16 OPTICS LETTERS / Vol. 33, No. 1 / January 1, 2008

Dual-frequency Brillouin fiber laser for opticalgeneration of tunable low-noise radio frequency/

microwave frequency

Jihong Geng,* Sean Staines, and Shibin JiangNP Photonics, Inc., 9030 S. Rita Road, Tucson, Arizona 85747, USA

*Corresponding author: jihong_geng@yahoo.com

Received August 1, 2007; revised October 29, 2007; accepted November 9, 2007;posted November 16, 2007 (Doc. ID 85922); published December 18, 2007

We demonstrate a new approach, i.e., a cw dual-frequency Brillouin fiber laser pumped by two independentsingle-frequency Er-doped fiber lasers, for the generation of tunable low-noise rf/microwave optical signals.Its inherent features of both linewidth narrowing effect in a Brillouin fiber cavity and common mode noisecancellation between two laser modes sharing a common cavity allow us to achieve high frequency stabilitywithout using a supercavity. Beat frequency of the dual-frequency Brillouin fiber laser can be tuned fromtens of megahertz up to 100 GHz by thermally tuning the wavelengths of the two pump lasers with tuningsensitivity of �1.4 GHz/ °C. Allan variance measurements show the beat signals have the hertz-level fre-quency stability. © 2007 Optical Society of America

OCIS codes: 060.2840, 060.5625, 140.3510, 290.5900.

There are numerous industrial and military applica-tions requiring rf or microwave modulated opticalsources that can generate tunable rf/microwave sub-carriers with high-frequency stability. These applica-tions include optical/wireless communications, high-speed optoelectronics characterization, millimeter/submillimeter-wave phased-array radar, remotedistributed antenna system, and hybrid lidar/radarsystem. The rf/microwave photonic technologies alsoenable optical fiber delivery of timing or rf/microwavefrequency references over long distances, which hasrecently received intense interest [1].

An obvious way of generating rf/microwave modu-lated optical signals is to optically heterodyne two la-ser lines with emission frequency separated by therequired modulation frequency. The optical hetero-dyne signals can be tuned if the laser lines are tun-able. Two different approaches have been used togenerate the optical heterodyne signal, either by twoindependent laser sources or by a single laser sourcewith dual-frequency output. The major advantage ofthe first approach is the flexibility in wavelength andpower control through two independent lasers; how-ever, it suffers from the fact that the performance ofthe generated rf/microwave signal is determined notonly by phase noise or linewidth of the two lasers butalso any relative frequency drift between them. Therelative frequency drift can be significantly reduced ifa frequency-offset locking scheme is implemented inthe system by using an external reference. The devel-opment of these kinds of rf/microwave optical sourcesincludes the use of distributed-feedback (DFB) semi-conductor lasers [2] and diode-pumped solid-state/fiber lasers [3]. Since a fiber-based source at the tele-com wavelength is especially attractive due to itscapability of long-distance delivery and distributionof timing or frequency references over optical fiber, inaddition to its compactness and reliability, semicon-ductor DFB lasers could be good candidates for such

a fiber-based source. However, their relatively high

0146-9592/08/010016-3/$15.00 ©

phase noise significantly limits the frequency stabil-ity of the DFB-laser-based rf/microwave sources. Al-though active frequency locking can be applied to theDFB lasers for noise reduction, requirements for thefrequency-locking loop in these lasers are extremelystringent due to the relative broad spectral linewidth��1 MHz� of DFB lasers. Diode-pumped single-frequency solid-state lasers have narrow spectrallinewidths (at the kilohertz level) that offer high fre-quency stability. Higher frequency stability can fur-ther be obtained in those diode-pumped single-frequency solid-state lasers if a frequency-lockingscheme (to either an external resonator or a fre-quency reference) is used. For example, the fre-quency stability at the subhertz level of a microwavemodulated laser source was demonstrated, in whichtwo diode-pumped Nd:YAG nonplanar ring oscilla-tors (NPRO) were frequency locked to adjacent axialmodes of a supercavity [a very high-finesse �F=22,000� interferometer] [4]. However, the require-ment for the frequency-locking scheme is stringent(need either a supercavity, or a highly stable fre-quency [5]). In the second approach, in which the twolaser lines come from two laser modes sharing a com-mon cavity and common gain [6], the major advan-tage is that most noise processes originated with thesame laser cavity can be canceled out. However, thepower and wavelength of the two laser modes thatshare the same cavity are not easily controlled inde-pendently.

