WO2003043149A1 - Electronically tunable laser using wavelength selective reflectors - Google Patents

Abstract

A laser cavity for generating a train of laser pulses at a selected wavelength comprises a gain medium (100); an input pulse generator for gating on an effective gain of the cavity, the generator generating pairs of pulses with a time tsel between the pulses and a time T between the pairs of pulses; a first and a second set of distributed wavelength selective reflectors (101,102) on each side of the gain medium (100), each of the reflections being spaced apart in time; the second (102) set being paired with the first set (101) wherein the pairing ensures that a total cavity round-trip delay of T is equal for all wavelengths; a delay modifier for adjusting the time tsel between the pulses and selecting a wavelength; and an output; whereby the output repetition rate of the train of laser pulses can be equal for all wavelengths reflected by the first and second sets.

Description

ELECTRONICALLY TUNABLE LASER USING WAVELENGTH SELECTIVE REFLECTORS

CROSS-REFERENCE TO RELATED APPLICATIONS

The application claims priority of US provisional patent application serial no. 60/331 ,413 filed November 15, 2001 by Applicant.

FIELD OF THE INVENTION

The invention relates to electronically tuned lasers and more particularly to an electronically tuned laser which generates pulses at a fixed repetition rate regardless of the selected wavelength of the pulses.

BACKGROUND OF THE INVENTION

Modern optical fiber communications systems use trains of laser pulses at different wavelengths to transmit digital information. The use of 100 or more wavelengths per optical fiber increases the information capacity and, in addition, offers the attractive possibility of wavelength routing. Through the use of dispersive optical network elements such as Arrayed Waveguide Gratings (AWGs), a transmitting node can route optical packets to the right destination through selection of the optical carrier wavelength.

The advantages of electronic tuning of a laser over other means, such as mechanical or micro-mechanical tuning, is the microsecond-range tuning speed that can be expected and the mechanical robustness attendant to all motion-free solid-state construction. The ability to tune a laser within microseconds would mean that a given packet of bits could route itself to the right destination in a given portion of a passive optical wavelength-routing network by virtue of the optical carrier wavelength having been properly selected for the desired destination. An example of a a passive optical wavelength-routing network element is the AWG which will route a set of input wavelengths on one input port to a plurality of distinct output ports, one port for each wavelength.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an electronically tunable short-pulse laser which ensures that the output repetition rate of the train of laser pulses can be equal for any selected wavelength.

An electronically tuned short-pulse laser (ETL) uses a gain medium which is gated on twice by picosecond-duration pulses during one half of the interpulse period. On the left-hand side the laser cavity comprises a set of waveguide grating segments, each one of which reflects light at a different wavelength. The double temporal gating selects which wavelength selective reflector will effectively reflect laser light, and it therefore selects the wavelength that will lase. On the right-hand side the laser cavity comprises another set of wavelength selective reflectors in which each reflector is spectrally and positionally paired with the corresponding left-hand side reflector. The pairing is such that the interpulse period is precisely the same for all selected wavelengths. The pair of reflectors to which the laser is tuned is selected by electronically adjusting the delay between the two driving pulses applied to the gain medium. Gratings, such as fiber Bragg gratings, can be used as wavelength selective reflectors and chirped gratings can provide for continuous wavelength tuning and/or to generate chirped laser pulses.

According to a broad aspect of the present invention, there is provided a laser cavity for generating a train of laser pulses at a selected wavelength from a plurality of wavelengths, comprising: a gain medium generating light having a plurality of wavelengths from pump energy; an input pulse generator for gating on an effective gain of the cavity, the input pulse generator generating pairs of input pulses with a time t_se| between the input pulses of the pairs and a time T between the pairs of input pulses; on a first side of the gain medium, a first waveguide coupled to the gain medium; on a second side of the gain medium, a second waveguide coupled to the gain medium; on the first waveguide, a first set of distributed wavelength selective reflectors, each reflector of the first set reflecting light at one of the plurality of wavelengths, each of the reflections being spaced apart in time; on the second waveguide, a second set of distributed wavelength selective reflectors, each reflector of the second set reflecting light at one of the plurality of wavelengths, each of the reflections being spaced apart in time and being paired with a corresponding one of the reflectors from the first set reflecting light at a same one of the plurality of wavelengths; wherein the pairing is adapted to ensure that a total cavity round-trip delay of substantially T is substantially equal for all wavelengths reflected by the first and second sets; a delay modifier for adjusting the time t_se| between the pulses of the pairs and thereby selecting a selected wavelength for the train of laser pulses; an output for outputting the train of laser pulses at the selected wavelength; whereby the output repetition rate of the train of laser pulses can be substantially equal for all wavelengths reflected by the first and second sets.

According to another broad aspect of the present invention, there is provided a telecommunications optical input for generating a signal to be transmitted on a transmission link, comprising a laser cavity for generating a train of laser pulses at a selected wavelength from a plurality of wavelengths as described above, comprising: an input for receiving a data signal to be transmitted on the transmission link at a selected wavelength; an output for outputting the train of laser pulses at the selected wavelength; a modulator for modulating the train of laser pulses; an output of the modulated being coupled to the transmission link for transmission; whereby the output frequency of the train of laser pulses can be substantially equal for all wavelengths reflected by the first and second sets.

According to still another aspect of the present invention, there is provided a method for generating a train of laser pulses at a selected wavelength from a plurality of wavelengths, comprising: generating light having a plurality of wavelengths by a gain medium using pump energy; gating on an effective gain of the cavity by generating pairs of input pulses with a time t_se| between the input pulses of the pairs and a time T between the pairs of input pulses; on a first side of the gain medium, coupling a first waveguide to the gain medium; on a second side of the gain medium, coupling a second waveguide to the gain medium; on the first waveguide, placing a first set of distributed wavelength selective reflectors, each reflector of the first set reflecting light at one of the plurality of wavelengths, each of the reflections being spaced apart in time; on the second waveguide, placing a second set of distributed wavelength selective reflectors, each reflector of the second set reflecting light at one of the plurality of wavelengths, each of the reflections being spaced apart in time and being paired with a corresponding one of the reflectors from the first set reflecting light at a same one of the plurality of wavelengths; wherein the pairing is adapted to ensure that a total cavity round-trip delay of substantially T is substantially equal for all wavelengths reflected by the first and second sets; adjusting the time t_se| between the pulses of the pairs and thereby selecting a selected wavelength for the train of laser pulses; outputting the train of laser pulses at the selected wavelength; whereby the output repetition rate of the train of laser pulses can be substantially equal for all wavelengths reflected by the first and second sets.

