A wavelength tunable stretched-pulse mode-locked all-fiber ring laser using single polarization fiber (SPF) was demonstrated. In this laser, a segment of SPF was used simultaneously as a polarizer and a tunable filter in the laser cavity. Self-starting mode-locking with femtosecond output pulses was demonstrated. A wavelength tuning of ~20nm was achieved by bending the SPF with different radii.
© 2006 Optical Society of America
All-fiber mode-locked lasers are an attractive alternative to solid-state lasers for generation of femtosecond pulses at infrared wavelengths. The potentially greater stability and the compact size of fiber lasers offer the possibility of widespread application. In addition, if the laser cavity consists entirely of optical fiber and fiber-based components, the device assembly is greatly simplified, since a fusion splicing replaces complicated optics alignment. One of the most common ways to achieve mode-locking in fiber lasers is to use the nonlinear polarization evolution (NPE) in optical fiber [1–5]. In such lasers, a component which can function as a polarizer is used in the laser cavity in order to achieve mode-locking. Such a component can be a polarization beam splitter [2–3], or a polarization dependent isolator [4–5]. Because the spectral bandwidth of femtosecond pulses is large, a bulk tunable filter, such as a birefringent tuning plate, is used in conventional femtosecond mode-locked fiber lasers to control the bandwidth or achieve wavelength tuning . Recently, a chirped fiber Bragg grating was used to control the output spectrum as well as the cavity dispersion in a compact femtosecond mode-locked fiber laser . In this paper, to the best of our knowledge, we demonstrate for the first time a wavelength tunable stretched-pulse mode-locked all-fiber ring laser in which SPF is used simultaneously as a polarizer and as a tunable filter.
2. Experimental setup
The experimental arrangement of the laser is shown in Fig. 1. As shown in Fig. 1, all components in the laser cavity are fiber-based or have fiber pigtails. The segment of positive dispersion fiber is 3.62m Er-doped fiber with a dispersion of β2=38.5 ps2/nm. The negative dispersion segment consists of 4.5m standard singlemode fiber and 3.93m HI1060 fiber pigtails of two wavelength division multiplexers (WDMs) used in the cavity. A 20cm piece of Corning’s SPF was inserted between the output coupler and the isolator. The estimated dispersion of SPF is about 60ps2/nm. The total cavity length is about 12.2 m, giving a fundamental frequency of 16.7 MHz. The estimated net dispersion of the laser cavity is about 0.01 ps2. The Er-doped fiber is bi-directionally pumped by one 980 nm and one 1470nm high power laser diodes through two wavelength division multiplexers (WDMs). The purpose of using bidirectional pump is to easily find the right position of the setting of the polarization controller in starting mode locking. When the laser starts the mode locking, the 1480nm pump is shut down to avoid the pulse breaking due to the excess nonlinearity. The unidirectional lasing in the ring is achieved by a fiber coupled polarization-insensitive isolator. The output is tapped just after 980nm/1550nm WDM by using a 50:50 optical coupler in the cavity. A single fiber polarization controller located before the SPF is used to adjust the polarization state of the cavity light.
3. Results and discussions
The SPF used here is a hole-assisted SPF that contains an elliptical core with two air holes placed next to the core and along the minor axis of the ellipse . The single polarization bandwidth of the 20cm SPF is ~60nm, and this single polarization window can be shifted by bending the SPF. Figure 2 shows the transmission spectra of the two linear polarization modes of the 20cm SPF with two different bending radii. It is clearly seen that the single polarization window shifts about 15nm to the short wavelength side when the bending radius changes from 11.5cm 5.5cm, while the SP bandwidth remains almost unchanged. Therefore, in addition to functioning as a polarizer, the SPF can also serve as a tunable low-bandpass filter to control the lasing wavelength in a mode-locked fiber laser. It should be noted that the cutoff wavelength shifts sensitively with the bend radius only when the radius is less than ~10cm. At such a radius, the laser can remain to be compact, which is an advantage over bulky solid state. It is obvious that this wavelength tuning feature could be not achieved by using a polarization sensitive isolator in a conventional ring laser. Figure 3 shows a typical polarization-dependent loss (PDL) versus wavelength curve which was measured with a bending radius of 6cm. The extinction ratio larger than 20 (40) dB was achieved in the 1502–1549 (1504–1543) nm wavelength range.
