1 Introduction

Oscillation wavelength control is a crucial technology for rare-earth doped fiber lasers in many practical applications, such as optical communications, medical treatment, optical sensing, and spectroscopy [1–4]. Generally, the wavelength control has been carried out by means of extra-cavity setups, such as the rotation of a diffraction grating with coupling optics. The extra-cavity systems require precise optical alignment, which is unfavorable for improving optical efficiency and robustness. In-fiber wavelength-selective operation can avoid such problems. Thus far, various ideas have been reported on the in-fiber process using cavity loss controllers, multimode interference effects, Mach–Zehnder interferometers, fiber Bragg gratings, and long-period fiber gratings (LPFGs) [5–10]. The mechanically induced LPFG is one of the useful tools to control the lasing wavelength; however, such wavelength control has been realized mostly by reshaping the gain spectrum of the rare-earth doped fiber using the inherent notch filter response of the LPFG. The LPFG becomes more practical as the lasing wavelength controller by making good use of the bandpass filter response. The transmission spectrum of the LPFG exhibits the passband inside the rejection band by inserting the phase shift of π into the middle of LPFG [11, 12]. In this article, we present the wavelength control on the fiber ring laser by using the π-shifted mechanical LPFG as an intra-cavity bandpass filter.

2 Principle of operation

The fiber ring laser is composed of the π-shifted LPFG (PS-LPFG), an Er-doped fiber amplifier (EDFA), and a fused-fiber output coupler. As shown in Fig. 1, the C-band EDFA (ILX Lightwave, MPS-8033) consists of the Er-doped fiber (EDF), a 980/1550 nm wavelength division multiplexer (WDM), a pump laser diode (LD) operating at 980 nm, and an optical isolator for making the optical gain biased to one direction in the ring resonator. The pump light is launched through the WDM into the backward pumping port of the EDF. We examined the laser oscillation in the C-band region with this configuration. A two-mode fiber (TMF) having a core radius of 5.5 µm and a relative index difference of 0.38 % is inserted between the EDF and the output coupler in order to introduce the PS-LPFG into the fiber ring resonator. The PS-LPFG is mechanically formed in the TMF along the length of 100 mm by the combination of a coil spring with an electromagnet. The TMF is pressed against the coil spring by magnetic attractive force so that a periodic stress pattern with the coil pitch is introduced to the fiber. The coil spring is filled with an iron rod to enhance the magnetic attractive force from the electromagnet. The TMF and the coil spring are put inside a U-shaped spring holder that is set on the electromagnet. The U-shaped holder is made of duralumin and aligns the coil spring with the fiber, so that it allows a line of magnetic force to parallel the centerline of the coil spring and the fiber. A thin spacer ring with a thickness of 380 μm is inserted in the coil spring having a wire diameter of 550 μm; it brings a wire-center distance of 930 μm in the middle of the spring. This means that the π phase shift for the typical grating period of 620 μm is formed inside the fiber grating geometry [13]. The coupling strength can be controlled by the applied voltage to the electromagnet and the grating period can be adjusted by pulling the coil spring from its both sides. The fundamental mode light from the single mode fiber (SMF) of the output coupler is converted into the forward-propagating LP11 core mode around the phase-matching resonance. Then the LP11 mode light is radiated at the connection with the SMF of the EDF side. On the other hand, the fundamental mode light at the resonance wavelength is restored through the PS-LPFG. This process generates the passband surrounded by the two split rejection bands in the transmission spectrum. Thus, the PS-LPFG allows the propagated light at the selected wavelength to circulate in the fiber ring resonator.

Fig. 1
figure 1

Schematic setup of the EDF ring laser using the PS-LPFG to provide wavelength control operation in the C-band range

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When we evaluated the transmission spectra of the PS-LPFG, the input and output sections of the TMF were connected with the SMFs to ensure pure LP01 launch and detection. The transmission spectrum at the grating period of 620 μm was measured with a white light source (Ando, AQ4303B) and an optical spectrum analyzer (Agilent, 70952A) as shown by the dotted spectral curve in Fig. 2. By applying the voltage of 25 V to the electromagnet, the passband appeared at the resonance wavelength of about 1530 nm in the center of the rejection band. We measured the attractive force of the electromagnet by lifting up the coil spring using a digital force gauge. The overall pressure force along the fiber length of 100 mm was measured by 25.7 N for the voltage of 25 V. The whole rejection bandwidth is dependent on the differential dispersion of the two core modes as well as the mode coupling coefficient. As for the PS-LPFG-based bandpass filter, the two split rejection bands surrounding the passband determines the effective tunable span. According to the bandpass filter response in Fig. 2, the separation of the loss peak wavelengths was measured by about 100 nm. The passband moved toward longer wavelengths by increasing the grating period and the separation of the loss peaks became gradually narrowed accordingly. The loss peak separation was measured by about 50 nm when the passband wavelength was set at 1630 nm. We estimated the effective operational window of the tunable laser system to be 25–50 nm as half of the loss peak separation.

