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We report on the generation and amplification of pulses with pulse widths of 400 – 1000 ps at 1064 nm. For pulse generation an ultra-fast semiconductor modulator is used that modulates a cw-beam of a DFB diode laser. The pulse lengths could be adjusted by the use of a voltage control. The pulses were amplified in a solid state Nd:YVO4 regenerative amplifier to an average power of up to 47.7 W at 100 – 816 kHz.
© 2012 Optical Society of America
1. Introduction
Today’s pulsed laser systems cover a wide range of pulse durations and repetition rates. However, most of the systems are not very flexible with respect to these parameters. Additionally, pulse durations of a few hundred picoseconds are typically hard to achieve. However, a flexible and easy to adjust laser source that covers this range of pulse durations is very interesting for development tasks where different pulse lengths have to be tested to find an application optimum.
Mode-locked lasers are typically not suited for this application. They generate pulses with a fixed pulse duration in the range of some ten picoseconds down to a few femtoseconds [1, 2]. Longer pulses with variable pulse durations could be achieved in the past by narrowing the gain spectrum of a Nd:YVO4 oscillator [3], but a fast change of the pulse length was not possible there.
Q-switched lasers, on the other hand, emit pulses with a duration of some ten nanoseconds [4] down to some ten picoseconds [5], depending on their resonator lengths. Especially passively Q-switched microchip lasers are capable of producing pulses with pulse lengths in the desired range [6], but for a given resonator the pulse length is fixed.
In terms of flexibility the semiconductor diode laser has the most advantages. Due to the fast build-up and decay of the inversion in the active region a diode laser can be directly driven with a wide range of pulse durations and repetition rates. However pulse durations below 1 ns are typically hard to achieve, because of the development of a gain-switch peak at the beginning of the pulse. Therefore we used a cw DFB diode laser that was modulated by a subsequent ultrafast semiconductor modulator, described in [7] and [8]. The modulator was capable of switching the optical beam within a few hundred picoseconds, generating pulses with pulse durations of 400 – 1000 ps.
Amplification was achieved by the use of a regenerative amplifier, published by Lührmann et. al [9]. The amplifier was based on a 888 nm pumped Nd:YVO4 crystal and provided a high gain with a small signal gain coefficient of 9. So even the low power pulses from the diode laser could be amplified to an average power of 47.7 W with a moderate number of round trips. The repetition rate could be varied between 100 and 816 kHz.
2. Experimental setup
The experimental setup of the laser system is shown in Fig. 1. As a seed source a DFB-diode laser, based on InGaAs, was used. The wavelength was temperature tuned to 1064.1 nm, which is the gain maximum of Nd:YVO4. The diode laser was driven by a 100 mA DC current. The output beam was collimated and, after passing through a Faraday isolator, focused into the two-section modulation chip. The first section, based on a ridge waveguide (RW), acts as an optical gate. It is driven by a high frequency GaN high-electron-mobility transistor (HEMT) with low capacitances and high current density. The transistor injects current pulses with a maximum current of 400 mA and a duration of a few hundred picoseconds into the waveguide section. During this time the RW section becomes transparent. The RW section is biased with −2 V, so the incoming beam of the DFB-diode is absorbed if no current is injected. The generated pulses are amplified in the following tapered section. This section is also driven by short current pulses of 2 – 4 ns with a peak current up to 5 A. The tapered section is turned on prior to the RW section to provide enough time for building up the necessary inversion. The exact delay of the switch-on times is crucial for getting clean optical pulses with high output power and should be chosen between 2 – 4 ns. If the tapered section is turned on too early, amplified spontaneous emission (ASE) can rise prior to the pulse. If it is turned on too late, the inversion and hence the gain is too low to extract enough power. The pumping of the tapered section should stop immediately after the pulse has passed to suppress ASE after the transit of the pulse. All the necessary timings are controlled by digital delay generator PCI cards.
Fig. 1 Experimental setup of the laser system. DFB: DFB-diode laser. FI: Faraday Isolator. MOD: semiconductor modulator. TA: Tapered amplifier. TFP: Thin film polarizer. FR: Faraday Rotator. BBO: BBO pockels cell.
The optical pulses are then coupled into a SM fiber to improve the beam quality, simplifying the coupling into the regenerative amplifier. A thin film polarizer separates the incoming and the outgoing pulses. A combination of half wave plate and BBO pockels cell switches the polarization state of the beam for a certain time and thus provides the necessary number of round trips in the cavity. The timings are controlled by the delay generator PCI cards. After the optimal time, the polarization state of the beam is switched back and the pulses are coupled out of the resonator. The Nd:YVO4 crystal is pumped by 110 W at 888 nm.
3. Results
The DFB diode laser was driven by 100 mA DC, which is 2.6 times above the threshold, enough to ensure stable single-mode operation. The output power was 42.2 mW. However, it turned out that a maximum power of 20 mW was enough for the modulation waveguide. A higher input power led to pulse distortions and could not raise the output power after the modulation chip. By using the half-wave plate in front of the Faraday isolator, the output power was therefore decreased to 20 mW.
