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18 W single-frequency operation of an injection-locked, CW, Nd: YAG laser
Volume 140, number 6
PHYSICS LETTERS A
2 October 1989
18 W SINGLE-FREQUENCY OPERATION OF AN INJECTION-LOCKED, CW, Nd:YAG LASER 0. CREGUT, C.N. MAN, D. SHOEMAKER, A. BRILLET Groupe de Recherche sur les Ondes de Gravitation, CNRS. Bat. 104, 91405 Orsay, France
A. MENHERT, P. PEUSER, N.P. SCHMITT, P. ZELLER MBBGmbH, ZTAJJ, Box 801109, 8000 Munich 80, FRG
and K. WALLMEROTH DLR, Institute of Optoelectronics, 8031 Oberpfaffenhofen, FRG Received 25 July 1989; accepted for publication 4 August 1989 Communicated by J.P. Vigier
We describe the design and operation of a single-frequency 18 W Nd: YAG laser, injection-locked to a diode-pumped Nd: YAG laser.
1. Infroduction
the master oscillator being a twisted-mode, actively stabilized, diode-pumped laser [4].
To illuminate the extremely sensitive interferometers which are being developed for the detection ofgravitational waves [1], one needsultrastable, high power, CW lasers. Argon ion lasers are used in the present prototypes, but their efficiency and their maximum output power are insufficient for the projected large scale interferometers. A promising alternative is Nd: YAG lasers. They have been very seldom used so far in this kind of precision experiments, but they do have intrinsically the right power and stability properties: they can deliver hundreds of watts in multimode operation for industrial applications, and low power diode-pumped systems are among the most frequency-stable lasers [2], the difficulty being to conciliate these properties in a single laser, We describe here the first results of our approach to this problem, which consists in the development ofan original high power single-mode Nd: YAG laser, in which single-frequency operation and frequency stabilization are obtained by injection-locking [3], 294
2. High power laser design The cavity of the high power laser has been designed for maximum efficiency in single-mode single-frequency operation, taking into account the important peculiarities ofa lamp-pumped Nd: YAG rod. Thermal lensing: the cavity geometry is calculated in order to ensure a large filling ratio ofthe rods by the TEM00 mode, as well as dynamical stability, —
for a given pump power. Thermal birefringence: this effect results in very high losses for the propagation of a linear polarization in the cavity [5]. Spatial hole burning: the presence of a standing wave inside the crystal generates a spatial modulation of the gain saturation, which makes it difficult to obtain single-frequency operation. The geometry of the high power laser cavity is described in fig. 1. We have chosen to use a ring cavity to avoid the spatial hole burning problem. The bi—
—
0375-9601/89/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 140, number 6
PHYSICS LETTERS A
crystals. It is possible to obtain a (rather unstable) single-frequency operation with an output power of
O~tPut
12 to 15 W by inserting in the cavity an uncoated etalon having a thickness of a few mm.
Quartz crystal rotator (89’) pluto
~II~~]
Magnet
Polarizer
3. Injection-locking
Nd:YAG rod ~~Nd:YAG_red
____________
1
m
2 October 1989
~.
Fig. 1. Geometry of the high power lamp-pumped Nd: YAG laser.
