WO2004073123A1

WO2004073123A1 - Multi-wavelength mode-locked laser source

Info

Publication number: WO2004073123A1
Application number: PCT/CA2004/000189
Authority: WO
Inventors: Sophie Larochelle; Jean-Noel Maran
Original assignee: Universite Laval
Priority date: 2003-02-14
Filing date: 2004-02-12
Publication date: 2004-08-26

Abstract

A multi-wavelength laser light generation device is provided for generating a multi-wavelength laser in a mode-locked regime. The device includes an input (802) for receiving a laser precursor signal, a laser cavity and an output for emitting a multi-wavelength laser in a mode-locked regime. The laser cavity includes a homogeneously broadened gain medium (804), a spectral filter module (806), a frequency shifter (808) and an amplitude modulator (810).

Description

TITLE: MULTI-WAVELENGTH MODE-LOCKED LASER SOURCE

FIELD OF THE INVENTION

The present invention relates generally to lasers and, more particularly, to a multi- wavelength mode-locked laser source. This invention is particularly application in the fields of telecommunications, optics, sensing and spectroscopy.

BACKGROUND OF THE INVENTION

Short pulses emission is generally obtained in a laser by synchronizing all the cavity modes in a given wavelength band, a phenomena called mode-locking. In the case of active mode-locking, mode-coupling is typically obtained by introducing an amplitude modulator in the laser cavity. To obtain a mode-locked regime, the modulation frequency value is adjusted so that it corresponds to a multiple of the cavity's fundamental frequency. Deviation of the modulation frequency from these values automatically induces the loss of the mode-locked emission regime (short pulses having a fixed repetition rate). From a temporal point of view, the mode-locked regime can be explained as follows: we consider that the laser begins to oscillate in a pulsed regime and that the pulses propagate through the cavity by passing through the amplitude modulator at the times when the amplitude modulator's transmission is maximum. Several pulses can propagate simultaneously in the cavity. In this case, the laser repetition rate is increased and the modulation frequency, f_mod, is now equal to a multiple of the fundamental frequency, f₀, such that f_mod=m.fo where m is an integer. This is the harmonically mode-locked regime frequently used with fiber lasers. The operation of mode-locked lasers can be described in the frequency domain by considering that the field of each mode with frequency f_p is partially injected in neighboring cavity modes with frequencies f_p ± n.f_mod where n is an integer. As a result of this injection, the different modes are locked in phase. A first difficulty in obtaining multi-wavelength mode-locked laser sources is that a stable multi- wavelength regime must be obtained at room temperature with a flat output spectrum (approximately 3 dB). A second first difficulty in obtaining multi- wavelength mode-locked laser sources is that simultaneous mode-locking of all wavelengths must be obtained with the same modulation frequency. Several techniques have been proposed in the literature to realize multi-wavelength mode- locked sources.

A first technique consists of using a gain medium that allows laser emission of different wavelengths at the same time. An example of such a medium is GaAs/AlGaAs commonly used to realize SOAs (Semiconductor Optical Amplifiers). The reader is invited to refer to the following publications for additional information related to this technique:

• Mielke, M., Alphonse, G.A. and Delfyett, P.J., "60 channel WDM transmitter using multiwavelength mode-locked semiconductor laser". Electron. Lett. 2002. Vol. 38, (8), pp. 370-371.

• Papakyriakopoulos, T., Stavdas, A., Protonotarios, E.N. and Avramopoulos, H., "10x10 GHz simultaneously mode-locked multiwavelength fibre ring laser" Electron. Lett., 1999. Vol. 35, (9), pp.717-718.

• United States Patent No. 6,334,011 issued to Galvanauskas, A., Wong, K.K. and Harter, D.J., "Ultrashort-pulse source with controllable multiple- wavelength output". • United States Patent No.6,192,058 issued to Abeles J.H., "

Multiwavelength actively mode-locked external cavity semiconductor laser".

