We propose a simple design of a saturable absorber Q-switched all-fiber ring laser. By locating a saturable absorber fiber in the intensity-enhanced section of a ring resonator, the laser is passively Q-switched. A set of location-dependent rate equations is established for Q-switching modeling. The design has been numerically and experimentally demonstrated using Er3+ -doped fiber at the emission wavelength of 1550 nm. A single-mode Q-switched pulse with pulse energy of 0.37 μJ and pulse duration of 218 ns was achieved with 980-nm pump power near 7 mW.
©2009 Optical Society of America
Q-switching operation is a useful technique employed in laser systems to produce short and high-intensity light pulses. There has been of great interest in Q-switched fiber lasers because of the advantages, including high efficiency, flexibility, compactness and high spatial beam quality. A Q-switched fiber laser with a smaller core diameter could directly provide a high intensity-density output of 0.1-10 MW/cm2 without external focusing components. Such a high intensity density is quite useful for micromachining, nonlinear optics studies and biomedical applications. Like most of the conventional bulk Q-switched lasers, Q-switched fiber lasers have been realized using similar active [1, 2] and passive [3-5] bulk Q-switches. These fiber lasers contain free-space sections in the resonators, and require sophisticated techniques of alignment for pump coupling and in-and-out light coupling between fibers, Q-switches and mirrors. Large-core double-cladding (DC) gain fibers and multimode laser diodes of several wattages are often used to ease the difficulty of pump coupling, and achieve great gain of tens -hundreds dB that can suitably compensate the fiber coupling loss.
Lately an increasing attention has been drawn upon the all-fiber Q-switched lasers that require no alignments and have inherently low cavity losses. An all-fiber laser is generally a single-mode system pumped with a single-mode laser diode of hundreds of milliwatts. The laser is composed of commercial fiber components as WDMs, FBGs, fibers and fiber-pigtailed components of low insertion losses. Although an all-fiber system has limited CW output power for the small core diameters and the low-power LD pump, pulses with comparably high intensity densities can still be achieved in an all-fiber Q-switched laser with a relatively low price.
A few all-fiber actively Q-switched lasers have been realized using acousto-optic modulators [6-9], piezoelectric (PZT) actuators [10, 11] and magnetostrictive transducers . A more compact and economic pulsed laser can be obtained using a saturable absorber Q-switch (SAQS). Nevertheless, because of the difficulty of finding fiber-type SAQSes, most of the literatures regarding all-fiber SAQS lasers were works of theoretical modeling and simulation [13, 14]. A low efficient and yet stable self-pulsed all-fiber erbium laser was reported in 2004 . The mechanism of self Q-fluctuation was attributed to the thermal lensing effect resulting from the excited state absorption of erbium. An all-fiber SAQS ytterbium laser was reported by Foriadi et al. in 2005 . In that study, a DC Yb3+-doped fiber was pumped by a 15W 976-nm LD and passively Q-switched at 1085 nm with a Sm3+-doped SAQS fiber. Peak pulse power of 30 W with pulse width of 0.65 μs was obtained at the repetition rate of 130 kHz.
In this paper, we propose a simple scheme of an all-fiber SAQS ring laser and establish the location-dependent rate equations to keep track of the non-uniform distributions of the resonant photons, the gain population inversion in the gain fiber Ng and the absorption population is the SAQS fiber, Na. The design was numerically and experimentally demonstrated using single-mode Er3+-doped fiber at the emission wavelength of 1550 nm. More than 90% extraction efficiency of Ng by a Q-switched pulse was obtained. It is important to note that the proposed design is applicable for self Q-switching performances of all the 3-level laser materials that can also serve as 2-level saturable absorbers at the emission wavelength.
2. Modeling and Simulation
Figure 1 shows the schematic design of an all-fiber SAQS ring laser. The gain region zg is for the gain fiber from zg1 to zg2, and the absorber region za for the absorber fiber from za1 to za2. The circulator and the FBG determine the roundtrip direction of lasing from zg1 to zg2, through the circulator, za1, za2, reflected by the FBG, back to za1, through the circulator, the WDM, and then starting from zg1 again. In the later experiment, the gain and the absorber were the same type of erbium-doped fiber. The 980-nm pump is coupled by the WDM into the gain fiber and blocked by the circulator. Thus, the absorber fiber is left unpumped. It is reasonable to expect that the photon density in the absorber fiber should be on average twice (or more than twice) that in the gain fiber. The more intense photon density would result in a fast bleaching of the absorber, and then lead to a passive Q-switching performance.
In most cases of dealing with laser dynamics, an assumption is commonly made in the rate equations that the photons, the gain population, the absorption population and the cavity losses are distributed uniformly in the resonator . This assumption is usually appropriate for standing-wave lasers. However, it is evidently not applicable for the proposed ring laser. To take account of non-uniformity, the photon densities, line densities of gain and absorption populations are defined to be functions of z location, indicated in Table 1.
