Fiber lasers which operate in burst-mode where densely spaced pulses occur inside bursts repeated at much lower repetition rates can be valuable tool for sensing and imaging. We introduce such lasers and propose possible applications. © OSA 2016

Akçaalan O.

Ilday F.O.

Kalaycioglu H.

Kesim D.K.

Kalaycioğlu H.

Bilkent University Institutional Repository

All-fiber laser systems that can operate in burst mode

Akçaalan Ö.

Ilday F.Ö.

This paper was not presented at the conference:Title:   All-Fiber Laser Systems That Can Operate in Burst ModeAuthors:   Denizhan Koray Kesim, Hamit Kalaycιoğlu, Önder Akçaalan, and FatihÖmer IldayEvent Name:   3D Image Acquisition and Display: Technology, Perception and ApplicationsYear:   2016JT3A.23.pdf 2016 Imaging and Applied Optics Congress (3D, AO, AIO,COSI, DH, IS, LACSEA, MATH) © OSA 2016All-Fiber Laser Systems That Can Operate inBurst ModeDenizhan Koray Kesim1,∗, Hamit Kalaycıoğlu2, Önder Akçaalan2, F. Ömer Ilday1,21Department of Electrical and Electronics Engineering, Bilkent Ankara, 06800, Turkey 2Department of Physics,Bilkent University, 06800 Ankara, Turkey*:dkkesim@ee.bilkent.edu.trAbstract: Fiber lasers which operate in burst-mode where densely spaced pulses occurinside bursts repeated at much lower repetition rates can be valuable tool for sensing andimaging. We introduce such lasers and propose possible applications.OCIS codes: 140.3510, 320.5550.1. Fiber LasersSpecially designed fibers can act as wave guide to laser light. Further, the amplification process can be done opticallypumping active fibers.Fibers, by nature, are quite flexible wave guides, except rod type fibers which are not in scope of this submission.They can be bent to certain degree even without losing polarization properties of the laser. The flexibility propertyof fibers allow them to be very compact. They can be fitted inside of a box much smaller than solid-state lasers withsimilar parameters. Furthermore, since the free space components are few and laser beam is mostly travelling insidea wave guide, little maintenance is necessary to operate fiber laser systems. Thanks to the recent developments, fiberlasers can go very high powers, which was limited before. Average power over 100W is reported for burst-mode fiberlaser with picosecond pulses [1].2. Burst ModeTraditionally, pulsed lasers operate in so-called uniform-mode where pulses are distributed equal in time. This modepractically limits the repetition rate for high energy pulses since the average power requirements raise which in turnproduces the need for more complex laser systems. To overcome this, we send pulse bursts containing densely spacedpulses which in turn are repeated at much lower repetition rates, compared to pulse-to-pulse repetition rate insideindividual bursts. This allows us to keep average power at sustainable levels. The potential of this mode of operationwas first demonstrated by Marjoribanks and co-workers [2].≈(a) (b)Fig. 1. Comparison of Burst-mode operation with uniform-mode operation. (a) The traditionaluniform-mode where the pulses are separated equally. (b) Burst-mode where the pulses are packedclosely and repeated at much lower repetition rates.Burst-mode operation enables us to achieve great pulse energies at high repetition rates without the need to increasethe laser complexity. The duty cycle is low in burst mode systems, and hence is the effective repetition rate of theJT3A.23.pdf 2016 Imaging and Applied Optics Congress (3D, AO, AIO,COSI, DH, IS, LACSEA, MATH) © OSA 2016system, which can be calculated as in Eq. 1, where fburst is the burst repetition rate and npulses is the number of pulsesinside each burst.fburst ∗npulses = fe f f ective (1)This way, we can use all the advantages of a high repetition rate laser without the need to go to high powers. Theeffectiveness of burst-mode operation was demonstrated for material processing by enabling ablation cooled regime,which was discovered in our group in relation to laser material processing [3]. We have developed specialized fiberlaser systems which use synchronized pulsed pumping to supress ASE and achieve high pulse energy at low burstrepetition rates [4].Furthermore, changing burst parameters such as number of pulses per burst and burst repetition rate can be seamlesssince bursts are generated by a fiber coupled AOM via pulse picking. An electronic circuit based on an FPGA synchro-nized with the seed signal from oscillator, that brings versatility to choose operation in burst-mode or in uniform-modealternatively.The pulses for bursts are picked