**Abstract**

An eye-safe all-fiber Coherent Doppler Lidar for wind sensing system has been developed and tested at the Remote Sensing Laboratory of the City College of New York, New York, NY. The system, which operates at a 20 kHz pulse repetition rate and acquires lidar return signals at 400 MSample/s, accumulates signals that are as much as 20 dB lower than the receiver noise power by using embedded programming techniques. Two FPGA embedded programming algorithms are designed and compared. In the first algorithm, power spectra of return signals are calculated and accumulated for different range gates. Line of sight wind speed estimates can then be calculated after transferring the range gated accumulated power spectra to a host computer. In the second FPGA algorithm, a digital IQ demodulator and down sampler allow an autocorrelation matrix representing a pre-selected number of lags to be accumulated. Precision in the velocity measurements is estimated to be on the order of 0.08 m/s and the precision in the measured horizontal wind direction is estimated to be to be about 2°.

**Keywords:** Doppler Lidar, wind sensing, heterodyne detection, FPGA, coherent laser, eye-safe

### **1. Introduction**

The first wind measurement device (anemometer) was invented in 1450 by an Italian architect named Leon Battista Alberti. Four hemispherical cups anemometer was later invented in 1846 by Dr. John Thomas Romney Robinson. Today, wind speed and direction can be measured by using classical anemometers, sonic anemometers, rawinsondes, SODAR (Sonic Detection and Ranging), RADAR (Radio Detection and Ranging), and LIDAR (Light Detection and Ranging). Sonic anemometers determines instantaneous wind speed and direction by measuring how much sound waves traveling between a pair of transducers are sped up or slowed down by the effect of wind. SODAR measures wind speed through measurements of the scattering of sound waves by atmospheric turbulence. Both radar and LIDAR use similar technique such as SODAR but instead of using sound waves, radar uses microwave, and LIDAR uses laser waves.

Sodars and radars are used in wind profilers to measure wind speed and direction at various altitudes above ground level. Wind speed can be estimated by transmitting five beams; one is vertical to measure vertical wind velocity, and the other beams are orthogonal to each other to measure horizontal components of the wind. The profiler's assumption to measure wind speed is that turbulent eddies that scatter probing signals are carried along by the mean wind. **Figure 1** shows radar and sodar wind profilers that are mounted at Liberty Science Center, New Jersey, and on top of the Metlife building in the center of Manhattan, New York, respectively, as part of the New York City meteorological network (NYC MetNet). These types of instruments are large and not portable.

oscillator power amplifier (MOPA) configuration. A notable feature of our system is

In this study, the development and operation of a CDL system for wind sensing is presented. In Section 2, the system configuration is presented and system's main components are described. In Section 3, transceiver noise analysis and coherent lidar signal range dependence are examined. In Section 4, signal processing and FPGA programming is introduced. In Section 5, wind measurement results are

In coherent Doppler lidar, laser pulses are transmitted into the atmosphere and interact with aerosols within the atmosphere. As a result of this interaction, laser signals are backscattered towards the laser source, which can be detected and measured through an optical detector. According to the movement of the atmospheric aerosols with respect to the laser source, backscattered signals may suffer a frequency shift (Doppler shift) that is proportional to velocity of moving aerosols.

*<sup>Δ</sup><sup>f</sup>* <sup>¼</sup> <sup>2</sup><sup>ν</sup>

In heterodyne optical detection, local oscillator and backscattered signals are optically mixed through an optical coupler. The resulting mixed signal is then incident upon a photodetector. Both local oscillator and backscattered fields can be

where ω<sup>0</sup> is the local oscillator frequency and ω<sup>s</sup> is the frequency shift that backscattered signals may suffer. The optical intensity as seen by the heterodyne

The terms: *2ω0*, *2*(*ω<sup>0</sup>* + *ωs*), and (*2ω<sup>0</sup>* + *ωs*) are at higher frequencies than detector's bandwidth and will not be seen by the detector. As a result, the generated

*<sup>λ</sup>* (1)

*xlo* ¼ *Alo cos*ð Þ *ωot* (2) *xs* ¼ *As cos*ð Þ *ωot* þ *ωst* (3)

*Id* ¼ *ηAIopt* (5)

<sup>2</sup> <sup>þ</sup> *ALOAs* cosð Þþ *<sup>ω</sup>st* High frequency component (4)

The Doppler shift *Δf* of laser signals with λ wave length is given by:

where ν is the velocity of the aerosols, i.e. wind velocity.

