1. Introduction

Cavity dumping has been used in the past as a means to remove the optical flux from laser oscillators [1]. The technique is particularly useful in the generation of higher powers in lasers where the gain medium does not accumulate population inversion for a long time, such as in gas [2], dye [3] and Ti:sapphire lasers [4]. Cavity dumping also finds applications in conjunction with mode-locking, in the generation of high power ultrashort pulses [5,6], where pulse recirculation is required in the shortening process and the Q-factor of the cavity is generally high. The development of new types of fiber lasers based on novel gain media [7,8] would benefit from the integrated functionality of cavity dumping. For this to be possible, a fiber-based intracavity switch should be used, with switching times much less than the cavity roundtrip. Various demonstrations of bulk optics-based systems have been reported, for example with acousto-optic modulation [10,9], single crystal photo-elastic modulation [11] or a MEMS system [12,13]. However these systems require tedious alignment and have high loss due to Fresnel reflections at the different glass, air and crystal interfaces. Therefore a component with fast switching of the guided light in an all-fiber laser setup would be favorable. Switching or modulating light in all-fiber lasers has also been demonstrated, usually with a piezoelectric actuator mechanically pressing on the fiber and introducing losses [14] or birefringence [15], or with modulation of fiber Bragg gratings (FBG) [16,17]. Problems with these methods include hysteresis, mechanical relaxation, slow rise and fall time and low extinction ratio.

As a proof of principle, this work shows how a fiber pulse-picker can be used to cavity dump an all-fiber erbium doped ring laser. All the laser radiation is dumped within one round trip without ring down, at a repetition rate continuously tunable up to 1.5 kHz. The component used [18] consists of a microstructured fiber with electrically driven internal electrodes for polarization rotation. The maximum average pulse energy extracted is 5.4 nJ with a peak power of ~54 mW. Although the gain medium used here has long lifetime and is therefore more useful for Q-switching, the use of an erbium-doped fiber amplifier is highly convenient for the performance characterization and optimization of the switching process. The cavity dumper developed here can be readily used with other laser systems where gain accumulation is not efficient.

2. The pulse-picker

The component described in detail earlier [18] is based on a microstructured fiber with four holes parallel to the core. The fiber is 125 μm in diameter with a core diameter of 8 μm. The device is manufactured to be single mode at 1.5 µm and compatible with standard telecommunication components. The 28 μm diameter holes are filled with BiSn alloy creating resistive electrodes inside the fiber, two of which are connected to SMA contacts, depicted in Fig. 1 .

 

Fig. 1 SEM picture of the cross section of the four-hole fiber used here.

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Driving a current through any of these electrodes results in heat generation, causing the metal to expand rapidly and to apply uniform mechanical stress to the core over the length of the electrode ~7 cm. This mechanical stress induces birefringence in the core that can be utilized to rotate the polarization state of the light guided in the core, as described in [19]. By applying a short high voltage (HV) pulse to one electrode the polarization of the guided light can be rotated in nanoseconds, and the light transmitted through an analyzing polarizer switched ON. This switching is much faster than the thermal relaxation, which takes place on a scale of a few 100 μs, caused by the heat dissipating in the fiber and surroundings. In order to switch OFF without having to wait for heat dissipation, another HV-pulse is applied to a second electrode placed at 90° relative to the first one. The birefringence introduced by the second electrode therefore cancels out the birefringence introduced by the first and the polarization is rotated back. The next switching ON can be performed after roughly 1 millisecond without significant polarization drift when operating at room temperature. Switching at higher rates leads to a polarization shift due to the heated environment around the fiber component. This can be compensated for with dynamic polarization control or active cooling, which should allow the operation rate to reach ~100 kHz.

The switching time for the component is ~4 times longer here than in previous work [18]. This is due to the HV-pulse having lower voltage, and thus requiring more time for the deposition of an equivalent amount of energy. The pulse-picker is driven with a solid state HV-switch delivering square electrical pulses with a rise- and falltime of 5 ns of up to ~600 V into 25 Ω. The HV-switch is triggered by a 5 V function generator and the duration of the HV-pulse is controlled by the function generator to make sure that the energy deposited is sufficient for full switching [20]. The output of the HV-switch is connected to two 50 Ω coaxial cables of different length. The shorter one is connected to the ON-switch electrode and the longer to the OFF-switch electrode. The ON/OFF time delay is chosen with subnanosecond precision by the difference in cable length.

3. Experimental

The laser system used in the experiment is a spliced all-fiber ring cavity, based on telecom 125/8 μm diameter/core fibers and components, depicted in Fig. 2 .

 

Fig. 2 Setup for all-fiber cavity dumping.

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It consists of a 1 m long highly doped Er³⁺ fiber (core diameter 8 µm and 44 dB/m absorption @ 978 nm) pumped in the backward direction by a fiber coupled 976 nm diode laser through a 0.98/1.55 μm wavelength division multiplexer (WDM) fusion coupler.

A circulator in combination with a fiber Bragg grating (FBG) is placed before the output port from the cavity, in order to filter out amplified spontaneous emission from the output signal and act as an unidirectional isolator. The FBG has high reflectivity (99.99%) at the laser wavelength 1546.96 nm and a bandwidth of 50 pm.

A fiber polarization splitter analyses the polarization state and after switching dumps vertically polarized light to the output port. The intracavity flux is monitored through a beam splitter (1% fiber tap). Power meters, 1- and 10 GHz photodiodes connected to an oscilloscope and an optical spectrum analyzer to analyze the performance of the laser.

