Per- formance in analog and digital signal transmission systems is also described for the most useful types of such devices: the standard. CDH [ 11, [2] and the ...
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Modulation Characteristics of Constricted Double-Heterojunction AlGaAs Laser Diodes
Abstract-Detailed experimental results are presented for three types 1) ConstrictedDouble-Heterojunction (CDH) [l],[2] : of constricted double-heterojunction (CDH) AlGaAs laser diodes. The These devices use a three-layer (double heterojunction) wavedynamic properties (pulsed andRF modulation, noiseresponse, and linguide structure (active layer and two cladding layers). Lateralearity characteristics) of these devices are given for the first time. Performanceinanaloganddigitalsignaltransmissionsystems is also mode control results from a thickness variation in the active layer. For single-mode operation, tight lateral optical confinedescribed for the most useful types of suchdevices:thestandard CDH [ 11, [2] and the high-power CDH large optical cavity (LOC) [3], ment gives a near-field spot size of 2-3 pm laterally. Trans[4]. With thelatter, an order-of-magnitudeincrease in modulated verse-mode control is also strong, resulting in a large confinepower capability over standard devices has been achieved (40-mW peak ment factor = 0.4-0.6) of the lasing mode within the active powerinto 0.2 NA, 50-pm coregraded-indexfiber).Finally,we layer. These devices are noteworthy for their extremely low present a detailed experimental analysis of self-oscillation phenomena threshold-current variation with temperature (To 200°C). inavariant typeof CDH structure (CDH-AMC) (asymmetric mode 2) Constricted Double-Heterojunction-LargeOptical Cavily confinement)[SI. Theseobservations,although uncommon in CDH and CDH-LOC devices, provide new experimental insight on the nature (CDH-LOC) [ 3 ] ,[4] : Here a four-layer waveguide structure is of self-pulsating laser diodes. produced by growing a passive waveguide layer (large optical-
(r
>
I. INTRODUCTION
M
OST APPLICATIONS of injection lasers involve modulating the optical output intensity by applying a corresponding modulation to the laser drive current. Considerable research has been devoted to the response of laser diodes to isolated pulses, digital data streams, and sinusoidal modulation, and to the optical noise and self-modulation produced by the laser independently of externalmodulation. These phenomena have generally been approached individually, and on that basis there is a growing understanding of their nature. There is lacking, however, complete data on all these in interrelated phenomena as they appear in typicaland well-characterized examples of specific device types. This paper gives the first detailed experimental results on the dynamic properties (modulationand noise) of various constricted double-heterojunction (CDH) single-mode AlGaAs laser diode structures. All of these structuresare based on epitaxial growth of cladding, active, and passive waveguiding layers over a pair of dovetail-shaped channels etched into the GaAs substrate. Orientation dependence ofthe epitaxial growth gives thickness variations to thelayers, thereby controlling the laser-modal properties. Devices produced by this process include both low- and high-power single-mode lasers, with significant commercial applicability infiber-opticcommunication and optical recording. Three structures are investigated in the work reported here.
ManuscriptreceivedSeptember 24, 1982; revisedNovember 23, 1982. This work was supported in part by the Naval Research Laboratory, under Contract N00173-80-C-0370. The authors are with RCA Laboratories, Princeton, NJ 08540.
cavity structure) adjacent to the active layer. Lateral thickness variation in both active and passive guiding layers give lateralmode control. Much less lateral-mode confinement is needed for single-mode operationthan in the CDH devices, so that large lateral-mode widths (5-7pm) can be achieved. Transverse-mode confinement is alsoweak (I'= 0.1-0.3) so that most ofthe laser mode rests in the passive layer. This increases the lasing spot size inthe transverse direction.The large cross sectionforthe lasing mode reduces the optical power densityatthe facets. This, in turn, reduces thetendency foroptical damage. As aresult, these lasers operate single modeat exceptionally high-power levels (20-40 mW/ facet). 3) Constricted Double-HeterojunctionAsymmetPic-Mode Confinement (CDH-AMC) [ 5 ] : This device also uses afourlayer waveguide structure with a passive layer adjacent to the active layer. The active layer is relatively thick (0.25-0.3 pm) and offers weak lateral-mode confinement. In contrast to the CDH-LOC devices, the transverse-mode confinement is strong, with a confinement factor I? = 0.5-0.7. This device (actually a variant of the LOC structure) does not offer either the lowthreshold-current-temperature dependence of CDH devices or the high single-mode power of CDH-LOCdevices. Yet its modulation characteristics are of scientific interest for the insight they give into properties of self-oscdlations in laser diodes. The focus of the work reported here is on dynamic phenomena (modulation and internal noise) that affect laser performance in high-frequency systems applications. CDH and CDH-LOCdevices are well suited for such applications. In particular, the CDH-LOC is shown to be capable of digital modulation at greater than 500 Mbit/s and of coupling modu-
0733-8724/83/0300-0l46$01.00 0 1983 IEEE
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~ ~ , emission-coupling faclated light into a communication-type optical fiber (0.2 NA, D. Photon lifetime ( T ~ ) spontaneous tor y i j , andstimulatedoptical gain Gij(N) are specified for 50-pm coregraded index) at a peak power of 40 mW. Another purpose of this work is to present data to support each mode. J represents the current density of injected carmore rigorous modeling of laser-diode dynamics. The experriers that pumps the carrier population inversion. imental data presented here representsthe small signal (linear), Most analyses start liy postulating the modal electric field large signal (pulse) and noise characteristics of the same de- distribution E j ( x , t )based on the waveguide geometry. In genvices. With detailed data such as this, theoretical models may eral, the waveguiding properties of the laser depend on the be evaluated in terms of their ability to give a self-consistent complex refractive index and gain profiles, which are in turn representation of all the observed phenomena. Such modeling influenced by Gii, N , and J . Ei(x, t ) may then be a variable. will be useful in specifyingand characterizing laser perfor- The gain itself is, in general, determined by both the carrier mance in high-frequency system applications. concentration N , and the photon concentrations. Several previously undescribed features of laser modulation Damping of ringing in the optical output, when the laser is are reported here. One of these is the impact of laser turn-off pulsed on, has beenshown to result from variousprocesses characteristics on digital modulationperformance. Previous represented by the rate