1. Introduction

Tunable narrow-linewidth lasers are of great interest for many applications, such as in LIDAR, optical communications, and sensors [1–4]. Especially, a fast-tuning low-noise laser is of significant importance in high-frequency modulated applications, like in interferometric sensors using the phase generated carrier (PGC) technique [5,6]. Semiconductor lasers are capable of tunable output light through temperature or current tuning [7]. However, the tuning range may have gaps in it, and the linewidth is large [8]. Moreover, temperature tuning is very slow, while fast current-tuning deteriorates the intensity noise which enhances the laser phase noise. The fast frequency tuning amplitude is also instable, which shall affect the long-term stability of the system with such a laser applied to. The external-cavity laser diode, with an external cavity coupled to the semiconductor laser, has a much narrowed linewidth and improved frequency tuning [9–11]. They are usually tuned by combining rotation and translation of a grating that is adjusted mechanically [9], or by using a fiber Fabry-Perot filters [10]. A fast-tuning external-cavity laser diode can be obtained by incorporating an intra-cavity electro-optic tuner [11]. Unfortunately, although those lasers have proved to be flexible in attaining wide tuning ranges and narrow linewidth, they have drawbacks in laser size, alignment, and mechanical stability. With the development of rare-earth doped fibers [12,13], fiber lasers present great superiorities in applications to modern fiber communications and sensors, for their inherent compatibility with optical fiber based systems and excellent efficiencies of pump light to laser emission as well as narrow linewidth (~kHz). Tunable fiber lasers usually need a tunable bandpass filter in the cavity [14], or with bulk external tuning grating [15]. With a piezoelectric transducer (PZT) implemented in the fiber laser cavity, a distributed Bragg reflector (DBR) Er:Yb codoped fiber laser with up to 10 kHz frequency modulation bandwidth and a narrow-linewidth ring-cavity erbium-doped fiber laser (EDFL) with up to 50 kHz modulation frequency have been reported [16,17]. Their fast-tuning output is stable, due to their modulation mechanism based on PZT effect. Nevertheless, the laser central frequency drifts severely, which is dental in high-accuracy applications [18].

As the development of nonlinear fiber optics, the Brillouin fiber laser (BFL) based on stimulated Brillouin scattering (SBS) in optical fibers has drawn much attention for their ultra-narrow linewidth (< kHz) [19]. Only few reports presented tunable BFLs [20,21]. And for the requirement of a critically pump-coupled resonator, there are not any reports about fast-tuning BFLs, to the best of the authors’ knowing. Brillouin/erbium fiber lasers (BEFLs), eliminating the need for a critical resonator, are still incapable of fast-tuning light, for their cavities are too long (> 100 m) to be efficiently modulated via a PZT [22]. Until recently, a series of novel-configured BEFLs with ultra-short cavity have been proposed in both theory and experiment [23–25]. They are indicated with low threshold (~30 mW), large output power (> 10 mW), low phase noise (−125 dB/Hz^1/2 @ 1 kHz frequency), single frequency, and single polarization (31 dB polarization extinction ratio) [24,25]. And they have been proved to have wide frequency tuning range and low phase noise under fast sinusoidal modulations [26].

Up to now, there have not been any theoretical researches on this kind of novel fast-tuning fiber laser. In this paper, we at first theoretically investigate on the relation of the frequency-tuning range with the parameters of the BEFL. And we analyze the mechanism of the novel fast-tuning fiber laser, in which both the lasing mode and the Brillouin gain spectrum (BGS) are affected by the PZT stretching effect. Then, experiments are carried out to validate the theoretical analysis and indicate the detailed output characteristics of this fast-tuning BEFL, including the response of the tuning range to the modulation frequency, the frequency-tuning amplitude against the driving voltage on the PZT, the relative intensity noise (RIN) and phase noise of the fast-tuning BEFL, and the fast-tuning stability of the BEFL. The potential applications of the BEFL in fiber sensors are also discussed. The work of this paper not only provides detailed study on this novel BEFL, but also suggests a choice high-performance laser source for coherent communication, sensors, frequency locking, and so forth.