In this Letter, we demonstrate a new approach,i.e., a cw dual-frequency Brillouin fiber laser pumpedby two independent single-frequency Er-doped fiberlasers, for the generation of tunable low-noise rf/microwave signals. The new approach combines allthe major advantages of the two above-mentioned ap-proaches together. In addition, the inherent featureof noise reduction and linewidth narrowing effect in aBrillouin fiber laser [7,8] allows us to achieve ex-

tremely high frequency stability without using a su-

2008 Optical Society of America

January 1, 2008 / Vol. 33, No. 1 / OPTICS LETTERS 17

percavity. The pump scheme in this approach is alsodifferent from that in the dual-wavelength Brillouinfiber laser recently reported by Dennis et al. [9],where two pump wavelengths were a pair of opticalsidebands of a single-frequency fiber laser generatedby a lithium niobate intensity modulator. Our newscheme provides a flexible tuning feature with amuch wider tuning range for the rf/microwave modu-lated optical signals.

The experimental setup is shown in Fig. 1, whichwas modified from a single-frequency Brillouin fiberlaser [7]. Two similar high-power ��150 mW� Er-doped single-frequency fiber lasers with a spectrallinewidth of a few kilohertz and a free-running fre-quency stability of better than 10 MHz/h at 1550 nmwere used to pump a single Brillouin fiber laser si-multaneously after beam combination with a 3 dB fi-ber coupler (C1). Both Er-doped fiber lasers are ther-mally and piezoelectrically tunable with tuningsensitivities of approximately 1.4 GHz/ °C and20 MHz/V, respectively. The Brillouin fiber ring laseris formed by another directional fiber coupler (C2)and a long piece of optical fiber ��20 m�, which areintegrated in a temperature-controlled and vibration-damped package.

Laser frequencies of the two Er-doped single-frequency fiber lasers were actively stabilized to beresonant with two cavity modes of the Brillouin fiberlaser by using two independent sets of feedbackservo, which are based on the Pound–Drever–Hallfrequency-locking technique as described in detail be-fore [7]. Dithering frequencies (f1 and f2) of the twofeedback servos were set to be different (for example,at 1 and 1.3 kHz, respectively) so that there was nointerference between the two feedback servos. Stabledual-frequency operation can be achieved simulta-neously by sharing the same Brillouin ring cavity.Output power of the Brillouin fiber laser at each fre-quency was balanced at �10 mW. Both frequencies ofthe laser can be independently tunable over multi-tens of gigahertz by thermally tuning the pumpwavelengths. Assuming that m1 and m2 are the modeorder of the Brillouin laser cavity of the stimulatedBrillouin Stokes beams generated by the two Er-doped fiber lasers, the modulated rf/microwave fre-quency of the Brillouin laser beam is given by frf=n�FSR, where n= �m1−m2� is the mode order differ-ence between the two Brillouin laser components,

Fig. 1. Experimental setup for the generation of low-noiserf/microwave optical signal. S, feedback servo; C, fiber cou-pler; PD, photodiode.

and FSR represents the free spectral range of the

Brillouin cavity. It is easy to tune the modulation fre-quency by changing either one of the mode orders m1and m2. This can be achieved simply by thermallytuning either wavelength of the two Er-doped fiberlasers.

In the experiment, the FSR of the Brillouin fiberring cavity was approximately 9.9 MHz. Figure 2shows a typical rf beat-note spectrum measured withan electrical spectrum analyzer (ESA) (AgilentE4401B). The rf frequency was measured at999.942 MHz, which corresponds to n=101. The ob-served linewidth was limited by the resolution band-width �1 kHz� of the ESA that we used.

