1250 OPTICS LETTERS / Vol. 21, No. 16 / August 15, 1996

Hybrid Brillouin/erbium fiber laser

Gregory J. Cowle

Sydney University Electrical Engineering and Australian Photonics Cooperative Research Centre, Building J03,University of Sydney, Sydney, NSW 2006, Australia

Dmitrii Yu. Stepanov

Australian Photonics Cooperative Research Centre, 101 National Innovation Centre, Australian Technology Park,Eveleigh, NSW 1430, Australia

Received March 4, 1996

We propose and demonstrate a novel hybrid Brillouin/erbium fiber laser that uses both Brillouin gain in single-mode optical fiber and gain in erbium-doped fiber. The lasing frequency was accurately determined by theBrillouin shift in single-mode optical fiber and was 10.35 GHz from the Brillouin pump. Large powers couldbe extracted because of incorporation of erbium-doped fiber in the laser resonator. An output power of 10 mWwas measured at a wavelength of 1532 nm. 1996 Optical Society of America

Brillouin fiber lasers have useful properties, such asnarrow linewidth, that make them of interest for useas f iber sensors.1 The general technique to produce aBrillouin fiber laser is to construct a critically coupledfiber resonator, which is necessary because of thesmall magnitude of the Brillouin gain.2 Althoughcritically coupled Brillouin f iber lasers exhibit usefulcharacteristics, their disadvantages include the smalloutput power that can be achieved, the requirement ofcavity matching to the pump signal, and the difficultyin incorporating intracavity elements because of theirassociated loss. This type of laser can be comparedwith erbium-doped fiber lasers, which operate with ahigh-gain medium that allows for efficient operation ofcomplex resonator structures, are suited to operationwith large output coupling, and can have an operatingquantum efficiency of .95%.3 We have overcome theneed for a critically coupled resonator in a laser basedon Brillouin gain by using an erbium-doped fiberamplifier to compensate for the resonator losses whilestill originating lasing action from the Brillouin gain.The wavelength of the resulting laser is an accuratelydetermined frequency shift from the Brillouin pump,and large output powers are achievable. The laseruses two gain media, the linear gain from erbium-dopedfiber and the nonlinear gain from Brillouin scattering,in a single-mode fiber. The laser operates in the1.5-mm telecommunications window and in addition tothe applications of Brillouin lasers in sensors may haveimportant applications in dense-wavelength-divisionmultiplexing communication systems.

The configuration of the Brillouin/erbium fiberlaser (BEFL) is illustrated in the experimental setupshown in Fig. 1. The resonator comprised a fusedfiber coupler that coupled 97% of the light out ofthe ring, a section of erbium-doped fiber (EDF), awavelength-division multiplexer (WDM) coupler topermit pumping of the EDF with 980-nm light from apigtailed semiconductor laser, an isolator, and a 100-mlength of single-mode optical f iber (SMOF). Withsuff icient 980-nm pump power applied, the ring laseroperated as a normal EDF ring laser. However, in

0146-9592/96/161250-03$10.00/0

this new laser we originated laser action from a signalshifted down in frequency by an amount determined bythe Brillouin shift in the glass fiber, generated from anarrow-linewidth signal, injected in the counterclock-wise direction as indicated in Fig. 1. The injectedBrillouin pump was nonresonant, as it was blockedby the isolator in the ring, and hence did not enterthe EDF; however, the Brillouin signal, generated inthe clockwise direction as in Fig. 1, was amplif iedby the EDF. The isolator prevented the ring fromoperating as an injection-locked laser locked to theBrillouin pump.

Fig. 1. Experimental laser configuration and measure-ment system: BPF, bandpass f ilter; MSA, microwavespectrum analyzer; OSA, optical spectrum analyzer.

