Electrically controlled fibers

Passively Q-switched and mode-locked Tm-Ho co-doped fiber laser using a WS2 saturable absorber fabricated by chemical vapor deposition

Experimental and theoretical study on an intracavity KTiOPO4 optical parametric oscillator pumped by a dual loss modulated Q-switched and mode-locked laser

FIrpic thin film as saturable absorber for passively Q-switched and mode-locked erbium-doped fiber laser

Summary form only given. Birefringent filters are routinely used as a wavelength tuning element in bulk optics, broadly tunable lasers such Ti:Sapphire [1] and dye lasers. A detailed description of birefringent filter operation can be found in ref. [2]. In this paper, we report the design, fabrication and characterisation of an integrated electro-optically tunable birefringent filter for wavelength and polarisation tuning of quantum cascade lasers (QCLs) [3].Our generic design for an integrated tunable birefringent filter (ITBF) for QCLs is shown in Fig. 1(a). It consists of 2 quarter waveplates with a differential phase shift (DPS) section between them. The integrated equivalents of waveplates are polarization mode convertors (PMC) [4] that converts the TM (Transverse Magnetic) polarised light from the gain section into circularly polarised light. In a QCL there is only gain for TM modes because of the intersubband transition selection rules. The DPS section adjusts the relative phase of the TM and TE (Transverse Electric) components of the circularly polarized light from the first PMC (PMC1 in figure 1(a)); this is because the birefringence of DPS section can be adjusted by injection current. The relative phase of the TE and TM components at the input into the second PMC (PMC 2 in figure 1(a)) determines the output polarization. In a QCL, only TM light will experience gain and the wavelengths that are rotated back to TM polarization are determined by the current in the DPS section. Thus we have a QCL that has its operating wavelength selected by an electro-optic tunable birefringent filter. For wavelength tuning the gain section was operated pulsed (100 ns pulse width at 60 kHz repetition rate) above threshold, at a current density of 6.8 kA/cm, and wavelength tuning was recorded (shown in Fig. 1(b)) for various values of DC injection current density into the DPS up to 24nm at 0.41kA/cm2. In our polarization control experiments at 0 kA/cm2 a maximum throughput polarisation angle of 37O was recorded and this angle increases up to 82O with an applied DC current of 0.97 kA/cm2 into the DPS and no further change in polarization angle recorded above 0.97 kA/cm2 current density (shown in Fig. 1(c)).In summary we have presented results on a QCL that includes an integrated birefringent filter. The results show that the output wavelength and polarisation can be electro-optically tuned by injecting a DC current into a differential phase shift section.

Q-switched fiber laser with a tunable wavelength output is experimentally demonstrated utilizing a nonlinear saturable absorption of vanadium pentoxide (V2O5) in conjunction with a tunable bandpass filter (TBF). The V2O5 saturable absorber (SA) was prepared by mixing V2O5 with polyethylene glycol. The SA film exhibits a modulation depth of 7% with a saturation intensity of 90 MW/cm2. By integrating the SA into an erbium-doped fiber laser cavity while controlling the passband wavelength of the TBF, a Q-switched pulsed laser is realized with a tunable wavelength range of 30 nm that extends from 1535 to 1565 nm. At 1565 nm operation, the laser exhibits the highest slope efficiency of 8.31%. The repetition rate varies from 13.32 to 15.25 kHz, while the pulse width shrinks from 21.3 10 12.75 μs when the pump power rises from 14.9 to 37.7 mW. The pulse energy is 102.6 nJ at the maximum pump power of 37.7 mW. This is the first demonstration of a tunable Q-switched fiber laser operating in a 1.5 μm wavelength region utilizing V2O5 SA.

We conducted a systematic investigation into the spectral and pulse characteristics of C and L-band Nonlinear Polarization Rotation (NPR) mode-locked fiber lasers effectively employing nonlinear polarization rotation technology. In our experimental setup, we achieved a stable mode-locked state at 1560.076 nm, exhibiting a 3 dB spectral bandwidth of 9.1 nm. As the pump power increased, we observed spectral shifts accompanied by shifts in the first Kelly sideband and the generation of new Kelly sidebands. In this paper, the phenomenon of spectral deviation is elucidated through the interplay of self-phase modulation, group velocity drift, and polarization-dependent isolator (PD-ISO) filter effect, with an analysis of the formation and deviation of Kelly sidebands. Notably, spectral shift persisted even when the pump power exceeded 200 mW. However, continuous pump power escalation led to soliton splitting, resulting in the formation of new soliton beams. Based on the simultaneous generation of spectral shift and pulse splitting, our study contributes to an enhanced understanding of soliton dynamics in ultrafast fiber lasers and lays a foundation for the application of high-repetition-frequency harmonic mode-locked lasers with tunable wavelengths.

Tunable multiwavelength mode-locked fiber laser using intra-cavity polarization and wavelength dependent loss

We review our studies on passively mode-locked fiber lasers using carbon nanotube (CNT) based saturable absorbers (SA). CNT-based SA offers several key advantages such as: ultra-fast response, robustness, tunability of wavelength, and compatibility to fibers.

The transient state of wavelength tuning in dispersion-tuned mode-locked lasers, which may introduce significant timing and amplitude jitters, is numerically and experimentally studied. The restriction on the tuning step of cavity length or modulation frequency for wavelength tuning to achieve an acceptable pulse performance is obtained.

