1. Introduction

Fiber lasers around 2 μm, corresponding to characteristic absorption lines of liquid water, greenhouse gases and some other chemical compounds, have attracted intense attention over the past few years [1–3]. Due to their great potential in practical applications such as differential absorption lidar (DIAL), medical diagnosis and terahertz difference frequency generation [4–6], dual-wavelength operation of fiber laser in this regime has been achieved with several approaches. For example, using two volume Bragg gratings (VBGs) [7], Wang et al. realized a tunable dual-wavelength thulium (Tm³⁺) doped fiber laser (TDFL), where the operating wavelength can be tuned independently with the incident angle of each VBG. Polarization hole burning enhanced by high-birefringence FBG was observed and applied by Peng et al. to generate a switchable dual-wavelength TDFL at 1.94 μm [8]. Ma et al. proposed a dual-wavelength TDFL incorporating a Sagnac loop mirror and tunable operation was achieved with a polarization controller in the cavity [9]. Recently, Soltanian et al. demonstrated a dual-wavelength fiber laser operating at 1.9 μm with a photonic crystal fiber-based Mach-Zehnder interferometer [10].

However, most previously reported dual-wavelength fiber lasers are based on Tm³⁺-doped fibers and their operating wavelengths are below 2 μm. It is more attractive to develop a compact dual-wavelength fiber laser operating above 2 μm because in this regime the atmosphere has relatively high transparency and some gases like CO₂ have strong absorption, which facilitates numerous applications including coherent Doppler wind lidars, CO₂ DIAL systems and other gas sensing systems. Because the unit gain of Tm³⁺-doped silica fiber decreases significantly with increased wavelength above 2 μm, it is difficult to achieve a dual-wavelength fiber laser in this regime. Ho³⁺-doped fiber or Tm³⁺:Ho³⁺ co-doped fiber is a good candidate for dual-wavelength laser operation above 2 μm because it can provide high gain through the transition between energy levels ⁵I₇ and ⁵I₈ [11].

The self-imaging effect of multimode interference (MMI) in multimode fiber (MMF) has been extensively investigated with the single-mode-multimode-single-mode (SMS) fiber structure. Different devices and lasers based on the SMS fiber structure have been experimentally demonstrated by several groups for various applications such as a wavelength tunable fiber lens [12], an all-fiber Q-switched fiber laser [13], a Bessel-like beam generator [14], high power multimode fiber lasers and amplifiers with single-transverse mode output [15], and all-fiber filters [16, 17]. Most recently, the SMS fiber structure has been used to achieve multiple-wavelength operation of a Tm³⁺-doped fiber laser around 1.9 μm [18].

In this paper, we present the investigation of the transmission properties of SMS fiber devices in the 2 μm wavelength region and the demonstration of a dual-wavelength fiber laser operating above 2 μm by using two cascaded SMS fiber structures. Here, one SMS structure with a short MMF acts as a long-pass filter to suppress the laser at shorter wavelengths, and the other SMS structure with a long MMF works as a band-pass filter to select specific lasing wavelengths. In the experiment, dual-wavelength operation of a Tm³⁺:Ho³⁺ co-doped fiber laser at 2002.8 and 2016.1 nm has been achieved with an SNR up to 50 dB. Stable operation was verified by monitoring the stability of the output power at both wavelengths for 30 minutes.

2. Theoretical analysis of the transmission properties of the SMS fiber structure

The SMS fiber structure consists of a piece of MMF spliced with two segments of single mode fiber (SMF). Due to the circularly symmetric characteristics of the fiber, the source light within the SMF is assumed to have a Gaussian-shaped field distribution of $E_s(r,0)$. Under the circumstance of ideal axis-alignment and in the linear polarization approximation, only the LP_0n modes, ${\psi }_n(r)$, are strongly excited when the single-mode light is launched into the MMF, and the field distribution at a length L can be expressed as [19]

When the light reaches the interface between the MMF and the output SMF, which is identical to the input fiber, it will be coupled to the core and cladding modes of the output SMF. As the light propagates in the output SMF, only the power coupled to the core mode survives and that coupled to the cladding modes will leak to the outer jacket eventually. Therefore, the transmission of the SMS structure can be written as [20]

For an SMS fiber structure with fixed multimode fiber length, the wavelength interval for self-imaging $\Delta {\lambda }_{im,n}$ can be expressed with the formula shown below [15]:

According to the equation above, one can conclude qualitatively that the wavelength interval for self-imaging decreases with the multimode fiber length. Furthermore, the same conclusion can be drawn for the wavelength spacing of the transmission peaks of SMS fiber structure. To definitively verify this conclusion, we simulated the transmission spectra of SMS structure with the Thorlabs MMF (AFS105/125Y), which has core and cladding diameters of 105 and 125 μm, respectively, and core NA of 0.22. SMF-28 fiber with the core diameter of 8.3 μm and NA of 0.14 was chosen as the single mode fiber.

