We propose an operation switchable ring-cavity erbium-doped fiber laser (EDFL) via intra-cavity polarization control. By using a semiconductor saturable absorber mirror in the EDFL cavity, stable Q-switching, Q-switched mode-locking, continuous-wave mode-locking, pulse splitting, and harmonic mode-locking pulses can be manipulated simply by detuning a polarization controller while keeping the pump power at the same level. All EDFL operation states can be obtained under the polarization angles detuning within 180°. Continuous-wave mode-locking of EDFL with 800-fs pulsewidth repeated at 4 MHz has been obtained, for which the output pulse energy is 0.5 nJ and the peak power is 625 W. Interaction between solitons and the accompanied non-soliton component will lead to either pulse splitting or 5th-order harmonic mode-locking at repetition rate of 20 MHz.
©2009 Optical Society of America
High peak power and short pulsewidth fiber lasers are of interested in laser science and technologies because they have many practical applications in various domains. The operation of short-pulse lasers can be divided into three major types: Q-switched [1–3], continuous-wave mode-locking (CML) [4–10], and Q-switched mode-locking (QML) [11–13]. Various active or passive technologies have been utilized in the construction of short-pulse fiber lasers. All active pulsed lasers contain bulk elements, which make their design rather complicated; therefore, much attention has been paid to the development of passive fiber lasers. An all-fiber passively Q-switched erbium laser with a Co2+:ZnSe crystal as a saturable absorber is demonstrated experimentally . Giant pulses with energy of 3.6 nJ and peak power of 0.7 mW have been obtained. Zenteno et al. report the generation of Q-switched mode-locked pulses from an Nd-doped fiber laser that uses a solid-state solution of BDN-I dye as the saturable absorber . For an absorbed pump power of 110 mW, pulses of 8-ns duration at a repetition rate of 14 MHz can be generated under an 800-ns-wide Q-switched envelope at a repetition rate of 10 kHz, yielding an average output power of 8 mW near 1.06 μm.
Hakulinen et al. demonstrate that resonant high-modulation-depth saturable absorbers allow efficient pulse shortening in Q-switched lasers . Using a 70% modulation depth resonant saturable absorber mirror (SESAM) they achieved 8 ns pulses that are close to the limit set by the cavity length. A resonant SESAM with a high reflectivity change also allows reliable start-up of passive mode locking in a wide range of normal or anomalous cavity dispersion [4–5]. With SESAMs, the mode-locked regime can be achieved for different values of cavity dispersion for a broad spectrum ranging from 800 to 1600 nm. Grudinin et al. studied passive harmonic mode-locking (HML) in soliton fiber lasers . They demonstrated that the laser performance could be further improved by the use of a SESAM in combination with a nonlinear amplifying loop mirror. The SESAM acts not only as a fast saturable absorber but also as a passive phase modulator. They demonstrated that such a laser is capable of generating 500-fs pulses at repetition rates exceeding 2 GHz. For diode-pumped solid-state lasers (DPSL), the laser operation state was found to be dependent on the pump power . At low pump powers, the laser operates in the continuous-wave (CW) state. As the pump power is increased, the laser reaches QML state. At even higher pump powers, the laser turns into CML state. However, the pump power for stable QML operation with regular QML pulses is limited within a small range. For certain applications, switching between different short-pulse states while keeping the same averaged output power will be useful.
In this paper, we propose a method for manipulating the output of a ring-cavity erbium-doped fiber laser (EDFL), by which the laser can be operated in CW and various short-pulse states. By use of a semiconductor saturable absorber mirror and polarization control in the laser, different operation states such as continuous-wave, stable Q-switching, Q-switched mode-locking, continuous-wave mode-locking, pulse splitting, and harmonic mode-locking can be obtained while the pump power remains unchanged. All EDFL operation states can be realized within polarization tuning angles of 180°. The role of polarization control in the formation of Q-switching, Q-switched mode-locking, and CML together with the mechanism bring about pulse splitting and harmonic mode-locking are discussed.
