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Diode-pumped FM modelocked fibre laser with coupled cavity bandwidth selection
Volume 75, number
OPTICS
1
DIODE-PUMPED WITH COUPLED
1 February
COMMUNICATIONS
1990
FM MODELOCKED FIBRE LASER CAVITY BANDWIDTH SELECTION
M.W. PHILLIPS a, A.I. FERGUSON b and D.B. PATTERSON a Department ofPhysics, The University, Southampton SO9 SNH, UK
’
b Department of Physics and Applied Physics, University of Strathclyde, Glasgow G4 ONG, UK ’ Department ofElectrical Engineering, Ginzton Laboratories, Stanford University, Stanford CA 94305. USA Received
3 October
1989
A diode-laser-pumped Nd’+-doped fibre laser has been actively mode-locked using an integrated fibre phase modulator. Pulses with a sub-80 ps duration are observed at a repetition rate of 4 I7 MHz. Near bandwidth-limited operation is achieved by restricting the laser bandwidth by feedback off a diffraction grating in an external coupled cavity. Low intracavity losses support a submilliwatt laser threshold and a slope efficiency in excess of 48%.
1. Introduction A number of recent publications have shown that FM (frequency modulation ) modelocking is a convenient method for the generation of short optical pulses in glass fibre lasers doped with rare earth elements. Using a bulk LiNb03 crystal as the phase modulator, we have demonstrated modelocking in a Nd3+-doped tibre laser producing bandwidth-limited pulses of 20 ps duration with a peak power of 1 W [ 11. The same modulator has yielded pulses of 70 ps duration and 90 mW peak output power in an Yb: Er fibre laser [ 21. In each case, it is necessary to extract light from the fibre gain medium and focus it through the modulator using an intracavity lens. This arrangement suffers from intracavity reflections which limit the useful gain bandwidth of the laser through etalon formation. Furthermore, the insertion of intracavity elements increases the overall loss in the system, thereby increasing the threshold for laser oscillation and preventing any reasonable attempt at diode-laser pumping. The problem of etalon formation has been overcome by directly butting a LiNb03: Ti waveguide modulator to the output end of the fibre gain medium. Both Nd3+-doped [ 31 and Er3+-doped [4] tibre lasers have been modelocked successfully in this manner. These systems, however, suffer from a mode 0030-4018/90/$03.50
0 Elsevier Science Publishers
mismatch at the fibre-waveguide interface giving rise to high intracavity losses and associated poor lasing characteristics. Broadband operation and minimal intracavity losses are simultaneously realised by modulating the phase of the intracavity field while light remains within the fibre gain medium. Using a modulator which achieves intrafibre phase modulation by setting up a standing acoustic wave in the fibre [5], we have modelocked a short length of Nd3+-doped tibre with mirrors butted directly to either end. The performance of this simple system when end-pumped by a R6G dye laser has been reported elsewhere [ 6 1. In this paper we report on the much improved performance of the modelocked system (shorter pulse duration ) when end-pumped with a diode laser. We also describe a novel scheme for controlling the lasing bandwidth using a diffraction grating in an external coupled cavity. This arrangement maintains a high finesse and low threshold for the main laser oscillator, while selecting the optimum bandwidth for modelocked operation through reinjection of a spectrally filtered output signal.
2. Experimental
details
The experimental configuration is depicted in fig. 1. For a detailed description of the basic laser os-
B.V. (North-Holland
)
33
Volume 7 5, number
1
OPTICS
COMMUNICATIONS
TO PUMP MONITOR
PUMP
OUTPUT TO MONOCHROYATOR
Fig. I. Schematic diagram of FM modelocked fibre laser with coupled cavity bandwidth control. Notation used are MO: microscope objective; HR: high reflector; OC: output coupler: AOM: acoustic-optic phase modulator; BS: beamsplitter; PZT: pieroelectric stack: HT: high tension: FPI: Fabry-Perot interferometer.
