What Narrow-linewidth semiconductor lasers can do for defense and security?

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1 See discussions, stats, and author profies for this pubication at: What Narrow-inewidth semiconductor asers can do for defense and security? Artice in Proceedings of SPIE - The Internationa Society for Optica Engineering Apri 2010 DOI: / CITATIONS 3 READS 1,134 8 authors, incuding: Miche Morin TeraXion 56 PUBLICATIONS 341 CITATIONS SEE PROFILE Simon Ayotte TeraXion 48 PUBLICATIONS 403 CITATIONS SEE PROFILE Maryse Aube Neoptix 12 PUBLICATIONS 67 CITATIONS SEE PROFILE Yves Painchaud TeraXion 105 PUBLICATIONS 1,117 CITATIONS SEE PROFILE Some of the authors of this pubication are aso working on these reated projects: Narrow Linewidth Laser View project A content foowing this page was upoaded by Simon Ayotte on 19 January The user has requested enhancement of the downoaded fie.

2 What Narrow-inewidth semiconductor asers can do for defense and security? M. Morin*, S. Ayotte, C. Latrasse, M. Aubé, M. Pouin, Y. Painchaud, N. Gagnon and G. Lafrance TeraXion, 2716 Einstein, Québec, Québec, CANADA G1P 4S8 ABSTRACT Sensing systems for defense and security operate in evermore demanding environments, increasingy eaving the comfort zone of fiber aser technoogy. Efficient and rugged aser sources are required that maintain a high eve performance under arge temperature excursions and sizabe vibrations. This paper first presents a sampe of defense and security sensing appications requiring aser sources with a narrow emission spectrum. Laser specifications of interest for defense and security sensing appications are reviewed. The effect of the aser frequency noise in interferometric sensing systems is discussed and techniques impemented to reduce phase noise whie maintaining the reative intensity noise performance of these sources are reviewed. Deveopments towards the size reduction of acousticay isoated narrowinewidth semiconductor asers are presented. The performance of a narrow-inewidth semiconductor aser subjected to vibrations is characterized. Simuation resuts of interferometric sensing systems are aso presented, taking into account both the intensity and phase noise of the aser. Keywords: Semiconductor asers, frequency noise, phase noise, interferometric sensing, fiber optic sensing, narrowinewidth, defense, security. 1. INTRODUCTION The present paper is concerned with semiconductor asers for defense and security sensing appications where the ight spectra content is of concern. The spectrum of emission of a semiconductor aser can be reduced by ocking its frequency to a frequency discriminator with a narrow spectra feature. This aows emission with a much narrower spectrum and reduced short-term phase noise, making these asers suitabe for interferometric sensing. This paper first presents in section 2 a sampe of defense and security appications requiring narrow-inewidth asers. Pertinent aser specifications for these appications are reviewed in section 3. Section 4 presents methods to contro the spectrum of emission of semiconductor asers, making them suitabe for interferometric sensing appications. Finay, section 5 reports simuation resuts of a time-domain mutipexed sensor array probed by either a DFB fiber aser or a narrow-inewidth semiconductor aser, taking into account both intensity and phase noise of the asers. 2.1 Free-space interferometric sensing 2. SPECTRALLY SENSITIVE APPLICATIONS Hard target detection can be performed by refection of a aser beam. The most straightforward approach is to measure the time-of-fight of a periodica train of aser puses refected by the target, which provides a direct evauation of the distance. Target speed is derived from the rate of change of the measured distance. More eaborate approaches are impemented to increase accuracy and provide a direct measure of the target speed. They rey on assessing the optica frequency of the refected ight beam, which is Dopper-shifted by a moving target. The ensuing fractiona change in the optica frequency is quite sma and an accurate speed measurement requires a narrow-inewidth aser with a stabe frequency of emission. Typicay, the output beam of the aser is spit in two, one part being used as a oca osciator (LO) within the optica receiver, whereas a second part is frequency-shifted for heterodyning and sent towards the moving target. The intensity beats produced by mixing the refected beam with the LO provides the desired information.

