Invited Paper

Applications of optical feedback in laser diodes

Peter de Groot

Hughes Danbury Optical Systems

1 00 Wooster Heights Road,

Danbury, CT 06810

Smali amounts of scattered light have strong effects on the behavior of laser diodes.in the present work, a wide range of applications of these effects Is considered,from laser radar to precision metroiogy of optical systems. The simplicity and highpertormance of these systems make them viable competitors to more traditional opti-cai instruments.

The gain medium of a semiconductor laser diode is a remarkably effective lightamplifier, and modem Fabry-Perot laser diodes have coated facets to take advantage ofthis high gain. Most commercial devices have front-facet refiectivities of a few percentthat permit high power outputs but, as a side effect, increase the sensitivity of these las-ers to small amounts of optical feedback. The amount of feedback need not be high forthe effects to be significant--feedback levels as small as as 1 0 times the emission intensi-ty are sufficient to measurably after the power output and frequency of the laser.Depending on the application, this extraordinary sensitivity is either a curse or a blessing.This paper is about the blessings.

The idea of using optical feedback in lasers deliberately for some practical appil-cation is almost as old as the laser itself. in the early 60's, the effects of feedback on therelative strengths of the 0.63j.t and 3.39j.t lines of a HeNe laser were used in the study ofplasma density [1 -3]. Veiocimetry using gas lasers has been proposed sporadicallysince that time [4-8].

The now widespread use of laser diodes in research laboratories has lead inevitablyto the rediscovery of optical feedback phenomena, due in large part to the almost unavoida-bie strong effect of scattered light on these devices. A generic and easily reproducibleexperiment is shown in fig.1 [9-1 1]. The naturally divergent diode beam is captured by asmall lens of fairly large numerical aperture, such as a microscope objective or a GRIN(gradient Index) optic, and the light is focused on a rotating disk, which could be the innerportion of a chopper wheel. The disk is tifted so that there is a velocity component alongthe line of sight. If the laser diode is packaged together with a photodiode for moni-toring the power output, the only extra equipment is a sensitive (lOiiV) oscilloscope orspectrum analyzer. The optical feedback effect resufts in a modulation of the photodiodesignal as shown in flg.2, with a modulation frequency that is directly proportional to the rota-tional speed of the disk. What we then have Is a very simple and easy to align laserradar.

SPIE Vol. 1219 Laser-Diode Technology andApplications II (19901/ 457

Figure 1 . Backscatter modulation velocimetry. Some of the doppIershiftedscattered light from the spinning disk is fed back into the laser, inducing apower modulation of the diode. The frequency of this modulation Is proportional to the rotation speed of the disk.

In trying to understand the source of the modulation for the setup in fig.1 ftls temptingto assume that a portion of the scattered light from the rough surface of the rotating diskis being projected back through the laser onto the detector, where interference occurs.The analysis is then reduced to a simple reproduction of the appropriate equations fortwo-beam interference [10,1 1]. Although this approach yields the correct fundamentalmodulation frequency, both the modulation waveform (fig.2) and the signal strength(flg.3) are dramatically different from what would be expected if the laser where playinga passive role in the transmission of the scattered light to the detector. Although these differences have occasionally been described as unexplained [10], the underlying causeof the mystery is just a case of mistaken Identity. The true detector in fig.1 Is not the pho-todiode but is the laser Itself.

Researchers working in the more esoteric domains of laser diode behavior haveknown for some time that small amounts of optical feedback lead to modulations such asthose shown In fig.2 [1 2-1 5]. These researchers where usually less interested in veloci-metry than In laser mode stablization and suppression of feedback noise for fibercommunications. The task of rewriting the relevant feedback equations in the context offlg.1 is easily achieved once we realize that the spinning disk must be included as anIntegral part of the laser resonator when calculating the diode's power output and modefrequency [8,16]. The concept is illustrated in fig.4, which shows how an external source offeedback, here modeled as a weakly reflecting mirror in a three-wail Fabry-Perot cavity,can be included In the effective reflectivity of the front facet of the diode resonator.This effective reflectivity has a modulus and phase that depend on the position of theexternal reflector. Since the threshold gain of the laser depends on the front-facet reflec-tivity, the power output of the laser will be depend on the position of the optical feed-back source. An addItional consideration is the coupling between the refractive index of

458 / SPIE Vol. 1219 Laser-Diode Technology andAppilcations 11(1990)

..OTA TIMp/i,,'

+80

>

LU

0>0

800 200

TIME( &s)

Figure 2. Oscilloscope trace of the signal produced by a backscattermodulation laser radar similar to flg.1 . The sawtooth waveform is verydifferent from what we would expect from a simple interference effect.From ref.[16J.

the gain medium to the photon density, which results In changes in optical frequencywith feedback. When all of these factors are considered, the modulation waveform (fig.2)and signal strength (fig.3) are readily obtained [1 6]. The result of optical feedback is abackscatter modulation of the laser diode itself.

