Pulse broadening in picosecond amplification by a 1.3 μm InGaAsP travelingwave amplifierHaiFeng Liu, Masaki Tohyama, Takeshi Kamiya, and Masato Kawahara Citation: Applied Physics Letters 63, 132 (1993); doi: 10.1063/1.110377 View online: http://dx.doi.org/10.1063/1.110377 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/63/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Experimental study of guidedwave modes in 1.3 μm InGaAsP lightemitting diodes Appl. Phys. Lett. 55, 343 (1989); 10.1063/1.101903 Amplification of high repetition rate picosecond pulses using an InGaAsP travelingwave optical amplifier Appl. Phys. Lett. 53, 1539 (1988); 10.1063/1.99949 Distortionless picosecond pulse amplification and gain compression in a travelingwave InGaAsP opticalamplifier Appl. Phys. Lett. 53, 1239 (1988); 10.1063/1.100025 Homogeneous gain saturation in 1.5 μm InGaAsP travelingwave semiconductor laser amplifiers Appl. Phys. Lett. 51, 381 (1987); 10.1063/1.98424 GaAs travelingwave polarization electrooptic waveguide modulator with bandwidth in excess of 20 GHzat 1.3 μm Appl. Phys. Lett. 51, 83 (1987); 10.1063/1.98982

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

155.33.16.124 On: Thu, 27 Nov 2014 18:20:23

Pulse broadening in picosecond amplification by a 1.3 pm InGaAsP traveling-wave amplifier

Hai-Feng Liu Photonics Research Laboratoq$, Department of Electronic Etlgineering, University oj’ Melbowne. Parkvilfe, Victoria 3052, Australia

Masaki Tohyama and Takeshi Kamiya Department of Electronic Engineering, ihGuer.sity of Tokyo, 7-3-I Hongo, Bttnkyo-ku, Tokyo 113. Japan

Masato Kawahara Research Lnboratory, OKt Electric Industy Co. Ltd., 550-5, Ifigashiasakau~a-the, Hachioji, Tokyo 193, Japan

(Received 2 June 1992; accepted for publication 28 April 1993)

Pulse broadening is observed for the amplification of a 2.2 ps pulse by a semiconductor laser traveling-wave amplifier when the input pulse energy is well below the saturation energy of the amplifier. The broadening is found to be much larger than that predicted on the basis of homogeneous gain saturation. It is shown from a traveling-wave rate equation simulation that t.he nonlinear gain compression is responsible for such a large temporal brotldening.

Amplification of short pulses using semiconductor la- ser traveling-wave amplifiers (TWAs) has been the subject of study in a number of experiments.13 When the input pulse energy is higher than the saturation energy Z& of the amplifier, which is defined as the pulse energy at. which the probabi1it.y of carrier recombination by stimulated emis- sion is l-l/e,? both pulse broadening and pulse compres- sion were observed,’ and the distortion was explained as a direct result of the gain saturation of the amplifier.’ MThen the input pulse energy was well below the saturation en- ergy level, as was the case for the pulses generated from semiconductor lasers, no observable temporal and spectral distortions were found for amplifying 1.5-25 ps pulses with InGaAsP TWAs,‘” and it was concluded that semicon- ductor laser amplifiers can be treated as homogeneously broadened two-level systems in the picosecond regime. In femtosecond experiments on a 1.55 pm mulitple quantum well amplifier, Hall et uZ. showed that the saturation out- put power of the amplifier for a 150 fs pulse was less than that for a 20 ps pulse, and explained the results by the dynamic carrier heating induced gain nonlinearity.’ How- ever, no detailed discussion of temporal changes of the input pulse was made. Recently, spectral distortion due to the gain depletion and hot carrier thermalization was ob- served in the picosecond amplification by a GaAs TWA,7 where again no information about temporal distortion of the input pulses was given.

In this letter, we report the observation of temporal broadening of a short pulse ( -2.2 ps) in the amplification by a 1.3 pm TW4 when the input pulse energy is well below the saturation energy. We examine the temporal dis- tortion of the pulse when passing through the TWA, and show the pulse broadening and spectral narrowing at var- ious input power levels and TWA bias current levels. It will be shown that the observed pulse broadening is much larger than that predicted on the basis of homogeneous gain saturation,’ and that nonlinear gain compression is responsible for such a large temporal broadening.