According to a further broad aspect of the present invention, there is provided a method for generating a signal to be transmitted on a transmission link, comprising the steps of the method for generating a train of laser pulses at a selected wavelength from a plurality of wavelengths, and further comprising outputting the train of laser pulses at the selected wavelength; modulating the train of laser pulses; an output of the modulated being coupled to the transmission link for transmission; whereby the output frequency of the train of laser pulses can be substantially equal for all wavelengths reflected by the first and second sets.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features, aspects and advantages of the present invention will become better understood with regard to the following description and accompanying drawings wherein:

FIG. 1 is an illustration of an arrangement for the Electronically Tuned Laser (ETL); FIG. 2 is an illustration of a gain temporal profile at the gain medium for selecting λ2 for laser oscillation; FIG. 3 is an illustration of long chirped Bragg gratings for theleft waveguide grating and theright waveguide grating in the ETL;

FIG. 4 is an illustration of a solution to positive chromatic dispersion due to propagation in a fiber, Fig. 4a shows the effect on a pulse with positive dispersion and Fig. 4b shows the effect on a pulse that has been pre-chirped; to offset positive dispersion (i.e. longer transmission delays for optical energy at longer wavelengths)

FIG. 5 is an illustration of an ETL with discrete chirped gratings for generating up- chirped laser pulses; FIG. 6 is an illustration of up-chirped laser pulses as solutions to positive chromatic dispersion, FIG. 6a shows the effect of chromatic dispersion on the pulse 131 , Fig. 6b shows the effect of chromatic dispersion on the pulse 133;

FIG. 7 is an illustration of an ETL pseudo-ring cavity;

FIG. 8 is an illustration of gain temporal profiles at different beginnings for selecting any wavelength, FIG. 8a shows the steady-state temporal positions of potential laser pulses at each wavelength λ-| , λ2, λ3, λ^ and λ5 for the ETL cavity illustrated in FIG. 1 , where the G1 gating pulse is at time trj = trj + 1000 ps and

FIG. 8b illustrates the same situation as FIG. 8a except that the G1~ gating pulse is right in the middle of the time interval between successive G1 gating pulses at time XQ~ = trj + 500 ps ;

FIG. 9 is an illustration of gain temporal profiles for doubled repetition rate operation;

FIG. 10 is an illustration of an external gating;

FIG. 11 is an illustration of an internal gating with pseudo-ring cavity; and FIG. 12 is an illustration of an internal gating with Fabry-Perot cavity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The principle of operation of the Electronically Tuned Laser (ETL) of the present invention can be understood by referring to FIGs. 1 and 2. For illustrative purposes, FIG. 1 describes a laser for operation on a choice of five selectable wavelengths labeled λ-i through λζ. Operation on many more wavelengths is possible by using wavelength selective reflectors for different wavelengths that can be partly spatially overlapped, and/or by using wave guide based chirped gratings in the case of a laser cavity comprising optical waveguides. As will be understood, the selectable wavelengths could be selectable wavelength ranges. In that case, the output train of pulses is within a specified wavelength range.

On the far left-hand side, the laser cavity comprises a set 101 of five distributed wavelength selective reflectors coupled to the gain medium 100. On the far right- hand side, the laser cavity comprises another set 102 of distributed wavelength selective reflectors in which each reflector is spectrally and positionally paired with the corresponding left-hand side reflector of the set 101. The distributed reflector set 101 is coupled to the gain medium 100 through a short clear waveguide 99 and the distributed reflector set 102 is coupled to the gain medium 100 through a longer clear waveguide 103 . The round-trip delay in waveguide 99 is t5 and in waveguide 103 it is D.

The round-trip times per segment t1, t2, t3, and t4 are substantially the same on the right and on the left hand sides and are labeled by the same symbol ti , where i runs from 1 to 4. The result is that the total cavity round-trip time Tj for any wavelength λj is substantially the same for any λj, and substantially equal to T, the period with which the gain medium (or the modulator) is driven. To simplify the description henceforth we will take Tj = T = t1 + t2 + t3 + t4 + t5 + D. Note that round trip time per segment t1 is not necessarily equal to t2, nor is t2 equal to t3, and so on for the others.

The gain medium 100 is typically a semiconductor that is pumped by either one of two means: a) through current injection as driven by an electronic pulse whose duration is in the picosecond to subnanosecond range; or b) through optical pumping by an ultrashort (picoseconds to tens of picoseconds) duration laser pulse of appropriate wavelength. We will refer to this as "internal gating" of the effective optical gain which is demonstrated in the Example section below and in which the gain of a commercial semiconductor optical amplifier (SOA) was electronically gated on. As mentioned earlier, an alternative way of gating on effective optical gain over an ultrashort time duration is to have a continuously pumped optical gain medium placed adjacent to an ultrafast light modulator that can be driven by an ultrashort electronic or optical pulse. We refer to this as "external gating" of the effective optical gain, demonstrated in the Example section below with an erbium-doped fiber as a continuously pumped gain medium and a Mach-Zehnder electro-optic modulator as a gate. Whether the effective optical gain is internally- or externally- gated on for a short time does not change the timing relationships involved for selecting a given wavelength. Therefore, this description will often be simplified by referring to the optical gain being internally-gated on for short time durations. The description for an externally gated gain medium is substantially the same. One notes that when a semiconductor optical amplifier is gated on either optically or electronically, the duration of the induced optical gain can be lengthened by the carrier lifetime. One assumes that the effective carrier lifetime is shorter than the pulses used to gate the optical gain on.

FIG. 2 shows the temporal profile of the net gain experienced by light going through the internally- or externally-gated optical gain medium 100. The gating being effected by a pair of drive pulses G1 and G2.

The first gating pulse 201 , labeled G1 , occurs at time trj and has a duration 108. Pulse G1 at time to triggers the emission of two oppositely directed bunches of photons 106 and 107 (see Fig. 1 ). Bunch 106 initially goes towards the left and bunch 107 initially goes towards the right, both of them containing many different wavelengths. Photon bunches 106 and 107 enter the two grating arrays placed to the left 101 and to the right 102 of the gain medium 100. The light 109 which comes back from set 101 is made up of reflected light pulses at wavelengths λ-| through λ$.

In the example case of FIGS. 1 and 2, lasing at wavelength λ2 is desired. It is obtained by first selecting a pulse of light from the left set 101. For this purpose the timing t_se| labeled 112 of the second pumping pulse G2 relative to G1 is chosen to be equal to t_se| = t2 + t3 + t4 + t5, so that when the photons 109 reflected from the left grating segment labeled λ2 in 101 return to the gain medium 100, the latter is at the peak of its temporal gain profile, as illustrated in FIG. 2. Photons reflected from other grating segments with labels other than λ2 in the left set 101 arrive either too early or too late to be optimally amplified. After the second gated amplification, in the gain medium 100, the photon bunch 110 goes on towards the right and is reflected from the λ2 grating on the right set

102; this now twice reflected pulse of light is labeled 111. The round-trip delay in the clear waveguide section 103 from the gain medium 100 to the λ-| grating on the right set 102 is D, so that the total round-trip delay T for the λ2 photon bunch is equal to t2+t3+t4+t5+D+t1. The λ2 photon bunch will therefore be transmitted again maximally by the following G1(trj + T) gating pulse occurring at trj + T.

