1. Introduction

Terahertz (THz) radiation sources have attracted considerable interest recently because of their potential applications to spectroscopy, communication, medicine, agriculture environmental monitoring, and security scan [1–3]. The combination of a photomixer and an optical beat source is a promising source for frequency-tunable continuous wave (CW) THz radiation. Optical beat sources are typically composed of two tunable distribution feedback (DFB) or distributed Bragg reflector (DBR) laser diodes (LDs) with different operation wavelengths [4–8]. However, considerable effort is required to stabilize the frequencies and match the two beams spatially. Recently, we used a monolithic 1.55-μm multi-section dual-mode DFB laser as a compact optical beat source for tunable CW THz radiation (0.17-0.49 THz) [9]. However, a wide tuning range beyond 0.5 THz has not been realized by typical 1.55-μm monolithic DFB lasers.

A dual-wavelength fiber laser is another promising candidate for a compact and low-cost optical beat source to obtain a wide tuning range for CW THz radiation. Various methods to fabricate fiber-based dual- or multi-wavelength lasers have been proposed and demonstrated [10–24]. Erbium-doped fiber (EDF) lasers are an attractive optical source for generating dual or multi wavelengths because they provide a wide gain bandwidth, low insertion loss, and high extinction ratio. However, EDF lasers exhibit the distinct characteristics of homogeneous gain broadening and multi-longitudinal-mode oscillation. The homogeneous gain broadening causes wavelength gain competition in EDF lasers. Therefore, it is not easy to achieve dual- or multi-wavelength lasing in EDF lasers even at room temperature. The gain-competition issue should be solved to achieve dual- or multi- wavelength lasing in EDF lasers. There are several approaches to solve this problem such as cooling the EDF in liquid-nitrogen [11,12], exploiting the polarization hole burning effect [13,14], and using a hybrid gain medium [15]. The multi-longitudinal-mode oscillation should be eliminated for practical applications such as narrow linewidth microwave or millimeter-wave beat signal generation. In order to obtain a single-longitudinal-mode oscillation in EDF lasers, several techniques have used an ultra-narrow bandpass filter [13,15–22]. Recently, a tunable and stable single-longitudinal-mode dual-wavelength EDF laser using Fabry-Perot laser diodes (FP-LDs) and fiber Bragg gratings (FBGs) has been proposed [16]. However, the spacing of the dual wavelengths is limited to within 1.3 nm.

In this paper, we propose a widely tunable dual-wavelength Er³⁺-doped fiber laser with FBGs and FP-LDs. Continuous tuning of the wavelength spacing is achieved from 3.2 nm to 9.6 nm. Using this laser as an optical beat source and a low-temperature-grown (LTG) InGaAs photomixer, we successfully demonstrate the generation of continuous THz radiations. The emitted CW THz frequency is continuously tuned from 0.3 to 0.8 THz.

2. Experiments

Figure 1 shows the experimental configuration for the operation of the proposed widely tunable and stable dual-wavelength EDF laser. It consists of a 10-m long EDF as a gain medium, two optical isolators for unidirectional operation, a 30% output coupler, three 3-dB optical couplers, two FBGs, and two FP-LDs monolithically integrated by a micro (μ)-heater. The EDF is pumped by a 980-nm laser diode (LD) through a 1550 nm/980 nm WDM coupler. The total cavity length of EDF laser is about 25 m, which corresponds to a longitudinal mode spacing of 8.15 MHz. The dual wavelengths are measured with an optical spectrum analyzer (OSA). The center wavelengths of the two FBGs are 1540 nm and 1546.4 nm, respectively. The reflectivity and 3-dB bandwidth of these FBGs are nearly 95% and 0.14 nm, respectively. In order to tune the reflected Bragg wavelength from the FBG, one side of the FBG is bound to the fixture while the other side is fixed to the translator stage, as shown in Fig. 1. The reflected optical signal from the FBG is self-injected into the FP-LD via the 3-dB fiber coupler. The FP-LDs are AR-coated in the front facet and HR-coated in the back facet. The reflectivities of the AR- and HR-coated of the FP-LDs are 0.5% and 89%, respectively.