equations: carrier lifetime r,, spontawork emphasized turn-on characteristics such as relaxation O S - neous emission into the lasing-mode proportionalto yij, lateral cillations. We find that the decay characteristics of the laser carrier diffusion described by the diffusion constant D , and are considerablymoreimportant to overall system perfor- optical saturation in stimulated gain Gj;. Of these processes, mance.Another new observationis the complexnonlinear only the last two appear large enough to account fully for the interactionsbetweenexternalmodulationand laser selfobserved laser-modulation behavior. oscillations. This interactioncanproduce distortions at freSelf-oscillations or output instabilities have been discussed in quencies far from the self-oscillation frequencyand have a termsof rate equations similar tothose presentedabove. significant impact on analog modulation applications. Some additional mechanisms are postulated to account for the self-oscillations. The most discussed mechanisms are saturable 11. BACKGROUND optical absorption due to defects or to the laser facets [25]There is an extensive publishedliteratureon laser-diode [29], carrier traps [30], or nonlinear gain phenomena [31]modulation phenomena, and this paper will give only a sum- [38]. Hakki [34],[35] discusses the possible relationship mary of the current understanding as it relates to the experi- between lasernoise and instability, and also the effect of mental program described here. Two features of laser modu- instability on the lasing optical spectrum. An extensive experlation havereceived most of theattention:1) relaxation imentalliterature on this subject [35]-[45] has also been oscillations, or ringing when the laser is pulsed into lasing and developed. 2) self-oscillations or instabilities in the laser output in the abThere is, in fact, an excess of plausible explanations for selfsence ofexternalmodulation.Considerable theoretical and oscillation. It has been hard to definitively associate instabilexperimental literature exists on these topics [6]- [ 181. Small- ity with a particular mechanism, and different laser structures signal RF modulation [ 191- [23] andharmonicdistortion and\material systems may be prone to different types ofoscil[24] have also been discussed. lation mechanisms. Experimentally, the problem is that the Current understanding of laser modulation is based on con- internal variables of the laser are not directly accessible for sideration of the coupled dynamics of the carrier concentra- study. tion N(x, t ) and the photon concentration S(x, t), within the The subject of RF noise in the lasing output has also been active region of the device. Since the active region in modern discussed. Most of the work has been directed toward a theodevices is very thin (-0.2 pm) compared to the carrier diffu- retical understanding of the phenomena[46]-[51].A desion length and the mode size, the lateral position x (parallel tailed comparison between theory and experiment is lacking. to the junction plane), is the only spatial variable required. Some explanation of the impact of laser noise on communiCoupled rate equations for these variables can be used to cal- cation system performance has been made [ 5 2 ] . This has not culate the dynamic properties of the laser.These rate equa- yet resulted in complete guidelines for the design of high S/N tions can be writtenas follows: systems using laser diodes.
-aN(x, _ - _ - t ) - J(x, t ) at
ea
N 7,
+
__ a2N ax
111. EXPERIMENTAL TECHNIQUES
The basic experimental approach to all the modulation and noise measurements wasas follows. The laser diodes were ij mounted on acopperlaboratorymountandoperatedina broad-band RF drive module which provided dc bias current d t = [ci;(N) sii + [ N ( x , t )& ( 2 ) and RF modulation to the laser. Light output was coupled to the photodetector with a single lens, using a variable aperture. The photodetector usedwas a silicon avalanche photodiode ij (AEG-TelefunkenS171P).lThephotodiodes were mounted In the equations, i and j refer to longitudinal and transverse in microwave “pill” packages and operated in a broad-band modes. The active region has thickness a and is composed of material with carrier lifetime r, and carrier diffusion constant AEG Telefunken Corp., USA, Somerviile, NJ.
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matching circuit that provided dc bias voltage and coupled the modulation or noise signals to a 5042 transmission line. Signal sources for the RF drive module were selected according to the particular experiment. For pulse-response measurements,a high-speedpulser(E-H 129)2with subnanosecond rise and fall times was used. Digital data transmission was studied with a 16-bit digital word generator (Tau Tron MG15).3 The AM modulation required a variable-frequency RF signal source (HP-8620C).4 The RF noise measurements required no signal source, only the dc bias. Signalsreceived bythe photodiode were amplified by a broad-band 26-dB gain amplifier (HP-8447D) and recorded from a sampling oscilloscope or a spectrum analyzer. Data was taken as consistently as possible among the various lasers so as to facilitate comparisons among the different laser types. A brief summary of the data collection approach for each principal measurement is given below. I ) Pulse Response: A current pulse of fixed amplitude and very low duty cycle (-0.1 percent) was applied to the laser. A series of photographs of the photodetector response was taken as the bias current was varied from below to above threshold. The dc threshold current was determined from static powercurrent characteristics of the laser. 2) RF Modulation: A fixed-amplitude low-level AM modulationcurrent was applied to the laser diode. The spectrum analyzer was tuned to themodulation frequency and thesignal was recorded as the bias current was swept from below threshold to the maximum safe-operating level. A series of curves were recorded with modulation frequencies between 50 and 1200 MHz. 3) Digital Data Transmission: A series of pulses in returnto-zero (RZ) format were applied to the laser. Distortion of the output data stream from the photodiode indicates the effect of laser-modulation phenomena on digital system performance. Two kinds of observations were made. In one,the datarate was set at 500 Mbit/s and the bias current varied from below to well above threshold. In the other, the bias current was set at a value above threshold that produced maximum interpulse effccts and the datarate wasvaried. Data output was photographed on the sampling oscilloscope. 4) RF Noise: No modulation was applied. The spectrum analyzer was tuned to the desired frequency and the output recorded as the laser bias was swept from below to well above threshold. The spectrum analyzer bandwidth was set at 1 MHz. The calibration reference was the shot noise level produced by an incandescent lamp in place of the laser units at a photocurrent of 0.2 mA, a photocurrent typical of that produced by the lasers in these experiments. This shot noise reference level gives an indication of the magnitude of the excess noise of the laser source compared to an incoherent source of the same optical power.