The contents of the above documents are incorporated herein by reference. An example of a multi-wavelength laser using a SO A (Semiconductor Optical Amplifier) is shown in Figure 1 of the drawings. With this type of cavity 100, the mode-locked regime is obtained with a direct modulation of the SOA's injection current. The wavelength selection is achieved by using a diffraction grating 102 combined with two spatial filters 104 106. Results obtained with this configuration are shown in Figures 2a and 2b of the drawings. In figure 2a, the spectrum for 60 wavelengths obtained with a 2200 lines/mm grating is shown. In figure 2b, a laser pulse train with at a 3.593 GHz repetition rate generated by the multi-wavelength laser of figure 1 is shown.

A first deficiency of the above-described cavity is that it is a free-space cavity prone to environmental perturbations and this architecture is less stable than an all-fiber cavity configuration. A second deficiency of the above-described cavity is that its losses are relatively large and the laser output power is low. As a consequence, the signal must be amplified by two additionals SOAs 108 110 at the laser output 112. The system also includes an optical time division multiplexing module (OTDM) 150 between the two SOAs 108 110 to increase the repetition rate. A third deficiency of the above-described cavity is that the direct modulation of the SOA's injection current limits the maximal modulation frequency to a few GHz. Typically, frequencies of the order of 10 GHz will generate pulses with large chirp (pulse frequency variation). This property is unsuitable for several applications including, for example, highspeed transmitters in telecommunication networks with transmission rates in the vicinity of 40 Gbit/s or higher, or with return to zero modulation formats.

Other techniques for obtaining a multi-wavelength mode-locked laser are based on using a homogenously broadened gain medium like erbium-doped medium. Erbium- doped glasses present several advantages. A first one is that its emission band corresponds to the main wavelength bands used in optical telecommunication systems. Moreover, it is possible to obtain erbium-doped optical fiber. It is therefore possible to realize all-fiber laser cavities thus increasing the stability and output power of these sources. A deficiency with such a gain medium is that, at room temperature, this medium behaves as an homogenously broadened gain medium that, because of gain cross-saturation, does not permit to obtain a stable multi-wavelength regime for a large number of laser lines. Typically, no more than two or three laser lines can be obtained, especially if the wavelengths are closely spaced. In fact, the Erbium-doped silica homogenous line width is typically 10 nm at room temperature, so two wavelengths cannot emit simultaneously if they are not separated by at least 10 nm. Multi-wavelength emission on closer wavelengths may be obtained if the gain spectrum is very flat, but this emission will often be unstable. An example of such a laser, developed by Bakhahi et al., is shown in Figure 3 and is described in Bakhshi, B. and Andrekson, P. A., "Dual-wavelength 10-GHz actively mode-locked erbium fiber laser" IEEE Photon. Technol. 1999. Nol 11, (11), pp. 1387-1389. The content of the above document is incorporated herein by reference. As shown, the cavity includes a LiΝb0₃ amplitude modulator 300 and a feedback electronic system 302 is used to stabilize the mode-locked emission and to limit the temporal jitter of the pulses. The results obtained with this configuration are shown in Figures 4a and 4b of the drawings. Figure 4a shows a pulse train with a repetition rate of 10 GHz generated by the laser of figure 3 and released at output 304. Figure 4b shows the output laser spectra. As shown, the wavelength spacing is 21.6 nm.

Cross-gain saturation resulting from the homogeneous linewidth of erbium-doped glasses presents a difficulty in developing dense multi-wavelength mode-locked sources corresponding to optical telecommunication specifications. For example, in optical telecommunications, the typical channel spacing is 100 GHz or 50 GHz, corresponding to wavelength spacing of 0.8 nm and 0.4 nm. Therefore, some techniques are needed to circumvent the homogeneous 'gain characteristic.

In the literature, three main groups of techniques have been developed to overcome the erbium gain homogeneity. The first group of techniques makes use of a different gain media, physically separated, for each wavelength. For further information, the reader is invited to refer to Pudo, D., Chen L.R., Giannone, D., Zhang, L. and Bennion L, "Actively mode-locked tunable dual-wavelength erbium-doped fiber laser" IEEE Photon. Technol. Lett. 2002. Vol.14, (2), pp. 143-145. The content of the above noted document is incorporated herein by reference. In this case, cross-gain saturation is avoided because each wavelength interacts with a single gain medium. The experimental setup of one of theses systems is shown in Figure 5 of the drawings. As shown, the laser cavity is divided in two arms 500 and 502 with a 3 dB coupler 504 and in each arm the signal is filtered with a Bragg grating 514 516 to select the emission wavelength. Each wavlength is then amplified by a respective erbium-doped optical fiber amplifier (EDFA) 512 and 510. The two wavelengths are then recombined by 3 dB coupler 506 before going through the electro-optic modulator 508.

