1. Introduction

Laser systems delivering pulses in the picosecond regime have a broad range of applications in material processing, nonlinear microscopy and biomedical research [1,2]. Typically, the required pulse durations and peak powers for these applications are achieved by laser systems based on mode-locked lasers. A modelocked laser was amplified in a three stage fiber amplifier chain to an average power of 96 W at a repetition rate of 478 MHz and pulse durations of 16 ps by S.P. Chen et al [3]. Recently, a compact laser system consisting of a fiber amplified microchip laser capable of producing 85 ps pulses with a peak power of 3 MW was demonstrated [4].

Gain-switched laser diodes are reliable laser sources for producing sub 100 ps pulses with excellent pulse to pulse stability and a very low timing jitter [5,6]. The driving electronics allows for control of the repetition rate and the pulse release time. This enables for synchronization to other systems leading to a high versatility. Due to the low output power compared to other solid-state lasers, amplification becomes necessary to reach aplicable power levels. The combination of a gain-switched laser diode and an all-fiber based amplifier therefore leads to a compact and stable laser source which can be adapted for many applications [6–8]. A fiber amplifier allows for a very high gain, while an excellent beam quality is maintained over a wide power range. This was demonstrated by the amplification of 20 ps pulses from gain-switched laser diodes in several stages of ytterbium doped fiber (YDF) amplifiers by Dupriez et al [6] and Chen et al [7]. An average power of 321 W at a repetition rate of 1 GHz [6] and an average power of 100 W at a repetition rate of 56 MHz [7] could be achieved. Applications such as material processing require high pulse energy and peak power preferable at repetition rates in the MHz or kHz regime because of the limited speed of scanning and switching devices. Recently, a gain-switched laser diode was amplified at a repetition rate of 1 MHz in a fiber based system resulting in an average output power of 3.5 W and a pulse duration of 90 ps [9]. However, no information on pulse energy and peak power was given. The low output power of gain-switched laser diodes requires the consideration of the effects of amplified spontaneous emission (ASE) generated in an unsaturated fiber amplifier. A high amount of ASE leads to inefficient amplification of the signal and a continuous-wave (cw) power background. If the amount of ASE is unknown, reliable information about the pulse energy cannot be extracted from the average output power and the repetition rate.

In this work, all-fiber based amplification of a gain-switched laser diode with detailed investigation of the signal to ASE ratio is demonstrated. Therefore, the emission of the system was analyzed with respect to average pulse and ASE power with an acousto-optic modulator (AOM) at a repetition rate of 1 MHz. From these measurements, reliable values of the pulse properties such as energy, peak power and optical spectrum could be obtained. Furthermore, temporal deformation of the pulses due to nonlinear effects was observed and is discussed.

2. Experimental setup

The experimental setup of the laser system is shown in Fig. 1 . For amplification of the low power gain-switched laser diode (PicoQuant GmbH, Berlin, Germany), a three stage ytterbium fiber amplifier was realized. All amplification stages were completely fiber based and directly spliced together allowing for efficient power coupling. Thus, free space losses between the amplifier stages were avoided and high mechanical stability was guaranteed, which was crucial for stable operation of the system. Usually power scaling in fiber amplifiers is limited by nonlinear effects such as stimulated Raman scattering (SRS) and self-phase modulation (SPM) [10]. To minimize these effects, reduction of the optical power density within the fiber was beneficial by increasing the fiber core diameter while maintaining single-mode propagation [11]. Thus, a successive power scaling from stage to stage was realized by increasing the mode-field diameter of each amplification stage to operate each amplifier under saturated conditions as far as possible and keep the ASE on a low level.

 

Fig. 1 Experimental setup of the laser system with fiber amplifier stages.

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The seed laser pulses had durations of 37 ps at a central wavelength of 1040 nm with a bandwidth of 20 nm. Repetition rates from 31.25 kHz up to 80 MHz could be selected by the associated driving electronics. A Faraday isolator protected the seed laser from back reflections. Pulse energies of about 20 pJ corresponding to an average power of 20 µW at a repetition rate of 1 MHz were launched into the first amplification stage. A wavelength division multiplexer (WDM) enabled pump light coupling of a cw laser diode (LD) with up to 600 mW average power at a wavelength of 976 nm into the core of an ytterbium doped single-clad fiber with a mode-field diameter of 4.4 µm and a length of 60 cm. The output of the first amplification stage was directly spliced to the second pre-amplification stage consisting of an 80 cm long YDF with a core diameter of 10 µm. A self made mode-field adaptor (MFA) allowed for efficient power coupling from the smaller core diameter fiber into the 10 µm core diameter fiber. The second YDF was also co-directionally core-pumped by a cw laser diode with up to 600 mW average power at a wavelength of 976 nm. The single clad pre-amplification stages were protected from back reflection by a fiber coupled isolator.