cillator we refer the reader to ref. [ 61. Here we restrict our description to the main features of the system. The fibre gain medium was a 24.55 cm length of Nd3+-doped. non-polarization preserving fibre with a numerical aperture &‘A= 0.2 1, a second mode cutoff wavelength i.,,= 945 nm and a dopant concentration of 220 ppm wt Nd. Consequently the fibre supported a single transverse mode at the laser wavelength >.L= 1.09 pm. The fibre cladding diameter of I25 + 0.5 pm was selected for optimum performance of the integrated phase modulator [ 51. The fibre was end-pumped by a single stripe AlGaAs diode laser operating at 820 nm with a maximum output power rating of 15 mW. Pump power was launched into the fibre through the butted high reflector mirror (HR at the laser wavelength) using a standard 10X microscope objective. Allowing for the 85% transmission of the high reflector at the pump wavelength, the overall launch efficiency was around 0.32. The high reflector substrate thickness of 0.17 mm minimised spherical aberration in the pump launching scheme but prevented wedging of the rear surface. The main laser cavity was completed by butting a 15% output coupling mirror to the fibre output endface. This mirror was wedged and AR coated to suppress back surface reflections detrimental to modelocked operation. The phase modulator transducer was pressed firmly against the outside surface of the fibre (protective coating removed) in close proximity to the output coupling mirror in order to maximise the mode-coupling efficiency ii [ 61. The fibre length was chosen 34
1 February
1990
to match the driving frequency for optimum modelocking to the peak response frequency of the modulator, v,,,= 4 17 MHz. This limited the pump absorption efficiency to just 0.41. Clearly, a larger dopant concentration in the fibre would improve this efficiency. The maximum single pass phase retardation at the laser wavelength, &,, was measured in situ by monitoring sideband generation on a single frequency HeNe laser beam (>.= 633 nm) launched into the fibre and subsequently allowing for the inverse dependence of &,, on optical wavelength. With 1 W of rf power applied at the peak response frequency, the measured phase retardation at the laser wavelength of 1.09 urn was 0.6 rad. Light emitted through the output coupling mirror was collected and collimated by an 18 x microscope objective onto a diffraction grating with a 600 lines/ mm pitch. With an arbitrarily chosen incidence angle of 70’, the power transmission into the zeroth, first and second diffraction orders was 0.52, 0.24 and 0.22 respectively. Transmission into the zeroth order and diffraction in the higher orders both increased on tilting the grating towards grazing incidence. The useful output coupling of the compound laser system was provided by the diffraction-free zeroth order beam. The first order was used to monitor the average output power while the more strongly diffracted second order was retroreflected back into the laser by a high reflecting mirror (wedged and AR coated on the rear surface), providing a spectrally filtered injection signal. The mirror was mounted on a translation stage with piezoelectric control to allow fine tuning of the coupled cavity path length. Focusing of the output lens was adjusted to maximise feedback from the coupled cavity. The filter bandwidth was determined by the number of lines illuminated on the grating and could be adjusted arbitrarily by altering the incidence angle on the grating or the focusing of the output beam. The temporal behaviour of the laser was monitored from the zeroth diffraction order using a fast InGaAs photodiode and sampling oscilloscope (overall response time is 56 ps). For convenience. the spectral content of the laser was determined from the weak beam coupled out of the input high reflector using a l/3 metre monochromator and a scanning Fabry-Perot interferometer with a finesse of 40 and an adjustable free spectral range. With the laser
Volume 75, number 1
OPTICS COMMUNICATIONS
1 February
1990
running continuous-wave (modulator deactivated), the threshold pump power absorbed in the fibre was just 520 uW (4 mW out of the diode laser). With respect to absorbed pump power, the slope efficiency of the main laser cavity was 48%. This corresponds to a slope efficiency for the compound system of 25% (for the arbitrarily chosen grating incidence angle of 70” ). All these parameter values remained unchanged when applying 1 W of rf power at the optimum modelocking frequency.
