1. Introduction

A high power single-wavelength, single-longitudinal-mode fiber ring laser operating at room temperature is attractive because of its applications in optical communication, interferometers, and sensing. Several methods were reported to produce a high power single frequency fiber laser. One of the techniques adopted was Master-Oscillator-Power-Amplifier (MOPA) configuration [1–3]. Although lasers produced high output power, application of the systems is limited due to complexity in design, lower noise performance compared to a stand-alone high power laser, and the stability. A unidirectional ring cavity configuration using erbium-doped fiber as a gain medium is suitable for developing a fiber laser, because it eliminates spatial hole-burning effect in the gain medium. However, the practical difficulty in developing a single-wavelength, single-longitudinal-mode, high power fiber laser arises due to the need for a longer gain medium (longer cavity) and this leads to instability of the laser wavelength due to smaller longitudinal-mode spacing. A few articles have been reported on high power single-frequency fiber laser in the ring cavity configuration with more than 700 mW output power [4,5]. In Ref. 4, a high power erbium-doped fiber amplifier was used inside the ring cavity to generate high output power from the laser. To eliminate mode hopping, an unpumped erbium-doped fiber was used inside the cavity as a saturable absorber [6]. In Ref. 5, an erbium-ytterbium co-doped phosphate glass fiber was used as an active medium in a unidirectional ring cavity. The active fiber was cladding pumped by four diode lasers using a contact side-pumping scheme. A tunable sub-cavity filter was used to eliminate mode hopping [7]. The filter was formed by two gratings, where the fiber separating two gratings was mounted on a PZT stretcher, and adjusted the cavity length by applying a DC voltage to the PZT. Further, a polarizing fiber (PZ- fiber) was used to ensure single-polarization mode operation of the laser. The use of a high power EDFA, saturable absorber, the contact side-pumping scheme, and PZT fiber stretcher to control the FP cavity length dynamically make systems complex and expensive.

In this article, a novel and simple medium power single-wavelength, single-longitudinal-mode fiber laser in a ring cavity configuration is demonstrated. A single-mode, polarization-maintaining, double-clad, erbium-ytterbium co-doped fiber (DC-EYDF, CorActive, Canada) of length 1 m was used as an active medium. The advantages of using a DC-EYDF are; (i) the large optical absorption cross-section and broader absorption band (910 nm −980 nm) of the ytterbium ions increase the efficiency of the pump power absorption; (ii) the double-clad structure of the fiber gives large pump wavelength guiding region and thus an inexpensive multimode laser diode can be used as a pump source; (iii) the polarization-maintaining property of the gain fiber provides better control of the polarization states of the light inside the cavity, otherwise, it can cause the instability of the laser output. To increase the longitudinal-mode spacing a Fabry-Perot filter (etalon) with large free spectral range (FSR), formed by two fused fiber couplers was used inside the cavity. The wider longitudinal-mode spacing eliminated mode hopping of the laser wavelength.

The novelty of the reported laser design are: i) use of conventional optical components; ii) no dynamic control was required to eliminate mode hopping; iii) a multimode diode laser together with a multimode fused fiber coupler (Gooch & Housego, UK) was used to pump the active fiber.

2. Results and discussion

The fiber ring laser is shown in Fig. 1 . A three port polarization independent optical circulator (CIR) guaranteed the unidirectional propagation of light waves, and the fiber polarization controller (PC) controlled the polarization state of the light inside the cavity. A fiber Bragg grating (FBG of band width, center wavelength, and reflectivity, 0.27 nm, 1570.26 nm, and 56%, respectively) selected the lasing wavelength. It is to be noted that the above grating was chosen after a series of experiments with gratings having different parameters. A Fabry-Perot filter (FP Filter) based on two 2 × 2 single-mode fused fiber couplers (split ratio 99:1) increased the effective FSR of the cavity. A DC-EYDF of length 1 m as the gain medium with core/clad dimensions, cladding ytterbium absorption at 915 nm, and numerical aperture of 9.8/134 μm, 2.9 dB/m, and 0.47, respectively. A multimode pigtailed laser diode at 976 nm (CPM-II, Alfalight, USA) was used to pump the active fiber through a multimode fused fiber coupler, which acts as a 976/1550 nm wavelength division multiplexing coupler. Output of the laser was measured simultaneously with an optical spectrum analyzer (OSA) of resolution 1.25 GHz, a scanning Fabry-Perot spectrum analyzer (SFPSA) of resolution 6.7 MHz, and a power meter. In order to protect the OSA from damage due to the high output power of the laser, 99% of the output power was fed into the power meter.

 

Fig. 1 Experimental fiber laser. OI: Polarization independent optical isolator.

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Figure 2 shows the Fabry-Perot (FP) filter formed by two fused fiber couplers [8]. The expressions for the transmitted and reflected fields are given by:

 

Fig. 2 Fabry-Perot filter formed by fused fiber couplers, where K₁ and K₂ are intensity coupling ratios. E_in, E_reflec. and E_trans are incident, reflected and transmitted electric fields, respectively. L_FP = L₁ + L₂ is the cavity length.