2. Laser structure and principle

The configuration of the proposed fast-tuning BEFL is shown in Fig. 1. The components of this laser are all polarization-maintaining (PM), guaranteeing the single-polarization state of the laser. A 1550 nm distributed feedback laser diode (DFB-LD) with 5 mW optical power provides the Brillouin pump (BP) for the BEFL through an optical circulator. A length of 4 m commercialized PM EDF (Coractive), pumped by a 980 nm laser diode through a 1550 nm/980 nm wavelength-division multiplexer (WDM), acts as both the Brillouin and linear gain media in the laser. The BP excites SBS in the EDF and provides the Brillouin gain for the Brillouin Stokes. The 980 nm pump generates the linear gain in the EDF, which compensates for the round-trip optical losses and eases the difficulty for SBS excitation by the BP. Due to the combined influence of the Brillouin gain and the linear gain of the EDF, milliwatt or even sub-milliwatt magnitude of Brillouin pump power is capable of exciting SBS lasing in this short EDF. And with the 5 mW BP, the 980 nm pump power threshold of the proposed BEFL is only about 30 mW [24]. About 3.5 m of this EDF is coiled on a PZT with a 4-cm diameter, which is applied with sinusoidal voltage for laser frequency modulation. A narrow-band (~0.3 nm) tunable optical filter (TOF) is set with its passband covering the Brillouin Stokes wavelength. This narrow-band TOF suppresses the amplified spontaneous emission (ASE) outside its passband and ensures the operation wavelength of the BEFL near the Brillouin Stokes wavelength. Meanwhile, once the wavelength of the BP and the TOF changes together, it allows the BEFL to operate in a wide wavelength range besides that where the peak EDF gain locates. The 3 dB optical fiber coupler extracts out the BEFL light.

 

Fig. 1 Configuration of the fast-tuning BEFL: DFB-LD, distributed feedback laser diode; WDM, wavelength-division multiplexer; EDF, erbium-doped fiber; PZT, piezoelectric transducer; TOF, tunable optical filter.

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When no driving voltage is applied on the PZT, the BEFL is a single-frequency fiber laser. The cavity length is$L_0$ and the output frequency is$f_0$. When a sinusoidal voltage is applied, the BEFL frequency is modulated at the same rate. Supposing the driving voltage is $U=u\cos \omega t$, the cavity length turns$L=L_0+\Delta L\cos \omega t$, and the BEFL frequency becomes$f=f_0+\Delta f\cos \omega t$. The BEFL frequency satisfies the cavity resonant condition:

The frequency tuning amplitude $\left|\Delta f\right|$can be calculated from Eq. (2), as the following:

The cavity length variation ΔL is determined by the PZT. The PZT diameter d and its variation $\Delta d$ under the driving voltage u satisfy the relation of $\Delta d/d=ku$, where k is a coefficient determined by the property of the PZT. The length of the EDF wrapped around the PZT is$L_W$. The relationship between$\Delta L$and the wrapped length $L_W$is that$\Delta L=L_{W}ku$. As a result, the frequency-tuning amplitude$\left|\Delta f\right|$satisfies the following relation:

The frequency tuning amplitude $\left|\Delta f\right|$is linear to the driving voltage of the PZT. It permits convenient frequency tuning through the voltage controls. As$\left|\Delta f\right|$is proportional to the coiled fiber length L_W, the EDF is expected to be coiled on the PZT as much as possible. The inverse proportional relation of $\left|\Delta f\right|$with the total cavity length L₀ points out the importance of a short cavity for efficient frequency modulations. The reason for that no fast-tuning BEFLs had been realized before is due to their long cavity lengths (> 100 m). Owning to the novel configures and principles, the BEFL is now able to output fast-tuning light for its ultra-short cavity (< 10 m). The coefficient k of the adopted PZT is about 1 × 10⁻⁷/V at modulation frequencies lower than the resonant frequency. The lasing mode frequency is varied by about 7 MHz with 1 V driving voltage on the PZT.