 

Fig. 1 Experimental configuration for dual-wavelength operation

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The mode spacing and threshold current of the FP-LD used are 0.8 nm and 22 mA, respectively. In the experiment, the FP-LD is operated under the threshold injection current. Figure 2(a) shows the optical spectrum when the FP-LD1 is operated at 20 mA and 25 °C. The output power level of the FP-LD is below −40 dBm around wavelengths between 1535 nm and 1550 nm. The μ-heater on the FP-LD is used to continuously tune the lasing modes of the FP-LD. Figure 2(b) shows the output spectra of the FP-LD with an operation current of 20 mA and various currents for the μ-heater. The longitudinal modes of the FP-LD can be tuned continuously by changing the current applied to the μ-heater as shown in Fig. 2(b). There is some degradation as the longitudinal modes move to the longer wavelength. This is due to the thermal effect on the FP-LD as the applied current to the μ-heater is increased.

 

Fig. 2 (a) Optical spectrum of FP-LD1 with injection current of 20 mA, and, (b) output spectra of FP-LD1 at operation current of 20 mA and various currents for μ-heater

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The lights reflected from FBG1 and FBG2 are injected into the FP-LD1 and the FP-LD2, respectively, as shown in Fig. 1. Since the EDF laser is self injection-locked by the FP-LDs and FBGs, the gain competition disappears in the EDF laser and consequently, stable dual wavelengths can be generated in the EDF laser. When not using the FP-LDs in Fig. 1, only one wavelength experiences a gain due to the gain competition in the EDF. The 3 dB linewidth of the dual wavelengths is less than 0.1 nm.

The FBGs are strained by translators to control the reflected Bragg wavelengths. Therefore, the reflected wavelengths from the FBGs can be tuned. The wavelength tuning is performed by applying static strains to the FBGs through the translators. When the reflected wavelength of the FBG is tuned continuously, it should be matched with one of the FP-LD longitudinal modes. Then, the lasing wavelength is self-injection locked with one of the FP-LD modes. The FP-LDs used have 0.8 nm mode spacing. Therefore, a 0.8 nm tuning step is determined by the longitudinal-mode spacing of the FP-LDs used. Each wavelength can be independently tuned by applying static strain to the respective FBG. First, the maximum strain is applied to FBG1. Therefore the reflected wavelength of FBG1 is moved to the longest wavelength of 1543.2 nm. Next, FBG2 is continuously strained up to 1549.6 nm in order to move to the longer wavelength for the reflected wavelength of FBG2 while FBG1 is fixed at 1543.2 nm as shown in Figs. 3(a) –3(e). Then, the reflected wavelength of FBG1 moves to the shorter wavelength as the strain applied to FBG1 is released, as shown in Figs. 3(f)–3(i). Therefore, the spacing of the dual wavelengths at the EDF laser can be tuned widely. Figure 3 shows the widely tuned dual-wavelength output spectra according to the strained FBGs. The minimum and maximum spacings of the dual wavelengths are 3.2 nm and 9.6 nm, respectively. The tuning range is limited by the damage of FBG under strain. It should be noted that there is no side mode in the spectra. The side-mode suppression ratio (SMSR) is more than 35 dB for both wavelengths.

 

Fig. 3 Output spectra of widely tuned wavelengths according to strained FBGs

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The μ-heater is monolithically integrated to the FP-LDs in order to achieve fine tuning in the laser [9]. By applying current to the μ-heater on the FP-LD, the position of longitudinal modes of the FP-LD can be tuned continuously, as shown in Fig. 2(b). The reflected wavelength of FBG1 achieved by continuously applying a static strain can be simultaneously matched with the longitudinal mode of the continuously tuned FP-LD1. Figure 4 shows the output spectra of the finely and continuously tuned dual wavelengths over 0.8 nm by simultaneously applying current to the μ-heater on the FP-LD1 and straining the FBG1. For the wavelength tuning range, the SMSR is larger than 40 dB.

