International Journal o f Infrared and Millimeter Waves, VoL 15, No. 7, 1994

FREQUENCY MEASUREMENTS OF FAR INFRARED LASER LINES BY MEANS OF MIM POINT CONTACT DIODES 1

G. Careili, 1 N. Ioli, A. Moretti, 1 D. Pereira, 2 and F. Strumia 1

1Dipartimento di Fisica dell'Universit~ Piazza Torricelli 2, 56126 Pisa, Italia

2Instituto de Fisica "Gleb Wataghin" UNICAMP 13.083/970, Campinas, S. P., Brasil

Received May 17, 1994

ABSTRACT

We present frequency measurements of 85 Far Infrared (FIR) laser lines

ranging from 2 748 GHz to 421 GHz. We use the heterodyne technique of

mixing FIR laser transitions and microwave radiation on MIM point contact

diodes to determine the FIR frequencies.

Key words: methanol, Far Infrared laser, frequency measurement

1. Work supported by Consiglio Nazionale delle Ricerche -Italia and INFM

(Consorzio Interuniversitario di Fisica della Materia)

1347

O195-9271/94/0700q 347507.00/0 0 1994 Plenum Publishing Corporation

1348 Careili et al.

1. INTRODUCTION

In optically pumped Far Infrared (FIR) lasers polar molecules are used as

active medium. Monochromatic radiation from an Infrared (IR) laser,

generally a CO2 laser, is focused on a low pressure molecular vapor

contained in a cavity resonator. The absorbed IR radiation excites roto-

vibrational transitions and population inversion is thus obtained between

some of the rotational levels of the excited vibrational manifold. In this

way, stimulated emission of radiation can be observed in correspondence of

the allowed transitions from the excited level. The monochromaticity of the

pump source, and the absence of a plasma in the cavity, with its inherent

fluctuations in the index of refraction, make the optically pumped molecular

lasers very similar in principle to a three level maser oscillator. The gain

curve, at the working pressure, is narrower than the linewidth of the

resonator modes, so that frequency stability can be better than 1 part in 109,

making these devices very interesting for applications in several fields of

high precision physics. Examples can be seen in high resolution atomic and

molecular spectroscopy [1,2]: astrophysics, where FIR molecular lasers are

used as local oscillators for heterodyne detection of atoms and molecules in

interstellar space [3,4]; and metrology, where FIR radiation is used in

optical frequency synthesis chains [5,6]. In this sense, frequency

measurements of FIR laser lines play a fundamental role

In 1986, Inguscio et al. [7] presented a review of frequency measurements

of optically pumped lasers from 0.1 to 8 THz. In that work a list of more

than 800 FIR laser lines frequency measured together with a survey of the

techniques of frequency measurement were presented. Since then a number

of new FIR laser frequency measurements were performed by several

groups. In particular, at Pisa, we have measured over 80 FIR laser

frequencies. The main motivation was to investigate possible candidates to

be used in metrological and astrophysical experiments. The frequencies of

all known FIR laser lines with wavelength close to the metastable triplet

transitions 3P0---~3P 1 of Ca (v = 1 563 630.0 MHz) and Mg (v = 601 277.t

MHz); a n d 3P0--)3P 2 of Ca ( V = 3 174 230.0 MHz) and Mg ( v = 1 220

575.1 MHz), and to the transition 3P2---~3P 1 of N + (v = 2 459 370.3 MHz)

were measured. The first ones are transitions suggested to be used in an

Far Infrared Laser Lines 1349

altemative atomic clock [8], with possibility of higher accuracy, precision

and stability than the Cesium atomic clock [9]. The second is an important

transition in astrophysics studies [4]. We present a brief description of the

technique used to perform the frequency measurements, and a Table of all

FIR laser frequencies, pointing out that several of them were previously

reported in specific works

2. EXPERIMENTAL APPARATUS AND RESULTS

We used a heterodyne technique for the frequency measurements: the FIR

laser radiation to be frequency measured is mixed on a Metal-Insulator-

Metal ( MIM ) point contact diode with one or two FIR laser lines

previously known in frequency and the radiation from a microwave (MW)

source. The beat signal, measured by means of a spectrum analyzer, is used

to determine the unknown frequency v L through the relation

V L = nlVL1 + n2VL2 -+ nMWVMW + ~V 1)

where rE1 and rE2 are the known FIR laser frequencies, vMW the

microwave frequency, 8v the beat frequency and n 1 , n 2 and nMW are

integers that define the mixing order.

