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Abstract
In this paper, we present a novel mechanism for the generation of laser pulses based on the phenomenon of thermocavitation. Thermocavitation bubbles were generated within a glass cuvette filled with copper nitrate dissolved in water, where the tip of an optical fiber was placed very close to the bubble generation region. Once the bubble is generated, it expands rapidly and the incoming laser light transmitted through the optical fiber is reflected at the vapor-solution interface and reflected back into the fiber, which is coupled to an erbium-doped fiber ring laser. Laser pulses were extracted from the ring cavity and detected by a fast photodetector, which corresponds to a single thermocavitation event, obtaining a pulse repetition rate from 118 Hz to 2 kHz at 1560 nm, with a pulse width ranging from 64 to 57 µs. The repetition rate can be controlled by adjusting the laser power to induce thermocavitation. To our knowledge, this novel mechanism of laser pulses has not been reported in the literature.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
An optical fiber laser is an optical system in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium, or holmium [1]. The fiber lasers possess several advantages over others laser such as light-weighted, flexible, compact and mechanical stability. They operate in continuous wave (CW) or pulsed mode. Currently, the research on the development of pulsed fiber lasers is of great interest due to their applications in fields such as medicine [2], optical time-domain reflectometry [3], optical instrumentations [4], remote sensing [5], materials processing [6], optical communications, among others.
Pulsed fiber lasers are either passive/active Q-switched or mode-locked [7,8]. The active method is based on the use of a modulator driven by an external electrical generator such as electro-optic modulators (EOM) [9] and acousto-optic modulators (AOM) [10]. The EOM is based on the Pockels effect, i.e. on the modulation of the refractive index with an externally applied field; while that the AOM modulates the refractive index through acoustic waves as it propagates through the medium. In the pulsed fiber laser regime, gain-switching is an alternative technique to generate a high-repetition-rate pulse train with nanosecond pulse duration beside Q-switching and external modulation of a CW laser [11]. Additionally, Master Oscillator Power Amplifier (MOPA) sources can be successfully implemented in the development of pulsed fiber lasers, where a high output power is required [12]. A typical MOPA source consist of several (from two to four) cascade fiber amplifiers connected, where each subsequent stage delivers increasing output powers [12].
Passive method is performed by using a saturable absorber element placed inside the cavity including nanoparticles of gold [13], silver [14], zinc [15], lead sulfide [16], graphene [17], tellurium oxide [18], among others. Thin layers of carbon nanotubes, in particular single-wall nanotubes have been used for mode-locking lasers [19]. The nanoparticles and nanotubes mentioned above have a similar physical principle, which was described by Gires and Combaud in 1965 [20], who reported for the first time the saturation phenomenon as a process that involving two energy levels. Particularly, for passive mode-locking, the laser resonator may contain a saturable absorber such as semiconductor saturable absorber mirror (SESAM) [21], it is also suitable for passive Q-switching at lower pulse energies. Other semiconductor saturable absorbers for mode-locking or Q-switching are based on quantum dots, for example, a Q-switched fiber laser was reported using quantum dots cadmium sulfide as a saturable absorber in an erbium-doped fiber laser [22]. Finally, there are also some types of artificial saturable absorbers, which are devices that exhibit decreasing optical losses for higher intensities. Such devices can be a Non-Linear Polarization Rotation or Non-linear Amplifying Loop Mirror (NALM) [23], often used for passive mode-locking.
Independently of the mechanism of pulse generation, what it is being controlled are the losses within the cavity. Here we propose a novel technique to control the losses using a time-varying reflectivity from a thermocavitation bubble. It was reported that thermocavitation is a self-organizing event with a repetition rate dependent on the intensity at the focal point. It was recently reported the generation of light pulses as the transmission through a cell containing an organic dye was controlled by the generation of bubbles. The duration of such pulses is basically controlled by the bubble lifetime [24]. Here we go further by using the bubbles as dynamics mirrors with variable reflectivity to produce microsecond-long laser pulses.
