NLM assisted SESAM mode-locking of an Yb:YAG thin-disk laserlaser

7 Mode-locked Yb:YAG thin-disk laser with multipass geometry

7.4 Experimental results: Laser specifications with intracavity second harmonic generationintracavity second harmonic generation

7.4.2 NLM assisted SESAM mode-locking of an Yb:YAG thin-disk laserlaser

SESAM QWP

TFP

DCEM NLC DM

MDMD

Active Multipass Cell Beam Forming Part

OC Beam

{

Figure 7.11: Schematic of the Yb:YAG thin-disk resonator for NLM assisted SESAM mode-locking. Modified laser resonator with the multipass geometry concept that is presented in detail in Section 7.3. Compared to the resonator given in Fig. 7.2 three changes were done: Firstly, a nonlinear optical crystal (NLC) was introduced. Secondly, with the help of the quarter wave plate (QWP) the polarization was adjusted for no output coupling through the thin film polarizer (TFP). Thirdly, in the multipass cell the end mirror was replaced with a dichroic end mirror (DCEM) for output coupling (OC). The number of passes through the active multipass cell was variable. For the sake of clarity 4 passes over the disk are drawn. Negative dispersion is introduced by 5 Gires-Tournois interferometer type dispersive mirrors (DM).

7.4.2 NLM assisted SESAM mode-locking of an Yb:YAG thin-disk laser

Based on the abovementioned extra-cavity experiments we incorporated the nonlinear mirror (NLM) consisting of a dichroic output coupling mirror and a 6 mm long LBO crystal in the existing Yb:YAG thin-disk laser resonator. Unfortunately, pure Stankov mode-locking could not be realized. We assume that the power intensity in cw mode is not high enough to generate efficient second harmonic for getting the nonlinear mirror for mode-locking. Thus, for starting mode-locking the SESAM as one of the resonator’s end mirrors was still needed. Nevertheless, we observed a kind of ’NLM assisted SESAM mode-locking’, since the nonlinear mirror stabilized and optimized the mode-locking behavior.

50 100 150 200

Figure 7.12: Experimental results of SESAM mode-locked Yb:YAG thin-disk laser with reduced output power and 22 AMC passes per cavity round trip. Left: Output power and optical efficiency as a function of pump power. The onset of self-starting pas-sive mode-locking is marked. Right: Pulse length of mode-locked pulses as a function of output power. The 1/x-relation between pulse duration and output power according to Eq. (2.13) confirms soliton mode-locking.

Resonator design

For Stankov mode-locking the Yb:YAG thin-disk laser resonator presented in Section 7.3 was modified. The new resonator design is illustrated in Fig. 7.11. The path of the OC beam was changed: By an appropriate choice of the quarter wave plate the polarization of the light was adjusted for s-Polarization so that no laser light was output coupled by the thin film polarizer. Instead, the end mirror in the active multipass cell was replaced by the dichroic OC mirror and the LBO crystal was set in front of it with the distance d=35 mm extracted from the extracavity measurements presented above. For output coupling a dichroic OC mirror was used with a reflectivity for the fundamental of R(1030 nm)=70 %. Depending on the experiment the SESAM was kept within the cavity or was replaced by a HR mirror for 1030 nm.

Experimental results

For reasons of comparison first of all the output characteristics for the configuration presented in previous sections with 11 passes over the disk and TFP output coupling is shown. The difference compared to the results given in Section 7.3 is another pump diode resulting in reduced output power. As it can be seen in Fig. 7.12 (left) a maxi-mum output power of 23 W corresponding to an optical to optical efficiency of nearly 15 % could be achieved. However, the pulse durations – plotted in Fig. 7.12 (right) – are comparable to the pulse durations given in Fig.7.3 (right). A pulse duration as short as 0.85 ps could be realized.

For NLM assisted SESAM mode-locking we tested different configurations: We used a dichroic OC mirrors with R(1030 nm)=70 %, resulting in an OC rate in the mode-locking regime of about 20 %. The reduced OC rate compared to the reflectivity of the

7.4 Experimental results: Laser specifications with intracavity second harmonic generation

AMC passes per round trip 22 18 14 10 6 Repetition rate (MHz) 3.47 4.17 5.22 6.98 10.53

Table 7.1: Number of passes through the active multipass cell (AMC) per cavity round trip and corresponding repetition rate for mode-locking regime.

mirror is due to SHG that reduces the infrared power after the crystal. The mirror is HR coated for 515 nm. We tried a variety of different numbers of AMC passes per round trip: 22, 18, 14, 10, and 6 passes resulting in a different amount of gain, SPM, and resonator length, respectively repetition rate. In Table 7.1 a list of AMC passes and corresponding repetition rates for mode-locking regime can be found.

