The nonlinear absorption properties of GaAs were measured at 1.06 and 1.34 µm by the open aperture Z-scan technique in this paper. Based on a neodymium doped calcium niobium gallium garnet (Nd:CNGG) disordered crystal grown by the Czochralski method, passively Q-switched lasers with the conventional GaAs wafer as the saturable absorber were demonstrated, operating on the transitions of 4F3/2 → 2I11/2 and 4F3/2 → 2I13/2. For the 4F3/2 → 2I11/2 transition, the laser operated at 1063 nm with a pulse duration of 546 ns and a repetition rate of 98.5 kHz. While for the 4F3/2 → 2I13/2 transition operating at 1340 nm, the minimum pulse width was 499.6 ns with the pulse repetition rate of 110.7 kHz.
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The neodymium-doped calcium niobium gallium garnet Ca3(NbGa)2−xGa3O12 (Nd:CNGG) crystal is a typical disordered laser material, suitable for the semiconductor laser diode pumping [1,2]. The lattice disorder comes from the random distribution of the Nb and Ga ions and vacancies at octahedral and tetrahedral lattice sites . Compared with the typical ordered crystal Nd:YAG, Nd:CNGG has a lower melting point of 1460°C ; therefore, it can be easily grown by the Czochralski method. Meanwhile, Nd:CNGG has a relatively broader absorption band around the diode-pumped wavelength of 808 nm, reducing the restraint on the temperature control of the diode in a Nd:CNGG laser, and a large emission bandwidth originating from the disordered nature of the crystal, which makes this crystal useful for the generation of ultrashort pulses [5,6]. So far, based on Nd:CNGG disordered crystal, lasers at 0.94, 1.06 and 1.32 µm have been demonstrated [1–3,7–10]. The previous works indicated that Nd:CNGG disordered crystal is an excellent candidate for the lasing.
GaAs is a conventional semiconductor which has the EL2 defect, so that it is widespread used as the saturable absorber in 1 µm lasers [11–13]. Recently, we found that GaAs also has the saturable absorption properties at 2 µm so that GaAs could be an excellent candidate for the pulsed 2 µm laser [14–16]. We attributed the saturable absorption at 2 µm to other defects in GaAs and the two-photon absorption (TPA). However, there are still essential questions lingering in our minds: Does GaAs exhibit nonlinear optical properties at other wavelengths, especially the nonlinear absorption properties? And could GaAs be implemented as the saturable absorber at other wavelengths?
These questions stimulated us to explore the nonlinear optical properties of GaAs at 1.06 and 1.34 µm by the open-aperture Z-scan technique. The experiment gave the positive results. As a consequence, GaAs was successfully to Q-switch the Nd:CNGG lasers at 1.06 and 1.34 µm in this paper. The shortest pulse width of 546 ns with the repetition rate of 98.5 kHz was achieved at 1063nm, and the shortest pulse width of 499.6 ns with the repetition rate of 110.7 kHz at 1340nm.
