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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 9 — May. 5, 2014
  • pp: 10105–10118
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Multimode regimes in quantum cascade lasers with optical feedback

L. L. Columbo and M. Brambilla  »View Author Affiliations


Optics Express, Vol. 22, Issue 9, pp. 10105-10118 (2014)
http://dx.doi.org/10.1364/OE.22.010105


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Abstract

We study the instability thresholds of the stationary emission of a quantum cascade laser with optical feedback described by the Lang Kobayashi model. We introduce an exact linear stability analysis and an approximated one for an unipolar lasers, who does not exhibit relaxation oscillations, and investigate the regimes of the emitter beyond the continuous wave instability threshold, depending on the number and density of the external cavity modes. We then show that a unipolar laser with feedback can exhibit coherent multimode oscillations that indicate spontaneous phase-locking.

© 2014 Optical Society of America

1. Introduction

Since long Quantum Cascade Lasers (QCLs) have triggered a widespread interest for several reasons: wavelength agility (1 – 100THz), high output power (> 100mW), continuous wave (CW) emission, narrow linewidth (< 10KHz), high speed modulation (up to several tens of GHz) [1

1. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum cascade laser,” Science 264, 553–556 (1994). [CrossRef] [PubMed]

, 2

2. R. F. Curl, F. Capasso, C. Gmachl, A. A. Kosterev, B. Mc Manus, R. Lewicki, M. Pusharsky, G. Wysocki, and F. K. Tittel, “Quantum cascade lasers in chemical physics,” Chem. Phys. Lett. 487, 1–18 (2010). [CrossRef]

, 3

3. J. Faist, Quantum Cascade Lasers (Academic, 2013). [CrossRef]

]. Such features led to a row of highly prized applications in imaging, communications technology, sensing, astrophysics and space-science [3

3. J. Faist, Quantum Cascade Lasers (Academic, 2013). [CrossRef]

].

From a theoretical point of view a complete understanding of the coherent multimode dynamics in QCLs is still to be achieved and this would eventually help to identify favorable phase relations for pulsed emission.

On the other side, QCLs entered with success the community of self-mixing interferometric sensing in which the laser source is used to simultaneously generate and detect electromagnetic radiation [9

9. P. Dean, Y. L. Lim, A. Valavanis, R. Kliese, M. Nikolić, S. P. Khanna, M. Lachab, D. Indjin, Z. Ikonić, P. Harrison, A. D. Rakić, E. H. Linfield, and A. G. Davies, “Terahertz imaging through self-mixing in a quantum cascade laser,” Opt. Lett. 36, 2587–2589 (2011). [CrossRef] [PubMed]

, 10

10. Y. L. Lim, P. Dean, M. Nikolić, R. Kliese, S. P. Khanna, M. Lachab, A. Valavanis, D. Indjin, Z. Ikonić, P. Harrison, E. Linfield, A. G. Davies, S. J. Wilson, and A. D. Rakić, “Demonstration of a self-mixing displacement sensor based on terahertz quantum cascade lasers,” Appl. Phys. Lett. 99, 081108 (2011). [CrossRef]

, 11

11. M. C. Phillips and S. Taubman, “Intracavity sensing via compliance voltage in an external cavity quantum cascade laser,” Opt. Lett. 37, 2664–2666 (2012). [CrossRef] [PubMed]

, 12

12. F. P. Mezzapesa, V. Spagnolo, A. Antonio, and G. Scamarcio, “Detection of ultrafast laser ablation using quantum cascade laser-based sensing,” Appl. Phys. Lett. 101, 171101 (2012). [CrossRef]

]. As an emitter, a QCL is characterized by a class A dynamics, where the carriers and the polarization variables are enslaved by the electromagnetic field. This is due to high values of the ratio between photon and carrier lifetime in unipolar lasers where the latter is dominated by non-radiative phonon scattering. This unique feature leads to the absence of relaxation oscillations in the evolution of a free running QCLs towards its steady-state [13

13. R. Paiella, R. Martini, F. Capasso, C. Gmachl, and H. Y. Hwang, “High-frequency modulation without the relaxation oscillation resonance in quantum cascade lasers,” Appl. Phys. Lett. 79, 2526–2528 (2001). [CrossRef]

]. In the case of conventional s.c. lasers subject to optical feedback (OF) it is well known that the amplification of relaxation oscillations under increasing feedback power is the fundamental mechanism of CW destabilization, leading to chaotic dynamics [14

14. D. M. Kane and K. A. Shore, Unlocking Dynamical Diversity: Optical Feedback Effects on Semiconductor Diode Lasers (John Wiley, 2005). [CrossRef]

]. Also, the linewidth enhancement factor (LEF) or Henry factor known to play a role in conventional s.c. lasers as evidenced in [15

15. J. Helms and K. Petermann, “A simple analytic expression for the stable operation range of laser diodes with optical feedback,” IEEE J. Quantum Electron. 26, 833–836 (1990). [CrossRef]

, 16

16. D. Weidmann, K. Smith, and B. Ellison, “Experimental investigation of high-frequency noise and optical feedback effects using a 9.7 μm continuous-wave distributed-feedback quantum-cascade laser,” Appl. Opt. 46, 947–953 (2007). [CrossRef] [PubMed]

], favoring stability for low values.

Recently, we predicted that a THz single mode QCL with OF, due precisely to the absence of relaxation oscillations and negligible LEF (α < 1) can exhibit an absolute stability against OF and experiments proved that it can sustain a feedback strength ∼ 70 times larger than that typically leading to the onset of chaos in a diode laser [17

17. F. P. Mezzapesa, L. L. Columbo, M. Brambilla, M. Dabbicco, S. Borri, M. S. Vitiello, H. E. Beere, D. A. Ritchie, and G. Scamarcio, “Intrinsic stability of quantum cascade lasers against optical feedback,” Opt. Express 21, 13748–13757 (2013). [CrossRef] [PubMed]

].