A deficiency of this configuration is the choice of the modulation frequency to obtain a mode-locked regime for the two wavelengths. In fact, the optical path lengths of the two arms 500 502 are not necessarily equal so the mode-locked regime is obtained at two different modulation frequencies for each wavelength. The problem therefore involves finding a common multiple of the two fundamental frequencies, which allows simultaneous mode-locking at the two wavelengths. This limits the number of possible frequencies and the likelihood that this frequency corresponds to the telecommunication standards are low. The results obtained with this configuration are shown in Figures 6a to 6c of the drawings. Figure 6a shows repeated scans for the output spectra of the laser. Figure 6b shows an output pulse train with both wavelengths contained in the signal generated by the laser shown in figure 5. Figure 6c shows an expanded view of output pulses shown for each independent laser wavelength.

Pudo et al. (see reference above) succeeded in obtaining a mode-locked regime for two wavelengths separated by 100 GHz. The multi- wavelength regime is stable, but achieving a dense multi- wavelength regime with this configuration (16 or 32 wavelengths) is impractical because of the difficulty associated with the modulation frequency choice.

Another means for achieving a multi-wavelength emission is to reduce the erbium homogeneous linewidth in immersing the optical fiber in liquid nitrogen at 77°K. A deficiency of this method is that it is impractical because of the maintenance required by the use of liquid nitrogen in the system. Another proposed configuration for obtaining a mode-locked laser emission for several wavelength bands with only a single gain medium is to introduce a temporal delay between the different wavelengths when the pulses pass through the gain medium. This technique is called the time-spreading technique. This solution has been demonstrated by Chen et al. who achieved simultaneous mode-locked emission for two wavelengths. For additional information regarding this technique, the reader is invited to refer to Chen, L.R., Town, G.E., Cortes, P.-Y., LaRochelle, S. and Smith, P., "W. E., Dual wavelength, actively mode-locked fibre laser with 0.7-nm wavelength spacing" Electron. Lett. 2000, Nol. 36, (23), pp. 1921 - 1923. The content of the above noted document is incorporated herein by reference. However, this configuration can allow only a limited number of wavelengths. The experimental setup is shown in Figure 7 of the drawings. A delay is introduced between wavelengths by the reflection of a set of Bragg gratings 702. The signal is then amplified by an erbium-doped optical fiber amplifier (EDFA) 706. The delay between wavelengths is then compensated by a second set of gratings 704 before passing trough the modulator 700.

In the context of the above, there is a need in the industry to provide a multiwavelength mode-locked laser that alleviates at least in part problems associated with the existing methods and devices.

SUMMARY OF THE INVENTION

In accordance with a first broad aspect, the invention provides a multi-wavelength mode-locked laser using a single homogenous gain medium.

In accordance with another broad aspect, the invention provides a multi-wavelength mode-locked laser including a laser cavity having an amplitude modulator, a frequency-shifter, a spectral filter and a homogenous gain medium. Advantageously, the combination of the amplitude modulator and the frequency- shifter in an homogenous gain medium allows for the generation of multiple wavelengths at a high repetition rate from a single laser cavity.

Advantageously, the invention provides a laser adapted for emitting multiple wavelengths simultaneously in a mode-locked regime.

In a specific example of implementation, the homogenous gain medium is selected from the set including erbium-doped glass and other rare earth doped glasses or crystals, and any other suitable homogenous gain medium.

In accordance with another broad aspect, the invention provides an optical transmitter including the above-described multi-wavelength mode-locked laser.

In accordance with another broad aspect, the invention provides a sensing system including the above-described multi-wavelength mode-locked laser.

In accordance with a broad aspect, the invention provides a multi-wavelength laser source generation device comprising an input, a laser cavity and an output. The input is for receiving a laser precursor signal. The laser cavity is in communication with the input and comprises a homogeneous gain medium, a spectral filter module, a frequency shifter and an amplitude modulator. The output is for emitting a multiwavelength laser in a mode-locked regime.

