116 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 2, JANUARY 15, 2012

High-Power, Low RIN 1.55-μm Directly ModulatedDFB Lasers for Analog Signal Transmission

Mickaël Faugeron, Michaël Tran, François Lelarge, Mourad Chtioui, Yannick Robert,Eric Vinet, Alain Enard, Joël Jacquet, and Frederic Van Dijk

Abstract— We report the demonstration of high-power andlow relative intensity noise (RIN) directly modulated distributedfeedback lasers at 1.55 µm. We have developed a structurewith asymmetrical cladding to reduce internal losses due tothe p-doped upper cladding. We obtain an output power of140 mW at 550 mA and an RIN below −157 dB/Hz in the0.1–20 GHz range along with a high side-mode suppression ratio(>55 dB). The modulation bandwidth was larger than 7 GHz.The characteristics of these lasers, namely a high electrical tooptical conversion efficiency (0.340 W/A at 350 mA), low noise,high power (98 mW at 350 mA), and linearity, make them theperfect candidates for high gain, high dynamic directly modulatedanalog links.

Index Terms— 1.5-µm lasers, directly modulated lasers,distributed feedback lasers, high-power diode lasers, low noiselasers.

I. INTRODUCTION

M ICROWAVE systems use coaxial cables to transportelectrical microwave signal. This type of cable has lots

of disadvantages as huge losses (∼10 dB/km @1 GHz), impor-tant weight and bad immunity against electromagnetic envi-ronment effects. Deployment of optical analog links for radarsystems or electronic warfare is in the pipeline. Advantagesof optical links are mainly related to the use of optical fibers:low weight, low losses (even at high frequency), flexibilityof fiber and low electromagnetic sensitivity. But in order tocompete with coaxial cables, optical links require key elementswhich are the electrical to optical and optical to electricalconversion devices being respectively the laser diode and thephotodetector.

In this letter we present the development of a directly modu-lated laser optimized for analog signal distribution. Comparedwith an external modulation link, it simplifies the transmissionsystem to 3 elements: a laser, an optical fiber and a photodiode

Manuscript received June 15, 2011; revised October 7, 2011; acceptedOctober 13, 2011. Date of publication October 25, 2011; date of currentversion January 5, 2012. This work was supported in part by the FrenchDirection Générale de l’Armement in the frame of the PEA-ORGE Project.

M. Faugeron and M. Chtioui are with Thales Air Systems, Limours91470, France (e-mail: mickael.faugeron@3-5lab.fr; mourad.chtioui@thalesgroup.com).

M. Tran, F. Lelarge, Y. Robert, E. Vinet, A. Enard, and F. Van Dijk are withIII–V Laboratory, a joint Laboratory of Alcatel Lucent Bell Laboratories,Thales Research and Technology, and CEA-LETI, Palaiseau 91767, France(e-mail: michael.tran@3-5lab.fr; francois.lelarge@3-5lab.fr; yannick.robert@3-5lab.fr; eric.vinet@3-5lab.fr; alain.enard@3-5lab.fr; frederic.vandijk@3-5lab.fr).

J. Jacquet is with the Laboratoire Matériaux Optiques Photonique etSystèmes, SUPELEC/LMOPS, Metz 57070, France (e-mail: joel.jacquet@supelec.fr).

Digital Object Identifier 10.1109/LPT.2011.2173479

are just required. An analog link with a large dynamic rangeusing this type of emitter requires a high power, low noise andhighly linear laser [1].

A lot of work has been done in the past on large bandwidth,medium power directly modulated lasers for digital and analogsignal transmission [2], [3]. Impressive work was also doneon continuous wave (CW) high power lasers for externalmodulation (140 mW @ 400 mA, RIN ≈ −161 dB/Hz @860 MHz, 400 mA) [4], (125 mW @ 1 A, RIN <−165 dB/Hzon the 0.1–20 GHz range @ 1 A) [5], pumping (1.2 W @ 5 A,λ = 1.48 μm) [6], (860 mW @ 4.8 A, λ = 1.5 μm) [7]and high power external cavity lasers (370 mW @ 4 A,RIN < −160 dB/Hz on the range 200 kHz−10 GHz) [8].The challenge is to combine high output power (>100 mW)with high modulation bandwidth (>4 GHz) [1]. The goalof this study is therefore to develop high power, low RIN(< −155 dB/Hz) and high efficiency DFB lasers compatiblewith direct modulation up to 6 GHz.

