Experimental investigation of high-frequency noise andoptical feedback effects using a 9.7 �m continuous-wavedistributed-feedback quantum-cascade laser

Damien Weidmann, Kevin Smith, and Brian Ellison

An experimental investigation of high-frequency noise, i.e., up to 3 GHz, exhibited by a 9.7 �m quantumcascade laser, is described. Noise characteristics and measurements of a liquid-nitrogen-cooled continuous-wave distributed-feedback laser are presented. Well defined sets of narrow and intense resonance peakshave been observed in the 10–300 MHz range. Measurements of relative intensity noise have been per-formed. It is also shown that quantum-cascade lasers are sensitive to optical feedback. The excess noisegenerated by the feedback has been investigated under well defined conditions. A description of theexperimental phenomenon is presented along with methods of minimizing optical feedback. © 2007Optical Society of America

OCIS codes: 140.5960, 140.3600, 270.2500.

1. Introduction

Since their initial experimental demonstration,1mid-IR quantum cascade lasers (QCLs) have become awidely used continuously tunable laser source span-ning applications from high-resolution molecular spec-troscopy to optical communications. Because of theunipolar nature of the laser transition occurring inQCLs, they can be modulated at high frequencies, upto the gigahertz range. As high-frequency laser noisemay impede performance of QCL-based systems, it isinteresting to investigate high-frequency intrinsiclaser-noise characteristics. Laser noise may be thedominant noise source in spectrometers based on fre-quency modulation detection schemes,2 in optical het-erodyne systems,3 or in laser systems for free spacecommunications using an optical link.4

Semiconductor laser-noise properties are usuallyaffected by optical feedback (OF), primarily becauseof their short cavity length. Intensity and phase noiseincrease or reduction, power modulation, laser lock-ing, optical bistability, and mode hopping are all OF-

related phenomenon observed with semiconductorlasers. Depending on the purpose of the QCL-basedsystem, an understanding of OF may help to mini-mize undesirable effects or even allow them to beexploited. Thus strategies to limit the amount of laserexcess noise injected within the instrument can bedeveloped.

The paper is organized as follows: the experimentaldetails of the setup are first introduced. Noise spectraare then presented along with results from relativeintensity noise (RIN) measurements. The last sectionbefore the conclusion describes an experimental in-vestigation of OF effects on a QCL.

2. Experiment

A. Setup

The laser investigated during this work is a liquid-nitrogen-cooled distributed-feedback (DFB) QCL op-erating in the continuous-wave mode (cw) with acentral wavelength at 9.7 �m. The laser ridge is27 �m wide, 1.5 mm long (hereafter the cavity opticallength is denoted L). The multi-quantum-well struc-ture is a bound-to-continuum design comprising 35cascade stages, as described in Ref. 5. The laser wasinstalled in a liquid-nitrogen cryostat with a temper-ature control better than 1 mK. The injection currentis supplied by a low-noise current source (noise andripple below 10 �A).

Figure 1 is a schematic of the optical configuration.Laser radiation exits through a wedged barium fluo-

The authors are with the CCLRC Rutherford Appleton Labora-tory, Space Science and Technology Department, Chilton, Didcot,Oxfordshire, OX11 0QX, UK. D. Weidmann can be reached bye-mail at d.weidmann@rl.ac.uk.

Received 14 August 2006; accepted 24 November 2006; posted 29November 2006 (Doc. ID 74060); published 2 February 2007.

