FIELD OF THE INVENTION

The present invention relates generally to mode-locked fiber lasers and in particular to semiconductor saturable absorber etalons used in transmission, and the use of such a device for mode-locking fiber lasers, for controlling the dispersion of the laser cavity and for setting the operation (lasing) wavelength. According to the present invention, a reliable mode-locking mechanism highly tolerant to the laser cavity dispersion is achieved.

BACKGROUND OF THE INVENTION

Semiconductor saturable absorbers are nonlinear optical elements that impose an intensity-dependent attenuation on a light beam incident upon it; an incident radiation of low intensity is preferably absorbed, while a high intensity radiation passes the saturable absorber with much less attenuation. These devices have found applications in a large variety of fields. In particular, passive mode-locking based on semiconductor saturable absorber is a powerful technique to produce short optical pulses simplifying in the same time the architecture of the laser cavities.

A semiconductor saturable absorber comprises semiconductor material(s) whose energy band-gap is small enough to absorb an optical signal to be controlled. The absorbing material is usually embedded within semiconductor material(s) with higher band-gap(s) that do not absorb the optical signal. The thickness of a single absorbing layer is typically in the range of few nanometers so that quantum-mechanical effects are enabled (in this case the absorbing layers are called quantum-wells, QWs). The whole absorber region may comprise a number of quantum-well layers representing the so- called multiple-quantum-wells structure. Typically, semiconductor saturable absorbers are fabricated on top of a high-reflective semiconductor, dielectric or metallic mirrors forming semiconductor saturable absorber mirrors (SESAMs). SESAMs with different designs

have been used for mode-locking of a large variety of lasers, see for example the works published by E. A. De Souza et al., IEE Electron. Lett., vol. 29, pp. 447-448, 1993, and F. X. Kartner et al., IEEE J. SeI. Top. Quantum Electron., vol. 2, pp. 540-556, 1996, and B. C. Collings et al., IEEE J. SeI. Topics Quantum Electron, vol. 3, pp. 1065-1075, 1997.

Additional design features of a semiconductor saturable absorber include placing the nonlinear absorbing layer within a Fabry-Perot cavity. The Fabry-Perot cavity is generally defined between a semiconductor mirror, i.e. distributed Bragg reflector (DBR), placed below the absorbing layer(s) (in other words, absorber region) and the semiconductor air interface or a dielectric or semiconductor mirror defining the top surface of the device. The design parameters of a Fabry-Perot saturable absorber (FPSA) include the choice of the cavity length and the position of the active material within the cavity. By changing the cavity length the absorber can have an anti-resonant, A- FPSA, or a resonant, R-FPSA, behaviour at the operation wavelength, see for example U. Keller, et al., IEEE J. SeI. Topics Quant. Electron., vol. 2, pp. 435-452, 1996. Antiresonance is achieved if the change in round-trip phase, φ _ή , experienced by a light beam propagating within the Fabry-Perot cavity comprising the absorber, satisfies the following relation:

φ _n = 2nkd + φ _b + φ _t = (2m + \)π ,

where k = 2π/λ, φ _t is the phase shift introduced by the top mirror, and φ _b is the phase shift introduced by the bottom semiconductor/dielectric or metallic mirror, n is the average refractive index of the cavity and d is the thickness of the Fabry-Perot cavity. The A-FPSA has a high reflectivity over a broad spectral range and negligible group-delay dispersion. However, the change in nonlinear reflectivity is relatively small because of the short effective interaction length between the optical field and the absorber (L.R. Brovelli, et al., JOSA B, vol. 12, pp. 311-322, 1995). An A-FPSA provides low loss but also exhibit a small modulation depth, usually below 1%. These features are desirable for oscillators with low single-pass gain, such as solid state lasers.

For high-gain lasers, e.g. fiber oscillators, higher modulation depth can be accommodated to improve the self-starting capabilities of mode-lock operation regime. The antiresonant absorber seems not to be a suitable solution. A substantial increase in modulation depth can be obtained by using R-FPSA, see for example J. F. Heffernan et al., Appl. Phys. Lett., vol. 58, pp. 2877-2879, 1991. In this case the thickness of the Fabry-Perot cavity is chosen so that the roundtrip phase near the signal wavelength is a multiple of 2π. The resonant type absorbers are incompatible with solid-state lasers because they introduce too much loss into the laser cavity when the operation wavelength is close to the Fabry-Perot resonance.

