CS-LDTC04 WAVELENGTH | Alldatasheet
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for Atmospheric Research developed an eye-safe lidar implementation for atmospheric investigation. to broaden the available locations for measurements, including more populated areas and near airports. the retina is one option for eye-safe lidar measurements. greater signal-to-noise ratio. non-eye-safe pump beam to a longer, eye-safe wavelength. various allowable wavelength shifts can be calculated. Figure 1. The maximum eye-safe energy varies as a with permission from Ref. [1], Applied Optics. transitions and control spatial beam parameters. laser's emission matched the first Stokes wavelength.
opposed to Raman scattering. Figure 2. Experimental layout for the Raman-shifted encountered issues with gas cell window transparency. Methods not utilizing SRS have also been developed. advantageous in that no methane needs to be handled.
Case Study CS-LDTC04 Rev. A Page 3 © 2019 • Sales & Technical Support: (406) 587-4910 • email: sales@teamWavelength.com • web: www.teamWavelength.com The researchers describe their method for reliably outputting the 1543 nm stimulated Raman scattered light as follows: "Typically, the Stokes field is initiated by the spontaneous emission of a photon, and therefore the energy and spatial characteristics will fluctuate. To prevent these fluctuations one can seed the cell with a stable tunable Stokes wavelength laser. We injection seed our Raman cell with a continuous-wave 20-mW telecom diode laser (Mistsubishi FU-68PDF/520M45B). The laser has a center wavelength of 1543.73 nm and approximately 3-nm tunability." Multiple steps were taken to ensure the first Stokes wavelength is the prevalent output from the pressurized gas cell. First, injection seeding was utilized, as described earlier. Secondly, specific optical consideration was made to suppress the second Stokes wavelength as well as the first anti-Stokes wavelength. Wavelength Electronics laser diode driver WLD3343 and temperature controller WTC3243 were used to control the output of the seed laser. These modules matched the requirements for the seed laser. The low power required also enabled operation of the driver away from the upper and lower bounds of drive current, where noise contributions could be greater. Both laser current and temperature were used to fine-tune the output wavelength to stimulate the desired Stokes wavelength. Here, the precision temperature controller and laser diode driver allowed for tight wavelength control. Thus, the emission at 1543 nm from the gas cell was optimized. In addition to injection seeding, other experimental parameters were also fine-tuned to maximize the efficiency of the 1543 nm output. These parameters include the pump laser's pulse energy, the gas cell pressure, and the path length within the gas cell. BEAM QUALITY Beam quality was also an important parameter for these measurements. The divergence of the beam (Θ) is given by Θ = 2M2λ (3) πw0 where M2 is the beam quality factor, λ the wavelength, and w0 the radius of the beam waist. To limit the divergence of the output beam and avoid the problems encountered with OPO systems, the researchers used two methods. First, the beam quality was improved via the seed laser. It is important that the seed laser emits a nearly perfect Gaussian beam. It is known that current noise from the driver can influence spatial beam properties. Thus, the low noise provided from the driver helped ensure ideal beam properties. Without these optimal spatial characteristics of the seed laser, the output beam from the gas cell would deteriorate. Second, after exiting the multi-pass cell, the resulting beam was expanded prior to being used to take measurements (see Figure 2). Both methods help to limit the divergence of the beam, and thus give this configuration an advantage over OPO systems.
Case Study CS-LDTC04 Rev. A Page 5 © 2019 • Sales & Technical Support: (406) 587-4910 • email: sales@teamWavelength.com • web: www.teamWavelength.com KEYWORDS lidar, aerosol, elastic backscatter, Raman, Stokes, seed laser, methane, atmospheric sensing, eye-safe PRODUCTS USED WLD3343, WTC3243 REVISION HISTORY Document Number: CS-LDTC04 REVISION DATE NOTES A April 2019 Initial Release REFERENCES aerosol lidar," Appl. Optics 43(19), 3915-3924 (2004). USEFUL LINKS
- WLD3343 Product Page
- WTC3243 Product Page WAVELENGTH SOLUTIONS The researchers have shown an improved aerosol lidar system, which utilized higher pump pulse power than previous systems. This, in combination with diode laser injection seeding allowed eye-safe lidar measurements of atmospheric composition as a function of both distance and time. The seed laser aided in the ability to collect this data, by optimizing the Stokes-shifted output beam. The use of a seed laser improved the beam quality, enhanced the conversion efficiency to the 1543 nm Stokes wavelength, and reduced the pulse-to-pulse energy fluctuations. Wavelength's WLD3343 and WTC3243 were used to control the seed laser. The seed laser's output wavelength was a crucial parameter to control in order to optimize the beam output. The WLD laser driver, with 200 ppm current stability, paired with the WTC temperature controller with stability better than 1 mK, allowed for precise tuning of the wavelength.