high-power, ultra-low noise hybrid lasers for microwave … · 2019-06-05 · high-power, ultra-low...

5048 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 36, NO. 21, NOVEMBER 1, 2018

High-Power, Ultra-Low Noise Hybrid Lasers forMicrowave Photonics and Optical Sensing

Paul A. Morton , Fellow, IEEE, Fellow, OSA, and Michael J. Morton

(Invited Paper)

Abstract—This paper describes the design, fabrication, andexcellent performance achieved with prototype hybrid lasersincorporating a high performance gain chip coupled into a fiberexternal cavity including a novel fiber Bragg grating (FBG)reflector. Packaged ultra-low noise (ULN) hybrid lasers operatingat 1550 nm and at 1319 nm with high output power, >100 mW, andextremely low relative intensity noise (RIN) are described. Devicesprovide extremely stable singlemode output with high side-modesuppression ratio (SMSR), typically above 70 dB, with worst casemeasured RIN at microwave frequencies (1–20 GHz) being below–165 dBc/Hz. Operation of these high power, low RIN deviceswithin an analog optical link demonstrates a Spurious FreeDynamic Range as high as 114.6 dB.Hz2/3. In addition to highpower and very low RIN, the ULN hybrid lasers provideextremely small low frequency phase noise, with Lorentzianlinewidths down to 15 Hz, enabling key Microwave Photonicsand Optical Sensing applications. A comparison of the phasenoise and Lorentzian linewidth of ULN lasers with different FBGdesigns / external cavity lengths is described, demonstrating thenovel hybrid approach for achieving extremely low phase noiselasers.

Index Terms—Microwave photonics, optical mixing, opticalsensing, phase noise, relative intensity noise, semiconductor laser,ultra-low noise laser.

I. INTRODUCTION

ANALOG optical links require a high-power, very low noiselaser in order to maximize the system Spurious Free Dy-

namic Range (SFDR), by limiting the effects of laser Rela-tive Intensity Noise (RIN) on system performance. Distributedfeedback (DFB) laser diodes, used in many analog opticallinks, provide relatively high power singlemode operation withrelatively low RIN [1], however, even this low level of RIN,e.g., −155 dBc/Hz provides a clear limitation in high perfor-mance, high SFDR links. More complex lasers, including fiber

Manuscript received February 9, 2018; revised March 9, 2018; acceptedMarch 14, 2018. Date of publication March 19, 2018; date of current versionOctober 19, 2018. This work was supported in part by DARPA #W31P4Q-08-C0427 for early hybrid laser development; in part by Morton Photonics SBIRprograms with the Navy #N00024-10-C-4123, NSF # 1248442, the Air Force# FA8650-15-C-1863 for ultra-low noise hybrid lasers; and in part by DARPAcurrent SBIR funding on integrated ultra-low noise lasers, # W911NF-16-C-0072. (Corresponding author: Paul A. Morton.)

The authors are with Morton Photonics Inc., West Friendship, MD 21794USA (e-mail:,[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2018.2817175

[2]–[4] and solid state lasers [5]–[7], can provide excellent phasenoise/frequency noise (FN) performance, and potentially higherpower levels, however, these devices have high size, weight andpower (SWaP) and are significantly more expensive (e.g., 10x)than a semiconductor device such as a DFB laser.

Fiber lasers, typically operating in the C-Band around1550 nm due to the Erbium doped fiber used for gain, can pro-vide very narrow linewidth due to long cavity lengths, however,these long cavity lengths with limited mode selectivity lead toreduced singlemode stability, and so these devices are often sen-sitive to environmental variations such as vibration, temperatureand pressure changes. Fiber lasers differ from semiconductorlasers in that they are not protected within a hermetically sealedpackage, which isolate semiconductor lasers from temperatureand pressure changes, humidity, etc., leading to increased stabil-ity, also to higher reliability and longer operating lifetimes. Thefiber laser system includes one or more semiconductor pumplaser, which itself will be protected within a hermetic package.Various commercial fiber lasers such as the Koheras laser canprovide very small low frequency phase noise and linewidth[3], especially in a frequency locked version [4], however, theyusually offer limited output power and poor RIN compared tostandard DFB lasers. Fiber lasers, and also semiconductor lasersoperating in the C-Band, can be amplified using an erbium dopedfiber amplifier (EDFA) to provide higher output power, thoughthat adds significant cost and SWaP, as well as additional noise,as demonstrated in Fig. 12 of this paper.

For fiber optic sensing applications, the nonplanar ring os-cillator (NPRO) solid state laser operating at 1319 nm is the‘gold standard’ for high power and extremely small FN, withoutput power up to 200 mW [6] and more [7]. However, the RINis poor at low frequencies and requires electronic feedback, a‘noise eater’, to be used in many applications; even then, the RINis higher than many semiconductor lasers. The NPRO has a verylimited operating wavelength range around 1319 nm, which pre-cludes its use in wavelength multiplexed systems. Additionally,the NPRO laser is a complex and high cost device, with verylarge power dissipation, which affect overall system size andcooling requirements.

