Appl Phys B (2011) 105:213–217 DOI 10.1007/s00340-011-4689-7
Phase-locked supermode emissions from a dual multicore fiber laser B.M. Shalaby · V. Kermene · D. Pagnoux · A. Desfarges-Berthelemot · A. Barthélémy
Received: 13 April 2011 / Revised version: 8 June 2011 / Published online: 23 August 2011 © Springer-Verlag 2011
Abstract We report on a fiber laser configuration, emitting two phase-locked and single mode beams at the output of two multicore fibers. The passive technique is based both on the selective excitation of the only in-phase supermodes of the multicore fibers and on an intra-cavity angular filtering of the emitted beams to provide phase control. As a proof of principle, we experimentally demonstrate the coherent combining of the fundamental in-phase supermodes of two parallel 7-cores fibers.
1 Introduction Multicore fiber (MCF) lasers are promising laser sources since they combine fiber advantages (reliability, ruggedness, compactness, high thermal dissipation, etc.) with a large mode field diameter. At high power level, large mode area is required to reduce nonlinear effects especially when fiber lasers operate in pulsed regime. Nevertheless, multicore fibers have similar modal behavior as standard multimodal fibers, leading to a poor spatial quality of the emitted beam in a general case. Different techniques to control the emission of multicore amplifying fibers have been proposed in recent years. They are often based on spatial filtering B.M. Shalaby · V. Kermene () · D. Pagnoux · A. Desfarges-Berthelemot · A. Barthélémy CNRS, XLIM Institut de Recherche, UMR No. 6172, Université de Limoges, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France e-mail:
[email protected] Fax: +(33)-555-457738 B.M. Shalaby Physics Department, Faculty of Sciences, Tanta University, Tanta, Egypt
[1–7]. They sometimes use more complex phenomena which are more difficult to manage such as gain nonlinearity [8] or core coupling by fiber curvature [9]. Some papers also report the possibility that a multicore fiber can only guide the fundamental mode thanks to a fine adjustment of its opto-geometrical characteristics (core size, pitch and numerical aperture) [10, 11]. Coherent combining applied to parallel fiber amplifiers is another way to extract high power level from fiber lasers. Many different passive techniques have been studied in the last past years to phase-lock fiber amplifier arrays [12–15]. Laser configurations leading to parallel emission of in-phase output beams [14] are more specifically appropriated to high power emission, because beam combining is performed out of the cavity, in the far field. In such a way, a total output power of 736 W has been obtained [15] in a laser configuration made of four parallel fiber amplifiers. Recently, spectral combining of four Q-Switched fiber lasers has been demonstrated [16] to reach MW peak power. The active fibers of the last amplifier stage had a 80 µm core diameter. This large value is close to the limit from which a single mode behavior is difficult to maintain because of inhomogeneous transverse gain, thermal effects, and environmental perturbations. The use of multicore fibers in connection with a combining technique has the potential to go further in terms of energy and peak power from fiber lasers. In this paper, we report on a laser configuration made of two multicore fibers that generates two in-phase fundamental supermodes, both beams interfering in the far field outside the cavity to provide a bright laser emission. The reported laser configuration, which is a proof of principle, is based (i) on the selective excitation of the fundamental mode of each multicore fiber as described in reference [3], (ii) and on a spatial filtering operation at the output which induces in-phase emissions from the two fibers, as shown in
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Fig. 1 Schematic drawing of the laser ring cavity; YDFA: Ytterbium doped fiber amplifier, SMFF: single mode feedback fiber, ISO: optical isolator, PC: polarization controller, P: polarizer, ISi (i = 1 or 3): imaging system, MCFi (i = 1 or 2): multicore fiber, OC: output coupler, L positive lens with focal length (f ), d and D are respectively the separation distance between the two centers of the MCFs and of the output beams
paper [14]. Let us note that the multicore fibers used in our experiments are not rare earth doped. Thus, the objective of the following study is to propose a fiber laser configuration fitted to the emission of high average power.
