**2. SYSTEM'S configurations**

In this section, the main system components are presented and an over view of the system's operation is explained. System's configurations and testing of optical and electrical components are also presented in this section.

#### **2.1 System overview**

The system configuration is shown in **Figure 2**, which consists of the following components: (i) Laser source (ii) Accousto-optic modulator (iii) Fiber amplifier (iv) Optical circulator (v) Optical antenna (vi) Balanced detector, and (vii) Signal

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

#### **Figure 2.**

<sup>¼</sup> *<sup>η</sup>AAlo*

since *Plo* > > *Ps*, then:

following relationship:

Signal power can be calculated as:

Plank's constant, ν is laser's frequency,

**2. SYSTEM'S configurations**

**2.1 System overview**

**6**

2 <sup>2</sup> <sup>þ</sup> *<sup>η</sup>AAs*

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

2

*PloPs*

where: *A* and *η* are detector's surface area and photo responsivity, respectively. *Id*

*PloPs*

*<sup>s</sup>* > ¼ *Id rms* ð Þ

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

*<sup>η</sup>* <sup>¼</sup> *<sup>e</sup>η<sup>q</sup>*

where; *e* is electron charge*, η<sup>q</sup>* is the quantum efficiency of the detector, *h* is

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.

In this section, the main system components are presented and an over view of the system's operation is explained. System's configurations and testing of optical

The system configuration is shown in **Figure 2**, which consists of the following components: (i) Laser source (ii) Accousto-optic modulator (iii) Fiber amplifier (iv) Optical circulator (v) Optical antenna (vi) Balanced detector, and (vii) Signal

*eηq hυ* � �<sup>2</sup>

<sup>¼</sup> <sup>2</sup>*η*<sup>2</sup>

<sup>¼</sup> *<sup>η</sup>Plo* <sup>þ</sup> *<sup>η</sup>Ps* <sup>þ</sup> <sup>2</sup>*<sup>η</sup>* ffiffiffiffiffiffiffiffiffiffi

consists of a dc component = *ηPlo + ηPs* and an ac component = 2*η* ffiffiffiffiffiffiffiffiffiffi

*Id ac* ð Þ <sup>¼</sup> <sup>2</sup>*<sup>η</sup>* ffiffiffiffiffiffiffiffiffiffi

<*i* 2

< *i* 2 *<sup>s</sup>* > ¼ 2

and electrical components are also presented in this section.

<sup>2</sup> <sup>þ</sup> *<sup>η</sup>AAloAs* cosð Þ *<sup>ω</sup>st* (6)

*Id dc* ð Þ ¼ *ηPlo* (8)

<sup>p</sup> cosð Þ *<sup>ω</sup>st* (9)

� �<sup>2</sup> (10)

*PloPs* (11)

*<sup>h</sup><sup>υ</sup>* (12)

*PloPs* (13)

<sup>p</sup> cosð Þ *<sup>ω</sup>st* (7)

*PloPs* <sup>p</sup> cosð Þ *<sup>ω</sup>st* .

*Coherent Doppler Lidar system's configuration.*

processor. Optical components are connected with a single mode polarized maintained (PM) optical fiber.

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. More detailed explanations of key components are presented below.

#### **2.2 Laser source**

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 accuracy of 0.2 cm s<sup>1</sup> , so the laser linewidth is enough for our specification.

#### **2.3 AOM**

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

AOMs on during the 300 ns of the 20 kHz RF driving pulse to generate a laser pulse with 200 ns Full Width at Half Maximum (FWHM).