As for the photons emitted at to in bunch 107 and reflected from the gratings 102 on the right, the round-trip delay D which they undergo is such that they come too late to catch the effect of gating pulse G2at time to + t_se| and too early to catch the effect of the next pumping pulse G1 at time tø + T. These photons therefore do not build up into a powerful laser pulse.

For operation at the other wavelengths the t_se| delay 112 is electronically adjusted to the following values: for λ-| to t_se| = t1 + t2 + t3 + t4 + t5, for λ3 to t_se| = t3 + t4 + t5, for λ4 to t_se| = t4 + t5, for λ5 to t_se| = t5. For all wavelengths, the total round-trip delay is dictated by the period T with which the optical gain is gated on for an ultrashort time.

As an alternative procedure, one could select lasing at wavelength λ2 by first selecting the delay t'_se| 98 so that the pulse reflected from the X^ grating in set 102 arrives at the gain medium as it is gated on. The delay t'_se| labeled 98 has to be electronically adjusted to the following values: for λ-j to t'_se| = D, for λ2 to t'_se| = D + t1 , for λ3 to t'sei = D + t1 + t2, for λ to t'_se| = D + t1 + t2 + t3, for λζ, to t'_se| = D + t1 + t2 + t3 + t4. This procedure creates a succession of temporal gatings which is in fact identical in steady-state to that described earlier. One can apply the concepts of FIGs. 1 and 2 to a 1Gb/s laser, that is where T is equal to 1 ns. Then the round-trip times t1 , t2, t3, t4 and t5 can be chosen to be respectively approximately equal to 80 ps, and the clear waveguide 103 can have a round-trip time D of 600 ps. For operation at all wavelengths the t_se| delay 112 is then electronically adjusted to the following values: for λ-| to t_se| = 400 ps, for ^λ2 ^to *sel ⁼ 3 0 ps, for λ$ to t_se| = 240 ps, for λ^ to t_se| = 160 ps and for λ§ to t_se| = 80 P^s-

With only five gatings as shown in FIGs. 1 and 2, the individual gratings can have a physical length of about 8 mm while avoiding physical overlap. Since the round- trip time within each grating upon reflection is about 80 ps, the 25-ps gating pulses G1 and G2 will slice out approximately bandwidth-limited laser pulses from the grating reflections. Outputs 104 and 105 will therefore be close to being Fourier-transform limited, i.e. the product Δv x Δt will be close to 0.44, where Δv is the spectral width of the optical power spectrum at half maximum and Δt is the pulse's duration at half maximum light intensity, the latter being defined as the instantaneous electromagnetic power averaged over one optical cycle.

When more gratings are put in to increase the number of wavelengths, physical overlapping of the gratings will occur, but it is possible through careful Bragg grating techniques known in the art to keep the reflection peaks spectrally separated and centered at the desired wavelengths. For example let us assume that operation on 40 different wavelengths is desired. The physical grating separation will now be about 2 mm. The delays between the different wavelengths will now be discretized in multiples of 20 ps instead of the earlier 80 ps multiples. With gating pulses about 25 ps in duration and with tens to hundreds of round-trips required in the laser cavity to build up laser oscillation, this 20-ps differential delay is sufficient to discriminate between the various wavelengths through proper selection of the t_se| delay 112 between the two gating pulses G1 and G2.

Bragg grating implementation of the ETL The gratings described above can be of the well-known and widely used Bragg grating type. In the fabrication of Bragg gratings using the UV interference method one has to pay attention to the wavelength displacements that occur when one Bragg grating overlaps entirely or partially other previously written Bragg gratings. A proper pre-compensation or post-compensation should be applied in order for one to obtain the precise wavelengths desired in the final set of fiber Bragg gratings.

Implementation of the ETL using other types of waveguide gratings

Other types of waveguide gratings could be used to implement the principle of the ETL described above, in particular the Segmented Waveguide Array Gratings (SWAGs) described in a co-pending patent application no * *** *** by Applicant. Chirped grating implementation of ETL

Instead of using sets of discrete Bragg gratings 101 and 102, each set for maximum reflectivity at one wavelength, one could use one long chirped Bragg grating 113 on the far left-hand side and a second one 114 on the far right-hand side as shown in Fig. 3. Working on the same principle of gated temporal selection described above, the two long chirped gratings would allow one to tune the laser wavelength in a spectrally continuous fashion instead of in discrete spectral steps. FIG. 3 illustrates a long chirped Bragg grating 113 which reflects light at wavelength λ|_ong at the Lλ|_ong proximate end 115 close to the gated gain medium 100, and which reflects light at wavelength λ_Short ^at the Lλ_short distant end 116. We can see that in reflecting an ultrashort pulse of light the Bragg grating 113 has an up-chirping effect on it, since the first spectral components to be reflected from it are at the low optical frequency end of the spectrum, whereas the last spectral components to be reflected from it are at the high optical frequency end of the spectrum. This means that the long light pulse 119 reflected by the left chirped Bragg grating 113 will be up-chirped.

On the right hand side the reverse process occurs: 114 is a long chirped Bragg grating which reflects light at wavelength λ_snort at the Rλ_s ort proximate end 117 closest to the gated gain medium 100, and which reflects light at wavelength λ|_ong at the distant Rλ|_ong end 118. Chirped grating 114 has a down-chirping effect upon reflecting a light pulse. An alternative set-up would be to have two long chirped Bragg gratings with a chirp sign opposite to that shown in FIG. 3.

In FIG. 3, when lasing at wavelength λj is desired, one sets the t_se| delay so that the gain is gated on by a G2pulse at time to + t_se| just as light energy at wavelength λj reflected from the left chirped Bragg grating 113 is going through. As with the embodiment of FIGs. 1 and 2, the pulse G1at time to causes two oppositely directed bunches of photons 106 and 107, having many different wavelengths, to leave the laser gain medium 100 and to enter the two Bragg gratings placed to the left (labeled 113) and to the right (labeled 114) of the gain medium 100. The light 109 which comes back is a long light pulse made up of wavelengths from λ_short through λ|₀ng corresponding respectively to reflections from the Lλ_short distant end 116 to the proximate Lλ|_0ng end 117 of Bragg grating 113.

The t_se| timing 112 is chosen so that gating of the reflected pulse's portion at wavelength λj is obtained. The photon bunch 120 emerging from the gated gain medium and going towards the right mostly contains spectral energy close to λj.