These considerations lead to the conclusion that what we have built in flg.1 is not somuch an interferometer as is ft is an extraordinarily long compound-cavity laser diode. Thephotodetector In flg.1 Is playing a secondary role as a monitor of the effects of movingone of the "walls" (the rotating disk) in this compound cavfty. This point is made strong-er noting that It is possible to detect optIcal feedback by monitoring the energy consump-tion of the laser directly using a sensitive current meter [7,9].

The dIfference between ordinary coherent detection and backscatter modulation ismore than philosophical. As is shown in fig.3, the best modulation depths are obtainedwith the laser running near Its threshold power levels. This is contrary to what we wouldexpect from an ordinary homodyne velocimeter. Thus in backscatter modulation there is lit-tIe to be gained by increasing the pump current to the laser, unless the return signal is soweak that we approach the quantum noise limit for detection.

The fact that most backscatter-modulation experimentors run their diodes well abovethreshold Is a consequence not of the need for strong feedback levels, as sometimessupposed [1 7J, but of the temporal coherence requirement for optical feedback phenome-na. In practice, the operational range of the system in flg.1 is about half the coherencelength of the laser. The spectral width of a laser diode's emission narrows with increas-ing power, and for target distances of a few meters, the coherence requirement is fulfilledonly when the diode is pumped at maximum current levels. This is because the sys-tem is coherence limited, not photon limited.

SPIE Vol. 1219 Laser-Diode Technology andAppilcationsil (1990)! 459

100

(V) __0

'U02z0I-4 1-J0

0 -34 36 38 40 42 44

PUMP CURRENT ( MA)

Figure 3. Modulation depth as a function of pump current for a backscatter-modulated laser diode veloclmeter. The modulation depth is here definedas the ratio of amplitude of the AC component to the DC component of thesignal measured by a photodiode monitoring the laser output. Theenhancement near the threshold current level is characteristic ofbackscatter modulation. The Ideal limit for conventional homodynecoherent detection is 1 x I O From ref.[20J.

Since in most cases the operational range of backscatter modulation of diodes isdetermined by the laser linewldth and not the power output, It Is natural to look for a way toreduce the phase noise In the laser. A number of options are available, Including elec-tronlc feedback (1 8] and high-finesse external resonators [1 9. The easiest solution, andone that lends Itself well to the present requirements, Is shown In fig.5. The addition ofa 40% external reflector within a few centimeters from the diode dramatically narrowsthe llnewldth of the resulting external-cavity laser, provided that the laser is pumped atcurrent levels below the ordinary threshold for the diode by Itself. This elementary exter-nal cavity configuration improves the coherence length by a factor of well over I 00 atthese low power levels. These improvements have been demonstrated with a standard40mW Sharp AlGaAs laser (LTO-1 5MDO) operating at 1 mW [20]. Without the externalcavity, the operational range for backscatter velocimetry at 1 mW is approximately 13cm,whereas the the external cavity velocimeter has an operational range of 40m.

True laser radar should give us both range and velocity. It turns out that it is rela-tively easy to do backscatter modulation laser ranging with the external cavity configurationof fig.5 by simply oscillating the 40% mirror along the optical axis over a 1 j.t range[21 ,221. One of the very nice things about diodes is that they tend to lock onto lasingmodes and track them as the overall length of the resonator varies. Of course, the onlyway this can be accomplished is if the diode frequency changes, which happensautomatically. Thus the oscillation of the 40% mirror then results in a frequency chirp thatcan be used for absolute ranging. The mechanical triangle-wave oscillation required forthis can be done by a plezo-electric translator. If the target is close enough so that

460 I SPIE Vol. 1219 Laser-Diode Technology andApplications II (1990)

I EXPERIMENT

— THEORY

I - —__ —

SERDIODE TARGET

R R -R

'IC

4 L—94-

R Z

Figure 4. TheoretIcal model of the laser system Including the diode cavity and aweak external reflector. The laslng properties of this system can bedetermined by taking the front facet and the target together as an effec-tive mirror with a complex reflection coefficient Z.

