DARK PULSE GENERATION

Abstract: An optical pulse generator comprises a FM mode locked laser that is configured to produce dark optical pulses. The laser comprises a cavity (M1, M2) and a Pr<3+> doped fluoride fibre (3) pumped by a source (1). The output is mode locked by a phase modulator comprising a silica fibre (2) in the cavity that receives modulating pulses from a DFB laser (7), so as to alter the refractive index of the fibre (2) and cross phase modulate the laser light. The phase modulation combines with the dispersive effect of the fibre (3) to produce dark output pulses.

Description: Dark Pulse Generation Field of the Invention This invention relates to an optical pulse generator for generating dark pulses such as dark soliton pulses. Background An optical pulse is usually considered to comprise a burst of optical carrier radiation with a given modulation envelope shape. When the pulse has a particular initial envelope shape e.g. U(t) - N sech(*), where N is an integer, the pulse can be transmitted as a soliton in an optical fibre. For such particular envelope shapes, the wavelength dispersion produced in the pulse by the fibre, or so-called chirp, is counterbalanced by the fibre's non-linear dependence of refractive index on amplitude, which produces a self phase modulation (SPM) by which the phase of the pulse is modulated by its own intensity. This counterbalance results in a self maintaining pulse or soliton, which tends to maintain its envelope shape with time as it is transmitted along the fibre. The non dispersive nature of solitons makes them attractive for data transmission through optical fibres over long distances. A pulse having the characteristics just described is known as a bright pulse. It is also possible to generate so called dark pulses such as dark solitons, which occur when an essentially continuous burst of optical radiation contains temporal gaps or regions of reduced intensity radiation. Such gaps are known as dark pulses. It can be shown that for the particular case of solitons, dark solitons may have a general envelope shape given by U(t) - Ntanh(r), where N is an integer. For a fuller discussion, reference is directed to Non¬ linear propagation effects in optical fibres: numerical studies - K.J. Blow & N.J. Doran Chapter 4, Optical Solitons - Theory and Experiment, edited by J.R. Taylor, Cambridge University Press 1992. As used herein, the term dark pulse includes both a black pulse in which the intensity drops to zero and a grey pulse in which the intensity drops only partially towards zero. Dark solitons have been produced experimentally for example as described on pages 394-396 of "Optical Solitons-Theory and Experiment'' Supra. In this arrangement, pulses from a dye laser have their frequency components spatially dispersed by means of a grating and then individually weighted by means of a mask. The resultant weighted amplitude components are then recombined by another grating. The pulse is accordingly imparted with a desired temporal profile according to a fourier transform of the desired pulse shape. Using this technique, dark pubes closely resembling the expected black and grey solitons have been generated. However, a problem with this prior arrangement is that the fourier transform performed by mask and gratings is not readily controllable. The use of a modulator, responsive to input modulating signals, to produce bright solitons is described in D.M. Pataca et al: "Actively Mode-locked Pr 3 *- doped fluoride fibre laser" Electronics Letters 9th June 1994, Vol.30, No.12, pp. 964-5. In this arrangement, the praseodymium (Pr + )-doped fibre is included in a resonant cavity pumped by a Nd:YAG pump laser. The cavity is de ined by a semi-reflective mirror at one end of the fibre and a fully reflective mirror at the other. The cavity also includes a electro-optical modulator. In use, the modulator is driven by a sinusoidal waveform which produces positive and negative sinusoidal variations in the refractive index of the modulator, so as to phase modulate light resonating in the cavity. If the period of the modulation is selected to correspond to the transit time for light resonating in the cavity, the cavity is said to be mode-locked. The sinusoidal phase modulation produced by the modulator causes positive and negative going chirp for successive half cycles of the modulation frequency. When the resulting chirp is negative going, it compensates for the dispersive characteristics of the Pr 3 * fibre so that bright solitons are produced during successive negative half cycles of the modulating waveform. For the other half cycles, the positive going chirp that is produced, adds to the dispersion produced by the fibre and as a result, broad unstable pulses are produced. Summary