In specific implementations, the laser precursor signal may be generated using any suitable energy source including, but not limited to, electrical energy sources (example: current injection, electric discharge) and optical sources (example: flash lamps, pump laser). In a non-limiting implementation, the laser precursor signal is generated by a pump laser unit. These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

Figure 1 is a block diagram of a laser cavity setup using an SOA (semiconductor optical amplifier) in accordance with a prior art configuration;

Figures 2a and 2b show results obtained with the cavity shown in figure 1. In 2a, the spectrum for 60 wavelengths obtained with a 2200 lines/mm grating is shown. In 2b, an output laser pulse train with at a 3.593 GHz repetition rate is shown;

Figure 3 is a block diagram of a laser cavity setup using a multi-wavelength fibre laser with a single gain medium in accordance with an alternate prior art configuration;

Figures 4a and 4b show results obtained with the configuration shown in figure 3. Figure 4a shows an output laser pulse train with a repetition rate of 10 GHz. Figure 4b shows the output laser spectra.

Figure 5 is a block diagram of a laser cavity setup using a different gain medium for each wavelength in accordance with another alternate prior art configuration;

Figures 6a, 6b, 6c show results obtained with the configuration shown in figure 5. Figure 6a shows repeated scans for the output laser spectra. Figure 6b shows an output laser pulse train with both wavelengths contained in the signal. Figure 6c shows an expanded view of output pulses shown for each independent laser wavelength. Figure 7 is a block diagram of a laser cavity setup using a time-spreading technique in accordance with another alternate prior art configuration;

Figure 8a is a block diagram of a multi-wavelength laser source generation device in accordance with a specific example of implementation of the present invention;

Figure 8b is a block diagram of a multi-wavelength laser source generation device in accordance with another specific example of implementation of the present invention;

Figures 9a, 9b and 9c are block diagrams of a laser cavity and its components for a continuous wave regime;

Figures 10a and 10b show results of numerical simulations obtained with the configuration shown in figure 9a for a continuous wavelength erbium-doped fibre laser. Figure 10a shows the laser spectra when the laser cavity uses a frequency shifter. Figure 10a shows the laser spectra when the laser cavity has no frequency shifter;

Figure 11 shows an experimental output spectra of a multi- wavelength laser source using a frequency shifter in the cavity shown in figure 9a;

Figure 12 is a block diagram of a multi-wavelength pulsed laser source generation device in accordance with another specific example of implementation of the present invention;

Figure 13 shows results obtained with the configuration shown in figure 12. Fig.13a shows the spectra of the output laser;

Figure 13b shows temporal results of an output laser released by the device of figure 12 measured with a fast photodiode and a sampling oscilloscope; Figure 13c shows a pulse train with a repetition rate of 3.113 GHz of an output laser released by the device of figure 12; Figure 13d shows the auto-correlation traces (λ=l 542.73 nm) of the pulse train;

Figure 14a shows an output spectrum of a laser using a spectral filter having a bulk Fabry-Perot etalon with a finesse of 2 and a FSR of 100 GHz in accordance with an alternative non-limiting example of implementation of the present invention;

Figure 14b shows temporal characteristics of the laser of figure 14a;

Figure 14c shown an RF spectrum for the laser of figure 14a measured for a single wavelength (1554.18 nm).

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

DETAILED DESCRIPTION

Before explaining the disclosed embodiment of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation.

With reference to Fig. 8a, there is shown a cavity suitable for use for generating a multi-wavelength pulsed laser in accordance with a first specific non-limiting example of implementation of the present invention. In the case of the configuration shown in figure 8a, the laser cavity is a ring cavity. As depicted in figure 8a, the laser cavity includes an amplitude modulator 810, a frequency-shifter 808, a spectral filter 806 and an homogeneously broadened gain medium 804. The cavity may also includes a set of reflectors namely 816 818 820 and 822. The ring cavity also includes and isolator unit 850 for controlling the direction of the laser within the cavity so that the laser travels in one direction within the cavity. In use, the laser cavity allows the generation of a laser with multiple wavelengths emitting simultaneously in a mode-locked regime with a high repetition rate. In the case of optical fiber laser, the reflectors 816 818 822 and 820 shown in figure 8a may be omitted.