3. Results Modelocked performance was assessed with 2 mW of absorbed pump power ( 15 mW out of the diode laser) and 1 W of rf power applied to the modulator at the optimum modelocking frequency of vML = 4 16.90499 MHz. With correctly butted mirrors and the feedback arm blocked, oscillation was observed simultaneously on both sets of interleaved modelocked pulses corresponding to either extremum of the phase modulation cycle. The stability of the pulse sets was much greater than observed in systems FM modelocked with a bulk phase modulator [ 1,2]. The two pulse sets had different optimum modelocked frequencies and minimum pulse duration. The difference in pulse duration is clearly illustrated in fig. 2 with the narrower pulses approaching the response limit of the diagnostics. The narrow pulses were preferentially selected by withdrawing the output coupler by a few interference fringes, introducing a broadband etalon to the system without significantly affecting the cavity finesse. Fig. 3 shows the pulse profile after fine tuning of the rf frequency to optimise modelocking of the narrow pulse set. Allowing for the response time of the detector, the measured pulse duration of 80 ps suggests an actual pulse duration around 60 ps. Baseline structure was due to detector ringing. The maximum absorbed pump power of 2 mW yielded an average output power (incident on the grating) of just 0.7 mW and prevented autocorrelation of the modelocked pulses. Assuming a 60 ps pulse duration, the peak output power was about 28 mW with a peak intensity in the fibre of 3 MW cme2. Significant changes in pulse duration were observed for a frequency detuning of 200 Hz. The pulse spectrum is shown in fig. 4, indicating a major peak with
Fig. 2. Oscilloscope trace showing stable simultaneous oscillation of both sets of modelocked pulses corresponding to either extremum of the phase modulation cycle (coupled cavity blocked). The difference in pulse duration for alternate pulses is clearly illustrated.
Fig. 3. Output signal after selecting single pulse set with shorter pulse duration by withdrawing output coupling mirror. Detector response time is 56 ps. Baseline structure is due to detector ringing.
two subsidiary peaks at shorter wavelength. The bandwidth (fwhm) of the major peak, centred at 1.087 urn, was 1.5 nm (or equivalently AuLz 380 GHz). These results are considerably better, in terms of reduced pulse duration, than those obtained when pumping the system with an R6G dye laser [ 6 1. The 35
Volume 75. number
I
OPTICS
1009 WAVELENGTH
(nm)
COMMUNICATIONS
I February
1990
11
Fig. 4. Power spectrum of FM modelocked tibre laser without coupled cavity bandwidth control. Major peak has a bandwidth (fwhm) of 1.5 nm (AvL=380GHz).
improved performance is due in part to the greater stability of the all solid-state system compared to the equivalent dye laser pumped system in which bubbles in the dye jet and 50 Hz ripple from the Ar ion laser cause fluctuations in the laser intracavity intensity. The values of pulse duration and spectral bandwidth indicate that, in the absence of feedback from the coupled cavity, modelocked operation was far from bandwidth-limited with insufficient phase modulation present to lock modes across the entire lasing profile. Bandwidth restriction was achieved by rebutting the output coupling mirror and unblocking the feedback arm. With feedback the laser exhibited random switching between the two sets of pulses due to the interferometric nature of the system causing random phase changes between the intracavity and reinjetted pulses. The output signal was stabilised by actively locking the coupled cavity and main laser cavity path lengths. The error signal was derived from fluctuations in the average power signal with changing pulsed states. This was fed to a high tension servolocking circuit which adjusted the feedback mirror position to maintain stable operation. By adjusting a dc offset voltage it was again possible to select and maintain the set of modelocked pulses with narrow pulse duration, as shown in fig. 5. Fig. 6 shows an oscilloscope trace of the pulse profile after optimising modelocking. The measured pulse duration of 36
Fig. 5. Preferential selection of short duration pulse set by synchronizing intracavity and reinjected pulses. (a) Without feedback both sets of pulses oscillate simultaneously. (b) With feedback, the long duration pulse set is totally extinguished.