As to the fast-tuning mechanism, it was primarily assumed in [26] that the BGS of the EDF changes together with the lasing cavity mode synchronously during the modulation process, since the strain of the EDF is correspondingly varied by the PZT stretching effect. The Brillouin frequency shift Δν_B of the EDF is varied by C_εΔL/L_E, where C_ε is the strain coefficient of the Brillouin frequency shift (~500 MHz/%) and L_E is the length of the EDF. With a certain driving voltage on the PZT (i.e., ΔL is fixed), the ratio between the variations of the lasing mode and the Brillouin frequency shift, equaling to f₀L_E/C_εL₀, is in the order of 10³. Compared with the lasing mode, the Brillouin frequency shift of the EDF is unchanged by the stretching effect from the PZT. As a result, the mechanism of this fast-tuning BEFL is that only the lasing cavity mode is modulated, while the Brillouin gain spectrum keeps quiescent. The tuning of the lasing mode within the BGS causes the tunable output of the proposed BEFL.

3. Experiments

3.1 Brillouin gain spectrum of the EDF

We used a distributed Brillouin optical time-domain analyzer (BOTDA) to measure the Brillouin gain spectrum of the 4-m EDF [27]. Both situations of the EDF with and without driving voltage on the PZT were investigated to study the fast-tuning mechanism. The BGS of the EDF are shown in Fig. 2, when the PZT is without and with ± 3.5V driving voltages, respectively. The peak intensity is a little decreased when the PZT is driven by voltages, owing to the minus strain coefficient of Brillouin optical power [28]. A slight central-frequency discrepancy is observed between the BGS of the EDF without and with ± 3.5 V voltages on the PZT. It is about 15 kHz according to the calculation. Considering the 1 MHz frequency resolution in the experiment, we cannot accurately measure this value. We can find it is in the order of ~10 kHz from Fig. 2. This discrepancy is neglectable compared with the over 20 MHz lasing mode variation with the 3.5-V voltage on the PZT in the BEFL. The experimental results show the presumption that the BGS shifts synchronously with the lasing mode during the modulation is incorrect. The BGS of the EDF is somewhat affected by the modulation but its central frequency can be regarded as unchanged. The fast-tuning output of the proposed BEFL is only based on the tuning of the lasing mode within the quiescent Brillouin gain spectrum.

 

Fig. 2 Brillouin gain spectra of the EDF without (the black solid line) and with ± 3.5 V driving voltages (the red and blue dashed line) on the PZT.

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The BGS bandwidth of the EDF is about 60 MHz. Considering the about 20 MHz free spectrum range of the BEFL (~10 m cavity length), there should be about 3 longitudinal modes in the BGS. The BEFL, however, operates in single-mode due to the homogeneously broadened SBS gain and EDF gain. Although the lasing mode may be instantaneously away from the peak gain during the modulation, it is too fast for another cavity mode to hop and establish [25]. Hence, the fast-tuning BEFL remains in single-mode state. It should be noted that the 60 MHz BGS bandwidth is wider than the typical value of 20 MHz for silica fibers in literatures. The broadening of the BGS of the EDF can be attributed to the inhomogeneities in the fiber-core cross section along the fiber, caused by non-uniform stain when wrapping the EDF. The numerical aperture of the EDF also plays an important role in BGS broadening [29]. As to the side peak on the right of the main peak in the BGS, it is caused by the splitting of the main longitudinal acoustic mode [30].

It is worth noting that the BGS measurement of an EDF is somewhat complicated, for the EDF properties are quite different from those of the passive fibers. The pump and probe signals can be mostly attenuated by the absorptions of the EDF, and this will lead to the failure of the BGS measure. Proper 980 nm pump powers should be injected into the EDF. Or, it can be considered to use a much shorter EDF for measurements.