With reference to Fig. 8b, there is shown an alternative configuration of a cavity suitable for use for generating a multi-wavelength laser in accordance with a second specific non-limiting example of implementation of the present invention. In the case of the configuration shown in figure 8b, the laser cavity is a linear cavity.

As depicted in figure 8b, the laser cavity includes an amplitude modulator 810, a frequency-shifter 808, a spectrally periodic filter 806 and an homogeneously broadened gain medium 804. The cavity may also include a set of reflectors namely 824 and 826. In use, the laser cavity allows the generation of a laser with multiple wavelengths emitting simultaneously in a mode-locked regime with a high repetition rate.

The stable multi-wavelength regime permitted through the use of the frequency shifter 808 inside the laser cavity.

The amplitude modulator 810 allows the cavity to achieve active mode-locking.

The wavelength selection is obtained with a periodic spectral filter 806 adjusted to select a set of desired wavelengths. The spectral filter 806 may be implemented using any suitable filter device including but not limited to spectrally selective filters including thin film filters, liquid crystal filters, filters based on optical Fourier transform, Bragg grating filters and interferometric filters. The gain medium 804 can be a bulk gain medium or can be made of an optical waveguide (optical fiber or planar optical guide).

The dispersion of the cavity, or equivalently the optical path difference between each wavelength, is mainly caused by refractive index and waveguide dispersion. This optical path variation is very small (typically a few %) and can be controlled by carefully choosing optical components with low chromatic dispersion or by balancing the dispersion of the cavity. Transmissive optical filters like Fabry-Perot have low dispersion values and consequently adjustment of the modulation frequency by modulation signal selector 812 is made easier in the cavity.

To obtain a high repetition rate, the amplitude modulator 810 is used with a frequency equal to a multiple of the cavity fundamental frequency.

The frequency shifter 808 allows a stable multi-wavelength emission by preventing the saturation of the gain medium 804 by a single wavelength. At each-round trip through the cavity, the frequency of the optical wave is modified. A non-limiting example of a frequency-shifter 808 is an acousto-optic element in which the optical wave is diffracted by the refractive index modulation induced by the sound wave. The diffracted wave undergoes a Doppler effect and its carrier frequency is shifted by the frequency of the sound wave. The combined effect of the frequency shifter 808 and the periodic filter 806 induce some loss at the laser wavelengths, including the laser wavelength having the highest gain and the laser operates with a non-saturated gain.

A specific implementation of the present invention makes use of a non-obvious combination of techniques for the generation of continuous wave emissions with other components in order to obtain actively mode-locked multi-wavelength fiber laser. For the purpose of clarity and simplicity, a specific implementation of a continuous wave emission setup will be described herein below prior to the description of a non- limiting implementation of a mode- locked multi-wavelength pulsed laser. Previous studies on multi-wavelength lasers with frequency-shifter have been focused on the continuous wave (CW) emission regime or the passively mode-locked regime with a repetition rate corresponding to the cavity fundamental frequency. For more information, the reader is invited to refer to the following documents whose contents are incorporated herein by reference:

• Bellemare, A., Karasek, M., Rochette, M., LaRochelle, S. and Tetu, M., "Room Temperature Multifrequency erbium-doped fiber lasers anchored on ITU frequency gridXJofLightwaveTechnol, 2000, Nol.18, (6), pp. 825-831.

• Maran, J-Ν., LaRochelle, S. and Besnard, P., C band multi-wavelength frequency-shifted erbium-doped fiber laser soumis a Opt. Comm.

• Slavik, R., LaRochelle, S., and Karasek, M., "High-Performance Adjustable Room Temperature Multiwavelength Erbium-Doped Fiber Ring Laser In The C- Band", Opt. Commun., 2002, Nol. 206 (4-6), pp. 365 - 371.

• Sabert, H., and Brinkmeyer, E., "Pulse generation in fiber lasers with frequency shifted feedback ", J of Lightwave Technol, 1994, Nol. 12, pp. 1360-1368.

