Setup of the four-wavelength Doppler lidar system with feedback controlled pulse shaping

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1 Setup of the four-wavelength Doppler lidar system with feedback controlled pulse shaping Albert Töws and Alfred Kurtz Cologne University of Applied Sciences Steinmüllerallee 1, Gummersbach, Germany Abstract: An all-fiber four-wavelength coherent Doppler lidar system in MOPA configuration with feedback controlled pulse shaping was developed. Four cw-lasers tuned to wavelengths of the ITU-grid near 1.55 µm are multiplexed and guided to the detector unit to serve as local oscillators (LO), and to the pulse-shaping unit. In this unit, the demultiplexed cw-wavelengths are individually modulated and collectively frequency shifted. The pulses seed a three-stage erbium-doped fiber amplifier. The feedback controlled pulse-shape unit (FCPS) controls pulse energy and the shape of the amplified pulses, and detects the onset of the back propagating SBS-wave. Application of FCPS results in a higher output energy, because of optimized pulse shapes and long term stability. The received multi-wavelength lidar signals are demultiplexed and mixed with the corresponding LO on a balanced detector and digitized. The data processing is performed by using the graphical processor of a PC. Keywords: all-fiber coherent lidar, multi-wavelength, multi-channel, pulse-shape control 1. Introduction Using fiber amplifiers in all-fiber coherent Doppler lidar systems has become more attractive due to many advantages in terms of stability under vibrations, maintenance, and compactness. The main pulse energy limiting effect of those fiber amplifiers is the stimulated Brillouin scattering (SBS). Due to the large spike on the leading edge of the output pulse, the SBS threshold is reached at very low possible pulse energy. Therefore, an adapted shape of the seed pulse is necessary to prevent SBS at very low energy. [1, 2] One important advantage of fiber amplifiers is that the output shape can be affected by the shape of the seed pulse. In this work, we present some properties of the four-wavelength Doppler lidar system [3] and the improved pulse-shape feedback control technique [4] which is capable of adjusting arbitrary pulse shapes for each channel of this lidar system. 2. Methodology The four-channel coherent Doppler lidar system with the main components is depicted in figure 1. This lidar system mainly consists of five sub-assemblies: the master oscillator unit, the pulse-shape control unit, the amplifier and transceiver unit, the detector unit, and the signal processing unit. All components of this lidar system are polarization maintaining and fiber-based. Four external cavity diode laser modules (LD) generate continuous-wave laser light in the master oscillator unit for each channel. The wavelengths are selected near 1.55 µm for low extinction in the atmosphere. The chosen channels from the ITU-grid (ITU: International Telecommunication Union) are channel 27, 31, 34, and 36. The laser modules parameters, such as temperature and laser diode current, were adjusted for highest side-mode suppression. All channels are simultaneously amplified in one erbium-doped fiber amplifier (EDFA), where the pump power is shared among those channels. However, the strongest channel with the highest gain becomes even stronger while passing through the fiber amplifier, reducing the gain of weaker channels. Due to those aspects, electronically changeable variable attenuators (VOA) are used to CLRC 2016, June 26 July 1 1

2 compensate those changes in intensity. The VOAs are feedback controlled by the local oscillator control unit (LOC). This control system balances the channel power and sets the local power to the maximum efficiency on power penalty (1.6 mw). The local power of every channel is measured, using the monitor outputs of the balanced detectors (BD) as the actual value. All four channels are combined by a wavelength division multiplexer (WDM) and amplified by one EDFA to a total power of 200 mw. A part of that laser light is used as the local oscillator and 80 % of the master oscillator are guided to the pulse-shape control unit. Figure 1. Setup of the multi-channel coherent Doppler lidar system. In the pulse-shape control unit the laser light is demultiplexed, in order to separately shape the pulses of each channel with an electro-optic modulator (EOM). The time scale of every channel has a resolution of 10 bit with a sampling rate of 100 MS/s. Within this time range of μs, arbitrary pulse shapes and sequences can be independently generated on every channel. The pulse amplitude of each channel has a CLRC 2016, June 26 July 1 2

