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Detection of spectrally narrow light emission by laser intra-cavity spectroscopy
Volume 62, number
6
OPTICS
COMMUNICATIONS
DETECTION OF SPECTRALLY NARROW LASER INTRA-CAVITY SPECTROSCOPY
LIGHT EMISSION
I5 June 1987
BY
V.M. BAEV ‘, K.-J. BOLLER, A. WEILER and P.E. TOSCHEK * I. Institut ftir Experimentalphysik, Universltiit Hamburg, D-2000 Hamburg 36, Fed. Rep. Germany Received
19 February
1987
The spectral distribution of the output of a multimode laser is extremely sensitive to narrow-band absorption, gain, or light injection. We demonstrate dramatic redistribution in such output spectra to depend on the time delay of the injection. This effect seems to be useful for spectrally resolved detection of weak light signals.
1. Introduction The emission of a broad-band (bb) laser depends extremely sensitively on narrow-band absorption or gain inside the laser cavity [ l-31. This sensitivity results from the intrinsic net gain of the broad-band laser being almost equal for all oscillating cavity modes. The light flux in modes which are affected by a narrow-band intra-cavity absorber decreases with the number of round trips of the light in the cavity, or with the pulse duration of the laser emission. For the detection of absorbance, (YZ,the ultimate limit of sensitivity determined by spontaneous emission, is on the order of 6( (wz) = 10 - ’ ’ ; it can be attained with laser pulses of 1 s duration. In practice, the sensitivity is limited to 10p8, as a consequence of the presence of various kinds of mode coupling [ 41. This substantial sensitivity emerges since, with a large number of round trips of the light in the cavity, even small spectral variations of the net gain show up as significant spectral contrast in the emitted light. Thus, the same sensitivity of detection applies to narrowband gain [ 5 1. The injection of weak light with narrow spectral distribution and pulse length dt into the broad-band ’ Permanent
address: Lebedev Physical Institute of the Academy of Sciences of the USSR, Moscow 117924, USSR. ’ Visiting Fellow (1986), Joint Institute for Laboratory Astrophysics, University of Colorado and National Bureau of Standards, Boulder, CO 80309-0440, USA.
380
laser causes analogous time-varying modifications of the broad-band emission spectrum. In this paper, we report on an experimental study of this phenomenon, which qualifies for a novel technique of laser intra-cavity spectroscopy.
2. The rate equation model Rate equations suffice for the description of the emission of the broadband laser under injection of narrow-band light. Analogous rate equations have been used to describe intra-cavity absorption [ 2-4,6] : dM,ldt=-yM,+B,N(M,+l)+ym,,
(1)
dNldt=p-NIT--N
(2)
i B,M,. q=l
Eq. (1) represents n equations, one for each laser mode 4, out of n modes, which contains M, photons. The broad-band loss of the cavity, y, equals the inverse lifetime, T,‘, of the photons in the cavity. The Einstein coefficient for stimulated emission is B,, N is the inversion of the laser medium, p is the pump rate, t is the lifetime of the inversion, and m4 is the number of photons injected into mode q, under the condition r,, < dt. The total number of photons in the cavity is M= 2 &, nii,. When B,= B does not depend on the particular mode and no light is injected, the stationary solutions of (1) and (2) are
0 030-4018/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Volume 62, number 6
15 June 1987
OPTICS COMMUNICATIONS
M= (Pthb)(dPth - I)>
(3)
N= ylB,
(4)
where pth= N/z is the pump rate at threshold, if spontaneous emission is neglected. The inversion N, determined by the broad-band loss of the cavity provides the same gain for all modes. The photons in the cavity are equally distributed among the individual modes, M~=(p,hlyn)(plp,h-l),
(5)
where fluctuations [ 71 are neglected. When injecting light into the laser mode q, we add m9 photons to its equilibrium number, M;. According to eq. ( 1)) and using (5), its instantaneous photon number is in first order, and with the inversion N kept quasi-constant, M,=M;
+ ym,t.
