Beam expander chosen for Doppler Wind Lidar system

15th December 2014
Mick Elliott

Optical Surfaces has been selected by Beijing Metstar Radar (Metstar) to supply a high precision beam expander that will form a key component in their next generation fibre-based pulse Doppler Wind Lidar system. Metstar`s charter is to combine the latest technological advancements in both meteorology and electronics to provide high-class and reliable weather radars and services to the domestic and oversea customers and to branch out into related fields.

Dr. Zhifeng Lin, a senior research scientist at Metstar commented: "In meteorological Lidar systems, beam expanders are often used to magnify the laser beam to reduce its divergence in the far field. An off-axis reflective beam expander greatly reduces the blind zone of Lidar detection and therefore was considered as the best solution to this problem. In Lidar systems, such as Doppler Wind Lidar, the wavefront performance of the beam expander is very important for the Lidar efficiency. For instance only 1 dB loss in total efficiency is experienced when the RMS wavefront deviation of the beam is between  lambda/10 and  lambda/20, whereas an optical wavefront performance between  lambda/5 and  lambda/10 will result into a 3 dB loss. So, a well-designed beam expander system with wavefront performance better than  lambda/20 RMS was specified as a critical component for our next generation Doppler Wind Lidar”.

Dr Lin added "These are the reasons why we choose a BEX series beam expander from Optical Surfaces Ltd for our fiber-based pulse Doppler Wind Lidar. These off-axis Laser Beam Expanders not only have no central obscuration and efficient transmission, but also have precision top quality surface accuracy of typically better than lambda/60 RMS (> lambda /10 PV). With this beam expander, alignment aids with sub-arcseconds repeatability are provided to ensure correct beam pointing. I am sure the BEX series beam expander from Optical Surfaces Ltd. will help enhance our Doppler Wind Lidar performance significantly".

Dr Aris Kouris, Sales manager for Optical Surfaces commented: “We are pleased to have received the order from Metstar which I believe will open a number of opportunities for further future collaborations. This is a challenging 100mm aperture Beam Expander based upon a Dall-Kirkham off-axis mirror design which, requires maintaining a high optical performance over a wide range of temperatures (-40° C to +50° C). In order to achieve this we are employing an INVAR rod solution together with a very high precision free-translating mechanism that exhibit, practically, negligible friction. In addition, in our efforts to better control the thermal behaviour, the off-axis mirrors have a short focal length resulting in a very robust assembly (the overall length expected to be around 0.5m) which is less sensitive to temperature changes, but makes the manufacturing of the highly aspheric primary and alignment a challenge as a result. Finally, the instrument has a relatively high magnification (M=20) that introduces further challenges during manufacturing, testing and assembly of the unit.”

Beijing Metstar Radar (Metstar) is a high-technology company formed in 1996 as a joint venture between the Lockheed Martin Corporation of the United States and China National Huayun Technology Development Corporation, a wholly owned subsidiary of the China Meteorological Administration.

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