Fast Wavelength switching from a tunable DFB laser with a non-linear optical feedback A. Fischer (1), J.P Goedgebuer (2) (1) Laboratoire de physique des lasers, UMR CNRS 7538, Université de Paris XIII, Av. J.B. Clément, 93340 Villetaneuse, France.
[email protected] (2) Laboratoire d'Optique P.M. Duffieux, UMR CNRS 6603, Université de Franche-Comté Route de Gray, La bouloie, UFR des sciences et techniques, 25030 Besancon, France
Abstract :Multistability in wavelength with nanosecond switching time is demonstrated. The device is based on a tunable DFB laser with an external cavity that contains a non-linear element in wavelength such as a birefringent slab between polarizers. Introduction All-optical bistable and multistable devices are among the basic elements that can be used to implement all-optical functions for signal processing. They can be used when fast wavelength switching is required such as in WDM systems for example. For this reason, multistability with both electro-optical (hybrid) and all-optical feedback loops has been intensively studied.[1]. A true controllable bistable device in wavelength has been demonstrated experimentally [2] with a 50µs-switching time, but despite those efforts, hybrid multistable devices are unable to exhibit fast wavelength switching times required for telecommunication applications when they are 4 or 5 orders of magnitude shorter. The thermal effects involved in the tuning process of the DFB lasers slow the switching mechanisms down. For these reasons, all-optical devices rather than hybrid bistable devices are preferable. The subject is of importance, as the underlying question is, how controlling light with light. We investigate here all-optical functions for optical networking purpose based on all-optical multistability in a semiconductor DFB laser. Description of the device The device is shown in Fig 1. It consists of a tunable DFB laser with a non-linear external cavity laser and a diagnostic branch. The DL used is a 5mW, λ0=1.545µm double section tunable DFB diode laser (DL). The tuning range is ∆λDFB=1.2nm tuning range when current I2 on the first electrode ranges from 20mA to 100mA. The threshold current is Ith=4mA for an I1=9mA current on the second electrode (free running). The external optical feedback loop consists of the following elements : an antireflection coated 5mm-focal length lens (L), a grey filter (F), a linear polariser (P), a calcite crystal birefringent slab (BS1) and a mirror (M).
Figure 1: Description of the device: DFB: tunable distributed feedback laser, P: polariser, BS1: birefringent slab, M: Mirror. The path of the light is the following. The light emitted by the DFB laser passes through a lens which is AR-coated, through a filter F, and through a polariser P whose direction of polarisation is the same as that of the DFB. Both P and F are slightly misaligned to avoid spurious reflections into the DFB laser. The light enters the birefringent slab BS1 of length l and whose ordinary and extraordinary axes are 45degrees tilted with respect to the direction of polarisation of P, resulting in an optical path-difference ∆=(ne-no)l= 9.6mm for l=100mm. The distance between the laser facet and the BS is 0.9m. Mirror M is a multicoated mirror with a reflection coefficient of 99,9% at 1.55µm at normal incidence. The total length of the external cavity is l+L, yieding longitudinal modes with a free spectral range. The light reflected back passes through BS1, undergoes a delay 2∆ and finally passes through P and F before being injected in the DFB. The role of filter F is to control the amount of light re-injected into the DFB laser. Due to P, the injected light has the same polarisation than the outgoing light. Finally, it turns out that the BS behaves as a spectral polarisation filter with a spectral transmission curve that is a channelled spectrum with a FSR expressed by ∆λ=λ02/2∆. This channelled spectrum corresponds to a very high wavelength non-linearity, the BS exhibiting 9 transmission peaks within the tuning range of the DFB laser (∆λDFB /λ02/2∆ ≈ 9 ), meaning that BS behaves as a multi-passband filter. The dynamics of such an extended-cavity laser diode can be modelized by use of rate equations [3].
Experimental results As the non-linearity in the external cavity acts as a filter with multiple transmission peaks (a channelled spectrum) which select a group of external cavity modes, the laser is lockeed on those selected modes [4]. Thus when scanning the wavelength of tunable laser, the system operates as a multistable device which hops from one steady wavelength state to another one, the wavelength of each of those steady-states being given by the wavelength of the transmission peaks of the BS, as defined before.
device makes use of a tunable 1,55um DFB laser with an extended cavity and an intra-cavity frequency selective birefringent element The system can be seen either as an external cavity laser with a multiple transmission frequency selective element, or as an all-optical nonlinear optical feedback loop. We showed that when varying continuously the pump current, the laser frequency remains locked to a given wavelength state, then jumps to another state, yielding hysteresis loops and multistability phenomena. The number of states in wavelength is defined by the number of peaks of the channelled spectrum within the tuning range of the DFB laser. The switching time is in the nanosecond scale (2 to 4ns). Potential applications deal with all-optical signal routing, wavelength switching and all-optical signal processing.
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Figure 2: Laser wavelength λ versus driving current I2. The experimental curve shows the hysteresis loops with nine stable states in wavelength Their wavelength periodicity is 0.125 nm (horizontal axis:10 mA/div., vertical axis :0.25 nm/div.)
When scanning the electrode current up and down [respectively], the wavelength follows the lower and the upper trace [respectively], describing a trajectory formed by 9 succesive hysteresis loops, as shown in Fig.2. Each loop corresponds to two states attached to the edges of the channelled spectrum transmission peaks where the system is locked. Two adjacent stable states in wavelength are separated one with each other by an unstable region. The set of steady-states thus obtained forms a comb of equispaced wavelengths that are determined only by the non-linear element and that are independent of the frequency drifts of the laser relative to the tuning current and temperature fluctuations. An advantage of such a setup is that those wavelength steady states are independent of mechanical or thermal instabilities of the laser cavity. Moreover such a locking mechanism prevents from frequency drifts. For an optical path difference ∆= 9.6mm, 9 states in wavelength separated by 16 GHz have been obtained. The switching time between two adjacent states in wavelength was measured to be between 2 to 4 when the amount of light injected in the chip was about 1% of the emitting light amout. For this amount of feedback light the relaxation oscillations (~1 to 2 GHz) remained damped. We speculate that the speed of the switching mechanism can be increased by improving the design of the cavity and by optimizing the feedback rate. Conclusion We have demonstrated experimentally all-optical multistability in wavelength. This wavelength multistable
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Optical bistability: Controlling light with light. Hyatt Gibbs. Academic Press.1985 A. Fischer, J. P. Goedgebuer, "High density wavelength switching from a tunable multisection laser diode for signal routing", Optics Lett. vol. 24, 1999, p. 745. M. Yousefi and D. Lenstra, “Dynamical behavior of a semiconductor laser wity filtered external optical feedback,” IEEE J. Quantum Electron., vol. 35, pp. 970–977, 1999. A. P. A. Fischer, O. K. Andersen, M. Y., S. Stolte, D Lenstra, "Experimental and Theoretical Study of Filtered Optical Feedback in a Semiconductor Laser " IEEE J. of Quantum Electron., Vol. 36, No. 3, 2000, pp-375-385.