Geometry for backscatter-modulatlon laser radar with an external cavi-ty. The external cavity Is used to Increase the coherence length of the diodewhen operated near the threshold current level. The frequency rampingrequired for chirp radar Is produced by the motion of the paillally-reflectivemirror, which Is actuated by plezo-electric transducers. From ref.[22].

r

EFFECTIVE

MIRROR

LASER/DETECTOR

PACKAGE i'zr

40 % MIRROR OBJECTIVE

Figure 5.

EXTERNAL CAVITY

SPIE Vol. 1219 Laser-Diode Technology andAppilcations II (1990) / 461

the external cavity can be removed, then a trianglewave modulation of the pump currentwill have the same effect [1 6,23]. In either case, the backscatter-modulation signal hasa fundamental frequency directly proportional to the distance of the target.

High-precision relative ranging is another useful application of optical feedback.The example shown in fig.6 is the continuous, optical monitoring of a piezo-electric trans-ducer for linearity and stroke. Using phase modulation we obtain the data shown in fig.7,which provides us with a measurement of the displacement to an accuracy of I nm. Thesharp peaks in the data correspond to the steeply sloped portions of the curve in fig.2.Note that this simple laser gauge does not require a specular surface on the object beingmonitored.

Figure 6. Optical geometry of an optical feedback laser gauge. The essential sim-plicity of the geometry is its primary virtue. The light reflected back from thetarget into the laser produces modulations in the laser power output thatcan be used for displacement sensing.

Metrology of optical systems using point-scanning interferometry is also possiblewith optical feedback [24]. Here again, the feedback lnterferometer shown in fig.8 has avery simple optical geometry. The diode is placed near the center of curvature of a con-cave mirror, and a movable sub-aperture mask is used to define the coordinates of thepoint being measured. The relative phase of the returned light is determined by moni-toting the power output of the laser. As the mask is moved across the surface, the differ-ence between the wavefront and the mirror surface is mapped out. Phase tracking byelectronic feedback can be used to automate the process, and an accuracy of A I I 0 hasbeen obtained [24,25]. A variation of this geometry using a sub-aperture lens can accom-modate slope errors on the order of 20i.t / mm [26].

So far we have discussed only those applications where the output power of asingle-mode laser is used. As might be expected, optical feedback can have strong

462 / SPIE Vol. 1219 Laser-Diode Technology andAppilcations 11(1990)

I

Figure 7. Signal obtained from the optical feedback laser gauge during a dis-placement of the PZT-actuated mirror shown in fig.6. Excellent signal tonoise was obtained by using phase modulation and active filtering tech-nlques, with a 1 kHz bandwidth. Dynamic centrolding of the peaks can beperformed with a resolution of 0.001

effects on the mode structure of the oscillation as well, particularly If we are workingwith a multimode diode. The phenomenon iswell known [14] and has recently been ana-lyzed In detail In the context of metrology [27]. The basic idea Is built around the obser-vatlon that when there Is no feedback, the threshold gain Is the same for all lasing modesand the laser simply choses the mode closest to the peak In the gain curve; whereas Inthe presence of feedback, the threshold gain Is different for each one of the possibleoscillation modes. Thus the mode structure of the laser Is In some sense controlled bythe feedback. The effect can be quite dramatic even for weak feedback ( 1 ci9 times theemission intensity), and manifests Itself as a shIfting of energy between the possible las-Ing modes. This TMmode modulation" phenomenon Is periodic with the position of thesource of feedback, and Is strongest at Intervals equal to the optical length of the diode(typically about 1 mm). Using wavelength-dependent detection (fig.9), It Is possible to gen-erate the curve shown In fig.1O. This curve Is useful for position monltoilng of a variety ofobjects, Including both stationary objects as well as rotating disks or spindles In machinery.

This oveiview of some of the many possible applications of optical feedback phe-nomena shows that In practice It may be easier to give in to the feedback rather than sup-press It. As Illustrated In fig.1 I , when a diode is adapted for Insertion Into more conven-tional optical systems the amount of expensive and delicate hardware that must beincluded can be very discouraging. What Is usually billed as a debilitating hypersensitivityto scattered lIght may In fact be an Indicator of the true destiny of the laser diode, as bothsource and coherent detector for veiocimetry, ranging and precision metrology.

SHE Vol. 1219 Laser-Diode Technology andAppilcations 11(1990)1 463

PIO ogmo (rósv...)