(6)
In a single-mode
laser, the response of the inversion with the characteristic time 7 sets in quickly, and the photon number does not vary appreciably from its stationary value. In contrast, a multi-mode laser shows little response of the inversion to the weak single-mode injection for a time whose length increases with the number of oscillating modes. The level of the light flux is preserved over a period of time which corresponds to the duration of the long pulse of the multi-mode laser, or to the characteristic time of its mode coupling, whichever is shorter. Up to this time, the net photon number generated in the bandwidth of injection, Au, grows in proportion with time, and eventually takes over the entire available inversion. Its maximum value is M-”4
= M/3IAu
(7)
photons, where p is the initial spectral width of the broad-band laser, and Au is the bandwidth of the injected light. The smallest injected signal detectable in the spectrum of the laser is equal to the noise of spontaneous emission [ 1,3], i.e.
m,al.
(8)
The flux of injected photons is j,=ym,l(l -R), where R is the reflectivity of the incoupling mirror. With y = c( 1 - R)/2L (where L is the cavity length, and c the speed of light), eq. (8) is written
Fig. 1.Experimental setup.
j4 > cl2L.
(9)
For L = 15 cm, the smallest detectable flux is 1O9s- ‘, or 3 x 10 - lo W at 600 nm wavelength. This detection requires the broad-band laser to display at least a minimum pulse length, which can be derived from eq. (6). For a narrow spectral signal twice as high as the background, one finds t ,,,=~ly=2~Llc(l-R). With MG =2x 10’ and R=35%,
tmin is 22 ms.
3. The experiment The setup of the experiment is shown in fig. 1. A pulsed xenon ion laser [8,9] excites a broad-band (p= 30 cm-‘) jet-stream Rhodamine-6G laser whose pulse length is about 1.5 ps. Its concentric cavity is formed by mirrors of 5 cm and 8 cm radius of curvature and of 35% and 99.8% reflectivity, respectively. Its light is analyzed by a I-m Czerny-Turner spectrograph in second order, with 0.005 nm resolution, and detected by a linear 23-mm long diode array (TH 7801, Thomson CSF) of 1728 channels. Since one channel corresponds to 3.6 pm spectral width, 14 channels contribute to one interval of spectral resolution of the spectrograph. The diode signal is recorded by a digital storage oscilloscope (NS-570 A, Tractor) and x-y recorder. The spectrally narrow light for injection ( Av= 0.8 cm-‘, 2 ns pulse length) is generated by a Coumar381
Volume 62, number
6
OPTICS
COMMUNICATIONS
15 June 1987
ine-153 laser excited by a N2 laser. The pulse shapes of both the broad-band and narrow-band light are detected by a fast photodiode (OC 3002, B&H Electronics) and recorded by a transient digitizer ( 79 12 AD, Tektronix). An essential feature of the experiment is that both lasers are pulsed, although on a different time scale. It is known that the development of the emission spectrum of a multi-mode laser after termination of any intra-cavity absorption - or light injection depends on the specific mechanisms of mode interaction [ 10,111. The characteristic time for mode interaction in a Rhodamine-6G laser is on the order of 100 ,us [ 4,111. Given its 1.5 ~LSpulse duration, we expect, therefore, that the spectral shape of the broadband laser, after its redistribution by the injected light, will stay unchanged up to the end of the emission. Although fluctuations - to large extent quantum fluctuations [ 71 - show up with recordings of single pulses, they have been neglected in the present context since we have usually recorded spectra by averaging over 128 pulses.
Fig. 2. Emission spectra of the broad-band laser (left), and time variation of superimposed broad-band and injection light (right), for increasing time of injection (a-e).