In this work we focus on the instabilities and dynamical behavior of the retroinjected QCLs, and investigate in particular the occurrence of regular oscillatory dynamics, that can be qualified as form of phase synchronization among modes. This can be an important heralding feature towards mode-locking and pulse generation. In absence of relaxation oscillations, the instability we deal with is characterized by the competition of modes, not defined by the free running laser, but by the external cavity (EC) formed by the feedback mirror and the laser output facet. As we will show, the LEF and EC length strongly influence the multimode dynamics and can either remove the instability altogether or favor a chaotic dynamics. We are of course interested in regimes comprised between the two, where regular dynamics can ensue from mode competition. To address this issue, we will adopt an essential and simple modelization of a QCL with OF, that allows us to focus on crucial dynamical parameters such as the LEF and characteristic time scales of the field and carriers dynamics. This is important to gain a physical insight towards the complex dynamics originating by the destabilization of the single mode CW emission, typical of lasers with OF [17

17. F. P. Mezzapesa, L. L. Columbo, M. Brambilla, M. Dabbicco, S. Borri, M. S. Vitiello, H. E. Beere, D. A. Ritchie, and G. Scamarcio, “Intrinsic stability of quantum cascade lasers against optical feedback,” Opt. Express 21, 13748–13757 (2013). [CrossRef] [PubMed]

].

The paper begins with the description and motivation of the adopted model (Sec. 2), it then proceeds with the analysis of the CW solutions and their spectral properties, with respect to the dependence on critical parameters (LEF and EC length). A Linear Stability Analysis (LSA) in Sec. 3 identifies the conditions for the loss of stability of the CW solutions and provides a simpler approximation valid for unipolar lasers. Section 4 is devoted to the analysis of the laser behavior above the instability threshold and shows the existence of regular oscillations where a number of CW solutions contributes to the emission. The conclusions are drawn in Sec. 5.

2. The model

We consider a QCL to which OF is provided by an external mirror placed at a distance L from the laser output facet as sketched in Fig. 1. The intensity of the back reflected radiation can be varied by an attenuator (not shown in the picture). We describe the system dynamics in the framework of the well known Lang-Kobayashi (LK) approach [18

18. R. Lang and K. Kobayashi, “External optical feedback effects on semiconductor injection laser properties,” IEEE J. Quantum Electron. 16, 347–355 (1980). [CrossRef]

]. With respect to more complex descriptions of the QCL dynamical behavior based on Maxwell-Bloch equations for two [4

4. A. Gordon, C. Y. Wang, L. Diehl, F. X. Kärtner, A. Belyanin, D. Bour, S. Corzine, G. Höfler, H. C. Liu, H. Schneider, T. Maier, M. Troccoli, J. Faist, and F. Capasso, “Multimode regimes in quantum cascade lasers: From coherent instabilities to spatial hole burning,” Phys. Rev. A 77, 053804 (2008). [CrossRef]

] o three level systems [19

19. T. Gensty, W. Elsäßer, and C. Mann, “Intensity noise properties of quantum cascade lasers,” Opt. Express 13, 2032–2039 (2005). [CrossRef] [PubMed]

, 20

20. M. Yamanishi, T. Edamura, K. Fujita, N. Akikusa, and H. Kan, “Theory of the intrinsic linewidth of quantum cascade lasers: hidden reason for the narrow linewidth and line broadening by thermal photons,” IEEE J. Quantum Electron. 44, 12–29 (2008). [CrossRef]

], it keeps a relatively simple formalism while still having remarkable success in describing the actual behavior of retroinjected lasers [14

14. D. M. Kane and K. A. Shore, Unlocking Dynamical Diversity: Optical Feedback Effects on Semiconductor Diode Lasers (John Wiley, 2005). [CrossRef]

]. Actually it has been recently used to derive relevant parameters of Mid-IR and THz quantum cascade lasers such as the LEF or to measure dynamical features of the target [21

21. J. Staden, T. Gensty, W. Elsäßer, G. Giuliani, and C. Mann, “Measurements of the α factor of a distributed-feedback quantum cascade laser by an optical feedback self-mixing technique,” Opt. Lett. 31, 2574–2576 (2006). [CrossRef]

, 22

22. R. P. Green, J. H. Xu, L. Mahler, A. Tredicucci, F. Beltram, G. Giuliani, H. E. Beere, and D. A. Ritchie, “Linewidth enhancement factor of terahertz quantum cascade lasers,” Appl. Phys. Lett. 92, 071106 (2008). [CrossRef]

, 23

23. A. D. Rakić, T. Taimre, K. Bertling, Y. L. Lim, P. Dean, D. Indjin, Z. Ikonić, P. Harrison, A. Valavanis, S. P. Khanna, M. Lachab, S. J. Wilson, E. H. Linfield, and A. G. Davies, “Swept-frequency feedback interferometry using terahertz frequency QCLs: a method for imaging and materials analysis,” Opt. Express 21, 22194–22205 (2013). [CrossRef]

].

Fig. 1 Schematic layout of the self-mixing configuration. The QCL radiation is focused on the external target and re-injected into the cavity laser after a cavity round trip.

