A self-Q-switched all-fiber erbium laser at 1530 nm using an auxiliary 1570-nm erbium laser

Fiber lasers have been Q-switched using traditional bulk Q-switches and fiber-pigtailed ones. Sophisticated alignment techniques were necessary for reducing the coupling losses between the bulk Qswitches and the fiber cores. All-fiber Q-switched lasers with fiber-pigtailed Q-switches are of great interest because of their alignment-free characteristics and low cavity loss that is essential for efficient Qswitching performance. The fiber-pigtail Q-switches can be active devices as acousto-optical modulators [1-3] , or passive ones as saturable-absorber fibers [4-8] . Passive Q-switching with a saturable absorber fiber is the simplest and most economical approach for producing high-power laser pulses. In addition, a solid-state saturable absorber Q-switch (SAQS) fiber has a high damage threshold and can hold an enormous laser gain for energy release into one Q-switched laser pulse. In spite of these advantages, the SAQS fibers are seldom considered in the literature. One of the criteria for a SAQS laser is that the absorption cross section, σa, of the SAQS should be larger that the emission cross section, σe, of the gain medium. Furthermore, to achieve sequentially Q-switched pulses using a continuous-wave (CW) pump source, the relaxation lifetime of the SAQS, τa2, should be shorter than that of the gain medium, τg2.

which corresponds to an emission and absorption wavelength range from 1.48 to 1.6 μm.According to the specifications of the SAQS erbium fiber provided by the manufacturer nLight, the emission and absorption cross sections are about the same: 6Χ10 -21 cm 2 at 1530 nm.They are 2.6Χ10 -21 cm 2 and 1.3Χ10 -21 cm 2 at 1570 nm, correspondingly.These values might differ slightly for different products and manufacturers.The cross-section ratio, σ a /σ e , is also the population ratio of N a2 to N a1 in the state of full saturation.When a large laser power at 1570 nm, I a,1570 , saturates the SAQS, the absorption population at 1570 nm, defined as N a,1570 =[ N a1 -(σ e /σ a )N a2 ], becomes zero and the population ratio of N a2 to N a1 is about 0.5.Such a population ratio indicates a reduction of the absorption population at 1530 nm, N a,1530 , by the I a,1570 to a value of one-third of the total erbium dopants of the SAQS fiber, N aT .Therefore, the initial value of N a,1530 , called N ai , for Q-switching is tunable with an I a,1570 applied on the SAQS fiber.More importantly, an I a,1570 can shorten the relaxation lifetime of N a,1530 .When the SAQS fiber is fully bleached by a Q-switched pulse at 1530nm, N a2 is equal to N a1 instantly.Here, N a,1530 will soon be modified by the I a,1570 back to the initial N ai for next Q-switching.To simplify the discussion on the effect of I a,1570 on the erbium lifetime, we assumed uniform distribution of N a2 , N a1 and I a,1570 in the SAQS and derived the effective lifetime of N a,1530 as: A a is the cross sectional area of the fiber core, Γ the confinement factor and I s,1570 is the saturation power of the SAQS fiber.Therefore, with the known ratioσ a,1570 / σ e,1570 ~0.5, the effective lifetime, τ a2 ', could be one order of magnitude shorter than the real lifetime τ a2 when the ratio I a,1570 /I s,1570 is larger than 6.