3 A frequency-moduated continuous-wave (FMCW) idar reies on the same principe of measurement 1. In this case, the aser emission optica frequency is moduated in time according to a periodica trianguar function, as shown in figure 1. The difference in frequency between the LO and probe beams can take on two vaues determined by the target distance and speed. The distance is cacuated from the average of these vaues, whereas speed is evauated from their difference. Figure 1: Measurement principe in a FMCW idar. 2.2 Fiber optic interferometric sensing A optica sensors utimatey rey on the measurement of optica power variations. Highest sensitivity is achieved when the measured optica power depends on the phase of the optica fied. This dependence can be created using interferometric sensors buit from singe mode fiber. A probe ight beam is divided between two fiber paths, an isoated reference arm and a sensing arm submitted to the measurand. Optica path ength variations induced in the sensor arm by the measurand ater the interference condition between the ight beams eaving the interferometer and hence the observed optica power. Sonar systems are a primary miitary appication of fiber optic interferometric sensing, the passive nature of the fiber sensors being ceary advantageous for underwater appications. The sensing arm then consists in a ength of fiber wound around an air-backed mandre designed to maximize path ength variations resuting from underwater vibrations. The sensitivity of an interferometric sensor, i.e. the power variation resuting from a phase change, is maximized when the interferometer is in quadrature, but can otherwise fade competey if the interferometer is impropery biased. Active bias contro of the interferometer can be avoided by moduating the phase of the probe ight, an approach termed phase generated carrier (PGC). The power refected (or transmitted) by an interferometer with a path mismatch then varies periodicay in time even in the absence of any externa perturbation. Detecting (down-converting and fitering) one odd and one even harmonic of the moduation frequency provides quadrature components from which the interferometer phase shift can be evauated. Heterodyning can aso be used, whereby ight puses are spit in two sub-puses, which are deayed and frequency-shifted from one another. The sub-puses are recombined by interferometric sensors with a matching path deay, their interference resuting in a power variation at the heterodyne frequency with a phase dependent on the interferometer phase shift. Optica fiber sensors are mutipexed to reduce costs. The PGC approach ends itsef to mutipexing in the frequency domain, whereby mutipe asers are phase-moduated at different frequencies. Mutipexing can aso be performed in the time domain, a periodica train of ight puses with a duty factor smaer than 1/N being refected, for exampe, by a series of N in-ine Micheson interferometers. The mutipe puse refections can be detected seriay by a singe detector. This configuration, discussed further in section 5, aows impementing heterodyning as discussed above. The eve to which aser phase noise corrupts interferometric measurements depends on the path mismatch incurred by interfering fieds. In the imit of zero differentia deay, it has no impact since both interfering fieds are affected by the same aser phase noise perfecty correated in time, which eaves their reative phase unchanged. This condition may be difficut to reaize in practice. In underwater acoustic sensors, hydrostatic pressure can produce a path mismatch as arge

4 as a meter. When phase generated carriers are used, a path mismatch is required to transate the aser phase moduation into a power moduation. The impact of aser phase noise depends aso on detais of the sensor interrogation. When a arge number of sensors are mutipexed in time, a detection bandwidth much arger than the samping rate of each sensor is required, in order to avoid overapping of returning puses. High frequency aser phase noise then gets aiased into the base band. The resuting increased noise eve often imits the mutipexing gain that can be achieved in practice. Further detais about fiber optic interferometric sensors can be found in a review paper by Kirkenda and Dandridge Fiber optic distributed sensing Distributed sensing takes advantage of the fact that backscattering of ight traveing aong an optica fiber varies with temperature and strain. Raman scattering resuts from moecuar vibrations and is thermay sensitive, whereas Briouin scattering, caused by attice vibrations, is sensitive to temperature and strain. Spatiay resoved measurement of the temperature and strain distributions aong an optica fiber can be reaized by anayzing the Briouin backscattering signa produced by a aser puse propagating down the fiber. Intrusion detection systems reying on such measurements can ensure the security of perimeters that are kiometers ong. Briouin backscattering in optica fibers produces Stokes and anti-stokes components that are shifted by ±11-13 GHz from the probe aser optica frequency. Their frequency shift and intensity provide information on the fiber strain and temperature. As spontaneous Briouin components are weak (20 db weaker than Rayeigh backscattered ight), the Briouin scattering signa can be enhanced using a pump-probe configuration, where two aser beams counter-propagate into the sensing fiber, an approach termed BOTDA (Briouin Optica Time Domain Anaysis) 3. The pump induces gain at the Briouin frequency, which depends on the fiber temperature/strain, and the probe is ampified when its frequency matches the Briouin frequency. By sweeping the probe frequency, the Briouin gain profie is recovered and temperature/strain information is deduced with great accuracy and resoution. As the Briouin gain spectrum is narrow (tens of MHz), a aser with a inewidth of severa tens of khz and continuous waveength tunabiity over severa GHz is required. 2.4 Fiber optic rotation sensing High performance rotation sensing appications are addressed with fiber optic gyroscopes 4. Of the various configurations that have been investigated, the interferometric fiber optic gyroscope (I-FOG) has achieved the widest commercia success. It uses a coi of optica fiber severa hundreds of meters in ength as the rotation sensing eement. As iustrated in the eft of figure 2, ight from a source is spit with a couper and propagates in the fiber oop in opposite directions (cockwise (CW) and counter cockwise (CCW)). When the fiber oop rotates, the CW and CCW beams experience a different path ength and undergo a reative phase shift proportiona to the rotation speed. In a resonant fiber optic gyroscope (R-FOG), ight is recircuated into the fiber oop, as iustrated on the right in figure 2. The same sensitivity can then be achieved with a much shorter ength of fiber. In this case, the resonance pattern corresponding to the CW and CCW directions of propagation are frequency-shifted by an amount proportiona to the rotation speed to be measured. This frequency difference can be measured by ocking two different asers to a resonance of the CW and CCW patterns. Figure 2: Principe of interferometric (eft) and resonant (right) fiber optic gyroscope. The phase change induced by rotation is very sma and any parasitic effect affecting the differentia phase shift becomes a source of error. The main contributors are non-inear Kerr effect, Rayeigh and stimuated Briouin scattering, as we as differentia temperature and poarization changes aong the fiber spoo. Their effect can be minimized by using a

