Compact lidar system using laser diode, binary continuous wave power modulation, and an avalanche photodiode-based receiver controlled by a digital signal processor

Compact lidar system using laser diode, binary continuous wave power modulation, and an avalanche photodiode-based receiver controlled by a digital signal processor Antoni Ardanuy Adolfo Comerón Antoni Ardanuy, Adolfo Comerón, “Compact lidar system using laser diode, binary continuous wave power modulation, and an avalanche photodiode-based receiver controlled by a digital signal processor,” Opt. Eng. 57(4), 044104 (2018), doi: 10.1117/1.OE.57.4.044104. Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 4/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use Compact lidar system using laser diode, binary continuous wave power modulation, and an avalanche photodiode-based receiver controlled by a digital signal processor Antoni Ardanuy and Adolfo Comerón* Universitat Politècnica de Catalunya, CommSensLab, Department of Signal Theory and Communications, Barcelona, Spain Abstract. We analyze the practical limits of a lidar system based on the use of a laser diode, random binary continuous wave power modulation, and an avalanche photodiode (APD)-based photereceiver, combined with the control and computing power of the digital signal processors (DSP) currently available. The target is to design a compact portable lidar system made all in semiconductor technology, with a low-power demand and an easy configuration of the system, allowing change in some of its features through software. Unlike many prior works, we emphasize the use of APDs instead of photomultiplier tubes to detect the return signal and the application of the system to measure not only hard targets, but also medium-range aerosols and clouds. We have developed an experimental prototype to evaluate the behavior of the system under different environmental conditions. Experimental results provided by the prototype are presented and discussed. ©The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.OE.57.4.044104] Keywords: lidar; pseudorandom modulation; clouds. Paper 171764 received Nov. 4, 2017; accepted for publication Mar. 28, 2018; published online Apr. 18, 2018. 1 Introduction The application of lidar systems began with the use of high peak-power lasers. Even today most of the lidar systems in use for atmospheric remote sensing are based on this principle. A pulse of light of high peak-power is emitted from a laser, backscattered by atmospheric particulates and molecules, and collected by a telescope, and the return optical power is converted to an electrical signal by a photodetector. The use of high peak-power lasers entails two disadvantages, namely the necessary power to operate them and the lack of eye safety. To overcome these problems, lasers with low peak-power and high pulse-repetition frequencies, in the tens of kHz, along with pulse-return accumulation can be used.1More efficient methods using continuous-wave (CW) low peakpower lasers power-modulated with sequences with properties similar to those of the sequences used in spread-spectrum communication systems2,3have been studied.4–6Detection using a correlation algorithm provides range resolution. Many of the systems described until now using CW laserdiode transmitters employ photomultiplier tubes in the receiver system. However, the fast increase in the performance of digital signal processors (DSP) permits the envisioning of the use of long coding sequences with short processing times—a 200-point correlation using a sequence of 2047 bits takes a maximum time of 400 ms approximately—making practical a low-power consumption, eye-safe, and compact-size all-semiconductor lidar system, with laser-diode transmitters and avalanche photodiodes (APDs) in the receiver system. The system can detect from low to high clouds, but it can be used in the detection of mediumrange aerosol (see Sec. 6). A limitation in these systems arises from the condition to constrain the laser peak power to a maximum value that leads to a reduction in the signal-to-noise ratio (SNR) when the spatial resolution is increased if the laser peak power is kept constant (see Sec. 3). The low-energy consumption makes these systems attractive from the portability point of view. For example, the maximum power necessary for the system presented in this paper is below 10 W