The microwave source is an HP 8350B sweeper operating in the region 0-

18 GHz. The signal from the diode is fed into a low noise preamplifier and

then sent to a HP 8355A spectrum analyzer to detect the intermediate

frequency.

The scheme of the experimental apparatus is shown in figure 1. We used

three different CO 2 pump lasers [10] and three FIR resonators [11], so that

we could choose the proper couples according to the requirements on the

line to be measured (pump power, pump offset, FIR wavelengths, etc.).

The first of the three CO 2 lasers is 300 MHz tunable in single line single

mode operation, and delivers an output cw power larger than I0 W on

many lines. The second is 200 MHz tunable with an output power of more

than 15 W and the third one is 150 MHz tunable with an output power

larger than 25 W on the strongest lines. All of them can work either in cw

or in synchronized long pulse regime, the latter being obtained by

1350 Careili et aL

electronically chopping the discharge current. In this mode of operation we

obtain pulses from 30 to 100 gs in duration with a repetition rate between 500 and 1000 Hz and peak powers in the range 50-300 W. An acoustooptic

modulator was also used in some cases to get an extension up to 480 MHz of the tuning range of the shortest CO 2 laser [12].

I ii

I Acoustooptic Modulator

4 \

Frequency lJ Counter I

4

I, synShW 'zer I Microwave

Source

){OpenResonator-~-] FIR Laser N~J_.]

>1 1 Waveguide FIR Lasers

-1 ..... I Figure 1- Experimental apparatus

Spectrum I Analyzer 4

0-6 GHz I Preamplifier

MIM Diode

/ ,7

/ ) /

!

2

As FIR resonators we used an open structure nearly confocal Fabry Perot

resonator and two waveguide resonators. A good stability in power as well

as in frequency, is achieved by using a thermally compensated frame. The

Fabry Perot resonator is a 100 cm pyrex cylinder terminated by two gold

coated copper mirrors of 115 cm curvature radius. The FIR power is

coupled out, through a silicon window, using an elliptical mirror obtained

by cutting at 45* a 6 mm diameter copper rode. This mirror can be moved

perpendicularly to the cavity axis, to optimize the output coupling at the

Far Infrared Laser Lines 1351

different wavelengths. The waveguide FIR resonators are pyrex tubes

terminated by plane copper mirrors. The guides are 150 cm long 40 mm

inside diameter and 134 cm long 35 mm i.d. respectively. The mirrors are

supported on a X-Y mount to carefully align the lasers. The CO 2 pump

radiation is coupled to the FIR cavity by a 2 mm axial hole in the input

mirror, while the FIR radiation is coupled out by an off-axis hole, whose

diameter lies in the range 2 - 12 ram, in the output mirror.

The FIR outputs were focused on the mixer diode together with the

microwave radiation. The signal to noise ratio of the beat note was in the

range 10 - 45 dB, depending on the power intensity of the two lines used

and on the order of the harmonic of the MW power. The values of the

frequency of the MW radiation and of the beat note were measured by

means of an HP5343A frequency counter. A good signal to noise ratio is

obtained when the power of the laser line is of the order of some 10 t.tW, at

least, and the MW power slightly saturates the diode. Since we can use few

mW of microwave radiation to saturate the mixer and to obtain a good S/-N

ratio, most of the lines can be measured using an harmonic of the MW.

This is the point that makes this technique very powerful: by using high

order mixing it is possible to measure laser lines tip to 80 GHz apart from

the reference one, making it possible to cover entirely the FIR region.

The value of n 1, n 2, nMw and the + or - sign in equation 1) are determined

experimentally by tuning the FIR laser cavities and the microwave

frequency, in order to get a small shift in the beat note on the spectrum

analyzer.