refringence is compensated by using a pair of identical amplifying rods, with a 900 polarization rotation between them [6]. The initial polarization is defined by a polarizing beamsplitter, and re-established by a halfwave plate after the second amplifier, All the mirrors of the cavity have maximum reflectivity; the polarizer is used as a tunable output coupler, the coupling coefficient depending on the rotation of the halfwave plate. The polarization rotator between the two amplifiers is a quartz crystal, cut for providing a rotation of 890; it is submitted to the axial field of a cylindrical magnet which provides 1 more degree of Faraday rotation for the wave which circulates in the counter clockwise direction in the upper part of the cavity [7]. In this system, the differential losses for the two opposite running waves vary (and change sign) with the rotation of the halfwave plate, but unidirectional operation is obtained in the normal operating conditions, The amplifiers are commercial laser heads (Micro-Controle, model 904), designed for industrial applications. They are pumped by two Krypton lamps each. The mechanical structure ofthe cavity consists in 3 Invar rods to which all the optical components are firmly cox~nected,except for the water-cooled amplifiers, which would induce vibrations, and are therefore isolated from the structure. For a total pump power of 8 kW, and an optimum output coupling of 20%, this laser delivers a single-mode unidirectional power of 18 W, spectrally spread on about ten lines, which indicates that the gain line broadeningis not completely homogeneousin the Nd: YAG
In order to achieve single-frequency operation more reliably, and without introducing losses, we chose the technique of injection-locking, which is often used with pulsed lasers, but was never used before in a CW operated high power Nd:YAG laser. The optical scheme of the injection-locking experiment is given by fig. 2. The light from a lowpower diode-pumped Nd: YAG laser is modematched to the high-power laser cavity and coupled by the polarizing beamsplitter. As is well known, injection-locking can occur at the condition that the difference A v between the frequencies of the master oscillator and the slave oscillator lies inside the locking range, which is of the order of A A ‘~ —
VL—
VcV
ml
where Av~is the linewidth of the slave laser cavity and Pm and P. are the powers of the master and the slave laser. A 11L represents both the width ofthe locking range and the unity gain bandwidth of the injection-locking loop when the frequency difference Av is null. In the typical conditions of the experiment (A v~ = 5 MHz, P 0 = 30 mW, and P. = 15 W), we find A ~L = 200 kHz. Since the thermal drifts and acoustical fluctuations ofthe slave laser cavity are larger than this value by more than two orders of magnitude, it is thus necessary to actively control the resonant frequency of the slave cavity in order to maintain it in the locking range. The servo-loop which realizes this function is similar to the servo-loop described in ref. [2] to lock a laser to a reference cavity. It uses the same high frequency phase-modulation, it monitors the output beam of the slave laser and feeds back the error signal to the piezoelectric transducers of the slave laser cavity. The unity gain frequency of this servo-loop is of the order of 20 kHz in the experiments we describe below. In a first set of experiments, the master oscillator was the low power diode-pumped laser described in 295
Volume 140, number 6
PHYSICS LETTERSA
2 October 1989
Acousto-optic frequency shifter
2
~\O
\\~O
Ae~9’
0SC
Faraday isolator
E-O
Quartz+ Faraday rotator
___________
Nd:YAG
~ Polari.zation
______
coupler
Halfwave plate
Nd:YAG)JJ
t
~Power meter
Fig. 2. The injection-locking set-up.
ref. [21. It never was possible to observe injectionlocking above threshold, but, when the halfwave plate was rotated in order to maintain the slave laser slightly below threshold, we could obtain an amplification of the 10 mW injected light by a factor 100. In these conditions, the high power laser operates no more as a slave oscillator, but as a regenerative amplifier. This regime is interesting, but does not provide, in the present conditions, a very high output power. The master oscillator was then replaced by the more powerful twisted-mode MBB/DLR laser [4], with which we were able to couple up to 100 mW in the TEM00 mode of the slave cavity. In these conditions, with careful mode-matching of the master oscillator beam to the slave cavity, and when the servo-loop is locked, we observe, as expected, that the spectrum ofthe slave oscillator collapses to a single line, while the full output power of 18 W is maintamed. Fig. 3 shows the spectrum of the slave laser, before and after the injection of the low power single-frequency beam. The main two lines of the first spectrum are reference lines provided by the beam extracted from the twisted-mode laser in the zeroth 296
order of the acousto-optic frequency shifter. They show the free spectral range of the analyzer. In the injection-locking regime, the error signal of the servo-loop gives a measure of the relative phase fluctuations of the master and slave oscillators (this phase difference varying from ~t/2 to + it/2 within the locking range). An upper limit of these phase fluctuations, limited by the measurement sensitivity we had in these first experiments, is 1 0~ rd Hz~/2, The real value of these fluctuations is probably much smaller, but this already confirms that the combination of the injection-locking and of the servo1oop is able to transfer the frequency stability of the master to the slave oscillator. Because of the relatively wide bandwidth of the injection-locking loop (a few hundred kHz) and of the large dynamic range of the electronic servo-loop (150 MHz), the system is very stable; it unlocks only in the case of strong acoustical or mechanical perturbations and usually relocks by itself. The results of these experiments suggest that there is a threshold in the master oscillator power necessary to reach injection-locking: this is not really surprising since we have observed that the gain line —
Volume 140, number 6
PHYSICS LETTERS A
2 October 1989
I
I
______
‘b
__...J~..____
---------~
Fig. 3. Spectral analysis of the high power beam (powervertical axis, frequency horizontal axis): (a) free running laser; (b) injectionlocked laser. The free spectral range of the Fabry—Perot analyzer is 3 GHz.
broadening is not completely homogeneous in the high power laser, but this requires further investigation.
