IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 38, NO. 2, APRIL 1989 541

FIR Frequency Measurement with a Waveguide- Optical Harmonic Mixer

JONG WOOK WON, JONG SU KIM, JAE HEUNG JO, MEMBER, IEEE, AND C. 0. WEISS

Absfracf-We report harmonic mixing results of FIR frequencies with a waveguide-optical harmonic mixer using a micrometer size Schottky-barrier diode which is able to generate high order harmonics at room temperature. The frequency measured of the 170.6-pm wave- length line of a methyl alcohol laser is 1757.529 f 0.005 GHz and that of 118.8 pm is 2522.786 & 0.005 GHz.

I. INTRODUCTION N FIR LASER is a useful source in diagnosis of fu- A sion plasma [ 11, laser magnetic resonance [2], [3], or

meter realization. In particular we use a methyl alcohol laser as a frequency transfer oscillator for connecting mi- crowaves to the CO2 laser in the laser frequency synthesis chain for the meter realization. Usually two FIR lasers are needed to transfer a microwave frequency to a CO2 laser frequency. To reduce the steps, Blaney et al. [4] used a Josephson point-contact harmonic mixer, which allows the production of very high order (43rd) harmonics. How- ever, it is relatively complicated, only able to be operated at low temperature. High S I N ratio mixing signals are not easily reproduced.

In this paper, we describe the frequency measurement of an alcohol laser by using a simple waveguide-optical harmonic mixer, which also allows to generate very high order (35th) harmonics at room temperature. We synthe- sized the alcohol laser frequencies of 118.8- and 170.6- pm wavelengths.

11. EXPERIMENTAL SETUP Fig. 1 shows the schematic diagram of the FIR laser

frequency measurement. Fig. 2 shows the hybrid harmonic mixer. It uses a

Schottky-barrier diode, which has capacitance of 2.5 f F, series resistance of 8.5 Q and epilayer doping of 1 X 1017 ~ m - ~ . The tungsten whisker of the Schottky-diode is per- pendicularly oriented to the waveguide. The diameter and the length of the tungsten whisker is 25 pm and 4 mm, respectively, so the whisker acts as a long-wire antenna compared with the wavelength of FIR laser. The coupling is increased by a 90" corner reflector.

The FIR radiation is focused on the mixer with a

Manuscript received June 10, 1988. This work was supported by the Ministry of Science and Technology, Korea.

J . W. Won, J . S. Kim, and J . H. Jo are with the Laser Laboratory, Korea Standards Research Institute, Taejon, 302-340, Korea.

C. 0. Weiss is with Laboratory 4.42, Physikalisch-Technische Bunde- sanstalt , Braunschweig , Germany.

IEEE Log Number 8825804.

Gunn Oscillator w

Fig. 1. Schematic diagram of the experimental setup of the FIR laser fre- quency measurement. Gunn oscillator frequency - 8 GHz, klystron fre- quency - 70 GHz.

SMA S

0

cc

' w-

x- Y

Fig. 2. Photo of waveguide-optical hybrid-harmonic mixer. W 70-GHz waveguide, S: backshort, CC: 90" comer reflector, D: Schottky-chip, SMA: IF-dc connector, X - Y: reflector adjustment (x - y ) .

polyethylene lens of 5-cm focal length. The Schottky- diode chip is flush mounted with the outer surface of the waveguide. It sits on a brass rod arranged perpendicularly to the 70-GHz propagation direction within the wave- guide. On the waveguide side opposite to the diode chip, the brass rod is connected to an SMA connector and fitted with a X/4 choke structure. The signal of the beat fre- quency is sent to a low noise amplifier and then to an RF spectrum analyzer.

The klystron has a typical output power of 250 mW. The FIR CH30H laser consists of a circular pyrex wave-

0018-9456/89/0400-0541$01 .OO O 1989 IEEE

542 lEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 38, NO. 2, APRIL 1989

guide of 180 cm length with a hole-coupled resonator. The IR pump radiation is coupled through a 3-mm diam- eter hole and the FIR radiation is coupled out through the same hole. The IR and the FIR radiation are spatially sep- arated by the reflection of a KCI plate which is located in front of the coupler.

A part of the klystron beam is mixed with a 8-GHz Gunn oscillator beam on another Schottky-diode mixer, and the beat frequency is also measured with the spectrum ana- lyzer. The precise frequency of the Gunn oscillator is measured simultaneously with a microwave counter.