5 broadband ight source. On the other hand, R-FOGs require reativey narrow-inewidth asers for frequency ocking to the resonance patterns, which increases the importance of non-inear effects. The recent deveopment of hoow core fiber (HCF) may renew interest in the R-FOG design for compact and ight weight gyroscopes. These fibers have a noninear Kerr effect reduced by one or two orders of magnitude 5, are fives times ess sensitive to temperature and dispay a temperature birefringence dependency (for poarization maintaining version) hundreds of times ower than standard singe mode fibers. Moreover, they are much ess sensitive to radiation and magnetic perturbations. However, they present higher oss (not too probematic when using a few meters of fiber for the oop) and higher Rayeigh backscattering for the moment. Aso, I-FOGs made of HCF may benefit from narrower inewidth asers and improve their ong term stabiity Optica characteristics 3. LASER SPECIFICATIONS The required waveength of emission depends on the appication. Waveengths ess attenuated by the atmosphere are advantageous for hard target detection. Lasers emitting in the teecommunications C-band ( nm) are preferred for optica fiber sensors reying on erbium-doped fiber ampifiers and waveength mutipexing. The state of poarization of aser emission is of importance in interferometric sensors, since visibiity of the interference beats depends on the aignment of interfering fieds poarization-wise. Semiconductor asers produce ight that is ineary poarized to a high degree. Free-space appications typicay require aser beams that do not spread too fast during propagation, whereas optica fiber appications require aser beams that coupe we to the optica fiber. Near-diffraction imited semiconductor aser diodes are avaiabe, that can provide tens of miiwatts of optica power in the core of a singe mode fiber. Erbiumdoped fiber ampifiers (EDFA) and semiconductor optica ampifiers (SOA) are avaiabe to further increase these powers if needed. 3.2 System characteristics Defense and security sensing appications put a premium on asers that are efficient, sma, ight, rugged and immune to externa perturbations. Semiconductor asers are the most efficient. Their ightness and sma voume are aso unmatched, as we as their immunity against vibrations. 3.3 Noise characteristics High performance sensing appications require asers that are quiet, both in intensity and phase. The infuence of intensity and frequency noise on interferometric sensing can be iustrated by considering the simpe case of a homodyne interferometer maintained in quadrature. The measured output power resuts from the interference between optica fieds e ( t) = P + δ P ( t) e e 1 jφ ( t) j2πν t jφi ( t ) jφ ( t τ I ) j 2πν t e2 ( t) = P + δ P ( t τ I ) e e e, (1) where P is the average power assumed equa for both fieds, δp represents aser power fuctuations, ν is the optica frequency and φ the aser phase noise, whereas φ I is the interferometer phase shift and τ I a differentia deay resuting from a path mismatch. The interferometer is maintained activey in quadrature such that and δφ I (t)~ 0. The optica power eaving the interferometer is thus given by π φi ( t) = + δφi ( t) (2) 2 P ( t) = 2 P + δ P ( t) + δ P ( t τ ) ± 2 ( P + δ P ( t))( P + δ P ( t τ )) sin( δφ ( t) + φ ( t τ ) φ ( t)). (3) out I I I I The ± sign depends on which output arm of the Micheson or Mach-Zehnder interferometer is considered. Ideay, baanced detection of the power eaving both arms shoud be performed, doubing the phase-sensitive power and eiminating common power terms. It is often not possibe to do so in practice, the detected power thus being equa to

