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Ne absorption tube in an alternating magnetic field inside a laser cavity as a frequency standard
Volume
6, number
1
OPTICS
Ne ABSORPTION INSIDE
TUBE
A LASER
September
COMMUNICATIONS
IN AN ALTERNATING
CAVITY
MAGNETIC
AS A FREQUENCY
1972
FIELD
STANDARD
V.BODLAJ Siemeru
AC;, Kesearck
Laboratories,
Received
Mtnicll,
Germatz.~
30 June 1972
A neon absorption tube placed in an alternating magnetic field is used as a frequency standard for the frequency stabilization of a He-Ne laser. This absorption tube, which is situated within the laser cavity, permits a very precise frequency adjustment (< 10-l 2) due to the formation of the inverse Lamb dip. An excellent beam coherence is also obtained because this method of stabilization does not superimpose any frequency modulation on the oscillation.
1. Introduction A neon absorption tube, at a low pressure (< 15 Pascal) has a very narrow absorption linewidth. Also the absorption of the tube is very low. When such a tube is placed together with a He-Ne laser tube within a short cavity, where the laser output depends strongly upon the losses, even this low absorption can have a considerable influence upon the laser power. Assuming that the laser power increases with increasing frequency, the absorption may rapidly reach saturation (Lamb dip) in the region of the ten tre frequency v. of the absorption transition and result in an abrupt increase in the laser power (see fig. 1). This increase in laser power is termed as the P
t
A
-?---Fig. 1. Frequency absorption
12
V
response of the inverse Lamb dip when an tube is inserted into the laser cavity.
inverse Lamb dip [l] Due to its inherent narrow line. width, this inverse Lamb dip provides a well defined frequency standard for the frequency stabilization of the laser [l-3] . The following section describes a new version for the frequency stabilization of such laser systems.
2. Generation
of the frequency
control signal
An axial magnetic field in a Ne-absorption tube splits the absorption transition line into two lines which are circularly polarised in opposite directions (Zecman effect). Hence the right-hand circularly polarised (RCP) laser wave of frequency vL,, travelling in a direction opposite to that of the magnetic field, interacts with the lower frequency RCP component of the absorption transition having a shifted centre frequency v, = v. ~ Au/7 (see fig. 2). When the magnetic field is reversed, the higher frequency component of the absorption transition becomes right-hand circularly polarised, i.e., the RCP laser wave now interacts with a frequency v2 = v. + AU/~. The displacement of the respective absorption transitions in the absorption tube also causes a shift of the inverse Lamb dip. Fig. 2 shows the laser power for a low magnetic field strength H of approximately 8X I Op4T in the absorption tube as a function of the laser frequency (curves (a) and (b) for H and -If resp.).
September 1972
OFWCS COMMUNICATIONS
Volume 6, number 1
P t
Fig. 2. Frequency response of the inverse Lamb dip: curve (a) denotes the axial magnetic field H and curve (b) the axial magnetic field -H in the absorption tube.
The intensity of the RCP laser wave in an absorption tube with an alternating magnetic field is a function of the time dependent magnetic field [4-61. The time dependence of the intensity of the RCP laser wave differs in phase by ?I for laser frequencies vL < vu and vL > uo. Here v. denotes the centre frequency of the inverse Lamb dip for the magnetic field H = 0. The left-hand circular polarisation LCP also shows a similar behaviour. The phase and amplitude of this intensity modulation, caused by the alternating magnetic field, can determine the position of the laser frequency in relation to the centre frequency v. of the inverse Lamb dip. Thus the intensi-
ty modulation can be utilized as a control criterion for the frequency stabilization. Fig. 3 shows the experimental set-up of the frequency stabilization. When the laser frequency deviates from vo, a correction signal is applied to the piezo-ceramic transducer PCT to maintain the cavity length L. This correction signal is obtained in the following manner: an RCP wave, for example, is obtained from the planar laser PL by means of a h/4 plate. This RCP laser wave undergoes the above described intensity modulation in the neon absorption tube. After demodulation of the intensity modulation by the photodiode PhD, the fundamental of the demodulated signal is amplified by the selective amplifier and fed into a phase sensitive detector. Here the phase and amplitude of the fundamental U, are compared with those of the reference signal U, for the alternating magnetic field. Only a magnetic field with an odd harmonic component has no influence upon the control signal, i.e., it does not cause an error in the frequency setting [6] . Such a field is best produced with a pushpull tuned amplifier. Due to the magnetic field of a tuned circuit, the intensity modulation is in phase quadrature with the control voltage. Therefore the reference voltage U, for the phase sensitive detector requires a phase correction cpto bring it in phase with the magnetic field. The phase sensitive detector produces a dc signal u& whose amplitude and polarity depend, analogous to the control signal UC,on the deviation v. - vL of the laser frequency.