0003-6935/07/060947-07$15.00/0© 2007 Optical Society of America

20 February 2007 � Vol. 46, No. 6 � APPLIED OPTICS 947

ride window and is collimated by a custom designed12.5 mm diameter ZnSe aspheric meniscus with an-tireflection (AR) coating on both surfaces. The menis-cus is opened at F�#0.5 to ensure full collection of thehighly divergent QCL beam. An intermediate imageis formed by a 90° off-axis paraboloid (101.6 mm focallength). This intermediate image was included forbeam manipulation purposes, for example, spatialfiltering or amplitude modulation by a mechanicalchopper. The beam is split into two parts by a 50�50ZnSe beam splitter. The transmitted beam is reservedfor characterization purposes, including power mea-surements and injection into a Fourier transformspectrometer. The reflected part is focused on a fastmercury cadmium tellurium (MCT) photodiode by a90° off-axis ellipsoid (79.06 mm focal length). Thedetector used during this work was a 100 �m �100 �m resonant optical cavity (ROC) MCT detector,exhibiting a 1.8 GHz 3 dB bandwidth and a 6 dBbandwidth over 3 GHz. The ROC has been designedfor peak sensitivity at 9.7 �m. The DC part of thephotocurrent is used for optical alignment and indi-rect power monitoring using an oscilloscope. Thehigh-frequency part of the photocurrent is fed into a35 dB gain, 3 GHz bandwidth preamplifier. The radiofrequency (RF) power contained in the signal is de-tected by a Schottky diode, or the signal is fed into aRF spectrum analyzer.

B. Laser Specifications

QCL spectral characteristics were investigated usinga Bruker IFS 125HR Fourier transform spectrometer(FTS) with a 6 m optical path difference. Figure 2shows an intensity-normalized spectrum recorded atthe maximum unapodized resolution, i.e., 0.0017 cm�1

(50 MHz). The spectrum indicates that the laser wasoperating in a single mode. At threshold (92 K, 650mA) the laser emited at 1035.85 cm�1. At higher cur-rent and temperature (1.12 A, 140 K) the laser fre-

quency decreased to 1025 cm�1. The maximumoutput power was approximately 35 mW. Currentand temperature tuning rates were measured to be�3.9 cm�1A�1 and �0.065 cm�1K�1.

The spectral resolution of the FTS was insufficientto provide an estimation of the laser linewidth. Toestimate this parameter, we performed tunable ab-sorption spectroscopy over a low-pressure carbonylsulfide (OCS) absorption line, the P30 transition lo-cated at 1035.4232 cm�1.6 A 52 mm long glass cellwas filled with 0.072 mbar of pure OCS. At such lowpressures the linewidth is determined solely by Dopp-ler broadening, and the absorption profile should ex-hibit a full width at half-maximum (FWHM) of 49.5MHz at 295 K.

Figure 3 shows the measured FWHM of the ab-sorption line was 45 � 3 MHz. As a first approxima-tion the observed FWHM, �� is given by theconvolution of the Doppler-broadened OCS line(FWHM �D) and the Gaussian lineshape (FWHM��L) of the laser emission:

��2 � �D2 ��L

2, (1)

Fig. 1. Schematic of the optical configuration for high-frequencyQCL noise measurements. OAPM, OAEM, BS, FTS, and PD areoff-axis paraboloidal mirror, off-axis ellipsoidal mirror, beam split-ter, Fourier transform spectrometer, and photodiode, respectively.

Fig. 2. Intensity-normalized spectrum of QCL emission recordedwith a Bruker IFS 125HR Fourier transform spectrometer. Theinset plot shows the �3 dB linewidth corresponding to the FTSresolution, the contribution from the laser linewidth being insig-nificant.

Fig. 3. Transmission spectrum of the Doppler-broadened P30OCS absorption line at room temperature and 0.072 mbar totalpressure.

948 APPLIED OPTICS � Vol. 46, No. 6 � 20 February 2007

as experimentally �� � �D, which means ��L2 �� �D

2,or 10��L

2 �D2. Hence the laser linewidth is below 16

MHz, most likely close to the 1 MHz range, as alreadyreported for free running QCLs operating at similarwavelengths.7 Using active stabilization techniques,reduction of linewidth down to the kilohertz rangehas been reported.8

The long-term frequency stability was also ob-served by monitoring the variation of the acquiredpeak position of the OCS absorption line with time. Aslow drift of 7 MHz over 30 minutes was observed,due to residual temperature variations.

The beam quality and alignment were verified andoptimized using an IR thermal imager.