of the Invention The present invention provides an improved way of producing dark pulses. Broadly stated, the present invention provides an optical pulse generator comprising the source of optical radiation, a modulator, and a source of modulating signals, the modulator being operative to phase modulate the optical radiation in accordance with the modulating signals in such a manner as to produce dark pulses in the optical radiation. The radiation source may comprise a laser that is mode-locked by the modulator. More particularly, the optical pulse generator according to the invention may comprise an optical cavity, an optically dispersive medium in the cavity, means for producing optical resonance within the cavity, and a modulator for cycl cally phase modulating optical signals in the cavity for mode locking the resonance to produce pulses of a given periodicity, wherein the dispersive characteristics of the cavity and the phase modulating characteristics of the modulator are selected to produce dark output pulses. The optically dispersive medium within the cavity may comprise an optical waveguide, for example an optical fibre doped to provide a particular wavelength dispersion characteristic. Whilst the generator according to the invention generates dark pulses, it can also be configured to produce bright pulses and, to this end the cavity may also include dispersive means having a predetermined wavelength dispersion characteristic so that radiation resonant in the cavity is subject to wavelength dispersion as a funαion of the dispersion charaαeristics of both the waveguide and the dispersive means. The dispersive means may comprise a grating with graticules that have a spatial frequency which varies along the length thereof. The modulator may comprise an optical modulator, for example an optical fibre conneαed in the cavity, with an optical modulating source for direαing optical modulating pulses into the fibre to modulate its optical charaαeristics. In this way, phase modulation, which produces mode locking, can be achieved. Brief Description of the Drawings In order that the invention may be more fully understood an embodiment thereof will now be described by way of example with reference to accompanying drawings in which: Figure 1 is schematic illustration of a prior art Pr^-doped fluoride fibre laser capable of generating bright pubes, Figure 2a illustrates the phase modulation produced by the modulator shown in Figure 1, Figure 2b illustrates the phase and chirp charaαeristics produced by the modulator in Figure 2a, Figure 2c illustrates the resulting output pulses for the successive half cycles of phase modulation produced by the apparatus of Figure 1, Figure 3 illustrates an embodiment according to the invention. Figure 4a illustrates the phase modulation produced in the apparatus of Figure 3, Figure 4b illustrates schematically the output from the laser of Figure 3, Figure 5 is a schematic illustration of the stepped chirped grating that can be used in Figure 3 to support bright pubes, Figure 6 illustrates the refleαion of the characteristic of the chirp grating shown in Figure 5, Figure 7a illustrates dark pubes produced by the apparatus of Figure 3 when a semi-refleαive mirror is used as an output, at 700 HMz, Figure 7b illustrates corresponding results with the mirror, at 2.8 GHz, and Figure 7c illustrates corresponding results when the mirror is replaced by the chirped grating, so as to provide bright output pubes. Description of the Embodiments Referring now to Figure 1, thb shows a prior art phase locked Pr 3+ -doped fluoride fibre laser as proposed by D. M. Pataca et al, Eleαronic letters June 1994, Vol. 30, No. 12 P.964 supra. The laser consists of a cavity between a fully refleαive mirror Ml and a partially refleαive mirror M2. Optical excitation is provided by a mode locked Nd: YAG laser 1 operating in the long wavelength wing of the Pr 3 * ion absorption (0.32dB/m absorption @ 1.064μm). The pumping energy is coupled through a wavelength division multiplexer WDM1 and a mode matching silica fibre 2 into a Pr 3+ -doped fibre 3. Because the upper state life time of the Pr 3 * ion is long (~ lOOμs) compared to the pumping period (— 10ms), the excitation is essentially continuous. Light leaving the Pr 3 * fibre is colli ated by a lens 4 and direαed to a 0.25mm thick glass tuning etalon 5, and thence to a modulator 6. In addition, the cavity includes a set of mechanical polarisation control disks 7. The modulator 6 is operative to achieve FM mode locking by phase modulating the laser emission of the Pr 3 * fibre within the cavity. The modulator 6 was operated with a periodicity corresponding to the transit time within the cavity between the mirrors (Ml, M2) and as a result, the