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Abstract
Solid-state single-frequency lasers around 1.6 µm are ideal sources for coherent Doppler wind lidars (CDWLs). A CDWL system with 1645 nm sing-frequency, injection-seeded Er:YAG ceramic laser is demonstrated. The Er:YAG laser based on an “M-shaped” ring resonator operates at pulse repetition frequencies (PRFs) of 300–1000 Hz at room temperature. The maximum single-frequency output energy is 10.1 mJ with a pulse width of 179 ns at 300 Hz. The 1645 nm Er:YAG laser is first used in a long-range CDWL system, and a line of sight (LOS) wind velocity up to 25 km is detected with 90 m range resolution in 0.5 s observation. To verify the reliability of the measurement results, the relationship between detection range, pulse energy, and accumulated numbers is also demonstrated.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Long-range detection with coherent Doppler lidars (CDLs) is attractive in aerosol detection, wind velocity measurements, and target identification. Recently, coherent Doppler wind lidars (CDWLs) have attracted more attentions because it is widely used in aviation safety [1], wind power forecasting [2], and meteorological research [3]. The CDWLs have been widely investigated since the 1970s [4]. The laser sources for CDWLs can be divided into three stages: 10.6 µm CO2 gas lasers [5,6], 1.06 µm Nd:YAG solid-state lasers [7,8] and 1.5-1.6 µm [9,10], 2.1 µm eye-safe lasers [11].
Eye-safe lasers with high atmospheric transmittance are ideal sources for CDWLs. Figure 1 shows the atmospheric transmittance and maximum permissible exposure (MPE) of near-infrared lasers from 1.4 to 2.2 µm. It can be seen that the wavelength of 1.5 µm, 1.6 µm, and 2.1 µm all have high transmittance, but the MPE of 1.5 µm and 1.6 µm is ten times higher than 2.1 µm. Nowadays, the eye-safe CDWLs which realize more than 10 km measurement range develop rapidly. Lockheed Martin Coherent Technologies (LMCT) WindTracer has devoted to the research of CDWLs over the past 30 years. In 2005, a 2 µm CDWL with LD pumped Tm:LuAG laser was reported [12]. The longest range of 12 km was demonstrated with a pulse energy of 2 mJ and a PRF of 500 Hz, and the diameter of the telescope was 10 cm. In 2007, WindTracer firstly demonstrated a 1617 nm Er:YAG CDWL, producing 2.3 mJ pulses at 750 Hz [10]. The longest measurement range was 33 km with a 12.5 cm telescope, and the typical range was 300 m to 15 km. Besides, the Mitsubishi Electric Corporation has developed the 1.5 µm long-range CWDLs. In 2001, Mitsubishi firstly reported the frequency-stabilized eye-safe laser of 1.5 µm for CDL with diode-pumped Er,Yb:glass [13]. The injection-seeded energy of 10.9 mJ at PRF of 15 Hz was obtained. In the same year, they developed a 1.5 µm eye-safe CDWL using the Er,Yb:glass laser source [14]. They finally realized high-average and high-peak-power light source at 1.5 µm by invention of Er,Yb:glass planar waveguide amplifier, of which the pulse repetition rate and pulse width could be easily fixed. In 2012, they reported a 1.55 µm CDWL using Er,Yb:glass planar waveguide as a laser amplifier producing 1.4 mJ pulses at 4 kHz [15]. The measurement range of more than 30 km was demonstrated with a 150 mm telescope. In 2019, they further improved the magnification capability of the planar waveguide [9]. The pulse energy of 3.2 mJ is achieved at a PRF of 4 kHz. National Institute of Information and Communications Technology (NICT) also reported their 2 µm CDWLs. In 2018, they firstly reported a 2 µm Ho:YLF CDWL, the LOS wind velocity up to 15 km was detected at a pulse energy of 12 mJ and a PRF of 300 Hz with a 100 mm telescope in 1 s observation time [11]. However, the 1.5 µm planar waveguide has a complex structure and needs advanced manufacturing techniques, and the detectors at 2 µm are generally not as sensitive as that in 1.5-1.6 µm. So the 1.6 µm Er:YAG laser sources are another ideal choice for CDWLs. Low Erbium-doped gain mediums could work at this wavelength range, such as Er:YAG has strong emission lines at 1617 and 1645 nm, and lasers working at 1645 nm are much easier to work stably at room temperature. As far as our knowledge, the 1645 nm Er:YAG long-range CDWL has not been reported yet.
Fig. 1. The atmospheric transmittance (Mid Altitude Summer, Rural-VIS=23 km) and human maximum permissible exposure (MPE) of near-infrared lasers.