LASER CAVITY .L-
COfNECTlX
DETECTQR
Fig. 3. Block diagram for the frequency stabilization of the He-Ne laser at the inverse Lamb dip with an axial alternating magnetic field in the neon absorption tube..
13
Volume 6, number I
OPTICS COMMUNICATIONS
This dc signal, after amplification by a dc amplifier, is applied to the piezo-ceramic transducer PCT for the correction of the cavity length L and hence the stabilization of the laser frequency. The automatic frequency stabilization of this method is realized in a manner similar to that described in [6,7] .
frequency modulated control signal.
September 1972
during the generation
of the
References [ l] P.H.Lee and M.L.Skoinik, Appl. Phys. Letters IO (1967)
3. Conclusion The described method of frequency stabilizing the He-Ne laser on the inverse Lamb dip yields a highly sensitive
frequency
discrimination
allowing
a very
high relative frequency setting accuracy (< 10Wt2). The coherence length is also considerably increased
because, contrary
14
to other methods, the beam is not
303. 121 W.G.Schweitzer, Appl. Phys. Letters 13 (I 968) 367. [ 31 S.N.Bagaev, Y.D.Kolomnikov, V.N.Lisitsyn and V.P. Chebotaev, IEEE J. Quantum Electron. QE-7 (I 971) 484. [4] V.Bodlaj, Frequenz 23 (1969) 374. [ 51 V.Bodlaj, Sixth Intern. Quantum Electron. Conf., Kyoto (1970); Opto-Electron. 2 (1970) 221. 161 V.Bodlaj, Z. Angew. Phys. 31 (1971) 97. i7] V.Bodlaj, Laser kngew. Strahhmastechnik 2 (I971 ) 21.
6, number
1
OPTICS
Ne ABSORPTION INSIDE
TUBE
A LASER
September
COMMUNICATIONS
IN AN ALTERNATING
CAVITY
MAGNETIC
AS A FREQUENCY
1972
FIELD
STANDARD
V.BODLAJ Siemeru
AC;, Kesearck
Laboratories,
Received
Mtnicll,
Germatz.~
30 June 1972
A neon absorption tube placed in an alternating magnetic field is used as a frequency standard for the frequency stabilization of a He-Ne laser. This absorption tube, which is situated within the laser cavity, permits a very precise frequency adjustment (< 10-l 2) due to the formation of the inverse Lamb dip. An excellent beam coherence is also obtained because this method of stabilization does not superimpose any frequency modulation on the oscillation.
1. Introduction A neon absorption tube, at a low pressure (< 15 Pascal) has a very narrow absorption linewidth. Also the absorption of the tube is very low. When such a tube is placed together with a He-Ne laser tube within a short cavity, where the laser output depends strongly upon the losses, even this low absorption can have a considerable influence upon the laser power. Assuming that the laser power increases with increasing frequency, the absorption may rapidly reach saturation (Lamb dip) in the region of the ten tre frequency v. of the absorption transition and result in an abrupt increase in the laser power (see fig. 1). This increase in laser power is termed as the P
t
A
-?---Fig. 1. Frequency absorption
12
V
response of the inverse Lamb dip when an tube is inserted into the laser cavity.
inverse Lamb dip [l] Due to its inherent narrow line. width, this inverse Lamb dip provides a well defined frequency standard for the frequency stabilization of the laser [l-3] . The following section describes a new version for the frequency stabilization of such laser systems.