111. RESULTS The transitions of 170.6- and 118.8-pm wavelengths of

the free-running alcohol laser optically pumped with a 9P(36) CO2 laser beam were used. Fig. 3 shows the beats between the 24th klystron harmonic and the 170.6-pm al- cohol laser, and between the 35th and the 118.8-pm laser.

Table I summarizes the results of the frequency mea- surements.

The beat S/N ratios of 25 and 15 dB in 300-kHz band- width were obtained, respectively.

The slope of the signal to the harmonic number after normalizing to a laser power of 10 mW is about 2.4 dB/harmonic number, whose value is quite similar to an- other experimental result [ 5 ] .

The statistical uncertainty of the laser frequency mea- surement is about 3 MHz and the estimated systematic uncertainty of our experiment is about 4 MHz. Adding these uncertainties quadratically yields an uncertainty of 5 MHz.

The resulting value of the measured frequency of 170.6- pm wavelength is 1757.529 i- 0.005 GHz and that of 118.8 pm is 2522.786 k 0.005 GHz. These results are consistent with [6] within the uncertainty. We are prepar- ing currently a more precise frequency measurement on the system by stabilizing oscillators and lasers. It is worth mentioning that the CH30H laser when tuned to FIR line center on both transitions emitted on two frequencies simultaneously (see Fig. 4) as reported in [7]. However, emission with a central carrier and two symmetric side- bands was, differently from [7], observed under no con- ditions.

In summary, we have measured alcohol laser frequen- cies with a waveguide-optical harmonic mixer at room temperature. The micrometer size Schottky-barrier diode mixer’s efficiency appears to be decreasing a ~ f - ~ roll-off in this frequency region, but can be extrapolated to higher frequencies.

REFERENCES [ l ] S . M. Wolfe, K. J. Button, J . Waldman, and D. R. Cohn, “Modulated

submillimeter laser interferometer system for plasma density measure- ments,” Appl. Opt. , vol. 15, pp. 2645-2648, 1976.

[2] R J. Saykally and K. M. Evenson, “Observation of pure rotational transitions in the HBr+ molecular ion by laser magnetic resonance,” Phys. Rev. Lett., vol. 43, pp, 515-518, 1979.

[3] J. R Anacona and P B. Davis, “Laser magnetic resonance of sulphur radicals,” Infrared Phys., vol. 25, pp. 233-237, 1985.

5 MHdDiv 20 MHz/Div

(a) (b) Fig. 3. RF beat spectrum in 300-kHz bandwidth. (a) Between the 24th

Klystron harmonic and the 170.6-pm methyl alcohol laser. (b) Between the 35th klystron harmonic and the 118.8-pm methyl alcohol laser.

10 M H Z ~ l V

Fig. 4. Heterodyne spectrum of 118.8-pm laser tuned to line center, in- dicating a symmetric pulsing laser instability. PCHIOH = 12 Pa, pump power: 13 W.

TABLE I CH,OH LASER LINE MEASUREMENT

Alcohol laser : wavelength 118.8 PM

power up t o 5 ON up to 8 ON I I

1 Klystron-Gunn osc i l la tor I I mixing ;

G u n osc i l la tor frequency 8.13464 GHz 8.w783 GHz hanmnic (m) beat frequency (fK-mfG)

I t I

I Laser-klystron mixing :

hanmnic (n) beat frequency ( f -nf 1 t161 Wz -33.5 wz signal to noise &tiOK

2522.786 GHz Measured frequency 1757.529 GHz

Uncertainty : s t a t i s t i c a l estimated systemtic t o t a l

3 l z 4 wz 5 w l z

I I

[4] T . G. Blaney, N . R. Cross, D. J. E . Knight, G. 3 . Edwards, and P. R. Pearce, “Frequency measurement at 4.25 THz (70.5 p m ) using a Josephson harmonic mixer and phase-lock techniques,” J . Phys. D: Appl. Phys. , vol. 13, pp. 1365-1370, 1980.

[5] C. 0. Weiss and A. Godone, “High-order harmonic mixing with Schottky-diodes in the FIR region,” Appl. Phys. B , vol. 35, pp. 199- 200, 1984.

[6] D. J. E. Knight, “Ordered list of far-infrared laser lines ( C . W., X > 12 pm) ,” NPL Rep. QU45, 1982.

[7] E. H. M. Hogenboom, W. Klische, C. 0. Weiss, and A. Godone, “Instabilities of a homogeneously broadened laser,” Phys. Rev. Leu., vol. 55, pp 2571-2574, 1985.