Abstract:
An hybrid laser source has a ring structure including the optical path in a semiconductor amplifier chip (1) and optical fibres (2, 3 and 4). The fibre (2) is rare-earth doped and provides an additional gain medium in the ring, the chip providing its own gain medium. The two gain media are interactive and when the chip input current is modulated the source produces relatively high peak power ultrashort pulses (FIG. 1).

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to laser sources and in particular to those incorporating rare earth doped fiber, for example erbium doped fiber. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an hybrid laser source comprising a semiconductor diode amplifier chip and a length of rare-earth-doped single mode optical fiber, the optical path within the chip forming part of a ring and another part of the ring being comprised by said fiber, which fiber provides an external cavity having distributed gain for the chip. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will not be described with reference to the accompanying drawings, in which: 
     FIG. 1 illustrates a doped fiber ring laser according to an embodiment of the present invention and 
     FIG. 2 illustrates the streak intensity profiles of pulses produced by an arrangement according to FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It has previously been proposed to employ erbium-doped fiber as a fiber amplifier. An optical signal input to one end of a length of erbium-doped fiber can be amplified during its passage therealong. 
     In GB Patent Application No. 8908671.3 (K. C. Byron - W. Sibbett 38-3) there is described a tuneable source employing such a length of fiber. It has also been proposed in our paper &#34;Amplification of mode-locked semiconductor diode laser pulsars in an erbium-doped fiber amplifier&#34; [R. A. Baker, D. Burns, K.C. Byron, W. Sibbett. Electronic Letters IEE 17th Aug. 1989, Vol. 25 No. 17 p 1131-3] to amplify picosecond pulses, from an actively mode-locked semiconductor diode laser having an external cavity, in a pumped erbium-doped fiber amplifier. In this case the output of the external-cavity laser was applied to one end of the fiber amplifier and the pump was applied to the other end, the output being taken from adjacent the other end by means of a coupler. 
     We now propose a laser configuration in which the amplifier fiber comprises an external fiber cavity of a semiconductor diode laser oscillator. In this proposed hybrid configuration the external fiber cavity provides access to distributed gain. An example of the configuration is illustrated in FIG. 1. An InGaAsP diode amplifier chip 1 is disposed in a ring cavity arrangement that includes a length 2 of rare-earth-doped, in particular erbium-doped, monomode optical fiber, and lengths 3 and 4 of monomode optical fiber. Couplers 5 and 6 serve to couple the ends of the erbium-doped fiber 2 to the fiber lengths 3 and 4. The diode amplifier chip may have its facets AR-coated or angled and AR coated. Emissions from the facets are directly coupled to fiber lengths 3 and 4. The erbium fiber is optically pumped by means 7, such as by a frequency-doubled (532 nm) Nd:YAG laser or a titanium-sapphire laser operating at 980 nm. In tests the length of the diode amplifier chip was 500 μm and that of the erbium-doped (300 ppm) fiber was 5 m. It should be noted that this leads to laser dynamics that are quite specific. The ring-cavity scheme should not be regarded as simply a mode-locked diode laser operating in conjunction with a distributed amplifier. When the injection current to the diode amplifier chip 1 is modulated, the pulse evolution processes develop in the system as a whole so that both gain media (chip and erbium-doped fiber) play important and interactive roles. The system, therefore, has hybridized operational characteristics and as such it has the inherent ability to produce relatively high peak power ultrashort pulses with the potential for exceptionally low phase-noise properties. 
     The combination of picosecond pulse durations, peak powers in the W regime and low phase noise suggest potential major importance of such sources in future optical systems. If the erbium-doped fiber is pumped by a strained-layer semiconductor diode laser that operates at 980 nm, a particularly practical, compact and reliable digital optical source results. 
     FIG. 2 illustrates the result obtained with the arrangement of FIG. 1 and the various parameters quoted above. The diode amplifier was injection current modulated at a frequency of 612 MHz. The time domain data of FIG. 2 confirms that high-quality picosecond pulses are produced by this arrangement. Specifically,. the streak intensity profiles (linear intensity scale) in FIG. 2 indicate that the pulses are discrete and have deconvolved durations of the order of 5 ps. Using second harmonic generation auto-correlation measurements that offer better time resolution, the pulse durations were confirmed to be of the order of 4 ps. The measured average laser power was in excess of 4 mW which represents a peak pulse power of more than 1.5W. At this level of peak power, optical non-linearities in fiber (such as the Kerr effect referred to in GB Patent Application No. 8909671.3) and other waveguide structures can become significant and potentially exploitable. 
     As will be appreciated from FIG. 1, the hybrid laser source has a ring structure comprised by the optical path within the chip 1 and the fibers 2, 3 and 4, and the fiber 2 provides an external cavity for the chip which has distributed gain. Such a source is high speed and stable and may be modulated.