1996 Optical Society of America

8743 (DLC)

August 15, 1996 / Vol. 21, No. 16 / OPTICS LETTERS 1251

The laser schematic is similar to that of a traveling-wave EDF laser4 and a high-finesse EDF ringresonator,5 but the mode of operation is different. Tocommence laser action requires that suff icient gainbe generated to overcome the resonator losses. Theconcept of this new laser mode of operation was toincrease the 980-nm pump entering the EDF to a leveljust below that required for the ring to operate as anEDF laser. The external narrow-linewidth laser wasthen injected in the nonresonant direction into the100-m length of SMOF in the ring. With sufficientintensity provided by the Brillouin pump in the SMOFa Brillouin signal was generated in the direction op-posite the injected signal, and additional narrow-bandgain was available to a signal traveling in the clockwisedirection. The nature of the Brillouin process in the1550-nm region in a SMOF is such that the Brillouingain has a linewidth typically less than 20 MHz, down-shifted ,10 GHz from the Brillouin pump. Maximumefficiency occurs if the Brillouin pump has a linewidthless than the Brillouin gain bandwidth.2 A signaltraveling in the clockwise direction at a frequencyseparated from the injected Brillouin pump by theStokes shift would have sufficient gain to reach lasingthreshold by virtue of the combination of the Brillouingain and the amplification in the doped fiber. A keyfeature of the BEFL is that it does not need a low-lossresonator, instead using the EDF to equalize theresonator losses. The isolator is a key element of thelaser, preventing depletion of the gain by the injectedsignal, and it also prevents injection locking to theBrillouin pump.

The combination of two gain media with differentproperties has been used in only a small number oflaser systems.6 In this laser we used a combinationof the broad-bandwidth gain in EDF and the narrow-bandwidth Brillouin gain in SMOF. The operationcould be viewed as a form of intracavity injectionlocking, but the EDF laser is injection locked to astimulated Brillouin signal rather than to a coherentlasing signal. An alternative view of the operationis that the lasing wavelength is that at which thetotal gain in the resonator f irst equals the loss inthe resonator as the gain is increased, and, with theinjected Brillouin pump and the gain that it generates,that wavelength is the Stokes-shifted wavelength fromthe Brillouin pump.

The pump source used to stimulate the Brillouin sig-nal was, for experimental convenience, a tunable semi-conductor laser followed by an EDF amplif ier. Thelarge input–output coupling was chosen in the initialdemonstration to generate a significant Brillouin sig-nal because the Brillouin pump is only single pass,not resonant as in a standard Brillouin fiber laser.With the injected signal in the ring the 980-nm pumpfor the EDF was increased, and the output from theclockwise direction of the ring was observed, combinedwith a monitor of the injected signal, as shown inFig. 2. As the 980-nm pump was increased a new sig-nal, shifted 10.08 nm from the injected signal, wasobserved in addition to the injected signal. Figure 2shows the mixed Brillouin pump and BEFL signals,measured on a monochromator-based optical spectrum

analyzer. With no pump injected into the EDF themixed signal contained only the Brillouin pump signal.As the pump was increased, the power in the Stokes-shifted signal increased. With sufficient pump powerapplied, the power in the BEFL output could easily ex-ceed the power of the Brillouin pump. The linewidthsof the signals are not resolvable in Fig. 2 (resolution0.08 nm), but the existence of two separate peaks in thespectra is quite evident. Individual narrow-linewidthpeaks were observed on a scanning Fabry–Perot spec-trum analyzer, as can be seen from Fig. 3. Note fromFig. 3 that the free spectral range is smaller than thespacing between the Brillouin pump and the BEFL sig-nal, and the spacing between the signals is 1.3 freespectral ranges. The frequency shift of 10.35 GHz(equivalent to 0.08 nm) was confirmed from the mixedsignal incident upon a high-speed photodetector, the

Fig. 2. Optical spectrum of mixed Brillouin pump andBEFL signals as 980-nm launched pump power was varied.Brillouin pump power, 5.7 mW.

