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Pressure shifts of the 3.51 μm line in xenon
Volume 71A, number 5,6
PhYSICS LETTERS
28 May 1979
PRESSURE SHIFTS OF THE 3.51 pm LINE IN XENON ~ David H. SCHWAMB Department of Physics, Bryn Mawr College, Bryn Mawr, PA 19010, USA Received 27 February 1979
36Xe laser amplifier. Optical heterodyne techniques are used to determine the 3.51 ~.imline shift versus pressure in a ‘ Measured shifts induced by helium and by xenon were —3.1 ±0.3 MHz/Torr and 7 ±3 MHz/Torr, respectively.
The output of a gas laser amplifier whose only input is the random spontaneous emissions occurring within the laser medium can have an intensity spectrum which is considerably narrower than the Doppler profile of the spontaneous emission process [1]. Further, the amplified spontaneous emission (ASE) intensity spectrum is centered on the atomic transition responsible for the lasing action. Hence, any shift in frequency of the ASE spectral profile peak corresponds to a shift in the atomic transition frequency. Optical heterodyne techniques have been used in this experiment to measure pressure shifts of the peak of the ASE spectral profile produced by a 3.51 pm 136Xe laser amplifier. The shifts were measured as a function of He pressure in a He—Xe discharge and as a function of Xe pressure in a pure Xe discharge. The ASE spectral profiles were obtained by a method sirnilar to that used by Cole and Sanders [2]. Fig. 1 depicts the experimental setup. The self-narrowed output of the laser amplifier cell was coherently mixed with the output of a single mode, tunable He—Xe laser, LI, on the face of an InAs photodiode. The laser was tunable over a range of approximately 100 MHz, while the ASE spectral width varied between 30 MHz and 50 MHz at half maximum, depending on discharge conditions. Because of the high gain and low saturation coefficient of the 3.51 pm line, pinholes were inserted in several places in the amplifier cell beam line to prevent nonresonant reflections off the surface of intermediate optics from enter°
This material is based upon work supported in part by the National Science Foundation under Grant No. ENG77-01118.
420
ing the amplifier cell and possibly distorting the ASE spectral profile. The pinholes also ensured that a single spatial mode in the center of the beam was selected. The photodiode acts as a square law detector and thus mixes the laser and amplifier cell signals. One of the heterodyne terms produced at the detector’s output has a radio frequency current spectrum whose algebraic square is proportional to the product of the laser intensity and the intensity spectrum of the amplifier cell [3]. The detector is followed by a high gain bandpass amplifier. The amplifier’s passband is approximately 100 kllz centered at 1 MHz. It thus selects from the above heterodyne term only certain frequency components which correspond to amplifier cell emissions near the laser frequency. It also rejects all dc terms. The amplified signal is then detected, squared and applied to the input of lock-in amplifier no. I. The reference input to the lock-in is provided by a mechanical chopper which modulates the beam from LI before it is mixed with the amplifier cell output. A fraction of LI ‘s output is simultaneously picked off from the main beam by a partially silvered mirror and monitored by lock-in amplifier no. 2. This provides a measure of the laser intensity. As LI is tuned across the amplifier cell intensity spectrum, lock-in no. 1 displays the spectral density of the squared heterodyne cross term between the laser and amplifier cell. Thus the ratio of the outputs of lock-in no. I and lock-in no. 2 as a function of the tunable laser’s frequency gives the ASE spectral profile of the amplifier cell. A second He—Xe laser, L2, is frequency stabilized to within 1 MHz and serves as a frequency reference.
Volume
71A, number 5,6
PHYSICS LETTERS
28 May 1979
H.-Xa LASER, L2
InAs DETECTOR
~
~ CHOPPER
H.-X. LASER, Li
\\
1
mA. DETECTOR
mA. DETECTOR
,/~
/~
~
CHOPPER~ AMPLIFIER CELL \______________ _____________________
-4
I-
~
SPECTRUM ANALYZER
]
A1~LIFIER
-~
*2
Fig. 1. Experimental apparatus.
