Background of the invention As new technologies are developed for next- generation optical transport equipment, cost reduction efforts become essential. For example, cost effective solutions are needed for assembly and aligning of optical components, production tests and process monitoring.

One widespread method is to use a tunable laser, well known in the prior art, to continuously scan through the wavelength range for which the components are to be tested. In conjunction with an optical spectrum analyzer (OSA), this set-up may be used e. g. to align components or test that they are functional within the specified wavelength range.

However, this prior art is associated with major drawbacks and limitations.

For instance, the scanning speed of a state-of-the- art widely tunable laser (tuning range of approximately 30 nm or more) is approximately 100 nm/s, which is insufficient in order to provide for proper real-time feedback.

Summary of the invention The present invention provides new methods and arrangements for a light source capable of scanning a large wavelength range (> 60 nm), wherein the scan time of the light source is drastically reduced.

The present invention has further advantages, which will be apparent from the specification set forth below.

It is a general object of the present invention to provide an ultra-fast scanning light source, and more particularly a narrow-linewidth wavelength scanning light source.

Yet another objective of the present invention is to provide for a cost effective scanning light source.

According to a first aspect of the present invention, an arrangement is provided comprising an optical cavity, in which a gain medium and an acousto- optic fiber Bragg grating filter have been arranged. An oscillating radiation field is generated in the cavity and by matching the round trip time of the radiation in the cavity with the scan time of the acousto-optic fiber Bragg grating filter, a wavelength scanning light source is provided.

According to a second aspect of the invention, an arrangement comprising means for changing the length or round trip time of the cavity is provided. In this way a desired match with the scan time of the acousto-optic fiber Bragg grating filter may be achieved.

The present invention is based on the general insight that an acousto-optic fiber Bragg grating filter (AOFBGF) arranged in an optical cavity may be used to create a sweeping light source if the cavity length is matched to the scan time of the acousto-optic fiber Bragg grating filter. An acousto-optic fiber Bragg grating filter uses a method of establishing transmission of light through a chirped Bragg reflector. According to the method, a certain wavelength component is transmitted through the Bragg-reflector at a certain (corresponding) instant in time. In an unperturbed state, the Bragg reflector is reflecting essentially all incident light within a predefined wavelength range. According to the method, light is incident into an optical waveguide incorporating a chirped Bragg-reflector. The reflective properties of said Bragg-reflector are altered by sending a longitudinal acoustic pulse into said waveguide for

propagation along the same. For each location of said acoustic pulse in the chirped Bragg-reflector, the reflectivity for a wavelength associated with said location in said Bragg-reflector is altered, thereby establishing transmission of the wavelength at issue.

Such a system is disclosed in the Swedish patent number 0002415-8 and the Swedish patent application number SE 0100868-9, which are incorporated herein by reference.

Brief description of the drawings The following detailed description of preferred embodiments is better understood when read in conjunction with the accompanying schematic drawings, in which: Fig. 1 is a preferred embodiment of the present invention according to a ring cavity configuration of tunable laser with acousto-optic fiber Bragg grating filter.

Fig. 2 shows an alternative embodiment according to a sigma cavity configuration of laser cavity with acousto-optic fiber Bragg grating filter.

Fig. 3 illustrates generation of trig pulses.

Fig. 4 shows a set-up for parallel characterization of components.

Detailed description of preferred embodiments In fig. 1, we show the first configuration of the device.

The radiation field is generated in a ring cavity, which consists of a gain medium (G), an optical isolator (01), a 16-km long delay line (D), a tap coupler (T), and an acousto-optic fiber Bragg grating filter (AOFBGF). The gain medium compensates for any passive losses in the ring cavity. The gain medium could, for instance, be a semiconductor optical amplifier (SOA) or a rare-earth doped fiber amplifier (RDFA). If the device is operated above the lasing threshold, an obvious requirement is that the saturation output power is adequate and that the small signal gain exceeds the passive losses (which are

estimated to 6-7 dB). Note, however, that it also is possible to operate the light source below threshold and make use of the amplified spontaneous emission. The tap coupler has the same function as the out-coupling mirror in a classical Fabry-Pérot configuration, i. e. it couples out a small fraction of the oscillating field. The isolator prevents bi-directional oscillation in the cavity. The acousto-optic fiber Bragg grating filter is continuously scanned during the operation of the device.

