Enhanced flatness of 20 GHz channel spacing multiwavelength Brillouin-Raman fiber laser with sub-millimeter air gap

Abstract

We discover the technique of controlling the flatness in signal amplitude of a multiwavelength Brillouin-Raman fiber laser by employing an air-gap outside of the cavity. The structure that is adjustable within sub-millimeter length behaves as flexible optical feedback that provides modifiable portions of multiple Fresnel reflectivities. This is the main benchmark that allows the efficient management of gain competition between self-lasing modes and Brillouin Stokes waves that is vital for self-flattening initiation. When setting the Brillouin pump wavelength at 1529 nm and the air-gap distance to 0.4 mm, 296 Stokes lines are produced with a channel spacing of 0.158 nm. The lasing bandwidth is 46.60 nm that covers from 1529.16 to 1575.76 nm wavelength. In this case at Raman power of 950 mW, the intense Brillouin pump power of 2 dBm saturates the cascaded higher-orders lasing lines. As a result, the overall peak power discrepancy is maintained at just 1.8 dB where an average optical-signal-to-noise ratio of 20 dB is realized. To date, this is the widest bandwidth with the flattest spectrum attained in multiwavelength fiber lasers that incorporate a single Raman pump unit.

Wide Tunability Microwave Photonic Filter Based on Brillouin-Raman Fiber Laser: A Modeling Study

“…Please note that in the previous BEFL work, a tuning bandwidth of ~24 MHz per optical channel was achieved [5]. The MPF in this setup is able to achieve tunability in terms of bandwidth as the number of optical channels can easily be changed by changing the pump power [8,11,[13][14][15]. Figure 6 shows the relationship of the 3 dB bandwidth and number of taps, which shows that the bandwidth decreases exponentially against the number of channels.…”

“…The wavelength spacing is set at 0.08 nm, number of optical taps at 50, and total dispersion of 450 ps/nm in a 25 km singlemode fiber (SMF) as the dispersive medium. A perfectly flat top or even optical channels is simulated by setting all relative optical power coefficients at 1, while the uneven optical taps were generated as random coefficients between 0.8-1.0, which is a reasonable amplitude variation in BRFL channels [8,11,[13][14][15]. By looking at Fig.…”

“…3 (a) and (b). The choices of wavelength spacing are based on commonly reported frequency spacing from multiwavelength fiber lasers of single wavelength spacing (10 GHz) [13], double spacing (20 GHz) [8,11,14,15] and triple spacing (30 GHz) [10,12], respectively. Tunability can be achieved by varying the wavelength, as evident in Fig.…”

“…The use of multiwavelength fiber laser in microwave photonic filter has been reported in [4,5]. There are several approaches of obtaining multi wavelength fiber laser operation which are Brillouin-Raman fiber laser (BRFL) [11,[13][14][15], Brillouin-erbium fiber laser (BEFL) [8,10], erbium-doped fiber laser (EDFL) [7], and Brillouin fiber laser (BFL) [9]. However, due to the low erbium gain and the spectral shape of the erbium gain, low number of optical channels and non-flat channels amplitude are generated in EDFL and BEFL [7][8][9][10].…”

“…[16]. It has several desirable traits such as stable operation at room temperature, flat and large gain bandwidth, compatibility with fiber properties, and design simplicity [8,[10][11][12][13][14][15]. BRFL generates multiple-channel output with wavelength spacing ~0.08 nm, ~0.16 nm, or ~0.24 nm, due to the single, double and triple Brillouin frequency spacing, respectively.…”