1. Introduction

Tunable narrow-linewidth single frequency fiber lasers have attracted great interest due to such promising applications as digital coherent communications, wavelength-division-multiplexing networks, optical sensors, spectroscopy and fiber-optic gyroscope [1–7]. Especially in the application of high-order orthogonal frequency-division multiplexing (OFDM) optical communication, the square quadrature amplitude modulation (QAM) formats require transmitting laser source with narrow linewidth and low frequency noise due to the larger necessary block lengths [8]. Also polarization stability is crucial in communication systems and relative intensity noise (RIN) imposes limitations on receiver sensitivity and bit error rate (BER). For example, in order to obtain a 40 Gbit/s optical system with BER lower than 10⁻⁴, the laser linewidth must be in the range of 1 kHz for square 64 QAM. However, these requirements cannot be fulfilled with current commercial available lasers [9].

Currently, tunable semiconductor laser is a popular commercial laser for its compact structure and stable properties. However, the eigen linewidth of semiconductor laser is wide and current techniques to suppress its linewidth such as stabilizing external cavity have drawbacks like the narrowest linewidth which can be achieved to date is in an order of kHz. For example, a 5 kHz linewidth tunable external cavity diode laser at 1.55μm has been reported by Bennetts et al in 2014, which is the best one of semiconductor tunable laser at present [10]. As for fiber laser, fiber ring cavity is a general structure for tunable laser for its attractive features including narrow linewidths and easy wavelength tunability, however this kind of fiber laser are prone to be sensitive to environment due to long cavity length, and always suffers from a higher laser noise and mode-hopping phenomena [11–15]. Short-cavity single-frequency fiber laser (SSFL) has been recognized as an attractive laser source with narrow linewidth, low noise and compact all-fiber structure [16]. Short-cavity length provides a wide space of longitude mode, and it contributes to generate stable single-longitude-mode laser [17]. Nevertheless, short-cavity configuration for C-band tunable fiber lasers is scarcely reported because of tuning difficulty. It will be an important challenge to solve the tuning method because it is hard to insert a tuning device in such a short cavity.

In this letter, we demonstrate a low noise single-frequency tunable fiber laser with ultra-narrow linewidth, which is based on the short-cavity self-injection locking scheme. The single-frequency laser could be flexibly tuned by injecting the tunable filtered light into a short-cavity. The laser linewidth of < 700 Hz and the measured relative intensity noise (RIN) of < −130 dB/Hz at frequencies of over 3 MHz are obtained based on self-injection locking [21,22].

2. Experimental setup

The schematic drawing of the self-injection locked tunable fiber laser is shown in Fig. 1.The laser cavity is constructed by fusion splicing a high reflectivity fiber chirped grating (HR-FCG) and a polarization maintaining low reflectivity fiber chirped grating (LR-FCG) on both endfaces of a 1.5-cm-long highly Er³⁺/Yb³⁺ co-doped phosphate fiber, respectively. The reflectivity of the LR-FCG is 60% at C-band, while that of the HR-FCG is over 99.9%. The bandwidth of LR-FCG and HR-FCG are both 60 nm, ranging from 1515 nm to 1575 nm. The whole cavity is temperature controlled by a cooling system with a resolution of 0.05 °C. The cavity is pumped by a 980 nm laser diode (LD) via a wavelength division multiplexer (WDM), and the laser signal is coupled out from the LR-FCG and the WDM. Then a 10/90 coupler is employed to split 90% of the laser power to the port 2 of an optical circulator (OC), while the remainder is outputted via an isolator (ISO). The port 3 of OC is connected by an Er³⁺/Yb³⁺ doped fiber amplifier (EDFA), which is used to amplify the source light of DBR fiber laser. The amplified laser light is then filtered by the fiber Fabry-Perot interferometer (FFP) with a free spectral range (FSR) of about 0.2 nm and a bandwidth of 5 pm. And an 88.9 nm-FSR fiber Fabry-Perot tunable filter (FFP-TF) with a bandwidth of 0.164 nm is applied to ensure only one transmission peak in the C-band and in the gain spectrum, facilitating the single longitudinal-mode operation of the laser. FFP is fixed and only the FFP-TF needs to be adjusted. A polarization controller (PC) is inserted between EDFA1 and FFP to adjust the polarization state of the laser. The filtered light is amplified by EDFA2 and injected back into the cavity. The gain of EDFA1 and EDFA2 are both 25 dB. Thus the LR-FCG in the laser cavity together with the OC ring form an external tunable resonator.

 

Fig. 1 Experimental setup of the self-injection locked tunable single-frequency fiber laser.

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3. Results and discussion

The lasing wavelength is tuned without spectral distortion under a 500-mA driven current of LD. And the laser spectrum at one of the tunable wavelength (1543.72 nm) is recorded with a spectrum resolution of 0.1 nm by OSA. The measured side mode suppression ratio (SMSR) is larger than of 62.0 dB. The single-frequency characteristics were confirmed by the scanning Fabry–Perot interferometer (SA210-9A), as shown in the inset of Fig. 2(a).

 

Fig. 2 (a) Laser spectrum of the fiber laser at 1543.72 nm which is one of the tunable wavelength. Inset is the longitudinal modes characteristics of the fiber laser measured by the scanning Fabry–Perot interferometer. (b) Superimposed spectra of the tunable single-frequency fiber laser at different wavelengths. (c) Different wavelengths in response to the change of the FFP-TF actuation voltage (red) and output power changes along with wavelength (blue). (d) Spectra of the tunable fiber laser taken at 30-min interval over 2 hours. Inset is power variance of the laser output over 2 hours.

