Design and modelling of modulated cw laser generation in lidar application

In this work, we designed a configuration that may be used for direct detection of light, detection and ranging sensing method based on generated continuous wave laser and characterized by range measurements. The generation of modulated laser distance is studied and analysed by several distances where modulation of laser signal is utilized. The modelling configuration consisted of transmission module that comprises of signal generator, modulator, and propagation module, object target, and receiver modules comprises of photodetector, amplifier, filter, and visualizers. The design and configuration were developed by Optisystem layout, and the study and analysis are done by simulation using the developed model. A subsystem of range measurement was designed to detect varied distances of detection of target object by the generated modulation of continuous wave laser at 10, 50, 100 and 200 meters respectively. The results discussed were based on the power received and frequency given by different detection ranges. Analysis shows that the detection scheme is sensitive to the received electromagnetic radiation and the signal is better at shorter haul based on the generated modulation of the continuous wave source and the signal deteriorated as target distance is increased.


Introduction
Distance detection and measurement through free space using light detection and ranging functions by transmitting and receiving electromagnetic energy.Light detection and ranging or lidar, can be presented by pulse radar or continuous wave radar, where pulse radar may represent a simpler system, and continuous wave has a very high time bandwidth when it is frequency modulated [1].In lidar, the signal generator or in transmission subsystem, the amplifier and laser determine the wavelength and waveform as well as the power of the transmitted signal.The receiver function will transform the propagating energy into an electrical signal that will be processed to extract the desired information [2].The range and time are related by equation (1), where c is the speed of light: In lidar, the system usually has smaller bandwidths than the target, whilst the target may be similar to a Lambertian or specular reflector or combination of both.There are many factors that may affect the reflected signal from the object target and back to the receiver module such as the laser module, environment factor, target reflectivity and angular dispersion [3].Distance measurement by using modulated wave method have been utilized in many applications such as noncontact surface profiling [4], optical sensing [5] and positioning due to the advantages of optimum resolution in short distance sensing.The signal may generate saw-tooth bias current in the modulator of the wavelength-tunable laser diode.The performance of the lidar may be measured by signal to noise ratio by considering the noise sources that may exist in the system such as thermal noise, dark noise and shot noise.This work presents modeling configuration by simulation using Optisystem for lidar by modulated continuous laser by analyzing the waveforms, detection signal and power by varied distances.

Design configurations
A configuration of lidar detection system was designed using modulated continuous wave by Optisystem simulation.The main subsystems are transmission module that are made up of signal generator, modulator, laser source, amplifier, where the signal is processed, and the propagating signal hit the target object and the receiver module that consists of photodetector, amplifier and filter processed the received signal.The built configuration is shown in Figure 1.
The carrier signal is produced by signal generator with frequency bandwidth of 300 MHz, and it is modulated with continuous wave laser.Modulator in transmission module modulates the signal generated by frequency modulation with continuous wave source.The modulated signal is then amplified, and the optical gain amplifier emitted the signal through where the signal then hit the target object.Parameters are set in the simulation for reflection process in terms of time.Varied distances can be set for each test and different frequencies may be observed after optical-electrical conversion.Several distances were set in the parameter layout from the transmission module to the target object, which in this work, the range of distances are set as dn = 10, 50, 100, and 200 meters.Optical delayer that is represented by time delay in the configuration is used and to obtain different distance from transmission module to the target object, the delayer parameters are varied.The reflected signal from the target is received by photodetector and amplified before being filtered and processed.After the signal passed through the filter, signal received is observed in frequency domain and time domain.
This work observed the differences of frequencies and signal power of the received signal by varying the distances of the target object and observed the sensitivity of the system in term of varied distance range.The reflected signal power can be determined analytically by equation ( 2), where the measurement considers the parameters such as transmission loss, atmospheric loss factor, object or target reflection and area and distance of the object or target.As shown in Figure 2, the distances set for the simulation model is 10 meters and 50 meters from the object target respectively where the frequency spectrum waveforms are depicted.The signal power is slightly lower for CW signal of distance of 10 meters compared 50 meters.The frequency spectrum of dn at 10 meters is around 8.00 × 10 6 Hz and 1.02 × 10 7 Hz for dn of 50 meters.On the other hands, when the distance is increased to 100 meters as shown in Figure 3, the signal power is increased whilst the frequency spectrum is also increased.Figure 4 shows the single peak of waveforms of target distance of 100 meters where the frequency spectrum is around 2.00 × 10 7 Hz.When the target distance is further increased to 200 meters, the signal power also increases as depicted in Figure 5.However, the target object is undetected when the distance is further increased to more than 450 meters.In the simulations done, thermal noise and shot noise are considered in the parameters.Since this work was carried out using simulations only, the ideal conditions are expected.However, in real environment for lidar, many factors should be considered that may influence the target detection such as the atmospheric conditions, noise, and interferences from other signals or target.

Conclusions
In this work, we showed detection configuration and modelling for simple detection using continuous wave where the laser was frequency modulated and the system was characterized by the several target distances.We described the basic concept of frequency modulated light detection and ranging by target distance through simulation.From the results shown, continuous wave modulated laser shows good sensitivity and signal power received at the short-haul distance, however, the optimum range detection is quite limited to less than 500 meters.In real environment, the signal power and frequencies might deviate slightly due to interference and various conditions of the environment, atmospheric, and noises between the transmission and receiver components.

3 .Figure 2 .
Results and discussionBy referring to the configuration as shown in Figure1above, we simulated the model using modulated continuous laser where the wavelength set is around 1552 nm, frequency of 300 MHz, power of 40 mW, threshold current of 40 mA, and amplifier of gain 20 dB.After setting the parameters as stated, we run the simulation at set distance ranges, dn = 10, 50, 100, 200 meters.The results obtained are shown for dn of 10, 50, 100 and 200 meters respectively from the target.The frequency spectrum at dn of (a) 10 meters and (b) 50 meters respectively.

Figure 3 .
Figure 3.The frequency spectrum at dn at 100 meters.

Figure 4 .
Figure 4.The frequency spectrum at dn at 100 meters.

Figure 5 .
Figure 5.The frequency spectrum at dn at 200 meters.