2. Nonlinear optical properties of GaAs
A thin GaAs wafer with the dimensions of 30 × 25 × 0.65 mm3 was utilized in the experiment. We measured the linear absorption properties of the GaAs wafer in the range of 800 nm to 1500 nm. Figure 1 depicts the linear absorption versus the wavelength. The strong sharp absorption change was attributed to the bandgap of GaAs (~1.42 eV, 870 nm). GaAs wafer exhibits the linear absorption properties in a wide spectral band range from 1 to 1.5 µm. We attributed the absorption properties to the defects in GaAs semiconductor , such as double Ga vacancies, the ternary defects and other quaternary defects. The initial transmissions at 1064 and 1342 nm were about 48.5% and 52.8%, respectively. To make sure whether GaAs semiconductor wafer has the saturable absorption properties or not, we carried out open aperture Z-scan experiment. Hereby, a laser operating at 1064 nm with a pulse duration of 190 fs and a pulse repetition rate of 1 kHz, and a laser operating at 1342 nm with a pulse duration of 30 ns with a pulse repetition rate of 10 kHz were employed as the pumping sources. The GaAs wafer was put on a stage after the lens so that we can easily change the incident laser fluence on GaAs wafer. The modulation depth, the nonlinear saturation loss, and the saturation intensity can be determined by Z-scan techniques. Figure 2 shows the transmission curves versus the incident pulse fluence at 1064 and 1342nm, respectively. The modulation depth was 5.01% at 1064 nm and that was 5.85% at 1342 nm. The nonlinear saturation loss was 46.49% at 1064 and 41.35% at 1342 nm. Meanwhile, it was found that the saturation started at pulse fluence of 17.8 mJ/cm2 and 3.1 mJ/cm2 at 1064 and 1342 nm, respectively. The Z-scan data indicated that GaAs wafer could also be a saturable absorber at both 1.06 and 1.34 µm spectral bands. Although the whole physical mechanism is still not fully understood, the defects in GaAs should play an important role in the nonlinear absorption properties at other wavelengths .
3. Experimental details
The disordered 0.5at.% Nd:CNGG crystal employed in the laser operation was grown by the Czochralski method. We first investigated the absorption and emission properties of the laser gain medium. As shown in Fig. 3, the peak absorption cross section for the InGaAs LD pumping is about 4.1 × 10−20 cm2. We also measured the emission fluorescence intensity excited by 808 nm LD, the transition of 4F3/2 → 4I11/2 had a spectral bandwidth of 15 nm while for the transition of 4F3/2 → 4I13/2 the full width at the half maximum (FWHM) was 27 nm, which shows great potential in the ultrafast laser pulses generation.
The Nd:CNGG laser cavity was a concave-plano resonator, shown in Fig. 4. The input coupler, M1, was a concave mirror having a radius of curvature of 200 mm. It was coated for high reflectance (HR) at 1063 and 1340nm. The output coupler (OC), M2, was a plano mirror. The full cavity length was 20 mm. The polished Nd:CNGG crystal was wrapped by a thin layer of Indium foil and mounted on a heat-sink maintained at 17 °C. The dimension of the <111>-cut Nd:CNGG crystal was 3 × 3 × 7 mm3. It positioned near the plane reflector, while the GaAs saturable absorber was placed close to the output coupler inside the resonator. Employed as the pump source was a commercial fiber-coupled 808-nm LD (FAP system, Coherent Inc., USA), whose fiber core diameter and NA were 400 μm and 0.22, respectively. The pump radiation was re-imaged first by a focusing optics and then coupled into the Nd:CNGG crystal through the input coupler (M1). The pulse temporal behavior was detected and recorded by a DPO 7104C digital phosphor oscilloscope (1GHz bandwidth and 20G sampling rate, Tektronix Inc., USA). A MAX 500AD (Coherent., USA) laser power meter was used to measure the average output power.
4. Experimental results and analysis
4.1 4F3/2 → 2I11/2 at 1063 nm
In this case, the OC M2 had a partial transmission of T = 4% at 1.06 µm. We first setup the continuous wave operation without GaAs inside the resonator. The maximum output power was 205.3 mW under an absorbed pump power of 1.77 W. By inserting the GaAs saturable absorber into the resonator, the pulse laser appeared with a threshold of 0.95 W, and under an absorbed pump power of 1.77 W, a maximum average output power of 49.2 mW was obtained. To avoid the pulses instability, the highest pump power was set as 1.77 W. The variations of the repetition rate and pulse width versus the absorbed pump power are shown in Fig. 5(b). The pulse width decreased monotonically from 1350 to 546 ns when the absorbed pump power increased from 0.95 to 1.77 W, while the repetition rate revealed an opposite trend from 32.07 to 98.5 kHz, which is typical in the passively Q-switched laser. The shortest pulse width was 546 ns and the maximum repetition rate was 98.5 kHz under an absorbed pump power of 1.77 W. Once the average power, the pulse duration and the pulse repetition rate were obtained, the pulse energy and the peak power could be calculated. Figure 5(c) shows the variations of the pulse energy and the peak power versus the absorbed pump power. The maximum pulse energy was 0.5 μJ and the maximum peak power was 0.92 W. The typical pulse profile and pulse train with a pulse width of 546 ns is shown in the Fig. 5(d). We noticed that the pulse duration was slightly larger when compared with other GaAs Q-switched lasers. Normally, the small round-trip, the large modulation depth, the large non-saturated loss and the large gain would result in the short pulse duration. In our case, the gain of Nd:CNGG laser crystal was low, as well as the pump level, leading to the large pulse duration.
