Tm-doped fiber laser delivering tunable single-wavelength and switchable dual-wavelength pulses

Accepted Manuscript Title: Tm-doped fiber laser deliverinE-mail address of corresponding author added from order viewer, kindly check.–>g tunable single-wavelength and switchable dual-wavelength pulses Author: M. Kim PII: DOI: Reference:

S0030-4026(18)31417-7 https://doi.org/10.1016/j.ijleo.2018.09.114 IJLEO 61553

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Please cite this article as: Kim M, Tm-doped fiber laser delivering tunable single-wavelength and switchable dual-wavelength pulses, Optik (2018), https://doi.org/10.1016/j.ijleo.2018.09.114 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Tm-doped fiber laser delivering tunable single-wavelength and switchable dualwavelength pulses

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M. Kim

Chungnam National University, 99 Daehak-ro Yuseong-gu, Daejeon, 34134, Republic

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Abstract

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of Korea

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We propose a tunable single-wavelength and switchable dual-wavelength Tmdoped fiber laser based on carbon nanotubes. The central wavelength of single-

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wavelength mode-locking operation can be tuned from 1892 to 1924 nm. The laser

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can also exhibit switchable dual-wavelength mode locking between the center

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wavelength of 1883/1894 nm and 1905/1910 nm. All the tunable or switchable operation is realized by rotating or squeezing the polarization controller. The

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flexible all-fiber laser can provide two different mode locking operations, which

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is convenient and attractive for practical applications.

Keywords: Tunable; Switchable; Fiber laser; Carbon nanotubes.

Introduction

In recent years, Tm-doped fiber lasers have been extensively investigated due to its wide

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applications in medicine, eye-save radar, remote sensing, telecommunications, and midinfrared generation. 1-2 Passively mode-locking is an effective technique to acquire pulses

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with high beam quality, excellent heat dissipation, good reliability, as well as robust mode

confinement.3-10 Currently, the typical technique for passively mode locking is incorporating a saturable absorber (SA) in the cavity, whose light absorbance decreases

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with the increase of the light intensity, such as nonlinear polarization rotation (NPR)

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technique, nonlinear optical loop mirror (NOLM), semiconductor saturable absorber

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mirror (SESAM), single walled carbon nanotube (SWNT), and graphene.11-45 SWNTs are

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particularly advantageous for fiber lasers compared to other SAs, because they are

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relatively simple, inexpensive to fabricate, insensitive to environmental perturbation and

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easily integrated into various fiber configurations.22 In particular, for the band structure

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of SWNT depends on its diameter and chirality, so broadband saturable absorption mode locker could be fabricated by mixing SWNTs with different diameters, offering

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possibilities for tunable or multi-wavelength mode locked fiber lasers. Passively mode-locked fiber lasers with spectral tuning or multi-wavelength capability

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have widespread applications in spectroscopy, optical communications, and biomedical research.23-25 Liu et al. reported a nanotube-mode-locked all-fiber ultrafast oscillator emitting tri-wavelength ultrafast pulses, which is also tunable by stretching the fiber Bragg gratings (FBGs) in the cavity.22 Tran et al. demonstrated a switchable multiwavelength erbium doped fiber laser with a nonlinear optical loop mirror based on a

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highly nonlinear dispersion shifted fiber.26 Huang et al. reported a tunable and switchable multi-wavelength dissipative soliton generation in an Yb-doped fiber laser.27 Nelson et al. obtained tunable sub-500 fs pulses from an additive-pulse mode-locked fiber ring laser by adjusting the birefringent plate in the cavity.28 Chamorovskiy et al. demonstrated the dual-wavelength soliton pulse in Ho-doped fiber laser by rotating the polarization

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controller (PC) in the cavity.29 NPR technique and NOLM are often used by many researchers to induce inhomogeneous effective gain spectral profile to obtain tunable

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mode-locking fiber lasers.30, 31 Some specialty components can be incorporated into laser cavities, such as optical filters, FBGs, or polarization maintaining fiber to generate

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tunable or multi-wavelength mode-locking operation, which however increase the

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complexity of the laser cavity.32-34 Until now, tunable and multi-wavelength fiber lasers

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mainly operate at the wavelengths of 1m and 1.55 m. Only few articles reported pulsed

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lasers with wavelength tunable and multi-wavelength capability in 2 m regime.46-48

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In this paper, we propose a passively mode-locked Tm-doped fiber laser based on

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SWNTs with spectral tuning and dual-wavelength switchable capability. The central wavelength of pulses in single-wavelength regime can be tuned from 1892 to and 1924

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nm by changing the polarization state of light in the cavity. Increasing the pump power,

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the dual-wavelength mode locking operation can be achieved. Furthermore, the central wavelength can be switched from 1883/1894 nm to 1905/1910 nm by rotating or

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squeezing the PC. The broadband saturable absorption of SWNTs and filtering effect caused by the combination of PC and birefringence in the cavity play key roles on the tunable and switchable mode-locking operation. The proposed fiber laser would be helpful for further understanding the mechanism of tunable wavelength.

