We present a brand-range self-sweeping Tm-Ho co-doped fiber laser experimentally. The laser’s center wavelength sweeps periodically around 1.9 μm with self-sweeping range of 4 nm ~17 nm, and the sweep rate can be changed in the range of 0.4 nm/s ~1.5 nm/s. The sweep range increases and the sweep rate declines when the pump power rises.
©2013 Optical Society of America
Fiber laser near 2 μm has attracted intense attentions for its various applications such as remote sensing [1, 2] and nonlinear frequency conversion . Many of the applications can benefit greatly if the 2 μm fiber laser’s wavelength is tunable. There are many demonstrations on wavelength-tunable lasers using active and passive methods. Actively wavelength-tuning lasers usually need external control device to scan the laser’s frequency continuously. Several useful techniques for actively tuning have been demonstrated using devices such as Fabry-Perot (F-P) tunable filter [4–10] and polygonal scanner [11–13]. Semiconductor optical amplifier (SOA) has been used to realize wavelength-sweeping [14, 15]. Wavelength-swept 2 μm laser has been demonstrated using actively tuning methods [8, 16, 17]. All these actively wavelength-tunable lasers need external control to make the laser’s wavelength scanning dynamically.
Self-sweeping, which does not need special control devices to provide scanning of the wavelength, can be an attractive approach for tuning the laser’s wavelength. Self-sweeping fiber lasers based on Yb-doped and Er-doped fibers have been reported only recently [18–22], although the self-sweeping in much narrower bandwidth was observed already in Ruby lasers [23, 24] However, there has been no demonstration (to our knowledge) on Tm-doped self-sweeping fiber laser so far.
In this letter, we present a Tm-Ho co-doped self-sweeping fiber laser near 1.9 μm without any actively scanning device. The maximum sweep range is about 17 nm and the sweep rate is about 0.4 nm/s ~1.5 nm/s. This is the first experimental report on the approach to realize self-sweeping near the range of 2 μm.
2. Experimental setup
Figure 1 depicts the experimental setup of the self-sweeping Tm-Ho co-doped fiber laser. The home-made 1570 nm continuous-wave fiber laser has a maximum output power of 333 mW. A wavelength division multiplexer (WDM, 1550/2000 nm) is employed to inject the 1570 nm pump laser into the Tm-Ho co-doped fiber laser cavity. The gain medium is a piece of 4 m long single-cladding Tm-Ho co-doped fiber (Coractive) with core diameter of 9 μm and cladding diameter of 125 μm (NA = 0.16). The active fiber is highly doped and the absorption coefficient is > 100 dB/m at 790 nm. A polarization controller (PC) and a 50/50 coupler (AFR, coupler 1, center wavelength at 2 μm) are used as a passive fiber loop mirror (FLM), which is one end of the self-sweeping fiber laser’s oscillation cavity. The flat-angle cleaved end of coupler 2 (AFR, 2 μm) provides the 4% Fresnel reflection, which is the other end of the laser cavity. The rest ports of the couplers are angle cleaved in order to prevent unnecessary feedback into the laser cavity. An isolator is also employed to avoid feedback from the output end of the fiber. A piece of 50 m long single-mode fiber (G652D) is employed to increase the length of the cavity.
3. Self-sweeping results
The laser output power’s temporal characteristics were measured using an InGaAs PIN photodetector (7 GHz bandwidth) and a digital oscilloscope (1.5 GHz bandwidth). It is found that the fiber laser operates in the self-pulsing regime, as Fig. 2 shows. The self-pulsing frequency is in the range of 15 kHz ~25 kHz, and the pulse train is modulated by high frequency intermode beating. The intermode beating frequency is determined by longitudinal mode frequency interval, Δυq = c/2nL, where c is the speed of light in vacuum space (~3 × 108 m/s), n is the refractive index (~1.45) and L is the length of laser cavity (~60 m). The measured intermode beating frequency of the pulse shape is 1.68 MHz, and the calculated Δυq equals 1.72 MHz.
The laser’s wavelength swept automatically when carefully tuning the PC and making the output power of the laser be the minimum value. The laser’s spectrum was studied using an optical spectrum analyser (OSA, YOKOGAWA), as Fig. 3 shows. One can see that the PC and coupler 1 together serve as a broad-band FLM with a reflective wavelength range of 1800 nm ~2000 nm. The signal-noise ratio is higher than 25 dB.