6 P ( t) = 2 P + δ P ( t) + δ P ( t τ ) + 2 ( P + δ P ( t))( P + δ P ( t τ )) sin( δφ ( t) + φ ( t τ ) φ ( t)). (4) out I I I I Assuming ow intensity and phase noise, this expression becomes to first order P ( t) 2 P + δ P ( t) + δ P ( t τ ) + 2 P ( δφ ( t) + φ ( t τ ) φ ( t)). (5) out I I I Interferometer phase fuctuations δφ I (t) are deduced from measured power variations with a proportionaity constant equa to the tota average power 2P. Other power fuctuations wi transate into a detected phase noise equa to δ P ( t) + δ P ( t τ I ) δφnoise ( t) + [ φ ( t τ I ) φ ( t) ]. (6) 2P The first term represents detected phase noise caused by the aser RIN. The second term in brackets resuts from aser phase noise and vanishes when paths are perfecty matched (τ I = 0). The RIN-reated phase noise can be expressed as a convoution with impuse response characterized by power transfer function δφ RIN ( t) = δ P ( t) g( t) (7) 1 g( t) = ( ( t) ( t I )) 2P δ + δ τ, (8) 2 2 cos ( πτ f ) I G( f ) =. (9) P The power spectra density of the detected phase noise is thus reated to the power spectra density of aser power fuctuations according to 2 S ( f ) Sδφ ( f ) = cos ( πτ f ) = cos ( πτ f )10. (10) RIN 2 δ P 2 RIN ( f )/10 I 2 I P On the other hand, the detected phase noise resuting from the aser phase noise can be written as δφ ( t) = φ ( t τ ) φ ( t) = 2 π δν ( t ) dt, (11) φ I t τ I where δν is the aser instantaneous frequency fuctuation. This integration is equivaent to ineary fitering with a transfer function G(f) such that 2 2 sin ( ) ( ) 4 f I 2 G f t πτ =. (12) f The power spectra density of the detected phase noise resuting from the aser frequency noise is thus equa to 2 sin ( πτ I ) S ( ) 4 f δφ f = S ( f ) δν 2 δν. (13) f Sow aser phase variations with a period onger than differentia deay τ I impact ess the detected phase. On the other hand, according to expression (10), the differentia deay has itte infuence on the detected phase noise resuting from aser RIN.

7 4. NARROW-LINEWIDTH SEMICONDUCTOR LASERS Teecommunications-grade DFB semiconductor asers offer inewidths beow 1 MHz and powers exceeding 80 mw in an optica fiber. Their frequency noise is dominated at ow frequencies by ficker (1/f) noise, which contributes significanty to their inewidth 7,8. By propery sensing this frequency noise and using a servo-oop correcting the aser injection current, the ficker noise can be reduced with a concomitant dramatic decrease of the inewidth 9. In this section, the main characteristics of state-of-the-art semiconductor asers are reviewed, from the singe packaged aser chip and its contro eectronics to more compex modues such as inewidth-reduced asers. 4.1 DFB semiconductor asers Distributed Feedback (DFB) asers incorporate a Bragg grating, with one or mutipe phase shifts 10, that favors osciation on a singe ongitudina mode. They exhibit typicay inewidths in the MHz range but optimized devices are avaiabe with inewidths of a few hundreds of khz. These asers are of great interest for appications discussed in this paper because of their fied-proven reiabiity and ifetime, ow power consumption together with monoithic and compact design. They present very good reative intensity noise, which is an advantage compared to fiber asers that exhibit a arge RIN peak (~ 110 dbc/hz) beow 1 MHz. Lasers with a fiber output power greater than 100 mw in the 1550 nm range can be found commerciay. Their frequency can be tuned by a few hundreds of GHz through temperature contro or tens of GHz using the injection current. Direct moduation is aso possibe at frequencies up to severa GHz. Very ow noise eectronics for the temperature contro and current source driving the semiconductor aser are critica to minimize frequency and intensity noise. The ocking oop of the temperature controer must be adjusted propery to maintain the aser temperature without osciations that coud degrade the aser free-running frequency stabiity. The aser inewidth is highy dependent on the noise of the current source driving the semiconductor aser. High performance eectronics have been devised and are part of TeraXion s soutions. TeraXion has deveoped a very sma form factor modue integrating a butterfy-packaged DFB aser and the aforementioned contro eectronics. It is driven at constant temperature and injection current, both parameters being adjustabe via a software interface. An externa input aows direct moduation of the aser injection current up to frequencies of 40 MHz. A typica reative intensity noise measured on a aser modue is shown in Figure 3. It presents a 1/f behavior up to about 100 khz and remains fat at a eve cose to 160 dbc/hz from 100 khz up to 1 GHz. Aso shown is the intensity noise measured on a commercia DFB erbium-doped fiber aser, which dispays a sizabe reaxation resonance around 250 khz DFB fiber aser DFB semiconductor aser RIN (dbc/hz) E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 Frequency (Hz) Figure 3: Reative intensity noise of a DFB semiconductor aser modue and of a commercia DFB fiber aser.

