The demand for increased bandwidth in fiber optic telecommunications has driven the development of sophisticated transmitter lasers usable for DWDM systems wherein multiple separate data streams propagate concurrently in a single optical fiber. Each data stream is created by the modulated output of a semiconductor laser at a specific channel frequency or wavelength, and the multiple modulated outputs are combined onto the single fiber. Telecom DWDM systems have largely been based on distributed feedback (DFB) lasers. To meet the demands for operation on the fixed grid of telecom wavelengths, also called the International Telecommunication Union (ITU) grid, DFBs have been augmented by external reference etalons and require feedback control loops. External cavity lasers have been developed to overcome the limitations of individual DFB devices.
The typical external cavity tunable laser design 100 as shown in FIG. 1 includes a gain medium 101 to provide light amplification, a periodic grid generator filter 103 (typically an etalon) to provide the correct channels frequency grid according to system specifications (typically C-band or L-band ITU frequency grid for DWDM systems), a channel selector filter 104 to be tuned with an appropriate tuning mechanism to select the lasing mode between the ones allowed by the channels grid and a collimating/adjusting lenses system 102 to provide the correct alignment and dimension of the collimated cavity beam. The cavity having a cavity length 110 is thus typically comprised between the end facet of the gain medium 101, which acts as the first mirror, and the channel selector element 104 which can work as the second mirror.
A design of such an external cavity laser can be found in U.S. Pat. No. 6,704,332 B2, where the channel selector filter 104 is implemented as a tunable element to feed-back light of a selected wavelength to the gain medium 101.
A possible solution to guarantee single mode lasing can be found in WO 2004070893, where the tunable mirror is a Guided Mode Resonance (GMR) mirror with a reflection peak that can be tuned by means of a suitable mechanism, which can be either electrical or thermal tuning of optical properties of one or more materials included in the GMR mirror structure. Practically, the effective gain range (EGR) of the gain medium is generally much wider than the required DWDM channels range, over which the channel selector element is required to be tunable. The gain profile also depends on the driving current, so a possible situation is that high gain is present at wavelengths outside the channels range. The tunable mirror described in WO 2004070893 does not have an infinite Free Spectral Range (FSR), thus besides the main reflection peak that can be used to select the channels over the grid, it always has secondary reflection peaks, and possibly a ground reflectivity noise. Both secondary peaks and ground noise can overlap outside the DWDM channels range with the etalon peaks to provide not negligible optical feedback.
FIG. 2 depicts a gain diagram 200 illustrating the gain 210 over wavelength of an external cavity laser that may generate a main lasing mode 201 and a secondary lasing mode 202. Due to the possible high gain outside the DWDM channels range 203, some unwanted lasing mode can arise in this frequency region, where secondary reflection peaks or ground reflectivity noise overlap with the etalon peaks. It results in an uncontrolled lasing frequency corresponding to longitudinal multimode oscillation of the laser cavity if the two modes, i.e., the main lasing mode 201 and the secondary lasing mode 202 as depicted in FIG. 2 are comparable in terms of gain/loss balance. This effect is much more critical when some frequency detuning exists between the etalon and the tunable mirror peaks as, for example, during the switch on procedure, in which the relative frequency alignment between the etalon and the tunable mirror is not actively controlled and thus the optical power feedback provided by the tunable mirror main reflection peak (corresponding to the main lasing mode 201), might be comparable with the one provided by the secondary peak, which allows the oscillation of the secondary lasing mode 202. Also, unwanted lasing modes can be excited by spurious reflections which can occur outside the channels range 203.