Tuneable Sagnac comb filter including two wave retarders

In this paper we study theoretically and experimentally a wavelength-tuneable Sagnac birefringence filter. The device is a Sagnac interferometer including a symmetric fibre coupler and a length of high-birefringence fibre in the loop. A wave retarder is inserted at each end of the birefringent fibre for absolute wavelength tuning. We show theoretically that wavelength tuning through wave plate orientation ensuring constant amplitude of the filtering function is possible only if a minimum of two wave retarders are included in the setup. The position of the transmission peaks then varies linearly with the angle of one of the retarders and can be adjusted over one entire channel spacing. This happens only when a quarter-wave retarder and a half-wave retarder are used, if the former is oriented at 45° with respect to the fibre birefringence axes, while the orientation of the latter serves as the adjustment parameter. The theoretical predictions are confirmed by the experimental results.     

Wavelength tunable high power laser using a double-clad Er:Yb doped fiber

We experimentally demonstrated a stable, wavelength-tunable fiber laser using a polarization-maintaining, double-clad Er:Yb doped fiber amplifier in the cavity. The output wavelength is tunable over the range from 1535 to 1567 nm using a fixed grating and the dichroic mirror placed on a rotational mount; under rotation of the dichroic mirror the tuning ratio of 50 nm/deg was found. We studied the wavelength tuning range dependence on the amplifier fiber length and achieved a maximal output power of 850 mW. This configuration can be Q-switched for high peak power and its narrow bandwidth is suitable for nonlinear optics applications, such as parametric teraherthz generator.     

Tunable dual-wavelength fiber laser based on a polarization-maintaining fiber Bragg grating and a Hi-Bi fiber optical loop mirror

We demonstrate experimentally the operation of a linear cavity dual-wavelength fiber laser using a polarization maintaining fiber Bragg grating (PM-FBG) as an end mirror that defines two closely spaced laser emission lines. The PM-FBG is also used to tune the laser wavelengths. The total tuning range is ∼8 nm. The laser operates in a stable dual-wavelength mode for an appropriate adjustment of the cavity losses for the generated wavelengths. The high birefringence (Hi-Bi) fiber optical loop mirror (FOLM) is used as a tunable spectral filter to adjust the losses. The FOLM adjustment was performed by the temperature control of the Hi-Bi fiber.     

Fine adjustment of cavity loss by Sagnac loop for a dual wavelength generation

We experimentally demonstrate a fine adjustment of cavity loss by Sagnac loop for a dual wave-length generation. The single or dual wavelengths are obtained by controlling the losses on both cavities through a fiber optical loop mirror (FOLM). Wavelength separation on the dual laser is 0.98 nm. The dual or single wavelength is obtained by changes in temperature in the order of 10⁻¹°C around the maximum in the FOLM. Also, we investigate energy fluctuations on signal level saturation effect in the cavity through different pumping power that act on the EDF, where we note that from the 60-mW pumping begins to generate dual-wavelength and 80-mW stabilizes.     

Demonstration of sharp switching from a power-symmetric NOLM and a polarizer

We demonstrate theoretically, experimentally and numerically that a steep all-optical transfer characteristic can be obtained from a power-symmetric NOLM, including a quarter wave retarder and highly twisted fibre, followed by a polarizer. We first develop a theoretical analysis to show that, if the input polarisation is chosen linear, then for a correct choice of the wave plate and input polarisation angles the transmission grows from zero with a 4 dB/dB slope as power is increased, two times faster than can be obtained with a NOLM alone. An experimental study is then realised, which supports the theoretical results. Steep switching is demonstrated for reasonably low peak power levels that do not exceed 8 W, less than one third of the NOLM critical power. In addition, using the same setup we demonstrate the possibility to obtain a sinusoidal transmission characteristic that cancels out for a particular nonzero value of input power, which can be easily adjusted simply through rotation of the wave plate and output polarizer. This setup would be very useful in the frame of optical pulse shaping and signal processing applications, among others.     

Dual-wavelength operation of a figure-eight fiber laser

We study numerically an erbium-doped figure-eight fiber laser including a double-band-pass optical filter for dual-wavelength pulse generation. Simulations are performed for several values of the filter band-width and wavelength separation between the transmission windows. The results show that dual-wavelength mode-locking is obtained in most cases, with a balanced energy distribution between wavelengths. Due to cavity dispersion, the pulses at each wavelength are asynchronous for a large wavelength separation, whereas they are synchronous for closely spaced wavelengths, as in this case cross-phase modulation is able to compensate for the dispersion-induced walkoff. In the asynchronous case, dual-wavelength operation is favored by the filter loss, whereas in the synchronous case it is favored by the saturable absorber action of the nonlinear optical loop mirror. Simulations also show that, thanks to those stabilization mechanisms, dual-wave-length pulsed operation does not require precise cavity loss equalization between the two oscillating wave-lengths.     

