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A synchronously pumped dispersion compensated fibre Raman ring laser around 1.4 μm
Volume 70, number 2
OPTICS COMMUNICATIONS
15 February 1989
A SYNCHRONOUSLY P U M P E D DISPERSION COMPENSATED FIBRE RAMAN RING LASER AROUND 1.4 Itm A.S. GOUVEIA-NETO
I,
P.G.J. WIGLEY and J.R. TAYLOR
Femtosecond Optics Group, PhysicsDepartment, Imperial College, Prince ConsortRoad, London SW72BZ, UK Received 30 August 1988
A synchronously pumped intracavity dispersion compensated fibre-Raman ring oscillator operating around 1.4 lira is described. Pulses as short as 1.8 ps and average output powers of 20 m W ( ~ 100 W peak) were obtained, by synchronously pumping a 400 m length of single mode, nonpolarization preserving dispersion shifted optical fibre (2 o = 1.46 ~tm) with the 100 ps pulses from a cw mode locked Nd: YAG laser at 1.32 Ixm. This laser provides a simple source of pulses for soliton studies.
The high gain associated with stimulated Raman scattering in silica based optical fibres [ l ], has attracted much attention in their application as laser media for ultrashort pulse generation in the near infrared region of the spectrum. Several schemes have been demonstrated which generate picosecond and subpicosecond pulses when operated in the region of normal chromatic dispersion of the gain fibre [ 2-4 ] and femtosecond pulses when in the regime where soliton compression takes place [ 5-9 ]. For operation in the anomalous dispersion regime, the fibre acts as the active medium and as a distributed compressor, balancing phase modulation and negative group velocity dispersion. In the normal dispersion regime, spectral broadening due to self phase modulation (SPM) together with positive group velocity dispersion ( P G V D ) gives rise to pulse temporal broadening. In order to overcome this problem, a negative dispersive delay line can be inserted in the fibre ring laser cavity to compensate for the combined effects of SPM and PGVD which permits the generation of subpicosecond pulses, as demonstrated by Kafka et al. [ 3 ] and recently by Dianov et al [ 4 ]. Both the reported systems described laser operation around I. 1 lira. In this communication, we describe a dispersion compensated Raman fibre ring laser similar to that Permanent address: Departamento de Fisica, Universidade Federal de Alagoas, Macei6 57000 AL Brasil.
128
described in refs. [3,4], utilizing a single mode dispersion shifted fibre with a dispersion minimum wavelength 2o at 1.46 ~tm which was operated in the region around 1.4 ~tm, delivering ~ 2.0 ps pulses with ~ 100 W peak power. This system is a potentially attractive source for soliton investigations in optical fibres around 1.4 ~tm, since the pulses generated are almost transform limited as compared with those from single-pass soliton-Raman generators or soliton-Raman fibre ring lasers [ 5-9]. The schematic of the synchronously pumped intracavity dispersion compensated fibre Raman ring laser is shown in fig. 1. A cw mode-locked Nd:YAG laser operated at 1.32 lxm, generating 100 ps pulses
Fibre ),o= 1./+6J4mI - -
EW MODE L O E K E D [ ~ Nd:YA6 132)Jm
;BS1
~ ~FILTER <~ ~======~>OUTPUT
APERTURE~~
G
M, " GI - " ~
BS2
2 ~¢MI
Fig. 1. Schematic of the fibre Raman ring laser.