100 ps suggests an actual pulse duration of 82 ps. Noise at the peak of the pulse was attributed to imperfect locking of the coupled cavity and main cavity path lengths. The degree of spectral reshaping by the relatively weak injection signal is quite considerable. Tilting the feedback mirror had the effect of shifting the diffraction grating filter profile, hence tuning the injection signal frequency across the laser spectrum. As a result, an enhancement peak was observed to pass through the spectrum shown in f”rg. 4, the width of which was determined by the filter bandwidth. Fig. 7 shows a plot of the laser spectrum monitored with ‘I~=
Volume 75. number
1
OPTICS
COMMUNICATIONS
1 February
1990
nificant homogeneous broadening in the laser. With the interferometer scanning over a single free spectral range of 60 GHz, the modelocked laser bandwidth was measured as Av,= 10 & 1 GHz, with a corresponding pulsewidth-bandwidth product of 0.82.
4. Summary
Fig. 6. Output signal after selecting single pulse set with shorter pulse duration by locking coupled cavity. Detector response time = 56 ps. Noise at the peak of the pulse is attributed to imperfect servo-locking of the coupled cavity.
Fig. 7. Power spectrum of FM pled cavity bandwidth control. a bandwidth (fwhm) of AuL= finesse = 40, free spectral range
modelocked fibre laser with couA single feature is observed with IO? 1 GHz (measured with FPI, = 60 GHz).
We have described the performance of a diodelaser-pumped Nd 3+-doped fibre laser actively modelocked with an integrated fibre phase modulator. Modulation occurs within the fibre itself allowing simple cavity design with low loss and broadband characteristics. Pulses of around 60 ps duration have been observed at a repetition rate of 417 MHz with a peak output power of 28 mW. The corresponding laser bandwidth is approximately Au,=380 GHz, indicating incomplete locking of axial modes over the laser profile. By coupling the laser to an external cavity incorporating a diffraction grating, we have restricted the laser bandwidth to Au,= 10 GHz, while maintaining an absorbed pump power threshold of just 520 uW. The compound system yields near bandwidth-limited modelocked pulses of about 82 ps duration, before optimising the laser bandwidth. This is the first demonstration, to our knowledge, of a modelocked fibre laser pumped by a low power, single stripe diode laser, reflecting the low threshold nature of the system despite a non-optimum dopant concentration in the fibre. Considerable improvement in system performance is envisaged with the use of a correctly doped polarization-selective fibre (6, is polarization-dependent [ 51) and with the bandpass of the coupled cavity optimised for bandwidth-limited operation.
Acknowledgements the Fabry-Perot interferometer, with the short duration pulses selected and the enhancement peak tuned roughly to the centre of the spectrum shown in fig. 4. The simultaneous enhancement of the major peak and the total extinction of secondary peaks suggested a dramatic channelling of energy over a spectral range of several nanometers, indicating sig-
This work has been partially funded within the Join Opto-Electronics Research Scheme. We are grateful to Janet Townsend of the Optical Fibre Group of the Department of Electronics, Information Engineering and Computer Science for providing the doped fibre. Mark Phillips is supported by a Science and Engi37
Volume 75, number
neering
I
Research
OPTICS
Council
postdoctoral
COMMUNICATIONS
fellowship.
References [ 1] M.W. Phillips, A.I. Ferguson and D.C. Hanna, Optics Lett. 14 (1989)
219.
[ 21 D.C. Hanna, A. Kazer, M.W. Phillips, D.P. Shepherd P.J. Suni, Electron.
38
Lett. 25 (1989)
95.
and
1 February
1990
[ 31 G. Geister and R. Ulrich, Optics Comm. 68 ( 1988) 187. [4] J.D. Kafka, T. Baer and D.W. Hall, CLEO 1989 Technical Digest, Paper FA3. [ 5 ] D.B. Patterson, A.A. Godil, G.S. Kino and B.T. Khuri-Yakub, Optics Lett. 14 (1989) 248. [6] M.W. Phillips, AI. Ferguson, G.S. Kino and D.B. Patterson, Optics Lett. 14 ( 1989) 680.