3.2 Frequency tuning of the BEFL

We used a high-sensitivity unbalanced fiber Michelson interferometer to study the output characteristics of the fast-tuning BEFL. The experimental setup is depicted in Fig. 3.The output of the fast-tuning BEFL was injected into the interferometer with 5 m optical path difference (OPD), in which the Faraday rotating mirrors (FRMs) are used to avoid polarization fading effect and to ensure high interferometric visibility. The interferometer was packaged and shielded in a housing specifically designed for environmental acoustic and thermal noise isolation. The interferometric signals were received by a low-noise photoelectric detector and an analog-to-digital (A/D) convertor, and then processed by a set of software in the computer. The frequency-tuning range, the phase noise, and the tuning stability of the fast-tuning BEFL were demodulated from the interferometric signals by the PGC technique.

 

Fig. 3 Experimental setup to measure the characteristics of the fast-tuning BEFL: DFB-LD, distributed feedback laser diode; TOF, tunable optical filter; FRM, Faraday rotating mirror; A/D, analog-to-digital convertor; PC, personal computer.

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The response of the fast-tuning BEFL to the modulation frequency was measured as shown in Fig. 4. The driving current of the 980 nm pump power is set to 500 mA (i.e. 288 mW 980 nm pump power) in the following experiments. The sinusoidal voltage amplitude applied to the PZT was 1 V. The frequency response is quite flat when the modulation frequency is lower than 15 kHz. The pronounced peak at 25 kHz indicates the resonance frequency of the PZT. The PZT operates instable at the resonance frequency so the modulation frequency should avoid it. The response curve returns flat after 35 kHz, and the maximum tuning rates achieve 48 kHz. However, the frequency modulation depth in this modulation frequency range is the smallest due to the PZT property. With a selected PZT, the modulation efficiency of the BEFL at higher frequencies can be further enhanced.

 

Fig. 4 Fast tuning response of the BEFL as a function of the modulation frequency with a sinusoidal input of $\pm$1 V.

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The relation of the frequency tuning amplitude with the driving voltage on the PZT was measured, as shown in Fig. 5. The modulation frequencies are 12.5 kHz, 32 kHz, and 48 kHz, respectively. All the three curves are linear to the driving voltage of the PZT, coinciding with the theoretical analysis. And the measured data at 12.5 kHz modulation frequency agree well with the calculation of Eq. (4). The experimental results under 32 kHz and 48 kHz modulations are a little different. The modulation efficiency at 32 kHz is higher than that at 12.5 kHz, while that at 48 kHz is the smallest of all. This phenomenon can be explained by the PZT response to the modulation frequencies, as indicated in Fig. 4. The coefficient k used in the simulation corresponds to the low-frequency state of the PZT. It is fit for the PZT modulated at 12.5 kHz. When the modulation rate is too fast for the PZT to response, the coefficient k shall decrease, as the case at 48 kHz. The modulation efficiency at 32 kHz is larger for this frequency is near the resonant frequency of the PZT. Although the modulation frequencies of 12.5 kHz and 32 kHz are capable for the requirement of the present interferometric fiber sensors based on PGC technique, the fast-tuning BEFL at above 100 kHz modulation frequency shall be realized by selecting a PZT with large high-frequency response in the future.

 

Fig. 5 Frequency modulating amplitude of the BEFL against the sinusoidal voltage amplitude at 12.5 kHz (〇), 32 kHz (□), and 48 kHz (◇) modulation frequencies on the PZT. The dashed line is the simulation result.

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The maximum frequency tuning amplitude of this BEFL is over 30 MHz, indicating the total tuning range is more than 60 MHz. Although in theory the tuning range could be further enhanced by increasing the voltage, we found that the interferometric signals became instable when the voltage amplitude was larger than 4 V at 32 kHz modulation frequency and 4.5 V at 12.5 kHz modulation frequency. This phenomenon is very interesting for the fast-tuning BEFL became instable when its frequency tuning range reached about 60 MHz in both situations. It can be explained by the tuning mechanism of this BEFL. The primary lasing mode is modulated by the PZT and it can only remain lasing in the BGS. As the increase of the driving voltage, the modulation depth of the lasing mode is also increased. Once it exceeds the bandwidth of the BGS, the primary lasing mode is extinguished from time to time and the fast-tuning BEFL thus becomes instable. The maximum frequency tuning range is about the bandwidth of the gain spectrum, which is found to be indeed about 60 MHz in Fig. 2. The experimental results here validate the theoretical analysis about the tuning mechanism in another aspect.

