Brillouin lasing in a single-longitudinal mode at 1.55 μm is demonstrated using As2Se3 single-mode fiber for the first time. The As2Se3 fiber provides sufficient Brillouin gain for the Stokes wave to initiate single frequency oscillation in a 2-m long fiber Fabry-Perot cavity with a non-resonant pump power of 56 mW. For a pump power of 78 mw, 12 mW of Stokes power was obtained, which corresponded a conversion efficiency of 15%.
©2006 Optical Society of America
Stimulated Brillouin scattering (SBS) in optical fiber  has many applications in the areas of optics and optical communications, such as narrow band amplification, lasing, sensing, phase conjugation, slow light generation and so on. SBS in optical fiber has been used for reduction of optical carrier of microwave signal [2–4]. When the power of a weakly intensity-modulated carrier exceeds the Brillouin threshold, it is converted to the backward propagating Stokes wave and is, therefore, selectively depleted. In such application, a single-mode Stokes wave is found to be more suitable compared with multimode in preventing the addition of noise .
Despite having a linewidth of only a few tens of MHz [5–7], it is rather difficult to achieve single mode Brillouin lasing using silica fibers due to the requirement of hundreds meters to kilometers of length. Indeed, short ring cavities, which are resonant to the pump wavelength, have also been realized in order to enhance the pump power inside the cavity by many times and effectively reduce the Brillouin threshold [4, 8]. Operation of such single-frequency laser, however, requires active control of the cavity length to fulfill the resonant condition with respect to the pump wave.
As an alternative, nonlinear materials that have large Brillouin gain coefficients [9, 10] compared with silica fibers should be useful in reducing the power required to achieved threshold in such Brillouin fiber devices. Chalcogenide glasses containing one or more of the chalcogen elements such as S, Se are optical materials reported to have enhanced optical nonlinearities. Experiments performed using multimode and single mode fibers drawn from As38Se62 chalcogenide glasses showed a nonlinear Kerr coefficients about 930 times and a Raman gain coefficient 780 times larger than those of silica glass . Recently, we have also observed strong stimulated Brillouin scattering in As39Se61 single mode fiber and experimentally measured a Brillouin gain coefficients of 6.0 × 10-9 m/W that is 134 times higher than that of silica fiber . Fiber with such a large gain coefficient should have enormous potential in the realization of compact Brillouin fiber devices.
In this paper, we report for the first time a Brillouin laser employing As2Se3 chalcogenide fiber that oscillates in a single longitudinal mode at 1.55-μm wavelength region with a non-resonant pumping scheme. Using only a 2-m-long As2Se3 fiber Fabry-Perot cavity, we achieved threshold for single frequency Brillouin lasing with a pump power of about 56 mW. Since the cavity is non-resonant to the pump, no feedback control was required which made the laser operation much easier.
The chalcogenide fiber used in the experiment had a core made from high-purity As39Se61 material and a cladding from one with a slightly reduced As-content . The core diameter of the fiber was 6 μm and the NA was 0.18, which allowed the single mode propagation in 1.55 μm wavelengths region. The transmission loss at 1.55 μm was measured to be 0.84 dB/m.
Experimental setup used for Brillouin lasing with As2Se3 fiber is shown in Fig. 1. The Fabry-Perot type Brillouin laser was made from a 2-m-long fiber the ends of which were flat-polished. The large Fresnel reflection of about 22% (refractive index; 2.82) yielded the optical feedback necessary for lasing in the Fabry-Perot type Brillouin resonator. A single mode pump wave tunable with in wavelength range of 1540 -1570 nm was used as a pump source. The effective length L eff = (1-exp-αL)/α of the 2m long fiber was 1.67m. Single mode silica fiber (SMF) with tapered end, was used to couple light to and from the chalcogenide fiber. The coupling loss was estimated to be 3.3 dB and the total round trip loss for the Stokes wave inside the Fabry-Perot cavity was 5.6 dB. A polarization controller was used to control the polarization of the pump wave. In addition to this, the As2Se3 fiber loops forming the Fabry-Perot cavity were adjusted manually so that the evolution of polarization state of the Stokes wave after one round trip becomes self-consistent, a requirement for efficient Brillouin lasing.
The optical spectrum of the laser output was observed using an optical spectrum analyzer with a resolution of 0.01 nm. A heterodyne measurement  was performed to study the longitudinal mode structure of the output the Brillouin laser. The output consisted of a backscattered Stokes wave and a pump waves that was reflected backward due to Fresnel reflection. By observing the beating produced by pump and Stokes using a photodetector and an RF spectrum analyzer, we could study the detailed mode structures.