TECHNOLOGY, VOL. LT-1, NO. 1, MARCH 1983
1.02
1.07
Current Pulse
Ip/Itb =o 18
CDH AI Go As LASER DIODE (DB-1088-67)
Fig. 1. Pulse response of a CDHlaser diode at various bias currents, showing typical behavior.
and pulse modulation systems. It also is a probe of the laser dynamic processes. Inthe measurements discussed here, a current pulse of fixed amplitude and duration is applied along witha bias currentof varying amplitude. This bias current ranges from about 10-20 percent below threshold to 10-20 percent above threshold. Fig. 1 shows the typical pulse-response characteristics of CDH AlGaAs lasers. At the lowest bias (zbiw/zth = 0.91) the laser shows a delay in the turn-on followed by a rapid increase in optical outputtoabout70 percent of the final value. Damped ringing oscillations follow the initial turn-on. These characteristics will be termed the onset phenomena. Following the onset, the output rises to its full value over a time interval of about 6 ns. This will be termedthe intermediate behavior. As the pulse current switches off, the laser output also drops rapidly. The pulse data show a rapid drop to about 10 percent of full output within less than 1 ns, followed by a slow dropoff to zero over about 5 ns. This slow drop-off is an experimental characteristic of the photodiode and not a property of the laser itself. The process taking place during and following thecurrent pulse turn-off will be termed the laser decay phenomena. Having identified these separate aspects ofthe pulse response, we follow their evolution as the laser bias is increased through threshold. The onset region shows a decrease in the ringing, which almost disappears when zbias/z* = 1. The intermediate regionis essentially unchanged. The decay process IV. BASIC MODULATIONCHARACTERISTICS shows the development of a “tail” of extended response perA. Pulse Response sisting for approximately 3-4 ns after the pulse current turns The response of the laser output to a current pulseis the off. This decay “tail” develops as the laser bias passes through basic characteristic for analyzing the performance of digital threshold and persists for higher bias currents. The typical pulse response of CDH-LOC lasers is shown in 2E-H International, Oakland, CA. Fig. 2. This behavior differs only in detail from that of the 3Tau Tron, Inc., Chelmsford, MA. typical CDH lasers shown in Fig. 1. Ringing in the onset phase 4Hewlett-Packard Corp., Palo Alto, CA.
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0
80
100
90
I IO
'bias Inn) Fig. 3. RF modulation of a CDHlaser diode at various frequencies, showing typical behavior.
CDH-LOC LASER
DIODE (DB-150A-122)
Fig. 2. Pulse response of a CDH-LOClaser diode at various bias currents, showing typical behavior.
is somewhat more intense and does not completely disappear as the bias increases above threshold. The tail in the decay region is also quite apparent. Observation of devices from seven different wafers establishes that the typical behavior indicated in Figs. 1 and 2 represents a well-defined subgroup ofthe CDH and CDH-LOC devices.Within this subgroup, pulse response is quite reproducible and consistent. Self-oscillations or enhanced ringing in the onset stage appears only in a small percentage of devices ( 0.85). However, very near threshold the valley cant factor is the presence or absence of a “tail“ in the decay amplitudes (between the pulses) begin to grow, as reflected in phase of the pulse response. Lasers showing this tail, such as the data of Figs. 18 and 19. the typical CDH and CDH-LOC devices, also show substantial With a reduced pulse current the process described above is digital power penalty when biased above threshold. compressed toward the threshold, as seen in Fig. 21. Only Most diode-laser pulse response studies have emphasized very near threshold (Ibias/Ith > 0.95) do the optical outputs ringing in the onset phase as alimitation on transmission become equal for all bit positions. capability at high data rates. However, this investigation shows that the decay phase is more significant as a source of B. High-PowerDigital Data Transmission signal degradation. One part of the problem is that the laser The available modulated power output from the laser diode does not get a chance to fully turn off before the next pulse provides a performance constraint on data transmission sys0
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a
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?
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Fig. 22. High power (20-mW average, 40-mW peak) fiber-coupled optical output of a CDH-LOC laser diode, with digital modulation.
. 7
r
0 0
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IO 'bm/q,
Fig. 20. Response of a CDH-LOC laser to 400 Mbit/s RZ 7-bit series at fixed-pulse current I p (200 mA).
D R I V E CURRENT 450 Mbit/s
OPTICALOUTPUT
Fig. 23. Digital modulation response of CDH-LOC laser at 450 Mbit/s withRZ (left)andNRZ (right) formats. Top pictures show drive current, bottom picturesshow optical output. Ibias/Ith
Fig. 21. Response of a CDH-LOC laser to 400 Mbitls RZ 7-bit series at fixed-pulse current Zp (50 mA).
