A gain-switched Tm-doped double-clad silica fiber laser operating at a wavelength of approximately 2µm with moderate output energy of 14.7mJ per pulse and a slope efficiency of 39.5% (with respect to launched pump energy) is realized pumped at 1.064µm from a Nd:YAG laser. The gain-switched fiber laser pulses are built up by a series of relaxation spikes, and every spike pulse duration is nearly 1µs. The output wavelength becomes longer, and the slope efficiency increases with the increase in fiber length.
© 2005 Optical Society of America
Rare-earth-doped fiber lasers offer an excellent combination of high efficiency and spatial beam quality. In many applications there is a requirement of short high peak-power pulses as sources for distributed and remote gas sensing . In the 2µm spectral region, the damage threshold of the human eye is high (about 1J/cm2), and lasers operating in that wavelength range are considered eye-safe and find practical application in laser range finding and eye-safe lidar, surgery in medicine, ultra-low-loss long-span communications, nonlinear-frequency conversion applications, long-distance optical time-domain reflectometry, and heterodyne detection applications.
Q-switched Tm3+-doped silica fiber lasers generate high-peak-power and relatively long-duration pulses, ranging from 100 ns to several hundred nanoseconds [3–5]. Mode-locked Q-switched Tm3+-doped silica fiber lasers can generate high-peak-power, high repetition frequency, and short-duration pulses of several hundred femtoseconds [6,7]. The pulse energy from the fiber laser is limited by power density considerations, and the intra-cavity elements used in the systems make the laser cavity complex. Gain-switched fiber lasers are convenient to use as pulse pump sources. For applications requiring a few hundred nanoseconds pulse duration in the 2µm spectral region, a feasible solution is use of gain-switched Tm3+-doped silica fiber lasers [8,9]. The goal of this investigation is to examine the characteristics of the gain-switched Tm3+-doped double-clad silica fiber lasers pumped at 1.064µm.
The absorption section of Tm3+-doped silica fiber at 1.064µm is 1.2×10-26 m2, but the absorption section of Tm3+-doped silica fiber at 790nm is 5×10-25 m2. Pumped at 1.064µm region, the Tm3+-doped fiber lasers use single-clad silica fiber, for example, CW output powers generates >1W at slope efficiencies of 37% and gain-switched pulse output generates 1.5mJ per pulse at slope efficiencies of 20%. Another major drawback pumped at 1.064µm is the strong pump ESA present. Pumped photons are absorbed from the 3F4 and 3H4 levels to the 3F2,3 and 1G4 energy levels (the absorption section is 3.2×10-25 m2 and 3.0×10-25 m2 respectively, and see the simplified energy level diagram in Fig. 1) .
As we know, this investigation is the first time to examine the gain-switched Tm3+-doped double-clad silica fiber lasers pumped at 1.064µm. In order to increase the absorption ratio of pumping laser pulse energy, we use a relatively long fiber.1 (35m, our fiber maximum length), a fiber.2 (8m), and a short fiber.3 (1m) which is used to be contrasted. The Tm3+-doped double-clad silica fiber is purchased from OFTC of Australian Photonics. Its core diameter is 18µm with a NA 0.158. The inner-clad diameter is 300µm with a shape of D and NA 0.37. The Tm3+-doped concentration is 1.3wt.% in the fiber core. The fiber has an effective absorption coefficient ~0.008m-1 (calculated according to ) at 1.064µm. The Nd:YAG pump laser pulse duration is ~100µs, and pumped with a single frequency flashlamp. The maximum total output energy is nearly 100mJ per pulse. No attempts are made to restrict oscillation to TEM00 and hence the laser output is quite multimode.
In the experiment, shown schematically in Fig. 2., the cavity uses Fresnel reflections from the cleaved ends of the fiber as feedback. A dichronic (R>90 at 1.064µm and T>99 at 1.8~2.1µm) butts to the distal end of the fiber, so the unabsorbed pump laser energy can be reflected to the fiber once more and the reflection is <0.1% at 1.8~2.1µm. With a lens f=50mm, the coupling efficiency can reach >50%. When the coupling position is best adjusted, the 467nm blue fluorescence associated with spontaneous emission from 1G4 energy level is visible.