4.2 4F3/2 → 2I13/2 at 1340 nm
In this section, the OC M2 was a plane mirror with PR coating at 1340 nm and HT coatings at 1.06 and 1.1 µm, so that the oscillations at 1.06 and 1.1 µm could be suppressed. And owing to the narrow coating spectral band of the OC and the low pump level, the lasing at 1329 nm was not observed in the experiment. The transmission of the OC was 3.8% at 1340 nm. Figure 6(a) shows the relationships between the absorbed pump power and the output power. A maximum output power of 134.9 mW was obtained in the CW operation. To explore the passive Q-switching pulse generation of the Nd:CNGG crystal at 1340 nm, we further studied the performance of the pulse laser by inserting a thin GaAs wafer as a saturable absorber in the cavity. By aligning the cavity mirrors carefully, the passive Q-switching operation was achieved and the characteristics of the pulse laser are assembled in Fig. 6. In order to obtain the pulses stable, we did not increase further the pump power. The maximum average output power was 27 mW under an absorbed pump power of 2.86 W as shown in Fig. 6(a).
The pulse repetition rate and pulse width versus absorbed pump power are shown in Fig. 6 (b). For the GaAs Q-switched laser, when the absorbed pump power increases from 2.07 to 2.86 W, the pulse width decreases from 1322 to 499.6 ns and the pulse repetition rate increases from 37.42 to 110.7 kHz. At the absorbed pump power of 2.86 W, the maximum pulse energy and pulse peak power are estimated to be 0.24 µJ and 0.48W, respectively. The pulse energy and peak power versus the absorbed pump power are shown in Fig. 6(c). The temporal pulse profile with the pulse width of 499.6 ns at the absorbed pump power of 2.86W is shown in Fig. 6(d). The inset of Fig. 6(d) gives the pulse train of Q-switched laser at a pulse repetition rate of 110.7 kHz, demonstrating excellent amplitude stability. For the further improve the performances of the Q-switching pulses, one possible approach is to use the MOPA system.
4.3 Optical spectra for laser transitions
To make sure the laser operating wavelength, for both cases, a spectrometer with a resolution of ~1 nm (Stellernet Inc. USA) was implemented to record the wavelength. Shown in Fig. 7, for the laser operating at the transition of 4F3/2 → 2I11/2, the operating wavelength is 1063 nm, while for the transition of 4F3/2 → 2I13/2, the working wavelength is 1340 nm.
In conclusion, we studied the nonlinear absorption properties of the GaAs semiconductor at 1 and 1.34 µm. The modulation depths were 5.01% and 5.85%, respectively. Subsequently, we experimentally investigated the CW and passively Q-switched performances of Nd:CNGG disordered crystal lasers at 1063 and 1340 nm. Stable pulses were achieved at both 1063 and 1340 nm using GaAs as the saturable absorber. The shortest pulse width was 546 ns with the repetition rate of 98.5 kHz at 1063 nm, while the minimum pulse duration was 499.6 ns with the repetition rate of 110.7 kHz at 1340 nm. This work convinced us that the GaAs could be an excellent saturable absorber in a wide spectral band.
National Natural Science Foundation of China (NSFC) (61575109, and 21872084); The Fundamental Research Funds of Shandong University (2018TB044); Foundation of President of China Academy of Engineering Physics (YZJJLX2018005).
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