4. Results
M,(Au, m,, to) IF J(t) dt T,h M,( Au, mq, 0) = Jr J(t) dt 7 ’
Fig. 2 shows spectra of the broad-band laser emission, and its pulse shapes, both for various values of the injection delay time. If injection precedes the broad-band laser pulse, the broad-band spectrum remains unchanged (a). When the injected light arrives in the early stage of the broad-band pulse development (b) almost the entire laser output is redistributed and condenses within the spectral range of the injected light. This behavior agrees with the essence of eq. (7). When the light injection is delayed, the fraction of redistributed light, as compared with the undisturbed emission, decreases (c-e). This happens because (i) the injected light modifies the broad-band spectrum only for the left-over fraction of the broad-band pulse following the injection, and (ii) as time lapses, the emission at the individual modes in the broad-band spectrum increasingly condenses into sections of narrower spectral bandwidth, leaving the fraction of injected light per oscillation bandwidth decreasing. These mechanisms are modeled by 382
(10)
where J(t) is the broad-band light flux at time t, and to is the delay time of injection. Fig. 3 shows the pho-
F Mq= ? 2108 z” d
INJECTION
DELAY
to/T@
-
Fig. 3. Number of photons condensed into one mode within the bandwidth of injected light, Au, after injection at delay time to: Experimental values (dots); model according to eq. (10) (solid line), or first and second factors separate (dashed and dot-dashed lines, respectively), normalized at t,lT,,=0.8.
Volume 62, number 6
OPTICS
12
0'I-
_/--
15 June 1987
COMMUNICATIONS
_____--_o---
,' 10
1
10
20 mW
LL
w
I
lb)
0
LO
L to
-l
Fig. 4. Number of photons in one mode of the injection bandwidth Au, versus power P of injected pulses, averaged over pulse length.
ton number M(dv, to) condensed into AU, as derived from the data of fig. 2, and a fit by the right side of eq. (1 O), where for J( t) the experimental broad-band pulse shape has been used. These data demonstrate that the spectrum is in fact most sensitive to injection immediately after the emission begins. Fig. 4 shows the detected number of photons M,(T) in each mode within the injection bandwidth, during the generation of time of the broadband laser, Ts 6t, long compared with the length 6t of the injected pulse. That quantity, with the firstorder solution of eq. (1)) and with N constant, is
where ym,(t’) is the instantaneous injected intracavity photon flux. The total flux, i.e. dM,ldt, takes on a quasi-stationary value after the injection, following the slow variation of the broadband laser. With the linear approximation of eq. (6), the injected pulse is modeled by a rectangular shape, and the injected photon number required to double the original photon number per mode is mi2’ =M(Av, 0, O)ly.dt. The total output peak power of the broad-band laser is 4 W, corresponding to M: = 2 x 10’ photons in each individual mode. Since Au includes 20 modes of the broad-band laser, and with 6t=2x 10e9 s, the total power of injected light per mode which would be
G-
,
I
80
120
I
160"s
-t Fig. 5. Time dependence of broad-band laser emission into interval of injection (a), and into an interval 0.3 nm off(b).
required for photon doubling in the broad-band oscillator is estimated to be 0.1 W. The actual peak power of the injected pulse is 2 W. The disparity of these figures accounts for spectral and optical mismatch of the injected light and the broad-band oscillator modes. We have also recorded the time dependence of the broad-band laser emission in selected narrow spectral intervals. Examples are shown in fig. 5 for the wavelength of injection (a), and for another interval 0.3 nm off that wavelength (b). The dramatic effect of condensation at the injection wavelength is obvious. The modulation is due to slight mode coupling in the broad-band laser. This coupling speeds the time variation of the emission spectrum, due to injectionless intrinsic condensation. Note, however, that this effect does not show up in the recordings, since its characteristic time, in our experiment, is still far beyond 1 PUS.
5. Conclusions The data presented above demonstrate the effects of the injection of narrow-band light pulses into a 383
Volume 62, number
6
OPTICS
COMMUNICATIONS
multi-mode laser, upon the time development of its broad-band emission spectrum. A small injected light flux of bandwidth du makes the broad-band emission condense into Au. This effect may be applied to the sensitive spectral detection of weak light signals. Up to 1O6 times higher sensitivity is expected for the detection of weak cw signals with the help of a broadband cw laser.
