A simple all-fiber widely tunable phosphosilicate Raman fiber laser (RFL) of high efficiency has been developed. The laser has more than 50 nm tuning range, and generates up to 3.2 W of output power with 72% maximum slope efficiency. The output power is almost constant in the range 1258–1303 nm. The width and the spectral power density of the RFL output spectrum can be controlled by the detuning of its cavity fiber Bragg gratings (FBGs) thus being optimized for efficient frequency doubling.
© 2007 Optical Society of America
Raman fiber lasers (RFLs) are attractive light sources providing almost any wavelength in the near-infrared region, see e.g. , that may be also tunable. Such unique features result in many applications, most of all are in telecom. Recently, frequency-doubled RFLs generating in the visible spectrum have been developed [2,3]. So the range of RFLs applications may be considerably extended. A combination of a frequency doubling and tuning in the RFLs may lead to a replacement of complicated tunable dye and Ti:Sa lasers generating in the yellow-red spectral range, which are also difficult in maintaining.
Various approaches have been implemented to develop a tunable RFL (TRFL). The wide tuning range of up to 110 nm at 1.1 μm can be achieved in the RFL with two auxiliary resonators comprising fiber Bragg gratings (FBGs) and a diffraction grating as a tuning element . However the efficiency of such laser is very low because of using bulk elements: Only 50 mW of output power from 7W pump power is generated. The advanced TRFL with almost continuous 120 nm tuning range in the all-fiber configuration has the efficiency of ~16% and the output power of up to 700 mW . Despite the large tuning range, the laser is very complicated as the tuning scheme involves 8 tunable FBGs at generation near 1.15 um. A simple TRFL based on only 1 tunable chirped FBG and a few-mode FBG has been proposed in paper . However, the tuning range of the laser is quite small (~15 nm). The most robust and flexible results are obtained in the ring fiber resonator by using tunable band pass filters for wavelength selection. In such way the TRFLs with tuning range of 40 nm at 1.3 μm  and 76 nm at 1.5 μm with 400 mW output power  have been developed.
All TRFL schemes proposed up to date have the same disadvantages important for a frequency doubling: The output power is not high enough and strongly depends on the wavelength. As the second harmonic power is proportional to the squared fundamental wave power, the power variations of a tunable frequency-doubled RFL will be much stronger. So, before a frequency doubling one needs to develop a tunable RFL with high output power (and high spectral power density), large tuning range and small power variations at the tuning.
In the present paper, we report on the development of a simple high-efficient all-fiber RFL with more than 50 nm tuning range near 1.3 μm. The TRFL generates more than 3 W of output power with 72% slope efficiency. The output power is almost constant at tuning in the range 1258–1303 nm. It has been also shown that the TRFL spectral width can be considerably reduced as well as the spectral power density can be increased by relative detuning of its cavity FBGs that is of high importance for the enhancement of the frequency doubling efficiency.
2. TRFL setup
There are several key points to make a simple all-fiber widely tunable RFL with high efficiency conversion from ~1.1 to ~1.3 μm.
First of all, one should reduce a number of stages for conversion of a pump wave to the Stokes wave at 1.3 μm, as each stage demands two FBGs resonantly reflecting an appropriate multiple-order Stokes wave. To increase output tuning range one should tune all the intermediate FBGs. In order to overcome this problem, we have chosen a phosphosilicate fiber having a large P2O5-related Stokes shift of ~1330 cm-1 as compared to the usual Stokes shift of 440 cm-1 in germanosilicate fibers . Thus, using conventional 1.1 μm pump radiation from the Ytterbium-doped fiber laser (YDFL) one can reach 1.3 μm region in one conversion stage only, that was demonstrated earlier at the fixed Stokes wavelengths near 1.3 μm .
Secondly, if the pump wavelength is fixed the value of the Raman gain (and the RFL power accordingly) depends strongly on the Stokes wave wavelength. Moreover, in this case the RFL tuning range is limited by the width of the Raman gain spectral profile that is only ~50 cm-1 for P2O5-related gain peak in a phosphosilicate fiber. Thus a possible tuning range is limited by ~8 nm for 1.3 μm region. Therefore one should apply for pumping a tunable pump laser, i. e. a tunable YDFL (TYDFL).
