Letter
OPTICAL REVIEW Vol. 14, No. 2 (2007) 75–77
Tunable Ytterbium-Doped Fiber Laser Based on a Mechanically Induced Long Period Holey Fiber Grating Gilberto A NZUETO-SA´ NCHEZ, Alejandro M ARTI´NEZ-R IOS, Ismael TORRES-G O´ MEZ, Daniel C EBALLOS-H ERRERA, Romeo S ELVAS-A GUILAR1 , and Victor D URAN-R AMIREZ2 ´ ptica A.C., Leo´n, Guanajuato 37150, Me´xico Centro de Investigaciones en O 1 Facultad de Ciencias Fı´sico Matema´ticas, UANL, Cd. Universitaria, N.L. 66450, Me´xico 2 Universidad de Guadalajara, Centro Universitario de los Lagos, Jalisco 47460, Me´xico (Received August 23, 2006; Accepted November 24, 2006) We describe a tunable double-clad Yb-doped fiber laser based on a long period fiber grating mechanically induced in a section of single mode holey fiber inserted into the laser cavity. The mechanically induced long period holey fiber grating acts as a wavelength-selective fiber filter whose central wavelength, linewidth, and strength can be tuned by changing the period, the length of the grating, and the applied pressure. The fiber laser gives a 12:6 nm tuning range, from 1079:4 {1092 nm, with slope efficiencies of 18.7 – 26.3% at this wavelength range, with respect to the launched pump power. # 2007 The Optical Society of Japan Key words: double-clad fiber laser, long period grating, wavelength tuning, ytterbium-doped fiber, holey fiber
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spectral position of the notch, while the pressure variation provides control of the strength of the notch. By adjusting the period of the mechanical pattern and the applied pressure on the holey fiber, we demonstrate a wavelength tuning range of 12.6 nm. Slope efficiencies in the range of 18.77 – 26.31% are obtained at the wavelength range of 1079.4 – 1092 nm.
Introduction
The search for simple and cost effective techniques for wavelength tuning of fiber lasers is the subject of continued interest. Fiber lasers offer broad absorption and emission wavelengths, and the possibility of obtaining high output laser power when a double clad geometry is used. The wavelength tuning in fiber lasers can be realized by external dispersive bulk elements,1,2) or in-line fiber elements.3–7) Tunable fiber lasers based on bulk elements require complicated arrangements which require expensive devices. Consequently, all-fiber systems are advantageous since they do not require free space components. One of the techniques used for all-fiber wavelength tuning is based on the insertion of in-line band-rejection filters into the laser cavity. In this case, the position of the stop band is selected such that it precludes oscillation at that particular wavelength range, modifying the overall gain spectrum of the laser cavity and allowing lasing at the resultant preferential wavelength. On the other hand, long period fiber gratings (LPFGs) have demonstrated their versatility as wavelength optical filters for applications where the equalization of the gain spectrum of doped fibers is required.8,9) Mechanically induced LPFGs that offer the advantages of broad tuning and the capability of being erased and reconfigured,10) are particularly useful for these applications. Several techniques of mechanically induced tunable LPFGs have been reported in standard communication fibers and holey fibers, where grooved plates (GPs), strings, and springs have been used to apply periodical mechanical stress on the optical single mode fiber to induce an effective index modulation.11,12) In this respect, holey fibers have shown an enhanced sensitivity to mechanical pressure that makes them suitable for implementation in tunable LPFGs.13) In this work, we present a tunable doubleclad Yb-doped fiber (DCYDF) laser, where a mechanically induced band rejection filter in a holey fiber is inserted into the laser cavity as the wavelength selective element. The adjustment of the grating period allows a change in the
2.