The now twice reflected pulse of light coming back from chirped grating 114 is labeled 121 and is composed of a band of spectral energy centered at the desired wavelength λj, the spectral width of this band being determined by the temporal widths Δtgi 108 and Δtg2 208 (see Fig. 2) of the gating pulses and by the effect of the chirped gratings upon the lasing pulses. For gratings with a relatively low chirp, i.e. a small wavelength difference between λ|_ong and λ_short- the spectral width Δf in frequency will be dominated by the effect of Δtgj (i = 1 or 2) according to the relation Δf = 0.44/Δtgj for Gaussian-shaped bandwidth limited pulses. For strongly chirped gratings, the laser pulses will have a larger spectral width due to the chirped gratings.

With the 113 / 114 chirped grating arrangement, any wavelength between λ_short and λ|_ong can be chosen by adjusting the t_se| delay 112 between the gating pulses G1 and G2. As before, the choice of t_se| 112 determines the center wavelength that will dominate the laser output. It is important to note that in the arrangement of FIG. 3 the pulse of light 105 emerging from output end 118 is up-chirped whereas the pulse of light 104 emerging form the 116 output end is down-chirped. In stating this, it is assumed that the chirped grating has a certain useful transmission coefficient, say 5% as an example, over its wavelength range. This will play a role below in the solution to chromatic dispersion.

Solution to chromatic dispersion

FIG. 4 illustrates another application of chirped gratings, namely overcoming fiber optic dispersion through laser pulse pre-chirping. In standard optical fiber links, chromatic dispersion in the 1500-nm band causes the transmitted laser pulses to become down-chirped because higher frequency components travel faster than lower frequency components. For example, in FIG. 4a an initially 25-ps bandwidth limited laser pulse 122 at 1550 nm will broaden approximately to about 100 ps after 40 km of standard optical communications fiber in which dispersion is typically 17 ps/nm-km. As the pulse emerges from a 40-km length of such a fiber, it will be down-chirped as illustrated by laser pulse 123 in FIG. 4a. If a transmitter's laser is made to generate essentially the same pulse except for a reversed chirp as in FIG. 4b, i.e. an up-chirped pulse 124 of about 100 ps duration in this example, that up-chirped pulse 124 will recompress to about a 25 ps pulse 125 as a result of going through 40 km of fiber. This illustrates how pre- chirping can effectively combat optical fiber dispersion, a technique well-known in the art.

FIG. 5 shows how an ETL employing chirped grating segments can generate up- chirped laser pulses, corresponding to the solution illustrated in FIG. 4. In Fig. 5, on the left-hand side, a set 126 of individual chirped Bragg gratings is provided and on the right-hand side, a set 127 of individual chirped Bragg gratings oriented in the same way as on the left is also provided. For both left (126) and right (127) sets of chirped gratings the sign of the chirp must be the same (up-chirp or down- chirp) as one goes towards the right. As with FIG. 1, each pair of wavelength- matched gratings forms a laser cavity with a round-trip time that is substantially equal to T, the period between pump pulses gating on the gain medium. As for the sign of the chirp on each individual grating, FIG. 5 illustrates a case where the Lλshort ^or R^λshort P^art of each grating is on the left and the Lλ|_ong or Rλ|_ong part is on the right. For the case shown in FIG. 5 the second gating pulse G2 has a duration Δt_g2 208 (see Fig. 2) which is longer than duration Δtg-| 108 of gating pulse G1. An example where the gating pulse G1 is 25 ps in duration while the G2 is about 100 ps will be described. Let us assume operation on a wavelength band centered at λ2- On the left-hand side, grating 126 is up-chirping and its length is such that the up-chirped reflected pulse 128 is about 100 ps in duration. As the light pulse 106 reflects from grating segment λ2 of the set 126, the longer wavelength components of 106 reflect first, and the shorter wavelength components reflect a little later. Upon reflection, pulse 106 is therefore transformed into a longer pulse of chirped light 128 directed back towards the gain medium 100. Gating pulse G2 is of sufficiently long duration in time that almost the entire chirped pulse 128 is amplified by the gain medium and becomes up-chirped light pulse 129 in FIG. 5. Up-chirped pulse 129 is partially transmitted through the grating set 127 and becomes the output pluse 105. In the present example, the dimensions and the chirps pertaining to gratings λ-| - λ5 on the set 126 are chosen so that the up-chirped output laser pulse 105 will recompress to about 25 ps after 40 km of standard communication fiber, thereby offsetting the positive dispersion encountered on this fiber.

On the right-hand side, grating λ2 in set 127 is chirped the opposite way to λ2 of the set 126 from the point of view of the incident light (but it still has the same sign of chirp from left to right). Upon reflection form the λ2 chirped grating on the right, the chirped light pulse 129 is therefore recompressed to an ultrashort duration of about 25 ps duration as reflected pulse 130 which can be amplified efficiently when the next gating pulse G1 arrives at time to + T. Gating pulse G1 can have a duration Δtg-j equal about to 25ps.

In FIG. 6, the effects of chromatic dispersion are illustrated for the two laser outputs (in the 1500-nm band) which are available. In FIG. 6a, pulse 131 comes from the output 104 which is the small fraction of laser light transmitted by 126 in the form of an ultrashort laser pulse 131 about 25 ps in duration. Pulse 131 will undergo dispersion in 40 km of fiber to become the relatively long down-chirped pulse 132. In FIG. 6b, pulse 133 from output 105 in FIG.5 is the small fraction of laser light transmitted by 127 and is an up-chirped laser pulse with a duration of about 100 ps and a pre-chirp designed to compensate for about 40 km of standard optical communications fiber. That pre-chirped pulse 133 is then recompressed after about 40 km into a pulse 134 of about 25 ps in duration., which is ideal for detection.

ETL pseudo-ring cavity FIG. 7 shows what is called a "pseudo-ring cavity" for the electronically tunable laser. This cavity uses the one set 135 of gratings instead of requiring two sets of matched gratings. This set 135 can be similar to the sets of gratings previously cited, i.e. 101 , 102, 113, 114, 126, 127. While this cavity has the appearance of a ring cavity laser, from the point of view of a light pulse, say at λ2, traveling back and forth and each time reflecting towards the right, and then towards the left, off the grating labeled λ2, there is virtually no difference with the embodiment shown in Fig. 1. This can be explained by the fact that light at λ2 does not go through the grating reflector labeled λ2, which is assumed to be highly reflecting for the purpose of this explanation. The λ2 light reflects off this grating towards the left and then towards the right just as in FIG. 1. That is why it is called a "pseudo-ring cavity." In this case two outputs 136 and 137 of the laser cavity can be obtained through the use of directional coupler 138.