However, the data rate is limited by the interface circuitry to about 75 Mbit/s. Fig. 22 shows the modulated laser drive current and fiberCDH-LOC laser using the driver tems. Substitution of CDH-LOC lasers for conventional types coupled optical output of a allows operation at powers greater by an order of magnitude. described above. The tests were made with Corning double The ability of these lasers to transmit digital data through op- window graded-index fiber with 50-pm core diameter and 0.2 tical fiber at high power and at significant data rates was re- numerical aperture. At SO percent duty cycle, the average optical power from the laser itself was 37 mW (75-mW peak outcently demonstrated [60]. The principal technical consideration in substituting CDH- put). From this, 20 mW (40-mW peak power) was measured LOC lasers for conventional types was found to be the high after coupling through 1 m of fiber (mode-stripped) and repvalues of modulated drive currents required. Pulses of approx- resents the highest power ever reported coupled into a teleimately 200 mA are required to fully modulate these devices. communications-type fiber. The cleaved end of the fiber was This requires the use of a special electronic driver circuit that prepared with an arc-formed hemispherical lens and resulted in uses .a dual-differential current switch based on high-power a 54-percent coupling efficiency from the CDH-LOC laser. microwave transistors (HP-5104). For improved efficiency, After 1 .l-km transmission through coiled fiber,the average the 5042 matching circuit of the driver, discussed in Section received power was 8.7 mW for an attenuation of 3.3 dB/km, 111, is eliminated. Instead, the laser is inserted directly into the in good agreement with the fiber manufacturer's specification. Digital data transmission witha CDH-LOClaser diode at collector circuit of one of the microwave transistors. This circuit is shown to be capable of driving a CDH-LOC laser with 450 Mbit/s with F U and N F Z data formats is demonstrated in 200-mA current pulses with rise and fall times at about 1 ns. Fig. 23. In these tests, the laser bias was just above threshold
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50
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I
I
I
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60
40 -
30
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I
I
I
I
10-3 APpp(W1
0 -
->
-2c
I
I
10-2
I
I
-10
0
I
+IO PRF (dBm)
I +20
50
Fig. 24. Analog modulation capabilities of CDHlaser diode,with fundamental receiver power (F1)and second harmonicpower (F2) Fig. 25. Analog modulation capabilities of CDH-LOC laser diode, with fundamental receiver power (Fl) and second harmonic power (F2) shown versus modulation power PRF (into 50 a) applied to laser. shown versus modulation power Pm (into 50 s2) applied to laser. Subsidiary scale shows peak-to-peak variation in optical power Subsidiary scale shows peak-to-peak variation in optical power (APpp) due to modulationat 200 MHz. (AI'*) due to modulation at 200 MHz.
(-120 mA) and themodulation current was limited to 20mA. This clearly illustrates the applicability of these high-power lasers to digital transmission systems with data rates in excess of 500 Mbit/s using above-threshold bias. Since the pulse tailing effect is evident here, we conclude that transmission at Gbit/s rates should be carried out with below-threshold bias as indicated in thepreceeding discussion.
(m),
(Fl), second harmonic and intrinsic laser noise level (at 1-MHz bandwidth). Intrinsic SNR is given by the separation (indB) between F1 and the noise level at a set P m . The signal-to-harmonic ratio (SHR2) is the separation between F1 and F2. Modulation levels for reliable operation are roughly indicated by thedata points. Examining the figure, one sees that for a maximum modulaC. Analog System Perfomance tion power P R of ~ +8 dBm, the values for PIeceiver of 60 dB The performance of laser diodes in analog applications is and a noise level of 12 dB give an SNR of 48 dB. Under these determined by the RF modulation response, RF noise, and conditions the second harmonic (F2) is at 34 dB for an SHRz harmonic-distortion characteristics described in Sections IV-B, of 26 dB. An optimum modulation level for maximizing both C, and D. This data is reduced to a form useful for predicting SNR and SHR2 would be a P m of -8 dBm to give both an system performance for CDH and CDH-LOC lasers. The for- SNR and SHR, of 39dB each. Fig. 25 gives analog performance capabilities for CDH-LOC mat is similar to that used in describing RF amplifiers. It, these devices, operationat higher modulation therefore, emphasizes the role of the laser diode as a system lasers.With powers and bias currents is possible. At a miximum modulaelement rather than as a highly specialized device. Fig. 24 gives the analog performance capabilities of CDH tion power, P w of t 2 0 dBm, the SNR is 66 dB and theSHR, lasers. Here the receiver power is shown on an arbitrary scale is 27 dB. The optimum modulation power for both SNR and as a function of RF modulation power P m into a 50-s1 load SHRz is at Pw of 0 dBm, where the values of both these measured in dBm. The modulation frequency was 200 MHz. quantities are 46 dB. The significant advantage of the CDH-LOC laser in analog A subsidiary scale shows the peak-to-peak modulated optical power (AP,) in watts corresponding to . , P The laser oper- applications is itsability to be modulatedat higher power levels than conventional lasers. In the examples given here, ating point is set at IbiaS/Z* = 1.3. The figure shows relative receiver power for the fundamental this amounts to an 18-dB improvement in SNR at the maxi-