3. Intrinsic Noise

To perform measurements of the laser noise in the0–3 GHz range, a mechanical chopper running at1.78 kHz was introduced at the intermediate focuscreated by the off-axis parabolic mirror (OAPM). Thepreamplifier output was connected to a 0.01–12 GHzspectrum analyzer. The Y channel output of the an-alyzer was fed into a lock-in amplifier for synchro-nous demodulation. This method ensured that onlythe laser noise was taken into consideration.

Initially, 2 GHz wide spectra were recorded with aresolution of 300 kHz, as shown in Fig. 4. The 2–3GHz frequency range was found not to exhibit anyspecific noise features. Unlike intraband semiconduc-tor lasers, QCLs do not exhibit a photon–electronresonance peak around 1 GHz. In addition to thenormal 1�f noise, clearly identified structures can beseen at frequencies up to 300 MHz. The most intenseof these occur below 200 MHz, as shown in Fig. 5.These peaks are very intense: between 10 and 20 dBabove the noise floor, and sharp. The inset of Fig. 4shows the spectral record of a single peak at 83 MHz,obtained with a 3 kHz resolution bandwidth. Thepeak exhibits a FWHM of about 10 kHz, yielding aresonance with a quality factor of around 8000.

Figure 5 shows noise spectra, recorded at 300 kHzresolution, but limited this time to the 0–200 MHz

region. Spectra for four different injection currentswere recorded: 672.6, 689.6, 721.9, and 775.2 mA. Forthese measurements a wire-grid polarizer was in-serted into the beam and adjusted so that the powermeasured at the detector remained constant. The ex-pected decrease in laser noise with injection currentwas observed.

The spectrum analyzer output (dBm) was then con-verted to relative intensity noise (RIN). The mechan-ical chopper and lock-in amplifier were removed toperform noise within the full measurement line. Thedetector and the RF line were fully shielded with a100 dB attenuation shielding material. The AC com-ponent of the RIN is given by

RIN�f � �1

Iph2� 10X�f��10

G2 � R � �f � 1000�, (2)

where X(f ) are the spectrum analyzer signals in deci-bels, G is the amplification gain, R is the load imped-ance, �f is the analysis bandwidth, and Iph is themeasured photocurrent.

Four frequencies were chosen to investigate the RINvariation with injection current and output power:200, 500, 1000, and 2000 MHz, respectively. Theresults are plotted in Fig. 6. The contribution fromthermal noise and shot noise is also plotted. This

Fig. 4. 2 GHz wide spectrum of the high-frequency component ofthe photocurrent under laser illumination. The resolution band-width was 300 kHz. The inset shows a well defined resonance peakat 83 MHz recorded at 3 kHz resolution.

Fig. 5. Noise spectra recorded in the 0–200 MHz region for fourdifferent injection currents.

Fig. 6. RIN measurements versus injection current and opticalpower, performed at four different frequencies.

20 February 2007 � Vol. 46, No. 6 � APPLIED OPTICS 949

contribution was calculated from

RINThsn �1

Iph2�4kB � T

R 2 � e � IPh�, (3)

where T is the noise temperature of the amplifier(102 K).

As expected, and as observed with interband semi-conductor lasers, the RIN is proportional to the laseroutput power raised to the scaling parameter expo-nent: P��. Rana et al. and Gentsy et al. have recentlypublished semiclassical models of QCL noise describ-ing the RIN behavior.9,10 Gentsy et al. have reported� values in the range of 2.1 for a 25-stage laser de-vice.11 From the data recorded during this study, thescaling factor � was observed to increase slightly asthe frequency increases: 1.67, 1.72, 1.83, and 1.96 for200, 500, 1000, and 2000 MHz, respectively. Thesevalues are smaller than those given in Ref. 11, whichis consistent as the QCL used in this work has 35cascade stages. A much smaller scaling factor of 1.1was measured for the noise peak at 83 MHz.