II. DESIGN OF THE STRUCTURE

In order to increase the maximum power and the efficiencythat can be generated by a semiconductor laser it is importantto reduce the losses encountered by the optical mode. At1.5 μm major source of internal losses is due to p-dopedlayers of the structure [9]. In the last ten years people havebeen using asymmetrical cladding to increase the power of1480 nm pump diode lasers [10], [11], [6] and more recently1.55 μm diode lasers [7]. An asymmetrical cladding permitsto reduce optical overlap between the optical eigenmode andthe p-doped layers. On the other hand it decreases the opticalconfinement factor with the quantum wells. To counterbalancethis effect we must increase the number of quantum wells tohave a sufficiently high confinement with gain layers to avoida prejudicial threshold current increase. This will also leads toa larger relaxation frequency what, by mean of consequencegives a lower RIN and a larger modulation bandwidth ofsemiconductor lasers [12].

So we need to find a compromise in the design of the laserto get:

1) a sufficiently large number of wells and a high confine-ment in the quantum wells to get a large modulationbandwidth, a low RIN and also a low threshold current,

2) a sufficiently low optical confinement factor withp-doped layers to increase the output power and externalefficiency.

In this letter we present a structure optimized according tothe previous arguments with �QW = 2.0%, and �p−InP =

1041–1135/$26.00 © 2011 IEEE

FAUGERON et al.: HIGH-POWER, LOW RIN 1.55-μm DIRECTLY MODULATED DFB LASERS 117

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Fig. 1. Power and net efficiency versus bias current for 1-mm-long DFBlaser.

6.2% and internal losses αi = 3.5 cm−1. These values havebeen calculated using the effective index method to solvecavity eigenmodes.

III. LASER DESCRIPTION

The multiple quantum well (MQW) distributed feedback(DFB) structure was grown by Gas-Source Molecular BeamEpitaxy (GS-MBE) on n-InP substrates. The structure hasan asymmetrical cladding and the active region is composedof 9 undoped InGaAsP quantum wells with a 1.52 μmphotoluminescence peak.

After a first epitaxy, first order gratings were defined by e-beam lithography and inductively coupled plasma reactive ionetching. The grating layer thickness (35 nm) is optimized inorder to obtain a coupling strength KL ∼ 1.4. This low valueof KL should limit spatial hole burning and the associatedoptical power saturation. Re-growth of p-doped top claddingwas then also done by GS-MBE. The dual-channel laser ridge-waveguide was formed by ion beam etching followed by wetchemical etch and proton isolation. The ridge-waveguide is3.5 μm wide. This value was found to minimize the thermalsaturation while preserving lateral single-mode operation. Barswere cleaved to form 1 mm-long cavities. Facets were HighReflectivity (HR) and Anti Reflectivity (AR) coated. Chipscleaved from the bars were mounted p-side up on AluminumNitride (AlN) submounts integrating a coplanar line but noresistive matching load.

IV. RESULTS

All measurements were performed at a regulated tempera-ture of 25 °C. The efficiency is a very important parameterfor directly modulated lasers because it sets the gain of thelink. If a direct modulation is applied, the thermal effectsdepend on the direct current (DC) bias point of the laserand dissipation of radio-frequency (RF) signal power. In ourcase we can neglect heating due to average microwave powerapplied to the laser because we use small signal modulation(−7 dBm). To correctly measure the efficiency at differentDC bias point we proceed in three steps. First we estimate thejunction temperature using the method proposed by Paoli [13].

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Fig. 2. Optical spectrum with a DFB peak at 1552.1 nm.