considering all the electronic and optoelectronic components. The telescope size would set the final limit to this portability. The use of a DSP permits the implementation of an allprogrammable system, with the possibility of changing the algorithms to detect the signals and the use of long sequences to extract the signal from the noise. A programmable system can adapt its performance to the needs of each moment or each application. Additionally, the use of DSPs allows the synchronization and total control of the transmitted and received signals, which is very important to avoid the degradation of the SNR. The paper is organized as follows: Sec. 2lays out the model of a system using pseudorandom sequences to modulate the power of a laser diode transmitter under the control of a DSP that is also in charge of performing the correlation operations to retrieve the range-resolved atmospheric backscatter. Section 3presents the SNR gain of the system and the SNR at the output. Section 4discusses the basic constraints of the system and the relation between the laser power, the electrical bandwidth, and the SNR. Section 5shows the *Address all correspondence to: Adolfo Comerón, E-mail: [email protected]pc .edu Optical Engineering 044104-1 April 2018 •Vol. 57(4) Optical Engineering 57(4), 044104 (April 2018) Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 4/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use effect of nonlinearities and offsets in the system, which to our knowledge has not been dealt with in previous related literature, and finally Sec. 6shows experimental results from the implemented prototype. Section 7presents conclusions and gives an outlook of possible developments using the discussed lidar architecture. 2 System Modeling The outline of a lidar system, independently of the used technology, is well known. Classically, single pulses are processed. The pulses can be either of high energy at a low repetition frequency or of low energy at a high repetition rate. Accumulating the returned pulses is equivalent to a processing gain. In the case of random binary CW modulation, the processing gain is obtained from the cross correlation of the received sequence with a reference sequence. In this way, the duty cycle, tending to 50%, is higher than in the other lidar systems, and the peak power can be reduced. The received power from a range zin a pulsed lidar is given by the lidar equation7 EQ-TARGET;temp:intralink-;e001;63;520PreceivedðzÞ¼P0AT cτ 2z2βðzÞexp−2Zz 0 αðxÞdx;(1) where P0is the transmitted laser-pulse power, ATis the telescope effective area, cis the speed of light, τis the pulse duration, zis the distance to the scattering atmosphere resolution cell, βis the atmospheric backscatter coefficient (units m−1sr−1), and αis the atmospheric extinction coefficient (units m−1), which is integrated along all the two-way path to the target at distance z. Geometrical overlap effects have been disregarded. For a CW pseudorandom sequence-modulated lidar (or CW-PRN lidar), it is convenient to cast the lidar equation as EQ-TARGET;temp:intralink-;e002;63;366PreceivedðtÞ ¼P0ATZ∞ 0 st−2z cβðzÞ z2exp−2Zz 0 αðxÞdxdz; (2) where P0is the maximum laser power and sðtÞis the powermodulating signal. In our case, sðtÞis a discrete pseudorandom sequence. Note that the integral in the right member of Eq. (2) is essentially a convolution of the sequence sðtÞwith a function hðzÞ¼βðzÞ z2exp½−2∫z 0αðxÞdxthat depends on the state of the atmosphere. We are interested in retrieving the attenuated backscatter βðzÞexp½−2∫z 0αðxÞdx¼z2hðzÞor at least something proportional to it, i.e., the in general so-called range-corrected signal. To evaluate the performance of the proposed CW lidar, we use the model of Fig. 1, where the input (the transmitted sequence) is known and the impulse response hðzÞis the function to evaluate. In this context, anticipating the digital signal processing to be carried out on the signal, the time functions are expressed as sample sequences, and, from now on, a discrete system will be assumed. According to Fig. 1, sðnÞis the transmitted sequence that yields an output xðnÞ after passing through the channel with impulse response hðnÞ. The channel also adds a background radiation that contributes a noise bðnÞ. The received signal is pre-processed by the optoelectronic receiver, physically consisting of a telescope with effective