4. Optical Feedback

Spurious OF can have a profound effect on semicon-ductor lasers operating in the cw mode.12,13 Depend-ing on the feedback conditions, effects such as laserline broadening, mode hops, and generation of excesslow- and high-frequency noise may degrade the laserperformance. Conversely, when the OF is controlled,phase and intensity noise reduction in the laser emis-sion can be achieved. OF effects on telecom laserdiodes and also lead salt lasers have been widelystudied.14–17

In most applications, OF is not controlled, and itoccurs due to unwanted reflections from optical com-ponents in the system. The feedback is then weak.Usually laser linewidth is not strongly affected underweak feedback conditions, and the effect on systemssuch as tunable diode laser spectrometers will beresidual etalon fringes. This may limit the sensitivityof the spectrometer if fringes are of similar strengthto the absorption signal. However, in applicationswhere photomixing and coherent detection are in-volved, like homodyne and heterodyne systems, themode mixing introduced by OF may be a severelylimiting factor.18

Continuous-wave DFB QCLs are likely to be par-ticularly sensitive to OF because of the following:

Y They exhibit a very good spectral purity (longphoton lifetime).

Y They are short cavity lasers (though 4 to 5times longer than telecommunications or lead saltdevices).

To first order, the optical feedback is likely to per-turb the laser behavior providing that the reinjectedfraction of power fEXT is such that12

fEXT �� ��f���2

, (4)

where �f is the laser linewidth and �� is the longi-tudinal mode spacing. For a typical cw QCL, if weconsider a 5 MHz laser linewidth and a 2 mm longcavity, it follows that a fraction of reinjected lightmuch greater than �70 dB will affect the QCL’s be-havior. Also, since the coherence length is on the scaleof hundreds of meters, reflections from very distantobjects are relevant.

The optical configuration shown in Fig. 1 has beendesigned to optimally reduce OF. By use of off-axisreflective optics, feedback-free and optimal align-ment can be achieved. The only transmitting opticalcomponent in the path is the ZnSe collimating me-niscus. The AR coatings on the meniscus have beenmeasured to transmit 98.5% of radiation at the work-ing wavelength. The potential effect of the reflectedradiation was investigated by running a simulationwith ray-tracing software (Zemax). Calculationsshow that for perfect alignment, the fraction of lightreinjected would be �67 dB. The sensitivity of thisfraction with respect to lens tilt (�� 1–2 degrees)and position along the optical axis (�� 10% of theeffective focal length) was found to be insignificant.Therefore we conclude that no significant OF wasintroduced by the collimating meniscus.

Whatever the optical configuration of an instru-ment, at some point the radiation is incident on adetector. If perfectly aligned, the surface of the detec-tor acts as an autocollimated partial reflector. Theuse of a ROC introduces more OF than would beexpected from a conventional MCT detector. TheROC has been fine tuned to the precise workingwavelength by addition of an uncoated germaniumlayer. Consequently, the feedback introduced by aperfectly aligned detector chip is expected to be 18%,considering the refractive index of Ge and the BSpresent in the path. This feedback level is far morethan �70 dB and should strongly affect the QCL las-ing properties. Unlike many previous studies14–17 onOF in semiconductor lasers, the weak feedback ap-proximation can no longer be made. Small-signalanalysis applied to the linearized rate equations is nolonger valid. Reference 19 provides a full numericalsolution of the rate equations applicable for a highlevel of feedback.

The presence of standing waves between the lasersource and the detector was first investigated by mov-ing the detector itself. The effect on the measuredpower is plotted in Fig. 7. The results are not indic-ative of a standing wave; rather, the observed powervariation indicates that the laser was being succes-sively locked to extended cavity modes. The slowlydecreasing envelope stems from defocusing of thebeam. The length Lext of the extended cavity formed is49.7 cm, approximately 200 times the length of thelaser cavity. The spatial separation between two con-secutive extended cavity modes derived from Fig. 8 is5.0 � 0.2 �m. This corresponds to laser emission at1000 � 40 cm�1, which is consistent with the actuallaser operating frequency of 1035.55 cm�1.