OPTICS
1
DIODE-PUMPED WITH COUPLED
1 February
COMMUNICATIONS
1990
FM MODELOCKED FIBRE LASER CAVITY BANDWIDTH SELECTION
M.W. PHILLIPS a, A.I. FERGUSON b and D.B. PATTERSON a Department ofPhysics, The University, Southampton SO9 SNH, UK
’
b Department of Physics and Applied Physics, University of Strathclyde, Glasgow G4 ONG, UK ’ Department ofElectrical Engineering, Ginzton Laboratories, Stanford University, Stanford CA 94305. USA Received
3 October
1989
A diode-laser-pumped Nd’+-doped fibre laser has been actively mode-locked using an integrated fibre phase modulator. Pulses with a sub-80 ps duration are observed at a repetition rate of 4 I7 MHz. Near bandwidth-limited operation is achieved by restricting the laser bandwidth by feedback off a diffraction grating in an external coupled cavity. Low intracavity losses support a submilliwatt laser threshold and a slope efficiency in excess of 48%.
1. Introduction A number of recent publications have shown that FM (frequency modulation ) modelocking is a convenient method for the generation of short optical pulses in glass fibre lasers doped with rare earth elements. Using a bulk LiNb03 crystal as the phase modulator, we have demonstrated modelocking in a Nd3+-doped tibre laser producing bandwidth-limited pulses of 20 ps duration with a peak power of 1 W [ 11. The same modulator has yielded pulses of 70 ps duration and 90 mW peak output power in an Yb: Er fibre laser [ 21. In each case, it is necessary to extract light from the fibre gain medium and focus it through the modulator using an intracavity lens. This arrangement suffers from intracavity reflections which limit the useful gain bandwidth of the laser through etalon formation. Furthermore, the insertion of intracavity elements increases the overall loss in the system, thereby increasing the threshold for laser oscillation and preventing any reasonable attempt at diode-laser pumping. The problem of etalon formation has been overcome by directly butting a LiNb03: Ti waveguide modulator to the output end of the fibre gain medium. Both Nd3+-doped [ 31 and Er3+-doped [4] tibre lasers have been modelocked successfully in this manner. These systems, however, suffer from a mode 0030-4018/90/$03.50
0 Elsevier Science Publishers
mismatch at the fibre-waveguide interface giving rise to high intracavity losses and associated poor lasing characteristics. Broadband operation and minimal intracavity losses are simultaneously realised by modulating the phase of the intracavity field while light remains within the fibre gain medium. Using a modulator which achieves intrafibre phase modulation by setting up a standing acoustic wave in the fibre [5], we have modelocked a short length of Nd3+-doped tibre with mirrors butted directly to either end. The performance of this simple system when end-pumped by a R6G dye laser has been reported elsewhere [ 6 1. In this paper we report on the much improved performance of the modelocked system (shorter pulse duration ) when end-pumped with a diode laser. We also describe a novel scheme for controlling the lasing bandwidth using a diffraction grating in an external coupled cavity. This arrangement maintains a high finesse and low threshold for the main laser oscillator, while selecting the optimum bandwidth for modelocked operation through reinjection of a spectrally filtered output signal.
2. Experimental
details
The experimental configuration is depicted in fig. 1. For a detailed description of the basic laser os-
B.V. (North-Holland
)
33
Volume 7 5, number
1
OPTICS
COMMUNICATIONS
TO PUMP MONITOR
PUMP
OUTPUT TO MONOCHROYATOR
Fig. I. Schematic diagram of FM modelocked fibre laser with coupled cavity bandwidth control. Notation used are MO: microscope objective; HR: high reflector; OC: output coupler: AOM: acoustic-optic phase modulator; BS: beamsplitter; PZT: pieroelectric stack: HT: high tension: FPI: Fabry-Perot interferometer.