K. Weingarten et al. in U.S. Pat. 6,538,298 B1 proposed a resonant design for SESAM to enhance nonlinear effects for devices comprising very thin active regions. Another R-FPSA design reported in U.S. Pat.

6,252,892 B1 combines the saturable absorption and power limiting function based on two photon absorption. An important characteristic of

R-FPSA is a large value of the dispersion exhibited near the resonant wavelength. It was recently shown that the dispersion induced by R-

FPSA operated in reflection could be high enough to balance the dispersion of a fiber laser cavity, see for example M. Guina et al., Appl.

Phys. B., vol. 74, pp. S193-S200, 2002 and L. Orsila et al., Appl. Opt. vol. 43, pp. 1902-1906, 2004. The dispersion properties of the R-FPSA are particularly attractive for mode-locking lasers with a normal average dispersion of the cavity where anomalous dispersion can be provided by R-FPSA.

It is important to note that these entire prior-art designs employed the resonant absorbers in reflection mode, i.e. in the form of a SESAM. Using R-FPSA in reflection has obvious problem, since the losses increase for near-resonant operation making difficult to achieve mode- locking close to Fabry-Perot resonant wavelength. Particularly, it was observed (D. Korf, et al., Opt. Lett., vol. 21 , pp. 486-488, 1996) that the absorption increase near resonant wavelength resulted in forces a laser to operate away from spectral range with anomalous dispersion thus preventing proper dispersion compensation to be achieved. In

order to force the laser to operate near resonant wavelength of the R- FPSA, the cavity architectures employs additional wavelength selective elements, e.g. optical filters.

Besides the usual approach to use semiconductor saturable absorbers in the form of SESAMs operated in reflection mode, the saturable absorption effect can also be exploited in transmission. Prior developments related to the use of semiconductor saturable absorbers in transmission for mode-locking fiber lasers have been reported by M. Zirngbil et al., IEE Electron. Lett., vol. 27, pp1734-1735 (1991), M. E. Fermann et al., U. S. Pat. 5,414,725 and, H. Lin et al., U. S. Pat. 5,436,925. However, in all these cases the advantages of employing R- FPSA designs in transmission have not been recognized.

SUMMARY OF THE INVENTION

According to this invention, a resonant Fabry-Perot semiconductor saturable absorber etalon is operated in transmission for achieving self-starting cw mode-locking. Unlike the reflection configuration, the lowest loss state for the laser operation occurs at the etalon resonance. Therefore, it is expected that the operation wavelength is spontaneously (without using intracavity filters) set near the resonance of the etalon where the device has the highest modulation depth and significant dispersion, which, consequently should improve self-starting capability of mode-locking. For lasers cavity with average dispersion in the normal regime the laser will automatically operate near cavity resonance at the wavelength where the dispersion of the Fabry-Perot etalon is anomalous. Indeed, appropriate dispersion compensation results in shorter mode-locked pulses and, consequently, in a higher peak power. Together with the high modulation depth, the possibility to achieve dispersion compensation makes the Fabry-Perot semiconductor saturable absorbers operating in transmission a promising component for ultrafast laser.

To put it more precisely, the resonant wavelength of the Fabry-Perot semiconductor saturable absorber etalon operating in transmission sets up the laser wavelength because it ensures low-loss operation at

this wavelength and provides high loss at the wavelengths outside the resonance. This firmly defines the operation wavelength without the need for additional spectral filter.

To put it more precisely, the resonant wavelength of the Fabry-Perot semiconductor saturable absorber etalon operating in transmission sets up the laser wavelength near the resonance, where Fabry-Perot cavity generates dispersion that may compensate the dispersion induced by other cavity elements.

To put it more precisely, the resonant wavelength of the Fabry-Perot semiconductor saturable absorber etalon operating in transmission is determined by the total thickness of the Fabry-Perot cavity and it can be set by a transparent spacer layer inserted in the cavity together with absorber region.