950 APPLIED OPTICS � Vol. 46, No. 6 � 20 February 2007

It is worth noting that under such strong OF condi-tions, maintaining the stability of the laser power be-comes critical. The slightest change in extended cavitylength produces significant power modulation. The re-fractive index of air changes at a rate of 10�6 per degreeKelvin, and the thermal expansion of the optical tablesteel is 10 ppm per degree. High temperature stabilityis then required to avoid power-level drifts.

Considering the description from Ref. 19, the feed-back coefficient C is given by

C �Lext

n � L1 � R

RfEXT, (5)

which yields C � 60 using n � 3.2 for the intracavityrefractive index. R is the laser facet reflectivity. Notsurprisingly, the feedback regime is strong. For con-ventional semiconductor lasers, the expression of Cshould include the contribution from the linewidthenhancement parameter �, which is proportional tothe variation of the refractive index with the carrierinjection. For a QCL this parameter was thought tobe essentially zero.1 Recent measurements show that� varies with the injection current, and values from�0.5 to 2 were reported.20 A � parameter close to zerohas a profound effect on the laser behavior under thestrong feedback regime since the coherence collapsecannot occur. Based on the definition of the differentfeedback regimes of Ref. 19, a QCL with � 0 cannever operate in regime IV but will remain in regimeIII, where it is locked on the extended cavity modeand exhibits a significant line narrowing.

Figure 8(a) shows measured laser output powerwhile applying a 10 mA current ramp to the laser,corresponding to a spectral scan of 3.9 � 10�2 cm�1.The influence of OF is obvious; rather than a linearincrease of power with current (as given with a ther-mopile), one can see a strong amplitude modulationcorresponding to the laser being “forced” to operate inextended cavity modes. Three different cases areshown: (1) with the detector well aligned and thelaser operating near threshold, (2) with a 5 degree tiltin the detector alignment and the laser operatingnear threshold, and (3) with the detector well alignedand the laser operating at 100 mA above threshold.

Also, Fig. 8(b) shows the corresponding RF powerwithin the full bandwidth of the detector during thecurrent scan. Between peaks 1 and 2, and 2 and 3, the

laser returns to below threshold, and hence the noisevanishes. Note that using extended cavity modes is amethod to decrease the laser threshold slightly. Be-tween lobes 3 and 4, and 4 and 5, we observed a sharpand intense peak of noise power. This is likely tocorrespond to a laser oscillation between the twoneighboring extended cavity modes, with a relaxationfrequency of between 120 and 180 MHz. This wasobserved with the spectrum analyzer when operatingthe laser between two extended cavity modes. It isworth noting that tilting the detector does reduce theOF since the contrast observed on power modulationdecreases. However, whatever the operating condi-tions of the laser, tilting the detector always resultsin RF noise enhancement; hence this is not the idealsolution to obviate OF when low-noise operation isrequired.

At higher current, a form of saturation occurs, andthe seed from the distant reflector is not as efficient.The measured power corresponds to a weak feedbacklevel, and only etalon modulation remains. This cor-responds to a shift toward higher values of the feed-back coefficient, the operating mode transiting fromregime II to III, as defined in Ref. 19. At the same

Fig. 7. Laser power modulation observed while translating thedetector along the optical axis.

Fig. 8. (a) Effect of the OF during a 10 mA current scan. (b)Corresponding recording of the RF power within the detector band-width showing noise peaks correlated to the switching from oneextended cavity mode to another.

20 February 2007 � Vol. 46, No. 6 � APPLIED OPTICS 951

time, as described in the previous section, the RIN isreduced with increased output power, and the RFpower level is considerably reduced.