The model consists in two rate equations for the spatio-temporal evolution of the electric field and the carrier density in the laser cavity; it considers a phenomenological refractive index and gain, independent from the frequency and linearly dependent from the carriers density and a single delay term in the equation for the field. The flat gain is a sustainable approximation at this investigatory stage, since the number of external cavity modes (ECMs) we will consider and the free spectral range of the EC (EFSR) will in general allow this approximation for a QCL, whose gain width easily exceeds few THz. Another fundamental limiting assumption in the LK model is the single roundtrip approximation, where multiple reflections in the EC are neglected assuming a moderate feedback [18

18. R. Lang and K. Kobayashi, “External optical feedback effects on semiconductor injection laser properties,” IEEE J. Quantum Electron. 16, 347–355 (1980). [CrossRef]

]. Of course, when the strength of the OF grows this approximation fails and more refined approaches must be used [14

14. D. M. Kane and K. A. Shore, Unlocking Dynamical Diversity: Optical Feedback Effects on Semiconductor Diode Lasers (John Wiley, 2005). [CrossRef]

]. We nevertheless checked that even when corrections accounting for multiple reflections are introduced, the relevant dynamics remains unchanged.

Looking for CW solutions of Eqs. (1) and (2) in the form E = Es expi(ωFω0)t and N = Ns we get:
Ns=12kτpτccos(ωFτ)
(3)
ωF=ω0kτpτc[αcos(ωFτ)+sin(ωFτ)]
(4)
|Es|2=Ip1(2kτp/τc)cos(ωFτ)1
(5)
As it is well known the s.c. laser with OF is an infinite dimensional system and its steady state characteristic might be strongly modified with respect to the free running laser case. We observe in particular that: 1) from Eq. (5) it directly follows that k is limited by kmax = τc/2τp; 2) Eq. (4) for the field frequency ωF is transcendental, so its multiple solutions cannot be determined in closed form.

If not otherwise specified we use a set of parameters for the THz QCL model reported in Table 1. They refer to typical GaAs/AlGaAs heterostructures and have been derived from experimental data [24

24. F. Mezzapesa, Internal CNR-IFN report (2013).

, 17

17. F. P. Mezzapesa, L. L. Columbo, M. Brambilla, M. Dabbicco, S. Borri, M. S. Vitiello, H. E. Beere, D. A. Ritchie, and G. Scamarcio, “Intrinsic stability of quantum cascade lasers against optical feedback,” Opt. Express 21, 13748–13757 (2013). [CrossRef] [PubMed]

]. Note the large value of the photon to carrier lifetime ratio γ, a unique feature of s.c. unipolar lasers.

Table 1. Physical parameters for a QCL in the LK model

table-icon
View This Table

While the complex structure of the CW stationary solutions of this model has been studied for decades, we now dwell to some extent thereon, in order to clarify the relation among the number of solutions, their ordering with respect to the ECMs, the feedback strength and to highlighten the role of the LEF in this model.

Choosing a pump 50% above free running laser thershold (Ip = 1.5), k = 0.5, and a cavity length of L = 146mm (i.e. τ = 30 that corresponds to a cavity round trip time of ≈ 1ns), we report in Fig. 2(a) the complete set of CW solutions in the intensity–frequency plane (ωω0, |E|2) for three different values of the LEF α = 0.35, 1.3, 3. Roughly speaking, the first two values can be considered appropriate for a THz QCL, a Mid-IR QCL [21

21. J. Staden, T. Gensty, W. Elsäßer, G. Giuliani, and C. Mann, “Measurements of the α factor of a distributed-feedback quantum cascade laser by an optical feedback self-mixing technique,” Opt. Lett. 31, 2574–2576 (2006). [CrossRef]

, 22

22. R. P. Green, J. H. Xu, L. Mahler, A. Tredicucci, F. Beltram, G. Giuliani, H. E. Beere, and D. A. Ritchie, “Linewidth enhancement factor of terahertz quantum cascade lasers,” Appl. Phys. Lett. 92, 071106 (2008). [CrossRef]

], while 3 is more typical of diode lasers. Moreover, in Fig. 2(b) we plot the frequencies of the CW solutions ωF vs k for the chosen values of α. We remind that the upper limit of k in Fig. 2(b) is imposed by the self-consistency of the LK model and it is given by kmax=τc2τp=0.58.

Fig. 2 (Continuous wave solutions (a) in the (ωω0, |E|2) plane for k = 0.5 and three values of α; (b) in the (ωω0, k) plane for three different values of α. The dashed lines represents a set of adjacent ECMs. The other parameters are: Ip = 1.5, τ = 30.

Figure 2(a) shows that the CW solutions are close but not exactly coincident with the ECMs (vertical dashed lines) that are separated by the EFSR = 2π/τ ≃ 0.21 (that corresponds to ≃ 6.45GHz in physical units).

Starting from the CW closest to ω0, the CW solutions separated by ≃ 2π/τ and associated with higher intensities are called ”modes” of the system and correspond to in–phase interference between the electric field in the laser cavity and the reinjected delayed field. The other solutions are called ”antimodes” and correspond to destructive interference; such antimodes are always unstable in standard operation as demonstrated by T. Erneux and coworkers for a diode laser [25

25. A. M. Levine, G. H. M. van Tartwijk, D. Lenstra, and T. Erneux, “Diode lasers with optical feedback: Stability of the maximum gain mode,” Phys. Rev. A 52, 3436–3439 (1995). [CrossRef]

]. Modes and antimodes appears in pairs via a saddle-node bifurcation as we increase the feedback strength.