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The expressions for transmitted and reflected intensities of the resonator obtained from Eq. (1) are given by

To obtain the resonance phase condition for the filter, we considered the reflected intensity to be zero. Thus, from Eq. (1),

When the real part of Eq. (3) is zero, $\sin \left(\beta L_{FP}\right)=0$ or $\cos \left(\beta L_{FP}\right)=\pm 1$. Since K₁ and K₂ are both positive, only $\cos \left(\beta L_{FP}\right)=-1$ can make the imaginary part of Eq. (3) zero, when K₁ = K₂. The resonance phase condition is then,$\beta L_{FP}=\left(2m+1\right)\pi$, where β is the propagation constant, L_FP is the filter cavity length and m is an integer. The separation between two transmission peaks which corresponds to the free spectral range (FSR) of the Fabry-Perot filter is given by:

It is evident from Figs. 3(a) and 3(b) that the presence of the Fabry-Perot filter inside the cavity provided the Vernier effect [9], i.e. some of the spectral orders of the ring cavity were suppressed, giving wider separations between transmission peaks. In other words, the output of the ring cavity was modulated by the Fabry-Perot filter. Thus, the separation between the two longitudinal-mode of the cavity changed to ~514 MHz. The theoretical resonance linewidth (FWHM) of the ring cavity and the FP filter were ~13 MHz and ~2 MHz, respectively. The optimum cavity lengths determined by simulation were verified experimentally. The longitudinal mode corresponding to the lasing mode satisfied the resonance conditions for the ring cavity and the FP cavity simultaneously (i.e. the transmission peak of the FP filter exactly matched with one of the transmission peaks of the ring cavity. It is to be noted that the accuracy of the FP cavity length should be within ± 0.02 m). Therefore, fluctuations of the ambient conditions did not destroy the stability of the laser.

 

Fig. 3 Theoretical output of the (a) ring cavity (obtained from Eq. (5)) for R_FBG = 56% and L_effcetive = 4.0 m. (b) FP filter (Eq. (2)) for K₁ = K₂ = 0.99 and L_FP = 0.4 m.

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Figure 4(a) shows the output of the laser at 1570.27 nm with an optical signal to noise ratio (OSNR) of ~44 dB, which was comparable to the previously reported values [3–5], measured by the residual amplified spontaneous emission (ASE). To study the modal structure of the laser, a scanning Fabry-Perot spectrum analyzer (FSR = 2GHz) of resolution 6.7 MHz and Nuview software (developed by EXFO) were used. Figure 4(b) shows the output of the laser obtained using the scanning Fabry-Perot Spectrum analyzer. The 2 GHz (FSR of the analyzer) apart narrow peaks in the figure correspond to the laser line, and this confirmed the single longitudinal-mode oscillation of the laser. The lasing wavelength was stable and free from mode hopping.

 

Fig. 4 Output of the laser obtained using (a) OSA and (b) Scanning Fabry-Perot spectrum analyzer.

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Figure 5 shows the output of the laser at 1570.27 nm vs. input pump power at 976 nm (The nonlinear behavior of the output could be due to the variation of pump wavelength with input pump current). The threshold pump power of the laser was 345 mW and a slope efficiency of ~7% was achieved. The maximum cavity loss was at the fusion splices between the DC-EYDF and the single-mode fiber. The slope efficiency can be increased by improving the quality of the fusion splices. The maximum output power of the laser was 250 mW due to the limited power handling capacity of the circulator (500 mW). The laser showed the tendency to oscillate in multimode beyond 250 mW output power. The threshold pump power for multimode oscillation was ~4 W. We could eliminate the extra mode from oscillation by adjusting the polarization controller plates. Figure 6 shows the output of the laser prior to the adjustment of the polarization controller. In Fig. 6, the small peak corresponds to the second longitudinal-mode of the laser cavity (~514 MHz apart). The polarization-maintaining property of the active fiber helped to control the polarization state of the wave inside the cavity. It may be possible to produce a linearly polarized output by using polarization-dependent optical components.

 

Fig. 5 Input-Output characteristics of the laser at 1570.27 nm. Length of the active fiber 1 m.

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Fig. 6 Output of the laser oscillated in multimode, prior to adjusting the polarization controller plates.

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The application of an unpumped erbium-doped fiber as a saturable absorber is a well established technique to reduce mode hopping form environmental fluctuations such as temperature variation. The above experiment was repeated with an unpumped erbium-doped fiber as a saturable absorber instead of the FP filter inside the cavity. Though the laser was free from mode hopping, the output power of the laser was low (<100 mW for the same input pump power as the laser reported in this article) due to the presence of the saturable absorber which produced high insertion loss. When the saturable absorber and FP filter were removed the laser produced high output power but suffered from severe mode hopping. The presence of a FP filter eliminated the mode hopping by increasing the longitudinal-mode spacing and keeping the high output power.

The 3-dB linewidth (FWHM = Full width at half maximum) of the lasing wavelength was ≈12 MHz, measured using a scanning Fabry-Perot spectrum analyzer (FSR = 2GHz) of resolution 6.7 MHz and Nuview software, developed by EXFO. The laser linewidth was very large compared to the theoretical value based on Schawlow-Townes formula [10]. Recent investigation shows that the wider linewidth in the erbium-ytterbium co-doped fiber laser is due to the temperature fluctuations induced by the pump intensity noise inside the core of the fiber [11,12]. We believe this might be the cause of broader linewidth of our laser.

3. Conclusions

A medium power single-wavelength single-mode fiber laser is reported. We achieved single-longitudinal mode oscillation by using an all fiber Fabry-Perot filter in the cavity. The laser output was modulated by the output of the filter. A polarization-maintaining, erbium-ytterbium co-doped fiber was used as the gain medium to increase pump power absorption efficiency. The high pump power increased the temperature of the core and this lead to fluctuation of polarization. The polarization-maintaining property of the active fiber provided better polarization control of the wave inside the cavity and increased the stability of the laser wavelength. The maximum output power of the laser was 250 mW.