2. Experimental setup
2.1 Thermocavitation experiment
Thermocavitation bubbles were created by focusing a CW near infrared laser (λ=980 nm) on the surface of a glass cuvette using a microscope objective (f=8mm) in an inverted microscope configuration, as shown in Fig. 1(a). The cuvette was filled with a saturated solution of copper nitrate dissolved in water (13.78 g of Cu(NO3)2 per 10 mL of water at room temperature), which is highly absorbent at the laser wavelength (α=135 cm−1). The absorbed light heats the solution up to its critical limit i.e. the temperature at which an explosive liquid-gas phase transition occurs. In the case of water, the critical limit is TCR ∼270–300 °C [25,26]. Around TCR, the superheated water is explosively converted to vapor producing a fast-expanding bubble, which eventually collapses emitting an acoustic wave. Due to the strong absorption of the laser light by the solution, the bubble is always attached to the glass surface taking a semi-spherical shape [26]. In a previous work, we showed that beam intensity i.e. the laser power or the beam focus position within the glass cuvette determine the bubble size and consequently, the frequency and amplitude of the acoustic wave [26]. In this work, the focus position was fixed at 30 µm above the glass-solution interface and the only way to control the frequency and amplitude of the acoustic wave was by changing the laser power. A full experimental characterization of thermocavitation can be found in Ref. [26].
Fig. 1. a) Experimental setup used for the generation of thermocavitation bubbles within a glass cuvette and b) Schematic representation of the light-vapor bubble interaction.
The tip of a single-mode optical fiber (125/8 µm) was submerged into the working solution, very close to the region where thermocavitation bubble is generated (1 mm above the glass-solution interface). Once the thermocavitation bubble is created, it expands rapidly and the incoming laser light transmitted through the optical fiber is reflected at the vapor-solution interface (surface S2) and transmitted back to the fiber, as shown in Fig. 1(b). The maximum bubble radius for this configuration was about 0.5mm [26], so the vapor-water interface never touches the fiber end. Finally, the reflected light from the optical fiber was coupled to a simple erbium-doped fiber ring laser (as explained below). It is important to mention that the optical fiber was placed inside a needle to avoid movements due to the emission of pressure waves at the collapse of the bubbles.
2.2 Generation of laser pulses
The experimental setup to generate laser pulses is shown in Fig. 2. The fiber laser was constructed using a simple ring cavity, in which 12 m of erbium-doped fiber (EDF) was used as the gain medium. The EDF has a cladding/core diameter of 125/6 µm, numerical aperture between 0.21–0.24 and core absorption at 980 nm from 11 to 13 dB/m (M12-980-125, Thorlabs). It was pumped by a 980 nm laser diode via a wavelength-division multiplexer (WDM), obtaining emission at 1550 nm, which passes through an isolator (ISO) to provide unidirectional operation and avoid backward reflection. The ISO is connected to a polarization controller (PC) in order to optimize the ring cavity. Later, the PC is connected to a 3-port fiber optic circulator (FOC), which constitute the main part of the ring cavity. The incoming laser light is transmitted from Port 1 to Port 2; the end tip of Port 2 was cleaved and spliced with the optical fiber shown in Fig. 1 and submerged within the working solution. Once the thermocavitation bubble is created, a fraction of laser light reflected at the vapor-solution interface is transmitted back to Port 2 and pass through Port 3 with minimal loss. Finally, the laser pulse is extracted from the ring cavity by a 90:10 optical coupled (OC), which fed 90% of the oscillated light power back into the EDF. The laser pulse is collected by a fast photodetector (PD) and measured as time-varying voltage at the oscilloscope.
Fig. 2. Scheme of the thermocavitation-based EDF laser. The pulsing mechanism is controlled by the time-varying reflectivity from the vapor-solution interface.
3. Results
In the ring cavity, the pump laser diode was fixed to a power of 235 mW and the temporal characteristics of the output pulses were measured as a function of the power of the CW laser that induces thermocavitation, which was varied from the threshold in this experimental setup (75 mW) up to the maximum power of the device (400 mW). In a previous work and under similar experimental conditions a bubble of maximum size of ∼470 µm radius and ∼150 µs of lifetime was reported [26]. Considering the above, the vapor bubble generated in our experiment never touch the tip of the optical fiber.