We now turn our attention to the dependence of the CW solution frequencies on the feedback parameter k. A qualitative picture can be garnered from Fig. 2(b). We now fix α and, in agreement with Eq. (6), we observe that a line parallel to the x–axis drawn for growing values of k, will exhibit an increasing number of intersections with the e.g. red curve (α=3); this means that the number of CW solutions increases with k and their frequencies always belong to an interval centered in ω0 with halfwidth Max(|ωFω0|)kmax = (α/2 + 0.5). Moreover we observe that, for k ≪ 1 (”bad external cavity limit”) the CW solutions reduce to a single one, close the free running laser frequency ω0, while for kkmax (”good external cavity limit”) the modes approach the EC resonances i.e. the ECMs. In the following we denote as CW0 the CW solution closest to ω0.

3. Linear stability analysis

While the CW solutions do not depend on the field and carrier decay rates, the linear stability analysis (LSA) we will perform in this section is pivotally centered on the QCL property γ ≫ 1. Although we focus here on the LSA of the CW0 solution, the same approach can be easily extended to any other CW solution.

By considering perturbations to the CW0 solution (Es, ωF, Ns) in the form:
δE(t)=δEsexp(λt)exp[i(ωFω0t)],δEEs
(7)
δN(t)=δNsexp(λt),δNNs
(8)
we get, after linearization, the characteristic equation for the complex eigenvalue λ
λ3+a2(λ)λ2+a1(λ)λ+a0=0
(9)
where:
a2=2kτpτc(1eλτ)cos(ωFτ)+γIpNs
(10)
a1=[kτpτc(1eλτ)]2+γ[2kτpIpNsτc(1eλτ)cos(ωFτ)+Es2Ns]
(11)
a0=γ{Es2Nskτpτc(1eλτ)[cos(ωFτ)αsin(ωFτ)]+IpNs[kτpτc(1eλτ)]2}
(12)

We observe that because of the phase invariance of Eqs. (1) and (2) we can choose Es real without loss of generality. We define the critical feedback kc as the minimum level causing Max(Re(λ)) > 0 and thus a CW instability.

At difference from T. Erneux and coworkers that in [26

26. T. Erneux, V. Kovanis, and A. Gavrielides, “Nonlinear dynamics of an injected quantum cascade laser,” Phys. Rev. E 88, 032907 (2013). [CrossRef]

] are able to analytically qualify the roots of the secular equation and the associated CW instability, in the case of a single mode QCL with OF, the delay-associated nonlocality renders Eq. (9) transcendental and this prevents any analytical solution. We found the complex eigenvalues λ looking for the zeros of Eq. (9) by implementing a very precise, though CPU-intensive, minimization algorithm based on ”simplex” methods [27

27. W. H. Press, B. P. Flannery, S. A. Teukolsky, and W. T. Vetterling, Numerical Recipes in Fortran 77: The Art of Scientific Computing (Academic, 1992).

]. We will use its results as a reference for the following approximate analysis.

In the limit γ ≫ 1 Eq. (9) can be reduced to the second order secular equation:
b2(λ)λ2+b1(λ)λ+b0=0
(13)
where:
b1=2kτpτc(1eλτ)cos(ωFτ)+Es2Ns2Ip
(14)
b0=Es2Ns2kτpIpτc(1eλτ)[cos(ωFτ)αsin(ωFτ)]+[kτpτc(1eλτ)]2
(15)
We note that the same result, can be obtained by first adiabatically eliminating the fast variable N in the LK equations and then performing the LSA of the CW solutions of this simplified model. As we demonstrated in [17

17. F. P. Mezzapesa, L. L. Columbo, M. Brambilla, M. Dabbicco, S. Borri, M. S. Vitiello, H. E. Beere, D. A. Ritchie, and G. Scamarcio, “Intrinsic stability of quantum cascade lasers against optical feedback,” Opt. Express 21, 13748–13757 (2013). [CrossRef] [PubMed]

], in this case the CW instability is due to the competition among the CW solutions close to the ECMs, because the fast medium suppresses the well known mechanism of destabilization via amplification of the relaxation oscillations. In the following we now further characterize the coherent character of this instability.

Fig. 3 Linear stability analysis of the CW solutions for Ip = 1.5, τ = 30.

We compare this prediction to that obtained from the complete characteristic equation (9) (full black line) and to the instability onset obtained numerically by integrating the LK equations (open square symbols). We observe a difference of few percents or less for α values in (1.5 – 2.5) (suitable for Mid-IR QCLs), among the complete and approximated LSA, while the numerics confirm the validity of the former one everywhere.

Finally, αc ≈ 0.35 is the critical value of α below which no instability is found; it represents the limit of the ultra stable regime for THz QCL identified in [17

17. F. P. Mezzapesa, L. L. Columbo, M. Brambilla, M. Dabbicco, S. Borri, M. S. Vitiello, H. E. Beere, D. A. Ritchie, and G. Scamarcio, “Intrinsic stability of quantum cascade lasers against optical feedback,” Opt. Express 21, 13748–13757 (2013). [CrossRef] [PubMed]

].

While the decrease of the threshold with α was described for conventional inter–band lasers in [15

15. J. Helms and K. Petermann, “A simple analytic expression for the stable operation range of laser diodes with optical feedback,” IEEE J. Quantum Electron. 26, 833–836 (1990). [CrossRef]

], we stress here that the underlying physics leading to the instability is quite different.

3.1. Variation of αc with τ

Fig. 4 Ip = 1.5. Numerically calculated values of αc against τ.

In Fig. 4 though, some values of τ (evidenced by circles) can be noted, where the threshold drops (i.e. the system is more prone to destabilize the stationary emission). By analyzing the values τ = 22.5, 52.5 and 90, we noted that the frequency of the free running laser frequency ω0 there falls close (less than 1/3 of the EFSR) to midway between two adjacent ECMs. This is a situation where the competition between the modes is strongest and thus leads to easier destabilization. In fact, by studying the distance of the closest (to the emitted one) CW mode from the neighboring ECMs, one sees that it diminishes with increasing k, keeping always smaller than for neighboring values of τ. This also tells us that the instability of this system is a complex mechanism where the number of solutions and the EFSR isn’t the only criterion involved, but the relative positions of the frequencies of the free-running laser, CW solutions and ECMs play a role.