Its output is coherently mixed with a fraction of LI’s output and the resulting beat note between the two lasers is displayed by a spectrum analyzer. This provides a direct measurement of the output frequency of Li as a function of cavity tuning. Stabilization of L2 was accomplished by applying an error signal to the piezoelectric transducer which controlled the resonant cayity length whenever the laser output drifted from the gain peak of an external He—Xe amplifier cell (not shown). The heterodyne techniques used here offer several advantages. Problems of optical feedback into the amplifier cell which would arise from interferometric types of measurements are avoided. Complications due to mode pulling and piezoelectric transducer nonlinearities in determining the tunable laser’s output frequency as a function of resonant cavity length are
eliminated by continuously observing the beat note between LI and the fixed-frequency reference laser oscillator, L2. The optical detectors used in this experiment were not required to have uniform, wide-band frequency response due to the fixed-bandpass detec. tion scheme. The amplifier cell upon which the line shift measurements were made was dc excited and of a cold cathode design. Anodes were spaced at regular intervals along the length of the 3.8 mm I.D. bore such that the active discharge length could be varied between 40 cm and 200 cm. A nonexcited return path was provided between the ends of the tube to reduce cataphoresis effects. The multi-anode amplifier cell was designed for a study of ASE spectral profiles versus discharge length which will be reported in a future publication. For the purposes of this experiment, the discharge 421
Volume 71A, number 5,6
PFIYSICS LETTERS
1
He PRESSURE I Torr I 2 3 4 5
6
7
I
I
I
I
I
I
I
28 May 1979
8
-5.
2 =
U-
=
U,
o z
U. -
20
~-l5
-
-
I 2 20
~4 Xe
.
PRESSURE
I 6 Torr
I
I
8
10
Fig. 3. Line shift versus Xe pressure. The straight line and the vertical scale are determined as in fig. 2. Fig. 2. Line shift versus He pressure. The straight line indicates a least squares fit to the data. The zero of the frequency scale has been chosen to coincide with the least squares line extrapolated to zero pressure.
length was varied to provide a relatively high amplifier cell output to compensate for the dependence of the gain coefficient on discharge pressure. This resulted in a ratio of the ASE signal peak to system electronic noise which was typically of order 100 1. For the 3.51 pm line shift versus helium pressure data, the amplifier cell was filled with 190 mTorr of 90% isotopically enriched 136Xe and operated at a discharge current of 2.5 mA. Helium was added to the discharge in small amounts up to a total pressure of 7 Torr. The ASE spectral profile was plotted for each fill. The data, shown in fig. 2, indicate a linear relationship between the location of the peak of the ASE spectrum and helium pressure. A least squares fit yields a fre-
quency shift of -—3.1 ±0.3 MHz/Torr. The sense of the shift is toward decreasing frequency with increasing helium pressure which is in agreement with a previous observation of the xenon 3.36 pm line shift as a function of helium pressure [4].
422
The 3.51 pm line shift as a function of pressure in a 36 pure 136Xe discharge was obtained similarly. The Xe pressure was varied between 165 mTorr and 950 mTorr. The resulting data are shown in fig. 3 and allow a least squares fit to a straight line with a slope of 7 ±3 MHz/Torr. This result is in agreement with a previously reported value obtained by a different experimental method [5]. No dependence of the 3.51 pin line shift on discharge current was observed in contradiction to a previous report [6]. The author wishes to express his thanks to S.R. Smith and N.B. Abraham for several helpful discussions regarding the preparation of this manuscript.
References [11 L.W. Casperson, 1. Appi. Phys. 48 (1977) 256. 121 J.B. Cole and J.l1. Sanders, J. Phys. B5 (1972) 371. 131 MC. Teich, Proc. IEEE 56 (1968) 37. [41 R.Vetter and D.Reymann,J.Phys. B7 (1974) 323.
[51J.
Brochard and R. Vetter, Phys. Lett. 33A (1970) 398.
[61 El. Gamo and S.S. Chuang, Final Report AFCRL-71-0612 (1971), unpublished.