The period time of the acousto-optic fiber Bragg grating filter sweep must be synchronized with the cavity round- trip time, so that the same part of the radiation field always experiences a transmission window at the same wavelength in each passage. The objective of the delay- line is to roughly match these two parameters-it takes about 80 ps for a light pulse to traverse 16 km of glass fiber. Active control of the delay line to maintain synchronization between the chirped pulse and the filter wavelength when the device is in operation is often required. It is often also necessary to apply some dispersion-compensation scheme, e. g. filtering the radiation by using a chirped grating in reflection. After a few passages, the amplified and filtered vacuum field has emerged into a long, chirped pulse that circulates in the cavity.

The ring cavity may also be configured in a slightly different way, without delay line. In this case, the cavity should be made as short as possible. When the transmission window of the acousto-optic fiber Bragg grating filter is at a certain position, an oscillating field of the corresponding wavelength is established in the entire ring. It is obvious that the so-called photon lifetime of the cavity in this case must be much smaller than the time required to scan the wavelength region of interest. For this configuration it is not necessary to maintain an accurate synchronization between the chirped

pulse and the filter wavelength, as the pulse is rebuilt during each wavelength sweep.

In fig. 2, we show another possible configuration, a so-called sigma cavity with an optical circulator (OC).

The radiation that enters the first port of the circulator exits at the second port, propagates through the gain medium and the delay line, and is then reflected by a Faraday Mirror (FM). The reflected radiation passes the delay line and the gain medium once again, and is then launched through the second port. The field exits the circulator at the third port and propagates through the acousto-optic fiber Bragg grating filter. Thereby, one round trip has been completed. Note that the delay line only should be half as long as in the previous case, as the light traverses this part of the cavity twice. It is not necessary to use an optical isolator in the sigma cavity, as the optical circulator ensures uni-directional operation. The field can be extracted anywhere in the cavity by a tap coupler, for instance, directly after the acousto-optic fiber Bragg grating filter. Also in the sigma cavity, the delay line can be removed, so that radiation of only one wavelength oscillates in the entire cavity at the same time.

It is important to have accurate timing between the electronic equipment that registers the radiation from the component under test and the tuned light source. In Fig. 3, we demonstrate how a train of trig pulses can be generated. A small part of the output radiation from the tunable light source is extracted in a tap coupler and transmitted through a comb filter (CF). The comb filter has narrow transmission peaks at a regular spacing in either frequency or wavelength domain. Such components are easy to find or to fabricate, e. g. Fabry-Perot cavities or superimposed multi-wavelength fiber-gratings.

It is also possible to use a filter with non-regular, but well-known absorption peaks, such as a gas cell. The filtered radiation is detected and converted to an

electrical signal by a standard detector (D), e. g. a photodiode. Every time the wavelength is scanned through one of the transmission/absorption peaks, a trig pulse is generated. The time delay between consecutive sweeps makes it possible to identify the first and the last wavelength in each pulse train.

One important application of a tunable light source is parallel characterization of many components. The tuning speed will be particularly important, if the components under test are to be adjusted interactively during the measurement. Fig. 4 shows an example of how this can be accomplished. A star coupler or a switch (S1) distributes the radiation from a tunable laser to a number of components under test (Ml-M4). The radiation from each output port is transmitted through one of the components, and then recombined in a switch (S2) and finally registered by a detector (D). The transmission spectrum of each device is recorded in 35 J. s. When a scan has been completed, the switch or the pair of switches selects the next component. In this way, all components are analyzed in the acousto-optic fiber Bragg grating filter period time (80 ps) times the number of components. State-of-the-art for widely tunable mode-hop free lasers is a tuning rate of 100 nm/s (external cavity diode lasers), which should be compared to around 500 pm/s for the acousto-optic fiber Bragg grating filter.

Evidently, the potential for improvement with an acousto- optic fiber Bragg grating filter-based ultra-fast tunable laser is huge.

Although the present invention has been described with reference to the drawings and by way of preferred embodiments, it is to be understood that the embodiments described can undergo several alterations and modifications without departing from the scope of the invention as defined in the accompanying claims.