collecting area AT, an APD-based photoreceiver with voltage responsivity RV,andanamplifier with gain g. Inevitably, the receiver also adds some noise represented by wðnÞat the input of the amplifier. The receiver output is sampled and digitized by the analogue-to-digital (A/D) converter, after which the processor produces an estimate of hðnÞbased on the knowledge of sðnÞ. After Fig. 1, the signal reaching the receiver is EQ-TARGET;temp:intralink-;e003;326;487yðnÞ¼P0sðnÞhðnÞþbðnÞþcs;(3) where sðnÞis the sequence modulating the transmitted power, hðnÞis the discrete impulse response of the channel (the atmosphere in this case), the symbol * denotes the convolution operation, bðnÞis the shot noise added by the channel because of the detection of background radiation, and csis the offset value associated to the background radiation. The discretized impulse response hðnÞcan be written from Eq. (2)as EQ-TARGET;temp:intralink-;e004;326;357hðnÞ¼cτbit 2 βcnτbit 2 cnτbit 22e−2Rcnτbit 2 0αðxÞdx;(4) where τbit is the duration of a bit in the sequence sðnÞ. The output of the photoreceiver is EQ-TARGET;temp:intralink-;e005;326;288yD¼fATRVP0½sðnÞhðnÞþbðnÞþwðnÞþcsrgg; (5) where ATis the telescope effective collecting area, Rvis the photodetector responsivity, gis the gain of an amplifier chain accommodating the output amplitude of the photodetector to the input of the A/D converter (accommodation gain), wðnÞ Fig. 1 Equivalent block chain of the lidar system. Optical Engineering 044104-2 April 2018 •Vol. 57(4) Ardanuy and Comerón: Compact lidar system using laser diode, binary continuous wave power. . . Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 4/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use is the noise added by the photoreceiver, including noise added by the photodetector and the amplifier, referred to the amplifier input, and csr is the total offset level due to the background radiation and the photoreceiver electronics referred to the photoreceiver output. If all the noise is included in one term, then EQ-TARGET;temp:intralink-;e006;63;686yD¼gfATRvP0½sðnÞhðnÞþwTðnÞþcsrg;(6) where wTðnÞ¼ATRvbðnÞþwðnÞ. In Secs. 3and 4,we analyze the SNR implied by Eq. (6) and the type of photoreceiver used, and we discuss the system basic constraints. 3 System Processing Gain and Signal-to-Noise Ratio The SNR in a photoreceiver operating in analogue mode can be expressed as8 EQ-TARGET;temp:intralink-;e007;63;555SNR ¼Ps ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PqðPsþPbÞþNEP2B q;(7) where Psand Pbare, respectively, the optical power of the signal and of the background radiation reaching the photodetector, Pq¼2FhcB ηλ , in which Fis the excess noise factor of the photodetector, his the Planck’s constant, cis the speed of light, ηis the photodetector quantum efficiency, λis the wavelength, and Bis the electrical bandwidth of the photoreceiver. NEP denotes the noise equivalent power of the photoreceiver. In the absence of background radiation, the optical signal power at which the operation regime of the photoreceiver changes from signal-shot-noise limited SNR to NEP-limited SNR is EQ-TARGET;temp:intralink-;e008;63;372Psk ¼NEP2ηλ 2Fhc :(8) For the used photoreceiver (see Table 1)NEP ¼ 1.2 10−13 WHz 1∕2, and with the values for its APD, F¼3.45 and η¼0.78 at λ¼785 nm, we find Psk ¼ 6.45 nW. For an atmosphere with typical optical coefficients and smallor medium-sized telescopes, the optical power in the photoreceiver is always well below this value if the transmitter is a laser diode. In this case, Eq. (7) can be approximated by EQ-TARGET;temp:intralink-;e009;63;241SNR ¼Ps NEP ffiffiffiffi B p:(9) Note that background radiation would cause an apparent increase in the NEP. Calling NEP0the effective noise equivalent power with background radiation, we have NEP02¼ NEP2þ2FhcPb ηλ .IfNpulses are accumulated, the resulting signal-to-noise ratio is SNRac ¼ffiffiffiffi N pSNR. Assuming that yQ≈yD—where yQand yDare the signals after and before the A/D converter (see Fig. 1) and yDis given by Eq. (6)—with a good degree of approximation, the processing leading to the recovery of hðnÞis accomplished by correlating yQðnÞwith a suitable signal related to the modulating signal sðnÞ. To retrieve the attenuated backscatter through the estimation of hðnÞ, the autocorrelation of the sequence sðnÞwith that “suitable”signal must be a delta function to