The 1645 nm single-frequency laser with narrow linewidth approaching the Fourier diffraction limit and relatively long pulse width for CDWLs can be obtained by injection locking technology. In 2019, a single-frequency Er:YAG laser with 5.52 mJ pulse energy and 500 ns pulse width at 1 kHz was reported by Zhang et al. [16]. In the same year, Shi et al. reported an Er:YAG laser adopting a double-crystals-end pumping architecture [17]. The maximum single-frequency output energy is 20.3 mJ with 110 ns pulse width at 200 Hz.
In this paper, a CDWL system based on a home-made 1645 nm single-frequency, Q-switched Er:YAG ceramic laser is developed. The maximum pulse energies of the laser are up to 10.1 mJ, 9.1 mJ, and 6.6 mJ at the PRFs of 300 Hz, 500 Hz, and 1 kHz, respectively. The measurement range capacity of the CDWL system is simulated through the lidar equation. The LOS wind measurements are performed with a pulse energy of 8 mJ at 300 Hz. The wind velocity up to 25 km is detected with 90 m range resolution in 0.5 s observation. To verify the reliability of measurement results, the relationship between detection range, pulse energy, and accumulated numbers is demonstrated. To the best of our knowledge, this is the first report of the long-range CDWL of more than 20 km utilizing a 1645 nm solid-state laser as the light source.
2. Experimental setup
A CDWL system using a 1645 nm Er:YAG ring laser is shown in Fig. 2. The main parts of the system are as follows: a seed laser, a slave laser, a control system, a data processing system, and transceiver optics.
Fig. 2. The schematic diagram of the CDWL system. LD, laser diode; NPRO, nonplanar ring oscillators; HWP, half waveplate; QWP, quarter waveplate; PBS, polarizing beam splitter; PZT, piezoelectric transducer; AOM, acousto-optic modulator; PD, photodiode; FPGA, field-programmable gate array.
The 1645 nm single-frequency Er:YAG NPRO is used as the seed laser. The NPRO laser which has the advantages of narrow linewidth and high stability is the ideal seed laser for injection-seeded system. The temperature of the NPRO is maintained at 18 °C by a thermoelectric cooler (TEC). Its linewidth is less than 8.8 kHz, measured by the delay self-heterodyne method [18]. The continuous-wave laser is divided into two parts by a half waveplate (HWP) and a polarizing beam splitter (PBS). One with p-polarization is frequency shifted of 68 MHz by an acousto-optic modulator (AOM) for the injection seeding, the other with s-polarization is utilized as a local oscillator (LO) for lidar detection and central frequency monitor.
An ‘M-shaped’ ring cavity is designed as the slave laser. With a ring cavity, the spatial hole-burning is avoided [11]. For the CDWL applications, the relatively long pulse width is essential to enhance the velocity accuracy. Thus, a folded cavity with a total length of 1.2 m is adopted to achieve long pulses. The ring resonator contains three flat mirrors (M1, M3 and M5) and two curved mirrors (M2 and M4). A piezoelectric transducer (PZT) mounted to M3 is used to adjust the cavity length. The output coupler M4 is coated with high reflectivity of 80% at 1645 nm. A 1532 nm fiber laser (ELR-30-1532-LP, IPG Photonics) is used as the pump source. The gain medium is a Ф 4 mm×60 mm Er:YAG ceramic with 0.25 at.% Er-doping concentration, which is conductively cooled at 18 °C. Both surfaces of the rod are anti-reflection coated at 1532 and 1645 nm. The pump beam is focused by lens f3 to 840 µm in diameter in the center of the rod. An AOM is employed to produce the pulsed operation.
Ramp-Fire (RF) technique is applied in the injection-seeded process [19]. The seed laser is injected into the slave laser from the first AOM diffraction order when the RF power is applied. Lens f2 is set to realize the spatially mode-matched between the seed laser and the slave laser. The cavity length is ramped by an HR mirror M3 glued to a PZT. The leaking resonance signal is detected by a photodiode (PD). The AOM Q-switch is triggered when the maximum resonance signal is detected, which means the resonant matching between the seed laser and the slave laser is achieved. In this case, the single-frequency Q-switched pulses are obtained.
The optical transceiver system is designed in free space. PBS1 is used to improve the polarization purity of the laser pulses. The beam expander with a beam expansion ratio of 1:2.5 is employed to expand the laser beam to about 4 mm. A coupler behind M13 is placed to collect the leaking light to monitor the central frequency of each pulse by a balanced detector. A PBS and a quarter waveplate (QWP) are utilized to compose a circulator to separate transmission light and backscattered light. Off-axis parabolic mirrors with a diameter of 10 cm are employed as a telescope. The diameter of the laser output from the telescope is 8 cm. A 45° mirror (M16) with a high precision surface is placed to reflect the collimated laser to the air with an elevation angle of about 2.4 deg. The focal length of the collimated laser can be adjusted by changing the distance precisely between the two lenses of the beam expander.