Using the dichroic mirror with R(1030 nm)=70 % it was obvious that mode-locking was stabilized by inserting the LBO. Stable mode-locking only based on the SESAM was hardly possible for all numbers of AMC passes. Without the LBO Q-switching dominated the laser regime, whereas with LBO the laser was running in stable cw mode-locking regime. Due to the fact that without LBO crystal no second harmonic is generated the dichroic mirror with R(1030 nm)=70 % corresponds to an OC rate of 30 %. Thus, inserting the LBO and generating second harmonic reduces the OC rate.

As an example for the performance with and without the LBO crystal Table 7.3 lists all important output characteristics for 14 AMC passes per cavity round trip. The different aspects will be explained in detail in the following. This example is picked, since with this configuration pulses as short as 0.7 ps could be realized. The general behavior of the laser and the development of the output characteristics was similar independently of the number of AMC passes.

The output characteristics in Table 7.3 show that for small pump power, respectively in cw mode, there is no difference between the configuration with and without LBO.

However, the output power with LBO decreases when mode-locking begins. This re-duction is caused by the fact that the LBO crystal converts parts of the fundamental light into second harmonic. Since second harmonic is reflected by the dichroic OC mirror, the output power decreases. The SHG also yields the general lower output power of the configuration with LBO than without LBO. Additionally, with LBO the output power saturates for high pump power. Increasing the pump power did not change the output power but reduces the pulse length. The saturation of the output power comes from the further reduction of the pulse length: The conversion efficiency into SHG increases with increasing incident power. Thus, the increase of the pump power leads to higher intracavity power leading to a higher conversion efficiency into SHG. This development may cause two points: Firstly, the output power of the funda-mental does not longer increase and secondly, a higher SHG yields shorter pulses. The latter observation is obvious in the plot of pulse length vs. output power. Here, for the same output power the reduction of the pulse length is clear. Due to the complex pulse shaping mechanism based on intensity dependent SHG the pulse duration does not follow the 1/x-development for increasing output power as it is the case for soliton mode-locking.

Infraredoutputpowervs.pump powerandpulselengthvs.in- fraredoutputpower.Thebe- ginningofstablemode-locking, respectivelyQ-switching(QS)is marked. 5010015020005

25 Pump Power (W)

Output Power (W)

stable mode−locking

with LBO without LBO 5101520250.61.11.62.12.63.1

3.6 Output Power (W)

Pulse Length (ps)

with LBO, stable mode−locking without LBO, Q−switching withLBOwithoutLBOAutocorrelation(AC)forpump powerof182W.WithLBOsta- blemode-lockingleadstoanac- curateACsignal,whereaswith- outLBOQ-switchingleadstoa noisyACsignal. 051015

00.5

1 Time (ps)

Intensity (a.u.)

τ AC=1.08 ps τ P =0.70 psAC signal sech2 −fit 051015

00.5

1 Time (ps)

Intensity (a.u.)

FWHM

τ AC=5.48 ps τ P =3.55 psAC signal sech2 −fit Correspondingspectrumofpulse presentedinlineabove.With LBOthelosswithinthespectrum duetoSHGisevident. 10261028103010321034

−60

−50

−40

−30 Wavelength (nm)

Log. Intensity (dBc)

10261028103010321034

−60

−50

−40

−30 Wavelength (nm)

Log. Intensity (dBc)

7.4Experimentalresults:Laserspecificationswithintracavitysecondharmonicgeneration

with LBO without LBO

Traces on oscilloscope indicating stable mode-locking with LBO and Q-switching without LBO.

Table 7.3: Output characteristics of NLM assisted SESAM mode-locked Yb:YAG thin-disk laser with 14 AMC passes per cavity round trip and a dichroic OC mirror with R(1030 nm)=70 %. The characteristics show that the LBO crystal is necessary for stable mode-locking. More details are given in the text.

105

50 100 150 200

Figure 7.13: Experimental results of NLM assisted SESAM mode-locked Yb:YAG thin-disk laser for different numbers of AMC passes per cavity round trip. Left: Output power as a function of pump power. The onset of self-starting passive mode-locking is marked. Right: Pulse length of mode-locked pulses as a function of output power.

As it is marked in the output characteristics stable mode-locking was just possible with the LBO. A pulse duration as short as 0.7 ps was possible for a pump power of 182 W and a corresponding output power of 17 W. The autocorrelation and the corresponding spectrum are plotted in the Table. In the spectrum the loss of the output coupled pulse due to SHG is evident. The shape of the spectrum does not allow either the extraction of a spectral bandwidth or the calculation of the time-bandwidth product.

