Characteristics of saturable absorbers

The Cr⁴⁺:YAG crystal has thickness of 3 mm and was highly doped with a small signal transmission of 28%. Both sides of the Cr⁴⁺:YAG crystal were coated for antireflection at 1030 ~1080 nm (R<0.2%). The AlGaInAs absorber was designed with 50 groups of three QWs as described in Ref [30]. Both sides of the semiconductor SA were coated for anti-reflecting to reduce back reflections and the couple-cavity effects.

Figure 2.6 shows the saturation transmission of the SAs, where the pump source was a nanosecond Nd:YAG Qswitched laser. The saturation energy density of AlGaInAs QWs and Cr⁴⁺:YAG crystal are estimated to about 1 mJ/cm² and 300 mJ/cm², respectively. The deduced absorption cross-section of the Cr⁴⁺:YAG crystal is in the order of 10^-19 cm² and agrees approximately with Ref. [31-33]. Besides, the cross-section for the AlGaInAs QWs was obtained in the order of 10−15 cm². The 95%

final transmission of AlGaInAs reveals the low nonsaturable loss induced by the facet reflection and absorption by the substrate. On the other hand, the final transmission of the Cr⁴⁺:YAG was only 85%, the lossy phenomenon was attributed mainly to the excited-state absorption (ESA) [34]. The final transmission influenced by the ESA effect could be express approximately as Tf =Tiβ, where Tf and Ti are the final and initial transmission, respectively, and the parameter β is the ratio of the absorption cross-section of the excited-state and the ground-state, i.e. β =σ es /σ gs . The values of β derived from Ref [31-33]. ranges from 0.1~0.28 and is 0.128 in our experiment. The modulation depth could be found to be 68% for AlGaInAs QWs and 57% for the Cr⁴⁺:YAG crystal. Furthermore, the relaxation time of the AlGaInAs QWs the Cr⁴⁺:YAG crystal were estimated to be on the order of 100 ns and 3 μs respectively.

Fig 2. 6 Saturation transmission of the AlGaInAs QWs and the Cr⁴⁺:YAG crystal.

2.2.3 Experimental setup

The cavity consists of a 3-m Yb-doped fiber and an external feedback cavity with a SA. Figures 2.7 (a) and (b) show the setups for PQS fiber lasers by use of a Cr⁴⁺:YAG crystal and a AlGaInAs semiconductor, respectively. The fiber has an absorption coefficient of 10.8 dB/m at 976 nm and a double-clad structure with a 350 μm octagonal outer cladding, a 250 μm inner cladding with a numerical aperture (NA) of 0.46, and 30μm circular core with a NA of 0.07. The use of the large-mode-area fiber with low NA is beneficial for storing higher pulse energies and sustaining excellent beam quality simultaneously. The external cavity in Fig. 2.7 (a) consists of a focusing lens of 25-mm focal length to focus the fiber output into the Cr⁴⁺:YAG crystal, a re-imaging lens to re-image the beam on a highly reflective mirror for feedback, and a thin film filter for controlling the resonant wavelength. The SA was wrapped with indium foil and mounted in a copper block without active cooling. Here we used a tight focusing configuration to enhance the energy inside the Cr⁴⁺:YAG crystal. The beam waist was about 20 μm and a translation stage was used to adjust the longitudinal position of the Cr⁴⁺:YAG saturable absorber for minimizing the beam volume inside the crystal and achieving the lowest Q-switching threshold. On the other hand, the low saturation energy density of the AlGaInAs QWs could allow a simple external cavity,

as shown in Fig. 2.7 (b), where the beam spot diameter was approximately 300 μm.

And the peak optical intensity allowed on the AlGaInAs QWs is estimated to be 300 MW/cm² without damage. The SA was tilted slightly to avoid facet reflection back to the gain fiber, which usually incurs parasitic fluctuation in pulse stability in high gain fiber lasers.

