Narrow Linewidth near-UV InGaN Laser Diode based on External Cavity Fiber Bragg Grating
Antoine Congar, Mathilde Gay, Georges Perin, Dominique Mammez, Jean-Claude Simon, Pascal Besnard, Julien Rouvillain, Thierry Georges, Laurent Lablonde, Thierry Robin, Stéphane Trebaol
LLetter Optics Letters 1
Narrow Linewidth near-UV InGaN Laser Diode based onExternal Cavity Fiber Bragg Grating A NTOINE C ONGAR , M ATHILDE G AY , G EORGES P ERIN , D OMINIQUE M AMMEZ , J EAN -C LAUDE S IMON , P ASCAL B ESNARD , J ULIEN R OUVILLAIN , T HIERRY G EORGES , L AURENT L ABLONDE ,T HIERRY R OBIN , AND S TÉPHANE T REBAOL Univ Rennes, CNRS, Institut FOTON - UMR 6082, F-22305 Lannion, France Oxxius, 4 rue Louis de Broglie, 22300 Lannion iXblue, rue Paul Sabatier, 22300 Lannion * Corresponding author: [email protected] February 9, 2021
We realize a fiber Bragg grating InGaN based laserdiode emitting at 400 nm and demonstrate its highcoherency. Thanks to the fabrication of a narrowband fiber Bragg grating in the near-UV, we can reachsingle-mode and single-frequency regimes for the self-injection locked diode. The device exhibits 44 dB side-mode-suppression-ratio and mW output power. De-tailed frequency noise analysis reveals sub-MHz inte-grated linewidth and 16 kHz intrinsic linewidth. Sucha narrow linewidth laser diode in the near-UV domainwith a compact and low-cost design could find appli-cations whenever coherency and interferometric resolu-tions are needed. © 2021 Optical Society of America http://dx.doi.org/10.1364/ao.XX.XXXXXX
The InGaN-based laser diodes (LD) market is mainly drivenby the Blu-ray industry, which requires powerful and low-costsources. The technology is now mature, providing reliable laserproducts in the blue/violet range and extends to UV. However,the need for narrow linewidth LDs is growing in a variety ofdomains ranging from industrial to metrological applications,where linewidth requirements extends widely from tens of GHzto sub-MHz. Compact and low-cost LDs are mandatory toaddress applications such as visible light communication [1],underwater LiDAR sensing [2], 2D and 3D holographic storage[3] and industrial spectroscopy [4]. Furthermore, fundamentalspectroscopic applications, optical clocks, atom cooling andatom interferometry applications require highly coherentsources with accurate wavelength for the pumping of particulartransitions or probing hyperfine atomic structures [5, 6]. Thoseapplications would benefit from low-cost, compact and highlycoherent LD. The research on coherence properties of GaNedge-emitting LDs is still in its infancy. Two main designs areconsidered in the literature to reach single-longitudinal-mode(SLM) regime. The first design called "monolithic approach"relies on the ridge waveguide effective index modulation.Recently, first electrically pumped SLM laser diode using ahigh order grating has been demonstrated [7]. In this work, the authors have reported 35 dB side-mode suppression ratio(SMSR). Despite encouraging results, this approach is not yetmature for mass production and requires demanding technicalresources such as high-resolution e-beam lithography.In a second approach, SLM operation is achieved thanks tothe filtering function of an external cavity. External-cavitydiode lasers (ECDL) are composed of a Fabry-Perot LD and anexternal mirror providing an optical feedback. Moreover, bytransferring its spectral purity to the diode, the use of higherquality factor cavities can induce drastic spectral narrowingenabling single-frequency (SF) operation. These cavities aretypically high finesse Fabry-Perot resonators [8] or whisperinggallery mode resonators [9]. In most commercially availableECDLs, optical feedback is provided by a diffraction grating[10] whose position and angle with respect to the LD shouldbe accurately controlled. These ECDLs are thus expensiveand quite large devices because they require implementationof high-quality electromechanical components and expensiveanti-reflection coating LDs to reach their performances.An