Fig. 2
figure 2

Transmission spectrum of the PS-LPFG showing the bandpass filter response, and the corresponding emission spectrum of the EDF ring laser

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3 Results and discussion

In the case of free-running oscillation, which was set up by turning off the electromagnet, the lasing occurred at the wavelength of 1566 nm. When the PS-LPFG was activated by turning on the electromagnet, the laser emission was moved to other wavelengths. As shown by the laser emission line in Fig. 2, the oscillation wavelength of 1533 nm is determined by the passband of the PS-LPFG. Figure 3 shows the laser emission spectra at various grating periods. Increasing the grating period of Λ from 619.1 to 624.6 μm moves the passband to the longer wavelength side, resulting in the red shift of the lasing wavelength from 1532.8 to 1565.1 nm. Figure 4 shows the laser output power as a function of launched pump LD power. The threshold pump power was measured by 2.8, 3.3, and 3.3 mW for the wavelength-controlled oscillation at 1553, 1532, and 1565 nm, respectively. On the other hand, the threshold pump power was measured by 2.3 mW at the free-running condition. The output laser power under wavelength control was decreased to 70–80 % of the free-running oscillation. They are attributable to the transmittance in the passband, i.e., the insertion loss of the PS-LPFG.

Fig. 3
figure 3

Laser emission spectra at different grating periods

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Fig. 4
figure 4

Output power of the fiber laser as a function of the pump LD power at the various oscillation wavelengths

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In order to examine the wavelength control in the L-band range, an amplified spontaneous emission source (Kyocera, ASE-001) was used as the L-band EDFA. As shown in Fig. 5, the EDF is bidirectionally pumped through the two 980/1550 nm WDMs and the optical isolator is also included in the L-band EDFA. Figure 6 shows the laser emission spectra when the launched pump power was divided to the forward pumping and backward pumping ports of the EDF at 86 and 12 mW, respectively. The free-running oscillation occurred at the wavelength of 1606 nm, which is shown by the dotted spectral curve in Fig. 6. As the grating period was increased from 627.4 to 632.9 μm, the emission wavelength shifted to longer wavelengths from 1586.8 to 1613.8 nm with a side-mode suppression ratio of ~40 dB. The spectral linewidth (full width at half maximum) of each line was measured by ~0.1 nm, which was determined almost by the resolution of the optical spectrum analyzer.

Fig. 5
figure 5

Schematic setup of the EDF ring laser using the PS-LPFG to provide wavelength control operation in the L-band range

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Fig. 6
figure 6

Laser emission spectra at different grating periods. The dotted spectral curve shows the lasing spectrum without the PS-LPFG

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We examined the relation of the lasing wavelength with the passband center wavelength of the PS-LPFG. The solid curve shows the quadratic fitting curve to the passband center wavelength as a function of the grating period from 619.5 to 632 μm. The extension of the coil spring required for tuning the wavelength corresponds to 12.6–14.9 % of the original pitch of 550 μm. The filled circles in Fig. 7 show the emission peaks of the fiber laser with the C-band EDFA when the grating period is changed between 619.1 and 624.6 μm. As for the fiber laser using the L-band EDFA, the filled squares show the emission peaks when the grating period is changed between 627.4 and 632.9 μm. The lasing wavelength follows the passband over the wavelength ranges of 32.3 and 27 nm in the C and L bands, respectively. The step-like shift in the lasing wavelength is considered to lack of smoothness in the movement of the coil spring. When the grating period was out of the ranges noted above, the lasing wavelength deviated from the tuning track and stayed around the free-running wavelength. It is attributed that the surrounding rejection bands could not suppress the free-running oscillation.

Fig. 7
figure 7

Oscillation wavelength shift with respect to the grating period. The solid curve is a quadratic fitting curve of the passband center wavelength

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4 Conclusions

We demonstrated prototype tunable fiber lasers that can be changed in the oscillation wavelength by all-fiber ring configuration. The π-shifted mechanical LPFG is constructed to be the in-fiber tunable bandpass filter that controls the fiber laser oscillation. Future effort should be put emphasis on broadening the rejection bandwidth around the passband in order to expand the tunable span of the fiber laser. We believe that the PS-LPFG-based bandpass filter can be applied not only for the tunable systems in EDFL but also in other fiber lasers, such as Yb-doped fiber lasers and Tm-doped fiber lasers.