Instead of solely using the waveguide section for pulse forming, it turned out that a combined use of the waveguide and the tapered section was better suited for generating pulses in a very simple way. First, pumping of the tapered section with a current of 5 A was started. This was necessary to ensure maximum inversion at pulse arrival. After 3 ns the waveguide section was pumped for 1 ns with a current of 400 mA. The generated pulse was amplified in the tapered section. Pumping of the tapered section was stopped at a desired time to absorb the tail of the pulse and hence pulse widths of 400 – 1000 ps could be generated, Fig. 2(a). The duration of the current pulse in the tapered section could be adjusted by a voltage that controlled the gate width of the driving transistor.
Fig. 2 (a) Pulse lengths of 400 – 1000 ps could be achieved by simply using a voltage control. (b) No broadband ASE was detectable.
By using this setup it was guaranteed that the amplifying section was pumped for no longer than necessary. A too long pump duration would lead to the development of unwanted ASE. The pulse length is therefore simply defined by a DC voltage control, which controls the length of the current pulse in the tapered section.
Figure 2(b) shows a time averaged optical spectrum of pulses with 1000 ps pulse duration. Due to the optimal timings of the waveguide section and the tapered section, ASE is suppressed by more than 54.9 dB.
Another important background issue is a possible cw background, resulting by insufficient suppression of the cw-seed. This would lead to a permanent offset power, independent of the repetition rate. By varying the repetition rate and measuring the output power, the extrapolation to a repetition rate of 0 kHz delivers the amount of cw-background. The measurement for a pulse width of 1000 ps is shown in Fig 3(a). The power was measured after the SM fiber. The repetition rate was varied between 100 and 816 kHz. A linear fit was used to extrapolate the measurement to 0 kHz and delivered a value of 0.33 μW, which is negligible for most of the applications. The pulse energy could therefore be estimated simply by using the equation Ep = Pavg/frep. The generated pulse energy of approximately 650 pJ is sufficient for an efficient seeding of the regenerative amplifier [9].
Fig. 3 (a) Estimation of cw-background by varying the repetition rate. The pulse width is 1000 ps. (b) Amplitude stability for 1000 pulses with 400 ps pulse width fitted by a bell-shaped curve.
An important characteristic of a laser system that is used for example in material processing is the amplitude stability. Strong fluctuations of the pulse amplitude or the pulse energy lead to insufficient processing quality. Therefore the amplitude stability is measured by recording 1000 consecutive pulses and measuring the amplitude of every single pulse. Figure 3(b) shows the result for an exemplary pulse width of 400 ps. The standard deviation is 0.015, which is sufficient for most of the applications. It should be noted that the amplification process in the regenerative amplifier will increase the standard deviation and therefore a very stable seed source is highly recommended.
To achieve a stable output of the regenerative amplifier, the number of round trips has to be chosen well. A too low number of round trips leads to a low output power, whereas a too high number of round trips leads to instabilities, mainly bifurcations up to chaotic behaviour of the pulse energy [10,11]. The right amount of cycles additionally depends on the repetition rate. In Fig. 4(a) the output power and the cycles in the amplifier are shown for pulses with 400 ps pulse width. At higher repetition rates the average inversion is reduced and hence more round trips are needed to reach the maximum output power. At lower repetition rates fewer round trips are sufficient to extract the maximum possible power. However, instabilities occur earlier at this repetition rate so the achievable average output power is decreased.
Fig. 4 (a) Output power of 400 ps pulses at different repetition rates. The number of round trips in the cavity has to be decreased for decreasing repetition rate. (b) The output power slightly increases for increasing pulse widths.
At the highest repetition rate of 816 kHz an output power of 47.7 W was achieved, which corresponds to a pulse energy of 58.5 μJ. At the lowest repetition rate of 99.8 kHz an output power of 26.4 W was achieved, which corresponds to a pulse energy of 264 μJ.
In Fig. 4(b) the variation of the pulse width is shown, exemplary for a repetition rate of 493 kHz. The output power increases very slightly for increasing pulse widths from 44.6 W at 400 ps to 45.5 W at 1000 ps. This is due to the lifetime of the lower laser level, which is in the same range as the pulse duration [12]. Longer pulses benefit from the depletion of the lower laser level during amplification.
In Figs. 5(a) and 5(b) the shortest and the longest pulses are shown before and after the amplification process at 99.8 kHz. No pulse deformation can be observed.
The beam quality was measured with the D4σ-Method [13]. At all repetition rates and all pulse widths the M2-value was below 1.25. In Fig. 6(a) the beam profile recorded with a CCD-camera is shown. The beam profile is symmetric and has a Gaussian shape.
Fig. 6 (a) CCD camera image of the beam profile of the regenerative amplifier. (b) Amplitude stability for 1000 pulses with 400 ps pulse width after amplification fitted by a bell-shaped curve.
The amplitude stability is decreased by the amplification process. The standard deviation for the normalized amplitude is increased from 0.015 to 0.027, Fig. 6(b).
4. Conclusion
We have demonstrated a hybrid laser system consisting of a semiconductor diode laser, a semi-conductor modulator and a Nd:YVO4 high power regenerative amplifier. The seed source enabled us to produce voltage controlled pulse widths between 400 and 1000 ps. The regenerative amplifier amplified the pulses to a maximum output power of 47.7 W and a maximum pulse energy of 264 μJ. The seed source is a very compact device and can be integrated on a very small footprint, to get a versatile master laser even for other kinds of amplifiers or as a stand alone system. In a next step the pulse length range will be expanded to shorter pulse durations.
Acknowledgments
This work was funded by the German Federal Ministry of Education and Research (BMBF) within the project “INDILAS” contract FKZ 13N9817.
References and links
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