Spectral Sculpting a Fiber Laser via Intracavity Spatial Light Modulator
Roman Iegorov1, F. ¨Omer Ilday1,2
1Department of Physics, Bilkent University, Ankara, Turkey
2Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey
Mode-locked fiber lasers have been intensely studied and utilized in applications as sources of ultrafast pulses during last decade as a result of their many practical features [1]. However, their complicated nonlinear dynamics render performance optimization quite difficult [1-3]. Spectral filtering and dispersion delay lines play a prominent role in most of the mode-locking regimes, including stretched-pulse, similariton, all-normal dispersion. However, there are practical limitations to optimizing mode-locked operation since the range of spectral filter profiles (Gaus-sian, rectangular, parabolic, etc.) or dispersion compensators available in practice is quite limited. At any rate, they are static and cannot be modified once placed into the cavity. The application of a filter with dynamically reconfig-urable spectral transmission profile can potentially lead the way to a new level of performance of the well-known mode-locking regimes.
Here, we introduce a spatial light modulator (SLM) into a mode-locked fiber oscillator with the intention of manipulating the nonlinear laser dynamics, for the first time to our knowledge. The oscillator has a standard dispersion-managed cavity (Fig. 1a). The back-mirror of a standard grating compressor was replaced by the SLM. Together with polarization selectivity of the gratings, it constitutes a high-resolution spectral filter, which allows us to realize a variety of transmission profiles. Using the SLM, we can implement almost any filter to maximize the bandwidth of the pulses. The SLM is dynamically reconfigurable through an attached computer.
Identification of the optimal transmission profile for the spectral filter is by itself a complicated task, which generally needs precise characterization of the cavity dispersion, absorption and nonlinearity maps. Therefore, we have chosen the simpler approach based on iterative algorithms to obtain the optimal regime using quasi real-time feedback of a cavity. To this end, we have used the bandwidth of the output-coupled pulses as a target-function for maximization process. The result of the optimization is presented at the Fig. 1b. The red curve is the initial spectrum of pulses extracted from the 10% coupler. The blue curve is the optimized output and the black dashed curve is the optimal spectral transmission pattern corresponding to this case. This way, the full width at half maximum of an output spectrum was changed from 29 nm to 44 nm, corresponding to a 50% increase in bandwidth. As can be seen from Fig. 1b, the central wavelength of the optimized spectrum is much closer to 1030 nm, which is roughly the center of the gain bandwidth. Thus, the optimization results in increased power efficiency in addition to the evident spectral expansion. Manipulations of the spectral transmission profile are similar to management of the cavity losses. It is possible to switch on/off the mode-locking via strong local changes of the transmission spectrum, while keeping the overall transmission at the nearly same level. In practice, our results show that the intra-cavity SLM can be used as an automated mode-locked for a non-self-starting cavity.
Spectral Sculpting a Fiber Laser via an Intracavity Spatial Light
Modulator
R. Iegorov1, F. Ö. Ilday1,2
1. Department of Physics, Bilkent University, Ankara, Turkey. 2. Department of Electrical Engineering, Bilkent University, Ankara, Turkey.
Mode-locked fiber lasers have been intensely studied and utilized in applications as sources of ultrafast pulses during last decade as a result of their many practical features [1]. However, their complicated nonlinear dynamics render performance optimization quite difficult [1-3]. Spectral filtering and dispersion delay lines play a prominent role in most of the mode-locking regimes, including stretched-pulse, similariton, all-normal dispersion. However, there are practical limitations to optimizing mode-locked operation since the range of spectral filter profiles (Gaussian, rectangular, parabolic, etc.) or dispersion compensators available in practice is quite limited. At any rate, they are static and cannot be modified once placed into the cavity. The application of a filter with dynamically reconfigurable spectral transmission profile can potentially lead the way to a new level of performance of the well-known mode-locking regimes.
