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Abstract
A homemade PQ:PMMA VBG was fabricated and characterized with a diffraction efficiency of about 8%. Mounting on an annular piezo-transducer, this VBG served as a narrow band feedback mirror of a 1060 nm external cavity diode laser (ECDL), which achieved a single longitudinal mode and frequency-tunable operation. With the laser source, an FMCW LiDAR system was demonstrated and successfully measuring target distance within 8.5 m.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
LiDAR (light detection and ranging) has long been important equipment and method in many fields. Laser pulse time-of-flight (ToF) and frequency-modulated continuous-wave (FMCW) are the two major configurations for mid-to-long range LiDAR [1,2]. ToF configuration is simple and mature and has long been dominating LiDAR applications. However, FMCW LiDAR catches more attention recently since it suffers less from cross talk and can achieve continuous measurement which allows vibration or Doppler velocity measurement [3,4]. Larger dynamic range, higher resolution in short-range sensing, and more robust against environmental disturbance are also major advantages of FMCW LiDAR [1,5]. The CW nature and commonly chosen 1550 nm wavelength make FMCW LiDAR consider eye-safer than ToF schemes [6].
In 2009, Satyan et al. utilized an optoelectronic feedback loop to control the ramping driving current and achieve 100 GHz broadband frequency tuning of a DFB single-mode diode laser. Using this laser, FMCW LiDAR was demonstrated [7]. Iiyama et al. proposed using VCSEL as the light source and FBG for calibration to achieve 10 μm accuracy at 2 m FMCW LiDAR distance measurement [8].
An FMCW LiDAR mainly consists of a wavelength tunable single longitudinal mode laser source, an interferometer, and a frequency counter or spectrum analyzer. By ramping the laser output wavelength/frequency, the returned laser light from the far target has a different frequency comparing with the local laser output (local oscillator, LO). Mixing the return signal and LO with the interferometer, the frequency difference can be obtained by a frequency counter or a spectrum analyzer. With the frequency difference, the distance to the far target can be evaluated. In general, longer coherence length, larger frequency sweeping range, and larger laser output power is preferred.
The laser frequency sweeps in a triangle waveform as a function of time as shown in Fig. 1. The solid and dashed lines indicate the local laser frequency and return laser frequency. $\Delta {f_{laser}}$ is the laser frequency sweep range. The laser frequency sweeps in a triangle waveform with a frequency of fs. D is the distance of the far target. Therefore, the return signal has a time delay of the following:
The frequency difference between the local and return laser frequencies is fB which can be obtained using heterodyne or beat signal. Therefore, the measured distance can be rewritten as the following:
As the fs and Δflaser are predetermined, the distance can be obtained by measuring fB.
External cavity diode laser (ECDL) configuration uses a grating to feedback a diode laser to achieve a narrow laser output spectrum. With careful design, the laser output can even reach a single longitudinal mode operation. The most common configurations are Littrow and Littman configurations which can also achieve frequency tuning. Therefore, an FMCW LiDAR can be constructed with such a light source [9].
Volume Bragg grating (VBG) is a free-standing optical element that has a narrow reflection spectrum. PTR (photo-thermal refractive) glass VBG has been reported serving as a narrow band laser mirror and wavelength selection element to control laser output spatial and spectral behaviors for solid-state lasers and diode lasers [10–12]. Since then, diode laser using PTR VBG to stabilize the laser output wavelength has been widely adopted [13–15]. In 2019, the first photopolymer PQ:PMMA (phenanthrenequinone doped polymethyl methacrylate polymers) VBG has been reported successfully achieving laser output spectrum narrowing of a 526 nm diode laser [16]. In other words, PQ:PMMA VBG provides an inexpensive and flexible alternative of diode laser wavelength and mode control. With careful ECDL design, single longitudinal mode laser output can be achieved.
To construct a FMCW LiDAR, wavelength/frequency tuning of a single longitudinal mode laser is essential. Precisely changing the cavity length leads to the change of the standing wave condition and allows tuning the laser output wavelength/frequency [17]. A subtle method is to modify the injection current of a diode laser. The laser gain spectrum, the thermal load of the diode laser, and the diode laser cavity length change with the injection current accordingly. In the meantime, laser output power is also modified which may lead to additional amplitude noise. If the cavity length changes too much, mode hopping could happen and results in unexpected frequency output. In ECDL FMCW LiDAR configuration, both laser injection current and external cavity mirror can be adjusted to change the single longitudinal mode wavelength/frequency [9].
Demanding laser performance leads to expensive and complicated FMCW LiDAR laser configuration and control system. In this work, a simple and inexpensive homemade PQ:PMMA VBG was fabricated and utilized to achieve frequency sweeping single longitudinal mode diode laser in an ECDL scheme by directly modify the cavity length with a piezo-electric transducer. This laser serves as the light source to demonstrate the potential of a low-cost FMCW LiDAR.