avoid a “blurred”profile and the masking of weak returns by strong ones, as would be the case for a cloud behind a weakly scattering aerosol layer. To modulate the laser transmitted power, pseudorandom M-sequences9have the delta-like character of their correlation with the derived sequences EQ-TARGET;temp:intralink-;e010;326;222s0ðnÞ¼2sðnÞ−1;(10) with sðnÞbeing the original sequence made of ones and zeros.4–6,9–11 If such a sequence is assumed in Eq. (3), a delta-function will be obtained after the correlation of sðnÞ with s0ðnÞ, with a maximum value of ðNþ1Þ∕2when the delay is zero and 0 for the rest of the sequence, Nbeing the sequence length.4,5,10 The processor block on Fig. 1will perform the correlation of the A/D converter output with s0ðnÞ for every time shift kto yield EQ-TARGET;temp:intralink-;e011;326;103zpðkÞ¼X N n¼1 yDðnÞs0ðnþkÞ:(11) Table 1 Parameters of the lidar system prototype. Parameters Laser diode peak power 0.125 W Wavelength 785 nm Receiver noise (NEP) (Hamamatsu_C5460SPL5343) 1.2×10−13 WHz −1∕2 Responsivity of the photodetector 1.02 ×107VW −1 APD diameter 3 mm SNR A/D converter 88 dB A-parameter [Eq. (13)] 6 Number of bits of the A/D converter 16 Telescope diameter (effective area) 0.2 m (0.031 m2) Telescope focal ratio 2 Chip duration (adjustable by software) 337 to 675 ns Bandwidth of optical filter 10 nm Transmittance of the narrowband optical filter 0.55 Photoreceiver electrical bandwidth 1.5 MHz Amplifiers based on LMH6624 gain bandwidth 1.5 GHz Range resolution (adjustable by software) 50 to 100 m Sequence length 1023 and 2047 bits Maximum distance (nonambiguous distance) 50 to 200 km Distance between the beam and telescope axes 0.25 m Overlap function Shown in Fig. 7 Optical Engineering 044104-3 April 2018 •Vol. 57(4) Ardanuy and Comerón: Compact lidar system using laser diode, binary continuous wave power. . . Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 4/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use Taking into account Eqs. (5) and (6), one finds EQ-TARGET;temp:intralink-;e012;63;741zpðkÞ≈Nþ1 2gATRvP0hðkÞþgX N n¼1 wTðnÞs0ðnþkÞþgcsr; (12) where PN n¼1s0ðnþkÞ¼1has been used. Clearly, the right side of Eq. (9) above shows that the output of the processor for a shift kcontains an estimate of the atmospheric “impulse response,”in the term SpðkÞ¼ Nþ1 2gATRvP0hðkÞ, affected by noise contained in the term NpðkÞ¼gPN n¼1wTðnÞs0ðnþkÞ. The offset term gcsr can be disregarded. However, the effect of a signal offset in practical systems using incomplete sequences in the correlation will be considered in Sec. 5.2. Assuming the samples of wTðnÞindependent random variables, the SNR in the estimate of hðkÞis, if a number Mof the cyclic sequences is processed and the results accumulated, EQ-TARGET;temp:intralink-;e013;63;542 SNRpðkÞ¼ ffiffiffiffiffi M pðNþ1ÞATB1∕2P0hðkÞ 2ffiffiffiffi N pNEP ≈ffiffiffiffiffiffiffiffi MN pATB1∕2P0hðkÞ 2NEP :(13) Equation (13) assumes the low-signal approximation for the SNR expressed by Eq. (9). 4 System Basic Constraints The resulting SNR will be the approximately the same as if a sequence of length MN was used, as other authors have already shown.4,5,10 The SNR depends on the total integration time rather than on the sequence length. However, using long sequences has an advantage with respect to employing short ones and increasing the accumulation number M because the long sequences increase the unambiguous distance and may have a beneficial effect on the effect of the offset value as discussed in Sec. 5. The unambiguous distance Rua is related to the sequence length through EQ-TARGET;temp:intralink-;e014;63;313Rua ¼cτbitN 2:(14) Therefore, the sequence length must be set to a minimum value Nmin ¼2Rmax∕ðcτbitÞ, where Rmax is the maximum range from which nonnegligible returns are expected. Because the range resolution ΔRis given by ΔR¼cτbit∕2, Nmin can be cast as Nmin ¼Rmax∕ΔR. Values of Nabove Nmin provide a safety margin against unexpectedly high returns (for instance, from clouds) coming from ranges above the nominal one. Assuming that each bit of the modulating sequence is τbit seconds long and the laser power in the ON state is P0, then the energy per ON bit (pulse) is Eb¼P0τbit. Using the parameters of the system described in Sec. 6, with a laser giving a peak power P0¼125 mW, and τbit ¼655 ns, the transmitted energy per pulse is 8.19 ×10−8J. The most