0 030-4018/89/$03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
Volume 70, number 2
OPTICS COMMUNICATIONS
at a 100 MHz repetition rate and with an average output power of ~ 2.0 W ( ~ 200 W peak) was used as the pump source. The pump radiation was directed towards the fibre input using the beam splitter BS1 with reflectivity nominally 100% at 1.32 ~tm for 45 ° angle of incidence and ~ 5% reflecting at 1.4 pm. A × 2 0 broad band anti-reflection coated microscope objective LI was used to focus the p u m p radiation into the fibre. The fibre used was 400 m long, nonpolarisation preserving, single mode at a 1.32 ~tm, with a core diameter of 7 ~tm. In the spectral region 1.2 ~m to 1.65 ~tm, the loss was less than 0.6 dB/km, with the exception in the region of the water absorption ~ 1.39 Ism where the loss rose to ~ 2 dB/ km. The fibre was tailored [ l0 ] to have its dispersion minimum in the region of 1.46 gm which ensured efficient Raman generation and positive group velocity dispersion in the region of lasing operation around 1.4 ~tm. On exiting the fibre, where typically an overall power coupling of ~ 40% was achieved, the radiation was collected and collimated with an identical microscope objective (L2) as used at the input, and the light was directed via the aluminum coated mirror M, through a negatively dispersive delay line, comprising a pair of diffraction gratings in a single pass configuration. The gratings were holographic type with 1200 l / m m and approximately 70% diffraction efficiency at 1.4 ~tm at a 75 ° angle of incidence. Mirror M 2 directed the radiation from the dispersive delay line through an aperture (which selectively defined the wavelength of interest), and via beam splitter BS~, back into the input focusing microscope objective. A 2 nm band pass dielectric filter centered at 1.4 ~tm was placed in the cavity in order to permit ultrashort pulse generation by restricting the oscillating bandwidth of the laser system. The beam splitter BS2 with ~ 20% reflectivity at 1.4 gm, provided the output coupling from this fibre ring oscillator. By mounting the fibre end-lens L2 assembly on a translation stage which was driven with micron precision, synchronism of the fed back Raman signal with the input p u m p pulse was achieved and this position was varied to obtain the maximum output signal. The output from the dispersion compensated fibre Raman ring laser was directed to the detection system comprised by vibrating background-free autocorrelator and a 1 m scanning spectrograph, which
15 February 1989
allowed continuous monitoring and optimisation of the laser system. For particular p u m p power, dispersion compensation was achieved by changing the distance between the grating pair readjusting the cavity length and continuously monitoring the output pulses. The optimum separation for the experimental situation described here was found to be 18 cm. For grating separation of 10% to 15% either longer or shorter than the optimum value, the fibre Raman ring laser pro,duced pulses much longer than the optimum duration of ~ 2.7 ps. The pump power coupled into the dispersion shifted optical fibre was kept at an power of ~ 400 mW, which is approximately the Raman threshold value for this fibre length. In the single pass arrangement no signal around 1.4 Ism was observable. The typical output from the optimized laser system was 20 mW in pulses with a spectral bandwidth of ~ 2 nm, which corresponded to the band pass of the intracavity filter. By coarsely varying the position of the aperture across the dispersed spectrum to select the wavelength region fed back and by finely tilting the angle of the filter, the output of the laser could be tuned from 1390 nm to 1415 rim. Fig. 2 shows a typical output spectrum for operation at a central wavelength around 1390 rim. In the time domain the dispersion compensated ring laser showed a behaviour very similar to that of synchronously pumped dye lasers, with a the critical adjustment of cavity length match for ultrashort pulse generation. Fig. 3 shows background-free autocorrelation traces of the output pulses from the fibre ring Raman laser. Shown in fig. 3 a is the case for optim u m cavity length match with the generation of
