1. Introduction

Photonic crystal fiber (PCF) technology has progressed rapidly in recent years and is of great interest for fiber device applications because of the wide range and novel optical properties that it offers [1]. One particularly interesting class of PCF combines a small-scale solid core with a large air-fraction cladding (i.e., closely spaced, large air holes). This type of fiber offers tight modal confinement of light and thus can provide an effective nonlinearity per unit length that is 10–100 times higher than that presented by a conventional fiber. Furthermore, with conventional optical fibers, typical non-linear fiber devices need to be several kilometers long for realistic operating powers, which make them rather impractical for anything other than laboratory usage. In contrast, similar performance levels can be achieved in PCF-based equivalents of such devices using fibers of a few tens of meters in length, making them a more realistic proposition for real-world applications [2,3].

Recently, Yang et al have demonstrated a PCF based multi-wavelength erbium-doped fiber laser (EDFL) using a PCF of 101 m long [4]. They reported the potentials of such EDFLs in optical test and measurement, optical wavelength-division-multiplexing communication and sensing systems. In their system, the multiple wavelength operation was achieved based on four-wave mixing in the PCF and a sample fiber Bragg grating. A multi-wavelength laser, which has small equal-wavelength spacing, large number of lasing lines within a broad wavelength band, and high output uniformity over the channels can also be achieved through Brillouin-erbium fiber lasers (BEFLs). A BEFL uses the combination of high broadband gain in erbium doped fiber (EDF) and narrow band nonlinear gain of stimulated Brillouin scattering (SBS) in both ring and linear configurations [5,6]. PCF based BEFL has been demonstrated recently [7], but only 6 output channels can be generated with signal-to-noise ratio (SNR) of much less than 20 dB. The poor performance was probably because the PCF used had a high SBS threshold due to guiding of several acoustic modes within a broad frequency range [8]. This is a high possibility since with a nonlinear coefficient of 11 (Wkm)^-1 and BP power of 220 mW, SBS should be quite efficient in a 20 m long optical fiber, which was not the case here. Great improvement may be observed if the PCF used has a low SBS threshold. Apart from that, the use of pre-amplified Brillouin pump (BP) [9] as well as linear cavity [10], instead of ring [5], can also help to improve the laser performance.

In this paper, as an enhancement to the laser device system, we present a Fabry-Perot linear cavity BEFL with 100 m of PCF utilized as the Brillouin gain medium for an improvement to the laser device size and hence, its practicality. In ensuring that the design could operate at modest power level, a pre-amplified BP is structured within the laser design [9]. Also, by incorporating a tunable band-pass filter (TBF) [10], large number of Brillouin Stokes lines could be tuned over a wide range by the laser system. Our results show that 14 Brillouin Stokes lines with more than 20 dB SNR and constant channel spacing of 0.079 nm could attain a tuning range of 29 nm.

2. Brillouin threshold and gain spectrum measurement of photonic crystal fiber

Brillouin threshold and gain spectrum of the 100 m long PCF used was first measured. Figure 1(a) illustrates the experimental setup for measuring the SBS threshold value with the inset showing the scanning electron micrograph (SEM) image of the PCF. The PCF exhibits a triangular core with average diameter of 2.1±0.3 µm and cladding diameter of 128±5 µm. The average air hole diameter of the fiber is 0.8 µm with 1.5 µm pitch. The PCF is made from pure silica with 17.4 wt% of Ge-doped core region. It is spliced to an intermediate fiber and then a single mode fiber (SMF) with a splice loss of 0.35 dB at each end. The nonlinear coefficient of the fiber is 11 (Wkm)^-1 with attenuation of less than 9 dB/km at 1550 nm region. In the setup, an external cavity tunable laser source (TLS) with 100 nm tuning range (1520–1620 nm) was used as the BP light source with the wavelength set to 1550 nm. Narrow linewidth option (linewidth <1 MHz) was selected to ensure efficient Brillouin Stokes line generation [11]. The BP signal was amplified by a high power erbium doped fiber amplifier (EDFA). The pump light was launched into the PCF through an optical circulator where Brillouin Stokes and anti-Stokes waves generated would propagate back into the circulator and measured by an optical power meter. In Fig. 1(b), the setup for Brillouin gain spectrum measurement is illustrated. The amplified BP was split by a 10-dB coupler (C) where 90% of the light power was injected into the PCF through an optical circulator to generate SBS. The other 10% of the BP light was coupled to a 3-dB coupler and would act as a reference light source. Brillouin Stokes and anti-Stokes lights originated from the PCF would travel back into the optical circulator where they would combine with the referenced BP light via the 3-dB coupler. The total combined optical power was converted into electrical power by a photodetector and the power spectrum over frequency range was analyzed via an electrical spectrum analyzer. In both setups, index matching liquid was applied at the PCF loose end to avoid any Fresnel reflection.

