High-level three-dimensional (3D) cameras have been developed that are capable of scanning a scene or object and developing a very accurate 3D model or image. Some 3D cameras use Lidar (light imaging, detection and ranging) that uses a pulsed laser to measure ranges or distances. The typical Lidar system, as shown in
Lidar uses frequency-modulated continuous-wave (FMCW) laser light.
R=1/2c·τR 1
The reflected light RX from the target is collected and combined with source light (as represented by the thin line between the TX and RX beams). The time delay between the reflected light and the source light causes a frequency difference fR that is proportional to the round-trip delay τR and the modulation slope γ, as shown in the graph in
In this example, the range error R depends on the measurement precision of the frequency fR. The range error R also depends on any error in the control (or alternatively observation) of the modulation slope γ. Examples of the effects of range error, as well as ramp non-linearity, in direct modulation systems are depicted in
In many applications of FMCW laser systems, such as fiber defect detection, the precise control of the modulation pattern (such as modulation slope γ for a linear frequency modulation) is of central importance for the system performance. For this reason a feedback technique has been utilized based on well-known electronic phase-locked loop (PLL) control, examples of which are described in the following publications, all of which are expressly incorporated herein by reference: K. Iiyama, L.-T. Wang and K.-i. Hayashi, “Linearizing Optical Frequency-Sweep of a Laser Diode for FMCW Reflectometry,” Journal of Lightwave Technology, pp. 173-178, 1996; A. C. Bordonalli, C. Walton and A. J. Seeds, “High-Performance Phase Locking of Wide Linewidth Semiconductor Lasers by Combined Use of Optical Injection Locking and Optical Phase-Lock Loop,” Journal of Lightwave Technology, vol. 17, no. 2, pp. 328-342, 1999; N. Satyan, A. Vasilyev, G. Rakuljic, V. Leyva and A. Yariv, “Precise control of broadband frequency chirps using optoelectronic feedback,” Optics Express, vol. 17, no. 18, pp. 15991-1599, 2009; and. Behroozpour, P. A. Sandborn, N. Quack, T.-J. Seok, Y. Matsui, M. C. Wu and B. E. Boser, “Electronic-Photonic Integrated Circuit for 3D Microimaging,” IEEE Journal of Solid-State Circuits, vol. 52, no. 1, pp. 161-172, 2017.
An exemplary PLL control system is shown in
fMZI=γ·τMZI 3
Since the MZI delay is fixed by the length of the waveguide, any change in the frequency fMZI can be interpreted as variations in the modulation slope γ. Therefore, ensuring that fMZI is fixed to a reference value in the feedback loop can ensure a constant modulation slope. The frequency fMZI can be fixed to a reference value fref using a well-known electronic phase-locked loop (PLL) circuit, as shown in
While the PLL controller shown in
Having a small value for number of cycles n is problematic because a PLL circuit usually samples the phase of its input signal at zero-crossings. Therefore, reducing n reduces the number of samples available to the feedback loop within one modulation ramp period. With a small number of samples the control loop does not have enough information to provide an accurate control signal through the feedback mechanism.
As an example, when using a laser with 1 GHz modulation depth (Δf=1 GHz) and an MZI of 0.5 ns delay (which corresponds to about 10 cm waveguide length on a silicon-photonic chip with a footprint slightly less than 1 sq-mm when laid out in a spiral shape) the number of cycles within one ramp period will be less than one (n=1 GHz×0.5 ns=0.5). In other words, the feedback loop may not have even a single sample of the MZI beat signal to observe the laser frequency slope and to provide any meaningful feedback to control it. In most practical cases the MZI signal should have at least tens of cycles within each ramp period to ensure a reasonable operation for the control loop. There is a significant need to address this problem by providing an improved feedback architecture to observe and control the laser frequency, as disclosed herein.
In one aspect of the disclosure, a Lidar (light imaging, detection and ranging) system is provided that comprises a tunable laser configured to generate an output light signal and a photodiode array for receiving light from the tunable laser reflected from a target object. The tunable laser includes a feedback loop including a Mach-Zender interferometer (MZI) receiving the output light signal from the tunable laser in which the MZI includes two optical paths receiving the output light signal. A phase shifter is provided in one optical path that is operable to produce a pre-determined shift in the phase angle of the light signal passing through the one optical path relative to the phase angle of the light signal passing through the other optical path. A photodiode configured to detect the interference signal generated by the MZI is operable to generate a photodiode current in response thereto. Circuitry converts the photodiode current to a control signal for controlling the tunable laser.