Numerous areas, both in science and industry level, have benefited in recent years from the application of ultrafast mode-locked lasers. The introduction of a saturable absorber (SA) in the cavity is a convenient and simple way to achieve passive self-starting mode-locking in fibre lasers. The applicability of such passively mode-locked ultrafast fibre sources is mainly limited by the achievable energy and peak power, the latter usually going up to only a few kW in the femtosecond pulse range. In our current study we rely on a novel InN-based SA, recently presented and studied in [1,2], to set-up an ultrafast, high peak power ultrafast passively mode-locked fibre laser with operation wavelength at 1.56 μm in telecommunication C-band. The fibre laser is implemented as a ring resonator. It is based on a commercial EDFA as gain medium and the aforementioned SA [1], that consists of InN grown by molecular beam epitaxy on a GaN-on-sapphire template, used in reflection configuration as SESAM, which is located in free space and connected to the ring through an optical circulator. The outstanding properties of this material are here further validated, especially its tolerance to extremely high fluences (>1TW/cm2). The cavity length is increased introducing a 1-km standard single mode fibre (SSMF). Note that no polarization control is needed in the system and, as shown in [2], the basic ring configuration around 40-m-long can achieve stable mode-locking with sub-250 fs pulses and a repetition rate around 5 MHz. However, if polarization control is introduced, it is possible to tune the temporal and spectral widths of the pulses, from 150 fs to 320 fs. With a 1 km-long cavity, fundamental mode-locking is easily achievable increasing cavity losses with a VOA. Since symmetric Gaussian pulse spectra are generated, very similar to those delivered in the basic ring configuration, it is possible to assume that all the energy is contained in the pulse spectrum, without significant contribution of continuous white noise. This results in pulses with a temporal width at FWHM of 239 fs and a spectral width of 25.4 nm. Since the repetition rate is 196.4 kHz and the average power is 30.5 mW, pulses with peak power of 650.5 kW and pulse energy of 155.3 nJ are produced. If losses in the cavity are gradually reduced, mode-locking to higher harmonics is obtained in cavities up to 6-km-long, proving the excellent stability of the system. This allows a broad choice of high peak powers and repetition rates. To summarise, for the first time to our knowledge, it has been experimentally demonstrated a polarization-independent passive ultrafast harmonic mode-locked ring fibre laser operating at 1.56 μm achieved with a simple low-cost set-up and the use of an InN-based SESAM, which in combination with a 1-km-long SSMF cavity, delivers femtosecond pulses with a peak power of 650.5 kW, the highest obtained to date, to our knowledge, in this kind of lasers.

A compact passively Q-switched and mode-locked erbium-doped fiber laser based on graphene saturable absorber was reported in this paper. A fiber ferrule, which had graphene deposited on the core region, was used as the saturable absorber. The Q-switched operation was initiated with a low pump threshold of about 50 mW at 974 nm and the repetition rate can be widely tuned from 14 kHz to 70 kHz along with the increase of the pump power. Moreover, the mode-locking state working at 1559.7 nm with a 0.4nm spectral bandwidth and about 3 ns pulse duration was also demonstrated in the same ring cavity when the pump power increased to about 150 mW. This is, to the best of our knowledge, the first report of the fiber laser, which had the both Q-switched operation with so wide repetition rate and mode-locking operation in the same ring cavity based on the same graphene saturable absorber.

We report a passively Q-switched and mode-locked erbium-doped fiber laser (EDFL) based on PtSe2, a new two-dimensional material, as a saturable absorber (SA). Self-started Q-switching at 1560 nm in the EDFL was achieved at a threshold pump power of 65 mW, and at the maximum pump power of 450 mW, the maximum single Q-switched pulse energy is 143.2 nJ. Due to the polarization-dependent characteristics of the PtSe2-based SA, the laser can be switched from the Q-switched state to the mode-locked state by adjusting the polarization state. A mode-locked pulse train with a repetition rate of 23.3 MHz and a pulse width of 1.02 ps can be generated when the pump power increases to about 80 mW, and the stable mode-locked state is maintained until the pump power reaches its maximum 450 mW. The maximum single mode-locked pulse energy is 0.53 nJ. This is the first time to our knowledge that successful generation of stable Q-switched and mode-locked pulses in an Er-doped fiber laser has been obtained by using PtSe2 as a saturable absorber.

A Q-switched mode-locking femtosecond all-fiber laser based on a 2 cm long homemade Er3+/Yb3+ codoped phosphate glass fiber has been reported. By using the nonlinear polarization evolution technique, a nearly 100% modulation depth of mode-locking pulse train is achieved. At a pump power of 410 mW, the energy of each Q-switched envelope, whose width is about 220 μs, is 10 μJ, while the duration of mode-locking pulse within the Q-switched envelope is 318-fs.

The development of efficient in-fiber amplitude modulators that operate in the MHz frequency range has enabled some recent advances in actively mode-locked all-fiber lasers. Our approach is based on the excitation of a standing acoustic wave along the fiber, which modulates the coupling between modes at twice the frequency of the acoustic wave. Among the remarkable features of these in-fiber modulators, we mention the high peak power damage, high modulation depth, broad bandwidth, easy tunability in optical wavelength and low insertion losses. The in-fiber modulation is crucial for the implementation of all-fiber compact and robust lasers. The experimental characterization of different cavity configurations as a function of the radio frequency voltage that controls the modulator, the length of the active fiber, the overall dispersion of the cavity, etc., has led to an improved operation of the lasers: output optical pulses of 34 ps temporal width, 1.4 W peak power and 4.7 MHz repetition rate, at the emission wavelength of erbium.

We review our studies on passively mode-locked fiber lasers and nonlinear optical devices using carbon nanotube (CNT). CNT-based devices offer several key advantages such as: ultra-fast response, robustness, tunability of wavelength, and compatibility to fibers.