cillator we refer the reader to ref. [ 61. Here we restrict our description to the main features of the system. The fibre gain medium was a 24.55 cm length of Nd3+-doped. non-polarization preserving fibre with a numerical aperture &‘A= 0.2 1, a second mode cutoff wavelength i.,,= 945 nm and a dopant concentration of 220 ppm wt Nd. Consequently the fibre supported a single transverse mode at the laser wavelength >.L= 1.09 pm. The fibre cladding diameter of I25 + 0.5 pm was selected for optimum performance of the integrated phase modulator [ 51. The fibre was end-pumped by a single stripe AlGaAs diode laser operating at 820 nm with a maximum output power rating of 15 mW. Pump power was launched into the fibre through the butted high reflector mirror (HR at the laser wavelength) using a standard 10X microscope objective. Allowing for the 85% transmission of the high reflector at the pump wavelength, the overall launch efficiency was around 0.32. The high reflector substrate thickness of 0.17 mm minimised spherical aberration in the pump launching scheme but prevented wedging of the rear surface. The main laser cavity was completed by butting a 15% output coupling mirror to the fibre output endface. This mirror was wedged and AR coated to suppress back surface reflections detrimental to modelocked operation. The phase modulator transducer was pressed firmly against the outside surface of the fibre (protective coating removed) in close proximity to the output coupling mirror in order to maximise the mode-coupling efficiency ii [ 61. The fibre length was chosen 34
1 February
1990
to match the driving frequency for optimum modelocking to the peak response frequency of the modulator, v,,,= 4 17 MHz. This limited the pump absorption efficiency to just 0.41. Clearly, a larger dopant concentration in the fibre would improve this efficiency. The maximum single pass phase retardation at the laser wavelength, &,, was measured in situ by monitoring sideband generation on a single frequency HeNe laser beam (>.= 633 nm) launched into the fibre and subsequently allowing for the inverse dependence of &,, on optical wavelength. With 1 W of rf power applied at the peak response frequency, the measured phase retardation at the laser wavelength of 1.09 urn was 0.6 rad. Light emitted through the output coupling mirror was collected and collimated by an 18 x microscope objective onto a diffraction grating with a 600 lines/ mm pitch. With an arbitrarily chosen incidence angle of 70’, the power transmission into the zeroth, first and second diffraction orders was 0.52, 0.24 and 0.22 respectively. Transmission into the zeroth order and diffraction in the higher orders both increased on tilting the grating towards grazing incidence. The useful output coupling of the compound laser system was provided by the diffraction-free zeroth order beam. The first order was used to monitor the average output power while the more strongly diffracted second order was retroreflected back into the laser by a high reflecting mirror (wedged and AR coated on the rear surface), providing a spectrally filtered injection signal. The mirror was mounted on a translation stage with piezoelectric control to allow fine tuning of the coupled cavity path length. Focusing of the output lens was adjusted to maximise feedback from the coupled cavity. The filter bandwidth was determined by the number of lines illuminated on the grating and could be adjusted arbitrarily by altering the incidence angle on the grating or the focusing of the output beam. The temporal behaviour of the laser was monitored from the zeroth diffraction order using a fast InGaAs photodiode and sampling oscilloscope (overall response time is 56 ps). For convenience. the spectral content of the laser was determined from the weak beam coupled out of the input high reflector using a l/3 metre monochromator and a scanning Fabry-Perot interferometer with a finesse of 40 and an adjustable free spectral range. With the laser
Volume 75, number 1
OPTICS COMMUNICATIONS
1 February
1990
running continuous-wave (modulator deactivated), the threshold pump power absorbed in the fibre was just 520 uW (4 mW out of the diode laser). With respect to absorbed pump power, the slope efficiency of the main laser cavity was 48%. This corresponds to a slope efficiency for the compound system of 25% (for the arbitrarily chosen grating incidence angle of 70” ). All these parameter values remained unchanged when applying 1 W of rf power at the optimum modelocking frequency.