The lines to be measured were selected from different molecules among

those laser lines whose wavelengths were known to be close to those of

interest. Table I resumes all of our frequency measurements ordered by

increasing wavelength. Columns from 2 to 6 refer to spectroscopic data of

the line to be frequency measured: the second column gives the wavelength,

the third the laser active molecule, the fourth the CO 2 pump line, the fifth

the polarization of the FIR laser line with respect to that of the CO 2 laser,

while the sixth column gives the IR absorption offset relative to the CO 2

laser frequency center measured using the Transferred Lamb Dip technique

[15]. This revealed to be an easy and powerful technique to distinguish

between laser lines of similar frequency from the same pump line. As an

1352 C a r e l l l et al.

example we describe the case of some transitions pumped by the 10R(10)

CO 2 laser line: from the literature two lines of wavelength of 191 gm were known to be pumped by this line and the frequency of one of them had

already been measured. We found that there are three different lines with

very close frequencies but different offsets, at -35 MHz, +45 MHz and

+100 MHz respectively, showing that it is not necessary to perform

frequency measurements to identify them, owing to their different offsets

(see figure-2).

a)

- - - " - - - ' - ' - - - - C )

d)

Fig. 2 - FIR laser lines around 191 gm and pumped by the C O 2

10R10 line: a) CO 2 output power, frequency tuning 300 MHz, b)

FIR laser at 1570160.8 MHz, c) FIR laser at 1564518.7 MHz, d)

FIR laser at 1563897.5 MHz,

Columns seven, eight and nine report data relative to the reference lines: the

seventh gives the laser active molecule, the eighth the CO 2 pump line and

the ninth the frequency. Finally the last column gives the result of the

frequency measurement, The accuracy of the measurements is affected by

Far Infrared Laser Lines 1353

the uncertainty in the setting of the FIR laser cavities at the centre of the

molecular gain width and by the uncertainty in the frequencies of the

reference lines[13]. When each FIR laser is accurately tuned to the

maximum of the FIR output power, the total error of the frequency

measurement lies in the range 0.5 to 1.5 MHz, depending on the laser lines

characteristics.

To check the accuracy of our method we remeasured the frequency of

several FIR laser lines previously determined by other methods [7]. The

well known 118.8 ~m line from CH3OH optically pumped by 9P(36) CO 2

laser line was used to demonstrate the extension of our heterodyne

technique to three FIR laser lines. In a first measurement (number 4 of

Table I), the t 18.8 ~m line was heterodyned with the second harmonic of

the 238.5 I-tm line of 13CH3OH ( 9P12 pump) plus a microwave radiation

of 8 GHz.The beat frequency was at 1 038.8 MHz. Using the known value

of 1 256 871.8 MHz [7] for the frequency of the reference line we get for

the 118.8 I.tm line the value of 2 522 782.4 MHz. The second measurement

(number 5 of Table I) was performed heterodyning the 118.8 ~m line with

the 184.7 t.tm line of CD3OD ( 10R24 pump) plus the 13CD3OH line at

333.3 I.tm ( 10P16 pump). The beat frequency is at 655.47 MHz. Using for

the frequencies of the reference lines the measured values 1 622 555.2 and

899 571.7 MHz 117] we get for the 118.8 ~m line the value of 2 522 782.4

MHz : both measurements are in good agreement with the value of 2 522

782.567 MHz obtained by Tunable FIR Spectroscopy[14].

Comparing the known frequency values of the Ca and Mg triplet metastable

transitions with the frequency values of table I we can see that the lines

closest to those of interest are at minimum 300 MHz apart, a difference too

large for metrological purposes to excite directly the atomic transition, but

suitable to be covered by a sideband modulator. On the other hand with

respect to the N + transition, we have FIR laser lines that can be used as

local oscillator in heterodyne experiments.

3 CONCLUSIONS

We used the heterodyne technique to measure the frequency of 85 FIR laser

lines. Three waveguide CO 2 laser were used to pump three FIR laser

Tab

le I

LIN

E T

O B

E M

EA

SUR

ED

W

avel

engt

h M

olec

ule

Pum

p R

el.

Off

set

(gm

) L

ine

Pol.