The pump source was a 35-W 976-nm fiber-coupled laser diode with a core diameter of 400 μm and a NA of 0.22. Focusing lens with 25 mm focal length and 92%

coupling efficiency was used to re-image the pump beam into the fiber through a dichroic mirror with high transmission (>90%) at 976 nm and high reflectivity (>99.8%) within 1030~1100 nm. The pump spot radius was approximately 200 μm. With launching into an undoped fiber, the pump coupling efficiency was measured to be approximately 80%.

Fig 2. 7 Schematic of diode-pumped PQS Yb-doped fiber lasers. (a) with Cr⁴⁺:YA crystal (b) with AlGaInAs QWs. HR: high reflection; HT: high transmission.

2.2.4 Results and discussions

Figure 2.8 shows the average output powers with respect to the launched pump power in cw and PQS operations. The cw operation was performed with an external cavity only comprising a re-imagining lens and a reflective mirror. In the cw regime, the laser had a slope efficiency of 74% and the output power reached 15.8 W at a launched pump power of 24 W. In the PQS regime, the maximum average output powers at a launched pump power of 24 W were up to 14.4 W and 13.8 W with the AlGaInAs QWs and with the Cr⁴⁺:YAG crystal, respectively. The Q-switching efficiencies were 91% and 87% for the lasers with with the AlGaInAs QWs and with the Cr⁴⁺:YAG crystal, respectively.

Fig 2. 8 Dependence of the average output power on the launched pump power for the cw and passive Q-switching operations.

The pulse temporal behavior was recorded by a Leroy digital oscilloscope (Wavepro 7100; 10G samples/sec; 4 GHz bandwidth) with a fast InGaAs photodiode.

Figure 2.9 shows the pulse characteristics including the pulse repetition rate and the pulse energy. Figure 2.9 (a) shows the pulse repetition rate versus the launched pump power. The repetition rates of both lasers increased monotonically with the pump power.

At a launched pump power of 24 W, the repetition rates were 38 kHz and 30 kHz for using the Cr⁴⁺:YAG crystal and the AlGaInAs QWs, respectively. Figure 2.9 (b) shows the pulse energy versus the launched pump power. The pulse energy with the Cr⁴⁺:YAG crystal was almost constant at 0.3 mJ for the pump power less than 20 W and slightly increased up to 0.35 mJ at a pump power of 24 W. On the other hand, the pulse energy with the AlGaInAs QWs increases gradually, from 0.25 mJ at the threshold to 0.45 mJ at a pump power of 24 W.

Another interesting characteristic of saturable absorbers is the wavelength- dependent absorption. In this investigation the thin film filter was tilted for controlling the lasing wavelength from 1055 nm to 1083 nm. Figure 2.10 shows the pulse energy versus the lasing wavelength at a pump power of 24 W. Since the absorption bandwidth of the AlGaInAs QWs was rather narrower, the variation of the pulse energy with the AlGaInAs QWs was more significant than that with the Cr⁴⁺:YAG crystal. Therefore, the Cr⁴⁺:YAG crystal is more suitable than the AlGaInAs QWs for using in tunable operation.

The temporal shapes of the Q-switched pulses for the maximum pulse energy were depicted in Fig. 2.11 The top of Fig. 2.11 shows the single Q-switched envelops. The pulse durations were 70 ns and 60 ns for using the Cr⁴⁺:YAG crystal and the AlGaInAs QWs, respectively. The bottom of Fig. 2.11 show the typical oscilloscope traces of Q-switched pulse train with the optimum alignment. The pulse-to-pulse stability was found to be noticeably better with the AlGaInAs QWs than with the Cr⁴⁺:YAG crystal under 30 °C because of the proper cooling ability by the copper sink. Without any cooling mechanism, the pulse-to-pulse stability and the laser output energy will be reduced.

Fig 2. 9 (a) Pulse repetition rate and (b) pulse energy versus the launched pump power.

Fig 2. 10 Pulse energy versus the resonant wavelength.

Fig 2. 11 Top: Oscilloscope traces of a typical Q-switched envelope; Bottom:

Oscilloscope traces of a train of Q-switched pulses.