4. Discussion of the results We have demonstrated earlier that we could servocontrol the frequency fluctuations of a low power diode-pumped Nd: YAG laser with a (shot-noise limited) precision of 102 Hz/Hz’ /2 corresponding to a theoretical laser linewidth of about 1 mHz. We show in this paper that this frequency stability can be transferred to a high power lamp-pumped Nd: YAG laser, using a combination of injectionlocking and of an electronic servo-loop. Further measurements are needed to confirm the reliability and the effective linewidth of the high power laser, but these first results make us very confident that the problem of the high power ultrastable laser needed for the interferometric detection of gravitational waves is nearly solved. In the near future, it will become realistic to pump the high power laser with laser diodes instead of lamps. This will have many advantages as concerns the reliability, the stability (less or even no water cooling of the laser crystal), and the efficiency. Fur-
thermore, this could give the possibility of increasing the locking range, by reducing the cavity length. Should the power need to be increased above the reasonable output power of one single laser, the phase stability resulting from injection-locking would suggest adding coherently a few lasers, as was demonstrated in ref. [8].
5. Conclusion We have demonstrated the possibility of using regenerative amplification, or injection-locking, to transfer the excellent frequency stability of a diodepumped Nd: YAG laser to a high power lamppumped laser. In the case of injection-locking, we obtained an output power of 18 W, and we verified the phase stabilization of the slave oscillator relative to the master oscillator with a precision of 1 Ø_4 rd Hz ‘/2 —
References [I] A. Brillet and Ph. Tourrenc, in: Gravitational radiation, eds. N. Deruelle and T. Piran (North-Holland, Amsterdam, 1982).
297
Volume 140, number 6
PHYSICS LETTERSA
[2] D. Shoemaker, A. Brillet, C.N. Man, 0. Cregut and G. Kerr, Opt. Lett. 14 (1989) 609. [3] R.Adler, Proc. IRE 34 (1946) 351 (reprinted in Proc. IEEE
61(1973)1380). [4] K. Wallmeroth and P. Peuser, Electron. Lett. 24 (1988)1088.
298
2 October 1989
[5] M.A. Karr, AppI. Opt. 10 (1971) 893. [6] W.C. Scott and M. de Wit, Appl. Phys. Lett. 18 (1971) 3. [7] A.R. Clobes and M.J. Brienza, AppI. Phys. Lett. 21(1972)
265. [8] C.N. Man and A. Brillet, Opt. Lett. 9 (1984) 333.
PHYSICS LETTERS A
2 October 1989
18 W SINGLE-FREQUENCY OPERATION OF AN INJECTION-LOCKED, CW, Nd:YAG LASER 0. CREGUT, C.N. MAN, D. SHOEMAKER, A. BRILLET Groupe de Recherche sur les Ondes de Gravitation, CNRS. Bat. 104, 91405 Orsay, France
A. MENHERT, P. PEUSER, N.P. SCHMITT, P. ZELLER MBBGmbH, ZTAJJ, Box 801109, 8000 Munich 80, FRG
and K. WALLMEROTH DLR, Institute of Optoelectronics, 8031 Oberpfaffenhofen, FRG Received 25 July 1989; accepted for publication 4 August 1989 Communicated by J.P. Vigier
We describe the design and operation of a single-frequency 18 W Nd: YAG laser, injection-locked to a diode-pumped Nd: YAG laser.
1. Infroduction
the master oscillator being a twisted-mode, actively stabilized, diode-pumped laser [4].
To illuminate the extremely sensitive interferometers which are being developed for the detection ofgravitational waves [1], one needsultrastable, high power, CW lasers. Argon ion lasers are used in the present prototypes, but their efficiency and their maximum output power are insufficient for the projected large scale interferometers. A promising alternative is Nd: YAG lasers. They have been very seldom used so far in this kind of precision experiments, but they do have intrinsically the right power and stability properties: they can deliver hundreds of watts in multimode operation for industrial applications, and low power diode-pumped systems are among the most frequency-stable lasers [2], the difficulty being to conciliate these properties in a single laser, We describe here the first results of our approach to this problem, which consists in the development ofan original high power single-mode Nd: YAG laser, in which single-frequency operation and frequency stabilization are obtained by injection-locking [3], 294
2. High power laser design The cavity of the high power laser has been designed for maximum efficiency in single-mode single-frequency operation, taking into account the important peculiarities ofa lamp-pumped Nd: YAG rod. Thermal lensing: the cavity geometry is calculated in order to ensure a large filling ratio ofthe rods by the TEM00 mode, as well as dynamical stability, —
for a given pump power. Thermal birefringence: this effect results in very high losses for the propagation of a linear polarization in the cavity [5]. Spatial hole burning: the presence of a standing wave inside the crystal generates a spatial modulation of the gain saturation, which makes it difficult to obtain single-frequency operation. The geometry of the high power laser cavity is described in fig. 1. We have chosen to use a ring cavity to avoid the spatial hole burning problem. The bi—
—
0375-9601/89/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 140, number 6
PHYSICS LETTERS A
crystals. It is possible to obtain a (rather unstable) single-frequency operation with an output power of
O~tPut
12 to 15 W by inserting in the cavity an uncoated etalon having a thickness of a few mm.