3. Results Modelocked performance was assessed with 2 mW of absorbed pump power ( 15 mW out of the diode laser) and 1 W of rf power applied to the modulator at the optimum modelocking frequency of vML = 4 16.90499 MHz. With correctly butted mirrors and the feedback arm blocked, oscillation was observed simultaneously on both sets of interleaved modelocked pulses corresponding to either extremum of the phase modulation cycle. The stability of the pulse sets was much greater than observed in systems FM modelocked with a bulk phase modulator [ 1,2]. The two pulse sets had different optimum modelocked frequencies and minimum pulse duration. The difference in pulse duration is clearly illustrated in fig. 2 with the narrower pulses approaching the response limit of the diagnostics. The narrow pulses were preferentially selected by withdrawing the output coupler by a few interference fringes, introducing a broadband etalon to the system without significantly affecting the cavity finesse. Fig. 3 shows the pulse profile after fine tuning of the rf frequency to optimise modelocking of the narrow pulse set. Allowing for the response time of the detector, the measured pulse duration of 80 ps suggests an actual pulse duration around 60 ps. Baseline structure was due to detector ringing. The maximum absorbed pump power of 2 mW yielded an average output power (incident on the grating) of just 0.7 mW and prevented autocorrelation of the modelocked pulses. Assuming a 60 ps pulse duration, the peak output power was about 28 mW with a peak intensity in the fibre of 3 MW cme2. Significant changes in pulse duration were observed for a frequency detuning of 200 Hz. The pulse spectrum is shown in fig. 4, indicating a major peak with
Fig. 2. Oscilloscope trace showing stable simultaneous oscillation of both sets of modelocked pulses corresponding to either extremum of the phase modulation cycle (coupled cavity blocked). The difference in pulse duration for alternate pulses is clearly illustrated.
Fig. 3. Output signal after selecting single pulse set with shorter pulse duration by withdrawing output coupling mirror. Detector response time is 56 ps. Baseline structure is due to detector ringing.
two subsidiary peaks at shorter wavelength. The bandwidth (fwhm) of the major peak, centred at 1.087 urn, was 1.5 nm (or equivalently AuLz 380 GHz). These results are considerably better, in terms of reduced pulse duration, than those obtained when pumping the system with an R6G dye laser [ 6 1. The 35
Volume 75. number
I
OPTICS
1009 WAVELENGTH
(nm)
COMMUNICATIONS
I February
1990
11
Fig. 4. Power spectrum of FM modelocked tibre laser without coupled cavity bandwidth control. Major peak has a bandwidth (fwhm) of 1.5 nm (AvL=380GHz).
improved performance is due in part to the greater stability of the all solid-state system compared to the equivalent dye laser pumped system in which bubbles in the dye jet and 50 Hz ripple from the Ar ion laser cause fluctuations in the laser intracavity intensity. The values of pulse duration and spectral bandwidth indicate that, in the absence of feedback from the coupled cavity, modelocked operation was far from bandwidth-limited with insufficient phase modulation present to lock modes across the entire lasing profile. Bandwidth restriction was achieved by rebutting the output coupling mirror and unblocking the feedback arm. With feedback the laser exhibited random switching between the two sets of pulses due to the interferometric nature of the system causing random phase changes between the intracavity and reinjetted pulses. The output signal was stabilised by actively locking the coupled cavity and main laser cavity path lengths. The error signal was derived from fluctuations in the average power signal with changing pulsed states. This was fed to a high tension servolocking circuit which adjusted the feedback mirror position to maintain stable operation. By adjusting a dc offset voltage it was again possible to select and maintain the set of modelocked pulses with narrow pulse duration, as shown in fig. 5. Fig. 6 shows an oscilloscope trace of the pulse profile after optimising modelocking. The measured pulse duration of 36
Fig. 5. Preferential selection of short duration pulse set by synchronizing intracavity and reinjected pulses. (a) Without feedback both sets of pulses oscillate simultaneously. (b) With feedback, the long duration pulse set is totally extinguished.