In one embodiment, the phase shifter is configured to provide a π/2 bias to the phase angle of the light signal passing through the one optical path. In another embodiment, the phase shifter is configured to bias the phase angle of the light signal passing through the one optical path as a function of the photodiode current, which is in turn a function of the interference signal generated by the MZI.
In another aspect, a method is contemplated for providing an output light signal for a Lidar (light imaging, detection and ranging) system including the steps of operating a tunable laser to generate an output light signal and directing the output light signal to a Mach-Zender interferometer (MZI), the MZI including two optical paths receiving the output light signal. A pre-determined phase shift is introduced into the phase angle of the light signal passing through one optical path relative to the phase angle of the light signal passing through the other optical path. The interference signal generated by the MZI is detected with a photodetector and a photodiode current is generated in response thereto. The photodiode current is processed to generate a control signal for controlling the tunable laser.
For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that the present disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles disclosed herein as would normally occur to one skilled in the art to which this disclosure pertains.
The present disclosure contemplates a Lidar system that uses a feedback system and method to map the frequency modulation (FM) of the laser light to an amplitude modulation or variation (AM) on the current of a photodetector to provide a feedback signal for a Lidar (rather than mapping it to a beat frequency like fMZI). One implementation of the FM to AM conversion is illustrated in
where kPD is the photodiode responsivity, P0 is the incident optical power, φ2 is the phase angle of the light signal passing through the second path 11 of the MZI and φ1 is the shifted phase angle of the light signal passing through the first path 10.
For a piecewise linear modulation (i.e., a constant modulation slope γ during the time delay τMZI), the argument of the cosine function at the end of Equation 5 can be simplified as shown in Equation 6 below:
The first term 2πγτMZI·t shows the variation of the signal at the MZI output that can be used as a measure in the feedback loop. The second term πγτMZI2 is an unwanted term that appears as a bias. The greatest sensitivity in the amplitude of the sinusoidal versus its phase occurs at its zero-crossing. Therefore, the argument of the cosine function should have a bias of π/2 for maximum sensitivity, and the phase shifter Δφ should be adjusted to ensure the following relationship in Equation 7:
For example, if the modulation slope γ is 1 GHz/10 μs and τ is 100 ps, then the term γτMZI2 would be 10−6 which can be neglected when compared to ½. In this case, the phase shifter in the first path 10 controls the phase bias of the MZI. In accordance with the present disclosure, the phase shifter 15 for the shorter MZI branch 11 can be implemented as disclosed in X. Tu, T.-Y. Liow, J. Song, X. Luo, Q. Fang, M. Yu and G.-Q. Lo, “50-Gb/s silicon optical modulator with traveling wave electrodes,” Optics Express, vol. 21, no. 10, pp. 12776-12782, 2013, or as described in S. Sharif Zadeh, M. Florian, S. Romero-Garcia, A. Moscoso-Martir, N. von den Driesch, J. Muller, S. Mantl, D. Buca and J. Witzens, “Low Vπ Silicon photonics modulators with highly linear epitaxially grown phase shifters,” Optics Express, vol. 23, no. 18, pp. 23526-23550, 2015, the entire disclosures of which are incorporated herein by reference. In particular, Section 2 of the Tu et al. article and Section 2 of the Zadeh et al. article are specifically incorporated herein by reference to describe the construction of an MZI phase shifter for use with the present disclosure.