laser produced output pluses, through the mirror M2, at a rate corresponding to the frequency of the modulator or sub-multiples thereof. The modulator 6 comprises a bulk lithium niobate eleαro-optic phase modulator, the peak phase shift of which is a few tenths of a radian, and the frequency of which (— 420MHz) was thus tuned to a harmonic of the cavity mode spacing (~ 10MHz). Thus, the laser produced bright pubes, mode locked at the modulation frequency of the modulator 6, at the lasing wavelength of 1.3μm associated with the Pr 3 * fibre 3. Further detaib of the experimental results are given in D.M. Pataca et al, supra. The bright output pubes consisted of a succession of short bright pubes, interleaved with broad unstable pulses, when the modulator was driven by a sinusoidal eleαrical signal. The reason for thb will now be explained with reference to Figure 2. In Figure 2a, the drive signal to the phase modulator 6 comprises a conventional sinusoidal eleαrical waveform. Thb produces a corresponding sinusoidal phase modulation of the light 0(t) in the cavity of the laser which, as shown in Figure 2b, in region I, produces a positive going rate of change of frequency or chirp in the FM mode locked pubes. Thb resultant positive going chirp combines with the positive dbpersion of the Pr 3+ fibre 3 so as to produce a pulse spreading effeα which results in a broad, relatively unstable bright output pulse, shown in Figure 2c. In contrast, for the negative going half cycle of the phase modulation shown in region II, for Figure 2a, a negative going chirp b produced which combines with the positive dispersion of the Pr 3 * fibre 3 so as to produce a relatively short duration compressed pube, as shown in Figure 2c. In accordance with the invention, it has been appreciated that the effeα shown in region 1 of Figure 2 can be modified in order to achieve dark output pubes and an embodiment of pube generator in accordance with the invention will now be described with reference to Figure 3. In Figure 3, a Pr 3 * doped fibre laser b shown, which instead of being mode locked by a eleαro-optical modulator as in Figure 1, b FM mode locked by an optical modulator. As in Figure 1, the Pr 3+ fibre 3 is coupled in the cavity between fully refleαive mirror Ml and a partially refleαive output coupler M2. The fibre was 10m long and pumped by a Nd:YAG laser 1 operating at 1.064μm through a wavelength division multiplexer WDMl. The end mirror Ml was a 100% refleαor butted to the Pr ϊ+ -doρed fibre 3. The output coupler M2 comprises either a ~92% refleαing mirror or an optically written fibre grating as will be described in more detail hereinafter. In the embodiment of Figure 3, the FM phase modulation is achieved in the silica fibre 2. To thb end, modulating pubes from a 1.56μm DFB laser 7 are coupled through a wavelength division multiplexer WDM 2 into the silica fibre 2. The modulating pulses produce changes in the refraαive index of the silica fibre 2 via a non linear ( err) effeα; sometimes known as cross phase modulation (XPM), and by seleαing the pube repetition rate in accordance with the resonant frequency of the cavity, mode locking can be achieved. In praαice, the pubes from the laser 7 have a duration of 35ps and are amplified in an erbium-doped fibre amplifier (not shown) to a maximum mean power of 30 mW. The pubes propagate along the length of 500 m of the silica fibre 2 between the wavelength divbion multiplexers WDM 2 and WDM 3, and then exit from the cavity. The modulating pulses produce changes in the refraαive index of the silica fibre so as to produce a positive going phase modulation of the Pr 3 * laser light that resonates in the cavity between the mirrors Ml, M2. As a result, dark pubes are produced in the output of the laser, as will now be explained with reference to Figure 4. In Figure 4a, the phase shift 0(t) produced by the modulating pubes in the silica fibre 2, b shown. It b to be noted that each successive modulation pulse produces the same positive phase window. The modulating fibre was chosen to have a dbpersion zero at — 1.44μm such that the group delays at 1.3μm (the resonant wavelength of the Pr 3 *) laser and at 1.565μm (the wavelength of the modulating pubes from DFB laser 7) were reasonably matched. The total dbpersion for the laser cavity was estimated at 14 ps/m resulting from a 4 ps/nm and 10 ps/nm contributions from the Pr 3 * fibre and the modulator fibre 3 and the modulator fibre 2 respeαively. The resulting positive going chirp produced by the phase modulation shown in Figure 4a, combines with the positive dbpersion of the cavity to produce dark pubes as shown in Figure 4b. Figure 7a illustrates an example of output dark pubes produced when modulating pubes from the laser 7 are applied