3.3 RIN and phase noise of the fast-tuning BEFL

For better understanding of the proposed BEFL, we measured the RINs of the BEFL with and without modulations, as shown in Fig. 6. The RIN of this BEFL without modulations reaches as low as about −132 dB/Hz, except at around 110 kHz which is caused by the relaxation oscillation, as the solid red curve shows. With 32 kHz modulation on the PZT, apparent peaks appear at the modulation frequency and harmonics, as the dashed blue line shows, indicating that the RIN of the fast-tuning BEFL is also modulated. This can be explained that the lasing mode is modulated in the Lorentzian Brillouin gain spectrum, causing the magnitude of the gain for the mode is tuned at the same rate. The output intensity of the BEFL is thus modulated, which is reflected by the peaks at the modulation frequency and its harmonics in the RIN spectrum. Other than these peaks, the RIN of the fast-tuning BEFL remains the same as that without modulations.

 

Fig. 6 RINs of the BEFL without modulation (the red solid line) and with 32 kHz modulation and 2.8 V voltage amplitude on the PZT (the blue dashed line).

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The phase noise of the fast-tuning BEFL was measured, as the blue curve shows in Fig. 7. The modulation frequency was 32 kHz, and the voltage amplitude was 2.8 V. The phase noise is about −110 dB/Hz^1/2 at 1 kHz frequency. For comparison, the phase noise of this BEFL without voltages on the PZT was also measured by modulating the OPD of the interferometer [31], as the red curve shows in Fig. 7. There is no apparent negative effect from the modulations on the phase noise of the proposed BEFL. The phase noises of the BEFL with and without modulations are nearly the same. This phenomenon can be explained as follows. The phase noise of the BEFL is determined by the phase noise of the Brillouin pump, the spontaneous emission, the spontaneous scattering, and environmental perturbations. The modulation of the PZT only changes the cavity length of the BEFL and modulates the BEFL frequency. Since the operation of the PZT is very stable, the induced noise of the modulated PZT is so small that it can be ignored compared with the original phase noise of the BEFL. Hence, the BEFL keeps its high-coherence properties under fast frequency modulations. To discard the impact of OPD on the result, phase noise values are generally normalized to 1 m OPD. As a result, the phase noise of this fast-tuning BEFL reaches −124 dB/Hz^1/2 at 1 m OPD. The remarkably low phase noise of this BEFL is comparable with the commercialized lasers representing the state-of-the-art laser technology [31].

 

Fig. 7 Phase noises of the BEFL without modulation (the red line) and with 32 kHz modulation and 2.8 V voltage amplitude on the PZT (the blue line), measured by the Michelson interferometer of 5 m OPD.

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3.4 Fast-tuning stability of the BEFL

Here, we study the fast-tuning stability of the BEFL for the first time. The experimental setup used is still the unbalanced Michelson fiber interferometer in Fig. 3. The BEFL with 32 kHz modulation was measured for about 45 minutes, and the experimental results are shown in Fig. 8(a). The ordinate C, equaling to 2πDΔf/c (D is the optical path difference, and c is the light velocity in vacuum.), shows the trend of the fast frequency tuning amplitude. The values of C keep flat during the measurement, demonstrating the stable fast tuning of the proposed BEFL. For comparison, we also measured two narrow-linewidth (~3 kHz) lasers, an EDFL and a laser diode, under the same experimental condition. The experimental results are shown in Figs. 8(b) and 8(c). The values of C of the EDFL also keep flat, while those of the laser diode are quite emanative. Whether the fast frequency tuning amplitude of a laser is stable or not depends on its fast-tuning mechanism. Fast-tuning fiber lasers perform well at fast-tuning stability due to the stable cavity modulation by the PZT, as shown in Figs. 8(a) and 8(b). Contrarily, a laser diode is tuned through modulating its operation current, which causes the fast tuning very instable, as shown in Fig. 8(c).The fast-tuning stability of the laser source is very important for the long-term accuracy of signal demodulations in the interferometric fiber sensors based on PGC technique. Hence, the experimental results demonstrate the priority of the fast-tuning BEFL to the LDs in the PGC-based fiber sensors.