The pseudo-ring cavity offers the advantage of increased compactness and avoids the need for perfectly matched grating reflectors. As far as the chirped grating operations are concerned, the pseudo-ring cavity works like the straight cavities. Assuming a chirped grating 113 of Fig. 3 in place of grating set 135 in Fig. 7, in the pseudo-ring cavity, the lasing pulse arriving from the left sees a grating with a down-chirping effect upon reflection, but when the laser pulse arrives on the same grating from the right it sees a grating with an up-chirping effect. So the operation is the same as in FIG. 3. Increasing the pulse repetition rate

Example of doubled repetition rate operation

The ETL version described above relied on a straightforward application of the gated ranging principle used in radar and in lidar. By looking at FIGs. 1 and 2 one can see however that there is a long period D during which no gating occurs. One apparent drawback of period D is to limit the pulse repetition rate because all optical components have a finite length and the net effect of delay line D seems to be to lengthen the period T by a factor of 2. What follows will show that this limitation can be overcome by introducing additional pairs of gating pulses during a period equal to the cavity round-trip time TRJ SO that the pulse repetition rate can be doubled and even quadrupled over what is possible with the operational regime described earlier where a single pair of gating pulses occurs with period T equal to TRJ.

TO illustrate how this is accomplished, FIG. 8a shows the steady-state temporal positions of potential laser pulses at each wavelength λ<| , λ2 , λβ , λ^ and λ^ for the ETL cavity illustrated in FIG. 1. Since lasing at λj would occur at these times if a gating G2 pulse were present, we will refer to these temporal positions as " λj

G2 time markers". The first gating pulse G1 occurs at tø, as before. FIG.δa illustrates two sets of λj G2 time markers: a) the one to the right of gating pulse G1 at time to + 1000 ps is the set of λj G2 time markers for light pulses reflected from the left grating set 101 ; and b) the one to the left of gating pulse G1 at time tQ + 1000 ps is the set of λj G2 time markers for light pulses reflected from the right grating set 102. The pair of gating pulses needed to select a desired wavelength λj comprises the G1 pulse, which is illustrated, and the G2 pulse which would be placed in time over one of the λj G2 time markers. The arrow in

FIG. 8a shows where the G2 pulse would be placed in time to select λ2, for example, as reflected from the left set 101.

The following numerical values for the various times in FIG. 8 have been chosen to be the following: the laser cavity round-trip time TRJ is 1000 ps; the gain medium gating duration is about 25 ps, round-trip times t1 , t2, t3, t4 are all equal to 80 ps, but t5 equals 30 ps; the round-trip delay time D is 650 ps. With a single pair of gating pulses the laser repetition rate is 1 GB/s.

Introducing other pairs of gating pulses We show in FIG. 8b how the relatively long 1000-ps TRJ period can be exploited to double the laser pulse rate by introducing a second pair of gating pulses labeled G1~ and G2~, in order to obtain operation at a 2 GB/s repetition rate.

The key idea is that the time slots used by the λj G2 time markers in FIG. 8a leave many unused time slots between these markers. This leaves room for the introduction of another sequence of pairs of gating pulses G1~ and G2~ with the same period T = TRJ which can be considered to be temporally independent of the sequence of pairs G1 and G2.

FIG. 8b illustrates the same situation as FIG. 8a except that the G1~ gating pulse is right in the middle of the time interval between successive G1 gating pulses at time to~ = to + 500 ps, and its accompanying set of "λj G2~ time markers" is arranged by design to coincide in time with empty time slots left unoccupied by the G1-G2 gating pair, as illustrated in FIG. 8b. The untilded G1-G2 and the tilded G1~-G2~ gating pairs form temporally independent sets. In interpreting FIG. 8b one must consider the action of the tilded gating pulses G1~ and G2~ to be independent of the presence of the G1 - G2 pair. The wavelength selection process is exactly the same as for the G1-G2 pair.

The question now arises as to whether a pulse of light gated through by G1~ -

G2~ will sneak through the temporal windows opened by the G1-G2 pair. The answer is negative as illustrated in FIG. 9. By proper choice of the delay D and of the round-trip delays tj between the grating reflectors, one can see that the peaks of all tilded gating pulses fall at times different from the untilded ones. The two sets are temporally distinct. The only interaction could be through memory effects in the gain medium, but this could be electronically compensated for through adequate laser power monitoring and through feed-back mechanisms to independently control the amplitude of the tilded and untilded sets of pump pulses.

By comparing the various gating times and light pulse reflection arrival times, one can see that the minimum time separation short of a coincidence is 30 ps.

Quadrupling the repetition rate One can go even further and quadruple the laser pulse repetition rate through careful adjustment of the individual delays tj between the grating reflectors and the delay D. For this purpose another "Quadruplet set" of two gating pairs G1Q- G2Q/G1Q—G2Q- is applied to the gain medium 100 with the G1Q pulses occurring at to + 250 ps, to + 1250 ps, etc... and G1Q^~ occurring at to + 750 ps, to + 1750 ps, etc. The guiding principle, as exemplified above, is to make sure that the gating times and the various light pulse reflections of the Q subset do not coincide with the on-gating times of the G1-G2/G1~-G2~ pulses applied to the gain medium 100.

Adjusting the delay time D to increase the separation of the Bragg gratings In the description of the ETL above, for example in FIG. 1 , the delay time D was chosen to be long enough to delay the arrival time of reflections from the right- hand side set of gratings 102 so that they arrive after the period covered by the λj

G2 time markers. This relatively long delay time D constrains the grating separation to be only 2 mm for operation on 40 wavelengths as explained before. To increase the grating separation one can greatly reduce delay time D and adjust its value so that the reflections from set 102 returning to the gain medium are interleaved with the reflections from set 101. Then with a proper choice of D for FIG. 1 the first reflected pulse in set 109 will come from the λζ grating of left- hand set 101 , the next (or second) reflected light pulse will come from the λ-| grating of right-hand set 102, the third reflected light pulse will come fromthe λ4 grating of the left-hand set 101 and so on. With such an adjustment of D the physical separation between Bragg gratings in sets 101 and in set 102 is doubled for the same repetition rate as in the example of Fig. 2.

Examples First experiment: Demonstration of external gating on 4 wavelengths

This first experiment, illustrated on FIG. 10, demonstrates the selection of one wavelength among four through gating external to the gain medium in a pseudo- ring cavity.

The pseudo-ring cavity shown in FIG. 7 was used here in order to make the experiment simpler by using a single set of Bragg gratings. A similar demonstration could be done with a Fabry-Perot cavity by using two nearly identical sets of gratings as shown in FIG.1.

The demonstration employed the following main components which are examples of components which could be used to carry out the invention:

The erbium-doped optical amplifier .fiber 139 is home-made with the main characteristics as indicated above. It is a highly erbium-doped fiber that amplifies light over a length under two meters. As in the usual erbium-doped fiber amplifier, a laser pump 140 is coupled to fiber amplifier 139 through an optical coupler 141 which couples input light at 980-nm wavelength from one of its input ports onto its output port. Coupler 141 lets light at 1550 nm on the other input port go through with little loss to its output port.

