Nowadays, to satisfy the increase of internet demands and requirement, two multiplexing techniques are used: WDM (Wavelength Division Multiplex) and TDM (Time Division Multiplex). WDM still more used than TDM. However, for practical system applications, such as optical CDMA (Code Division Multiplex Access) and OTDM (Optical Time Division Multiplex) systems, high speed optical communications require light sources with a repetition rate control. In this area, pulsed fiber lasers have become very attractive.
Fiber lasers have a number of qualities which make them very attractive for ultra short pulses generation via Q-switching, active or passive mode locking mechanisms. The gain bandwidth of rare-earth-doped fibers is large, typically tens of nanometers, which allows the generation of femtosecond pulses. The high gain efficiency of active fibers makes possible such lasers to operate with fairly low pump powers and tolerate intra cavity optical elements with relatively high optical losses. Fiber laser setups are very compact and can be done with a low cost. Furthermore, mode locked fiber lasers can rely on telecom components.
2. Q-switching mechanism
Storing ions in a higher energy level can be achieved by limiting ions flow to the bottom level. So, it’s necessary to prevent stimulated emission prevalence.
By means of light modulators able to generate high optical powers when transiting between the off and on states, we prevent light propagate within the laser cavity. For a radiative transition, the only possible drain to the bottom level is caused by spontaneous emission (see Fig. 1). The
At a given time, there is no way for stimulated emission to happen and the cavity is emptied by resulting losses of the output mirror (see Fig. 3).
The abrupt variation of the number of photons into the cavity results in emitting a high peak power optical pulse. Generally, several journeys between the two mirrors are necessary to completely depopulate the up-level and empty the cavity. So, the pulse width would be higher than the time of a coming and going through the cavity. With lengths lower than one meter, it is possible to generate nanosecond pulses. The repetition rate varies between few hundreds of
3. Mode locking mechanism
In a laser cavity, frequencies circulating into the resonator and having more gain than losses are called longitudinal modes. They can be considered as an assembly of independent oscillators. These modes gain increases after each round trip through the cavity. These modes are separated by
If we consider
The resulting light intensity is:
Fig. 6 shows the resulting output pulses sequence of a mode locked laser cavity allowing the oscillation of
4. Pulsed fiber laser
In case of fiber laser, the
Passively mode locked fiber lasers have the advantage of being entirely consisted of optical components. They do not require external electrical components and the mode locking mechanism in the cavity is carried out automatically [3-4-5]. However, these lasers can’t reach high pulses repetition rates. In fact, the repetition rate of generated pulses depends mainly on the cavity length [6-7]. The laser resonator may contain a saturable absorber such as SESAM (Semiconductor Saturable Absorber Mirror) to passively mode lock the cavity (see Fig. 8).
A nonlinear loop mirror is used in a “figure-of-eight laser”. A schematic diagram of the 8FL (Eight Fiber Laser) is shown in Fig. 10. The 8FL overall design is that of a ring cavity with a Sagnac interferometer with a gain medium placed asymmetrically in the loop. By addition of pulses through the central coupler, the NALM (Non linear Amplifying Loop Mirror) transmits highest intensities of pulse and reflects the lowest ones [9-10]. The nonlinear fiber loop amplifies, shapes and stabilizes the circulating ultra short pulse . With the P-APM (Polarization-Additive Pulse Mode-Locking), the polarization state of a pulse propagating through an optical fiber differs from the peak to the wings and the transmission through a polarizer can be adjusted to eliminate the wings [12-13]. The SAs act as intensity dependent elements. The wings of the pulse exhibit more losses than the peak .
The PCs (Polarization Controllers) set the input signal in an arbitrary polarization state. The azimuth and elliptical parameters define the polarization state of the output signal. Considering
In actively mode locked fiber lasers, as shown in Fig. 12, the pulses frequency depends on the electro-optic or the acousto-optic modulator inserted in the cavity [16-17-18]. Generally, these types of laser cavities provide typically pulses larger than those provided by a passively locked laser. This can be explained by the fact that no compression techniques are applied . The most used optical modulator to actively mode lock the different modes oscillating into a fiber laser cavity is the MZM (Mach Zehnder modulator). It’s an intensity modulator based on an interferometer principle. It consists of two
Aiming to profit at the same of the two configurations advantages: a rather low width and a sufficiently high repetition rate of pulses, new prospects and configurations of fiber lasers, using both the passive and active mode locking techniques, have been proposed. This new generation of pulses generator is called hybrid type mode locked fiber laser . Fig. 13 shows hybrid type mode locked fiber laser using both a machZehnder modulator to actively mode lock the cavity and a non linear amplifying loop mirror to passively mode lock the cavity.
Being 90% made of fiber; light propagation through a fiber laser can be modeled by the Split Step Fourier Method.
5. Split step fourier method
Light propagation within optical ﬁber may be expressed by the Generalized Non Linear Schrödinger Equation (GNLSE) as follow:
The gain of the Erbium Doped Fiber Ampliﬁer (EDFA) can be estimated as
The SSFM consists on transforming the GNLSE as the sum of linear and nonlinear operators:
The SSFM relies on that propagation in each segment of the optical fiber is divided in three steps: two linear and one non linearsteps (see Fig. 14). The nonlinear step is inserted between the two linear steps [21-22].
So, linear and nonlinear effects are supposed to be applied in the whole segment of the fiber. The linear operator is used in the frequency area and the non linear one is used in time area.