The 1.3 [lrn InCaAsP TWA used in this experiment is

a state-of-the-art fiber-to-fiber module,” which consists of a TWA laser chip with antireflection coatings on both facets, a pair of ball lenses for input and output optical coupling, a pair of isolators, a narrow bandpass filter, a polarization beam splitter, and two polarization maintaining tibers, The threshold current of the laser chip before coating was 20 mA. This amplifier module can provide approximately 11 dB fiber-to-fiber small signal optical gain centered at 1.305 kern with a 3 dB bandwidth of 15 nm (limited by the filter). Both the input and output coupling loss were 4 dU. The gain ripple was found to be less than 0.5 dR at typical operating conditions (Ibias= 100 mA): owing to the ex- tremely low residual reflectivities ( ~0.02% ).

The 2.2 ps pulses were prepared by compressing 18 ps gain-switched pulses generated from it 1.3 pm distributed feedback laser diode with a dispersion-shifted fiber.“’ Las- ing wavelength of the pulses was centered at 1.303 /lrn with a spectral width of 1.1 nm, which just falls at the flat gain region of the TW.4. The repetition period of the pulses was 10 ns, which is long enough for the amplifier to recover its gain between pulses.

We launched the 2.2 ps pulses into the TWA and mea- sured the amplified pulses with a noncollinear autocorre- lator (0.1 ps resolution j at various launching power levels and TWA bias currenk The experimental accuracies were ensured by using the same fiber input for the autocorrela- tor to perform all the measurements. Figure 1 shows the typical temporal and spectral characteristics of the pulses before and after amplification, where the amplifier was op- erated just at 3 dB gain compression point (Z~i;ir= 100 mA and the input pulse energy at the facet of the amplifier was - 100 El). From Figs. 1 (a) and 1 (b) we can see that after amplification the full width at half-masimum (FWIIM) of the pulses at the second-harmonic intensity (SHTj trace was broadened from 4.25 to 4.95 ps. Since these autocor- relation traces fit well with the Lorentzian profile,“’ the broadening in actual pulse width should be 0.3-0.4 ps. In correspondence with this temporal broadening, change in spectrum also occurred, As shown in Figs. 1 (c) and 1 (d),

132 Appl. Phys. Lett. 63 (2j, 12 July 1993 0003~6951/93/63(2)/ 132/3/$6.00 $1 1993 American institute of Physics 132 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

155.33.16.124 On: Thu, 27 Nov 2014 18:20:23

I 2 .g z? @ E 5 (W -6 -4 -2 0 2 4 6 -6 -4 -2 0 2 46

1 ffip”t

,^ ; FWW - 1 1 nm -

0.5 nn, P d c--i g 6.5 .$

i

c E c

ii i 0 11a1.3

wweh?ng1lr inm1

Delay Time (ps)

ic) o ,343 3

kvawlength (nm)

Cd)

FIG. 1. hxkgrcxmd-free autocomelation traces of the pulse before (a) and after amplification (h). Since these traces fit well with Lorentzian prc~filc, the trmporal broadening in pulse width was estimated to be 0.3- 0.4 ps. Avaxged sptvtra of the pulse before and afkr amplification arz shown in cc) amd (dj, respectively, where the spectral width WGS nar- rowed due to the self-phase modulation effect.

the time-averaged spectral width (FWHM) was narrowed from 1.1 to 0.52 nm after amplification and the substantial part of the encr;iy was shifted slightly to the longer wave- length side, consistent with Ref. 7. This change in spec- trurll can be considered as a result of the self-phase mod- ulation effect in the amplifier, which originates from the index change induced by the carrier density variations dur- ing the amplification.“~’ 1*12 The temporal broadening and the spectral narrowing were examined at various input power levels and the TWA bias current levels. The results were summarized in Figs. i ? and 3, respectively, where Ar is the temporal broadening, defined as the difference in FWHM of the pulses before and after amplification at the SHI trace. As the input pulse energy was increased, the temporal broadening grew exponentially and the broaden- ing was typically about several hundred femtoseconds. It should also be noted that at the identical input power level, and increase in dc bias current levels of the TWA will enhance the pulse broadening.

Previous experiments on femtosecond gain dynamics

a TWA bias: 125 mA o

I j .,.,._.., $,f ,...I ~ .,*.I “.W

1

Input Pulse Energy (fJ)

FIG. 2. Summsry ofthc nbi;etved pulse broadening as a function of input pdse mrr#_v Of the x2 ps lx~lse estimated at the facet of the amplifier at various TWA hias current lev& where the pulse broadening is dcliaed as time differense in FWIlM found at SHI traces before and after amplilication.