By properly adjusting the PC, the mode-locking self-started by means of NPE in the cavity fiber. For one setting of the PC, self-starting mode-locking at a single pulse per round trip was achieved with a pump power of ~40 mw at 980 nm (and no pumping at 1470 nm). Since the output was tapped just after the Er-doped fiber, the output had a large positive chirp. The chirp of the pulses was partially compensated by the anomalous dispersion single mode fiber of the pigtail of the output coupler.
Figure 4 shows the spectra of the output pulses for the SPF with four different bending radii. As expected, with decreasing bend radius of the SPF, the spectrum of the output pulses moves toward shorter wavelength due to the shift of the single polarization window. The center wavelength of the output spectrum shifts from ~1556nm to ~1539nm when the bending radius of the SPF is changed from the 50cm to 4cm. A wavelength tuning range of 17nm is achieved. The spectral width of the output pulses is near 20nm over the whole tuning range (20nm, 19nm, 18.5nm 20nm respectively for the bending radii of 50cm, 10cm, 5cm, 4cm). As expected, the output spectrum doest not shift linearly with the bending radius, and the wavelength shift is more sensitive to the bending when the bending radius is less than ~10cm. Therefore, at such a radius, the laser can remain to be compact, which is an advantage over bulky solid state. In our present setup, when the bending radius of the SPF was changed, the cavity fibers which were connected to the SPF were also moved. This caused the changes of the polarization state of cavity light. Therefore, in the tuning range, sometime the mode-locking was broken due to the change of the polarization state of cavity light, and the re-setting of the PC was needed. However, the mode-locking could be sustained in most of time. This problem can be solved by using proper way to bend the SPF without disturbing the rest of cavity fibers.
The corresponding autocorrelation traces of the output pulses for the SPF with different bending radii are shown in Fig. 5. The autocorrelation traces were captured without taking average over multiple sampling. The measured pulse widths were 750fs, 700fs, 660fs, and 680fs for the bending radii of 50cm, 10cm, 5cm, and 4cm respectively, which correspond the time-bandwidth product values of the output pulses 1.87, 1.66, 1.53, and 1.70, respectively. The output pulses were still chirped because the pigtail fiber of the output is shorter than the length required to provide enough negative dispersion to compensate the positive chirp of the output pulses. By optimizing the length of the external compensation fiber, the pulses with a width as short as 150fs should be achievable. For all above cases, the laser is mode-locked at a single pulse per round trip with a repetition rate of 16.7MHz. The output average power of the laser is 5.2mW, 6.6mW, 6.1mW, and 5.9mW respectively for the bending radii of the SPF of 50cm, 10cm, 5cm, and 4cm.
In this paper, we have focused on exploring the role of the SPF as a polarizer and wavelength tunable filter. Since the SPF can have novel dispersion properties similar to photonic crystal fibers, it can also potentially play an intriguing role of balancing the overall fiber dispersion. We have calculated the dispersion as a function of the wavelength for the SPF based on the design parameters and shown the results in Fig. 6. The modeling results clearly show that SPF can provide a much higher dispersion than conventional fibers of similar core size. The normal dispersion region of the optical cavity is typically composed of standard erbium doped fiber with dispersion around -30ps/nm/km. If a ring laser is implemented so that the SPF is also doped with high concentration erbium, the use of the SPF can significantly shorten than the use of standard erbium fiber by a factor of about 3–5, and thus reduces the fiber nonlinearity in the cavity. Consequently, the output pulse energy can be increased. In addition, as shown in Fig. 6, the dispersion slope of the SPF is negative, which is opposite to that of standard single mode fiber, the use of SPF can also be used to manage the third order dispersion of the laser cavity. The change of dispersion of the SPF in the tuning range from 1539nm to 1556nm is ~16 ps2/nm, corresponds a change of the net cavity dispersion of ~0.003ps2. Moreover, this idea can also be generalized to 1060nm mode locking fiber laser by designing a SPF with anomalous dispersion, which is not possible for conventional fiber.
In summary, we reported an all-fiber wavelength-tunable stretched-pulse mode-locked erbium ring laser in which the 20cm long segment of the SPF was simultaneously used as a fiber polarizer and a broadband tunable filter. The use of SPF provides a low cost, all-fiber approach to achieve wavelength tuning in NPE based mode-locked fiber laser. Femtosecond pulses with a wavelength tuning range of 20nm were demonstrated. The optimizing of the performance of the laser, such as pulsewidth and energy, could be an interesting subject of future effort when combined with the consideration of dispersion handling of SPF to manage the overall dispersion and the high order dispersion of the laser cavity.
References and links
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