Experiments
Fig. 1 shows the schematic design of a self-Q-switched, erbium, all-fiber laser, pumped with a continuous-wave (CW) 980-nm laser diode (LD).The Q-switched laser was stabilized, and the pulse repetition rate was tunable using a 1570-nm laser that was also an erbium fiber laser pumped with a 980-nm LD (not shown).Therefore, the laser system could be simplified using only one high-power pump LD and a pump splitter with a properly designed ratio.The gain medium was a 210-cm erbium-doped fiber with a core diameter of 14 μm and an absorption loss of 19 dB m -1 at 1530 nm, manufactured by the company Coractive.The SAQS was a 20-cm Er fiber, made by the manufacturer nLight, with a relatively smaller core diameter of 4 μm and an absorption loss of 110 dB m -1 at 1530 nm.The mismatch of the mode field areas (MFA) between the gain and the SAQS resulted in high photon density and fast absorption saturation in the SAQS fiber, thereby giving rise to Qswitching action.The numerical aperture (NA) number and the mode field diameter of the SAQS were 0.2 and 6.5 μm, indicating a confinement factor Γ of about 0.53.Thus, the saturation power, I s,1570 , was determined by Eq. (1) to be 1.2 mW.The 980/1530 nm WDM inside the resonator was used to protect the SAQS from the pump power.Similarly, a 1530/1570 nm WDM was employed to prevent the gain fiber from being stimulated by the 1570-nm laser.All components were core-fusion spliced.The length of the resonator was about 4 meters.Fig. 1.Schematic diagram of a self-Q-switched, all-fiber erbium laser at 1530 nm with a tunable repetition rate using an auxiliary 1570 nm laser.
The Q-switching performances related to the pulse repetition rate, R pr , with I a,1570 =0 and I a,1570 =10 mW are compared and shown in Fig. 2 (a) and (b).Both the cases with and without I a,1570 were stable when R pr was within 0.9-10 kHz and unsteady when R pr was below 0.9 kHz.At the end of the experiment, the case with the I a,1570 was stabilized in a low-R pr operation of 0.1-1.5 kHz by doubling the length of the SAQS fiber.Instead of stabilizing the laser at low R pr (<1 kHz) as the function of a saturable-amplifier pump switch (SAPS) [10] , the tuning source, I a,1570 , primarily improved the Q-switching efficiency at high R pr by affecting the relaxation lifetime.Without the I a,1570 , a pulse had a full-width-at-half-magnitude (FWHM) of about 0.9 μs and an energy of 1.1 μJ at a repetition rate, R pr , of 0.9 kHz near the laser threshold.The 0.9 kHz value of R pr indicated a 1.1ms recovery time (i.e.τ a2 /9) for N a,1530 after being bleached by a Q-switched pulse.Assuming the SAQS was fully bleached by each pulse, it can be calculated that N a,1530 switched between 0 to about 0.1N aT .A higher pump power would give less time for N a,1530 recovery, leading to a higher repetition rate and smaller pulse energy.The pulsing output still remained with a pump power larger than 100 mW where the pulse had a pulse width of microseconds and a very low pulse energy.Such low-efficiency Q-switching is referred to as "Qfluctuation" and can only hold a small amount of energy in the gain medium and will have low extraction efficiency of the gain population.When the 1570-nm laser was turned on, the Q-switching performance was much improved.By comparing the two cases (i.e. with and without I a,1570 ) at the same R pr in Fig. 2(a), the improved pulse energy with the I a,1570 demonstrates faster and more extensive recovery of N a,1530 .Due to this increased N a,1530 recovery, a higher pump power was required with the I a,1570 .Furthermore, the improved recovery of N a,1530 denoted a higher hold-off ratio of the gain population in the gain fiber, in turn leading to a better Q-switching performance with a shorter pulse width and higher peak power as clearly demonstrated in Fig. 2(b).
We further improved the Q-switching performance by doubling the length of a SAQS fiber.The SAQS had an absorption loss of 44 dB at 1530 nm that was even larger than that of the gain fiber.Thus, the laser could not reach the threshold by pumping alone without the assistant of the I a,1570 .Stable, sequential, Q-switched pulses were achieved using a 10-mW I a,1570 , as shown in Fig. 3.The pulse had a very stable shape, a FWHM of about 40 ns, and a peak power of larger than 100 W along the pump range from 75-200 mW.The maximum pulse energy of 6 μJ and peak power of 165 W was achieved at the lowest R pr of 0.1 kHz.The R pr was steadily proportional to the pump power and limited by the maximum 980-nm LD output.The high Q-switching efficiency was attributed to the high hold-off ratio of the gain population by the large N ai , which, in turn, lead to a high extraction efficiency of the pumped gain.Since more pumping time was needed for the large gain population, the Q-switching was stabilized into a low-R pr range from 0.1-1.5 kHz.Efficient Q-switching at higher R pr should be achievable using a more intense pump LD.The results demonstrated here were better and more stable than what can be achieved using a saturable-amplifier pump switch [10] .Fig. 3. (a) Q-switching performance using a 10-mW I a,1570 and a SAQS erbium fiber with an absorption loss of 44 dB at 1530nm.(b) Sequential Q-switched pulses at 1.5 kHz captured on an oscilloscope, and (c) the corresponding pulse with a peak power of 105 W.

