Numerous applications in laser absorption spectroscopy [1], remote sensing [2], atom cooling [3], isotope separation [4], gravitational wave detection [5], and coherent communication systems [6] rely on single-frequency lasers, i.e., lasers operating in a single longitudinal mode. Distributed feedback (DFB) laser diodes, distributed Bragg reflector (DBR) laser diodes, fiber lasers with a short resonator formed by a pair of narrowband fiber Bragg gratings [7], fiber lasers with a $\pi$-phase shifted fiber Bragg grating [8], and microchip solid-state lasers [9] are commonly used if the frequency is either fixed or slightly adjustable. Broadly tunable single-frequency lasers are necessary in swept-frequency optical coherence tomography [10], optical vector network analyzers [11], and difference frequency generators [12]. Very broad tunability is achieved with external-cavity laser diodes covering a range of 1500–1600 nm, but significantly smaller tunability is obtained around a wavelength of 1060 nm. A promising approach is based on vertical cavity surface emitting lasers with micro-electromechanical mirrors, which can cover a 100 nm band around a wavelength of 1060 nm [13]. Broadly tunable single-frequency fiber lasers rely on a narrowband wavelength selective filter. The fiber laser resonator is usually long and the longitudinal mode pitch is small, and, therefore, it is not easy to achieve a single-frequency regime. A pair of Fabry–Perot etalons (FPEs) with large (4020 GHz) and small (10.2 GHz) free spectral ranges was used to provide sufficient wavelength selectivity of a fiber ring laser [14]. Two optical isolators were necessary to suppress unwanted reflections from the etalons in such a fiber ring. The Vernier effect in a compound fiber resonator is another approach [15]. A tracking filter based on saturable absorption in the unpumped segment of a doped fiber was used to ensure the single-frequency operation while providing tuning over a spectral range of 1.7 nm with a step of 50 GHz [16]. The comb transmission spectrum of a loop mirror consisting of a polarization controller and polarization-maintaining fiber made it possible to achieve a single-frequency regime with a laser linewidth of 2 kHz and relatively coarse tuning step of 1 nm over a spectral range of 17.5 nm [17].

In this Letter, we report on a broadly tunable single-frequency fiber ring laser in which a fiber ring resonator filter (FRRF) is used to provide additional wavelength selectivity required for achieving the single-frequency regime. Its scheme is shown in Fig. 1. It is a unidirectional fiber ring laser based on the ytterbium-doped fiber (Nufern) pumped by a fiber-pigtailed laser diode wavelength stabilized at 976 nm. The pump power is coupled into the core of the active fiber by a hybrid component that integrates functions of the wavelength division multiplexer, optical isolator, and 10% output coupler (Optosun Technology). Such an approach allows us to build a short and stable resonator with low intrinsic losses. All fibers and components used in the laser are polarization maintaining. The wavelength of the fiber laser is determined by a narrowband, computer-controlled grating filter (WL Photonics) with a bandwidth of 0.3 nm, which is tunable in a spectral range from 1023–1112 nm. However, the bandwidth of the grating filter is too large for the laser to achieve the single-frequency regime. The fiber ring laser with such a grating filter operates in several adjacent longitudinal modes with random phases, which manifests itself in fluctuating output power. For the pump power of 300 mW, which is the maximum allowable power for the optical hybrid component, the laser operates in the whole tuning range of the grating filter, as can be seen in Fig. 2. We converted that few-mode laser into a single-longitudinal-mode laser by inserting a FRRF (Fig. 1). The FRRF is a fiber analogy of the FPE that was used for achieving the single-frequency regime by Vahala et al. [14]. Its main advantage over the FPE is the absence of reflections. These reflections should be otherwise blocked by another optical isolator to preclude the formation of standing waves and dynamic gratings in the active fiber. Adding another isolator increases spectrally dependent losses in the resonator and limits the tunability range of the laser. FRRF was already used in an erbium-doped fiber laser by Gloag et al. [18]. These authors used low-finesse FRRF made of 50% fiber couplers and demonstrated the single-longitudinal-mode operation at a fixed wavelength.

 

Fig. 1. Setup of the fiber laser. LD, pump laser diode; YDF, ytterbium-doped fiber; PMTIWDM, hybrid component integrating polarizing isolator, output coupler, and wavelength division multiplexer; VDL, piezo-driven variable delay line; TF, tunable grating filter; C1,2, couplers; PD, photodetector; LPF, low-pass filter; PID, proportional-integral-derivative controller; VCO, voltage-controlled oscillator; FRRF, fiber ring resonator filter.

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Fig. 2. Average output power of the fiber ring laser (blue) and fiber ring laser with FRRF (red) at a pump power of 300 mW.

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The transmission function of the FRRF can be derived as

 

Fig. 3. Peak transmission of the FRRF versus bar-coupling coefficients $R_{\mathrm{1,2}}$ of couplers C1,2.

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Fig. 4. Instantaneous frequency of the laser with open (blue) and closed (red) feedback loop.