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mum modulation level. Of course, actual performance in fiber-optic systems requires consideration of receivernoise level and modal noise phenomena. The information presented in Figs. 24 and 25 offers guidelines for determining the ultimate capability of analog systems. VI. CONCLUSIONS Comprehensive data has been presented on the modulation and noisebehavior of CDH-type laser diodes, together with studies on digital and analog-signal-transmissionperformance. These results give for the first time a picture of thecapabilities of this important class of devices for system applications at high frequencies. This section summarizes the results and conclusions of these investigations. Both CDH and CDH-LOC lasers show a pulse-modulation response (on a nanosecond time scale) characterized by the following: minimal or no ringing in the onset phase; growth in output during the intermediate phase; and bias-dependent behavior in the decay phase. With bias current below threshold, turn-off in the decay phase is rapid (subnanosecond) as expected from stimulated carrier recombination. With bias set at or above threshold a “tail” appears in the decay phase that extends for several nanoseconds. We find that this decay-phase tailing degrades digital transmission performance at high frequencies. The failure of the laser to completely extinguish beforea subsequent pulse is applied reduces the overall modulation efficiency. The system optical power penalty (in dB) deduced from the measurements was seen to increase linearly with data rate, and was in the range of 2-3 dB at 500 Mbit/s. A possible explanation of the pulse tailing phenomena is lateral carrier diffusion of injected carriers in the active layer into the lasing mode, as proposed by Ikegami [15]. This is lkely to occur in CDH structures since they provide for lateral mode confinement but not lateral carrier confinement. The injected current distribution is substantially wider (10 pm) than theoptical mode in the lasing region. After the drive current turns off and stimulated recombination depletes the carriers in the lasing mode, diffusion of the external carriers maintains the output with a time constant of the order of lo-’ s. Previous studies of laser modulation have emphasized ringing in the onset phase as an undesirable aspect of laser-modulation behavior. Our work indicates that tailing in the decay phase may have a more significant effect on digital system performance at high data rates. In the CDH and CDH-LOC lasers studied here, optimum performance is obtained when the laser bias is held below threshold, Data presented here on this mode of operation indicates that good performance can be optimized only if both the pulse amplitude and bias current are carefully‘controlled. No detailed discussion of decay-phase tailing phenomena in other laser types has yet been reported. Evidence for tailing effects occurring in other lasers is suggested in pulse response pictures presented on separated-multiclad-layer-stripe lasers [67] and channeled-substrate narrow-stripe lasers [62]. Another expected consequence of lateral carrier diffusion is in damping of the internal laser resonance. Our studies show that CDH and CDH-LOC lasers show considerable damping of
this resonance, both in the low ringing level in the pulse response and in the RF modulation and noise data. We note thatthelatter analog measurements show clear differences between CDH and CDH-LOC lasers. The pulse response data do not show pronounced differences between the two laser types. This appears paradoxical, since both analog and pulse phenomena are thought to be represented by the same rate equations. However, the RF modulationtechnique, because of its wide dynamic range, may be a more sensitive and detailed probe of laser dynamic processes than is pulse response. Use of CDH-LOC lasers has been shown to provide an orderof-magnitude increase in themodulatedoptical poweravailable for system applications. In these studies we coupled a 20-mWaverage(40-mW peak) optical power at 50-percent duty-cycle digital modulationinto50-pm core graded-index optical fiber. Analog modulation of CDH and CDH-LOC lasers has been characterized over a wide range of SNR and harmonic distortion levels.Maximum SNR values of 48 dB for CDH and 66 dB for CDH-LOC lasers were obtained at 200-MHz modulation and 1-MHz bandwidth. The improvement for the CDHLOC laser results from its ability to operate reliably at highpower (signal) levels. Noise at low frequency(1-100 MHz) showed an unstable character in both CDH and CDH-LOCdevices.The spectral intensity decreased with increasing frequency in a way suggestive of l/f noise. Carewas taken to avoid optical feedback which is known to stimulate noise in laser diodes. It issuggested that this noise component may be associated with mode hopping among the longitudinal modes. This would be consistent with the observed instability of the noise spectrum and its tendency to appear within limited ranges of laser bias current. An effort was made to investigate and characterize selfoscillations in CDH-type lasers, since this phenomena can have an important impact on high-frequency transmission systems. Of the three laser types studied, the CDH devices showed no self-oscillations; the CDH-LOC devices only occasionally (1020 percent after burn-in), and only the CDH-AMC devices had a large fraction showing self-oscillation. This encouraging result shows that the types most useful in other respects are also highly resistant to self-oscillations. The prevalence of selfoscillation only in the AMC structure suggests that asymmetry in the confinement creates conditions favorable to instability. In some respects the situation in the AMC devices resembles that of wide-stripe gain-guided lasers, in that the laser mode, while strongly confined transversally to the active layer, has only weak lateral confinement. While such gain-guided stripe lasers are known to show self-oscillations, especially after aging [42] ,[43], the consistency of instability we see in the AMC lasers is unusual. The relationship between structure and output stability clearly deserves further attention. Using lasers of the CDH-AMC structure we show for the first time interaction phenomena between self-oscillations and externalmodulationof the laser current. These arebasically nonlinear phenomena, the interpretationof which would require analytical tools not yet applied to laser-diode modulationtheory. Because this phenomena involves coupling between widely differing frequencies, they represent a potential