Large amplitude noise reduction in ultrashort pulse trains using a power-symmetric nonlinear optical loop mirror

We analyse numerically the capabilities of a power-symmetric nonlinear optical loop mirror (NOLM) in the ultrashort pulse regime for high-quality amplitude regeneration of an optical signal. The device, which operates through nonlinear polarisation rotation, includes twisted, anomalous-dispersion fibre and a quarter-wave retarder. For particular adjustments of the retarder orientation, and a circularly polarised input beam, the output energy characteristic flattens near the switching energy, a property that can be used to eliminate large amplitude fluctuations in an optical signal. The group velocity mismatch between polarisation components introduced by twist is mitigated by the interplay between anomalous dispersion and the nonlinear Kerr effect, although strong twist should be avoided as it still introduces substantial pulse distortion. Contrary to other designs, where a plateau characteristic requires a large power imbalance between the counter-propagating beams, both pulses in the present scheme can be simultaneously close to fundamental solitons, which allows a substantial widening of the plateau for particular pulse parameters. Good quality, nearly transform-limited pulses are obtained in this case at the NOLM output. The device is applicable for the regeneration of ultrafast data streams in which the signal-to-noise ratio is severely deteriorated.     

Optical pulse shaping at moderate power using a twisted-fibre NOLM with single output polarisation selection

We demonstrate theoretically and experimentally that efficient signal shaping operation can be obtained at moderate power by using the transmission characteristic of a power-symmetric nonlinear optical loop mirror (NOLM) including highly twisted fibre and operating through nonlinear polarisation rotation, when the circular polarisation state orthogonal to the input polarisation is selected at the NOLM output. By adjusting the angle of the quarter-wave retarder inserted in the loop, the phase bias of the transfer characteristic can be adjusted precisely to enable proper signal shaping for moderate values of input power, remaining well below switching power. The tolerance of the procedure to deviations of the input polarisation from the ideal circular case is investigated numerically. We demonstrate experimentally the capabilities of this setup for both power equalisation and extinction ratio enhancement. Finally, we show that this setup is also useful to shape ultrashort optical pulses from the relaxation oscillations of a DFB semiconductor laser. In comparison with other NOLM-based techniques, the proposed approach allows to reduce by a factor of 8-10 the peak power required for pulse shaping, for the same fibre length and Kerr coefficient.     

Generation of high-energy pulses from an all-normal-dispersion figure-8 fiber laser

We propose and study numerically an all-normal-dispersion Ytterbium-doped figure-eight fiber laser scheme for generation of high-energy pulses. The monotonous pulse stretching that takes place in the fiber under the combined actions of normal dispersion and nonlinear Kerr effect is compensated by the amplitude modulation effect of a bandpass filter inserted in the ring section of the laser. The Nonlinear Optical Loop Mirror (NOLM) also contributes to shorten the pulses. An output coupler with a large output coupling ratio is inserted at the amplifier output in order to extract the maximal energy from the laser. A short segment of Ytterbium-doped fiber compensates for the losses. Stable single-pulse operation is predicted over a wide range of values of the laser parameters. If the laser parameters (ring and NOLM length, dispersion, filter bandwidth, output coupling ratio) are optimized, pulses with several tens of nanojoules energy are readily obtained, with picosecond duration and a large positive chirp which is linear near the peak. If small-signal gain is large enough, the use of very large output coupling ratios opens the way to pulse energies close to 100 nJ and, after dechirping outside the laser, to durations of ˜50 fs and peak powers of 1 MW.     

Experimental investigation of self-starting operation in a F8L based on a symmetrical NOLM

We experimentally analyze the self-starting operation of a figure-eight mode-locked fiber laser. The design is based on a power-balanced nonlinear optical loop mirror (NOLM) with highly twisted low-birefringence fiber and a quarter-wave (QW) retarder in the loop. The NOLM operates by nonlinear polarization rotation. Self-starting mode-locking requires a careful adjustment of the NOLM low-power transmission, which is easily realized with our setup by adjusting the angle of the QW retarder. The laser is capable of generating ∼20 ps pulses at the fundamental repetition frequency of 0.78 MHz.