To put it more precisely, the dispersion induced by the cavity of the Fabry-Perot absorber is set to the required value by changing the value of mirrors' reflectivities defining the Fabry-Perot cavity and the thickness of the Fabry-Perot cavity.

To put it more precisely, the cavity of the Fabry-Perot semiconductor saturable absorber generates both anomalous and normal dispersion located at opposite slopes of the cavity resonance. The absolute value of dispersion is determined by the finesse and free spectral range of the Fabry-Perot cavity.

To put it more precisely, in one preferred embodiment, the reflectivity of the top and bottom DBRs are equal providing symmetric Fabry-Perot structure.

To put it more precisely, a Fabry-Perot semiconductor saturable absorber etalon operated in transmission mode ensures a reliable self- starting passive mode-locked operation in a compact laser system.

To put it more precisely, in one preferred embodiment, the resonate Fabry-Perot semiconductor saturable absorber etalon is used in a ring

fiber laser cavity that improves mode-locking capability by minimizing the spurious reflections.

To put it more precisely, in one embodiment, the resonate Fabry-Perot semiconductor saturable absorber etalon is used in a standing-wave linear cavity to achieve high repetition rate harmonic mode-locking by exploiting optical pulse collision within the saturable absorbing region.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the invention is provided by the description of several specific embodiments and the corresponding drawings in which:

Fig.1 shows a cross-section description of a general structure of a resonant semiconductor saturable absorber etalon designed according to this invention. The functional differences between operation in reflection mode and transmission mode are also presented.

Figs.2a and 2b present two design embodiments of a resonant semiconductor saturable absorber etalon for operation in transmission.

Fig.3 shows an example of a low intensity transmission and dispersion characteristics of a resonant semiconductor saturable absorber etalon used according to this invention for applications at 1050 nm.

Fig.4a illustrates one embodiment of unidirectional ring cavity passively mode-locked fiber lasers employing a semiconductor saturable absorber etalon used according to this invention.

Fig.4b illustrates one embodiment of bi-directional ring cavity passively mode-locked fiber lasers employing a

semiconductor saturable absorber etalon used according to this invention.

Fig.5 illustrates one embodiment of linear-cavity passively mode- locked fiber lasers employing a semiconductor saturable absorber used according to this invention to initiate harmonic mode-locking through pulse collision.

DETAILED DESCRIPTION OF THE INVENTION

With reference to Fig. 1 , the general structure of a semiconductor saturable absorber etalon 12 designed according to this invention includes a first distributed Bragg reflector (DBR) 1 and a second distributed Bragg reflector 2 that define the Fabry-Perot cavity comprising the absorber region 3 and spacer layers 4 and 5.

The DBR layers have thicknesses of a quarter of the optical wavelength at which the DBR is designed to have a certain reflection. Reflectivity can be adjusted by changing the number of constituting layers. The absorbing multi-layers region (i.e. the absorber region 3) comprises layer(s) with energy band-gap small enough to absorb an optical signal and provide a nonlinear interaction with the signal. Those skilled in the art would recognize that certain technological measures should be implemented for reducing the absorption recovery time. Compound semiconductor layers 4 and 5 are transparent at the operation wavelength. They are used to control the thickness and, therefore, the resonant wavelength of the Fabry-Perot cavity defined by the DBR mirrors as well as the position of the absorber region in respect to the maxima of the standing-wave pattern of the optical field existing within the cavity.

Figs. 2a and 2b reveal two design embodiments of the resonant semiconductor saturable absorber etalon. According to a first design, Fig. 2a, the device is grown on a semiconductor substrate 6 whose surface is antireflection coated (AR) to avoid unwanted Fabry-Perot effects between the absorber's mirrors 1 and 2 and the substrate/air

interface. According to a second embodiment, Fig. 2b, the substrate is removed until the first layer of DBR 2.

As those skilled in the art could recognize, by suitable choice of the materials the design presented in this invention can be used to fabricate resonant semiconductor saturable absorber etalons at different wavelengths. For example, the active material consists of InGaAs or InGaAsP or InGaAsN , the substrate is GaAs or InP and the DBRs preferably consists of AIGaAs/GaAs pairs.