2. Generation
of the frequency
control signal
An axial magnetic field in a Ne-absorption tube splits the absorption transition line into two lines which are circularly polarised in opposite directions (Zecman effect). Hence the right-hand circularly polarised (RCP) laser wave of frequency vL,, travelling in a direction opposite to that of the magnetic field, interacts with the lower frequency RCP component of the absorption transition having a shifted centre frequency v, = v. ~ Au/7 (see fig. 2). When the magnetic field is reversed, the higher frequency component of the absorption transition becomes right-hand circularly polarised, i.e., the RCP laser wave now interacts with a frequency v2 = v. + AU/~. The displacement of the respective absorption transitions in the absorption tube also causes a shift of the inverse Lamb dip. Fig. 2 shows the laser power for a low magnetic field strength H of approximately 8X I Op4T in the absorption tube as a function of the laser frequency (curves (a) and (b) for H and -If resp.).
September 1972
OFWCS COMMUNICATIONS
Volume 6, number 1
P t
Fig. 2. Frequency response of the inverse Lamb dip: curve (a) denotes the axial magnetic field H and curve (b) the axial magnetic field -H in the absorption tube.
The intensity of the RCP laser wave in an absorption tube with an alternating magnetic field is a function of the time dependent magnetic field [4-61. The time dependence of the intensity of the RCP laser wave differs in phase by ?I for laser frequencies vL < vu and vL > uo. Here v. denotes the centre frequency of the inverse Lamb dip for the magnetic field H = 0. The left-hand circular polarisation LCP also shows a similar behaviour. The phase and amplitude of this intensity modulation, caused by the alternating magnetic field, can determine the position of the laser frequency in relation to the centre frequency v. of the inverse Lamb dip. Thus the intensi-
ty modulation can be utilized as a control criterion for the frequency stabilization. Fig. 3 shows the experimental set-up of the frequency stabilization. When the laser frequency deviates from vo, a correction signal is applied to the piezo-ceramic transducer PCT to maintain the cavity length L. This correction signal is obtained in the following manner: an RCP wave, for example, is obtained from the planar laser PL by means of a h/4 plate. This RCP laser wave undergoes the above described intensity modulation in the neon absorption tube. After demodulation of the intensity modulation by the photodiode PhD, the fundamental of the demodulated signal is amplified by the selective amplifier and fed into a phase sensitive detector. Here the phase and amplitude of the fundamental U, are compared with those of the reference signal U, for the alternating magnetic field. Only a magnetic field with an odd harmonic component has no influence upon the control signal, i.e., it does not cause an error in the frequency setting [6] . Such a field is best produced with a pushpull tuned amplifier. Due to the magnetic field of a tuned circuit, the intensity modulation is in phase quadrature with the control voltage. Therefore the reference voltage U, for the phase sensitive detector requires a phase correction cpto bring it in phase with the magnetic field. The phase sensitive detector produces a dc signal u& whose amplitude and polarity depend, analogous to the control signal UC,on the deviation v. - vL of the laser frequency.
LASER CAVITY .L-
COfNECTlX
DETECTQR
Fig. 3. Block diagram for the frequency stabilization of the He-Ne laser at the inverse Lamb dip with an axial alternating magnetic field in the neon absorption tube..
13
Volume 6, number I
OPTICS COMMUNICATIONS
This dc signal, after amplification by a dc amplifier, is applied to the piezo-ceramic transducer PCT for the correction of the cavity length L and hence the stabilization of the laser frequency. The automatic frequency stabilization of this method is realized in a manner similar to that described in [6,7] .
frequency modulated control signal.
September 1972
during the generation
of the
References [ l] P.H.Lee and M.L.Skoinik, Appl. Phys. Letters IO (1967)
3. Conclusion The described method of frequency stabilizing the He-Ne laser on the inverse Lamb dip yields a highly sensitive
frequency
discrimination
allowing
a very
high relative frequency setting accuracy (< 10Wt2). The coherence length is also considerably increased
because, contrary
14
to other methods, the beam is not
303. 121 W.G.Schweitzer, Appl. Phys. Letters 13 (I 968) 367. [ 31 S.N.Bagaev, Y.D.Kolomnikov, V.N.Lisitsyn and V.P. Chebotaev, IEEE J. Quantum Electron. QE-7 (I 971) 484. [4] V.Bodlaj, Frequenz 23 (1969) 374. [ 51 V.Bodlaj, Sixth Intern. Quantum Electron. Conf., Kyoto (1970); Opto-Electron. 2 (1970) 221. 161 V.Bodlaj, Z. Angew. Phys. 31 (1971) 97. i7] V.Bodlaj, Laser kngew. Strahhmastechnik 2 (I971 ) 21.