2 The dual multicore fiber laser setup Efficiency of coherent combining techniques depends on the spatial quality of the elementary beams. Thus, the laser configuration has to limit the number of modes guided by the multicore fibers used as amplifiers. Selective injection is a common practice to reduce the number of excited modes in a multimode waveguide [17, 18] and in some cases it can be efficient enough to excite only the fundamental mode. We had previously obtained single mode emission from a 19 cores fiber thanks to a proper excitation beam [3]. In the present paper, we report on a laser architecture based on a unidirectional ring cavity including two MCFs (Fig. 1). The guiding structure of these MCFs (so-called MCF1 and MCF2 in Fig. 1) is based on a hexagonal arrangement of 7 elementary single mode cores: a central core surrounded by a ring of 6 identical cores (Fig. 2). The mode field radius of the fundamental mode in each core (core diameter Φ = 6.3 µm and numerical aperture NAcore ∼ 0.075) is ωic = 5.2 µm at λ = 1080 nm. The pitch, i.e., the distance between the centers of neighboring cores, is 9 µm and the cladding of the 7-cores fiber has a diameter of 200 µm. Numerical computations based on the finite element method show that each MCF can guide up to 7 supermodes at the 1080 nm wavelength. The intensity patterns of these supermodes are shown in [2]. The computed effective area of the fundamental supermode of each MCF is 381 µm2 . The two MCFs are seeded in parallel by a Gaussian beam of 12 µm FWHM from a single mode fiber (6.2 µm core diameter, 0.14 numerical aperture). The two input Gaussian
beam are resized by means of imaging systems (IS1 and IS2) in order to fit the fundamental supermode of the MCFs. Both output beams from the MCFs are partially reflected by an output coupler (OC) to interfere in the far field at the focal plane of a converging lens L. The magnification of the imaging system IS3 and the focal length (f ) of the lens L are chosen so that the single mode feedback fiber (SMFF) only collects the central part of this far field pattern. The feedback signal is then amplified through an ytterbium doped fiber amplifier (YDFA). It must be noted that, in this demonstration of principle, the YDFA is the only amplifier of the laser configuration as the MCFs are passive fibers. The net gain of the amplifier is high enough to compensate for the filtering process induced by the feedback single mode fiber (SMFF). In the configuration described in Fig. 1, the fiber amplifier delivers 150 mW whereas the feedback fiber collects only 1 mW. A couple of optical isolators (ISO) assigns a single traveling direction of the radiations in the cavity. The SMFF is spliced to a 50:50 coupler providing two identical beams feeding each MCF. Two polarizers (P) and polarization controllers (PC) manage the polarization states in the laser cavity. 30% of the overall power is extracted from the laser through the output coupler. MCF1 and MCF2 are respectively 65 cm and 35 cm long. The fiber lengths are chosen different to avoid any unstable operating regime as described in reference [19]. However, this length difference is not critical. It only requires being longer than the correlation length of the laser radiation which is about 5 mm. The fibers are kept straight line to avoid inner mode coupling.
3 Selective excitation The coupling coefficient Ci between the input beam and the supermode labeled “i” of a MCF is the normalized overlap
Phase-locked supermode emissions from a dual multicore fiber laser
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tems IS1 and IS2. In these conditions, the fundamental supermode I is selectively excited.
