cw Brillouin laserK. O. Hill, B. S. Kawasaki, and D. C. Johnson

Citation: Applied Physics Letters 28, 608 (1976); doi: 10.1063/1.88583 View online: http://dx.doi.org/10.1063/1.88583 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/28/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A new method of coherent Rayleigh–Brillouin scattering using frequencytunable cw lasers J. Acoust. Soc. Am. 100, 2624 (1996); 10.1121/1.417707 A stable cw HCN laser Rev. Sci. Instrum. 45, 767 (1974); 10.1063/1.1686732 Losses in cw dye lasers J. Appl. Phys. 44, 263 (1973); 10.1063/1.1661872 PREMIXED cw CHEMICAL LASER Appl. Phys. Lett. 17, 514 (1970); 10.1063/1.1653290 THE RUBY LASER AS A BRILLOUIN LIGHT AMPLIFIER Appl. Phys. Lett. 5, 127 (1964); 10.1063/1.1754082

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cw Brillouin laser K. O. Hill, B. S. Kawasaki, and D. C. Johnson

Department of Communications, Communications Research Centre, Ottawa, Canada K2H 8S2 (Received 30 January 1976)

The observation of efficient continuous-wave Brillouin laser action in an optical-fiber ring resonator is reported. Intemallaser conversion efficiencies of SO% and a Brillouin-shifted output of 20 mW have been achieved.

PACS numbers: 42.S0.+g, 42.60.Lh, 42.6S.Dr, 42.80.Mv

Optical fibers are known to provide a particularly suitable medium for the study of stimulated scattering processes. 1-5 The strong transverse confinement of the optical fields by the fiber and the long interaction lengths that are possible significantly reduce the pump­power levels required to observe nonlinear processes. 1

Stimulated Brillouin scattering (SBS) has been observed previously in glass fibers using a xenon-laser pump at 5355 A with power levels less than 1 W in 600-nsec pulses. 3

We report here the first observation of Brillouin laser action in an optical-fiber ring resonator. The main attributes of the Brillouin ring laser (BRL) are a lower threshold than has previously been reported for SBS, a high continuous conversion efficiency, the narrowest output linewidth yet observed for this effect, and continuous -wave operation.

The form of the BRL is illustrated in Fig. 1. The argon -ion laser which acts as the pump is tuned to the 5145-A oscillation; an etalon internal to the laser cavity is used to select a single longitudinal mode that has a short-term oscillation linewidth of 25 MHz at output powers up to 750 mW. The Single-mode fiber of the BRL has losses less than 100 dB/km at 5145 A, a core of diameter 2.4 f,.Lm, a numerical aperture N A = O. 1, and is 9.5 m long. The ring resonator consists of the optical fiber together with the beam path defined by the 4% and 50% (nominal) reflectors. The fiber is coupled to the beams by 20x microscope objectives. The 4~

ARGON ION LASER

PUMP BEAM

DETECTOR

1000/0 REFLECTOR

I APERTURE

5lA5A

50'l'. REflECTOR

AND DISPLAY

APPARATUS

OPTICAL FIBER

FIG. 1. Schematic of Brillouin ring-laser apparatus. The rin~ consists of the optical fiber together with the beam paths de­fined by the 4% and 50% partial reflectors. The pump-beam throughput is monitored by the pump-beam detector.

608 Applied Physics Letters, Vol. 28, No.1 0, 15 May 1976

reflector allows the pump beam from the argon -ion laser to enter the ring and also serves to redirect a portion of the backward -scattered Stokes wave to the 50% reflector. The 50% reflector serves to close the ring into the fiber and to provide an exit route for a por­tion of the Stokes wave. The output is monitored with a scanning Fabry-Perot interferometer.

The feedback losses due to the partial reflectors are nominally 98%. This high-loss resonator configuration dictates that at most 2% of the pumn beam leaving the fiber is returned to the input (pumped) end of the fiber. The low coupling thus reduces pump-beam enhancement effects that can arise as a consequence of the resonator geometry.

The BRL is a true traveling-wave oscillator in that oscillation occurs in only one direction around the ring. Oscillation is at the Stokes-shifted frequency character­istic of the glass fiber, 34 GHz down shifted from the frequency of the pump. Figure 2 is a frequency scan of the output of the BRL. The Brillouin-shifted signal has a width in frequency of 20 MHz (FWHM), a value which approaches the resolution limit of our scanning Fabry­Perot interferometer at the maximum etalon spacing of 14 cm. The output is a Single peak; it is noted that we have not observed BRL longitudinal modes of oscil­lation which are expected to have a frequency separa­tion of approximately 10 MHz.

