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Amplified spontaneous emission in distributed feedback dye lasers
Volume 50, number 3
OPTICS COMMUNICATIONS
1 June 1984
AMPLIFIED SPONTANEOUS EMISSION IN DISTRIBUTED FEEDBACK DYE LASERS I.A. MclNTYRE and M.H. DUNN Department of Physics, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, Scotland Received 17 January 1984
It is shown that amplified spontaneous emission (ASE) in pulsed distributed feedback dye lasers can be reduced by pumping with higher powers. An explanation for this is made in terms of the cavity decay time, r e. It is also shown that ASE can be reduced by operation at a wavelength close to gain maximum and by using an external reflector.
1. Introduction Dye lasers pumped by pulsed lasers are widely used as tunable sources of narrow bandwidth visible radiation. Depending on the cavity configuration and operating pump level above threshold, the single pass unsaturated gain in a pulsed dye laser may be between 1 0 3 - 1 0 7 . With such high gain, spontaneous fluorescent emission from the dye is amplified as it propagates along the laser axis and emerges along with the desired, tunable laser beam, but with a much larger divergence, of the order of 2 0 - 1 0 0 mrad [ 1 ]. Amplified spontaneous emission (ASE) is therefore a source of noise and an important characteristic of a dye laser. Measurements of ASE from dye lasers have been reported by Bakos [2] and Duarte [3,4]. A comparison between different cavity types for ASE content has been made by Bor [5] and Trebino [6]. The problem of ASE in dye lasers and amplifiers has been treated by Ganiel [1] and Haag [7]. Bor [5] showed that the fraction o f ASE produced by a distributed feedback dye laser (DFDL) is considerably lower than that o f Littrow grating or grazing incidence tuned dye lasers. The reasons for this are as follows. In a conventional cavity, the laser pulse is delayed with respect to the growth o f the gain because several round trips are required to build up the intensity of laser radiation. During this buildup time the laser strongly emits ASE. After the laser radiation has had time to build up to a high level, 0 030-4018/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
the gain becomes saturated and the laser ceases to emit ASE. The DFDL produces less ASE because the build up time for radiation is shorter than for a conventional cavity. Also, the single pass gain at threshold is low for a DFDL, reducing ASE further [5]. The purpose o f this paper is to examine the former mechanism in reducing ASE from DFDLs i.e. the effect of the cavity build up time.
2. Theoretical The time for circulating radiation to grow or die off in lasers is determined by the cavity decay time, r c. For a conventional cavity with mirror reflectivities R 1 and R2, optical length r/L, ~'c is given by r c = 2nL/c(1 - R 1 R 2 ) ,
(1)
where c is the speed of light. It is worthwhile noting that the factor (1 - R 1 R 2 ) is simply equal to the ratio of extracted power over intracavity power. Using this together with two expressions for the power extracted from and the intracavity power in a distributed feedback (DFB) laser, given by Haus [8], Pout = 2 ( A n / r L ) 2,
(Ca)
Pin = A2c°s2~ z,
(2b)
where g is the distributed feedback coupling constant [9], 169
Volume 50, number 3
OPTICS COMMUNICATIONS
(3)
K =nr/1/~ 0 + i a l / 2 ,
and A is the maximum intracavity amplitude of the DFB laser o f length L, it is possible to obtain an expression for the cavity decay time for a DFB laser. 771 and a 1 in eq. (3) are the modulations in refractive index and gain respectively and X0 is the Bragg wavelength which is twice the modulation period. The resuiting expression for r c in the DFB laser is (4)
r c = (r/L3/2c)(K/rr) 2,
(setting l = r/L), which is exactly that obtained bY Chinn [10], omitting scattering and internal losses. The cavity decay time plays a crucial role in the dynamics of the DFDL. The decay time may be rewritten for the DFDL (assuming a pure gain grating) as 7"c = (r/L 3 [ 8 c ) ( a 1/Tr)2.