100 ps suggests an actual pulse duration of 82 ps. Noise at the peak of the pulse was attributed to imperfect locking of the coupled cavity and main cavity path lengths. The degree of spectral reshaping by the relatively weak injection signal is quite considerable. Tilting the feedback mirror had the effect of shifting the diffraction grating filter profile, hence tuning the injection signal frequency across the laser spectrum. As a result, an enhancement peak was observed to pass through the spectrum shown in f”rg. 4, the width of which was determined by the filter bandwidth. Fig. 7 shows a plot of the laser spectrum monitored with ‘I~=
Volume 75. number
1
OPTICS
COMMUNICATIONS
1 February
1990
nificant homogeneous broadening in the laser. With the interferometer scanning over a single free spectral range of 60 GHz, the modelocked laser bandwidth was measured as Av,= 10 & 1 GHz, with a corresponding pulsewidth-bandwidth product of 0.82.
4. Summary
Fig. 6. Output signal after selecting single pulse set with shorter pulse duration by locking coupled cavity. Detector response time = 56 ps. Noise at the peak of the pulse is attributed to imperfect servo-locking of the coupled cavity.
Fig. 7. Power spectrum of FM pled cavity bandwidth control. a bandwidth (fwhm) of AuL= finesse = 40, free spectral range
modelocked fibre laser with couA single feature is observed with IO? 1 GHz (measured with FPI, = 60 GHz).
We have described the performance of a diodelaser-pumped Nd 3+-doped fibre laser actively modelocked with an integrated fibre phase modulator. Modulation occurs within the fibre itself allowing simple cavity design with low loss and broadband characteristics. Pulses of around 60 ps duration have been observed at a repetition rate of 417 MHz with a peak output power of 28 mW. The corresponding laser bandwidth is approximately Au,=380 GHz, indicating incomplete locking of axial modes over the laser profile. By coupling the laser to an external cavity incorporating a diffraction grating, we have restricted the laser bandwidth to Au,= 10 GHz, while maintaining an absorbed pump power threshold of just 520 uW. The compound system yields near bandwidth-limited modelocked pulses of about 82 ps duration, before optimising the laser bandwidth. This is the first demonstration, to our knowledge, of a modelocked fibre laser pumped by a low power, single stripe diode laser, reflecting the low threshold nature of the system despite a non-optimum dopant concentration in the fibre. Considerable improvement in system performance is envisaged with the use of a correctly doped polarization-selective fibre (6, is polarization-dependent [ 51) and with the bandpass of the coupled cavity optimised for bandwidth-limited operation.
Acknowledgements the Fabry-Perot interferometer, with the short duration pulses selected and the enhancement peak tuned roughly to the centre of the spectrum shown in fig. 4. The simultaneous enhancement of the major peak and the total extinction of secondary peaks suggested a dramatic channelling of energy over a spectral range of several nanometers, indicating sig-
This work has been partially funded within the Join Opto-Electronics Research Scheme. We are grateful to Janet Townsend of the Optical Fibre Group of the Department of Electronics, Information Engineering and Computer Science for providing the doped fibre. Mark Phillips is supported by a Science and Engi37
Volume 75, number
neering
I
Research
OPTICS
Council
postdoctoral
COMMUNICATIONS
fellowship.
References [ 1] M.W. Phillips, A.I. Ferguson and D.C. Hanna, Optics Lett. 14 (1989)
219.
[ 21 D.C. Hanna, A. Kazer, M.W. Phillips, D.P. Shepherd P.J. Suni, Electron.
38
Lett. 25 (1989)
95.
and
1 February
1990
[ 31 G. Geister and R. Ulrich, Optics Comm. 68 ( 1988) 187. [4] J.D. Kafka, T. Baer and D.W. Hall, CLEO 1989 Technical Digest, Paper FA3. [ 5 ] D.B. Patterson, A.A. Godil, G.S. Kino and B.T. Khuri-Yakub, Optics Lett. 14 (1989) 248. [6] M.W. Phillips, AI. Ferguson, G.S. Kino and D.B. Patterson, Optics Lett. 14 ( 1989) 680.