A negative feedback controlled hybrid mode locked Nd : YAG laser using a semiconductor quantum dot saturable absorber

The generation of ultrashort laser pulses from compact and reliable devices and their applications are of great interest in the study of ultrafast processes in many domains of physics, chemistry and biology [1, 2]. Nowadays, almost all ultrashort lasers are completely solid-state, continuously pumped systems and based on mode-locking techniques. Usually, continuous wave (CW) gas, CW diode or diode-pumped CW solid-state lasers are used for pumping ultrashort laser oscillators. However, until recently, the peak powers obtainable from the femtosecond laser oscillators were still lower than that required for many applications, in particular for those involving nonlinear wavelength conversion. In order to obtain high power, a high-repetition rate (80 MHz) and low-power femtosecond laser oscillator must be combined with one or more amplification stages pumped by the other powerful solid-state pulsed laser of low-repetition rate (10–20 Hz). Such femtosecond laser systems are complicated, inefficient and expensive. Thus it is clearly preferable to achieve system simplicity and higher power performance directly with a laser oscillator or/and high compatibility in operation between the laser oscillator and its amplifiers. For this purpose, we proposed a method of femtosecond pulse generation from the Ti : sapphire laser synchronously pumped by a pulsed (not CW) Nd : YAG laser [3]. However, the liquid dye-type saturable absorber (SA) used in this system for passive mode-locking of the pump ^Nd:^YAG laser is criticized by low photostability and complexity in the setup. In this paper, we present the results obtained in the development of the pulsed negative feedback controlled hybrid mode-locked ^Nd:^YAG laser using a solid-state PbS quantum dot (QD) saturable absorber. The experimentally obtained results demonstrated that the PbS semiconductor in the form of nanoparticles doped into glass matrices deserves to be employed as a near infrared (IR) solid-state saturable absorber, in which saturation is based on the first excitonic absorption peak (the lowest quantum confined transition). As a result, the ^Nd:^YAG laser provided highly stable microsecond pulse trains of about 5000 ps pulses and 3 mJ total train energy at a 10 Hz repetition rate. This all solid-state mode-locked ^Nd:^YAG laser was successfully used for synchronous pumping of a Kerr-lens mode-locked femtosecond Ti : sapphire laser.

Semiconductor nanoparticles belong to a quantum confined system, thus it is possible to shift progressively the semiconductor nanoparticle absorption edge to shorter wavelengths (when the quantum dots are narrowly size distributed) and the semiconductor compound used specifies the longest absorption edge wavelength. To obtain a semiconductor quantum dot saturable absorber for Q-switching and/or mode locking of the near IR laser operations, IV–VI semiconductors are of interest as they possess a relatively narrow band-gap and large exciton Bohr radii. In our research, lead sulfide (PbS) QDs were considered.

The samples were prepared and made of a ^SiO ₂–^Al ₂ ^O ₃–NaF–^Na ₂ ^O–ZnO glass system doped with PbS QDs of different sizes. The chosen conditions of a glass thermal treatment allowed us to obtain the samples with narrowly size-distributed PbS QDs of average radii of 2, 2.4, 2.8 and 3 nm. The room-temperature optical absorption spectra of these samples were measured and are presented in figure 1. It is clear that the first excitonic absorption peak shifted to a longer wavelength. This effect is due to the increase in PbS QDs in size prolonging thermal treatment.

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The obtained absorption spectrum resulted from the superposition of the spectra of single PbS nanoparticles. Thus the spectral broadening is in accordance with the size distribution of the PbS QDs: the narrower the size distribution, the narrower the spectral width. Using both our calculations of average QD size and the experimentally obtained results, the narrow width of the size distribution around the average QD's radius for the glass was about 5%.

A saturable absorber is characterized by its key parameters, such as ground state absorption cross section, residual (nonsaturable) absorption, bleaching relaxation (absorption recovery) time, saturation intensity and saturation energy density. Some of them can be evaluated from experimental date on bleaching relaxation kinetics. First, we investigated the spectral-kinetic properties of the semiconductor PbS QDs in a solid-state matrix.