In contrast to the stable mode-locking regime with LBO crystal, extracting the crys-tal leads to higher output power but Q-switching tendencies. Without the cryscrys-tal only noisy mode-locking with Q-switching was observed. Despite the Q-switching the measurements of noisy autocorrelations were possible, leading to pulse lengths in the Q-switching regime of 2.5-3.5 ps. The autocorrelation for a pump power of 182 W and an output power of 25 W and the corresponding spectrum are plotted in Table 7.3.

The noisy autocorrelation and the narrow bandwidth of the spectrum reflect the Q-switching regime.

Furthermore, Table 7.3 shows the traces of an oscilloscope when the light is partially detected with a fast photo-diode. Here the differences of stable mode-locking with LBO and unstable mode-locking with Q-switching if the LBO is extracted is clear, too.

To exclude that the mode-locking regime with LBO is just due to a change in disper-sion based on the incorporation of the crystal we also measured the laser characteristics when the LBO was misaligned resulting in no phase matching. In this configuration the output characteristics were similar to the characteristics without any crystal. That proofs that the mode-locking process is stabilized by second harmonic generation and not just due to the incorporation of the crystal as a dispersive element.

Table 7.3 focuses on 14 AMC passes per cavity round trip. The output character-istics for different numbers of AMC passes are given in Fig. 7.13. In the left panel

7.4 Experimental results: Laser specifications with intracavity second harmonic generation

it is apparent that the development of output power follows the same characteristic, as it is already described for 14 AMC passes: The beginning of stable mode-locking implies a reduction of the output power due to second harmonic generation. The laser configuration with 6 AMC passes started mode-locking for a pump power of 124 W, whereas all other configuration started molocking at 103 W. Furthermore, for de-creasing number of AMC passes the dip in output power gets less pronounced and disappears completely for 6 AMC passes. Since the dip in the output power results directly from SHG, the reduced shaping shows that with decreasing number of AMC passes the efficiency of SHG decreases, too. This effect is a consequence of the increased pulse length. As it can be seen in the right panel of Fig. 7.13 the pulse length increases with decreasing AMC passes, except for 14 AMC passes. The number of AMC passes determines the gain per cavity round trip, the cavity length, respectively the self phase modulation of air, and the amount of negative dispersion due to dispersive mirrors in the active multipass cell. Since the pulse shaping mechanism is mainly influenced by the latter two parameters (see Section 2.1 for details about self phase modulation and dispersion), a reduced number of AMC passes may lead to longer pulses. Since SHG is intensity dependent, longer pulses yield lower SHG, resulting in a less pronounced dip in output power.

The extraordinary short pulses for a cavity configuration of 14 AMC passes stick out in the right panel of Fig. 7.13. They can be explained by the fact that the generation of short pulses is a sensitive interplay of SPM, negative GDD, and intracavity power, respectively OC rate. Furthermore, efficient SHG for the nonlinear mirror is essential for the stabilization of mode-locking. The results obtained with 14 AMC passes and a dichroic OC mirror with R(1030 nm)=70 % indicate that all parameters lead to an ideal configuration.

For all numbers of AMC passes the pulse lengths do not follow the 1/x-development for increasing output power as it is for pure soliton mode-locking. A general trend is not to be noticed. However, this behavior shows that the crystal and the nonlinear mirror affect the pulse shaping mechanism and the pulse lengths. The fact that the LBO crystal influences the mode-locking mechanism became additionally obvious in the ob-servation that without the crystal mode-locking was very unstable and perturbed by Q-switching or it was not possible at all. Of course, stable mode-locking is influenced i.a. by the OC rate. Incorporating the LBO leads to SHG and reduces the OC rate of the dichroic OC mirror with R(1030 nm)=70 % from 30 % without SHG to approx-imately 20 % with SHG. Since the mode-locking regime and its stability is inter alia determined by the OC rate, this change in OC rate may be the reason for the differ-ences of mode-locking behavior with and without LBO crystal. To check this point, we also characterized the laser with the following configuration: We eliminated the LBO, exchanged the dichroic OC mirror by a HR mirror, and used the quarter wave plate and the polarizer for output coupling, as it is described in Section 7.3.1 and illustrated in Fig. 7.2. The OC unit based on the polarizer and the quarter wave plate allows for adjustable OC rates. Thus, we set the OC rate to 30 % and 20 % for different numbers of AMC passes per cavity round trip and compared the mode-locking behavior. In Fig.