MIRROR

IThe essential optical components of the laser-feedback interferometer

are a movable aperture and the laser diode Itself. A portion of the lightfrom the diode passes through the aperture and Is fed back into the diode.The interferometer operates by observing the effect of the feedback light onthe lasing properties of the diode as the aperture is scanned across the sur-face. Adapted from ref .[24J.

Figure 9. Apparatus for detecting mode-modulation phenomena. The grating Is usedto observe the modulation of specific lasing modes of the diode under opticalfeedback. From ref.[27].

MOVABLEAPERTURE

AMP POWER

DETECTOR LASER

DIODE

Figure 8.

DIFFRACTION

GRATING

TARGET

DETECTORS

BEAM

SPLITTER

464 / SPIE Vol. 1219 Laser-Diode Technology andApplications 11(1990)

30

10

0

TARGET DISTANCE( mm)

504

Figure 10. Strength of mode-modulation effects onfunction of the distance of a target.dependence has a peiiodiclty equal to theregion of the laser diode. From ref .[27J.

a muWmode laser diode as aThe modulation depth range

optical length of the active

Figure 11. A diode laser pretending to be a gas laser. Is this what nature intend-ed?

SPIE Vol. 1219 Laser-Diode Technology andApplications II (1990) I 465

20

501 502 503

8LAtI

S"( $DMj/Al

REFERENCES

1 . D. E. T. F. Ashby and D. F. Jephcott, "Measurement of plasma density usinga gas laser as an infrared interferometer," Appi. Phys. Let. 3 1 3 (1963).

2. D. E. T. F. Ashby, D. F. Jephcott, A. Malein, and R. A. Raynor, "Performance ofthe HeNe gas laser as an interferometer for measuring plasma density," J. Appi.Phys, 36, 29 (1965).

3. J. B. Gerardo and J. T. Verdeyen, "Plasma refractive index by a laser phasemeasurement," Appi. Phys. Left. 3, 121 (1963).

4. M.J. Rudd, "A Laser Doppler Velocimeter Employing the Laser as a Mixer-Oscillator," J Phys. E 1 , 723 (1968).

5. T. A. Lawrence, D. J. Wilson, C. E. Craven, J. Jones, A. M. Huffaker, and J. A.L. Thomson, "A laser velocimeter for remote wind sensing," Rev. Sd. Instrum.43, 512 (1972).

6. N. L Abshire, R. L Schwiesow, and V. E. Derr, uppler lidar observations ofhydrometeors," J. Appi. Meteorol. I 3, 951 (1974).

7. J.H. Chumside, "Laser Doppler Velocimetry by Modulating a C02 Laser withBackscattered Light," Appl. Opt. 23, 61 (1984).

8. J.H. Churnside, "Signal-to-Noise in a Backscatter-Modulated Doppler Velocime-ter," AppI. Opt. 23, 2097 (1984).

9. S. Shinohara, A. Mochizuki, H. Yoshida, and Masao Sumio, "Laser Doppler Vel-ocimeter Using the Self-Mixing Effect of a Semiconductor Laser Diode," Appl.Opt. 25, 1417 (1986).

1 0. E.T. Shimizu, NDirectional Discrimination in the Self-Mixing Type Laser DopplerVelocimeter," Appi. Opt. 26, 4541 (1987).

1 1 . H. W. Jentink, F. F. M. do Mul, H. E. Suichies, J. G. Aamoudse and J. Greve,"Small Laser Doppler Velocimeter Based on the Self-Mixing Effect in a DiodeLaser," Appl. Opt. 27, 379 (1988).

I 2. P. Zorabedian, W. R. Trutna and L. S. Cutler, "Bistability in Grating-TunedExternal-Cavity Semiconductor Lasers," IEEE J. Quantum Electron., QE-23, 1855(1987).

1 3. G. A. Acket, D. Lenstra, A.J. den Boef , and B. H. Verbeek, "The Influence ofFeedback Intensity on Longitudinal Mode Properties and Optical Noise in Index-Guided Semiconductor Lasers," IEEE J. Quantum Electron., QE-20, 1163(1984).

14. A. Lang and K. Kobayashi, "External Optical Feedback Effects on Semiconduc-tor Injection Laser Properties," IEEE J. Quantum Electron. QE-16, 347 (1980).

15. H. Sato, Y. Matsui, J. Ohya, and H. Serizawa, "Bistabllity and intensity noiseof semiconductor lasers due to weak optical feedback," J. AppI. Phys. 63,2200 (1988).

466 / SP/E Vol. 1219 Laser-Diode Technology andApplications 11(1990)

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