The electro-optic Mach-Zehnder modulator 142 is the externally driven gate in the cavity. Its bias is set at 7 volts and it is driven by a broad-band RF amplifier 143.

Fiber Bragg gratings 144, 145, 146 and 147 were chosen to have a high reflectivity of approximately 99%. These gratings are made in SMF-28 Corning fiber which is a well-known standard telecommunications fiber.

The polarization controller 148 was introduced in order to optimize the performance of the polarization-sensitive electro-optic modulator 142.

The 50/50 coupler 149 supplies an output port for the laser cavity.

Pattern generator 150, driven by clock 151 , is set to generate voltage pulses which are amplified by the broad-band RF amplifier which feeds into the electro- optic light modulator.

On the pattern generator 150, the series of voltage pulses is symbolized by a word made up of bits where a voltage pulse is a "1 " bit and the absence of a pulse is a "0" bit. The generator 150 continuously repeats the chosen word. The bias of the modulator 142 is chosen to gate the modulator on when it receives a 1-bit and off when it receives a "0" bit.

The laser pulse coming out the cavity via coupler 149 is divided into two signals thanks to a 90/10 coupler 152. One output is fed into an optical spectrum analyzer 153 and the other output is sent into a photodiode 154 for instantaneous power monitoring on a fast oscilloscope. The electrical signal generated by the photodiode 154 is displayed on an oscilloscope 156 after being amplified by a broad-band RF amplifier 155.

Description of the experimental protocol: Step 1 : Assembly of laser cavity.

Before assembling the laser cavity, one can estimate the propagation time of light by measuring the physical length of each component in the cavity. Here, the length of erbium doped fiber 139 plus the coupler 141 is about 935 centimeters. The length of the fiber in which the fiber Bragg gratings 144, 145, 146 and 147 have been inscribed, is about 212.6 centimeters. The fiber on which the polarization controller 148 is mounted has a length of about 320 centimeters; the 50/50 coupler 149 is about 325 centimeters long including its input/output fibers, and the modulator 142 is about 300 centimeters long including its input/output fibers. The total physical length is estimated to 20.92 meters and the one-way propagation time of light considering the effective index of about 1.45 is approximately 101 ns.

To reproduce the results of this experiment it is important to respect the approximate lengths of each component, including the physical separations 160, 161 and 162 between the fiber Bragg gratings 144, 145, 146 and 147 (as illustrated in FIG. 10) which have to be respectively about 30.1 cm, 31 cm and 31.7 cm. Concerning the separation 159 between grating 144 and the left-hand fiber connector 157 and the separation 163 between grating 147 and the right- hand fiber connector 158, the fiber lengths have to be approximately 41 cm and

78.8 cm respectively. Step 2 : Determination of the laser's period TRJ.

The pulse repetition rate of the laser cavity is determined by the period T between two G1 gatings, which must be very close to the cavity round-trip time TRJ. In the pseudo-ring cavity the round-trip time of the cavity is two times the one-way propagation time of a light pulse in the cavity. Thanks to the estimation of the physical cavity length we calculated that half the cavity round-trip time was about 101 ns, giving TRJ = 202 ns, as an estimate for the cavity round-trip time. Given this estimated TRJ, we created a 120-bit word with the pattern generator 150 which consisted of a binary 1 followed by 119 binary zeros. The bits were clocked at frequency 1.188 GHz by clock 151. This clock frequency sets the duration of each bit to (1.188 x 10⁹/s)^"1 = 841.75 ps. This makes a 120-bit long word have a duration of 101.01 ns, which is very close to the estimated TRJ.

In order to find more precisely the cavity round-trip time, one varies the clock frequency and one observes on the oscilloscope the amplitude of the spontaneous emission light pulses traveling only one way through the cavity right through the gratings (the latter only reflects light over narrow spectral bands which cover a small percentage of the total spontaneous emission from erbium). The pulse amplitude varies with the clock frequency. In the experiment we found that at clock frequency 1.1794 GHz the pulse amplitude became maximized. This gives the result that half the cavity round-trip time was 120 x (1.1794 x 10⁹/s)^"1 = 101.75 ns.

Thanks to the optical spectrum analyzer 153, one could then optimize the polarization by adjusting the polarization controller 148 to obtain a spectrum with as wide a spectral peak as possible as observed on the optical spectrum analyzer 153. When this is done one knows that the modulator will modulate equally well all wavelengths that we want to select from. Step 3 : Determination of t_s | for each wavelength.

The word on the pattern generator is set to consist of 240 bits so that the word will repeat it_se|f every 2 x 101.75 = 203.5 ns. The word is made to consist of a pair of binary 1 bits separated by a number m of inserted binary zeros; this number m therefore determines the time interval t_se| between the G1 and G2 gating pulses. The latter is varied by changing the number of binary 0 bits between the two binary 1 bits. At some precise value of t_se| a very strong spectral peak is observed on the optical spectrum analyzer corresponding to the selected wavelength.

In this experiment, the numbers m that select specific wavelengths are:

Minor adjustments on the polarization controller were needed at times to optimize the wavelength selection. When the numbers m have been determined one can then choose the wavelength at will by changing m.

2^nd experimentation : Demonstration of internal gating on 3 wavelengths

This experimentation demonstrates the internal gating (i.e. direct gating of the optical gain), the pseudo-ring cavity configuration, the Fabry-Perot cavity configuration and increasing the pulse repetition rate (by doubling and tripling).

The setup of the demonstration is shown in FIGs. 11 and 12. The demonstration employed the following main components which are examples of components which could be used to implement the invention:

In this case, the semiconductor optical amplifier 164 plays the role of gain medium and gate. It is driven by a broad-band RF amplifier 165 which receives a signal from the pattern generator 166 composed of a repeated word of 0 and 1- bits of 730mV amplitude. The pattern generator 166 is driven by a clock 167 which determines the bit duration.

The fiber Bragg gratings 168, 169 and 170 were chosen to have a high reflectivity of approximately 99%. These gratings are engraved in HI-980 Corning fiber which is a well-known standard fiber in telecommunications. The choice of fiber is not optimum because the core diameter is quite different from the core diameter of the SMF-28 fiber which makes up other components in the laser cavity. In the future these losses due to core mismatch could be avoided by using fiber Bragg gratings in SMF-28 Corning fiber. There is an additional patchcord 176 in the laser cavity which is a link between coupler 152 and the fiber in which the fiber Bragg gratings 168, 169 and 170 are engraved. In that case, the two components have incompatible types of well- known connectors FC/PC and FC/APC. The patchcord 176 has a connection FC/APC 177 with the coupler 152 and a connection FC/PC 172 with the fiber on which the fiber Bragg gratings 168, 169 and 170 are engraved.