2.2.5 Conclusion

In conclusion, we have demonstrated comparative studies for the Cr⁴⁺:YAG crystal and the AlGaInAs QWs used as a SA in efficient high-pulse-energy PQS Yb-doped fiber lasers. The two SAs were designed to exhibit nearly identical small-signal transmission of ~28%. Under a pump power of 24 W, the average output powers were up to 14.4 W and 13.8 W obtained with the AlGaInAs QWs and with the Cr⁴⁺:YAG crystal, respectively. The maximum pulse energies obtained with the AlGaInAs QWs and with the Cr⁴⁺:YAG crystal were 0.45 mJ and 0.35 mJ, respectively.

The pulse-to-pulse stability was found to be noticeably better with the AlGaInAs QWs than with the Cr⁴⁺:YAG crystal. Nevertheless, the Cr⁴⁺:YAG crystal has a broader absorption band that is beneficial to the tunable operation. It is believed that the efficient Q-switched fiber lasers should be useful light sources for technical applications because of its high average power as well as high pulse energy.

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Chapter 3

Passively Q-switched photonic

crystal fiber lasers

3.1 Passively Q-switched photonic crystal fiber laser with AlGaInAs quantum wells

3.1.1 Introduction

High-power diode-pumped double-clad rare-earth doped fiber lasers have been proved to be efficient and compact with excellent beam quality, high efficiency, and good thermal management [1-3]. Q-switched fiber lasers are practically useful in a variety of applications in virtue of their high pulse energy, such as remote sensing, industrial processing, and medical needs [4-6]. Compared with active Q-switching techniques, passive Q-switching methods that employ saturable absorbers can considerably enhance the compactness and simplify the operation [7-10]. By enlarging the active volume of the gain medium, corresponding to the doped core size of the fiber, one can achieve the merit of the high pulse energy. However, the conventional large-core fibers suffer from mode-quality degradation and their long lengths usually lead to long pulse widths and low peak powers. For improving these deficiencies, photonic crystal fibers (PCFs) have been developed to provide large single-mode cores and high absorption efficiencies. The PCF was recently employed to demonstrate a passively Q-switched laser with a Cr⁴⁺:YAG crystal as a saturable absorber in which under a pump power of 14.2 W, an average output power of 3.4 W with a repetition rate of 5.6 kHz was generated, corresponding to a pulse energy of 630 μJ [11]. However, the scale-up of the pulse energy is hindered by the nonsatuarble loss of the Cr⁴⁺:YAG crystal [9].

In recent years, an AlGaInAs semiconductor material with a periodic quantum-well (QW) structure grown on a Fe-doped InP structure has been successfully used as a saturable absorber in an Yb-doped fiber laser to produce pulse energy up to 450 μJ [9]. It was found that the saturation fluence of the AlGaInAs QW absorber was two orders of magnitude smaller than that of Cr⁴⁺:YAG crystal. This property enables the AlGaInAs QW devices to be appropriate saturable absorbers for high-gain lasers.

More importantly, experimental results also revealed that the AlGaInAs QW absorber has a lower nonsatuarble loss than the Cr⁴⁺:YAG crystal with the same initial transmission. This result indicates that AlGaInAs QW absorbers have a potential to generate much higher pulse energies. So far, AlGaInAs QWs have not been employed to passively Q-switch Yb-doped PCF lasers.

We demonstrate a millijoule-level passive Q-switched Yb-doped photonic crystal

fiber laser with AlGaInAs QWs as a saturable absorber. We fabricate three types of AlGaInAs devices with different QW numbers to investigate the performance of passively Q-switched PCF lasers. With 50 groups of three AlGaInAs QWs as a saturable absorber and under a pump power of 16 W, the PCF laser generates an average power of 7.1 W at the pulse repetition rate of 6.5 kHz, corresponding to a pulse energy of approximately 1.1mJ. The overall pulse-to-pulse amplitude fluctuation and the temporal jitter are found to be well below 10% in root mean square (rms). I also calculated the peak power by integrating the photodiode traces and found its maximum value to reach 110 kW.