Fig. 3. Optical spectrum measured with a Fabry–Perotspectrum analyzer. FSR, free spectral range.

1252 OPTICS LETTERS / Vol. 21, No. 16 / August 15, 1996

Fig. 4. Power characteristics of a BEFL as functions of980-nm pump and Brillouin pump signals.

current from which was analyzed on a microwave spec-trum analyzer.

The output power characteristic of the BEFL isillustrated in Fig. 4 for four different injected powers.In the best case 10-mW output power from the BEFLwas measured; however, the best eff iciency of Brillouinpump to BEFL signal conversion occurs for lowerlevels of injected Brillouin pump powers. This canbe seen by the bunching of the upper three tracesfor injected power of .5 mW. An increase of theBrillouin pump power from 0.5 to 5.5 mW increasedthe output power by 4 mW, but the next increase,from 5.5 to 10 mW, increased the output power byonly 0.5 mW. Once threshold was reached, the outputpower was dominated by the conversion of 980-nmpump rather than the Brillouin pump to the BEFLsignal. Although without the injected signal applied,the 980-nm pump power had to be kept small so thatthe gain was just less than the loss to prevent the laserfrom operating as an EDF laser, once the BEFL wasoperating the 980-nm pump could simply be increasedto increase the output power, as the gain was thenclamped to the threshold value, the total gain arisingfrom the Brillouin gain, and the gain from the EDF.

The BEFL configuration is quite robust. Withoutthe seed signal applied, the drive on the 980-nmpump for the EDF could be increased such that anEDF laser was realized. The application of the seedsignal suppressed the spurious lasing of the EDF laser,and the BEFL began laser action at the Stokes shift

from the injected signal. Although the wavelength atwhich the BEFL operated was approximately matchedto the wavelength of operation of the free-runningEDF laser, tuning of at least 2 nm about the peakoperating wavelength was observed on tuning of theinjected signal wavelength. Operation in the 1550-nmregion should easily improve this tuning range over therange observed in the 1532-nm region, because of thef latter gain curve in the longer-wavelength region. Inaddition, a filter could be incorporated within the ring;then matching of the seed signal to the f ilter passbandwould permit operation across a large portion of theEDF gain bandwidth.

We believe that this laser can easily be cascadedto generate a multiple-wavelength source, possibly at-tractive for application in dense-wavelength-divisionmultiplexing networks with 10-GHz channel spacing.Other possible applications for the laser are pulse gen-eration from a dual-frequency source7 and microwavegeneration.8

In conclusion, we have demonstrated a novel f iberlaser that uses Brillouin gain and gain from an erbium-doped fiber. The lasing signal had a fixed wavelengthshift from the pump signal owing to the Brillouin gain,and large output power was possible because of the gainin the erbium-doped fiber.

The authors acknowledge the support of the Aus-tralian Research Council.

References

1. S. P. Smith, F. Zarinetchi, and S. Ezekiel, Opt. Lett. 16,393 (1991).

2. G. P. Agrawal, Nonlinear Fiber Optics (Academic, SanDiego, Calif., 1989), Chap. 9, p. 263.

3. G. J. Cowle, D. N. Payne, and D. Reid, Electron. Lett. 27,229 (1991).

4. P. R. Morkel, G. J. Cowle, and D. N. Payne, Electron.Lett. 26, 632 (1990).

5. H. Okamura and K. Iwatsuki, J. Lightwave Technol. 9,1554 (1991).

6. L. Yan and L. Ding, Appl. Phys. Lett. 67, 3679 (1995).7. S. V. Chernikov, J. R. Taylor, and R. Kashyap, in Optical

Fiber Communication Conference, Vol. 4 of 1994 OSATechnical Digest Series (Optical Society of America,Washington, D.C., 1994), p. 25.

8. D. Culverhouse, K. Kalli, and D. A. Jackson, Electron.Lett. 27, 2033 (1991).