Coherent ultrabroad orbital angular momentum supercontinuum generation in an AsSe₂-As₂S₅ microstructured fiber with all-normal dispersion

Highlights

• A novel fiber is proposed which can achieve HE_2,1 dispersion from −13 to −0.7 ps/nm/km over 3.3–$8.6\mathrm{\mu }\mathrm{m}$.

• A method to get nearly-zero flattened dispersion of different high-order modes is summarized.

• Several orbital angular momentum supercontinuum spectra spanning from 1.3 to $11\mathrm{\mu }\mathrm{m}$ at −40 dB are obtained.

Abstract

We design and simulate a hollow AsSe₂-As₂S₅ microstructured fiber surrounded by air holes with nearly-zero flattened all-normal dispersion profile. Simulation results show that the coherent first-order orbital angular momentum (OAM${}_{\mathrm{1,1}}$) supercontinuum (SC) spectrum extending from 1.3 to 11 $\mathrm{\mu }$m at −40 dB level, which can be achieved by pumping a chirp-free hyperbolic secant pulse with 100 fs pulse duration and 20 kW peak power at the wavelength of 6 $\mathrm{\mu }$m into an 11 mm proposed fiber. Due to the flexible controllability for chromatic dispersion of this fiber, several coherent SC spectra generated by different OAM modes are analyzed.

Introduction

Since the concept of orbital angular momentum (OAM) was proposed in 1992  [1], OAM beams with spiral phase front exp(il $\theta$) (l is the topological charge number) have come to the foreground in recent years. The unique properties of OAM beams make it can be used in a variety of applications such as optical communication [2], [3], super-resolution microscopy [4], optical sensing [5] and laser machining [6]. Therefore, expanding its band is an issue worth of research. OAM modes can be described as combinations of its two eigenmodes with a phase shift of $\pm$ $\mathrm{\pi }$/2 [7]. In air-core fibers, the four-fold degeneracy of OAM modes can be separated because of different propagation constants between HE and EH mode. Using a spatial light modulator to get desired OAM modes and a quarter-wave plate to choose polarization state, after propagation in an air-core fiber, the output OAM supercontinuum spectrum at all the nonlinearly generated frequencies is same as the launched pump state [8].

Mid-infrared supercontinuum (MIR SC) has been widely investigated in the past few years [9], [10], [11]. In order to obtain broader SC spectra, heavy elements such as tellurite and chalcogenide are used to replace silica because of a wider transparency window and a higher nonlinearity. Compared with traditional SC, the research on OAM SC gets a relatively later start. In 2012, As₂S₃ OAM-supported photonic crystal fiber (PCF) was proposed [12] and OAM SC spanning from 950 to 1850 nm at −20 dB in a slotted vortex fiber was reported in the following year [13]. In 2019, OAM SC spanning from 696 to 1058 nm at −20 dB was obtained in an SiO₂ annular core PCF [14]. In the same year, high-order OAM ($l=8$) SC generation was experimentally demonstrated in an air-core fiber [8]. Recently, by using chalcogenide As₂S₃ material, 959–2905 nm coherent OAM SC and 1560–6250 nm OAM ($l=17$) SC was generated in a PCF and an air-core ring fiber, respectively [15], [16]. In most of the studies reported above, broadband SC spectra were achieved by pumping in anomalous dispersion regime, which is near the zero-dispersion wavelength (ZDW). In this pumping condition, the mechanism responsible for the SC generation is dominated by soliton dynamics, and it may amplify the input pulse noise and degrade the coherence of the SC light because of the modulation instability (MI) [17]. To improve the coherence in SC generation, a common approach is pumping nonlinear medium with all-normal dispersion (ANDi) by femtosecond lasers [18], [19] at the expense of the spectrum bandwidth. In ANDi regime, the SC spectrum broadening is dominated by self-phase modulation (SPM) and optical wave breaking (OWB) induced four-wave mixing (FWM) [20]. Since ANDi is introduced in the nonlinear process, there is no soliton formation, the spectrum is relatively flat and a single pulse is maintained at the time domain. Consequently, SC with high coherence is vital in practical applications.

Considering SPM is dominated in ANDi regime, high peak power and high nonlinearity is indispensable. Material AsSe₂ (core) and As₂S₅ (cladding) are selected because of the good compatibility [21]. Furthermore, AsSe₂ glass possesses a wide transparency from 0.83 to 18.9 $\mathrm{\mu }$m [21], a high nonlinear refractive index $n_2$ which is $1.1\times 10$⁻¹⁷ (m²W⁻¹) [22] and a sufficient laser induced damage threshold (LIDT) more than 0.2J/cm² [23]. Hollow ring-core with large effective refraction index difference between adjacent modes which is suitable for OAM modes is necessary. In order to maximize the SC spectrum, a low and flat dispersion profile is highly desirable. The waveguide dispersion needs to be adjusted to balance the material dispersion by changing the fiber structure, and a method to flexibly adjust the waveguide dispersion is adding air holes by rotation in the cladding. With the design of circular symmetry, walk-off between even and odd modes can be dismissed. At present, all-fiber system to generate OAM ($l=1$) [24], [25], [26] is limited by order number l, and OAM${}_{\mathrm{1,1}}$ exhibits lower loss and more stability in a same fiber structure. Therefore, we focus on OAM${}_{\mathrm{1,1}}$ and investigate the dispersion regulation and influence of pump conditions on the broadening.

In this paper, a novel microstructured fiber is designed to generate OAM SC. The simulation results of OAM${}_{\mathrm{1,1}}$ are as follows: a low and flat dispersion with the variation from −13 to −0.7 ps/nm/km over 3.3–8.6 $\mathrm{\mu }$m and a broad SC spectrum with 9650 nm bandwidth extending from 1295 to 10,945 nm at −40 dB level (1452–9675 nm at −20 dB). Owing to the excellent ability to engineer the dispersion of the proposed fiber, a 9892 nm SC of OAM${}_{\mathrm{5,1}}$ and a 10,020 nm SC of OAM${}_{\mathrm{9,1}}$ with all-normal dispersion is formed at −40 dB level. These SC spectra are all generated from an available tunable optical parametric amplifier (OPA) at the pump wavelength of 6 $\mathrm{\mu }$m [23] and they are highly coherent.