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Rare Earth Doped Fibers for use in Fiber Lasers and Amplifiers

Nov 4, 2013
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ABOUT THIS WEBINAR
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Additional Questions and Answers from the Webinar are Detailed Below


Rare Earth Doped Fibers for use in Fiber Lasers and Amplifiers

The advances in performance of fiber lasers have been dramatic in the last 10 years, making fiber lasers a commercial business worth over $500M/year. All of this is made possible by the unique performance attributes of rare earth doped silica fibers which is at the heart of each of these laser systems. In this webinar we will describe the key parameters of these fibers, including the use of large mode area (LMA) fibers for high power CW lasers as well as pulsed fiber lasers, along with the key other fiber-based components needed to make a fiber laser. The common rare earth dopants for silica fibers, namely Ytterbium, Erbium, Thulium and Holmium will also be described, along with some of the commercially relevant devices that are enabled by these fibers.

Presenters:

Fiber Product Line Manager, Nufern
Dr. George Oulundsen

Dr. Oulundsen joined Nufern from OFS where he served as Distinguished Member of Technical Staff in the areas of MM fiber design and processing focusing on fibers for 10/40/100 GBE.

As part of his role he served on several IEEE standards committees and was Secretary of the IEEE P802.3ba 40Gb/s and 100Gb/s Ethernet Task Force. Prior to OFS, he worked for Lucent Technologies and started his career at Spectran Corporation. He received his Ph.D. in Chemical Engineering from University of Massachusetts-Amherst and his B.S. degree in Chemical Engineering from Worcester Polytechnic Institute.

He is an inventor on several patents in optical fiber design as well as fiber manufacturing processes and has been published in numerous industry journals.

VP Business Development, Nufern
Dr. Bryce Samson

Dr. Samson joined Nufern from Corning where he served as Senior Research Scientist in the areas of doped fibers, fiber amplifiers and fiber lasers.

Prior to that, he worked as a Research Fellow at the University of Southampton focusing on novel fibers and fiber device physics. He received his Ph.D. in Physics from Essex University in the UK and his B.S. degree in Applied Physics from Heriot-Watt University in Edinburgh, UK.

He is an inventor on several patents in the amplifier and fiber laser field and has been published in numerous industry journals.



Additional Questions and Answers from the Webinar

Q: Can the speaker make any comments about the potential for radiation damage for these rare-earth fibers? Or ways to radiation harden these fiber?
A: Yes there is plenty of radiation test data in the literature on rare earth doped fiber performance and some optimization that can be done to alter the performance in different environments. An example of this is discussed in this paper: http://www.nufern.com/library/item/id/254/

Q: Are there triple or 4 clad fibers?
A: Yes triple clad fibers are sometimes used and are available in various standard configurations (core and clad size). They are mostly used in high power Yb-doped lasers at 1µm or in some resonantly pumped fiber lasers at longer wavelengths (1.5µm and 2µm) where the pump wavelength would be absorbed by the polymer coating. There are articles on these types of triple clad fibers available at www.nufern.com/library and specific fibers at: www.nufern.com/pam/optical_fibers/

Q: Cooling issues of doped fibres? e.g. from what range of powers do you require water cooling?
A: Water cooling is primarily needed for the laser diodes that pump the rare earth doped fiber, rather than the fiber itself. By maintaining the laser diodes in a suitable temperature range the lifetime can be predicted and the specifications such as power and wavelength maintained. Air cooled fiber lasers operating >100W CW power at 1um can readily be designed and are sold in the market.

Q: Do you see any merit in using Holey Fibers for Fiber Laser Application?
A: In the last few slides we talk about state of the art fibers such as PCF and PBG fibers, which are part of the holey fiber family. Most of the development in that area is towards ultra-fast fiber lasers where the performance of LMA fibers limits the pulse energy due to non-linear interactions in the rare earth doped fiber. A: The spectrum for the rare earth ions does not strongly depend on the host glass composition.

Q: For double cladding fibers, how thick are the 1st and 2nd cladding 2? What is the index difference?
A: Typical cladding sizes are between 130µm and 400µm for the double clad fibers described in the webinar. That is the glass inner cladding where the pump light propogates. The NA with respect to the low index polymer coating surrounding this is around 0.46. For lots of examples look at the fiber data sheets on the website: http://www.nufern.com/pam/optical_fibers/

Q: Hello. Thanks for the nice presentation. I would like to know whether it is possible to purchase fibers with end-caps or just the end-caps?
A: Yes the examples we used in the webinar were ITF labs commercially available pigtails with end-caps.