The transmission spectra of the SMS fiber structure with different MMF lengths were simulated and are shown in Fig. 1. When the length of MMF in the SMS fiber structure increases from 50 to 200 mm, the wavelength spacing of the transmission peaks decreases gradually, which indicates that an SMS fiber structure with a long MMF can be employed in a fiber laser system to act as a wavelength selector, as presented in Fig. 1(c), to achieve dual-wavelength and even multi-wavelength operation. Furthermore, for an SMS structure with a short MMF, the transmission bandwidth becomes very broad and the transmission spectrum is flat, as shown in Fig. 1(a). As defined in Ref. 15, an SMS fiber structure with a fixed MMF length has the best self-imaging quality at the highest transmission peak, where the quasi-reproduction of the input field occurs. As the wavelength deviates from the transmission peak, the phase differences between excited modes at the end of MMF increase and consequently self-imaging quality degrades [15], which results in increased transmission loss. This can be understood intuitively by looking at the transmission spectrum of an SMS device with 50 mm MMF in Fig. 1(a), where the loss increases with the wavelength decreasing from 2050 to 2000 nm. Accordingly, an SMS structure having short MMF length can work as a long-pass filter to suppress the competitive laser at shorter wavelengths. Therefore, as illustrated by the spectra shown in Fig. 1(d), we can combine the two SMS devices with MMFs length of 50 and 200 mm in a cascaded fashion in a Tm³⁺:Ho³⁺ co-doped fiber ring laser to achieve the dual-wavelength operation above 2 μm.

 

Fig. 1 Transmission spectra of the SMS fiber structure for different multimode fiber lengths: (a) L_MMF = 50 mm; (b) L_MMF = 100 mm; (c) L_MMF = 200 mm; (d) Comparison of the transmission spectra of the SMS structure for L_MMF = 50 mm and 200 mm.

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3. Experimental setup and results

The experimental schematic of the dual-wavelength fiber laser is shown in Fig. 2. A piece of Tm³⁺:Ho³⁺ co-doped fiber (CorAcitve TH512) was chosen as the gain medium. The Tm³⁺:Ho³⁺ co-doped fiber has the core and cladding diameter of 9 and 125 μm, respectively, associated with a core absorption coefficient of ~23 dB/m at 1570 nm, and was pumped by a homemade 1570 nm CW fiber laser through a 1570/2000 nm wavelength division multiplexer (WDM). An isolator was spliced after the active fiber to force the unidirectional propagation of the laser in the ring cavity. 30% of the laser power was extracted from the ring cavity by a 30/70 fiber coupler. The SMS fiber devices used in the experiment were fabricated by splicing two SMF-28 fibers to both ends of a piece of MMF.

 

Fig. 2 Schematic of the dual-wavelength fiber laser based on cascaded SMS fiber devices.

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Generally, the active fiber length is chosen for sufficient pump absorption and thus 1 m of Tm³⁺:Ho³⁺ co-doped fiber can absorb the pump with a coefficient of 23 dB. However, since the laser wavelength red-shifts with the fiber length due to the re-absorption process [21], long active fiber is usually used in the experiment to achieve laser operation above 2 μm. The operating wavelength of the free-running ring laser (without the two SMS structures) with different Tm³⁺:Ho³⁺ co-doped fiber lengths was investigated at a launched pump power of 1.5 W. As shown in Fig. 3, the operation wavelength of the Tm³⁺:Ho³⁺ co-doped fiber laser increases with the increased gain fiber length. When the gain fiber is 7 meter, laser operation with a broad spectrum above 2000 nm was obtained, which makes it possible to achieve dual-wavelength operation at long wavelength. Therefore, a 7-m Tm³⁺:Ho³⁺ co-doped fiber was used to achieve the dual-wavelength fiber laser.

 

Fig. 3 Optical spectra of the Tm³⁺:Ho³⁺ co-doped fiber laser with different gain fiber lengths without SMS devices in the cavity.

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The transmission spectra of the two cascaded SMS fiber devices were measured with a supercontinuum source (NKT SuperK COMPACT) and are shown in Fig. 4. One SMS (named as SMS1) with an MMF length of 4.5 cm acted as a long-pass filter to suppress competitive lasing around 1950 nm. The other SMS (named as SMS2) with a 20.3-cm MMF worked as band-pass filter to select specific operating wavelengths. It should be pointed out that, compared to the results in [22, 23], the transmission spectra we obtained here were not associated with perfect self-imaging. The minimum loss of the SMS device is about 6 dB, which significantly limits the efficiency of the fiber laser. Reducing the propagation loss of the SMS devices by improving the self-imaging is currently under way for the power scaling of the dual-wavelength fiber laser above 2 μm.

 

Fig. 4 Measured transmission spectra of the SMS1, SMS2 as well as cascaded SMS fiber devices.