4. Dynamical simulations

We studied the dynamical regimes when the feedback k is increased across the threshold kc for different values of α.

4.1. CW-solution switching and multimode regimes of stationary emission

In Fig. 5 we plot for α = αsw = 1.3 the maximum and minimum values of the intensity I = |E|2 during a ramp in k from 0 to 0.5. To avoid transient latency, the system was allowed to relax to steady state before changing k by each step.

Fig. 5 Ip = 1.5, τ = 30, α = 1.3. Maximum and minimum value of the intensity obtained by gradually increasing k during the system dynamical evolution. The corresponding values of k are reported on the right vertical axis.

On the overall, in all cases where αcα ≤ 1.3, all instabilities met by increasing k cause the switching of a stationary emission from a CW mode (for k ≪ 1 the laser starts from CW0) to the adjacent one. This corresponds to the system transitions from region I to region II and from region II to region III in Fig. 5. The switches are accompanied by a transient oscillation that can be seen in Fig. 5 immediately after the thresholds.

4.2. Multimode regimes: irregular dynamics and locked states of regular high contrast oscillations

Fig. 6 Ip = 1.5, τ = 30, α = 3. (a), (b) Maximum and minimum value of the intensity and intensity power spectrum obtained by gradually increasing k during the system dynamical evolution. (c)–(e) Power spectrum and temporal variation of the intensity for k = 0.12 (c), k = 0.163 (d) and k = 0.2 (e). (f) Correlation plot for Δt = 1.2τ = 35 and three different values of k.

A detailed behavior of the oscillations in the regular regimes is shown in Fig. 6(f) where we plot the correlation diagram for three different values of k belonging to the regions II, III and IV. The intensity I(t + Δt) is plotted against I(t) with Δt = 1.2τ. The points corresponding to regular intensity oscillations in region II and IV depict two distinct limit cycles corresponding to slightly different periods, while those corresponding to the irregular oscillations of region III are more scattered but their density is higher around the two limit cycles, since the dominant contribution in system dynamics is linked to the CW modes.

The stable periodic oscillations in Fig. 6(e) for k = 0.2 represent the phenomenon of coherent dynamics involving the largest number of modes (≃ 5 in the first decade of the power spectrum) that we were able to simulate for the chosen values of Ip and τ and we believe that it can be considered an interesting phenomenon of coherent synchronization of multimode emission. Here, without external modulation of laser gain (as in active mode-locking [5

5. C. Y. Wang, L. Kuznetsova, V. M. Gkortsas, L. Diehl, F. X. Kärtner, M. A. Belkin, A. Belyanin, X. Li, D. Ham, H Schneider, P. Grant, C. Y. Song, S. Haffouz, Z. R. Wasilewski, H. C. Liu, and F. Capasso, “Mode-locked pulses from mid-infrared quantum cascade lasers,” Opt. Express 17, 12929–12943 (2009). [CrossRef] [PubMed]

, 6

6. S. Barbieri, M. Ravaro, P. Gellie, G. Santarelli, C. Manquest, C. Sirtori, S. P. Khanna, H. Linfield, and A. G. Davies, “Coherent sampling of active mode-locked terahertz quantum cascade lasers and frequency synthesis,” Nat. Photonics 5, 306–313 (2011). [CrossRef]

]), the system shows a self-organization of its emission with a (still limited) comb of ECMs. This is a condition that may hint towards a spontaneous mode-locking in a QCL with feedback.

Since we are interested in QCLs, we scanned smaller values of α. Of course the reduction in the number of solutions this implies, must be compensated by an increase of the EC length (i.e. of τ) in order to have a sizable number of modes to compete and possibly lock in phase. We found that the regime of coherent phase synchronization is rather widespread and it even occurs close to the first instability window. We report here on α = 2, a value that is still compatible with experimental evidences (a value as high as 2.5 is reported in [21

21. J. Staden, T. Gensty, W. Elsäßer, G. Giuliani, and C. Mann, “Measurements of the α factor of a distributed-feedback quantum cascade laser by an optical feedback self-mixing technique,” Opt. Lett. 31, 2574–2576 (2006). [CrossRef]

]) and shows phase coherence to appear for still moderate EC lengths.

As shown in Fig. 7 for τ = 60 and k = 0.13, close to the instability threshold of the CW0 solution and before any chaotic crisis, a regime of nontrivial regular oscillations appears where the ≃ 5 CW modes are present in the first decade. For k > 0.2 a complex multimode competition causes the first abrupt transition to a highly irregular regime.

Fig. 7 Ip = 1.5, τ = 60, α = 2. Power spectrum (a) and temporal variation (b) of the field intensity showing a regime regular multimode dynamics for k = 0.13.

5. Conclusions

In conclusion, we analyzed the stationary solutions of a QCL with optical feedback and showed their dependencies on critical parameters such as the LEF and the EC length, we provided an exact LSA and a simpler, approximated one, valid for unipolar lasers which allows to study the destabilization of the CW0 mode and also validates and extends the prediction of a regime of absolute stability of THz QCLs against OF. By studying the behavior of the laser above the instability threshold we could evidence the multimode dynamics typical of unipolar lasers where the mechanism of amplification of relaxation oscillations is absent and the emission is determined by the competition of several modes; in particular at threshold or in the windows of regular dynamics between chaotic islands, we could prove the existences of regimes of coherent multimode oscillations emerging from very simple physical processes in a 2-level model with feedback, which could possibly indicate a path towards spontaneous mode-locking.