Q: How stable are the BGRs? Lifetime?
A: Very stable and using the commercially available gratings there are examples of high power (kW) 1µm fiber levels lasers operating with thousands of hours with grating degradation.

Q: I cannot understand how the light travels inside a double-clad fibers or how the light can travel inside the core and clad without suffer a diffraction when the light pass through the core. Thank you!
A: The NA of the pump waveguide is much higher than the core waveguide and mostly is primarily absorbed by the core with the absorption factor scaling roughly as the core to cladding area ratio.

Q: Is there any dopant available to obtain visible-light lasers?
A: None of the rare earth dopants that lase on the visible transitions work in silica glass fibers because of the phonon energy of the glass, but work in Fluoride glasses. Examples are Er, Pr and Tm doped Fluoride fibers. Some of these are becoming commercially available.

Q: Is there any fiber laser or amplifier in visible wavelengths?
A: Yes some of the rare earth doped fluoride fibers lase in the visible (see other question). With Silica fibers it is commercially available to have frequency doubled fiber lasers using external non-linear conversion to operate at 488nm and 1064nm wavelengths amongst others.

Q: Why 2um fiber lasers and not 3 or other number?
A: The earth ion Tm-lases at 2µm in silica glass fibers,which do not transmit light very well at longer wavelengths such as 3µm.

Q: REE doped benefits?
A: Unlike in crystals, transition metal ions (such as Cr)do not lase in glass fibers and only a few of the rare earth ion transitions work well in silica glass fibers. The benefits of fiber over other medium are discussed in the webinar.

Q: What are the pros and cons of using pedestal SM or LMA?
A: The pedestal fiber designs are mostly used for Er:Yb doped and Tm-doped fibers where the core NA is high with respect to the silica cladding glass, and in order to reduce that NA to value around ~0.1, requires a second doped layer referred to as a pedestal payer. See for example: http://www.nufern.com/library/item/id/165/

Q: What is the loss in Pump combiners?
A: That depends of the design but typical values less than 0.5dB are the norm with high power combiners closer to 0.1dB.

Q: Is it possible to extend the operation of the fiber laser for example lower than 1220nm in SM?
A: Yb-doped fiber lasers operating as short as 976nm have been demonstrated and are commercially available. See for example: http://www.nufern.com/library/item/id/160/

Q: What is the advantage/disadvantages of the PLMA compared to LMA?
A: PLMA is a polarization maintaining version of the large mode area (LMA) fiber technology and is useful for linear polarized light transmission or amplification. See for example: http://www.nufern.com/library/item/id/374/

Q: What is the maximum power they can handle?
A: PLMA fiber amplifiers have demonstrated over 1kW CW output power and the non PM versions even higher powers. See for example: http://www.nufern.com/library/item/id/383/

Q: What is the prospective market size for this rare earth fibers?
A: Total market for fiber lasers is >$500M and almost100% of these lasers use rare earth doped fibers somewhere on the device. There has been no accurate, publicly available study of the market size for the fibers themselves, at least that we are aware of.

Q: What is your opinion on Bi-doped fibers? It is said that it can cover the spectrum from 1-1.5 microns. Can it be as efficient as Rare-Earth doped fiber technology?
A: Progress has been slow on commercializing Bi-doped fibers and it still looks like much of the basic research is still needed to optimize these new fibers, although the properties look interesting.

Q: What is your recommendation on splice between round cladding fiber and octagonal cladding fiber?
A: For a discussion on splice losses and fiber designs. See for example: http://www.nufern.com/library/item/id/391/

Q: Why not just put a thin film coating of a light emitting material like a 5nm filmf cadmium phosphide between the core and cladding?
A: That would not be easily compatible with the MCVD process for depositing silica glass used in these fibers.

Q: Would there be big advantages for industrial applications for 2 micron wavelength compared to the 1.06 micron range?
A: Industrial applications for 2µm fiber lasers are still the subject of research, but plastic welding and processing is starting to look interesting because of the material properties at this wavelength. Also eye-safety considerations are very different at 2µm compared with 1µm.
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