With a bias of
Equation 5 can be approximated as given below in Equation 8:
iPD(t)=i0·(1+sin(2πγτMZI·t)) 8
where kPD(t)·P0 from Equation 5 has been replaced with i0. As an example, for Δf=1 GHz and τMZI=500 ps, the photocurrent will then look like a half period of a sinusoidal as shown in
The laser frequency-tuning method of the present disclosure can be implemented in a mixed-signal electronic feedback system as shown in
The frequency tuned output light is shown in the graph of
One challenge in implementing this MZI loop is that the extra phase shift Δφ is needed in the MZI to ensure maximum sensitivity of the photocurrent versus laser frequency tuning. While this phase shift can be calibrated to be at the optimum point at the beginning of the operation for a given system, the phase shift might have temperature dependency or drift due to aging during the operation of the tunable laser circuit. In one aspect of the present disclosure implemented to avoid this issue, a servo loop can be provided to observe the peak-to-peak value of the photocurrent during each ramp period and to maximize the photocurrent by adjusting the phase change Δφ as a function of the changing VB. Thus, in one embodiment illustrated in the schematic of
Unlike the prior approaches for FM laser control that use electro-optic PLL, the system and method of the present disclosure can achieve a high density of samples in the feedback loop independent of the MZI length or of the laser tuning depth. The systems shown in
Known frequency-modulated continuous-wave (FMCW) laser light has multiple applications including ranging and 3D imaging, fiber defect detection, etc. Such systems operate based on modulating the laser frequency with a known pattern and measuring the delay by which this pattern is observed in an interferometric optical detector. Precision of these systems has a strong dependency on the control or, alternatively, on observation of the laser frequency and its modulation pattern. The present disclosure provides a feedback system with better dynamics compared to the existing FMCW systems. In particular, the present disclosure relies on amplitude variation of the photocurrent in a feedback loop to control the tunable laser TL.
The present disclosure should be considered as illustrative and not restrictive in character. It is understood that only certain embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the disclosure are desired to be protected.
This application is a 35 U.S.C. § 371 National Stage Application of PCT/EP2018/083990, filed on Dec. 7, 2018, which claims the benefit of priority to U.S. provisional patent application No. 62/608,667, filed on Dec. 21, 2017, the entire disclosures of which are incorporated herein by reference.
Filing Document | Filing Date | Country | Kind |
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PCT/EP2018/083990 | 12/7/2018 | WO |
Publishing Document | Publishing Date | Country | Kind |
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WO2019/121069 | 6/27/2019 | WO | A |
Number | Name | Date | Kind |
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4900112 | Kawachi | Feb 1990 | A |
9705283 | Deppe | Jul 2017 | B1 |
20100085992 | Rakuljic | Apr 2010 | A1 |
Number | Date | Country |
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2 980 954 | Sep 2016 | CA |
Entry |
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International Search Report corresponding to PCT Application No. PCT/EP2018/083990, dated Mar. 21, 2019 (5 pages). |
Satyan, A. et al., “Precise control of broadband frequency chirps using optoelectronic feedback,” Optics Express 15991, vol. 17, No. 18, Aug. 31, 2009 (9 pages). |
Tu, X. et al., “50-Gb/s silicon optical modulator with traveling wave electrodes,” Optics Express 12776, vol. 21, No. 10, May 20, 2013 (7 pages). |
Sandborn, P. et al., “Linear Frequency Chirp Generation Employing Opto-electronic Feedback Loop and Integrated Silicon Photonics,” CLEO 2013, Optical Society of America (2 pages). |
Behroozpour, B. et al., “Electronic-Photonic Integrated Circuit for 3D Microimaging,” IEEE Journal of Solid-State Circuits, vol. 52, No. 1, pp. 161-172, Jan. 2017 (12 pages). |
Behroozpour, B. et al., “Lidar System Architectures and Circuits,” IEEE Communications Magazine, vol. 55, No. 10, pp. 135-142, Oct. 2017 (8 pages). |
Iiyama, K. et al., “Linearizing Optical Frequency-Sweep of a Laser Diode for FMCW Reflectometry,” Journal of Lightwave Technology, vol. 14, No. 2, pp. 173-178, Feb. 1996 (6 pages). |
Bordonalli, A. C. et al., “High-Performance Phase Locking of Wide Linewidth Semiconductor Lasers by Combined Use of Optical Injection Locking and Optical Phase-Lock Loop,” Journal of Lightwave Technology, vol. 17, No. 2, pp. 328-342, Feb. 1999 (15 pages). |
Azadeh, S. S. et al., “Low Vπ Silicon photonics modulators with highly linear epitaxially grown phase shifters,” Optics Express 23526, vol. 23, No. 18, Sep. 7, 2015 (25 pages). |
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20210083449 A1 | Mar 2021 | US |
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62608667 | Dec 2017 | US |