to fibre 2 with a repetition frequency of ~ 700 MHz. The combination of the positive frequency shifts produced by the modulating pubes from laser 7, and the normal dbpersion of the cavity tends to "push" light of the modulation time slots into the unmodulated regions, giving rise to a broad, essentially continuous wave output, separated by dark optical pubes. The apparatus shown in Figure 3 additionally can be reconfigured to produce bright pubes. Thb can be achieved by changing the cavity dbpersion by replacing the mirror M2 by a so-called "chirped" grating. Such a grating may be produced as described in R. Kashyap et al: Novel method of producing all fibre photoinduced chirped gratings", Eleαronics letters, Vol.30, No.12, pp. 996-998, 1994. Briefly, different gratings are produced by establishing standing wave in a photosensitive fibre to produce a grating pattern. The pattern is repeated in the fibre at different leveb of applied stress so that, when the stress is released, gratings of different spacings are recorded in the same fibre. A schematic illustration of a fibre including two such gratings b shown in Figure 5, with its output charaαeristics being shown in Figure 6. Referring to Figure 5, the grating includes two spaced grating portions A, B, which exhibit peak refleαivities at λ, and λ 2 respeαively. It can thus be seen that the spacing between the regions A, B will introduce a chirp into the pubes that resonate in the cavity, which by appropriate seleαion of the graticule spacings for regions A, B, can result in a negative going dbpersion, in order to compensate for the positive dbpersion of the Pr 3 * fibre 3. Figure 7b and c show the laser output for the device of Figure 3, when firstly the semi-refleαing mirror M2 is used (Figure 7b) and when the mirror is replaced by the chirped grating (Figure 7c). In both cases, modulating pubes from laser 7 were fed into the fibre 2 with a repetition rate of ~ 2.8GHz. In absence of the grating as shown in Figure 7b, dark pubes were produced and the results were qualitatively similar to those shown in Figure 7a. However, when the grating was introduced, a net positive group delay was produced within the fibre, capable of supporting bright solitons and as a result, a stream of narrow — 50ps bright optical pubes were produced. Many modifications and variations of the described examples are possible. For example, it may be possible to replace the phase modulator by an amplitude modulator and achieve FM mode locking, and generate the dark pulses. In all of the described examples, the dark pubes may be solitons. As used herein, the term optical radiation includes visible light and non-visible radiation such as ultraviolet and infra-red radiation.

Claims: Claims 1. An optical pulse generator comprising: a source of optical radiation (1, WDMl, 3); a modulator (2); a source of modulating signals (7); said modulator being operative to phase modulate said optical radiation in accordance with the modulating signals in such a manner as to produce dark pubes in said optical radiation. 2. An optical pube generator according to claim 1 wherein the radiation source comprises a laser (Ml, 1, 3, M2) that is mode locked by the modulator. 3. An optical pulse generator for generating dark pulses, comprising: an optical cavity (Ml, M2), an optically dispersive medium (3) within the cavity, means (1, 2, 3) for producing optical resonance within the cavity, and a modulator (2, 7) for cyclically modulating optical signals in the cavity for mode locking the resonance to produce pubes of a given periodicity, wherein the dbpersive charaαeristics of the cavity and the modulating charaαeristics of the modulator are seleαed so as to produce dark output pulses. 4. An optical pube generator according to claim 3 wherein the modulator (2, 7) is a phase modulator 5. An optical pulse generator according to claim 3 or 4, including a pumping energy source (1), and means (WDMl) for coupling pumping energy from the source into the cavity. 6. An optical pube generator according to claim 5 wherein the pumping energy source (1) comprises a Nd:YAG laser pump. 7. An optical pube generator according to any one of claims 3 to 6 wherein the optically dbpersive medium within the cavity comprises an optical waveguide (3). 8. An optical pulse generator according to claim 7 wherein the optical waveguide comprises an optical fibre (3) doped to provide a particular wavelength dbpersion charaαeristic. 9. An optical pube generator according to claim 8 wherein the fibre is a praseodymium-doped fibre. 10. An optical pulse generator according to any one of claims 3 to 9 wherein the cavity abo includes dbpersive means (M2) having a predetermined dbpersion charaαeristic so that radiation resonant in the cavity is subjeα to wavelength dbpersion as a funαion of the dbpersion charaαeristics of both the waveguide (3) and the dbpersive means (M2). 