As in FIG. 10, the laser pulse coming out the laser cavity via coupler 152 is then divided in two signals by means of a 50/50 coupler 149. One signal is directed to the optical spectrum analyzer 153 and the other to a photodiode 154 for display on the oscilloscope screen after having been amplified by a broad-band RF amplifier 155.

Internal gating in pseudo-ring cavity (FIG. 11)

Step 1 : Assembly of laser cavity.

As in FIG. 10, before assembling the laser cavity one can estimate the light propagation time by measuring the physical length of each component in cavity. The fiber in which the fiber Bragg gratings 168, 169 and 170 are engraved is about 133.3 centimeters long . The SOA 164 and its input/output fibers are about 26.5 centimeters long . Coupler 152 is approximately 13.7 centimeters long and the patchcord 176 is about 26.5 centimeters long . One has also to take into account a 1.06 meter long fiber between the SOA 164 and the coupler 152. The total cavity length is then about 306 centimeters and the one-way propagation time of light taking into account the effective index of about 1.45 is approximately 14.8 ns.

To repeat the results of this experiment it is important to respect the approximate lengths of each component, including the physical separation 174 between the fiber Bragg gratings 168, 169 and 170 as illustrated on the FIG. 11 which is about 30.1 cm long . Concerning the separation 173 between the grating 168 and the left-hand connector 171 of the fiber, and the separation 175 between the grating 170 and the right-hand connector 172 of the fiber, the respective lengths have to be approximately 27.6 cm and 39.55 cm. Step 2 : Determination of the laser's period TRJ.

The pulse repetition rate of the laser cavity is determined by the period T between two G1 gatings, which must be very close to the cavity round-trip time TRJ. In the pseudo-ring cavity the round-trip time of the cavity is two times the one-way propagation time of a light pulse in the cavity. Thanks to the estimation of the physical cavity length one calculated that half the cavity round-trip time was about 14.8 ns, giving TRJ = 29.6 ns, as an estimate for the cavity round-trip time. Given this estimated TRJ a 20-bit word was created with the pattern generator 150 which consisted of a binary 1 followed by 19 binary zeros. The bits were clocked at frequency 1.35 GHz by clock 151. This clock frequency sets the duration of each bit to (1.35 x 10⁹/s)^"1 = 740.74 ps. This makes a 20-bit long word have a duration of 14.8 ns, which is very close to the estimated TRJ.

In order to find more precisely the cavity round-trip time one varies the clock frequency and one observes on the oscilloscope the amplitude of the spontaneous emission light pulses traveling only one way through the cavity right through the gratings (the latter only reflect light over narrow spectral bands which cover a small percentage of the total spontaneous emission from erbium). The pulse amplitude varies with the clock frequency. In the experiment we found that at clock frequency 1.246 GHz the pulse amplitude became maximized. This gives the result that half the cavity round-trip time was 20 x (1.246 x 10⁹/s)^"1 = 16.05 ns, which is in approximate agreement with the value for TRJ estimated above..

Step 3 : Determination of t_s | for each wavelength.

At the optimized frequency found above, one can look for the time delay t_se| between gating pulses G1 and G2 in order to select a wavelength. Then one inserts a 1-bit in the word and moves it along the word. When that inserted 1-bit is at the correct position, the selected wavelength appears on the OSA 153.

In that case, the bytes that selects a wavelength are:

As for pattern generator 150, the numbering of bytes begins at 0 for the pattern generator 166.

When these positions are determined, on can change wavelength as one likes.

Internal gating in Fabry-Perot cavity (FIG.12)

Step 1 : Assembly of laser cavity.

In the case of the Fabry-Perot cavity laser, the two sets of gratings have to be substantially identical both in reflected wavelengths and in grating spacings.

With this in mind, the fibers of each Bragg grating were measured, cleaved and fused with about 33.1 cm between consecutive Bragg gratings.

To introduce the round-trip delay time D, a patchcord 178 was introduced between the connections 180 and 181. Its length 179 was about 26.5 cm. Step 2 : Determination of t_se| for the first wavelength 1530 nm.

In the experiment with the configuration shown in FIG. 12, the results of the experiment with the pseudo-ring cavity were used to establish that m = 4 was the value for t_se| for selecting the light reflected from grating 168 (1530 nm wavelength). This was for a clock frequency close to 1.246 GHz.

In the experiment in FIG. 12 the physical length between the two identical gratings 168 was approximately 253.9 cm which translated to a round-trip time of about 24.56 ns. For a 31 -bit word this gives a frequency of 1.26 GHz for clock 167. For this word length one could estimates that a choice of m = 4 will select the 1530-nm wavelength.

Step 3 : Determination of the laser's period TRJ.

The word length was then set to 31 with all bits at zero except the first bit (i.e. the G1 gating pulse) set to binary 1 , the following 4 inserted bits set to zero, and the 6^th bit (i.e. the G2 gating pulse) to binary 1.

By searching for the right frequency in the vicinity of 1.26 GHz, an optimum frequency of 1.238 GHz was revealed by the fact that one could see a strong peak on the optical spectrum analyzer 153 at the 1530-nm wavelength. This frequency corresponds to a 25.04-ns long word, which is the laser cavity round- trip time TRJ. By changing t_se| (i.e. by changing the number m of inserted binary zeros) which sets the position of the second binary bit 1 in the word, one experimentally finds the m values for the other two wavelengths.

In this experiment, the m values which select the three wavelengths are:

Increasing the pulse repetition rate

An increased pulse repetition rate was also demonstrated with the Fabry-Perot cavity described in FIG. 12. Doubled and tripled repetition rates operation were obtained.

Doubling the repetition rate To achieve doubling of the frequency, the clock 167 and the length of the word were doubled. Then the pattern generator 166 was set to repeat a word of 62 bits driven by a clock 167 at 2.476 GHz. The word length was the same as before (25.04 ns) and no parameters needed to be re-adjusted. Two adjacent binary 1 bits made up the gating pulses G1 and G2. The number m of bits inserted between G1 and G2 was also doubled. To generate a second set of laser pulses (the tilded set) two adjacent binary 1 bits were placed at word center to create a G1~ pulse, followed by two adjacent 1 bits after a time t'_se| to create a G2~ gating pulse.

The words that selected the various wavelengths at the doubled pulse repetition rate were:

Tripling the repetition rate :

As in the doubling case, the clock frequency and the word length (length measured in bits) are tripled to be respectively 3.714 GHz and 93 bits. With tripling, the laser repetition rate is about 120 Mb/s.