Setup
Figure 1 shows the experimental setup of the tunable fiber laser. It consists of 30-m-long DCYDF with core/cladding dimensions of 6/125 mm, and 0.14/0.45 of numerical aperture. The DCYDF is end pumped by a 905 nm pigtailed diode laser connected to a fiber collimator via an SMA (SubMiniature version A) adapter. The collimated pump is coupled to the input end of the DCYDF by an aspheric lens. Between the fiber collimator and the aspheric lens there is a 45 dichroic mirror with a high transmission at the pump wavelength and a high reflection from 1050 {1100 nm, which prevents back reflection to the pump diode and serves as output coupler for the laser signal. The other end of the DCYDF is spliced to a 60-cm-length single mode holey fiber which has core/cladding diameter of 11/125 mm. The cladding presents a hexagonal pattern of holes with lattice pitch of 11 mm and hole diameter of 5 mm. Figure 2 shows a photograph of a transversal section of this holey fiber. The holey fiber is mounted between two 50 50 mm2 square
Fig. 1. Experimental setup. 75
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Fig. 2. Cross sectional image of the holey fiber.
Fig. 3. Sketch of the device used to mechanically induce the LPG in the holey fiber.
metallic plates, one of which had a radial grooved periodical pattern with 60 step grooves and each of which is 360 mm in width and 900 mm in depth. The mechanically induced tunable LPFG is obtained by pressing an adjustable half semicircular section of a single mode holey fiber on a radial GP by a flat plate as is shown in Fig. 3. A detailed description of the tunable long period holey fiber grating was reported by Ceballos-Herrera et al. in a previous work.14) This design in particular allows changing of the period () of the mechanically induced LPG by adjusting the circular bending radius (R) of the holey fiber from 15 to 50 mm according to the expression ¼ ð=120ÞR. Therefore, the center wavelength of the notch is adjusted by changing the period of the LPG and the depth of the notch depends directly on the pressure of the flat plate. In this way, the fiber laser cavity is defined by the DCYDF, the spliced holey fiber, and each of the two perpendicular cleaved ends at the sides of the cavity gives 4% of Fresnel reflection. The output laser light is detected from the output end of the holey fiber and the reflected light at the dichroic mirror. 3.
Experimental Results and Discussion
The first step in the implementation of the tunable fiber laser was the characterization of the mechanically induced LPG on the holey fiber. Figure 3 shows a sketch of the mechanically induced LPG. As already mentioned, it consists of one radial corrugated GP, on top of which is placed a flat plate where the mechanical pressure is applied. The holey fiber is placed between the two plates such that when the pressure is applied the fiber is subject to periodic microbending that introduces an index modulation via the photoelastic effect. The central wavelength of the notch filter created by this mechanical device depends on the size period of the induced LPG. White light transmission measurements in the mechanically induced long period grating showed that the dependence of the center wavelength on the grating period can be fitted by a fifth order polynomial, i.e., center ¼ c0 þ c1 þ c2 2 þ c3 3 þ c4 4 þ c5 5 , where is the grating period and the coefficients ci are given by: c0 ¼ 109733, c1 ¼ 1026:03, c2 ¼ 3:89799, c3 ¼ 0:00738438, c4 ¼ 6:94664 106 , and c5 ¼ 2:59216 109 . This relation does not take into account variation with the applied pressure, as the main effect observed with increasing pressure is the depeening of the notch depth. Figure 4 shows the experimental white light transmission
1080.6 nm Λ=603.475 µm
1084.8 nm Λ=600.363 µm
1091.4 nm Λ=595.78 µm
-84 1040 1050 1060 1070 1080 1090 1100 1110 1120 1130
Wavelength [nm]
Fig. 4. White light transmission spectra of the LPG at three different grating periods.