6. Erbium doped fiber amplifier
The EDFA is based on a two-level
Where the optical powers are expressed in units of number of photons per unit time, τ is the metastable spontaneous emission lifetime,
7. Interaction between mode locking mechanism and non linear effects in fiber laser
Normally, when designing extremely high output average and peak power fiber laser generating ultra short pulses, the best solution that can be adopted is to enhance the non linear effects in the cavity. This can be achieved either by pumping the piece of doped fiber amplifier with a high input power rate or enhancing the SPM, XPM and FWM effects by reducing the average dispersion of the cavity and the effective area of the different fibers used. In this section, managing the pumping input powers level, the dispersion and the effective area of different microstructured optical fibers inserted into a passively and an hybrid type mode locked 8FLs, we prove that enhancing non linear effects does not lead necessarily to better results. It depends also on the type of mode locking mechanism used. The highest peak powers and the narrowest pulse widths are obtained only for specific parameters.
In spite of their singularities and particularities in managing linear and non linear effects, the exploitation of MOFs in laser cavities has remained a subject of research bit addressed. In fact, MOFs offer many degrees of freedom in the management of dispersion and effective area
A schematic diagram of the first passively mode locked 8FL is shown in Fig.15. It consists of two loops: a ring cavity and a non linear amplifying loop mirror NALM connected to each other through a
The second configuration, shown in Fig.16, is a hybrid type mode locked 8FL. It differs from the first one by the presence of a MZM (Mach Zehnder Modulator) as an electro-optical modulator into the linear ring cavity.
By modelling the light propagation through the various components by the SSFM (Split Step Fourier Method), we studied the influence of varying nonlinear parameters of the cavity on the output pulses shape. Light pulse propagation in the 8FL may be expressed by the NLGSE (Non Linear Generalised Schrödinger Equation) and the transfer function of the different components used . The central coupler is a cross-coupler for combining or splitting the optical signal. It is bidirectional, with wavelength independent coupling, insertion loss and return loss. If we consider
A single secant hyperbolic input pulse with
A second approach to study the non linear effects impact in a fiber laser cavity is to use longer portion of the non linear optical fiber used. Fig.19 and Fig.20 show the output pulses peak power and width for different lengths and effective areas of MOF. The pump power delivered by each laser diode is equal to
As shown in Fig.19 and Fig.20, enhancing dramatically the non linear effects, by increasing the MOF length and decreasing its effective area, does not lead necessarily to optimal results. In fact, for each length of one selected fiber there are two optimal effective areas. The first corresponds to the one leading to the highest peak power and the second corresponds to the one leading to the lowest pulse width and conversely. However, there is always an intermediate value of the effective area leading to a high peak and a low pulse width. For
Thus, by reducing the mean dispersion of the cavity with an appropriate choice of the MOF optimal length and effective area, generated ultra short pulses would have the highest peak power and the lowest width.
Unlike the passively mode locked 8FL carried out above, in case of hybrid type 8FL shown in Fig.16, no input pulse is inserted in the cavity to release the cavity oscillation. The first handling aimed to study the average pulses output power fluctuation according to the pump powers of the two lasers diode for different MOF’s effective areas. The MOF length and dispersion are respectively
The repetition rate and the width of output pulses are fixed by the electro-optical modulator characteristics.
The repetition rate of pulses depends directly on the frequency of the electrical signal injected into the MZM. Fig.24 illustrates the variation of the width of output pulses according to the electrical signal frequency.
Fig.25 shows hybrid type output pulses with a repetition rate of 20GHz.The second handling aimed to study the average pulses output power fluctuation from a hybrid type 8FL according to non linear effects by varying the length and the effective area of the MOF.
Curves shown in Fig.26 illustrate that more the MOF is long and its effective area is small more the exit power of the laser is significant. However, a significant increase of the MOF length and the effective area leads to a fast power fall. We can also notice that for all different MOF’s lengths there is a particular value of the effective area leading always to the same result. In this case, it corresponds to
Thus, increasing the average exit power of hybrid type 8FL, operating at any pulses repetition rate, can be reached by choosing a rather long MOF having small effective area and normal dispersion.
We summarized different techniques used to generate ultra short pulses from a fiber laser. Using the Split Step Fourier Method algorithm to model light propagation within a loop cavity, we described some operating process of different kind of mode locked fiber lasers. We also focused on some optical components operating process used in fiber laser to passively or actively mode lock the different modes oscillating within a laser cavity. In addition, we focused on Erbium Doped Fiber Amplifier operating process. We highlighted the improvement of fiber laser performances does not depend only on the management of the non linear parameters of the cavity. In fact, it depends tightly on the mode locking mechanism used. A passively mode locked 8FL and a hybrid type 8FL do not respond the same way to non linear effects increase. In fact, in case of passively mode locked 8FL, for each length of the high non linear fiber, correspond two associated optimal effective areas: one leading to the highest peak power and one leading to the lowest pulse width. Whereas, increasing the non linear effects by using a rather long high non linear fiber having a reduced effective area leads to the best output results in case of hybrid type 8FL. Moreover, contrarily to hybrid type 8FL, reducing the average dispersion of the cavity leads necessarily to better output passively mode locked 8FL pulses shape. In fact, this work aims to illustrate the existing interaction between non linear effects and mode locking mechanism in fiber laser.