Narrow-linewidth multi-wavelength fiber lasers are of significant interests for fiber-optic sensors, spectroscopy, optical communications, and microwave generation. A novel narrow-linewidth dual-wavelength random fiber laser with single-mode operation, based on the semiconductor optical amplifier (SOA) gain, is achieved in this work for the first time, to the best of our knowledge. A simplified theoretical model is established to characterize such kind of random fiber laser. The inhomogeneous gain in SOA mitigates the mode competition significantly and alleviates the laser instability, which are frequently encountered in multi-wavelength fiber lasers with Erbium-doped fiber gain. The enhanced random distributed feedback from a 5km non-uniform fiber provides coherent feedback, acting as mode selection element to ensure single-mode operation with narrow linewidth of ~1kHz. The laser noises are also comprehensively investigated and studied, showing the improvements of the proposed random fiber laser with suppressed intensity and frequency noises.
© 2017 Optical Society of America
Multiwavelength single longitudinal mode (SLM) fiber lasers have drawn considerable research interests for applications in optical communications, optical fiber sensing, modern instrumentation, and microwave photonic systems [1–9]. Particularly, dual-wavelength lasers are of significant importance for generation of beat signals at microwave frequencies or THz for electronic signal processing systems , generation of high bit rate soliton pulse trains , and differential absorption measurement of trace gases . Various gain mechanisms have been implemented in the dual-wavelength fiber lasers, including erbium doped fiber amplifier (EDFA), semiconductor optical amplifier (SOA), stimulated Raman scattering (SRS), and hybrid gain mechanism. However, Raman fiber lasers suffer from high pump power consumption and a very long cavity length is needed in order to trigger the SRS with lower power [13,14]. While dual-wavelength erbium doped fiber lasers have poor performance in lasing stability at room temperature due to the strong homogeneous gain broadening and cross-gain saturation, especially for dual-wavelength lasing with small wavelength spacing. Approaches to making the EDF inhomogeneous, such as cooling the EDF down to the cryogenic temperatures , using hybrid gain medium , using a frequency shifter within the laser cavity , exploiting the polarization-hole-burning effect of the PM fiber , and tunable comb filter , are not fully practically applicable. Therefore inhomogeneous broadening gain materials, such as SOA, could be an excellent alternative to generate stable multi-wavelength lasing at room temperature [17,18]. Narrow-linewidth SLM operation in dual-wavelength fiber lasers is also vital for microwave generation with low phase noise. Many ultranarrow mode selecting mechanisms have been proposed, including utilizations of saturable absorbers , phase-shifted fiber Bragg gratings (FBGs) and regular FBGs [20,21], multiple ring-cavities , Fabry-Perot like filters [22,23]. These techniques, however, show some design complications and lasing instability due to the highly sensitive fiber devices.
Random fiber lasers have been an important subject of scientific interest due to their unusual properties and promising applications. They take advantages of the optical gain and random feedback mechanisms that occur in one-dimensional optical fiber waveguide, being a solution that has successfully addressed the confinement and directionality issues encountered in traditional random lasers in bulk materials. The reported configurations of random fiber lasers could be mainly divided into two categories based on EDF gain, Brillouin gain or Raman gain. One is the linear open cavity configuration with active EDF or Brillouin/Raman-amplifying passive fiber, which meanwhile act as random feedback medium [24–29]. In this configuration, the optical gain and the random feedback are from the same fiber. The other one is the fiber ring laser structure with a random medium spatially separated from the gain fiber [30–34], which is similar to a random medium as an injection to a fiber ring laser. The random feedback in most random fiber lasers is from the Rayleigh scattering in optical fibers, which has been demonstrated as an effective mechanism to compress the linewidth of the fiber ring laser [35–38]. Thus random fiber lasers with prominent merits such as single-mode operation and narrow linewidth could be realized for applications in high-resolution spectrometers, coherent light sources, and microwave photonic systems. Compared with conventional semiconductor lasers with fixed cavities and strong feedback, random fiber lasers usually have higher threshold and lower lasing efficiency, which is mainly due to its open cavity feature with less stronger optical feedback. The lasing stability of random fiber laser is also compromised due to the severe mode competition and mode hopping effects induced by the large number of random modes. However, the properties of random fiber laser could be improved through various techniques, such as utilizing special fiber with enhanced Rayleigh scattering feedback , manually modifying standard single-mode fiber with enhanced feedback and fewer random modes , insertion of narrow-bandwidth filter for frequency selection  and so on. Recently multi-wavelength random fiber lasers have drawn intensive attention and turned to be a promising light source with simple design [42–48]. An ultra-narrow linewidth dual-wavelength fiber ring laser with Rayleigh distributed feedback was realized with a dual-wavelength lasing spacing of ~20nm . However, the proposed dual-wavelength laser is not a truly random fiber laser as two fixed ring cavities were formed by the two FBGs. SLM operation was realized by adjusting the cavity loss using an attenuator and side modes were still observed when increasing the pump power. The linewidth measurement is inaccurate as 50km delay line in delayed self-heterodyne method limits the linewidth measurement accuracy to 4kHz.