Quartz crystal rotator (89’) pluto
~II~~]
Magnet
Polarizer
3. Injection-locking
Nd:YAG rod ~~Nd:YAG_red
____________
1
m
2 October 1989
~.
Fig. 1. Geometry of the high power lamp-pumped Nd: YAG laser.
refringence is compensated by using a pair of identical amplifying rods, with a 900 polarization rotation between them [6]. The initial polarization is defined by a polarizing beamsplitter, and re-established by a halfwave plate after the second amplifier, All the mirrors of the cavity have maximum reflectivity; the polarizer is used as a tunable output coupler, the coupling coefficient depending on the rotation of the halfwave plate. The polarization rotator between the two amplifiers is a quartz crystal, cut for providing a rotation of 890; it is submitted to the axial field of a cylindrical magnet which provides 1 more degree of Faraday rotation for the wave which circulates in the counter clockwise direction in the upper part of the cavity [7]. In this system, the differential losses for the two opposite running waves vary (and change sign) with the rotation of the halfwave plate, but unidirectional operation is obtained in the normal operating conditions, The amplifiers are commercial laser heads (Micro-Controle, model 904), designed for industrial applications. They are pumped by two Krypton lamps each. The mechanical structure ofthe cavity consists in 3 Invar rods to which all the optical components are firmly cox~nected,except for the water-cooled amplifiers, which would induce vibrations, and are therefore isolated from the structure. For a total pump power of 8 kW, and an optimum output coupling of 20%, this laser delivers a single-mode unidirectional power of 18 W, spectrally spread on about ten lines, which indicates that the gain line broadeningis not completely homogeneousin the Nd: YAG
In order to achieve single-frequency operation more reliably, and without introducing losses, we chose the technique of injection-locking, which is often used with pulsed lasers, but was never used before in a CW operated high power Nd:YAG laser. The optical scheme of the injection-locking experiment is given by fig. 2. The light from a lowpower diode-pumped Nd: YAG laser is modematched to the high-power laser cavity and coupled by the polarizing beamsplitter. As is well known, injection-locking can occur at the condition that the difference A v between the frequencies of the master oscillator and the slave oscillator lies inside the locking range, which is of the order of A A ‘~ —
VL—
VcV
ml
where Av~is the linewidth of the slave laser cavity and Pm and P. are the powers of the master and the slave laser. A 11L represents both the width ofthe locking range and the unity gain bandwidth of the injection-locking loop when the frequency difference Av is null. In the typical conditions of the experiment (A v~ = 5 MHz, P 0 = 30 mW, and P. = 15 W), we find A ~L = 200 kHz. Since the thermal drifts and acoustical fluctuations ofthe slave laser cavity are larger than this value by more than two orders of magnitude, it is thus necessary to actively control the resonant frequency of the slave cavity in order to maintain it in the locking range. The servo-loop which realizes this function is similar to the servo-loop described in ref. [2] to lock a laser to a reference cavity. It uses the same high frequency phase-modulation, it monitors the output beam of the slave laser and feeds back the error signal to the piezoelectric transducers of the slave laser cavity. The unity gain frequency of this servo-loop is of the order of 20 kHz in the experiments we describe below. In a first set of experiments, the master oscillator was the low power diode-pumped laser described in 295
Volume 140, number 6
PHYSICS LETTERSA
2 October 1989
Acousto-optic frequency shifter
2
~\O
\\~O
Ae~9’
0SC
Faraday isolator
E-O
Quartz+ Faraday rotator
___________
Nd:YAG
~ Polari.zation
______
coupler
Halfwave plate
Nd:YAG)JJ
t
~Power meter
Fig. 2. The injection-locking set-up.