133 Appl. Phys. Left., Vol. 63, No. 2, 12 July 1993

1.2 . ‘..“-‘r . .“.‘.‘I . * ‘..- P E. 0.9 - ; =. s

g o.s- 0 0

z n TWA bias: 50mA 8

t $

0.3 - 0 TWA bias: IODmA

cn” - l TWA blss: f25mA

0.0 ’ * * .+*‘.I . * ’ ..*‘.I . - ‘*“LL 1

Input klse Engage (fJ) 1000

PIG. 3. Summary of the change in spectral width as a function of input pulse txrrgy of the 2.2 ps pulse at various TWA bias current levels.

of TWAs did not show changes in pulse width after am- plification.” This ditl’erence may be due to the weak input pulses they used. From the data in Ref. 6, we estimated at the 3-dB gain compression point the input pulse energy Ei” of the 150 fs pulses was 2.5 Ll and the corresponding peak power was 16.7 mW? both of which were much weaker than the pulses used in this experiment (Ei,zz 100 ff, peak power - -45 mW). For those weak and short pulses, one may expect that temporal broadening in a TWA with 16 dB gain is insignificant in comparison with the broadening induced by the material dispersion,13 as will be shown later.

Two mechanisms have been considered to give tempo- ral broadening of short pulses in an amplifier. One is the material dispersion, and the other is gain saturation. It is shown’” that the dispersion-induced temporal broadening depends highly on the input pulse width and the amplifier. Using typical data of TWAs, we estimated that the broad- ening for a 150 fs pulse is about 100 fs and that the broad- ening is less than 20 fs for a 2.2 ps pulse. Thus, the bro- dening induced by the group velocity dispersion of the material is obviously not responsible for the observed re- sults of several hundred femtoseconds. The pulse broaden- ing induced by gain saturation when the input pulse energy is far above the saturation energy had been demonstrated experimentally’ and theoretically.’ However, when the in= put pulse energy E,, is well below the saturation energy E,.t, the gain saturation is known to give little influence to the pulse broadening. From the measurement of saturation characteristics of the TWA,9 we estimated that the satura- tion energy E,,, for the 2.2 ps pulse was 5.1 pJ. Thus, the maximum ratio of Ei,/Esat in this experiment was 0.02. Following the standard approach,s we calculated the tem- poral broadening of the 2.2 ps pulses when the saturation energy Esat and the initial gain of the amplifier were 5 pJ and 22 dB, respectively. When the input pulse energy was 100 fl, as the case of this experiment, the broadening for the 2.2 ps pulses was less than 100 fs. Therefore, the gain saturation is not likely to be the dominant mechanism for the observed several hundred femtoseconds broadening in this experiment.

On the other hand, the nonlinear gain may cause a reduction in gain (gain compression) even when the input pulse energy is well below the saturation energy.9,*5 This reduction in gain will certainly give influence to the dy-

Liu ei a/. 133 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

155.33.16.124 On: Thu, 27 Nov 2014 18:20:23

namic response of the pulses. Because this effect is only pronounced when the photon density inside the cavity is very high, it will cause little change to the leading edge and the trailing edge of the pulses, but will lower the pulse peak significantly, which in turn results in a wider pulse width with respect to FWHM. This speculation was confirmed by examining the amplification of a longer pulse ( 18 ps j with the same energy, where no observable broadening was re- solved from the autocorrelator measurements. Since the gain compression was found to be larger at. higher input power levels and higher bias current levels, it will lead to greater distortion when TWAs are biased at higher current levels, and/or, when the pulses are of higher peak power. This result indicates that although semiconductor laser TWAs can provide higher output power when they are biased at higher current levels, temporal distort.ion will also be enhanced due to the larger gain compression.

To conform this nonlinear gain-induced pulse broad- ening, we simulated pulse amplification in a semiconductor laser TWA using a set of traveling-wave rate equations,” where the nonlinear gain is expressed in the form of gNL =g,/( 1+ ES) by the linear gain g,. , gain compression fac- tor E( = lo- i7 cm3), and the photon density S. The calcu- lated results of input pulse energy dependent pulse broadening were plotted in Fig. 4 by solid circles, where the initial gain of the amplifier and the input pulse width were assumed to be 22 dR and 2.2 ps, respectively. For comparison, calculated pulse broadening under the same conditions using the theory in Ref. 8 is also shown in Fig. 4 (open circles). From these results we can see that the rate equation simulations with a phenomenological gain compression parameter are in good agreement with the experimental observations in Fig. 2.