The spectral behavior under strong OF was furtherinvestigated by coupling the transmitted part of thelaser power with the Bruker IFS 125HR FTS. Thelaser current was scanned from threshold (T � 94 K,I � 653.5 mA) up to I � 662.8 mA with 0.2 mA steps.At each of the steps a 0.01 cm�1 resolution spectrumwas recorded to accurately determine the emissionfrequency. Figure 9 shows a plot from this data set. Itshows the laser frequency as a function of current.Rather than a continuous tuning, steps are observedthat are coincident with drops in power between twoextended cavity modes. This is due to the sudden hopfrom one of the extended cavity modes to the next one.The theoretical free spectral range of the extendedcavity is 0.010 cm�1. Assuming that the central pointof each segment in Fig. 9 corresponds to the peakpower emission within the given extended cavitymode, the free spectral range was found to be consis-tent with the theoretical value.

Due to the unstable nature of the laser emissionwhen operating between two consecutive extendedcavity modes, it was not possible to record high-resolution spectra of the corresponding emission withthe FTS. A high-resolution FTS measurement re-quires a few minutes, but the unstable operation ofthe laser usually only lasts for a few seconds, asindicated by the apparition of a peak in the 120 to 180MHz range on the spectrum analyzer. However, inFig. 9 there are three anomalous increases in theFWHM of the emission spectrum corresponding to atransition between two extended cavity modes. Thelineshape was also observed to have an unusual tri-angular shape.

Several methods of reducing feedback have beeninvestigated. Tilting the detector did not appear to bea suitable solution since it reduced the power at thedetector and did not prevent RF noise enhancement,as witnessed by Fig. 8.

The introduction of pinholes has also been tried un-successfully. Though machined with a conical profile,they introduced their own OF that perturbed the laser.

Of the methods investigated during this work, thebest was the introduction of a quarter-wave platebefore the detector. Reflection from the detector chipintroduces a � phase shift, and a polarizer could thenbe used to block the backreflected radiation. The po-larizer extinction ratio was 1�300, and Fig. 10 showsthe observed suppression of feedback. Once theorientation of the quarter-wave plate had beenoptimized, the amplitude modulation due to OF van-ished, and the residual amplitude modulation of thelaser power was decreased to 0.1%. We have alsomeasured a similar amount of RF power when reduc-ing the detection bandwidth to 200 MHz, showingthat the OF induced noise remains within that bandand is insignificant at higher frequencies. A moreefficient way of reducing OF would be the introduc-tion of a dedicated Faraday isolator,21 with a higherextinction ratio.

5. Conclusion

We have presented an experimental investigation ofthe intrinsic and OF-related high-frequency noise ina cw DFB QCL. The scaling parameters deducedwere in good agreement with those observed previ-ously11 for a different QCL device. Within the noisespectrum, a series of intense sharp peaks have beenobserved, mainly in the 0–200 MHz range. Thesepeaks have a much smaller scaling parameter,around 1.1. OF effects have also been characterized.As expected, under strong OF, no coherence collapseregime was observed. The use of a quarter-waveplate–polarizer system was found to be the most ef-ficient isolation technique. It is worth noting that,even though this work emphasizes the use of a ROCdetector, conventional detectors (MCT with AR coat-ing) and immersed lens technology detectors havebeen observed to show similar behavior.

OF may not always be a problem, since it could beused under controlled conditions to lock laser fre-quencies. Considerable linewidth reduction, up to thekilohertz range, would also be expected.

The authors wish to thank Gary Williams andBrian Landowsky from the Space Science and Tech-

Fig. 9. Evolution of the peak position (left scale) and the FWHM(right scale) of the laser emission spectrum with the injectioncurrent, starting from threshold. Transitions from one extendedmode to another correspond to a visible step in frequency and in asignificant increase of the FWHM in three occasions.

Fig. 10. Plot similar to Fig. 8, showing the effectiveness of thequarter-wave plate in suppressing OF.

952 APPLIED OPTICS � Vol. 46, No. 6 � 20 February 2007

nology Department of the Rutherford AppletonLaboratory (RAL) for their technical support. TheEngineering and Physical Science Research Councilinstruments loan pool is also acknowledged for pro-vision of a thermal imaging system. This work wassupported by the Natural Environment ResearchCouncil (grant NER�Z�S�2003�00642) and the EarthObservation and Atmospheric Science Division ofRAL.

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