Figure 3 shows the relationship between the Brillouin-

FIG. 2. Frequency trace of the Brillouin-shifted output (large­amplitUde signal) and a portion of the unshifted pump (small signal). The horizontal scale is 100 MHz per large division.

Copyright © 1976 American Institute of Physics 608

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200

~ i ISO

3 ! E t- .. .. ., .

i ~ § 100 ..... 10...1 .. 9! '" 8i :z: .. t-

~ . . ~a "- 50 !i ~ ...I

~~ .. 0 ~i

0 I0IO 200 300 400 500 600 700 800 LASER PUMP POWER (MIUIWATTS I

FIG. 3. Graph showing the variation with laser pump power of (1) throughput pump with feedback path broken between 4% and 50% reflectors (x); (2) throughput pump with feedback path in­tact (e); (3) Brillouin-shifted output power through 500/ partial reflector (G).

shifted output power of the BRL and the input pump power. Also shown is the variation in the pump power passing through the fiber (pump-power throughput) that is detected via the 50~ reflector as a function of input pump power. There is a clear laser oscillation thresh­old at 250 mW of input pump power and a subsequent linear increase of Brillouin-shifted output with pump power. Coincident with BRL threshold the throughput pump power saturates; any increase in pump power beyond the saturation point is almost completely con­verted to Brillouin -shifted light in the ring resonator. If the feedback path is broken between the two partial reflectors, the pump power throughput remains propor­tional to the input power until the threshold for super-: radiant SBS is reached at 550 mW. At pump powers above this point unstable Brillouin-shifted output can be observed with no feedback in the resonator.

A high conversion efficiency was achieved with this resonator configuration. A maximum conversion to Brillouin-shifted light of 50% can be deduced from Fig. 3 by taking the ratio of the pump-beam throughput with feedback to the extrapolated throughput without feedback

609 Appl. Phys. Lett., Vol. 28, No. 10,15 May 1976

for a laser pump power of 750 mW. Approximately 500 mW of pump power is coupled into the fiber; 250 mW of this is converted into Brillouin-shifted light at the pump­put fiber end. This value is reached for a BRL output of 8 mW. 6 There is an obvious trade-off between BRL output from the resonator and the efficiency of coupling the pump power to the fiber. In another resonator con­figuration the 4% pump-input reflector was replaced by a 50% reflector. This change decreased the threshold of the BRL and increased the BRL output to 20 mW with the argon-ion pump at its maximum 750 mW of single­mode power.

Cratering of the input end of fibers in the presence of SBS has been reported and attributed to the high in­tensity levels reached by the backward -traveling stimu­lated Brillouin pulse. 3 In the work reported here the optical-fiber ends have not been damaged during con­tinuous operation of the laser at high output-power levels over periods of several hours.

In summary, we have achieved cw Brillouin laser action in a ring-resonator configuration and have shown that the use of feedback reduces SBS threshold levels considerably. We have not attempted to optimize the resonator configuration used in our preliminary ex­periments but tentatively conclude that reduction of laser threshold is possible by employing a longer length of lower-loss fiber, and by increasing the coupling efficiencies.

We thank T. Ozeki for his interest in our work and for stimulating discussions on SBS. We thank F. Dabby of Fiber Communications Inc. for supplying the optical fiber used in this work.

lE. P. Ippen, Appl. Phys. Lett. 16, 303 (1970). 2R.H. Stolen, E.P. Ippen, and A.R. Tynes, Appl. Phys. Lett. 20, 62 (1972).

3E.P. Ippen andR.H. Stolen, Appl. Phys. Lett. 21,539 (1972). ~.H. Stolen and E. P. Ippen, Appl. Phys. Lett. 22, 276 (I 973).

5J. Stone, Appl. Phys. Lett. 26, 163 (1975). 6We note that use of the nominal reflector reflectivities yields 8 x1/0. 04 x 1/0. 5 ~400 mW of Brillouin-shifted light at the pump-input fiber end which is considerably larger than the correct value of 250 mW.

Hill, Kawasaki, and Johnson 609

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