4 Coherent combining process
Fig. 2 Image of the 7-core fiber facet. In the upper right corner a magnified image of the 7-core region is shown including the dimensions in microns
Fig. 3 (a) Calculated intensity distributions of supermode I (fundamental) and supermode II guided by each 7-core fiber under on axis excitation; (b) Coupling coefficient Ci between a Gaussian input beam and the supermodes I and II of the 7-core fiber versus the input beam mode field radius (ωi )
integral between the input beam and this supermode. The fraction of the input power coupled into the supermode “i” is equal to Ci2 . When a Gaussian beam is launched on axis into one 7-core fiber, the overlap integral with the supermodes exhibiting an azimuthal modulation is zero, because of symmetry considerations. Thus, such supermodes are not excited. In other words, the only supermodes to be excited are those with an axial symmetry. In such a configuration, only the two supermodes shown in Fig. 3a can be excited in the MCFs (supermode I = fundamental supermode with mode field radius ω0I = 12.5 µm, and supermode II). The coupling coefficient obviously depends on the mode field radius ω0 of the input Gaussian beam, as shown in Fig. 3b. When ω0 = 12 µm, almost all the input power is coupled to the in-phase supermode (I) ∼97% while only ∼3% is coupled to the supermode II. In the experimental setup, the Gaussian beam launched into each MCF is then carefully centered. Its size on the MCF input face is set to about 12 µm by adjusting the magnification of the imaging sys-
The output faces of the two MCFs are set in the same plane, their centers being spaced by a distance d = 250 µm (the diameter of cladding in each MCF is 200 µm). The parallel output beams from the MCFs interfere in the far field and the SMFF core acts as a spatial filtering element in the input face plane of this fiber. In order to optimize the filtering efficiency, the imaging system IS3 and the lens L (focal length 8 mm) are chosen so that the width of the interference fringes in the far field pattern (λf/D) is nearly equal to the diameter of the fundamental mode of the SMFF. Thus, a constant phase relationship between the two output beams is strictly imposed. The dual multicore fiber laser self adjusts its resonance frequencies to maintain a maximum power level coupled into the SMFF, minimizing the intracavity losses. This constant adjustment compensates for possible environmental perturbations on phase relationship between both laser outputs in such interferometric configuration. Let us notice that the large distance between the MCFs centers is due to the cladding thickness. Without specific double imaging system, the filling factor FF (output beam diameter compared to the output beam spacing: FF = 2ω0I /d = 10%) was very poor leading to numerous fringes in the far field pattern. Then, the SMFF only collects a very small part of the whole beam giving rise to large intracavity losses. To increase the Q factor of the cavity, we used an output coupler which reflects a significant part of the emitted power (70%). In an upgraded configuration, the filling factor FF should be improved so that intracavity loss should fall, allowing decreasing of the output coupler reflectivity. This reflectivity could be reduced to some percents like in the configuration of [14], protecting the SMFF from high power level. Figure 4a displays the computed near field of the in-phase supermode at the output faces of the two MCFs while Fig. 4b shows the corresponding far field pattern where both beams are coherently combined. Figures 5a and 5b show the experimental patterns of the in-phase supermodes recorded at the output of each MCF and Fig. 5c is the corresponding experimental far field pattern. As can be seen in Figs. 4 and 5, the calculated and experimental results are in very good agreement. The experimental far field pattern (Fig. 5c) shows that the profile and the divergence of its envelope are similar to those of the MCF fundamental supermode. This result proves that the proposed configuration is able to select the in-phase supermode of both MCFs. This combined far field exhibits also contrasted fringes proving the efficient phase-locking of the
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Fig. 4 (a) Theoretical near field of the in-phase supermode at the exit of the MCFi (i = 1, 2); (b) and coherently combined far field
Fig. 5 Experimental near field patterns recorded at the output of the MCF1 (a) and MCF2 (b) and far field (c)
two output beams. Position of the fringes is stable over time and it is adjustable by shifting the SMFF input face in the focal plane of the lens L. The stability of the modulated far field is the signature that both output beams oscillate on the same resonance frequencies of the laser cavity. However, in such a configuration, the loss due to the SMFF filtering is high due to the large number of fringes in the far field pattern. It should be significantly reduced by improving the filling factor FF and by the use of a 2D array of MCFs. The measured spectrum is peaked about ∼1076 nm with a narrow line-width of about 0.3 nm FWHM.
5 Conclusion In this paper, we demonstrated, for the first time to our knowledge, coherent combining in a fiber laser including two multicore fibers. The hybrid laser architecture is based on a ring laser cavity with two arms made of two pieces of a seven-core fiber. The cavity geometry is characterized by a specific selective injection in the MCFs leading to the excitation of the in-phase supermode and by a specific angular filtering that phase-locks both supermodes. As a consequence, coherent combining of the two MCFs radiations is obtained
in the far field. Observations in terms of near field and far field are in good agreement with the expectations deduced from computations. The filling factor remains poor here but it can be improved by decreasing the separation distances (d) between the two output beams with an appropriate imaging system and by using a 2D array of multicore fibers. Extension to a larger number of MCFs, to Q-switched regime, and use of actively doped fibers are planned for the future.
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