Dual-wavelength mode-locked Er-doped fiber laser in a spectral filter free cavity

Highlights

• A dual-wavelength passive mode-locked in a spectral filter free cavity with NPR is presented.

• The dual-wavelength is obtained by decreasing linear and circular birefringence in the cavity by twisting the fiber.

• The double-twist fiber allows to eliminate the spectral dependence of the saturable absorber.

Abstract

We present the experimental investigation of a dual-wavelength passively mode-locked Er-doped fiber laser with strict polarization control. The configuration includes a double-pass amplifier with a Faraday mirror and twisted fiber. We observe that including double-twist fiber cancels circular and linear birefringence and allows obtaining dual-wavelength emission varying the losses in the cavity. In contrast, results including single-twist fiber which only emits single-wavelength due to large circular birefringence.

Introduction

Dual-wavelength lasers find applications in many areas such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering microscopy [1], [2], sum frequency generation, and difference frequency generation [3], and among others. Most of the researches on dual-wavelength lasers and their applications are based on solid-state lasers. However, passively mode-locked fiber lasers can present cost effective, compact, and robust alternative to solid-state lasers. Depending on the laser characteristics such as birefringence, dispersion, nonlinearities, amplification of gain media, and attenuation the passive mode-locked fiber lasers (PMLFL) allows the generation of different types of pulses like conventional solitons (CSs), bound solitons, soliton molecules (SM) and dissipative soliton resonance (DSR), noise-like pulses (NLP), etc. [4]. Dual-wavelength and multi-wavelength generation were demonstrated considering different techniques for mode-locking including nonlinear polarization rotation (NPR) [5], [6], [7] and the figure-eight configuration [8]. Dual-wavelength mode-locked generation was observed also using semiconductor saturable mirror (SESAM) [9], carbon nanotube mode-locker [10], and tungsten disulfide saturable absorber [11]. To achieve dual-wavelength generation a spectral filter can be inserted into the cavity. For example, the Lyot filter was used in [6]. The dual-wavelength generation was observed also without any specific spectral filter [9], [11], [12], [13], [14], however, the authors of [9], [12], [13] suppose that some filtering effect, because of the residual birefringence of the elements comprising a laser cavity, is important for dual-wavelength formation. The authors of [14] explained the erbium-doped fiber laser (EDFL) dual-wavelength operation via net gain cross-section variations caused by changes in the population inversion rates in the gain medium as a function of the pump power.

The important issue is the synchronization of the pulses with different wavelengths traveling in the same cavity. Usually, the pulses are not synchronized because of GVD. The authors of [15] experimentally studied the ring cavity configuration using a single-wall carbon nanotube saturable absorber as a mode-locker. The generated solitons were not synchronized if the total dispersion was higher than 8 fs/nm. At smaller dispersion, the synchronization of pulses was observed. The authors of [16] studied a dual-branches laser cavity in which the pulses with different wavelengths travel through different branches. The lengths of branches have to be adjusted with a tolerance of 0.46 mm to obtain synchronization of the pulses.

One of the most important parameters to control is the polarization state of the pulses traveling in the cavity. Practically all fiber laser configurations include polarization controllers which are adjusted somewhat randomly to achieve the desired regime of generation. Random and usually unknown residual birefringence of the fiber elements in the cavity makes it complicated to link the operating regime with the other experimental conditions. One way to control the polarization is to use highly birefringent (Hi-Bi) fibers [17], [18], however, this technique does not allow the nonlinear polarization rotation to be exploited to generate mode-lock. Another technique is to use twisted fibers. The twisted fiber eliminates residual linear birefringence and induces circular birefringence. Unlike the linear birefringence, circular birefringence does not cause the change of the polarization ellipticity that is very important for mode-locking technique base on NPR. The circular birefringence results in polarization azimuth rotation which is less sensitive to environmental conditions. This technique allows stable generation in different regimes and was used to link the regimes of generation with the polarization state of pulses in the cavity [19], [20], [21], [22]. As was mentioned, fiber twist adds circular birefringence and as a result, the azimuth of the polarization becomes dependent on the wavelength. This effect produces some spectral filtering which is important for dual-wavelength generation.

In this work, we report the experimental investigation of dual-wavelength mode-locked operation in the EDFL with strict polarization control. We have found the ellipticity vs azimuth area where dual-wavelength generation was observed. We canceled the wavelength dependence of the polarization rotation in the twisted fiber by using two fiber spans twisted in opposite direction and verify the absence of any spectral dependence in the cavity to make sure the mechanism of dual-wavelength formation is based only on the properties of the EDF.