(5)
The gain modulation, a l , is time dependent, being proportional to the density o f dye molecules in the upper laser level [ 11 ], and so r c is also time dependent. It is this time dependence o f z c during a pump pulse which gives rise to the self Q-switching in DFDLs and the production of a train o f picosecond pulses [11,12]. Bor has shown [ 1 1] that the time o f arrival of the first output pulse from a DFDL during pumping shortens as the level of pumping increases. So, it is expected that the level of ASE produced also decreases with increasing pump power. ^
~
tr
1 June 1984
3. Experimental The setup used to examine the output from a DFDI_ is shown in fig. 1. A Quantel YF 480 frequency-doubled Nd : YAG laser produces pulses o f 100 mJ at 532 nm with a repetition rate o f 10 pps. This radiation is passed through a variable attenuator and used to pump in DFDL in the style of Bakos [13]. The pump beam is passed through a cylindrical lens, split and recombined in a dye cell via a prism. The dye cell has a solution of Rhodamine B in ethanol (about 4 × 10 - 4 M) circulated through it. The DFDL was around 10% efficient. The radiation emitted by the DFDL is passed through a Zeiss prism SPM2 monochromator and detected by an ITL TF 1850 vacuum photodiode connected to an HP 1727A oscilloscope. The detection system has a risetime of 1.4 ns, so it is not possible to resolve the picosecond pulses from the D F D L The prism monochromator was used to separate the DFB radiation from the ASE. Consequently, the DFDL had to operate at a wavelength well away from the gain maximum in order to resolve the DFB from the ASE. Care was taken to ensure that all the ASE passed through the exit slit of the monochromator and was detected. The ASE consisted of wideband radiation (FWHM 20 nm) centred around 590 nm. DFB radiation was centred at 617 nm and had a FWHM o f less than 0.05 nm. The results of the experiments are given in figs. 2-4.
dc bm /
cl
I l atf
~
I
Nd:YAGpump
I
Fig. 1. Schematic of experimental apparatus used; att = variable attenuator, cl = 30 cm focal length cylindrical lens, bs = beamsplitter, bm= beam stearing mirror, de = dye cell, tr = 100% reflector, pd = vacuum photodiode. The prism coupling pump light into the dye cell has been omitted for clarity. The 100% reflector was not normally in place.
170
Volume 50, number 3
OPTICS COMMUNICATIONS
1 June 1984
B
DFB with ASE DFB without 6 ASE
x
30 DFB power ASE power
1
2
3 pump energy (m J)
Fig. 4. Graph showing i m p r o v e m e n t of the DFB/ASE ratio with the 100% reflector in place over operation without the reflector.
4. Discussion
i
i
i
i
;
i
pump energy (mJ)
Fig. 2. Graph showing dependence of the ratio of DFB power/ ASE power on pump energy. DFB wavelength is 617 nm.
100 80 50 t,0 30 20 DFBpower ASE power 10 B 5 (, 3 2
I
,
610
620
630
640 650 6'60 wavelength(nm)
Fig. 3. Graph showing dependence o f the ratio o f DFB power/ ASE power on the DFB operating wavelength. P u m p energy is 4.5 mJ. Note the logarithmic scale on the y-axis.
As can be seen from fig. 2, the content o f ASE follows the expected trend and decreases with an increase in pump power. The turnover in the curve at around 4 mJ pump energy can be attributed to the fact that the amount b y which the arrival time o f the first pulse shortens decreases with increasing pump power, so that the reduction in ASE content is less at higher pump powers than it is at low powers, given a certain increment in pump power. The ratios of DFB power/ASE power obtained are considerably less than the best achieved b y Bor [5]. This is a consequence o f operating the DFB at a wavelength from the gain maximum. Fig. 3 shows how the ASE content increases if the DFB operates further away from maximum gain. This can be explained by lower gain increasing the time for laser radiation to build up, thus allowing more ASE to be emitted. The ASE content may be further reduced by operating the D F D L with feedback from a 100% reflector (see fig. 1). Studies o f feedback into DFDLs have been carried out b y Bor [14] and Golub [15]. ASE is reduced in the presence of added feedback because the build-up time for laser radiation is further reduced. This is particularly helpful when operating a D F D L at low powers where the ASE content is large. Fig. 4 shows a comparison o f the DFB/ASE power 171
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OPTICS COMMUNICATIONS
ratios for operation with and without the external reflector, for various pump energies. It is seen from this that the beneficial effect of an external reflector in reducing ASE is diminished as the pump energy increases i.e. as the cavity build up time reduces. Further reduction in the ASE content may be obtained by passing the emitted radiation through an aperture. Since the divergence of ASE is much greater than that of DFB radiation, a substantial fraction of ASE could be blocked by an aperture of suitable dimension which did not attenuate the desired DFB light.