 

Fig. 4 Output spectra of finely and continuously tuned dual wavelengths from fiber laser

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In order to confirm the possibility of using this laser as a THz optical beat source, a photomixer chip based on an LTG InGaAs [25] is pumped by the dual wavelength fiber laser. Figure 5 shows the experimental setup for the CW THz generation using the LTG-InGaAs photomixer with the dual-wavelength EDF laser as an optical beat source. The output of the dual wavelength EDF laser is amplified through an erbium-doped fiber amplifier (EDFA) to increase the output power to 13 dBm. It is focused onto the photomixer gap through a free-space lens system. Thanks to the dual-wavelength EDF laser, no mechanical alignment between the two beams is needed. To fabricate the LTG-InGaAs photomixer, a 0.7-μm-thick Be-doped (1x10¹⁸ cm⁻²) InGaAs layer is grown on a semi-insulating InP substrate at 250°C using a molecular beam epitaxy (MBE) system. Then, the wafer is in situ annealed in the same MBE chamber at 550°C in an AsH₃ environment. A broad-band bowtie antenna and an interdigitated gap structure are simultaneously defined by using a stepper system followed by the evaporation of the Ti/Au metals. The fabricated InGaAs photomixer chip is mounted onto a collimating Si lens for the THz emission experiments. A cryogenic bolometer operating at 4.2 K is used to measure the THz emission power. In order to enhance the detection of the THz signal, a lock-in amp is used with a sinusoidal function generator, which modulates the photomixer bias.

 

Fig. 5 Experimental setup for CW THz generation using LTG-InGaAs photomixer based on dual-wavelength fiber laser

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Figure 6 shows the continuous frequency tuning of the CW THz radiation up to 0.8 THz using the LTG-InGaAs photomixer with the dual-wavelength EDF laser as an optical beat source. The number of interdigitated fingers, the dark resistance of the photomixer, the total optical input power, and the operation bias voltage on the photomixer are 3, ~40 kΩ, ~20 mW, and 2.4 V, respectively. The THz output power falls to the thermal radiation level of the photomixer beyond 0.8 THz possibly due to the long carrier lifetime (~7 ps) of the LTG-InGaAs layer, as well as the background amplified spontaneous emission (ASE) from the EDFA. The detected THz output power from the bowtie-antenna photomixer is about 1 nW at 0.3 THz in this measurement. In the inset of Fig. 6, one can see the clear square-law property of the bias dependent THz output power at the input beating frequency of 0.4 THz. The single longitudinal mode in the dual-wavelength laser would be essential to obtain high quality THz radiation. It can be realized using an unpumped EDF segment in the laser cavity. Furthermore, we believe that if material and process optimizations are achieved for the InGaAs photomixers, the THz output power would be greatly enhanced more than one order of magnitude with our dual-wavelength fiber laser.

 

Fig. 6 Continuous tuning of CW THz emission using LTG-InGaAs photomixer with dual-wavelength fiber laser. (Inset: square-law property of bias dependent THz output power at input beating frequency of 0.4 THz.)

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3. Summary

The operation of a widely tunable and stable dual-wavelength EDF laser that uses two FP-LDs and two FBGs was successfully demonstrated. The tuning range of 3.2 nm to 9.6 nm was achieved for the dual-wavelength spacing. The SMSR was more than 35 dB for both wavelengths. In addition, fine wavelength tuning was achieved by using the integrated μ-heater on the FP-LD. Each wavelength could be independently and finely tuned with an FP-LD and an FBG. Tunable CW THz emission from an LTG-InGaAs photomixer using the dual-wavelength fiber laser as an optical beat source was demonstrated. The emitted CW THz frequency is continuously tuned from 0.3 to 0.8 THz. The tuning range could be extended by employing a widely tunable filter such as a polymer Bragg grating.