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For the evaluation of the laser linewidth, a coherent delayed self-heterodyne interferometer (DSHI), shown in Fig. 5, was used. Coherent DSHI was selected because it is difficult to achieve interferometer delay ${\tau }_D$ several times longer than the coherence time of the laser for a wavelength of 1060 nm, at which the fiber losses are far away from its minimum, and the interferometer input power is limited by the stimulated Brillouin scattering. The laser signal was amplified in the semiconductor optical amplifier (SOA) and split into the short and long branches of the interferometer by a polarization-maintaining coupler. The long branch was made of a standard nonpolarization-maintaining fiber. Its residual birefringence was compensated for in a double-pass configuration with a Faraday mirror (FM) and polarization beam splitter (PBS). A fiber pigtailed acousto-optical frequency shifter (AOFS) was employed to shift the beatings from the baseband to avoid the $1/f$ noise of detection circuits. It shifts the frequency by 150 MHz for each pass of the signal. Signals from the short and long branches are combined in the polarization-maintaining coupler, detected by a detector (MenloSystems FPD310) and analyzed by a radio frequency spectrum analyzer (Rohde Schwarz FSU). A frequency-noise spectrum model [19],

 

Fig. 5. Phase noise measurement setup. LUT, laser under test; SOA, semiconductor optical amplifier; PBS, polarization beam splitter; AOFS, acousto-optical frequency shifter; Rf-gen, radio-frequency generator; FM, Faraday mirror; D, detector; FSA, frequency spectrum analyzer.

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DSHI spectra for the laser with opened loop are shown in Figs. 6(a) and 6(b) with a blue line for delays of 6.91 μs and 57.3 μs, respectively. We measured the spectra with a resolution bandwidth ${\delta }_{\mathrm{FSA}}$ of 2 kHz, and a video bandwidth of 100 Hz. Red curves show results of fitting for white component $\gamma /2\pi =600\mathrm{Hz}$. The $1/f$ component of frequency noise was set to zero in Eq. (3), $\xi =0$. The model underestimates the frequency noise of the laser for short delays [Fig. 6(a)] and overestimates it for long delays [Fig. 6(b)]. This is another confirmation that frequency noise of the fiber laser is not white in nature. A proper model should include low-frequency noise, arising from pump-noise-induced temperature fluctuations [21]. The frequency noise measurements of the FRRF laser were compared with measurements of a commercial, semiconductor external-cavity laser (CTL 1050, Toptica), as shown in Fig. 7(a) and Fig. 7(b) with a blue line. The red curve shows results of fitting with a white component $\gamma /2\pi$ of 600 Hz and $1/f$ component $\sqrt{\xi }/2\pi$ of 45 kHz. We already achieved an incoherent DSHI regime in Fig. 7(b). Spectra measured with short and long delays are consistently described by the model.

 

Fig. 6. Delayed self-heterodyne interferometer spectra measured for YDFL-FRRF with a delay of (a) 6.91 μs and (b) 57.3 μs.

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Fig. 7. Delayed self-heterodyne interferometer spectra measured for the Toptica laser with a delay of (a) 6.91 μs and (b) 57.3 μs.

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When the feedback loop in FRRF laser is closed, the linewidth increases. We optimized the amplitude and frequency of the modulating signal in order to simultaneously achieve the mode-hop-free regime and narrow linewidth. The DSHI spectrum of the laser measured with the delay of 57.3 μs can be seen in Fig. 8. The linewidth is broadened by a deterministic signal, and the frequency noise model given by Eq. (2) is not applicable. We therefore estimate the laser linewidth from a Gaussian fit of the central peak of the DSHI spectrum. The DSHI peak has the full bandwidth at half-maximum bandwidth of 23.3 kHz, and the laser linewidth is half this value, 11.7 kHz. It can be increased to several hundreds of kilohertz without destabilizing laser simply by changing the modulation signal amplitude.

 

Fig. 8. Interferometer spectrum measured for the FRRF laser with closed feedback loop employing DSHI with a delay of μs.

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In conclusion, we presented a single-frequency ytterbium-doped fiber laser with a FRRF, tunable in a broad spectral range of 1023 nm to 1107 nm. The broadband tuning is made possible by low losses of the resonator. The laser wavelength is determined by a grating filter, and the single frequency operation is enforced by using a fiber ring resonance filter. The frequency noise of the laser was determined by the DSHI method and compared to a commercial, continuously tunable semiconductor laser from Toptica. The white noise component was estimated to be the same in both lasers with a value of $600{\mathrm{Hz}}^2/\mathrm{Hz}$. The main difference is in the $1/f$ noise component, which dominates in the Toptica laser while it is negligible in the fiber laser. However, our fiber laser lacks the continuous tunability of a semiconductor laser. In the closed-loop operation, the mode hopping of the fiber laser is prevented, and the linewidth increases to 11.7 kHz. Such lasers may find use in combination with mode-hop-free tunable lasers in difference frequency generators for infrared laser absorption spectroscopy.