Finally, a simple and robust technique for a wide tuning of FBGs forming YDFL and RFL cavities should be developed. Despite of its simplicity, a thermal tuning of FBGs is not appropriate because of a very small tuning range for standard fibers and coatings (only ~1 nm per 100 C has been achieved in 1.24 μm RFL ). A FBGs stretching does not also provide a sufficient tuning range: Only 5 nm tuning has been demonstrated for 1.48 μm RFL . At the same time, it has been shown that a FBG compression by the beam bending technique can provide up to 110 nm tuning range in the region of 1.5 μm . Based on this technique, we have developed a simple and robust FBGs tuning mechanical setup in which the fiber is glued on the plexilgas beam that may be bent in both directions thus providing tuning by the compression and tension at the same time (Fig. 1). Moreover, in order to make a TRFL more simple, we have glued on each plexiglas beam two FBGs simultaneously. Thus both FBGs forming the laser cavity can be tuned synchronously.
As a result, the high-efficient all-fiber widely tunable RFL has been realized (Fig. 2). The RFL linear cavity is formed by two synchronously tunable FBGs and 370 meters of the phosphosilicate fiber with 13 mol% of P2O5 in the core and the MFD of 6.3 μm at 1060 nm. Both FBGs are assembled into one mechanical setup providing the synchronous tuning. The input FBG has more than 98% reflectivity (HR FBG) at its spectral maximum whereas the output FBG is highly transmissive (HT FBG) with reflectivity of ~30 %. The FBGs reflection spectral profiles are initially centered at the same wavelength (~1300 nm).
As a tunable pump source we have used the widely tunable YDFL, for details see . The TYDFL cavity is formed by the 25-m long Yb-doped fiber analogous to the GTWaveTM fiber and two tunable FBGs assembled into one mechanical setup for the synchronous tuning. The FBGs central wavelengths in the uncompressed state are ~1110 nm. The reflectivity of the output FBG (15%) is optimized for the maximum TYDFL output power. The end FBG has 90% reflectivity. The TYDFL generates up to 7 W from 15 W of the multimode 980 nm pump power. The TYDFL output power is nearly constant in the tuning range of 1075–1110 nm.
3. TRFL performances
The output RFL radiation comprises the generated Stokes wave with some addition of the residual pump wave. To measure the output power precisely, the RFL radiation has been collimated by a lens, and different spectral components has been selected by means of a prism. The Stokes wave generation starts at 2 W of the YDFL pump power, and above the threshold grows up linearly with 72% slope efficiency at increasing input pump power (Fig. 3a). At the same time, the residual pump power exhibits saturation at the threshold and the reduction with increasing Stokes wave power owing to the pump power depletion through the SRS process, see e.g. [15, 16]. The maximum generated Stokes wave power takes the value of 3.2 W that is limited by the available pump power from the YDFL. The estimated 72% slope efficiency of the pump to Stokes wave conversion is close to the quantum efficiency of 85%.
The measured RFL output spectrum is shown in Fig. 2b. The spectrum has wide exponential wings accumulating essential part of the output power that is unlikely for an efficient frequency doubling . Moreover, the spectrum width is growing rapidly while the generated power increases that is also undesirable. It has been shown recently that the spectral widening in a long high-Q RFL cavity is caused by the fundamental reasons: Quasi-degenerate four-wave mixing processes involving multiple Stokes wave longitudinal modes which induce stochasticity [17,18]. Such turbulent broadening mechanism leads to the square-root growth of the spectral width with increasing power as well as defines the specific exponential shape of the spectral wings. The turbulence-based spectral broadening mechanism proposed in papers [17,18] seems applicable not only to the high-Q but also to the low-Q cavity realized here. In contrast to the bell-shaped intracavity profile, the output spectrum at high powers has a central dip (Fig. 3b) that is shown to be caused by the combination of the intra-cavity spectrum and the output FBG transmission profile .