2.Conclusion
We have demonstrated a self-Q-switched, all-fiber erbium laser emitting at 1530 nm through the use of an auxiliary laser at 1570 nm, I a,1570 , that allowed tunable and optimizable Q-switching performance.The I a,1570 was applied to a SAQS erbium fiber to shorten the relaxation lifetime.The wavelengths of 1530 and 1570 nm are in the same band of energy transition between 4 I 13/2 and 4 I 15/2 .A SAQS fiber with a 22-dB absorption loss was employed for demonstrating the effect of lifetime shortening, and the improvement on Q-switching at the repetition rate from 0.9 to 10 kHz.By doubling the length of the SAQS fiber and applying a 10-mW I a,1570 , sequential pulses with pulse energy of 6-4 μJ, steady pulse width of 38-40 ns and peak power of 165-105 W were achieved at repetition rate of 0.1-1.5 kHz.Efficient Q-switching at higher repetition rates is expected when a more intense pump source is employed.

Reference
The heterostructures of Zn-ZnO coaxial nanocables and ZnO nanotubes were synthesized by simple pyrolysis of zinc acetylacetonate in a two-temperature-zone furnace.Zinc acetylacetonate hydrate placed on a cleaned silicon susceptor was loaded into the low temperature zone of the furnace which was controlled at 130-140 °C to vaporize the solid reactant.The evaporated gaseous species were carried by a 500 sccm N2 flow at a total pressure of 200 Torr into the higher temperature zone of the furnace which was set at 500 °C.Black products were formed on the wall of the quartz tube in the region down stream out of the higher temperature zone with a temperature of 230 °C.
Figure 1a shows the SEM image of the 1D ZnO nanostructures with an average diameter of 30 nm.Typical TEM image, as shown in Figure 1b, reveals that some tubular nanostructures are observed among the nanowires.The nanowire possesses a coaxial structure, that is, a thin sheath with lighter contrast is formed outside the surface of nanowire-like structure of dark contrast.Moreover, some of the tubular nanostructures are connected with the coaxial nanocables.EDS analyses reveal that the coaxial nanocable is composed of the Zn core and the ZnO sheath.As shown in Figure 2, high-resolution TEM analyses reveal that the Zn core and the ZnO sheath of the nanocables have an epitaxial relationship with their longitudinal axis oriented along the <001> direction.ZnO nanotubes with a wall thickness of 4 nm possess a single-crystal structure and appear to be the extension of the ZnO sheath of the coaxial nanocables.It is suggested that the ZnO nanotubes are formed by partial evaporation of Zn core of the Zn-ZnO coaxial nanocables.The Zn nanowires could be considered as the templates for subsequent spontaneous formation of single crystalline ZnO nanotubes.With globalization and technological development, major cities in the world actively transform themselves into information and communication technology cities (ICT Cities) through master plans (i.e.Finland's Helsinki Arena 2000, Singapore's IT2000, Japan's E-Japan and Cyber Taipei).The initiative of ''Cyber Taipei'' refers to the ICT content adopted by the Taipei City government to handle citizens' affairs and information transmission.The policy's purpose is to reduce the time involved in handling affairs manually, to increase service efficiency and effectiveness, and to effectively enhance service qualities of city affairs.Previously studies mostly use satisfaction and cost-benefit as the reference for measuring the effectiveness of ICT policy promotion, but are still lack of a more systematic process evaluation, outcome evaluation, and particularly evaluations regarding users' attitudes and behavioral perspectives.This study use user cognition analysis to understand users' intention and service evaluation for various ICT systems, with the aim of consultation for future cyber city construction.
This study use Technology Acceptance Model (TAM) to effectively analyze the use behavior of users regarding technology acceptance and clarify information system users' use intention.TAM applies perceived usefulness (PU), perceived ease-of-use (PEOU), the attitude of the user, and external factors to illuminating users' intention.The hypotheses proposed in this study to verify PU, PROU and the attitude of the user.Furthermore, this study adopts structural equation modeling (SEM) to verify the proposed model and the relevant assumptions.The potential variables include security, efficiency, service, education, convenience and equality etc.
The results show that the direct effects that impact the behavioral intention of use, PEOU (0.9774) is more important than PU (0.27).For the overall effects caused by various paths, PEOU has a more significant impact than PU.Thus, ease-of-use leads the behavior of users.And the main factor that directly impacts PEOU is the ''convenience'' (0.51) aspect of the Cyber Taipei initiative.The results can be as the reference for future cyber city construction.

Fig. 2 .
Fig. 2. The Q-switching performances with I a,1570 =0 and I a,1570 =10 mW.(a) Pulse energy and pump power related to pulse repetition rate, (b) pulse peak power and pulse FWHM related to pulse repetition rate.

Figure 1 (
Figure 1 (a) SEM image of the 1D ZnO nanostructures.(b) TEM image of the heterostructures of the coaxial nanocables and the nanotubes.

Figure 2
Figure 2 High-resolution TEM image of the heterostructure of coaxial nanocable and nanotube.