Morton Photonics (MP) has developed a semiconductor basedultra-low noise (ULN) Hybrid Laser (ULN-HL) based on earlywork by its founder on hybrid lasers for mode-locking; using again chip plus external silica Bragg grating [8] and a fiber Bragggrating (FBG) [9], [10], plus research on single frequency lasers

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MORTON AND MORTON: HIGH-POWER, ULTRA-LOW NOISE HYBRID LASERS FOR MICROWAVE PHOTONICS AND OPTICAL SENSING 5049

using a similar gain chip / FBG hybrid geometry [11], [12]. MPsULN-HL [13]–[15] provides excellent RIN performance, lowerthan DFB lasers and the more complex, higher SWaP fiber andsolid-state devices [2]–[7]. The ULN-HL also has very low FN,similar to many fiber lasers, and approaching that of the bestfiber and solid state lasers. Frequency locking of the MP ULN-HL, as demonstrated for a similar kind of semiconductor laserdevice [16], can reduce its low frequency FN below that of freerunning fiber and solid state lasers, making it a candidate forthe highest performance optical sensing systems [17], includ-ing interferometric sensing systems for oil and gas discovery,sonar sensing, or for high performance distributed sensing sys-tems. MPs 1550 nm ULN-HL has been licensed to Thorlabsand is currently in the pilot production stage before being fullycommercialized and generally available in the summer of 2018[18]. Similar performing lasers at other operating wavelengths,around 1310 nm (O-Band) and around 2 microns, have beendemonstrated by MP, and additional wavelength ULN-HL de-vices can be developed wherever high performance gain chipsand high performance fiber Bragg gratings (FBGs) can be fab-ricated. This paper includes initial results at both 1319 nm andnear 2 microns.

A key requirement for ULN lasers used in RF Photonic sys-tems that include RF mixing is for the laser to have extremelysmall low frequency FN and very low Lorentzian linewidth, asthese directly affect the fidelity of the RF electrical signal com-ing from the system. Applications include the generation of amicrowave signal by the beating of two ULN laser signals in ahigh speed photodetector [19], the use of a similar approach foroptical down-conversion of a modulated carrier signal [20], plusapplications in Electronic Warfare systems [21]. For ULN lasers,the Lorentzian linewidth represents the intrinsic frequency widthof the optical signal, however, that is broadened typically by me-chanical, thermal and other technical noise effects, e.g., noisefrom the laser bias current source, which give rise to the broad-ening of the linewidth (often 1/f noise etc) that limit the perfor-mance of the laser in a given application. Frequency locking thelaser to a very stable filter, e.g., a Fabry-Perot cavity with min-imized low frequency thermal and mechanical variations, canreduce the low frequency fluctuations of the ULN laser down tothat of the frequency locking filter [22], [23]; however, draw-backs of this approach are increases in system complexity, costand SWaP.

Alternative semiconductor laser based ULN lasers includethe RIO Planex external cavity laser (ECL) [24], which offersa Lorentzian linewidth down to �2 kHz with 20 mW outputpower and shot noise limited RIN, and the Pure Photonics lownoise version [25] of Neophotonics tunable ECL [26], whichprovides a Lorentzian linewidth of �10 kHz with up to 60 mW ofoutput power, and with a RIN of �−145 dBc/Hz. The TeraxionPureSpectrum NLL narrow linewidth laser [27], which includesa DFB laser frequency locked to a sharp FBG filter, provides<5 kHz Lorentzian linewidth, up to 80 mW of output power,and RIN <−150 dBc/Hz. By comparison, the extended versionof the MP laser provides a Lorentzian linewidth as low as 15 Hz,power output >100 mW, and RIN �−165 dBc/Hz; the Standardversion provides a Lorentzian linewidth of 64 Hz with the same

Fig. 1. (a) A photograph of the Thorlabs packaged ULN laser product and (b)a photograph of a prototype Extended ULN laser.

power and RIN as the longer version, however, in a shorterpackage and at lower cost.