In this paper, we propose and demonstrate a novel stable dual-wavelength random fiber laser with narrow-linewidth single-mode operation based on SOA gain at room temperature. The inhomogeneous gain in SOA significantly mitigates the mode competition which is a common problem in multi-wavelength erbium doped fiber lasers and ensures stable dual-wavelength lasing. Our theory demonstrates that such kind of random fiber laser has high probability of single-mode operation with narrow linewidth due to the coherent Rayleigh scattering. With a 5km non-uniform fiber providing enhanced coherent random distributed feedback in the ring cavity, the single-mode operation with a 3dB linewidth of ~1kHz is achieved. The intensity and frequency noises of such a dual-wavelength random fiber laser are investigated and discussed comparing with the EDF-based random laser for the first time to the best of our knowledge.
2. Experimental setup
The configuration of the SOA-based dual-wavelength random fiber laser is shown in Fig. 1. Two SOAs (QSOA-1550, QPhotonics LLC) pumped by electric currents both provide a broadband gain spectrum centered at 1534nm with a spectral range of 40nm. Each SOA has a maximum gain of 25dB for a small input signal (~10μW). The two SOAs compensate the large loss induced by the open cavity and weak Rayleigh scattering feedback in the ring resonator. An enhanced Rayleigh feedback is provided by a 5km non-uniform fiber with the mode field diameter from 5μm at one end to 7μm at the other end and consequently with a stronger refractive index inhomogeneity along the fiber axis . It has been demonstrated that the Rayleigh scattering coefficient is enhanced to ~-34dB/km, which is 2 orders higher than standard telecom single mode fibers . An isolator is connected at the output end of the non-uniform fiber in order to eliminate the light reflection from the air/fiber interface. Two FBGs with Bragg wavelengths of 1541.892nm and 1544.948nm and a bandwidth of 0.04nm act as optical filters to select the two lasing wavelengths. A polarization controller (PC) varies the state of polarization of the intra-cavity light so as to adjust cavity loss and improve lasing efficiency. The lasing output is detected through the 5% port of a 95/5 optical coupler.
3. Theoretical model
In the proposed laser system, the SOA gain is tailored by the inserted FBG with a specific profile. The 5km non-uniform fiber acts as random distributed feedback medium and provides elastic Rayleigh scattering feedback, which preserves the phase coherence between the incoming and scattered light waves, thus leading to coherent random feedback with constructive interference. The random lasing builds up through the combination of the coherent Rayleigh scattering in the non-uniform fiber, round-trip amplification by the SOA, and filtering effect of the FBG. The resonant lasing spike would mainly emerge at frequency regions with highest optical gain determined by the un-flattened gain profile through the FBG filtering effect. Note that the intra-cavity light in the lasing system experiences the coherent Rayleigh backscattering from different ensembles of scattering centers in the non-uniform fiber every round-trip during the lasing build-up process. Hence the effective laser cavity length is not constant for the random fiber resonator every round-trip, which physically means that the photons recaptured in the open cavity follow complex and various trajectories, while the phase correlation among them results in multiple and complex interference peaks. Therefore, non-periodic interference patterns and randomly resonant spikes in the lasing spectra are expected due to the random distribution of the optical path length for arbitrary backscattered lights. If the polarization effect and the phase noise terms are omitted, the lasing output from the proposed random fiber laser could be expressed as
Figure 2 shows the simulated interference patterns of the proposed random fiber laser and the corresponding lasing spectra during the lasing build-up process using Eq. (1) for the lasing characteristics. As expected, non-periodical interference patterns are generated due to the field superposition of multiple backscattered light waves with different optical path lengths. When those backscattered lights are amplified by optical gain with un-flattened profile, which is determined by the FBG filters with a bandwidth of ~5GHz in our setup, resonant lasing spikes would be observed in the frequency region where the gain could compensate the cavity loss. Figure 2 also illustrates the evolution of the lasing spectrum during the lasing build-up process. The spectral envelope tends to be narrowed with the increased round-trips. This could be justified qualitatively through the analysis of the coherence evolution of the intra-cavity light, i.e. a process of coherence enhancement of the intra-cavity light through the coherent Rayleigh backscattering and un-flattened gain amplification. As the original light seed is the ASE from the SOA with relatively low coherence, during the initial round-trips, only short section of the non-uniform fiber near the input end would provide effective coherent Rayleigh feedback, i.e. the Rayleigh backscattered lights scattered within that section can be constructively interfered. This is due to not only the limited coherence length of the initial light seed, but also stronger reflection at section near the input end of the non-uniform fiber as light intensity attenuates when propagating along the fiber. The backscattered lights after the coherence length would only contribute incoherent feedback to the lasing system as their phase correlation with the backscattered lights from within the coherence length is lost. The lucky photons that are coherently scattered back will be recaptured and amplified by the gain medium, leading to amplified intra-cavity light with enhanced coherence. Consequently, as the number of round-trip times increases, longer section of the non-uniform fiber with more scattering centers would provide coherent Rayleigh feedback. As resonances preferably occur at frequency positions where gain overcomes the loss, the increasing number of scattering centers every round-trip is of a much higher probability to provide coherent feedback to enhance the existing resonances. Thus through competition for gain and provided that an un-flattened gain profile is adopted for amplification, lasing with narrow bandwidth is expected.