7.14 the pulse lengths for different laser configurations are plotted. For both OC rates

5 10 15 20 25 30

Figure 7.14: Pulse length vs. output power of SESAM mode-locked Yb:YAG thin-disk laser without LBO for different numbers of AMC passes per cavity round trip and different OC rates. Left: OC rate of 30 %. Right: OC rate of 20 %. Some results are fitted with 1/x-relation according to Eq. (2.13) for soliton mode-locking.

and varying number of AMC passes mode-locking could be achieved. For an OC rate of 30 % (see Fig. 7.14 (left)) the range of stable mode-locking decreases with decreasing number of AMC passes. For 6 AMC passes no mode-locking at all could be realized.

Furthermore, with this OC rate the minimal pulse length gets shorter as the number of AMC passes increases. This comes from the interplay of SPM, negative dispersion – both parameters change with the number of AMC passes – and OC rate.

With a reduced OC rate of 20 % (see Fig. 7.14 (right)) three points are obvious com-pared to an OC rate of 30 %: Firstly, the range of stable mode-locking is less sensitive to the number of AMC passes. Secondly, this leads to the realization of mode-locking even for just 6 AMC passes. However, the mode-locking got noisy for 10 and 6 AMC passes and the development of pulse length with increasing output power does not follow the 1/x-theorem, as it can be seen for 10 AMC passes. The reasons therefore are the disbalance between gain, OC rate, GDD, and negative dispersion. Thirdly, the shortest pulses could be achieved with 14 AMC passes per cavity round trip. This is in best agreement with the experiments with the LBO crystal, where the shortest pulses could also be achieved with 14 AMC passes (see figure of pulse lengths in Table 7.3).

The experimental results without LBO but varying OC rate show that the mode-locking behavior is strongly dependent on the OC rate. Thus, the realization of mode-locking with and without LBO crystal is a question of the OC rate. Due to the LBO the OC rate reduces allowing for mode-locking. But nevertheless, the incorporation of the LBO, respectively the nonlinear mirror, stabilized the mode-locking regime. It was very stable, very insensitive to misalignment, and therefore extremely easy to adjust.

7.5 Summary and conclusion

In this Chapter a mode-locked high-power Yb:YAG thin-disk laser is presented. The resonator design follows the active multipass geometry from Neuhaus et al. [Neu08b].

Basis of the multipass geometry is an increase of the gain by passing the laser beam several times over the gain media within one cavity round trip. This increases the gain and allows for OC rates of up to 70 % [Neu08a, Bau12a] leading to a moderate intracavity power. This, in turn, results in reduced self phase modulation of air that is a limiting factor for mode-locked high-power lasers.

Whereas Neuhauset al. and Baueret al. focused on pushing the limits of pulse energies emitted directly from an oscillator [Neu08b, Bau12b], the focus of the laser presented in this Chapter was reliability and stability for long-term use.

Two techniques for mode-locking were realized: Pure SESAM mode-locking and SESAM mode-locking assisted by a nonlinear mirror. In the first configuration with 22 passes over the disk per cavity round trip an output power as high as 44 W with an op-tical to opop-tical efficiency of 31 % could be achieved. A repetition rate of 3.47 MHz yields pulses with a pulse energy of up to 13µJ at a wavelength of 1030.0 nm. Due to soliton mode-locking the pulse lengths decrease with increasing output power. Corresponding to the highest output power a minimal pulse length of 0.87 ps could be achieved. The time-bandwidth product of 0.32 indicates nearly bandwidth limited pulses. The OC rate was approximately 50 %.

For starting and stabilizing the mode-locking an uncoated InGaAs-QW SESAM with a modulation depth of 2.0 % and a saturation fluence of 45µJ/cm² was used.

After the power-up phase of the laser with Q-switching instabilities, stable self-starting mode-locking was observed. Long-term tests over a time period of several weeks proofed the reliability and stability of the laser.

However, as it is discussed in Chapter 5, the saturable absorber is vulnerable for heat induced damage. Thus, for an alternative to SESAM mode-locking we tried to real-ize a Stankov mode-locked thin-disk laser. Extracavity wave mixing measurements showed that the phase relation between the fundamental and the second harmonic is determined by the free pathlength, respectively the dispersion of air, leading to a further conversion of the fundamental into second harmonic or a reconversion of the second harmonic into the fundamental again during the second pass through the crystal [Sta88b]. By an appropriate choice of free pathlength in air the second harmonic light is reconverted into fundamental.

The realization of a purely Stankov mode-locked Yb:YAG thin-disk laser failed because the intracavity power in cw mode was not high enough to start the mode-locking. Thus, we built a laser resonator with a nonlinear mirror and a SESAM and called it NLM assisted SESAM mode-locking. We characterized the laser performance for different

Im Dokument Characterization and optimization of mode-locking of a high-power Yb:YAG thin-disk laser (Seite 113-123)