A major use of ULN lasers, in particular requiring extremelylow FN, is in the fiber optic sensing field, e.g., in interferomet-ric acoustic sensing systems for exploration or sonar sensingsystems, distributed sensing systems e.g., in the power industryfor gas and oil well management, and for wind turbine stresssensing, plus sensing along critical infrastructure such as build-ings, power stations, bridges, tunnels, plus pipelines and electri-cal distribution systems. Another sensing application is in freespace sensing such as LIDAR/LADAR systems [28], where thelow frequency FN again directly impacts system performance.All of these systems require a laser with high power, low RIN,excellent FN and Lorentzian linewidth, while also being a lowcost, low SWaP device that is highly reliable. Additionally, forinclusion on commercial or military platforms, the ULN lasermust be immune to variations in external temperature and pres-sure, have very high singlemode stability, and be able to with-stand shock and vibration. While high performance solid state[5]–[7] and fiber lasers [2]–[4] can approach some or all of theseperformance requirements, they are high cost and also typicallyfall down on one or more SWaP or environmental requirements,leaving an unfilled need for a rugged, low cost, low SWaP ULNlaser.

This paper describes such a device, developed by MP byhybrid integration of a high performance semiconductor gainchip, a novel fiber Bragg grating (FBG) and laser cavity design,housed in a hermetic butterfly style package to provide a lowcost, rugged ULN laser [13]–[15]; two versions of the packageddevice are shown in Fig. 1. These ULN lasers can be designedand manufactured across a variety of wavelengths, limited onlyby the availability of suitable gain chips and the fabrication ofhigh performance FBGs; ULN-HL devices to date have beenfabricated by MP in the C-Band, O-Band, and at 2 microns.This paper includes experimental results from prototype pack-aged 1319 nm ULN lasers, demonstrating similar performanceto the more mature 1550 nm devices, however with higher op-


Fig. 2. Schematic of the ULN-HL device.

tical output power as expected at this shorter and more efficientsemiconductor laser wavelength. Experimental measurementstaken on an optical table for the first 2 micron hybrid lasers,with the gain chip aligned/coupled to the fiber to form the ex-ternal caity, are also included in the paper.

This paper describes measurements of two different versionsof the ULN-HL devices operating at 1550 nm, with differentFBG/laser cavity designs, one a ‘Standard’ design [13] and onewith an ‘Extended’ external cavity [14], both devices provid-ing the performance needed for a variety of RF photonics andsensing systems. The Standard ULN-HL is housed in a 65 mmlong hermetic package, as shown in the photograph in Fig. 1(a),whereas the Extended ULN-HL is housed in a 125 mm longhermetic package to support the longer cavity, as shown in thephotograph in Fig. 1(b). Significant further reductions in lowfrequency FN have been measured by frequency locking theseULN-HLs (both 1550 nm and 1319 nm versions), using thePound Derver Hall (PDH) frequency locking technique [22], toan ultra-narrow fiber Fabry-Perot cavity filter.

II. HYBRID ULN LASER DESIGN AND FABRICATION

A schematic of the ULN-HL is shown in Fig. 2. The hybridlaser is composed of a high-performance gain chip coupled intoan external cavity including a custom FBG. The gain chip is acustom version of the single angled facet (SAF) 1550 nm gainchip sold by Thorlabs [29]. This includes a high reflectivity(HR) straight facet on one end of the gain chip and an angledfacet with an anti-reflection (AR) coating on the other facet. Theoutput from this angled facet is coupled into the external fibercavity through a lensed fiber end. The gain chip is optimizedfor high-power operation together with extremely low Fabry-Perot reflections, plus high coupling through a fiber lens to theexternal cavity.

The external fiber cavity includes a proprietary custom-designed FBG which is optimized to provide extremely stablesingle mode operation together with very narrow linewidth. Var-ious FBG designs were tested as part of the development of thisproduct, including FBGs with uniform grating pitch and ampli-tude designs, amplitude apodized designs, as well as a numberof different wavelength (grating pitch) apodized designs. Thegoals of the FBG designs are to minimize the size and effectsof grating sidemodes while keeping a relatively short FBG, alsooptimizing the apodization to increase the singlemode operatingrange and stability of the laser, and to reduce the laser linewidth.The temperature of the gain chip and the FBG are held constantby two independent thermo-electric cooler (TEC) / thermistor

Fig. 3. SM and MM L/I characteristics and SM wavelength of a Standard1550 nm laser measured by stepping the bias current between 0 and 500 mA,(a) upwards and (b) downwards (showing hysteresis).

temperature control loops. The temperature of the laser cavityis controlled, so that the laser cavity phase is held constant,keeping the laser at the same operating point with its associatedoutput characteristics.

The Standard ULN laser provides high-power operation withextremely low RIN and very narrow linewidth. The ExtendedULN laser has a longer external cavity in order to further im-prove the laser linewidth and its low frequency FN. This papercompares the Lorentzian linewidth and low-frequency FN ofthese two laser designs, demonstrating the trade-off betweenthese parameters and the laser cavity length.

