# Investigation of Intensity Noise in an Interband Cascade Laser Epitaxially Grown on Silicon and Designed for High-speed Applications

Hyunah Kim, Pierre Didier, Sara Zaminga, Heming Huang, Daniel Andrés Díaz-Thomas, A. N. Baranov, Jean-Baptiste Rodriguez, Eric Tournié, Hedwig Knötig, Benedikt Schwarz, et al.

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Hyunah Kim, Pierre Didier, Sara Zaminga, Heming Huang, Daniel Andrés Díaz-Thomas, et al. Investigation of Intensity Noise in an Interband Cascade Laser Epitaxially Grown on Silicon and Designed for High-speed Applications. 2024 Conference on Lasers and Electro-Optics (CLEO 2024), May 2024, Charlotte, NC, United States. hal-04996211

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# Investigation of Intensity Noise in an Interband Cascade Laser Epitaxially Grown on Silicon and Designed for High-speed Applications

H. Kim,<sup>1,†</sup> P. Didier,<sup>1,2</sup> S. Zaminga,<sup>1</sup> H. Huang,<sup>1</sup> D. A. Díaz-Thomas,<sup>3</sup> A. N. Baranov,<sup>3</sup> J. B. Rodriguez,<sup>3</sup> E. Tournié,<sup>3</sup> H. Knötig,<sup>4</sup> B. Schwarz,<sup>4</sup> L. Cerutti,<sup>3</sup> O. Spitz,<sup>5</sup> and F. Grillot<sup>1,6</sup>

**Abstract:** The high-speed parameters of an interband cascade laser grown on silicon are analyzed through the prism of relative intensity noise. The evolution of the relaxation frequency allows deriving a modulation bandwidth in the GHz range. © 2023 The Author(s)

## 1. Introduction

Interband lasers have played a key role in the recent development of high-speed communication. When the transmitted data is injected through direct electrical modulation, the electrical bandwidth of the laser is of prime interest as this is related to the maximum transmission speed [1]. A good estimation of the laser's electrical bandwidth can be derived from relative intensity noise (RIN) measurements where the relaxation oscillation frequency ($f_{ro}$) is observed for several bias currents [2]. Despite the invention of interband cascade lasers (ICLs) dating back to 1995 [3], studies about their relaxation oscillations are rare and the first clear experimental observation of such oscillations was unveiled in 2021 [2]. The particularity of ICLs is that they combine interband technology, broken bandgap and cascaded structure to emit in the mid-infrared. ICLs are better candidates than quantum cascade lasers (QCLs) [4] for integration on silicon because of their low-power consumption and they could thus pave the way for novel free-space mid-infrared communication systems. Mid-infrared wavelengths hold several advantages for optical wireless propagation and those, like reduced beam scattering and resistance to adverse weather conditions, are well established [5]. So far, the study of ICLs epitaxially grown on silicon is mainly limited to the description of their material properties [6], without any demonstration of real-field applications. In this work, we describe a Fabry-Perot (FP) ICL epitaxially-grown on silicon with a wavelength of emission close to 3.5 µm at room temperature. The ICL is designed for radio-frequency (RF) electrical operation and relevant parameters toward high-speed applications are presented by focusing on the intensity noise of the ICL.

## 2. Relative intensity noise experiment

The ICL epitaxially-grown on silicon used in this work is identical to the one described in [7], and Fig. 1 a) describes the experimental setup for the analysis of the RIN. The ICL is DC-biased with a low-noise current source, being kept at room temperature thanks to a temperature controller. The modulated mid-infrared optical signal from the ICL impinges the active area of the ICIP. This photodetector, which has been previously studied in [8], is biased with a DC voltage source around 4 V. It exhibits a high responsivity of 110 mA/W at 3.5 µm as well as a 1.5-GHz bandwidth and thus does not limit the frequency analysis of the ICL grown on silicon, as shown in the following. The high-frequency electrical signal from the ICIP is first amplified before being fed to the input of an electrical spectrum analyzer (ESA) with 43-GHz bandwidth. The raw data from the ESA takes into account the intrinsic intensity noise of the laser as well as non-relevant noise contributions from the setup. By getting rid of the latter [2], one can retrieve the RIN of the ICL and observe the relaxation oscillation frequency, as shown in Fig. 1 b) for bias currents from 2.19 × $I_{th}$ to 2.41 × $I_{th}$ (or in other words, from 129 mA to 142 mA). In order to guide the eye of the reader, the relaxation frequency resonance is fitted with a black dashed line for each value of the bias current. In Fig. 1 b), one can see that the value of the relaxation oscillation frequency increases when the ICL's bias current increases, as expected.

