Graphene Assisted Tunable Semiconductor Laser Based on Vernier Effect
Seyyed Moin Alden Mostaan
1, Hassan Rasooli Saghai
21Department of Electrical Engineering, Tabriz Branch, Islamic Azad University, Tabriz, Iran, 5157944533,
2Department of Electrical Engineering, Tabriz Branch, Islamic Azad University, Tabriz, Iran, 5157944533, [email protected]
Abstract- A graphene assisted tunable semiconductor laser based on Vernier effect is presented and simulated. The combination of Fabry-Perot and Vernier ring resonators provides a strong mode selection filter and could be shown that demonstrate a wide tuning range.
The side-mode suppression ratio, the tuning range, and threshold current are dramatically improved comparing to distributed feedback, distributed Bragg reflector, and other ring resonator lasers.
Keywords: graphene, semiconductor laser, tuning range, Vernier effect, ring resonator
1. Introduction
Silicon-On-Insulator (SOI) is getting significant fondness as a modern platform for integration of optical circuit, because of ultra-compact integration capability which can be achieved through its large reflective index difference. The heterogeneous integration of different semiconductors such as III-V on silicon using a wafer bonding technique is the most promising solution for the fabrication of laser sources on silicon.
Lots of applications requires single frequency and tunable laser operation. Numerous single frequency laser based on silicon platform have been investigated [1] [2]. The demonstrated heterogeneous DBR lasers consist of two Bragg reflector mirrors created on silicon waveguide. Single mode was demonstrated, with a lasing threshold of 65 mA and output power of 11 mW [3]. In DFB laser investigation, the optical mode is guided by a silicon waveguide and the tail of optical mode overlap with multiple quantum well region.
Its threshold current is around 25 mA and slop efficiency are around 0.05 W/A [4].
This paper reports on a study of modern heterogeneous graphene assisted tunable Vernier effect laser. Double ring resonator is used to allow single frequency operation of laser cavity. In the other hand exerting a single layer graphene sheet allows use to tune its operating frequency with a range of 0.1 THz. The paper is organized as follow: section 2 is devoted to present the theory and method of investigation. Section 3 focuses on the simulating results of the designed laser.
2. Theory and Method
The density of carrier and photon related to a typical laser diode, in both continuous wave and directly modulated regime, are presented by the coupled rate equations [5], [6]:
∂N
∂t =𝜂𝑖𝐼
𝑞𝑉− (𝐵𝑁2+ 𝐶𝑁3) − 𝑔𝑣𝑔𝑁𝑝 (1)
∂𝑁𝑝
∂t = 𝛤𝑔𝑣𝑔− 𝛤𝛽𝑒𝑠𝑝𝐵𝑁2− 𝑔𝑣𝑔𝑁𝑝−𝑁𝑝 𝜏𝑝 (2) Where 𝑔 = 𝑔0ln(𝑁
𝑁𝑡𝑟). The DC electrical performance follow a diode equation:
𝐼 = 𝐼0𝑒𝑥𝑝𝑞(𝑉 − 𝐼𝑅)
𝑛𝑘𝑇 (3)
The list of laser parameters which is used in this paper is shown in table 1. The specified values are extracted from different kinds of numerical and experimental investigation of semiconductor laser [7] [8] [9]. These set of laser parameters are passed as input to the built-in laser models in specialized photonic circuit simulator which is lumerical INTERCONNECT in our investigation. Lemerical’s 1D gain element model which runs in the time domain tracking the slowly varying envelop of the optical mode amplitude as well as the time sampling of carrier distribution along the direction of power follow in the gain medium [10]. The amplitude and shape of the gain and spontaneous emission profile are corresponded to the local carrier density. The carrier densities are altered to be able to respond the radiative and nonradiative recombination, injection of carrier density, and stimulated emission. As a result, having a device model in the photonic circuit simulator help to meet the need for an accurate analyse of both active and passive components in an optical link.