3.1.2 AlGaInAs QWs absorber and experimental setup

Similar to the previous structure [9] the saturable absorbers that offered by TrueLight Corporation were AlGaInAs QW/barrier structures grown on a Fe-doped InP substrate by metalorganic chemical-vapor deposition. The saturable absorbers were designed to consist of many groups of several QWs, spaced at half-wavelength intervals by InAlAs barrier layers with the band-gap wavelength around 806 nm and with the luminescence wavelength near 1064 nm. The thickness of the saturable absorbers was approximately 400 μm. Compared with other similar QWs devices, AlGaInAs material has the advantages of lattice match with the substrate InP over InGaAs/GaAs that output pulse energy of the passive Q-switch and the conversion efficiency are limited as a result of the lattice mismatch. AlGaInAs materials is also superior to InGaAsP material which can be grown on InP substrate because of its better electron confinement covering the wavelength range in 0.84-1.65μm provided by the larger conduction band offset [12,13]. In this work we fabricated three types of AlGaInAs QWs that posses 50 groups of three QWs (3 × 50 QWs), 30 groups of three QWs (3 × 30 QWs), and 30 groups of two QWs (2 × 30 QWs). Figures 3.1(a)–(c) depict the schematic diagrams of three periodic AlGaInAs QWs structures. Figure 3.2 shows the measured results for the low-intensity transmittance spectrum of the three QW saturable absorbers. The initial transmissions of the absorbers near the wavelength of 1030 nm can be seen to be 18%, 36%, and 48% for the devices of 3 × 50 QWs, 3 × 30 QWs, and 2 × 30 QWs, respectively. With the z-scan method [9], I found that the modulation depths between low and high intensities were approximately 77%, 59%, and 47% for the absorbers of 3

× 50 QWs, 3 × 30 QWs, and 2 × 30 QWs, respectively. We also found that the

nonsaturable losses for three devices were less than 5%. The low nonsaturable losses indicate the quality of the QW devices to be rather high. Furthermore, the saturation fluence of the QW absorbers was measured to be in the range of 1 mJ/cm2 and the relaxation time to be on the order of 100 ns [14]. The damage threshold for the AlGaInAs QWs was found to be approximately 300 MW/cm2. Both sides of the semiconductor absorber have a simple single layer coating to reduce back reflections and the couple-cavity effects. The scheme of the experimental setup is shown in Fig.

3.3 (a). The cavity is composed of a 0.55 m polarization maintaining Yb-doped PCF (NKT photonics) that is the same one described in Ref. 11 and an external feedback cavity with a saturable absorber. Figure 3.3 (b) depicts the image of the cross section of the PCF pumped by a 532 nm light source. Since the absorption coefficient of the PCF was approximately 30 dB/m at 976 nm, the overall absorption efficiency could reach 95%. The rod-type PCF has a mode field diameter of 55 μm and a low numerical aperture (NA) of 0.02 to sustain the excellent beam quality. The pump cladding of the PCF has a diameter of 200 μm and an air-cladding to maintain a high NA of 0.6. The PCF was surrounded with a 1.7-mm thick outer cladding and was sealed with end-caps for protection. The boron doped stress-applying parts near the core were adopted to induce birefringence that produces diverse spectral losses to form a linearly polarization state for the fundamental mode.

The external cavity incorporates with a focusing lens of 50-mm focal length to focus the fiber output into the AlGaInAs QW absorber and a high reflective mirror behind the absorber for feedback. The AlGaInAs QW absorber was mounted in a copper block as a heat sink and with water cooling. The mode diameter on the saturable absorber was approximately 200 μm. The pump source was a 20-W 976-nm fiber-

The external cavity incorporates with a focusing lens of 50-mm focal length to focus the fiber output into the AlGaInAs QW absorber and a high reflective mirror behind the absorber for feedback. The AlGaInAs QW absorber was mounted in a copper block as a heat sink and with water cooling. The mode diameter on the saturable absorber was approximately 200 μm. The pump source was a 20-W 976-nm fiber-