r-
>-F--
z~- 0 _
nm
Z LtJ
i
I
i
I
I
I
I
I
I
I
I
1392 1386 WAVELENGTH (nm) Fig. 2. Typicaloutput spectrum from the fibre Raman ring laser. 1398
129
Volume 70, number 2
OPTICS COMMUNICATIONS
Ca)
"
~
"l~t= 1 3 2
=
.Tps
ps
Fig. 3. Background-freeautocorrelation traces of the pulses around 1.4 tam for (a) the optimized cavity length and (b) cavity length mismatched by + 10 tam, similar pulse structure occurs for negative mismatch of the same magnitude. pulses o f 2.7 ps duration, assuming gaussian pulse shape. F o r a cavity length m i s m a t c h o f + 10 ~tm, the fibre ring laser generated pulses with the t e m p o r a l structure characteristic o f a burst o f noise, as shown in the autocorrelation trace o f fig. 3b. Similar profiles were obtained when the cavity length was shorter by the same a m o u n t . W h e n o p e r a t e d with p u m p powers close to the lasing threshold ( ~ 350 m W average powers in the fibre), pulses as short as 1.8 ps were generated with output powers still a r o u n d 20 mW. In o r d e r to achieve o p t i m a l o p e r a t i o n o f the laser it was necessary to ensure that the relaunched polarization was similar to that o f the i n p u t p u m p . This required the straining o f the o u t p u t end o f the fibre in order to obtain polarization control [ 11 ] and the gain was a sensitive function o f this control. The 2 n m b a n d w i d t h o f the fibre ring laser was supportive o f 1.4 ps pulses, while pulses which were about two times the transform limit were typically generated. It is most likely that the p r o d u c t i o n o f a nonlinear chirp, which cannot be c o m p e n s a t e d for by p r o p a g a t i o n in a grating pair, occurred in the Ram a n spectra. This arises through cross phase m o d 130
15 February 1989
ulatton [ 12 a n d references there in ] where the nonlinearity experienced by the Stokes signal is a d d i t i o n a l l y i n d u c e d b y the field o f the p u m p pulses at 1.32 ~tm. This n o n l i n e a r shirp o f the R a m a n comp o n e n t has been previously directly m e a s u r e d [ 13 ]. In conclusion, a synchronously p u m p e d intracavity dispersion c o m p e n s a t e d fibre R a m a n ring oscillator o p e r a t e d a r o u n d 1.4 Ixm has been presented for the first time. Pulses as short as 1.8 ps with powers o f ~ 100 W were obtained, by p u m p i n g at 1.32 ttm a n d using a dispersion shifted single m o d e fibre as the gain m e d i u m . In this wavelength regime a n d at the power levels p r o d u c e d by this system, this simple laser system m a y find useful applications in the study o f soliton p r o p a g a t i o n in optical fibres a n d semic o n d u c t o r spectroscopy. The financial support for this work by British Telecom a n d SERC is gratefully acknowledged. P.G.J. Wigley is s u p p o r t e d by a S E R C / B r i t i s h Telecom CASE studentship a n d A.S. G o u v e i a - N e t o is partially s u p p o r t e d by CAPES, a Brazilian agency.
References [ 1] R.H. Stolen and E.P. Ippen, Appl. Phys. Left. 22 (1973 ) 276. [ 2 ] R.H. Stolen, Fibre Raman lasers, Fibre and Integrated Optics 3 (1980) 21. [3] J.D. Kafka, D.F. Head and T. Baer, Springer series in Chemical Physics, Utrafast Phenomena V, 46 ( 1986 ). [4] E.M. Dianov, P.V. Mamyshev, A.M. Prokhorov and D.G. Fursa, Pizma Zh. Eksp. Teor. Fiz. 45 (1987) 469. [ 5 ] M.N. Islam, L.F. Mollenauer and R.H. Stolen, Springer series in Chemical Physics, Ultrafast Phenomena V, (1986) 46. [6] B. Zysset, P. Beaud, W. Hodel and H.P. Weber, Springer series in Chemical Physics, Ultrafast Phenomena V, (1986) 54. [ 7 ] J.D. Kafka and T. Bear, Optics Lett. 12 ( 1987 ) 181. [8] A.S. Gouveia-Neto, A.S.L. Gomes and J.R. Taylor, Elect. Lett. 23 (1987) 537. [9 ] A.S. GouveiaoNeto, A.S.L. Gomes, J.R. Taylor, B.J. Ainslie and S.P. Craig, Optics Lett. 12 (1987) 295. [ 10 ] B.J. Ainslie and C.R. Day, J. Lighwave Techn. LT-4 (1986) 967. [ 11 ] B.G. Koehler and J.E. Bowers, Appl. Optics 24 ( 1985 ) 349. [ 12] M.N. Islam, L.F. Islam, L.F. Mollenauer, R.H. Stolen, J.R. Simpson and H.T. Shang, Optics Lett. 12 (1987) 625. [ 13] A.S.L. Gomes, V.L. da Silva and J.R. Taylor, J. Opt. Soc. Am. B 5 (1988) 373.
