Volume 22 Issue 1 - April 13, 2012 PDF
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A self-Q-switched all-fiber erbium laser at 1530 nm using an auxiliary 1570-nm erbium laser
Tzong-Yow Tsai, Yen-Cheng Fang
Institute of Microelectronics, College of Electrical Engineering and Computer Science, National Cheng Kung University
 
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1.Introduction

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 Q-switches 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 Q-switching 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.

Erbium fiber lasers emitting at the so-called eye-safe wavelength of 1.5-1.6 μm are the preferred laser devices for light detection and ranging (LIDAR) and other applications that require Q-switched operation. There has been no fiber-type SAQS material demonstrated for erbium fiber lasers except for erbium fiber itself. Recently, we demonstrated self-Q-switched, all-fiber erbium lasers in a ring scheme [9] and a standing-wave resonator [10] , where erbium fibers served as both 3-level lasers and 2-level saturable absorbers. Since the cross sections, σa and σe, of erbium are comparable, techniques of photon density enhancement in the SAQS fibers, such as double-passing routes and mismatch of mode field areas (MFA), were developed to activate SAQSing functions. Due to lifetime matching, τa2g2, in a self-Q-switched laser, a saturable-amplifier pump switch (SAPS) was introduced for acquiring sequentially Q-switched pulses [10] .

In this paper, we demonstrate a self-Q-switched, all-fiber, erbium laser that emits at 1530 nm using an auxiliary 1570 nm erbium laser applied to the SAQS erbium fiber. Both wavelengths, 1530 and 1570 nm, are in the same transition band of erbium. The dopant Er3+ has a broad band of energy transition between 4I13/2 and 4I15/2, 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 cm2 at 1530 nm. They are 2.6Χ10-21 cm2 and 1.3Χ10-21 cm2 at 1570 nm, correspondingly. These values might differ slightly for different products and manufacturers. The cross-section ratio, σa /σe, is also the population ratio of Na2 to Na1 in the state of full saturation. When a large laser power at 1570 nm, Ia,1570, saturates the SAQS, the absorption population at 1570 nm, defined as Na,1570=[ Na1-(σe /σa)Na2], becomes zero and the population ratio of Na2 to Na1 is about 0.5. Such a population ratio indicates a reduction of the absorption population at 1530 nm, Na,1530, by the Ia,1570 to a value of one-third of the total erbium dopants of the SAQS fiber, NaT. Therefore, the initial value of Na,1530, called Nai, for Q-switching is tunable with an Ia,1570 applied on the SAQS fiber. More importantly, an Ia,1570 can shorten the relaxation lifetime of Na,1530. When the SAQS fiber is fully bleached by a Q-switched pulse at 1530nm, Na2 is equal to Na1 instantly. Here, Na,1530 will soon be modified by the Ia,1570 back to the initial Nai for next Q-switching. To simplify the discussion on the effect of Ia,1570 on the erbium lifetime, we assumed uniform distribution of Na2, Na1 and Ia,1570 in the SAQS and derived the effective lifetime of Na,1530 as:
(1)

Aa is the cross sectional area of the fiber core, Γ the confinement factor and Is,1570 is the saturation power of the SAQS fiber. Therefore, with the known ratioσa,1570 / σe,1570~0.5, the effective lifetime, τa2’, could be one order of magnitude shorter than the real lifetime τa2 when the ratio Ia,1570 /Is,1570 is larger than 6.

2. 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 Q-switching 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, Is,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, Rpr, with Ia,1570=0 and Ia,1570=10 mW are compared and shown in Fig. 2 (a) and (b). Both the cases with and without Ia,1570 were stable when Rpr was within 0.9-10 kHz and unsteady when Rpr was below 0.9 kHz. At the end of the experiment, the case with the Ia,1570 was stabilized in a low-Rpr operation of 0.1-1.5 kHz by doubling the length of the SAQS fiber. Instead of stabilizing the laser at low Rpr (<1 kHz) as the function of a saturable-amplifier pump switch (SAPS) [10], the tuning source, Ia,1570, primarily improved the Q-switching efficiency at high Rpr by affecting the relaxation lifetime. Without the Ia,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, Rpr, of 0.9 kHz near the laser threshold. The 0.9 kHz value of Rpr indicated a 1.1-ms recovery time (i.e. τa2/9) for Na,1530 after being bleached by a Q-switched pulse. Assuming the SAQS was fully bleached by each pulse, it can be calculated that Na,1530 switched between 0 to about 0.1NaT. A higher pump power would give less time for Na,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 “Q-fluctuation” and can only hold a small amount of energy in the gain medium and will have low extraction efficiency of the gain population.
Fig. 2. The Q-switching performances with Ia,1570=0 and Ia,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.

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 Ia,1570) at the same Rpr in Fig. 2(a), the improved pulse energy with the Ia,1570 demonstrates faster and more extensive recovery of Na,1530. Due to this increased Na,1530 recovery, a higher pump power was required with the Ia,1570. Furthermore, the improved recovery of Na,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 Ia,1570. Stable, sequential, Q-switched pulses were achieved using a 10-mW Ia,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 Rpr of 0.1 kHz. The Rpr 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 Nai, 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-Rpr range from 0.1-1.5 kHz. Efficient Q-switching at higher Rpr 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 Ia,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, Ia,1570, that allowed tunable and optimizable Q-switching performance. The Ia,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 4I13/2 and 4I15/2. 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 Ia,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.

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