Acknowledgments

This research has been funded by the Italian Ministry of Research (MIUR) through the Futuro in Ricerca FIRB-grant PHOCOS ( RBFR08E7VA). The authors also acknowledge support from MIURPON02-0576 INNOVHEAD and MASSIME.

References and links

1.

J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum cascade laser,” Science 264, 553–556 (1994). [CrossRef] [PubMed]

2.

R. F. Curl, F. Capasso, C. Gmachl, A. A. Kosterev, B. Mc Manus, R. Lewicki, M. Pusharsky, G. Wysocki, and F. K. Tittel, “Quantum cascade lasers in chemical physics,” Chem. Phys. Lett. 487, 1–18 (2010). [CrossRef]

3.

J. Faist, Quantum Cascade Lasers (Academic, 2013). [CrossRef]

4.

A. Gordon, C. Y. Wang, L. Diehl, F. X. Kärtner, A. Belyanin, D. Bour, S. Corzine, G. Höfler, H. C. Liu, H. Schneider, T. Maier, M. Troccoli, J. Faist, and F. Capasso, “Multimode regimes in quantum cascade lasers: From coherent instabilities to spatial hole burning,” Phys. Rev. A 77, 053804 (2008). [CrossRef]

5.

C. Y. Wang, L. Kuznetsova, V. M. Gkortsas, L. Diehl, F. X. Kärtner, M. A. Belkin, A. Belyanin, X. Li, D. Ham, H Schneider, P. Grant, C. Y. Song, S. Haffouz, Z. R. Wasilewski, H. C. Liu, and F. Capasso, “Mode-locked pulses from mid-infrared quantum cascade lasers,” Opt. Express 17, 12929–12943 (2009). [CrossRef] [PubMed]

6.

S. Barbieri, M. Ravaro, P. Gellie, G. Santarelli, C. Manquest, C. Sirtori, S. P. Khanna, H. Linfield, and A. G. Davies, “Coherent sampling of active mode-locked terahertz quantum cascade lasers and frequency synthesis,” Nat. Photonics 5, 306–313 (2011). [CrossRef]

7.

A. K. Wójcik, P. Malara, R. Blanchard, T. S. Mansuripur, F. Capasso, and A. Belyanin, “Generation of picosecond pulses and frequency combs in actively mode locked external ring cavity quantum cascade lasers,” Appl. Phys. Lett. 103, 231102 (2013). [CrossRef]

8.

N. Yu, L. Diehl, E. Cubukcu, D. Bour, S. Corzine, G. Höfler, A. K. Wójcik, K. B. Crozier, A. Belyanin, and F. Capasso, “Coherent coupling of multiple transverse modes in quantum cascade lasers,” Phys. Rev. Lett. 102, 013901 (2009). [CrossRef] [PubMed]

9.

P. Dean, Y. L. Lim, A. Valavanis, R. Kliese, M. Nikolić, S. P. Khanna, M. Lachab, D. Indjin, Z. Ikonić, P. Harrison, A. D. Rakić, E. H. Linfield, and A. G. Davies, “Terahertz imaging through self-mixing in a quantum cascade laser,” Opt. Lett. 36, 2587–2589 (2011). [CrossRef] [PubMed]

10.

Y. L. Lim, P. Dean, M. Nikolić, R. Kliese, S. P. Khanna, M. Lachab, A. Valavanis, D. Indjin, Z. Ikonić, P. Harrison, E. Linfield, A. G. Davies, S. J. Wilson, and A. D. Rakić, “Demonstration of a self-mixing displacement sensor based on terahertz quantum cascade lasers,” Appl. Phys. Lett. 99, 081108 (2011). [CrossRef]

11.

M. C. Phillips and S. Taubman, “Intracavity sensing via compliance voltage in an external cavity quantum cascade laser,” Opt. Lett. 37, 2664–2666 (2012). [CrossRef] [PubMed]

12.

F. P. Mezzapesa, V. Spagnolo, A. Antonio, and G. Scamarcio, “Detection of ultrafast laser ablation using quantum cascade laser-based sensing,” Appl. Phys. Lett. 101, 171101 (2012). [CrossRef]

13.

R. Paiella, R. Martini, F. Capasso, C. Gmachl, and H. Y. Hwang, “High-frequency modulation without the relaxation oscillation resonance in quantum cascade lasers,” Appl. Phys. Lett. 79, 2526–2528 (2001). [CrossRef]

14.

D. M. Kane and K. A. Shore, Unlocking Dynamical Diversity: Optical Feedback Effects on Semiconductor Diode Lasers (John Wiley, 2005). [CrossRef]

15.

J. Helms and K. Petermann, “A simple analytic expression for the stable operation range of laser diodes with optical feedback,” IEEE J. Quantum Electron. 26, 833–836 (1990). [CrossRef]

16.

D. Weidmann, K. Smith, and B. Ellison, “Experimental investigation of high-frequency noise and optical feedback effects using a 9.7 μm continuous-wave distributed-feedback quantum-cascade laser,” Appl. Opt. 46, 947–953 (2007). [CrossRef] [PubMed]

17.

F. P. Mezzapesa, L. L. Columbo, M. Brambilla, M. Dabbicco, S. Borri, M. S. Vitiello, H. E. Beere, D. A. Ritchie, and G. Scamarcio, “Intrinsic stability of quantum cascade lasers against optical feedback,” Opt. Express 21, 13748–13757 (2013). [CrossRef] [PubMed]

18.