 

Fig. 8 Fast-tuning stability of the BEFL (a), an EDFL (b), and a laser diode (c).

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4. Discussions on potential applications in fiber sensors

Fast-tuning lasers are of importance in many applications, particularly in interferometric fiber sensors using the PGC technique. The PGC technique requires the laser frequency to be modulated with ~10 kHz modulation frequency. And the modulation frequency determines the upper-limit of the dynamic range of the fiber sensor. The fast-tuning BEFL has been demonstrated efficiently modulated with 12.5 kHz and 32 kHz, which can satisfy the requirements for most of the present interferometric fiber sensors. To further enhance the modulation frequency, a PZT with more efficient high-frequency response shall be selected to implement in the BEFL.

In the interferometric fiber sensors using the PGC technique, sufficient path difference (~20m) between the two arms should be provided for signal demodulations. A low phase noise laser can reduce the phase-induced intensity noise of the interferometer, allowing the sensor to achieve higher measurement sensitivity. The remarkably low phase noise of the fast-tuning BEFL can lower the minimum detectable signals and enhance the maximum detection distance of the sensing systems.

Presently, the popular laser sources are laser diodes and EDFLs. Both types of lasers can obtain a linewidth as narrow as ~3 kHz through advanced linewidth-narrowing techniques. Laser diodes have drawbacks in fast-tuning stability, and the EDFL has severe central frequency drifts [18]. Both of the fast-tuning instability and the central frequency drifts are negative to the long-term accuracy of the sensor. The BEFL presents stable fast tuning, and its central frequency stability has already been demonstrated in our precious work [25]. The long-term performance shall be much enhanced with the proposed BEFL applied in the sensor.

It should be noted that the TOF in this BEFL is a Fabry-Perot filter which is with relatively large insertion losses (~5 dB) and large size. In contrast, a fiber-Bragg-grating (FBG) filter is all-optical-fiber, and its reflection can easily reaches over 90%. If the FBG is used to replace the Fabry-Perot filter, the 980 nm pump threshold shall be reduced and the optical power shall be enhanced due to the decreased cavity loss. The size of the BEFL can be also reduced. Hence, we shall adopt a FBG filter in the BEFL and package this fiber laser to isolate the environmental and thermal noises in the next study. We shall also further enhance the maximum modulation frequency of the fast-tuning BEFL by adopting a selected PZT in the future work.

5. Conclusion

In summary, a novel fast-tuning Brillouin/erbium fiber laser is studied on its fast-tuning mechanism and output characteristics. Frequency modulations are realized through the PZT stretching effect on the EDF which acts as both the Brillouin gain and linear gain media. The mechanism of the fast-tuning BEFL is demonstrated in both theory and experiment that the lasing mode is modulated in the BGS of the EDF, instead of the previous assumption of the synchronous movements of the lasing mode together with the BGS. The frequency tuning amplitude of the BEFL increases linearly with the driving voltage of the PZT. Maximum over 60 MHz frequency tuning range is obtained which is limited by the bandwidth of the BGS. The maximum tuning rates reach up to 48 kHz, which can be further enhanced by using a selected PZT. The RIN is affected by the modulation at the modulation frequency and the harmonics. The phase noise, however, suffers no effect from the modulation. The BEFL remains high coherence property under fast frequency modulations. This BEFL can find a wide range of applications, particularly in the frequency modulated sensing systems.