We demonstrate an enhanced architecture of Brillouin-Erbium fiber laser utilizing the reverse-S-shaped fiber section as the coupling mechanism. The enhancement is made by locating a common section of Erbium-doped fiber next to the single-mode fiber to amplify the Brillouin pumps and the oscillating Stokes lines. The requirement of having two Erbium gain sections to enhance the multiple Brillouin Stokes lines generation is neglected by the proposed fiber laser structure. The mode competitions arise from the self-lasing cavity modes of the fiber laser are efficiently suppressed by the stronger pre-amplified Brillouin pump power before entering the single mode fiber section. The maximum output power of 20 mW is obtained from the proposed fiber laser with 10 laser lines that equally separated by 0.089 nm spacing.
©2008 Optical Society of America
Stimulated Brillouin Scattering (SBS) is a nonlinear process that occurs when the optical power launched into the fiber exceeds a threshold level . It manifests through the generation of backward-propagating Stokes wave which its frequency is downshifted from that of the incident light by an amount set by the nonlinear medium. Its unique characteristic of narrow frequency shift has fostered an interesting research area of generating multiple wavelengths from single laser cavity.
The generation of Brillouin Stokes line in optical fiber had been utilized as single longitudinal fiber laser [2-5]. Since the Brillouin gain in optical fiber is practically low, it is difficult to achieve efficient operation of a fiber laser with its own cavity . Thus, the SBS effect must be integrated with other amplifying medium to allow large output powers and to avoid the requirement for critically coupled resonator. The amplified medium provides a primary gain to compensate cavity loss and SBS is utilized as the frequency-shifted mechanism. This idea was successfully demonstrated by combining the SBS effect with Erbium gain medium to create a ring fiber laser with reasonable output powers . In this case, only one channel was obtained from the proposed ring fiber laser structure.
This hybrid technique led to the development of multi-wavelength Brillouin-Erbium fiber lasers (BEFL’s) by feeding back the Brillouin Stokes lines into the laser cavity via the non-resonant direction; famously known as the reverse-S-shaped fiber section . In this enhanced architecture of BEFL, two 3-dB couplers were deployed to tap a portion of the oscillating lasers to be injected into the single-mode fiber. However, the construction of the reverse-S-shaped fiber section was achieved at the expense of higher cavity loss. Therefore, the total output power was low for this type of BEFL. In order to enhance the BEFL performance, an Erbium-doped fiber amplifier (EDFA) was inserted in the reverse-S-shaped fiber section to enhance the lasers intensity as the subsequent Brillouin pump (BP) . Two EDFA sections were required to achieve the objective which increased the operational complexity. Although that a significant number of multiple lines was successfully obtained, it required stringent optimization procedures in order to produce 53 lines at the peak gain of the laser cavity . The achievement was obtained in balancing the mode competition between the Brillouin Stokes lines and self-lasing cavity modes (super-luminescence spectrum). Therefore the generation of multiple lines was not robust in terms of tuning range because it required the self-lasing cavity modes to be close enough to the injected Brillouin pump.
The effect of mode competition between Brillouin Stokes lines and self-lasing cavity modes must be handled properly in order to achieve wide tuning range operations. The requirement of higher Brillouin pump power to suppress the mode competition from the self-lasing cavity modes was critically important as reported in . Owing to this study, the Brillouin pump must be amplified first in order to suppress the build-up of the self-lasing cavity modes. The concept of Brillouin pump pre-amplification in BEFL with linear-cavity was already reported recently . The injected BP into the laser cavity was amplified by the EDFA before entering the single-mode fiber, therefore higher intensity of BP and Brillouin Stokes line were generated in the laser cavity. Thus it led to the homogenous gain saturation of Erbium gain medium in which the self-lasing cavity modes were suppressed in a wider wavelength range. In this case, the technique creates a more stable operation in a wider tuning range in contrast to the conventional technique of injecting BP directly into the single mode fiber. Base on this study, the EDFA location is critically important in determining the characteristics of BEFL. Nevertheless, this enhanced technique has not been investigated for the ring-cavity BEFL to the best of authors’ knowledge.