The group index of the fiber was measured using a chromatic dispersion analyzer and was found to be about 2.93, which yielded cavity round trip frequency of 25.6 MHz. This was larger than the Brillouin linewidth of 13.2 MHz, obtained from our previous measurements , which thus favored single frequency Brillouin lasing.
As the pump power into the chalcogenide fiber was gradually increased, and polarization state adjusted, Brillouin lasing could be observed when the power exceeded about 120 mW (launched power 56 mW). At this power level, a small signal gain of 31 dB was estimated from exp(gBLeffKP/Aeff), using a gain coefficient gB = 6×10-9m/W , effective length Leff = 1.67 m, effective area Aeff =39 μm2 (calculated from the core diameter and the numerical aperture ) and a factor K=0.5. Constant K depends on the polarization property of the fiber, which is shown to be 1 if the polarization is maintained and 0.5 otherwise, and here we assumed it to be 0.5 .
Figure 2 shows the optical spectrum of the back-scattered light observed for a pump power of 167 mW (power launched in the fiber: 78 mW). The Stokes wave was seen to red shift by about 0.06 nm from the pump. Pump component that appears in the optical spectrum is due to light that is reflected from the uncoated-fiber ends. For a pump power of 78 mW, Stokes light of 12 mW was available in the backward direction, which yielded a conversion efficiency of 15%. Although the Stokes light circulating inside the Brillouin laser was amplified only when propagated in the direction opposite to the pump, Stokes wave also came out from the other facet with the residual pump. The Stokes wave output power in the forward direction could be calculated from the loss inside the fiber (0.84 dB/m) and Fresnel reflection (22%), and estimated to be 53% of the Stokes wave available in the backward direction.
Figure 3 shows the RF spectrum of the output from the laser, which shows the beat signal between the Stokes and the pump waves. The single line in the RF spectrum frequency is an indication of single-frequency Brillouin oscillation occurring within the Fabry-Perot cavity. The beat frequency was also measured for a number of pump wavelengths over the range 1540-1570 nm and is plotted against the inverse of the pump wavelength. As shown in Fig. 4 the beat signal (= νB) varied inversely with the pump wavelength, in accordance with the relation, νB = 2nνA/λ p. From the slope of νB vs. (1/λ p) curve, and using n = 2.82 we determined the acoustic velocity in As2Se3, νA= 2258 m/s which was in well agreement with that reported in Ref. 15.
We could see a drift in the RF frequency of the beat signal and occasional mode hop. This was due to a random fluctuation of the optical frequency of the pump laser and also due to a variation of the length of the Fabry-Perot cavity (which changed the frequency of the longitudinal modes of the Brillouin laser) due to ambient temperature fluctuation. Figure 5 shows the beat frequency observed in an experiment performed with a laser that had poorer frequency stability. Discrete lines with almost constant amplitude could be seen around 7.975 GHz as a result of these fluctuations. The two neighboring longitudinal modes of the Brillouin laser with intensity more than 20 dB lower than the central peak could also be noted in the spectrum. The separation between the side-modes from the center of the peak was about 25.7 MHz, which was in consistent with a cavity length of 2 m.
Currently, as the fiber was not polarization maintaining, the polarization state of the circulating Stokes wave changed with ambient temperature, which required occasional readjustment of the polarization of the input light. Such fluctuation in the polarization state of light was more prominent with increased pump power (presumably due to heating), which created difficulty in obtaining I-L curve and in stable operation with higher output power. This could be eliminated by use of polarization maintaining chalcogenide fiber.
Furthermore, by appropriate control of the Fabry-Perot cavity length using temperature stabilization or incorporation of optoelectronic feedback, it should be possible to eliminate the variation in the absolute frequency of the Stokes lines. A cavity stabilized against such fluctuations, might be useful in producing Stokes lines with fixed frequencies at an arbitrary wavelength by pumping with a second laser, over the transparency range of the As2Se3 fiber which extends to as far as 10 μm. The optical frequency of a single-frequency laser could be stabilized by using the Stokes wave it would generate through Brillouin lasing.
In conclusion, we demonstrate for the first time a Fabry-Perot Brillouin fiber laser oscillating in a single longitudinal mode by using As2Se3 chalcogenide fiber. The large Brillouin gain coefficient of the As2Se3 fiber enabled Brillouin lasing to occur in a 2-m-long Fabry-Perot fiber cavity for a pump power of 56 mW. A compact single frequency Brillouin laser that operates as demonstrated in this paper will have potential applications for microwave photonics and for laser line stabilization.
References and links
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