An ultralow-repetition-rate, all-polarization-maintaining (PM), Er-doped, ultrashort-pulse fiber laser was demonstrated using a single-wall-carbon-nanotube polyimide film. Using a ring cavity configuration, output pulses with pulse energy of 0.7–2.6 nJ were obtained at an ultralow repetition rate of 943–154 kHz for a fiber length of 0.1–1.3 km. A novel θ (theta) cavity configuration was proposed, which enabled us to reduce the required fiber length by half. A repetition rate of 132 kHz was achieved using this configuration with 909 m of PM fiber.
© 2010 OSA
High-energy ultrashort pulse sources are important in the field of ultrashort-pulse laser processing. One promising way to generate high-energy pulses is to use a combination of a low-repetition-rate pulse source and amplifier. If we assume an ideal amplifier with saturation power, the lower the repetition rate is, the higher the energy of the amplified pulse is. Therefore, an ultralow-repetition-rate pulse source is attractive for high-energy pulse generation. So far, a pulse picker and mode-locked pulsed laser source have been mainly used to obtain a stable low-repetition-rate pulse source . In such a system, however, high performance is required for the pulse picker, such as excellent on-off ratio, fast response time, and accurate synchronization. If these performance requirements are not satisfied, sub-pulses accompany the main pulses, and they cause serious degradation of the amplified pulse characteristics .
Extension of the cavity length in a passively mode-locked laser is one way to realize a stable low-repetition-rate ultrashort pulse source. Kowalevicz et al. demonstrated a 5.85 MHz, low-repetition-rate, high-energy Ti:Sapphire laser using a Herriott-type multi-pass cavity . Since it uses a free-space configuration, it is difficult to extend the cavity length in solid state lasers. For fiber lasers, an extremely long cavity is feasible simply by extending the fiber length in the cavity. So far, there have been several studies of ultralow-repetition-rate, passively mode-locked fiber lasers [4–10]. Since the fiber length is extremely long in those lasers, they are sensitive to environmental variations. However, there have been no studies of all-polarization-maintaining (PM), ultralow-repetition-rate fiber lasers. Besides, in terms of laser physics, there is some interest in how far we can decrease the repetition rate of passively mode-locked fiber lasers. A weak point of the long cavity laser is that the amplified spontaneous emission (ASE) component arises at the long time between the pulses. So the characteristics of the ultra-low repetition rate fiber laser are also interesting in terms of both practical applications and laser physics.
Recently, single wall carbon nanotubes (SWNTs) have been attracting a great deal of attention as novel and effective saturable absorbers for passive mode-locking [11–21]. They show saturable absorption properties with a recovery time of ~1 ps, and a transparent saturable absorber can be made to work as an effective and practical mode-locker. We have been investigating an all-PM, passively mode-locked, ultrashort-pulse, Er-doped fiber laser using an SWNT polyimide film [20,21]. The polyimide film is easily inserted between PM fiber connectors so that there is no need for free-space optical alignment.
In this work, we demonstrated an all-PM, ultralow-repetition-rate, high-energy, passively mode-locked, Er-doped fiber laser using an SWNT polyimide film. The dependences of the repetition rate (cavity length) and the noise performance were examined in a ring cavity configuration.
The demonstration of ultralow repetition rate in ring cavity configuration requires a relatively long PM fiber. So far, the sigma-cavity configuration has been used for extending the cavity length [10,22]. In this work, we propose a novel θ (theta) cavity configuration for an all-PM fiber laser. The two birefringent axes are used and the laser emits an output pulse once every two circulations. We achieved an ultra-low repetition rate of 132 kHz using the proposed theta cavity configuration.
A similar theta-cavity configuration has also been proposed for a highly chirped pulse source . In that scheme, however, a chirped fiber Bragg grating was used in the center part, and the theta configuration was used for the chirp control, not for extending the cavity length. In this work, we have independently proposed the theta cavity configuration for an all-PM fiber laser, enabling us to extend the cavity length by almost a factor of two.