Since each of the Er-doped fiber lasers has at least50 GHz thermal tuning range (corresponding to 35°Ctemperature change), the modulated rf/microwavefrequency from the Brillouin laser can be easilytuned over 100 GHz. Higher beat frequency (up tothe terahertz region) can also be generated in thesame way if the wavelengths of the two Er-doped fi-ber lasers are set to be far enough. Figure 3 showsthat the beat-note frequency can be set to be any fre-quency at an integral multiple (n, i.e., the mode orderdifference of the dual-frequency components) of thecavity FSR. Interestingly, however, the “effectiveFSR” of the dual-frequency Brillouin fiber laser,which is defined as the measured beat frequency di-vided by the integer n, is not a constant over integraln. When the integer n is small, that means that thetwo laser frequencies get close, and the effective FSRis reduced. The effective FSR approaches a constantwhen the two laser frequencies get separated farenough. This phenomenon can be readily explainedby the Brillouin slow-light effect [10]. When the fre-quency difference between the two lasers is compa-rable to, or smaller than, the Brillouin gain band-width in fiber, either one of the dual-frequencyBrillouin laser outputs can experience an additionalBrillouin gain generated by the other pump laser. Ac-cording to the Kramers–Kronig relations, this addi-tional Brillouin gain results in a substantial changein refractive index, thereby yielding a reduced groupvelocity of light in the Brillouin fiber laser. This ex-planation is supported by the fact that the 40 MHzbandwidth of the effective FSR profile observed inFig. 3 is consistent with the natural Brillouin band-width in the single-mode fiber at 1.55 �m, which is

Fig. 2. Typical rf beat-note spectrum. The rf frequency

was measured at 999.942 MHz, corresponding to n=101.

18 OPTICS LETTERS / Vol. 33, No. 1 / January 1, 2008

also the bandwidth of Brillouin slow light [10]. Fromthe data in Fig. 3, we know that the maximumchange in refractive index in the Brillouin fiber laseris �0.4%. The Brillouin slow-light effect has no im-pact on the stable operation of the dual-frequencyBrillouin fiber laser.

Performance of the rf beat signals generated fromboth the Brillouin fiber laser and the two Er-dopedpump lasers was characterized by using a frequencycounter (Stanford Research System, SRS620). Figure4 shows the root Allan variance of typical rf beat sig-nals (at 148.5 MHz) as a function of sampling gatetime. The number of samples for the measurementswas 100. As a comparison, the root Allan variances ofthe rf beat signals between the two Er-doped pumplasers are also shown in Fig. 4. The two pump laserswere operated under two conditions, respectively, i.e.,free-running operation and frequency-locking (to cav-ity modes of the ring cavity) operation. The root Allanvariance of the rf beat signals generated from theBrillouin fiber laser goes down to a few hertz whenthe sampling gate is longer than 10 s, which is 3 or-ders of magnitude smaller than that of the two Er-doped pump lasers even in the operation mode of fre-quency locking. Although common-mode noisecancellation can also function in the two frequency-

Fig. 3. Measured effective FSR and beat frequency as afunction of mode order difference.

Fig. 4. Root Allan variance of the rf beat signals (at148.5 MHz) generated from the Brillouin laser (solidcircles), the two Er-doped pump lasers in free-running(cross points), and frequency-locking mode (triangles).

locked Er-doped pump lasers because they werelocked simultaneously to the single ring cavity, thelow finesse of the passive ring cavity and low band-width of the frequency-locking feedback servos usedin the two frequency-locked Er-doped pump lasersgreatly limit its capability for frequency stability im-provement. As compared with the free-running la-sers, the frequency-locked Er-doped fiber lasers canonly eliminate some slow relative frequency drift be-tween them. The high frequency stability (at hertzlevel) of the beat signal generated from the Brillouinfiber laser is attributed to the inherent features ofboth noise reduction [7,8] and noise cancellation be-tween two laser modes in the Brillouin fiber laser.

This approach provides an easy way to generatewidely tunable, extremely stable rf/microwave fre-quency; however, it should be noted that the gener-ated beat signal is stepwise tunable, instead of con-tinuous tuning. As a result, the commonly usedoptical phase-locked loops cannot be implemented di-rectly in this approach, thereby limiting its useful-ness in those applications where phase locking is re-quired for the rf/microwave signal.

In conclusion, we demonstrate a cw dual-frequencyBrillouin fiber laser for the generation of tunable low-noise rf/microwave signals. Beat frequency of thedual-frequency Brillouin fiber laser can be tunedfrom megahertz up to 100 GHz by thermally tuningthe wavelengths of its pump lasers independently,i.e., two Er-doped single-frequency fiber lasers, withtuning sensitivity of 1.4 GHz/ °C. Allan variancemeasurement shows the hertz-level frequency stabil-ity of the rf/microwave beat signals is generated fromthe dual-frequency Brillouin fiber laser without usinga supercavity.References

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