The static output spectra of the self-sweeping Tm-Ho co-doped fiber laser at different time are depicted in Fig. 4. The data were obtained by single-shot recording of the spectra from the OSA at different time while the laser was operating in the self-sweeping regime. The 1570 nm pump power was 333 mW, and the fiber laser’s wavelength swept in the range of 1904.7 nm ~1921.8 nm. It can be found from Fig. 4 that the powers of each wavelength are similar, and the wavelength-sweeping range has reached as wide as 17 nm.
Some typical temporal dynamics of the fiber laser’ spectra with different pump powers are shown in Fig. 5. The OSA in our experiment is not a very fast scanning one. However, when shorten the OSA’s scan range (~30 nm), the OSA can read the selected spectra at a frequency of ~5 Hz. It is fast enough to analyze the self-sweep of the laser wavelength. Actually, the passive single-mode fiber in the laser cavity was employed to slow the sweep rate , thus our OSA is capable of analyzing the spectra. It can be found from the curves that the Tm-Ho co-doped fiber laser is in the regime of self-sweeping. The wavelength of the laser starts from a shorter wavelength and sweeps to a longer one periodically without using any actively scanning device. The sweep range is around 1900 nm and relatively stable. It should be noted that, by turning the PC, the sweep range of the laser can also vary manually. As seen in Fig. 5, the sweep range with 333 mW pump power [Fig. 5(d)] does not overlap with the others [Fig. 5(a)-5(c)]. That’s because the PC’s rotation position of Fig. 5(d) is different with those of the three others.
This self-sweeping result can be explained by the theory of the non-stationary spatial hole burning (SHB) in the longitudinal distribution of gain for a small number of generated modes [18–21, 24]. Suppose that there were a few longitudinal modes in every self-pulsation of the fiber laser. The generated modes results in dynamic SHB in the fiber gain medium, and the dynamic SHB will suppress these existing corresponding longitudinal modes during the next self-pulsation. These processes will generate the self-scanning of the fiber laser’s longitudinal modes, which means the wavelength of the laser is in the regime of self-sweeping. The SHB theory in  estimated that the sweep rate increases as the laser power scales up and decreases as the laser cavity length decreases, but does not discuss the relationship between the sweep range and the pump power.
Figure 6 depicts the sweep range and sketch of gain contours versus wavelengths of the Tm-Ho co-doped laser at different pump powers. One can see from Fig. 6(a) that the laser’s sweep range increases as the pump power increases. It is in accordance with observations in Yb-doped fiber lasers reported in . A reasonable explanation is that the gain contour of the Tm-Ho co-doped fiber lifts up as the pump power increases [Fig. 6(b)], which results in a wider spectral range for lasing. Hence more longitudinal modes with different frequencies can take part in the lasing progress and results in SHB, the laser’ center wavelength can sweep in a broader range.
The sweep rate of Tm-Ho co-doped fiber laser decreases as the pump power increases, as shown in Fig. 7(a). Meanwhile, the laser’s frequency of self-pulsation (relaxation oscillation) increases as the pump power scales up [Fig. 7(b)]. This result is not consistent with the SHB analysis in Yb-doped fiber laser . We think that the increasing number of longitudinal modes by scaling the pump power up may play an important role in sweep progress. It is possible that the detailed mechanism of self-sweeping in fiber laser should be studied thoroughly and improved.
It should be noted that the Ho ions in active fiber may play an important role in the sweep wavelength of our experiment. Tm-doped fibers have high gain in the wide spectrum of 1.8 ~2.0 μm, and the maximum emission cross section located around 1.84 μm. The Ho ions may extend the laser’s wavelength beyond 2 μm . In our experiment, the self-sweep wavelengths of the laser were generally longer than 1.84 μm. So we think the Ho ions increases the gain of active fiber at longer wavelengths, thus the laser’s self-sweep wavelengths may move to longer spectra, even the sweep range may be broadened, too.
The discussion above is just a qualitative explanation, more detailed and theoretical analysis should be performed using numerical simulation, which is out of the frame of our research. However, the experiment in this paper provides an effective approach for self-sweeping fiber laser near 2 μm. The laser’s performance and parameters can be improved in further endeavors.
A self-sweeping Tm-Ho co-doped fiber laser around 1.9 μm is demonstrated. The laser cavity is formed by a flat cleaved end of a fiber coupler and a FLM consists of a PC and a coupler. All the unnecessary feedbacks into the laser cavity are avoided. The fiber laser operates in the regime of self-sweeping without any actively scanning device, and the laser has a maximum sweep range of 17 nm, the sweep rate is 0.4 nm/s ~1.5 nm/s. Self-pulsation is observed while the wavelength of the laser is self-sweeping. The sweep range increases as the pump power rises, and meanwhile the sweep rate decreases. Detailed and thorough theoretical analysis of the experimental results should be performed in the future work.
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