important constraint in this system is the lowenergy per transmitted pulse. A transmitter figure of merit, related to the capability of achieving a fixed SNR in a given accumulation time, can be defined as Fme ¼Effiffiffiffiffiffiffiffiffi PRF p, where Eis the energy per pulse and PRF is the effective pulse repetition frequency. In our system, the effective pulse repetition frequency is 1∕ð2τbitÞ. Taking into account that the energy and the spatial resolution are related by Eb¼P0τbit ¼2P0ΔR∕c, where ΔR¼cτbit∕2is the spatial resolution, the merit factor can be written as EQ-TARGET;temp:intralink-;e015;326;697Fme ¼P0ffiffiffiffiffiffiffi ΔR c r:(15) This relationship shows that if the laser power is divided by a given value, to maintain the figure of merit (hence a given SNR if everything else remains unchanged) the spatial resolution must be reduced by the square of this value. This is a handicap in aerosol detection, especially for long-range detection, yet the use of a low peak-power laser is a practical option to increase the life and reduce the cost of the laser component, in addition to increasing eye safety. 5 Effect of Nonlinearity and Offset on the Correlation In this section, we demonstrate the detrimental effects of receiver nonlinearity in this type of system, which does not seem to have been described in related previous literature. 5.1 Nonlinear Effects In this type of lidar system, when very low signal is received from the atmosphere, the input yDof the A/D converter is largely dominated by noise. Even so, if the gain in the receiver amplification parts preceding the A/D converter is too high (or, more unlikely, if the received signal is too strong) there is a risk to driving the amplifiers and converter into the nonlinear region or saturation. The most important problem related to the nonlinearities is the appearance of products between the returned (with different delays) sequences. The results of these products are new sequences that do not meet the correlation criteria of the original sequences. As shown by Eq. (2), the echo signal consists of a superposition of sequences with different amplitudes and delays. Under saturation or nonlinear behavior of the reception chain, and depending on the sequence delays and amplitudes, the output of the cross-correlation process can show peaks where nothing exists (“ghost”targets), as some authors have reported from employing these pseudorandom sequences in audio applications.12,13 We have as well observed this phenomenon under specific circumstances in the prototype described in this work. In the general case, if we have a received signal yðnÞ, the output of the receiver chain before the processing and considering possible nonlinear effects will be EQ-TARGET;temp:intralink-;e016;326;184ynlðnÞ¼a0yðnÞþa1y2ðnÞþa2y3ðnÞþ:::; (16) where a0;a 1;a 2::: are the coefficients of the linear and nonlinear parts of the response. The received signal yðnÞis composed of a sum of replicas of the emitted sequence with the correspondent delay and amplitudes according to Eq. (2). For clarity of the subsequent analysis, we simplify the notation and write yðnÞ¼b0sðn−n0Þþb1sðn−n1Þþb2sðn−n2Þþ:::. Optical Engineering 044104-4 April 2018 •Vol. 57(4) Ardanuy and Comerón: Compact lidar system using laser diode, binary continuous wave power. . . Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 4/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use A simple simulation is shown in Fig. 2. Four cases can be found related to the combination of sequences and nonlinearities and its consequences. These are (1) Only one sequence delayed (bk¼0for k>0) and linear amplification: the result is correct. (2) Only one sequence delayed (bk¼0for k>0) and nonlinear amplification: the result is correct [Fig. 2(a)]. (3) Two or more delays (bk>0for some k>0) and linear amplification: the result is correct [Fig. 2(b)]. (4) Two or more delays (bk>0for some k>0)and nonlinear amplification: possibility of false (ghost) echoes after the correlation process with s0ðnÞ [Eq. (7)] [Fig. 2(c)]. The first case of the list is trivial and is not represented in the figure. The other cases correspond to the application of Eq. (16)witha0¼1,a1¼0, and a2¼0(linear case) and with a0¼1,a1¼0.1, and a2¼0(nonlinear case), to a delta-like temporal received signal (simulating a solid target or very thin target) and with a received signal consisting of five consecutive deltas with same amplitude (simulating a wider layer of clouds and aerosols). The most interesting situation is that of Fig. 2(c).