measuring targets located in short distances. With this two-pulse-generator system to drive the laser diode, the laser chirp and waveform are reduced with some background noise. Therefore, the disclosed technology can be used to improve the system sensitivity, receiver detection accuracy, jandj Kii_] dynamic range of a LiDAR system when such a pulsed optical transmitter is used.

[0021] In another aspect of the pulsed optical transmitter in FIG. 3 A, the pulse from Pulse Gen 2 (303) and the pulse from Pulse Gen 1 (304) can be tuned in amplitude and in the delay in time to pre-shape the driving pulse to the laser diode 310, thus shaping the optical waveform of the pulsed laser output from the laser diode 310 and compensating for laser chirp and pulse distortion in various applications. Examples of applications for such pre-shaping include, for example, during free space optical transmission because of the presence of hydrogen from water. This tuning can be achieved by Rising a control circuit coupled to the first and second pulse generators 304 and 303 [BAi2]|[jKi3]to control the operations of the generators, including shutting them off together after completion of generation of the first and second electrical pulse signals, respectively, in response to the same clock signal. In the illustrated example, the same clock circuit 302 can be used to perform this tuning. Specifically, the amplitude and pulse duration of the pre-pulse signal can be tuned or adjusted while maintaining the pulse shape, pulse amplitude or pulse duration of the operation pulse as is based on a particular design for generating the laser pulse produced by the laser diode 310.

[0022] FIG. 3B further shows the relationship between the operation pulse and pre-pulse signals produced by the two pulse generators 304 and 303 in FIG. 3 A. The operation pulse and pre-pulse signals produced by the two pulse generators 304 and 303 can be voltage or current signals and are mixed together to produce a common signal to the LLD 306 which, in response, produces a driving current pulse that drives the laser diode 310 for producing the laser pulse 312. As shown in FIG. 3B, both pulse and pre-pulse signals produced by the two pulse generators 304 and 303 have signal amplitudes that correspond to a driving current to the laser diode 310 higher than the lasing threshold current for the threshold laser output power. However, their amplitudes are different. The amplitude of the pre-pulse signal is set to be just above the level corresponding to the lasing threshold current for the threshold laser output power so that, in absence of the operation pulse, the pre-pulse signal alone can cause the LDD 306 to drive the laser diode 310 in the lasing mode but at a low power output in the lasing mode. The amplitude of the operation pulse signal is also set to be above the level corresponding to the lasing threshold current for the threshold laser output power but is set higher than the amplitude of the pre-pulse signal. Also as shown in FIG. 3B, the pre-pulse signal’s leading edge is launched first and has a pulse duration longer than the desired pulse duration for the operation pulse to generate a desired laser pulse by the laser diode. As such, the pulse-pulse’s leading edge causes the laser diode to undergo through the transition from the spontaneous emission to the lasing emission at a relatively low optical power just above the laser threshold level. Under this design, the arrival of the operation pulse signal after the laser diode is already in the lasing mode caused by the pre pulse signal will convert the energy of the driving current added by the operation pulse signal more efficiently into the laser output.

[0023] FIG. 3B further shows that time difference between the leading edge tl of the pre pulse signal and the leading edge t2 of the operation pulse signal can be tunable or adjusted.

Since both signals have the same tailing edge t3, the duration of the pre-pulse signal can be tuned to tune or adjust the time difference between tl and t2. In addition, the amplitude of the pre pulse signal can be also be tuned. For a given operation pulse signal, those two tunings on the pre-pulse signal can be used to optimize the reduction in the waveform distortion or optical chirp in the laser diode output pulse. The pulse shape, amplitude or duration of the operation pulse can be specifically designed for a desired pulse shape, amplitude or duration of a laser output pulse.

[0024] In implementations, the same clock circuit 302 can be used to trigger or control Gen 1 and Gen2. The pulse amplitude and width of Gen 1 is to generate an optical pulse for desired optical signal. The pulse amplitude of Gen 2 is to generate a small laser current that just above the laser threshold. Pulse width of Gen 2 is wider than the one from Gen 1. By turning amplitude and pulse width of Gen 2 one can generate the best optical transmitter signal with small optical chirp and beam width.

[0025]

[0026] jFlG. 4 {BAi4]:pKi5]i 11 ustrates an example of the chirp compensation based on the tuning of the two-pulse generator optical transmitter in FIG. 3. The wavelength is drifted when applying the pulse from Pulse Gen2, but the peak power is very small compared to that of the principal pulse generated by Pulse Gen 1. When applying the principal pulse from Pulse Genl, the wavelength becomes constant with high peak optical power. Therefore, this transient chirp is very small and can be neglected. Similar to FIG. 2, FIG. 4 includes FIGS. 4A, 4B, 4C and 4D for comparison with FIGS. 2A, 2B, 2C and 2D, respectively. Comparing FIGS. 4C and 2C, the optical pulse in FIG. 4C shows a significant less distortion than the optical pulse in FIG. 2C due to the use of the pre-pulse signal for driving the laser diode in the lasing mode in advance before the arrival of the operation pulse that is responsible for generating the desired laser output pulse.