8 Figure 4 compares the power spectra densities of the frequency noise (PSDFN) measured on a aser modue and a commercia DFB fiber aser. Frequency wise, the fiber aser is ceary ess noisy. The non panar ring Nd:YAG osciator is aso quieter but does not emit at a waveength compatibe with EDFA ampification. The semiconductor aser dispays ficker noise at ow frequency and white frequency noise beyond 100 khz. The corresponding optica spectrum of the semiconductor aser modue is presented in Figure 5. Typica inewidths of TeraXion aser modues are khz. Other curves presented in Figure 4 and Figure 5 reate to the semiconductor aser controed by a inewidth-reduction feedback oop and wi be discussed in section 4.2. PSDFN (Hz 2 /Hz) 1.E+12 1.E+11 1.E+10 1.E+09 1.E+08 1.E+07 1.E+06 1.E+05 1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 1.E-01 DFB semiconductor aser Narrow-inewidth DFB semiconductor aser DFB fiber aser 1.E-02 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 Frequency (Hz) Figure 4: Power spectra densities of frequency noise (PSDFN) measured on a semiconductor aser modue, either free-running or controed by a inewidth-reduction feedback oop, and on a commercia DFB fiber aser Free-running NLL Optica power density (a.u.) Frequency (Hz) x 10 5 Figure 5: Optica spectra of the semiconductor aser modue, with and without inewidth reduction. 4.2 Linewidth-narrowed DFB semiconductor asers Various techniques have been used to further narrow the emission inewidth of DFB semiconductor asers. Optica feedback consists in returning part of the aser emission in the diode with a narrow band externa refector (a diffraction or a Bragg grating), which forces the aser to emit ight at the seected waveength 11. This technique is aso used with Fabry-Perot semiconductor asers. A imiting case is the externa cavity aser (ECL), where the externa refector faces an anti-refection coated facet of the diode and thus performs as a mirror of the aser cavity. Optica feedback aows a wideband reduction of the frequency noise and a inewidth in the few khz to few tens of khz but may require a contro

9 of the feedback phase. The avaiabe output power of the aser is usuay ower after impementing this technique. Another concern is the occurrence of mode hops when the aser is sighty tuned through its temperature or injection current. In some cases, these mode hops can be avoided by simutaneousy changing the current and the temperature using caibration tabes. Finay, the monoithic character of the semiconductor aser cavity is ost, which makes the aser more sensitive to vibrations. Aternativey, the aser inewidth can be reduced using a frequency discriminator and negative eectrica feedback, a technique that preserves the monoithic character of the aser cavity. In this technique, the injection current of a DFB semiconductor aser is used to maintain the optica frequency of emission on the side of the spectra response of a narrow optica fiter 9, 12. The fiter, acting as a frequency discriminator, converts the aser frequency noise into intensity fuctuations, which are measured by a photodetector, and a frequency correction signa is sent back to the aser injection current. In cosed oop operation, this aows a strong reduction of the optica frequency fuctuations. The aser therefore inherits the ow frequency noise of the reference within the bandwidth of the ocking oop, whereas the frequency noise outside the oop bandwidth remains that of the free-running aser. This technique does not invove sensitive optica feedback and does not ead to mode hops when the frequency discriminator is sighty tuned. Impementation of this scheme is iustrated in Figure 6. Part of the ight coming from a DFB semiconductor aser is aunched into the frequency discriminator using an optica couper. The detection signa of the ight fitered by the frequency discriminator is compared to an optica power reference signa provided by the interna photodiode of the DFB aser package. This renders the system operation insensitive to aser power fuctuations. The ampitudes of these two signas are adjusted to provide a nu error signa when the aser frequency is centered in the side of the transmission or refection profie of the frequency discriminator. A oop fiter generates a frequency correction signa that is appied to the DFB aser injection current. When the ocking oop is enabed, the DFB aser frequency tracks the frequency of the FBG haf-transmission/refection point. In order to obtain the best phase noise performance from the aser, the ocking oop is optimized to yied the maximum ocking bandwidth possibe considering the FM response of the aser and the oop deays. In the current set-up, a bandwidth of approximatey 1 MHz is typicay achieved 9. Figure 6: Schematic iustration of the inewidth reduction system. The frequency discriminator used in our system is a speciay designed π-phase shifted fiber Bragg grating (FBG). Such FBG provides a narrow transmission notch centered in a broad refective band. The FBG is designed with a grating-free centra region that reduces the intensity buid-up compared to that observed in a standard π-shifted FBG of equa ength 13. In a standard π-shifted FBG, the intensity peaks at the center of the fiter where the phase shift is ocated (see Figure 7, top eft). This coud make the fiter spectra response sensitive to optica power through heating. In the inewidth reduction system, this is ess of an issue since ony a sma part of the aser power (ess than 1 mw) is directed to the FBG. However, introducing a grating-free region enabes designing a shorter but more refective FBG, thus maintaining the width of the transmission notch, without significanty increasing the intensity buid-up. This is an important benefit since the frequency discriminator is the most vibration-sensitive eement of the narrow-inewidth aser assemby, as discussed in section 4.3. This aowed shortening the FBG from 34 to 8.5 mm whie maintaining the width of the transmission notch at 15 MHz. The spatia apodisation profies and spectra characteristics of both fiters are shown in Figure 7. These FBGs are written in poarization maintaining (PM) fiber.