Figure 3 shows the oscilloscope traces of the laser output for four different laser power (75, 175, 282 and 400 mW). In this figure it is possible to observe that the amplitude of the laser pulses extracted from the ring cavity remains constant, but the generation frequency increases as the laser power increases. The last one is due to the fact that in thermocavitation, the volume of superheated water is larger for lower power due to the heat diffusion. The heated volume determines the water volume available for vaporization and consequently, larger bubbles are produced but at low frequency [26]. On the contrary, larger power means smaller superheated volume, and thus smaller bubbles but at higher generation frequencies. This behavior of the light pulses agrees well with the dynamics of the thermocavitation bubble reported in [26].
Fig. 3. Oscilloscope traces of the laser output for four laser powers: a) 75, b) 175, c) 282 and d) 400 mW. In the four graphs, the intensity of the pulses is normalized to the same value.
Figure 4(a) shows the repetition rate and temporal width of the pulses extracted from the EDF ring laser as a function of the CW laser power. In this figure, it is possible to observe that the repetition rate of the pulses increases with the power from 118 Hz up to 2 kHz and, the pulse width decreases moderately from 64 to 57 µs. It is important to mention that the maximum repetition rate is limited by the power of our laser, in principle it could reach several tens of kHz. Figure 4(b) represents the laser output spectrum for four laser pulses shown in Fig. 4(a), obtained with an optical spectrum analyzer (MS9740A, Anritsu). In this figure, we can see that the spectrum width decreases (7.54, 5.03, 3.27 and 3.02 nm) as the laser power increases (75, 175, 282 and 400 mW), obtaining a central peak at 1560 nm. The spectrum width was measured as full width at half maximum (FWHM).
Fig. 4. a) Repetition rate (blue circles) and pulse width (black squares) as a function of the laser power to induce thermocavitation and b) output spectrum of the pulses extracted from the EDF ring laser.
In order to observe the variation of the temporal characteristics of the output pulses when the pump power is varied in the ring cavity, the laser power of bubble generation was fixed to 272 mW. In this case, the pulse width moderately decreases with the pump laser, but the repetition rate remained almost constant (∼1.2 kHz) as expected, since the light absorption at 1550 nm by the solution is negligible and also its contribution to the thermocavitation process, i.e., the repetition rate is independent to the pump laser, as shown in Fig. 5(a). Figure 5(b) represents the laser output spectrum for four laser pulses, obtained for a pump laser power of 106, 131, 199 and 235 mW. In this figure, it is possible to observe that the spectrum width remains almost constant around 3.4 nm (FWHM), obtaining a central peak again at 1560 nm.