In summary, temporal broadening of an extremely short ( ~2.2 ps) nearly transform-limited pulse was ob- served for the first time when it was amplified by a 1.3 pm semiconductor laser TWA under the condition that the input pulse energy is well below the saturation energy of the amplifier. The pulse broadening was exdmirled at var- ious input power levels and the TWA bias current levels, and was found to be related to the nonlinear gain-induced gain compression. By including the gain compression pa- rameter in a set of traveling-wave rate equations, we suc- cessfully simulated this large broadening and the calcu- lated results were in good agreement with the experiments.

Gain = 22 dB FWHM = 2.2 ps 0

l

0 Nonlinear Gain l

0 Gain SaturatEon 6 I

p ..i., :.:,,,,; ;,J 10-I 10° 10' 103

Input Pulse Energy (fJ)

FIG. 4. Calculatt+ temporal brosdenings (solid circles) of a 2.2 ps pulse in a semiconductor laser TWA with an initial gain of 22 dH using a bet of traveling-wave rate equations, where the gain compression parameter t; IO ” cm’ was incorporated. For comparison, broadening induced by gain saturation calculated using the theory in Ref. S are also rhown (open circles).

The authors wish to thank Dr. Y. Ogawa and Dr. T. Kunii for discussions.

‘I. W. Marshall and D. M. Spirit, Conference on Lasers and Electm- Optics, CLE.0 ‘SS, Los Angeles, CA, 19% (unpublished), paper TUM- 64.

?I. W. Marshall and W. J. Delvin, Conference on Lasers and Electro- Optics. Cl.EO ‘89, Washington, DC, 1989 (unpublished), Post dead- Iine paper, PD 16-I.

‘J. hf. Wiesenfeld, G. Eisenstein, R. S. Tucker, G. Raybon, and P. B. Hansen, Appl. Phys. Lett. 53, 1239 (19%).

‘G. Eisenstein, P. B. Hansen, J.M. Wiaenfeld, R. S. Tucker, and G. Raybon, Appl. Phys. Lett. 53, 1539 (198X).

‘A. J. Lowery, IEE Proc. 136, Pt. J, I41 (1989). “K. L. Hall, E. P. Ippen, J. Mark, and G. Eisenstein, Appl. Phys. Lctt.

56, 1740 (1990); Y. L:d, K. L. Hall. E, P. Ippen, and G. Eisenstein, 1EEE Photon. Tech. L&t. 2 711 (1990).

’ P. J. Delfyett and Y. Silberberg, Appl. Phyr, L&t. 59, 10 ( 199 I ). ‘G. P. .49ranal and N. A. Olsson, IEElI? J. Quantum Electron. QE-25,

2297 (1$89). “H. F. Liu, M. Tohyama, T. Kamiya, and hi. Kawahara, Jpn J. Appl.

Phys. 31, Pt. B, 1.4X7 ( 1992). “‘II. F. Liu, Y. Ogawa, and S. Oshiba, Appl. Phys. Lett. 59, 1284 (1991 ). “N. A. Olsson and G. P. Agrawal, Appl. Phys. L&t, 55. 13 i 19%)). “Ii. F. Liu, Y. Ogawa, S. Toshiba, L ~md ‘1~. Nonaka, IEEE J. Quantum

Electron. QE-27, 1655 ( lc)91 ). “hf. P. Kesler and E. P. Eppen, Appl. Phys. Lett. 51, 1765 ( 1987). ” 4 E Siegman. I..&%‘~.~ (Oxford University Press, Oxford, England, ‘, ~

19X6), Chap. 9. “A.. Uskov, J. Mark, and J. Mark, IEEE Photon. Tech. L.&t. 4, 443

(lusr). IhM. Tohyama, R. Tahahashi, and T. Kamiya, IEEE J. Quxitum Elec-

tron. QE-27, 2201 (1091).

134 Appl. Phys. Lett., Vol. 63, No. 2, 12 July 1993 Liu et al. 134 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

155.33.16.124 On: Thu, 27 Nov 2014 18:20:23