2. PQ:PMMA VBG fabrication and characterization
PQ:PMMA is a photopolymer known for the potential to be the recording medium of hologram and holographic storage [18–20]. Commonly, 532 nm is used as the recording wavelength to achieve thicker volume grating. A two-beam interference scheme can be utilized to record a PQ:PMMA VBG [21]. Such VBG can serve as the wavelength selection element for the ECDL scheme to achieve output spectrum narrowing at the same time [16,22]. With properly chosen configuration and operation parameters of the ECDL, a single longitudinal mode operation can even be achieved.
Two 2 mm thick PQ:PMMA samples were fabricated using a two-step polymerization process [19]. The PQ:PMMA VBG was recorded using a two-beam interference configuration with a 532 nm single longitudinal mode laser (Coherent, Verdi V-2) as shown in Fig. 2. The PQ:PMMA sample was sandwiched between two roof prisms with silicone oil as the index matching fluid. By adjusting the mirrors before the sample, the recorded Bragg wavelength on the sample can be adjusted. The clear aperture of the recorded VBG was about 10 mm in diameter. A 650 nm diode laser was used to probe the diffraction light of the sample to monitor the recording process. The 532 nm laser irradiance was 60 mW/cm2 with recording time about 6 mins.
The Bragg wavelengths recorded for the two PQ:PMMA VBGs are about 1066 nm and 1061 nm which will be referred to as 1066-VBG and 1061-VBG respectively in the following paragraphs. The nominal refractive index of the PQ:PMMA is 1.482 at about 1064 nm. Therefore, the corresponding recorded grating periods are 359.6 nm and 358.0 nm, respectively. The 1066-VBG was utilized to construct a V-shaped wavelength-tunable ECDL single longitudinal mode CW laser which is shown as the green box in Fig. 3. A single transverse mode 1060 nm diode laser (Thorlabs M9-A64-0300) served as the gain medium. The nominal output spectral width of this diode laser is about 0.5-2 nm. An aspherical lens L (Thorlabs C240TME-C, f=8 mm) collimated the diode laser beam. The collimated beam was diffracted by the 1066-VBG which served as the wavelength selection element, output coupler as well as a mirror. The diffracted beam then reached the end mirror M of the V-shaped ECDL cavity. The ECDL laser output was monitored by a scanning Fabry-Pérot interferometer (Thorlabs SA200-9A, 1.5 GHz FSR). The laser output achieved a single longitudinal mode operation with a maximum output power of 85 mW. By adjusting the orientation of 1066-VBG and end mirror M simultaneously, the ECDL laser output wavelength could be fine adjusted. The 1061-VBG was then inserted between the laser and the scanning Fabry-Pérot interferometer. A wedge prism W (Thorlabs BSF2550) was inserted between the two VBGs to monitor the power of incident and diffracted beams which were measured by a powermeter (Ophir 1Z01601 laserstar). By tuning the ECDL output wavelength, the 1061-VBG peak diffraction efficiency was measured to be 8.2% and the diffraction spectral width is less than 1 nm. The estimated VBG refractive index contrast was about 5 × 10−5. Since diode lasers have extremely high gain, small amount of feedback is enough to dictate the operation of a diode laser. 5-10% of diffraction efficiency is enough to lock the laser output wavelength without the penalty of significant output power reduction. By replacing the Fabry-Perot interferometer with an OSA (Agilent HP 86142B), the Bragg wavelength of the 1061-VBG was determined at about 1060.7 nm.
3. FMCW setup and measurement result
3.1 Frequency sweeping linear ECDL using the PQ:PMMA VBG
The frequency sweeping laser configuration is shown in the red box in Fig. 4. The diode laser and the aspherical lens were the same as the V-shaped ECDL above-mentioned. 1061-VBG was attached to a ring-shaped piezo actuator (Noliac NAC2124-A01) as the linear ECDL feedback mirror/grating of the diode laser and the total length was about 2 cm. The narrow diffraction spectral width of VBG suppressed the generation of multi-longitudinal mode and ensure single longitudinal mode output with laser wavelength locked at the vicinity of the Bragg wavelength of VBG. The single mode laser linewidth is mainly determined by the finesse of the ECDL Fabry-Pérot cavity. The laser output linewidth was measured to be 18.6 MHz or 0.0696 pm as shown in Fig. 5 by the same scanning Fabry-Pérot interferometer above-mentioned. The corresponding coherence length was about 16.14 m. The laser wavelength was 1060.7 nm and the output power was about 100 mW. The piezo actuator was driven by a triangle function which leads to the laser output frequency modulation accordingly. The mode-hop-free single longitudinal mode frequency tuning range was measured to be about 0.28 GHz or 1.05 pm.
