OPTICAL WAVEGUIDE LASER COMPRISING BRAGG GRATINGS

Abstract: A Bragg Grating optical waveguide laser source comprising at least one Bragg grating in a rare earth doped waveguide and an optical pump source coupled to said doped waveguide, said Bragg gratings having at least two different peak reflection wavelengths and at least one of said Bragg gratings comprising a phase-shift and a phase-shift actuator being coupled to the phase-shift for controlled application of changes in the phase-shift thus activating or deactivating the corresponding Bragg grating waveguide laser.

Description: OPTICAL WAVEGUIDE LASER COMPRISING BRAGG GRATINGS This invention relates to a Bragg grating optical waveguide laser source comprising two or more Bragg gratings in a rare earth doped waveguide and an optical pump source coupled to said doped fibre. In wavelength division multiplexed (WDM) telecommunication systems as well as WDM sensing systems there is a need for wavelength selectable and/or tunable narrowband laser sources . In future dense WDM (DWDM) telecommunication systems and networks wavelength selectable lasers seem very attractive both as back-up lasers in multiwavelength laser transmitter modules and as wavelength switchable sources for wavelength routing/switching in optical networks. The first application typically requires very wide wavelength tuning such that one tunable laser can be a back-up for many fixed wavelength sources, but does normally not require very fast tuning. The latter application requires fast wavelength switching, typically <lms. Future DWDM systems will have a large number of wavelength channels with very narrow channel spacing (50GHz or less) . This requires very tight laser wavelength control combined with robust single mode laser operation with narrow linewidth (less than a few MHz) and high side-mode suppression ratio (SMSR) . In addition tunable lasers should ideally provide wide tuning (>40nm) combined with high power (>20mW) . There are several tunable multi-section semiconductor lasers under development which are aiming at the applications mentioned above [1] . However, these multi section lasers are complex, with up to 4 different currents required to control both wavelength and power and ensure mode-hop free single mode operation, and are difficult to make with both high power and wide tuning. A wavelength selectable, mode-hop free single mode laser with high SMSR can be realised by combine several semiconductor DFB laser cavities with different wavelengths on a single chip in a linear gain-coupled [2] or a parallel index-coupled [3] DFB array, with separate drive currents such that only one laser is pumped and oscillating at a time. All laser cavities can be temperature tuned simultaneously. Exact wavelength control of such lasers require wavelength lockers. DFB fibre lasers are another attractive alternative for wavelength accurate, mode-hop free tunable single mode laser source, which seem able to satisfy the requirements described above. A DFB fibre laser typically consist of a phase-shifted fibre Bragg grating (FBG) written in a rare earth doped (eg. erbium doped) optical fibre, which when pumped by a semiconductor pump laser provides lasing at wavelength determined by the grating [4] . With an optical phase-shift of Φ=π/2 at the centre of the grating the round-trip phase condition of n*2π (n is an integer) of the laser cavity is satisfied at the centre (Bragg) wavelength of the grating, providing the lowest lasing threshold. Detuning the phase-shift away from the optimum value of π/2 will increase the threshold and possibly prevent laser operation [4] . The wavelength of a DFB fibre laser can be set with great accuracy and repeatability in the writing of the grating, and with proper annealing the wavelength remains stable over time. DFB fibre lasers will inherently operate at two polarisations, but single-polarisation operation can be obtained for example by making a grating with differential strength for the two polarisation states. The temperature sensitivity of the laser wavelength is one order of magnitude lower than of a semiconductor DFB laser, relaxing the requirements on temperature stabilisation. The wavelength of a DFB fibre laser can be tuned continuously without mode-hopping through uniform strain/compression and/or temperature tuning of the whole laser cavity [5] , [6] . Such mechanical or thermal tuning is relatively slow (typically > 0.1s). The tuning range with temperature is typically <l-2nm due to the low temperature sensitivity. Strain/compression tuning allows tuning over typically 5- 15nm, mainly limited by the mechanical reliability of the fibre. Good control of strain/compression and temperature can provide highly accurate wavelength control without the need for closed loop control and wavelength lockers. Note that DFB fibre laser can not be directly modulated at telecommunication speeds due to the slow gain medium, and require external modulators . A wavelength selectable fibre laser has been demonstrated using a laser cavity comprising a widely Φ ,β 4-J Φ TJ > Φ rd υ JJ β TJ CQ β n -H φ CQ 4-J rd β r-{ Φ H φ Φ i 4-1 4-J 4H -H H TJ rH .3 A 4-J • -H CQ ^ 0) β 4-1 -H β 4-J A td rl « tn β o -H φ 4-1 CQ -H rd CQ rH o rH rd 4-1 β 4-1 TJ -H -rl Jβ CQ -H 1 TJ φ β n -H rd A Φ .