10 Figure 7: (eft) Apodization and intensity profies, (center) refection and transmission spectra, and (right) transmission and group-deay spectra over the notch for a 34 mm-ong π-shifted FBG (top row) and an 8.5 mm-ong π-shifted FBG with a grating-free centra region (bottom row). T, R: Transmissivity, refectivity. Figure 4 (bue curve) shows the effect of the eectrica feedback on the power spectra density of the aser frequency noise (PSDFN). When the inewidth reduction system is activated, the frequency noise is significanty reduced over severa hundreds of khz, by up to 2-4 orders of magnitude. The corresponding optica spectrum of the aser, computed from the measured PSDFN, is shown in Figure 5. The FWHM of the semiconductor aser spectrum is narrowed from about 350 khz to better than 1.8 khz when estimated for a measurement time of 1 ms. The coherence ength is improved from 200 m up to 15 km as shown in Figure 8. Figure 8: Coherence of a narrow-inewidth semiconductor aser. Using the sma-form factor FBG aowed a very rugged and compact narrow-inewidth aser modue to be designed. The size of the modue is 20 x 64 x 90 mm. As optica fiber sensing systems typicay require mutipe aser sources, a mutichanne patform that eases the management of mutipe asers operating at different waveengths has been deveoped,

11 which houses up to ten narrow-inewidth asers in a 19" by 2U frame. A power suppy and a contro modue compete the system that is operated using appication software deveoped by TeraXion. Both modues are shown in Figure 9. Figure 9 : Compact narrow-inewidth aser modue comprising the ow-noise contro eectronics and the negative eectrica feedback system aong with a muti-channe patform comprising ten narrow-inewidth DFB semiconductor asers. 4.3 Immunity to acoustic vibrations In the schemes described above, the eement most sensitive to vibrations is the FBG used as the frequency discriminator. This sensitivity was characterized by mounting the FBG package onto a mechanica shaker. A sinusoida excitation of 2.2 g was appied to the shaker and the effect on the power spectra density of the aser frequency noise was measured. This was repeated with excitation signas of different frequencies. The resuts are shown in Figure 10 in comparison with the resuts obtained with a 34-mm ong packaged FBG, which was used in previous impementations. The sensitivity of a commercia DFB fiber aser is aso shown for comparison. Reducing the fiter ength resuted in a significant improvement in the acoustic isoation efficiency. The resuts shown in Figure 10 dispay the normaized frequency deviation induced by the shaker, in MHz/g. It must be noted that the frequency deviation measured with the 8.5-mm ong FBG was sometimes imited by the noise foor of the measurement system. In a second experiment, the sinusoida excitation was repaced by a random vibration profie (in the Hz range). The resuts shown in Figure 11 ceary demonstrate a very ow sensitivity of the inewidth-narrowed aser to externa vibrations, cose to that of the unperturbed aser (grey curve) 13. Such a system is therefore very we adapted for operation in harsh environment. 1 Sensitivity to vibrations (MHz/g) mm FBG 34 mm FBG Commercia fiber aser Frequency (Hz) Figure 10 : Sensitivity to vibrations measured point-by-point with a sinusoida excitation of different frequencies.

12 1.E+10 1.E+09 1.E mm FBG 34 mm FBG No vibrations Commercia fiber aser PSDFN (Hz 2 /Hz) 1.E+07 1.E+06 1.E+05 1.E+04 1.E+03 1.E+02 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 Frequency (Hz) Figure 11 : Power spectra density of the aser frequency noise submitted to a random vibration profie. 4.4 Offset-frequency phase-ocked DFB semiconductor asers The inewidth narrowing technoogy by frequency-ocking to a frequency discriminator can be modified to phase-ock a semiconductor save aser to a master aser with a frequency offset 14. In this case, the error signa is derived by mixing the signa of the beat note between the two asers with the signa from a precision RF synthesizer that sets the frequency offset between the asers. A Laser Synthesizer reying on this principe of operation was manufactured for, among others, the Atacama Large Miimeter Array (ALMA) internationa radio astronomica faciity currenty under construction in Chie. ALMA requires a highy stabe LO reference signa that can be tuned between GHz and distributed to each of the antennae through optica fiber. Antennae can be up to 15 km away from the centra buiding. Other appications for the Laser Synthesizer incude Radio-over-fiber and RF photonics in genera requiring transmission of high frequency RF signas over sizabe distances with very ow residua phase noise 15. A simpified schematic of the Laser Synthesizer instrument is presented in Figure 12. The ight from a semiconductor aser (SCL) acting as a save aser is combined with the master aser output in a 50/50 PM couper. Two identica copies of the dua waveength signa are therefore obtained: one is used to perform the interna phase ocking, whie the second is the optica output of the instrument. The beat frequency between the master and the seected save asers is detected by a high speed photodetector. A ow noise RF synthesizer drives an active harmonic mixer to down convert the miimeter wave beat note at an IF frequency of 125 MHz. Since the down conversion is done using the 2nd, 5th or 7th harmonic of the RF synthesizer signa, its frequency remains in the GHz range whie the optica waves can be spaced by as much as 122 GHz. The IF frequency is phase-compared to a 125-MHz IF reference provided by a fixed-frequency ownoise osciator. The phase error signa is sent to a oop fiter which appies a correction to the injection current of the aser to cose the phase ocking oop. To increase the oop bandwidth, part of the error signa is aso sent to an eectrooptic phase moduator paced in the path of the save aser, cose to the beam combining couper. For the ALMA radio-teescope system, the master aser is not a semiconductor aser, but rather a high performance frequency-stabiized fiber aser deveoped especiay for this project. At each antenna, the photonic LO reference signa is recovered by detecting the beat note between the two optica waves using a high-speed photodetector. In order to aow the radio teescope to achieve the desired imaging resoution, the tota deay fuctuations of the LO reference provided by the Laser Synthesizer must not exceed 27 fs rms over the 65 to 122 GHz band, after a 1 MHz ow-pass fiter is appied to the beat signa. This corresponds to a phase noise of 1x10-4 rad 2 (11 mrad) at 65 GHz and 4x10-4 rad 2 (20.6 mrad) at 122 GHz. Figure 13 shows the absoute phase noise for different offset frequencies between the asers (f b ), with and without the appication of the 1 MHz ow-pass fiter.