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Figure 2(b) shows different wavelengths laser spectra obtained by changing the actuation voltage of FFP-TF. The wavelength can be continuously tuned from 1527 to 1563 nm, which is limited by the Er³⁺/Yb³⁺ doped fiber (EDF) gain spectral region and the bandwidth of LR-FCG. The spectra quality degraded comparatively at the wavelength around the margin of EDF gain spectrum, where the SMSR is in the range of 50-55 dB, due to the decrease of the optical gain. It is believed that a higher SMSR of the laser can be realized by inserting a gain flattening filter (GFF) between the WDM and coupler to provide the same value of gain coefficient for the full C-band. The tuning precision is determined by the FSR of FFP, which is 0.2 nm in this experiment. Figure 2(c) demonstrates the change of the FFP-TF actuation voltage from 12.5 to 22.0V is in response to full C-band wavelengths. The wavelength tuning has a roughly linear relation with the decreasing of the actuation voltage across the whole tuning range. The output power can be relatively constant at 2.4 mW at the wavelength from 1534 to 1560 nm by adjusting the PC. And the output power decreases at the wavelength around the margin of EDF gain spectrum because of the low optical gain, as shown in the blue curve in the Fig. 2(c). The spectra were measured at a 30-min interval over 2 hours as shown in the Fig. 2(d) displaying a stable operation. The output power fluctuations of < ± 0.05 dB over 2 hours is shown in the inset of Fig. 2(d). Finally, it is believed that the power stability could be further optimized by changing all the devices in the feedback loop to polarization-maintaining type.

The linewidths for full C-band wavelengths are measured by the self-heterodyne method involving a 48.8-km fiber delayed Mach-Zehnder interferometer and 40-MHz fiber-coupled acousto-optical modulator (AOM). The linewidth measurement results for arbitrarily selected wavelengths are almost the same, as shown in Fig. 3(a), and inset shows the linewidths at three representative wavelengths of 1540.56 nm, 1550.12 nm, and 1560.61 nm. The laser linewidth was measured to be less than 700 Hz, indicating that a compressing effect was realized by self-injection locking the laser to an external resonator with high-quality (Q) factors [18]. The laser frequency noise was measured by a fiber Michelson interferometer with 100 m optical path difference and an optical phase demodulator (OPD-4000). The measured power spectrum of frequency noise from 0 to 25 kHz, in which the term “dB re Hz/Hz^1/2” denotes the logarithmic computing of the noise amplitude. As shown in Fig. 3(b), the laser frequency noise is less than 40 dB re Hz/Hz^1/2 at low frequencies (<100 Hz) and reaches −5 dB re Hz/Hz^1/2 at around 25 kHz, which indicates a considerable low frequency noise level. We further evaluated the linewidth of the fiber laser by exploiting the estimation line ${\nu }_{rms}^2=f/\pi$ in the measured frequency noise spectrum (where v_rms and f denote the noise amplitude and electrical frequency, respectively) [19], as shown in Fig. 3(b). In principle, the electrical frequency of the intersection between noise spectrum and estimation line could estimate an approximation of linewidth, given that the noise spectrum is reasonably frequency independent around the intersection [20]. The obtained value of the Lorentz linewidth is about 650 Hz, indicates a good agreement with the heterodyne measurement.

 

Fig. 3 (a) Linewidths of the tunable fiber laser at arbitrarily selected wavelengths. Inset are three typical linewidths at 1540.56 nm, 1550.12 nm, and 1560.61 nm. (b) Measured frequency noise power spectrum of the fiber laser and estimation line for evaluation of linewidth.

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The RIN in full tuning range is measured by an InGaAs PD with the bandwidth of 150 MHz and using bandwidth resolution 2 kHz of the electrical spectrum analyzer. Figure 4 illustrates the RIN performance of the laser from 0 to 20 MHz, and results comparison shows that the laser RIN are basically the same across the tuning range. It is observed in the figure that a series of gradually weakened harmonic peaks arise in the noise spectra, with the harmonic frequency of multiple of 2.8 MHz, and this is attributed to the recursion dynamics of the laser light in the self-injection locked laser system, and more interpretation about the RIN harmonic peaks for this kind of fiber laser has been reported in [21]. Additionally, the first peak of −120 dB/Hz at the frequencies of 2.3 MHz is in fact the relaxation oscillation peak of the laser. The RIN is less than −130 dB/Hz for frequencies over 3 MHz, which meets the application requirement of optical communication with medium haul.

 

Fig. 4 RIN from 0 to 20 MHz of the single-frequency tunable fiber laser base on the short-cavity self-injection locking. The inset is SOP of the fiber laser (red dot) represented by a Poincare sphere.

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The state of polarization (SOP) of the fiber laser was measured using an optical polarization analyzer, as shown in the inset of Fig. 4, confirming the stable linear polarization of the laser output. The degree of polarization (DOP) is measured to be 99.85%, which gives a linear polarization extinction ratio (LPER) of >28 dB, indicating a fully linearly polarized laser operation and it greatly exceeds the application requirement of optical specifications for ultra-long haul.

4. Conclusion

In conclusion, by injecting the narrow-band FFP and FFP-TF filtered light into a short-cavity, an ultra-narrow linewidth tunable single-frequency linear-polarization fiber laser has been demonstrated. The laser has a wavelength tuning range from 1527 to 1563 nm, while the linewidth is less than 700 Hz. The best result of SMSR is larger than 62.0 dB. The laser frequency noise less than 40 dB re Hz/Hz^1/2 at low frequencies (<100 Hz) and reaches −5 dB re Hz/Hz^1/2 at around 25 kHz. The measured RIN is less than −130 dB/Hz with regard to frequencies of over 3 MHz, while the obtained LPER is higher than 28 dB. This ultra-narrow linewidth low-noise tunable single-frequency linear-polarization fiber laser is suitable for advanced applications of high-order OFDM optical communication.