Fig. 4. FMCW experimental setup. The red box indicates the wavelength sweeping laser section, and the blue box indicates the interferometer section
Fig. 5. Scanning Fabry-Pérot trace of PQ:PMMA feedback 1060 nm diode laser. The green line is the ramp voltage of the scanning Fabry-Pérot interferometer.
3.2 Interferometer
The distance measurement part of the FMCW setup was essentially a Michelson interferometer as shown in the blue box in Fig. 2. To ensure the stability of the laser frequency sweeping, an optical isolator (Thorlabs, IO-5-1064-LP) was placed right after the laser output. A 70:30 (T:R) beam splitter (Thorlabs BS050) was used to separate the reference arm and the test arm of the Michelson interferometer. The reference arm has an end mirror located 5 cm away from the beam splitter. The laser beam in the test arm was expended by a beam expander (Thorlabs BE05M) right after the beam splitter. A plane mirror served as a cooperative target and was placed at the far end of the test arm. A photodetector (Thorlabs DET110) received the heterodyne interference signal which was recorded by a computer for post-processing.
The driving signal for the piezo actuator was a triangular function that swept the laser output wavelength/frequency. The triangular function frequency, ${f_s}$, was 500 Hz to compromise with the sweeping speed and frequency response of the piezo actuator. The laser sweep frequency range, $\Delta {f_{laser}}$, was chosen to be 0.18 GHz. This sweep range was about 60% of the maximum mode-hop-free single frequency sweeping range to ensure stable single longitudinal mode operation.
3.4 Post-processing and the FMCW distance measurement result
By taking the FFT (fast Fourier transform) of the heterodyne signal, the corresponding power spectrum can be obtained as well as the maximum spectral component frequency, ${f_B}$. With all the above parameters given or obtained, the FMCW LiDAR measured distance D can be evaluated using Eq. (2). The fast Fourier transformed (FFT) heterodyne signal spectrum of a target located at a 3 m distance is shown in Fig. 6 as an example. The maximum frequency component, ${f_B}$, was at 900 Hz. By using Eq. (2), the FMCW LiDAR measured distance could be evaluated as 3 m which agreed with the test target location. By changing the distance of the target, the FMCW measured distance matched well with the nominal target distance as shown in Fig. 7. The measured distance uncertainty was about 5 cm obtained by multiple measurements. Since the DC signal was kept, the visibility of the heterodyne signal at a different distance could also be evaluated as shown in Fig. 8. The visibility dropped rapidly when the distance exceeds 8.5 m which agreed with the 16.4 m coherence length of the linear ECDL laser linewidth estimated by the scanning Fabry-Perot interferometer measurement. The reason the visibility did not reach 1 at a short target distance was due to the uneven power ratio between the test arm and reference arm.
4. Conclusion
A low-cost PQ:PMMA VBG with Bragg wavelength of 1060.7 nm and diffraction efficiency of 8.2% was fabricated and served as the wavelength-selection output coupler of a 100 mW frequency-swept linear ECDL laser. The laser output reached a single longitudinal mode with a coherence length exceeds 16 m. The mode-hop free frequency sweeping range could reach 0.28 GHz. With this laser source, an FMCW LiDAR system was demonstrated and successfully measured target with a distance of less than 9 m with good linearity.
Funding
Ministry of Science and Technology, Taiwan (MOST 108-2221-E-008-085-MY3).