“Ghost”echoes appear when the received signal contains more than an echo and the amplifier is nonlinear (case 4 of the list). These “ghost”echoes appear at any unpredictable position (the analysis is very complex and is out of the scope of this paper). Fig. 2 Simulation of different situations in the received signal and the correlation with the sequence s0ðnÞ. An M-sequence of 1023 bits is used. (a) A pure delta function is not sensitive to distortion, (b) a wider function than a delta under linear conditions shows a nondistorted result too, and (c) when this wider signal is correlated after having undergone distortion, an unpredictable number of ghost peaks can appear. Optical Engineering 044104-5 April 2018 •Vol. 57(4) Ardanuy and Comerón: Compact lidar system using laser diode, binary continuous wave power. . . Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 4/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use The simulated effects can be observed under specific circumstances in the test process of the real prototype. Figure 3 shows a real measurement obtained in a test against a solid target close to the lidar to obtain the zero reference (in this case the returned signal is very strong). The first peak is the solid target, and the secondary peak is the ghost peak at the distance of 76 samples. This distance is characteristic of the used length sequence that we have been able to observe under similar situations. 5.2 Effects of Signal Offset The property of obtaining a delta-like function when the pseudorandom sequence sðnÞis correlated with the derived sequence s0ðnÞ(Sec. 3) is true when applying the circular correlation. In the lidar system, it is necessary to capture more samples in the reception process than the transmitted sequence length because of the delay time to measure in the return signal. If Nis the sequence length and Dis the number of samples waiting for the maximum delay to measure, the correlation (circular) is done between two sequences of length L¼NþD. The received and sampled signal contains replicas of the transmitted sequence (length N) that delayed the round-trip time to the scattering volume with superimposed noise and offset; Dis assumed to be large enough for the last samples of the captured return to contain only noise and offset. The correlation operation in this case will take the form EQ-TARGET;temp:intralink-;e017;326;511zpðkÞ¼X NþD n¼1 yQðnÞs0ðnþkÞ:(17) Fig. 3 A real measurement where the effect of the ghost peak is visible. In the 1-D profile the peak is situated 76 samples after the principal peak (equivalent length of 7.6 km). Fig. 4 Simulation of the offset effect on the correlation between incomplete sequences: (a) with N¼2047 and D¼300, (b) with 2N(two complete sequences), (c) and (d) using very long sequences with N¼16;535 and D¼300 bits, and N¼262;143 and D¼300 bits, respectively. A delta-like target is assumed to be at sample number 20 and the offset level has been taken to be equal to the amplitude of the received signal. Optical Engineering 044104-6 April 2018 •Vol. 57(4) Ardanuy and Comerón: Compact lidar system using laser diode, binary continuous wave power. . . Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 4/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use The difference between the number of ones and minus ones in different sections of length Dof the sequence s0ðnÞvaries, which can give rise to spurious oscillations in zpðkÞin the presence of an offset level (whether originating from the receiver electronics, induced by background radiation, or by both causes) in yQðnÞ. This is illustrated in the simulations of Fig. 4, where there is assumed a delta-like target at sample number 50 and an offset level equal to the amplitude of the received signal. This undesired effect can be avoided, increasing the receiving time to acquire 2Nsamples, i.e., making D¼N, which ensures that the difference between the numbers of ones and zeros in the interval is constant. This solution has the drawback of being energetically inefficient, as ∼50% of the time the system is idle. Another, more energetically efficient solution to this problem is the use of very long sequences, with Nin the order of 105bits, which is feasible with a short processing time with the current DSP state of the art, while keeping D constant. In this way, the effect of the varying number of ones and minus ones in sections of Dbits in s0ðnþkÞin the presence of a signal offset keeps limited, whereas the signal increases approximately as N[Eq. (9)]. The effects of the proposed solutions in the crosscorrelation function can be observed in Fig. 4. The offset effect disappears for L¼2N[Fig. 4(b)] and is greatly reduced for N≫D[Figs. 4(c) and 4(d)]. The offset effect on a real measurement is shown in Fig. 5. 