(MH

z)

RE

FE

RE

NC

E L

INE

M

olec

ule

Pu

mp

Fr

eque

ncy

Lin

e (M

Hz)

ME

AS

UR

ED

F

RE

QU

EN

CY

(M

Hz)

1 10

9,07

C

H31

8OH

10

R04

//

68

2 11

4,80

D

2180

10

R10

_1

_ -1

65

3 1

18

,55

13

CD

3OH

9R

14

//

-20

4 1

18

,83

C

H3O

H

9P36

_1

_ 24

5

11

8,8

3

CH

3OH

9P

36

_1_

24

6 1

19

,01

~

3CD

3OH

10

P42

]]

5 7

119,

82

CH

318O

H

9P26

//

10

8

122,

30

CD

3OD

10

R28

3_

-5

5 9

122,

40

13C

H3O

H

9P20

//

-1

15

10

122,

88

13C

H3O

H

10R

30 /

/ -1

40

1 1

123,

86

CH

318O

H

9P30

//

60

12

124,

25

13C

D3O

H

10P

24 /

/ 21

13

12

9,41

C

H20

10

R18

_1

_ 20

14

13

2,11

13

CH

2F2

9R28

//

11

7

CH

2F 2

9P24

2.

742.

946,

0 13

CH

3OH

1

0R

16

2.

588.

361,

7 C

H3O

H

9P36

2.

522.

782,

6 13

CH

3OH

9P

12

1.25

6.87

1,8

13C

D3O

H

10

P1

6

899.

571,

7 C

D3O

D

10R

24

+1.6

22.5

55,2

C

H3O

H

9P36

2.

522.

782,

6 C

H3O

H

9P36

2.

522.

782,

6 C

H2F

2 9R

22

2.44

7.96

8,5

CD

3OD

10

R28

2.

451.

203,

1 C

D3O

D

10R

28

2.45

1.20

3,1

CH

2DO

H

10P

34

2.40

9.29

3,3

CH

2DO

H

10P

34

2.40

9.29

3,5

CH

3OH

10

R34

2.

314.

111,

3 C

D3O

H

10R

32

2.27

8.70

3,0

2.74

8.49

9,8

2.61

1.41

8,5

2.52

8.77

3,8

2.52

2.78

2,4

2.52

2.78

2,4

2.51

9.12

2,0

2.50

2.06

5,3

2.45

1.20

2,3

2.44

9.24

5,9

2.43

9.62

3,2

2.42

0.34

1,6

2.41

2.75

7,4

N 2

.316

.525

,9

2.26

9.19

7,5

m

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

145,

12

151,

55

153,

54

157,

20

162,

40

176,

37

182,

11

182.

12

188,

97

189,

94

189,

94

189,

94

190,

93

190,

93

190,

99

191,

68

191,

68

191,

70

205,

14

205,

76

Tab

le I

(C

ont.)