CHANNIN e t al. : MODULATION CHARACTERISTICS OF LASER DIODES
source of distortion in analog systems, by causing harmonics of a hgh-frequency self-oscillation frequency and a low-frequency signal to appear in the low-frequency information band. In conclusion, this experimental work demonstratesthe utility of CDH-type lasers for applications involving modulationat high frequencies. In particular, the CDH and CDHLOC structures show excellent digital and analog characteristics. Use of CDH-LOClaserscan provide an order-ofmagnitude increase in the modulated optical power available for fiber-optic communication systems, with no detriment to other modulation performance characteristics. REFERENCES [ 11 D. Botez, “Single mode CW operation of ‘double-dovetail,’ constricted DH (A1Ga)As diode lasers,” Appl. Phys. Lett., vol. 33,
pp, 872-874, NOV.1978. [2] -, “Constricteddouble-heterojunction AlGaAs diode lasers: Structures and electro-optical characteristics,” ZEEE J. Quantum Electron., vol. QE-17, no. 12, pp. 447-451, Dec. 1981. [3] -, “CW high power single-mode operation of constricted double-heterojunction AlGaAs lasers with a large optical cavity,” Appl. Phys. Lett., voi. 36,pp. 190-192,Feb. 1980. “High-power single-mode semiconductordiode lasers,” in [4] -, IEDMTech. Dig., pp. 447-451, Dec. 1981. [5] D. Botez and J.C. Connolly, “Single-mode positive-index guided CW constricted double heterojunction large-optical cavity AIGaAs lasers withlowthreshold-currenttemperature sensitivity,” Appl. Phys. Lett., vol. 38, pp. 658-660, May 1981. [6] G. Arnold and P. Russer, “Modu.lation behavior of semiconductor injection lasers,” Appl. Phys., vol. 14, pp. 255-268, Nov. 1977. [7] N. Chinone, K. Aiki, M. Nakamura, and R. Ito, “Effects of lateral modeand carrierdensityprofile on dynamicbehavior of semiconductor lasers,” ZEEE J. Quantum Electron,, vol. QE-14, pp. 625-631, Aug. 1978. [SI K. Furuya, Y. Suematsu, and T. Hong, “Reduction of resonancelite peak in direct modulation due to carrier diffusion in injection lasers,” Appl. Opt., vol. 17, pp. 1949-1952, June 1978. [9] D. Wilt, K. Y. Lau, and A. Yariv, “The effect of lateral carrier diffusion on the modulation response of a semiconductor laser,” J. Appl. PhyS., V O ~ .52, pp. 4920-4974, Aug. 1981. 101 K. Y. Lau and A. Yariv, “GBit/s rate bipolar pulse modulation of semiconductor injection lasers,” Opt. Cornmun., vol. 35, pp. 337431, Dec. 1980. 111 M. Ito, T. Ito, and T. Kimura, “Dynamic properties of semiconductor lasers,” J. Appl. Phys., vol. 50, pp. 6158-6174, Oct. 1979. 121 3. Buus and M. Danielsen, “Carrier diffusion and higher order transversalmodesinspectraldynamic of the semiconductor laser,” IEEE J. QuantumElectron., vol. QE-13, pp. 669-674, Aug. 1977. [ 131 . M. Yamada and T. Mizukami, “An analysis of direct modulation in undoped injection laser with consideration of inhomogeneous gain broadening,” Trans. IECE Japan, vol. E63, pp. 795-802, Nov. 1980. [14] D. J. Channin, “Effect of gain saturation on injection laser - switching,” J. Appl. Phys., vol. 50, pp. 3858-3860, June 1979. [15] T. Ikegami, “Spectrum broadening and tailing effect in directly modulatedinjection lasers,” in Pvoc. 1stEuropeanConf:Opt. Fiber Commun., (London, England), pp. 111-113, Sept. 1975. [16] K. Kajiyama, S. Hata, and S. Sakata, “Dynamic mode interaction in multi-mode laser diodes,” Appl. Phys., vol. 16, pp. 155-158, June 1978. [17] R. S. Tucker, “Large-signal circuit model for simulation of injection-laser modulation dynamics-Part I,” IEEE Proc., vol. 128, pp. 180-184, Oct. 1981. [18] I. Habermager,“Nonlinearcircuitmodel for semiconductor lasers,” Opt. QuantumElectron., vol. 13, pp. 461-468, Nov. 1981. [19] H. Nishi, H. Kuwahera, K. Hanamitsu, M. Takusagawa, and T. Kudo, “A semiconductor laser with flat‘frequency response up to 2 GHz,” Trans. IECE Japan, vol. E61, pp. 128-132, Mar.
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1978. [20] K. Nagano, M. Maeda, K. Saito, M. Tunaka, and R. Ito, “sinusoidal modulation characteristics of buried-heterostructure lasers,” Trans. ZECE Japan, vol. E61, pp. 4 4 1 4 4 5 , June 1978. [21] J. M. Dumant, Y. Guillasseau,and M. Monerie,“Small signal modulation ofDH laser diodes:Effect of the junction capacitance,” Opt. Commun., vol. 33, pp. 188-192, May 1980. [22] K.Y. Lau, C. Harder,and A. Yariv, “Ultimatefrequencyresponseof GaAs injection lasers,” Opt.Cornmun., vol. 36, pp. 472-474, M a . 