Fig. 5. a) Repetition rate (blue circles) and pulse width (black squares) versus the pump power and b) output spectrum of the pulses extracted from the EDF ring laser
4. Discussion
The physical principle of commutation (active Q-switching) in the proposed fiber optic laser is based on the change of the light reflecting back to the ring laser cavity. The reflected light can be caused by two different ways: i) the growth and collapse of the vapor bubble and ii) the acoustic pressure wave emitted immediately after the bubblés collapse. The optical fiber immersed in the solution can behave like a fiber optic hydrophone, where its physical principle is based on the change of light reflectivity at the end-face of the fiber, when an incident acoustic wave impinges on the fiber tip [27–29]. The acoustic wave changes the density of the solution, which in turn modulate the refractive index. A positive pressure change corresponds to a compression of the liquid, causing an increase in its refractive index, while that a negative pressure change is related to the rarefaction of the liquid, causing a decrease in its refractive index. Therefore, this change of refractive index modulates the intensity of reflected light at the glass-solution interface (surface S1) and it is transmitted back to the fiber. The change of the refractive index of the glass fiber can be neglected because of its low compressibility. With these assumptions, the coefficient of reflection R can be calculated from the refractive indices of the solution ${n_s}$ and the fiber core ${n_f}$ using the Fresnel equations [27]. A good approximation can be achieved if normal light incidence to the fiber end-face is considered:
When the acoustic wave passes the fiber tip (glass-solution interface), ${n_s}$ is modulated in time,
where $n_s^0\; $ is the refractive index of the static solution in the absence of any acoustic wave, $\delta $ is the variation of the refractive index due to the acoustic wave [27] and P is the pressure. In our experiment, the refractive index of the fiber core is ${n_f} = 1.4682\; $ (Corning SMF-28) and the refractive index of the working solution is $n_s^0 = 1.4168$, which was measured using an Abbe Refractometer (NAR-1T-Solid, ATAGO). According to [27], the variations in the refractive index due to the pressure variations of an acoustic wave is given by $\delta \approx \frac{{\partial {n_s}}}{{\partial P}} \approx 1.4x{10^{ - 4}}\textrm{MP}{\textrm{a}^{ - 1}}$.The amplitude of the acoustic wave emitted by the collapse of a thermocavitation bubble is ∼1 MPa [25], obtaining a variation of the refractive index of $\delta P(t )= 1.4x{10^{ - 4}}$ and consequently ${n_s} = 1.4169$, i.e., the change in the index refraction of the working solution is altered up to the fourth decimal point, therefore the reflectivity of the light is negligible. H.S. Yadav et. al., measured the refractive index of water under high dynamic pressures by a geometrical refraction method [30]. According to this study, in order to obtain a variation of the refractive index of water from 1.33 to 1.41 a pressure wave of approximately 10 kBar (1GPa) is necessary; however, these pressures are only obtained for shock waves emitted by the collapse of cavitation bubbles induced by short-pulsed lasers [31].
Now, if we calculate the reflectivity of the light at the surface S1 and S2 (see Fig. 1(b)) in the absence of an acoustic wave, we obtain a value of RS1 = 0.00031, which indicates that only 0.03% of the light is reflected back at the glass-solution interface and RS2 = 0.0297 (considering the refractive index of the vapor bubble as ${n_{vapor}} = 1.0003$), which indicates that approximately 3.0% of the light is reflected at the vapor-solution interface. Thus, the cavity is completed with the vapor-solution interface. Typically, the bubble lifetime lies between 100–300 µs depending on the bubble size (controlled with the CW laser). So, the mirror lasts an even shorter time since the light reflected towards the cavity from smaller bubbles is smaller given its curvature. Above certain bubble diameter, the vapor-solution interface is practically plane and the reflectivity constant. With this information in hand, it is possible to consider that the physical principle of switching of the laser pulses is due to the change of the optical reflection coefficient at the vapor-solution interface, when the thermocavitation bubble growth and collapse and not by the acoustic wave.
5. Conclusions
In this paper, we present a novel mechanism for the generation of laser pulses based on the phenomenon of thermocavitation. Here, vapor bubbles were induced using a CW laser at 980 nm (which is an inexpensive energy source) focused into a saturable solution of copper nitrate dissolved in water. Each laser pulse corresponds to a single thermocavitation event, obtaining a pulse repetition rate from 118 Hz to 2 kHz at 1560 nm, with a pulse width ranging from 64 to 57 µs. The repetition rate can be controlled by adjusting the laser power to induce thermocavitation bubbles and it is limited by the power of our device (400 mW); however, it could reach several tens of kHz. Although the maximum repetition rate obtaining in this paper is lower compared to the reported by Q-switched and mode-locked fiber lasers (up to MHz), the physical mechanism of pulse generation is totally different to them, which is based on the change of the optical reflection coefficient at the vapor-solution interface, when a thermocavitation bubble is created. This kind of pulsed fiber laser, to our knowledge, has not been reported in the literature.
Funding
Vicerrectoría de Investigación y Estudios de Posgrado, Benemérita Universidad Autónoma de Puebla (100526492-VIEP2019); Consejo Nacional de Ciencia y Tecnología (FOINS-2319).
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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