alternative, called fiber Bragg grating LD (FGL) scheme,has been proposed and extensively studied with applicationsin the telecom band [11]. Here, the LD is coupled to a narrowband fiber Bragg grating (FBG). The stability lies on the in-fiberoptical function and no mechanical component is needed like inconventional ECDL [12]. The main advantage of FGL devicesis their versatility. By design, the Bragg mirror characteristics(center wavelength, peak reflectivity and bandwidth) canbe easily and accurately modified so that external cavityparameters can be tailored to optimize SLM operation, wherehigh SMSR and high optical output power are mandatory, orSF operation where narrow linewidth is requested in particularfor metrological applications. In the following, we considerthat the SLM operation is obtained when one mode of the laserdiode is selected by the Bragg mirror and reflected back in thelaser. On top of that, to reach SF operation, a careful tuning ofthe external cavity length should be done to put in phase oneexternal cavity mode with the previously selected LD mode.Then strong narrowing of the laser emission can be observed.This two regimes address the large range of linewidth satisfyingthe application needs expressed above. a r X i v : . [ phy s i c s . op ti c s ] F e b etter Optics Letters 2 In this paper, we demonstrate near-UV (NUV) FGL exhibitingstable SLM emission at 400 nm with 1.3 mW optical power,44 dB SMSR and intensity noise (IN) below -130 dB/Hz above10 kHz. SF operation can even be obtained by accurately settingup the FBG cavity. In this last configuration, we report intrinsiclinewidth down to 16 ± Fig. 1.
Experimental setup. The device under test (DUT) iscomposed of the LD, optical beam shaping lenses and a fiberBragg grating (FBG). The external cavity length (about 7 cmlong) is adjusted using a piezo actuator (PI1). Path 1 is usedfor intensity noise (IN), low resolution spectrum using anoptical spectrum analyzer (OSA) and power measurements(POW). For OSA and POW measurements, a fraction of thesignal is extracted. On path 2, the movement of one mirrorof the Fabry-Perot etalon (FPE) is used for laser line scanningand frequency noise measurement, thanks to a piezo actuator(PI2). Electrical signals generated by photodiodes (PD) aresent to an oscilloscope (OSC) and an electrical spectrum an-alyzer (ESA) through transimpedance amplifiers (TIA) andDC-blocks (DCB). DC photocurrents are measured using amultimeter (A). Coupling of light in the fiber and collimationof the output beam are made using APC fiber couplers (C).cavity is based on a Bragg mirror, photo-inscribed in the core ofa single-mode fiber. To eliminate parasitic reflections, fiber endsare polished at an angle of 8° (APC connectors). The overalllength of the cavity is 7 cm, including the free space section used for beam-shaping and the few centimeters fiber sectionextending to the Bragg mirror. The Bragg mirror inscriptionrelies on the photosensitivity of germanosilicate single-modefiber (core diameter 2.4 µ m at 400 nm). The fiber is transversallyexposed to a UV fringe field. No phase-mask is commerciallyavailable to reach Bragg wavelengths lower than 405 nm bydirect inscription technique. We thus used a Talbot interfer-ometer allowing Bragg mirrors with reflectivity wavelengthswithin the range 375-405 nm [13]. To our knowledge, it is thefirst realization of FBG at such short Bragg wavelengths.To ensure the selection of only one longitudinal mode of theLD cavity, the 3 dB-reflection-bandwidth of the FBG has to besmaller than the FSR. We then designed the FBG bandwidthto be around 20 pm. Moreover, to reach strong feedbackregime [14], the following expression should be satisfied : I R Bragg > R LD , where I = R LD = R Bragg the Bragg mirror reflectivity. Coefficient R Bragg should thus begreater than 0.4. Hence, the choice of a low-cost non-AR coatedLD implies the use of a quite high reflectivity Bragg mirror.Thus, the peak reflectivity of the Bragg mirror is chosen to be R Bragg = Fig. 2.