CH

3OH

C

H31

8OH

C

H31

8OH

13

CD

3OH

C

H3a

SOH

C

H31

8OH

C

H31

8OH

C

HzF

2 C

D3O

D

CH

3OH

C

H3O

H

CH

3OH

C

H3O

H

CH

3OH

C

H31

8OH

C

H3O

H

CH

3OH

C

H3O

H

CH

3OH

C

H3O

H

10R

20

9P22

9P

36

10R

40

9P38

10

R04

9P

I4

9R32

10

R20

9R

10

9R10

9R

10

10R

10

10R

10

9P42

9R

16

9R16

10

R10

9P

18

9P34

# 2.

// _L

L

± # l # // // // # // // # // # // ±

147

13C

H3O

H

9P10

17

C

H2

DO

H

10R

34

-60

13C

H3O

H

9P12

13

2 13

CH

3OH

9P

12

-20

CH

3OH

10

R38

68

]3

CD

3OD

10

R20

70

C

H2D

OH

9P

10

5 C

H2F

2 9R

32

74

CD

3OH

10

P42

-140

C

H3O

H

10R

10

-140

C

H3O

H

10R

10

-140

C

H3O

H

10R

10

100

CH

2F 2

9P22

10

0 C

H3O

H

10R

10

-72

CH

3OH

10

R10

-8

C

H3O

H

10R

10

-8

CH

3OH

10

R10

-3

5 C

H3O

H

10R

10

163

13C

H3O

H

10R

16

125

13C

H3O

H

10R

16

2.05

2.00

4,1

1.98

7.79

8,9

1.89

8.27

9,9

1.89

8.27

9,9

1.83

8.83

9,3

1.72

6.54

8,5

1.63

2.66

6,9

1.62

6.60

2,6

1.59

1.05

3,2

1.56

4.51

8,7

1.56

3.89

7,5

1.56

4.51

8,7

1.56

2.65

5,9

1.56

4.51

8,7

1.56

4.51

8,7

1.56

4.51

8,7

1.56

3.89

7,5

1.56

4.51

8,7

1.47

2.19

9,3

1.47

2.19

9,3

2.06

5.85

9,4

1.97

8.12

9,4

1.95

2.51

5,3

1.90

7.05

7,8

1.84

6.02

0,5

1.69

9.74

7,1

1.64

6.20

5,8

1.64

6.19

6,6

1.58

6.45

5,2

1.57

8.33

9,6

1.57

8.33

9,4

1.57

8.33

9,6

1.57

0.16

1,2

1.57

0.16

0,3

1.56

9.69

2,5

1.56

4.05

4,7

1.56

4.05

3,6

1.56

3.89

7,5

1.46

1.37

8,7

1.45

7.00

3,7

Q, r

Tab

le I

(C

ont.)

C~

U

rl

35

205,

76

36

205,

96

37

205,

96

38

208,

27

39

208,

27

40

210,

31

41

216,

36

42

218,

56

43

219,

80

44

220,

69

45

221,

86

46

227,

66

47

227,

88

48

232,

79

49

242,

26

50

243,

10

51

243,

38

52

243,

85

53

244,

38

CH

3OH

C

H31

8OH

C

H31

8OH

C

H3O

H

CH

3OH

C

D3O

D

13C

D3O

H

CH

318O

H

CH

318O

H

CH

318O

H

CH

318O

H

13C

H2F

2 13

CD

3OH

C

H3O

H

CH

318O

H

CH

3NH

2 C

D3C

1 C

D20

C

D20

9P34

±

9P

20

H

9P20

ff

9P

34

±

9P34

±

10

P18

# 10

P24

±

9P30

±

10

R20

ff

9P

26

±

9P30

±

9P

18

±

10P2

4 #

9R22

ff

9P

22

ff

10P1

2 ff

9P

32

±

9R24

H

10

P16

±

125 60

60

125

125

-51 21

60

21

10

-15 27

99

15

17

45

30

-80

13C

H3O

H

13C

H3O

H

CH

2DO

H

13C

H3O

H

13C

H3O

H

CH

3OH

C

H3O

D

CH

2DO

H

CH

2DO

H

CH

2DO

H

CH

2DO

H

CD

3OD

CD3OD

CH3O

H CH

3OH

CH3O

H CH

3OH

CH3O

H CH

3OH

10R

16

10R

16

9P14

10

R16

10

R16

10

R04

9R

14

9R24

9R

24

9R24

9R

24

10R

10

10R

10

9R10

10

R32

10

R32

10

R32

10

R32

10

R32

1.47

2.19

9,3

1.47

2.19

9,4

1.45

0.46

3,1

1.47

2.19

9,3

1.47

2.19

9,3

1.41

9.04

9,3

1.39

1.97

2,1

1.36

8.31

5,4

1.36

8.31

5,4

1.36

8.31

5,4

1.36

8.31

5,4

1.31

6.83

8,7

1.31

6.83

8,7

1.28

6.99

9,5

1.23

4.49

0,4

1.23

4.49

0,4

1.23

4.49

0,4

1.23

4.49

0,4

1.23

4.49

0,4

1.45

7.00

0,0

1.45

5.57

5,6

1.45

5.57

5,5

1.43

9.42

2,8

1.43

9.41

9,6

1.42

5.47

3,6

1.38

5.64

5,7

1.37

1.65

5,8

1.36

3.92

5,4

1.35

8.43

8,0

1.35

1.28

9,6

1.31

6.86

0,7

1.31

5.59

4,8

1.28

7.83

2,0

1.23

7.50

4,5

1.23

3.19

4,4

1.23

1.78

0,6

1.22

9.42

1,4

1.22

6.76

0,1

_=

54

244,

38

5 5

245,

06

5 6

246,

33

5 7

246,

46

58

251,

58

5 9

253,

72

60

262,

21

6 1

262,

21

6 2

275,

57

63

280,

18

64

280,

28

65

2813

9 66

28

4.76

67

28

5 27

68

29

091

69

297

63

70

2994

8 71

31

562

72

316.