III. ULN-HL OPERATING CHARACTERISTICS

The light output versus bias current (L/I) characteristics for a1550 nm Standard ULN laser are shown in Fig. 3(a) showing theL/I when going upwards from 0 to 500 mA, and (b) showing themeasurement going down from 500 mA to 0. The L/I character-istics are extremely clean examples of the expected operationof an external cavity semiconductor laser; the device operatessinglemode (SM) over a range of bias currents, then moves intoa multimode (MM) state when the designed laser cavity cannotsupport SM operation. The changes from SM at threshold, toMM, then back to SM and repeating are due to the changingphase within the laser cavity that accompanies the change incurrent of the gain chip. It is the index change of the gain chipdue to heating (from the bias current) that gives rise to most of


Fig. 4. Multiple optical spectra at various bias currents; below threshold(black) 20 and 40 mA, Singlemode Output (red) 50, 100, 130, 180, 220, 260,330, 360, 390, 440, and 490 mA, Multimode output (blue) 150, 160, 290, 320,400 and 420 mA. Spectra from lowest current on left to highest on right, eachspectrum moved 0.2 nm further to the right to allow the progression to be clearlyseen.

the phase change in the ECL, and so the ‘period’ (mA range)of repeating the SM/MM behavior becomes smaller at highercurrents due to the higher change in heating (mostly I2R).

As can be seen in Fig. 3(a), there are large and consistent SMranges where the device can be operated within a system. Forfixed operating conditions i.e., constant gain chip and FBG tem-peratures and fixed bias current, the device remains in very sta-ble singlemode operation, at the same operating position (cavityphase). The L/I characteristic in Fig. 3(b) shows the strong hys-teresis of this device, in which, as the bias current is reducedthe device stays in SM operation to a much lower bias current.This shows that once the device is operating with an SM output,this operating condition strongly favors the continuation of SMoperation, leading to the extremely stable singlemode operationof the ULN-HL.

The green circles in Fig. 3 show how the operating wavelengthof the device changes with the bias current to the device. Thisis shown in the plots as a delta wavelength, or variation fromthe mean wavelength in pm, as an easier way to understanddevice operation. Considering Fig. 3(a), this L/I includes fourSM ranges, the first from threshold upwards to 140 mA, followedby an MM range, then repeated periods of SM and MM ranges upto the highest SM range from 430 mA to 500 mA. At threshold,the operating point/cavity phase is somewhat random, set by thetemperatures of the gain chip and FBG, and so its wavelengthis also somewhat random. As the bias current increases abovethreshold, heating in the gain chip increases the cavity phaseand moves the operating point to a longer wavelength.

The ULN-HL typically starts lasing near the top of the FBGreflection characteristic, with the lasing mode moving to thelong wavelength side of the grating as the current increases. Thelasing mode moves down the long wavelength side of the FBG,the wavelength variation determined by the FBG and cavitydesign, until SM operation can no longer be continued, at whichpoint the laser enters the MM region, as shown in Fig. 4.

The MM spectrum broadens as the current increases withincreasing bias, until the cavity phase is increased to the point

Fig. 5. Optical spectrum; 1550 nm Standard PM laser; 490 mA, 101 mW.

where the device snaps back to SM operation near the top ofthe grating reflection. In the second and subsequent SM ranges,the lasing mode moves from a position near the top of the grat-ing reflection down the long wavelength side of the FBG. Theprogression of the lasing spectrum from below threshold (black)where the grating removes part of the amplified spontaneousemission (ASE) from the gain chip, to SM (red) and MM (blue)ranges, is shown in Fig. 4; spectra are offset from each other inwavelength for better visibility.

For very long-term operation, e.g., many years, a feedbackloop can be used to keep the ULN-HL at the same operating po-sition on the SM curve. This can be accomplished by comparingthe current from the back facet monitor (BFM), which measureslight out of the HR facet of the gain chip, and a measure ofthe light output from the laser, which passes through the FBG.The FBG provides a very strong filter characteristic to comparewith the BFM output, allowing the lasing mode position on thegrating to be fixed.

The optical spectrum from a 1550 nm Standard ULN laser,providing over 100 mW optical power after an external opticalisolator, is shown in Fig. 5. The output, shown over 100 nm,is provided in a polarization maintaining (PM) fiber, and has aside-mode suppression ratio (SMSR) of over 70 dB.

A version of the ULN-HL allows the FBG temperature to bevaried over a large range, allowing the laser wavelength to betuned. Examples of the laser spectra with the FBG temperaturevaried from 10 °C up to 70 °C, with the bias current changedat each temperature to provide �100 mW of SM output power,are shown in Fig. 6. The laser wavelength tunes at 12 pm/°C,Fig. 6 showing 0.71 nm tuning for 60 °C temperature change.