R. Lang and K. Kobayashi, “External optical feedback effects on semiconductor injection laser properties,” IEEE J. Quantum Electron. 16, 347–355 (1980). [CrossRef]

19.

T. Gensty, W. Elsäßer, and C. Mann, “Intensity noise properties of quantum cascade lasers,” Opt. Express 13, 2032–2039 (2005). [CrossRef] [PubMed]

20.

M. Yamanishi, T. Edamura, K. Fujita, N. Akikusa, and H. Kan, “Theory of the intrinsic linewidth of quantum cascade lasers: hidden reason for the narrow linewidth and line broadening by thermal photons,” IEEE J. Quantum Electron. 44, 12–29 (2008). [CrossRef]

21.

J. Staden, T. Gensty, W. Elsäßer, G. Giuliani, and C. Mann, “Measurements of the α factor of a distributed-feedback quantum cascade laser by an optical feedback self-mixing technique,” Opt. Lett. 31, 2574–2576 (2006). [CrossRef]

22.

R. P. Green, J. H. Xu, L. Mahler, A. Tredicucci, F. Beltram, G. Giuliani, H. E. Beere, and D. A. Ritchie, “Linewidth enhancement factor of terahertz quantum cascade lasers,” Appl. Phys. Lett. 92, 071106 (2008). [CrossRef]

23.

A. D. Rakić, T. Taimre, K. Bertling, Y. L. Lim, P. Dean, D. Indjin, Z. Ikonić, P. Harrison, A. Valavanis, S. P. Khanna, M. Lachab, S. J. Wilson, E. H. Linfield, and A. G. Davies, “Swept-frequency feedback interferometry using terahertz frequency QCLs: a method for imaging and materials analysis,” Opt. Express 21, 22194–22205 (2013). [CrossRef]

24.

F. Mezzapesa, Internal CNR-IFN report (2013).

25.

A. M. Levine, G. H. M. van Tartwijk, D. Lenstra, and T. Erneux, “Diode lasers with optical feedback: Stability of the maximum gain mode,” Phys. Rev. A 52, 3436–3439 (1995). [CrossRef]

26.

T. Erneux, V. Kovanis, and A. Gavrielides, “Nonlinear dynamics of an injected quantum cascade laser,” Phys. Rev. E 88, 032907 (2013). [CrossRef]

27.

W. H. Press, B. P. Flannery, S. A. Teukolsky, and W. T. Vetterling, Numerical Recipes in Fortran 77: The Art of Scientific Computing (Academic, 1992).

28.

L. Columbo, M. Brambilla, M. Dabbicco, and G. Scamarcio, “Self-mixing in multi-transverse mode semiconductor lasers: model and potential application to multi-parametric sensing,” Opt. Express 20, 6286–6305 (2012). [CrossRef] [PubMed]

OCIS Codes
(140.4050) Lasers and laser optics : Mode-locked lasers
(190.3100) Nonlinear optics : Instabilities and chaos
(140.5965) Lasers and laser optics : Semiconductor lasers, quantum cascade

ToC Category:
Nonlinear Optics

History
Original Manuscript: January 22, 2014
Manuscript Accepted: March 17, 2014
Published: April 21, 2014

Virtual Issues
Physics and Applications of Laser Dynamics (2014) Optics Express

Citation
L. L. Columbo and M. Brambilla, "Multimode regimes in quantum cascade lasers with optical feedback," Opt. Express 22, 10105-10118 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-9-10105