*Iopt* <sup>¼</sup> ½ � *Alo* cosð Þþ *<sup>ω</sup>ot As* cosð Þ *<sup>ω</sup>ot* <sup>þ</sup> *<sup>ω</sup>st* <sup>2</sup>

*As* 2

photodiode electric current will equal to:

maintaining of polarization state between both local oscillator and back scattered fields. The advantage of using a 1.5 μm laser source is the eye-safety feature, which allows for operation in urban areas. In addition to eye-safety feature, the usage of a 1.5 μm source allows the system to benefit from the technology and component

that it utilizes polarized maintained (PM) fiber optics, which ensures the

development driven by the telecommunication industry, which results in

significant cost deductions.

*Coherent Doppler Lidar for Wind Sensing DOI: http://dx.doi.org/10.5772/intechopen.91811*

reported in both vertical pointing and scan modes.

**1.1 Coherent Doppler lidar theory**

**1.2 Heterodyne detection theory**

represented as:

detector is given by:

<sup>¼</sup> *Alo* 2 2 þ

**5**

Coherent Doppler Lidar (CDL) has proven to be a powerful tool for remote sensing of the atmosphere, and has been widely adopted in applications such as measuring atmospheric wind velocity, turbulence, aerosol concentration, cloud height and velocity, and detection of atmospheric constituents and pollutants. The CDL systems have been developed for remote sensing measurements since the late 1960's. The first CDL wind-sensing system was reported by Huffaker et al. [1], where a 10.6 μm cw CO2 laser was used. CO2-laser-based CDL systems have been used for airborne clear air wind and hard target measurements such as ranging and produced valuable results for a long time. Since the late 1980s, CDL systems with newly developed solid-state lasers attracted a lot of researchers due to advantages of size, weight, reliability, and lifetime [2].

Operation at shorter wavelengths allows for higher spectral resolution, which means higher velocity resolution. A lot of effort has been put into 2 μm pulsed systems mainly intended for wind measurements [3–5]. Kavaya et al. [6] developed a 1.06 μm pulsed CDL system that realized a measurable range of a few tens of kilometers used for launch-site wind sensing. Karlsson et al. [3] reported a 1.5 μm cw all-fiber wind sensing CDL system, which utilized optical fiber components used in telecommunication systems.

In this study, we report on the design, measurements, and performance of a CDL system for wind measurements [7]. The design involves a very low energy per pulse (12 μJ/pulse), because it employs all-fiber optic laser components for availability, cost affordability, robustness, and size compactness. As a result of this low energy per pulse, a very high frequency repetition rate (FRP) is used, which produces a very large volume of returned signals. Acquiring such a large volume of data at a very high sampling rate and processing it cannot be achieved using a classical data acquisition and processing hardware. Therefore, signal pre-processing has to be carried out on hardware level by means of the FPGA, which allows for real time processing and a moderate data transfer rate from the data acquisition card to the host PC. In our design, signal pre-processing was implemented to produce the power spectrum of time gated received signals by calculating fast Fourier transform FFT, which produces fixed spatial resolution gates. Another pre-processing technique was implemented that involved the calculation of received signals' autocorrelation, which can be used to find the power spectrum at any desired range resolution.

The system consists of a distributed-feedback (DFB) semiconductor laser emitting at a 1.5 μm in addition to an erbium-doped fiber amplifier (EDFA) in a master

#### **Figure 1.**

*A radar wind profiler (left) mounted on the liberty science center and a sodar wind profiler (right) mounted on a NYC high rise [8].*

#### *Coherent Doppler Lidar for Wind Sensing DOI: http://dx.doi.org/10.5772/intechopen.91811*

scatter probing signals are carried along by the mean wind. **Figure 1** shows radar and sodar wind profilers that are mounted at Liberty Science Center, New Jersey, and on top of the Metlife building in the center of Manhattan, New York, respectively, as part of the New York City meteorological network (NYC MetNet). These

*Spatial Variability in Environmental Science - Patterns, Processes, and Analyses*

Coherent Doppler Lidar (CDL) has proven to be a powerful tool for remote sensing of the atmosphere, and has been widely adopted in applications such as measuring atmospheric wind velocity, turbulence, aerosol concentration, cloud height and velocity, and detection of atmospheric constituents and pollutants. The CDL systems have been developed for remote sensing measurements since the late 1960's. The first CDL wind-sensing system was reported by Huffaker et al. [1], where a 10.6 μm cw CO2 laser was used. CO2-laser-based CDL systems have been used for airborne clear air wind and hard target measurements such as ranging and produced valuable results for a long time. Since the late 1980s, CDL systems with newly developed solid-state lasers attracted a lot of researchers due to advantages of

Operation at shorter wavelengths allows for higher spectral resolution, which means higher velocity resolution. A lot of effort has been put into 2 μm pulsed systems mainly intended for wind measurements [3–5]. Kavaya et al. [6] developed a 1.06 μm pulsed CDL system that realized a measurable range of a few tens of kilometers used for launch-site wind sensing. Karlsson et al. [3] reported a 1.5 μm cw all-fiber wind sensing CDL system, which utilized optical fiber components used