spectra of the LPG at three different center wavelengths, 1080.6 (solid curve), 1084.8 (dashed curve), and 1091.4 nm (dotted curve), which according to the polynomial fit corresponds to grating periods of approximately 603, 600, and 595 mm, respectively. These values were in agreement with experimental data.14) As observed with other mechanically induced LPGs in holey fibers,15) the center wavelength of the notch is reduced as the grating period is increased, while a change in the number of periods varies the bandwidth of the notch and variation of the applied pressure changes the depth. The fiber laser was first operated as a free running fiber laser without the holey fiber spliced to the DCYDF. Under these conditions the laser operated at a wavelength of 1089 nm with a threshold pump power of 131 mW and 61.52% of slope efficiency with respect to the launched pump power. When we spliced the piece of holey fiber to the DCYDF, the threshold pump power increased up to 324 mW, while the slope efficiency decreased to 27.1%. From these data, we estimated the splice loss between the DCYDF and the holey fiber plus the intrinsic holey fiber losses as 3:97 dB. As we will see, when the holey fiber is under pressure, there is an increase in the loss which further reduces the laser efficiency. The dotted line in Fig. 5 shows the transmission spectrum of the LPG when the center wavelength and the depth of the notch are 1089.6 nm ( ¼ 596:995 mm) and 16 dB, respectively. As can be observed, the effect of the LPG is to introduce an excess loss around the peak fluorescence, thus modifying the original spectrum (dashed line in Fig. 5), so that the wavelength at which the gain is maximum is altered. This results in a different free-running laser wavelength. By varying slightly the position of the center wavelength of the notch, we were able to tune the fiber laser in a range of 12.6 nm, from 1079.4 to 1092 nm. Figure 6 shows the output laser spectra at four different wavelengths obtained by tuning the mechanically induced LPG, namely at 1079.4, 1084.8, 1087.8, and 1092 nm. It is worth mentioning that under certain conditions the laser can operate at multiple wavelengths where the laser is clearly unstable, mainly due to problems of mechanical stability of the device and to nonuniform pressure that result in slight
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OPTICAL REVIEW Vol. 14, No. 2 (2007)
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LPG transmission spectrum Fluorescence spectrum Modified fluorescence spectrum
Output laser power [mW]
Normalized amplitude
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700 600 500 400 300 200 100 600
900 1200 1500 1800 2100 2400 2700
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Fig. 5. Modification in the fluorescence spectrum induced by the LPG.
Normalized amplitude [u.a.]
P1079.4=0.1877(Pp-434.867) P1084.8=0.21522(Pp-440.93) P1087.8=0.23784(Pp-407.133) P1092.0=0.26313(Pp-391.126)
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Fig. 7. Output laser power as a function of the pump power at four different wavelengths.
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obtained. The main advantage of this scheme for wavelength tuning of fiber lasers is the simplicity and low cost of the device.
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Acknowledgments
1084.8 nm
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1087.8 nm 1092.0 nm
1079.4 nm
The authors acknowledge support from CONACYT (Grant SEP2004-C01-47237/A1) and CONCYTEG (Grant 05-*04-K117-015) in the realization of this work.
0.4 0.2 0.0 1060
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1090
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Wavelength [nm]
Fig. 6. Output laser spectra at four different wavelengths.
variations in the position of the center wavelength of the notch that cause gain competition between different lasing lines due to homogeneous broadening. We have designed a mechanical fixture that in principle can improve the mechanical stability of our device and can provide a more uniform pressure on the holey fiber. Figure 7 shows the total output laser power as a function of the launched pump power for the four laser wavelengths referred to above (1079.4, 1084.8, 1087.8, and 1092 nm). Their respective slope efficiencies are 18.77, 21.52, 23.78, and 26.31%. As we approach to longer wavelengths, the slope efficiency increases, which can be explained by the fact that, without pressure on the microstructured fiber, the laser operates at 1089 nm. Further improvements in the laser efficiency can be obtained by inserting a low loss microstructure fiber, and optimization of the splice loss between the DCYDF and the microstructured fiber through mode matching techniques. 4.
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Conclusions
In conclusion, we have reported a simple method for wavelength tuning of fiber lasers based on the use of a tunable mechanically-induced LPG in a holey fiber. The center wavelength and depth of the notch are adjusted by changing the period and the applied pressure on the holey fiber. Using this approach, we have demonstrated a wavelength tuning range of 12:6 nm. Between this wavelength range, slope efficiencies in the range of 18.7 – 26.3% are
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