by an AOM driven by FPGA which is triggered directly from laser oscillator. Thus,it is guaranteed to catch pulses properly with minimal loss.3. Current SystemsThese are the burst-mode fiber lasers systems that has been developed in our group.3.1. Compact Burst-Mode Fiber Laser SystemOriginally developed for improving laser assisted cataract surgery by exploiting ablation cooled regime [5], it is atabletop system that can be operated by medical doctors for further experiments. Yb fiber laset can generate repetitionrate of 50 kHz for bursts with intra-burst repetition rate of 91 MHz. Each burst can contain up to 48 pulses. Pulsedurations are less than 300 femtosecond. The output can go up to 3 W which is distributed equally among pulses.3.2. High Repetition Rate SystemsThis Yb fiber laser system exceeds GHz repetition rate barrier by applying burst-mode operation [6]. Pulse-to-pulserepetition rate is increased to 3.5 GHz from a 108 MHz oscillator by using cascaded 50/50 couplers. Bursts have 215µJ energy distributed among 50 pulses which are repeated at 1 kHz while keeping the average power as low as 200mW.4. LIDAR ApplicationWe propose burst-mode lasers for LIDAR sensing and imaging systems. Sensing in atmospheric medium through longdistances requires high pulse energies for coherent detection. Only a fraction of light manages to return to be collectedby sensors [7]. By using burst-mode operation, such high pulse energies are made possible for low average powerswithout giving up high repetition rate to get more information out of a single burst. This would reduce the cost ofLIDAR systems for both building the light source and supplying power to the system.5. ConclusionRobustness and low maintenance makes fiber lasers desirable for various applications including imaging. The cost ofbuilding and maintaining them is lower compared to solid-state lasers. Recently, output of fiber lasers is increaseddramatically so that even material processing applications are made possible. There are tunable fiber lasers that can beused for various applications.Additionally, advantages of burst-mode operation has been recently illustrated experimentally. In this mode, pulsesare gathered very closely in groups of bursts instead of distributing them equally temporally. It has been shown toachieve greater ablation rates compared to traditional uniform mode and employed to exploit ablation-cooled materialprocessing. Burst-mode operation is especially useful for high repetition rate applications where average power wasoriginally limiting.Imaging technologies can benefit from either of them. Especially burst-mode operation is a promising tool forsensing. By using some pulse-to-pulse modulation, the possible applications can reach even further. We propose onepossible application, LIDAR imaging and sensing, for burst-mode fiber lasers, though the application areas can beexpanded.JT3A.23.pdf 2016 Imaging and Applied Optics Congress (3D, AO, AIO,COSI, DH, IS, LACSEA, MATH) © OSA 2016References1. S. Yılmaz, P. Elahi, H. Kalaycıoğlu, and F. Ö. Ilday, “Amplified spontaneous emission in high-power burst modefiber lasers,” J. Opt. Soc. Am. B, vol. 32, no. 12, p. 2462, 2015.2. M. Lapczyna, K. P. Chen, P. R. Herman, H. W. Tan, and R. S. Marjoribanks, “Ultra high repetition rate (133MHz) laser ablation of aluminum with 1.2-ps pulses,” Appl. Phys. A Mater. Sci. Process., vol. 69, no. 7, 1999.3. F. Ö. Ilday, “Ultrafast burst-mode fiber lasers: Source development and material processing,” Lasers and Electro-Optics (CLEO), pp. 1-1, 2015 Conference on, San Jose, CA, 2015.4. H. Kalaycıoğlu, K. Eken, and F. Ö. Ilday, “Fiber amplification of pulse bursts up to 20 µJpulse energy at 1 kHzrepetition rate,” Opt. Lett., vol. 36, no. 17, p. 3383, 2011.5. D. K. Kesim, H. Kalaycıoğlu, C. Kerse, and F. Ö. Ilday. “All-Fiber Burst Mode Femtosecond Laser SystemIntegrated with OCT for Cataract Surgery,” In The European Conference on Lasers and Electro-Optics, p.CL 5 1. Optical Society of America, 2015.6. C. Kerse, H. Kalaycıoğlu, P. Elahi, Ö. Akaalan, and F. Ö. Ilday, “3.5-GHz intra-burst repetition rate ultrafastYb-doped fiber laser,” Opt. Commun., vol. 366, pp. 404–409, May 2016.7. Weitkamp, Claus, ed. “Lidar: range-resolved optical remote sensing of the atmosphere,” Springer Science &Business, Vol. 102, 2006.

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All-fiber laser systems that can operate in burst mode

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Bilkent University Institutional Repository

Bilkent University Institutional Repository