2. Long-cavity-length, all-polarization-maintaining, Er-doped fiber ring laser with SWNT polyimide film
First, we investigated the ring cavity configuration of an ultralow-repetition-rate, passively mode-locked, Er-doped fiber laser with an SWNT polyimide film. Figure 1 shows the configuration of the fiber laser developed in this work. The laser cavity consisted of all-PM fiber devices. The total cavity length could be adjusted to give a repetition rate of 943–154 kHz. The fiber section consisted of 100–1200 m of PM single-mode fiber (SMF) with anomalous dispersion and 100 m of PM SMF with normal dispersion, followed by 1.2 m of PM Er-doped fiber (EDF). The peak absorption of the EDF is 55 dB/m at a wavelength of 1550 nm and it has anomalous dispersion property. The total cavity dispersion was –1.2 to –25 ps2. A polyimide film containing SWNTs synthesized by the laser ablation method was used as the mode locker [14,21] and inserted between the FC/APC connectors. In order to reduce the irradiation power on the SWNT film, the output coupling ratio at the variable PM fiber coupler was set to about 97.1–98.8%. Most of the pulse energy was emitted from the output coupler to obtain a high pulse energy output. The high output ratio was also effective in reducing nonlinear effects in the cavity, so as to achieve high-energy single-pulse mode-locking. A wavelength filter was used to suppress unwanted oscillation at the shorter wavelength side. For the long PM-SMF, we designed the cavity so that the pulse propagated as a fundamental soliton in the 100–1200 m PM-SMF. Owing to the soliton propagation and PMF, we could obtain stable mode-locking for a fiber laser with an extremely long cavity. After propagation in the long PM-SMF, the optical pulse was temporally broadened in the normal-dispersion PM-SMF, and the peak power was reduced before amplification. Then the optical pulse was amplified in the PM-EDF.
Self-start stable operation could be obtained owing to the all-PM configuration. The output pulses were observed with a combination of a fast photodiode and a digital oscilloscope, an optical spectrum analyzer, and an SHG frequency-resolved optical gating (FROG) system.
Next, we examined the dependence of the output performance on the repetition rate when the output coupling ratio was 97.1%. Figure 2(a) shows the maximum pulse energy and pulse duration of the output pulse as a function of repetition rate. As the repetition rate was decreased, the full width at half maximum (FWHM) pulse duration increased monotonically. The pulse energy was almost constant and independent of the repetition rate. The maximum pulse energy and the pulse duration were limited by the nonlinear effect and soliton condition. The pump power was also almost constant at ~100 mW for the maximum pulse energy. Since we used an all-PM configuration, the laser performance was stable and the extinction ratio of the output pulse was 16 dB.
Figure 2(b) shows the optical spectra of the output pulses. For all of the conditions, a soliton pulse spectrum with Kelly sidebands was stably observed. We demonstrated stable soliton mode-locking at an ultralow repetition frequency of 943–154 kHz at a wavelength of about 1545 nm. As the repetition rate was decreased, the FWHM bandwidth was decreased, and the ASE spectral component was increased. Since the fiber laser was in the soliton mode-locking operation, the sech2 shaped spectrum was assumed for mode-locked pulse and the energies of output pulse and ASE component were estimated. When the repetition rate was 943 kHz, the ratio of ASE component was ~10% for output power. As the repetition rate was increased, the ratio of ASE component was increased and finally the energy of ASE component became almost equal to the pulse energy.
Figure 3 shows the characteristics of the output pulses from the fiber laser when the cavity length was 1309.5 m and the output coupling ratio was 98.8%. Figure 3(a) shows the observed temporal waveform of the pulse train. The temporal pulse separation was measured to be 6.5 μs, corresponding to a repetition rate of 154 kHz. Single-pulse oscillation with a pulse energy of 2.6 nJ was obtained when the pump power was 134 mW. The observed optical spectrum of the output pulse is shown in Fig. 3(b). The central wavelength was 1546 nm, and the FWHM bandwidth was 0.65 nm. The ASE component was observed at shorter wavelength side. Figure 3(c) shows the temporal pulse shape and instantaneous wavelength observed with the SHG-FROG system. A clean pulse without any pedestal component was observed. The pulse duration was 6.20 ps FWHM, and the pulse was almost linearly chirped due to the normal dispersion.
Next, we examined the RF noise of the developed fiber laser using the single sideband method. A fast photodiode, bias-T, and RF spectrum analyzer were used for the measurement. The observed RF noise is shown in Fig. 4 . Despite the long cavity length, there was no large noise component. Equally spaced sharp harmonic spectral peaks with almost constant power were observed, which means that the single pulse mode-locking was obtained. Since the repetition frequency and average power were low, the RF background noise level was almost the same as the system noise level and it was difficult to estimate the actual RF background noise level. From the previous work , it is considered that the relative background noise level is increased as the repetition rate is decreased.
In order to demonstrate the dispersion compensation of the output pulse, since the magnitude of chirping is large and pulse energy is high, it is not easy to demonstrate it. It is considered that the usage of a pair of grating or long length of large mode area photonic crystal fibers is possible candidate for the dispersion compensation.
