A fiber ring cavity laser based on an InP/InGaAsP multiple-quantum-well (MQW) semiconductor optical amplifier (SOA) is proposed and experimentally demonstrated in this paper. The laser employs an InP/InGaAsP MQW as the gain medium and a fiber Bragg grating (FBG) as the wavelength selector.
The experimental results show that the central wavelength of the amplified spontaneous emission (ASE) spectrum exhibits a blue shift as the injection current of the InGaAsP MQW increases. Under the conditions of an injection current of 220 mA and an FBG operating temperature of 23 °C, the laser achieves an output with a central wavelength of 1549.66 nm, a maximum output power of 1.524 mW, and an electro optical conversion efficiency of 1.1%. The threshold current of the laser is 78 mA. When the FBG operating temperature rises from 8 °C to 28 °C, the central wavelength of the output laser varies from 1549.27 nm to 1549.59 nm, indicating excellent temperature stability of the laser.
1. Introduction
Fiber lasers operating at 1.55 μm are widely used in optical fiber communications, fiber sensing, microwave and millimeter wave photonics, optical information processing, laser machining, and other fields [1–4]. Fiber lasers in the 1.55 μm band are mainly classified into erbium doped fiber lasers [5], semiconductor optical amplifier based fiber lasers [6,7], and Raman fiber lasers. Among them, erbium doped fiber lasers and Raman fiber lasers adopt all fiber configurations, whereas fiber lasers based on erbium doped fiber amplifiers and semiconductor optical amplifiers use discrete active components (optically amplified by electrical injection) as the gain medium within the laser resonant cavity.
To the best of our knowledge, relatively few studies have been reported on semiconductor optical amplifier based fiber lasers. In recent years, extensive research has been conducted on the amplified spontaneous emission of InP/InGaAsP MQW semiconductor materials. In 2012, Yeh et al. realized an ultra broadband ASE source using a superstructured semiconductor optical amplifier and an erbium doped fiber amplifier, achieving broadband gain amplification over the 1464.0–1578.0 nm wavelength range [8]. In 2015, Lin et al. simulated and analyzed the influence of InGaAs MQW structures on their spontaneous emission characteristics [9]. In 2016, Xia and Ghaffouri Shiraz investigated the effect of strain on the spontaneous emission of quantum wells using a quantum well transmission line model, showing that the influence of strain on the spontaneous emission intensity can be neglected under specific conditions [10]. In our previous work, we studied the spontaneous emission lineshape function of InP/InGaAsP MQW structures and found that it follows a Gaussian distribution [11].
In this paper, a fiber ring cavity laser based on an InP/InGaAsP MQW semiconductor optical amplifier is proposed, with a fiber Bragg grating serving as the wavelength selector. The effects of the FBG operating temperature and the SOA injection current on the output characteristics of the laser are investigated.
2. Experimental Setup
Figure 1 shows a schematic diagram of the semiconductor optical amplifier based fiber ring cavity laser. The ring cavity laser consists of the following components: a semiconductor optical amplifier (THORLABS, S9FC), a polarization controller, a fiber circulator, a fiber Bragg grating, a temperature controller, and a 90:10 fiber coupler. The total length of the ring cavity is approximately 6 meters.
The fiber circulator is used to couple the fiber Bragg grating into the laser cavity; the fiber Bragg grating acts as a wavelength selector, and the temperature controller is employed to regulate the operating temperature of the fiber Bragg grating; the polarization controller is used to adjust the polarization direction of the optical field inside the cavity.
In the experiment, a optical spectrum analyzer (Anritsu, MS9710C) is used to measure the output spectrum of the laser.
3. Fundamental Principle
Neglecting nonradiative recombination, the logarithmic relationship between the optical gain g of a multiple quantum well semiconductor and the carrier concentration N can be expressed as
where g0 is the gain coefficient of the quantum-well structure, and N0 is the transparency carrier concentration. Equation (1) shows that the optical gain of the multiple-quantum-well semiconductor tends to saturate with increasing carrier concentration.
Under the mean-field approximation, the carrier rate equation of the laser based on a multiple-quantum-well semiconductor optical amplifier can be expressed as
where (S) is the photon density, (eta) is the current injection efficiency, (q) is the elementary charge, (tau_{text{ph}}) is the photon lifetime (related to the intra-cavity loss), (tau_{text{sp}}) is the carrier lifetime, (beta) is the spontaneous emission factor, (Gamma_0) is the confinement factor of a single quantum well, (V) is the active region volume, and (M) is the number of quantum wells. The dynamic rate equation given by Eq. (2) can be solved using the fourth-order Runge–Kutta method. Relaxation oscillations between the carrier and photon densities can be observed during the solution process, and the system will eventually reach a steady state [14]. After the current (I) is injected into the laser, the electron and photon densities reach a steady state following a transient process, at which point
The threshold current is defined under the specific condition of β, at which the threshold current Ith can be expressed as
The threshold current is determined by both the structural parameters of the laser and the photon lifetime, which in turn depends on the cavity loss.