In this paper, an enhanced ring-cavity BEFL architecture is investigated utilizing the original reverse-S-shaped fiber section. The enhancement is made by optimizing the location of the EDFA section in the ring-cavity fiber laser. The maximum output power of 20 mW is recorded with 10 laser lines that are equally separated by 0.089 nm line spacing. The performance is recorded using only single EDFA in the BEFL structure.
2. Brillouin-Erbium fiber laser architecture
The architecture of ring-cavity BEFL is illustrated in Fig. 1. The BEFL structure consists of 9.5 m long of Erbium-doped fiber (EDF), 10 km long of single-mode fiber (SMF), two optical circulators (Cir1 and Cir2), two 3-dB couplers (C1 and C2) and a 980/1550nm wavelength selective coupler (WSC). The linear gain medium is provided by the EDF and pumped by a 980-nm laser pump. The pump and the oscillating lasers are multiplexed by the 980/1550-nm WSC. The EDF has 785 ppm of Er3+ ion concentration, numerical aperture of 0.22, cutoff wavelength of 900 nm and peak absorption of 7.38 dB/m at 1531nm. Meanwhile, the Brillouin gain medium is provided by the 10-km long of SMF.
A narrow linewidth light of 200 kHz from the external cavity tunable laser source (TLS) is used as the Brillouin pump (BP) that can be tuned from 1520 nm to 1620 nm. The BP is injected into the 3-dB coupler (C1) via the Cir1 in a counter clockwise (CCW) direction and is amplified by the EDF section. The amplified BP is injected into the SMF to create a narrow bandwidth of Brillouin gain. The first-order Brillouin Stokes (BS) line is generated at a wavelength shifted by 0.089 nm from the BP wavelength and it propagates in the opposite direction to the BP (clockwise direction). This BS line is then amplified in the EDF section and circulates in the laser cavity in the clockwise direction. The BS line becomes laser if its roundtrip gain is equal to the cavity loss (threshold condition is satisfied). If the total gain produced by the SBS and EDF equivalent to the laser cavity loss, a laser is formed and oscillates in the ring cavity in the clockwise direction. This lower-order BS line can be utilized as the BP for a cascading generation of additional higher-order BS lines.
In order to utilize this idea, the lower-order BS line is partially redirected into the SMF by another 3-dB coupler (C2). The re-directed BS line propagates back into the ring-cavity BEFL through Cir2 and C1 in the same direction as the initial BP. There is no additional EDFA used to amplify this portion of BS line in the reverse-S-shaped . Nevertheless, the amplification occurs in the common EDF section to increase the intensity of the redirected BS line to act as the next (higher order) BP. The same process of generating Brillouin Stokes lines is repeated and will terminate when the higher-order BP intensity is below its SBS threshold in the SMF. The output of the BEFL system is measured by an optical spectrum analyzer through port 3 of Cir1.
3. Results and discussions
The proposed BEFL structure acts as a normal fiber laser when the BP is not injected into the cavity. This behavior is important to specify its peak wavelength which determines the highest gain region of the proposed laser cavity. Since the laser cavity peak gain occur at around 1560-1561 nm, the oscillating modes within these peak gain region are always dominant. Therefore, unstable oscillation modes are observed as a result of strong mode competition between self-lasing modes within the peak gain of the laser cavity. With adequate BP power injected into the laser cavity, it is amplified by the EDF section before propagating into the SMF. The amplified BP generates higher intensity of the down-shifted Stokes line in the opposite direction and makes a complete round-trip oscillation in the clockwise direction. In order to get the most stable output, the BP wavelength must be carefully optimized within the peak gain of the laser cavity. In our experiment, the optimized BP wavelength is found at 1560.5 nm.