PhYSICS LETTERS
28 May 1979
PRESSURE SHIFTS OF THE 3.51 pm LINE IN XENON ~ David H. SCHWAMB Department of Physics, Bryn Mawr College, Bryn Mawr, PA 19010, USA Received 27 February 1979
36Xe laser amplifier. Optical heterodyne techniques are used to determine the 3.51 ~.imline shift versus pressure in a ‘ Measured shifts induced by helium and by xenon were —3.1 ±0.3 MHz/Torr and 7 ±3 MHz/Torr, respectively.
The output of a gas laser amplifier whose only input is the random spontaneous emissions occurring within the laser medium can have an intensity spectrum which is considerably narrower than the Doppler profile of the spontaneous emission process [1]. Further, the amplified spontaneous emission (ASE) intensity spectrum is centered on the atomic transition responsible for the lasing action. Hence, any shift in frequency of the ASE spectral profile peak corresponds to a shift in the atomic transition frequency. Optical heterodyne techniques have been used in this experiment to measure pressure shifts of the peak of the ASE spectral profile produced by a 3.51 pm 136Xe laser amplifier. The shifts were measured as a function of He pressure in a He—Xe discharge and as a function of Xe pressure in a pure Xe discharge. The ASE spectral profiles were obtained by a method sirnilar to that used by Cole and Sanders [2]. Fig. 1 depicts the experimental setup. The self-narrowed output of the laser amplifier cell was coherently mixed with the output of a single mode, tunable He—Xe laser, LI, on the face of an InAs photodiode. The laser was tunable over a range of approximately 100 MHz, while the ASE spectral width varied between 30 MHz and 50 MHz at half maximum, depending on discharge conditions. Because of the high gain and low saturation coefficient of the 3.51 pm line, pinholes were inserted in several places in the amplifier cell beam line to prevent nonresonant reflections off the surface of intermediate optics from enter°
This material is based upon work supported in part by the National Science Foundation under Grant No. ENG77-01118.
420
ing the amplifier cell and possibly distorting the ASE spectral profile. The pinholes also ensured that a single spatial mode in the center of the beam was selected. The photodiode acts as a square law detector and thus mixes the laser and amplifier cell signals. One of the heterodyne terms produced at the detector’s output has a radio frequency current spectrum whose algebraic square is proportional to the product of the laser intensity and the intensity spectrum of the amplifier cell [3]. The detector is followed by a high gain bandpass amplifier. The amplifier’s passband is approximately 100 kllz centered at 1 MHz. It thus selects from the above heterodyne term only certain frequency components which correspond to amplifier cell emissions near the laser frequency. It also rejects all dc terms. The amplified signal is then detected, squared and applied to the input of lock-in amplifier no. I. The reference input to the lock-in is provided by a mechanical chopper which modulates the beam from LI before it is mixed with the amplifier cell output. A fraction of LI ‘s output is simultaneously picked off from the main beam by a partially silvered mirror and monitored by lock-in amplifier no. 2. This provides a measure of the laser intensity. As LI is tuned across the amplifier cell intensity spectrum, lock-in no. 1 displays the spectral density of the squared heterodyne cross term between the laser and amplifier cell. Thus the ratio of the outputs of lock-in no. I and lock-in no. 2 as a function of the tunable laser’s frequency gives the ASE spectral profile of the amplifier cell. A second He—Xe laser, L2, is frequency stabilized to within 1 MHz and serves as a frequency reference.
Volume
71A, number 5,6
PHYSICS LETTERS
28 May 1979
H.-Xa LASER, L2
InAs DETECTOR
~
~ CHOPPER
H.-X. LASER, Li
\\
1
mA. DETECTOR
mA. DETECTOR
,/~
/~
~
CHOPPER~ AMPLIFIER CELL \______________ _____________________
-4
I-
~
SPECTRUM ANALYZER
]
A1~LIFIER
-~
*2
Fig. 1. Experimental apparatus.