5. Conclusion It has been discussed that the content of ASE in a dye laser pulse depends on the build up time of laser radiation in the dye laser. It has been demonstrated theoretically that the cavity build up time is dependent on the power of the pump pulse and this has been verified experimentally.
Acknowledgement The authors wish to acknowledge Mr. R. McGraw and his staff for assistance in constructing the DFDL.
172
1 June 1984
One of us (IAM) is supported by an SERC CASE studentship held in collaboration with Barr and Stroud. This work was carried out under SERC grant no. GR/C/48578.
References [1] U. Ganiel, Y. Hardy, A.H.G. Neumann and D. Treves, IEEE J. Quant. Electron. QE-11 (1975) 881. [2] J.S. Bakos and Zs. Sorlei, Optics Comm. 22 (1977) 258. [3] F.J. Duarte and J.A. Piper, Optics Comm. 35 (1980) 100. [4] F.J. Duarte and J.A. Piper, Appl. Optics 20 (1981) 2113. [5] Zs. Bor, Optics Comm. 39 (1981) 383. [6] R. Trebino, J.P. Rolles and A.E. Siegman, IEEE J. Quant. Electron. QE-18 (1982) 1208. [7] G. Haag, M. Munz and G. Marosky, IEEE J. Quant. Electron. QE-19 (1983) 1149. [8] H.A. Haus, Appl. Optics 14 (1975) 2650. [9] H. Kogelnik and C.V. Shank, J. Appl. Phys. 43 (1982) 2327. [10] S.R. Chinn, Optics Comm. 19 (1976) 208. [11] Zs. Bor, IEEE J. Quant. Electron QE-16 (1980) 517. [12] Zs. Bor, A. Muller, B. Racz and F.P. Schafer, Appl. Phys. B 27 (1982) 9. [13] J.S. Bakos, Z. Fuzessey, Zs. Sorlei and J. Szigeti, Phys. Lett. 50A (1974) 227. [14] Zs. Bor, B. Racz, L. Kozma, A.N. Rubinov and T.Sh. Efendiev, Optics Comm. 24 (1978) 265. [I5] I. Golub, G. Erez and R. Shuker, Appl. Phys. B 31 (1983) 75.
OPTICS COMMUNICATIONS
1 June 1984
AMPLIFIED SPONTANEOUS EMISSION IN DISTRIBUTED FEEDBACK DYE LASERS I.A. MclNTYRE and M.H. DUNN Department of Physics, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, Scotland Received 17 January 1984
It is shown that amplified spontaneous emission (ASE) in pulsed distributed feedback dye lasers can be reduced by pumping with higher powers. An explanation for this is made in terms of the cavity decay time, r e. It is also shown that ASE can be reduced by operation at a wavelength close to gain maximum and by using an external reflector.
1. Introduction Dye lasers pumped by pulsed lasers are widely used as tunable sources of narrow bandwidth visible radiation. Depending on the cavity configuration and operating pump level above threshold, the single pass unsaturated gain in a pulsed dye laser may be between 1 0 3 - 1 0 7 . With such high gain, spontaneous fluorescent emission from the dye is amplified as it propagates along the laser axis and emerges along with the desired, tunable laser beam, but with a much larger divergence, of the order of 2 0 - 1 0 0 mrad [ 1 ]. Amplified spontaneous emission (ASE) is therefore a source of noise and an important characteristic of a dye laser. Measurements of ASE from dye lasers have been reported by Bakos [2] and Duarte [3,4]. A comparison between different cavity types for ASE content has been made by Bor [5] and Trebino [6]. The problem of ASE in dye lasers and amplifiers has been treated by Ganiel [1] and Haag [7]. Bor [5] showed that the fraction o f ASE produced by a distributed feedback dye laser (DFDL) is considerably lower than that o f Littrow grating or grazing incidence tuned dye lasers. The reasons for this are as follows. In a conventional cavity, the laser pulse is delayed with respect to the growth o f the gain because several round trips are required to build up the intensity of laser radiation. During this buildup time the laser strongly emits ASE. After the laser radiation has had time to build up to a high level, 0 030-4018/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
the gain becomes saturated and the laser ceases to emit ASE. The DFDL produces less ASE because the build up time for radiation is shorter than for a conventional cavity. Also, the single pass gain at threshold is low for a DFDL, reducing ASE further [5]. The purpose o f this paper is to examine the former mechanism in reducing ASE from DFDLs i.e. the effect of the cavity build up time.