[2] [3]
[4]
[ 51 [6]
Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft. One of the authors (V.M.B.) acknowledges support by the Alexander von Humboldt Foundation.
[ 71 [8] [9] [IO]
References [ 111 [ I] L.A. Pakhomycheva,
384
E.A. Sviridenkov,
A.F. Suchkov,
L.V.
15 June 1987
Titova and S.S. Churilov, Zh. ETF Pisma Red. 12 (1970) 60 [JETPLett. 12 (1970) 431. T.W. Hlnsch, A.L. Schawlow and P.E. Toschek, IEEE J. Quant. Electron. QE-8 (1972) 802. V.M. Baev, T.P. Belikova, E.A. Sviridenkov and A.F. Suchkov, Zh. Eksp. Teor. Fiz. 74 (1978) 43 [ Sov. Phys. JETP (1978) 211. Yu.M. Ajvasjan, V.M. Baev, V.V. Ivanov, S.A. Kovalenko and E.A. Sviridenkov, Kvantovaya Elektron.. to be published. V.M. Baev, H. Schrdder and P.E. Toschek, Optics Comm. 36 (1981) 57. S.A. Kovalenko and S.P. Semin, Kvantovaya Elektron., to be published. V.M. Baev, G. Gaida, H. Schroder and P.E. Toschek, Optics Comm. 38 (198 1) 309. T.W. Hansch, A.L. Schawlow and P.E. Toschek, IEEE J. Quant. Electron. E-9 (1973) 553. H. Schroder, K. Schultz and P.E. Toschek. Optics Comm. 60 (1986) 159. N.A. Raspopov, A.N. Savchenko and E.A. Sviridenkov, Kvantovaya Elektron. 4 (1977) 736 [Sov. J. Quantum Electron. 7 (1977) 4091. H. Atmanspacher, H. Scheingraber and V.M. Baev, Phys. Rev. A, to be published.
6
OPTICS
COMMUNICATIONS
DETECTION OF SPECTRALLY NARROW LASER INTRA-CAVITY SPECTROSCOPY
LIGHT EMISSION
I5 June 1987
BY
V.M. BAEV ‘, K.-J. BOLLER, A. WEILER and P.E. TOSCHEK * I. Institut ftir Experimentalphysik, Universltiit Hamburg, D-2000 Hamburg 36, Fed. Rep. Germany Received
19 February
1987
The spectral distribution of the output of a multimode laser is extremely sensitive to narrow-band absorption, gain, or light injection. We demonstrate dramatic redistribution in such output spectra to depend on the time delay of the injection. This effect seems to be useful for spectrally resolved detection of weak light signals.
1. Introduction The emission of a broad-band (bb) laser depends extremely sensitively on narrow-band absorption or gain inside the laser cavity [ l-31. This sensitivity results from the intrinsic net gain of the broad-band laser being almost equal for all oscillating cavity modes. The light flux in modes which are affected by a narrow-band intra-cavity absorber decreases with the number of round trips of the light in the cavity, or with the pulse duration of the laser emission. For the detection of absorbance, (YZ,the ultimate limit of sensitivity determined by spontaneous emission, is on the order of 6( (wz) = 10 - ’ ’ ; it can be attained with laser pulses of 1 s duration. In practice, the sensitivity is limited to 10p8, as a consequence of the presence of various kinds of mode coupling [ 41. This substantial sensitivity emerges since, with a large number of round trips of the light in the cavity, even small spectral variations of the net gain show up as significant spectral contrast in the emitted light. Thus, the same sensitivity of detection applies to narrowband gain [ 5 1. The injection of weak light with narrow spectral distribution and pulse length dt into the broad-band ’ Permanent
address: Lebedev Physical Institute of the Academy of Sciences of the USSR, Moscow 117924, USSR. ’ Visiting Fellow (1986), Joint Institute for Laboratory Astrophysics, University of Colorado and National Bureau of Standards, Boulder, CO 80309-0440, USA.