Generally speaking, the long cavity fiber laser is easy to oscillate the multiple pulses inside the cavities . For the fiber laser in this work, owing to the designed configuration, we could demonstrate stable single pulse soliton mode-locking operation by the precise pump power control.
3. Ultralow-repetition-rate, all-polarization-maintaining, passively mode-locked, Er-doped fiber laser using theta (θ) cavity configuration
Demonstrating an ultralow repetition rate in the ring cavity configuration requires a relatively long PM fiber, more than one kilometer. In order to reduce the necessary fiber length, here we propose a new cavity configuration, which we call a θ (theta) cavity. The two birefringent axes are used, and the cavity length is almost twice as long as the length of the long PM fiber. The laser emits an output pulse once every two circulations.
Figure 5 shows the configuration of the proposed all-PM theta cavity fiber laser with an SWNT film. The cavity consists of a long PM-SMF, two polarization beam combiners (PBCs), and the same PM fiber devices used in the ring cavity configuration shown above.
The operating principle of this theta cavity laser is as follows. The mode-locked pulse polarized along the slow axis is amplified in the PM-EDF and passes through the PM-isolator, the output coupler, the SWNT film, and the wavelength filter. Then, the birefringent axis of the PMF is inclined by 90 degree before PBC1, and the mode-locked pulse polarized along the fast axis is introduced into port 2 of PBC1. The output pulse polarized along the fast axis comes from port 3 of PBC1 and is introduced into the long PM-SMF. The ultrashort pulse polarized along the fast axis propagates along the long PM-SMF and enters port 3 of PBC2. Then, the optical pulse exits from port 2 of PBC2. Here, the birefringent axis of the PMF is inclined by 90 degree, and the polarization direction is along the slow axis. Then, the pulse enters port 1 of PBC1, exits from port 3, and is introduced into the long PM-SMF. At this time, the polarization direction of the propagating pulse is along the slow axis. After the 2nd propagation round, the optical pulse again enters port 3 of PBC2, and at this time, since the polarization direction is along the slow axis, it exits from port 1 and enters the PM-WDM and PM-EDF. The propagating pulse is amplified in the PM-EDF, and 97% of the pulse exits from the output port and serves as the output pulse.
Figure 6(a) shows the optical spectrum of the output pulse when the length of the PM-SMF was 609 m. A clear spectrum with Kelly side bands was observed. The center wavelength was 1546.2 nm. The observed repetition rate was 165 kHz, which corresponds to a cavity length of 1209 m. The pulse energy was 1.5 nJ.
Figure 6(b) shows the optical spectrum of the output pulse when the length of the PM-SMF was 909 m. A mode-locked pulse spectrum with Kelly side bands was observed stably at a wavelength of 1532.5 nm. For this long fiber condition, single-pulse mode-locking was not obtained in the 1555 nm wavelength region.
Figure 7 shows the observed pulse train for the condition in Fig. 6(b). A clear pulse train was stably observed. The temporal separation of the pulses was 7.6 μs, which corresponds to a repetition frequency of 132 kHz. The corresponding cavity length was ~1800 m, which means that the theta-cavity laser emits an output pulse once every two circulations. The observed pulse energy was 1.1 nJ and the corresponding pump power was 60 mW.
The extinction ratio is important factor for the theta cavity laser. When we used normal-dispersion PM fiber, the mode-locking was not obtained owing to the insufficient extinction ratio of the fiber. If we could use normal-dispersion PM fiber with high extinction ratio, we might obtain higher pulse energy by the dispersion control in the cavity.
Using the theta cavity configuration, we confirmed the mode-locking for the repetition rate of 919-132 kHz. For the RF noise measurement, there was no large noise component and the RF background noise level was below the system noise level. Since the average power was low, we could not demonstrate the autocorrelation and FROG measurements.
We demonstrated an ultralow-repetition-rate, all-polarization-maintaining (PM), passively mode-locked, Er-doped, ultrashort-pulse fiber laser with a single-wall-carbon-nanotube polyimide film. Using a ring cavity configuration, single-pulse passive mode-locking was achieved for a repetition rate of 943–154 kHz. The pulse energy was 0.7–2.6 nJ. The RF noise of the output pulses was examined, and no large noise component was observed. We also proposed a new θ (theta) cavity configuration for an all-PM fiber laser. We achieved a ultralow repetition rate of 132 kHz.
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