Figure 1 shows the experimental setup of the EDFL. The output port of a tunable C-band erbium-doped fiber amplifier (EDFA, SDO Corp.) was connected to its input port through a 2×2 3-dB coupler, a polarizer and a polarization controller, by which a ring cavity EDFL was constructed. The cavity length of EDFL is about 50 m. All the fiber components in the laser cavity are linked via single-mode fibers with FC/PC or FC/APC connectors. One of the coupler ports was connected to the SESAM (Batop, SAM 1550-30-10ps-FC/APC), which has an unsaturated absorption of A 0 = 30% and modulation depth of ΔR = 18% at 1550 nm, with saturation fluence of 70 μaJ/cm2. The low intensity spectral reflectance of this SESAM is monotonically increasing with respect to wavelength between 1530 nm and 1620 nm. Another port of the 3-dB coupler was used as the EDFL output, which was then connected to the measurement instruments.
The EDFL output was measured by a power meter (ILX OMM-6810B), a high speed InGaAs detector (Electro-Physics Technology, ET 3000) that was connected to an oscilloscope (Tektronix TDS 2022), and an optical spectrum analyzer (Ando AQ6317B). While manually tuning the polarization controller, the laser operating states were monitored by using the InGaAs detector and the oscilloscope, and both of the temporal and spectral behavior of the EDFL was recorded. A noncollinear autocorrelator (Femtochrome FR-103XL) was used to measure the width of mode-locked pulses.
3. Results and discussion
The EDFL can firstly be tuned to operate in the CW state [Fig. 2(a)]. With laser diode (LD) pump current of 70 mA (corresponding to LD pump power of 42 mW), the EDFA output power is 10.2 dBm. When operated in CW state, the EDFL output power is 3.3 dBm and the laser wavelength is 1559 nm, which is different from the ASE peak of this EDFA at 1532 nm. By manually adjusting the polarization controller, the EDFL can be operated in the Q-switching state [Fig. 2(b)], and the pulse repetition rate is dependent on the LD pump power. The repetition rate of passively Q-switched laser pulses increases with the EDFL intracavity power. We have observed that even with fixed LD pump power, the Q-switched pulse repetition rate can be slightly tuned by adjusting the polarization controller. For cavity length of 50 m and LD pump power of 42 mW, the range of repetition rate for Q-switched pulses is 18.4–20.8 kHz, and the pulsewidth is 4.1 μs. The center wavelength of Q-switched EDFL is 1532 nm, which is also the ASE peak of the EDFA. As the polarization controller was adjusted while maintaining the LD pump power, the EDFL operation state gradually changed from Q-switching to Q-switched mode-locking state [Fig. 2(c)]. The repetition rate for the Q-switching envelope is about 20 kHz, while the QML pulse repetition rate can be 4 MHz or higher-order harmonics (64 MHz has been achieved). The QML sate can either be operated at center wavelength of 1532 nm or 1558 nm, depending on the polarization adjustment. This adjustment is capable of manipulating central wavelength dynamics in such EDFLs . For short cavity configurations and low pump powers, the optical feedback from fiber connections outside the EDFL cavity could switch the QML to Q-switched state, which is improved by connecting the output port of 3-dB coupler to an optical isolator. As shown in Fig. 2(c), a pedestal is superimposed on the QML pulses. This can be improved by increasing the LD pump current to 100 mA and removing the polarizer and polarization controller.
The result for polarizer-free Q-switched mode-locking EDFL are shown in Fig. 3, where the QML envelope repetition rate has been increased to about 115 kHz due to the increased intracavity power and the QML pulse spacing is reduced to 200 ns due to the shortening of laser cavity length. Further adjustment of the polarization controller eventually turns the QML state into continuous-wave mode-locking state [Fig. 2(d)]. The pulse repetition rate is again 4 MHz, corresponding to cavity round-trip time of 250 ns and cavity length of about 50 m. Figures 4(a) and 4(b) show the autocorrelation trace and spectrum of the CML pulses respectively. The EDFL operated at CML mode exhibits central wavelength of 1531 nm associated with two narrow-band peaks at 1524.2 and 1528.5 nm. Since we do not incorporate dispersion compensation components in the cavity, the total intracavity dispersion is anomalous at 1550 nm band. The measured CML-EDFL pulsewidth is 800 fs with a spectral linewidth of 8 nm, giving rise to a time-bandwidth product deviated from transform-limit situation. Therefore, the CML-EDFL pulses are not transform-limited at current stage, and a shorter pulsewidth can be expected by utilizing intracavity/extracavity dispersion compensation method. The EDFL output pulse energy is 0.5 nJ and the peak power is calculated to be 625 W. At some particular polarization adjustments, we have observed that the mode-locked EDFL output spectrum spans from 1530 nm to 1560 nm, but this operation state depends strongly on the environments.