In this study, we report on the design, measurements, and performance of a CDL system for wind measurements [7]. The design involves a very low energy per pulse (12 μJ/pulse), because it employs all-fiber optic laser components for availability, cost affordability, robustness, and size compactness. As a result of this low energy per pulse, a very high frequency repetition rate (FRP) is used, which produces a very large volume of returned signals. Acquiring such a large volume of data at a very high sampling rate and processing it cannot be achieved using a classical data acquisition and processing hardware. Therefore, signal pre-processing has to be carried out on hardware level by means of the FPGA, which allows for real time processing and a moderate data transfer rate from the data acquisition card to the host PC. In our design, signal pre-processing was implemented to produce the power spectrum of time gated received signals by calculating fast Fourier transform FFT, which produces fixed spatial resolution gates. Another pre-processing technique was implemented that involved the calculation of received signals' autocorrelation, which can be used to find the power spectrum at any desired range resolution.

The system consists of a distributed-feedback (DFB) semiconductor laser emitting at a 1.5 μm in addition to an erbium-doped fiber amplifier (EDFA) in a master

*A radar wind profiler (left) mounted on the liberty science center and a sodar wind profiler (right) mounted on*

types of instruments are large and not portable.

size, weight, reliability, and lifetime [2].

in telecommunication systems.

**Figure 1.**

**4**

*a NYC high rise [8].*

oscillator power amplifier (MOPA) configuration. A notable feature of our system is that it utilizes polarized maintained (PM) fiber optics, which ensures the maintaining of polarization state between both local oscillator and back scattered fields. The advantage of using a 1.5 μm laser source is the eye-safety feature, which allows for operation in urban areas. In addition to eye-safety feature, the usage of a 1.5 μm source allows the system to benefit from the technology and component development driven by the telecommunication industry, which results in significant cost deductions.

In this study, the development and operation of a CDL system for wind sensing is presented. In Section 2, the system configuration is presented and system's main components are described. In Section 3, transceiver noise analysis and coherent lidar signal range dependence are examined. In Section 4, signal processing and FPGA programming is introduced. In Section 5, wind measurement results are reported in both vertical pointing and scan modes.

### **1.1 Coherent Doppler lidar theory**

In coherent Doppler lidar, laser pulses are transmitted into the atmosphere and interact with aerosols within the atmosphere. As a result of this interaction, laser signals are backscattered towards the laser source, which can be detected and measured through an optical detector. According to the movement of the atmospheric aerosols with respect to the laser source, backscattered signals may suffer a frequency shift (Doppler shift) that is proportional to velocity of moving aerosols. The Doppler shift *Δf* of laser signals with λ wave length is given by:

$$
\Delta f = \frac{2\nu}{\lambda} \tag{1}
$$

where ν is the velocity of the aerosols, i.e. wind velocity.

#### **1.2 Heterodyne detection theory**

In heterodyne optical detection, local oscillator and backscattered signals are optically mixed through an optical coupler. The resulting mixed signal is then incident upon a photodetector. Both local oscillator and backscattered fields can be represented as:

$$x\_{lo} = A\_{lo} \cos\left(o\_o t\right) \tag{2}$$

$$\mathbf{x}\_t = A\_t \cos \left(a\_0 t + a\_i t\right) \tag{3}$$

where ω<sup>0</sup> is the local oscillator frequency and ω<sup>s</sup> is the frequency shift that backscattered signals may suffer. The optical intensity as seen by the heterodyne detector is given by:

$$\begin{aligned} I\_{opt} &= \left[ A\_{lo} \cos \left( \omega\_o t \right) + A\_s \cos \left( \omega\_o t + \alpha\_i t \right) \right]^2 \\ &= \frac{A\_{lo}}{2} + \frac{A\_s}{2} + A\_{LO} A\_s \cos \left( \omega\_o t \right) + \left( \text{High frequency component} \right) \end{aligned} \tag{4}$$