1981. [23] K. Seki, M. Yano, T. Kamiya, and H. Yanai, “The phase shift of light outputin sinusoidallymodulatedsemiconductorlasers,” ZEEE J. QuantumElectron., vol. QE-15, pp. 791-798, Aug. 1979. [24] K. Stubkjaar and M. Danielsen, “Nonlinearities of GaAlAs lasersharmonic distortion,” ZEEE J. Quantum Electron., vol. QE-16, pp. 531-537, May 1980. [25] R. W. Dixon and W. B. Joyce, “A possible model for sustained oscillations(pulsations) in (Al, Ga)As double-heterostructure lasers,” ZEEE J. Quantum Electron., vol. QE-15, pp. 470-474, June 1979. in the self-pulsing [26] T. L. Paoli, “Saturableabsorbtioneffects (A1Ga)As junction lasers,” Appl. Phys. Lett., vol. 34, pp. 652: 655, May 1979. [27] C. H. Henry, “Theory of defect-induced pulsations in semiconductor injection lasers,” J. Appl. Phys., vol. 51, pp. 3051-3061, June 1980. [28] H. Kuwahara,“Simulation onintensity self-pulsationin CW semiconductor lasers,” Appl. Phys., VOL 20, pp. 67-73, Sept. 1979. [29] K.D. Chik, J. C. Dyment, and B. A. Richardson, “Self-sustained pulsations in semiconductor lasers: Experimentalresults and theoretical codurnation,” J. Appl. Phys., vol. 51, pp. 40294037, Aug. 1980. [30] J. A. Copeland,“Semiconductor-laser self-pulsing dueto deep level traps,” Electron. Lett., voL 14, pp. 809-810, Dec. 1978. [31] J. P. van der Zeil, “Self-focusing effects in pulsating AlXGal-,As double-heterostructure lasers,” ZEEE J. Quantum Electron., vol. QE-17, pp. 60-68, Jan. 1981. [32] B. S. Poh and T. W. Rossi, “Intrinsic instabilities in narrow stripe geometry lasers caused bylateralcurrent spreading,” IEEE J. Quantum Electron., vol. QE-17, pp. 723-731, May 1981. [33] R.P. Brouwer, C.H. F. Velzel, and B. Yeh, “Lateral modes and self-oscillations in narrow-stripe double-heterostructure GaAl-As injection lasers,” ZEEE J. Quantum Electron., vol. QE-17, pp. 694-700, May 1981. [34] B. W. Hakki, “Optical and microwave instabilities in injection lasers,”J. Appl. Phys., vol. 51, pp. 68-73, Jan. 1980. [35] -, “Instabilities in output of injection lasers,” J. Appl. Phys., vol. 50, pp. 5630-5637, Sept. 1979. [36] J. P. van der Ziel, J. L. Merz, and L. T. Paoli, “Study of intensity AI,Gal,As lasers,” J. Appl. Phys., vol. 50, pp. 4620-4637, July 1979. [37] F. R. Nash, R. L. Hartman, T. L. Paoli, and R. W. Dixon, “Stabilization of aging-induced self-pulsations andthe elimination of an initialtemperallysaturablemode of degradation in (Al, Ga)As lasers by means of facet coatings,” AppI. Phys. Lett., vol. 35, pp. 905-909, Dec. 1979. [38] R. J. Nelson and N. K. Dutta, “Self-sustainedpulsationsand negative-resistance behavior in InGaAsP (A = 1.3 pm)doubleheterostructure lasers,” Appl. Phys. Lett., voL 37, pp. 769771, Nov. 1980. Hseih, and A. J. Foyt, “Self[39] J. N. Walpole, T. A.Lind,J.J. Appl. Phys. sustainedpulsationsin GaInAsP diodelasers,” Lett., vol. 36, pp, 240-242, Feb. 1980. [40] J. C. Dyment and K.D. Chik,“Suppressionofsemiconductor J. Appl. laser pulsations using optical feedback from a fiber,” Phys., V O ~ .51, pp. 5252-5256,Oct. 1980. [41] R. L. Hartman, R. A. Logan, L. A. Koszi, and W. T.Tsang, “Pulsations and absorbing defects in(AI, Ga)As injection lasers,” J. Appl. P h y ~ .Vol. , 50, pp. 4616-4619, July 1979. [42] D. J. Channin, G. H. Olsen, and M. Ettenberg, “Self-oscillations and dynamic behavior of aged InGaAsP laser diodes.” IEEE J. Quantum Electron.,vol. QE--17,pp. 207-210, Feb. 1981. [43] D.J. Channin, M. Ettenberg,and H. Kressel, “Self-sustained oscillations in (A1Ga)As oxide-definedstripe lasers.” J. Aaol. PhyS., VOI.50, pp. 6700-6706, NOV. 1979.
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[44] T. L. Paoli, “Changes in the optical properties of CW (A1Ga)As junction lasers during accelerated aging,” IEEEJ.Quantum Electron., vol. QE-13, pp. 351-359, May 1977. [45] E. S. Yang, P. G. McMullin, A. W. Smith, J. Blum, and K. K. Shih, “Degradation-induced microwave oscillations in double-heterostructure laser diodes,” Appl. Phys. Lett., vol. 24, pp. 324-327, Apr. 1974. [46] D.E.McCumber, “Intensityfluctuations in the output of CW laser oscillators I,” Phys. Rev., vol. 141, pp. 306-322, Jan. 1966. [47] D. J. Morgan and M. J. Adams, “Quantum noise in semiconductor lasers,’’ Phys. Status Solidiall,pp. 243-263, 1972. [48] H. Huag, “Quantum-mechanical rate equations for semiconductor lasers,” Phys. Rev., vol. 184, pp.338-348, Aug. 1969. [49] T. L. Paoli, “Near-threshold behavior of theintrinsicresonant frequency in a semiconductor laser,” IEEE J. Quantum Electron., V O ~ .QE-15, pp. 807-812, Aug. 1979, [50] T. Kobayashi, Y. Takanashi, and Y . Furukawa,“Reduction of quantum noise in very narrow planar stripe lasers,” Japan J. Appl. Phy~.,V O ~ .17, pp. 535-540, Mar. 1978. [51] J. W. M. Diesterhos and A. J. Den Boef, “High-frequency noise in the output of DH (A1Ga)As injection lasers with different stmctures and waveguiding mechanisms,” IEEE J. Quantum Electron., vol. QE-17, pp. 701-705, May 1981. [52] G. Arnold and K. Petermann, “Intrinsic noise of semiconductor lasers in opticalcommunication systems,” Opt. and Quantum Electron., vol. 12, pp. 207-219, May 1980. [5 3J J. E. Ripper and T. L. Paoli, “Frequency pulling and pulse positionmodulation of pulsing CW GaAs injection lasers,” Appl. Phys. Lett., voL 15, pp. 203-205, Oct. 1969. [54] K. L. Lau and A. Yariv, “Nonlinear distortions in thecurrent modulation ofnon-self-pulsing and weaklyself-pulsingGaAs/ GaAlAs injection lasers,” Opt.Comm., vol. 34, pp. 424-428, Sept. 1980. [55] W. Harth, “Subharmonic resonance in the direct modulation of injection lasers,” Arch. Elek. Ubertrag., vol. 28,pp. 391-392, Sept. 1974. [56] D. Siemsen, “Observation of inherent oscillations and subharmonic resonance in the light output of GaAs DH lasers,” Int. J. Electron., vol. 45, pp. 63-70, 1978. of injection lasers,’’ 1571 W. Harth, “Largesignaldirectmodulation Electron. Lett., vol. 9, pp. 532-533, Nov. 1979. [58] -, “Properties of injection lasers to large-signal modulation,” Arch. Elek. Ubertrag., voL 29, pp. 149-152, Apr. 1975. 1591 R. G. Smithand S . D. Personick, “Receiver design foroptical fiber communication systems,” in Semiconductor Devices for Optical Communication, H. Kressel,Ed.New York: Springer, 1980. [60] D. J. Channin, D. Botez, J. C. Connolly, J. 0. Schroeder, J. P. Bednarz, and M. Ettenberg, “High poweroptical fiber data transmission usingCDH-LOC AlGaAs laser diodes,” in IEDM Tech. Dig., pp. 452-455, Dec. 1981. [61] H. Ishikawa, K. Hanamitsu, N. Takagi, T. Fujiwara, and M. Takusagawa, “Separated multiclad-layer stripe-geometry GaAlAs DHlaser,” IEEE J. Quantum Electron., vol. QE-17, pp. 12261233, July 1981. 1621 P.A. Kirkby, “Semiconductor laser sources for optical communications,” Radio and Elect.Eng., vol. 51, pp. 362-376, July/ Aug. 1981.