19

73

327

28

Tab

le I

(C

ont.)

CD

20

CH

3Br

CD

20

CD

3C1

CH

318O

H

CD

3OH

C

H31

8OH

C

H31

8OH

C

H31

8OH

C

H31

8OH

13

CD

3OH

13

CD

3OH

C

H31

8OH

C

H31

8OH

13

CD

3OH

C

D3O

H

13C

H2F

2 13

CH

2F2

13C

D3O

H

13C

D3O

H

10P1

6 _1

_ 9P

28

//

10P2

6 .1_

10

R14

//

9P32

_1

_ 10

R36

/_

9P

26

// 9P

26

# 10

R20

//

9P

38

//

10P2

4 //

10R

08

// 9P

30

//

10R

04

//

10P2

4 //

10R

34

//

9P18

//

9P

20

//

10R

26

//

10R

10

//

-8O

160

-34 90

18

10

10

-30

-20 99

40

6O

68

21

-17 27

4 -2

5 40

CD

3OD

C

H3O

H

CD

20

CH

3OH

C

D3O

H

13C

D3O

H

CH

2F2

CD

20

CD

20

CD

20

CD

20

CD

20

CD

20

CH

2F2

CH

2F 2

CH

2DO

H

CD

3OD

C

D3O

D

CD

3OD

D

CO

OH

10R

28

10R

32

10P1

6 10

R32

IO

R36

10

P08

9P38

10

P08

10P0

8 10

P08

10P0

8 10

P08

10P0

8 9P

24

9R34

10

R34

10

R24

10

R10

10

R10

10

P22

2.45

1.20

3,1

1.23

4.49

0,4

1.22

6.76

0,1

1.23

4.49

0,4

1.18

1.58

9,8

2.36

0.17

4,8

1.14

5.43

0,1

1.07

6.84

2,8

1.07

6.84

2,8

1.07

6.84

2,8

1.07

6.84

2,8

1.07

6.84

2,8

1.07

6.84

2,8

1.03

6.84

4,8

1.04

2.15

0,4

1.01

4.04

7,7

1.00

3.53

6,6

952.

203,

9 95

2.20

3,9

912.

729,

7

1.22

6.76

0,1

1.22

3.31

8,2

1.21

7.03

7,9

1.21

6.39

4,8

1.19

1.64

5,7

1.18

1.58

9,1

1.14

3.32

8,7

1.14

3.32

7,9

1.08

7.88

6,7

1.06

9.99

7,6

1.06

9.61

4,9

1.06

5.40

7,3

1.05

2.80

8,2

1.05

0.90

7,4

1.03

0.53

5,6

N 1

.007

.272

,6

1.00

1.05

4,0

949.

857,

4 94

8.14

1,4

916.

009,

9

o.

,-t i"

C,

Tab

le I

(C

ont.)

tJI

74

75

76

77

78

79

80

81

82

83

84

85

86

87

3282

1 33

2 75

33

7.77

34

3.00

34

3.25

34

5.98

36

113

3636

3 36

3.75

41

9.39

45

419

4960

7 49

615

711,

20

13C

H2F

2 13

CH

2F2

CH

2F2

CD

3OD

C

D3O

D

CD

3OD

C

D3O

D

CH

318O

H

CH

318O

H

13C

D3O

H

CD

3OD

C

H3F

(K=

I)

CH

3F (K

=3)

13C

D3O

H

9P20

9P

38

9R14

10

P18

10R

22

10Pl

0 10

R22

10

R20

10

R20

9P

32

10R

10

9P20

9P

20

9P32

// _1_ // // i

i

i //

//

//

//

i

i

i

-120

11

8 98

-51

-62

-92

-62

-30 21

77

72

-58

208 77

DC

OO

H

DC

OO

H

13C

H3O

H

CD

3OD

C

D3O

D

CD

3OD

C

D20

C

D20

CD

zO

HC

OO

H

HC

OO

H

CH

3F (K

=2)

CH

3F (K

=2)

DC

OO

H

10P2

2 10

P22

9P22

10

R04

10

R04

10

R04

10

P28

10P2

8 10

P28

9R22

9R

38

9P20

9P

20

10R

34

912.