The terms: *2ω0*, *2*(*ω<sup>0</sup>* + *ωs*), and (*2ω<sup>0</sup>* + *ωs*) are at higher frequencies than detector's bandwidth and will not be seen by the detector. As a result, the generated photodiode electric current will equal to:

$$I\_d = \eta A I\_{\text{opt}} \tag{5}$$

*Spatial Variability in Environmental Science - Patterns, Processes, and Analyses*

$$\frac{\eta A A\_{lo}}{2} + \frac{\eta A A\_s^2}{2} + \eta A A\_{lo} A\_s \cos\left(\alpha\_i t\right) \tag{6}$$

$$=\eta P\_{lo} + \eta P\_s + 2\eta \sqrt{P\_{lo}P\_s} \cos\left(\alpha\_i t\right) \tag{7}$$

where: *A* and *η* are detector's surface area and photo responsivity, respectively. *Id* consists of a dc component = *ηPlo + ηPs* and an ac component = 2*η* ffiffiffiffiffiffiffiffiffiffi *PloPs* <sup>p</sup> cosð Þ *<sup>ω</sup>st* . since *Plo* > > *Ps*, then:

$$I\_{d(dc)} = \eta P\_{lo} \tag{8}$$

$$I\_{d(ac)} = 2\eta \sqrt{P\_{lo}P\_s} \cos\left(o\_it\right) \tag{9}$$

Signal power can be calculated as:

$$ = \left(I\_{d(rms)}\right)^2\tag{10}$$

$$\mathbf{p} = 2\eta^2 \mathbf{P}\_{lo} \mathbf{P}\_s \tag{11}$$

processor. Optical components are connected with a single mode polarized

More detailed explanations of key components are presented below.

The laser source is a distributed feedback erbium doped fiber laser (DFB-EDFL) from NP Photonics. The laser's wavelength is 1545.2 nm, and it has two outputs; first output, used as a seed laser, and second output has an adjustable output power up to 500 mW. The spectrum of the delayed heterodyne detected signal as measured by a spectrum analyzer has a full width at half-maximum (FWHM) of a few kHz. The laser linewidth is approximately 3 kHz, which corresponds to a velocity estimation

The continuous wave (CW) laser input is frequency shifted and pulsed through

the AOMs, where an ultrasonic pulse is generated at a piezoelectric device by driving RF signals. Two AOMs are connected in series to obtain a very low extinction ratio. Each AOM shifts the frequency by 42 MHz, which leads to a total frequency shift of 84 MHz. The purpose of shifting the frequency of transmitted signals by 84 MHz is to shift the frequency of the zero velocity, so that both positive and negative Doppler shifts could be recognized. The driving RF signals, turn the

, so the laser linewidth is enough for our specification.

Our laser source has two outputs: a low power seed laser that is used as a local oscillator (LO), and a high power output (0.5 W) that is modulated, pulsed, and frequency shifted using an acousto-optic modulator (AOM). Electronic circuits drive the AOM to shift laser signals by 84 MHz and generate 200 ns Gaussian shaped laser pulses. These laser pulses are amplified through an erbium doped fiber amplifier (EDFA) then transmitted from port 1 to port 2 of the optical circulator. To minimize the back reflection from port 2 back to port 1, the fiber tip at port 2 is angled and polished. Laser pulses are transmitted into the atmosphere and aerosol particles scatter the laser signals back into the lens, which in turn are transmitted from the optical circulator's port 2 to port 3. Backscattered and LO signals are optically mixed using an optical coupler. Optically mixed signals are heterodyne detected through an optical balanced detector, which generates RF signals. These RF signals are acquired at a 400 MHz sampling rate using an analog to digital converter card (ADC), which is equipped with an on-board field programmable gate array (FPGA) to allow for real time analysis. Digital data is then streamed to a host PC for further processing.

maintained (PM) optical fiber.

*Coherent Doppler Lidar system's configuration.*

*Coherent Doppler Lidar for Wind Sensing DOI: http://dx.doi.org/10.5772/intechopen.91811*

**Figure 2.**

**2.2 Laser source**

accuracy of 0.2 cm s<sup>1</sup>

**2.3 AOM**

**7**

Detector's responsivity is related to detector's quantum efficiency through the following relationship:

$$
\eta = \frac{e\eta\_q}{h\nu} \tag{12}
$$

where; *e* is electron charge*, η<sup>q</sup>* is the quantum efficiency of the detector, *h* is Plank's constant, ν is laser's frequency,

$$\quad = 2\left(\frac{e\eta\_q}{h\nu}\right)^2 P\_{lo} P\_s \tag{13}$$

In this system, backscattered signals are sampled at 400 MHz using a 14-bit ADC equipped with an on-board FPGA. The laser pulse frequency rate (PFR) is 20 kHz, which limits the maximum measurement range to 7.5 km. To estimate wind velocity, the frequency shift of scattered signals (Doppler shift) has to be extracted. Backscattered signals are broken into time gates to represent desired range distances and a power spectrum of each range gate is calculated by Fast Fourier Transform (FFT). A gate length of 128 data samples is chosen, which corresponds to 48 m range distance. Due to low pulse energy (14 μJ/pulse), power spectrum accumulation is needed to improve detection probability and velocity estimation accuracy. The wind velocity of each range gate is estimated from the calculated mean frequency of a post processed power spectrum around the peak frequency.