[0027] While this patent document contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple

embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.

[0028] Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.

Claims

CLAIMS What is claimed is:

1. A pulsed optical transmitter, comprising:

a laser to generate a laser pulse in response to an electrical laser control pulsed signal; a laser driver circuit coupled to the laser to apply the electrical laser control pulsed signal to the laser for generating the laser pulse;

a first electrical pulse generator to generate a first electrical pulse signal, the first electrical pulse signal having a first pulse amplitude and a first pulse width in time;

a second electrical pulse generator to generate a second electrical pulse signal which has a second pulse amplitude less than the first pulse amplitude and sufficiently high to cause the laser drive circuit to trigger lasing operation in the laser, a second pulse width in time greater than the first pulse width, a leading pulse edge ahead of a leading pulse edge of the first electrical pulse signal; and

a signal mixer coupled to the first and second electrical pulse generators to receive the and combine the first and second electrical pulse signals to produce a laser driver control pulse signal, the signal mixer further coupled to the laser driver circuit to apply the laser driver control pulse signal to the laser driver circuit to cause the laser driver circuit to produce the electrical laser control pulsed signal.

2. The pulsed optical transmitter as in claim 1, comprising a control circuit coupled to the first and second pulse generators to turn off the first and second pulse generators together after completing generation of the first and second electrical pulse signals to reduce noise.

3. The pulsed optical transmitter as in claim 1, comprising a control circuit coupled to the first and second pulse generators to adjust the first and second pulse amplitudes and a time delay between the first and second electrical pulse signals to reduce a jchirpu kib_] or distortion in the generated laser pulse.

4. The pulsed optical transmitter as in claim 1, wherein the second electrical pulse generator is structured to operate in tuning the second amplitude and a time difference between a time delay between the first and second electrical pulse signals.

5. A light detection and ranging (LiDAR) system including an optical transmitter to produce laser pulses to optically sensing one or more targets, the optical transmitter including: a laser to generate a laser pulse in response to an electrical laser control pulsed signal; a laser driver circuit coupled to the laser to apply the electrical laser control pulsed signal to the laser for generating the laser pulse;

6. The LiDAR system as in claim 5, wherein the optical transmitter includes a control circuit coupled to the first and second pulse generators to turn off the first and second pulse generators together after completing generation of the first and second electrical pulse signals to reduce noise.

7. The LiDAR system as in claim 5, wherein the optical transmitter includes a control circuit coupled to the first and second pulse generators to adjust the first and second pulse amplitudes and a time delay between the first and second electrical pulse signals to reduce a chirp|[ _Ki7] or distortion in the generated laser pulse.

8. The LiDAR system as in claim 5, wherein the second electrical pulse generator is structured to operate in tuning the second amplitude and a time difference between a time delay between the first and second electrical pulse signals.

9. An optical time-domain reflectometer (OTDR) system including an optical transmitter which comprises an optical transmitter to produce laser pulses for optical sensing, the optical transmitter including:

10. The OTDR system as in claim 9, wherein the optical transmitter includes a control circuit coupled to the first and second pulse generators to turn off the first and second pulse generators together after completing generation of the first and second electrical pulse signals to reduce noise.

11. The OTDR system as in claim 9, wherein the optical transmitter includes a control circuit coupled to the first and second pulse generators to adjust the first and second pulse amplitudes and a time delay between the first and second electrical pulse signals to reduce a chirp|_[jKi8] or distortion in the generated laser pulse.

12. The OTDR system as in claim 9, wherein the second electrical pulse generator is structured to operate in tuning the second amplitude and a time difference between a time delay between the first and second electrical pulse signals.

13. A method for operating a pulsed optical transmitter to generate laser pulses, comprising:

operating a first electrical pulse generator to generate a first electrical pulse signal having a first pulse amplitude and a first pulse width in time and a second electrical pulse generator to generate a second electrical pulse signal having a second pulse amplitude less than the first pulse amplitude and a second pulse width in time greater than the first pulse width;

turning off the first and second electrical pulse generators together after generation of the first and second electrical pulse signals;

combining the first and second electrical pulse signals to produce a laser driver control pulse signal;

applying the laser driver control pulse signal to a laser diode to generate a laser pulse; and applying a leading edge and a front portion of the second electrical pulse signal to produce the laser driver control pule signal to drive the laser diode to lase without the first electrical pulse signal while delaying the first electrical pulse signal in time to a later time to combine the first and the second electrical pulse signals to drive the laser diode to produce a laser pulse in response to the delayed first electrical pulse signal.

14. The method as in claim 13, comprising:

adjusting the first and second pulse amplitudes and a time delay between the first and second electrical pulse signals to reduce a |chirp|[ Ki9] or distortion in the generated laser pulse.

15. The method as in claim 13, comprising:

adjusting the second amplitude to reduce a jchirpjjiao] or distortion in the generated laser pulse.

16. The method as in claim 13, comprising:

adjusting a time delay between the first and second electrical pulse signals to reduce a |chirp|[jK2i] or distortion in the generated laser pulse.