β 4-J φ 4-1 ε rd 0 CQ strain/compression with a capacitance positioning sensor and the temperature with a temperature sensor, or alternatively by measuring the combined effect of strain/compression and temperature with an FBG sensor imprinted in the strained/compressed fibre section [Norwegian patent application 1999.5485] . The phase-shift of each laser can be set to the correct magnitude by introducing a pre-determined local increase in refractive index and/or strain at the centre of the grating, or alternatively by tuning the phase- shift to maximise the output power using a feedback loop. The invention will be described in detail below with reference to the accompanying drawings, illustrating the invention by way of examples. Fig. 1 shows three overlaid DFB fibre lasers having the same phase-shift position but different phase- shifts . Fig. 2 illustrates schematically the wavelength spectrum of three overlaid DFB fibre lasers having the same phase-shift position, with tuning of the induced central phase-shift φ combined with strain/compression tuning, and with only one laser lasing at a time. Fig. 3 illustrates schematically the reflection spectra of three overlaid DFB fibre lasers having the same phase-shift position, with different phase-shifts. Fig. 4 shows the calculated output power from five overlaid Er 3+ -doped DFB fibre lasers with 5nm wavelength spacing as a function of the induced central phase-shift φ, where only one laser is lasing at a time. Fig. 5 shows three partially overlaid DFB fibre lasers having separated phase-shift positions and common strain/compression tuning. Fig. 6A illustrates schematically the wavelength spectrum of three partially overlaid DFB fibre lasers, having separated phase-shift positions, with tuning of the phase-shifts combined with strain/compression tuning, and with only one laser lasing at a time. Fig. 6B illustrates schematically the wavelength spectrum of three partially overlaid DFB fibre lasers, having separated phase-shift positions, with tuning of the phase-shifts such that one to three lasers are lasing at a time. Fig. 7 shows three serially multiplexed (single or overlaid) DFB fibre lasers along the same fibre with individual phase-shift and strain/compression control . Fig. 8A shows three parallel multiplexed (single or overlaid) DFB fibre lasers along the same fibre with individual phase-shift control and common strain/compression control. Fig. 8B shows three parallel multiplexed (single or overlaid) DFB fibre lasers in separate fibres with individual phase-shift control and common strain/compression control. Fig. 9A and B illustrates schematically the wavelength tuning operation of a wavelength swept DFB fibre laser array (overlaid or separate) . Fig. 1 shows n overlaid DFB fibre lasers 1 with different Bragg wavelengths in a rare-earth doped fibre, which can be an erbium or erbium-ytterbium doped fibre, which is pumped by an optical pump source, typically a semiconductor laser, through an input end 2 and emitting single mode, single wavelength laser light at output end 3 and also f om end 2. Typically the lasers are made asymmetric such that most of the laser light is emitted from one end. This is standard technique and will not be described in any further detail here. The lasers have the same phase-shift position but different inherent (UN-induced) phase-shifts. With the output from end 3 the residual pump light can be used to pump an amplifying fibre (eg. erbium) to boost the laser power. All laser gratings have different phase-shifts at the same position in the fibre, with values such that none or only one laser is lasing when pumped. The phase-shift at the centre can be changed by means of a first actuator, here called the phase-shift actuator, e.g. comprising a local threshold will increase such that lasing might not be reached. Fig. 4 shows the calculated output power P in mW from five fully overlaid Er 3+ -doped DFB fibre as a function of the induced central phase-shift φ, where only one laser is lasing at a time. The difference in Bragg wavelength between the individual lasers is 5nm. The difference in initial phase-shift φ 0 between the individual lasers is 72° (=360°/5) . The laser length is 10cm and the fibre small signal absorption and gain at the laser wavelength are 12dB/m and 16dB/m, respectively. The grating coupling strength K = πδn eff /λ of each laser is 55m" 1 , corresponding to an effective index modulation δn eff for each laser of 2.75*10" 4 , while the threshold coupling strength is 47.5m -1 . The maximum index modulation of 5 overlaid lasers is 5*δn eff = 1.4*10" 4 . If the overlaid DFBs can be strained or compressed by 0.5%, corresponding to a 6nm shift in Bragg wavelength, a total tuning range of 30nm can be obtained. Fig. 5 shows three partly overlaid DFB fibre lasers 7 with different Bragg wavelengths in a rare-earth doped fibre, which is pumped by an optical pump source through an input end 8 and emitting laser light at an output end 9 and also from end 8. All laser gratings have different phase- shifts at different positions along the fibre. The phase- shifts of the individual lasers can be changed by means of a local heaters or stretchers 10,11,12, which can be tuned electrically such that the total phase-shift is correct for any of the individual lasers and such that one or more than one laser can lase at a time. All laser (Bragg) wavelengths can be tuned simultaneously by straining or compressing the whole laser grating, for example by fixing each end of the laser at fixing points 13,14, and moving 14 mechanically with the wavelength actuator mechanism. This laser configuration can allow tunable single wavelength operation, as illustrated in Fig. 6A, ilustrating the wavelength scan with a first, second and third phase-shift actuators Al, A2, A3, respectively operative, or multi-wavelength operation as illustrated in Fig. 6B, by separately controlling all heaters/stretchers individually and thus using the phase- shift actuators Al, A2, A3 for activating the lasers at the CQ 4-1 β Φ β O ft ε O CJ P s S3: P Φ > -H CQ CQ rti ft TJ β rd A S3 O fH fH rti β 4H 0 β O -H 4-1 rti CQ -H fH Φ 4-J U fti fH (ti Jβ V References : [1] Amman, M.