By the synchronous compression and stretching of the FBGs forming YDFL and RFL cavities, a continuous tuning of TRFL in >50 nm range has been obtained (Fig. 4a). The TRFL wavelength depends linearly on the pump laser (TYDFL) wavelength. In the wavelength range 1275–1303 the TRFL output power is constant with ~1 % accuracy, and in 1258–1303 nm range it has ~10% variations, see Fig. 4b. Thus, the main disadvantage of the previously developed TRFLs has been overcome. However, the output power decreases at further compressing down to the 1250 nm. We have checked that the RFL output power decrease is caused by a mismatch of the FBGs spectral profiles in the twin FBG beam bending setup, arising because of inhomogenities of the bent beam at high compression degrees. This supposition has been confirmed in the special experiment in which the output and the end FBGs of the RFL cavity have been tuned independently. One can see the power decrease if the FBGs detuning is nonzero (Fig. 5a). It is also clear that the width (~1 nm) of the detuning curve is determined by the width of highly-reflective FBG spectral profile.
The TRFL output power is not constant in the large tuning range if the RFL is tuned at the fixed pump wavelength (Fig. 5b). In this case the RFL output power nearly copies the Raman gain spectral profile that depends strongly on the wavelength. By such independent RFL tuning only ~1–2 nm RFL tuning with more or less constant output power can be realized.
4. TRFL spectrum control
The FBGs mismatch impacts not only the output power but the generated spectrum also. The typical RFL output spectrum at different FBGs mismatch is shown in Fig. 6. If the FBGs spectral profiles are centered at the same wavelength, the RFL output spectrum is wide, and the power is high. At large FBGs spectral displacement, the spectral width is small, but the output power as well as the spectral power density are also small. At the same time, there are some intermediate values of the mismatch when the output spectrum is narrower (Fig. 6, red curve) than the spectrum at zero detuning (blue curve). Moreover, despite of the fact that the total RFL output power is by ~20% less than the power at zero FBGs mismatch, the spectral power density in the ~0.2 nm bandwidth becomes higher (30% higher in the spectral maximum).
It is well-known that the spectrum width and the spectral power density are key parameters for a frequency doubling. The typical RFL spectrum is growing with the power reaching 1–2 nm width at the multiWatts level that is too broad for the efficient frequency doubling. As a result, the second harmonic power grows linearly with increasing fundamental wave power instead of the conventional quadratic low . The possibility to control the RFL output spectrum, especially to reduce its width and to increase simultaneously the spectral power density in the narrow bandwidth of ~ 0.2 nm by the introducing some mismatch between FBGs spectral profiles may led to an essential increase of the second harmonic power and the generation efficiency.
Let us note that the typical mismatch at which the spectrum narrowing and spectral power density increasing are possible is about 0.3 nm or less. Therefore the RFL output spectrum can be easily on-line controlled by the changing the FBGs temperature  in addition to the developed mechanical setup for wide-range tuning by the compression.
Thus, we have developed the simple all-fiber widely tunable RFL having high efficiency. The TRFL has more than 50 nm tuning range in 1.3 μm region. The TRFL generates up to 3.2 W of output power that is almost independent on the generation wavelength. The TRFL slope efficiency of 72 % is close to the quantum efficiency. For achieving the constant output power in the wide tuning range, the TRFL has been pumped by the widely tunable Ytterbium-doped fiber pump laser. An application of synchronously tuned FBGs in the RFL cavity let us develop the simple TRFL design, that may be further improved by placing into single mechanical setup both RFL and YDFL cavity FBGs.
It has been shown that the decrease of the RFL output power at high FBGs compression degrees is caused by the FBGs center wavelengths mismatch. To make wider the TRFL tuning range of constant output power, one should choose a special geometrical shape for plexiglas beams in order to provide more uniform mechanical stress distribution along both fibers. Using FBGs with wider spectral profiles can also eliminate the problem of power decreasing while tuning in the wide spectral range.
We have also experimentally shown that by external driving of the FBGs mismatch, the RFL output spectrum can be controlled. It is possible to reduce the spectrum width and to increase the spectral power density at the same time. The proposed RFL spectrum control technique should be useful for frequency doubling of Raman fiber lasers radiation.
The authors acknowledge financial support by the Integration program of the Siberian Branch of the Russian Academy of Sciences, the programs of the Presidium and the Departmrnt of Physical Sciences of the Russian Academy of Sciences, CRDF grant RUP1-1509-NO-05; and the Fiber Optic Research Center (Moscow, Russia) for supply of the fibers. We also thank V. A. Akulov, A. E. Ismagulov, and I. S. Shelemba for technical assistance.
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