729,

7 91

2.72

9,7

884.

438,

0 86

9.52

2,7

869.

522,

7 86

9.52

2,7

822.

512,

2 82

2.51

2,2

822.

512,

2 71

6.15

6,8

653.

821,

4 60

4.29

7,3

604.

297,

3 42

1.53

2,0

913.

423,

3 90

0.94

6,2

887.

551,

1 87

4.02

8,7

873.

396,

1 86

6.50

0,2

830.

156,

8 82

4.43

3,6

824.

181,

2 71

4.82

4,2

660.

053,

6 60

4.33

4,6

604.

236,

8 42

1.53

2,0

Far Infrared Laser Lines 1359

resonators. The FIR laser radiation to be frequency measured is mixed with one or two FIR laser radiations previously known in frequency plus, if necessary, a microwave radiation. We use a MIM point contact diode as harmonic mixer, and conventional microwave equipment to analyze the beat signal. The technique is very powerful since using high order mixing it is possible to cover all the FIR region.

REFERENCES

1- "Infrared and Millimeter Waves"- K.J. Button ed., Plenum (1982) 2- "Hanle Effect and Level Crossing Spectroscopy"- G. Moruzzi and F.

Strumia eds., Plenum (1991) 3- D.M. Watson, in "Galactic and Extragalatic Infrared Spectroscopy"-

M.F. Kessler and J.P. Phyllips edts.- Reidel, Dordrecht (1984) 4- G. Carelli, N. Ioli, A. Moretti, D. Pereira, F. Strumia and R. Densing

- Appl. Phys. B45, 97-100 (1988) 5- D.A. Jennings, F.R. Petersen and K.M. Evenson, in "Laser

Spectroscopy IV"- H. Walther and K.M. Roth edts.- Springer (1979),

pp.39-48 6- K.M. Evenson, J.S. Wells, F.R. Petersen, B.L. Danielson and G.W.

Day - Appl. Phys.Lett. 22, 192,195 (1973) 7- M. Inguscio, G. Moruzzi, K.M. Evenson and D.A. Jennings- J.Appl.

Phys. 60, R161-R192 (1986) 8- F. Strumia- Metrologia 8, 85-90 (1972) 9- F. Strumia- in "Laser Science and Technology", A.N. Chester and S.

Martellucci edts.- Plenum Press, N.Y. (1988), pp. 367-401 10- M. Inguscio, N.Ioli, A.Moretti, G. Moruzzi and F.Strumia - Opt.

Commun. 37,211-216 (1981) F. Strumia and N. loli: "High Power C.W. Waveguide CO2 Lasers" in Physics of New Laser Sources, ed. by N.B. Abraham, F.T. Arecchi, A. Mooradian, A. Sona (Plenum N.Y. 1985) pp.189-199.

1 1 - M. Inguscio, A. Moretti, G. Moruzzi and F. Strumia - Int. J. of

Infrared and Millimeter Waves 2, 943-986 (1981) G. Carelli, N. Ioli, A. Moretti, D. Pereira and F. Strumia- Appl.

Phys. B44, 111-117 (1987)

1360 Carelli et al.

12- G. Carelli, N. Ioli, A. Moretti, G. Moruzzi, D. Pereira and F. Strumia-

Int. J. of Infrared and Millimeter Waves 12, 449-471 (1991)

13- M. Inguscio, N. Ioli, A. Moretti, F. Strumia and F. D'Amato: Appl.

Phys. B40, 165-169 (1986)

14- K.M. Evenson, D.A. Jennings, L.R. Zink and M. Inguscio-Proc. of

the l l th Int. Conf. on Infrared and Millimeter Waves - Pisa (1986),

G.Moruzzi edt. 98-100

15- M. Inguscio, A. Moretti and F. Strumia - Opt. Commun. 30, 355-360

(1979)