IV. SFDR MEASUREMENTS OF ANALOG OPTICAL LINK

A simple analog optical link is made up of a high-power,low noise laser, followed by a high linearity optical modulator;typically a Mach Zehnder interferometer (MZI) modulator. Themodulator impresses an RF signal on the optical carrier, the out-put is often passed through optical fiber before being receivedat an optical photodetector which provides the RF output ofthe system. The SFDR is a key parameter defining the dynamic


Fig. 6. Spectra for 100 mW output as laser is tuned via FBG temperature.

Fig. 7. RIN versus bias current for a JDSU CQF935/5619H DFB laser.

range of an analog optical link. It includes a measurement ofthe linearity of the link as well as the noise floor of the link;which is made up from thermal noise, shot noise of the pho-todetector, plus the RIN of the laser. High optical power at thephotodetector increases the output signal until the shot noisedominates thermal noise, providing ‘shot noise limited’ opera-tion; as the optical power level is increased further, the signalto noise ratio (SNR) and therefore the SFDR also increases,highlighting the need for a high-power laser and photodetec-tor, as well as a low loss modulator and system. However, asthe optical power at the photodetector increases, the laser RINeventually dominates the system noise, at which point increas-ing the optical power at the photodetector does not improvesystem SFDR. Laser RIN is therefore extremely important inhigh performance analog optical links because reducing laserRIN can significantly improve SFDR.

The laser RIN measured from a high-power (40 mW) DFBlaser, versus bias current, is plotted in Fig. 7. RIN is mea-sured with the received photodetector current set to ∼10 mA,unless the optical power is too low, providing a shot noise of∼ −160 dBm, which is calculated from the photodetector biascurrent and subtracted from the measured noise. As is typicalfor DFB laser, the RIN is high at low bias, with a resonancefrequency at a few GHz; increasing the bias pushes up the res-

Fig. 8. RIN for a Standard 1550 nm ULN laser a) across top SM range atdifferent bias levels, and b) near optimum bias within each SM range - includingthe lowest DFB measured RIN from Fig. 7 (190 mA, 44 mW).

onance frequency to beyond 10 GHz and flattens the RIN. Themaximum bias produces the lowest peak RIN, which for thisand other DFB lasers is typically around −155 dBc/Hz, withsome selected devices providing as low as −160 dBc/Hz.

The ULN–HL has extremely low RIN, as can be seen fromthe two plots in Fig. 8. The RIN is lowest when the laser isoperated near the peak of the grating reflection, and as the lasingmode is moved down the long wavelength side of the gratingsmall RIN peaks can be seen from mode interactions betweenthe lasing mode and a close-by cavity mode. In most cases thesmall peaks, e.g., as in Fig. 8(a) between 2 and 3 GHz are stillbelow −155 dBc/Hz, i.e., as good as the best DFB laser. Whenoperated near the peak of the grating reflection, as shown inFig. 8(b), the RIN is very low even close to threshold and fallsbelow the noise floor for this measurement (which includes shotnoise subtraction) of −165 to −170 dBc/Hz.

The experimental setup used to measure SFDR is shown inFig. 9. Two RF tones were combined using a power combinerand RF pads and applied to the device. The fundamental andthird order intermodulation products were measured on a highperformance electrical spectrum analyzer. The noise was mea-sured using the spectrum analyzer and a low noise pre-amplifier(LNA).

Using a standard lithium niobate MZI modulator, an SFDRmeasurement for a modulation frequency of 1 GHz (tones sep-


Fig. 9. Experimental setup used to measure SFDR.

Fig. 10. SFDR measurement at 1 GHz modulation frequency, in a noisebandwidth of 1GHz (54.6 dB), or 114.6 dB.Hz2/3 .

arated by 10 MHz) is shown in Fig. 10. The noise is shownfor a 1 GHz bandwidth, providing an SFDR of 54.6 dB, whichcorresponds to an SFDR in 1 Hz bandwidth of 114.6 dB.Hz2/3 .

In order to demonstrate the effect of optical loss (attenuation)within the analog optical link, SFDR and photodetector currentmeasurements were taken for the link as the level of opticalattenuation was increased from 0 dB up to 20 dB. These mea-surements were taken with a modulation frequency of 10 GHz,and results are shown in Fig. 11. The SFDR with no attenuationis 52.3 dB in a noise bandwidth of 1 GHz, or 112.3 dB.Hz2/3 .The SFDR falls fairly linearly with optical attenuation, fallingquicker at high attenuations (low photodetector currents) as ther-mal noise becomes important.

Higher optical attenuation values can be included in the link ifan EDFA is included in order to increase the optical power on thephotodetector. Measurements taken with the optical attenuationvaried from 0 to 45 dB, with the EDFA included for higherattenuation values, are shown in Fig. 12. The lower attenuationvalues without the EDFA (blue) are the same as in Fig. 11,with SFDR values with the EDFA (red) added. Over the wholerange a linear variation in SFDR versus attenuation is found,the slope showing that the SFDR falls at ∼2/3 the attenuationvariation, i.e., almost 30 dB drop in SFDR for 45 dB increase inattenuation. This variation was confirmed by internal theoretical

Fig. 11. SFDR measurement at 10 GHz modulation frequency; SFDR andPhotocurrent vs. Link optical attenuation. For zero attenuation, 52.3 dB in anoise bandwidth of 1GHz, or 112.3 dB.Hz2/3 .