An important feature of the resonant semiconductor saturable absorber etalon operating in transmission is that low-loss spectral window near resonance corresponds to the wavelength range of the highest dispersion that may be sufficient to compensate the dispersion supplied by the other cavity elements. To justify these claims, Fig. 3 shows the calculated transmission and dispersion characteristics of an exemplary resonant semiconductor saturable absorber etalon with a resonance of the Fabry-Perot around 1050 nm. The curves are plotted for different values of the absorption.

The resonant semiconductor saturable absorber etalon operating in transmission mode becomes a natural solution for mode-locking fiber lasers with ring cavity configuration, as shown for example in Fig. 4a and 4b. It is well established that contrary to the standing-wave linear cavity, a unidirectional ring cavity, shown in Fig. 4a, is less sensitive to the spurious intracavity reflections and, therefore, has an enhanced potential to self-starting mode-locking. The use of the resonant semiconductor saturable absorber etalon in bi-directional ring cavity, as shown in Fig. 4b, allows to exploit pulse collision in the absorber and thus to achieve additional pulse shortening owing to colliding-pulse mode-locking, as initially presented in U. S. Pat. 5,414,725 to Fermann et al.

In another application example, the resonant semiconductor saturable absorber etalon operating in transmission is placed in a linear cavity such that it defines two subcavities, as shown in Fig. 5, with optical lengths L _A and L ₆ chosen such that L _A =nl_ _B . In this case harmonic

mode-locking is enhanced owing to pulse collision within the absorbing region.

Here the gain medium, for example Erbium or Ytterbium doped fiber, is pumped optically to generate a signal beam. The pump generates the pump signal which is launched into the fiber by a coupling element, e.g. fiber wavelength multiplexer. The ring laser cavity may contain optical isolators to ensure unidirectional propagation in the laser cavity. The

Fabry-Perot absorber operating in transmission butt-coupled or lens coupled with the fiber section of the cavity.

REFERENCES CITED U.S. PATENT DOCUMENTS

6,538,298 3/2003 Weingarten 257/436 5,701 ,327 12/1997 Cunningham 372/99 6,252,892 06/2001 Jiang 372/11 5,414,725 05/1995 Fermann 372/18 5,436,925 05/1995 Lin 372/92

OTHER PUBLICATIONS

E. A. De Souza et al., "Saturable absorber ", IEE Electron. Lett. , vol. 29, pp. 447-448, 1993.

F. X. Kartner et al., "Soliton mode-locking with saturable absorbers", IEEE J. SeI. Top. Quantum Electron., vol. 2, pp. 540-556, 1996.

B. C. Collings et al., "Short cavity Erbium/Ytterbium fiber laser mode- locked with a saturable Bragg reflector", IEEE J. SeI. Topics Quantum Electron, vol. 3, pp. 1065-1075, 1997.

U. Keller et al. "Semiconductor Saturable Aborber Mirrors (SESAMs) for femtosecond to nanosecond pulse genartion in solid-state lasers", IEEE J. SeI. Top. Quantum Electron., 2, pp. 435-452, 1996.

L. R. Brovelli, et al. "Design and operation of antiresonant Fabry-Perot saturable semiconductor absorbers for mode-locked solid-state lasers", JOSA B, vol. 12, pp. 311-322, 1995.

J. F. Heffernan M. H. Moloney et al., "All optical, high contrast absorptive modulation in an asymmetric Fabry-Perot etalon", Appl. Phys. Lett., vol. 58, pp. 2877-2879, 1991.

1 .1:

M. Guina, et al., "Stretched-Pulse Fiber Lasers Based on Semiconductor Saturable Absorbers", Appl. Phys. B., 74, pp. S193- S200, 2002.

L. Orsila et al., "Mode-locked ytterbium fiber lasers", Appl. Opt. vol. 43, pp. 1902-1906, 2004.

D. Korf, et al., "All-in-one dispersion-compensating saturable absorber mirror for compact femtosecond laser sources", Opt. Lett., vol. 21 , pp. 486-488, 1996.

M. Zimgbil et al., "1.2 ps pulses from passively mode-locked laser diode pumped Er-doped fiber ring laser", IEE Electron. Lett., vol. 27, pp1734-1735, 1991.