Spectrum of the solitary laser diode (black curve) andFGL (red curve) for a pump current close to 95 mA. Inset) L-Icurves of the solitary laser diode (black curve) and FGL (redcurve).central wavelength is close to 400.5 nm and the spectrumspreads over (cid:39) etter Optics Letters 3 maximum, in order to maximize the SMSR. In our experimentalconditions, we obtained an SMSR of 44 dB and an optical powerof 1.3 mW as depicted in figure 2. It is to notice that the finestructure of the spectrum is not resolved by the 10pm resolutionOSA used and characterisations are completed by furthermeasurements as described in the following. Comparable SMSRperformance, close to 40 dB, has been obtained with a bulkdiffraction grating configuration in reference [16]. A value of25 dB has been measured for an equivalent structure withoutAR coating in the telecom range [17]. It is to notice that bestresults for SLM lasers obtained by the monolithic approachat NUV wavelengths are 25 dB SMSR [7]. Futhermore, FBGstretching can be used to tune the SLM central frequency over0.5 nm with mode hops between adjacent LD longitudinalmodes separated from an FSR. To this aim, one FBG extremity isfixed, and the other is attached on a translation plate connectedto a piezo actuator. Applying a longitudinal strain onto the fibershifts the FBG wavelength with a sensitivity of 0.3 pm/µstrain.This configuration provides stable SLM operation over a periodof hours. The device fulfills requirements for SLM applications.To get single-frequency (SF) operation, the external cav-ity length must be precisely controlled in order to reach goodspectral overlap between a mode of the LD cavity and a modeof the external cavity. Figure 1 displays the linewidth and
Fig. 3.
FGL linewidth estimation obtained by characteriz-ing the Fabry-Perot etalon (FPE) transmission for 63 mA ofpump current. The red dashed line is a Lorentzian fit of themeasurement. One FPE FSR is shown in the inset to ensuresingle-frequency operation and for frequency graduation ofthe abscissa axis. During FN measurement, the FPE, used as afrequency discriminator, is maintained at the quadrature point(blue point) where response is linear (blue linear fit).frequency noise measurement benches we used. The laserfibered output is separated into two paths. Path 1 is used tomeasure intensity noise, optical power and low-resolutionspectrum while path 2 gives access to frequency noise andreal-time high-resolution spectrum measurement. For noisemeasurements (IN and FN), the signal is focused on a photo-diode and measured as a power spectral density (PSD) usingan electrical spectrum analyzer (ESA) through a variable gaintransimpedance amplifier.High resolution spectrum measurement is performed using a Fabry-Perot etalon (FPE) (FSR 1GHz, finesse>500) on path2. A FPE resonance is scanned over the laser line, applying avoltage ramp to a piezoelectric actuator and the output signal isobserved as a function of time on an oscilloscope.Measurement of one FSR of the 1 GHz FP etalon (inset infigure 3) is used to ensure single-frequency operation and scalethe abscissa axis as a function of frequency. From a close look onthe laser line shown on figure 3, the linewidth is estimated to be ∆ ν =2.4 MHz, which corresponds to the FPE resolution.We then performed FN characterization to get more insights onthe SF laser frequency dynamics and obtain an estimation of thelaser linewidth for selected integration times. The FP voltageramp is removed and the moving mirror is now connected tothe output of a PID controller, which parameters are set in sucha way that the laser is maintained at the quadrature point (bluedot in figure 3). In such a configuration, intensity fluctuations ofthe output signal are proportional to frequency fluctuations ofthe input signal and the proportionality coefficient is given bythe resonance flank slope ( P = ± Fig. 4.