<footnote n="1"><sup>1</sup>LTCI Télécom Paris, Institut Polytechnique de Paris, 19 place Marguerite Perey, Palaiseau, 91120, France</footnote>

<footnote n="2"><sup>2</sup>mirSense, Centre d'intégration NanoInnov, 8 avenue de la Vauve, Palaiseau, 91120, France</footnote>

<footnote n="3"><sup>3</sup>IES, Université de Montpellier, CNRS UMR 5214, Montpellier, 34000, France</footnote>

<footnote n="4"><sup>4</sup>Institute for Solid State Electronics, TU Wien, Gußhausstraße 25-25a, 1040 Vienna, Austria</footnote>

<footnote n="5"><sup>5</sup>The College of Optics and Photonics (CREOL), University of Central Florida, Orlando, FL 32816, USA</footnote>

<footnote n="6"><sup>6</sup>CHTM, University of New-Mexico, 1313 Goddard SE, Albuquerque, NM 87106, USA</footnote>

<footnote>†hyunah.kim@telecom-paris.fr</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1: a) Experimental setup for the extraction of the RIN. b) RIN measurement for selected pumping currents of the ICL when the temperature is stabilized at 293K. Relaxation oscillation frequencies are highlighted with black dashed line fittings. The value of $f_{ro}$ varies from 0.1 GHz at $2.19 \times I_{th}$ to 0.2 GHz at $2.41 \times I_{th}$.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2: a) Damping factor $\gamma$ of the ICL on silicon for ten experimental measurements between $2.19 \times I_{th}$ and $2.41 \times I_{th}$. The linear trend of $\gamma$ with $f_{ro}^2$ allows extracting the damping offset $\gamma_0$ and the K-factor. b) High-frequency response of the ICL on silicon when directly modulated with a low-amplitude electrical signal. The 3-dB bandwidth is found at 0.8 GHz and is promising for mid-infrared high-speed communication.</figcaption></figure>

### 3. High frequency characterization and application

By knowing $f_{ro}$ at various bias currents, it is possible to extract the damping factor $\gamma$ that gives access to the K-factor (details of the extraction process can be found in [2] for another ICL structure), which in turns gives an estimation of the modulation bandwidth of the laser. Figure 2 a) shows the evolution of $\gamma$ with respect to $f_{ro}^2$ and allows one to derive a damping offset $\gamma_0$ of $0.23 \text{ ns}^{-1}$ and a K-factor of $4.44 \text{ ns}$. The maximum modulation bandwidth being $2\sqrt{2\pi}/K$, it is estimated to be 2.0 GHz. A direct electrical modulation experiment, as the one presented in [8] for another ICL structure, allows measuring the true electrical bandwidth of the ICL on silicon and is presented in Fig. 2 b). As one can observe, the value derived from the formula with the K-factor slightly overestimates the true electrical bandwidth of 0.8 GHz but remains in the right order of magnitude. With this modulation response, it is possible to achieve direct-modulation free-space data transmission of 8 Gbit/s as described in [7]. This highlights the potential of novel mid-infrared technology such as ICLs epitaxially grown on silicon for high-speed applications.

### References

1. Petermann, K., *Laser diode modulation and noise*, Springer Science & Business Media (1991).

2. Didier, P., et al., *Applied Physics Letters* 119 (17) 171107 (2021).

3. Yang, R. Q., *Superlattices and Microstructures* 17 (1) 77-83 (1995).

4. Faist, J., et al., *Science* 264 (5158) 553-556 (1994).

5. Colvero, C. P., et al., *Electronic Letters*, 41 (10) (2005).

6. Tournié, E., et al., *Light: Science & Applications* 11 (1) 165 (2022).

7. Zaminga, S., et al., *2023 IEEE Photonics Conference (IPC)*, Orlando, USA MB4.2 (2023).

8. Didier, P., et al., *Photonics Research* 11 (4) 582-590 (2023).

<footnote>This work is supported by the Agence Nationale de la Recherche (ANR-11-EQPX-0016) and Délégation Générale de l'Armement (DGA) as well as the European Office of Aerospace Research and Development (FA8655-22-1-7032).</footnote>