Fig. 12. SFDR vs. Link attenuation, with and without an EDFA.

analysis, which showed that the required gain from the EDFAis accompanied by an increase in noise.

Calculations were carried out to determine the effects of laserRIN on the overall noise of an analog optical link, demonstratingthe effect of laser RIN on the system SFDR. Results of thesecalculations are shown in Fig. 13; they include the photodetectorthermal and shot noise, assuming the photodetector has a 50 ohmload resistance, plus the laser RIN for three different cases;−160, −170, and −180 dBc/Hz. The noise power is plottedversus the received optical power at the photodetector. Thermalnoise is constant at −173 dBm, shot noise increases linearlywith received optical power, shown by the thick red line; it canbe seen that for �0 dBm optical power the photodetector isshot noise limited. However, at higher received optical powerlevels the laser RIN is seen to dominate the total noise. Fig. 13includes three dotted vertical lines which show the receivedoptical power in a typical analog optical link for laser outputpower levels of 50 (black), 100 (red) and 200 mW (blue). Usinga selected 100 mW DFB laser, with −160 dBc/Hz RIN, thetotal system noise is dominated by laser RIN which is about8 dB larger than the shot noise. The laser such as the ULN-HLthat may have RIN less than −170 dBc/Hz provides an SFDR


Fig. 13. The effects of thermal noise, shot noise and RIN on the total noise inan analog photonic link; for RIN values of −160, −170 and −180 dBc/Hz, andshowing typical received optical powers for a 50, 100 and 200 mW laser.

improvement in such a link of ∼6 dB, which is significant. Inorder to take advantage of a 200 mW laser, it’s RIN should beless than −170 dBc/Hz.

V. LOW FREQUENCY RIN AND FN

As shown in Fig. 8, the ULN-HL high-frequency (microwavefrequency range) RIN is optimized by biasing the laser to operatenear the peak of the grating reflection, or at the beginning of anSM range. However, the ULN-HL is designed to take advantageof the large slope on the long wavelength side of the grating toreduce the laser linewidth [30]–[33] so that the optimum biasposition for smallest low-frequency FN is near the top of anSM range. Depending on the specific application of the ULN-HL, the chosen bias position may be towards the bottom or thetop of the SM range, or a compromise position with both goodhigh-frequency RIN and low linewidth can be found towardsthe center of the SM range.

Measurements of the low-frequency RIN and FN of variousULN-HLs were taken using an OEwaves Phase Noise measure-ment system (OE4000). Examples of the RIN measurementsfrom 1 Hz to 10 MHz for a Standard 1550 mm ULN-HL at var-ious bias currents across a top SM range are shown in Fig. 14.Over the entire offset frequency range from 10 Hz to 10 MHzthe measured RIN is equal to the noise floor of the equipment,thus only providing an upper limit to the RIN of the ULN-HL. The increased optical power as the bias is increased from500 to 560 mA reduces the instrument noise floor down to−160 dBc/Hz above 100 kHz. Between 1 and 10 Hz the RINis above the instrument noise floor; this noise is due to low-frequency variations in the TEC control loops.

Measurements of low-frequency FN from 1 Hz to 10 MHzfor various bias levels within the top SM range of a Standard1550 nm ULN-HL are shown in Fig. 15. The FN has a similarshape in all the curves, however, it is reduced as the lasing modeposition is moved from the top of the grating peak down the longwavelength side, as expected. The Lorentzian linewidth of thisdevice, found from the flat section of the frequency noise curve,of 4.5 Hz/rt(Hz), is 64 Hz at 520 mA bias current (4.52 × π),

Fig. 14. Low frequency RIN of Standard 1550 nm ULN laser at differentcurrents in top SM range - basically follows the noise floor of measurementequipment (shot noise floor reducing to −160 dBc/Hz at highest bias level.

Fig. 15. FN of Standard 1550 nm ULN-HL at different currents in the top SMrange. At 520 mA the Lorentzian linewidth is 64 Hz.

increasing by ∼3x for the lower bias currents within this SM.Similar, slightly higher linewidths are seen on the lower SMranges. In the frequency range 30 to 100 Hz several noise peakscan be seen; these are inherent to the noise from the electronicsof the OEwaves measurement system. Additional broader peaksat 700 and 1500 Hz are likely from small mechanical vibrationsof the optical fiber within the laser; these prototype lasers havethe FBG lying in a groove rather than being mechanically held,which can allow them to vibrate.