Experimental Setup

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Figure 1 shows the configuration of tunable and switchable Tm-doped fiber laser with a SWNTs-SA. A 2-m Tm-doped fiber (Nufern, core/cladding diameter: 9/125 nm, NA: 0.15) serves as the gain medium in the oscillator, whose absorption coefficient at 1570 nm is ~12 dB/m. A 1570-nm continuous wave (CW) source from an Erbium-doped fiber amplifier (EDFA) is used to provide the pump power via a 1570-nm/1940-nm

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wavelength-division multiplexer (WDM). The laser output from the oscillator is coupled out through the 30% port of a 30/70 fiber coupler. The PC is utilized to adjust the linear

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birefringence and the loss of the cavity by imposing pressure or twisting on the fiber. A

polarization insensitive isolator (PI-ISO) with 10% insertion loss and over 35-dB

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isolation at 2-m wavelength was fusion spliced into the cavity to enforce a clockwise

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unidirectional ring. With about 10-m single mode fiber (SMF) pigtails of the optical

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components, the total cavity length is around 12 m. The dispersion parameters of the Tm-

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doped fiber and SMF at 1940 nm are about -0.076 ps2/km and -0.068 ps2/km,

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respectively. Thus, the net group delay dispersion of the resonator is anomalous and equal

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to approximately -0.832 ps2. The SWNTs with the outside diameter ranging from 1.4 to 2.0 nm are made by the catalytic chemical vapor decomposition method.35, 36 Polyvinyl

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alcohol (PVA) is chosen as the composite matrix for its mechanical property and solvent

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compatibility.37 The mixture of SWNT and PVA is deposited on a Petri disk until a freestanding polymer film is formed. The SA based on the mixture of SWNTs and PVA is

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sandwiched between two fiber connectors in the ring cavity. An optical spectrum analyzer with an operating bandwidth of 1200-2400 nm (Yokogawa AQ6375) is used to monitor the optical spectrum of the laser output. An InGaAs photodetector (~12 GHz) and a 4GHz real-time digital storage oscilloscope (Rohde&Schwarz RTO2044) are combined to measure the pulse trains.

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The nonlinear absorption of the SWNT-PVA absorber is experimentally measured with a homemade ultrafast laser. According to a simplified two-level saturable absorber model,22 the experimental data are fitted as the solid curve of Fig. 2. The insertion loss is ~ 62%, the modulation depth is ~7%, and the saturation intensity is ~ 24.5 J/cm2. Experimental Results and Discussions

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With appropriate pump power, mode locking of the fiber laser can be achieved by adjusting the PC. Once the proper orientation and pressure settings of the PC are found

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and fixed, the laser always self-starts when the pump power exceeds the threshold value of 110 mW. During operation, the pump power might be further increased up to 150 mW

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without losing the mode locking. As the pump power is 120 mW, the output spectrum of

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stable single-wavelength mode locking is shown in Fig. 3(a) with clear Kelly’s sidebands

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on the spectrum, which is the typical characteristic of standard soliton in anomalous

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dispersion fiber laser.38-44 The center wavelength of the spectrum is ~1916 nm and the

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full width at half-maximum bandwidth is about 3.7 nm. A simple phase-matching

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argument is responsible for the special structure on the spectrum. When the soliton propagates in the fiber laser, it is periodically perturbed by gain and loss due to splices or

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output couplers. The perturbed soliton sheds dispersive radiation over the entire

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bandwidth of the spectrum and reshapes back into a soliton. Each frequency of the dispersive radiation propagates at its own phase velocity. Only the certain frequency

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components are phase match coupled to the soliton and result in significant sidebands, whose positions are determined by the cavity dispersion and length.45 Additionally, characteristic dips can be seen in the spectrum, originating from water absorption lines, which are densely located in the 1.9-μm region. The output pulse train is recorded by the real-time oscilloscope to manifest the single-pulse operation, as depicted in Fig. 3(b). The

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separation of adjacent pulses is ~59.1 ns, corresponding to the cavity length of ~12 m. The inset of Fig. 3(b) shows a close-up view of the output pulse, indicating that the temporal width of the output mode-locking pulses must be less than ~139 ps, which is the minimum measurable pulse width of our real-time oscilloscope. Assuming that the pulses were in a transform-limited pulse form, the output pulse width was estimated to be ~1.04

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ps. In fact, the autocorrelation trace of output pulses could not be obtained due to the sensitivity limitation of our homemade autocorrelator and the relatively low peak power

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of the output pulses. The output power is approximately 1 mW due to the loss of SMF,

SWNTs-SA and the 30/70 coupler. Figure 4 shows the RF spectrum with the fundamental

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frequency of ~16.9 MHz at a resolution of 10 Hz. The signal noise ratio (S/R) is ~ 50 dB.