 

Fig. 1. (a) Brillouin threshold measurement setup. Inset, SEM image of the PCF. (b) Brillouin gain spectrum measurement setup.

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Figure 2(a) illustrates the transmitted and backscattered powers as a function of the input pump power. The plotted values correspond to optical powers just inside the PCF at the beginning and at the end of the PCF as the splice loss between the PCF and SMF has been taken into account. As highlighted in Fig. 2(a), the PCF Brillouin threshold power equals to 19.04 dBm of the input pump power, which is the point where the backscattered power reaches ~1% of the input pump power [12]. The measured Brillouin threshold value obtained is about the same as the calculated value using standard models [11–14]. The typical high SBS threshold feature normally reported for highly nonlinear PCF [3,8] was not observed in this case. This is confirmed with the measurement of Brillouin gain spectrum linewidth as in Fig. 2(b) which shows comparable width to that of standard Ge-doped fiber [11].

 

Fig. 2. (a) Transmitted and backscattered power of the PCF as a function of the input power. (b) Brillouin gain spectrum linewidth.

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3. Multi-wavelength Brillouin-erbium photonic crystal fiber laser

The experimental setup for the widely tunable BEFL is illustrated in Fig. 3 with the Brillouin gain medium provided by a 100 m long PCF. The same TLS, as used in the previous setup, was applied as the BP light source with a 3-dB coupler (C) performs the coupling of the BP light into the laser system. A 980 nm laser diode (LD) with 200 mW maximum pump power acted as the primary pump light for the 10 m long EDF. A wavelength selective coupler (WSC) was used in the setup to multiplex the pump and signal lights. A fiber mirror reflector (FMR) and a circulator (Cir2) were applied to form the linear cavity of the laser system. A TBF (1 nm 3-dB bandwidth) which was placed within Cir2 would permit only selective wavelengths to loop back into the cavity of the laser system and remove all others. Output spectrums were taken from the coupler-circulator (Cir1) link, and analyzed via an optical spectrum analyzer (OSA) with the resolution set to 0.015 nm.

 

Fig. 3. Experimental setup of Fabry-Perot linear cavity Brillouin-erbium photonic crystal fiber laser.

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Without any BP injection and with the 980 nm LD power above its threshold value, the laser system would operate as a bi-directional EDFL. Absence of any TBF in the setup would fix the EDFL self-lasing cavity modes at certain wavelengths and it depends on the length of the EDF in use. Large number of Brillouin Stokes lines could not be generated over a wide tuning range due to the fact that the region of self-lasing cavity modes provides the largest EDF gain. However, by utilizing a TBF in the design, EDFL self-lasing cavity modes could be tuned anywhere within the EDF gain profile, subject to the tuning range of the TBF in use. Thus, a large number of Brillouin Stokes lines could be tuned over a wide range of wavelength in conjunction with modifying the BP wavelength. To ensure modest power BEFL operation, the laser system was designed to acquire a pre-amplified BP source. This was accomplished (Fig. 3) through amplification of the BP by the EDF before it is injected into the Brillouin gain medium where sufficiently high BP power (launched through the 3-dB coupler) would commence the SBS phenomenon in the PCF. The first Brillouin Stokes line would then be downshifted by 0.079 nm from the BP wavelength and propagate in opposite direction of the BP. It would travel back into the EDF for double-pass amplification in the linear cavity before being re-injected into the PCF to create higher order Brillouin Stokes lines. Generation of cascade Brillouin Stokes lines would continue until the total gain in the laser cavity is less than the cavity loss at the operating wavelength. A stable laser will be formed at the steady-state condition which made up by the BP and its Brillouin Stokes lines.

Optimum point for launching the BP within the self-lasing cavity modes must be determined beforehand so that the maximum number of Brillouin Stokes lines could be generated. Figure 4(a) shows the tuned self-lasing cavity modes by the TBF centered at 1550.5 nm without any BP injection and with 980 nm LD power of 200 mW. Brillouin Stokes lines will be generated along the long-wavelength end of the injected BP. Thus, the BP must be launched at the short-wavelength end of the self-lasing cavity modes to generate the maximum Brillouin Stokes possible while suppressing the self-lasing effect contributing to the output instability of the laser system. Based on our observations, the optimum point of launching the BP is at 1550.0 nm, which is near to the starting-edge of the self-lasing cavity modes region. Up to 14 Brillouin Stokes lines having SNR of more than 20 dB and equal channel spacing of 0.079 nm could be generated with 7 dBm of BP power, as seen in Fig. 4(b). Launching the BP at any shorter wavelengths and lower power would cause the self-lasing cavity modes to be too efficient to be suppressed by the BP. We clearly observe from Fig. 4(b) that the PCF utilized provided enough nonlinear effect to generate even higher number of Brillouin Stokes lines. However, limitation of the TBF bandwidth used caused the generation of further Brillouin Stokes lines to exhibit the SNR of less than 20 dB. This could be overcome if a TBF with a wider bandwidth was in use.