OPTICS COMMUNICATIONS
15 February 1989
A SYNCHRONOUSLY P U M P E D DISPERSION COMPENSATED FIBRE RAMAN RING LASER AROUND 1.4 Itm A.S. GOUVEIA-NETO
I,
P.G.J. WIGLEY and J.R. TAYLOR
Femtosecond Optics Group, PhysicsDepartment, Imperial College, Prince ConsortRoad, London SW72BZ, UK Received 30 August 1988
A synchronously pumped intracavity dispersion compensated fibre-Raman ring oscillator operating around 1.4 lira is described. Pulses as short as 1.8 ps and average output powers of 20 m W ( ~ 100 W peak) were obtained, by synchronously pumping a 400 m length of single mode, nonpolarization preserving dispersion shifted optical fibre (2 o = 1.46 ~tm) with the 100 ps pulses from a cw mode locked Nd: YAG laser at 1.32 Ixm. This laser provides a simple source of pulses for soliton studies.
The high gain associated with stimulated Raman scattering in silica based optical fibres [ l ], has attracted much attention in their application as laser media for ultrashort pulse generation in the near infrared region of the spectrum. Several schemes have been demonstrated which generate picosecond and subpicosecond pulses when operated in the region of normal chromatic dispersion of the gain fibre [ 2-4 ] and femtosecond pulses when in the regime where soliton compression takes place [ 5-9 ]. For operation in the anomalous dispersion regime, the fibre acts as the active medium and as a distributed compressor, balancing phase modulation and negative group velocity dispersion. In the normal dispersion regime, spectral broadening due to self phase modulation (SPM) together with positive group velocity dispersion ( P G V D ) gives rise to pulse temporal broadening. In order to overcome this problem, a negative dispersive delay line can be inserted in the fibre ring laser cavity to compensate for the combined effects of SPM and PGVD which permits the generation of subpicosecond pulses, as demonstrated by Kafka et al. [ 3 ] and recently by Dianov et al [ 4 ]. Both the reported systems described laser operation around I. 1 lira. In this communication, we describe a dispersion compensated Raman fibre ring laser similar to that Permanent address: Departamento de Fisica, Universidade Federal de Alagoas, Macei6 57000 AL Brasil.
128
described in refs. [3,4], utilizing a single mode dispersion shifted fibre with a dispersion minimum wavelength 2o at 1.46 ~tm which was operated in the region around 1.4 ~tm, delivering ~ 2.0 ps pulses with ~ 100 W peak power. This system is a potentially attractive source for soliton investigations in optical fibres around 1.4 ~tm, since the pulses generated are almost transform limited as compared with those from single-pass soliton-Raman generators or soliton-Raman fibre ring lasers [ 5-9]. The schematic of the synchronously pumped intracavity dispersion compensated fibre Raman ring laser is shown in fig. 1. A cw mode-locked Nd:YAG laser operated at 1.32 lxm, generating 100 ps pulses
Fibre ),o= 1./+6J4mI - -
EW MODE L O E K E D [ ~ Nd:YA6 132)Jm
;BS1
~ ~FILTER <~ ~======~>OUTPUT
APERTURE~~
G
M, " GI - " ~
BS2
2 ~¢MI
Fig. 1. Schematic of the fibre Raman ring laser.