2. Theoretical The time for circulating radiation to grow or die off in lasers is determined by the cavity decay time, r c. For a conventional cavity with mirror reflectivities R 1 and R2, optical length r/L, ~'c is given by r c = 2nL/c(1 - R 1 R 2 ) ,
(1)
where c is the speed of light. It is worthwhile noting that the factor (1 - R 1 R 2 ) is simply equal to the ratio of extracted power over intracavity power. Using this together with two expressions for the power extracted from and the intracavity power in a distributed feedback (DFB) laser, given by Haus [8], Pout = 2 ( A n / r L ) 2,
(Ca)
Pin = A2c°s2~ z,
(2b)
where g is the distributed feedback coupling constant [9], 169
Volume 50, number 3
OPTICS COMMUNICATIONS
(3)
K =nr/1/~ 0 + i a l / 2 ,
and A is the maximum intracavity amplitude of the DFB laser o f length L, it is possible to obtain an expression for the cavity decay time for a DFB laser. 771 and a 1 in eq. (3) are the modulations in refractive index and gain respectively and X0 is the Bragg wavelength which is twice the modulation period. The resuiting expression for r c in the DFB laser is (4)
r c = (r/L3/2c)(K/rr) 2,
(setting l = r/L), which is exactly that obtained bY Chinn [10], omitting scattering and internal losses. The cavity decay time plays a crucial role in the dynamics of the DFDL. The decay time may be rewritten for the DFDL (assuming a pure gain grating) as 7"c = (r/L 3 [ 8 c ) ( a 1/Tr)2.
(5)
The gain modulation, a l , is time dependent, being proportional to the density o f dye molecules in the upper laser level [ 11 ], and so r c is also time dependent. It is this time dependence o f z c during a pump pulse which gives rise to the self Q-switching in DFDLs and the production of a train o f picosecond pulses [11,12]. Bor has shown [ 1 1] that the time o f arrival of the first output pulse from a DFDL during pumping shortens as the level of pumping increases. So, it is expected that the level of ASE produced also decreases with increasing pump power. ^
~
tr
1 June 1984
3. Experimental The setup used to examine the output from a DFDI_ is shown in fig. 1. A Quantel YF 480 frequency-doubled Nd : YAG laser produces pulses o f 100 mJ at 532 nm with a repetition rate o f 10 pps. This radiation is passed through a variable attenuator and used to pump in DFDL in the style of Bakos [13]. The pump beam is passed through a cylindrical lens, split and recombined in a dye cell via a prism. The dye cell has a solution of Rhodamine B in ethanol (about 4 × 10 - 4 M) circulated through it. The DFDL was around 10% efficient. The radiation emitted by the DFDL is passed through a Zeiss prism SPM2 monochromator and detected by an ITL TF 1850 vacuum photodiode connected to an HP 1727A oscilloscope. The detection system has a risetime of 1.4 ns, so it is not possible to resolve the picosecond pulses from the D F D L The prism monochromator was used to separate the DFB radiation from the ASE. Consequently, the DFDL had to operate at a wavelength well away from the gain maximum in order to resolve the DFB from the ASE. Care was taken to ensure that all the ASE passed through the exit slit of the monochromator and was detected. The ASE consisted of wideband radiation (FWHM 20 nm) centred around 590 nm. DFB radiation was centred at 617 nm and had a FWHM o f less than 0.05 nm. The results of the experiments are given in figs. 2-4.
dc bm /
cl
I l atf
~
I
Nd:YAGpump
I
Fig. 1. Schematic of experimental apparatus used; att = variable attenuator, cl = 30 cm focal length cylindrical lens, bs = beamsplitter, bm= beam stearing mirror, de = dye cell, tr = 100% reflector, pd = vacuum photodiode. The prism coupling pump light into the dye cell has been omitted for clarity. The 100% reflector was not normally in place.
170
Volume 50, number 3
OPTICS COMMUNICATIONS
1 June 1984
B
DFB with ASE DFB without 6 ASE
x
30 DFB power ASE power
1
2
3 pump energy (m J)
Fig. 4. Graph showing i m p r o v e m e n t of the DFB/ASE ratio with the 100% reflector in place over operation without the reflector.