The switching between EDFL operation states could be attributed to the incorporated loss modulation given by the intensity-dependent polarization evolution under the control of intracavity polarizer and SESAM. In our case, the effective intensity-dependent loss modulator results from the combining effects of the polarization controller, the erbium-doped fiber (EDF), the intracavity single-mode fiber (SMF) links, and the polarizer, as schematically shown in Fig. 5. In principle, the nonlinear phase shift induced polarization evolution in the EDFL configured with SMF is mainly caused by self-phase modulation (SPM) and cross-phase modulation (XPM) . In addition, the polarized electric field of the pumping source also introduces a birefringence in the EDFL for intensity-dependent phase retardation. As a result, the CW and short-pulse states will experience different phase retardation during intracavity circulation. The polarization controller works as a manually tunable phase retarder to provide pre-defined phase retardation, while the polarizer works as an analyzer. The intensity-dependent loss will be encountered when passing the optical field through such an effective modulator, providing different transmission losses for the low-and high-intensity components. By properly tuning the polarization controller, one kind of the operation states can benefit a smaller loss for lasing. While operating in CML or HML states, the EDFL should be considered as a passive mode-locked laser with dual or hybrid mode-locking mechanism. Recently, Xu et al. calculated the cavity transmission coefficient for a CML-EDFL using the nonlinear polarization rotating technique , in which the cavity transmission coefficient is express as a function of the net phase delay. In the numerical calculation they have taken into account the nonlinear phase change, the phase delay induced by the polarization controller, and the fiber birefringence. They have observed experimentally the evolution of center wavelength with different operation state. Using this concept, we can explain the wavelength switching of the QML sate between 1532nm and 1558 nm, as well as the observation that the mode-locked EDFL output spectrum could span from 1530 nm to 1560 nm at particular polarization adjustments. The mechanism responsible for the nonlinear polarization evolution can also explain that the EDFL operation states strongly depend on the initial bending or twisting of the intracavity fiber components.
In more detail, Okhotnikov et al. have stated that the pulse evolution in EDFL cavity can be described by modifying the rate equation of time-dependent saturable loss of the SESAM with Ginzburg-Landau equation :
where δs(t) and ΔR denote the time-dependent and stable saturable losses of SESAM, τrec is the recovery time of SESAM, ψ(t) is the optical intensity, and Esat,A is the saturation energy of SESAM. According to the results given by Okhotnikov and co-workers, the critical intracavity pulse energy Ec required for the stable operation of CML against Q-switching/QML should obey the criterion of
where Esat,G is the gain saturation energy of the EDFL. For a stable CML operation, the intracavity pulse energy must be larger than Ec. As estimated, the parameters for the SESAM used in the EDFL are Esat,A = 0.18 nJ and ΔR = 0.18. and the gain saturation energy is Esat,G = 10 μJ (adopted from Ref. 16). The value of Esat,G Esat,AΔR is calculated to be 324 nJ2, and the critical pulse energy is Ec = 18 pJ. The maximum intracavity CML pulse energy Ep obtained for this EDFL is 2.5 nJ. Obviously, Eq. (2) cannot be satisfied if a SESAM alone is used as the absorber, while the EDFL will not self-start from Q-switching instability into continuous-wave mode-locking. In this case, either an increase in the pulse energy (by increasing the pump power or decreasing the pulse repetition rate), or an additional mode-locking mechanism must be introduced into the EDFL cavity for stable CML operation. In our wok, the transition from Q-switched to mode-locked regime is carried out via the polarization control under constant pump power.
Further adjustment of the polarization controller enable us to split a CML-EDFL pulse into several pulses [Fig. 6(a)], and the EDFL operation can evolve into the harmonic mode-locking under proper polarization control [Fig. 6(b)]. In our experiment, stable 5th-order HML has been observed, with pulse repetition rate of 20 MHz and pulse spacing of 50 ns. By examining the EDFL output with a half-wave plate and a polarizer, we find that the splitted pulses are of the same polarization state. As shown in Fig. 4(b), the spectrum for fundamental mode-locked EDFL presents nonsoliton components in the form of narrow-band peaks located at 1524.2 nm and 1528.5 nm. This nonsoliton component plays a significant role for the EDFL to operate in either pulse-splitting or HML regime . The interaction between solitons and the accompanying nonsoliton component may attract or repel adjacent solitons to form either pulse splitting or HML. In our experiments, pulse splitting with temporal spacings of about 15 ns has been observed. However, HML higher than 6th order are not stable against environments and we believe that higher-order HML will be obtained by increasing intracavity laser powers and using more delicate polarization tuning mechanisms. In particular, the polarization tuning angles for various EDFL operation states depends on the initial bending/twisting of intracavity fiber components. Nevertheless, all operation state, from CW state to harmonic mode-locking state, can be routinely obtained by tuning the polarization controller. Table 1 shows typical polarization tuning angles and related output powers for various EDFL states by using an electronic lightwave polarization controller (FiberPro PC4002), where the angles are measured at the center of each operation range with the CW tuning angle as a reference. We observed that all of the EDFL operation states can be obtained within polarization tuning angles of 180°. The output powers depend slightly on the initial bending/twisting of intracavity fiber components. Even a single polarization controller in the EDFL cavity can maintain the operation state stable as long as the intracavity fiber jumpers are not twisted or translated.