A comparison of the FN of the 1550 nm Standard ULN-HLand Extended ULN-HL is shown Fig. 16. While having similarshapes, the longer cavity of the Extended device provides forlower FN over most of the frequency range, and significantlylower linewidth. The Lorentzian linewidth of the Standard ULNlaser is 64 Hz, while the Lorentzian linewidth of the ExtendedULN laser is only 15 Hz (an oscillator Q factor of 13 × 1012).


Fig. 16. FN of Standard and the Extended 1550 nm ULN lasers. The ExtendedULN device has a Lorentzian linewidth of 15 Hz.

Fig. 17. Free-space (optical bench) spectra of �2000 nm ULN-HLs, basedon FBGs in the 1940 nm to 2000 nm range.

VI. ADDITIONAL WAVELENGTHS

Additional wavelength versions of the ULN-HL can be de-veloped over a wide frequency range as long as the two keycomponents of the ECL can be obtained; a high performancegain chip optimized for use in an external cavity, and a highperformance custom FBG fabricated in the appropriate fiber.

MP carried out a small R&D program to validate the approachat around 2 microns, using modifications to available gain chips,plus FBG designs around the wavelength range of those gainchips, i.e., 1940 nm to 2000 nm. Initial testing was carriedout by aligning the gain chip to the lensed fiber on an opticalbench and holding them in alignment while laser characteristicswere measured. FBG’s around 1940, 1950, 1970 and 2000 nmwere used in these ‘free space’ measurements. Excellent resultswere found from these first experiments, as can be seen from thelasing spectra taken with each different FBG based laser shownin Fig. 17, with output powers up to ∼10 mW, and SMSRsaround 50 dB. Significantly higher powers can be achieved withan optimized gain chip, and at wavelengths out to 2040 nmand beyond; meeting the requirements for a high power, very

Fig. 18. Standard 1319 nm ULN laser L-I up from 0 to 600 mA, showing SM(red), MM (blue), plus delta wavelength (pm) in green dots.

Fig. 19. Optical spectrum at 590 mA bias current, Standard 1319 nmULN-HL.

low RIN, ultra-low linewidth laser for use in LIDAR/LADARsystems.

MP has developed prototype ULN-HLs at various O-bandwavelengths, 1319 nm being chosen to provide a low-cost, lowSWaP replacement option for sensing systems that currentlyuse the ‘gold standard’ NPRO based solid state laser [5]–[7].Semiconductor lasers and gain chips operate far better in theO-band around 1310 nm than those operating at longer wave-lengths, e.g., in C-Band, L-Band and longer, providing higherefficiency, higher output powers, and lower temperature sen-sitivity. A measured L/I and the SM peak wavelength versusbias current characteristics for a prototype Standard 1319 nmULN-HL are shown in Fig. 18. These results demonstrate sim-ilar operating characteristics to the 1550 nm ULN-HL devices,although providing higher power levels; this device produced184 mW SM output (after a spliced dual isolator) at a biascurrent of 590 mA.

Optimized gain chips for O-band ULN-HLs provide the op-portunity for far higher output power devices, e.g., �250 mW,which will be particularly useful in analog optical links; pro-viding improved system SFDR from the higher optical powerlevels combined with extremely low RIN. The optical spectrumfrom this Standard 1319 nm ULN-HL at 590 mA is shown inFig. 19.


Fig. 20. RIN at various bias currents for a Standard 1319 nm ULN laser.

Fig. 21. FN of Standard and the Extended 1319 nm ULN lasers. Lorentzianlinewidths are 95 Hz (Standard) and 28 Hz (Extended).

The RIN characteristics of a Standard 1319 nm ULN-HLmeasured at bias currents across an SM range close to 500 mAare shown in Fig. 20. In these measurements the RIN is mostlybelow −170 dBc/Hz across the frequency range of 1 to 20 GHz,except for a small peak near 13 GHz that is still well below−160 dBc/Hz, plus peaking from the mode interactions seen athigher bias levels around 3.5 GHz. The high mode interactionpeak in this measurement, near −140 dBc/Hz, is for operationclose to the transition from SM to MM operation.

Measurements of the low-frequency FN for 1319 nm pro-totype Standard and Extended ULN-HL devices are shown inFig. 21. They are similar to 1550 nm devices, however, boththe lower frequency 1/f noise and the Lorentzian linewidths aresomewhat larger. This is believed to be due to higher carriernoise within the first generation gain chips used for these pro-totypes. Optimized 2nd generation O-band gain chip designsare being developed to reduce this additional noise, as well asto increase the output power. Lorentzian linewidths of 95 Hz(Standard) and 28 Hz (Extended) were found for these 1319 nmULN-HL prototype devices.