• Okhnotnikov, O. G., "Multi-wavelength picosecond frequency-shifted feedback laser with pulse control by a shaped-gain fiber amplifier", Opt. Lett., 1998, Nol. 23, pp. 1459-1461.

A specific experimental setup used for continuous wave emission is shown in Figure 9. The laser shown in an Erbium doped optical fiber laser pumped by a laser diode 902 at 980 nm. The periodic spectral filter 904 may be a Fabry-Perot (FP) micro etalon (shown in figure 9c), a filter based on fiber Bragg grating technology (shown in figure 9b) or any other suitable filter.

With reference to figure 9c, a Fabry-Perot 950, with a free spectral range (FSR) of 100 GHz, is used with a band pass filter 952, which in this non-limiting implementation has been embodied as a chirped fiber Bragg grating, to limit the operation spectral band of the laser (here 1542-1558 nm).

With reference to figure 9b, the second type of filter is a succession of fiber Bragg gratings written into the same photosensitive fiber with a Sagnac type interferometric writing setup. For additional information the reader is invited to refer to LaRochelle,S., Cortes, P.-Y., Fathallah, H., Rusch, L.A. and Ben Jaafar, H., "Writing and application fibre Bragg grating arrays" Proceedings of SPIE, 2000. (4087), pp.140. The content of the above document is incorporated herein by reference. The full width at half maximum (FWHM) and the reflectivity of each grating can be adjusted and controlled using well-known techniques. Typically, the gratings FWHM is 25 GHz and the reflectivity is close to 95 %.

The variable attenuator 920 allows the control of the total cavity losses.

Isolators 922 and 924 are used to reduce the amplified spontaneous emission (ASE) level and to induce a unidirectional propagation within the cavity that limits the problems due to spatial hole burning. The frequency shifter 930, which in a non- limiting implementation is in the form of an acousto-optic element, induces a frequency shift of 80 MHz at each round-trip through the cavity.

Advantageously, the presence of a frequency shifter 930 allows obtaining a stable multi-wavelength regime at room temperature. The reader is invited to refer to Bellemare, A., Karasek, M., Rochette, M., LaRochelle, S. and Tetu, M., "Room Temperature Multifrequency erbium-doped fiber lasers anchored on ITU frequency grid." J of LightwaveTechnol, 2000, Nol.18, (6), pp. 825-831. The content of the above-noted document is incorporated herein by reference.

The results of numerical simulation of the output of the configuration shown in figure 9a are shown in Figures 10a and 10b. In figure 10a, the spectra of the output laser is shown when the frequency shifter 930 is present in the cavity while figure 10b the spectra of the output laser is shown when the frequency shifter 930 is omitted from the cavity. As can be observed, without the frequency shifter 930, the laser emits only at the wavelength having the highest net gain. Advantageously, the frequency shifter inside the laser cavity allows obtaining a stable multi-wavelength regime. In a non-limiting implementation, the cavity shown in figure 9a includes a spectral filter 904 composed of 18 fiber Bragg gratings written into the same photosensitive fiber, which generated a laser source emitting on 17 wavelengths distributed on the C band (1525-1565 nm). The results obtained with this configuration are shown in Figure 11 of the drawings. The reader is invited to refer to Maran, J-N., LaRochelle, S. and Besnard, P., "C-band multi-wavelength frequency-shifted erbium-doped fiber laser", Optics Communications, vol. 218, pp. 81-86 (2003). The content of the above document is incorporated herein by reference.

In an alternate implementation, the cavity shown in figure 9a includes a spectral filter 904 with bulk Fabry-Perot micro-etalons, output power uniformity of 3 dB or better may be achieve for either 18 lines in the 1543-1560 nm region or for 13 lines in the 1538-1548 nm region. The reader is invited to refer to Slavik, R., LaRochelle, S., and Karasek, M., "High-Performance Adjustable Room Temperature Multiwavelength Erbium-Doped Fiber Ring Laser In The C-Band", Opt. Commun., 2002, Vol. 206 (4- 6), pp. 365 - 371. The content of the above document is incorporated herein by reference.

