Relative intensity noise of multiwavelength fibre laser J. Poe ¨tte, S. Blin, G. Brochu, L. Bramerie, R. Slavı ´k, J.-C. Simon, S. LaRochelle and P. Besnard Relative intensity noise of a six-channel multiwavelength distributed Fabry-Perot fibre laser is compared to the noise of a singlemode distributed feedback fibre laser. The noise characteristics of individual channels are similar to that of the singlemode laser, offering a low-cost and compact solution for wavelength multiplexing applications with- out noise penalty. Introduction: Distributed feedback (DFB) erbium-doped fibre lasers exhibit many attractive features such as narrow linewidth, tunable frequency, and simple fabrication. Singlemode (longitudinal and polarisation) DFB erbium-doped fibre lasers [1] are useful in metro- logy, telecommunications and sensors systems. To increase their capacity and capability, several popular multiplexing schemes, such as wavelength division multiplexing (WDM) or code division multiple access (CDMA), use a plurality of single-wavelength sources [2, 3]. Multiwavelength lasers offer an alternative for the generation of an optical frequency comb, but the homogenous line broadening of erbium at room temperature unfortunately leads to unstable behaviour. We recently reported on an original configuration of an ultra-compact multiwavelength laser, based on a distributed Fabry-Perot filter written into an active fibre [4]. In this Letter, we compare the relative intensity noise (RIN) of this kind of laser with a single-frequency DFB laser made in the same fibre. Fibre lasers: The active fibre (University of Southampton, UK) has a photosensitive inner cladding and an erbium-ytterbium codoped core. The fibre is deuterium loaded to increase its photosensitivity. The gratings are written with a phase-mask scanning method, using a continuous wave UV beam from a 244 nm frequency-doubled Ar-ion laser. The laser polarisation is orthogonal to the axis of the fibre (s-polarisation) to favour a single-polarisation operation by increasing the photo-induced birefringence. The fibre laser is pumped using a 980 nm semiconductor diode, the signal of which is counter- propagating relative to the measured laser output. Lasers are placed in the groove of a copper piece filled with silicone heatsink paste. Lasers are also optically isolated to ensure single frequency operation. The multiwavelength laser consists in two 15 mm-long chirped gratings, which are superimposed but shifted by 2 mm along the fibre axis. Transmission of the first grating is 26 dB, that of the second one is estimated at 55 dB. The shift creates a Fabry-Perot cavity with a 50 GHz free spectral range. The chirp is obtained using a 1.25 nm=cm chirped phase-mask (TeraXion Inc., Canada). As a consequence, the Fabry-Perot is distributed and a multiwavelength stable operation is allowed due to the spatial distribution of the cavities corresponding to the different laser lines. The optical spectrum of the laser (Fig. 2) presents six lines (L1 to L6) with a spectral power flatness better than 3.7 dB. L6 is closer to the pump than L1. The laser threshold occurs for a pump power of 14.5 mW and the power efficiency is 20% (average- per-line of 3.3%). For comparison with the multiwavelength laser, that has 2 mm-long cavities, we fabricated a short (5 mm-long grating) DFB fibre laser [1] with a grating strength (product of the coupling coefficient with the grating length) of 11. To ensure singlemode operation, a p=2 rad phase- shift is included during the inscription by moving the 1066 nm-period phase-mask relative to the fibre. The phase-shift splits the grating into two unequal parts (44–56%) to maximise the output power at the shorter grating side. The laser threshold occurs for a 30 mW pump power and the power efficiency is 3.3%. For a 5.7 pumping rate r (ratio of the pumping power to that at threshold), the output power is 1.5 mW at a wavelength of 1544.2 nm with a sidemode suppression ratio (SMSR) greater than 34 dB (experimental measurement limit). Noise measurement: The power spectral density of the signal gener- ated by a photodetector illuminated by a continuous laser is composed of the three following contributions: thermal noise, shot noise (proportional to the optical power), and laser excess noise (quantified by the RIN). To measure the RIN, thermal and shot noises have to be estimated. The electrical noise, without optical signal, allows the determination of the thermal noise. The shot noise is then obtained using a very low RIN reference laser. Two reference lasers are used: one to study RIN in the 100 kHz–10 MHz band, the other one for the 10 MHz–20 GHz band. The first reference is a 1552 nm active- feedback semiconductor laser (Dicos Technologies Inc., Canada) with RIN lower than 165 dB  Hz 1 below 10 MHz. The other one is a 1319 nm ring-cavity Nd:YAG laser (Lightwave Inc., USA), the RIN of which is 170 dB  Hz 1 at 10 MHz and less than 180 dB  Hz 1 above 10 MHz. Fig. 1 shows RIN measurements of the singlemode DFB laser. Continuous lines are direct measurements (using an electrical spectrum analyser), squares (or triangles) are obtained using a method similar to Cox et al. [5]. The RIN presents a classical behaviour: a floor for low frequencies, a peak due to relaxation oscillations (the frequency square of which depends linearly on r), and a decrease for higher frequencies. The relaxation oscillation frequency (ROF) peak occurs at 2 MHz with a 96.2 dB  Hz 1 amplitude. Note that the ROF is usually lower than 1 MHz for DFB fibre lasers [6]. The RIN is not displayed for frequencies higher than 1 GHz, since it is below our measurement limit of 169 dB  Hz 1 . Nevertheless we may observe a peak around 6.0 GHz of 130 dB  Hz 1 due to a beating between longitudinal modes, even when the SMSR is higher than our experimental measure- ment limit of 34 dB. line 6 DFB DFB -100 MHz 10 -120 -140 -160 -180 RIN, dB/Hz frequency , MHz 10 -1 10 0 10 1 10 2 10 3 6035 Fig. 1 RIN of DFB laser, and one filtered line (L6) of multiwavelength laser Optical power: 1.5 mW for both (before filter for L6). DFB: r ¼ 5.7; L6: r ¼ 5.3. Measurement floors: 169 dB  Hz 1 (DFB) and 166 dB  Hz 1 (L6) u DFB n L6 -100 line 2 line 6 all lines -120 -140 -160 -60 1546 1547 l, nm optical power spectr al density , dBm 1548 1549 1550 -40 -20 0 -180 RIN, dB/Hz frequency , MHz 10 -1 10 0 10 1 10 2 10 3 L1 L6 Fig. 2 RIN measurements of multiwavelength laser for r ¼ 11.9 Optical powers (before filter for L2 and L6): 18.1 mW (total), 1.81 mW (L2), 3.2 mW (L6). Measurement floors: 176 dB  Hz 1 (total) and 166 dB  Hz 1 (L6) u total n L6 Inset: Optical spectra for all-lines (solid line) and only one filtered line (shaded line) Fig. 2 presents RIN measurements of the multiwavelength laser for all-lines, and for two of the six lines filtered using a 0.2 nm-wide grating optical filter. The optical spectrum of the filtered L6 line is shown in the inset of Fig. 2. The RIN of the entire multiwavelength laser is much lower than the RIN obtained with a filtered line ELECTRONICS LETTERS 10th June 2004 Vol. 40 No. 12