In simulation, time is scaled by t=k×tr, where tr is the roundtrip transit time and k is a positive integer. In every time slice, Ng(z) and Na(z) are solved iteratively by the rate equations:
The pump rate and the population relaxation are negligible in the Q-switching duration and ignored in the equations. In every time step, once Ng(z,k) and Na(z,k) are obtained, n(z,k) is calculated accordingly along the propagation direction by
The simulation starts from the threshold condition when the gain is equal to the total loss. Before lasing (t<0), the pump intensity Ip(zg) and the corresponding Ng(zg) can be numerically solved for the initial condition:
The other initial variables and parameters are ng(zg1,0)=1×102, A=1.26×10-7 cm2, tr=18 ns, σg=σa=5×10-21 cm2 and NT=1.38×1019 cm-3. The parameters are based on the characteristics of the erbium fiber employed in the experiment. The transmission losses of 0.6 dB are assumed (i.e. α1=α2=0.1382 correspondingly). The reflection loss of the FBG output coupler is 0.6 dB. The length of the absorber is 10 cm and that of the gain fiber is 70 cm. The simulation result is shown in Fig. 2. A passively Q-switched pulse has peak power of 1.72 W and pulse duration of 198 ns. If the output beam diameter is 10 μm, an output intensity density of 2.2 MW/cm2 is obtained. Three temporal points ta, tb and tc are chosen for later observation, when nai(za2, tb) reaches the pulse peak, and nai(za2, ta)=nai(za2, tc)=nai(za2, tb)/100. Figure 3 shows Ng(zg) and Na(za) at the moments ta, tb and tc, normalized by the initial Ng(zg1,0) and Na(za2,0). It is clear that the SAQS fiber is saturated faster than the gain fiber, and well bleached at the end of the pulse. The saturation intensity of an erbium fiber with a core diameter of 4 μm is about 0.5 mW at 1550 nm. The inner peak intensity through the SAQS is 25 W. Thus the SAQS is about 10% saturated at the observation point ta when the passing intensity reaches 0.25 W, about 90% saturated with 25 W and completely saturated after the pulse. The extraction efficiency of the total gain population (Ng integral over zg) can be calculated to be 0.93, indicating a highly efficient Q-switching performance.
In the simulation, the pump is ignored during the 20-μs Q-switching action, and a Q-switched pulse occurred passively. After Q-switching, the recovering time of Ng is determined by the relaxation lifetime τg2, and that of Na is by τa2. For achieving next Q-switching and preventing from free-running lasing, Na needs to recover faster than Ng. Thus when τg2 is close to or smaller than τg2, modulated pump is required for sequentially Q-switched pulses.
The experiment was arranged as depicted in Fig. 1. The erbium fiber used for the gain and the SAQS in the experiment had an absorption loss of 30 dB/m at 1550 nm, and a core diameter of 4μm. The length of the absorber was 10 cm and that of the gain fiber was 70 cm. The reflectivity of the FBG output coupler was about 90% at 1550nm with a bandwidth less than 0.3 nm. The total roundtrip length of the ring resonator was about 360 cm. All the applied characteristics were similar to the parameters used in the previous simulation. The laser output was detected by a 175-ps fast EOT ET-3000 InGaAs photodetector and plotted using an Agilent 300-MHz MegaZoom oscilloscope. The output power was measured using a Newport 818-IR power detector that was assembled with an integrating sphere and an energy meter.
A light-modulated 980-nm single-mode LD was employed as a pump source. The pump intensity Ip was modulated near the threshold Ipth, alternating between 0.7×Ipth and 1.05×Ipth. The applied Ip was between 5-7 mW. The low-level pump less than the Q-switching threshold could slow down the recovering of Ng, prevent free-running lasing, and give time for full recovery of Na. The relaxation lifetime of erbium τ2 is about 10 ms. By experience, it was better that the duration of the low-level pump in modulation was longer than ~3τ2, giving rise to the limitation of the pulse repetition rate near 30Hz.
Steady sequentially Q-switched pulses were obtained, as shown in Fig. 4. The modulation rate was set to be 25 Hz. Each pulse appeared about 12 ms after the rising edge of the pump. The pulse had pulse energy of 0.37 μJ and pulse FWHM of 218 ns. The peak pulse power of 1.69 W was achieved, in very good agreement with the simulation result. Pulse power degraded quickly with a modulation frequency over 30 Hz. Increasing the high pump level (> 1.05×Ipth) would shift the pulse closer to the rising edge of the pump. The duty-cycle should be reduced accordingly for avoiding free-running lasing. However, it could not increase the pulse power, energy and repetition rate. In addition, the Q-switched pulses were similar with the low-level pump in the range of 0~0.7×Ipth, and Q-switching performance disappeared and free-running lasing occurred when the low-level pump power was larger than 0.7 × Ipth. The average pump power could be much saved by using On/Off pump (i.e. making the low-level pump zero) and increasing the upper-level pump intensity with a shorter duty cycle. In other words, better pump efficiency could be expected if pulsed pump was employed. Please do not mistake the proposed SAQSing mechanism for gain switching by pulsed pump. As demonstrated in the previous simulation, the pump factor is ignored in Eq. (2), and the Q-switching mechanism is passively performed by quick bleaching of Na.
We have proposed a simple design for all-fiber saturable absorber Q-switched lasers, and numerically and experimentally demonstrated the high efficiency of Q-switching performance using single-mode erbium-doped fibers. A set of location-dependent rate equations was established for modeling the Q-switching dynamics and the time variations of n(z), Ng(z), and Na(z). In the experiment, peak pulse power of 1.69 W with pulse FWHM of 218 ns was achieved with 7-mW pump power of a 980-nm LD. The experimental data was in solid agreement with the simulation result. The proposed laser scheme is compact, economic, efficient and applicable for various laser materials.
This work was financially supported by the National Science Council of Taiwan (Project No. NSC-97-2221-E-006-023) and by NCKU Project of Promoting Academic Excellence & Developing World Class Research Centers (D97-3360).
References and links
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