4. Experimental results and discussion
The lasing threshold was measured by connecting the laser output with the power meter. During the measurements, the driven current of SOA 1 was fixed at the normal operating current of 300mA. The laser output power was measured as a function of SOA 2’s driven current as shown in Fig. 3(a). Below lasing threshold, the output power is very low as the loss is larger than the gain in the ring cavity. The laser output suddenly rises up as the driven current increases to 135mA and remains linearly increased with the driven current. Note that lasing at 1541.892nm is firstly triggered as it is nearer to the peak wavelength of the SOA gain spectrum with higher optical gain. Further increasing the driven current to 235mA triggers the lasing at 1544.948nm, which is clearly indicated in Fig. 3(a) in the form of two curves with different slopes, one for single-wavelength lasing and the other for dual-wavelength lasing. The spectra of route to lasing are illustrated in Fig. 3(b) by measuring the laser output with an optical spectrum analyzer (OSA), where two lasing thresholds are found for single- and dual-wavelength lasing.
The lasing stability of the proposed dual-wavelength random fiber laser was also investigated in the experiments. Both the lasing wavelength stability and lasing output power stability were measured as shown in Fig. 4. The optical spectra of the dual-wavelength random fiber laser were recorded every one minute using the OSA with a resolution of 0.02nm. Figure 4(a) shows the lasing wavelength stability when the driven currents of the two SOAs are 300mA. It is illustrated that both lasing wavelengths experienced little spectral shifts during the measurement time and are relatively stable in the lab environment. Lasing line 2 (1544.948nm) suffers a more pronounced spectral shift within a range of 0.016nm, which may be mainly due to the more significant mode competition and the resultant spectral variation because of lower gain provided by the SOA at that wavelength. Compared with the known dual-wavelength SOA-based fiber laser with a maximum wavelength variation of 0.024nm , our proposed random fiber laser has a slightly smaller wavelength shift and comparable spectral stability. The stability of lasing output power is shown in Fig. 4(b), where the lasing output powers of SOA- and EDF-based dual-wavelength random fiber lasers are measured every one minute. The EDF-based dual-wavelength random fiber laser is constructed by simply replacing the SOAs in the setup with an EDFA and operated well above the lasing threshold. It is noted that compared with EDF-based dual-wavelength laser, which suffers a huge power fluctuation of up to 112.7%, the output power of the SOA-based dual-wavelength laser is more stable at both wavelengths with a power fluctuation of only 13.1%. This indicates that the inhomogeneous gain in SOA suppresses the mode competition between different lasing wavelengths; whereas the homogeneous property of the EDF gain intensifies this competition for gain, especially for relatively narrowly-spaced multi-wavelength lasing. It is noted that the proposed fiber random laser has a higher lasing power fluctuation than that (estimated to be ~4%) in ref , which is probably due to the intensified mode competition effect induced by the large number of random modes in the Rayleigh scattering based random fiber lasers. This problem can be solved by utilizing random feedback medium with reduced number of random modes such as fiber random grating .