Polarization controller 1 (PC1) is adjusted so that light arrives at the pulse-picker linearly polarized and with the appropriate orientation for full switching when the pulse-picker switches ON. PC2 is then adjusted so that continuous wave (CW) lasing can start in the forward direction when the pulse-picker is OFF, and all the flux passes through port H of the polarization splitter.

When the pulse-picker is switched ON, the circulating radiation is directed to port V of the polarization splitter, i.e., emptying the cavity and creating an output pulse, as shown in the oscilloscope trace of Fig. 3 . The optical pulsewidth corresponds to the cavity roundtrip time. The pulse-picker is switched OFF some nanoseconds after the cavity flux is emptied, allowing laser oscillation to rebuild.

 

Fig. 3 (a) Electrical pulses applied to the pulse-picker causing it to switch ON and OFF and (b) the output optical pulse.

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On a long time-scale, the signal at the output port consists of a single cavity dumped pulse, followed by low amplitude noise, as seen in Fig. 4(a) . Most of the spiking and relaxation oscillations that follow during the gain recovery phase of the laser seen in Fig. 4(b) (intracavity flux) are blocked from the output signal with a measured rejection ratio of ~25 dB. It should be noted that the relaxation oscillations converge on an even longer time scale to CW operation, at the amplitude level seen at t < 0 in Fig. 4(b).

 

Fig. 4 (a) Averaged trace of optical output when cavity dumping at time t~0. (b) Corresponding intracavity signal showing the cavity being emptied at time t~0 and the laser action recovery with spiking and relaxation oscillations.

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The laser system is operated successfully at repetition rates continually tunable up to 1.5 kHz. However, at rates above 150 Hz the relaxation oscillations shown in Fig. 4(b) have not yet settled into CW operation, resulting in unstable dumping, unless the timing for the application of the HV pulse to the polarization rotator is fine tuned to guarantee stable operation.

Figure 5(a) displays the pump power versus average peak power and pulse energy, for a pulse of full width half maximum (FWHM) 100 ns (seen in the inset). Here, the output pulses have a FWHM ~100 ns and rise- and falltime ~45 ns. The optical pulse energy is 5.4 nJ and the peak power 54 mW, when pumping with 550 mW (~500 mW is absorbed by the Er-doped fiber).

 

Fig. 5 (a) Average pulse energy and peak power of cavity dumped laser at 40 Hz. The inset shows the pulse shape. b) Output over time for two different cavity lengths.

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Since the optical flux is removed within one roundtrip, an output peak power of 54 mW indicates that the intracavity lasing is running CW with a quantum efficiency of ~10% before the dumping. This relatively low value is mainly due to the excess loss of the non-optimal components employed.

The peak power of this setup is limited by cavity losses and the available pump power. Scaling the CW power to the limit of the most vulnerable component - the circulator specified to 500 mW – should be possible. The pulse-picker ought to be able to withstand as much optical flux as a standard 125/8 μm cladding/core diameter fiber. The output pulsewidth could be altered by inserting a length of fiber between the beam-splitter and the Erbium-doped fiber. Figure 5(b) shows output pulses from two cavities of different lengths. The pulse acquires a top-hat shape when the cavity roundtrip time is significantly longer than the risetime of the pulse-picker. The shortest pulse switched is limited in practice by the length of the leads of the various components employed.

The laser cavity used in this study does not consist of polarization maintaining fiber. Some instability occurred, particularly when longer cavity lengths and higher pump powers are employed. It is possible to observe, in some cases, rapid oscillations within one cavity dumped pulse, as illustrated in Fig. 6 . The typical period is in the range 2-10 ns, much faster than the roundtrip time of the cavity, which is equal to the envelope width of the pulse switched.

 

Fig. 6 Rapid oscillations (2.6 ns period) observed in a single cavity dumped pulse.

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A full physical explanation for the behavior observed is not available at present. Similar rapid oscillations have been reported in the literature before, associated with polarization effects [21], Risken-Nummedal-Graham-Haken instability [22], the Kerr effect [23] and electrostriction [24,25]. In a CW-mode, i.e. no switching, the rapid oscillations persist over ~0.1–10 ms. The period of these oscillations is not changed by introducing a section of high birefringence fiber into the cavity, but generally shorter periods are observed with spectrally wider FBGs. Removing all polarizing elements from the cavity and analyzing the polarization state at the 1% fiber tap reveals polarization rotation with a period comparable to the rapid oscillations.

4. Conclusion

In conclusion, the first cavity dumped all-fiber laser system based on an integrated polarization rotation pulse-picker is reported. The active component is driven by solid state electronics, has rise- and falltime ~45 ns and is single mode at 1.5 µm. The erbium-doped fiber chosen as gain medium to simplify the characterization of the pulse-picker here can readily be replaced by other types of fiber amplifier where cavity dumping is more advantageous, in particular in a mode-locked regime. Repetition rates up to 1.5 kHz are studied, and 100 ns pulses produced, limited by the cavity roundtrip. The maximum energy measured in the pulses is 5.4 nJ. The optical flux is removed from the cavity within one roundtrip and the amplified spontaneous emission, spiking and relaxation oscillation at the laser output create a background 25 dB lower than the dumped pulses. The laser dynamics occasionally experiences a fast pulsing behavior, and the cavity dumped pulses then consist of a rapidly varying signal within the cavity roundtrip time.