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Inset of Fig. 4 illustrates the broad RF spectrum with a scanning range of 650 MHz and

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resolution of 10 KHz. The absence of modulation in such broad RF spectrum indicated

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that the oscillator operated at stable mode-locking regime.

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By rotating or squeezing the PC without changing the pump power, the central

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wavelength of mode-locking operation can be tuned from 1892 ~ 1924 nm, as shown in Figure 5. We believe larger tuning rang of the central wavelength from 1860 to 1940 nm

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would be achieved by fabricating SWNTs with broader saturable absorption range and

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further adjusting the pump power and the loss of the cavity, due to the broad gain

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spectrum of Tm-doped fiber. Further adjusting the PC state and increasing the pump power to 150mW, stable dual-

wavelength mode-locking at the wavelength of 1883/1894 nm was achieved and the spectrum is presented in Fig. 6(a). When finely tuning the PC state, the laser can be switched to the other dual-wavelength mode locking operation with the center wavelength of 1905/1910 nm, as illustrated in Fig. 6(b).

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The laser will emit at the wavelength with the maximum cavity effective gain due to gain competition, which is single-wavelength laser emission. Due to the birefringent effect of SMF, the cavity transmission can be changed by rotating or squeezing the PC. The variation of central wavelength is due to the inhomogeneous effective gain profile in the cavity, which is caused by the modulation of cavity effective gain for the changing of

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cavity transmission. Additionally, in order to obtain the dual-wavelength lasing operation, the cavity transmission should be modulated to have two maximum points for the cavity

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effective gain. This can be easily achieved by coarsely adjusting the PC state. And the switchable operation will be got with fine adjustment of the PC.

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Conclusions

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We present a spectral tunable and switchable mode locked Tm-doped fiber laser with

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carbon nanotubes. The tunable single-wavelength and switchable dual-wavelength operation result from wavelength-dependent loss of the cavity induced by the

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birefringence. For the single-wavelength mode locking, the central wavelength of pulses can be tuned from 1892 to and 1924 nm. For the switchable dual-wavelength operation,

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we demonstrate two pairs of dual-soliton with center wavelengths of 1883/1894 nm and 1905/1910 nm. The tunable and switchable operations are achieved by changing the

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polarization state using the PC. We believe this flexible and compact fiber laser can find

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important applications in the future.

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(a)

Fig. 1. Schematic diagram of the SWNTs mode locking Tm-doped fiber laser system. EDFA, Erbium-

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doped fiber amplifier; WDM, wavelength-division multiplexer; TDF, Tm-doped fiber; PC,

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polarization controller; PI-ISO, polarization insensitive isolator; SWNT, single walled carbon

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nanotube.

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Experimental data Fit curve

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Absorption

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0.62

S/

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20

40

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Incident fluence (J/cm )

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Fig. 2. (c) Nonlinear absorption characterization of the SWNT-PVA saturable absorber. The solid curve is fitted from the experimental data (circle symbols).

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2.5

Intensity (a.u.)

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(b)

Intensity (a.u.)

(a) 2.0

Intensity (dB)

-50 -55

1.5

=3.7 nm

59.1 ns

1.0

139 ps

0.5 0.0 -400

0 Time (ps)

400

1.0

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0.5

-65 1910 1915 1920 Wavelength (nm)

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(a)

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0 Time (ns)

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200

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Fig. 3. Mode locking operation at 1916 nm. (a) Output optical spectrum and (b) oscilloscope trace

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200

S/N ~ 50 dB

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-30

300

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-75

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-60

0

400

500

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 ~ 27.1 ps

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RF Power (dBm)

together with a close-up view of a pulse.

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-120 16.85

16.90

16.95

17.00

17.05

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Frequency (MHz)

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Fig. 4. RF spectrum with the resolution bandwidth of 10 Hz. Inset is the corresponding RF spectrum

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with scanning range of 650 MHz.

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FWHM=38.3ps

Intensity (5 dB/div.)

1892 nm

1895 nm

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1916 nm

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1922 nm

1870

1880

1890

1900

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1924 nm

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1920

1930

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Wavelength (nm)

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Fig. 5. Tunable mode-locking operation with tuning range from 1892 nm to 1924 nm.

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1940

-45 1883/1894 nm

-55 -65 -75 1870

1880

1890

1900

Wavelength (nm) (b)

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1905/1910nm

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-55 -65 -75 1900

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1905

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Intensity (dB)

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Intensity (dB)

(a)

1910

1915

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Wavelength (nm)

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Fig. 6. Switched dual-wavelength mode-locking operation. Dual-wavelength mode locking with the

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center wavelength of (a) 1883/1894 nm; (b) 1905/1910 nm.

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