Laser
Laser documentation in PDF form:Datei:Laser Module.pdf
Contents
Synopsis
This module is designed to provide information on the available laser options in lidar systems. The Laser module is used as a source that emits radiation to the target in the atmosphere, whose reflected light will be analyzed.
Glossary of Terms
1. Nd:YAG: Neodymium doped Yttrium Aluminum Garnet
2. LuLiF: Lutetium Lithium Fluoride
3. VECSELs: Vertical-External-Cavity Surface-Emitting-Laser
4. ECL: External Cavity Diode Laser
Design Requirements
This section will discuss the main functions of the Laser module and its constituent components(1).
1. The emitted radiation must be eyesafe
a. Class 1M type lasers are preferred unless additional optics are used to lower the optical power to an eyesafe level b. Maximum permissible energy density: 1J/cm2
2. The Laser module should emit radiation in a bandwidth which is commonly used in commercial society
a. A spectral range of near infrared (1510-1560nm) is commonly used in telecommunications, allowing for cheap infrastructure
3. The pulsed length of the laser must be between 1ns ~ 1μs (with a pulsed laser)
4. The Laser module should have a small size and low power consumption
Safety
This section describes the safety concerns of the Laser module, and suggestions for mitigating any possible risks.
The Laser module is one of the most dangerous modules in the OpenLidar system, due to the potentially harmful radiation that it emits. To ensure that no radiation is leaving the laser before the optics and the optional scanner modules are installed properly into the system, the Laser module should receive its power through outlets on the optics and optional scanner modules. In this case, if any component in either the optics or scanner module is removed or malfunctioned, the power to the laser will be cut off and the laser will not emit any radiation. Furthermore, the Laser module should be isolated from the outside world so that no possibly reflective surface can find its way into the beam path, deflecting the harmful radiation into the outside world.
High Level Overview
This section will discuss the main functions of the Laser module and its constituent components.
File:Laser System Diagram.jpg
Figure 1: System diagram of the Laser module
The laser module is separated into two major categories, laser source, the source of the laser beam needed for operation, and safety. Safety, as discussed in the safety section of this document, has two major components. These are power safety, to keep the module from receiving power when the focused beam is exposed, and shielding to encase the module and prevent the focused beam from escaping.
Low Level Information
This section will discuss the details of the Chassis module, including suggestions that may be implemented to accomplish its roles in the lidar system. The specification of each type of laser will be discussed in this section.
There are two types of laser that have been most commonly used in lidars, the solid state laser and the fibre laser. Within the solid state laser category, it can be further broken down into two different laser choices, Nd:YAG laser and LuLiF laser. Furthermore, the fibre laser category can be broken down into two different laser choices, VECSELs or ECL.
It is important to note that some lasers require specific optical devices in order to operate most efficiently. A list of the optical devices used for each laser can be found in the optics module.
Solid State Laser
1. Nd:YAG(2)
-Wavelength: 1064nm -Wavelengths of 532, 355, and 266nm can be generated by frequency doubling, frequency tripling, and frequency quadrupling
2. LuLiF(3)
-Wavelength: 2000nm -Output energy: 100mJ -Output power: 10mW
Fibre Laser
A fibre laser consists of a continuous wave diode laser emitting radiation of single frequency and a laser amplifier. Diode lasers have lower power output levels compared to solid state lasers, which is why a laser amplifier is generally used with the laser(1). The amplifier converts a fraction of laser radiation into high power pulses. Suggested fibre lasers are given below.
1. VECSELs(4)
-Wavelength: 960 or 1030nm (GaAs) -Output power: < 1W
2. ECL(1)
-Wavelength: 1550.12nm -Bandwidth: 3kHz -Output power: 20mW
Interfaces
Figure 2: Dependency Map of Laser module; details in the table below
Figure 2 shows the dependencies of the Laser Module with the rest of the lidar system. The inputs and outputs are described in detail below. The descriptions are colour coded for ease of reading and transferring from diagram to text.
| Inputs | Outputs |
| Control
– Timing cues – Emergency power cutoff |
Control
– Timing, wavelength, power, and module atmosphere sensor data to internal controller |
| Optics/Scanner
– Laser beam |
Table 1: Inputs and outputs of Laser module
Progress
This module is currently in the abstract definition phase.
Testing
Type description of testing on module here.
Comments
Software and power diagrams should be created as needed.
Laser module colour (Hex code): #38761d
Revision History
| Description of Work | Date | Author |
| Document started | 21.07.15 | Alan Yeh |
| Added High Level Overview, Low Level Overview, System Diagram, Dependency Map, Interfaces | 21.07.15 | Alan Yeh |
| General edits to grammar and style | 31.07.15 | Joshua Calafato, Katherine Maul, Frank Modruson IV, Alan Yeh |
| Added new System Diagram, updated High Level Overview, moved former high level to Low Level | 12.08.15 | Frank Modruson IV |
References
1. Hofmeister, P., Bollig, C., Fayed, S., Kunze, M., & Reuter, R. (2015). A COMPACT DOPPLER WIND LIDAR FOR CONTROLLING THE OPERATION OF WIND TURBINES. EARSeL EProceedings, (14). Retrieved August 2, 2015.
2. Paschotta, R. (n.d.). In RP-Photonics.com. YAG Lasers. Retrieved August 2, 2015 from http://www.rp-photonics.com/yag_lasers.html
3. Koch, G., Beyon, J., Barnes, B., Petros, M., Yu, J., Amzajerdian, F., . . . Singh, U. (2007, November 20). High-energy 2μm Doppler lidar for wind measurements. In Opticalengineering.Spiedigitallibrary.org. Retrieved August 2, 2015 from http://opticalengineering.spiedigitallibrary.org/mobile/article.aspx?articleid=1088598
4. Paschotta, R. (n.d.). In RP-Photonics.com. Vertical External-cavity Surface-emitting Lasers. Retrieved August 2, 2015 from http://www.rp-photonics.com/vertical_external_cavity_surface_emitting_lasers.html