Here, we introduce a spatial light modulator (SLM) into a mode-locked fiber oscillator with the intention of manipulating the nonlinear laser dynamics, for the first time to our knowledge. The oscillator has a standard dispersion-managed cavity (Fig. 1a). The back-mirror of a standard grating compressor was replaced by the SLM. Together with polarization selectivity of the gratings, it constitutes a high-resolution spectral filter, which allows us to realize a variety of transmission profiles. Using the SLM, we can implement almost any filter to maximize the bandwidth of the pulses. The SLM is dynamically reconfigurable through an attached computer.
Identification of the optimal transmission profile for the spectral filter is by itself a complicated task, which generally needs precise characterization of the cavity dispersion, absorption and nonlinearity maps. Therefore, we have chosen the simpler approach based on iterative algorithms to obtain the optimal regime using quasi real-time feedback of a cavity. To this end, we have used the bandwidth of the output-coupled pulses as a target-function for maximization process. The result of the optimization is presented at the Fig. 1b. The red curve is the initial spectrum of pulses extracted from the 10% coupler. The blue curve is the optimized output and the black dashed curve is the optimal spectral transmission pattern corresponding to this case. This%way,%the%full%width%at% half%maximum%of%an%output%spectrum%was%changed%from%29%nm%to%44%nm,%corresponding%to%a%50%%increase% in% bandwidth.% As% can% be% seen% from% Fig.% 1b,% the% central% wavelength% of% the% optimized% spectrum% is% much% closer%to%1030%nm,%which%is%roughly%the%center%of%the%gain%bandwidth.%Thus,%the%optimization%results%in% increased%power%efficiency%in%addition%to%the%evident%spectral%expansion.%%
a) b)
Fig. 1 The% scheme% of% experimental% setup% (a)% and% the% spectrums% of% a% laser% output% (b)% before% (red)% and% after% the% application%of%an%optimal%filtering%(blue)%together%with%the%optimizing%filtering%transmission%curve.%At%the%scheme% one%can%see%typical%design%of%dispersionHmanaged%laser%with%modified%dispersion%management%stage.%
%
Manipulations%of%the%spectral%transmission%profile%are%similar%to%management%of%the%cavity%losses.%It%is% possible%to%switch%on/off%the%modeHlocking%via%strong'local'changes%of%the%transmission%spectrum,%while% keeping% the% overall% transmission% at% the% nearly% same% level.% In% practice,% our% results% show% that% the% intraH cavity%SLM%can%be%used%as%an%automated%modeHlocked%for%a%nonHselfHstarting%cavity.%
In conclusion, we present the first SLM-optimized mode-locked operation to our knowledge. Preliminary results show possibility to increase spectral bandwidths by 50%, turn on/off mode-locking and virtually unlimited possibilities for manipulation and exploration of the intra-cavity pulse evolution dynamics.%
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[1] M. Fermann, I. Hartl, "Ultrafast fibre lasers", Nature Photon. 7, 868–874 (2013). [2] G. P. Agrawal, Nonlinear Fiber Optics, 3rd. ed., (Academic Press, Boston, 2001).
[3] F. O. Ilday and F. W. Wise, "Nonlinearity management: a route to high-energy soliton fiber lasers", J. Opt. Soc. Am. B 19 (3), 470 (2002).
Fig. 1 (a) Experimental setup showing the typical design of dispersion-managed laser with the dispersive delay line modified to include the SLM. (b) Spectra of the laser output before (red) and after (blue) the application of optimized filter together with the optimized filter’s transmission curve are shown.
In conclusion, we present the first SLM-optimized mode-locked operation to our knowledge. Preliminary results show possibility to increase spectral bandwidths by 50%, turn on/off mode-locking and virtually unlimited possibilities for dynamically reconfigurable manipulation and exploration of the intra-cavity pulse evolution. References
[1] M. Fermann, I. Hartl, “Ultrafast fibre lasers,” Nature Photon. 7, 868 (2013). [2] G. P. Agrawal, Nonlinear Fiber Optics, 3rd. ed. (Academic Press, Boston, 2001).
[3] F. O. Ilday and F. W. Wise, “Nonlinearity management: a route to high-energy soliton fiber lasers,” J. Opt. Soc. Am. B 19, 470 (2002).