3. Results and discussion
The temporal characteristics of the gain-switched fiber laser pulses from the fiber.1 and fiber.2 are measured by a PIN detector (monitoring the pump pulses) and a HgCdTe detector (monitoring the fiber laser pulses). The results are showed in Fig. 3.~Fig. 6. The fiber laser pulses from the fiber.1 and fiber.2 are a series of relaxation spikes and the duration of the relaxation spike pulses is about 600~800ns from the Fig. 4. and 700ns~1µs from Fig. 6.. The quite chaotic temporal characteristics of the output from the gain-switched fiber laser are related to mode hopping and mode competition . The fiber laser duration is longer than the result <500ns from single-clad Tm3+-doped silica fiber laser , and perhaps because the double-clad fiber is longer and has the lower concentration than the single-clad fiber used in ref. which has the length <1m and the concentration 1.5wt.%. The fiber laser pulse time delays from fiber.1 and fiber.2 are about 48µs from Fig. 3. and 25µs from Fig. 5. relative to the pump pulses.
An enhancement of different length of the fiber.1, fiber.2 and fiber.3 leads to a variation in the fiber laser spectrums (given in Fig. 7.). The spectrum characteristics are measured by the monochromator WDG30 with a grating of 300 strip/mm and blaze wavelength 2µm. A PbS detector is used to monitor the pulses energy from the monochromator output slit. The output laser peak wavelength of fiber.1, fiber.2 and fiber.3 is 2.04µm, 2.0µm and 1.92µm respectively. In the approximately 2µm spectral region, the shorter wavelength laser has higher reabsorption losses with the silica fiber length increasing . The intrinsic absorption peak of silica is at 1.94µm, so the relatively shorter laser wavelength has more intrinsic loss with the fiber length increasing . Because the reabsorption losses and the intrinsic absorption losses are relatively less in the fiber.3, the output laser is similar to the spontaneous radiation fluorescent spectrum . The output laser spectrums from fiber.1 and fiber.2 are concordant to the general laser spectrum characteristics, and have a relatively narrow line width, though there are no specific cavity mirrors.
The fiber laser pulse energy (measured with the Newport energy meter 818-J25) from the three fiber lasers as a function of the launched pump energy is shown in Fig. 8.. The launched pump energy is measured by the cutback method. The output laser pulse energy from fiber.1 reaches the maximum 14.7mJ per pulse. (For the Fresnel cavity, it is assumed that equal amounts of laser energy are emitted from each end of the fiber and therefore pulse energy measurements made at the distal end of the fiber are doubled .). The variation in the slope efficiencies and thresholds for the three fiber lasers is shown in Fig. 9. The slope efficiency increases with the fiber length increasing. The absorption ratio of the double-clad fiber is much smaller than the single-clad fiber and there exists amounts of unabsorbed pump energy, so the longer fiber can absorb more pump pulse energy and acquire more gain than the shorter one.
In conclusion, we have presented >14mJ of total gain-switched fiber laser output from a Nd:YAG cladding-pumped Tm-doped silica fiber laser. The maximum slope efficiency is 39.5% (with respect to the launched pump energy), which to our knowledge is the highest slope efficiency that has been reported pumped at 1.064µm in a similar cavity. A series of relaxation spikes from the gain-swirched fiber lasers are observed and have potential applications in surgery and others areas requiring pulse duration of a few hundred nanoseconds. The output peak wavelengths are 2.04, 2.0, and 1.92µm with fiber lengths of 35, 8, and 1m. Because the pump pulse characteristics and cavity are similar to those in Ref. , it is convenient to contrast our results to single-clad fiber results from Ref. . The output energy from fiber 1 and fiber 2 have not saturated appearances during the launched pumped pulse energy range from 27 to 73mJ, but in Ref.  there are saturated appearances during the launched pumped pulse energy range from 6 to 24mJ; thus with Tm3+-doped double-clad fiber, the pulse energy can be greater than with single-clad fiber. There are more fiber laser spike pulses from the double-clad fiber than from the single-clad fiber during a pump pulse duration. The high-energy pulse is easily coupled into the double-clad fiber. With heavily Tm3+-doped big-core-diameter fiber to increase absorbed ratio and optimal length, the fiber laser will further increase the output energy and slope efficiency.
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