The backscattered signal from the aerosols is gathered by the same telescope and focused into a polarization-maintaining (PM) fiber by an aspheric lens with a focus of 18.4 mm, and it is mixed with the local signal in a 2×2 PM fiber coupler with a 50:50 coupling ratio. The mixed signals are detected by a balanced detector with a bandwidth of 200 MHz, and digitized on a 14-bit 400 MHz analog to digital (AD) card and processed by the FPGA (Field Programmable Gate Array). Because of the dither of cavity length, the frequency jitter is inevitable. The improvement of signal-to-noise ratio (SNR) and the accuracy of measurement will be reduced with the accumulation of pulses. Therefore, the central frequency of each pulse is corrected according to the monitor heterodyne signal which is also connected to the FPGA. Because of the limits of the FPGA resource utilization, the central frequencies of the injection-seeded pulses from 56 MHz to 80 MHz are chosen to be corrected, and the frequency scope is enough for our laser system. The control system triggers the FPGA when the AOM driver is triggered.
3. Results and discussion
3.1 Results for laser system
The 1645 nm single-frequency pulses are acquired when the seed laser is injected into the slave laser. The characteristics of output pulses at PRFs of 300-1000 Hz are shown in Fig. 3. The pulse energies decrease from 10.1 mJ to 6.6 mJ when the PRFs increase from 300 Hz to 1000 Hz. Correspondingly, the pulse widths rise from 179 ns to 271 ns.
Fig. 3. (a) Single-frequency pulse energies at PRFs from 300 Hz to 1 kHz versus the pump power. (b) Single-frequency pulse widths at PRFs from 300 Hz to 1 kHz versus the pump power.
The linewidth of the pulses is measured through the beating signal of pulses and the seed laser under the maximum pump power at 300 Hz. The measured linewidth is 2.82 MHz, which is 1.14 times Fourier-transform-limited. The M2 factors of the oscillator at 300 Hz are measured to be about 1.51 and 1.55 in horizontal and vertical directions, respectively. Time-domain data of each pulse is collected by a high-speed data acquisition card (Agilent U5303A), then the central frequencies are obtained through FFT and Gaussian fitting. Figure 4 shows the short-time (6s) frequency stability of the injection-seeded laser pulses at 300 Hz. The mean value is 71.57 MHz, while the root-mean-square (RMS) error is 1.97%. The bias of the central frequency between the slave laser and the seed laser is caused by the ramp-fire process because there is a delay from the control system finding the maximum resonance signal to trigger the AOM driver. The frequency jitter is resulted from the dither of cavity length and other disturbance [20].
3.2 Detection range estimation
For short-noised-limited coherent detection, the signal-to-noise ratio (SNR) of a pulsed system can be expressed as [21,22]
where λ is the laser wavelength, η is the total efficiency, E is the transmitting pulse energy, β is the backscatter coefficient, T is the one-way atmospheric transmittance, Aeff is the effective telescope area, h is the Planck’s constant, R is the range, and B = 1/τ is the receiver noise-equivalent bandwidth with matched filter assumption [23], where τ is the pulse duration.The Detectability (The ratio of the signal height above the noise floor to the noise standard deviation), which is used to evaluate the maximum measurement range, is expressed as [24]
With the presence of atmospheric turbulence, the effective telescope area Aeff is given as [22,25]
Based on the Mie scattering theory at the U.S. Standard Atmosphere, the backscatter coefficient β can be expressed as [28]
The atmospheric transmittance of distance R is given by [29]
For the wavelength of 1645 nm in sunny weather conditions, the values of a, b, c are 0, 0.16337 and 1.77257, respectively.The total efficiency η, including the system efficiency ηs, the antenna efficiency ηa, and the collimation efficiency ηc can be expressed as
where ηs is about 52.5% of the lidar system. ηa is about 40.1%, and ηc is about 90.0% [24].The rest of the parameters in the lidar equation is given in Table 1. Figure 5 shows the simulation of the detectability depends on the range of this CDWL system. The simulated detection range of the LOS wind velocity is about 25 km when the detectability is above 3 dB.
Table 1. The parameters of the CDWL system in detection range estimation
The simulation result is based on the Mie scattering theory at the U.S. Standard Atmosphere in the sunny weather with weak-turbulence conditions. The model ensures the detection ability of the CWDL system, but the actual measurement results will be affected by the weather and geographical location.