Nonlinear transient absorption of the samples was investigated by the pump–probe technique using femtosecond laser pulses. This technique is based on the effect that the powerful excitation of a sample by pump pulses leads to a change in the sample absorbance for probe pulses. The intensity of the probe pulse is much less than the pump intensity and the influence of the probe pulse upon the absorption change is negligible. The absorption change depends on the delay time after the excitation pulse enters the sample [1]. We use the pump–probe setup, as shown in figure 2, based on a home-made femtosecond Ti : sapphire laser oscillator and a regenerative amplifier, both operated at a 10 Hz pulse repetition rate. The Ti : sapphire laser oscillator was synchronously pumped at 532 nm by the frequency-doubled output of the mode-locked Nd : YAG laser [3]. The pulse width and energy of the Ti : sapphire laser after the amplifier were measured to be about 150 fs and 0.5 mJ, respectively. The laser wavelength is tunable over the spectral range 760–820 nm. For the present investigation, the fundamental output of the Ti : sapphire laser was set at 790 nm and split into two beams by a semi-transparent mirror of the ratio 1:4 (figure 2). The more intense beam after passing through a PC-controlled optical delay line was utilized to pump the sample (pump pulse). The second beam of fundamental frequency was used to produce a femtosecond super-continuum (by focusing into a 1 cm ^H ₂ ^O cell), which served as probe pulses. The super-continuum beam was split into two parts nearly identical in intensity and focused on the sample by mirror optics as reference and signal pulses. The spectra of both probe pulses were recorded for every flash shot by a polychromator equipped with a silicon CCD matrix and transferred to the computer. Temporal resolution of the pump–probe setup is only limited by the pump and probe pulse duration, but not by the time constant of the detector, and estimated as 0.2 ps.

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As presented in figure 1, sample #1 (doped with PbS QDs of 2 nm average radius) showed the most promising features as a saturable absorber for laser operation around 1064 nm. It is seen that the maximum of its first excitonic absorption band is almost exactly at the emission wavelength of the 1064 nm Nd : doped solid-state laser operation.

Information about spectroscopic parameters of PbS QDs can be obtained from different studies [4, 6, 7], such as typical values of ground-state absorption cross section for the PbS QDs of 2–3 nm in radii, which are ∼10^{−17  cm 2}, and their absorption coefficients in the range of the first excitonic peak are of several ^{cm −1}, which gives a concentration of PbS QDs in glass of 10¹⁶–10^{17  cm −3}.

Transient absorption spectra of sample #1 were measured by femtosecond pump–probe absorption spectroscopy and are presented in figure 3(a). One can observe a bleaching in the first excitonic absorption peak around 1064 nm and induced absorptions in all other spectral ranges. In particular, it is seen that this PbS QD-doped glass possesses fast bleaching relaxation times suitable for usage as a saturable absorber for mode locking of solid state laser operation around 1064 nm. Generally, the PbS-doped glass exposes two-component relaxation [4]. The bleaching relaxation of sample #1 is well analyzed and expressed by the two-exponential function with decay times of 3.2 and 35 ps (figure 3(b)). It is noted that the bleaching relaxation times depend on the pump light intensity, under the high powerful excitation (high intra-cavity field intensity), the faster relaxation takes place and becomes dominant and the slower component is negligible. This was taken into account in our laser experiment. The contribution of the fast component is estimated to be more important (>60%). Furthermore, the ratio of the PbS QD's size to its exciton Bohr radius (18 nm) in sample #1 is nearly 0.1, so the very strong confinement exists and makes the bleaching relaxation time of sample #1 faster than that of the rest. As the features, sample #1 can be exploited as a saturable absorber possessing quasi-single component ultrafast bleaching relaxation and the shorter mode locked laser pulses are expected with higher intracavity field intensities in the saturable absorber.

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Experimentally, sample #1 could be used as a solid-state PbS QD saturable absorber for passive mode-locking of a flash-lamp pumped Nd : YAG laser at 1064 nm when the SA transmittance is about 70% at the laser wavelength. The passively mode locked laser operation with such SA was sometimes less stable and not all pulses were mode locked. Moreover, one of the targets of this research is to develop a picosecond Nd : YAG laser for synchronous pumping of a Kerr-lens mode-locked femtosecond laser. Thus we improved the passive mode locked Nd : YAG laser operation using hybrid mode-locking based on a negative feedback control circuit.

Figure 4 presents the scheme of a negative feedback controlled and hybrid mode-locked flash-lamp pumped Nd : YAG laser with a PbS QD saturable absorber and the femtosecond Ti: sapphire laser pumped by this Nd : YAG laser.