3 dynamic range of 8 bit, in which a value of 0 corresponds to maximum attenuation within the limits of those EOMs (30-35 db). A value of 255 sets the amplitude to minimum attenuation, which corresponds to the insertion loss of such EOMs (~6 db). Then, all pulses are simultaneously frequency shifted by an acoustooptic modulator (AOM) to enable positive and negative radial wind velocity measurements. Using both modulators for pulse shaping, the minimum needed extinction ratio of 85 db is achieved to prevent additional peaks in the Doppler spectrum. The pulses are additionally shaped by this AOM with a 10 bit dynamic range to compensate the pulse distortion due to gain saturation of the following three EDFAs. The insertion loss of this AOM is 4 db, resulting in a 12 db overall insertion loss of the pulse shaping unit, including the loss of the WDMs. The pulse-shape control unit is configured as the master trigger of the lidar system, where the PRF can be set to a value between 100 Hz and 90 khz. The weak pulses are amplified to a peak power of around 2 W by means of a two-stage standard EDFA in order to seed the LMA EDFA. To monitor the pulses to be transmitted, a coupler tapes around 0.5 % of the output pulse power. After attenuation, the wavelength-channels are demultiplexed and four photo diodes (PD) measure the shape of the pulses for each channel separately. The pulses are digitized within the feedback control unit (FCU) with a sampling rate of 100 MS/s. This unit controls the EOMs, the AOM, and the third EDFA stage. For additional shaping with the AOM, which compensates the gain saturation of the three-stage fiber amplifier, a constant second degree polynomial function is applied to the driver voltage. The set values of the EOMs and the EDFA are controlled by the feedback control unit, as shown in figure 2. Figure 2. Schematic of the feedback controlled pulse shaping. The pulse-shape control system consists of two nested controller loops. The EOM control loop controls the amplitude values of each sample point of the pulse and the EDFA control loop controls the overall pulse energy via the pump power. Starting the control system, the EDFA is turned on at low pump power, the EOMs provide a squared shaped pulse on every channel, and the AOM shapes those pulses. Each pulse shape is measured and digitized with M sampling points. The measured actual pulse shapes are converted (A C) and each sampling point is compared to the set value of the target pulse shape. The resulting M control errors are sent through a PI controller. The control signals of all sampling points per channel are filtered to prevent oscillation due to clock jitter. The corrected M values are normalized to the maximum amplitude of all channels within the limits of the EOMs. This ensures maximized seed power for the amplifier chain, because at least one set point of the EOMs is 255. Those corrected values are converted to a voltage and applied to the EOM driver. The actual value of the EDFA control loop is the measured pulse energy, which is proportional to the sum of the sampling amplitudes of all channels. This actual value is compared to the CLRC 2016, June 26 July 1 3