This type of laser (shown in figure 9a) was also shown to spontaneously emit short pulses at repetition rate corresponding to the cavity fundamental frequency. Simultaneous emission at a few number of wavelengths were thus observed at low repetition frequency, e.g. 3 wavelengths and 9 MHz repetition rate. This system however does not provide active mode-locking capabilities and the repetition rate is relatively low. In addition, only a limited number of wavelengths were simultaneously supported this laser.

The reader is invited to refer to the following documents whose contents are hereby incorporated by reference:

• Sabert, H., and Brinkmeyer, E., "Pulse generation in fiber lasers with frequency shifted feedback ", J of Lightwave Technol, 1994, Vol. 12, pp.

1360-1368; and Old-notnikov, O. G., "Multi-wavelength picosecond frequency-shiffted feedback laser with pulse control by a shaped-gain fiber amplifier", Opt. Lett, 1998, Nol. 23, pp. 1459-1461.

Specific Physical Implementation

A specific example of implementation of an actively mode-locked multi-wavelength fiber laser in accordance. with the invention is shown in Figure 12. As shown, the actively mode-locked laser in accordance with this implementation is similar to that shown in figure 9a for the continuous wave (CW) regime with the addition of an amplitude modulator 1000. The modulation frequency (3.113 GHz) of the amplitude modulator 1000 corresponds to several times the cavity fundamental frequency. This configuration allows obtaining active mode-locking and harmonic mode-locking. The spectral filter 904 includes an all fiber Fabry-Perot with a finesse of 2 and a FSR of 50 GHz. Further details on the experimental procedure used to make the filter can be found in Doucet, S., Slavik, R. and Larochelle, S. ,High-finesse large band Fabry- Perot fibre filter with superimposed chirped Bragg gratings Electron. Lett., 2002. Nol 38, (9), pp.402-403. The content of the above document are hereby incorporated by reference.

A band pass filter 1002 may also be used to limit the laser spectral band. The isolator 1004 located between the two filters 904 1002 avoids the coupled cavity effects that could be caused by the reflections between the two filters.

The results obtained with this laser source are shown in Figures 13a, 13b, 13c and 13 d. In a specific implementation, the laser delivers 24 wavelengths with an output spectrum flatness of 7 dB. The amplitude variation between the peaks comes from gain equalization imperfections (in order of 0.15 dB in the operating band). The multi-wavelength regime is stable. In Figure 13b, it can be observed that the intensity of each peak is relatively low. In a variant, the intensity of the peaks could be improved using a plurality of suitable techniques, such as, for example, by causing a reduction of the loss associated to the components in the laser cavity, using a high laser pump power, using a set of laser pumps (2 or more pump lasers), using an active medium with higher gain resulting from a higher dopant concentration or the use of other doping elements. The above techniques are well-known in the art and as such will not be described further here.

In another variant, the 90/10-output coupler 1006 is replaced by a 3 dB coupler placed before the modulator 1000. The pulse train released by the laser of figure 12 is displayed in Figure 13c. For the purpose of the experiment, the pulse train was measured using an averaging over 16 acquisitions, which is normally considered to constitute a good indication of low temporal pulse jitter. The spectral width for each wavelength band is 14 GHz (0.11 nm). It was measured with an optical spectrum analyzer with a 0.06 nm resolution. For this particular implementation, the pulse width is typically between 30 and 40 ps as can be deduced from the temporal trace shown in Figure 13c (measured on a sampling oscilloscope with a 30 GHz bandwidth) and the auto-correlation trace presented in Figure 13 d.

In yet another variant, the all fiber Fabry-Perot spectral filter 904 is replaced by a bulk Fabry-Perot etalon with a finesse of 2 and a FSR of 100 GHz. The results obtained with this broader filter are shown in figures 14a, 14b and 14c. The laser delivers 25 wavelengths (Fig. 14a) distributed between 1540 and 1560 nm. The output spectrum flatness is 6.8 dB and the total average power is 1 mW with a signal to noise ratio (SNR) better than 20 dB. In this specific implementation, the average pulse width is about 18 ps (depicted in fig. 14b). The time bandwidth product, noted Δv.Δτ is 0.48 indicating the pulses are nearly unchirped. The RF spectrum measured for a single wavelength (1554.18 nm) is shown in figure 14c. The SNR of the RF spectrum is better than 30 dB indicating that the pulse train is quite uniform.