-C, and Buus, J., Tunable Laser Diodes, Artech House, London, 1998. [2] Hong, J., et.al., "Cascaded strongly gain-coupled (SGC) DFB lasers with 15-nm continuously-wavelength tuning," IEEE Photon. Technol. Lett., Vol. 11, pp. 1214-1216, 1999. [3] Young, M.G., et.al., "Six wavelength laser array with integrated amplifier and modulator," Electron. Lett., Vol. 31, pp. 1835-1836, 1995. [4] Kringlebotn, J. T., Archambault, J.-L., Reekie, L., and Payne, D.N, "Er 3+ :Yb 3+ -codoped fibre distributed-feedback laser," Optics Letters, Vol. 19, pp. 2101-2104, 1994, and US patent 5,771,251. [5] Ball, G., and Morey, W. W. , Opt. Lett., Vol. 17, pp. 420-422, 1992. [6] Kringlebotn, J. T., US patent 5,844,927. [7] Libatique, N. J. C. and Jain, R. K. , "Pricisely and rapidly wavelength-switchable narrow-linewidth 1.5-μm laser source for wavelength division multiplexing applications," IEEE Photon. Technol. Lett., Vol. 11, pp. 1584-1586, 1999. [8] Conradi, J. , US patent 6,061,369.

Claims: C l a i m s 1. A Bragg grating optical waveguide laser source comprising at least two fully, partly or non-overlapping Bragg gratings in the same waveguide, each grating defining one laser in a rare earth doped waveguide or waveguide section and an optical pump source coupled to said doped waveguide, said at least two Bragg gratings having different grating wavelengths and corresponding phase-shifts, wherein the laser source further comprises a first actuator means for the controlled application of changes in the phase-shifts thus activating or deactivating the corresponding Bragg grating lasers. 2. A laser source according to claim 1, wherein the phase- shifts are located at a common position in the waveguide and the first actuator means being located at said common position of the phase-shifts for the controlled application of changes in the phase-shifts thus activating or deactivating the corresponding Bragg grating waveguide lasers . 3. A laser source according to claim 2, wherein the first actuator means is provided with means for altering the temperature locally in said phase-shifts, thus providing a controlled change in the phase-shifts. 4. A laser source according to claim 2, wherein the first actuator means is provided with means for providing a strain or stress in the fibre, and thus a controlled change in the phase-shifts. 5. A laser source according to claim 2, wherein the first actuator means is provided with means for altering the refractive index of the fiber, and thus a controlled change in the phase-shifts. 6. A laser source according to claim 1, wherein the phase- shifts are different, such that only one laser at a time has a total phase-shift for yielding lasing conditions. 7. A laser source according to claim 1, wherein the Bragg gratings are overlaid in the same length of waveguide, being written separately or as one complex sampled grating with an index profile equal the sum of two or more individual gratings. 8. A laser source according to claim 1, wherein the Bragg gratings are separated or partially overlapping , each grating having separate phase-shifts each being provided with a separate actuator means, thus providing means for individual activation of each grating. 9. A laser source according to claim 1, further comprising a second actuator means for the controlled application of changes in at least one grating and the corresponding grating wavelength. 10. A laser source according to claim 9, wherein the second actuator means comprises a wavelength actuator means being common for two or more Bragg gratings . 11. A laser source according to claim 10, wherein the second actuator means is provided with means for altering the temperature of the gratings, thus providing a controlled change in the grating wavelengths. 12. A laser source according to claim 10, wherein the second actuator means is provided with means for altering the strain or compression of the grating, thus providing a controlled change in the grating wavelength. 13. A laser source according to claim 9, wherein the second actuator means is adapted to provide a wavelength sweep by continuously varying the strain/compression or temperature of the gratings. Itf . A laser source according to claim 1, wherein the output of the laser is monitored by an analysing means and wherein the actuators are coupled to said analysing means. IS A laser source according to claim 1, wherein the wavelenghts of the gratings correspond to chosen wavelengths in an optical communication network, said phase-shift actuators providing means for shifting between said chosen wavelengths . lb. Use of a laser source according to claim 9 for spectral characterization of optical components comprising a detector for measuring the transmitted or reflected spectral power of the optical component.

Inventors: KRINGLEBOTN JON THOMAS, LOEVSETH SIGURD WEIDEMANN, KRINGLEBOTN, JON, THOMAS, LOEVSETH, SIGURD, WEIDEMANN
Applicants: KRINGLEBOTN JON THOMAS, LOEVSETH SIGURD WEIDEMANN, OPTOPLAN AS, KRINGLEBOTN, JON, THOMAS, LOEVSETH, SIGURD, WEIDEMANN
Application year: 2001
Main classification: H01S3
IPC codes: H01S3