4. Results and Discussion
Figure 2 shows the amplified spontaneous emission (ASE) characteristics of the InP/InGaAsP multiple-quantum-well structure under different injection currents. At a given injection current and operating temperature, the ASE spectral profile of the InP/InGaAsP MQW can be well described by a Gaussian function [11]. As the injection current increases gradually, the peak ASE power tends to saturate (as shown in Figure 3). This phenomenon is caused by the gain saturation effect described by Eq. (1), which is consistent with the theoretical derivation. With increasing injection current, the central wavelength of the ASE from the InP/InGaAsP MQW exhibits a blue shift, and the bandwidth of the ASE spectrum broadens.
At an operating temperature of 20 °C, the transmission spectrum of the fiber Bragg grating is shown in Figure 4: the central wavelength is 1549.38 nm, and the 3 dB bandwidth is 0.49 nm. When the fiber Bragg grating operates at 23 °C and the injection current of the semiconductor optical amplifier is 100 mA, the output spectrum of the fiber laser is shown in Figure 5: the central wavelength is 1549.66 nm (lasing state), the average output power is −5 dBm, the signal-to-noise ratio is 45 dB, and the laser linewidth is approximately 2.3 MHz.
With the operating temperature of the fiber Bragg grating fixed at 23 °C, the relationship between the laser output power and the injection current of the semiconductor optical amplifier is shown in Figure 6. The results indicate that the threshold injection current is 78 mA. At an injection current of 220 mA, a maximum output power of 1.524 mW is achieved, corresponding to an electro optical conversion efficiency of 1.1%. The relatively low efficiency is attributed to the high insertion loss of the connectors used in the experiment and the inherently low electro optical conversion efficiency.
The relationship between the central output wavelength of the laser and the operating temperature of the fiber Bragg grating is presented in Figure 7. As the FBG temperature increases from 8 °C to 28 °C, the central output wavelength shifts only from 1549.27 nm to 1549.59 nm, a total shift of 0.32 nm. This demonstrates that the laser exhibits excellent temperature stability.
With the operating temperature of the fiber Bragg grating fixed at 23 °C, the relationship between the laser output power and the injection current of the semiconductor optical amplifier is shown in Figure 6. The results indicate that the threshold injection current is 78 mA. At an injection current of 220 mA, a maximum output power of 1.524 mW is achieved, corresponding to an electro optical conversion efficiency of 1.1%. The relatively low efficiency is attributed to the high insertion loss of the connectors used in the experiment and the inherently low electro optical conversion efficiency.
The relationship between the central output wavelength of the laser and the operating temperature of the fiber Bragg grating is presented in Figure 7. As the FBG temperature increases from 8 °C to 28 °C, the central output wavelength shifts only from 1549.27 nm to 1549.59 nm, a total shift of 0.32 nm. This demonstrates that the laser exhibits excellent temperature stability.
5. Conclusion
The central wavelength of the amplified spontaneous emission from the InP/InGaAsP multiple quantum well structure exhibits a blue shift with increasing injection current, while the bandwidth of the ASE spectrum broadens accordingly. The key laser performance parameters obtained in this study are as follows: the lasing central wavelength is 1549.66 nm, and the threshold injection current is 78 mA. At an injection current of 220 mA, the maximum output power reaches 1.524 mW, the signal to noise ratio is 45 dB, and the electro optical conversion efficiency is 1.1%. The proposed laser demonstrates excellent temperature stability.
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Tianjin Janhoo Optoelectronics Co., Ltd. is a high-tech enterprise specializing in the R&D and manufacturing of chinese semiconductor optical amplifiers (SOAs). At present, the company has launched a full range of SOA products operating at wavelengths of 850 nm, 1060 nm, 1270 nm, 1310 nm, 1550 nm and 1625 nm, as well as reflective semiconductor optical amplifier (RSOA) gain chips available at 850 nm, 1310 nm and 1550 nm.
We have established a Class 10,000 cleanroom laboratory and equipped with a comprehensive set of equipment for the fabrication, testing and packaging of optical chips, boasting the capability of hybrid integrated micro-packaging for optical chips. Currently, the company is conducting R&D on hybrid integrated devices including NLL/ECL+SOA and high-power SOA devices. In addition, we provide contract services for the testing, packaging and processing of various optoelectronic devices for external customers.