In a specific implementation, the multi-wavelength laser source generation device may be used in a plurality of applications including but not limited to the field of telecommunication and metrology. It can also be used for optical components characterization. Other fields of interest include spectroscopy and sensing.

For example, the multi-wavelength laser source generation device may be integrated in:

• a high speed optical transmission device for use in a communication system;

• a laser source for optical sensing systems;

• a laser source for optical components characterization; • a laser source for temporal spectroscopy;

• a laser source for material characterization using non-linear effect.

Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, variations and refinements are possible without departing from the spirit of the invention. For example, the number of wavelengths or the optical output power may differ from that in the specific implementation described in this specification without detracting from the spirit of the invention. Therefore, the scope of the invention should be limited only by the appended claims and their equivalents.

Claims

CLAIMS:

1. A multi-wavelength mode-locked laser using a single homogenous gain medium.

2. A multi-wavelength laser source generation device, said device comprising: a) an input for receiving a laser precursor signal ; b) a laser cavity, in communication with said input, said laser cavity comprising: i. a homogeneous gain medium; ii. a spectral filter module; iii. a frequency shifter;and iv. an amplitude modulator; c) an output for emitting a multi-wavelength laser in a mode-locked regime.

3. A multi-wavelength laser source generation device as defined in claim 2, wherein the laser precursor signal is generated by a pump laser unit.

4. A multi-wavelength laser source generation device as defined in claim 2, wherein said laser cavity is linear.

5. A multi- wavelength laser source generation device as defined in claim 2, wherein said laser cavity is a ring cavity.

6. A multi-wavelength laser source generation device as defined in claim 2, wherein said homogeneous gain medium includes a bulk gain medium.

7. A multi-wavelength laser source generation device as defined in claim 2, wherein said homogeneous gain medium includes an optical waveguide.

8. A multi-wavelength laser source generation device as defined in claim 7, wherein said optical waveguide includes an optical fiber.

9. A multi-wavelength laser source generation device as defined in claim 7, wherein said optical waveguide includes a planar optical guide.

10. A multi-wavelength laser source generation device as defined in claim 2, wherein the homogenous gain medium is selected from the set consisting of erbium-doped glass, rare earth doped glasses and crystals.

11. A multi-wavelength laser source generation device as defined in claim 2, wherein said spectral filter includes a Fabry-Perot filter unit.

12. A multi-wavelength laser source generation device as defined in claim 2, wherein said spectral filter includes a band-pass filter.

13. A multi- wavelength laser source generation device as defined in claim 11, wherein said spectral filter includes a band-pass filter.

14. A multi-wavelength laser source generation device as defined in claim 11, said device comprising an isolator module between said band-pass filter and said Fabry-Perot filter unit.

15. A multi-wavelength laser source generation device as defined in claim 2, wherein the amplitude modulator is adapted to apply a frequency substantially equal to a multiple of the fundamental frequency of the cavity.

16. A multi- wavelength laser source generation device as defined in claim 2, wherein the frequency shifter includes an acousto-optic component.

17. An optical transmitter apparatus comprising the multi- wavelength laser source generation device described in claim 2.

18. A device suitable for providing optical components characterization comprising the multi-wavelength laser source generation device described in claim 2.

19. A device suitable for providing temporal spectroscopy functionality comprising the multi- wavelength laser source generation device described in claim 2.

20. A device suitable for providing material characterization for non-linear effects comprising the multi-wavelength laser source generation device described in claim

2.

21. A multi- wavelength laser source generation device, said device comprising a laser cavity comprising: i. a homogeneous gain medium; ii. a spectral filter module; iii. a frequency shifter;and iv. an amplitude modulator;

22. An method suitable for generating a multi-wavelength laser source, said method comprising: a) receiving a laser precursor signal; b) providing a laser cavity comprising: i. a homogeneous gain medium; ii. a spectral filter module; iii. a frequency shifter;and iv. an amplitude modulator; c) providing the laser precursor signal to said laser cavity thereby generating a multi-wavelength laser in a mode-locked regime.