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The pump laser resonator consists of a Nd : YAG active medium, a solid-state PbS QD saturable absorber (SA) like sample #1, flat mirrors (M1–M3), a lens L1 (f=14 ^cm), a polarization mirror (PM), an electro-optical crystal (EOC) and an electronic feedback circuit (FC) with a fast photodiode FD (figure 4). The mirrors M1 and M2 are of high reflection at 1064 nm, and the output mirror M3 of 55% reflectivity at this wavelength. The Nd : YAG laser resonator length is 125 cm. The PbS QD saturable absorber has transmittance at 1064 nm of 75% without antireflection. It was placed near the mirror M1 and the focus of the lens L1. The stability zone of such a resonator is in the range of 11.8–13.2 cm with respect to the M1–L1 distance. The lens L1 is placed 13.1 cm from M1, so the resonator is nearly in the edge of stability and this favors one mode laser operation at a high pumping level. Moreover, the lens L1 also provides the SA with a large light flux, which makes the QD saturable absorber bleachable and ensures its quasi-single component ultrafast bleaching relaxation. The saturable absorber is placed near the mirror M1 and simultaneously near the L1 lens focus. It is possible to adjust the light flux to the SA by changing its position along the optical axis. At the optimal adjusting mode-locked laser pulse parameters were nearly unvaried during a few hundred hours of laser operation at a 10 Hz repetition rate.

Electronic FC, FD, electro-optical crystal and PM constitute the negative feedback control. The latter plays a key role in the creation of a required regime of pulse generation. The negative feedback insert loses proportional to the intracavity light field, so the active medium is confined and maintained at the specified operation level. The population inversion is consumed for a longer period, so the generation of a pulse train is prolonged up to 40–50 μ^s. Such a pulse train is considerably longer than that generated without the negative feedback (only passive mode locking) [6], and it is long enough to achieve high efficiency in the synchronous pump of a femtosecond Ti : sapphire laser. Whereas the negative feedback is governed by the periodic intracavity signal (e.g. a noise spike), it creates periodic losses with a cavity roundtrip time. For the mode locking operation, we have to adjust the correct feedback phase by finely tuning the resonator length. It is also possible to make it with an optical delay line placed before photodiode. The hybrid mode locked pulse duration in this case is expected to be about a few tens of picoseconds. In fact, the QD saturable absorber additionally shortens the laser pulses and a train of 27 ps pulses is registered. Of course, this increases the peak laser power at 1064 nm and, therefore, favors the second harmonic generation of higher conversion efficiency.

When the parameters of the saturable absorber, optical elements, resonator and negative feedback circuit of the Nd : YAG laser were well adjusted, stable microsecond pulse trains (40–50 μ^s) containing about 5000 ps pulses and 3 mJ pulse train energy were produced at 1064 nm with a 10 Hz repetition rate (figure 5). Thus an average energy per pulse was estimated to be about 0.6 μ^J, corresponding to a peak power of 22 kW and a time interval between successive picosecond mode-locked laser pulses of 8.3 ns.

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The output beam of the Nd : YAG laser is focused in a potassium titanyl phosphate (KTP) crystal by the lens L2, converted into the second harmonic at 532 nm and then transmitted to the lens L3 to synchronously pump the Ti : sapphire laser (figure 4). This femtosecond laser oscillator in turn consists of two curved mirrors (M6 and M7) with 10 cm radius, three flat mirrors (M8–M10), an active medium of Ti : sapphire crystal cut at the Brewster angle, a pair of dispersion prisms and a wedge glass plate.

All five mirrors have high reflection at 740–840 nm. The laser cavity is constituted of the classical astigmatism-free Z-configuration, but this is a strong nonsymmetric resonator avoiding two-pulse generation at intensive synchronous pumping. By the self-consistent nonlinear ABCD method [5], the stability regions and the optimal parameters of the Ti : sapphire laser for the best Kerr-lens mode locking (KLM) were calculated. The prism pair as usual creates negative group velocity dispersion in the cavity. The 1° wedge plate was used as a coupler extracting laser emission from the cavity. The information about the femtosecond Ti : sapphire laser operation and characteristics will be reported in another publication.

The ^SiO ₂–^Al ₂ ^O ₃–NaF–^Na ₂ ^O–ZnO glass doped with PbS QDs has been investigated as a solid-state saturable absorber for mode-locking of near IR laser operation. We have developed the negative feedback controlled hybrid mode-locked Nd : YAG laser at 1064 nm using the glass doped with PbS QDs of 2 nm average radius as a solid-state saturable absorber. The Nd : YAG laser produced stable microsecond pulse trains (40–50 μ^s) containing about 5000 ps pulses, 3 mJ total train energy at a 10 Hz repetition rate. This pulse train was successfully used for synchronously pumping a Ti : sapphire femtosecond laser oscillator. Such a solid-state semiconductor saturable absorber can be fabricated and used for mode locking of near IR laser operation within the 1–2 μ^m wavelength range.

The present research was supported by a joint Belarus-Vietnam grant from the National Academy of Sciences of the Belarus and Vietnam Academy of Science and Technology (Project F10V- 005).