13 125 MHz (IF reference) Figure 12 : Phase-ocking of two semiconductor asers with a variabe offset frequency. Absoute Phase Noise (mrad) Fitered Not fitered fb (GHz) Figure 13 : Integrated phase noise from 10 khz to 10 MHz on beat note frequency (crosses: no fiter, dots: 1 MHz fiter). 5.1 Architecture 5. A CASE STUDY: TIME-MULTIPLEXED SENSOR ARRAY A preferred approach for mutipexing optica fiber sensors, which minimizes hardware requirements, is iustrated in Figure 14. The output of a narrow-inewidth aser is moduated as a train of ow duty factor puses at sensing frequency f s. Each puse is spit in two sub-puses, one being deayed by T and frequency-shifted by f b for heterodyning. The subpuses are refected by a series of in-ine Micheson interferometers with matching round-trip path deay T. Refection of the eading sub-puse by mirror 1 and of the agging sub-puse by mirror N+1 produces unmoduated puses in the return signa. A other refections fi avaiabe time sots with N puses that are intensity-moduated at frequency f b because of the superposition of the sub-puses from a given sensing period. The eectrica detection bandwidth must accommodate the received puses to avoid distortion and overapping and is much arger than the samping frequency of each sensor f s when a arge number of sensors are mutipexed. Anti-aiasing fitering is thus not possibe and high frequency noise gets aiased into base band.

14 5.2 Simuations Figure 14 : Time-mutipexed sensor array. Figure 15 presents the mode used to evauate the effect of aser RIN and frequency noise in such a system. Interference beats at 68 khz are detected and fitered according to the power transfer function given in Figure 16, designed to accommodate 100 ns puses. The fitered signa is then samped at 204 khz and mixed with eectrica osciators to generate quadrature component from which the phase of the interference beats is deduced. These parameters are representative of a system deveoped for ocean-bottom oi fied monitoring 16. The modeing was performed according to the genera approach outined in section 3.3 but taking into account the effect of high frequency noise aiasing and the phase extraction procedure iustrated in Figure 15. Rayeigh scattering taking pace aong fiber eads and detection noise were eft out of the mode. Figure 17 and Figure 18 present, for three differentia deays, the power spectra densities cacuated respectivey for a DFB fiber aser and a narrow-inewidth semiconductor aser using eectrica feedback. Resuts are presented at frequencies of interest for oi fied monitoring, i.e. from DC to 500 Hz. The phase noise spectrum shows itte dependence on the interferentia deay in the case of the fiber aser, indicating that aser RIN is the dominant contributor in this case. The predicted phase noise spectra density (24 µrad 2 /Hz) corresponds cosey to measurements obtained with a fiber aser in a system once Rayeigh scattering was suppressed 16. In the case of the narrow-inewidth semiconductor aser, the phase noise spectra density varies as the square of interferentia deay τ I, indicating that aser phase noise is dominant. According to these simuations, the narrow-inewidth semiconductor aser outperforms the fiber aser for interferentia deays smaer than 0.35 ns. Figure 15 : Mode of a time-domain mutipexed sensor array used to evauate the effect of aser RIN and phase noise on the demoduated phase.

15 Figure 16 : Eectrica fiter transfer function. Figure 17 : Power spectra density of the detected phase noise cacuated with a DFB fiber aser. Figure 18 : Power spectra density of the detected phase noise cacuated with a narrow-inewidth DFB semiconductor aser. 6. CONCLUSIONS Semiconductor asers are very attractive sources for defense and security appications, given their high efficiency, sma voume and weight and their immunity to vibrations. For some high performance interferometric sensing appications, DFB semiconductor asers dispay a troubesome eve of frequency noise. Distributed-feedback fiber asers outperform semiconductor asers from this point of view, but they are more sensitive to vibrations. Fortunatey, the frequency noise at ow frequencies of a DFB semiconductor aser can be sizaby reduced by ocking to a fiber Bragg grating frequency