380
laser causes analogous time-varying modifications of the broad-band emission spectrum. In this paper, we report on an experimental study of this phenomenon, which qualifies for a novel technique of laser intra-cavity spectroscopy.
2. The rate equation model Rate equations suffice for the description of the emission of the broadband laser under injection of narrow-band light. Analogous rate equations have been used to describe intra-cavity absorption [ 2-4,6] : dM,ldt=-yM,+B,N(M,+l)+ym,,
(1)
dNldt=p-NIT--N
(2)
i B,M,. q=l
Eq. (1) represents n equations, one for each laser mode 4, out of n modes, which contains M, photons. The broad-band loss of the cavity, y, equals the inverse lifetime, T,‘, of the photons in the cavity. The Einstein coefficient for stimulated emission is B,, N is the inversion of the laser medium, p is the pump rate, t is the lifetime of the inversion, and m4 is the number of photons injected into mode q, under the condition r,, < dt. The total number of photons in the cavity is M= 2 &, nii,. When B,= B does not depend on the particular mode and no light is injected, the stationary solutions of (1) and (2) are
0 030-4018/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Volume 62, number 6
15 June 1987
OPTICS COMMUNICATIONS
M= (Pthb)(dPth - I)>
(3)
N= ylB,
(4)
where pth= N/z is the pump rate at threshold, if spontaneous emission is neglected. The inversion N, determined by the broad-band loss of the cavity provides the same gain for all modes. The photons in the cavity are equally distributed among the individual modes, M~=(p,hlyn)(plp,h-l),
(5)
where fluctuations [ 71 are neglected. When injecting light into the laser mode q, we add m9 photons to its equilibrium number, M;. According to eq. ( 1)) and using (5), its instantaneous photon number is in first order, and with the inversion N kept quasi-constant, M,=M;
+ ym,t.
(6)
In a single-mode
laser, the response of the inversion with the characteristic time 7 sets in quickly, and the photon number does not vary appreciably from its stationary value. In contrast, a multi-mode laser shows little response of the inversion to the weak single-mode injection for a time whose length increases with the number of oscillating modes. The level of the light flux is preserved over a period of time which corresponds to the duration of the long pulse of the multi-mode laser, or to the characteristic time of its mode coupling, whichever is shorter. Up to this time, the net photon number generated in the bandwidth of injection, Au, grows in proportion with time, and eventually takes over the entire available inversion. Its maximum value is M-”4
= M/3IAu
(7)
photons, where p is the initial spectral width of the broad-band laser, and Au is the bandwidth of the injected light. The smallest injected signal detectable in the spectrum of the laser is equal to the noise of spontaneous emission [ 1,3], i.e.
m,al.
(8)
The flux of injected photons is j,=ym,l(l -R), where R is the reflectivity of the incoupling mirror. With y = c( 1 - R)/2L (where L is the cavity length, and c the speed of light), eq. (8) is written
Fig. 1.Experimental setup.
j4 > cl2L.
(9)
For L = 15 cm, the smallest detectable flux is 1O9s- ‘, or 3 x 10 - lo W at 600 nm wavelength. This detection requires the broad-band laser to display at least a minimum pulse length, which can be derived from eq. (6). For a narrow spectral signal twice as high as the background, one finds t ,,,=~ly=2~Llc(l-R). With MG =2x 10’ and R=35%,
tmin is 22 ms.