Frequency noise measurement. Peaks in the curve aredue to external perturbations. The green dashed line showsthe low frequency 1/ f trend, the red dashed line the highfrequency plateau. The servolocking bandwidth is limited to20 Hz to insure no parasitic contributions to the FN measure-ment.In a laser, frequency noise can usually be described by twomain contributions (see Fig. 4) expressed by S ν = h α f α + h . Ahigh frequency white noise, which is related to the Lorentzianshape of the intrinsic laser-linewidth (dashed red line). Thiswhite noise level is identified by the coefficient h . This noise isdue to the random phase fluctuation of spontaneous emission[19]. At lower frequencies the device is under influences ofvarious contributions like acoustic, thermic and electromagneticperturbations. Their spectral signature usually follows a f − α evolution on the frequency noise with α bounded between 1 etter Optics Letters 4 and 2. This second contribution gives a Gaussian shape to theintegrated optical linewidth [20] that exhibits a Voigt profile inthe general case [21].FN measurement plotted in figure 4 is taken at pump currentI=63 mA. We find the two behaviors mentioned above. At lowfrequency (<10 kHz), the FGL FN displays a 1/ f tendencywith h = × Hz /Hz (green dashed line in the figure).Strong perturbations probably coming from the current sourcelead to an increase of FN beyond the 1/ f line (100 Hz-1 kHz).Above 10 kHz, the FN reaches a plateau at h = × Hz /Hz,corresponding to the white intrinsic noise of the laser.We use the beta-line approach [20] to estimate the integratedoptical linewidth of the device. Short-time (t=1ms) and longer-time (t=10ms) linewidths are 250 kHz and 950 kHz respectively.In this calculation, peaks from external perturbations have beenneglected. The former value is in the same order of magnitudethan the linewidth of 420 nm grating-ECDL laser presented in[6] and 405 nm self-injection locked LD in [9]. Using the relation ∆ ν = π h [20], we can estimate the intrinsic linewidth of thelaser to be 16 ± et al. [22] have reporteda similar value ( ≈
30 kHz) for self-injection locked LD at 370nm. ECDL frequency stabilizations based on external cavityoptical feedback [8, 23] allow to narrow the linewidth down tofew kHz but the large external cavity (meter long) and movingparts limit the observation time from minutes to seconds.To reach stable SF operation, a careful consideration should begiven to the setup. External perturbations upon the externalcavity are indeed detrimental to the device linewidth perfor-mances. Acoustic perturbations are drastically reduced usingan anti-vibration table and a Peltier module situated below theexternal cavity that provides a precise temperature stabilization.However, for the sake of compactness, improvements canbe implemented. To ensure stability of the device undersingle-frequency operation, the amount of reinjected light intothe LD after a round trip in the cavity must be as constantas possible. Because LD emission is linearly polarized, theuse of polarization-maintaining fiber during the light roundtrip may ensure higher performances without demandingimplementation of other external parts. The cavity lengthshould be reduced as much as possible to decrease externalperturbations on the fiber. However, a trade-off has to be foundbetween the length and the quality factor of this cavity [24].GaN based LDs still suffer from epitaxial growth imperfec-tions that revealed nonlinear gain behaviors [15]. Moreover,working in the strong feedback regime contributes to favor theappearance of nonlinear effects since a relatively large amountof light is reinjected in the laser diode. To observe SF regime,we thus operate the laser device at low power to prevent theapparition of unstable regimes. In a future work, optimizationof the Bragg reflectivity should allow to reach higher powerwhile maintaining SLM or SF regimes.We demonstrate a reliable, all-fibered optical output, compact,FBG NUV laser source. We have shown that single longitudinalmode operation can be achieved using non-antireflectioncoated GaN LD coupled to a fiber Bragg grating designedat 400 nm. We measured side-mode suppression ratio up to44 dB with 1.3 mW output power and an intensity noise levelbelow -130 dB/Hz above 10 kHz. Furthermore, transferringthe spectral purity of the external cavity to the diode leads tosingle-frequency operation with integrated sub-MHz linewidthand 16 ± FUNDING
The present work is supported under projects DeepBlueand UV4Life by the Region Bretagne (contracts N° 16008022,19005486) and the European Regional Development Fund (con-tracts N° EU000181, EU000998).
DISCLOSURES
The authors declare no conflicts of interest.
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