The output spectra of the proposed BEFL configuration, measured as a function of 980 nm pump power at fixed BP power of 3.25 mW are shown in Fig. 2. The optical output spectra from this BEFL configuration is measured by the optical spectrum analyzer with a resolution bandwidth of 0.015 nm. The first-order Stokes line is clearly measured at the 980-nm pump power of 20 mW and the wavelength of the Stokes signal generated shifted by 0.089 nm from the Brillouin pump with about -0.74 dBm Stokes power. As the pump power increases, the number of Stokes lines is also increased due to adequate pump power to amplify the higher order Stokes lines to reach its threshold for the process of oscillation in the laser cavity. At the pump power of 101 mW, five Stokes lines are generated with the first-order Stokes peak power at 5.64 dBm. The number of Stokes lines increases to seven when the pump power is increased to 156 mW. Since a portion of circulating Stokes lines is amplified before entering the single-mode fiber, it creates a higher gain efficiency of generating the higher-order Stokes lines. At the same time, this higher intensity of Stokes lines suppresses the energy extraction by the self-lasing cavity modes. Thus the output spectrum is clean from any spurious self-lasing cavity modes.
The characteristics of output power with respect to pump power are investigated as shown in Fig. 3(a). In our study, the pump power threshold of the multi-wavelength BEFL is around 16 mW. The measured pump threshold is the lowest reported value of multi-wavelength ring-cavity BEFL to the best of our knowledge. The slope efficiency of the proposed BEFL is about 10% with its maximum output power of 20 mW. Even though at the maximum pump power, there are no spurious self-lasing modes appear within the wavelength range of laser line as indicated in Fig. 3(b). There are 10 clean laser lines with a constant spacing of about 0.089 nm and their optical signal-to-noise ratio value is averaged at 23 dB.
The wavelength tuning range characteristic of the proposed BEFL system as a function of 980 nm pump power is illustrated in Fig. 4(a). In this experiment, the BP power is fixed to 3.25 mW and its BP wavelength is tuned at different pump power values. There are two parameters measured from this experiment, the tuning range and also the number of laser lines generated. In general, the tuning range of the BEFL system decreases as the 980 nm pump power increases. At 20 mW pump power, the tuning range is about 14.8 nm and this value decreases to only 5 nm at 156 mW pump power. This decrement is due to the strong mode competition between the self-lasing cavity modes and the Stokes lines as the 980 nm pump power increases. In order to obtain a wider tuning range, the 980 nm pump power should be reduced to a lower value so that the mode competition can be minimized . Although higher 980 nm pump power decreases the wavelength tuning range, it increases the number of Stokes lines generated within the tuning range. At 980 nm pump power of 156 mW, the average number of Stokes lines generated is nine within the tuning range of 5 nm. It can be seen clearly that there is a trade-off between the tuning range and the number of Stokes lines similar to the previous report . In overview, the tuning range is inversely proportional to the number of Stokes lines. Figure 4(b) depicts an output spectrum of the proposed BEFL configuration at 156 mW of 980-nm pump power. The tuning range of 5 nm is obtained from 1558 nm to 1563 nm. In between these two wavelengths, there is no self-lasing cavity modes captured from the optical spectrum analyzer. It indicates that the generation of multiple Stokes lines is able to suppress the competition arises from the self-lasing cavity modes. However, the tuning range can be improved by deploying a few spectral filtering techniques [12,13]. The improved result of tuning range is achieved at the expense of its operational simplicity in which the tunable filter must be synchronized with the BP wavelength. Hence, the complexity of the BEFL operation is increased that leads to the issue of robustness.
An enhanced Brillouin-Erbium multi-wavelength fiber laser based on the reverse-S-shaped fiber section is successfully demonstrated. The optical amplification is provided by a common EDF section that amplifies the oscillating Stokes lines and also the Brillouin pumps simultaneously. Since the Brillouin pump is amplified before propagating into the Brillouin gain medium of long single-mode fiber, the mode competition arises from the internal laser cavity modes is efficiently reduced. A clean output spectrum without any spurious self-lasing cavity modes is recorded and its maximum output power reaches 20 mW with 10 laser lines at a spacing of 0.089 nm. A tuning range of 14.8 nm is achieved at the Brillouin pump power of 3.25 mW and the 980-nm pump power of 20 mW. The tuning range is much wider at smaller 980-nm pump powers and higher value of Brillouin pump powers at the cost of the number of Stokes lines generated.
References and links
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