VII. CONCLUSION

This paper has described high performance ULN-HL devicesoperating at various wavelengths, including 1550 nm (C-Band),

1319 nm (O-Band) and near 2 microns. The most mature de-vices, at 1550 nm, have been through several generations ofprototype devices in order to optimize both the subcomponentperformance (gain chip and FBG) and also the overall ULN-HL performance. These 1550 nm ULN-HL devices have beendeveloped into a commercial product that will soon be generallyavailable for use in a wide range of RF photonics and opticalsensing applications.

The ULN-HL devices provide high output power in a PMfiber, with extremely low RIN and FN, plus extremely narrowlinewidth. Devices at both 1550 nm and 1319 nm demonstrateRIN that is �−165 dBc/Hz at microwave frequencies from1 to 20 GHz, with small mode interaction peaks increasingthe RIN between 2 to 3 GHz when the laser is operated onthe long wavelength side of grating, towards the MM region.The high power operation, �100 mW (after an external isolator),and extremely low RIN make these devices ideal for use in RFphotonics systems, in particular for use in analog optical links.The prototype 1319 nm ULN-HLs demonstrated even higherpower levels, up to 184 mW SM operation, and with a secondgeneration gain chip it is expected that this power can be pushedmuch higher, enabling high performance analog optical links inthe O-Band.

The low frequency FN of Standard and Extended ULN-HLdevices, operating at both 1550 nm and at 1319 nm, werecompared. 1550 nm ULN-HL devices demonstrated Lorentzianlinewidths of 64 Hz (Standard) and 15 Hz (Extended), whereasinitial 1319 nm devices demonstrated Lorentzian linewidths of95 Hz (Standard) and 28 Hz (Extended). The increased linewidthof the 1319 nm devices is thought to be due to increased carriernoise in this first generation of O-Band gain chips. The nextgeneration gain chips are being designed to reduce this addi-tional noise and also to increase the output power further. TheseULN-HL lasers with extremely low FN and linewidth are idealcandidates for use in high performance optical sensing systems,such as interferometric or distributed sensing systems, as well asin RF photonics applications that include optical mixing, suchas microwave signal generation and optical down-conversion.

The ULN-HL devices can be developed over a wide wave-length range, wherever high performance gain chips and FBGscan be fabricated. As an example, initial free space measure-ments were described for ULN-HL devices operating at variouswavelengths between 1940 and 2000 nm, with up to 10 mWoutput power and 50 dB SMSR.

Future work includes increasing the output power of ULN-HLdevices, and developing devices and products at different wave-lengths, e.g., at 2040 nm for LIDAR/LADAR systems. Work al-ready underway at both 1550 nm and 1319 nm to frequency lockthe ULN-HL devices to ultra-narrow bandwidth fiber Fabry-Perot locking filters, to further reduce the low-frequency FN,will be extended to alternative locking filters to allow theseULN-HL devices to be used in all applications. Additionally,MP is currently developing integrated photonics ULN lasers,based in part on the design and performance achieved withthe ULN-HL devices. These are based on a silicon photonicsplatform enabled by heterogeneous integration of III-V gainelements and ultra-low loss waveguides.


ACKNOWLEDGMENT

The authors would like to acknowledge Jill Morton for tech-nical discussions and help preparing this manuscript, and ZemerMizrahi for help packaging prototype devices.

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Paul A. Morton (F’11) received the B.Sc., M.Eng. and Ph.D. degrees in elec-trical engineering from the University of Bath, U.K. He was a postdoctoralresearcher with the University of California Santa Barbara, before becoming amember of technical staff with the AT&T Bell Laboratories, Murray Hill, NJ,USA, where he made fundamental research contributions in the areas of high-speed laser diodes, mode-locked optical pulse sources, and photonic integration.At CIENA, he was a technical leader with the development of commercial highcapacity DWDM transmission and switching systems. He cofounded MortonPhotonics, which develops photonic components, photonic integrated circuitsutilizing heterogeneous integration, and subsystems based on these devices, forapplications of high performance analog RF Photonic links, sensing, and pho-tonic processing for phased array sensors. He has authored or coauthored 2 bookchapters, more than 100 journal and conference papers, and holds 11 patents.He is a Fellow of the Optical Society of America.

Michael J. Morton received the B.A. degree from the University of Maryland,College Park, in 2013. He works at Morton Photonics where he carries outtesting of high performance ULN lasers, including “free space” optical testingof gain chips and FBGs in an external cavity geometry, plus FN and RINmeasurements of packaged ULN lasers. He is experimenting on the frequencylocking of ULN lasers to ultra-narrow Fabry-Perot filters, and measurements ofthe resulting laser FN and RIN characteristics.

http://dx.doi.org/10.1016/S1079-4042(03)80017-6

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