 

Fig.4. (a) BEFL self-lasing cavity modes at 200 mW of 980 nm LD power (b) Generation of 14 Brillouin Stokes lines with 7 dBm of BP power and 200 mW of 980 nm LD power.

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Figure 5 illustrates the number of Brillouin Stokes lines generated at fixed BP wavelength of 1550 nm and BP power of (a) 5 dBm, (b) 6 dBm, and (c) 7 dBm as a function of launched 980 nm pump. With 5 dBm of BP power, 20 mW of the 980 nm pump power was required to generate the first Brillouin Stokes line with 20 dB SNR. Also, with such Brillouin pump power, 13 maximum Brillouin Stokes lines could be generated at minimum 980 nm pump power of 140 mW. With 6 dBm of injected BP power, the 980 nm pump power required to generate the first Brillouin Stokes line with 20 dB SNR increased to 30 mW. The increase in Brillouin laser threshold is expected as higher BP power launched into the laser system would increase the 980 nm LD power requirements of generating the Brillouin Stokes lines. Maximum number of output channels that could be generated maintains at 13 Brillouin Stokes. However, the minimum 980 nm pump power required to generate the 13 Brillouin Stokes increased to 160 mW, as a result of increased Brillouin laser threshold. The total gain (EDF linear gain and SBS nonlinear gain) is not enough at these powers to generate higher number of output channels. Since EDF gain is already fixed at 200 mW of the 980 nm LD, the only means left to generate higher number of Brillouin Stokes lines is by increasing the SBS nonlinear gain. This was achieved through launching 7 dBm of BP power into the laser system where 14 Brillouin Stokes lines were able to be generated at 200 mW of the 980 nm LD power. As with previous BP power, 30 mW of the 980 nm LD power was required to generate the first Brillouin Stokes with 20 dB SNR. The same procedure was repeated over 44 nm bandwidths from 1526–1569 nm (TBF tuning range). The generation of Brillouin Stokes lines at these wavelengths is illustrated in Fig. 5(d) at fixed BP and 980 nm LD powers of 7 dBm and 200 mW respectively. Up to 14 uniform Brillouin Stokes lines could be generated and tuned over 29 nm tuning range from 1540–1569 nm. Even with fixed BP and 980 nm LD powers, the number of Brillouin Stokes lines generated would still drop at certain shorter wavelengths. These drops were caused by low EDF gain profile at such wavelengths; hence, the laser system does not able to maintain the same number of generated Brillouin Stokes lines.

Without utilizing a TBF in the design, the operation at these different wavelengths would require different pumping powers of 980 nm LD and BP to achieve the same number of Brillouin Stokes lines as the gain provided by the EDF remains unequal through all the wavelengths. The output spectrum of the tuning process is illustrated in Fig. 6 where seven groups of 14 Brillouin Stokes lines with BP wavelengths at 1540, 1545, 1550, 1555, 1560, 1565 and 1569 nm are shown.

 

Fig. 5. Number of Brillouin Stokes lines generated at (a) 5 dBm, (b) 6 dBm and (c) 7 dBm of BP power with various 980 nm LD power. (d) Number of Brillouin Stokes lines generated at over 44 nm tuning range with fixed BP power of 7 dBm and 980 nm LD power of 200 mW.

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Fig. 6. Output Spectra of the tunable BEFL with 14 Brillouin Stokes lines over 29 nm tuning range.

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4. Conclusion

A widely tunable low SBS threshold PCF based multi-wavelength Brillouin-erbium fiber laser has been demonstrated. Utilization of 100 m of PCF and a tunable band-pass filter within the pre-amplified BP configuration has resulted in the generation of 14 Brillouin Stokes lines which are tunable over 29 nm range. Constant channel spacing of 0.079 nm and signal to noise ratio of more than 20 dB for each Brillouin Stokes line were also observed from the proposed design. The laser system was able to operate at consistent and modest BP power of 7 dBm and 980 nm LD power of 200 mW.