6 Experimental Prototype and Results A complete lidar system has been built and tested in a real environment and under real conditions of operation. The core of the lidar system is a DSP and the software to control all the processes. The use of a software-defined lidar allows for changing the operation mode of the system with the same hardware. Sequence lengths of 1023 and 2047 bits have been considered initially in the experimental prototype, but these can be modified very easily. Even using long sequences of tens Fig. 5 Real measurement showing the signal offset effect that occurs when a noninteger number of sequences is used in the correlation process (N¼4096 bits, D¼300 bits). Fig. 6 Experimental setup layout. A buffer between DSP and laser emitter is necessary to adapt the logical levels of the DSP output to the amplitude necessary to modulate the laser emitter. Both amplifiers in emission and reception are wide bandwidth (1.5 GHz) and ultralow noise op-amp. High bandwidth is necessary to have a fast commutation in the emitted pulses and a low noise amplifier is good in the receiver chain. Fig. 7 The lidar prototype mounted in the enclosure. Optical Engineering 044104-7 April 2018 •Vol. 57(4) Ardanuy and Comerón: Compact lidar system using laser diode, binary continuous wave power. . . Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 4/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use or hundreds of thousands of bits, the processing power of the state of the art DSPs makes it possible to neglect the processing time. The time dominating the system is almost exclusively the integration time, except if very short sequences are used, when the processing time dominates. The lidar system parameters are listed in Table 1. The layout of the experimental setup is detailed in Fig. 6. The equipment, which is operated day and night in an autonomous mode, is shown in Fig. 7. Note that the scale reference in the figure gives an idea of its compactness. The photodetector is in the primary focus of a Schmidt– Cassegrain telescope with the secondary mirror removed to simplify the alignment of the laser inside the field of view (wider field of view), resulting in an f∕2telescope. Figure 8shows the calculated overlap function of the system. Figure 9shows different examples of cloud detection. Figure 9(a) shows a medium-high cloud evolving between 3and 5-km altitude, and at the end of the measurement interval a high cloud at around 8 km appears. Figures 9(b) and 9(c) show high altitude clouds between 7 and 8 km. Figure 9(c) is the 1-D profile resulting from the integration of the profiles in the measurement interval of Fig. 9(b).In Fig. 9(d), the detection of a low cloud and rain is noticed. The measurements of Fig. 9have been obtained using M-derived A-sequences,11 which have properties close to those of M-sequences, the circular correlation of an A-sequence with a shifted one yielding a peak for zero shift and a small ripple of constant amplitude for other shifts. Fig. 8 Calculated overlap function of the experimental lidar system. Fig. 9 Cloud detection with the constructed prototype. (a) Medium height clouds between 3 and 5 to 6 km, and at 8 km at the end of measurement period, (b) high clouds at 7 to 8 km, (c) profile 1-D resulting from the integration in the time interval of panel (b), and (d) low clouds and rain detected at the end of the night in the presence of a low cloud layer. Temporal resolution: 4 min for (a) and (b), 30 s for (d). Spatial resolution is 100 m in all the cases. Sequence: A1type of 2047 bits.11 Optical Engineering 044104-8 April 2018 •Vol. 57(4) Ardanuy and Comerón: Compact lidar system using laser diode, binary continuous wave power. . . Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 4/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use