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LT-1, NO. 1 , MARCH 1 9 8 3
and modulation, electro-optics waveguidedevices, and liquid crystals. His work on liquid crystals has included the developmcnt of an integrated-circuit inspection process and a new technique for scanned displays.During 1977-1978,hespent a year at the Univcrsity of SSo Paulo, Srio Carlos, Brazil, as a Visiting Professor. Since his return to RCA Laboratories he has been doingresearch on modulation and high-frequency effects in injection lasers. He is the author of approximately 25 technical publications and holds nineU.S. patents. Dr. Channin has received two RCA Laboratories Outstanding Achievement Awards. He is a member of Sigma Xi, Tau Beta Pi, the American Physical Society, and theOptical Society of America.
*
JOURNAL O F LIGHTWAVE LT-1, TECHNOLOGY, VOL.
NO. 1, MARCH 1983
ment of waveguide materials, electrooptic modulators, and holographic surface gratings. He is presently working with various optical communication links and coupling junction lasers to optical fibers. He has published twelve technical papers and holdsfive U.S. patents. He is a memberof the Optical Society of America.
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junction large-optical-cavity (CDH-LOC) lasers for optical-disc recording and long-distance highdata-rate fiber-opticalcommunication. He has co-authored morethan ten scientific paperson hiswork. Mr. Connolly is a member of the American Vacuum Society and the American Society of Mechanical Engineers.
* John C. Connolly ("79) received the A.S. degreeinmechanicalengineeringtechnology from Middlesex County College, Edison,NJ, in 1973 and the B.S. degree in mechanical engineeringfromRutgersUniversity, New Brunswick, NJ, in1977. He ispresentlyworking towards the M.S. degree in materals science at Polytechnic Institute of New York, Brooklyn, NY. He joined RCA Laboratories, Princeton, NJ, in 1979. As an Associate Member of Technical Staff, he has been involved in the growth of multiple-layer heterojunction structures for the development of the constricted double-heterojunction (CDH) laser, anovel low-power single-mode-stabilized CW diode laser. More recently, he contributed to the development of a new type of high-power single-mode-stabilized CW diodelaser: the terracedheterojunction large-opticalcavity (TH-LOC) laser: Currently, he is involved in the development of high-power constricted double-hetero-
Daniel W. Bechtle received the B.A. degree with honors in physics from the University of Oregon, Eugene, in 1971 and a Ph.D. in Physics in 1977 from the University of Colorado, Boulder. As part of his graduate work, he designed and built an automatically stabiLized scanning Fabry-P6rotinterferometer. HisPh.D. dissertation was concerned with experimental laserlight Brillouin scattering in a transparent semiconductor. In 1978 he joined RCA Laboratories at Princeton, NJ, as a Member of the Technical Staff. There he has developed analog and digital fiber-optic communication systems, and is currently developing fiber-optic linksoperating at GHz rates. Heis the author of several papers on Brillouin scattering and fiber optics, Dr. Bechtle is a memberof Phi Beta Kappa.
Dynamic Single-Mode Semiconductor Lasers with a Distributed Reflector YASUHARU SUEMATSU, FELLOW,
IEEE,
SHIGEHISA A M I ,
AND
KATSUMI KISHINO
(Invited Paper)
Ahtract-Recent progress in the dynamic single-mode (DSM) semiconductor lasers in the wavelength of 1.5-1.6 pm are reviewed and the basic principle of DSM operation is given. Study of the DSM laser is originated for application to the wide-bandoptical-fibercommunication in the lowest loss wavelength region of 15 to 1.65 gm. A DSM and atransverse-modelaser consists of amode-selectiveresonator controlled waveguide, such as the narrow-stripeddistributed-Braggreflector (DBR) laser, so as t o maintain a fixed axial mode under the of monolithicintegration rapid direct modulation.Thetechnology for optical circuits is applied to realize some of DSM lasers. Structures, static and dynamic characteristics of lasing wavelength, output power, Manuscript received September 27, 1982. The authors are with the Department of Physical Electronics, Tokyo Institute of Technology, Tokyo, 152Japan.
and reliabiiity of the art DSM lasers are reviewed. The dynamic spectral width of 0.3 nm, the output pow= of a few milliwatts, and the reliability over a few thousand hours are reported for experimental DSM lasers.
I. INTRODUCTION
INGLE-WAVELENGTH-OPERATION or single-longitudinal-mode operation of a rapidly modulatedinjection laser in the wavelength of 1.5 to 1.6 pm is very attractive for a light source, especially for single-mode fiber communications in that minimum loss wavelength region [l] , [ 2 ] - [14] , because of the fact that the chromatic dispersion [15] ,due to
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0733-8724/83/0300-Ol61$01.OO 0 1983 IEEE