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Fig. 3. RIN of a typical laser at different bias current.

After that we regulate the laser at this estimated temperatureand we do a pulsed L-I measurement (10 μs pulse width, 1%duty cycle). Then we calculate the efficiency as defined informula (1) [14].

ηd (I ) =(

�Pout

�I

)I bias point

. (1)

The continuous output power and the efficiency of the laser asa function of the current are shown on the same plot in Fig. 1.

We obtained at the same time an output power better than123 mW and an efficiency of 0.233 W/A at 450 mA. Themaximum output power is 146 mW at 600 mA. For a currentramp of 0–380 mA at 25 °C, the output power shows a goodlinearity with a limited influence of thermal rollover.

In Fig. 2 we superimpose the large span optical spectrumof a DFB laser diode at 450 mA with a zoom of DFB peakmaximum (1552.1 nm). We measure a side-mode-suppression-ratio (SMSR) better than 55 dB.

We also measure the optical far-field of the laser. Thefull width at half maximum (FWHM) divergence is 12° inhorizontally and 33° vertically, which is standard for shallowridge lasers. Despite the ellipticity of the optical beam wecoupled more than 65% of the optical power into a standardantireflection-coated single mode lensed fiber.

In order to evaluate the dynamic performances of ourdevices, we performed RIN and modulation response. RIN

118 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 2, JANUARY 15, 2012

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Fig. 4. Modulation bandwidth of a typical laser at different bias current.

measurements with different bias currents are plotted in Fig. 3.At 450 mA we found a RIN below −157 dB/Hz in the0.1–20 GHz range with a relaxation frequency of 7 GHz, therelaxation frequency corresponding directly to the resonancemeasured on the RIN spectrum. At 550 mA the RIN isaround −165 dB/Hz in the 0–3.5 GHz range. The modulationresponse of the laser was measured using in a link with a U2TXPDV2140R photodiode in order to close the optical link.The signal was attenuated in order to limit the photocurrentto a few mA due to the power limitation of the photodiode.No correction for the response of the photodiode was appliedbecause −3 dB photodiode bandwidth is superior to 45 GHz.

Fig. 4 represents the small signal modulation bandwidth. Forbias currents higher than 400 mA we have observed a −3 dBbandwidth better than 7 GHz. With this value of modulationbandwidth we cover VHF, UHF, L, S and a large part of Cfrequency bands. The smaller values of resonance frequencycompared to RIN measurements are due to the transit timeand RC (resistor-capacitor) parasitic of the laser and the AlNsubmount. It is interesting to estimate the gain of the analoglink using (2) in the case of a link without impedance matching[15]. We obtain Ghyp,dB = −7.1 dB with a photodiode load(Z0) of 50 �, a laser resistance RL = 1.5 �, a photodiodeefficiency ηPD = 1 A/W, a laser efficiency ηL = 0.340 W/A(Ibias = 350 mA) and optical losses Lopt = 0.67 % whichcorrespond to 70% coupling efficiency for laser and one opticalconnector.

Ghyp,d B = 10 log

(8Z0

(RL + Z0)2

(ηL LoptηP D

)2 1

2Z0

). (2)

V. CONCLUSION

We have fabricated and characterized high power, low noiseDFB lasers which have a modulation bandwidth better than7 GHz. Thanks to the reduced losses due to the asymmetricalcladding of our structures we were able to obtain very goodstatic performances (output power better than 120 mW andefficiency better than 0.23 W/A) without sacrificing dynamicperformances (RIN below −157 dB/Hz). These lasers are verygood candidates for directly modulated links. Combined witha high power photodiode it should permit to reach very highSFDR (Spurious-Free Dynamic Range) [16]. We expect to

improve the fiber coupling by using more specific couplingsystems and by circularizing the optical mode [5]. Lastlysome modifications in process and mounting should permit toincrease current polarization point and decrease thermal roll-over at high power.

We wish to thank the reviewers for their helpful commentsand Guilhem De Valicourt from III–V lab for fruitful technicalsupport.

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