The final amplifier stage consisted of an all-fiber multimode pump combiner with signal feed through, spliced to a double clad large mode area (LMA) YDF with a core diameter of 30 µm, a cladding diameter of 250 µm and a length of 2.5 m. Again the mode-field of the 10 µm fiber was matched to the 30 µm fiber by a mode-field adaptor. The power amplification stage was co-directionally pumped by multimode laser diodes operating at a wavelength of 976 nm. Overall the amplifier was completely fiber based enabling for a maximum of mechanical stability and efficiency in terms of power coupling.

3. Results

The required pump power and hence the average output power of the first and the second amplification stages were roughly determined by the fixed output power of the seed diode (20 µW at 1 MHz) and the saturation threshold of the third amplifier stage, which was measured to be 100 mW. Additionally, an optimization of the pump power for the first two stages was made by maximizing the signal to ASE ratio while simultaneously minimizing nonlinear effects. Finally, optimum average output powers of 4 mW and 90.5 mW for the first and the second amplification stage were determined, respectively, for a seed laser repetition rate of 1 MHz. The shape and optical spectrum of the pulses after the second stage are shown in Fig. 2 . The investigation of the pulse shape was realized by a measurement with a fast photodiode (New Focus 12 ps NFO-1024) and a 40 GHz sampling oscilloscope. The pulse duration stays constant after amplification in the preamplifier stages as shown in Fig. 2a) and no spectral deformation due to nonlinearities is observed at 90.5 mW average power (Fig. 2b). For comparison, the optical spectrum of the seed source is depicted as inset in Fig. 2b). The centre wavelength is at 1040 nm. The spectrum contains narrow spikes from 1050 nm to 1060 nm. Spectral filtering experiments have shown a relation between theses spikes and the tail of the pulse in Fig. 2a). The main pulse is related to the spectral components below 1050 nm. Due to the amplification in YDF the spectrum is shifted to the gain maximum at 1030 nm which leads to a suppression of the pulse tail.

 

Fig. 2 Pulse shapes (a) and optical spectra (b) of the seed laser and after the second amplifier stage.

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The achieved output power of the complete system after the last amplification stage with a core diameter of 30 µm is shown for a repetition rate of 1 MHz in Fig. 3 . The M-square value was measured to be about 1.23 at the horizontal and 1.25 at the vertical beam axis. The corresponding measurement is depicted as inset in Fig. 3. The maximum extracted output power was 14.9 W at an absorbed pump power of 25.3 W, whereas about 3 W of the pump light was transmitted through the fiber.

 

Fig. 3 Average output power vs. absorbed pump power at 1 MHz repetition rate and corresponding beam profile.

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At the repetition rate of 1 MHz the launched average power of the seed source was 20 µW. At these low power levels saturation of the first YDF with a mode-field diameter of 4.4 µm was not achieved, since the calculated saturation power for this fiber is about 4 mW. Therefore, the first amplification stage determined the performance of the whole laser system in terms of ASE emission and self-lasing, since the subsequent stages were all operated in a saturated regime. To ensure an efficient and stable regime of operation, the ratio of signal to ASE had to be quantified. For seed sources with narrow spectral distribution, information on the signal to ASE ratios can be obtained from the optical spectrum of the amplifiers output radiation. Due to the broadband nature of the used seed source and the overlap of the optical spectra with the ytterbium emission spectrum a distinction between ASE and signal was difficult. However, ASE results in a cw emission allowing for a determination of the signal to ASE ratio in the temporal domain. Therefore, the output radiation was analyzed with respect to pulse and cw power utilizing an AOM for gating. As shown in Fig. 4a ), the output radiation was reflected by a dichroic mirror for removal of the residual pump light. A part of the light was reflected by a glass plate and coupled into the AOM. Its first order deflection was set to a time window of about 40 ns containing the pulses. Afterwards, the time window was inverted as sketched in Fig. 4b). The average power was measured. The pulse duration of about 40 ps was much smaller than the AOM time window. So the amount of ASE radiation in the 40 ns time window had to be accounted for as well. At a repetition rate of 1 MHz this resulted in about 4% additional ASE power, which was considered in the calculations. Reliable information about the signal to ASE ratio could be extracted from these measurements.