0 030-4018/89/$03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
Volume 70, number 2
OPTICS COMMUNICATIONS
at a 100 MHz repetition rate and with an average output power of ~ 2.0 W ( ~ 200 W peak) was used as the pump source. The pump radiation was directed towards the fibre input using the beam splitter BS1 with reflectivity nominally 100% at 1.32 ~tm for 45 ° angle of incidence and ~ 5% reflecting at 1.4 pm. A × 2 0 broad band anti-reflection coated microscope objective LI was used to focus the p u m p radiation into the fibre. The fibre used was 400 m long, nonpolarisation preserving, single mode at a 1.32 ~tm, with a core diameter of 7 ~tm. In the spectral region 1.2 ~m to 1.65 ~tm, the loss was less than 0.6 dB/km, with the exception in the region of the water absorption ~ 1.39 Ism where the loss rose to ~ 2 dB/ km. The fibre was tailored [ l0 ] to have its dispersion minimum in the region of 1.46 gm which ensured efficient Raman generation and positive group velocity dispersion in the region of lasing operation around 1.4 ~tm. On exiting the fibre, where typically an overall power coupling of ~ 40% was achieved, the radiation was collected and collimated with an identical microscope objective (L2) as used at the input, and the light was directed via the aluminum coated mirror M, through a negatively dispersive delay line, comprising a pair of diffraction gratings in a single pass configuration. The gratings were holographic type with 1200 l / m m and approximately 70% diffraction efficiency at 1.4 ~tm at a 75 ° angle of incidence. Mirror M 2 directed the radiation from the dispersive delay line through an aperture (which selectively defined the wavelength of interest), and via beam splitter BS~, back into the input focusing microscope objective. A 2 nm band pass dielectric filter centered at 1.4 ~tm was placed in the cavity in order to permit ultrashort pulse generation by restricting the oscillating bandwidth of the laser system. The beam splitter BS2 with ~ 20% reflectivity at 1.4 gm, provided the output coupling from this fibre ring oscillator. By mounting the fibre end-lens L2 assembly on a translation stage which was driven with micron precision, synchronism of the fed back Raman signal with the input p u m p pulse was achieved and this position was varied to obtain the maximum output signal. The output from the dispersion compensated fibre Raman ring laser was directed to the detection system comprised by vibrating background-free autocorrelator and a 1 m scanning spectrograph, which
15 February 1989
allowed continuous monitoring and optimisation of the laser system. For particular p u m p power, dispersion compensation was achieved by changing the distance between the grating pair readjusting the cavity length and continuously monitoring the output pulses. The optimum separation for the experimental situation described here was found to be 18 cm. For grating separation of 10% to 15% either longer or shorter than the optimum value, the fibre Raman ring laser pro,duced pulses much longer than the optimum duration of ~ 2.7 ps. The pump power coupled into the dispersion shifted optical fibre was kept at an power of ~ 400 mW, which is approximately the Raman threshold value for this fibre length. In the single pass arrangement no signal around 1.4 Ism was observable. The typical output from the optimized laser system was 20 mW in pulses with a spectral bandwidth of ~ 2 nm, which corresponded to the band pass of the intracavity filter. By coarsely varying the position of the aperture across the dispersed spectrum to select the wavelength region fed back and by finely tilting the angle of the filter, the output of the laser could be tuned from 1390 nm to 1415 rim. Fig. 2 shows a typical output spectrum for operation at a central wavelength around 1390 rim. In the time domain the dispersion compensated ring laser showed a behaviour very similar to that of synchronously pumped dye lasers, with a the critical adjustment of cavity length match for ultrashort pulse generation. Fig. 3 shows background-free autocorrelation traces of the output pulses from the fibre ring Raman laser. Shown in fig. 3 a is the case for optim u m cavity length match with the generation of