The words that selected wavelengths are described below :

It will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense. It will further be understood that it is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features herein before set forth, and as follows in the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A laser cavity for generating a train of laser pulses at a selected wavelength from a plurality of wavelengths, comprising: a gain medium generating light having a plurality of wavelengths from pump energy; an input pulse generator for gating on an effective gain of said cavity, said input pulse generator generating pairs of input pulses with a time t_se| between the input pulses of said pairs and a time T between the pairs of input pulses; on a first side of the gain medium, a first waveguide coupled to said gain medium; on a second side of the gain medium, a second waveguide coupled to said gain medium; on the first waveguide, a first set of distributed wavelength selective reflectors, each reflector of said first set reflecting light at one of said plurality of wavelengths, each of said reflections being spaced apart in time; on the second waveguide, a second set of distributed wavelength selective reflectors, each reflector of said second set reflecting light at one of said plurality of wavelengths, each of said reflections being spaced apart in time and being paired with a corresponding one of said reflectors from said first set reflecting light at a same one of said plurality of wavelengths; wherein said pairing is adapted to ensure that a total cavity round-trip delay of substantially T is substantially equal for all wavelengths reflected by said first and second sets; a delay modifier for adjusting the time t_se| between the pulses of said pairs and thereby selecting a selected wavelength for the train of laser pulses; an output for outputting the train of laser pulses at the selected wavelength; whereby the output repetition rate of the train of laser pulses can be substantially equal for all wavelengths reflected by said first and second sets.

2. A laser cavity as claimed in claim 1 , wherein said reflectors are Bragg gratings.

3. A laser cavity as claimed in claim 1 , wherein at least one of said first set and said second set of reflectors is at least one of a chirped grating and a set of chirped gratings.

4. A laser cavity as claimed in claim 1 , wherein said input pulse generator is an electro-optic gate placed between said gain medium and one of said first and said second set.

5. A laser cavity as claimed in claim 1 , wherein said first set and said second set share a common set of reflectors; a far end of said first waveguide is coupled to a near end of said second waveguide, a furthest reflector of said first set from said gain medium being a closest reflector of said second set from said gain medium; said output is an optical coupler; whereby said laser cavity is a pseudo-ring laser cavity.

6. A laser cavity as claimed in claim 1 , wherein said input pulses have subnanosecond duration.

7. A telecommunications optical input for generating a signal to be transmitted on a transmission link, comprising: an input for receiving a data signal to be transmitted on said transmission link at a selected wavelength; a gain medium generating light having a plurality of wavelengths from pump energy; an input pulse generator for gating on an effective gain of said cavity, said input pulse generator generating pairs of input pulses with a time t_se| between the input pulses of said pairs and a time T between the pairs of input pulses; on a first side of the gain medium, a first waveguide coupled to said gain medium; on a second side of the gain medium, a second waveguide coupled to said gain medium; on the first waveguide, a first set of distributed wavelength selective reflectors, each reflector of said first set reflecting light at one of said plurality of wavelengths, each of said reflections being spaced apart in time; on the second waveguide, a second set of distributed wavelength selective reflectors, each reflector of said second set reflecting light at one of said plurality of wavelengths, each of said reflections being spaced apart in time and being paired with a corresponding one of said reflectors from said first set reflecting light at a same one of said plurality of wavelengths; wherein said pairing is adapted to ensure that a total cavity round-trip delay of substantially T is substantially equal for all wavelengths reflected by said first and second sets; a delay modifier for adjusting the time t_se| between the pulses of said pairs and thereby selecting said selected wavelength for a train of laser pulses to be generated; an output for outputting the train of laser pulses at the selected wavelength; a modulator for modulating said train of laser pulses; an output of said modulated being coupled to said transmission link for transmission; whereby the output frequency of the train of laser pulses can be substantially equal for all wavelengths reflected by said first and second sets.

8. A method for generating a train of laser pulses at a selected wavelength from a plurality of wavelengths, comprising: generating light having a plurality of wavelengths by a gain medium using pump energy; gating on an effective gain of said cavity by generating pairs of input pulses with a time t_se| between the input pulses of said pairs and a time T between the pairs of input pulses; on a first side of the gain medium, coupling a first waveguide to said gain medium; on a second side of the gain medium, coupling a second waveguide to said gain medium; on the first waveguide, placing a first set of distributed wavelength selective reflectors, each reflector of said first set reflecting light at one of said plurality of wavelengths, each of said reflections being spaced apart in time; on the second waveguide, placing a second set of distributed wavelength selective reflectors, each reflector of said second set reflecting light at one of said plurality of wavelengths, each of said reflections being spaced apart in time and being paired with a corresponding one of said reflectors from said first set reflecting light at a same one of said plurality of wavelengths; wherein said pairing is adapted to ensure that a total cavity round-trip delay of substantially T is substantially equal for all wavelengths reflected by said first and second sets; adjusting the time t_se| between the pulses of said pairs and thereby selecting a selected wavelength for the train of laser pulses; outputting the train of laser pulses at the selected wavelength; whereby the output repetition rate of the train of laser pulses can be substantially equal for all wavelengths reflected by said first and second sets.

9. A method as claimed in claim 8, wherein said reflectors are Bragg gratings.

10. A method as claimed in claim 8, wherein at least one of said first set and said second set of reflectors is a chirped grating.

11. A method as claimed in claim 8, wherein at least one of said first set and said second set is a set of chirped gratings.

12. A method as claimed in claim 8, wherein said first set and said second set share a common set of reflectors; a far end of said first waveguide is coupled to a near end of said second waveguide, a furthest reflector of said first set from said gain medium being a closest reflector of said second set from said gain medium; said output is an optical coupler; whereby said laser cavity is a pseudo-ring laser cavity.

13. A laser cavity as claimed in claim 8, wherein said input pulses have subnanosecond duration.

14. A method for generating a signal to be transmitted on a transmission link, comprising: an input for receiving a data signal to be transmitted on said transmission link at a selected wavelength; generating light having a plurality of wavelengths by a gain medium using pump energy; gating on the gain medium with pump energy, said pump energy being generated using pairs of input pulses with a time t_se| between the input pulses of said pairs and a time T between the pairs of input pulses; on a first side of the gain medium, coupling a first waveguide to said gain medium; on a second side of the gain medium, coupling a second waveguide to said gain medium; on the first waveguide, placing a first set of distributed wavelength selective reflectors, each reflector of said first set reflecting light at one of said plurality of wavelengths, each of said reflections being spaced apart in time; on the second waveguide, placing a second set of distributed wavelength selective reflectors, each reflector of said second set reflecting light at one of said plurality of wavelengths, each of said reflections being spaced apart in time and being paired with a corresponding one of said reflectors from said first set reflecting light at a same one of said plurality of wavelengths; wherein said pairing is adapted to ensure that a total cavity round-trip delay of substantially T is substantially equal for all wavelengths reflected by said first and second sets; adjusting the time t_se| between the pulses of said pairs and thereby selecting a selected wavelength for the train of laser pulses; outputting the train of laser pulses at the selected wavelength; modulating said train of laser pulses; an output of said modulated being coupled to said transmission link for transmission; whereby the output frequency of the train of laser pulses can be substantially equal for all wavelengths reflected by said first and second sets.