11. An optical pube generator according to claim 10, wherein the dbpersive means (M2) comprises a grating (A, B). 12. An optical pube generator according to claim 11 wherein the grating has graticules with a spatial frequency that varies along the length thereof. 13. An optical pulse generator according to any preceding claim wherein the modulator includes a modulator optical fibre (2) conneαed in the cavity, and an optical modulating source (7) for direαing optical modulating pulses into the modulating fibre to modulate the optical charaαeristics thereof. 14. An optical pube generator according to claim 13 wherein the modulating optical fibre (2) comprises a silica fibre. 15. An optical pube generator according to any preceding claim wherein the modulator b additionally configurable to produce bright pubes. 16. An optical pube generator according to any preceding claim wherein the pubes are soliton pubes. AMENDED CLAIMS [received by the International Bureau on 17 January 1996 (17.01.96)- original claims 1,2 amended; remaining claims unchanged (3 pages)]' 1. An optical pulse generator comprising: 5 a resonant source of optical radiation (1, WDMl, 3, Ml, M2); a modulator (2); a source of modulating signals (7); said modulator being operative to mode lock said resonant source and phase modulate said optical radiation in accordance with the 10 modulating signals in such a manner as to produce dark pulses in said optical radiation. 2. An optical pulse generator according to claim 1 wherein the radiation source comprises a laser (Ml, 1, 3, M2). 15 3. An optical pube generator for generating dark pulses, comprising: an optical cavity (Ml, M2), an optically dispersive medium (3) within the cavity, means (1, 2, 3) for producing optical resonance within the 20 cavity, and a modulator (2, 7) for cyclically modulating optical signals in the cavity for mode locking the resonance to produce pulses of a given periodicity, wherein the dispersive charaαeristics of the cavity and the modulating charaαeristics of the modulator are seleαed so as to produce dark 25 output pulses. 4. An optical pulse generator according to claim 3 wherein the modulator (2, 7) is a phase modulator 30 5. An optical pulse generator according to claim 3 or 4, including a pumping energy source (1), and means (WDMl) for coupling pumping energy from the source into the cavity. 6. An optical pube generator according to claim 5 wherein the pumping energy source (1) comprises a Nd:YAG laser pump. 7. An optical pulse generator according to any one of claims 3 to 6 j wherein the optically dispersive medium within the cavity comprises an optical waveguide (3). 8. An optical pube generator according to claim 7 wherein the optical waveguide comprises an optical fibre (3) doped to provide a particular 10 wavelength dispersion charaαeristic. 9. An optical pulse generator according to claim 8 wherein the fibre is a praseodymium-doped fibre. 15 10. An optical pube generator according to any one of claims 3 to 9 wherein the cavity also includes dbpersive means (M2) having a predetermined dbpersion charaαeristic so that radiation resonant in the cavity is subject to wavelength dbpersion as a funαion of the dispersion charaαeristics of both the waveguide (3) and the dispersive means (M2). 20 11. An optical pulse generator according to claim 10, wherein the dispersive means (M2) comprises a grating (A, B). 12. An optical pulse generator according to claim 11 wherein the grating 2 has graticules with a spatial frequency that varies along the length thereof. 13. An optical pulse generator according to any preceding claim wherein the modulator includes a modulator optical fibre (2) conneαed in the cavity, and an optical modulating source (7) for direαing optical modulating pubes 0 into the modulating fibre to modulate the optical charaαeristics thereof. 14. An optical pulse generator according to claim 13 wherein the 7 5 - modulating optical fibre (2) comprises a silica fibre. 15. An optical pulse generator according to any preceding claim wherein the modulator is additionally configurable to produce bright pubes. 16. An optical pulse generator according to any preceding claim wherein the pulses are soliton pulses.

Inventors: DE LACERDA ROCHA MONICA, PATACA DANIEL MOUTINHO, SMITH KEVIN, DE LACERDA ROCHA, MONICA, PATACA, DANIEL, MOUTINHO, SMITH, KEVIN
Applicants: BRITISH TELECOMM, LACERDA ROCHA MONICA DE, PATACA DANIEL MOUTINHO, SMITH KEVIN, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY, DE LACERDA ROCHA, MONICA, PATACA, DANIEL, MOUTINHO, SMITH, KEVIN
Application year: 1995
Main classification: H01S3
IPC codes: G02F1, H01S3