16 discriminator: inewidth reduction to sub-khz vaues is thus achievabe. Such narrow-inewidth semiconductor asers dispay very good immunity against vibrations when propery packaged. This inewidth reduction approach eaves unatered the frequency noise at high frequencies, which can prove detrimenta in time-mutipexed sensor arrays where aiasing takes pace. Care must then be taken to minimize the differentia path deay between interfering fieds. This inewidth reduction technoogy makes avaiabe the efficient, compact and rugged DFB semiconductor aser for interferometric sensing appications. For exampe, TeraXion narrow-inewidth aser has met the optica requirements set forth by NASA Langey Research Center for a FMCW idar being deveoped for enabing precision safe anding on the Moon and Mars under the ALHAT (Autonomous Landing and hazard Avoidance Technoogy) project 17. This technoogy can aso be adapted for spectroscopic appications where the ong-term stabiity of the average frequency of emission is of concern, by ocking the aser to an absoute waveength reference such as a moecuar ine of absorption 18. Two semiconductor asers can aso be ocked to one another with a frequency offset for the generation of RF signas for microwave photonic appications 14,15. REFERENCES [1] Pierrottet, D., Amzajerdian, F. and Peri, F., "Deveopment of an a-fiber coherent aser radar for precision range and veocity measurements", MRS Symposium Proc. 883, (2005). [2] Kirkenda, C. K. and Dandridge, A., "Overview of high performance fibre-optic sensing", J. Phys. D: App. Phys. 37(18), R197-R216 (2004). [3] Kurashima, T., Horiguchi, T. and Tateda, M., Distributed-temperature sensing using stimuated Briouin scattering in optica siica fibers, Opt. Lett. 15(18), (1990). [4] Barbour, N. and Schmidt, G., "Inertia sensors technoogy trends", IEEE Sensors J. 1(4), (2001). [5] Ying, D., Demokan, S., Zhang, X. and Jin, W., "Anaysis of Kerr effect in resonator fiber optic gyros with trianguar wave phase moduation", App. Opt. 49(3), (2010). [6] Kim, H. K., Digonnet, M. J. F., Gordon, S. and Kino, G. S., "Air-Core Photonic-Bandgap Fiber-Optic Gyroscope", J. Lightwave Tech. 24(8), (2006). [7] Kikuchi, K., Effect of 1/f-type FM noise on semiconductor-aser inewidth residua in high-power imit, IEEE J. Quantum Eectron. 25(4), (1989). [8] Stéphan, G. M., Tam, T. T., Bin, S., Besnard, P. and Têtu, M., "Laser ine shape and spectra density of frequency noise", Phys. Rev. A 71, (2005). [9] Ciche, J.-F., Aard, M. and Têtu, M., "High-power and utranarrow DFB aser: the effect of inewidth reduction systems on coherence ength and interferometer noise", Proc. SPIE 6216, 62160C (2006). [10] Whiteaway, J. E. A., Garrett, B., Thompson, G. H. B., Coar, A. J., Armistead, C. J. and Fice, M. J. "The static and dynamic characteristics of singe and mutipe phase-shifted DFB aser structures", IEEE J. Quantum Eectron. 28(5), (1992). [11] Tkach, R. W. and Chrapyvy, A. R., "Regimes of feedback effects in 1.5-µm distributed feedback asers", J. Lightwave Tech. LT-4(11), (1986). [12] Ohtsu, M. and Kotajima, S., "Linewidth Reduction of a Semiconductor-Laser by Eectrica Feedback", IEEE J. Quantum Eectron. 21(12), (1985). [13] Pouin, M., Painchaud, Y., Aubé, M., Ayotte, S., Latrasse, C., Brochu, G., Peetier, F., Morin, M., Guy, M. and Ciche, J.-F., Utra-narrowband fiber Bragg gratings for aser inewidth reduction and RF fitering, Proc. SPIE 7579, 75791C (2010). [14] Ciche, J.-F., Shiue, B., Têtu, M. and Pouin, M., "A 100-GHz-tunabe photonic miimeter wave synthesizer for the Atacama Large Miimeter Array radioteescope", IEEE/MTT-S Int. Microwave Symp., (2007). [15] Middeton, C. and DeSavo, R., "Baanced coherent heterodyne detection with doube sideband suppressed carrier moduation for high performance microwave photonics inks", Avionics, fiber-optics and photonics technoogy conference (AVFOP), (2009). [16] Rønnekeiv, E., Waagaard, O. H., Thingbø, D. and Forbord, S., "Suppression of Rayeigh scattering noise in a TDM mutipexed interferometric sensor system", Optica fiber communication conference (OFC), paper OMT4 (2008). [17] Amzajerdian, F., private communication (2010). [18] Latrasse, C., Breton, M., Têtu, M., Cyr, N., Roberge, R. and Vieneuve, B. "C 2 HD and 13 C 2 H 2 absorption ines near 1530 nm for semi-conductor aser frequency ocking", Opt. Lett. 19(22), (1994). View pubication stats