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References

  1. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, A. Y. Cho, “Quantum cascade laser,” Science 264, 553–556 (1994). [CrossRef] [PubMed]
  2. R. F. Curl, F. Capasso, C. Gmachl, A. A. Kosterev, B. Mc Manus, R. Lewicki, M. Pusharsky, G. Wysocki, F. K. Tittel, “Quantum cascade lasers in chemical physics,” Chem. Phys. Lett. 487, 1–18 (2010). [CrossRef]
  3. J. Faist, Quantum Cascade Lasers (Academic, 2013). [CrossRef]
  4. A. Gordon, C. Y. Wang, L. Diehl, F. X. Kärtner, A. Belyanin, D. Bour, S. Corzine, G. Höfler, H. C. Liu, H. Schneider, T. Maier, M. Troccoli, J. Faist, F. Capasso, “Multimode regimes in quantum cascade lasers: From coherent instabilities to spatial hole burning,” Phys. Rev. A 77, 053804 (2008). [CrossRef]
  5. C. Y. Wang, L. Kuznetsova, V. M. Gkortsas, L. Diehl, F. X. Kärtner, M. A. Belkin, A. Belyanin, X. Li, D. Ham, H Schneider, P. Grant, C. Y. Song, S. Haffouz, Z. R. Wasilewski, H. C. Liu, F. Capasso, “Mode-locked pulses from mid-infrared quantum cascade lasers,” Opt. Express 17, 12929–12943 (2009). [CrossRef] [PubMed]
  6. S. Barbieri, M. Ravaro, P. Gellie, G. Santarelli, C. Manquest, C. Sirtori, S. P. Khanna, H. Linfield, A. G. Davies, “Coherent sampling of active mode-locked terahertz quantum cascade lasers and frequency synthesis,” Nat. Photonics 5, 306–313 (2011). [CrossRef]
  7. A. K. Wójcik, P. Malara, R. Blanchard, T. S. Mansuripur, F. Capasso, A. Belyanin, “Generation of picosecond pulses and frequency combs in actively mode locked external ring cavity quantum cascade lasers,” Appl. Phys. Lett. 103, 231102 (2013). [CrossRef]
  8. N. Yu, L. Diehl, E. Cubukcu, D. Bour, S. Corzine, G. Höfler, A. K. Wójcik, K. B. Crozier, A. Belyanin, F. Capasso, “Coherent coupling of multiple transverse modes in quantum cascade lasers,” Phys. Rev. Lett. 102, 013901 (2009). [CrossRef] [PubMed]
  9. P. Dean, Y. L. Lim, A. Valavanis, R. Kliese, M. Nikolić, S. P. Khanna, M. Lachab, D. Indjin, Z. Ikonić, P. Harrison, A. D. Rakić, E. H. Linfield, A. G. Davies, “Terahertz imaging through self-mixing in a quantum cascade laser,” Opt. Lett. 36, 2587–2589 (2011). [CrossRef] [PubMed]
  10. Y. L. Lim, P. Dean, M. Nikolić, R. Kliese, S. P. Khanna, M. Lachab, A. Valavanis, D. Indjin, Z. Ikonić, P. Harrison, E. Linfield, A. G. Davies, S. J. Wilson, A. D. Rakić, “Demonstration of a self-mixing displacement sensor based on terahertz quantum cascade lasers,” Appl. Phys. Lett. 99, 081108 (2011). [CrossRef]
  11. M. C. Phillips, S. Taubman, “Intracavity sensing via compliance voltage in an external cavity quantum cascade laser,” Opt. Lett. 37, 2664–2666 (2012). [CrossRef] [PubMed]
  12. F. P. Mezzapesa, V. Spagnolo, A. Antonio, G. Scamarcio, “Detection of ultrafast laser ablation using quantum cascade laser-based sensing,” Appl. Phys. Lett. 101, 171101 (2012). [CrossRef]
  13. R. Paiella, R. Martini, F. Capasso, C. Gmachl, H. Y. Hwang, “High-frequency modulation without the relaxation oscillation resonance in quantum cascade lasers,” Appl. Phys. Lett. 79, 2526–2528 (2001). [CrossRef]
  14. D. M. Kane, K. A. Shore, Unlocking Dynamical Diversity: Optical Feedback Effects on Semiconductor Diode Lasers (John Wiley, 2005). [CrossRef]
  15. J. Helms, K. Petermann, “A simple analytic expression for the stable operation range of laser diodes with optical feedback,” IEEE J. Quantum Electron. 26, 833–836 (1990). [CrossRef]
  16. D. Weidmann, K. Smith, B. Ellison, “Experimental investigation of high-frequency noise and optical feedback effects using a 9.7 μm continuous-wave distributed-feedback quantum-cascade laser,” Appl. Opt. 46, 947–953 (2007). [CrossRef] [PubMed]
  17. F. P. Mezzapesa, L. L. Columbo, M. Brambilla, M. Dabbicco, S. Borri, M. S. Vitiello, H. E. Beere, D. A. Ritchie, G. Scamarcio, “Intrinsic stability of quantum cascade lasers against optical feedback,” Opt. Express 21, 13748–13757 (2013). [CrossRef] [PubMed]
  18. R. Lang, K. Kobayashi, “External optical feedback effects on semiconductor injection laser properties,” IEEE J. Quantum Electron. 16, 347–355 (1980). [CrossRef]
  19. T. Gensty, W. Elsäßer, C. Mann, “Intensity noise properties of quantum cascade lasers,” Opt. Express 13, 2032–2039 (2005). [CrossRef] [PubMed]
  20. M. Yamanishi, T. Edamura, K. Fujita, N. Akikusa, H. Kan, “Theory of the intrinsic linewidth of quantum cascade lasers: hidden reason for the narrow linewidth and line broadening by thermal photons,” IEEE J. Quantum Electron. 44, 12–29 (2008). [CrossRef]
  21. J. Staden, T. Gensty, W. Elsäßer, G. Giuliani, C. Mann, “Measurements of the α factor of a distributed-feedback quantum cascade laser by an optical feedback self-mixing technique,” Opt. Lett. 31, 2574–2576 (2006). [CrossRef]
  22. R. P. Green, J. H. Xu, L. Mahler, A. Tredicucci, F. Beltram, G. Giuliani, H. E. Beere, D. A. Ritchie, “Linewidth enhancement factor of terahertz quantum cascade lasers,” Appl. Phys. Lett. 92, 071106 (2008). [CrossRef]
  23. A. D. Rakić, T. Taimre, K. Bertling, Y. L. Lim, P. Dean, D. Indjin, Z. Ikonić, P. Harrison, A. Valavanis, S. P. Khanna, M. Lachab, S. J. Wilson, E. H. Linfield, A. G. Davies, “Swept-frequency feedback interferometry using terahertz frequency QCLs: a method for imaging and materials analysis,” Opt. Express 21, 22194–22205 (2013). [CrossRef]
  24. F. Mezzapesa, Internal CNR-IFN report (2013).
  25. A. M. Levine, G. H. M. van Tartwijk, D. Lenstra, T. Erneux, “Diode lasers with optical feedback: Stability of the maximum gain mode,” Phys. Rev. A 52, 3436–3439 (1995). [CrossRef]
  26. T. Erneux, V. Kovanis, A. Gavrielides, “Nonlinear dynamics of an injected quantum cascade laser,” Phys. Rev. E 88, 032907 (2013). [CrossRef]
  27. W. H. Press, B. P. Flannery, S. A. Teukolsky, W. T. Vetterling, Numerical Recipes in Fortran 77: The Art of Scientific Computing (Academic, 1992).
  28. L. Columbo, M. Brambilla, M. Dabbicco, G. Scamarcio, “Self-mixing in multi-transverse mode semiconductor lasers: model and potential application to multi-parametric sensing,” Opt. Express 20, 6286–6305 (2012). [CrossRef] [PubMed]

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