3. The experiment The setup of the experiment is shown in fig. 1. A pulsed xenon ion laser [8,9] excites a broad-band (p= 30 cm-‘) jet-stream Rhodamine-6G laser whose pulse length is about 1.5 ps. Its concentric cavity is formed by mirrors of 5 cm and 8 cm radius of curvature and of 35% and 99.8% reflectivity, respectively. Its light is analyzed by a I-m Czerny-Turner spectrograph in second order, with 0.005 nm resolution, and detected by a linear 23-mm long diode array (TH 7801, Thomson CSF) of 1728 channels. Since one channel corresponds to 3.6 pm spectral width, 14 channels contribute to one interval of spectral resolution of the spectrograph. The diode signal is recorded by a digital storage oscilloscope (NS-570 A, Tractor) and x-y recorder. The spectrally narrow light for injection ( Av= 0.8 cm-‘, 2 ns pulse length) is generated by a Coumar381
Volume 62, number
6
OPTICS
COMMUNICATIONS
15 June 1987
ine-153 laser excited by a N2 laser. The pulse shapes of both the broad-band and narrow-band light are detected by a fast photodiode (OC 3002, B&H Electronics) and recorded by a transient digitizer ( 79 12 AD, Tektronix). An essential feature of the experiment is that both lasers are pulsed, although on a different time scale. It is known that the development of the emission spectrum of a multi-mode laser after termination of any intra-cavity absorption - or light injection depends on the specific mechanisms of mode interaction [ 10,111. The characteristic time for mode interaction in a Rhodamine-6G laser is on the order of 100 ,us [ 4,111. Given its 1.5 ~LSpulse duration, we expect, therefore, that the spectral shape of the broadband laser, after its redistribution by the injected light, will stay unchanged up to the end of the emission. Although fluctuations - to large extent quantum fluctuations [ 71 - show up with recordings of single pulses, they have been neglected in the present context since we have usually recorded spectra by averaging over 128 pulses.
Fig. 2. Emission spectra of the broad-band laser (left), and time variation of superimposed broad-band and injection light (right), for increasing time of injection (a-e).
4. Results
M,(Au, m,, to) IF J(t) dt T,h M,( Au, mq, 0) = Jr J(t) dt 7 ’
Fig. 2 shows spectra of the broad-band laser emission, and its pulse shapes, both for various values of the injection delay time. If injection precedes the broad-band laser pulse, the broad-band spectrum remains unchanged (a). When the injected light arrives in the early stage of the broad-band pulse development (b) almost the entire laser output is redistributed and condenses within the spectral range of the injected light. This behavior agrees with the essence of eq. (7). When the light injection is delayed, the fraction of redistributed light, as compared with the undisturbed emission, decreases (c-e). This happens because (i) the injected light modifies the broad-band spectrum only for the left-over fraction of the broad-band pulse following the injection, and (ii) as time lapses, the emission at the individual modes in the broad-band spectrum increasingly condenses into sections of narrower spectral bandwidth, leaving the fraction of injected light per oscillation bandwidth decreasing. These mechanisms are modeled by 382
(10)
where J(t) is the broad-band light flux at time t, and to is the delay time of injection. Fig. 3 shows the pho-
F Mq= ? 2108 z” d
INJECTION
DELAY
to/T@
-
Fig. 3. Number of photons condensed into one mode within the bandwidth of injected light, Au, after injection at delay time to: Experimental values (dots); model according to eq. (10) (solid line), or first and second factors separate (dashed and dot-dashed lines, respectively), normalized at t,lT,,=0.8.
Volume 62, number 6
OPTICS
12
0'I-
_/--
15 June 1987
COMMUNICATIONS
_____--_o---
,' 10
1
10
20 mW
LL
w
I
lb)
0
LO
L to
-l
Fig. 4. Number of photons in one mode of the injection bandwidth Au, versus power P of injected pulses, averaged over pulse length.