In brief, the optimal SESAM characteristics are quite different for mode-locked and Q-switched laser operations . Although the SESAM used in our experiment is optimized for CML operation, we can still obtained stable CW, Q-switching, or HML operation with the help of loss modulation mechanism induced by intracavity polarization control. Alternatively, the EDFL operation states can be expected to be changed by tuning the intracavity laser power, as that for diode-pumped solid-state lasers , or by changing the total cavity length. While increasing the pump power, so that the intracavity pulse energy being larger than Ec, the transition from Q-switched to mode-locked regime is realized. Without the insertion of polarization and polarization controller, the EDFL can be tuned to operate in Q-switched or QML states for shorter cavity lengths and lower laser powers; however, the laser cannot be tuned to CML or HML states. Under these circumstances, back reflection from outside cavity connections becomes important and an optical isolator should be used to stabilize the operation states. As the LD pump power is increased for an elongated cavity, the EDFL will be gradually switched from CW through Q-switching, QML, and finally to CML, which is similar to the solid-state lasers. However, the polarization controlling method enable us to change the EDFL from CW state to various pulsed states using the same LD pumping power and cavity configuration, and the EDFL output powers are almost unchanged. The constant pumping scheme is an advantage in many applications where a laser with different pulsewidth (or peak power) but almost the same average power is required. Moreover, the EDFL with hybrid saturable absorber we presented in this work can realize stable CML at lower pump power as well as shorter pulsewidths in comparison with the EDFL using only the SESAM for mode-locking. Therefore, it can be used in low power lasers or when the pump power is limited. Different from conventional ring-cavity fiber lasers using two polarization controllers in the nonlinear polarization rotating technique [16–17], only one polarization controller is used in our EDFL laser and the operation of each laser state is quite stable. This versatile tunable EDFL could be used as a laser oscillator stage for subsequent power amplification. It would also be useful for studying the nonlinear interaction of optical pulses in nonlinear optical materials or devices, such as photonic crystal fibers and supercontinuum generation .
We have demonstrated a versatile tunable erbium-doped fiber laser by using semiconductor saturable absorber mirror and intra-cavity polarization control, in which the laser can be switched from CW state to various short-pulse states while keeping the pump power at the same level. By tuning the intra-cavity polarization, Q-switching, QML, CML, pulse-splitting, and HML have been obtained for almost the same laser output powers. All EDFL operation states can be obtained within polarization tuning angles of 180°, and the switching between EDFL operation states could be attributed to the overall loss modulation induced by polarizer and polarization controller, in combination with the saturable loss by SESAM. The passively mode-locked EDFL has 800-fs pulsewidth and 4-MHz repetition rate, with output pulse energy of 0.5 nJ and peak power of 625 W. The spectrum for fundamentally mode-locked EDFL presents non-soliton components in the form of narrow band peaks. Further adjustment of the polarization controller begins to split the CML pulses and eventually lead to pulse-splitting and harmonic mode-locking of the EDFL, which is attributed to the interaction between solitons and the accompanying non-soliton component. Stable 5th-order HML has been observed, with pulse repetition rate of 20 MHz and pulse spacing of 50 ns. In contrast to conventional ring-cavity fiber lasers using two polarization controllers in the nonlinear polarization rotating technique, only one polarization controller is used in our EDFL laser and the operation of each laser state is quite stable.
This work is supported by the Natural Science Council of Taiwan, Republic of China, under grants NSC 97-2221-E-002-055, 97-2221-E-133-001, and 97-2112-M-029-001-MY3.
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