 

Fig. 4 Setup (a) and time scheme (b) of the AOM measurement for pulse analysis.

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The ASE ratios and the average power versus pump power of the main amplification stage were measured. The pulse energy can be calculated from the average power, if the signal to ASE ratio is taken into account. The ASE ratios and corresponding pulse energies are shown in Fig. 5 . The highest extracted pulse energy was 13 µJ at 14.9 W average output power with a signal to ASE ratio of 88%.

 

Fig. 5 (a) Signal to ASE ratios with corresponding pulse energies and (b) pulse duration with corresponding peak power at 1 MHz repetition rate.

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The pulse duration of the laser system was measured with the fast photodiode, while the peak power of the pulses was calculated from the corresponding pulse energies and pulse durations. For increasing pulse energy, the full width at half maximum (FWHM) pulse duration broadened from 37 ps to 46 ps at 13 µJ. This evolution is shown in Fig. 5b). It was accounted for pulse broadening in the calculation of the pulse peak power, which led to a maximum pulse peak power of 270 kW being, to the best of our knowledge, the highest achieved peak power by fiber based amplification of a gain-switched laser diode. The normalized temporal pulse shapes at pulse energies of 0.5 µJ and 13 µJ are shown in Fig. 6a ). The pulses were detected on a sampling oscilloscope triggered by a synchronization signal of the seed laser electronics, which allowed for a precise determination of the temporal delay of the pulses. It is obvious that in the temporal domain power is shifted into the leading edge of the pulses. The corresponding optical spectra are shown in Fig. 6b). For increasing pulse energies spectral power was shifted from 1040 nm to longer wavelengths due to intra-pulse Raman scattering, which is assumed to be the dominant nonlinear effect [10]. The negative peaks at 1155 nm and 1185 nm were caused by the spectral transmission of the dichroic mirror placed at the output of the power amplification stage for the separation of transmitted pump light.

 

Fig. 6 Temporal pulse shapes (a) and optical spectra (b) after AOM filtering of the pulses at a pulse energy of 0.5 µJ and 13.1 µJ corresponding to 17.3 kW and 270 kW peak power.

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The temporal deformation of the pulses is directly related to the spectral deformation caused by SRS. To understand the pulse broadening mechanism due to SRS, the effect of slow and fast light generation during SRS must be taken into account. The pulse peak power levels in the kW regime and the temporal overlap of the different spectral components of the unchirped pulses are sufficient for delays of several picoseconds [12,13]. However, this requires further experimental and numerical investigations exceeding the frame of this paper and will be subject of future work.

The optical spectra in Fig. 6b) furthermore prove stable pulsed operation of the laser system even at energies of 13.1 µJ, since no peaks resulting from narrow bandwidth cw laser operation were evident. Further power scaling of the system was not limited by damage of the fiber or self lasing. Due to the temporal and spectral broadening further scaling of the pulse peak power became inefficient, since higher pulse energies resulted in stronger temporal broadening of the pulses.

4. Conclusion

In conclusion, an all-fiber based three stage YDF amplifier was developed to amplify 37 ps pulses from a gain-switched laser diode at 1040 nm. The process of ASE generation in a single-pass setup at low seed average power was accounted for by analyzing the amplified pulses in terms of pulse and cw power by using an AOM. This allowed for accurate measurements of the pulse properties. Stable operation of the laser system at a repetition rate of 1 MHz was verified. The pulses were amplified from pulse energies of 20 pJ to 13.1 µJ corresponding to an overall gain of 58.15 dB. Spectral broadening due to SRS and related temporal deformation of the pulses was observed. The pulse duration (FWHM) at energies of 13.1 µJ therefore increased to 46 ps corresponding to a peak power of approximately 270 kW. To the best of our knowledge, the achieved peak power and pulse energy is the highest extracted from a laser system based on a gain-switched laser diode.