The tunable Vernier based semiconductor laser, as shown in Fig. 1, included a III–V semiconductor optical amplifier (SOA) section with the length of 80 μm, two ring resonators for single mode selection with the coupling of 20% and rings radii are 9.55
and 7.96 μm. Two mirrors is located at the two facets with the reflectivities of 100%
and 98%. A graphene sheet is located beneath the two rings to let use to tune the effective reflective index of the resonators through changing of Fermi level. The model, which is used here for the tuning mechanism, is based on [11]. Here we consider the two effective reflective indexes of 2.45 and 2.451 corresponding to Fermi levels of 0.1 and 0.9 eV, respectively. The effective index of strait waveguides set to be 3.4.
In the feedback section the resonator has a free spectral range (FSR) of 10 THz. A small difference
Table 1. Semiconductor laser parameter list
parameter value Unit
Active region volume 40 μm3
Radiative recombination 2.5e+08 1/s
Gain coefficient 1.9×10-20 m2
Gain shape quality factor
100 -
Transparent carrier density
1.5×1018 m-3
Confinement factor 0.8 -
Spontaneous emission factor
0.01 -
Group index 4 -
Gain central frequency 195.25 THz
between FSRs of two rings allows us to take advantages of Vernier effect for single mode operation and wavelength tuning. In addition, the bandwidth of Vernier filter is designed to select only on Fabry-Perot mode of the cavity.
3. Simulation and result
Initially, it is of paramount importance to be able to demonstrate high wavelength accuracy and single-longitudinal mode (SLM) operation. To this end, the cold cavity measurement is performed. For each resonance peak, the full width at half maximum is estimated to be 6 GHz and the semiconductor laser feedback circuit is design to work around frequency 194.9 THz. Such narrow resonance peak shows high accuracy, and is crucial to obtain large tuning range.
Fig 1. A schematic view of the tunable laser with external cavity. The tuner is yellow, graphene, and SOA is red. The output power is measured at the end facet with
the reflectivity of 90%.
Fig 2. Lasing spectrum at a bias current of 50 mA.
The static lasing spectral characteristic of the designed structure is calculated and shown for current injection of 50 mA in Fig. 2. The side mode suppression ratios (SMSRs) is around 35 dBm, which shows an improvement in comparison to other counterparts, for example [2] [3] [4], in such a low operating current. Having high SMSR guarantees the single frequency operation of the laser.
The laser operates in the specified band and are tunable over a 0.1 THz range from 194.89 to 194.79 THz with maintenance of desired SMSR in the shifted spectrum (Fig. 3).
The demonstrated tunability is obtained through Vernier effect of two ring with different radii. Chang of Femi level in graphene, which can be achieved using electrostatic gating, leads to a change in effective reflective index of propagating mode in the ring resonators, so this alter the resonance condition of our whole resonator.
Fig 3. Super-imposed laser spectra for two different Fermi level which demonstrate the shifting of central frequency of the operation.
Fig 4. Plot of carrier dynamic in active medium of laser.
To describe the dynamic behavior of carrier density in the semiconductor laser diode, the solving of equations of 1 and 2 with the consideration of specific laser parameter is required. The carrier density were calculated and plotted for three different injected currents. As it can be seen in Fig. 4, the carrier densities for pumping currents reach steady state after undergoing relaxation oscillation at the early stages of operation.
The relaxation oscillation represent interaction between energy levels occupation and generation of carriers and photons in the cavity. As current increase, carrier density increases and lasing began. The increasing will continue until the density reaches a constant value after that extra current does not change the steady state carrier density.
One of the most significant characteristics of a laser diode is P-I profile. The P-I carve can be sketched by calculation of laser output power for different injection currents.
Fig 5. The P-I curve for the designed laser which shows threshold current is 0.01 A.
Threshold and slop efficiency, dP/dI, can be obtained using this profile. Below the threshold, the output power is negligible, but above the threshold, the output shows linearly increasing behavior. The P-I carve for designed laser is shown in Fig. 5, which indicate a threshold of 1 mA and slop efficiency of 0.37 W/A.
4. Conclusion
In this paper, a tunable semiconductor laser diode based on Vernier effect feedback were modeled and simulated through solving the coupled rate equation in lumerical INTERCONNECT software. The output spectrum characteristics were investigated. It is shown that spectrum of the laser can be tuned using a change in Fermi level of graphene sheet which is located beneath the rings. In addition the carrier density dynamic and P-I characteristics was presented which shows good agreement with the intuitive views.
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