3.3. Results for wind sensing
The wind measurements are performed with 8 mJ pulses at 300 Hz. The performances of CDWL for wind sensing are evaluated. The dates reported in this paper are July 11th, 2019 (Figs. 6–9.), and June 14th, 2019 (Fig. 10). The CDWL system locates in Beijing Institute of Technology, Beijing, China.
Fig. 7. The power spectrum and Gaussian fitting of the beating signal at different range gates: (a) 60th range gate. (b) 120th range gate. (c) 180th range gate. (d) 240th range gate.
Fig. 9. (a) The maximum measurement range versus the pulse energy. (b) The maximum measurement versus the accumulated number of pulses.
The detectability depends on the range and LOS wind velocity near the horizontal direction (with an elevation angle of 2.4 deg) is shown in Fig. 6. The duration of the range gate is 600 ns, corresponding to 90 m range resolution. The accumulated pulses are 150 for each wind data, which means the accumulated time is 0.5 s. Gaussian fitting and peak recognition algorithms are used for data processing. The detectability, which is denoted in the vertical axis, is the ratio of the signal height above the noise floor to the noise standard deviation. The detectability of 3 dB is set as the detection line, which empirically corresponds to the signal detection probability of 90%. It is shown that there is no random outlier in LOS wind velocity and the detectability is above 3 dB before 25 km. In this case, the measurement range of about 25 km is confirmed.
Figure 7 shows the power spectrum and Gaussian fitting of the beating signal at different range gates with an accumulation of 150 pulses. The spectrum data was selected from 40 MHz to 100 MHz because a band-pass filter (70 MHz±30 MHz) is used before the FPGA, which corresponding to a maximum detectable wind velocity of about ±25 m/s. It is shown that the intensity of the signal decreases as the distance increases. The ratio of the signal peak to the noise average is 6.08 dB at the 60th range gate (5.4 km) and decreases to 1.27 dB at the 240th range gate (21.6 km). The LOS wind velocity can be acquired from the Doppler shift of each range gate.
The reliability of the measurement results is also confirmed. 20 measuring results are plotted in Fig. 8. at the output energy of 8 mJ. The slight fluctuation of the detection range is due to the change of the environment and the statistical fluctuation characteristics of the noise itself. Figure 8 shows that most of the results are more than 22 km (90%), and the maximum measurement range is over 25 km. The consistency of wind velocity confirms stable measurements.
According to Eq. (2), the maximum measurement range is affected by the pulse energy and accumulated number. Firstly, the relationship between the detection range and pulse energy is demonstrated. An HWP and a PBS are used in front of the beam expander to change the emission energies, and the pulse energies are detected in front of the telescope. The range gate of 90 m, accumulated time of 0.5 s and the local signal power of 1.7 mW remain unchanged. The detection ranges of LOS wind velocities increase from 7.68 km to 23.94 km while the pulse energies increase from 0.5 mJ to 7 mJ. Each data is an average result of ten measurements. Figure 9(a) shows the relationship between measuring ranges and the pulse energies.
Next, the accumulated number of pulses is changed while the pulse energy remains at 7 mJ. The detection ranges increase from 17.64 km to 24.96 km, while the accumulated number increases from 30 to 150. The relationship between the measuring ranges and the accumulated numbers are depicted in Fig. 9(b). The results mean that there is also ability to enhance the measurement range by increasing the pulse energy and the accumulated number.
When the elevation angle of the 45° mirror is about 0.5 degrees, an SNR peak is observed at about 1.3 km, where the laser pulses shot on a building. The expected wind velocity is 0 m/s at the building location distance. Figure 10 shows the histogram of 200 measuring velocities of building from 150 shots (0.5 s) accumulation. The median speed is -0.026 m/s while the interquartile range (IQR) is 0.628 m/s. The accuracy of measurements could be improved by further optimization of the data processing algorithm.
4. Conclusion
A 1645 nm single-frequency, Q-switched Er:YAG ceramic laser with changeable PRFs is demonstrated. The detection range of the CDWL is estimated according to the lidar equation. Then the LOS wind velocity experiments are carried out. With the pulse energy of 8 mJ at 300 Hz, the wind velocity up to 25 km is detected with 90 m range resolution in 0.5 s observation time. To verify the reliability of measurement results, the consistency of wind velocity is verified. The relationship between detection range, pulse energy, and accumulated number is also demonstrated. To the best of our knowledge, the 1645 nm Er:YAG laser is firstly used for a long-range CDWL to detect the wind velocity of more than 20 km, and the results show the highest figure of merit (FOM) for the CDWL in 1.6 µm.
Funding
National Natural Science Foundation of China (61627821).
Disclosures
The authors declare no conflicts of interest.
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