Its output is coherently mixed with a fraction of LI’s output and the resulting beat note between the two lasers is displayed by a spectrum analyzer. This provides a direct measurement of the output frequency of Li as a function of cavity tuning. Stabilization of L2 was accomplished by applying an error signal to the piezoelectric transducer which controlled the resonant cayity length whenever the laser output drifted from the gain peak of an external He—Xe amplifier cell (not shown). The heterodyne techniques used here offer several advantages. Problems of optical feedback into the amplifier cell which would arise from interferometric types of measurements are avoided. Complications due to mode pulling and piezoelectric transducer nonlinearities in determining the tunable laser’s output frequency as a function of resonant cavity length are
eliminated by continuously observing the beat note between LI and the fixed-frequency reference laser oscillator, L2. The optical detectors used in this experiment were not required to have uniform, wide-band frequency response due to the fixed-bandpass detec. tion scheme. The amplifier cell upon which the line shift measurements were made was dc excited and of a cold cathode design. Anodes were spaced at regular intervals along the length of the 3.8 mm I.D. bore such that the active discharge length could be varied between 40 cm and 200 cm. A nonexcited return path was provided between the ends of the tube to reduce cataphoresis effects. The multi-anode amplifier cell was designed for a study of ASE spectral profiles versus discharge length which will be reported in a future publication. For the purposes of this experiment, the discharge 421
Volume 71A, number 5,6
PFIYSICS LETTERS
1
He PRESSURE I Torr I 2 3 4 5
6
7
I
I
I
I
I
I
I
28 May 1979
8
-5.
2 =
U-
=
U,
o z
U. -
20
~-l5
-
-
I 2 20
~4 Xe
.
PRESSURE
I 6 Torr
I
I
8
10
Fig. 3. Line shift versus Xe pressure. The straight line and the vertical scale are determined as in fig. 2. Fig. 2. Line shift versus He pressure. The straight line indicates a least squares fit to the data. The zero of the frequency scale has been chosen to coincide with the least squares line extrapolated to zero pressure.
length was varied to provide a relatively high amplifier cell output to compensate for the dependence of the gain coefficient on discharge pressure. This resulted in a ratio of the ASE signal peak to system electronic noise which was typically of order 100 1. For the 3.51 pm line shift versus helium pressure data, the amplifier cell was filled with 190 mTorr of 90% isotopically enriched 136Xe and operated at a discharge current of 2.5 mA. Helium was added to the discharge in small amounts up to a total pressure of 7 Torr. The ASE spectral profile was plotted for each fill. The data, shown in fig. 2, indicate a linear relationship between the location of the peak of the ASE spectrum and helium pressure. A least squares fit yields a fre-
quency shift of -—3.1 ±0.3 MHz/Torr. The sense of the shift is toward decreasing frequency with increasing helium pressure which is in agreement with a previous observation of the xenon 3.36 pm line shift as a function of helium pressure [4].
422
The 3.51 pm line shift as a function of pressure in a 36 pure 136Xe discharge was obtained similarly. The Xe pressure was varied between 165 mTorr and 950 mTorr. The resulting data are shown in fig. 3 and allow a least squares fit to a straight line with a slope of 7 ±3 MHz/Torr. This result is in agreement with a previously reported value obtained by a different experimental method [5]. No dependence of the 3.51 pin line shift on discharge current was observed in contradiction to a previous report [6]. The author wishes to express his thanks to S.R. Smith and N.B. Abraham for several helpful discussions regarding the preparation of this manuscript.
References [11 L.W. Casperson, 1. Appi. Phys. 48 (1977) 256. 121 J.B. Cole and J.l1. Sanders, J. Phys. B5 (1972) 371. 131 MC. Teich, Proc. IEEE 56 (1968) 37. [41 R.Vetter and D.Reymann,J.Phys. B7 (1974) 323.
[51J.
Brochard and R. Vetter, Phys. Lett. 33A (1970) 398.
[61 El. Gamo and S.S. Chuang, Final Report AFCRL-71-0612 (1971), unpublished.