r-
>-F--
z~- 0 _
nm
Z LtJ
i
I
i
I
I
I
I
I
I
I
I
1392 1386 WAVELENGTH (nm) Fig. 2. Typicaloutput spectrum from the fibre Raman ring laser. 1398
129
Volume 70, number 2
OPTICS COMMUNICATIONS
Ca)
"
~
"l~t= 1 3 2
=
.Tps
ps
Fig. 3. Background-freeautocorrelation traces of the pulses around 1.4 tam for (a) the optimized cavity length and (b) cavity length mismatched by + 10 tam, similar pulse structure occurs for negative mismatch of the same magnitude. pulses o f 2.7 ps duration, assuming gaussian pulse shape. F o r a cavity length m i s m a t c h o f + 10 ~tm, the fibre ring laser generated pulses with the t e m p o r a l structure characteristic o f a burst o f noise, as shown in the autocorrelation trace o f fig. 3b. Similar profiles were obtained when the cavity length was shorter by the same a m o u n t . W h e n o p e r a t e d with p u m p powers close to the lasing threshold ( ~ 350 m W average powers in the fibre), pulses as short as 1.8 ps were generated with output powers still a r o u n d 20 mW. In o r d e r to achieve o p t i m a l o p e r a t i o n o f the laser it was necessary to ensure that the relaunched polarization was similar to that o f the i n p u t p u m p . This required the straining o f the o u t p u t end o f the fibre in order to obtain polarization control [ 11 ] and the gain was a sensitive function o f this control. The 2 n m b a n d w i d t h o f the fibre ring laser was supportive o f 1.4 ps pulses, while pulses which were about two times the transform limit were typically generated. It is most likely that the p r o d u c t i o n o f a nonlinear chirp, which cannot be c o m p e n s a t e d for by p r o p a g a t i o n in a grating pair, occurred in the Ram a n spectra. This arises through cross phase m o d 130
15 February 1989
ulatton [ 12 a n d references there in ] where the nonlinearity experienced by the Stokes signal is a d d i t i o n a l l y i n d u c e d b y the field o f the p u m p pulses at 1.32 ~tm. This n o n l i n e a r shirp o f the R a m a n comp o n e n t has been previously directly m e a s u r e d [ 13 ]. In conclusion, a synchronously p u m p e d intracavity dispersion c o m p e n s a t e d fibre R a m a n ring oscillator o p e r a t e d a r o u n d 1.4 Ixm has been presented for the first time. Pulses as short as 1.8 ps with powers o f ~ 100 W were obtained, by p u m p i n g at 1.32 ttm a n d using a dispersion shifted single m o d e fibre as the gain m e d i u m . In this wavelength regime a n d at the power levels p r o d u c e d by this system, this simple laser system m a y find useful applications in the study o f soliton p r o p a g a t i o n in optical fibres a n d semic o n d u c t o r spectroscopy. The financial support for this work by British Telecom a n d SERC is gratefully acknowledged. P.G.J. Wigley is s u p p o r t e d by a S E R C / B r i t i s h Telecom CASE studentship a n d A.S. G o u v e i a - N e t o is partially s u p p o r t e d by CAPES, a Brazilian agency.
References [ 1] R.H. Stolen and E.P. Ippen, Appl. Phys. Left. 22 (1973 ) 276. [ 2 ] R.H. Stolen, Fibre Raman lasers, Fibre and Integrated Optics 3 (1980) 21. [3] J.D. Kafka, D.F. Head and T. Baer, Springer series in Chemical Physics, Utrafast Phenomena V, 46 ( 1986 ). [4] E.M. Dianov, P.V. Mamyshev, A.M. Prokhorov and D.G. Fursa, Pizma Zh. Eksp. Teor. Fiz. 45 (1987) 469. [ 5 ] M.N. Islam, L.F. Mollenauer and R.H. Stolen, Springer series in Chemical Physics, Ultrafast Phenomena V, (1986) 46. [6] B. Zysset, P. Beaud, W. Hodel and H.P. Weber, Springer series in Chemical Physics, Ultrafast Phenomena V, (1986) 54. [ 7 ] J.D. Kafka and T. Bear, Optics Lett. 12 ( 1987 ) 181. [8] A.S. Gouveia-Neto, A.S.L. Gomes and J.R. Taylor, Elect. Lett. 23 (1987) 537. [9 ] A.S. GouveiaoNeto, A.S.L. Gomes, J.R. Taylor, B.J. Ainslie and S.P. Craig, Optics Lett. 12 (1987) 295. [ 10 ] B.J. Ainslie and C.R. Day, J. Lighwave Techn. LT-4 (1986) 967. [ 11 ] B.G. Koehler and J.E. Bowers, Appl. Optics 24 ( 1985 ) 349. [ 12] M.N. Islam, L.F. Islam, L.F. Mollenauer, R.H. Stolen, J.R. Simpson and H.T. Shang, Optics Lett. 12 (1987) 625. [ 13] A.S.L. Gomes, V.L. da Silva and J.R. Taylor, J. Opt. Soc. Am. B 5 (1988) 373.