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When only the SMS2 device was used in the ring cavity, in addition to a laser at 2021.2 nm, another laser at a shorter wavelength of 1982.4 nm was observed as shown in Fig. 5(b). The short wavelength lasing usually cannot be suppressed by adjusting the transmission peak of the SMS2 with different MMF lengths because an SMS fiber device with a long multimode fiber length is associated with a relatively small transmission peak spacing and small loss differences between these peaks, which is essential for dual-wavelength or multi-wavelength operation. The transmission spectrum of the SMS1 with 4.5-cm MMF length is shown in Fig. 5(a), which shows that the SMS1 can be used as a long-pass filter to allow the lasing above 2 μm to operate and suppress the lasing below 2 μm. Therefore, we put the two SMS fiber devices in a cascaded fashion into the ring cavity to achieve dual-wavelength operation above 2 μm. The transmission spectrum of the cascaded SMS device was measured and is shown in Fig. 4, which is the combined transmission of the two SMS fiber devices, i.e. the wavelength space between two transmission peaks is small while the transmission loss of the light below 2 μm is significantly increased.

 

Fig. 5 (a)Transmission spectrum of SMS1 and corresponding optical spectrum of the fiber laser when it was inserted in the ring cavity. (b) Transmission spectrum of SMS2 and corresponding optical spectrum of the fiber laser when it was inserted in the ring cavity.

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When the two SMS fiber devices were incorporated into the ring cavity, the output spectrum of the fiber laser was measured with an optical spectrum analyzer (OSA, YOKOGAWA AQ6375). Single wavelength operation of the laser at 2002.8 nm was obtained at a pump power of 480 mW. When the pump power increased to 1.1 W, dual-wavelength operation with a second laser wavelength at 2016.1 nm was achieved, as shown in Fig. 6(a). However, dual-wavelength operation was not stable at this pump level, especially for the long wavelength laser at 2016.1 nm. The stability of the dual-wavelength operation was improved when the pump power was increased. Figure 7(b) shows the dual-wavelength operation of the fiber laser at a pump power of 1.5 W. The SNRs at both wavelengths were larger than 50 dB. The 3-dB bandwidth at the wavelengths of 2002.8 and 2016.1 nm were measured to be 0.21 and 0.06 nm, respectively. In order to know the individual output powers corresponding to the two wavelengths, we integrated their spectra separately and calculated their individual powers. When the pump power was 1.5 W, the output powers for the two laser wavelengths were 2.4 and 0.8 mW, respectively. The difference of the output powers at the two laser wavelengths is due to their different thresholds. Dual-wavelength operation with equal output power can be achieved by tailoring the transmission spectrum of the cascaded SMS device.

 

Fig. 6 (a) Spectra of single wavelength operation of the fiber laser at a pump power of 480 mW (black line) and dual-wavelength operation at a pump power of 1.1 W (red line), and the transmission spectrum of the cascaded SMS fiber device. (b) Spectrum of stable dual-wavelength operation of the fiber laser at a pump power of 1.5W.

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Fig. 7 (a) The optical spectrum and (b) output power stability of the dual-wavelength fiber laser measured in 30 minutes.

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To investigate the stability of the output power at the two laser wavelengths, we monitored the output spectra of the fiber laser at a pump power of 1.5 W for 30 minutes. The optical spectra of the laser measured at a time interval of 5 minutes are shown in Fig. 7(a). Figure 7(b) shows the output power stability of the two lasers at 2002.8 and 2016.1 nm in 30 minutes. Our experimental results show that stable dual-wavelength operation of a Tm³⁺:Ho³⁺ co-doped fiber laser above 2 μm can be achieved by using a cascaded SMS fiber device. However, the output power of the dual-wavelength fiber laser is only 4.9 mW at a launched pump power of 2 W and the slope efficiency was measured to be around 0.3%. In addition to the relatively small gain of the Tm³⁺:Ho³⁺ co-doped fiber at the laser wavelengths, the low output power and efficiency are mainly attributed to the high cavity loss, including the large loss of the silica fiber and fiber devices at a wavelength above 2 μm and the propagation losses of the SMS fiber devices. For instance, the insertion losses of the WDM and isolator at the laser wavelengths were measured to be 0.9 and 1 dB, respectively. The propagation loss of the cascaded SMS fiber device was measured to be 8.5 and 7 dB at the two laser wavelengths of 2002.8 and 2016.1 nm, respectively, as shown in Fig. 6(a). The efficiency of the dual-wavelength fiber laser will be improved by reducing the losses of the two SMS devices through optimization of the parameters of multimode fiber (core diameter, fiber length, etc.) and using graded-index MMF as well [22, 23].

4. Conclusion

In conclusion, dual-wavelength operation of a Tm³⁺:Ho³⁺ co-doped fiber laser operating above 2 μm based on wavelength filtering effects of two cascaded SMS fiber devices has been proposed and demonstrated. One SMS fiber device with a short (4.5 cm) multimode fiber acted as a long-pass filter to suppress the competitive laser below 2 μm and the other one with a long (20.3 cm) multimode fiber worked as a band-pass filter to select the specific lasing wavelengths. A dual-wavelength fiber laser operating at 2002.8 and 2016.1 nm has been achieved with an SNR >50 dB. Stable operation of the dual-wavelength fiber laser has been verified by monitoring the individual output power at the two wavelengths for 30 minutes. The efficiency of this dual-wavelength fiber laser can be significantly improved by optimizing the SMS fiber devices.