ton number M(dv, to) condensed into AU, as derived from the data of fig. 2, and a fit by the right side of eq. (1 O), where for J( t) the experimental broad-band pulse shape has been used. These data demonstrate that the spectrum is in fact most sensitive to injection immediately after the emission begins. Fig. 4 shows the detected number of photons M,(T) in each mode within the injection bandwidth, during the generation of time of the broadband laser, Ts 6t, long compared with the length 6t of the injected pulse. That quantity, with the firstorder solution of eq. (1)) and with N constant, is
where ym,(t’) is the instantaneous injected intracavity photon flux. The total flux, i.e. dM,ldt, takes on a quasi-stationary value after the injection, following the slow variation of the broadband laser. With the linear approximation of eq. (6), the injected pulse is modeled by a rectangular shape, and the injected photon number required to double the original photon number per mode is mi2’ =M(Av, 0, O)ly.dt. The total output peak power of the broad-band laser is 4 W, corresponding to M: = 2 x 10’ photons in each individual mode. Since Au includes 20 modes of the broad-band laser, and with 6t=2x 10e9 s, the total power of injected light per mode which would be
G-
,
I
80
120
I
160"s
-t Fig. 5. Time dependence of broad-band laser emission into interval of injection (a), and into an interval 0.3 nm off(b).
required for photon doubling in the broad-band oscillator is estimated to be 0.1 W. The actual peak power of the injected pulse is 2 W. The disparity of these figures accounts for spectral and optical mismatch of the injected light and the broad-band oscillator modes. We have also recorded the time dependence of the broad-band laser emission in selected narrow spectral intervals. Examples are shown in fig. 5 for the wavelength of injection (a), and for another interval 0.3 nm off that wavelength (b). The dramatic effect of condensation at the injection wavelength is obvious. The modulation is due to slight mode coupling in the broad-band laser. This coupling speeds the time variation of the emission spectrum, due to injectionless intrinsic condensation. Note, however, that this effect does not show up in the recordings, since its characteristic time, in our experiment, is still far beyond 1 PUS.
5. Conclusions The data presented above demonstrate the effects of the injection of narrow-band light pulses into a 383
Volume 62, number
6
OPTICS
COMMUNICATIONS
multi-mode laser, upon the time development of its broad-band emission spectrum. A small injected light flux of bandwidth du makes the broad-band emission condense into Au. This effect may be applied to the sensitive spectral detection of weak light signals. Up to 1O6 times higher sensitivity is expected for the detection of weak cw signals with the help of a broadband cw laser.
[2] [3]
[4]
[ 51 [6]
Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft. One of the authors (V.M.B.) acknowledges support by the Alexander von Humboldt Foundation.
[ 71 [8] [9] [IO]
References [ 111 [ I] L.A. Pakhomycheva,
384
E.A. Sviridenkov,
A.F. Suchkov,
L.V.
15 June 1987
Titova and S.S. Churilov, Zh. ETF Pisma Red. 12 (1970) 60 [JETPLett. 12 (1970) 431. T.W. Hlnsch, A.L. Schawlow and P.E. Toschek, IEEE J. Quant. Electron. QE-8 (1972) 802. V.M. Baev, T.P. Belikova, E.A. Sviridenkov and A.F. Suchkov, Zh. Eksp. Teor. Fiz. 74 (1978) 43 [ Sov. Phys. JETP (1978) 211. Yu.M. Ajvasjan, V.M. Baev, V.V. Ivanov, S.A. Kovalenko and E.A. Sviridenkov, Kvantovaya Elektron.. to be published. V.M. Baev, H. Schrdder and P.E. Toschek, Optics Comm. 36 (1981) 57. S.A. Kovalenko and S.P. Semin, Kvantovaya Elektron., to be published. V.M. Baev, G. Gaida, H. Schroder and P.E. Toschek, Optics Comm. 38 (198 1) 309. T.W. Hansch, A.L. Schawlow and P.E. Toschek, IEEE J. Quant. Electron. E-9 (1973) 553. H. Schroder, K. Schultz and P.E. Toschek. Optics Comm. 60 (1986) 159. N.A. Raspopov, A.N. Savchenko and E.A. Sviridenkov, Kvantovaya Elektron. 4 (1977) 736 [Sov. J. Quantum Electron. 7 (1977) 4091. H. Atmanspacher, H. Scheingraber and V.M. Baev, Phys. Rev. A, to be published.