ref. [21. It never was possible to observe injectionlocking above threshold, but, when the halfwave plate was rotated in order to maintain the slave laser slightly below threshold, we could obtain an amplification of the 10 mW injected light by a factor 100. In these conditions, the high power laser operates no more as a slave oscillator, but as a regenerative amplifier. This regime is interesting, but does not provide, in the present conditions, a very high output power. The master oscillator was then replaced by the more powerful twisted-mode MBB/DLR laser [4], with which we were able to couple up to 100 mW in the TEM00 mode of the slave cavity. In these conditions, with careful mode-matching of the master oscillator beam to the slave cavity, and when the servo-loop is locked, we observe, as expected, that the spectrum ofthe slave oscillator collapses to a single line, while the full output power of 18 W is maintamed. Fig. 3 shows the spectrum of the slave laser, before and after the injection of the low power single-frequency beam. The main two lines of the first spectrum are reference lines provided by the beam extracted from the twisted-mode laser in the zeroth 296
order of the acousto-optic frequency shifter. They show the free spectral range of the analyzer. In the injection-locking regime, the error signal of the servo-loop gives a measure of the relative phase fluctuations of the master and slave oscillators (this phase difference varying from ~t/2 to + it/2 within the locking range). An upper limit of these phase fluctuations, limited by the measurement sensitivity we had in these first experiments, is 1 0~ rd Hz~/2, The real value of these fluctuations is probably much smaller, but this already confirms that the combination of the injection-locking and of the servo1oop is able to transfer the frequency stability of the master to the slave oscillator. Because of the relatively wide bandwidth of the injection-locking loop (a few hundred kHz) and of the large dynamic range of the electronic servo-loop (150 MHz), the system is very stable; it unlocks only in the case of strong acoustical or mechanical perturbations and usually relocks by itself. The results of these experiments suggest that there is a threshold in the master oscillator power necessary to reach injection-locking: this is not really surprising since we have observed that the gain line —
Volume 140, number 6
PHYSICS LETTERS A
2 October 1989
I
I
______
‘b
__...J~..____
---------~
Fig. 3. Spectral analysis of the high power beam (powervertical axis, frequency horizontal axis): (a) free running laser; (b) injectionlocked laser. The free spectral range of the Fabry—Perot analyzer is 3 GHz.
broadening is not completely homogeneous in the high power laser, but this requires further investigation.
4. Discussion of the results We have demonstrated earlier that we could servocontrol the frequency fluctuations of a low power diode-pumped Nd: YAG laser with a (shot-noise limited) precision of 102 Hz/Hz’ /2 corresponding to a theoretical laser linewidth of about 1 mHz. We show in this paper that this frequency stability can be transferred to a high power lamp-pumped Nd: YAG laser, using a combination of injectionlocking and of an electronic servo-loop. Further measurements are needed to confirm the reliability and the effective linewidth of the high power laser, but these first results make us very confident that the problem of the high power ultrastable laser needed for the interferometric detection of gravitational waves is nearly solved. In the near future, it will become realistic to pump the high power laser with laser diodes instead of lamps. This will have many advantages as concerns the reliability, the stability (less or even no water cooling of the laser crystal), and the efficiency. Fur-
thermore, this could give the possibility of increasing the locking range, by reducing the cavity length. Should the power need to be increased above the reasonable output power of one single laser, the phase stability resulting from injection-locking would suggest adding coherently a few lasers, as was demonstrated in ref. [8].
5. Conclusion We have demonstrated the possibility of using regenerative amplification, or injection-locking, to transfer the excellent frequency stability of a diodepumped Nd: YAG laser to a high power lamppumped laser. In the case of injection-locking, we obtained an output power of 18 W, and we verified the phase stabilization of the slave oscillator relative to the master oscillator with a precision of 1 Ø_4 rd Hz ‘/2 —
References [I] A. Brillet and Ph. Tourrenc, in: Gravitational radiation, eds. N. Deruelle and T. Piran (North-Holland, Amsterdam, 1982).
297
Volume 140, number 6
PHYSICS LETTERSA
[2] D. Shoemaker, A. Brillet, C.N. Man, 0. Cregut and G. Kerr, Opt. Lett. 14 (1989) 609. [3] R.Adler, Proc. IRE 34 (1946) 351 (reprinted in Proc. IEEE
61(1973)1380). [4] K. Wallmeroth and P. Peuser, Electron. Lett. 24 (1988)1088.
298
2 October 1989
[5] M.A. Karr, AppI. Opt. 10 (1971) 893. [6] W.C. Scott and M. de Wit, Appl. Phys. Lett. 18 (1971) 3. [7] A.R. Clobes and M.J. Brienza, AppI. Phys. Lett. 21(1972)
265. [8] C.N. Man and A. Brillet, Opt. Lett. 9 (1984) 333.