The linewidth of the proposed SOA-based dual-wavelength random fiber laser was measured by a conventional delayed self-heterodyne (DSH) method. The DSH technique was employed by connecting the lasing output with the input of (C) in Fig. 1. The laser output was divided into two parts by a 95/5 coupler, with the larger part launched into the fiber delay line and the smaller part through an AOM, which downshifted the optical frequency of laser light by 40 MHz. A photo detector (PDB130C, Thorlabs) was put after the recombination of the two light beams to detect the beat spectrum centered at 40 MHz by using an electrical spectrum analyzer (ESA) (E4446A, Agilent). The length of the delay fiber is selected with 200 km in the linewidth measurement to ensure that the two light beams are uncorrelated with each other when recombined and obtain an accurate Lorentzian line shape measurement. The experimental results for both lasing lines are given in Fig. 5 where the linewidth profiles are acquired by averaging over 20 beat spectra to remove undesired noises. It is illustrated that the 20-dB linewidths of the two lasing lines are 19.0 ± 0.6 kHz and 25.8 ± 0.8 kHz, respectively, which correspond to 0.95 ± 0.03 kHz and 1.29 ± 0.04 kHz for 3-dB linewidth. The inset figures in Fig. 5 show the radio frequency spectra of the DSH technique with 10MHz span for both lasing lines. Only single lasing peak is observed for both lasing wavelengths with a contrast around 40dB, indicating effective suppression of cavity modes by the distributed Rayleigh feedback in the open ring cavity. The linewidth measurement was also conducted for the EDF-based dual-wavelength random fiber laser. The results show similar linewidth values to that of the SOA-based random fiber laser when the EDF-based random fiber laser was operating relatively stable. However, the strong mode competition induced power fluctuation makes the measurement results suffer from large fluctuations, leading to high linewidth uncertainty.
The relative intensity noise (RIN) of the proposed SOA-based random fiber laser was also investigated in the experiments by recording the laser output with a photo detector (PDB130C, Thorlabs) and an oscilloscope (DS081204B, Agilent). The output of each lasing wavelength was monitored separately by using an optical filter. Figure 6 shows the RIN comparison between SOA- and EDF-based dual-wavelength random fiber lasers. It is illustrated that the SOA-based random fiber laser exhibits RIN two orders lower than the EDF-based dual-wavelength random fiber laser in the low Fourier frequency region (<1 kHz). This is mainly attributed to the mitigation of mode competition thanks to the inhomogeneous gain in SOA. The homogeneous feature of the EDF gain forces the two lasing lines to compete for the limited gain and thus intensifies the RIN for each lasing line. Another factor that contributing to the higher RIN in EDF-based dual-wavelength random fiber laser is the relaxation oscillation noises at lower frequency region, which is determined by the intrinsic lifetime (~ms) of the upper state in the EDF gain medium and cavity damping time of the lasing system. As clearly shown in Fig. 6, relaxation oscillation induced RIN peaks are found within frequency range from 1 to 2 hundreds of Hertz to kHz, which agrees well with the lasing characteristics of the EDF-based fiber lasers with long cavity length. However, the SOA-based random fiber laser shows higher RIN in the high Fourier frequency region, which is due to the high frequency relaxation oscillation noises in the SOA. Moreover, two SOAs were used in the lasing system, which further strengthens the high frequency RINs. An optimized system could be constructed with one SOA with higher amplification and output power.
The frequency noise measurement of the proposed SOA-based dual-wavelength random fiber laser was conducted with an imbalanced symmetric 3 × 3 coupler Michelson interferometer with a delay fiber of 4.3-km-long in one arm. After performing the phase demodulation scheme on the measured data and averaging over 20 measurements, the frequency noise spectra of both SOA-based and EDF-based dual-wavelength random fiber lasers are obtained as plotted in Fig. 7. Note that although both random fiber lasers have shown comparable frequency noise spectra within the detected frequency range with similar trends, EDF-based random fiber laser has a much larger error bar of a standard deviation of ± 7dB for the 20 frequency noise measurements. This is due to the severe mode competition between the two lasing wavelengths, which renders unstable gain for each lasing wavelength and intensifies the mode hopping effect and the laser frequency noise. As opposed to the EDF-based dual-wavelength random fiber laser, the SOA-based one is more stable in lasing gain and wavelengths, leading to measured frequency noise spectra with smaller error bar of a standard deviation of ± 2dB. After averaging, it is also observed that the SOA-based random fiber laser has suppressed frequency noise level especially in the low-frequency range, which is attributed to the alleviated gain competition in the lasing system.
In summary, we have demonstrated a novel SOA-based dual-wavelength random fiber laser based on coherent Rayleigh feedback. The proposed dual-wavelength random fiber laser shows much higher stability and robustness compared to the EDF-based counterpart thanks to the inhomogeneous broadening feature of the SOA gain. A simplified theoretical model was established to reveal the linewidth narrowing effect induced by the coherent Rayleigh scattering and un-flattened gain profile during the lasing build-up process. Experimental results show that the proposed random fiber laser possesses narrow linewidth of ~1kHz with single mode operation and suppressed cavity modes. The intensity and frequency noises of the laser were also measured and analyzed. Compared with the EDF-based random fiber laser, the proposed SOA-based laser shows the improvements in the lasing noise. Such a high-quality narrow-linewidth dual-wavelength laser could find interesting applications in microwave photonic systems, telecommunication wavelength division multiplexing systems, fiber-optic sensors, and spectroscopy.
Canada Research Chairs; Natural Sciences and Engineering Research Council of Canada (NSERC) (06071/ FGPIN/2015).
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