4 set value, which is often the SBS limited pulse energy. The control error is sent through a PI controller and the resulting control signal is limited to the lower and upper limit. The range limited control signal is converted to a voltage, which is applied to the pump power of the EDFA. Via a circulator (CI) the amplified and feedback controlled pulses are directed to the telescope (TC). The polarization sensitive transceiver is realized by a quarter-wave plate and a polarization beam splitter (PBS). With this configuration a transmit pulse isolation of ~80 db is achieved at the expense of a higher loss of the returned signal of about 0.4 db. The backscattered light is guided to the detection unit, where the wavelengths of the local oscillator and the backscattered light are demultiplexed, thus, every channel can be mixed separately onto its balanced detector (BD). The amplified differential signal eliminates the DC and amplifies the AC component of the heterodyne signal. After analog signal processing, the heterodyne signal is digitized and the raw data are sent to the personal computer (PC). An algorithm on the graphical processor extracts the information on wind velocity and signal strength in real time. 3. Results The multi-channel lidar system is capable of controlling arbitrary pulse shapes and sequences. To show one possible example, figure 3 depicts four shifted and shaped pulses, which are simultaneously amplified in one EDFA with a PRF of 10 khz. Since this lidar system runs with four channels in parallel, the effective PRF is 40 khz with the corresponding range ambiguity to 10 khz of 15 km. The advantages of shifting the pulses by 25 µs, compared to the amplification without shifting, are the reduced amplified spontaneous emission (ASE) in the power amplifier and the independent SBS threshold for each channel. At the beginning of the pulse the SBS threshold is higher than at the end of the pulse, which was measured with a SBS detection module. Therefore, it is suitable to create pulse shapes which are higher on the leading edge. We investigated that for pulse durations between 100 ns and 1 µs a ratio of 0.7 is ideal for maximum pulse energy. The possible improvement in pulse energy is about 10 % compared to squared shaped pulses. Figure 3. Measurement of the simultaneously amplified pulse train with energy optimized pulse shapes. The wavelength-channel dependent SBS thresholds for a standard and a LMA EDFA are listed in table 1. Those values correspond to a pulse duration of 300 ns. The SBS threshold increases with longer wavelength due to the lower SBS gain coefficient compared to a shorter wavelength [5]. Therefore, the possible peak power is highest for channel 27 and lowest for channel 36 for both types of amplifiers. During the start-up and due to changes in the ambient temperature, the gain of an EDFA varies until the thermal equilibrium is reached. During that time the averaged output power of the fiber amplifier is not stable at constant pump power. In consequence, those systems have to operate below the SBS threshold. CLRC 2016, June 26 July 1 4

5 Those downsides can be compensated by using this pulse-shape control system, which ensures a reliable and stable long term operation close to the SBS limit. Table 1. SBS limited peak power of the feedback controlled amplified pulses. channel Standard EDFA LMA 1 st Gen. LMA 2 nd Gen. CH nm 7.7 W 88 W CH nm 8.8 W 92 W CH nm 9.1 W 115 W ~ 350 W CH nm 10.9 W 128 W 4. Conclusion and future work We present a feedback control technique which is capable of adjusting any given pulse shape for each channel of a four-wavelength Doppler lidar system. This control system enhances the performance and the long term stability of coherent Doppler lidar systems. In future we will apply this control system to the 2 nd generation LMA EDFA, where a peak value of around 350 W is expected. 5. References [1] S. Kameyama, T. Ando, K. Asaka, Y. Hirano, and S. Wadaka, Compact all-fiber pulsed coherent Doppler lidar system for wind sensing, Appl. Opt., 46, (2007). [2] N. S. Prasad, R. Sibell, S. Vetorino, R. Higgins, and A. Tracy. An all-fiber, modular, compact wind lidar for wind sensing and wake vortex applications, in Laser Radar Technology and Applications XX; and Atmospheric Propagation XII, (Society of Photo-Optical Instrumentation Engineers, Baltimore, 2015), 94650C. [3] A. Töws and A. Kurtz, A multi-wavelength LIDAR system based on an erbium-doped fiber MOPA-system, in Lidar Technologies, Techniques, and Measurements for Atmospheric Remote Sensing X, (Society of Photo-Optical Instrumentation Engineers, Amsterdam, 2014), 92460T. [4] A. Töws and A. Kurtz, Pulse-shape control in an all-fiber multi-wavelength Doppler lidar, (27 th International Laser Radar Conference, New York, 2015). [5] L.V. Kotov, A. Töws, A. Kurtz, K.K. Bobkov, S.S. Aleshkina, M.M. Bubnov, D.S. Lipatov, A.N. Guryanov, and M. Likhachev, Fiber Lasers XIII: Technology, Systems, and Applications, in Fiber Lasers XIII: Technology, Systems, and Applications, (Society of Photo-Optical Instrumentation Engineers, San Francisco, 2016), 97282U. CLRC 2016, June 26 July 1 5