Acknowledgment
The author would like to thank Prof. Lin, Shiuan-Huei for his generous help and valuable discussion.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
References
1. M. C. Amann, T. Bosch, M. Lescure, R. Myllyla, and M. Rioux, “Laser ranging: a critical review of usual techniques for distance measurement,” Opt. Eng. 40(1), 10–19 (2001). [CrossRef]
2. G. Berkovic and E. Shafir, “Optical methods for distance and displacement measurements,” Adv. Opt. Photonics 4(4), 441–471 (2012). [CrossRef]
3. E. Shafir and G. Berkovic, “Compact fibre optic probe for simultaneous distance and velocity determination,” Meas. Sci. Technol. 12, 943–947 (2001). [CrossRef]
4. H. J. Yang, J. Deibel, S. Nyberg, and K. Riles, “High-precision absolute distance and vibration measurement with frequency scanned interferometry,” Appl. Opt. 44, 3937–3944 (2005). [CrossRef]
5. B. Behroozpour, P. A. M. Sandborn, M. C. Wu, and B. E. Boser, “Lidar System Architectures and Circuits,” IEEE Commun. Mag. 55(10), 135–142 (2017). [CrossRef]
6. Y. Li and J. Ibanez-Guzman, “Lidar for autonomous driving: The principles, challenges, and trends for automotive lidar and perception systems,” IEEE Sig. Process. Mag. 37, 50–61 (2020). [CrossRef]
7. N. Satyan, A. Vasilyev, G. Rakuljic, V. Leyva, and A. Yariv, “Precise control of broadband frequency chirps using optoelectronic feedback,” Opt. Express 17(18), 15991–15999 (2009). [CrossRef]
8. K. Iiyama, S. Matsui, T. Kobayashi, and T. Maruyama, “High-Resolution FMCW Reflectometry Using a Single-Mode Vertical-Cavity Surface-Emitting Laser,” IEEE Photonics Technol. Lett. 23(11), 703–705 (2011). [CrossRef]
9. S. F. Collins, W. X. Huang, M. M. Murphy, K. T. V. Grattan, and A. W. Palmer, “A Simple Laser-Diode Ranging Scheme Using an Intensity-Modulated Fmcw Approach,” Meas. Sci. Technol. 4(12), 1437–1439 (1993). [CrossRef]
10. T. Y. Chung, A. Rapaport, V. Smirnov, L. B. Glebov, M. C. Richardson, and M. Bass, “Solid-state laser spectral narrowing using a volumetric photothermal refractive Bragg grating cavity mirror,” Opt. Lett. 31(2), 229–231 (2006). [CrossRef]
11. B. L. Volodin, S. V. Dolgy, E. D. Melnik, E. Downs, J. Shaw, and V. S. Ban, “Wavelength stabilization and spectrum narrowing of high-power multimode laser diodes and arrays by use of volume Bragg gratings,” Opt. Lett. 29(16), 1891–1893 (2004). [CrossRef]
12. G. B. Venus, A. Sevian, V. I. Smirnov, and L. B. Glebov, “High-brightness narrow-line laser diode source with volume Bragg-grating feedback,” in High-Power Diode Laser Technology and Applications III, Bellingham, WA, USA, (2005), pp. 166–176.
13. A. Gourevitch, G. Venus, V. Smirnov, and L. Glebov, “Efficient pumping of Rb vapor by high-power volume Bragg diode laser,” Opt. Lett. 32(17), 2611–2613 (2007). [CrossRef]
14. P. Crump, C. Schultz, A. Pietrzak, S. Knigge, O. Brox, A. Maaßdorf, F. Bugge, H. Wenzel, and G. Erbert, “975-nm high-power broad area diode lasers optimized for narrow spectral linewidth applications,” Proc. SPIE 7583, 75830N2010. [CrossRef]
15. S. Heinemann, H. Fritsche, B. Kruschke, T. Schmidt, and W. Gries, “Compact high brightness diode laser emitting 500W from a 100μm fiber,” Proc. SPIE 8605, 86050Q (2013). [CrossRef]
16. T. Y. Chung, W. T. Hsu, Y. H. Hsieh, and B. J. Shin, “Recording 2nd order PQ:PMMA reflective VBG for diode laser output spectrum narrowing,” Opt. Express 27(6), 8258–8266 (2019). [CrossRef]
17. W. Koechner, Solid-State Laser Engineering1, 5th rev. and updated ed. (Springer, 1999).
18. J. E. Castillo, J. M. Russo, and R. K. Kostuk, “Response of axially stressed edge-illuminated holographic gratings in PQ/PMMA photopolymers,” IEEE Photonics Technol. Lett. 20(13), 1199–1201 (2008). [CrossRef]
19. S. H. Lin, P. L. Chen, Y. N. Hsiao, and W. T. Whang, “Fabrication and characterization of poly(methyl methacrylate) photopolymer doped with 9,10-phenanthrenequinone (PQ) based derivatives for volume holographic data storage,” Opt. Commun. 281(4), 559–566 (2008). [CrossRef]
20. P. Liu, F. W. Chang, Y. Zhao, Z. R. Li, and X. D. Sun, “Ultrafast volume holographic storage on PQ/PMMA photopolymers with nanosecond pulsed exposures,” Opt. Express 26(2), 1072–1082 (2018). [CrossRef]
21. B.-J. Shih, C.-W. Chen, Y.-H. Hsieh, T.-Y. Chung, and S.-H. Lin, “Modeling the diffraction efficiency of reflective-type PQ-PMMA VBG using simplified rate equations,” IEEE Photon. J. , 10, 6101307 (2018). [CrossRef]
22. Y.-H. Hsieh, T.-Y. Chung, L.-C. Du, and S.-H. Lin, “A high power, narrow linewidth and wavelength tunable semiconductor laser using a photopolymer Bragg grating as one of the cavity mirrors,” presented at Frontiers in Optics 2014/Laser Science XXVII, Tucson, AZ, USA, 2014.