4. Discussion
i
i
i
i
;
i
pump energy (mJ)
Fig. 2. Graph showing dependence of the ratio of DFB power/ ASE power on pump energy. DFB wavelength is 617 nm.
100 80 50 t,0 30 20 DFBpower ASE power 10 B 5 (, 3 2
I
,
610
620
630
640 650 6'60 wavelength(nm)
Fig. 3. Graph showing dependence o f the ratio o f DFB power/ ASE power on the DFB operating wavelength. P u m p energy is 4.5 mJ. Note the logarithmic scale on the y-axis.
As can be seen from fig. 2, the content o f ASE follows the expected trend and decreases with an increase in pump power. The turnover in the curve at around 4 mJ pump energy can be attributed to the fact that the amount b y which the arrival time o f the first pulse shortens decreases with increasing pump power, so that the reduction in ASE content is less at higher pump powers than it is at low powers, given a certain increment in pump power. The ratios of DFB power/ASE power obtained are considerably less than the best achieved b y Bor [5]. This is a consequence o f operating the DFB at a wavelength from the gain maximum. Fig. 3 shows how the ASE content increases if the DFB operates further away from maximum gain. This can be explained by lower gain increasing the time for laser radiation to build up, thus allowing more ASE to be emitted. The ASE content may be further reduced by operating the D F D L with feedback from a 100% reflector (see fig. 1). Studies o f feedback into DFDLs have been carried out b y Bor [14] and Golub [15]. ASE is reduced in the presence of added feedback because the build-up time for laser radiation is further reduced. This is particularly helpful when operating a D F D L at low powers where the ASE content is large. Fig. 4 shows a comparison o f the DFB/ASE power 171
Volume 50, number 3
OPTICS COMMUNICATIONS
ratios for operation with and without the external reflector, for various pump energies. It is seen from this that the beneficial effect of an external reflector in reducing ASE is diminished as the pump energy increases i.e. as the cavity build up time reduces. Further reduction in the ASE content may be obtained by passing the emitted radiation through an aperture. Since the divergence of ASE is much greater than that of DFB radiation, a substantial fraction of ASE could be blocked by an aperture of suitable dimension which did not attenuate the desired DFB light.
5. Conclusion It has been discussed that the content of ASE in a dye laser pulse depends on the build up time of laser radiation in the dye laser. It has been demonstrated theoretically that the cavity build up time is dependent on the power of the pump pulse and this has been verified experimentally.
Acknowledgement The authors wish to acknowledge Mr. R. McGraw and his staff for assistance in constructing the DFDL.
172
1 June 1984
One of us (IAM) is supported by an SERC CASE studentship held in collaboration with Barr and Stroud. This work was carried out under SERC grant no. GR/C/48578.
References [1] U. Ganiel, Y. Hardy, A.H.G. Neumann and D. Treves, IEEE J. Quant. Electron. QE-11 (1975) 881. [2] J.S. Bakos and Zs. Sorlei, Optics Comm. 22 (1977) 258. [3] F.J. Duarte and J.A. Piper, Optics Comm. 35 (1980) 100. [4] F.J. Duarte and J.A. Piper, Appl. Optics 20 (1981) 2113. [5] Zs. Bor, Optics Comm. 39 (1981) 383. [6] R. Trebino, J.P. Rolles and A.E. Siegman, IEEE J. Quant. Electron. QE-18 (1982) 1208. [7] G. Haag, M. Munz and G. Marosky, IEEE J. Quant. Electron. QE-19 (1983) 1149. [8] H.A. Haus, Appl. Optics 14 (1975) 2650. [9] H. Kogelnik and C.V. Shank, J. Appl. Phys. 43 (1982) 2327. [10] S.R. Chinn, Optics Comm. 19 (1976) 208. [11] Zs. Bor, IEEE J. Quant. Electron QE-16 (1980) 517. [12] Zs. Bor, A. Muller, B. Racz and F.P. Schafer, Appl. Phys. B 27 (1982) 9. [13] J.S. Bakos, Z. Fuzessey, Zs. Sorlei and J. Szigeti, Phys. Lett. 50A (1974) 227. [14] Zs. Bor, B. Racz, L. Kozma, A.N. Rubinov and T.Sh. Efendiev, Optics Comm. 24 (1978) 265. [I5] I. Golub, G. Erez and R. Shuker, Appl. Phys. B 31 (1983) 75.